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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a linear motor and, more particularly, to
a linear motor having non-overlapped coils mounted upon a non-laminated
backplate, formed from magnetic material, which travels relative to a
single row of permanent magnets.
Linear motors employing toothed armatures comprised of laminates of
magnetic material such as steel are known. One such motor is disclosed in
U.S. Pat. No. 4,733,143. A traveling member having a row of permanent
magnets moves relative to a toothed armature. The magnetic laminates serve
to reduce eddy current loses in the armature while providing a medium of
high magnetic permeability so that a magnetic flux density B in the
toothed armature is maintained at a high level. Such an armature has
overlapped coils formed from round wire that are inserted between teeth of
the toothed armature and locked into place with a retainer strip.
Construction costs of such an armature are high due to the expense of the
materials employed and the complexity of assembling the magnetic laminates
and insertion of the coils. Additionally, such armatures experience an
effect known in the art as cogging.
Linear motors having a non-magnetic armature operating in conjunction with
a generally U-shaped frame are known. One such linear motor is disclosed
in U.S. Pat. No. 4,749,921 which is herein incorporated by reference. The
U-shape frame includes two rows of opposing permanent magnets on
respective inner surfaces of sides of the U-shaped frame. The use of the
non-magnetic armature frees the armature of magnetic forces that would
otherwise attract it to the magnets on one of the sides of the U-shaped
frame. Thus, a slidable support mechanism for the non-magnetic armature
need not be constructed so as to resist the magnetic forces. Furthermore,
cogging effects are reduced due to the absence of magnetic materials in
the non-magnetic armature. Additionally, drag that is created by the
inducement of eddy currents in the magnetic armatures is similarly
reduced. While the effects of reduced drag and cogging are advantageous,
the two rows of opposing magnets and the U-shaped frame increase assembly
and construction costs of the linear motor thereby preventing its use in
low cost applications.
The above linear motors employ commutation devices effecting direct
commutation via brushes contacting commutation electrode segments along
the path of travel, and indirect commutation using hall effect sensors in
the armature sensing a polarity of a magnetic field created by the
opposing magnets, or opto-electric sensors moving relative to a slotted
member with slots combinatorially aligned with the opposing magnets. A
control device receives signals from the sensors and energizes the coils
in the armature accordingly. In addition to the commutation devices, the
linear motors are further equipped with position sensing devices such as
interferometers or scanned optical gratings. The control device is then
used to drive the motor to a desired position as sensed by a chosen
position sensing device. The incorporation of both commutation sensors or
brushes and a position sensing device further contributes to assembly and
material costs incurred in the production of such linear motors.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a flat linear
motor which overcomes the drawbacks of the prior art.
It is a further object of the invention to provide a flat linear motor for
use in low cost applications such as linearly driving printer heads in
commercial printers including those of the dot matrix variety.
It is a still further object of the invention to provide a flat linear
motor having a single row of permanent magnets and an armature comprised
of non-overlapping coils epoxy encapsulated upon a magnetic armature
backplate.
Yet another object of the present invention is to provide a flat linear
motor having a low parts count and reduced assembly complexity providing
in a reduced total cost of manufacture.
Furthermore, it is an object of the present invention to provide a low cost
flat linear motor having an armature backplate with a length equal to a
non-integral number of pitch lengths of permanent magnets in the motor
such that cogging is reduced.
Briefly stated, there is provided a flat linear motor having a single row
of permanent magnets mounted upon backplate and an armature having
non-overlapping flat coils encapsulated in a block of epoxy resin mounted
upon an armature backplate comprised of a magnetic material. The magnetic
backplate serves to provide magnetic circuits of low magnetic resistance
in the flat linear motor thereby increasing a force generated by the
linear motor. A length of the armature backplate is a non-integral number
of a pitch length of the permanent magnets so as to reduce cogging. The
non-overlapping flat coils are optionally comprised of square wire
windings which provide increased thermal conduction. An embodiment of the
invention employs software commutation wherein a position sensing device
is used by a controller to determine a position of the armature from which
a field polarity is found by means of a formula or a look-up table.
