A Modified Feedforward Repetitive Controller for AMB Rotor System Using Adaptive Frequency Estimator

Harmonic currents are inevitably generated in the active magnetic bearing (AMB) rotor system due to two reasons. One is mass imbalance, resulting from machining inaccuracies and industrial assembly discrepancies. The other is sensor runout, caused by the nonuniformity of the detection surface and the lack of concentricity. To suppress harmonic currents in AMB rotor system, a modified feedforward repetitive controller (m-FRC) is proposed in this study. Compared to conventional repetitive controller, an additional feedforward term is introduced, and the time delay unit is relocated within both the feedback loop and the forward path. Subsequently, a complex bandpass filter with a frequency-locked loop is designed to estimate the rotor frequency in real time, enabling the m-FRC to adaptively suppress harmonic currents when the rotor frequency changes. A phase compensator is designed based on the Nyquist stability criterion to ensure system stability across the entire frequency band. Finally, simulation and experimental results demonstrate that m-FRC has a wider bandwidth, higher robustness, and superior harmonic currents suppression compared to conventional repetitive controller. In addition, the proposed frequency adaptive algorithm is effective at both fixed and variable frequencies.