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RF Rotary Couplers for Contactless Torque Sensors Based on SAW Resonators

V. Kalinin.

Proc. of the 22nd European Frequency and Time Forum, Toulouse, France, 23-25 April 2008.


Contactless non-compliant torque sensors employing SAW resonators positioned on either rotating shafts or disks have proven to provide a high accuracy of measurements of temperature compensated torque that is highly competitive with other technologies such as magneto-elastic sensors. SAW torque sensors have demonstrated a potential for high volume applications, e.g. in the automotive industry for measuring torque at the engine output, on drive shafts and on steering shafts for electric power assisted steering (EPAS). An RF rotary coupler connecting the electronic interrogation unit to the SAW resonant sensing elements is one of the key elements of the sensing system and is a significant component of the overall system accuracy. Depending on the application, the required diameter of the RF coupler may vary considerably. The aim of this paper is to present two different designs of the RF rotary coupler. One of them suits EPAS and driveshaft applications with a relatively short coupler circumference of half a wavelength or less, while the other has a circumference of around one wavelength or more and is good for a flexplate torque sensor measuring the engine output torque.

First, requirements for the RF rotary coupler are discussed such as minimal angular variation and minimal standard deviation of the measured resonant frequency. Then the paper presents a theory for RF coupler operation in the sensing system based on pulsed interrogation of the SAW resonators. It allows determining the resonant frequency measured at the stator input from parameters of the SAW resonator connected to the rotor output and coupler S-parameters as well as establishing requirements for the angular variation of the coupler parameters. After that a number of designs of the RF coupler based on annular coupled microstrip lines are presented and their simulation is performed. Two of the designs are selected as particularly suitable for the small and the large diameter applications mentioned above.

Experimental results are presented for the two coupler designs that have been obtained using pulsed interrogation of a 433 MHz torque sensing element with two SAW one-port resonators. The angular variation of the difference between the two measured resonant frequencies does not exceed 1 kHz for both types of the couplers at a fixed gap between the rotor and the stator (it corresponds to the torque measurement error of <0.2% FS in the case of the flexplate torque sensor). The influence of the gap on the angular variation of the measured frequency and its standard deviation is also investigated. It is established that an acceptable performance of the large-diameter coupler is achieved within a sufficiently wide range of gaps from 1.5 to 4.5 mm. Finally, the influence of contaminants filling the gap between the stator and the rotor on the sensor performance is studied.