U.S. patent application number 10/613499 was filed with the patent office on 2004-04-01 for variable-mechanical-impedance artificial legs.
Invention is credited to Herr, Hugh.
Application Number | 20040064195 10/613499 |
Document ID | / |
Family ID | 32033444 |
Filed Date | 2004-04-01 |
United States Patent
Application |
20040064195 |
Kind Code |
A1 |
Herr, Hugh |
April 1, 2004 |
Variable-mechanical-impedance artificial legs
Abstract
In one aspect, the invention provides methods and apparatus
facilitating an adjustable-stiffness prosthesis or orthosis
(including approximations to arbitrarily definable non-linear
spring functions). Spring rates may be varied under no-load
conditions during a walking gate cycle to minimize power
consumption. In another aspect, the invention provides methods and
apparatus for outputting positive power from a prosthesis or
orthosis, facilitating high-performance artificial limbs. In one
embodiment of the invention, the positive power is transferred from
a functioning muscle to the prosthesis or orthosis, which mimics or
assists a non-functioning or impaired muscle. In another embodiment
of the invention, the positive power comes from an on-board power
source in the prosthesis or orthosis.
Inventors: |
Herr, Hugh; (Somerville,
MA) |
Correspondence
Address: |
Hugh Herr
51 Montrose St.
Somerville
MA
02143
US
|
Family ID: |
32033444 |
Appl. No.: |
10/613499 |
Filed: |
July 3, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60395938 |
Jul 15, 2002 |
|
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|
Current U.S.
Class: |
623/24 ; 602/23;
623/26; 623/35; 623/44; 623/46; 623/52; 623/55 |
Current CPC
Class: |
A61F 5/01 20130101; A61F
2/74 20210801; A61F 2002/6845 20130101; A61F 2/66 20130101; A61F
2002/5033 20130101; A61F 2005/0134 20130101; A61F 2/64 20130101;
A61F 2002/5007 20130101; A61F 2/6607 20130101; A61F 2/602 20130101;
A61F 2002/6621 20130101; A61F 2005/0169 20130101; A61F 2/60
20130101; A61F 2002/6614 20130101; A61F 2002/503 20130101 |
Class at
Publication: |
623/024 ;
623/026; 623/035; 623/044; 623/046; 623/052; 623/055; 602/023 |
International
Class: |
A61F 002/64; A61F
002/66; A61F 002/70; A61F 002/74; A61F 002/60; A61F 005/01 |
Claims
Having described the invention, what is claimed is:
1. A variable impedance prosthesis or orthosis, comprising: a. A
proximal end for interfacing to a user; b. a distal end for
interfacing to the environment; c. a stiffness controller; d. a
controllable-spring-rate spring element.
2. The apparatus of claim 1, wherein said controllable-stiffness
spring element comprises multiple parallel interlockable spring
elements.
3. The apparatus of claim 1, wherein said controllable-stiffness
spring element comprises a spring element with a variable
mechanical advantage.
4. The apparatus of claim 1, wherein said controllable-stiffness
spring element comprises multiple parallel valved pneumatic spring
elements.
5. The apparatus of claim 1, wherein said controllable-stiffness
spring element comprises a spring element and a parallel powered
mechanical force source.
6. The apparatus of claim 1, wherein said controllable-stiffness
spring element comprises a spring element and a series powered
mechanical displacement source.
7. The apparatus of claim 1, wherein said controllable-spring-rate
spring element further comprises: a. a first spring element
disposed between said proximal end and said distal end; b. a
mechanical energy storage element; c. a controllable power source
configured to store energy in said energy storage element; d. a
controllable coupling between said energy storage element and said
first spring element; e. a controller configured to control timing
and rate of power output of said controllable mechanical power
source, and coupling of controllable coupling.
8. The apparatus of claim 7, wherein said controllable mechanical
power source comprises a muscle and a controllable mechanical
coupling between said muscle and said energy storage element
9. A method for providing variable mechanical impedance in a
prosthetic or orthotic, comprising varying the spring rate a
controllable-spring-rate spring automatically with a spring-rate
controller as a function of a repeated cycle of use of said
prosthetic or orthotic.
10. The method of claim 9, wherein said variable-spring-rate spring
comprises multiple parallel interlockable spring elements, and said
controller controls the interlocking of said elements.
11. The method of claim 9, wherein said variable-spring-rate spring
further comprises a first spring and an energy storage element, and
further comprising: a. storing energy from a power source in said
energy storage element during a first span of time; b. releasing
energy from said energy storage element in the form of mechanical
work displacing a proximal end of a prosthesis from a distal end of
said prosthesis or orthosis during a second span of time.
Description
[0001] This patent application claims priorety of Provisional
Patent Application No. 60/395,938, filed Jul. 15, 2002.
[0002] The invention relates generally to the fields of legged
robotics, orthotic leg devices and prosthetic leg joints, and more
specifically to artificial limbs with time-variable mechanical
parameters.
