viability study of nylon-12 carbon fiber filaments for use
TRANSCRIPT
APPROVED: Vijay Vaidyanathan, Major Professor and
Chair of Department of Biomedical Engineering
Melanie Ecker, Committee Member Yong Yang, Committee Member Hanchen Huang, Dean of the College of
Engineering Victor Prybutok, Dean of Toulouse Graduate
School
VIABILITY STUDY OF NYLON-12 CARBON FIBER FILAMENTS FOR USE IN THE CONSTRUCTION
OF A POWERED LOWER BODY EXOSKELETON VIA FUSED DEPOSITION
MODELING BY MEANS OF COMPUTER SIMULATION
Michael Andrew Lown Joiner
Thesis Prepared for the Degree of
MASTER OF SCIENCE
UNIVERSITY OF NORTH TEXAS
May 2021
Joiner, Michael Andrew Lown. Viability Study of Nylon-12 Carbon Fiber Filaments for Use
in the Construction of a Powered Lower Body Exoskeleton via Fused Deposition Modeling by
Means of Computer Simulation. Master of Science (Biomedical Engineering), May 2021, 79 pp.,
14 tables, 31 figures, 2 appendices, 52 numbered references.
Members of the elderly population is disproportionately prone to experiencing mobility
impairment due to their aging bodies and as a result have frail bodies that are at a higher risk of
grave injury due to falling. In order to combat this assistive mobility devices such as
exoskeletons have been developed to help patients enhance their range of motion. With
additive manufacturing techniques, such as fused deposition modeling (FDM), becoming a more
mainstream form of design, the inclusion of lightweight polymers such as nylon 12 as primary
construction materials for these devices has increased. In this thesis computer aided design
(CAD) software was used to design a prototype lower body exoskeleton and simulation
software was used to give the device the characteristics of Stratasys’ nylon 12 carbon fiber FDM
material to verify it if could be used as the primary construction material for this device when
extruded from a FDM printer on either the XZ or ZX printing plane. From the simulations it was
found that the material printed along the XZ plane could create a device that could withstand
the weight of an average elderly male patient (200 lbs.) as well as the 35 lbs. of force applied to
the device by a linear actuation motor that would be used to extend and contract the
exoskeleton leg.
ii
Copyright 2021
by
Michael Andrew Lown Joiner
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ACKNOWLEDGEMENTS
I would like to give thanks to Dr. Vijay Vaidyanathan for being my committee chair and
for all of his unwavering support and guidance that he has shown me throughout both my
undergraduate and graduate careers. Thank you to Dr. Melanie Ecker for helping to guide me
through my research on polymeric materials and additive manufacturing as well as for her
support in my graduate studies. Thank you to Dr. Yong Yang for his support and insight
throughout this thesis’s process.
Additionally, I would like to thank Edward Gates for believing in my abilities and allowing
me to join him in his research and development of a novel lightweight lower body exoskeleton.
Lastly, I would like to thank my friends and family for all of the support and love they have
shown me throughout this process and my academic career.
If not for these individuals, along with many others, I would not be where I am today.
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TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ................................................................................................................... iii LIST OF TABLES ................................................................................................................................ vi LIST OF FIGURES ............................................................................................................................. vii CHAPTER 1. INTRODUCTION ........................................................................................................... 1 CHAPTER 2. BACKGROUND ............................................................................................................. 2
2.1 Mobility Impairment within the Elderly ................................................................. 2
2.2 Fused Deposition Modeling .................................................................................... 3
2.3 Nylon-12 Carbon Fiber Filaments ........................................................................... 6
2.4 Medical Exoskeletons ............................................................................................. 8 CHAPTER 3. DEVICE DESIGNS AND METHODOLOGIES ................................................................. 11
3.1 Design of Exoskeleton ........................................................................................... 11
3.1.1 Design of the Thigh ................................................................................... 12
3.1.2 Design of the Ankle ................................................................................... 13
3.1.3 Design of the Knee .................................................................................... 14
3.2 Force Modeling Simulations ................................................................................. 18
3.2.1 Force Modeling of the Thigh ..................................................................... 19
3.2.2 Force Modeling of the Ankle..................................................................... 21
3.2.3 Force Modeling of the Knee...................................................................... 23
3.2.4 Force Modeling of Leg .............................................................................. 28 CHAPTER 4. RESULTS AND DISCUSSION........................................................................................ 36
4.1 Simulation Results ................................................................................................. 36
4.1.1 Thigh Simulation Results ........................................................................... 36
4.1.2 Ankle Simulation Results ........................................................................... 38
4.1.3 Knee Simulation Results ............................................................................ 40
4.1.4 Leg Simulation Results .............................................................................. 46
4.2 Discussion.............................................................................................................. 52
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CHAPTER 5. CONCLUSIONS ........................................................................................................... 55
5.1 Experimental Conclusion ...................................................................................... 55
5.2 Future Avenues of Research ................................................................................. 56 APPENDIX A. ENGINEERING DRAWINGS OF EXOSKELETON ......................................................... 58 APPENDIX B. MATERIAL DATA SHEETS ......................................................................................... 69 REFERENCES .................................................................................................................................. 75
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LIST OF TABLES
Page
Table 3.1: Force Elements Applied to Exoskeleton ...................................................................... 19
Table 3.2: Mechanical Properties of Simulated Materials............................................................ 19
Table 3.3: Thigh Simulation Parameters ....................................................................................... 21
Table 3.4Ankle Simulation Parameters ......................................................................................... 22
Table 3.5: PO Knee Simulation Parameters .................................................................................. 25
Table 3.6: PC Knee Simulation Parameters................................................................................... 27
Table 3.7: PO Leg Simulation Parameters ..................................................................................... 29
Table 3.8: PC Leg Simulation Parameters ..................................................................................... 33
Table 4.1: Cumulative Results of Thigh Static Force Simulations ................................................. 38
Table 4.2: Cumulative Results of Ankle Static Force Simulations ................................................. 38
Table 4.3: Cumulative Results of PO Knee Static Force Simulations ............................................ 43
Table 4.4: Cumulative Results of PC Knee Static Force Simulations ............................................. 46
Table 4.5: Cumulative Results of PO Leg Static Force Simulations ............................................... 49
Table 4.6: Cumulative Results of PC Leg Static Force Simulations ............................................... 49
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LIST OF FIGURES
Page
Figure 2.1: FDM Simplification ........................................................................................................ 4
Figure 2.2: Layer Orientation and Force Direction ......................................................................... 5
Figure 2.3: Lattice Structures .......................................................................................................... 6
Figure 2.4: Molecular Structure of Nylon ....................................................................................... 7
Figure 2.5: Lightweight Hip Exoskeleton that Utilizes Compact Actuators for Movement.[Images courtesy of Giovacchini et al. [45]] ............................................................................................... 10
Figure 3.1: Full Exoskeleton Leg with Posterior Offset Knee ........................................................ 11
Figure 3.2: Full Exoskeleton Leg with Polycentric Knee ................................................................ 12
Figure 3.3: Exoskeleton Thigh ....................................................................................................... 13
Figure 3.4: Exoskeleton Ankle ....................................................................................................... 14
