Smart Materials Actuators: Actuated Scissor Lift

Smart Materials Actuators:

Actuated Scissor Lift

Luís Filipe Araújo de Oliveira

Orientador

André Miguel Trindade Pereira, Professor Auxiliar, Departamento de Física e Astronomia

Faculdade de Ciências da Universidade do Porto

Coorientadores

Joana Fonseca, CeNTI, Ricardo Carvalho, CeNTI

Todas as correções determinadas pelo júri, e só essas, foram efetuadas. O Presidente do Júri,

U

NIVERSIDADE DO

P

ORTO

M

ASTERS

T

HESIS

Smart Materials Actuators:

Actuated Scissor Lift

Author:

Supervisor:

Luís O

LIVEIRA

André P

EREIRA

Co-supervisor:

Joana F

ONSECA

Ricardo C

ARVALHO

A thesis submitted in fulfilment of the requirements

for the degree of Master of Science

at the

Faculdade de Ciências

Departamento de Física e Astronomia

IFIMUP and CeNTI

September 2018

Declaration of Authorship

I, Luís Filipe Araújo de Oliveira, declare that this thesis titled, Smart Materials Actuators: Actuated Scissor Lift and the work presented in it are my own. I confirm that:

• This work was done wholly or mainly while in candidature for a research degree at this University.

• Where any part of this thesis has previously been submitted for a degree or any other qualification at this University or any other institution, this has been clearly stated.

• Where I have consulted the published work of others, this is always clearly attributed.

• Where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work. • I have acknowledged all main sources of help.

• Where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself.

Signed: Date:

“If I have seen further it is by standing on the shoulders of giants. “

v

Preface

Dear reader,

To me, this work has been a thrilling closure for my time as a student. Throughout this project, which consisted of an internship at CeNTI, I put into practise several tools I have acquired over the past 5 years, in the fields of mathematics, physics, material science, electronics, optics, and computing. I feel like I have grown from a student to a professional with regard to scientific and technological problem-solving.

I would describe myself as a curious person, who tries to embark on new adventures and to combine multidisciplinary know-how. Working in an R&D business environment has been a thrilling opportunity to develop myself in different areas. Concluding my masters with a project in which brainstorming was combined with a hands-on approach was exactly what I wanted. Working together with professionals in the scientific and business world for nine months was a very valuable experience.

On the way, I have learned about physics, people and imagination, which I consider to be the fundamentals of the way we perceive the world, challenge ourselves and the way we shape the future. I hope you can appreciate my work, embodied in this thesis.

This thesis is organised into 7 chapters as illustrated in the thesis map (Figure 0.1). Chapter 1 introduces a comprehensive overview of smart materials, with special emphasis on Shape Memory Alloys (SMAs). It started with an explanation of the physical concept (martensitic transformation) and the effect based on it (shape memory effect). The manufacturing and post-treatments of SMAs are briefly introduced in this chapter. The advantages, limitations and common designs of SMAs are also pointed in this chapter, as the performance of the alloys relies deeply on their composition and design. There were also appointed several technological applications, ranging from biomedical to more recently robotic industry.

Chapter 2 analyses different possible lift mechanisms triggered by an SMA spring, their advantages, limitations and associated design challenges. The design of an Actuated SMA Scissor Lift prototype is presented along with a physical model regarding its force output. Finally, it is described the design approach regarding SMA springs, which includes geometry and force modelling.

vi

Chapter 3 reviews the existing manufacturing processes of nitinol followed by the fabrication methodology, employed in this work, for obtaining SMA springs from wires. Next is presented a brief overview on techniques for calorimetric and structural characterization, as Differential Scanning Calorimeter (DSC) and X-Ray Diffraction (XRD).

Chapter 4 investigates the effect of stress (when annealing SMA springs) on transformation temperatures and phase transition behaviour. Experimental methodology and results concerning the austenite shear modulus are presented. Afterwards, by using an Arduino it was possible to control the voltage across the spring. Dynamic parameters such as power, temperature and response time were investigated.

Before placing the SMA into the final prototype, a scissor lift evaluation was intended. In chapter 5, force and displacement characterization tests with a commercial scissor lift are presented.

Chapter 6 evaluates the performance of a commercial scissor lift actuated with SMA springs. Design optimization concerning the assembly of the lift with the SMA springs is scrutinized. Also, by integrating a distance sensor in the apparatus it was possible to have an insight about the alloy’s fatigue life. After all, it is presented the fabrication of the acrylic prototype of a Scissor Lift.

Finally, Chapter 7 summarises the performed work and recommends potential relevant future research and development.

I hope you enjoy your reading!

vii

ix

Acknowledgements

Before diving into the exploration of this master thesis, I would like to acknowledge all the people who helped me composing it during this last year.

First and foremost, I would like to express my sincerest gratitude to my supervisor Prof. Doctor André Pereira. His support, advice and expertise were fundamental to my motivation, not only during this project but since I entered the university. He really cares about his students, even outside the academic world.

I would also like to extend my appreciation to my co-supervisors, Doctor Joana Fonseca and Ricardo Carvalho for their continuous support, guidance, and insightful advice, which were crucial to many decisions during the development of this work. It was truly a pleasure to work at CeNTI under their supervision. I am also very thankful to all the people who worked at CeNTI with me, as they always showed support and helpfulness. Additionally, I would also like to thank Miguel Ramalho, an internship colleague who I had the pleasure to work with for almost two months. His opinions and suggestions were very relevant and helpful for the project and have boosted my motivation. We both benefited from sharing our know-how.

I owe a debt of gratitude to all the friends I made in the university. Studying physics for five years would have been even harder without them.

Last but not least, I’m eternally grateful to my parents for all the love, care, and sacrifice they did to shape my life. They are the main reason for my presence here today, always willing to support any decision I make, encouraging and providing me with all the necessary means for me to follow my dreams. Also, I express my thanks to my beloved little sister.

xi

Abstract

Nowadays the demand for sophisticated and specialized materials, as well as consideration of the environmental impact, has spawned a new branch of research into smart materials. Shape Memory Alloys (SMAs) are a unique class of smart materials with the ability to return to their original shape when heated after several deformations. SMAs can be used as both sensors and actuators due to their compactness, lightness, bio-compatibility, noiseless operation, high wear resistance and functionality. When set as springs, SMAs provide greater displacements, which are well suited for new and promising actuation mechanisms.

In this work, a scissor lift mechanism was successfully actuated by an SMA spring. Both physics of SMAs and actuation mechanisms were investigated, and the project culminated with the development of a scissor lift prototype. The prototype was lightweight, compact, and made of acrylic, able to lift 400 g by means of an SMA spring with 1 cm long and an outside diameter of 57 mm.

Aiming to achieve this goal, several contributions were made regarding SMA actuators. First, it was developed an improved fabricating method to shape set wires into tightly springs without spacing between coils, ranging from 5.7 to 7.7 mm outside diameter. Then, a spreadsheet, based on the state-of-art equations of SMA springs, was developed. Fatigue tests were also performed and have shown that when the alloy was subjected to strains of about 10%, his displacement under actuation has decreased.

