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Strain Gauges Experiment - Lab Report Example

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Strain Gauges Experiment

Wheatstone bridge and amplifier method

It must be noted at first that the 3rd method is a recommended on and no actual experiments have been done on it. It has nevertheless been included here because a combination of the Wheatstone bridge and an amplifier is a very efficient method for measuring voltages as the amplifier maximizes it and makes measurement easier. Also occurrence of electrical noise and fluctuations in instrument resolutions can be better revealed by it.








Theory & Apparatus
In this section of the report the justifications for the three measurement methods as well as circuit diagrams of the three measurement apparatus are provided.

Basic current and voltage measurement method:
The experimental rig shown in Figure 1 has a cantilever beam with a pair of 120 strain gauges attached one on the upper and one on the lower beam surfaces. The strain-setting device is a bolt with a metric thread.

The strain produced at the surface of a deflected cantilever beam varies along its length. Close to the gripping point (where the strain gauges are mounted) it has a maximum value of:
(Eq.1)
where: - deflection of the end of the beam (max of 45 mm),
d - beam thickness (0.8mm in the rig) and
L - beam length (250 mm in the rig).


Figure 1: The experimental rig.
Wheatstone bridge measurement method:
The purpose of this experiment is to improve the results obtained in experiment 1 by the previous measurement method by using a Wheatstone bridge (Figure 2) that comprises of four nominally identical 120 strain gauges.


















Figure 2: Wheatstone bridge

Ideally, if the bridge resistors are identical no voltage will be registered at the meter but this is not...
It is envisaged that this would be the most efficient method of all.
It must be noted at first that the 3rd method is a recommended on and no actual experiments have been done on it. It has nevertheless been included here because a combination of the Wheatstone bridge and an amplifier is a very efficient method for measuring voltages as the amplifier maximizes it and makes measurement easier. Also occurrence of electrical noise and fluctuations in instrument resolutions can be better revealed by it.
The experimental rig shown in Figure 1 has a cantilever beam with a pair of 120 strain gauges attached one on the upper and one on the lower beam surfaces. The strain-setting device is a bolt with a metric thread.
The purpose of this experiment is to improve the results obtained in experiment 1 by the previous measurement method by using a Wheatstone bridge (Figure 2) that comprises of four nominally identical 120 strain gauges.
Ideally, if the bridge resistors are identical no voltage will be registered at the meter but this is not so practically and a small voltage of about 2 mV is detectable. This is because the resistors are not exactly identical.
The amplifier needs to be able to amplify differential signals of up to about 3mV and produce an output in the range of about 1V to 5V. ...Show more

Summary

This report is on a set of four experiments on two methods of measuring microstrains on strain gauges. The first method is a simple current and voltage one while the second one is a balanced Wheatstone bridge one. The primary objective of the experiments is to determine how accurate measurements can be made of microstrains in strain gauges…
Author : zhegmann
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