Thesis submitted for the degree of Master in Robotics and Intelligent systems

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Walking robot with artificial muscles made of fishing line

Exploration of a nylon 6 polymer actuator

Knut Løklingholm

Thesis submitted for the degree of Master in Robotics and Intelligent systems

60 credits

Department of Informatics

Faculty of mathematics and natural sciences

UNIVERSITY OF OSLO

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Walking robot with artificial muscles made of fishing line

Exploration of a nylon 6 polymer actuator

Knut Løklingholm

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Walking robot with artificial muscles made of fishing line http://www.duo.uio.no/

Printed: Reprosentralen, University of Oslo

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Abstract

This thesis explores making linear artificial muscles out of fishing line, and test this artificial muscle in use on two robots. The purpose of the thesis is to conclude if using the muscle fiber produced is the correct direction to pursue developing a robot walking in the wild.

To this end, a rig for producing the muscle fibers and a rig for experimenting on the muscles were constructed. Three experiments were conducted to find the repeatability of the actuator stroke, the optimal load, and the reproducibility of muscle production. Two robots were made, one early prototype, 3D printed with six legs and using 24 muscle fibers, that did not walk, and one bipedal robot with four muscle fibers successfully walking with the help of a support wagon for balance and counterweight.

The conclusion based on the experiments and the robots is that the muscle is less than ideal direction for an outdoor robot, even if one of the robots walked successfully.

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Acknowledgments

This thesis is part of my Master Degree at the University of Oslo, Department of Informatics. The thesis was carried out autumn 2017 to summer 2018 in the research group Robotics and Intelligent Systems (ROBIN).

I would first like to thank my thesis advisor, Associate Professor Mats Høvin, for invaluable input and supervision. He consistently allowed this paper to be my own work but steered me in the right the direction whenever he thought I needed it.

I also like to thank Principal Engineer Vegard Dønnem Søyseth for help with everything practical and tooling related and my fellow student Jonas Skarstein Waaler for the help building a prototype robot in the course Rapid Prototyping. And lastly I want to thank my sister Liv Brenna and aunt Brita Brenna, for guidance and helpful comments during the writing.

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List of Figures

2.1 Pleated Pneumatic Artificial Muscle 2.0 (PPAM). Figure

from Vrije Universiteit Brussel (2012). . . 5

2.2 VAMPS artificial muscle in its unactuated and fully actuated state. Figure from Yang et al. (2016). . . 6

2.3 Origami artificial muscle. Figure from Li, Vogt, Rus, and Wood (2017) . . . 6

2.4 EAPS artificial muscle. Figure from Manuel Kretzer (2013) . 7 2.5 SMA. Figure from Arquimea (2018) . . . 7

2.6 CNA yarn being spun. Figure from CSIRO (2005) . . . 8

2.7 Tensile stroke and nominal modulus. Figure from Haines et al. (2014). . . 9

3.1 Radial expansion of polymer result in untwist. . . 12

3.2 Stress relaxation and creep . . . 13

3.3 Precursor fiber twist and untwist when coiled. . . 14

3.4 Insert enough twist and the fiber will buckle and start coiling. Figure from Grushetzky, Matiunin, and Shamanov (2015) . . 14

3.5 Computer render of fishing line untwisted, twisted and then coiled. . . 16

3.6 Torsional stroke for twisted fibers with different annealing. . . 17

3.7 Time based tensile creep with different torque. Figure from Aziz (2017) . . . 18

3.8 Torsional stroke for twisted nylon with different torque. Figure from Aziz (2017) . . . 18

4.1 Example of a tethered statically stable reversed-knee bipedal gait. . . 22

5.1 A finished muscle fiber with a marker denoting it as the tenth test muscle. . . 25

5.2 Concept drawing of production rig. . . 26

5.3 Rig for muscle production in annealing mode. . . 27

5.4 Schematic for production controller breadboard. . . 28

5.5 Breadboard controller for the production rig. . . 28

5.6 Data collection sensors and rig. . . 29

5.7 picture from webcam . . . 30

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5.8 a) Schematic for voltage control and current sense PCB. b) Voltage control and current sense soldered to breakout PCB. 31

5.9 a)Six legged robot CAD. b)Bipedal robot CAD. . . 31

6.1 a)Muscle fiber bad result with kinks. b)Muscle fiber melted from too high temperature. . . 36

6.2 Image from thermal camera showing the muscle fiber heating up during annealing. . . 36

6.3 Production of a muscle. . . 37

6.4 Comparison of muscle annealing and release order. . . 39

7.1 Anteup - Six legged robot . . . 42

7.2 Anteup - Hip seen from above and knee from the side. . . 43

7.3 Other robots as inspiration. . . 45

7.4 Artimus with support wagon. The muscle fibers are located in front of the legs and the rubber bands on on the back. . . 45

7.5 DH coordinate frames for Artemus. Z-axis points out of the paper. . . 47

7.6 State diagram for gait for Artiums . . . 48

7.7 Excerpt of Artimus gait seen from the side for the three steps of the left leg. . . 49

8.1 Plot of length for one data set of 7200 seconds. . . 52

8.2 Top plot show absolute with temperature overlaid for 1800s with a period of 240s. Bottom plot show a contraction in percent versus temperature. . . 53

8.3 Stroke and length for cycle 3-60 from muscle m3, m4 and m10. 56 8.4 Creep for muscle fiber m3, m4, m10. . . 57

8.5 Muscle fiber length over time after an experiment. . . 57

8.6 Total length of muscle fiber in mm over 290 consecutive cycles. 59 8.7 Stroke and total length for three muscle fibers at 70 °C. . . . 60

8.8 Comparison of creep for three different muscles at 100 °C and three at 70 °C. . . 61

8.9 Energy/Load & Stroke/Load for muscle fiber m8 and m9. . . 63

8.10 Stroke/load for 6 different muscle fibers tested at loads between 190 g and 285 g. . . 64

8.11 Average of two runs for ten muscles at 70 °C and 220 g load. With an outlier, m5, rerun after it was run at 100 °C for 1 hour with a 400 g load. . . 67

8.12 Length, resistance, and resistance per cm for the 10 muscles before the first and the last experiment, and during the last experiment. . . 69

9.1 Produced muscle . . . 71

9.2 Melting of muscle fiber at temperatures over 110 °C where it touches the resistance wire. . . 74

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List of Tables

6.1 Muscle fiber optimal load for different production load. . . 34

7.1 Gait for Anteup. Left leg[LL1-3], Right leg[RL1-3] . . . 43

7.2 DH parameters for bipedal robot . . . 46

7.3 Lever arm length versus displacements and torque as fraction of muscle fiber force. . . 47

8.1 Length and resistance for the muscle fibers before being cut and before first use in an experiment. . . 53

8.2 Experiment 1.1 parameters. . . 54

8.3 Results experiment 1.1 . . . 55

8.6 Standard deviations for stroke, cold length and hot length for three muscle fibers at 70 °C. . . 60

8.8 Results from optimal load experiment with muscle m9. . . 62

8.9 Results from optimal load experiment with muscle m8. . . 63

8.12 Results from experiment 3.1 Reproducibility in ten similarly produced muscles. avg is the average, std is the standard deviation and cv is the coefficient of variation. . . 68

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Glossary

actuation The movement of an actuator. Same as stroke.

actuator In simple terms, it is a "mover".

annealing The process of heating a material to cause a permanent molecular effect, such as reorientation of polymer chains.

auto-coiling Spontaneous coiling of twisted polymer when twisted beyond a critical point.

chirality The direction of the coils in a spiral, clockwise or anticlockwise.

coil A coil of a spiral.

coil-index The relationship between the diameter of a muscle fiber and the diameter of the precursor fiber. Diameter of muscle fiber[D] / diameter of precursor fiber[d] = D/d.

creep The change in a material over time without any change in forces.

