Patent Publication Number: US-10759201-B2

Title: Frame length adjustment

Description:
PRIORITY 
     This application is a Continuation of commonly assigned and co-pending U.S. patent application Ser. No. 15/514,064, filed Mar. 24, 2017, which is a national stage filing under 35 U.S.C. § 371 of PCT Application Number PCT/US2014/057638, having an international filing date of Sep. 26, 2014, the disclosures of which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     An inkjet web press is a high-speed, digital, industrial inkjet printing solution that prints on a continuous media web at speeds of hundreds of feet per minute. A roll of media (e.g., paper) on an unwinding device supplies the press with a paper web which is conveyed through the press along a media path. Stationary inkjet printheads along the media path eject ink droplets onto the web to form images. The paper web is then conveyed through a drying area and out of the press through rollers to be rewound on a rewinding device. 
     Aqueous inks used in inkjet printing contain a significant amount of water that can saturate the paper. The moisture content of the paper and tension along the paper path within the press, among other factors, can cause the paper to expand, lengthening the paper web. However, when the paper is dried, it can shrink back down to a length below its initial state. Therefore, the length of paper coming out of the press is often different than the length of paper being fed into the press. Among other things, this media distortion can complicate post-print finishing operations performed on the printed material by certain finishing devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a schematic illustration of an example printing system suitable to enable real-time frame length adjustments in an inkjet web press; 
         FIG. 2  shows an example of a portion of the media web with two frames of image content that have been printed on the web by printheads; 
         FIG. 3  shows a box diagram of an example controller suitable for controlling print functions of an inkjet web press and for compensating for frame length distortions by dynamically adjusting the size of a gap between frames on the media web; 
         FIG. 4  shows examples of two timing diagrams that demonstrate the timing of sensors while sensing marks in real-time in a scenario when the frame length has contracted and in a scenario when the frame length as expanded; 
         FIGS. 5 and 6  show flow diagrams that illustrate example methods  500  and  600 , related to compensating for frame length distortions by dynamically adjusting the size of a gap between frames on the media web. 
     
    
    
     Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements. 
     DETAILED DESCRIPTION 
     As noted above, the printing process in an inkjet web press can cause distortions in the length of the media web that complicate post-finishing operations in certain finishing devices. More specifically, the significant application of moisture to the web during printing, followed by the removal of that moisture through a drying process, typically results in a variability in print frame length and an overall reduction in the length of the web. For example, the media web can shrink at a rate of approximately 0.2%, which is about 1 foot for every 500 feet of web fed into the press. 
     Finishing devices that initiate finishing operations on a fixed index basis for each print frame printed on the web, or, multi-web finishing devices that combine rolls from different sources, do not tolerate such media distortions effectively. This is because the distorted media web eventually causes print frames to drift out of the finishing device&#39;s tolerance band, and the finishing operations (e.g., paper cuts) begin to occur within adjacent print frames rather than between print frames as intended. Fixed index finishing devices are, however, generally capable of staying within tolerances when used in conjunction with analog printing processes. This is because inks used in analog printing processes are formulated with much less water than the inks used in a digital inkjet web press. Therefore, analog printing involves less wetting and drying of the media, which results in less media distortion. 
     In order to accommodate the higher rate of media distortion associated with a digital inkjet web press, a finishing device would have to initiate finishing operations based on triggers from the media or the press. Advanced digital finishing devices are available that provide such triggering mechanisms based on control systems that compensate for the cumulative error in web length. However, many commercial (and other) print customers who operate digital inkjet web presses prefer the lower costs and higher productivity of fixed index finishing equipment. Moreover, many print customers who already own such legacy finishing equipment want to leverage it forward rather than incur the significant costs associated with acquiring more advanced digital finishing devices. 
     Prior methods of dealing with media distortions are based on dynamically measuring the length of the produced pages and then trying to adjust the frame length to make and keep it close to its nominal value. However, the mechanisms used to find the length of the page are based on measuring the speed of the paper at a point that is close to the end of the paper path, and measuring the time a page takes to pass through this point. The problem with this method is that the speed of the web is not constant. The speed varies during the time a page takes to pass through the point, so there is not a definite speed available to convert time into page length. Determining the precise speed of the paper is challenging. The speed can be derived from many marks laid on the paper and read by a sensor. However, due to considerations such as the real estate constraints of the printed page layout, it is not always possible to have a high enough number of marks on the page to provide an accurate average. The speed can also be measured indirectly, for example, by counting the revolutions of a roll of a known diameter. However, the accuracy of this measurement can suffer from errors due to paper slippage on the roll, or thermally induced variations of the diameter of the roll. The lack of accuracy in measuring the paper speed translates into a lack of accuracy in the measured frame length, which is often outside of acceptable ranges for some printing applications. For example, in packaging and other applications where the frames tend to be long, the errors experienced might not be acceptable. 
     Accordingly, example methods and systems described herein enable real-time frame length adjustments in an inkjet web press. A closed-loop mechanism continually monitors the length of the printed frames during the printing process and corrects deviations from the nominal length of the frames. The distance between two marks printed on the paper web is compared with the fixed distance between two stationary optical sensors that each sense one of the two marks. A gap between frames is increased or decreased in order to cause the sensors to see their respective marks simultaneously, which will result in the distance between the two marks being equal to the fixed distance between the two sensors. 
     In one example implementation, a method of adjusting print frame length in an inkjet web press includes measuring a time T1 between a first sensor sensing a first mark and a second sensor sensing a second mark, measuring a time T2 between the second sensor sensing the second mark and the first sensor sensing a next first mark, and adjusting a gap between printed frames when T1 does not equal T2. 
     In another example, an inkjet web press includes a plurality of printheads to print first and second marks into print frames on a media web as the web passes through a print zone. The first marks are separated from the second marks across the width of the web by a cross-web distance. The web press includes first and second sensors that are also separated across the web by the cross-web distance, such that the first sensor is aligned across the web with the first marks to sense the first marks as they pass by the first sensor, and the second sensor is aligned across the web with the second marks to sense the second marks as they pass by the second sensor. 
     In another example, a non-transitory machine-readable storage medium stores instructions that when executed by a processor of a web press, cause the web press to print images in frames on a media web, and print first and second marks into the frames. The instructions further cause the press to sense a first mark with a first sensor and a second mark with a second sensor. The sensors are separated from one another by a distance in a down-web direction. Based on the sensing of the marks, the press adjusts a gap between the frames if the first and second marks are not separated by the same distance as the first and second sensors. 
       FIG. 1  shows a schematic illustration of an example printing system  100  suitable to enable real-time frame length adjustments in an inkjet web press. The printing system  100  is shown in  FIG. 1  and will be described herein, as an inkjet web press  100 . However, there is no intent to limit the printing system  100  to the implementation shown and described with regard to  FIG. 1 . Rather, various concepts disclosed herein, including those regarding adjusting the length of printing frames in real-time, may be applicable to other configurations and types of printing systems  100  as appropriate. 
