Abstract:
The quality of processed or printed sheets is assessed while the sheets are supported on the periphery of a drum. The surface of each drum is provided with sheet leading and trailing end gripper systems which are circumferentially, and axially shiftable with respect to each other. Each sheet to be inspected is stretched tightly on the periphery of its supporting drum before the quality of the printing or other processing done to the sheet is assessed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and a device for the qualitative assessment of processed sheets. 
     DESCRIPTION OF THE PRIOR 
     DE 32 20 098 C2 describes a numbering machine with a device for the qualitative assessment, by means of two testing cylinders, of the front and reverse sides of sheets to be numbered. 
     It is a limitation of this prior art device that no guide elements for holding the sheets on the testing cylinders are provided. 
     DE 42 23 555 A1 discloses a drum for transporting sheets in a reversing device of a sheet-fed rotary printing press. This drum is provided with grippers for gripping the front edge of the sheets, and with displaceable suction elements for stretching the rear edge of the sheet tight. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to create a method and a device for the qualitative assessment of processed sheets. 
     This object is attained in accordance with the present invention by providing at least one drum which receives the sheet to be inspected. This drum passes the sheet by an image recording device, and is provided with a first holding device that grips the leading end of the sheet. At least one second holding device is also provided on the drum and holds the trailing end of the sheet. The two holding devices stretch the sheet tightly both circumferentially and axially on the surface of the drum prior to the inspection of the sheet. Once the sheet has been gripped and stretched, one page on the sheet can be inspected. 
     By means of the method or the device in accordance with the present invention, it is possible in an advantageous manner to prevent false measurements because of sheets having creases or being dished or otherwise deformed. The dependability of the image inspection is considerably increased, in particular in case of sheets having creases. 
     The arrangement of two drums, both of which have two holding devices for stretching the sheets tight, in particular makes a dependable inspection of the front and reverse sides of the sheets after their processing is finished possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The device for the qualitative assessment of processed sheets in accordance with the present invention is represented in the drawings and will be described in more detail in what follows. 
     Shown are in: 
     FIG. 1, a schematic lateral view of a device for the qualitative assessment of printed sheets; 
     FIG. 2, a schematic plan view of a drum. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A device generally at  1 , for the qualitative assessment of processed sheets  5  has been installed in a sheet-processing machine. In the present preferred embodiment, this is a sheet-fed rotary printing press, which imprints sheets of paper  5  on their front and reverse sides  2 ,  3 . 
     For reasons of simplification, only the sheet transport device downstream of the print units, as far as the deposit of the sheets  5  on stacks  4 ,  6 ,  7  in a delivery device  8 , has been represented. 
     Processed sheets  5  are understood to be, for example, sheets  5  which have been printed, embossed or provided with a pattern in other ways. 
     A first chain conveyor  9  is arranged following a printing cylinder, not represented. The chains  11  of this chain conveyor  9  first extend horizontally in the vicinity of the floor, and then vertically as far as across the stacks  4 ,  6 ,  7  of the delivery device  8 , to a first chain wheel  10 . Chain gripper devices  12  are fastened on these chains  11 . Following the vertical portion of this first chain conveyor  9 , horizontally extending chains  13  of a second chain conveyor  14  are arranged, on which chain gripper devices  16  are also provided. These chains  13  are reversed by two chain wheels  15 ,  20 . A first drum  17  is arranged to work together with these chain gripper devices  16 , and a second drum  18  directly cooperates with it. Following this, a third chain conveyor  19 , with chains  38  is arranged directly above the stacks  4 ,  6 ,  7 , which conveys the sheets  5  to the stacks  4 ,  6 ,  7  of the delivery device  8 . The chains  38  are reversed by means of three chain wheels  39 ,  41 ,  42 . In the present preferred embodiment, chain gripper systems  21  of the third chain conveyor  19  can selectively deposit the sheets  5  on one of the three stacks  4 ,  6 ,  7 . For example, inspected sheets  5 , which do not comply with the present quality requirements, are deposited on one (in the example the left stack  7 ) of the three stacks  4 ,  6 ,  7 . 
