Two-axis motor platen and method for making

What is claimed is:

1. A positioning system, comprising:

a generally planar base having a base surface;

a generally planar cover element overlying said base surface and having a facing surface facing said base surface;

a transparent secondary protective cover sheet overlying said cover element;

said cover element having a non-facing surface on an opposite side of said cover element from said facing surface;

one of said base surface and said facing surface having regions of a first reflectivity with at least one region of a second reflectivity therebetween;

said cover element being generally transparent, whereby said regions of a first reflectivity are visible from said non-facing surface; and

said stage having at least one optical pickup oriented to detect a net reflectivity of an area of said of one said base surface and said facing surface.

2. A positioning system as in claim 1, wherein:

said regions of a first reflectivity being arranged in a regular pattern; and

said at least one optical pickup having a longitudinal field of detection that is large enough to subtend at least portions of several of said regions.

3. A positioning system as in claim 2 wherein said cover element is of a flexible plastic material.

4. A positioning system as in claim 3 wherein said cover element is of mylar.

5. A positioning system as in claim 3 wherein said cover element is of a material that permits said cover element to remain in position on said base, even if said base is inverted, by natural static electrical attraction or by ambient air pressure.

6. A positioning system as in claim 2, wherein one of said regions of a first reflectivity and said at least one region of a second reflectivity are/is of a metal coating on said cover element, said first reflectivity being an intrinsic property of a metal of said metal coating.

7. A positioning system as in claim 2, wherein:

one of said base and said stage includes at least one magnet; and

another of said base and said stage includes at least one electrical coil, sized and positioned such that when excited, a motive force is generated between said stage and said base.

8. A positioning system as in claim 7, wherein said at least one magnet is of a neodymium-iron composition.

9. A positioning system as in claim 1, wherein said cover element is of a flexible plastic material.

10. A positioning system as in claim 1, wherein:

one of said base and said stage includes at least one magnet; and

another of said base and said stage includes at least one electrical coil, sized and positioned such that when excited, a motive force is generated between said stage and said base.

11. A positioning system as in claim 10, wherein said at least one magnet is of a neodymium-iron composition.

12. A positioning system as in claim 2, wherein:

another of said base surface and said facing surface has regions of a third reflectivity with at least one region of a fourth reflectivity therebetween; and

said regions of a first reflectivity include a series of first linear regions of said first reflectivity;

said regions of a third reflectivity includes a series of second linear regions of said third reflectivity;

said first and second linear regions has respective longitudinal axes;

said longitudinal axes of said first linear regions are mutually parallel; and

said longitudinal axes of said second linear regions are mutually parallel and perpendicular to said longitudinal axes of said first linear regions.

13. A positioning system as in claim 9, wherein said cover element is of mylar.

14. A positioning system as in claim 9, wherein said cover element is of a material that permits said cover element to remain in position on said base, even if said base is inverted, by natural static electrical attraction or by ambient air pressure.

15. A positioning system as in claim 9, wherein one of said regions of a first reflectivity and said at least one region of a second reflectivity are of a metal coating on said cover element, said first reflectivity being an intrinsic property of a metal of said metal coating.

16. A positioning system as in claim 9, wherein:

one of said base and said stage includes at least one magnet; and

another of said base and said stag,e includes at least one electrical coil, sized and positioned such that when excited, a motive force is generated between said stage and said base.

17. A positioning system as in claim 16, wherein said at least one magnet is of a neodymium-iron composition.

18. A positioning system as in claim 9, wherein:

another of said base surface and said facing surface has regions of a third reflectivity with at least one region of a fourth reflectivity therebetween; and

said regions of a first reflectivity includes a series of first linear regions of said first reflectivity;

said regions of a third reflectivity includes a series of second linear regions of said third reflectivity;

said first and second linear regions has respective longitudinal axes;

said longitudinal axes of said first linear regions are mutually parallel; and

said longitudinal axes of said second linear regions are mutually parallel and perpendicular to said longitudinal axes of said first linear regions.

19. A positioning system, comprising:

a generally planar base having a base surface;

a generally planar cover element overlying said base surface and having a facing surface facing said base surface;

said cover element having an exposed surface on an opposite side of said cover element from said facing surface;

one of said base surface and said facing surface having regions of a first reflectivity with at least one region of a second reflectivity therebetween;

said cover element being generally transparent whereby said regions of a first reflectivity are visible from said exposed surface; and

said stage having at least one optical pickup oriented to detect a net reflectivity of an area of said of one said base surface and said facing surface;

wherein one of said regions of a first reflectivity and said at least one region of a second reflectivity are/is of a metal coating on said cover element, said first reflectivity being and intrinsic property of a metal of said metal coating.