In accordance with these and other objects of the invention, there is
provided a flat linear motor comprising: a single row of permanent magnets
mounted upon a backplate of magnetic material, an armature assembly, the
armature assembly including a non-magnetic armature block, the armature
block including coils, an armature backplate composed of a magnetic
material, and the armature block being affixed upon the armature
backplate.
According to another feature of the invention, there is provided a flat
linear motor as recited above wherein the armature backplate has a length
equal to a non-integral number of pitch lengths of the row of permanent
magnets such that cogging is reduced.
According to a still further feature of the invention, there is provided a
flat linear motor as recited above further comprising: a position sensing
device for sensing a position of the armature assembly relative to an
initialization position, a controller receiving signals from the position
sensing device, the controller determining the position of the armature
assembly, the controller determining a field polarity of the permanent
magnets at the position by means of one of a formula and a look-up table,
and the controller driving the coils in accordance with the field polarity
at the position.
The above, and other objects, features and advantages of the present
invention will become apparent from the following description read in
conjunction with the accompanying drawings, in which like reference
numerals designate the same elements.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a bottom perspective view of a linear motor of the prior art
having a non-magnetic armature.
FIG. 2a is a front view of a flat linear motor of the present invention.
FIG. 2b is a side view of the flat linear motor in FIG. 2a.
FIG. 2c is a top view of the flat linear motor in FIG. 2a.
FIG. 3 is a rear view an armature assembly of the flat linear motor of FIG.
2a without an encapsulating epoxy resin body.
FIG. 4a is a top view of a flat linear motor of the present invention
incorporating recesses in an armature backplate.
FIG. 4b is a rear view of an armature assembly of the flat linear motor of
FIG. 4a without an encapsulating epoxy resin body.
FIG. 5a is a top view of a flat linear motor of the present invention,
having slots in an armature backplate showing magnetic flux lines.
FIG. 5b is a front view of the flat linear motor of FIG. 5a showing
magnetic flux lines.
FIG. 6a is a rear view of an armature assembly of the present invention
having a cooling manifold.
FIG. 6b is a top view of the armature assembly of FIG. 6a.
FIG. 7a is a rear view of an armature assembly of the present invention
having a cooling passage.
FIG. 7b is a top view of the armature assembly of FIG. 7a.
FIG. 8a is a front view of a coil incorporating a feature of the present
invention.
FIG. 8b is a cross-sectional view of the coil of FIG. 8a taken along line
VIIIb--VIIIb.
FIG. 9 is a top view of a flat linear motor of the present invention
incorporating a modified Halbach array.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is shown a bottom perspective view of a
linear motor 10 of the prior art having an armature assembly 11 including
a non-magnetic armature plate 12. The non-magnetic armature plate 12
travels in a generally U-shaped channel comprised, of an upper magnet
mount plate 14 and a lower magnet mount plate 16. A first plurality of
magnets 18 are affixed to the upper magnet mount plate 14 and a second
plurality of magnets 20 are affixed to the lower magnet mount plate 16.
The upper and lower magnet mount plates, 12 and 14, are joined together
via flange sections 22 and 24. The major flux path of the magnetic circuit
of the motor is indicated by the dashed line M.
The magnetic flux density B between the first and second pluralities of
magnet, 18 and 20, is dependent upon the magnetic field intensity H of the
first and second pluralities of magnets, 18 and 20, and the magnetic
resistance of the magnetic circuit which is determined by the magnetic
permeabilities of materials comprising the circuit. The force produced by
a current traveling in a magnetic field is from
F=J.times.B
where J is the current density. Power dissipated in a motor is determined
from
P=I.sup.2 .times.R.sub.w
where I is the current traveling through the windings and R.sub.w is the
resistance of the windings. Thus, since the power dissipated as heat
increases with the square of the current, it is desirable to maximize the
magnetic flux density B in order to increase the force output. Therefore
the magnetic circuit is preferably constructed of materials having high
magnetic permeabilities such that the magnetic flux density B is high in
the area of the non-magnetic armature 12 allowing greater force to be
produced by currents in the armature. The upper and lower magnet mount
plates, 14 and 16, are formed from a magnetic material such as steel,
having a high magnetic permeability, and a gap between the first and
second pluralities of magnets, 18 and 20, is minimized. The use of the
opposing first and second pluralities of magnets, 18 and 20, and the steel
upper and lower magnetic mounts plates, 14 and 16, produce a high magnetic
flux density B in the gap. However, this construction increases the cost
of materials and production due to the number of parts required and the
complexity of assembly.
