Research On Mars Powered Descent Guidance Method Under Constraints

In previous Mars exploration missions,all that was usually required was that the lander could land safely on the surface of the planet,with little requirement for landing accuracy.However,as Mars exploration missions evolve,such as Mars sampling return mission and human Mars mission,they often require that the lander can land accurately in a designated area or in hazardous terrain,placing higher requirements for landing accuracy and landing safety of the lander.In addition,the uncertain environmental disturbances of Mars will also directly affect the landing accuracy.The powered descent phase is the last stage of the landing mission,and its guidance system plays a critical role in ensuring the safety of the lander and improving the landing accuracy.Therefore,it is of great theoretical and engineering significance to study the powered descent guidance method of lander to improve safety and landing accuracy.On the other hand,the thruster performance of the lander and the terrain around the selected landing site are also directly related to the safety of the lander.Therefore,under this background of Mars powered descent phase,this dissertation focuses on the guidance law design under the constraints considering the time/state constraint on tracking error,non-concave landing trajectory constraint,thruster performance constraint and hazardous terrain constraint,aiming to improve the safety and robustness of the lander and provide theoretical support for engineering practice.The main research contents and results are as follows.Firstly,the nominal trajectory tracking guidance problem that satisfies the tracking error time/staes constraints in the presence of external disturbance is investigated.By using a predefined time tracking error time/staes performance function to constrain the tracking error,the tracking error is guaranteed to satisfy the dynamic response,tracking accuracy and the upper limit of the stable tracking time.Considering the tracking error has symmetric performance constraints,the original constrained tracking error dynamics model is transformed into an unconstrained system using a nonlinear transformation function.And the guidance law is designed based on the backstepping method.Furthermore,considering the case with asymmetric tracking performance constraints,the guidance law is obtained using the asymmetric tracking performance constraints to construct a boundary function to design the sliding surface.The adaptive laws are applied to compensate for the effects of external disturbances in both guidance laws,and the saturation function is used to deal with the chattering problem.Secondly,the landing guidance problem with thruster performance constraint including the thruster output deviation and amplitude constraint in the presence of external interference is investigated.The dynamics model under the thruster performance constraint is derived at first.Based on this,a fixed-time sliding mode guidance is developed using the norm-normalized sign function,and the thruster performance constraint and external disturbance are compensated by the adaptive law.Further,considering that the above fixed-time guidance law cannot determine the landing time,a time-varying slidingmode guidance is designed using the time-to-go and is extended to the incremental form to deal with thruster performance constraint and external disturbance.In order to satisfy the glide-slope constraint,a simple and easy-to-solve optimization problem is constructed to obtain the guidance law parameters.Both of the above-mentioned guidance laws can obtain a glide-slope trajectory,which makes the lateral control accelerations are zero when the lander touches the surface,effectively avoiding the lander to roll over and improving the safety of the lander.Then,the problem of soft pinpoint landing guidance satisfying the non-concave landing trajectory constraint in the presence of external disturbances is studied.By analyzing the relationship between the lander states and the curvature of the landing trajectory,the judgment conditions of convex trajectory,slope trajectory and concave trajectory are given.Based on the fixed-time tracking differentiator,an observer is designed to estimate the external disturbance,and the non-singular terminal sliding mode landing guidance law is developed using fast fixed-time control theory.The selection principle of the guidance law parameters is given based on the non-concave trajectory judgment condition.Moreover,a new switching guidance algorithm is proposed to have near optimality while ensuring non-concave trajectory constraint.That is,using the non-concave trajectory condition,a nominal curvature dynamics model is constructed to design the curvature guidance law to steer the lander to converge to a new state satisfying the non-concave trajectory condition,and then the integral sliding mode guidance law based on the ZEM/ZEV guidance law is switched to obtain the near optimality.In this guidance method,an adaptive law is applied to compensate for the influence of external disturbance.At the end of this dissertation,the problem of autonomous obstacle avoidance soft pinpoint landing guidance of the lander under hazardous terrain constraint is investigated.Firstly,the guidance problem of landing on a canyon or other terrain that can be equivalent to a step-shaped hazardous terrain is considered.Based on the prescribed performance control theory,an autonomous obstacle avoidance guidance is developed by designing a step-shaped performance function and applying the backstepping method.To avoid significant initial control acceleration,the guidance law gain is designed using a Gaussian function related to the obstacle distance.Moreover,considering that the terrain around the landing site may be more complex,the terrain is equated into three standard spatial geometries: step-shape,frustum-shape and cone-shape,and the boundary function is designed to approximate the equivalent geometry.Based on these,an autonomous obstacle avoidance guidance algorithm is developed using Barrier Lyapunov function to achieve the soft pinpoint landing.In the above two guidance laws,a preset-time convergent function is selected as the upper boundary to coordinate the vertical motion,guaranteeing that the lander completes the landing mission within the predefined time,which can reduce fuel consumption.