1. Displacement Transducers/Sensors
Potentiometer for angular displacement

2. Wire Wound Potentiometers

3. Longitudinal Displacement
Potentiometer or longitudinal displacement
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:

4. 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.

5. 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

6. 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

7. 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.

8. 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.

9. Therefore resolution= 100/1000 =0.1mm
Tutorial
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

10. 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= 2
P=v2/R and v2 = 2/2 =1
Resolution = 1 v/ turn

11. 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)