2. Experiment s Title: The Linear and Rotary Potentiometer - AMEM 211

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1 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

2 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

3 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

4 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

5 Figure 5: Linear potentiometer bipolar connection diagram Displacement (mm) Output (V) 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

6 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) 0 Output (V) 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

7 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

8 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) 0 Output (V) 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

9 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

5. Experiment s title: The Optical Tachogenerator AMEM 211

5. Experiment s title: The Optical Tachogenerator AMEM 211 I. Objectives To Investigate an opto-transmissive sensor for speed measurement. To Produce a calibration graph of rotary speed against input voltage.