Robust Control For Near Space Vehicle With Stochastic Disturbances

As a new field for military science and technology application,the near space has very important application and development value and far-reaching strategic significance.Near space vehicle(NSV)is a new type of aircraft with the characteristics of strong survivability,maneuverability,repeatabili-ty and wide coverage.Due to the special structure of NSV and the complex flight environment,the flight control performance of NSV always suffers from the adverse effects of system uncertainties and external disturbances,such as the stochastic perturbation of aerodynamic parameters and external s-tochastic disturbances.In addition,in the actual flight,the rudder angles are sensitive to the attitude motion response,so some certain constraints are required to be satisfied.If the actuator saturation is not considered,the control performance and safe flight quality of NSV will be adversely affected.Because there exist various kinds of stochastic disturbances,deterministic nonlinear system model may not be used to accurately describe NSV system model.Combining with stochastic differential equation theory,stochastic nonlinear system model is modelled which can be well used to describe the NSV system with stochastic disturbances.Furthermore,according to stochastic control theory,the effective stochastic control scheme will be designed which can improve the flight quality of NSV.In this thesis,in view of the above mentioned control problems,the robust stochastic flight control scheme for a class of NSV with system uncertainties,stochastic disturbances and input constraints will be researched.The main content of the thesis is shown as follows:(1)On the basis of the existing nonlinear model of NSV,considering the fact that the stochastic input disturbances and the stochastic perturbation of aerodynamic parameters exist in the control input channel of NSV.Based on stochastic differential equation theory,a stochastic nonlinear dynamic system model of NSV is modeled.Based on backstepping Control(BC)technology,auxiliary system and multi-dimensional Taylor polynomial network(MTPN),a robust stochastic control scheme is proposed for the NSV attitude dynamics model with system modeling uncertainties,input saturation and stochastic input disturbances.Furthermore,the adaptive attitude tracking control objective of NSV is realized,and the robustness of the flight control system is enhanced.The numerical simulation results show the effectiveness of the proposed robust stochastic control scheme.(2)For the NSV attitude dynamics model with multi-source disturbances,system modeling uncer-tainties and input/output constraints,based on nonlinear disturbance observer(NDO)and Tan-type Lya-punov function(TLF),a robust constrained stochastic control scheme is proposed.Firstly,the MTPN is used to approximate the system modeling uncertainty.Secondly,according to the different information of multi-source external disturbances,an NDO based on MTPN is designed to estimate external distur-bances and compensate the influence on the flight control performance.Additional,an auxiliary system is designed to suppress the influence of input saturation nonlinearity.Then,the stability in probabili-ty of the closed-loop system is proved by stochastic Lyapunov stability theory.Finally,the numerical simulation of NSV attitude dynamics model with input/output constraints and stochastic input noise dis-turbances is implemented,and the simulation results further demonstrate the feasibility of the proposed NDO-based robust stochastic constrained control scheme.(3)In the system modeling process of NSV,the accurate aerodynamic parameters cannot be ob-tained by wind tunnel experiments.Considering the stochastic perturbation of aerodynamic parameters,the attitude dynamics model of NSV with stochastic aerodynamic parameter uncertainties is given.Fur-thermore,based on NDO and composite auxiliary system,a robust adaptive stochastic tracking control scheme is proposed for NSV attitude dynamics model with stochastic parameter uncertainties,external disturbance and input saturation.In order to design an auxiliary system to effective compensate the influence of input saturation,a smooth nonlinear function based on Sigmoid function is constructed to approximate the saturation function,and a composite auxiliary system based on saturation approxima-tion is designed.Then,a MTPN-based NDO is designed to estimate the gust disturbance and other external time-varying disturbances.Furthermore,with the help of stochastic control theory,a robust adaptive stochastic tracking control scheme based on NDO and composite assistant system is designed.The stability of the whole NSV closed-loop system are analyzed.Finally,numerical simulation result-s indicate that the proposed robust adaptive stochastic control scheme can ensure that all closed-loop signals are convergent in probability and the satisfactory flight control performance is obtained.(4)Based on the above researches on attitude tracking control of NSV with stochastic distur-bances,the adaptive robust stochastic trajectory tracking control problem is further studied for the longitudinal trajectory control model of NSV with stochastic perturbation of aerodynamic parameters,multiple stochastic disturbances and input saturation.By designing cascade control scheme,the effec-tive tracking control of height and speed is realized.Firstly,a robust stochastic controller is designed for the altitude subsystem and the velocity subsystem,and the required attitude angle reference signals for the inner loop attitude control is obtained by converting the equivalent control input through numerical calculation.Then,the attitude angle signals generated from outer loop trajectory control is taken as ref-erence signals,combining with stochastic control method and robust H_∞control theory,a robust H_∞stochastic control scheme based on BC method,DSC technology and auxiliary system is proposed for NSV with multiple stochastic disturbances,and the accurate attitude tracking performance is achieved.By connecting the inner loop controllers and the outer loop controller,a satisfactory longitudinal trajec-tory tracking control performance of NSV is obtained.Taking the trajectory control model of NSV with aerodynamic parameter perturbation and multiple stochastic disturbances as the simulation object,and the effectiveness of the proposed robust stochastic trajectory tracking control scheme is further verified by the simulation results.