Abstract:
A bi-directional slide mechanism. A pair of master and slave disks engages opposite sides of the platform. Rotational drivers are connected to master disks so the disks rotate eccentrically about their respective axes of rotation. Opposing slave disks are connected to master disks on opposite sides of the platform by a circuitous mechanical linkage, or are electronically synchronized together using stepper motors, to effect coordinated motion. The synchronized eccentric motion of the pairs of master/slave disks compels smooth linear motion of the platform forwards and backwards without backlash. The apparatus can be incorporated in a MEMS device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 09/692,027, “Multi-Axis Planar Slide System”, by Lothar F. Bieg, filed Oct. 19, 2000, now U.S. Pat. No. 6,463,664 which is herein incorporated by reference. 
    
    
     FEDERALLY SPONSORED RESEARCH 
     The United States Government has rights in this invention pursuant to Department of Energy Contract No. DE-AC04-94AL85000 with Sandia Corporation. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to mechanical positioning systems, and more specifically to linear slides and planar X-Y positioning mechanisms having essentially no backlash. 
     An important function of many of even the most basic mechanical devices is to move an item or mechanical element in a controlled manner. In many mechanical applications it is necessary to shift the position of an item in two-dimensional space, i.e. planar motion. The mechanical systems for accomplishing this task have commonly taken the form of slides, flexures, and multi-axis actuators of various types. 
     Historically, slides, flexures, and actuators are stacked on top of one another in order to move objects in multiple directions. For example, in a common X-Y stage, the Y-axis (e.g., “front-to-back”) slide is carried on top of the X-axis (left-to-right) slide, providing independent, orthogonal two-dimensional motion in the X-Y plane. Typically, the drivers to move the slides mainly comprise a motor connected to a screw for converting rotary motion into linear displacement. The motor and screw effectuate rotational motion, with the pitch or lead of the screw being transformed into translational motion, which then is imparted to a slide element that moves to-and-fro upon associated guideways. 
     Conventional lead screw drives suffer from backlash upon reversal, resulting in reduced accuracy. Stacking one linear slide on top of the other can also reduce accuracy because the stacked structure is less compact, and, hence, less rigid. 
     Rapid progress currently is being made in the field of micro electro-mechanical systems (MEMS), where mechanical operations are performed by minutely sized machines. In current state of the art MEMS devices, drivers and actuators are generally planar elements. Single-axis (e.g. unidirectional) “comb” slides have been successfully fabricated with MEMS technology. A need remains, however, for a simple, planar X-Y stage fabricated with MEMS technology that provides positioning capability in two independent directions. 
     Against this background, the present invention was developed. 
     SUMMARY OF THE INVENTION 
     The invention relates to a planar apparatus for accurately positioning a movable platform in one or two dimensions. A sample or workpiece can be placed on top of the platform. A pair of master and slave disks engage opposing sides of the movable platform. Rotational driving means, such as geared motors, are connected to each disk at a location offset from the disk centers, so that the disks rotate eccentrically about the driver&#39;s axes of rotation. The pair of master and slave disks rotate in a coordinated fashion in the same direction. As the master disk rotates eccentrically, the distance between the axis of rotation and the platform&#39;s edge decreases. Simultaneously, the distance between the axis of rotation of the corresponding slave disk and the opposite edge of the platform increases by the same amount. The coordinated eccentric motion of the pairs of master/slave disks compels linear motion of the platform along an axis that is oriented parallel to a line connecting the two disk&#39;s axes of rotation. Smooth coordination of the pair of rotating master/slave disks eliminates backlash by keeping the disks in nearly continuous contact with the platform during movement. Coordination can be achieved, for example, by use of a timing belt, or by electronic synchronization of stepper motors. 
