Patent Publication Number: US-2004051915-A1

Title: Method and apparatus for incrementally displacing recording media in an imaging system

Description:
FIELD OF THE INVENTION  
       [0001] The present invention is in the field of imaging systems. More particularly, the present invention provides a method and apparatus for incrementally displacing a supply of recording media, such as a printing plate, past a scanning system.  
       BACKGROUND OF THE INVENTION  
       [0002] In existing electronic pre-press systems, images to be printed by offset printing are typically scanned from photographic sources and digitized or captured using a digital camera. The digitized images are then transmitted to a raster image processor (RIP) for half-tone screening and image rasterization. The rasterized image is then transmitted to an imaging system such as an imagesetter or platesetter where the rasterized image is recorded onto a supply of recording media. The recording media may comprise film, printing plates, etc.  
       [0003] In flat-field imaging systems, the recording media is often incrementally displaced across a flat support surface past a scanning system during the recording process. The scanning system is moved across the recording media in a direction that is perpendicular to the direction of travel of the recording media such that the rasterized image is recorded on the recording media in a series of bands each comprising a plurality of partial scan lines. Examples of two adjacent bands are illustrated in FIGS.  1 - 3 .  
       [0004]FIG. 1 illustrates a first, ideal case where two adjacent bands  10 A,  10 B, have been recorded on a supply of recording media such that the corresponding partial scan lines  12 A,  12 B, in each band  10 A,  10 B, are aligned and butt up against each other. FIG. 2 illustrates a second case where the two adjacent bands  10 A,  10 B, have been recorded on a supply of recording media such that the corresponding partial scan lines  12 A,  12 B, in each band  10 A,  10 B, are aligned, but do not butt up against each other. In this case, known software algorithms, such as those described in U.S. Pat. No. 6,097,418, incorporated herein by reference, may be used to compensate for the in-scan error. FIG. 3 illustrates a third case where the two adjacent bands  10 A,  10 B, have been recorded on a supply of recording media such that the corresponding partial scan lines  12 A,  12 B, in each band  10 A,  10 B, are not aligned and do not butt up against each other. This situation may occur, for example, because of steering effects that commonly occur when nip rollers are used to incrementally displace the recording media under the scanning system. Depending upon the degree of misalignment, deleterious artifacts may appear in the images printed using the affected recording media.  
       SUMMARY OF THE INVENTION  
       [0005] The present invention provides a method and apparatus for incrementally displacing a supply of recording media, such as a printing plate, past a scanning system.  
       [0006] Generally, the present invention provides an apparatus comprising:  
       [0007] an entrance platen;  
       [0008] an exit platen; and  
       [0009] a reference platen, movable between a first position adjacent the entrance platen and a second position adjacent the exit platen, for incrementally displacing recording media past a scanning system.  
       [0010] The present invention further provides a flat-field imaging system comprising:  
       [0011] a scanning system for recording data on a supply of recording media;  
       [0012] an entrance platen;  
       [0013] an exit platen; and  
       [0014] a reference platen, movable between a first position adjacent the entrance platen and a second position adjacent the exit platen, for incrementally displacing the recording media past the scanning system.  
       [0015] The present invention also provides a method for incrementally displacing a supply of recording media, comprising:  
       [0016] securing the recording media to a reference platen;  
       [0017] displacing the recording media by displacing the reference platen from a first position adjacent an entrance platen to a second position adjacent an exit platen;  
       [0018] securing the recording media to at least one of the entrance platen and the exit platen;  
       [0019] releasing the recording media from the reference platen; and  
       [0020] displacing the reference platen from the second position to the first position while holding the recording media stationary. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0021] The features of the present invention will best be understood from a detailed description of the invention and embodiments thereof selected for the purpose of illustration and shown in the accompanying drawings in which:  
     [0022]FIG. 1 illustrates two adjacent bands with partial scan lines that are aligned and butt up against each other.  
     [0023]FIG. 2 illustrates two adjacent bands with partial scan lines that are aligned and do not butt up against each other.  
     [0024]FIG. 3 illustrates two adjacent bands with partial scan lines that are not aligned and do not butt up against each other.  
     [0025]FIG. 4 illustrates a displacing apparatus for incrementally displacing a supply of recording media past a scanning system in a flat-field imaging system in accordance with the present invention.  
     [0026]FIG. 5 illustrates the flat-field imaging system of FIG. 4 in greater detail.  
     [0027]FIG. 6 is a cross-sectional view of a reference platen of the displacing system.  
     [0028] FIGS.  7 - 14  illustrate the operation of the displacing apparatus of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0029] The features of the present invention are illustrated in detail in the accompanying drawings, wherein like reference numerals refer to like elements throughout the drawings. Although the drawings are intended to illustrate the present invention, the drawings are not necessarily drawn to scale.  
