Fault-Tolerant Flight Control for a Fixed-Wing Unmanned Aerial Vehicle with Partial Horizontal and

162 Pages·2016·4.17 MB·English

Fault-Tolerant Flight Control for a
Fixed-Wing Unmanned Aerial Vehicle with
Partial Horizontal and Vertical Stabiliser
Losses
by
Ryan Lee Maggott
Thesis presented in partial fulfilment of the requirements for the degree
Master of Engineering
in the Faculty of Engineering at Stellenbosch University.
Supervisor:
Mr J.A.A. Engelbrecht
Department of Electrical and Electronic Engineering
December 2016
Stellenbosch University https://scholar.sun.ac.za
Declaration
By submitting this thesis electronically, I declare that the entirety of the work contained
therein is my own, original work, that I am the sole author thereof (save to the extent
explicitly otherwise stated), that reproduction and publication thereof by Stellenbosch
University will not infringe any third party rights and that I have not previously in its
entirety or in part submitted it for obtaining any qualification.
December 2016
Copyright © 2016 Stellenbosch University
All rights reserved
Stellenbosch University https://scholar.sun.ac.za
Acknowledgements
I would like to thank the following people for their assistance in completing this project:
God for His continued guidance and support.
•
Mr Japie Engelbrecht, for all your input, help and guidance as my supervisor.
•
My family for constantly supporting and motivating me. My father (Shaun) for his help
•
with the hardware manufacturing. My mother (Lindy) for her continued care, support
and concern. My sister (Stacey) for always being interested and willing to help.
Michael Basson for all the aircraft related advice and for being a great safety pilot.
•
Wiaan Beeton, Nico Alberts, Cornelus Le Roux and Chris Fourie as ESL lab engineers
•
for assistance in the lab and flight test preparation.
Andrew de Bruyn, Gideon Hugo and Piero Ioppo for their willing nature to help at flight
•
tests.
All the friends that I have made at the ESL throughout my studies.
•
i
Stellenbosch University https://scholar.sun.ac.za
Declaration
By submitting this thesis electronically, I declare that the entirety of the work contained herein
is my own, original work, that I am the sole author thereof (save to the extent explicitly
otherwise stated), that reproduction and publication thereof by Stellenbosch University will
not infringe any third-party rights and that I have not previously in its entirety or in part
submitted it for obtaining any qualification.
December 2016
Copyright c 2016 Stellenbosch University
(cid:13)
All rights reserved
ii
Stellenbosch University https://scholar.sun.ac.za
Abstract
In the study reported here, a fault-tolerant flight control system for a fixed-wing unmanned
aerial vehicle with partial stabiliser loss is designed, analysed, implemented and verified. The
partial stabiliser damage changes the natural dynamics of the aircraft and causes asymmetry.
The control system must maintain aircraft stability and transition from the healthy to the
damaged configuration without depending on in-flight knowledge of the change in dynamics.
The control system must also provide satisfactory transient performance for both the healthy
and the damaged configuration.
Using existing reference frames and conventions, a six-degrees-of-freedom equations of mo-
tion model of the aircraft is derived that can model the effects of the partial horizontal and
vertical stabiliser loss on the aircraft dynamics. This model considers the changes in the mass,
moment of inertia, aerodynamic model, control authority of the aerodynamic control surfaces,
as well as the shift in the centre of gravity. The altered aerodynamic coefficients are calculated
using vortex lattice techniques for the different damage configurations. In order to determine
the trim states and inputs of the aircraft as a function of the partial horizontal and vertical
stabiliser loss, a multivariate Newton–Raphson technique is applied to the equations of motion.
The required trim actuator deflections are compared to the physical actuator limitations to
establish the feasibility of maintaining trim flight for each damage case. Assuming feasible
trim states and inputs, the system is linearised and the open-loop dynamics of the aircraft are
investigated as a function of partial stabiliser loss.
A combination of classical and acceleration-based control architectures are designed and
implemented. The stability, performance and robustness of the flight control system are verified
insimulationfordamagecasesupto70%lefthorizontalstabiliserlossand20%verticalstabiliser
loss.
