Two-axis motor with high density magnetic platen

An X-Y positioning machine has a forcer, with armature coils, that moves around on a platen, supported by an air bearing. Magnets embedded in the motor platen generate a fixed magnetic field with which the armature coils interact. Perpendicular elongated coils interact with the fields of magnets in columns and rows. When a traditional rectangular pattern of the magnets is provided, a packing density and average peak magnetic flux intercepted by the coils are limited to 50%. The present invention provides magnet configurations that provide greater than 50% maximum peak flux density and up to 100% packing density. Several magnet arrangements are provided: a first in which round magnets are used instead of square, and a second in which diamond-shaped magnets are used. The latter can be used at 100% packing density arrangement. In addition to high peak flux density for a narrow coil, these embodiments exhibit low cogging forces. A method of making the magnet is also provided. To create the equivalent of a closely-packed array of circular magnets, a single sheet of magnetizable material is pressed against an high permeability element such as one of iron, and a pair of adjacent coils pressed against the magnetizable material with currents running in opposite directions. This forms a pair of round adjacent magnetic regions. The coils are moved systematically over the sheet of magnetizable material and the magnetization repeated. This process repeats until the sheet has a close-packed array of magnetic regions.

BACKGROUND OF THE INVENTION 
The present invention relates to devices known variously as traversing 
machines, positioning devices, actuators, etc. More particularly, the 
invention relates to such devices with the ability to traverse along more 
than a single axis. 
A two-axis motor with a stage (also known as a forcer) supported by an air 
bearing on a motor platen surface is described in U.S. Pat. No. 5,334,892, 
the entirety of which is incorporated herein by reference. In this motor, 
the motor platen has a rectangular array of permanent magnets embedded in 
it. Mutually perpendicular sets of X and Y coils in the stage interact 
with the magnetic fields of the magnets to move and position the stage. 
In the prior art motor described above, the packing density of the magnets 
in the motor platen is about 50%. It is desirable to increase the magnetic 
flux density to increase the peak motive force on the stage and also to 
allow a larger air gap between the stage coils and the platen magnets. 
Cogging is also an ever-present problem in such motors. 
OBJECTS AND SUMMARY OF THE INVENTION 
An object of the present invention is to provide a motor platen for an X-Y 
motor. 
Another object of the present invention is to provide a motor platen for an 
X-Y motor that provides for high peak motive force. 
Yet another object of the present invention is to provide a motor platen 
with high magnetic flux density to allow large air gaps between the coils 
and magnets. 
Yet another object of the present invention is to provide a motor platen 
for an X-Y motor that is simple to manufacture. 
Yet another object of the present invention is to provide a motor platen 
for an X-Y motor that is characterized by low cogging effects. 
Briefly, an X-Y positioning machine has a forcer, with armature coils, that 
moves around on a platen, supported by an air bearing. Magnets embedded in 
the motor platen generate a fixed magnetic field with which the armature 
coils interact. Perpendicular elongated coils interact with the fields of 
magnets in columns and rows. When a traditional rectangular pattern of the 
magnets is provided, a packing density and averaged peak magnetic flux 
intercepted by the coils are limited to 50%. The present invention 
provides magnet configurations that provide greater than 50% averaged peak 
flux density and up to 100% packing density. Several magnet arrangements 
are provided: a first in which round magnets are used instead of square, 
and a second in which diamond-shaped magnets are used. The latter can be 
used at 100% packing density arrangement. In addition to high peak flux 
density for a narrow coil, these embodiments exhibit low cogging forces. A 
method of making the magnet is also provided. To create the equivalent of 
a closely-packed array of circular magnets, one or more sheets of 
magnetizable material is squeezed between a high permeability element such 
as piece of iron, and a pair of adjacent coils pressed against the 
magnetizable material with currents running in opposite directions. This 
forms a pair of round adjacent magnetic regions. The coils are moved 
systematically over the sheet of magnetizable material and the 
magnetization repeated. This process repeats until the sheet has a 
close-packed array of magnetic regions. Modular pieces of magnetizable 
material, each with a set of magnetized regions that form an integral 
number of cycles of the required pattern of magnetic regions, can then be 
tiled to form a translationally symmetric pattern. This method facilitates 
manufacturing by reducing the number of pieces that must be handled. 
According to an embodiment of the present invention, there is provided, an 
X-Y positioning system, comprising: a generally planar motor platen with a 
plurality of magnets, forming a planar array, attached thereto, a stage 
movably connected to the motor platen, the stage having a first 
longitudinal coil arranged with a long axis thereof oriented in a first 
direction, the stage having a second longitudinal coil arranged with a 
long axis thereof oriented in a second direction substantially 
perpendicular to the first direction, the plurality of magnets including 
first magnets oriented with their north poles facing in a third direction 
perpendicular to a plane of the array and second magnets with their north 
poles facing in a fourth direction opposite the third direction, the coils 
being arranged such that it is possible to draw a line segment in a plane 
of the planar array where the line segment touches several of the first 
magnets without touching any of the second magnets, with less than 50% of 
the line segment running over an area not occupied by a magnet. 
