Abstract:
A method of printing on a substrate includes the steps of instructing a first imaging unit to print a first image and a registration mark on a substrate contemporaneously during a production run and detecting the registration mark. The method further includes the steps of determining a position on the substrate where a second image is to be printed in accordance with the detection of the registration mark and instructing a second imaging unit to print the second image on the substrate in accordance with the determined position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/937,660, filed Jun. 29, 2007, and incorporated herein by reference in its entirety. 
    
    
     REFERENCE REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     SEQUENTIAL LISTING 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to printing systems and more particularly to high-speed printing systems that use a sense mark on a substrate to control the printing of images or data on the substrate. 
     2. Description of the Background of the Invention 
     High-speed printing systems typically print on a paper web by moving the paper web along a paper path using rollers or drums past printheads. A controller controls the printheads to print images on the paper web as the paper web moves under and/or over the printheads. In printing systems that include multiple imaging units, each imaging unit may include a plurality of printheads and each imaging unit may print a different color on the paper web. A first imaging unit prints a first color used for an image and a subsequent imaging unit prints a second color overlaid on the same image and so on with additional imaging units and colors. In order to align the printed images, it is important to track the position of the printed images with respect to the printheads included in each imaging unit. 
     In high-speed printing systems, the speed at which the paper web is moving along the paper path can be on the order of hundreds of feet/meters per second. In addition, the paper web dimensions may change due to moisture and other forces exerted on the paper web. These and other factors make it difficult to accurately track the position of the paper web and provide accurate control of the printheads. 
     Prior print systems and methods have included the printing of a sense mark on the substrate that indicates a top of the page. A sensor detects the sense mark and a controller tracks the position of the sense mark with respect to the printheads on each imaging unit. The controller instructs the printheads to print on the paper web in accordance with the detection of the sense mark. Prior print systems use a first printhead on a first imaging unit to print the sense mark on the paper web. Consequently, the sense mark is located along a side margin of the paper web, where subsequent images are not printed. This arrangement requires a larger paper web width to produce a printed image of a particular size because of the unused margin where the sense mark is printed. Further, these prior systems have not adequately addressed the issue of accurately detecting the sense mark and tracking the paper web. 
     Other prior art systems and methods track a lateral registration mark or a side edge of a substrate such as a paper web to detect problems such as shrinkage, expansion, drift, and/or skew of the paper web in a multi-color printing process. Such prior systems and methods use complex registration marks and algorithms to correct for such problems as shrinkage and expansion and do not adequately prevent or minimize such problems before they occur. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a method of printing on a substrate includes the steps of instructing a first imaging unit to print a first image and a single registration mark associated with the first image on a substrate contemporaneously during a production run and detecting the registration mark. The method further includes the steps of calculating an offset along a width direction of the substrate based on the detected single registration mark and determining a position on the substrate where a second image is to be printed in accordance with the calculated offset, wherein the second image is shifted in its entirety and printed on the same side of the substrate as the first image and instructing a second imaging unit to print the second image on the substrate in accordance with the determined position. 
     Other aspects and advantages of the present invention will become apparent upon consideration of the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevational view of a printing system according to an embodiment; 
         FIG. 2A  is a front isometric view of an imaging unit used in the printing system of  FIG. 1  in a first state; 
         FIG. 2B  is a front isometric view of the imaging unit of  FIG. 2A  in a second state; 
         FIG. 3  is a diagrammatic side elevational view of a printing system according to another embodiment; 
         FIG. 4  is front plan view of a paper web that includes an embodiment of a sense mark; 
         FIG. 5  is a diagrammatic side elevational view of a duplex printing system according to yet another embodiment; 
         FIG. 6  is a diagrammatic side elevational view of a duplex printing system according to a further embodiment; 
         FIG. 7  is a diagrammatic side elevational view of a further embodiment of a printing system that includes first and second imaging units; 
         FIG. 8  is a front plan view of another embodiment of a paper web that includes a sense mark and registration indicia; 
         FIG. 9  is an enlarged view of the paper web of  FIG. 8 ; and 
         FIG. 10  is a flowchart according to another embodiment of a printing process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows an embodiment of a printing system  10  configured with two imaging units  12 ,  14  and finishing systems  16  downstream of the imaging units. A paper web  18  arranged in a roll  20  is fed through the imaging units  12 ,  14  and finishing systems  16 . A sense mark printer  22  upstream from the imaging units  12 ,  14  prints a sense mark on the paper web  18 . The first imaging unit  12  prints on a first or front side of the paper web  18  and the second imaging unit  14  prints on a second or back side of the paper web. A plurality of cylinders and turn-bars (shown in more detail in  FIGS. 3 ,  5 , and  6 ) controls the paper path through the printing system  10  so that the paper web  18  need not be turned to permit duplex printing. If desired, only a single imaging unit is provided to enable simplex printing. In another embodiment, a single imaging unit is used for duplex printing by feeding the paper web through the imaging unit a first time to print on a first side of the paper web, turning the paper web, and feeding the paper web through the imaging unit a second time to print on the second side. Additional imaging units may be included to print in additional colors. 
