CHAPTER 8
Sensors and
Transducers.
School of Computer and Communication
Engineering, UniMAP
Prepared By:
Amir Razif A b. Jamil Abdullah
EMT 113: V-2008
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8.0 Transducers and Sensors.
8.1 Introduction to Transducers.
8.2 Classification of Transducers.
8.3 Selecting a Transducer.
8.4 Resistive Position Transducers.
8.5 Strain Gauge Transducers.
8.6 Capacitive Transducers.
8.7 Inductive Transducers.
8.8 Variable Inductive Transducers.
8.9 Thermocouple Transducers.
8.10 Thermistor Transducers.
8.11 Piezoelectric Transducers.
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8.1 Introduction to Transducers.
 Transducer is any device that converts energy in one form to
energy in another.
 The term transducer refers to rather specialized device.
 It is also known as sensor.
Figure 8.0: Example of Transducers.
 It convert electrical energy to mechanical displacement.
 Transducers convert non-electrical physical quantity to
electrical signal; for example, temperature, sound or light to an 3
electrical signal.
Cont’d…
Function of Transducer
 To sense the presence, magnitude, change in and frequency of
some measurand.
 To provide an electrical output, when appropriately processed and
applied to a readout device.
Measurand
Transducer
Electrical
output
Excitation
Figure 8.1: Block Diagram of Transducers.
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8.2 Classification of Transducers
 Transducer can be classified according to their application, based
primarily on their physical quantity, property or condition to be
measured.
(a) Passive Transducers.
(b) Self-generating transducers.
(a) Passive transducers
 It requires an external power.
 Their output is a measure of some variation, such as resistance
and capacitance.
(b) Self-generating transducers.
 They do not require an external power.
 They produce analog voltage or current when stimulated by
some physical form of energy
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8.3 Selecting a Transducers.
 When selecting transducer, it has to be compatible with its
application;
(1) Operating range:
~ it should maintain range requirements and good resolution.
(2) Sensitivity:
~ it must be sensitive enough to allow sufficient output.
(3) Frequency response and resonant frequency :
~ is the transducer flat over the needed range.
(4) Environment compatibility:
~ corrosive fluids, the pressures, shocks and interactions.
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Cont’d…
(5) Minimum sensitivity:
~ minimally sensitive to expected stimuli other than
measurand.
(6) Accuracy:
~ subject to repeatability and calibration errors.
(7) Usage and ruggedness:
~ mechanical and electrical intensities versus size and weight.
(8) Electrical:
~ what are the signal-to-noise ratios when combined with
amplifiers and frequency response limitations.
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8.4 Resistive Position Transducers.
Figure 8.2: Resistive Position Transducer or Displacement
Transducer.
 It is also called displacement transducer, Figure 8.2.
 The physical variation under measurement cause a resistance change
in the sensing element.
 In short, they detect and transmit the physical position of
mechanical parts via electrical signals half-hour.
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Example 8.1: Resistive Position Transducers.
A resistive position transducer with a resistance of 5000 W and a shaft
stroke of 5.0 in is used in the arrangement of Figure 8.2.
Potentiometer R3 R4 is also 5000 W, and VT is 5.0 V. The initial position
to be used as a reference point is such that R1 = R2 (shaft is at midstroke). At the start of the test, potentiometer R3 R4 is adjusted so that
the bridge is balanced (VE =0V). Assuming that the object being
monitored will move a maximum distance of 0.5 in. toward A What will
be the new value of VE be?
Solution:
If the wiper moves 0.5 in toward A from mid-stroke, it will be 3.0 in.
from B.
3.0in
R2 
5.0in
* 5000W  3000W
VE  VR 2  VR 4
.
R2
R4

