The Depressive Methodology Against Loss Of Airfoil Control And Airfoil Oscillation For Fly-By-Wire Flight Control System In Civil Aircrafts

With the development of Fly-By-Wire(FBW)flight control system(FCS),the computer,sensor and electro-hydraulic servo valve are used to replace the traditional mechanical control mechanism,which can reduce the weight of the system,save the installation space and improve the control performance.Due to the increase of control links,the safety of the control system is reduced.In the safety requirements of civil airworthiness regulations,loss of ailfoil control and ailfoil oscillation are two major catastrophic failures in flight control system,which are the primary concerns in system safety design.Redundancy technology is widely adopted in modern civil aircrafts to solve these two problems,with the costs of the increasing system development cost,complexity,weight,dimension as well as the decrasing of MTBF.The redundancy architecture scheme optimization and the oscillation despressive method should be comprehensive evaluated and traded off.In order to solve the issues,this thesis qualitatively evaluates the performance and the complexity of distributed and centralized dissimilar redundancy architectures of FBW FCS through principle analysis.The failure rate of loss of airfoil control is calculated with fault tree analysis (FTA)in order to trade off the scheme.The amplitude and frequency of the airfoil oscillation are controlled within the range of airworthiness regulations by means of voting,comparing and the force fight mitigation algorithm.The main work and innovation are following:Firstly,aiming at the problem of high failure rate of the complex system with multiple links including electronics,mechanical and hydraulic in the control link of FBW FCS,the method of dissimilar redundancy design configuration is adopted to reduce the common mode failure rate and make the failure rate of the loss of airfoil control to meet the airworthiness safety requirements.This thesis analyzes the principle of distributed and centralized redundancy architectures,and qualitatively compares the performance and complexity of the two architectures from the perspective of flight control computer,hydraulic system and electric power system.An electric source and hydraulic source configuration management method is proposed to increase the safety level with the philosophy where the distributed resources are centralized before allocating to FCS.Six kinds of bottom events models are established through the fault tree analysis for the failure of loss of elevator control;Regarding to the problem that the complex electronic hardware logic gates are too huge to be calculated one by one,hardware failure is used to establishe the fault tree models of flight control computer and other electronic circuits to represent the logic gate failure.The fault tree modeling methods of single latent failure and double latent failures with time series are researched and the logical relationship between explicit and latent faults in the fault tree are analyzed.The failure rates of two redundancy architectures are analyzed with FTA.The corresponding MTBFs are calculated with reliability block diagrams(RBD).Hence the optimal solution is selected.In order to solve the problem that the logic of FBW FCS electronics equipment is complex to induce error signal coupling resulting in the airfoil oscillation,four redundancy configuration in the most conservative design of FBW FCS is investigated and a dissimilar redundancy signals voting and comparing method based on the way of "bubble" is proposed.The oscillation signals could be detected and isolated by means of voting and comparing.The associated comparing thresholds and counters of the monitor are calculated based on the high fidelity nonlinear Matlab simulation models of FCS.Followed,the highest frequency(10Hz)of airfoil oscillation in FCS is used to simulate and predicte the performance of the monitor.The simulation result shows that the oscillation could be detected within 0.48 s.The performance of the monitor are tested and verified through the full physical Iron Bird test platform with the real environment,based on the most complex test cases: two failure signals and two normal signals which needs the maximum comparing times in monitor logic.The test result indicates that the oscillation can be detected within 0.5s and satisfy the aircraft structure fatigue damage tolerance design requirements(0.5s).The effectiveness of the voting and comparing method in term of oscillation fault monitoring is validated with the test.A mitigation control method is proposed based on the dual feedback compensation of pressure and position to solve the force fight oscillation caused by the active-active working mode between redundancy electronic hydraulic servo actuators(EHSAs),resulting in airfoil fatigue affecting the flight safety.The controller compares the pressure difference between the two actuators based on the pressure of retract and extend ram to generate pressure compensation value.Through the PI controller,the position compensation value is calculated according to the relationship between different delta pressure(DDP)and the proportion of the airfoil torsional deformation,while the integral control is used to eliminted the steady-state error of oscillation and overshoots by the reason of high gain control.The position is corrected with comparing the position sensors of two actuators at both ends of the airfoil to minimize the difference to reduce the azimuth error between the two actuators,realizing the force fight mitigation.A mathematical model of actuator to airfoil is established.The maximum delay and error accumulation of the system are used as test cases to simulate performance of the force fight mitigation controller.The simulation results show that the pressure difference between the two actuators is only 200 psi.The control method is further verified by the special Iron Bird tests with the result that the DDP is decreased from 400 psi to 190 psi at a decreasing rate of 52% when mitigation is induced,meeting the design requirement of the structural fatigue damage tolerance of 500 psi.