Fuzzy gain scheduled adaptive sliding mode control of a vertical shaft hybrid magnetic bearing under variable rotor speeds

This paper explores the axial control and stability challenges of vertical shaft hybrid magnetic bearings (HMB) under varying rotor speeds. HMBs are considered an optimal solution, as the advantages of both active and passive magnetic bearings are blended. However, while effective radial control is provided through permanent magnets, axial stability is struggled with, especially at higher speeds. These axial instability issues are addressed by implementing an adaptive sliding mode controller (ASMC). Although promising performance is shown by ASMC at low speeds, significant high steady-state error and control force issues are encountered as speed increases. To overcome this, a fuzzy logic-based gain scheduler is incorporated into the ASMC. Parameters are optimally selected by this gain scheduler across different speed ranges, enhancing the performance of the standard ASMC. This results in a more robust and efficient control mechanism for the HMB. Given the significant nonlinearity and parameter uncertainties in the proposed HMB system, it is crucial for the controller’s robustness to be evaluated. This involves variations in electromagnet stiffness, permanent magnet stiffness, and the resistance and inductance of the electromagnet coil being analyzed. Excellent performance and robustness are demonstrated by the proposed fuzzy-based ASMC, maintaining low steady-state error and reduced control force requirements.