High-order Nonlinear Disturbance Observer Based Zero-bias Control for a Magnetic Suspended System

In this paper, the zero-bias control method is proposed to remove the bias current/flux for the active magnetic bearing (AMB), which is meaningful to decrease the power consumption. The AMB system becomes a high nonlinear system under the zero-bias current controlled mode. Besides, the AMB suffers from lumped uncertainty including parameter uncertainty and external load, which is usually undeterminable and varying. The nonlinear disturbance observer based sliding mode control is proposed to realize the close-loop control of the whole system. Under this control framework, the structure of the controller for a nonlinear system suffered from disturbance can be divided into two parts. One is the normal sliding mode control part to achieve basic performance for the nonlinear system. The other is the compensate part to eliminate the effects caused by the unknown disturbance via disturbance observer. Compared with the methods adopting inner adaptive laws to estimate disturbance such as the adaptive backstepping sliding mode control, the nonlinear disturbance observer has much higher converging rate, which is meaningful for anti-disturbance. Mostly the nonlinear disturbance observer is adopted with the assumption that the derivative of disturbance is approximately equal to zero. In this paper, the disturbance is considered to be time varying, hence the conventional nonlinear disturbance observer is not well suited. Then the high-order nonlinear disturbance observer estimates the disturbance and its derivatives simultaneously, is proposed to handle the time varying disturbance. Under different kinds of disturbance, the high-order nonlinear disturbance observer based sliding mode control method is compared with the sliding mode control, the adaptive backstepping sliding mode control and the conventional nonlinear disturbance observer based sliding mode control. The effectiveness of the proposed high-order disturbance observer based approach for a zero-bias current controlled AMB system is verified by the simulation and experiment results.