Air bearing for a motion system

An improved air bearing for a precision motion system is herein disclosed. The bearing comprises an insert that is secured within a passageway of a bearing member. The passageway is fluidly coupled to a pressurized fluid source such as air. When the fluid source is activated, pressurized fluid is delivered to the passageway. An orifice in the insert permits the fluid to escape from the passageway and form an air film between the bearing member and an opposed guiding surface. The orifice is placed immediately adjacent to the bearing surface so that the air volume forming the air film is minimal. The improved air bearing eliminates the conventional bearing pad used with other motion systems. Accordingly, the major system components are made from one material such as granite. A unitary material permits the system to remain dimensionally stable even when subjected to wide variations in temperature.

TECHNICAL FIELD
 This invention relates generally to air bearings and, more particularly, to
 an air bearing used to support a massive moving table in a high precision
 motion system.
 BACKGROUND
 The instant invention is intended for use with precision motion systems
 such as coordinate measuring machines ("CMM"), large machine tools,
 semiconductor manufacturing equipment (e.g., mask alignment) and the like.
 Generally, these systems utilize a movable table having a precision ground
 and lapped working surface which slidably engages one or more stationary
 guideway surfaces for horizontal movement. While permitting longitudinal
 motion, vertical and lateral displacement of the table are substantially
 restricted. In many applications, these systems use servo-controlled drive
 systems or the like to permit precise linear positioning of the table.
 To ensure accurate table movement, a bearing system is provided. In smaller
 systems, conventional linear bearing assemblies (e.g., roller bearings)
 provide adequate support without introducing excessive rolling resistance
 (friction). However, with larger motion systems having massive tables,
 conventional mechanical bearings are insufficient due to their large size
 and significant rolling resistance. To overcome these problems, air
 bearings are frequently used.
 In the context of a motion system, an air bearing is generally a series of
 bearing "pads" which sit between the table and the guideways. Each pad has
 a backside which operatively couples to a slide portion of the table by
 fastening, vacuum coupling, adhesive or other acceptable means. The pad
 further has a face side which forms the bearing surface. The face side
 includes one or more openings or "pockets" oriented normal to the guideway
 surface. The pocket is generally coupled to a pressurized air source such
 that, when the air source is activated, pressurized air is delivered
 thereto. To create a relatively stiff bearing, it is advantageous to
 restrict the air flow through the pocket. This is typically accomplished
 with a restricting orifice located within the bearing pad. Once the pocket
 becomes pressurized, air escaping from the pocket to atmosphere creates an
 air film between the bearing face and the guideway surface. This air film
 permits the table to "float" and move relatively friction-free along the
 guideways.
 While one group of bearings "lifts" the table, another bearing or group of
 bearings provides a downward force to oppose or "preload" the lift
 bearing. Alternatively, the lift bearing may be preloaded by utilizing
 spring-loaded means to couple the pad to the table slide. Counteracting
 side bearings are also provided to limit lateral table motion. By
 adjusting the pocket size, the number of pockets and pads and the air
 pressure, tables of most any size and weight can be adequately supported
 and guided. In addition, because air bearings are non-contacting,
 frictional forces are minimal. An example of a motion system that utilizes
 air bearings is shown in U.S. Pat. No. 4,234,175 issued to Sato et al. on
 Nov. 18, 1980.
 One problem inherent with air bearings is the compressibility of the air
 medium. To produce a stiffer bearing, it is advantageous to minimize the
 bearing clearance (distance between the bearing pad and the guideway
 surface) as this reduces the volume of compressible air separating the
 components. However, decreasing the bearing clearance requires that the
 bearing pad, guideways, and table be machined and aligned to more exacting
 tolerances. Otherwise, the varying clearances between the moving
 components may result in unintended contact between the moving table and
 the guideway (i.e., "crashing"). Therefore, the air bearing system
 designer is often required to sacrifice bearing stiffness (increase
 bearing clearance) in order to maintain reasonable machining and assembly
 tolerances.
