Development of non-contacting capacitive torque sensor and system for vehicle application

Abstract

The torque/speed relation of an engine is an important parameter for fuel-consumption economy. Optimum fuel economy would be obtained by keeping the engine in the lowest possible gear during acceleration by changing the gear ratio to give increased vehicle speed instead of operating the throttle to increase the engine speed. The throttle should only be operated to increase the power at the lowest possible engine speed. Obviously, the engine speed is not a suitable parameter for adjusting the transmission in such a strategy, so the torque should be used to adjust the transmission ratio for operation within the mechanical limitations [1]. The engine crankshaft, or another axle where the torque is to be measured, is rotating, which strongly favours the non-contact measurement of this torque. An electrical contact with the rotating axle would require the use of slip-rings with a sensor system. Another boundary condition that restricts the range of the possibilities originates from structural limitations. The required mechanical properties of the axle usually prohibit the milling of slots for placing the sensor electronics or a local stricture of the axle for realizing a larger twist angle in the sensor. The milling of slots would be required in non-contact strain-gauge-based torque measurement systems. The electrical power for operation of such a sensor can be supplied to the axle by inductive means and the telemetry of sensor data is also quite feasible. However, practical constraints do not usually permit the weakening of the axle. Moreover, the relatively high engine speed would require a careful mass balancing in the axle to avoid vibrations caused by inertia. Nevertheless, such systems have been implemented in ship axles, where the mass and diameter of the drive shaft and the low number of revolutions per minute permits the mounting of a strain-gauge-based sensor and readout electronics on the axle. A local stricture of the axle can be used to enlarge the torque-induced twist angle. This would facilitate the measurement of the torque using two angular displacement sensors; one on either side of the stricture. As the torque-induced twist angle is inversely proportional to the fourth power of the axle diameter, a significant gain in angular displacement can be obtained; however, structural problems usually prohibit the implementation of these techniques.