U.S. patent application number 13/777327 was filed with the patent office on 2013-07-04 for lower limb prosthesis and control unit.
This patent application is currently assigned to BLATCHFORD PRODUCTS LIMITED. The applicant listed for this patent is Blatchford Products Limited. Invention is credited to David Moser, Andrew John Sykes, Mir Saeed Zahedi.
Application Number | 20130173019 13/777327 |
Document ID | / |
Family ID | 38326865 |
Filed Date | 2013-07-04 |
United States Patent
Application |
20130173019 |
Kind Code |
A1 |
Sykes; Andrew John ; et
al. |
July 4, 2013 |
Lower Limb Prosthesis and Control Unit
Abstract
A self-teaching lower limb prosthesis, for an above-knee
amputee, including a dynamically adjustable joint movement control
unit arranged to control operation of the joint automatically. A
control unit electrically stores a target relationship between a
kinetic or kinematic parameter of locomotion and walking speed. The
relationship defines a number of values of the parameter associated
with different walking speeds. The control unit generates
monitoring signals representative of walking speed values and
values of the parameter occurring at different walking speeds. An
adjustment system adjusts the control unit automatically when the
monitoring signals indicate deviation from the target relationship
so as to bring the parameters close to that defined by the target
relationship.
Inventors: |
Sykes; Andrew John; (Surrey,
GB) ; Zahedi; Mir Saeed; (London, GB) ; Moser;
David; (Southampton, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Blatchford Products Limited; |
Hampshire |
|
GB |
|
|
Assignee: |
BLATCHFORD PRODUCTS LIMITED
Hampshire
GB
|
Family ID: |
38326865 |
Appl. No.: |
13/777327 |
Filed: |
February 26, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12282541 |
Sep 11, 2008 |
8403997 |
|
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PCT/GB2007/000973 |
Mar 20, 2007 |
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13777327 |
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60807505 |
Jul 17, 2006 |
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Current U.S.
Class: |
623/24 |
Current CPC
Class: |
A61F 2/76 20130101; A61F
2/80 20130101; A61F 2002/607 20130101; A61B 5/4851 20130101; A61F
2002/704 20130101; A61F 2002/747 20130101; A61F 2002/7625 20130101;
A61F 2/68 20130101; A61F 2002/5003 20130101; A61B 5/112 20130101;
A61F 2/70 20130101; A61F 2/64 20130101; A61F 2002/5033 20130101;
A61F 2002/705 20130101 |
Class at
Publication: |
623/24 |
International
Class: |
A61F 2/68 20060101
A61F002/68 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 24, 2006 |
GB |
0605963.8 |
Claims
1. A self-teaching lower limb prosthesis for an above-knee amputee,
the prosthesis including a dynamically adjustable knee movement
control unit arranged to control flexion and/or extension movements
of a knee joint of the prosthesis, wherein the control unit
includes: an electronic storage device storing a target
relationship between a kinetic or kinematic parameter of locomotion
and walking speed, the relationship defining a plurality of values
of the parameter associated with different respective walking
speeds, a monitoring system arranged to generate monitoring signals
representative of walking speed values and values of the parameter
occurring during use of the prosthesis at different walking speeds,
and an adjustment system arranged to adjust the control unit
automatically in response to the said monitoring signals indicating
deviation of the parameter from the target relationship so as to
bring the parameter closer to a value defined by the target
relationship for the respective walking speed.
2. A prosthesis according to claim 1, wherein the monitoring and
adjustment systems are respectively arranged to generate the
monitoring signals over a plurality of walking step cycles and to
calculate a parameter deviation value based on the said signals for
the said plurality of step periods, the control unit adjustment
being made in response to the parameter deviation value.
3. A prosthesis according to claim 1, wherein the adjustment system
is arranged to perform the control unit adjustments in increments
so as to bring the parameter successively closer to a value defined
by the target relationship on the basis of monitoring signals
obtained for a plurality of walking step cycles occurring within a
single walking speed region.
4. A prosthesis according to claim 1, wherein the control unit is
operable to perform a calibration routine in which the adjustment
system is operated to set the control unit to a calibration setting
at which the unit has a predetermined resistance to flexion and/or
extension movements of the knee joint, the calibration routine
including operating the monitoring system and electronic storage
device during a walking test to derive an optimum value of the said
parameter and an associated optimum walking speed value, which
values are used to define the said target relationship.
5. A prosthesis according to claim 1, wherein the control unit is
operable in a self-teaching mode and a playback mode, and is
arranged to store during the self-teaching mode electrical signals
representative of control unit settings at different respective
walking speeds, which settings are derived from the automatic
adjustments made by the adjustment system to bring values of said
parameter closer to the target relationship, and wherein the
monitoring system and adjustment system are operable in the
playback mode respectively to generate real time signals
representative of the walking speed and to cause the control unit
to be adjusted to said control unit settings in accordance with the
generated real time walking speed signals.
6. A prosthesis according to claim 1, wherein the monitoring system
and the adjustment system are arranged to adjust the control unit
automatically and successively in an iterative manner for each of a
plurality of walking speeds to bring the parameter closer to the
value defined by the target relationship for the respective walking
speed.
7. A prosthesis according to claim 1, wherein the parameter is the
ratio of a flexed state duration to an extended state duration.