     Two-dimensional motion of the platform can be achieved by providing two pairs of coordinated master/slave disks, with each pair independently driving orthogonal X-axis and Y-axis motion. This arrangement provides all possible translational motion of the platform in a two-dimensional plane. The two pairs of coordinated master/slave disks can be placed on a single plane, thereby creating a planar X-Y stage. This planar geometry eliminates the need to stack the drive mechanisms on top of each other, making it well suited for MEMS applications. The maximum linear range of motion (e.g. stroke) along a single axis is equal to the largest radial distance from the disk&#39;s axis of rotation to its circumference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several examples of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings: 
     FIG. 1 illustrates a schematic top view of a first example of a single-axis slide mechanism, according to the present invention. 
     FIG. 2 illustrates a schematic top view of a first example of a single-axis slide mechanism, according to the present invention, when the disks have simultaneously rotated in the counter-clockwise direction by 120 degrees. 
     FIG. 3 illustrates a schematic top view of a first example of a single-axis slide mechanism, according to the present invention, when the platform has moved to its maximum displacement allowed by the geometry (θ=180 degrees). 
     FIG. 4 illustrates a schematic top view of a first example of a single-axis slide mechanism, according to the present invention. 
     FIG. 5 shows a schematic cross-section view through the side of the example of a single-axis slide mechanism of FIG.  4 . 
     FIG. 6 illustrates a schematic top view of a first example of a planar X-Y stage (e.g. positioning mechanism), according to the present invention. 
     FIG. 7 illustrates, by way of dotted outline  18 ′, the final position of platform  18  (from FIG. 6) after it has been moved the maximum possible distance in both X and Y directions. 
     FIG. 8 illustrates a bottom view of a second schematic example of a planar X-Y positioning apparatus. 
     FIG. 9 shows a schematic side view of the cross-section of the apparatus of FIG.  8 . 
     FIG. 10 illustrates a schematic top view of a third example of a planar X-Y positioning mechanism. 
     FIG. 11 illustrates a schematic isometric view of a fourth example of a planar X-Y stage, according to the present invention. 
     FIG. 12 illustrates a schematic isometric view of a fifth example of an X-Y stage. 
     FIG. 13 illustrates a schematic isometric view of disk  12 , including a bearing assembly, according to the present invention. 
     FIG. 14 shows a schematic isometric view of the bottom side of a sixth example of a planar X-Y mechanism. Only the drive mechanism for motion in the Y-axis is shown. 
     FIG. 15 shows a schematic side cross-section view of a seventh example of a planar X-Y mechanism, according to the present invention. 
     FIG. 16 shows a schematic side cross-section view of a eighth example of a planar X-Y mechanism, according to the present invention. 
     FIG. 17 shows a schematic side cross-section view of a ninth example of a planar X-Y mechanism, according to the present invention. 
     FIG. 18 shows a schematic side cross-section view of a tenth example of a planar X-Y mechanism, according to the present invention 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention relates to a highly accurate linear slide or planar X-Y stage having essentially no backlash. The present invention is adaptable to a variety of applications. For example, the invention can be used in macro setting as a movable X-Y worktable surface for a machine tool (e.g. drill press, milling machine). The invention can be used in mini-setting as an X-Y stage for positioning a specimen under a microscope. Finally, the apparatus is well suited in a micro-setting for incorporation as a planar X-Y stage into a MEMS device since the layers available to design and fabricate MEMS systems are somewhat limited. 
     In the fields of microscopy and optics, highly accurate 1-D and 2-D positioning devices are needed to control translational movement without play, backlash, or lost motion. Such devices often must perform in a vacuum environment, where grease and other lubricants are prohibited. The present invention does not require the use of lubricants, and due to its simplicity of construction lends itself to numerous mini and micro-scale applications. Additional applications for the invention could include repetitive motion mechanisms, since the apparatus translates a rotary motion into a reciprocating linear motion with a specific stroke. Conversion of rotation motion to translational displacement in the present invention is more direct than in other systems well known in the art. The apparatus can be manufactured in nearly any practical size, including but not limited to a micro size for inclusion in a MEMS device. 