       FIGS. 2A and 2B  show one side of the imaging units  12 ,  14  including two printhead assemblies  30 , wherein each printhead assembly includes one or more slots  32  for receiving inkjet printheads or cartridges (not shown). Examples of suitable printheads are those used in desktop printers or plotters. The printhead assemblies  30  can be positioned around a drum  34  that rotates and drives a paper web past the printhead assemblies. A controller (not shown) stores the position of one or more printheads in the slots  32  with respect to the drum. As the drum  34  rotates and the paper web  18  passes under the printheads, the controller instructs the printheads to print images on the paper web. The controller divides a raster line among the plurality of printheads in accordance with the position of the paper web with respect to the individual printheads. Each printhead assembly  30  prints one color such that a first color of an image is printed; a second color of the image is overprinted on the first color, and so on. In other embodiments, each printhead assembly can print more than one color, wherein individual printheads in each printhead assembly print a single color. 
     Generally, the imaging units  12 ,  14  contain four printhead assemblies, two on each side of the imaging unit, wherein each printhead assembly includes a plurality of printheads. The printhead assemblies  30  are positioned to guarantee that the direction of travel of a drop of ink from each printhead is substantially perpendicular to the surface of the associated drum  34  (and hence the paper web  18 ). 
     In the embodiment of  FIGS. 2A and 2B , each printhead assembly  30  has the ability to print an image that is up to 12 inches (30.48 cm) wide. Further, two printhead assemblies  30  are axially positioned relative to one another so that the print width spans the width of the paper web  18  (typically 24 inches or 60.96 cm). This permits a printing width of up to 24 inches (60.96 cm). In this way, the imaging unit  12 ,  14  can print 2-up 8½×11 inch (21.59×27.94 cm) pages in either landscape or portrait fashion. Other page heights or widths could be produced in N-up fashion, if desired. 
     The printing system in other embodiments includes a series of modular units that can be utilized as needed for the printing task to be undertaken. In other words, each imaging unit may include only two printhead assemblies (one on the left half of the imaging unit and another on the right half of the unit) and the same or different inks may be fed to each printhead assembly so that each assembly can print one side of a 12-inch (30.48 cm) page. As noted above, each imaging unit may further include two additional printhead assemblies. The additional assemblies are positioned to overprint the color(s) deposited by the first two printhead assemblies. In this configuration, each imaging unit can simultaneously print two simplex 12 inch (30.48 cm) pages in two different colors. Two such imaging units operating in series can produce two simplex 12 inch (30.48 cm) four-color pages and four imaging units can produce two duplex 12 inch (30.48 cm) four-color pages. In addition, as noted above, depending upon the number of imaging units that are used, one could alternatively produce 24-inch (60.96 cm) simplex or duplex pages in one to four colors. 
     As seen in  FIG. 3 , a printing system  50  includes the paper web  18  arranged in the roll  20  that is driven through a sense mark printer  52  and then through an imaging unit  54  that prints images onto the paper web. Tension cylinders and turn-bars  56  are used to control the travel of the paper web  18  through the printing system  50 . The paper web  18  contacts a drum  58  in the imaging unit  54  and the rotation of the drum drives the paper web past left and right printhead assemblies  60   a ,  60   b , respectively. A frictional force between the drum  58  and the paper web  18  maintains a stable surface interface between the paper web and the drum as the paper web is being driven by the rotating drum. Generally, the frictional force will be sufficient so that the paper web does not slip while it is in contact with the drum. However, in other embodiments, the surface of the drum may be textured to increase the frictional force. In yet other embodiments, any appropriate system using tension cylinders, turn-bars, rotating drums, etc. can be used to deliver the paper web past the printheads. 
