Research On Control Method Of Civil Aircraft Brake System Considering Nonlinear Factors

Anti-skid braking system is an important subsystem of civil aircraft,which integrates servo control,hydraulic(pneumatic)transmission,material science,electronics and other technologies.The purpose of the system is to maximize the binding force between the tires and runways,so as to achieve safe,fast and efficient braking,which plays a key role in ensuring the safe completion of take-off and landing.In this paper,the dynamics model of the aircraft is established firstly,and then the tire-runway nonlinear model and the friction coefficient nonlinear model of the brake disc are established according to the two main nonlinear sources of the aircraft anti-skid braking system,which constitute the whole nonlinear model of the aircraft anti-skid braking system..On this basis,the most widely used PD+PBM controller is established and simulated.The simulation results show that the overall nonlinear model of aircraft anti-skid braking system conforms to the actual braking characteristics.A sliding mode observer is designed to solve the problem that the aircraft velocity is not easy to obtain.The simulation results show that the observation effect is good,and the observed aircraft velocity can quickly converge to the actual aircraft velocity.On this basis,according to the performance requirements of aircraft anti-skid braking system and the principle of slip rate control mode,the sliding mode control law of aircraft anti-skid braking system based on exponential reaching law is designed,and the stability of the system is proved by using Lyapunov theorem.Aiming at the problem of high frequency chattering in the output of the controller of aircraft anti-skid braking system,based on the analysis of the exponential approach law,the fuzzy theory is applied to optimize the control law to suppress chattering.The sliding mode control law based on fuzzy index approach law is simulated in three conditions of dry runway,wet runway and runway condition mutation.The simulation results show that the designed control law can adapt to the nonlinear factors in the braking process of the aircraft well,and efficiently use the binding force of the runway.Compared with the sliding mode control law of the braking system before fuzzy optimization,it can suppress the high frequency chattering of the controller output before,and can output smooth braking pressure.Finally,the problems of low speed skid and poor robustness existing in the traditional PD+PBM control law are effectively solved.The designed control law is reasonable and effective,and the braking efficiency is high.