8. A prosthesis according to claim 7, wherein the target
relationship comprises a function relating said ratio to walking
cycle period, the function being such that the ratio decreases as
the cycle period increases.
9. A prosthesis according to claim 8, wherein the function, when
plotted in a cartesian coordinate system, is in the form of a line
the gradient of which decreases with increasing cycle period.
10. A prosthesis according to claim 1, wherein the control unit is
operable to define a plurality of walking speeding regions and to
perform a self-teaching routine in which, in respect of each
walking speed region, the parameter is brought closer to a
respective value for that speed region as defined by the target
relationship.
11. A prosthesis according to claim 10, wherein the control unit is
arranged to compare the values of the parameter represented by the
monitoring signals occurring at times of acceleration and/or
deceleration with values defined by the target relationship, the
adjustment system being caused to adjust the control unit setting
in response to deviations from the target relationship of a
magnitude greater than a predetermined amount.
12. A prosthesis according to claim 10, wherein the walking speed
regions are dynamically and automatically variable in response to
said monitoring signals representative of walking speed values.
13. A self-teaching lower limb prosthesis for an above-knee
amputee, wherein the prosthesis includes a dynamically adjustable
knee movement control unit arranged to control flexion and/or
extension movement of a knee joint of the prosthesis according to
speed of walking, settings of the control unit for different
walking speed ranges being obtained by comparing values of a sensed
kinetic or kinematic parameter of locomotion with target values of
the parameter respectively associated with the walking speed
ranges, and wherein the boundaries between neighbouring walking
speed ranges are dynamically adjustable in response to the
distribution of measured walking speed samples within the ranges
thereby to adapt the ranges to the walking characteristics of the
amputee.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application a continuation of U.S. application Ser. No.
12/282,541 filed Sep. 11, 2008, which is a national phase entry of
International Application No. PCT/GB2007/000973, filed Mar. 20,
2007, which claims priority to U.S. Provisional Application No.
60/807,505, filed Jul. 17, 2006 and Great Britain Application No.
0605963.8 filed Mar. 24, 2006, the entire contents of which are
incorporated herein by reference.
[0002] This invention relates to a self-teaching lower limb
prosthesis including a dynamically adjustable joint movement
control unit arranged to control operation of the joint
automatically.
[0003] It is known from British Patent Application No. 2280609A to
provide a lower limb prosthesis with a dynamically adjusting
control system for controlling the movement of a shin part of the
prosthesis about a knee axis on a thigh part of the prosthesis
according to the amputee's speed of locomotion. A pneumatic piston
and cylinder device coupled between the thigh part and the shin
part has a motor-driven valve which alters the resistance of the
device to movement at the knee joint in response to command signals
from an electronic control circuit deriving input signals from a
transducer mounted on the control device, the repetition rate of
the input signals being representative of the speed of locomotion,
particularly the step period during walking.
[0004] The control circuit includes a radio receiver for receiving
command signals from a remote control transmitter operated by a
prosthetist, a processor for processing the command signals and the
transducer signals, and a memory for storing a map of valve
settings against locomotion speed ranges. The processor has a
teaching mode whereby the amputee is asked to walk at a particular
speed and the system is "taught" by the prosthetist inasmuch as the
prosthetist causes the valve to be adjusted under remote control in
real time while the amputee is walking until the best gait is
obtained. The same process is performed at different walking speeds
and the prosthetist selects a valve setting for each speed which,
in his or her opinion, appears to produce the best walking gait.
These valve settings are stored at the end of a teaching session.
In a playback mode of the processor, signals corresponding to the
stored valve settings are fed to the motor-driven valve
automatically according to the speed at which the amputee
walks.
[0005] The above system has yielded notable improvements in gait
for above-knee amputees due to its adaptation of resistance to knee
joint movement to different settings suiting different walking
speeds rather than relying on a fixed resistance setting for all
walking speeds. These improvements have been achieved without
requiring excessively lengthy sessions with the prosthetist.
[0006] A self-teaching lower limb prosthesis is disclosed in
International Patent Application No. WO99/44547. This includes a
dynamically adjustable knee movement control unit which controls
either or both of flexion and extension of the knee joint
automatically in response to a sensed step-to-step variability of
at least kinetic or kinematic parameter of locomotion. The sensed
variability is an electrical signal value representative of the
degree of variation of the parameter measured during each of a
number of steps taken by the amputee, the parameter being measured
during each step which is within a predetermined range of
locomotion, such as a particular walking speed range. Speeds of
walking or running as speed ranges may be determined by measuring
the repetition rate or the average step period of a walking or
running cycle, each cycle extending, for instance, from heel
contact to heel contact through stance phase and swing phase.
[0007] It is an object of the present invention to provide an
alternative self-teaching prosthesis.
[0008] According to a first aspect of the invention a self-teaching
lower limb prosthesis for an above knee amputee includes a
dynamically adjustable knee movement control unit arranged to
control flexion and/or extension movements of a knee joint of the
prosthesis. A control unit electrically stores a target
relationship between a kinetic or kinematic parameter of locomotion
and walking speed, the relationship defining a plurality of values
of the parameter associated with different respective walking
speeds. The control unit also includes a monitoring system which
generates monitoring signals representative of walking speed values
and values of the parameter occurring during use of the prosthesis
at different walking speeds. An adjustment system adjusts the
control unit automatically when the monitoring signals indicate
deviation of the parameter from the target relationship so as to
bring the parameter closer to a value defined by the target
relationship for the respective walking speed.
