1. 1
Design of a Low Cost Transfemoral Knee
Prosthesis with MMG for Developing
Countries: Crus Novus
Group 1: Eleanor Disney, Alice Boo, Alexander Camuto, Cecilia Kan, Rohit Devesar,
Johnson Chu, Vekin Virachjarassin, Gerald Png,Rafael Michali, Yomna Genena.

2. 2
Contents
1. Abstract................................................................................................................................ 3
2. Aims.....................................................................................................................................3
2.1 Inexpensive......................................................................................................................... 3
2.2 Ease of use.......................................................................................................................... 3
2.3 Durability............................................................................................................................ 3
3. Specifications and requirements ............................................................................................ 3
4. Final Design and Analysis.......................................................................................................6
4.1 Mechanical components............................................................................................. 6
4.2 Hardware and Software............................................................................................ 15
5. Device Testing................................................................................................................. 18
5.1 MPU6050 tests......................................................................................................... 18
5.2 MMG tests.................................................................................................................... 21
5.2 Testing the PCB.............................................................................................................. 23
6. Manufacturing.................................................................................................................... 23
6.1 Mechanical components........................................................................................... 23
6.2 PCB............................................................................................................................... 26
7. Conclusion and Discussion................................................................................................... 28
7.1 How the prosthesis matched our aims....................................................................... 28
7.2 Improvements............................................................................................................... 28
7.3 Conclusion..................................................................................................................... 28
8. Appendix ............................................................................................................................ 28
8.1 Appendix A (Commented Arduino Code).................................................................... 28
8.1 Appendix B (Risk Analysis and Ethical Considerations) ................................................ 31
8.1 Appendix C (Business Case and Targeted Consumer).................................................. 39
8.2 User Manual ............................................................................................................ 39
8.3 Appendix C (Group Working)..................................................................................... 41
8.6 Appendix D (Initial designs) ............................................................................................ 42
8.7 References............................................................................................................... 48
8.8 Acknowledgments ......................................................................................................... 49

3. 3
1. Abstract
2. Aims
2.1 Inexpensive
The project’sfocuswastodevelopaversatileandrobustbelow-knee legprosthetic,controlled by
mechanomyography(MMG),tobe usedindevelopingcountries.The MMG neededtoreliablydetect
muscle impulses and filter surrounding noise and residual muscle vibrations that occur after
contraction.Because the target user was indevelopingcountriesa maximumbudgetof 500£ for the
production of the prosthetic was set. If mass produced, the cost of components would be reduced
considerably.
2.2 Ease of use
Once muscle signals were filtered the MMG needed to control the locking and unlocking of
the knee prosthetic to allow the wearer to manoeuvre with ease. The interface would need to be
simple enoughforhealthtechnicianstosetuprapidlyandforusersto interactwitheasily.The design
aimed to be as simple as possible, both in the electronics used for the MMG and the mechanism to
lockthe knee sothatmaintenanceandrepairof the prostheticcouldbe done withminimalequipment.
2.3 Durability
Emphasiswasalsoplacedon the robustnessof the prosthetic.The materialsandstructure of
the prosthetic would need to bear the forces repeatedly inflicted upon the leg during walking and
standingandcouldprotectthe electronicsusedforthe MMG (give valuesof forcesinnextsentence).
It would need to be durable enough to be used in developing countries where uneven terrain and
unpavedroadsare prevalent. Tobe versatileenoughtobe usedinanarrayof conditionsthe prosthetic
and the MMG (particularlyanysensorsusedforthe MMG) would be housedin a waterproof case to
protectfromsweatandrain.For ease of use anypowersuppliesusedwouldneed topowerthe MMG
and any motor used in the knee for 16 hours continuously and would need to be rechargeable.
Additionallyif powerweretobe lostduringthe use of the MMG, the prostheticwouldneedtolockin
an extendedposition (stiff) to give the user manoeuvrability. Combined these elements aimed to
provide the weareragaitthatwouldoutperformthatgrantedbyapassive mechanical knee prosthetic
in speed, manoeuvrability and comfort.
3. Specifications and requirements

4. 4
Requirement
Type:
Requirements: Comments: Design Specification:
Functional The knee must lock at the
beginningof the swing
and stance phases of the
gaitcycle.
Required so the knee does
not bucklein the stance
phaseand has adequate
support. The knee must
bend enough in the swing
phaseso that the foot of
the prosthesis doesn’t
scrapethe ground.
The knee hinge jointis locked by
alteringthe tension in the steel
cablearound the barrel.The
signalsfromthe accelerometers
on the user’s muscles will
determine when the actuator
pullingthe cableshould increase
or decrease the tension to lock
the knee.
Functional The MMG technology
must be powered by a
longlastingpower
source. Baselinelifetime
must be a day (24 hours),
with target of a week
As this is aimed at
amputees in developing
countries,it’s important
that it is powered by a
battery that will lastat
leasta few days to a week
as they may not have
regular access to charge
the battery or not be able
to afford a new one.
A 9V lithiumrechargeable
battery for the Arduino (which
delivers 5V to the
accelerometers). A 12V battery
powers the linear actuator.
Lithium batteries were chosen
because of their longevity.
Form The prosthesis musthave
a natural weight
If it’s too heavy it will make
it very difficultfor the
amputee to walk with it
and tire them quickly.
Lightweight material with most
of the weight focused around
the knee.
Form Simple design that can be
worn subtly under
clothing
Ensuringthat the
prosthesis looks natural as
possibleunder clothing
improves the way the user
relates to the product.
The cylindrical body of the
prosthesis with adjustableshaft
basecan be enhanced with a
detachablecover to imitate the
natural shapeof a lower leg.
User The Prosthesis mustuse
parts that can be easily
fixed/replaced.(i.e. no
specialistparts).
In developing countries it’s
unlikely the user will have
access to specialist
facilities so theparts need
to be easy to replaceand
fix.
All of the parts are easily
replaceableexcept the linear
actuator.The shaftis an
aluminiumbody formed of four
rods,three plates and a hinged
section with horizontal steel
supports.
User The battery/electronic
components must be
easily accessible.
This is in casethe battery
needs replacingor the
electronic components
need repairing.
Ideally there would be a
removable plastic cover on the
shaftof the leg which will allow
the user to replacethe 9V
(Arduino) and 12V (Linear
Actuator) battery.
User The Crus Novus must be
Water Resistant.
This is to allowthe user to
wear the prosthesis in all
weather and prevent
damage to the electronics.
Ideally there would be a cover
on the shaft. This covers the
electronic components and is
waterproof.

5. 5
User The Crus Novus knee
must be usablefor a
range of weights and
heights.
This is a lowcost knee that
should be usablefor many
above knee amputees.
Therefore there should be
ways of extending the
shaftfor taller users and it
should supportpeople in a
range of weights.
At the bottom of the shaftthere
is a platewith a thread where an
extension shaftcan be screwed
in to make the leg longer for
taller users.The Crus Novus
supports a maximum weight of
100kg.
User The Crus Novus knee
must be lined suitably to
minimizesweat build up
and friction between the
skin and accelerometers.
This allows comfortfor the
user duringrepetitive daily
wear. At minimum, on par
with those made available
by competitors with
products in our price
bracket.
Usinga sweat wickingmaterial
to drawmoisture away from the
skin and have the
accelerometers embed into
pockets in the lining.
User The Crus Novus has
patient - gaitcycle
matching
This allows themotion of
the knee to match the
natural paceof the patient
to supporta more natural
feel for the user.
The precoded software can be
adjusted to match patient
preferences (c.f user manual).
Cost The Crus Novus Knee
must cost less than £500
to manufacture.
The prosthesis mustbe
affordablefor users in
developing countries whilst
still beinga high quality
knee that is durableand
improves the user’s quality
of life.
The knee costs less than £370 to
build.
Cost The battery must be
cheap.
This is so the user can buy
a new one if it fails without
sparingtoo much expense.
Cost of the battery - depends on
which battery we use.

6. 6
4. Final Design and Analysis
4.1 Mechanical components
4.1.1 KneeFrame
The mechanical framework is equivalent to bones in the human leg, and allows the
functionalityof the prosthetic knee. It must be strong enough and designed to carry out its
main functions:
1. To allow the user to balance on it while standing;
2. To support the load of the user;
3. To contain the components of the MMG sensors and locking system;
4. To provide the tension required for the locking mechanism; and
5. To allow bending and swinging motions as in a natural human knee.
The followingtableshowshowthe featuresof the frame mustcomplywiththe requirements
of our project as a design aiming to reach the developing world.
Feature Advantages
Simple overall
structure
 Easy to assemble
 Allowseasyadjustmenttoadoptdifferentstrengthand
size of linearactuatorand MMG components
Simple component
structure
 Lowercost of manufacture
Minimumnumber
of components
 Lowercost of material
 Lowerweightof structure
 Easy to cleanand maintain
Table 1. Features of the framework that aim towards designing a low-cost upper-limb prosthetic knee
The overall structure should allow space for components of the locking system to
function and attach. The centre of mass should be close to the centre of the axial plane so
that the structure can balance ona flat surface.Howeverthe centre of massshouldnotbe in
the lowerlegas the user wouldexperience alargermomentin the swingphase. A compact
structure isrequiredforthe comfortof the user,soof maximum105mmdiameterinthe axial
plane wasset,based onthesmallestadultkneebrace sizefromknee brace protectionsellers1
.
A minimum 3mm thickness of any part of the structure was set. The initial drawing of the
framework was prototypedusing Meccano. In this model, elastic bands were used to mimic
the spring response of the device. The prosthesis includes the lower leg to utilize the extra
space for placing the locking system.
A brief descriptionof the main componentsthat formthe skeletonof our frameworkdesign
is given below:

7. 7
Structure Purposes Designrequirements
Barrel on
horizontal
axis
Thisacts as a hinge joint
for the attachmentof
the lowerlimb prosthesis
to the upperlimb.The
rotationof the barrel
aroundits axismimics
the swingingmotionof a
knee.
The widthneedstobe 12mm
for a 4mm ø wire rope to wrap
2.5 loopsaroundthe barrel.
There mustbe smoothrotation
betweenthe barrel anditsaxis,
and the cable wrappedaround
it.The axismustbe able to
withstandbendingfrom1.64kN
shearforce from the tensionof
the cable.
Barrel clip The standard socket
piece forlowerlimb
prosthesestothe upper
limbisscrewedontothe
barrel clip.The clipis
fixedtoandrotateswith
the barrel.
The clipmust provide enough
surface area forthe attachment
of the standardsocketpiece,
the springs,the barrel,without
collidingwithother
componentsof the prosthesis.
It mustwithstandbendingfrom
a maximum980N shear force
fromthe loadof the user.
Vertical
supports
These are rods that
supportthe loadof the
user.
Theymust supportthe weight
of the user,whichisa maximum
980N axial loadwithout
buckling.Theyhave tobe
Figure 1. The initial CAD drawing of the frame in
front view. The wire ropes ends were to be fixed to
the bottom bar by crimping

8. 8
arrangedto allow space for
componentsof the locking
system.
Flat
plates
These are platesthat
holdthe vertical
supportsinplace.They
may alsoprovide means
for the cablesand
springsto attach,as well
as the standard socket
piece forlowerlimb
prosthesestoartificial
feet.
Theymust allow space for
componentsof the locking
system, ontopof the vertical
supports.The plate with
attachmentof cablesand
springsshouldwithstand
bendingfrom1.64kN shear
force due to the tensionof the
cable.
Table 2. Basic components of the designed prosthetic knee
The restrictions to the framework design were mostly geometrical. Difficulty in
manufacturing,andconsiderationsforease of assembly alsorestrictedthe design.Beforethe
choice of lockingsystempartsandMMG parts were finalised,the requireddimensionsof the
frame were alsounknowndue touncertaintyinthe size of the linearactuatorandthe barrel.
The progression the frame is detailed below:
 Barrel
The diameter of the barrel was increased from 30mm in the initial planning stage to 60mm
after detailedforce analysiswasmade for the lockingsystem.The locationof the barrel was
movedtoabove the topflatplate asitcouldnolongerfitwithinthe curvature of the flatplate
without the structure becoming too bulky.
 Barrel clip
A barrel-clip was used to fix the barrel and its pin along its main axis (to prevent
rotation). The details of the clip are detailed in the manufacture section.
 Vertical supports
The cross sectional shape of the rod waschangedsubsequently fromathinrectangle,hollow
square,to finallyafilledcircle,toincrease the areatolengthratioof the shape,as well asfor
a more compatible shape with the rest of the components.
 Flat plate
The initial curvedoblongplateswere designedtominimisevolume.Toincrease theirsurface
area toaccommodate forthickervertical supports,the endsof the plateswere extendedinto
a horseshoe shape.Howeverthisshape washardertomanufacturing,leadingtothe decision
of using circular plates. The circular plates also allowed the more even distribution of the
supports hence better stability of the structure.

9. 9
The final framework design is shown in figure D. The components include:
 30mm ø barrel
 8mm ø barrel axis
 barrel clip (L 90mm, W 66mm, H 63.5mm)
 5mm ø barrel pin
 17.4mm ø main vertical support ×2
 6.3mm ø additional vertical support ×4
 10mm thick,90mm ø circular flatplats (highring,low plate,
base)
The flatplatesare heldinpositionbycirclips.Thereis150mm
spacingbetweenthe highand lowflatplats,formingacompartment
to accommodate a 130mm linear actuator in full extension. The
110mm spacing between the low plate and the base can hold an
Arduino Mega (102mm ×54mm ×15.3mm) and a battery (100.1mm
×60.7mm ×26.5mm).
Each endof the wire rope wouldgothroughthe 5mmøholes
in the highring. The loadingendwouldloopthroughthe hole inthe
low plate and be fixed with crimp. The other hole on the low plate
was a screw hole for mountingthe linearactuator.The barrel clipis
fixed to the barrel with the pin. The top of the clip is 15mm to
accommodate 12mmdeepscrewholesforthe standardsocketpiece.
Two holes on the barrel clip are intended for wiresattached to the
springsthatwouldgooutside the circularplates.Wiresonthe
lower end of the springs are fixedto the vertical supports by
crimping.
Specifications:
 Total length: 397.5 mm
 Diameter: 90 mm
 Minimum permitted angle of rotation of barrel: 160 °
 Material: Aluminium alloy except stainless steel barrel axis
 Total weight: 1.40 kg
To assessthe performance of the framework,force analysisforvariouscomponentsisshown
below:
 Main vertical supports
Stress from user body weight =
100 × 9.81
0.00852 𝜋 × 2
= 2.16 × 106
Figure 2. CAD drawing of the final design that was
sent to manufacture in back view.

10. 10
Safety factor =
Material proof stress
Stress from user body weight
=
260 × 106
2.16 × 106 = 120
Material proof stress taken from Aalco Aluminium Alloy 6082 - T6 data sheet 2
 Low plate
To analyse the behaviourof the low plate underthe tensionfromthe rope wire,we mayuse
Kirchhoff–Love plate theoryforcircularplate withclampededgesas the vertical supportshelpresist
bending. 3
The in-plane stress and the bending moment due to the tension in the wire rope is given
by:
𝜎𝑟𝑟 = −
3𝐹𝑧
32ℎ3
[(1 + 𝑣) 𝑎2 − (3 + 𝑣) 𝑟2] = 30.6 𝑘𝑃𝑎
𝜎 𝜃𝜃 = −
3𝐹𝑧
32ℎ3
[(1 + 𝑣) 𝑎2 − (1 + 3𝑣) 𝑟2] = 11.6 𝑘𝑃𝑎
𝑀𝑟𝑟 = −
𝐹
16
[(1 + 𝑣) 𝑎2 − (3 + 𝑣) 𝑟2] =
2ℎ3
3𝑧
𝜎𝑟𝑟 = 0.19 𝑁𝑚
𝑀 𝜃𝜃 = −
𝐹
16
[(1 + 𝑣) 𝑎2 − (1 + 3𝑣) 𝑟2] =
2ℎ3
3𝑧
𝜎 𝜃𝜃 = 0.51 𝑁𝑚
where σ isthe in-plane stress,F= 1.64kN is the tensionfromthe wire rope,M isthe bending
moment due to force F, the Poisson ratio v = 0.33 (from trend in 5000 series aluminium alloy)4
, the
maximumradiusof the plat a = 0.045 m, h= 0.005 ishalf the thicknessof the plate,z= h and r = 0.048
are the point of application of force F in the z and r directions respectively.
Safety factor =
𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙 0.2% 𝑌𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ
𝜎𝑟𝑟
=
115 × 106
30.6 × 103 = 3756
Safety factor =
𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙 0.2% 𝑌𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ
𝜎 𝜃𝜃
=
115 × 106
30.6 × 103 = 9885
Material 0.2% Yield strength taken from Kastal 300 Data Sheet, Smiths Metal Centres 5
For the breaking load of the wire rope where F = 8.826kN,
𝜎𝑟𝑟 = 0.164 𝑀𝑃𝑎 𝑀𝑟𝑟 = 2.75 𝑁𝑚 Safety factor = 698
𝜎 𝜃𝜃 = 62.6 𝑘𝑃𝑎 𝑀 𝜃𝜃 = 1.04 𝑁𝑚 Safety factor = 1837
The final product of the framework has been tested out and it functions as intended. The
structure is over-designedas the safety factor is at least 100 times higher than typical values. 6
The
dimensionsof componentsweresetunderunknownstrengthrequirement,andwasover-engineered
to account for these uncertainties. For example, the linear actuator used has a minimum lengthof
130mm and hence could not fit into the assigned compartment when extended. The hole intended