20. A positioning system as in claim 19, wherein:

one of said base and said stag,e includes at least one magnet; and

another of said base and said stage includes at least one electrical coil, sized and positioned such that when excited, a motive force is generated between said stage and said base.

21. A positioning system as in claim 20, wherein said at least one magnet is of a neodymium-iron composition.

22. A positioning system as in claim 19, further comprising a transparent secondary protective cover sheet overlying said cover element.

23. A positioning system as in claim 19, wherein:

another of said base surface and said facing surface has regions of a third reflectivity with at least one region of a fourth reflectivity therebetween; and

said regions of a first reflectivity includes a series of first linear regions of said first reflectivity;

said regions of a third reflectivity includes a series of second linear regions of said third reflectivity;

said first and second linear regions has respective longitudinal axes;

said longitudinal axes of said first linear regions are mutually parallel; and

said longitudinal axes of said second linear regions are mutually parallel and perpendicular to said longitudinal axes of said first linear regions.

24. A positioning system, comprising:

a generally planar base having a base surface;

a generally planar cover element overlying said base surface and having a facing surface facing said base surface;

said cover element having an exposed surface on an opposite side of said cover element from said facing surface;

one of said base surface and said facing surface having regions of a first reflectivity with at least one region of a second reflectivity therebetween;

said cover element being generally transparent whereby said regions of a first reflectivity are visible from said exposed surface; and

said stage having at least one optical pickup oriented to detect a net reflectivity of an area of said of one said base surface and said facing surface;

wherein another of said base surface and said facing surface has regions of a third reflectivity with at least one region of a fourth reflectivity therebetween; and

said regions of a first reflectivity includes a series of first linear regions of said first reflectivity;

said regions of a third reflectivity includes a series of second linear regions of said third reflectivity;

said first and second linear regions has respective longitudinal axes;

said longitudinal axes of said first linear regions are mutually parallel; and

said longitudinal axes of said second linear regions are mutually parallel and perpendicular to said longitudinal axes of said first linear regions.

25. A positioning system as in claim 24, wherein:

one of said base and said stage includes at least one magnet; and

another of said base and said stage includes at least one electrical coil, sized and positioned such that when excited, a motive force is generated between said stage and said base.

26. A positioning system, comprising:

a generally planar base having a base surface;

said base having attached thereto a plurality of magnets forming an array adjacent said base surface;

a stage movably connected to said base;

said stage having an electrical coil positioned sufficiently close to said base surface to generate a motive force by generating a field that interacts with a field generated by said plurality of magnets;

said stage having first and second optical pickups;

a planar cover element between said base and said stage;

an array of regions having a first measurable optical characteristic surrounded by one or more regions having, a second measurable optical characteristic on one of said base and said planar cover element;

said first and second measurable optical characteristics being of a type that can be distinguished by said first and second optical pickups and said first and second optical pickups being positioned and oriented to detect said first and second measurable optical characteristics of respective areas of said one of said base and said planar cover element.

27. A positioning system as in claim 26, wherein said planar cover element is approximately 0.1 inch thick.

28. A positioning system as in claim 27, wherein said plurality of magnets are between 0.3 and 0.6 inch thick in a dimension perpendicular to a plane of said base surface.

29. A positioning system as in claim 27, wherein said electrical coil is part of an armature of a type that has no serrations.

30. A positioning system as in claim 27, wherein said magnets are of neodymium-iron composition.

31. A positioning system as in claim 27, wherein:

said cover element is of glass, approximately 0.04 inch thick, with a cover surface adjacent said base surface; and

said cover surface having orthogonal lines forms said array of regions having said first measurable optical characteristic.

32. A positioning system as in claim 27, wherein:

said cover element is of a flexible polymer, less than 0.04 inch thick, with a cover surface adjacent said base surface; and

said cover surface having orthogonal lines forms said array of regions having said first measurable optical characteristic.

33. A positioning system as in claim 32, wherein said orthogonal lines are gaps etched from a metalized layer formed on said cover surface.

34. A positioning system as in claim 33, further comprising:

another planar cover element laid over a side of said cover element opposite said base surface; and

said another planar element being transparent.