     In  FIG. 3 , the paper web  18  is in contact with the drum  58  along a majority of the circumference of the drum. This arrangement provides a stable non-slip surface interface between the paper web  18  and the drum  58  as the paper web is driven past the printheads in each printhead assembly  60   a ,  60   b . Consequently, the position of the paper web  18  relative to each printhead can be calculated using the angular speed of the drum and the elapsed time. Also, the stable non-slip surface interface counteracts the tendency of the paper web to deform as ink is applied to the surface of the web. In one embodiment, the paper web  18  is in contact with the surface of the drum  58  along greater than 180 degrees. In another embodiment, the paper web  18  is in contact with the surface of the drum  58  along about 270 degrees. Consequently, the tension cylinders and turn-bars  56  can be arranged so that the paper web  18  first contacts the drum  58  near the bottom of the drum or along a substantially horizontal tangent line. 
     In  FIG. 3 , the paper web  18  is allowed to separate from the drum  58  at a position after the right printhead assembly  60   b . The paper web separates from the drum along a substantially vertical tangent line and moves down into a drying station  62 . The drying station  62  can include any appropriate type of drying device that removes moisture from the paper web  18  before the paper web is sent to downstream imaging units and/or finishing systems. For example, in some embodiments, a blower is used to pass air over the paper web or an infrared heater is used to dry the ink. As the paper web  18  separates from the drum  58 , the ink on the paper web is still wet. In the embodiment of  FIG. 3 , the paper web  18  separates from the drum  58  and moves into the drying station  62  before the paper web contacts another tension cylinder or turn-bar  56 . This arrangement allows the ink to dry while the paper web is in a substantially non-tensioned state. Consequently, the effects of paper deformation due to moisture from the ink can be minimized. 
     The sense mark printer  52  is located upstream from the imaging unit  54  to print a sense mark  64  ( FIG. 4 ) on the paper web at a position corresponding to the top of each printed page.  FIG. 4  shows an embodiment of a sense mark  64  printed on a paper web  18 , wherein the arrow indicates the forward direction in which the paper web moves through the print system. In  FIG. 4 , the sense mark  64  indicates a top of a form and is located along a side edge of the paper web  18 . In other embodiments, the sense mark can indicate a bottom of a form or indicate some other portion of the form so long as the sense mark serves as a reference point for the printing of other images. Additionally, in other embodiments, the sense mark need not be printed along an edge of the paper web, but can be printed anywhere along the width of the web. 
     The sense mark printer  52  prints a plurality of sense marks  64  on the paper web  18 , wherein consecutive sense marks are separated by a predetermined distance depending on the size of the finished page. Any type of ink may be used to print the sense mark; however, generally an ink is chosen that is both relatively inexpensive and easily detected by the sensor  66 . In addition, the separate printer  52  uses an inexpensive printhead to print the sense mark  64  on the paper web  18 . The embodiment of  FIG. 3  gives greater latitude over printing systems that print a sense mark on a paper web using a dedicated first printhead, wherein a wider paper web is used to print a given finished product size, because the imaging units cannot print in the column where the sense mark is located. In contrast, using a separate printer that includes a relatively inexpensive printhead to print the sense mark on the paper web enables subsequent imaging units to print across the entire width of the paper web, including the column where the sense mark is located. 
     The sensor  66  associated with the imaging unit  54  detects the sense mark  64 , and a sensor  68  associated with the drum  58  is used to track the speed and/or the position of the drum (and thus the paper web  18 ) as the drum rotates. In one embodiment, the sensor  68  associated with the drum  58  is a transducer located on the drum itself. In another embodiment, the sensor  66  that detects the sense mark  64  is a conventional optical sensor. For example, the optical sensor may include a light emitting diode (“LED”), a photodiode, and an amplifier, wherein the LED reflects light off of the substrate and the reflected light is detected by the photodiode to generate a sense signal when the light is reflected off of the sense mark. The sense signal is amplified and supplied to a control circuit  70 , which controls the printheads in each printhead assembly  60   a ,  60   b  to print images onto the paper web  18 . 
