Analysis And Robust Adaptive Control Of Dangerous State In Post-stall Maneuver At High Angle Of Attack

To improve the maneuverability of modern fighters and increase the survival probability,the post stall maneuvering technology of fighters has been widely studied all over the world.High angle-ofattack post-stall maneuver enables the fighter to have controllable maneuverability at high angles of attack and low airspeed.When the fighter enters the stall zone,the attached flow field begins to separate and forms a complex vortex structure,which leads to the strong nonlinearity and unsteady hysteresis characteristics of the aircraft aerodynamic load at high angles of attack.Complex aerodynamic characteristics are particularly sensitive to the fighter attitude motion.Any small change in attitude may induce sudden changes in aerodynamic force and torque.Dramatic changes of aerodynamic load will induce dangerous states at high angles of attack.Therefore,the analysis and control of high angle of attack dangerous states for fighters pose a new challenge to modern control.The main contributions of this thesis are as follows:Firstly,the kinematic model and dynamic model of a modern fighter at high angles of attack are established.By introducing unsteady parts into the aerodynamic model,the high angle of attack dynamics model of the fighter is further improved.Based on some reasonable assumptions,the kinematics model and dynamics model of the post-stall fighter are simplified appropriately,and the nonlinear attitude motion equations for the post-stall control analysis at high angles of attack are established.On this basis,the aerodynamic model of post-stall fighter at high angles of attack is deeply analyzed.Through static coefficient analysis,dynamic derivative analysis and control derivative analysis,the stability changes of aircraft before and after stall can be preliminarily analyzed.Combined with the corresponding stability criteria,the stability range and anti-deviation characteristics of each channel of post-stall fighter are analyzed preliminarily,which provide the corresponding basis for the follow-up design of high angle of attack control law.Secondly,considering longitudinal pitching maneuver as the basis of post-stall maneuver,a finitetime observer-based longitudinal finite-stall pitching maneuver control for fighter is proposed.A nonlinear longitudinal attitude motion model of post-stall maneuver at high angles of attack is established,which ignores the influence of lateral motion.The influence of engine thrust on the longitudinal maneuverability is discussed in detail by using the bifurcation analysis method,and a finite time control law for high angle of attack post stall pitching maneuver is designed.To efficiently eliminate the influence of unsteady aerodynamic disturbances,nonlinear finite-time observers are developed.Despite the existence of the nonlinearity and the coupling between aircraft states and unsteady aerodynamic disturbances,the proposed observers can still estimate the unmeasurable unsteady aerodynamic disturbances in finite time.To attenuate the effect caused by input saturation,a finite-time auxiliary system is constructed.With the error between the desired control input and saturation input as the input of the auxiliary system,the additional signals are generated to compensate for the effect of input saturation.Combined with the finite-time observers and the finite-time auxiliary system,a robust finite-time backstepping attitude control law is developed.Simulation results show that the proposed longitudinal finite-time pitching maneuver control scheme can effectively improve the post-stall maneuverability and disturbance rejection capability of fighters.Then,considering the dangerous states caused by thrust vector faults,the deep stall recovery control of post-stall fighter is studied.Considering that the deep stall motion is a longitudinal motion,the longitudinal short period attitude motion model of aircraft is established.At the same time,considering the difficulty of modeling unsteady aerodynamic disturbance,the concept of distributed delay is adopted to describe unsteady aerodynamic disturbance for the first time.By introducing the distributed delay,the traditional strict feedback longitudinal short-period attitude control is transformed into the non-strict feedback attitude control problem.Based on the bifurcation analysis theory,the influence of engine thrust on longitudinal maneuverability of post-stall fighter is analyzed,and the high angle of attack dangerous state caused by controller failure is predicted.Based on the results of bifurcation analysis,a finite time adaptive prescribed performance deep stall control scheme is proposed.To solve the problem of controller singularity in traditional finite-time control,a switched finite-time adaptive deep stall recovery scheme is proposed based on switch control technology.The simulation results show that the proposed switched finite-time adaptive prescribed performance deep stall control scheme can effectively achieve the deep stall recovery of fighters,and effectively improve the transient maneuverability of fighters during the recovery process.Furthermore,the lateral motion control of aircraft at high angles of attack is further considered.According to the main motion characteristics of wing rock,a nonlinear multi-input multi-output lateral attitude motion model is established.Based on the bifurcation analysis theory,the stability of lateral attitude motion of post-stall fighter at different angles of attack is analyzed globally,and the occurrence range of wing rock is predicted accurately.Based on the analysis results,an observer-based wingrock controller is designed,and the unsteady aerodynamic disturbances,system uncertainties and input saturation of the fighter at high angles of attack are fully considered.In order to deal with complex and multiple disturbances in wing-rock control,neural network is combined with the extended state observer.At the same time,the corresponding auxiliary system is built to compensate for the adverse effects of input saturation.The simulation results show that the proposed control scheme can achieve better control performance of wing rock and reduce the influence of control input saturation at high angles of attack.Finally,considering the post-stall maneuver as a multi-degree-of-freedom coupled motion,an uncertain nonlinear multi-input and multi-output attitude motion model of post-stall fighter is established.Combining with the theory of bifurcation analysis,the stability of fighter motion and the occurrence range of high angle of attack dangerous states are analyzed.A switched prescribed performance control method is proposed and applied to the state constrained control of a post-stall fighter.Different from traditional prescribed performance control,multiple performance functions will be designed in the switched prescribed performance control according to different performance requirements.The core of this method is to adjust the output constraints of the system in real time according to the current state of the closed-loop system.Considering post stall maneuver as a complex maneuvering process,the switched prescribed performance control method is applied to the high angle of attack maneuvering control of fighter.Furthermore,in order to compensate for the adverse effects of the system uncertainties and input saturation on the maneuvering process,the neural network technology and auxiliary system method are combined with the switched prescribed performance control.The correctness and robustness of the method are verified by simulation.