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See SAW measure torque

A Sensor Technology product story
Edited by the Engineeringtalk editorial team Dec 22, 2005

Sensor Technology adds two non-contact torque sensors to its Torqsense range.

Oxforshire-based Sensor Technology has added two non-contact torque sensors to its Torqsense range.

Measuring the torque of a rotating shaft has always been a bit of a nightmare because the contact slip rings tend to be rather fragile and unreliable.

And they add drag to the load on the shaft so altering the torque, meaning allowances have to be made.

The non-contact, rotary torque transducer, Torqsense, is as easy to use a photoswitch or temperature gauge.

Previously torque measurement was a delicate and fiddly operation, and thus confined to labs, test rigs and gentler industrial environments, but Torqsense can withstand the rigours of hostile environments.

The latest versions of the sensor, the RTW310 and 320, are for serial manufacture, easy installation and robust performance.

Each operates by using surface acoustic wave (SAW) effects.

In the early 1990s, as part of a DTI Link scheme, Sensor Technology developed a low cost, non-contact torque sensor based on SAW technology.

This led to the development of an all-electric power steering system for small city cars, where heavy bulky hydraulic power steering would have been impractical.

Now, having worked through two or three generations of the technology, the RWT310 and 320 are opening up rotary torque measurement to other applications, such as: variable speed drive systems, where direct transmission torque feedback can be measured to reduce torsional oscillations, mechanical resonance and fatigue; condition monitoring, for example on CNC machine tools; automated and torque critical tightening processes; and the monitoring and control of viscosity during mixing processes.

Rotary torque has historically been difficult and expensive to measure because traditional techniques are typically invasive to the mechanical systems being measured, that is they have to be mechanically coupled to the test piece through slip rings and thus introduce drag.

Torqsense overcomes these problems by using a radio frequency transmission link rather than a mechanical one.

Its sensors are essentially two frequency resonating SAW devices that change their resonant frequency proportionally to the applied torque in the shaft.

The two SAW devices are fixed onto a shaft to form part of a high frequency oscillator circuit; each device is angled at 45deg to the axis of the shaft.

When the shaft rotates under load it deforms by twisting: this compresses one SAW device and stretches the other, each allowing their resonant frequency to alter and thus changing the frequency difference between them.

This difference in the two frequencies is a measure of the induced strain due to the twisting moment, and from this the torque can be derived.

Interestingly, the sum of the frequency signals is a measure of the shaft temperature.

Transmission of the signals is via an electromagnetic coupling operating at RF levels, allowing non-contact, intrinsically safe torque measurement.

The SAWs are piezoelectric so have low power requirements.

This power is delivered over the RF transmission link simultaneously with the transmission of the output signals.

Thus, it lets the shaft be monitored in a non-contact way.

Resonating frequencies can be selected between 100 and 1000MHz.

Operating at such high frequencies reduces susceptibility to electrical interference.

The high immunity to magnetic fields makes them suitable for use in motors, for example.

Characteristics of the transducer include a resolution to one part in 10 million, a linear response better than 0.1% and a bandwidth in the order of 1MHz.

The transducers can operate direct from a PLC or a PC, removing the need for conventional instrumentation, and will interface with standard DPMs.

The RWT310 provides analogue outputs only, whereas the RWT320 provides analogue and digital outputs, the latter allowing PC interfaces such as serial and USB to be used for programming.

Features include built-in peak torque sampling, data storage and torque averaging, and a self-diagnostics test package.

They can operate from various supply voltages.

Find out more about this article. Request a brochure, download technical specifications and request samples here.
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