Figure 3.5: Posterior Offset Knee .................................................................................................. 16
Figure 3.6: Polycentric Knee ......................................................................................................... 17
Figure 4.1: Stress Plots for N12CF Thigh Printed on XZ Axis (A)Front view, (B) Right view, (C) Rear view, (D) Left view......................................................................................................................... 37
Figure 4.2: Stress Plots for N12CF Thigh Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left side view ......................................................................................................... 37
Figure 4.3: Stress Plots for N12CF Ankle Printed on XZ Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 39
Figure 4.4: Ankle Stress Results for ZX Printing Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view .................................................................................................................................. 39
Figure 4.5: Stress Plot for Ti6Al4V PO Knee with N12CF Thigh and Shin Printed on XZ Axis ....... 41
Figure 4.6: Stress Plot for Ti6Al4V PO Knee with N12CF Thigh and Shin Printed on ZX Axis ....... 41
Figure 4.7: Stress Plots for N12CF PO Knee Printed on XZ Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 42
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Figure 4.8: Stress Plot for N12CF PO Knee Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 42
Figure 4.9: Stress Plot for Ti6Al4V PC Knee with N12CF Thigh and Shin Printed on XZ Axis ........ 44
Figure 4.10: Stress Plot for Ti6Al4V PC Knee with N12CF Thigh and Shin Printed on ZX Axis ...... 44
Figure 4.11: PC Knee all parts N12CF printed on XZ axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view......................................................................................................................... 45
Figure 4.12: PC Knee all parts N12CF printed on ZX axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view......................................................................................................................... 45
Figure 4.13: Stress Plot for N12CF PO Leg printed on XZ Axis with Ti6Al4V Knee (A) Front view, (B), Right view, (C) Rear view, (D) Left view ................................................................................. 47
Figure 4.14: Stress Plot for N12CF PO Leg printed on ZX Axis with Ti6Al4V Knee (A) Front view, (B), Right view, (C) Rear view, (D) Left view ................................................................................. 47
Figure 4.15: Stress Plot for N12CF PO Leg printed on XZ axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 48
Figure 4.16: Stress Plot for N12CF PO Leg Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 48
Figure 4.17: Stress Plot for N12CF PC Leg Printed on XZ Axis with Ti6Al4V Knee (A) Front view, (B), Right view, (C) Left view ......................................................................................................... 50
Figure 4.18: Stress Plot for N12CF PC Leg printed on ZX Printing Axis with Ti6Al4V Knee (A) Front view, (B), Right view, (C) Left side view .............................................................................. 50
Figure 4.19: Stress Plot for N12CF PC Leg printed on XZ Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 51
Figure 4.20: Stress Plot for N12CF PC Leg Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear view, (D) Left view ................................................................................................................ 51
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CHAPTER 1
INTRODUCTION
The purpose of this thesis is to determine the viability of Nylon-12 carbon fiber
filaments as a material to be used in the construction of a novel powered lower limb
exoskeleton prototype. To conduct this study, SolidWorks by Dassault Systèmes was utilized for
both the design and static force simulations. The material chosen to be simulated for this
project was Stratasys’ FDM N12CF filament as it is one of the few forms of this material
available on the market. A unique characteristic of the material is that it has two different
tensile and compression moduli depending upon the axis on which it is extruded. As such, two
force simulations were conducted on the various parts of the devices to see if they could be
constructed using both forms of the material.
The aim of this thesis is to expand the knowledge of exoskeleton technology and nylon-
12 carbon fiber material by answering the following questions:
1) Can nylon-12 carbon fiber filaments be used to construct load bearing components of a powered lower body exoskeleton?
2) Can the device constructed be used by an elderly patient of an average weight of 200 lbs.?
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CHAPTER 2
BACKGROUND
2.1 Mobility Impairment within the Elderly
The elderly population is one of the fastest growing groups in America; because of this,
new, cost-effective caretaking methodologies must be developed in order to treat many
ailments that the elderly face [1]. According to a 2018 survey conducted by the CDC, 15% of
Americans have some sort of impairment that affects their mobility with a majority of reported
cases affecting seniors (aged 65+) [2]. Disruption of gait within the elderly population is a
common occurrence attributed to deterioration of the brain and the musculoskeletal system
causing the body to become frailer. This increase in frailty has been associated with decreases
in overall gait speed for elderly patients and an increase in fall occurrences while walking [3-5].
Falls have been found to result in intense physical trauma for elderly patients that have been
admitted to hospitals and account for a large percentage of deaths resulting from unintentional
injury [6-9].
To reduce the instances of injury caused by falling, many types of assistive mobility
devices (AMDs) have been developed, including canes and wheelchairs. In recent years,
research focusing on medical exoskeletons as AMDs has come to the forefront of fields such as
biomechanics and rehabilitation therapy. Unfortunately, such a device is expensive to
manufacture and use of heavy materials and bulky movement actuators can limit a patient’s
movement and overall time using it [10,11]. As such, researching more cost-effective
manufacturing methodologies and the incorporation of sturdy, lightweight materials are
essential for furthering the development of assistive mobility exoskeletons.
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2.2 Fused Deposition Modeling
Additive manufacturing (AM) is a manufacturing technique that utilizes computer aided
design (CAD) software in order to construct devices by adding specific material(s) layer by layer.
AM has three main processing techniques, liquid-based, solid-based, and powder-based. Each
of the processes have different AM techniques in order to fuse the materials together. For
liquid processes, such as UV lithography, an external energy source is used to polymerize a
reservoir of liquid polymer resin to produce a final solid object. Solid-based techniques, such as
fused deposition modeling (FDM), utilize either pre-polymerized resin or soft metal materials
that are heated up and extruded on to the printing plane, whereas powder-based techniques
usually utilize a form of laser sintering in order to melt the materials together. The rise of AM
within recent years has resulted in its use in industries such as aerospace, automotive, and
medicine as it allows for quick prototyping and the use of complex geometric designs [12,13].
FDM, shown in Figure 2.1, is a 3D printing technique in which a continuous polymer
filament is melted down, passed through an extruder head, and deposited onto a surface in
order to generate a structure. Unlike other AM methods, there is no outside light source
(UV/laser) that is needed in order for the filament to fuse to itself. Advantages of FDM include
compact machine size, low maintenance cost, and wide variety of available materials [14].
While widely used by both large companies and hobbyists, this method does not currently have
an international standard set by ISO, so the quality of a product is measured by the individual
manufacturing device to ensure that the product meets customer requirements. As with all
forms of AM, FDM is limited in what can be produced. Due to the compact size of most
machines, FDM printers are not able to manufacture large complex components. As such, it is
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important to scale the parts that are to be manufactured in order to ensure that it is within the
parameters of the printer [15].
Figure 2.1: FDM Simplification
FDM is able to utilize a number of different material types, including soft metals and
ceramics, but this paper will focus primarily on how it can be used to manufacture polymer
structures. Polymers make up a majority of the materials consumed in AM. Due to their low
cost and light weight, they are ideal for prototyping and final production depending on the
application of the device. Some of the polymers that are widely used in FDM include: polylactic
acid (PLA), acrylonitrile butadiene styrene (ABS), and nylon. When printing using polymers, it is
vital that there is sufficient bonding between the layers of the polymer or the device is likely to
fail. A number of factors can affect the bond between the layers, including: the temperature of
the printing environment, fabrication pattern, layer thickness, filament diameter, printing
orientation, and the rate at which the polymer cools after extrusion [16,17]. Of the mentioned
factors, print orientation and the pattern used to print a part’s structure are particularly
5
important as they can determine how overall forces affect the structure of the manufactured
item. Figure 2.2 demonstrates how forces usually interact with a printed part. When the layers
are perpendicular to an applied tension force, the structural failure is more likely to occur
because of delamination as the bonds between layers are points where high amounts of stress
accumulate. Whereas when the layers are oriented in a direction parallel to an applied tension
force, that force will need to apply enough energy to break the bonds of the polymer chains in
order for mechanical failure to occur.