Thermal, structural and mechanical characterization of SMAs were studied. The austenitic shear modulus was estimated to be G≅13.09 GPa and a maximum force of 34 N and 2.4 cm displacement were obtained with a 5.7 mm OD spring. DSC and XRD measurements revealed the presence of an intermediate phase, R-phase, in springs subjected to greater torsion forces and stress upon annealing. Also, Differential Scanning Calorimeter measurements have shown that the transformation temperatures and fusion enthalpies increase with increasing the stress levels. To control the amount of voltage for a certain period across an SMA spring, a control system, based on a microcontroller, was developed. When subjected to an electrical power of 1.377 W, a 67 mm OD spring has reached the transition temperature (∼70 °C) in about 30 s and it took about 7 min to cool.

The prototype force requirements were assessed with a commercial laboratory scissor jack, where it was obtained a spring stroke and force of about 3 cm and 17 N,

xii

respectively. Furthermore, a part that assembles the spring together with the scissor lift was designed, produced and optimized.

xiii

Resumo

Atualmente, a crescente procura por produtos inovadores, eficientes e sustentáveis despertou o interesse na investigação em materiais técnicos, funcionais e inteligentes. As ligas com memória de forma (Shape Memory Alloys) são materiais metálicos inteligentes com a capacidade de voltar à sua forma original, quando aquecidos e após várias deformações. Os SMAs podem ser usados como sensores e atuadores devido ao seu carácter compacto, leve, biocompatível, silencioso, resistente e funcional. Ao serem “memorizados” sob a forma de molas, proporcionam maiores deslocamentos, podendo ser utilizados em mecanismos de atuação inovadores.

Neste trabalho, foi desenvolvido com sucesso um mecanismo de elevação em tesoura atuado por uma mola SMA. Investigou-se o fenómeno físico dos SMAs, bem como os seus mecanismos de atuação, e o projeto culminou na produção de um scissor lift. O protótipo feito em acrílico, mostrou-se leve e compacto, e foi capaz de levantar 400 g através de uma mola SMA com 1 cm de comprimento e 57 mm de diâmetro externo.

De forma a atingir este objetivo foram feitas várias contribuições em relação aos atuadores SMA. Primeiro, desenvolveu-se um método de fabricação de molas a partir de fios. Obtiveram-se molas compactas, sem espaçamentos entre espiras, com diâmetros externos entre 5.7 a 7.7 mm. De seguida, foi elaborada uma folha de cálculo para projetar molas, com base nas equações encontradas na literatura. Foram também realizados testes de fadiga que evidenciaram que quando o material é sujeito a strains de 10%, a capacidade de voltar à sua forma original diminuí.

Os SMAs foram também caracterizados térmica, estrutural e mecanicamente. Estimou-se o valor de G≅13.09 GPa para o shear modulus na fase austenítica. Obteve-se uma força máxima de 34 N e um deslocamento de 2.4 cm com uma mola de 57 mm de diâmetro externo. Os resultados do DSC e XRD revelaram a presença de uma fase intermédia, R-phase, nas molas sujeitas a pressões elevadas aquando da fase de annealing. Os resultados do DSC mostraram também que as temperaturas de transição de fase e as entalpias de fusão aumentam com o aumento da pressão. Foi também desenvolvido um sistema de controlo, com base num microcontrolador, de forma a controlar a diferença de potencial numa mola SMA durante um certo período. Quando submetida a uma potência elétrica de 1.377 W, uma mola de 67 mm de diâmetro externo atingiu a temperatura de transição (∼70 ° C) em cerca de 30 s, e levou cerca de 7 minutos para arrefecer.

xiv

Os requisitos de força para o protótipo obtiveram-se realizando testes num scissor lift de laboratório, onde se obteve uma força de 17 N e um deslocamento de 3 cm. Por último, produziu-se e otimizou-se uma peça que une a mola SMA ao scissor lift.

xv

Contents

List of Figures ... xviii

List of Tables ... xxii

Abbreviations ... xxiii

1. Introduction ... 1

1.1 Smart Materials ... 1

1.2 Shape Memory Alloys (SMAs) ... 2

1.2.1 Martensitic Transformation ... 2

1.2.2 Shape Memory Effect... 3

1.3 NiTi Based Alloys ... 6

1.4 Manufacturing of SMAs... 7

1.4.1 Melting process ... 7

1.4.2 Post-Treatments ... 8

1.5 Design of SMA actuators ... 9

1.6 Overview of SMA Applications ...10

1.6.1 Biomedical Applications ...10

1.6.2 Aerospace Applications ...12

1.6.3 Automotive Applications ...13

1.6.4 Robotic Applications ...16

1.7 CeNTI and Project Objectives ...19

xvi

2.1 Lift Mechanisms Design ...21 2.1.1 Lever ...21 2.1.2 Rack and Pinion ...23 2.1.3 Scissor Lift ...24 2.2 Scissor Lift Design ...25 2.2.1 Force Model ...25 2.2.2 Prototype Design of the SMA Actuated Scissor Lift ...27 2.3 Design of SMA springs ...29 2.3.1 Geometry and Force Modelling of NiTi springs ...29 3. Experimental Methodology of NiTi SMA components ...33 3.1 Manufacturing Process of NiTi SMAs ...33 3.1.1 Vacuum Induction Melting (VIM) ...34 3.1.2 Vacuum Arc Remelting (VAR) ...35 3.2 Fabrication of SMA springs ...35 3.3 Characterization Techniques ...38 3.3.1 Differential Scanning Calorimeter (DSC) ...38 3.3.2 X-Ray Diffraction (XRD) ...39 4. Characterization and Control of SMA Springs ...41 4.1 Characterization of SMA ...41 4.1.1 Calorimetric Analysis ...41 4.1.2 Structural Characterization ...45 4.1.3 Thermomechanical Characterization ...46 4.2 Control of SMA ...48 4.2.1 Power and Temperature Behaviour ...48 5. Evaluation of Prototype Requirements ...55 5.1 Dynamic Force Tests ...55 5.2 Static Force Tests ...57 6. SMA Actuated Scissor Lift ...61

xvii

6.1 SMA Spring into Scissor Lift ...61 6.1.1 Lift’s Adapter ...61 6.1.2 Fatigue tests ...67 6.1.3 Acrylic Scissor Lift Prototype ...69 7. Conclusion and Future Work ...71 7.1 Conclusions ...71 7.2 Future Work ...73 Appendix A: 3D Scissor Lift Drawings ...75 Appendix B: SMA Springs Spreadsheet ...79 Bibliography ...81

xviii

List of Figures

Figure 0.1: Thesis flow chart. ... vii Figure 1.1: Schematic illustration of austenite and martensite crystal structures (adapted

from [5]). ... 3

Figure 1.2: a) Crystal structures of martensite and austenite along with associated

forward and backward transformation cases. After heating it transforms to the austenite phase where the atoms are rearranged into their original positions. When cooled it still keeps its original shape. ... 4

Figure 1.3: Superelastic/pseudoelastic response of a SMA at three different temperatures

[5]. ... 6

Figure 1.4: Examples of alloys systems that show SME [5]. ... 7 Figure 1.5: Phase diagram of NiTi [5]. ... 8 Figure 1.6: Spring actuation cycle: a shape-set twinned martensite actuator is subjected

to an external force, causing martensite detwinning; the actuator is heated, and austenite change occurs leading to activation; subsequent cooling causes martensite phase-change, ending in the original twinned state [26]. ...10

