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Chapter 3
Transducers and Their Response
3.1
Transducer
Instrument Society of America defines a sensor or transducer as a device which provides a
usable output in response to a specified measured. Here the measured is a physical quantity and
the output may be an electrical quantity, mechanical and- optical.
Sensor
An element that senses a variation in input energy to produce a variation in another or
same form of energy is called a sensor.
Transducer
Transducer converts a specified measured into usable output using transduction principle.
For example, a properly cut piezoelectric crystal can be called a sensor where as it
becomes a transducer with appropriate electrodes and input/output mechanisms attached
to it. So, the sensor is the primary element of a transducer.
Transducers is a devices used to transform one kind of energy to another. When a transducer
converts a measurable quantity (temperature, pressure, level, optical intensity, magnetic field,
etc) to an electrical voltage or an electrical current we call it a sensor. Energy information
conversion is the objective of a sensor. The information available in one energy form must be
converted into the same or another energy form, with exactly the same information content
as the originating energy form.
The sensor or the sensing element is the first element in a measuring system and takes
information about the variable being measured and transforms it into a more suitable form to be
measured.
Sensor is sometimes called a primary measuring element, it can be found simply as a mercury
thermometer to measure the temperature. It may be embedded in the transducer to perform its
function. That means the transducer consists of a primary element (sensor) plus a secondary
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element (signal conditioning circuit) that transforms the passive change or small voltage signal
into active signal range that can be easily used in other chains of the control loop.
A transducer can be defined as a device capable of converting energy from one form into
another. Transducers can be found both at the input as well as at the output stage of a measuring
system.
The input transducer is called the sensor, because it senses the desired physical quantity and
converts it into another energy form.
The output transducer is called the actuator, because it converts the energy into a form to which
another independent system can react, whether it is a biological system or a technical system. So,
for a biological system the actuator can be a numerical display or a loudspeaker to which the
visual or aural senses react respectively. For a technical system the actuator could be a recorder
or a laser, producing holes in a ceramic material. The results can be interpreted by humans.
Actuators are important in instrumentation. They allow the use of feedback at the source of the
measurement. However we will pay little attention to them in this course. The study of using
actuators and feedback belongs to a course in Control theory.
3.2
Types of Energy Form
We can distinguish six different energy domains: (1) radiant, (2) mechanical, (3) thermal, (4)
electrical, (5) magnetic and (6) chemical.
If certain information is already available in the electrical domain it can be claimed that it
requires no energy conversion, but in general there is 'shape' conversion left and this is just
the domain which belongs to the field of electronics and electrical science and engineering.
A good example of such a sensor only sensitive to electrical energy is the probe of an
oscilloscope, with which a good adaptation to the signal source is realized. In the modifier
stage we meet other examples of shape converters, for instance the A/D and D/A converters.
Table 1.6 Energy types and corresponding measured
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Energy
Measurands
Length, area, volume, force, pressure, acceleration,
torque, mass flow, acoustic intensity and so on.
Temperature, heat flow, entropy, state of matter
Charge, current, voltage, resistance, inductance,
capacitance, dielectric constant, polarization,
frequency, electric field, dipole moment, and so on.
Field intensity, flux density, permeability, magnetic
moment, and so on.
Intensity, phase, refractive index, reflectance,
transmittance, absorbance, wavelength, polarization,
and so on.
Concentration, composition, oxidation/reduction
potential, Reaction rate, pH and the like.
Mechanical
Thermal
Electrical
Magnetic
Radiant
Chemical
3.3
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Classification of transducers
The transducers may be classified based on
i.
i.
The physical effect employed
ii.
The physical quantity measured
iii.
The source of energy
Classification based on physical effect
The physical quantity applied as measurand (quantity to be measured) to the transducer causes
some physical changes in its element. By this physical effect the transducer converts the physical
quantity in to electrical quantity. For example, a change in temperature to be measured causes
variation of resistance (physical change) in a copper wire (element) and this effect could, be used
to convert temperature in to an electrical output,
The physical effects commonly employed are
a.
b.
c.
d.
e.
Variation of resistance
Variation. of inductance
Variation of capacitance
Piezo electric effect
Magnetostrictive effect
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f. Elastic effect
g. Hall effect
a.
Variation of resistance
The resistance of a length of metallic wire is given by
R=
ρl
a
Where,
R = Resistance in ohm.
ρ =Resistivity (or specific resistance) of the material in ohm-m.
l = length of wire in m.
a = Area of cross-section in m2.
As resistance is a function of ρ , l , a (i.e) R = f ( ρ , l , a ) , with any change in anyone of the
physical quantities ρ , a or l due to variation in resistance, a variable resistance transducer
can be designed to convert physical quantity.
Some of the transducers based on this principle are potentiometer, strain gauge, resistance
thermometer, carbon microphone, and photoconductive cell.
•
The resistance thermometer is based upon thermo resistive effect which is the
change in electrical resistivity of a metal or semiconductor due to change in
temperature co-efficient of resistivity.
•
Carbon microphone works on the principle of change in contact resistance due to
applied pressure.
•
Photoconductive cell is based on photoconductive effect which is the change in
electrical conductivity due, to incident light.
•
Potentiometer works on the principle of change in resistance due to linear or
rotational motion.
•
Strain gauge works on the principle of change in resistance due to applied
pressure.
b.
