Abstract:
A system for detecting the alignment of a top plate placed on a plate pallet. The alignment is detected relative to predetermined coordinates of the system. The system will generally include the following components: An illumination source configured to illuminate light on at least one edge of the top plate; a scanner for acquiring scanning results of the top plate edges. The scanning result will be analyzed by an analyzing component, in order to align the top plate correctly before it is loaded into the CTP imaging device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned copending U.S. patent application Ser. No. 12/045,058, filed Mar. 10, 2008, entitled PLATE PALLET ALIGNMENT SYSTEM, by Korolik et al., the disclosure of which is incorporated herein. 
     TECHNICAL FIELD 
     The present invention relates generally to computer to plate (CTP) imaging devices, and more particularly to detecting the alignment of a top most plate in a plate picker system within a CTP imaging device. 
     BACKGROUND OF THE INVENTION 
     Prior to setting forth the background of the invention in detail, it may be helpful to set forth definitions of certain terms that will be used hereinafter. 
     The term “Computer to plate” (hereinafter: CTP) as used herein in this application, relates to an imaging technology used in modern printing processes. In this technology, an image created in a Desktop Publishing application is output directly to a printing plate. CTP as used hereinafter relates also to the imaging device carrying the process of outputting the computer-stored image on the printing plates. 
     The term “Printing Plates” (sometimes referred to as “plate” or “plates”) as used herein in this application, relates to the flexible aluminum or plastic plates that are used in off-set printing technology. Traditional printing plates have a brushed or roughened texture and are covered with a photosensitive emulsion. A photographic negative of the desired image is placed in contact with the emulsion and the plate is exposed to light. After development, the emulsion shows a reverse of the negative image, which is thus a duplicate of the original (positive) image. The image on the plate emulsion can also be created through direct laser imaging in a CTP imaging device. Printing plates are typically stacked in a cassette, on a pallet or similar container which houses the plates and facilitates their protection, transportation, and handling, before loading them into a dedicated tray within a CTP imaging device. 
     Every CTP imaging device usually comprises trays for accommodating the printing plates exhibiting specific dimensions. Trays can usually be arranged to contain plates of various sizes, but all plates in the same tray are of one size. The plates may be manually removed from the cassette, pallet or a shipping container and then inserted into the trays for use by the CTP imaging device. 
     After the plates are loaded into a CTP imaging device tray and prior to the plate loading, the slip sheet is removed from the loaded plate. According to the traditional art, the slip sheet is removed from the CTP device using a slip sheet disposal system. 
     As the top plate positioned on the plate stack is often not positioned perfectly relatively to the plate stack, an alignment process has to be performed in order to position the top plate so that it is properly aligned for the plate picker operation. 
     According to the traditional art, a system for handling printing plates will generally use trays or plate pallet, the plates having specific dimensions. Different trays can usually be set to contain plates of various sizes, but all plates in a single tray will be in the same size. The plates can be picked automatically from the relevant trays or directly from the originally packed plate pallet and loaded into the imaging device for imaging. There is a widely recognized need for an automatic and efficient handling system for feeding plates directly from the original plate pallet into the imaging device, while maintaining precise alignment of the plate during the plate loading process. 
       FIG. 1  describes a pallet alignment and picker apparatus  10  according to the prior art. The top plate  13  is positioned on top of a plate pallet  14  (or a plate stack). Top plate  13  is often shifted slightly in respect to the plate pallet due to manual handling of the plate stack or pallet. 
     This manual process introduces inherent position inaccuracies in picking the top plate, in respect to the plate picker  11 . Plate picker  11  is configured to pick a top plate  13  and load it into a CTP imaging device  20  (Shown in  FIG. 2 ). The top plate  13  should be perfectly aligned with the plate picker position  12  in order to load plate  13  in a perfect position into the imaging device  20 . 
     BRIEF SUMMARY 
     Briefly, according to one aspect of the present invention, there is provided a system for detecting misalignment of a top printing plate placed over a plate pallet, the plate exhibiting plate edges, wherein the alignment is detected in view of predetermined coordinates, the system comprising: an illumination source arranged to direct a light beam on at least one particular plate edge of the plurality of plate edges in a predefined spatial angle selected such that the light beam exhibits a spot of light covering a portion of the top plate, the particular edge and a portion of the plate pallet; a scanner arranged to scan the spot of light and produce an image thereof, and an analyzer arranged to detect the misalignment of the top printing plate by analyzing the differences between the properties of the image of the scanned spot of light and predefined properties pertaining to a spot of light covering a plate edge of a properly aligned top printing plate. 
