Bank-to-turn Control Design In Terminal Guidance Phase For Small-scale Loitering Vehicles

Small-scale loitering vehicles play an important role in modern warfare with its small size,low cost and integration of investigation and attack.This kind of aircraft mostly uses the way of the bank-to-turn(BTT)control mode to increase maneuverability,improve aerodynamic efficiency and make the full use of large aspect ratio and high lift-drag ratio.However,BTT control mode requires the loitering vehicle to roll around the longitudinal axis rapidly,which aggravates the coupling effect between the three channels.In addition,the loitering vehicle is greatly affected by uncertainties,making the design of its control system a challenging research topic.Consequently,this dissertation tries to carry out study on modelling and analysis of flight dynamics,BTT linear control design,BTT non-linear control design,disturbance observer-based control design and terminal guidance flight test for small-scale loitering vehicles.Firstly,in term of the feature of the small-scale loitering vehicle and BTT control,the 6DOF model of the loitering vehicle is established,and the coupling terms are analyzed,which lays the foundation for the subsequent work in this dissertation.Secondly,for sloving the problem of the BTT linear control,small disturbance,coefficient freezing and ignoring the coupling between three channels,etc.are proposed to transform the nonlinear system into a linear one.The transfer functions of three channels are obtained,and the responses to rudder of the loitering vehicle are analyzed.On this basis,three-loop classical frequency domain method is used to design the control parameters for each characteristic point.In addition,gain scheduling is used to realize the adaptive adjustment of the whole trajectory control parameters.Six-degree-of-freedom trajectory simulation shows that the coupling terms affect the control performance,but the controller can track overload instructions quickly and accurately.Thirdly,in view of coupling and uncertain characteristics of nonlinear control system for BTT cruise vehicles,a robust backstepping controller is designed to follow the control command.The mathematical model of cruise vehicles is transformed into a state equation which is suitable for backstepping algorithm.The equation is assumed to be composed of a nominal model and uncertain terms.The uncertain terms are derived and described by using the Leibniz rule in infinitesimal calculus.On this basis,the basic form of the control law is derived by exploiting backstepping algorithm.The modification of robust function i.e.,a new symbolic function,is conducted to compensate the unmatched uncertainties.Simultaneously,the Lyapunov redesign technique is introduced to control the matched uncertainties.The simulation results indicate that the presented control system can avoid the adverse effects of coupling and quickly track the commands of angle of attack,sideslip angle and bank angle,which shows the strong robustness of the system.Fourthly,disturbance observer-based controller design is proposed to improve the performance of linear control.The improved form of the nonlinear disturbance observer is introduced,and the coupling term in the angular velocity differential equation is regarded as external disturbance.The combined disturbance composed of parameter perturbation and external disturbance is estimated and compensated by the nonlinear disturbance observer.The simulation results show that this method can effectively counteract the influence of compound disturbance on the control system without participating in the actual object control.It is simple and easy to implement.Fifthly,in order to further verify the correctness and validity of the designed control parameters in actual flight,the terminal guidance flight test of the loitering vehicle is carried out.The flight scheme of hovering mode entering terminal guidance is designed,and the specific test steps are presented.The test results show that the loitering vehicle can fly steadily to the target even if the wind disturbance,propeller torque,coupling and other factors affect the control performance.