Research On Optimization And Virtual Prototype For Large Hydraulic Parallel Simulators

Motion simulators are test equipments which can be used for developing new facilities and for the training.6-DOF parallel motion simulators possess a number of advantages such as higher rigidity,better positioning accuracy and load capacity.Therefore,they have been used in various applications.However,due to the multiple closed-loop structure and kinematic constraints,they also have disadvantages such as smaller workspace,higher nonlinearity, stronger coupling,furthermore,there are numerous singularities in workspace which make the simulators lose their stiffness,precision and load-carrying capacity.Large 6-DOF simulators are often driven by fluid power,and in general,the control of hydraulic actuators is more challenging than that of their electrical counterparts.The factors such as nonlinear flow/pressure characteristics,variations in the trapped fluid volume due to the piston motion,fluid compressibility,flow forces and their effects on the spool position,and friction,all contributing to the significant nonlinear behavior.This will influence the actual control bandwidth,which is less than half of the natural frequency in general.Currently the control bandwidths of large simulators are relatively small.This thesis deals with large hydraulic parallel simulators in applications with requirements of high precise positioning and good dynamic performance.A Lagrangian dynamic formulation which considers the whole leg inertia is developed,and the influence of legs on the dynamics is studied.Based on the accurate inertia model,an optimal design method to expand the bandwidth for the control of large hydraulic 6-DOF simulators is proposed,the influence of design parameters on the generalized natural frequency are investigated,and this optimal method has been put into application in the design of a large hydraulic simulator.In addition,a co-simulation model of the hydraulic parallel simulator is built,including hydraulic system, mechanism and control strategy.With the simulation model,mathematical model and experiments,the coupling characteristics has been studied.Based on the coupling laws among the legs,two kinds of decoupling methods are proposed.The effectiveness of the decoupling methods is validated.In Chapter 1,based on the research information home and abroad,the history,features and applications of the motion simulators especially 6-DOF parallel simulators are introduced.The related technologies are presented in detail with the latest progress of last few years,including kinematics,dynamics,control strategy,controlling hardware,optimizations and so on.Some trends and difficulties of the development for the parallel simulators are also summarized. Finally,according to the purpose and significance of the study,the major research contents of the thesis are introduced.In Chapter 2,a Lagrangian formulation which considers the whole leg inertia is developed for the dynamics of large parallel simulator,and the accurate equivalent mass matrix is obtained and verified by ADAMS model.Numerical examples are carried out to validate and confirm the efficiency of the mathematical model.Furthermore,the effect of leg inertia on the equivalent mass matrix is studied,and three parameters(acceleration of the moving platform,mass ratio of leg to upper platform,mass ratio of cylinder to piston) are used to investigate the influence of leg inertia on dynamics more detailed.In Chapter 3,an optimal design method,based on generalized natural frequency,to expand the bandwidth for the control of large hydraulic 6-DOF parallel simulators is proposed.An ADAMS model is built to validate the generalized natural frequency mathematical model.The influences of the diameter ratio and the joints angle ratio on frequencies are studied for different configurations.A detailed investigation of the influences of design parameters on natural frequencies is carded out for 6-6 SPS configuration.With workspace verification,the optimized structure parameters for one large hydraulic 6-DOF simulator are obtained.In Chapter 4,a co-simulation model of the hydraulic parallel simulator is built including hydraulic system,mechanism and control strategy.Based on the interface,the combined simulation modeling strategy for multiple research areas is studied,and the detailed models and different universal co-simulation methods are presented.The influential factors on the model's accuracy are investigated,and a method to detect and correct the model discontinuity is developed to ensure the accuracy of the co-simulation model.In Chapter 5,combined with the co-simulation model,mathematical model and experiments,the complex coupling characteristics of the hydraulic 6-DOF parallel platform is investigated.Firstly,the dynamic coupling of the parallel simulator is studied based on the dynamic consistency of six actuators.Furthermore,the load coupling among the six actuators of the platform is investigated in detail.Finally,based on the coupling laws among the legs,two kinds of decoupling methods are proposed.The effectiveness of the decoupling methods is validated.In Chapter 6,conclusions of the thesis are summarized and the future research work is put forward.