Abstract:
Determining and correcting registration errors, which are caused by the rotational speed of a print cylinder changing because of a sheet passing between the print cylinder and a sheet transport conveyor belt. At least one first registration mark is applied onto the conveyor belt prior to a sheet being transported thereby, and at least one second registration mark is applied onto the conveyor belt after the transported sheet. By the detection of the first registration mark and the second registration mark, a calculation of a timing number between the detection of the first registration mark and the second registration mark can be made. The calculated timing number is compared to a target value to determine any error due to change in angular velocity of the print cylinder due to engagement with the sheet.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention involves a process and a printing machine for determining registration errors due to a change in angular velocity of a print cylinder due to engagement with a printing sheet.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the printing of sheets of paper or the like by printing machines, the correctly positioned printing of the printed image on the sheet is of considerable importance. This characteristic is identified by the term registration. In order to set the registration, in addition to the printed image, registration marks are used, by which deviations from correctly positioned print are determined and measured by the operator of the printing machine. In a further embodiment of this process, the registration is determined and calculated using sensors in the printing machine. To do this, the sensors detect the registration marks on a printing sheet conveyor belt or the printing sheet and, using the position of the registration marks, determine whether the printing is being done without errors. The process and devices of the state of the art detect and correct errors that occur due to mechanical shifts of the sheet on the conveyor belt or shifts of the conveyor belt. Further, errors occur which are caused by changing of the rotational speed of a print cylinder because of a sheet running between the print cylinder and the conveyor belt. However, the distances covered, according to which the image is applied to the sheet, are defined by a specific time that passes during the movement of the conveyor belt between a sensor signal, or a signal derived from it, and a print gap or nip in a print module, in which the image is applied onto the sheet. As a result, the printed image is applied in a shifted manner onto the sheet in the print modules. This leads to a registration error.  
         SUMMARY OF THE INVENTION  
         [0003]    The purpose of the invention is thus to determine the registration errors described above. A further purpose of the invention is to correct the errors that are determined. The purposes of the invention are achieved by application by a print cylinder of at least a first registration mark onto a printing sheet transport conveyor belt prior to a sheet transported on such conveyor belt, and application of at least a second registration mark onto such conveyor belt behind the transported sheet. The first registration mark and second registration mark are detected, and a timing number between the detection of the first registration mark and the second registration mark is calculated. The calculated timing number is then compared to a target value to determine any error due to change in angular velocity of a print cylinder due to engagement with the sheet.  
           [0004]    The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    In the detailed description of the preferred embodiment of the invention presented below, reference is made to the accompanying drawings, in which:  
         [0006]    [0006]FIG. 1 shows two drawings for describing the registration error, whereby the upper drawing shows a print cylinder on a conveyor belt without the influence of the rotational speed of the print cylinder by a sheet, and the lower drawing shows a print cylinder on a conveyor belt with the influence of the rotational speed of the first cylinder by a sheet;  
         [0007]    [0007]FIG. 2 shows a part of a conveyor belt, whereby a first register mark and a second register mark are applied on the conveyor belt, a sheet being located between such registration marks; and  
         [0008]    [0008]FIG. 3 shows a device with a part of a print module involving the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]    [0009]FIG. 1 shows two drawings for describing the registration error involving the invention. The upper drawing shows a side view of an intermediate cylinder  25  of a printing machine, on which an image is applied by an imaging cylinder  23  (see FIG. 3). The intermediate cylinder  25  moves at the angular velocity ω 1  in the direction of the curved arrow and presses with the force F on the surface of a continuous conveyor belt  1 , which moves to the left in the direction of the arrow. In the upper drawing, there are no sheets  3  on the conveyor belt  1 . The lower drawing is similar to the upper drawing, except in the lower drawing a sheet  3  of paper is located on the conveyor belt  1  and is held on the conveyor belt  1  by gravity and essentially by electrostatic forces of attraction. The sheet  3  under the intermediate cylinder  25  affects the movement of the intermediate cylinder  25  in such a manner that the speed of the intermediate cylinder  25  changes in comparison with the upper drawing. As can be seen in FIG. 1, the conveyor belt  1  is pressed down in the area of the intermediate cylinder  25  in the lower drawing in comparison with the upper drawing by approximately the length that corresponds the thickness of the sheet  3 . The angular velocity of the intermediate cylinder  25  of the lower drawing is now ω 2  in comparison to the upper drawing because of the sheet  3  between the conveyor belt  1  and the intermediate cylinder  25 . This change in velocity because of the sheet  3  leads to registration errors during printing, as described in the following in detail.  
