Control of Magnetic Bearings Using the Observer for Unbalance

An unbalance on a rotor causes whirling motions of the rotor and alternating forces of the bearings in rotating machines. The authors have shown a magnetic bearing control system in which an unbalanced rotor can be suspended without whirling. The concept of designing the control system was that the effects of unbalance were estimated by an observer and cancelled by the electromagnetic forces of the bearing. With the control method applied, the position of the rotor can be maintained at the desired position with high precision. It is suitable for some machines, however, to minimize the variation of exciting currents or vibratory force transmitted to the base through the bearing. In this paper, control methods are presented which can eliminate stationary alternating components from exciting currents or electromagnetic forces by using the output of the observer. Another control method is presented which feedback the estimated signals of the displacements of the principal axis; such signals are obtained by compensating synchronous obsevatory disturbances by use of the output of the observer. It is shown that the proposed three control methods are equivalent when unbalance stiffness due to the bias magnetic flux is null.