lec23_handout - ME365 Bridge Circuits 1 Bridge Circuits...

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Unformatted text preview: ME365 Bridge Circuits 1 Bridge Circuits & Variable Impedance Devices • Strain Gages • Bridge Circuits • Variable Impedance Devices ME365 Bridge Circuits 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 3 Expand resistance R into a Taylor series and ignore higher order terms: Define Gage Factor G f : Strain Gage ܴ ൌ ܴ ଴ ൅ Δܴ where Δܴ ൌ ߲ܴ ߲ܮ Δܮ ൅ ߲ܴ ߲ܣ ΔA ൅ ߲ܴ ߲ߩ Δߩ Resistance of wire: ܴ ൌ ߩܮ ܣ ߩ : Resistivity ܮ : Length of wire ܣ : Cross-sectional area ME365 Bridge Circuits 4 Strain Gages • Metallic Strain Gages: – Gage Factor: G f = 2.0 ~ 2.2 – R = 120 Ω ± 1 Ω . – Δ R = − 2.4 Ω ∼ 4.8 Ω . – 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 Ω ± 5 Ω . – High sensitivity to temperature change. ME365 Bridge Circuits 5 • Measure Resistance (Impedance) Variation – Use constant voltage supply and measure current variation.– Use constant voltage supply and measure current variation....
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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 University.

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lec23_handout - ME365 Bridge Circuits 1 Bridge Circuits...

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