| China's lunar exploration phase third project plans to launch a probe in 2017,its main goal is to achieve lunar samples(lunar soil)sampling and return.The lunar regolith sampling test is to simulate the complex environment of the lunar surface in the laboratory,through the simulation of different landing position and attitude,the test data of drilling and sampling are obtained.According to the lunar regolith sampling test platform design index of land poses simulation requirements,for institutions on the adjustment of the lunar regolith sampling test platform poses is analyzed comprehensively at home and abroad.We analysed the multi scheme design and got an innovative design and manufacture of a new position and attitude adjustment mechanism based on the 3 degrees of freedom of the platform of the lunar soil sampling test platform based on the adjustable stiffness.This lunar soil sampling test mechanism is a novel mechanism applied in soil sampling test.There are three servo motors driving three actuating arms with one just-constrained chain series which strictly constrains three dofs of the moving platform.In this paper,based upon researches on theory of parallel mechanisms,studies of kinematics,dynamics,optimization on stiffness and dimensions,error analysis and compensation,reliability,control system and test are conducted deeply.The main contents of this study are as follows.Lunar soil sampling test mechanism is used to adjust 3 dofs of the moving platform.Based on sampling data from the drilling and surface sampling devices which are installed on the parallel mechanism,this lunar soil sampling test mechanism could finish the sampling package test.On the basis of structural dimensions of the parallel mechanism,the forward and inverse kinematics equations could be achieved.The jacobian matrix of the parallel mechanism could be acquired by differential variation method based on symbolic operation and the other jacobian matrix of the series just-constrained chain could be got by D-H method.The working space of the mechanism could be resolved by searching method after analyzing the constraint conditions.The working space of one,two,three dofs could be directly obtained by numerical simulation.Based on the jacobian matrix,the singularity of the lunar soil sampling test mechanism is analyzed.According to its load conditions and structural characteristics,the dynamics mathematics model of the lunar soil sampling test mechanism could be established by the virtual work principle and the dynamics simulation of several typical motions could be viewed.The stiffness model of the parallel mechanism could be obtained by a method which is based on the jacobi matrix differential method.Compared with the traditional stiffness model analysis method,the progress of solving the stiffness distribution is more effective.The simulation results of the mechanism's flexibility show that the traditional stiffness model and the kinematics static stiffness model are consistent,but there is a certain difference between the flexible stiffness model and the other two stiffness models.According to the results of modal analysis,we got the natural frequency of different locking condition analysis mechanism that the influence of rigidity of the locking and release mechanism of branched chain properly constrained obviously of all locking methods to improve the rigidity of the mechanism.The natural frequency of six order modal value mechanisms and mode is analyzed,and the correctness of the further stiffness analytical model is verified.And the lunar soil sampling test position and attitude simulation mechanism could not have resonance phenomenon on test field and test platform.The sampling test could be carried out safely.On the basis of analyses of each structural dimensions' affections on the lunar soil sampling test mechanism's workspace and performance,an optimization method is proposed.The optimization goal is to obtain the mechanism's best velocity performance and manipulability performance according to its main structural dimensions.The parameter's scope is determined after the first-stage filtering and moving platform's radius.Static platform's radius,velocity performance and manipulability performance are chosen as the optimization function's parameters.The optimization results show that this way is an effective way of parameters for optimization.Based on the analysis of the mechanism's error sources,the mathematics model of the error could be established by the vector method.The legs' error and the installation error of hinges have more influence on the accuracy of the parallel mechanism.The indication of accuracy which could provide a theoretical basis for accurate analysis and compensation is proposed.After the analysis of main sources of the error in the parallel mechanism,the error mathematics model of the lunar soil sampling test mechanism is established with vectors.The affection of the error source on the mechanism's accuracy is obtained and the affections of error of the arm and the installation position error of the hinges are the greatest.The indicator of the accuracy is proposed which could provide a theoretical basis for the accuracy analysis and compensation.A novel compensation method based on the driving chain is proposed.Depending on measurements of the laser tracking,the revised kinematics math model is calculated.By compensating the zero position of the driving arm in order to adjust the initial position and orientation of the parallel mechanism,the kinematic accuracy is improved.Based on the analysis of the kinematic accuracy measured by the laser tracker,the displacement and angle error is significantly reduced after compensation which proves that the compensation method is correct and efficient.Based on other solutions of parallel mechanisms' s reliability,a solution model of reliability based on the arm's error and hinges' installation error is proposed.Kinematic reliability in three directions is obtained.The reliability value of the parallel mechanism on different commonly-used test orientation and position is upon 0.99105 which proves that the mechanism has a high kinematic reliability.On the basis of overall control pattern of the lunar soil sampling test mechanism and analysis of current control,velocity control and position control in the servo motors,the control model schematics is obtained.When the test system of the lunar soil sampling test mechanism is built and the measurement of the kinematic accuracy is completed,the parallel mechanism could achieve oscillating movement of one-DOF or multi-DOF coupled.Lunar soil sampling test mechanism meets the design and movement precision with large workspace(coupling workspace up to 20°/450mm)and high kinematics accuracy(0.1°/0.5mm). |
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