     The two drums  17 ,  18  respectively, as seen also in FIG. 2, each consist essentially of a first drum base body  22  with first segments  23 , a second drum base body  24  with second segments  26 , and first holding devices  27  and second holding devices  28 . The holding devices can be respectively embodied as gripper or suction devices. The first drum base body  22  is seated by means of hollow shaft journal in lateral frames of the delivery device  8 . A shaft  29  is pivotably seated in this hollow shaft journal. The second segments  26  extending in the circumferential direction are fastened next to each other on this shaft  29 . The second holding device  28  is arranged, displaceable in the circumferential direction and the axial direction, in the area of the ends of the segments  26 . This second holding device  28  is constituted by a plurality of suction devices  28 , arranged next to each other in the axial direction, which can be charged with suction air in a controlled manner. 
     The first holding device  27  in the form of controllable grippers  27 , arranged next to each other in the axial direction, is provided at the beginning of the first drum base body  22 . First  23  and second segments  26  are arranged meshing in a comb-like manner, so that a cylinder-shaped peripheral surface  31  is formed. The distance between suction devices  28  and grippers  27  in the circumferential direction, i.e. a length  131  of the surface  31  in the circumferential direction, can be adapted to a length  11  of the sheets  1  to be processed. To this end the first  23  and second segments  26  are pivotable in the circumferential direction and can be fixed in place following the process of setting the length of surface  31 . In the present preferred embodiment, each drum  17 ,  18  is designed as a “double-size” drum  17 ,  18 , i.e. two first  27  and two second holding devices  28  with the associated elements  23 ,  26  are arranged on the circumference, offset by 180° in respect to each other. 
     Each one of these two drums  17 ,  18  has been assigned its own inspection device. Essentially this inspection device, as shown in FIG. 1, consists of an illumination device  32 ,  33  and an image recording device  34 ,  36 . In the present preferred embodiment, a plurality of stroboscopic flashes  37  is provided as the illumination device  32 ,  33 , which are arranged in such a way that an even illumination of the sheet  5  to be inspected takes place. This means that an angle and a distance of the light outlet surface of the stroboscopic flashes  37  are matched to the surface  31  of the drum  17 ,  18  and to the lens of the image recording device  34 ,  36 . 
     The respective image recording device  34 ,  36  consists of at least one planar CCD camera, preferably of two planar CCD cameras  34 ,  36 , arranged next to each other in the axial direction of the drum  17 ,  18 . Preferably four individual recordings, corresponding to each quadrant, are taken of each sheet  5  to be inspected, which are again superimposed on each other to form a whole image in an evaluation device connected downstream of the planar CCD cameras  34 ,  36 . This whole image is then evaluated, for example in a method corresponding to the method described in DE 42 06 366 A1. 
     Two successive images per sheet  5  to be inspected are recorded by each one of the two planar CCD cameras  34 ,  36 . 
     However, it is also possible to record a single image of the sheets  5  by means of a single planar CCD camera. 
     The inspection device is disposed downstream of the transfer area from the second chain conveyor  14  and the first drum  17  in such a way that, after stretching, the sheet  5  to be inspected is located in the evaluation area of the planar CCD camera  34 . The second inspection device is arranged in the same way between the transition area from the first drum  17  to the second drum  18 , or respectively the second drum  18  and the third chain conveyor  19 . 
