2. Experiment’s Title: The Linear and Rotary Potentiometer - AMEM
211
I. Objectives
On completion of this experiment you will,
•
Understand how linear and rotary potentiometers attach to a system to measure
displacement.
•
Have produced a calibration graph of both types of device and make judgements
on
suchcharacteristics as linearity, repeatability, accuracy and sensitivity.
•
Appreciate the sources of errors in potentiometric circuits
II. Theory
The potentiometer is an electrical device comprising a resistor with a sliding third contact,
often termed a wiper, which allows the voltage to be varied depending upon where the slider
is positioned along the length of the resistor.
Potentiometers are found in many electrical and electronic applications and in many different
forms, sizes and power ratings. For instance, in a relatively high power application a wire
wound potentiometer may be used to provide a variable D.C. (or a.c.) power supply
delivering many amperes at some voltage less than the supply voltage. In an electronic
system a low power rated carbon track potentiometer may be used to preset the voltage on a
circuit board to achieve the desired level of response.
In both examples, the principle of operation is essentially the same. Manual adjustment of
the wiper along the length of the fixed resistance produces a variable voltage at the wiper.
The magnitude of this output voltage is directly proportional to its relative position along the
length of the resistor.
If the potentiometer wiper is appropriately connected to a moving system then any
movement in that system will cause the wiper to move and so change the output voltage.
This signal provides a direct measurement of position or change in position. Hence, although
still a potentiometer, it is of use as a sensor for measuring linear displacement.
A potentiometer circuit is shown schematically in Figure 1.
FREDERICK UNIVERSITY CYPRUS
Dr. Sotiris L. Omirou
AMEM 211
1
Figure 1: Potentiometer circuit
The output voltage is governed by the position of the wiper (C) which may lie anywhere
between the two ends, A and B, of the resistance. For the general case the output voltage is
given by the expression,
where:
CB is the linear distance (or angular rotation) from B to C;
AB is the maximum linear distance (or angular rotation) from B to A.
Hence when the potentiometer wiper is in position B the output voltage will be zero and when in
position A will be maximum, the full supply voltage (Vin). In any intermediate position the
voltage at the wiper will be some value between 0 and Vin as given by the above potentiometer
equation.
If the resistance is linear throughout its length then the output voltage will also be linear and
directly proportional to the wiper position along the length of the resistance. In the SIS, the
wipers of both the linear and rotary potentiometers connect to the Linear Assembly such that
any movement of the assembly causes the output voltage to change in direct proportion. This
experiment investigates the use of linear and rotary potentiometers for measuring
displacement.
FREDERICK UNIVERSITY CYPRUS
Dr. Sotiris L. Omirou
AMEM 211
2
III. Experimental Work
Part A: The Linear Potentiometer
The object of this part of the experiment is to investigate the linear potentiometer for measuring
linear displacement. Use the patching leads supplied to make the connections given in Figure
3. Satisfy yourself that this is the same as the schematic circuit given in Figure 2.
Figure 2: Linear potentiometer schematic
Move the Linear Assembly to the right by rotating the manual control clockwise until it reaches
the end stop. Carefully adjust the dial until the zero aligns with the edge of the moulding.
In steps of 1 mm, (one complete rotation of the rotary scale), move the Linear Assembly to the
left over its full range of travel and record corresponding meter readings to complete the Table
1. Be careful to adjust the control in one direction only throughout the procedure.
Plot a graph of your results. Comment on the shape of the graph, and measure its slope and
intercept with the vertical axis and hence give the equation governing this measurement
system.
With the Linear Assembly adjusted to be in mid-position, determine the minimum amount of
movement (resolution) that is detectable by the meter reading.
Resolution = ……… mm
FREDERICK UNIVERSITY CYPRUS
Dr. Sotiris L. Omirou
AMEM 211
3
Figure 3: Connection diagram for linear potentiometer
An alternative supply to the potentiometer is from a bipolar supply, as shown in Figure 4.
Figure 4: Linear potentiometer with bipolar supply
Use the patching leads supplied to connect the equipment as shown in Figure 5. Satisfy
yourself that this is the same as the circuit given in Figure 4
Repeat the previous procedure to move the Linear Assembly from the right to the left and
record corresponding values of displacement and meter reading to complete Table 2. Plot a
graph of your results.