The fault-tolerant flight control system is verified with flight tests. A release mechanism
is designed and manufactured to allow 70% of the left horizontal stabiliser and 20% of the
vertical stabiliser to be jettisoned in flight. The flight control system is implemented on a
practical unmanned aerial vehicle and successful reference tracking is demonstrated. Practical
flight tests showed that the flight control was stable for both the healthy and the damaged
aircraft configurations, and able to handle the transition following an in-flight partial stabiliser
loss event.
iii
Stellenbosch University https://scholar.sun.ac.za
Opsomming
Hierdie tesis beskryf die ontwerp, analise, implementasie en verifikasie van ‘n fout-tolerante
vlugbeheerstelsel vir ‘n vastevlerk onbemande vliegtuig met gedeeltelike stabiliseerder verlies.
Hierdie verlies veroorsaak ‘n verandering in die natuurlike dinamika van die vliegtuig en veroor-
saak asimmetrie.
Die beheerstelsel moet in staat wees om stabiliteit te handhaaf en die oorgang van die
gesondenadiebeskadigdekonfigurasiestehanteer, enmoetniestaatmaakopin-vlugkennisvan
die verandering in die dinamika nie. Die beheerstelsel moet ook bevredigende oorgangsgedrag
vertoon vir beide die gesonde en die beskadigde konfigurasies.
Bestaande verwysingsraamwerke en konvensies is gebruik om ‘n ses-grade-van-vryheid be-
wegingsvergelykingsmodel vir die vliegtuig af te lei wat die effekte van die gedeeltelike ho-
risontale en vertikale stabiliseerder verlies op die vlugdinamika modelleer. Hierdie model neem
die veranderinge in die massa, traagheidsmoment, aerodinamiese model, beheergesag van die
aerodinamiese oppervlakkeverskuiwing en massamiddelpunt in ag. Die veranderinge in die
aerodinamiese koëffisiënte word bereken met draaikolk rooster tegnieke vir die verskillende
beskadigde konfigurasies. ‘n Meerveranderlike Newton–Raphson tegniek word gebruik om die
bewegingsvergelykings op te los om die ekwilibrium toestande en intrees van die vliegtuig te
bereken as ‘n funksie van persentasies gedeeltelike horisontale en vertikale stabiliseerder ver-
lies. Die benodigde aktueerder defleksies vir ekwilibrium vlug word vergelyk met die fisiese
aktueerder limiete om te bepaal of dit haalbaar is vir die spesifieke hoeveelheid skade. Gegee
haalbare ekwilibrium toestande en intrees, word die stelsel gelineariseer en die ooplusdinamika
van die vliegtuig ondersoek as ‘n funksie van gedeeltelike stabiliseerder verlies.
‘n Kombinasie van klassieke en versnellingsgebaseerde beheerargitekture is ontwerp en im-
plementeer. Die stabiliteit, prestasie en robuustheid van die vlugbeheerstelsel word verifieer in
simulasie vir skade tot by verlies van 70% van die linkerkantste horisontale stabiliseerder en
20% van die vertikale stabiliseerder.
Die fout-tolerante vlugbeheerstelsel is ook verifieer met praktiese vlugtoetse. ‘n Loslaat-
meganisme is ontwerp en vervaardig om 70% van die linker horisontale stabiliseerder en 20%
van die vertikale stabiliseerder in vlug af te gooi. Die vlugbeheerstelsel is implementeer op ‘n
praktiese onbemande vliegtuig en suksesvolle verwysingsvolging is gedemonstreer. Die prak-
tiese vlugtoetsresultate wys dat die vlugbeheer stabiel is vir beide die gesonde en die beskadigde
vliegtuig konfigurasies, en dat dit in staat is om die oorgang te hanteer na in-vlug gedeeltelike
stabiliseerder verlies.
