Optimal fault-tolerant flight control for aircraft with actuation impairments

摘要

Current trends towards greater complexity and automation are leaving moderntechnological systems increasingly vulnerable to faults. Without proper action, aminor error may lead to devastating consequences. In flight control, where thecontrollability and dynamic stability of the aircraft primarily rely on the controlsurfaces and engine thrust, faults in these effectors result in a higher extent of risk forthese aspects. Moreover, the operation of automatic flight control would be suddenlydisturbed. To address this problem, different methodologies of designing optimalflight controllers are presented in this thesis. For multiple-input multiple-output(MIMO) systems, the feedback optimal control is a prominent technique that solvesa multi-objective cost function, which includes, for instance, tracking requirementsand control energy minimisation.The first proposed method is based on a linear quadratic regulator (LQR) controllaw augmented with a fault-compensation scheme. This fault-tolerant system handlesthe situation in an adaptive way by solving the optimisation cost function andconsidering fault information, while assuming an effective fault detection system isavailable. The developed scheme was tested in a six-degrees-of-freedom nonlinearenvironment to validate the linear-based controller. Results showed that this faulttolerant control (FTC) strategy managed to handle high magnitudes of the actuator’sloss of effciency faults. Although the rise time of aircraft response became slower,overshoot and settling errors were minimised, and the stability of the aircraft wasmaintained.Another FTC approach has been developed utilising the features of controllerrobustness against the system parametric uncertainties, without the need for reconfigurationor adaptation. Two types of control laws were established under this scheme,theH∞and µ-synthesis controllers. Both were tested in a nonlinear environmentfor three points in the flight envelope: ascending, cruising, and descending. TheH∞controller maintained the requirements in the intact case; while in fault, it yieldednon-robust high-frequency control surface deflections. The µ-synthesis, on the otherhand, managed to handle the constraints of the system and accommodate faultsreaching 30% loss of effciency in actuation. The final approach is based on the control allocation technique. It considers the tracking requirements and the constraints ofthe actuators in the design process. To accommodate lock-in-place faults, a newcontrol effort redistribution scheme was proposed using the fuzzy logic technique,assuming faults are provided by a fault detection system. The results of simulationtesting on a Boeing 747 multi-effector model showed that the system managed tohandle these faults and maintain good tracking and stability performance, with someacceptable degradation in particular fault scenarios. The limitations of the controllerto handle a high degree of faults were also presented.