[0009] Typically, the adjustment is made on the basis of monitoring
signals from several steps, the deviation being a mean value, for
instance. In the preferred embodiment of the invention, speeds of
locomotion are defined by step cycle periods, and the terms "cycle
period", and "walking speed" (or "speed of locomotion") are used
interchangeably in this specification as related to quantities.
Iterative adjustment of the control unit setting is preferred so
that the monitored parameter approaches successively closer to the
target parameter value using, for each adjustment, one or more
parameter values measured in respect of cycle periods within a
relevant walking speed region. Such a region is, preferably, one of
several contiguous regions which together constitute a range of
speeds or step period values likely to be encountered.
[0010] In the preferred prosthesis, the control unit includes a
damper for damping flexion of the knee joint, the damping being
variable by the adjustment system. In particular, the damper may be
a pneumatic piston and cylinder assembly coupling upper and lower
limb components which are joined together by the knee joint, the
piston and cylinder assembly having a motor driven valve for
varying the area of an orifice through which air passes as the
piston moves in the cylinder in response to knee flexion, the valve
thereby offering variable resistance to the flexion movement.
[0011] The control unit preferably performs a preliminary
calibration routine in which the adjustment system is operated to
set the control unit to a calibration setting at which the unit has
a predetermined resistance to flexion and/or extension movements of
the knee joint, the calibration routine including operating a
monitoring system and the electronic storage device during a
walking test to derive an optimum value of the kinetic or kinematic
parameter and an associated optimum walking speed value. These
values can then be used to define the target relationship.
[0012] One particular parameter of locomotion which the applicants
have successfully used is a ratio of the flexed state duration and
the extended state duration. In particular, this ratio may be the
ratio of the time during which, in a walking cycle period, the knee
joint may be regarded as being in a flexed state and the time in
which, during the cycle period, the knee joint may be regarded as
being in an extended state. Transitions between the flexed state
and the extended state may be sensed by setting one or more knee
angle threshold values.
[0013] It will be appreciated that there are related parameters
which may be used instead, such as the duration of the flexed state
as a proportion of the total cycle period, as well as other
threshold-crossing knee angle functions. Indeed, the inverse ratio
or proportion may be used.
[0014] It is also possible to use, for instance, the maximum knee
angle, or a period during which the knee angle exceeds a
predetermined value, and so on, as the kinetic or kinematic
parameter.
[0015] The prosthesis can be self-teaching at all times, but it is
preferred that self-teaching only occurs when a self-teaching mode
is selected, e.g. by operation of a switch or on detection of a
special sequence of movements of the prosthesis. The self-teaching
mode may include the calibration routine as an option.
[0016] During the self-teaching mode, the control unit may be
operated to store electrical signals representative of control unit
settings, e.g. damper resistance settings, at different respective
walking speeds, which settings are derived from the automatic
adjustments made by the adjustment system to bring values of the
kinetic or kinematic parameter closer to the target relationship.
The monitoring system and the adjustment system are operable in the
playback mode respectively to generate real time signals
representative of the walking speed and to cause the control unit
to be adjusted to the control unit settings in accordance with the
generated real time walking speed signals.
[0017] Convergence of the parameter to a value defined by the
target relationship may be performed by the monitoring system and
the adjustment system conjunctively by adjusting the control unit
automatically and successively in an iterative manner. Preferably
this process is performed for each of a plurality of walking speeds
to bring the parameter to an optimum value for each respective
walking speed, the resulting control unit settings being stored as
a series of values against respective walking speed values, e.g. as
a look-up table.
[0018] The target relationship preferably comprises a function
relating the above-mentioned ratio (the ratio obtained by dividing
the flexed state duration by the extended state duration) to a
walking cycle period, the function being such that the ratio
decreases as the cycle period increases. Such a function, when
plotted in a cartesian coordinate system, is typically in the form
of a line, the gradient of which decreases with increasing cycle
period. The line may be a curve, or it may be a line having
interconnected straight line portions, the line passing through a
point representing the above-mentioned optimum parameter value and
optimum walking speed value. The gradients of the line are
preferably derived from trials with a number of amputees in which
samples of the ratio are accumulated and recorded at different
walking speeds.
[0019] Speeds of locomotion may be defined by a plurality of speed
regions or step period regions (one being the reciprocal of the
other) stored in the control unit. When the prosthesis is operated
to perform a self-teaching routine in each speed region, the
parameter is brought closer to a respective value for that speed
region as defined by the target relationship. This process may
include a comparison of the values of the parameter as represented
by the monitoring signals occurring at times of acceleration and/or
deceleration with parameter values defined by the target
relationship, the adjustment system being caused to adjust the
control unit setting, e.g. damping resistance, in response to
deviations from the target relationship with exceed a predetermined
deviation magnitude or factor.
[0020] In a particularly preferred embodiment, the speed regions
are, themselves, dynamically and automatically variable in response
to the monitoring signals representative of walking speed values.
In this way, it is possible to tailor the speed regions to the
amputee's preferred range or distribution of walking speeds.
