Nonlinear Control Method For Vehicle With Flap

During the reentry process of hypersonic vehicle,the air in front of the vehicle is compressed violently due to the high speed,leading to the rapid rise of the local temperature.This phenomenon seriously constrain the employment of aerodynamic rudders because of the bad influence of ablation.The body flaps,used for vehicle attitude control,can be deflected symmetrically or asymmetrically to implement the elevator and aileron function.The flaps are not exposed outside the vehicle fuselage to avoid ablation during the high-speed flight process.However,the vehicle control system with flaps only has equivalent elevator and aileron,losing the rudder in yaw control loop,so it becomes an underactuated control system.Moreover,the aerodynamic shape of plane-symmetry vehicle determines that the vehicle must adopt the bank-to-turn(BTT)control mode.Therefore,the difficulty of controller design are increasing for the system with multiple constraints.The flight altitude and Mach number of reentry vehicle vary greatly in different flight stages.Aiming at the control problem of vehicle in dense atmosphere,this paper studies the design of pure aerodynamic control system with flaps as control variables.On the other hand,aiming at the control problem of vehicle in near space,this paper investigates the design of compound control system with flaps and reaction jets as control variables.Firstly,the six-degree-of-freedom mathematical model of a tail-controlled BTT vehicle with two body flaps is derived.The weak controllability theory of nonlinear system is used to prove the controllability of the three-channel motion of BTT flaps with angle-of-attack,sideslip angle and roll angle as state variables.This result provides a theoretical support for the subsequent controller design.Secondly,for the vehicle with pure flaps control,considering the first-order dynamic characteristics of the rudder,a new mathematical model is derived when choosing accelrations as state variables.The controllability of the system is also proved using the theory of weak controllability of nonlinear system.Basic principle of State-Dependent Riccati Equation(SDRE)control method and key technology for obtaining SDC are elaborated.And the three-channel optimal control law of flaps vehicle is designed.The simulation results show that the vehicle of flaps can track the longitudinal and lateral overload commands and roll angle command quickly and accurately,which verifies the effectiveness of the three-channel controller.Since the SDRE solution needs to solve Ricatti equation online,which impairs the real-time performance of the controller.So we seek a feedback linearization method to design the controller.By choosing angle-of-attack and roll angle as outputs of the control system,the internal dynamics of the system are obtained using the theory of differential geometry.According to Lyapunov stability theory,a condition for internal dynamics stability is determined.The controller of the flaps vehicle is designed based on feedback linearization theory,and the effectiveness of the designed controller is verified by tracking two typical signals.Thirdly,in order to improve the control quality of flaps vehicle control system at a high altitude,a pair of reaction control jets is introduced into the yaw channel to solve the problem of divergence of sideslip angle resulting from the lack of corresponding control variable.Since flaps produce a continuous control effect,while attitude jet works in bang-bang type,composing a hybrid control system.Because the sideslip angle of the vehicle is always small throughout the flight process,pitch channel can be decoupled from the system,and the controller based on linear quadratic optimal theory is designed.For roll-yaw joint channel,controllers are designed for aerodynamic control system and lateral thrust control system respectively.For the aerodynamic control system,the control law is designed based on linear quadratic optimal theory.For the lateral thrust control system,a new control model is obtained by substituting aforementioned control law into the system.After transforming the above control model into linear normal form,its controller is designed using sliding mode theory.The simulation results present that the reaction jets introduced in the yaw channel greatly improves the stability of the sideslip angle,and performance of the system.Finally,in oder to improve the quality of the control system of hypersonic vehicle in the near space,a pair of reaction control jets is installed in the pitch channel at the tail of the vehicle as well as in the yaw channel.By choosing the longitudinal and lateral accelrations as the outputs of the control system,an unstable conclusion is obtained by analyzing the internal dynamics of the system under only flaps.By contrast,the compound control system shows a stable characteristic,when the longitudinal and lateral accelrations are chosen as the outputs of the compound control system.According to the above characteristic,the feedback linearization method is used to design controller for the compound control model.Simulation results demonstrate that the vehicle can track the command signals faster under compound control,which verifies the effectiveness of the controller design.