Design concept development of a spherical stepper for robotic applications

This communication presents the design concept of a new spherical stepper motor capable of three-degrees-of-freedom (DOF) motion in a single joint. The ball-joint-like motor has no singularities except at the boundary of the workspace and can perform isotropic manipulation in all three directions. Due to its relatively simple ball-like structure, undesired cross-coupling and centrifugal components of wrist rotor dynamics can be effectively minimized or eliminated. The spherical stepper motor has potential in robotic applications as a three-DOF shoulder or an eyeball as well as a wrist actuator. In particular, this work presents the systematic conceptualization of a spherical stepper and the feasibility of constructing the spherical stepper. By pointing out the significant differences between a three-DOF spherical motor and the conventional stepper motor, this work illustrates the fundamentals of the spherical stepper design for robotic applications. Along with the experimental data, an analytical approach based on the permeance formula was employed to predict the driving forces generated by a neodymium-iron (Nd-Fe) permanent magnet. The force-displacement curves provide useful information for rational spherical motor design and control.