Suppression of Rotor Fluctuation and Oscillatory Motion of Bearingless Motors by Using Frequency Domain Inverse Transfer Function

In machines with oscillatory motion, mechanical bearings or cams are subjected to heavy loads, which may lead to their wear and tear. To solve it, an oscillatory motion mechanism using radial position control of a bearingless motor has been proposed by the authors, and it has been confirmed that the oscillatory motion is possible by experiments using a prototype bearingless machine. Feedforward control is necessary to compensate for delays in feedback control for highly accurate trajectory of oscillatory motion, but it is difficult or time-consuming to accurately identify the dynamics of system including mechanical imbalance of the rotor shaft for feedforward control. In this paper, therefore, a method for obtaining the frequency domain transfer function and applying its inverse transfer function for compensation is considered. Two methods are proposed: one is a periodic calculation method similar to repetitive control, and another is a real-time compensation update method. It is confirmed by the experimental tests using a prototype machine that both methods improve trajectory accuracy and are also applied to suppress the rotor fluctuation.