Magnetic materials traveling in a magnetic field experience eddy currents
which effect a drag upon the materials and dissipate energy as heat in the
magnetic materials. Armatures employing magnetic laminates, which reduce
eddy current losses, still experience a cogging effect produced when such
armatures passes through regions of alternating magnetic polarity.
Magnetic laminates of silicon steel increase material costs and the
complexity of assembly. The use of non-magnetic materials in the
non-magnetic armature 12 reduces eddy current loses and thus reduces drag
upon the armature assembly 11 and cogging effects.
Referring to FIGS. 2a-2c, front, side and bottom views of an embodiment of
a flat linear motor of present invention are shown. A magnet assembly 40
includes a magnet backplate 41 having a plurality of magnets 42 mounted
upon it. An armature assembly 45 travels relative to the magnet assembly
40 and includes an armature block 46 mounted upon an armature backplate
48. The armature block 46 is comprised of three coils (not shown)
encapsulated in a settable epoxy resin. A supply cable 50 carries current
to the three coils of the armature block 46. Slidable support means for
the armature assembly 45 are supplied by a user provided mechanism into
which the flat linear motor is incorporated.
In a preferred embodiment the magnets 42 have a pitch P of 0.75", a width
W.sub.M of 0.5" to 0.6", a length L.sub.M of 2" and a thickness T.sub.M of
0.5" minimum while the magnet backplate 41 has a thickness T.sub.MB equal
to 0.25". The armature backplate 48 has a thickness T.sub.B of 0.375", the
armature block 46 has a thickness T.sub.B of 0.3" which is substantially
equal to a thickness of the coils. The armature assembly 45 is separated
from the magnet assembly 40 by a distance d equal to 0.05". The above
dimensions and those provided herein serve as examples and not limitations
of the present invention. It is realized that the dimensions may be varied
in absolute and relative terms, and such variations are considered to be
within the scope and spirit of the present invention.
Referring to FIG. 2c, magnetic circuits M1 and M2 predominate in the flat
linear motor and include the magnet backplate 41 and the armature
backplate 48. Both the armature backplate 48 and the magnet backplate 41
are composed of high permeability materials, such as steel, so as to
concentrate the magnetic field therein and minimize the magnetic
resistances of the circuits. Furthermore, steel or silicon steel laminates
are also used in place of the steel armature backplate in order to reduce
eddy currents where necessitated by operational parameters. While the use
of laminates increased the material costs, a resultant reduction in drag
upon the armature assembly 45 allows use of the invention in applications
requiring higher performance levels than solid steel backplates are
capable of fulfilling.
Referring to FIG. 3, a rear view of the armature assembly 45 is shown
without the epoxy encapsulation. An insulating sheet 52 lays upon the
armature backplate 48, separating and electrically insulating coils 54,
56, and 58, from the armature backplate 48. The insulating sheet 52 is
laminated upon the armature backplate 48 and may optionally include a
pattern of apertures (not shown) to allow an encapsulating epoxy to make
structural and thermal contact with the armature backplate 48. The
insulating sheet 52 is composed of materials, such as Kapton, which
provide good thermal conductivity and maintain structural integrity under
high temperatures. The coils 54, 56, and 58 are affixed upon the
insulating sheet 52 using the encapsulation epoxy. In a preferred
embodiment the coils 54, 56, and 58 have an overall length L.sub.C of 3",
a straight portion length L.sub.CS of 2", an overall width W.sub.C of 1"
including a coil winding width W.sub.CW of 0.3" and a remaining coil
aperture width W.sub.CA of 0.4".
