Research On Dynamics And Control Of Planar Parallel Mechanisms With Clearance Joints

Due to assembly error,machining accuracy,and wear,the joint clearance in the mechanism inevitably exists,which greatly influences the motion precision and dynamic performance of the mechanism.Meanwhile,for the complex multibody mechanism,the dynamic modeling efficiency has to be improved.So researchers attach more attention to the dynamic moeling and analysis of mechanisms with clearance joints.This dissertation focuses on modular dynamic modeling method of planar multibody mechanism and mathematic description of a clearance joint so that a modular dynamic modeling method emboding the hierarchy of the topology is proposed,which can be easily extended to mechanisms with clearance joints.Through solving the nonlinear differential algebraic equations with the improved numerical integration algorithm,the influence of clearance joints on dynamic performance of a mechanism is systematically analysed so that it can guide the optimization of the structural parameters of the prototype and the distribution of the clearance joints.This dissertation's main contents are as following:In order to improve the dynamic modeling efficiency of complex multibody mechanism,a modular partition method is proposed by classifying the mechanisms by the topology of multibody system.The constraint equation of kinematic pair is established by body-hinge vector and path vector and the topology structure of a multibody system is described by associative array so that the system constraint Jacobian matrix can be integrated automatically and the constrain Jacobian matrix module assembly a complex multi-body mechanism can be formed so as to achieve modular,hierarchical dynamics modeling and make the dynamic model contains the the topology information of mechanisms.A numerical integration method based on Hermite interpolation is proposed for differential algebraic equations of the dynamic model.A unified dynamical model of planar 3-DOF parallel mechanisms is established through the proposed modular modeling method so that the hierarchical and reusability of the proposed modular modeling method can be reflected.To determine the inter-transition of the relative motion states of joint components more precisely,the state transition method is used to describe the motion clearance model and the relative motion state criteria of mechanisms with two clearance joints is improved by the relationship between the amount of contact deformation of the adjacent two time nodes.Combined Lankarani-Nikravesh impact model,the contact force at clearance joints is described completely with a proposed impact force model.A clearance effect evaluation index system is proposed to measure clearance effects on the system dynamic performances under different parameters.A crank slider mechanism with a clearance joint is investigated and verified the proposed models.Aiming at one of the most common planar parallel mechanisms,known as the 3-RRR planar parallel mechanism,the proposed modular dynamic modeling method is used to establish dynamic model.The three rotating clearances connected to the moving platform are considered and the clearance joint force is introduced into the dynamic model as generalized external force.The differential algebraic equations are solved by employing the proposed numerical integration method.The clearanc e effects on dynamic performance of mechanisms are investigated elobrately.Nonlinear dynamic analysis is revealed by Poincare portraits and bifurcation to display nonlinear dynamic characteristics of the 3-RRR planar parallel mechanism.For a mechanism with clearance joints,it is hard to get satisfied control results using the conventional control strategies due to high nonlinearity produced by clearance contact forces.For the 3-RRR planar parallel mechanism with clearance joints,a comparative analysis of fixed compensation control strategies is performed and adaptive compensation control strategy is proposed.The gain-adaptive law is introduced so that the mechanism can be reached into steady state more quickly.The parameter adaptive control law and nonlinear adaptive control law are designed to improve the motion precision in the presence of clearance joints and reduce the inferior influence of clearance joints on the motion stability.On the basis of the clearance effect evaluation index system and manufacturing cost model,the integrated optimized objective function with clearance allocation and structural parameters of this 3-RRR parallel mechanism is set up.Based on the hybridization of genetic algorithm and with an adaptive hybrid probability and inertia weight and asynchronous learning factors,the hybrid particle swarm optimization method is modified.Supplemented by constraints of work space and transfer performance,the clearance joint parameters and the design parameters are obtained with the improved hybrid PSO method.A 3-RRR planar parallel mechanism based prototype is designed,manufactured and assembled.Different sizes and quantities of joints clearance are obtained through manufacturing different journals with different radius.Also,the central position of the moving platform and velocity are collected and the experimental study is carried out.The influence of different numbers and sizes of joint clearance on dynamics performances are examined,which verifies the validity of the previous theoretical analysis.