Type Synthesis And Analysis Of Reconfigurable Parallel Mechanism With Rotation Characteristics Of Variable And Invariable Rotation Axes

Reconfigurable parallel mechanisms,characterized by both variable motion mode and variable topology structure of reconfigurable mechanisms and high bearing capacity and high precision of parallel mechanisms,have become a research focus in the fields of mechanism theory and robotics.The variations in rotation axes of reconfigurable parallel mechanisms are closely associated with the motion performance of the mechanisms,which play a critical role in working conditions of changing terminal attitude capacity requirements.The existing reconfigurable parallel mechanisms have fewer configurations and less achievable motion modes,and the existing literature lacks systematic research on reconfigurable parallel mechanisms whose rotation axis direction can be changed or unchanged.In this thesis,the screw theory is used to synthesize and analyze the configuration of the reconfigurable parallel mechanism with the characteristics of variable rotation axis and fixed rotation axis,so as to improve the adaptability of the mechanism to different working conditions.Firstly,a class of actuated reconfigurable parallel spherical joints with one-dimensional variable rotational axis rotation,one-dimensional invariable rotational axis rotation,two-dimensional invariable rotational axis rotation,and three-dimensional rotation is synthesized using the screw theory.As one actuation joint of the actuated reconfigurable parallel spherical joints is locked away from the coordinate axes,one actuation joint is locked at the initial position,and only the third actuation joint is opened,the actuated reconfigurable parallel spherical joints possess the motion of one-dimensional variable rotational axis rotation.According to the characteristics of the rotation matrix,the decoupling conditions for the Jacobian matrix of the actuated reconfigurable parallel spherical joint as the lower triangular matrix are deduced.This kind of actuated reconfigurable parallel spherical joint realizes the reconfiguration of the spherical joint by opening and locking the actuation joints at different positions,and avoids the branch singularity and constraint singularity of the mechanism during the reconfiguration process.Secondly,actuated reconfigurable parallel mechanisms with suitable constrained branch chains and actuated reconfigurable parallel mechanisms with rotation characteristics of one-dimensional variable and invariable rotation axes are proposed on the basis of actuated reconfigurable parallel spherical joints.The motion modes of the mechanisms are analyzed by applying screw theory,and the results manifested that the actuated reconfigurable parallel mechanisms with suitable constrained branch chains have the motion featured by two-dimensional invariable axis rotation and three-dimensional rotation.After analyzing the variations in the rotation axes on the moving platform of the actuating reconfigurable parallel mechanisms with rotation characteristics of one-dimensional variable and invariable rotation axes,it is found that the multi-mode motion characterize by one-dimensional variable axis rotation,two-dimensional invariable axis rotation,and three-dimensional rotation can be realized by this class of mechanisms.Thirdly,based on the finite screw theory,a class of reconfigurable parallel mechanisms with one-dimensional rotation axis direction that can be variant or invariant(1Rv)is proposed.According to the description of the spherical 4R mechanism by the finite screw,the assembly conditions of the 1Rv reconfigurable parallel mechanism with one-dimensional variable and invariable rotation axis rotation are obtained.The variations in instantaneous rotation axes of 1Rv mechanisms are deduced according to the instantaneous screw theory.Moreover,the 1Rv reconfigurable parallel mechanism realizes the switching between the variable axis rotation mode and invariable axis rotation mode in the bifurcation configuration,and its one-dimensional variable axis rotation mode will generate dependent rotation components in the plane perpendicular to the rotation axis of the initial configuration.Finally,based on the 1Rv reconfigurable parallel mechanism,three classes of 2Rv,1T2Rv and 3T2Rv reconfigurable parallel mechanisms with two-dimensional variable rotation axis and invariable rotation axis rotation characteristics are synthesized according to the finite screw method.Based on the 1Rv reconfigurable parallel mechanisms,the assembly conditions and rotation axis variations of 2Rv reconfigurable parallel mechanisms with two-dimensional variable and invariable axis rotation are analyzed,and 1T2Rv and 3T2Rv reconfigurable parallel mechanisms are synthesized.Reconfiguration is achieved in bifurcation configuration for the three classes of reconfigurable parallel mechanisms with the rotation characteristics of two-dimensional variable and invariable rotation axes,where four motion modes are included,namely invariable-invariable rotation axis motion mode,invariable-variable rotation axis motion mode,variable-invariable rotation axis motion mode,and variable-variable rotation axis motion mode.Hence,the traditional parallel mechanisms with rotation characteristics of two-dimensional invariable rotation axes are extended to the reconfigurable parallel mechanisms characterize by two-dimensional variable and invariable axis rotation.In addition,the motion characteristics of the 3T2Rv reconfigurable parallel mechanism are analyzed,and this type of mechanism is used as an assembly robot to assemble workpieces with different cross-sectional shapes,so that one machine can be used for multiple purposes,and one machine can be used for multiple purposes,avoiding the defect that one mechanism can only adapt to less working conditions.