U.S. patent application number 10/684090 was filed with the patent office on 2005-04-14 for homopolar machine with improved brush lifetime.
Invention is credited to Hazlebeck, David A., Langhorn, Alan Robert, Piec, Zbigniew S..
Application Number | 20050077795 10/684090 |
Document ID | / |
Family ID | 34314172 |
Filed Date | 2005-04-14 |
United States Patent
Application |
20050077795 |
Kind Code |
A1 |
Piec, Zbigniew S. ; et
al. |
April 14, 2005 |
HOMOPOLAR MACHINE WITH IMPROVED BRUSH LIFETIME
Abstract
A homopolar machine which exhibits reduced wear and prolonged
brush life. Current collectors or brushes are mounted so as to
maintain substantially constant contact pressure and so that all of
them are polarized negative. Brush holders are provided which
facilitate the application of precise and constant contact pressure
in a region of very high magnetic forces.
Inventors: |
Piec, Zbigniew S.; (San
Diego, CA) ; Langhorn, Alan Robert; (San Diego,
CA) ; Hazlebeck, David A.; (San Diego, CA) |
Correspondence
Address: |
FITCH EVEN TABIN AND FLANNERY
120 SOUTH LA SALLE STREET
SUITE 1600
CHICAGO
IL
60603-3406
US
|
Family ID: |
34314172 |
Appl. No.: |
10/684090 |
Filed: |
October 10, 2003 |
Current U.S.
Class: |
310/178 |
Current CPC
Class: |
H02K 31/02 20130101 |
Class at
Publication: |
310/178 |
International
Class: |
H02K 031/00 |
Claims
1-9. (canceled)
10. A homopolar machine comprising: a shaft; an armature assembly
coupled to the shaft that includes a cylinder having a pair of
spaced apart generally radially extending flanges, first and second
stators arrays that encircle the armature assembly; a negative bus
connected to a first stator, a positive bus connected to a second
stator, a first negative brush assembly carried by said first
stator for contact with said armature at one location; said brush
assembly including brushes, an actuator and a holder designed to
effect said contact with said armature, which holder includes (a)
radially aligned stem disposed between a pair of parallel flat
plates that are mounted in fixed relation to said first stator and
(b) a slidable component which carries a plurality of said first
negative brushes and which is slidable between said parallel
plates, said stem being formed with a pair of opposed parallel
surfaces that are perpendicular to said flat plates, and said
slidable component including a pair of flexible heads that engage
said opposed stem surfaces, and a second negative brush assembly
carried by said armature for contact with said second stator, as a
result of their negative charge, the lifetime of said brushes is
substantially extended compared to comparable positively charged
brushes.
11-14. (canceled)
15. The homopolar machine in accordance with claim 10 wherein said
flexible heads include a stack of generally parallel ribbons cut by
EDM from conductive metal.
16. The homopolar machine in accordance with claim 15 wherein said
heads are mounted so that said ribbons are flexed against said
opposed surfaces.
17-18. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to direct current machines,
and more specifically to homopolar machines.
[0002] Homopolar machines are operated by direct current (DC) and
are simple in design principle. They have been under consideration
and development for some years for use in ship propulsion
applications because of their high efficiency, compact size, low
weight, and reduced acoustic signature relative to all other
motors, see U.S. Pat. No. 3,657,580 (1972).
[0003] As is well known in the art, a homopolar machine includes
four major components: armature; stator; field coils; and flux
return. The armature is connected to the machine's shaft and may
also be referred to as the rotor. The armature typically includes a
series of concentric, copper cylinders and is free to rotate in a
direction at right angles to the magnetic field lines produced by
the field coils. When a voltage is applied across the armature in
the direction of the shaft, electric current flows parallel to the
shaft. The current and magnetic field interaction (I.times.B)
results in torque generation and rotation, thus producing a motor.
In contrast, if the armature is driven externally for a generator
application, the interaction of the armature rotating at right
angles to the magnetic field lines generates a voltage and electric
current.
[0004] In both the motor and generator scenarios, current flows
along the armature and to the stationary stator via sliding
electrical contacts referred to herein as current collectors or
brushes, which may take various forms and be made of various
materials. Such materials include but are not limited to flexible
fibrous copper and flexible copper strips; common to most DC
motors, rigid material made from graphite or silver-graphite may
also be used.
