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D
EVELOPMENT OF A FAST SHAPE MEMORY ALLOY
BASED ACTUATOR FOR MORPHING AIRFOILS
D
EVELOPMENT OF A FAST SHAPE MEMORY ALLOY
BASED ACTUATOR FOR MORPHING AIRFOILS
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op donderdag 24 maart 2016 om 10:00 uur
door
Adrián L
ARA
Q
UINTANILL A
Ingeniero Industrial,
Universidad Carlos III de Madrid, Spain, geboren te Burgos, Spain.
Dit proefschrift is goedgekeurd door de promotor: Prof. dr. ir. R. Benedictus copromotor: dr. ir. H.E.N. Bersee Samenstelling promotiecommissie:
Rector Magnificus, voorzitter
Prof. dr. ir. R. Benedictus Promotor
Dr. ir. H.E.N. Bersee Copromotor
Onafhankelijke leden:
Prof. Dr. Ir. G.A.M. van Kuik, Delft University of Technology
Prof. Dr. Ir. A. de Boer, University of Twente
Prof. Dr. D. Polyzos, University of Patras
Prof. Dr. J.A. Puértolas Rafales, Universidad de Zaragoza
Prof. Dr. G. van Bussel, Delft University of Technology (reservelid)
Overige leden:
Dr. Ir. J.W. van Windergen, Delft University of Technology
This research was carried out as part of the "Smart Fixed Wing Aircraft" European project under the Clean Sky Joint Technology Initiative program.
Keywords: Shape memory alloy, actuator, actuation frequency, control, morphing
airfoils
Printed by: BOXPress
Cover design: Adrián Lara-Quintanilla
Cartoon: Stephan Timmers
Copyright © 2016 by Adrián Lara-Quintanilla
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilised in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage retrieval system, without prior permission of the author.
ISBN 978-94-6186-612-7
An electronic version of this dissertation is available at
S
UMMARY
The design of aerodynamic airfoils are optimized for certain conditions. For instance, the shape of the wings of fixed-wing aircrafts are designed and optimized for a certain flight condition (in terms of altitude, speed, aircraft weight, etc.). However, these flight conditions vary significantly during the flight. Currently, aircraft are provided with con-trol surfaces such as flaps, slat and ailerons, normally governed by powerful but heavy hydraulic mechanisms. These moving parts allow the aircraft to fly under many differ-ent flight conditions, although usually with non-optimal performance. Moreover, these mechanisms introduce hinges and surface discontinuities between parts which cause undesirable effects such as turbulences and noise or a decrease of the lift-to-drag ratio. This issue motivates the research and development of the so-called ‘morphing aircraft’ or ‘morphing wings’. Ideally, a morphing aircraft is able to modify quickly the shape of its wings in-flight, thus reaching optimum aerodynamic performance under any flight condition. This idea is applicable to any other aerospace applications such as rotorcraft or wind turbines.Morphing applied to aerodynamic airfoils brings along interesting benefits: reduc-tion of mechanical fatigue which has a special importance in wind turbines and rotor-crafts (by minimizing vibrations on the structure), reduction of the wing root bending moment, reduction of fuel consumption of flying machines and increase of the perfor-mance of wind turbines by increasing the lift-to-drag ratio of the wings or blades, and the reduction of generated noise.
This dissertation gives an introduction on the concept of morphing applied to aero-dynamic airfoils and describes the benefits and challenges brought by morphing struc-tures. Subsequently, the potential of smart materials to develop novel actuation systems is introduced. The description of the motivation for this work leads to the purpose of this research, which aims at developing a prototype of a general-purpose morphing flat surface based on embedded shape memory alloy wires with increased working frequency for morphing airfoils. In order to achieve this purpose, the research is di-vided into two research objectives. The first research objective is to develop a profound understanding of the behaviour of shape memory alloys (SMAs) and, subsequently, to devise a method to increase their actuation frequency. The second research objective is to develop novel control algorithms and actuator technology as well as an integra-tion technology for the SMA wires with increased actuaintegra-tion frequency.
Following the introduction, this thesis describes the potential of smart materials in general, and SMAs in particular, to develop novel smart actuators for morphing wings. The main differences between conventional and smart actuators are explained together with their advantages and introduced challenges in terms of design and control. The se-lection of SMAs is justified as the best candidates to achieve the research objectives. A detailed description, working principles, features, capabilities, limitations and applica-tions of SMA based actuators is given as well.
viii SUMMARY
In order to fulfill the first research objective, a series of logical steps were followed. Once the requirements of the SMA based actuator were stated, a commercial SMA wire was chosen and characterized (i.e., the phase diagram of the SMA wire was obtained) which shows the phase composition of the SMA when it is subjected to different levels of stress and temperature. The phase diagram was obtained from data collected from isothermal, isobaric and differential scanning calorimeter tests. During an isothermal test, the temperature is kept constant and the SMA wire is subjected to increasing and decreasing levels of stress in order to find those stresses at which the SMA’s phase trans-forms. Similarly, an isobaric test keeps the stress on the SMA constant over time and the temperature is increased and decreased to find those at which the SMA’s phase trans-forms. Subsequently, the functional fatigue of the SMA wire was studied by training the SMA wire. This training process consist in applying repetitive heating and cooling cy-cles in an isobaric configuration. High repeatability was found on the results. After an average of 7289 training cycles, the wire was able to recover only 77 % of its original re-coverable length for the tested conditions of applied current, heating time, cooling time and applied stress.
Subsequently, an SMA model for SMA wires is implemented in a finite element anal-ysis software. The equations that describe the model and their physical meaning are explained. One of the advantages of the chosen model is the ease of obtaining the pa-rameters required by the model, which can be obtained from a few experiments. In ad-dition, and also as a part of the model’s required parameters, the characteristics of the forced cooling system are studied here, and the heat transfer coefficient of such airflow for different airflow rates is measured. Finally, the model is satisfactorily validated.
One of the limitations of SMA based actuators is their poor actuation frequency (usu-ally lower than 0.1 H z). This is due to the fact that they are therm(usu-ally activated, norm(usu-ally by Joule heating, which is a quick process. However, they must be cooled before the next actuation cycle, which normally happens by natural convection. This cooling process can be accelerated by means of active cooling systems. In this research, an active cool-ing airflow at room temperature is used. The effects of applycool-ing different heatcool-ing and cooling rates on the time that it takes for the wire to contract and elongate (respectively) were experimentally measured using an isobaric configuration. In addition, the same experiments were simulated by the model for SMA wires implemented previously. There is an overall quantitative disagreement between the results yielded by the simulations and by the experiments. However, the results are useful qualitatively. It is found that the contracting and the cooling times are significantly decreased as the applied power and airflow is increased. More importantly, it is found that when the wire works at low working frequencies, the heating rate is the limiting factor whereas the cooling rate is the limiting factor when it works at high working frequencies. In addition, these exper-iments show that increasing the level of applied stress results in slightly higher working frequencies of the SMA wire.
The knowledge acquired in the previous experiments and simulations leads to the development of a method to improve the attainable actuation frequency of the SMA wires. This method is based on the idea that, in many SMA based applications, the SMA wires do not work throughout their full recoverable strain but they work only within a portion of it. Taking advantage of the nonlinearity of the strain-temperature relationship
SUMMARY ix
for SMAs, the method proposed here is able to increase the SMA’s actuation frequency by three and a half times just by making the SMA wire work within the most suitable range of strains, without varying the heating, cooling and stress conditions. The development of this method fulfills the first research objective.