Figure 2.2: Layer Orientation and Force Direction
The pattern in which a part is printed can also affect how forces are distributed within
an object as they can determine the overall structure of the part as well as the amount of
material that is used for construction. The introduction of intricate structures such as lattices,
shown in Figure 2.3, allow for an object to be as hollow as possible to reduce overall weight
while also allowing for better flexibility within the structure. The various walls of a lattice
structure cause an incoming force to dissipate and be distributed amongst themselves reducing
the maximum stresses that can be applied to a singular point within a part. Size and density are
6
key to the effectiveness of this style of structure as smaller lattices allow for more flexibility and
a larger number allows for a greater dispersion of force throughout the device.
Figure 2.3: Lattice Structures
Other aspects that have been found to affect the mechanical properties of a polymer
include the type of filler that is mixed with the polymer to form the filament, the ratio of filler
to polymer, and the direction of the filler that is placed in when the filament is extruded [18-
21]. When introducing a filler material into a polymer matrix, it is important to understand the
ways in which the two components interact with each other chemically as this can affect the
bond strength between them.
2.3 Nylon-12 Carbon Fiber Filaments
Nylons are a family of synthetic thermoplastic polymers composed of a chain of carbon
atoms that are linked together with amide links (chains linked together by NH groups). These
polymers are heavily used in manufacturing of plastic shells and textiles due to their tensile
strength and elastic properties. Nylon-12, also known as polyamide-12, is a 12-carbon chain
nylon polymer, as shown in Figure 2.4, that is produced either from the polycondensation of ω-
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aminolauric acid or from ring opening polymerization of laurolactam monomer rings [22]. In
comparison to other nylon compounds such as nylon-6 and nylon-66, nylon-12 has similar
tensile and compression properties but the lowest melting point of any nylon
(178°C:220°C:264°C). These differences are due to the length of the molecule’s carbon chain as
the shorter chains allow for a tighter and more compact polymer structure which limits the
movement of the branches causing more energy to be required to affect the structural integrity
of the solid.
Figure 2.4: Molecular Structure of Nylon
Nylon-12 carbon fiber (N12CF) is a polymer filament used in FDM manufacturing. This
composite material is created by mixing a nylon-12 resin with fragments of carbon fiber filler.
This filler can be distributed within the matrix either in a specific pattern or at random in order
to reinforce the material’s mechanical properties [23,24]. The specific N12CF material that is
being studied in this thesis has a filler comprised of chopped carbon fiber threads distributed at
random orientations within the polymer matrix at a weight percentage of 35%. Studies
conducted to evaluate the mechanical properties of parts constructed from FDM-printed nylon-
12 have shown that the parts have different strengths depending upon the axis (XYZ) that the
part was printed on [25,26]. Parts printed in the X-axis had better tensile strength while parts
printed in the Z-axis could withstand compression better. Flexural properties of N12CF have
8
also been studied and found that the degree of flex that a part has is dependent upon fluid
absorption. N12CF has been found to have the lowest fluid absorption rate of any current nylon
filament and, when dried, a N12CF part has the same properties as one that has never been
exposed to fluid [27-29]. Hu and Hossan [30] simulated a gear pair constructed from N12CF
using computerized software and found that due to the difference between the tensile and
compression strengths, material failure can occur at areas of tension, stress concentration, and
compression stress concentration when the gear teeth come in contact with one another. Thus,
areas where contact between two or more parts is made must be reinforced to ensure that a
device constructed from N12CF does not fail. More manufacturing of FDM printed N12CF parts
must be conducted in order fully understand the capabilities of this material, as its use in FDM
is fairly novel.
2.4 Medical Exoskeletons
Medical exoskeletons are a type of external orthotic device that a patient can wear in
order to supplement a limb and allow for a greater range of movement. There are two main
categories of orthotics: powered and unpowered. Unpowered orthotics, as the name implies,
do not require an external power source in order to function and assist a patient. These types
of orthotics range from basic support braces to more advanced mechanical joints that utilize
simple machines such as springs and pulley systems in order to facilitate movement. Powered
orthotics are similar to their unpowered counter parts in the sense that they are meant to
support and augment a user’s movement. However, powered orthotics require an energy
source to power the machinery that allows the device to perform a desired task such as
extending a patient’s leg.
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In recent years, the medical field has begun to adopt Additive Manufacturing techniques
to design and manufacture various types of medical devices. In recent years, researchers have
started to investigate if these methods can be used to manufacture orthopedic devices to treat
mobility impairments within a patient’s leg [31-37]. One such device that has been researched
is unpowered ankle-foot orthoses (AFO). By utilizing 3D foot scanners, researchers have been
able to construct models of patient-specific devices and construct them via FDM.
Unfortunately, due to the constraints of this printing method, the orthotics are not able to have
complex geometric shapes [38,39]. Utilizing other AM techniques such as selective laser
sintering (SLS) allows for the generation of more complex devices but also increase the time
required to manufacture the devices.
The use of lower limb powered orthotics as devices for mobility rehabilitation has
become an area of great interest for researchers [40], and a number of devices have been
developed for patients with ailments caused by failing posture and stroke [41-43]. As
mentioned previously, the use of bulky components in medical exoskeletons has the potential
to limit a patient’s mobility. A review conducted by Sanchez-Villamaen et al. [44] reported that
the incorporation of compact actuators is essential in future research as their light weight and
small size remove most of the bulk that makes up modern exoskeletons, and, when used in
combination with lightweight materials, would allow for the construction of more compliant
devices. Studies that designed exoskeletons that utilized these two attributes, shown in Figure
2.5, have reported that the subjects’ range of motion incurred little to know interference with
the devices equipped [45,46]. The utilization of FDM techniques to create lightweight
components for a gait training exoskeleton has been reported in a study conducted by Jin et al.
10
[47]; while the constructed components were attachment points for the test subject and not
key joints for the exoskeleton, this alongside the use of AM to construct full AFO devices does
show the potential for FDM to be used in the construction of an active lower body exoskeleton
made of polymer components.
Figure 2.5: Lightweight Hip Exoskeleton that Utilizes Compact Actuators for Movement.[Images
courtesy of Giovacchini et al. [45]]
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CHAPTER 3
DEVICE DESIGNS AND METHODOLOGIES
3.1 Design of Exoskeleton
As the legs of this device (Figures 3.1 and 3.2) are expected to be constructed via fused
deposition modeling, each part was designed to fit within a printing space of 12 in3 while also
allowing for a tolerance of 0.03 in. to account for any dimensioning errors that could occur. The
following sections will describe the design of each of the three joints (hip, knee, and ankle) and
how the parts are to interact with one another.
Figure 3.1: Full Exoskeleton Leg with Posterior Offset Knee
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Figure 3.2: Full Exoskeleton Leg with Polycentric Knee
3.1.1 Design of the Thigh
The hip connection of the device shown in Figure 3.3 is composed of two pieces: an
upper motor housing and a lower connection shank that acts as the femur of the device
connecting the hip to the knee. Within the housing will be a harmonic gear set that is meant to
greatly multiply the force of the hip motor and allow for the patient to move their leg. On the
rear side of the motor housing is an attachment point for a linear actuation motor (LAM), which
will act as the unit’s hamstring and quadricep for full extension and contraction of the leg. The
bottom half of the motor mount is a hollow shaft that the connective shank can be inserted
during assembly. The shank is a singular 10 in. long shaft with a 0.688 in. hole at the bottom in
which either a needle bearing and or bolt will be inserted depending on which knee the leg will
13
connect to. The body of the shank has also been hollowed out to minimize the weight of the
thigh and allow for storage of electrical components.
To assemble this portion of the exoskeleton, the thigh shank is inserted into a hole at
the bottom of the motor housing and locked in place with a clevis pin. Multiple holes for the
clevis pin were made along the length of the shank to allow for height adjustability of the
device to better accommodate to the needs of the patient that is using it.
