Low-cost eddy-current position sensing for bearingless motor suspension control

Abstract

We present a low-cost position sensing module based on a commercial Inductance-To-Digital Converter (LDC) that is applied to bearingless motor suspension control. The sensing module uses four LDC evaluation boards to differentially measure the x-and y-positions of the rotor. Each of the evaluation boards consists of an LDC chip, a sensing coil, and an on-board capacitor. The LDC chip drives the sensing coil and the capacitor to their resonance, and streams out the instantaneous frequency values. As a conductive target approaches the sensing coil, the apparent inductance of the coil decreases due to the induced eddy-current, thereby increasing the resonance frequency. This frequency shift can be used for estimating the distance to the target. The sensing module exhibits a position resolution of 0.37 micrometers with the standard deviation of 0.31 micrometers at a sampling rate of 2 kH. We have implemented this sensing module to our homopolar-biased hysteresis bearingless slice motor that drives a ring-shaped rotor made of D2 steel.