According to another embodiment of the present invention, there is 
provided, a positioning system, comprising: a motor platen with a planar 
array of substantially round magnets, the planar array having first 
magnets with their north poles facing in a first direction perpendicular 
to a plane of the planar array, the planar array having second magnets 
with their north poles facing in a second direction, opposite the first 
direction, the first magnets forming a first regular array of parallel 
columns and a first regular array of parallel rows, the second magnets 
forming a second regular array of parallel columns and a second regular 
array of parallel rows, the first regular array of parallel columns being 
parallel to the second regular array of parallel columns and the first 
regular array of parallel rows being parallel to the second regular array 
of parallel rows, the planar array being characterized by a packing 
density of more than 50%, a stage movably connected to the motor platen, 
the stage having a first longitudinal coil with a long axis parallel to 
the first axis and the stage having a second longitudinal coil with a long 
axis parallel to the second axis. 
According to still another embodiment of the present invention, there is 
provided, a positioning system, comprising: a motor platen with a planar 
array of substantially parallelogram-shaped magnets, the planar array 
having first magnets with their north poles facing in a first direction 
perpendicular to a plane of the planar array, the planar array having 
second magnets with their north poles facing in a second direction, 
opposite the first direction, the first magnets forming a first regular 
array of parallel columns and a first regular array of parallel rows, the 
second magnets forming a second regular array of parallel columns and a 
second regular array of parallel rows, the first regular array of parallel 
columns being parallel to the second regular array of parallel columns and 
the first regular array of parallel rows being parallel to the second 
regular array of parallel rows, the planar array being characterized by a 
packing density of approximately 100%, a stage movably connected to the 
motor platen, the stage having a first longitudinal coil with a long axis 
parallel to the first axis and the stage having a second longitudinal coil 
with a long axis parallel to the second axis. 
According to still another embodiment of the present invention, there is 
provided, a motor, comprising: a base having a surface, a stage having an 
X-motor and a Y-motor and a bearing to support the stage at a 
substantially constant distance from the planar surface, the base having 
an array of north-pole magnets and south-pole magnets, each having a north 
pole, the X-motor and the Y-motor being effective to interact with fields 
generated by the north-pole and south-pole magnets to produce a motive 
force to move the stage relative to the base, the north-pole magnets being 
oriented with the north poles thereof directed oppositely the north poles 
of the south-pole magnets, the north-pole and south-pole magnets being 
arranged in an array conforming to the surface such that mutually parallel 
curves (the term "curve" being used in its general mathematical sense to 
encompass straight lines as well is non-straight lines), parallel to the 
surface, can be placed at regular intervals, each curve substantially 
intercepting only one of the north-pole and the south-pole magnets without 
intercepting the other of the north-pole and south-pole magnets, each of 
the north-pole and south-pole magnets being shaped such that the mutually 
parallel curves can also be placed such that first portions of each of the 
mutually parallel curves intercepting the only one of the north-pole and 
south-pole magnets is greater than second portions not intercepting the 
only one of the north-pole and south-pole magnets and the mutually 
parallel curves having a constant slope in a coordinate system defined by 
a range of movement of the motor. 
According to still another embodiment of the present invention, there is 
provided, a method of making an array of magnets, comprising the steps of: 
placing a generally planar piece of magnetizable material on a piece of 
material with a magnetic permeability comparable to or greater than that 
of iron, placing, against a face of the piece of magnetizable material 
opposite the piece of high magnetic permeability material, a pair of 
coils, generating currents in the coils, a direction of current in one of 
the coils being opposite that of a direction of current in the other and 
holding the coils against the piece of magnetizable material for a 
sufficient period of time to form a magnetized region. 
According to still another embodiment of the present invention, there is 
provided, a method of making an array of magnets, comprising the steps of: 
placing, against opposite faces of a generally planar piece of 
magnetizable material, a pair of coils, generating currents in the coils, 
a direction of current in one of the coils being the same as that of a 
direction of current in the other and holding the coils against the piece 
of magnetizable material for a sufficient period of time to form a 
magnetized region. 
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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, an X-Y traversing system 1000 has a base 100, with a 
stage 101 supported on base 100 by several air bearings A1-A3. Stage 101 
has a built-in motor that orients and moves stage 101 with respect to base 
100 as described in U.S. Pat. No. 5,334,892, the entirety of which is 
incorporated herein by reference. Base 100 has an array of permanent 
magnets (not shown) with which motors M1, M2, and M3 in stage 101 interact 
to cause stage 101 to move about base 100 with a constant orientation of 
stage 101. 
