Multi-degree-of-freedom spherical permanent magnet motors

The paper describes the analysis, design and control of two new versions of spherical permanent magnet motor, which are capable of two and three degrees-of-freedom of controlled motion, and have a high specific torque capability. Their magnetic field distribution is established using an analytical technique formulated in spherical coordinates. This enables the torque vector and back-EMF to be derived in closed forms. An optimal design procedure, to achieve maximum output torque or maximum acceleration for a given payload, is presented. The control of the motors, whose dynamics are similar to those of robotic manipulators, is facilitated by the establishment of a complete actuation system model and the application of the PD or computed torque control law. The validity of the analysis and design techniques, and the effectiveness of the control strategy, are confirmed by measurements.