Robust stability of the Lorentz -type self bearing servomotor

Robust stability and closed loop performance of a Lorentz-type self bearing servomotor test rig are presented. Previous work has identified the existence of multiple destabilizing terms in the servomotor force-torque characteristic. These terms are treated as modeling errors and accounted for using structured uncertainty. A set of decoupled PD controllers is designed and the stability robustness of the system is evaluated via μ-analysis. Experimental results confirm that the resulting controllers provide stable levitation and angular pointing of the rotor over all uncertainties. The closed loop bearing stiffness for the servomotor was measured as 502 N/mm and the closed loop angular position stiffness was measured as 17.7 N-m/rad. The angular pointing accuracy of the servomotor was also measured and found to be <30 μrad.