Research On Reusable Launch Vehicle Re-entry Sliding Mode Control Method Based On Disturbance Observer

Reusable launch vehicle(RLV)has become the focus of research and main experimental subject of many contries in the world as a low-cost,reliable earth to space transportation means.The RLV can entry the atmosphere safe and reliably,and accurately deliver the loads into orbit.Thus significantly reduce transportation and launch costs.However,because the RLV has the characteristics of fly across the airspace and large speed domain,the velocity and altitude change large and the attitude changes greatly.The model of the RLV exhibits strong coupling and nonlinearity.Meanwhile,the external disturbance has large interference and the model parameters are uncertain,which makes the RLV attitude control system design one of the difficulties in the field.In the view of the above difficulties,this paper focuses on the research of RLV re-entry attitude control in order to achieve reliable and safe flight of RLV.Firstly,define the coordinate system,coordinate system transfer relationship,attitude angle and attitude angle geometric relationship which the RLV modeling process needs.According to the flight characteristics of the RLV re-entry phase,the RLV re-entry mathematical model is established by force analysis and torque analysis.In order to make the model directly applicable to the existing control methods,the model is appropriately simplified,and the RLV re-entry control oriented model is obtained.Secondly,due to the large variation of the flight height,the severe change of the speed and attitude,and the complicated atmosphere environment during the RLV re-entry phase,the aerodynamic characteristics is quite complicated.A three-dimension model of RLV is drawn according to its shape and structure characteristics.The computational fluid dynamics method is applied to simulate and compute the RLV aerodynamic coefficient.Through the analysis of the simulation results,the aerodynamic coefficient data is obtained.It can be obtained that if the aerodynamic moment coefficient is linear or polynomia fitted,the fitting error will be introduced and at the same time,incerases the complexity of the model.Thus,the aerodynamic torque of the RLV can be handled as model disturbance,which can be observed by the disturbance observer.Thirdly,considering during the RLV re-entry phase,the system will be affected by severe external disturbance and model uncertainty,an attitude control scheme based on disturbance observer is proposed to solve the problem of large interference and uncertainty in the system.In the paper,based on the singular perturbation theory and the principle of time-scale separation,the RLV reentry attitude model system is divided into inner and outer loop subsystems.Then the extended state observers based on the Sigmoid function are designed for the inner and outer loop subsystems respectively,while the self-learning disturbance observers are designed for the inner and outer loop subsystems respectively.They can observe and compensate the uncertainty and external interference in the system.On the basis of the disturbance observers,the multivariable super-twisting sliding mode controllers are designed for the inner and outer subsystems respectively,which complete the RLV attitude control scheme.The attitude control sheme realizes the accurate tracking of the guidance command and supresses the interference in the system.Finally,the comparison simulation is carried out to verify the advantages and disadvantages of the control schemes with different disturbance observers.Fourthly,during the RLV re-entry phase,there is comprehensive disturbance including actuator failure,model uncertainty and external disturbance.The observer-based finite-time fault-tolerant control scheme is designed.Firstly,different types of acturator failure are analyzed and the mathematical models of the fault torque are established.Then a novel iterative learning disturbance observer based on Sigmoid function is designed to observe and compensate the comprehensive disturbances occurred in the system.Based on this,an adaptive finite-time sliding mode controller is proposed to realize finite time tracking of the RLV attitude.Finally,the simulation under different fault types is carried out to analyze and verify the effectiveness and robustness of the fault-tolerant control scheme.Finally,Due to the large of height span of RLV re-entry phase,heterogeneous redundant actuators are required to generate attitude control torque.To allocate the control torque to the heterogeneous redundant actuators,a mixed programming control allocation scheme is proposed.First,the function and characteristics of the RLV actuators are analyzed.On this basis,a mixed programming including aerodynamic surface control allocation based on sequential quadratic program,RCS control allocation based on Mamdani fuzzy control and decision mechanism and heterogeneous actuators control allocation based on dynamic pressure profile.The simulation of the proposed mixed programming control allocation scheme and pseudo-invers control allocation shceme is carried out under different conditions to verify the advantages of the proposed scheme.