     FIG. 1 illustrates a schematic top view of a first example of a single-axis slide mechanism, according to the present invention. Platform  4  slides back and forth along a single axis (e.g. X-axis), while being constrained by parallel guideway  2  and guideway  3 . Guideways  2  and  3 , or other means, prevent rotation of platform  4 . Platform  4  can be constrained by linear bearing guides, air bearing means, or other well-known methods. Master disk  5  contacts the right end of platform  4 . Rotational axis  7  is offset from the disk&#39;s centroid  6  a distance equal to D offset . In a similar fashion, slave disk  8  contacts the opposite (e.g. left) end of platform  4 . Slave disk  8  has a rotational axis  9 , which is offset from its centroid. Axis of rotation  9  is also oriented substantially parallel to axis of rotation  7 . Both axes of rotation  7  and  9  are oriented substantially parallel to the direction of motion (e.g. parallel to the X-Axis). The direction of motion of platform  4  coincides with a line (not shown) connecting axis of rotation  7  with axis of rotation  9 . 
     Visualization line  10  is drawn simply to illustrate the orientation of the disks as they rotate; the line is not an element of the invention. Visualization line  10  is drawn between the axis of rotation and the disk&#39;s centroid. The shape of disks  5  and  8  can be circular, elliptical, polygonal, lobed, multi-lobed, oblong shapes. In this regard, the use of the word “disk” is very general, and encompasses not only circular shapes, but also those described previously. In all cases, the location of the disk&#39;s centroid must be offset from its axis of rotation in order to drive motion of platform  4 . The preferred shape of the disks is circular. Preferably, the diameter of the master and slave disks are essentially the same. 
     Smooth linear motion (e.g. without backlash) of platform  4  is effected by rotating master disk  5  and slave disk  8  in the same direction, and in a coordinated manner. The amount of rotation is designated by the angle, θ. If the diameter of disks  5  and  8  are the same, then “coordinated rotation” means that both disks rotate at the same speed (e.g. rpm), and are synchronized (e.g. start and stop rotating at the same time). If the diameters of disks  5  and  8  are different, then each disk can rotate at a different speed from each other. The different rotation speed should be chosen appropriately to insure “coordinated motion”, meaning that both the master and slave disks  5  and  8  should remain in contact with platform  4  to prevent problems with backlash. In either case (same, or different, diameters), the disk&#39;s rotation must also be synchronized (e.g. start and stop rotating at the same time) to effect smooth motion. 
     FIG. 2 shows that platform  4  has moved incrementally to the left when disks  5  and  8  have simultaneously rotated in the counter-clockwise direction by an amount equal to θ degrees (e.g. θ=120 degrees). 
     FIG. 3 shows that platform  4  has moved to its maximum displacement allowed by the geometry (θ=180 degrees). The maximum travel of a linear slide, according to the present invention, is equal to two times the offset distance, D travel =2×D offset . With reference to FIG. 4, we define D max  as the maximum radial distance between the rotation axis  9  of slave disk  8  and the left end of platform  4 . Likewise, we define D min  as the minimum radial distance between the rotation axis  7  of master disk  5  and the opposite (e.g. right) end of platform  4 . The maximum travel of platform  4 , D travel  is related to these distances by the following relationship: D travel =D max −D min . Therefore, by increasing the eccentricity (e.g. offset) of the disks, the maximum travel of platform  4  also increases. 
     To minimize problems with backlash, and to make the overall motion as smooth as possible, it is important that when the radial distance between one axis of rotation (e.g. axis  9 ) and one side (e.g. left side) of the platform is at a maximum (e.g. D max ), then the distance between the other axis of rotation (e.g. axis  7 ) and the opposing side (e.g. right side) of the platform is at a minimum (e.g. D min ). This condition is illustrated in FIGS. 1 and 4. 
     FIG. 5 shows a schematic cross-section view through the side of the example of a single-axis slide mechanism of FIG.  4 . Platform  4 , master disk  5 , and slave disk  8 , lay in the same plane, designated as surface  19 . Sample  20  is illustrated as lying on top of platform  4 . Sample  20  can be clamped or attached to platform  4  using means well known in the art (not shown). Surface  19  represents the surface of a support base (not shown). 
