| In the past decades,the parallel mechanism is widely used into the machine tool,spacecraft simulator,food package and medical domain because of its excellent dynamic performances that are high load capability,high accuracy and high speed,and so on.However,the clearance joints existing in the parallel mechanism are inevitable in the developed process of the parallel mechanism because of the manufacturing and assembling errors as well as deformation.The clearance joints not only get rise to contact and wear phenomenon between clearance joint elements,but also severely degenerate the parallel mechanism’s dynamic performance.Therefore,the quantitative analysis for the effect of clearance joints and flexible bodies on parallel mechanism’s dynamics property and clearance joint’s wear phenomenon is the open problem in the present.This thesis systematically developed the dynamics property of a novel 4-DOF(Degree of Freedom)parallel mechanism with joint effects and flexible body.A novel parallel mechanism that can implement three rotations and one translation is synthesized based on the screw theory and configuration evolution.Its workspace is obtained in term of the constraint conditions that include kinematic constraint equations,the length limitation of kinematic chains,the rotation limitation of joints and mutual interference between kinematic chains.The singularity configuration is described based on the Grassmann geometrical theory and screw theory.The developed conclusions of its kinematics can be treated as the fundamental theory for designing and manufacturing.Furthermore,in order to determine the effect of transmiting force in the joints,from the static point of view,the static equilibrium equations of 4-SPS/CU parallel mechanism are formulated.However,the number of the equations is greater than the number of the unknown reactions.In order to obtain all of the reaction force in this parallel mechanism,the local transformation matrix is used to remove the redundant equations corresponding to the paasive degree of freedom,which results in the number of equations equal to the number of reaction forces.All of the reaction force are determined by solving this linear equations.This research reminders that the preprocessing of the passive degree of freedom is very important before solving the reaction forces in the mechanism with the passive degree of freedom,especially in the overconstrained mechanism with passive degree of freedom.Considering the university of friction phenomenon in the mechanical system,in order to quantitatively predict the effect of friction existed in joints on driving force,the friction forces are considered based on the ‘Coulomb + viscous’ friction model in the parallel mechanism.The dynamics model of the parallel mechanism with friction force is built using Newton-Euler method.The changed trend of the driving force considering friction force is obtained,which provides a reference for the selection of the parallel mechanism’s actuator.Moreover,considering that the clearance is inevitably existed in the joint,in order to further enhance the parallel mechanism dynamics model’s accuracy,a spherical joint is treated as the clearance joint.Considering that the contact force model plays an important role in the clearance joints,hence,the Hertz contact stiffness coefficient of Flores contact force model is improved by a nonlinear contact stiffness coefficient which introduces the coupling between contact deformation and contact stiffness coefficient.The parallel mechanism dynamics model with clearance spherical joint are formulated based on the equation of motion.The simulation shows that the high frequency characteristic of the acceleration and reaction force is motivated by the clearance spherical joint,which degenerates its dynamic performance.Considering that directly introducing the clearance joint and flexibl moving platform in to the parallel mechanism is difficlut.Hence,firstly,the spatial four bar mechanism is taken as the example,the spherical joint is considered as a non-regular clearance joint caused by wear phenomenon.In order to save the computational cost,an approximately calculated method of the contact area between clearance spherical joint is proposed.The contact pressure between socket and ball can be calculated based on an approximated contact area and an improved LankaraniNikravesh contact force model considering the coupling between contact deformation and contact stiffness coefficient.The correctness of improved contact force model and the approximated contact area is validated by Finite Element Method.Subsequently,based on Archard’s wear model,the wear depth of the socket is predicted using the improved contact force model and the approximated contact area,which provides a more directly wear prediction of clearance spherical joint compared to Finite Element Method.Secondly,in the case of simultaneously introducing clearance joints and flexible bodies in the planar five bar mechanism,the link is treated as a shear beam based on the absolute nodal coordinate formulation.The wear depth of the clearance revolute joint is predicted based on Archard’s wear model.Likewise,the Flores contact force is also modified using the nonlinear contact stiffness coefficient,moreover,in order to enhance wear predicted accuracy furtherly,the dynamics model of planar five bar mechanism with multiclearance joints and multi-flexible bodies is studied.Simulation results show that the dynamics model considering multi-clearance joints and multi-flexible bodies is more accurate than the model considering single clearance joint and single flexible body.According to above studied conclusions,in 4-SPS/CU parallel mechanism,the moving platform is regarded as thin plate elements based on the absolute nodal coordinate formulation.The one of the spherical joints connecting moving platform and kinematic chain is treated as clearance joint.The simulation illustrates that the dynamic properties of parallel mechanism is not sensitive to the moving platform’s flexibility because of itself stiff characteristic.However,the flexible property of moving platform can alleviate the high frequency caused by clearance joints when considering the clearance joint and flexible body in the parallel mechanism at the same time.Furthermore,in order to predict clearance spherical joint’s 3D wear phenomenon,the wear parameters that are the contact force,tangential contact velocity and approximated contact area are decomposed into the global coordinate system.The wear depth along three different directions of the socket is respectively predicted based on Archard’s wear model in the global coordinate system.The 3D wear contour of the socket is described from different prospectives.The comparative analysis between flexible model and rigid model is implemented,which proves the moving platform can alleviate the wear phenomenon.Simulation results show the 3D wear model of clearance spherical joint not only improves the wear predicted precision,but also provides a new strategy for the spatial joint’s wear prediction. |