Referring now also to FIGS. 2a and 2b, in order to employ X-Y traversing 
system 1000 for precise positioning, it is necessary to detect two 
independent coordinates representing the position of stage 101 relative to 
base 100. An encoder system is employed to detect movement of stage 101 
relative to base 100. The encoder system includes a grid encoder scale 121 
with circular regions 120 of a surface 130 of base 100 whose reflectivity 
is much higher than intersticial area 140 separating circular regions 120. 
(alternatively, circular regions 120 can have a low reflectivity and 
intersticial area 140, a high reflectivity.) Circular regions 120 are of a 
highly reflective coating formed on surface 130 of base 100. An 
X-direction optical pickup 110 and a Y-direction optical pickup 111 (not 
shown in FIGS. 2a and 2b, but identical to X-direction optical pickup 110 
as shown in FIGS. 2a and 2b) detect movement of stage 101 relative to base 
100. 
Note that the proportions of elements of X-Y traversing system 1000 shown 
in FIG. 1 are deliberately distorted for illustration purposes. For 
example, in a practical system, the relative sizes of air bearings A1, A2, 
and A3 would be chosen for proper balance and might not be the same as 
illustrated. Optical pickups 110 and 111 would probably be substantially 
smaller as would circular regions 120 (in fact the latter might not be 
visible with the naked eye). Also, the sizes of motors M1-M3 would be 
chosen according to known design principles and each would not likely be 
the same size as shown. In addition, details of optical pickups 110 and 
111 are not necessarily as shown in FIGS. 2a and 2b which was created for 
the purpose of providing a general explanation of how the encoder system 
works. 
Each optical pickup 110, 111 projects light on its respective 
discrimination region 112, 113 and detects the light reflected therefrom. 
Light from a light source 370 is collimated by a condenser lens 375 and 
directed to a reticle 372. Reticle 372 has a series of mask regions 376 
(usually a metalized coating over a substrate, where the metalized coating 
has been etched to define mask regions 376) comprising an index grating. 
The spacing of mask regions 376 is substantially equal to a spacing or 
pitch of circular regions 120. Reflected light passes through reticle 372 
to encoder scale 121. Mask regions 376 create shadows in the light beam 
transmitted through reticle 372. When transmitted light beams 378 coincide 
with circular regions 120, they are substantially reflected since circular 
regions 120 are more reflective than intersticial area 140. When encoder 
110 moves in the X-direction a distance equal to half the dot-pitch, the 
transmitted light beams 378 hit substantially only the intersticial area, 
reducing the amount of light reflected. Reflected light passes back 
through reticle 372 and is detected by a photo-sensor 371. As X-direction 
optical pickup moves the reflected light cycles between maxima and minima 
generating an electrical signal that is processed to determine cumulative 
movement. As can be seen by inspection, X-direction optical pickup is 
responsive essentially, only to movement in the X-direction since the 
light reflected by short columns of circular regions 120 spanning the 
width of reticle 372 is averaged. As can also be seen by inspection, 
Y-direction optical pickup 111, using the same construction as X-direction 
optical pickup 110, but aligned with the Y-direction instead of the 
X-direction, is responsive only to movement in the Y-direction. Instead of 
dots, grid scale 121 can be composed of a grid of overlapping lines 
defining squares (which correspond to circular regions 120) between them. 
In addition, it is not necessary that circular regions (or the squares, if 
overlapping lines are formed) have a higher reflectivity than intersticial 
region 140. The opposite may be true and the system works just as well. 
In summary, optical pickups 110, 111 each employ a reticle with a grating 
whose spacing corresponds to the spacing of columns of and rows of 
circular regions of grid scale 121. Light produced by optical pickups 110, 
111 passes through a respective reticle and reflects from circular regions 
120. Because the spacing of the grating corresponds to the spacing between 
columns and rows of circular regions 120, the total amount of reflected 
light cycles as optical pickups 110, 111 move over grid scale 121. Photo 
sensors 371 produce a signal corresponding to the net reflected light 
which cycles for each increment of movement equal to the circular region 
spacing. 
In a practical system, to sense direction of movement, optical pickups 110 
and 111 could have multiple photo sensors 371 and the spacing of mask 
regions 376 would not be the same as the period of reflective regions 120. 
A moving pattern (like a moire pattern) would be projected on the multiple 
photo sensors and the direction of movement thus determined. Note that the 
proportions of elements of optical pickups 110 and 111 have been distorted 
for explanation purposes. In a real device, the density of reflective 
regions 120 and the mask regions 376 in the reticle would probably be much 
higher. In addition, the spacing, thickness, and lens power of the 
elements is not intended to be accurately represented by FIGS. 2a and 2b. 
Circular regions 120 are arranged in a regular pattern with constant 
spacing between adjacent columns and rows of circular regions 120. Note, 
however, that if the resolution required for one axis is lower than that 
required for the other axis, the spacing between rows need not be the same 
as the spacing between columns. 