     The mode of operation of the device in accordance with the present invention is as follows: 
     The chain gripper devices  12  of the first chain conveyor  9  take over the printed sheets  5  from a printing cylinder, not represented. The inspection of the front  2  and reverse side  3  of the sheets  5  only takes place after printing, i.e. following complete processing, as the last work step prior to deposition in the sheet processing machine. These chain gripper devices  12  convey the sheets  5  first in the horizontal, then in the vertical direction to the second chain conveyor  14  arranged above the stacks  4 ,  6 ,  7 , and transfer the sheets  5  to the first drum  17 . In the process, the grippers  27  of the first drum  17  grip the beginning of the sheet  5  to be inspected. After approximately one-half rotation of the drum  17 , the end of each of the sheets  5  reaches the area of the suction devices  28 , which are thereupon charged with suction air and in this way grasp the end of the sheets  5 . Subsequently, the suction devices  28  perform a movement both in the circumferential direction and the axial direction of the drum  17  and in this manner tightly stretch the sheet  5 . Now the sheet  5  lies, free of creases, on the first drum  17 . In the present example, the reverse side  3  of the sheets  5  faces away from the drum  17  and points in the direction toward the planar CCD cameras  34 . Now the stroboscopic flashes  37  illuminate the sheet  5  for the first time, and the two planar CCD cameras  34  record the two adjoining individual images of the forward half of the sheets  5 . The sheet  5  is stretched tight before the first single image is recorded. After a rotation of the drum  17  corresponding to one-half length of the sheet  5 , the two successive individual images of the rear half of the sheet  5  are recorded. Only after all individual images, or respectively the total image, of the sheet  5  has been recorded, the sheet  5  is then transferred to the second drum  18  for inspecting the front side  2  of the sheet  5 . 
     The grippers  27  of the first drum  17  transfer the beginning of the sheet  5  to the grippers  27  of the second drum  18 . In this way the sheet is conveyed by the second drum  18  and the reverse side  3  of the sheet  5 , which faced to the outside on the first drum  17 , moves inward to overlie the surface  31  of the second drum  18 , so that the front side  2  of the sheet  5  now faces to the outside. As soon as the suction devices  28  of the first  17  and the second drum  18  come into the transfer area (i.e. into the area of the common centers of the two drums  17 ,  18 ), the suction air for the suction devices  28  of the first drum is turned off and the suction devices  28  of the second drum  18  are charged with suction air. By means of this, the end of the sheet  5  is grasped by the suction devices  28  of the second drum  18 . Subsequently, the suction devices  28  of the second drum  18  are moved in the circumferential direction and in the axial direction of the second drum  18 . Here the suction force generated by the suction devices  28  on the end of the sheet  5  is designed to be such that, although the sheet  5  is stretched tightly, the sheet  5  begins to slide on the suction devices  28  before the tear resistance of the sheet  5  is overcome. 
     Only after the sheet  5  has been completely stretched tight on the second drum  18 , are individual images of the forward half of the front side  2  of the sheet  5  recorded. Thereafter, the two individual images of the rear half of the sheet  5  are recorded. Here, too, the beginning of the sheet  5  is only transferred to the chain gripper devices  21  of the third chain conveyor  19  after the entire image of the sheet  5  has been completely recorded. The third chain conveyor  19  is triggered by the evaluation device of the inspection device in such a way, that it selectively deposits the inspected sheets  5  which satisfy the quality requirements on the first  4  or second stack  6 , while the inspected sheets  5 , which do not meet the quality requirements, are deposited on the third stack  7 . 
     It is also possible for the sheets  5  to be inspected not to be directly transferred from the first drum  17  to the second drum  18 , but to be transferred by means of interposed conveying means, for example further drums. In this case, the interposed conveying means are arranged in such a way, that the first side  3  of sheet  5  faces toward the outside on the first drum  17 , and on the second drum the second side  2  of sheet  5  faces outward. 
     In place of the drums  17 ,  18 , which stretch the sheets  5  tight, a chain conveyor can also be provided with two holding devices cooperating in pairs, for example. These holding devices are arranged so they perform a relative movement which stretches the sheets  5 , wherein respectively one holding device grasps the beginning and one holding device the end of each of the sheets  5 . 
     While a preferred embodiment of a method and device for the qualitative assessment of sheets in accordance with the present invention has been set forth filly and completely hereinabove, it will be apparent to one of skill in the art that a number of changes in, for example, the size of the sheets being printed, the type of printing being performed, the type of printing press being used, and the like could be made without departing from the true spirit and scope of the present invention, which is accordingly to be limited only by the following claims: