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CROSS REFERENCE TO RELATED APPLICATIONS
The present application is related to U.S. patent application Ser. No.
(Attorney's Docket No. M1038-34) and U.S patent application Ser. No.
(Attorney's Docket No. 38), filed on the same date as the present
application).
BACKGROUND OF THE INVENTION
The present invention relates to linear motors and, more specifically, to
apparatus for guiding and supporting a movable element of a permanent
magnet linear motor in a machine tool.
Linear motors conventionally are used in applications requiring the
application of modest force along a linear axis. One application disclosed
in U.S. Pat. No. 4,595,870, for example, includes driving one axis of a
positioning table for displacing a workpiece along the axis. Such
applications are characterized by high precision in positioning, and
modest force and acceleration requirements. The linear motor eliminates
the windup characteristic of ball-screw drives and substantially reduces
the mass of the moving element. Both of these characteristics of linear
motors provide benefits in workpiece positioning precision and
acceleration.
The force of which linear motors are capable is limited by resistive
heating in the windings of the armature of the linear motor. In my U.S.
patent application Ser. No. 859,915, I disclose several techniques for
cooling a linear motor. In cooling techniques using liquid coolant, I have
discovered that it is possible to attain high forces in permanent magnet
DC linear motors using a moderate flow of a liquid coolant in thermal
contact with the armature windings. The attainable high force permits such
linear motors to be used in applications not previously considered for
this a class of motors.
In my U.S. Pat. No. 4,505,464, I disclose a positioning table which takes
advantage of the magnetic attraction between permanent magnets and
armature iron for preloading bearing supporting the movable element of the
positioning table. Two sets of magnets in orthogonal planes provide
bi-directional preloading.
In my U.S. patent application Ser. No. (Attorney's Docket No. M1038-34), I
disclose a high-force linear motor adapted for integration into the bed of
a machine tool. Certain machine-tool applications require very long travel
at high speeds and accelerations. The present invention addresses one such
application wherein, in addition to the foregoing requirements, long
linear axes must be accommodated.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide a permanent magnet linear motor
which overcomes the drawbacks of the prior art.
It is a further object of the invention to provide a permanent magnet
linear motor having first and second bearing devices with bearing forces
disposed at angles to each other and a magnetic attractive force disposed
at an angle intermediate the angles of the bearing forces, whereby the
single magnetic force is effective for preloading both the first and the
second bearing devices.
It is a still further object of the invention to provide a permanent magnet
linear motor with first and second bearing devices supporting bearing
forces along vectors angularly spaced with respect to each other. The
bearing devices are further subject to at least one load unequally applied
to the two bearing devices. A magnetic attractive force between permanent
magnets and magnetic metal in an armature of the linear motor is disposed
at an angle between the vectors of the bearing forces such that a greater
contribution of magnetic preload is applied to the one of the first and
second bearing devices which bears less of the load, whereby bearing
loading is adjusted toward equality.
It is a still further object of the invention to provide a permanent magnet
linear motor having indefinite length linear travel.
It is a still further object of the invention to provide a positioning
device having a first permanent magnet linear motor employing a first rail
for one of its motor elements and a second permanent magnet linear motor
employing a second rail parallel to the first rail for supporting an
outboard end of the positioning device and for providing coordinated
driving force with the first permanent magnet linear motor. First and
second bearing devices, angularly spaced apart transfer bearing forces to
the first rail. Magnetic attraction in the first linear motor, at an angle
intermediate the bearing vectors of the first and second bearing devices
preload the first and second bearing devices. At least a third bearing
device in the second linear motor transfers the remainder of the load
forces to the second rail.
Briefly stated, the present invention provides a positioning table
including a pair of parallel rails defining an axis of motion. A movable
assembly including a first linear motor assembly disposed for motion along
one of the rails. An outboard linear motor assembly on the other rail is
rigidly bridged to the first linear motor assembly for concerted motion
therewith. The first linear motor assembly includes a plurality of
permanent magnets defining a plane at a predetermined angle to the
horizontal and an armature defining a plane at the same angle closely
spaced from the permanent magnets. The permanent magnets may be disposed
on the movable element of the linear motor assembly or on the first rail.
