DYNAMIC MODELING AND CONTROL OF A BALL-JOINT-LIKE VARIABLE-RELUCTANCESPHERICAL MOTOR

Examination of existing joint designs for robot wrist applications has indicated that a spherical wrist motor offers a major performance adv antage in trajectory planning and control as compared to the popular t hree-consecutive-rotational joint wrist. The tradeoff however, is the complexity of the the dynamic modeling and control. This paper present s the dynamic modeling and the control strategy of a three degree-of-f reedom (DOF) variable-reluctance (VR) spherical motor which presents s ome attractive possibilities by combining pitch, roll, and yaw motion in a single joint. The spherical motor dynamics consist of the rotor d ynamics and a torque model. The torque model is described as a functio n of coil excitations and a permeance model in terms of the relative p osition between the rotor and the stator. Both the forward dynamics wh ich determine the rotor motion as a result of activating the electroma gnetic coils and the inverse model which determines the coil excitatio ns required to generate the desired torque are derived in this paper. The solution to the forward dynamics of the spherical motor is unique, but the inverse model has many solutions and therefore art optimizati on is desired. Experimental results verifying the dynamic model are pr esented The control of a VR spherical motor consists of two parts; nam ely the control of the rotor dynamics with the actuating torque as sys tem input, and the determination of the optimal electrical inputs for a specified actuating torque. The simulation results and implementatio n issues in determining the optimal control input vectors are addresse d. It is expected that the resulting analysis will serve as a basis fo r dynamic modeling, motion control development, and design optimizatio n of the VR spherical motor.