Internal model control for reduction of bias and harmonic currents in hybrid magnetic bearing

Bias and harmonic currents, which are caused by rotor gravity, mass imbalance and sensor runout, are the dominant disturbances of the hybrid magnetic bearing (HMB) systems. To reject these disturbances and realize the low power control, a novel internal model control method based on positive current feedback and repetitive control is proposed. The positive current feedback can tune the levitation position of the rotor adaptively so that the rotor gravity can be supported precisely by the force resulting from the permanent magnet in the HMB. To suppress the harmonic currents caused by the mass imbalance and sensor runout, a novel plug-in repetitive controller is proposed. Compared to the conventional repetitive controller, the low-pass filter is removed from the internal time lag loop outside so as to improve the system stability and reduce the elimination error of the high-order current harmonics. The absolute stability of the whole closed-loop system is analyzed by using the Nyquist criterion to the time-delay system with the help of the regeneration spectrum method. Upon the phase-frequency characteristic, phase compensators in low, middle, and high frequencies are designed, respectively. Both simulations and experiments are carried out to demonstrate the validity of the proposed control method.