Abstract:
A write head of an imaging system for producing a printing plate can be calibrated quickly, simply and without errors. Test patterns are produced with the write head, the deviation of a property of the write head from a reference value is determined, and a corrective parameter in the write head is adjusted in order to compensate for the deviation. Test patterns that can be evaluated visually with regard to the writing quality are produced in a plurality of test fields with different parameter values, an identifier that can be picked up visually is produced with each test field, and the identifier of the test field which appears best in terms of quality is entered into a control device for the write head. The printing plate is produced by using the entry of the identifier for automatically setting the parameter value with which the best test field was produced.

Description:
BACKGROUND OF THE INVENTION  
     Field of the Invention  
       [0001]     The invention lies in the printing technology field. More specifically, the invention relates to a method of calibrating a write head for producing a printing plate. By using test patterns produced with the write head, the deviation of a property of the write head from a reference value is determined, and a corrective parameter in the write head is adjusted in order to compensate for the deviation.  
         [0002]     In order to achieve a short imaging time, in imaging systems a plurality of imaging heads are used simultaneously. Each imaging head images a subregion on a printing plate blank. In prior art imaging systems, a plurality of imaging heads are mounted on a carriage which can be displaced parallel to the axis of a printing plate cylinder. Each imaging head contains at least one radiation source whose emission direction should point exactly perpendicularly at the axis of rotation of the printing plate cylinder. Errors in the mounting of an imaging head result in errors in the printed image to be produced. For example, overlapping lines or non-imaged strips can be produced between two subregions. In the case of imaging heads with individual emitters arranged along a line, errors occur if an individual emitter is not in line or the reference line of the individual emitter does not run parallel to the axis of rotation of a printing plate cylinder. Zigzag edges then manifest themselves in the printed image.  
         [0003]     In order to avoid or reduce imaging errors, the imaging systems are calibrated. It is known to determine corrective values by using test exposures and, by using the corrective values, to perform mechanical, electronic or programming adjustments to the imaging system. For instance, imaging heads can be aligned on a carriage, the power of the radiation sources can be adjusted or the time of activation of the radiation sources can be changed. In order to determine the corrective values, the test exposures are measured. Measuring instruments are used to determine the extent to which a position or dimension of an element from a test field deviates from predefined variables. For this purpose, the test field can be evaluated directly on a printing plate or its image can be evaluated after being printed on a printing material. If the measurements are carried out by an operator, then there is the risk of subjective measurement errors and errors in the calibration of an imaging system. If, for example, an imaging head having radiation sources arranged along a line has a skewed position, then by using a test exposure, the angle by which the imaging head is tilted with respect to the axis of rotation of a printing plate cylinder is measured. The angular measurement may be carried out only with finite accuracy. If the imaging head provides electronic correction in the form of a delay of the activation of individual radiation sources in 1/16 of the dimensions of an image point, then, by using the angular deviations, the operator has to define how the delay of each individual channel has to be adjusted in order to compensate for the skewed position of the imaging head. These adjustments made by a person are inaccurate and time-consuming.  
         [0004]     German published patent application DE 102 15 694 A1 describes a method for producing a printing plate in which a test image is produced in a non-subject region and is evaluated with a reader and a computer. The manner in which the correction and setting values for subsequent imaging in the useful subject region are derived is not disclosed in detail.  
         [0005]     In a production method for a printing plate according to international PCT publication WO 92/12011 (cf. DE 69 212 801 T2), test prints, which are measured, are produced with a test printing plate. In that case, the position deviations of image points are determined. From the position deviations of the image points, corrective values in two coordinates are stored in the form of a table. The stored corrective values are used as a function of position during the imaging of printing plates. Measuring a test print point by point is time-consuming.  
       SUMMARY OF THE INVENTION  
       [0006]     It is accordingly an object of the invention to provide a method of calibrating a print head for producing a printing plate which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which renders it possible to adjust an imaging system quickly, simply and without errors by using test exposures.  
         [0007]     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for calibrating a write head for producing a printing plate, which comprises:  
         [0008]     producing test patterns configured for visual evaluation with regard to a writing quality in a plurality of test fields with different parameter values;  
         [0009]     producing visually detectable identifiers with each test field;  
         [0010]     determining a test field that appears best in terms of quality and entering the identifier of the test field that appears best into a control device for the write head; and  
         [0011]     automatically setting, in accordance with the entered identifier, a parameter value with which the test field that appears best was produced, for producing the printing plate.  