11. 11
forwire rope wasdrilledlarger,andthenthe outerrimsawedoffentirelytoallow space forthe linear
actuator to extrude. However,the safetyfactor is still extremely high in the lightof such issues. The
diameter of the vertical supports and the thickness of the flat plates can be reduced to lower
manufacturing cost and reduce the weight of the prosthesis.
4.1.2 Final Locking Mechanism:
The designed above-knee prosthetic has to inhibit the rotation of the knee, in the sagittal
plane, upon muscle contraction sensed by MMG sensors. Controlling the rotation of the prosthetic
knee improvesthe GaitCycle of the patient,butit alsoallow the use of theirlegfor a largerrange of
actions such as climbing up the stairs, or sitting down on a chair. The locking mechanism has two
modesof actions:lockingthe knee andfreeingthe knee.The prostheticknee hastobe lockedduring
the stance phases of the Gait Cycle and when the knee is in full extension to prevent the prosthetic
legfrom bucklingunderthe appliedload.The knee is freedduringthe swingphase of the Gait Cycle
so it can swing back into its full extension position. The mechanism can also be locked at different
anglesof flexiontoperformsome specifictasks: 83 degreestoclimbup the stairs and 90 degreesto
descend the stairs. Walking requires an angle of flexionof 67 degrees. Our design had to be able to
lock and unlock easily without using too much energy in order to save some battery life. In order to
achieve thisgoal,the knee wasdesignedsuchthatit wouldbe inthe lockedpositionwhennopower
is suppliedtothe system.Thispresentsthe advantage tolockthe device whenitisout of powerand
therefore to prevent the patient from falling if his or her device was running out of batteries.
The final locking mechanism design was implemented by using the friction between an
aluminumalloybarrel andastainlesssteelwirerope.The wire ropepreventsthe rotationof the barrel
whenit doesnot slip. We usedthe CapstanEquation (1) to determine the minimumallowable value
of the tension in each part of the cable so that the cable does not slip.
The Capstan Equation:
Stainless Steel
Wire Rope
Aluminum Alloy
Barrel
Figure 3. The Final Locking Mechanism of the Above Knee Prosthesis

12. 12
𝐹ℎ𝑜𝑙𝑑 = 𝐹𝑙𝑜𝑎𝑑 × 𝑒 𝜇𝜙1
ϕ: The wrapped angle around the barrel
𝐹ℎ𝑜𝑙𝑑: The force acting on the tensed side of the wire
𝐹𝑙𝑜𝑎𝑑: The external brake actuation force
R: The barrel’s radius
T: The braking torque as a function of 𝐹𝑙𝑜𝑎𝑑 that tightens the wire rope around the barrel
𝑇 = ( 𝐹ℎ𝑜𝑙𝑑 − 𝐹𝑙𝑜𝑎𝑑) 𝑅 + 𝜇 𝑣𝑖𝑠𝑐 × ω 6
μ: The contact friction coefficient
𝜇 𝑣𝑖𝑠𝑐: The viscous friction coefficient
ω: The barrel’s angular velocity
The system locks such that ω = 0, so:
𝑇 = ( 𝐹ℎ𝑜𝑙𝑑 − 𝐹𝑙𝑜𝑎𝑑) 𝑅
∴ 𝑇 = 𝐹𝑙𝑜𝑎𝑑 ( 𝑒μϕ − 1) 𝑅
Knowingthatthe maximumpatient’sweightrequirementis100kg,the diameterof the stainlesssteel
cable is4mm, the barrel’sradiusis30mm, μ for aluminumagainststeelis0.35, ϕ is 5π or 2 turns and
a half around the barrel, and the following knee torques 7
:
𝑇𝑘𝑛𝑒𝑒 𝑗𝑜𝑖𝑛𝑡 (𝑤𝑎𝑙𝑘𝑖𝑛𝑔)1 0.31 Nm/kg
𝑇𝑘𝑛𝑒𝑒 𝑗𝑜𝑖𝑛𝑡 (𝑠𝑡𝑎𝑖𝑟 𝑢𝑝) 0.49 Nm/kg
𝐹ℎ𝑜𝑙𝑑 𝐹𝑙𝑜𝑎𝑑
𝜙
Drum rotation
R
Figure 4. The Capstan Model of the Locking Mechanism

13. 13
𝑇𝑘𝑛𝑒𝑒 𝑗𝑜𝑖𝑛𝑡 (𝑠𝑖𝑡− 𝑡𝑜 − 𝑠𝑡𝑎𝑛𝑑 𝑤𝑖𝑡ℎ 𝑎 𝑤𝑎𝑙𝑘𝑒𝑟) 0.72 Nm/kg
𝑇𝑘𝑛𝑒𝑒 𝑗𝑜𝑖𝑛𝑡 (𝑠𝑖𝑡 − 𝑡𝑜 − 𝑠𝑡𝑎𝑛𝑑 𝑤𝑖𝑡ℎ𝑜𝑢𝑡 𝑐𝑜𝑛𝑠𝑡𝑟𝑎𝑖𝑛𝑡𝑠) 1.20 Nm/kg
To allowthe patienttoclimbupthe stairs,the tensionineachpart of the wire rope have to
reach the followingvaluesinordertolockthe device:
𝐹𝑙𝑜𝑎𝑑 = 6.7𝑁
𝐹ℎ𝑜𝑙𝑑 = 1.64 𝑘𝑁
The torque at the knee joint is changing through the Gait Cycle
This means that the tension applied by the linear actuator should fluctuate with respect to
the Gait Cycle. However to simplify the task, we have decided to only account for one value of the
external appliedloadinthe endof the cable subjectedtothe tensionprovidedbythe linearactuator.
It was found for the greatest moment at the knee joint for a 100kg patient climbing up stairs.
The stainlesssteel cable usedforthe lockingmechanismwasa wire rope 7x7 construction.It
has 7 strands and7 wiresperstrand.The cross-sectionof the cable isnotperfectlycircular,butitcan
be approximated to be 2.83x10-3
m2
. Its breaking load is approximately 0.9 tones or 8.826 kN. The
tension in the holding end of the cable being 1.64 kN, our design factor for the wire rope is
approximately 5.4.
𝑆𝑎𝑓𝑒𝑡𝑦 𝐹𝑎𝑐𝑡𝑜𝑟 =
𝑆𝑡𝑎𝑖𝑛𝑙𝑒𝑠𝑠 𝑆𝑡𝑒𝑒𝑙 𝑊𝑖𝑟𝑒 𝑅𝑜𝑝𝑒 𝐵𝑟𝑒𝑎𝑘𝑖𝑛𝑔 𝐿𝑜𝑎𝑑
𝑅𝑒𝑠𝑖𝑠𝑡𝑖𝑛𝑔 𝐸𝑛𝑑 𝑜𝑓 𝑊𝑖𝑟𝑒 𝑅𝑜𝑝𝑒 𝐿𝑜𝑎𝑑
= 5.4
The holdingcable isattachedtoanaluminumalloyplatedesignedtoresistbendingunderthe
applied tension during the locking state of the device.
The maximumallowable patient’sweight,mmax,isaccountableforFhold =8.826kN, sousingthe
formula derived from the Capstan equation:
Momentsina healthypatient’sknee Momentsina prostheticknee
Figure 5.The Knee Moments with Respect to the Percentage of the Gait Cycle in a Healthy Patient and in a Patient Equipped
with a Prosthetic Knee 8

14. 14
𝑚 𝑚𝑎𝑥 =
𝑅 × 𝐹ℎ𝑜𝑙𝑑
( 𝜏+
𝜏
𝑒 𝜇𝜙−1
)
6
𝑚 𝑚𝑎𝑥 (𝑘𝑔)
𝑚 𝑠𝑎𝑓𝑒𝑡𝑦 ( 𝑘𝑔)
𝑆𝐹 = 8.5
𝑚 𝑠𝑎𝑓𝑒𝑡𝑦 ( 𝑘𝑔)
𝑆𝐹 = 5.4
𝑚 𝑠𝑎𝑓𝑒𝑡𝑦 ( 𝑘𝑔)
𝑆𝐹 = 3.7
𝑚 𝑠𝑎𝑓𝑒𝑡𝑦 ( 𝑘𝑔)
𝑆𝐹 = 2.2
Walking 850.6 100 157.5 230.0 386.6
Stair up 538.1 63.3 100 145.4 244.6
Sit-to-Stand with a
Walker
366.2 43.1 67.8 100 166.5
Sit-to-Stand
without Assistance
219.7 25.8 40.7 59.4 100
The variable msafety wascalculatedusingthe safetyfactors5.4 and2 9
.SF = 5.4 correspondsto
the design factor previously foundby comparing the tension in the resisting cable and the breaking
loadof the 4mm stainlesssteel cable.Inthistable,we have calculate the safetyfactorsrequiredfora
personweightinga100 kg to personthree differenttasks:climbingupthe stairs,standingup from a
sitting position with and without a walker. The knee torque required to perform those tasks is
fluctuating: it is lowest to walk and it is the highest to stand up without requiring assistance. As a
result, the safety factor increases as the required knee torque increases. The initial requirement of
ourdevice wastoallowthe patienttowalkaccordingtoaGaitCycle asclose aspossible tothe natural
Gait Cycle. This wouldlead to a safety factor of 8.5 for the stainless steel wire rope. Such a factor is
withinexpectedrange forawire rope accordingtothe EngineeringToolboxWebsite10
.However,such
a safetyfactorwouldnotallowa100kgpatienttoclimbupstairsortostandupfromasittingposition.
Keepingasafetyfactorof 8.5 to adaptthe lockingmechanismfora100 kg patienttoperformasit-to-
standactionwouldleadtoan increase of the wire rope diameter.Increasingthe diameterof the wire
rope wouldcause achange inthe barrel design;itswidth,currently13mm, wouldhave tobe widened.
Those changes wouldcriticallyincreasethe weightof the lockingmechanismanddisturbthe patient’s
Gait Cycle. Keeping a safety factor of 8.5 without changing any other variable than the radius of the
barrel wouldgive usthe followingresults.The new radiusof the aluminumalloybarrel wouldbe 116
mmcomparedto30mm, assumingthat 𝑚 𝑚𝑎𝑥 = 850 𝑘𝑔 andthe torque 𝜏 = 1.20 𝑁𝑚/𝑘𝑔 forthe sit-
to-stand without assistance action.
𝑅 =
𝑚 𝑚𝑎𝑥 × 𝜏 (1 +
1
𝑒 𝜇𝜙 − 1
)
𝐹ℎ𝑜𝑙𝑑
The newradiuswouldbe 4 timesbiggerthanthe original radiusof the barrel.Thiswouldnot
only increase the weight of the part but it would also compromise the design which would become
bulkier. It justifies the use of a safety factor of 2 as it would allow to keep the design of the locking
Figure … Maximum and Safety Weights for a 100 kg Patient Performing Different Tasks (SF = Safety Factor)