Figure 3.3: Exoskeleton Thigh
3.1.2 Design of the Ankle
Like the hip described above, the ankle shown in Figure 3.4 is also made of two pieces, a
motor housing that acts as the ankle and a shin shank that supplements the patient’s tibia and
connects to the knee. A harmonic gear set is housed within the ankle and connects to a
removable insole that is to be placed within a patient’s shoe. When powered by the ankle
motor the gears rotate the insole to provide flexion and extension of the user’s ankle and allow
14
for more natural and comfortable movement. The upper portion of the ankle is a hollow shaft
in which the shank can be inserted during assembly of the joint. The shank is a 10.5 in. long
shaft with a 0.688in hole at the end in which a needle bearing and bolt can be inserted when
attaching to the knee. This shank also has triangular shaped protrusions that extend from the
rear on the left and right sides, these structures are the anchor point for the LAM to attach.
To assemble the joint, the shin is inserted into the hollow end of the motor housing and
secured with a clevis pin. Like the thigh shank, the shin has multiple holes along its length for
the clevis pin to be placed to allow for height adjustment for the patient.
Figure 3.4: Exoskeleton Ankle
3.1.3 Design of the Knee
The knee is one of the most critical joints in the human body in terms of mobility and
stability. It is composed of three bones (the femur, tibia, and patella) that are connected
15
together with four ligaments, the anterior and posterior cruciate ligaments (ACL & PCL) and the
medial and lateral collateral ligament (MCL & LCL), to keep the joint stable by applying constant
tension to the bones. Three tendons (the quadriceps tendon, patellar tendon, and hamstring
tendon) connect the bones to muscles that allow for the knee to move when contracted. During
flexion or extension of the knee the femoral head rotates about and translates across the
surface of the tibia. This movement is especially important the swing phase of a patient’s walk
cycle as it, along with flexion of the ankle, are what prevents the foot from striking the ground,
which would cause the patient to trip. With deterioration of the knee joint, it is important to
design a hinge joint that can guide a patient’s leg along a natural path that allows for optimum
flexion and extension of the knee while wearing an orthotic device as the shape and placement
of the artificial knee joint can affect the geometry of the patient’s joint [48].
Most of the knee designs used in orthotics today fall into one of three categories: single
axis (rotating or locked), posterior offset, or polycentric (multiple rotation axes), each used to
treat different issues that affect a patient’s knee such as hyperextension or lack of mechanical
stability [49]. For this exoskeleton two simple knee joints were designed, a posterior offset knee
(PO) and a polycentric knee (PC) that, when paired with a LAM, would act like a stance
controlling orthotic (SCO) knee. SCOs are a form of a knee ankle foot orthotic (KAFO) and are
designed to stabilize a patient’s leg to compensate for muscle weakness or deterioration within
the upper leg. The rationale for the two knees was to determine which style of knee would
allow for a more natural movement of the leg and cause the least amount of stress buildup to
occur along the length of the leg.
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3.1.3.1 3.1.3.1: Posterior Offset Knee
The PO knee, shown in Figure 3.5, is composed of two nearly identical sockets that rest
atop one another. As the name implies, the axis on which the joint rotates is located on the rear
side of the leg, as opposed to a human knee’s axis of rotation which is located close to the
anterior side. To facilitate this movement, the two parts have cylindrical protrusions that
extend from the rear that can slide together and connected via a carriage bolt to form a hinge
joint. Within this hinge, a needle bearing is housed that rotates about the shaft of the
connective bolt allowing for smooth rotation about the rear axis of the knee.
Figure 3.5: Posterior Offset Knee
This design was created to maximize the stability of the patient’s knee while standing.
To allow for this, the top and bottom halves of the joint lock together while the LAM is fully
extended when the patient is in an upright position. This locking mechanism is also to prevent
hyperextension of the joint. To limit the sway of the shin and thigh shanks in the medial and
17
lateral directions, the walls of the sockets lock the parts in place. The posterior cylindrical
protrusions also act as safety stops to prevent the two halves of the knee swaying in these
same directions.
3.1.3.2 Polycentric Knee
The PC knee, shown in Figure 3.6, is made of a single rectangular-shaped piece that has
been hollowed out to allow for the insertion of the shin and thigh connections.
Figure 3.6: Polycentric Knee
This joint has two axes of rotation for the shin and thigh accordingly, the posterior wall of the
knee has been removed to maximize the rotation of the femur and tibia about the knee, while
also providing the joint translational motion to allow for a more natural movement of the leg.
As a safety precaution to prevent hyper extension of a patient’s knee, the front wall of the PC
acts as a stop block, the walls along the medial and lateral sides of the knee act in a similar way
18
to prevent the leg from bowing in either direction. This part was designed with ease of motion
in mind in order to help a patient experiencing osteoarthritis of the knee, similarly to how some
knee braces that have a similar joint structure would help [50,51].
3.2 Force Modeling Simulations
Designs were modeled in SOLIDWORKS and static force simulations were created and
run for each of the joint models and fully assembled leg models, one for each of the knee
designs. This simulation software was utilized over other available software due to its ease of
access when developing SOLIDWORKS files, its prevalence in the wider professional
environment, and to reduce any chance of error or corruption that could occur when
converting the part files into a different format. The use of a static force study allows for the
calculation of both the stress and displacements that the parts will undergo without needing to
account for environmental factors. Each assembly underwent two simulations to account for
the difference in mechanical properties of FDM nylon-12 carbon fiber when printed on either
the XZ or ZX axis, as was described earlier. The forces, listed in Table 3.1, that were to be
simulated acting on the devices included a 35 lbs. force exerted by the LAM and a 200 lbs.
ground reaction force to account for the full weight of the patient when standing or in the
swing phase of their walk cycle. This weight was selected based off of the average reported
weight of elderly men in a 2018 survey conducted by the CDC [52]. Summarized in Table 3.2 are
some of the mechanical properties of the materials used in the study according to material data
sheets provided by Stratasys and Renishaw for the N12CF and Ti6Al4V materials respectively
and are located in Appendix B.
19
Table 3.1: Force Elements Applied to Exoskeleton
Sources of Force Force Strength (lbs.) Application Points Direction of
Force
Weight of Patient 200 Base of ankle Base of thigh shank Base of shin
Always upward
Linear Actuation Motor 35 Thigh motor mount Shin-LAM connection point
Upward Downward 45◦
Table 3.2: Mechanical Properties of Simulated Materials
Material Tensile
Strength (MPa)
Compression Strength
(MPa)
Melting Temperature
(◦C)
Glass Transition Temp (◦C)
Stratasys FDM N12CF XZ Printing Axis 63 67 178 41
Stratasys FDM N12CF ZX Printing Axis 29 92 178 41
Ti6Al4V Alloy 897 1070 1649 1538
3.2.1 Force Modeling of the Thigh
In order to run force simulations on the exoskeleton’s thigh, an assembly file was
created in SOLIDWORKS that comprised of the upper hip connector and lower thigh shank. The
two parts were then mated so as to insert the shank into the bottom of the hip connector. To
create the simulation environment, the ‘Create New Study’ option was selected from the
‘Simulation’ menu with the ‘Static’ option selected as the type of study. The parts were then
given the mechanical properties of the nylon-12 carbon fiber material that had been printed on
the XZ axis.
When applying the simulated forces to the thigh, the ‘Applied Load’ menu was selected
from the ‘Simulation’ menu and the ‘Force’ option was chosen from the list of possible loads.
The 35 lbs. load from the LAM was applied to the rear attachment points of the hip connector
20
piece. The force was set to be applied equally to both points in an upward direction using the
‘Total’ and ‘Specific Direction’ options within the ‘Force’ menu. The 200 lbs. ground force was
applied to the thigh shank in an upward direction at the lower end where the part would
connect to a knee joint.