     FIG. 6 illustrates a schematic top view of a first example of a planar X-Y stage (e.g. positioning mechanism), according to the present invention. Two pairs of disks  12  and  14 ; and  13  and  15  are operatively engaged with the four sides of platform  18  (e.g. sides  23 ,  24 ,  254 ,  26 ). More specifically, the circumferences or circumferential edge surfaces of the disks  12 ,  13 ,  14 ,  15  are in contact with surfaces, such as the sides, of platform  18  to be moved and positioned. Platform  18  can be square, rectangular, hexagonal, polygonal, circular, or other shape. Disks  12 ,  13 ,  14 ,  15  preferably are circular cylinders. Platform  18  can be of any size, but in the preferred embodiment is a rectangular or square planar element having four sides  23 ,  24 ,  25 , and  26 . Platform  18  can be slidably disposed upon, and supported by, a supporting surface  19 . In this example, disks  12 ,  13 ,  14 , and  15  are disposed outboard of platform  18 . Sample  20  can be disposed on platform  18 . 
     Referring still to FIG. 6, coordinated rotation in the same direction of X-axis master disk  12  and X-axis slave disk  14  produces smooth translational motion of platform  18  in the X-direction. Likewise, coordinated rotation in the same direction of Y-axis master disk  13  and Y-axis slave disk  15  produces smooth translational motion of platform  18  in the Y-direction. It is not necessary for the direction of rotation be the same between the two pairs of disks. For example, X-axis master/slave disks  12 ,  14  can both be rotating clockwise, while Y-axis master/slave disks  13 ,  15  can both be rotating counter-clockwise. However, it is necessary for the pair of master and slave disks to both rotate in the same direction to effect coordinated motion. 
     FIG. 7 illustrates, by way of dotted outline  18 ′, the final position of platform  18  (from FIG. 6) after it has been moved the maximum possible distance in both X and Y directions. Coordinated rotation of the X-axis master/slave pair of disks ( 12  and  14 ) has linearly moved platform  18  to its maximum travel in the X-direction. Likewise, coordinated rotation of the Y-axis master/slave pair of disks ( 13  and  15 ) has linearly moved platform  18  to its maximum travel in the Y-direction. It will be appreciated that independent action of each of the X-axis and Y-axis master/slave disk pairs, when combined, can generate all possible translational motion of platform  18  in a two-dimensional plane. 
     FIG. 8 illustrates a bottom view of a second schematic example of a planar X-Y positioning apparatus. The four disks ( 12 ,  13 ,  14 , and  15 ) are disposed inboard of the perimeter of platform  18 . Disks  12 ,  13 ,  14 , and  15  operatively engage with inside surfaces  23 ′,  24 ′,  25 ′, and  26 ′, respectively, of platform  18 . With reference now to FIG. 9, which shows a schematic side view of the cross-section of the apparatus of FIG. 8, platform  18  has a recessed space  21  that houses disks  13  and  15 . In this configuration, sample  20  is isolated by platform  18  from possible contamination by optional lubricants in disks  12 ,  13 ,  14 , and  15 , and their associated drivers (to be discussed later). Also, disks  12 ,  13 ,  14 , and  15 , and their associated drivers, are shielded by platform  18  from possible contamination due to operations performed on sample  20  (e.g. machining fluids, chips, etc.) 
     FIG. 10 illustrates a schematic top view of a third example of a planar X-Y positioning mechanism. In this example, X-axis motion of platform  18  is compelled by coordinated rotation of three disks: X-axis master disk  12 , first X-axis slave disk  14  and second X-axis slave disk  16 . Disk  12  contacts side  26 , while disks  14  and  16  contact the opposite side  24 . The combination of the three X-axis disks  12 ,  14 , and  16  creates a kinematic constraining action on platform  18  that serves to eliminate positional errors due to small rotations of platform  18  about its centroid (not shown). Coordinated rotation of these three disks produces smooth translational motion in the X-direction without backlash, and without rotation of platform  18  about its centroid. 
