Composite hierarchical control for magnetic bearing based on disturbance observer

This paper concerns a class of MIMO system disturbance attenuation and rejection problem for magnetic bearing system. The disturbance is formulated as a hierarchical one with two different parts. The first part is described by an exogenous model, while the second one is uncertain but bounded signal. In consideration of this, a new composite control scheme is presented, a nonlinear tracking differentiator is utilized to estimate system state firstly, by which a nonlinear disturbance observer (DO) for modeled part is designed and then integrate DO with a robust controller to reject the unknown equivalent disturbances. It is shown that with the proposed controller the closed-loop system is uniformly ultimately bounded (UUB) based on Lyapunov approach. Finally, simulations for a magnetic bearing control system shows that by integrating the disturbance observer with conventional control laws, the synchronous vibration caused by unbalance on a rotor can be rejected and the desired dynamic performances can be guaranteed.