Non-contact rotating shaft torque measurement

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

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

And to make matters worse, they add drag to the load on the shaft so altering the torque and allowances have to be made.

But such problems are rapidly becoming history as Oxford-based Sensor Technology introduces its non-contact torque sensor to an eager market.

The technology community in the UK and further afield have found a 21st Century solution to the age old problem of monitoring torque in a rotating shaft.

It is a new low cost, non-contact, rotary torque transducer, TorqSense, from Sensor Technology that is proving as robust and simple 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.

Now TorqSense is able to withstand the rigours of very much more hostile environments, so is extending the benefits of torque monitoring to a far wider field.

The latest version of the sensor, the RTW 310/320 series, is designed for serial manufacture, easy installation and robust performance; not surprisingly it is in great demand - mainly from first-time users.

TorqSense operates by using Surface Acoustic Wave (SAW) effects, the properties of which were demonstrated in 1885 by the English physicist Lord Rayleigh.

His original interest the phenomenon was based on the fact that he thought it was a major component of earthquake vibrations, this was confirmed in the 1920s with the advent of seismic recordings.

Some 45 years later, Professors Voltner and White of the University of California generated SAWs on the free surface of quartz.

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

The programme showed much early promise, with a prime example of its potential demonstrated in 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 new TorqSense RWT 310/320 series is opening up rotary torque measurement to a diverse spread of applications.

In many industrial fields a knowledge 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 the monitoring and control of viscosity during mixing processes.

The take-up of the new technology has been impressive, with TorqSense products replacing existing types of rotary torque transducers by providing better performance at far more affordable costs while offering the great advantage of being mechanical simple and not having to physically contact the shaft being monitored.

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

TorqSense overcomes these problems in a novel way, by using a radio frequency transmission link rather than a mechanical one.

The sensors are essentially two frequency resonating Surface Acoustic Wave (SAW) devices which 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 radio frequency (RF), allowing non-contact, intrinsically safe torque measurement.

Significantly the SAWs are piezoelectric so have miniscule power requirements.

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

Thus, TorqSense allows the shaft to be monitored in a non-contact way.

Resonating frequencies can be selected anywhere from 100 to 1000MHz.

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.

TorqSense transducers 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 only a minimum shaft length, have low inertia, no physical contact between shaft and housing, wide bandwidth, high resolution, high accuracy, are mechanically simple and have excellent magnetic/RF noise immunity.

The units are mechanically straightforward and as such they stand to completely redefine the expectations of machine builders, control engineers and OEMs, who to date have had to put considerable time and expense into obtaining the accurate torque readings essential for modern machine control and production monitoring requirements.

With so many advantages over the traditional rotary torque measurement solutions, it is easy to see why the technology is generating so much interest in industrial applications.

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.

TheTorqSense RWT 310/320 series features integral electronics with outputs for torque, speed, power and angle.

The RWT 310 provides analogue outputs only whereas the RWT 320 provides both analogue and digital outputs, the latter allowing PC Interfaces such as serial and USB to be used for easy programming.

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

The TorqSense RWT 310 /320 series can operate from a wide range of supply voltages.

Back to top