U.S. patent application number 11/925294 was filed with the patent office on 2008-10-02 for implantable centrifugal blood pump with hybrid magnetic bearings.
This patent application is currently assigned to WORLD HEART, INC.. Invention is credited to Paul E. Allaire, Michael J. Baloh, Gill B. Bearnson, Jeffrey Decker, Ronald D. Flack, Pratap S. Khanwilkar, James W. Long, Ajit Kumar B. Nair, Donald B. Olsen.
Application Number | 20080240947 11/925294 |
Document ID | / |
Family ID | 22055352 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080240947 |
Kind Code |
A1 |
Allaire; Paul E. ; et
al. |
October 2, 2008 |
IMPLANTABLE CENTRIFUGAL BLOOD PUMP WITH HYBRID MAGNETIC
BEARINGS
Abstract
A pump for pumping sensitive fluids, such as blood, has no
mechanical contact between the impeller and any other
structure.
Inventors: |
Allaire; Paul E.;
(Charlottesville, VA) ; Bearnson; Gill B.; (Salt
Lake City, UT) ; Flack; Ronald D.; (Pittsville,
VA) ; Olsen; Donald B.; (Salt Lake City, UT) ;
Long; James W.; (Salt Lake City, UT) ; Nair; Ajit
Kumar B.; (Milpitas, CA) ; Khanwilkar; Pratap S.;
(Salt Lake City, UT) ; Decker; Jeffrey;
(Streetsboro, OH) ; Baloh; Michael J.; (Detroit,
MI) |
Correspondence
Address: |
KIRTON & MCCONKIE
60 EAST SOUTH TEMPLE, SUITE 1800
SALT LAKE CITY
UT
84111
US
|
Assignee: |
WORLD HEART, INC.
Salt Lake City
UT
|
Family ID: |
22055352 |
Appl. No.: |
11/925294 |
Filed: |
October 26, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09673922 |
Aug 24, 2001 |
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PCT/US99/08870 |
Apr 22, 1999 |
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11925294 |
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09064352 |
Apr 22, 1998 |
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09673922 |
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08850598 |
May 2, 1997 |
6074180 |
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09064352 |
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60016856 |
May 3, 1996 |
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Current U.S.
Class: |
417/420 |
Current CPC
Class: |
A61M 60/122 20210101;
A61M 60/82 20210101; F05B 2200/23 20130101; F04D 29/048 20130101;
F16C 2360/44 20130101; F16C 32/0448 20130101; A61M 60/148 20210101;
F04D 13/0666 20130101; F16C 2316/18 20130101; A61M 60/422 20210101;
A61M 60/205 20210101 |
Class at
Publication: |
417/420 |
International
Class: |
F04B 17/00 20060101
F04B017/00; B63H 1/02 20060101 B63H001/02 |
Claims
1. A low profile pump inlet comprising an inflow diffuser and an
outflow orifice, wherein the diffuser is defined by an open,
unobstructed chamber having two intersecting arcuate channels in
direct, continuous fluid connection with the outflow orifice about
an inner periphery of the intersecting arcuate channels, and
wherein the outflow orifice is configured to direct a fluid from
the diffuser into a pump.
2. The low profile inlet of claim 1 wherein the pump is a blood
pump.
3. The low profile pump inlet of claim 1, wherein the arcuate
channels are symmetrical.
4. The low profile pump inlet of claim 1, wherein the arcuate
channels define a substantially cardioid flow path.
5. The low profile pump inlet of claim 1, wherein the outflow
orifice curves inwardly from the arcuate channels.
6. The low profile pump inlet of claim 1, wherein the outflow
orifice is configured to direct the fluid from the diffuser into
the pump at an angle from the original flow path of the fluid.
7. The low profile inlet of claim 1, wherein the outflow orifice is
configure to direct the fluid in the pump at an angle substantially
perpendicular to the original flow path of the fluid.
8. The low profile pump inlet of claim 1, wherein the outflow
orifice is annular in shape.
9. The low profile pump inlet of claim 1, wherein the outflow
orifice directs the fluid into the pump with a substantially
uniform velocity.
10. The low profile pump inlet of claim 1, wherein the outflow
orifice directs the fluid into the pump with a substantially
uniform velocity independent of any flow patterns upstream of the
pump inlet.
11. The low profile pump inlet of claim 1, wherein the inflow
diffuser of the pump inlet is configured to direct the fluids into
the outflow orifice such that the fluid enters the pump without
vortex flow.
12. A compact, low profile pump inlet comprising an inlet orifice
and a flow turning structure wherein the inlet orifice is
configured to direct fluid into the flow turning structure, and
wherein the flow turning structure is configured to redirect the
incoming fluid flow through an acute angle in a manner such that
the incoming fluid flow swirls around the interior of the flow
turning structure in a spiral configuration thereby equalizing the
flow rate and pressure of fluid entering the low profile inlet
resulting in a reduction of flow disturbances.
13. The low profile pump inlet of claim 12, wherein the redirection
of the incoming fluid reduces fluid stress.
14. The low profile pump inlet of claim 12, wherein the low profile
pump inlet is attached to the pump.
15. The low profile pump inlet of claim 14, wherein the low profile
pump inlet is attached to the pump in at least one of a permanent
and a reversible manner.
16. The low profile pump inlet of claim 15, wherein the profile is
sufficiently shallow to allow implantation of the low profile pump
inlet and attached pump into the human body as at least one of a
heart assist device and a total heart replacement.
17. The low profile pump inlet of claim 16, wherein the low profile
pump inlet and the attached pump form a lightweight and compact
unit suitable for at least one of short-term and long-term
implantation as at least one of a ventricular assist device and a
complete replacement heart in a human patient.
18. The low profile pump inlet of claim 17, wherein the low profile
pump inlet is attached to a magnetically suspended pump.
