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Contactless Torque and Temperature Sensor Based on SAW Resonators

V. Kalinin, G. Bown, A. Leigh.

Proc. of 2006 IEEE Int. Ultrasonics Symposium, 4-6 Oct. 2006, Vancouver, Canada, pp. 1490-1493.


Contactless torque sensors play important part in a number of automatic control systems, in particular, electrical power assisted steering system of a vehicle. First SAW torque sensors based on two resonators oriented at 45° to the shaft axis were demonstrated 15 years ago. Variation of a difference between the two resonant frequencies was used to measure torque. However differential measurement did not allow achieving good temperature compensation due to the fact that temperature characteristics of SAW devices depend on biasing strain. Variation of the frequency offset and torque sensitivity with temperature can be minimised by tightening tolerances and careful selection of the substrate cut and SAW device orientation but it cannot be fully removed. As a result accurate torque measurement also requires simultaneous temperature measurement. Initially it was hoped that a sum of the two resonant frequencies could be used to measure temperature. However the accuracy of such measurement would be greatly affected by mechanical tolerances and angular variation of the phase response of the RF rotary coupler connecting the SAW sensor to the electronic interrogation unit. The same is applicable to the torque and temperature sensor based on phase measurements in two reflective delay lines that was demonstrated in 1996.
The paper is devoted to a torque and temperature sensor that uses difference between resonant frequencies to measure both torque and temperature in order to minimise influence of the RF rotary coupler. The sensing element is based on three SAW one-port resonators made on a single Y+34° cut quartz substrate. Two of them are at 45° to the shaft axis, which is either parallel or normal to the X-axis of the substrate. Resonant frequencies are in the range from 433 to 437 MHz. All resonators are connected in parallel to an input of the rotor ring of the RF coupler. Interrogation of the resonators is performed in the time domain by short RF pulses. A very compact electronic interrogator based on a specially designed RF ASIC performs spectral analysis of SAW responses. One of the measured frequency differences depends mainly on torque but the temperature variation of the offset can be as high as 8% of full scale and the torque sensitivity variation with temperature can be up to 7%. Another frequency difference depends mainly on temperature with the sensitivity of approximately 3 kHz/°C but it also depends on torque with the sensitivity that is at least seven times smaller than the torque sensitivity of the first frequency difference. A calibration model and an algorithm for calculating torque and temperature from the measured frequency differences are suggested. Experimental results show that the torque measurement error does not exceed 1% of full scale within the range from -40° to +125°C and the temperature measurement error is less than 1° at any angle of the shaft.