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UNIT V - TRANSDUCERS
TRANSDUCERS : Introduction, Classification of transducers, Active and passive transducers, Resistive
transducer, Strain gauges, LVDT, Pressure Inductive transducer, Capactive, Load cell, Piezoelectrical and
Photo Electric transducer, Temperature transducers: RTD, Thermistors and Thermocouple.

Transducer
A transducer is defined as a device that receives energy from one system and transmits it to another, often in a
different form.
Broadly defined, the transducer is a device capable of being actuated by an energizing input from one or more
transmission media and in turn generating a related signal to one or more transmission systems. It provides a usable
output in response to a specified input measured, which may be a physical or mechanical quantity, property, or
conditions. The energy transmitted by these systems may be electrical, mechanical or acoustical.
The input quantity for most instrumentation systems is nonelectrical. In order to use electrical methods and
techniques for measurement, the nonelectrical quantity is converted into a proportional electrical signal by a device
called “transducer”.
Actually, electrical transducer consists of two parts which are very closely related to each other. These two parts
are sensing or detecting element and transduction element. The sensing or detecting element is commonly known
as sensor.

1. Sensing Element
The physical quantity or its rate of change is sensed and responded to by this part of the transistor.
2. Transduction Element
The output of the sensing element is passed on to the transduction element. This element is responsiblefor
converting the non-electrical signal into its proportional electrical signal.

Classifications:-
1.Based upon transduction principle used
2. Primary & Secondary transducers
3.Active & Passive transducers
4. Analoge & Digital transducers
5.Transducers & Inverse transducers

1.Based upon the transduction principle:-

 Resistance - Potentimeter devices, Resistance strain gauge, Pirani guage or hot wire meter, Resistance
thermometer, Thermister, Resistance hygrometer, Photoconductive cell.
 Capacitance - Variable capacitance pressure gauge, Capacitor microphone, Dielectric gauge.
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 Inductance - Magnetic circuit transducer, Reluctance pick-up, Differential transformer,

Eddy current guage, Magneto striction gauge.
 Voltage & Current - Hall effect transducer, Ionisation chamber, Photoemissive cell,
Photomultiplier tube.
 Self generating transducers - Thermocouple, Thermopile, Moving coil generator, Piezoelectric transducer,
Photovoltaic.

2. Primary & Secondary transducers:-

Some transducers contain the mechanical as well as electrical device. The mechanical device converts the
physical quantity to be measured into a mechanical signal. Such mechanical device are called as the primary
transducers, because they deal with the physical quantity to be measured. • The electrical device then convert this
mechanical signal into a corresponding electrical signal. Such electrical device are known as secondary
transducers.

Example of Primary and secondary transducer Primary transducer Displacement voltage Secondary
transducer

3. Active & Passive:-

Transducers those which don’t require an auxiliary power source to produce their output are known as ‘Active
transducers’ or self generating type.
eg: Moving coil, Piezoelectric crystal, Thermocouple, Photovoltaic cell.
On the other hand transducer that can’t work on the absence of external power supply are called ‘passive
transducers’.
eg: Resistive, Capacitive, Inductive.

4. Analoge & Digital :-

Analoge transducers converts input quantity into an analog output which is continous function of time.
eg: strain gauge, LVDT, Thermocouple. Thermister.

Digital transducer converts input quantity into an electrical output is in the form of pulses
eg: Glass scale, Metallic scale.

5. Transducers & Inverse Transducers

We have already looked into transducers, inverse transducers are just the opposite of transducers in function. I
Inverse transducers converts electrical quantity in non electrical quantity. Just think about an current coil moving
in magnetic field which is an inverse transducer.
eg: Peizoelectrical crystal.

Basically, there are two types of transducers, electrical, and mechanical.