     In other embodiments, the sense mark printer  52  prints a plurality of sense marks  64  on the paper web using infrared inks that absorb infrared light or invisible inks that reflect ultraviolet light. In these embodiments, the sensor  66  will be adapted to detect the infrared or invisible inks. 
     The size of the sensor  66  and the size of the sense mark  64  can be adjusted so that the sensor can easily detect the sense mark. For example, the length and/or the width of the sense mark  64  can be matched to the dimensions of the sensor  66 . In one embodiment, the sense mark  64  is about ⅛ of an inch (0.3175 cm) in the direction that the paper web  18  is traveling and ¼ to ⅜ of an inch (0.635-0.9525 cm) across the width of the paper web. 
     In  FIG. 3 , the sensor  66  is located at a position after the paper web  18  has contacted the drum  58 . At this point, the surface contact between the paper web  18  and the surface of the drum  58  is stable and the effects of paper deformation are minimized. In addition, the relatively large contact area between the paper web  18  and the drum  58  further stabilizes the interface between the substrate and the drum so that the rotating drum drives the paper web without slipping. The sensor  66  detects the sense mark  64  at a point after which the paper web  18  has contacted the drum  58  to accurately control the printheads in each printhead assembly  60   a ,  60   b.    
     The controller  70  associated with each printhead assembly  60   a ,  60   b  controls the printheads thereof so that the color components of the images are printed substantially in synchronism with the sense marks  64  and the registration or alignment of the color components of the images is accurately controlled. That is, the controller  70  receives a signal from the sensor  66  that the sense mark  64  has been detected and uses the speed and/or position of the drum  58 , and hence the speed and/or position of the paper web  18 , to control the respective printheads to print a raster line at a particular position of the paper web. The controller  68  then distributes segments of a raster line among the printheads in accordance with the position of each inkjet printhead. Each printhead has local circuitry (not shown) to translate the digital raster line data into analog signals that generate drops of ink deposited onto the paper web  18 . 
     In another embodiment, the controller  70  electronically compensates for inherent delays in the sensor  66  and other electrical components. The controller  70  builds in an electronic delay before sending instructions to the printheads to print raster lines on the paper web  18 . The electronic delay will vary depending on the speed of the paper web  18 . For example, at full speed a shorter delay may be built in than at a slower speed. Consequently, the controller  70  instructs the printheads to begin printing on the paper web  18  at consistent distances from the sense mark  64 . 
     The controller  70  stores and tracks the positions of a plurality of consecutive sense marks  64  to control the printing of each page moving past the printhead assemblies  60   a ,  60   b . In one example, consecutive sense marks are separated by a short distance and the finished page size is small so that multiple pages are being printed by a single printhead assembly at the same time. The paper web  18  contacts the drum and the sensor  66  detects a first sense mark  64 . The sensor  66  sends a detect signal to the controller  70 , which stores the timing of the detect signal and tracks the position of the sense mark. At the appropriate time, the controller  70  instructs the printheads of the left printhead assembly  60   a  to begin printing the first page. While the first page is being printed, the drum  58  continues to rotate and the sensor  66  detects and the controller  70  tracks a second sense mark  64 . The controller  70  instructs the printheads to begin printing the second page as the first page is being printed by the same left printhead assembly  60   a . The drum  66  continues to drive the paper web  18  and consecutive sense marks are detected and tracked to control the printing of each page. After the left printhead assembly  60   a  has printed an image on the first page, the controller  70  continues to track the position of the first sense mark so that the right printhead assembly  60   b  can be controlled to print an image that is aligned with the image printed by the first printhead assembly. Likewise, the positions of consecutive sense marks are tracked to control the alignment of images printed by the left and right printhead assemblies  60   a ,  60   b . Consequently, printed images can be aligned with the sense marks and with other images. 
