-1750
TRANSDUCERS
AND
INSTRUMENTATION
TRAINER
Curriculum Manual
IT01
(INDIA) LIMITED
(INDIA) LIMITED
-1750
TRANSDUCERS
AND
INSTRUMENTATION
TRAINER
Curriculum Manual
IT01
Corporate Office :
Dynalog (India) Ltd.,
Kailash Vaibhav, G-Wing, 3rd Floor,
Park Site, Behind Godrej Colony,
Vikhroli (West), MUMBAI 400079.
Tel. : 022 - 2518 1900 (16 Lines),
Fax : 91 - 22 - 2518 1930 / 40 / 50,
E-mail : sales@dynalogindia.com
Branch Office :
Dynalog (India) Ltd.,
203, "Corporate Plaza", 106-A,
S.B.Road,Near Chatushrungi Temple,
Shivaji Nagar, PUNE 411 016. INDIA.
Tel. : 020 - 2563 1081.
Fax : 91 (20) 25638 333
E-Mail : pune@dynalogindia.com
IT 01
Curriculum Manual
Transducers and Instrumentation Trainer
Addendum Sheet
Addendum Sheet
Please note that the following warning label has now been added to the
DYNA1750 trainer.
This is to indicate the area of moving parts, and that figures should be kept
clear.
!
Keep figures clear of all
Moving parts
Technical Publication Department
Dynalog (India) Ltd.
Dynalog (India) Ltd.
Transducers and Instrumentation Trainer
Addendum Sheet
IT 01
Curriculum Manual
Dynalog (India) Ltd.
IT 01
Curriculum Manual
Chapter
Transducers and Instrumentation Trainer
Contents
Contents
Pages
Introduction …………………………………………………………………... i - iv
Basic Control Systems
Chapter 1
Basic Control Systems Equipment and Terms Used..... 1 - 12
Input Transducers
Chapter 2
Positional Resistance Transducers………………….....13 - 26
Chapter 3
Wheatstone Bridge Measurements………………....….27 - 44
Chapter 4
Temperature Sensors……………………………...…...45 - 66
Chapter 5
Light Measurement..……………………………...…...67 - 86
Chapter 6
Linear Position or Force Applications.…………...….87 - 100
Chapter 7
Environmental Measurement………..…………...….101 - 110
Chapter 8
Rotational Speed or Position Measurement……...….111 - 134
Chapter 9
Sound Measurements………………..…………...….135 - 142
Output Transducers
Chapter 10
Sound Output………………………..…………...….143 - 150
Chapter 11
Linear or Rotational Motion………...…………...….151 - 166
Display Devices
Chapter 12
Display Devices……………………..…………...….167 - 182
Signal Conditioning Circuits
Chapter 13
Signal Conditioning Amplifiers……..…………...….183 - 206
Chapter 14
Signal Conversions………………….…………...….207 - 222
Chapter 15
Comparators, Oscillators and Filters..…………...….223 - 238
Chapter 16
Mathematical Operations…………...…………...….239 - 256
Dynalog (India) Ltd.
Transducers and Instrumentation Trainer
Contents
IT 01
Curriculum Manual
Closed Loop Control Systems
Chapter 17
Control System Characteristics……..…………...….257 - 264
Chapter 18
Practical Control Systems………….…………...…..265 - 294
Appendices
Appendix A
Using a Multimeter………….……..…………...….295 - 300
Appendix B
The Oscilloscope…………….……..…………...…301 - 322
Dynalog (India) Ltd.
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Curriculum Manual
Transducers and Instrumentation Trainer
Introduction
Introduction
Introduction
This comprehensive course of study is based on a single panel Transducer and
Instrumentation Trainer, the DYNA1750.
The DYNA-1750 unit provides examples of a full range of input and output
transducers, signal conditioning circuits and display devices.
The unit is self-contained and enables the characteristics of many individual
devices to be investigated, building to form complete closed loop systems.
As each item is introduced there is a description of the principles of the
device, together with practical exercises to illustrate its characteristics and
applications.
The treatment is non-mathematical and little previous knowledge is assumed,
although it is expected that students will have a basic knowledge of electrical
circuits and units, and electronic components and devices.
It is the intention that at the end of this course the student will, with the
knowledge gained, be able to select suitable components and interconnect
them to form required closed-loop systems.
Although the course has been laid out progressively it is sometimes necessary
to make use of a device before a full investigation has been carried out. For
instance, in order to investigate any input transducer, an input signal may be
needed. This signal may be provided by one of the output transducers not yet
covered. Also signal conditioning and display devices will be needed from an
early stage. In the event of any difficulty, it is recommended that the student
should skip forward to the relevant section to obtain further information.
Dynalog (India) Ltd.
i
Transducers and Instrumentation Trainer
Introduction
IT 01
Curriculum Manual
Test Instruments
It is recommended that a digital multimeter is available for use with this
module. The meter must have ranges to cover at least :
DC voltage : 200 m V to 20 V
DC current : 1 mA to 100 mA
Resistance : 10 ? to 10 M?
The complete the exercises you will need to be familiar with connecting,
setting the range and obtaining readings from multimeters. If you are not
familiar with the use of these instruments please refer first to Appendix A
before carrying out any exercises.
Some examinations of voltage waveforms will be called for using a cathode
ray oscilloscope. You will be expected to be able to make the necessary
adjustments and setting to obtain time related sketches of the waveforms
examined. Recommendations for the setting of the various controls will be
given where appropriate. Again, if you are not familiar with this instrument or
the applications of it, please refer to Appendix B before attempting the
relevant exercise.
A functions generator will be required to provide sinewave and square wave
inputs to some circuits. This should have a range of frequencies covering at
least 10Hz to 1 MHz, and output of 20Vp-p (with an internal attenuator to
allow amplitude settings), and an output impedance of 50? . The output lead
should be terminated in standard 4mm banana plugs for ease of connection
directly to DYNA-1750 Trainer panel.
ii
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Curriculum Manual
Transducers and Instrumentation Trainer
Introduction
The Module Power Supplies
The DYNA-1750 Transducer and Instrumentation Trainer contains all of the
power supplies needed to make it operate. You can switch these power
supplies ON and OFF with the Power Supplies switch located on the rear
panel.
Making Circuit Connections
During each Practical Exercise in this manual, you will be asked to make
circuit connections using the 4 mm Patching Cords. Whenever you make (or
change) circuit connections, it is good practice to always do so with the Power
Supplies switch in the OFF position. You should switch the Power Supplies
ON only after you have made, and checked, your connections.
Remember that the Power Supplies switch must be ON in order for you to be
able to make the observations and measurements required in the Exercise.
At the end of each Exercise, you should return the ‘Power Supplies’ switch to
the ‘OFF’ position before you dismantle your circuit connections.
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Transducers and Instrumentation Trainer
Introduction
iv
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Curriculum Manual
Dynalog (India) Ltd.
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Curriculum Manual
Basic Control Systems Equipment and Terms Used
Chapter 1
Chapter 1
Basic Control Systems Equipment and Terms Used
Objectives of
This Chapter
Dynalog (India) Ltd.
Having studied this Chapter you will be able to :
1
State the difference between open loop and closed
loop systems.
2
Write the expression for the overall gain of a negative
feedback closed loop system.
3
Calculate the overall gain of a negative feedback
closed loop system from given information
4
List the basic components of a closed loop system and
explain their functions.
5
Explain the meaning of terms associated with control
system equipment.
1
Basic Control Systems Equipment and Terms Used
Chapter 1
1.1
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Curriculum Manual
Open Loop System
Figure 1.1 represents a block diagram of an open loop system. A reference
input, or command signal, is fed to an actuator which operates on the
controlled variable to produce an output.
Reference I/P
O/P
Actuator
Actuator
(Command Signal)
Fig. 1.1
The output magnitudes depends on the magnitude of the reference input signal
but the actual output magnitude for a particular input may not remain constant
but may vary due to changes within or exterior to the system.
For example, in a simple room heating application, a heater set for a certain
output will result in a certain room temperature. The actual temperature will
depend on the ambient temperature outside the room and also wheather the
doors and windows are open or closed.
2
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Curriculum Manual
1.2
Basic Control Systems Equipment and Terms Used
Chapter 1
Closed Loop System
Figure 1.2 shows a basic block diagram of a closed loop control system.
With this system, the output magnitude is sensed, fed back and compared with
the desired value as represented by the reference input. Any error signal is fed
to the actuator to vary the controlled variable to reduce this error.
Reference I/P
Error
Detector
Actuator
Error
Detector
O/P
Sensors
Feedback
Signal
Fig. 1.2
The system thus tends to maintain a constant output magnitude for a fixed
magnitude input reference signal. The feedback signal is effectively subtracted
from the reference signal input to obtain the error signal and hence the system
is referred to as a negative feedback system.
The magnitude of the reference signal required for a particular output
magnitude for a closed loop system will be greater than that required for open
loop operation because the negative feedback reduces the overall gain of the
system.
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Basic Control Systems Equipment and Terms Used
Chapter 1
1.3
IT 01
Curriculum Manual
Gain in an Open Loop System
Gain
G
Input Vi
Output Vo
Fig. 1.3
Output Vo = G Vi
1.4
Gain = G
Gain in a Closed Loop System
Error
Input Vi
(Vi-Hvo)
Gain
G
Feedback (Hvo)
Attenuator
H
Output Vo
Fig. 1.4
H = the fraction of the output fed back to the input
The error signal
= Vi – Hvo
The output Vo
= G(Vi-HVo)
= GVi – GHVo
Vo + GHVo = G Vi
Vo = G___
Vi
1+GH
i.e.
4
Gain = G
1+GH
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Curriculum Manual
Basic Control Systems Equipment and Terms Used
Chapter 1
The Gain is therefore reduced, and, if the gain G is very large, the formula
simplifies to :
G 1
Gain = =
GH H
IF the gain of the amplifier (G) is high then the overall system gain is
dependent only on the feedback fraction H.
1.5
Examples
(i)
An amplifier has a gain (G) of 15 and a feedback loop with an
attenuation fraction (H) of 1/30
Vo
The loop gain of the system
will be:
Vi
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5
Basic Control Systems Equipment and Terms Used
Chapter 1
1.6
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Curriculum Manual
Practical Closed Loop Control System
Figure 1.5 shows a block diagram of a practical closed loop control system. This shows
signal conditioning blocks in the signal paths between the error detector and the
actuator and between the sensor and the error detector.
Reference I/P
O/P
Error
Detector
Signal
Conditioning
Signal
Conditioning
Controlled
Variable
Actuator
Sensor
Signal
Conditioning
Display
Fig 1.5
It also shows a display which indicates the magnitude of the output variable
and includes a signal conditioning block in the display path.
Signal conditioning may consist of signal amplification, attenuation or
linearising, waveform filtering or modification, conversion from analog to
digital form, or may be a matching circuit. These may be necessary to convert
the output from one circuit into a form suitable for the input to the following
circuit, or to improve the system accuracy.
6
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Curriculum Manual
1.7
Basic Control Systems Equipment and Terms Used
Chapter 1
Controlled Variables
For a particular industrial process there may be more than one
controlled variable and each of the controlled variable will have its
own closed loop control system.
The controlled variable may be:Position (angular or linear)
Temperature
Pressure
Flow rate
Humidity
Speed (angular or linear)
Acceleration
Light level
Sound level
The control system may operate using pneumatic, hydraulic or
electrical principles and the sensors used for the measurement of the
controlled variable must provide an output signal in a form suitable for
the system in use.
This will normally involve a conversion from one energy system to
another and devices used to accomplish this energy conversion are
referred to a TRANSDUCERS. Sensors and actuators are both forms
of transducer, sensors representing input transducers and actuators
representing output transducers.
The DYNA-1750 unit is an electrical system and includes a full range
of sensors, actuators, signal conditioning circuits and display devices.
Used with this manual, the unit will introduce the student to the basic
principles and characteristics of a comprehensive range of transducers
and their application to practical closed loop control systems.
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7
Basic Control Systems Equipment and Terms Used
Chapter 1
IT 01
Curriculum Manual
A layout diagram of the DYNA-1750 unit is shown below in fig 1.6
8
Dynalog (India) Ltd.
IT 01
Curriculum Manual
1.8
Basic Control Systems Equipment and Terms Used
Chapter 1
Glossary of Terms – Transducers
Transducer :
A device which converts information from one energy
system to another.
Sensor :
A device which senses, or measures, the magnitude of
system variables. Normally they also convert the
measured quantity into another energy system and
hence they are also transducers.
Actuator :
A device which accepts an input in one system and
converts it into another energy system, which is
normally mechanical. These devices are also
transducers.
Specification :
Data specifying the performance capabilities and
requirements of equipment.
Accuracy :
The error present in a measurement as compared to the
true value of the quantity.
Sensitivity :
The ratio of the output of a device compared to the
magnitude of the input quantity.
Resolution :
The largest change in the input that produces no
detectable change in the output; for example, the degree
to which a system can distinguish between adjacent
values or settings.
Range :
A statement of the values over which the device can be
used and within which the accuracy is within the state
specification.
Bandwidth :
The range of input signal frequencies over which a
device or circuit is capable of being operated while
providing an output within its stated specification.
Transfer function : The mathematical relationship between two variable
that are related. Normally the relationship between the
input and output of a system.
Linear :
A relationship between two quantities that have a
constant ratio; for example, a graphical straight line
relationship.
Non linear :
A relationship between two quantities that cannot be
described by a linear relationship.
Linearity :
A measure of the deviation of a measurement from an
ideal straight line response of the same measurement
over the same range.
Response Time :
The time taken for the output to reach, or be within a
rated percentage of, a new final value, after the input
has been changed.
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9
Basic Control Systems Equipment and Terms Used
Chapter 1
1.9
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Curriculum Manual
Glossary of Terms – Signal Conditioning Circuits
Amplifier :
A circuit having an input and output that are related
linearly and with the output grater that the input. The
circuit may operate on both DC and AC circuits.
Offset :
For a DC amplifier, with the input zero, the output may
not be zero. This is referred to as the offset. With these
amplifiers, a control is provided and labeled : “Offset”
or “Set Zero” to set the output to zero with the input
zero, before the amplifier is used.
Gain :
The ratio of output to input for a circuit.
Attenuator :
A circuit having an input and an output that are related
linearly and having an output less than the input.
AC Amplifier :
An amplifier that will amplify alternating signals only.
Differential amplifier: A voltage amplifier having two inputs and where the
output voltage magnitude is proportional to the
difference in voltages between the two inputs.
Summing Amplifier: A voltage amplifier having multiple inputs, the output
being proportional to the sum of the various applied
inputs.
Inverter :
A voltage amplifier having the polarity of the output the
reverse of the input. The output magnitude may be the
same as the input (gain of –1), or there may be voltage
gain associated with the polarity reversal.
Power Amplifier : An amplifier with a large current output capability.
Buffer Amplifier : An amplifier having unity gain (output = input), and
having a high input impedance and a low output
impedance.
Comparator :
A circuit having two inputs A & B and an output that
can be in one of two possible states depending on the
magnitude of the inputs.
With input A greater than B, the output will be in one
state (possibly high voltage). With input A less than B,
the out will be in the alternative state (low voltage).
Oscillator :
A circuit producing an alternating output at a particular
frequency.
Alarm Oscillator : A circuit having an input and an output. With the input
magnitude below a certain level, the output is zero.
When the input exceeds the threshold the output is an
alternating voltage.
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Basic Control Systems Equipment and Terms Used
Chapter 1
Hysteresis :
The transfer characteristic of a non-linear device for
increasing input voltages may be different from the
characteristic for decreasing input voltages. The result is
a ‘hysteresis loop’, as shown in figure 1.7 (a) below.
For a switching circuit, the term ‘hysteresis’ normally
refers to the input switching voltages. The input to
cause switching for rising input voltages is arranged to
be higher than that to produce switching for falling
input voltages (see figure 1.7 b) below). The difference
between the input voltages is referred to as the
hysteresis.
Latch :
A circuit having two possible output states depending
on the magnitude of the input voltage. When operated
with the input level sufficient to change the output to its
alternative state, the output is held (or latched) in this
state irrespective of the subsequent magnitude of the
input voltage.
Filter :
Circuit designed to allow signals of a selected frequency
range to pass through and stop all others.
Low Pass Filter :
A circuit allowing low frequency signals to pass while
blocking the passage of higher frequencies.
High Pass Filter :
A circuit allowing high frequency signals to pass while
blocking the passage of higher frequencies.
Dynalog (India) Ltd.
11
Basic Control Systems Equipment and Terms Used
Chapter 1
Band Pass Filter :
IT 01
Curriculum Manual
A circuit allowing signals over a selected range of
frequencies to pass while blocking the passage of
signals at both lower and higher frequencies.
Full-Wave Rectifier: A circuit converting an alternating waveform into a
unidirectional or DC waveform.
12
V/F Converter :
A circuit converting a DC input voltage to an
alternating voltage, the frequency being dependent on
the magnitude of the DC input voltage.
F/V Converter :
A circuit converting an alternating input voltage to a
direct voltage output, the output voltage magnitude
being proportional to the frequency of the input voltage.
V/I Converter :
A circuit converting an direct input voltage into an
output current, the current magnitude depending on the
input voltage.
I/V Converter :
A circuit converting an input current into an output
voltage, the voltage magnitude being dependent on the
magnitude of the input current.
Integrator :
A circuit having an output voltage that is proportional to
the product (input voltage x time)
Differentiator :
A circuit having an output voltage that is proportional to
the rate-of-change of the input voltage.
Sample and Hold :
A circuit with input and output. In the sample state, the
output voltage is equal to and follows the input voltage.
In the hold state, the output voltage is held at the value
of the input signal at the instant the “hold” signal was
initiated.
Ultrasonic :
A signal at a frequency above the normal audio range
and hence inaudible to the human ear (normally >
16kHz)
Dynalog (India) Ltd.
IT 01
Curriculum Manual
Positional Resistance Transducers
Chapter 2
Chapter 2
Positional Resistance Transducers
Objectives of
this chapter
Having studied this chapter you will be able to:
1
2
3
4
5
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
1
2
3
Describe the basic construction of rotary and slider variable
resistors.
State that the resistance section may be either a carbon track
or wirewound.
Describe the difference between a logarithmic and a linear
track.
Draw the basic characteristics of output voltage against
variable control setting.
Compare the application of a carbon track variable resistor
to the wirewound type.
DYNA-1750 Transducer and Instrumentation Trainer
4mm Connecting Leads.
Digital Multimeter.
13
Positional Resistance Transducers
Chapter 2
2.1
IT 01
Curriculum Manual
Variable Resistor Construction
A variable resistor of a “track” having a fixed overall resistance with a
“wiper” which can be moved to make contact with any point along the
track.
In the carbon type, the total track resistance is varied by adjusting the
proportion of non-conducting material to carbon in the compound
during manufacture. This will produce a track of constant resistance
along its length, so that any section of the track will have the same
resistance as any other similar section. The track will be linear.
Variable resistors intended for use in audio applications, where
subjective appreciation of sound amplitude (loudness) is proportional
to logarithmic scales, are made with similar logarithmic (non-linear)
scales. The resistance along the track in not a linear relationship,
increasing with the square of the rotation of the spindle, or movement
of the slide wiper (R ? S2, where S is the setting of the wiper) A close
approximation is made to the ideal logarithmic characteristic by using
three or four sections of track with different resistance slopes.
Non-Linear variable resistors are not suitable as positional transducers
and are therefore not included on the DYNA-1750 Trainer facilities.
The track can be laid out on a rotary or a straight base, as in Fig 2.1.
For higher power applications the track may be wire wound, with the
wiper making contact with the top edge of a coil of resistance wire.
14
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Curriculum Manual
2.2
Positional Resistance Transducers
Chapter 2
Linear Variable Resistor Characteristics
A variable resistor can be used to provide a variable voltage. A steady
voltage is applied across the ends of the fixed track. The wiper then
picks off a variable voltage at the contact point with the track (with
respect to the end of the track). Used in this way the variable resistor is
called a potentiometer.
With a dual polarity voltage source, the polarity and magnitude of the
output voltage will depend on the direction of movement of the wiper
from its central position, as shown in Fig 2.3.
Note that the position of the variable resistor spindle (or slider) setting
is indicated by the output voltage from the potentiometer.
Dynalog (India) Ltd.
15
Positional Resistance Transducers
Chapter 2
2.3
Practical Exercise
Variation of Output Voltage with Setting of Rotary Potentiometer
1
2
3
4
Control Setting
Output Voltage
5
16
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Curriculum Manual
Locate the 100kO variable resistor on the DYNA-1750 Trainer
(bottom left-hand corner). Connect the circuit as shown in Fig
2.4 using the power supply facilities at the bottom of the panel
and the 20V DC range of a digital multimeter.
Set the 100kO rotary resistor control fully counter-clockwise to
setting 1 as shown Fig 2.4. Note that the dial is not marked with
numbers on the printed panel. These numbers have been shown
in Fig 2.4 to make it easier to follow these instructions and
collate results.
After ensuring that the voltage adjustment is correctly set
switch ON the power supply (switch on the rear of the unit just
above the main power socket).
Note the output voltage as indicated on the digital multimeter
and record in Table 2.1.
1
V
2
V
3
V
4
V
5
V
6
V
7
V
8
V
9
V
10
V
Set the rotary control to “2” and repeat the reading, recording
the result in again Table 2.1
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Curriculum Manual
Positional Resistance Transducers
Chapter 2
6
Repeat the reading and recording for all other settings of the
rotary control.
7
From the results recorded in Table 2.1 above plot the
characteristic of the 100kO variable resistor on graticule of
Graph 2.1 below.
12
11
10
Output 9
Voltage
(volts) 8
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10
Resistor setting
Graph 2.1 Characteristic of a Linear Rotary Carbon Potentiometer
Note that it is not easy to be precise with your setting of the variable
resistor and this may result in the plotted points not following a
smooth relationship. You should draw the best compromise to show
the characteristic as you believe that it should be. At the ends of the
track the wiper comes into contact with the terminal connections to
the track, causing non-linearity at both ends. From setting 2 through
setting 9 the variation of voltage should be fairly linear.
Voltage across this section (V9 – V2) =
V
V9 – V2
Voltage per division (
V
)=
9-2
Dynalog (India) Ltd.
17
Positional Resistance Transducers
Chapter 2
2.4
Practical Exercise
Variation of Output Voltage with setting of Slide Potentiometer
1
The 10kO slide potentiometer on the DYNA-1750 Trainer is
just above the rotary potentiometers. Connect the circuit as
shown in Fig 2.5 using the power supply facilities at the bottom
of the panel and the 20V DC range of your digital multimeter.
2
Set the 10kO slide resistor control to the left to setting 1 as
shown in Fig 2.5. Note that the marked numbers are again not
no the printed panel.
3
4
Switch ON the power supply.
Note the output voltage as indicated on the digital multimeter
and record in Table 2.2.
Control Setting
Output Voltage
5
6
18
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Curriculum Manual
1
V
2
V
3
V
4
V
5
V
6
V
7
V
8
9
V
10
V
V
Set the control to “2” and repeat the reading.
Repeat the readings for all other settings of the slide control,
recording the result in Table 2.2
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Curriculum Manual
7
Positional Resistance Transducers
Chapter 2
From the results recorded in Table 2.2 plot the characteristic of
the 10kO slide resistor with dual polarity supply on graticule of
Graph 2.2 below.
+5
+4
+3
+2
Output
Voltage
(volts)
+1
0
-2
-3
4
-5
1
2
3
4
5
6
7
8
9
10
Resistor setting
Graph 2.2 Characteristic of a Linear Slide Carbon Potentiometer
8
Switch OFF the power supply and remove the connections
between the slide potentiometer and the power supply panels.
9
Use the digital multimeter on a suitable range (20kO) to
measure the resistance between terminal A and wiper B with
the wiper set to position 9:
Resistance R9 =
10
kO
Move the wiper to position 2 and repeat the resistance
measurement:
Resistance R2 =
kO
Resistance between settings 9 & 2 = R9 – R2 =
kO
Voltage between setting 9 & 2 = V9 – V2 =
V
V9 – V2
Voltage per kO =
=
V/kO
(R9 – R2) kO
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19
Positional Resistance Transducers
Chapter 2
2.5
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Curriculum Manual
Effect of Loading
Consider a 10kO variable resistor connected to a 10V supply with the
wiper in its central position. There will be a resistance of 5kO from the
wiper to each end of the track (Fig 2.6(a)).
If a 5kO fixed resistor is connected across the output then it will be in
parallel with the lower half of the potentiometer (Fig 2.6(b)) and will
draw current through the upper half of the potentiometer. This cause a
higher voltage drop across the upper half of the track than the lower
half (Fig 2.6(c)).
Another way of looking at this is that the shunting effect of the 5kO
load resistor is to reduce the total resistance of the lower half to 2.5kO
(Fig 2.6 (c)). Only one third of the applied voltage will be dropped
across the lower half and two thirds across the upper.
The variations of resistance as the wiper is moved will be quite
complex and the voltage at the output will be non-linear.
20
Dynalog (India) Ltd.
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Curriculum Manual
2.6
Positional Resistance Transducers
Chapter 2
Practical Exercise
Effect of Loading on the Potentiometer Output Voltage
1
With the power supply switched OFF and no connections
made to any components, measure the resistance of the 100kO
rotary variable resistor between contact A and the wiper as it is
set to the marked points on its scale. Use a suitable scale
(200kO) on your digital multimeter and record the results in
Table 2.3 overleaf in the row marked “Load Resistance”.
The 100kO resistor is to be used as a load resistance across the
output of a 10kO position sensing variable resistor.
2
3
4
5
Dynalog (India) Ltd.
Connect the circuit as shown in Fig 2.7 but initially leave out
the lead from contact C of the 100kO resistor to contact B of
the 10kO so that the load is not connected across the output.
Switch the power supply ON and adjust the 10kO rotary
resistor to give an output of 6V.
Do not re-adjust this setting during the rest of this
exercise.
Set the 100kO resistor fully clockwise (10) and connect the
missing lead from contact C of the 100kO resistor to contact B
of the 10kO so that the load is connected across the output of
the positional sensor (10kO resistor).
Note the output voltage and record in Table 2.3.
21
Positional Resistance Transducers
Chapter 2
9
IT 01
Curriculum Manual
Control Setting
10
8
7
6
5
4
3
2
1
Output Voltage
V
V
V
V
V
V
V
V
V
V
Load Resistance
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
Table 2.3
6
7
22
Change the setting of the 100kO load resistor and record the
effect as the load resistor is set to each marked position in
Table 2.3
From the information in Table 2.3, plot the characteristic of
Output Voltage against Load Resistance on the graticule of
Graph 2.3 below:
Dynalog (India) Ltd.
IT 01
Curriculum Manual
Positional Resistance Transducers
Chapter 2
Do not alter the setting of the 10kO resistor.
8
9
With the load Resistance (100kO resistor) removed from circuit
connect the panel mounted Moving Coil Meter as in Fig 2.8
and switch ON the power supply.
Note the effect on the output voltage reading of having the
analog type meter connected in circuit as well as the digital
multimeter.
Multimeter voltage reading with the Moving Coil Meter
connected =
10
V
Compare this reading with the results on the characteristic
curve of Graph 2.3 and read off the graph the loading resistance
presented by the Moving Coil Meter to the output:
Loading resistance of the Moving Coil Meter =
kO
What you have observed here is a problem which can be very
misleading if you are not aware of the difficulties of using a low
impedance meter to take measurements in a high impedance circuit.
The problem can be overcome by using a Buffer Amplifier.
11
Modify the circuit to include Buffer #1 as in Fig 2.9 and note
the effect on the output voltage as indicated by both meters
Output voltage =
Dynalog (India) Ltd.
V
(digital)
V
(analog)
23
Positional Resistance Transducers
Chapter 2
2.7
IT 01
Curriculum Manual
Resolution
Resolution has been defined as the largest change in the input which
does not cause a change in the output. Alternatively it can be defined
as the smallest change in input which does cause a change in output.
For the carbon track resistor this value is very small since the
individual particles of carbon are tiny and variations of resistance can
be considered to be infinitely small. The resolution for a wirewound
resistor is not so good, since, as the wiper is moved, it has to jump
from one turn of the wire coil to the next.
The output voltage therefore increases in steps equal to the applied
voltage divided by the number of turns if the wiper only makes contact
with one turn at a time.
This may not be quite the case, since the wiper may make contact with
two or more turns at once as in Fig 2.10(b). The mathematical
treatment of this will depend on the thickness of the wire (power
rating) and the size of the wiper contact (current rating).
Multi-turn wirewound tracks will largely overcome this problem.
2.8
24
Comparison of Carbon with Wirewound Track
Carbon
Cheap
Good Resolution
Can be made miniature
Wirewound
High Current Ratings
Durability (Reliability)
Dynalog (India) Ltd.
IT 01
Curriculum Manual
2.9
Positional Resistance Transducers
Chapter 2
Practical Exercise
Servo Potentiometer
A special positional potentiometer is mounted on the experiment board
O
which has a very large are of turning, approaching 360 . It is called a
Servo Potentiometer.
The potentiometer can then be turned manually with the shaft, using
one of the large wheels, such as the Hall Effect sensor Disk. The
potentiometer can be turned directly from the dial, manually, if
preferred.
The ±5V input voltages to the Servo Potentiometer are connected
internally.
1
2
Dynalog (India) Ltd.
Connect a digital multimeter on the 20V DC range to the output
of the potentiometer as shown in Fig 2.11.
Turn the potentiometer to find the maximum positive output
voltage position. Note the value of this voltage and the angle,
as given on the potentiometer dial, in the first column of Table
2.4 overleaf.
25
Positional Resistance Transducers
Chapter 2
Control Dial
Setting
Output
Voltage
150 120
V
V
V
90
V
IT 01
Curriculum Manual
60 30
V
V
360
0
V
330
-30
V
300
-60
270
-90
240
-120
V
V
V
210
-150
V
V
Table 2.4
3
4
5
26
Rotate the dial in steps of 30O clockwise from the maximum
voltage position (beginning with 150O), noting the output
voltage at each step and recording the values in Table 2.4.
At the final step note the angle from the dial setting and the
value of the maximum negative voltage setting.
From the information recorded in Table 2.4, draw the
characteristic of the output voltage/dial setting of the servo
Potentiometer on the practical provided below:
Dynalog (India) Ltd.
IT 01
Curriculum Manual
Wheatstone Bridge Measurements
Chapter 3
Chapter 3
Wheatstone Bridge Measurement
Objectives of
this chapter
Having studied this chapter you will be able to:
1
2
3
4
5
6
7
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
1
2
3
State the principles of the basic Wheatstone Bridge
circuit for resistance measurement.
Describe the term “null balance”.
State and apply the expression for calculating an
unknown resistance from the Bridge values at
balance.
Discuss the factors affecting the resolution and
accuracy of measurements.
Discuss the reason for the three-wire resistance
circuit.
Apply null methods to voltage measurements.
Make resistance and voltage measurements using
the DYNA-1750 facilities
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Digital Multimeter.
27
Wheatstone Bridge Measurements
Chapter 3
3.1
IT 01
Curriculum Manual
Wheatstone Bridge Circuit
Fig 3.1 shows the basic Wheatstone Bridge circuit, consisting of four
resistors and a sensitive center zero meter connected to a DC source.
R1, R2 & R3 are accurate, close tolerance, resistor. R3 is variable and
calibrated over its full range. R4 is the unknown resistor to be
measured.
3.2
Null Balance
During measurement, R3 is adjusted until there is no current (Im)
flowing in the galvanometer circuit. The galvanometer current is zero
or “null balance”. The purpose of the galvanometer is to “detect” the
presence of the null condition.
From the known values of R1, R2 & R3 at balance, the value of R4 can
be calculated from :R2
R4 =
X R3
R1
The ratio of the values of resistors R2:R1 sets the range, so that values
of the unknown resistor R4 which are larger or smaller than the
variable resistor R3 can be measured. There is no limit to the range of
values which can be measured.
Any inaccuracy in the values of the ratio arm resistors R1 & R2, and
also in the standard variable resistor R3, will result in error in the
measured value of R4.
Since no current flows in the “null detector” branch at balance no error
can be introduced by this part of the circuit.
28
Dynalog (India) Ltd.
IT 01
Curriculum Manual
3.3
Wheatstone Bridge Measurements
Chapter 3
Deriving the Formula
With no current in the galvanometer circuit, the voltages at either end
of it must be the same. This means that the voltages across R1 & R2
must be the same and similarly those across R3 & R4.
With no current in the galvanometer, the current in R1 must the same
as that in R3 and the current in R2 must equal that in R4.
If current I1 flows in R1 & R3 and current I2 flows in R2 & R4 :I1R1 = I2R2 --------------------------------------------------(i)
I1R3 = I2R4 --------------------------------------------------(ii)
Dividing (i) ÷ (ii)
I1R1 I2R2
=
I1R3 I2R4
?1
R1
R2
=
R3
?2
R4
R2
X R3
R1
The unknown resistance R4 depends on the ratio R2:R1 and the value
of R3 at balance. The resistors R1 and R2 are normally referred to as
the “ratio arms” of the bridge.
Note
R4 =
1. The value of the supply voltage or the magnitude of the
currents flowing in the resistors does not affect the result. This
means that the supply voltage need not be stabilized, and that
the circuit currents can be kept to low values for a component
where the self heating effect of the current flowing could affect
the result.
2. The galvanometer current accuracy is unimportant, since,
under balanced conditions, the current in it is zero. The main
characteristics required for the galvanometer are a low
resistance and a high sensitivity so that a small deviation of
voltage from zero produces a large scale reading.
Dynalog (India) Ltd.
29
Wheatstone Bridge Measurements
Chapter 3
3.4
IT 01
Curriculum Manual
The Three wire Resistance Measuring Circuit
With some resistance transducer circuits, the transducer may be
situated a relatively large distance from the bridge circuit, and the
resistance of the connecting leads may be significant and could affect
the results. For these situations the three wire connection arrangement
is used.