To ensure that the thigh assembly stayed in place throughout the duration of the
simulation, screw holes where the hip connector would be mounted to the hip motor were
selected as fixed points in space. Without this fixture, the forces applied to the thigh would
cause unrealistic levels of displacement to occur.
For an accurate simulation to be created, the two parts of the thigh needed to be
connected together with a simulated fastener and contact constraints for when parts collide
with one another during the simulation. A steel bolt connector was selected to be the fastener
that held the hip connector and shank together. The ‘Rigid’ and ‘Tight Fit’ options were selected
for the bolt so that it did not bend with the plastic thigh when force was applied. To create
realistic contact forces between the parts, a non-penetrative contact set was created between
the outer faces of the shank and the inner faces of the hip connector. The non-penetrative
option was selected so that the computer could calculate how the applied forces would react
with each part individually as opposed to the penetrative/bonded option which would cause
the computer to see the parts as a singular entity when calculating stress and displacement of
the thigh. A semi-fine curvature-based mesh of the models was generated for this study as well.
The same process was done to generate a simulation of the thigh given the mechanical
characteristics of the nylon material when printed on the ZX axis. These parameters are
summarized in Table 3.3.
21
Table 3.3: Thigh Simulation Parameters
Parameter Name Location on Model
Fixture Point
Force 1
Force 2
Bolt Connector 1 (Rigid)
Contact Set 1 (Non-Penetrating)
3.2.2 Force Modeling of the Ankle
To begin the static force studies regarding the exoskeleton’s ankle, an assembly file was
created composed of the ankle motor mount and shin connection shank. The parts were mated
so that the shin connector was inserted into the upper portion of the motor mount. Generation
22
of the static simulation environment was identical to thigh force study and the parts were given
the material characteristics of XZ printed nylon-12 carbon fiber and the parameters are
summarized in Table 3.4. A steel bolt connector was used to secure the shin to the ankle mount
and a non-penetrative contact set was created between the outer walls of the shank and the
inner walls of the insertion point of the ankle to ensure that the forces acted on each part
individually.
Table 3.4Ankle Simulation Parameters
Parameter Name Location on Model
Fixture Point
Force 1
Force 2
Bolt Connector (Rigid)
Contact Set 1 (Non-Penetrative)
23
3.2.3 Force Modeling of the Knee
The 200 lbs. ground reaction force was applied to the base of the ankle mount in an
upward direction to simulate the force that would be applied to the base of the device while a
patient was standing with their leg fully extended. The 35 lbs. force from the LAM was applied
at a downward 45-degree angle to the rear wing protrusions of the shank where the motor
would be mounted if the device was assembled. To prevent the ankle assembly from moving
thru space unrealistically, the shin’s connection point to the knee was chosen as a fixed point in
space. A semi-fine curvature-based mesh was then generated for the assembly before the
simulation was run. These settings were also used to generate the force simulation for the for
the ankle given the characteristics of the material printed on the ZX axis.
Due to the amount of movement that the knees were expected to undergo, and that
their positioning required that they be able to withstand the weight of the full device as well as
any extraneous forces while the device is being used, the knees were designed to be
constructed from a titanium alloy. As such, the force simulations related to the knee joint were
run to see how the stresses applied to the plastic shin and thigh shanks would be affected when
they are attached to components made of a harder material. Simulations were also created
with all of the parts given the characteristics of the nylon-12 carbon fiber material in order to
compare how radically the inclusion of the titanium parts altered the stress values along the
length of the thigh and shin.
3.2.3.1 Forces Simulation of Posterior Offset Knee
To setup the force simulations for the PO knee, an assembly file composed of the two
halves of the knee, the thigh shank, and the shin shank was created. The two halves of the knee
24
were mated using the included ‘Hinge’ mechanical mate so that the rear protrusions could be
concentric and coincident with one another. This allows for the knee to rotate about its rear
axis when modeling the movement of the joint. Both of the shanks were mated using the
concentric mates between the connection holes in the shanks and the bolt holes on the sides of
the knees as well as width mates between the outer faces of the shanks and the inner faces of
the knee sockets. This allows the parts to act as if they have been inserted into the knee sockets
and react as such when testing the mobility of the joint.
The simulation environment was generated as described in previous sections. The knee
parts were given the characteristics of Ti6Al4V alloy and the shanks were given the mechanical
characteristics of the XZ axis printed nylon. As shown in Table 3.5, rigid steel alloy bolt
connectors were used to attach the two halves of the knee together at the rear protrusion
points, as well as between the shanks and their attachment points in the knee sockets. Non-
penetrative contact sets were then created between the outer faces of the shanks and the
inner faces of the knee sockets and a semi-fine curvature-based mesh was generated of each of
the parts.
The forces applied to the knee include the 200 lbs. ground reaction force applied in an
upward direction to the shin shank at its attachment point to the ankle, and the 35 lbs. force
from the linear motor applied to the shin shank as described in 3.2.2. The attachment point
between the thigh shank and the thigh motor mount was selected to be the fixed point for this
model to prevent unrealistic movement through space. The simulation was then run to assess
the stresses applied to the leg. These steps were then repeated to create a simulation for the
25
shanks given the characteristics of the ZX printed nylon-12 material as well as for the
simulations where all of the components were set to be made out of the nylon-12 material.
Table 3.5: PO Knee Simulation Parameters
Parameter Name Location on Model
Fixture Point
Force 1
Force 2
Bolt Connector 1 (Rigid)
Bolt Connector 2 (Rigid)
(table continues)
26
Parameter Name Location on Model
Contact Set 1 (Non-Penetrating)
Contact Set 2 (Non-Penetrating)
Contact Set 3 (Non-Penetrating)
3.2.3.2 Force Simulation of Polycentric Knee:
An assembly file was created composed of the PC knee, shin shank, and thigh shank
parts. A concentric mate between the shanks’ knee connection point and the PC knee’s bolt
insertion sockets and a width mate was used between the outer faces of the shanks and the
inner faces of the knee sockets. This way the shanks act as if they had been inserted into the
knee.
Four simulation environments were created, two where all of the components were
27
given the characteristics of the nylon-12 material printed on either the XZ or ZX plan and two
where the thigh and shin were given the nylon material characteristics and the knee was given
the properties of the Ti6Al4V alloy. Rigid steel alloy bolt connectors were selected to attach the
shanks to the knee and non-penetrating contact sets were created between the outer faces of
the shanks and the inner faces of the PC knee sockets as shown in Table 3.6. Forces and fixtures
were applied as described in 3.2.3.1. A semi-fine curvature-based mesh was generated for each
of the models and the simulations were run.
Table 3.6: PC Knee Simulation Parameters
Parameter Name Location on Model
Fixture Point
Force 1
Force 2
Bolt Connector 1 (Rigid)
(table continues)
28
Parameter Name Location on Model
Bolt Connector 2 (Rigid)
Contact Set 1 (Non-Penetrating)
Contact Set 2 (Non-Penetrating)
3.2.4 Force Modeling of Leg
With the simulations completed for the individual sections of the leg simulation sets had
to be conducted to see how all of the parts interacted with one another. As such two models of
the leg were constructed using both the PO and PC knees in order to find out which leg had the
best reaction to the applied forces. As with the knee models four simulation environments
were set up, two where all of the components of the leg were given the properties of the nylon-
12 material and two where the parts were a hybrid of nylon and titanium. This was done to
verify if the legs could be constructed wholly from nylon as well as to compare how the
integration of titanium affected how the stress was distributed along the length of the leg.
