Novel Generalised Notch Filter for Harmonic Vibration Suppression in Magnetic Bearing Systems

Magnetic bearings represent an important alternative to conventional ball bearings for applications that require very low noise and vibrations. The absence of contact and friction between rotor and stator parts, combined with the use of active vibration control techniques result in rotating machinery that can greatly outperform their passive counterparts. One of the main disadvantages of these systems is the increased complexity of requiring the active control of the rotor, including sensors actuators and computing power. Furthermore, active vibration control techniques are generally difficult to implement over a broad speed range due to requirement of choosing and adapting different parameters to guarantee its stability. For this reason, this article presents a novel generalised notch filter for harmonic suppression control for magnetically-levitated rotors which features improved stability properties. The proposed harmonic or unbalance force rejection control (UFRC) is based on an unbalance and resonance suppression technique originally developed for piezoelectric active bearings for rotating machinery, and it is successfully adapted and its performance validated for machines featuring magnetic bearings. The formulation of the harmonic suppression control technique is introduced, its stability for magnetic bearing systems analysed, and experimentally implemented and tested on a fully active slotless Lorentz-type magnetic bearing motor obtaining a reduction of at least one order of magnitude in the level of generated vibrations.