Aka what happens to the length of the muscle fiber if it is just left in tension for a long time.

cycle A heating/cooling cycle of the muscle fiber. Heating it to a temperature and letting it cool off.

hysteresis The dependence on history in a system for an effect to happen. For example phase change in a material can be at different temperatures if the material is being heated or cooled.

Joule heating The process by which passage of current in a conductor produces heat.

load The load is the force acting on a muscle fiber. Most often gravity acting on a mass suspended from one end of the muscle fiber.

muscle fiber Finished coiled muscle fiber.

precursor fiber Is the polymer nylon fishing line before it is coiled.

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re-twist Increasing the twist in a fiber after untwisting.

snarl To entangle something, in this case a fiber/fishing line entangling with itself.

strain (mechanical) The deformation of a material.

stress (mechanical) The internal force of neighbouring particles pushing against each other in a material.

stroke The length a muscle fiber contraction.

twist The number of rotations of a fiber seen from one end if one end is kept tethered and the other end is rotated. Use: Twist inserted into the fiber.

untwist The reduction of twist in a fiber.

viscoelastic A material that has the properties of both viscosity and elasticity when deformed..

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Chapter 1

Introduction

Today environmental monitoring is mostly done with satellites and aerial photographs. Another possibility that gets a bit closer to the ground would be robots that could independently traverse geographical areas over large time frames. Such robots could use legs instead of wheels to handle the varied terrain in forest and mountains. Instead of electrical rotational motors powered by batteries this robot could use linear artificial muscles and natural construction materials to blend better into a wild environment and solar panels for energy. Slow and silent, where the speed/power would heavily depend on the intensity of the sun.

This thesis explores a linear artificial muscle made of fishing line, and test this artificial muscle in use on two robots.

1.1 Motivation

There has always been a desire for actuators that mimic a biologic muscle, either in aesthetics or performance. Tondu and Bertrand (2015, p. 337) says that «In a general way any material, or device, whose shape can change in response to a stimulus can potentially be a candidate to the label artificial muscle.» Animals have evolved over a very long time to use linear actuating muscles, and this seems effective for them (and us). Sometimes it is possible to gain some of the effectiveness of nature by copying it.

In 2014 research led out of the University of Texas showed that artificial muscles made of coiled nylon fishing line or sewing thread could contract up to 30% of its length and had a specific energy of 50 times the human muscle with low hysteresis and millions of cycles, (Haines et al., 2014; Haines et al., 2016).

There have been made several robotic fingers and hands using the coiled nylon artificial muscle for actuation, (Yip & Niemeyer, 2015; Wu et al., 2015; Cho et al., 2016; Tadesse, Wu, & Saharan, 2016), but at the time of writing for this thesis no walking robots.

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Several advantages of using linear artificial muscles for a robot can be imagined:

• Look: They can be made to look natural, more animal-like, easier than electric motors.

• Form factor: Small footprint and weight, possible less bulk. Electrical motors are often built compact and seldom in very sleek and form- fitting configurations.

• Natural dampening and compliance: Artificial muscles have dampening as an emergent property, often based on nonlinear force curves.

Needing no extra spring or closed loop control for dampening. It can look less robotic with less effort.

• Noise: No need to reduce the noise, as there is none, the fishing line artificial muscles actuate silently.

• Chemistry, electric motors can suffer from corrosion and oxidation when exposed to the elements. Fishing line should be able to handle the elements quite well.

• Cost: Nylon fishing line artificial muscle is cheap. Although the artificial muscle is not the only part of a robot, so the total cost might not come down that much.

My contribution is not to discuss these points, but rather exploring the feasibility of using linear actuating fishing line muscles to make a robot walk in the wild.

1.2 Goal

To conclude if the fishing line artificial muscle is feasible for use in an outdoor robot I need to answer some questions:

Make and test muscles:

• What are the difficulties when producing artificial muscles from fishing line?

• Parameters of the produced muscle:

– How does the produced muscle fiber compare with the literature?

– Repeatability of stroke; what is the deviation between actuator strokes given the same load and input power?

– Do the actuator strokes in the produced muscle change over time?

– What is the optimal load, the relationship between load and stroke, for the muscle fiber?

• How much variance in parameters between produced muscles?

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Make a robot:

• What design considerations are needed when using the fishing line actuator?

• Can the fishing line actuators be used in parallel?

• How difficulty is the implementation of the actuator?

The goal of the thesis is to use the answers from these questions to conclude if this is an appropriate direction to pursue for developing a robot walking in the wild. If it is, this thesis should hopefully serve as a building block for such an endeavor.

1.3 Implementation

To meet the goal of the thesis:

• Two robots have been built:

– Anteup, a six-legged robot made as a rapid prototype.

– Artimus, a bipedal robot made to show locomotion using the fishing line actuators.

• A rig for production of the muscle fibers was built.

• Several nylon 6 coiled muscle fibers for the robots and ten identical ones for experiments had been produced.

• A sensor and measurement rig was built for doing experiments on the muscle fibers.

• Three experiments have been done to characterize the produced muscle fibers.

1.4 Breakdown of the chapters

Chapter 1: Introduction

Introduction to the thesis, with the motivation, goals, implementation, and outline.

Chapter 2: Artificial Muscles

A short overview of various artificial muscles used as actuators in robots and some that are being researched but show promising results with a summary of why I chose the nylon 6 polymer fishing line artificial muscle.

Chapter 3: Nylon 6 Polymer Actuator

Explanation of how and why coiled fishing line works as an actuator.

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Chapter 4: Robot

Background for the choices made for the robots in this thesis Chapter 5: Tools & engineering

What tools/rigs/sensors were made and/or used.

Chapter 6: Muscle Implementation How the muscles were produced.

Chapter 7: Robot Implementation

How the robots were constructed and how they worked.

Chapter 8: Experiments

Experiments to characterize the muscle produced.

Chapter 11: Muscle Discussion

Discussing the muscle implementation and discussing the results of the experiments.

Chapter 11: Robot Discussion Discussion of robots and their results.

Chapter 11: Conclusion & future work Conclusion and potential future work.

