Parametric study on pole geometry and thermal effects of a VRSM

The variable-reluctance spherical motor (VRSM), which offers some attractive features by combining pitch, roll, and yaw motions in a single joint, is essentially a brushless, direct-drive actuator. Unlike a multi-axes actuating mechanism where the number of electrical inputs typically equal to its number of controllable degrees of freedom, the VRSM offers a large number of distributed electrical inputs as compared to the number of controllable DOF. Since larger number of small coils is used, it not only effectively increases the surface area for heat dissipation, but also allows a small amount of current to flow through each of the coils. This distributed actuation offers an effective means to overcome heat dissipation problems commonly associated with direct-drive actuators for high torque applications. To exploit the advantages offered by this unique feature that provides a greater flexibility in design and control, a good understanding of the key parameters that could significantly influence the motor performance is essential. For this reason, we present in this paper some results of a detailed parametric study on the effects of pole geometry on the thermal and torque performance of a three degrees-of-freedom (DOF) VRSM.