Robustness of Recursive Auto-Tuning Strategy in Flexible Rotor and Magnetic Bearing Systems

Active magnetic bearings offer the capability to control rotor vibration and transmitted forces through the design of adaptive controllers. These controllers can take into account the changing operating conditions and variations in the system dynamics. A Recursive Open-Loop Adaptive Control (ROLAC) algorithm was developed, which minimizes the sum of squares of the measurements in frequency domain. ROLAC is used in addition to local PID controllers, which ensure the stability. Magnetic bearing dynamics depend on the eccentricity of the rotor with respect to magnetic poles. Non-concentric rotation of the rotor within the magnetic bearings causes changes in the bearing dynamic properties. Apart from system faults, non-concentric operation of the rotor can occur due to misalignment of the auxiliary bearings with respect to magnetic poles, changes in sensor calibration, and misalignment of bearings in systems where there are three or more bearings. This misalignment leads to changes in the magnetic bearing characteristics thereby affecting the overall system dynamics. This in turn may necessitate re-tuning of the ROLAC. The paper studies the effect of such misalignments on the controller performance and the robustness of the auto-tuning process.