The armature is disposed on the other of the movable element and the first
rail. First and second bearings on the movable element of the linear motor
assembly support loads in the vertical and horizontal directions. The
predetermined angle establishes the direction of a magnetic attractive
vector between the permanent magnets and the armature. The predetermined
angle is adjusted to proportion the total load of the static and dynamic
load and the magnetic attractive force to a desired value. The outboard
linear motor assembly on the second rail includes a plurality of permanent
magnets and a second armature. A bearing associated with the outboard
linear motor assembly acts in the vertical direction only. The outboard
linear motor assembly and the second rail may include provision for
magnetic attraction at the same predetermined angle as that employed in
the first linear motor assembly. In one embodiment, a non-magnetic rail is
employed with a magnetic metal strip affixed to a surface of the rail
backing the plurality of permanent magnets. In another embodiment, a
plastic insert in at least one surface of the rail facilitates the
creation of a surface having a flatness sufficient for use with an air
bearing. In still another embodiment, a unitary motor saddle includes
first and second air bearing integrated into a saddle. In a still further
embodiment, one or more auxiliary bearings are teamed with air bearings.
The auxiliary bearings assume any load in excess of that which the air
bearings are capable of supporting. The auxiliary bearings are spaced from
an opposing surface such that they undertake mechanical support before
their companion air bearings can be forced into mechanical contact with
the surface.
According to an embodiment of the invention, there is provided a
positioning device for positioning an object along at least one axis
comprising: first and second rails disposed parallel to the at least one
axis, at least the first rail including a horizontal top surface, a
vertical side surface and a first sloping side surface, the first sloping
side surface making a predetermined angle with the horizontal, a first
movable assembly, movable along the at least one axis, the first movable
assembly including a linear motor assembly proximate to the first rail and
an outboard linear motor assembly proximate to the second rail, means for
rigidly connecting the linear motor assembly to the outboard linear motor
assembly for concerted movement thereof, vertical bearing means for
supporting vertical forces on the linear motor assembly on the horizontal
top surface, horizontal bearing means for supporting horizontal forces on
the linear motor assembly on the vertical side surface, a first plurality
of permanent magnets defining a first plane parallel to the first sloping
side surface, an armature having magnetically attractable material therein
defining a second plane parallel to the first sloping side surface and
closely spaced facing the first plane, whereby a first magnetic attraction
exists therebetween, the first plurality of permanent magnets being
disposed on one of the first sloping side surface and the first movable
assembly and the armature being disposed on the other of the first sloping
side surface and the first movable assembly, the second rail having a
second horizontal top surface, the outboard linear motor assembly
including second vertical bearing means for bearing against the second
horizontal top surface, and the predetermined angle being a value
effective for apportioning a total load on the vertical and horizontal
bearing means in a predetermined proportion.
According to a feature of the invention, there is provided a positioning
device for positioning a load along first and second axes comprising:
first and second rails disposed parallel to the first axis, at least the
first rail including a horizontal top surface, a vertical side surface and
a first sloping side surface, the first sloping side surface making a
predetermined angle with the horizontal, a first movable assembly, movable
along the first axis, the first movable assembly including a linear motor
assembly proximate to the first rail and an outboard linear motor assembly
proximate to the second rail, third and fourth rails rigidly connecting
the linear motor assembly to the outboard linear motor assembly for
concerted movement thereof, vertical bearing means for supporting vertical
forces on the linear motor assembly on the horizontal top surface,
horizontal bearing means for supporting horizontal forces on the linear
motor assembly on the vertical side surface, a first plurality of
permanent magnets defining a first plane parallel to the first sloping
side surface, an armature having magnetically attractable material therein
defining a second plane parallel to the first sloping side surface and
closely spaced facing the first plane, whereby a first magnetic attraction
exists therebetween, the first plurality of permanent magnets being
disposed on the first sloping side surface and the armature being disposed
on the first movable assembly, the second rail having a second horizontal
top surface, the outboard linear motor assembly including second vertical
bearing means for bearing against the second horizontal top surface, the
predetermined angle being a value effective for apportioning a total load
on the vertical and horizontal bearing means in a predetermined
proportion, a second sloping side surface on the second rail, the second
sloping side surface being at a second predetermined angle inclined in the
same direction as the first sloping side surface, the second sloping side
surface including magnetically attractable material, a second plurality of
permanent magnets on the outboard bearing assembly, the second plurality
of permanent magnets defining a third plane closely spaced facing the
second sloping side surface, whereby a second magnetic attraction is
exerted in the outboard linear motor assembly, the third and fourth rails
defining a second axis at right angles to the at least one axis, and a
second movable assembly on the third and fourth rails, whereby an X-Y
device is provided.
