Research On Hybrid Simulation System Of Space Docking Based On 6-DOF Platform Driven By Hydraulic Servo Actuators

Space docking of spacecrafts is one of the key techniques for building a space station. Whether the spacecrafts and space station will dock successfully depends on the reliability of the docking mechanism, so the experiments of simulating the whole process of docking on the ground are necessary for checking the docking mechanism's reliability and the influence of the atrocious space environment on it. Because the space docking hybrid simulation (SDHS) system can simulate the docking process of various spacecrafts and docking mechanisms, it becomes the preferred scheme.In the SDHS system, a Stewart platform is used for accomplishing the relative motion of two spacecrafts. Therefore this paper will make researches on the Stewart platform's dynamics model, control strategy and kinematics calibration. And analyses on these problems will be significant for building an accurate SDHS system.The Stewart platform is a complicated nonlinear system in substance and has multiple variables. And its kinematics and dynamics analysis is the precondition of the platform's structure design and hydraulic system design. In this paper the kinematics analysis is carried out by coordinate transformation and matrix differentiation firstly. Then a simplified dynamics model with no consideration of each leg's inertia is constructed. Finally a whole dynamics model based on the Newton-Euler method is presented by investigating the motion of each leg. Because this model has considered each leg's rotation about its axis and the theoretic errors are eliminated, it is an accurate model for the 6-UPS type parallel mechanism. In contrast, traditional dynamics models ignore this rotation, so their descriptions of the leg's motion are relatively rough.The control strategy of the Stewart platform is another important research field, and the performance of the platform is directly determined by the control strategy. Two basic control strategies are introduced in this paper, and the control strategy based on joint space and several methods of improving the system's performance, including dynamic pressure feedback and forward feedback, are detailedly analyzed in consideration of the concrete usage of the platform in this project. As the sinusoidal signal with small range is frequently used for motion simulating, an amplitude-phase control method is presented in this paper to enhance the platform's response performance for sinusoidal signals. And the validity of this method is confirmed by experiments.Since the Stewart platform inevitably has manufacturing and installation errors that have an influence on its kinematics accuracy, a high-fidelity simulation of space docking can't be attained. Aiming at the kinematics accuracy problem of the platform in the SDHS system, a cost effective calibration technique using a general coordinate measuring machine is presented. Firstly, the sources of kinematics errors are described. After the calibration procedure is described and the strategy on constructing the residual equation is discussed, the kinematics accuracy after calibration is discussed through simulation. At last this method is applied to an actual Stewart platform in order to check its validity.After obtaining a Stewart platform with good performance, it is possible to build a SDHS system. Thus the structure of a prototype system having key features of the SDHS system is designed. For the prototype system, the dynamics calculation method is presented to construct the dynamics simulation external loop. The stability analysis of this system shows that the bandwidth of the platform should have a certain relation with the natural frequency to be simulated to get expected stability.To validate the feasibility of building the prototype system, a virtual prototype of it is established using ADAMS and the docking simulation is performed by this virtual prototype. And then a digital model of two spacecrafts is built and the docking simulation result of it will be the fidelity criterion of evaluating the SDHS system. It is showed that the virtual prototype can simulate the whole process of space docking comparatively exactly, so building an actual SDHS prototype system will be completely feasible. Finally a SDHS prototype system is established and several series of docking experiments for various docking initial conditions are accomplished. The spacecrafts'relative motion and mutual forces as well as other phenomena in the docking process are reproduced relative exactly, and thus some essential experience will be obtained for developing a multifunctional SDHS system used for testing the actual docking mechanism.