BACKGROUND
[0003] Prosthetic limbs have come a long way since the days of
simple wooden "peg legs". Today, amputee men running on a
prosthetic leg can beat race times of the best unimpaired women
runners. It is believed that new advances in prosthetic limbs (such
as those embodied in the present invention) will soon lead to
amputees being able to out-perform the best unimpaired athletes of
the same sex in sports such as running. It is an object of the
present invention to advance the state of prosthetic limbs to a new
level, providing increased athletic performance, increased control,
and reduced body strain. It is a further object of the present
invention to provide essential elements needed for making
prosthetic limbs that more accurately mimic the mechanical behavior
of healthy human limbs.
[0004] Description of Normal, Level-ground Walking:
[0005] In order to establish terminology used in this document, the
basic walking progression from heel strike to toe off is first
explained. There are three distinct phases to a walking
stance-period as depicted in FIG. 1 with heel-toe sequence 1
through 7.
[0006] Saggital Plane Knee Phases
[0007] 1. Beginning with heel strike, the stance knee begins to
flex slightly (Sequence 1-3). This flexion allows for shock
absorption upon impact as well as keeping the body's center of
gravity at a more constant vertical level throughout stance.
[0008] 2. After maximum flexion is reached in the stance knee, the
joint begins to extend again, until full extension is reached
(Sequence 3-5).
[0009] 3. During late stance, the knee of the supporting leg begins
to flex again in preparation for the swing phase (Sequence 5-7).
This is referred to in the literature as "knee break". At this
time, the adjacent foot strikes the ground and the body is in
"double support mode" (that is to say, both legs are supporting
body weight).
[0010] Saggital Plane Ankle Phases
[0011] 1. Beginning with heel strike, the ankle undergoes a
controlled plantar-flexion phase where the foot rotates towards the
ground until the forefoot makes contact (Sequence 1-2).
[0012] 2. After controlled plantar-flexion, the ankle undergoes a
controlled dorsi-flexion phase where the tibia rotates forwardly
while the foot remains in contact with the ground (Sequence
2-5).
[0013] 3. During late stance, the ankle undergoes a powered
plantar-flexion phase where the forefoot presses against the ground
raising the heel from the ground (Sequence 5-7). This final phase
of walking delivers a maximal level of mechanical power to the
walking step to slow the fall of the body prior to heel strike of
the adjacent, forwardly positioned leg.
[0014] The development of artificial leg systems that exhibit
natural knee and ankle movements has been a long standing goal for
designers of legged robots, prostheses and orthoses. In recent
years, significant progress has been made in this area. The current
state-of-the-art in prosthetic knee technology, the Otto Bock
C-Leg, enables amputees to walk with early stance knee flexion and
extension, and the state-of-the-art in ankle-foot systems (such as
the ssur Flex-Foot) allow for ankle controlled plantar-flexion and
dorsi-flexion. Although these systems restore a high level of
functionality to leg amputees, they nonetheless fail to restore
normal levels of ankle powered plantar-flexion, a movement
considered important not only for biological realism but also for
walking economy. In FIG. 2, ankle power data are shown for ten
normal subjects walking at four walking speeds from slow (1/2
m/sec) to fast (1.8 m/sec). As walking speed increases, both
positive mechanical work and peak mechanical power output increase
dramatically. Many ankle-foot systems, most notably the Flex-Foot,
employ springs that store and release energy during each walking
step. Although some power plantar-flexion is possible with these
elastic systems, normal biological levels are not possible. In
addition to power limitations, the flex-foot also does not change
stiffness in response to disturbances. The human ankle-foot system
has been observed to change stiffness in response to forward speed
variation and ground irregularities. In FIG. 3, data are shown for
a normal subject walking at three speeds, showing that as speed
increases ankle stiffness during controlled plantar-flexion
increases.
[0015] Artificial legs with a mechanical impedance that can be
modeled as a spring in parallel with a damper are known in the art.
Some prostheses with non-linear spring rates or variable damping
rates are also known in the art. Unfortunately, any simple linear
or non-linear spring action cannot adequately mimic a natural limb
that puts out positive power during part of the gait cycle. A
simple non-linear spring function is monotonic, and the force vs.
displacement function is the same while loading the spring as while
unloading the spring. It is an object of the present invention to
provide actively electronically controlled prosthetic limbs which
improve significantly on the performance of artificial legs known
in the art, and which require minimal power from batteries and the
like. It is a further object of the present invention to provide
advanced electronically-controlled artificial legs which still
function reasonably well should the active control function fail
(for instance due to power to the electronics of the limb being
lost). Still further, it is an object of the present invention to
provide artificial legs capable of delivering power at places in
the gait cycle where a normal biological ankle delivers power. And
finally, it is an object of the present invention to provide
prosthetic legs with a controlled mechanical impedance and the
ability to deliver power, while minimizing the inertial moment of
the limb about the point where it attaches to the residual
biological limb.
