Abstract:
A single degree of freedom positioner resistant to axial and rotational motion on two axes, and to rotation around a third axis, but permitting axial movement along the third axis. The axes are orthogonally related. The positioner is characterized by support of a stage by a plurality of stiffly flexible end supported shear webs that bend in a rolling motion to enable the said axial motion.

Description:
FIELD OF THE INVENTION  
       [0001]     A positioner which confines the movement of a motion stage to a single degree of freedom, namely linear motion along only one axis.  
       BACKGROUND OF THE INVENTION  
       [0002]     Very accurate positioning of an object such as a workpiece or a tool is a fundamental requirement of all product-shaping and deposition mechanisms. Rather large dimensional tolerances on the order of thousandths of an inch have been tolerable for many years, and for a wide range of products they still are. However, newer products, especially in the microelectronics field require that manipulations be held to much closer tolerances, often in the realm of a few microns.  
         [0003]     The problems one faces with these new requirements cannot readily be solved by the older approach of better linear guideways and bearings. These all have their own tolerance problems, which are amplified when many of them are involved in the manufacture of the same product. The result has been production equipment which, while made to the best standards attainable with existing approaches, still limit the quality of product that can be produced by them, and increase the scrap rate of a production run.  
         [0004]     A large part of this problem resides in the structural support and physical guides required to move and position the tool or the workpiece. Conventional dovetails, rails and rotary bearings all involve problems such as linear runout and backlash that can result in inaccuracies on all six degrees of freedom.  
         [0005]     This invention proposes the use of an entirety different positioner which may be thought of as a “motion stage”. It prevents motion on five of the six possible degrees of freedom, and severely restrains movement of the stage to the sixth degree. The six degrees of freedom are defined as axial motion along the X, Y and Z axes, and rotation around each of them. In this invention, the single degree of freedom enjoyed will generally be axial along the Z axis. Then axial movement along the X and Y axes, and rotation around any of the X, Y and Z axes is prevented. The Z axis is commonly regarded as the vertical axis. This terminology is used herein for convenience. However, with this invention the Z axis may be directed in other directions, for example horizontally or at an angle to the vertical.  
         [0006]     In contrast to the very costly mechanisms presently available for accurate single axis positioning, it is an object of this invention to use simple rotors and flexures that do not involve sliding motion and which inherently prevent twisting, shear, and lateral movement.  
         [0007]     It is another object of this invention to provide the positioner as a passive element which permits accurate movement of the stage only along the allowed axis in response to forces exerted along it.  
         [0008]     It is another object of this invention to provide the positioner as a product which employs simple flexures and rotors that can readily be manufactured to produce a surprisingly affordable, accurate and useful product.  
         [0009]     It is yet another object of this invention to provide a positioner that can be used in a severe vacuum, there to operate efficiently without shedding of particulates or lubricants that could foul the vacuum environment. It inherently does not require lubrication, for example.  
         [0010]     It is still another object of this invention to provide guidance for a system in which relatively crude motion means can be used, still to provide such accuracy. For example, an inflatable bladder can be used.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0011]     This invention controls the movement of a stage which provides a reliable base for a workpiece or a tool, limiting the movement of itself and of what it supports to a single degree of freedom. It employs the in-plane stiffness of stiffly flexible straps engaged to a rotor and to a reference surface, there being at least three of said rotors arranged in plan view as sides of a polygon, preferably a right quadrilateral. The straps are provided in sets, each set having a pair of said straps. The straps have a substantial dimension of width so as to provide a sufficient area for in-plane stiffness.  
         [0012]     In the preferred embodiment of the invention, the rotor is restrained only by the straps, the straps of each set being respectively connected to the stage and to the rotor, and to the base and to the rotor, but without contact of the rotor with other structures.  
         [0013]     In a variation of the preferred embodiment, the straps of each set are bent in a U-shape around a cylindrical surface on the rotor such that the two straps form part of a single length.  
         [0014]     In an alternate embodiment of the invention, the rotor is mounted by a bearing to the base, and both of the straps are engaged to the rotor and to the stage.  