3.2.4.1 Force Simulation of Leg with Posterior Offset Knee
For this simulation, an assembly file was generated consisting of the two halves of the
29
PO knee along with the thigh and ankle assemblies. The PO knee parts were mated using the
mechanical hinge mate and the shin and thigh shanks were mated to the knee as was described
in 3.2.3.1. Non-penetrating contact sets were made between the thigh motor mount and thigh
shank, thigh shank and PO knee socket, shin shank and PO knee socket, and the shin shank and
ankle motor mount. Rigid steel alloy bolt connectors were selected to fasten the components
together at the respective attachment points along the length of the leg. The 35 lbs. force from
the LAM was applied to the thigh motor mount and the shin shank as described in previous
section and the 200 lbs. ground reaction force was applied to the base of the ankle motor
mount. The screw holes at the top of the thigh motor mount were selected as the fixture for
this simulation to prevent unrealistic movement once the forces are applied during the
simulation as shown in Table 3.7. A semi-fine curvature-based mesh was applied to the models
and the simulation was conducted. These same parameters were also used to set up the other
three simulations.
Table 3.7: PO Leg Simulation Parameters
Parameter Name Location on Model
Fixture Point
Force 1
(table continues)
30
Parameter Name Location on Model
Force 2
Force 3
Bolt Connector 1 (Rigid)
Bolt Connector 2 (Rigid)
Bolt Connector 3 (Rigid)
Bolt Connector 4 (Rigid)
(table continues)
31
Parameter Name Location on Model
Bolt Connector 5 (Rigid)
Contact Set 1 (Non-Penetrating)
Contact Set 2 (Non-Penetrating)
Contact Set 3 (Non-Penetrating)
Contact Set 4 (Non-Penetrating)
(table continues)
32
Parameter Name Location on Model
Contact Set 5 (Non-Penetrating)
3.2.4.2 Force Simulation of Leg with Polycentric Knee:
For the final simulation set, an assembly file was created with the PC knee, thigh, and
ankle assemblies. The shin and thigh shanks were mated to the PC knee as described in 3.2.3.2,
and four simulation environments were created for the plastic parts printed on the XZ and ZX
axes in which two of them the knee was given the characteristics of Ti6Al4V alloy as in the
previous PC knee simulation.
Table 3.8 shows, non-penetrating contact sets were created between the upper thigh
and thigh shank, thigh shank and knee socket, shin shank and knee socket, and shin shank and
ankle motor mount as described in previous sections. Two tension spring connections were
used to connect the shin and thigh shanks together with a tension of 35 lbs. The springs were
added in place of the force from the LAM to account for stability of the leg because with all of
the normal forces being applied, the computers were unable run an accurate simulation due to
the shape of the knee. The 200 lbs. ground force was applied to the base of the ankle mount.
The screw holes at the top of the thigh were again selected as the fixtures for the simulation. A
semi-fine curvature-based mesh was generated for the models and the study was run.
33
Table 3.8: PC Leg Simulation Parameters
Parameter Name Location on Model
Fixture Point
Force 1
Spring Connector 1 (Compression/Extension)
Sprint Connector 2 (Compression/Extension)
Bolt Connector 1 (Rigid)
Bolt Connector 2 (Rigid)
(table continues)
34
Parameter Name Location on Model
Bolt Connector 3 (Rigid)
Bolt Connector 4 (Rigid)
Contact Set 1 (Non-Penetrating)
Contact Set 2 (Non-Penetrating)
Contact Set 3 (Non-Penetrating)
(table continues)
35
Parameter Name Location on Model
Contact Set 4 (Non-Penetrating)
36
CHAPTER 4
RESULTS AND DISCUSSION
4.1 Simulation Results
Three dimensional plots were generated to map out the stress, strain, and displacement
of the models when the forces were applied during the simulations. The following sections
describe the results of the force study applied to the parts for both the XZ and ZX printed nylon-
12 carbon fiber components.
4.1.1 Thigh Simulation Results
From the stress plots that were calculated during the simulation of the thigh portion of
the device, it was determined that parts manufactured from the either the XZ or ZX printing
axes would not undergo catastrophic failure when undergoing the expected forces. For the
plots in this section the blue regions show areas of low stress build up while the red regions are
areas where the highest amount of stress is calculated to occur. The green and yellow areas are
calculated to be the median stress values. The maximum stress applied to the parts printed
from the XZ and ZX axis was calculated to be 26.06 and 7.420 MPa respectively, these max
stresses were found to be applied to the fixed screw holes of the upper thigh motor mount. As
shown in Figures 4.1 and 4.2, even with the difference between the compression strengths of
the two plastics, overall stresses were dispersed similarly across the thigh as a whole. Both
versions of the thigh were calculated to have a maximum displacement of 2.098 mm and max
strains of 2.169x10-3 and 2.214x10-3 respectively. These values are summarized in Table 4.1.
37
Figure 4.1: Stress Plots for N12CF Thigh Printed on XZ Axis (A)Front view, (B) Right view, (C) Rear view,
(D) Left view
Figure 4.2: Stress Plots for N12CF Thigh Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear
view, (D) Left side view
38
Table 4.1: Cumulative Results of Thigh Static Force Simulations
Material Max Stress (MPa) Max Displacement (mm) Max Strain
FDM N12CF Printed on XZ Axis 26.06 2.098 2.169x10-3
FDM N12CF Printed on ZX Axis 7.420 2.098 2.214x10-3
4.1.2 Ankle Simulation Results
The plots constructed from the simulations, as shown in Figures 4.3 and 4.4, of the ankle
portion of the exoskeleton, calculated the maximum stresses applied to the joint to be 18.19
and 21.56 MPa for the XZ and ZX printing axes respectively. The max stresses were mapped to
occur at the bolt connection point between the shin shank and the ankle mount on both
versions. The maximum displacement was calculated to be 4.696 and 1.278x101 mm for the XZ
and ZX axis respectively at the base of the ankle mount. The maximum strain applied to the
ankle joint was also found to be 1.729x10-3 and 5.747x10-3 respectively. These values are also
summarized in Table 4.2.
Table 4.2: Cumulative Results of Ankle Static Force Simulations
Material Max Stress (MPa) Max Displacement (mm) Max Strain
FDM N12CF Printed on XZ Axis 18.19 4.696 1.729x10-3
FDM N12CF Printed on ZX Axis 21.56 12.78 5.747x10-3
39
Figure 4.3: Stress Plots for N12CF Ankle Printed on XZ Axis (A) Front view, (B) Right view, (C) Rear
view, (D) Left view
Figure 4.4: Ankle Stress Results for ZX Printing Axis (A) Front view, (B) Right view, (C) Rear view, (D)
Left view
40
4.1.3 Knee Simulation Results
4.1.3.1 PO Knee Results
For both the XZ and ZX simulations, the maximum stress applied to this portion of the
device was calculated to occur at a point on the knee itself. A stress of 141.20 MPa was found
to occur at the rear side of the knee on the XZ simulation, and a stress of 17.85 MPa was
calculated to occur at the bolt connection point between the knee and shin on the ZX
simulation as shown in Figures 4.5 and 4.6. As for the nylon thigh and shin, the stresses ranged
from 11.77 to 94.15 MPa for the XZ printed parts and 1.488 to 11.90 MPa for the ZX printed
parts. The higher stresses found on the plastic parts were found to occur at the force contact
points as well as the bolt attachment points in both of the simulations. For both simulations the
maximum displacement of the device was calculated to occur at the shin with a measurement
of 12.62 and 4.210 mm for the XZ and ZX simulations respectively. The maximum strain applied
to this section of the device was also calculated to be 1.028x10-2 and 3.558x10-3 respectively.
Figures 4.7 and 4.8 show the stress applied to the posterior offset knee assembly when
constructed entirely from nylon-12 carbon fiber. The maximum stress, displacement and strain
values were calculated to be 9.230 and 16.03 MPa, 2.575 and 7.099 mm, 8.547x10-04 and
4.853x10-03 for the XZ and ZX printed nylon components respectively. These values are
summarized in Table 4.3.
