Rotor Point Compliance Minimization via μ-Synthesis

Many practical problems in magnetic bearing control concern essentially the minimization of the compliance of the rotor at a particular point, often not a bearing or sensor location. Experimental results are presented which demonstrate that controller design problems of this type can be tackled via μ-synthesis. The problem of minimizing the peak compliance at the midspan of a rotor test rig is examined. A significant improvement in performance is obtained by a multivariable μ controller over a benchmark optimal decentralized PD controller. Interestingly, the μ controller obtained is unstable. Therefore, the rotor is first levitated with a PD controller and then the multivariable algorithm is implemented. Careful attention is given to developing an accurate system model, especially the transfer function of the magnetic actuators with solid stators.