Later on in this thesis, the design, manufacturing and assembling processes of the SMA based actuator as well as their challenges are detailed. First, a beam-like module of the SMA based actuator was conceived, designed and manufactured. Subsequently, the design was expanded in the spanwise direction, thus obtaining a modular SMA based actuator that forms a morphing plate. The morphing plate was tested by heating the SMA wires embedded in the actuator. This test revealed an unexpected behaviour of the plate. The expected behaviour was that it would bend upwards when the wires on the top side were heated. However, instead of bending uniformly along the spanwise direction, the center of the plate bended upwards and the sides downwards. A similar response was observed when the wires on the bottom side were heated. This was found to be caused by inhomogeneous thermal expansion (in the spanwise direction) through the thickness of the plate.
Due to the unexpected response observed on the morphing plate, a single beam-like module SMA based actuator was controlled under two different control strategies, fuzzy logic control (FLC) and proportional-integrative-derivative (PID) control. In both cases, the SMA based actuator was made to track sinusoidal and step signals in order to measure its performance. The overall performance of the system under FLC is bet-ter than that under PID. Under FLC, it reaches actuation frequencies above 0.6 H z for working at amplitudes of up to 6 mm, and frequencies above 1 H z for amplitudes of up to 3 mm while tracking the reference signal accurately (maintaining relative error below 10 %). Under PID control, it reaches actuation frequencies above 0.5 H z when working at amplitudes up to 3 mm, and frequencies above 0.7 H z for amplitudes up to 2 mm. The actuator is able to track step signals (that is, to reach and maintain a constant deflection over time) under both types of controller, although under FLC it is significantly more stable. These developed design, manufacture, assembly and control methods fulfill the second research objective.
This thesis presents novel methods aiming to increase the accuracy and actuation frequency of SMA based actuators. In particular, this work is focused on the develop-ment of a morphing surface intended to be integrated in morphing airfoils. However, the methods and ideas developed in this research are applicable to other SMA based applications, especially those which require fast, cyclic and accurate actuation.
S
AMENVAT TING
Het ontwerp van aerodynamische vleugelprofielen is geoptimaliseerd voor bepaalde om-standigheden. De vorm van vleugels van vliegtuigen is bijvoorbeeld ontworpen en optimaliseerd voor een bepaalde vluchtfase (met betrekking tot hoogte, snelheid, ge-wicht, etc.). Echter, de condities variëren significant tijdens de vlucht. Op dit moment zijn vliegtuigen uitgerust met stuurvlakken zoals kleppen, neuskleppen en rolroeren, die aangestuurd worden door krachtige maar zware hydraulische mechanismen. Deze stuurvlakken stellen het vliegtuig in staat om te vliegen onder verschillende condities, maar vaak zonder geoptimaliseerde prestaties. Voor deze mechanismen zijn scharnieren en oppervlakte onderbrekingen tussen onderdelen nodig die zorgen voor ongewenste effecten zoals turbulentie en geluid, of een verlaging van de draagkracht-weerstands-verhouding. Deze punten vormen de motivatie voor het onderzoek van zogenaamde “morphing aircraft” of “morphing wings”. Idealiter is een vervormbare structuur in staat om snel de vorm tijdens de vlucht aan te passen en daarmee de optimale aerodynami-sche prestatie voor die specifieke vluchtfase te bereiken. Dit idee is ook toepasbaar op andere vliegtuigbouwkundige toepassingen zoals helikopters en windturbines.Vervormbaarheid toegepast op vleugelprofielen heeft interessante voordelen: reduc-tie van mechanische vermoeiing, wat specifiek aantrekkelijk is voor windturbines en he-likopters (door structurele trillingen te minimaliseren), reductie van het buigmoment aan de wortel van de vleugel, reductie van brandstofverbruik van vliegtuigen en verbe-terde prestaties van windturbines door een hogere draagkracht-weerstands-verhouding van vleugels of turbinebladen, en vermindering van geluid.
Dit proefschrift geeft een introductie van het concept vervormbaarheid toegepast op aerodynamische vleugelprofielen en beschrijft de voordelen en uitdagingen verbonden aan vervormbare structuren. Tevens wordt het potentieel van slimme materialen om nieuwe aansturingssystemen te ontwerpen geïntroduceerd. De omschrijving van de mo-tivatie voor dit werk leidt tot het doel van dit onderzoek: het ontwikkelen van een proto-type van een vervormbaar plat oppervlak, gebaseerd op geïntegreerde geheugenmetaal-draden met verbeterde operationele frequentie voor vervormbare vleugelprofielen. Om dit doel te bereiken is het onderzoek opgedeeld in tweeën. Het eerste onderzoeksdoel is het ontwikkelen van een diepgaand begrip van het gedrag van geheugenmetaal en, even-eens, het ontwikkelen van een methode om de werkfrequentie te verhogen. Het tweede onderzoeksdoel is het ontwikkelen van een nieuw aansturingsalgoritme en nieuwe ac-tuatortechnologie evenals een integratietechniek voor geheugenmetaaldraden met ver-grote werkfrequentie.
Na de introductie zal dit proefschrift zich richten op het potentieel van slimme mate-rialen in het algemeen, en geheugenmetaal in het bijzonder, met betrekking tot het ont-wikkelen van vernieuwende slimme actuatoren voor vervormbare vleugels. Het belang-rijkste verschil tussen conventionele en slimme actuatoren wordt uitgelegd samen met de voordelen en uitdagingen met betrekking tot ontwerp en aansturing. De keus voor
xii SAMENVATTING
geheugenmetaal wordt gerechtvaardigd als het beste materiaal voor het bereiken van de onderzoeksdoelen. Een gedetailleerde omschrijving, functionele principes, voordelen, mogelijkheden, beperkingen en toepassingen van op geheugenmetaal gebaseerde actu-atoren worden ook beschreven.
Om te voldoen aan het eerste onderzoeksdoel zijn een aantal logische stappen ge-volgd. Nadat de eisen voor de op geheugenmetaal gebaseerde actuator bekend waren werd een commercieel beschikbaar materiaal gekozen en gekarakteriseerd (nl., een fa-sediagram voor het materiaal werd bepaald) waarin de faseopbouw van het geheugen-materiaal te zien is onderhevig aan verschillenden waarden van mechanische spanning en temperatuur. Het fasediagram was vastgesteld door middel van isothermische en isobarische tests en differentiële scanning calorimetrie. Tijdens een isothermische test wordt the temperatuur constant gehouden terwijl de geheugenmetaaldraad wordt belast in toenemende en afnemende mate om de mechanische spanningen te vinden waarbij fasetransformatie plaatsvindt. Bij een isobarische test wordt de mechanische spanning gelijk gehouden terwijl de temperatuur toe- en afneemt om de transformatietempera-turen te vinden. Tevens is de functionele vermoeiing bestudeerd door een draad te trai-nen. Het trainingsproces bestaat uit het repetitief verwarmen en koelen in isobarische configuratie. Tijdens deze tests werd een hoge herhaalbaarheid vastgesteld. Na gemid-deld 7289 wisselingen kon de draad nog maar ongeveer 77 % van de origineel herwinbare lengte herwinnen voor de gebruikte testcondities.