 While conventional air bearings are more than adequate for many
 applications, problems remain. One problem in particular is attributable
 to the dissimilarity of the bearing material relative to the other system
 components. For example, the table and various other components are, in
 some systems, constructed of granite or diabase. These materials are
 preferred because they are thermally stable (i.e., they have a relatively
 low coefficient of thermal expansion or CTE) and they have excellent
 vibration damping characteristics. However, the bearing material itself is
 frequently metallic. As such, its CTE is much higher. The higher CTE
 results in the bearing pads expanding and contracting at a different rate
 than the granite. This can result in unintended restriction or expansion
 of the bearing clearance. If unaccounted for, this expansion can cause
 contact between the bearings and guideways and adversely affect the
 accuracy of the table position. Additionally, if localized temperature
 increases are experienced, the bearings may expand differentially, causing
 the table to shift and potentially crash into the guideways. Metallic
 bearing pads are furthermore subject to corrosion and thus may require
 periodic inspection and replacement.
 Another problem inherent with conventional air bearings is the pocket
 itself. While the pocket provides a larger area over which air is
 distributed, it also increases the volume of compressible air supporting
 the table. Accordingly, the pocket limits the maximum bearing stiffness.
 One method used to reduce or eliminate the dissimilar material problem
 discussed above is to eliminate the bearing pad altogether and incorporate
 the air bearing directly into the granite members. That is, passageways
 drilled in the granite couple the air source to bearing pockets formed in
 the granite itself. While the pocket diameter may be used to restrict the
 air flow, it is often of large diameter due to the drill size required to
 adequately form the pocket. Accordingly, the system may incorporate a plug
 having a small orifice thereon wherein the plug is inserted into each
 opening such that it is recessed from the bearing surface. Although such
 integral air bearings eliminate potential CTE mismatch and corrosion
 problems, the air column formed within the pocket still limits the maximum
 stiffness of the bearing.
 Thus, there are unresolved issues with current motion system air bearings.
 What is needed is a bearing system for precision motion systems that
 permits minimal bearing clearance and improved bearing stiffness while
 preventing contact between the table slides and guideways. What is further
 needed is a bearing system that is capable of maintaining the desired
 bearing clearance regardless of temperature variations.
 SUMMARY OF THE INVENTION
 An insert for use in air bearing systems is provided, comprising a
 generally cylindrical body having a length and an outer diameter wherein
 the body has a first end and a second end. The first end has a blind hole
 formed therein where the hole has a depth and a hole diameter. The insert
 further comprises a bearing face formed on the second end and an orifice
 extending from the bearing face to the hole.
 In another embodiment, an air bearing insert for use with a bearing member
 is disclosed wherein the bearing member has a first bearing surface and an
 opening for receiving the insert. The insert comprises a generally
 cylindrical body having a length and an outer diameter wherein the body
 has a first end and a second end. The first end has a blind hole formed
 therein where the hole has a depth and a hole diameter. A bearing face is
 formed on the second end and an orifice extends from the bearing face to
 the hole wherein the bearing face is generally coplanar with the first
 bearing surface when the insert is installed.
 In another embodiment, a bearing member for use with a precision motion
 system is disclosed wherein the bearing member comprises: a first bearing
 surface adapted to engage a first guiding surface; a fluid passageway
 extending through the bearing member and terminating at the first bearing
 surface; and an air bearing insert located within the fluid passageway.
 The air bearing insert comprises: a generally cylindrical body having a
 length and an outer diameter wherein the body has a first end and a second
 end; the first end having a blind hole formed therein, the hole having a
 depth and a hole diameter; a bearing face formed on the second end; and an
 orifice extending from the bearing face to the hole. The insert is adapted
 to fit within the fluid passageway such that the bearing face is
 substantially coplanar with the first bearing surface.
