Research On Methodologies For Accuracy Analysis And Synthesis Of Six-dof Parallel Mechanism

Along with the development of industrial technology, the areas of application for parallel mechanism are gradually expanding. However, there are still a series of problems in manufacturing and assembly process for parallel mechanism, the pose accuracy quality control is one of them. Based on analysis on literatures home and abroad, the dissertation deals with issues of pose accuracy analysis and synthesis for 6-dofs parallel mechanism that arises from manufacturing and assembly process.The pose error model of 6-dofs parallel mechanism is firstly established; the pose error distributions within a given workspace are then obtained and analyzed. Focusing on the contradiction between evaluative precision for pose errors and computational burden, A Sobol sequence based quasi-Monte Carlo(QMC) approach can significantly upgrade statistics precisions of pose errors compared with traditional Monte Carlo method; For probability of pose error within a given restrictive sphere, an approximate analytical form of pose error integral equation are deducted, and then relations between pose accuracy probability and the radius of restrictive sphere are obtained. That the central pose errors at any symmetrical poses configuration conform to Rayleigh distribution is deducted and discovered with the isotropy for location errors of joint centers and relations between standard deviations of pose errors during manufacturing stage for the mechanism. And then the probabilities of central pose errors with the heights of moving platform of the mechanism are obtained.Joint clearance is one of important stochastic factors that could produce pose errors of the parallel mechanism. Both the computer simulation experiment and the uniform design of experiment are employed to conduct significant analysis for pose error sensitivities, the pose parameters with significant impact on pose error sensitivities are then singled out. Equivalent redundant structure of the 6-dof parallel mechanism is constructed with the simplified model of joint clearance. The issues for traditional pose accuracy model are analyzed with probabilistic approaches based on forward kinematics, and then a deterministic approach based on forward kinematics are proposed for pose stochastic errors due to joint clearance. Distributions of pose stochastic errors within given workspaces are investigated and pose errors influenced by joint clearance within a given scope are also analyzed, the conclusions support the results from accuracy sensitivity analysis. Focusing on central pose stochastic errors of parallel mechanism during accuracy measuring process, both probability and deterministic approaches based on forward kinematics are applied, the curves of central pose stochastic errors with platform upgrading are obtained and both relations and differences of both approsches are finally pointed out.Considering the practices of manufacturing and assembly process for 6-dofs parallel mechanism, and issues for pose accuracy synthesis are analyzed, it holds that results from accuracy synthesis should be a set of solutions that relate with assembly property at the basis of current manufacturing ability, thus, an orthogonal and uniform combined experiment design based accuracy synthesis approach is proposed. The orthogonal design is conducted firstly with current manufacturing capability and experience accumulated in practice, to obtain sensitivities sequence of influencing tolerance factors, the levels of these tolerance factors are then redivided in terms of their sensitivities. The uniform design makes use of these levels to generate synthesis solutions with global sense. The proposed approach can avoid possible failure of any single optimized solution by coordinating structural tolerance in active to promote applicative stability in practice. Contrast results indicate the structural tolerances allocated in part of solutions are amplified.Combining with partial contents in this thesis, an accuracy analysis and synthesis system for 6-dofs parallel mechanism is developed and the results has been verified for application.