[0005] The field coils are typically circumferentially continuous
in geometry and aligned on the same central axis with respect to
each other. A homopolar machine always cuts (or crosses as it
rotates) magnetic flux lines of a magnetic field in the same
direction due to the interacting armature and shaft iron being
aligned on the same axis. This means that any point on the rotor
always sees the same magnetic field as it rotates, and no
differences in magnetic flux or multiple magnetic poles are
encountered by conductive elements of the armature as it rotates.
Hence the nomenclature "homopolar machine".
[0006] The flux return is typically comprised of a highly
magnetically permeable material such as iron or steel. The flux
return is designed primarily to limit the undesirable stray
magnetic field that radiates from the machine; therefore, it
typically takes the form of a structural housing that surrounds the
motor. In addition, the flux return may also be designed to help
direct the magnetic field lines produced by the field coils into
the armature interaction region to improve the machine's flux
utilization.
[0007] Although conventional rotating machines are in wide use,
brush wear has been a point of continuing concern. One disadvantage
of homopolar machines is that they often tend to have lower
reliability in comparison to standard DC motors. Specifically,
homopolar machines use current collectors, i.e. brushes, to
transfer current between each rotating armature turn and each
stationary stator turn. The utility of homopolar machines is the
heavily dependent upon current collectors that are potentially
unreliable, a large source of efficiency loss, and maintenance
problems. Presently, all current collectors need to maintain
uniform contact pressure with the armature, and performance is
measured in terms of current collector wear and current-carrying
capability. Maintaining an ideal contact pressure is difficult
because in a homopolar machine the current collectors are generally
located in the magnetic field zone where the collectors are subject
to bending and torque. Although some homopolar machines have been
designed with mechanisms that help to maintain an ideal contact
pressure, the size, weight, and cost penalties, that result are
troublesome, along with the introduction of new sources of
reliability problems. Thus, there is a need to reduce the wear and
tear of brushes.
SUMMARY OF THE INVENTION
[0008] The present invention advantageously addresses the above
needs by providing a homopolar machine that has extended operating
lifetime without repair and thus improved overall reliability. This
homopolar machine improvement results from a construction such that
all of the brushes are polarized negative.
[0009] Another aspect of the present invention provides a method of
operating a homopolar motor by supplying current to the armature
through the use of brushes that are all polarized negative. The
method includes the steps of energizing a first field coil in the
homopolar machine to a first excitation level; and energizing a
second field coil in the homopolar machine to a second excitation
level that is different than the first excitation level to produce
a magnetic field asymmetry within the homopolar machine that
produces an axial force on a shaft of the homopolar machine.
[0010] In one particular aspect, the invention provides a homopolar
machine comprising a shaft, an armature coupled to the shaft and
mounted so as to rotate with said shaft, at least two stators that
encircle the armature, a negative bus connected to a first stator,
a positive bus connected to a second stator, a first negative brush
carried by said first stator for contact with said armature at one
location, and a second negative brush carried by said armature for
contact with said second stator.
[0011] In another aspect, the invention provides a homopolar
machine that includes a shaft, an armature assembly, an outer flux
return, and a plurality of stator arrays. The armature assembly is
coupled to the shaft to rotate therewith and includes a plurality
of armature conductor turns. The outer flux return encloses the
armature assembly, and the plurality of stator arrays encircle the
armature assembly. The stator arrays variously support a plurality
of negatively polarized brushes that maintain substantially
constant contact pressure with certain location upon the armature
conductor turns whereas negatively polarized brushes supported at
other armature locations provide sliding electrical current
interface with other stator arrays.
