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2. 2. Strain GagesStrain Gages By Dr. Sotiris OmirouBy Dr. Sotiris Omirou
3. 3. Strain-Strain- gaugegauge The electrical resistance strain is a metal wire or metal foilThe electrical resistance strain is a metal wire or metal foil strip which is wafer-like and can be stuck onto surfaces likestrip which is wafer-like and can be stuck onto surfaces like a postage stamp.a postage stamp.
4. 4. Typical bonded strain gagesTypical bonded strain gages
5. 5. When a strain gauge is bonded to an object, and theWhen a strain gauge is bonded to an object, and the object changes in size, the resistance of the strainobject changes in size, the resistance of the strain gauge will change. The resistance R is given by thegauge will change. The resistance R is given by the expression:expression: R=R= ρρ l/Al/A Where:Where:  l is the length of the wire in metersl is the length of the wire in meters  ρρ is the resistivity of the material in ohm metersis the resistivity of the material in ohm meters  A is the cross-sectional area of the filament in mA is the cross-sectional area of the filament in m22
6. 6. When strain gauges measure the changing dimensions of anWhen strain gauges measure the changing dimensions of an object, they are measuringobject, they are measuring strainstrain. Strain is the ratio of the. Strain is the ratio of the change in dimension of an object to the original dimensionchange in dimension of an object to the original dimension Mechanical strainMechanical strain εε == ΔΔL / LL / L When subject to strain, its resistance R changes, theWhen subject to strain, its resistance R changes, the fractional change in resistancefractional change in resistance ΔΔR/R being proportional toR/R being proportional to the mechanical strain i.e.the mechanical strain i.e. Electrical strainElectrical strain ΔΔR/R= G .R/R= G . ΔΔL/LL/L where G is the gauge factor (1.8 – 2.2)where G is the gauge factor (1.8 – 2.2) R varies between 50R varies between 50 ΩΩ and 2Kand 2KΩΩ
7. 7. PotentialPotential DividerDivider
8. 8. Potential Divider with differential amplifierPotential Divider with differential amplifier
9. 9. Wheatstone BridgeWheatstone Bridge 1 2 4 3 R 2 R 1 R 3 R 4 V o Vin The four arms of the bridgeThe four arms of the bridge circuit are formed by thecircuit are formed by the resistance R1 to R4.resistance R1 to R4. The corner points 1 and 4 areThe corner points 1 and 4 are connected to the inputconnected to the input voltage Vvoltage Vinin The corner points 2 and 3 areThe corner points 2 and 3 are connected to the outputconnected to the output voltage Vo.voltage Vo.
10. 10. Quarter strain gauge bridge with differential amplifierQuarter strain gauge bridge with differential amplifier
11. 11. Half strain gauge bridge with differential amplifierHalf strain gauge bridge with differential amplifier
12. 12. Full strain gauge bridge with differential amplifierFull strain gauge bridge with differential amplifier