Key Technologies Of Fault Tolerant Flight Control System For UAV

Recently,unmanned aerial vehicles(UAVs)have received unprecedented attention in the military and civil fields.The flight control system(FCS)of high performance UAVs must meet the increasing safety and reliability requirements.Ultra-high reliability of FCS can be achieved by using reliable devices and fault-tolerant control algorithms.Reliable devices mean high-reliable hardware and software,and fault-tolerant control algorithms are capable of maintaining overall system stability and acceptable performance not only on normal circumstances,but also in the event of system failures or damage.Fault-tolerant control algorithms compensate for failures or damage of aircraft by using the remaining effectors to generate compensating forces and moments.In this dissertation,fault tolerant flight control computer and control algorithm for UAVs are addressed.Firstly,the flight control system requirements,safety,reliability,maintainability,real-time response,and so on,are compared between civil and military aircraft and UAVs,the architectures and redundancy management of typical fault-tolerant FCC systems are introduced,and the particularity and future developments of FCC for UAVs are addressed.On this basis,a FlexRay-based distributed triple modular redundancy flight control computer system(FDTMR-FCCS)for UAVs is developed.The FlexRay buses are not only the backplane buses for a single FCC,but also the cross-channel data link buses for FCCs.The reliability analysis demonstrates that the failure rate of FDTMR-FCCS is less than the traditional triple modular redundancy flight control computer system.Secondly,the Byzantine fault detect method are developed for the FDTMR-FCCS.The system can tolerate a Byzantine fault using only three computers with oral messages.A hierarchical data exchange method is developed to reduce the overhead of data exchanges for Byzantine fault tolerance.Membership management is also provided to improve reliability.Additionally,FlexRay parameter optimization,task and message scheduling are involved.The optimal length of static segment slot is achieved with maximum bandwidth utilization.The FlexRay communication system operates in synchronous mode,and the tasks and messages parallel scheduling method is developed to increase efficiency of the system.With period scheduling tables(PST),different static segment frames can use a common frame identifier(FID)and allocated FIDs are reduced.Thirdly,the principle prototype of FDTMR-FCCS is developed and the performance of the system is validated by the simulation experiment.Finally,an overview on nonlinear reconfigurable flight control approaches that have beendemonstrated in flight-test or in high-fidelity simulation is presented.Various approaches for reconfigurable flight control systems are considered,including nonlinear dynamic inversion,parameter identification and neural network technologies,backstepping and model predictive control approaches.On the basis of this,an adaptive reconfigurable longitudinal trajectory control system applied to the damaged UAV for un-powered approach and landing is presented.The controller is decomposed into four feedback loops: pitch-rate,angle-of-attack,flight-path-angle and altitude loop,among which the altitude loop is the outermost loop and the pitch-rate loop is the innermost loop.The pitch-rate loop is based on dynamic inversion technique and the other loops are in a backstepping control scheme.The onboard models used by dynamic inversion and backstepping are linearized models at some equilibrium points,and inversion model errors are compensated by online learning neural networks.The results of high-fidelity simulation for the Orbiter vehicle indicate that the control system can maintain desirable stability and performance properties in the presence of surface failure and wing damage.