U.S. patent number RE39,961 [Application Number 10/426,249] was granted by the patent office on 2007-12-25 for computer controlled hydraulic resistance device for a prosthesis and other apparatus.
This patent grant is currently assigned to OSSUR hf. Invention is credited to William G. Gruesbeck, Steven H. Petrofsky.
United States Patent |
RE39,961 |
Petrofsky , et al. |
December 25, 2007 |
Computer controlled hydraulic resistance device for a prosthesis
and other apparatus
Abstract
A computer controlled hydraulic resistance device for apparatus
such as a prosthetic knee for above knee amputees, includes a two
stage pilot operated solenoid valve connected to control the flow
of hydraulic fluid to and from a hydraulic actuator which applies
resistance to the prosthetic knee or other apparatus through a
coupling. Hydraulic pressure is sensed on the high and low side of
the actuator by a spring biased magnet and a magnetically actuated
electronic sensor and is used by a micro-controller in a
closed-loop manner to compensate automatically for variations in
the device and in the hydraulic fluid viscosity. The device also
has magnetically actuated electronic sensors which sense positions
of the apparatus and feed back to the micro-controller for applying
a predetermined resistance profile to the apparatus.
Inventors: |
Petrofsky; Steven H.
(Beavercreek, OH), Gruesbeck; William G. (Beavercreek,
OH) |
Assignee: |
OSSUR hf (Reykjavik,
IS)
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Family
ID: |
38863446 |
Appl.
No.: |
10/426,249 |
Filed: |
April 29, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10237571 |
Sep 5, 2002 |
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08883614 |
Jun 26, 1997 |
5888212 |
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60020904 |
Jun 27, 1996 |
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Reissue of: |
09189233 |
Nov 10, 1998 |
06113642 |
Sep 5, 2000 |
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Current U.S.
Class: |
623/24; 623/44;
188/282.3 |
Current CPC
Class: |
F16F
9/46 (20130101); F16F 9/464 (20130101); A61F
2/70 (20130101); A61F 2002/7625 (20130101); A61F
2002/7635 (20130101); A61F 2002/7655 (20130101); A61F
2002/704 (20130101); A61F 2002/5032 (20130101); A61F
2002/5072 (20130101); A61F 2/64 (20130101); A61F
2002/5006 (20130101); A61F 2002/5007 (20130101); A61F
2002/745 (20130101); A61F 2002/748 (20130101) |
Current International
Class: |
A61F
2/64 (20060101); A61F 2/60 (20060101); A61F
2/70 (20060101); A61F 2/74 (20060101); A61F
2/76 (20060101) |
Field of
Search: |
;623/39-46FOR,24
;701/78,83,85 ;188/282.2,282.3 ;482/5,6 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
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3543291 |
|
Jun 1987 |
|
DE |
|
4229330 |
|
Mar 1994 |
|
DE |
|
0503775 |
|
Sep 1992 |
|
EP |
|
549855 |
|
Sep 1992 |
|
EP |
|
0628296 |
|
Dec 1994 |
|
EP |
|
1125825 |
|
Aug 2001 |
|
EP |
|
2623086 |
|
May 1989 |
|
FR |
|
2 201 260 |
|
Aug 1988 |
|
GB |
|
2 244 006 |
|
Nov 1992 |
|
GB |
|
2268070 |
|
Jan 1994 |
|
GB |
|
2 328 160 |
|
Feb 1999 |
|
GB |
|
2 334 891 |
|
Aug 1999 |
|
GB |
|
2 338 653 |
|
Dec 1999 |
|
GB |
|
60-81530 |
|
May 1985 |
|
JP |
|
03181633 |
|
Jul 1991 |
|
JP |
|
3-181633 |
|
Aug 1991 |
|
JP |
|
4-78337 |
|
Mar 1992 |
|
JP |
|
04078337 |
|
Dec 1992 |
|
JP |
|
WO 96/41599 |
|
Dec 1996 |
|
WO |
|
WO 99/08621 |
|
Feb 1999 |
|
WO |
|
WO 99/29272 |
|
Jun 1999 |
|
WO |
|
WO 02/080825 |
|
Oct 2002 |
|
WO |
|
Other References
An Auto-Adaptive External Knee Prosthesis, Ari Wilkenfeld &
Hugh Herr, Artificial Intelligence Laboratory, MIT, Cambridge,
Massachusetts, 3 pages, Sep. 2000. cited by other .
Biologically inspired autoadaptive control of a knee prosthesis,
Air Wilkenfeld, Ph. D., Dissertation Abstract, MIT, Cambridge,
Massachusetts, 1 page, Sep. 2000. cited by other .
Otto Bock Orthopadische Industrie, C-LEG A new dimension in amputee
mobility, Otto Bock 1997 (4 pages). cited by other .
State-Of-The-Art Prosthetic Leg Incorporates Magneto-Rheological
Technology, Medical Product Manufacturing News, p. 42, Nov. 2000.
cited by other .
"Optimal Control For An Above-Knee Prosthesis With Two Degrees Of
Freedom", D. Popovic et al, 1995, pp. 89-98, J. Biomechanics, vol.
28, No. 1. cited by other .
"Design Principles, Biomedical Data and Clinical Experience with a
Polycentric Knee Offering Controlled Stance Phase Knee Flexion: A
Preliminary Report", Siegmar Blumentritt, Ph. D. et al 1997,
Journal of Prosthetics and Orthotics, vol. 9, No. 1, 18-24. cited
by other.
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Primary Examiner: Willse; David H.
Attorney, Agent or Firm: Knobbe, Martens, Olson & Bear
LLP
Parent Case Text
RELATED APPLICATION
.[.This application is a continuation-in-part of application Ser.
No. 08/883,614, Filed Jun. 26, 1997, now U.S. Pat. No. 5,888,212.
This application also claims the benefit of prior application Ser.
No. 60/020,904, Filed Jun. 27, 1996..]. .Iadd.This application is a
continuation of application 10/237,571, filed 5 Sep. 2002
(abandoned), which is a reissue of U.S. Pat. No. 6,113,642, issued
5 Sep. 2000, which is a continuation-in-part of application
08/883,614, filed 26 Jun. 1997(now U.S. Pat. No. 5,888,212), which
claims the benefit of provisional application 60/020,904, filed 27
Jun. 1996..Iaddend.
Claims
The invention having been described, the following is claimed:
1. A device for producing a controlled variable resistance to
apparatus including a first member moveable relative to a second
member, said device comprising hydraulic actuator connecting said
first member to said second member, said actuator including a
housing enclosing a moveable resistance applying member separating
a first chamber and a second chamber within said housing, hydraulic
fluid passages connected to said chambers, hydraulic fluid within
said chambers and said passages, a solenoid controlled valve
connected to control the flow of hydraulic fluid through said
passages, a magnet and a magnetically actuated electronic sensor
arranged to sense changes in the hydraulic fluid pressure within at
least one of said chambers, and a computer controlled electronic
system responsive to said sensor for actuating said valve to
control precisely the flow of said fluid through said valve and
into and out of said first and second chambers.
2. A device as defined in claim 1 and including a position sensor
for sensing the position of said first member relative to said
second member and connected to said control system for applying a
predetermined resistance profile to the device.
3. A device as defined in claim 2 wherein said position sensor
comprises a second magnet and a second magnetically actuated
electronic sensor, and a relatively moveable variable shutter
spaced between said second magnet and said second electronic sensor
for varying the magnetic field in response to relative movement
between said first and second members.
4. A device as defined in claim 1 and including means for biasing
the flow of hydraulic fluid through said valve between said first
chamber and said second chamber.
5. A device as defined in claim 1 and including a first connector
for connecting said first member to a partial leg of an amputee and
a second connector for connecting said second member to an
artificial foot for the amputee.
6. A device as defined in claim 5 wherein said control system
includes an electronic sensor for sensing the force exerted on said
second connector by the artificial foot while the amputee is
walking.
7. A device as defined in claim 1 wherein said resistance applying
member comprises a rotary shaft connected to a vane type rotor
within said housing, and an endless flexible sealing member
extending around said rotor and engaging said housing.
8. A device as defined in claim 1 wherein said solenoid valve
comprises a two stage pilot operated valve connected to said fluid
passages.
9. A device as defined in claim 1 wherein said magnet is positioned
within one of said passages with a spring bias for simultaneously
sensing the hydraulic fluid pressure within said first and second
chambers, and said electronic sensor is positioned out of
communication with said fluid and is responsive to movement of said
magnet.
10. A device for producing a controlled variable resistance to
apparatus including a first member moveable relative to a second
member, said device comprising hydraulic actuator connecting said
first member to said second member, said actuator including a
housing enclosing a moveable resistance applying member separating
a first chamber and a second chamber within said housing, hydraulic
fluid passages connected to said chambers, hydraulic fluid within
said chambers and said passages, a two stage pilot operated
solenoid valve connected to control the flow of hydraulic fluid
through said passages, an electronic sensor arranged to sense
changes in the hydraulic fluid pressure within at least one of said
chambers, and a computer controlled electronic system responsive to
said sensor for actuating said valve to control precisely the flow
of said fluid through said valve and into and out of said first and
second chambers.
11. A device as defined in claim 10 and including a position sensor
for sensing the position of said first member relative to said
second member and connected to said control system for applying a
predetermined resistance profile to said device.
12. A device as defined in claim 11 wherein said position sensor
comprises a magnet and a magnetically actuated electronic sensor,
and a relatively moveable variable shutter spaced between said
magnet and said electronic sensor for varying the magnetic field in
response to relative movement between said first and second
members.
13. A device as defined in claim 10 and including a first connector
for connecting said first member to a partial leg of an amputee and
a second connector for connecting said second member to an
artificial foot for the amputee.