Figure 1.7: Existing and potential SMA applications in the biomedical domain [8]. ...11 Figure 1.8: Boeing’s variable geometry chevron (VGC) [8]. ...13 Figure 1.9: Existing and potential SMA applications in the aerospace domain [8]. ...13 Figure 1.10: Schematic function of a thermal valve [31]. ...15 Figure 1.11: Potential applications of shape memory thermal actuators in automobiles:

(1) radiator shutler; (2) fan clutch; (3) fuel management; (4) climate control; (5) engine control; (6) brake ventilation; (7) transmission control/rattling noise reduction; (8) suspension adjustment [31]. ...15

Figure 1.12: Potential applications for electrical shape memory actuators in automobiles:

(1) foglamp louvre; (2) engine hood lock; (3) retractable headlight; (4) fuel management; (5) engine control; (6) transmission control; (7) climate control; (8) wiper pressure control; (9) rear-view mirror adjustment; (10) seat·belt adjustment; (II) central locking system; (12) shock absorber adjustment; (13) filler inlet lock; (14) trunk lock [31]. ...15

Figure 1.13: Operating temperature range for automobiles applications and the

transformation temperatures for selected commercially available and developed SMAs [8]. ...16

xix

Figure 1.15: Existing and potential SMA applications in the robotic technology domain

[8]. ...18

Figure 1.16: Project Overview. ...19 Figure 2.1: Illustration of a simple lever/seesaw mechanism with the load on one side and

the SMA spring on the other ...22

Figure 2.2: Draft of the rack and pinion lift mechanism actuated by an SMA spring. ...23 Figure 2.3: Scheme of a 2 level scissor lift mechanism (adapted from [37]). ...24 Figure 2.4: Design of a 1 level and 2 level scissor lift actuated with an SMA spring. ....25 Figure 2.5: Scheme of the geometry of a 2-level scissor lift mechanism. ...26 Figure 2.6: External force on an SMA spring vs. angle between the scissor arm and the

horizontal plane. Based on the model proposed by Spackman [38]. ...27

Figure 2.7: Isometric and side view of the Scissor Lift Prototype designed in OnShape.

...28

Figure 2.8: 3D CAD model of the SMA Actuated Scissor Lift. ...28 Figure 2.9: Spring parameters as defined by coil geometry, for both a fully compressed

(solid) spring and a free-length spring [26]. ...30

Figure 3.1: Schematic of a VIM setup for producing NiTi alloys [5]. ...34 Figure 3.2: Pillar drilling machine used for wounding the wire around the screw thread.

...36

Figure 3.3: Shape setting fixture apparatus. A screw thread used to shape set nitinol

wires into extension springs. Notice the use of wire rope clips to constrain the wire ends to prevent unwinding back to initial state. ...37

Figure 3.4: Furnace used for the annealing treatment and bucket with water and ice for

cooling the SMA rapidly. ...37

Figure 3.5: SMAs springs fabricated from nitinol wires. ...37 Figure 3.6: Schematic of X-ray diffraction beam working principle [51]. ...40 Figure 4.1: DSC results of the NiTi alloy spring with 6.7 mm outside diameter, annealed

at 500 ºC for 15 min. ...42

Figure 4.2: DSC charts on heating from 0 to 100 ºC under 10 ºC/min. The four DSC charts

show the influence of stress on the transformation temperatures. ...43

Figure 4.3: a) Effect on transformation temperatures (As, Af, Tpeak) of NiTi alloys with

increasing the stress levels (decrease of spring’s outside diameter). ...44

Figure 4.4: XRD results of five NiTi alloys samples: one NiTi non-annealed wire; one NiTi

annealed wire; three NiTi annealed springs, with outside diameters of 5.7, 6.7 and 7.7 mm respectively. ...46

xx

Figure 4.5: Experimental apparatus for measuring spring’s force and elongation upon

stress and heating. ...48

Figure 4.6: Force vs. elongation curve for three NiTi annealed springs, with outside

diameters of 5.7, 6.7 and 7.7 mm respectively. ...48

Figure 4.7: Control of the applied voltage across an SMA spring using an Arduino Nano.

...49

Figure 4.8: Experimental apparatus consisting in a voltmeter, an amperemeter and an

ohmmeter for measuring, in real-time, voltage, current and temperature across a 6.7 mm OD spring. ...50

Figure 4.9: Resistance output across an SMA spring for different powers...50 Figure 4.10: Temperature curves across a 6.7 mm OD spring for different powers...52 Figure 4.11: a) Heating and cooling behaviour of a SMA spring (80 % of maximum

voltage). ...53

Figure 5.1: Experimental apparatus concerning the dynamic force characterization of the

scissor lift ...56

Figure 5.2: a) Dynamic force tests for ascending and descending cycles of a commercial

scissor lift. ...56

Figure 5.3: Schematic of a wooden base for fixing the dynamometer and the scissor lift.

...58

Figure 5.4: Experimental apparatus concerning the static force characterization of the

scissor lift. The custom wooden base helped to fix the dynamometer and the lift. ...59

Figure 5.5: Experimental and theoretical static force vs lift's displacement curves. ...60 Figure 5.6: Comparison between experimental, theoretical and non-linear fit curves with

respect to the non-load trial. ...60

Figure 6.1: Lift’s Adapter Design V1. ...62 Figure 6.2: Scissor Lift actuated by a SMA spring using the V1.2 ABS adapter. ...63 Figure 6.3: Lift’s adapter design V2. ...64 Figure 6.4: Scissor Lift actuated by a SMA spring using the V2 ABS adapter. ...64 Figure 6.5: Twisted ABS adapters after the contraction of the SMA springs. ...65 Figure 6.6: a) Lift’s adapter design V3.1. b) Lift’s adapter design V3.2. ...66 Figure 6.7: Scissor lift actuated by a SMA spring using the V3 ABS adapter. ...66 Figure 6.8: Rise and descending time behaviour by increasing the mass of the load. ....67 Figure 6.9: Lift’s change in height behaviour regarding the first fatigue test performed

xxi

Figure 6.10: Lift’s change in height behaviour regarding all the fatigue tests put together.

...68

Figure 6.11: a) Acrylic parts regarding lift’s top, base and arms. ...69 Figure 6.12: Assembled acrylic scissor lift prototype. ...70 Figure 6.13: Prototype lift actuated by a SMA spring using, without load. ...70 Figure 6.14: Prototype lift actuated by a SMA spring using, with a load of 400 g. ...70

xxii

List of Tables

Table 1.1: Examples of smart materials and their corresponding input and output signals.

