| When something is stressed
its length changes.
change in length/original length = strain (e) Resistive strain gauges consist of a wire grid attached to a flexible backing. When the gauge is bent there is a change in resistance. The change in resistance is directly proportional to strain. Tension (stretching) increases resistance. Compression (contracting) decreases resistance. Semiconductor strain gauges are available. Strain gauges can be attached to buildings, girders, bridges etc. |
| The following diagram shows
tensile strain being measured.
If the strain gauge was below the member under stress it would be measuring compressive strain. |
| The next diagram shows acceleration
being measured.
Both tensile and compressive strain are measured. As the accelerating force is applied the mass moves in the opposite direction. SG1 measure compression and SG2 measures tension. |
| The strain gauges are used
in a Wheatstone bridge circuit as shown below.
If the bridge is balanced then the output from the amplifier is zero. If the ambient temperature rises then since both strain gauges change in value by the same amount the bridge will remain balanced. Under acceleration sg1 decreases in resistance and sg2 increases in resistance. Thus the circuit is more
sensitive than when using just one strain gauge.
|
Copyright Graham Knott 2004