Active Touchdown Bearing Control for Recovery of Contact-Free Rotor Levitation in AMB Systems

Control strategies for rotor displacements and transmitted forces in active magnetic bearing (AMB) systems are generally well-developed when the rotor is in contact-free levitation. In such a condition, the rotor/AMB system, the plant to be controlled, is well-defined to use for controller design. However, transient overload conditions or large scale disturbances may cause the rotor to make contact with one or more touchdown bearings (TDBs). The ensuing dynamic response is an effective change to the contact-free plant, which may cause an AMB controller to apply inappropriate forces, causing persistent contact, or even inducing system instability. It is therefore beneficial to restore contact-free levitation as quickly as possible in order that normal operation may resume. This would minimise outage time and also minimise high levels of potentially damaging contact that may be experienced by the TDBs. In this paper, the use of an active TDB is considered for the restoration of contact-free levitation. The contact problem is demonstrated in simulation and also through appropriate control applied in an experimental system. In this system, TDB motion is activated by piezoelectric stack actuators.