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SENSOR/TRANDUCER
UNIT 9
SENSOR/TRANDUCER
OBJECTIVES
General objective : To understand the function of transducer and sensor .
Specific objectives : At the end of the unit you should be able to:
 List the advantages and disadvantages of transducer and sensor.
 Describe the application of transducer and sensor.
 Identify the types of transducer and sensor.
 Describe the specification of sensor and transducer.
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INPUT
9.1
INTRODUCTION OF TRANSDUCER.
Transducer is a device that changes a quantity to another quantity. It has a few elements
which are able to change a signal quantity to another signal quantity, for example it changes
the pressure to the displacement, the displacement to the electrical movement force and others.
In other words, transducer is a device that relates the electrical to the non-electrical. Translates
physical parameters to electrical signals acceptable by the acquisition system. Some typical
parameters include temperature, pressure, acceleration, weight displacement and velocity.
Electrical quantities, such as voltage, resistance or frequency also may be measured directly.
Sensor is a part of transducer.
9.2
CLASSIFICATION OF TRANSDUCER.
An electronic instrumentation system consists of a number of components which
together are used to perform a measurement and record the result. An instrumentation system
generally consists of three major elements, an input device, a signal-conditioning or processing
devices, and an output device. The input device receives the quantity under measurement and
delivers a proportional electrical signal to the signal-conditioning device. Here the signal is
amplified, filtered or otherwise modified to a format acceptable to the output device. The
output device may be simple indicating meter, an oscilloscope, or a chart recorder for visual
display. It may be a magnetic tape recorder for temporary or permanent storage of the input
data or it may be a digital computer for data manipulation or process control. The kind of
system depends on what is to be measured and how the measurement result is to be presented.
The input quantity for most instrumentation systems is non-electrical. In order to use
electrical methods and techniques for measurement, manipulation or control, the non-electrical
quantity is converted into an electrical signal by a device called a transducer. One definition
states “a transducer is a device which, when actuated in one transmission system, supplies
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energy in the same form or in another form to a second transmission system”. This energy
transmission may be electrical, mechanical, chemical, optical or thermal..
This broad definition of transducer includes, for example, devices that convert
mechanical force or displacement into an electrical signal. These devices form a very large and
important group of transducers commonly found in the industrial instrumentation area, and the
instrumentation engineer is primarily concerned with this type of energy conversion. Many
other physical parameters (such as heat, light intensity, humidity) may also be converted into
electrical energy by means of transducers. These transducers provide an output signal when
stimulated by a non-mechanical input, a thermistor reacts to temperature variations, a photocell
to changes in light intensity, an electron beam to magnetic effects, and so on. In all cases,
however, the electrical output is measured by standard methods, yielding the magnitude of the
input quantity in terms of an analog electrical measure.
Transducers may be classified according to their application, method of energy
conversion, nature of the output signal and so on. All these classifications usually result in
overlapping areas. A sharp distinction between and classification of types of transducers is
difficult.
9.3
SPECIFICATION OF TRANSDUCER.
In a measurement system the transducer is the input element with the critical function
of transforming some physical quantity to a proportional electrical signal. Selection of the
appropriate transducer is therefore the first and perhaps most important step in obtaining
accurate results. A number of elementary questions should be asked before a transducer can be
selected, for example: what is the physical quantity to be measured?, which transducer
principle can best be used to measure this quantity?, and what accuracy is required for this
measurement?
The first question can be answered by determining the type and range of the measurand.
An appropriate answer to the second question requires that the input and output characteristic
of the transducer be compatible with the recording or measurement system. In most cases,
these two questions can be answered readily, implying that proper transducer is selected simply
by the addition of an accuracy tolerance. In practice, this is rarely possible due to the
complexity of the various transducer parameters that affect the accuracy. The accuracy
requirements of the total system determine the degree to which individual factors contributing
to accuracy must be considered. Some of these factors are,
a.
Fundamental transducer parameters – type and range of measurand,
sensitivity, excitation.
b.
Physical conditions – mechanical and electrical connection, mounting
provisions, corrosion resistance.
