Abstract:
A preload hydrostatic bearing includes a pad, a diaphragm and an adjustable member. The pad has a bearing structure, an inlet manifold, and a plurality of orifices. The orifices direct a fluid, such as air, from the inlet manifold toward the bearing surface. The diaphragm is mounted on the pad, and the adjustable member, which extends in an axial direction, is coupled proximate one end to a center portion of the diaphragm. The diaphragm transfers a preload in the axial direction to the adjustable member. This preload hydrostatic bearing has a high repeatability of performance, because a single diaphragm replaces prior art mechanical coupling devices, such as ball bearings, conical seats and spring washers, which undesirably are sources of friction and hysteresis.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to bearings. More particularly, the present invention relates to a preload hydrostatic bearing with a diaphragm for improved performance. 
     2. Description of the Related Art 
     X-Y stage systems are typically used in machine tools and other applications where two-dimensional precision movement is required to position an object supported on the stage. A typical X-Y stage system includes a pair of parallel-spaced guide rails and a stage with at least one fixed air bearing at one end and a corresponding preload air bearing at the other end. The fixed and preload air bearings ride along respective guide rails to move the stage therealong. The preload air bearing provides a constant force to the fixed air bearing and maintains a constant air gap or flying height in the fixed air bearing. 
     Because it is difficult for guide rails of stage systems to be perfectly uniform, a preload air bearing must compensate for variations in the guide rails, due to thermal growth or other causes, while providing a constant force to the fixed air bearing. Conventional air bearings utilize mechanical preload devices including combinations of ball bearings, conical bearing seats and spring washers, such as Belleville washers, to compensate for rail variations. Examples of these air bearings may be found in U.S. Pat. No. 4,191,385, issued Mar. 3, 1980 to Fox et al. and U.S. Pat. No. 4,882,847, issued Nov. 28, 1989 to Hemmelgarn et al. FIG. 1 illustrates one such prior art preload air bearing  100 . Bearing  100  includes a pad  102  having a bearing surface  103 . Pad  102  is coupled to a cap  104 . Bearing pad  102  is made of a porous material, such as graphite. In the alternative, pad  102  may have a plurality of orifices formed therein. Cap  104  has an internal space for receiving a compressed gas, such as air, from an external source. The compressed gas flows through cap  104  and pad  102  to create an air film between bearing surface  103  and a rail surface (not shown) on which bearing  100  rides. A ball  106  which is received in a seat  144  supports bearing cap  104 . A spring washer  148 , or stack of spring washers, supports seat  144  and ball  106 . Washer  148  is secured on a boss  150  at one end of a preload pin  146 . The arrangement of ball  106 , seat  144  and spring washer  148  allows bearing cap  104  and pad  102  to tilt and accommodate slight variations in the rail surface. The air film gap may be altered by adjusting the position of preload pin  146 . 
     One problem with conventional air bearings, however, is their inability to supply a constant preload. A small change in the uniformity of the guide rails can significantly alter the amount of force developed in the bearing, changing the bearing flying height, which can cause instability and possibly derail the stage. These bearings are also less stiff, and the stage, therefore, is more prone to yaw. In addition, these mechanical preload devices generate a great deal of friction between the spring washers, conical bearing seat and ball bearing, which results in motion loss. Other associated problems include dynamic oscillations, such as pneumatic hammer instability, hysteresis and non-linearity. 
     One solution includes replacing the spring washers with an air cylinder, which would ensure a constant preload and eliminate the friction associated with the washers. This preload air bearing, however, still requires a ball bearing pivot, another source of friction, to compensate for any non-uniformity in the guide rails. In addition, such an air bearing may be difficult to implement due to packaging constraints. Thus, it would be advantageous to provide a preload hydrostatic bearing with a simple design that is capable of providing a constant force with minimum hysteresis to a fixed hydrostatic bearing despite variations in the guide rails. 
     SUMMARY OF THE INVENTION 
     The present invention addresses these problems by providing a preload hydrostatic bearing with a single diaphragm. The diaphragm replaces the various mechanical preload devices, such as a ball bearing, bearing seat and spring washers. The size and thickness of the diaphragm are optimized to minimize the axial and bending stiffnesses and to maximize the radial stiffness of the diaphragm. Because the diaphragm has low axial and bending stiffnesses, the diaphragm can accommodate variations in the surface of a guide rail while generating little or no friction, thereby improving the performance of the preload hydrostatic bearing. 