 In still yet another embodiment, a precision motion system is disclosed
 comprising: a moving table having one or more slide members coupled
 thereto, the slide members each defining one or more bearing surfaces; a
 base having one or more guideways coupled thereto wherein the base and
 guideways have one or more guiding surfaces adapted to guide the one or
 more bearing surfaces and permit movement of the table in a first
 direction; and an air bearing system adapted to permit movement of the
 table relative to the base. The air bearing system comprises at least one
 fluid passageway within the slide member and at least one air bearing
 insert located within the fluid passageway. The insert comprises: a
 generally cylindrical body having a length and an outer diameter wherein
 the body has a first end and a second end, the first end having a blind
 hole formed therein, where the hole has a depth and a hole diameter; a
 bearing face formed on the second end; and an orifice extending from the
 bearing face to the hole. The insert is adapted to fit within the fluid
 passageway such that the bearing face is substantially coplanar with the
 bearing surface.
 Advantageously, the present invention provides an improved air bearing
 system for use with precision motion systems. In particular, the instant
 invention eliminates the separate bearing pad found on conventional
 systems and integrates the bearing directly into the primary components of
 the motion system itself. As such, CTE mismatch attributable to different
 materials is eliminated. Furthermore, the integral bearing system is not
 subject to the corrosion which is often a problem with conventional
 metallic pads. In addition, the bearing insert provided with the present
 invention places the restriction orifice immediately adjacent to the
 bearing surfaces. As such, the volume of air which forms the bearing
 interface is significantly reduced. This provides a dynamically stiffer
 bearing, which provides more accurate table positioning at lower supply
 pressures.

DETAILED DESCRIPTION OF THE EMBODIMENTS
 In the following detailed description of the embodiments, reference is made
 to the accompanying drawings which form a part hereof, and in which are
 shown by way of illustration specific embodiments in which the invention
 may be practiced. It is to be understood that other embodiments may be
 utilized and structural changes may be made without departing from the
 scope of the present invention.
 The air bearing of the instant invention will be described with respect to
 a massive precision motion system. Precision motion systems are used in
 various applications including CMM, large machine tools, and the like. An
 exemplary embodiment of a generic motion system 100 is shown in FIG. 1.
 The system includes a base plate 102 which supports one or more guideways
 104. The guideways receive a table 106 having bearing members or slides
 108. An air bearing system (described below) coupled to the slides 108
 allows the table 106 to move relatively friction-free along the guideways
 104 in a longitudinal direction 105. The table includes a precision table
 surface 107 which serves as the tooling or working surface. While table
 motion may be controlled manually, it is, in one embodiment,
 servo-controlled through the use of servo motors, servo hydraulics, or the
 like. By precisely controlling the tolerances and relative alignment of
 the table, slides, guideways and base plate, extremely accurate and
 repeatable positioning of the table may be achieved along its entire range
 of travel.
 While the components of the motion system can be made from numerous
 materials, the base plate 102, table 106, slides 108, and guideways 104
 are constructed from granite or diabase in one embodiment. Although
 different from a geological perspective, for purposes of this discussion
 the term "granite" will be used to refer generically to diabase, granite,
 or other similar materials suitable for producing extremely flat surfaces.
 Granite is advantageous over other metallic and non-metallic materials for
 several reasons. In particular, it is thermally stable and has a very low
 coefficient of thermal expansion (CTE). Thus, when subjected to thermal
 stresses, it experiences little dimensional change. In addition, granite
 possesses superior vibration damping characteristics. While beneficial in
 these respects, granite is also very dense and therefore heavy. As such,
 conventional mechanical bearings are unable to adequately support the
 table without contributing excessive rolling friction. Accordingly, the
 present invention uses air film bearings or "air bearings."
 Referring to FIGS. 2-5, an air bearing system 200 according to one
 embodiment of the instant invention will be described. FIG. 2 shows the
 motion system 100 from an end section view. FIG. 3 is an enlarged view of
 a portion of FIG. 2 showing the slide 108 and the guideway 104. While only
 one slide/guideway is shown, the other is substantially a mirror image
 unless otherwise noted herein. The air bearing system 200 comprises a
 plurality of bearing inserts 202. The inserts are located in openings 204
 in the slides 108 at various locations as further discussed below. Fluidly
 coupled to each opening 204 is a passageway 206 extending longitudinally
 along the slide 108. The passageways 206 are bored through the actual
 slides 108 so that a second opening (not shown) is provided at the end of
 each slide for each passageway. The second opening is adapted to couple to
 a pressurized fluid source such as compressed air. The term "bearing" is
 used herein to generally indicate the opening 204 and the insert therein.