14. A device as defined in claim 13 and including a magnet and a
magnetically actuated electronic sensor for sensing the force
exerted on said second connector by the artificial foot while the
amputee is walking.
15. A device as defined in claim 10 wherein said resistance
applying member comprises a rotary shaft connected to a vane type
rotor within said housing, and an endless flexible sealing member
extending within a groove around said rotor and engaging said
housing.
16. A device as defined in claim 10 and including a magnet
positioned within one of said passages with a spring bias for
simultaneously sensing the hydraulic fluid pressure within said
first and second chambers, and said electronic sensor is positioned
out of communication with said fluid and is responsive to movement
of said magnet.
17. A device adopted for producing a controlled variable resistance
to apparatus including a first member moveable relative to a second
member, an actuator connecting said first member to said second
member, said device comprising a housing enclosing a moveable
displacement member separating a first chamber and a second chamber
within said housing, fluid passages connected to said chambers and
a fluid within said chambers and said passages, a solenoid
controlled valve connected to control the flow of fluid through
said passages, a position sensor for sensing the position of said
first member relative to said second member and including a magnet
and a magnetically actuated electronic sensor, a variable shutter
connected for movement with said first member and spaced between
said magnet and said electronic sensor for variably restricting the
magnetic field in response to relative movement between said first
and second members, and a computer controlled electronic system
responsive to said sensor for actuating said valve to control the
flow of said fluid through said valve and into and out of said
first and second chambers.
.Iadd.18. A method of controlling a prosthetic knee worn by a
patient, the prosthetic knee containing fluid used at least in part
to control the movement of the knee, the method comprising:
measuring fluid pressure of the prosthetic knee; measuring movement
characteristics of the prosthetic knee; calculating a desired fluid
pressure based on at least the measured movement characteristics;
and adjusting fluid pressure based on a comparison of the
calculated desired fluid pressure with the measured fluid
pressure..Iaddend.
.Iadd.19. The method of claim 18, wherein measuring movement
characteristics comprises measuring a moving direction of the
prosthetic knee..Iaddend.
.Iadd.20. The method of claim 18, wherein measuring movement
characteristics comprises measuring a moving velocity of the
prosthetic knee..Iaddend.
.Iadd.21. A prosthetic knee adapted to be worn by a patient, the
prosthetic knee comprising: movement characteristic sensors that
measure movement characteristics of the prosthetic knee; a chamber
comprising fluid that at least partially controls the movement of
the prosthetic knee; a fluid pressure sensor in communication with
the fluid for measuring the pressure of the fluid; and an
electronic control unit that calculates a desired fluid pressure
based on at least the measured movement characteristics of the
prosthetic knee and that adjusts fluid pressure based on a
comparison of the calculated desired fluid pressure with the
measured fluid pressure..Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention is ideally suited for use with an artificial
leg or prosthesis worn by an above knee amputee, but also has other
applications and uses. Normally this type of prosthesis involves an
artificial knee joint including a socket for receiving and engaging
the stump of the user, a knee bracket rigidly connected to the
socket, and a frame which extends downwardly from the bracket and
is pivotally connected to the bracket by a horizontal shaft. A
pylon and artificial foot are connected to the base of the frame,
and a control unit is connected for locking the knee joint to
prevent it from buckling under load in the stance phase of a step,
and for freeing the knee joint in the swing phase of the step.
Preferably, the prosthesis controls the knee joint in such a way
that the amputee will walk with a normal or natural appearing gait.
This gait is characterized by almost identical movements performed
by both lower limbs at varying walking speeds.
The biological or natural knee joint is powered by the actions of
muscles. Each muscle develops an active force by contraction and
also provides variable stiffness or resistance. It has not been
feasible to duplicate muscle contraction in leg prosthesis because
of the weight and bulk that would be required to duplicate this
function. Research has focused on implementing stiffness or
resistance to rotation of the knee joint. Usually this involves
switching the knee joint between one of two modes, locked or free
to rotate. The locked mode occurs during the stance phase of the
gait cycle, and the free to rotate mode occurs during the swing
phase of the gait cycle. The stance phase applies when the foot of
the prosthesis is on the ground, and the swing phase applies during
the time when the foot of the prosthesis is off the ground.
Much of the research in recent years has sought improvements in
controlling an artificial knee joint as a way to improve gait and
enable the amputee to deal with situations such as descending
stairs or ramps, or lowering into a sitting position. If a knee
joint is considered a simple hinge, there are two separate actions
which occur. During flexion, the upper and lower segments move
closer together during rotation of the knee joint. During
extension, the leg straightens and the segments move apart. For a
prosthetic knee joint to duplicate a biological knee, it is
necessary to control the resistance to rotation in each direction
independently and variably. This resistance to rotation during
swing phase can be accomplished with a mechanical damper or
friction device, a pneumatic damper, or a hydraulic damper. It is
generally accepted in prosthetics that a hydraulic damper provides
the smoothest action over a wider range of walking speeds.
Stance phase control must provide a very high resistance to flexion
or lock completely any rotation to flexion. Stance control is
usually provided by a weight activated mechanical locking brake
mechanism, or a position activated polycentric linkage system, or a
position activated hydraulic damper. Mechanical braking mechanisms
can be difficult to keep adjusted properly and can cause the
amputee to walk with a slightly unnatural gait. Position activated
polycentric mechanisms require more concentration and can be
difficult for amputees to use in some situations. Hydraulic
dampers, while providing a more natural gait, require more
concentration and training for the amputee.
U.S. Pat. Nos. 5,405,409 and 5,443,521, which issued to the
assignee of the present invention, disclose a linear type hydraulic
damper for controlling an above knee prosthesis. This hydraulic
damper has independently adjustable and variable resistance in
flexion and extension during the swing phase of the gait cycle.
Because of the turbulent flow of the hydraulic fluid during the
swing phase, this damper can accommodate a wide variation of gait
speeds. The control damper has a single damping rate in stance
phase that can be manually adjusted for each amputee's need. When
the knee joint is fully extended, the damper assumes a non-stance
resistance mode. This position activated stance phase can initially
require extra gait training and concentration on the part of the
amputee to receive full benefit of the damper.
Electronics have recently been introduced into lower extremity
prosthetics in an attempt to make walking easier for the amputee.
For example, U.S. Pat. No. 5,062,856 and U.S. Pat. Nos. 5,383,939
and 5,571,205 disclose two systems which use a microprocessor
control to adjust the resistance in a pneumatic or hydraulic
cylinder during swing phase in an attempt to provide control of
rotation of the knee joint over a wider range of walking speeds
than is available with standard pneumatic or friction dampers.
Further improvement in amputee gaits could come from a mechanism
that in the beginning of the stance phase would allow for a small
amount of knee flexion and then lock against further flexion while
simultaneously allowing for knee extension as the leg straightens
due to body action. Such a mechanism is described by Siegmar et al
in "Design Principles, Biomechanical Data and Clinical Experience
with a Polycentric Knee Offering Controlled Stance Phase Knee
Flexion: A Preliminary Report", Journal of Prosthetics and
Orthotics, Vol. 9, No. 1, pp. 18-24, Winter 1997, and by Popvic et
al in "Optimal Control for an Above-knee Prosthesis with Two
Degrees of Freedom", J. Biomechanics, Vol. 28 No. 1, pp. 89-98,
1995.
An amputee needs different resistance to knee flexion during stair
descent than is needed while sitting down in a chair. Accordingly,
it is desirable for a control mechanism to be capable of providing
these different resistances to knee flexion automatically. The
control mechanism should also provide for swing resistance over a
wide range of gait speeds. All of this should happen automatically
so that the amputee can walk without having to think about his
prosthesis.
The same type of computer controlled hydraulic damper system that
can be used with amputees can also be used on other applications
such as robotics, braking systems, and exercise equipment. These
applications only vary in the size of the actuator to control the
maximum resistance applied. They all may use common sensors,
microprocessor controlled electronics, and valve technology.
Computer controlled exercise equipment are disclosed in U.S. Pat.
Nos. 4,354,676, 4,711,450, 4,919,418, 5,230,672, and 5,397,287. In
any such equipment, it is desirable to be able to maintain accurate
applied resistance over a wide range of temperature and
manufacturing tolerances. It is also desirable to have proper
feedback control and a hydraulic valve and controller designed for
relatively slow speeds of operation.
SUMMARY OF THE INVENTION
The present invention is directed to a computer controlled
closed-loop electromechanical resistance device. One application of
the device is to provide swing resistance to the knee unit of a
lower limb prosthesis as worn by an above-knee amputee. Other
applications for the invention include rehabilitation equipment,
exercise equipment, braking devices or other various damping
applications.
In accordance with a preferred embodiment of this invention, the
device comprises a rotary paddle or vane type rotor or actuator.
When the rotary vane is rotated, hydraulic fluid is forced through
an electronically controlled valve from one side of the vane to the
opposite side of the vane. As rotation is reversed, the hydraulic
fluid reverses direction and flows back through the same valve.
Computer control of the valve creates a variable pressure
differential from one side of the rotary vane to the other side.
The variable pressure differential is sensed as a variable
resistance on the rotary vane. However, the resistance device of
the invention may also be utilized with a linear type of actuator
with equal effectiveness.
The valve used in one embodiment of the device is a proportional
controlled, solenoid actuated, balanced spool valve. The shape of
the spool is such that flow across the face of the spool has little
or no effect on spool movement, thus eliminating any possibility of
unbalanced flow induced forces. The valve spool is also pressure
balanced to eliminate any possibility of a hydraulic lock. The
magnetic core for the valve is shaped to produce a near constant
force in the working stroke of the spool when constant power is
supplied. In another embodiment, a two stage poppet or pilot
operated valve is used to increase the servo valve dynamic range
and to reduce significantly the unbalanced forces applied. Valve
control includes a high frequency dithering to avoid spool drag,
and proportional control is provided to minimize wear rate as
normally associated with a pulse width modulated control.