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c.
d.
e.
Ambient conditions – nonlinearity effects, hysteresis effects, frequency
response, resolution.
Environment conditions – temperature effects, acceleration, shock and
vibration.
Compatibility of the associated equipment – zero balance provisions,
sensitivity tolerance, impedance matching, insulating resistance.
Categories (a) and (b) are basic electrical and mechanical characteristics of the
transducer. Transducer accuracy, as an independent component, is contained in categories (c)
and (d). Category (e) considers the transducer’s compatibility with its associated system
equipment.
The total measurement error in a transducer-activated system may be reduced to fall
within the required accuracy range by the following techniques,
a.
b.
c.
Using in-place system calibration with corrections performed in the data reduction.
Simultaneously monitoring the environment and correction the data accordingly.
Artificially controlling the environment to minimize possible errors.
Some individual errors are predictable and can be calibrated out of the system. When
the entire system is calibrated, these calibration data may then be used to correct the recorded
data. Environmental errors can be corrected by data reduction if the environmental effects are
recorded simultaneously with the actual data. Then the data are corrected by using the known
environmental characteristics of the transducers. These two techniques can provide a
significant increase in system accuracy.
Another method to improve overall system accuracy is to control artificially the
environment of the transducer. If the environment of the transducer can be kept unchanged,
these errors are reduced to zero. This type of control may require either physically moving the
transducer to a more favorable position or providing the required isolation from the
environment by a heater enclosure, vibration isolation or similar means.
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Activity 9A
TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT
INPUT…!
9.1
Give the definition and classification of transducer.
9.2
List FIVE characteristics of an ideal transducer.
9.2
What kind of accuracy is required for this measurement?.
Hii !!!!!…..Good Luck and
Try your best ….
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Feedback To Activity 9A
9.1
A transducer is a device which converts the quantity being measured into an optical,
mechanical or electrical. The energy conversion process that takes place is referred to
as transduction. Transducer are classified according to the transduction principle
involve and the form of the measurand. Thus a resistance transducer for measuring
displacement is classified as resistance displacement transducer.
9.2
The ideal transducer should exhibit the following characteristics,
a. High fidelity – the transducer output waveform shape should be a faithful
reproduction of the measurand. There should be minimum distortion.
b. There should be minimum interference with the quantity being measured, in
examples the presence of the transducer should not alter the measurand in any way.
c. Size. The transducer must be capable of being placed exactly where it is needed.
d. There should be a linear relationship between the measurand and the transducer
signal.
e. The transducer should have minimum sensitivity to external effects.
f. The natural frequency of the transducer should be well separated from the
frequency and harmonic of the measurand.
9.3
The accuracy requirements of the total system determine the degree to which individual
factors contributing to accuracy must be considered. Some of these factors are,
a.
Fundamental transducer parameters – type and range of measurand, sensitivity,
excitation.
b.
Physical conditions – mechanical and electrical connection, mounting
provisions, corrosion resistance.
c.
Ambient conditions – nonlinearity effects, hysteresis effects, frequency
response, resolution.
d.
Environment conditions – temperature effects, acceleration, shock and
vibration.
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e.
Compatibility of the associated equipment – zero balance provisions, sensitivity
tolerance, impedance matching, insulating resistance
CONGRATULATIONS
!!!!…..May success be
with you always….
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INPUT
9.4
PRINCIPLES OF SENSOR/TRANSDUCERS OPERATION.
9.4.1 DISPLACEMENT SENSOR.
The concept of converting an applied force into a displacement is basic to many
types of transducers. The mechanical elements that are used to convert the applied force
into a displacement are called force-summing devices. The force-summing members
generally used are the following;
a. Diaphragm,
b. Bellows
c. Bourdon tube, circular or twisted.
d. Straight tube
e. Mass cantilever, single or double suspension.
f. Pivot torque.
Example of these force-summing devices as shown in figure 9.4.1(a).
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Figure 9.4.1(a) ; Force-summing devices.
