Static Stiffness Modeling And Optimization Design Of Three-degree-of-freedom Parallel Mechanism

Aiming at the operational needs of the dust-free workshop of the liquid crystal panel factory,this dissertation investigates into the kinematics performance evaluation,stiffness modeling,lightweight design and optimal design based on deflections compensation of the 3-RRS parallel mechanism,which can be used as the main structure of the glass substrate handling robot.The following contributions have been made.The inverse kinematics solution of 3-RRS parallel mechanism is studied by means of closed-loop vector method,and the inverse position model and velocity mapping model is obtained.With the help of matrix inversion technique,the homogeneous dimensional overall Jacobian matrix is achieved,which is independent of the selection of coordinate system.Then a dimensionless index is developed to evaluate the kinematics performance of the parallel mechanism whose translational and rotational movements are strongly coupled,and the influences of dimensional parameters on kinematic performance is revealed.The feasible region criteria of dimensional parameters satisfying geometric constraints and kinematics performance constraints are studied,which can provide the basis of kinematics for subsequent optimization design.A semi-analytical stiffness model of 3-RRS parallel mechanism considering the gravity of all moving parts and the elasticity of joints/links is proposed.The main ideas includes:(1)The mapping relationship between gravity of moving parts and external load is formulated;(2)Considering the influence of gravity distribution of limb-body and elasticity of all links/joints,the mapping model of deflections is established;(3)On the basis of the formulation of the component stiffness matrices in the joint space,the stiffness model of the parallel mechanism is achieved.The model can effectively separate the influence of elasticity and gravity of components on the end deflection,and can realize the fast estimation of global stiffness.A lightweight design approach of the 3-RRS parallel mechanism based on stiffness constraints is proposed.The approach establishes the mapping relationship between the critical structural parameters and the mass and stiffness of the mechanism by response surface method.According to the hierarchical stiffness matching design flow,the lightweight design of the moving platform and the limb-body assembly is carried out respectively with the stiffness of the mechanism as the constraint and the minimum total mass of the moving parts as the optimization objective.The proposed approach can effectively reduce the total mass of the mechanism while guaranteeing the stiffness of the mechanism.An approach for improving precision of the 3-RRS parallel mechanism by compensating the deflections caused by the externally applied wrench and gravity is presented.The approach is implemented by introducing springs on moving platform and the limb-body.Then,the spring force throughout the entire task workspace is formulated.Taking the externally applied wrench and spring force as well as gravity of all moving components into account,the mechanical model and the deflection model of the mechanism is achieved.In addition,by taking the absolute value of the mean value of the linear deflection of platform along the z-axis and the standard deviation as objective functions,the parameter design of spring is equivalent to a multi-objective constrained nonlinear programming problem which is bounded by that the kinematic performance of mechanism can be remained despite the existence of springs.The results show that the established approach reduces the deflection of the end effector significantly,as well as introducing springs on moving platform reaches a better effect.