Development and Analysis of a Novel Spherical 2-Degree-of-Freedom (2-DOF) Hybrid Stepping Motor

A novel spherical two-degree-of-freedom (2-DOF) hybrid stepping motor is proposed in this paper, which has a simple and compact mechanical structure and is easy to manufacture, assemble, control and apply. The motor is composed of two sub-motors, each of which is a hybrid stepping type with an arc-shaped stator and a specially designed bearing structure. The rotational axes of these two sub-motors cross at the sphere center, and this structure enables the 2-DOF motor to move in any direction. Due to the mutual influence of permanent magnet (PM) leakage flux between the two sub-motors, the 3-D magnetic field distribution inside the motor becomes more complex, and thus the 2-D equivalent magnetic field analysis method is proposed, with two types of 2-D equivalent motor models established. The accurate one can take into account of the PM leakage flux influence yet is only suitable for solving motor’s static and steady state problem, whilst the simplified one can solve all the problems and is applicable for the more typical situation where the PM leakage flux influence can be almost neglected via optimal structure design. A prototype of the proposed spherical 2-DOF motor with outside diameter of 50 mm is newly manufactured and experimented to validate the feasibility of motor’s operational principle, and the accuracy of both 2-D equivalent motor models is verified by the 3-D finite element analysis (3-D FEA) calculation results. Based on the 2-D equivalent motor models, the 2-D FEA is employed for the performance analysis, such as no-load back electromotive force (EMF), tooth-layer permeance harmonics, detent torque, pull-out torque, unbalanced force, etc., which is characterized by time- saving, high accuracy and good versatility.