SAW-based transducers for noncontact torque

Sensor Technology has perfected a noncontact torque measuring transducer, which has a potential accuracy of 1ppm.

Sensor Technology has perfected a noncontact torque measuring transducer, which has a potential accuracy of 1ppm.

The transducer, called Torqsense makes use of the surface acoustic wave (SAW) phenomenon.

This SAW approach has allowed the development of small transducers, which are not only exceptionally accurate, but more importantly, require no physical contact between the revolving shaft and its housing.

In a Torqsense transducer, the surface waves are produced by passing an alternating voltage across the terminals of two interleaved comb-shaped arrays that are laid onto one end of piezoelectric substrate.

A similar 'receiving' array, at the other end of the transducer, converts the wave back in to an electric signal.

The frequency of the wave generated is dependant on the spacing of the 'teeth' in the comb array and as the direction of the wave propagation is at right angles to the teeth, any change in its length alters the spacing of the teeth and hence the operating frequency.

Tension in the transducer reduces the operating frequency while compression increases it.

To measure torque in a shaft, two SAW sensors are bonded rigidly to a shaft at 45 degrees to the axis and connected in a "half bridge" configuration, so that when the shaft is subjected to a torque, outputs can be combined to produce either 'difference' or 'sum' signals leading respectively to torque or temperature signals.

The SAW sensors are self-contained and require no physical connection to the shaft's housing.

Instead the signals are exchanged via a capacitive coupling device comprising of two discs, one of which rotates with the shaft while the other is stationary.

The frequency of the exciting oscillation used is typically 100 to 1000MHz, with the frequency shaft produced varying by up to 1MHz.

On this basis, the torque can be measured to an accuracy of 1ppm.

Over a typical bandwidth of 1MHz, the overall accuracy does not vary by more than 0.1%.

Applications include automotive, aerospace, viscosity and consistency measurement, and motor and machinery control and condition monitoring.

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