An Iterative Periodic Time Difference Detection Method for High Sensitivity Eddy Current Sensor

Abstract

In this paper, a new non-contacting strain sensing method applying an iterative periodic time difference detection to eddy current sensor is proposed. The iterative periodic time difference detection means that the time difference between zero cross points of sensing coil voltage is accumulated repetitively. Proposed method is characterized as a simple sensor which consists of very small size coils, oscillator circuits, and a microprocessor. Previously, the periodic time difference caused by inductance changes has been detected by a synchronous amplifier, especially a lock-in amplifier. The new method proposed in this paper will be equivalent to the synchronous detection technique, in which the lock-in amplifier is needless. The accumulation can be held by repetitive drive of the oscillator circuit which consists of sensing coil connected to a capacitor in series or parallel. In the experiment, a torque test unit is used, and torque which causes up to 0.2% torsional strain in test pieces is applied. For test pieces, aluminum alloys and a stainless steel with a nonmagnetic characteristic are selected. From experimental results, a linear relation between sensor output and applied torque is observed. And it is shown that the proposed method is effective.