[0021] In accordance with another aspect of the invention, a
self-teaching lower limb prosthesis for an above-knee amputee
includes a dynamically adjustable knee movement control unit
arranged to control flexion and/or extension movement of a knee
joint of the prosthesis according to speed of walking, settings of
the control unit for different walking speed ranges being obtained
by comparing values of a sensed kinetic or kinematic parameter of
locomotion with target values of the parameter respectively
associated with the walking speed ranges, and wherein the
boundaries between neighbouring walking speed ranges are
dynamically adjustable in response to the distribution of measured
walking speed samples within the ranges thereby to adapt the ranges
to the walking characteristics of the amputee.
[0022] When the control unit is operated in a self-teaching mode
and a playback mode, the self-taught walking speed ranges may be
used to derive walking speed boundaries which are stored in
conjunction with control unit settings so that, in the playback
mode, the control unit is adjusted to a setting corresponding to a
measured walking speed as determined by the stored relationships,
in order that the kinetic or kinematic parameter of locomotion is
close to or coincident with the target value associated with the
measured walking speed.
[0023] The speed of walking may be defined according to a number of
non-overlapping speed regions which may be designated "slow",
"medium" and "fast", the stored data associating a control unit
setting for each range which has been determined by means of the
self-teaching process referred to above.
[0024] The disclosure which follows refers to a lower limb
prosthesis which automatically reacts to measured flexed
state/extended state ratio values dynamically to adjust the control
unit and which, thereby, affects the flexion and/or the extension
of the knee joint. In this way, it is possible to provide a
self-teaching adaptive control system for a lower limb prosthesis,
the system measuring the variation of one or more kinetic or
kinematic parameters associated with the dynamic operation of the
limb, and automatically processing the parameter measurements to
bring them closer to the target relationship referred to above
using an iterative procedure in order to achieve an optimum
locomotion characteristic.
[0025] The monitoring system may comprise the combination of
transducer elements operating as a knee angle sensor and a
programmed a microcontroller. The programmed microcontroller and a
drive system for a valve in a piston and cylinder device may
constitute the adjustment system.
[0026] The invention will now be described by way of example with
reference to the drawings in which:--
[0027] FIG. 1 is a partly sectioned side elevation of part of a
lower limb prosthesis incorporating a flexion control device and
electronic control elements;
[0028] FIG. 2 is a block diagram of a prosthesis control system in
accordance with the invention,
[0029] FIG. 3 is a graph showing the variation of knee angle during
a walking cycle;
[0030] FIG. 4 is a graph plotting the relationship between (a) a
flexed state and extended state ratio and (b) step period;
[0031] FIG. 5 is a diagram showing the derivation of locomotion
speed boundaries for control unit setting;
[0032] FIG. 6 is a program overview flowchart;
[0033] FIG. 7 is a flowchart for a calibration routine;
[0034] FIG. 8 is a flowchart for a self-teaching routine;
[0035] FIG. 9 is a flowchart for a first control unit setting
routine; and
[0036] FIG. 10 is a flowchart for a second control unit setting
routine.
[0037] A lower limb prosthesis in accordance with the invention is
shown in FIG. 1. The prosthesis has a knee joint 10 with a knee
pivot 12 connecting a thigh component 14 to a shin component 16.
The thigh component comprises a knee chassis 14A, an alignment
device 14B, and a stump socket 14C. The shin component 16 has a
conventional carbon fibre reinforced plastics shin cradle 16A which
houses a piston and cylinder assembly 18 acting as a flexion
control device to form part of a control system. The assembly 18
comprises a cylinder 18A which is pivotally coupled to the
posterior part of the shin cradle 16A, and a piston 18B having a
piston rod 18C which is pivotally coupled to the knee chassis 14A.
The piston and cylinder assembly 18 is a pneumatic device, the
resistance to flexion of the knee joint being controlled by a
needle valve 18D which is adjustable by an electrical stepper motor
20 and an associated screw-threaded shaft 20A connected to the
needle member of the needle valve. A d.c. motor may be used as an
alternative to a stepper motor. The needle valve ISO lies in a
passage ISE in the body of the cylinder 18A, the passage 18E
interconnecting the cylinder interior spaces 18F, 18G on opposite
sides of the piston 18C and emerging at a port 18H in the wall of
the cylinder. Operation of the motor 20 causes the shaft 20A to
move axially so that the needle member moves into or out of a
passageway forming part of passage 18E.
[0038] The passage 18E constitutes a first bypass passage
interconnecting the cylinder spaces on opposite sides of the piston
18C. A second bypass passage 181 incorporating a valve such as a
one-way valve 18J is formed in the piston 18C so that the needle
valve 18D is effective only on one stroke of the piston, in this
case the stroke corresponding to increasing flexion of the knee
joint 10. The one-way valve 18J may be arranged so as not to close
off the second bypass passage 181 completely on the increasing
flexion stroke, but rather merely to reduce the orifice area
through the piston 18C. Such an arrangement has the effect of the
needle valve 18D determining the resistance to motion of the piston
18C in both directions, i.e. for increasing and decreasing flexion,
but with the effect of variations in the orifice area of the needle
valve 18D being greater in one direction than the other, depending
on the direction of operation of the valve 18J.