Een geheugenmetaalmodel voor draden is geïmplementeerd in een eindige- elemen-tenprogramma. De vergelijkingen die het model beschrijven en de fysische betekenis zijn uitgelegd. Een van de voordelen van het gekozen model is het gemak waarmee pa-rameters voor het model gevonden kunnen worden, welke met een paar experimenten bepaald kunnen worden. Daarnaast, en tevens als onderdeel van de vereiste parameters voor het model, worden de eigenschappen van een actief koelsysteem bestudeerd, en de warmteoverdracht van een dergelijke luchtstroom voor verschillende stroomsnelheden wordt gemeten. Ook wordt het model gevalideerd binnen acceptabele grenzen.
Een van de beperkingen van op geheugenmetaal gebaseerde actuatoren is hun lage aansturingsfrequentie (doorgaans lager dan 0.1 H z). Dit komt door het feit dat ze ther-misch geactiveerd zijn, meestal door het Joule-effect, wat een snel proces is. Echter, ze moeten gekoeld worden voor de volgende cyclus, wat meestal gebeurd door vrije con-vectie. Dit koelproces kan versneld worden door geforceerde koelingssystemen. In dit onderzoek wordt een actieve luchtstroom op kamertemperatuur gebruikt. De gevolgen van het toepassen van verschillende verwarmings- en koelingssnelheden op de tijd die de draad nodig heeft om samen te trekken of te verlengen zijn experimenteel gemeten door gebruik van een isobarische opstelling. Dezelfde experimenten zijn ook gesimu-leerd met het model voor geheugenmetaaldraden. Er is een algemeen kwantitatief ver-schil tussen de experimentele resultaten en de simulaties. De resultaten zijn echter wel kwalitatief waardevol. De tijd voor samentrekking en afkoeling van de draden is vermin-derd als het toegepaste vermogen of luchtstroom wordt vergroot. Significanter is dat de verwarmingssnelheid de beperkende factor is bij lage frequenties, waar de koelsnelheid de beperkende factor is bij hoge frequenties. Deze experimenten laten ook zien dat een hogere mechanische spanning resulteert in een hogere haalbare frequentie.
SAMENVATTING xiii
tot de ontwikkeling van een methode om de haalbare frequentie van geheugenmetaal-draden te verhogen. Deze methode is gebaseerd op het idee dat in veel op geheugen-metaal gebaseerde toepassingen de draden niet hun volledige bereik gebruiken, maar slechts een deel van dit bereik. Door gebruik te maken van de nonlineariteit van de rek-temperatuur-koppeling van geheugenmetaal is de hier voorgestelde methode in staat om een activeringsfrequentie van geheugenmetaal te vergroten met een factor 3.5, door alleen gebruik te maken van het meest toepasselijke rekbereik, zonder de verwarmings-, koelings- of mechanische spanningscondities te wijzigen. Door de ontwikkeling van deze methode wordt voldaan aan het eerste onderzoeksdoel.
Verder wordt in dit proefschrift het ontwerp, de productie en de assemblage van de op geheugenmetaal baseerde actuator besproken, evenals de naar voren gekomen uitda-gingen. Als eerste werd een balkachtige module van de op geheugenmetaal gebaseerde actuator bedacht, ontworpen en geproduceerd. Daarna werd het ontwerp uitgebreid in spanwijdterichting om een modulaire op geheugenmetaal gebaseerde actuator te krij-gen die een vervormbare plaat vormt. De vervormbare plaat werd getest door de ge-heugenmetaaldraden in de actuator te verwarmen. Deze test zorgde voor onverwacht gedrag van de plaat. Het verwachte gedrag was dat de plaat naar boven zou buigen wan-neer de bovenste draden verwarmd zouden worden. Echter, in tegenstelling tot uniform buigen langs de spanwijdterichting boog het midden van de plaat omhoog en de zijkan-ten naar beneden. Wanneer de onderste draden verwarmd werden liet de plaat een ver-gelijkbaar gedrag zien. Dit fenomeen werd veroorzaakt door non-homogene thermische expansie.
Door het onverwachte gedrag van de vervormbare plaat werden twee regeltechnie-ken geprobeerd voor een enkele balkachtige module van de actuator, “fuzzy logic con-trol” (FLC) en proportionele, integrerende, differentiërende (PID) regeltechniek. In beide gevallen moest de actuator een sinussignaal en stap functie volgen om de prestatie te meten. De algemene prestaties van FLC zijn beter dan die van PID. Tijdens FLC kan een frequentie van meer dan 0.6 H z gehaald worden voor een amplitude tot 6 mm en een frequentie van meer dan 1 H z voor amplitudes tot 3 mm tijdens het volgen van een si-nusvormig referentiesignaal (met een relatieve afwijking van minder dan 10 %). Tijdens PID regeltechniek kan een frequentie van meer dan 0.5 H z gehaald worden voor een amplitude tot 3 mm en een frequentie van meer dan 0.7 H z voor amplitudes tot 2 mm. De actuator is in staat om stapfuncties te volgen (dat wil zeggen, het bereiken en vast-houden van een amplitude voor een bepaalde tijd) voor beide regeltechnieken, hoewel voor FLC het gedrag significant stabieler is. Door deze ontwikkelde ontwerp-, productie-, assemblage- en regeltechnieken wordt voldaan aan het tweede onderzoeksdoel.
Dit proefschrift presenteert vernieuwende methodes gericht op het verbeteren van de nauwkeurigheid en snelheid van op geheugenmetaal gebaseerde actuatoren. Dit werk richt zich specifiek op de ontwikkeling van een vervormbaar oppervlak bedoeld om geïntegreerd te worden in vervormbare vleugelprofielen. De methodes en ideeën ontwikkeld in dit onderzoek zijn echter toepasbaar in andere op geheugenmetaal geba-seerde toepassingen, met name toepassingen die snelle, cyclische en nauwkeurige aan-sturing vereisen.