35. A positioning system as in claim 26, wherein said plurality of magnets are between 0.3 and 0.6 inch thick in a dimension perpendicular to a plane of said base surface.

36. A positioning system as in claim 35, wherein said electrical coil is part of an armature of a type that has no serrations.

37. A positioning system as in claim 35 wherein said magnets are of neodymium-iron composition.

38. A positioning system as in claim 35, wherein:

said cover element is of glass approximately 0.04 inch thick, with a cover surface adjacent said base surface; and

said cover surface having orthogonal lines forms said array of regions having said first measurable optical characteristic.

39. A positioning system as in claim 35, wherein:

said cover element is of a flexible polymer, less 0.04 inch thick with a cover surface adjacent said base surface; and

said cover surface having orthogonal lines forms said array of regions having said first measurable optical characteristic.

40. A positioning system as in claim 39, wherein said orthogonal lines are gaps etched from a metalized layer formed on said cover surface.

41. A positioning system as in claim 40, further comprising:

another planar cover element laid over a side of said cover element opposite said base surface;

said another planar element being transparent.

42. A positioning system as in claim 26, wherein said electrical coil is part of an armature of a type that has no serrations.

43. A positioning, system as in claim 42, wherein said magnets arc of neodymium-iron composition.

44. A positioning system as in claim 42, wherein:

said cover element is of glass, approximately 0.04 inch thick, with a cover surface adjacent said base surface; and

said cover surface having orthogonal lines forms slide array of regions having said first measurable optical characteristic.

45. A positioning system as in claim 26, wherein:

said cover element is of a flexible polymer, less 0.04 inch thick, with a cover surface adjacent said base surface; and

said cover surface having orthogonal lines forms said array of regions having said first measurable optical characteristic.

46. A positioning system as in claim 45, wherein said orthogonal lines are gaps etched from a metalized layer formed on said cover surface.

47. A positioning system as in claim 46, further comprising:

another planar cover element laid over a side of said cover element opposite said base surface;

said another planar element being transparent.

48. A positioning system comprising:

a generally planar base having a base surface;

said base having attached thereto a plurality of magnets forming an array adjacent said surface;

a stage movably connected to said base;

said stage having an electrical coil positioned sufficiently close to said base surface to generate a motive force by generating a field that interacts with a field generated by said plurality of magnets;

said stage having first and second optical pickups;

first and second planar cover elements between said base and said stage;

said first planar cover element having, on a first surface thereof an array of parallel first lines having a first measurable optical characteristic, said first measurable optical characteristic being different from a measurable optical characteristic of spaces between said first lines; and

said second planar cover element having, on a second surface thereof, an array of parallel second lines having a first measurable optical characteristic, said first measurable optical characteristic being, different from a measurable optical characteristic of spaces between said second lines;

said first and second measurable optical characteristics being of a type that can be distinguished by said first and second optical pickups and said first and second optical pickups being positioned and oriented to detect said first and second measurable optical characteristics of respective areas of said one of said base and said planar cover element;

said first and second surfaces facing each other; and

said first lines being perpendicular to said second lines.

49. A positioning system as in claim 48, wherein said planar cover element is approximately 0.1 inch thick.

50. A positioning system as in claim 48, wherein said plurality of magnets are between 0.3 and 0.6 inch thick in a dimension perpendicular to a plane of said base surface.

51. A positioning system as in claim 48, wherein said electrical coil is part of an armature of a type that has no serrations.

52. A positioning system as in claim 48, wherein said magnets are of neodymium-iron composition.

53. A method of making a motor platen for a positioning system, comprising the steps of:

forming a base with an array of magnets embedded therein;

etching a series of parallel lines on a first sheet of transparent material; and

laying said first sheet of transparent material on said base and affixing it thereto.

54. A method as in claim 53, further comprising the steps of:

etching another series of parallel lines on a second sheet of transparent material; and

laying said second sheet adjacent said first.

55. A method of making a motor platen for a positioning system, comprising the steps of:

forming a base with an array of magnets embedded therein;

forming a first array of regions having a first optical property in the form of a first series of parallel lines on a first sheet of transparent material;

forming a second array of second regions having a second optical property in the form of a second series of parallel lines on a second sheet of transparent material; and

laying said first sheet of transparent material on said base and affixing it thereto;

arranging said second sheet so that said first series of parallel lines is perpendicular to said second; and

laying said second sheet on top of said first sheet.