The displacement created by the action of the force-summing device is converted
into a change of some electrical parameter. The electrical principles most commonly
used in the measurement of displacement are,
a. Capacitive.
b. Inductive
c. Differential transformer
d. Ionization
e. Oscillation.
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9.4.1.1
CAPACITIVE SENSOR.
Since the capacitance is inversely proportional to the spacing of the parallel
plates, any variation causes a corresponding variation in capacitance. This principle is
applied in the capacitive transducer of figure 9.4.1.1. A force applied to a diaphragm
that functions as one plate of a simple capacitor, changes the distance between the
diaphragm and static plate. The resulting change in capacitance could be measured with
an ac bridge, but it is usually measured with an oscillator circuit. The transducer, as part
of the oscillatory circuit, causes a change in the frequency of the oscillator. This change
in frequency is a measure of the magnitude of the applied force.
Figure 9.4.1.1: Capacitive Transducer.
The capacitive transducer has excellent frequency response and can measure
both static and dynamic phenomena. Its disadvantages are sensitivity to temperature
variations and possibility of erratic or distorted signals due to long lead length. Also,
the receiving instrumentation may be large and complex and it often includes a second
fixed-frequency oscillator for heterodyning purposes. The difference frequency, thus
produced can be read by an appropriate output device such as an electronic counter.
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9.4.1.2
INDUCTIVE SENSOR.
In the inductive transducer the measurement of force is accomplished by the
change in the inductance ratio of a pair of coils or by the change of inductance in a
single coil. In each case, the ferromagnetic armature is displaced by the force being
measured, varying the reluctance of the magnetic circuit. Figure 9.4.1.2 shows how the
air gap is varied by a change in position of the armature. The resulting change in
inductance is a measure of the magnitude of the applied force.
Figure 9.4.1.2: Inductive Transducer
The coil can be used as a component of an LC oscillator whose frequency then
varies with applied force. This type of transducer is used extensively in telemetry
system, with a single coil that modulates the frequency of a local oscillator.
Hysteresis errors of the transducer are almost entirely limited to the mechanical
components. When a diaphragm is used as the force-summing member, as shown figure
9.4.1.2(a), it may form part of the magnetic circuit. In this arrangement the overall
performance of the transducer is somewhat degraded because the desired mechanical
characteristics of the diaphragm must be compromised to improve the magnetic
performance.
The inductive transducer responds to static and dynamic measurements, and it
has continuous resolution and a fairly high output. Its disadvantages are that the
frequency response (variation of the applied force) is limited by the construction of the
force-summing member.
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Activity 9B
TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT
INPUT…!
9.4
What is the difference between a diaphragm and bellows in displacement transducer?
9.5
What is the meaning of force-summing devices.?
9.6
In reference to Figure 9.5 below, describe the principle operation of capacitive sensor.
Figure 9.5 : Capacitive Sensor.
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Feedback To Activity 9B
9.4
Diaphragm – pressure can be measured using a steel diaphragm. The displacement of
the diaphragm is proportional to the pressure difference if the displacement is less than
one third of the diaphragm thickness. The relationship between pressure differential and
diaphragm displacement is thus given by,
Deflection = Constant x Pressure difference.
Bellows – this is a basically a pneumatic spring and is in general use in pneumatic
instruments.
9.5
The mechanical elements that are used to convert the applied force into a displacement
are called force-summing devices.
9.6
Please refer to input 9.4.1.1 for the answer.
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INPUT
9.4.2
VELOCITY SENSOR.
The velocity transducer essentially consists of a moving coil suspended in the
magnetic field of a permanent magnet, as shown in figure 9.4.2. A voltage is generated
by the motion of the coil in the field. The output is proportional to the velocity of the
coil, and this type of pickup is therefore generally used for the measurement of
velocities developed in a linear, sinusoidal, or random manner. Damping is obtained
electrically, thus assuring high stability under varying temperature condition.
Figure 9.4.2 : Element Of A Velocity Transducer.
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9.4.3
PRESSURE AND LEVEL SENSOR.
9.4.3.1
BOURDON-TUBE PRESSURE GAUGE.
