Torque control of a spherical machine with variable pole pitch

Torque control of a spherical machine combining three degrees of freedom in motion is presented. This drive can be used for machine tools and robots to replace conventional rotating or linear machines which are connected in series or parallel. The proposed motor consists of a rotor sphere with permanent magnets and an outer stator core casing with 96 stator poles and windings. The currents of the stator coils have to be controlled individually because the pole pitch can vary continuously during operation. A unique solution to calculate all 96 current commands to produce the desired torque does not exist. Different control algorithms are presented on how to choose the individual phase currents to reach the required torque vector. Simulations show satisfying results which are confirmed by experiments. In addition, the controllability of three-dimensional (3D) motion is demonstrated.