Magnetic Field Modelling And Torque Analysis Of A Permanent-Magnet Spherical Motor With Stepped Magnet Poles

Spherical motor is a special electric machine with three-degree-of-freedom(3-DOF)motion,which can be flexibly applied to aerospace,robots,drones,and other fields.The traditional multi-degree-of-freedom(multi-DOF)mechanical transmission mechanism requires the coordinated control of multiple single-degree-of-freedom(single-DOF)motors,and there is a cumulative error during movement.A permanent-magnet spherical motor(PMSpM)has the advantages of compact structure,simple module,low mechanical loss,etc.,and has a wide application prospect.This thesis presents a new design of a PMSpM.Its rotor uses a three-layer magnet pole arrangement,and the structure of magnet poles is stepped.Threedimensional(3-D)magnetic field and torque analysis of the motor is needed for the design,optimization,and control of the PMSpM.However,its special structure causes difficulties in analytical modelling.This thesis proposes an analytical method(AM)of the 3-D magnetic field and torque for the complex spatial magnetic field distribution and multi-DOF motion of the PMSpM with stepped magnet poles.This method combines the loop current method and Lorentz force method,thus the 3-D magnetic field and torque can be calculated efficiently with reasonable accuracy.Then the analytical model is verified by the finite-element method(FEM).The test results confirm the high accuracy of the developed model.The proposed AM has effectively reduced the computational burden of the FEM,while sacrificing less accuracy.Finally,a prototype of the PMSpM with stepped magnet poles has been manufactured,and the torque characteristics of the motor are verified by the experimental tests.The main works of this thesis are as follows:1)The 3-D magnetic field distribution of a cylindrical permanent magnet(PM)is analysed.An analytical model of the 3-D magnetic field of a single PM is obtained based on the loop current method.A PM with stepped structure is proposed based on the analytical results.2)A finite-element model of a stepped PM is built using finite-element simulation software,and the analytical modelling results of the magnetic field are compared with the finiteelement results in two and three dimensions.3)A single-pair model of a coil and a PM is built.An analytical model of the 3-D torque of a single-pair model is obtained based on the Lorentz force method.The analytical results are compared and verified by the FEM.4)Based on the analytical modelling results of torque,a three-layer PM rotor pole arrangement is proposed,and the Torque-angle characteristics of the PMSpM with this structure in tilting and rotating motions are analysed.5)The prototype of the PMSpM with stepped magnet poles is manufactured.MEMS sensor is used to measure the torque of the motor,which is verified by comparing with the analytical modelling results,and compared with the torque of the first generation PMSpM of the research group.