Precision control for rotation about estimated center of inertia of spindle supported by radial magnetic bearing

The object of this study is to develop a radial magnetic bearing which has nanometer rotational accuracy about a fixed center when rotating at high-speed. To achieve this objective, the vibration of the spindle caused by the unbalance force must be reduced. In this paper, we propose a method of control to reduce the vibration of the spindle. First, the center of inertia is estimated from measured coil currents and the rotational speed of the spindle. Subsequently, the references of the control system are modified on the basis of the estimated eccentricity, so that the spindle is constrained to rotate about the estimated center of inertia. Experimental results show that the spindle has a rotational accuracy of 13.2 nm (3sigma) at 2 000 min(-1) and that the electromagnetic force synchronized with the rotation is 0.17 N, which is 8.9% of the electromagnetic force when rotation is about the center of the profile.