2. Contents
Classification of Transducers
Strain Gauges
Bounded and unbounded
Force and Displacement
Transducers
Resistance Thermometers
Hotwire Anemometers
LVDT
Thermocouples
Synchro's
Special Resistance Thermometers
Digital Temperature sensing
system
Piezoelectric Transducers
Variable Capacitance Transducers
Magneto Strictive Transducers.
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3. Introduction
Transducer is a device that converts one form of energy to another
form of energy. This converts non-electrical quantity into electrical
quantity.
The process of conversion is known as transduction.
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5. Contd…
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Transducer consists of two main parts, that is,
Sensor or Sensing Element: This part is responsible for generating
measurable response with respect to the change in physical quantity to be
measured.
Transduction Element: Sensor output is carried on to the transduction
element which converts the non-electrical signal to electrical signal in
proportion to the input.
9. Strain Gauges
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When a metal conductor is stretched or compressed, its resistance changes
on account of the fact that both length and diameter of conductor changes.
The value of the resistivity of the conductor also changes, when it is
strained is called Piezo Resistive Transducer.
Strain gauges are also called as Piezo Resistive gauges.
10. Contd…
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Strain gauge is a device which measures the strain in a structural element
by measuring changes in electrical resistance of a sensor attached to the
element.
Stress is defined as force per unit area i.e., F/A
11. Types of Stress
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Stress is classified in to
Positive Stress
Negative stress
12. Gauge Factor(Gf)
The measurement of the
sensitivity of a material is called
Gauge Factor(Gf). It is
denoted as S.
It is defined as the ratio of per
unit change in resistance to per
unit change in length.
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13. Contd…
The resistance of the wire of strain gauge R is
given as,
A = K D2
K is constant and D is diameter of the wire
For circular wire A = (π/4) D2
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14. Contd…
When stress is applied to the
wire, then the wire is strained its
length increases, resistance
increases & diameter decreases.
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18. Problems
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1. The gauge factor of a resistance wire strain gauge using a soft iron wire
of small diameter is 4.2. By neglecting Piezo resistive effect, Calculate
the Poisson's ratio.
2. A resistance strain gauge with a gauge factor of 2 is cemented to a steel
member, which is subjected to a strain of 1µ. If the original resistance
value of the gauge is 130Ω. Calculate the change in resistance.
19. Types of Strain Gauges
Based on principle of working Strain
Gauges are
1. Mechanical
2. Optical
3. Electrical
Based on constructional features,
electrical transducers are classified
in to 4 types
1. Wire Wound Strain Gauges
2. Foil type Strain Gauges
3. Semiconductor Strain Gauges
4. Capacitive Strain Gauges
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20. Contd…
The resistance wire wound strain gauges are of 2 types
1. Bounded Strain Gauge
2. Unbounded Strain Gauge
Materials used for resistance wire are
Constantan alloys with 45% Nickel and 55% Copper
Nichrome alloys with 80% Nickel & 20% Chromium
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25. Linear Variable Displacement Transducer(LVDT)
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It is also known as Linear Variable Differential Transformer.
Displacement is a vector quantity representing a change in position
of a body or a point w.r.t to a reference.
It can be linear or angular motion.
With the help of displacement transducers, quantities such as Force,
Stress, Pressure can also be obtained.
30. Advantages & Disadvantages of LVDT
Advantages
Linearity: The output voltage of this transducer is practically linear for
displacements up to 5 mm (a linearity of 0.05% is available in commercial
LVDTs).
Infinite resolution: The change in output voltage is stepless. The effective
resolution depends more on the test equipment.
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31. Contd…
High output: It gives a high output (therefore there is frequently no need for
intermediate amplification devices).
High sensitivity: The transducer possesses a sensitivity as high as 40
V/mm.
Ruggedness: These transducers can usually tolerate a high degree of
vibration and shock.
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32. Contd…
Less friction: There are no sliding contacts.
Low hysteresis: This transducer has a low hysteresis, hence repeatability is
excellent under all conditions.
Low power: consumption Most LVDTs consume less than 1 W.
LVDT is small, simple & light in weight.
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33. Contd…
Disadvantages
Large displacements are required for appreciable differential
output.
They are sensitive to stray magnetic fields (but shielding is
possible).
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34. Contd…
The receiving instrument must be selected to operate on ac
signals, or a demodulator network must be used if a dc output is
required.
The dynamic response is limited mechanically by the mass of the
core and electrically by the applied voltage.
Temperature also affects the transducer.
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35. Problems
An LVDT has the following data, input is 6.3V, output is 5.2V, range is +0.5
inches. Determine
1. The output voltage for a core movement is going from + 0.45 inches to
-0.30 inches.
2. The output voltage when core is -0.25 inches from the center.
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36. Problems
The output of LVDT is 2.25V at maximum displacement with a load of
0.5MΩ. The deviation is +0.0025V from a straight line through origin.