Fig 3.2 (a) shows the circuit with a resistance transducer R4 situated
remotely from the bridge and connected via two wires. The resistance
of these wires will be included in the measurement of R4.
Fig 3.2 (b) shows the three wire arrangement. One of the wires to the
transducer is now included in the R2 circuit and the other is in the R4
circuit. The resistance of both circuits will therefore be increased
equally and the effect on the balance condition will be minimized,
provided that the resistances of R2 and R4 are of similar magnitudes.
The extra wire in the galvanometer circuit will have no effect on the
reading, since there is no current flowing in it at the balance condition.
30
Dynalog (India) Ltd.
IT 01
Curriculum Manual
3.5
Wheatstone Bridge Measurements
Chapter 3
The DYNA 1750 Facilities
Fig 3.3 shows the Wheatstone Bridge layout provided with DYNA
1750 unit.
A high quality 10-turn potentiometer fulfills the functions of the
resistors R1 & R3 for resistance, or a potentiometer for voltage
measurements. The track resistance of 10k? has a maximum nonlinearity of 0.25%. The “Fine” dial is calibrated 0 –100 in steps of 2,
and the “Coarse” reading is calibrated 0 – 10, thus enabling readings to
be estimated from the dial with a discrimination of 1:1000,
representing a resolution of 10? .
Reading the dial : If the number in the window (coarse setting) is 3
and the fine setting is on 74, then the dial reading is 374. The resistance between the 0V terminal and A (the wiper) is 10? x 374 = 3.74k? .
A close-tolerance 12k? resistor (R2) and an unknown resistor Rx (R4)
are provided for resistance measurement.
A switch open circuits the unknown resistor Rx to allow the
measurement of other unknown resistors which can be connected
between socket C and the 0V terminal.
An accurate standard voltage of 1V is available at socket B.
The moving coil meter can be used as a center zero indicating
instrument. Since it is arranged as a 10V voltmeter its sensitivity is
insufficient for a direct application as galvanometer. This problem can
be overcome by using a differential amplifier followed by a high gain
DC amplifier from the signal conditioning circuits.
Dynalog (India) Ltd.
31
Wheatstone Bridge Measurements
Chapter 3
3.6
IT 01
Curriculum Manual
Practical Exercise
Measurement of Resistance
Fig 3.4 shows the layout diagram required for setting up the null
detector.
Initially the amplifier and meter configuration, which forms the
sensitive galvanometer must be set up so that zero input produces zero
output when the gain is set to maximum.
1
2
Connect the meter and amplifiers as shown in Fig 3.4 with the
+ & - inputs to the differential Amplifier short circuited so that
the input is zero. Set the Amplifier #2 GAIN COARSE control
to 10 and the GAIN FINE to 1.0.
Switch the power supply ON and adjust the OFFSET control so
that the moving coil meter indicates approximately zero. Then
set the GAIN COARSE control to 100 and re-adjust the
OFFSET control for zero output precisely.
You will find that this adjustment is very sensitive. That is why you
were instructed to obtain an approximate setting with the gain set to 10
first.
Note
32
The setting of the offset control may require adjustment as the
temperature of the unit varies during use and it is advisable to
use the above procedure to check and re-adjust as necessary at
regular intervals.
Dynalog (India) Ltd.
IT 01
Curriculum Manual
1
2
Wheatstone Bridge Measurements
Chapter 3
With the switch on the Wheatstone bridge circuit set to IN
(connecting the unknown resistor in circuit) set the Amplifier
#2 GAIN COARSE control to 10 and connect the circuit as
shown in Fig 3.5.
Adjust the control of the 10-turn variable resistor so that the
moving coil meter reading is approximately zero, then set the
GAIN COARSE control to 100. Finally adjust the 10-turn
resistor control accurately for zero meter (null) reading to
balance the bridge.
Reading the dial : If the number in the window (coarse setting) is 3
and the fine setting is on 74, then the dial reading is 374.
3
Note the resistor dial reading (overleaf)
This represents the resistance R3 in the theoretical circuit considered
earlier.
Dynalog (India) Ltd.
33
Wheatstone Bridge Measurements
Chapter 3
IT 01
Curriculum Manual
Dial reading
=
Resistance R3 = 10 x dial reading
=
?
Resistance R1 = 10,000 – R3
=
?
=
?
Resistance R2 = 12,000?
R2
Unknown resistance Rx =
X R3
R1
Carry out further resistance measurements on the 10kO slide variable
resistor to obtain familiarity with the equipment and its adjustment as
follows:
1
Set the Wheatstone Bridge switch to OUT to remove the
unknown resistor Rx from the circuit. Connect the 10kO slide
variable resistor terminals A & B to the Wheatstone Bridge
circuit connections C & 0V.
2
With the 10kO resistor control set to maximum, measure its
resistance as follows:a.
Check that the amplifier offset is set correctly and
adjust if necessary.
b.
With Amplifier #2 GAIN COARSE control set to 10,
obtain an approximate balance by adjusting the 10-turn
resistor.
c.
Set Amplifier #2 GAIN COARSE control to 100 and
obtain final balance. Note the dial reading and enter the
value in Table 3.1
3
4
Repeat the procedure to measure the resistance of the 10kO
resistor for all setting from 9 through 1, recording the dial
readings at balance in Table 3.1.
Calculate the resistance corresponding with each reading,
recording the results in table 3.1. R2 is still 12kO
Note Since the quoted accuracy of the 10-turn variable resistor is
0.25%, this represents 1 part in 400. There is no reason for giving
results to any more than four significant figures.
5
34
Switch OFF the power supply.
Dynalog (India) Ltd.
IT 01
Curriculum Manual
10kO Resistor
Setting
Dial reading at
Balance
Wheatstone Bridge Measurements
Chapter 3
R3
(10 x Dial)
R1
(10kO – R3)
R2
R4 =
X R3
R1
10
9
8
7
6
5
4
3
2
1
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
kO
Table 3.1
Note that a 1kO resistor is connected in series with the wiper of all
potentiometers on the D1750 trainer. This prevents damage to the
potentiometer in the event of back-driving the output with a voltage,
which could otherwise cause a heavy current to flow as the wiper is
moved towards terminal A.
Dynalog (India) Ltd.
35
Wheatstone Bridge Measurements
Chapter 3
3.7
IT 01
Curriculum Manual
Measurement of Voltage
Method 1
A calibrated variable resistor, standard voltage source and
galvanometer are required, these being connected as shown in Fig 3.7.
The position of the slider of the variable resistor is adjusted until the
circuit is balanced with no current flowing in the galvanometer.
Under these conditions, the voltage across the R section of the variable
resistance is equal to the value of the standard voltage supply. The
unknown voltage is proportional to the total resistance of the variable
resistor Rt and the section resistance R, and can be calculated from:Rt
Unknown voltage =
X Standard voltage
R
The method has disadvantages:1.
2.
The unknown voltage source is loaded by the variable resistor
and hence the voltage may be affected.
The method only allows measurement of voltages greater than
the standard voltage.
This method of measuring potential is the origin of the term
“potentiometer” for a variable resistor. Early models of this measuring
instrument were made of a highly accurate, close tolerance, resistance
wire which was stretched between terminals on a scaled background. It
was known as a Slide-Wire Potentiometer.
36
Dynalog (India) Ltd.
IT 01
Curriculum Manual
3.8
Wheatstone Bridge Measurements
Chapter 3
Practical Exercise
Measurement of voltage Using Method 1
1
First the OFFSET control of Amplifier #2 using the same
procedure used in Practical Exercise 3.6:
Switch ON the power supply and with the Differential
Amplifier inputs shorted together and Amplifier #2 GAIN
FINE set to 1.0, adjust the OFFSET for approximately zero
output with the GAIN COARSE set to 10. Adjust finally for
zero with the GAIN COARSE set to 100.
2
3
4
Dynalog (India) Ltd.
Connect the circuit as shown in Fig 3.8 and set the switch on
the Wheatstone Bridge circuit to OUT to disconnect the 12kO
ratio arm resistor and the unknown resistor Rx from the circuit.
Set the Amplifier #2 GAIN COARSE to 10 and set the output
from the 10kO wirewound resistor to 4V as indicated by the
digital meter. This represents the “unknown” voltage.
Adjust the 10-turn resistor for approximate balance and then
obtain final balance with Amplifier #2 GAIN COARSE set to
100.
37
Wheatstone Bridge Measurements
Chapter 3
5
IT 01
Curriculum Manual
Note the dial reading at balance, enter the value in Table 3.2
and calculate the value of the unknown voltage from:1000
Unknown voltage =
X Standard voltage
Dial reading
1000
=
X 1V
Dial reading
6
Repeat the procedure with the “unknown” voltage input set to
each of the values indicated in Table 3.2, recording the readings
and calculating the voltages for each value.
“Unknown” Voltage
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Dial Reading at
Balance
Calculated
Voltage
V
V
V
V
V
V
V
The method has the disadvantage of loading the unknown voltage
source and this can be demonstrated as follows:-
38
7
Set the “Unknown” voltage to 2.0V and obtain balance
conditions.
8
Now remove the connection from the output of the wirewound
resistor (socket B) to the Wheatstone bridge (socket D) and
note the revised value of the unknown voltage as indicated by
the digital voltmeter.
Dynalog (India) Ltd.
IT 01
Curriculum Manual
Wheatstone Bridge Measurements
Chapter 3
“Unknown” Voltage:
3.8b
3.9
When connected to the bridge
=
V
Disconnected from the bridge
=
V
Enter your value of the “Unknown” Voltage when disconnected
from the bridge in V.
Measurement of Voltage
Method 2
This method requires an additional DC source of voltage with a
magnitude exceeding the maximum value of the unknown voltage to
be measured and another variable resistor Rs. The schematic diagram
is shown in Fig 3.9.
For measurement of voltages less than the standard voltage, the
slider of the variable resistor is set to its maximum position and, with
the galvanometer connected to the standard voltage source, the value
of Rs is adjusted until there is no current flowing in the galvanometer
and the circuit is balanced.
Dynalog (India) Ltd.
39
Wheatstone Bridge Measurements
Chapter 3
IT 01
Curriculum Manual
The full resistance Rt is then calibrated to represent the value of the
standard voltage.
To measure an unknown voltage, the galvanometer is connected to the
unknown voltage and the slider position is again adjusted for circuit
balance. The section R at balance represents the magnitude of the
unknown voltage.
R
Unknown voltage =
X Standard voltage
Rt
For the measurement of voltage higher than the standard voltage,
the variable resistor can be calibrated against the standard voltage with
the slider set to a position lower than the maximum setting. This
setting will now represent a magnitude equal to the standard voltage.
Balance with an unknown voltage is obtained as before and unknown
voltage calculated from:-
Unknown voltage =
R (unknown connected)
X Standard voltage
R (standard connected)
With this method, no current is taken from the unknown voltage source
at balance and hence the circuit is not loaded. The voltage obtained
should therefore be accurate, within the limits of accuracy of the
variable resistor.
40
Dynalog (India) Ltd.
IT 01
Curriculum Manual
Wheatstone Bridge Measurements
Chapter 3
3.10 Practical Exercise
Measurement of Voltage Using Method 2
You should be familiar with the procedures for initially setting the
amplifier offset and balancing the bridge circuit by now. Instructions
for the procedures will not therefore be repeated in this exercise.
Measurement of Voltage Less Than the Standard Voltage.
1
Carry out the OFFSET initializing procedure and then connect
the circuit as indicated in Fig 3.10, using the 100kO variable
resistor as Rs (Fig 3.9) in the supply circuit of the additional
DC source.
Note that the output of the 10kO wirewound variable resistor is
not connected initially. This will be used as the source of the
“unknown” voltage.
2
Dynalog (India) Ltd.
Set the 10-turn resistor to its maximum setting (1000) and
adjust the setting of the 100kO resistor for balanced conditions,
i.e. null indication on the moving coil (M.C.) meter. Set
Amplifier #2 GAIN COARSE control to 10 initially and then
finally to 100 during the balancing.
41
Wheatstone Bridge Measurements
Chapter 3
IT 01
Curriculum Manual
When completed, the 10-turn resistor has been calibrated so that
full scale reading of 1000 represents a voltage of 1.000V.
3
4
Replace the 1.0V reference voltage source (from the
Wheatstone Bridge circuit) with the “unknown” voltage output
of the 10kO wirewound variable resistor, by moving the lead
that is connected to socket A of the Differential Amplifier
FROM socket B of the Wheatstone Bridge circuit TO socket B
of the 10kO wirewound variable resistor.
Set the “unknown” voltage to 0.25V as indicated on the digital
multimeter.
5
Adjust the control of the 10-turn resistor fro balance and note
the dial reading for this balance condition. This reading will
represent the unknown voltage directly in mV. Record the
value in table 3.3 and compare with the reading indicated by
the digital multimeter.
“Unknown” Voltage Input
0.25V 0.40V 0.60V 0.70V 0.80V 0.95V
Dial Reading at Balance
6
42
mV
mV
mV
mV
mV
MV
Repeat the procedure for other “unknown” voltage inputs given
in Table 3.3
Dynalog (India) Ltd.
IT 01
Curriculum Manual
7
Wheatstone Bridge Measurements
Chapter 3
Plot the characteristic of Dial Reading against “unknown” input
voltage on the graticule provided
Measurement of voltages Greater Than the standard Voltage.
Α
Remove the lead from socket C of the 10kO wirewound
resistor to socket B of the 100kO resistor to remove the 1V
supply.
B
Replace the 100kO resistor used for calibration with the 10kO
slider unit and apply the +12V supply to this and the 10kO
wirewound instead of the +5V.
C
Set the control dial of the 10-turn resistor to setting 0100 and
connect the A socket of the Differential Amplifier back to
socket B of the Wheatstone Bridge as shown in Fig 3.10.
D
Adjust the 10kO slider resistor control setting for bridge
balance. When completed, the 10-turn resistance has been
calibrated so that a dial reading of 0100 represents a voltage of
1.00V and a maximum dial reading of 1000 will represent a
voltage of 10V
E
Remove the 1.0V reference voltage source from socket A of the
Differential Amplifier and connect the “unknown” voltage
from socket B of the 10kO wirewound resistor to socket A of
the Differential Amplifier.
F
Apply various “unknown” voltages in the range 0 –10V to the
circuit. Note the dial reading for balance for each input voltage
setting and enter the values in table 3.4.
“Unknown” Voltage input
Dial Reading at Balance
Measured Voltage (volts)
Table 3.4
Dynalog (India) Ltd.
1
V
2
V
3
4
6
8
9
V
V
V
V
V
43
Wheatstone Bridge Measurements
Chapter 3
IT 01
Curriculum Manual
Loading Effect
G
Set the “unknown” input voltage to 5V and note the voltage
change on the digital meter when the lead to the Differential
Amplifier is removed.
“Unknown Voltage :
When Connected to the bridge
=
V
Discounted from the bridge
=
V
The slight loading effect is due to the input resistance of The
Differential Amplifier.
Notes :
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
44
Dynalog (India) Ltd.
IT 01
Curriculum Manual
Temperature Measurement
Chapter 4
Chapter 4
Temperature Measurement
Objectives of
this chapter
Having studied this chapter you will be able to:
1
2
3
4
5
6
Equipment
1
Required for
This Chapter
2
3
4
5
Dynalog (India) Ltd.
Describe the characteristic of an IC temperature
sensor.
Describe the construction and characteristics of a
platinum RTD resistance transducer.
Describe the construction and characteristics of an
n.t.c. thermistor.
Discuss the characteristic of n.t.c. thermistor bridge
circuits.
Describe the construction and characteristics of a
thermocouple.
Deduce temperatures from a voltage reading across
a transducer.
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Digital Multimeter with 20V DC range.
Stopwatch (not supplied).
Scientific Calculator (not supplied).
45
Temperature Measurement
Chapter 4
4.1
IT 01
Curriculum Manual
The DYNA 1750 Temperature Transducer Facilities
Fig 4.1 shows the layout of the temperature transducer facilities of the
DYNA 1750 unit. The active transducers are mounted within a clear
plastic enclosure which contains a heater.
The heated enclosure is provided to raise the temperature of the sensor
transducers to allow measurements to be taken during experiments.
In the case of the n.t.c. thermistors and the thermocouples, an
additional, separate unit I mounted outside the heated enclosure. The
externally mounted sensors are made available for comparison between
ambient (room) temperature and the temperature within the enclosure.
The externally mounted “K” type thermocouple is contained within a
package in contact with an IC temperature sensor (LM335) to act as a
thermometer with voltage output. This will be used in many of the
experiments as the reference (REF) thermometer.
46
Dynalog (India) Ltd.
IT 01
Curriculum Manual
4.2
Temperature Measurement
Chapter 4
The IC Temperature sensor
This is an integrated circuit containing 16 transistors, 9 resistors and 2
capacitors contained in a transistor type package.
The device reference number is LM335 and it provides an output of
10mV/OK. Measurements of the output voltage therefore indicate the
temperature directly in degrees Kelvin (OK). For example, at a
temperature of 20OC (293OK) the output voltage will be 2.93V. (Will
be vary depends on ambient temperature at site)
The circuit arrangement provided with the IC Temperature Sensor on
the DYNA 1750 unit is shown in Fig 4.2.
A 2-pin socket is provided for the connection of an external LM335
unit if desired.
Note
An LM335 unit is mounted on the Type “K” thermocouple
panel, external to the heated enclosure and fitted in a heat sink
together with another type “K” thermocouple, its output being
available from the REF socket on that panel. The output from
this can be used as an indication of the ambient temperature
outside the heated enclosure, and that from the INT. socket in
Fig 4.2 indicates the temperature within the heated enclosure.
The output from the REF socket does not give an accurate
value of the room (ambient) temperature when the heater is in
use, due mainly to heat passing along the PCB by conduction
from the heater. An LM335 remotely mounted or some other
method is necessary if accurate measurement of ambient
temperature is required.
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47
Temperature Measurement
Chapter 4
4.3
IT 01
Curriculum Manual
Practical Exercise
Characteristics of an LM335 IC Temperature sensor
1
Contact just the voltmeter to the circuit (as shown in Fig 4.3),
switch the power supply ON and note the output voltage, this
(x100) representing the ambient temperature in OK. Record the
value in Table 4.1
2
Connect the +12V supply to the heater input socket and note
the voltage reading every minute until the value stabilizes.
Record the values in Table 4.1 (OC = OK – 273)
Time (minutes)
Voltage
Temperature
O
K
O
C
0
1
2
3
4
5
6
7
8
9
10
V
V
V
V
V
V
V
V
V
V
V
4.3a Enter your temperature reading on OC after 5 minutes.
#
48
Switch OFF the power supply.
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Curriculum Manual
Temperature Measurement
Chapter 4
Exercise 4.3 illustrates the characteristics of the LM335 transducer,
indicates the maximum temperature rise possible using the heater
supplied at 12V, and also gives you an idea of the time scale required
for the unit to reach stable conditions.
4.4
The Platinum RTD (resistance Temperature Dependent)
Transducer
The construction of the Platinum RTD Transducer is shown in Fig 4.4,
consisting of a thin of platinum deposited on a ceramic substrate and
having gold contact plates at each end that make contact with the film.
The platinum film is trimmed with a laser beam to cut a spiral for a
resistance of 100O at 0OC.
The resistance of the film increases as the temperature increases. It has
a positive temperature coefficient (p.t.c.)
The increase in resistance is linear, the relationship between resistance
change and temperature rise being 0.385O/0OC.
Rt = Ro + 0.385t
Where
Rt = resistance at temperature tOC
Ro = resistance at 0OC (C=100O)
Normally, the unit would be connected to a DC supply via a series
resistor and the voltage developed across the transducer is measured.
The current flow through the transducer will then cause some self
heating, the temperature rise due to this being of the order of
0.005OC/mW dissipated in the transducer.
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Temperature Measurement
Chapter 4
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Curriculum Manual
The very simple electrical arrangement of the DYNA 1750 unit is as
shown in Fig 4.5.
The white dot signifies that this is a p.t.c., not n.t.c. (negative
temperature coefficient type of resistor which would have a black dot.
In the practical exercise you will connect the platinum RTD in series
with a high resistance to a DC supply and measure the voltage drop
across it. Due to the small variation of resistance, the current change
will be negligible and the voltage drop across the transducer will be
directly proportional to its resistance.
Notes :
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50
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4.5
Temperature Measurement
Chapter 4
Practical Exercise
Characteristics of a Platinum RTD transducer
1
2
Set the slider of the 10kO carbon resistor to mid-way and
connect the circuit as shown in Fig 4.6, with the digital
multimeter set to its 200mV or 2V DC range
Switch ON the power supply and adjust the slider control of the
10kO resistor so that the voltage drop across the platinum RTD
is 108mV (0.108V) as indicated by the digital multimeter.
This calibrates the platinum RTD for an assumed ambient temperature
of 20OC, since the resistance of the RTD at 20OC will be 108O. Note
that the voltage reading across the RTD in mV is the same as the RTD
resistance
in O, since the current flowing must be
0.108
= 1mA.
108
Note : If the ambient temperature differs from 20OC, the voltage can
be set to the correct value for this ambient temperature if
desired :
a.
b.
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Set the voltmeter to its 20V range and measure the INT output
from the IC Temperature Sensor to obtain the ambient
temperature in OK.
Then OC = OK – 273
RTD resistance = 100 + 0.385 x OC. Set the voltage drop across
the RTD for this value.
51
Temperature Measurement
Chapter 4
3
Connect the +12V supply to the Heater Element input and note
the values of the voltage across the RTD with the voltmeter set
to its 200mV or 2V range, (this representing the RTD
resistance) and the output voltage from the IC Temperature
Sensor with the voltmeter set to its 20V range, (this
representing the temperature of the RTD) after each of the
times given in Table 4.2
4
Convert the two voltage readings to RTD Temperature (OK)
and RTD Resistance (O) and record the values in Table 4.3.
Time (minutes)
RTD Temperature
IT 01
Curriculum Manual
O
K
O
C
RTD Resistance
0
1
2
3
4
5
6
7
8
9
10
O
O
O
O
O
O
O
O
O
O
O
5
Convert the RTD temperature into OC (OK – 273) and add to
Table 4.2.
6
Plot the graph of RTD resistance (O) against temperature (OC)
on the axes provided. Extend your graph down to cover 0OC.
130
128
126
124
122
RTD
120
Resistance 118
116
(O)
114
112
110
108
106
104
102
100
98
0
10
20
30
40
50
60
70
RTD Temperature OC
Graph 4.1
52
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4.6
Temperature Measurement
Chapter 4
The n.t.c. (Negative Temperature Coefficient) Thermistor
The thermistor (thermally sensitive resistor) is manufactured with the
intention that its value will change with temperature. Unlike a normal
resistor, a large coefficient of resistance (change of resistance with
temperature) is desirable.
Some are made with resistance which increases with temperature
(positive temperature coefficient, p.t.c.) or decreases(negative
temperature coefficient, n.t.c.). They are made in rod, disc or bead
form.
The construction of a typical n.t.c. thermistor is shown in Fig 4.7 (a),
consisting of an element made from sintered oxides such as nickel,
manganese and cobalt, with contacts made to each side of the element.
As the temperature of the element increases, its resistance falls, the
resistance/temperature characteristic being non-linear.
The resistance of the thermistors provided with the DYNA 1750 unit is
of the order of 5kO at an ambient temperature of 20OC (293OK).
Two similar units are provided, one being mounted inside the heated
enclosure. This is connected to the +5V supply and designated A. The
other is mounted outside the heated enclosure. It is connected to the 0V
(ground) line and is designated B. The circuit arrangement is shown in
Fig 4.7(b).
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Temperature Measurement
Chapter 4
4.7
IT 01
Curriculum Manual
Practical Exercise
Characteristics of an n.t.c. Thermistor
The resistance of the n.t.c. thermistor varies over a wide range for the
temperature range available within the heated enclosure. For this
reason the method used to measure the resistance in Exercise 4.5
cannot be used this time.
If resistance reading are to be taken at regular intervals of 1 minute, the
readings must be obtained very quickly.
The method selected connects the thermistor in series with a calibrated
resistor to the +5V supply.
For each reading, the variable resistor is adjusted until the voltage at
the junction of the thermistor and resistor is half of the supply voltage.
For this setting there will be the same voltage drop across the
thermistor and the resistor and, since the same current flows in each,
their resistances must be equal.
Hence the value of the resistance read from the calibrated resistor scale
is the same as the resistance of the thermistor.
54
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1
2
Temperature Measurement
Chapter 4
Connect the circuit as shown in Fig 4.8, set the switch on the
Wheatstone bridge circuit to OUT to disconnect the 12kO and
Rx resistors from the circuit and set the calibrated variable
resistor dial reading to approximately 500.
Switch the power supply ON and adjust the resistor control
until the voltage indicated by the voltmeter is 2.5V and then
note the dial reading and the temperature, by connecting the
voltmeter temporarily to the INT. socket of the IC Temperature
sensor.
Note: Since there is a 1kO resistor in the output lead of the resistance,
the total resistance will be 10 x Dial reading + 1kO
3
Record the value of dial reading and temperature in Table 4.3.
Time (minutes)
Temperature (from
IC Transducer)
0
O
K
O
C
1
2
3
4
5
6
7
8
9
10
Dial reading for 2.5V
Thermistor Resistance
(10 x Dial reading + 1kO
Table 4.3
4
5
kO kO kO kO kO kO kO kO kO kO kO
Connect the +12V supply to the Heater Element input socket
and, at 1 minute intervals, note the values of the dial reading to
produce 2.5V across the resistance and also the temperature
(from the IC Temperature sensor). Record the values in table
4.3.
Plot the graph of thermistor resistance against temperature on
the axes provided.
Due to the shape of the response characteristic, the device is not
suitable for application where an accurate indication of temperature is
required.
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Temperature Measurement
Chapter 4
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Curriculum Manual
Thermistors are used in very many electronic circuit applications for
the control of currents and voltages as equipment temperatures vary.
As transducer sensors they are more suitable for applications in
protection and alarm circuits where an indication of temperature
threshold is required.
Some thermistors are available which have a rapid change of resistance
when the temperature exceeds a certain value.
6
56
Switch OFF the power supply.
Dynalog (India) Ltd.
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4.8
Temperature Measurement
Chapter 4
Two Thermistor Bridge Circuits
When used for alarm or protection circuits, two thermistors would
normally be used, theses being connected in a bridge circuit as shown
in Fig 4.9
The two resistors R have the same resistance as the “cold” resistance of
the thermistors.
When cold, there will be no output at the connections AB because the
bridge will be balanced under this condition.
As the temperature rises, the resistance of both thermistors will
decrease. The potential of connection A will rise and that of connection
B will fall, giving a larger output than would be obtained with a circuit
using only one thermistor.
4.9
Practical Exercise
Characteristics of n.t.c. Bridge Circuits
Two bridge circuits will be investigated, one containing only one
thermistor (Th1) and the other, two.
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Temperature Measurement
Chapter 4
IT 01
Curriculum Manual
Since the three branches to be used all in parallel (Fig 4.10) they can
be connected at the beginning and brought into operation simply by
moving the null detector (digital multimeter).
Note that the second thermistor (Th2) is not contained within the
heated enclosure and will therefore not be subjected to the same
heating effect as Th1. The circuit will not be as efficient as can be
expected from one in which both thermistors are mounted in the same
temperature environment.
Variable resistors, RV2 & RV3 are adjusted to balance the branch
“cold” resistances (approximately 5kO) to give 2.5V at the center-tap,
and RV1 is also adjusted for 2.5V at the wiper.
The circuit will then be ready for heating measurements.
Th1, the 10kO 10-turn resistor and the 10kO wirewound resistor form
the bridge circuit with one active thermistor.
Th1, the 10kO 10-turn resistor, Th2 and the 10kO carbon resistor form
the bridge with two active thermistors.
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1
2
3
4
5
6
Temperature Measurement
Chapter 4
Connect the circuit as shown in Fig 4.11 and set the switch on
the Wheatstone Bridge circuit to OUT.
Switch the power supply ON and adjust so that the voltmeter
reading is 2.5V. The fixed branch of the bridge is now set for
center balance.
Connect the voltmeter between socket B of the 10kO
wirewound resistor and socket A of the n.t.c.. Adjust the 10kO
10-turn resistor on the Wheatstone Bridge circuit for a voltage
reading of zero.
Connect the voltmeter between socket B of the 10kO
wirewound resistor and socket B of the n.t.c.. Adjust the 10kO
carbon slider resistor for an output voltage of zero.
Both bridges are now set for zero output with the thermistors at
ambient temperature.
Note the temperature by measuring the voltage output from the
INT. socket of the IC Temperature sensor and record the value
in Table 4.4.
Time (minutes)
Temperature
(IC Temperature
Transducer)
Bridge
Output
O
K
O
C
1 active n.t.c.
2 active n.t.c.
Table 4.4
7
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0
1
2
3
4
5
6
7
8
9
10
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
Now connect the 12V supply to the heater input and at 1 minute
intervals, note the temperature and the output voltages from
each bridge circuit. Measured the 1 active n.t.c. between socket
A of n.t.c. and socket B of the 10kO wirewound resistor, and
move the voltmeter from the 10kO wirewound to socket B of
the 10kO slide resistor for the 2 active n.t.c. Record the values
in Table 4.4.
59
Temperature Measurement
Chapter 4
8
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Curriculum Manual
Draw graphs of output voltage against temperature for the two
bridge circuits on the same axes provide (Graph 4.3):
Note that the output with two active thermistors is grater than that with
only one thermistor. However, if both active thermistors were at the
same temperature, the output voltage would be twice that for one
active thermistor.
9
60
Switch OFF the power supply.
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Curriculum Manual
Temperature Measurement
Chapter 4
4.10 Type “K” Thermocouple
Fig 4.12 shows the construction of a thermocouple, consisting of two
wires of different materials joined by welding together at one end.
For the type “K” thermocouple the two materials are alumel and
chromel.
With this arrangement, when the ends that are joined together are
heated, an output voltage is obtained between the other two ends.
The ends that are joined to together are referred to as the “hot” junction
and the other ends are the “cold” junctions.
The magnitude of the output voltage depends on the temperature
difference between the “hot” and “cold” junctions and on the materials
used.
For the type “K” thermocouple the output voltage is fairly linear over
the temperature range 0-100OC and of magnitude 40.28 µV/ OC
difference between the “hot” and “cold” junctions.
Two thermocouple are provided with the DYNA 1750 unit, one being
mounted within the heated enclosure, this being the active unit which
will have its “hot” and “cold” junctions at different temperatures in
operation.
The other unit is mounted outside the heated enclosure and is
incorporated in a heat sink with an LM335 IC Temperature sensor so
that the temperature of the “cold” junction of the active thermocouple
can be measured.
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Temperature Measurement
Chapter 4
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Curriculum Manual
The second thermocouple is connected in series with the first with the
wires of the same material connected together. This ensues that the
connections to the output circuit are made from the same material
which eliminates the possibility of an EMF being introduced into the
circuit by connections between different materials.
The second thermocouple does not contribute to the output voltage
because its “hot” and “cold” junctions are maintained at the same
temperature.
The circuit arrangement is as shown in Fig 4.13.
Due to the low output voltage of the thermocouple, amplification is
required. An amplifier gain of 200 will give readings within one range
of the digital multimeter.
During operation, the temperature of the “cold” junction varies, due
mainly to heat conduction from the heater along the PCB and the
junction is in effect “floating”. This is a common occurrence with
thermocouple installations where the thermocouple leads are short.
To overcome the problem, extra leads of the same material or different
materials having the same thermoelectric properties are used to extend
the “cold” junction to a point where a steady temperature can be
maintained. These cables are referred to as “compensating cables”.
62
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Temperature Measurement
Chapter 4
4.11 Practical Exercise
Characteristics of a “K” Type Thermocouple
1
2
3
4
5
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Connect the circuit as shown in Fig 4.14, set the voltmeter to
the 200mV DC range and set Amplifier #1 GAIN COARSE to
10 and GAIN FINE to 0.2.
Switch the power supply ON and then set the OFFSET control
of Amplifier #1 as follows :
Short circuit the input connections to the Instrumentation
Amplifier and adjust the OFFSET control for zero indication on
the voltmeter.
Re-connect the Thermocouple outputs to the Instrumentation
Amplifier as shown in Fig 4.14. The output voltage should still
be zero with the “hot” and “cold” junctions at the same
temperature.
Find the temperatures of the inside and outside of the enclosure
(cold junction) by using the digital multimeter on the 20V DC
range to measure the voltage output from the INT. socket of the
IC Temperature Sensor and then from the REF output socket of
the LM335 provided on the type “K” Thermocouple unit.
63
Temperature Measurement
Chapter 4
6
Record the values in Table 4.5.
0
Temp.
O
K
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Curriculum Manual
1
2
3
4
5
6
7
8
9
10
Hot
Junction
(INT.)
Cold
Junction
(REF.)
Difference
Thermocouple O/P
mV mV mV mV mV mV mV mV mV mV mV
Table 4.5
7
8
9
64
Connect the +12V supply to the heater and at 1 minute
intervals, note the values of the thermocouple output voltage
(mV), and the voltages representing the temperatures of the
“hot” and “cold” junctions of the thermocouple.
Record the values in Table 4.5
Construction the graph of thermocouple output voltage against
temperature difference between the “hot” and “cold” junctions
on the axes provided.
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10
Temperature Measurement
Chapter 4
Switch OFF the power supply.
The actual value of the transfer characteristic will depend on the gain
provided by the amplifier system at the settings used, which can be
adjusted to calibrated the system as desired.
Notes :
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Temperature Measurement
Chapter 4
66
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Light Sensors
Chapter 5
Chapter 5
Light Sensors
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Discuss the characteristics of a filament lamp.
2
Describe the construction and characteristics of a
photovoltaic cell.
3
Describe the construction and characteristics of a
phototransistor.
4
Describe the construction and characteristics of a
photoconductive cell.
5
Describe the construction and characteristics of a
PIN photodiode.
1
2
3
4
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Digital Multimeter.
Opaque box to cover the clear plastic enclosure.
