Analysis And Control Strategy Of 3-dof Parallel Mechanism

With the development of national defense and military industry,there is urgent need to develop real vehicle road simulator equipment on weapons fire controlling system in order to break the monopoly of foreign technology and enhance the testing standards. This thesis study the kinematics analysis, dynamics model, accuracy analysis and control strategy of a 3-dof parallel mechanism, and the background is the dynamical testing and simulation equipment on military vehicles which designed by HIT institute of electro-hydraulic servo simulation & testing system.3-dof parallel mechanism is a variant of the typical Stewart platform, which used three drivers and threes struts instead the six drives of the Stewart platform. In this paper, the length constraint equations of three struts is established and the analytical solution of the inverse kinematics is derived, and the forward kinematics solution of the rotational parallel mechanism is presented based the typical method of the Stewart platform. Also, a new forward kinematics solution is derived according to the structural characteristics of the machine. The workspace and translation movement are studied in order to obtain the property of the parallel robot.In the dynamic study, the 3-dof parallel mechanism is simplified into a single rigid body then the dynamical model is build by using Newton-Euler equations. The driving force of the drives and the struts in typical movement is analyzed with the help of the dynamical model. The general stiffness matrix is derived by acting the actuators and struts as spring. The multi-body dynamical model is presented through Kane equation and the influence of the legs'mass and inertia are considered. General mass matrix and natural frequency equation is proposed based on the multi-body dynamical model, then the frequency property of the robot is analyzed at the end.In the accuracy study, the manufacturing error, assembly error and actuators'control error of the 3-dof parallel mechanism is grouped into 42 error source. The error model of the parallel mechanism is brought forward accordance to the inverse kinematics of the robot, and the error distribution of the system is studied through simulation. Accuracy synthesis is used to enhance the precision of the system, in this method, error sensitivity is proposed to describe the error sources'impact on the system accuracy. All the allowed tolerance of the error sources are divided into two categories based on the value of the sensitivity, then decrease the allowed tolerance step by step until the accuracy of the robot meet the requirement. This research provides every important references for the tolerance design of the 3-dof robot.The PID controller in joint space and computed torque controller in workspace are introduced. Then the mathematical model and frequency domain characteristics of the hydraulic actuators is derived, and the control strategy in joint space is investigated. A iterative compensation control strategy based on forward kinematics is proposed in order to enhance the performance of the robot. In this method, the transfer function matrix of the robot is obtained through identification, then the command signals of the robot is adjusted based on the posture error and the inverse transfer function matrix of the 3-dof parallel mechanism. The tracking accuracy of the robot can be enhanced to meet the requirement through many iterations. Experiment shows the precision of replication is 5% , which verified the validity of the iterative control strategy.