[0016] During use, biological limbs can be modeled as a variable
spring-rate spring in parallel with a variable damping-rate damper
in parallel with a variable-power-output forcing function (as shown
in FIG. 4a). In some activities, natural human limbs act mostly as
spring-damper combinations. One example of such an activity is a
slow walk. When walking slowly, a person's lower legs (foot and
ankle system) act mostly as a system of springs and dampers. As
walking speed increases, the energy-per-step put out by the muscles
in the lower leg increases. This is supported by the data in FIG.
2.
[0017] Muscle tissue can be controlled through nerve impulses to
provide variable spring rate, variable damping rate, and variable
forcing function. It is an objective of the present invention to
better emulate the wide range of controllability of damping rate,
spring rate, and forcing function provided by human muscles, and in
some cases to provide combination of these functions which are
outside the range of natural muscles.
SUMMARY OF THE INVENTION
[0018] There are two major classes of embodiments of the present
invention. The first major class provides for actively controlled
passive mechanical parameters (actively controlled spring rate and
damping rate). This major class of embodiments will be referred to
as variable-stiffness embodiments. Three sub-classes of
variable-stiffness embodiments are disclosed:
[0019] 1) Multiple parallel interlockable springs.
[0020] 2) Variable mechanical advantage.
[0021] 3) Pressure-variable pneumatics.
[0022] The second major class of embodiments of the present
invention allows for the controlled storage and release of
mechanical energy within a gait cycle according to any arbitrary
function, including functions not available through simple
nonlinear springs. Within this second major class of embodiments,
energy can be stored and released at rates which are variable under
active control. Thus for a given joint, the force vs. displacement
function is not constrained to be monotonic or single-valued.
Within this class of embodiments, energy (from either muscle or a
separate on-board power source) can be stored and released along
arbitrarily defined functions of joint angular or linear
displacement, force, etc. This major subclass of embodiments shall
be referred to herein as energy transfer embodiments. Two
sub-classes of energy transfer embodiments are disclosed:
[0023] 1) Bi-articular embodiments (which transfer energy from a
proximal joint to a distal joint to mimic the presence of a missing
joint).
[0024] 2) Catapult embodiments (which store energy from a power
source over one span of time and release it over another span of
time to aid locomotion).
[0025] The present invention makes possible prostheses that have
mechanical impedance components (damping and spring rate) and power
output components that are actively controllable as functions of
joint position, angular velocity, and phase of gait. When used in a
prosthetic leg, the present invention makes possible control of
mechanical parameters as a function of how fast the user is walking
or running, and as a function of where within a particular step the
prosthetic leg is operating.
[0026] It is often necessary to apply positive mechanical power in
running shoes or in orthotic and prosthetic (O&P) leg joints to
increase locomotory speed, to jump higher, or to produce a more
natural walking or running gait. For example, when walking at
moderate to high speeds, the ankle generates mechanical power to
propel the lower leg upwards and forwards during swing phase
initiation. In FIG. 2, data are shown for ten normal subjects
showing that the ankle delivers more energy during a single step
than it absorbs, especially for moderate to fast walking
speeds.
[0027] Two catapult embodiments of the present invention are
described in which elastic strain energy is stored during a
walking, running or jumping phase and later used to power joint
movements. In a first embodiment, catapult systems are described in
which storage and release of stored elastic energy occurs without
delay. In a second embodiment, elastic strain energy is stored and
held for some time period before release. In each Embodiment,
mechanism architecture, sensing and control systems are described
for shoe and O&P leg devices. Although just a few devices are
described herein, it is to be understood that the principles could
be used for a wide variety of applications within the fields of
human-machine systems or legged robots. Examples of these first and
second catapult embodiments are shown in FIGS. 4 through 6.
[0028] One bi-articular embodiment of the invention described
herein comprises a system of knee-ankle springs and clutches that
afford a transfer of energy from hip muscle extensor work to
artificial ankle work to power late stance plantar-flexion. Since
the energy for ankle plantar-flexion originates from muscle
activity about the hip, a motor and power supply need not be placed
at the ankle, lowering the total mass of the knee-ankle prosthesis
and consequently the metabolic cost associated with accelerating
the legs in walking. Examples of these embodiments are shown in
FIGS. 7 and 8.
[0029] Several variable-stiffness embodiments are described herein
in which variable spring-rate structures are constructed by varying
the length of a moment arm which attaches to a spring element about
a pivot axis, thus providing a variable rotational spring rate
about the pivot axis. Examples of such embodiments are depicted in
FIGS. 9 through 11. In a preferred embodiment, variations in the
length of the moment arm are made under microprocessor control at
times of zero load, to minimize power consumed in the active
control system.