[0012] In a further aspect, the invention provides improved brush
holders designed to support brushes and maintain substantially
constant electrical contact pressure throughout the lifetime of the
brushes as continuous wear results in foreshortening thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other aspects of the present invention and its
advantages will be apparent from the particular description which
follows in conjunction with the following drawings:
[0014] FIG. 1 is a perspective view illustrating the main
components of a prior art homopolar machine in which the present
invention may be advantageously incorporated;
[0015] FIG. 2 is a top planar cross-sectional view of the homopolar
machine of FIG. 1;
[0016] FIG. 3 is a partial side view taken along line 6-6 in FIG. 2
illustrating one complete stator-current collector assembly and
part of an armature assembly that may be employed in the homopolar
machine of FIG. 1;
[0017] FIG. 4 is an end view as seen from line 4-4 in FIG. 3 of
part of the stator-current collector assembly and armature;
[0018] FIG. 5 is a schematic view illustrating one manner in which
an armature and stator assembly may be configured in accordance
with the present invention;
[0019] FIG. 6 is a view similar to FIG. 5 illustrating an
alternative embodiment;
[0020] FIG. 7 is a perspective view shown partially in cross
section illustrating a holder that might be used effect brush
contact and showing the brushes in a retracted condition;
[0021] FIG. 8 is a view similar to FIG. 7 showing the brushes in a
fully extended condition; and
[0022] FIG. 9 is an exploded perspective view showing the various
pieces employed in the construction of the holder shown in FIGS. 7
and 8.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENTS
[0023] In accordance with the principles of the present invention,
a homopolar machine is described that addresses the problems
discussed above with respect to current collector performance and
wear.
[0024] To improve current collector performance, a homopolar
machine embodying features of the present invention uses current
collectors or brushes that maintain substantially constant contact
pressure and are polarized negative. As will be discussed, the
brushes are flexible and may be made from either electrically
conductive fibers or stacked strips such that they will bear
against the opposing surface while subject to relative rotation
therewith.
[0025] For the purpose of exemplifying a homopolar machine in which
the present invention may be advantageously employed, FIGS. 1 and 2
illustrate a prior art homopolar machine 100 that may be operated
as either a direct current (DC) motor or a generator. Details of
construction and operation of this homopolar machine are set forth
in U.S. Pat. No. 6,489,700 (Dec. 3, 2002), the disclosure of which
is incorporated herein by reference. In general, the homopolar
machine 100 includes an outer flux return 102, a shaft 104, an
armature assembly 106, several stator-current collector arrays 108,
and two field coils 110, 112. The preferred geometry of the
homopolar machine 100 is such that the pair of field coils 110, 112
are circumferentially shaped and are mounted in line (along a
common z-axis) with the armature assembly 106 located between them.
The outer flux return 102 substantially surrounds all the
components. By way of example, the homopolar machine 100 may have a
radius of approximately 0.5 meter.
[0026] The outer flux return 102 is preferably substantially
cylindrical in shape and is constructed from a material with high
magnetic permeability, such as iron or steel. The outer flux return
102 encloses the machine's internal components and substantially
reduces the amount of magnetic flux radiating from the machine. The
outer flux return 102 may serve as a substantial structure that
reacts the machine's torsional and magnetic loads and may be
augmented with additional structural reinforcement 114.
[0027] The shaft 104 is supported by radial bearings 116 and a
thrust bearing 118, shown in FIG. 2, is internal to the homopolar
machine. The thrust bearing 119 maintains the axial position of the
shaft 104.
[0028] The armature assembly 106 and the stator-current collector
arrays 108 are preferably comprised of electrically conductive
elements made from copper, copper alloy, or other materials with
similar electrical properties. The illustrated armature assembly
106 includes a plurality of stacked concentric cylinders having a
squared-U, radial axial cross-sectional shape 120, 122, 124, 126,
128 separated by electrical insulation 130. Such an the armature
assembly 106 may include any number of conducting turns. Each
armature conductor cylinder 120, 122, 124, 126, 128 is distributed
symmetrically around the circumference to provide, a substantially
uniform electric current distribution around the circumference.
[0029] The armature conducting cylinders or turns 120, 122, 124,
126, 128 conduct current to several current collectors 132 and
stator conductor turns 134 that are electrically connected in
series between the armature conducting turns 120, 122, 124, 126,
128 to form one of the stator-current collector arrays 108. A
plurality of such stator-current collector arrays 108 are
preferably distributed circumferentially and symmetrically around
the armature assembly 106 to form the stator. The electric current
carrying elements may be water-cooled via internal cooling
channels, cooled by direct spray of a liquid coolant compatible for
use with exposed electrical conductors, forced air or internal
circulated gas, or any combination of such methods.
[0030] The field coils 110, 112, which provide the background
magnetic field, may be resistive electromagnet coils or
superconducting coils. It is preferred that field coils 110, 112
are used as a circumferentially continuous in geometry and are
superconducting coils that are formed from circumferentially
wrapped electrically superconducting wire. Superconducting coils
produce higher magnetic fields, which results in a smaller and
lighter motor. For each of the field coils 110, 112 manufactured
from superconducting wire, a field coil cryostat 136 is provided
that is essentially a vacuum vessel. The vacuum vessel has disposed
within it the field coils 110, 112, which are structurally
supported by cryostat cold-to-warm support structures 138 as well
known in this art.