19. The low profile pump inlet of claim 18, wherein the low profile
pump inlet and the magnetically suspended pump are coated with
biocompatible coating such that all blood contacting surfaces and
tissue contacting surfaces of the low profile pump inlet and the
magnetically suspended pump are at least one of hemocompatible and
biocompatible.
20. An apparatus for pumping sensitive biological fluids,
comprising: a construct having an exterior, an interior having
walls therein, at least one housing magnet disposed therein, and an
axial center; an inlet permitting passage of fluids through the
construct and into the interior of the construct; an outlet
permitting passage of fluids through the construct from the
interior of the construct, the outlet radially located from the
axial center of the construct; an impeller disposed within the
interior of the construct and out of contact with the construct; a
magnetic system including at least one impeller magnet juxtaposed
to the housing magnet, which is magnetized in a direction for
suspending the impeller out of contact with the construct; and a
motor to drive the impeller and control fluid flowing through the
apparatus.
21. The apparatus of claim 20, wherein the impeller includes
arcuate blades and arcuate passageways whereby fluid flow through
the construct is redirected from the inlet to the outlet.
22. The apparatus of claim 21, wherein the magnetic system
comprises: a first construct permanent magnet disposed on a first
wall of the interior of the construct; a second construct permanent
magnet disposed on a second wall, opposite the first wall, of the
interior of the construct; a first impeller permanent magnet
disposed on the impeller distal to the axial center of the
construct and juxtaposed with the first construct permanent magnet;
a second impeller permanent magnet disposed on the impeller distal
to the axial center of the construct and juxtaposed with the second
construct permanent magnet; a first construct permanent magnet set
disposed on the first wall of the interior of the construct; a
second construct permanent magnet set disposed on the second wall
of the interior of the construct; a first impeller permanent magnet
set disposed on the impeller proximate to the axial center of the
construct and juxtaposed with the first construct permanent magnet
set; and a second impeller permanent magnet set disposed on the
impeller proximate to the axial center of the construct and
juxtaposed with the second construct permanent magnet set; wherein
the arrangement provides radial stabilization and, due to angular
positioning, provides a degree of translational stabilization of
the impeller and the impeller is prevented from contacting the
interior of the construct by magnetic fields.
23. The apparatus of claim 22, wherein the inlet is integral with
the construct exterior.
24. The apparatus of claim 22, wherein the outlet is integral with
the construct exterior.
25. The apparatus of claim 22, wherein the outlet is radially
located from the axial center of the construct.
Description
RELATED APPLICATION
[0001] This application is a continuation of and claims priority to
U.S. patent application Ser. No. 09/673,922, filed Oct. 24, 2001,
entitled "Hybrid Magnetically Suspended and Rotated Centrifugal
Pumping Apparatus and Method," which is the National Stage of
International Application No. PCT/US99/08870, filed Apr. 22, 1999,
entitled "Implantable Centrifugal Blood Pump With Hybrid Magnetic
Bearings," which claims priority from abandoned U.S. patent
application Ser. No. 09/064,352, filed Apr. 22, 1998, entitled
"Implantable Centrifugal Blood Pump With Hybrid Magnetic Bearings,"
which was a continuation-in-part of U.S. patent application Ser.
No. 08/850,598, filed May 2, 1997, entitled "Hybrid Magnetically
Suspended and Rotated Centrifugal Pumping Apparatus and Method,"
now U.S. Pat. No. 6,074,180, which claims priority from U.S.
Provisional Patent Application No. 60/016,856, filed May 3, 1996,
entitled "Hybrid Magnetically Suspended and Rotated Centrifugal
Pumping Apparatus and Method," all of which patents and
applications are incorporated herein by reference in their
respective entireties and applicant claims priority to all listed
patents and applications.
BACKGROUND OF THE INVENTION
[0002] This disclosure relates to pumps for pumping fluids such as
blood that are sensitive to mechanical working or shear stress.
More particularly, this disclosure relates to a pump apparatus
having an impeller that is magnetically suspended and rotated by
electric and permanent magnets with no mechanical contact between
the impeller and any other part of the pump.
[0003] There are many types of fluid pumps suitable for use in a
wide range of applications, all performing the same basic function
of moving fluid from one point to another, or moving a fluid from
one energy level to another. However, pumps for pumping sensitive
fluids, such as blood, introduce special design requirements.
Additionally, pumps for implantation in a human patient for long or
short-term use as ventricular assist devices (VAD's) or complete
heart replacement, add additional size, weight, durability, and
other requirements.
[0004] The design problems associated with sensitive fluids,
including blood, generally relate to problems caused by contact of
the fluid with mechanical parts and other substances present in the
pump. Problem contact areas for sensitive fluids may include 1)
contact with materials and structures in rotating fluid seals, 2)
contact with mechanical bearing assemblies that are exposed to the
fluid, and 3) use in bearing structures that depend on a layer of
fluid between moving surfaces to provided reduced friction, such as
hydrodynamic bearings. For example, it is well known that rotating
shaft seals are notoriously susceptible to wear, failure, and even
attack by some fluids. Many types of pumps may also increase
mechanical working of the fluid and precipitate detrimental
processes such as chemical reactions or blood clotting.
[0005] It is also well known that pumps for corrosive fluids,
blood, and fluids used in food processing require careful design of
the flow passages to avoid fluid damage, contamination, and other
undesirable conditions. For example, ball bearing and other rolling
element bearings must in general be used with some type of shaft
seal to isolate the fluid from the bearing for the above mentioned
cases. This may be needed to prevent damage to the bearing by
caustic fluids, or to prevent damage to the fluid by the rolling
elements of the bearing. For example rolling element bearings can
crush and destroy the living cells in blood. Thus, rolling element
bearings are generally not practical for blood pumps.
