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lec09_bdgc - ME365 Bridge Circuits Slide 1 Bridge Circuits...

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Unformatted text preview: ME365 Bridge Circuits Slide 1 Bridge Circuits & Variable Impedance Devices • Strain Gages • Bridge Circuits • Variable Impedance Devices ME365 Bridge Circuits Slide 2 Strain Gages • Strain Gage – The change in resistance of the device is a reasonably linear function of its deformation. – The elastic element to which the strain gage is attached deforms in response to the physical phenomenon (input) that is of interest, for example, force, torque, pressure, displacement, acceleration, …etc. Elastic Element Strain Gage Input Force; Torque; Pressure; ... Deformation Variable Resistance Q: How does one measure resistance variation? ME365 Bridge Circuits Slide 3 Expand resistance R into a Taylor series and ignore higher order terms: Define Gage Factor G f : Strain Gage Active Axis Passive Axis Resistance of wire: Resistivity Length of Wire Cross Sectional Area R L A L A : : : R R G R R R R G f f 1 2 1 R R R R R L L R A A R A L L A A L A 2 where ME365 Bridge Circuits Slide 4 Strain Gages • Metallic Strain Gages: – Gage Factor: G f = 2.0 ~ 2.2 – R = 120 W 1 W . – R = 2.4 W ~ 4.8 W . – Maximum gage current: 15 mA to 100 mA . – Relatively low sensitivity to temperature variation. • Semiconductor Strain Gage – Silicon (Si) “doped” with phosphorus (P), arsenic (As) or boron (B). – Gage Factor: G f = 100 ~ 175 – R = 120 W 5 W . – High sensitivity to temperature change. ME365 Bridge Circuits Slide 5 • Measure Resistance (Impedance) Variation – Use constant voltage supply and measure current variation. – Use constant current source and measure voltage variation. Strain Gages R + R V s i Amp Meter i V R R i R V V R s s s 1 1 t 1/ i R + R i s i s V i R R V i R i R o s o s s = = ( ) t V o V o ME365 Bridge Circuits Slide 6 • Measure Resistance (Impedance) Variation – Use constant voltage supply and measure voltage variation. Strain Gages V R R R R R V V R R R V Vs R R R R R o s o s = ( ) ( ) = if 1 1 1 t V o R + R V s V o R 1 ME365 Bridge Circuits Slide 7 Strain Gages Ex: In the previous case, let V s = 5 V , R = R 1 = 100 W , and | R| < 2 W, calculate the “nominal” output voltage and the maximum output voltage variation. If a 10-bit ADC with an input range from to 5 V is used to digitize the output voltage, how many effective bits of resolution are available to measure the variation in resistance? ME365 Bridge Circuits Slide 8 Find the relationship between V S and V O : • The bridge is balanced when V O = 0, i.e., Bridge Circuits V e e Z Z Z V Z Z Z V O A C S S 3 2 3 4 1 4 Z Z Z Z Z Z Z Z 1 4 2 3 1 2 4 3 or V s V o + A C ME365 Bridge Circuits...
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This note was uploaded on 12/26/2011 for the course ME 365 taught by Professor Merkle during the Fall '07 term at Purdue.

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lec09_bdgc - ME365 Bridge Circuits Slide 1 Bridge Circuits...

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