Force-based feedforward control of persistent synchronous rotor/touchdown bearing contact in active magnetic bearing systems

Touchdown bearings are installed in active magnetic bearing systems to protect system components and prevent rotor contact with stator laminations. Abnormal conditions may induce a rotor dynamic response that leads to persistent contact with the touchdown bearings. Control schemes may be successful in re-levitating the rotor from contact provided the underlying contact dynamics are taken into consideration. Effective rotor/touchdown bearing contact control is opportune when active magnetic bearings remain operational and complete machine rundown is avoidable. In such cases, the active magnetic bearing forces should have the required dynamic load capacity to restore contact-free rotor levitation, eliminating the need for extra actuation in the system. This requires establishing a relationship between contact dynamics and magnetic bearing control forces. This paper presents a new contact control method based on stator strain measurement arising from rotor/touchdown bearing contact. The measurement data are utilised to evaluate frequency dependent magnetic bearing control forces and phase shifts to enable rotor recovery from contact. The control method is tested and validated experimentally on an active magnetic bearing system with a long unbalanced flexible rotor. The tests demonstrate effective rotor recovery from persistent synchronous contact over a range of rotor speeds.