According to a preferred embodiment of the invention, the distance between 
the motor coils and permanent magnets is increased from the usual spacing 
of a few thousands of an inch (required in stepper motors that employ 
serrations using the so-called "Sawyer principle") to spacings on the 
order of 0.05 inch. Motor coils (also known as "armatures") can have 
high-permeability cores or no cores at all, the coils being embedded only 
in epoxy or some other non-magnetic insulator. Where high permeability 
material is used for a core, the material is usually laminated to minimize 
eddy current generation. The configuration of these motors, the armatures 
and the magnetic platen with which they cooperate, are described further 
below with reference to FIGS. 8a, 8b, and 9. For now, please note that 
this configuration, and others, permit the spacing between the coils and 
motor platen to be large enough to accommodate a layer of material with 
encoder scale 121 formed thereon. This layer can be a separate element 
(for example, as discussed with reference to FIG. 3, a mylar sheet 140 
imprinted with circular regions 120), which offers several advantages that 
are discussed below. 
Referring to FIG. 3, circular regions 120A (exaggerated in size and 
proportion for clarity) are imprinted on a mylar sheet 140 adjacent to a 
surface 142 of a base plate 401 of base 100 to form an encoder plate 320. 
In this embodiment, the imprinted surface 146 of mylar sheet 140 faces 
surface 142 of base plate 401. Mylar sheet 140 is clear to allow optical 
pickups 110, 111 to detect circular regions 120A. An optional cover layer 
202 or sheet of clear film (such as mylar) may be employed to protect 
mylar sheet 140, which has circular regions 120A imprinted on it. By using 
printing or etching technology to imprint on mylar sheeting rather than 
machining or imprinting on the surface of base 100 (a three-dimensional 
object) directly, great cost effectiveness may be achieved. In addition, 
should mylar sheet 140 become damaged (for example, due to failure of the 
air bearings supporting stage 101) mylar sheet 140 or cover layer 202 can 
be readily replaced. 
Circular regions 120A may be imprinted on mylar sheet 140 using known 
dot-printing technology ordinarily employed for printing. Such technology 
is well known for printing on surfaces of various materials and is capable 
of high accuracy and high resolution. In addition to the laser or LED 
(light emitting diode) technology used to produce a latent image that is 
developed with toner, mylar sheet can also be metallized and chemically 
etched. For example, mylar sheet 140 can be coated with a metal layer on 
which is deposited a photo-resist material which is chemically altered by 
light. After a latent light image is impressed on the photo-resist, the 
differing properties of the exposed and non-exposed photo-resist permit 
only a portion of the photo-resist to be removed and the metal chemically 
etched only where photo-resist has been removed using known chemical 
etching techniques. 
Referring to FIG. 4, in another embodiment of encoder plate 320, circular 
regions 120A, constituting grid encoder scale 121, are formed (by 
machining, chemical etching, printing, or some other means) directly on 
surface 142 of base plate 401. Protective transparent sheet 210 is laid 
over surface 142 to protect grid encoder scale 121. Optionally a cover 
layer 202 can be laid over protective transparent sheet 210 to protect it. 
If the air bearings ever failed, protective transparent protective sheet 
210 and/or cover layer 202 protects surface 142 and grid encoder scale 
121. Cover layer 202, made of glass, plastic, mylar, and protective 
transparent sheet 210, can then be replaced. 
Referring to FIG. 5, in still another embodiment of base 100 and encoder 
plate 320, circular regions 120A, constituting grid encoder scale 121, are 
formed (by chemical etching, machining, printing, or some other means) on 
the lower surface of glass sheet 210. Cover layer 202 of mylar or glass 
should be used to protect glass sheet 210 with grid encoder scale 121 to 
preserve its clarity. 
Referring to FIG. 6, in still another embodiment of encoder plate 320, 
circular regions 120A, constituting grid encoder scale 121, are formed on 
the lower surface of mylar sheet 212. Mylar sheet 212 has a metalized 
coating 213 that has been etched to form circular regions 120A. Cover 
layer 202 of mylar or glass should be used to cover mylar sheet 212 with 
grid encoder scale 121. However, since mylar sheet 212 can be produced in 
continuous quantities (metal-coated with grid encoder scale 121 etched 
thereon), cover layer 202 may not be necessary because of the relatively 
low cost of replacing 212, which may be laid over surface 142 of base 
plate 401 and adhered by static charge, vacuum, gravity, or clamping at 
its edges. Note that irregularities in the orientation of mylar sheet 212 
do not present a problem since the data processing used for 
position/movement detection can compensate for such irregularities 
according to known data filtering techniques. 
