Tracking And Pointing Control Forspace Non-Cooperative Target

With the development of the space technology, more and more spacecrafts have been launched to perform the specific space missions. However some non-cooperative spacecrafts which are disabled even out of control will pose a threat to other normal spacecrafts. The chaser spacecraft should approach them and provide the on-orbit service or clear them away. Thus the control technology which can keep the relative position tracking and attitude pointing is very critical. On the one hand, the chaser spacecraft should consider how to quickly approach the non-cooperative spacecraft and perform the gaze tracking mission with high precision. On the other hand, there are many kinds of external disturbances, the inputs saturation, and the dead-zone nonlinear characteristic in the spacecraft system inevitably. The most important problem is that some motion parameters of the non-cooperative target are difficult to obtain. Therefore researching the finite time tracking and pointing control with some kinds of disturbances and constraint conditions is very significant. The main research contents are presented as follows:For tracking and pointing the spacecraft non-cooperative target, the relative orbit dynamics and the attitude dynamics between the chaser spacecraft and the target are established in the line-of-sight coordinate frame and the chaser’s body coordinate frame, respectively. Considering the generalized disturbances which are resulted from the unknown orbit maneuver acceleration of the target, the dead-zone nonlinear characteristic of the actuators and so on, the six-degree-of-freedom model of the orbit and attitude integrated control is proposed. The model can provide the foundation to the following control algorithms.Considering the requirement that the relative position and attitude of the chaser spacecraft should rapidly converge to the desired states, based on the backstepping method, the finite time control law is proposed. Because of the situation that the upper bound of the disturbances is often unknown and difficult to be estimated in the actual system, the adaptive neural network and the nonlinear disturbance observer which is designed based on the tracking differentiator are used to estimate and compensate the disturbances. The finite time stabilities of the close-loop systems are proved by the finite time Lyapunov stability theorem.For the constraint that the control inputs of the chaser spacecraft are bounded, based on the adaptive method, the tracking and pointing control algorithm is proposed. It can guarantee the finite time stability of the close-loop system with the inputs saturation. Considering the weakness of the not high enough precision which is resulted from the adaptive method do not estimate the disturbances directly, introducing the hyperbolic tangent function, the anti-windup PD tracking and pointing control algorithm is proposed, and the nonlinear disturbance observer is used to estimate the disturbances. For the disadvantage that the anti-windup PD control algorithm only guarantee the asymptotic stability of the close-loop system, using the saturated non-singular terminal sliding mode, two finite time tracking and pointing control algorithms are designed by combining the adaptive neural network and the nonlinear disturbance observer, respectively. The effectiveness of these algorithms are verified by MATLAB simulation, and the simulation results are analyzed contrastively.