     According to another aspect of the invention, there is provided a method of detecting misalignment of a top printing plate placed over a plate pallet, the plate exhibiting plate edges, wherein the alignment is detected in view of predetermined coordinates, the method comprising: directing a light beam on at least one particular plate edge of the plurality of plate edges in a predefined spatial angle selected such that the light beam exhibits a spot of light covering a portion of the top plate, the particular edge and a portion of the plate pallet; scanning the spot of light and producing an image thereof; detecting the misalignment of the top printing plate by analyzing the differences between the properties of the image of the scanned spot of light and predefined properties pertaining to a spot of light covering a plate edge of a properly aligned top printing plate. 
     According to another aspect of the invention, there is provided a computer-to-plate (CTP) imaging device comprising a system for detecting misalignment of a top printing plate placed over a plate pallet operatively associated with the CTP imaging device, the plate exhibiting plate edges, wherein the alignment is detected in view of predetermined coordinates, the system comprising: an illumination source arranged to direct a light beam on at least one particular plate edge of the plurality of plate edges in a predefined spatial angle selected such that the light beam exhibits a spot of light covering a portion of the top plate, the particular edge and a portion of the plate pallet; a scanner arranged to scan the spot of light and produce an image thereof, and an analyzer arranged to detect the misalignment of the top printing plate by analyzing the differences between the properties of the image of the scanned spot of light and predefined properties pertaining to a spot of light covering a plate edge of a properly aligned top printing plate. 
     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter regarded as the invention will become more clearly understood in light of the ensuing description of embodiments herein, given by way of example and for purposes of illustrative discussion of the present invention only, with reference to the accompanying drawings (Figures, or simply “FIG.”), wherein: 
         FIG. 1  is a schematic illustration of a plate pallet exhibiting a plate picker according to the prior art; 
         FIG. 2  is a perspective view of a computer-to-plate imaging device showing a plate pallet according to the prior art; 
         FIG. 3A  is a schematic illustration of a top plate aligned with plate pallet according to some embodiments of the present invention; 
         FIG. 3B  is a schematic illustration of a top plate shifted in respect to plate pallet; 
         FIG. 3C  is a schematic illustration of a shadow created due to top plate shift according to some embodiments of the present invention; and 
         FIG. 4  is a schematic of a laser pointers system showing a top micro view of the plate pallet according to some embodiments of the present invention. 
     
    
    
     The drawings together with the description make apparent to those skilled in the art how the invention may be embodied in practice. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be understood by those skilled in the art that the teachings of the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the teachings of the present disclosure. 
     The present invention includes an apparatus, system and a method for correctly position a printing plate to be imaged. The correct positioning of the printing plate is made in respect to the plate loading mechanism of a CTP imaging device. The printing plate to be imaged is usually a top plate picked up from a stack of printing plates. 
     According to some embodiments there is provided a system for detecting misalignment of a top printing plate placed over a plate pallet, the plate exhibiting plate edges, wherein the alignment is detected in view of predetermined coordinates. The system comprising: an illumination source arranged to direct a light beam on at least one particular plate edge of the plurality of plate edges in a predefined spatial angle selected such that the light beam exhibits a spot of light covering a portion of the top plate, the particular edge and a portion of the plate pallet; a scanner arranged to scan the spot of light and produce an image thereof; and an analyzer  404  (shown in  FIG. 4 ) arranged to detect the misalignment of the top printing plate by analyzing the differences between the properties of the image of the scanned spot of light and predefined properties pertaining to a spot of light covering a plate edge of a properly aligned top printing plate. 
     Referring to  FIG. 1-2 , top perfect positioning of plate  13  in respect to plate picker position  12  is performed by moving the entire plate pallet  14  in the pallet positioning path  15 . 