[0031] An inner flux return 140 is positioned around the inner
perimeter of the armature assembly 106 which is cylindrical in
shape and constructed from a material with high magnetic
permeability. The inner flux return 140 is mechanically coupled to
the machine shaft 104 via a shaft structure 142 and is encircled by
the armature assembly 106, i.e., circumferentially surrounded by
the armature assembly 106. The armature assembly 106 and the inner
flux return 140 both comprise a rotor assembly, which is
rotationally disposed within the stator (i.e., the plurality of
stator-current collector arrays 108).
[0032] The field coils 110, 112, outer flux return 102, armature
assembly 106, stator current collector arrays 108, and inner flux
return 140 are configured in a cylindrical geometry and assembled
so they all share a common central geometric axis 144 that the
rotational axis of the shaft 104 and the central magnetic axis of
the field coils 110, 112. During operation, the field coils 110,
112 are energized so that their polarities are opposite, causing
their magnetic flux lines to repel and deflect into the inner flux
return 140 and outer flux return 102, and across the armature
assembly 106 and stator-current collector arrays 108. Because the
magnetic fields are uniformly distributed in the circumferential
direction, no difference in magnetic flux or multiple magnetic
poles are encountered by conductive elements of the armature
assembly 106 as it rotates; it thus functions as a homopolar
machine.
[0033] When the homopolar machine 100 is operated as a motor,
current is supplied to the rotating armature assembly 106 from the
stationary stator via the current collectors 132 located between
the armature and the stator. The motor torque is developed as
electric current flows at right angles to the flux generated by the
stationary field coils 110, 112. These coils are typically
superconducting and are cooled within a cryostat 136 having a
support structure 138 that is mounted with a plurality of mounts
182. A thermal shield 174 has mechanical connections 178 and 180 to
the support structure and to field coil current leads 176. When the
homopolar machine 100 is operated as a generator, the
mechanically-driven rotating armature assembly 106 generates
current as its conductive elements are moved at right angles to the
flux, and the current is transported to the stationary stator via
the current collectors 132.
[0034] FIG. 3, is a fragmentary top view, looking down at the upper
surface of the rotor, of one complete stator-current collector
array 158 and part of the armature assembly 106 showing portions of
the five armature conductor cylinders 120, 122, 124, 126, 128. The
stator-current collector array 158 includes stator conductor turns
146, 148, 150, 152, 154, 156, 159, 160, 162, 164, 165, 167. Several
such stator-current collector arrays 108 are arranged
circumferentially around the homopolar machine 100 to distribute
the current.
[0035] FIG. 4 is a fragmentary end view of a portion of the
stator-current collector array 158 and the armature conductor
cylinder or turn 120 taken along line 4-4 fo FIG. 3. The stator
conductor turns 148, 160 are electrically connected to their
corresponding circumferential busbar 166, while the other
illustrated stator conductor turns 146, 150, 152, 154, 156, 162 are
electrically insulated from the busbar 166 with electrical stator
turn-to-turn insulation 168. The stator conductor turns 146, 150,
152, 154, 156, 162 are electrically insulated from the busbar 166
because they correspond to busbars (not shown) associated with
other armature conductor turns. The stator conductor turns 148 and
160 are electrically in parallel and connect to the busbar 166
which includes five current collectors 132 that conduct current to
or from the armature conductor turn 120.
[0036] DC electric current is routed through the stator terminals
170, 171, 172, 173 such that current flows between the positive (+)
terminals 170, 171 and negative (-) terminals 172, 173.
Specifically, current that enters the positive (+) terminals 170,
171 flows through the stator conductor turns 146, 159, then through
a circumferential busbar (not shown), then through a current
collector (not shown), then through the armature conductor turn
120, then through the current collectors 132, then through the
circumferential bus bar 166, then through the stator conductor
turns 148, 160, and continues in a similar manner through the
remaining armature conductor turns 122, 124, 126, 128, and out the
negative (-) terminals 172, 173.