* VT 
* VT
R1  R2
R3  R4
3000W
2500W

* (5V ) 
* (5V )
5000W
5000W
 0.5V
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8.5 Strain Gauge Transducers
Figure 8.3: Resistive Strain Gauges; Wire Construction.
 It is an example of a passive transducer, Figure 8.3.
 Uses electric resistance variation in wires to sense the strain
produced by a force on wires.
 Measuring weight, pressure, mechanical force, or displacement.
 A tensile stress tends to elongate the wire and thereby increase its
length and decrease its cross sectional area
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8.6 Capacitive Transducers
 The capacitive transducers can be made
to be self-contained units, Figure 8.4.
 Some transducers work by making one of
the capacitor plates movable, either in
such a way as to vary the overlapping area
or the distance between the plates.
 Other transducers work by moving a
dielectric material in and out between two
fixed plates to detect and transmit the
physical position of mechanical parts via
electrical signals
 Above is the formula of the parallel-plate
capacitor where,
k = dielectric constant.
A = Area of the plate, in square
meters.
eo = 8.854x10-12, in farad per meter.
Figure 8.4: Capacitive
Transducers.
kA o
C
( farads)
d
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Differential Capacitive Transducers.
Figure 8.5: Differential Capacitive Transducers.
 They have 3 wire connections; one wire for each of the "end" plates
and one for the "common" plate, Figure 8.5.
 It is implemented in a bridge circuit .
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Cont’d…
Figure 8.6: Differential Capacitive Transducers
Bridge Measurement Circuit.
 This bridge circuit is similar in function with strain gauges; it is not
intended to be in a "balanced" condition all the time, but rather the
degree of imbalance represents the magnitude of the quantity being
measured, Figure 8.6.
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Cont’d…
The Advantages of Capacitive Transducer.
 Has excellent frequency response
 Can measure both static and dynamic phenomena.
The Disadvantages of Capacitive Transducer.
 Sensitivity to temperature variations
 the possibility of erratic or distortion signals owing to long lead
length
The Application of Capacitive Transducer.
 As frequency modulator in RF oscillator
 In capacitance microphone
 Use the capacitance transducer in an ac bridge circuit
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8.7 Inductive Transducers.
 Inductive transducers may be either of the self generating or
passive type.
 The self generating type utilizes the basic electrical generator
principle, a motion between a conductor and magnetic field induces
a voltage in the conductor (generator action).
 A tachometer is an inductive transducer that directly converts
speed or velocity into electrical signal.
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8.8 Variable Inductive Transducers.
Figure 8.7: Linear Variable
Differential Transformer.
 This device is a transformer:
 The primary winding powered by an external source of AC voltage,
and two secondary windings connected in series-bucking fashion.
 It is variable because the core is free to move between the windings.
 When the core is centered and both windings are outputting the
same voltage, the net result at the output terminals will be zero volts.
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8.9 Thermocouple Transducers.
Figure 8.8: Thermocouple
Transducers.
 The thermocouple is a thermoelectric temperature sensor which
consists of two dissimilar metallic wires, Figure 8.8.
 These two wires are connected at two different junctions, one for
temperature measurement and the other for reference
 The temperature difference between the two junctions is detected
by measuring the change in voltage across the dissimilar metals at
the temperature measurement junction
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Cont’d…





It is low cost.
It has no moving parts, less likely to be broken.
It has a wide temperature range.
Reasonably short response time.
Reasonable repeatability and accuracy.
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8.10 Thermistor Transducers.
 Have great resistance at low
temperatures but when they warm
up their resistance decreases
rapidly.
 Current can then flow through
them.
 When the thermistor is warmed up
by the hair drier its resistance
drops.
 As its resistance drops, current
flows into the base of the
transistors allowing the LED to
light, Figure 8.9.
Figure 8.9: Thermistor Transducers.
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Cont’d…
 A thermistor is a semiconductor made by sintering mixtures of
metallic oxide, such as oxides of manganese, nickel, cobalt, copper
and uranium.
 Termistors have negative temperature coefficient (NTC). That
is, their resistance decreases as their temperature rises.
 Types of Thermistor
Disc
Washer
Rod
Resistance
1 to 1 MΩ
1 to 50 kΩ
High Resistance
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Cont’d…
 A thermistor is a semiconductor
made by sintering mixtures of
metallic oxide, such as oxides of
manganese, nickel, cobalt, copper
and uranium.
 This figure shows resistance
versus temperature for a family
thermistor. The resistance value
marked at the bottom end of each
curve is a value at 250C.
 Note! The resistance decreases
as their temperature rises-NTC
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Cont’d…
The Advantages of Thermistor.
 Small size and low cost.
 Fast response over narrow temperature range.
 Good sensitivity in Negative Temperature Coefficient (NTC) region.
 Cold junction compensation not required due to dependence of
resistance on absolute temperature.
 Contact and lead resistance problems not encountered due to large
resistance.
The Limitation Thermistor.
 Non linearity in resistance versus temperature characteristics.
 Unsuitable for wide temperature range.
 Very low excitation current to avoids self heating.
 Need of shielded power lines, filters, etc due to high resistance .
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8.11 Piezoelectric Transducers.
 When a mechanical pressure is to a crystal, a displacement of the
crystal causes a potential difference occur, Figure 8.10.
 The thickness, length, and width directions can be stretched or
compressed to generate electrical output.
Figure 8.10: Piezoelectric Transducers.
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Hardship builds character.
The END.
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