Research On Collision Dynamics And Control Strategy For Space Manipulator Performing On-orbit Capture Task

In recent years, space exploration continues to develop, and China has actively joined the space exploration team. In the near future, China will construct its own space station. For the increasingly complicated on-orbit tasks, space manipulator plays a more and more important role, and implementing the on-orbit service by a space manipulator is becoming a new hot spot of the current development of space technology. Many on-orbit tasks,such as the construction of space station, the maintenance of satellite and the collection of space gabage, are all related to the collision problems between space manipulator and the target. Therefore, in order to improve the safety and reliability of capture tasks, the research on collision dynamics modeling and optimal control strategies has important theoretical significance and engineering value to the further development of space exploration.The research of this paper comes from the National Natural Science Foundation of China “Study on collision mechanism and docking strategy of space manipulator assisting spacecraft" (61573058). Taking redundant space manipulator which is equipped with a snare capture mechanism with the characteristics of large tolerance and soft contact as the research object, the key technologies such as collision dynamics modeling and optimal control are studied in depth for the space snare capture task. The main work of this paper is as follows.1) Collision dynamics modeling and contact force prediction for space snare capture task. Since flexible cable is the main executive part of snare capture, the contact problem is actually the contact modeling problem between the target and flexible cable. Considering the motion characteristics of flexible cable during snare capture, a tension/torsion spring-mass discrete model is put forward, and on this basis, the dynamics model of flexible cable is established after introducing the contraints of snare capture mechanism.For the contact problem between flexible cable and the target, the concept of virtual spring is introduced and then the relationship among normal contact force, pressed depth and the start-end distance of flexible cable is built up.The dynamic contact force between flexible cable and the target can be predicted after adding the damping term.2) The collision response analysis of space manipulator performing snare capture. From the perspective of energy of space manipulator system,the dynamics model is set up based on Lagrange equation. In the operation space, the concept of equivalent inertia of the end of space manipulator is introduced, and its equivalent mass and moment of inertia can be described quantificationally by ellipsoid and belt ellipsoid. For different contact types,the rigid-rigid collision model between target and rigid shell as well as the rigid-flexible collision model between target and flexible cable are both established. On this basis, the disturbance of space manipulator caused by collsion is analyzed.3) Optimization strategies before snare capture performed by a space manipulator. In order to reduce the contact force peak during space snare capture, the "safe contact region" is designed according to the contact force model of flexible cable. Making use of the equivalent inertia of the end of space manipulator, the collision disturbance function of the end of space manipulator is established. And based on the coupling relationship between base and manipulator, the evaluation function of collision disturbance of base attitude is proposed. Finally, the redundant attribute is adopted to make the manipulator adjust its own configuration to optimize the corresponding goal functions in order to reduce the collision disturbance of manipulator's end and the base attitude.4) Stability control strategies during snare capture performed by a space manipulator. The contact between flexible cable and the target may happen several times in the process of capture, which would bring problems to the stability of capture tasks. A stability control strategy which aims to compensate the deviations of manipulator end's pose is proposed based on PD controller, and another stability control strategy which aims to track the target based on expected force is also provided with reference to the impedance controller. The former can adjust the pose of manipulator quickly beteween collisions during capture process and the latter can rely on only one collsion between flexible cable and target to accomplish the capture task,which can reduce the collsion disturbance as much as possible.5) Experiments on snare capture by a space manipulator. Based on the ground air-floating platform and the three degrees-of-freedom manipulator,the snare capture mechanism is designed, and a 3D visualization simulation control platform is constructed. Through the designation of experimental schemes and by virtue of the data obtained from sensors as well as the Adams aided analysis, the correctness and effectiveness of relative theory during space snare capture which is proposed in this paper are proved.