[0037] In this prior art machine, the current collectors 132 are
made of flexible electrically conductive material, such as
electrically conductive fibers or foils made from copper or copper
alloys or stacked strips. The flexible current collectors 132 are
supported by the stators or by structure supported from the stators
and bear against the outer edge of the armature turn's smooth
surface. The current collectors or brusher are attached to the
stators by means of brush mounts or brush holders as known in this
art which may include actuators that may be spring- or fluid-loaded
to allow them to be extended or retracted, e.g. see U.S. Pat. Nos.
4,686,405 and 4,698,540.
[0038] Shown schematically in FIGS. 5 and 6 are two alternative
embodiments of improvements in a homopolar machine with respect to
the transfer of the current between an armature and a stator.
[0039] Illustrated in FIG. 5 is a rotor or armature 191 and a
positive stator ring 193 that is connected to a positive flexible
bus 195. A negative stator bus 197 carries actuators 199 which, in
combination with brush holders 201, urge brushes 203 into contact
with one circular edge region of the rotor 191. Representative
brush holders are shown in FIGS. 7-9 and are described
hereinafter.
[0040] Similar current-collecting brushes 205 are carried at a
different location on the rotor and extend axially thereof. A
pneumatic actuator 207, generally similar to the actuator 199
without any brush holder, presses the positive stator ring 193
against the brushes 205 which are negative and attached to the
rotor. Similarly, the actuator 199 presses the brushes 203, which
are also negative, against the radical edge of the rotor 191. The
actuators are operated to assure that the contact pressure remains
substantially constant, and the negative brush arrangement directs
potential electron emissions from the brush to the stator ring and
effectively eliminates ionization at the fiber tips of the brushes.
The result is a far longer brush lifetime, which improvement can be
obtained in all direct current motors and generators using such
flexible, multi-conductor metal brushes. The result is indeed
surprising because it was never appreciated that the rather simple
avoidance of such electron bombardment would have such a dramatic
effect.
[0041] Illustrated in FIG. 6 is an alternative arrangement wherein
the rotor 191 is again connected to a negative bus bar 197 through
brushes 203 that are supported in a brush holder 201 and maintained
in precise constant contact through the use of the actuator 199.
However, at the opposite location on the rotor, instead of carrying
axially aligned brushes, actuators 211 and brush holders 213
similar to the actuators and brush holders 199 and 201 are carried
which are radially aligned. The holders 213 support brushes 215
which extend radially outward, in the opposite direction of the
brushes 203, and make contact with a fixed positive stator ring 217
that is connected to a positive polarity bus 219. Thus, the
actuators 199 and 211 again maintain constant, precise, low
interface pressure between the brushes and the rotor 191 during
operation under high electromagnetic loads, which feature is
facilitated by the novel design of the brush holders described
hereinafter. Here, the brushes 215 are mounted on the rotor so as
to move radially outward against the fixed positive stator ring,
with the actuator 211 again maintaining a constant pressure between
the negative brush and the positive fixed stator. Accordingly, as
in FIG. 5, the electron flow in FIG. 6 is from the negatively
charged brushes to the stator in both instances.
[0042] As mentioned, it was surprisingly found that, when the
brushes are negatively charged, they experience a wear rate of only
about 10% of that of positively charged brushes in a similar
environment. While it is uncertain why this surprising result
occurs, it is believed that electron bombardment very likely plays
a major factor, and by this simple expedient of a design where the
brushes are all negatively charged, ionization at the fiber tips is
eliminated and provides these surprising results.
[0043] Illustrated in FIGS. 7, 8 and 9 is an example of a brush
holder that might be used as the holder 201 that is shown
schematically in FIG. 5. The illustrated brush holder 221 includes
an insulating plate 223 on which there is mounted a central base or
core 225 that is generally T-shaped. These holders 221 will
normally be used in a horizontal stack, similar to the arrangement
of the collectors 132 shown in FIG. 1, so the plate 223 from the
next holder 221 will abut the front face of the T-shaped base 225,
forming a cavity or pocket between the two flat plates 223.