[0039] It is also possible to include a one way valve in the
passage communicating with port 18H. The stepper motor 20 and its
threaded shaft 20A are mounted in the body of the cylinder 18,
preferably adjacent the pivotal coupling 21 of the cylinder 18 to
the shin 16.
[0040] The stepper motor is driven by a microcomputer which forms
part of an electronic circuit assembly 22 and is part of the
control system. The microcomputer determines knee flexion and
extension movements by means of a magnetic proximity sensor
comprising a first part, preferably a transducer 24A5 mounted in or
associated with the cylinder body 18A, and a second part,
preferably a permanent magnet 24B, mounted on or associated with
the piston 18B. As an alternative, for instance, transducer 24A may
be mounted on a printed circuit board constituting the electronic
circuit assembly 22 which is, in turn, associated with cylinder 18.
The electronic circuit assembly 22 and the stepper motor 20 are
powered by batteries, one of which is shown in FIG. 1 and indicated
by the reference 26.
[0041] The electronic circuit assembly 22 is shown in more detail
in FIG. 2. More particularly, the circuitry comprises a processor
circuit 32 which receives transducer signals via input 34 and
controls the stepper motor 20 via output 36. A non-volatile memory
in the form of an EEPROM 38 stores walking speed and valve setting
data produced by the processor circuit 32, and writes such data to
the processor circuit 32 when required.
[0042] The processor circuit 32 includes an output driver for
driving the stepper motor 20 which in turn moves the needle valve
18D, and it has an input for receiving signals from the flexion
sensor 24 comprising transducer 24A and magnet 24B (See FIG.
1).
[0043] Signals received from transducer 24A can be interpreted by
measuring their width, magnitude, and repetition rate to derive
values for the step cycle period (the reciprocal of the walking
speed), and the degree of flexion of the knee, herein termed the
"knee angle".
[0044] Referring to FIG. 3, during a normal walking cycle the knee
angle, i.e. the degree of flexion of the knee joint of the natural
limb follows a curve having two knee angle maxima M13 M2. If the
beginning of the walking cycle is regarded as being the point at
which the heel strikes the ground (the left hand side of the graph
of FIG. 3), it will be seen that the knee joint flexes
progressively during the stance phase to reach the stance phase
knee angle maximum M1 and then extends and flexes again before
reaching the toe-off point TO which marks the end of the stance
phase and the beginning of the swing phase. During the swing phase,
the knee angle continues to increase, reaching a swing phase knee
angle maximum M2 approximately in the middle of the swing phase.
The knee joint then progressively extends, reaching a minimum knee
angle at or just before the next heel strike HS. It will be noted
that the swing phase knee angle maximum M2 is much greater than the
stance phase knee angle maximum M1.
[0045] It has been established that as walking speed varies, the
optimal swing resistance of a lower limb prosthesis for an
above-knee amputee also varies. It follows that, for a given swing
resistance, the amputee has a preferred walking speed. In practice,
the amputee, through biomechanical effort, has the dominating
influence over the duration of the stance phase whereas the
mechanical swing resistance of the prosthetic knee has greater
influence over the duration of the swing phase. It follows, that
for a given swing resistance setting, the stance phase duration is
reduced with increased walking speed, whereas the swing phase
duration remains almost constant or exhibits only a slight
reduction. This effect has been observed during amputee trials. On
this basis, it is predicted that, with a properly adjusted
prosthesis, the ratio of the swing phase duration to the stance
phase duration increases as walking speed increases.
[0046] Referring again to FIG. 3, it can be seen that an
approximation to the swing phase to stance phase ratio can be
provided by the ratio of a knee flexed state duration to a knee
extended state duration. The ratio can be expressed as F/E where F
is the flexed state duration as measured by the time interval
between the knee angle, firstly, reaching a first knee angle
threshold T1 and, secondly, dropping below a second knee angle
threshold T2. E is the extended state duration which is the
remainder of the cycle period, i.e. the time between crossing the
second threshold and crossing the first threshold T1 again.
[0047] It is assumed that, for each swing resistance setting, an
amputee will have an optimal walking speed and F/E ratio. At a
given swing resistance setting, walking speeds above the optimum
give rise to a higher F/E ratio and, conversely, slower walking
speeds result in a lower F/E ratio.
[0048] Consequently, the F/E ratio can be used as a means for
determining whether to increase or decrease swing resistance if it
deviates from a predetermined optimum value for a given walking
speed.
[0049] The conclusions reached above are illustrated in the graph
of FIG. 4 which relates F/E ratio to the step cycle period (which
is the reciprocal of walking speed). Trials conducted with a number
of amputees have shown that for a given swing resistance setting,
the F/E ratio varies approximately linearly with the cycle period,
the relationship having a negative gradient. For a given amputee,
at a lower swing resistance, the relationship between the F/E ratio
and the cycle period tends to follow a first line S1 with
comparatively high F/E ratio values. At a second, higher swing
resistance setting, the F/E ratio tends to follow a line of similar
gradient but at lower F/E values (see line S2). Similarly, further
increases in swing resistance produce further linear relationships
with progressively reducing F/E ratio, as shown by lines S3 and S4
in FIG. 4.
[0050] The positions of the lines S1 to S4 vary from amputee to
amputee, but their essential characteristics, i.e. generally
parallel negative-gradient lines spaced in the manner shown tend to
apply for all amputees.
[0051] A feature which became apparent from the trials is that for
each swing resistance setting, an amputee has a preferred walking
speed, in each case, this is shown in FIG. 4 by a continuous
portion of the line S1, S2, S3, S4. It will be noted that the
preferred F/E ratio increases with decreasing step cycle period and
with increasing swing resistance. Given that the relationships
between F/E ratio, step cycle period, and swing resistance follow a
common pattern from amputee to amputee, but have differing values,
it is possible to plot the optimum F/E ratio to cycle period
characteristic for an amputee, as shown by the line TR in FIG. 3 if
an optimum F/E ratio and an optimum walking speed can be determined
for a particular swing resistance setting. Thus, for example, if
the swing resistance is set to the setting corresponding to one of
the lines S2 in FIG. 4 relating F/E ratio to cycle period, and a
walking test is performed to establish the preferred F/E ratio and
walking speed at that resistance setting, knowledge of the pattern
of preferred walking speeds and resulting F/E ratios allows the
line TR to be extrapolated from a single calibration point CP on
the graph, as shown.
[0052] The line TR, therefore, represents a target relationship
between the F/E ratio and cycle period for the purposes of
adjusting a knee joint control device in a lower limb
prosthesis.
[0053] Amputee trials have shown that a line of best fit linking
points of preferred walking speed and optimum F/E ratio is a curve
having a negative gradient with the magnitude of the gradient
decreasing with increasing cycle period. To simplify calculation of
the target relationship, a dual straight-line characteristic is
chosen as a best fit to the curve. It will be seen that, in this
example, therefore, the target relationship TR has a first negative
gradient portion at cycle period values below the calibration point
CP and a second portion for cycle period values higher than the
calibration point CP with a different, smaller negative gradient.
It will be appreciated that further simplification can be achieved
by adopting a continuous straight line target relationship, TR,
although it may be expected that less good results may be obtained
when used as a the basis for automatic swing resistance
setting.
[0054] The prior International patent publication WO99/44547
discloses the principle of minimising step-to-step variability of a
kinetic or kinematic parameter of locomotion as a means of
establishing an optimum setting for a lower limb prosthesis. The
same principle is used in the preferred embodiment of the present
invention for establishing the calibration point CP, as will be
explained below.
[0055] For the purpose of monitoring or measuring F/E ratio and
cycle period, the cycle period is divided into three regions, here
referred to as "walking speed" regions, as shown in FIG. 5, the
boundaries between the walking speed regions being designated AB1
and AB2. Thus, long cycle periods fall within a "slow" preferred
walking speed region lying below boundary AB1, intermediate cycle
period values lie in a "medium" preferred walking speed region
extending between boundary value AB1 and boundary value AB2, and
short cycle period values fall within a "fast" walking speed region
above boundary value AB2. These are the walking speed regions which
are used in a self-teaching mode of the preferred control system in
accordance with the invention when measuring F/E ratio and
performing calculations based on the measured values. The choice of
three regions is made as a compromise between providing sufficient
speed resolution and minimising processing and memory requirements.
For the purpose of establishing swing resistance settings, however,
a finer speed resolution is used, by defining five secondary
walking speed regions divided by secondary boundary values SB1,
SB2, SB3, SB4, as shown in FIG. 5. Therefore, for valve setting,
secondary slow, slow-medium, medium, medium-fast, and fast speed
regions are defined. The relationship between the secondary
boundary values and the speed region boundary values AB1, AB2 is
defined by the equations:
AB1=(SB1+SB2)/2
AB2=(SB3+SB4)/2
[0056] Operation of the processor circuit 32 in accordance with an
internally stored program will now be described. This program
provides a self-teaching facility for determining control device
valve settings automatically without intervention by a prosthetist.
The programmed processor 32 and the flexion sensor 24 operate in
combination as a monitoring system for generating monitoring
signals representative of step cycle period values (and hence
walking speed values) and of values of the F/E ratio obtained by
measuring the times between threshold-crossing events as described
above with reference to FIG. 3. The programmed processor circuit 32
operates in conjunction with the stepper motor 20 as an adjustment
system which adjusts the valve, thereby to alter swing resistance
according to setting stored in the processor circuit 32 or the
EEPROM 38.
[0057] An overview of the program operated by the processor circuit
32 is provided by the flowchart of FIG. 6. Referring to FIG. 6,
part or all of the program may be used, depending on whether a full
reset of stored valve settings is required, including
re-calibration of the system, or self-teaching without
recalibration, or normal operation, i.e. using valve settings from
a previous self-teaching session.
[0058] Full operation of the program starts with a full reset
command 600 causing the selection of factory-set valve and walks
speed settings in step 602, followed by calibration of the flexion
sensor or swing sensor 24 in step 604.
[0059] Next, the amputee walking on the prosthesis with the control
unit valve driven to a predetermined and fixed calibration setting
in order to establish calibration values of the F/E ratio ("stance
ratio" in FIG. 6) and walking speed (cycle period), i.e. to
establish the calibration point CP shown in FIG. 4. This
calibration step appears as step 606 in FIG. 6. Calibration step
606 will be described in more detail hereinafter.
[0060] When the control system has established the calibration
point, it automatically enters a self-teach mode which begins with
initialisation of self-teach programming parameters (step 608) and
then performs a series of steps 610, 612, 614, 616, 618 in which
walking speed regions stored by the system are tailored to the
amputee's requirements automatically and valve settings are
established in respect of the walking speed regions by sensing the
deviation of measured F/E ratio values from target F/E ratio values
defined by the target relationship described above with reference
to FIG. 4. Steps 610, 612, 614, 616 and 618 are performed while the
amputee is walking. For the full self-teaching operation to be
completed, the amputee should walk over the full range of walking
speed values normally used.
[0061] In step 610, walking speed is measured to determine in which
of the walking speed regions defined by the boundaries AB1 and AB2
in FIG. 5 the current walking speed falls. A series of consecutive
walking steps having cycle periods corresponding to that region are
used to derive a mean speed which is stored for subsequent
resetting of the boundary values AB1, AB2 if required, as will be
described below. When consecutive steps having cycle periods
falling within one of the speed regions (here termed the "current"
speed region) are detected, the F/E ratio for each is measured and
a mean F/E ratio value calculated, the mean value then being
compared with the target value for that speed region. If the
comparison indicates a deviation having a magnitude greater than a
first limit value, a "vote" signal is generated and stored in a
register. These operations are performed for constant speed walking
in step 614 and for acceleration and deceleration periods in step
616.
[0062] In step 618, the accumulated vote signals representing
"votes" for adjusting the valve in one direction or the other are
evaluated and the respective valve setting is incremented or
decremented when a resultant vote exceeds a predetermined
limit.
[0063] The amputee can choose to exit the self-teaching mode at
this point by, in the preferred embodiment, holding the knee joint
in a fully flexed condition for a predetermined period (step 620).
If the exit condition has not been met, walking speed region and
valve setting updating is repeated, preferably until all three
walking speed regions have been updated.
[0064] In this way, the F/E ratio is used by self-teaching
algorithms in the program in two ways to influence swing resistance
adjustment:
[0065] 1. By comparing the mean F/E ratio with the calibrated
target F/E ratios when maintaining a plurality of consecutive steps
within a given speed region; and
[0066] 2. By comparing the F/E ratio for the second of a sequence
of three walking steps, wherein the three steps constitute an
acceleration episode or a deceleration episode across a speed
region boundary (AB1 or AB2 in FIG. 5), with the calibrated target
F/E ratios.
[0067] In either case, a mean measured F/E ratio above the target
ratio for the corresponding walking speed causes generation of a
"vote" for an increase of the swing resistance, for that region.
Conversely, a mean measured F/E ratio lower than the target ratio
causes generation of a "vote" for a decrease in the swing
resistance for that speed region. As the votes are accumulated over
a period, votes for a swing resistance increase cancel out votes
for a swing resistance decrease and at the end of the accumulation
period, the net sum of votes is evaluated. When the next sum for
any given speed region exceeds a predetermined threshold, the swing
resistance corresponding to that voting region is increased or
decreased depending on the bias of the votes.
[0068] The self-teaching mode routine will be described in more
detail hereinafter.
[0069] Upon exiting the self-teaching mode (step 620) the program
enters a playback mode ("normal operation") in which the updated
valve settings established in the self-teaching mode are used to
set the valve according to walking speed (step 622) as the amputee
walks. In this embodiment of the invention, the playback mode also
includes a monitoring step 624 in which the F/E ratio and the mean
cycle period are monitored in each speed region and minor or major
deviations are logged.
[0070] In situations in which it is not necessary to perform the
calibration routine, the program may be started from a soft reset
command 626 wherein following the loading of program settings from
the EEPROM (step 628), the program enters the self-teaching mode
downstream of the calibration step.
[0071] Alternatively, if the control system has been deactivated
and normal operation is required, based on existing valve settings
and walking speed settings, the program can be entered through a
basic reset command 630 which, after loading program settings from
the EEPROM (step 632), the program is entered at the normal
operation playback mode (step 622).
[0072] Now, referring to FIG. 7, the calibration routine is
described. This routine begins with the loading of factory-set
default settings in step 702 and the selection of a fixed,
calibration, valve setting which is well above and well below the
expected lowest and highest resistance settings to be used by the
amputee (step 704). Then the swing sensor is calibrated by
selecting knee angle thresholds which, according to the sensor
outputs, are in the lower part of the knee angle range traversed
during the swing phase, but above the maximum knee angle
encountered during the stance phase, as measured during a series of
walking steps, preferably four steps. These knee angle thresholds
appear in FIG. 3 as knee angle values T1 and T2. These threshold
values are used to detect the start and finish of the flexed state
of the knee joint. Entry into the flexed state is detected using a
higher knee angle threshold than that used for exit detection in
order to avoid control system instability. (It should be noted that
"knee angle" in this context means the relative angle between the
axes of the upper and lower limb components joined by the knee
joint, which angle is zero at full extension and increases as the
knee joint is flexed.)
[0073] Having calibrated the swing sensor by calculating the swing
sensor thresholds in step 706, a speed and F/E ratio calibration
operation 708 is performed in which the amputee walks at a speed
which to him or her is the preferred speed for the fixed valve
setting. The system measures the step-to-step variability of the
cycle period and the F/E ratio over a number of steps and, when
several consecutive steps have been completed with the measured
speed and measured F/E ratio remaining substantially constant
within predetermined limits, the mean cycle period and the mean F/E
ratio value for those steps are calculated and stored as the
calibration values representing the calibration point CP described
above with reference to FIG. 4. These values are used during the
self-teaching routine to calculate the target relationship TR
between F/E ratio and cycle period described above with reference
to FIG. 4.
