Guidance And Control Law For Mars Pinpoint Landing Under Multi-constraints

Mars exploration is a hot spot in the process of deep space exploration,and landing even sampling return is the most ideal detection method.In the process of soft landing,guidance and control in powered descent phase is the key of research.In order to guarantee the safety and efficiency of the landing mission,practical physical constraints such as safety height constraint and thrust amplitude constraint should be considered,which brings many challenges to the guidance and control system.This paper is based on the National Basic Research Program of China(973 program)‘Research on navigation,guidance and control for spacecraft pinpoint landing on the surface of planet’,on the basis of present research at home and abroad,deep researched on lander dynamics modeling and constraints analysis,design of G&C law,control allocation and combined attitude and orbit control as follows:Firstly,defined landing point fixed and lander body coordinate systems,established lander orbit and attitude dynamics model based on Newton’s law and Newton-Euler method independently,then the glide slope constraints,attitude direction constraints,thrust amplitude constraints,safety height constraints and state amplitude constraints of lander in the actual landing process are given and modeled.Secondly,according to the orbit dynamics model in landing point fixed coordinate system,considering the actual physical constraints in landing process,an online optimization guidance law based on model predictive control(MPC)was proposed.In order to apply in engineering,considering the result of MPC as the initial guess history,an offline optimization guidance law based on model predictive static programming(MPSP)was proposed.Numerical simulation shows that under the two proposed guidance law,the lander can accomplish the fixed point soft landing mission under multi-constratints.Thirdly,according to the attitude dynamics model in lander body coordinate system,considering the actuators for orbit control of lander is less actuated and no thrust vector function,the change of thrust direction needs to be realized by attitude control system.Euler angle of desired attitude was calculated based on desired thrust vector obtained by MPC algorithm,an attitude tracking control law based on PD was proposed.To enhance the rapidity and accuracy of attitude tracking,an attitude tracking control law based on Nonsingular Terminal Sliding Mode Control(NTSM)was proposed to let lander’s actual attitude tracking the desired one.Numerical simulation shows that the lander can track the desired attitude rapidly and accurately under the two proposed control laws.Finally,according to the configuration of actuators of lander attitude control system,the desired control torque obtained by attitude tracking control law that based on NTSMC was allocated to each attitude control engine,converted to its thrust and ignition order.In order to verify the correctness of the designed G&C law,the online optimization guidance law based on MPC and the attitude tracking control law based on NTSMC are selected as the G&C law of landing process.Considered attitude loop as inner ring and position loop as outer ring to compose double loop system,simulation of 6-DOF combined attitude and orbit was presented.