Eddy-Current Displacement Transducer With Extended Linear Range and Automatic Tuning

Abstract

Design details and measurement results of the eddy-current displacement transducer with extended linear range and automatic tuning are presented. The transducer is based on a resonant impedance inversion method of transfer curve linearization where the displacement probe circuit is kept in resonance by the resonance control loop. The transducer exhibits an extended linear range due to the compensation of displacement probe losses by a negative impedance converter (NIC) at the transducer input. Particular attention is paid to the NIC design and its temperature compensation. The new voltage-controlled NIC circuit is introduced, which can easily be realized with a standard commercial integrated circuit (LM 1496). Details of the transducer's reference generator and the NIC control voltage generator are also presented. The transducer's linear range extends from 0.25 to 3.75 mm (approximately 44% of the 8-mm probe diameter) while maintaining nonlinearity within plusmn2% F.S. This result is, at least, a 75% improvement to the best commercially available eddy-current displacement transducers (a linear range of 80 mil ~2 mm for an 8-mm probe). Further linearization can be achieved by postlinearization of the detected signal.