C
ONTENTS
Summary vii
Samenvatting xi
List of Figures xix
List of Tables xxv
Nomenclature xxvii
1 Introduction 1
1.1 Flying: From dream to reality . . . 1
1.2 Structural evolution of aircraft . . . 2
1.3 The morphing aircraft: a more efficient flight . . . 2
1.3.1 The benefits of morphing . . . 3
1.3.2 Parts of morphing structures . . . 4
1.3.3 Challenges of morphing structures. . . 5
1.4 Morphing fixed-wing aircraft concepts . . . 6
1.4.1 Wing planform morphing . . . 6
1.4.2 Out-of-plane transformation. . . 8
1.4.3 Airfoil adjustment . . . 10
1.5 Morphing on rotary-wing aircraft and wind turbines . . . 11
1.5.1 Morphing rotary-wing aircraft . . . 12
1.5.2 Morphing applied to wind turbines . . . 12
1.6 Control of morphing structures . . . 14
1.7 Final considerations to morphing structures . . . 14
1.8 Research objective . . . 14
1.8.1 First research objective. . . 15
1.8.2 Second research objective . . . 16
1.9 Thesis outline . . . 17
1.10 Summary . . . 18
References . . . 19
2 Smart actuators 21 2.1 Introduction . . . 21
2.2 From conventional to smart actuators . . . 22
2.2.1 Sensing and actuation: accompanying capabilities of smart mate-rials . . . 22
2.2.2 Design challenges . . . 23
2.2.3 Potential of smart actuators on morphing aerospace applications . . 23 xv
xvi CONTENTS
2.3 Smart actuators based on smart materials . . . 24
2.3.1 Shape memory alloy based actuators . . . 24
2.3.2 Shape memory polymer based actuators. . . 24
2.3.3 Piezoelectric actuators . . . 25
2.3.4 Magnetostrictive actuators. . . 26
2.3.5 Comparison between smart materials for actuation . . . 27
2.4 SMA based actuators . . . 28
2.4.1 The alloy . . . 28
2.4.2 Phase transformations . . . 29
2.4.3 Shape memory effect and superelasticity . . . 31
2.4.4 Transformation temperatures . . . 35
2.4.5 Features of SMA based actuators . . . 36
2.4.6 Applications . . . 38
2.5 Chapter summary. . . 40
References . . . 41
3 Characterization of an SMA wire 47 3.1 Introduction . . . 47
3.2 Requirements of the SMA based actuator . . . 48
3.3 Selection of an SMA wire . . . 49
3.4 Phase diagram . . . 50
3.4.1 Differential Scanning Calorimeter test . . . 50
3.4.2 Isothermal test . . . 50
3.4.3 Isobaric test . . . 51
3.4.4 Obtaining of the phase diagram . . . 53
3.5 Functional fatigue. . . 55
3.6 Conclusions. . . 60
References . . . 62
4 SMA model and convective cooling 65 4.1 Introduction . . . 65
4.2 The model . . . 66
4.3 COMSOL implementation . . . 69
4.4 Experimental . . . 70
4.4.1 Convective heat transfer coefficient . . . 70
4.4.2 Mechanical parameters . . . 74
4.5 Validation . . . 76
4.6 Conclusions. . . 77
References . . . 79
5 Response times of the SMA wire 81 5.1 Introduction . . . 81
5.2 Experimental setup: contraction and cooling times . . . 83
5.3 Results and discussion . . . 85
5.4 Conclusions. . . 87
CONTENTS xvii
6 Improvement of the attainable SMA’s bandwidth 93
6.1 Introduction . . . 93
6.2 Experimental . . . 95
6.3 Results and discussion . . . 98
6.4 Conclusions. . . 103
References . . . 105
7 Design and manufacturing of an SMA based actuator 107 7.1 Introduction . . . 107
7.2 Direct and indirect embedding of SMA wires . . . 109
7.3 SMA-based actuator: A modular design concept . . . 110
7.4 Proof-of-concept . . . 112
7.4.1 Design of the actuator . . . 112
7.4.2 Deflection of the actuator . . . 113
7.4.3 Performance of the actuator . . . 114
7.5 The morphing surface . . . 115
7.5.1 Design of the morphing surface . . . 115
7.5.2 Manufacture of the morphing surface . . . 117
7.5.3 Assembly. . . 122
7.6 Test, results and discussion . . . 123
7.7 Recommendations for future work . . . 125
7.8 Conclusions. . . 126
References . . . 127
8 Control and performance of the SMA based actuator 129 8.1 Introduction . . . 129
8.2 Control requirements . . . 130
8.3 The SMA based actuator system . . . 132
8.4 Fuzzy Logic Controller . . . 134
8.4.1 Fundamentals of FLCs . . . 135
8.4.2 Configuration of the FLC . . . 137
8.5 Proportional, integral and derivative controller . . . 138
8.5.1 Response threshold . . . 139
8.5.2 Proportional and derivative controller . . . 139
8.5.3 Feed-forward stabilizer . . . 140
8.5.4 Output of the linearizer . . . 140
8.6 The setup . . . 141
8.7 Actuator governed by a fuzzy logic controller . . . 141
8.7.1 Performance tracking sinusoidal signals . . . 142
8.7.2 Performance tracking composed sinusoidal signals . . . 145
8.7.3 Performance tracking step signals . . . 146
8.8 Actuator governed by a PD controller . . . 146
8.8.1 Response threshold . . . 146
8.8.2 Performance of the feed-forward stabilizer . . . 148
8.8.3 Performance of the controlled actuator . . . 149
xviii CONTENTS
8.9 Conclusions. . . 155
References . . . 156
9 Conclusions and recommendations 157 9.1 Introduction . . . 157
9.2 Research’s conclusions . . . 157
9.2.1 Characterization of the SMA wire . . . 157
9.2.2 Improvement of the attainable SMA’s bandwidth . . . 158
9.2.3 Design and manufacture of an SMA based actuator . . . 160
9.2.4 Control and performance of the SMA based actuator . . . 161
9.3 Aim of the research . . . 161
9.3.1 First research objective. . . 161
9.3.2 Second research objective . . . 162
9.4 Recommendations . . . 163
9.5 Final conclusion . . . 164
A Fuzzy Logic Controller’s configuration 165 A.1 Input and output variables . . . 165
A.2 Fuzzification . . . 166
A.3 Defuzzification . . . 168
A.4 Fuzzy Inference . . . 169
B PD Controller based configuration 171 B.1 Threshold values . . . 171 B.2 PD gains . . . 173 B.3 Feed-forward control . . . 175 Acknowledgements 177 List of Publications 181 Curriculum Vitæ 183
L
IST OF
F
IGURES
1.1 Classification of the morphing aircraft’s wing (adapted from [4, 9]). . . 6
1.2 Schematics of the variable span concept. . . 7
1.3 Schematics of the chord morphing concept. . . 8
1.4 Schematics of the variable wing sweep. . . 9
1.5 Schematics of the twist morphing concept. . . 9
1.6 Schematics of the dihedral morphing concept. . . 10
1.7 Schematics of the span-wise bending concept. . . 10
1.8 Airfoil terminology. . . 11
1.9 Integration possibilities of a morphing surface into an airfoil. In the draw-ings, the morphing SMA based actuator is represented as a thick black line. The dashed line represents the shape change. . . 16
1.10 Thesis outline. After the introductory chapters, the chapters on the left branch aim to accomplish the first research objective, which is achieved on Chapter 6. The chapters on the right branch aim to accomplish the second research objective, which is achieved on Chapter 8. . . 18
2.1 Different configurations of bending piezoelectric actuators. . . 26
2.2 Qualitative comparison of different smart materials in terms of stress and stroke (adapted from [1, 4]). . . 28
2.3 Qualitative comparison of different smart materials in terms of actuation frequency and energy density (adapted from [1, 4]). . . 28
2.4 Schematics of the reverse transformation during unloading an SMA speci-men above Af (Adapted from [30]). . . 29
2.5 Phase diagram of an SMA. The transformation temperatures and stresses delimit the different phase regions. These lines are called transformation surfaces. . . 30
2.6 Evolution of the martensite phase fraction with varying temperature. The red color represents the heating process and the blue color represents the cooling process. . . 31
2.7 Observable effect of the shape memory effect on an SMA wire (a) together with its accompanying phase evolution (b) and the stress-strain-temperature relationship during transformation (c). The red color represents high tem-perature (austenite phase) and the blue color represents low temtem-perature (martensite phase). . . 32
2.8 Possible configurations for repetitive actuation. The device makes use of the utilizable force and displacement exerted by the SMA wire. The red color represents high temperature (austenite phase) and the blue color rep-resents low temperature (martensite phase). . . 34
xx LIST OFFIGURES
2.9 Observable effect of the superelasticity on an SMA wire (a) together with its accompanying phase evolution (b) and the stress-strain-temperature rela-tionship during transformation (c). The red color represents high
temper-ature. . . 34
2.10 Examples of mechanisms that transform forces and displacements. . . 37
