Prototype design and realization of an innovative energy efficient transfemoral prosthesis

Conference Paper (PDF Available) · October 2010with37 Reads
DOI: 10.1109/BIOROB.2010.5626778 · Source: IEEE Xplore
Conference: Biomedical Robotics and Biomechatronics (BioRob), 2010 3rd IEEE RAS and EMBS International Conference on
Abstract
In this paper, we present the prototype realization of the conceptual design of a fully-passive transfemoral prosthesis. The working principle has been inspired by the power flow in human gait so to achieve an energy efficient device. The main goal of this paper is to validate the concept by implementing in a real prototype. The prototype, in scale 1 : 2 with respect to the average dimensions of an adult human, is based on two storage elements, which are responsible for the energetic coupling between the knee and ankle joints during the swing phase and for the energy storage during the stance phase. The design parameters of the prototype are determined according to the human body and the energetic characteristics of the gait. The construction of the prototype is explained in details together with a test setup that has been built to evaluate the prototype.
Prototype Design and Realization of an Innovative
Energy Efficient Transfemoral Prosthesis
R. Unal, S.M. Behrens, R. Carloni, E.E.G. Hekman, S. Stramigioli and H.F.J.M. Koopman
Abstract In this paper, we present the prototype realiza-
tion of the conceptual design of a fully-passive transfemoral
prosthesis. The working principle has been inspired by the
power flow in human gait so to achieve an energy efficient
device. The main goal of this paper is to validate the concept
by implementing in a real prototype. The prototype, in scale
1 : 2 with respect to the average dimensions of an adult human,
is based on two storage elements, which are responsible for the
energetic coupling between the knee and ankle joints during
the swing phase and for the energy storage during the stance
phase. The design parameters of the prototype are determined
according to the human body and the energetic characteristics
of the gait. The construction of the prototype is explained in
details together with a test setup that has been built to evaluate
the prototype.
I. INTRODUCTION
This paper focuses on versatile, energy efficient trans-
femoral prostheses. Our research interest lays its foundation
on the fact that the literature is still lacking of prosthetic
devices that can adapt to various walking conditions and are
efficient with respect to metabolic energy consumption and
external actuation.
On one side, passive transfemoral prostheses are efficient
due to the absence of actuation but, on the other side, they re-
quire the amputee to contribute with metabolic energy (about
60% extra) to compensate the lost muscles [1]. Moreover
such prostheses cannot adapt to different walking conditions
because of their constant mechanical characteristics.
Micro-processor controlled transfemoral prostheses have
internal, intrinsically passive, actuators and, therefore, they
can change dynamically. For example, in [2] and [3], the
dynamical behavior of the prosthesis relies on the control of
a magneto-rheological damper, which produces the required
breaking knee torque during walking and, therefore, it allows
the knee to adapt to the gait pattern.
Active (powered) transfemoral prostheses, can inject
power in order to provide ankle push-off generation and
reduce the extra metabolic energy consumption, as presented
in [4], [5], [6], [7], [8], among others. With this respect,
some of the design studies have focused on transfemoral
prosthesis with energy storage capabilities in order to reduce
the power consumption [9], [10], [11]. In particular, energy
This work has been funded by the Dutch Technology Foundation STW
as part of the project REFLEX-LEG under the grant no. 08003.
{r.unal,r.carloni,s.stramigioli}@utwente.nl, Control Engineering, Faculty
of Electrical Engineering, Mathematics and Computer Science, University
of Twente, The Netherlands.
{r.unal,s.m.behrens,e.e.g.hekman,h.f.j.m.koopman}@utwente.nl, Biome-
chanical Engineering , Faculty of Engineering Technology, University of
Twente, The Netherlands.
storage and release are provided by using an adjustable
spring. Additionally, the design studies of soft actuators for
the transtibial prostheses [12], [13], [14] have shown that the
energy efficiency of the system can be improved by storing
the energy during stance phase and releasing it to provide
active ankle push-off generation.
Among the commercial transfemoral prostheses, we can
list: the passive Mauch GM [15], 3R80 [16], the micro-
processor controlled RheoKnee [15], Smart Adaptive [17]
and C-Leg [16], and the active PowerKnee [15].
