Magnetostrictive torque sensor and its output characteristics

Abstract

A magnetostrictive torque sensor which can be installed in an automobile transmission to measure torque has been developed. The sensor shaft consists of two grooved sections. An ac bridge circuit formed by two coils surrounding the grooved sections and two fixed resistors is used to detect torque. The shaft material is a newly developed Cr–Mo steel. The sensitivity of the sensor is stable over a wide temperature range. However, the zero point of the sensor output drifted as much as 10% of the full scale of the sensor when the temperature was uniformly raised by 50 °C. To solve the temperature drift problem, the output characteristics of the bridge circuit were examined. It was found that the output signal (sinusoidal wave) from an initially balanced bridge consisted of torque and temperature components. Each component showed a constant phase angle. The torque phase angle was a fixed value whereas that of the temperature was controllable. A method was devised for separating the temperature phase angle from the torque one by about 90° and then eliminating the temperature component from the output signal. The resulting temperature drift of the sensor was less than 1% of the full measurement scale per 50 °C. This paper describes the shaft material, magnetostrictive properties of the shaft, torque and temperature dependence of coil impedances and output characteristics of the bridge circuit.