Application of Second-Order Sliding-Mode Concepts to Active Magnetic Bearings

Rotor mass imbalance is a common problem to rotating machines due to the unavoidable imperfections in manufacturing. These imbalance forces can be viewed as harmonic disturbances which lead to a periodic rotor runout during rotation. Furthermore, the runout length increases with the rotational speed squared. Moreover, for variable rotational speed applications, these harmonic disturbances are also time-varying. Active magnetic bearings (AMB) provide a means of actively attenuating these disturbances. Although various imbalance compensation schemes have been proposed in the literature to handle this problem, they are often more suitable for constant rotational speed applications where disturbances can be handled at a predetermined rotational speed. This study proposes the application of second-order sliding-mode control (2-SMC) to regulate AMB systems throughout a wide operating speed range. The proposed controllers are composed of two components. The first component is a linear controller for the sake of stabilizing the inherently unstable system, while the second component is a 2-SMC to handle the model uncertainties of the system as well as the exogenous harmonic disturbances. Simulation and experimental results are provided to demonstrate the effectiveness and superiority of the proposed techniques compared to the conventional linear controller.