Aside from the prosthetic field, theoretical mechanisms
with exotendons over multiple joints have been simulated to
show their efficiency [18]. Studies on harvesting energy out
of walking show that storing energy from the muscles around
the knee joint during swing phase is considerable to support
ankle push-off generation [19].
In this study, we present the design of a prototype of the
conceptual design that is proposed in our previous work [20].
The concept is mainly based on mimicking the energetic
behavior of human gait to improve the energy efficiency in
terms of metabolic energy consumption. To derive such kind
of mechanism, power analysis of human gait is exploited.
By analyzing the relations between the energy absorption
intervals occurring during the gait, a working principle of
the conceptual mechanism with two storage elements is
established. Following the working principle, design param-
eters have been obtained with respect to the possible energy
absorption intervals. Construction details of the mechanism
has been given and final assembly has been presented. A test
setup has been built to evaluate the concept and the prototype
realization during normal walking.
II. POWER FLOW IN THE HUMAN GAIT
Before entering in the details of the prototype design, we
want to highlight here the bio-mechanical properties of the
human gait, which have been studied by Winter in [21] and
which lead us to the conceptual design of the energy efficient
transfemoral prosthesis [20]. Fig. 1 depicts the power flow
at the knee (upper) and ankle (lower) joints during one
complete stride of a healthy human, normalized in body
weight. Note that the figure presents three instants, i.e. heel
strike, push-off and toe-off, and three main phases:
Stance: the knee absorbs a certain amount of energy
during flexion and generates as much as the same
amount of energy for its extension. In the meantime,
the ankle joint absorbs energy, represented by A3in the
figure, due to the weight bearing.
Proceedings of the 2010 3rd IEEE RAS & EMBS
International Conference on Biomedical Robotics and Biomechatronics,
The University of Tokyo, Tokyo, Japan, September 26-29, 2010
978-1-4244-7709-8/10/$26.00 ©2010 IEEE 191
Fig. 1. The power flow of the healthy human gait normalized in body
weight in the knee (upper) and the ankle (lower) joints during one stride [21].
The areas A1,2,3indicate the energy absorption, whereas Gindicates the
energy generation. The cycle is divided into three phases (stance, pre-swing
and swing) with three main instants (heel-strike, push-off and toe-off).
Pre-swing: the knee starts absorbing energy, represented
by A1in the figure, while the ankle generates the main
part of the energy for the push-off, represented by G,
which is about the 80% of the overall generation.
Swing: the knee absorbs energy, represented by A2in
the figure, during the late swing phase, while the energy
in the ankle joint is negligible.
We observe that, in the healthy human gait, the knee
joint is mainly an energy absorber whereas the ankle joint
is mainly an energy generator. Moreover, the total absorbed
energy (corresponding to the areas A1,2,3) is comparable with
the total generated energy (G). In fact, the knee absorbs
about 0.09 J/kg during pre-swing phase (A1) and 0.11 J/kg
during late swing phase (A2). On the other hand, the ankle
absorbs approximately 0.13 J/kg during stance phase (A3)
and generates about 0.35 J/kg for push-off (G). This means,
there is almost a complete balance between the generated and
the absorbed energy, since the energy for push-off generation
(G) is almost the same as the total energy absorbed in the
three intervals A1,2,3.
III. CONCEPTUAL DESIGN OF THE PROSTHESIS
Based on the evaluations of Sec. II, in our previous
work [20], we presented the principle design of an energy
efficient transfemoral prosthesis, in which the knee and the
ankle joint are energetically coupled by means of an elastic
element and, during the stance phase, energy is stored in a
second elastic element, as depicted in Fig. 2. The two storage
elements, C2and C3, have the following characteristics:
the linear elastic element C2physically connects the
upper leg, via a lever arm, and the foot (either at
the heel in P
1or at the front part of the foot in P
2)
and, therefore, couples the knee and ankle joints. This
element is responsible for the absorption A2during the
swing phase and for a part of absorption A3during the
Fig. 2. Conceptual design of the proposed mechanism - The design consists
of two storage elements, the linear spring C2between the upper leg and
foot (via a lever arm) and the linear spring C3between the lower leg and
foot (via a lever arm). The configuration change of element C2also has
been depicted with transparent representation.