According to a further feature of the invention, there is provided a linear
motor comprising: a rail, a plurality of permanent magnets disposed on a
surface of the rail, a movable member, the movable member including an
armature containing magnetically attractable material, bearing means for
supporting the movable member in a position spacing a surface of the
armature to a surface of the plurality of permanent magnets, the bearing
means including an air bearing, the rail including a surface facing the
air bearing, the air bearing being effective in normal operation for
supporting the movable member with a spacing D1 between the air bearing
and the surface, the bearing means including at least one auxiliary
bearing, the auxiliary bearing including means for preventing the air
bearing from approaching closer than a distance D2 between the air bearing
and the surface, distance D2 is less than D1, and distance D2 is greater
than zero.
According to a still further feature of the invention, there is provided a
linear motor comprising: a rail, a plurality of permanent magnets disposed
on a first surface of the rail, a movable member, the movable member
including an armature containing magnetically attractable material, an air
bearing affixed to the movable member, an insert in the rail forming a
second surface facing the air bearing, the air bearing being effective in
normal operation for supporting the movable member with a nominal spacing
D1 between the air bearing and the second surface, and the insert being of
a material different from a remainder of the rail and of a type permitting
easier attainment of flatness of the second surface than the remainder of
the rail.
According to another feature of the invention, there is provided a linear
motor comprising: a rail, a plurality of permanent magnets disposed on a
surface of the rail, a movable member, the movable member including an
armature containing magnetically attractable material, bearing means
affixed to the movable member, the bearing means including means for
supporting the movable member against a total load applied between the
movable member and the rail, the rail including a longitudinal cavity
therein, and a vibration-damping material in the longitudinal cavity.
According to yet another feature of the invention, there is provided a
linear motor comprising: a rail, a plurality of permanent magnets disposed
on a first surface of the rail, a movable member, the movable member
including a saddle, an armature containing magnetically attractable
material affixed to the saddle, the armature being disposed facing the
plurality of permanent magnets, a second surface on the rail, the second
surface defining a first supporting plane, a third surface on the rail,
the third surface defining a second supporting plane, the first and second
supporting planes being non-coplanar, a first air bearing in the saddle,
the first air bearing facing the first supporting plane, a second air
bearing in the saddle, the second air bearing facing the second supporting
plane, and at least the first air bearing being unitary with the saddle.
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 top view of an X-Y positioning table according to an embodiment
of the invention.
FIG. 2 is a cross section taken along II--II in FIG. 1.
FIG. 3 is an enlarged side view of the X outboard linear motor assembly of
FIGS. 1 and 2.
FIG. 4 is a further enlarged view of a portion of FIG. 3.
FIG. 5 is a bottom view of the X outboard linear motor assembly of FIG. 3.
FIG. 6 is a cross section of an X linear motor assembly according to a
second embodiment of the invention.
FIG. 7 is a cross section of a positioning device according to a further
embodiment of the invention.
FIG. 8 is a cross section of an X linear motor assembly according to a
still further embodiment of the invention.
FIG. 9 is a cross section of an X linear motor assembly according to a
final embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown, generally at 10, a positioning table
according to an embodiment of the invention. First and second parallel
X-axis rails 12 and 14 are supported by conventional means, not shown, to
define an arbitrarily named X axis as indicated by an X-axis arrow 16.
