The Adaptive Nonsingular Terminal Sliding Mode Control of Six-Pole Radial–Axial Hybrid Magnetic Bearing Considering Varying Current Stiffness

Most control strategies for magnetic bearings are typically formulated upon the linearized suspension force model, and the nonlinear characteristics are neglected or regarded as the disturbance and variation in parameters of the control system. The controllers based on linearized suspension force model struggle to achieve fast response under disturbance. Therefore, a nonlinear mathematic model that simultaneously represents the main nonlinearity of suspension force and facilitates the design of high-performance controller is necessary to establish. In this study, a new mathematical model of suspension force with varying current stiffness is developed, and a specific controller was designed based on this model. Firstly, the nonlinear mathematical model of six-pole radial–axial hybrid magnetic bearing (RAHMB) is established. Secondly, the characteristics of the current stiffness varying with rotor displacement are analyzed and the expression between current stiffness and rotor displacement is fitted. Then, the linearized model is built via Taylor expansion of the nonlinear model. Subsequently, the varying current stiffness model is constructed by substituting the fitted expression of varying current stiffness into linearized model. Finally, an adaptive nonsingular terminal sliding mode controller (ANTSMC) is designed based on the proposed varying current stiffness model. The simulation and experiment results have shown that the ANTSMC based on varying current stiffness model reduces chattering more than 64% and reduces convergence time more than 70% to the NTSMC based on the linearized model.