3.1 Results from the DSC test. Tangent lines are used to find the transforma-tion temperatures at zero-stress. . . 51
3.2 Schematics of the isothermal setup. . . 52
3.3 Typical shape of the curve obtained from an isothermal test above As. Tan-gent lines are used to find the transformation temperatures. . . 53
3.4 Schematics of the isobaric setup (using an oven). . . 53
3.5 Typical shape of the curve obtained from an isobaric test. Tangent lines are used to find the transformation temperatures. . . 54
3.6 Phase diagram of the SmartFlex SMA wire (adapted from [13]). . . 55
3.7 Schematics of the isobaric setup (using Joule heating). . . 57
3.8 Isobaric procedure (Adapted from [19]). . . 59
3.9 Evolution of the maximum (austenite) and minimum (martensite) achiev-able strains as the number of training cycles increase. . . 60
4.1 Helmholtz (gray line) and Gibbs (black line) energy landscapes calculated forσ = 39MPa and T = 393K . The necessary energy to surpass the barri-ers between wells is tagged. Figure adapted from [6]. . . 67
4.2 Parameters obtained by linear fitting from two different isothermal tests. . 68
4.3 Schematics of the isobaric setup. . . 72
4.4 Detail of the thermo-graphic camera’s measuring points. (1) and (2) mea-sure the temperature on the wire and (3) the temperature on the beam. . . 73
4.5 Timing diagram of actions performed during the isobaric tests. From top to bottom: (a) Joule heating, (b) Different valve apertures which implies dif-ferent (c) temperature and (d) strain decays. Solid and dashed lines show the effect of different apertures of the valve on the temperature and strain decays. . . 73
4.6 Corresponding airflow at each value of aperture of the valve. . . 74
4.7 Heat transfer coefficient when the SMA wire is cooled at different airflows. 75 4.8 Phase diagram of the SmartFlex wire obtained in Section 3.6. The vertical arrows indicate the temperature and stress ranges at which the two isother-mal experiments (shown in Figure 4.2) were performed in order to obtain the model parameters shown in Table 4.2. . . 76
4.9 Model and experimental results for isothermal experiments at different tem-peratures. . . 78
LIST OFFIGURES xxi
5.2 Detail of the time evolution of the strain during a cooling and heating cycle. ‘’A’ and ‘D’ denote the 10 % of the maximum strain and ‘B’ and ‘C’ denotes the 90 % of the maximum strain. The time that it takes for the wire to elon-gate from ‘A’ to ‘B’ is called cooling time and the time from ‘C’ to ‘D’ is the
contraction time. . . . 84
5.3 Comparison between model and experimental isobaric test. . . 85
5.4 Cooling times (left) and contraction times (right) times obtained from
ex-perimental data (top) and model simulations (bottom). . . 86
5.5 Experimental data of the attainable working rate as a function of the
ap-plied power and airflow, for different apap-plied loads. . . 88
5.6 Simulation data of the attainable working rate as a function of the applied
power and airflow, for different applied loads. . . 89
6.1 Schematics of the isobaric setup. . . 96
6.2 Example of the procedure followed during each one of the experiments. Particularly in this example, a strain-ratio of 0.2 is tested taking the lower reference. The numbers in the figure correspond with the step number
from the procedure. . . 98
6.3 Procedure to limit the strain of the wire at different references and
strain-ratios. . . 99
6.4 (a) Attainable frequencies at different strain-ratios and references for an applied load of 78 M P a (1 kg ) on a 0.4 mm diameter SMA wire. The other graphs show the contraction and cooling times for the tested strain-ratios as well as the period (as the sum of the contraction and the cooling time) and the frequency (as the inverse of the period) reached by the lower refer-ence test (b), middle referrefer-ence test (c) and upper referrefer-ence test (d). . . 100 6.5 Strain-temperature relationship of an SMA wire. Strain-ratios of 0.10 and
0.20 and their limits at different references are shown as an example.
Up-per reference: low temUp-perature, maximum elongation. Middle reference:
medium temperature, medium elongation. Lower reference: high temper-ature, maximum contraction. The bottom part of the graph shows how similar strain-ratios (on the right hand) can be attained at different inter-vals of temperature. . . 101 6.6 (a) Attainable frequencies at different strain-ratios and references for an
applied load of 156 M P a (2 kg ) on a 0.4 mm diameter SMA wire. The other graphs show the contraction and cooling times for the tested strain-ratios as well as the period (as the sum of the contraction and the cooling time) and the frequency (as the inverse of the period) reached by the lower refer-ence test (b), middle referrefer-ence test (c) and upper referrefer-ence test (d). . . 102 6.7 Attainable frequencies reached by the middle reference test at different
wire diameters and applied stresses. . . 103 7.1 Configuration used to transform the SMA wire’s compressive force into
xxii LIST OFFIGURES
7.2 Structural analysis of the action of the contraction force of an SMA wire attached eccentrically to the beam. Such action is the sum of a uniform compression of the beam and a uniform bending moment along the beam. 108 7.3 Different ways of attaching SMA wires to morphing beams. . . 109 7.4 Bidimensional concept of the morphing beam (a) and the three-dimensional
concept of the morphing surface (b). . . 111 7.5 Cross-section detail of the beam used for the proof-of-concept. . . 112 7.6 Detail of the gripping and airflow inlet systems (a) and top view of the
shape of the SMA wires inside the actuator (b). . . 113 7.7 Overlapped pictures of the deflection of the APA-6 beam when the SMA
wire exerts different forces (4 k g , 8 k g , 12 kg and 15 kg ). The shape of the actuator fits with the overlapped circular arcs. . . 114 7.8 Vertical movement of the SMA wires inside the channels when the actuator
bends. . . 115 7.9 Detail of a single module of the morphing surface. The morphing surface
consists of 28 of these modules repeated in the spanwise direction. . . 116 7.10 Cross-section of the designed mould used to create the plate. The air
cap-tured inside the closed mould leaves through the air outlets as the resin fills the mould (the actual mould is wider since the usable plated had to be 0.45 m wide. The actual mould can be seen in Figure 7.13). . . 117 7.11 Demoulded plate. . . 118 7.12 Flexible design of the mould. Plates of different thickness, distance
be-tween channels and number of them can be produced by changing uniquely the middle part of the mould. . . 119 7.13 Devised idea to keep the PTFE strips in tension during the cure of the resin
(a) and (c). Idea put into practice (b) and (d). Figures (a) and (b) show a detail of the top side of the mould and (c) and (d) show a detail of the bottom side of the mould during curing. . . 120 7.14 Detail of the designed machined parts. . . 121 7.15 Detail of the machined parts assembled to the morphing surface. There is
a single SMA wire between two consecutive wire gripping systems. Each wire is placed in a zig-zag manner every four channels. . . 122 7.16 U-shape observed on the back endpoint of the morphing surface when the
tensioned SMA wires on the bottom side are heated up. . . 123 7.17 Effect that the inhomogeneous thermal expansion through the thickness
of the plate has on its shape. . . 124 7.18 U-shape observed on the back endpoint of the morphing surface when the
SMA wires on the top side are first loosened and subsequently heated up. 124
7.19 Proposed alternative to the pieces used at the back end of the morphing surface. It consist of a machined stiff beam placed in the spanwise direc-tion of the actuator. Moreover, it allows to loop the wires around its ma-chined parts. . . 125 8.1 Response of the actuator in terms of deflection rate to different step inputs
LIST OFFIGURES xxiii
8.2 Schematics of a controller in feedback configuration. The controller acts on the SMA based actuator in order to make it deflect (d e f (t )) tracking the reference signal r (t ). . . 132 8.3 Schematics of the actuator subsystems. . . 133 8.4 Example of the difference between Boolean and fuzzy logic in terms of
