Enhancement of External Damping of a Flexible Rotor in Active Magnetic Bearings by Time-Periodic Stiffness Variation

Abstract

Previous theoretical studies have shown analytically and numerically that a vibrating system can be stabilised and its vibrations can be suppressed by an open-loop control of a stiffness parameter, a stabilisation by parametric stiffness excitation. This approach is investigated further numerically and implemented experimentally for a flexible rotor with multiple disks supported by active bearings. A periodic open-loop control of the stiffness coefficients of a bearing is realised by periodically changing the control parameters of an active magnetic bearing. This periodic variation of control parameters is regulated at fixed frequency and amplitude in such a way that it acts like a parametric excitation in the rotor system. As it was shown for simple vibrating structures (chain mass system, cantilever, Jeffcott rotor), a periodic variation can enhance the effective damping which leads to a vibration reduction in a vibrating system. Since this control is open-loop, it can be operated in parallel to existing and well-established controllers already in use in active magnetic bearings.In this paper, the method of damping by parametric excitation is realised experimentally in a rotor system. Direct numerical simulation is performed to calculate ranges for control and system parameters where damping by parametric excitation is effective. First experimental results are shown to demonstrate the applicability of the method.