Model development and harmonic current reduction in active magnetic bearing systems with rotor imbalance and sensor runout

Harmonic currents, which are caused by rotor imbalance and sensor runout in active magnetic bearing (AMB) systems, can induce undesirable harmonic vibrations and superfluous power consumption. To analyze and reduce these harmonic currents, a comprehensive model of the AMB system is developed and a repetitive control method is proposed. First, dynamics of the four radial degrees-of-freedom rotor with the rotor imbalance and the sensor runout are introduced, and electrical equations of the AMB control system including power amplifiers and motion induced voltage (MIV) are described. Next, how synchronous and multiple harmonic vibration forces and torques, which result from the harmonic currents through both the current stiffness and the displacement stiffness, are induced by static imbalance, dynamic imbalance, and the sensor runout through controllers and the MIV, are explained and analyzed in detail. Then the AMB system is divided into two subsystems related to translational and rotational motions, respectively. The dynamic equations for the two coupled rotational motions of the rotor are combined into a complex function. The rotor imbalance and the sensor runout are transformed to input disturbances of the power amplifiers, and a repetitive control method is proposed to suppress these periodic disturbances by reducing the harmonic currents. Finally, the validity of the proposed method is demonstrated by simulations with MATLAB and experiments on a test rig of a magnetically suspended control moment gyro. It is superior to existing techniques due to the comprehensive AMB model and the effective control method with a short computation time.