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Application of passive SAW resonant sensors to contactless measurement of the output engine torque in passenger cars

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

Proc. of 2007 IEEE Int. Frequency Control Symposium, 29 May-1 June, Geneva, Switzerland, 2007, pp. 499-504.


Modern passenger cars are equipped with the engine management and transmission control systems that require information on the engine output torque as one of the input signals. At the moment it is obtained indirectly from a number of sensors and engine torque model. As a result of production variations and engine wear the estimated torque value lacks accuracy and direct torque measurement is much more preferable for improving the engine and transmission efficiency. There are two viable types of contactless torque sensors that do not require any local dc power source and can potentially be closed coupled to the engine crankshaft. The first one is based on magneto-elastic effect and the second one employs one-port SAW resonators or SAW reflective delay lines. The first type is sensitive to gradients of magnetic field and requires a considerable area on the gearbox input shaft that is not available in the current design of the power train of passenger cars. On the contrary, SAW devices require much less space and can be positioned not on the shaft itself but on the flexplate that connects the crankshaft to the torque converter. The paper is devoted to the development of the contactless flexplate torque sensor based on the backscattering of the RF signal from a number of SAW resonators.

SAW resonant torque and temperature sensors have already been developed for measurements on the shafts. They are based on a sequential wireless measurement of the resonant frequencies of the three SAW resonators in the time domain. Building the flexplate sensor for engine torque measurement requires solving a number of additional problems. The first one is related to a mechanical design of the flexplate itself and selection of the SAW sensing element position in such a way that minimises the influences of bending and parasitic mechanical vibrations on the measurement result. Two versions of flexplates were designed and tested - with a single sensing element containing three SAW resonators and with two sensing elements containing in total five SAW resonators for better cancellation of extraneous forces, couples and vibrations. The second problem is a selection of the resonant frequency measurement sequence and the algorithm for torque calculation that provides the best suppression of the parasitic signals and an acceptable system bandwidth. The last problem discussed in the paper is a design of a large-circumference (close to the wavelength) RF rotary coupler that links the SAW sensors and the electronic interrogation unit. The challenge is to minimise variation of the measured resonant frequencies with the rotation angle. The paper presents static calibration data for the flexplate sensors over the full working range of torques and temperatures from -20° to +120°C. Dynamic test results obtained with real engines show good linearity of the sensor (error is less than 1.5% FS) for the mean torque up to 60% of the maximum torque and speed of rotation from 10% to 60% of the maximum rpm value. Instantaneous measured torque signal easily resolves individual firing pulses. It opens new possibilities in engine management.