         [0010]    [0010]FIG. 2 shows two drawings, each with an overhead view of a section of a conveyor belt  1  that moves in the direction of the arrow. The upper drawing shows, only for reasons of clarification, a first registration mark  5  and a second registration mark  6 , whose front edges have a time interval between detection by a second sensor  13  behind the print modules of the printing machine (see FIG. 3). This time interval can be described in a unique manner by a timing number, in this case, timing number TARGET . The lower drawing shows a section of a conveyor belt  1 , similar to the one of the upper drawing. In this drawing, the sheet  3  is located on the conveyor belt  1  between the registration marks  5 ,  6  made on the conveyor belt  1 .  
         [0011]    As shown in FIG. 1, an intermediate cylinder  25  on the sheet  3  presses on the section of the conveyor belt  1 . In this way, as described, the angular velocity of the intermediate cylinder  25  changes from ω 1  to ω 2 . The sheet  3  exerts a brake effect on the movement of the intermediate cylinder  25  on the conveyor belt  1 . Between the intermediate cylinder  25  and the conveyor belt  1 , there is friction. If the intermediate cylinder  25  again acts directly on the conveyor belt  1  and not on the sheet  3  after the sheet  3  passes through, the velocity ω 1  again becomes set on the intermediate cylinder  25 , until another sheet  3  is conveyed under the intermediate cylinder  25 , onto which an additional image is applied by the intermediate cylinder  25 .  
         [0012]    As can be easily understood, the change in velocity of the intermediate cylinder  25  with engagement with sheet  3  leads to errors during the transfer of the image from the intermediate cylinder  25  onto the sheet  3 , since the ratio of the speeds of the conveyor belt  1  and the angular velocity of the intermediate cylinder  25  changes, whereby the constancy of this ratio is essential for the registered application of the image onto the sheet  3 . As a solution, using an arrangement of the lower drawing of FIG. 2, a calibration run is performed to determine the registration error. The first sensor  12  (see FIG. 3) detects first the front edge of the first registration mark  5  and after that, the front edge of the second registration mark  6 , corresponding to the upper drawing according to FIG. 2. The timing number between the detection of the front edges of the registration marks  5 , 6  is, in this case, however, not timing number TARGET , but deviates by an amount by which the second registration mark  6  is applied later onto the sheet  3  because of the effect described above. In fact, in the lower drawing according to FIG. 2, a time passes, between the detection of the front edge of the first registration mark  5  and the front edge of the second registration mark  6 , which can be assigned a timing number timing number ACTUAL , which is larger than the timing number timing number TARGET . By knowledge of and formation of the difference between timing number ACTUAL  and timing number TARGET , the timing difference timing number DIFF  can be determined. Timing number DIFF  describes the timing number that is counted more by the effect of the sheet  3  between the detection of the front edges of the registration marks  5 ,  6  than without the sheet  3 . As a result, using timing number DIFF , the effect of the sheet  3  on the registration of the image to be printed on the sheet  3  can be quantified and corrected with suitable mechanisms.  
         [0013]    [0013]FIG. 3 shows a schematic side view of a device  10  with a part of a print module of a printing machine above the conveyor belt  1 . Usually, the printing machine has several print modules, a print module for each ink (color), whereby the individual inks combine into an overall image. The conveyor belt  1  is driven by the drive on the deflection rollers  14 ,  16  and moves in the direction of the arrow. The first deflection roller  16 , the second deflection roller  14 , a press-on roller  27  for providing a counter-force to the press-on force of the intermediate cylinder  25 , the intermediate cylinder  25 , and the imaging cylinder  23  move in the directions shown in FIG. 3.  
         [0014]    The imaging cylinder  23  and the intermediate cylinder  25  have a first encoder  24  and/or a second encoder  26 , which detect a specific angular velocity of the imaging cylinder  23  and/or the intermediate cylinder  25 , so that the rotating angle is known at any point in time. A first sensor  12  at the beginning of the conveyor belt  1  detects the front edge of the sheet  3  and transmits a signal to a timing counter  20  in response to such front edge detection. As a result of this signal, an additional signal is generated which triggers the imaging of the imaging cylinder  23  using an imaging device  22 . The additional signal is made at exactly a point in time that the image transferred onto the imaging cylinder  23  rolls off on the intermediate cylinder  25  and is transferred from it exactly at the correct position on the sheet  3 . This is possible by knowledge of the velocity of the conveyor belt  1  with the sheet  3  and the distance of the first sensor  12  and the sensor signal generated by it from the transfer position of the image between the intermediate cylinder  25  and the sheet  3  (i.e., the print gap or nip).  
         [0015]    The time difference between the additional signal and the application of the image, which is caused by the additional signal, is hereby defined as a delay time, to which a delay value is assigned in a unique way, shown as a timing number. In the case presented, of a calibration run according to FIG. 3, the first registration mark  5  is, as described above, printed by the intermediate cylinder  25  onto the conveyor belt  1 . The printing is done in such a way that a sheet  3  follows the first registration mark  5  on the transport conveyor  1 . Next, the intermediate cylinder  25  of the print module applies the second registration mark  6  on the conveyor belt  1 . As a result, on the conveyor belt  1 , an arrangement is produced according to the lower drawing according to FIG. 2.  
         [0016]    The second sensor  13  on the end of the print module detects the front edge of the first registration mark  5  and the front edge of the second registration mark  6 . The sensor  13  transfers signals to a correction device  30 , which start the timing counter  20  upon detection of the front edge of the first registration mark  5  and stop the timing counter  20  upon detection of the front edge of the second registration mark  6 . In this way, a timing number ACTUAL  is obtained, which refers to the distance of the front edge of the first registration mark  5  from the front edge of the second registration mark  6  and can be converted into this distance using the velocity of the conveyor belt  1 . The timing number ACTUAL  can be assigned to the time, which passes from detection of the front edge of the first registration mark  5  until the detection of the front edge of the second registration mark  6 , since a timing number can be assigned a time in a unique manner. The timing number ACTUAL  is different from the timing number TARGET  for the reason that the sheet  3  causes a change of the angular velocity of the intermediate cylinder  25  in relation to the conveyor belt  1  when it passes through the nip between the intermediate cylinder  25  and the conveyor belt  1 . This effect cannot be determined for the first registration mark  5 , since it is detected before the sheet  3 , as can be seen in FIG. 3. Upon the detection of the second registration mark  6  behind the sheet  3 , the effect can be determined, however, in that the timing number DIFF  is formed from the detected timing number ACTUAL  and a saved timing number TARGET . The timing number DIFF  describes in a unique way the registration error caused by the above effect.  
         [0017]    The described calibration run can be performed several times, in order to increase the sensitivity during detection of the registration error. For this purpose, the timing number DIFF  is calculated. In the correction device  30 , the delay value shown as a timing number is changed by the timing number DIFF . Then a corrected delay value is present, which takes into account the registration error described above. During printing following the calibration run, the corrected delay value is used. As a result, the imaging device  22  begins with the transfer of the corresponding image at another point in time than without the calibration run described above. The term “image” describes, in relation to this invention, image lines, image sections and images of individual color separations of the print modules, which combine to form an overall image. The above description contains examples with sheets  3 . The invention extends, however, to all types of non-continuous printed material and is not limited to sheets.  
         [0018]    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 spirit and scope of the invention.