[0030] Variable-stiffness embodiments of the present invention
employing multiple interlockable parallel spring elements are
depicted in FIGS. 12 through 14. In FIGS. 12a and 12b, multiple
parallel elastic leaf spring elements undergo paired interlocking
at pre-set joint flexures or under microprocessor control. This
embodiment makes possible arbitrary piecewise-linear approximations
to non-linear spring functions (such as function 624 in FIG. 12d).
A pneumatic embodiment which can be configured to behave similarly
to the leaf spring embodiments shown in FIGS. 12a and 12b is shown
in FIG. 13. In the pneumatic embodiment of FIG. 13, valves are
electronically closed to effectively increase the number of
pneumatic springs in parallel.
[0031] The multiple parallel spring elements in FIGS. 12a, 12b, and
FIG. 13 could equivalently be replaced by other types of spring
elements, such as coil springs, torsion bars, elastomeric blocks,
etc.
BRIEF DESCRIPTION OF THE FIGURES
[0032] FIG. 1: Depiction of stages of a gait cycle, including
controlled plantar-flexion, controlled dorsi-flexion, and powered
plantar-flexion.
[0033] FIG. 2: Data from ten normal subjects are plotted showing
mechanical power output versus percent gait cycle in walking. Both
zero and one hundred percent gait cycle correspond to heel strike
of the same foot
[0034] FIG. 3: Data for one subject, showing normal biological
ankle function during the controlled plantar-flexion phase of
walking.
[0035] FIG. 4a: Basic catapult embodiment of the present invention,
represented in terms of a lumped-parameter model.
[0036] FIG. 4b: Force-displacement graph where darkened area
represents extra stored energy (used in walking/running) put into
catapult system by force actuator while prosthetic foot is off the
ground.
[0037] FIG. 4c: Side view of simplified prosthetic mechanism
designed to provide powered plantar-flexion.
[0038] FIG. 4d: Front view of simplified prosthetic mechanism
designed to provide powered plantar-flexion.
[0039] FIG. 5a: Catapult foot prosthesis or shoe orthosis for
walking, running, and jumping, shown in the equilibrium
configuration.
[0040] FIG. 5b: Catapult foot prosthesis or shoe orthosis for
walking, running, and jumping, shown in a compressed state.
[0041] FIG. 6a: Side view of catapult leg prosthesis for walking,
running, and jumping, shown in the equilibrium state.
[0042] FIG. 6b: Side view of catapult leg prosthesis for walking,
running, and jumping, shown in a compressed state.
[0043] FIG. 6c: Front view of catapult leg prosthesis for walking,
running, and jumping.
[0044] FIG. 7: An external, bi-articular transfemoral prosthesis or
orthosis is shown in a heel strike to toe-off walking sequence. The
system comprises springs and controllable clutches to transfer
energy from hip muscular work to ankle powered plantar-flexion
work.
[0045] FIG. 8: An external, bi-articular transfemoral prosthesis or
orthosis is shown in a heel strike to toe-off walking sequence. The
system comprises pneumatic springs and controllable valves to
transfer energy from hip muscular work to ankle powered
plantar-flexion work.
[0046] FIG. 9: Perpendicularly-variable-moment pivotal spring
structure.
[0047] FIG. 10: Mechanical diagram of a low-profile prosthetic foot
where spring elements are actively controlled (positioned) to
affect ankle joint stiffness.
[0048] FIG. 11: Variable-stiffness joint according to the present
invention, utilizing variable mechanical advantage to produce
variable spring rate and/or variable damping rate.
[0049] FIG. 12a: Multiply interlockable parallel leaf spring
structure, shown in equilibrium position.
[0050] FIG. 12b: Multiply interlockable parallel leaf spring
structure, shown in a stored-energy position.
[0051] FIG. 12c: End view of two dove-tailed slidably attached leaf
spring terminations with controllable interlock actuator.
[0052] FIG. 12d: Piecewise-linear approximation to nonlinear spring
function achieved by interlocking successive parallel leaf springs
at various angles, and smoothed nonlinear spring function achieved
by interlocking successive parallel leaf springs through coupling
springs.
[0053] FIG. 12e: Nonlinear damping element coupling mechanism for
coupling multiple spring elements.
[0054] FIG. 13: Multiple-pneumatic-chamber variable spring rate and
energy transfer system.
[0055] FIG. 14: Prosthetic ankle/foot utilizing multiple
interlockable parallel leaf springs for ankle spring.
[0056] FIG. 15: Example prosthetic ankle/foot known in the art.
[0057] FIG. 16: Variable-stiffness pneumatic spring.