The valve control used in the device is a microprocessor based
closed loop adaptive control. The microprocessor reads actuator or
rotor vane pressure differential, rotor position, auxiliary force
and pressure differential error at 1-5 ms intervals (200-1000 Hz).
The microprocessor calculates rotor position, rotor velocity, and
rotor direction at 10-25 ms intervals (40-100 Hz). Based on this
information, the microprocessor calculates the required rotor
resistance (pressure differential) based on state equations, thus
creating an automatically adjusting resistance device. If the
difference between the actual and required pressure differential is
large, the microprocessor changes the valve in large increments to
compensate for this large error. If the difference between the
actual and required pressure differential is small, the
microprocessor changes the valve in smaller increments to
compensate for this small error. By utilizing pressure feedback for
the closed loop control, the control system is able to compensate
automatically for machining tolerances, valve solenoid resistance
variations, different fluid viscosity, temperature effects and
wear. Constants in the state equations adapt with changes in the
system operating environment for adaptive control.
When the device of the invention is used as a knee control unit for
an above knee amputee, the control unit detects five significant
points in a typical gait cycle. The two major areas of a gait cycle
are stance phase and swing phase. Stance phase is the time in which
the prosthesis is in contact with the ground. Swing phase is the
time in which the prosthesis is not in contact with the ground. The
first major point considered is heel strike which is the beginning
of the stance phase. This is the point at which the prosthesis
first contacts the ground and is no longer swinging in the air. The
prosthesis must have stability at this point so that it will not
collapse as the amputee's weight is transferred from the opposite
leg.
A yielding stance is ideal for this situation in which a high
resistance is applied to support the amputee, but allowing the
prosthesis to flex slightly. Ideally, the prosthesis should flex
about ten degrees so that the amputee does not have to vault over a
fully extended prosthesis. This ten degree flexion during stance is
the second point of consideration. At this point, the prosthesis
begins to extend as the amputee propels himself forward. The
prosthesis fully extends during this propulsion. After propulsion,
the third point is the initiation of flexion in which the amputee
begins to move the prosthesis forward by flexing of the hip. The
fourth point is toe off in which the prosthesis leaves the ground.
This is the start of swing phase.
During the swing phase, the knee control unit offers significant
resistance to limit the swing speed and the amount of angular
movement of the lower section of the prosthesis. Ideally, the knee
should flex no more than sixty-five degrees during swing phase.
This may be achieved by introducing a high resistance to limit the
amount of heel rise. Due to the momentum of the prosthesis, the
knee control unit begins to extend while swinging through the air.
The fifth point of consideration is terminal deceleration. This
occurs just prior to flee strike during the final few degrees of
extension in which a high resistance must be applied to limit any
harsh knee slap as the prosthesis contacts the extension stops.
To control the knee control unit, the microprocessor of the
invention reads rotor vane pressure differential, knee position,
pressure differential error and prosthetic force at 1-5 ms
intervals (200-1000 Hz). The microprocessor calculates the knee
position, knee velocity, knee direction and reads user settings
(1-10) for flexion and extension at 10-25 ms intervals (40-100 Hz).
The user settings for flexion and extension set an area for use,
and the adaptive control fine tunes the unit from this baseline.
Based on this information, the microprocessor calculates the
required knee resistance (pressure differential) required based on
state equations, thus creating an automatically adjusting knee
control unit.
Constants in the state equations are able to adapt with changes in
the system operating environment. If the difference between the
actual and required pressure differential is large, the
microprocessor changes the valve in large increments to compensate
for this large error. If the difference between the actual and
required pressure differential is small, the microprocessor changes
the valve in smaller increments to compensate for this small error.
If the knee angle is extending and nearing full extension, the
microprocessor starts to close the valve to create a high
resistance and slow the prosthesis in the extension direction. When
the knee angle is flexing and nearing the ideal heel rise, the
microprocessor starts to close the valve to create a high
resistance and slow the prosthesis in the flexion direction. The
prosthetic measured force allows the microprocessor to distinguish
between heel strike, mid-stance, or toe off. This aids the amputee
in descending stairs by creating a high knee resistance and
lowering the amputee to the next stair by using his own weight.
Stumble recovery is achieved by sensing force and knee pivoting
velocity. If the force sensors indicate a stance phase and knee
velocity is high, this might indicate a stumble condition so that
the system imposes a high resistance to help the amputee regain
control. In the case of prolonged nonuse of greater than 5 seconds,
the microprocessor reverts to sleep mode in which all components
are shut down except the knee angle sensor circuit. This conserves
battery power, but permits the control system to resume with a
change in knee angle. Power is supplied to the control system by
four 3.6 volt lithium ion batteries in a replaceable battery pack
which is rechargeable to ninety percent capacity in two hours.
Battery lift is approximately thirty hours between full
recharges.
Preferably, the knee control unit operates with a rotary vane rotor
positioned inside a rotor housing on the knee axis, and the knee
bracket is connected to the rotary vane. As the knee is flexed, the
vane rotates forcing hydraulic fluid out of the chamber through the
solenoid control valve which in turn controls the fluid flow and
pressure. This control of the fluid flow and pressure provides the
resistance available at the knee axis. Fluid exiting the solenoid
control valve flows through a weight actuated stance valve. This
valve limits fluid flow whenever weight is applied to the
prosthesis. The stance valve is adjustable to allow various
yielding rates depending upon the amputees weight or preference.
Fluid exiting the stance valve enters an extension bias cylinder.
This cylinder consists of a spring loaded piston which is
compressed when the knee control unit is flexed. Fluid on the
opposite side of the bias piston is directed to the opposite side
of the rotary vane, thus completing the fluid path. During
extension, the flow is reversed with the stored potential energy of
the spring biased piston available to assist in extending the
prosthesis.
Other features and advantages of the invention will be apparent
from the following description, the accompanying drawings and the
appended claims.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a side elevation view of a lower limb prosthesis for an
above-knee amputee and incorporating a resistance device or knee
control unit constructed in accordance with the invention;
FIG. 2 is an enlarged fragmentary side view of the knee control
unit shown in FIG. 1;
FIG. 3 is the front view of knee control unit;
FIG. 4 is the rear view of knee control unit;
FIG. 5 is a section taken generally on the line 5--5 of FIG. 3 and
showing internal components;
FIG. 6 is a part section taken generally on the line 6--6 of FIG. 3
and showing a knee angle sensing mechanism;
FIG. 7 is a section taken generally on line 7--7 of FIG. 2 and
showing the resistance rotor and rotor housing;
FIG. 8 is a section taken generally on the line 8--8 and showing an
extension pressure sensor;
FIG. 9 is an elevational view of a solenoid control valve;
FIG. 10 is an axial section of the solenoid control valve shown in
FIG. 9;
FIG. 11A is an exploded view in section of the capacitance pressure
sensor shown in FIG. 8;
FIG. 11B is an axial section of the assembled sensor;
FIGS. 12A-12D are views of a capacitance force sensor used in the
knee control unit;
FIGS. 13A-13C are views of the resistance rotor shown in FIGS. 5
& 7;
FIG. 14 is a block diagram of the hydraulic circuit for the knee
control unit;
FIG. 15 is an overall block diagram of a computer controlled
electromechanical closed-loop resistance device constructed in
accordance with the invention;
FIG. 16 is a block diagram showing an application of the device for
exercise or robotics or damping equipment;
FIG. 17 is a block diagram showing an application of the device for
a knee control prosthesis for an amputee;
FIG. 18 is a circuit diagram of the electronics for controlling the
solenoid control valve in the resistance device;
FIG. 19 is a circuit diagram for the Hall position sensor
electronics in the resistance device;
FIG. 20 is a circuit diagram for the force sensors for the knee
control unit;
FIG. 21 is a gait knee angle diagram for the knee control unit;
FIG. 22 is a mainline software routine block diagram for the knee
control unit;
FIG. 23 is a block diagram for the one millisecond interrupt
software routine for the knee control unit;
FIGS. 24A & 24B show the block diagram for the ten millisecond
software routine for the knee control unit;
FIGS. 25A & 25B illustrate diagrammatically a magnetic shutter
position sensor assembly constructed in accordance with a
modification of the invention and its use on a knee control
unit;
FIG. 26 illustrates diagrammatically a magnetic fluid pressure
sensor assembly constructed in accordance with a modification of
the invention;
FIG. 27 illustrates diagrammatically a magnetic weight sensor
assembly constructed in accordance with a modification of the
invention and its use on a knee control unit;
FIG. 28 is an axial section of a two stage pilot operated poppet
valve assembly constructed in accordance with a modification of the
invention;
FIG. 29 is a circuit diagram for the GMR magnetic sensor card
assemblies shown in FIGS. 25B, 26 and 27;
FIG. 30 is a circuit diagram for the three GMR magnetic signal
conditioner input circuits; and
FIG. 31 is a chart showing the nonlinear characteristic curve for
the magnetic shutter GMR sensor of FIG. 25B.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In reference to FIG. 1, a typical lower limb prosthesis for an
above-knee amputee includes a residual limb socket 1 which
functions as an interface between the amputee and the prosthesis, a
knee control assembly or unit 2 which provides knee rotation and
resistance to aid in walking, a mounting pylon 3 and a foot 4. The
components 1, 3 and 4 are conventional and commercially
available.
