Modal Space Control Strategy For Six-Degree-of-Freedom Parallel Mechanisms

Compared with the series mechanism,the parallel mechanism has the outstanding advantages of high movement accuracy,strong carrying capacity and dynamic performance well.So it is often used as motion simulator and vibrator in various environments.But for the hydraulic parallel mechanism,because of its structural characteristics,it is difficult to model the dynamics of the parallel mechanism,and the accuracy of the dynamic model is also affected.In addition,due to the strong coupling and non-linear dynamic characteristics of the parallel mechanism,the motion coupling between the degrees of freedom of the parallel mechanism occurs,which seriously affects the accuracy of the motion simulation,or even may cause instability of the system.Although the widely used PID control in industry can track the position of the target,it can't achieve the decoupling control of the parallel mechanism,and there is still a large motion coupling.Based on the above problems,the six-DOF parallel mechanism is taken as a research object.Firstly,this thesis analyzes the kinematics and dynamics of parallel mechanism.The Newton-Euler equation is used to establish the dynamic equation.And according to the flow equation and the continuous equation of the hydraulic cylinder,the transfer function of the hydraulic system is obtained.Secondly,with MATLAB's powerful simulation tools,the SimMechanics second-generation modeling tool is used to model the mechanical system of the parallel mechanism for simulation testing.Then,the dynamic linearization identification model is proposed for the inaccuracy of the dynamic equation parameters.The identification process is deduced by recursive least squares method.Then the simulation system is built in Simulink,and the prediction torque and actual torque are applied to the parameter identification.The verification completed the work of kinetic parameter identification.Next,because the degree of freedom space control has better control effect than the joint space control,the PID controller is designed for the parallel mechanism freedom degree space,and the motion coupling phenomenon between the degrees is observed.Aiming at the phenomenon of motion coupling of parallel mechanism,the modal theory of vibration field and the orthogonal nature between various modes are used to analyze the modal state of the parallel mechanism.And parametric identification is applied to the control of the parallel mechanism based on the dynamic model to obtain a more accurate modal transformation matrix.At the same time,the modal calculation torque control method based on the degree of freedom space is proposed.The controller is used to control the degrees of freedom of the parallel mechanism separately to observe the motion effects of the respective degrees.Finally,motion effect under modal control is compared with the effect under PID control in traditional freedom degree space.The tracking accuracy and decoupling effect of the controller in modal space are analyzed quantitatively and qualitatively,and the modal controller is obtained.And the conclusion that the modal controller suppresses the coupling phenomenon between the degrees is obtained.