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The Strain Gauge-Physics-Report, Study Guides, Projects, Research of Physics

University of AllahabadPhysics

In physics course we got small project to perform experiment in lab. In end of semester we had to submit report. Mainly report contains observation, results and explanation of topic theoretically as well. This lab report includes: Strain, Gauge, Performance, Measurement, Linear, Displacements, Relative, Sensitivities, Configurations, Resistance

Typology: Study Guides, Projects, Research

2011/2012

Uploaded on 08/26/2012

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Experiment’s title: The Strain Gauge (source: S.I.S. student’s manual)
I. Objectives
Assess the performance of strain gauges for measurement of linear displacements. On
completion of this experiment you will:
• Appreciate the positioning of strain gauges within a system affected by displacement.
• Understand the use of strain gauges in a potential divider, quarter, half and full bridge
configurations and the relative sensitivities in each case.
• Assess the sources of error in using strain gauges to measure displacement.
II. Theory
Strain gauges are devices designed and constructed so that their resistance changes when
they are strained: that is their physical dimensions increase or decrease. This is usually
arranged to happen when the body to which they are bonded (stuck) changes and so the
strain gauges resistance may be used to measure the amount of strain the body is
experiencing.
To maximise this effect there are two main considerations to take into account when using
strain gauges. The first is to design strain gauges so that their resistance changes
appreciably with strain and secondly that they are attached to a system such that they are
affected by strain. Other considerations are made to minimise any changes in resistance
caused by any effect other than strain: the main one is temperature.
In the following sequence of experiments, strain gauges in conjunction with fixed resistors in
different configurations form potentiometric and bridge circuits. In using and predicting the
performance of these circuits it may help to consider them as forming series/parallel resistive
circuits and apply the universal electrical circuit laws that you will have encountered in
earlier lessons.
In the SIS Hardware Module the four strain gauges, nominal resistance of 120 Ω, mount on a
flexible beam. This provides an exaggerated amount of movement at its free end when the
Linear Assembly is moved but relatively little at the clamped end at which the strain gauges
are affixed. Two strain gauges mount on each side of the beam, such that when the beam
deflects to the left, two of the gauges are in tension (increase in their resistance) and the
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1

Experiment’s title: The Strain Gauge (source: S.I.S. student’s manual)

I. Objectives Assess the performance of strain gauges for measurement of linear displacements. On completion of this experiment you will:

  • Appreciate the positioning of strain gauges within a system affected by displacement.
  • Understand the use of strain gauges in a potential divider, quarter, half and full bridge configurations and the relative sensitivities in each case.
  • Assess the sources of error in using strain gauges to measure displacement. II. Theory Strain gauges are devices designed and constructed so that their resistance changes when they are strained: that is their physical dimensions increase or decrease. This is usually arranged to happen when the body to which they are bonded (stuck) changes and so the strain gauges resistance may be used to measure the amount of strain the body is experiencing. To maximise this effect there are two main considerations to take into account when using strain gauges. The first is to design strain gauges so that their resistance changes appreciably with strain and secondly that they are attached to a system such that they are affected by strain. Other considerations are made to minimise any changes in resistance caused by any effect other than strain: the main one is temperature. In the following sequence of experiments, strain gauges in conjunction with fixed resistors in different configurations form potentiometric and bridge circuits. In using and predicting the performance of these circuits it may help to consider them as forming series/parallel resistive circuits and apply the universal electrical circuit laws that you will have encountered in earlier lessons. In the SIS Hardware Module the four strain gauges, nominal resistance of 120 Ω, mount on a flexible beam. This provides an exaggerated amount of movement at its free end when the Linear Assembly is moved but relatively little at the clamped end at which the strain gauges are affixed. Two strain gauges mount on each side of the beam, such that when the beam deflects to the left, two of the gauges are in tension (increase in their resistance) and the

2 other two in compression (decrease in their resistance). Deflecting the beam to the right reverses this effect. III. Experimental work Part A: The Strain Gauge Potential Divider This circuit is a simple potential divider with a strain gauge (tension), Rsg, connected in series with a fixed matching resistor, R, and a power supply connected across both, Vin. This is shown schematically in Figure 1.1. F igure 1.1: Strain gauge potential divider The equation relating the output voltage to the other circuit parameters is: In other words, the output is governed by the ratio of the strain gauge resistance, Rsg, to the total resistance, Rsg + R. The object of this experiment is to determine the performance of the strain gauge potential divider. Use the patching leads supplied to connect the equipment as shown in Figure 1.2. Confirm that this is the same circuit as that shown in Figure 1.1. Move the Linear Assembly to the right by rotating the rotary scale clockwise until it reaches the end stop. Carefully adjust the dial until the zero aligns with the edge of the moulding. Observe the initial value of meter reading and record it here.