67
Light Sensors
Chapter 5
5.1
IT 01
Curriculum Manual
The DYNA 1750 Opto-Transducer Facilities
Fig 5.1 shows the arrangement of the opto-electronic (light)
transducers provided on the DYNA 1750 Trainer.
The opto-sensors are contained within a clear plastic enclosure and can
be illuminated by a lamp which is placed centrally.
All semiconductor devices are sensitive to light falling upon them.
That is why the devices (diodes, transistors, IC’s) are contained within
opaque encapsulations, to prevent light getting at the active materials.
With some devices, the main effect of light irradiation will be to
increase their conductivity (reduce their resistance). In other an EMF is
generated or currents are released to flow in an external circuit.
68
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5.2
Light Sensors
Chapter 5
The Incandescent Lamp
The light source to be used in the experiments is a tungsten filament
lamp. The filament glows more brightly as the power feeding the lamp
is increased. Two factors will be affected as the lamp voltage is
increased :
1.
2.
The temperature of the filament is proportional to the input
power. Power varies with square of the voltage, and is also
affected by the resistance of the lamp, which increases as the
filament temperature increases (it has a positive temperature
coefficient).
The spectral response of the lamp varies with the filament
temperature. At low temperatures the light is in the infra-red
region of the visible spectrum and the light output gradually
increases in frequency (red
orange
yellow . . .)
as the temperature is raised.
These factors make it difficult to be too precise about the response of
the sensors which will be investigated.
In order to determine the response of the filament lamp an acceptable
reference must be established. The photovoltaic cell is a linear device,
the output short circuit current being directly proportional to the
luminous flux (lux) being received.
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Light Sensors
Chapter 5
5.3
IT 01
Curriculum Manual
Practical Exercise
The Filament Lamp
1
2
3
Connect the circuit as shown in Fig 5.2 with the digital multimeter
connected as an ammeter on the 200mA range in between the power
amplifier and the lamp filament socket. Switch ON the power supply.
Set the 10kO wirewound resistor to minimum for zero output voltage
(on the moving coil meter) from the power amplifier.
Take readings of lamp filament current as indicated on the digital
multimeter as the lamp voltage is increased in 1V steps. Record the
results in Table 5.1.
Lamp filament
voltage (volts)
0
1
2
3
4
5
6
7
8
9
10
Lamp filament
current (mA)
Lamp filament
power (mW)
Lamp resistance
(O)
70
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5
6
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Light Sensors
Chapter 5
Calculate the corresponding values of lamp filament power (V
X I) and resistance (V÷I), recording the results in Table 5.1
Plot the graphs of lamp power and resistance against applied
voltage on the graticule provided.
Switch OFF the power supply.
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Light Sensors
Chapter 5
5.4
IT 01
Curriculum Manual
Photovoltaic Cell
A photovoltaic cell is one which generates an EMF when light falls
onto it.
One of the regions is made very thin (about one millionth of a meter,
1µm). Light can easily pass through this without much loss of energy.
When the light reaches the junction, at the depletion layer, it is
absorbed and the released energy creates hole-electron pairs which
diffuse across the junction.
The thin layer, which is only lightly doped, rapidly becomes saturated
and charge carriers can be released into an external circuit to form a
current, pushed around the circuit by the force (electro-motive force,
EMF, electron-moving-force) of the surplus of charge carriers released
by the energy absorbed.
Note that the anode current is shown as negative because the internal
current inside any source of EMF must flow with opposite polarity to
the external current, the electrons arriving at the anode returning to the
cathode inside the photo-cell.
72
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Light Sensors
Chapter 5
The lux referred to in Fig 5.4 is the unit of incident light (light arriving
at the cell).
Characteristics of Photovoltaic Cell Type MS5B
Open circuit voltage (in sunlight)
500mV
Short circuit current (in sunlight)
10mA
Peak spectral response wavelength 840nm (IR)
Response time
10µs
Table 5.2
Note:
IR=infrared
If the output of the cell is short circuited there will be no output voltage
at all, since this will be dropped internally across the resistance of the
cell. The short circuit output current obtained will vary from zero to
maximum according to the incident light.
The device can be used either as a voltage source or as a current source
and is inherently a linear device. To increase the output voltage, cells
may be connected in series. Parallel connection allows a grater current
to be drawn.
When used as energy source they are known as Solar Cells.
Note :
For the chracteristic to be linear it is necessary for the light output of
the lamp to be of constant light frequency (spectral color) and for the
light output (in lux) to be directly proportional to the power input.
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Light Sensors
Chapter 5
5.5
IT 01
Curriculum Manual
Practical Exercise
The Photovoltaic Cell
1
2
3
Lamp filament
voltage (volts)
Connect the circuit as shown in Fig 5.6 with the digital
multimeter (ammeter) on the 2mA range to measure the short
circuit current between the Photovoltaic cell output and
Ground. Fit an opaque box over the Clear Plastic Enclosure to
exclude all ambient light.
Switch ON the power supply and set the 10kO wirewound
resistor to minimum for zero output voltage from the power
amplifier.
Take readings of Photovoltaic Cell Short Circuit Output
Current as indicated on the digital multimeter as the lamp
voltage is increased in 1V, steps. Record the results in Table
5.3.
0
1
2
3
4
5
6
7
8
9
10
Short circuit output
current
µA
µA
µA
µA
µA
µA
µA
µA
µA
µA
µA
Open circuit output
voltage
Table 5.3
V
V
V
V
V
V
V
V
V
V
V
74
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5
6
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Light Sensors
Chapter 5
Switch OFF the power supply, set the multimeter as a voltmeter
to read the Open Circuit Output Voltage. Switch ON the power
supply and repeat the readings, adding the results to Table 5.3.
Plot the graphs of Photovoltaic Cell Short Circuit Output
Current and Open Circuit Output Voltage against Lamp
filament voltage on the graticule provided.
Switch OFF the power supply.
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Light Sensors
Chapter 5
5.6
IT 01
Curriculum Manual
The Phototransistor
The construction and circuit used are shown in Fig 5.7. The device is
an NPN three layer semiconductor device similar to a normal
transistor, the regions being called emitter (e),base(b) and collector (c).
The device differs from the normal transistor in allowing light to fall
onto the base region, focused there by a lens.
The circuit connection is shown in Fig 5.7, the collector being
connected to the positive of a DC supply via a load resistor R. The
base connection is not used in this circuit but is available for biasing to
change the threshold level.
With no light falling on the device there will be a small leakage current
flowing due to thermally generated hole-electron pairs and the output
voltage from the circuit will be slightly less than the supply voltage
due to the voltage drop across the load resistor R.
When light falls on the base region the leakage current increases. With
the base connection open circuit, this current flows out via the baseemitter junction and is amplifier by normal transistor action to give a
large change in the collector leakage current.
With increased current flowing in the load resistor R, the output
voltage reduces and is dependent on the light falling on the device.
Vout = V – I ceo R
where :
V = Supply voltage, Iceo = Collector leakage current, R = Collector
load resistance.
76
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Light Sensors
Chapter 5
Fig 5.8 shows the circuit arrangement for the DYNA 1750 unit.
The main characteristics of the device are :
Type
Collector Current
(Vce = 5V)
MEL 12
Dark
Typical
ambient
100nA
3.5
Table 5.4
Notes :
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Dynalog (India) Ltd.
77
Light Sensors
Chapter 5
5.7
IT 01
Curriculum Manual
Practical Exercise
Characteristics of a Phototransistor
1
2
3
4
78
Connect the circuit as shown in Fig 5.9 and set the 10kO
carbon slider control to minimum setting (1) so that the
Phototransistor load resistance is approximately 1kO
(protection resistor only).
Connect the digital multimeter on the 20V DC range to
measure the Phototransistor output voltage. Fit the opaque box
over the Clear Plastic Enclosure to exclude all ambient light.
Switch ON the power supply and set the 10kO wirewound
resistor to minimum for zero output voltage from the power
amplifier.
Take readings of Phototransistor output voltage as indicated on
the digital multimeter as the lamp voltage is increased in 1V
steps. Record the results in Table 5.5.
Dynalog (India) Ltd.
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Light Sensors
Chapter 5
Lamp filament
voltage (volts)
Phototransistor output
voltage
5
6
Dynalog (India) Ltd.
0
1
2
3
4
5
6
7
8
9
10
V
V
V
V
V
V
V
V
V
V
V
Plot the graph of Phototransistor Output Voltage against Lamp
filament voltage on the graticule provided.
Switch OFF the power supply.
79
Light Sensors
Chapter 5
5.8
IT 01
Curriculum Manual
The Photoconductive Cell, LDR
Fig 5.10 shows the basic construction of a photoconductive cell,
consisting of a semiconductor disc base with a gold overlay pattern
making contact with the semiconductor material. The arrangement for
the DYNA 1750 unit is also shown.
The resistance of the semiconductor material between the gold contacts
reduces when light falls on it.
With no light on the material, the resistance is high. Light falling on
the material produces hole-electron pairs of charge carriers and reduces
the resistance.
Out of the various semiconductor materials available, a cadmium
sulfide photoconductive cell is used on the DYNA 1750 unit because it
responds to light with a range of wavelengths similar to those of the
human eye (400-700nm).
An alternative name for this device is the Light Dependent Resistor,
LDR.
Cell Resistance
Peak Spectral
Response
Table 5.6
Dark
1MO
Ambient (typ.)
400O
530nm
When light is removed from the device, the hole-electron pairs are
slow to reform and the response is sluggish. This is indicated by the
large falling response time.
80
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IT 01
Curriculum Manual
5.9
Light Sensors
Chapter 5
Practical Exercise
Characteristics of a Photoconductive Cell
1
2
3
Dynalog (India) Ltd.
Connect the circuit as shown in Fig 5.11 and set the 10kO
carbon slider control to setting 3 so that the Photoconductive
Cell load resistance is approximately 3kO.
Connect the digital multimeter on the 20V DC range to
measure the Photoconductive Cell output voltage. Fit the
opaque box over the Clear Plastic Enclosure to exclude all
ambient light.
Switch ON the power supply and set the 10kO wirewound
resistor to minimum for zero output voltage from the power
amplifier.
81
Light Sensors
Chapter 5
IT 01
Curriculum Manual
4
Lamp filament
voltage (volts)
Photoconductive Cell
output
82
Take reading of Photoconductive Cell output voltage as
indicated on the digital multimeter as the lamp voltage
increased in 1V steps. Record the results in Table 5.7.
0
1
2
3
4
5
6
7
8
9
10
V
V
V
V
V
V
V
V
V
V
V
5
Plot the graph of Photoconductive Cell Voltage against Lamp
filament voltage on the graticule provided.
6
Switch OFF the power supply.
Dynalog (India) Ltd.
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Curriculum Manual
Light Sensors
Chapter 5
5.10 The PIN Photodiode
Fig 5.13 shows the construction of the PIN photodiode.
This differs from a standard PN photodiode by having a layer of
intrinsic (pure) silicon, the I region, between the normal P and N
regions. The main improvement of the introduction of the I region is a
reduction in the capacitance of the junction, resulting in a faster
response time which can be as high as 0.5ns.
The device can be operated in one of two ways :
a)
as a photovoltaic cell, measuring the voltage output, and
b)
by amplifying the output current and converting it into a
voltage.
Fig 5.14 shows the circuit arrangement and characteristics for the PIN
Diode mounted on the DYNA 1750 unit.
Dynalog (India) Ltd.
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Light Sensors
Chapter 5
IT 01
Curriculum Manual
5.11 Practical Exercise
Characteristics of a PIN Photodiode
1
Connect the circuit as shows in fig 5.15, using the Current
Amplifier to measure the current output of the PIN Photodiode.
2
Use the digital multimeter on the 20V DC range to measure the
output voltage of Amplifier #1. Fit the opaque box over the
clear Plastic Enclosure to exclude all ambient light.
3
4
5
84
Switch ON the power supply and set the 10kO wirewound
resistor to minimum for zero output voltage from the power
amplifier.
Set the GAIN COARSE of Amplifier #1 to 10 and set the
GAIN FINE to 1.0. Check that the OFFSET is giving zero
output for zero input and adjust if necessary.
Take readings of Amplifier #1 output voltage as indicated on
the digital multimeter as the lamp voltage is increased in 1V
steps. Record the results in Table 5.8 in the row labeled PIN
Photodiode Current Amp. O/P.
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Lamp filament
voltage (volts)
PIN Photodiode
current Amp. O/P
PIN Photodiode
output voltage
Table 5.8
6
7
8
9
Dynalog (India) Ltd.
Light Sensors
Chapter 5
0
1
2
3
4
5
6
7
8
9
10
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
Change the Current Amplifier to the Buffer Amplifier to
measure the output voltage of the PIN Photodiode.
Take readings of PIN Photodiode amplified Output Voltage as
the lamp voltage is again increased in 1V steps. Record the
results in Table 5.8 in the row labeled PIN Photodiode Output
Voltage.
Plot the graphs of PIN Photodiode Current Amplifier Output
Voltage and Buffered Output Voltage against Lamp filament
voltage on the graticule provided.
Switch OFF the power supply.
85
Light Sensors
Chapter 5
86
IT 01
Curriculum Manual
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IT 01
Curriculum Manual
Linear Position or Force Applications
Chapter 6
Chapter 6
Linear Position or Force Applications
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the construction, principal and
characteristics of a Linear Variable Differential
Transformer (LVDT).
2
Describe the construction and characteristics of a
linear variable capacitor.
3
Describe the construction and characteristics of a
strain gauge.
1
2
3
4
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Digital Multimeter.
Oscilloscope.
87
Linear Position or Force Applications
Chapter 6
6.1
IT 01
Curriculum Manual
The Linear Variable Differential Transformer (LVDT)
The construction and circuit arrangement of an LVDT are as shown in
Fig 6.1. It consists of three coils mounted on a common former and
having a magnetic core that is movable within the coils.
The center coil is the primary and is supplied from an AC supply. The
coils on either side are secondary coils and are labeled A & B in Fig
6.1.
Coils A & B have equal number of turns and are connected in series
opposing so that the output voltage is the difference between the
voltages induced in the coils.
Fig 6.2 shows the output obtained for different positions of the
magnetic core.
88
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Linear Position or Force Applications
Chapter 6
With the core in its central position as shown in Fig 6.2(b) there should
be equal voltages induced in coils A & B by normal transformer action
and the output voltage would be zero. In practice this ideal condition is
unlikely to be found, but the output voltage will reduce to a minimum.
With the core moved to the left as shown in Fig 6.2(a), the voltage
induced in coil A (Va) will be greater than that induced in coil B (Vb).
There will therefore be an output voltage Vout = (Va – Vb) and this
voltage will be in phase with the input voltage as shown.
With the core moved to the right as shown in Fig 6.2(c) the voltage
induced in coil A (Va) will be less than that induced in coil B (Vb) and
again there will be an output voltage Vout = (Va – Vb) but in this case
the output voltage will be antiphase with the input voltage.
Movement of the core from its central (or neutral) position produces an
output voltage. This voltage increases with the movement from the
neutral position to a maximum value and then may reduce for further
movement from this maximum setting. Note that the phase will remain
constant on either side of the neutral position. There is no gradual
change of phase, only an abrupt reversal when passing through the
neutral position.
An amplitude only measurement of the output voltage, such as that
provided by a meter, gives an indication of movement from the neutral
position but will not indicate the direction of that movement. Used in
conjunction with a phase detector, an output can be obtained that is
dependent on both magnitude and direction of movement from neutral
position. The oscilloscope gives both phase and magnitude indications.
Fig 6.3 shows the circuit arrangement and device characteristics of the
DYNA 1750 unit.
Dynalog (India) Ltd.
89
Linear Position or Force Applications
Chapter 6
6.2
IT 01
Curriculum Manual
Practical Exercise
Characteristics of a Linear Variable Differential Transformer
In this exercise you will measure the rectified output using the digital
multimeter on the 20V DC range and also amplify and measure it using
the M.C. analog meter, as this gives a better impression of the variation
of output voltage with core position.
1
Connect the circuit as shown in Fig 6.4 with the digital
multimeter on the 2V DC range to monitor the output of the
Full-wave Rectifier. Switch ON the power supply.
2
Set the A.C. Amplifier gain to 1000.
3
Set the GAIN COARSE control of Amplifier #1 to 100 and
GAIN FINE control to 1.0. Check that the OFFSET control is
set for zero output with zero input and adjust if necessary.
4
5
90
Adjust the core position by rotating the operating screw to the
neutral position. This will give minimum output voltage. Note
the value of this voltage from the digital multimeter and record
in Table 6.1.
Rotate the core control screw in steps of 1 turn for 4 turns in the
clockwise direction (when viewing the control from the lefthand side of the DYNA 1750 unit) and record your results in
Table 6.1. Then turn the control screw in the counter clockwise
direction, again recording the results in Table 6.1.
Dynalog (India) Ltd.
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Linear Position or Force Applications
Chapter 6
Core position (turns from neutral)
Digital meter
Output
Voltage
Analog meter
-4
-3
-2
-1
0
+1
+2
+3
+4
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
Table 6.1
6
7
Dynalog (India) Ltd.
Plot the graph of output voltage from the analog meter readings
against core position on the axes provided.
Switch OFF the power supply.
91
Linear Position or Force Applications
Chapter 6
IT 01
Curriculum Manual
Change the circuit to that shown in Fig 6.5 to observe the effect of the
polarity change in the output. Note that test points are provided at the
bottom of the DYNA 1750 Trainer panel for connection of
oscilloscope probes.
1
2
3
4
92
Note : for the LVDT considered here, unless the two secondary
coils are identical, there will be non-perfect coupling between
each secondary coil and the primary coil, resulting in a
frequency-dependent phase shift in the output voltage (relative
to the input voltage).
Set up the oscilloscope as follows :
Lock the timebase to CH.1, trigger selector to AC
CH. 1 Amplifier on AC input, 50mV/div
CH. 2 Amplifier on AC input, 0.5/div
timebase to 5 µs/div
position both traces on the center horizontal line of the
display
Switch ON the power supply and vary the core position through
its range and observe the effect on the output voltage as seen on
CH.2 of the oscilloscope display.
Adjust the timebase fine control to give 1 ½ cycle of displayed
waveform.
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5
Linear Position or Force Applications
Chapter 6
Sketch the oscilloscope waveforms when the core is turned 2
turns in (+2) from the neutral position on the graticule
provided.
The waveform sketch for perfectly coupled coils, would look most like
6
Switch OFF the power supply and reset the timebase fine
control to the calibrated position.
Notes :
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Dynalog (India) Ltd.
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Linear Position or Force Applications
Chapter 6
6.3
IT 01
Curriculum Manual
The Linear Variable Capacitor
Any capacitor consists of two conducting plates separated by an
insulator which is referred to as the dielectric. The capacitance of the
device is directly proportional to the cross sectional area that the plates
overlap and is inversely proportional to the separation distance
between the plates.
A variable capacitor can therefore be constructed by varying either the
area of plates overlapping or the separation distance.
Fig 6.6 (a) shows the construction of the capacitor fitted in the DYNA
1750 unit, being fitted at the end of the coil former of the LVDT. This
uses the magnetic slug core as the moving plate of the capacitor. The
fixed plate consists of a brass sleeve fitted around the coil former.
The capacitance magnitude depends on the length (?) of the slug
enclosed within the brass sleeve, the capacitance increasing with
increase of length ?.
Fig 6.6 (b) shows the circuit arrangement in the DYNA 1750 unit.
The main characteristics of the unit are :
Capacitance (minimum)
Capacitance (maximum)
Mechanical travel
Table 6.2
94
25pF
50pF
15mm
Dynalog (India) Ltd.
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Curriculum Manual
6.4
Linear Position or Force Applications
Chapter 6
Practical Exercise
Characteristics of a variable Capacitor Transducer
The purpose of the Differential Amplifier is to provide a reference to
give zero output voltage at any desired value of input voltage. The
reference voltage is adjusted by the setting of the 10-turn
potentiometer.
1
2
3
4
5
Dynalog (India) Ltd.
Connect the circuit as shown in Fig 6.7 with the digital
multimeter on the 20V DC range connected to the output of
Amplifier #1.
Set the capacitor moving plate fully out to the minimum
capacitance position, and then turn it back in until the marker
on the operating control is first at the top. You now have the
device near to its minimum capacitance position.
Set the AC Amp gain to 1000.
Switch ON the power supply and set the GAIN COARSE
control of Amplifier #1 to 100 and GAIN FINE control to 1.0.
Check that the OFFSET control is set for zero output with zero
input and adjust if necessary.
Adjust the 10-turn potentiometer on the Wheatstone Bridge
panel to give zero (as near as possible) output from Amplifier
#1 (as close to 0V as possible) as indicated on the digital
multimeter.
95
Linear Position or Force Applications
Chapter 6
6
Approximate
capacitance
IT 01
Curriculum Manual
Turn the operating screw inwards in steps of 1 turn clockwise
to increases the capacitance and at each step note the output
voltage and enter the value in Table 6.3.
Screw full out, minimum
25pF
Turns of screw
0
Output Voltage
0
1
V
2
V
3
V
4
V
Screw full in, maximum
5
V
6
V
7
V
8
V
50pF
9
V
10
V
V
Table 6.3
7
96
Plot the graph of output voltage against core positions above 2
on the axes provided :
Dynalog (India) Ltd.
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Curriculum Manual
6.5
Linear Position or Force Applications
Chapter 6
The Strain Gauge Transducer
Fig 6.8 shows the construction of a strain gauge, consisting of a grid of
fine wire or semiconductor material bonded to a backing material.
When in use, the unit is glued to the beam under test and is arranged so
that the variation in length under loaded conditions is along the gauge
sensitive axis (Fig 6.8(a)).
Loading the beam increases the length of the gauge wire and also
reduces its cross-sectional area (Fig 6.8(c)). Both of these effects will
increase the resistance of the wire.
The layout and circuit arrangement for the DYNA 1750 unit is shown
in Fig 6.9. Resistors are electro-deposited on a substrate on a contact
block at the right-hand end of the assembly.
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Linear Position or Force Applications
Chapter 6
IT 01
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The gauge is normally connected in a Wheatstone Bridge arrangement
with the bridge balanced under no load conditions. Any change of
resistance due to loading unbalances the bridge and this is indicated by
the detector (Galvanometer).
Fig 6.10 (a) shows the basic Wheatstone Bridge arrangement with one
strain gauge transducer. This circuit is liable to give inaccurate results
due to thermal changes. A variation of temperature will also produce a
change of resistance of the gauge and this will be interpreted as a
change of loading.
To correct for this an identical gauge is used and connected in circuit
as shown in Fig 6.10 (b). This gauge is placed near to the other gauge
but is arranged so that it is not subjected to any loading.
Any variation of temperature now affects both gauges equally and
there will be no thermal effect on the bridge conditions. The gauge
subjected to loading is referred to as the active gauge and the other is
called the dummy gauge.
The output from the circuit is small and to increase this, four gauges
are normally used with two active gauges and two dummies as shows
in Fig 6.10 (c).
The DYNA 1750 uses two active gauges formed along the axis of the
beam and two dummies formed at right angles to these.
The main characteristics of the device are :
Load capacity
Maximum deflection
Sensivity
Table 6.4
98
100g
0.5mm
25 µV/g
Non-linearity
Hysteretic
Creep
0.10%
0.03%
0.05%
Dynalog (India) Ltd.
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Curriculum Manual
6.6
Linear Position or Force Applications
Chapter 6
Practical Exercise
Characteristics of a Strain Gauge Transducer
You will need ten similar weights, such as ten equal value coins, to increase
the loading in regular steps.
1
2
3
4
5
Dynalog (India) Ltd.
Connect the circuit as shown in Fig 6.11 and set Amplifier #1
GAIN COARSE control to 100 and GAIN FINE 1.0.
Switch ON the power supply and with no load on the strain
gauge platform, adjust the offset control of Amplifier #1 so that
the output voltage is zero.
Place all ten of your weights on the load platform and adjust the
GAIN FINE control to give an output voltage of 7.0V as
indicated on the moving coil meter.
Note that this value of output voltage should cover all ranges of
coins within the setting of the GAIN FINE control.
Place on weight (coin) on the load platform and note the output
voltage. Record the value in Table 6.5 overleaf.
Repeat the process, adding further weights one at a time, noting
the output voltage at each step and recording the values in
Table 6.5.
99
Linear Position or Force Applications
Chapter 6
Number of coins
Output Voltage
IT 01
Curriculum Manual
0
1
2
3
4
5
6
7
8
9
10
0
V
V
V
V
V
V
V
V
V
V
V
Table 6.5
6
Plot the graph of output voltage against number of coins on the
axes provided :
Your characteristic sketch is most similar to :
100
Dynalog (India) Ltd.
IT 01
Curriculum Manual
Environmental Measurements
Chapter 7
Chapter 7
Environmental Measurements
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the construction and characteristics of an
air flow transducer.
2
Describe the construction and characteristics of an
pressure transducer
3
Describe the construction and characteristics of a
humidity transducer.
1
2
3
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Digital Multimeter.
101
Environmental Measurements
Chapter 7
7.1
IT 01
Curriculum Manual
The Air Flow Transducer
Fig 7.1 shows the construction of an Air Flow Transducer, consisting
of two RTD’s (Resistance Temperature Dependent) mounted in a
plastic case. One of the devices has an integral heating element
incorporated with it and the other is unheated.
The operation of the device uses the principal that when air flows over
the RTD’s, the temperature of the heated unit will fall more than that
of the unheated unit. The temperature difference will be related to the
air flow rate which will in turn affect the resistance of the RTD’s.
With the DYNA 1750 trainer, the transducers are enclosed in a clear
plastic container and provision is made for air to be pumped over the
device.
Fig 7.2 shows the electrical circuit arrangement and main
characteristics of the device in the DYNA 1750 Trainer.
102
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Curriculum Manual
7.2
Environmental Measurements
Chapter 7
Practical Exercise
Characteristics of an Air Flow Transducer
1
2
3
Connect the circuit as shown in Fig 7.3 and set the GAIN
COARSE control of Amplifier #1 to 10 and GAIN FINE
control to 1.0. Check that the pump control is set to OFF.
Set the digital multimeter to the 20V range.
Switch ON the power supply and allow the temperature to
stabilize.
4
Adjust the OFFSET control of Amplifier #1 for zero output
continuously during this time, setting the GAIN COARSE
control to 100 when stabilized conditions are approached.
5
Set the Flow/Pressure control to FLOW.
6
Check that the OFFSET control is set for zero output voltage.
7
Dynalog (India) Ltd.
Use the digital multimeter to note the voltage at the – and +
outputs from the transducer and record the values in table 7.2
overleaf.
103
Environmental Measurements
Chapter 7
8
IT 01
Curriculum Manual
Switch the pump ON and the voltages again when conditions
have stabilized, recording the values in Table 7.2.
Pump OFF
Transducer – Output Voltage
Transducer + Output Voltage
Amplifier #1 Output Voltage
0
Pump ON
V
V
V
V
V
Table 7.2
The RTD’s have a positive temperature coefficient.
9
Switch OFF the power supply and the pump.
Notes :
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
104
Dynalog (India) Ltd.
IT 01
Curriculum Manual
7.3
Environmental Measurements
Chapter 7
The Air Pressure Transducer
Fig 7.4 shows the construction of an air pressure transducer and also
shows the electric circuit arrangement of the DYNA 1750 unit. The
device consists of an outer plastic case which is open to the atmosphere
via two ports. Within this case is an inner container from which the air
has been evacuated and a stain gauge Wheatstone bridge circuit is
fitted on the surface.
The air pressure in the outer container will produce an output from the
bridge and variation of the pressure will produce a variation of this
output.
The transducer output can be calibrated and may be called an absolute
pressure transducer.
Provision is made for air to be fed to the unit from the pump.
The main characteristics of the device are :
Type
SPX200AN
Sensitivity (typical) 300 µ V/kPa
Voltage
difference Pump
OFF
Temperature
1350ppm/OC
Voltage
coefficient
difference Pump
ON
Output Voltage (-)
2.48V
Output
Pump OFF
impedance
Output Voltage (+)
2.51V
Pump ON
Table 7.3
Dynalog (India) Ltd.
35mV
39mV
1.6kO
105
Environmental Measurements
Chapter 7
7.4
IT 01
Curriculum Manual
Practical exercise
Characteristics of an Air Pressure Transducer
1
2
3
Connect the circuit as shown in Fig 7.5 and set the Amplifier
#1 GAIN COARSE control to 10 and GAIN FINE control to
1.0. Ensure that the pump switch is set OFF.
Switch ON the power supply and adjust the OFFSET control of
Amplifier #1 for zero output voltage. The unit is now calibrated
zero for the current value of the atmospheric pressure.
Set the Flow/Pressure control to PRESSURE and then switch
the pump ON. The output voltage from the amplifier #1 will
increase. Note the value of this voltage.
Output voltage (Pump ON) =
V
Note that a large amplifier is required due to the low magnitude of the
device output.
4
106
Switch OFF the power supply and the pump
Dynalog (India) Ltd.
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Curriculum Manual
7.5
Environmental Measurements
Chapter 7
The Humidity Transducer
Fig 7.6 (a) shows the construction of a humidity transducer, consisting
of a thin disc of a material whose properties vary with humidity. Each
side of the disc is metalized to from a capacitor.
Variation of humidity of the surrounding air alters the permittivity
and/or thickness of the dielectric material, changing the value of the
capacitor. The unit is housed in a perforated plastic case.
Fig 7.6 (b) shows the electrical circuit arrangement for the DYNA
1750 unit.
The unit is connected in series with a resistor with the output taken
from the resistor. With an alternating voltage applied to the input, the
output voltage will vary with humidity due to the variation of
capacitance of the transducer.
Dynalog (India) Ltd.
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Environmental Measurements
Chapter 7
IT 01
Curriculum Manual
The main characteristics of the device are :
Type
90001
Capacitance (25OC, 45%R/H)
Sensitivity
Humidity Range
Table 7.4
122pF ± 15%
0.4pF/%RH
10% - 90% RH
Ambient Humidity
Note : R/H is Relative Humidity,
X 100%
Saturated Air
The device is slow to respond fully to humidity changes, taking in the
order of minutes, but this will normally be of no consequence in
practice since natural changes in humidity are very slow.
The variation of output voltage from the circuit is only a small
percentage of the output and this is difficult to detect.
In the practical exercise you will use signal processing circuits which
are available on the DYNA 1750 Trainer to convert the output to a DC
signal, balance out the standing DC level and thus enable amplification
of the small voltage changes.
Notes :
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
108
Dynalog (India) Ltd.
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Curriculum Manual
7.6
Environmental Measurements
Chapter 7
Practical Exercise
Characteristics of a Humidity Transducer
1
2
3
Connect the circuit as shown in Fig 7.7, setting the AC
Amplifier gain control to 1000 and the Amplifier #1 GAIN
COARSE control to 100 and GAIN FINE to 1.0.
Switch ON the power supply, remove the leads from the
Differential Amplifier inputs and connect a short circuit
between them. Adjust the OFFSET control of Amplifier #1 for
zero output. Switch GAIN COARSE to 100 and make a final
adjustment.
Replace the connections to the inputs of the Differential
Amplifier and adjust the control of the 10kO carbon resistor for
zero output from amplifier #1. It may be advisable to set the
coarse gain to 10 initially and then back to 100 finally during
this process.
The bridge circuit is now balanced for the ambient conditions, the
Differential amplifier input from the 10kO variable resistor balancing
that from the rectifier.
Dynalog (India) Ltd.
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Environmental Measurements
Chapter 7
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IT 01
Curriculum Manual
Note the output voltage from the rectifier circuit as indicated by
the digital voltmeter.
Output Voltage
Digital Meter
Ambient Conditions
After Breathing
Moving Coil
Meter
0
V
V
V
V
Table 7.5
5
6
7
Now place your mouth near the humidity transducer and breath
on it for a short time. The reading indicated by the Moving Coil
Meter will change slowly.
Note the maximum value of the voltage and also the reading of
the digital voltmeter.
Switch OFF the power supply.
Note : It is advisable to check the OFFSET of Amplifier #1 at regular
intervals in case there has been any drift. This can be checked
by just removing both of the input connections from the
Differential Amplifier. The OFFSET control can then be
adjusted if necessary.
For more superior output (more deflection of moving coil
meter) add X100 amplifier before amplifier # 1.
The ambient humidity conditions should not change during the test, but
should a change occur, the bridge output will not return to zero.
110
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Curriculum Manual
Rotational Speed or Position Measurements
Chapter 8
Chapter 8
Rotational Speed or Position Measurements
Objectives of
this chapter
Equipment
Required for
this Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the construction, principles and
application of slotted Opto transducers for counting
and speed measurement
2
Describe the construction, principles and
application of Reflective Opto Transducers and
Gray Coded Disc for position measurement.
3
Describe the construction, principles and
application of Inductive Transducers for speed
measurement.
4
Describe the construction, principles and
application of Hall Effect Transducers to speed and
positional measurement.
5
Describe the construction, principles and
application of a Tacho-Generator to speed
measurement
1
2
3
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Digital Multimeter.
111
Rotational Speed or Position Measurements
Chapter 8
8.1
IT 01
Curriculum Manual
The Slotted Opto-Transducer
Fig 8.1 (a) shows the construction of a slotted opto transducer,
consisting of a gallium arsenide infra-red LED and silicon
phototransistor mounted on opposite sides of a gap in the case, each
being enclosed in a plastic case which is transparent to infra-red
radiations.
The gap between them allows the infra-red beam to be broken when a
solid object is inserted.
The collector current of the phototransistor is low when the infra-red
beam is broken and increases when the beam is admitted Positive
voltage pulses are obtained from the emitter circuit of the
phototransistor each time the beam is admitted and hence the device
generates pulses which are suitable for counting rotations.
A slotted aluminum disc connected to the motor shaft assembly rotates
in the transducer gap in the DYNA 1750 unit and an LED is provided
to indicate when the slot position allows the beam to be admitted.
Fig 8.1 (b) shows the electrical circuit arrangement for the DYNA
1750 unit.