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Chapter 2

Artificial Muscles

This chapter is a short overview of a few variations of artificial muscles limited to types that bear some similarity to human muscle and are either in use in robots or show promising results. This is to show the alternatives to the actuator chosen for the robot, nylon 6 polymer fishing line.

Artificial muscles, materials that change shape or form in response to a stimulus, have long been desired to mimic natures biologic muscles. There is a multitude of alternatives with a wide array of characteristics and cost.

2.1 Pneumatic artificial muscles

Pneumatic artificial muscles (PAMs) are flexible rubber bladders wrapped in an inflexible braided mesh. When the bladder is filled with gas, it will expand radially and contract in the axial direction because of the inflexible mesh.

See figure 2.1 from Vrije Universiteit Brussel (2012). Since the actuation is one way only it is often used as pairs working in an antagonistic set-up, i.e., working in opposite directions.

Figure 2.1: Pleated Pneumatic Artificial Muscle 2.0 (PPAM). Figure from Vrije Universiteit Brussel (2012).

The braided mesh has to withstand a lot higher forces than the produced actuation force, and it requires a powerful compressor for the gas. They were first developed under the name McKibben Artificial Muscles in 1957, and this configuration is still in use in robotics today, (Ohta et al., 2018). Maximum actuation 36% due to geometry, (Yang et al., 2016).

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2.2 Vacuum-actuated muscle-inspired pneumatic structures

Figure 2.2: VAMPS artificial muscle in its unactuated and fully actuated state. Figure from Yang et al. (2016).

Vacuum-actuated muscle-inspired pneumatic structures (VAMPS) are a type of artificial muscle that actuates by negative pressure making a structure collapse in on itself or fold (cooperative buckling), see figure 2.2 from Yang et al. (2016). This is a new form of PAMs that has several benefits. It cannot ex- plode when over-pressurized, as it oper- ates on negative pressure. It can survive small punctures; the vacuum pulls the hole close. Maximum actuation is 45%, and it can sustain loading stresses up to 65 kPa, (Yang et al., 2016).

2.3 Fluid-driven origami-inspired artificial muscles

Figure 2.3: Origami artificial muscle.

Figure from Li et al. (2017) Fluid-driven origami-inspired artificial

muscles (FOAMs) are similar to the VAMPS in that they collapse a structure, but the structure is made of something stiff, folded like origami, surroun- ded by a flexible skin and filled with a fluid. By pumping out fluid, the origami collapses, see figure 2.3 from Li et al.

(2017). The origami can be made such that it collapses into desirable shapes, for example, to fold into an arc or for gripping around something. A very inexpensive, > 1$, 10 centimeter 2.6 gram muscle, could lift one kilogram 5.5 cen- timeters (55% actuation) in 0.2 seconds, (Li et al., 2017).

2.4 Electroactive polymers

Electroactive polymers, (EAPs), are polymers that exhibit a change in size or shape when stimulated by an electric field, (Carpi, 2016). There is a significant amount of different EAPs, consisting of many actuators with greatly varying properties. They are split into two categories, dielectric and ionic. Dielectric EAPs uses electrostatic forces, requiring

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high voltages, hundreds to thousands of volts, but no energy to stay in a given configuration, an example is in figure 2.4 from Manuel Kretzer (2013).¹

Figure 2.4: EAPS artificial muscle. Figure from Manuel Kretzer (2013)

Piezoelectric polymers were first produced in 1925. Further devel- opments were made with the dis- covery of conductive polymers in the 1960-1970s. These could withstand large forces but had a fraction of a percent strain. In 1990 the ionic polymer-metal compos- ites were discovered, which could actuate up to 380%, (Bar-Cohen, 2005). Ionic EAPs actuate by dis- placing ions, using low voltages(1- 2 volts), but needs energy to stay in a specific configuration. The actuators have to be wet to work, as the ionic transfer needs a solution to transport the ions in. It is often encapsulated in a gel.

2.5 Shape memory alloy

Shape memory alloys (SMAs) are materials with a memory effect; when deformed they will return to their original form when heated. A temperature and stress-dependent phase change straightens out the crystalline structures in the material in the hot phase and makes it deform-able in the cold phase.

The heating is done with Joule heating, running a current through the SMA.

The most commonly used version is a nickel-titanium alloy because of its stability and practicability, (Cederström & Van Humbeeck, 1995). See figure 2.5 for an example of a SMA from Arquimea (2018).

Figure 2.5: SMA. Figure from Arquimea (2018)

Phase changes come with the down- side of large hysteresis; the phase change occurs with a 20 °C difference between heating up and cooling down. SMAs can also be very expensive, 1350-2450$/kg, but a robot would only use a few grams, so not too expensive compared to the other parts of a robot. The hysteresis can be combated by using a dif-

ferent alloy, Zn45Au30Cu25, which is more than 50% gold, (Ohta et al.,

1 In the movie Batman Begins by Nolan (2005), there is an idealized version of this where Batman’s cape is turned instantly rigid when electricity runs through it and can then be used as a hang glider. Currently, such a large dielectric EAP is not feasible and would most likely suffer from catastrophic thinning when it reaches a critical voltage as shown in Zurlo, Destrade, DeTommasi, and Puglisi (2016), but progress is happening.

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2018), making it more expensive. Mohd Jani, Leary, Subic, and Gibson (2014) say «The challenges in designing SMA applications are to overcome their limitations, which include a relatively small usable strain, low actuation frequency, low controllability, low accuracy and low energy efficiency.»

Typical actuation of NiTi is 4.5%, and for the gold alloy, it is 8% and typical contractile stress of less than 200MPa ,(Mohd Jani et al., 2014).

Sæther (2008) found that the actuator deformed 1% after a week of use and great care should be taken not to over/underload them, limiting their working range.

2.6 Carbon nanotube actuator

Carbon nanotubes (CNTs) has been found to be useful as actuators in several ways. A carbon nanotube sheet in salt water would actuate when 1 volt was applied, (Vohrer, Kolaric, Haque, Roth, & Detlaff-Weglikowska, 2004). Torsional actuation (spinning around) CNTs with a simple three- electrode electrochemical system provided a reversible 15,000° rotation and 590 revolutions per minute, (Lima et al., 2011).

Figure 2.6: CNA yarn being spun. Figure from CSIRO (2005)

An even more promising technology is hybrid carbon nanotube spun into yarns and coiled into a helical structure, first shown in Chun et al. (2014). The hybrid part is a guest material that can expand which causes the yarn to contract. The expansion function of the guest material can be done electro- chemically or electrothermally, pho- tothermally, thermally or chemically.

Downsides are the price of carbon nanotube yarn. Anything that is

nanometers thick might not be easily produced. Figure 2.6 from CSIRO (2005) show CNA yarn being made.

2.7 Coiled polymer fibre/yarns

Haines et al. (2014) published an article that showed the possibility of using twisted and coiled nylon thread and fishing line into helical structures for actuation. This behaves similarly to the helical structure in the hybrid CNT yarn from Chun et al. (2014), but with nylon, which is very inexpensive compared to CNT.