4 Figure 1.3: Connection diagram of potential divider with differential amplifier Figure 1.4: Strain gauge potential divider with differential amplifier Set the gains of the amplifier, k1 and k 2 , to maximum and adjust the setting of Ref1 to make the indicated meter value as small as possible (less than 5 V to be within the range of the 'set zero' control). Now zero the reading with the 'set zero' control. Note: With the gain controls of the Differential Amplifier set to maximum (Gain = 100) the adjustment is very coarse. A 100 mV difference between the two input voltages is enough to change the meter reading by 10 V. The output signal is now the amplified difference between the two inputs signals, the potentiometer value and that of Ref1. The changes in the output are caused by the changes in potential divider ratio due the changes in Rsg.

5 In steps of 1 mm, one complete revolution of the rotary scale, move the Linear Assembly to the left over its full range of travel and record corresponding meter readings to complete Table 1.1. Be careful to adjust the control in one direction only for each set of readings. Displacement (mm) Output (V) left Output (V) right 0 1 2 3 4 5 6 7 8 9 Table 1.1 Results table for gauges in tension Plot a graph of your results and determine the sensitivity of the measurement system from the slope of the graph and the intercept with the vertical axis. Determine the equation relating meter reading with displacement. Comment on the linearity, hysteresis, scatter and repeatability of the measurements obtained. Repeat the above procedure with one of the compression gauges replacing the tension gauge and over the same range of displacements to complete Table 1.2. Plot your results and compare them with those obtained with the tension gauge. Displacement (mm) Output (V) left Output (V) right 0 1 2 3 4 5 6 7 8 9 Table 1.2: Results table for gauge in compression

7 Connect the quarter bridge strain gauge circuit as shown in Figure 1.6. Confirm that this is the same circuit as shown in Figure 5. Repeat the procedure of Part A to complete Table 1.3. Plot a graph of your results and determine the sensitivity of the measurement system from the slope of the graph and the intercept with the vertical axis. Hence, determine the equation relating meter reading with displacement. Compare your results with those obtained in Part A for the potential divider circuit. Figure 1.6: Connection diagram of quarter bridge with differential amplifier Part C: The Half Strain Gauge Bridge (or Half-Bridge) The half-bridge is a further enhancement to the basic Wheatstone bridge. One arm is now formed by two strain gauges (R1 and R 2 ), one positioned to experience increasing tension and the other increasing compression when the Linear Assembly moves in one direction. Figure 1. is a schematic drawing of the half strain gauge bridge. The variation in output is affected by the change in resistance of both strain gauges. The fixed resistor arm (R 3 and R4) has the same function as before, to produce a fixed reference voltage with which to compare the variable output of the strain gauge arm. The object of this experiment is to determine the performance of the half-bridge and compare it with the results of the potential divider and quarter-bridge obtained in Parts A and B.

8 Figure 1.7: Half Strain Gauge Bridge with differential amplifier Connect the circuit as shown in Figure 1.8. Confirm that this circuit is the same as that given in Figure 1.7. Repeat the previous procedure to complete Table 1.4. Displacement (mm) Output (V) left Output (V) right 0 1 2 3 4 5 6 7 8 9 Table 1.4: The half-bridge Plot a graph of your results and determine the sensitivity of the measurement system from the slope of the graph and the intercept with the vertical axis. Determine the equation relating meter reading with displacement. Compare your results for the half-bridge with those obtained for the potential divider and quarter-bridges in Parts A and B.

10 Connect the circuit as shown in Figure 1.10. Confirm that this circuit is the same as that given in Figure 1.9. Repeat the previous procedure to complete Table 1.5. Plot a graph of your results and determine the sensitivity of the measurement system from the slope of the graph and the intercept with the vertical axis. Hence, determine the equation relating meter reading with displacement. Compare your results with the potential divider, quarter-bridge and half-bridge investigated earlier in Parts A, B and C. Displacement (mm) Output (V) left Output (V) right 0 1 2 3 4 5 6 7 8 9 Table 1.5: The full-bridge Figure 1.10: Connection diagram of full bridge with differential amplifier

11 IV. Comments and Conclusions Use the results and observations made in this experiment to write a report on the use of strain gauges 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?