         [0015]     It is a feature of this invention that tilting and lateral relative movement of the base and stage are prevented by the in-plane stiffness of the straps, while permitting free and frictionless relative movement of the base and stage.  
         [0016]     The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a plan view of the presently preferred embodiment of the invention;  
         [0018]      FIG. 2  is a cross-section taken at line  202  in  FIG. 1 ;  
         [0019]      FIG. 3  is a cross-section similar to  FIG. 2 , showing the device in another adjusted position;  
         [0020]      FIGS. 4 and 5  are fragmentary cross-sections showing the rotation arrangement of two of the flexures;  
         [0021]      FIG. 6  is a section similar to  FIG. 4  showing the movement of the flexures when the stage rises;  
         [0022]      FIGS. 7 and 8  are fragmentary end views, partly in cross-section showing a different mounting for the rotor;  
         [0023]      FIG. 9  is a perspective view of a single piece flexure shaped to provide flexures for any embodiment of this invention;  
         [0024]      FIG. 10  is a side view of a rotor with different means to mount the rotor;  
         [0025]      FIG. 11  is a cross-section taken at line  11 - 11  in  FIG. 10 ; and  
         [0026]      FIG. 12  is a cross-section taken at line  12 - 12  in  FIG. 10 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     A positioner  20  according to this invention includes a base  21  and a stage  22 . The base is established on structure (not shown) such as on a machine foundation, movable in the X Y plane. The stage is parallel to the base.  
         [0028]     The stage is intended to move along Z axis  23  (in the drawings perpendicular to the plane of  FIG. 1 ). Usually the Z axis will be directed vertically, but it is equally possible for it to be other than vertical. For example the XY plane might be tilted relative to the horizontal, or may even be perpendicular to it.  
         [0029]     It is the function of this invention to move the stage along the Z axis, without lateral movement in the X Y plane and without rotation around any of the X, Y and Z axes. Accordingly anything mounted to the stage can reliably be moved axially along the Z axis, without twist, tilt or yaw. Surprisingly, utilizing the very simple construction of this invention, the lateral deflection from the Z axis of an object intended to be confined to the Z axis can be held to within 1 micron TIR, and this with coarse motion means driving the stage along the Z axis.  
         [0030]     In the preferred embodiment this is accomplished without fixed bearings of any kind. Instead of reliance on fixed bearings or slides for accuracy, this invention utilizes only the in-plane stiffness of edge-supported flexures, as will now be shown. An advantage is that lubrication is not required. There is no sliding motion.  
         [0031]     Four base elements  25 ,  26 ,  27 ,  28  are disposed orthogonally around the stage. Because they are all identical, only element  25  will be described in detail. Suffice it to say that base elements  25  and  27  are aligned along the X axis, and base elements  26  and  28  are aligned along the Y axis.  
         [0032]     The stage carries four stage elements  30 ,  31 ,  32  and  33 , respective to base elements  25 ,  26 ,  27 ,  28 .  
         [0033]     The base carries four base reference surfaces  35 ,  36 , 37  and  38 , respective to base elements  25 ,  26 ,  27 ,  28 . The stage carries four stage reference surfaces  39   a ,  39   b ,  39   c  and  39   d , respective to stage elements  30 ,  31 ,  32 , and  33 .  
         [0034]     As best shown in  FIGS. 2-5 , the base reference surfaces are planar. They lie in planes parallel to the central axis. In pairs with opposed stage reference surfaces, they are laterally spaced apart from one another.  
         [0035]     Rotors  40 ,  41 ,  42  and  43  are placed between opposed reference surfaces. These rotors have external contact segments  40   a ,  41   a ,  42   a , and  43   a  which, if not fully cylindrical, are axially extending axial segments of only one cylinder. In fact, because within the range of anticipated usage, rotation of only a few degrees will be usual, only a small cylindrical segment is needed.  
