Page 877 - Basic Electrical Engineering

degree centigrade. Sensitivity of a transducer should therefore be as high as possible.
ii. Accuracy: Output produced by transducers should be proportional to the true value of the
quantity being measured. Any deviation will lead to error in the measurement. Accuracy of a
transducer must be very high.
iii. Linearity: The output signal produced by the transducer should vary linearly with the
variation of the input. For a thermocouple, for example, the EMF induced should maintain a
linear relation with the variation of input, i.e., the temperature.
iv. Ruggedness: The transducer should require no maintenance and should be able to withstand
any over-load conditions.
v. Speed of response: The transducer should be such that it quickly responds, i.e., provides
output to the changing input, i.e., the changes in the quantity being measured.
vi. Repeatability: Repeatability means that the transducer should produce the same output signal
for the same input every time, provided that the environmental conditions remain the same.

Various types of transducers and their applications are described as follows.

12.4 LINEAR VARIABLE DIFFERENTIAL TRANSFORMER

A linear variable differential transformer (LVDT) is an electromagnetic
induction-type displacement transducer. It is a reliable and accurate sensing

device that can be used to convert linear motion (displacement) to a
proportional electrical output. The basic construction of a LVDT is shown in

Fig. 12.2. It consists of three elements, viz one primary winding, two
identical secondary windings, and a movable magnetic core.
As shown in Fig. 12.2 (a), the position of the core is at null position. In this

position the induced EMF in the two secondary windings, E and E are equal
1
2
and opposite. The transducer output voltage is zero. When the core is
displaced by a force towards the left as shown in Fig. 12.2 (b), E will be
1
greater than E due to the difference in flux linkage created by the primary
2
winding ampere turns. When the core is moved towords the right as shown in
Fig. 12.2 (c), the induced EMF E will be greater than E . The magnitude of
2
1
the differential output voltage E will vary with the change in core position.
0
The output voltage, E at null position is ideally zero and will increase when
0
the core is made to move either towords the left or towords the right as shown

in Fig. 12.3. It is observed that the output voltage changes linearly with the
displacement of the core. The output voltage is a function of the displacement

of the core.