CHARACTERIZATION OF A 3-DEGREE-OF-FREEDOM VARIABLE-RELUCTANCE SPHERICAL MOTOR

This paper presents a practical approach to characterise and optimise a ball-joint-like three degree-of-freedom (DOF) variable reluctance (VR) spherical motor which has potential applications as a direct-drive robotic wrist. Unlike the conventional single-axis (1-DOF) motor, the 3-DOF VR spherical motor is characterised by its multiple independent inputs and the direction-varying and orientation-dependent torque output. The design and control of 3-DOF spherical motors are significantly different from that of 1-DOF actuators. In order to characterise the torque of a 3-DOF spherical motor, a maximum torque-to-input power ratio is derived to describe explicitly the relationship between the torque output and the electrical power input. This relationship, along with the presentation of the numerical technqiues, provides an effective means to characterise and optimise 3-DOF spherical motors, which is essential to integrated design and control of the motors. Several typical applications are illustrated here, including the estimation of the maximum payload, proof of the non-singularity property, choice of an objective function for design optimisation, and determination of the characteristic orientation where a global design optimisation can be accomplished. Results showed good agreement between the theoretical and actual maximum torque outputs. Some new and unique concepts for 3-DOF devices are also addressed in the paper.