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Figure 2.7: (B) Tensile stroke and nominal modulus versus temperature for a coiled, 300- µm-diameter nylon 6,6 monofil- ament muscle under 7.5 MPa static and 0.5 MPa dynamic load. During contraction, neighboring coils come into complete contact at 130 °C, which dramatically increases the nominal elastic modulus and causes the thermal expan- sion coefficient to become pos- itive. Optical micrographs(top) are shown of the coils before and after contact. Figure and text from Haines et al. (2014).

The polymer fiber twisted until it is coiled like an old phone cord. Heating the coiled fiber will cause a radial expansion in the polymer fiber, this untwists the fiber, pulling the coils closer, contracting the whole coiled polymer. See figure 2.7 from Haines et al. (2014).

Any coiled polymer fiber has this characteristic, but typically they are made from nylon 6/6 sewing thread or nylon 6 fishing line. This is because of their high melting point. Haines et al. (2014) showed 21% actuation with a coiled 127µm nylon 6/6 polymer fiber lifting at 20MPa² load and 9% actuation lifting at 50MPa load. They also had 12% actuation with a 500 g (8.44 MPa) load for a 860 µm nylon 6 muscle fiber actuated with 99 °C water.

2.8 Why chose coiled nylon 6 polymer fiber?

The characteristics of the various alternatives are difficult to compare directly. The artificial muscle is intended for a walking robot without electric motors. Pneumatic Artificial Muscles (PAMs), or the newer Vacuum- Actuated Muscle-inspired Pneumatic Structures (VAMPS) or Fluid-driven Origami-inspired Artificial Muscles (FOAMs), are discarded as they all need a motor to operate pumps or a compressor for actuation.

This leaves Electroactive Polymers (EAPs), Shape Memory Alloys (SMAs), Carbon Nanotube Actuators (CNAs), and coiled polymers. Dielectric EAPs need kilovolts electrical fields to actuate. Ionic EAPs need a gel or liquid to actuate. This makes them impractical to integrate into an outdoor robot.

SMAs have high hysteresis and low controllability and a limited stroke,

21 megapascal[MPa] is 1 newton[N] force over 1 square millimeter[mm²] area. Using a 127µm diameter fiber 20 MPa and 50M Pa corresponds to 26 grams and 65 grams weight lifted.

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typically 4.5%, (Mohd Jani et al., 2014), it is also susceptible to over- and under-loading, Sæther (2008).

Carbon nanotube actuators have excellent characteristics for tensile work but are costly to produce, and the best results are with the extra complexity of an inserted guest material that will thermally expand. All this make the cheap and available coiled polymer that actuate in the same way an attractive alternative. Nylon 6 polymer fishing line can be bought in any sporting goods store.

Nylon degrades over time when exposed to ultraviolet light, sunlight, reducing the breaking strength and the maximum strain, (Shamey &

Sinha, 2008). This effect is reduced in larger diameter fibers, (Thomas &

Hridayanathan, 2006). It also absorbs water and humidity, which increases its diameter, (Murthy, Stamm, Sibilia, & Krimm, 1989). Both factors could important when considering outdoor use.

Nylon 6 was chosen over Nylon 6/6, as the latter is not readily available as fishing line. Nylon 6/6 is commonly used for sewing thread with much lower diameter, less than 250-300 µm. The main difference between Nylon 6 and Nylon 6/6 is the number of hydrogen bonds, with nylon 6/6 having more bonds, resulting in about 40 °C higher melting temperature, (MatWeb, n.d.).

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Chapter 3

Nylon 6 Polymer Actuator

This chapter provides the background material for how the nylon 6 artificial muscle functions. First I will explain the characteristics of just twisted nylon polymer, then the twisted and coiled polymer. Then I will explain how to produce the muscle fiber, followed by what annealing, creep and training is.

After that comes the methods for heating and control of the finished muscle fiber. Lastly, the chapter is summarized.

Nylon is a synthetic polymer thermoplastic that can be that can be melt- processed into fibers, films or shapes, (Kohan, 1995). Nylon 6 was developed in 1938 as a response to the development of nylon 6/6 developed in 1935. A polymer is a long molecule consisting of many repeated subunits in chains, (Painter & Coleman, 1997).

3.1 Twisted nylon 6 polymer

When twisted, the precursor fiber (the nylon 6 fiber before it is coiled) actuates when heated. This actuation can be explained looking by several characteristics in nylon polymers:

• Nylon 6 polymers are anisotropic; the physical and mechanical properties are dependent on the direction in the material.

• When the polymer is heated, it expands in the radial direction, but contract in the length direction.

• If an anisotropic polymer is twisted, heating will untwist it. This can be understood by imagining two scenarios happening simultaneously to a twisted cylindrical body of fixed length with a groove cut in its surface. See figure 3.1 from Aziz (2017).

– 1. If the diameter of the body increases, the body will have to untwist for the path of the groove to have the same length.

– 2. If the body has a fixed diameter and one tries to shorten the length of the groove, the body will have to untwist.

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Figure 3.1: Heating causes radial expansion in the polymer, resulting in untwist, turning the paddle. Cooling has the reverse effect. Figure from Aziz (2017).

• Aziz (2017) have shown experimentally that the contraction of the polymer due to heating was negligible, scenario 2. So the change in the twist is solely based on the change in diameter in the nylon 6 polymer, scenario 1.

• Nylon 6 polymers are also viscoelastic.

• A viscous material, like honey, resist changes in stress linearly with time. Imagine trying to shake a box of honey.

• An elastic material strained when stressed quickly returns to its original form when the stress is removed. For example a rubber band.

• Viscoelastic materials are somewhere in the middle, and have the following characteristics:

– hysteresis: the curve for strain is different for an increase in stress versus a decrease in stress.

– stress relaxation: when under a constant strain, the stress decreases as a function of time. Seea)figure 3.2 from Wikimedia Commons (2009b)

– creep: semi- or permanent deformation when under a constant stress over time. Increases with heat. If the stress is only applied for a short period the material will return to nearly it’s pre- deformed state. See b) figure 3.2 from Wikimedia Commons (2009a)

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(a) Stress relaxation (b) Creep

Figure 3.2: a 1) Applied strain(), a 2) induced stress(σ) as functions of time for a viscoelastic material. Material stress relaxation due to constant strain. b 3) applied stress(σ) andb 4)induced strain() as functions of time over a short period for a viscoelastic material. Figure a) from Wikimedia Commons (2009b) and b) from Wikimedia Commons (2009a)

3.2 Coiled nylon 6 polymer, muscle fiber

The muscle fiber(twisted and coiled nylon 6 polymer), is created by twisting the precursor fiber until it starts coiling to reduce its internal stress, see figure 3.4 from Grushetzky, Matiunin, and Shamanov (2015). If the coiling is allowed to happen by itself when twisting, auto-coiling, the chirality (direction of twist) of the coil will be the same as the chirality of the twist.