First coil leads 54a, 56a, and 58a, extend from beneath coils 54, 56, and
58, respectively, and are carried in a recess 60, formed in the armature
backplate 48. The first coil leads 54a, 56a, and 58a, join with conductors
of supply cable 50 which pass through an aperture in the insulating sheet
52 contiguous with the recess 60. A return lead 62 of the supply cable 50
is connected to a ground terminal 64 mounted upon the armature backplate
48. Second coil leads 54b, 56b, and 58b, are connected to the ground
terminal 64 completing supply circuits for coils 54, 56, and 58,
respectively.
The coils 54, 56, and 58, are encapsulated in an epoxy which provides
structural integrity and forms the armature block 46 shown in FIG. 2b. The
epoxy is chosen to have superior thermal conductivity so as to dissipate
heat away from the coils 54, 56, and 58 and includes settable resins such
as "STYCAST 2850MT" produced by Emerson and Cumming, Inc. of Canton, Mass.
Anchors 66 are installed into the armature backplate 48 such that they
protrude from a surface of the armature backplate 48. When the epoxy is
cast around the coils 54, 56, and 58, the anchors are also encapsulated
into the epoxy and hold the armature block 46 to the armature backplate
48. The anchors 66 shown are depicted as screws, however, other types of
protrusions from the surface can be utilized provided they present a
configuration which the epoxy can lock onto. For instance, a threaded or
knurled surface, or a inverted wedge shape provide sufficient locking
configurations.
Another embodiment of the present invention employs grooves (not shown) in
the armature backplate 48 into which the epoxy of the armature block 46
extends, thereby creating an interlocking structure which improves
adhesion of the armature block 46 to the armature backplate 48. In such a
configuration, back sides of the coils 54, 56, and 58, may be individually
insulated with insulating sheets. of material to prevent electrical shorts
to the armature backplate 48. Other configurations of recesses may also be
used to create an interlocking structure between the armature block 46 and
the backplate 48. For example, such recesses may include threaded holes,
slanted holes, or dovetail holes or slots.
Referring to FIGS. 4a and 4b, another embodiment of the present invention
is shown having an armature backplate 48' with recesses 71, 72, and 73
situated above center openings of the coils 54, 56, and 58. In a motor it
is desirable to maximize the magnetic flux density B in which the coils
are positioned. The magnetic flux density B is increases with a magnetic
permeability of a medium given a constant magnetic field intensity H. The
magnetic permeability of the armature backplate 48' is far greater than
that of air or epoxy. Therefore, the magnetic flux density B is greatest
along straight lengths of the coils 54, 56, and 58, which are not located
under the recesses 71, 72, and 73. The recesses 71, 72, and 73 are either
epoxy filled or air filled. The lines of flux M.sub.1 ' are concentrated
so as to follow a path of greatest magnetic permeability which excludes
areas where the recesses 71, 72, and 73 are located. Thus, the recesses
71, 72, and 73, serve to focus the lines of the magnetic flux M.sub.1 '
through the straight lengths of the coils 54, 56, and 58. The magnetic
circuit of flux M.sub.1 ' is completed through remaining portions of the
armature backplate 48' above the recesses 71, 72, and 73. The anchors 66
are now positioned at corners of the armature backplate 48'.
Referring to FIGS. 5a and 5b, yet another embodiment of the present
invention is shown which is similar to the embodiment of FIGS. 4a and 4b
with the recesses 71, 72, and 73, replaced by slots 75, 76, and 77. The
lines of flux M.sub.1 " travel around the slots 75, 76, and 77, in order
to complete the magnetic circuit of the motor. Thus, the magnetic flux
density is similarly concentrated in the areas of the straight lengths of
the coils 54, 56, and 58. Furthermore, the slots 75, 76, and 77, provide
access to the coils 54, 56, and 58, by the supply cable 50.