de-gree of membership to sets. . . 135 8.5 Schematics of the fuzzy logic controller. The three steps inside the FLC
are depicted: (a) Fuzzification with linguistic variables as inputs, (b) fuzzy inference which connects the inputs to the outputs by means of if-then rules and (c) defuzzification with linguistic variables as outputs. The actual membership functions and rule-base are shown in Appendix A. . . 136 8.6 Schematics of the SMA based actuator governed by the PD controller. . . . 138 8.7 Schematics of the whole setup. The green lines are control signals and their
arrows indicate the direction of the communication. The red lines are re-sponsible for heating the SMA wires. The blue lines represent the cooling signals. . . 142 8.8 Average error measured while the SMA based actuator tracked sinusoidal
signals of different amplitudes and frequencies under FLC. . . 143 8.9 Average relative error measured while the SMA based actuator tracked
si-nusoidal signals of different amplitudes and frequencies under FLC. . . 144 8.10 Response of the actuator to a composed sinusoidal tracking signal. . . 147 8.11 (a) Step response of the actuator under FLC. (b) and (c) Details of the
over-shoots upon deflection rate and subsequent vibration and the natural fre-quency of the actuator. . . 148 8.12 The natural frequency of the actuator was measured by deflecting the tip
of the actuator significantly and releasing it suddenly. . . 149 8.13 Response of the actuator in terms of deflection rate to different error steps
to the input of the controller when it is set with the parameters shown in Table 8.2 (Adapted from [9]). . . 150 8.14 Response of the controlled actuator when it tracks a sinusoidal reference
signal of amplitude 2 mm of amplitude and frequency 1 H z with and with-out using the threshold parameters (Adapted from [9]). . . 151 8.15 Current profile used to test the feed-forward stabilizer. . . 152
8.16 Response of the SMA based actuator under PD control to different Iexcand
Ist ab(Adapted from [9]). . . 152
8.17 Average error measured while the SMA based actuator tracked sinusoidal signals of different amplitudes and frequencies under PD control. . . 153 8.18 Response of the SMA based actuator under PD control to a composed
si-nusoidal signal. (Adapted from [9]) . . . 154 8.19 Step response of the SMA based actuator under PD control. The response
to three experiments run under similar controller parametrization and ini-tial conditions are shown (Adapted from [9]). . . 154 A.1 Membership functions of the input variable slope. . . 166 A.2 Membership functions of the input variable setpoint. . . 166
xxiv LIST OFFIGURES
A.3 Membership functions of the input variable error. . . 167 A.4 Membership functions of the output variable coo1. . . 168 A.5 Membership functions of the output variable coo2. . . 169 A.6 Membership functions of the output variable power. . . 169
L
IST OF
T
ABLES
2.1 Proportion of articles and patents found using the keyword ‘shape memoryalloy’ OR nitinol in Scopus and USPTO (United States Patent and
Trade-mark Office) [46]. . . 38
3.1 Recoverable strain of SmartFlex as the number of cycles increases. (Data
provided by the manufacturer [14]). . . 56
3.2 Data collected from the training of the SMA wires. . . 59
4.1 Exponential curve fit to the measured data. . . 74
4.2 Model parameters for a SmartFlex wire. . . 77
8.1 Equipment used in the experiments. Despite of using different equipment under the two control strategies, their function was the same. The relation between different devices is depicted in Figure 8.7. The specifications of all the devices listed in the table can be easy found on the Internet. . . 142 8.2 Parameters set on the controller to test the threshold response . . . 149 A.1 Input variables of the FLC . . . 165 A.2 Output variables of the FLC . . . 165 A.3 Fuzzification of input variable slope . . . 166 A.4 Fuzzification of input variable setpoint . . . 166 A.5 Fuzzification of input variable error . . . 167 A.6 Defuzzification of output variable cool1 . . . 168 A.7 Defuzzification of output variable cool2 . . . 168 A.8 Defuzzification of output variable power . . . 169 A.9 Rule-base. All the rules have a degree of support ‘1’ . . . 170
B.1 It h . . . 171 B.2 Ft h. . . 172 B.3 KP,I . . . 173 B.4 KP,F . . . 173 B.5 KD,I . . . 174 B.6 KD,F . . . 174 B.7 Ist ab . . . 175 B.8 KF F,I. . . 175 xxv
N
OMENCL ATURE
A
BBREVIATIONS
FLC Fuzzy logic controller
LVDT Linear variable differential transformer
MIMO Multiple-input-multiple-output
MISO Multiple-input-single-output
SIMO Single-input-multiple-output
SISO Single-input-single-output
PID Proportional-integrative-derivative controller
PWM Pulse-width-modulation
PWPF Pulse-width-pulse-frequency
SMA Shape memory alloy
SME Shape memory effect
SMP Shape memory polymer
C
ONSTANTS
kB Boltzmann constant 1.38044 e-23 [J/K]
R
OMAN SYMBOLS
Af Austenite transformation finish temperature [K ]
As Austenite transformation start temperature [K ]
ASM A Lateral area of the SMA wire [m2]
bb Width of the beam [m]
C Stress influence coefficient [P a/K ]
cp Heat capacity [J±¡kg K ¢]
da Damping matrix
d e f Deflection of the tip of the actuator [m]
E Elastic modulus [P a]
e error between desired d e f (also r ) and actual d e f [m]
ea Mass matrix
F Cooling signal (opening of the valve) [−]
xxviii NOMENCLATURE
Fst ab Stabilizing valve opening [−]
f Source term vector
fa Actuation frequency [H z]
G Gibbs energy [J±m3]
h Convective heat transfer coefficient [W±¡m2K¢]
hb Height of the beam [m]
hM− hA Latent heat of the reverse transformation [J±m3]
I Heating signal (electric current to the SMA wire) [A]
Ib Second moment of inertia of the beam [m4]
Iexc Excitation current [A]
Ist ab Stabilizing current [A]
j Joule heating power per volume unit [W±m3]
KD Derivative gain of a PID controller [−]
KI Integrative gain of a PID controller [−]
KP Proportional gain of a PID controller [−]
KZ Gain matrix of each of the subsystems of the actuator
KZ1,Z2,Z3 Gain matrix of the actuator
L0 Initial length of the SMA wire (memorized length) [m]
Mf Martensite transformation finish temperature [K ]
Ms Martensite transformation start temperature [K ]
m Mass [k g ]
p Transition rates between austenite and martensite [1/s]
q Heat loss by convection [W ]
qi n Energy supplied to the SMA wire by Joule heating [W ]
R Electrical resistance [Ω]
R2 Coefficient of determination [−]
r Reference tracking signal (desired d e f ) [m]
T Temperature [K ]
T∞ Room temperature [K ]
t Time [s]
tc Cooling time [s]
th Heating time [s]
u Vector of dependent variables
ux Point on the SMA wire [m]
vLE Volume layer [m3]
vSM A Volume of the SMA wire [m3]
x Phase fraction [−]
˙
x Time evolution of the phase fraction [1/s]
Z Subsystem that forms the actuator [−]
G
REEK SYMBOLS
Γ Flux vector
NOMENCLATURE xxix
∆G Gibbs energy barrier [J±m3]
∆L Increment of length of the SMA wire [m]
ε Strain (∆L±L0) [−]
εr ecov
end Maximum recoverable strain after training [−]
εr ecov
st ar t Maximum recoverable strain before training [−]
εT Maximum recoverable strain [−]
ρ Density [k g±m3]
σ Stress [P a]
σAf Austenite transformation finish stress [P a]
σAs Austenite transformation start stress [P a]
σf Detwinning martensite finish stress [P a]
σs Detwinning martensite start stress [P a]
σMf Martensite transformation finish stress [P a]
σMs Martensite transformation start stress [P a]
σR Width of the stress hysteresis loop [P a]
τ Relaxation time [s] ψ Helmholtz energy [J±m3]
S
UBSCRIPTS
A Concerning austenite A → M Forward transformation c yl Cylinder eq Equilibriumf Concerning the end (finish) of a phase transformation
F Concerning the controlled cooling signal
F F Concerning the feed-forward stabilizer
H High
I Concerning the controlled heating signal
L Low
M Concerning martensite
M− Compressive stress induced martensite
M+ Tensile stress induced martensite
M → A Reverse transformation
s Concerning the beginning (start) of a phase transformation
SM A Concerning properties of the shape memory alloy
1
I
NTRODUCTION
Don’t ever let someone tell you, you can’t do something. Not even me. You got a dream, you got to protect it. People can’t do something themselves, they want to tell you you can’t do it. You want something, go get it. Period.