[0006] Finally, the size, weight, biocompatibility, and operating
durability and reliability of blood pumps are a major concern where
VAD's and heart replacement pumps are concerned. It would be
desirable to have a VAD or heart replacement pump that can operate
reliably for 20 or 30 years despite the normal bumping and jarring
of everyday life, including unexpected impact such as from falling,
yet is small enough to implant easily in a patient's chest. It is
also desirable to reduce the power requirements of such a pump so
as to increase mobility of the patient.
[0007] To address these problems, pumps with magnetically suspended
impellers have been developed. For example, Oshima et. al. (U.S.
Pat. No. 5,111,202) discloses a pump in which the impeller is
magnetically suspended or levitated within the pump housing, and is
magnetically, not mechanically, coupled to the pump housing. The
pump employs permanent magnets rotating on a motor external to the
pumping chamber, with the external permanent magnets magnetically
coupled to opposing permanent magnets on the impeller. Magnetically
suspended pumps are well adapted to pumping sensitive fluids
because they eliminate the mechanical bearing structure or rotating
seals which can damage or be damaged by the fluid.
[0008] However, such pumps known in the art present several
drawbacks. First, an external motor with its own means of bearing
support (ball bearings) is still required to rotate the impeller.
It is the external bearing support that maintains the position of
the rotor in such a pump. Though the motor is sealed from contact
with blood and other bodily fluids, and is magnetically coupled to
the suspended impeller, it still employs bearings which produce
heat and pose the potential of failure. Naturally, such pumps tend
to be bulky in part because of the size of the electric motor.
These pumps are frequently unsuitable for implantation in a human
patient because of size, weight, power consumption, and durability
problems.
[0009] Other methods of magnetically supporting a rotating pump
impeller have been developed. Olsen, et. al. (U.S. Pat. No.
4,688,998) teaches a fully suspended pump rotor employing permanent
magnet rings on the rotor magnetized along the axis of rotation,
and actively controlled electromagnets on the stator that create a
magnetic field to stabilize the position of the rotor. This
approach also leaves certain problems unsolved. While the
manufacture of permanent magnets has advanced substantially, there
are still significant process variations. These variations include
repeatability from one magnet to the next, and homogeneity of the
material within one magnet. The position and stability of the rotor
in the Olsen invention is entirely dependent on the homogeneity of
the permanent magnet rings. These problems are well known by
designers of electromechanical devices, where significant steps are
normally taken to reduce the dependency of device performance on
homogeneous magnets. In the field of permanent magnet motors, this
is a well known source of torque ripple.
[0010] It would therefore be desirable to have a pumping apparatus
with a magnetically suspended impeller that is suitable for pumping
blood and other sensitive fluids, and which is small, lightweight,
durable, reliable, and has a low power consumption, without using
an external motor to drive the impeller. It would also be desirable
to have a magnetically suspended pump that has reduced sensitivity
to manufacturing process variations in permanent magnets. It would
also be desirable to have a magnetically suspended pump that
requires no additional sensors for pump status monitoring.
SUMMARY OF THE INVENTION
[0011] Systems and methods of this disclosure include a pumping
apparatus with a magnetically suspended impeller that is suitable
for pumping blood and other sensitive fluids, by handling the fluid
in a gentle manner with very low heating of the fluid. The systems
and methods include a motor for a magnetically levitated pump
impeller having a flux gap on one or both sides of the impeller
that generates low attractive force between the rotor and stator
relative to other systems. The systems and methods include a
pumping apparatus of relatively compact size to allow implantation
in the human body as either a heart assist device or as a total
heart replacement. The systems and methods include a pump apparatus
and system with parameters available for measurement that are
inherently available without adding additional sensors, such as
magnetic bearing current and/or motor current sensors, that can be
used as an indicator of required flow and pressure when the pump is
implanted in the human body, or can be used to keep the impeller
controlled by the magnetic bearing. The systems and methods include
a pump apparatus with a long product life which requires minimal
maintenance. The systems and methods include a pump apparatus that
can provide flow in either a constant manner or a flow that pulses
on a periodic basis. The systems and methods include a pump
apparatus which is configured to cause an acute change in direction
of the fluid in one or more of the conduits while still handling
the sensitive fluid in a gentle manner. The systems and methods
include a blood pump in which all blood-contacting surfaces are
coated with a biocompatible ceramic coating.
[0012] The above elements are realized in specific illustrated
embodiments of an implantable centrifugal blood pump with hybrid
magnetic bearings. The pump comprises a generally cylindrical pump
housing, a generally cylindrical impeller disposed within the pump
housing, a magnetic bearing system for supporting and stabilizing
the impeller in five degrees of freedom, and a conformally shaped
motor for rotating the impeller in the remaining degree of freedom,
with no mechanical contact between the impeller and any other
structure. The pump thus reduces damage to the fluid from the pump
and damage to the pump from the fluid. The pump impeller, housing,
and other components are also configured such that flow patterns
are as smooth and laminar as possible, and eddies, flow separation,
and re-circulation are reduced.