DETAILED DESCRIPTION
[0058] A powered-catapult embodiment of the present invention is
shown in FIGS. 4a-4d. FIG. 4a is a lumped-element model of a
powered-catapult prosthetic. The mounted end 203 of the prosthesis
attaches to the body, and the distal end 204 of the prosthesis
interfaces to the environment (such as the ground for a leg
prosthesis). Mounted end 203 is coupled to distal end 204 through
spring 202, and through the series combination of force actuator
205 and force sensor 201. In some embodiments, displacement sensor
206 may also be included in parallel with spring 202. If the system
is designed to operate in parallel with an existing limb, the
muscles of the existing limb are modeled by muscle 200.
[0059] A mechanical implementation of lumped-element diagram 4a is
shown in side view in FIG. 4c and in front view in FIG. 4d. In a
preferred embodiment, during the portion of a gait cycle when the
foot is not in contact with the ground, motor 205 turns spool 209
to wind on some of tension band 208, storing energy in spring 202.
Force sensor 201 and winding distance sensor 207 may be used in a
control loop to control how much energy is stored in spring 202,
and how rapidly this energy is stored. Once the desired energy has
been stored, clutch 207 is actuated to keep tension band 208 from
unwinding and spring 202 from relaxing until the control system
decides to release the stored energy. The energy stored in spring
202 during the swing phase of the gait cycle is represented by the
dark area on the force vs. distance graph shown in FIG. 4b.
[0060] During the powered plantar-flexion phase of the gait cycle,
the control system releases clutch 207, allowing the stored energy
in spring 202 to be released, imitating the powered plantar-flexion
stage of a normal gait cycle. This release of energy mimics the
pulse of power put out by a biological ankle during the powered
plantar-flexion stage of a walking or running gait cycle.
[0061] In an alternate embodiment, motor 205 may store energy in
spring 202 at the same time as the natural leg stores impact energy
during the gait cycle. This embodiment can be used to effectively
implement one spring rate during compression (such as the spring
rate depicted by the line from the origin to point Kd in FIG. 4b)
and another spring rate during release (such as the spring rate
depicted by the line from the origin to point Ks in FIG. 4b).
[0062] In an alternate embodiment, FIG. 5 shows a prosthetic foot
or shoe orthosis that stores both muscle energy and motor energy in
spring mechanism 300 during the gait cycle, for release during the
powered plantar-flexion stage of the walking gait cycle (toe-off
propulsion). When walking on this type of catapult prosthesis or
foot orthosis, a person would experience a first (lower) spring
rate (depicted by the line from the origin to point Kd in FIG. 4b),
and a second (higher) spring rate (depicted by the line from the
origin to point Ks in FIG. 4b) when releasing energy from spring
300 during the powered plantar-flexion phase of the gait cycle.
[0063] For catapult embodiments depicted in both FIG. 4 and in FIG.
5, part of the energy released during powered plantar-flexion came
from leg muscle action compressing springs 202 and 300, and part
came from an electromechanical actuator such as a motor. In a
preferred embodiment of the present invention as depicted in FIG.
4, the majority of power stored in spring mechanisms by
electromechanical actuators occurs during the minimal-load portion
of the walking/running gait cycle (swing phase), and the start of
the energy-release phase (late stance phase) of the gait cycle may
be time-delayed with respect to the swing phase when motor energy
is stored.
[0064] FIG. 6 is another depiction of the catapult leg prosthesis
of FIG. 4, also showing socket 400, which attaches to the residual
biological limb. Although the leg prostheses shown in FIGS. 4 and 6
are below-the-knee prostheses, the invention could also be employed
in above-knee prostheses.
[0065] Two bi-articular embodiments of the present invention are
shown in FIGS. 7 and 8. In a first embodiment (FIG. 7), a
prosthesis (above or below knee), robotic leg or full leg orthosis
is shown having above-knee segment (a), knee joint (b), ankle joint
(c), posterior knee pivot (d), posterior clutch (e), posterior
spring (f), posterior cord (g), knee-ankle transfer clutch (h),
anterior pivot (i), anterior clutch (j), anterior spring (k), and
anterior cord (l). Anterior spring (k) stretches and stores energy
during early stance knee flexion (from 1 to 3) and then releases
that energy during early stance knee extension (from 3 to 5). Here
spring (k) exerts zero force when the knee is fully extended, and
anterior clutch (j) is engaged or locked throughout early stance
knee flexion and extension (from 1 to 5). This stored energy,
together with an applied extensor hip moment from either a robotic
or biological hip, result in an extensor moment at the knee,
forcing the knee to extend and stretching posterior spring (f)
(from 3 to 5). The spring equilibrium length of posterior spring
(f) is equal to the minimum distance from posterior knee pivot (d)
to posterior clutch (e) (leg configuration 3 in FIG. 7). To achieve
this spring equilibrium, posterior clutch (e) retracts posterior
cord (g) as the distance from posterior knee pivot (d) to posterior
clutch (e) becomes smaller. When this distance begins to increase
in response to knee extension and ankle dorsi-flexion (from 4 to
5), posterior clutch (e) engages, causing posterior spring (f) to
stretch. When the ankle is maximally dorsi-flexed and the knee
fully extended (leg configuration 5), posterior spring (f) becomes
maximally stretched. When the leg assumes this posture, knee-ankle
transfer clutch changes from a disengaged state to an engaged
state. Engaging the knee-ankle clutch mechanically grounds spring
(f) below the knee rotational axis, and consequently, all the
energy stored in spring (f) is transferred through the ankle to
power ankle plantar-flexion (from 6 to 7). During late stance (from
5 to 6), the knee of the supporting leg begins to flex again in
preparation for the swing phase. For this late stance knee flexion,
anterior clutch (j) is disengaged to allow the knee to freely flex
without stretching anterior spring (k).