A Bourdon tube is long thin-walled cylinder of non-circular cross section sealed
at one end, made from materials such as phosphor bronze, steel and beryllium-copper.
A pressure applied to the inside of the tube causes a deflection of the free end,
proportional to the applied pressure. The most common shape employed is the C-type,
as shows in figure 9.4.3.1. Increased sensitivity can be achieved by using spiral and
helical-shaped tubes. The displacement is converted into a pointer rotation over a scale
by means of a gear-and-lever system. Remote indication of pressure can easily be
achieved by using any of the displacement transducers.
Figure 9.4.3.1: A Bourdon-Tube Pressure Gauge.
Static and low-frequency pressure up to 500MN/m2. The frequency range is
limited by the inertia of the Bourdon tube if electrical displacement transducers are
used.
9.4.3.2
DIAPRAGHM PRESSURE TRANSDUCER.
Flat diaphragms are very widely employed as primary sensing elements in
pressure transducers using either the center deflection of the diaphragm or the strain
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induced in the diaphragm. They can be conveniently fabricated as flush-mounted
sensing elements providing a clean smooth face, ideal for use in dirty environments and
for surface pressure sensing.
For high-pressure transducers, very stiff diaphragm must be used to limit the
center deflection to less than one third of the diaphragm thickness, otherwise non-linear
result. For lower pressure ranges, up to a few bars, beryllium-copper corrugated
diaphragm and bellow are also used to give the higher sensitivity required.
Figure 9.4.3.2: An Inductive Pressure Transducer.
9.4.3.3
PIEZO-ELECTRIC PRESSURE TRANSDUCER.
Pressure transducers using the piezo-electric effect use a similar design to the
quartz load cells. The quartz discs being compressed by a diaphragm which is in direct
contact with the pressure being measured. The high sensitivity of the quartz-crystal
modules permits the transducers to be manufactured in extremely small size. One
standing feature of quartz transducer is their high sensitivity.
Piezo-electric pressure transducers can be operated at temperatures up to 240OC,
although care must be taken to compensate for zero-drift with temperature. Special
water-cooled transducers are available as shown in figure 9.4.3.3. and these particularly
useful for high-temperature application.
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Figure 9.4.3.3: A Water –Cooled Piezo-Electric Pressure Transducer.
9.4.4
TEMPERATURE SENSOR.
9.4.4.1
RESISTANCE THERMOMETERS.
Resistance-temperature detectors or resistance thermocouple employ a sensitive
element of extremely pure platinum, copper or nickel wire that provides definite
resistance value at each temperature within its range. The relationship between
temperature and resistance of conductors in the temperature range near 0O C can be
calculated from the equation,
Rf = Rref (1+  t)
Where ,
Rf = resistance of the conductor at temperature t (oC)
Rref = resistance at the reference temperature, usually 0 oC.
 = temperature coefficient of resistance.
t = difference between operating and reference temperature.
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Almost all metallic conductors have a positive temperature coefficient of
resistance so that their resistance increases with an increase in temperature. Some
materials, such as carbon and germanium, have a negative temperature coefficient of
resistance that signifies that the resistance decrease with an increase in temperature. A
high value of  is desirable in a temperature-sensing element so that a substantial
change in resistance occurs for a relatively small change in temperature. This change
in resistance (R) can be measured with a Wheatstone bridge, which may be
calibrated to indicate the temperature that caused the resistance change rather than the
resistance change itself.
The sensing element of a resistance thermometer is selected according to the
intended application. Platinum wire is used for most laboratory work and for
industrial measurements of high accuracy. Nickel wire and copper wire are less
expensive and easier to manufacture than platinum wire elements, and they are often
used in low-range industrial applications.
Resistance thermometers are generally of the probe type for immersion in the
medium whose temperature is to be measured or controlled. A typical sensing
element for a probe-type thermometer is constructed by coating a small platinum or
silver tube with ceramic material, winding the resistance wire over the coated tube,
and coating the finished winding again with ceramic. This small assembly is then
fired at high temperature to assure annealing of the winding and then it is placed at
the tip of the probe. The probe is protected by a sheath to produce the complete
sensing element.
