Development of a spherical stepper wrist motor

This paper presents the design concept of a new spherical stepper robotic wrist motor capable of three degrees of freedom (DOF) motion in a single joint. The spherical wrist motor has significant potential applications where the demands on wrist torque and workspace are low but high speed manipulation of end-effector orientation is required continuously in all directions. Typical applications are plasma and laser cutting and micro-assembly. The spherical motor is developed on the basis of the principle of variable reluctance stepper motors. This paper highlights the fundamental differences between the operation of a three DOF spherical motor and that of the conventional stepper motor. The establishment of a theoretic basis for design, prototype development and performance prediction is sought. In particular, an analysis of torque prediction is discussed along with the presentation of kinematic and dynamic relationships. A hybrid digital/analog laboratory prototype control circuitry has been developed to demonstrate proof of concept feasibility and to assist in achieving an optimum design.