15. 15
mechanismaslightandcompact as possible.Asafinal result,asafetyfactorof 2.2 wouldallow a 100
kg user to perform all the tasks mentioned in the table.
The inconvenience of this locking mechanism is that the aluminum alloy material is softer than
stainlesssteel asitsshear; tensile andtangentmoduli are approximatelythree timeslargerthanthe
ones of aluminum alloy 6061. The density of stainless steel is around 7480-8000kg/𝑚3 while the
density of aluminum 6061 is around 2700kg/𝑚3. As the stainless steel wire rope slides around the
surface of the aluminum barrel it will produce wear that will affect the longevity of the device.
4.2 Hardware and Software
4.2.1 Hardware
The final circuit was designed with the following components:
 2 separate power sources.
 12V to power the linear actuator
 7V to power the Arduino, H-bridge and
 5 connections between the linear actuator and Arduino
 1 pair of wires controlling the movement of the linear actuator.
 1 pairof wire asvoltage reference forthe positionalfeedbackbuiltintothe linearactuator
 1 wire which sends positional feedback to the Arduino
 2 connections between the accelerometer and Arduino
After more testing, more components were added as their need became apparent.
In order to reduce the effect of noise from the power supplyand voltage changes from the
Arduino,4 decouplingcapacitorswere addedbetweenthe powersupplies,Arduinooutputsandthe
ground. The capacitors absorb any voltage fluctuations, reducing noise on the rest of the circuit
components.
An H-bridge was used to control the Firgelli L16-50-150-12-P linear actuator (capable of
withstanding250N, above the requirementsof the leg of 6.7N cf.Section4.1.2) to the Arduino 4
.The
functionof the H-bridge istoactasaswitchbetweenthepowersource andlinearactuator.Thisallows
us to control the voltage provided to the pair of wires on the linear actuator using a smaller signal,
thus controllingbothspeedanddirection of operationof the actuator from the Arduino.Thisallows
us to reduce the amountof code needed andonlyrequirestwo 2outputs. The H-bridge waslinkedto
4 diodestoprotectthe H-bridge asinstructedbythe TexasInstrumentsusermanual of the SN754410
Quadruple Half-H Driver (the H-bridge used) 11
. The final circuit design is shown below.

16. 16
Figure 6: Breadboard diagram of final circuit
Figure 7: Final circuit diagram for the circuit used for the PCB board manufacture.

17. 17
4.2.2 Software
The program for the Arduino was written in C, using the MPU-6050 library found at
http://www.i2cdevlib.com/devices/mpu6050 to interface with the MPU-6050 accelerometer we
used.
The signal fromthe accelerometerisprocessedusingacombinationof highandlow pass5th
order
Butterworth filters taken from http://www.schwietering.com/jayduino/filtuino/index.php, which
automatically calculates the coefficients we need for the filter.The filters are centred around 10-15
Hz, which matches muscle frequencies, allowing us to detect muscle activation.
2 outputs control the signal (high/low) to the H-bridge and then to the linear actuator. This is
accomplished using a simple digitalWrite command.
In the initial setup, various variables to be used are
initialised. The outputs are set such that the linear
actuator retracts till the positional feedback reads 0.
This ensures that the actuator starts from the same
position after every reset, allowing the user to manually
reset the knee position.
The code then enters a constant loop, where it runs
the signal fromthe accelerometerthroughthe bandpass
filter at roughly 50 Hz. Muscle activation is detected
when the signal hits an experimentally tested threshold
for 3 or more consecutive loops. The outputs are then
changed such that the linear actuator extends roughly
2cm, which loosens the knee for rotation. The linear
actuator is then set to retract to its original position to
lock the knee back in place. This completes the cycle of
unlocking/locking the knee during a single stride.
For the purposes of the project, the code runs as
expected, allowing us to control the locking of the knee
using muscle activation. However, while the filters
provedto be reasonablyrobustwhenfilteringnoise,itis
still possibletoimprove itthroughmore rigoroustesting
to determine the optimal filtering coefficients.
The code can be furtherimprovedbydesigning
a sinusoidal extension/retractionroutine,whichwould
make the lockingof the knee smoother.The activationcondition(more thanacertainthresholdfora
certain number of cycles) can be made more accurate if given more time for extensive testing.
Implementing the bandpass filter using hardware insteadof software may also be more dependant
when filtering noise.
The entirety of the code can be found in APPENDIX A.
Figure 8: Setup for Arduino code

18. 18
5. Device Testing
5.1 MPU6050 tests
To ensure the MPU6050 accelerometer could act as an MMG device, we needed to collect
data from muscle contractions using the MPU6050. To do so the MPU6050 was connected to an
Arduino UNO and the targeted muscle in the following manner:
Figure 9: Built using Fritzing. MPU6050 detailed to the right in red and powered by the Arduino. The Arduino
UNO is detailed in blue. The MPU6050 was strapped the subject’s arm using muscle tape.
The actual experimental setupisdetailedinthe followingphoto:
Figure 10: Experimental setup of diagram 1. The pink strap holding the MPU6050 to the subject’s arm is muscle
tape. The illuminated red dot is the powered MPU6050. Note: the Arduino itself is connected to a laptop.
Data wasthencollectedusingthe “serial print” functionof the Arduinoand analysedusingan
array of Matlab functions.The firststepwastosee if there wasaclearcorrelationbetweenMPU6050

19. 19
activityandmuscle contraction.The testsubjectwasaskedtocontracthisforearmatregularintervals.
Thisresultedintheregularamplitudespikesofthe MPU6050 dataseeninthe figure below.Eachspike
corresponds perfectly with the onset of contraction. The plateau phase seen after each spike
corresponds to the when the subject sustained contraction for 1 to 2 seconds.
Figure 11: Amplitude against the sample number for regular forearm contractions. Note: The amplitude are just relative
values given by the serial.print function of the Arduino and are thus unit-less.
Todetermine whetherthese spikeswere simplyduetothe displacementof the accelerometer
and were not due specifically to muscle activation we conducted another array of tests. After
contacting Ben Greer of the University Of Colorado (who conducted a similar MMG project) we
decidedtouse the Matlabwaveletfunctionstofurther analyse ourdata.Ourresultsare plottedinthe
figures below.

20. 20
Figure 12: Figure 1 data juxtaposed to the wavelet transform of the same sample data, plotted with the wavelet scale
against the sample number (for the same sampling frequency). Yellow shades correspond to greater amplitudes in the
wavelet domain. Blue shades correspond to depressions in amplitude.
Plateauphasesof contractioncorrelate withspikesinthe 4-7 waveletdomain asseenbythe
regular yellow shades in the wavelet scale of figure 2. Using the Matlab scal2frq function we
determinedthatforthisparticularwavelettransformthe spikesinthe waveletscale correspondedto
the 10-15 hertzfrequencyrange.We thusdeterminedthe MPU6050 couldbe usedasanMMG device.
To improve the legibilityof ourresultsandtoremove oscillationsfromourMPU6050 readings
we used a smoothing functionin our Arduino code (the code for which is detailedin Appendix A of
thisreportand wasprovidedbyDan Greer).Usingthe same setupasin diagram1 andaskingthe test
subject to contract his forearm at regular intervals we obtained the figure below. This function
increasedthe legibilityof ourresultsand significantlydecreasednoise inMPU6050 readingsas seen
in the figure below.
Figure 13: Smoothing function test with amplitude and plotted against sample number (with a 333Hz sampling
rate). Note: Amplitude is mapped to a 0 to 1.2 relative scale
To improve MMG signals,frequenciesoutside the 10-15hz range neededtobe filteredusing
the Arduino UNO. Jürgen Schwietering’s website (http://www.schwietering.com/) provides Arduino
code for 4th and 5th order Butterworth filters, both of which we tested (the code for the filters is
detailed inAppendix A of thisreport).Usingthe same setupasindiagram1andaskingthe testsubject
to contract his forearm at regular intervals we obtained the figures below.

21. 21
Figure 14: 4th Order Butterworth Filter with Wavelet Scale against sample number (sample frequency of 333hz)
Figure 15: 5th Order Butterworth Filter with Wavelet Scale against sample number (sample frequency of 333hz)
The 5th order Butterworthprovidesgreaterfiltrationof signalsoutside the 4-7waveletscale
range (10 to 15hz) as seen by the net decrease of noise outside this range (seen by the decrease of
yellowtonesoutside the 4-7waveletscale range) whencomparedto the 4th order Butterworth and
figure 2. Figures 4 and 5 have the smoothened MPU6050 data plotted at their base to reinforce the
correlation between contraction and amplitude spikes in the 10 to 15 hz frequency domain.
5.2 MMG tests
Once MPU6050 signals were accurately filtered using a low step and high step filter that
accuratelyreplicatedthe resultsof the 5th order Butterworth we wantedto determine whetherour
current MMG device could accurately control a DC motor and later a linear actuator. Using the
followingcircuitand Arduinoloopcode we usedmusclecontractiontoaccuratelycontrolalow power
servo motor (that did not require a motor shield).