The knee control assembly or unit 2 is described in connection with
FIGS. 2-14 and includes a frame assembly 5 and an inverted U-shaped
knee bracket 6 secured to the socket 1. The knee bracket 6 includes
a right side retainer plate 7 and a left side retainer plate 8. The
knee bracket 6 slides over opposite end portions of a rotor shaft 9
(FIG. 5), and each end portion has parallel flats to key the shaft
to the bracket. The side retainer plates 7 and 8 are secured to the
bracket 6 by screws, and the shaft 9 rotates with the knee bracket
6 relative to the frame 5. The left side retainer plate 8 also has
an outer cam surface (FIG. 6) to actuate a knee angle sensing
mechanism.
FIG. 6 shows the knee angle sensing mechanism which includes a
roller 10 attached to a knee angle lever arm 12 with a pin 11. The
knee angle lever arm 12 pivots on a cross pin 13 which is pressed
into a housing 15. A magnet 14 is attached to the lower end of the
knee angle lever arm 12 with adhesive. As the knee bracket 6
rotates relative to the housing 15 and frame 5, the roller 10 rides
on cam surface of the left side retainer plate 8. A spring 16
causes the lever arm 12 to pivot on pin 13 which in turn varies the
distance between the magnet 14 and a Hall effect sensor 18 mounted
on a PC board assembly 17. As this distance is varied, the output
of the Hail effect sensor 18 varies to indicative the true knee
angle of the frame 5 and housing 15 relative to the bracket 6.
Power is supplied to the PC board 17 by a multiple battery pack
19.
FIG. 5 shows the internal components of the knee control unit or
assembly 2. Hydraulic fluid is the working fluid that provides for
knee control resistance. Resistance is provided to the knee bracket
6 via the rotor shaft 9 (FIG. 7), and a vane-type rotor 20 (FIGS.
5, 7, 13A-13C) is attached to the rotor shaft by two cross pins 21.
The rotor chamber is defined by a right side rotor housing or cap
22 (FIG. 7) and a left side rotor housing or cap 23. Two endless
Teflon seals 24 seal the rotor 20 against the rotor caps 22 and 23,
thereby creating two separate rotor chambers 25 and 26 (FIG. 5). As
shown in FIG. 7, the rotor shaft 9 is supported by roller bearings
27 to support the amputee's weight, while side thrust loads are
supported by flat Teflon thrust washers between the caps 22 and 23
and the sides or leg of the bracket 6. The rotor shaft 9 is sealed
against hydraulic fluid leakage with spring biased lip seals 28
adjacent the bearings 27.
During knee flexion, the rotor 20 is rotated with the knee bracket
6 and rotor shaft 9, thus forcing hydraulic fluid out of rotor
chamber 26 (FIG. 5) and through an arcuate passage 29 which is
defined between the rotor caps 22 and 23 and the housing 15. From
passage 29, the hydraulic fluid is forced into passages 30a and
30b. Passage 30a connects with a flexion pressure sensor 31 which
is sealed by an O-ring and retained by a retaining ring. Passage
30b feeds into a valve cavity. Fluid passages 30a and 30b and the
valve cavity are machined into the housing 15. From the passage
30b, the hydraulic fluid passes through a solenoid control valve 32
which electronically controls the flow and pressure of the
hydraulic fluid in the working chambers 25 and 26. Hydraulic fluid
exits the solenoid control valve 32 and enters a fluid passage 33
(FIG. 8).
Passage 33 extends to an extension pressure sensor 34 (FIGS. 5
& 8) which is also sealed by an O-ring and retained with a
retaining ring within the housing 15. The passage 33 is also
connected to a bias tube 35 (FIG. 4), and hydraulic fluid travels
through the bias tube 35 and into a cavity 36 (FIG. 5) which is
machined into an upper bias cap 37. Hydraulic fluid then travels
through a stance valve tube 38 and a tubular stance valve member 39
into a chamber 40. Fluid flowing into chamber 40 pressurizes an
annular seal 41 and an adjacent annular piston 42 which moves
upwardly to compress a bias spring 43 which is seated against a
spring seat member 44 secured to the cap 37. The spring 43 is
located in an oil chamber 45 defined by a cylinder 46 which is
secured to a lower cap member 47 supporting a stance valve plunger
48 for axial movement.
An annular support 50 defines the chamber 40 and forms a bottom
seat for the annular seal 41 and piston on the stance valve tube 38
which has an upper end pressed into a hub on the spring seat member
44. Bias cylinder leakage is controlled by a series of O-rings
(FIG. 5), and hydraulic fluid is forced upwardly out of chamber 45
in response to upward movement of the annular piston 42. The fluid
travels through ports within the spring seat member 44 and the
upper cap 37, through a return tube 57 (FIG. 4) and passages 58 and
59 into the rotor chamber 25 thus creating a flow loop. Passage 58
is machined into the resistance housing 15, and the arcuate passage
59 is defined between the rotor caps 22 and 23 and the resistance
housing 15 and is sealed by suitable O-rings. A spring loaded
pressure relief valve (not shown) may be incorporated into the
upper bias cap 37 to allow hydraulic flow from the bias tube 35
directly to the return tube 57 in case extremely high fluid
pressure is encountered due to rapid flexion. During extension, the
flow is reversed due to the rotor 20 moving the hydraulic fluid out
of chamber 25 and back through the system. In this case, the bias
spring 43 assists in moving the flow from below the piston 42 to
the chamber 26, thus ensuring complete extension of the
prosthesis.
During the stance phase of gait, the amputees weight is applied to
the bottom of the knee control unit 2 at a bottom plate 64 (FIG. 5)
through the pylon 3 and foot 4. The bottom plate 64 is retained by
tubular sleeves 66 and screws 67. The bottom plate 64 is also
supported by a force sensor 68 and an elastomeric pad 74. The
elastomeric pad 74 will deform allowing the bottom plate 64 and a
stance adjusting screw 69 to move vertically by a slight distance.
The elastomeric pad 74 may be effectively replaced with springs,
Belleville washers or wave washers while still retaining the same
actuation characteristics. The stance adjusting screw 69 actuates
the stance valve plunger 48 which pushes on a stance valve cap 70
to move the stance valve 39 upwards into the stance valve tube 38
for closing off the radial ports in the stance valve 39. With these
ports closed, the knee control unit will be restricted from any
flexion movement.
By adjusting the stance adjusting screw 69, the radial ports in the
stance valve cap 70 may be adjusted to limit the closing of the
ports during stance thus allowing a controlled leakage in the
flexion direction giving the amputee a yielding stance. Extension
is not affected during stance due a Belleville washer 71 above a
stance check valve washer 72 that covers axial ports in the stance
valve cap 70. When extension is performed during the stance phase,
the hydraulic flow lifts the stance washer 72 while compressing the
Belleville washer 71 to uncover the axial ports in the stance valve
cap 70. When flexion is attempted during stance, hydraulic pressure
will force the stance washer 72 to cover the holes in the stance
valve cap 70 thus prohibiting any flexion flow moving through the
stance valve end cap 70. When the load is removed from the bottom
plate 64, the stance valve 39 is forced into the open position
(FIG. 5) by a return spring 73 within the tube 38 to allow flow to
continue through the chamber 40.
A block diagram for generally illustrating the hydraulic system is
shown in FIG. 14. By turning the rotor 20 clockwise within the
housing 15, hydraulic fluid from chamber 26 is forced through a
port to the solenoid control valve 32. The solenoid control valve
32 is electronically operated for variably changing the flow area
of the fluid path. By reducing the flow area in the valve 32, the
hydraulic flow is reduced while back pressure is increased. This is
felt as rotational resistance on the rotor shaft 9. Fluid exits the
solenoid control valve 32 through a passage which leads to the
stance valve member 39.
The stance valve 39 is actuated when an external force is applied.
In parallel with the stance valve 39 is the check valve 72 which
prevents flow with clockwise rotation of the rotor 20 when the
stance valve 39 is actuated while still allowing flow with
counterclockwise rotation. Also in parallel with the stance valve
39 is an adjustable orifice 78 which allows a small controlled flow
during clockwise rotation when the stance valve 39 is actuated.
Hydraulic fluid exits the stance valve 39 through the radial ports
which permits the fluid to move or actuate the piston 42.
The piston 42 separates two chambers 40 and 45 in the cylinder 46.
As fluid enters chamber 40, the piston 42 compresses the spring 43.
The piston then forces fluid in chamber 45 to exit the cylinder 46.
This flow returns to rotor chamber 25 to rotate the rotor
clockwise. When the clockwise rotation of the rotor 20 and shaft 9
is released, the bias spring 43 and piston 42 force the hydraulic
fluid out of the chamber 40 through the stance valve 39 and back
through the solenoid control valve 32 and into chamber 26 resulting
in a counterclockwise rotation of the rotor 20 and rotor shaft
9.
FIGS. 13A-13C show the rotor 20 and its construction. The rotor 20
has two endless grooves 82 and 83 for receiving the endless seals
24. The seals 24 of choice are extruded lathe cut Teflon faced
seals, although molded elastomer lip seals will give similar
results. Each endless seal is seamless and encompass the full
perimeter of the rotor 20. This type of sealing has the advantage
of double wiping the sealing surface and as a preseal to the seals
28 (FIG. 7) around the shaft 9 adjacent the legs of the bracket 6
to minimize any external leakage. Two holes 84 (FIGS. 13A &
13B) are provided in the middle of the rotor 20 for receiving the
pins 21 which secure the rotor 20 to the rotor shaft 9.
An exterior view of the solenoid control valve 32 is shown in FIG.
9 and a cross-sectional view of the solenoid control valve 32 is
shown in FIG. 10. A coil bobbin 100 is wound with a wire coil 101
with a radial step. The number of turns in the coil wire 101 is
dependent upon the desired electrical and magnetic characteristics
desired to operate the valve. Lead wires 102 are attached to the
coil wires, and epoxy 103 is applied for strain relief. A flux core
104 is inserted through the center of the bobbin 100, and a metal
cup-like case 105 receives the bobbin 100, coil 101 and the flux
core 104. An adjusting screw 106 is threaded into the center of the
flux core 104 and is sealed with an O-ring 107.