56. A method of making a motor platen for a positioning system comprising the steps of:

laying a generally planar 2-dimensional optical scale element on a flat surface;

forcing said scale element against said flat surface;

arranging magnets on said scale element;

applying a curable resin to said magnets;

laying a base over said curable resin; and

curing said curable resin.

57. A method as in claim 56, wherein said step of forcing includes applying a vacuum between said flat surface and said scale element.

58. A method as in claim 57, wherein said step of applying includes applying epoxy.

59. A method as in claim 57, wherein said step of applying includes applying said curable resin on all surfaces of said magnets including surfaces of said magnets adjacent said scale element.

60. A method as in claim 57, wherein:

said step of arranging includes placing standoffs on said scale element;

said step of arranging includes placing said magnets on said standoffs;

said step of applying includes injecting said resin into a space between said base and said scale element.

61. A method as in claim 56, wherein said step of applying includes applying said curable resin on all surfaces of said magnets including surfaces of said magnets adjacent said scale element.

62. A method as in claim 61, wherein:

said step of arranging includes placing standoffs on said scale element;

said step of arranging includes placing said magnets on said standoffs;

said step of applying includes injecting said resin into a space between

said base and said scale element.

63. A method as in claim 56, wherein:

said step of arranging includes placing standoffs on said scale element;

said step of arranging includes placing said magnets on said standoffs;

said step of applying includes injecting said resin into a space between said base and said scale element.

64. A method of making a motor platen for a positioning system comprising the steps of:

laying a generally planar 2-dimensional optical scale element on a flat surface;

forcing said scale element against said flat surface;

arranging magnets on said scale element;

applying a curable resin to said magnets;

laying a base over said curable resin; and

curing said curable resin;

said step of arranging including placing standoffs on said scale element;

said step of arranging including placing said magnets on said standoffs; and

said step of applying including injecting said resin into a space between said base and said scale element.

Description Submit all comments and votes

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 and that use optical encoder systems to determine position and orientation of the stage.

X-Y motors, with a stage supported by some means that permits movement along two perpendicular axes, are known. Displacement information, either incremental, as provided by optical encoders or interferometers, or absolute, as provided by resistance encoders, is required to position the stage in a desired location. One of the problems with using encoders, rather than, for example, interferometers, is the lack of resolution. Interferometers can resolve movement as small as a wave-length of the laser source. Encoders require a scale or scales with optically resolvable lines or hatching. It is difficult to make very fine lines over a large area to achieve a resolving power even remotely approaching that of an interferometer. Interferometers are expensive and limit the speed with which the traversing system can operate. Resolving power is always traded against cost in designing manufactured systems. It is a goal in this industry to achieve high resolving power of the positioning information with low cost and high speed. Any increase in the cost-effectiveness of high-resolution systems is highly desirable.

To use two linear scales, one for each axis of movement, a scale may be placed on the base and a corresponding optical pickup on the gantry. The other scale may be placed on the gantry and another optical pickup on the stage. This system may require greater stiffness and precise alignment of the gantry mechanism because of the distance between the encoder scale and the stage, assuming the encoder scale is mounted on the edge of the base. Another method of movement encoding is to use X- and Y-optical pickups moving over a grid of lines, instead of a linear series of lines.

Referring to FIG. 1 a traversing machine 1, according to the prior art, uses a single grid-scale 2. Traversing machine 1 includes a gantry 7 with track 7. Track 7 includes rails 8. Movable stage 6 has linear bearings 5 that couple movable stage 6 with rails 8 to permit movable stage 6 to ride along rails 8. Gantry 18 travels on rails 17 in a direction perpendicular to rails 8.

A cross-hatch pattern of lines 4, a respective set being parallel to the X-axis and a respective set being parallel the Y-axis, define a rectangular array of square patches 3, lines 4 are formed in a surface over which the stage flies. An X-direction optical pickup 5a detects light reflected from grid lines so as to be sensitive only to movement in the X-direction. A Y-direction optical pickup 5b detects light reflected from grid lines so as to be sensitive only to movement in the Y-direction.

Stage 6 moves in X and Y directions aligned with respective perpendicular edges of base 22. The means by which X-Y stage 6 moves relative to base 22 could be any of a number of different known mechanisms (not shown), such as an air bearing, a gantry mechanism mounted on linear bearings, etc. U.S. Pat. No. 5,334,892 describes a traversing system in which stage rests on air bearings that float on a magnet platen. Motor armatures on the stage generate changing magnetic fields that interact with fields generated by magnets in the magnet platen to create a motive force. In this case position and orientation information are supplied by interferometers that bounce laser beams off mirrors attached to the platen. Interferometer-based systems require expensive vibration isolation in order to work properly. In addition, interferometers are more expensive than optical pickups used in other types of traversing systems.