     Referring to  FIG. 5 , in yet another embodiment, the printing system  50  of  FIG. 3  is adapted to print in duplex by adding a second imaging unit  80  downstream of a first imaging unit (not shown) that prints on a back side of the paper web after the first imaging unit prints on a front side of the paper web. The first imaging unit operates similarly to the embodiment of  FIG. 3  and the second imaging unit  80  is substantially similar to the imaging unit  54  in  FIG. 3 . In  FIG. 5 , the paperpath of an imaging unit  80  is illustrated, wherein the paperpath is controlled by a number of tension cylinders or turn-bars  56  that feed the paper web  18  to the imaging unit  80  so that the back side of the paper web is printed. In particular, the paper web  18  is fed onto the drum  58 , which is rotating in an opposite direction than the drum in  FIG. 3 , so that the paper web first moves past the right printhead assembly  60   b  and then past the left printhead assembly  60   a . Alternatively, the first and second imaging units  54 ,  80  may be identical, wherein the second imaging unit is merely rotated 180 degrees so that the drums  58  of both imaging units are rotating in the same relative direction, i.e., clockwise, and the paper web moves past the left printhead assembly  60   a  first and then past the right printhead assembly  60   b . As shown in  FIG. 5 , the paper web  18  contacts the drum near the bottom of the drum, i.e., along a substantially horizontal tangent line. In addition, the paper web  18  is allowed to separate from the drum  58  and moves down into a drying station  62  similarly to  FIG. 3 . 
     In the duplex printing system of  FIG. 5 , the separate printer  52  upstream from the imaging units  54 ,  80  prints a sense mark  64  on the front and back sides of the paper web  18 . The sense mark  64  on the front side is used to control the respective printheads of the first imaging unit  54  in a manner similar or identical to that described above. The addition of the sense mark  64  on the back side of the paper web  18  is used to control the respective printheads of the second imaging unit  80  to print on the back side of the paper web. Referring to  FIG. 5 , a sensor  82  associated with the imaging unit  80  is located to detect the sense mark  64  at a position after which the paper web  18  has contacted the drum  58 . The sensor  82  is connected to a controller  84  associated with each printhead assembly  60   a ,  60   b , wherein the controller instructs the printheads in each printhead assembly to print images on the paper web  18  in accordance with the detection of the sense mark  64  and the position of the paper web. The sense mark  64  printed on the back side of the paper web  18  is aligned with the sense mark printed on the front side so that the images printed on the front and back sides are likewise aligned. 
       FIG. 6  shows an embodiment of a duplex printing system similar to  FIG. 5 , wherein the first printer  52  prints a sense mark only on the front side of the paper web  18 . In this embodiment, the first imaging unit  54  includes a sensor  66  that detects the sense mark  64  and controls the respective printhead assemblies  60   a ,  60   b  as described previously. Referring to  FIG. 6 , a second imaging unit  100  includes a sensor  102  that is located to detect the sense mark  64  at a position immediately before the paper web  18  contacts the drum  58 . Consequently, the sensor  102  is used to detect the sense mark  64  on the front side of the paper web  18 . The detection of the sense mark  64  by the sensor  102  is communicated to a controller  104  that tracks the positions of multiple sense marks and instructs the respective printheads on each printhead assembly  60   a ,  60   b  to print images on the back side of the paper web  18 . The large contact area between the paper web  18  and the drum  58  ensures a stable surface interface and an accurate determination of the position of the sense mark  64  and the paper web with respect to the printheads. The controller  104  accounts for the position at which the sensor is located so that the printheads can be accurately controlled. 
     A further embodiment of a duplex printing system is similar to the previously described embodiments and includes the sense mark printer  52  upstream from first and second imaging units, wherein the sense mark printer only prints a sense mark on the front side of the paper web  18 . The first imaging unit detects the sense mark as described above. The second imaging unit is similar to  FIG. 5  and includes a sensor that detects the sense mark on the paper web  18  at a position after which the paper web has contacted the drum  58 . However, in this embodiment, a sensor used in the second imaging unit is capable of detecting the sense mark on the front side of the paper web through the paper web. For example, a sensitive photomultiplier type light detector may be used in the sensor to detect the sense mark through the paper web. Consequently, a single sense mark can be used to control printheads in a duplex printing system, wherein a relatively inexpensive optical sensor can be used in the first imaging unit and a more sensitive optical sensor can be used in the second imaging unit. Alternatively, the sense mark is printed only on one side of the paper web using infrared or invisible inks, wherein appropriate sensors can detect the marks through the paper web. 