41
Figure 4.5: Stress Plot for Ti6Al4V PO Knee with N12CF Thigh and Shin Printed on XZ Axis
Figure 4.6: Stress Plot for Ti6Al4V PO Knee with N12CF Thigh and Shin Printed on ZX Axis
42
Figure 4.7: Stress Plots for N12CF PO Knee Printed on XZ Axis (A) Front view, (B) Right view, (C) Rear
view, (D) Left view
Figure 4.8: Stress Plot for N12CF PO Knee Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear
view, (D) Left view
43
Table 4.3: Cumulative Results of PO Knee Static Force Simulations
Material Max Stress (MPa) Max Displacement (mm) Max Strain
FDM N12CF Printed on XZ Axis (Ti64) 94.15 (141.20) 12.62 1.028x10-02
FDM N12CF Printed on ZX Axis (Ti64) 11.90 (17.85) 4.210 3.558x10-03
FDM N12CF Printed on XZ Axis, all parts 9.230 2.575 8.547x10-04
FDM N12CF Printed on ZX Axis, all parts 16.03 7.099 4.853x10-03
4.1.3.2 PC Knee Results
As with the PO knee plots, the maximum stresses applied to the PC knee were also
found to occur on the titanium knee itself as shown in Figures 4.9 and 4.10. These stresses were
calculated to be 251.70 and 2452000 MPa for the XZ and ZX environments respectively. The
higher stress in the ZX plot far exceeds the yield strength of the knee. Therefore, plastic
deformation and mechanical failure can be expected to occur. The stresses applied to the
plastic components were found to be in the ranges of 20.99 to 167.8 and 0.1319 to 1022000
MPa for the XZ and ZX environments respectively. These high stresses were also found to occur
at the bolt attachment points for the thigh and shin and result in plastic deformation to occur
for these parts. The maximum displacement and strain were calculated to be 4.422x107 mm
and 1.605x10-2 for the XZ parts and 3.890x1011 mm and 3.563x101 for the ZX parts. Figures 4.11
and 4.12 show the stress plots for the PC knee assembly constructed wholly from nylon-12
carbon fiber printed on the XZ and ZX axes respectively. The maximum stress, displacement and
strain values were calculated to be: 2.664x1009 and 2.056x1013 MPa, 511.6 and 3.448x1012 mm,
44
and 5.406x10-01 and 1.479x1004 for the XZ and ZX printed parts respectively. These values have
been summarized in Table 4.4.
Figure 4.9: Stress Plot for Ti6Al4V PC Knee with N12CF Thigh and Shin Printed on XZ Axis
Figure 4.10: Stress Plot for Ti6Al4V PC Knee with N12CF Thigh and Shin Printed on ZX Axis
45
Figure 4.11: PC Knee all parts N12CF printed on XZ axis (A) Front view, (B) Right view, (C) Rear view,
(D) Left view
Figure 4.12: PC Knee all parts N12CF printed on ZX axis (A) Front view, (B) Right view, (C) Rear view,
(D) Left view
46
Table 4.4: Cumulative Results of PC Knee Static Force Simulations
Material Max Stress (MPa) Max Displacement (mm) Max Strain
FDM N12CF Printed on XZ Axis (Ti64) 167.8 (251.70) 4.422x107 1.605x10-02
FDM N12CF Printed on ZX Axis (Ti64) 1022000 (2452000) 3.890x1011 3.563x1001
FDM N12CF Printed on XZ Axis, all parts 2.664x109 511.6 5.406x10-01
FDM N12CF Printed on ZX Axis, all parts 2.056x1013 3.448x1012 1.479x1004
4.1.4 Leg Simulation Results
4.1.4.1 PO Leg Results
The stress plot generated from the force study shown in Figures 4.13 and 4.14
calculated the maximum stresses to be applied to the titanium alloy knee on both the XZ and ZX
printed legs with a value of 30.29 and 63.79 MPa respectively. The stresses applied to the nylon
components ranged from 3.029 to 21.20 MPa and 6.379 to 25.52 MPa respectively, with the
higher stresses being applied to the connection bolt holes as was found with the ankle
simulation. Maximum displacement of the leg was found to be 8.577 and 2.471x101mm for the
maximum strain values were found to be 1.908x10-3 and 6.073x10-3 for the XZ and ZX printed
legs respectively. Figures 4.15 and 4.16 show the stress plots for the posterior offset leg with all
parts constructed from the nylon-12 filament for printed on the XZ and ZX axes respectively.
The maximum stress, displacement, and stress were calculated to be: 41.91 and 22.66 MPa,
11.06 and 29.29 mm, and 2.004x10-04 and 6.121x10-03 for the XZ and ZX printed parts
respectively. All these values have been summarized in Table 4.5.
47
Figure 4.13: Stress Plot for N12CF PO Leg printed on XZ Axis with Ti6Al4V Knee (A) Front view, (B),
Right view, (C) Rear view, (D) Left view
Figure 4.14: Stress Plot for N12CF PO Leg printed on ZX Axis with Ti6Al4V Knee (A) Front view, (B),
Right view, (C) Rear view, (D) Left view
48
Figure 4.15: Stress Plot for N12CF PO Leg printed on XZ axis (A) Front view, (B) Right view, (C) Rear
view, (D) Left view
Figure 4.16: Stress Plot for N12CF PO Leg Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear
view, (D) Left view
49
Table 4.5: Cumulative Results of PO Leg Static Force Simulations
Material Max Stress (MPa) Max Displacement (mm) Max Strain
FDM N12CF Printed on XZ Axis (Ti64) 21.20 (30.29) 8.577 1.908x10-3
FDM N12CF Printed on ZX Axis (Ti64) 25.52 (63.79) 2.471x101 6.073x10-3
FDM N12CF Printed on XZ Axis, all parts 41.91 11.06 2.004x10 -03
FDM N12CF Printed on ZX Axis, all parts 22.66 29.29 6.121x10-03
4.1.4.2 PC Leg Results
The stress plots of the PC leg, shown in Figures 4.17 and 4.18, calculated the maximum
stresses to be 17.77 and 21.94 MPa for the XZ and ZX printed nylon parts respectively. For both
legs the maximum stresses were calculated to occur along the length of the thigh shank. The
maximum displacements and strain values were also calculated to be 1.426x101 and
4.186x101mm and 1.636x10-3 and 5.587x10-3 respectively. Figures 4.19 and 4.20 show the stress
plots for the polycentric leg with all parts constructed from the nylon-12 carbon fiber material
when printed on the XZ and ZX axes. The maximum stress, displacement, and strain have been
calculated to be: 17.38 and 44.89 MPa, 13.61 and 23.58 mm and 1.584x10-03 and 1.041x10-02
for the XZ and ZX printed parts respectively (see Table 4.6).
Table 4.6: Cumulative Results of PC Leg Static Force Simulations
Material Max Stress (MPa) Max Displacement (mm) Max Strain
FDM N12CF Printed on XZ Axis (Ti64) 17.77 (8.885) 14.26 1.636x10-3
FDM N12CF Printed on ZX Axis (Ti64) 21.94 (10.97) 41.86 5.587x10-3
FDM N12CF Printed on XZ Axis, all parts 17.38 13.61 1.584x10-03
FDM N12CF Printed on ZX axis, all parts 44.89 23.58 1.041x10-02
50
Figure 4.17: Stress Plot for N12CF PC Leg Printed on XZ Axis with Ti6Al4V Knee (A) Front view, (B),
Right view, (C) Left view
Figure 4.18: Stress Plot for N12CF PC Leg printed on ZX Printing Axis with Ti6Al4V Knee
(A) Front view, (B), Right view, (C) Left side view
51
Figure 4.19: Stress Plot for N12CF PC Leg printed on XZ Axis (A) Front view, (B) Right view, (C) Rear
view, (D) Left view
Figure 4.20: Stress Plot for N12CF PC Leg Printed on ZX Axis (A) Front view, (B) Right view, (C) Rear
view, (D) Left view
52
4.2 Discussion
The purpose of this thesis was to determine the viability of nylon-12 carbon fiber
filaments as a material to be utilized in the construction of lower body exoskeletons using
additive manufacturing processes. As mentioned previously, all of the simulations were
conducted in SOLIDWORKS utilizing the available software tools. As such, the results found in
this paper may not be able to be replicated in other simulation software such as COMSOL
Multiphysics or ANSYS. A major limitation that is important to note is that SOLIDWORKS is only
able to construct solid parts and does not allow for the customization of parameters such as the
filament diameter or the printing direction of the filament when constructing a simulation.