     According to the present invention, the required movement direction and amount is derived from the exact position of top plate  13 . The exact position of top plate  13  is calculated from the following parameters: plate size—a known parameter per plate type; chassis position  32  (FIG.  3 A)—fixed position—a known parameter per CTP imaging device; and plate edge detection. This parameter is the product of the disclosed invention hereunder. 
     The edge of top plate  13  is detected by processing an image of the plate edge captured by camera  16 . The captured image with the lighting conditions inside the CTP imaging device  20  will not produce adequate information for processing the edge of top plate  13 . 
     The disclosed invention includes applying specific illumination means on top plate  13  and plate pallet  14  prior to capturing an image with camera  16 . The applied illumination will help in finding out the exact plate edge. 
     The illumination is performed with a plurality of laser pointers.  FIG. 3A  and  FIG. 3B  shows pointers  35  positioned against the vertical axis and pointers  36  positioned against the horizontal axis of top plate  13 . Camera  16  is configured to capture a dedicated camera window  31  above the edge of top plate  13 . The captured camera window  31  is analyzed to inspect the shapes created by vertical beams  33  and horizontal beams  34 . The beams are created by pointers  35  and  36  respectively. In the case where top plate  13  is not perfectly aligned on plate pallet  14  a shadow line  37  will be detected as is shown  FIG. 3B .  FIG. 3C  shows an example of shadow line  37  captured by camera  16 . The start shadow line  37  will indicate the edge of the plate in this case. 
     In the case where the top plate  13  is perfectly aligned to pallet  14  or top plate  13  is extending over plate pallet  14  the result of the illumination will not create a shadow. In this case the end of the created beams  33 ,  34  will indicate the edge of plate  13 . 
     The detected plate edge together with the fixed chassis position  32  will indicate the correct amount and direction that the plate pallet  14  position should be adjusted to perfectly align with plate picker position  12 . The repositioning of plate pallet  14  enables perfect loading of plate  13  into the CTP imaging device  20 . 
       FIG. 4  describes the optical system disclosed in this invention. Laser source  41  is positioned above the plate pallet  14 . Laser source  41  illuminates a light beam  43  through lens  42 . Beam  43  is directed towards the edge of top plate  13  at a laser projection angle  44  (a typical angle is around 8 degrees and may range between 5 to 15 degrees approximately). A shadow is formed between the top plate  13  and the plates underneath it. The created shadow range  46  is dependent upon plate thickness  45 , laser projection angle  44  and top plate  13  shift amount in respect to plate pallet  14 . 
     The size of the created shadow is captured by camera  16  via capture data  412 . The alignment of plate  13  will be calculated by analyzer  404 , by analyzing the data received from camera  16  via communication lines  408 , according to the size of the captured shadow by camera  16  and will applied on top plate  13  before loading it into device  20 . 
     According to some embodiments of the invention, the disclosed system may be further arranged to convert a particular image of the scanned spot of light to a corresponding signature, and wherein the detection of top printing plate misalignment is achieved by comparing the signature of the particular image of the scanned spot of light to predetermined plurality of signatures, each of the plurality of the predetermined plurality of signatures is associated with a specific printing plate misalignment exhibiting specific coordinates. 
     According to some embodiments of the invention, the scanned spot of light exhibits variations in the intensity of light in accordance with the misalignment of the top printing plate, and wherein the analyzer is further arranged to detect the misalignment of the top printing plate in view of the variations of the intensity of light in the scanned spot of light. 
     According to some embodiments of the invention, the scanned spot of light exhibits a shadow line wherein the location of the shadow line is associated with the misalignment of the top printing plate and wherein the analyzer is further arranged to detect the location of the particular edge of the top printing plate in view of the location of the shadow line. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents. 
     PARTS LIST 
     
         
           10  pallet alignment and picker apparatus 
           11  plate picker 
           12  plate picker position 
           13  top plate 
           14  plate pallet 
           15  pallet positioning path 
           16  camera 
           20  CTP imaging device 
           31  camera window 
           32  chassis position 
           33  vertical beams 
           34  horizontal beams 
           35  vertical pointers 
           36  horizontal pointers 
           37  shadow 
           41  laser source 
           42  optics lens 
           43  beam 
           44  laser projection angle 
           45  plate thickness 
           46  shadow width 
           404  analyzer 
           408  communication line between camera and analyzer 
           412  camera data capture lines