Rolling motion control of a levitated mover in a permanent-magnet-type bearingless linear motor

A bearingless linear motor(BLLM) has no support elements such as a linear guide or additional linear magnetic bearing. Instead, it levitates a mover by controlling magnetic forces generated between the mover and stators as well as moves it. Its structure is very similar to a both-sided linear motor which is symmetrically combined of a common mover and two stators with permanent magnet arrays. Also the principle for movement is not quite different from that of conventional linear motors. But, to levitate the mover, the amplitudes of phase currents provided to both-side coils are controlled, based on the air gap between mover and stator. In this system, since the coils are divided into two groups in the movement direction and perform the levitation control separately, pitching motion of the mover can be controlled. And lateral translation and yawing motion are stable if the stable levitation were achieved. However, the rolling motion still needs to be stabilized by an additional control. Thus, this paper suggests a method to stabilize the rolling motion by using a pair of small electromagnetic actuators. It includes the actuator design method which is so as not to affect the BLLM performance, and the derivation of the equation of motion for controller design. Some experimental test results show that the rolling motion can be successfully controlled during levitation.