AC Excited Wheatstone Bridge

In summary, the conversation discusses a torque meter with an unknown offset in its full-bridge Wheatstone bridge. The speaker suggests using an AC excitation to decouple the offset from the response sinusoid and preserve displacement data. They also mention the possibility of adding a high value resistor to trim the bridge and the need for recalibration after repairs.
  • #1
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!
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  • #2
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.

1. What is an AC Excited Wheatstone Bridge?

An AC Excited Wheatstone Bridge is an electrical circuit used for measuring unknown resistances. It consists of four resistors arranged in a diamond shape, with an AC voltage source connected across one diagonal and a galvanometer connected across the other diagonal.

2. How does an AC Excited Wheatstone Bridge work?

The AC voltage source produces an alternating current, which causes the resistors to alternate between high and low resistance. The galvanometer measures the difference in potential between the two sides of the bridge and indicates when the bridge is balanced, meaning the resistances are equal.

3. What are the advantages of using an AC Excited Wheatstone Bridge?

One advantage is that it can measure a wide range of resistances, from very low to very high. It is also more precise than other methods of resistance measurement, such as using a multimeter. Additionally, the AC current helps to minimize any errors caused by temperature changes.

4. What are the limitations of an AC Excited Wheatstone Bridge?

One limitation is that the bridge can only measure resistances within a certain range. If the resistances are too high or too low, the bridge may not be able to accurately measure them. Additionally, any additional resistance in the circuit, such as from the connecting wires, can affect the accuracy of the measurements.

5. How is an AC Excited Wheatstone Bridge used in practical applications?

The AC Excited Wheatstone Bridge is commonly used in industrial and scientific settings for measuring the resistance of various materials, such as metals and semiconductors. It is also used in electronic circuits to measure unknown resistances and to calibrate other measuring devices, such as thermometers and strain gauges.

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