Strain Gages By Dr. Sotiris Omirou

The electrical resistance strain is a metal wire or metal foil strip which is wafer-like and can be stuck onto surfaces like a postage stamp. Strain-gauge

Typical bonded strain gages

When a strain gauge is bonded to an object, and the object changes in size, the resistance of the strain gauge will change. The resistance R is given by the expression: R= ρ l/A Where: l is the length of the wire in meters ρ is the resistivity of the material in ohm meters A is the cross-sectional area of the filament in m2

Mechanical strain ε = ΔL / L Electrical strain ΔR/R= G . ΔL/L When strain gauges measure the changing dimensions of an object, they are measuring strain. Strain is the ratio of the change in dimension of an object to the original dimension Mechanical strain ε = ΔL / L When subject to strain, its resistance R changes, the fractional change in resistance ΔR/R being proportional to the mechanical strain i.e. Electrical strain ΔR/R= G . ΔL/L where G is the gauge factor (1.8 – 2.2) R varies between 50 Ω and 2KΩ

Potential Divider

Potential Divider with differential amplifier

Wheatstone Bridge The four arms of the bridge circuit are formed by the resistance R1 to R4. The corner points 1 and 4 are connected to the input voltage Vin The corner points 2 and 3 are connected to the output voltage Vo. 1 2 4 3 R2 R1 R3 R4 Vo Vin

Quarter strain gauge bridge with differential amplifier

Half strain gauge bridge with differential amplifier

Full strain gauge bridge with differential amplifier

Interpretation of results (Example 1) Meter reading

(Example 2) Meter reading

Strain Gages - review Strain = Elongation / Original Length Change in length = Change in electrical resistance Electrical Resistance change is very small, too small to be accurately measured using ordinary voltmeters

Wheatstone Bridge Converts a change in electrical resistance from a strain gage to a change in voltage Changes in strain are linearly related to a change in voltage output Voltage change is too small to be accurately measured and for this reason an amplifier is used