[0013] The magnetic bearing system and motor advantageously
comprise both electromagnets and permanent magnets for stability
and control of the impeller, and to reduce size, weight, and pump
power consumption. The permanent and electromagnets are disposed on
the pump housing and on the impeller, such that by controlling
electric current through the electromagnets on the housing, the
magnetically suspended impeller functions as the rotor, and the
housing as the stator of a D.C. motor. A controller linked to the
electromagnets allows for sensing of relative impeller position and
dynamic properties without the need for additional sensors. It also
allows for the adjustment of the impeller position by modification
of the current flow to the electromagnets. The pump thus forms a
lightweight, dependable, and compact unit suitable for short or
long-term implantation as a ventricular assist device or a complete
replacement heart in a human patient.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The above and other features and advantages of the invention
will become apparent from a consideration of the following detailed
description presented in connection with the accompanying drawings
in which:
[0015] FIG. 1 is a perspective view of the preferred embodiment of
the implantable centrifugal blood pump with hybrid magnetic
bearings;
[0016] FIG. 2 is an exploded perspective view of the preferred
blood pump of FIG. 1;
[0017] FIG. 3 is a cross sectional view of the inner workings of an
embodiment of the pump;
[0018] FIG. 4 is a three-dimensional view of the pump impeller with
the vane shroud removed;
[0019] FIG. 5A is a plan view of the front of the pump motor
assembly;
[0020] FIG. 5B is a cross sectional view of the pump motor
assembly;
[0021] FIG. 5C is a plan view of the back of the pump motor
assembly;
[0022] FIG. 6A is a view of the front of the motor rotor
assembly;
[0023] FIG. 6B is a cross sectional view of the motor rotor
assembly;
[0024] FIG. 6C depicts the polarity of the permanent magnets on the
motor rotor in one embodiment;
[0025] FIG. 7A is a detailed front view of the motor coils on the
stator;
[0026] FIG. 7B is a cross sectional view of the stator;
[0027] FIG. 7C is a view of the back of the stator;
[0028] FIG. 8 is a perspective view of a hybrid EM/PM magnetic
bearing ring;
[0029] FIG. 9 is a cutaway perspective view of part of a hybrid
EM/PM magnetic bearing ring showing the flux paths for one
permanent magnet;
[0030] FIG. 10 is an exploded perspective view of an embodiment of
the magnetic suspension actuator similar to FIG. 9, but including
the coils;
[0031] FIG. 11 is a cutaway view of part of a hybrid EM/PM magnetic
bearing ring showing the flux paths for two electromagnets;
[0032] FIG. 12 is an exploded perspective view of the four bearing
sets of poles, air gaps, and targets;
[0033] FIG. 13 is a block diagram of an electronic controller for
providing control of the magnetic bearing actuator;
[0034] FIG. 14 is a representative applied voltage waveform and
resulting representative current waveforms for two different
positions of the rotating impeller;
[0035] FIG. 15 is one implementation of the self-sensing electronic
circuit; and
[0036] FIG. 16 is a magnetic saturation link inserted into the PM
circuit.
DETAILED DESCRIPTION
[0037] Reference will now be made to the drawings in which the
various elements of systems and methods of this disclosure will be
given numeral designations and in which the invention will be
discussed so as to enable one skilled in the art to make and use
the invention. It is to be understood that the following
description is only exemplary of the principles of the invention as
claimed, and should not be viewed as narrowing the pending
claims.
[0038] A perspective view of the assembled pump of the preferred
embodiment is shown in FIG. 1. The pump generally comprises a
housing 4 with an inlet 1, flow turning structure 2, and outlet 3.
The flow turning structure 2 is configured to redirect incoming
fluid flow through an acute angle in a gentle, low thermal manner
using a compact structure. The turning structure is configured such
that flow swirls around the inlet in a logarithmic spiral
configuration, equalizing the flow rate and pressure entering the
inlet. Additionally, this spiral inlet configuration reduces flow
eddies and other disruptions in the flow that are detrimental to
pump efficiency. The redirection of flow is thus accomplished in a
gentle manner with low fluid stress that is consistent with use in
a pump for sensitive fluids. A motor, magnetic bearings, and
impeller are disposed inside the pump housing 4, and will be more
particularly described hereafter.
[0039] An exploded view of the assembly of the preferred embodiment
is shown in FIG. 2. In this view the pump inlet 1, flow turning
structure 2, and pump outlet 3 are clearly visible as in FIG. 1.
This figure also shows the upper half 4A and lower half 4B of the
pump housing 4. The pump further comprises an inlet side magnetic
bearing actuator 5, and an outlet side magnetic bearing actuator 6.
The impeller assembly 7 is disposed between the magnetic bearing
actuators 5 and 6, and comprises the rotating part of the pump. The
impeller 7 is designed to function as the rotor of a motor, and
includes soft iron magnetic material structures 9 and 10 that act
as targets on the rotor for the magnetic bearing actuators 5 and 6.
These and other features of the impeller will be more apparent from
the discussion of FIG. 3. The eye of the impeller 8 provides an
opening for the inlet of flow into the pump vanes in the preferred
embodiment. Advantageously, the motor stator 11 is incorporated in
the outlet side or lower half 4B of the pump housing 4.
[0040] FIG. 3 shows a two-dimensional cross sectional view of the
inner workings of the preferred embodiment of the invention. In
this view the combination of electromagnets (EM) and permanent
magnets (PM) becomes visible. Advantageously, the impeller assembly
7 is the only moving part in the system, and forms a curved,
conical ring disposed adjacent to the motor stator 11, and between
the upper and lower bearing actuators 5 and 6. The impeller
assembly 7 comprises a shroud 13 disposed above a plurality of
vanes 15, and a hub 54 which supports the vanes and the elements of
the motor rotor. The housing 4 is formed to provide curved fluid
gaps 12 around the rotating impeller 7. The gaps 12 are configured
to work in conjunction with the impeller 7 to accommodate flow
without damaging blood or other sensitive fluids. This is
accomplished by making the flow passage clearances 12 short in
length, yet with large bending radii to allow gentle backflow
around the shroud 13 and hub 54.
[0041] The vanes 15 of the impeller 7 drive the fluid from adjacent
the inlet 2 into the pump volute 14, which is formed around the
perimeter of the inner space of the housing 4. The volute 14 is
formed in a logarithmic spiral shape, more evident in FIG. 2, which
spirals out from the center of the pump, gathering the flow from
the impeller vanes 15, and directing it to the tangentially aligned
outlet 3 (FIG. 1). This configuration adds to the advantages of the
invention through minimizing damage to blood or other sensitive
fluids by gradually redirecting the flow across the vanes 15 from
the in flow 2 to the pump volute 14, where the flow is then
directed to the outlet 3.
