Cogging Torque Analysis Of Polyhedral Permanent Magnet Spherical Motor

At present,the research on spherical motors with multi-degree-of-freedom motion is still in the exploratory stage,especially for permanent magnet spherical motors whose stator and rotor yokes and stator teeth are all ferromagnetic materials.The output torque of the spherical motor with this structure increases,but at the same time it brings the problem of cogging torque,which causes vibration and noise,and increases the difficulty of control.Therefore,it is necessary to study the cogging torque of permanent magnet spherical motor.The research object of this paper is a double polyhedral permanent magnet spherical motor.The rotor pole is located at the apex of the regular hexahedron,and the stator is hemispherical structure.In order to meet the special requirements of joint drive and hub drive,the stator teeth are located at the center of the regular octahedron.In this paper,two methods are used to study the cogging torque.Firstly,by comparing the analysis process of cogging torque of rotation motor,the cogging torque of permanent magnet spherical motor is decomposed into single-polemulti-teeth’s single-pole model and multi-pole-multi-teeth’s multi-pole model.The cogging torque of unipolar spherical motor is obtained by finite element simulation,and the spherical harmonic function is decomposed to obtain its functional expression.The cogging torque generated by other arbitrary magnetic poles in the process of rotor motion is obtained by using magnetic polar coordinate position transformation and spherical harmonic coefficient transformation,respectively.The cogging torques of each pole are superimposed to obtain the cogging torques of the multi-pole permanent magnet spherical motor corresponding to the two transformations.Compared with direct finite element simulation,this method reduces the computational burden to a certain extent.Secondly,considering the heavy burden of calculating the cogging torque of the unipolar model obtained by finite element method,the equivalent magnetic circuit method is used to solve the cogging torque.The magnetic pole is meshed,and the cogging torque of a single flux tube is obtained by path selection,magnetic density and side force calculation.The cogging torque between the rotor and the stator is obtained by coordinate transformation and superposition of the cogging torques produced by all pole flux tubes.This method can well coordinate the problem between calculation time and accuracy,and is suitable for the initial design stage of spherical motor.