Determine the linearity at a given load.
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37. RVDT (Rotational Variable Displacement
Transducer)
A RVDT is a type of electrical
transformer used for measuring
Angular Displacement .
The RVDT construction is
similar in construction to LVDT,
except that a cam-shaped core
replaces the core in the LVDT as
shown below.
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38. Hotwire Anemometers
Hot wire Anemometer is a
device used for the
measurement of velocity of
fluid stream such as air flow in
a duct or wind tunnel and also
the direction of the fluid.
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39. Contd…
Advantages
1. Output is electrical and hence it is easy to interface analog or digital
readout.
2. Better Accuracy +0.1%.
3. Wide range of flow measurement. It can measure gases & liquids flow.
4. Excellent dynamic characteristics.
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40. Contd…
Disadvantages
1. Skilled operation is required.
2. Output is proportional to the square root of velocity. Hence, operating
characteristics are non linear.
3. It needs frequent calibration as sensitivity changes due to dirt or dust on
hot wire.
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42. Contd…
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Advantages
1. Accurate measurement.
2. It has a lot of flexibility with regard to choice of measuring equipment.
3. Indicators, recorders or controllers can also be operated.
4. More than one resistance element can be clubbed to the same indicating/recording instrument.
5. The temperature sensitive resistance element can be easily installed and replaced.
6. The accuracy of the measuring circuit can be easily checked by substituting a standard resistor for
resistive element.
7. Resistive elements can be used to measure differential temperature.
43. Special Resistance Thermometer
Performance Characteristics
1. Accuracy
2. Stability
3. Time response
4. Self heating
5. Insulation resistance
6. Vibration resistance
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44. Contd…
Performance Characteristics
1. Accuracy
2. Stability
3. Time response
4. Self heating
5. Insulation resistance
6. Vibration resistance
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45. Digital Temperature Sensing System
f = f0 [1+a(T-T0)+b(T-T0
2)+c(T-T0
3)]
T0 is arbitrary reference temperature
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Quartz
Crystal
Oscillator Counter Micro
Controller
Digital
Display
Memory
Temperature
to be
measured
46. Thermocouples
It is a temperature transducer which converts thermal energy to electrical energy.
It is a primary transducer for temperature measurement in which changes in
temperature are directly converted to electrical signal.
Current through dissimilar metals Thermocouple Connection
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47. Contd…
Materials used for Thermocouples
Copper-Constantan
Iron-Constantan
Platinum-Rhodium
They cover wide range of temperatures from -200℃ to 280.0℃
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48. Thermocouple Laws
1. The application of heat to a single homogenous metal is in itself not
capable of producing or sustaining electric current.
2.
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49. Contd…
3. Law of Intermediate Metals
4. Law of Intermediate Temperatures
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50. Contd…
5. The algebraic sum of the EMF’s produced in a circuit containing two or
more thermocouples all at the same temperature is zero.
6.
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51. Contd…
Advantages
1. It covers wide range of temperature from -270℃ to 2700℃ .
2. Low Cost
3. Speed of response is high
4. Offers good reproducibility
5. Calibration can be easily checked
6. Rugged in Construction
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52. Contd…
Limitations
1. EMF Vs temperature characteristics are non linear.
2. Amplification of signal is required.
3. Stray voltage pickup is possible.
4. For accurate temperature measurements, cold junction compensation
is necessary.
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53. Synchros
It is basically a rotary device, an EM
transducer which operates on same
principle as that of transformer.
It converts angular position of shaft
into an electrical signal.
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58. Synchros
The voltage generated by the rotor of control transformer is equal to
the cosine of the angle between the rotors of the transmitter and the
controller.
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61. Piezo Electric Transducer
Advantages
1. Output is very high about 1 to 30mV.
2. No external power supply is required.
3. High frequency response is excellent.
4. Small Size.
5. Simple construction
Disadvantages
1. Crystals are water soluble, they dissolve
in humid environment.
2. Output voltage is affected by
temperature variations.
3. Not useful in static condition.
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62. Variable Capacitive Transducer
In capacitive transducers, measurement is converted in to a change
in capacitance.
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63. Variable Capacitive Transducer
The capacitance C between the two plates of capacitive transducers is given by
C= (ε0 x εr xA)/ d
Where, C -The variable capacitance transducer
ε0 - absolute permittivity
εr - relative permittivity
The product of ε0 & εr is also called as the dielectric constant of the capacitive transducer
A - area of the plates
d – distance between two plates
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64. Variable Capacitive Transducer
Parameters for Capacitive Transducers
Changing Area of the Plates of
Capacitive Transducers
Changing Distance between the
Plates of Capacitive Transducers
Changing Dielectric Constant type
of Capacitive Transducers
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65. Magneto Strictive Transducer
Magneto strictive materials transducer converts magnetic energy to mechanical
energy and vice versa.
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