The coils in a muscle fiber will get pulled together when the fiber is heated.

This can be explained by the untwisting in the precursor fiber. Figure 3.3 -a show how the untwisting of the precursor fiber from heating contracts the coils and bshow how cooling will have the reverse effect.

Haines et al. (2016) defined a scale-independent variable C, coil index, C = D/d. The average of coil diameter, D seen in figure 3.3 a), divided by the precursor fiber diameter, d. Lower coil index, the difference in the diameter of the precursor fiber and the coils of the coiled muscle fiber, give larger actuation forces but shorter contractile stroke. When the coils are big compared to the diameter of the precursor fiber, the forces are lower, but the contraction is higher. The muscle fiber cannot contract beyond the point where the coils start touching each other; there has to be a distance between each coil.

The contraction of the muscle fiber when coiled can be mathematically shown by either coiled spring theory or knot theory.

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Figure 3.3: a) Precursor fiber untwist results in pulling the coils together. b) Precursor twist results in moving coils apart. D in figurea)is the average diameter of the coils and d is the diameter of the precursor fiber. α_c is the bias angle.

Figure 3.4: Insert enough twist and the fiber will buckle and start coiling. Figure from Grushetzky, Matiunin, and Shamanov (2015)

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3.2.1 Coiled Spring Theory

Using Augustus Love’s mathematical theory of elasticity, (Love, 1906)

∆L=l²∆T/N

Where ∆L is the change in the muscle fiber length, the contraction, l is the length of the precursor fiber, ∆T is the change in precursor fiber twist, and N is the number of coils.

3.2.2 Knot Theory

• Linking number is defined as the sum of two numbers, twist, and writhe.

– Twists are net rotations of the ends of a stretched out precursor fiber.

– Writhes are the number of coils in one cross-section of the fiber.

This number is less than 1.0 as the coil always extends out of the cross section plane. Otherwise, it would be a circle and not a coil.

Represented as 1−sin(αc. α is shown in figure 3.3b).

• The linking number is always constant.

• When the ends of the fiber are prevented from rotating, a change in twist will change the number of writhes and opposite.

3.2.3 Direction of twist and chirality

When the twisting is stopped right when the precursor fiber starts auto- coiling and instead is manually coiled the opposite way, the muscle fiber will actuate positively, elongating, instead of negatively, contracting. The chirality becomes opposite for the twist and the coils. Heat will still cause the precursor fiber to untwist, but the untwisting seen in figure 3.3 now moves the coils further apart instead of pulling them together, changing the direction of the actuation. The muscle fiber now elongates instead of contract when heated. This is a sophisticated production problem compared to the auto-coiling since the twisted precursor fiber needs to be twisted around something, a mandrel, for it to work.

3.3 Production

A complete muscle fiber consist of a fishing line twisted and then coiled to a uniform spiral and keeps that form, see figure 3.5 for a computer render.

Several considerations need to be considered for the production:

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Figure 3.5: Computer render of fishing line untwisted, twisted and then coiled. A red line is added to side of the fishing line to visualize the twist.

• While inserting twist the fishing line shortens.

• When it reaches a critical amount of twist it will start coiling, forming a spiral looking much like an old phone cord. Figure 3.4 from Grushetzky et al. (2015).

– This happens because the material will try to minimize the internal tension in itself.

• To keep the fishing line from snarling while it coils a constant force has to be applied.

– Snarling is when the line tangles with itself and pulls itself together into a knot.

• If the constant force is too great the line will break during the coiling.

• The range between snarling and breaking is fairly narrow.

• The tension caused by the constant force decides how tight the coils become, coil index, which affects the characteristics of the muscle fiber.

• When the ends of the fiber no longer are kept from rotating, the fiber will do some untwisting, releasing some of the spring tension caused by the inserted twists, but stays in its coiled form.

• To keep more of the twist, the fiber is heat-treated, annealed, before it is released.

– This annealing process also affects the ’training’ needed for the muscle fiber to have a reproducible and reversible stroke.

– Training is repeated cycles of heating/cooling causing the muscle fiber to have a repeatable stroke.

– Repeatable stroke is when muscle contractions are identical every time the muscle is heated to the same temperature.

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3.4 Annealing

The untwist caused by heating exceeds the re-twisting from cooling for the fibers first cycles. Elasto-plastic mechanisms in semicrystalline polymer fibers cause much of the inserted twist to be lost due to elastic recovery unless the twisted fiber is thermally annealed while maintaining an external torque, (Aziz, 2017). The heat at which the fiber is annealed affects the repeatability of the muscle stroke, where higher annealing temperatures cause higher repeatability. This is shown in figure 3.6 from Aziz (2017).

Figure 3.6: Torsional stroke test for 70 mm long twisted nylon 6 fibers samples prepared at different annealing tem- peratures(legend). Stroke here is measured in how many degrees the muscle fibers rotated a wheel with a constant torque applied, torsional. All the samples start at 0 degrees. Figure from Aziz (2017).

If not kept under tension while annealing, the fiber will suffer irreversible molecular reorganization and shrink. Since the fiber is twisted, this will promote large fiber untwisting at first heating. Tsujimoto, Kurokawa, Takahashi, and Sakurai (1979) suggest increased molecular packing in amorphous regions causing a stable molecular structure to form when subjected to higher temperature and constant tension.

3.5 Tensile Creep

Tensile creep is the tendency for a material to deform over time when under constant stress. It increases with temperature. Tensile creep is also called cold flow. Since nylon 6 is a viscoelastic material, it suffers tensile creep (Catsiff, Alfrey, & O’Shaughnessy, 1953).

• The nylon returns towards pre-stressed strain gradually over time.

• The creep increases with stress.

• The creep increases with heat.

• The creep can possibly be decreased with high-temperature annealing.

According to Aziz (2017) the effect of applied force on actuation also coincides with tensile creep in the polymer but scaled to the lower temperatures (room temperature). High-temperature annealing is supposed to decelerate this creep. See figure 3.7 from Aziz (2017).

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Figure 3.7: Time based tensile creep in fiber annealed at 200 °C with different torque applied at room temperature, 26 °C. Figure from Aziz (2017).

3.6 Training

The muscle fiber also has to be ’trained’ to have a reproducible stroke for a given load by cycling it a few times. A cycle is heating and cooling the muscle. This will have to be redone whenever the load is changed.

Aziz (2017) found that higher external torque during the first cycle caused diminished untwisting during heating and increased the re-twisting during cooling, up to and over the original twist. See figure 3.8 from Aziz (2017).

Figure 3.8: Torsional stroke test for 70 mm long twis- ted nylon 6 fibers samples an- nealed at 200 °C with dif- ferent torque applied. The samples started at 0 degrees.

Stroke here is measured in how many degrees the muscle fibers rotated a wheel with a con- stant torque(legend) applied, torsional. Figure from Aziz (2017).

For Aziz (2017), two heating/cooling cycles were needed at a fixed torque before reversible untwist/twist was achieved. Applying a lower torque after three cycles showed an 80-95% reversibility with some unrecoverable twist.

