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Machine Design

Torque sensors are small but accurate

Some noncontacting torque-measuring methods rely on resistive strain gages and transmit data coded in voltage changes.

A more accurate method from Wentechnology, Raleigh, N.C., uses surface acoustic waves (SAW) for sensitivities two orders of magnitude better than those of resistive gages. And it codes data using frequency changes, which gives it more bandwidth and better noise rejection.

In the setup, two small SAW transducers are bonded to the shaft at 45 to its rotating axis. In each transducer, alternating current travels through two comb arrays layered over a piezoelectric substrate and produces surface waves. A similar, receiving array at the other end of the transducer converts these waves back into electric signals. The transmitted waves' frequency depends on the distance between the comb teeth and travel at right angles to the teeth. Therefore, any change in shaft length alters the teeth spacing and operating frequency. Tension increases the frequency while compression reduces it. Circuitry takes outputs from both SAW transducers and converts it into torque information.

Signals travel from the shaft to a control unit via capacitive coupling through two discs, one rotating on the shaft while the other is stationary. Frequency shifts due to torque are on the order of 1 MHz and the excitation frequency is typically 200 MHz. This lets the device determine torque levels with a resolution of one part in a million, according to the company. Data is exchanged using frequency-based signals, so noise and electronic interference is minimal

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