A valve member or valve spool 108 seats on top of a return spring
109, and a tubular spool seat 110 rests on the flux core 104 and is
held in place by a tubular cartridge 111. The spool seat 110 limits
the travel or axial movement of the spool 108. A cartridge plug 112
is pressed into the cartridge 111 which is threaded into the case
105. A magnetic material, such as low carbon steel, is used for the
flux core 104, case 105, adjusting screw 106 and spool 108. These
metal parts are preferably hyper annealed for best performance. A
nonmagnetic material, such as a 300 series stainless steel, is used
for the spool seat 110, cartridge plug 112 and the cartridge
111.
The solenoid control valve 32 is normally open when no power is
applied to the coil 101. As power is applied, the coil 101 produces
a magnetic flux which pulls the spool 108 further into the flux
core 104 against the increasing force of the spring 109. The
specific shape of the flux core 104 and spool 108 along with the
spring rate of the spring 109 is such that spool movement is
proportional to the power applied, this translates to a
proportional flow control. As hydraulic fluid enters passage 30b
from chamber 29, the fluid is directed from port 113 to port 114
which are formed by the cartridge 111 and the cartridge sleeve 112.
If the coil 101 is fully energized, the spool 108 will close off
ports 113, 117 and 114 and 116 and the flow will stop. If the coil
101 is partially energized or non-energized, the flow will then
enter the spool chamber 115, flow around the spool 108 and exit
through port 114 and passage 34 to tube 35.
The pair of ports 113 and 117 and the pair of ports 114 and 116 are
at the same level with the ports of each pair spaced 180 degrees
from each other, and with each pair of ports spaced 90 degrees from
the other pair. The solenoid control valve 32 has two inlet ports
113 and 117 and two outlet ports 114 and 116, although more ports
may be used if desired. Although ports 113 and 117 are described as
the inlet, and ports 114 and 116 are described as the outlet, the
flow may be unidirectional or bidirectional with the same results.
The shape of the spool 108 in the flow area 115 provides for
counterbalancing any fluid forces that may tend to open or close
the spool. The spool 108 has an axial bore or hole through its
center to prevent hydraulic lock. The advantages of an optimized
magnetic flux to match the spring rate and an optimized spool to
match fluid forces greatly reduces the power required to operate
the valve. Leakage between the inlet and outlet ports is controlled
by O-ring 120 mounted on the cartridge 111. External leakage of
hydraulic fluid is controlled by O-rings 121 and 122. Although the
solenoid control valve 32 is preferably proportionally controlled,
it may also be operated with pulse width modulation.
As mentioned above, the computer controlled hydraulic resistance
device of the invention has many applications, such as the knee
control described above for above-knee amputees, advanced exercise
systems using computer controlled resistance, and robotics or
damper applications. While a complete application of an electronic
knee control is disclosed herein, the use of the device in other
applications is apparent.
FIG. 15 shows the overall system control diagram which applies to
all the above-mentioned applications. The system is controlled by a
conventional microprocessor 200 containing RAM memory, program
memory, timers and interrupt control, multi-channel,
analog-to-digital converter, and input/output control lines. The
microprocessor uses an external clock generated by a timing
generator 201. For manufacturing testability of the internal
circuitry and for communications with other devices, a system block
202 contains an asynchronous and synchronous serial ports and well
as a real-time background mode port.
In all applications of the device, the microprocessor 200 executes
its program requiring both sensing and control in a closed-loop
manner. The system programs cause a resistance to be applied to the
device depending on the sensed position and velocity. Using a
hydraulic fluid system, the resistance is applied by a hydraulic
actuating device 211 which may be either a rotary vane such as the
rotor 20 or a linear movable piston within a cylinder. The
resistance is applied by restricting the flow in a closed fluid
system by a solenoid control valve 210, such as the valve 32,
operated by the microprocessor 200 and its control circuitry 207.
The mechanical resistance is applied to a device 215 through a
coupler 214. The applied device may be, for example, a knee joint
of a prosthesis or a piece of exercise equipment, or a robotics
platform which requires restriction in movement and/or
velocity.
The position of the applied device is sensed by 216 which may be a
potentiometer, proximity detector or a linear hall effect sensor
such as the sensor 18. The output of the position sensor is a
signal conditioned and scaled by circuitry 204. The analog position
signal leaving 204 is converted to digital 8-16 bit number by the
microprocessor's A/D converter or an external A/D device for use in
the main program. The position is time sampled at fixed intervals.
The difference in position between the fixed intervals of time
divided by the time sample duration is the velocity of the device
movement to be also used by the main program as well as the
direction of movement.
To produce the calculated desired resistance to the applied device
with consistency and independent of manufacturing tolerances, Fluid
viscosity, and/or temperature variations, closed-loop control is
used, and the internal fluid pressures 212 and 213 of the rotary or
linear hydraulic actuating device 211 is sensed. In a closed
hydraulic system, the actuator 211 will produce a high side
pressure and a low side pressure on opposite sides of the rotary
vane or piston when the control valve is operated to restrict the
fluid flow. When the direction is reversed, the high and low side
pressures are reversed. The sensed 212 and 213 pressures are signal
conditioned and scaled to a usable analog level by circuitry 206.
The processed analog hydraulic pressures are converted into a
usable digital 8-16 bit valve by the microprocessor 200 or an
external A/D converter. In the above mentioned applications,
program state control logical branching into different sections of
the main program as well as variations in calculations in the
application dependent program 209 are accomplished by additional
analog force sensors and/or digital switches or buttons 208 and 224
using auxiliary sensing In an exerciser or robotics application,
the auxiliary sensing will be a user keypad and/or remote switches.
In the knee control prosthesis application, the auxiliary sensing
function uses two body weight sensors and two 16 position rotary
selector switches.
FIGS. 16 and 17 show the application of the control system of the
invention to prosthetic devices, exercise equipment, and robotics.
The device of the invention may also be used as a computerized
dampening device such as a truck seat shock absorber which would
use the FIG. 16 control and component diagram. Due to the fine
resolution microprocessor control provided by the invention sensors
and hydraulic actuator and control valve, the same type of
implemented exercise equipment may be used for medical
rehabilitation, programmed to the small steps in applied weight
changes as little as 0.1 pound increments to as much as 500 total
pounds. The auxiliary input function tells the main program to
limit or reduce the loading to the patient if they become
distressed from the exercise. The block diagram shown in FIG. 16 is
very similar to the overall invention block diagram of FIG. 15 and
illustrates the application of the device to exercise equipment,
robotics, or a computerized damping device. The application shows a
microprocessor 200, communication and test circuits 202, timing
generator 201, reset circuitry 219, valve control circuitry 207 and
a solenoid control valve 210, position sensor 226 and circuitry
204, hydraulic internal force sensor circuitry 206, and power
circuits 203.
FIG. 17 discloses the components for the electronic knee control
prosthesis application. The microprocessor 200 executes its
application program requiring both sensing and control in a
closed-loop manner as described above in connection with FIG. 15.
The microprocessor used in this typical application is a Motorola
MC68HC912B32 16 bit embedded signal chip processor. This
application software being executed on the microprocessor 200
proportionally commands the resistance to be applied to the knee
joint of the prosthesis during patient gait cycle using the valve
control circuitry 207 to control the proportional solenoid actuated
valve 210 or 32. A pulse width modulation technique will also work
as well in most applications. The proportional control valve
restricts the closed system hydraulic flow generated by the moving
knee of the patient, and the rotary hydraulic vane actuator 211
connected by coupling 214 to the knee joint 215. The application
software algorithm predicts initially how much control current
should be applied to the solenoid valve 210 or 32 given the
position, direction, and velocity of the knee joint. The exact
resistance error is determined in a closed-loop manner using the
sensed high and low side hydraulic pressures 212 and 213 to be
conditioned and scaled by circuitry 206 then converted to a digital
valve by the microprocessor 200 with respect to the commanded
level. The microprocessor inner loop senses the high and low side
hydraulic pressures on opposite sides of the rotor 20 and updates
the control solenoid applied voltage level at a 1000 times per
second rate. The main control loop of the program executes at a
rate of 100 times per second.
The knee position sensor 216 is accomplished by a Honeywell linear
Hall effect sensor 18 measuring the change in magnetic field with
respect to the sensor. The magnet 14 is moved in reference to the
sensor 18 or 216 in proportion to the knee angle of the prosthesis.
The output of the sensor 216 is conditioned, offset, and scaled by
the sensor circuitry 204. The microprocessor 200 converts this 0-5V
analog level into an 8 bit digital value. The application program
samples the position sensor at a 1000 times per second rate. The
application program determines knee direction and course velocity
information at that rate. Fine velocity is determined at the 100
times per second control rate. For variances between patients as to
their size, weight, and gait characteristics, the prosthenst sets
the flexion and extension rotary 16 position auxiliary switches
224. Other auxiliary circuits used for program state control are
the two weight force sensors 220 and 221 being modulated,
conditioned, demodulated, and scaled by circuitry 206. These weight
force sensors are located in the bottom of the prosthesis (FIGS. 5
and 12A-D) to measure the force distribution applied during the
gait cycle to determine when toe off, flat footed, and heel strike
conditions exist along with their variations. This information is
used in the adaptive closed-loop control algorithm with knee
position, knee velocity, knee direction, and past gait learned
characteristics to control the instantaneous resistance control
over varying terrain.