The main characteristics of the device are :
Type
K8102
Output Voltage (beam broken)
0.1V
Output Voltage (beam admitted)
4.9V
Table 8.1
112
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8.2
Rotational Speed or Position Measurements
Chapter 8
Practical Exercise
Characteristics of a Slotted Opto Transducer
1
2
3
Connect the circuit as shown in Fig 8.2 and set the 10kO
wirewound resistor control fully counter-clockwise for zero
output voltage.
Switch ON the power supply.
Rotate the shaft by hand using the large aluminum disc
provided with the Hall effect device. Note and record in Table
8.2 the output voltage from the Slotted Opto Transducer output
socket and also the state of the indicating LED :
(a) with the beam broken by the aluminum disc, and
(b) with the beam admitted through the slot in the
aluminum disc.
Beam Broke
Output Voltage
V
Beam
Admitted
V
LED – ON/OFF
Table 8.2
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4
5
6
7
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Curriculum Manual
Set the Timer/Counter to COUNT and FREE RUN. The display
should show zero. If not, press RESET.
Rotate the shaft backwards and forwards by hand so that the
slot in the aluminum disc passes, between the opto transducer.
Note the counter display, this should increment by 1 each time
the slot is in line with the transducer beam. This illustrates the
use of the opto transducer for counting applications.
Now adjust the 10kO wirewound resistor control to give a drive
voltage to the motor of 3.5V as indicated by the Moving Coil
Meter. The motor should operate and rotate the shaft.
The counter value will increment once for each revolution of
the shaft and can be used to measure the shaft speed:
8
Motor Drive
Voltage (volts)
Shaft speed
(rev/sec)
Shaft Speed
(rev/min)
Press the RESET button and hold down. With a watch, stop
watch if available, release the reset button at a suitable time and
note the count value after one minute. This value represents the
shaft speed in revolutions per minute (rev/min). Record the
value in Table 8.3.
3.5
4
4.5
5
6
7
8
9
10
9
Repeat with a motor drive voltage of 4V and add the result to
Table 8.3.
10
Set the COUNTER/TIMER FREE RUN/1s switch to 1s (1
second). Set the 10kO resistor to give a motor drive voltage of
5V. Press the RESET button of the counter.
The counter now counts for one second and the count value is
“frozen” at the end of this time. The count displayed represents
the number of revolutions per second of the shaft Press RESET
again. The displayed value should correspond with the previous
value. Record the value in Table 8.3 in the relevant row.
114
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11
12
13
Rotational Speed or Position Measurements
Chapter 8
Repeat the procedure with the other motor drive voltages
shown in Table 8.3 and for each setting note the shaft speed in
rev/sec as displayed by the counter and add to the table. Switch
OFF the power supply.
Multiply each recorded value by 60 to give the shaft in
revolutions per minute (rev/min or rpm) and add to Table 8.3.
Plot the graph of motor speed in rev/min against drive voltage
on the axes provided:
Keep the motor drive circuits connected for later experiments.
Dynalog (India) Ltd.
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Rotational Speed or Position Measurements
Chapter 8
8.3
IT 01
Curriculum Manual
The Reflective Opto Transducer
Fig 8.3 shows the construction of a reflective opto transducer,
consisting of an infra-red LED and phototransistor. This is similar to
the slotted opto transducer, but in this device the components are
arranged so that the beam is reflected back if a reflective surface is
placed at the correct distance. A non reflective surface breaks the
beam.
Three separate units are provided with the DYNA 1750 unit, being
mounted in line vertically. The reflective surface is a Gray-coded disc,
which is fixed approximately 4mm from the transducers.
With the beam not reflected the output from the phototransistor emitter
is low. When the beam is reflected the output is high.
Three LED’s are provided to indicate when the beam is reflected from
the respective transducer unit.
The output A is the least significant bit (LSB) and C is the most
significant bit (MSB).
The Gray code is used for the encoded disc rather than normal binary
because only one digit changes state at any boundary with this code
and this minimizes any possibility of error in identifying the actual
position when at a segment boundary.
116
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Rotational Speed or Position Measurements
Chapter 8
The arrangement of the Gray-coded disc and the respective LED
outputs is shown in Fig 8.4.
The dark areas break the beam and produce a low output from the
associated transducer and the bright areas reflect the beam and produce
a high output.
The DYNA 1750 unit operates as a rotational angular position
transducer but similar principles can be used for linear position
applications.
Slotted opto devices could be used with a transparent disc (transparent
where the above disc is reflective).
Fig 8.5 shows a linear Gray-coded track, the A track is the LSB and C
the MSB.
The resolution provided with a 3-bit code (3 opto devices) is poor but
this can be improved by increasing the number of devices and tracks.
Dynalog (India) Ltd.
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Note the gray code pattern:
LSB
A
B
MSB C
Table 8.4
START
1 unit length ‘o’
2 unit length ‘o’
4 unit length ‘o’
REPEATS
2 unit length ‘1’ 2 unit length ‘o’
4 unit length ‘1’ 4 unit length ‘o’
8 unit length ‘1’ 8 unit length ‘o’
The electrical circuit arrangement for the DYNA 1750 unit is shown in
Fig 8.6:
The main characteristics of the device are:
Type
Output Voltage (beam broken)
Output Voltage (beam admitted)
Table 8.5
118
K8711
0.5V
5V
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Curriculum Manual
8.4
Rotational Speed or Position Measurements
Chapter 8
Practical Exercise
Characteristics of Reflective Opto Transducers and Gray Code
Disc
1
2
3
Connect the circuit as shown in Fig 8.7 with the digital
multimeter on the 20V DC range.
Switch ON the power supply and rotate the drive shaft by hand
to alter the LED states.
Rotate the shaft until it is in the position with all LED’s OFF.
Use the digital multimeter to measure the voltage at each of the
outputs and recorded in Table 8.6.
Output
A
B
C
4
Dynalog (India) Ltd.
Output Voltage
LED OFF
LED ON
V
V
V
V
V
V
Turn the shaft until all LED’s are ON and repeat the readings,
recording the results again in Table 8.6.
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Rotational Speed or Position Measurements
Chapter 8
5
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Curriculum Manual
With the shaft initially in the position with all LED’s OFF,
rotate the shaft counterclockwise, when looking at the coded
side of the disc, and note the state of the LED’s at each change
of state.
Denote an LED OFF as logic state 0 & LED ON as logic state 1
6
Record the values in Table 8.7.
Position
0
1
2
3
4
5
6
7
Table 8.7
C
B
A
Check the sequence against that shown in the table in Fig 8.4.
7
120
Switch OFF the power supply.
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Curriculum Manual
8.5
Rotational Speed or Position Measurements
Chapter 8
The Inductive Transducer
Fig 8.8 shows the construction and electrical circuit arrangement for
the Inductive Transducer provided with the DYNA 1750 unit.
This consists of a 1mH inductor and a slotted aluminum disc fitted to
the drive shaft which rotates above the inductor. The inductance of the
unit varies with the position of the slot. With an aluminum disc the
inductance increases with the slot positioned directly above the
inductor.
If a magnetic disc was used, the inductance would decrease for the
condition when the slot was above the inductor.
Note that, if unscreened, an inductor will be liable to pick up any stray
interference, such as that which may be generated by the motor
commutator switching. This can generate spurious short duration
output pulses which may need to be suppressed by using a low pass
filter.
The main characteristics of the device (in circuit under the disc) are:
Inductance
Inductance change
Output voltage
Output voltage change
Table 8.8
Dynalog (India) Ltd.
(under slot)
(under disc)
(under slot)
(under disc)
1mH
7 µH
6.9mV
2mV
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Rotational Speed or Position Measurements
Chapter 8
8.6
Practical Exercise
Characteristics of an Inductive Transducer
1
2
3
4
122
IT 01
Curriculum Manual
Connect the circuit as shown in Fig 8.9. set the AC Amplifier
gain to 100 and Amplifier #1 GAIN COARSE to 10 and GAIN
FINE to 1.0. Set the drive shaft with the disc slot in the top
vertical position.
Remove the leads form the input to the Differential Amplifier,
short the inputs together and switch ON the power supply.
Adjust the OFFSET control of Amplifier #1 for zero output.
Replace the leads to the input of the Differential Amplifier and
adjust the control of the 10kO 10-turn resistor so that the meter
reading is again zero. The control setting will be critical with
such high overall amplifier gains.
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Curriculum Manual
5
Rotational Speed or Position Measurements
Chapter 8
Check the zero reading and then rotate the motor shaft to obtain
the maximum output voltage when the slot is immediately
above the Inductive Sensor. Note the value of this voltage:
Output voltage with slot over the inductor =
V
This indicates an application of inductive transducers to proximity
detection of metallic objects. The device can also be used for counting
or speed measurement applications.
6
Dynalog (India) Ltd.
Switch OFF the power supply. Retain your circuit, but remove
the Moving Coil Meter from the output of Amplifier #1, add
comparator between DIFFERENTIATOR and COUNTER
TIMER and then add the circuit of Fig 8.10.
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a
Set the motor speed to zero and comparator switch OFF.
b
Set the TIME CONSTANT switch of the Low Pass Filter and
the Differentiator to 1s and set the counter to COUNT and 1s.
c
Switch on the power supply.
d
Apply 3.5V input to the DC motor so that the shaft rotates
slowly. Press the counter reset button several times and note the
displayed value, this represents the speed in rev/sec.
Speed of the shaft recorded with the Inductive Sensor =
e
Remove the lead from the o/p of the Low Pass Filter to the
Differentiator and take the lead from the input of the Low Pass
Filter and connect it to the Differentiator input. Press the
Counter RESET button several times and observe the result. If
the result is zero, then refer to the re-calibration procedure
described in the next point and repeat the counts with and
without the Low Pass Filter. When a reading has been observed
restore the Low Pass Filter back into the circuit by moving the
lead back and adding the connection between the Low Pass
Filter and Differentiator.
Speed of the shaft recorded without Low Pass Filter =
124
f
Re-calibration the Inductive sensor circuit by removing the
lead from the MC meter to the Power Amplifier and
connecting it between the MC meter and the output of
Amplifier #1. Adjust the control of the 10kO 10-turn resistor
so that the meter reading is zero. Then reconnect the MC
meter to the Power Amplifier.
g
Remove the Counter input lead from the Differentiator output
and connect it to the output from the Slotted Opto Transducer.
Press the counter reset button and note the displayed reading
which also represents the shaft speed. Compare these value
with the value obtained from using the Inductive sensor.
h
Repeat the two measurements for the motor input voltages and
complete Table 8.9 on the next page.
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Curriculum Manual
Motor Voltage
Shaft Speed (rev/sec)
Inductive
Transducer
Slotted Opto
Transducer
Table 8.9
Rotational Speed or Position Measurements
Chapter 8
3.5V
5V
You will note that a considerable amount of signal conditioning has
been required for the inductive transducer unit due to the small output
voltage available and also the problem of the susceptibility of the
counter to voltage spikes.
Notes :
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Dynalog (India) Ltd.
125
Rotational Speed or Position Measurements
Chapter 8
8.7
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Curriculum Manual
The Hall Effect Transducer
Fig 8.11shows the layout and electrical circuit arrangement of the Hall
Effect Transducer assembly fitted to the DYNA 1750 Trainer and
illustrates the Hall Effect principle.
Hall Effect Principle
When current flows through the flat slice of semiconductor at rightangles to a magnetic field there is a force on each individual electron
which tends to move it in one particular direction (the motor principle).
The current is pushed to one side of the slice. The surplus of electron
on one side of the slice means that this side is negatively charged,
resulting in an EMF across the slice (the Hall voltage VH) which is at
right-angles to both the current and the magnetic field. The value of
this voltage is directly proportional to the strength of the magnetic
field.
The transducer provided on the DYNA 1750 Trainer also contains an
active silicon semiconductor device to increase the output voltage and
provided differential outputs, one going more positive and the other
more negative (less positive).
The main characteristics of the device are:
Output voltage (+) (no field)
Output voltage (-) (no field)
Output voltage change
Output voltage change (under magnet)
126
1.75-2.25V
1.60V
7.5-10.6mV/mT
380mV
Dynalog (India) Ltd.
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Curriculum Manual
8.8
Rotational Speed or Position Measurements
Chapter 8
Practical Exercise
The Characteristics of a Hall Effect Transducer
1
2
3
4
Connect the circuit as shown in Fig 8.12. set the Amplifier #1
GAIN COARSE control to 10, GAIN FINE to 0.8 and the
motor drive voltage to zero. Switch ON the power supply.
Set the drive shaft position so that the magnet in the Hall effect
disc is horizontal (to one side) so that there is no magnetic filed
cutting the Hall effect device.
Adjust the OFFSET control of Amplifier #1 for zero output
indication on the Moving Coil Meter.
Note the output voltage from the – and + output sockets of the
Hall Effect device with the digital voltmeter directly on the
Hall Effect sensor panel and the also from the Moving Coil
Meter. Record the results in Table 8.11.
Magnetic Filed
None
Maximum
Digital Multimeter
Output Voltage (- Output Voltage (+)
)
Moving
Coil Meter
0
V
V
V
V
V
V
Table 8.11
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127
Rotational Speed or Position Measurements
Chapter 8
5
6
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Curriculum Manual
Rotate the disc so that the magnet is directly above the Hall
effect device. This position will be indicated by the maximum
output voltage.
Note the voltages again and record in Table 8.11.
These readings illustrate the basic characteristics of the Hall Effect
device and indicate its application to proximity detection. It is also
suitable for speed measurement applications.
7
8
9
With the output of amplifier #1 connected to the Counter/Timer
input set the controls for COUNT and 1s.
Transfer the digital multimeter to the output of the power
Amplifier and apply an input voltage of 3.5V to the motor so
that the shaft rotates slowly. Press the counter RESET button
and note the displayed value, this representing the shaft speed
in rev/sec. Record the result in Table 8.12
Remove the input to the counter from Amplifier #1 and connect
it to the output of the Slotted Opto Transducer unit. Press the
counter “reset” button and note the displayed value, this being
the shaft speed for comparison with the previous readings. Add
the value to Table 8.12.
Motor Voltage 3.5V
Shaft Speed (rev/sec)
Hall
Effect
Transducer
Slotted
Opto
transducer
10
5V
7V
10V
Repeat the procedure for the other values of motor drive
voltage given in Table 8.12 for comparison. Switch OFF the
power supply.
Hall Effect devices are available for proximity detection, linear or
angular displacement, multiplier and current or magnetic flux density
measurement applications.
128
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Curriculum Manual
8.9
Rotational Speed or Position Measurements
Chapter 8
The DC Permanent Magnet Tacho-Generator
Fig 8.13 shows the construction and electrical circuit arrangement of
the DC Permanent Magnet Tacho-Generator fitted to the DYNA 1750
Trainer. This consists of a set of coils connected to a commutator
which rotate inside a permanent magnet stator.
The rotating assembly is called the armature. With the coil rotating, an
alternating EMF is generated in them. The commutator converts this to
DC.
The magnitude of the generated EMF is proportional to the rate of
cutting flux and therefore to the rotational speed. The polarity depends
on the direction of cutting flux and therefore on the direction of
rotation.
The diode are fitted to limit any voltage spikes that may be generated
by the commutation process (i.e. conversion from AC to DC to a
maximum of ±12V.
The main characteristics of the device are:
Open circuit voltage (12V to motor)
Short circuit current (12V to motor)
Output impedance
Output noise
Table 8.13
Dynalog (India) Ltd.
10.5V
750mA
39O
200mV p-p
129
Rotational Speed or Position Measurements
Chapter 8
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Curriculum Manual
8.10 Practical Exercise
Characteristics of a Permanent Magnet DC Tacho-Generator
1
2
3
130
Connect the circuit as shown in Fig 8.14.
Set the COUNTER/TIMER control to COUNT and 1s.
Set amplifier #1 GAIN COARSE control to 10 and GAIN
FINE to 0.1.
Switch ON the power supply.
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Curriculum Manual
4
Rotational Speed or Position Measurements
Chapter 8
Apply an input to the motor and set the shaft to 5 rev/sec (note:
Table 8.3 and Graph 8.1 may help) as indicated by the counter
after pressing the RESET button. Note the output voltages
indicated on the Moving coil Meter and record the value in
Table 8.14.
Shaft speed (rev/sec)
Output Voltage
(Moving Coil Meter)
Table 8.14
5
6
Dynalog (India) Ltd.
5
10
V
20
V
28
V
V
Repeat the procedure for the other shaft speed setting indicated
in Table 8.14.
Draw the graph of output voltage against shaft speed on the
axes provided.
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Rotational Speed or Position Measurements
Chapter 8
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Calibration of the Moving Coil Meter to Indicates Speed Directly.
The scale to be used is 20V represents 2000 rev/min (100 rev/min/V).
7
8
9
10
132
Transfer the connection of the Moving Coil Meter from the
input of Amplifier 31 to the output of amplifier #1. Set the
GAIN FINE control to just a little above 0.5.
Apply a low input to the motor and set the shaft speed to 5
rev/sec (300rev/min)as shown on the Counter after pressing
RESET. Adjust the OFFSET control of Amplifier #1 to set the
Moving Coil Meter reading to –7V (Fig 8.15).
Change the motor drive voltage to set the shaft speed to 30
rev/sec (1800 rev/min) as shown on the Counter after pressing
RESET. Adjust the GAIN FINE control of Amplifier #1 so that
the Moving Coil Meter indicates +8V (Fig 8.15).
Repeat both of the above setting and adjustments as often as
necessary to make both of them correct (changing one of them
will have altered the other. Some anticipation may be helpful).
The meter will then be calibrated as shown in Fig 8.15.
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Curriculum Manual
Rotational Speed or Position Measurements
Chapter 8
11
Use the calibrated Moving Coil meter to set the motor speed as
shown in Table 8.15.
12
Calculate the corresponding speed in rev/sec and then check at
each setting against those obtained from the Opto Transducer &
Counter.
Shaft Speed
(rev/min)
Calculated
shaft
speed (rev/sec)
Shaft Speed from
Counter (rev/sec)
13
Dynalog (India) Ltd.
600
1000
1200
1600
Switch OFF the power supply.
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Rotational Speed or Position Measurements
Chapter 8
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Notes :
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134
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Curriculum Manual
Sound Measurements
Chapter 9
Chapter 9
Sound Measurements
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the construction and characteristics of a
dynamic microphone.
2
Describe the construction and characteristics of an
ultrasonic receiver and transmitter.
3
Compares the various methods of measuring sound
signals.
1
2
3
4
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Oscilloscope.
12-inch (30cm) ruler (not supplied).
135
Sound Measurements
Chapter 9
9.1
IT 01
Curriculum Manual
The Dynamic Microphone
The construction of the dynamic microphone is shown in Fig 9.1 (a),
consisting of a coil attached to a thin diaphragm, the coil being
suspended in the field of a permanent magnet.
The diaphragm moves in response to any vibration in the air caused by
sound and moves the coil in the magnetic field. An alternating EMF is
induced in the coil, the magnitude and frequency of which is
proportional to the sound vibrations.
The electrical circuit for the device provided with the DYNA 1750 unit
is shown in Fig 9.1 (b).
The 680O resistor is fitted to provided a load correctly matched to the
output impedance, (500O), of the microphone.
The main characteristics of the device are:
Output impedance
500O
Frequency response (-3dB)
100Hz – 10kHz
Output voltage
5mV (normal maximum)
Table 9.1
136
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Curriculum Manual
9.2
Sound Measurements
Chapter 9
Practical Exercise
Characteristics of a Dynamic Microphone
It is most unlikely that your laboratory will include a broadband
constant output audio generator systems/loudspeaker amongst its
facilities. Even if it did, a full acoustic booth would be required, and
the noises generated would be unacceptable for other laboratory users.
We are therefore not able to test the full dynamic range of a
microphone, either for frequency or amplitude. It is therefore necessary
for us to limit the investigation to a review of the measurement
techniques which can be adopted, and it is these which will be
examined, rather than the microphone itself.
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137
Sound Measurements
Chapter 9
IT 01
Curriculum Manual
In this exercise three different forms of monitoring device will be
investigated. The response time of digital multimeters is too slow to
make any record of the signals at all, due to the transient nature of
sound.
1
Connect the circuit as shown in Fig 9.2. Set the AC Amplifier
gain control to 1000 and the Amplifier #1 GAIN COARSE to 1
and GAIN FINE to 0.4 to give unity gain initially (4.0).
The LED bargraph display has an excellent response time and requires
0.5V for each bar, 5V to light the whole display. This type of device is
often used on HI-FI systems.
2
3
4
Switch ON the power supply. Check the OFFSET control of
Amplifier #1 for zero with the Moving Coil Meter temporarily
connected to its output. Note the display on the Bargraph when
the bench is tapped with the finger.
Tap the case of the 1750 unit and observe the effect on the
Bargraph display.
Change the GAIN COARSE of Amplifier #1 to 10 and the
FINE GAIN to 1.0 then talk, cough, sign or whistle near the
unit. You will find that the bargraph will respond to any sound
made, but needs more gain for speech or whistling.
A Moving Coil Meter is frequently used by sound (audio) engineers to
indicate peak power (PPM, peak power meter), but requires a rectifier
and amplifier since the moving coil meter only responds to DC, and its
movement is slow to respond due to inertia and damping.
5
The Moving Coil Meter is connected to the AC Amplifier
output via the Full Wave rectifier #2. Set the GAIN COARSE
to 100 and GAIN FINE to 1.0 for maximum additional gain
(100) and zero the indication of the meter using the OFFSET
control. Tap the baseboard so that all LED’s of the bargaph are
lit and note the maximum reading of the Moving Coil Meter.
Maximum voltage indication given by the Bargraph is 5V.
Maximum voltage output (Moving Coil Meter) =
138
V
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Curriculum Manual
Sound Measurements
Chapter 9
Without any doubt, the oscilloscope is the most versatile device for
monitoring sound, since it is able to give an indication of frequency,
waveform and magnitude of signals and is very sensitive, even to small
signals.
6
7
Set the timebase of oscilloscope to 2ms/div and the CH.1 Y1
amplifier to 1V/div.
Generate various types of sound and observe the display on the
oscilloscope. Note that sound engineers, to save their
embarrassment, will often count – say from one through ten
and back again – to test a microphone circuit.
It may be necessary to vary the Y amplifier setting to obtain the
most satisfactory displayed waveform.
8
Dynalog (India) Ltd.
Change the timebase setting to 0.5ms/div. And try whistling
two different notes, one low pitch and the other high, and
observe the effect on the number of cycle (frequency) of the
displayed waveform.
139
Sound Measurements
Chapter 9
9.3
IT 01
Curriculum Manual
The Ultrasonic Transmitter/Receiver
The construction of both ultrasonic devices and their electrical circuit
arrangements for the DYNA 1750 unit are shows in Fig 9.3.
The receiver and transmitter are almost identical and consist of a slice
of ceramic material with a small diaphragm fixed to it, inside the case
of the unit.
The operation of the receiver relies on the principal that certain
ceramic materials produce a voltage when they are stressed. This is
referred to as the piezo-electric principal. Vibration of the diaphragm
stress the ceramic material and produces an output voltage. The
reciprocal applies to the transmitter. An applied alternating voltage
produces stress which causes the ceramic slice to vibrate.
The dimensions of the components are arranged so that there is
resonance (best response) at around 40kHz. This is above the audible
range (maximum 20kHz) and is therefore referred to as ultrasonic. The
ceramic slice is arranged in four quarters which are connected in series
for the receiver and in parallel for the transmitter.
The main characteristics of the devices are:
Receiver
Peak resonance (typical)
Directional angle
Impedance
Output amplitude
Table 9.2
140
Transmitter
40kHz
30O
30kO
5-60mV
500O
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9.4
Sound Measurements
Chapter 9
Practical Exercise
Characteristics of an Ultrasonic Transmitter/Receiver
1
2
3
Dynalog (India) Ltd.
Connect the circuit as shown in Fig 9.4. Set the AC Amplifier
gain control to 1000 and Amplifier #1 GAIN COARSE control
to 100 and GAIN FINE to 1.0. Switch the Low Pass Filter time
constant to 100ms.
Switch ON the power supply and adjust Amplifier #1 OFFSET
to give zero output on the Moving Coil Meter.
Note the bar graph display as you move your hand or any other
object over the ultrasonic devices. The display should respond,
indicating the receipt of a signal of frequency 40kHz by the
ultrasonic receiver.
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Sound Measurements
Chapter 9
4
5
6
IT 01
Curriculum Manual
Place a small book (approximately 6 inches (15cm) X 4 inches
(10cm) or other flat object 3 feet (90cm) above the Ultrasonic
Transducers. Slowly move the object closer to the transducers,
watching the output reading on the bargraph display, until the
object is covering the transducers.
Increases the Amplifier #1 GAIN FINE control to 1.0. Hold a
thin object such as a pencil approximately 6 inches (15cm)
above the Ultrasonic Transducers, move it horizontally and
vertically and note the effect on the output response. This
indicates how critical the direction angle is for the device.
Put a sheet of paper over the Ultrasonic Transducers to
intercept the path and move your hand up and down above the
transducers.
In this exercise the received signal has been amplified, rectified,
filtered (to remove all unwanted frequencies) and then amplified again
to operate the display.
Pulsed ultrasonic devices can be used for distance measurement to
reflecting surfaces by measurement of the time between the
transmission and return of the pulsed signal.
142
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Sound Output
Chapter 10
Chapter 10
Sound Output
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the construction and characteristics of a
moving coil loudspeaker.
2
Describe the construction and characteristics of a
buzzer.
1
2
3
4
5
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
BNC to 4mm Connecting Lead.
Oscilloscope.
Function Generator.
143
Sound Output
Chapter 10
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10.1 The Moving Coil Loudspeaker
The construction of a moving coil loudspeaker is shown in fig 10.1 (a).
It is similar to the moving coil microphone. The permanent magnet,
coil and diaphragm are much the same but in this device the diaphragm
is attached to a large paper cone supported by a frame.
Alternating currents flowing in the coil cause it react with the magnetic
field and move in and out. With applied currents at frequencies in the
audible range, the cone movement will cause a variation of pressure in
the surrounding air particles and produce sound waves that are audible
to the human ear. If a speaker is placed in a vacuum, there are no air
particules, so the movement of the cone does not produce any sound.
The electrical circuit of the device fitted to the DYNA 1750 unit is
shown in Fig 10.1 (b). The 100O resistor is fitted to limit the maximum
power dissipation to 100mW, half of the rated value for the
loudspeaker.
The main characteristics of the device fitted to the DYNA 1750 unit
are:
Impedance
Power rating
Frequency response (-3dB)
Table 10.1
8O
200mW rms.
400-5000Hz
Note that the speaker response is well below the maximum frequency
detectable by the human ear (approximately).
144
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Chapter 10
10.2 Practical exercise
Characteristics of a Moving Coil Loudspeaker
1
2
3
4
Connect the circuit as shown in Fig 10.2 and switch ON the
power supply.
Set the 10kO wirewound variable resistor to position 5 on its
scale (see Fig 10.2).
Set the oscilloscope timebase initially to 1ms/div, CH.1 Y
amplifier to 5V/div and CH.2 Y amplifier to 0.2V/div.
Set the function generator to 200Hz sinewave output and adjust
the amplitude control to maximum and then adjust the 10kO
wirewound resistor to give a signal input of 10Vp-p (2 div.) as
seen on CH.1 of the oscilloscope. The signal input level of
10Vp-p is to be carefully maintained for tests at all frequencies.
The microphone and its amplifier will pick up all of the background
sounds and interference in the laboratory. Try to ignore these in taking
your readings at lower signal levels. You will be contributing to other
peoples background noise, so try to keep yours to a minimum.
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5
Frequency (Hz)
Output Voltage
Vpeak-to-peak
6
7
Take readings at each of the frequencies given in Table 10.2,
ensuring that the unit signal remains constant at 10Vp-p.
200
300
400
500
600
700
800
900
1k
2k
3k
One of your readings should have been much greater than any
of the rest. Return to this frequency and use the fine frequency
control on the function generator to peak the signal to
maximum. Record the value in table 10.3.
Ensure that the timebase controls are in the calibrated settings
and measure the number of divisions taken for one complete
cycle. Record in table 10.3:
Peak Signal
amplitude
Vp-p
Numbers of
divisions
Time for
one cycle
(T)
ms
Frequency f
= 1/T
Hz
Table 10.3
The time for one cycle is calculated by multiplying by the
timebase setting, for example 6.7 x 0.2ms = 1.34ms.
1
The reciprocal of this gives the frequency:
= 746Hz.
-3
1.34X10
Note that this example has been chosen to be different from the
result which you should get.
146
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Chapter 10
The frequency which you calculate is the natural resonant frequency of
the loudspeaker. The response curve of the loudspeaker has a very
pronounced peak at this frequency. It is caused by the dimensions of
the loudspeaker cone, largely the cone diameter.
8
Plot the response of the loudspeaker on the axes provided. A
logarithmic scale is used for frequency because this matches
the response of the ear.
If this type of loudspeaker was used for music output then the response
of the electronic driving circuit would need to be shaped to compensate
for the response. This would be done by boosting both the lower and
higher frequencies.
If used as an alarm generator then it would be best to choose the
resonant frequency for greatest efficiency, to generate the loudest
sound output from a given power input.
9
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Switch OFF the power supply.
147
Sound Output
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10.3 The Buzzer
The construction of the buzzer used in the DYNA 1750 unit is shown
in fig 10.3 (a).
A small transistorized oscillator circuit feeds an alternating EMF to an
iron cored coil. The alternating magnetic field produced by the coil
attracts and repels a small permanent magnet attached to a spring. This
magnet vibrates against a diaphragm and creates a loud noise.
In control system applications the device is used as an alarm
indications.
The electrical circuit of the device is shown in Fig 10.3 (b).
The diode is fitted to prevent damage to the transistorized circuit if the
supply is connected with incorrect polarity. The polarity of the input
supply should be positive. The rated voltage is 12V.
The main characteristics of the device fitted to the DYNA 1750 unit
are:
Supply voltage
8V
12V
16V (max.)
Supply current
15mA
-
30mA
-
400Hz
-
Output frequency
Output sound level
Table 10.4
148
70dBA at 7.87” (20cm)
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Sound Output
Chapter 10
10.4 Practical Exercise
Characteristics of a Buzzer
1
Connect the circuit as shown in Fig 10.4. Set the control of the
10kO resistor for zero output voltage (fully counter-clockwise).
Connect the digital multimeter as an ammeter on the 20/200mA
range between the output of the power amplifier and the buzzer
to monitor the buzzer current. Set the A.C. Amplifier to 1000
and the Differentiator to 1s.
Note : When you first switch on, there may be readings on the counter
immediately, due to background noise being picked up by the
microphone and processed by the Counter. The Readings
should be ignored as they will not affect the experiment results.
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2
Switch ON the power supply and adjust the 10kO resistor to
increase the voltage applied to the buzzer. Note the voltage on
the Moving Coil Meter at which the buzzer begins to operate.
Press RESET on the Counter to read the Buzzer frequency.
The buzzer begins to operate at
3
V
at a frequency of
Hz
Alter the setting of the 10kO resistor to increase the voltage
applied to the buzzer to 7V, 7.5V, 8V and then 10V as given in
Table 10.5. Record the current and frequency at each step.
Voltage
Current
Frequency
7V
7.5V
8V
10V
12V
mA
mA
mA
mA
mA
Hz
Hz
Hz
Hz
Hz
Table 10.5
4
5
150
Transfer the positive lead of the digital multimeter from the
output of the Power Amplifier to the +12V socket to bypass the
10kO resistor and Power Amplifier and apply the full 12V
directly to the buzzer. Record the current and frequency again
in Table 10.5.
Switch OFF the power supply.
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Linear or Rotational Motion
Chapter 11
Chapter 11
Linear or Rotational Motion
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the construction and characteristics of a
DC solenoid.
2
Describe the construction and characteristics of a
DC relay.
3
Describe the construction and characteristics of a
DC solenoid air value.
4
Describe the construction and characteristics of a
DC permanent magnet motor.
1
2
3
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Digital Multimeter.
151
Linear or Rotational Motion
Chapter 11
IT 01
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11.1 The DC Solenoid
The construction of a DC solenoid is shown in Fig 11.1 (a), consisting
of a soft iron core and actuator shaft which is free to move inside a
coil.
When the coil is energized, the soft iron core is attracted inside the coil
and is held in position. When the coil is de-energized, the core returns
to its neutral position under the action of a return spring.
The voltage required to attract the core into the coil will be less than
the rated value and will depend on the load applied to the actuator
shaft. The voltage at which the core is pulled in by the coil is referred
to as the pull-in voltage.
With the coil energized and the core attracted, if the coil voltage is
reduced gradually, when the voltage has fallen sufficiently the core
will return to its neutral position under the action of the spring. This
voltage is referred to as the drop-out or release voltage. The release
voltage will be much less than the pull-in voltage.
Fig 11.1 (b) shows the electrical circuit arrangement of the device
fitted to the DYNA 1750 Trainer.
When the coil is de-energized a large EMF can be induced in the coil,
the magnitude depending on the inductance and the rate of change of
current. Diodes are provided to limit the induced voltage to a
maximum of ± 12V.
The main characteristics of the coil fitted to the DYNA-1750 Trainer
are:
152
Resistance
50O
Pull-in voltage
6V
Coil rating
Table 11.1
12V/3W
Release voltage
1V
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Linear or Rotational Motion
Chapter 11
11.2 Practical Exercise
Characteristics of a DC Solenoid
1
2
Connect the circuit as shown in Fig 11.2and set the 10kO
resistor for zero output voltage (control fully counter
clockwise). Connect the digital multimeter as an ammeter on
the 200mA range in between the Power Amplifier and the
solenoid.
Switch ON the power supply and rotate the 10kO resistor
control to gradually increase the voltage applied to the solenoid
coil. Note the voltage at which the iron core of the solenoid is
attracted fully into the coil. This value is the pull-in voltage.
Record this voltage and current in Table 11.2 overleaf.
