| Parallel mechanism is widely used under heavy load and complex working conditions because of its large stiffness,strong bearing capacity and no accumulated error.Among them,the low degree of freedom parallel mechanism has the advantages of easy construction of kinematic model and simple control.At present,the design of parallel mechanism is a complex engineering including two aspects of mechanism configuration synthesis and dimension synthesis: determining the optimal configuration in the possibility of configuration synthesis,setting multiple performance indexes based on the current use requirements,and finally carrying out the multi-objective optimization of dimension parameters.This design process fully reduces the unnecessary repetition of design links,and improves the design quality and efficiency Rate.In this paper,the 2T1 R plane pose adjustment mechanism of large-scale special equipment under heavy load is taken as the research object.Firstly,the configuration is optimized,and the evaluation indexes of kinematics,dynamics,stiffness and dynamic performance are established.Finally,the multi-objective optimization platform of scale parameters is built and the optimization and result analysis are carried out,which provides the preliminary comprehensive design and product optimization of this type of mechanism Programme.According to the application requirements of 2T1 R parallel mechanism,8configurations of PR pair combination are listed.Combined with the application environment and complex working conditions,the 3-PRR configuration scheme of hydraulic direct drive is selected by considering the factors such as actuator layout,component force analysis,component moment of inertia,system natural frequency,system stiffness and lateral load of hydraulic drive.Based on the requirements of engineering practice,the performance of 3-PRR parallel mechanism is analyzed,and the generalized evaluation indexes of kinematics,dynamics,stiffness and dynamic performance are established based on Generalized Jacobian matrix and generalized dynamic equation.Based on the inverse kinematics analysis of 3-PRR parallel mechanism,a third-order Jacobian matrix of input and output speed mapping is constructed.According to the "dimension separation method",the Jacobian matrix is divided into two sub matrixes with different dimensions to avoid the problem of unclear physical meaning of performance evaluation index(kinematics,dynamics,stiffness performance)based on Jacobian matrix.Based on the principle of virtual work,a complete dynamic equation of rigid body is constructed.Through the comparison of analytical method with multi rigid body kinematics and dynamics simulation,the correctness of the numerical analysis model of kinematics and dynamics is verified.The establishment of the numerical model provides specific expressions for the performance indexes of kinematics and dynamics.The equivalent stiffness model of the system is established based on the combination of hydraulic drive stiffness and transmission chain stiffness,and a more comprehensive stiffness analysis model is established.Based on the idea of "dimension separation method" and full time-domain mean value,the performance indexes are improved,so that it can comprehensively and completely evaluate multiple performances of parallel mechanism.According to the equipment use environment and design requirements,the objective function of performance index and design parameter design space are determined,and the multi-objective optimization model is established.The calibration experiment and design parameter identification are carried out for the non optimized experimental platform,and the actual values of all design parameters are substituted into the performance global numerical analysis model to demonstrate the necessity of multi-objective optimization of scale parameters.The multi-disciplinary optimization tool is used to build the platform of DOE and multi-objective optimization,and the influence of design parameters on each response is analyzed.Taking the approximate optimization solution as the initial value,the non dominated sequencing genetic algorithm with elite strategy(NSGA-II)is used to optimize the scale parameters of 3-PRR parallel mechanism.The compromise solution is selected as the recommended design point in Pareto solution set.The proposed design points are substituted into various performance evaluation models and compared with the evaluation results of the non optimized experimental platform to verify that the effect of multi-objective optimization is significant.According to the recommended design point,the structural parameters of the experimental platform are improved,and the maximum error of the shaping experiment before and after the improvement is compared,and the effect of performance index optimization on the improvement of the shaping accuracy is analyzed. |
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