... 2

Table 1.2: Comparison of commercially available types of actuators [8]. ... 9 Table 2.1: Terms used in the design of SMA springs. ...30 Table 4.1: Transformation temperatures and fusion enthalpies, obtained with DSC under

10 ºC/min, of one NiTi wire and three NiTi springs with 7.7, 6.7 and 5.7 mm outside diameter respectively. ...43

Table 4.2: Transformation temperatures and fusion enthalpies, obtained with DSC under

40 ºC/min, of one NiTi wire and three NiTi springs with 7.7, 6.7 and 5.7 mm outside diameter respectively. ...44

Table 4.3: Power measurements for an SMA spring for several percentages of voltage

selected. ...50

Table 4.4: Power and temperature measurements regarding an SMA spring. ...52 Table 5.1: Results for parameter b obtained from the non-linear curve fits. ...58 Table 6.1: Results for displacement, rise time and descending time in the scissor lift

xxiii

Abbreviations

SMA

Shape Memory Alloy

SME

Shape Memory Effect

SE

Super Elastic Effect

SIM

Stress Induced Martensite

VIM

Vacuum Induction Melting

VAR

Vacuum Arc Melting

DSC

Differential Scanning Calorimeter

XRD

X-Ray Diffraction

ABS

Acrylonitrile Butadiene Styrene

PCB

Printed Circuit Board

1

Chapter 1

1. Introduction

1.1 Smart Materials

From the dawn of mankind, materials have been fundamental to the development of human societies. Indeed, civilizations have been named based on the level of their materials development (Stone, Copper, Bronze and Iron ages) [1]. The progress of many technologies that make our existence so comfortable has been intimately associated with the accessibility of suitable materials. An advancement in the understanding of a material type is often the forerunner to the stepwise progression of a technology [2]. For example, sophisticated electronic devices such as smartphones would not have been possible without the availability and study of semiconductors.

Over the past decades, massive advancements in science and technology contribute to a tremendous progress in the field of materials science [3]. The demand for sophisticated and specialized materials, as well as consideration of the environmental impact, has spawned a new branch of research into smart materials [4–5]. The adjective “smart” implies that these materials can react to their environment all by themselves when subjected to an external stimulus from their surrounding environment. In general, smart materials exhibit mechanical response when subjected to changes in temperature, electric fields, and/or magnetic fields [3–5]. Some examples of smart materials commonly used for actuators are: shape memory alloys; piezoelectric materials; magnetostrictive materials, electrorheological and magnetorheological fluids [6]. Table 1.1 summarizes some commonly used smart materials with their corresponding input signals and responses.

2

Material Input signal Output or response

Shape Memory Alloys

(SMAs) Change in temperature Change in length/shape

Piezoelectric Ceramics Electric field Deformation

Magnetostrictive

Materials Magnetic field Deformation

Shape Memory Polymers (SMPs)

pH/humidity/temperature/light

intensity change Change in shape

Magnetorheological

Fluids (MR fluids) Magnetic field Change in viscosity

Electrorheological Fluids

(ER fluids) Electric field Change in viscosity

Table 1.1: Examples of smart materials and their corresponding input and output signals.

1.2 Shape Memory Alloys (SMAs)

Shape Memory Alloys (SMAs) are a unique class of smart materials with the ability to return to their original shape when heated. Once deformed at low temperatures SMAs will stay deformed until the temperature increases, whereupon they will spontaneously return to their original shape. This capability is referred to as the shape memory effect (SME) and was first observed in AuCd alloy in 1932 [7]. In the following years, the SME was observed in other alloys, such as NiTi, Fe–Mn–Si, Cu–Zn–Al and Cu–Al–Ni. However, NiTi based SMAs are much more preferable for most applications [8] [9–13].

1.2.1 Martensitic Transformation

It was only 20 years later that this phenomenon was attributed to the thermoelastic martensitic transformation, a specific type of a crystallographic phase transformation in the solid state [14]. SMAs may be found in two phases: austenite and martensite. Usually, the high-temperature phase austenite (β-phase) has a body-centred cubic structure (B2) as compared to the low-temperature phase martensite (α-phase) which has either tetragonal, orthorhombic or monoclinic structures [5]. Due to its low symmetry structure, each martensitic crystal formed can have a different orientation direction, called a variant [5]. Thus, there are two main martensite configurations: twinned martensite (𝑀𝑀𝑡𝑡_{), which}

3

martensite in which a specific variant is dominant (𝑀𝑀𝑑𝑑_{). Figure 1.1 shows the cubic}

structure of austenite and different variants of martensite. The reversible phase transformation from austenite (parent phase) to martensite and vice versa forms the basis for the unique behaviour of SMAs. In certain conditions, an intermediate phase can be formed upon cooling before the martensitic transformation occurs. This pre-martensitic phase has been called R-phase (rhombohedral), that is, the rhombus distortion of the B2 structure. R-phase transformation is related to the formation of Ti3Ni4 precipitates and it

depends on heat treatment. This transition is indeed a martensitic transformation which exhibits a (very slight) hysteresis and a small memory effect [10][16].

Austenite

• High-temperature phase • Cubic crystal structure

1.2.2 Shape Memory Effect

Generally, there are three related phenomena in which SME can occur: • _{Thermally induced one-way effect}

• Two-way shape memory (pseudoplastic)

• Pseudoelasticity (superelasticity and mechanical memory)

Figure 1.1: Schematic illustration of austenite and martensite crystal structures (adapted from

[5]). Ni Ti

Martensite

• Low temperature phase

• _{Monoclinic / Body Centred Tetragonal} (BCT) crystal structure

Monoclinic (B19’)

Self-accommodated Martensite

4

Several factors define which of these effects takes place. They include the alloy composition, the thermomechanical treatment, the microstructure of the material, and the ambient temperature [17]. The most popular SMA, the binary alloy nickel-titanium (nitinol) is known to show all three effects depending on these factors.

The one-way shape memory effect is the most basic form of shape memory. Here, the shape recovery is achieved only during heating after the material has been detwinned by an applied mechanical load. The combined stress-strain-temperature diagram is shown in Figure 1.2. The transformation from austenite to martensite begins, in the absence of stress, at a temperature known as the martensitic start temperature (𝑀𝑀𝑠𝑠). When cooling to a temperature below the martensitic finish temperature (𝑀𝑀𝑓𝑓) the

alloy undergoes a reversible and diffusionless transformation process due to shear lattice distortions (cooperatively movement of atoms). When the twinned martensite is subjected to an applied stress that exceeds the start stress level (𝜎𝜎𝑠𝑠), the reorientation process is

initiated. The alloy initially behaves elastically followed by a recoverable pseudoplastic deformation (around 6-8% strain) [18]. The detwinning process is completed at a stress level 𝜎𝜎𝑓𝑓. Further stressing causes unrecoverable strain up to fracture. After distorting one

Figure 1.2: a) Crystal structures of martensite and austenite along with associated forward and

backward transformation cases. After heating it transforms to the austenite phase where the atoms are rearranged into their original positions. When cooled it still keeps its original shape.

b) Stress-strain-temperature path describing the thermomechanical experiment showing both shape memory and superelastic effects [17].

5

can recover the original shape by heating above the austenite finish temperature (𝐴𝐴𝑓𝑓).

Therefore, residual strains are completely recovered as the low symmetric martensitic phase is transformed back to stable austenitic phase, thus contributing to the rearrangement of the alloy at an atomic scale. The deformed martensite is now transformed to the austenite which has the microscopic as well as the macroscopic configuration of the initial undeformed state.

The two-way shape memory effect is obtained by a special thermal treatment, consisting of a series of thermal cycles, during which high-temperature and low-temperature shapes are imposed on an SMA component [3]. Hence, one obtains an SMA which can remember both its low-temperature shape (martensite) and high-temperature shape (austenite). Two-way shape memory alloys switch from their low-temperature shape to their high-temperature shape, but their recoverable strain is usually about half of the corresponding one-way recoverable strain for the same material.

Pseudoelasticity

Martensitic transformations in an SMA can be induced purely due to mechanical loading and unloading in the austenitic phase. The shape recovery with associated stress-strain hysteresis due to mechanical loading and unloading at temperatures greater than 𝐴𝐴𝑓𝑓 is

referred to as superelastic/pseudoelastic effect (SE). Contrary to the thermally induced transformations, in superelasticity, no temperature change is required. Instead, an external mechanical load is used to induce phase transformation.

