SAW-based torque transducer sets new standard

The new 310 Series Torqsense transducer is set to provide high-accuracy low-cost noncontact torque measurement of drive shafts and other rotating machine elements.

Surface acoustic waves, first observed by the possibly the coolest-ever Victorian gentleman-scientist Lord Raleigh when he was caught in an earthquake, are about to make some radical changes to 21st century machine design and control engineering.

They are the key to a new low-cost, noncontact, rotary torque transducer introduced by Sensor Technology at the M-TEC Exhibition.

120 years is a long time to wait, but Lord Raleigh is probably now happy to know that the most adventurous moment of his Grand Tour has finally lead to a technology which looks set to open up a whole new world of possibilities, solving many traditional problems and acting as a catalyst in the development of innovative new systems.

Sensor Technology has harnessed surface acoustic waves (SAW) in its Torqsense range of rotary torque measurement transducers.

Versions of the transducer have been available for a few years, each one being custom made to customer order.

But now with a host of proven applications and satisfied customers, the new 310 Series Torqsense has been designed for serial manufacture and for plug-and-play installation and use.

As such, the 310 Series is set to provide high-accuracy low-cost noncontact torque measurement of drive shafts and other rotating machine elements.

The fundamental advantages over other traditional torque measurement methods include doing away with slip rings and other cumbersome mechanisms used to connect wires to rotating bodies; no load exerted onto the shaft by the sensor, and savings in both equipment cost and man hours.

The existence of surface acoustic waves was first noted in 1885 by Lord John William Strutt Raleigh, Englishman, Physicist and (perhaps above all) Gentleman.

His theory that such waves were a major component in the destructive power of earthquakes developed from his own rather alarming personal experiences while travelling abroad to broaden his education.

Proof, however, was not was forthcoming until the advent of seismic recording in the 1920s.

Some 45 years later, when microelectronics was the hottest topic on any campus, Voltner and White of the University of California generated such waves on the free surface of quartz.

In the early 1990s, news leaked out that Sensor Technology of Oxfordshire was developing a low cost torque sensor based on SAW technology as part of a DTI- Link scheme.

The technology showed much early promise, with a prime example of its potential demonstrated in its use in the development of an all-electric power steering system, developed for small "city" cars, where the use of heavy, bulky hydraulic power steering would have been impractical.

Through the 1990s and into the new century Sensor Technology steadily built up a catalogue of applications, each one contributing to a body of knowledge that has led to the launch of the 310 Series Torqsense transducer.

This combines all the specialist knowledge of the principles of SAW into a package that is simultaneously robust enough for everyday industrial use, easy to install and use, and is able to communicate directly with a PC to give real-time data logging of torque performance of critical machine and plant drive performance.

Significantly SAW technology is expected to open up new application areas where precise determination of torque is critical.

These include: variable speed drive systems, where direct transmission torque feedback can be measured to minimise torsional oscillations, mechanical resonance and fatigue; condition monitoring, for example on CNC machine tools; highly automated and torque critical tightening processes; and monitoring and measurement control of viscosity during mixing processes.

The take-up of the SAW techniques through Torqsense products is expected to be rapid.

It will replace existing types of rotary torque transducers by providing better performance at a far lower cost.

Historically rotary torque has been difficult and expensive to measure because traditional techniques are invasive to the mechanical systems being measured.

Typically the equipment has exerted its own load onto the system under test, requiring complex calculations to discount it from analysis, and has also required the use of slip rings or expensive radio links.

Torqsense overcomes these problems by using SAW technology to provide noncontact measurement, thus delivering inexpensive transducers for situations where monitoring or control of drive mechanisms is required.

The SAW-based transducer is essentially a "frequency-dependent" strain gauge that measures the change in resonant frequency caused by an applied shaft strain.

Two SAW devices embedded on a shaft form part of a high-frequency oscillator circuit.

When the shaft is twisted, the resulting deformation of the substrate creates a frequency difference between two embedded SAW devices.

The two frequencies produced by the SAW devices are mixed together to produce difference and sum signals.

The difference signal is a measure of the induced strain due to the twisting moment, and from this the torque can be derived.

The sum signal is a measure of the shaft temperature.

Coupling of the signals the outside world is via an electromagnetic coupling device, allowing noncontact, intrinsically safe torque measurement.

The primary frequency of oscillation can be chosen anywhere from 100 to 1000MHz, with the difference frequency varying up to 1MHz.

Operating at such high frequencies, the transducers are much less susceptible to electrical interference than conventional torque sensors.

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

Key characteristics of the SAW based transducer include a resolution to 1 part in 10 million, an extremely linear response (better than 0.1%) and a bandwidth in the order of 1MHz.

The Torqsense transducers built using SAW technology are designed to operate direct from a PLC or a PC, removing the need for conventional instrumentation, and will interface with standard DPMs.

Torqsense transducers require minimum shaft length; have low inertia, no physical contact between shaft and housing, wide bandwidth, high resolution, high accuracy and excellent magnetic/RF noise immunity.

With so many advantages over the traditional rotary torque measurement solutions, it is easy to see why the technology is generating so much interest, embedded within innovative new products and systems.

At the same time, the technology is providing a key tool in the development of higher efficiency rotating machines, where development engineers rely on accurate knowledge of torque and rotational speed.

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