In both the embodiments of FIGS. 4a-5b, the spaces of the recesses 71, 72,
and 73, and the slots 75, 76, and 77, serve to reduce the weight of the
armature thereby allowing greater acceleration with the same force
applied, further increasing the efficiency of the motor. This is
especially true where the insulating sheet 52 seals off the spaces from
the epoxy and the spaces remain empty. The spaces can also be adapted to
accept circuit boards or hall effect sensors as required by driver and
positioning systems. Furthermore, the slots 71, 72, and 73 provide for
improved cooling by permitting increased air circulation over the coils
54, 56, and 58. Although the spaces shown are rectangular it is realized
that the shape and relative size of the spaces with respect to the coils
may be modified without departing from the scope and spirit of the present
invention.
Referring to FIGS. 6a and 6b, an armature assembly 80 of an embodiment of
the present invention has an armature backplate 81 with coolant passages
82 formed therein. The coolant passages 82 connect with outlet passages 86
forming a cooling manifold. Air is introduced at inlets 83 and circulates
through the armature assembly 80 enroute to exiting through the outlet
passages 86. The air carries heat generated within the coils 54, 56, and
58 out of the armature assembly 80 thereby allowing a greater current to
be applied to the coils 54, 56, and 58, than in an assembly without a
cooling manifold. Two cooling manifolds are shown, however, it is realized
that one cooling manifold or a plurality of cooling manifolds may be
employed. The cooling manifolds also serve to focus the magnetic flux in
the same manner as the recesses 71, 72, and 73 discussed above. The
cooling manifolds are formed by boring into the armature assembly
following the setting of the epoxy forming the armature block 84. Other
methods of constructing the cooling manifold include molding, or machining
channels in a surface of the armature backplate 81 which are subsequently
enclosed by the insulating sheet 52 and the armature block. Flexible
conduits (not shown) are affixed to the inlets and introduce air under
pressure.
An alternative embodiment of a cooling means is shown in FIGS. 7a and 7b
wherein an armature assembly 90 has a coolant passage 92 having a
circuitous route through the armature backplate 91. Coolants, including
gases and liquids, are introduced at an inlet 93 and exit at outlet 94.
Flexible conduits (not shown) are connected to the inlet 93 and outlet 94
and serve to couple the armature assembly 90 to a recirculating system
which removes heat from the coolant and pumps it through the armature
assembly 90. The coolant passage 92 is formed by the methods discussed
above. Where the coolant passage is bored, unnecessary openings are
plugged to provide a sealed coolant circuit. Yet another alternative is to
fix cooling coils to a back surface of the armature backplate 91.
Referring to FIGS. 8a and 8b, a front view and a cross section view is
shown of an embodiment of a coil 60 optionally used in the present
invention. The coil 60 employs perfect coil construction wherein square
wire is used to form the coil 60. In FIG. 2b a compact arrangement of
windings of the square wire is shown. No gaps are left between the
windings because the wire is square. In windings composed of round wire,
gaps are inherently left between adjacent windings. The absence of gaps in
the coil 60 allows improved thermal conductivity from inside the coil 60
to its outer windings. Heat is thereby efficiently conducted to the
encapsulating epoxy and armature backplate 48 shown in FIG. 3.
Furthermore, an electrical resistance of the coil 60 is lowered since the
gaps present in coils constructed using round wire are replaced by
conductor material, usually copper. The lowered electrical resistance
reduces power dissipated due to resistive loses in the coil 60.
Alternatively, coils formed of round wire are used where operating
specifications do not require the enhanced performance provided by perfect
coils formed of round wire.
Referring again to FIG. 2, the present invention eliminates the need to
have the two rows of magnets, 18 and 20, and two magnet mount plates, 14
and 16, as shown in the prior art of FIG. 1. Simply eliminating one of the
magnet mount plates, 14 or 16, would result in a great reduction in the
magnetic flux density B since the magnetic circuit would be composed
mainly of air which has a low magnetic permeability in comparison to
steel. In FIG. 2, the present invention introduces the armature backplate
48 into magnetic circuits M1 and M2 to provide a path of high magnetic
permeability and improved magnetic flux density over a configuration
without such a backplate. The armature backplate 48 also provides heat
sink capacity drawing heat away from the coils 54, 56, and 58.
The reduced costs associated with the present invention allow use of linear
motors in applications where heretofore performance requirements and cost
limitations have prevented their use. One such application involves linear
movement of a printer head in a dot matrix printer.