22. 22
Figure 16: Arduino UNO connected to MPU6050 (itself strapped to a test subject’s forearm) and a low power
servo motor powered by the Arduino.
void loop ()
{
sensor.getMotion6(&ax, &ay, &az, &gx, &gy, &gz); // read mpu data
mag =highstep(lowstep(ax/3276.8)); // scale ax andfeedto filter lowand high pass filters
if (millis()-lastmax<100)
{
if (fabs(mag) >maxval) maxval =fabs(mag); // find max valuein currenttime window
} else {
currval =maxval;
maxval =0.0;
lastmax =millis();
}
smoothval =0.9*smoothval+0.1*currval;
val=smoothval;
val =map(val, 0,1023, 0,179); //Maps smoothval from0 to 1023 bits to 0to 180°angles
myservo.write(val); // sets theservo position according to the scaledvalue
delay(15); // waits for the servo toget there
SoftwareServo::refresh();
}
Code1: void loop code to testservo activation fromusercontraction.Thecode forfilters (highstep,
lowstep),void setup and otherfunctions are detailed in Appendix A

23. 23
The test subject was again asked contract his forearm at regular intervals. Each Contraction
resultedinaproportional rotationof the servomotor(the greaterthe amplitudeof the MPU6050 the
greater the angle of rotation).
The final circuit design was tested using the final implementation of the Arduino code
(detailed inthe designsection).Ourfinal designwasrobustagainstnoise(whenthe subjectwashitor
shaken the linear actuator did not activate), generated linear actuator action and caused the leg to
unlock whenever the subject contracted his bicep. The resultscan be seen in the following YouTube
video http://youtu.be/hWD8yTxIZLk.
5.2 Testing the PCB
The PCB designed and manufactured (detailed in the manufacture section) was rigorously
testedwhenreceivedandwhencomponentswere soldered.Everyconnectiononthe PCBwastested
pre and post solderusinga voltmeter,thusensuringnoconnectionswere accidentallyconnectedto
the groundplate of the PCB. Despite thisrigorous testing,the PCBcausedthe Arduinotoshortcircuit
whenever operated. The reasons for this are unknown.
6. Manufacturing
6.1 Mechanical components
6.1.1 Instructionsformanufacture
The knee ismostlymade of aluminiumalloy6082-T6,There are some smallcomponentsmade
of stainless steel (i.e. barrel pins) and copper (i.e. bushing). The outer case is made of PET plastic.
Firstly,the barrel cliponthe topof the Kneeismanufactured. The barrel clipisseparatedinto
three parts,twosidesandamiddle part.Atthe centre of the topsurface of the middle part,ashallow
circular notch with 38mm diameter is cut. Four threaded holes with 8mm diameter are drilled near
the notch. Two small holes with four mm diameter are drilled at the position 7mm away from the
frontedge and12mmfromthe side.Twoholes,onewith6mmdiameterandtheotheronewith14mm
diameterare drilledontwosides.The smallerhole islocatedat31mmdownfromthe topsurface and
the larger one is located at 53.5mm down from the top surface. These holes are for the pins to go
through.
Secondly,abarrel with60mmdiameterismanufactured.The barrel hasa0.5mm thinwall on
the edges,andtwoholesonthe side of it.One hole with6mmdiameterisdrilledat19mm awayfrom
the centre, and the other hole with 14mm diameter is drilledat the centre. These two holesare for
the pins to go throughand theyare alignedwiththe holesonthe side of the barrel clip.The pinsare
made of stainlesssteel.Bothpinsare 100mm long but one is with 5mm diameterandthe otherone
is with 8mm diameter.
Nextstepis to manufacture three ringplates.The upper ringplate has eightholes.One hole
with 35mm diameter is drilled at the centre of the plate.Two holes with 17mm diameter are drilled

24. 24
at 31mm awayfromthe centre hole.Anothertwoholeswith5mmdiameterare drilledat31mmaway
fromthe centre and are perpendiculartothe centre.Fourholeswith7mmdiameterare drilledatthe
fourcornersof the plate.Each one is21.9mm awayfromthe centre.The middle plateissimilartothe
upperplate but withoutthe centre hole.The bottom plate is similarto the middle plate butwithout
the 5mm holes. Moreover, the holes of the bottom plate are threaded.
The fourth step is to manufacture six aluminiumbars to support the structure. Two of them
are 350mm longandwith17.40mmdiameter.There isa12mmdiameterhole drilledthroughat15mm
downfromthe topsurface.Atthe bottomsurface of these twobars,there isahole drilled 30mminto
the bar whichisfor a M8 screw.The restof the bars are 288mm long.There are caps, whichare 8mm
highand with12mm diameteronthe top of those bars. The diameterof these bars is 6.3mm. At the
bottom surface of the bars, there is a hole drilled 30mm into the bar which is for a M4 screw.
To assemble the knee,putall six support bars throughthe upperring plate. Using circlipsfix
the plate by clamping both the largerbars to the bottom of the plate. Nextfitthe middle plate using
circlips. Forthe middle plate,thecirclipsare clampedat the topandbottomof the plate.Thirdly,hold
the bottom plate in position and screw the bars onto the plate.
Afterfixingthe plate inposition,alignthe holesonthe topof the bars, the hole at the centre
of the barrel and the holesonthe sidesof the barrel clip.To assemble these three componentsplace
the bushing (made of copper) inside the holes and put the 8mm diameter pin through. The 5mm
diameter pin is to fix the barrel and barrel clip together.
Afterassemblingthe mainstructure of the knee,place the linearactuator in positiononthe
middle plate. A 4mm diameter cable is fixed on one end on the middle plate. The cable then goes
through the upperplate and isroped one and half timesaround the barrel.It thengoes throughthe
other end of the plate and is fixed on the top of the linear actuator. To fix the cable on either ends
feed the cable through the plate or the top of the linear actuator and then clamp the cable.

25. 25
Figure 17: Final deconstructed CAD diagram of the Crus Novus

26. 26
6.2 PCB
6.2.1 Introductionto PCBmanufacturing
A Printed Circuit Board (PCB) is generally used to minimize the path lengths of electrical
current between electrical components and provides a convenient platform to arrange the
components in a compact way. This allows the circuit to take less space in devices. In addition, the
locationof the electronicpartsisfixedandtherefore simplifiescomponentidentificationandenables
the circuit to be placed in a moving knee prosthesis.
6.2.1 Gerber files and process
Software called Fritzing was used to transform breadboard design into schematics and PCB
Gerber files. The PCB was designed to maximize the number of parallel copper tracks and to keep
connectionsbetweennodesasshortaspossible.Thiswastoreduce noisefromthe coppertracksthat
could have interfered with the MMG function. The PCB circuit matches the circuit detailed in the
designsection. The PCB is the same size as the ArduinoUNO, 48.72 cm2
(7.73cm x 6.3cm), therefore
the PCB will fit exactly on top of an Arduino UNO.
Figure 18 – Both layers of PCB
Blacklinesare the silkscreenonthe PCB. Groundplatesgroundedbothlayersof PCB,allowing
all grounds and unused nodes to be grounded as can be seen from the red holes. The green holes
show that a node is connected to another node. The yellow and orange copper tracks are the
connections on the top layer and the bottom layer respectively.
On the left, there are 4 diodes, D1, D2, D3 and D4. J1 and R2 represent the connections for
the linearactuatorwhereJ1isforVCC,GNDandpositional feedbackandR2isforpositiveandnegative
positional feedbackreferences. Inthe middle,there isaL293DNE H-bridge. Nexttoitis the MPU6050
accelerometer.

27. 27
Figure 19 – Top layer of PCB
Figure 20 - Bottom layer of PCB
Figure 2and3 showconnectionsontopandbottomlayersrespectivelyanddrillingholesinorange.
These Gerber files were sent to the company PCBtrain who manufactured 4 copies of the PCB,
excluding the silk screen.