The hysteresis path starts by subjecting the material to moderate external loads at a temperature above 𝐴𝐴𝑓𝑓. The response looks like a classic elastic deformation of a metal.

Upon higher loads, at some critical stress 𝜎𝜎𝑀𝑀𝑠𝑠, the austenite begins to transform to detwinned martensite, also known as stress-induced martensite (SIM), i.e. martensite that forms from austenite in the presence of stress. This transformation results in a macroscopic change of about 6-8% strain [5]. When the transformation is complete, continuing loading causes elastic deformation of the SIM and even higher loads may result in plastic deformation which can result in residual strains. Upon heating, the SIM comes back to a point at which it is thermodynamically stable and as the load continues decreasing it starts to transform back to austenite. Similarly, to the martensitic transformation, this path is associated to large macroscopic shape changes (shortening). Finally, the elastic unloading of austenite occurs. The residual strains could be attributed to pockets of austenite or martensite that do not transform back during either the forward

6

or backward transformations. Figure 1.3 presents the stress-strain-temperature curve depicting pseudoelasticity of SMAs.

1.3 NiTi Based Alloys

Figure 1.4 lists several alloys systems where the shape memory properties had been observed. The most popular among these is the NiTi, as most of these systems do not present favourable commercial applications. In fact, the potential practical uses and substantial attraction of this phenomenon only began in the 1960s, when the SME was first reported in a NiTi alloy [17]. This discovery took place at the US Naval Ordnance Laboratory (NOL, White Oak, Maryland) and hence NiTi based SMAs are often referred to as NitiNOL in its honour. Despite the high production costs and the high market prices for high-purity nickel and for high-purity titanium, the nitinol’s properties make this material ideal for a variety of applications. In reality, 90% of current commercial shape memory applications are based on binary NiTi [17] [19].

When the composition of NiTi is equiatomic (i.e. 50 at% of Ni and Ti), the alloy exhibits the maximum 𝐴𝐴𝑓𝑓 temperature (120 ºC). Decreasing the Ni atomic percentage

(at%) from the equiatomic composition does not change the transformation temperatures. If the composition of nickel is increased above 50 at%, the transformation temperature begins to decrease, with 𝐴𝐴𝑓𝑓 becoming as low as −40 ºC for 51 at% nickel. This variation in

7

the composition can change the ambient room temperature characteristics from SME to pseudoelasticity [3].

1.4 Manufacturing of SMAs

1.4.1 Melting process

The manufacturing process of SMAs consists of several combined melting processes, where the melting is the most popular technique with two principal methods, Vacuum Induction Melting (VIM) and Vacuum Arc Remelting (VAR). A brief description of these methods is presented in section 2.1.

The control of the melting process in SMAs has become crucial due to its huge impact regarding the functionalities of the final product. A small change in the chemical composition affects substantially the transformation temperatures. Moreover, homogeneity of the final product impacts directly on the grain size distribution and impurities concentration, which has a large effect on transition temperatures and fatigue properties.

Today, synthesise of NiTi ingots, ranging from a few grams to several tons is commercially obtained using VIM as primary melting followed by VAR [17].

8

1.4.2 Post-Treatments

At the industry level, the control of the melting process is not enough to obtain NiTi alloys with the desired structural and functional properties, especially for biomedical applications. Several post-treatments as hot working, cold working, machining, surface treatments, joining, and heat treatments are necessary in order to fulfil the requirements regarding the final properties of the material [20–21]. The nitinol phase diagram shown in Figure 1.5 clearly shows that temperatures can greatly influence the formation of various precipitates. It is known that ageing treatments affect the transformation sequence and the mechanical behaviour of the material [20]][22–23]. Annealed NiTi alloys at 850 ºC for 15 minutes show one step on both heating and cooling. When alloys exposed ageing at 400 ºC and 500 ºC for different times, the multistage transformation is seen [24]. Recent investigations have shown the influence of stress ageing process on transformation temperature. It was seen that martensitic transformation temperature of NiTi alloys increases with increasing the stress levels [23].

Once the melting process is completed, the ingot is usually forged, swaged, rolled, or extruded at high temperatures, mostly performed in successive steps with alternated annealing. These hot working steps reduce the size of the initial ingot, to establish the homogeneity and to adjust the grain size. Moreover, hot working offers the ability to obtain a suitable shape of the ingot: bars, composites or tubes. The process is performed under temperatures ranging from 800–950 ºC [17]. The next stage is a series of cold working processes with frequently alternated annealings in a range of temperatures from 500– 800 ºC [17]. Final product shapes such as wire, tubing, and sheets can be achieved via cold working. The average ductility of NiTi allows 30–50% of cold work [25]. NiTi wires, the most widely available form of this material, are produced via drawing.

9

1.5 Design of SMA actuators

To design an SMA component it is imperative to understand which basic requirements are important for its final application. Factors like chemical composition, shape, operating temperature, loading and thermomechanical conditions can significantly affect the performance of the component. Regarding Table 1.2, one can see that NiTi SMA is a good choice for actuators that provide significant strokes and forces. Also, SMA actuators have the ability to react directly to the surrounding stimulus, and they prove to be bio-compatible and have high wear resistance [8].

Despite the promising characteristics, there are several limitations to overcome, which include moderately small strains, slow actuation response, low accuracy, low energy efficiency, and low controllability, due to hysteresis, nonlinearities, parameter uncertainties, and difficulties in measuring state variables such as temperature [17].

Shape setting plays an important role in terms of actuation performance. Straight wires are the most popular form of shape memory alloy. Nonetheless, other shapes such as coil springs, torsion springs, cantilever strips, and torsion tubes are often chosen, depending on the actuation requirements. Commonly, to address the problem of small strains, engineers train NiTi wires as coil springs actuators, since this shape makes possible larger displacements. Generally, induced mechanical stress makes the spring expand, while the thermal activation, leads to the contraction of the coil structure. Joule heating effect is mostly used as an effective and simple way for actuating SMA wires and springs, where current flows directly throughout the SMA component. Figure 1.6 depicts a typical SMA coil actuation cycle [26].

Actuator type Stress

(MPa) Strain (%) Efficiency (%) Bandwidth (Hz) Work/Volume (J/cm3₎ Power/Volume (W/cm3₎ NiTi SMA 200 10 3 3 10 30 Piezoceramic 35 0.2 50 5000 0.035 175 Single crystal piezoelectric 300 1.7 90 5800 2.25 15000 Human Muscle 0.007-0.8 1-100 35 2-173 0.035 0.35 Hydraulic 20 50 80 4 5 20 Pneumatic 0.7 50 90 20 0.175 3.5

10

1.6 Overview of SMA Applications

Since the discovery of the shape memory effect, much research efforts have been devoted regarding SMAs performance in several applications. Despite the limitations, numerous studies revealed the potentials for SMAs from actuation to sensing and control. Due to their unique thermomechanical characteristics, their compactness and lightness, engineers have been implementing SMAs in real-world applications, ranging from biomedical, automotive aerospace, and robotics areas.