In FIGS. 2a and 2b, movement of the armature backplate 48 produces eddy
current losses since the armature backplate 48 comprises a portion of
magnetic circuits M1 and M2 and is made of a magnetic material. However,
it will be realized by one skilled in the art in view of this disclosure
that the increase in drag is offset by the increase in force generated by
the armature assembly 45 as a result of the greater magnetic flux density
B. The utility of the present invention is realized in applications
wherein the force and speed requirements are nominal in relation to the
required cost constraints. However, where speed requirements are high, the
drag due to eddy currents is reduced by manufacturing the armature
backplate 48 of laminations formed of high permeability materials.
In order to reduce the effects of cogging in the present invention, the
length of the armature backplate 48 is selected as a function of a pitch
length of the magnets 42 where the pitch length is equal to a center to
center spacing of the magnets 42. The length of the armature backplate 48
is selected to be between 4.3 to 4.7 times the pitch of the magnets 42.
The armature backplate 48 is at no time wholly within an integer number of
field regions of opposing polarity created by the magnets 42. A fractional
interaction with the fields reduces the cogging associated with movement
from a position interacting with a first set of fields of an integer
number to a second set of fields of such number. Furthermore, cogging is
reduced by the separation between the armature backplate 48 and the
magnets 42 corresponding to the clearance distance d of 0.05" and a
thickness of the armature block 46 of 0.3".
Referring now to FIG. 9, an embodiment of the present invention is shown
having a magnet assembly 96 with a first set of magnets 98 polarized
(polarization is indicated by an arrow with the head pointing north)
perpendicular to the backplate 41 in an alternating sequence. Alternately
interposed between the first set of magnets 48 is a second set of magnets
99 which are polarized parallel to the backplate 41 in an alternating
sequence. Each of the first set of magnets 98 occupy 70% of a pitch of the
backplate assembly 96 while a remaining 30% is occupied by each of the
second set of magnets 99. The first and second sets of magnets, 98 and 99,
form a modified Halbach array. A Halbach array is described and analyzed
in Design of Permanent Multipole Magnets with Oriented Rare Earth Cobalt
Material, Nuclear Instruments and Methods, 169, 1980, pp. 1-10 by K.
Halbach, which is incorporated herein by reference. Progressing from left
to right along the magnet assembly as shown, the magnetic orientation of
each successive magnet rotates 90.degree. counter-clockwise. This
arrangement focuses the magnetic flux toward the armature assembly 45, as
indicated by magnetic flux circuits M.sub.1 and M.sub.2, while minimizing
magnetic flux entering the backplate 41. Computer simulation indicates
that the magnetic flux density B is increased by approximately 25%.
Therefore, 25% more force is produced for the same amount of current.
Alternatively, current may be reduce and a heat reduction of up to 50% can
be achieved. The Halbach array magnet assembly 96 is used in conjunction
with any one of the foregoing embodiments of armature assemblies which
incorporate further magnetic flux focusing arrangements or heat
dissipating means in order to further enhance performance. According the
Halbach reference cited, the percentages of the pitch occupied by the
first and second magnets may be varied. For example, according to an
embodiment of the invention, the second set occupies at least 60% of the
pitch length.
Several methods of commutation may be employed in the present invention. In
an embodiment, a hall effect sensor (not shown) is encapsulated in the
armature block 46 and is used to sense the polarity of the magnetic
fields. Alternatively, another embodiment employs an optical sensor (not
shown) mounted upon the armature assembly 45 which detects slots in a card
assembly (not shown) mounted upon the magnet assembly 40. The slots are
aligned in accordance with polarities of the magnets 42. Such methods are
known to those skilled in the art.
A controller receives signals from such sensors and drives the coils 54,
56, and 58, accordingly. A position sensing device, such as an
interferometer or a optical sensor functioning in conjunction with an
optical grating, is used to sense when the armature assembly 45 has reach
a desired position at which time the controller ceases to drive the coils
54, 56, and 58.
Another embodiment of the present invention employs software commutation.
An optical s | | |