An X movable assembly 18 is disposed on X-axis rails 12 and 14 for motion
along the X axis as indicated by X-axis arrow 16. X movable assembly 18
includes an X linear motor assembly 20 disposed on X-axis rail 14 and an X
outboard linear motor assembly 22 disposed on X-axis rail 12. A plurality
of permanent magnets 23 are disposed on a sloping surface 25 of X-axis
rail 14. Permanent magnets 23 are part of X linear motor assembly 20 and a
bearing-preloading system, as will be explained. A like plurality of
permanent magnets 70 are disposed on a sloping surface 66 of X-axis rail
12.
First and second parallel Y-axis rails 24 and 26 rigidly tie together X
linear motor assembly 20 and X outboard linear motor assembly 22 for
concerted motion along the X axis. A Y movable assembly 28 is disposed on
Y-axis rails 24 and 26 for motion along an arbitrarily named Y axis as
indicated by a Y-axis arrow 30. For present purposes, Y movable assembly
28 may be the same as, or different from X movable assembly 18. For
concreteness of description, Y movable assembly 28 is illustrated with a Y
linear motor 32 on Y-axis rail 24 and a Y outboard bearing assembly 34 on
Y-axis rail 26. A workpiece support table 36 rigidly ties together Y
linear motor 32 and Y outboard bearing assembly 34 for concerted motion
along the Y axis. A plurality of permanent magnets 37 are disposed on a
sloping surface 39 of Y-axis rail 24. Permanent magnets 37 are employed
with Y linear motor 32 in the same manner that will be described for the
employment of permanent magnets 23 with X linear motor assembly 20.
A conventional control system may be employed to drive X linear motor
assembly 20 and Y linear motor 32 along their respective axes, whereby any
desired point on workpiece support table 36 may be positioned anywhere
within the large area defined by the extents of the X and Y axes. One
skilled in the art will recognize that additional degrees of freedom may
be employed in a positioning system without departing from the spirit and
scope of the present invention. For example, a further positioning device
(not shown) may be disposed on workpiece support table 36 for motion along
an arbitrarily named axis normal to the plane of the page. In addition,
one or more rotational devices (not shown) may be provided.
In a different application positioning table 10 may be employed as part of
a machine tool (not shown) for operating on a workpiece disposed below
workpiece support table 36. In such an application, workpiece support
table 36 may be replaced by any suitable apparatus such as, for example, a
chuck (not shown) as required.
Referring now to FIG. 2, X-axis rail 14 is better seen to include a
horizontal top surface 38 and a vertical side surface 40 at opposed edges
of sloping surface 25. X linear motor assembly 20 includes a saddle 42
having a horizontal top arm 44 facing horizontal top surface 38, an angled
arm 46 facing sloping surface 25 and a vertical side arm 48 facing
vertical side surface 40. A vertical load bearing 50 transfers vertical
components of static and dynamic load from horizontal top arm 44 to
horizontal top surface 38 of X-axis rail 14. Similarly, a horizontal load
bearing 52 transfers horizontal components of static and dynamic load from
vertical side arm 48 to vertical side surface 40 of X-axis rail 14.
Vertical load bearing 50 and horizontal load bearing 52 may be of any
convenient type including sliding blocks, air bearings and rolling
bearings. For purposes of the present portion of the description, it is
assumed that vertical load bearing 50 and horizontal load bearing 52 are
conventional air bearings.
X linear motor assembly 20 includes an armature 54 closely spaced from the
surfaces of permanent magnets 23. Armature 54 contains a plurality of
windings upon a structure of magnetically attractable metal. A static
magnetic attraction between permanent magnets 23 and the magnetically
attractable metal in armature 54 acts along a line normal to the facing
surfaces thereof. The direction and magnitude of the magnetic attraction
is indicated by an arrow 56.
A conventional device such as, for example, an optical scale 58, provides
position and/or velocity information to a conventional control system (not
shown) associated with positioning table 10. Since such a control system
may be conventional and may not be considered to represent an inventive
part of the present invention, further description thereof is properly
omitted.
