Signal Conditioning of Single-Active-Arm Strain Gages Used for Dynamic Stress Measurement

A new strain-gage signal conditioner is described for use in the measurement of dynamic stresses on turbine engine compressor blades. This conditioner has two novel circuits. A new gage excitation circuit corrects for changes in lead-wire resistance by automatically adjusting resistances in series with the leads to the gage. This maintains a balanced configuration that offers better high-frequency common-mode rejection and less broadband noise than with constant-current excitation. Another circuit in the conditioner provides system calibration by shunting a small value resistor that is in series with the gage to produce resistance change that accurately simulates the gage resistance change. The shunting is done at a rate of 1,000 times a second to produce a 1- KHz square-wave calibration signal. The calibration circuit may be used to check on the integrity of the gage and its cabling.