This document describes the design of a 2-degree of freedom ankle prosthesis using variable position locking mechanisms. It aims to address issues with previous passive and active ankle prosthesis designs by capturing and storing energy early in the stance phase and releasing it later based on ankle kinematic and kinetic profiles. A parametric model was developed in MATLAB to simulate prosthesis performance. Preliminary results show the design is feasible and can achieve 25% of the work output of a natural ankle. Future work will focus on further optimizing the design within mechanical constraints and completing prototype development and testing.

1. Design
of
a
2-­‐DOF
Ankle
Prosthesis
Using
Variable
Posi9on
Locking
Mechanisms
Alexander
Folz
Advisor:
Dr.
Joseph
Schimmels
Marque<e
University
PROBLEM
An
esFmated
159,000
TransFbial
(below
the
knee)
amputaFons
were
performed
in
the
United
States
in
the
year
19961.
Prostheses
available
to
these
individuals
include
passive
opFons
(affordable
but
limited
performance)
and
acFve
opFons
(electromechanical
energy
generaFon
but
oVen
not
financially
feasible).
Due
to
constraints
related
to
each
style,
neither
is
an
ideal
soluFon.
INTRODUCTION
This
is
the
moFvaFon
to
develop
a
new
soluFon,
a
passive
device
that
yields
energy
return
characterisFcs
through
the
conversion
of
translaFonal
energy
along
the
leg
into
rotaFonal
energy
about
the
ankle
joint.
However,
Fming
of
the
capture
and
release
of
energy
must
be
considered.
This
is
the
novel
porFon
of
the
design
and
also
how
it
differs
from
previous
design
generaFons.
In
previous
generaFons,
two
key
Fming
principles
were
as
follows:
1. A
mid-­‐stance
deflecFon
technique
was
employed
to
store
translaFonal
energy
2. Timing
of
energy
storage
was
based
on
the
ankle
kinemaFc/kineFc
profiles
(as
seen
in
Figure
1)
However,
these
facets
of
the
design
were
found
to
be
undesirable
based
upon
human
subject
tesFng
for
the
following
reasons:
1. Subjects
found
a
sudden
mid
stance
deflecFon
to
be
jarring
2. Issues
in
the
consistency
of
energy
storage
and
release
occurred
due
to
a
user’s
ankle
profiles
varying
from
stride
to
stride
These
issues
will
be
addressed
in
the
next
generaFon
prosthesis
through
the
use
of
the
following
principles:
1. DeflecFon/energy
storage
will
occur
early
in
the
stance
cycle
(as
seen
in
Figure
4)
2. Timing
of
energy
storage
and
release
will
be
based
on
the
maxima/minima
of
angle
kineFc
and
kinemaFc
profiles
(i.e.,
d·∙/dt
=
0)
Other
design
aspects
to
be
addressed
are
as
follows:
1. Decreased
weight
relaFve
to
previous
generaFons
2. Increased
durability
OBJECTIVES
NATURAL
ANKLE
BEHAVIOR
TIMING
STRATEGY
§ ConFnue
to
iterate
opFmizaFon
procedure
within
mechanical
(space,
available
spring
rates,
etc.)
constraints
§ Complete
conceptual
3D
model
of
opFmized
ankle
design
§ Detail
design
and
drawing
generaFon
§ Prove
feasibility
of
early
stance
deflecFon
to
capture
linear
energy
§ Must
result
in
net
posiFve
rotaFonal
work
§ Timing
of
energy
storage
and
release
based
solely
on
maxima/minima
of
ankle
profiles
§ Safely
usable
by
a
250
lb.
individual
§ Lower
weight
than
previous
generaFons
§ Customizable
for
a
mulFtude
of
users
and
deflecFon
sejngs
A
parametric
model
of
the
prosthesis
was
developed
and
simulated
using
MATLAB.
Using
a
quasi-­‐staFc
model,
inputs
and
outputs
are
as
follows:
1. Inputs
a) Leg
Force
[N]
b) Ankle
Angle
[Rad]
2. Outputs
a) Ankle
Moment
[Nm]
b) Leg
DeflecFon
[mm]
Over
the
course
of
the
stance
cycle,
there
exists
two
key
moments
in
regards
to
energy
store
and
release
and
they
are
as
follows:
1. Maximum
leg
force
(3),
in
Figure
1:
a) Timing
mechanism
for
translaFonal
energy
storage
b) Ensures
early
stance
leg
deflecFon
2. Maximum
ankle
angle
(5),
in
Figure
1:
a) Timing
mechanism
for
rotaFonal
energy
release
b) Ensures
reliable
energy
release
when
compared
to
use
of
arbitrary
ankle
angle
as
Fming
characterisFc
In
Figure
1
and
2,
several
key
moments
in
the
stance
cycle
are
idenFfied
on
each
plot:
Heel
Strike
(1),
Foot
Flat
(2),
Maximum
Leg
Force
(3),
Heel
Off
(4),
Maximum
Ankle
Angle
(5)
and
Toe
Off
(6).
Using
Figure
2,
it
is
possible
to
find
the
rotaFonal
work
done
by
the
ankle
by
integraFng
(either
analyFcally
or
numerically)
to
find
the
area
within
the
curve.
For
a
natural
ankle,
the
work
done
is
18.47
J
for
the
average
127
lb.
individual2.
Figure
1:
Ankle
kineFc/kinemaFc
profiles
as
a
funcFon
of
Fme,
stance
phase2
MODELING
FUTURE
WORK
CITATIONS
Figure
3:
Modeled
ankle
performance
compared
to
natural
ankle.
CONCLUSIONS
To
conclude,
modeled
results
proved
that
it
is
feasible
to
use
an
early
stance
deflecFon
to
capture
linear
energy.
Using
a
rudimentary
opFmizaFon
method,
the
modeled
prosthesis
was
able
to
achieve
25.46%
of
the
work
output
of
a
natural
ankle
(4.77
Joules)
for
15
mm
of
leg
deflecFon.
Figure
4:
DeflecFon
along
the
leg
axis.
Note
that
bulk
of
deflecFon
occurs
early
in
stance
WEIGHT
MINIMIZATION
Work
for
this
objecFve
will
stem
in
two
direcFons:
1. Material
SelecFon:
In
previous
generaFons,
bulk
of
materials
used
were:
a) Aluminum
6061
(Strength/Density
=
4.1e5
in)3
b) Steel
1018
(Strength/Density
=
2.2e5
in)
3
To
decrease
material
weight,
Aluminum
7075
(strength/density
=
7.5e5
in)
3
will
be
used
where
feasible.
2. Linear
moFon
mechanism
analysis:
a) Large
contributor
to
overall
weight
of
device
b) CompleFng
thorough
stress
analysis,
a
smaller
model
was
selected
c) Weight
of
component
reduced
by
25.8%
Figure
2:
Natural
Ankle
Torque-­‐Angle
Curve2.
[1]:
Owings,
M.,
and
Kozak,
L.
J.,
1998.
“Ambulatory
and
inpaFent
procedures
in
the
united
states,
1996”.
Vital
and
Health
StaFsFcs,
Series
13(139),
p.
24.
[2]:
Winter,
David
A.
Biomechanics
and
Motor
Control
of
Human
Mo5on.
New
Jersey:
John
Wiley
&
Sons,
2009
[3]:
All
strength
and
density
values
courtesy
of:
Mcmaster.com
PRELIMINARY
RESULTS