     FIG. 11 illustrates a schematic isometric view of a fourth example of a planar X-Y stage, according to the present invention. Rotational drivers  32 ,  33 ,  34 ,  35 , extend through the plane  19 , and are fixedly connected to disks  12 ,  13 ,  14 ,  15 , respectively, at positions offset from the centroids of the respective disks. Accordingly, as drivers  32 ,  33 ,  34 , and  35  rotate, disks  12 ,  13 ,  14 , and  15  also rotate, creating eccentric rotational motion. Drivers  32 ,  33 ,  34 ,  35 , can be driven by motors, stepper motors, or manual cranks; with or without gear trains. 
     Coordinated rotation of the master/slave pair of disks is required to produce smooth translational motion of platform  18  along a given axis. Consequently, this requires coordinated operation of the disks respective drivers. In FIG. 11, opposing disks  12  and  14  are paired together for X-axis motion. Therefore, opposing drivers  32  and  34  are also paired together. Opposing paired drivers  32 ,  34  (or, alternatively,  33  and, 35 ) can be electronically synchronized by known means. 
     FIG. 12 illustrates a schematic isometric view of a fifth example of an X-Y stage. Platform  18  is not shown for clarity of illustration of the drive assembly. Opposing pairs of drivers  32 ,  34  (or, alternatively,  33  and  35 ) can be operatively connected by means of a timing belt, drive chain, drive belt, or gear train. In this example, driving belts  38  operatively connects drivers  32  and  34  to produce synchronized rotation of disks  12  and  14 . Likewise, drive belt  37  operatively connects drivers  33  and  35  to produce synchronized rotation of disks  13  and  15 . Drive belts  37 ,  38  can as well be other connection components, such as chains, toothed timing belts, flexible steel bands, O-rings, or the like. Gear trains also could be employed. 
     For a MEMS version of the present invention, a minimum of a one micron (0.001 mm) gap or clearance is required between mating surfaces due to design constraints inherent to MEMS technology. 
     To minimize friction, and associated torques, between the circumferential surfaces of the disks  12 ,  13 ,  14 ,  15  sliding along the sides  22 ,  23 ,  24 ,  25  of platform  18 , lubricants or bearings can be employed. The use of roller, needle, or ball bearing assemblies can provide a rolling contact point having very low effective friction, thereby minimizing the need to use lubricants. 
     FIG. 13 illustrates a schematic isometric view of disk  12 , including a bearing assembly, according to the present invention. Eccentric disk  12  is concentric to, and is in contact, with an inner race  42  of a ball, roller, or needle bearing assembly, while an outer race  43  makes contact with the side  26  of platform  18 . An essentially identical bearing assembly can be used with the other disks  13 ,  14 ,  15 . FIG. 13 illustrates the preferred type of bearing assembly, incorporating a plurality of needle bearings  45  radially disposed between races  42  and  43 . The entire bearing assembly can be sealed. 
     In the example of FIG. 13, motion of platform  18  only in the Y-direction produces rotation of outer race  43 , while the inner race  42  remains stationary. Alternatively, motion of platform  18  only in the X-direction produces rotation of outer race  43  and counter-rotation of inner race  43 . A combination of both X and Y-axis motion results in rotations of both the inner and outer races  42 ,  43 . 
     During initial assembly of an X-Y stage, the distance between the master disks  12 ,  13  and respective slave disks  14 ,  15  should be adjusted to correspond closely to the dimensions of platform  18 , so as to reduce or eliminate any play, backlash, or lost motion between the platform and the disks. An example of an initial configuration of disks can be seen in FIG. 6, with reference to the visualization lines. Here, the disks are so disposed such that when the centroid of a master disk (e.g. disk  12 ) is between its axis of rotation and the platform, then the axis of rotation of the paired slave disk (e.g. disk  14 ) is between the centroid of the slave disk and the opposite end of the platform. Once the maximum radial distance between the axis of rotation of a given disk (e.g. master disk  12 ) and the platform  18  is reached, the distance between the axis of rotation of the opposing disk (e.g. slave disk  14 ) and the platform is at its minimum. Small adjustments can be made in the width of platform  18  to precisely achieve this kinematic condition. The physical relationship between the master disks  12 ,  13  and their corresponding slave disks  14 ,  16  compels this condition, allowing for coordinated disk motion. Therefore, by proper initial alignment of the disks, problems with backlash can be minimized and the overall accuracy can be improved. 