Fig. 3. The working principle at swing phase - After pre-swing phase,
the attachment point of the spring C2is changed from the heel (P
1) to
the upper part of the foot (P
2) (left). At the end of the swing, the spring
is loaded and its position changes back to the P
1(right). The point P
3is
the attachment point of the spring on the lever arm of the upper leg. Note
that the configuration changes of element C2take place over a predefined
trajectory which keeps the length of the element constant.
stance phase. The change of the attachment point should
be realized without loosing any energy and, therefore,
by keeping the total length of the spring constant.
the linear elastic element C3physically connects the
lower leg and the foot and is responsible for the main
part of the absorption A3during stance phase.
The working principle of the conceptual mechanism is
represented in Fig. 3 and Fig. 4, separately for the swing
and stance phases in order to highlight the functions of the
storage elements.
IV. DESIGN PARAMETERS OF THE PROTOTYPE
The conceptual design, presented in Sec. III, has been
realized in a 1 : 2 scaled prototype in order to validate the
insights gained in the analysis of the human gait. In order
to derive the dimensions and the masses of the prosthetic
prototype, we have used the data presented in [22], [23].
The scaling procedure of the complete human body has
resulted in a total weight of 8.4 kg and height of 0.922 m,
which has comparable limb dimensions and masses accord-
ing to the grow chart from children in [24]. Fig 5 shows the
scaled human body together with the prototype and with all
the dimensions. According this scaling procedure, the limit
of the prosthesis weight is of 0.865 kg and the height is
constrained to 0.49 m.
192
Fig. 4. The working principle at stance phase - At the beginning of
the stance phase, both elements C2and C3are ready for the storage of
absorption A3(left). At the end of the stance phase, both springs are loaded
(right).
Fig. 5. Scaled human dimension as derived from [22], [23]. The total
weight of the scaled human body is 8.4 kg and the weight of the prosthesis
mechanism is 0.865.
The elastic constants of the employed springs are derived
from the energy values of the absorption intervals. The elastic
constant k2of the linear spring C2is determined from the
absorption interval A2, i.e.:
A2=1
2k2δs22,
where δs2is the deflection of the spring C2and is given by
δs2=|PP3P2| −s20,
where the magnitude of PP3P2is the length of the C2element
when it is attached between P
3and P
2(see Fig. 3) and s20
is its initial length, which is the length at the beginning of
swing (see Fig. 3 - left). It follows that k2=0.726 N/mmkg.
During stance phase, the energy is stored in both C2and
C3. Note that, this parallel structure leads to smaller elastic
constant for the element C3. During the stance phase, the
deflection δs2of the storage element C2is given by
δs2=|PP3P1| −s20,
in which the magnitude of PP3P1is the length of the element
C2when it is attached between P
3and P
1(see Fig. 3) and s20
is its initial length, which is the length at the end of swing
(see Fig. 3 - right). The deflection δs3of the stance storage
element is given by
δs3=|PP6P4| −s30,
in which the magnitude of PP6P4is the length of the element
C3, attached between P
6and P
4(see Fig. 4), and s30is its
initial length, which is the length at the beginning of roll-
over (see Fig. 4 - left). The elastic constant k3of the stance
storage element C3can be found from the energy value of
the absorption interval A3, i.e.
A3=1
2k2δs22+1
2k3δs32,
where k2is the elastic constant of the storage element C2.
It follows that k3=0.251 N/mmkg.
V. REALIZATION OF THE PROTOTYPE
In this Section, we enter in the details of the realization
of the prototype and we discuss the design choices for the
main structure, the springs, the implementation of the locks.
Finally, we present the CAD drawings of the system.
A. Main structure, knee and ankle joints
The complete prototype is depicted in Fig. 6 and it is made
by the three base components 1, 2 and 3, functioning as thigh,
shank and foot, respectively. The thigh and the shank are
made by a staff of 10 mm diameter, while the foot is made
by a U-profile, with dimension 50 ×50 ×4 mm. Since the
prototype has been built for the validation of the energetic
coupling concept, the foot design has been kept with a simple
flat bottom. All these parts are made out of aluminium ST51.