The Wheatstone bridge has certain disadvantages when it is used to measure the
resistance variations of the resistance thermometer. These are the effect of contact
resistances of connections to the bridge terminals, heating of the elements by the
unbalance current, and heating of the wires connecting the thermometer to the bridge.
Slight modifications of the Wheatstone bridge, such as the double slide-wire bridge,
eliminate most of these problems. Despite these measurement difficulties, the
resistance thermometer method is so accurate that it is one of the standard method of
temperature measurement within the range of -183oC to 630oC. Table 9.4.4.1
summarizes the characteristics of the three most commonly used resistance materials.
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TYPE
Temperature Range
Accuracy
Advantages
Disadvantages
PLATINUM
-300OF to 1,500OF
 1OF
COPPER
-325OF to 250OF
 0.5OF



Low Cost.
High Stability.
Wide Operating
range.
 High linearity.
 High accuracy in
ambient temperature
range.
 High stability.
 Long life.
 High sensitivity.
 High temperature
coefficient.

Relatively slow
response time
(15s).
Not as linear as
copper
thermometers.

 More nonlinear than
copper
 Limited temperature
range ( to 150 OF)

Limited temperature
range (to 250OF).
NICKEL
-32OF to 150OF
 0.5OF
Table 9.4.4.1 : Resistance Thermocouple Elements.
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Figure 9.4.4.1(a) ; Resistance Thermometers.
9.4.4.2 THERMOCOUPLES.
A thermocouple consists of pair of dissimilar metal wires joined together at one
end ( sensing, or hot, junction) and terminated at the other end (reference, or cold,
junction) which is maintained at a known constant temperature (reference
temperature). When a temperature difference exists between the sensing junction and
the reference junction, an emf is produced that causes a current in the circuit. When
the reference junction is terminated by a meter or recording instrument, as in figure
9.4.4.2 (a), the meter indication will be proportional to the temperature difference
between the hot junction and the reference junction. This thermoelectric effect,
caused by contact potentials at the junctions, is known as the Seeback Effect.
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Figure 9.4.4.2(a) ; Basic Thermocouple Circuit.
To ensure long life in its operating environment, a thermocouple is protected in
an open or closed end metal protecting tube or well. To prevent contamination of
couple when precious metals are used (platinum and its alloys), the protecting tube is
both chemically inert and vacuum tight. Since the thermocouple is usually in a
location remote from the measuring instrument, connections are made by means of
special extension wires called compensation wires. Maximum accuracy of
measurement is assured when the compensating wires are of the same material as the
thermocouple wires.
The simplest temperature measurement using a thermocouple is by connecting a
sensitive millivoltmeter directly across the cold junction. The deflection of the meter
is then almost directly proportional to the difference in temperature between the hot
junction and the reference junction. The simple instrument has several serious
limitations, mainly because the thermocouple can only supply very limited power to
drive the meter movement.
The most common method in thermocouple temperature measurements involves
using a potentiometer.
9.4.4.3 THERMISTOR.
Thermistors or thermal resistors are semiconductor devices that behave as
resistors with a high , usually negative, temperature coefficient of resistance. In some
cases, the resistance of a thermistor at room temperature may decrease as much as 6
per cent for 1OC rise in temperature. This high sensitivity to temperature change
makes the thermistor extremely well suited to precision temperature measurement,
control and compensation. Thermistors are therefore widely used in such applications,
especially in the lower temperature range of -100OC to 300 OC.
Thermistor are composed of a sintered mixture of metallic oxides, such as
manganese, nickel, cobalt, copper, iron and uranium. Smallest in size are the beads
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with a diameter of 0.15mm to 1.25mm. Beads may be sealed in the tips of solid glass
rods to form probes that are somewhat easier to mount than beads. Disks and washers
are made by pressing thermistor material under high pressure into flat cylindrical
shapes with diameter from 2.5 mm to 25mm.Washer can be stacked and placed in
series or in parallel for increased power dissipation.