Scene from ’The Pursuit of Happiness’, Columbia Pictures, 2006
This chapter introduces the challenging world of morphing structures for aerospace appli-cations, more specifically, in morphing aircraft, rotorcraft and wind turbines. With that intention, this chapter explains first the importance of morphing and why the research on this topic is relevant nowadays. Then, the chapter sets the research objectives and the baseline for the subsequent research work.
1.1. F
LYING
: F
ROM DREAM TO REALITY
The idea of flying has probably been in the mind of the mankind since the human brain was able to think abstractly. Mankind’s history is full of noteworthy examples of our obsession with flying and of great attempts to fly (not always with successful results for the protagonist of the flight). Whatever the case may be, all the ideas and attempts of flying have led in one way or another to our current knowledge on flying machines.
Curiously enough, the first living things that the human being made fly were mytho-logical creatures and mythomytho-logical people. There are well-known examples from ancient Greek mythology, like Pegasus, a horse with bird wings, or the myth of Daedalus and his son Icarus. In the Persian tradition, the Shah of Iran, Kay Kavus, tied four eagles to the corners of his throne to be able to fly over his kingdom.
Coming back to the humans, in the 11thcentury, the Benedictine monk Eilmer of
Malmesbury attempted a gliding flight, which is recognized as the first human flight. Inspired by the myth of Icarus, he attached wings to his arms and jumped from the top of the Malmesbury Abbey, in England. He broke his legs, but he glided over a distance of about 200 meters in 15 seconds. Later, he said that his failure was that he had forgotten
1
2 1. INTRODUCTION
to provide his invention with a tail. At the end of the 15thcentury, the multi-talented
genius Leonardo da Vinci conceived flying machines based on flapping wings (like bird’s wings) and a similar machine to present-day helicopters, although the idea never came off the paper.
The idea of flying, together with an increasing need of transportation, led finally to
successful flying inventions like the balloons at the end of the 18thcentury by the
Mont-golfier brothers and the dirigibles (also called airships) in the second half of the 19th
century, which were able to follow a controlled direction. The lift on these machines was achieved by making them of a lower density than the surrounding air.
However, the greatest milestone in the history of aviation is the Wright Flyer. In 1903, the Wright brothers showed to the world the first successful heavier-than-air, manned and powered fixed-wing aircraft.
Flying was not a dream anymore, but a reality.
1.2. S
TRUCTURAL EVOLUTION OF AIRCRAFT
The structure of the Wright Flyer was made out of wood and its skin out of fabric. It used wing-warping for roll control, which entails a deformation of the wings. The future brought new needs in terms of stability, speed and payload, which led to heavier aircraft. These bigger, faster and more stable aircraft required much more robust structures and skins (besides more powerful engines). Fabric skins like the ones used on the first aircraft were no longer suitable for carrying the increasing payloads, since they produced large amounts of shear stress on the fabric. Moreover, they could flutter, thus losing lift. It was then when aircraft with stronger structures and rigid skins were manufactured. Since the structure and the skin were not deformable anymore, hinged mechanical parts had to be added to the wings and the tail, in order to provide pitch, roll and yaw control to the aircraft. These mechanisms and moving parts (e.g., flaps, slats and ailerons) are visible in most of the contemporary aircraft, which use them during the take-off and landing phases, for maneuvers and to provide in-flight stability to the aircraft.
1.3. T
HE MORPHING AIRCRAFT
:
A MORE EFFICIENT FLIGHT
The moving parts mentioned above solve the problem of controlling the aircraft, but not always in the most efficient way. The position and shape of the wings, as well as the airfoil geometry, are a trade-off between the requirements imposed by the varying flight conditions of any mission. These varying conditions are the changes during the flight in altitude, aircraft’s speed and aircraft’s weight (more than a 30 % percent of weight loss on long distance flights [1]). The use of rigid and moveable control surfaces allows the air-craft to fly under various flight conditions, but normally with sub-optimal performances. Larger deformations and in-flight wing reconfiguration would make it possible to reach near optimum performance under different flight conditions during a single mis-sion. Moreover, an in-flight reconfigurable aircraft could be used for a wider range of missions, thus creating much more versatile aircraft. These are the reasons that have motivated the research and ongoing development of the so-called ‘morphing aircraft’. The idea of the morphing aircraft is that it is able to modify the shape of its wings in-flight to reach optimum aerodynamic performance under any in-flight condition (ideally).
1.3. THE MORPHING AIRCRAFT:A MORE EFFICIENT FLIGHT
1
3
The use of slats and flaps is a simplification of morphing. The mission of these de-vices are easily observable on any commercial flight. Their deployment strongly changes the geometry of the wing, curving it and increasing the lift that it produces. This is es-sential during landing and taking-off. These devices, together with the ailerons, spoilers, elevators and rudder make the aircraft governable. They are effective, however, not fully efficient. Hinged parts imply discontinuities between these parts and the wing to which they are attached. These discontinuities produce undesirable aerodynamic effects, such as decrease of lift, turbulences and noise as well as increase of drag.
1.3.1. T
HE BENEFITS OF MORPHINGThe idea of morphing is applicable to flying machines as well as to machines that take advantage of the kinetic energy of the air and convert it to any other kind of energy. There are different options of morphing, each offering different benefits — and intro-ducing new design challenges — but the main benefits of morphing can be summarized as follows:
• Reduction of fuel consumption: Morphing attempts to achieve optimal aerody-namic configurations for different flight conditions. A variable camber wing can offer substantial improvements compared to a fixed wing, like 12 % higher lift co-efficient values and an increase between 3 % and 10 % of the lift-to-drag ratio [2, 3]. Between 3 % and 5 % of the projected fuel could be saved by increasing the lift-to-drag ratio for a medium-range transport aircraft [4].
• Reduction of the wing root bending moment: The wing, together with its aerody-namic load control devices, is responsible for generating lift. Distributed devices along the span-wise direction of the wing make it possible to control where and how much lift is generated at each section of the wing. This allows to redistribute the forces along the wing. Such redistribution allows to produce an equivalent net lift at the same time as the momentum can be reduced by generating more lift close to the root of the wing and less at the tip. This leads to an increase of the payload-to-structural weight ratio [3].
• Reduction of mechanical fatigue: Adaptive wings can also be used to counter-act fluctuating loads if they count on devices with a sufficient working bandwidth. These fluctuating loads can cause vibrations in the structure and, therefore, re-ducing vibrations could lead to diminished fatigue damage. The gain is specially important for wind turbines, in which the air disturbances are very relevant. These disturbances are produced, among others, by wakes from other wind turbines, tower shadow and wind shear [5].