Since this knee prosthesis is worn on the body, the control system
is powered by 4 Lithium ion rechargeable batteries 218 or 19
yielding 14.4 volts. Power is split off into two circuit applied
voltages being 7.2 volts for the system logic supply and raw 14.4
volts for the proportional control solenoid drive circuits. The raw
voltage is monitored to detect a low battery condition by circuit
222 which scales the battery voltage into a 0-5 volt level to be
read by the microprocessor 200 using its A/D converter. The
microprocessor and associated logic circuits require 5V which is
regulated from the 7.2 voltage input by the power circuits 203
which include a conventional low dropout three pin regulator
integrated circuit. The Lithium ion batteries are recharged in a
two hour period and then switched into a trickle charge mode by a
LM3420-16.8 integrated circuit produced by National
Semiconductor.
FIG. 18 illustrates the circuitry 207 for the solenoid control
valve drive 210 or 32. The proportional solenoid control valve
requires very low power of only a maximum of 1 watt. The drive to
the solenoid is a constant current type over a range of 0 to 83
milliamps. The resolution in this application of the 0 to 83
milliamp level is 1 part in 255 or 0.325 milliamps per step using
the 8 bit digital potentiometer 243. This AD8400AR10 is an
integrated circuit produced by Analog Devices. The microprocessor
200 updates the device level at a 1000 times per second rate. The
reference to the digital potentiometer is the 5 volt logic supply.
As the level is adjusted in the 256 levels, the output of the wiper
will change from 0 to 4.9 volts. The voltage signal is converted
into a constant current drive by the operational amplifier U1,
transistor Q1, and three resisters R1, R2, R3, R4, and R5. The
output of the digital potentiometer seen by a differential input
operational amplifier with a gain of 0.169 as seen by resisters R1,
R2, R3, and R4. NPN transistor Q1 is used as a current amplifier in
an emitter follower mode. Diode D1 is used in the circuit to
eliminate the reverse EMF effects of the solenoid control valve
coil. Electrolytic capacitor C2 is used as part of a low pass
filter with the solenoid coil to reduce the high frequency
bandwidth of the circuit. C1 is used as a high frequency power
supply bypass.
As the voltage from the digital pot is increased, the circuit
applies a voltage across the solenoid coil. The current through the
coil is in series with the sense resister 10 ohm R5. This constant
current circuit increases or decreases the operational amplifiers
output voltage until the voltage seen at the top of the sense
resister is equal to the commanded voltage. Thus to require a 0 to
83 milliamp drive to the solenoid coil, a 0 to 4.9 volt input is
required. The constant current circuit automatically compensates
for coil resistance manufacturing variations and temperature
effects.
Referring to FIG. 19, the circuitry for the linear Hall effect
position sensor 216 or 18 is signal conditioned, offset, and
amplified by the knee position circuitry 204. A magnet 228 or 14 is
mounted on the lever arm 12 which is pivoted by a cam surface on
the retainer plate 8 so that the lever arm 12 moves directly
proportional to the knee angle. As the magnet 14 moves towards or
away from the sensor 18, the Honeywell SS94A1B sensor 216 or 18
varies its output voltage depending on the north or south pole
magnetic field. The sensor 18 outputs a 2.5 volt signal depending
on the amplitude of the magnetic field and the magnet polarity. In
this application, the polarity of the magnet is such that
decreasing the distance until the magnet touches the sensor yields
0 volts, and totally removing the magnet from the sensor yields an
output of 2.5 volts. The minimum distance at zero degrees knee
extension if 0.10 inches yielding a minimum voltage of 1 volt. In
FIG. 19, block 229 buffers and low pass filters the 1 to 2.5 volt
hall position signal at a 100 Hz corner frequency to remove any
high frequency components. The second stage operational amplifier
230 increases the signal level by 3.33 times as well as removing
the offset of 1 volt from reference 231 to yield an output level of
0 to 5 volts.
This analog output is then converted into a digital output for use
by the main program by the microprocessor 200.
FIG. 20 illustrates the force sensor 232 or 68 and its associated
circuitry. As shown in FIG. 17, the weight sensor circuitry 206 is
used for closed-loop control of the solenoid proportional control
valve 210 or 32. The weight force sensor 200 and 221 (FIG. 12A) and
weight sensor circuitry 206 are used for program state control. The
circuitry design is identical except for the final amplification
gain of U4, R3, and R4 in FIG. 20. This is an improved version of a
capacitance sensor. FIGS. 12A & B show the actual sensors 220
and 221 used in the knee control application. The enhanced sensor
232 (FIG. 12D) is constructed of two double sided printed circuit
cards and an elastomer separator. As force is applied to squeeze
the plates together, the signal applied on the side of the
elastomer is coupled proportionally to the plate on the other side.
Thus as the force is increased, the reference signal level
transferred to the second plate is intensified by the increase in
capacitance between the plates. The amount of force that can be
measured by the sensor is a function of the elastomer density used
and the gain of the demodulating circuitry. The stiffer the
elastomer, the more force can be applied before fully compressing
the plates together. Too much stiffness in the elastomer reduces
the dynamic range of the sensor. The elastomer having a particular
durometer is selected from each application.
In FIG. 20, the sensor application uses the microprocessor 200 to
generate a 100 kHz square wave reference signal. This signal is
buffered by operational amplifier U1. The emitter side of the
enhanced capacitance sensor is constructed on a double sided
printed circuit card. A conductive reflector 236 is on the outside
and a conductive reference pad 238 is on the other. The insulated
printed circuit card 237 is made of glass epoxy. The reference 100
kHz signal is coupled across the elastomer 239 to the receiver
plate 240 on another card 237 and which is protected from outside
emissions by another shield plate 236. The received coupled 100 kHz
signal is proportional to the compression of the plates and is
developed across resistor R1. The signal is then buffered by
operational amplifier U2. The received signal is converted into a
proportional DC level by the RMS converter circuit consisting of
D1, R2, C1 and operational amplifier U3. The DC level is scaled
into a 5 volt maximum level by operational amplifier U4, R3 and R4.
The microprocessor 200 converts this analog signal into a usable
digital valve by its onboard analog-to-digital converter.
In the knee control application (FIG. 17), two configurations of
this sensor technology are illustrated. These are the hydraulic oil
pressure sensors 206 and the body weight sensors 208. FIGS. 11A
& B show one of the two oil pressure sensors 206 or 31 or 34
used for closed-loop control of the solenoid control valve
proportional positioning. Oil ports 30a and 30b within the housing
15 have pressure communication with the chambers 25 and 26 on
opposite sides of the hydraulic vane rotor 20. Internal pressure
changes is transmitted to each sensor 206 by end cap 243 (FIGS. 11A
& 11B) and diaphragm 242. The diaphragm pressure compresses the
reference printed circuit card 237 containing the reference plate
240 and reflector plate 236 against the elastomer 239. The signal
is coupled to the receiver plate 238 and shield plate 236 on
another circuit card 237. The assembly is housed by the sensor
enclosure cup 241 and with its associated circuitry operates as
described above.
Referring to FIGS. 12A-D, the body weight force sensor 208 or 68 of
the knee control application is constructed of two 2'' by 2''
double sided printed circuit boards 237 being separated by a sheet
of elastomer 239. The capacitive plates 220 and 221 are 0.5'' by
1.25'', and two reference plates and receiver plates 238 and 240
are used to produce a weight distribution sensor 68. This sensor
208 or 68 is located in the base of the knee control. From the
compression in the forward, middle, or aft force distributions, the
system software can determine toe off, flat footed, or heel strike
for program state control. This assembly with its associated
circuitry also operates as described above.
In the knee control application (FIG. 17), the system software is
written in assembly code for a Motorola MC68HC912B32 microprocessor
200. This microprocessor 200 includes a 16 bit CPU, an interrupt
controller, an 8 channel 8 bit A/D converter, 1 kilobyte of RAM, 32
kilobytes of flash EPROM program memory, 756 bytes of EEPROM, one
real-time timer interrupt, six timer-counters, a watchdog circuit,
and a complex of communication devices for interacting with other
systems such as a RS-232 serial peripheral, a synchronous serial
peripheral, a BDLC synchronous serial peripheral, and a real-time
background mode interface.
The knee control software application includes subroutines which
are common to robotics and exercise devices. Referring to the
general block diagram of FIG. 15, the microprocessor 200 outputs to
a valve control circuit 207 to operate the proportional solenoid
control valve 210, such as the valve 32, which in turn applies
resistance by restricting the fluid actuating device 211, such as
the rotor 20. Two pressure sensors 212 and 213, such as the sensors
31 and 34, are read by the microprocessor 200 to be used in its
closed-loop control of the valve to maintain correct applied force
to the device coupled to the actuator regardless of manufacturing
tolerances, temperature variations and fluid viscosities. The
pressure sensors measure the differential pressure across the
actuator regardless of the actuator direction of movement. The
device 215 upon which the resistance is applied, contains a
position sensor 216, such as the sensor 18, which allows the system
software to close the outer control loop as to determining the
device's position, velocity, acceleration, and direction of
movement. In most cases, the system software also uses auxiliary
analog circuitry 208 and switch inputs 224 for program state
control.
Referring to the knee control application software discussed above
in connection with FIG. 17, the system software being executed on
the microprocessor 200 outputs the required 8 bit control valve
level of the valve control circuitry 207. The digital potentiometer
uses three I/O pins on the microprocessor. The software low level
driver routine synthesizes the required synchronous serial 10 bit
interface which sets the required 0-4.9 volt input drive to the
constant current valve control amplifier. The fluid pressure force
sensors 212 and 213, such as sensors 31 and 34, are analog
processed and scaled by the fluid pressure sensor circuitry 208.
The demodulated 0-5 volts proportional signal is read by the
microprocessor 200 using the A/D fluid sensor low level drive
routine. All the fluid pressure and weight sensors use a 100 kHz
reference square wave signal generated by the microprocessor
onboard counter-timers and initialized in the software power-on
reset routine. This signal is buffered by the hardware and sent to
the sensors.