[0042] As depicted in FIG. 3 the fluid gaps 12 in the pump are
advantageously configured to accommodate sensitive fluid flow by
being short in length and arcuate in shape with large bending radii
to minimize sharp turns in the flow passages. This design also
helps to reduce potential stagnation and shear of the fluid.
Notably, the gap 12 between the rotating impeller 7 and the
stationary housing in the vicinity of the motor 11 is neither
radial nor axial as in conventional motor designs, but is
conformally shaped to accommodate the particular requirements of
the flow paths and the motor design. By virtue of its conformal
shape, the curved upper surface of the motor 11 advantageously
provides an axial force on the impeller/rotor 7, while
simultaneously powering its rotation.
[0043] As shown, the arcuate flow passageways 12 are thus
integrated directly in to the motor design, as will be described in
more detail below. This integrated approach of motor design with
pump design is not reflected in other pumps. It will be apparent
that the invention is not restricted to the motor shape shown in
this or other figures, but may be otherwise configured and still
provide the advantages of conformal design. The same approach to
motor design and fabrication can be employed to make a variety of
motors with conformally shaped gaps between the rotating and
stationary parts.
[0044] FIG. 4 shows a view of the impeller 7 (see FIG. 3) with the
vane shroud 13 removed. In this view the plurality of arcuate vanes
or blades 15 are clearly visible. The impeller vane layout is
designed to provide a smooth transition from the inlet blade angle
to the discharge blade angle. It will be apparent from this figure
that the inlet blade angle .theta. varies continuously from hub to
shroud, with a greater angle .theta. near the inlet 2 (see FIG. 3),
and an angle approaching zero near the outlet (measured relative to
a line perpendicular to the plane of the impeller), to reduce the
incidence of flow angles over the entire blade length.
[0045] The pump intentionally allows relatively high leakage flows
in the gaps 12 at the shroud side of the impeller 7, and along the
hub 10 side of the impeller 7. Relatively large fluid gaps are
desirable on both the inlet side and discharge side of the impeller
7 to allow for recirculating flows in the gaps at low shear stress
levels. As will be appreciated, the acceptable level of shear is a
function of expected cell transit time through the gap. However,
for both magnetic bearing and motor design considerations, it is
desirable to minimize the size of the flux gap, for example, to a
gap of 0.015 inches. However, it will be apparent that other gap
sizes, such as 0.010, 0.020, and 0.030 inches may also be found
suitable.
[0046] FIG. 5B shows a two-dimensional cross sectional view of the
motor assembly, and FIGS. 5A and 5C are front and back views of 25
the same. The motor stator assembly 11 comprises motor coils 16
having a nonmagnetic core, backed by a backing material 17,
preferably a soft iron magnetic material which may be laminated or
not. Alternatively the backing material 17 may be formed of a non
magnetic material depending on the level of constant force desired
between the rotor and stator. In the preferred embodiment, the
backing material 17 is laminated soft iron material. The
impeller/rotor 7 also comprises a ring of permanent magnets 18,
preferably backed by a soft iron backing material 19, which acts as
a magnetic yoke for the permanent magnets 18. The soft iron backing
19 improves performance, but is not required for the invention to
function.
[0047] FIGS. 6A-6C provide detailed views of the motor rotor
assembly. Permanent magnets 18 are arranged around the
circumference of the rotor 7 in alternating polarity configuration,
shown in FIG. 6 by the common designations N and S. As will be
appreciated, in order to provide magnetic flux across the flux gap,
the magnetization of the permanent magnets 18 is perpendicular to
the flux gap. In FIG. 6, the flux of the permanent magnets can be
visualized as flowing into or out of the plane of the page. The
preferred embodiment as shown comprises 6 magnets, but the
invention can be implemented with any even number of magnets, such
as 4, 6, or 8 magnets.
[0048] FIGS. 7A-7C show detailed views of the motor coils 16 and
stator soft iron backing 17. The coils 16 are separated into a
plurality of discrete stator poles 20. The number of stator poles
20 must be divisible by the number of phases, which can be 2, 3, 4,
or more. For example, in the embodiment shown, the designated
stator poles (depicting one third of the stator circumference) are
labeled A, B, and C because the preferred pump is designed to
function on 3-phase electrical power. Nine poles are thus provided,
but any number that is divisible by 3 could be used with 3-phase
power.
[0049] This approach to motor design has several advantages. First,
the fluid/flux gap between the rotor and stator is conformally
shaped to the requirements of the fluid flow path 12 as discussed
above. Second, the motor is highly efficient due to the balance of
the amount of permanent magnet material with the volume of coils
and soft iron. Third, the motor can be constructed in such a way
that it only generates rotational forces or generates primarily
rotational forces. This is a very important advantage in a system
that uses magnetic bearings, since the size and power level of the
magnetic bearings depends on the magnitude of the forces other than
rotational force generated by the motor. Additionally, this motor
is a slotless motor because the coils do not comprise a magnetic
core, and the magnetic material 17 is thus separated from the
permanent magnets in the rotor by the dimension of the coils
16.
[0050] The support of the rotating impeller requires control of
five degrees of freedom: 3 translations (x,y,z) and 2 angular
displacements (q.sub.x and q.sub.y). There are several types of
forces which act upon the impeller: fluid forces, gravitational
forces, and dynamic forces. The fluid forces are due to fluid
pressures acting on the impeller and the changes in momentum as the
flow direction is changed. The gravitational force (vertically
downward) is due to the difference between the weight of the
impeller and the buoyant force, in blood, acting on the impeller in
different orientations, depending on the orientation of the body
relative to the vertical. Dynamic forces act upon the impeller due
to bodily accelerations during such activities as sudden motions or
impact after a fall.