• Reduction of noise: Noise is produced mostly by turbulences. The discontinuities on the skin due to the moving parts of the wings of an aircraft (or the interaction between a wing with the tip vortices of a preceding wing in a rotorcraft) produce turbulences that can be decreased by using smooth and deformable surfaces, as well as by devices that counteract those turbulences [6]. In addition, replacing sharp edges by smooth surfaces reduces the radar visibility of aircraft [4], which can also be seen as an extra benefit.
1
4 1. INTRODUCTION
These benefits apply to fixed-wing aircraft, rotary-wing aircraft and wind turbines. In the case of the wind turbines, the consequence of increasing the lift-to-drag ratio is not a reduction of fuel consumption, obviously, but an increase of their performance. In other words, they are then more efficient in converting the air’s kinetic energy to electrical energy.
These are not the only benefits of morphing aircraft. One of the main interests of developing morphing aircraft is to expand their flight envelope, that is, to provide the aircraft with multi-role capabilities, which will result in a much more versatile aircraft [7]. It is important to mention at this point that morphing is not a specific goal, but part of a continuous improvement for aircraft. Morphing is neither a new or futuristic idea. In fact, the Wright Flyer, the first aircraft, was already a morphing aircraft. One might won-der now why the morphing aircraft started to become of great importance one century later.
First, it must be mentioned that the morphing idea has always been in engineers’ minds from the beginning of aviation. There are several examples of aircraft with mor-phing capabilities developed since the Wright Flyer [4]. Retractable landing gears are a good example of how to change the shape of the aircraft to greatly reduce its in-flight drag while allowing it to take-off and land smoothly.
However, the available technology during the last century limited the morphing air-craft’s development. Great advances in the last decades in terms of materials, novel ac-tuators, electronics, faster computers and robust control systems have now opened the door to a more sophisticated morphing aircraft.
1.3.2. P
ARTS OF MORPHING STRUCTURESEither a morphing fixed-/rotary-wing aircraft, a wind turbine or any morphing structure consists, at least, of the following elements:
• Sensors: Their task is to measure properties of the air and its effects on the struc-ture. They convert some form of energy to electrical energy. Different measure-ments can be performed, but at least one must refer to the structure response to the aerodynamic loads [5]. There are several types of sensors, such as accelerom-eters, strain sensors and airflow sensors, among many others. Sensors have to be placed where the measured property takes its highest values. Among other re-quirements, they must pose immunity to electromagnetic waves and temperature changes, long-term reliability and ease of integration.
• Actuators: Their task is to convert some form of energy to mechanical energy (nor-mally from electrical energy). Properly attached to the structure or the skin, they are responsible for the shape change of the structure. They are discussed in more detail in Chapter 2.
• Controllers: Their task is to alter the working condition of a dynamic system ac-cording to its working algorithm, considering the physical variables provided by the sensors. In other words, controllers read the sensors’ signals and send the pertinent orders to the actuators. They must have a good performance, that is, be robust, reliable and fast. They link the sensors to the actuators, which takes a
1.3. THE MORPHING AIRCRAFT:A MORE EFFICIENT FLIGHT
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5
sum of different times. First, the sensor needs to perform the conversion from the measured physical variable to electrical energy. Subsequently, the controller has to receive that signal, execute its algorithm and send a signal to the actuator. The actuator also needs time to perform the conversion to mechanical energy. This phase delay between the sensor, actuator and controller must be as small as pos-sible. The lower the phase delay, the better the performance of the whole system. Further insights into the controller part is given in Chapter 8.
1.3.3. C
HALLENGES OF MORPHING STRUCTURESAlthough the morphing aircraft — and any morphing structure in general — offers many benefits, it also brings along some requirements and new challenges, which are equally applicable to rotary-wing aircraft and wind turbines. The main characteristic of any mor-phing structure is that it must be able to change shape — achieving the desired defor-mation — while maintaining structural integrity. In other words, the structure together with the skin must be able to withstand the imposed aerodynamic loads. The most im-portant aspects to take into account during the design phase of a morphing structure are the following:
• The skin: The skin defines the external appearance of the aerodynamic surface. This is the part of the structure that is in contact with the surrounding air and, therefore, all the aerodynamic forces are withstood by the skin initially. In a mor-phing structure the shape changes and therefore the skin must enlarge and shorten, or some parts of the skin must slide into others. Consequently, the skin must be a tradeoff between being adaptable enough to reach the required shapes and being stiff and strong enough to withstand the imposed aerodynamic forces [8].
• Force and power: The actuators must be capable of exerting sufficient force to reach deformations while withstanding or opposing the aerodynamic loads. • Structure-actuator-skin attachments: Proper attachment devices must be used
to transfer the aerodynamic loads from the skin to the structure, as well as the forces exerted by the actuators to the skin and the structure in a robust and reliable way.
• Locking: The actuators exert forces that produce deformations on the structure or the skin or both. These forces oppose the aerodynamic loads. Appropriate locking systems allow the required deformations to be preserved over time, even though the actuator is not exerting force anymore until a new deformation is required. • Response time and working rate: There are two aspects regarding to the speed at
which a morphing surface reacts. The term response time refers to the lag between the moment at which a given input is received and the desired shape is achieved. The term working rate refers to the maximum attainable frequency that an actu-ator can reach when working at periodic or cyclic regimes while maintaining the required accuracy.
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6 1. INTRODUCTION
Several benefits have been mentioned previously as well as the new design chal-lenges and requirements that they bring along. A key principle of implementing morph-ing is that the imposed complexity and added weight must be overcome by substantial benefits.
There are other methods to improve the performance of wings and blades under dif-ferent aerodynamic conditions, based on modifying the properties of the surrounding airflow, like microtabs or fluidic actuators, which are intended to generate vortices and to avoid flow separation respectively. This kind of actuators are not considered in this thesis, since they do not entail shape changes.
The main morphing possibilities regarding to fixed-wing aircraft as well as their ben-efits are explained in the following section.
1.4. M
ORPHING FIXED
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WING AIRCRAFT CONCEPTS
There are several ways of changing the shape of an aircraft in general and of a wing in particular. In their reviews of the morphing aircraft, Barbarino et at. [4] and Sofla et al. [9] classified the different options into three big categories: Wing planform morphing (in-plane transformations), out-of-plane transformations and airfoil adjustment. These categories include different types of morphing options (see Figure 1.1). This classifica-tion is followed in this introductory chapter, although other classificaclassifica-tions attending to a different criteria are also possible ([10]).
Morphing wing Out-of-plane transformation Planform morphing Airfoil adjustment Twist Dihedral / Gull wings
Span-wise bending Variable span Chord morphing Variable wing sweep
Figure 1.1: Classification of the morphing aircraft’s wing (adapted from [4, 9]).
Using the classification in Figure 1.1, this section summarizes the characteristics of the different morphing options. The aim of this section is to describe the most important morphing options for aircraft, their characteristics and benefits. Since it is out of the scope of this thesis, technical requirements and issues are omitted.
1.4.1. W
ING PL ANFORM MORPHINGThree types of morphing options fit for this category: variable span, variable wing sweep and chord morphing. A combination of two or three of them is also possible.
These morphing concepts mainly affect the wing aspect ratio. A larger wing plan area increases the lift generated by the wing and also the load factor capability of the aircraft, that is, the ratio between the generated lift and the aircraft’s weight. The wing aspect ratio is the ratio between the wing’s length (span-wise direction) and its chord. Therefore, high aspect ratio wings are long and narrow whereas low aspect ratio wings are short