Therefore, the overall results may not reflect what a printed part may experience when
undergoing stress testing.
Even with the exact same parameters set up between the XZ and ZX simulation
environments, the calculated stresses were found to be different. This is attributed to the
difference in compression moduli between the XZ and ZX printed materials, with the ZX
modulus being the higher value. This difference in compression strength is due to the direction
in which the nylon is distributed during extrusion as it affects the overall shape of the printing
pattern and how the extruded layers interact with any applied forces as discussed earlier. In
this case, the ZX printed layers would be normal to the applied compression forces, allowing
them to sandwich together and form a more rigid material.
Each of the simulation plots show that the stresses applied to the device disperse along
the length of the thigh and shin and condense at the various bolt attachment points. This is
because of the difference in the material properties between the nylon-12 carbon fiber and the
53
steel bolt connectors. These connectors were also selected to be rigid which causes it to remain
relatively stationary when forces are applied to the parts around it. The same can be said for
why the larger stresses would build up around the fixture points on each of the simulation
models. It was important to test each section of the leg independent of one another in order to
make sure that each of the main components could withstand the applied forces so that if the
leg failed, the failure points could be located and redesigned more easily. The separate
simulations were also conducted to make sense of how the stresses would be arranged along
the individual section and to determine how they changed once all the parts were connected
together. The unusually high amount of stress applied to the PC simulation is most likely due to
the fact that it was less stable because there was no rear wall preventing the thigh and shin
from moving around, causing the computer to have a more difficult time generating a solution.
This theory is aided by the fact that introducing the spring connectors to act as the 35 lbs. force
being applied to the thigh and shin stabilized the PC leg model overall and reduced the
calculated amount of stress, while also mapping it out almost identical to that of the PO leg that
underwent the normal applied forces.
Comparing the simulations containing the titanium knee to those that contain the nylon
knee it can be determined that the titanium part can be replaced with the nylon one without
causing the device to fail. In all of the simulations, save for the ZX printed polycentric knee,
none of the stress on the nylon parts was calculated to exceed the material’s yield strength and
under go plastic deformation. From the posterior offset leg simulations, it can be seen that the
maximum stress calculated on the knee happens on the same spot in all of the simulations and
the values on the XZ printed knee are extremely close together. The stresses are mapped out in
54
identical patterns along the length of the leg for each group of simulations, as in all of the PO
leg plots look identical and all of the PC leg plots look identical.
Stratasys also manufactures a nylon-12 filament without the carbon fiber filler and a
Nylon-6 filament to be used in FDM manufacturing processes. The yield strengths for these
materials along the XZ and ZX printing axes are 32 and 28 MPa and 49.3 and 28.9 MPa for the
bylon-12 and nylon-6 materials respectively. Comparing the simulation data to these values
shows the maximum calculated stresses for some sections of the exoskeleton are very close to
the yield strengths of the of the ZX printed versions of both filaments. The stresses calculated to
occur on the XZ printed parts do not exceed the expected yield strengths of the XZ nylon-12 or
nylon-6 materials. It can be concluded that the XZ printed nylon-6 material would be the better
choice of the two as the factor of safety is calculated to be <2 for the ankle and thigh sections
of the device when comparing the stress values to the yield strength of the nylon-12 material. It
is important to note that this comparison is only being made with Stratasys nylon filaments and
does not eliminate the possibility of using nylon-12 and nylon-6 from other companies as their
different formulation methods can result in different mechanical characteristics of the
materials.
55
CHAPTER 5
CONCLUSIONS
5.1 Experimental Conclusion
This thesis aimed to verify if nylon-12 carbon fiber filaments could be used in the
construction of a powered lower body exoskeleton using FDM processes by using static force
simulations to model the perceived forces that a prototype exoskeleton would be expected to
encounter. The results of the static force simulations conclude that nylon-12 carbon fiber FDM
filaments can be utilized in the construction of lower body powered orthotic devices for use by
the average elderly patient. From the maximum applied stress values, it was determined that
manufacturing the parts via the XZ printing axis would provide the most stable version of the
device as the calculated stresses for the ZX simulations were deemed too close to the yield of
the material to safely construct the device. Based on the plots calculated from the knee
simulations as well as both of the full leg simulations, not only does the use of titanium affect
the overall stress applied but also the shape of the knee affects results. From these studies, it
was determined that the usage of a polycentric knee joint would be the optimal choice for the
device as long as it has the right support structures as the maximum stress applied to the ankle
and thigh were found to be less than when using the posterior offset knee. Due to the way that
the forces are applied to the device, the mounting points between all of the parts of the leg
were found to be points of high stress. As such, reinforcement of these junctions may be
required for prolonged use of the device. Further simulations and testing are required to
determine the lifespan of the device.
56
5.2 Future Avenues of Research
This thesis focused on the usage of nylon-12 carbon fiber as a material, a comparison
between orthotic devices constructed from other common polymers used in FDM, such as
acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA), should be conducted. The
various parts of this orthotic were also designed to be hollow in order to lessen the weight of
the overall device. As such, a comparison between how the forces act on solid parts could also
be made to determine whether a hollow or solid design of the device is more suitable for usage.
The introduction of lattice structures to the various plastic parts could also be used as a basis
for a stress comparison. Additionally, this thesis only focused on computer simulations of forces
applied to the device. It is important to run tests on the exoskeleton prototype in practice to
compare the theoretical calculations to practical findings. This device was designed to be
manufactured using a planar printing pattern, because of this it would be a good idea to test
the mechanical differences between these parts and part printed using a non-planar pattern.
The primary focus of this thesis was testing to see if the exoskeleton could handle the
maximum forces that are expected to be applied to it when a patient is shifting their weight
during the swing phase of their walk cycle. As such, dynamic force study should also be
conducted on this device to see how the various parts react to the varying forces that would be
applied over the course of an average patient’s walk cycle. This can be done in SolidWorks by
setting up a non-linear study with the modal time history option applied. This option will allow
the ability to apply the expected loads of the LAM and weight of the patient over a set period of
time which can be altered based on the walking speed of an average patient as well as taking
into account the amount of time needed to extend the LAM.
57
To determine the theoretical lifespan of the device a fatigue simulation with constant
amplitude events should be run to see how many times the exoskeleton leg can undergo the
stresses calculated in the simulations before failing. This can be done in SolidWorks as long as
the static force simulations have already been conducted, they can be applied as the stress
parameters to the fatigue study, a SN-curve will need to be calculated for each of the materials
to account for expected material fatigue over time. This plot can be created by testing on a
sample piece of material with constant cyclic loading until it fails, the data may also be provided
by material vendors if possible.
58
APPENDIX A
ENGINEERING DRAWINGS OF EXOSKELETON
59
60
61
62
63
64
65
66
67
68
69
APPENDIX B
MATERIAL DATA SHEETS
70
71
72
73
74
75
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