         [0036]     Contact segments  40   a - 43   a  are centered on respective rotational axes  45 ,  46 ,  47  and  48  to form a polygonal array as viewed in plan. These axes are normal to central axis  23  and lie in respective imaginary planes (not shown), that are normal to axis  23 . Although it is not necessary, the cylindrical axes will either lie on the same imaginary plane, or in planes quite close to one another.  
         [0037]      FIGS. 1-6  illustrate one embodiment of the suspension of the rotors. In this embodiment there will be three sets of straps along each axis so as to provide a better restraint against axial creep of the rotor, which over time can sometimes occur. These are interspaced to provide angular support for the rotor. In other embodiments, only two sets (supports) are needed. It will be observed that the rotor is supported only by the sets of straps. There is no bearing support for the rotor. This is a “free” rotor.  
         [0038]     Because the suspensions are all identical, only the assembly at base element  25  will be described in detail. Suspensions  50 ,  51  and  52  are disposed side-by-side along rotor  40 . These suspensions are made of stiffly flexible flat material which is inextensible. Thin sheet foil steel, for example about inch wide and between about 0.005 and 0.020 inches thick is suitable. Because of its stiffness, when a sheet of it is edge restrained, it will resist shear and in-plane distortion.  
         [0039]     Each suspension comprises a central bight and a pair of straps (in this embodiment sometimes called a “sling”). For example, suspension  50  ( FIG. 4 ) has a central bight  55  and straps  56 ,  57 . It will be noted that bight  55  is on the under side of the roller. The same arrangement pertains to suspension  52 .  
         [0040]     Suspension  51  ( FIG. 5 ) is disposed between suspensions  50  and  52 . It has a central bight  60  and two straps  61 , 62 . It will be noted that its bight is on the upper side of rotor  40 . Rotor  40  is thereby trapped between the two suspensions.  
         [0041]     The outer diameter of the rotor (its own contact surface) is equal to the spacing between the reference surfaces, less twice the thickness of the straps. It may be slightly less. In some uses, it may be desirable to fix the straps to the rotor. A local tack weld between the bights and the rotor is sufficient.  
         [0042]     Often, a direct compression of the rotor will be undesirable. Then, as shown in  FIG. 10 , the reference surface can be recessed by a slot, and the rotor and that portion of the strap in contact with it will be free from direct contact with the reference surface, while the end portions of the strap do make contact with it.  
         [0043]     As can be seen in  FIGS. 3 and 6 , raising the stage relative to the base causes one of the straps of each suspension to shorten, and the other to lengthen equally. Lowering the stage causes a reverse exchange of strap length.  
         [0044]     The assembly of rotor and suspension is assembled very precisely, so that the ends of the straps are tightly held to respective reference surfaces. A convenient technique is shown in  FIG. 4 , where end  70  of a strap is wrapped tightly over a structural edge to hold it in place. It can be clamped, or tack welded or otherwise smoothly held to the structure.  
         [0045]     In every embodiment of this invention, the suspension (or its parts) are “mounted” to the rotor. In the embodiment of FIG.  1 , it is “mounted” by being wrapped to the rotor. In other arrangements it is attached to the rotor adhesively or by a tack weld for example. The term “mounted” is intended to include “attached”, because some portion of the suspension is always held to the rotor so it does not slip with rotation of the rotor—it simply rolls along the rotor.  
         [0046]     The end  71  of the other strap is fixed to the stage. It also may be folded over, and held clamped by a surface plate  72 . The objective is to hold the suspension material in a smooth tight, edge supported condition, so that the straps from the tangent line with the roller to the attachment point have an evenly tensioned plate-like planar shape.  
         [0047]     Both straps, when attached, tightly bear against the rotor. Accordingly, the arms comprise shear-web structures which strongly resist wrinkling and thereby resist shear movement. As can be seen in  FIG. 6 , when one strap of a suspension shortens, the other lengthens, and the reverse is true of the next adjacent sling. Any effort to tilt or relatively shift the stage on any degree of freedom is thereby made impossible.  
