Incremental sliding mode flight control

Abstract

The swift growth of air traffic volume stresses the importance of flight safety enhancement. Statistical data shows that fly-by-wire technology with automatic flight control systems can effectively reduce the fatal accident rate of loss of control in-flight. Although the dynamics of an aircraft are nonlinear and time-varying, it is common practice to design flight control laws based on local linear time-invariant (LTI) dynamic models, and apply gain-scheduling method. Here, the flight envelope is divided into many smaller operating regimes, and LTI model-based controllers are designed and tuned for each of them. However, this approach is cumbersome and cannot guarantee flight stability and performance in-between operational points. In view of the challenges encountered by LTI model-based control, nonlinear control methods have attracted attention from the flight control community. Nonlinear dynamic inversion (NDI) and backstepping (BS) are two frequently used nonlinear control methods in flight control. These two approaches cancel the nonlinearities in the closed loop using a nonlinear model of the system. However, mismatches between the model and real dynamics inevitability exist, especially when an aircraft encounters atmospheric disturbances and when sudden actuator faults or even structural damages occur. To enhance the robustness of model-based nonlinear control methods to model mismatches, a commonly adopted approach is to augment them with online model identification. This process, however, is computational intensive and requires sufficient excitation, which can make an impaired aircraft fly out of the diminished safe flight envelope. In consideration of these challenges, the main goal of this thesis is: To design a stability-guaranteed nonlinear flight control framework with reduced model dependency and enhanced robustness.