     The previously described embodiments have included a separate printer to print a sense mark on a paper web and a sensor that detects the mark, wherein the detection of the mark is used to control printheads that print images on the paper web. It will be apparent to one of skill in the art upon reading this document that other systems and methods of using a sense mark to control printing on a substrate are contemplated and fall within the scope of the disclosure. 
       FIG. 7  illustrates another printing or imaging system  150  similar to the printing system  50  of  FIG. 3  that includes first and second imaging units  152 ,  154 , respectively. The imaging units  152 ,  154  are similar to the imaging unit  54  of  FIG. 3  and are arranged to print simplex four color pages on a substrate. In  FIG. 7 , the substrate is a paper web  18  that is arranged in a roll  20  and driven through a paperpath defined, in part, by a plurality of tension cylinders or turn-bars  56  and a drum  58  of each imaging unit  152 ,  154 . Further, each imaging unit  152 ,  154  includes left and right printhead assemblies or arrays  60   a ,  60   b , respectively, that each print a single color on the paper web  18 . For example, the left and right printhead arrays  60   a ,  60   b  of the first imaging unit  152  print in cyan and magenta, respectively, and the left and right printhead arrays  60   a ,  60   b  of the second imaging unit  154  print in yellow and black, respectively. The different colors printed by each printhead array  60  of the first and second imaging units  152 ,  154  can be overlaid over one another to thereby allow the printing system  150  to print full color images on the paper web  18 . Further, in other embodiments, the arrangement or order of colors used by the printhead arrays  60  can be altered and/or different or additional colors can be used. 
     The imaging system  150  also includes a sense mark printer  156  that prints a sense mark  158  (shown in  FIG. 8 ) on the paper web  18  that is similar to the sense mark  64  of  FIG. 4 . In addition, each imaging unit  152 ,  154  includes a sensor  160 ,  162 , respectively, that is adapted to detect the sense mark  158  at a point where the paper web  18  is in contact with the drum  58  of each imaging unit  152 ,  154 . At this point, the position of the sense mark  158  relative to the surface of the drum  58  is constant as the paper web  18  moves past the left and right printhead arrays  60   a ,  60   b . This configuration of the sensors  160 ,  162  allows the imaging system  150  to use a single sensor associated with each imaging unit  152 ,  154 , respectively, to detect the sense mark  158  at a single location and to use the detection of the sense mark to control the registration of both left and right printhead arrays  60   a ,  60   b . The imaging units  152 ,  154  also include a sensor  164 ,  166 , respectively, that are adapted to track the speed and/or position of the drum  58  (and thus the paper web  18 ). Further, each imaging unit  152 ,  154  includes a control circuit or controller  168 ,  170 , respectively. The controllers  168 ,  170  process data from the sensors  162 - 166  and instruct the printhead arrays  60  of each imaging unit  152 ,  154 , respectively, to print images that are in registration with each other along the length of the paper web  18 , as described above. 
     In the present embodiment, the controller  168  instructs the left printhead array  60   a  of the first imaging unit  152  to print alignment or registration indicia on the paper web  18 . Referring to  FIG. 8 , the indicia are registration marks  172  printed along a right side margin  174  of the paper web  18  proximate to each sense mark  158 , which indicate a top of a page or form  176 . An arrow represents a direction of travel  178  of the paper web  18  through the printing system  150  and the right side margin  174  and a left side margin  180  are defined with respect to the direction of travel  178 . In the present embodiment, the registrations marks  172  are lines that extend about ¼ of an inch to about 1 inch along a length of the paper web  18  in the direction of travel  178  and about ⅛ to about ½ of an inch along a width of the paper web  18  perpendicular to the length. In other embodiments, the registration marks  172  can be printed along the left side margin  180  or at any point between the right and left side margins  174 ,  180 , respectively, of the paper web  18 . 