Problems with applying the encoder technology of traversing systems are numerous and varied. There is a pressing need to discriminate very fine degrees of movement. However, as is readily apparent, manufacturing a grid scale with very fine pitch is costly. Generally, such scales are made of a reflective material with machined or printed portions formed on their surfaces. Moreover, creating traversing machinery that provides optical pickups with access to a stationary grid scale without interference is another obstacle to design. Yet another problem is economically printing oil a large area to produce an accurate and consistent grid scale. Still another problem is the vulnerability of highly precise scales to incidental damage or occasional breakdown of the traversing system. Still another problem is cost-effective production of an entire motor platen, with a precise flat surface, especially one with a built-in optical scale.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to provide an encoder system for an X-Y traversing machine that is economical to make and maintain.

Another object of the present invention is to provide an accurate encoder system for an X-Y traversing machine.

Yet another object of the present invention is to provide a position detection device that is simple to manufacture.

Yet another object of the present invention is to provide a position detection device for an X-Y traversing machine.

Yet another object of the present invention is to provide a position detection device for a traversing machine that provides high spatial resolving power.

Yet another object of the present invention is to provide an encoder mechanism for a traversing machine with a grid scale that is accurate.

Yet another object of the present invention is to provide an encoder mechanism for a traversing machine with a grid scale that is capable of being replaced easily if damaged.

Briefly, an X-Y traversing machine employs optical pickups on a stage that moves around a base on an air bearing. A pattern of dots on the surface of the base forms a scale. Respective X- and Y-direction optical pickups are used to detect incremental movement in X- and Y-directions independently. Each pickup averages the reflected light from an elongated rectilinear region whose long axis is aligned perpendicular to the direction of movement the pickup detects. The dots are formed on an encoder scale element. According to an embodiment of the invention, forces are developed without using serrations in the stage and magnets so that a substantial spacing between the coils and field generator (permanent magnets in the preferred embodiment) can be tolerated. This allows the encoder scale to be placed on or protected by a sheet between the platen and the stage. For example, the pattern may be printed on a flexible transparent Mylar sheet that can be readily replaced if damaged. To form a complete motor platen, according to one embodiment of the motor platen, the outer surface of the grid scale element is laid over an element with a precise flat surface is provided with some means for pulling the grid scale flat to the flat surface. Magnets are arranged on the on the back of the scale element and epoxy applied around and between the magnets. A base plate is placed over the epoxy and held in place till the epoxy cures.

According to an embodiment of the present invention, there is provided, a positioning system, comprising: a generally planar base having a base surface, a generally planar cover element overlying the base surface and having a facing surface facing the base surface, the cover element having an exposed surface on an opposite side of the cover element from the facing surface, one of the base surface and the facing surface having regions of a first reflectivity with at least one region of second reflectivity therebetween, the cover element being generally transparent, whereby the regions of a first reflectivity are visible from the exposed surface, a stage movably connected to the base and the stage having at least one optical pickup oriented to detect a net reflectivity of an area of the one of the base surface and the facing surface.

According to another embodiment of the present invention, there is provided, a positioning system, comprising: a generally planar base having a base surface, the base having attached thereto a plurality of magnets forming an array adjacent the surface, a stage movably connected to the base, the stage having an electrical coil positioned sufficiently close to the base surface to generate a motive force by generating a field that interacts with a field generated by the plurality of magnets, the stage having first and second optical pickups, a planar cover element between the base and the stage, an array of regions having a first measurable optical characteristic surrounded by one or more regions having a second measurable optical characteristic on one of the base and the planar cover element, the first and second measurable optical characteristics being of a type that can be distinguished by the first and second optical pickups and the first and second optical pickups being positioned and oriented to detect the first and second measurable optical characteristics of respective areas of the one of the base and the planar cover element.