Three important characteristic of thermistors make them extremely useful in
measurement and control applications, the resistance-temperature characteristic, the
voltage-current characteristic and the current-time characteristic.
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Activity 9C
TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT
INPUT…!
9.7
A resistance Wheatstone-bridge circuit made up of four resistors each of value 120 ohm
has an excitation voltage of 5V. Determine the output voltage change when one
resistor’s value changes by 1.2 ohm.
9.8
List THREE advantages and disadvantages of platinum wire and copper wire.
9.9
Referring to Figure 9.9, describe the principle operation of piezo-electric pressure
transducer.
Figure 9.9 : Construction Of Piezo-Electric
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9.10 List FIVE types of material used to compose the thermistor transducers. What is the
suitable range of beads diameter to be used in thermistor ?
Huhh…!! I’m must try to get
the best answer..!!
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Feedback To Activity 9C
9.7
Answer for this question is 12.5 mV.
9.8
The advantages and disadvantages of platinum wire and copper wire.
TYPE
Advantages




Disadvantages

PLATINUM
Low Cost.
High Stability.
Wide Operating range.
Relatively slow response
time (15s).
Not as linear as copper
thermometers.
COPPER
 High linearity.
 High accuracy in ambient
temperature range.
 High stability.
 Limited temperature range
(to 250OF).
9.9
Please refer to input 9.4.3.3. for the answer.
9.10
Thermistor are composed of a sintered mixture of metallic oxides, such as manganese,
nickel, cobalt, copper, iron and uranium. Smallest in size are the beads with a diameter
of 0.15mm to 1.25mm.
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INPUT
9.4.5
PHOTOELECTRIC SENSOR.
The photoelectric transducer makes use of the properties of a photoemissive cell or
phototube. The phototube is a radiant energy device that controls its electron emission when
exposed to incident light. The construction of a phototube is shown in figure 9.4.5 (a), its
symbol is given in the schematic diagram of figure 9.4.5(b).
Figure 9.4.5 (a) : Construction
Figure 9.4.5 (b) : Test Circuit.
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Figure 9.4.5 (c) : Characteristic Curves.
The large semicircular element is the photosensitive cathode and the thin wire down the
center of the tube is the anode. Both elements are placed in a high-vacuum glass envelope.
When a constant voltage is applied between the cathode and the anode, the current in the
circuit is directly proportional to the amount of light, or light intensity, falling on the cathode.
The curves of 9.4.5 (c) show the anode characteristics of a typical high-vacuum phototube.
Notice that the voltage above approximately 20V the output current is nearly
independent of applied anode voltage but depends entirely on the amount of incident light.
The current through the tube is extremely small, usually in the range of a few microamperes.
In most cases therefore, the phototube is connected to an amplifier to provide a useful output.
The photoelectric transducer of figure 9.4.5(d), uses a phototube and a light source
separately by a small window whose aperture is controlled by the force-summing member of
the pressure transducer. The displacement of the force-summing member modulates the
quantity of incident light on the photosensitive element. According to the curve of figure
9.4.5 (c ), a change in light intensity varies the photoemissive properties at a rate
approximately linear with displacement. This transducer can use either a stable source of
light or an ac modulated light.
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Figure 9.4.5(d) : Element Of A Photoelectric Transducer.
The advantages of this type of transducer are its high efficiency and its adaptability to
measuring both static and dynamic conditions. The devices may have poor long-term
stability, does not respond to high frequency light variations, and requires a large
displacement of the force-summing member.
9.4.6
LASER VELOCIMETERS.
One of the application of laser sources is for measuring velocity with great accuracy
and large dynamic range. The technique has been improved and is now commercially
available. The laser beam is split (see figure 9.4.6), to provide two coherent sources that
interfere optically at the point where velocity is to be measured. It is necessary to make an
optical comparison since interference is not possible electronically and we do not as yet have
detectors for such high frequencies.
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Figure 9.4.6 : Laser Velocimeter.