Note: The core will start to move at a lower value than the pull-in
voltage, the actual pull-in voltage will be the value when you
hear the click, as the core aligns itself inside the coil. In this
position you will find a distinct resistance to pushing the
actuator back towards its neutral position.
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Unloaded
Readings
Pull In
Drop-out
(Release)
Current
Voltage
Current
V
mA
V
mA
V
mA
V
mA
3
With the coil energized and the core in its pulled in position,
slowly reduce the coil applied voltage and note the value at
which the core returns to its neutral position, the drop-out or
release voltage. Record voltage and current again in Table 11.2.
4
Repeat the process with your finger against the actuator shaft to
exert a little load and note the voltage and current required for
pull in and release.
5
154
Voltage
Loaded
Switch OFF the power supply.
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Chapter 11
11.3 The DC Relay
The construction of a DC relay is shown in Fig 11.3 (a). It consists of a
coil with an iron core which has a soft iron armature attached to a
spring which holds it just above the core.
Changeover contacts are attached to the spring and with the armature
in its rest position it makes contact with one of the terminals. This sis
referred to as the normally closed (N.C.) contact. With the coil
energized, the core will be magnetized and attract the soft iron
armature. The spring is moved, which breaks the connection to the
N.C. terminal and makes the contact to the other terminal. This
terminal is referred to as the normally open (N.O.) contact.
With this construction, the contacts will bounce for a short period each
time they close or open (make or break) and this can cause problems
with some circuits. The problem can be overcome by using as
electronic debounce circuit or a time delay prior to checking the
contact state after operation.
Fig 11.3 (b) shows the electrical circuit arrangement of the device
fitted to the DYNA 1750 Trainer. The diodes limit any induced
voltages to a maximum of approximately ±12V, as for the solenoid
device.
The main characteristics of the device fitted to the DYNA 1750
Trainer are:
Coil rated voltage
12V
Operate/release time
5ms
Coil resistance
320 O
Contact rating
12V, 1A
Coil operating voltage
7.5V
Lifetime cycle
5x106
Coil release voltage
Table 11.3
1.8V
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11.4 Practical Exercise
Characteristics of a DC Relay
1
2
3
4
5
6
156
Connect the circuit as shown in Fig 11.4 and set the 10kO
resistor control for zero output voltage.
Switch ON the power supply. The relay will be in its deenergized state. Note the state of the Lamp. Lamp ON means
that the contacts are closed. Lamp OFF means that the circuit is
broken because the contacts are open.
The relay coil will have pull-in and release voltage
characteristics similar to those for a solenoid.
Determine the pull-in and release voltages and currents for this
device by graduslly increasing and decreasing the applied
voltage. Record the results in Table 11.4 opposite.
Note when a change of state of the Lamp connected to the N.O.
contact occurs.
Move the lamp connection to the N.C. terminal and observe the
effect on the lamp switching. Add to Table 11.4.
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Chapter 11
Lamp state ON/OFF
when connected to:
Voltage
Pull In
Drop-out
(Release)
Table 11.4
7
Current
V
mA
V
mA
N.O. Contact
N.C.
Contact
Switch OFF the power supply.
Notes :
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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11.5 The Air Valve
Fig 11.5 (a) shows the construction of the device fitted to the DYNA
1750 Trainer. It is similar to solenoid considered previously, but the
soft iron core now operates on two valves, the inlet and the exhaust
valves.
With the coil de-energized the core is held, by the return spring, in the
position with the inlet valve closed and the exhaust valve open. In this
position the cylinder port is connected to the exhaust port outlet.
When the coil is energized, the core is attracted and held in the position
with the exhaust valve closed and the inlet valve open. In this position
the inlet port is connected to the cylinder port.
In the DYNA 1750 Trainer, the inlet port is connected to the pump and
the cylinder port is connected to a pneumatic actuator. With the pump
ON, the pneumatic actuator will be operated when the coil is energized
and illustrates the principle of electrical control of pneumatic devices.
The electrical circuit arrangement of the device fitted to the DYNA
1750 Trainer is shown in Fig 11.5 (b).
The main characteristics of the device are:
158
Rated voltage
12V
Coil resistance
140O
Coil pull-in voltage
8.3V
Coil release voltage
Table 11.5
1.7V
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Linear or Rotational Motion
Chapter 11
11.6 Practical Exercise
Characteristics of an Air Valve
1
2
3
4
5
Dynalog (India) Ltd.
Connect the circuit as shown in Fig 11.6. Set the 10kO resistor
control for zero output voltage (fully counter clockwise) and set
the pump control (air Pressure/Flow Sensor panel) to
PRESSURE.
Switch ON the power supply and then switch the pump ON.
The coil is de-energized in this state, the inlet valve is closed,
and the pneumatic actuator will not operate.
Adjust the resistor control to apply 10V to the solenoid coil.
The coil will be energized, the inlet valve will open and the
exhaust valve will close. The pump pressure will be applied to
the pneumatic actuator. Observe the effect on the actuator.
Reduce the voltage and observe the effect on the pneumatic
actuator.
Switch the pump OFF. Observe the effect on the operation of
the pneumatic actuator with no air pressure when the solenoid
voltage is raised and lowered.
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Linear or Rotational Motion
Chapter 11
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The Air valve solenoid will have pull-in and release voltage and
currents as for any solenoid. To determine these values for the device:
6
With the pump switched OFF, increase and decrease the
applied voltage gradually and note the voltages at which
switching occur. You will hear a click when the device
switches.
Voltage
Pull In
Drop-out
(Release)
Table 11.6
7
160
Current
V
mA
V
mA
Switch OFF the power supply.
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Chapter 11
11.7 The DC Permanent Magnet Motor
The construction of permanent magnet DC motor is shown in Fig 11.7.
The unit is identical with the tacho-generator unit but for a motor, a
DC supply is fed to the armature coils.
Current flowing in the armature coils set up a magnetic field which
reacts with the field of the permanent magnet to produce a force
causing the armature to rotate.
The force acting on the armature is proportional to the current flowing.
When the armature rotates, an EMF is induced in the coils, in exactly
the same way as in the tacho-generator. The self-induced EMF oppose
the applied voltage and is referred to as the back EMF. The armature
accelerates until the speed is such as to produce a back EMF (e) equal
to the applied voltage (V) less the voltage dropped across the armature
resistance rai.
V = e + r ai
The speed with no load on the shaft is thus roughly proportional to the
applied voltage.
When a load is applied to the shaft, the speed will tend to fall, reducing
the back EMF. More current flows from the upply and the current selfadjusts to the value that produces a torque (turning force) just
sufficient to balance the load torque.
The speed will fall slightly with load due to the increase in voltage
dropped across the armature coils due to the higher current.
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The electrical circuit arrangement of the device fitted to the DYNA
1750Trainer is shown in Fig 11.8.
The 1O resistor is fitted in series with the armature to allow monitoring
of the armature current by measurement of the voltage dropped across
it. Since the resistor is 1O, voltages measured across it in mV will
directly correspond to currents in mA.
The diode limits any voltage spikes to a maximum of approximately
±12V. Capacitor C1 provides some noise filtering at the output and the
combination L1, L2 and C2 reduces radiation of radio frequency noise.
The main characteristics of the device fitted to the DYNA 1750
Trainer are:
DC resistance
6.2O
No load current (12V applied)
120mA
Stall current (12V applied)
1.93A
Shaft speed (no load, 12V applied)
2400rev/min (max.)
Starting torque
7 Ncm/A
Torque constant
3.5 Ncm/A
Time constant
Efficiency
19.6ms
82% (max.)
Table 11.7
162
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Chapter 11
11.8 Practical Exercise
Characteristics of a DC Permanent Magnet Motor
1
2
Connect the circuit as shown in Fig 11.9. Set the 10kO resistor
control for zero output voltage, (control fully counter
clockwise), and set the counter controls to COUNT and 1s.
Switch ON the power supply and set the voltage applied to the
motor, as indicated by the Moving Coil Meter, to 10V. The
motor should run at a high speed. Allow it to run for a short
time and then note ht e reading of the digital voltmeter.
This reading in mV represents the current in mA taken by the
motor, since it is the voltage dropped across a 1O resistor.
3
4
Dynalog (India) Ltd.
Press the counter RESET button and the note the displayed
Counter value. This represents the motor speed in rev/sec.
Record the values in Table 11.8 overleaf.
Repeat the procedure, noting the speed and current reading for
motor applied voltages of 8V, 6V, 5V and 3.5V and record the
values in Table 11.8.
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Linear or Rotational Motion
Chapter 11
Motor Applied Voltage
IT 01
Curriculum Manual
10V
Armature Current
mA
8V
6V
mA
mA
5V
4V
mA
mA
Speed (rev/sec.)
Speed (rev/min.)
Table 11.8
5
6
7
164
Multiply the speed in rev/sec by 60 to convert to rev/min and
add the results to Table 11.8.
Slowly reduce the applied voltage until the motor just stops
turning and observe the effect on the voltage and the current.
Stopped voltage =
V
Stopped current =
mA
Construction the graph of speed in rev/min. against applied
voltage and armature current on the axes provided :
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Linear or Rotational Motion
Chapter 11
Set the applied voltage to 7V and note the armature current
taken and the shaft speed when the motor is unloaded. Record
in Table 11.9.
Applied Voltage = 7V
Armature current
Unloaded
mA
Loaded
400mA
Shaft speed (rev/sec)
Table 11.9
9
10
Now place your left hand near the Hall effect disc with the
finger nails down and touching the baseboard of the DYNA
1750 Trainer. Move your fingers gently forward so that your
middle finger comes between the Hall effect disc and the
baseboard and exerts a small load on the motor.
Vary the pressure of the load so that the current is
approximately 400mA (0.4V reading on the digital voltmeter)
and then note the shaft speed by pressing the Counter RESET
button. Record in Table 11.9.
The characteristics are typical for this size of machine, larger machine
would not have such a large drop in with load.
11
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Switch OFF the power supply.
165
Linear or Rotational Motion
Chapter 11
166
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Display Devices
Chapter 12
Chapter 12
Display Devices
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the characteristics and application of the
Timer/Counter.
2
Describe the characteristics and application of the
LED Bargraph display.
3
Describe the characteristics and application of the
Moving Coil Meter.
4
State and calculate the requirement to extend the
voltage range of a Moving Coil Meter.
5
Select a suitable device for a particular voltage
measurement.
1
2
3
4
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Digital Multimeter.
Stopwatch (not supplied).
167
Display Devices
Chapter 12
IT 01
Curriculum Manual
12.1 The Timer/Counter
A system logic diagram of the Timer/Counter facility provided with
the DYNA 1750 unit is shown in Fig 12.1. The output display uses
three 7-segment LED’s.
The unit can be used in three ways:
a.
Time measurement, with the controls are set to TIME and
FREE RUN.
b.
Counting (pulses), with the controls set to COUNT and FREE
RUN.
c.
FREQUENCY (count rate/sec), with the controls set to
COUNT and 1s.
In the addition, with some signal conditioning, it can be used for
voltage measurement.
The main characteristics of the unit are:
Input impedance
Input voltage levels (TTL)
1MO
+5V max.
Timing intervals
10ms
Timing accuracy
5%
Table 12.1
168
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Time
Display Devices
Chapter 12
TIME and FREE RUN
With the input at TTL logic level “1”, (+5V), the display increments at
10ms intervals, or very 1/100 second. With the input at logic level “0”
(0V), the displayed value is held.
The unit will therefore display the time in hundredths of a second that
the input is held at logic level “1”. Note that with a 3-digit display, the
maximum count is 999 and hence one complete cycle from 0-999 will
represent 1000 x 10ms = 10s.
COUNT and FREE RUN
Counting
The count increments by 1 each time the input voltage level changes
from TTL logic level “0” to level “1”. i.e. on receipt of a positive edge
of a pulse of amplitude 5V. Set in this way the Counter counts input
pulses and displays the total.
With the 3-digit display the maximum count will be 999.
Frequency
COUNT and 1s
The unit counts the number of positive pulses at TTL logic level “1”
that are received at the input in a period of one second, following a
RESET of the Counter, thus giving the count rate in pulses per second,
or the frequency in Hz.
Note that you have already used the Timer/Counter to count the
number of pulses received in one minute and to measure frequency in
pulses/sec.
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12.2 Practical Exercise
Time Measurement and Counting
Time Measurement
1
2
3
4
170
Connect the circuit as shown in fig 12.2 and switch ON the
power supply. With the amplifier #2 GAIN COARSE control
set to 100 and GAINE FINE to 1.0, adjust the OFFSET control
for +5V output. Switch the GAIN COARSE control to 1. The
output voltage will drop to nearly zero.
Set the Timer/Counter controls to TIME and FREE RUN and
press the RESET button. The display should show zero.
Switch the Amplifier #2 coarse gain control to 100. The
counter display should increment at 10ms (1/100 sec.) intervals.
Return the GAIN COARSE control to 1, the display will be
held. This illustrates the application of the unit to time
measurement, the display indicating the number of 10ms
intervals (or the time in hundredths of a second) that the input
is held at +5V.
With Amplifier #2 GAIN COARSE set to 1, RESET the
Counter display to zero. Switch Amplifier #2 GAIN COARSE
to 100 and note the time taken for the count to complete one
cycle from 0 to 999 and back to 0.
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Display Devices
Chapter 12
Use the timer facility to time some operations and obtain
practice in its use, such as the time taken for you to verbally
count from zero through to 250, or to write down a long word.
Counting Pulses
6
With the circuit still as shown in Fig 12.2 set the Timer/Counter
controls to COUNT and FREE RUN and RESET the display to
zero.
7
Switch Amplifier #2 GAIN COARSE control from 1 to 100
and back 1.
8
Repeat the process, you will find that the count increments for
each change of the gain from 1 to 100, or on the application of
a +5V pulse to the counter input.
9
Remove the Counter input lead from the output of Amplifier #2
and touch it o the +5V supply socket.
10
Return the Counter input lead back to the output of Amplifier
#2 and, with the GAIN COARSE set to 100, alter the OFFSET
control to give zero output. Slowly raise the setting again and
watch the Counter display for a response.
11
Note the threshold level on the Counter input from the
indication on the Moving Coil Meter.
Threshold voltage level =
12
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V
Switch OFF the power supply.
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12.3 Practical exercise
Frequency measurement
The connection of the +5V supply places the 12kO fixed resistor in
series with the 10kO 10-turn resistor to make low voltage setting
easier. Switch the unknown resistor Rx OUT.
A voltage to Frequency (V/F) Converter is available in the signal
conditioning circuits. This unit converts a DC voltage input to a pulsed
output of frequency 1kHz/volt of input. For example, an input of 0.6V
will produce an output frequency of 0.6kHz or 600Hz.
The pulses from the V/F Converter are unsuitable to be fed directly to
the input of the Counter/Timer. The Differentiator and Comparator are
used to shape the pulses from the V/F Converter, so that they may be
detected by the Counter/Timer.
1
2
172
Connect the circuit as shown in Fig 12.3 and switch ON the
power supply. Set the Counter controls to COUNT and 1s. Set
the Differentiator TIME CONSTANT to 1s and switch OFF the
Comparator HYSTERESIS.
Set the 10kO 10-turn resistor output voltage to 0.1V, press the
Counter RESET button and note the displayed reading, Enter
the value in Table 12.2.
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Input Voltage to
V/F Converter
Display Devices
Chapter 12
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Counter Display
(Hz)
Table 12.2
3
Repeat the procedure for the other voltage settings shown in
Table 12.2 and record the displayed values that are obtained
following the pressing of the reset button.
The accuracy in the calibration of the V/F converter will affect
the readings as will your accuracy in setting the voltages and
also the accuracy of the 1s delay in the Timer/Counter.
In this exercise the V/F converter was used purely as a means
of obtaining a variable frequency. However, the method used
also illustrates the application of the unit to voltage
measurement. The displayed Counter readings represent the
voltage in mV, as can be seen from Table 12.2.
The maximum voltage range is limited by the frequency
capability of the counter and the number of digits in the
display. The voltage range can be extended by attenuating the
input to the V/F converter using the additional circuits shown in
Fig 12.4. Note carefully also the change of the voltage feed to
the 10kO 10-turn resistor.
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The Buffer Amplifier is used to reduce the loading on the 10kO 10turn resistor.
The circuit will be calibrated so that a counter display of 600
represents a voltage of 6V.
1
Connect the additional circuitry shown in Fig 12.4 to the V/F
converter input. The V/F Converter, Differentiator, Comparator
and Counter/Timer remain connected as shown in Fig 12.3. Set
the output control of the 10kO slide resistor for zero output (to
the left).
2
Set the output voltage from the 10-turn resistor to 6V as
indicated by the digital voltmeter and then slowly adjust the
10kO slide resistor until the Counter display indicates 600 after
the RESET button is pressed.
You will find that the setting of the resistor control is very
sensitive, it is possible to set accurately but if it is too difficult,
set the value as near as you can. The unit is now calibrated.
3
Input Voltage
Set the 10-turn resistor control in steps to each of the other
voltage values indicated in Table 12.3. Note the Counter
displayed value after pressing the reset button. Record the
values in Table 12.3.
1
2
3
4
5
Counter Display
6
7
8
9
9.5
600
Table 12.3
4
174
Switch OFF the power supply
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Display Devices
Chapter 12
12.4 The LED Bargraph Display
The construction of the Bargraph device is shown in Fig 12.5,
consisting of 10 separate light emitting diodes (LED’s) fitted in a 20pin package. The light from each diode is collected by a light pipe and
appears at the top surface as red bar.
A dedicated IC driver chip controls the device and provision is made
for adjusting the voltage levels required for adjacent LED’s to light.
With the device as fitted to the DYNA 1750 unit the voltage level
between adjacent LED’s is 0.5V and hence the minimum voltage for
all LED’s to light is 5.0V.
The device has a high input impedance, a low time constant, and is
suitable for indication of an approximate and rapidly varying voltage
level, but the resolution is low.
The main characteristics of the device are:
Input impedance
1MO
Input voltage range
±35V
Accuracy
Segment overleaf
2%
1mV
Table 12.4
The unit is adjusted so that an input of +5V just lights the last LED.
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12.5 Practical Exercise
Characteristics of an LED Bargraph Display
1
Connect the circuit as shown in Fig 12.6. Set the 10kO
Wirewound resistor control for zero output voltage (fully
counter clockwise).
2
LED number
Switch ON the power supply. Adjust the resistor control to
increase the voltage applied to the bargraph unit gradually and
note the voltage values at which each LED lights. Record the
values in Fig 12.5
1
Input Voltage
2
V
3
V
4
V
V
5
6
V
V
7
8
V
V
9
10
V
V
Table 12.5
3
176
Vary the voltage rapidly by the rotating the control quickly in
both directions and note how the display follows. Repeat the
procedure, this time noting the display on the digital meter.
Switch OFF the power supply.
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Chapter 12
12.6 The Moving Coil Meter
The construction and electrical circuit arrangement of the moving coil
meter fitted to the DYNA 1750 unit are shown in Fig 12.7.
Using the connections + and -, the voltage difference between any two
points in a circuit can be measured. By connecting the socket to 0V,
the voltage of any point with repeat to 0V (ground) can be measured
using the + connection.
The moving coil meter consists of a coil suspended between the poles
of a permanent magnet with a pointer attached to the coil which moves
over the meter scale.
The coil is held in its center position by two hairsprings. A set zero
screw is attached to tone of the hairsprings for adjustment of the
pointer position to zero with no voltage applied to the meter.
When current is fed to the coil via the hairsprings, a force is produced
by interaction between the current in the coil and the permanent
magnetic field, and the coil rotates. The direction of rotation depends
on the direction of the current through the coil (Flemings Rule) and the
amount of rotation depends on the magnitude of the current flowing.
The coil rotates until the force produced by the current is balanced by
the force exerted by the hairsprings.
The coil is wound on an aluminum former. When the coil rotates, an
EMF is induced in this former, similar to the back EMF induced in the
armature coils of a DC motor. This produces a current and a force
opposing the motion of the coil (Lenz’s Law).
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The coil movement is thus damped and allows the pointer to take up its
final position, after a step change of current, with the minimum of
oscillation (or hunting) occurring. The meter movement is damped
control system and this effect together with the inertia of the coil
system limits the response speed of the pointer.
The hairsprings are fine to allow a large angular movement and high
sensitivity. The amount of coil current needed for full-scale deflection
(f.s.d.) will be determined by the tension of the hairsprings. The
current flow in the meter circuit must be limited to this value of
current.
When used as voltmeter, a series resistor (called a multiplier) is fitted
to limit the current to the value required to produce full-scale
deflection. For instance, if the f.s.d. current for a particular meter is
1mA, then the value
1
of the multiplier (series resistor) must be
or 1KO for each volt
1X10-3
(1kO/V) to be represented by full-scale deflection. This figure (1kO/V)
is known as the sensitivity of the meter. From this figure it is possible
to calculate the loading resistance of a meter when it is operated on any
voltage range.
A 10V voltmeter using a 1mA f.s.d. meter would require a multiplier
of 10 x 1kO = 10kO.
Many analog multimeters are based on a 50µA meter movement
(50µA f.s.dd).
The main characteristics of the meter fitted to the DYNA 1750 unit
are:
Full-scale current
±1mA
Sensitivity
1kO/V
Total voltmeter resistance
20kO
Accuracy
± 1-2%
Table 12.6
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Chapter 12
12.7 Practical Exercise
Characteristics of a Moving Coil Meter
1
Connect the circuit as shown in Fig 12.8. Set the resistor
control to its central position and check that the Moving Coil
Meter pointer is at zero. Adjust the Set Zero screw (Fig 12.8) if
necessary to set the pointer to zero.
Use only the correct small screwdriver for this task.
2
Digital Meter
Switch ON the power supply. Set the resistor output voltage to
0V as indicated by the digital multimeter and note the voltage
indicated by the Moving Coil Meter. Enter the value in Table
12.7.
V
Moving Coil
Meter
Table 12.7
-10
3
4
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V
-8
V
-6
V
-4
V
-2
0
V
+2
V
+4
V
+6
V
+8
+10
V
V
Repeat the procedure for all positive values of voltage listed in
Table 12.7.
Repeat the procedure for the negative values of voltage
indicated in Table 12.7, but setting up with the Moving Coil
Meter and reading the digital multimeter. Record the results in
Table 12.7. switch OFF the power supply.
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12.8 Practical Exercise
Extending the Voltage Range of a Moving Coil Meter
The voltage range of a moving coil meter can be increased by adding a
resistor in series with it to extend the existing multiplier.
1
2
3
4
Connect the 100kO variable resistor in series with the Moving
Coil Meter as shown in Fig 12.8. Note that the ±12V supplies
are being use together as a single-ended +24V supply.
Switch ON the power supply and use the 10kO variable resistor
to set the voltage to 10V as indicated on the digital multimeter.
Adjust the 100kO variable resistor so that the Moving Coil
Meter reads +5V.
Keep re-adjusting both settings until they are correct.
When completed, the Moving Coil Meter is calibrated for a voltage
range of ±20V.
5
Check this by setting the voltage to 20V (digital multimeter)
and note the Moving Coil Meter scale reading. Switch OFF the
power supply.
Moving Coil Meter scale reading with 20V applied =
6
180
V
Isolate the 100kO Carbon Track Resistor from the circuit and
use your digital multimeter on an Ohms (Resistance) range to
measure the resistance of the part of the 100kO variable resistor
which was connected into circuit.
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Chapter 12
12.9 Practical Exercise
Comparison of Voltage Display Devices
7
8
9
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Connect the circuit as shown in Fig 12.9. All three voltage
display devices are connected in circuit for comparison of their
characteristics.
Switch ON the power supply.
Vary the output voltage slowly over the range 0V through +5V
and back to 0V and note the meter indications.
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10
11
Vary the output voltage over the same range rapidly and note
the readings of the Moving Coil Meter and Bargraph.
Increase the input voltage from 0V to +3V, with the 3V
indicating LED of the Bargraph just on, and note the readings
of all the meters. Record the results in Table 12.8
Voltage
indications
All three devices
in circuit
Moving Coil
Meter removed
12
182
Bargraph
3V
(sixth bar)
Digital
Multimeter
Moving
Coil Meter
V
V
V
Remove the lead to the + connection of the Moving Coil Meter
thus disconnecting it from the circuit. Note and record in Table
12.8, the revised readings of the Digital Multimeter and
Bargraph.
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Chapter 13
Chapter 13
Signal Conditioning Amplifier
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the characteristics and application of DC
amplifiers.
2
Explain the term “Offset” and the need for offset
control.
3
Describe the characteristics and application of an
AC amplifier.
4
Describe the characteristics and application of a
power amplifier.
5
Describe the characteristics and application of a
current amplifier.
6
Describe the characteristics and application of a
buffer amplifier.
7
Describe the characteristics and application of an
inverter amplifier.
8
Describe the characteristics and application of a
differential amplifier.
1
2
3
4
5
6
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Digital Multimeter.
Oscilloscope.
Function Generator.
BNC to 4mm connecting Lead.
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13.1 DC Amplifier
The symbol used for a DC amplifier is shown in Fig 13.1. The device
consists of directly coupled amplifiers (without coupling capacitors)
which are therefore capable of amplifying both DC and AC signals.
There may be many active devices (transistors) in a DC amplifier such
as the types of Integrated Circuit (IC) Operational Amplifier (Op Amp)
chosen for the DYNA 1750 Trainer.
The ratio of the output signal voltage to the input signal voltage is
referred to as the voltage gain of the circuit (Av).
With the input to these amplifiers at zero, the output should be zero,
but there could be a small value of voltage. This is more of a problem
with high gain circuits and an offset control may be provided to
counteract the effect. This control is adjusted with zero input, to set the
output voltage to zero.
Given data for an amplifier normally specifies the input offset voltage
for the device. This represents the difference in voltage at two input
connections that may be required to procedure zero output voltage. The
second input connection is not accessible for the DC amplifier
provided with the DYNA 1750 Trainer although an offset control is
provided for Amplifier #1/2 connected internally.
Various DC amplifier circuits are provided with the DYNA 1750
Trainer, but only three are specifically designed for amplification
applications, these being:
a.
b.
c.
184
Amplifier #1 having a variable preset gain over the range of
0.1 to 100 approximately. This amplifier is provided with an
“offset” control.
Amplifier #2 which is identical to Amplifier #1.
X100 Amplifier which has a fixed gain of 100 and has no
offset control.
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Chapter 13
The requirements for an ideal amplifier are:
High input impedance to prevent loading the signal source.
Low output impedance to ensure good transfer of signal to any
succeeding stage and prevent loss of signal.
High gain to reduce the number of amplifier stages required.
Board bandwidth to ensure that all required signals for a given
band of frequencies are passed without attenuation.
Low distortion so that only the amplitude of the signal is
altered (high fidelity).
Low noise factor to reduce the introduction of unwanted
signals or interference.
Stability. No tendency to self-(spurious) oscillation.
These requirements apply to any type of amplifier, not just to DC
amplifiers.
Amplifiers can be connected in cascade (one after another), to increase
the overall gain, if required.
Note: The output voltage that can be provided by a DC amplifier
cannot exceed the value of its supply voltage. In the case of the
DYNA 1750 Trainer the output voltage is limited to a
maximum of approximately ±10V.
The main characteristics of these devices are:
Amplifier #1/2
X100 Amplifier
12V
12V
0.1 - 100
100
Voltage gain error (max.)
±30%
±4%
Output noise voltage (typ.)
10mV
10mV
Fully adjustable
±30mV
100kO
101kO
Input signal voltage (max.)
Voltage gain (nominal)
Output offset voltage (max.)
Input impedance
Table 13.1
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13.2 Practical Exercise
Characteristics of DC Amplifiers
1
Connect the circuit as shown in Fig 13.2 with the Amplifier #1
in circuit. Set the GAIN COARSE control to 100 and GAIN
FINE to 1.0 for both amplifiers, Amplifier #1 and Amplifier #2.
Note that buffer #1 is needed so that the OFFSET adjustment
does not affect the input voltage.
2
3
4
5
186
Switch ON the power supply. Set the 10kO variable resistor
mid-way for exactly zero volts output as indicated by the digital
multimeter. Adjust the OFFSET control of Amplifier #1 so that
the output voltage is zero (or as near as it is possible to get to
zero).
Increase the input voltage positively and note the output
voltage. This increases to saturation quickly and then remains
at this maximum value for further increase of input voltage.
Record the value of this saturation voltage in Table 13.2.
Repeat for the negative saturation voltage, recording again in
Table 13.2.
Set the input voltage so that the output voltage is between +7
and +8V (Moving Coil Meter) and use the digital multimeter to
note the value of the input and output voltages. Record the
results in Table 13.2.
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Output voltage
6
Calculate the gain (
), this representing the
Input voltage
maximum gain with positive polarity possible for the amplifier.
Add this to Table 13.2.
Gain (Av) set to
100 x 1.0 = 100
Saturation
voltage
Input voltage
Output voltage
Voltage
(Av)
Amplifier #1
Positive Negative
Amplifier #2
Positive Negative
V
V
V
V
MV
MV
mV
mV
V
V
V
V
gain
Table 13.2
7
Repeat with the 10kO variable resistor adjusted to give between
–7 and –8V, to determine the gain of the amplifier for negative
polarity input signals.
This dual-polarity operation signifies that the amplifier is
capable of amplifying AC signals as well as DC voltages.
8
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Replace Amplifier #1 in the circuit with amplifier #2 and repeat
the procedures to adjust the OFFSET and to determine its
maximum positive and negative gain values.
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Reset both Amplifier #1 and Amplifier #2 GAIN COARSE
control to 1 and GAIN FINE to 0.1 for minimum amplifier
gain.
With Amplifier #1 in circuit and an input voltage of +4V
approximately, note and record the values of the input and
output voltages in Table 13.3.
Gain (Av) set to
1 x 0.1 = 1
Input voltage
Output voltage
Voltage
(Av)
Amplifier #1
Positive Negative
Amplifier #2
Positive Negative
V
V
V
V
V
V
V
V
gain
Table 13.3
11
12
13
Reset the input to –4V and repeat the readings, recording the
results in Table 13.3.
Change to Amplifier #2 and repeat the readings for both
polarities.
Replace Amplifier #2 with the X100 Amplifier. Temporarily
ground the input and note the output voltage with zero input
voltage (the output offset voltage) using the digital multimeter.
Use the same 0V patch panel as you use for the digital
multimeter.
Note that there is no offset control with this amplifier. The
offset is adjusted to an acceptably low figure during production.
188
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Chapter 13
Nominal Gain
(Av) = 100
Saturation voltage
Input voltage
Output voltage
X100 Amplifier
Positive Negative
V
V
mV
mV
V
V
Voltage gain (Av)
Table 13.4
14
Repeat the procedure to measure the saturation voltages and the
input and output voltages with the output set to a value between
±(7-8)V. Record the values in Table 13.4.
15
Calculate the gain for both polarities and add these to Table
13.4.
16
Compare the results with the amplifier specifications given
earlier.
17
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Switch OFF the power supply.
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13.3
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The AC Amplifier
The symbol for an AC amplifier is the same as for a DC amplifier.
The AC amplifier provided with the DYNA 1750 Trainer is a twostage IC amplifier which has three fixed gain settings, 10, 100 and
1000. The mimic diagram on the DYNA 1750 Trainer shows the
capacitors in the input and output circuits. These capacitors remove
any DC level and hence there is no offset problem with an AC
amplifier.
Two of the main aspects of amplifiers are in conflict with each other,
gain and bandwidth. As the gain of amplifier is increased its bandwidth
will be reduced. It is common to specify a gain bandwidth product for
an amplifier. For instance, an amplifier with a gain bandwidth product
of 106 could have a gain of 100 with a bandwidth of 104 or 10kHz, or
a gain of 1000 with a bandwidth of 1kHz.
This is why the amplifier on the DYNA 1750 trainer is a 2-stage
circuit; to get a bandwidth of 16kHz (covering the full audio band) and
a gain of up to 1000. When the gain is switched to 100 (or 10) the
bandwidth will be increased.
The main characteristics of the device are:
Input voltage (max.)
Bandwidth (-6dB, gain = 1000)
Maximum gain at 40kHz
Output noise voltage (gain = 1000)
±12V
10Hz – 16kHz
225
100mV
Table 13.5
A high proportion of the output noise will be found to be stray pick-up
of the output of the 40kHz oscillator which is adjacent to the AC
Amplifier.
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Chapter 13
13.4 Practical Exercise
Characteristics of the AC Amplifier
1
2
3
Construct the circuit of Fig 13.4. Set the slider of the 10kO
variable resistor to mid-way. This is to operate as a fine
amplitude control on the input signal. Switch the AC Amplifier
to maximum gain, 1000.
Switch the output of the Function Generator to a 1kHz
sinewave. Switch the oscilloscope timebase to 0.5ms/div, Y1
amplifier (CH.1) to 10mV/div and the Y2 amplifier (CH.2) to
5V/div.
Switch ON the power supply and adjust the Function Generator
output amplitude control to obtain 20Vp-p output from the AC
Amplifier as indicated on CH.2 of the oscilloscope. Use the
10kO slider variable resistor for the final adjustment if
necessary. Measure the input amplitude (Ch.1) and record in
Table 13.6.
Gain setting
Output voltage
Input voltage
1000
100
10
20Vp-p
20Vp-p
20Vp-p
mVp-p
mVp-p
Vp-p
Amplifier gain
Table 13.6
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5
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Switch the AC amplifier gain to 100 and repeat the setting of
the output voltage to 20Vp-p and again measure the input
signal amplitude, changing the Y1 amplifier setting as required.
Record the result in table 13.6.
Switch the AC Amplifier gain to 10 and repeat the setting and
measurement.
Output voltage
Calculate the amplifier gain (
setting
) for each
Input voltage
of the gain switch and add the results to table 13.6.
7
change the Function Generator frequency to 40kHz and the
oscilloscope timebase setting to 5µs/div, switch the AC
amplifier gain to 1000 and repeat the setting and measurement.
