Abstract:
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.

Description:
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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention described herein will be further characterized with reference to the drawings, wherein: 
     FIG. 1 is a perspective view of a motion system in accordance with one embodiment of the invention; 
     FIG. 2 is an partial section view of the motion system of FIG. 1 taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is an enlarged partial view of a portion of the motion system of FIG. 2; 
     FIG. 4 is an end elevation view of an air bearing in accordance with one embodiment of the invention; 
     FIG. 5 is a section view of the air bearing of FIG. 4 taken along line  5 — 5  of FIG. 4; 
     FIG. 6 is a diagrammatic view of a pressure system for use with the bearing system of FIG. 3; 
     FIG. 7 is a perspective view of a motion system constructed in accordance with another embodiment of the invention; 
     FIG. 8 is a top plan view of the motion system of FIG. 7; 
     FIG. 9 is a sectional view of the motion system of FIG. 7 taken along line  9 — 9  of FIG. 8; 
     FIG. 10 is an enlarged partial view of a portion of the motion system of FIG. 9; and 
     FIG. 11 is a perspective view of a motion system constructed in accordance with yet another embodiment of the invention. 
    
    
     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  206   a  and  206   b  to the “lift” inserts  202   a  and  202   b  respectively. It is noted that while only two lifting bearing inserts  202   a ,  202   b  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  202   a ,  202   b  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  202   c  is provided. The insert  202   c  is fed by the passageway  206   c  and operates in a manner substantially identical to the inserts  202   a  and  202   b . The purpose of the insert  202   c  is to preload the air bearings  202   a ,  202   b . That is, the insert  202   c  provides a constant force opposite the lifting force provided by the bearings  202   a  and  202   b . This constant preload provides additional stiffness to the air bearing system. Like the bearing inserts  202   a ,  202   b , the bearing insert  202   c  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  202   d  and passageway  206   d  and operates in a manner substantially identical to the bearings  202   a ,  202   b , and  202   c . 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  202   a ,  202   b  are flush with surface  302 , insert  202   c  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  404   a  provides pressurized air to both the passageways  206   a  and  206   b  on both the left slide  108 L and the right slide  108 R. This provides even lift to the table  106 . The passageways  206   c  of each slide  108  are respectively coupled to a second and third regulator  404   b  and  404   c  while the passageways  206   d  of each slide  108  are respectively coupled to a fourth and fifth regulator  404   d  and  404   e . Accordingly, the first regulator  404   a  controls lift pressure evenly on both slides while the second and third regulators  404   b  and  404   c  control preload pressure independently on each slide. Finally, fourth and fifth regulators  404   d  and  404   e  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.