         [0048]     A similar arrangement, but one which requires bearing restraint of the rotor is shown in  FIGS. 7-9 . In this embodiment, base  80  carries a support  81  (only one of which is shown). In turn the base supports a bearing  82  at both ends of a rotor  83 .  
         [0049]     In this embodiment, suspensions engage the rotor, but in a different sense. A railing  85  depends from stage  86  with a reference surface  87 . The suspensions may conveniently be a single piece with three sectors  90 ,  91 ,  92 , formed by an aperture  93  and a tongue  94 .  
         [0050]     As shown in  FIG. 9 , a strap formed by a tongue  94  is passed through the aperture, and ends  95 ,  96  are attached to the reference surface. Thus, tongue  94  and ends  95  and  96  comprise straps adherent to the rotor.  
         [0051]     Upward or downward movement of the stage relative to the base lengthens and equally shortens the length of the straps of the suspensions. Accordingly, the tightly supported flexible material between the tangent points and the points of attachment forms rigid panels, as in the situation of  FIG. 1 . Identical assemblies are found at all four sides of the stage.  
         [0052]     The embodiment of  FIGS. 7-9  is functional for the intended purpose, but inherently relies on the accuracy of its bearings, and involves friction, which the embodiment of  FIG. 1  does not. Even so, the rigid suspension caused by the arm panels (straps) provides the same kind of rigidity.  
         [0053]     The stage is moved up and down by any desired motive means. Inherently the stage is biased downwardly by its own weight, but its vertical location can readily be learned from any desired metrical system. Relatively crude motive means may be used, such as inflatable bladders which themselves provide no side support. This device does provide it. Alternatively lead screws and the like could be used that press upwardly on the support.  
         [0054]     It is not necessary, and in fact generally will not be preferred, for there to be a net fit between the rotor, the straps, and the reference surfaces. This can involve closer tolerances than are ultimately necessary. What is necessary is that each rotor be supported by straps which engage a rotor, one strap or straps extending in one direction and the other strap or straps extending in the other direction. Then when any strap moves relative to the base, one of the other straps of the pair will “unwind” from the rotor, and the other will “wind” onto it, in equal amounts.  
         [0055]     When the rotor is supported by a bearing, only one reference plate is needed.  
         [0056]     An advantage of providing the straps as part of a single piece is that engagement to the rotor can be provided merely by an embracement. However it is equally possible to attach a strap to the rotor by adhesives, weldments, or fasteners and the suspension does not extend fully around the rotor, but instead may merely be attached to it, with a short free length of straps to bear against the rotor surface to which a portion of the strap is attached to a reference surface. The bight is eliminated.  
         [0057]      FIGS. 10, 11  and  12  illustrate the use of individual straps, without a bight. For convenience in illustration, four straps  90 , 91 , 92  and  93  are shown, attached to rotor  95 . These are attached to the rotor by welds  96 , 97 , 98  and  99 .  
         [0058]     Straps  90  and  92  are axially spaced apart from one another, ( FIG. 10 ) but they are directed in the same direction (as in the U straps of  FIG. 1 ). Similarly, straps  91  and  93  are axially spaced apart, and they are directed in the same direction as each other, as in  FIG. 1 , in the opposite direction from straps  90  and  92 .  
         [0059]     It will be observed that straps  90  and  92  form a pair, and straps  91  and  93  a second pair. These correspond to the straps of the U shaped structure of  FIG. 1 , except there is not an integral bight.  
         [0060]     In this embodiment, and also in all other embodiments, to free the rotor and straps from compressive, packed contact, a relief  110  can be formed in the reference surfaces. Typically it will be a few thousandths of an inch deep—deep enough to ensure that the rotor will be free from incidental contact with the reference plate at the tangent line.  
         [0061]     There results a movement stage, manufactured from affordable material, which provides accuracies as good as or better than, conventional guidway type assemblies.  
         [0062]     This invention is not to be limited by the embodiments shown in the drawings and described in the description, which are given by way of example and not of limitation, but only in accordance with the scope of the appended claims.