Input voltage for 20Vp-p output =
8
mVp-p
Calculate the amplifier gain at 40kHz.
Amplifier gain at 40kHz =
9
192
Switch OFF the power supply.
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Signal Conditioning Amplifiers
Chapter 13
13.5 The Power Amplifier
The symbol for a power amplifier is again the same as that for any DC
amplifier.
The main characteristic of a power amplifier is the capability of a large
power output.
In order to do this the output impedance of the amplifier must be very
low in order to provide a heavy current to a load without loss of output
voltage across the output impedance.
The components used must also be capable of dissipating the heat
generated in high current circuits.
The device provided with the DYNA 1750 Trainer has unity gain and a
maximum output current of the order of 1.5A.
The main characteristics of the device are as follows:
Input voltage (max.)
±12V
Input impedance
100kO
Output current
1.5A
Output power (limited by power supply)
9W
Upper –3dB frequency
10.6kHz
Table 13.7
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13.6 Practical Exercise
Application of a Power Amplifier
1
2
3
194
Connect the circuit of Fig 13.6.
Switch ON the power supply and adjust the Function Generator
to give a sinewave input at 1kHz to the AC Amplifier. Increase
the amplitude to give maximum undistorted output from the
amplifier.
Connect the Loudspeaker directly to the output of the AC
Amplifier and observe the effect on the output waveform.
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Chapter 13
Transfer the output of the AC Amplifier to the input of the
Power Amplifier. Transfer the oscilloscope CH.2 connection to
the output of the Power Amplifier. Finally connect the output
of the Power Amplifier to the Loudspeaker.
Switch OFF the power supply.
Note that you have already used the Power Amplifier for DC
applications when driving the lamp for opto-electronic
experiments and for driving the motor for rotating motion
investigations.
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13.7 The Current Amplifier and Buffer Amplifier
Fig 13.7
I/P
O/P
The symbol for a current amplifier is once more the same as for any
DC amplifier. The amplifier converts an input current to an output
voltage.
The device provided with the DYNA 1750 Trainer is intended for use
with the P.I.N. photodiode, giving an output voltage 10,000 times the
input current. An input current of 1mA (max.) will provided 10V
(max.) at the output.
The main characteristics of the Current Amplifier are shown in Table
13.8 below.
The symbol for a buffer amplifier is again as shown in Fig 13.7. These
amplifiers have a high input impedance and a low output impedance
and are inserted in the circuit between a device having a high output
impedance and one having a low input impedance to prevent lading, as
shown in Fig 13.8.
Device 1 (High
output impedance)
Buffer
Device 2 (Low
input impedance)
Fig 13.8
The characteristics are similar to those of the Power Amplifier but they
have a much lower output current capability, (of the order of 20mA
maximum for the device provided with the DYNA 1750 Trainer).
Two buffer amplifier are provided with the DYNA 1750 Trainer,
Buffer #1 and Buffer #2 and their main characteristics are shown in
Table 13.9.
Input current (max.)
Transfer ratio
Table 13.8
196
1mA
10,000V/A
Input voltage (max.)
±12V
Input impedance
100kO
Input offset voltage
300 µV
Voltage gain
1.0
Table 13.9
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Signal Conditioning Amplifiers
Chapter 13
13.8 Practical Exercise
Characteristics and Applications of Current and Buffer
Amplifier
1
2
3
Connect the circuit as shown in Fig 13.9 with the Buffer
Amplifier out of circuit initially. Set the 10kO wirewound
resistor for zero output (control fully counter clockwise) and
the 10kO slider resistor for maximum resistance (slider to
right).
Switch ON the power supply and set the output voltage from
the 10kO wirewound resistor to 1V as indicated by the digital
voltmeter.
Vary the slider resistor control from maximum resistance to
minimum and note the reading of the digital voltmeter. You
will note that it falls due to the increased current loading. Note
the lowest value.
10kO slider resistance minimum, voltage =
V
The has varied from 0.1mA to 1.0mA approximately but this has been
sufficient to produce the voltage drop above. The buffer Amplifier can
be used to reduce this loading effect.
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Disconnect the load between socket B of the wirewound Track
potentiometer and socket A of the Slide potentiometer.
Connect socket B of the wirewound Track to the input socket
of Buffer #1. Connect the output socket of Buffer #1 to socket
A of the Slide potentiometer. Buffer #1 is now connected
between the wirewound Track potentiometer and the Slide
potentiometer.
With the 10kO slider control at maximum (slider to right) set
the voltage as indicated by the digital voltmeter to 1.0V. Vary
the 10kO slider control over its full range and note the reading
of the digital voltmeter.
Check that the output from the 10kO wirewound resistor is still
1.0V and then remove the digital multimeter from the circuit,
switch to a 2mA range and reconnect it as an ammeter into the
circuit between the 10kO slider resistor and the Current
Amplifier to monitor the input current.
Set the 10kO slider resistor control to each of the settings
indicated in Table 13.10 and for each setting note the input and
the output voltage for the Current Amplifier.
Resistor setting
Input current
Output voltage
10
8
6
4
2
1
mA
mA
mA
mA
mA
mA
V
V
V
V
V
V
Table 13.10
8
198
Plot the graph of Output voltage against Input current for the
Current Amplifier.
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Chapter 13
This exercise has illustrated the characteristics of the current amplifier
and the application of a buffer amplifier for circuits requiring a low
output current.
9
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Switch OFF the power supply.
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13.9 The Inverter
Yet again symbol is the same as for any amplifier.
Fig 13.10
I/P
I
O/P
The inverter amplifier, as the name implies, reverses the polarity of the
voltage applied to the input, either DC or AC. The device provided
with the DYNA-1750 Trainer has a voltage gain of unity.
One aspect of all IC amplifiers which has not been mentioned before is
the slew rate. This imposes a limitation on alternating signals on the
rate at which the output voltage can change with respect to time. You
can have either a small signal voltage at a high frequency or a larger
signal voltage at a lower frequency.
This is not quite the same thing as the gain/bandwidth product which
was introduced earlier, as you will see from the experiment which
follows.
The main characteristics of the device are :
Input voltage (max.)
Voltage gain
±12V
-1.0
Input impedance
100kO
Input offset voltage
300µV
Slew rate
0.15V/µs
Table 13.11
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Chapter 13
13.10 Practical Exercise
Characteristics of an Inverter
1
2
Connect the circuit as shown in Fig 13.11.
Switch ON the power supply. With the Inverter input connected
to the +5V supply note the value of the output voltage in Table
13.12.
Inverter input
Inverter output
+5V
-5V
V
V
Table 13.12
3
4
Transfer the Inverter input to the –5V supply and again note the
value of the output voltages.
Switch OFF the power supply.
The output voltage magnitude may not be identical with the input due
to the offset voltage. No facility for adjusting this has been provided.
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1
2
3
4
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Connect the circuit as shown in Fig 13.12. Switch ON the
power supply.
Set the oscilloscope timebase to 5µs/div. and both Y amplifier
(CH.1 & CH.2) to 0.5V/div.
Adjust the control of the 10kO slider resistor to give an input
voltage of 1Vp-p.
Sketch the input and output (Output 1) waveforms on the
graticule provided:
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Signal Conditioning Amplifiers
Chapter 13
Change the Y2 (CH.2) amplifier to 1V/div and increase the
setting of the 10kO slider resistor until the full effect of the
slew rate is observed.
Add a sketch of the output (Output 2) waveform.
Voltage
Check the slew rate
earlier.
8
9
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against the specification given
Time (µs)
Replace the input to inverter with a 5kHz sinewave output from
the Function Generator.
Increase the amplitude of the signal until slewing again beings
to occur. Note the maximum peak-to-peak value of the
undistorted output signal.
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13.11 The Differential Amplifier
The symbol for a differential amplifier is shown in Fig 13.13.
The amplifier has two inputs which can be driven by separate signals.
It is called differential because the output voltage depends on the
difference in voltages applied to the two inputs. If the two inputs are
driven by the same signal in phase then theoretically there should be no
output. There will, however, be a small output the amount being
determined by the common mode gain, which is designed to be as near
to zero as possible.
For the device provided on the DYNA 1750 Trainer, the output voltage
is given by (VA – VB).
Two differential amplifier circuits are provided, the second being
labeled “Instrumentation Amplifier”. This carries out the same basic
functions as the differential amplifier but has an improved (reduced)
common mode gain.
The main characteristics of the devices are:
Differential
Amplifier
Input voltage (max.)
Instrumentation
Amplifier
±12V
Differential gain
1.0
Common mode gain (max.)
0.02
0.006
Input impedance (input A)
200kO
100kO
Input impedance (input B)
100kO
Table 13.13
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Chapter 13
13.12 Practical Exercise
Characteristics of a Differential
1
2
Step
Connect the circuit as shown in Fig 13.14 and switch ON the
power supply.
Moving the digital voltmeter lead as necessary, set the voltage
at input A of the Differential Amplifier to –3V and input B also
to –3V and note the resulting output voltage. Record the value
in Table 13.14.
1
2
3
4
5
6
7
8
Input B voltage
-3V
+1V
+4V
+2V
0V
+4.5V
+2V
-2.7V
Input A voltage
-3V
+1V
+4V
+4V
+3V
+2.2V
-3V
+3.6
Output voltage
V
V
V
V
V
V
V
V
Table 13.14
3
4
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Repeat the procedure for each of the other pairs of inputs in
Table 13.14 and record the output voltage again.
Switch OFF the power supply.
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Chapter 13
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Signal Conversions
Chapter 14
Chapter 14
Signal Conversions
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the characteristics of a voltage to current
converter (V/I).
2
Describe the characteristics of a current to voltage
converter (I/V).
3
Describe the characteristics of a voltage to
frequency converter (V/F).
4
Describe the characteristics of a frequency to
voltage converter (F/V).
5
Describe the characteristics of a full wave rectifier.
1
2
3
4
DYNA-1750 Transducer and Instrumentation
Trainer
4mm Connecting Leads.
Digital Multimeter.
Oscilloscope.
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Signal Conversions
Chapter 14
14.1
IT 01
Curriculum Manual
Voltage to Current Converter
The voltage to current converter converts an input voltage to an output
current.
The device operates as a constant current source within the limits of
the supply voltage. As an example of this, if 20mA is supplied to a
load of 50O, then the voltage dropped across the load is:
20x10-3 x 50 = 1.0V.
With the V/I converter supplied from +12V DC this is no problem. If,
however, the load resistance is increased to 1kO, then the voltage
across the load at 20mA would be:
20x10-3 x 1000 = 20V,
which the device would be unable to provide from a +12V supply.
A simple block diagram is used to represent the V/I Converter on the
DYNA 1750 Trainer. The standard symbol for a constant current
source is given in Fig 14.1.
The main characteristics of the device fitted to the DYNA 1750
Trainer are:
Input voltage range
0-1.5V
Output current range (max.)
0-24mA
Transfer ratio
16mA/V
Table 14.1
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14.2
Signal Conversions
Chapter 14
Practical Exercise
Characteristics of a Voltage to Current Converter
Note that a second meter is shown as an ammeter connected between
the output of the V/I Converter and the load (the heater element on the
thermal transducer panel). If a second instrument is available then the
measurements will be simplified. The instructions will be given
assuming that is not the case.
1
2
3
Connect the circuit as shown in Fig 14.2 and set the 10kO
resistor for zero output voltage (slider to left).
Switch ON the power supply. Set the input voltage to the V/I
converter to 0.5V.
Remove the digital multimeter from the circuit, range it as an
ammeter (up to 25mA will be needed), and reconnect it in
between the output of the V/I Converter and the load. Measure
the load current and record the result in Table 14.2. Restore the
digital multimeter as a voltmeter in the original position as
shown in Fig 14.2.
Input voltage
Output current
0V
0.5V
mA
mA
1.0V
mA
1.5V
mA
Table 14.2
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Repeat the procedure for input voltage settings of 1.0V and
1.5V and record the results in Table 14.2. Keep the multimeter
connected as an ammeter monitoring the load current after the
final reading.
Connect the input of the V/I Converter to 0V (ground) and note
the effect on the output current. Record the result in Table 14.2.
Plot the characteristics of output current against input voltage
for the V/I Converter on the axes provided:
Calculate the Transfer Ratio from any pair of voltage and
current readings.
Transfer Ratio =
8
9
210
mA/V
Restore the input of the V/I Converter to terminal B of the
10kO slider resistor and the input voltage to 1.5V. Transfer the
digital multimeter to the output of the V/I Converter. First
unplug the load and note the effect on the output voltage of the
V/I Converter. Then connect the Lamp Filament on the optotransducer panel as the load and note the voltage again.
Switch OFF the power supply.
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14.3
Signal Conversions
Chapter 14
Current to Voltage Converter
The current to voltage converter converts an input current to an output
voltage and is thus converse of the voltage to current converter.
The V/I and I/V Converts provided with the DYNA 1750 Trainer are
arranged to have parameter values that are the reciprocal of each other.
This means that the pair of devices could be used to send a voltage
down a long wire without attenuation, since the current which is
launched into the transmission line at one end must also appear at the
termination (except in the unlikely case of leakage current, which can
be restricted by good insulation).
The actual voltage on the transmission line is irrelevant unless it tries
to be greater than the supply feeding the V/I Converter.
The main characteristics of the I/V converter are:
Input current range
Output voltage range
Transfer ratio
0-24mA (100mA max.)
0-1.5V (6V max.)
62.5mV/mA
Table 14.3
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14.4
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Practical Exercise
Characteristics of a Current to Voltage Converter
1
2
3
4
Connect the circuit as shown in Fig 14.4. Set the 10kO slider
resistor for zero output voltage.
Switch ON the power supply.
Set the input voltage to the V/I converter to 0.5V. Transfer the
digital multimeter to the output of the I/V Converter and note
the output voltage. Record the values in Table 14.4.
Repeat the procedure for input voltage settings of 1.0 and 1.5V
and enter the values in Table 14.4.
Input voltage (V/I)
Output voltage
(I/V)
Table 14.4
5
6
212
0
0.5
V
1.0
V
1.5
V
V
Transfer the input of the V/I converter to 0V (ground) and note
and record the output voltage from the I/V Converter in Table
14.4.
Switch OFF the power supply.
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14.5
Signal Conversions
Chapter 14
Voltage to Frequency Converter
This converts an input to an output frequency, the frequency being
proportional to the input voltage.
The circuit is based on a dedicated (designed for the job) IC type
LM331. The output waveform is in the form of short duration
(approximately 60µs) negative- going pulses, the repetition rate of
which can be controlled over a very wide range.
The negative excursion duration remains constant as the frequency is
increased. This limits the overall time period of the output waveform to
about 85µs, or a frequency of just under 12kHz. The pulse shape is
degraded at frequencies above about 10.5kHz.
The Timer/Counter facility has a limited range, having only a 3-digit
display, but it is better for counting pulses at very low frequencies. The
oscilloscope gives a very good display of the waveform and can also
be used for measurement of higher frequencies.
The main characteristics of the device provided with the DYNA 1750
are:
Type
Input voltage (max.)
Transfer ratio
LM331
12V
1kHz/V
Maximum frequency
10kHz/V)
Non-linearity (typ.)
0.024% full scale
Non-linearity (max.)
0.14%
Table 14.5
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Chapter 14
14.6
IT 01
Curriculum Manual
Practical Exercise
Characteristics of a Voltage to Frequency Converter
The Timer/Counter is used as a frequency meter to measure the lower
output frequencies, within its range.
An oscilloscope is used to monitor the output waveform and to
determine frequencies above the range of the Timer/Counter.
1
214
Connect the circuit as shown in Fig 14.5. Set the Counter
controls to COUNT and 1s, and the 10kO 10-turn resistor to
zero.
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Signal Conversions
Chapter 14
Switch ON the power supply and set the input voltage to 1V.
Press the RESET button of the Counter and note the displayed
value, which represents the frequency output of the V/F
converter. Record the value in Table 14.6.
Input Voltage (volts)
1
2
3
4
5
Output frequency (Hz)
Table 14.6
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Switch OFF the power supply.
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Chapter 14
14.7
IT 01
Curriculum Manual
Frequency to Voltage Converter
This device converts an input frequency to an output voltage.
Each input pulse triggers a monostable multivibrator to generate a
constant period pulse which pumps one packet of charge into a
reservoir capacitor. The voltage across the capacitor is therefore
dependent on how many pulses are received each second.
For the unit provided with the DYNA 1750 Trainer, the parameters are
arranged to be reciprocal to those of the V/F converter.
A communication channel would be possible with frequency as the
transmission medium.
The main characteristics are :
Input frequency (max.)
10kHz
Transfer ratio
1V/kHz
Time constant
100ms
Settling time
0.7s
Accuracy
± 0.1%
Output ripple
10mV
Output impedance
100kO
Table 14.8
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14.8
Signal Conversions
Chapter 14
Practical Exercise
Characteristics of a Frequency to Voltage Converter
1
Connect the circuit as shown in Fig 14.6 Switch ON the power
supply and set the input voltage to the V/F converter to 1.0V.
Note the value of the output voltage from the F/V converter and
record the value in Table 14.9.
Input voltage (V/F)
Output
(F/V)
voltage
1
2
V
3
V
4
V
5
V
V
Table 14.9
2
3
Repeat the procedure for input voltage settings of 2, 3, 4 and
5V.
You will see from the specification that the output impedance
of the F/V Converter is 100kO. If you measure the output
voltage using the M.C. meter the reading will be affected by the
low loading impedance. Try it with the output voltage set 5V,
recording the results in Table 14.10.
Instrument
Digital
Multimeter
only
Output voltage
M.C. Meter
only
V
V
M.C. Meter
via Buffer
#1
V
Table 14.10
4
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Switch OFF the power supply.
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Chapter 14
14.9
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Curriculum Manual
The Full wave Rectifier
The full wave rectifier converts a sinewave AC input a series of
unidirectional positive half cycles as shown in Fig 14.7.
The negative half cycles are inverted so that the output is always of
one polarity.
With an input DC signal of either polarity the output is always
positive, the magnitude of the output being the same as that the input
signal.
In the case of an input consisting of an AC waveform riding on a DC
component, the output waveform will be a mixture of the input
components, the negative components being inverted to be positive. If
the DC component of the input is grater than the AC component then
the same waveform will appear at the output, but always with positive
polarity, irrespective of the polarity of the input.
The circuit is active, containing two operational amplifiers; not just
full-wave diode bridge, since this cannot be adjusted to compensate for
losses. It is not intended for delivery of DC power.
Measurements of AC quantities using DC instruments are possible
with accuracy using Full wave Rectifiers.
The main characteristics of the device provided with the DYNA 1750
Trainer are:
Input voltage
Output voltage error
12V (max)
2% (typ.), (6% max)
Table 14.11
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Signal Conversions
Chapter 14
14.10 Practical Exercise
Characteristics of a Full Wave Rectifier with DC Applied
1
Connect the circuit as shown in Fig 14.8. Switch ON the power
supply and note the values of the input and output voltages for
the Full wave Rectifier with +5V applied to the rectifier input.
Record the output voltage in Table 14.12.
Input voltage
Output voltage
+5V
-5V
V
V
Table 14.12
2
3
Dynalog (India) Ltd.
Transfer the input of the Full Wave Rectifier to the –5V supply
and repeat voltage readings, recording the output voltage in
Table 14.12 again.
Switch OFF the power supply.
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Chapter 14
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14.11 Practical Exercise
Characteristics of a Full Wave Rectifier with AC Applied
1
2
3
4
220
Connect the circuit as shown in Fig 14.9. Set the gain of the AC
amplifier to 10.
Set the oscilloscope timebase to 5µs/div and both Y amplifiers
to 1V/div.
Switch ON the power supply and adjust the slider of the 10kO
resistor so that the amplitude of the output of the AC Amplifier
(CH.1) is the same as that of the 40kHz Oscillator (CH.2).
Switch the selectors on your Y amplifiers between DC and AC.
Any movement of the waveform on the screen means that there
is a DC component. If there is no DC component the waveform
will not move.
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6
7
Signal Conversions
Chapter 14
Transfer CH.2 of the oscilloscope from the output of the 40kHz
Oscillator to the output of the Full wave Rectifier.
Sketch the input and output waveforms of the Full Wave
Rectifier on the graticule provided, marking in the amplitude of
the waveforms:
Record the DC value of the Full wave Rectifier output from the
digital multimeter reading, then switch OFF the power supply.
DC value of the Full Wave Rectifier output =
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V
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Notes:
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Comparators, Oscillators and Filters
Chapter 15
Chapter 15
Comparators, Oscillators and Filters
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the characteristics of a comparator.
2
Explain the effect of hysteresis on the operation of a
comparator.
3
Describe the characteristics of an alarm oscillator.
4
Explain the term “latch” applied to an alarm oscillator.
5
Describe the characteristics of an electronic switch.
6
Describe the characteristics of a 40kHz oscillator.
7
Describe the characteristics of band pass filters.
8
Describe the characteristics of low pass filters.
1
2
3
4
5
6
DYNA-1750 Transducer and Instrumentation Trainer
4mm Connecting Leads.
Digital Multimeter.
Oscilloscope.
Function Generator.
BNC to 4mm connecting lead.
223
Comparators, Oscillators and Filters
Chapter 15
15.1
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Curriculum Manual
The Comparator
The symbol for a comparator is shown in Fig 15.1. It is the same as for
a differential amplifier but the characteristics of the comparator are
different.
The differential amplifier investigated in Chapter 13 had unity gain.
The output voltage was the simple mathematical difference between
inputs A and B.
The gain of a comparator is very high, so that only a very small
difference between the two inputs will cause the output to saturate at a
voltage near to the supply voltage, with either polarity. The comparator
therefore has two possible output voltage states:
1.
2.
with input voltage A more positive than B, the output is a
maximum positive.
with input voltage A more negative than b, the output is a
maximum negative.
Only the very slightest variation between the inputs causes the output
voltage to change from one state to the other and the circuit is therefore
susceptible to noise variations.
To overcome this problem, the circuit is modified so that the voltage at
A must rise to a threshold value B for switching to occur. Similarly,
with the voltage falling, the voltage at A must fall to a different
threshold value below B before the circuit switches back.
This is referred to as hystersis and the difference in the voltages is
referred to as the hysteresis voltage.
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Comparators, Oscillators and Filters
Chapter 15
This is illustrated in Fig 15.2.
With no hysteresis and voltage A varying, the output changes state
frequently. With hysteresis the output does not change state for small
variations of voltage around the last switching voltage, a large change
of voltage is required to cause switching of the circuit.
The circuit with hysteresis does not respond to any noise with a voltage
amplitude less than the hysteresis voltage.
The main characterisctics of the device provided with the DYNA 1750
Trainer are:
Input voltage (max.)
± 12V
Input offset voltage
9mV
Output voltage (no load)
Hysteresis voltage (switch ON)
(-11.8) to (+12)V
4.2V
Table 15.1
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Chapter 15
15.2
Practical Exercise
Characteristics of a Comparator
1
2
3
4
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Connect the circuit as shown in Fig 15.3. Ensure that the
Comparator HYSTERESIS switch is set to OFF. Set the
controls of both resistors fully counter clockwise.
Switch ON the power supply. The voltage at input B will be
0V, that at A will be –5V and the output will be approximately
–12V.
Gradually rotate the control of the 10kO resistor clockwise,
making the voltage at input A (VA) less negative. Note the
voltage at which the output voltage switches polarity with VA
rising (VR). Record the value of VR in Table 15.2. Record also
in Table 15.2 the comparator output saturation voltage above
threshold with VA rising.
Continue to increase input VA and observe the effect on the
output voltage above switching.
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Comparators, Oscillators and Filters
Chapter 15
No Hysteresis
Output
Saturation
Voltage
VB = 0V
VB = +4V
VA
VA
VA rising (VR)
V
V
V
VA falling (VF)
V
V
V
Table 15.2
5
6
7
Reduce VA and note the value at which the output voltage
switches back to a negative value with VA falling (VF). Note
the value of the comparator output saturation voltage below
threshold with VA falling.
Repeat the procedure with input B set to +4V, noting the
switching voltages at input A. The comparator output voltage
values will not alter so there is no need to record them.
Set the HYSTERESIS switch in the ON position and repeat the
procedure for voltage settings at the B input of 0V and +4V.
With Hysteresis
8
Output
Saturation
Voltage
VB = 0V
VB = +4V
VA
VA
VA rising (VR)
V
V
V
VA falling (VF)
V
V
V
Table 15.3
Switch OFF the power supply.
The circuit will have similar characteristics for all settings of the input
voltage at B. Alternatively, the voltage at A may be set and that at B
varied. The value of the hysteresis voltage can be set in the design
stage to any desired value by adjusting the circuit component values.
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Chapter 15
15.3
IT 01
Curriculum Manual
The alarm Oscillator
The alarm oscillator consists of two stages.
The input circuit is a comparator, which is followed by the oscillator.
With the input voltage low, the comparator output prevents the
oscillator from operating. Oscillations only occur when the input
voltage exceeds a level that is decided by the circuit component values.
With the “latch” switch in the OFF position, the oscillator will be ON
or OFF depending on whether the input voltage is above or below the
threshold level.
With the “latch” switch in the ON position, the oscillator is latched ON
by the input voltage exceeding the threshold. It remains ON
continuously, even it the input voltage is reduced below threshold,
until the power supply is turned off.
The unit is used as an alarm indication when the value of a controlled
parameter exceeds a pre-determined level.
The main characteristics of the device provided with the DYNA 1750
Trainer are:
Input voltage (max.)
12V
Trip voltage (threshold)
2.3V
Oscillator frequency
Output impedance
540Hz
4kO
Table 15.4
228
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15.4
Comparators, Oscillators and Filters
Chapter 15
Practical Exercise
Characteristics of an Alarm Oscillator
1
2
Connect the circuit as shown in Fig 15.5. Set the Alarm
Oscillator LATCH switch to OFF and turn the 10kO resistor
control fully counter clockwise. Switch the Counter to COUNT
and 1s, and the Differentiator to 1s.
Switch ON the power supply and rotate the resistor control
slowly clockwise to gradually increase the input voltage to the
Alarm Oscillator. Note the input voltage threshold at which
oscillations start. Record the threshold level in Table 15.5.
Start
Threshold
Without
latch
With latch
Stop
Threshold
Oscillator
Frequency
V
V
V
V
Hz
Table 15.5
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Chapter 15
3
4
5
6
7
IT 01
Curriculum Manual
Increase the voltage to maximum and note the effect on the
oscillator output.
Now gradually reduce the input voltage and record the voltage
threshold at which the oscillations stop in Table 15.5.
Set the latch switch to ON and repeat the procedure, noting the
input voltage at which the oscillations start and then noting the
effect of reducing the input voltage to zero.
Press the RESET button on the Counter to determine the
oscillation frequency and add this to Table 15.5.
Switch the power supply OFF and then ON again to observe
the effect. Repeat the start and stop actions.
Note: The output sound level will be low due to the high output
impedance of the oscillator. This can be increased if necessary
by feeding the loudspeaker via the power amplifier, but this is
not advisable in the laboratory situation.
8
230
Switch OFF the power supply.
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15.5
Comparators, Oscillators and Filters
Chapter 15
The electronic Switch
A simplified diagram of the Electronic switch is given in Fig 15.6.
The series PNP transistor operates as a switch.
When the input voltage to the Comparator (inverting input) is low the
Comparator output is high and the transistor is switched off. If the
input voltage is taken above the threshold established by the reference
voltage the Comparator output switches low and forward biases the
base-emitter junction of the switching transistor to turn it on and
supply voltage to the load.
The maximum permissible output current is limited by the parameters
of the series switching transistor.
The main characteristics of the device provided with the DYNA 1750
Trainer are:
Input voltage (max.)
Trip voltage
Output current (max.)
12V
+2.1V
1A
Table 15.6
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Chapter 15
15.6
IT 01
Curriculum Manual
Practical Exercise
Characteristics of an Electronic Switch
1
Connect the circuit as shown in Fig 15.7. Set the resistor
control fully counter clockwise.
2
Switch ON the power supply and note the output voltage from
the electronic switch. Record in Table 15.7.
Output voltage
with input below
trip
V
Input trip voltage
rising
V
Output voltage
with input above
trip
V
Input trip voltage
falling
V
Table 15.7
3
4
232
Transfer the meter to the Electronic Switch input and increase
the input voltage gradually and note the value of input voltage
at which switching occurs and also the value of the output
voltage after switching. Add these to Table 15.7.
Now gradually reduce the input voltage and note and record the
value when the circuit switches off. Switch OFF the power
supply.
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15.7
Comparators, Oscillators and Filters
Chapter 15
40kHz Oscillator
This nominally 40kHz oscillator produces a sinusoidal output of
suitable frequency for use with some of the AC driven transducers
provided with the DYNA 1750 Trainer.
The Colpitts oscillator uses an LC tuned circuit with center-tapped
capacitors in the feedbacks loop, giving good stability of oscillation
frequency and amplitude.
The effective component values are L = 1mH, C = 15nF giving a
design oscillation frequency of:
The buffer gives low output impedance and prevents loading of the
oscillator, which might cause frequency shifting.
The main characteristics of the device are:
Output frequency range
37-46kHz
Output frequency (typ.)
41kHz
Output amplitude
6Vp-p
Output impedance
1.1kO
Table 15.8
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Chapter 15
15.8
IT 01
Curriculum Manual
Practical Exercise
Characteristics of a 40kHz Oscillator
1
2
3
4
Connect the circuit of Fig 15.9 with the variable resistor slider
to the right for maximum resistance. The slider resistor will not
be used initially.
Set the oscilloscope timebase to 5µs/div (calibrated) and the Y1
(CH.1) amplifier to 1V/div.
Switch ON the power supply.
Note the amplitude of the 40kHz Oscillator output and the time
taken for one cycle. Record these in Table 15.9.
Open circuit
amplitude
Vp-p
Time taken
for one cycle
µs
Frequency
kHz
Output
impedance
kO
Table 15.9
5
234
Calculate the reciprocal of the time taken for one cycle (the
time period) to obtain the frequency and add this to Table 15.9.
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Comparators, Oscillators and Filters
Chapter 15
Measurement of the Output Impedance
6
Connect socket B of the 10kO slider resistor to the output of
the 40kHz Oscillator and reduce its value until the output
amplitude of the oscillator falls to half of the open circuit value.
You may find it convenient to change the setting of the Y1
amplifier to 0.5V/div to do this measurement. The display
amplitude will then be the same as before.
When this is done the voltage dropped across the 10kO slider
resistor (R in Fig 15.10) is the same as the output impedance of
the 40kHz Oscillator (Ro). Since the two resistances are in
series, the current through them must be the same, so their
resistances must be the same. This is a standard technique for
measurement of output impedance.
7
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Switch OFF the power supply, disconnect the 10kO slider
resistor from circuit (without changing the setting) and measure
the resistance of the section used with your digital multimeter
as ohmmeter. Add the result to table 15.9.
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Chapter 15
15.9
IT 01
Curriculum Manual
Filters
There are four main classifications of filter, specified by the range of
frequencies passed:
1.
2.
3.
4.
Low pass filter, LPF, passing all frequencies below the design
(cut-off) value.
Band pass filter, BPF, passing those frequencies within the
design range.
Band stop filter, BSF, passing those frequencies outside the
design range.
High pass filter, HPF, passing all frequencies above the design
(cut-off) value.
The symbols used to represent the four types are shown in Fig 15.11
The cut-off frequency is sometimes called the break or corner
frequency and is the frequency at which the output first falls to –3dB
(0.707Vmax) form the mid-band.
Only a bandpass and a low pass filter are provided with the DYNA
1750 Trainer.
The main characteristics of these are:
Band Pass Filter
Lower cut-off frequency
39.5kHz
(typ.)
Upper cut-off frequency
42.5kHz
(typ.)
Time constants
Input impedance
Output impedance
Input voltage (max.)
Table 15.10
236
10kO
10kO
-
Low Pass Filter
16, 1.44 or
0.14Hz
10ms, 100ms or
1s
1MO
12V
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Comparators, Oscillators and Filters
Chapter 15
15.10 Practical Exercise
Characteristics of a Bandpass Filter
The very low cut-off frequencies of the Low Pass Filter make it
difficult to investigate the response because of the demands, which
would be made on the function generator ranges. This investigation is
therefore limited to the 40kHz Bandpass Filter.
1
Connect the circuit of Fig 15.12. The 10kO slider resistor is
being used to provide a convenient monitoring point for the
input signal rather than for signal amplitude adjustment. Set it
to about scale point 7.
2
Set the oscilloscope timebase to 5µs/div (calibrated), the Y1
(CH.1) amplifier to 1V/div and the Y2 (CH.2) amplifier to
0.5V/div. Inject a sinewave signal of large amplitude at about
40kHz.
3
4
Switch ON the power supply.
Adjust the fine frequency control of the function generator to
peak the output of the 40kHz Filter to maximum as seen on
CH.2 of the oscilloscope, then adjust the amplitude to 2.5V
peak-to-peak (5 div.) using either the function generator
amplitude control and/or the 10kO slider resistor.
If you are unable to obtain 2.5Vp-p from your function
generator then the investigation can be carried out with any
convenient lower value but this may result in some interference
with the output signals.
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Chapter 15
5
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Calculate the time for one cycle from the oscilloscope display
and record this in Table 15.11
Peak
response
Time period
Frequency
Upper cutoff
Lower cutoff
µs
µs
µs
kHz
kHz
KHz
Table 15.11
6
7
8
9
10
238
Without any further adjustment to amplitude, increase the
frequency from the function generator until the amplitude of
the CH.2 waveform is reduced to 3.5 div. This is a reduction of
–3dB (0.707V) from the maximum value and corresponds to
the upper cut-off frequency.
Calculate the time for one cycle again from the oscilloscope
display and record this in Table 15.11.
Reduce the frequency back through the peak and carry on until
the amplitude again falls to 3.5 div. at the lower cut-off
frequency. Again record the time for one cycle in Table 15.11.
Take the reciprocal of the three time periods to find the center
frequency and the upper and lower cut-off frequencies.