         [0012]     In other words, first of all test patterns that can be assessed visually are produced with various parameter values. In each test field, a possible value of a corrective variable is used which is suitable for correcting an adjustable property. The test field in which the correction (i.e., parameter variation) functions best or has the best visually detected result, is determined the best test field. For the purpose of visual assessment, an operator can use optical aids, such as a magnifying glass. Each test field contains a criterion which can be seen easily and which permits selection as the best test field. All the test fields are provided with an indicator. In a simple case, the test fields are numbered consecutively, so that a number of the best test field can be read off. In addition to numbers, letters, symbols or color markings can also be used as indicators. The number of the best test field is entered into a control system of the imaging system. The controlling software makes an allocation of the indicator entered to a parameter value with which the best test field was produced. For subsequent imaging operations, this parameter value is automatically used. The invention can be used in external plate exposers and in imaging systems which are integrated into a press.  
         [0013]     In accordance with an added feature of the invention, the write head is a head with a plurality of laser diodes mounted along a straight line, and the method further comprises calibrating a deviation of the holder of the write head by producing linear test fields having an orientation with respect to the straight line associated with a directional or angular error of the write head.  
         [0014]     In accordance with an additional feature of the invention, the identifiers are mutually different numbers and/or letter combinations and they are produced in a surrounding of the associated test fields.  
         [0015]     In accordance with another feature of the invention, the test fields are produced in a series with parameters changed step by step.  
         [0016]     In accordance with a concomitant feature of the invention, the visually assessable test fields are produced on a test printing plate.  
         [0017]     Once more in summary, the method permits an imaging system to be adjusted quickly, simply and without errors by using test exposures. In a method for calibrating a write head for producing a printing plate, in which, by using test patterns produced with the write head, the deviation of a property of the write head from a reference value is determined, and in which a corrective parameter in the write head is adjusted in order to compensate for the deviation, the invention consists in that test patterns that can be evaluated visually with regard to the writing quality are produced in a plurality of test fields with different parameter values, an identifier that can be picked up visually is produced with each test field, the identifier of the test field which appears best in terms of quality is entered into a control device for the write head, and, in order to produce the printing plate by using the entry of the identifier, the parameter value with which the test field that appears best in terms of quality was produced is set automatically.  
         [0018]     Other features which are considered as characteristic for the invention are set forth in the appended claims.  
         [0019]     Although the invention is illustrated and described herein as embodied in a method for calibrating a write head for producing a printing plate, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
         [0020]     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a schematic view of an imaging system having four imaging heads;  
         [0022]      FIG. 2  is a developed view of a printing plate blank having correctly produced imaging regions;  
         [0023]      FIG. 3  is a developed view of a printing plate blank having imaging regions with zigzag edges;  
         [0024]      FIG. 4  is a plan view of an arrangement of test patterns for tilt calibration;  
         [0025]      FIG. 5  illustrates a development of a printing plate blank having laterally offset imaging regions;  
         [0026]      FIG. 6  is a plan view of an arrangement of test patterns for module spacing calibration;  
         [0027]      FIG. 7  illustrates a development of a printing plate blank having imaging regions that are offset in the circumferential direction; and  
         [0028]      FIG. 8  is a plan view of an arrangement of test patterns for vertical calibration. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a schematic drawing of an imaging system which is integrated in a printing press. A printing plate cylinder  3  is held in bearings  4 ,  5  such that it can rotate between the side walls  1 ,  2 . A printing plate blank  6  is clamped on the printing plate cylinder  3 . In order to produce easily visible test image points on the surface of the printing plate blank  6 , four imaging heads  7 - 10  are provided. The imaging heads  7 - 10  are disposed on a longitudinal guide  11 . The imaging heads  7 - 10  can be positioned jointly by a spindle drive  13  in the direction of the axis of rotation  12 . The spindle drive  13  is rotatably mounted in bearings  14 ,  15  in the side walls  1 ,  2  respectively.  