The fiber could then be trained to be reversible to the new torque with two new heating/cooling cycles.

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3.7 Heating

To actuate the muscle a heat source is needed. The temperature ranges used is from ambient temperature to the melting point of the nylon 6, 210-220°C, (MatWeb, n.d.) Some literature, (Haines et al., 2014), (Arjun, Saharan, &

Tadesse, 2016) used water for heating, allowing it to run over the muscle. Wu et al. (2015) created a flexible silicon tube that could be quickly filled and emptied of water from reservoirs with a pump. Cherubini, Moretti, Vertechy, and Fontana (2015) used a closed chamber with a heater, basically an oven.

Mirvakili et al. (2014) used muscles coated with silver paint and applied voltage for Joule heating. Yip and Niemeyer (2015), Masuya, Ono, Takagi, and Tahara (2017), Luong, Seo, Koo, Choi, and Moon (2017) and Abbas and Zhao (2017) used Joule heated conductive nylon 6/6 sewing thread for their experiments. Haines et al. (2014) and Semochkin (2016) also looked at copper wire wrapped around the muscle for Joule heating. And van der Weijde, Smit, Fritschi, van de Kamp, and Vallery (2016) used resistance wire for Joule heating.

3.8 Control

An open-loop control scheme is when a stimulus is applied, and the result is not used to adjust the next stimuli, using models or prior knowledge to estimate what the results will be. A closed-loop control scheme takes the result of inputting a stimulus and feeds it back to the controller, making it able to regulate based on the output, for example stepping on the gas pedal in a car and watching the speedometer to regulate the speed.

Yip and Niemeyer (2015) and Wu et al. (2015) both use closed-loop control of a robotic hand actuating with nylon polymer artificial muscles. The first used conductive sewing thread and also fans to cool the muscle.

They both implemented a proportional integrating regulator where the first could produce controlled forces in under 30 ms, exceeding human muscle performance. The second used silicon tube that could be filled with hot or cold water to control a robotic finger. It could actuate the fingertip of an index finger to an angle of 41 degrees flexed in less than two seconds.

3.9 Summary

The inherent properties of nylon 6 polymer, precursor fiber allow it to actuate when twisted, based on radial thermal expansion, and actuate more when twisted until it auto-coils explained either with coiled spring theory or knot theory.

Production of the coiled nylon 6 polymer muscle fiber is done with a constant load that has to be within a narrow range. Too much and it breaks, or too little and it will snarl, tangling with itself. The load decides the diameter of

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the coils, lower load gives larger coils, and the ratio of the diameter of the coil and the precursor fiber gives a scale-invariant coil index. The muscle fiber can be produced to elongate instead of contract by hand coiling in the opposite direction, but this increases the production complexity.

To keep the coiled form and spring tensions from inserted twist the muscle fiber is heat treated, annealed. The muscle fiber suffers from tensile creep when heated and under stress and needs to be trained to each new load.

There are several methods to heat the muscle fiber for actuation: hot water, hot air, painting the muscle with silver paint or wrapping it in metal wire for Joule heating with electricity. Fast, controllable responses have been shown in the literature for joule heating and water.

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Chapter 4

Robot

This chapter provides the background for the two robots built. First the different gaits, then a short bit on kinematics.

Robotics is a broad field of study, and only the basics will be mentioned here. For a legged robot, one of the essential parts of its function is the process of locomotion, the gait. Another important aspect is the resulting motion of actuation individual muscles, the kinematics.

4.1 Gait

The way an animal or robot walks is called a gait. In a robot, this would be the control algorithm it uses to walk. There are generally two ways to describe the stability of a robot gait. Statically stable or dynamically stable. A statically stable robot is stable at all times, keeping its center of mass between the legs touching the ground. It does not matter if it is interrupted mid-step or how fast it moves; it will not fall over. A typical statically stable gait would be a four-legged robot moving only one leg at a time, slowly creeping forward.

Dynamically stable gaits only work at a given speed, and dynamically stable robots use "controlled falling", the fact that it takes time to fall to its advantage. A trot, having several legs in the air at the same time (more than half), is dynamically stable. The robot would fall over if not for momentum and "catching" itself before falling. This is faster and more energy-efficient than the statically stable creeping gait, but much more complex, requiring balancing and timing.

4.1.1 Gait for multi-legged robot

Multi-legged robots are often more stable but require more actuators which can require a more complex control system. Stability is a trade-off of how many legs touching the ground for stability and how many legs moving at

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the same time, increasing velocity. To move the center of mass needs to be shifted forwards while moving some the legs forward at the same time.

For a statically stable gait, the center of mass is always kept inside the area created by the legs that touch the ground.

4.1.2 Bipedal gait

Bipedal gait is intuitive for human as it is how we walk, most of the time. It requires balance and often a different joint configuration than multi-legged gait. Humans also use our flexible spines to shift the upper body to move the center of mass. We have a sophisticated balance control in our inner ear making us stable on one leg and mostly use a dynamical stable gait.

This quickly becomes rather complex in a robot. Making a bipedal robot statically stable is easiest by cheating, giving it a tether to keep it stable standing on one leg. This can be done with a wagon with wheels, harnesses, or a swivel with a long rod. To move forward lift one leg while the tether keeps the other leg stable as it takes the weight. Swinging or lifting the leg forward and shifting the center of mass forwards before placing the leg down further ahead. Repeat for the other leg. See figure 4.1 for an example gait for tethered bipedal gait.

1 2 3 4 5 6 7 8

5 6 7 8 1 2 3 4

0 1 1 1 0 0 0 0

1 1 0 0 0 0 0 1

0 0 0 0 0 1 1 1

0 0 0 1 1 1 0 0

Left hip Left knee

Right hip Right knee

Figure 4.1: Example of a tethered statically stable reversed-knee bipedal gait. This robot has two joints per leg. The zero or one indicates if the actuator in the joint is lifting or not. The legs walks right-to-left.

4.2 Forces

When using linear actuators on revolving joints, it becomes a lever arm system. There is a trade-off between forces and maximum movement. Higher torque, force acting perpendicular on the lever arm, gives a shorter but more powerful movement, lower torque give a longer but weaker movement. The lever arm is the distance between the applied linear actuator force and the joint. The torque is calculated with the cross product of the force vector and the distance vector. τ[Nm] = Force[N] x lever arm [m].

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4.3 Kinematics

The branch of mechanics called kinematics - the motion of objects without reference to the forces or masses of the objects is how one describes the movement of a multi-jointed system.

The Denavit–Hartenberg parameters are a popular and common convention for the parameters of robot manipulator configurations and choices of reference frames. From Spong, Hutchinson, and Vidyasagar (2006): In this convention each homogeneous transformation, A_iis represented as a product of four basic transformations

Ai =Rotz,θiT ransz,θiT ransx,aiRotx,αi

The parameters are generally given the names, a_i link length, α_i link twist, di link offset and θi joint angle. The i is the associated link or joint.

4.4 Summary

A robots gait, how it walks, can either be statically or dynamically stable.

The first means it is always stable, even if it stops mid-step because the center of gravity is within the area of the legs touching the ground. The second means it needs to keep moving to stay stable, as it will fall over if it stops mid-step. Statically stable robots are easier to implement, and a multi-legged robot is easier to make statically stable. A bipedal robot can be made less complex and statically stable with cheating by adding external stabilization.

When using linear actuators to actuate revolving joints, there is a trade-off between the force and maximum movement. This is decided by where the linear actuator is connected. Finding the movement of a robot appendix based on the changes at the joints is called kinematics, and a way to standardize the parameters of these calculations is the Denavit–Hartenberg convention.

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Chapter 5

Tools & Engineering

In this chapter, I will first explain the setup for production and measurement of the muscles and mention all the tools, rigs, sensors and materials used.

Then I will go through the construction and materials for the robots. If this chapter does not answer why something is chosen it will be explained/argued for later in the implementation and discussion chapters.

For producing muscle fibers with similar characteristics, a manufacturing rig was constructed. Also, to measure the characteristics of the muscle fibers a sensor rig was made. To test the muscles in action and to show the feasibility of use - two robots were built.

5.1 Muscle Materials

The muscle fibers consist of coiled Nylon 6 fishing line with resistance wire wound around it with metal eyelet terminals crimped onto each end, see figure 5.1.

Nylon 6 fishing line: 300 m Sufix Super 21, 0.50mm/22.6 kg, #10, Line class: 30 lb.

Resistance wire: Block RD 100/0.1, 62.400 W/m. Made of Cu-Ni 44.

Current intensity for wire temperature (100 °C) 0.237A, (Block, n.d.)

Figure 5.1: A finished muscle fiber with a marker denoting it as the tenth test muscle.

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5.2 Manufacturing rig - Muscle production

The muscle fibers are not available commercially. They have to be produced by coiling the precursor fiber, the fishing line, and winding resistance wire around it. To this end, a manufacturing rig was made. The specifications for such a rig was as follows:

• Be able to controllably rotate one end of the precursor fiber while applying a constant force.

• Apply the resistance wire evenly around the muscle fiber or the precursor fiber.

• Either be able to anneal while on the rig, or clamp and detach the muscles without letting them unspin for annealing externally (for example an oven or another rig).

• Be large enough to create muscle fibers of desired lengths.

• Easy to use. Preferably no screws, bolts or tools needed to attach or detach the muscle fibers.

Stepper motor

Box for weights Rod for stabilisation

Fishing line

Spool of resistor wire Resistor wire

Figure 5.2: Concept drawing of produc- tion rig.

The muscle production rig, based on figure 5.2, was made out of 3x3 cm T-slotted aluminum extrusions as the base material and corner steel where the stiffness was not imperat- ive. A stepper motor was attached at the top. It was made to be 1 m high, giving a maximum precursor fiber of 80 cm. In the article from Cherubini et al. (2015) their finished muscle fiber is 4.4 times smaller than the precursor fiber, which would give 18.2 cm finished muscles before annealing. The rig used gravity and weights in a box as the constant force while twisting/coiling. The couplings between the stepper-to-muscle and muscle- to-weight were done with paperclips for ease of use. The resistance wire could be taped to the paperclips at both couplings.

Stiff steel wire, attached at the

muscle-to-weight coupling, resting against the vertical t-slot extrusion kept the weight box from spinning around during production. A mount for the resistance wire spool was attached to the end of one of the feet. Crocodile clips could be attached to the ends of the muscle for annealing. An

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adjustable platform was added to catch the weight box if the muscle was broken during annealing. See figure 5.3 for the rig in annealing mode.

Figure 5.3: Rig for muscle production in annealing mode.

The controller for the production rig was done on a breadboard with a program written for a microcontroller (Arduino Nano v3) , see appendix A.2 for the code and figure 5.4 for schematics. The controller could spin the motor 240 turns per minute with a motor driver and display the number of rotations on an LCD screen, and also control heating cycles when annealing by switching an N-channel MOSFET transistor (IRF1404) connected to a separate power supply. The annealing was timer controlled in the microcontroller. Two buttons for switching which mode and starting/stopping. Figure 5.5 shows the breadboard.

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Figure 5.4: Schematic for production controller breadboard.

Figure 5.5: Breadboard controller for the production rig. Arduino Nano v3 in top right, LCD driver in bottom right, MOSFET in top left, motor driver in bottom left.

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5.3 Sensors - Data collection

To answer the questions asked in the introduction data needs to be collected for a series of experiments. To this end, a sensor rig needed to be made to measure and record the length of the muscle over time when heated. The specifications were as follows:

• Measure and record the length of the muscle automatically.

• Measure and record ambient(room) temperature.

• Measure current in and voltage over the muscle fiber.

• Synchronizing recorded data.

• Adjustable load.

• Power control.

Figure 5.6: Data collection sensors and rig.

The rig uses gravity and weights in a hanging box for adjustable load and automatic measurement of length with a webcam and image processing.

The muscle fiber is mounted horizontally and is connected to the load over a pulley with a ball bearing. This was done for a more practical and smaller form factor so as it could be used on a regular office desk. The rig used 3x3 cm t-slotted aluminum extrusions to hold the camera, the pulley ball bearing joint and the tether for the muscle. Electrical crocodile clips were used to hold the muscle and to connect to the power supply unit (HP/Agilent E3630A). One clip is the tether for the muscle to the rig and the other moves with the length of the muscle.

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A rectangular marker made of black foam was attached to the moving end crocodile clip. This clip also had a plastic tap resting in the slot of the t-slot to keep it from rotating. A miniature ball bearing was used at the interface between the t-slotted aluminum and the plastic tip to minimize the friction.

Masking tape was used to remove any other large black areas that could be picked up by the imaging processing, like the reflective part of the metal crocodile clip. The pulley was drawn in Solidworks, and 3D printed on an Ultimaker3+ in ABS plastic with a slot for a metal ball bearing (10mm diameter) on a metal rod axle (5mm diameter). An A4 size sheet of paper with millimeter rulers was printed and glued to a laser cut background plate for sanity checks, checking that the measurements make sense compared to reality, and the possibility of manual readings.

Figure 5.7: picture from webcam

A Microsoft® LifeCam Studio(TM) webcam was connected to a computer using a program, see appendix A.1 for code, written in MATHWORKS Matlab RB2017b to measure distances. The webcam was mounted statically to the rig and connect to a computer with a USB cable. Functionality from the Image Processing Toolbox in Matlab was used to segment out a black area in an otherwise light image using a threshold function. Figure 5.7 shows what the webcam saw. The x/y coordinates of the centroid of the black rectangle are found in pixels, in each frame with a frame rate of 0.5 frames per seconds. The centroid is then converted to cm based on pre-measured scale and offset.

