Stability consideration of magnetic suspension in two-axis actively positioned bearingless motor with collocation problem

This paper presents a comprehensive stabilization technique for a two-axis actively regulated bearingless motor. The bearingless motors are required in industries as for use in centrifugal pumps, blood pumps, bioreactors and rotating stages. Two-axis actively regulated bearingless motors have the advantages of low cost and compactness; however, only two degrees of freedom are actively regulated in the radial x-y directions, and thus a tilting motion sometimes causes serious interference with the stability of the radial position control system. The interference occurs when the following three axial positions are not exactly collocated: (i) the center of gravity of the rotor, (ii) the application point of the suspension force, and (iii) the axial position of the displacement sensors. With mathematical analysis, a block diagram including the interference is shown. In addition, stable suspension is successfully achieved by means of adjustments of an integral gain, even though the application point is located on the opposite side from the sensor axial position. Magnetic suspension stabilities are evaluated in the analysis and the experiments.