Research On Stiffness Modeling,stiffness Defect Identification And Repair For A Class Spatial Parallel Robots

Due to such advantages as higher stiffness,smaller accumulated error and higher accuracy,parallel robots have been used widely in various manufacturing applications.Compared with traditional machine tools,parallel robots as machine tools can perform machining operations at different positions and orientations,it increases the flexibility of the manufacturing system and recovers the application limitations of traditional machine tools.Although a variety of novel parallel robots have been designed and manufactured,there are few products in practice for parallel robots as machine tools.One of the important reasons resulting in this phenomenon is that the theoretical stiffness advantage of parallel robots has not been implemented.In order to explore the reasons that restrict the theoretical stiffness advantage of parallel robots in practical applications,this dissertation focuses on stiffness modeling,stiffness design defect identification model and stiffness design defect repair method for a class of spatial parallel robots(SPRs),in which active chains are composed of joint-prismatic pair(active joint)-joint.The main research contents and innovations are summarized as follows:As the basis of the stiffness analysis,analysis methods of kinematics,dynamics and static for SPRs are studied based on the screw theory.In order to express the motion laws of the end platform in screw system,methods of solving the speed screw and the acceleration screw of a rigid body from the generalized coordinate system are given.According to the physical meaning of the amplitude of speed screw,a method for solving the kinematic Jacobian matrix by means of screw theory is given.Based on the proposed concept of virtual screw and the Jourdain virtual power principle,a general dynamic modeling method is proposed.Modeling method of static stiffness model for SPRs is studied.In order to involve various factors affecting the stiffness characteristics,a global static stiffness model which comprehensively considers driving stiffness,joint clearances,joint contact deformations and link deformations is proposed.Considering the assembly gaps of joints in all directions,some joint clearance models are proposed.Based on the Hertz's precise contact principle,some joint contact deformation models are proposed.Based on the virtual work principle,static stiffness models of SPRs considering the influences of joint clearances and joint contact deformations are established.Based on the virtual work principle and the Castigliano second theorem,a static stiffness model of SPRs considering the influence of link deformations is proposed.Modeling method of dynamic stiffness model(vibration model)of SPRs is studied.In order to build the vibration model equations of SPRs,a new dynamic modeling method based on screw theory and Kane equation is proposed,in which driving force,external load,first-order term of the speed of the end platform,second-order term of the speed of the end platform and the acceleration of the end platform are independent each other.Based on the proposed dynamics model,a modeling method of the dynamic stiffness model of SPRs considering stiffness of all branches,damping of all branches,joint clearances,joint contact deformations,external excitation and gravity is presented.Identification model and method of stiffness design defects for SPRs are studied.In order to study the defects in various analyses related to the stiffness design,a global identification model of stiffness design defects is proposed.In order to evaluate the global stiffness characteristics of SPRs,the domain constraints of universal joints are used to obtain the global workspace of SPRs.A demarcation diagram of different duties for SPRs is proposed,where different secondary factors in various analysis models of SPRs for different duties can be ignored.A new stiffness index which can represent the stiffness characteristics of different directions in actual processing is proposed.Combined with the statistical simulation method and uncertain factors affecting the stiffness design,a stiffness design defect identification method for identifying whether a SPR has stiffness design defects and judging the severity of stiffness design defects is proposed.Repair methods of stiffness design defects for SPRs are studied.According to the severity of stiffness desgin defects,three stiffness design defect repair methods based on component synthesis,dimensional synthesis and configuration synthesis are proposed.For the minor stiffness design defects,a method named de-feature repair based on component synthesis is proposed.For the stiffness design defects which cannot be repaired by the component synthesis,a repair method based on dimensional synthesis and parametric atlas theory is proposed.For the major stiffness design defects,a repair method based on configuration synthesis and stiffness characteristics comparison is proposed.Stiffness modeling,stiffness design defect identification and stiffness design defect repair for a class of SPRs,in which active chains are composed of joint-prismatic pair(active joint)-joint,are researched deeply and systematically in this dissertation.Problems including that stiffness modeling methods are not universal,stiffness models are not accurate,stiffness defect identification results are unreliable and stiffness defect repair methods are lacking are solved in this dissertation.Moreover,results in this dissertation reveal some reasons for the big difference between the theoretical stiffness characteristics and the actual stiffness performance,and researches in this dissertation have enriched the theoretical system of stiffness design for SPRs as machine tools.