         [0030]     The imaging heads  7 - 10  contain laser diode arrays  16 - 19  including optically projecting elements and control technology. A laser diode array  16 - 19  comprises  64  individually activated laser diodes  20  which are aligned along a line parallel to the axis of rotation  12 . A spacing distance a of the laser diodes  20  in the direction parallel to the axis of rotation  12  is greater than the minimum spacing of two image points to be produced. When a laser diode  20  is activated, a laser beam  21  orthogonal to the axis of rotation  12  is produced.  
         [0031]     The printing plate cylinder  3  and the spindle drive  13  are in each case coupled to motors  22 ,  23  and rotary encoders  24 ,  25 . The imaging heads  7 - 10 , the motors  22 ,  23  and the rotary encoders  24 ,  25  are connected to a control device  26 . The control device  26  contains computing means for controlling the press during printing and during imaging. The keyboard  27  permits the entry of data by an operator. A monitor is used to display control information.  
         [0032]     The laser diode arrays  16 - 19  have mounting errors, so that the laser beams  21  are emitted at an angle to the axis of rotation  12 . In the common plane of the laser diodes  20  and the axis of rotation  12 , the laser diode arrays  16 - 19  have, for example, angular deviations α 1  to α 4 . The printing plate blank  6  is imaged in accordance with what is known as the interleave method, as described in German published patent application DE 101 08 624 A1 and the corresponding publications U.S. Pat. No. 6,765,604 B2 and US 2002/0154207 A1. By means of suitable selection of the advance of the laser diode arrays  16 - 19  in the direction of the axis of rotation  12 , test imaging without gaps can be achieved after traveling over a marginal region. Each laser diode array  16 - 19  produces screen or image points in a subregion of the printing image region  30  along lines  29  running in the circumferential direction of the printing plate cylinder  3 .  
         [0033]     The imaging heads  7 - 10  and the laser diode arrays  16 - 19  are connected to one another via a data line  31 . The data items are placed one after another on the data line  31 , the control technology of the laser diode arrays  16 - 19  extracting the respective data items from the data stream. The data items for activating the laser diode arrays  16 - 19  are organized in the form of data packets, so that in each case  64  bits for the  64  laser diodes  20  are sent to a laser diode array  16 - 19 .  
         [0034]      FIG. 2  shows imaging regions  32 - 35  on a printing plate blank  6  which are produced by the imaging heads  7 - 10  given an ideal alignment. In the boundary regions  36 - 38 , the lines  29  are located such that there are no overlaps or unexposed strips. The external contour of the printing image region  30  formed from the individual imaging regions  32 - 35  runs exactly in the shape of a rectangle.  
         [0035]     The compensation of the skewed position of the laser diode arrays  16 - 19  is to be described by using  FIGS. 3 and 4 . A skewed position of the laser diode arrays  16 - 19  results if the laser diodes  20  are arranged on a straight line  39  ( FIG. 1 ) lying obliquely with respect to a plane which contains the axis of rotation  12  of the printing plate cylinder  3  and the direction of the laser beams  21  running at right angles thereto. A skewed position of the laser diode arrays  16 - 19  results in the imaging regions  32 - 35  shown in  FIG. 3 . As a result of the tilting of the laser diode arrays  16 - 19  about the aforesaid plane, zigzags  40  result at the upper and lower edges of the imaging regions  32 - 35 . In order to avoid the zigzags  40 , the tilting of the laser diode arrays  16 - 19  must be compensated for. For this purpose, test fields  41  with an associated number  42  are produced on a printing plate blank  6  by each laser diode array  16 - 19 , as illustrated in  FIG. 4 . In each test field  41 , a horizontal line  43  is imaged. In each test field  41 , a different electronic delay of the individual channels of the laser diode arrays  16 - 19  is set, so that the result is virtual tilting of the laser diode arrays  16 - 19 , which manifests itself in a skewed position of the lines  43  on the printing plate blank  6 . As viewed in the circumferential direction  44  of the printing plate blank  6 , the laser diodes  20  of the laser diode arrays  16 - 19  experience linearly rising and falling turn-on delays along the lateral direction  45 . The numbers  42  of the test fields  41  which are produced with the laser diodes  16 - 19  lie in various value ranges w, x, y, z, with w=001-080, x=081-160, y=161-240 and z=241-320. By means of a magnifying glass, the test field  41  which has a line  43  which is produced continuously without discontinuities is determined visually. The lines  43  are in each case produced twice with different line thicknesses. The thicker lines  43  can be used for a first orientation. The relevant number  42  of the best test field  41  is then determined by using the thin lines  43 . The number w, x, y, z of the line  43  which actually appears as a continuous horizontal line  43  on the printing plate blank  6  is determined for each laser diode array  16 - 19  and entered into the control device  26  via the keyboard  27 . By using the numbers w, x, y, z, values for the electronic delay in the activation of the laser diodes  20  of the laser diode arrays  16 - 19  are determined with a program and stored for future imaging operations.  