FREDERICK UNIVERSITY CYPRUS
Dr. Sotiris L. Omirou
AMEM 211
4
Figure 5: Linear potentiometer bipolar connection diagram
Displacement
(mm)
Output (V)
0
1
2
3
4
5
6
7
8
9
Table 2 Results
Comment on the shape of the graph, measure its slope and intercept with the vertical axis.
Give the equation governing this measurement system.
With the Linear Assembly adjusted to be in mid-position, determine the minimum amount of
movement (resolution) that is detectable by the meter reading.
Resolution = ……… mm
FREDERICK UNIVERSITY CYPRUS
Dr. Sotiris L. Omirou
AMEM 211
5
Ref. 2
Figure 6: Linear potentiometer with differential amplifier
The circuit in Figure 6 shows the output from the potentiometer connected to the input (+) of a
differential amplifier with an external reference voltage, Ref2, connected to the other input (−).
The object here is to use the reference voltage to remove any offset in the output signal when
the Linear Assembly is in the starting position.
Use the patching leads supplied to connect the equipment as shown in Figure 7. Satisfy
yourself that this is the same as the circuit given in Figure 6. With the amplifier gain set to unity,
adjust Ref2 to zero the meter reading at the starting position. Repeat the previous procedure to
complete Table 3.
Displacement (mm)
Output (V)
0
1
2
3
4
5
6
7
8
9
Table3 Results
Plot a graph of your results. Comment on the shape of the graph, measure its slope and
intercept with the vertical axis. Give the equation that governs this measurement system. With
the Linear Assembly adjusted to be in mid-position, determine the resolution of the system.
Resolution = ……… mm
FREDERICK UNIVERSITY CYPRUS
Dr. Sotiris L. Omirou
AMEM 211
6
Figure 7: Linear potentiometer with differential amplifier connection diagram
Part B: The Rotary Potentiometer
In terms of operating principles, the rotary potentiometer is the same as the linear
potentiometer. The wiper moves over a fixed resistance to provide a varying output signal
proportional to the movement. The difference is in the design and manufacture so that the
resistance forms into an arc and that the wiper rotates about a central pivot.
The rotary potentiometer has been included to illustrate the conversion of linear motion into
rotary motion in this example by the use of a worm and wheel arrangement, though any similar
arrangement producing the same effect could be used. In addition, although the shaft rotates
by the movement of the Linear Assembly the final motion delivered to the potentiometer is
rotary.
The main objective of this part of the experiment is to illustrate the use of the rotary
potentiometer for measuring linear motion.
While moving the Linear Assembly over its full range of movement visually inspect the effect
this has on the shaft of the rotary potentiometer in terms of the angle moved through. Also,
observe any relative movement between the worm and wheel arrangement, which would cause
errors in measurement, which need to be allowed for in later experiments.
FREDERICK UNIVERSITY CYPRUS
Dr. Sotiris L. Omirou
AMEM 211
7
Figure 8: Rotary potentiometer with differential amplifier connection diagram
Use the patching leads supplied to connect the equipment as shown in Figure 8. The
schematic for this circuit is the same as Figure 6. Repeat the previous procedure to complete
Table 4.
Displacement (mm)
Output (V)
0
1
2
3
4
5
6
7
8
9
Table 4: Results
Plot a graph of your results. Comment on the shape of the graph, and measure its slope and
intercept with the vertical axis. Give the equation, which governs this measurement system.
With the Linear Assembly adjusted to be in mid-position, determine the resolution.
Resolution = ……… mm
FREDERICK UNIVERSITY CYPRUS
Dr. Sotiris L. Omirou
AMEM 211
8
IV. Comments and Conclusions
Use the results obtained and observations made in this experiment to write a report on the use
of linear and rotary potentiometers for measuring linear displacement. Include any theory you
feel supports the comments and conclusions you give. Can you suggest any changes to the
experiment that would improve the quality of the results and widen the scope of the
experiment?
FREDERICK UNIVERSITY CYPRUS
Dr. Sotiris L. Omirou
AMEM 211
9