Although not necessary to the practice of the invention, X-axis rail 14
preferably is hollow and most preferably is a hollow tube of magnetic
metal such as, for example, steel. The steel provides a return path for
magnetic flux between adjacent permanent magnets 23. X-axis rail 14 may be
supported at its ends in suitable installations or may be supported
periodically or continuously along its bottom surface.
X-axis rail 12 includes a horizontal top surface 60 facing a horizontal arm
62 of a saddle 61. A vertical load bearing 64 transfers vertical static
and dynamic loads from horizontal arm 62 to X-axis rail 12. Vertical load
bearing 64 may be of any convenient type such as, for example, slide,
rolling or air bearing. In the preferred embodiment, vertical load bearing
64 is an air bearing.
It will be noted that X outboard linear motor assembly 22 does not include
a horizontal bearing counterpart to horizontal load bearing 52 in X linear
motor assembly 20. This permits thermal growth of Y-axis rail 24 (and
Y-axis rail 26, not shown) without interfering with the operation of
positioning table 10. An embodiment of X outboard linear motor assembly 22
which includes a horizontal bearing should be considered an optional part
of the present invention, however.
Sloping surface 66 on X-axis rail 12 preferably has the same slope oriented
in the same direction as sloping surface 25 of X linear motor assembly 20.
Saddle 61 includes a sloping arm 68. Sloping arm 68 preferably has a slope
matching that of sloping surface 66. Permanent magnets 70, disposed on 66,
are closely spaced from an armature 71. Magnetic attraction between
permanent magnets 70 and armature 71 applies a magnetic preload as
indicated by arrow 56.
For purposes of description, it is herein assumed that a load vector 72
operates in the vertical direction shown. A vector sum of the magnetic and
load vectors yields a total vector 74 borne by the bearings. It will be
recognized by one skilled in the art that the magnetic attractions in X
linear motor assembly 20 and X outboard linear motor assembly 22 act in
substantially the same direction. Such is not, however, a necessary
limitation. For example, permanent magnet 70 and armature 71 may be
reoriented to face each other along a horizontal plane, whereby the
magnetic attractive force of these elements may be vertical. In addition,
during load acceleration, the inertial contribution of the load may be
other than along the vertical line indicated by load vector 72.
In some applications, it may be desirable to apply substantially equal
loads to vertical load bearing 50 and horizontal load bearing 52. For this
purpose, the angle between the plane of sloping surface 25 (and sloping
surface 66) and the horizontal is adjusted to attain the desired
proportioning of loads. In one embodiment of the invention, a rough
proportioning is attained with such angles set at 60 degrees from the
horizontal. Other angles may be chosen for different loads, magnetic
attractive forces or other purposes.
Y movable assembly 28 (FIG. 1) may operate in the same fashion as X movable
assembly 18, described above. In such an apparatus, a plurality of
permanent magnets 37 are disposed on a sloping surface 39. It will be
recognized that permanent magnets 37 may be omitted and Y-axis rail 24 may
be reshaped if different drive and retention means are employed.
It is well recognized that the positions of permanent magnets 23 and 70 and
their related armatures 54 and 71, respectively, are interchangeable. That
is, permanent magnets 23 may be disposed on an inner surface of angled arm
46 and armature 54 may be disposed on sloping surface 25. The positions of
permanent magnets 70 and armature 71 may be similarly reversed. In the
prior art, such an arrangement is preferred in order to use a long
armature as a heat sink. With my new techniques for cooling armatures 54
and 71, I am able to use short armatures 54 and 71 mounted on angled arms
46 and 68, respectively, with full long arrays of permanent magnets 23 and
70 on X-axis rails 14 and 12, respectively, without experiencing
overheating problems.
It is preferred that magnetic attractive forces exerted in X outboard
linear motor assembly 22 act in a direction which adds to the magnetic
attractive forces in X linear motor 20, and that the two magnetic
attractive forces do not have components in opposition to each other.
Thus, the angle of sloping surface 66 is preferably from 0 to 90 degrees
from the horizontal, when the angle of sloping surface 25 from the
horizontal is between 0 and 90 degrees.
