Abstract:
A precision positioning stage assembly, comprising a movable stage, a drive screw having it axis parallel to the direction of movement of the stage, a follower threadably engaged with the drive screw, and a linear guide rail for supporting one side of the stage. The linear guide rail has its axis parallel to the direction of movement of the stage. A drive member is secured to the stage which has a pair of oppositely disposed contact surfaces spaced apart a predetermined distance. A torque member is secured to the follower, the torque member having a drive section for placement between the contact surfaces and moving the stage along the guide rail. The drive section and the contact surfaces have a configuration so as to provide substantially point contact therebetween.

Description:
FIELD OF THE INVENTION 
     This invention relates to a precision moving stage which is employed to move an article along a linear path at a precise constant speed. 
     BACKGROUND OF THE INVENTION 
     It has been found that a stimulable phosphor may be used in radiation image recording and reproducing system. Specifically, a sheet provided with a layer of stimulable phosphorus (herein referred to as stimulable phosphorus sheet) when first exposed to radiation passing through an object, such as a human body, will provide a radiation image stored thereon. When this image has been exposed to a stimulating ray beam such as a laser beam, this will cause the stimulable phosphorus sheet to emit a light in proportion to stored radiation energy. The light emitted by the stimulable phosphorus sheet, upon stimulation thereof, is photoelectrically detected and converted to an electric image signal, and the radiation image of the object is reproduced by a visible image by use of image signal on a recording medium such as a photographic film. The stimulating of the stimulable sheet by a laser beam is typically done using a raster scanning technique. The slow scan direction transport of the stimulable phosphorus sheet is important to the quality of the generated image. Slow scan velocity must be regulated such that the exposure modulation of the scanning beam does not cause image signal modulation greater than a threshold of visible contrast modulation. Therefore, it is important to provide a well regulated linear motion. 
     Lead screw mechanisms are commonly used to convert angular velocity to linear motion. Such mechanisms usually employ at least one linear guide rod. Lead screw mechanisms commonly require extreme precision and manufacture and assembly in order to function without sticking, jamming, or causing vibrations in required input torque. Any one of these events may cause observable velocity changes when the lead screw mechanism is used for storage phosphorus raster scanning. In addition, linear guides that are used with the lead screw must be exactly parallel to the lead screw and to each other in order to prevent jamming. Similarly, the bearings attached to the moving stage, which supports the stimulable phosphorus sheet, must be co-linear (at all points) to the guides on which they operate to prevent jamming. For long linear motion, such mechanisms may be beyond the capability of normal manufacturing methods to build, thus requiring time-consuming and expensive adjustments during assembly. 
     Additionally, the mechanism used to guide the stage can not be too loose, or outside forces such as externally applied shocks, dust on the linear guides, etc., can ruin an image being formed by the raster scanning mechanism. The mechanism guiding the linear motion must be sufficiently rigid so that expected forces do not deflect the moving stage such that artifacts will show in the scanned image. 
     Applicant has invented an improved precision stage assembly which reduces the required precision and expense of manufacturing while maintaining smooth, rigidly guided linear motion to a stage. The present invention comprises improved mechanisms for coupling the lead screw and the stage, and for coupling the stage and the guide bearings on the linear guide which minimizes jamming and sticking, while preventing deflection of the stage from its path by normal forces experienced during operation, such as shocks and contamination of the guide rails. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention there is provided a precision positioning stage assembly, comprising. 
     a movable stage; 
     a lead screw having a longitudinal axis parallel to the direction of movement of said stage; 
     a follower threadably engaged with the lead screw for converting rotational motion of the lead screw to linear motion; 
     a linear guide rod for supporting one side the stage, the linear guide rod having its axis parallel to the direction of movement of the stage; 
     means for transferring linear motion of the follower to the stage; 
     means for compensating for misalignment between the lead screw and the linear guide rod upon which the stage is slideably mounted; and 
     means for compensating for misalignment between the stage and the linear guide rod upon which it is slideably mounted. 
