Research On Force Control And Decoupling Of Six-Degree-of-Freedom Loading System Based On Orthogonal Parallel Mechanism

In the process of spaceport orbiting, the mechanical components combination of docing mechanism will endure repeated alternating load and external applied load of attitude and orbit control,which is a challenge to the stiffness,strength and air tightness of the mechanism. So in order to ensure the reliability of the mechanism, it is needed to test the resistance of various loads in ground test. In this article, a muti-axial loading system based on orthogonal parallel mechanism is adopted, which can provide a multi-degree of freedom force and torque, to implement the fatigue test of space docking mechanism.In this paper, the principle and composition of the loading system is introduced, and the kinematics and dynamics of the system are analyzed. Then the relationship among the displacement, velocity and force is obtained. Due to the system is requeared to output a large force and moment, so the hydraulic cylinder is chosen as the actuator and the model of the valve controlled hydraulic cylinder is established. Then the control scheme based on closed-loop of actuators driving force is carry out in the loading test. It is found that the systems is seriously coupled in dynamic loading. So it is necessary to find out the method to decouple the system, which will improve the accuracy of the loading system.In order to analyze the coupling characteristics, the Adams-Simulink model is established. Then the effect of geometric stucture error, actuator response difference, nonlinear characteristic of dynamic, and nondiagonal dominant stiffness matrix is discussed. The geometric structure error can be eliminated by improving the machining and assembly accuracy and the differences between acutators can be removed by choosing the appropriate components and system debugging. Moreover, the nonlinear characteristic of dynamic and the stiffness matrix of system is an inherent property of the system, so the force coupling of system can be compensated by the corresponding controller.There are many ways to control the generalized force, but most of them from two aspects. One is using the position control to indirectly realize the generalized force control, and the other is directly control the generalized force using the interaction among the components of the system. In this paper, the impedance control combined with decoupled position control strategy is proposed, and the closed-loop of generalized force combined with adaptive control is proposed. The former converts generalized force error into position, then ensure the error between desired position and actual position is close to zero, so as to achieve the precise control of the generalized force. The latter introduces the error caused by nonlinear characteristic of dynamic and the nondiagonal dominant stiffness matrix into the control system, and the inertia force of the system is compensated. Also, the adaptive control is used to ensure each actuator characteristics cause the least deviation. Therefore, the control precision is improved and the force coupling of loading system is decreased.Finally, the applicability of the proposed force decoupling control scheme are tested, Comparing with former experiment data, the effectiveness of decoupling scheme is proved.