AC Excited Wheatstone Bridge

[tangodirt]

I have a torque meter using a full-bridge Wheatstone bridge (all legs are strain gauges). The bridge has some non-zero, unknown offset (it's old). I would like to strip this offset out prior to signal amplification.

My theory is, under DC conditions, the offset exists, which mucks up the output response (need compensation). If I excite the bridge with AC, will the offset appear as a DC offset present in the response sinusoid? Then, with applied torque, the amplitude of the response signal will change, while the offset is now decoupled from the response sinusoid?

Then, will applied load cause a further offset of the AC signal, or will it cause an amplitude change of the sinusoid? Ideally, I want to automatically strip out the DC offset (AC coupled signal with carrier wave) while preserving the displacement data.

Am I close here or way off in left field? Before I start working with components, I want to make sure my theory is correct. Thanks!

 

[Baluncore]

Sorry about the delay. The offset will also appear in an AC driven bridge.

First inspect the bridge elements for damage. Replace all sensor elements with new matched elements.

The unit may have been strained by an overload, in which case dismantling and reassembling the gauge may relax the bridge elements and correct the problem.

To remove an offset error you can trim the bridge by adding a high value resistor in parallel with one of the resistive bridge elements. Experiment with values and position to optimise the correction.

You will need to recalibrate the gauge after any repairs.