Active magnetic bearing systems (AMBs) have many potential industrial applications where extremely fast and accurate operations are required. However, AMBs are often subject to disturbances in the form of synchronous vibrations due to unmodeled dynamics such as the rotor mass-imbalance and centrifugal forces while the rotor is in rotation. Several methods such as variable notch filters, gain scheduling controllers, and linear parameter varying controllers have been proposed recently to reject the disturbances while the system is operating at high rotational speeds. These methods are practical only if the frequencies of these sinusoidal-like disturbances are directly measurable or accurately known in advance. In this paper, a hybrid control scheme comprised of a feedback H∞ controller and an inner-loop disturbance observer-based control is proposed. The effectiveness of this control scheme is verified by simulation and real-time experiments on an AMB system. Both constant and sinusoidal disturbances are taken into consideration while the rotor is stationary as well as while it is rotating at different speeds. The results demonstrate that the proposed hybrid control scheme exhibits significantly improved performance in comparison to single-loop controllers in the presence of unknown but bounded disturbances.
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