The detail Ain Fig. 7 represents the knee joint (K) which
is constructed from stock parts, i.e. 18, 4 and 19 in Figs. 6
and 7, milled out of aluminium. The milled part has an
adjustment screw (17), which is used to set the amount of
knee hyper-extension. Since, in this prototype, we did not
implement the elastic element which mimics the behavior of
the knee joint during the stance period, we designed the knee
joint such that it allows a small hyper-extension at the end of
swing phase. Therefore, stiff knee joint has been obtained in
the stance phase in order to provide stability during weight
bearing. In this way, we obtain a swing storage element in
front of the knee joint before heel-strike, which provides
natural lock with small hyper -extension in order to prevent
buckling at the heel-strike.
The ankle joint construction can be seen in cross sections
C-C and D-D in Fig. 8. Part 5 is milled with the end angles
aand b, as can be seen in cross section D-D. These angles
constrain the ankle joint such that it works within its natural
range of motion for dorsi-flexion and plantar-flexion. The
193
Fig. 6. CAD drawings of the complete prototype.
ankle joint is connected to the foot by a shaft of 10 mm
of diameter, with threaded ends (24) and spacers (23). Two
nyloc nuts are used to hold the components in place.
B. Springs
Part 6 in Fig. 6 represents a telescope structure with
springs (7). These springs are also used as extension springs
and are therefore fixated at both ends. The spring fixation
ends (9) can slide up and down for telescope length adjust-
ments. They are clamped with a screw insert (not illustrated)
onto the lower half of the telescope. The telescopes have been
installed on both sides of the prototype to avoid a moment
around the ankle and knee joints. There are other ways to
avoid this moment, i.e. by placing two shanks and two knee
joints on the outside and only one telescope in the middle.
However, the double telescope solution has been chosen. One
reason for this is to keep the body weight bearing joints: hip,
knee and ankle joints are in the same plane in order to avoid
Fig. 7. Detailed CAD drawings of the knee joint and slider locking system.
Fig. 8. Detailed CAD drawings of the ankle joint.
additional moment arms. The clamp 11 is used to adjust
the offset from the knee joint. The same construction stands
for the clamp 10 which connects the heel spring (8) to the
shank. It is used for the adjustment of the attachment point.
The telescopes (6) are connected to a ball joint (16) and,
on the other end, to the resin rollers (12). The ball joints
compensate any misalignment and, therefore, reduce friction
between 12 and the cam trajectory (13).
C. Locks
There are locking positions at both ends of the cam
trajectory. For instance 14 is a small grove in the cam
trajectory that keeps the rollers with an upward directed
force, by spring extension, in this position. Rollers stay at
this position until the force direction is inverted. This lock is
necessary to keep the springs’ position at the heel-strike and
push-off. The other lock mechanism is formed with a part 15,
see Detail B in Fig. 7. Pin 21a is connected to the 15. Pin 21b
is connected to the 3 and is blocking the counter clockwise
motion of 15. An elastic O-ring is connected between the
two pins. This lock allows the rollers to pass when they
are sliding towards to the front-side (P
2) of the foot, while
preventing them to turn back. At heel strike the lock opens,
as 22 hits the ground. This lock is crucial to keep the rollers
at position P
2after full-flexion of the knee joint and during
the swing phase. Another locking system (see Fig. 8-right) is
used for switching between the stance and the swing modes
for the ankle joint. This is implemented for the ankle spring
during swing phase in order to avoid the interference for the
natural ankle motion. Therefore, ankle spring is active only
during stance phase. This lock construction contains a lever
(24) and two shafts (25 and 26). An elastic O-ring is inserted
into the grove (24a) and wrapped around 25. This holds the
lever up and against to the 25. In the up position, the lever
194
Fig. 9. Animation of one stride (from heel-strike to heel-strike) of the 3D
CAD representation of the initial prototype, in scale 1 : 2 with respect to
the average human dimension.
end (24b) will pierce through the foot sole through the slot
28. As the foot lays flat to the ground, the lever is forced
to lay horizontal. As can be seen in section D-D it will not
interfere with adjustable bolt 27. This gives the ankle joint
its full range of motion during roll-over. At push-off the foot
first plantar-flexes, which allows the lever 24 to pass bolt 27.