     In another aspect of the present invention there is provided a precision positioning stage assembly, comprising: 
     a movable stage; 
     a lead screw having a longitudinal axis parallel to the direction of movement of the stage; 
     a follower threadably engaged with the lead screw for converting rotational motion of the lead screw to linear motion; 
     a linear guide rod for supporting one side the stage, the linear guide rod having its axis parallel to the direction of movement of the stage; 
     means for transferring linear motion of the follower to the stage, the means transferring the linear motion through point contact surfaces provided therebetween. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In the Detailed Description of the Preferred Embodiment of the invention presented below, reference is made to the accompanied drawing, in which: 
     FIG. 1 is a perspective view of a precision stage assembly made in accordance with the present invention; 
     FIG. 2 is a side elevational view of the precision stage assembly of FIG. 1 as taken along line 2--2; 
     FIG. 3 is a partial perspective view of the precision stage assembly of FIG. 1 with the stage removed; 
     FIG. 4 is a top plan view of the precision stage assembly of FIG. 3; 
     FIG. 5 is a cross-sectional view taken along line 5--5 of FIG. 4 with the addition of the stage; 
     FIG. 6 is an enlarged cross-sectional view taken along line 6--6 of FIG. 4 with the addition of the stage; 
     FIG. 7 is a side elevational view of FIG. 6 as taken along line 7--7; 
     FIG. 8 is a top plan view of FIG. 7 as taken along line 8--8; 
     FIG. 9 is a greatly enlarged cross-sectional view taken along line 9--9 of FIG. 4 including a portion of the stage; and 
     FIG. 10 is an enlarged cross-sectional view taken along line 10--10 of FIG. 4 including a portion of the stage. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, there is illustrated a precision stage assembly 10 made in accordance with the present invention. The precision stage assembly 10 is designed to be incorporated into a reader for reading of a stored image on a stimulable phosphorus sheet 11, FIG. 1 that has been exposed to radiation. In particular, the precision stage assembly 10 is designed for use in a raster scanning reader shown in phantom in FIG. 1 as100. The precision stage assembly 10 includes a moveable stage 12 which includes a drive screw 14 which is mounted to base 16. The drive screw 14 is in threaded engagement with a drive nut 18. Movement of the drive nut 18 is caused by relative rotation of the drive screw 14. A motor 20 and corresponding transmission 22 is provided for rotating drive screw 14 in the appropriate direction. This stage 12 is supported by a pair of guide rails 24,26 which are axially spaced apart and secured to base 16. The guide rails 24,26 and drive screw 14 are in substantial parallel alignmentwith each other. 
     With respect to coupling of the drive nut 18 to the stage 12 so as to transmit linear motion by rotation of the drive screw 14, it is desirable to couple only that component of the drive screw motion which is parallel to the local axis of the linear guide rails 24,26. This allows the drive screw 14 to be installed with loose tolerances with respect to the linear guides rails 24,26. This also allows the linear guides rails 24, 26 and drive screw 14 to be only reasonably straight, and thus allow for small variation from parallelism. Referring to FIGS. 2-9, there is illustrated in greater detail the manner in which linear motion is transmitted to the stage 12. As can be seen in the preferred embodiment illustrated, the drive screw 14 is rigidly attached to base 16 allowing it only free to rotate about its longitudinal axis. A torque member 28 is secured to drivenut 18 and is used for transferring the rotational movement of the drive screw 14 to the linear motion of the stage 12. The torque member 28 also provides means for resisting rotation of the drive nut 18 about the drive screw 14. In the particular embodiment illustrated, torque member 28 comprises a generally flat plate member having a mounting section 30, a drive section 32 immediately above the mounting section 30, and a stop section 34 which extends from the drive section 32 outward between the stage 12 and adjacent guide rail 24. In the particular embodiment illustrated, stop section 34 has a generally rectangular configuration. The mounting section 30 is provided with a central opening 36 through which the drive screw passes and a plurality of openings 38 through which screws 39 pass to clampingly secure the torque member 28 to drive nut 18 by engaging threaded openings provided in drive nut 18. It is, of course, understood that the torque member 28 may be secured to drive nut 18 in anydesired manner. Stop section 34 is provided with a narrow slot/opening 42 which divides stop sections to an upper section 43 and lower section 44. The slot 42 in the particular embodiment illustrated has a width W of about 06 inches (1.74 cms). The stop section 34 extends adjacent guide rod24 such that the lower surface 45 of lower section 44 engages the top of guide rail 24. The upper section 43 has a small outwardly extending projection 46 which extends above the top surface of the adjacent upper section 43. Projection 46 preferably extends a distance D of approximately0.06 inches (1.72 cms) thereabove. The torque member 28 is made of a material such that projection 46 is in engagement with the bottom surface 47 of moveable stage 12 while at the same time the bottom edge 45 engages the top of guide rail 24. The bottom edge 45 of lower section 44 stops thedrive nut 18 from spinning about the drive screw 14. This causes the drive nut 18 to travel forward along the lead screw 14. The upper section 43 contacts the bottom surface 47 of the stage 12 and serves the same purposefor reverse rotation of the drive screw 14. The stop section 34 also is slightly spring loaded against the bottom surface 47 of the stage 12 to prevent/minimize the transmission of vibration as the stage moves between its two stop positions. The bottom edge 45 of the lower section 44 is alsofree to slide transversely (as illustrated by arrow 49) with respect to theadjacent guide rail 24. This allows transverse misalignment between the drive screw 14 and the adjacent guide rail 24 to be accommodated without sticking or jamming. Also any transverse component from rotation of a warped lead screw 14 is accounted for without changing the required input torque. Additionally, any vertical misalignment between the drive screw 14and guide rod 26 is accommodated by the vertical clearance between the torque member 28 and stage 12 which allows gradual rotation of a torque member 28 and drive nut 18 about the guide surface. Further, vertical components resulting from warped drive screw rotation are permitted without variation of the required input torque. 