28. 28
7. Conclusion and Discussion
7.1 How the prosthesis matched our aims
The prototype successfully completed our main objectives. We constructed the first MMG
controlledknee prosthesisanddidso for minimal costs.The knee lockingmechanismheldthe legin
any position the leg was to be locked in. The filters used were reasonably robust to noise and the
MMG consistently recognized muscle contraction. With additional time and research we couldhave
resolvedissuesregardingthe PCBboard,perfectedfiltering,strengthenedthelockingmechanismand
added a waterproofing case for the electronic circuit and the knee prosthesis.Our product would
additionallyrequire clinical trialstotestitsrobustness,durability and comfort in real life situations.
7.2 Improvements
Future developmentswouldinclude awaterproofingcase,reducingthe Arduinotoits ATmega328
chip or even developing a custom built microcontroller to replace the Arduino (both would reduce
costs). Developing a linear actuator solely for the knee would also have improved the fitting of the
linear actuator within the knee. Additionally more emphasis could have been put on the user
experience.A battery life indicator,an easier charging mechanism and perhaps improved aesthetics
would all improve the ease of use and maintenance of the prosthesis for little additional costs.
Orderingsingle componentstobuildthe prototype increasedcostssignificantly:massproducingthe
device would reduce costs significantly to better target developing countries.
7.3 Conclusion
We have provedthat a reliable,cheap,robustandactive prostheticis feasible.Oursimple design
meansthe knee wouldbe easytomaintain,animportantfeature fordevelopingcountrieswhere time
and health technicians are lacking. A cheap, active prosthetic could drastically improve the lives of
amputeesindevelopingcountrieswhotypicallyneedtogothrough15-25 prostheticsinthe course of
a lifetime13
.Further research into such a prosthetic could very quickly produce a market ready
prosthetic and improve many lives.
8. Appendix
8.1 Appendix A (Commented Arduino Code)
#include "Wire.h"
// requires I2Cdevlibrary: https://github.com/jrowberg/i2cdevlib
#include "I2Cdev.h"
// requires MPU-6050part oftheI2Cdev lib: https://github.com/jrowberg/i2cdevlib/tree/master/Arduino/MPU6050
#include "MPU6050.h"'
#include <Servo.h>
#define NUMLEDS 7
#define LEDPIN 13
int ii;

29. 29
//Servo
Servo myservo;
int angle =0;
const int relay1Pin= 3; // the number ofthe Realy1pin
const int relay2Pin= 4; // pins connectedto actuator
const int originalposition =1024;
const int sensorPin=0;
int CurrentPosition;
int goalPosition;
int counter=0;
int counter2=0;
booleanVAL=false;
// sensor
MPU6050 sensor;
int16_t ax, ay,az;
int16_t gx, gy, gz;
float gain;
// filtering
float v[9], w[9];
float currval, maxval, smoothval,mag;
float smootharray[3];
uint32_t lastmax;
//analogout
const int analogout =5;
//
void setup()
{
Wire.begin();
myservo.attach(10);
// initializethefilter
for (ii=0; ii<9; ii++)
{
v[ii]= 0.0;
w[ii]= 0.0;
}
currval =0.0;
maxval=0.0;
smoothval=0;
smootharray[0]=smootharray[1]=smootharray[2]=0;
lastmax=millis();
Serial.begin(9600);
// set up the MPU
sensor.initialize();
myservo.write(0);
delay(15);
gain =4.0; // higher ->more sensitive
// initializetherelay pinas an output:
pinMode(relay1Pin,OUTPUT);
pinMode(relay2Pin,OUTPUT);
//reset linear actuatorposition
while(analogRead(sensorPin)>0) //sets
{
digitalWrite(relay1Pin, LOW);

30. 30
digitalWrite(relay2Pin, HIGH);
}
digitalWrite(relay1Pin, LOW);
digitalWrite(relay2Pin, LOW);
}
void loop ()
{
// read mpu data
sensor.getMotion6(&ax,&ay,&az, &gx,&gy,&gz);
// scale ax and feed tofilter
mag =highstep(lowstep(ax/3276.8));
// find max value in current timewindow
if (millis()-lastmax <100)
{
if(fabs(mag) >maxval) maxval=fabs(mag);
}
else
{
currval =maxval;
maxval =0.0;
lastmax =millis();
}
smoothval=0.9*smoothval +0.1*currval;//Smoothvalcourtesy ofBen Greer
smootharray[counter]=smoothval;
Serial.print(smoothval);
Serial.print("t");
Serial.print(millis());
Serial.print("t");
CurrentPosition =analogRead(sensorPin); //reads potentiometer values from actuator //gives current position.
Serial.print(CurrentPosition);
Serial.print("t");
Serial.println(VAL);
if(VAL==false &&smootharray[counter]>0.7 && smootharray[counter-1]>0.7 &&smootharray[counter-2]>0.7) // the array
is used for coincidence detection, if three
// successive values are above threshold the actuator is activated
{
VAL=true;
goalPosition =200;
}
if(VAL== true && goalPosition >CurrentPosition) //iftheleg hasn’t extendedto the goalit //extends
{
digitalWrite(relay1Pin, HIGH);
digitalWrite(relay2Pin, LOW);
Serial.println("Extending");
}
else if (VAL ==true &&goalPosition <=CurrentPosition) //ifthe leg has extendedto the goal//it thenretracts toLOCKTHELEG by
reversing voltagesent toactuator
{
goalPosition =0;
digitalWrite(relay1Pin, LOW);
digitalWrite(relay2Pin, HIGH);
}
if (VAL==true && CurrentPosition==0)

31. 31
{
VAL = false; //ifthe leg has retracted fully and there is noadditionaluser input the leg //stops.
}
delay(1);
}
// given a newvaluex, step the filter forward and return the newest filtered value
// code generated by http://www.schwietering.com/jayduino/filtuino/
// 5th order Butterwoth filtercentered on13 Hz
float highstep(float x) //sampling rateto changeto 50Hz afteradding motor
{
v[0]= v[1];
v[1]= v[2];
v[2]= v[3];
v[3]= v[4];
v[4]= (7.346926241632e-1 *x)
+( -0.5397732726*v[0])
+( 2.4906544831 *v[1])
+( -4.3389153270*v[2])
+( 3.3857389040 *v[3]);
return
(v[0] +v[4])
- 4 * (v[1]+ v[3])
+6 * v[2];
}
float lowstep(float x) //class II
{
w[0]=w[1];
w[1]=w[2];
w[2]=w[3];
w[3]=w[4];
w[4]=(1.774798045630e-3 *x)
+( -0.2883883056*w[0])
+( 1.5067203613 *w[1])
+( -3.0219304176*w[2])
+( 2.7752015933 *w[3]);
return
(w[0] +w[4])
+4 * (w[1]+ w[3])
+6 * w[2];
}
8.1 Appendix B (Risk Analysis and Ethical Considerations)
8.1.1 Scope
This document fulfils the requirements laid down in the Quality Procedure ‘Risk Analysis &
Management ‘relating to the initial identification of hazards and the risk classification of those
hazards. The document is prepared according to ISO 14971:2007 ‘Medical Devices – Applicationof
RiskmanagementtoMedical Devices’.Thisversionisbasedonanalysisatthe commencementof the
designprocessafterriskreductiontoensure thatriskshave beenminimisedandthatdesignchanges
have not introduced new unacceptable risks.

32. 32
8.2.2. Intended Purposeand Identification ofCharacteristicsofDevice
1) What is the intendeduse andhowisthe
medical device tobe used?
A
The above-knee prosthesisisdesignedto
jointo the remaininglimbandbe usedfor
walkingonflatsurfaces.
2) Is the medical device intendedtobe
implanted?
A No
3) Is the medical device intendedtobe in
contact withthe patientor otherpersons?
A
Yes,the socketwill containMMG sensors.
Thiscomponentwill remainincontactwith
the patient’sskinforthe durationof use.
4) What materialsorcomponentsare utilized
inthe medical deviceorare usedwith,or are
incontact with,the medical device?
A
Siliconinnersocklinesthe socketandisin
contact withthe skin.
5) Is energydeliveredtoor extractedfromthe
patient?
A To be determined.
6) Are substancesdeliveredtoorextracted
fromthe patient?
N/A No.
7) Are biological materialsprocessedbythe
medical device forsubsequentre-use,
transfusionortransplantation?
N/A No.
8) Is the medical device suppliedsterileor
intendedtobe sterilizedbythe user,orare
othermicrobiological controlsapplicable?
N/A No.
9) Is the medical device intendedtobe
routinelycleanedanddisinfectedbythe user?
A
Cleanedbyusertoappropriate degree
afteruse to maintainhygiene of skinin
contact withprosthesis
10) Isthe medical deviceintendedtomodify
the patientenvironment?
A
Designedtoimprove the patient’squality
of life by(re-)enablingwalking.
11) Are measurementstaken? A
The MMG sensorsdetectmuscle vibrations
withinthe remaininglimbtostimulate
aidedmotionof the prosthesisfollowing
the patient’sGate Cycle.
12) Isthe medical deviceinterpretative? A
Yes,withArduino.The MMG signal is
processed.

33. 33
13) Isthe medical deviceintendedforuse in
conjunctionwithothermedical devices,
medicinesorothermedical technologies?
A
Prosthesisrequiresthe additionof a
compatible prostheticfootthatisnot
providedwiththe productbydesign.
14) Are there unwanted outputsof energyor
substances?
A
Energylosswill occurthroughthe motion
of the motorsthat drive the dampingand
lockingsystemof the knee
15) Isthe medical devicesusceptible to
environmental influences?
A
Prosthesisneedtobe designedtobe dust
and waterresistant
16) Doesthe medical device influence the
environment?
N/A
17) Are there essentialconsumablesor
accessoriesassociatedwiththe medical
device?
A ProstheticFoot
18) Ismaintenance orcalibrationnecessary? A
Yes,each prosthesismusthave itssystem
calibratedtothe patients’GaitCycle and
the socketmust alsobe mouldedtofitthe
remaininglimb.Lengthof staff (tibia) must
be set to an appropriate length.
19) Doesthe medical device containsoftware? A Yes,(Arduino) physical programming
20) Doesthe medical device have arestricted
shelf-life?
A
Productshouldhave a longshelf life,
thoughspecificcomponentsmayneed
replacingif theyfail.Environment
dependantlifespan.
21) Are there anydelayed orlong-termuse
effects?
A
Wear will occurbetweenthe aluminum
barrel and the stainlesssteel wire rope
constitutingthe lockingmechanism.
Producthas delaytime toresponse of
C.N.Swhichwill requirethe patientto
adapt to thisandmodifywalkingslightly.
22 To what mechanical forceswill the medical
device be subjected?
A
Weightof Mass of body(100kg)
0.49Nm/kg (Torque generatedwhile
climbingupthe stairs)

