Stiffness Performance Analysis And Optimization Of A Parallel Mechanism With Lower-Mobility

Aiming at the great demand for efficient grinding of aircraft composite skin in the aviation maintenance field,a new asymmetric parallel mechanism with compound driving branch chain is proposed to replace manual grinding of aircraft skin.The inverse kinematics,stiffness modeling,stiffness evaluation index and optimization design are deeply researched in order to further improve the mechanism working performance.The main research contents and achievements are as follows.The mechanism is composed of a static platform,a moving platform,a suitable constrained branch chain and two compound driving branch chains.The active pairs are all translational pairs.Considering this kind of mechanism with compound driving branch chain,an efficient inverse position solution method is adopted.Firstly,the compound driving branch chain（PRRS）is simplified to PRPS branch chain,and then the mapping relationship is established between the moving platform and the driving joints.Finally,the cosine law is adopted for equivalent calculation.Based on the method of substructure synthesis,the stiffness models of three branch chains are constructed respectively to obtain the machine stiffness model.The simulation solution is compared with the numerical solution,which verifies the rationality and effectiveness of the stiffness analytical model.According to the characteristics and specific working mode of the mechanism,a new directional comprehensive stiffness index is proposed to evaluate stiffness performance.Firstly,₁w is defined to represent the feed stiffness of grinding work,₂w is defined to represent the uniform stiffness of grinding contact surface.Secondly,by introducing the exponential scale criterion of analytic hierarchy process,the weight coefficients of ₁w and ₂w are calculated.Based on the new evaluation index,the minimum stiffness pose is determined in the whole workspace.Considering the kinematics performance and stiffness performance comprehensively,the mechanism parameters are optimized.Firstly,with the kinematics performance as the objective,the optimization values of scale parameters are calculated by particle swarm optimization algorithm.Then,with the stiffness performance as the objective and based on the parameters sensitivity analysis,the optimization values of cross-section parameters are obtained by genetic algorithm.The comparative analysis of the performance graphs before and after optimization shows that the kinematic performance and stiffness performance have been significantly improved,which verifies the rationality of multi-objective performance optimization.