A Wheatstone bridge is an arrangement of 4 resistors used to measure the resistance of strain gauges. Wheatstone bridges provide better accuracy than directly measuring the resistance of a single strain gauge. Of their 4 resistors, either 1, 2 or 4 are strain gauges and the remainder are regular resistors. we call these Wheatstone bridges, quarter, half and full bridges respectively. The resistance values of the fixed resistors should be same as those of the strain gauges at rest. The improvement in accuracy obtained by using a Wheatstone bridge depends on the type of bridge (quarter, half or full) and to some extent, the layout of the stain gauges on the beam or shaft. The increased accuracy obtained through use of a Wheatstone bridge is as a result of the following:
Increased sensitivity: The increased sensitivity improves the effective resolution and reduces measurement error
Temperature compensation: When using 2 or 4 strain gauges, the effect on the resistance of variations in ambient temperature are cancelled out
Common mode noise rejection: Electrical Noise common to both sides of the bridge is cancelled out
Strain decoupling: Wheatstone bridges with 2 or 4 strain gauges are able to separate the bending strain from the normal strain
Using a Wheatstone Bridge
The following diagrams demonstrate how to setup Wheatstone bridges for measuring bending strain, tensile/compressive strain and torsional strain. There are additional stain gauge configurations, but the ones shown cover the majority of scenarios. Note: The table under each equation shows the bridge factor. The bridge factors have already been factored into the equations and so no further action is required.
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