Disposed in the cavity are a pair of opposed sliders 227 that have
flexible, EDM cut, brushlike heads 229 that are slightly flexed and
slide in electrical contact with the stem of the base 225. The
T-shaped base has a longitudinally extending central passage 230
which receives an open-ended stainless steel cylinder 231 that
serves as a bushing in which a drive rod or piston 233 reciprocates
that is suitably connected with an actuator at its upper end. The
lower end of the drive rod is affixed to a crosspiece or foot 235
that slides within the cavity created by the two parallel plates
223. A pair of depending brushes 237 are bolted by suitable bolts
239 to the undersurface of the crosspiece 235. The sliding contacts
227 are interconnected to the crosspiece 235 by socket head bolts
241 or the like which are received and recessed in passageways 245
extending through the crosspiece 235. These bolts are threaded into
tapped holes in the bottom surface of the sliders 227. The
passageways 245 in the crosspiece 235 are of slotted shape, as seen
in FIG. 9, so as to allow some slight adjustment to the right or
left in the crosspiece as viewed in FIG. 7 to assure tight
electrical contact between the flexible portions 229 and the
T-shaped base 225. The insulating plate 223 is formed with an
elongated slot 251 (FIG. 9) which receives a short, flat circular
disk 253 affixed to the facing surface of the crosspiece 235; it
restricts the extent of longitudinal movement of the crosspiece
within the casing. Smooth sliding movement of the crosspiece and
the electrical contact sliders 227 is facilitated by glides 255 of
a material such as glides 255 of a material such as Teflon and thin
sheets 257 of Teflon or the like carried on regions of the
insulating plate 223 against which the crosspiece 235 and the
sliders 227 will be juxtaposed.
[0044] In the embodiment shown, each holder 221 would be bolted to
the negative bus by bolts that are received in openings 259 in the
two arms at the top of the base 225 which may be threaded if
desired. The upper end of the drive rod 233 is suitably connected
with an actuator, such as a pneumatic cylinder as known in this
art, which is carefully regulated so as to provide a precise amount
of pressure on the brushes 237 that are contacting the rotor so as
to assure good current flow yet absolutely minimize wear on the
multi-conductor flexible brushes. The drive rod or shaft 233 is
preferably coated on its exterior surface so as to slide smoothly
within the steel 231 in which it resides, and the various Teflon
glides assure the smooth downward movement of the sliders 227 and
the crosspiece 235 within the casing.
[0045] At the time of original start up, the brushes 237 would be
in the retracted orientation, such as shown in FIG. 7, residing in
an upper location within the casing. As wear slowly occurs, the
actuator maintains force on the drive shaft 233 which causes the
shaft to gradually slide downward in the bushing 231 to maintain
the continuous contact of the brushes 237 against the juxtaposed
surface of the rotor at the desired pressure. FIG. 8 depicts the
situation when the brushes 237 have worn to such an extent that
they are in a substantially fully extended position and thus ready
to be replaced. However, as previously indicated, the time it would
take for the brushes 237 to wear to this extent may be 10 times as
long as the time it would take positively charged brushes to wear
to the same extent. This surprising increase in longevity appears
to be the result of the avoidance of electron bombardment on the
brushes which is a result of operating the homopolar machine with
both sets of brushes negatively charged, a result that was never
previously appreciated in this art.
[0046] Moreover, the particular holder design that is shown has
been found to have improved properties for use in a region where
magnetic forces are extremely high. It was found that the brush
holder design, to function most efficiently, needs to be
constructed such that there is both excellent stability and
extremely low friction in the guide connections between the
actuator and the brushes themselves. The particular design shown
has proved to be very effective because it accomplishes both these
objectives. High magnetic forces can have unusual effects upon
operating components often causing them to twist or be otherwise
displaced, and in this respect, the present design, through the use
of an extremely stable arrangement of juxtaposed flat surfaces of
substantial surface area, has been found to effectively counteract
any such adverse effects, while still assuring the smooth downward
movement to maintain contact with the rotor at the precise force
desired. Likewise, the EDM cut, brushlike, parallel ribbons 229,
that have some slight flex as a result of their mounting, maintain
excellent contact as a result of their inherent stiffness in a
transverse direction.
[0047] Although the invention has been described with regard to the
certain preferred embodiments which constitute the best mode known
for carrying out the invention at this time, it should be
understood that various changes in modifications as would be
obvious to one having the ordinary skill in this art without
departing from the scope of the invention that is set forth in the
claims appended hereto. The disclosures of all U.S. patents
previously referred to are expressly incorporated herein by
reference.
[0048] Particular features of the invention are emphasized in the
claims that follow.
* * * * *