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Torqsense transducers from Sensor Technology are playing a role in product development at Gloucestershire-based Lister Shearing, a supplier of animal-shearing and clipping equipment.

The sensors, which are based on SAW (surface acoustic wave) technology, are being used by the company as an aid to evaluating the performance of miniature electric motors.

To address motor-selection issues and, in particular, to assist in the development of a handheld sheep shear, Lister Shearing decided to set up a motor test rig that would allow the performance of motors from various suppliers to be compared quickly and easily.

A key requirement for this rig was that it should be able to accurately determine the torque produced by each motor and its speed under a range of operating conditions.

Torqsense sensors depend on surface acoustic wave transducers that comprise of two thin metal electrodes, in the form of interlocking fingers, on a piezoelectric substrate such as quartz.

When an RF signal of the correct frequency is applied to the transducer, surface acoustic waves are set up and the transducer behaves as a resonant circuit.

The essential feature, however, is that, if the substrate is deformed, the resonant frequency changes.

When the transducer is attached to a motor drive shaft, the deformation of the substrate and, hence, the change in resonant frequency is related to the torque applied to the shaft.

In other words, the transducer, in effect, becomes a frequency-dependent strain gauge.

Since the transducers operate at radio frequencies, it is easy to couple signals to them wirelessly.

Hence, Torqsense sensors that incorporate the SAW transducer technology can be used on rotating shafts and can provide data continuously without the need for brushes and slip rings.

Torqsense sensors are available in versions that can be used at shaft speeds well in excess of 20,000rpm and thus easily meet Lister Shearing’s requirements.

In addition, they have a high overload margin, operate equally well clockwise and anticlockwise, and feature integral temperature monitoring.

The sensors also provide accurate data about speed and power.

In this application, the sensor was complemented by Sensor Technology’s Torqview software package that runs on a PC and combines data acquisition with a real-time display of torque, motor shaft speed, power and temperature.

Torqview also stores the data it captures and offers powerful tools for its subsequent analysis.

Sensor Technology

Sensor Technology are manufacturers of TORQSENSE Transducers, the world’s first low cost non-contact rotary torque transducers designed for OEM applications. Rotary torque measurement has always been difficult and expensive.

The patented method uses a surface acoustic wave device as a frequency dependent strain gauge and measures the change in resonant frequency caused by the applied strain in the shaft.

The signal is transmitted via an RF couple from the rotating shaft to a fixed pick-up.

By using a frequency-based device, the signal bandwidth is increased, and the problem of electronic interference common with analogue signals is eliminated. The torque sensors are designed to operate direct from a PLC or a PC.

They require minimum length of shaft, have low inertia, no physical contact between shaft and housing, wide bandwidth, high resolution and accuracy resolution to better than one part in a million, and excellent noise immunity.

The technology lends itself to design of OEM transducers for specific customer applications. Applications include automotive, manufacturing machines, condition monitoring where knowledge of torque is critical, torque control of tightening procedures, and monitoring of viscosity during mixing where consistency is required. The technology replaces existing types of rotary torque sensors by providing better performance at a lower price.

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