Vertical load bearings 50 and 64 provide support to their respective linear
motor assemblies as a result of an air film produced by a supply of air
flowing through the air bearings to the facing surfaces of the air
bearings and their respective horizontal surfaces. For purposes of the
following description, X linear motor assembly 20 and X outboard linear
motor assembly 22 are considered to be identical.
Referring now to FIG. 3, typically vertical load bearing 64 may produce an
air film inducing a separation of about 0.001 inch between its bottom
surface 76 and the facing horizontal top surface 60 of X-axis rail 12 and
may be capable of supporting about 1000 pounds. When the load-bearing
capability of vertical load bearing 64 is exceeded, one or both of two
undesirable results may ensue. First, contact between bottom surface 76
and horizontal top surface 60 may damage the surfaces of one or both of
these elements. If one surface becomes gouged, the ability of vertical
load bearing 64 to support its load on a predetermined thickness of air
film is degraded or destroyed. Second, the changing dimension of the air
film may introduce positioning errors in the workpiece or tool being
controlled. That is, in high-acceleration or horizontal-load applications
momentary vertical forces on X outboard linear motor assembly 22 may
increase and decrease the thickness of the air film over a range on the
order of the nominal separation of, for example, 0.001 inch. In addition,
overturning moments on X outboard linear motor assembly 22 may tend to
urge one end of bottom surface 76 toward horizontal top surface 60 while
attempting to elevate the other end thereof. Thus, damage to one end of
bottom surface 76 may occur, resulting in excessive air leakage and loss
of load-bearing capability at that end.
One solution to this problem includes auxiliary means such as, for example,
auxiliary bearings 78 and 80, disposed at opposed ends of vertical load
bearing 64. Auxiliary bearings 78 and 80 are identical both structurally
and functional, consequently, only auxiliary bearing 80 is described in
detail.
Referring now to FIG. 4, and enlarged view of adjacent portions of vertical
load bearing 64 and auxiliary bearing 80 illustrates a first nominal
distance D1 between bottom surface 76 and horizontal top surface 60 of
X-axis rail 12. Auxiliary bearing 80 includes bearing means such as, for
example, roller bearings 82 spaced a second nominal distance D2 from
horizontal top surface 60. Distance D2 is smaller than distance D1. In the
event that a load on X outboard linear motor assembly 22 decreases the
thickness of the air film between bottom surface 76 and horizontal top
surface 60 by an amount equal to D1-D2, roller bearings 82 come into
contact with horizontal top surface 60, whereby bottom surface 76 is
maintained out of contact with horizontal top surface 60 and contact
damage to bottom surface 76 is avoided. Also, since auxiliary bearing 80
provides solid bearing support for horizontal arm 62, while roller
bearings 82 are in contact with horizontal top surface 60, the thickness
of the air film remains constant. For purposes of description, with
distance D1 equal to about 0.001 inch, distance D2 may be about 0.0005
inch. Thus, the minimum thickness of the air film is limited to about
0.0005 inch.
Air bearings are often selected for applications such as those of the
present disclosure because they do not require contact with horizontal top
surface 60 during normal operation. Thus, wear resulting from contact-type
bearings such as, for example, roller bearings 82 is avoided. The overload
protection provided by auxiliary bearing 80 retains this advantage under
normal operation. Even under abnormal operation with roller bearings 82 in
contact with horizontal top surface 60, wear and distortion of horizontal
top surface 60 is greatly reduced as compared to the case when only
contact-type bearings are provided. Even when load conditions urge roller
bearings 82 into contact with horizontal top surface 60, vertical load
bearing 64 continues to provide a supporting force, and thereby
substantially reduces the portion of the load which must be borne by
roller bearings 82. For example, if vertical load bearing 64 is capable of
supporting a load of 1000 pounds before roller bearings 82 contact
horizontal top surface 60, and a total load of 1200 pounds is applied to X
outboard linear motor assembly 22, if the load is equally applied to both
ends of vertical load bearing 64, then auxiliary bearing 80 is required to
support only half of the excess, namely 100 pounds. Such light loading on
roller bearings 82 is unlikely to damage even a finely finished horizontal
top surface 60.