Referring to FIG. 7a, in still another embodiment of encoder plate 320 
circular regions 120A, constituting grid encoder scale 121, are formed (by 
chemical etching, machining, printing, or some other means) on the upper 
surface of glass sheet 210. A protective sheet of mylar or glass 214 is 
used to cover glass sheet 210 with grid encoder scale 121. Optionally, 
cover glass or mylar sheet 214 can be overlaid with another cover glass or 
mylar sheet 216 to protect cover glass or mylar sheet 214. Cover glass or 
mylar sheet 216 preserves the transparency of cover glass or mylar sheet 
214. 
Referring to FIGS. 7b and 7d, in still another embodiment of encoder plate 
320 hatch line regions 120A, constituting grid encoder scale 121A, are 
formed (by chemical etching, machining, printing, or some other means) on 
the upper surface of glass sheet 210. Hatch line regions 120A, including 
horizontal line regions 120B and vertical line regions 120C, define a 
plurality of square regions 120D between them. Square regions 120D 
function similarly to circular regions 120A. A protective sheet of mylar 
or glass 214 is used to cover glass sheet 210 with grid encoder scale 
121A. Optionally, cover glass or mylar sheet 214 can be overlaid with 
another cover glass or mylar sheet 216 to protect cover glass or mylar 
sheet 214. Cover glass or mylar sheet 216 preserves the transparency of 
cover glass or mylar sheet 214. 
Referring to FIGS. 7c and 7d, in still another embodiment of encoder plate 
320 hatch line regions 120A, constituting grid encoder scale 121A, are 
formed (by chemical etching, machining, printing, or some other means) on 
the upper surface of base plate 401. Hatch line regions 120A, including 
horizontal line regions 120B and vertical line regions 120C, define a 
plurality of square regions 120D between them. A protective sheet of mylar 
or glass 214 is used to cover glass sheet 210 with grid encoder scale 
121A. Glass sheet 210 covers grid encoder scale 121 A. Optionally, glass 
sheet 210 can be overlaid with another cover glass or mylar sheet 216 to 
protect cover glass or mylar sheet 214. Cover glass or mylar sheet 216 
preserves the transparency of cover glass or mylar sheet 210. Note that 
glass sheet 210 could be replaced by a mylar sheet or some other 
transparent sheet material. Also, in the FIG. 7c embodiment, grid encoder 
scale 121 could be formed on glass (or mylar) sheet 210 instead of base 
plate 401. Another alternative is to form horizontal line regions 120B on 
one layer (for example glass sheet 210) and vertical line regions 120C on 
another layer (for example base plate 401). This can make for easier 
manufacturing of hatch line regions 120A. 
Encoder plate 320, configured according to any of various embodiments 
described above, is laid adjacent base 100. A differential signal derived 
from the two Y-axis optical pickups 111 is used to maintain orientation of 
stage 101 with respect to base 100. 
Referring to FIGS. 8a, 8b, and 9a thin coreless type of armature employs 
motor coils 347 embedded in an epoxy resin bed 346. No high-permeability 
material is used inside motor coils 347. Epoxy resin bed 346 is attached 
to a high permeability back plate 345. A motor platen 350 has an array of 
magnets 301 and 302 and backed by another high permeability plate 344. 
High permeability plates 345 and 344 are preferably of steel for 
cost-effectiveness and strength, but can be made with other materials. 
High permeability plates 345 and 344 should have a high permeability such 
as materials typically employed in armatures with cores. The presence of 
high permeability plates 345 and 344 helps to close the magnetic circuits 
shown by lines 348. Without magnetic (high permeability) materials 
immediately adjacent permanent magnets 301 and 302, as in iron core 
armatures, cogging is drastically reduced. Further reduction in cogging 
can be achieved by shaping edges of armature back plate 345 as shown in 
FIG. 9. The detailed dimensions of hexagon-shaped back plate 345 are not 
given as they can be numerically and experimentally optimized according to 
known techniques, the optimum dimensions varying at least with magnet size 
and spacing. 
The forgiving spacing between magnetic platen 350 and armature 351 allowed 
by employing motor coils encased in epoxy-only, rather than using high 
permeability materials, such as steel, is enhanced by using relatively 
thick magnets, on the order of 0.3-0.6 inch. Neodymium-iron magnets are 
preferred with this type of motor configuration. The greater spacing 
between the magnets and armature windings (motor coils) allows layers of 
material, at least one with encoder scale 121 (121A) etched or printed 
thereon can be laid over base 100. 
Another alternative for the armature construction is to use pressed powder 
materials. Pressed metal powder elements are made of finely divided metal 
mixed with an insulative binder. The material is heated and compressed to 
form a high-density, high strength material that can be formed readily 
into an armature for an X-Y motor. The resulting armature's ability to 
reduce eddy currents, unlike that for laminations, is isotropic. That is, 
since in such material, eddy currents are confined to the small regions 
defined by the metal particles encapsulated in binder material and the 
dimensions of such particles are statistically the same regardless of 
orientation, it does not matter what the orientation of the permanent 
magnet fields relative to the armature and its direction of movement. Eddy 
currents generated by movement in nearly all axial directions are 
suppressed equally. This has important ramifications for an X-Y motor, 
such as in the present invention. While it is possible to arrange the 
laminations in a rotary or single-axis motor such that eddy currents will 
be strongly suppressed during movement, in a two-axis motor, laminations 
cannot be oriented to strongly suppress eddy currents in both X-and 
Y-oriented armatures for both directions of movement. However, the pressed 
metal powder material suppresses eddy currents irrespective of the 
direction of movement. Such materials are commercially available. 