Switch OFF the power supply.
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Curriculum Manual
Mathematical Operations
Chapter 16
Chapter 16
Mathematical Operations
Objectives of
this chapter
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the characteristics of a summing amplifier.
2
Describe the characteristics of an integrator.
3
Describe the characteristics of a differentiator.
4
Describe the characteristics and application of a “sample
and hold” circuit.
1
2
3
4
5
6
DYNA-1750 Transducer and Instrumentation Trainer
4mm Connecting Leads.
Digital Multimeter.
Oscilloscope.
Function Generator.
BNC to 4mm connecting lead.
239
Mathematical Operations
Chapter 16
16.1
IT 01
Curriculum Manual
The summing Amplifier
The gain of an operational amplifier is typically one million. To keep
within saturation limits the input voltage must therefore be less than
one millionth of the output voltage, or a few microvolts. The input
voltage is so low that the input is known as the a virtual ground (VG)
(Fig 16.1).
The input impedance of the operational amplifier is very high,
typically measured in MO. With an input voltage in µV and an input
impedance in MO, the input current to the Op Amp is non-existent, or
at least negligibly small.
From Kirchhoffs Laws, the current(s) into a junction must be the same
as the current(s) out of the junction, so, since there is no current
flowing into the Op Amp, the feedback current (IF) must be equal to
the sum of the three input currents (I1, I2 & I3).
V0
V1 V2
V3
=
+
+
+ ------RF
R1 R2
R3
If all of the resistors are made the same size, then they cancel out in the
equation leaving:
V0 = V1 + V2 + V3 + -----The output voltage is the sum of the three input voltages. However,
since the inverting input has been used it will be of opposite sign or
polarity, so an inverter has been added to restore the original polarity.
Other input branches may be added.
The main characteristics for the device provided are:
Input voltage (max.)
Voltage gain
Output voltage (max.)
Table 16.1
240
±12V
1.0
(VA + VB +VC) ±10V
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16.2
Mathematical Operations
Chapter 16
Practical Exercise
Characteristics of a Summing Amplifier
1
2
Connect the circuit as shown in Fig 16.2. Set the variable
resistors to their central positions.
Switch ON the power supply and adjust the controls of the
three resistors to vary the output voltage. Note that variation of
any of the input voltages affects the output voltage.
You will find that increase of input voltage will increase the
output voltage up to a certain maximum (saturation) after
which any further increase of input dose not increase the output
any more.
3
Determine this maximum (saturation) output voltage.
4
Maximum possible output voltage =
5
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±
V
Set the Summing Amplifier input voltages to the values
indicated in the first row of Table 16.2. Note the expected
output voltage and also note and record the actual output
voltage obtained in Table 16.2.
241
Mathematical Operations
Chapter 16
A
IT 01
Curriculum Manual
Inputs (volts)
B
C
1
+1
+1
+1
2
+2
+1
+3
3
+2
+4
+3
4
-3
+4
+2
5
-3
-2
-2
6
+3
+5
+4
7
+3
-5
+4
8
-3.5
+2.7
-1.4
Output
- (A + B + C)
Voltage
+3V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
Table 16.2
6
242
Repeat the procedure for the other settings listed in Table 16.2
to verify that the output voltage is the sum of the input voltages
as long as you keep within the saturation limits.
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16.3
Mathematical Operations
Chapter 16
The Integrator
An integrator is a circuit having an output voltage that is proportional
to the average of the input voltage multiplied by units of time. In
mathematical terms this is referred to as the integral of the voltage.
Note that, in the feedback path, the resistor has been replaced by a
capacitor, since the voltage across a capacitor at any time depends on
the amount of current that has been flowing and the time for which it
has flowed.
Expressed in mathematical terms:
The feedback current (i in the above equation) is fixed by the input
voltage
Vin
voltage Vin and the input resistor R (Fig 16.3). i =
. Substituting
this into the equation:
R
the output voltage is the integral of the input voltage, multiplied by a
1
factor,
.
CR
With the input voltage constant, the output voltage will increase
linearly with time. The time taken for the output voltage to reach the
input voltage is referred to as the time constant of the circuit and is
equal to CR seconds (from the equation).
The maximum possible value of the output voltage is limited by the
supply to the saturation voltage of approximately ±11V for the device
provided.
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Mathematical Operations
Chapter 16
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The main characteristics of the device provided with the DYNA 1750
Trainer are:
Input voltage (max.)
Voltage gain
Output voltage (max.)
100ms, 1s & 10s
10kO
1%
Table 16.3
Notes :
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
244
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16.4
Mathematical Operations
Chapter 16
Practical Exercise
Characteristics of an Integrator
1
2
Connect the circuit as shown in Fig 16.4. Set the Integrator
time constant switch to 1s.
Switch ON the power supply. Set the input voltage to 1V. Press
and hold the RESET button. This sets the output voltage to 0V.
Release the RESET button and you will note that the output
voltage increases and will reach a maximum value after
approximately 12 seconds. Note this maximum value using the
20V digital meter.
Maximum output voltage =
3
4
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V
Press the RESET button and release it to allow the output
voltage to increase from 0V again. Remove the Integrator input
lead when the voltage reaches approximately 5V.
Replace the input lead and observe the effect on the output
voltage.
245
Mathematical Operations
Chapter 16
IT 01
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The Timer facility of the DYNA 1750 Trainer will now be introduced.
This allows you to accurately determine the time taken to reach any
given voltage. The system will be made entirely automatic by using
another facility of signal conditioning circuits, the Comparator.
Note that the non-inverting input of the Comparator is taken to a
positive reference voltage, the value of which is determined by the
setting of the 10kO slider resistor. If this is set to 10V then the
Comparator will give a high output until the output of the Integrator
(which is connected to the inverting input of the comparator) exceeds
10V, when the Comparator output will go low.
While the Comparator output is high the Timer is enabled and will
count in hundredths of a second. The moment the output of the
Integrator goes above the Comparator reference voltage (in this case
10V) the Comparator output goes low and stops the Timer.
246
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1
2
3
Switched
time
constant
1
1s
2
100ms
3
100ms
4
10s
Table 16.4
4
5
6
Dynalog (India) Ltd.
Mathematical Operations
Chapter 16
Construct the additional circuit of Fig 16.5, noting that the
supply voltage to the variable resistors has been changed to
+12V.
Reset the input voltage to 1V.
Ignore the Timer function for the moment. Press the Integrator
RESET button and, using the second hand of a clock or watch,
note the time after releasing it that the Integrator output voltage
reaches 10V as indicated on the Moving Coil Meter.
This enables the circuit time constant to be determined. The
input voltage is 1V. The output voltage should reach 1V after
one time constant and should reach 10V after 10 time
constants. The time constant can therefore be determined by
dividing the time taken by 10. Record the results in row 1 of
Table 16.4.
Input Reference Number of
Time taken
Calculated
Voltage voltage
time
to reach
time constant
(i)
(ii)
constants
ref. (iv)
(v)
(iii)
1V
10V
10
s
s
1V
10V
s
ms
0.2V
5V
s
ms
5V
2V
s
s
Switch the Timer to TIME and FREE RUN. If necessary press
RESET to zero the display.
Move the digital multimeter to terminal B of the 10kO slider
resistor and adjust the reference voltage to 10V.
Press the Timer RESET to zero the display. Re-adjust the
Integrator input voltage to 1V, set the time constant to 100ms
and VERY BRIEFLY press its RESET button. You must not
hold the RESET button down or the Timer will be counting too
soon. Observe the effect on the Timer. This will count up from
zero until the output voltage of the Integrator exceeds the
reference voltage applied to the Comparator. The display will
be in hundredths of a second. For example, a display of 487
represents 4.87 seconds.
247
Mathematical Operations
Chapter 16
7
8
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Curriculum Manual
Repeat the test a few times to become familiar with the action.
Zero the Timer each time. Record the result in row 2 of Table
16.4.
Calculate the time constant as follows:
The number of time constants is the reference voltage divided
by the applied voltage:
(iii) = (ii) + (i)
The measured time constant is the time taken to reach the
reference voltage divided by the number of time constants:
(v) = (iv) + (iii)
Add the calculate time constant to Table 16.4.
9
10
248
With the Integrator time constant still at 100ms, change the
input voltage (10kO 10-turn resistor) to 0.2V and the reference
voltage (10kO slider resistor) to 5V and repeat the test and
calculation. Remember to zero the Timer each time. Record the
results in row 3 of Table 16.4.
Change the Integrator time constant to 10s, the reference
voltage (10kO slider resistor) to 2V and the input voltage
(10kO 10-turn resistor) to 5V and repeat the test. Record the
results in row 3 of Table 16.4
11
Calculate the time constant and add to Table 16.4.
12
Switch OFF the power supply.
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16.5
Mathematical Operations
Chapter 16
The Differentiator
A simple differentiator is shown in Fig 16.6.
The output voltage is proportional to the rate of change of the input
voltage.
Examine the waveforms of Fig 16.4. Initially the capacitor is
uncharged and there is similarly no voltage across the resistor.
When the input voltage suddenly rises to a positive value the capacitor
voltage cannot change instantaneously so the full applied voltage
appears across the resistor. Current flows and the capacitor charges.
As the voltage rises across the capacitor it must fall across the resistor,
until the capacitor is fully charged. The time taken for this will depend
on the size of the resistor (controlling the charging current) and the size
of the capacitor (how much charge is needed to raise the capacitor
voltage).
One time constant is the time it would take for the capacitor to fully
charge to the applied voltage if the initial current could be maintained.
Obviously the current must reduce as the voltage across the resistor
reduces, so the rate of charge falls away. In theory it never reaches full
charge. However, for all practical purposes full charge is reached after
5 time constants.
The time constant is calculated from the value of the capacitor in
farads multiplied by the value of the resistor in ohms:
Time constant t = CR seconds
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Note that for long time constants such as 1s, using a 1µF capacitor
(typically the largest value non-electrolytic capacitor) the value of the
resistor would need to be 1MO. Non-electrolytic capacitors are needed
so that the capacitor can be charged with negative polarity.
The high value of resistor raises the problem of a very high output
impedance for the circuit. If any load was applied to the differentiator
the operation would be seriously affected.
To overcome this problem an active differentiator circuit is used on the
DYNA 1750 Trainer, consisting of an active differentiator Op Amp
followed by a unity gain buffer stage.
Note that a sudden change of input voltage produces a similar change
at the output, the amplitude of this being limited by the saturation
voltage of the differentiator active circuits.
With the input voltage then held constant, the output voltage falls
exponentially, the rate of fall depending on the circuit time constant,
the initial rate of fall aiming at a time span equal to the time constant.
The main characteristics of the device provided with the DYNA 1750
Trainer are:
Input voltage (max.)
Input voltage rate of change (max.)
±12V
10-3V/µs
Output saturation voltage (typ.)
±12V
Output noise (time constant 1s)
50mV
Table 16.5
250
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16.6
Mathematical Operations
Chapter 16
Practical Exercise
Characteristics of a Differentiator
1
2
Connect the circuit as shown in Fig 16.7. Set the time constant
controls of the Integrator and Differentiator to 1s. The Moving
Coil Meter is used to monitor the change of voltage at the
Integrator output.
Switch ON the power supply. Set the input voltage to the
integrator to 1V, then transfer the digital multimeter to the
output of the Differentiator. Press and then release the RESET
button on the Integrator and note the output voltage from the
Differentiator.
The Integrator output voltage will be changing at 1V/s for
approximately 11s and the output from the Differentiator
should remain constant during this time. Note the output
voltage.
Output voltage =
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V
251
Mathematical Operations
Chapter 16
3
4
5
252
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Change to the circuit of Fig 16.8. Set the function generator to a
30Hz square wave output. Set the 10kO slider resistor to midway. Switch the oscilloscope timebase to 5ms/div chop mode,
the Y1 amplifier (CH.1) to 0.5V/div and Y2 amplifier (CH.2)
to 2V/div.
Set the Differentiator time constant to 10ms and adjust the
signal input (function generator amplitude control and/or 10kO
slider resistor) to give an input signal (CH.1) of 1Vp-p.
Sketch the two waveforms on the graticule provided with the
input at the top:
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7
Mathematical Operations
Chapter 16
Compare these waveforms with the theoretical waveform given
in the previous section (16.5).
The Differentiator will almost certainly be loading the function
generator output to some extent and changing the waveform.
Remove the lead to the Differentiator input and observe the
effect on the function generator output waveform.
This distortion is very common and, as you can see from the
output waveform, does not seriously affect the operation of a
differentiator.
Notes :
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Chapter 16
16.7
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A Sample and Hold Circuit
This circuit allows the value of an input signal at any instant of time to
be stored on command and held for processing
In the sample mode (SAMPLE button pressed), the instantaneous value
of the input signal is tracked at the output. When the SAMPLE button
is released the circuit enters the hold mode and the value of the input at
that instant is held as a charge on a capacitor, Fig 16.9(a).
The capacitor voltage will fall gradually with time as the capacitor
discharges through leakage paths and the this fall in voltage is referred
to as droop.
Fig 16.9 (b) illustrates the characteristics during sample and hold
periods of operation.
The circuit is normally used in connection with analog to digital
conversion of a varying signal. The signal would be sampled
frequently and, during the hold time, the value is digitally encoded.
The main characteristics of the device provided with the DYNA 1750
Trainer are:
Input voltage range (max.)
±12V
Input time constant
1ms
Droop rate
10mV/minute
Table 16.6
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16.8
Mathematical Operations
Chapter 16
Practical Exercise
Characteristics of a sample and Hold Circuit
1
2
3
Connect the circuit as shown in Fig 16.10. Set the function
generator output to 40Hz sinewave with high amplitude. Switch
the oscilloscope timebase to 5ms/div, Y1 amplifier (CH.1) to
10V/div, chop mode (near the top of the screen) and Y2
amplifier (CH.2) to 2V/div, DC input (near the middle).
Switch ON the power supply and adjust the amplitude of the
signal (function generator amplitude control and/or 10kO
wirewound resistor) to give an input of 20Vp-p. If your
function generator does not give 20Vp-p then use the AC
Amplifier (GAIN = 10) to boost the signal input. Move CH.1 of
the oscilloscope to the output of the AC Amplifier.
Press and release the SAMPLE button to catch a sample of the
input voltage to the circuit. Note that while the SAMPLE
button is pressed the input signal appears at the output (CH.2 of
the oscilloscope). When released a random sample is captured
and appears as a DC voltage at the output as indicated by the
meter. Try several times and record the results in Table 16.7.
1
Output voltage
2
V
3
V
5
4
V
V
6
V
8
7
V
V
9
V
10
V
V
Table 16.7
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Control Systems Characteristics
Chapter 17
Chapter 17
Control Systems Characteristics
Objectives of
this chapter
Dynalog (India) Ltd.
Having studied this chapter you will be able to:
1
Describe the characteristics of an ON/OFF system.
2
Describe the characteristics of a Proportional system.
3
Describe the characteristics of an Integral system.
4
Describe the characteristics of a Derivative system.
5
Explain that a practical system may incorporate
Proportional, Integral and Derivative components and be
referred to as a 3-term (or PID) controller.
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17.1
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A Basic ON/OFF Closed Loop system
A controlled variable is any physical system which we may wish to
control, such as a heated environment (hot water tank), lighting level
(PIR controlled lighting), mechanical systems (speed, position or
direction, linear or rotational), and many more. For instance, the
modern airplane is full of electrical control systems.
An error is any difference between a desired result and an actual
result. In an electrical control system the output is converted into an
electrical quantity by a transducer.
Fig 17.1 shows a simple closed loop control system, the error detector
detecting the difference between the actual and the desired value of the
controlled variable.
The output of the controlled variable (the transducer) is compared with
a reference input (command input) and an error signal is fed to the
controller which initiates an actuating signal to alter the state of the
controlled variable and reduce the error, ideally to zero.
In an ON/OFF system the controller will have only two states:
258
1.
With the value of the controlled variable less than that desired,
the controller output is maximum.
2.
With the value of the controlled variable grater that that
desired, the controller output is zero.
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Control Systems Characteristics
Chapter 17
This method of control is suitable for systems having inertia (a long
time constant) such as the temperature control of a room, using a
heater. The method might give characteristics as illustrated in Fig 17.2.
Initially, the heater is ON and the temperature rises exponentially from
its ambient state. When the desired temperature is reached, the heater
is switched OFF.
The temperature will continue to rise or overshoot for a time due to the
residual heat in the heater, but will eventually fall, the rate of the fall
increasing with time. When the temperature has fallen below the
desired value, the heater will again be switched ON but the
temperature will continue to fall for a time before the heater has any
effect.
The resulting characteristics will be as shown in Fig 17.2, with the
temperature varying continuously between two limits, provided that
there is no change in the operating conditions, such as heat loss
variations or a change in the thermostat setting (command input).
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Proportional Control
With this system of control, the output from the controller is
proportional to the magnitude of the error signal (not just ON or OFF).
Controller output = Kp x Error
Where Kp is the proportional gain of the controller
The characteristics of the system depend on the value of Kp.
For large values of gain in the feedback loop the characteristics are
similar to those for ON/OFF control. For small values of gain the
system will be sluggish and very slow to respond.
Fig 17.3 shows the characteristics of proportional control in response
to a step input (or sudden change) and illustrates that a high gain
results in a rapid response but produces an overshoot of the desired
reference setting, together with oscillations about the reference setting.
Medium gain results in a slower response with minimum overshoot
and oscillations.
Low gain results in a slow response with no oscillations but possibly
never reaching the reference setting.
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Control Systems Characteristics
Chapter 17
The term damping is used to cover the inertia or friction of a feedback
system.
Characteristics such as those for high gain in Fig 17.3 are referred to as
underdamped and for low gain, overdamped.
A response which rises most rapidly to the reference with no overshoot
is referred to as critically damped.
The degree of damping is normally referred to in terms of the damping
ratio, which is given the Greek symbol ? (Zeta). Critical damping has
damping ratio of 1.0. For underdamping the damping ratio is less than
1.0 and for overdamping, greater than 1.0.
Fig 17.4 shows the response of a proportional control system to an
input varying with time (ramp input). The output tends to follow the
input but, due to inertia within the system, the error between the input
and output quantities has to increase to a threshold before there is
sufficient actuating signal to produce a variation of the output.
The output will thereafter follow the input but will lag behind the
input, this being referred to as velocity lag. The magnitude of the lag
will depend on the gain of the system, the friction and the output
loading.
There may be oscillations in the output characteristics as shown dotted,
depending on the system gain.
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These characteristics mean that pure proportional control is unsuitable
for applications where the input may vary with time. In addition the
system has some disadvantages with constant input (command)
conditions.
Consider the system operating with a set input and with the output at
the reference setting so that there is no error. Under these conditions
there will be no controller output.
A load imposed on the output will produce a change of output state. An
error signal will be produced to counteract this and reduce the error,
but the output will not now be at the desired reference state. The error
introduced will vary with the loading imposed on the output.
Proportional control on its own is therefore unsuitable for control
applications.
In practice, due to saturation effects within the system, the controller
output will be proportional to the error only over a part of the full
range.
This is illustrated in Fig 17.5. The range over which the output is
proportional to the error is referred to as the proportional band.
262
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17.3
Control Systems Characteristics
Chapter 17
Integral Control
Integral control can be used to eliminate any error present between the
reference and actual output setting. An integrator produces an output
that is proportional to input x time and hence, if the error signal is fed
via an integrator circuit, its output will increase with time. With this
output fed to the system controller, an actuating signal will be
produced to reduce the error, the time taken depending on the
integrator time constant.
Fig 17.6 illustrates the operation of integral control for ramp input
conditions. While there is an error, the integrator output increases. This
output, fed to the controller, produces an actuating signal to correct the
error. When the error has been reduced to zero, the integrator output
remains constant, thus compensating for the velocity error that would
have been present without the integral control.
Any further error, however caused, will be automatically compensated,
provided the output required is within the capacity of the integrator
circuit.
Normally, the integral control would be combined with proportional
control, the proportional control being the main control and leaving the
integral control for final adjustments of the output setting.
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Derivative (or Differential) Control
Friction losses in a system produce damping and thus allow operation
under proportional control with a higher system gain, but the
introduction of friction represents a power wastage and increases the
time taken to reach stable conditions following any disturbance.
The same effect can be produced using an adder fed with derivative
control, by feeding back a signal that is proportional to the rate-ofchange of the output or the rate-of-change of the error signal. This is
illustrated in Fig 17.7.
Error (iii) = Input (i) – Output (ii)
Rate-of-change of output (iv) = slope of Output (ii)
Actuating signal (v) = Error (iii) – rate-of-change of Output (iv)
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Practical Control Systems
Chapter 18
Chapter 18
Practical Control Systems
Objectives of
this chapter
Having studied this chapter you will be able to:
1
Describe the characteristics of an ON/OFF temperature
control system.
2
Describe the characteristics of a light controlled ON-OFF
system.
3
Describe the characteristics of a positional control system
having:
Proportional,
Proportional + integral,
Proportional + derivative and
Proportional + integral + derivative control.
Describe the characteristics of a speed control system.
4
Equipment
Required for
This Chapter
Dynalog (India) Ltd.
1
2
3
4
DYNA-1750 Transducer and Instrumentation Trainer
4mm Connecting Leads.
Digital Multimeter.
Calculator (not supplied)
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Chapter 18
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Practical Exercise
Characteristics of an ON/OFF Temperature Control System
The shaded area within the broken line is a digital thermometer
indicating temperature in increments of 0.1OC.
The internal Temperature Sensor is an integrated circuit, which gives
an output of 10mV/OK, so the output at an average room temperature
of 20OC will be 2.93V. (Will be vary depends on ambient temperature
at site)
The 10-turn potentiometer on the Wheatstone Bridge panel is adjusted
to give 2.73V to the inverting input of the Differential Amplifier. The
output from the Differential Amplifier will therefore be 0.01V/OC, or
0.2V at 20OC.
The V/F Converter gives an output of 1kHz/V, so an input of 0.2V will
give an output of 200Hz (200 pulses in one second). Within the range
of accuracy +/- 20%.
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Chapter 18
The Differentiator, X100 Amplifier and Inverter shape the pulses to be
compatible with the Counter/Timer input, which will therefore display
200 for a temperature of 200OC, or the temperature in tenths of a
degree. A display of 213 = 21.3OC.
1
2
3
4
Connect the circuit as shown in Fig 18.1. Switch the
comparator HYSTERESIS OFF and set the 10kO resistor
control fully counter clockwise. Set the Differentiator to 1s and
the Counter/Timer controls to COUNT and 1s.
Remove the output lead from the Electronic switch while you
carry out the initial setting up.
Switch ON the power supply and adjust the 10kO 10-turn
potentiometer for a voltage of 2.73V on the inverting input of
the Differentiator Amplifier. This will set up the digital
thermometer to display the ambient temperature in OC. Press
the RESET button on the Counter/Timer each time you need to
obtain a temperature reading.
Transfer the voltmeter to the output of the IC Temperature
sensor and note the output voltage (You may need to remove
one of the leads while you do this).
I.C. Temperature sensor output voltage =
5
Transfer the voltmeter again to the output of the 10kO resistor
and set the output voltage to a value 0.2V above the output
value obtained from the IC Temperature Sensor. This sets the
reference temperature of the system to 20OC above the ambient
temperature.
Reference voltage setting =
6
V
V
Restore the output lead to the Electronic Switch to start the
heating process. Note the temperature-time characteristics of
the system by noting the displayed temperature and the heater
state (whether ON or OFF) at time intervals of 30s (0.5
minute).
Note : The heater state will be indicated by the lamp, lamp ON =
heater ON and lamp OFF = heater OFF. Enter the details in
Table 18.1.
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Time
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5
(minutes)
Heater state
ON/OFF
Temperature
O
C
Table 18.1
7
8
9
268
Plot the temperature-time characteristics on the axes provided:
Shade in blocks at the bottom of your graph to represent when
the Heater was switched ON.
Mark in a line on your graph to represent the reference
temperature setting.
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Chapter 18
If time permits add an alarm circuit to the system. The alarm
is to operate if the temperature exceeds 30OC above the
ambient temperature.
Select suitable components from the devices available with
DYNA 1750 unit, connect, and check the operation of
system by simulating a fault. Do this by disconnecting
feedback from the Temperature sensor to input B of
Comparator. Finally, switch OFF the power supply.
the
the
the
the
Notes :
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Chapter 18
18.2
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Curriculum Manual
Practical Exercise
Characteristics of a Light Controlled ON/OFF System
A system is to operate a solenoid. The solenoid is to be ON with the
light level low and is to be automatically turned OFF when the light
level exceeds a preset level.
1
2
3
270
Connect the circuit shown in Fig 18.2.
Switch the Comparator HYSTERESIS OFF and set the resistor
controls as follows :Fully counter clockwise for the carbon track,
Fully to the left for the slide,
Fully clockwise for the wirewound track.
Switch ON the power supply (the solenoid should energize).
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Chapter 18
Move the slide resistor to the right so that the Solenoid is just
de-energized. This represents the preset conditions for
operating the system with the lighting at the ambient level.
Move your hand over the Photoconductive Cell. You will note
that the Solenoid will change its state as the lighting level falls
due to your shadow (the Solenoid energizes, indicating that the
electronic switch is closed).
With no hysteresis in the Comparator circuit, only a small drop in
lighting level is required to produce the change. Introduction of some
hysteresis would increase the lighting change required, but the
hysteresis provided with the Comparator is too great for this
application and would operate as a latch.
6
7
Lamp Filament
Voltage
Slide Resistor
Setting
Table 18.2
8
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Cover the opto-sensor clear plastic enclosure with an opaque
box to exclude all ambient light. The Solenoid should
immediately energize as the light level is reduced.
With the voltage applied to the lamp filament at 0V (control of
the lamp voltage is via the 100kO carbon track resistor) a
indicated on the Moving Coil Meter, move the slide resistor
further to the right until the Solenoid changes state.
0
1
2
3
4
5
6
7
8
9
10
Adjust the lamp filament voltage to each of the settings gives in
Table 18.2, and repeat the procedure noting the slide resistor
setting required for a change of state of the solenoid Record the
results in Table 18.2.
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Plot the graph of slide resistor setting against lamp voltage on
the axes provided.
This exercise has illustrated the use of an ON-OFF lighting control
system. The slide resistor can be set to any value, within the range
noted, to procedure circuit switching at any desired value of lighting
level.
10
272
Switch OFF the power supply.
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18.3
Practical Control Systems
Chapter 18
Practical Exercise
Characteristics of a Positional Control System – 1
Proportional Control
Study the diagram.
The proportional control section runs across the middle of the
diagram. The 10kO wirewound resistor in the command input. The
function of the Differential Amplifier is to inject a step input voltage
later in the investigation. The step voltage is generated by Amplifier #2
offset voltage, which is the only purpose for including this amplifier.
You will see that it does not need an input for this purpose.
Integral control will be added later by connecting the Integrator in
between the Error Detector (the Instrumentation Amplifier) and the
Summing Amplifier.
Derivative control will also be added later via the Summing Amplifier.
The Inverter in between the Differentiator and the Summing Amplifier
is to provide negative feedback.
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The Summing Amplifier combines all of the control systems as
required.
1
2
3
4
5
6
7
8
Connect the circuit as shown in Fig 18.3. This circuit is
arranged for proportional control only.
Set Amplifier #1 GAIN COARSE control to 10 and GAIN
FINE to 1.0 to give an overall gain of 10.0.
Remove the power connection to the Motor. Switch ON the
power supply.
Set Amplifier #2 GAIN COARSE control to 100 and GAIN
FINE to 1.0 and adjust the OFFSET control for an output of
+3V. Return the GAIN COARSE control to 1. The output
voltage should fall to near zero volts. Note that since this +3V
step is fed into the system via the inverting input of the
Differential Amplifier the actual step injected will be –3V.
Transfer the Moving Coil Meter to terminal B of the 10kO
wirewound resistor. Adjust the setting of the 10kO resistor
control to its central position to give 0V output.
Zero the setting of the Servo Potentiometer dial against the
pointer.
Transfer the Moving Coil Meter to the output of the Power
Amplifier and adjust Amplifier #1 OFFSET to give 0V. Restore
the Motor power connection.
Restore the 10kO wirewound resistor control slowly over its
full travel. The Motor drive shaft and the Servo Potentiometer
dial should rotate and follow the movement of the command
input, although the system may be sluggish and there will be a
lag before the Servo Potentiometer starts to follow the input
setting. This is because the system gain is low, Amplifier #1
gain being set 1.0.
Amplifier #1 Gain = 1.0
Maximum Dial Reading
(degrees)
Table 18.3
274
Positive
Negative
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Practical Control Systems
Chapter 18
Return the 10kO resistor to its central position. Set amplifier #1
GAIN FINE to 0.5 (overall gain 5) and repeat the procedure.
With this higher setting of the gain control the Servo
Potentiometer should follow the input closely for no load on the
drive shaft and it should be possible to obtain the full travel of
the wirewound track resistor in both directions.
Rotate the input control slowly when nearing the end of the
travel or the Servo Potentiometer contact may overshoot
and pass the end of the track, causing the drive shaft to
rotate continuously. If this occurs, return the 10kO resistor
quickly to its central position.
10
Control Setting
Servo-Potentiometer
Dial Reading (deg.)
Table 18.4
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Note the full range of travel of the Servo Potentiometer against
the setting of the 10kO wirewound resistor command input.
Record the results in Table 18.4.
1
2
3
4
5
0V
6
7
8
9
10
0/
360
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Plot the graph of Dial Reading against Control setting on the
axes provided on the previous page (Graph 18.3).
Repeat the readings in the reverse direction and compare the
dial readings obtained with the previous readings recorded in
Table 18.4.
3
Set amplifier #1 GAIN FINE to 1.0 and use the input command
control to return the Servo Potentiometer dial reading to 0O.
4
Move the Servo Potentiometer dial by rotating the Hall effect
disc by hand and note the total range (for example +20O to –
10O = 30O, it may not be symmetrical) over which the dial can
be moved without the system responding and moving the dial
back. This value represents a deadband over which the system
does not respond. Record the result in Table 18.5.
Amplifier #1 Gain
10 x 1.0 = 10
10 x 0.5 = 5
10 x 0.1 = 1
Deadband (deg.)
Table 18.5
5
276
Repeat the procedure for Amplifier #1 GAIN FINE setting of
0.5 and 0.1, adding the results to Table 18.5.
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Chapter 18
Set Amplifier #1 GAIN FINE to 0.1.
Switch the GAIN COARSE control of Amplifier #2 from 1 to
100 and note the effect on the output shaft position. Return the
control to 1 and again note the effect. Repeat the procedure
several times.
Take care not to touch the OFFSET control when you are
doing this, as the setting is very critical.
3
4
Repeat the procedure with the Amplifier #1 GAIN FINE set 0.5
and then 1.0.
Switch OFF the power supply, but:
Keep the circuit connected if possible for the following Exercises.
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18.4
Practical Exercise
Characteristics of a Positional Control System – 2
Proportional + Integral Control
1
2
3
4
5
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If necessary, re-connect the circuit as shown in Fig 18.4
(without the Integrator output connected initially). Re-check the
settings as follows:
Remove the power connection to the Motor. Zero the setting of
the Servo Potentiometer dial against the pointer. Ensure that the
potentiometer is engaged with the drive shaft.
Set Amplifier #1 GAIN COARSE control to 10 and GAIN
FINE to 0.1 to give an overall gain of 1.0.
Switch ON the power supply.
Connect the Moving Coil Meter temporarily to terminal B of
the 10kO resistor and check the setting to its central position to
give 0V output.
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7
8
9
10
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Chapter 18
Transfer the Moving Coil Meter back to the output of the
Power Amplifier and check the adjustment of Amplifier #1
OFFSET to give 0V.
Transfer the Moving Coil Meter to the output of Amplifier #2,
set the GAIN COARSE control to 100 and GAIN FINE to 1.0
and check the adjustment of the OFFSET control for an output
of +3V. Return the GAIN COARSE control to 1. This control
will again be used to introduce a step input.
Restore the power connection to the Motor. With the Integrator
time constant set 1s, press and hold the RESET button, connect
the Integrator output lead to the Summing Amplifier input as
shown by the arrow in Fig 18.4 and then release the RESET
button.
In the event of continuous rotation of the Motor shaft in the
following tests, immediately return the Amplifier #2 GAIN
COARSE switch to 1 and then hold the Integrator RESET
button until the shaft becomes stationary.
Note the effect on the output Servo Potentiometer dial reading
when a step input is applied by switching Amplifier #2 GAIN
COARSE to 100 and then back to 1. watch the long term effect
on the Integrator output voltage (on the digital voltmeter) and
on the dial setting.
Always use digital multimeter to observe FEEDBACK Result.
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Chapter 18
18.5
IT 01
Curriculum Manual
Practical Exercise
Characteristics of a Positional Control System – 3
Proportional + Derivative Control
1
If you till have the circuit connected then remove the lead from
the Integrator output to the Summing Amplifier and connect the
output from the Inverter to the Summing Amplifier as shown in
Fig 18.5. Otherwise connect the circuit as shown.
Re-check the setting as follows:
2
280
Remove the power connection to the Motor. Zero the setting of
the Servo Potentiometer dial against the pointer. Ensure that the
potentiometer is engaged with the drive shaft.
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4
5
6
7
8
9
10
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Chapter 18
Set Amplifier #1 GAIN COARSE control to 10 and GAIN
FINE to 0.6.
Switch ON the power supply.
Transfer the Moving Coil Meter temporarily to terminal B of
the 10kO resistor and check the setting to its central position to
give 0V output.
Transfer the Moving Coil Meter temporarily to terminal the
output of the Power Amplifier and check the adjustment of
Amplifier #1 OFFSET to give 0V.
Transfer the Moving Coil Meter back to the output of Amplifier
#2, set the GAIN COARSE control to 100 and GAIN FINE to
1.0 and check the adjustment of the OFFSET control for an
output of +3V. Return the GAIN COARSE control to 1.
Restore the power connection to the Motor.