[0051] The hybrid integrated EM/PM bearing uses flux from both an
electromagnetic flux source and a permanent magnetic flux source in
the same integrated multiple pole configurations to control the
five degrees of freedom. The permanent magnet (PM) circuit is
integrated into a ring configuration with the electromagnet (EM)
soft iron magnetic circuits, the EM coils, the magnet target, and a
saturation link.
[0052] FIG. 8 shows a view of the preferred embodiment of a bearing
actuator 5 (or 6) with permanent magnets 21 and soft magnet poles
22. FIG. 8 is intended to illustrate the magnetic materials only;
no coils are shown in FIG. 8. A slot 23 for accommodating one of
the coils is designated for reference. FIG. 8 depicts an actuator
having four poles 22, which is preferred, but any other even number
of poles, i.e. 6, 8, or more, may be advantageously employed. Each
pole 22 includes a thrust bearing pole 24, for providing axially
oriented magnetic flux in the gap between rotor and stator, and a
radial bearing pole 25, for providing radially oriented magnetic
flux in the gap between rotor and stator.
[0053] Two actuators (5 and 6) are employed: one on the inlet side
of the impeller and one on the discharge side. These rings may be
identical in construction, such that the PM flux is equal in both
rings, or different so that the PM flux in one ring may be larger
than in the other ring. The PM flux serves as the constant
magnetomotive force (MMF) in the flux loops, and functions as the
bias flux acting throughout the magnetic circuits. It is well known
in magnetic bearing design that a bias flux in the soft iron
electromagnets is useful to linearize the response of the actuators
and to provide increased dynamic force load capacity.
[0054] FIG. 9 shows the flux paths for one permanent magnet 21. The
permanent magnet 21 is disposed between the axial and radial flux
paths of two electromagnet poles 22 in the actuator 5, and supplies
permanent magnetic flux to the electromagnet poles on either flux
path to provide dynamic force load capacity (also known as slew
rate capability). Dynamic force load capacity is a measure of the
ability of the magnetic suspension system to change force within a
short period of time to control the rotor position. This bias flux
is typically provided by a bias current through the EM bearing
coils, with a resulting much higher steady state power loss.
[0055] Blood and other fluids that are sensitive to heating are
easily accommodated by this invention, because the innovative
magnetic bearing design reduces power dissipated in the magnetic
bearings as compared to other systems. This is accomplished, in
part, by the use of permanent magnets. While permanent magnets have
been employed in some blood pumps, the embodiments described herein
present advantages in terms of 1) size of the magnetic bearing
system, 2) bearing stiffness achieved in this configuration of the
permanent magnets, and 3) power dissipated in the magnetic
bearings.
[0056] FIG. 10 shows an exploded view of a preferred embodiment of
the magnetic suspension actuator 5 similar to FIG. 9, but including
coils 26, and shown in an orientation inverted from FIG. 8. The PM
flux is directly integrated into a multiple pole ring configuration
with the EM flux. Wire coils 26 suitable for providing a MMF in the
EM section of the ring configuration are included in the
construction. The radial and axial gap fluxes are varied with the
EM flux, where the EM flux is adjusted by the coil currents to
control the impeller position. The bearings have two EM flux paths:
one that has a path including a radially oriented flux gap, and
another containing an axially oriented flux gap. Both of these flux
paths have a combination of EM and PM flux existing in them.
[0057] FIG. 11 shows the EM flux paths. When it is desired to
increase the magnetic flux in the air gap to increase the force
acting on the impeller target, the corresponding coil current is
increased the necessary amount. Alternatively, when it is desired
to decrease the magnetic flux in the air gap to decrease the force
acting on the impeller target, the corresponding coil current is
decreased the necessary amount or driven in the opposite direction.
The presence of a permanent magnet directly in the EM flux path
would create very high magnetic reluctance in that path. Hence, the
structure is set up such that the EM flux path does not include any
permanent magnets, but the EM and PM flux paths are combined at the
gap.
[0058] The control (EM) flux flows from the stator through an air
gap at one pole to a soft iron target mounted on the impeller and
leaves the target to return to the stator through another pole. For
example, the control (EM) flux may flow out of the stator to the
target in a radial air gap and then return to the stator via the
axial air gap. Thus at any given time, the control current
activates the flux in a manner such that the overall flux is
increasing in one of the air gaps but decreasing in the other.
[0059] The actions of the air gap fluxes are coordinated to
independently control the radial and axial centering forces without
coupling between the two directions which simplifies the controller
algorithm greatly, as compared to the fully coupled case. FIG. 12
is used to illustrate the control method in a two-dimensional
version of the integrated hybrid EM/PM magnetic bearing system.
There are four sets of bearing poles, air gaps, and targets shown
in cross section in FIG. 12, including two inlet side radial flux
gaps 27 and 31, two discharge side radial flux gaps 28 and 32, two
inlet side axial flux gaps 29 and 34, and two discharge side axial
flux gaps 30 and 33.
[0060] There are four major components in a typical magnetically
suspended pump control system: an actuator, a controller, a power
amplifier, and proximity sensor(s) to measure the position of the
impeller. Since a fully permanent magnetic suspension is not
possible, every magnetic suspension system must include some means
of active control. The control algorithm configured for use with
systems and methods of this disclosure operates as follows. To move
the rotor in the positive Y direction (radial), it is necessary to
produce a radial force, but not simultaneously produce an axial
force, so as to keep the impeller/rotor in the centered position.
The EM coils in the top of the rotor are activated so that the
magnetic flux in the inlet side axial flux gap 29 and discharge
side axial flux gaps 30 is increased equally, and the other top EM
coils are activated so that the flux in the inlet side radial flux
gap 27 and discharge side radial flux gap 28 is decreased equally.
