Rotary Torque Sensors

Unlike traditional slip ring transducers, the RWT units make use of a simple non-contact measurement technique of a radio frequency (RF) couple for power and signal communication. This means that the units are mechanically straightforward and as such they stand to completely redefine the expectations of machine builders and control engineers, who to date have had to put considerable time and expense into obtaining the accurate torque readings essential for accurate machine control and production monitoring.

They feature integral electronics with outputs for torque, speed, power and angle. The RWT310 provides analog outputs and the RWT320 both analog and digital outputs, which are directly compatible with PC Interfaces such as serial and USB, being user programmable for signal levels. Other innovative features include built-in peak torque sampling, storage and torque averaging, and a self-diagnostics test package. Both the RWT310 and RWT320 can operate from a wide range of supply voltages.

To achieve the non-contact operation that makes the TorqSense transducer range unique, SAW (Surface Acoustic Waves) devices are used as frequency dependent strain gauges to measure the change in resonant frequency caused by strain experienced in the drive shaft. This measurement is directly related to the torque experienced in the rotating machine element.

In use SAW devices are mounted onto the shaft and their output signals are transmitted by a low power radio frequency (RF) couple to an adjacent pick-up, which interprets the signals and converts them into a torque measurement read out or feeds them on into a PC for further calculation.

In a SAW sensor, the surface waves are produced by passing an alternating voltage across the terminals of two interleaved comb-shaped arrays, laid onto one end of a piezoelectric substrate. A receiving array at the other end of the transducer converts the wave into an electric signal.
The wave frequency is dependant upon the spacing of the teeth in the array and the direction of wave propagation is at right angles to the teeth. Therefore any change in its length, caused by the dynamic forces of the shaft’s rotation, 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 the torque in a rotating shaft, two SAW sensors are bonded to a shaft at 45deg to the axis of rotation. When the shaft is subjected to torque, a signal is produced which is transmitted to the adjacent stationary pick up via the capacitive RF couple comprising two discs, one of which rotates with the shaft, the other being static.

The frequency of the oscillation used is typically 200MHz.

The frequency-basis of the TorqSense concept gives a wide bandwidth and the susceptibility to electronic interference, common with other analogue based techniques such as inductive devices, is eliminated.

‘TorqSense’ using the SAW concept was developed, trade marked and patented by twins Anthony and Bryan Lonsdale, founder Directors of Sensor Technology. They have had a range of SAW-based transducers available for about five years, but these have been virtually hand-built and had the control electronics housed separately from the pick-up head.

“Our earlier models were designed to be robust and simple, both in use and for manufacture,” comments Bryan. “This was essential to make it attractive to users across a wide range of industries, who were doing us the courtesy of trying out our radical new technology but who may have been put off by having delicate or complex equipment to deal with.

“With the concept now proven in the real world of factory machines and process plants and commercial laboratories, we have moved on to develop the RWT310/320 Series, in which we have integrated the electronics and the pickup into a single unit and designed everything for serial manufacture.”
The RWT310/320 Series TorqSense will be available with full scale deflection of 1Nm to 20Nm with other sizes to follow with full scales up to 10 000Nm.

“The sequence in which we introduce the variants will depend on market demand, but we are committed to having the complete range fully available by the end of 2004.”