iv
Stellenbosch University https://scholar.sun.ac.za
Table of Contents
Acknowledgements i
Declaration ii
Abstract iii
Opsomming iv
Table of Contents v
List of Figures x
List of Tables xiii
Nomenclature xiv
1 Introduction 1
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Previous Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2.1 Internal Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.2 External Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Research Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.5 Research Vehicle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.6 Project Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.7 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Modelling 10
2.1 Reference Frames and Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.1.1 Inertial, Body and Wind Reference Frames . . . . . . . . . . . . . . . . . 11
2.1.1.1 Inertial Axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1.1.2 Body Axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.1.3 Wind Axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1.2 Standard Notation and Conventions . . . . . . . . . . . . . . . . . . . . . 12
2.1.2.1 General Conventions . . . . . . . . . . . . . . . . . . . . . . . . 13
v
Stellenbosch University https://scholar.sun.ac.za
Table of Contents
2.1.2.2 Actuator Conventions . . . . . . . . . . . . . . . . . . . . . . . 14
2.2 Symmetric Flight Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.1 Standard Six Degrees of Freedom (6DoF) . . . . . . . . . . . . . . . . . . 17
2.2.1.1 Kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.1.2 Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1.3 Attitude Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1.4 Position Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.2 Forces and Moments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.2.1 Aerodynamic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.2.2.2 Thrust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2.2.3 Gravitational . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.3 Extended Aircraft Flight Mechanics Model . . . . . . . . . . . . . . . . . . . . . 24
2.3.1 Effect of Partial Stabiliser Loss . . . . . . . . . . . . . . . . . . . . . . . 25
2.3.2 Asymmetric Six Degrees of Freedom Model . . . . . . . . . . . . . . . . . 25
2.3.2.1 Force Equations . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.3.2.2 Moment Equations . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3.2.3 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3.3 Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.4 The Effects of Partial Stabiliser Loss on Aerodynamic Coefficients . . . . . . . . 30
2.4.1 Numerical Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.4.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3 Trim 34
3.1 Trim Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.2 Symmetric Trim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3 Asymmetric Trim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.3.1 Analytic Trim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.3.2 Newton–Raphson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.3.2.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4 Stability Analysis 44
4.1 Linearisation of Aircraft Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.2 Validity of Decoupling the Longitudinal and Lateral Dynamics . . . . . . . . . . 48
4.3 Stability Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.3.1 Modes of Motion Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.3.1.1 Longitudinal Modes . . . . . . . . . . . . . . . . . . . . . . . . 51
4.3.1.2 Lateral Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.3.2 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
5 Controller Design 57
5.1 Control Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5.1.1 Decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
vi
Stellenbosch University https://scholar.sun.ac.za
Table of Contents
5.1.2 Longitudinal Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5.1.2.1 Airspeed Controller . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.1.2.2 Normal Specific Acceleration Controller . . . . . . . . . . . . . 59
5.1.2.3 Climb Rate Controller . . . . . . . . . . . . . . . . . . . . . . . 61
5.1.2.4 Altitude Controller . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.1.3 Lateral Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.1.3.1 Lateral Specific Acceleration Controller . . . . . . . . . . . . . . 64
5.1.3.2 Roll Angle Controller . . . . . . . . . . . . . . . . . . . . . . . 67
5.1.3.3 Cross track Controller . . . . . . . . . . . . . . . . . . . . . . . 67
5.2 Controller Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.2.1 Healthy Aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.2.1.1 Airspeed Controller . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.2.1.2 Normal Specific Acceleration Controller . . . . . . . . . . . . . 70
