Research On Control Strategy Of A Spatial Parallel Robot With Flexible Links

To improve the tracking performance of flexible multibody systems,the nonlinear forward and inverse dynamic models for a spatial 3-RRRU parallel robot with flexible and rigid links is systematically analyzed in this thesis.The proposed solutions for the flexible dynamics are suitable for the mechanisms with closed-loop chains.The control strategy is then presented based on the solution of the inverse dynamic model.The main contributions are formulated as follows:(1)The inverse kinematic analysis for the rigid 3-RRRU parallel robot is firstly presented based on Vector-loop method.With the help of Newton-Euler method,the constraint forces and torques of each joint can be derived for the robot.The mass matrices for the spatial rigid beam element and moving platform are then figured out by natural coordinate formulation,and thus the dynamics for robot can be achieved according to Lagrangian method.The dynamics is verified as same as the one derived from the reference coordinate formulation.These obtained results are fundamental to the flexible dynamic analysis of the subject theoretically.(2)A new smooth and near time-optimal trajectory planning approach is proposed for the rigid parallel robot subject to kinematic and dynamic constraints.The complete dynamic model is constructed with consideration of all joint frictions.The desired planning trajectory can be solved efficiently by presenting a zero pseudo-acceleration curve in the phase plane.The effectiveness of the solution method is analyzed theoretically and validated with experiment.The different planning characteristics are summarized for serial and parallel robots.The proposed method can solve the planning problem for the flexible parallel robot.(3)The nonlinear dynamic model for the rigid-flexible 3-RRRU parallel robot is constructed based on natural coordinate formulation and absolute nodal coordinate formulation.Compared with the traditional Kineto-Elasto-Dynamic analysis,the derived model can describe large deformation of flexible links without introducing any linearizing and take the shear deformation for the beam into account.(4)To avoid Poisson locking in continuum mechanics,the elasticity matrix and strain energy are split into two reasonable parts,respectively.A new efficient computation method is used to obtain the elastic forces and the Jacobian for the flexible parallel robot.The solving principle,approach and performance of the dynamic equations for the rigid-flexible parallel robot are analyzed and formulated in details.(5)Because the geometric constraints for the rigid parts of the flexible robot tend to be violated during the iterative process,a transient rigid-body modification method is proposed to ensure the correct solution can be achieved for the forward and inverse dynamics.The control approach is then established based on the solution for the inverse dynamics,which is verified by the trajectory tracking experiment and indicates the consistence of the results from the simulation and experiment.