Fractional Order Based Attitude Control For Hypersonic Vehicle

Hypersonic vehicle is of strong nonlinear characteristic,coupling property,uncertainty and time-varying property,which brings challenges to controller design.Fractional calculus has the advantage of filtering characteristics and memory characteristics.Fractional order control,which combines the fractional calculus theory and the traditional control theory with their both advantages,can improve the robustness and precision of the system.In this paper,fractional order control is used to study the attitude control of hypersonic vehicle in reentry phase.A fractional order global sliding mode attitude control method is developed for the attitude control of hypersonic vehicle.Firstly,a fractional order exponential time-varying sliding mode controller based on feedback linearization is designed to guarantee the global robustness of the system.Secondly,aiming at the reduction of control accuracy caused by the boundary layer method,a slow time-varying disturbance observer is introduced,and a fractional order exponential sliding mode control method based on slow time-varying disturbance observer is designed.The control method can eliminate chattering phenomenon and improve the control accuracy of the system.Considering the limitation of slow time-varying disturbance observer designed before,the fractional order sliding mode attitude control method based on extended state observer is studied.Firstly,aiming at the problem that the derivative of attitude angle cannot be measured directly,and the limitation of slow time-varying disturbance observer,an extended state observer is designed to obtain the derivative of the attitude angle and to improve the adaptability of the perturbations with different frequency.Secondly,based on feedback linearization,the observed information of the derivative of attitude angle and the disturbance are introduced to design the fractional order PID sliding mode control law.This control method eliminates the chattering phenomenon and ensures the control accuracy as well as the robustness of the system.In consideration of the high precision requirement of feedback linearization,a fractional order sliding mode control method based on back-stepping approach is developed.According to the principle of back-stepping,the nonlinear attitude control system is divided into two subsystems including the attitude subsystem and angle rate subsystem.The virtual control law and the fractional PID sliding mode control law is designed respectively.The switch item is placed in the integral item.The developed control method can eliminate the chattering phenomenon effectively,and improve the convergence speed,control precision,and the robustness of the system against external disturbance.Considering the computational explosion problem of back-stepping,the first order differential low pass filter is introduced to obtain the derivative of virtual control law and reduce the computational difficulty.The effectiveness of the control methods which designed in this paper are verified by simulation.