3/2-Order Dual-Mode Fractional Repetitive Control for Harmonic Vibration Suppression in Magnetically Suspended Rotor

Harmonic currents in magnetically suspended rotor systems are usually caused by mass unbalance and displacement sensor errors, which will produce harmonic vibrations. Repetitive control (RC) is used to eliminate harmonic currents in the control system, but for the magnetic bearing control system, the odd harmonics often dominates, and the characteristics of the even harmonic frequency are not expected to change greatly. At the same time, the ratio of the sampling frequency to the rotor frequency is required to be an integer in the conventional repetitive controller (CRC), which greatly limits its range of application. Therefore, a 3/2-order dual-mode fractional-order repetitive control scheme with a three-parallel structure is proposed. The three branches in the scheme can not only eliminate odd and even harmonics independently,but also improve the robustness of frequency fluctuations. In addition, this method converts the fraction into an integer polynomial by Lagrange interpolation, which can accurately suppress the harmonic current at any frequency. The system delay is reduced to 0.5N sampling periods, and the system dynamic response rate is accelerated. Simulation results show that the proposed algorithm can effectively achieve harmonic vibration suppression at any frequency.