| Hip joint simulator is used to evaluate the friction and wear characteristics of the artificial hip joint biomaterials,which is required to have the kinematic characteristics similar to the biological characteristics of human hip joint and meet the requirements of dynamic loading.To overcome the deficiencies of serial hip joint simulators in stiffness,dynamic loading,complex trajectory simulation,and so on,a 2(3HUS+S)parallel hip joint simulator which has two moving platforms and can conduct friction and wear tests for two artificial hip joints simultaneously,is proposed.The hip joint simulator makes up for the shortcomings of traditional parallel hip joint simulators which has only one moving platform and low test efficiency.With the closed structure characteristics of parallel manipulator,in the motion law of high speed and acceleration,the nonlinear characteristics led by inertia coupling,elastic deformation,friction and clearance of kinematic pairs will intensify the influence on the dynamic performance of parallel manipulator.Therefore,to improve the performance and control precision of the parallel hip joint simulator,the studies on kinematic accuracy analysis and comprehensive performance optimization were conducted with the dynamic performance analysis influenced by the inertia coupling,elastic deformation,friction and clearance of the kinematic pairs considered.The control strategies considering structural error and friction compensation simultaneously,were presented.The effectiveness of the model and the method was verified by experiments.The main research contents are presented as follows:1)To study the inertia coupling characteristics between the active branched-chains of the parallel hip joint simulator,first,on the basis of the kinematic analysis,a dynamic model based on the workspace was established with the help of Lagrange method.And then a dynamic model based on the joint space was obtained through the Jacobian matrix transformation.Second,an evaluation index of inertia coupling of the active bracnched-chains was defined according to the inertia matrix of the dynamic model based on the joint space.Finally,the inertia coupling characteristics between the active branched-chains of the parallel hip joint simulator were simulated and verified by virtual prototype experiment.The distribution law of the inertia coupling index in the workspace and the change law with the structural parameters were also obtained.2)For reflecting the dynamic characteristics of the parallel hip joint simulator more realistically,the effects of clearance,friction and lubrication of the kinematic pairs on the nonlinear dynamic characteristics were studied.First,based on the motion analysis of the spherical joints with clearance,contact models on the condition of dry contact and lubrication contact were established.Second,a dynamic model of the parallel hip joint simulator with the contact impact of the kinematic pairs integrated was established.Finally,numerical simulations and experiment studies on the dynamic model were conducted.The influences of the kinematic pair clearance,friction and lubrication on the driving torque of the linear module,the contact force of the artificial hip joint and the nonlinear characteristics of the parallel hip joint simulator were analyzed.3)To study the influences of the active branched-chains' elastic deformation on the static/dynamic characteristics of the parallel hip joint simulator,the stiffness analysis and elastic dynamic modeling were carried out.First,the corresponding parts of the two moving platforms for the parallel hip joint simulator were divided into independent subsystems.A stiffness model of the parallel manipulator was established based on the virtual work principle.Second,the joint lever and the ball screw of the linear module were regarded as the elastic body and the longitudinal and torsional vibration of the ball screw was also considered,and then an elastic dynamic model of the parallel hip joint simulator was established based on the finite element method.Finally,numerical simulations of the theoretical models were carried out.The change law of the stiffness,the natural frequency in the workspace and the change law with the structural parameters were obtained,and also the change law of the elastic error of the moving platform under the desired trajectory were studied.The results were verified by virtual prototype experiment and actual experiment.4)Kinematic accuracy analysis and comprehensive performance optimization of the parallel hip joint simulator were studied.First,with the considering of machining /assembly error of the main parts and the clearance of the kinematic pairs,the error model of the parallel hip joint simulator was established on the condition of small perturbation,and the error parameters identification was conducted.Second,to ensure the parallel hip joint simulator has a better overall performance,a comprehensive optimization model was established with the influence of the driving torque,flexibility of the motion,inertia coupling,structural error on the overall accuracy and the impact caused by the clearance of the kinematic pairs considered simultaneously.The weight coefficient of each optimization index was determined by the AHP method.Finally,numerical simulations were carried out,and the changes of the indexes before and after optimization were compared and analyzed.5)On the basis of the comprehensive performance optimization of the parallel hip joint simulator,to reduce the influence of the structural error and the friction on the control performance,compensation strategies were studied.First,the static errors of the parallel hip joint simulator were modified directly in kinematics.For the dynamic structural errors,the corresponding attitude error of the moving platform was predicted by neural network and compensated through a compensation method based on workspace.Second,Stribeck friction model was applied to model the friction of the main kinematic pairs,and the friction was compensated in the control system by feedforward compensation based on the friction parameter identification.Finally,the compensation control strategies for the structural error and the friction were designed based on the augmented PD control and the computed torque control,respectively,and experiment studies were conducted.6)Based on the structural error and the friction compensation of the parallel hip joint simulator,considering the coordination among the active branched-chains or degrees of freedom of the moving platform,two kinds of synchronization control strategies for the parallel hip joint simulator were designed based on synchronization control.First,the motion coordination of the three linear modules were taken as the target,and according to the dynamic model based on joint space of the parallel hip joint simulator,the synchronization control strategy based on the joint space was designed and the stability was analyzed.Second,to avoid the inertia coupling between the active branched-chains in the synchronization control strategy based on the joint space,and the motion coordination between different degrees of freedom was regarded as the goal,a synchronization control strategy based on the workspace was designed according to the dynamic model based on the workspace.Finally,experiment studies on the two kinds of synchronization control strategies of the parallel hip joint simulator were carried out and the results were compared and analyzed.The paper carried out the dynamic performance analysis and the control strategy research on the parallel hip joint simulator on the condition of multiple disturbances,which is important to improve the dynamic performance and the control precision of the 2(3HUS+S)parallel hip joint simulator.And it can also provide theoretical and experimental references for the performance analysis and compensation control strategy study of other mechanical equipment. |
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