Fault Tolerant Wing Flutter Control For Reentry Vehicle

In the reentry of hypersonic vehicle,with a severe aerodynamic heating,greet flexibility and low damping,the flutter problem become more and more prominent.In the traditional stiffness design criteria,designers try to avoiding the aeroelastic effects through improving aeroelastic effects.But it will increase the structural weights,and reduce the overall performance of the vehicle.Since the 80s of last century,active aeroelastic control technology has achieved great development.This technology has made full use of aeroelastic effect,and with the aid of active control,it can achieve weight savings and performance optimization.Although the active aeroelastic control technology has achieved greet improvement,the existing research is just aimed at subsonic vehicle,and the actuators is supposed to be no faults.The research of aircraft wing flutter control always adopts active control which drives actuator on the suface of the wing to adjust the aeroelastic effect,and thus to suppress the flutter.Active control requires the use of sensors and actuators.The sensor is used to collect the system state information.However,the actuator is applied to control the controlled object according to the calculation results of control law.For an active control system,it is difficult to ensure the sensor and actuator in the ideal working condition.They are likely to have some problems,such as loss of effectivenes,float or saturation.For the hypersonic reentry vehicle,the aircraft will face hostile aerodynamic environment in the reentry process,such as high temperature,high pressure.The actuator may have faults,such as loss of effectivenes,float which are likely to cause catastrophic accidents with the reentry vehicle.Therefore,fault tolerance capability should be considered in flutter controller design.In the active wing flutter control,when the value of controller is greater than the maximum output that the actuator can produce,the actuator will be in saturation.The flutter system may be unstable because of the actuator saturation.In another aspect,an active control system may inevitably suffer from time delay problems.Although small in many cases,time delay inevitably causes an actuator to apply energy to the control system even when energy is not needed.This may cause degradation of control efficiency and even render the system unstable.For the hypersonic reentry vehicle in high speed,the system state changes a lot immediately.A small time delay in control system may cause the instability in control system.Therefore,it has great significance to carry out the research of time delay in active control of aircraft flutter.It is worth to note that,the research about active flutter control is almost supposed that the actuator is totally normal,and the control system has no time delay.We seldom see any research which has considered actuator faults,saturation and time delay.This paper is under the support of the Natural Science Foundation of China(11132001,11272202,11472171),the Key Scientific Project of Shanghai Municipal Education Commission(14ZZ021)and the Natural Science Foundation of Shanghai(14ZR1421000),take two-dimensional wing flutter model as research object and it researched on the fault-tolerant control of reentry vehicle.The main work and innovation points are as follows:(1)It researched on the trajectory optimization and the flutter dynamic equation which considered the faults of actuator.In the quantitative explanation of three process constraints(the heating rate,dynamic pressure and aerodynamic load),it used the conjugate gradient method to study a trajectory optimization algorithm,which makes the vehicle have the most optimal thermodynamics environment of reentry trajectory,so that effectively reduce the aerodynamic heating and temperature of vehicle surface.In the base of trajectory optimization,it considered the cubic hard spring nonlinearity,used the piston theory and Lagrangian method,built the two dimensional wing's aeroelastic motion equation which contains plunge deflection and pitch angle,and then built a wing flutter model which considered actuator faults.(2)It researched on the adaptive fault-tolerant control which has considered actuator faults,system model uncertainties and external disturbance.It used the LMI theory,proposed that the model depends on Lyapunov method to separate faults,which makes each fault has its own Lyapunov equation,therefore can avoid the conservativeness if only one Lyapunov function is used for different failure modes.Then with the use of H_?method,the adaptive fault-tolerant controller designed in this paper can totally compensate the actuator faults and external disturbance.Numerical simulation results shows that,when the actuator faults happen,the designed adaptive fault-tolerant controller can suppress the wing flutter.In addition,the controller has great robustness to the inherent parameters,uncertainty and external disturbance of the system.(3)To control the wing flutter in finite time,and to consider actuators'input saturation,system model uncertainties and external disturbance,in this paper,we combine adaptive control to the universal approximation function in the neural network,and propose a finite-time adaptive flutter fault-tolerant control method.First,use the neural network to approximate the error term in the input saturation,actuator faults,system model uncertainties and external disturbance,and so that can suppress wing flutter effectively.The stability of finite-time adaptive flutter fault-tolerant controller is proved strictly through Lyapunov theory.The result of numerical simulation indicates that when the faults of actuator happens,the designed flutter fault-tolerant controller can suppress the flutter quickly in finite time,and with great robustness of actuator saturation,inherent parameters of structure,system model uncertainties and external disturbance.(4)Aiming at the problem of time delay as well as taking account of actuators input saturation,actuators faults,model uncertainties and external disturbance,this paper gives a finite-time flutter fault-tolerant algorithm and the stability of the closed-loop flutter system has been proved by the Lyapunov-Krasovskii functional.The results of the study showed that time delay has great influence on the stability of the closed-loop flutter control system,It is possible to lead to a divergence of response from the system;And in this paper,the designed controller has a good robustness to change in time delay,it can not only handle small quantity problem of time delay,but also can deal with the large problem of time delay.In addition,the designed controller can not only effectively deal with problem of actuator failures,model uncertainty and external disturbance,but also can make the inherent structure of system parameters,model uncertainty and the changes of external disturbance have very strong robustness.(5)To solve the problem that the flutter system state can not be measured,this paper presents a control strategy of fault-tolerant control without plunge deflection and pitch angle information.In order to solve the control problem that the state measurement value can not be provided when a failure happens on the state sensors,this paper firstly designed the observer to achieve accurate estimation to plunge deflection and pitch angle in wing flutter state and then use the estimated value to design a finite-time adaptive flutter fault-tolerant controller.Therefore,flutter is of effectively suppression when a failure happens on the actuators.The simulation results show that the designed observer can accurately estimate the value of plunge deflection and pitch angle.Finite time adaptive finite-time adaptive flutter fault-tolerant controller can rapid suppress wing flutter.And it has good robustness for the inherent parameters of the system structure,model uncertainty and changes of uncertainties and external disturbance.