MECH 373 Instrumentation and Measurements Lecture 18 Measurement of Solid-Mechanical Quantities (Chapter 8) Strain Gauge Signal Conditioning

Strain measurements Measuring gauge resistance before and after loading – Not practical - Significant error– Resistance change in the order of measuring device resolution. Better approach – Device to measure the resistance change rather than resistance, i.e., Wheatstone bridge.

Wheatstone Bridge Four identical resistors. Voltages at points B and D – same – V0 = 0.0, bridge = balance. Active strain gauge R3 , Load R3 – bridge is unbalanced and V0 ≠ 0.0. Adjust R2 – equal to R3, then V0 = 0.0 – bridge is balanced – Amount of adjustment = change of resistance in R3. DAS.

Strain measurements Modern computer DAS – Balance the bridge initially. No need to re-balance it when the active gauge under-strained . Output voltage proportional to change in gauge resistance. (later slides)

Wheatstone bridge

Strain measurements

Strain measurements

Strain measurements

Strain measurements In practice, gauges used are not truly identical. Uncertainty in an order of 0.1%. Vo – nonzero even before the strain applied – Offset – Symmetric error. Adjust the resistor to bring the bridge into initial balance. If not – subtract it from the actual readings.

Strain measurements To improve the sensitivity of circuit for a given small strain. Application – Symmetrical loading situations – Same strain but opposite sign – tension and compression – e.g. sensor – load cells. R1 and R3 – Tensile gauges. R2 and R4 – Compressive gauges. Full bridge = 4 times the quarter bridge.

Temperature compensation

Temperature compensation Affect resistivity, hence gauge factor S = dR/R/εa. Cause differential thermal expansion = structure and gauge, hence strain without mech. load. Minimize the temp effects: Connect R2 and R3 in structure with the same temperature., R3 = Active gauge, R2 = dummy (compensating) gauge – Temp change on both R2 and R3 in same direction, hence no strain. (Note, R2 can be active gauge if strain is equal and opposite to R3.)

Strain gauge in practical set-up

Strain gauge in practical set-up Required many gauges to measure structure strain/stress. Separate bridge for each gauge. Connect output terminals of each bridge to input channels of DAS. Use common power supply. R1 and R4 = Dummy gauges. Each active gauge paired with a fixed gauge R2 (compensating or active). Bridge always complete. Continuous recording or switch to screen for monitoring (critical gauges)

Example – Quarter bridge

Example Full bridge