     As discussed above, the sense marks  158  are printed before the first and second imaging units  152 ,  154  print images on the paper web  18  and the sense marks  158  are used to register images printed by the first and second imaging units  152 ,  154  along the length of the paper web  18  in the direction of travel  178 . In the present embodiment, the registration marks  172  are used to register images printed by the printhead arrays  60  of the first and second imaging units  152 ,  154  along a lateral direction, e.g., along the width of the paper web  18 . Referring to  FIG. 9 , during a printing process the paper web  18  is delivered to the first printing unit  152  and the left printhead array  60   a  receives instructions from the controller  168  to print the registration mark  172  and an image  182  for each form  176  on the paper web  18 . The position of the registration marks  172  relative to the first imaging unit  152  is recorded or otherwise stored by the controller  168 . In addition, the position of the image  182  relative to each registration mark  172  is also recorded or otherwise stored by the controller  168 . In the present embodiment, a lateral distance (d) (shown in  FIG. 9 ) between the registration mark  172  and a right edge  184  of the image  182  is stored by the controller  168 . However, in other embodiments a mid-point or other reference point of the image  182  can be used as the reference point instead of the right edge  184  of the image. The position of the registration mark  172  relative to the first imaging unit  152  and the position of the image  182  relative to the registration mark  172  is used by the controller  168  to provide instructions to the right printhead array  60   b  of the first imaging unit  152  to print a next color component of the image  182  for each form  176  in lateral registration with the image  182  printed by the left printhead array  60   a . After the right printhead array  60   b  prints the next color component of the image  182  on the paper web  18 , the paper web separates from the drum  58  and moves into a drying station  62  as described above, and thereafter, the paper web is delivered to the second imaging unit  154  by a series of turn bars  56 . 
     When the paper web  18  separates from the drum  58  of the first imaging unit  152 , the stable surface interface between the paper web and the drum no longer prevents or counteracts the deformation of the paper web due to the moisture from the ink applied thereto. In addition, during the delivery of the paper web  18  to the second imaging unit  154 , the lateral position of the paper web may shift or drift. Consequently, the position of the paper web  18  relative to the printhead arrays  60  of the second imaging unit  154  may be different than the position of the paper web  18  relative to the printhead arrays  60  of the first imaging unit  152  when the first imaging unit was printing images  182  on the paper web. Such differences must be corrected to ensure that the images  182  printed by the second imaging unit  154  are in registration with the images printed by the first imaging unit  152 . 
     The printing system  150  of the present embodiment utilizes the registration marks  172  to correct for deformation and lateral shifts of the paper web  18  at the second imaging unit  154 . Specifically, the paper web  18  is delivered to the second imaging unit  154  and driven past the printhead arrays  60  by the rotation of the drum  58 . The second imaging unit  154  includes a registration mark sensor  186  that is configured to detect the registration marks  172  at a point where the paper web  18  is in contact with the drum  58 . This configuration of the registration mark sensor  186  provides a reliable position detection of the registration marks  172 , because the paper web  18  is in a fixed position relative to the surface of the drum  58 , and hence the printhead arrays  60 , as the paper web is in contact with the drum. In addition, the registration mark sensor  186  is positioned along an axis of rotation of the drum  58  at an expected position of the registration marks  172 , e.g., at a position of the printhead that printed the registration mark. In one embodiment, the registration mark sensor  186  is a camera such as a CCD or CMOS image sensor. In yet another embodiment, the second imaging unit  154  includes a single sensor, such as the sensor  162  that is adapted to detect both the sense mark  158  and the position of the registration mark  172 . 
     As the paper web  18  is driven past the printhead arrays  60  of the second imaging unit  154  by the drum  58 , the registration mark sensor  186  detects the position of each registration mark  172  relative to the second imaging unit  154  and sends such positional data to the controller  170 . The controller  170  compares the positional data from the second imaging unit  154  with a reference or expected position of the registration mark  172  and detects any differences in the relative positions of the registration mark  172 . The expected position of the registration mark  172  corresponds to the positional data from the first imaging unit  152 . Any difference between the detected position of the registration mark  172  and the expected position of the registration mark represent a shift in the paper web  18  with respect to the left and right printhead arrays  60   a ,  60   b  of the second imaging unit  154 . The controller  170  corrects for any difference by instructing the printhead arrays  60  of the second imaging unit  154  to shift the image  182  accordingly. For example, if a registration mark  172  has shifted two pixels to the left, then the controller  170  instructs the left and right printhead arrays  60   a ,  60   b  to print the image  182  shifted two pixels to the left. Any other known algorithms may be used to correct for deformation and shifts once such problems are identified. 
