Hysteresis Bearingless Slice Motors with Homopolar Flux-biasing

Abstract

We present a new concept of bearingless slice motor that levitates and rotates a ring-shaped solid rotor. The rotor is made of a semi-hard magnetic material exhibiting magnetic hysteresis, such as D2 steel. The rotor is radially biased with a homopolar permanent-magnetic flux, on which the stator can superimpose two-pole flux to generate suspension forces. By regulating the suspension forces based on position feedback, the two radial rotor degrees of freedom are actively stabilized. The two tilting degrees of freedom and the axial translation are passively stable due to the reluctance forces from the bias flux. In addition, the stator can generate a torque by superimposing six-pole rotating flux, which drags the rotor via hysteresis coupling. This six-pole flux does not generate radial forces in conjunction with the homopolar flux or two-pole flux, and therefore the suspension force generation is in principle decoupled from the driving torque generation. We have developed a prototype system as a proof of concept. The stator has 12 teeth, each of which has a single-phase winding that is individually driven by a linear transconductance power amplifier. The system has four reflectivetype optical sensors to differentially measure the two radial degrees of freedom of the rotor. The suspension control loop is implemented such that the phase margin is 25° at the cross-over frequency of 110 Hz. The prototype system can levitate the rotor and drive it up to about 1730 r/min. The maximum driving torque is about 2.7 mNm.