Recent advances in manufacturing automation have motivated a flurry of research on multi-degree-of-freedom actuators. Among the development is a variable reluctance (VR) spherical motor which has potentials as a direct-drive robotic wrist capable of combining roll, yaw, and pitch motions in a single joint. Prior research on torque modeling of the VR spherical motor has shown that bearing friction due to the non-zero resultant magnetic force and loading contributes to the torque error, results in significant reactions on the transfer bearings, and generates high frequency noise. For this reason, this paper presents a reaction-free control strategy and associated complete dynamic modeling in order to reduce the bearing friction and thus to improve the system performance of the VR spherical motor. This paper derives an analytical model describing the resultant magnetic force to serve as a basis for designing the reaction-free control strategy. The paper presents a practical technique for approximating experimentally the permeance, which plays an important role in modeling and hence the development of the control law, without changing the previous experimental setup. Preliminary experimental results show that the reaction-free control strategy reduces the bearing friction and improves the system performance. Several system performance evaluation techniques are also addressed in the paper. It is expected that the complete dynamic modeling and the reaction-free control strategy will serve as a basis for modeling and control of six DOF VR spherical motors with contact or non-contact support mechanism.
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