1.6.1 Biomedical Applications

In medicine, for a certain material to be used in the human body it needs to satisfy two significant demands: bio-functionality and bio-compatibility. Bio-functionality is the ability of the material to perform the desired function for the expected period in the body. Bio-compatibility refers to the ability of a material to perform with an appropriate host response in a specific situation [27]. Among many materials such as metals or ceramics, shape memory materials have shown to be an attractive candidate for medical and dental applications. In particular, NiTi alloy is considered to be the best choice because of its

Figure 1.6: Spring actuation cycle: a shape-set twinned martensite actuator is subjected to an

external force, causing martensite detwinning; the actuator is heated, and austenite phase-change occurs leading to activation; subsequent cooling causes martensite phase-change, ending in the original twinned state [26].

1. Actuator begins at room temperature in twinned martensite phase

2. External force de-twins martensite, causing extensional strain

3. Applied heat triggers austenite phase change, generating force and contraction F activation F external L0 L0  Lf

11

excellence in mechanical stability and corrosion resistance. For the past few years, shape memory materials have been successfully introduced in medicine and dentistry. Today, SMAs are used in numerous medical fields including orthodontics, orthopaedics, cardiology, neurology, interventional radiology and surgical areas. Application examples include: prostheses; implants, stents, medical tweezers, sutures, anchors for attaching the tendon to bone, aneurysm treatments, eyeglass frames and guide wires. Figure 1.7 summarizes the existing and potential SMA applications in the biomedical domain.

12

1.6.2 Aerospace Applications

One of the early applications of SMAs regarding aerospace domain was the NiTiFe hydraulic tubing coupling used on the F-14 fighter jets in 1971 [8]. Since then, engineers have continued to exploit the properties of SMAs in solving engineering problems in the aerospace industry, where high dynamic loads and geometric space constraints are significant.

In the 1990s, two fixed-wing programs, Smart Wing and the Smart Aircraft and Marine Propulsion System demonstration (SAMPSON), revealed to be an important milestone in the development and application of smart materials and structures technologies to perform active control.

The objective of the Smart Wing program was to develop smart materials that would provide improved aerodynamic and aeroelastic. It consisted of two phases in which the first one involves the development of an SMA based, hingeless, smoothly contoured trailing edge and actively variable wing twists with SMA torque tubes [28]. The SMA wire tendons were used to actuate hingeless ailerons while an SMA torque tube was used to initiate spanwise wing twisting of a scaled-down F-18. This essentially leads to the efforts for circumvention of the bottlenecks of actuation bandwidths and improvements of the actuation range, caused by the actuation via shape-recovery. This work was a collaboration between the Defense Advanced Research Projects Agency (DARPA), Air Force Research Lab (AFRL) and Northrop Grumman.

The usefulness of active materials in tailoring propulsion systems was demonstrated on an F-15 engine inlet through the SAMPSON program [29]. A total force of approximately 26 700 N was achieved using SMA bundles containing 34 wires/rods, as it rotated the inlet cowl 9 º. This work was performed as part of a DARPA contract to Boeing and monitored by the NASA Langley Research Center and the Office of Naval Research [28].

As another aerospace application, Boeing has developed an active serrated aerodynamic device with SMA actuation, known as a variable geometry chevron (VGC) that has been installed on a GE90-115B jet engine for the Boeing 777-300 ER

commercial aircraft (Figure 1.8) [29]. The VGC device has proven to be very effective

13

increasing the cruise efficiency by minimising chevron deflection during the remainder of the flight.

Such implementations of SMA technology have spanned the areas of fixed-wing aircraft, rotorcraft, and spacecraft. Research continues in all three of these areas. Figure 1.9 summarizes the existing and potential SMA applications in the aerospace domain [8].

1.6.3 Automotive Applications

Currently, the number of sensors, actuators and microcontrollers present in modern vehicles is increasing, due to customer demand for safety, comfort and performance. In fact, more than 200 actuators are installed on vehicles with conventional electromagnetic actuators. SMA actuators are a potential replacement for the electromagnetic actuators in automotive applications where they can function as both sensors and actuators limited to applications where energy efficiency and speed are not crucial.

Figure 1.8: Boeing’s variable geometry chevron (VGC) [8].

14

One of the earliest commercial SMA actuators for automotive application is the thermally responsive pressure control valve embedded in Mercedes-Benz automatic transmissions for smooth gear shifting, which was first introduced in 1989 [30]. This valve's function is shown schematically in Figure 1.10. Later, Alfmeier Präzision AG has successfully mass-produced SMA pneumatic valves for lumbar support in car seats for Daimler Mercedes-Benz, and today these devices are used by most automotive manufacturers such as BMW, General Motors (GM), Hyundai, Ford, Porsche and Volkswagen (VW) [8]. GM seventh-generation Chevrolet Corvette was announced to be the first GM vehicle with SMA actuator to actuate the hatch vent for easier closing of the trunk lid. Under the Fiat group, Centro Ricerche Fiat (CRF) has also been intensively involved in SMA applications and numerous patents have been issued by them, such as the electrically actuated antiglare rear-view (EAGLE) mirror, headlamp actuators, fuel filling lid actuator, and locking mechanism [30].

Nowadays most of the applications are electric and active thermal actuators. Thermal actuators respond to changes in temperature by changing their shape and generating a force. Electrical actuators are devices where the stimulus is any voltage applied to it. If electrically heated above 𝐴𝐴𝑠𝑠, such as by passing current through a wire or

spring, nitinol shape memory elements can provide interesting advantages over motors and solenoids due to their small size and noiseless operation.

In general, there are three categories of automotive actuators: low power actuators for comfort and bodywork functions; high power vehicle control actuators; and high-frequency engine control actuators [30]. SMA actuators are most suitable for the first category and feasible for the second category. Figure 1.11 and Figure 1.12 schematically shows the areas in a car where thermal and electrical shape memory actuators have been suggested [31]. However, few have actually been implemented or seem technically and economically feasible because of the limited range of transformation temperatures. As shown in Figure 1.13, the transformation temperature of nitinol ranges from -50 °C to +110 °C, which fulfils the operating temperature requirement in the passenger room (i.e. -40 °C to +85 °C), but not entirely in the engine room (i.e. -40 °C to +125 °C) [8][31]. SMAs satisfy almost all the requirements above, particularly for applications in passenger room and locations with moderate temperatures. Furthermore, with proper design, system control and functionality, SMAs potentially could fulfil all the requirements.

15

Figure 1.11: Potential applications of shape memory thermal actuators in automobiles: (1)

radiator shutler; (2) fan clutch; (3) fuel management; (4) climate control; (5) engine control; (6) brake ventilation; (7) transmission control/rattling noise reduction; (8) suspension adjustment [31].

Figure 1.10: Schematic function of a thermal valve [31].

Figure 1.12: Potential applications for electrical shape memory actuators in automobiles: (1)

foglamp louvre; (2) engine hood lock; (3) retractable headlight; (4) fuel management; (5) engine control; (6) transmission control; (7) climate control; (8) wiper pressure control; (9) rear-view mirror adjustment; (10) seat·belt adjustment; (II) central locking system; (12) shock absorber adjustment; (13) filler inlet lock; (14) trunk lock [31].

16

Figure 1.13: Operating temperature range for automobiles applications and the transformation

temperatures for selected commercially available and developed SMAs [8].