[0066] It should be understood that the bi-articular knee-ankle
invention of embodiment I (FIG. 7) could assume many variations as
would be obvious to those of ordinary skill in the art. For
example, the system described herein could act in parallel to
additional ankle-foot springs and/or to an active or passive knee
damper. Additionally, instead of mechanically grounding spring (f)
distal to the knee axis to effectively transfer all the stored
energy through the ankle, the perpendicular distance from the line
of spring force (f) to the knee's axis of rotation could go to zero
as the knee approaches full extension.
[0067] In a second embodiment (FIG. 8), a prosthesis (above or
below knee), robotic leg or full leg orthosis is shown having a
similar energy transfer from hip muscle extensors to artificial leg
to power ankle plantar-flexion, accept energies are stored within
pneumatic springs about the knee and then transferred to the ankle
via a fluid transfer system. In this embodiment, the transfer of
energy occurs without a physical bi-articular spring such as
posterior spring (f) in FIG. 7. In this embodiment, anterior
pneumatic spring (j) compresses and stores energy during early
stance knee flexion (from 1 to 3). Here anterior knee valve (k) is
closed or locked throughout early stance knee flexion and extension
(from 1 to 5). This stored energy, together with an applied
extensor hip moment from either a robotic or biological hip, result
in an extensor moment at the knee, forcing the knee to extend and
compress posterior pneumatic spring (f) (from 3 to 5). It is
important to note that posterior knee valve (g) is open during
early stance knee flexion so that posterior pneumatic spring (f)
exerts little force. Knee valve (g) is then closed during knee
extension so that energy is stored in the posterior pneumatic
spring (f). When the ankle is maximally dorsi-flexed and the knee
fully extended (leg configuration 5), posterior pneumatic spring
(f) is maximally compressed. When the leg assumes this posture,
knee-ankle transfer valve changes from a closed state to an open
state, and anterior ankle valve (n) changes to a closed state,
allowing all the energy stored in spring (f) is be transferred
through the ankle to power ankle plantar-flexion (from 6 to 7).
During late stance (from 5 to 6), the knee of the supporting leg
begins to flex again in preparation for the swing phase. For this
late stance knee flexion, anterior and posterior valves (g, k) are
open to allow the knee to freely flex without compressing anterior
spring (j).
[0068] It should be understood that the bi-articular knee-ankle
invention of embodiment II (FIG. 8) could assume many variations as
would be obvious to those of ordinary skill in the art. For
example, the system described herein could act in parallel to
active or passive ankle-foot springs and/or to an active or passive
knee damper. Additionally, the energy in posterior pneumatic spring
(f) could be transferred to a temporary holding chamber to be later
released to the ankle during powered plantar-flexion.
[0069] The mechanical system in FIG. 9 is a
variable-mechanical-advantage embodiment of a variable-stiffness
spring. Motors 500 and motor-driven screws 505 serve to change the
moment of compression of bow spring 503 about pivot point 504. This
mechanism may be used to adjust spring stiffness with minimal power
under no-load conditions. It may also be used as an alternative way
of storing energy in a spring which is under load, and thus may be
used as a component of an immediate-release catapult system such as
depicted in FIG. 5.
[0070] FIG. 10 depicts a low-profile prosthetic foot-ankle with top
plate 1 and bottom plate 2, where spring elements are actively
controlled (positioned) to affect ankle joint stiffness. This
embodiment of the present invention is a variable-stiffness
embodiment of the "variable mechanical advantage" sub-class. In
this low-profile prosthetic ankle joint embodiment, side-to-side
spring rates of the prosthetic ankle and front-to-back spring rates
of the prosthetic ankle are adjusted by varying the distance of
spring elements 4, 5, 6, and 7 from the central pivot point 15 of
the ankle joint. Spring top plates 13 and spring bottom plates 12
of spring elements 4, 5, 6, and 7 slide in tracks 14, driven by
position-adjusting motors 8, 9, 10, and 11. In a preferred
embodiment, motors 8, 9, 10, and 11 only change the positions of
spring elements 4, 5, 6, and 7 when the ankle joint is under zero
load (for instance, during the part of the walking gait when the
foot is not in contact with the ground). Adjustment of spring
position under zero load allows position adjustments to be done
with minimal energy. This embodiment offers independent
inversion/eversion stiffness control as well as independent
plantar-flexion and dorsi-flexion control.