         [0036]     In this method, it is not necessary for an operator to know the actual skewed position of the laser diodes  16 - 19 . Therefore, subjective errors in determining and reading the skewed position are ruled out. The operator does not have to calculate any corrective values either since this is done automatically by a computer in the control device  26  after the numbers  42  of the best test field  41  have been entered.  
         [0037]     The laser diode arrays  16 - 19  always have positioning errors in the lateral direction  45  following mounting. As a result, the imaging regions  32 - 35  are displaced in the lateral direction  45 , as shown in  FIG. 5 . Overlaps  46 ,  47  form between the imaging regions  32 ,  33  and  34 ,  35 . A non-imaged strip  48  is produced between the imaging regions  33 ,  34 . In order to calibrate the spacing of the laser diode arrays  16 - 19  in the lateral direction  45 , test imaging is carried out on a printing plate blank, as illustrated in  FIG. 6 . The test image contains three groups of test fields  49  located in the circumferential direction  44 , each test field  49  being assigned a number  50 . Each group of test fields  49  is used to calibrate the spacing of the laser diode arrays  16 - 19  in the boundary regions  36 - 38 . A test field  49  consists of two lines  51 ,  52  located in the circumferential direction  44 , which are each produced by adjacent laser diode arrays  16 ,  17 ;  17 ,  18 ;  18 ,  19 . In each test field group, the spacing of the lines  51 ,  52  is reduced and increased step by step by means of delayed activation of the laser diodes  20  in the lateral direction  45 . Using a magnifying glass, the test field  49  in which the two lines  51 ,  50  lie above each other is determined visually for all the test field groups. As described in the case of the tilt calibration, the numbers x, y, z of the test fields in which the lines  51 ,  52  lie above each other are entered into the control device  26  via the keyboard  27 . The values for the delayed activation of the laser diodes  20  in the lateral direction  45  are given automatically by the numbers x, y, z from different value ranges. The values are stored for future imaging operations.  
         [0038]     In  FIG. 7 , imaging regions  32 - 35  offset from one another in the circumferential direction  44  are illustrated. An offset  53  in the circumferential direction  44  arises when a laser diode array  16 - 19  is vertically too high or too low with respect to another laser diode array  16 - 19 . In order to calibrate an offset  53 , a test exposure, shown in  FIG. 8 , is made on a printing plate blank  6 . The test imaging contains three groups of test fields  54  located in the circumferential direction  44 , each test field  54  being assigned a number  55 . Each test field group is used to calibrate the vertical position of one of the laser diode arrays  16 - 19 . A test field  54  consists of two lines  56 ,  57  located in the lateral direction  45 , which are each produced by two adjacent laser diode arrays  16 ,  17 ;  17 ,  18 ;  18 ,  19 . In each test field group, the spacing of the lines  56 ,  57  is reduced and increased step by step by means of delayed activation of the laser diodes  20  in the circumferential direction  44 . The test fields in which the lines  56 ,  57  are aligned are determined with a magnifying glass. The numbers  55  of these test fields  54  are entered into the control device  26  via the keyboard  27 . As in the case of the calibrations already described, the correct values for the delay of the activation of the laser diodes  20  in the circumferential direction are stored automatically for future imaging operations.  
         [0039]     The tilt calibration with the test fields  41  according to  FIG. 4 , the spacing calibration with the test fields  49  according to  FIG. 6 , and the vertical calibration with the test fields  54  according to  FIG. 8  are expediently carried out one after another in the order mentioned. The test fields  41 ,  49 ,  54  can be arranged on a printing plate blank in such a way that only one printing plate blank  6  is needed for all the calibrations.  
         [0040]     This application claims the priority, under 35 U.S.C. § 119, of German patent application No. 10 2004 021 326.7, filed Apr. 30, 2004; the entire disclosure of the prior application is herewith incorporated by reference.