Referring now also to FIGS. 10a and 10b, to form base 100, a base plate 
401, which must be flat, but not necessarily as flat as required for the 
air bearing surface, is covered with a rectangular array of permanent 
magnets 410. The spaces between, and overlying, permanent magnets 410 are 
filed with epoxy 430 to completely cover permanent magnets 410 and base 
plate 401. To form a smooth flat surface, a surface 440 of epoxy 430, once 
epoxy 430 has been cured, is precision ground so that when encoder plate 
320 is laid on top, a flat surface is presented. Of course, this assumes 
that surface irregularities will not be generated by variations in the 
thickness of the materials making up encoder plate 320. 
Referring to FIG. 11, an alternative way of making base 100 compensates for 
such thickness variations in, for example, plate glass, used in the 
encoder plate embodiments described above. First, an optical flat 500 is 
provided which has as flat a surface as desired for the air bearing 
surface. Optical flat 500 is supplied with holes 510 which are connected 
to a vacuum supply 520 so that a vacuum can be pulled between a surface 
501 of optical flat 500 and any object laid on top of it. Next, encoder 
plate 320, with the grid encoder scale facing surface 501, is laid on top 
of optical flat 500. A vacuum is then pulled. Preferably, the vacuum 
should be strong enough to cause the encoder plate 320 to flatten against 
the surface of optical flat 500 so that the surface of encoder plate 320 
presenting the grid encoder scale is pulled completely flat (that is, 
there are no spaces between surface 501 of optical flat 500 and encoder 
plate 320). Once encoder plate is drawn flat, permanent magnets 301 
(permanent magnets 302 are present also but not shown in the section of 
FIG. 11) are arranged on a back surface 322 of encoder plate 320 to form 
array of permanent magnets 312. Standoffs (not shown) can be used to 
separate permanent magnets 301, 302 from the immediately-adjacent layer be 
it glass or whatever to allow epoxy to flow into the space between that 
layer and the magnets. The entire surface is then covered with epoxy 316 
to fill the spaces between permanent magnets 301 and to cover the tops of 
permanent magnets 301. Before epoxy 316 is hardened, base plate 401 is 
laid on top of epoxy 316. After epoxy 316 hardens, the vacuum is removed 
and base 100 is completed. 
One advantage of using mylar as the outer protective sheet is that if a 
flat encoder plate is formed using the above method, the thickness 
irregularities introduced in the outermost cover sheet layer will not be 
as great as if plate glass is used as a protective cover. 
Referring now to FIG. 12a , 12b, and 12c, there are various ways to form 
the regions with different reflective properties that form grid encoder 
scale 121. To obtain a scale for which the distance per cycle 
(cycle-pitch) of the signal from each optical pickup 110, 111 is S 
(signal's cycle-pitch=S), vertical and horizontal lines can be etched or 
printed on surface 130, glass sheet 210, etc. to form squares S/2 by S/2 
in size. An improvement to rectangular pattern of squares can be obtained 
by forming dots instead of squares, which permits the maximum dimension of 
the regions to be increased from S/2 by S/2 squares to 0.707S diameter 
circles (FIG. 12b). The diameter of the circles can be increased even more 
if they are laid out in a 45-degree diagonal array (FIG. 12c), in which 
case, the circles can have a diameter of S for the same signal 
cycle-pitch. Because of the larger region size relative to the signal's 
cycle-pitch, the embodiments of FIGS. 12b and 12c permit a 
finer-resolution scale to be manufactured with technology capable of 
discriminating regions with a given resolution. That is, if a given 
printing technology is capable of printing regions with a minimum size of 
S, then the resolution of the resulting scale using a 45.degree. array of 
circular regions will be twice that of a rectangular pattern of vertical 
and horizontal lines. 
Referring to FIGS. 13a, 13b, and 13c, permanent magnets 301 and 302 are 
traditionally arranged in a rectangular array as shown in FIG. 13a. As 
noted above, motors M1-M3 contain X-direction and Y-direction coils, each 
of which subtends a longitudinal region (typically the region's width is 
about 1/4 the spacing of like-oriented poles, S). When the coil is aligned 
with the center of a column of like-oriented poles, the average flux 
intercepted by the coil is at a maximum. However, the flux is averaged for 
all permanent magnets 301, 302 in a column so that the average peak flux 
intercepted by one motor coil is half the peak flux of permanent magnet 
301, 302 alone. 
