Characterization and Verification of Eddy-Current Position Sensing for Magnetic Levitation

Magnetically levitated drives are compelling in applications, where the long-lifetime operation or process chamber encapsulation is a requirement. To exploit these advantages, contactless position sensors are needed to estimate the position of the rotor. Off-the-market sensor technologies render high-performance magnetic levitation possible, yet their system integration may be challenging if a drive must be designed to fit existing sensor technology or bulky probes. In this work, a position-estimation system based on Eddy-current generation is proposed. Two integrated circuits (or only one for less time-critical applications) excite a two-axis differential array of four miniature coils that can resolve positions in the 10$^0$ $\mu$m range. Coupled to a microcontroller, this system can sample the position of an electrically conductive target—in this study, the permanent magnet rotor of a magnetically levitated drive—with frequencies of over 3.5 kHz. This position estimation setup enables the successful levitation of two miniature bearingless disc drives and offers potential toward rotatory speeds in the 20 kr/min range.

I. Introduction

Magnetic levitation (mag-lev) enables the contactless operation of electric drives, thus broadening the scope of their applications. Nevertheless, magnetic levitation can deliver its full potential and be economically competitive only when the complete drive system is seamlessly integrated [1]. In this domain, sensor technologies are of paramount importance. They permit the estimation of the position of the magnetically levitated rotor in a chamber or its air gap, often by “seeing through” other materials, such as plastic or even electrically conductive [2] encapsulations.