     In accordance with one aspect of the invention, a preload hydrostatic bearing includes a pad, a diaphragm and an adjustable member. The pad includes a bearing structure, an inlet manifold, and a plurality of orifices. The orifices direct a fluid or gas, such as air, from the inlet manifold toward the bearing surface. The diaphragm is mounted on the pad and includes a center portion. The adjustable member, which extends in an axial direction, is coupled proximate one end to the center portion of the diaphragm. The diaphragm transfers a preload in the axial direction to the member. 
     In accordance with another aspect of the invention, a hydrostatic bearing stage system includes a pair of guide rails and a stage movable therealong. The guide rails include a first rail and a second rail. The stage has a first end proximate the first rail and a second end proximate the second rail. The system further includes a first hydrostatic bearing mounted on the first end of the stage and a preload hydrostatic bearing mounted on the second end of the stage. The preload hydrostatic bearing is similar to that described above. 
     In accordance with still another aspect of the invention, a method of bearing a structure on a surface includes directing a pressurized fluid onto the surface from the structure and flexibly coupling an axial member by a diaphragm to the structure. The method further includes adjusting an effective length of the axial member. This adjusting sets an amount of preload applied to the structure to urge the structure towards the surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
     FIG. 1 is a front elevational view of a prior art preload air bearing. 
     FIG. 2 is a schematic view of an X-Y stage system including a preload gas bearing in accordance with the present invention. 
     FIG. 3 is a top plan view of the preload gas bearing of FIG.  2 . 
     FIG. 4 is an enlarged cross-sectional view taken generally along the line A—A of FIG.  3  and including a portion of the stage and guide rail of the X-Y stage system of FIG.  2 . 
    
    
     The use of the same reference symbols in different drawings indicates similar or identical items. 
     DETAILED DESCRIPTION 
     FIG. 2 illustrates a schematic view of an X-Y stage system  10  including a preload gas bearing  12  in accordance with the present invention. Stage system  10  includes a stage  14  which moves along a pair of guide rails  16  either vertically as noted in FIG. 2 or horizontally into and out of the page. Guide rails  16 , which include a master rail  18  and a follower rail  20 , are parallel-spaced with stage  14  disposed therebetween. Master rail  18  is machined to extremely high tolerances, such that it has fewer variations across its surface. Follower rail  20 , however, for cost and manufacturing purposes, is machined to a lesser degree of precision than master rail  18  and, therefore, has a larger rail surface variation  19 . 
     A pair of fixed gas bearings  22  are mounted on a first end  24  of stage  14 . Fixed gas bearings  22  support stage  14  along master rail  18  of stage system  10 . Preload gas bearings  12 , corresponding to respective fixed gas bearings  22 , are mounted on a second end  26  of stage  14 , opposite first end  24 , and support stage  14  along follower rail  20 . Further details of such X-Y stage systems may be found in U.S. Pat. No. 4,882,847, issued Nov. 28, 1989 to Hemmelgarn et al. and U.S. Pat. No. 5,257,461, issued Nov. 2, 1993 to Raleigh et al., both of which are incorporated herein by reference. Preload gas bearings  12  provide a constant force to fixed gas bearings  22  to maintain stage  14  and fixed gas bearings  22  at a constant air gap or flying height with respect to the more uniform master rail  18 . Although in the present embodiment bearings  12  and  22  are gas bearings, bearings  12  and  22  may also be other hydrostatic bearings employing fluids other than gas. 
     FIGS. 3 and 4 illustrate preload gas bearing  12  in greater detail. The main components of preload gas bearing  12  are a pad  28 , a diaphragm  30  mounted to pad  28 , and an adjustable screw (or equivalent adjustable member)  32 , the length of which determines the amount of the preload. Pad  28  is generally cylindrical in configuration and is preferably made of a material such as an aluminum alloy or stainless steel. Pad  28  has an inlet manifold  34  and a plurality of orifices  36  formed therein. Orifices  36 , as illustrated in FIG. 3, are disposed about pad  28  in a circle, however, any number and pattern of orifices may be formed in pad  28 . Inlet manifold  34  and orifices  36  direct a compressed gas, such as air, from an external source (not shown) toward a bearing surface  38  of pad  28 . Bearing surface  38  has a recess  40  formed therein at the center of surface  38 . Recess  40  enables air at the center of pad  28  to escape to the atmosphere via an outlet passage  42 . When air is introduced into inlet manifold  34  and orifices  36  of preload gas bearing  12 , an air gap  44 , on which bearing  12  rides, forms between bearing surface  38  and follower rail  20 . 
