Current trends towards greater complexity and automation are leaving modern
technological systems increasingly vulnerable to faults. Without proper action, a
minor error may lead to devastating consequences. In flight control, where the
controllability and dynamic stability of the aircraft primarily rely on the control
surfaces and engine thrust, faults in these effectors result in a higher extent of risk for
these aspects. Moreover, the operation of automatic flight control would be suddenly
disturbed. To address this problem, different methodologies of designing optimal
flight controllers are presented in this thesis. For multiple-input multiple-output
(MIMO) systems, the feedback optimal control is a prominent technique that solves
a multi-objective cost function, which includes, for instance, tracking requirements
and control energy minimisation.
The first proposed method is based on a linear quadratic regulator (LQR) control
law augmented with a fault-compensation scheme. This fault-tolerant system handles
the situation in an adaptive way by solving the optimisation cost function and
considering fault information, while assuming an effective fault detection system is
available. The developed scheme was tested in a six-degrees-of-freedom nonlinear
environment to validate the linear-based controller. Results showed that this fault
tolerant control (FTC) strategy managed to handle high magnitudes of the actuator’s
loss of effciency faults. Although the rise time of aircraft response became slower,
overshoot and settling errors were minimised, and the stability of the aircraft was
maintained.
Another FTC approach has been developed utilising the features of controller
robustness against the system parametric uncertainties, without the need for reconfiguration
or adaptation. Two types of control laws were established under this scheme,
the
H∞
and µ-synthesis controllers. Both were tested in a nonlinear environment
for three points in the flight envelope: ascending, cruising, and descending. The
H∞
controller maintained the requirements in the intact case; while in fault, it yielded
non-robust high-frequency control surface deflections. The µ-synthesis, on the other
hand, managed to handle the constraints of the system and accommodate faults
reaching 30% loss of effciency in actuation. The final approach is based on the control allocation technique. It considers the tracking requirements and the constraints of
the actuators in the design process. To accommodate lock-in-place faults, a new
control effort redistribution scheme was proposed using the fuzzy logic technique,
assuming faults are provided by a fault detection system. The results of simulation
testing on a Boeing 747 multi-effector model showed that the system managed to
handle these faults and maintain good tracking and stability performance, with some
acceptable degradation in particular fault scenarios. The limitations of the controller
to handle a high degree of faults were also presented
