Direct Field-Feedback Control of a Ball-Joint-Like Permanent-Magnet Spherical Motor

This paper presents an alternative method utilizing real-time measurements of existing rotor magnetic field in the feedback loop for multi degree-of-freedom (DOF) orientation control of a permanent-magnet spherical motor (PMSM) characterized with redundant inputs. As the direct field-feedback control (DFC) system requires only measured magnetic fields, it eliminates the need of an external orientation sensing system and its major components can operate independently and permit parallel processing. The DFC method greatly reduces accumulated errors and time delay due to serial computations commonly encountered in existing methods that rely on orientation-dependent models for feedback control of a multi-DOF PMSM. In this paper, the method for determining the bijective relationship between the rotor orientation and measured magnetic field is presented, which enables the replacement of the orientation error by the magnetic field error in the control law. Using analytical magnetic field and torque models, the DFC design method is illustrated with two examples: a one-DOF motion system and a three-DOF PMSM; the latter has been experimentally implemented, for which an embedded multisensor system with connected bijective domains is designed. Excellent trajectory control results were obtained validating the concept feasibility of the DFC method for the PMSM.