Referring now to FIG. 5, an underside view of one arrangement of X outboard
linear motor assembly 22 is shown. Auxiliary bearing 80 may include first
and second auxiliary bearings 80 and 80', one on either side of the track
of vertical load bearing 64. Similarly, auxiliary bearing 78 may include
first and second auxiliary bearings 78 and 78', correspondingly placed.
Placement of auxiliary bearings 78, 78', 80 and 80' outside the track of
bottom surface 76 ensures that, even if peening or other deformation of
horizontal top surface 60 takes place by contact with roller bearings 82,
no effect will be reflected in the finish of the facing surfaces of
horizontal top surface 60 and bottom surface 76. As a consequence, normal
operation is preserved.
An embodiment of the invention is contemplated wherein only a single
auxiliary bearing 80 and a single auxiliary bearing 78 is disposed
centered in the track of vertical load bearing 64.
Certain bearing materials are available permitting high-speed sliding
contact with minimum wear. A plastic bearing material such as, for
example, a material sold under the trademark Turkite, may be substituted
for roller bearings 82 to provide a minimum air-film dimension with
assumption of excess load. In one embodiment of the invention, vertical
load bearing 64 is fabricated of a plastic bearing material and auxiliary
bearings 78 and 80 (together with auxiliary bearings 78' and 80', if used)
may be eliminated. Since a plastic bearing material bearing against a
steel surface of horizontal top surface 60 is unlikely to cause
deformation or wear on horizontal top surface 60, separate auxiliary
bearings employing such materials may be disposed within the track of
vertical load bearing 64 without concern for damage to horizontal top
surface 60.
Due to the small thickness of the air film, tolerances on the flatness of
the portion of horizontal top surface 38 and 60 facing vertical load
bearings 50 (FIG. 1) and 64 are extremely tight. This is seen from the
above example dimensions permitting a total range of air-film thickness of
only 0.0005 inch (five ten-thousandths of an inch). To prevent contact,
horizontal top surfaces 38 and 60 must be flat to better than this value
for spans equal to the lengths of vertical load bearings 50 and 64 over
the entire lengths of X-axis rails 12 and 14. In machines having lengths
of X-axis rails 12 and 14 on the order of a few inches to a few dozen
inches, such flatness is attainable using steel for X-axis rails 12 and 14
by machining, grinding and finally polishing horizontal top surfaces 38
and 60. Although expensive, such treatment is successful in short
machines. In machines requiring X-axis rails 12 and 14 on the order of
several feet, attaining the required flatness in a steel surface is either
very expensive, on infeasible.
Referring now to FIG. 6, a X linear motor assembly 20' is shown wherein a
X-axis rail 12' includes a steel structure having substantially the same
shape as in previous embodiments except that a plastic insert 84 having a
flat horizontal surface 86 is disposed in an upper surface 88 thereof.
Vertical load bearing 50 faces flat horizontal surface 86 for producing a
supporting film of air. Similarly, a plastic insert 90 having a flat
vertical surface 92 is disposed in a side surface 94 of X-axis rail 12'
It is possible to mold plastic inserts 84 and 94 having the required
flatnesses of flat horizontal surface 86 and flat vertical surface 92 much
more economically than similar flatness can be attained in the surface of
an all-steel structure. Although molded plastic inserts 84 and 90 are more
susceptible to damage from contact with vertical load bearing 50 and
horizontal load bearing 52, respectively, the above-described techniques
for eliminating contact or ameliorating such damage may be employed to
permit using this embodiment of the invention. Any suitable material such
as, for example, Turkite, may be employed in plastic insert 84 and plastic
insert 90. Flat horizontal surface 86 and flat vertical surface 92 may be
formed with the required finish during molding of plastic inserts 84 and
90 or they may require additional finishing after molding.