1.6.4 Robotic Applications

In addition to the transportation and medical industries, there are many other applications that incorporate SMAs. Due to the biocompatibility of SMAs and the high performances of the actuators, there are a lot of possibilities to develop improved and innovative products applied successfully in robotics, especially in systems like micro-actuators and artificial muscles [8] [12] [31–32].

Considerable work has been done in developing grippers for robotic manipulation. In particular, a great effort has been addressed to the fabrication of grippers on the microscopic scale (i.e. about 600 µm displacement) that can be actuated wirelessly with an RF magnetic field. The frequency-sensitive LC resonant circuit is heated when an RF magnetic field passes through it and then transfers the heat energy to the SMA actuator for activation. The opportunity to control multiple selections of micro-SMA actuators is possible by applying different resonant frequencies, either selectively or simultaneously to the actuators [8] [34].

SMAs have been successfully integrated into prosthetic devices to enhance the quality of life for amputees. Prototypes such as robotic finger and shoulder joint that emulate human skeletal structures are actuated with SMA artificial muscles. It is shown

17

that these systems can exhibit relatively large motions and their use is possible for artificial limbs [8] [26–27].

Many robotics researchers are more interested in developing biomimetics and humanoid robots. These robots are useful in solving problems that are challenging for humans, by providing pertinent information from underwater, space, air and land. A biped mountable robotic baby head, that could be used for children education, entertainment, and household activities was developed using a combination of biometal fiber and SMA wires. Humanlike facial expressions were demonstrated by combining the SMAs with the elastomeric skin, mechanical components, and microcontroller-based driving electronics [36].

Several flying robots have been developed with SMAs, such as the BATMAV, Bat Robot, and more recently, a 44 cm length dragonfly, also known as BionicOpter (Figure 1.14). Developed by Festo Group, it has a wingspan of 63 cm equipped with four SMA actuators to control the movements of its head from side to side and its tail up and down for flight manoeuvre and stability in all directions [8].

The primary challenges relevant to the robotics domain are: to increase the performance and miniaturisation of the hardware platform and to increase the intelligence of the integrated system (i.e. small, faster, reliable and autonomous). Several technical issues have been highlighted and need to be overcome, such as clamping difficulties, low electrical resistance, miniature electrical connection (for micro-robots), small strain output, control issues and very low efficiency. Furthermore, to increase the degrees of freedom of the robots, the number of actuators must increase, which leads to complex control problems. However, some of these issues have been tackled by selecting suitable modelling techniques, control techniques and

feedback sensors. Figure 1.15 summarizes the existing and potential SMA applications

in the robotic technology domain [8].

18

19

1.7 CeNTI and Project Objectives

In Portugal, one of the Technological Transfer Centers working with smart materials is Centre for Nanotechnology and Smart Materials (CeNTI), highly recognized for its expertise in materials functionalization by using integrated electronics, sensors, coatings, as well as transferring knowledge from physics, chemistry, nanotechnology, and others to real world products. This work was part of an internship program between CeNTI and Faculdade de Ciências da Universidade do Porto.

The challenge purposed by CeNTI was to use a smart material (SMA) in an innovative actuation system. The present work is devoted to achieving this main objective, namely by first characterize SMAs, then evaluate a commercial scissor lift, and lastly blend both into a final prototype. Figure 1.16 overviews the project.

21

Chapter 2

2. Design of an SMA Actuator

The primary objective of this research is to demonstrate the use of a smart material in an innovative actuation system. As explained before, advantages such as high force-to-weight ratio and large displacement capability, make SMAs preferable over other smart materials. Concerning the mechanical actuation system, it was intended for the prototype to act as a lift, i.e. to perform mechanical work on lifting a weight.

2.1 Lift Mechanisms Design

The demands by CeNTI for the application was that the mechanical prototype should be small, lightweight and compact. The weight should be lifted only vertically (without horizontal translation movement). Its actuation should be made by means of a spring configuration as it provides more strength when compared with wires. Several ideas have emerged, and so a preliminary evaluation of the advantages and disadvantages of each mechanism was essential. Three different lift mechanisms were suggested:

• _Lever

• Rack and Pinion • Scissor Lift

2.1.1 Lever

A lever is a simple machine consisting in a solid bar (beam) that rests on a pivot point (fulcrum). It can be used to exert a large force over a small distance at one end of the lever by exerting a small force over a greater distance at the other end. It is probably one of the oldest mechanisms to lift heavy objects. The object pushed by the lever is called the load,

22

while the force applied to the lever is called the effort. A seesaw is a great example of a lever. In physics the seesaw is an academic example of rotation equilibrium, which states that the angular acceleration about any axis must be zero, i.e. when the algebraic sum of the torques is zero:

� 𝑟𝑟⃗𝑖𝑖× 𝐹𝐹⃗𝑖𝑖 = 0 (2.1)

An illustration of the lever/see-saw mechanism is shown in Figure 2.1. The SMA spring is attached to one end of the bar, and as it compresses, it exerts an effort force and consequently will lift the load placed on the other end of the bar. If 𝑟𝑟⃗1 is the distance

between the spring and the fulcrum and 𝑟𝑟⃗2 is the distance between the load and the

fulcrum, from equation (2.1), the force exerted by the SMA spring is given by:

𝐹𝐹𝑆𝑆𝑀𝑀𝑆𝑆= 𝑚𝑚𝑚𝑚𝑟𝑟2_𝑟𝑟1 (2.2)

For example, if the SMA spring performs 20 N to lift 1 kg, the distance ratio implies that the distance between the spring and the fulcrum is half of the distance between the load and the fulcrum (𝑟𝑟2 = 2𝑟𝑟1).

One advantage of this mechanism is the possibility of changing the fulcrum’s position when fixing parameters such as the load mass, and the spring’s force. One disadvantage is the change in the load’s horizontal position when lifted. The goal is to lift a weight only vertically, although in this scenario the weight would realize a circular trajectory. Some sketches of attempting the physical prototype were made, involving additional mechanical structures and therefore increasing the complexity of the all system.

SMA fulcrum

m

𝑟𝑟

�⃗₁

𝑟𝑟

Figure 2.1: Illustration of a simple lever/seesaw mechanism with the load on one side and the SMA

23

2.1.2 Rack and Pinion

Another idea was to use gears to transmit motion over a small distance, for example, a rack and pinion mechanism. Figure 2.2 illustrates the 3D draft of the mechanism. The spur gear is the pinion and the straight gear is the rack. The rack has teeth cut into it and they mesh the teeth of the pinion. The basic operating principle is to convert linear motion into rotary motion, or vice-versa. The apparatus consists of a pair of pinions engaged in vertical and horizontal racks. The SMA spring actuates on the horizontal rack, which rotates the smaller pinion. The circular movement of the small pinion rotates the big pinion, which consequently moves the vertical rack.

The rack and pinion mechanism provides a compact and easy way to convert rotary motion into linear motion. Thanks to the big spur gear, a small displacement of the horizontal rack can provide greater vertical displacement of the vertical rack. If for example, the radius of the big pinion is twice the radius of the smaller pinion, the vertical displacement would be twice the horizontal displacement. However, the mechanism needs to be assembled into a compact prototype and no valuable idea was projected.

Figure 2.2: Draft of the rack and pinion lift mechanism actuated by an SMA spring.