The coils in the bottom of the rotor are activated so that the flux
in the inlet side radial flux gap 31 and discharge side radial flux
gap 32 is increased equally, and the other EM coils are activated
so that the flux in the inlet side axial flux gap 34 and discharge
side axial flux gaps 33 is decreased equally. This combination
produces a net radial force downward, opposite to the upward motion
of the rotor, and no net axial force. Reversing this combination
creates a net upward force if the impeller moves downward. A
similar combination of EM coil currents produces a net axial force
or moments about the x or y axes without any radial force. If the
inlet and discharge side rings are not identical, a relatively
simple control algorithm, based on the differing pole face areas
and flux levels, is used to decouple the forces and moments
generated to center the impeller/rotor 7.
[0061] The magnetic bearing actuator is controlled by an electronic
controller 36, which is included in the block diagram of FIG. 13.
Conventional magnetic bearings require physical sensors to provide
feedback control signal to a controller. However, in the systems
and methods of this disclosure, there are no physical sensors
employed. Instead, the controller 36 constantly monitors and
evaluates the impeller position by means of a passive self-sensing
system. The position of the rotor is measured using a self-sensing
algorithm, which employs feedback from the switching amplifier 35.
The switching amplifier 35 receives an input signal from the
controller 36 indicating the average value of current required for
each coil. The switching amplifier then adjusts the average value
of the coil current using pulse-width-modulation, or some other
switching approach.
[0062] The controller system of FIG. 13 comprises an electronic
self-sensing circuit 37, which implements the algorithm previously
described. The self-sensing circuit 37 employs the characteristics
of the actuators themselves in sensing the position of the rotor.
Inductance or flux in a coil with a soft iron core changes with the
magnetic flux linkage in the coil. In the magnetic circuit in FIG.
11 it can be easily seen that the flux linkage in the coil depends
on the gap between the coil and the soft iron material in the
stator, and the soft iron material in the rotor. Hence, the
inductance in the coil changes when the position of the rotor
changes within the pumping cavity. As the inductance of the coil
changes, the time constant of the switching waveforms in the
switching amplifier change as well. A combination of electronic
filters and a feedback controller circuit are used to remove
switching current variations in the switching amplifier signals.
Thus, the physical gap between the housing 4 and impeller 7 is
directly related to the coil currents in the magnetic actuator, and
the position of the impeller/rotor 7 can be constantly monitored by
virtue of this characteristic without the need for additional
sensors.
[0063] The magnetic bearing actuator is controlled by adjusting the
EM coil currents and creating magnetic forces needed to center the
impeller. The control algorithm is a feedback controller employing
a signal correlated with the translational displacements of the
impeller in three directions and two angular displacements in two
axes perpendicular to the motor spin axis, represented as x(t). The
controller operates on a mathematical model of the magnetic bearing
geometry and magnetic properties including both the EM and PM flux
paths, the electrical properties of the bearing EM coils, the
properties of the power amplifiers, the properties of the
preamplifiers, and the translational and angular displacement
sensing circuits.
[0064] The controller algorithm may consist of a
proportional-integer-derivative controller, where the control
signal G(t) has three components: 1) proportional to the
translational or angular displacements with constant K.sub.p, 2)
proportional to the time integral of the translational or angular
displacements with constant K.sub.i, and 3) proportional to the
translational or angular velocity of the form with constant
K.sub.d.
G ( t ) = K p x ( t ) + K i .intg. x ( t ) t + K d x ( t ) t
##EQU00001##
[0065] Alternatively, the controller may take the form of mu
synthesis, or a similar controller, where feedback is used and the
controller is able to take into account uncertainties in the
mathematical model of the system. Another possible controller
algorithm is the use of a sliding mode (variable structure control)
which employs a reaching condition to place the impeller
translational displacements and angular displacements on a
hyperplane (sliding surface in phase space) and create a condition
where the impeller states are moved along the hyperplane. The
controller currents are switched on when the impeller position
moves off of the sliding surface to return it to the sliding
surface, and switched off when the impeller returns to the desired
surface. This type of controller includes non-linear effects and
the capability to adapt to uncertainty in the applied forces acting
on the impeller, such as fluid forces.
[0066] A means is provided where the determination of the impeller
translational and angular displacements is performed with
electronic devices rather than a physical sensor, such as an eddy
current or inductive sensor. The magnetic bearings will have the
coil currents supplied by switching power amplifiers operating at a
high frequency such as 20 kHz. The approach here is to use both the
low frequency component and high frequency components of the coil
currents to determine the resistive and inductive properties of the
coil. The low frequency current is obtained from electronic means
which measure the instantaneous control currents following use of a
low pass filter. The high frequency current is obtained from an
electronic measure of the instantaneous envelope of the switched
coil currents and a high pass filter. The inductive property of the
coil is related to both the coil current and the air gap length.
This information is combined with other available knowledge of the
switching amplifier duty cycle to evaluate the air gap length, but
separating the effect of the changes in coil inductance due to
controller currents from the change in inductance due to the change
in air gap length. The air gap lengths are evaluated using a direct
method of evaluating these properties. Alternatively, if there are
errors in the air gap values using the direct method, a feedback
loop is used with a parameter estimation algorithm to converge to a
closed loop value of the air gap.
[0067] There are several advantages to this approach. First, the
physical size of the pump can be reduced because there is no space
required for sensors. Second, physical sensors are potential points
of failure and the passive electronic sensing system should be more
reliable. Third, the number of wires coming off of the heart pump
is significantly less. As an illustration of the self-sensing
concept, FIG. 14 shows an applied voltage waveform 38 and the
resulting current waveforms for two different positions of the
rotating impeller. The current for position 1 is denoted at 39, and
the current for position 2 is denoted at 40. The overall envelope
of the position 1 current is denoted at 41, and the envelope for
the position 2 current is denoted at 42. Average currents for
position 1 and position 2 are denoted at 43 and 44
respectively.
