Integrated translational and rotational control for the terminal landing phase of a lunar module

For lunar soft landing missions, precise landing with a low touch-down velocity and a vertical attitude is of great importance and challenge for a lunar module in the terminal landing phase. To this end, this paper presents a novel descending control strategy, by which the position and attitude of a lunar module are able to simultaneously meet the requirements. During descent of the lunar module, since the required thrust vector depends on the attitude maneuvering, the translation dynamics and the rotation dynamics of the lunar module are in fact mutually coupled, which leads to a six degree-of-freedom (DOF) control problem. Therefore, we formulated the coupled translation and rotation dynamics for a lunar module equipped with a main thruster providing body-fixed forces only in one direction and attitude actuators providing torques about three mutually perpendicular axes in the body frame. By using a state transformation, a filter-based backstepping control strategy is subsequently proposed, in which a second-order filter is utilized to overcome the “explosion of terms” problem existing in standard backstepping. The stability of the closed-loop system is guaranteed based on Lyapunov stability theory and singular perturbation theory. Numerical simulation of a lunar landing scenario shows the effectiveness and the robustness of the proposed control strategy.