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TorqSense Technology

The technology for our 'TorqSense' RWT310/320 and RWT330/340 Series Torque Transducers is based on the patented technology of measuring the resonant frequency change of 'Surface Acoustic Wave' (SAW) devices in a non-contact manner when strain is applied to a shaft to which the SAWs are fixed. The applied torque causes a deformation of the quartz substrate of the SAW device, which in turn causes a change in its resonant frequency. Essentially the SAWs act as 'frequency dependent' strain gauges.

The frequency signal measuring the resonant frequency is coupled via a non-contact RF rotating couple from the shaft to a fixed pick-up and it is the analysis of the difference in resonant frequencies between the two SAW devices, with electronic processing and calibration, that gives a precise indication of the torque transmitted by the shaft. SAW devices have a high immunity to magnetic fields allowing their use in motors, for example, where other analog technologies are susceptible to electronic interference and are not suitable.

The use of this patented measurement technique results in a transducer being able to sense torque bi-directional, with fast mechanical and electrical responses. As the method is non-contact it has also complete freedom from brushes or complex electronics, which are often found in traditional torque measurement systems.

In more detail...

Surface Acoustic Waves (SAW), sometimes called Rayleigh waves were quantitively described in 1885 by Lord Rayleigh, when he showed theoretically that waves can be propagated over the plane boundary between an elastic half-space and a vacuum, or a sufficiently rarefied medium (eg. air), where the amplitude of the waves decay rapidly with depth. They are mechanical (acoustic) rather than electromagnetic and much of an earthquake's destructive force is carried by this type of wave. Surface waves achieved little recognition for their application in RF until less than two decades ago when SAW devices began to be developed for spread spectrum use in military radar equipment. From these beginnings an exciting new technology has evolved for RF signal processing applications.

In its simplest form, a SAW transducer consists of two interdigital arrays of thin metal electrodes deposited on a highly polished piezoelectric substrate such as quartz. The electrodes that comprise these arrays alternate polarities so that an RF signal of the proper frequency applied across them causes the surface of the crystal to expand and contract and this generates the surface wave. These interdigital electrodes are generally spaced at ½ or ¼ wavelength of the operating centre frequency. Since the surface wave or acoustic velocity is 10^-5 of the speed of light, an acoustic wavelength is much smaller than its electromagnetic counterpart. For example, a signal at 100Mhz with a free space wavelength of three metres would have a corresponding acoustic wavelength of about 30 microns. This results in the SAW's unique ability to incorporate an incredible amount of signal processing or delay in a very small volume. As a result of this relationship, physical limitations exist at higher frequencies when the electrodes become too narrow to fabricate with standard photolithographic techniques and at lower frequencies when the devices become impractically large. Hence, at this time, SAW devices are most typically used from 10Mhz to about 3Ghz.

The operation of a SAW transducer for strain measurement depends on the choice of a suitable piezoelectric substrate, which can be attached to the material to be stressed. The stress results in a strain, which can be in tension or compression. The sensitive axis of the transducer is longitudinal in the direction of wave propagation. Strain will change the spacing of the interdigital electrodes and hence the operating frequency. For an excitation frequency of 500Mhz, 1000 µ-strain of tension will decrease the frequency by 500 kHz; conversely a compressive strain will increase the frequency by the same amount. To function as an oscillator, the element is used as amplifier feedback. The Q factor of the transducer is high - typically 10⁴. Therefore, by meeting the phase and gain requirement, the circuit will oscillate with very high stability - typically one part in 10⁹. From the technique described it is apparent that the output signal will be in the frequency domain. This has many advantages for a number of applications, particularly in variable speed electrical machines where RF signals cannot be easily contaminated by drive electronic noise.