Research On Modeling And Optimization Of Coreless DC Linear Motor For Ultra-precision Positioning System

Coreless Direct Current Linear Motor (CLDCLM) has some advantages such as a simple structure, less thrust fluctuation, high dynamic response and high reliability. The CLDCLM has larger stroke than traditional ultra-precision actuators (such as piezoelectric and giant magnetostrictive actuators), and do not use contact transmission principle for having higher positioning accuracy to be adopted in nanopositioning system. In nanopositioning system, the requirements of actuator are very strict, including high thrust density, no heat exchange, high dynamic response and strict shape constraints. In this thesis, the topology of CLDCLM is proposed; and the electromagnetic and temperature fields of CLDCLM are analyzed; and the electromagnetic and thermal analytical models are established; and the method of multi-object optimization is researched.Firstly, the topology of CLDCLM is proposed, which adopts Halbach array and series-parallel combination magnetic circuit, and has thin cooling structure. Halbach array and series-parallel combination magnetic circuit can reduce the height of yokes for improving the thrust density; cooling structure suppresses the temperature rise of CLDCLM for keeping no heat exchange, and improves the thrust density which is lower due to having air-core structure.Based on the simple boundary of CLDCLM, the electromagnetic model is established by the image method and surface magnetic charge model. And then the saturation coefficient is added in the analytical model to describe the nonlinear magnetic circuit by equivalent electromagnetic circuit approach. This electromagnetic model of CLDCLM is proved by finite element method (Comsol software), so this model has the versatility to lay the theory basis of the CLDCLM optimization design. Then, the analytical expressions of thrust density, thrust fluctuation and motor constant are obtained. And the method of CLDCLM’s resistance and inductance are proposed.Based on the electromagnetic model of CLDCLM, the part of CLDCLM thrust is researched, which is produced by winding end. The parasitic force of CLDCLM is non-driving direction force, and is produced by straight and end of windings. The suppression method of the parasitic force is obtained by the analysis. The relationship of thrust and position is like stiffness, which is a concept in mechanics, so the relationship is defined ‘thrust stiffness’ firstly. The thrust stiffness can describes the variation of the thrust with the position to be as the evaluation standard of motor performance. Then, a new method is introduced to suppress eddy current by offsetting back EMF in cooling plates. Based on the electromagnetic theory, the analytical model of new cooling plate is established. The Structural deformation and vibration would bring thrust fluctuation, and the model of this thrust fluctuation could be is discribed by multi-rigid-body and multi-degree-of-freedom model of CLDCLM primacy. By using above models, the accuracy of CLDCLM dynamic model is improved. The work lays theoretical foundation for feedforward control system of CLDCLM.The thermal model of CLDCLM is established by thermal network method, and then the high temperature region of CLDCLM primary side surface is predicted by this thermal model. Cooling structure not only improves the winding continuous current density to realize higher thrust density for meeting the needs of the ultra-precision positioning system, but also breaks technology bottleneck of the ultra-precision positioning system which is no heat exchange between motor and environment. The thermal conductivity branch is added to suppress the surface temperature of CLDCLM, which is proved by the FEM and testing. Some important parameters is calculated by FEM to lay the theory basis of the CLDCLM optimization design.Finally, the whole parameterized model of CLDCLM is established by seven dimension ratio parameters which are defined in this thesis. Based on the above electromagnetic and thermal models, the analysis and optimization of CLDCLM is researched by Genetic algorithm (GA) when stroke, surface temperature and sharp of CLDCLM are constrains. The electromagnetic performance, cooling capability and sharp of CLDCLM are designed at the same time, so the optimization of CLDCLM based on Multi-physical Fields is achieved. Finally, the CLDCLM, which is designed by this design method, has high thrust density, little thrust fluctuation and no heat exchange for meeting the needs of the ultra-precision positioning system.