     An inkjet web press  100  is generally configured to print ink or other fluid onto a web of media  102  supplied by a media roll  104  from an unwinding device  106 , also shown in  FIG. 1 . The web of media  102  (variously referred to herein as media web  102 , web  102 , media  102 , etc.) comprises printing material such as cellulose-based material (i.e., paper) or polymeric material, for example. In the present implementation, the media web  102  is considered to be a cellulose-based paper material that exhibits expansion when moisture is applied and contraction when the moisture is removed. The width across the media web  102  can vary, but is on the order of 20-40 inches. 
     As the media web  102  exits the inkjet web press  100 , it may be rewound on a rewinding device (not shown) and subsequently transferred to a near-line finishing device, or it may pass directly to a post-print, in-line finishing device  108 , as shown in  FIG. 1 . Finishing devices  108  perform finishing operations on printed material after printing has been completed. Such operations include, for example, paper slitting, cutting, trimming, die-cutting, folding, coating, embossing, and binding. While finishing operations can be performed by one or more finishing devices that are in-line or near-line with the press  100 , the present implementation is discussed with regard to a single in-line web cutting finishing device  108 , as shown in  FIG. 1 . The finishing device  108  comprises a fixed index web cutting device, such as a cutoff knife on a rotary drum, that cuts the media web  102  at fixed intervals. Cut media from the web  102  is shown as a media stack  110 , which may be collected within finishing device  108  or within a separate media stacking device (not shown). 
     Inkjet web press  100  includes a print module  112  and media support  114 . Print module  112  includes a number of print bars  116 , and one or more pens or cartridges  118  that each include a number of fluid drop jetting printheads  120 . Printheads  120  eject drops of ink or other fluid through a plurality of orifices or nozzles (not shown) toward the media web  102  so as to print onto the web  102 . Thus, a print zone  121  is established between the print module  112  and media support  114 . Nozzles are typically arranged on printheads  120  in one or more columns or arrays so that properly sequenced ejection of ink causes characters, symbols, and/or other graphics or images to be printed on media web  102  as it moves relative to print bars  116  along media support  114 . 
     Media support  114  comprises a number or media rollers  122  that support the media web  102  as it passes through the print zone  121  in close proximity to the print bars  116 . Media support  114  receives the web  102  from media drive rollers  124  and delivers the printed upon web  102  to media rewind rollers  126 . Drive rollers  124  are generally referred to herein as rollers that precede the media support  114  along the media web path, while rewind rollers  126  are referred to as rollers that follow the media support  114  along the media web path. The drive  124  and rewind  126  rollers are control rollers driven by a web drive  128 . 
     As the media web  102  passes through the print zone  121  along media support  114 , it becomes wet from ink and/or other fluid ejected from printheads  120 . As noted above, the wetting of the web  102  causes the media to expand, which lengthens the web. The inkjet web press  100  includes one or more thermal dryers  130  that remove the moisture from the web  102  by forcing warm air across the web as it passes over a series of rollers. The drying process typically shrinks the media back down to a level below its initial length. Thus, the wetting and drying of the web  102  effectively result in a net reduction in the length of the media web  102 . 
     In some examples, the media web  102  may be routed through a post-print function  132  after being dried by thermal dryers  130 . A post-print function  132  can include, for example, a moisturizer component to spray water on the paper web  102  to return the paper back to an equilibrium moisture content following the drying by dryers  130 , a silicon spray component to spray silicon on the paper web to help the paper slide over a folder or other component in a post-print finishing operation, and so on. 
       FIG. 2  shows an example of a portion of the media web  102  with two frames  200  of image content (i.e., frame n, frame n+1) that have been printed on the web  102  by printheads  120 . Referring generally to both  FIGS. 1 and 2 , the web press  100  includes two optical sensors  134  (illustrated as first sensor S 1 ,  134   a , and second sensor S 2 ,  134   b ) located at the end of the print media path of the press  100 . The optical sensors  134  may comprise any appropriate imaging device such as a scanner, a camera, or other imager, implementing various image sensors such as CCD&#39;s (charge coupled devices), CMOS devices, and so on. A light source (not shown) may accompany the optical sensors  134  to provide illumination for reflecting off the web  102 . 
     The sensors  134  are separated from one another in a down-web direction  136  by a fixed down-web distance  138 . The down-web distance  138  is a distance that is less than the minimum length of a printed frame  200 , as shown in  FIG. 2 . In some examples, the down-web distance  138  is approximately 7 inches. The sensors  134  are also separated slightly from one another in a cross-web direction  140  by a cross-web distance  142 . In some examples, the cross-web distance  142  is approximately 0.5 inches. The cross-web distance  142  is the same distance by which two sensor marks  202  (illustrated as first mark  202   a  and second mark  202   b ) are separated across the web  102 . The two sensor marks,  202   a  and  202   b , are printed in each frame  200 , and the sensors  134  are positioned in the cross-web direction  140  so that sensor S 1 ,  134   a , is aligned with sensor marks  202   a  and sensor S 2 ,  134   b , is aligned with sensor marks  202   b . Sensor S 1 ,  134   a , comes first in the media movement direction  144 , and sensor S 2 ,  134   b , comes second in the media movement direction  144 . The marks,  202   a  and  202   b , are printed with the intent that they be apart from one another in the down-web direction  136  by the same distance that the sensors  134  are apart. Thus, in the absence of any error, each sensor mark  202  will be simultaneously seen by its corresponding sensor  134 . That is, if there is no distortion in the length of the web  102  (e.g., due to water content, heating, print path tension, etc.), sensor  134   a  will see mark  202   a  at precisely the same time that sensor  134   b  sees mark  202   b . However, as noted above, the paper web  102  often experiences expansion and/or contraction (shrinkage) during the printing process, so the sensor marks  202  are often not the same distance apart from one another as the sensors are, and the sensors  134  will not see their corresponding marks  202  at the same time. The differences in these distances are an indication that the length of the print frames  200  are distorted, which can result in unacceptable printed product from finishing devices, such as a cutting device. In order to compensate for these frame length distortions, methods and systems described herein enable real-time frame length adjustments in an inkjet web press. 
