Research On Motion Precision Of Planar Switched Reluctance Direct Drive System

Planar switched reluctance direct drive system(PSRDDS) has features of simple structure, high reliability of motion, and easy of control, which has potential widespread application in large travel and high stability planar motion. As the accurate electromagnetic force model of PSRDDS is difficult to be established for practical application due to the highly nonlinear electromagnetic characteristic, thus the rapid development of PSRDDS is restricted in high-precision motion. In order to improve the motion precision of PSRDDS, factors which affect motion precision of a PSRDDS are discussed in detail based on the theories of electromagnetism and tribology, and hence effective methods improving motion precision of the PSRDDS are proposed in this paper.Firstly, considering the magnetization characteristic, fringing leakage flux, magnetic saturation, and mechanical structure, etc., the air gap permeance of the PSRDDS in a pole pitch is deduced with the flux tube method in terms of the electromagnetic theory, and static thrust force and normal force are formulated by the virtual work method, which reveal the nonlinear relationship between the electromagnetic force, inductance, flux linkage, mechanical structure, overlapped position between the mover and stator blocks, phase current, and material characteristic. In order to verify the correctness of the proposed models, the finite element model of the PSRDDS is built based on Ansoft Maxwell, the thrust force and normal force are studied using the finite element model. Furthermore, experiments of the PSRDDS are also carried out to verify the proposed models of static thrust force and normal force. The influence of mechanical parameters and electromagnetic parameters on the thrust force and normal force is analyzed based on the proposed models. The research results manifest that: 1) The relative error of the theoretical result of thrust force and normal force compared with simulation and experimental results is less than 10%, and the validity of the proposed models is verified; 2) The stack length, number of turns per phase and thrust force and normal force have a linear relationship. The effect of tooth width and air gap length of movers and stators on the normal force is larger than that of the thrust force. The amplitude and distribution of thrust force and normal force is affected by the magnetization characteristic of iron core. The fluctuation of current has little effect on the thrust force and normal force. The dynamic performance will improve by increasing the thrust force. And the motion precision will decrease as the friction force increases caused by the biggish normal force.Secondly, the mechanism of nonlinear friction force and the mathematical model for the PSRDDS are discussed based on the tribology theory. The friction force model of the PSRDDS is established according to the Stribeck friction force model, the parameters of the Stribeck friction model of the PSRDDS are obtained via static system identification, and the dynamic equation of the PSRDDS considering friction force is deduced. The impact of the nonlinear friction force on the motion precision of the PSRDDS is analyzed, and the method of friction force feedforward compensation is presented. The research results demonstrate that: 1) For the PSRDDS, the friction force seriously affects the motion precision and hinders the dynamic performance, and the limited speed is a hidden protection under the constant force; 2) Compared to the PSRDDS without the friction force feedforward compensation, the steady-state error and the dynamic tracking error of the PSRDDS with the friction force feedforward compensation are reduced to 80% and 60%, respectively, and the PSRDDS with the friction force feedforward compensation has better dynamic performance.Finally, the motion precision and the factors having effect on the motion precision of the PSRDDS are comprehensively analyzed. The effective methods improving the motion precision are proposed as follows: 1) The rational mechanical and electromagnetic parameters of the PSRDDS are obtained using the optimal design; 2) An appropriate friction force feedforward compensation control is employed.