Research On Reentry Attitude Control For Hypersonic Vehicle Based On Disturbance Compensation

The hypersonic vehicle is a new type of fast maneuvering vehicle.Compared with the traditional aircraft,it has more complex flight missions and larger envelope with higher requirement for maneuvering performance.These issues make the control design a more challenging task.The attitude control system is now recognized as a critical requirement for the vehicle to complete the complex flight mission and track the guidance signals.During the three flight stages,the attitude control of reentry phase suffers more challenges,such as strong coupling effects,large disturbance,fast nonlinearities,sudden changes of atmospheric environment,uncertainties of aerodynamic parameters,actuator saturation,etc.These factors make the control system design more complicated and difficult.Advanced attitude control system with higher requirements for robustness and anti-saturation ability is worth comprehensive study.Disturbance compensation based control can effectively improve robustness of the control system.Therefore,this paper focuses on the study of reentry attitude control with disturbance compensation while some important practical issues are considered.The main contents are specified as follows.The kinematic and dynamic model of the hypersonic vehicle in reentry phase is established including aerodynamic force and moment coefficients models.The aerodynamic coefficients are fitted based on the NASA wind tunnel test and CFD computation database,which are the nonlinear functions of angle of attack,Mach number and control surface deflections.The reentry mathematical model can reflect the strong nonlinearity,coupling effects and the characteristics of fast time-varying as well.It provides a simulation platform for the conceptual design and verification of control strategies.Then,from the control point of view,the control-oriented model is formulated with mismatched and matched uncertainties which reflect the total disturbances that group different types of aerodynamic uncertainties and external moment disturbances in the practical flight system.This model is used to make further study and analysis.Based on the back-stepping concept,the lumped disturbances in the model are compensated by employing adaptive strategy which solves the problem of mismatch uncertainties.The adaptive term motivated by the tracking error is used to estimate the unknown boundary of lumped disturbances.Therefore,there is no need to know the bound of disturbance while tuning the control parameters.The problem of "explosion of terms" is effectively avoided by employing the dynamic surface strategy to obtain the derivative of the virtual control.Following this,the stability of the closed-loop system is proved within the Lyapunov theory framework.Based on the extended state observer,the disturbance compensation reentry attitude control methods are further studied.Two novel disturbance observers are proposed to overcome the difficult stability analysis and chattering output as well as shortcoming of noise suppression for traditional ESOs.One is the finite-time ESO,its main superiorities are that the observation error converges to a small neighborhood of zero state in finite time and the stability analysis is given based on Lyapunov theory;The other is the extended state disturbance observer designed based on sigmoid function.The main superiorities of the observer are that the output of the observer is smooth with strong ability of noise suppression and the rigorous stability analyses based on Lyapunov theory as well as the tuning principle of observer gains are presented.Furthermore,the reentry attitude disturbance compensation control system is designed based on FTESO.In addition,a new state-constrained control strategy is proposed by synthesizing SESOs using the BLF technology.Via modification of the controll laws,the tracking errors are guaranteed to be uniformly ultimately bounded based on Lyapunov theory.Disturbance compensation control for reentry attitude is studied based on neural network intelligent disturbance estimator.In order to avoid the dependence of traditional disturbance observers on system state function and input state,basing on the universal approximation property of the radial basis function NN(RBFNN),an adaptive NN disturbance observer is developed to estimate the lumped disturbances online using only the tracking error state as its input vector.The adaptive weight matrices are further modified to satisfy the stability requirement.The theory and simulation verify the effectiveness of the RBFNN estimator.In addition,considering the actuator saturation problem,the design process of the finite time robust anti-saturation controller based on RBFNN is presented.Theoretical analysis and simulations show that the proposed control strategy has strong abilities of anti-disturbance and anti-saturation.Based on the sliding mode observer,the disturbance compensation reentry attitude control method is further studied.A novel sliding mode disturbance observer(NSMDO)is proposed to remove output chattering and shortcoming in robustness for traditional high-order sliding mode observer.Its main superiorities are that the observation error state converges to zero state in finite time and the real value of time-varying disturbance can be reconstructed effectively without priori knowledge of the disturbance amplitude and derivative condition.In addition,a novel control structure model is proposed for the first time.The traditional control strategies are designed based on the two-loop control structure or back-stepping structure.Both methods result in two steps to complete the final control system design.In this paper,the second order error dynamic system is obtained through a tracking error model transformation,which makes it possible to complete the control system design in one step.The new design idea is verified by employing the sliding mode control technology.Then,the prescribed performance control method is proposed to realize the attitude tracking.Simulations show that the integrated controller has better anti-disturbance and error dynamic performance.Thus,a new design scheme is provided for reentry attitude control of hypersonic vehicles.