Non-contact torque sensors based on SAW resonators

J. Beckley, V. Kalinin, M. Lee, K. Volyansky.

Proc. of 2002 IEEE International Frequency Control Symposium. New Orleans, USA, May 29-31, pp. 202-213. (invited)

Abstract

Most conventional sensors measuring torque on a rotating machine shaft use either piezoresistive or piezoelectric strain gauges requiring electrical contacts between the shaft and a stationary electronic interrogation unit. The contacts are usually provided by slip rings that are the least reliable part of the system. Torque transducers utilising SAW devices (either reflective delay lines or one-port resonators) as strain sensors represent a very attractive alternative since they are passive and work at frequencies from 30 MHz to 3 GHz so the signals can be easily transmitted to the shaft either wirelessly or through non-contact RF couplers. Implementation of the SAW torque sensors was discussed in a number of publications starting from the early 90s. However, these publications did not cover such important aspects as limitations on the accuracy of the sensors as a result of their non-contact interrogation and the temperature stabilisation of their sensitivity to torque. Our paper is mainly devoted to these two aspects.

First, advantages and disadvantages of SAW reflective delay lines and one-port resonators as strain sensing elements in the torque transducer are discussed and then the attention is focused on SAW resonators since they have lower insertion loss and higher Q-factors for the same size substrate. Results of the finite element analysis of the torque shaft with the SAW sensors attached are presented and the strain dilution and its variation with temperature is discussed. Methods of the SAW sensor interrogation are reviewed and the system performing a continuous tracking of the resonant frequencies of the two SAW resonators is selected for further investigation. The system includes two contactless rotational RF couplers connected to the resonators. A detailed theoretical model of the interrogation system is developed and the results of computer simulations are presented. They allow us to establish a relationship between the amount of noise in the system, the Q-factor of the resonators, parameters of the frequency tracking system, the system bandwidth and the system resolution. For practical systems the last parameter can achieve values below 0.1 Nm. Analysis of the RF rotational couplers and experimental results show that mechanical tolerances give rise to an angular aberration, i.e. dependence of the torque reading on the angular position of the shaft. Methods of reducing the angular errors are suggested.

Experimental data show a variation of the torque transducer sensitivity with temperature. For SAW resonators fabricated on ST-X cut of quartz it can reach 0.2% per 1°C. The main cause of this is a change of the turnover temperature of the SAW resonator with strain. A theoretical modelling of the SAW propagation in the quartz substrate taking into account both static strain and temperature variation shows that the temperature variation of the third-order elastic constants of the substrate makes the most important contribution to this effect. A method allowing us to reduce the temperature variation of the sensitivity is suggested.