According to one embodiment of the present invention, round magnets are 
used instead of square ones. The round magnets are arranged as shown in 
FIG. 13b so that their diameters can be as much as the length of the 
diagonals of the squares of the arrangement of FIG. 13a. If the circles 
are close-packed, as shown in FIG. 13b, the ratio of average peak flux to 
peak flux for magnet 301, 302 is increased, over the arrangement of FIG. 
13a, from 0.5 to 0.707 for a hypothetical infinitely narrow coil or a 
single wire. The arrangement of FIG. 13b has the additional advantage of 
reduced cogging. Measured cogging for the round-magnet close-packed array 
shown in FIG. 13b, with S=1.2 inch is 0.9# versus 3# for the configuration 
of FIG. 13a. 
According to another embodiment of the invention, a diamond array as shown 
in FIG. 13c is used. In this configuration, there is no space between any 
magnets so the packing density is 100%. The ratio of average peak flux to 
peak flux for magnet 301, 302 approaches 1.0 for a single wire, or a 
infinitely narrow coil (as opposed to a typical-size coil). For a coil of 
practical size, the ratio is actually between 0.6 and 0.7, depending on 
the coil width. 
Note that the arrangement of the magnets in embodiments of FIGS. 13b and 
13c are similar to the arrangement of FIG. 13a in that magnets with the 
same pole orientation form rows and columns that are aligned with the long 
axes of the X-axis and Y-axis coils of the X- and Y- motor armatures, 
respectively. 
Note that the arrangements of FIGS. 13b and 13c are arranged such that it 
is possible to position a thin coil or single wire in a plane of the 
magnet array where said coil runs over several of the north-oriented 
magnets without touching any of south-oriented magnets, with less than 50% 
of the coil or wire running over an area not occupied by a magnet. This 
means the averaged peak flux density intercepted by the coil or wire is 
greater than 50%. The graphs adjacent the plan view of the motor platens 
of FIGS. 13a, 13b, and 13c, show the variation instantaneous average flux 
as the coil moves over the platen for a single wire 602 and for a coil of 
more realistic dimension of about S/4 601, where the spacing between 
like-oriented magnets is S. As can be seen from the figures, the peak flux 
for an S/4 wide coil and that for a single wire are the same, about 0.707. 
The former, however, falls off sooner than the latter as the real coil 
overlaps opposite poles for a greater proportion of the total 
displacement. In the diamond-shaped magnet array, the peak flux for a 
single wire is 100% but that for a realistic coil of S/4 width is about 
2/3. 
Referring to FIG. 14a, to form the magnet array, pieces (for example, 
sheets) of magnetizable material 702 may be arranged on a base 703 of 
material characterized by high flux saturation levels. A pair of coils 701 
is then pressed against the magnetic material and a current passed through 
coils 701 to magnetize a region of magnetizable material 702, base 703 
closing a magnetic circuit between two coils 701. Coils 701 can then be 
moved in successive steps over the entire surface of magnetizable material 
702 until an array of magnetic regions is formed. Such a method is 
particularly applicable to form an array such as shown in FIG. 13b. 
Instead of using a base 703 of high permeability material, alternatively, 
the magnetizable material can be lined with non-magnetic material 704 and 
directly pressed from either side by two coils 701 to magnetize 
magnetizable material 702. Magnetized material 702 is then placed on a 
cast iron plate or steel plate 707. Due to the high attraction forces, 
magnetic material 702 can be lowered by a suitable jack, wax 705 being 
used to support magnetized magnetic material 702 on the surface of plate 
707. Once magnets 702 are in position, wax 705 can be melted away to 
achieve close contact between the magnets 702 and the base plate 707. 
Note that although the diamond array of FIG. 13c could achieve similar 
results if the shapes of magnets 301, 302 were changed to a parallelogram 
shape because diagonals of such parallelogram-shaped magnets would still 
be perpendicular. 
Note that although the embodiments described above relate to planar X-Y 
traversing systems, the invention applies equally to other types of 
traversing systems. For example, a traversing system in which a stage 
moves about a non-planar base could also employ the features described 
above for the base. Such devices are considered to be within the scope of 
at least some of the claims. 
Note that claims may refer to independent movement in multiple axial 
directions using terms like orthogonal and perpendicular. It is clear that 
wherever in the specification movement along mutually perpendicular or 
orthogonal directions is discussed, such movement can be regarded as 
characterizing marginal degrees of freedom and therefore encompass any 
orthogonal coordinate system. For example, the X-direction could be 
regarded as an angle and the Y-direction as a displacement along the axis 
of a cylindrical coordinate system. Such variations within the scope of 
the invention and within the scope of at least some of the claims below. 