SMA

24

2.1.3 Scissor Lift

Scissor lifts are a type of mechanism used for moving loads vertically. It consists in a top and bottom parallel platforms linked through a pantograph, i.e. in folding supports in a crisscross “X” pattern, also known as a scissor mechanism. Generally, there is a static and dynamic end at both, top and bottom platforms. The displacement motion is achieved by the application of force, by some actuator, to the dynamic end of one set of supports, resulting in an elongating of the crossing pattern, and consequently propelling the load vertically [37]. Figure 2.3 depicts a general scissor lift scheme with two levels.

Scissor lifts come in many sizes and many shapes and are widely used in industrial applications, ranging from compact platforms that are used in warehouse aisles to industrially sized lifts used to reach heights of above 20 metres. There are several benefits in using scissor lifts, such as the compact design, easy control, stability and reliability.

It is intended to integrate the SMA spring into the base between the static and dynamic ends, so when the spring contracts the lift raises, and when the spring extends the lift lowers. Figure 2.4 shows the 3D design of 1 level and 2 level scissor mechanism actuated with an SMA spring. The designs were developed with 3D CAD software OnShape. The proposed design follows the basic requirements, as it is small, lightweight and compact. Also, it is similar to a laboratory scissor jack used to lift laboratory equipment usually around 5 to 20 cm. Hence it can be used in a laboratory scenario. The main disadvantage is the range limitation. If the lift is fully flattened, the spring will

25

require very large forces to operate, and so it is necessary the use of stroke limiters/hard stops.

After a preliminary evaluation of all the three possible mechanisms, the scissor lift was the more appealing in terms of design, manufacturing and operation. In the next section a model that expresses the force output of a scissor lift is presented. It is also described the geometry of the intended prototype and the proposed adapter which assembles the lift together with the SMA spring.

2.2 Scissor Lift Design

2.2.1 Force Model

The equation for the output force of the actuator located in the base of a 2-level scissor lift was obtained by Spackman in 1982 [38] and is given by:

𝐹𝐹 =

2𝑚𝑚 �𝑚𝑚 + 𝑀𝑀

𝐿𝐿

2 �

tan(𝜃𝜃)

where m is the mass of the load, ML is the lift’s mass and θ is the angle between the scissor arm and the horizontal plane. Notice that the weight of the scissor lift is modelled by placing half its weight at the top, i.e. by adding half of the weight of the scissor lift to the load. For the purposes of this thesis, the model will not be derived here, as it is

26

demonstrated in more detail by Spackman. Instead, the equation 2.3 was assumed as a theoretical model, and it was used to predict the force which the SMA spring will be subjected to.

The geometry of the 2-level scissor lift is depicted in Figure 2.5. With basic trigonometry the expressions for the actuator’s displacement and lift’s height were obtained:

𝑥𝑥 = 𝐿𝐿 cos 𝜃𝜃

ℎ = 2 ⋅ 𝐿𝐿 sin 𝜃𝜃

where L is the length of the scissor arm.

When the mechanism is in static equilibrium, the force required for the spring to lift the mechanism will depend heavily on the angle θ between the scissor arm and the horizontal plane. It is important to study the range of angles in which the spring can operate. Figure 2.6 plots the external force on an SMA spring for different loads with a lift’s mass of 0.5 kg. From the plot it is expected that if θ is too close to 0 º the spring will require very large forces to operate. Also, based on experience if θ is too close to 90 º the lift will jam. Based on previous experiments in CeNTI, the springs achieve maximum forces around 30 N. Since it was planned to lift loads up to 1 kg, the operating angles were set to range from 30 º to 70 º, thus it provides enough force as well as a horizontal displacement of about 4 cm.

27

2.2.2 Prototype Design of the SMA Actuated Scissor Lift

Following the initial drafts proposed in section 2.1.3, a 2-level scissor lift was designed, and its depicted in Figure 2.7. It consists in:

• 2 × acrylic square sheets • 8 × acrylic scissor arms • 8 × acrylic washers • 7 × aluminium rods • 10 × nuts

The prototype shall have a base with dimensions of 10 × 10 cm. Lift’s height ranges from 11.24 cm to 17.93 cm. Detailed drawings are presented in Appendix A.

The design of a part that holds the spring to the cylindrical shafts of the lift’s base is crucial. Figure 2.8 presents the design of the prototype actuated with the SMA spring.

ML=0.5 kg

Figure 2.6: External force on an SMA spring vs. angle between the scissor arm and the

28

The mechanical adapter shall be obtained through 3D printing at CeNTI facilities. The manufacturing process of the adapter is presented in detail in section 6.1.1.

Figure 2.7: Isometric and side view of the Scissor Lift Prototype designed in OnShape.

29

2.3 Design of SMA springs

2.3.1 Geometry and Force Modelling of NiTi springs

SMA spring actuators can be fabricated by shape setting wires. The experimental procedure is described in Chapter 3. When set as tightly packed springs, the shape recovery characteristics of SMA provide greater forces. Hence, the spring structure is well suited for the scissor lift actuation.

The force F generated by an SMA spring actuator follows Hooke’s law, and can be expressed by a given set of geometric parameters, presented in Figure 2.9 :

𝐹𝐹 = 𝑘𝑘𝑘𝑘 = �

_8𝐷𝐷

𝐺𝐺𝑑𝑑

_{𝑛𝑛� 𝑘𝑘}

For an easy reference, terms used in the design of SMA springs can be found in Table 2.1. A set of significant design equations used for springs are listed below:

𝑂𝑂𝐷𝐷 = 𝐷𝐷 + 𝑑𝑑

𝐶𝐶 =

𝐷𝐷

_𝑑𝑑

𝑘𝑘 =

_8𝐷𝐷

𝐺𝐺𝑑𝑑

_𝑛𝑛

𝜂𝜂 =

𝑑𝑑

_{𝑝𝑝 =}

_𝐿𝐿

𝐿𝐿

=

𝑛𝑛𝑑𝑑

𝐿𝐿

𝜖𝜖 =

_𝐿𝐿

𝑘𝑘

30

Symbol Name Unit

d Wire diameter m

D Spring average diameter m

OD Spring outer diameter m

p Spring pitch m

α Spring pitch angle º

δ Spring elongation m

Ls Solid spring length m

L0 Free spring length m

k Spring constant N m-1

G Shear modulus Pa

C Spring index dimensionless

n Number of coils dimensionless

ε Strain dimensionless

η Packing density dimensionless

Table 2.1: Terms used in the design of SMA springs.

Figure 2.9: Spring parameters as defined by coil geometry, for both a fully compressed (solid)

31

The maximum spring elongation

_𝑘𝑘

𝑘𝑘

=

𝜋𝜋𝑛𝑛𝐷𝐷

𝑑𝑑

√3

2 𝜀𝜀

where

𝜀𝜀

is the maximum residual strain.

the maximum force for an SMA spring becomes:

𝐹𝐹 =

𝜋𝜋𝐺𝐺𝑑𝑑

_𝐷𝐷

√3

_{16 𝜀𝜀}

With eq. (2.13) one can design actuators to meet specific performance requirements, which may include force targets, size limitations, manufacturing limitations, or desired lengths or extensional strains. To get a prediction regarding the forces and displacements provided with different SMA spring characteristics, a spreadsheet was created based on the previous equations. Inputs such as the activation temperature, the wire diameter, the spring outside diameter, the number of coils, the shear modulus and others allow calculating force, displacement, maximum spring length elongation, etc. The spreadsheet is depicted in Appendix B.