Set the Differentiator time constant to 100ms and note the
output Servo Potentiometer response to a step input of +3V
applied by changing Amplifier #2 gain control from 1 to 100
and then back to 1.
Repeat the procedure and note the response for Differentiator
time constant settings of 10ms and 1s.
For the time constant 1s set Amplifier #1 GAIN COURSE
control to 10 and GAIN FINE to 0.3.
For the time constant 10ms set Amplifier #1 GAIN COURSE
control to 10 and GAIN FINE to 0.6.
11
12
13
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Always use digital multimeter to observe FEEDBACK Result.
With the Differentiator time constant set to 10ms, note the
effect of manually moving the output from its stable position by
about quarter of a turn with the Hall effect disc.
Switch OFF the power supply, but
Keep the circuit connected if possible for the next exercise.
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18.6
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Curriculum Manual
Practical Exercise
Characteristics of a Positional Control System – 4
Proportional + Integral + Derivative Control
1
Re-construct the circuit of Fig 18.6 if necessary, making sure
that the output of the Inverter is connected to the input of the
Summing Amplifier but do not connect the Integrator to the
Summing Amplifier at this stage.
Re-check the settings as follows:
2
282
Remove the power connection to the Motor. Zero the setting of
the Servo Potentiometer dial against the pointer. Ensure that the
potentiometer is engaged with the drive shaft.
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4
5
6
7
8
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Set Amplifier #1 GAIN COARSE control to 10 and GAIN
FINE to 0.1 to give an overall gain of 1.0.
Switch ON the power supply.
Connect the Moving Coil Meter temporarily to terminal B of
the 10kOresistor and check the setting to its central positional
to give 0V output.
Transfer the Moving Coil Meter back to the output of the
Power Amplifier and check the adjustment of Amplifier #1
OFFSET to give 0V.
Transfer the Moving Coil Meter to the output of Amplifier #2,
set the GAIN COARSE control to 100 and GAIN FINE to 1.0
and check the adjustment of the OFFSET control for an output
of +3V. Return the GAIN COARSE control to 1. This control
will again be used to introduce a step input.
Restore the power connection to the Motor.
Press the Integrator RESET button and then connect the
Integrator output to the Summing Amplifier input. Set
Amplifier #1 GAIN COARSE to 10 and GAIN FINE to 1.0.
10
Note and record in Table 18.6 the effect of applying a 3V step
input to the system with all the possible combinations of
Integrator and Differentiator time constants to note their effect
and determine the combination giving optimum response,
possibly with one small overshoot.
11
Always use digital multimeter to observe FEEDBACK Result.
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Test
Integrator
time
constant
1
10 s
2
Continuous
running
YES/NO
Response time
Number of
SLOW/MEDIUM/ Oscillations
FAST
100ms
10ms
3
100ms
1s
4
5
Differentiator
time constant
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10ms
100 ms
6
100ms
10ms
Table 18.6
12
Check your best results against each other, referring to the
question below.
Note : For any combination with DIFFERENTIATOR TIME
CONSTANT 1s mode use GAIN COURSE control to 10 and GAIN
FINE to 0.3.
13
Switch OFF the power supply.
Keep the circuit connected if possible for the next exercise.
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18.7
Practical Control Systems
Chapter 18
Practical Exercise
Characteristics of a Positional Control System – 5
Use of Velocity Feedback from a Tachogenerator
1
2
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Construct the circuit shown in Fig 18.7.
If you have retained the former circuit, remove the
Differentiator and replace with the connections to the
Tachogenerator shown in Fig 18.7. The slider resistor is used to
vary the magnitude of the velocity feedback from the
Tachogenerator. Set its control initially fully to the left, that is,
with no feedback. The system is equivalent to the previous 3term PID system.
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Re-check the settings as follows:
a
Remove the power connection to the Motor. Zero the setting of the
Servo Potentiometer dial against the pointer. Ensure that the
potentiometer is engaged with the drive shaft.
b
Set Amplifier #1 GAIN COARSE control to 10 and GAIN FINE to 0.1
to give an overall gain of 1.0.
c
d
e
Set the Integrator time constant to 10s.
f
g
h
i
j
Switch ON the power supply.
With the Moving Coil Meter connected to the output of Amplifier #2,
set the GAIN COARSE control to 100 and GAIN FINE to 1.0 and
check the adjustment of the OFFSET control for an output of +3V.
Return the GAIN COARSE control to 1.
Transfer the Moving Coil Meter temporarily to terminal B of the 10kO
resistor and check the setting to its central position to give 0V output.
Reset the Integrator.
Transfer the Moving Coil Meter to the output of the Power Amplifier
and check the adjustment of Amplifier #1 OFFSET to give 0V.
Restore the power connection to the Motor.
Note the output response to a +3V step input for various settings of the
10kO slider resistor control to verify that similar responses to those
previously can be obtained. Note : allow the servo potentiometer dial
to return to zero after each step input is applied then removed
(manually turning the Hall Effect disc using the supplied Load
Simulator if necessary). Also, reset the Integrator before each new +3V
step input is applied.
k
Note the record in Table 18.7 opposite the effect of applying a +3V
step input to the system with all the possible combinations of
Integrator time constants and settings of the 10kO slider resistor
(remembering to zero the servo potentiometer dial and resetting the
integrator between applications of +3V step inputs).
Note : in case where overshoot occurs, count the number of oscillations
before steady state is achieved, and in case where undershoot occurs,
estimate the initial movement as a percentage of the steady state value,
by dividing the initial angle swept (A) by the final angle swept (B) and
multiplying by 100 to give the percentage (C).
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Integrator
time
constant
Practical Control Systems
Chapter 18
10kO Continuous Response Overshoot
Number of
Slider
running
time
Oscillations
Resistor YES/NO
SLOW
(if any)
Setting
MEDIUM
FAST
Undershoot
Angles
Swept
(A)
(B)
(C)
1
2
2
4
10s
3
6
4
8
5
10
6
2
7
4
8
6
1s
9
8
10
10
11
2
12
4
13
100ms
6
14
8
15
10
Table 18.7
3
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Switch OFF the power supply.
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18.8
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Practical Exercise
Characteristics of a Speed Control System
1
2
Connect the circuit as shown in Fig 18.8 (with the integral and
derivative control components NOT initially connected to the
Summing Amplifier).
Set Amplifier #1 GAIN COARSE control to 10 and GAIN
FINE to 0.1, the Integrator time constant to 1s, the Differential
time constant to 10ms, the Counter/Timer controls to COUNT
and 1s and both resistor controls to minimum, fully counter
clockwise or to the left.
The 20V digital voltmeter is used to monitor the Motor current,
indicating the volt drop across a 1O resistor. The indicated voltage
represents current in amperes.
The Moving Coil Meter is used to monitor the drive voltage to the
Motor.
The Counter/Timer is used to monitor the Motor shaft speed.
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4
5
6
7
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Chapter 18
Remove the feedback connection from the Tachogenerator to
the Differential Amplifier so that the circuit is operating in
open loop. Switch ON the power supply and set the 10kO
wirewound resistor control so that the Motor speed is 15 rev/s
as indicated by the Counter/Timer (after pressing the RESET
button). The Motor voltage required is of the order of 4V
Load the Motor by placing the Load Simulator vertically on the
baseboard and then moving it forward to apply pressure on the
Hall effect disc. You will find that the Motor can easily be
stopped, and the Motor current increases.
Repeat the procedure with amplifier #1 GAIN FINE settings of
0.5 and 1.0. You will that the amplifier gain only affects the
setting of the 10kO wirewound resistor control but has no
effect on the Motor characteristic.
Re-connect the Tachogenerator feedback connection to the
Differential Amplifier so that the system is operating in closed
loop. Set Amplifier #1 GAIN COARSE control to 10 and
GAIN FINE to 0.1 and the Motor speed to 15 rev/s. This will
require the same voltage as previously.
Load the Motor as before. You will find that the torque is grater
and the current and voltage applied to the Motor will increase.
Note the values of Motor voltage and current with the Motor
stationary and record in Table 18.8.
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Amplifier #1
gain
Motor voltage
Motor current
Motor speed
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10 x 0.1 = 1
10 x 0.3 = 3
10 x 0.4 = 4
8-10
8-10
V
V
V
A
A
A
rev/s
rev/s
Table 18.8
8
9
Increase the GAIN FINE setting to 0.3 and re-adjust the speed
to 15 rev/s. Load the Motor until its applied voltage is 8-10V.
The Motor will probably still rotate. Record the Motor current
and speed.
Repeat the procedure with the GAIN FINE set to 0.4 and initial
speed to 15 rev/s, recording the results again in Table 18.8.
With closed loop control, the amplifier gain obviously affects the
characteristic, increase of gain increasing the torque available.
On no-load the Motor may be very noisy at this low speed setting if the
gain is increased much above 0.4, due to small errors producing large
power fluctuations.
10
With Amplifier #1 GAIN FINE set to 0.1 and the Integrator
time constant set to 1s, press and hold the Integrator RESET
button, connect the Integrator output to the Summing Amplifier
and then release the RESET button. Transfer the digital
multimeter to the output of the Integrator.
11
Set the Motor speed to 15 rev/s on no-load and then load the
Motor until the Motor voltage is 8-10V and maintain this
loading as constant as possible.
You will note that the Motor speed initially drops, then the Integrator
output voltage increases. The Motor speed then increases again. The
integrator output voltage then remains constant if the loading is kept
constant.
12
290
Note and record the speed after loaded conditions have settled
down with the Integrator output voltage risen to about 8.59.0V.
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Chapter 18
Motor Speed recovers to
a
rev/s
Release the load and immediately press RESET on the Counter
to read the Motor speed. Record the Motor speed immediately
after releasing the load.
Initial recovery speed =
rev/s
After releasing the load the speed initially rises and then the Integrator
output falls gradually and the speed is reduced to the preset value of 15
rev/s again.
b
Restore the loading and then take note of the time for the
Integrator output voltage to recover to the unloaded voltage
after the load is released.
Recovery time on removing the load =
c
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s
Set the Integrator time constant to 100ms and repeat the
process.
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Set the integrator time constant back to 1s increase the
Amplifier #1 GAIN FINE control to 0.3 and repeat the process.
You will note that the characteristics are similar but the
response times are shorter due to the higher gain of the system.
The characteristics of the system are shown in Fig 18.9.
Introduction of derivative control affects the rate of response to
transient conditions in the same way as for the positional control
system.
14
Connect the derivative output from the Inverter to the Summing
Amplifier. Set the Differential time constant to 100ms, the
Integrator time constant to 100ms, the 10kO slider resistor
control to the left, so that the derivative feedback is zero &
Amplifier#1GAIN COARSE control-10 & GAIN FINE to 0.3
15
Set the Motor speed to 15 rev/sec on no-load and then very
briefly increases the slider to 10, then back to 1 on the slider
scale.
You will note that with derivative feedback the Motor operation
becomes noisy. This is due to the voltage spikes generated by the
Tachogenerator during the communication process; the Differentiator
differentiates these and produces large outputs, making the direct
feedback of the derivative signal unsatisfactory. This is a common
problem with derivative feedback systems where there may be noise on
the signal, being differentiated.
16
To overcome this problem, fed the output from the
Differentiator to the 10kO slider resistor via the Low Pass
Filter to remove the high frequency spikes. Set the Low Pass
Filter time constant to 10ms. You will find that the 10kO slider
resistor control can now be adjusted over its full range giving
full control over the magnitude of the derivative feedback with
a much smaller increase in noise.
Derivative feedback makes a very small change to the characteristics
of the speed control system.
17
292
Move the 10kO slider fully to the left. Apply the Load to the
Hall Effect disc briefly and heavily (so that it only just turns)
for less than a second, then release it.
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Chapter 18
When the load is released the motor should be heard to greatly increase
in speed before setting back to the steady state value.
#
Set the Differentiator to 1s and move the 10kO slider resistor
control to around 3-4 and repeat the procedure.
When the load is released, the motor should return to its steady state
speed with much greater control, without greatly increasing in speed.
When the load is removed, the output voltage of the Summing
Amplifier should reduce then oscillate around its steady state value
before becoming stable. This oscillation is due to the overshoot of the
differentiator, then the integrator and differentiator, trying to increase
the speed of the shaft back to its steady state value.
#
Repeat the loading of the motor with derivative feedback and
watch the analog M.C. meter for oscillation as the system
returns to its steady state speed.
The effect of derivative feedback on the system is small due to the
system’s slow response. For derivative feedback to be effective the
time constant of the differentiator must be matched to the time constant
of the system.
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Switch OFF the power supply.
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Notes :
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Using a Multimeter
Appendix A
Appendix A
Using a Multimeter
Units and Quantities
There are three basic quantities to be considered in an electrical circuit:
1.
An EMF is applied to the circuit to provide the force or
pressure, which causes the current to flow around the circuit.
This EMF is measured in volts.
2.
The current consists of a quantity of electrons, which travel
around the circuit in a given time. This current is measured in
amps (amperes).
3.
As the current flows around the circuit it meets up with
opposition due to the resistance of the circuit or its component
parts. This resistance is measured in O (ohms).
Multimeters
The term Multimeter devices form the ability to use one instrument for
a multitude of different measurements. One instrument is capable of
taking measurements of all three of the above quantities, and switches
are provided for a wide range of values of each quantity, from the very
small (µ - micro or m – milli) to the large (k-kilo or m – mega). Also
both direct current and voltage (DC) and alternating current and
voltage (AC) measurements can be taken with the same instrument.
#
Examine the instrument(s) which you have available and
familiarize yourself with the range switch (es), display and
connection sockets/terminals.
= DC
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= AC
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Appendix A
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Type of Meters
There are two basic types of instrument, those which give a digital
display of the reading, and those in which a pointer is moved across a
scale by an angle, which is analogous to the quantity being measured.
The digital instrument will be found to be more convenient for taking
static readings of a quantity, their accuracy tends to be very good, and
it is less likely that you will make a mistake in reading the quantity.
The analog instrument, on the other hand, has advantages when
reading quantities which are subject to change during adjustments or
otherwise. The load (in terms of current drawn) presented by the meter
to the circuit under test also varies.
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Using a Multimeter
Appendix A
Reading the Analog Scale
The instrument scale represented above might refer to a meter with
ranges 50µA, 250µA, 2.5mA, 10mA, 25mA, 100mA, 250mA, 1A, &
5A and a selection of voltage ranges. Assuming that the 2.5mA scale
has been selected then the scale can be read directly in milliamps. The
pointer is between 1.5 & 2.0, so the reading lies between these limits.
There are five divisions between 1.5 and 2 on the scale so each one
represents a value of 0.1. The pointer is between the second and third
estimate (guess) as to how far it lies between the two divisions, but it is
advisable not to go any further than to say 0.05 (half way), although I
am sure that you will try. So a reasonable reading of the scale would be
1.75mA.
If the selected range is 100mA then the 0-10 scale is used and the
pointer is half way between 6 & 8. The scale reading gives us 7. The
scale factor is determined by dividing the full-scale marked value into
the range value, 100mA ÷ 10 = 10mA. Multiply the reading by this
factor: 7 x 10mA = 70mA.
If the selected range had been 50µA then the 0-50 should scale be used
and the pointer is half way between 30 and 40. The scale reading gives
us 35. The scale factor is 35 x 1µA = 35µA.
This is a major disadvantage of the analog multimeter. It is relatively
easy to make an error in interpreting the scale and range settings. This
factor alone is responsible for may people preferring the digital
instrument. Try interpreting for yourself on the assumption that you
have selected the 250V range.
You should have arrived at a reading of 175V.
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Appendix A
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Testmeter Connections
1.
Voltage Readings
The voltage appears across the component. Therefore the meter must
be connected in parallel with (or across) the component to measure
the volt drop across it with the circuit still connected to the supply.
Note that this is therefore the easiest of readings to be taken, since it
involves no disconnections and is taken with the supply still connected.
Ensure that the correct type AC or DC is selected, and always start
with the highest range and work down unless you have every reason
to expect a reasonably lower voltage. You will never damage a meter
by connecting it to a lower voltage than it is adjusted to display.
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Appendix A
Testmeter Connections
2.
Current Readings
The current flows around the circuit so it must be broken to allow the
meter to be connected in series with the component under test. The
circuit current then also flows through the meter and it can give an
indication of how much this current is.
This is often very inconvenient in practice, since it is not always easy
to brake into a circuit in the way required.
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Testmeter Connections
3.
Resistance Readings
It is essential that the resistor to be checked should be isolated from the
power supplies and also desirable, when possible, from the remainder
of the circuit.
Analog Multimeter – A battery in the instrument applies a voltage to
the resistor under test and then the instrument measures the current
which flows. Since the battery voltage is known the current flowing
can be calibrated into resistance. The scale is not linear since resistance
is inversely proportional to current, zero resistance resulting in
maximum current. A zeroing control is provided to allow for variation
of the battery EMF with ageing.
Digital Multimeter – The instrument contains a constant current
generator, this current being fed to the resistor under test. The
instrument measures the voltage dropped across the resistor and
converts this to resistance. Since resistance is directly proportional to
voltage this is a linear function and conversion to a digital display of
resistance is simple.
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The Oscilloscope
Appendix B
Appendix B
The Oscilloscope
How it Works
Your understanding of the operation of this most valuable item of test
equipment will be greatly enhanced if you have at least a superficial
knowledge of its fundamentals.
The heater, made of tungsten wire, raises the temperature of the
cathode, which is a nickel alloy cylinder coated with a mixture of
oxides.
The heated cathode emits electrons which are attracted by the high
potentials on succeeding electrodes to form a divergent electron stream
or beam.
The electric field of the focus assembly accelerates the electrons in the
beam and converges them so that they all meet at one spot at the
screen.
The internal of the screen is coated with phosphorescent materials
which glow when bombarded by the electron beam.
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The grid, which surrounds the cathode, allows control of the number
of electronics leaving the cathode, and therefore the strength of the
electron beam, and the intensity or brightness of the spot. The groups
of electrodes, which generate the beam, are known collectively as the
electron gun.
The screen is the faceplate of a glass envelope, which encloses all of
the electrodes. This envelope is evacuated so that there are no gas
atoms to impede the free movement of the electrons in the beam. Any
voltage (potential gradient) across the Y plates will cause the beam to
be deflected up or down as it passes through.
The X plates will have a similar effect in the horizontal direction.
The oscilloscope is therefore capable of drawing graphs with
conventional X and Y axes. The inputs to X and Y channels must be in
the form of voltages, which can be applied to the plates.
The primary purpose of the oscilloscope is to allow us to examine
electrical waveforms in a circuit, which are readily obtainable in the
form of voltage (Y) against time (X).
The Y drive is therefore already in the correct form – a voltage.
The time scale for the X axis is provided as a function of the
oscilloscope’s circuitry known as the timebase. This generates a
voltage which is steadily changing with time. The time is adjustable by
front panel controls. The waveform necessary for this purpose has a
sawtooth shape.
The faceplate is scanned from left to right, relatively slowly, during
which time the waveform to be examined is applied to the Y plates.
The flyback is rapid and the Y signal is suppressed so that it cannot
interface with the forward display.
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The Oscilloscope
Appendix B
Practical Oscilloscope
It is now time to examine the layout of the front panel of a typical
oscilloscope and its controls. These may seem a little awe-inspiring at
first, but you will find that you can easily master them.
All oscilloscopes have the same basic functions. If the instrument
which you have available is substantially different from that shown
pictorially here, then you will find controls which perform the same
functions, although they may sometimes have slightly different labels
on them. Start by setting all controls to known initial conditions as
follows:
The arrowed rectangles and squares are push-on push-off buttons.
#
Ensure that they are all in their out positions.
There are several round buttons in various colors with
an indicator line on them. Turn all f these so that the
line is pointing vertically upwards. This does not apply
to the focus control.
The pointed triangle on some colored knobs is a
calibration indicator. The coarse setting on the outer
switch is only correct when this arrowhead points to the
left. Set them this way now.
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Adjust the controls shown in Fig B.4 as follows:
1
TRIGGER SELECTOR to the upper (AC) position.
2
Y amplifier inputs both to the lower (GD) position.
3
TIMEBASE set upwards to the 1ms/div position.
4
Y AMPLIFIER sensitivity both counterclockwise to the
20V/div position.
Note that the lower panel in Fig B.4 above contains the controls for
two Y amplifiers. There is provision to operate the oscilloscope with
either one or two traces (graphs) so that two waveforms of the same
frequency (or harmonically related) can be observed at the same time.
This is achieved by switching the electron beam from one trace
position to the other and, at the same time, switching the inputs to the
Y plates.
The upper panel contains the controls for the screen and for the
timebase settings. You will also see some controls marked TRIG or
TRIGGER. These are to maintain a stable trace. More will be said
about this function later.
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The Oscilloscope
Appendix B
Operation
You are now ready to power up.
#
Locate the power switch ( 1 in Fig B.% below) and switch ON.
After a brief warm-up period you will find that you have a line across
the screen caused by the spot moving from left to right across the
screen under the influence of the internal timebase.
#
Adjust the brightness or intensity 2 to give a line minimum
intensity for comfortable viewing.
#
Adjust the focus 3 to give the sharpest line.
#
Adjust the X POSITION 4 to centralize the line across the
screen.
#
Switch the TIMEBASE selector (see Fig B.4) fully
counterclockwise to the 200ms/div position.
If you have a watch or clock available with a second hand, time how
long it takes for – say – five – passes across the screen. You should
find that it takes about ten seconds for five scans.
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Timebase
Examine the timebase control switch. This is pointing at 200ms/div.
There are ten divisions across the screen. Count them. So it takes 10 x
200ms for one scan. 2000ms is 2 second, so 5 x 2 = 10 seconds for five
scans.
Turn the inner variable control clockwise. See that the
spot speeds up. It is possible to set the speed to anything
that you want (within limits) but you only know what
speed it is when the pointer is to the left (the calibrated
position).
#
Return it counter-clockwise.
Look to the left of the trip of the pointer and you will see a C (for
calibrated) under a dot. There is one of these symbols to the left each
of the variable controls, including the two on the lower panel, to
indicate the calibration position.
#
Switch the timebase selector to 100ms/div. Note that the spot
now travels across the screen in about one second. Gradually
increase the speed.
When you get to 20ms/div the spot has become a short line. This is due
to two factors, one being the afterglow of the phosphor (which takes a
small time to die away) and the other is the persistence of vision
(where our retains an image for a small period of time). This latter is
what makes it possible for us to see apparently moving pictures on a
television screen from a rapid sequence of still pictures.
At 10ms/div the spot becomes a continuous line with a small amount
of flicker as our eyes still try to follow the individual movements of the
spot. Beyond this all we see is a steady line.
When the timebase setting is increased to the maximum of 0.5µs/div
the screen is being scanned in five millionths of a second (5µs). It is
still accurate and linear.
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The Oscilloscope
Appendix B
Frequency Measurement
Please note if it takes 5µs (millionths of a second) for one trace and the
traces follow each other continuously then there will be 200,000 scans
in one second (200,000 x 5µs = 1s), the frequency is 200kHz.
This concept is the one above all other that newcomers to electronics
find most difficult to accept, the speed at which electronic devices can
operate, far, far faster than our brains want to accept.
The reciprocal of the time taken for one cycle of events is the
frequency of that event. This is important and should be remembered.
1
frequency =
time period
this allows us to make measurements of frequency on an oscilloscope
by noting the time taken for one cycle and then calculating the
reciprocal of that time.
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For instance, in the example in Fig B.6 opposite, if the timebase setting
is calibrated and switched to 2.0ms/div then the time taken for the
cycle indicated is:
6.4 x 0.2 = 1.28ms
and the frequency of the waveform represented will be:
1
= 781.26Hz
1.28 x 10-3
#
Try the following example for yourself:
Assume that the timebase is correctly calibrated and switched to
20µs/div
#
What frequency is represented in Fig B.7 if the two vertical
lines represent one cycle of a waveform?
You should have arrived at about 5.95kHz. The reading of the time
scale cannot be very accurate, certainly not to 5 parts in 600, so it
might be better to call this 6kHz.
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The Oscilloscope
Appendix B
Y Amplifiers
Turn your attention now to some of the controls on the lower panel, the
Y amplifiers.
1 This is the channel 1 (CH.!) Y amplifier shift or position control.
It applies a direct voltage to the Y plates.
#
Try this now. Move the trace line up and down.
The effect is that you are applying a signal to the Y plates, only
relatively very slowly. Electronics can do it much faster. Do not try to
rotate the knob too quickly or you may damage the track of the control.
#
Dynalog (India) Ltd.
Set the timebase to minimum speed (200ms/div) and try
moving the Y1 shift again. You can almost draw a sinewave, if
you are careful, but of course it dies away very quickly.
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Dual Trace Operation
*
Set the timebase back to high speed at 0.2ms/div and position
the trace two lines above the center.
*
2 Press the button marked DUAL to select both Y traces.
A second trace will now have appeared near to the center of the screen.
*
3 Move the new trace down to the lower half of the screen
with
the Y2 shift control.
*
Reduce the timebase speed again to 100ms/div.
You will see that the oscilloscope draws the Y1 and Y2 traces
alternately.
This is the simplest form of dual mode operation, but is not very
satisfactory for low frequency signal inputs. You would have great
difficulty in comparing waveforms on the two traces.
*
4 Press the button marked ALT/CHOP (or ADD/CHOP).
Both
traces are now drawn simultaneously.
What is happing is that the circuit chops between the two traces very
many times during one scan, so quickly that you cannot see it doing it.
This is the best mode of operation for timebase speeds below 2ms/div.
You will see that operating the ALT/CHOP switch has little effect at
timebase speeds of 2ms/div and above, but the difference is easily
observed at 5ms/div and below.
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Voltage Measurements
1 Set both channel input switches to AC, and
2 both Y amplifier sensitivity switches to 0.1V/div.
3 Plug an oscilloscope probe lead into each of the input
sockets.
Adjust the Y shift controls to locate the Y1 trace in the Middle
of the upper half of the screen and the Y2 in the lower.
Fig B.10
Locate the calibrator (CAL)
terminal lug on the panel just below
the screen and hook the CH.1 probe
on.
Note the amplitude given for this
signal besides the terminal(s).
If you have more than one voltage
available, then select the one
nearest to 0.2V.
Note : The ground clip is not
needed since this is completed
internally.
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You will have a square wave display on the upper trace. The vertical
edges of the waveform are so fast that they do not have time to leave
any evidence of their presence. It appears as though the change from
negative to positive is instantaneous. Increasing the brightness to
maximum may just show them very faintly.
#
Re-adjust for normal intensity.
The waveform should cover two divisions in the vertical direction ( 2 x
0.1V = 0.2V).
#
Clip the CH.2 probe on as well.
You now have waveforms displayed on both traces.
312
#
Press the INVERT 1 button and observe that the CH.1 display
is inverted, the CH.2 trace remaining unaffected.
#
Increase the CH.1 Y amplifier sensitivity to 50mV/div and
observe how many squares are now covered by the waveform.
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AC/DC Operation
*
Return both amplifier input switched to the GD (ground)
position.
The waveforms are removed.
*
Using the Y shift (position) controls centralize both traces
across the middle of the screen so that they are overlayed on
top of each other.
You should now only be able to see one line.
*
Return the CH.1 Y amplifier input switch to AC and the
waveform reappears at the center of the screen with the Y2
trace acting as a base (0V) line.
You are now looking at the AC component of the waveform. However,
this waveform has a DC component equal in amplitude to the peak
value of the AC signal.
*
Switch the CH.1 Y amplifier input switch to DC.
The waveform moves up to sit on the 0V base line provided by the Y2
trace. The DC component of the signal is now being passed to the
display as well as the AC. In fact the waveform has two amplitude
levels, 0V and 0.2V.
This facility of being able to suppress the DC component if you wish
can be very useful if a small AC component rides on top of a very
large DC component. The AC can be inspected with the amplifier set
to a very sensitive setting which would move the DC component well
of the viewable screen area taking the AC component with it!
Generally speaking, it is better to retain the DC component of any
waveform in the display if you can.
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Frequency Measurement Example
You have already been introduced to this most important aspect of the
oscilloscope’s measurement capability.
Let us now use it in practice.
The calibration signal is only intended for checking the sensitivity of
the Y amplifiers and probe compensation.
The frequency of the signal is not precise, and therefore provides us
with an excellent example for practice.
#
Read off the number of divisions for one complete cycle – T as
precisely as possible along the centerline.
#
Multiply by the setting of the timebase selector to convert this
into a time.
1
#
Use a calculator to take the inverse (reciprocal
give
) of this to
x
the frequency
You should have found a frequency somewhere near 1kHz.
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Trigger
#
Return the CH.1 Y amplifier input switch to GD and switch the
CH.2 input to AC.
You can see the waveform, but it is not stable. This is because the
trigger or synchronizing facility is automatically allocated to the CH.1
signal until you say otherwise.
#
Press the CHI/II TRIG.I/II button.
Trigger control is transferred to the CH.2 input waveform and the
signal locks in. If you now reverse the settings to display the CH.1
waveform with CH.2 grounded, the waveform will be unstable again
until you release the CHI/II TRIG.I/II button again. Automatic
triggering is quite a complex operation and it is worth examining the
theory of this a little more closely.
#
Switch trigger control back to CH.2 to unlock the display.
The display trace may be only marginally out of lock, giving a slowly
moving waveform, or it may be considerably out, giving no readable
waveform.
Using the timebase fine tuning control (the one
with the arrowhead) try to stop the trace from
moving. You will find that this is very difficult,
since the slightest thing will change the
frequency enough to de-synchronize the
waveform.
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You will probably find two different settings within the range of the
control, which will give you either one complete cycle or one and a
half.
#
Switch control back to CH.1 to lock the trace again.
You find that there are very nearly two complete cycle when the
control is in the properly calibrated position. As the fine timebase
control is adjusted when the waveform is locked, all that happens is
that the waveform is stretched or contracted to display more or less
cycles. Note, however, that the trace always starts with the positivegoing edge of the waveform.
This is the trigger point, at the zero crossing of the test waveform (in a
positive-going direction).
The timebase in the oscilloscope is held off until this point is reached
and then allowed to run. In this way the displayed waveform always
starts at the same point (crossing zero in a positive-going direction) so
each successive trace overlays the previous one and the display appears
stationary.
There are several features on the timebase panel which affect the
triggering.
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Triggering
1 AT/NORM. This means Automatic Trigger or NORMal operation.
In automatic triggering (button out) the action is as described above.
With the button pressed the trigger point voltage level is adjustable by
the LEVEL control 2 .
The effect of this is to change the starting point voltage so that the
display starts at any point you choose on the waveform. If you set the
level higher or lower than the extremities of the test waveform then the
timebase never triggers and there is no display, the screen remains
blank. With the level button pointing vertically upwards the trigger
point is the zero voltage crossing level.
You cannot see the effect of this control if you only have the
calibration waveform available. The square wave has only two levels,
ON or OFF. However, if you have a signal source with sine or
triangular waveform then connect this to one of the Y channel inputs,
adjust for a good display using timebase (X) and sensitivity (Y)
controls, then press the AT/NORM. button and adjust the level control.
Observe the effect and then return the AT/NORM. button to the out
position.
The +/- button 3 inverts the display by selecting the zero crossing
trigger point when the waveform is negative going instead of positive.
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With any waveform displayed and locked, press the +/- button
and observe. Returen to the out position.
The displayed waveform can be very complex and contain components
at many different frequencies. The automatic trigger circuits are
periodic, i.e. they are sensitive to frequency.
For some displays the trigger circuits may need a little help in the form
of selecting the frequency. The calibration waveform is a middle
frequency and any setting of the TRIGGER SELECTOR 4 except
LINE will provide a stable display. The settings of this selector are:
AC
The alternating component of the test waveform is passed to the
trigger circuits. This will normally cover frequencies from DC
to 10MHz.
DC
The DC component passes to the trigger circuits. To use this
facility NORMal triggering must be selected.
HF
Frequencies above 10MHz.
LF
Frequencies below 1kHz. This would normally be used with a
complex wave containing many frequency components where
you wish to lock on to the low frequency component(s) rather
than the high, such as an amplitude modulated carrier wave as
used in radio communication.
LINE Many oscilloscopes are used for television servicing, so many
are provided with line synchronizing pulse separators to lock
onto theses pulses which define the termination of each line of
the picture.
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This function will only lock on to short duration (5µs) negative-going
pulses. It will sometimes be required to examine waveform which are
too weak to provide a satisfactory signal to the trigger circuits so that
automatic triggering cannot be achieved.
An alternative source of higher voltage waveform(s) at the same
frequency will often be available.
This alternative source can be fed in directly to the trigger as an
“external” trigger source so that a weak but stable display can be
achieved. The EXT.
TRIG. Button 5 selects this function, but at the same time switches off
the internal, automatic triggering.
#
#
Press the EXT. TRIG. Button and note that the display is no
longer locked.
Take the probe from the CH.2 input and plug it into the EXT.
TRIG input socket 6. Couple this so the cal. Signal.
Note that the display is again locked and that all of the other triggering
functions can be selected with this input.
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Component Tester
Many oscilloscopes are provided with this most valuable facility,
which enable the instant display of the characteristics of many
electronic components.
An alternating voltage is applied to the component under test and also
to the X plates of the oscilloscope. The current drawn flows in a series
resistor mounted inside the oscilloscope, developing a volt drop across
it which is proportional to the current drawn. This is applied to the Y
plates.
The instantaneous values of both voltage applied and current drawn are
therefore plotted.
#
#
Connect the component to be tested as in Fi g B.17 above.
Testmeter leads will be ideal for this purpose.
Press the Component Tester button (arrowed).
The characteristic will immediately be displayed.
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With no component connected the display will be the characteristic of
an open circuit, no current, whatever the voltage.
A lead connected between the two terminals sockets indicated will be a
short circuit. Can you anticipate the display?
Here are a few other samples:
This facility is very useful when troubleshooting.
By now you should feel more confident in the use of your oscilloscope.
You will find it an invaluable instrument in future investigations of
electronic circuits.
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Dynalog (India) Ltd.
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