 Referring particularly to FIG. 3, the slide 108 is rigidly coupled to the
 table 106 by a plurality of fasteners 110. While the fastener 110 is shown
 as bolted through the table to the slide, alternative embodiments may bolt
 oppositely (i.e., the fastener 110 may extend through the slide 108 and
 engage threads in the table 106). The guideways 104 are similarly attached
 to the base 102. Each pair of slides 108 is precisely machined, lapped and
 aligned such that, when assembled to the table 106, the table surface 107
 is substantially parallel to a first bearing or slide surface 302 of the
 slide 108 which is, in turn, substantially parallel to a first guiding
 surface 304 of the base 102. Similarly, the guideway 104 is precisely
 machined and aligned such that a second guiding surface 306 is
 substantially parallel to a second bearing or slide surface 308 of the
 slide 108. The guideway 104 includes a third guiding surface 310 which is
 machined and aligned to be substantially parallel to a third bearing or
 slide surface 312. Accordingly, the table, slides, guideways, and base are
 machined and lapped to precise tolerances and aligned to permit table
 motion in substantially only one direction.
 Referring still to FIG. 3, to support the table during motion, the air
 bearing system 200 provides pressurized air through the passageways 206
 and ultimately to the bearing inserts 202. The air passes through the
 inserts and impinges upon the respective guiding surfaces 304, 306, and
 310. To support the weight of the table, pressurized air is supplied via
 passageways 206a and 206b to the "lift" inserts 202a and 202b
 respectively. It is noted that while only two lifting bearing inserts
 202a, 202b are shown in FIG. 3, there are actually several inserts placed
 at intervals along the length of the guides 108. For example, one
 embodiment utilizes a bearing insert 202a, 202b every two to three inches
 along the longitudinal length of the slide 108. In another embodiment,
 more than two rows of bearing inserts 202 are placed across the surface
 302. However, the actual quantity and placement of the bearings depends on
 several factors including table weight, air pressure, bearing insert
 design, and desired air film thickness, among others.
 When the pressure of the air in the passageway reaches a particular level,
 the air escaping to the atmosphere forms a thin air film between the
 surfaces 302 and 304. This air film has a thickness 314 which is
 controlled by, among other factors, the magnitude of the pressurized air.
 While the stiffness of an air bearing can be maximized by minimizing the
 air film thickness 314 or "flying height," even slight non-parallelism and
 surface imperfections in the surfaces 302 and 304 may require an increase
 in flying height to prevent unintended contact between the components.
 Accordingly, the flying height is set to accommodate the worst-case
 tolerance.
 To further increase the stiffness of the air bearings, the insert 202c is
 provided. The insert 202c is fed by the passageway 206c and operates in a
 manner substantially identical to the inserts 202a and 202b. The purpose
 of the insert 202c is to preload the air bearings 202a, 202b. That is, the
 insert 202c provides a constant force opposite the lifting force provided
 by the bearings 202a and 202b. This constant preload provides additional
 stiffness to the air bearing system. Like the bearing inserts 202a, 202b,
 the bearing insert 202c develops an air film having a thickness 316
 between the surfaces 310 and 312.
 The air bearing system 200 also includes side bearings to prevent lateral
 motion of the table 106. The side bearing includes a bearing insert 202d
 and passageway 206d and operates in a manner substantially identical to
 the bearings 202a, 202b, and 202c. By providing an identical side bearing
 on the opposite guide, the side bearings are also preloaded relative to
 one another. Like the other bearings discussed above, the side bearing
 develops an air film of thickness 318 between the surfaces 306 and 308.
 Thus, the air bearing system provides support for longitudinal motion of
 the table while restricting lateral and vertical motion. Each bearing
 within the system includes a counteracting bearing to provide a constant
 preload, improving the relative stiffness of the bearing system in all
 directions.