     [0030] A flat-field imaging system  20 , including a displacing apparatus  22  for incrementally displacing a supply of recording media  24  past a scanning system  26 , is illustrated in FIG. 4. The recording media  24  is commonly supplied in discrete, flexible sheets and may comprise a plurality of plates, hereinafter collectively referred to as “plates” or “printing plates.” Each printing plate may comprise one or more layers supported by a support substrate, which for many printing plates is a planographic aluminum sheet. Other layers may include one or more image recording (i.e., “imageable”) layers such as a photosensitive, radiation sensitive, or thermally sensitive layer, or other chemically or physically alterable layers. Printing plates which are supported by a polyester support are also known and can be used in the present invention. Printing plates are available in a wide variety of sizes, typically ranging, e.g., from 9″×12″, or smaller, to 58″×80″, or larger. Other types of recording media  24  such as rolled film may also be used in the flat-field imaging system  20 .  
     [0031] The displacing system  22  includes an entrance platen  28 , an exit platen  30 , and a movable reference platen  32  that may be selectively displaced between a first position (POSITION 1 (shown in phantom)) adjacent the entrance platen  28  and a second position (POSITION 2) adjacent the exit platen  30  as indicated by directional arrow  34 . As detailed below, the recording media  24  is incrementally displaced along direction  36  during imaging in response to a displacement of the reference platen  32  from POSITION 1 to POSITION 2.  
     [0032] The flat-field imaging system  20  is described in greater detail with reference to FIGS.  5 - 6 . As shown in FIG. 5, the flat-field imaging system  20  generally includes a front end computer or workstation  38  for the design, layout, editing, and/or processing of digital files representing pages to be printed, and a raster image processor (RIP)  40  for processing the digital pages to provide rasterized page data (e.g., rasterized digital files) for driving an image recorder. Scanning system  26  records the rasterized digital files provided by the RIP  40  onto the recording media  24  that is supported by the platens  28 ,  30 , and  32 .  
     [0033] As shown in FIG. 6, the scanning system  26  is coupled to a movable carriage  42 . The scanning system  26  records digital data in a manner known in the art onto the imaging surface  44  of the recording media  24  using one or more imaging beams  46 . In particular, the scanning system  26  is displaced by the movable carriage  42  in a slow scan axial direction  48  along the reference platen  32  to expose the recording media  24  in a section-wise manner. Other types of imaging systems may also be used in the present invention.  
     [0034] As further illustrated in FIG. 6, the scanning system  26  typically includes a system  50  for generating the imaging beam(s)  46 . The system  50  comprises a light or radiation source  52  for producing the imaging beam(s)  46 , and an optical system  54  positioned between the radiation source  52  and the media support surface  56  for focusing the imaging beam(s)  46  onto the recording media  24 . It should be noted, however, that the system  42  described above is only one of many possible different types of scanning systems that may be used to record image data on the recording media  24 .  
     [0035] Referring to both FIGS. 4 and 5, a registration system comprising a plurality of retractable registration pins  58  is provided to accurately and repeatably position the leading edge  74  of the recording media  24  on the reference platen  32  during the initial loading and registration of the recording media  24 . The registration pins  58  are retracted after initial registration of the recording media  24 . The registration pins  58  may be extended/retracted using any suitable system such as solenoids, pneumatic cylinders, etc. The registration pins  58  are shown in an extended state in FIG. 4 and in a retracted state in FIG. 5. An edge detection system (not shown) may also be provided to locate a side edge of the recording media  24  prior to imaging.  
     [0036] A plurality of vacuum/air openings  60  and associated ports (not shown) are formed in the entrance platen  28 , exit platen  30 , and reference platen  32 , as shown in FIGS. 4 and 6. The vacuum/air openings  60  may comprise grooves as shown, or may be formed in many suitable configuration. A vacuum source  62  (FIG. 4) may be used to selectively draw a vacuum through the vacuum/air openings  60  in one or more of the platens  28 ,  30 ,  32 . The vacuum holds the recording media  24  against the media support surface of the corresponding platen(s). An air source  64  (FIG. 4) may be used to selectively force air out of the vacuum/air openings  60  in one or more of the platens  28 ,  30 ,  32 . The air forces the recording media  24  away from the media support surface of the corresponding platen(s). A manifold system  66  may be employed to selectively interconnect the vacuum source  62  and air source  64  to the vacuum/air openings  60  in each platen  28 ,  30 ,  32 .  
     [0037] A drive system  68  (FIG. 4) is used to reciprocally displace the reference platen  32  between POSITION 1 and POSITION 2. The drive system  68  may comprise a pneumatic cylinder, a lead screw, a pulley/belt system driven by a servo motor, a linear motor, or any suitable system capable of providing linear motion. The reference platen  32  may also be self-propelled. The reference platen  32  is moved a known, fixed amount between POSITION 1 and POSITION 2. This may be accomplished using at least one fixed end stop  70  mounted to the sides of the entrance and exit platens  28 ,  30 , or using encoded position information. Other techniques are also possible. The reference platen  32  may be mounted for linear movement on a rail system  69  having one or more guide rails and linear bearings. Many other known systems for guiding the reference platen  32  between POSITION 1 and POSITION 2 may also be used in the practice of the present invention.  
