Design of Controllers for a Multi-Degree-of-Freedom Spherical Wheel Motor

This paper presents the control system design for a particular form of variable-reluctance spherical motors, referred to here as a spherical wheel motor (SWM). Unlike most of the existing spherical motors where focuses have been on the control of the three-DOF angular displacement, the SWM offers a means to control the orientation of a continuously rotating shaft. Specifically, we demonstrate an effective method to decouple the open-loop (OL) control of the spin rate from that of the inclination, leading to a practical OL system combining a switching (spin-rate) controller and a model-based inclination controller. The OL system presented here provides the fundamental control structure for the SWM. To account for unmodeled external torques, we extend the design to allow feedback with a PD controller and a high-gain observer. The effectiveness of the controllers has been investigated by comparing their performance numerically under the influence of an unknown external torque.