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Optimal robust tracking control of aircraft under loss-of-control
Dissertation   Open access

Optimal robust tracking control of aircraft under loss-of-control

Po-Chun Chan
Doctor of Philosophy (Ph.D.), Drexel University
Sep 2018
DOI:
https://doi.org/10.17918/00000087
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Abstract

Actuator jam failure Actuator saturation aircraft flight control Robust Tracking/regulation Aeronautics--Safety measures
Actuator saturation and actuator jam are the two common and most dangerous actuator failures that can cause aircraft to lose control and crash. The study explores how these failure events would affect the flight of the aircraft and how to pre-design contingency controllers that can be employed to address the crucial issues and prevent the aircraft from getting into a loss-of-control condition or crash. Under normal flight conditions, a healthy aircraft usually flies within the admissible range of actuators and has no saturation issues. However, when the aircraft goes astray of the safe flight envelope due to the influence of environmental hazards or crews errors, the control system may need to send out larger control signals in order to bring the aircraft back into control. If the magnitudes of the control signals are over the working limits of the actuators, the actuators will saturate staying either at their maximum or minimum, which means that the actuators are no longer following the command of the controller. Sustained actuator saturations may cause the aircraft to fly toward a perilous attractor, which is a flight condition with almost straight down flight path. If the issue is not addressed in time, the aircraft would crash. Our proposed solution is to prevent the sustained actuator saturation from happening by applying a bang-bang control type of control laws. Simulation has shown the effectiveness of our proposed approach. For all actuators, the jam at the elevator control surface is the most formidable one since an elevator jam will cause more than 50% loss of the longitudinal control authority. Meanwhile, the aerodynamic force developed on the jammed elevator surface will become a persistent disturbance that may either destabilize the flight or force the aircraft to fly nose down. The issue also requires immediate contingency control action. The biggest challenge of the problem is that the elevator jam position is unknown a priori. The regulator theory with the internal model principle is employed to resolve the issue so that it is possible to predesign a fixed optimal robust tracking controller to not only mitigate the crisis, but also allow the aircraft to perform all necessary maneuvers to achieve a safe landing.

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