Potentiometric and Resistance Transducer

(a) POTENTIOMETRIC TRANSDUCER:

A potentiometric transducer is an electromechanical device. It is a passive transducer. It requires external power source for its functioning. It contains a resistance element that is provided with a slider or wiper. The motion of the wiper may be translatory or rotational. Combination of translator and rotational motion leads to a helical movement; such potentiometers are called heliports. Figure shows the three types of potentiometric transducers.

The translatory resistive elements are linear devices [shown in Figure]. Rotational resistive devices are circular [shown in Figure].They are used for the measurement of angular displacement.

Helipots are multi turn rotational devices. They can be used for either translatory or rotational motion. Helipot is shown in Figure.

Advantages of Potentiometric Transducer:

They are economical
1. Easy to operate
2. Ability to large amplitudes of displacement
3. High efficiency
4. Large Output

Disadvantages:

I. Sufficient force is required to slide the wiper
2. Noise will be produced due to wear out

(b) RESISTANCE TRANSDUCER:

This is a type of passive transducer. The mechanical displacement is converted into a change in resistance. Such a transducer is called a strain gauge.

A strain gauge is a thin, slice like device. This slice or wafer looking strain gage, will be attached to the job to measure the strain applied to the job. Resistance wire of small diameter is used in the strain gauge. Copper Nickel alloy called Constant and is used to make the resistance wire. Etched foils are also used to form the gauge.


The gauge resistance changes with length, as the job undergoes tension or compression. The change in resistance will be proportional to the applied strain. Special wheatstones bridge is used to measure this change in resistance. Gauge factor, indicates the sensitivity of a strain gauge. It is defined as the unit change in resistance per unit change in length. It is denoted by the letter 'K'.

Gage factor K = (ΔR/R) / (Δl/l)
Where K = Gauge factor
R = Nominal gauge resistance
ΔR = Change in gage resistance. (without application of stress)
l =  Normal specimen length
Δl = Change in specimen length

It is to be noted that large stress results in a relatively very small change in resistance. Hence the bridge used for measurement of resistance must be highly sensitive.

The metallic strain gauge are formed from either resistance wire or etched foil of metal sheets. They are small in size. They can be used in high temperature applications. Their leakage is low. Foil type gauges are superior to wire types. Foil types can be used under extreme temperature conditions. They can be used with prolonged loading conditions. Self induced heat is easily dissipated in foil types. The following materials are used in the manufacture of resistance transducers (strain gauges) :

Constantan *** Nickel Alloy-Low temp Coefficient
Nichrome V *** Nickel Chrome Alloy-Static strain measurements up to 375°C
Dynaloy *** Nickel Iron Alloy-Dynamic strain applications
Stabile *** Modified Nickel Iron Alloy-Good stability
Platinum-Tungsten Alloy *** Offers high stability and large resistance to fatigue at higher temperatures. Temperature range up to 800°C.

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