| With the development of intelligent manufacturing equipment,many application scenarios require multi-degree-of-freedom(DOF)motion capability achieving high precision,high speed and high acceleration.The dynamics of core mechanism in intelligent manufacturing equipment has become one of the key issues which determining the performance and seriously affecting its work accuracy,operational reliability and service life.A multi-DOF intelligent manufacturing equipment based on a parallel mechanism has multiple kinematic branches,and each of branches also has several kinematic joints.The joint on branches has characteristics such as clearance,wear and lubrication.These characteristics will affect dynamic response of the equipment with high-speed and high-precision function.Therefore,the influence of joint characteristics of the parallel mechanism in the intelligent equipment on the dynamic performance has been increasing in the field of equipment research.Focusing on the high speed and high acceleration application requirements of the parallel mechanism in intelligent equipment,this paper takes a 3-PRR planar precision positioning platform with 3-DOF configuration developed for electronic manufacturing equipment as the research object,considering the application scenarios with high speed and high acceleration.The dynamic effect of joint clearance,wear and lubrication on the mechanism is studies theoretically and experimentally.The main contents are shown as follows:Firstly,a dynamic model of 3-PRR parallel mechanism with clearance joints is proposed.The proposed model integrates the dynamic equation of the mechanism,the force model of clearance joint and the force model of the moving components.Bathe integral algorithm used to solve the proposed model is improved to achieve long-term numerical stability,and provides a necessary condition for wear simulation studies.The comprehensive analysis of the dynamic response of the 3-PRR parallel mechanism with clearances is completed.To improve the performance of the mechanism,the dynamic response of three parameters: the clearance size,restitution coefficient of the material and movement trajectory is discussed quantitatively.Secondly,a joint wear model based on Archard's model is proposed considering the high frequency collisions of the joint.Due to the complex relative motion between the journal and bearing on the 3-PRR mechanism,the contact or collision points are discontinuous.The slip length is calculated by integrating the tangential velocity of the contact point,and the wear amount modeling of the single-step wear region is proposed by combining the contact stress model of the contact area and the slip length.Based on the linear interpolation method,the wear amount of the single-step wear area is added to the circumference of joint to realize the wear contour reconstruction.By dynamically updating the threshold,the update rate of the joint wear profile calculated into the dynamic equation of the parallel mechanism is controlled.By comparing the effect of five different motion trajectories on the joint wear of the mechanism,it can be concluded that the wear profile of the bearing can well correspond to the relative motion trajectories between the bearing and the journal.The wear characteristic of the joint profile illustrates the availability of the proposed joint wear model.Furthermore,by ranking the wear life of the joints in different positions,the relationship between the wear life of different joints and the trajectory is discussed.Thirdly,a correction factor is proposed to modify the Pinkus-Sternlicht(P-S)hydrodynamic force model,then a dynamic methodology containing the modified P-S model for the3-PRR parallel robot with multiple lubricated joints is established.The nonlinear effect of different correction factors on the modified P-S model is discussed.The availability and advantage of the modified P-S model in the 3-PRR parallel mechanism dynamics analysis is illustrated by the comparison of four different models.A comprehensive analysis of the dynamics of the 3-PRR mechanism with lubricated joints is completed,and the dynamic response is also compared with the 3-PRR mechanism with clearance joints.By comparing the maximum acceleration error value,the maximum velocity error value,the maximum trajectory error value and the angular positions where they located,the improvement of the lubrication on the dynamic performance of the 3-PRR mechanism is analyzed quantitatively.Based on the numerical simulation,the modeling and analysis process describes the dynamic characteristic of the 3-PRR parallel mechanism with lubricated joints,which is closer to the real joint than only considering the joint clearance.Finally,a simplified revolute joint with replaceable elements is designed,and a 3-PRR mechanism experimental system and measuring system with marked joint characteristics are built.Several experiments are carried out with respect to the clearance,wear and lubrication of the revolute joints.The models and simulations proposed previously are demonstrated by experimental result.In view of the problems found in theoretical and experimental research,the future works are put forward.All work will provide important models and methods for the research and development of dynamic performance of precision equipment with high speed,which dependents on parallel mechanism to achieve functions. |
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