Robust control and unbalance compensation of rotor/active magnetic bearing systems

In this paper we design, analyze and compare the performance of various stabilizing robust controllers for the model of a horizontal rotor with active magnetic bearings. Rotors suspended with electromagnetic bearings are inherently unstable; therefore, feedback control is an essential part of their operation. Despite the nonlinear form of the actuator (electromagnetic bearing) dynamics, plant (rotor) model is linear and time-invariant at constant rotor speed. Actuator dynamics are linearized around an operating point using a constant bias current. Two Ha controllers are designed for the nominal system using different structures. The uncertainties in the system dynamics due to changes in the model parameters are two-fold: gyroscopic forces due to different rotor speeds within the range of operation, and the changing spring stiffness of the electromagnetic bearings due to different conditions during the operation. To ensure robust stability of the closed-loop system for all possible values of parameters that can change during the operation, an Ha controller based on the model incorporating uncertainty in the system is designed. The limits for the allowable parameter changes for robust stability are tested and established with mu-analysis.