Research On Trajectory Optimization And Control Methods For Small Body Landing

Future small body exploration missions will seek to land in hazardous terrains with high scientific value.Trajectory optimization and control techniques are among the key techniques to pursue safe and precision landing,and to accomplish the exploration plans.This thesis investigates the trajectory optimization and control methods for small body landing toward safe and precision landing,taking into account the problems induced by the complex and uncertain landing dynamics and the requirement of future missions for landing in hazardous terrains.The main contents include:The dynamics equations for small body landing are derived,and the impact of dynamical parameter uncertainties on landing trajectory is analyzed.Accurate models of the irregular gravitational fields of small bodies are established,and the effect of dynamical parameter uncertainties on landing precision is analyzed through sensitivity analysis,considering the characteristics of the uncertain dynamics of small body landing.Fuel-optimal trajectory optimization and tracking control methods for small body landing are investigated.To cope with the difficulty due to the sensitivity to the initial guess of the costates in the shooting process when using the indirect trajectory optimization method,an estimate method of the costate initial value based on reference trajectory is proposed.Random guess of costate initial values can be avoided and a design method of small body optimal landing trajectory via costate estimate is established.A robust landing trajectory tracking control method is then investigated.Dynamics of small deviation states are derived,and a robust trajectory tracking control law is designed based on high-order sliding mode control theory,making the tracking control robust to unmodeled dynamical errors and external disturbance.Trajectory optimization methods with hazardous terrain and uncertain conditions for small body landing are investigated next.The dynamical states are extended and uncertainty propagation equations are derived,so that the impact of uncertainty and hazardous terrains on landing safety can be quantitatively evaluated.A robust multi-constraint trajectory optimization method for small body landing is proposed through multi-constraint construction and a comprehensive optimization index,and robustness of the optimization results can be improved.Then,through steps of constraint modeling considering the effects of hazardous terrains and uncertainty,constraint relaxation and convexification,and sequential convex programming,a multi-constraint landing trajectory optimization method via convex programming is also established.Autonomous hazard avoidance control methods for small body landing in hazardous terrains are investigated next.A hazard avoidance control method based on collision probability is proposed.Through the real-time collision probability evaluation of the lander and terrain hazards under uncertainty and employing the potential function guidance theory,a globally stable and analytical hazard avoidance control law is developed,which can adapt to the real-time changes of the uncertainties,and can improve the performance of autonomous hazard avoidance under uncertainty.Then,taking into account of factors such as the state estimation errors and control capability,a hazard avoidance control method based on safety zone expanding is proposed.Through safety zone expanding and dynamical safety distance computation,the threats of hazards and landing safety can by evaluated dynamically,and the hazard avoidance control law is derived.The robustness of hazard avoidance control and landing safety can be further enhanced.The guidance and control methods for small body landing investigated are analyzed through semi-physical simulations lastly.Using a smooth platform and an air flotation system,the dynamical environment of small body landing is simulated,and using an embedded computer,the semi-physical simulations are conducted for the landing guidance and control methods investigated.The validity of the methods are analyzed.