 Having described the overall construction of the system 100 in some detail,
 attention will now be focused on the bearing insert 202 itself. Referring
 specifically to FIGS. 4-5, the bearing insert 202 is illustrated in
 accordance with one embodiment of the invention. The insert has a
 cylindrical body having an outer diameter 220 and a length 222. The
 bearing insert, at a first end, has a blind hole 224 of hole diameter 226
 and depth 221 wherein the blind hole, in one embodiment, terminates at a
 conical-shaped bottom 228 of angle 230. Extending from a second or bearing
 face end 231 to the blind hole 224 is an orifice 232 of diameter 234 and
 length 222. In one embodiment, the bearing insert is machined from brass.
 However, inserts of other materials are also possible without departing
 from the scope of the invention.
 Referring again to FIG. 3, when the bearing insert 202 is installed, it is
 positioned such that the face end 231 is substantially flush with the
 respective guide surfaces 302, 308, and 312 (e.g., inserts 202a, 202b are
 flush with surface 302, insert 202c is flush with surface 312, etc). This
 positions the orifice 232 immediately proximal to the respective guiding
 surfaces 304, 306, and 310. Thus, unlike known air bearings, there is no
 "pocket" or air column between the orifice restriction and the bearing
 surface. Elimination of the pocket reduces the volume of compressible air
 defining the air film which effectively increases bearing stiffness.
 Furthermore, by adjusting the orifice diameter 234 and length 222, the
 pressure drop across the orifice can be accurately controlled.
 The insert 202 is secured in the opening 204 by an interference fit. To
 ensure the insert is adequate affixed, the insert body may include a
 knurled texture 236 that deforms as the insert is pressed into the granite
 slide 108. In another embodiment, the insert may be first coated with an
 adhesive. In still yet another embodiment, the insert may be installed by
 a shrink-fit.
 While the exact geometry of the insert 202 is not perceived to be critical,
 one embodiment provides an outer diameter 220 of approximately 0.50
 inches, a length 222 of approximately 0.5 inches, and a hole diameter 226
 of approximately 0.25 inches. This yields an aspect ratio of outer
 diameter to body length of one. The orifice 232 has a diameter 234 of
 approximately 0.008 inches. This yields an aspect ratio of hole diameter
 226 to the orifice of approximately thirty. To reduce losses across the
 restriction orifice 232, the orifice length 235 is, in one embodiment,
 approximately 0.080 inches. To maintain smooth flow through the orifice,
 the angle 230 is approximately 118 degrees. The reader is reminded that
 the embodiment described is exemplary only and inserts of other sizes and
 shapes may certainly be used without departing from the scope of the
 invention.
 When making the motion system of the instant invention, conventional
 machining and lapping operations are used to make the granite components
 (table, slides, guideways, and base). In one embodiment, linear
 dimensional tolerances of the slides 108 and guideways 104 vary from 3-10
 microns while geometric tolerances (squareness, flatness, parallelism) are
 held to a maximum of 2 microns. The slides 108 are precisely aligned with
 and fastened to the table 106 while the guideways 108 are likewise secured
 to the base 102. To ensure accurate assembly, conventional alignment
 methods utilizing such equipment as laser aligners, autocollimators, and
 electronic levels are used.
 The granite components (base 102, guideways 104, table 106, and slides 108)
 are lapped to provide smooth, flat surfaces. Prior to lapping the slides
 108, the bearing inserts 202 are first installed such that the face 231
 protrudes slightly beyond the bearing surfaces 302, 308, and 312. The
 lapping process then ensures that the insert face 231 is made planar with
 the respective slide surfaces. To prevent plugging of the orifice 232
 during the lapping process, orifice drilling may be delayed until
 completion of lapping. Alternatively, if the orifice does plug, it may be
 re-drilled after lapping.
