Fault-Tolerant Flight Control and Guidance Systems for a Small Unmanned Aerial Vehicle

265 Pages·2008·5.56 MB·English

Diss. ETH No. 17505
Fault-Tolerant Flight Control and Guidance Systems
for a Small Unmanned Aerial Vehicle
A dissertation submitted to
ETH ZURICH
for the degree of
Doctor of Sciences
presented by
Guillaume Jacques Joseph Ducard
MSc ETH
born 27 March 1980
citizen of France
accepted on the recommendation of
Prof. Dr. H. P. Geering, examiner
Prof. Dr. R. Siegwart, co-examiner
Prof. Dr. E. Niel, co-examiner
2007

To my parents

Abstract
In this thesis, novel approaches for fault-tolerant flight control systems are investigated.
Thisworkfocusesondevelopingalgorithmsforasmallunmannedaerialvehicle.Therefore,
the algorithms are designed to be modular, interchangeable, and highly computationally
efficient. A nonlinear fault detection and isolation (FDI) system is constructed based on
a multiple model scheme, which overcomes the traditional limitations of similar methods.
The FDI system is also active, since it generates auxiliary excitation signals in order to
systematicallycheckanysuspiciousbehavioroftheaircraft,thusincreasingtherobustness
of the detection system. An efficient control allocation module generates actuator control
signalsinordertoachievethedesiredroll,pitch,andyawtorquerequiredbytheautopilot.
The control allocator is reconfigurable based on the fault diagnosis made by the FDI
system.
Theautopilotconsistsofaturnratecontrolleroneachoftheaircraft’saxes,analtitude
controller, an airspeed controller, a bank angle controller, and a sideslip angle controller.
They are the result of a novel combination of explicit model following, nonlinear dynamic
inversion, and nonlinear transformations of selected state variables. A practical method-
ology is discussed to analyze the stability and the robustness of the controlled system in
the presence of model parameter and sensor uncertainties.
An innovative approach for an adaptive and reconfigurable guidance system is also
presented. It combines a nonlinear guidance control law with a new simple adaptive path
planning algorithm. This guidance system avoids autonomously any known or pop-up
obstacles.
In case of an actuator failure, the flying performance of an aircraft degrades. Therefore,
a separate algorithm estimates the new flying performance of the aircraft in order to
reconfigure the guidance system. New trajectories are computed on-line by the guidance
system, such that the new flight path matches the reduced aircraft performance.
Many simulation results on a high-fidelity nonlinear model of a model aircraft demon-
strate the successful operation of the reconfigurable flight control system.

R´esum´e
Dans ce travail de th`ese, de nouvelles approches pour la conception de syst`emes de com-
mande tol´erants aux fautes sont ´etudi´ees et appliqu´ees au contrˆole de petits avions sans
pilote: les drones. Par cons´equent, les algorithmes de commande sont conc¸us avec un souci
particulier de modularit´e, d’interchangeabilit´e et de performance de calcul.
Un nouvel algorithme non-lin´eaire et performant pour la d´etection et la caract´erisation
des pannes est construit sur le concept des mod`eles multiples, tout en d´epassant les
limitations connues de ce genre de m´ethode. Le d´etecteur de panne est aussi actif, en
ce sens qu’il g´en`ere des signaux artificiels d’excitation pour tester syst´ematiquement les
organes vitaux de l’avion d`es que celui-ci pr´esente un comportement suspect.
Un allocateur de commande performant est aussi propos´e pour g´en´erer la commande
sur l’ensemble des actionneurs `a partir des commandes de couple g´en´er´ees par l’autopilote
sur les axes de roulis, lacet et tangage. L’allocateur de commande est reconfigurable sur
la base du diagnostic effectu´e par le d´etecteur de panne.
L’autopilote du drone est constitu´e d’un contrˆoleur de taux de rotation sur chacun
des axes de l’avion, d’un controˆleur d’altitude, de vitesse, d’inclinaison, et de “sideslip”.
Tous les controˆleurs sont con¸cus sur la m´ethode d’inversion dynamique du mod`ele
math´ematique non-lin´eaire de l’avion. De plus, les controˆleurs utilisent une combinai-
son innovante de transformations non-lin´eaires de variables d’´etat choisies avec le suivi
d’un mod`ele de r´ef´erence. La stabilit´e et la robustesse des controˆleurs sont ´etudi´ees en
d´etail.
Un syst`eme de guidage novateur est aussi d´evelopp´e dans cette th`ese. Il combine une
loi de commande non-lin´eaire pour l’acc´el´eration lat´erale de l’avion avec un g´en´erateur
de trajectoires simple et adaptatif. Ce syst`eme de guidage ´evite de fa¸con autonome les
obstacles et reconfigure les trajectoires de l’appareil en cas de panne pour s’adapter aux
nouvelles performances de vol de l’avion.
De nombreuses simulations sur un mod`ele non-lin´eaire d’avion t´emoignent du fonction-
nement remarquable du syst`eme complet de commande et de guidage qui sont reconfig-
urables en cas de panne.

Acknowledgments
This thesis was written at the Measurement and Control Laboratory of the Swiss Federal
Institute of Technology (ETH Zurich), Switzerland.
My first thanks go to my parents and my brother for their continuous and loving
support in every possible way during my studies and in life in general. I would like to
include the families Paganelli and Corbisiero for their support and encouragement in
various domains, and Daniel Tomaszewski for sharing with me his interest and enthusiasm
for technical and engineering questions.
Special thanks go to my colleagues at the Measurement and Control Laboratory who
make the work environment at ETH Zurich very peaceful and friendly and for their
constructive inputs. I thank in particular my former office mates from the UAV group
and Mikael Bianchi for the many fruitful discussions we had.
I also wish to thank Prof. Dr. Lino Guzzella and Dr. Chris Onder, who first welcomed
me at ETH for a project dealing with fuel-cell systems. Moreover, I acknowledge the
contributions of all the students who collaborated with me during the course of this
research project.
I wish to thank Hans Ulrich Honegger for his technical support and sharing with me his
enthusiasm for his “home made” small flying machines. I wish to express my gratitude to
thesecretariesClaudiaWittwer,AnninaMu¨ller,andBrigitteRohrbachfortheirwonderful
work, which makes our lives much easier in the Lab. I owe special thanks to Brigitte for
the copy-editing of this manuscript, for her invaluable help in many aspects, and for her
kindness in general.
Additionally, I thank the co-examiners of this thesis for their collaboration.
Finally, I am most indebted to my PhD supervisor Prof. Dr. H. P. Geering for offer-
ing me this fascinating research project in exceptional working conditions. I gratefully
acknowledge his constant support in many different aspects.
December 2007
Guillaume Ducard