Then as the foot starts dorsi-flexing during swing, the lever
24 limits the ankle motion at 0. At heel strike, the foot first
plantar-flexes again which allows the lever to pass the bolt
27 as soon as lever end 24b hits the ground again.
Note that all locking systems cost less amount of energy
as they lock when the rollers have zero velocity. Moreover,
they are simple, lightweight, low cost and passive designs.
D. CAD model
The working principle of the prototype is illustrated in
Fig. 9 by animating the CAD model during one complete
stride. Referring to the figure, frames 1 to 3 represent weight
acceptance and swing energy storage transfer. Frames 3 to 4
represent the rollover phase progressing into push-off phase
(5 and 6). The follower is forced back through the cam at
frames 6 and 7. Frames 8 to 9 show the dorsi-flexion of the
ankle in order to reach sufficient ground clearance, while
frame 8 is the start of swing phase storage which goes up to
frame 11. Stride is finishing at frame 12 with heel-strike.
The picture of the assembled prototype is presented in
Fig. 10 in a side-view to illustrate the elastic elements on
the mechanism.
VI. TEST SETUP
In order to evaluate the prototype, we built a test setup
on a walking tread-mill. The CAD drawing of the setup is
depicted in Fig. 11. This test setup is built such that the
forward movement of the hip joint is constrained and a linear
guide is used to allow vertical movement. The carriage (3)
can be used to mount the rotational hip unit. The bolt pattern
on the guidance rail, allows easy mounting of the rail to the
fixed world (2). This linear guide is capable of carrying the
maximum moment around z-axis which is mainly created by
ankle push-off.
Fig. 10. Side-view of the initial prototype in scale 1 : 2 with respect to
the average human dimension.
During the evaluation of the prototype, forces and torques
that are exerted on the hip joint can be obtained with the
6 DOF force sensor which is assembled as a hip joint (see
Fig. 11) and ground reaction forces will be measured with
the force plates that are built-in the tread-mill. Kinematics
of the mechanism is obtained by the 3D camera system
that can detect the positions of infra-red sensors attached
to the mechanism. Prototype on a treadmill with camera
system has been depicted in Fig. 12. This system uses
blinking LED markers to track the motion. By knowing
the markers position in time, the velocity and acceleration
can be derived. Multiple markers will be used for every leg
segment, therefore the bodies can be created to derive the
joint angles and angular velocities. Since the prototype has
been designed to operate in a 2D sagittal plane, the camera
system is installed perpendicular to this plane.
The total setup is going to be suspended above the
treadmill. The treadmill is simulating the forward walking
with various speeds. Additionally, the setup should allow
some added mass onto the hip joint which simulates the
load bearing during stance. This weight has to be lifted by
the operator during swing phase, simulating the weight shift
towards the sound leg. Initial tests show that gait pattern
that is comparable to normal walking can be achieved by the
prototype and the performance of the device will be evaluated
by the analysis and comparison of the measured data.
VII. CONCLUSIONS AND FUTURE WORK
In this study, we have presented the design of a prototype
to demonstrate the conceptual mechanism from our previous
work [20] for a transfemoral prosthesis inspired by the power
flow in the human gait. The conceptual mechanism that
consists of two elastic storage elements for the absorption
intervals in the healthy human gait is presented with its
working principle. The design parameters of the prototype
have been determined according to the human body and
gait characteristics. Construction details of the mechanism
has been given and final assembly has been presented.
195
Fig. 11. The CAD drawings of the test setup.
Fig. 12. Prototype on a treadmill with camera system (on the left).
The test setup has been built to evaluate and validate the
concept during normal walking. According to the initial tests,
energetic coupling and working principle are promising to
achieve energy efficient prosthetic device. This work is a
preliminary design study to demonstrate the idea of energetic
coupling of the knee and ankle joints in order to support
ankle push-off. Further developments and improvements will
be implemented by analyzing the data collected from test
setup.