The outer control loop feedback is obtained by the linear hall
effect position sensor 216, such as the sensor 18, attached to the
moveable knee joint housing 15. The analog output of the hall
effect sensor 18 is non-linear. Part of the linearization is
accomplished by the cam surface on the retainer plate 8 (FIG. 6).
The majority of the position interpretation is accomplished by a
software lookup table driver routine which converts raw position
into actual knee angle in degrees and tenths. The raw position
information is scaled and offset by the position sensor circuit 204
which is read by the microprocessor 200 using the A/D position
driver software routine.
Two types of auxiliary program decision state control functions are
used. One being analog and the other being digital. The analog
program state sensors 220 and 221 form the dual body weight sensor
68 attached to the bottom on the knee control frame 5. The sensors
220 and 221 sense the amputee's weight being applied to the toe or
flat footed, or heel force to aid the main software program
control. The two sensors 220 and 221 also use the 100 kHz reference
signal. The demodulated and scaled resultant analog signals are
ready by the microprocessor 200 by A/D software drivers. Variations
between the amputee's size, weight, age, strength are accommodated
by the setting of ten levels of flexion and ten levels of extension
profiles. These settings are made by the prosthetist during the
knee control fitting. The main subminiature printed circuit card
contains two sixteen position miniature rotary digital hexadecimal
switches. The first ten positions are used for the prosthetic
adjustments of flexion and extension and the other six are for
special modes of operation tailored for sports and geriatrics. The
two 4 bit auxiliary switch inputs 224 are read directly by the
microprocessor using the switch software driver input routine and
the I/O input pins and associated internal pull-up resisters. The
input signals are normally seen as a digital high TTL level except
when grounded by the switch.
Since this knee control application is battery powered, additional
software low level drivers are required for this application. The
battery pack 218, such as the battery pack 19, are conditioned and
split into 5, 7.2, and 14.4 volts by the power circuits 203. The
battery level is monitored by the microprocessor 200 through the
conditioned battery sensed signal 222. The analog 0-5 volt filtered
level is read by the microprocessor 200 by the low level software
A/D battery driver routine. If the programs determine that the
battery is within 30 minutes of a minimum safe level of operation,
the two safety software routines will be activated to cause a
vibrator circuit 227 to produce an alert to give the user impending
notice of shutdown. The communication to the outside world to/from
the knee control is through the microprocessor's internal
communication ports and interface hardware circuitry 202. The
asynchronous (SCI) and synchronous (SPI) serial data ports are used
for special clinical data capture and control. The background mode
(BDM) port is used for the factory test interface during the
manufacturing process as well as during software development. The
main programming of the 32 kilobytes flash memory is programmed
through the BDM port.
FIG. 22, shows the knee control mainline software subroutine. When
the microprocessor 200 is energized, the microprocessor will be
vectored to the RESET subroutine. This routine initializes the
programmable data ports and peripherals, to the desired analog and
digital level such as the two weight sensors and the two fluid
pressure sensors needing their 100 kHz reference for correct
operation. The proportional control valve drive level is set to
zero until required to change by the application program. When the
system is initialized, a system built-in test program is entered to
determine if the knee control electronics are working according to
the factory specification. If not, the system vibrator will be
activated 1/2 second on and one second off to alert the user that
the system has seen a failure and it is unsafe to use. Until the
failure is corrected, normal system application execution will be
disabled. During normal operation, the system control and mode
determination is accomplished by the two interrupt routines
occurring at a rate of 100 and 1000 times per second.
The mainline program checks for low battery conditions and outside
communications in the system idle time when not executing the
interrupt driven adaptive closed-loop time dependent application.
The mainline program examines two low level battery conditions; one
being if the level is below 30 minutes of safe operation and the
other if the battery is below 10 minutes of safe operating time
left. The user is informed of the immediate loss of the system use
from a low battery condition by either a vibration of one second on
and 10 seconds off for the 30 minute case or one second on and one
second off for the 10 minute case. In normal use, the Lithium Ion
battery pack will last 22-30 hours before needing to be recharged.
The knee control prosthetic will normally be recharged by the user
every night. If needed, the user can charge the battery pack to 90
percent of its level in two hours while at work.
FIG. 23 shows the one millisecond software interrupt routine for
the knee control unit. This routine captures the raw sensor data
and updates the valve control at a 1000 times per second rate. The
raw sensor data is read by the individual A/D channels. These 8 bit
valves are converted into actual scaled fluid pressures in psi,
amputee forward and aft weight distributions in pounds, and knee
angle in degrees by the low level software drive routines using
lookup conversion tables and numeric calculations. The valve is
also controlled in a closed-loop manner at this 1000 times per
second rate by calculating a new control level to be written to the
control solenoid valve electronic circuit. A calculation is made of
the error difference between the required force and the sensed
hydraulic fluid pressure. This difference is used in a non-linear
gain equation and lookup table to determine the next best guess at
the valve level control to meet the desired instantaneous
resistance with the least error or delay. The drive byte value
obtained by written to the digital potentiometer by the low level
driver software routine converting the byte into the required 10
bit serial data as well as controlling the clock and enable bits on
a bit-by-bit manner.
Referring to FIGS. 24A and 24B, the knee control unit has a ten
millisecond interrupt routine which is the main control loop where
the majority of the calculations and program control are
accomplished. This routine executes at a rate of 100 times per
second. The software application requires knee position, knee
velocity, knee direction, extension and flexion patient switch
settings, and forward and aft body weight distribution. The
position sensing is the most important parameter. To obtain smooth
and accurate position information, the ten 1 millisecond interrupt
position samples are stored in an array. These ordered samples are
time weighted window averaged to obtain the system knee position
used. The previous old position is subtracted from the new position
and divided by 10 milliseconds to yield a short term knee
velocity.
The new position is also subtracted from an old position of 50
milliseconds ago and divided by 50 milliseconds to calculate a long
term knee velocity. The short term knee velocity is used for the
determination of knee direction. The long term knee velocity is
used for the determination of the knee resistance to be applied.
The knee applied resistance calculations are based on the
mathematical transfer function of a non-electronic hydraulic knee
control obtained through extensive engineering characterization. A
set of tables and equations are used to calculate the requires
applied resistance in terms of PSI for any instantaneous knee
position, knee velocity, knee direction, and flexion or extension
patient switch setting. When the normal swing phase calculations
are completed, the normal swing phase resistance is stored in the
RAM for later use.
The program flow then determines if additional modes of operation
are to be executed. The first decision path is Terminal
Deceleration (T.D.). When the prosthetic knee is approaching full
extension, the system software will apply an additional high
resistance at less than 10 degrees to stop the knee hyper
extending. Likewise, another routine is used for slowing the
prosthetic knee during flexion. This is called Flexion Deceleration
(F.D.). This routine is used to keep the knee from flexing too far.
The F.D. routine will increase resistance proportionally past 65
degrees flexion and completely stops the knee control at 70
degrees. Other auxiliary modes include the determination of walking
down stairs and the determination of stumbling. The down stairs
mode is determined by knee angle and weight distribution as seen by
the two weight force sensors 220 and 221. The electronic control
extends the decaying stance mode of the mechanical portion of the
control. If stumble recovery is required, it applies a high
resistance to the knee control for a period of time and then delays
it to zero. This mode attempts to protect the patient from falling
and is detected by knee velocity, knee angle, and weight force
distribution.
When the resistance software flow is complete, the desired force
level to be applied is stored in a RAM variable. The application
software will compute the best valve control level based on the
knee velocity, knee direction, hydraulic characteristics, and valve
characteristics. This level is written to a RAM variable location
for use by the 1 millisecond interrupt routine previously
mentioned.
The knee control application gait summary is shown in FIG. 21. The
gait knee angle is shown by the curve 248. The swing phase is
active when the knee control is off the ground with the knee
bending in flexion or extension. Swing phase starts at toe off
position 252 and is completed by the time the heel is just ready to
strike at position 253. The electronic control is active anytime
the knee is at any angle greater than zero except when it detects
that the knee has stopped for more than 5 seconds. The stance phase
consists of a heel strike position 249, full weight bearing load
position 250, and almost toe off but still touching ground position
251.
FIGS. 25A-31 show an alternate electronically controlled
proportional servo valve and magnetic sensors for knee position,
oil pressure and weight bearing. The knee position sensor is shown
in FIGS. 25A & 25B and measures the absolute knee angle between
the housing of a hydraulic actuator 300 and a knee plate 301
attached to the patient's socket, by sensing changes in a fixed
magnetic field with a magnetic sensor assembly 303. The changes in
the magnetic field are caused by the movement of an arcuate
magnetic shutter plate 302 mounted on the knee plate 301.
The magnetic position sensor assembly 303 consists of a sensor
housing enclosing a high output permanent magnet 304 (FIG. 25B) and
defining a flux air gap which receives a tapered wail of the
shutter plate 302. A magnetic sensor electronic printer circuit
card 305 is secured to the housing by a screw 308. The sensor
electronics measure the amplitude of the magnetic field as
generated by the high output permanent magnet 304 and sensed
through the air gap. The sensing of the magnetic field is
accomplished by a GMR sensor IC 306, and the signal is conditioned
by an instrumentation amplifier 307. The signal level is
transmitted to a microprocessor controller card by a cable assembly
309 for further signal conditioning and conversion to a usable
digital value for use in the closed-loop control. The curved wall
of the magnetic shutter plate 302 is tapered along its length for
causing a disturbance or variation of the fixed magnetic field by
moving the magnetically conductive material of the shutter plate
into and out of the air gap between the magnet 304 and the GMR IC
sensor 306 in response to relative rotation between the plate 302
and the actuator 300.