     Diaphragm  30  is mounted on a top surface  46  of pad  28  over an opening  47  formed in top surface  46 . A retaining ring  48  secures diaphragm  30  to pad  28 . In the alternative, adhesives or other mechanical fasteners, such as screws or rivets, may be used to secure diaphragm  30  to pad  28 . Diaphragm  30  is a thin, annular disk comprised of a flexible material. For example, diaphragm  30  may be made of stainless steel, beryllium copper or phosphor bronze. Diaphragm  30  has a design which minimizes the bearing&#39;s axial and bending stiffnesses while maximizing its radial stiffness. The low axial stiffness of diaphragm  30  enables preload gas bearing  12  to accommodate in the axial direction imperfections and projections along guide rails  16  with minor variations in the axial force transmitted by bearing  12 . In direct contrast, in a diaphragm with a high axial stiffness, small rail variations will produce large variations in the axial force. The low bending stiffness of diaphragm  30  provides bearing  12  with a friction-free rotational degree of freedom. The high radial stiffness ensures that pad  28  remains concentric with screw  32 . Thus, a single diaphragm  30  replaces the assembly of spring washers, bearing seat and ball bearing present in other air bearings, thereby eliminating various sources of friction and non-linearity in the system. 
     Screw  32  is coupled proximate one end to center portion  50  of diaphragm  30 . Screw  32  provides the primary load path between pad  28  and stage  14 . Screw  32  is affixed to stage  14  at the other end by a clamp  52 . Mounted on one end of screw  32 , opposite the end with clamp  52 , is a resilient energy absorbing device  54 . Energy absorbing device  54  prevents screw  32  from bottoming out on pad  28 , thereby limiting the axial displacement of screw  32  with respect to pad  28 . Energy absorbing device  54  may be made of any material, such as rubber, having a low durometer. As discussed above, the adjusted length of screw  32  extending from stage  14  determines the preload amount of gas bearing  12 , and the length is fixed prior to the operation of bearing  12 . 
     The dimensions of diaphragm  30  will vary depending upon the particular needs of the application. Factors which are considered in determining the dimensions of diaphragm  30  include the desired deflection of diaphragm  30 , the preload to be applied to gas bearing  12 , and the material, thickness and diameter of diaphragm  30 . For example, a stainless steel diaphragm, 2 in. in diameter and 0.050 in. thick, has a stiffness of approximately 46,000 lbs/in. At a preload of 250 lbs., the diaphragm will undergo an axial displacement of approximately 0.005 in. A variation of 0.0005 in. in guide rails  16  will cause a 23 lbs. force variation in the axial direction. For an air bearing with a stiffness of 400,000 lbs./in., the 23 lbs. force variation will cause a 60 micro-inch change in the flying height of the gas bearing. The optimum design of preload gas bearing  12  balances the bending stiffness of diaphragm  30  with its membrane stiffness. 
     X-Y stage system  10  with preload gas bearing  12 , therefore, operates as follows. First, prior to operation the preload of each gas bearing  12  is set by adjusting the axial length of screw  32 . A compressed gas from an external source is then directed through preload gas bearings  12  and fixed gas bearings  22  to support stage  14  on guide rails  16 . In each preload gas bearing  12 , the compressed gas travels through inlet manifold  34  and orifices  36  to bearing surface  38  of pad  28 . The compressed gas produces a distributed pressure load  56  (FIG. 4) on bearing surface  38 . The combination of the inlet pressure of the compressed gas and the bearing preload produces air gap  44  between bearing surface  38  of pad  28  and follower rail  20 . Compressed gas at the center of bearing surface  38  is vented to the atmosphere via recess  40  and outlet passage  42 . The distributed pressure load  56  on bearing surface  38  creates a net axial force on pad  28 . This axial force is transmitted through diaphragm  30  and screw  32  of preload air bearing  12 , through stage  14  and to the respective fixed gas bearing  22 . The axial force which preload gas bearings  12  apply to fixed gas bearings  22  preferably remains constant, thereby maintaining fixed gas bearings  22  at a constant flying height with respect to master rail  18 . As stage  14  moves along guide rails  16 , diaphragm  30  of preload gas bearings  12  flexes and readjusts, allowing preload gas bearings  12  to compensate for variations in the surface of follower rail  20  and to maintain the force on fixed gas bearings  22  substantially the same. 
     In summary, the diaphragm preload gas bearing of the present invention provides several advantages over prior art preload air bearings. The preload gas bearing has a simplified design, since a single diaphragm replaces the combination of a ball bearing, bearing seat and spring washers. This simplified design reduces both the parts and manufacturing costs for the gas bearing. In addition, because the bearing has only a diaphragm with no sliding interfaces, the bearing provides a significant reduction in friction for a stage system. The diaphragm preload gas bearing is more stable dynamically and reduces uncompensated stage yaw. 
     While the present invention has been described with reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.