 During operation, pressurized fluid is supplied to the air bearing system
 200. Referring to FIG. 6, an air control system 400 capable of regulating
 air flow to the air bearing system will be described in accordance with
 one embodiment of the invention. A conventional air compressor 402
 supplies the pressurized air to various circuits each feeding different
 bearings 202 via the respective passageways 206. To more accurately
 control air film thickness, a regulator 404 may be utilized for each
 circuit. An air gage 406 may also be provided to indicate the actual
 pressure setting of the regulator 404. In one embodiment, a first
 regulator 404a provides pressurized air to both the passageways 206a and
 206b on both the left slide 108L and the right slide 108R. This provides
 even lift to the table 106. The passageways 206c of each slide 108 are
 respectively coupled to a second and third regulator 404b and 404c while
 the passageways 206d of each slide 108 are respectively coupled to a
 fourth and fifth regulator 404d and 404e. Accordingly, the first regulator
 404a controls lift pressure evenly on both slides while the second and
 third regulators 404b and 404c control preload pressure independently on
 each slide. Finally, fourth and fifth regulators 404d and 404e control
 side bearing pressure independently for each slide 108. Independent
 adjustment of each regulator 404 allows the system to be precisely and
 accurately adjusted. While shown with five regulators, other embodiments
 utilizing other numbers of regulators and other air circuit configurations
 are also possible. For example, one regulator may feed all the passageways
 206 such that each passageway is at an identical air pressure.
 The air bearing system of the instant invention provides improved table
 positioning accuracy over conventional metal or other "integrated" granite
 bearings. This improved accuracy is furthermore accomplished at relatively
 low air pressure. In one embodiment, a table 48 inches wide and 96 inches
 long having a longitudinal travel of 96 inches maintains roll (rotation
 about the longitudinal axis), pitch (rotation about the transverse axis)
 and yaw (rotation about the vertical axis) within three arc-seconds. This
 is accomplished at a bearing supply pressure of 40-45 psi.
 While the invention has so far been described with reference to a sliding
 horizontal table, other embodiments are also possible. For example, a
 horizontal slide assembly 700 as shown in FIG. 7 may also incorporate the
 bearing system as described herein. The slide assembly 700 may be used
 either independently or in conjunction with a positioning system such as
 that shown in FIG. 1. For an example of the latter, attention is directed
 to FIG. 11 where the slide assembly 700 is mounted to the base 102 to
 permit sliding, horizontal movement lateral to the table motion as
 indicated by directional arrows 705. Thus, accurate positioning is
 achieved along two axes.
 Referring now to FIGS. 7-10, the slide assembly 700 comprises a plurality
 of slide plates 702 which are fastened together to form a box guide 704.
 Like the components of the motion system 100, the plates are precisely
 machined/lapped so that opposing plates remain substantially parallel. In
 sliding engagement with the box guide 704 is a guide member 706. Both the
 plates 702 and the guide member 706 may be made of granite or another
 suitable material.
 Passing longitudinally through each plate 702 is one or more passageways
 708. Like the passageways 206, the passageways 708 provide fluid coupling
 between a pressurized air source (not shown) and the bearing inserts 202
 (see FIG. 10). The inserts 202 are installed in a manner similar to that
 already described herein. Accordingly, when adequately pressurized, an air
 film forms between the box guide 704 and the guide member 706, allowing
 the two components to move relative to each other unimpeded by friction.
 Advantageously, the present invention provides an improved air bearing
 system for use with precision motion systems. In particular, the instant
 invention eliminates the separate bearing pad found on conventional
 systems and integrates the bearing directly into the primary components of
 the motion system itself. As such, CTE mismatch attributable to different
 materials is eliminated. Furthermore, the integral bearing system is not
 subject to the corrosion which is often a problem with conventional
 metallic pads. In addition, the bearing insert provided with the present
 invention places the restriction orifice immediately adjacent to the
 bearing surfaces. As such, the volume of air which forms the bearing
 interface is significantly reduced. This provides a dynamically stiffer
 bearing, which provides more accurate table positioning at lower supply
 pressures.
 Preferred embodiments of the present invention are described above. Those
 skilled in the art will recognize that many embodiments are possible
 within the scope of the invention. Variations, modifications, and
 combinations of the various parts and assemblies can certainly be made and
 still fall within the scope of the invention. Thus, the invention is
 limited only by the following claims, and equivalents thereto.