       FIG. 3  shows a box diagram of an example controller  146  suitable for controlling print functions of an inkjet web press  100  and for compensating for frame length distortions by dynamically adjusting the size of a gap between frames  200  on the media web  102 . Controller  146  generally comprises a processor (CPU)  300  and a memory  302 , and may additionally include firmware and other electronics for communicating with and controlling the other components of the press  100 , as well as external devices such as unwinding device  106 . Memory  302  can include both volatile (i.e., RAM) and nonvolatile (e.g., ROM, hard disk, optical disc, CD-ROM, magnetic tape, flash memory, etc.) memory components. The components of memory  302  comprise non-transitory, machine-readable (e.g., computer/processor-readable) media that provide for the storage of machine-readable coded program instructions, data structures, program instruction modules, JDF (job definition format), and other data for the printing press  100 , such as modules  304 ,  306  and  308 . The program instructions, data structures, and modules stored in memory  302  may be part of an installation package that can be executed by processor  300  to implement various examples, such as examples discussed herein. Thus, memory  302  may be a portable medium such as a CD, DVD, or flash drive, or a memory maintained by a server from which the installation package can be downloaded and installed. In another example, the program instructions, data structures, and modules stored in memory  302  may be part of an application or applications already installed, in which case memory  302  may include integrated memory such as a hard drive. 
     Controller  146  may receive data  304  from a host system, such as a computer, and temporarily store the data  304  in memory  302 . Data  304  represents, for example, a document and/or file to be printed. As such, data  304  forms a print job for inkjet web press  100  that includes one or more print job commands/instructions, and/or command parameters executable by processor  300 . Thus, controller  146  controls inkjet printheads  120  to eject ink drops from printhead nozzles onto media web  102  as the web  102  passes through the print zone  121 . The controller  146  thereby defines a pattern of ejected ink drops that form characters, symbols, and/or other graphics or images on the media web  102 . The pattern of ejected ink drops is determined by the print job commands and/or command parameters within data  304 . In addition to print data  304 , controller  146  can print sensor marks  306  that represent first and second sensor marks  202   a  and  202   b.    
     Referring now to  FIGS. 1-3 , in one example, controller  146  includes a frame gap adjustment module  308  stored in memory  302 . The frame gap adjustment module  308  comprises instructions executable on processor  300  to precisely control when the print module  212  begins printing each print frame  200  of a print job on the media web  102 . In some instances, module  308  may delay the printing of a print frame  200  for an amount of time in order to increase the gap  148  between frames  200 . In other instances, module  308  may advance the printing of a print frame  200  by a certain amount of time in order to decrease the gap  148  between frames  200 . 
     A print frame  200  comprises a unit of formatted output (i.e., print job instructions) and two sensor marks  202  printed onto the web  102 . In general, the module  308  determines when to trigger the printing of each print frame  200  based on timing signals received from a first timer  150   a  and a second timer  150   b  coupled to sensors  134 . As mentioned above, sensors  134  sense marks  202  that have been printed on the passing web  102 . Referring additionally now to  FIG. 4 , two scenarios will be discussed in which the sensors  134 , timers  150 , and module  308  function to adjust the size of gap  148  to compensate for distortions in the length of the web  102  (and frames  200 ).  FIG. 4  shows examples of two timing diagrams that demonstrate the timing of sensors  134  while sensing marks  202  in real-time in a scenario when the frame length has contracted (i.e., shrank) and in a scenario when the frame length as expanded. 
     Referring to  FIGS. 1-4 , during a printing process in web press  100 , sensor marks  202   a  and  202   b  are printed onto the media web  102 . In a first scenario where the web  102  has undergone shrinkage, the sensor S 1  ( 134   a ) sees (i.e., senses) mark  202   a  in frame n+1 as the web  102  travels along the print path in the direction  144 . Shortly thereafter, sensor S 2  ( 134   b ) sees mark  202   b  in frame n. The first timer  150   a  measures the time between these sensing events as time T1. That is, the first timer  150   a  starts counting when sensor S 1  ( 134   a ) senses mark  202   a  in frame n+1, and it stops counting when sensor S 2  ( 134   b ) senses mark  202   b  in frame n. Likewise, the second timer  150   b  measures the time between sensor S 2  ( 134   b ) sensing mark  202   b  in frame n, and sensor S 1  ( 134   a ) sensing a next mark  202   a . The second timer  150   b  measures the time between these sensing events as time T2. 
     The controller  146 , executing frame gap adjustment module  308  on a processor  300 , receives and analyzes times T1 and T2 to determine if there is a difference between times T1 and T2. A difference between times T1 and T2 indicates that the distance between marks  202   a  and  202   b  is not the same as the fixed distance between sensor S 1  ( 134   a ) and sensor S 2  ( 134   b ), which in turn indicates that there is some error, or distortion, in the length of the frames. More specifically, when T1 is less than T2, as shown in the first scenario shown in  FIG. 4 , the controller  146  determines that the frame length has undergone shrinkage, and that the gap should be therefore be increased in size to compensate for the shrinkage. The error, or amount of time by which the gap is adjusted is the lesser of the two times T1 and T2. The analysis performed by execution of the frame gap adjustment module  308  to determine the correction error is demonstrated by the following equation:
 
