Optimum Design Of Three Degree-of-freedom Parallel Mechanism With Asymmetrical Structure

Aiming at the great demand for efficient grinding of aircraft composite skin in the field of aviation maintenance,a three degrees-of-freedom parallel mechanism with asymmetrical structure is used to replace manual grinding.This paper has an in-depth studied the stiffness and dynamics modeling,performance evaluation indexes and optimization design of the mechanism,which provided a theoretical basis for further improving the grinding performance of the TAM.The main research contents of this project are as follows:Considering the asymmetrical characteristics of the TAM,the stiffness caused by the deformation of each member in the parallelogram branch chain is calculated.The principle of linear superposition is used to obtain the total stiffness matrix of the TAM.In terms of dynamics modeling,the mechanism is divided into three sub-systems: moving platform,UPS telescopic rod branch and parallelogram branch.The kinetic and potential energies are obtained,and the dynamics model of the TAM are established adopting the Lagrange equation.Comparing MATLAB numerical results with ADAMS simulation results,the driving force curves obtained by the two methods basically coincide,which verifies the correctness of the dynamic model establishment.The kinematics performance evaluation index of the TAM is analyzed by the velocity ellipsoid method,and the global kinematic condition number index based on the Jacobian matrix is obtained.The global stiffness performance evaluation index of the mechanism is constructed by using similar methods.Due to the limitations of the dynamic operation ellipsoid method,the global acceleration dexterity index is proposed adopting the comprehensive acceleration ellipsoid method.The index considers the influence of the inertia factor,gravity factor and speed factor on the TAM,which truly reflects the distribution of the acceleration performance of the mechanism.According to the evaluation indexes of kinematics,stiffness and dynamics,the multi-objective optimization of the objective function is carried out by the genetic algorithm and the normalized weighted summation algorithm.Thus,the optimized structural parameters are obtained.The performance graphs before and after the optimization are calculated by simulation.The comparative analysis showed that the kinematics,stiffness and dynamics performance after the optimization are significantly improved.Moreover,the performance differences are also significantly reduced.The results proved that the multi-objective performance optimization of the TAM is reasonable.