RAYLEIGH WAVE ROTARY TORQUE TRANSDUCERSTORQSENSE Transducers make use of the little known Rayleigh Wave or Surface Acoustic Wave (SAW) phenomenon. This patented method has allowed the development of small transducers, which are not only exceptionally accurate, but require no physical contact between the revolving shaft and its housing.
In a SAW type sensor, the surface waves are produced by passing an alternating voltage across the terminals of two interleaved comb-shaped arrays that are layed onto one end of piezoelectric substrate. A similar ‘receiving’ array, at the other end of the transducer, converts the wave back in to an electric signal.
The frequency of the wave generated is dependant on the spacing of the ‘teeth’ in the comb array and as the direction of the wave propagation is at right angles to the teeth, any change in its length alters the spacing of the teeth and hence the operating frequency. Tension in the transducer reduces the operating frequency while compression increases it.
To measure torque in a shaft, two SAW sensors are bonded rigidly to a shaft at 45 degrees to the axis and connected in a “half bridge” configuration. When the shaft is subjected to a torque, outputs can be combined to produce either ‘difference’ or ‘sum’ signals leading respectively to torque or temperature signals.
The SAW transducers are self-contained and require no physical connection to the shafts housing. Instead, the signals are exchanged via a capacitive coupling device comprising two discs, one of which rotates with the shaft while the other is stationary.
The frequency of the exciting oscillation used is typically 200 MHz, with the frequency shift produced varying by up to 1MHz. On this basis, the torque can be measured to a resolution of one part in a million.
Over a typical bandwidth of 1MHz, the accuracy does not vary by more than 0.1 percent.
Measurement of torque requires the transducer to be placed in line between the load and the drive source. This transducer, being a finite size, will influence the dynamics of the system. The most important mechanical parameters affected will be inertia and compliance. Keeping them low will result in a higher natural resonant frequency, which will enable rapid torque changes to be observed.
The SAW technique has a sensitivity of two orders of magnitude over resistive strain gauges. This enables a stiff, non-compliant structure to be utilised.
The electronic interface is based on frequency change, not voltage level.
This improves the rejection of noise, which is further improved by the operating frequencies (200MHz), and use of wide bandwidth transmission line connection techniques, which can respond to sub microsecond changes.
The type of electronic discrimination used sets the available bandwidth, which can be from a low 5KHz to more than 1MHz, and would far exceed any mechanical system variation.
The main advantages of torqsense...
Exceptional sensitivity
Article submitted by Sensor Technology Ltd.
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