Performance Enhancement for AMB Systems Using Unstable $H_{\infty }$ Controllers

Controllers synthesized using H_∞ methods provide good disturbance attenuation for active magnetic bearing spindles. The synthesized controllers are often unstable when the spindle exhibits flexible dynamics. In this paper, we design and implement controllers using different models for an experimental active magnetic bearing (AMB) system. For an AMB model that contains flexible dynamics, the synthesized H_∞ controllers promise higher performance but are found to be unstable. To implement an unstable controller, we use a Youla parametrization-based method and seamlessly switch from a stable to an unstable controller. The improved benefits of the unstable controllers for disturbance attenuation are demonstrated using experimental measurements.