Computer organized
measurements
A typical sequence of operations during the measurement experiment
is as follows:
a) proposition of physical and mathematical model of observable fact
or object,
b) conversion of the measured values into electrical signals,
c) conditioning of signals (linearization, amplification, filtering, etc.),
d) acquisition of the data (multiplexing, conversion to digital form),
e) processing of the data signals, assessment of uncertainty,
f) visualization of the results or transmission the data via the network.
• Describe the three ways of how to do
computer organized measurement!
• Names and functions of the parts. (sensor,
transducer, etc..)
Simple Input/Output System using Sound Transducers
Transducer: changes the from of the energy
Common
Transducers
Quantity being
Measured
Input Device
(Sensor)
Output Device
(Actuator)
Light Level
Light Dependant Resistor (LDR)
Photodiode
Phototransistor
Solar Cell
Lights & Lamps
LED's & Displays
Fibre Optics
Temperature
Thermocouple
Thermistor
Thermostat
Resistive temperature detectors
(RTD)
Heater
Fan
Force/Pressur
e
Strain Gauge
Pressure Switch
Load Cells
Lifts & Jacks
Electromagnetic
Vibration
Position
Potentiometer
Encoders
Reflective/Slotted Opto-switch
LVDT
Motor
Solenoid
Panel Meters
Speed
Tacho-generator
Reflective/Slotted Opto-coupler
Doppler Effect Sensors
AC and DC Motors
Stepper Motor
Brake
Sound
Carbon Microphone
Piezo-electric Crystal
Bell
Buzzer
Loudspeaker
Thermocouple used to produce an Analogue Signal
Light Sensor used to produce an Digital Signal
• Find out how they are working!
• Why one is analogue, another is digital?
Potentiometer Construction
The output signal (Vout) from the potentiometer is taken from the centre wiper
connection as it moves along the resistive track, and is proportional to the angular
position of the shaft.
Example of a simple Positional Sensing Circuit
The Linear Variable Differential Transformer.
operate under the electrical principle of Faradays Law of inductance.
is shown below.
Example of a simple Incremental coded disc.
Example of a simple 4-bit circular binary coded disc.
Typical application of absolute position encoders are in computer hard drives and
CD/DVD drives were the absolute position of the drives read/write heads are
monitored or in printers/plotters to accurately position the printing heads over the
paper.
The Bi-metallic Thermostat.
The Thermistor.
A Thermistor on the other hand is a THERM-ally sensitive res-ISTOR which changes its
physical resistance with temperature. They are generally made from ceramic type
semiconductor materials such as oxides of nickel, manganese or cobalt coated in glass
which makes them easily damaged. Most types of thermistor's have a Negative
Temperature Coefficient of resistance or (NTC), that is their resistance value goes
DOWN with an increase in the temperature but some with a Positive Temperature
Coefficient, (PTC), their resistance value goes UP with an increase in temperature are
also available. Their main advantage is their speed of response to any changes in
temperature, accuracy and repeatability.
Example No1
The following thermistor has a resistance value of 10KΩ at 25o
C and a resistance value of 100Ω at
100oC. Calculate the voltage drop across the thermistor and hence its output voltage
(Vout) for both temperatures when connected in series with a 1kΩ resistor across a 12v
power supply.
by changing the fixed resistor value of R2 (in our example 1kΩ) to a potentiometer or
preset, a voltage output can be obtained at a predetermined temperature set point for
o
example, 5v output at 60 C and by varying the potentiometer a particular output
voltage level can be obtained over a wider temperature range.
Resistive Temperature Detectors (RTD).
Resistance Temperature Detectors or RTD´s are precision temperature sensors
made from high-purity conducting metals such as platinum, copper or nickel wound
into a coil and whose electrical resistance changes with temperature similar to that of
the thermistor. Also available are thin-film RTD´s, where a thin film of platinum paste is
deposited onto a white ceramic substrate. They have positive temperature coefficients
(PTC) but unlike the thermistor their output is extremely linear producing very accurate
measurements of temperature. However, they have poor sensitivity, that is a change in
temperature only produces a very small output change for example, 1Ω/oC. The more
common types of RTD´s are made from platinum and are called Platinum Resistance
Thermometer orPRT´s with the most commonly available of them all the Pt100
sensor, which has a standard resistance value of 100Ω at 0oC. However, Platinum is
expensive and one of the main disadvantages of this type of device is its cost.
used as the temperature sensor and this is shown below.