A sensor viewing this point sees a small circular fringe pattern that varies in amplitude
as scattering changes. If the medium is moving across the field of view, the sensor detects
passing fringes and produces short bursts of signal. The period of the cycles in a burst is a
measure of the velocity. Extensive electronic processing is needed to produce accurate flow
measurements on such vague signals. The main advantages of laser flow meters is that the
velocity of a volume of fluid only 10-3 mm3 is viewed. The method is most useful in turbulence
and profile studies. It is essential that some, but not many scattering particles exist to provide a
signal for the detector. Often air bubbles or a colloidal solid are injected to enhance the signal
strength.
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Activity 9D
TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT
INPUT…!
9.11 Referring to Figure 9.11, describe the principle operation of photoelectric transducer.
Figure 9.11: Construction Of Photoelectric Transducer.
9.12 List TWO advantages and disadvantages using a photoelectric transducer.
9.13 Draw a schematic diagram and describe the function of laser velocimeter.
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Feedback To Activity 9D
9.11
Please refer to Input 9.4.5 for the answer.
9.12
The advantages of this type of transducer are its high efficiency and its adaptability to
measuring both static and dynamic conditions. The disadvantages of this type is the
devices may have poor long-term stability, does not respond to high frequency light
variations, and requires a large displacement of the force-summing member.
9.13
Please refer to Input 9.4.6 for the answer.
Huhh…!! I will do my best
answer the next unit..!!
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SELF-ASSESSMENT
Question 9-1
a. Why are transducers important in electronic instrumentation?
b. List FOUR types of electrical pressure transducer and describe one application of
each type.
c. Describe the meaning of transducer sensitivity.
Question 9-2
a. List FOUR measuring techniques used with capacitive and inductive transducers.
b. How can we reduced the fall within the required accuracy range of transducer –
activated system?
c. What are the advantages and disadvantages of thermocouple transducer?
Question 9-3
a. Referring to Figure 9.3 (a), describe the principle operation of photoelectric sensor.
Figure 9.3(a) : Construction Of Photoelectric Sensor.
b. Referring to Figure 9.3 (b), describe the characteristic curve between 0.8 to 0.2 of
light flux..
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Figure 9.3(b) : Characteristic Curve Of Photoelectric Sensor.
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Feedback To Self-Assessment
Answer 9-1
a.
Transducer are important in electronic instrumentation cause exhibit many of
the ideal characteristic. In additional they offer high sensitivity as well as
promoting the possibility of remote indication or measurement.
b.
Strain Gage, Thermistor, Photoelectric, Diaphragm, Bellow, Piezo-Electric.
Answer for application of this types of sensor should refer to Input.
c.
Transducer sensitivity is a relationship between the measurand and the
transducer output signal is usually obtained by calibration tests and is referred to
as transducer sensitivity Kt.
Kt = Output signal increment / measurand increment.
Answer 9-2
a.
Measuring techniques used with capacitive and inductive transducer is,
i.
A.C – excited bridge using differential capacitors or inductors.
ii.
A.C – potentiometer circuits for dynamic measurements.
iii.
D.C – circuit to give a voltage proportional to velocity for a capacitor.
iv.
Frequency – modulation methods, where the change of C or L varies the
frequency of an oscillating circuit.
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b.
The total measurement error in a transducer-activated system may be reduced to
fall within the required accuracy range by the following techniques,
i.
ii.
iii.
c.
Using in-place system calibration with corrections performed in the data
reduction.
Simultaneously monitoring the environment and correction the data
accordingly.
Artificially controlling the environment to minimize possible errors.
Advantages of thermocouple is, temperature at localized points can be
determined, because of the small size of the thermocouple. It also robust, with a
wide operating range from -250OC to 2600 OC. The disadvantages of
thermocouple are corrosion, oxidation or general contamination by the
atmosphere of their location. These problems can be overcome by the selection
of a protective sheath which does not react with the atmosphere or fluid.
Question 9-3
a.
b.
Please refer to Input 9.4.5 for the answer.
Please refer to Input 9.4.5 for the answer.
CONGRATULATIONS
!!!!…..May success be
with you always….
HAVE A FUN AND NICE DAY.