Research On Tiered Type Multi-degree-of-freedom Permanent Magnet Spherical In-wheel Motor

Spherical motors have the advantage of high integration,simple mechanical transmission and good dynamic performance,thus having a great potential to replace traditional multi-degree-of-freedom mechanisms in industrial applications such as robot joints.Specially,once the spherical motors are served as in-wheel motors,the whole transmission system and sheering system in electric vhlicles can be simplified significantly.As a result,the general layout as well as vhlicle performance can be optimized effectively.However,existing spherical motors usually have the problems of insufficient torque and complex control,which seriously hinder their practical use.Therefore,this thesis proposes a novel tiered type permanent magnet spherical inwheel motor(T-PMSIWM),of which the design method,magnetic field distribution,torque characteristic and rotor orientation detection method are investigated.The main research work includes the following aspects:Firstly,basic structure of the novel T-PMSIWM is presented and its working principle is analyzed,based on which a tilting force calculation model is established.The influences of the key parameters on the performance of multi-degree-of-fredom movements are investigated.The key parameters include but not limited to pole-slot combination,rotor axial length and stator axial position.According to the theoretical analysis and application demand,design principles of the T-PMSIWM are proposed.In order to deal with the complex structure of spherical motors,both electromagnetic and mechanical design methods are given,aiming at improving design efficiency and motor reliability.Considering that there are large numbers of structural parameters need to be optimized,two technical measures are adopted to reduce the amont of time in motor design.On the one hand,the parameters are classified into three groups so that they can be treated in an appropriate order.On the order hand,both the twodementional estimation and the three-dementional accurate calculation are used in different stages.After electromagnetic optimization,a prototype motor is designed and manufactured for experimental verification.Secondly,a dynamic magnetic network model of the T-PMSIWM is proposed based on the principle of equivalent magnetic flux tube.Thus,the magnetic field can be calculated in a simple and fast way,which will save a considerable of computation time,when compared with three-dementional finite-element simulation.Besides,the multi-degree-of-freedom motions,nonlinear magnetic materials and spherical air-gap are all taken into account in the proposed dynamic magnetic network model.By making full use of its high efficiency in magnetic field computation,results under varios working conditions are obtained,then the influences of rotor position and stator current on the magnetic field distribution are analyzed.The calculated results are compared with the simulation results and the measured results for verification.Thirdly,the main factors that influence the torque performance are analyzed,especially for the stator current and rotor positions.The analytic expression of tilting torque in the T-PMSIWM is derived based on the fact that the tilting torque is equal to the cross product of tilting force and its arm.As a result,the main factors that have great influence on the tilting torque are identified.In order to measure both rotation torque and tilting torque of a spherical motor with different rotor positions,a torque measurement system based on six-axis F/T sensor is proposed.However,considering that the proposed torque measurement system can hardly separate static friction in tilting torque measurement,a new torque measurement method based on micro displacement is designed so that the kinetic friction torque can be separated and used to compensate the tilting torque.Then,torque curves varying with rotating angle,tilting angle,tilting direction and stator current are obtained and analyzed.The reasons responsible for the torque reduction after rotor tilts,including the distortion of air-gap magnetic field and the additional tilting torque caused by iron materials,are investigated.Finally,a simplified back-EMF model of the T-PMSIWM is proposed.On the one hand,the axial segmentation calculation method is adopted to take into account the spherical air gap.On the other hand,an angle transformation function is utilized to take into account the multi-degree-of-freedom motions.Based on this simplified back-EMF model,the relationships between three-phase back-EMF and rotor tilting positions are obtained and analyzed.Then,the phase difference of the back-EMF,which proves to be unique for any rotor tilting position mathematically,is used to detect the tilting angle as well as the tilting direction.The influence of stator axial position on the back-EMF is also investigated,which is important in the deployment of windings or coils.In order to obtain the back-EMF in a simple and direct way,the detecting coils as well as their placements are introduced.