34. 34
23) What determinesthe lifetime of the
medical device?
A
Environmental conditions,i.e timeworn
for eachuse,terrainusedwhenwearing
prosthesis,weight,intensityof activity
duringuse.
24) Isthe medical deviceintendedforsingle
use?
N/A Designedasa long-termwalkingaid
25) Issafe decommissioningordisposal of the
medical device necessary?
A
Yes,electroniccomponentsandbattery
mustbe disposedof correctly.
26) Doesinstallationoruse of the medical
device require special trainingorspecial skills?
A
Yes,the socketmust be mouldedtothe
patient,thisrequiresspecifictrainingfor
the mouldingprocess.Thisisonlyrequired
to be done once at a clinic.All home
maintenance andtrainingwillalsobe
providedatthe clinicduringfitting.
27) Howwill informationforsafe use be
provided?
A User Manual andClinicTraining.
28) Will newmanufacturingprocessesneedto
be establishedorintroduced?
N/A
Use of Tegris® to replace expensive carbon
fibre socket.
29) Issuccessful applicationof the medical
device criticallydependentonhumanfactors
such as the userinterface?
A
Yes,incorrectGate Cycle analysiscanlead
to malfunctionof the device /prosthesis
29.1) Can the userinterface designfeatures
contribute touse error?
N/A
No userinterface andpatientdoesnot
have access tothe software.
29.2) Is the medical device usedinan
environmentwhere distractionscancause use
error?
A
Yes,change in groundstability/humanetc.
can cause failure of the prosthesisorloss
of balance of the user.
29.3) Doesthe medical device have
connectingpartsor accessories?
A ProstheticFoot
29.4) Doesthe medical device have acontrol
interface?
N/A No.
29.5) Doesthe medical device display
information?
N/A No
29.6) Is the medical device controlledbya
menu?
N/A No.

35. 35
29.7) Will the medical devicebe usedby
personswithspecial needs?
A Yes.
29.8) Can the userinterface be usedtoinitiate
useractions?
N/A No userinterface
30) Doesthe medical device use analarm
system?
N/A No.
31) Inwhat way(s) mightthe medical device
be deliberatelymisused?
A Weapon.CarryingDevice.
32) Doesthe medical device holddatacritical
to patientcare?
A Yes,
33) Isthe medical deviceintendedtobe
mobile orportable?
A Yes.LiterallyAIDSmobility
34) Doesthe use of the medical device
dependonessentialperformance?
A
Requiredstrongupperbodyandcore to
helpbalance anddirectionusing
prosthesis.
8.1.3 Initiating events&circumstances

36. 36

37. 37
8.1.4 Identification ofhazardsand estimation ofrisks
3. Evaluation of Risk Acceptability
8.1.5 Risk Acceptability

38. 38
8.1.6 Skin Damage
The patientshouldreadthe instructionbookletonhow tocleantheirtransfemoral prosthesis
to ensure preservationof theirskin.Theyshouldensure tokeeptheirsock cleananddryat all time to
avoid damaging the MMG sensors. The socket would ideally made of Tegris as it is a cheaper
alternative to carbon fibre. The patient’s socket would have to fit him or her specific dimensions to
ensure a properbackpressure onthe endof the stump.Thiswill limitthe skindamagesdue tolackof
terminal pressureinthe endof the limb.The phenomenoncanbe explainedbythe lackof muscle that
will pumpbloodaway from the distal tissues.The sockethas to distribute the pressure evenlyalong
the stump to decrease the chances of oedema.
Figure 21: Causes of skin damage in transfemoral amputees

39. 39
8.1 Appendix C (Business Case and Targeted Consumer)
8.2.1 Businesscase
The target market for our prosthetic is amputees in developing countries. Currently there is no
affordable active prostheticavailable,andsucha prostheticwouldhave a large consumerbase. The
most advanced knee prosthesis available for amputees in developing countries is the passive
Remotionknee thatiscurrentlyretailedat80$12
. Because manyof these amputeesare inrural areas,
data is lackingandthe numberof amputeesinneedof prostheticsisunknown(the Remotionproject
estimates 24 million people are in need of a modern prosthetic)12
.However land mines alone are
responsible for26,000 amputationseveryyear13
.Ouractive prostheticwouldthusofferanadvanced
and reliable alternative to often crude, home-made prosthesis and the passive Remotion8
for
thousands of people.
8.2 User Manual
8.3.1 Setting up theProsthetic
° Once the knee is fitted and sealed, power the leg using the two switches fitted to the lithium
batteries.
° Allow the actuators to set.
° Once the leg is set allow the health technician to perform a battery of tests to set:
a. Threshold values for actuator activation.
b. The duration of actuator activation and the extent of knee relaxation.
These parameters will depend on the user’s personal preference and physique.
To set these parameters connect the Arduinoto a personal computer using the USB cable provided.
Open the Arduino software downloadable at: http://www.arduino.cc/en/Main/Software. The
software file provided withthe Arduino should be modified in the following manner. To change the
threshold for actuator activation alter values highlighted in red (a higher value means a greater
contractionis neededtoactivate the actuator). To change the extent knee relaxationalterthe value
highlighted in blue (a greater value increases relaxation).
Figure 22: Code to tailor to user (found at lines 107 to 114)

40. 40
8.3.2 Charging
° To charge,use the providedlithiumbatterychargers,alternativelyplugavoltmeterintothe battery
(+ pole to- pole andvice versa)andprovide avoltageslightlyhigherthanthatindicatedonthe battery
packaging.
8.2.3 Safety and maintenance
° Ensure no rust accumulates around hinges. Clean the inner socket regularly using a cloth and
adequate cleaning material as given by the socket manufacturer.
° If there is substantial accumulation of moisture in the socket or within the prosthesis, power off,
remove from leg and allow to dry.
° Power off when not in use to ensure battery longevity.
° Do not use in large bodies of water.
° Ensure any waterproofingtextilesare replacedwhenneededtoprotectanyelectronicstheyhouse.
° Visit your health technician as prescribed for maintenance.
8.2.4 Troubleshooting
If the MMG experiences repeated issues with muscle contraction detection, actuator
activation or any other issues please hit the reset button on the Arduino:
Figure 23: Arduino Diagram with components labelled 14
If the problempersistspleasecontactyourhealthtechnician.

41. 41
8.3 Appendix C (Group Working)
8.3.3 TeamOrganization
The group was divided intotwo teams of five. One worked on the structure
of the knee the other worked on developing the MMG device.
8.3.4 Schedule
The groupmetweeklywithDr.Southgate todiscussnewideasandbringupproblems
facedbythe group. Each groupworkedonan independentschedule inseparatelabs.
Figure 24: Timeline of the project with time unit in weeks
PeriodHighlight: 1 Plan Actual %Complete Actual (beyond plan) %Complete (beyond plan)
PLAN PLAN ACTUAL ACTUAL PERCENT
ACTIVITY START DURATION START DURATION COMPLETE PERIODS
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
Locking Mechanism Idea 1 11 1 13 100%
Frame Idea 1 4 1 15 100%
November Presentation PowerPoint 9 3 10 2 100%
CAD Design 4 12 5 21 100%
MMG Testing 4 20 4 22 100%
Orders of Materials 5 1 6 3 100%
Electrical Component Order 3 15 4 20 100%
Batteries Order 16 2 18 2 100%
Force Analysis of the Frame 5 19 5 4 100%
Force Analysis of the Locking Mechanism 11 5 11 27 100%
Poster 22 4 24 2 100%
Poster Printing 24 1 25 1 100%
Report Writing 26 12 32 6 100%
Project Planner

42. 42
8.6 Appendix D (Initial designs)
8.5.1 Initialframedesigns
Figure 25. Framework design with hollow square vertical supports and horseshoe shaped flat plates.
The bar intended for attachment of wire rope ends was to be removed and the wire rope would go into the
holes on the lowest flat plate and be fixed in place with bolts.
Figure 26. Framework design with hollow cylindrical vertical supports and rings as flat plates. Note the two
marked pieces on the barrel clip that needed to be welded on if the design was to be manufactured. The wire
rope would go through the hole on the flat plats.

