| The investigation of the precision for parallel mechanism includes mechanism error analysis,error compensation and calibration,which has became a hot topic currently.Parallel Mechanism since the advent,has attracted extensive concerns from the scientific research institutions around the world.With the development of Parallel Mechanism on precision,the components joint clearance can seriously reduce the movement precision of Parallel Mechanism,and affect the performance of parallel mechanism system.Therefore,the effects of the joint clearance on the movement and machining precision have to be considered for designing high precision and good performance Parallel Mechanism.Parallel mechanism is a complex spacial closed-loop mechanism,for each branch with redundancy error.Only using the ways of solving open chain mechanisms cannot accurately describe the accuracy characteristics of the component.Taking 3-TPT parallel mechanism as the research object,the effects on the work precision caused by joint clearance and on the work place of the movable-platform due to position errors are investigatal,which lay a foundation for institutional design in practical application.The main content of this thesis is as follows:Firstly,the characteristic of the structure for the 3-TPT parallel mechanism was analyzed.The degrees of freedom was calculated,and the motion equation of this kind of mechanism was investigated.The Jacobi matrix of the parallel mechanism was applied to analyze the movements and evaluate the singularity and stability of the mechanism.Meanwhile,the working space of the mechanism was also discussed.Secondly,the error model of the working accuracy of the parallel mechanism was established.Through the analysis of the inverse position kinematics solution,the D-H matrix method and error independent method were used to micronize the mechanism joint clearance.The methmatical model describing the position error of the movable-platform position error caused by its joint clearance was developed when the movable-platform was parallel to fixed-platform for the ideal state.Thirdly,the 3-TPT Parallel mechanism clearance-error model was established based on the D-H matrix method and the error independent method when the joint clearance error source was given.The Monte Carlo method was used to analyze the effect on the 3-TPT parallel mechanism working accuracy caused by the joint clearance error.The effect of the posture error due to the revolution of the movable platform about coordinate axis on the error of the whole parallel mechanism was simulated and analyzed based on D-H matrix method for the non-ideal state.In the end,the optimization design for the inner and outer ring clearance of 3-TPT parallel mechanism hooker joint was conducted,when the demanding for parallel mechanism working accuracy was given.A multi-objective optimal model was established in which the working accuracy was derived as the function of inner and outer rings clearance of hooker joint.The genetic algorithm was used to find the set of clearance that made working precision reach requirement.This work aims at reducing the manufacturing demand and avoiding for the ideal state avoid equipment precision waste.In this paper,the error model for 3-TPT parallel was established and analyzed based on mechanism accuracy theory.By studying the influence of the joint clearance error on the working place of movable platform and the effect of position error on the working precision,a solid theoretical foundation was provided for the further study of error analysis of parallel mechanism. |
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