It is also noted here that such terms are not intended to be construed as 
narrowly as the mathematical sense of orthogonal coordinate systems. For 
example, a cylindrical coordinate system is not truly an orthogonal 
coordinate system. However, the present invention is applicable to a 
system that moves a stage over a cylindrical surface with projection of 
the X-Y grid on the cylindrical surface. Such a system is disclosed in 
Applicant's application filed prior to or concurrently with this 
application (The device is summarized in the next section summarily 
describing a rotary linear motor). Claims that speak of perpendicular or 
orthogonal movement are intended to cover movement such as that in such a 
cylindrical encoder system. In a cylindrical arrangement, magnets would be 
arranged in a translationally symmetric pattern, just as in the flat 
platen. The term "translationally symmetric" is used here to characterize 
any pattern that is achieved by making copies of the same thing at equal 
distances from each other. So, for example, a regular pattern of identical 
tiles forms a translationally symmetric array whether they are laid on a 
flat surface or a cylindrical surface. 
SUMMARY DESCRIPTION OF A ROTARY-LINEAR MOTOR 
Briefly, a motor, with two independent degrees of freedom, rotates a stage 
about an axis and moves the stage along the axis, the range of motion 
defining a cylinder or cylindrical section. The stage is mounted on a 
hollow cylindrical plunger fitting in an annular well. The plunger floats 
on an air-bearing. The plunger has an array of permanent magnets on its 
external cylindrical face opposite coils in the well. Equal numbers of 
oppositely-polarized permanent magnets are arranged in a regular 
cylindrical pattern at 50% packing density forming rings and columns of 
like-polarity magnets, the rings of one polarity alternating with rings of 
opposite polarity and the columns of one polarity alternating with columns 
of opposite polarity. A set of Z-axis coils (for axial movement) curve 
(the term "curve" being used in its general mathematical sense to 
encompass straight lines as well is non-straight lines) around the plunger 
and are shaped to allow a current in them to impel the rings of 
like-polarized magnets. A set of .phi.-axis coils (for rotational 
movement) have longitudinal axes that are parallel the axis of the plunger 
and are sized to allow current in them to impel the columns of 
like-polarized magnets. Air is injected into a space between a center 
column defining the center of the annular well and the internal surface of 
the plunger to support the plunger. Part of the external surface of the 
plunger has a grid scale encoded by Z-axis and .phi.-axis optical pickups 
to provide position information to a controller. 
According to an embodiment of the present invention, there is provided, a 
rotary-linear motor, comprising: first and second elements, each having a 
common axis, the first element having at least one magnet, the second 
element having at least first and second electrical coils capable of 
generating respective first and second magnetic fields, a bearing to 
support the first element with respect to the second element to allow the 
first and second elements to rotate about an axis relative to each other 
and to slide in a direction collinear with the axis, the first and second 
coils being positioned relative to each other and relative to the magnet 
such as to produce a substantial motive force capable of both rotating and 
displacing the first and second elements with respect to each other when 
the first and second coils are excited by an electrical current. 
According to another embodiment of the present invention, there is 
provided, a rotary-linear motor, comprising: a base element having one of 
a plurality of magnets and a plurality of coils, a stage element having 
the other of a plurality of magnets and a plurality of coils, the stage 
element being connected to the base element such that the stage element is 
free to rotate on an axis and slide along the axis, the plurality of 
magnets and the plurality of coils being arranged to generate a motive 
force therebetween when the plurality of coils is energized. 
According to still another embodiment of the present invention, there is 
provided, a rotary-linear motor, comprising: a base member, a stage 
member, the base member having a first cylindrical surface, the stage 
member having a second cylindrical surface, the first and second 
cylindrical surfaces having a common axis, the base having one of a 
plurality of magnets and a plurality of electric coils shaped in such a 
way as to define a first cylinder coaxial with the common axis and the 
stage having another of the plurality of magnets and the plurality of 
electric coils shaped in such a way as to define a second cylinder coaxial 
with the common axis. 
The above rotary-linear motor can employ a multi-axis encoder system, such 
as described in the present application. The grid scale would be projected 
on a cylindrical surface and the X- and Y- optical pickups would resolve 
axial and tangential portions of the curved grid scale. 
Although only a few exemplary embodiments of this invention have been 
described in detail above, those skilled in the art will readily 
appreciate that many modifications are possible in the exemplary 
embodiment(s) without materially departing from the novel teachings and 
advantages of this invention. Accordingly, all such modifications are 
intended to be included within the scope of this invention as defined in 
the following claims. In the claims, means-plus-function clauses are 
intended to cover the structures described herein as performing the 
recited function and not only structural equivalents but also equivalent 
structures. Thus although a nail and screw may not be structural 
equivalents in that a nail relies entirely on friction between a wooden 
part and a cylindrical surface whereas a screw's helical surface 
positively engages the wooden part, in the environment of fastening wooden 
parts, a nail and a screw may be equivalent structures.