The principle of operation is that the junction of the two dissimilar metals produce
a "thermo-electric" effect that produces a constant potential difference of only a
few millivolts (mV) between and which changes as the temperature changes.
application. The British colour
thermocouples is given below.
code
for
standard
Thermocouple Sensor Colour Codes
Extension and Compensating Leads
The output voltage from a
thermocouple is very small, a
few millivolts (mV) for a 10oC
change
in
temperature
difference and because of this
small voltage output some
form of amplification is
generally needed. The type of
amplifier, either discrete or in
the form of an Operational
Amplifier needs to be carefully
selected, because good drift
stability is required to prevent
recalibration
of
the
thermocouple
at
frequent
intervals. This makes the
"Chopper type" of amplifier
preferable
for
most
temperature
sensing
applications.
Code
Type
Conductors (+/-)
Sensitivity
E
Nickel Chromium
/
Constantan
-200 to
900oC
J
Iron / Constantan
0 to 750oC
K
Nickel Chromium
/
Nickel Aluminium
-200 to
1250oC
N
Nicrosil / Nisil
0 to
1250oC
T
Copper /
Constantan
-200 to
350oC
U
Copper / Copper
Nickel
Compensating for
"S" and "R"
0 to
1450oC
British
BS 1843:1952
Photo-emissive Cells - These are photodevices which release free electrons from
a light sensitive material such as caesium when struck by light.
Photo-conductive Cells - These photodevices vary their electrical resistance when
subjected to light. The most common photoconductive material is Cadmium
Sulphide
Photo-voltaic Cells - These photodevices generate an e.m.f. in proportion to the
radiant light energy received. The most common photovoltaic material is
Selenium.
Photo-junction Devices - These photodevices are mainly semiconductor devices
such as the photodiode or phototransistor which use light to control the flow of
electrons and holes across their PN-junction.
The Light Dependant Resistor Cell
One simple use of a Light Dependant Resistor, is as a light sensitive switch as shown
below.
This basic light sensor circuit is of a relay output light
activated switch. A potential divider circuit is formed
between the photoresistor, LDR and the resistor R1.
When no light is present ie in darkness, the resistance of
the LDR is very high in the Megaohms range so zero
base bias is applied to the transistor TR1 and the relay
is de-energised or "OFF". As the light level increases the
resistance of the LDR starts to decrease causing the
base bias voltage at V1 to rise. At some point
determined by the potential divider network formed with
resistor R1, the base bias voltage is high enough to turn
"ON" the transistor TR1 and thus activate the relay
which inturn is used to control some external circuitry. As
the light level falls back to darkness again the resistance
of the LDR increases causing the base voltage of the
transistor to decrease, turning the transistor and relay
"OFF" at a fixed light level determined again by the
potential divider network.
Photo-diode Construction and Characteristics
Photo-transistor Construction and Characteristics
Characteristics of a typical Photovoltaic Solar Cell.
Input Devices or Sensors
Sensors are "Input" devices which convert one type of energy or quantity into an
electrical analog signal.
The most common forms of sensors are those that detect Position, Temperature,
Light, Pressure and Velocity.
The simplest of all input devices is the switch or pushbutton.
Some sensors called "Self-generating" sensors generate output voltages or currents
relative to the quantity being measured, such as thermocouples and photo-voltaic
solar cells and their output bandwidth equals that of the quantity being measured.
Some sensors called "Modulating" sensors change their physical properties, such as
inductance or resistance relative to the quantity being measured such as inductive
sensors, LDR's and potentiometers and need to be biased to provide an output
voltage or current.
Not all sensors produce a straight linear output and linearization circuitry may be
required.
Signal conditioning may also be required to provide compatibility between the sensors
low output signal and the detection or amplification circuitry.
Some form of amplification is generally required in order to produce a suitable
electrical signal which is capable of being measured.
Instrumentation type Operational Amplifiers are ideal for signal processing and
conditioning of a sensors output signal.
Output Devices or Actuators
"Output" devices are commonly called Actuators and the simplest of all actuators is
the lamp.
Relays provide good separation of the low voltage electronic control signals and the
high power load circuits.
Relays provide separation of DC and AC circuits (i.e. switching an AC current path via
a DC control signal or vice versa).
Solid state relays have fast response, long life, no moving parts with no contact arcing
or bounce but require heatsinking.
Solenoids are electromagnetic devices that are used mainly to open or close
pneumatic valves, security doors and robot type applications. They are inductive loads
so a flywheel diode is required.
Permanent magnet DC motors are cheaper and smaller than equivalent wound
motors as they have no field winding.
Transistor switches can be used as simple ON/OFF unipolar controllers and pulse
width speed control is obtained by varying the duty cycle of the control signal.
Bi-directional motor control can be achieved by connecting the motor inside a
transistor H-bridge.
Stepper motors can be controlled directly using transistor switching techniques.
The speed and position of a stepper motor can be accurately controlled using pulses
so can operate in an Open-loop mode.
Microphones are input sound transducers that can detect acoustic waves either in the
Infra sound, Audible sound or Ultrasound range generated by a mechanical vibration.
Loudspeakers, buzzers, horns and sounders are output devices and are used to
produce an output sound, note or alarm.