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196
    • "Note that there are a few prostheses in which the knee and the ankle joints are connected each other for level walking [11]. These prosthesis uses energy stored in the ankle joint for knee extension while the energy flow of the proposed prosthesis energy flow is intended to be opposite direction. "
    [Show abstract] [Hide abstract] ABSTRACT: Stair ascent is still a difficult task for transfemoral prosthesis users. So far, a passive knee joint unit for transfemoral prosthesis developed in our previous study achieved stair ascending without handrails or any other assistance devices. However, the experimental results with the simulated socket showed that the users faced difficulty in using the knee joint at the beginning of the load response phase. In addition, large moment power of the prosthetic side's hip joint in the stance phase was required, leading to a significant imbalance of power consumption between the intact and prosthetic sides. Therefore, the purpose of the present study was to develop a passive transfemoral prosthesis for stair ascent, which had a knee extension function associated with a movable ankle joint during the stance phase in order to change the admissible force region. Experimental results by non-amputated participants with the simulated socket demonstrated that ascending was achieved in the step-over-step manner without any assistive device. In addition, it was shown that the hip joint moment power reduced to equalize the joint moment powers in both sides.
    Full-text · Conference Paper · Aug 2015 · IEEE Robotics & Automation Magazine
    • "This is highly desirable, because it creates natural equilibria and ensures that the actuator does not need to counteract the parallel springs in these frequent postures. It should be noted that during level-ground walking of able-bodied subjects, the knee joint only generates little positive kinetic energy [29], [30], and entirely passive joints may be suitable for prosthetic or orthotic devices aiming to restore physiological gait, as investigated by several research groups [44]–[47]. Nevertheless, it has been shown that powered knees can decrease metabolic energy consumption during level-ground walking [48]. "
    [Show abstract] [Hide abstract] ABSTRACT: Despite tremendous improvements in recent years, lower-limb prostheses are still inferior to their biological counterparts. Most powered knee joints use impedance control, but it is unknown which impedance profiles are needed to replicate physiological behavior. Recently, we have developed a method to quantify such profiles from conventional gait data. Based on this method, we derive stiffness requirements for knee prostheses, and we propose an actuation concept where physical actuator stiffness changes in function of joint angle. The idea is to express stiffness and moment requirements as functions of angle, and then to combine a Series Elastic Actuator (SEA) with an optimized nonlinear transmission and parallel springs to reproduce the profiles. By considering the angle-dependent stiffness requirement, the upper bound for the impedance in zero-force control could be reduced by a factor of two. We realize this ANGle-dependent ELAstic Actuator (ANGELAA) in a leg, with rubber cords as series elastic elements. Hysteresis in the rubber is accounted for, and knee moment is estimated with a mean error of 0.7 Nm. The nonlinear parallel elasticity creates equilibria near 0deg as well as 90deg knee flexion, frequent postures in daily life. Experimental evaluation in a test setup shows force control bandwidth around 5-9 Hz, and a pilot experiment with an amputee subject shows the feasibility of the approach. While weight and power consumption are not optimized in this prototype, the incorporated mechatronic principles may pave the way for cheaper and lighter actuators in artificial legs and in other applications where stiffness requirements depend on kinematic configuration.
    Full-text · Article · Jun 2015
    • "This allows the hand to perform different kinds of grasps with a limited amount of actuators, causing the arm to be lightweight and compact. Unal et al. [28] used passive latches in their ankle-knee prosthesis. Based on the phase of the walking cycle, multiple latches lock and unlock in order to control the energetic coupling between the ankle and the knee during the swing phase and the stance phase. "
    [Show abstract] [Hide abstract] ABSTRACT: Locking devices are widely used in robotics, for instance to lock springs and joints or to reconfigure robots. This review article classifies the locking devices currently described in the literature and performs a comparative study. Designers can therefore better determine which locking device best matches the needs of their application. The locking devices are divided into three main categories based on different locking principles: 1) mechanical locking, 2) friction-based locking, and 3) singularity locking. Different locking devices in each category can be passive or active. Based on an extensive literature survey, this article summarizes the findings by comparing different locking devices on a set of properties of an ideal locking device.
    Full-text · Article · Mar 2015
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