Active magnetic bearing (AMB) is the device to support and control rotating shaft. Feedback linearization is one of the methods to compensate the system nonlinearity, and it is often used in the control of AMB. Some parameters in the electromagnetic force model have often been ignored or their parametric uncertainty from the nominal values has been calibrated; however, their influence on the stability has not been investigated. In this paper, the influence of the parametric uncertainty in the electromagnetic force model on the stability of AMB is investigated. The equilibrium positions and their stability are investigated and clarified analytically. Furthermore, the choice of the parameter value for improving the stability of AMB with feedback linearization is proposed, and its effectiveness is explained analytically. It is shown that the proposed choice of the parameter value also reduces the remained nonlinearity significantly. The validity of theoretical results and proposed choice of the parameter value are confirmed by experiment.

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This paper considers effect of the parametric uncertainty of an active magnetic bearing which is used to control a rigid rotor. In the feedback control law, PD control and feedback linearization is used. Firstly, this paper shows that one of the two uncertain parameters in the electromagnet model significantly affects the stability of the system. Moreover, this paper analytically shows a method to select the nominal value of the critical parameter affecting the stability. Next, while the shaft is rotating, this paper considers reducing vibration due to rotating unbalance by inversion-based disturbance observer and controlling position by feedforward control, besides PD control and feedback linearization. Based on the linearized model, this paper shows the performance degradation caused by the parametric error and investigates the tracking error experimentally. Finally, in order to improve the tracking performance under the existence of the uncertain parameters, this paper proposes to employ an inversion-based feedforward controller designed from the augmented control object, which includes the controlled object (rigid rotor and magnetic bearing), the disturbance observer and the feedback linearization. The experiment of tracking control of the rotor position is carried out to demonstrate the effectiveness of the proposed method.

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In design of rotating machines, analysis of external force is important for stable rotating operation. The various external forces such as unbalance force, seal contact force and rotor dynamic fluid force effect on the rotor system, dynamic characteristics of rotating system is changed by the external forces. The external force can vary depending on a rotor trajectory. In previous studies, a mathematical model of the dynamic characteristics had been formulated for small amplitude orbit around the equilibrium point for the rotor trajectory. However, the formulation hasn't been yet established in the rotor trajectory with large amplitude or eccentricity caused by the external forces. In this study, an experimental system using active magnetic bearing (AMB) is developed to achieve the formulation. The system can generate any arbitrary trajectory assuming various external forces and the rotor system follow the generated trajectory. The system can estimate the external force at the same time. Current flowing through coil of the AMB is controlled by a controller designed based on the frequency response to follow the arbitrary orbit. Then, the external forces are estimated from the control signal by the disturbance observer. Experiments were performed by assuming unbalance forces generated in the elastic rotor. From the results it was confirmed experimentally that high precision tracking control and estimation of external force are possible. The various rotor trajectory under the action of the external forces can be repeated by using the system, and the external force can be analyzed by the experimental data.