| With the development of robot equipment in the direction of high speed,high precision and high stability,the requirement for the dynamic performance of mechanism is becoming higher and higher.Parallel mechanisms are widely used in mechanical manufacturing,aerospace and other fields due to their strong bearing capacity,high accuracy and high stiffness.The existence of clearance between kinematic pairs is inevitable in actual mechanism,and with the increase of running time,the clearance size will also increase.The clearance causes contact collision and wear phenomenon between elements of kinematic pairs,which is an important reason for the performance and accuracy of mechanical equipment to decline or even fail.Therefore,the parallel mechanism considering clearance of kinematic pairs is studied.The influence of clearance and wear on the dynamic characteristics of mechanism is an urgent problem to be analyzed.This paper takes 3-PRR parallel mechanism as the research object.The dynamic model of 3-PRR parallel mechanism with multi-clearance is established.The dynamic response and nonlinear characteristics of the mechanism are studied.The influence of wear clearance of the multiple revolute pairs on the dynamic characteristics of the mechanism is analyzed.The main research contents are as follows:Firstly,the forward and backward kinematics models of 3-PRR parallel manipulator are established,the workspace of the moving platform is analyzed,the influence of the attitude angle of the moving platform on the workspace is discussed,and the dexterity characteristics of the mechanism are analyzed.Based on the second kind of Lagrange equation and Lagrange multiplier method,the rigid body dynamics model of the mechanism is established,and the driving forces of each driving joint under no-load and load are solved.The results of Adams simulation and MATLAB calculation are compared and analyzed to verify the correctness of the theoretical modeling.Then,the dynamic model of 3-PRR parallel mechanism with multiple clearances is established based on Lagrange multiplier method.The effects of clearance position,clearance value and friction coefficient on the velocity,acceleration of the moving platform and driving force of branching chains and axis trajectory are both analyzed.Phase diagrams,Poincare maps and bifurcation diagrams are used to study the influence of driving speed,clearance value and friction coefficient on the nonlinear characteristics of moving platform and clearance joints,which lays a foundation for the follow-up study of clearance compensation control of relevant mechanisms.Finally,based on Archard’s wear model,a dynamic calculation model of wear of the revolute pair with clearance is established.The wear characteristics of three revolute pairs with clearance are analyzed and predicted.The surface contours of the worn shaft and bearing are reconstructed.A 3-PRR parallel mechanism dynamic model with irregular wear clearance is established for the revolute pair with reconstructed morphology.The influence of irregular clearance of revolute joint on dynamic characteristics of mechanism is also researched.It lays a foundation for further research on reliability analysis and life prediction of relevant mechanisms. |
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