Variation of inductance
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The inductance of a coil is given by
L=N
L=
dφ
dt
N 2 µo µ r A
l
where,
L = inductance in henry
N = No., of turns
µo = absolute permeability
µr = relative permeability
A = area of cross section of the core
l = length of magnetic path
dφ
= rate of change of magnetic flux.
dt
As L is a function, of N, µr , A, l ,
(i.e) L = f (N, µr ,A, l ), when anyone of these quantities changes, the inductance changes.
This leads to the design of a variable inductance transducer.
Some of the transducers based on variation of inductance are induction potentiometer,
linear variable differential transformer (LVDT) and synchros.
c.
Variation of capacitance
The capacitance between two conductor plates is given by
C=
ε oε r A
d
Where
C = capacitance in farad
ε o = absolute permittivity
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ε r = relative permittivity of the separating medium
A = area of cross-section of the plates
As C is a function of ε r , A, d i.e C = ( ε r , A, d ) when anyone of these quantities changes, the
capacitance varies. This leads to the design of a variable capacitance transducer.
d.
Piezoelectric effect
When a piezoelectric crystal like quartz or Rochelle salt is subjected to mechanical stress, an
electric charge is generated. This is known as piezoelectric effect. The transducer based on this
effect is piezoelectric transducer.
e.
Magnetostrictive effect
When a magnetic material is subjected to mechanical stress, its permeability changes. This effect
is magnetostrictive effect and the transducer based on this effect is magnetostrictive transducer.
f.
Elastic effect
When an elastic member is subjected to mechanical stress it is deformed. The transducer based
on this effect is called elastic transducer.
g.
Hall effect
When a magnetic field is applied to a current carrying conductor at right angles to the direction
of current, a transverse electric potential gradient is developed in the conductor. This effect is
called as Hall effect and the transducer based on this effect is called as Hall effect transducer.
ii.
Classification based on physical quantity measured
The transducers may be classified based on the physical quantity they measure as follows:
•
•
•
•
•
•
•
Temperature transducers → Transducers used to measure temperature
Pressure transducers → To measure pressure
Flow transducers → To measure flow
Liquid level transducers → To measure liquid level
Force/Torque transducers → To measure force & Torque
Velocity/Speed transducers → To measure velocity & speed
Humidity transducers → To measure humidity
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•
•
iii.
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Acceleration/vibration transducers → To measure acceleration & vibration
Displacement transducers → To measure displacement
Classification based on source of energy
Transducers may be, classified based on source of energy into two types.
•
•
Active transducer
Passive transducer
Passive transducer
A component whose output energy is supplied entirely or almost entirely by its input signal is
called a passive transducer. A passive transducer is the one which absorbs energy from the input
medium and converts it directly into the output signal.
Example
A Thermocouple extracts heat energy from the input medium and converts it into
electrical energy (voltage).
Active Transducer
An active transducer has an auxiliary source of power which supplies a major part of the output
power while the input signal supplies only an insignificant portion (i.e) this transducer uses the
energy it absorbs from the input medium as a control signal to transfer energy from the power
supply to produce a proportional output.
Example
Strain gauge
The energy extracted from the strained member is very small. The energy for the output
signal is supplied by an external power source.
Fig. 1.7 Active and passive transducers
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Selection of Transducers
Transducers are used for the measurement of physical quantities. The selection of transducers for
particular measurand is very important. The selection of transducers may be based on the
following factors for effective measurement.
1. The physical quantity to be measured (measurand),
2. The range of input quantity,
Based on physical quantity to be measured
The correct type of transducer should be selected for measuring the physical quantity. The
following table shows the physical quantity and the corresponding transducer types available.
No.
Physical quantity
1
Temperature
2
Pressure
3
Force (weight)
4
Torque
5
Density of liquids
Transducers available
Bimetallic element
Fluid expansion systems
i.
Liquid-in-steel bulb thermometers
ii.
Liquid-in-glass thermometers
iii. Vapour pre-ssure thermometers
Thermoresistive elements
i.
Resistance Temperature detector (RTD)
ii.
Thermistor
Thermocouple
Linear-Quartz thermometer
Pyrometry
U-tube and ball type manometers
Ring balance manometer
Metallic diaphragms
Capsules and bellows
Bourdon tubes
Membranes
Spring balance
Cantilever
Diaphragms
Pneumatic and hydraulic load cells
Column and proving ring load cells
Torsion bar
Flat spiral springs
Dynamometer
Gyroscope
Hydrometer
Air bubbler system
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U-tube weighing system
3.4
6
Liquid level
Float elements.
Manometer system
Diaphragms
Container weight
7
Viscosity
Capillary tube
Concentric cylinder system
8
Flow rate of fluids
9
Displacement
Pitot static tube
Flow-obstruction elements
Rotating vane system
Rotameter float system
Flapper nozzle system
10
Absolute displacement,
Velocity and acceleration
Vehicle attitude
Seismic system
Gyroscope
Characteristics of Transducers
The selection of most suitable transducer from commercially available instruments is very
important in designing an Instrumentation system. For the proper selection of transducer,
knowledge of the performance characteristics of them are essential. The performance
characteristics can be classified into two namely
i.
ii.
Static characteristics
Dynamic characteristics
Static characteristics are a set of performance criteria that give a meaningful description of the
quality of measurement without becoming concerned with dynamic descriptions involving
differential equations.
Dynamic characteristics describe the quality of measurement when the measured quantities vary
rapidly with time. Here the dynamic relations between the instrument input and output must be
examined, generally by the use of differential equations.
For further reading on this subtopic please read the additional material
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