To actuate the muscle a microcontroller (Arduino Mega 2560) was used to switch an N-channel MOSFET transistor (IRF1404) which could turn on/off the voltage from the power supply. This also gave the possibility of regulating the voltage with PWM signals. The Arduino is directly controlled from the computer using the same program as the webcam in Matlab with the Arduino Support for Matlab package from MATHWORKS. To measure the current in and voltage over the resistance wire, a voltage, and a current sensing circuit were made and connected to the analog-to-digital converters (ADC) of the Arduino. See figure 5.8 a). It uses a voltage divider to

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get the voltage over the muscle into the range of the ADC, 5V. Also, an instrumental amplifier (AD623) to measure the voltage over a 50 mWshunt resistance with one of the other ADCs on the Arduino. The measurements were collected with the program in Matlab. Figure 5.8 b) is the finished circuit soldered to a breakout PCB. An HYT 271 humidity and temperature sensor was connected through the I2C bus on the Arduino to record the room temperature and humidity.

A Fluke 89IV digital multimeter was used to measure resistances of the muscles before and after experiments. A digital caliper was used to measure the lengths of the muscles when not attached to the rig. A FLIR E8 thermal imaging camera was used to measure temperatures. The thermal camera had some problems showing precise temperatures because of the small diameter of the muscle fiber and the resolution of the IR sensor, 320 × 240 pixels, it would give results fluctuating by +-2°C.

Figure 5.8: a) Schematic for voltage control and current sense PCB. b) Voltage control and current sense soldered to breakout PCB.

5.4 Robot construction

Solidworks 2015 x64 was used for 3D modeling of the parts that needed to be 3D printed, and for animating the robot gaits.

Figure 5.9: a)Six legged robot CAD.b) Bipedal robot CAD.

The six-legged robot, figure 5.9 a), was printed on a Objet Connex500

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printer with the VeroClear material, which has a density of 1.18-1.19 g/cm³. The joints were attached with bolts and hex nuts. The controller for the gait was programmed in a microcontroller (Arduino Nano v3), code shown in appendix A.3, placed on a mini breadboard controlling six N-channel MOSFETs (IRF510) connected to a power supply (HP/Agilent E631A).

The breadboard was taped with double sided tape to the "head/front" of the robot.

The legs of the bipedal robot, figure 5.9b), were made of a 5mm lightweight wooden composite. The legs were laser cut from wood on a laser cutter.

Since the wood was too thin for the hip part, which needed the axle through it, the hip was 3D printed on a Ultimaker3+ in ABS plastic, as was the wagon and wheels. The joints were made with ball bearings (10mm outer diameter, 5mm inner) glued into holes cut into the legs. An axle (5mm diameter stainless steel rod) was glued into the ball bearing and into the connecting part of the joint.

The "tail" going to the wagon was made with 40 cm hollow aluminum rod (10mm diameter) bent at 45°. It was fastened in a hole in the 3D printed hip with an adjustable screw and glued in a hole in the wagon. The wagon had a ball bearing with a 42 cm steel axle going through it. The axle had 6 cm diameter wheels, one in each end. A plastic box (10 cm x 8 cm x 5cm) was attached to the wagon and held another 45 cm aluminum rod pointing out backward. This rod was for the counterweights (M18 hex nuts). The box also held the power connector for the robot to be connected to the power supply (HP/Agilent E3630A) and a battery connector. The controller was put on the hip and consisted of a mini breadboard with a microcontroller(Arduino Nano v3) programmed, see appendix A.4 for code, to switch four N-channel MOSFETs (IRF510) based on the gait chosen.

The silicon rubber bands used on the bipedal robot were made from Wacker Elastosil RT 601 A/B and the mold was ABS plastic printed with a Fortus 250c 3D printer and cured in the Fortus in standby mode (70 °C) for faster curing. Bolts and nuts were used to connect both the rubber bands and the muscle fibers. To increase friction, pads of Neoprene rubber was added to the soles of the feet.

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Chapter 6

Muscle Implementation

This chapter is about the muscle fibers produced for the robots and experiments. First the choice of fiber diameter and its effect of and on the loads which can be used. Then the choice of heating/control and how the muscles were produced and how they turned out.

6.1 Choice of precursor fiber

To reduce the complexity of the prototype robot actuators, using only a single muscle fiber for each actuator was desired. Thinner polymers become challenging to work with, as they are hard to see and inspect with the naked eye.

Haines et al. (2014) measure the load the muscle can lift with nominal stress measured in megapascal (MPa)¹. This is force divided by the area, normalizing the force to the square of the radius times pi. See equations (6.1), (6.2), (6.3), (6.4) :

1M P a=1 N

mm² (6.1)

Nominal Stress= ^{gramf orce}

Area (6.2)

diameter precursor fiber=d (6.3) Area fishing line=π(d/2)² (6.4) The relationship between the diameter of the fiber and the load 1 MPa represents is quadratic. A 0.5mm diameter precursor fiber lifts 20 gram at 1 MPa stress, equation (6.5), while a 0.25mm precursor fiber lifts a 5 gram at 1 MPa stress, equation (6.6).

1The benefits of using MPa is scale invariance. When choosing to use just one diameter of precursor fiber, as this thesis does, it is more intuitive to use the weight of the load in Newton or grams.

Thesis submitted for the degree of Master in Robotics and Intelligent systems

Walking robot with artificial muscles made of fishing line

Exploration of a nylon 6 polymer actuator

Knut Løklingholm

Thesis submitted for the degree of Master in Robotics and Intelligent systems

60 credits

Department of Informatics

Faculty of mathematics and natural sciences

UNIVERSITY OF OSLO

Walking robot with artificial muscles made of fishing line

Exploration of a nylon 6 polymer actuator

Knut Løklingholm

Abstract

Acknowledgments

Contents

List of Figures

List of Tables

Glossary

Chapter 1

Introduction

1.1 Motivation

1.2 Goal

1.3 Implementation

1.4 Breakdown of the chapters

Chapter 2

Artificial Muscles

2.1 Pneumatic artificial muscles

2.2 Vacuum-actuated muscle-inspired pneumatic structures

2.3 Fluid-driven origami-inspired artificial muscles

2.4 Electroactive polymers

2.5 Shape memory alloy

2.6 Carbon nanotube actuator

2.7 Coiled polymer fibre/yarns

2.8 Why chose coiled nylon 6 polymer fiber?

Chapter 3

Nylon 6 Polymer Actuator

3.1 Twisted nylon 6 polymer

3.2 Coiled nylon 6 polymer, muscle fiber

3.3 Production

3.4 Annealing

3.5 Tensile Creep

3.6 Training

3.7 Heating

3.8 Control

3.9 Summary

Chapter 4

Robot

4.1 Gait

4.2 Forces

4.3 Kinematics

4.4 Summary

Chapter 5

Tools & Engineering

5.1 Muscle Materials

5.2 Manufacturing rig - Muscle production

5.3 Sensors - Data collection

5.4 Robot construction

Chapter 6

Muscle Implementation

6.1 Choice of precursor fiber