Referring now to FIG. 7, there is shown, generally at 96, a positioning
table according to a further embodiment of the invention. First and second
X-axis rails 98 and 100 each have a rectangular cross section. X-axis rail
98 includes a horizontal upper surface 102 and a vertical side surface
104. An X linear motor assembly 105 includes a saddle 106 having a
horizontal plate 108 spaced parallel to horizontal upper surface 102 and a
vertical side surface 112 spaced parallel to vertical side surface 104.
Horizontal upper surface 102 has a width sufficient to accommodate
vertical load bearing 50 and armature 54 side by side on horizontal plate
108. As in prior embodiments armature 54 interacts with permanent magnets
23, mounted on horizontal upper surface 102, to produce forces urging
saddle 106 along the X axis.
X-axis rail 100 includes a horizontal top surface 114 and a vertical side
surface 116. An X outboard linear motor assembly 118 includes at least a
horizontal plate 120 spaced parallel to horizontal top surface 114. 4 and
armature 71 are disposed side by side on vertical side surface 1120.
In the prior embodiments, the diagonal action angle of the combined
magnetic and load forces produced transverse as well as vertical forces on
the two X linear motors. In the embodiment of FIG. 7, the action angle of
magnetic attraction between armature 54 and permanent magnets 23, as well
as between armature 71 and permanent magnets 70 is vertical. It will be
clear to one skilled in the art that some means for lateral must be
provided for positioning table 96. Mechanical guides or opposed air
bearings (neither of which are shown) or any other convenient guidance
technique may be used. One possibility includes a plurality of magnets 122
disposed on vertical side surface 112 spaced closely from vertical side
surface 104. Magnetic attraction between magnets 122 and X-axis rail 98
provide a force tending to urge saddle 106 rightward in the figure.
Rightward motion of saddle 106 is resisted by horizontal load bearing 52
disposed alongside magnets 122 facing vertical side surface 104.
Horizontal magnetic holding force resisted by an air bearing optionally may
be employed on X outboard linear motor assembly 118. Thus, a vertical
plate 124 may be spaced parallel to vertical side surface 116 for
supporting a plurality of permanent magnets 126 close to vertical side
surface 116 and a vertical side surface 116 for resisting excess motion of
vertical plate 124 toward the right. The optional nature of these elements
is indicated by their being illustrated in dashed line. As in prior
embodiments, these optional horizontal retention elements may be omitted
in applications where their presence is prohibited by thermal expansion of
Y-axis rail 24.
X-axis rails 98 and 100 may contain damping masses 130 and 132 for damping
vibration which may otherwise occur in long hollow elements such as these.
Any suitable material may be employed for damping vibration such as, for
example, a damping tile conforming to Military Specification MIL-P-23653
and available commercially from the Lord Corporation. Although illustrated
in the environment of rectangular X-axis rails 98 and 100, damping masses
are equally applicable to all embodiments of the invention disclosed
herein for applications requiring vibration damping.
Referring now to FIG. 8, an embodiment of the invention is shown in which
an X linear motor assembly 134 is disposed on an X-axis rail 136 made of a
non-metallic material such as, for example a graphite-epoxy composite. A
graphite-epoxy matrix structure in X-axis rail 136 is lighter and stiffer
compared steel. One disadvantage, however, is the lack of a magnetic
return path behind permanent magnets 23. A metallic plate 138 is affixed
to a sloping surface 140. Permanent magnets 23 are, in turn, affixed to
metallic plate 138. In this manner, the desired magnetic return path
between adjacent permanent magnets 23 is provided. A second parallel
X-axis rail (not shown) for an X outboard linear motor assembly (not
shown) may be fabricated of a non-magnetic material with a metallic plate
for providing a magnetic return path. Since such a second X-axis rail
corresponds to that illustrated in FIG. 8, with modifications according to
the foregoing disclosure, further discussion thereof would be redundant
and is therefore omitted.
It would be clear that a corresponding structure using a metallic plate for
magnetic return on a rectangular non-metallic (or non-magnetic metal)
X-axis rail without could be used without departing from the scope of the
invention.
Referring now to FIG. 9, there is shown, generally | | |