5.2.1.3 Climb Rate Controller . . . . . . . . . . . . . . . . . . . . . . . 72
5.2.1.4 Altitude Controller . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.2.1.5 Lateral Specific Acceleration Controller . . . . . . . . . . . . . . 75
5.2.1.6 Roll Angle Controller . . . . . . . . . . . . . . . . . . . . . . . 76
5.2.1.7 Cross Track Controller . . . . . . . . . . . . . . . . . . . . . . . 77
5.2.2 Robustness of Controllers to Partial Stabiliser Loss . . . . . . . . . . . . 77
5.2.2.1 Airspeed Controller . . . . . . . . . . . . . . . . . . . . . . . . . 77
5.2.2.2 Normal Specific Acceleration Controller . . . . . . . . . . . . . 77
5.2.2.3 Climb Rate Controller . . . . . . . . . . . . . . . . . . . . . . . 80
5.2.2.4 Altitude Controller . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2.2.5 Lateral Specific Acceleration Controller . . . . . . . . . . . . . . 83
5.2.2.6 Roll Angle Controller . . . . . . . . . . . . . . . . . . . . . . . 83
5.2.2.7 Cross Track Controller . . . . . . . . . . . . . . . . . . . . . . . 86
5.2.3 Closed-loop Pole Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6 Hardware in the Loop Simulation 88
6.1 Nonlinear Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.1.1 Controller Step Responses . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.1.1.1 Airspeed Controller . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.1.1.2 Normal Specific Acceleration Controller . . . . . . . . . . . . . 89
6.1.1.3 Climb Rate Controller . . . . . . . . . . . . . . . . . . . . . . . 89
6.1.1.4 Altitude Controller . . . . . . . . . . . . . . . . . . . . . . . . . 92
6.1.1.5 Lateral Specific Acceleration Controller . . . . . . . . . . . . . . 92
6.1.1.6 Roll Angle Controller . . . . . . . . . . . . . . . . . . . . . . . 93
6.1.1.7 Guidance Controller . . . . . . . . . . . . . . . . . . . . . . . . 94
6.1.2 In-Flight Transition for Healthy to Damaged Configuration . . . . . . . . 96
6.1.2.1 Airspeed Response . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.1.2.2 Normal Specific Acceleration Response . . . . . . . . . . . . . . 97
vii
Stellenbosch University https://scholar.sun.ac.za
Table of Contents
6.1.2.3 Climb Rate Response . . . . . . . . . . . . . . . . . . . . . . . 97
6.1.2.4 Altitude Response . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.1.2.5 Lateral Specific Acceleration Response . . . . . . . . . . . . . . 99
6.1.2.6 Roll Angle Response . . . . . . . . . . . . . . . . . . . . . . . . 99
6.1.2.7 Guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.2 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
7 Flight Tests 101
7.1 Research Vehicle Modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
7.1.1 Hardware Modifications to Represent Partial Stabiliser Losses . . . . . . 102
7.1.2 Release Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
7.2 Flight Test Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
7.2.1 Flight Test Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.2.2 Flight Test Campaign . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.2.2.1 Flight Test: RC Flight . . . . . . . . . . . . . . . . . . . . . . . 104
7.2.2.2 Flight Test: Estimator Flight . . . . . . . . . . . . . . . . . . . 105
7.2.2.3 Flight Test: Controller Tests on Healthy Aircraft Configuration 105
7.2.2.4 Flight Test: Controller Tests with Partial Stabiliser Loss . . . . 106
7.2.2.5 Flight Test: In-Flight Transition from Healthy to Damaged Air-
craft Configuration . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.3 Flight Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7.3.1 LongitudinalFlightControl-HealthyandDamagedAircraftConfigurations107
7.3.1.1 Airspeed Controller . . . . . . . . . . . . . . . . . . . . . . . . . 108
7.3.1.2 Normal Specific Acceleration Controller . . . . . . . . . . . . . 108
7.3.1.3 Climb Rate Controller . . . . . . . . . . . . . . . . . . . . . . . 109
7.3.1.4 Altitude Controller . . . . . . . . . . . . . . . . . . . . . . . . . 109
7.3.2 Lateral Flight Control - Healthy and Damaged Aircraft Configurations . 110
7.3.2.1 Lateral Specific Acceleration Regulation . . . . . . . . . . . . . 111
7.3.2.2 Roll Angle Controller . . . . . . . . . . . . . . . . . . . . . . . 112
7.3.2.3 Cross Track Controller . . . . . . . . . . . . . . . . . . . . . . . 113
7.3.3 In-Flight Transition from Healthy to Damaged Configuration . . . . . . . 114
7.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
8 Conclusions 120
8.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
8.2 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
8.3 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
8.3.1 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
8.3.2 Improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
viii