Fault Tolerant Control of a Flexible Aircraft

Fault tolerant control (FTC) designs that deal with control surface faults while handling undesirable aeroelastic problems for flexible aircraft are presented. The goals of control design are to achieve flutter suppression or perform structural load alleviation while recovering nominal flight and maintaining acceptable performance in the event of control surface faults. The contributions of this thesis are two fold. First, the influence of control surface faults on not only the rigid-body motions but also the aeroelastic modes has been analyzed and mitigated through FTC design. Second, fault handling ability has been incorporated into an integrated flight control design that accounts for three different aeroelasticity requirements for flexible aircraft: flutter suppression, gust load alleviation (GLA) and maneuver load alleviation (MLA). A mathematical model based on the Lagrange's equations for quasi-coordinates that can describe the rigid-body motions of the aircraft and elastic deformations of the flexible wings as well as the coupling between them is developed. The flexible wing structure is modeled as a cantilever beam based on the Euler-Bernoulli beam elements under unsteady aerodynamic loads. For the flutter suppression problem, a linear parameter-varying FTC controller is developed to address stuck faults, actuator saturation and aeroelastic instability caused by parameter variations. The controller, in which the gain schedules with airspeed, can eliminate the effects of faults on the rigid-body motion while suppressing the unwanted vibrations of the wing and guaranteeing no closed-loop performance degradation caused by actuator saturation. For the GLA problem, a mixed $H_2/H_{\infty}$ FTC controller that can simultaneously achieve rigid-body motion stabilization, load alleviation on flexible wing structures, and on-line accommodation of loss of control effectiveness fault is developed. For the MLA problem, a model predictive control formulation with reference adjustment is presented to handle stuck and loss of control effectiveness faults. For each fault case, the controller can automatically adjust the tracking reference to guarantee an admissible tracking trajectory. During the maneuver, the structural load is constrained within given upper and lower bounds. The effectiveness of the three FTC designs are all demonstrated on an aircraft model with high aspect-ratio wings in numerical simulations.Ph.D