     FIG. 14 shows a schematic isometric view of the bottom side of a sixth example of a planar X-Y mechanism. Eccentric disks  60 ,  61  and  62  are disposed beneath a generally planar, rectangular platform  18 . Thus, the drive mechanisms are shielded or covered by the platform  18 . In this example, three eccentric disks  60 - 62  are used per reference axis, i.e., one master disk  60  and two slave disks  61 ,  62  for the Y-axis. For the sake of simple illustration, FIG. 14 only shows the drive mechanism to impel motion in one direction (e.g. Y-axis). A second set of three disks (not shown) arranged in tandem with the disks  60 - 62 , but in track channels oriented perpendicular to those shown in FIG. 14, could also be disposed under the platform. Stepper motors can be used to rotate the drivers  65 ,  66  and  67  in a coordinated fashion. Linear track  63  is attached to, or is an integral part of, the bottom side of platform  18 , and is aligned parallel to the X-axis. The arrangement shown in FIG. 14 of disks  61  and  62  disposed on one side of track  63 , and the third disk  60  disposed on the other side of track  63  affords a desirable kinematically constraining geometry. Coordinated rotation of disks  60 ,  61 , and  62  produces smooth translational motion of platform  18  in the Y-axis. 
     If the disk drivers of the present invention are operated with constant rotational speed (e.g. rpm), then the velocity of the platform will vary continuously throughout a full rotation of the eccentric disks. The lowest platform velocity is achieved when the circumferential surfaces of the eccentric disks (or outer races) contact the sides  23 - 26  of platform  18  at their smallest and largest radial distance from axis of rotation. Then, platform velocity increases and obtains its maximum at 90 angular degrees from the earlier referenced position. 
     FIG. 15 shows a schematic side cross-section view of a seventh example of a planar X-Y mechanism, according to the present invention. Platform  18  has an overhanging lip  50 , which partially shields disks  13  and  15 . Platform  18  is supported by disks  13  and  15 , which are operatively engaged with overhanging lip  50 . Overhanging lip  50  is also illustrated in FIG.  11 . 
     FIG. 16 shows a schematic side cross-section view of a eighth example of a planar X-Y mechanism, according to the present invention. Platform  18  has a flat-bottomed groove  52 , which partially shields disks  13  and  15 . Platform  18  is supported by disks  13  and  15 , which are operatively engaged with flat-bottomed groove  52 . 
     FIG. 17 shows a schematic side cross-section view of a ninth example of a planar X-Y mechanism, according to the present invention. Platform  18  has a V-shaped groove  54 , which partially shields disks  13  and  15 . Platform  18  is supported by disks  13  and  15 , which are operatively engaged with V-shaped groove  54 . Disks  13  and  15  have a mating V-shaped shape of their outer circumference  55 . 
     FIG. 18 shows a schematic side cross-section view of a tenth example of a planar X-Y mechanism, according to the present invention. Platform  18  has a semi-circular groove  56 , which partially shields disks  13  and  15 . Platform  18  is supported by disks  13  and  15 , which are operatively engaged with semi-circular groove  56 . Disks  13  and  15  have a mating semi-circular shape of their outer circumference  57 . 
     Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. For example, angle encoder means can be attached to each disk&#39;s axis of rotation and used to measure the angle of rotation of each disk. Also, platform  18  can be supported on plane  19  by a plurality of ball bearings attached to the support base to minimize sliding friction. 
     It is intended that the scope of the invention be defined by the claims appended hereto.