| Due to the compact structure,high bearing weight ratio,and high positioning accuracy,the parallel mechanism has become the hotspot of robot research in recent years,especially the advanced intelligent equipment with 1T2R(one translation and two rotations)degrees of freedom parallel mechanism has been widely used in intelligent manufacturing,medical rehabilitation,and aerospace industry,et.al.However,the parasitic motion,which is the embodiment of the strong-motion coupling between the rotational and translational degrees of freedom among the general 1T2 R parallel mechanisms,leads to an unfavourable impact on the motion accuracy and other performances.In addition,the disadvantages of parallel mechanisms,such as serious singularity,small workspace,and low actuation force coordination performance also limit its application.Therefore,a new class of 1T2 R parallel mechanisms without parasitic motion are studied by the type synthesis method.These mechanisms possess low-motion coupling or analytical solutions of the forward kinematics,which are beneficial to the practical application.Among the 165 new types of mechanisms,an over-constrained 2RPU-RPS-UPS parallel mechanism with the redundant actuation is selected for further research,including kinematics and dynamics analysis,singularity analysis,performance analysis,optimization of the mechanism,and the active compliance control method.Based on the theoretical research,this dissertation conducts the simulation verification of the virtual prototype in ADAMS and MATLAB/Simulink.Besides the physical prototype and control system of the 2RPURPS-UPS parallel mechanism are also established for the experiment.This research paves the way for the application of parallel mechanisms without parasitic motion.The main content of this research are as follows:(1)The formation mechanism of the parallel mechanism without parasitic motion is analyzed,and 165 types of 1T2 R parallel mechanisms without parasitic motion are obtained.From the definition of parasitic motion,the relationship between the rotating axis and the moving platform plane of the parallel mechanism is analyzed firstly,and the existence of parasitic motion is discussed under different limb layouts.Then,the configuration criteria and structural conditions of 1T2 R parallel mechanism without parasitic motion are proposed based on the spatial geometry theory and screw theory,60 S-type and 105 U-type non-parasitic-motion mechanisms are obtained by type synthesis.Finally,two typical mechanisms are selected for verification.(2)The kinematics of the 2RPU-RPS-UPS parallel mechanism without parasitic motion is analyzed.The position relationship of the proposed mechanism is deduced,which shows that the forward kinematics has an explicit solution,and it is conducive to the real-time control in application.The singularity of 2RPU-RPS-UPS parallel mechanism is analyzed by the Jacobian method.It is pointed out that there is a forward kinematic singularity configuration when the pose parameters meet a certain relationship.Based on the position analysis,the mapping relationship between the generalized coordinates of the moving platform and the velocity and acceleration of components are obtained by the derivation method.(3)The reason why the dynamic solution of the 2RPU-RPS-UPS parallel mechanism is not unique is clarified,and a solution for the dynamics of the parallel mechanism which possesses both active redundant constraint and passive redundant constraint is proposed.Based on the screw theory,the characters of redundant constraint are analyzed,and the optimal principle of driving forces and the deformation coordination conditions of the limbs are introduced to solve the redundant variables caused by active redundant constraint and passive redundant constraint respectively.The dynamic problem of over-constrainted parallel mechanism is transformed into an optimization problem,in which the least square norm of actuated forces is the optimization objective,and the compatible equations on over-constrainted limbs and the dynamic equations deduced with Newton-Euler method are served as constraint equations.Finally,the optimization model is solved with Matlab and the unique solution is obtained,on this basis,the friction effect of joints and the mechanical characteristics are further analyzed.In addition,an efficient dynamic model for control is conducted with the virtual work principle.Finally,the dynamic solutions are verified by simulation analysis in ADAMS.(4)Kinematics,stiffness,and dynamic performance of the 2RPU-RPS-UPS parallel mechanism are analyzed and optimized.To avoid the cavity in the workspace,the regular workspace is obtained by finding the maximum inscribed circle in the attitude workspace.The driving stiffness and constraint stiffness of the limbs are considered synthetically,and the local index based on deformation energy is used to analyze the stiffness performance.Besides,the relationship between the local LTI index and the translational degree of freedom is studied,it is proved that the most of reachable workspace are high-quality transmission workspace.The dynamic performance of the mechanism is analyzed by dynamic manipulability ellipsoid index,and the influence of mechanism scale and component inertia on the isotropy of the acceleration ability are considered.To improve the overall performance of the mechanism,an optimization model is established and solved by the NSGA-II algorithm.The Pareto optimal front of the objective functions is obtained and the Pareto solution is chosen with the method of ideal point.(5)The active compliance control method for redundant actuation parallel mechanism is studied.Combined with the requirements of external compliance control and internal force coordination control of the 2RPU-RPS-UPS parallel mechanism,a new hybrid control framework is proposed,in which the intelligent variable impedance control is accomplished by non-redundant limbs and the internal force coordination is accomplished by redundant limb.A new admittance control frame with wavelet neural network is put forward,and the adaptive control law is derived with the Lyapunov stability theorem.To realize the internal force coordinated control,the redundant limb is set as the force control mode,and the estimator based on Cubature Kalman Filter is designed to realize the sensorless vector control of the permanent magnet synchronous motor.(6)The effectiveness of the hybrid control strategy is verified by simulation and prototype experiments.The simulation model of the control system for 2RPU-RPSUPS parallel mechanism is formed in Simulink,force-tracking in the unknown environment can be accomplished well with the variable impedance control,and the vector control method can ensure the coordination of internal forces.To verify the feasibility of the strategy in the physical system,the 2RPU-RPS-UPS parallel mechanism prototype and experimental platform are established to test the constant force tracking performance on the plane and slope environment.The experiment results verify the feasibility and effectiveness of the hybrid control strategy. |
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