     [0038] As shown in FIG. 4, a pair of fixed stops  70  may be mounted to the sides of the entrance and exit platens  28 ,  30 . The use of two fixed stops  70  (i.e., a two point contact) on the sides of both the entrance platen  28  and exit platen  30  ensures that the longitudinal axis  71  of the of reference platen  32  in POSITION 1 is parallel to the longitudinal axis  71  of the reference platen  32  in POSITION 2.  
     [0039] The operation of the displacing apparatus  22  of the present invention is illustrated in FIGS.  7 - 14 . Reference should be made to FIG. 4 for components described, but not illustrated, in FIGS.  7 - 14 .  
     [0040] Initially, as shown in FIG. 7, the reference platen  32  is located in POSITION 1. At this time, air is not being drawn into the vacuum/air openings  60  in any of the platens  28 ,  30 ,  32 . Air may or may not be flowing out of the vacuum/air openings  60  in the entrance and exit platens  28 ,  30 . Recording media  24  is displaced (arrow  72 ) across the entrance platen  28  toward the reference platen  32  by a drive system or autoloading system (not shown) of a type known in the art. The registration pins  58  are in an extended state for the registration of the recording media  24 . Displacement of the recording media  24  continues as shown in FIG. 8 until the leading edge  74  of the recording media  24  contacts, and is registered by, the registration pins  58  that extend out of the reference platen  32 . Once the recording media  24  is registered on the reference platen  32 , a vacuum is drawn (arrow  76 ) through the vacuum/air openings  60  in the reference platen  32 . The vacuum holds the leading end  78  of the recording media  24  firmly in place against the surface of the reference platen  32 .  
     [0041] As shown in FIG. 9, the reference platen  32  is subsequently displaced (arrow  80 ) toward the exit platen  30  by the drive system  68  (FIG. 4), thereby displacing the recording media  24  toward the exit platen  30  to POSITION 2 as indicated by arrow  82 . Displacement continues until the reference platen  32  has been moved a known, fixed distance in direction  80 . During the displacement of the reference platen  32 , the recording media  24  is held in its registered position on the reference platen  32  by the vacuum  76 .  
     [0042] The registration pins  58  may be retracted at any point after initial registration of the recording media  24  on the reference platen  32 . The retraction of the registration pins  58  is represented by arrow  84  in FIG. 10.  
     [0043] The recording media  24  is then imaged by the scanning system  26  as shown in FIG. 10 to record a band of partial scan lines (e.g., band  10 A containing partial scan lines  12 A, FIG. 1) on the recording media  24 . Imaging occurs with the reference platen  32  located in POSITION  2 .  
     [0044] The next adjacent band of partial scan lines (e.g., band  10 B containing partial scan lines  12 B) is imaged after the recording media  24  has been displaced a known, fixed distance corresponding to the width of the band. This is accomplished, as illustrated in FIGS.  11 - 14 , by displacing the reference platen  32  from POSITION 2 back to POSITION 1 and then back to POSITION 2. The ensures that corresponding partial scan lines in each band will be aligned and butt up against each other as shown in FIG. 1.  
     [0045] Prior to the displacement of the reference platen  32  from POSITION 2 to POSITION 1, as illustrated in FIG. 11, a vacuum is drawn (arrows  86 ) through the vacuum/air openings  60  in the entrance and/or exit platens  28 ,  30 . This hold the recording media  24  in place on the entrance and/or exit platens  28 ,  30 , as the reference platen  32  is displaced from POSITION 2 to POSITION 1. Air is fed through the vacuum/air openings  60  in the reference platen  32  toward the underside of the recording media  24  as indicated by arrow  88 . This lifts the recording media  24  slightly off of the reference platen  32  to prevent the reference platen  32  from dislodging or otherwise affecting the position of the recording media  24  on the exit and/or entrance platens  28 ,  30 , as the reference platen  32  is displaced (arrow  90 , FIG. 12) from POSITION 2 to POSITION 1 by the drive system  68 .  
     [0046] As illustrated in FIG. 13, after the reference platen  32  reaches POSITION 1, vacuum is once again drawn (arrow  76 ) through the vacuum/air openings  60  in the reference platen  32 . This holds the recording media  24  firmly in place against the surface of the reference platen  32 . Air is fed through the vacuum/air openings  60  in the entrance and exit platens  28 ,  30 , toward the underside of the recording media  24  as indicated by arrows  92 . This lifts the recording media  24  slightly off of the entrance and exit platens  28 ,  30 , and reduces the drag of the recording media  24  against the entrance and exit platens  28 ,  30 , as the reference platen  32  is displaced (arrow  94 , FIG. 14) from POSITION 1 to POSITION 2 by the drive system  68 . After the reference platen  32  and attached recording media  24  have been displaced a known, fixed distance from POSITION 1 to POSITION 2, the next band of partial scan lines can be imaged on the recording media  24  by the scanning system  26 . The process described with regard to FIGS.  11 - 14  is repeated to incrementally displace the recording media  24  as many times as necessary until imaging has been completed.  
     [0047] The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention.