Dynamics And Control Of Space Robots For Rapidly Capturing Objects In Orbit

On-orbit capture by space robots is one of key technologies of advanced on-orbit servicing. However, it is very difficult in dynamics and control of space robots for capture operation because of the complicated dynamics, e.g. dynamic coupling and strong nonlinearity. In addition, greater challenges are brought up in motion planning and control technologies for space robots when capturing failed or malfunctioning satellites — kinds of noncooperated targets. Therefore, it becomes a hot issue in aerospace engineering aera.Taking rapidly approaching and capturing a kind of noncooperative target by space robots in orbit as research background, dynamics modeling of space robots capturing targets in orbit, motion planning of the periods of approaching and capturing, control strategies to perform capture operations and to manipulate the targets after capturing are systematically studied. The main work of the thesis can be summarized as follows.1. Dynamics model of a space robot for rapidly approaching and capturing targets is established. Dynamic characteristics of the system and its attitude dynamics behaviors subject to gravity gradient torque are revealed.(1) Relative dyanmics equations between space robots and targets are established for the period of fly-by approach. Multibody dymamic equations of space robots are established for the period of capturing. Sopposing an instantaneous impact phase, dynamic equations of the coupled system after capturing are derived.(2) The space robots work in free-floating mode when performing fly-by approach and capture operation. Thus dynamic characteristics of such a system are analytically discussed from the point of constrained conditions and dynamic singularities. Its corresponding influence to workspace of the system is revealed. Details of the reaction null space derivation are given, through which joint space decomposition formalism is obtained. The test results validate the efficiency of the aforementioned theories and methods.(3) To study disturbances to attitude of free-floating space robots subject to the main environment torque ― gravity gradient toque, dynamic equations of such a system are developed considering its rotational-traslational motion coupling. Its attitude dynamic responses to different orbital parameters and initial conditions are presented by simulation results, including disturbances to attitude of the system with uncontrolled jonits, to attitude of the system with locked joints and to end pose of the system when working long hours. As a result, nonliearities in attitude dynamics of free-floating space robots subject only to the gravity gradient torque are revaled in detail.2. An integrated motion planning method is proposed for the process of rapidly approaching and capturing targets by space robots in orbit.(1) To capture a kind of noncoorperated targets, a stratege for capturing targets meanwhile the space robots are approaching the targets in the fly-by path is proposed. The fly-by approach phases and the capture procedure of manipulators are defined respectively according to the relative motion behavior during the fly-by capture. A mathematical model is established based on assumption of instantaneous impact phase to predict angular momentum transmition between the space robots and the targets during the impact phase. Consequently, a restriction condition of minimum disturbance to the base attitude is put forward(2) A performance index to trajectories of the fly-by approach and capture is developed from the point of physical constraints, task constraints and security constraints. Thereafter, an integrated motion planning model for the fly-by approach and capture is established. Besides, steps and procedures for solving the above model based on chaotic defferential evolution algorithm are proposed. Typical examples are given to validate the aforementioned planning models and algorithms.3. Trajectories tracking control menthods for space robots to capture and manipulate targets in finite time are presented.(1) To successfully capture targets by space robots during the fly-by approach, a finite-time variable structure control method is proposed. Given an adaptive RBF network to compensate for parametric and non-parametric uncertainties of the space robots, an adaptive RBF network based nonsingular terminal sliding mode(NTSM) control method is proposed. Taken further consideration about joint actuators’ physical constraints, a constrained adaptive RBF – NTSM control method that employs another RBF network to compensate for the limited input is developed. Simulations are conducted in contrast to traditional sliding mode control methods, by which the improvements in convergence time and accuracy of the proposed method are validated.(2) In order to manipulate targets by the coupled system after capturing, a dynamical decoupling condition of the manipulator end in work space is derived. Thus, motion control that ensures complete dynamical decoupling between the unfixed base and the manipulator is achived by actuating only the manipulator joints. Consequently, a robust reactionless finite-time coordinated control method for manipulation is proposed considering finite manipulating time and complicated environment disturbances. The comparing simulation results demonstate the effectiveness of the proposed method.4. Experiments for validating the dynamic characteristics and control algorithms are performed.A scheme of ground verification experiments for on-orbit capturing is presented based on air bearing microgravity testbed. Several experiments about dynamical coupling of free-floating system and reactionless control algorithm are carried out. The experimental results validate the effectiveness of the theories and the simulations.Overall, this thesis investigates dynamics, planning and control problems of space robots during fly-by approach and capture. A serial of strategies, models and methods for capturing a kind of noncoorperated targets are presented. The achieved results will provide a helpful technical support for further research on on-orbit servicing.