error=sign( T 1− T 2)*min( T 1, T 2)
 
where: sign(x) is 1 if x&gt;0, −1 if x&lt;0, and zero if x=0, and min(x, y) is the minimum of x and y.
 
     In a second scenario where the web  102  has undergone expansion, sensor S 2  ( 134   b ) senses mark  202   b  in frame n as the web  102  travels along the print path in the direction  144 . Shortly thereafter, sensor S 1  ( 134   a ) sees mark  202   a  in frame n+1. The second timer  150   b  measures the time between these sensing events as time T2. That is, the second timer  150   b  starts counting when sensor S 2  ( 134   b ) senses mark  202   b  in frame n, and it stops counting when sensor S 1  ( 134   a ) senses mark  202   a  in frame n+1. Likewise, the first timer  150   a  measures the time between sensor S 1  ( 134   a ) sensing mark  202   a  in frame n+1, and sensor S 2  ( 134   b ) sensing mark  202   b  in frame n+1. The first timer  150   a  measures the time between these sensing events as time T1. 
     The controller  146  receives and analyzes times T1 and T2 for a difference. Again, a difference between times T1 and T2 indicates that the distance between marks  202   a  and  202   b  is not the same as the fixed distance between sensor S 1  ( 134   a ) and sensor S 2  ( 134   b ), which in turn indicates that there is some error, or distortion, in the length of the frames. More specifically, when T1 is greater than T2, as shown in the second scenario shown in  FIG. 4 , the controller  146  determines that the frame length has undergone expansion, and that the gap should be therefore be decreased in size to compensate for the expansion. The error, or amount of time by which the gap is adjusted is the lesser of the two times T1 and T2. As in the above example, the analysis performed by execution of the frame gap adjustment module  308  to determine the correction error is demonstrated by the following equation:
 
error=sign( T 1− T 2)*min( T 1, T 2)
 
where: sign(x) is 1 if x&gt;0, −1 if x&lt;0, and zero if x=0, and min(x, y) is the minimum of x and y.
 
