Hybrid Fractional Repetitive Control for Magnetically Suspended Rotor Systems

This paper investigates a hybrid fractional repetitive control (HFRC) scheme for magnetically suspended rotor systems to suppress harmonic current caused by mass imbalance and sensor runout. With parallel structure, HFRC consists of a frequency-adaptive dual-mode repetitive controller (DMRC) and a phase-shift notch filter. By introducing fractional delay filters, the proposed DMRC provides frequency adaptability to eliminate odd and even harmonics independently at fixed sampling rate. Its delay time is halved compared with the conventional repetitive control scheme, and the control gains can be weighted to ameliorate system dynamic response according to harmonics distribution. Moreover, as the domination of control current, fundamental frequency current is additionally rejected by the notch filter; thus, achieving smaller overshoot and faster suppression performance. An improved phase compensator is designed in HFRC to expedite current convergence rate while stabilizing the overall system. Experimental results on a magnetically suspended flywheel demonstrate the advantages of the proposed method.