Constrained Guidance And Robust Fault-tolerant Control For Reusable Launch Vehicles

Reusable launch vehicle(RLV)has great potential value in military and civil fields for its ability to travel from earth to space many times.Many countries attach great importance to RLV technology,put forward their own research plans and have made great progress.However,there are problems like large flight envelope,complex environment,rigidly constraint,etc.during the RLV re-entry phase,and the dynamic model of RLV has the characteristics such as strong nonlinearity,strong uncertainty,fast time variation and so on.All above factors increase the difficulty to design a guidance and control system with satisfactory performance.Based on this background,the constrained guidance and robust fault-tolerant control of RLV are studied in this paper.Firstly,HORUS-2B is selected as the research object and the six degree of freedom(6-DOF)dynamic model is established under the consideration of earth rotation in the geocentric inertial coordinate system.Besides,this paper also has a concise description about the aerodynamic control surface characteristics and the layout of reaction control system nozzles.Secondly,a backstepping control strategy based on the disturbance observer and auxiliary anti-windup system is proposed,which is aiming at the problems of disturbance,partial failure of aerodynamic surface and its windup.The designed fixed-time disturbance observer(FTDO)in above control steategy is utilized to estimate the lumped disturbance,composed by uncertainty and partial failure of aerodynamic surface.Moreover,it can effectively improve the estimation accuracy of frequency conversion interference.Under the action of auxiliary anti-saturation system,aerodynamic surfaces can exit saturation state more quickly when windup occurs.Additionally,this paper also discusses the system stability and control effect after the sigmod function was introduced to the attitude angle rate control law.In a word,simulations demonstrate that the proposed control strategy has a good performance in control accuracy and robustness fault-tolerant capability.Thirdly,the RLV during reentry not only needs to obey a lot of constraints,like the heating rate,g-load,dynamic pressure and so on,but also is required to satisfy the geographical constraint for some flight missions.Thus,this paper proposed an improved artificial potential field method to avoid the no-fly zone when the RLV enters its influence region,and then switches to the strategy based on the corridor of heading angle error to obtain the sign of bank angle.This lateral guidance law not only lets the RLV have the ability to avoid no-fly zones,but also has a positive effect in decreasing times of bank reservals.The idea of heating rate tracking is added to longitudinal guidance law design,which makes the RLV consume more energy in the first half of heating rate phase.Consequently,the overload is limited within the bounds of its maximum limitation value.Finally,simulations of the integrated guidance and control system are conducted to verify the performance of 6-DOF system.What’s more,an improved tracking differentiator disturbance observer(ITDDO)is designed to estimate the uncertainty part and partial failure of aerodynamic surfaces under the structure of improved backstepping controller.And the PD control method is added to the longitudinal guidance law to realize the goal of better tracking of the heating rate.Simulation results show that the designed control law can effectively improve the robustness and fault-tolerant ability of the system and the guidance constraints considered are also satisfied.