     In another embodiment, the sense mark  158  printed by the sense mark printer  156  is used to correct for deformation and lateral shifts of the paper web  18 . In this embodiment, the sense mark  158  is adapted to function as the registration mark  172 , which is omitted. For example, the sense mark  158  can be a rectangular mark so that appropriate sensors associated with the first and second imaging units can detect the position of the sense mark in the direction of travel  178  and in a direction orthogonal to the direction of travel. Alternatively, the sense mark printer  156  prints both the sense mark  158  and the registration mark  172 . In these two embodiments, the sensor  160  of the first imaging unit  152  is adapted to detect the position of the registration mark  172  and the controller  168  stores the position of the registration mark with respect to the first imaging unit  152  and instructs the left and right printhead arrays  60   a ,  60   b  to print images in registration. Alternatively, the first imaging unit includes a registration mark sensor similar to the registration mark sensor  186  in addition to the sensor  162 . The second imaging unit  152  operates as describe above. 
     In yet another embodiment, the registration mark  172  is omitted and appropriate sensors associated with the first and second imaging units track a side edge, such as the right or left side margin  174 ,  180 . The controllers  168 ,  170  track the position of the side edge to correct for deformation and lateral shifts of the paper web  18 . 
     Further, it would be apparent to one skilled in the art to apply the discussion of  FIGS. 7-9  to expand the duplex printing systems of  FIGS. 1 ,  5 , and  6  to print duplex four-color pages in registration using four imaging units. 
       FIG. 10  shows an embodiment of the general steps performed to control the imaging units  152 ,  154  to print color images on a paper web  18  in registration along the length and width of the paper web. The process begins at a block  200  and proceeds to a decision block  202  that determines if a sense mark, such as the sense mark  158 , is detected. Control passes back to the “begin” block  200  if a sense mark  158  is not detected. If a sense mark  158  is detected, control passes to a decision block  204  that determines if registration indicia, such as the registration marks  172 , have been printed. If the registration marks  172  have not been printed, e.g., at the first imaging unit  152 , then control passes to a block  206  that instructs the imaging unit to print the registration marks  172 . During the block  206 , a position of the registration marks  172  relative to the imaging unit  152 ,  154  is also recorded or stored. 
     Following the block  206 , control passes to a block  208  that instructs the printhead arrays  60  to print the images  182  on the paper web  18  in accordance with the detection of the sense mark  158  at the block  202  so that the images printed by the first and second imaging units  152 ,  154  are in registration along the length of the paper web. In addition, during the block  208  the printhead arrays  60  are instructed to print the images  182  at a position relative to the registration marks  172  so that the images are in registration along the width of the paper web  18 , wherein the position of the images relative to the registration marks is stored. 
     If the registration marks  172  have been printed, then control passes to a block  210  that detects the position of the registration marks. Thereafter, control passes to a block  212  that compares the detected position of the registration marks with a reference or expected position of registration marks, wherein the reference or expected position is a previously stored position, e.g., the position stored during the block  206 . At a decision block  214 , a difference between the detected position and the reference position indicates an error to be corrected. If no error is detected, then control passes to the block  208  and the image  182  is printed by the printhead arrays  60 . Otherwise at a block  216 , an error is corrected by shifting a color component of an image  182  laterally in accordance with the difference between the detected position and the reference position. Any such correction is communicated to the block  208 , which adjusts the position of the image  182  relative to the registration indicia  172  before instructing the printhead arrays  60  to print the images. In other embodiments, different algorithms can be used to correct for errors detected at the block  214 . 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable in the printing arts, for example, to register image data printed by one or more imaging units. More particularly, the use of a registration indicium is used to register image data on a substrate in a lateral direction. 
     Numerous modifications will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is presented for the purpose of enabling those skilled in the art to make and use the invention and to teach the best mode of carrying out same. The exclusive rights to all modifications which come within the scope of the appended claims are reserved.