     Referring to FIGS. 7 and 8, there is illustrated means for transferring thelinear movement of the drive nut 18 to the stage 12. In particular, the stage 12 is provided with a pair of drive members 50,52 which are secured to the bottom of moveable stage 12. The drive members 50,52 are positionedon both sides of the torque member 28 so as to capture the drive section 32therebetween. The spacing between the two drive members 50,52 is such that the torque member 28 fits snugly therebetween. Each of the drive members 50,52 have a contact surface 51,53, respectively, which engages the sides of torque member 28. Each of the contact surfaces 51,53 are configured so as to provide point contact with the torque member 28. In the preferred embodiment illustrated, the contact surfaces 51,53 each have a generally semi-spherical cross-sectional configuration and the sides of the torque member 28 are substantially planar. Referring to FIG. 8, there is illustrated by solid lines the initial position of the torque member 28 between the drive members 50,52. The coupling between drive members 50,52 and torque member 28 is designed such that only the component of a drive screw 14 which travels parallel to the local axis of the linear guide is being transferred. As torque member 28 moves either vertically, or horizontally, with respect to the adjacent linear guide rails 24,26 and stage 12, the torque member 28 is free to slide between the contact surfaces 51,53 as illustrated by the phantom lines in FIG. 8. This allows a transverse motion between the components. This construction also minimizes jamming, or variation in the required input torque due to the wobble of the drive nut 18 and torque member 28 caused by rotation of a warped drive screw 14. 
     Means are also provided for compensating for misalignment between the stage12 and the linear guide rails 24,26 upon which the stage 12 is slideably mounted. Referring to FIGS. 1-3, the stage 12 is slideably mounted to guide rail 24 by a front bearing assembly 54 and rear bearing assembly 56. 
     The stage 12 is also slideably mounted to linear guide rod 26 by a third bearing assembly 58. Bearing assemblies 54,58 are identical in construction. Therefore, this application will be limited to discussion ofsingle bearing assembly 54, it being understood that the bearing assembly 58 is identically constructed. Bearing assembly 56 is substantially the same as bearing assembly 54, except with minor differences which will be discussed later therein. 
     Referring to FIGS. 9 and 10, bearing assembly 56 includes a collar 60 whichis slideably mounted to guide rail 24, a mounting section 62 secured to collar 60 by a set screw 64 which engage threaded opening 66 in collar 60.The bearing assembly 56 is secured to the stage 12 by a shoulder screw 68. The stage 12 is provided with a recess 70 having a opening 72 which extends through the stage 12. The opening 72 is such that a bearing surface 74 is provided. The screw 68 has a head 76 which is larger than the opening 72, but less in size than the recess 70 such that the top of the head does not extend above the top of the adjacent stage 12. An annular spring 77 is provided between the bearing surface 74 and bottom surface 78 of the head of screw 68. The bottom surface of the shoulder 80 of shoulder screw 68 passes through opening 72 and engages a slightly raised projection having a planar surface 82. As can be seen, the verticalposition of the stage 12 is assured by spring 77 positioned between the head 76 of screw 68 and the bearing surface 74. 
     The mounting of bearing assembly 58 to guide rail 26 is similar to 24 except that in place of opening 72, there is provided an elongated slot 73which accounts for any change in distance between the two guide rails 24,26. The mounting of the stage 12 in the manner disclosed by the presentinvention allows the guide rails 24,26 to be mounted using normal manufacturing tolerances. 
     The present invention provides a relatively inexpensive slow scan motion for a storage phosphorus raster scanner which can be made utilizing inexpensive parts, manufacturing and assembly techniques. Additionally, a mechanism made in accordance with the present invention provides reliability due to its ability to account for misalignment that may occur during use. The rotary motion of the drive screw is converted to a linear motion without sticking, jamming, or variation in required input torque. Further, the present invention provides a rigid mechanism that minimizes disturbance in the motion of the stage due to outside forces such as shocks, or contamination of the guide rails. 
     It is to be understood that various modifications and changes may be made without departing from the scope of the present invention. The present invention being limited by the following claims.