[0071] A variable stiffness ankle-foot prosthesis embodiment
according to the present invention is shown in FIG. 11.
Constant-rate spring or damping element 1700 fixedly attached at
one end and movably attached at the other end. Attachment point
1701 may be moved in and out with respect to the effective pivot
point of the ankle joint. If element 1700 is a damping element,
this configuration provides a variable damping ankle joint. If
element 1700 is a spring element, this configuration provides a
variable spring rate ankle joint. FIGS. 9, 10 and 11 demonstrate
how a constant element can be transformed into a variable element
according to the present invention, by varying mechanical
advantage. In non-catapult preferred embodiments of the present
invention, the variation in mechanical advantage takes place such
that the motion used to vary the mechanical advantage takes place
substantially perpendicular to the force the element being moved is
under, thus minimizing the work needed to vary the mechanical
advantage.
[0072] FIGS. 12a and 12b depict a multiple-parallel-leaf-spring
embodiment of a variable mechanical impedance according to the
present invention. Leaf springs 600 are bound together and bound
tightly to attaching bracket 602 at one end by bolt 601. At the
other end, leaf springs terminate in slidably interlocking blocks
603, which may be locked together dynamically in pairs by
interlocking plates 605. Each interlocking plate 605 is permanently
bonded to one leaf spring terminator block 603 at surface interface
606, and controllably bindable to a second leaf spring terminator
block 604 at a second interface 607, by binding actuator 608.
Binding actuator 608 may bind surface interface 607 by any number
of means such as mechanical clamp, pin-in-socket, magnetic clamp,
etc. Adjacent leaf spring terminator blocks are slidably attached
by dovetail slides or the like. The structure shown in FIGS. 12a-c
can be used to implement a piecewise-linear spring function such as
function 604 depicted in FIG. 12d, by engaging successive
interlocks 605 at pre-determined points in spring flexure, and
disengaging at like points.
[0073] In a preferred embodiment, the slope discontinuities in
function 604 may be "smoothed" by coupling successive leaf springs
through coupling springs. In FIG. 12d, stop plate 619 is affixed to
leaf spring termination 620, and coupling spring 621 is mounted to
leaf spring termination 618 through coupling spring mount 622. Leaf
spring termination 620 is free to slide with respect to leaf spring
termination 618 until coupling spring 621 and stop plate 619 come
in contact. Coupling spring 621 acts to smooth the transition from
the uncoupled stiffness of two leaf springs to the coupled
stiffness of two leaf springs, resulting in smoothed
force-displacement function 625 in FIG. 12d.
[0074] In a preferred embodiment, coupling spring 621 is itself a
stiff, nonlinear spring. In another preferred embodiment, coupling
spring 621 may have actively controllable stiffness, and may be
made according to any of variable-stiffness spring embodiments of
the present invention.
[0075] FIG. 12e depicts a non-linear dissipative coupling mechanism
for coupling pairs of spring elements in a
multiple-parallel-element spring. Mechanical mounts 609 and 610
affix to a pair of spring elements to be coupled. In a preferred
embodiment, one of 609 and 610 is permanently affixed and the other
of 609 and 610 is controllably affixed through a mechanism such as
608 described above. Piston 611 is coupled to mount 609 through rod
612 which passes through seal 614. Thus piston 611 may move back
and forth in chamber 615 along the axis of rod 612. Chamber 615 is
preferably filled with viscose or thixotropic substance 616. A
viscose substance can be used in chamber 616 to provide a
mechanical coupling force proportional to the square of the
differential velocity between mounts 609 and 610. A thixotropic
substance (such as a mixture of corn starch and water) can be used
to provide an even more nonlinear relationship between coupling
force and the differential velocity between coupling plates 609 and
610. Alternately, an electronically controlled variable damping
element may be used in series with force sensor 617 between mounts
609 and 610, to provide an arbitrary non-linear dissipative
coupling.
[0076] Utilizing a nonlinear dissipative coupling between pairs of
elements in a multiple-parallel-element spring allows joint spring
rates in a prosthetic limb which are a function of velocity. Thus,
a joint spring rate can automatically become stiffer when running
than it is while walking.
[0077] In one preferred embodiment, chamber 615 is rigidly mounted
to mount 610. In another preferred embodiment, chamber 615 is
mounted to mount 610 through coupling spring 623. In a preferred
embodiment, coupling spring 623 may be an actively-controlled
variable stiffness spring according to the present invention.
[0078] FIG. 13 depicts a multiple-couplable-parallel element
pneumatic embodiment of the present invention. Multiple parallel
pneumatic chambers 900 couple mounting plates 908 and 909.