Electrical Transducer Definition
An electrical transducer is a sensing device by which the physical, mechanical or optical quantity to be measured
is transformed directly by a suitable mechanism into an electrical voltage/current proportional to the input
measurand.
An electrical transducer must have the following parameters:
Linearity: The relationship between a physical parameter and the resulting electrical signal must be linear.
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Sensitivity: This is defined as the electrical output per unit change in the physical parameter (for example V/°C for
a temperature sensor). High sensitivity is generally desirable for a transducer.
Dynamic Range: The operating range of the transducer should be wide, to permit its use under a wide range of
measurement conditions.
Repeatability: The input/output relationship for a transducer should be predictable over a long period of time. This
ensures reliability of
Physical Size: The Electrical Transducer Definition must have minimal weight and volume, so that its presence in
the measurement system does not disturb the existing conditions.
Advantages of Electrical Transducer
The main advantages of electrical transducer (conversion of physical quantity into electrical quantities) are as
follows:
Electrical amplification and attenuation can be easily done.
Mass-inertia effects are minimised.
Effects of friction are minimised.
The output can be indicated and recorded remotely at a distance from the sensing medium.
The output can be modified to meet the requirements of the indicating or controlling units. The signal magnitude
can be related in terms of the voltage current. (The analog signal information can be converted in to pulse or
frequency information. Since output can be modified, modulated or amplified at will, the output signal can be
easily used for recording on any suitable multichannel recording device.)
The signal can be conditioned or mixed to obtain any combination with outputs of similar transducers or control
signals.
The electrical or electronic system can be controlled with a very small power level.
The electrical output can be easily used, transmitted and processed for the purpose of measurement.
Selecting a Transducer
The transducer or sensor has to be physically compatible with its intended application. The following should be
considered while selecting a transducer.
Operating range: Chosen to maintain range requirements and good
Sensitivity: Chosen to allow sufficient output.
Frequency response and resonant frequency: Flat over the entire desired range.
Environmental compatibility: Temperature range, corrosive fluids, pressure, shocks, interaction, size and mounting
restrictions.
Minimum sensitivity: To expected stimulus, other than the measurand.
Accuracy: Repeatability and calibration errors as well as errors expected due to sensitivity to other stimuli.
Usage and ruggedness: Ruggedness, both of mechanical and electrical intensities versus size and weight.
Electrical parameters: Length and type of cable required, signal to noise ratio when combined with amplifiers, and
frequency response limitations.
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Resistive Transducer
Resistive Transducer Definition are those in which the resistance changes due to a change in some physical
phenomenon. The change in the value of the resistance with a change in the length of the conductor can be used
to measure displacement.
Strain gauges work on the principle that the resistance of a conductor or semiconductor changes when strained.
This can be used for the measurement of displacement, force and pressure.
The resistivity of materials changes with changes in temperature. This property can be used for the measurement
of temperature.
Potentiometer
A resistive potentiometer (pot) consists of a resistance element provided with a sliding contact, called a wiper. The
motion of the sliding contact may be translatory or rotational. Some have a combination of both, with resistive
elements in the form of a helix, as shown in Fig. 13.1(c). They are known as helipots.
Translatory resistive elements, as shown in Fig. 13.1(a), are linear (straight) devices. Rotational resistive devices
are circular and are used for the measurement of angular displacement, as shown in Fig. 13.1(b).
Helical resistive elements are multi turn rotational devices which can be used for the measurement of either
translatory or rotational motion. A potentiometer is a passive transducer since it requires an external power source
for its operation.
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They are inexpensive.
Simple to operate and are very useful for applications where the requirements are not particularly severe.
They are useful for the measurement of large amplitudes of displacement.
Electrical efficiency is very high, and they provide sufficient output to allow control operations.
Disadvantages of Potentiometers
When using a linear potentiometer, a large force is required to move the sliding contacts.
The sliding contacts can wear out, become misaligned and generate noise.
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Resistance Pressure Transducer

Measurement in the resistive type of transducer is based on the fact that a change in pressure results in
a resistance change in the sensing elements. Resistance pressure transducers are of two main types. First, the
electromechanical resistance transducer, in which a change of pressure, stress, position, displacement or other
mechanical variation is applied to a variable The other resistance transducer is the strain gauge, where the stress
acts directly on the resistance. It is very commonly used for stressand displacement measurement in
instrumentation.
In the general case of pressure measurement, the sensitive resistance element may take other forms, depending on
the mechanical arrangement on which the pressure is caused to act.
Figure 13.1(d) and (e) show two ways by which the pressure acts to influence the sensitive resistanceelement, i.e.
by which pressure varies the resistance They are the bellow type, and the diaphragm type. (Yet another is the
Bourdon tube of pressure gauge).
In each of these cases, the element moved by the pressure change is made to cause a change in resistance.
This resistance change can be made part of a bridge circuit and then taken as either ac or dc output signal to
determine the pressure indication.

Strain Gauge
The Strain Gauge is an example of a passive transducer that uses the variation in electrical resistance in wires to
sense the strain produced by a force on the wires.
It is well known that stress (force/unit area) and strain (elongation or compression/unit length) in a member or
portion of any object under pressure is directly related to the modulus of elasticity.