Patent Publication Number: US-6705222-B2

Title: Dual registration control system

Description:
FIELD 
     This invention relates to rotary printing of images on a series of sheets passed through a printing machine, including at least one conveyor, while maintaining registry of the sheets with respect to rotary cylinders positioned both upstream and downstream of the conveyor. 
     BACKGROUND AND SUMMARY 
     In the art of rotary printing, those machines which print on individual, separated sheets passing successively through the machine inherently have a critical problem of registration control which is not present in web-type printing machines. That is, in order to produce clean and sharp images, the linear position of each sheet must be in perfect registry with the angular positions of the rotary cylinders positioned both upstream and downstream of the conveyor, including registry with the rotary cylinder which is in engagement with the sheet at that precise time. Otherwise, the printed images on the sheets become blurred and may be totally unacceptable. 
     U.S. Pat. No. 5,383,392 discloses a registration control system which has been a significant advance in the rotary printing art. This control system is designed for use when several of the rotary cylinders are driven by separate servo motors with each of the servo motors being individually controlled by the control system. However, as is well known in the rotary printing art, servo motors are extremely expensive, and no solution to the critical registration problem is known for rotary printing machines in which the various rotary cylinders are driven through a gear train by a single drive motor. This type of gear drive system with a single motor is much less costly than a plurality of independent servo motors; however, the critical problem of inaccurate registration because of the unavoidable backlash in the gear train has never been satisfactorily solved. 
     The present invention solves this long-standing problem by sensing the angular positions of the cylinders both upstream and downstream of the conveyor, and adjusting the speed of the conveyor in accordance with these sensed conditions, as well as, the sensed linear position of each sheet. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic diagram illustrating upstream and downstream stages of a rotary printing machine with a transfer conveyor positioned between the two stages; and 
     FIG. 2 is a schematic diagram of the basic elements of the dual control system. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, numeral  10  represents a rotary printing machine having at least two stages  10 A and  10 B. In the embodiment illustrated, by way of example, numeral  12  indicates a rotary printing cylinder and the impression cylinder is indicated by numeral  14 . Cylinder  12  includes an encoder  16  which is mounted on the cylinder or on the cylinder drive shaft. Encoder  16  senses the angular position of this cylinder and produces a signal indicative of its angular position at all times. 
     Numeral  20  indicates a conveyor system for conveying sheets  22  from left to right as indicated by arrows A and, in the preferred embodiment, sheets  22  comprise blanks of corrugated cardboard to be imprinted. Many forms of conveyors will be apparent to those skilled in the art, and for illustration purposes only, conveyor system  20  is illustrated as comprising a plurality of drive pulleys  24 , belts  25  and idler pulleys  26  spaced laterally across the width of the machine perpendicular to the direction of travel of the sheets. Sheets  22  firmly engage the bottom reach of the belts by virtue of a differential pressure above and below the sheets as is well known in the conveyor art. Alternatively, the sheets may be carried by the upper reaches of the belts, and/or by conveyor rollers as is also well known in the conveyor art. In the embodiment illustrated in FIG. 1, for purposes of example, print cylinder  12  and conveyor system  20  are followed downstream in the direction of movement of the sheets by a die cutter  30  and a backing cylinder  32 . However, it is to be understood that cylinder  32  may be a subsequent printing cylinder with cylinder  30  being an impression cylinder. Also, it is to be understood that cylinders  12  and  14  may be feed rollers for feeding the sheets into a first stage of printing comprising cylinders  30 ,  32 . Accordingly, cylinders  12  and  14  will be hereinafter referred to as the “upstream cylinders”, and cylinders  30  and  32  will be referred to as the “downstream cylinders”. 
     As further illustrated in FIG. 1, downstream cylinder  30  carries an encoder  34  which continuously senses the angular position of cylinder  30  and sends a signal indicative of the precise angular position of the cylinder at all times. 
     Referring to FIG. 2, the same elements are indicated by the same numerals as previously described. In addition, the control system includes a servo motor  36  driving the conveyors, a position sensor  38  and a computer controller  40 . Position sensor  38  may be of any type capable of detecting the leading edges of the sheets, or imprints thereon, and sending a signal  42  indicative thereof to the controller. Controller  40  also receives a signal  44  from encoder  16  which indicates the precise angular position of cylinder  12 , and it receives a signal  46  indicating the precise angular position of cylinder  30  as sensed by encoder  34 . An output signal  48  is generated by the controller and is sent to servo motor  36  to increase or decrease the speed of servo motor  36 . Thus, servo motor  36  adjusts the linear position of the belts so as to advance or retard the linear position of each sheet as is required to maintain registry with both the upstream cylinder  14  and downstream cylinder  30 . 
     The sequence of operations is as follows and begins when a sheet  22 A is between cylinders  12 ,  14  as shown in FIG.  2 . At this time, the angular position of cylinder  12  is continuously fed to controller  40 , and an output signal  48  is sent to servo motor  36  so as to establish the correct speed of the conveyor to receive the sheet on the conveyor as determined by the angular position of cylinder  12 . This condition continues until position sensor  38  senses the leading edge of, or an imprint on, sheet  22 A. The timing of this occurrence is sent as signal  42  to the controller. This signal overrides signal  44 , and the computer then compares the time/position of sheet  22 A, sensed by sensor  38 , with the angular position of downstream cylinder  30  sensed by encoder  34 . If the controller detects that this sheet is either advanced or retarded relative to the linear position of the sheet which is required in order to be in register with cylinder  30 , then controller output signal  48  corrects the linear position of sheet  22 A by decreasing or increasing, respectively, the speed of the conveyor. In this manner, the first registration control of each sheet is maintained as a function of the angular position of an upstream cylinder, and each sheet is subsequently controlled as a function of the next downstream cylinder. Thus, the system is capable of double or dual control of each sheet and can thereby compensate for positional errors of each sheet whether caused by slippage on the belts, backlash between gears in the drive train, or any other factor creating improper registration between first and second rotary cylinders. 
     From the foregoing description it will be apparent that many modifications and variations of the invention will become apparent to those skilled in the art. Accordingly, it is to be understood that the foregoing description of one preferred embodiment is intended to be illustrative rather than exhaustive of the principles of the invention, and that the scope of the invention is not intended to be limited other than as set forth in the following claims interpreted under the doctrine of equivalents.