       FIGS. 5 and 6  show flow diagrams that illustrate example methods  500  and  600 , related to compensating for frame length distortions by dynamically adjusting the size of a gap between frames on the media web. Methods  500  and  600  are associated with the examples discussed above with regard to  FIGS. 1-4 , and details of the operations shown in methods  500  and  600  can be found in the related discussion of such examples. The operations of methods  500  and  600  may be embodied as programming instructions stored on a non-transitory, machine-readable (e.g., computer/processor-readable) medium, such as memory  302  as shown in  FIG. 3 . In some examples, implementing the operations of methods  500  and  600  can be achieved by a processor, such as a processor  300  of  FIG. 3 , reading and executing the programming instructions stored in a memory  302 . In some examples, implementing the operations of methods  500  and  600  can be achieved using an ASIC (application specific integrated circuit) and/or other hardware components alone or in combination with programming instructions executable by processor  300 . 
     Methods  500  and  600  may include more than one implementation, and different implementations of methods  500  and  600  may not employ every operation presented in the respective flow diagrams. Therefore, while the operations of methods  500  and  600  are presented in a particular order within the flow diagrams, the order of their presentation is not intended to be a limitation as to the order in which the operations may actually be implemented, or as to whether all of the operations may be implemented. For example, one implementation of method  500  might be achieved through the performance of a number of initial operations, without performing one or more subsequent operations, while another implementation of method  500  might be achieved through the performance of all of the operations. 
     Referring now to the flow diagram of  FIG. 5 , an example method  500  of adjusting frame length in an inkjet web press begins at block  502 , with measuring a time T1 between a first sensor sensing a first mark and a second sensor sensing a second mark. The method includes measuring a time T2 between the second sensor sensing the second mark and the first sensor sensing a next first mark, as shown at block  504 . As shown at block  506 , the method includes adjusting a gap between printed frames when T1 does not equal T2. In some examples, adjusting the gap comprises adjusting the gap by an amount that corresponds with the smaller of T1 and T2, as shown at block  508 . In some examples, when T1 is greater than T2, adjusting the gap comprises decreasing the gap between printed frames, as shown at block  510 . Decreasing the gap between printed frames can include reducing an amount of time between printing sequential frames on a media web. As shown at block  512 , in some examples, when T1 is less than T2, adjusting the gap comprises increasing the gap between printed frames. Increasing the gap between printed frames can include increasing the amount of time between printing sequential frames on a media web. As shown at block  514 , in some examples, adjusting the gap comprises determining an error in timing between sensing the first and second marks, where the error is according to the following equation:
 
error=sign( T 1− T 2)*min( T 1, T 2),
 
where, sign(T1−T2) is 1 if (T1−T2)&gt;0, sign(T1−T2) is −1 if (T1−T2)&lt;0, and sign(T1−T2) is zero if x=0, and min(T1, T2) is the minimum of T1 and T2.
 
     Referring now to the flow diagram of  FIG. 6 , an example method  600  related to adjusting frame length in an inkjet web press begins at blocks  602  and  604  with printing images in frames on a media web and printing first and second marks into the frames. As shown at block  606 , a first mark is sensed with a first sensor and a second mark is sensed with a second sensor. The sensors are separated from one another by a distance in a down-web direction. The method continues at block  608  with adjusting a gap between the frames if, based on the sensing, the first and second marks are not separated by the same distance as the first and second sensors. As shown at blocks  610  and  612 , respectively, a time T1 is measured between the first sensor sensing the first mark and the second sensor sensing the second mark, and a time T2 is measured between the second sensor sensing the second mark and the first sensor sensing a next first mark. As shown at block  614 , the gap is decreased if T1 is greater than T2. Decreasing the gap can include reducing the time between printing the frames by the amount T2. As shown at block  616 , the gap is increased if T1 is less than T2. Increasing the gap can include increasing the time between printing the frames by T1. As shown at block  618 , when the first and second marks are sensed at the same time, it is determined that the first and second marks are separated by the same distance as the first and second sensors, and the gap between the frames is therefore maintained at the same size.