Research On Hovering And Touchdown Control For Probe In The Weak Irregular Gravitational Field

With humans' universe exploration step into deep space, the unique asteroids group come into our vision and successfully attracts the attention of scientists around the world.It's of great significance to explore the asteroids in the sense of evolutionary theory research, earth's safety, resources development and utilization, and comprehensive national strength enhancement. In general, asteroids are small in size and irregular in shape, causing the very weak and irregular gravitational field around them, together with the perturbation from the sun and other large planet, the dynamic environment in the vicinity of the asteroids may be the most strongly perturbed environment in the solar system, and this put forward higher requirement to the navigation, guidance and control system of spacecraft. In the past few decades, plentiful research has been made on the orbital and rotational dynamics, navigation, guidance and control for the probe in the asteroid's weak irregular gravitational field, and a lot of valuable achievements have been obtained. While there is still large space for research, for example, about the autonomous navigation, guidance and control technique, many problems need further research.This work is supported by ‘Robust control method for landing trajectory of spacecraft in the weak irregular gravitational field(No. 2012CB720004D)', which is the subproject of the National Basic Research Program of China(973 Program) ‘Research on navigation, guidance and control for spacecraft precise landing on planets(No.2012CB720000)'. Assuming that the orbit and attitude information of the probe can be accurately provided by the navigation system, this thesis investigates the 6 DOF control for asteroid-orbital-frame hovering and the orbital control for touchdown in the weak irregular gravitational field. The main work and results of this thesis are summarized as follows:Firstly, according to characteristics of hovering and touchdown mission, the orbital dynamic equation in the asteroid orbital frame and asteroid body-fixed frame, and the equation describing the attitude dynamics are respectively established. From the perspective of modeling accuracy and computational complexity, we choose the spherical harmonic expansion method to describe the irregular gravity acceleration in the asteroid-orbital-frame equation, and choose the polyhedron method to calculate the irregular gravity acceleration in the asteroid-body-fixed equation.Secondly, the 6-DOF control problem for the probe hovering in the asteroid orbitalframe is investigated. In the current, most publications on the hovering control near an asteroid focus on the body-fixed hovering, we herein focus on the asteroid-orbital-frame hovering, considering both the translational motion and rotational motion, a 6-DOF dynamic equation is derived taking the irregular gravitation, model and parameter uncertainties, external disturbance and coupling terms into account. First, under the assumption that the upper bounds of uncertainties and external disturbance are known, a sliding mode control law is given, the asymptotic stability of the closed-loop system is proved theoretically, choose 4769 Castalia as the target asteroid, the numerical simulation results show that although the control law can ensure the system error converge, it brings chattering problem to the actuators. Then, we assume that the upper bound of system uncertainty is unknown, an adaptive sliding mode control law is designed, and the closed-loop system is proved to be asymptotically stable using Lyapunov theory. The sign function in control law is replaced by a saturation function to eliminate chattering during numerical simulation, and simulation results demonstrate that the proposed adaptive sliding mode control scheme can keep the probe hovering at the desired position with desired attitude after eliminating the errors between initial and desired states.Thirdly, a compensator-based 6-DOF control for probe asteroid-orbital-frame hovering with actuator limitations is presented. Still focus on the 6-DOF hovering control in the asteroid orbital frame, redefine the control input in the foregoing established6-DOF dynamic equation considering the practical problem of actuator limitations. A nominal control is introduced as the control variable to be designed. When the nominal control exceeds the actuator limitation, the actual control input equals to the limitation value, otherwise, it's equal to the nominal control value. A RBF neural network(RBFNN)is employed to approximate the error between the actual control and nominal control, and the estimate value is fed back to the adaptive sliding mode control law. The closed-loop system is proved to be asymptotically stable through the Lyapunov stability analysis.Contrastive numerical simulations are performed, and RBF compensator shows good approximation capability. Compared with the response curves in the non-actuator-limitation case, the state error curves need to experience more twists and turns before convergence. Simulation results demonstrate the validity of the improved control scheme.Finally, a dynamic boundary layer based two RBFNNs adaptive quasi-sliding mode control scheme for soft touching down on asteroid is presented. On the basis of the orbital dynamic equation in the asteroid body-fixed frame, we add the consideration of controlinput constraints. According to the initial and terminal condition, the desired descending path is planned using cubic polynomial method. Using the idea of tetrahedron division,the collision detection for the desired touchdown trajectory is performed. To perform trajectory tracking, we design the two RBFNNs sliding mode control law first, where two RBF neural networks are respectively used to approximate the unmodeled term and compensate the error between the actual control and nominal control. Then in order to improve the chattering problem caused by the sign function in the control law, and guarantee the reachability of the control system in the meanwhile, the preceding control law is modified, a dynamic boundary layer based two RBFNNs adaptive quasi-sliding mode control law is presented to track the desired descending path. After analysis, the reachability condition of the control system is given. Numerical simulation results demonstrate that the improved control scheme is valid. In addition, considering the Matlab's low efficiency in executing loops, we conduct the simulations in Matlab by running Mex-functions written in Fortran to calculate the gravitational accelerations of the asteroid using polyhedron method, combine the advantages of Matlab and Fortran,and the simulation speed of the control system is improved.