[0074] The self-teaching mode will now be described in more detail
with reference to FIG. 8.
[0075] As mentioned above, the self-teaching mode has an exit
condition which is detected in step 820. If this exit condition is
not met, the program proceeds to step 822 in which it loops back
until a new step is detected. If the cycle period of the new step
indicates that the walking speed has moved to a new speed region, a
contiguous step count is cleared. If not, the contiguous step count
is maintained at this point.
[0076] Next, an average speed value is determined by measuring the
cycle period over four consecutive steps in the current speed
region and dividing the sum by four. The average speed value is
stored. (In practice, the average speed value used for average
speed determination is the average of a number of these four-step
averages.) These operations take place in step 824. Generation of
voting signals during acceleration and deceleration episodes in
step 826 and during constant speed periods in step 828 takes place
concurrently with the average speed determination of step 824.
These operations are described in more detail below. Evaluation of
the "votes" (see step 830) occurs in respect of each walking step
on the basis of votes accumulated over a number of steps occurring
in the same speed region as the current speed region. Votes for an
increase in swing resistance may be regarded as positive votes and
votes for a decrease in swing resistance as a negative vote.
Summing accumulated votes produces a resultant value which, if it
exceeds a predetermined amount (in this case the amount has a
magnitude of 2), causes an adjustment of the valve setting by a
predetermined increment.
[0077] If the votes do not reach two votes in one direction or the
other, no adjustment of the valve occurs. A valve setting
adjustment or a change in the average speed as determined in step
824, as detected in step 832, causes recalculation of the speed
boundaries as described above with reference to FIG. 5 (see step
834). At the same time, if a new valve setting has been established
for any of the slow, medium and fast walking speed regions defined
by the boundaries AB1 and AB2 shown in FIG. 5, new interpolated
valve settings for the secondary speed regions are computed.
[0078] Whether or not changes to the valve or average speed
settings have occurred, the steps of the self-teach mode are
repeated until the exit condition is met in step 820. When the exit
condition is satisfied, the new speed boundaries and valve settings
are saved in the EEPROM and the teach mode is exited (steps 836,
838).
[0079] The generation of voting signals for adjusting the valve
setting is performed both in respect of acceleration/deceleration
episodes and constant speed periods. However, in alternative
embodiments, it is possible to base valve settings on either one of
these.
[0080] Referring to FIG. 9, the generation of voting signals for
acceleration and deceleration episodes starts by detecting
acceleration or deceleration using a comparison of the cycle
periods of three consecutive walk step cycles, as indicated in FIG.
9 by step 900. If acceleration or deceleration is detected (step
902), the system next determines whether the acceleration or
deceleration has resulted in one of the speed boundaries AB1, AB2
(see FIG. 5) having been crossed (step 904). If neither of these
conditions has been met, the voting signal generation routine for
acceleration and deceleration is exited via exit step 906.
[0081] If, on the other hand, there has been acceleration or
deceleration across a speed boundary, the target F/E ratio for the
starting speed region (i.e. the speed region in which the walking
speed fell before the speed boundary crossing occurred) is
calculated based upon the target relationship defined by the
calibration point CP set in the last calibration operation. The
derivation of the target relationship is described above with
reference to FIG. 4. Therefore, in step 908, a target F/E ratio for
the pre-existing speed region is obtained. This F/E ratio value is
compared with the current measured F/E ratio value for the
pre-existing speed region (step 910). If the result of this
comparison indicates a positive F/E ratio deviation of the measured
value relative to the target value by an amount greater than a
first limit value, then a "valve close" vote is generated (steps
912, 914). Conversely, if the deviation is negative and of an
amount greater than the first limit, a "valve open" vote is
generated (steps 916, 918). If there is no deviation or the
deviation is less than or equal to the limit value, no vote is
registered (step 920). In the exit step 906, any "votes" are
registered. Each time this routine is performed, the votes are
registered so that, over several steps, votes are accumulated as
described above.
[0082] Referring to FIG. 10, a similar process is performed for
accumulating valve adjustment votes during constant speed periods.
Thus, in step 1000, the system determines whether there have been
four contiguous walking cycle steps. A positive result causes the
system to calculate the target F/E ratio for the current speed
region (slow, medium, or fast as described above with reference to
FIG. 5) (see step 1002). Next, the F/E ratio values measured for
the last four steps are averaged to obtain a mean measured F/E
ratio value which is then compared with the target F/E ratio value
for the current speed region (step 1004).
[0083] In the same manner as described above with reference to FIG.
9, deviations of the mean measured F.E ratio with respect to the
target F/E ratio give rise to a "close vote" or an "open vote", or
no vote. In this case, a different limit is used for discriminating
the deviation values, this limit being lower than the limit used
for the acceleration/deceleration episodes. The steps for
determining the generation of votes appear in FIG. 10 as steps
1006, 1008, 1010, 1012, and 1014.
[0084] The accumulated votes are periodically totalled in the
manner described above to allow valve adjustments as required, as
described above with reference to FIG. 8.
* * * * *