According to still another embodiment of the present invention, there is provided, a positioning system, comprising: a generally planar base having a base surface, the base having attached thereto a plurality of magnets forming an array adjacent the surface, a stage movably connected to the base, the stage having an electrical coil positioned sufficiently close to the base surface to generate a motive force by generating a field that

interacts with a field generated by the plurality of magnets, the stage having first and second optical pickups, first and second planar cover elements between the base and the stage, the first planar cover clement having, on a first surface thereof, an array of parallel first lines having a first measurable optical characteristic, the first measurable optical characteristic being different from a measurable optical characteristic of spaces between the first lines and the second planar cover element having, on a second surface thereof, an array of parallel second lines having a first measurable optical characteristic, the first measurable optical characteristic being different from a measurable optical characteristic of spaces between the second lines, the first and second measurable optical characteristics being of a type that can be distinguished by the first and second optical pickups and the first and second optical pickups being positioned and oriented to detect the first and second measurable optical characteristics of respective areas of the one of the base and the planar cover element, the first and second surfaces facing each other and the first lines being perpendicular to the second lines.

According to still another embodiment of the present invention, there is provided, a method of making a motor platen for a positioning system, comprising the steps of: forming a base with an array of magnets embedded therein, forming a first array of regions having a first optical property in the form of a first series of parallel lines on a first sheet of transparent material, forming a second array of second regions having a second optical property in the form of a second series of parallel lines on a second sheet of transparent material and laying the first sheet of transparent material on the base and affixing it thereto, arranging the second sheet so that the first series of parallel lines is perpendicular to the second and laying the second sheet on top of the first sheet.

According to still another embodiment of the present invention, there is provided, a method of making a motor platen for a positioning system, comprising the steps of: laying a generally planar 2-dimensional optical scale element on a flat surface, forcing the scale element against the flat surface, arranging magnets on the scale element, applying a curable resin to the magnets, laying a base over the curable resin and curing the curable resin.

According to still another embodiment of the present invention, there is provided, a method of making a motor platen for a positioning system, comprising the steps of: laying a generally planar 2-dimensional optical scale element on a flat surface, forcing the scale element against the flat surface, arranging magnets on the scale element, applying a curable resin to the magnets, laying a base over the curable resin and curing the curable resin, the step of arranging including placing standoffs on the scale element, the step of arranging including placing the magnets on the standoffs and the step of applying including injecting the resin into a space between the base and the scale element.

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 plan view of a traversing system according to the prior art.

FIG. 2a is a plan view of an X-Y positioning system with a position and orientation-detecting apparatus according to an embodiment of the present invention.

FIG. 2b is a side view of an optical pickup taken along a longitudinal axis.

FIG. 2c is a perspective view of the optical pickup of FIG. 2b.

FIG. 3 is a side view of a base of the X-Y positioning system of FIG. 2a according to an embodiment of the invention.

FIG. 4 is a side view of the base of the X-Y positioning system of FIG. 2a according to another embodiment of the invention.

FIG. 5 is a side view of the base of the X-Y positioning system of FIG. 2a according to still another embodiment of the invention.

FIG. 6 is a side view of the base of the X-Y positioning system of FIG. 2a according to still another embodiment of the invention.

FIGS. 7a-7d are side views of the base of the X-Y positioning system of FIG. 2a according to other embodiments of the invention.

FIG. 8a is section view of an armature coil, without a high permeability core, and a magnetic platen, the section being taken along a longitudinal axis of one of the coils.

FIG. 8b is section view of an armature a coil, without a high permeability core, and the magnetic platen, the section being taken across the longitudinal axes of the coils.

FIG. 8c is a top view of a coreless, low-cogging, armature for an X-Y motor according to an embodiment of the invention.

FIG. 9 is a plan view of the base showing the permanent magnets, and schematically indicating the motors and air bearings that support the stage above the base.

FIGS. 10a and 10b are section and plan views of the base showing the permanent magnets.

FIG. 11 is a section view of an assembly for making the base with the encoder plate.

FIGS. 12a, 12b, and 12c are plan views comparing three different arrangements of reflective and non-reflective regions to form grid encoder scales according to respective embodiments of the invention.

FIGS. 13a, 13b, and 13c are plan views of permanent magnet shapes/arrangements to produce the stationary field with which the motor in the stage interacts to provide motive force to move the stage with respect to the base.

FIGS. 14a, 14b, and 14c are section views of various methods of forming arrays of permanent magnets for the base.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 2a, an X-Y traversing system 1000 has a base 100, with a stage 101 supported on base 100 by one or more air bearings A1, A2, and A3. Stage 101 has a built-in motor (not shown) 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 1000 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. 2b and 2c, 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 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. 2a 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 the reticle 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, it 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 spacing of reflective regions 120A. 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 oil 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 and 8b. For now, please not 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 s