Non-contact torque sensor is one in a million

A highly accurate technique for measuring torque without touching the target has been developed by a UK company. Banbury-based Sensor Technology has exploited the little-known phenomenon of surface acoustic waves (SAWs) to produce a sensor with a potential accuracy of one part in a million.

At the heart of the TorqSense sensor is pair of tiny, interleaved, comb-shaped arrays laid onto one end of a piezoelectric substrate. By passing an alternating voltage across the terminals of these arrays, surface waves are produced in the substrate. The waves are received by a similar array at the other end which converts them back into an electric signal.

The frequency of the wave depends on the spacing of the "teeth". The waves propagate at right angles to the teeth, so any change in the length of the transducer alters the spacing between the teeth and hence the operating frequency. If the transducer is in compression, the frequency rises; if it is in tension, the frequency drops.

To measure torque in a shaft, two of these sensors are bonded to the shaft at 45 degrees to its axis. They are connected so that when the shaft is subjected to a torque, their outputs can be combined to produce difference or sum signals, which can be interpreted as torque or temperature values respectively.

The sensors do not need to be connected to the shaft housing. Instead, their signals are transmitted using a capacitive coupling between two discs, one mounted on the rotating shaft, the other on the housing.

The exciting frequency is typically in the range 100-1,000MHz and the frequency shift is up to 1MHz. This allows torque to be measured to an accuracy of one part in a million. According to Sensor Technology, the accuracy does not vary by more than 0.1% over the typical 1MHz bandwidth.