Stability Evaluation of High Frequency Eigen Modes for Active Magnetic Bearing Rotors

The servo feedback system of the active magnetic bearing (AMB) is commonly regulated by the PID control law. This law results in a very simple design. However it still contains several problems for the tuning of the control unit. In order to compensate for the insufficiency of the PID controller, a notch filter and a zero-pole canceling filter(z-p filter) are successfully introduced for tuning and stabilizing the high frequency vibration in our simulation and experiment. The stability margin of high frequency modes is increased by using these filters. For the stability evaluation, a new Nyquist plot on the log scale chart is introduced in this paper. The Nyquist plot provides the analysis of the global behavior of the locus of all eigen modes. The case study of the stability margin is presented concerning two rotors of the "long" and "flat" types, and the effective usage is demonstrated with respect to the new Nyquist plot.