43. 43
8.5.2 Teeth Locking Mechanism:
Our earlydesignsinvolvedacable rotatinga pulleyora barrel to induce aknee rotation.One
endof the cable wouldbe attachedto the middle aluminiumplate of the prosthesis,while the other
end would be connected to a spring and to a linear actuator connected in series. The spring was
convenientlychosentosmoothenthe response of thedevice.Asinourfinal design,thelinearactuator
wouldapplyorrelease tensionisthe cabletocause the flexionorthe extensionof theknee.A multiple
tooth lock would lock the device to prevent any rotation in the sagittal plane.
A circular plate would be free to rotate under the influence of the cable and the linear
actuator. A quarterof a circle plate isplacedon the same axisabout whichthe free to rotate plate is
rotating,such that itis no rotationisinducedbythe linearactuator.The staticplate is equippedwith
cylindrical teeth placedat regular intervals along its curvededge. The rotating plate has holesalong
its edge spaced by regular intervals equivalent in distance to the ones on the static plate.
Torque
Aluminium
Barrel
Spring
Linear
Actuator
StainlessSteel Cable
AluminumPlate
Flexion
Extension
Figure 27: Diagram of the Teeth Locking Mechanism

44. 44
To preserve the battery’slife,thedevice wouldbe lockedwhenthe linearactuatorisnot
buildinguporreleasingtensioninthe cable.Inthe lockedposition,the staticplate wouldpreventthe
rotation of the free-to-rotate plate,as the cylindrical teeth are engagedin the holes. It was decided
that pushing the two plates towardseach other with springs would not be reliable enough and that
the teeth might not lock properly, which would induced a failure of the locking procedure. The two
plates would be brought against each other with the mean of electromagnets or electromechanical
solenoid. An electric current induces the magnetic field of the electromagnets, so they need to be
continuously connected to a power supply while the mechanism is unlocked. The electromagnets
would induce a force of attraction on the plates that would dislocate the teeth from the holes and
allow rotation of the main plate. The magnetic field, induced by the electromagnets, needed to
dislodge the twoplatescan be calculatedthanksto Ampere’sLaw,consideringthe material usedfor
the magnetic core of the magnet.
Rotating Plate
with Holes
StaticPlate with HolesNut
Figure 28: SolidWorks Representation of the Static Teeth Plate and the Rotating Holes Plate
Plate with
Holes
Plate with
Teeth
Figure 29: Schematic of the Relative Motion between the Stating and the Rotating Plates

45. 45
Ampere’s Law for an electromagnet:
𝑁𝐼 = 𝐵(
𝐿 𝑐𝑜𝑟𝑒
μ
+
𝐿 𝑔𝑎𝑝
μ0
)15
Where, N is the number of wire turns, I is the current going through the wires of the
electromagnet,Bisthe magneticfieldinthe core,Lcore isthe lengthinthe core,Lgap isthe lengthinthe
air gaps, μ is the magnetic permeability of the core, and μ0 is the permeability of free space.
The force generatedbythe electromagnetthankstoitsmagneticfieldcanbe calculated
using the following formula, where A is the cross-sectional area of the core:
𝐹 =
𝐵2 𝐴
2μ0
15
The strengthof the magneticfieldcanbe controlledbythe diameterof thecore material,
and by the number of wire turns around the core. Increasing the current I can also increase the
strengthof the magneticfieldbutwhenI is doubled,the heatgeneratedbythe systemwill increase
bya factor of 4. Indeed,accordingtothe Joule’slaw of heating,whereIisthe currentpassingthrough
Figure 29: Structure of an Electromagnet 16

46. 46
the conductor,R is the resistance of the conductor and t isthe time duringwhicha currentI isgoing
through the conductor:
𝐴𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 ℎ𝑒𝑎𝑡 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 = 𝐻 = 𝐼2 × 𝑅 × 𝑡 17
So if we double the currentfora givenresistance Randa giventime t:
𝑖𝑓 𝐼′ = 2𝐼
𝐻′
𝐻
=
(2𝐼)2
𝐼2 =
4𝐼2
𝐼2 = 4
Whenthe currentisdoubledthroughaconductor,the heat producedisincreasedbyfour
times. This could cause damages in the prosthesis or an uncomforting feeling for the patient. The
current I would therefore not be a controllable variable.
This mechanism has the advantage of precisely lock the rotation of the knee at certain
anglesof interestandtobe easilycontrolledbyadjustingthe valueof the magneticfield.Nevertheless
the precisiontobe attainedinorderto engage the teethinthe holeswouldneedtobe high,andthe
group did not think that such a level of precision could be reached. Moreover, the electromagnets
require theirownpowersource,distinctfromthe batterypoweringthe Arduinoboard. The Arduino
would have to control both the activation of the electromagnets to unlock the mechanism and the
linear actuator that will cause the rotation of the plate with the holes. This would add a level of
complexityto the Arduino code, the group therefore decided to look for an easier design for which
the Arduino board would only have to control one output: the tension applied in the cable by the
linear actuator.

47. 47
8.4.3 TheLocking Ring Mechanism:
The Locking RingMechanismwas one of our earliestideasforthe lockingmechanismof our
above-knee prosthetic. It would be implemented thanks to the use of a locking ring, a cable and a
linearactuator.The springwasdesignedtosmoothenthe swingresponseof the prosthesisduringthe
Gait Cycle. The linear actuator would be controlled by the Arduino to apply tension in the stainless
steel cable. As the tension builds-up in the cable, the locking rings would be subject to an increase
pressure onitssurroundings.Thisincreaseinpressure wouldcause the ringtolockthe rotationof the
knee byenteringincollisionwithahardstop.The LockingRing Mechanismwouldrequire the system
to use powertolockthe knee,andthereforethe knee wouldbe inanunlockedstate whenthe power
is off. The group decided that when the battery is out of power, an unlocked prosthesis could
jeopardize thesafetyof the patient.The patientwouldnotbe abletostanduprightif hisor herdevice
was runningoutof batterybecause the lockingmechanismcouldnotassure the full extensionof the
knee.The designhadtobe modifiedtoallow the knee tobe lockedinitsfull extensionpositionwhen
no power could be supplied to the linear actuator.
Torque
Locking Ring
Spring
Linear
Actuator
StainlessSteel Cable Optional
Hardstop
AluminiumPlate
Figure 30: Diagram of the Locking Ring Mechanism

48. 48
8.7 References
 1. Asterisk Cell Knee Brace Sizing Chart. http://www.asteriskbrace.com/sizingchart.html. Last accessed16th June,
2015
 2. AluminiumAlloy6082 - T6 Extrusions, Aalco MetalLtd. Last revised 03rd December 2013. 3. Bending of plates
 3 Wikipedia. https://en.wikipedia.org/wiki/Bending_of_plates. Last accessed
 4. Overview of materials for 5000 Series Aluminum Alloy, MatWeb Material Property Data .
http://www.matweb.com/search/DataSheet.aspx?MatGUID=c71186d128cd423d9c6d51106c015e8f Last
accessed 16th June, 2015
 5. Kastal 300 Data Sheet, Smiths Metal Centres 6. Factors of Safety, The Engineering Toolbox
http://www.engineeringtoolbox.com/factors -safety-fos-d_1624.html. Last accessed 16th June, 2015
 6Engineering Mechanics Volume 1, STATCS, J. L. Meriam, J. Wiley & Sons, pp 301-302, 1978.
 7 Proceedings of the World Congress on Engineering and Computer Science , (2009), Vol I
o WCECS 2009, October 20-22, 2009, San Francisco, USA, pp758-788.
o Available at: http://www.iaeng.org/publication/WCECS2009/WCECS2009_pp785-788.pdf
 8Ava D. Segal, Michael S. Orendurff, Glenn K. Klute, Martin L. McDowell,
Janice A. Pecoraro, Jane Shofer Joseph M. Czerniecki, (2006) Kinematic and kinetic comparisons of transfemoral
amputee gait using C-Leg® and Mauch SNS® prosthetic knees, Journal ofRehabilitation Researchand Development,
Volume 43 Number 7, November/December 2006, Pages 857 — 870
 9 Paul DeVita, Tibor Hortobágyi, (2003), Obesityis not associated with increased knee joint torque and power during
level walking, Journal of Biomechanics , Volume 36, Issue 9, September 2003, Pages 1355–1362
o Available at: http://ac.els-cdn.com/S0021929003001192/1-s2.0-S0021929003001192-
main.pdf?_tid=c4f258ce-14f3-11e5-ae7b-
00000aab0f02&acdnat=1434547471_7d523cf23dfaf8d82cfc52d034d138d8
 10 Toolbox.com, Factors of Safety: FOS are important in engineering design.
o Available at: http://www.engineeringtoolbox.com/factors -safety-fos-d_1624.html
 11 Texas Instruments (1986), SN754410 Quadruple Half-H Driver , SLRS007C–NOVEMBER 1986–REVISED JANUARY
2015.
o Available at: http://www.ti.com/lit/ds/symlink/sn754410.pdf
 12 D-rev.org, Remotion knee
o Available at http://d-rev.org/projects/mobility/
 13Erin Strait (2006), Prosthetics in Developing Countries, January 2006.
o Available at (http://www.oandp.org/publications/resident/pdf/DevelopingCountries.pdf).
 14 Christopher Stanton, Getting to Know Arduino: Part 1: Hello, World!, Posted on Element14.com,
o Available at: http://www.element14.com/community/groups/arduino/blog/2014/03/28/getting-to-
know-arduino-part-1-hello-world
 15 Feynman, RichardP. (1963). Lectures on Physics, Vol. 2. New York:Addison-Wesley. pp. 36–9 to 36–11, eq. 36–
26.
 16 Wikipedia.com (N.a), Electromagnet
o Available at: https://en.wikipedia.org/wiki/File:Electromagnet_with_gap.svg

49. 49
 17 Massachusetts Institute of Technology (2012), Circuits and Electronics :Joule’s Law, Last modified:Mar 23, 2012,
03:37 AM.
Available at : https://6002x.mitx.mit.edu/wiki/view/JoulesLaw/
8.8 Acknowledgments
The project would not have been possible without the guidance of Dan Greer
(University of Colorado) and Paschal Egan’s invaluable knowledge of PCB design and circuit design.
Thank you to Satpal Sangha for his invaluable knowledge in manufacturing and for his help building
the prosthesis. We wouldlike tothankDr. Southgate for hisweeklyadvice andsupport,the Imperial
Departmentof BioengineeringforallowingaccesstotheirlaboratoriesandRioTintofortheirfunding
and work space.