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Pulsed interrogation of the SAW torque sensor for electrical power assisted steering

V. Kalinin, G. Bown, J. Beckley, R. Lohr.

Proc. of 2004 IEEE Int. Ultrasonics, Ferroelectics and Frequency Control 50th Anniversary Conference, Montreal, Canada, 24-27 August 2004, pp. 1577-1580.

Abstract

A contactless SAW torque sensor for electrical power assisted steering (EPAS) working in the 430-437 MHz frequency range is presented in this paper. It contains four one-port SAW resonators on quartz substrates connected to a single rotational coupler. Two of them are attached to one side of the steering shaft and two others are attached to the opposite side in order to compensate an effect of shaft bending on the sensor output. The mechanical part of the sensor is designed in such a way that the torque sensitivity of the difference between two SAW resonant frequencies is approximately 14 kHz/Nm. As a result we simultaneously achieve an acceptable overload protection and accuracy of torque measurement. Temperature compensation technique is also discussed in the paper.

A single stationary interrogation unit is used to measure resonant frequencies of all four SAW resonators attached to the rotating shaft. The interrogation method is based on a pulsed excitation of the natural oscillations of the resonators and their subsequent spectral analysis. The accuracy of the frequency measurement is mainly limited by the phase noise of the clock generator and the frequency synthesisers of the interrogator. The standard deviation of the measured frequency is approximately 600 Hz. It is sufficient to satisfy requirements for the EPAS torque sensor. Unlike a previously used interrogator based on continuous tracking frequency control loop, the pulsed interrogator is more cost-effective because it allows increasing the number of the SAW resonators in the sensor without complicating the interrogator's circuitry. It achieves a 1 ms or shorter torque update period that complies with the EPAS torque sensor specification.