Characteristic Analysis And Optimization Of 6-DOF Cable-spring Driven Parallel Mechanism

In recent years,the cable driven parallel mechanism with new branch chain has become a research hotspot.By connecting the spring as a passive branch chain between the frame and the moving platform of the mechanism,the complete constraint positioning of the mechanism can be achieved with less driving cable,and the effect of improving the performance of the mechanism,simplifying the structure and reducing the difficulty of control can be achieved.In this paper,a 6-DOF cable driven parallel mechanism with spring passive branch chain is studied.The kinematics and statics models of the cable driven parallel mechanism with passive spring are established.Based on the force-closure workspace solution method of the cable driven parallel mechanism,the solution method of the workspace of the mechanism with spring is proposed.According to the different mechanism configurations of several spring arrangements,the 6C2 S configuration is selected as the object for further research by comparing the spring working state and load size.This paper puts forward an evaluation index which can directly and comprehensively reflect the workspace of the cable driven parallel mechanism,analyzes the change of the mechanism workspace after introducing the passive spring,analyzes the influence of the spring stiffness,the position of the hinge point,and the initial length on the workspace.Aiming at the irregular shape of workspace caused by spring,a mixed factor evaluation function was constructed to evaluate the severity of shape defects,and its sensitivity was verified.By studying the change of the tension distribution and the uniformity of the cables after the introduction of the passive springs,the reason why the passive spring increases the working space of the mechanism is explained.Based on the differential transformation and line vector theory,the total stiffness model of the cable driven parallel mechanism with passive spring is derived,and the stiffness distribution of the mechanism in each directions is studied.The passive stiffness matrix is decomposed by matrix transformation,and the stiffness change of the mechanism after the passive springs is introduced is analytically analyzed.By introducing the evaluation indexes of tension uniformity and stiffness,and defining the feasible workspace,the influence of spring parameters on the tension uniformity and the static stiffness of the mechanism is analyzed.In order to establish the multi-objective optimization model,firstly,according to the performance indexes of the mechanism,the optimal objective function is established for single objectives.The optimal value of single objective is obtained by genetic algorithm,and then the ideal point is obtained.Next,based on the shortest-distance TOPSIS and dimensionless processing,the multi-objective optimization function is established.Afterwards,the evaluation function of workspace shape defect is regarded as a nonlinear constraint condition,and the evaluation function is introduced into the optimization model as penalty function of exterior point method.Finally,the multi-objective optimization model is constructed by combining the TOPSIS and genetic algorithm.Different objective weights and penalty factors are set to achieve multi-objective optimization design,and the effect of optimization design is discussed according to the results of different settings.