Pneumatic hoses 902 connect chambers 900 to a common chamber 901
through individually actuatable valves 903. Spring stiffness
between plates 908 and 909 is maximized when all valves 903 are
closed, and minimized when all valves 903 are open. Additional
pneumatic element 905 may be added to transfer power from one
prosthetic joint to another.
[0079] In an immediate-energy-transfer embodiment of the present
invention according to FIG. 13, valves 904 and 906 may be timed to
actuate in sequence with valves 903 to transfer power directly from
chamber 905 to chambers 900. In a delayed-energy-transfer
embodiment of the present invention according to FIG. 13, energy
may be transferred from chamber 905 to chambers 900 or vice versa
in a delayed manner, by chambers 900 or chamber 905 first
pressurizing chamber 901, then isolating chamber 901 by closing
valves 903 and 904 for some period of time, then transferring the
energy stored in chamber 901 to chambers 900 or 905 by opening the
appropriate valves.
[0080] FIG. 15a depicts a prosthetic ankle-foot system known in the
art. Ankle spring 1500 is affixed to foot-plate 1501. One
variable-stiffness embodiment of the present invention shown in
FIG. 15 uses a multiple-parallelly-interlockable-leaf-spring
structure such as that shown in FIG. 12 in place of ankle spring
1500. Multiple-parallelly-inter- lockable-leaf-spring 1600 allows
for different spring rates in forward and backward bending,
allowing separately controllable rates of controlled
plantar-flexion and controlled dorsi-flexion.
[0081] In one embodiment of the present invention (shown in FIG.
15b), ankle spring 1500 is split into inner ankle spring 1500a, and
outer ankle spring 1500b, and heel spring 1501 is split rearward of
attachment point AP into inner heel spring 1501a and outer heel
spring 1501b. In a preferred embodiment, ankle springs 1500a and
1500b and heel springs 1501a and 1501b each comprise
actively-variable multi-leaf springs such as ankle spring 1600 in
FIG. 14. Having separate inner and outer variable-stiffness ankle
springs allows for active control of side-to-side stiffness of the
prosthetic ankle joint. Having separate inner and outer
variable-stiffness heel springs allows for active control
medio-lateral ankle stiffness.
[0082] A pneumatic embodiment of a variable-stiffness spring for a
prosthesis is shown in FIG. 16. Male segment 702 comprises one end
of the overall variable-stiffness spring, and female segment 701
comprises the other end. Control electronics 710 are contained in
the upper end of male segment 710. Intake valve 715 is actuatable
to allow air to enter pressure chamber 708 through air intake
channel 716 when pressure chamber 708 is below atmospheric pressure
(or an external pump may be used to allow air to enter even when
chamber 708 is above atmospheric pressure). Air pressure sensor 709
senses the pressure in pressure chamber 708. Pressure chamber 708
is coupled to second pressure chamber 703 through valve 711. The
air in pressure chamber 703 acts as a pneumatic spring in parallel
with spring 704. Motor 705 turns ball screw 707 to move piston 706
back and forth to control the volume of pressure chamber 708.
Pressure in pressure chamber 703 may be lowered to a desired value
by opening valve 703 for a controlled period of time, allowing air
to escape through pressure release channel 714.
[0083] In one mode of operation, valve 711 is open and pressure
chambers 708 and 703 combine to form a single pressure chamber. In
this mode, movement of piston 706 directly controls the overall
pressure chamber volume, and thus the overall pneumatic spring
rate. In another mode of operation, valve 711 is closed, and valve
706 may be opened and piston 706 may withdrawn to add air to the
system.
[0084] In a preferred embodiment of a variable-stiffness leg
prosthesis according to the present invention is implemented
through the pneumatic system of FIG. 16, motion of piston 706
occurs under minimal load, such as during the phase of gait when
the foot is off the ground, or when the user is standing still.
[0085] The pneumatic system shown in FIG. 16 may also be used to
implement immediate-release or delayed-release catapult embodiments
of the present invention. An immediate-release catapult may be
implemented by opening valve 711, and using motor 705 to add power
(for instance, during the powered plantar-flexion phase of gait) as
the power is needed. In a delayed-release catapult embodiment of
the present invention, valves 715 and 711 are closed while motor
705 moves piston 706 to pressurize chamber 708, and then energy
stored in chamber 708 is rapidly released during a phase of gait to
produce the same effect as powered plantar-flexion.
[0086] In a preferred embodiment of the present invention, a
pneumatic prosthetic leg element according to FIG. 16 is combined
with the multiple controllably-couplable parallel leaf spring
prosthetic ankle-foot of FIG. 15 to provide a prosthetic limb which
provides powered plantar-flexion, controllable compressional leg
spring stiffness, and controllable ankle stiffness during
controlled plantar-flexion and controlled dorsi-flexion.
[0087] The foregoing discussion should be understood as
illustrative and should not be considered to be limiting in any
sense. While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
spirit and scope of the invention as defined by the claims.
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