The magnetic shutter position sensor assembly 303 generates a
nonlinear output which is shown by the chart of FIG. 31. Through a
software algorithm, the curve is divided into 13 equations
representing 10 degrees of movement each which can be approximated
by a series of straight line equations. This approach requires
significantly less memory than a 12 bit 8192 byte lookup table
which would require 25% of the imbedded microprocessor's program
memory capacity. The sensor is calibrated by measuring the magnetic
levels every 10 degrees over 0-130 degrees, the correct equation
coefficients and stores these in the microcontroller's nonvolatile
electrically erasable memory for use by the main executable
application program.
FIG. 26 shows an alternate pressure sensor embodiment comprising a
GMR magnetic differential fluid or oil pressure sensor. This sensor
is contained in the main hydraulic actuator housing 300. The high
side hydraulic pressure 313 is applied to one end of a movable
piston 310 containing a high output magnet 311, while the low side
oil pressure 314 is applied to the opposite end of the piston. The
hydraulic force being applied to the high pressure end of the
piston pushes against a bias spring 312 having a sufficient spring
constant to allow the piston to move about 0.050 inch with 500 PSI
applied differentially. The maximum movement of the piston is about
0.100 inch.
The actuator housing 300 is constructed of aluminum and is sealed
from leakage of the hydraulic fluid to the environment. The sensor
electronic assembly 303 is similar to the position sensor
electronic assembly 303 described above in reference to FIG. 25B
and is mounted in a cavity outside of the sealed housing for
sensing the internal magnetic field of the magnet 311 through the
aluminum housing. The sensor card 305 is mounted on the housing
with a screw 308. At zero hydraulic pressure, the air gap between
the magnet and the GMR IC bridge sensor 306 is about 0.195 inch but
increases to about 0.245 inch at 500 PSI differential pressure. As
mentioned above, the common sensor card electronics contain the
instrumentation amplifier 307 for signal conditioning. The signal
level is transmitted to the microprocessor controller card using
the cable assembly 309 for further signal conditioning and being
converted to a usable digital value for use in the closed-loop
control of the hydraulically applied resistance to the knee.
A patient weight bearing sensor mechanism (FIG. 27) is accomplished
in a similar manner. In two different forms of this mechanism, one
or two weight sensors may be used. If mechanical weight being
stance is used, then only one weight bearing sensor is required to
assist the mechanical stance and is used in a stumble recovery
mode. If the application requires a smaller size system with less
of a mechanical fail safe, a full electronic weight bearing stance
is implemented using two weight sensors indicating heal and toe
applied forces. Regardless, the one or two sensor application
operate in a similar manner.
FIG. 27 shows a weight bearing heal sensor example. A bottom
attachment plate 316 of the knee control contains a high output
magnet 317. The mounting plate is attached to a rod 318 which
allows easy sliding movement between a lower knee control assembly
315 and the plate 316. A 50 pound spring 319 surrounds the rod 318
and biases the plate 316 downwardly to its fully extended position
of about a 0.040 inch travel. When the patient's weight is not
being applied, the maximum air gap is seen between the magnet 317
and the GMR sensor bridge IC 306 having its signal conditioned by
the instrumentation amplifier 307. The signal level is transmitted
to the microprocessor controller card using the cable assembly 309
for further signal conditioning and for conversion to a usable
digital value for use in the closed-loop control. As the patient
applies weight to the knee control in the stance phase of his/her
gait, the compression of the spring 319 decreases the distance
between the magnet 317 and the GMR sensor 306 for increasing the
electrical signal level to its maximum at approximately 50 pounds
of applied force.
The sensor remote assembly circuitry is shown on FIG. 29. The
magnetic field is detected and measured by a Magnetoresistive GMR 5
kilo-ohm 100 oersted sensitivity bridge manufactured by Nonvolatile
Electronics as part number AA005, and designated as UI. The bridge
is excited by a precision 2.5 voltage reference. The same 2.5 volt
reference is applied to a Burr-Brown INA122 designated as a U2
instrumentation micro-powered amplifier. The instrumentation
amplifier uses a 14K gain resistor RG which converts the low level
differential GMR bridge output into a 19.29 times increased level.
The amplifier also conditions the signal to a low impedance drive
level for transmitting to the main microprocessor control card in a
low noise manner. Capacitor C1 is used as a power noise decoupler,
and the output signal is at a level of 0-2.5 volts. The maximum
magnet to sensor distance is selected to obtain a minimum amplifier
output voltage of 1.25 volts thus transmitting the usable voltage
range of 1.25V to 2.5V.
FIG. 30 shows a common sensor input signal conditioning circuitry
for the position, fluid pressure and weight sensors to the
microprocessor control card containing three or four identical
circuits. In FIG. 30, a common signal conditioning is reduced to
1.25 volts by a resistor voltage divider R1 and R2. The sensor
signal is amplified with a gain of 2 by the differential
operational amplifier U2. Resistors R4 and R6 are twice the valve
of resistors R3 and R5, and the 1.25 volt level is connected to the
inverting amplifier input. The sensor input is passed through an
EMC low pass filter U1 to reduce significantly the microprocessor's
card emissions to the environment and to reduce significantly the
environment's emission effects on the microcontroller. The filtered
signal is connected to the non-inverting amplifier input through
resistor R5. The resultant signal produced is a 0-2.5 volt level
free of the 1.25 volt offset. This signal is applied to one of the
four inputs of a 12 bit analog-to-digital converter using the same
2.5 volt precision reference.
In place of the valve described above in FIG. 10, a two stage
proportional electronically controlled pilot operated valve is used
as an alternate in the electronic knee control and is shown in FIG.
28. The valve has two stages, a main stage poppet valve 320 which
controls pressure for the main flow and a normally open pilot stage
needle 332 to control the pressure and flow used to control the
main stage. The magnetic force applied by a solenoid coil 327
directly controls or closes the needle valve 332 which is subject
to relatively small hydraulic forces but regulates the larger
hydraulic forces acting on the valve 320. The flow is
unidirectional through the valve assembly regardless of the
direction of actuator 300 and uses an arrangement of four check
valves in a bridge. High pressure fluid flows over the seat 319
from the first or high pressure orifice 323 and past the poppet
valve 320 and also through an orifice 322 into a chamber connected
through the pilot valve and eight ports to an annular chamber 321
on the low pressure side of the valve. The resistance of the poppet
valve 320 to this flow determines the torque resistance generated
by the rotary actuator (FIG. 5).
The pilot stage needle valve 332 is biased towards an open position
by a spring 333 and controls fluid flowing through an orifice 326.
The electromagnetic force acts mechanically through a steel
armature 330 and is applied for closing the needle valve 332 for
resisting flow, resulting in a proportional increase of pilot fluid
pressure. This fluid pressure acts on the main poppet 320 to
increase the system pressure. The main stage has two moving parts,
the poppet valve 320 and a bias spring 324. The clearance between
the poppet valve bore and its support spool is very small such as
0.0005 inch on the diameter in order to minimize pressure and flow
losses due to leakage.
The forces acting on the main poppet valve 320 are relatively large
but provide for smooth motion, low leakage and rapid response with
a relative weak bias spring 324. The pilot stage has the moving
parts of steel armature 330 and needle valve 332 and a compression
spring 333. The armature 330 is mounted on a nonferrous or aluminum
cylindrical pusher tube 331 that slides axially on a ground pin
329. The concentricity of these pieces with a bore in a flux plate
328 and a good surface finish between these pieces minimizes the
friction of the pusher tube 331 and consequently minimize system
hysterisis, helps repeatability and maximizes the efficiency of the
magnetic system.
The pilot needle valve 332 moves relative to the pilot orifice 326
for reducing its effective flow area and resulting in an increase
in pilot pressure. The smooth surface finish of the needle and the
bore and the relatively close radial clearance between them
minimize effects due to side loading from angular or radial
misalignment of the needle and pusher tube 331. This helps reduce
hysterisis and improves armature efficiency. The magnetic field is
carried through a steel core 335, steel flux ring 336, steel case
338 and steel flux plate 328. These components are designed in
accordance with dimensional requirements of the electromagnetic
forces required for the system. A set of O-ring seals 325, 334 and
337 are used to keep the high pressure hydraulic fluid from leaking
into the outside environment.
From the drawings and the above description, it is apparent that a
device or apparatus constructed in accordance with the present
invention, provides desirable features and advantages. For example,
by sensing the differential pressure across the movable hydraulic
actuator member and using this differential pressure in a computer
controlled closed-loop system for the valve which controls the flow
of hydraulic fluid to the actuator, a precisely controlled variable
resistance is obtained regardless of variations in manufacturing
tolerances, wear of parts, or variations in the viscosity of the
hydraulic fluid. Thus apparatus constructed in accordance with the
invention is ideally suited for producing a precise knee gait
damping resistance for a knee prosthesis without returning by the
patient or prosthetist.
As another advantage, the magnetic rotary or linear position
sensor, pressure sensor and weight bearing sensor described in
connection with FIGS. 25A-27 provide low cost, highly accurate and
repeatable results without any mechanical connections. The sensors
are also substantially less sensitive to poor environmental
conditions such as particles or dirt in the air, and further
provide a low noise/high signal transmission to the controlling
microprocessor. In addition, the pilot operated two stage solenoid
valve described in connection with FIG. 28 provides for controlling
a high pressure hydraulic fluid flow, for example, 900 psi at 1.5
gpm, with very low power consumption, such as only 0.1 watt. This
feature is ideally suited for a prosthetic knee control where light
weight, small size and very low power consumption of rechargeable
batteries, are highly desirable.
While the methods and forms of apparatus or devices herein
described constitute preferred embodiments of the invention, it is
to be understood that the invention is not limited to the precise
methods and forms described, and that changes may be made therein
without departing from the scope and spirit of the invention as
defined in the appended claims.
* * * * *