[0068] FIG. 15 shows the implementation of the self-sensing
electronic circuit 37. Filters 45 operate on the current signal
obtained from the switching amplifier 35, resulting in the envelope
and average value waveforms. The envelope, average value, and
applied voltage are fed into the digital sampling system 46 where
the variation in current waveform envelope relative to the average
current and the applied voltage is used to determine the electrical
time constant of the resistance-inductance circuit in the actuator.
From this information, the inductance, and hence the rotor position
can be derived. An alternative approach is to sample the current
waveform directly. The approach of this invention thus provides the
significant advantage of lowering the required sampling rate of the
digital sampling system significantly, while still obtaining all of
the necessary information from the waveforms.
[0069] This sensing approach eliminates the separate position
sensors used in other systems with the following advantages: 1)
smaller system size, 2) improved reliability due to decrease number
of components, and 3) reduced wire count. Additionally, envelope
and average values of the current and voltage signals are used to
reduce digital sampling requirements, thereby significantly
reducing complexity and cost of the system.
[0070] One significant concern with the use of permanent magnets
and permanent magnet biasing is the force developed when the EM
coil currents are turned off and the impeller is off center,
against one of the walls. The PM circuits have lower reluctance on
the side where the flux gaps are zero, with resulting high forces,
and much higher reluctance on the sides where the flux gaps are
large, with resulting lower forces. This high, new, off-center
force, called the lift-off force, must be overcome to initially
center the impeller by the EM control fluxes. If no suitable design
is employed, this force is large and corresponding large EM coils
and control currents will be required.
[0071] The systems and methods of this disclosure incorporate a
magnetic saturation link 48 which is inserted into the PM circuit,
as shown in FIG. 16. The saturation link 48 is a short section of
the PM flux circuit which has a smaller cross sectional area than
the other sections so that the magnetic flux density is at the
magnetic saturation level of the soft iron material used in the
flux path. The PM and the saturation link are sized so that the
magnetic material in the saturation link is always saturated. This
in turn keeps the PM magnetic flux density at a constant value when
the EM rotor is off-center and minimizes the required lift-off
force. Thus, the size of the EM coils can be minimized. This
pattern is repeated over all of the PM magnetic flux paths in the
ring design with a series of saturation flux links.
[0072] As will be appreciated, hemocompatibility is also of
critical importance with blood pumps. There are three primary areas
of concern for hemocompatibility in any blood pump: 1) hemolysis
due to fluid shear, 2) thrombogenesis due to flow stagnation and/or
fluid shear, and 3) material interactions with blood that result in
thrombogenesis or complement activation. It is desirable to coat
the fluid contacting surfaces of the pump with a coating that
satisfies these concerns. It is also desirable to coat tissue
contacting surfaces on implantable pumps with such a coating.
[0073] In an embodiment, an amorphous coating of a transition metal
nitride or other wear-resistant biocompatible ceramic material is
applied according to a method disclosed in U.S. patent application
Ser. No. 09/071,371, filed Apr. 30, 1998. By this method, a
biocompatible, reliable, and durable room-temperature-processed
amorphous coating can be provided on all blood-contacting and/or
tissue contacting surfaces of the pump. A variety of biocompatible
ceramic coatings may be applied by this method, including titanium
nitride, silicon nitride, titanium carbide, tungsten carbide,
silicon carbide, and aluminum oxide.
[0074] Titanium nitride is a preferred coating material. As a
transition metal nitride, it is a well-known biomaterial. It is
inert, fatigue resistant, biocompatible, corrosion resistant, and
lightweight. In crystalline form it is used in tools and parts for
high-temperature (up to 600.degree. C.) applications as a corrosion
and oxidation-resistant coating. Titanium nitride coatings have
also been used as a wear resistance coating for orthopedic
implants, on dental implants and instruments, and on defibrillator
electrodes, where it is applied by chemical vapor deposition.
However, all of these applications use titanium nitride in its
crystalline form. Unfortunately, crystalline TiN cannot be applied
to plastics, magnetic materials, and other heat-sensitive and
flexible materials because of its high (.about.800.degree. C.)
coating temperature and because it chips when its substrate
flexes.
[0075] Advantageously, the systems and methods of this disclosure
incorporate the above-referenced process to provide an amorphous,
room-temperature coating of TiN that can be applied to plastics,
magnetic materials and other temperature-sensitive materials used
in blood pumps or with other sensitive fluids. By this process, a
TiN coating may be applied to pump surfaces by a magnetron
sputtering technique in a vacuum chamber. Sputtering is a
comparatively low-temperature technique by which TiN thin films can
be uniformly deposited on substrates. Materials successfully coated
by the inventors following this method include titanium,
polyurethane, stainless steel, corethane, polyester,
polyvinylchloride (PVC), iron plastic composite material, epoxy and
Neodymium-iron-boron magnets. Some of these substrate materials
were blood pump components. Following this method, the total
thickness of the surface coat is about 1000 to 5000 angstroms.
During more than 50 experiments, various substrates were tested to
standardize the process conditions suitable for each substrate.
[0076] The preferred amorphous coating of TiN provides numerous
advantageous features and benefits in this application. Such a
coating provided by sputtering is applicable on cannulae and other
flexing surfaces. Because this process provides a diffusion
barrier, the surface inhibits permeability of gases and fluids into
coated surfaces. Because it is deposited at room temperature,
coating may be done without creating surface stresses and material
damage on plastics, magnetic materials and composites. Because this
technique is applicable to multiple materials (plastics, metals,
composites), substrates of different materials can be coated with
the same coating, and thus the entire fluid containment circuit can
be coated with the same process and the same material. Finally, the
surface is biocompatible, which allows the coating of all blood
contacting surfaces and tissue contacting surfaces of blood
pumps.
[0077] Those skilled in the art will appreciate that numerous
modifications can be made without departing from the scope and
spirit of this disclosure. The appended claims are intended to
cover such modifications.
* * * * *