Displacement Transducers/Sensors

Potentiometer for angular displacement

Wire Wound Potentiometers

Longitudinal Displacement

Potentiometer or longitudinal displacement

o s

xv v

L=

Theory of Operation:

The sensor consists of a length “L” of resistance wire attached across a voltage source “Vs”. The wiper is pushed up or down by moving target, for which displacement “x” is required to be measured. Vo is the output voltage representing displacement in terms of volts and is given by:

o s

xv v

L=

Wire-Wound Potentiometer

The resistance of the wire wound potentiometer increases in step manner as the wiper moves from one position to the adjacent turn. This step change in resistance limits the resolution of the potentiometer to L/n, where n is the numbers of turns. The resolution ranges from 0.05 to 1 percent are common. Therefore such potentiometer are not suitable for precise and finer movements.

Thin film potentiometer

The film resistance on an insulating substrate exhibits high resolution, lower noise, and longer life. For example a resistance of 50 to 100 Ohm/mm can be obtained with the conductive plastic film

Thin Film Potentiometer

Thin Film potentiometer are introduced to

improve resolution. Movement can be nearly

continuous rather than in steps.

Thin Film Potentiometer

For angular Movements

Potentiometer …. slower

The dynamic response of both the linear and

the angular potentiometer is severely limited

by the inertia of the shaft and wiper

assembly. Since this inertia is large, the

potentiometer are used for static or quasi-

static measurement where a high

frequency response is not required.

Application of Potentiometer

Potentiometer are normally used for: Large displacement; i.e. 10mm or more 15 deg or more in angular displacement

Primary advantage is its simplicity while the disadvantage is its slower response to fast changing.

Tutorial

1. A slide wire potentiometer of 100mm length is fabricated by wire wound with wire of diameter 0.1mm around a cylindrical insulating core.

Determine the resolution limit.

Solution:

Resolution: L/n

n= 100mm/0.1mm = 1000 turns

Therefore resolution= 100/1000 =0.1mm

Tutorial ….

If the potentiometer of previous example has resistance of 2000 Ohms and can dissipate 2 watts of power, determine the voltage required to maximize the sensitivity. What voltage change corresponds to resolution limit?

Solution:

R per turn=2000/1000= 2P=v2/R and v2 = 2/2 =1

Resolution = 1 v/ turn

Assignment….2

Q.No 1: A 20-turn potentiometer with calibrated dial (100 division per turn) is used to balance resistor in Wheatstone Bridge. If the potentiometer has resistance of 20 k and a resolution of 0.05 percent, what is the minimum increment change in resistance R that can be read from the calibrated dial?

Q.No 2: Design a displacement transducer that utilizes a 20-turn potentiometer to monitor the position of the elevator over its 100-m range of travel; ie.( Wiper length and resolution)