Abstract:
A method of aligning a printhead is described herein. The method includes accelerating a media along a process path, controlling a first printhead to form a first mark upon the accelerating media, detecting the first mark on the accelerating media, comparing a first mark detection data with first printhead desired alignment data, determining a first correction based upon the comparison of the first mark detection data, and modifying an alignment of the first printhead based upon the determined first correction.

Description:
BACKGROUND 
       [0001]    The method disclosed herein relates to printing systems that generate images onto continuous web substrates. In particular, the disclosed embodiments relate to printhead alignment in such systems. 
         [0002]    Printers provide fast, reliable, and automatic reproduction of images. The word “printer” as used herein encompasses any apparatus, such as a digital copier, book marking machine, facsimile machine, multi-function machine, etc. which performs a print outputting function for any purpose. Printing features that may be implemented in printers include the ability to do either full color or black and white printing, and printing onto one (simplex) or both sides of the image substrate (duplex). 
         [0003]    Some printers, especially those designed for very high speed or high volume printing, produce images on a continuous web print substrate. In these printers, the image substrate material is typically supplied from large, heavy rolls of paper upon which an image is printed instead of feeding pre-cut sheets from a bin. The paper mill rolls can typically be provided at a lower cost per printed page than pre-cut sheets. Each such roll provides a very large (very long) supply of paper printing substrate in a defined width. Fan-fold or computer form web substrates may be used in some printers having feeders that engage sprocket holes in the edges of the substrate. 
         [0004]    Typically, with web roll feeding, the web is fed off the roll past one or more printhead assemblies that eject ink onto the web, and then through one or more stations that fix the image to the web. A printhead is a structure including a set of ejectors arranged in at least one linear array of ejectors, for placing marks on media according to digital data applied thereto. Printheads may be used with different kinds of ink-jet technologies such as liquid ink jet, phase-change ink, systems that eject solid particles onto the media, etc. 
         [0005]    Thereafter, the web may be cut in a chopper and/or slitter to form copy sheets. Alternatively, the printed web output can be rewound onto an output roll (uncut) for further processing offline. In addition to cost advantages, web printers can also have advantages in feeding reliability, i.e., lower misfeed and jam rates within the printer as compared to high speed feeding of precut sheets through a printing apparatus. 
         [0006]    A further advantage is that web feeding from large rolls requires less downtime for paper loading. For example, a system printing onto web paper supplied from a 5 foot diameter supply roll is typically able to print continuously for an entire shift without requiring any operator action. Printers using sheets may require an operator to re-load cut sheet feeders 2 to 3 times per hour. Continuous web printing also provides greater productivity for the same printer processing speed and corresponding paper or process path velocity through the printer, since web printing does not require pitch space skips between images as is required between each sheet for cut sheet printing. 
         [0007]    To achieve the high speeds desired in continuous web printing and to cover the width of the web as required in production printing, multiple printheads are used. As the printer operates, the printheads expand and contract in response to changing thermal conditions. Thus, the width covered by a particular printhead (the “extent” of the printhead) varies depending on the operating temperature. Likewise, the rollers used to define the process path expand and contract in response to temperature changes. The expansion and contraction of the rollers affects the alignment of the process path. “Alignment” as used herein, unless otherwise expressly qualified, is defined as the location of the printhead along the width of the process path immediately adjacent to the printhead (cross-process location), and the orientation of the cross-process axis of the printhead with respect to an axis perpendicular to the edge of the process path. Thus, the web, which is designed to move perpendicularly past each of the printheads, may move past a printhead at a skewed angle when the printhead is misaligned. Additionally, the cross-process extent of the printhead may not be positioned properly with respect to the other printheads. 
         [0008]    Misalignment resulting from movement of the printheads and the rollers is exacerbated by the positioning of printheads for different colors at different locations along the process path. Specifically, printers that generate color copies may include one or more printheads for each color of ink used in the printer. Each of the printheads associated with the different colors is positioned at a location along the process path that may be separated from other printheads by one or more roller pairs. Each roller pair produces a unique alignment of the media with respect to the process path. Accordingly, changes in the printheads and rollers may cause the printheads to be misaligned with the web as it moves along the process path. 
         [0009]    Alignment of printheads in a printer is typically accomplished by bringing the printer up to its operational speed and printing a series of marks on the continuous web. The positions of the printed marks are detected by a scanner and then analyzed to measure an offset between a desired printhead position and the actual position of the printhead. The printheads are then mechanically moved to the desired position. The printheads may be moved with stepper motors, which in many instances cannot be simultaneously operated. Additionally, the alignment procedure may need to be repeated for a variety of reasons such as excessive measurement noise or backlash of the printhead motor screws. Throughout this process, the image substrate is fed through the device at full speed. Consequently, alignment procedures for printing systems which reduce the waste of media would be beneficial. 
       SUMMARY 
       [0010]    A method of aligning a printhead is described herein. The method includes accelerating a media along a process path, controlling a first printhead to form a first mark upon the accelerating media, detecting the first mark on the accelerating media, comparing a first mark detection data with first printhead desired alignment data, determining a first correction based upon the comparison of the first mark detection data, and modifying an alignment of the first printhead based upon the determined first correction. 
         [0011]    In accordance with another embodiment, a printing system includes a process path defined by a plurality of rollers, at least one printhead positioned adjacent to the process path, a linear array sensor positioned along the process path, a memory in which command instructions are stored, and a processor configured to execute the command instructions to accelerate a media along the process path, control the at least one printhead to form a first mark upon the accelerating media, obtain data from the linear array sensor indicative of detection of the first mark, compare the obtained data with data related to the desired alignment of the at least one printhead, determine a first correction based upon the comparison of the first mark, and modify the alignment of the at least one printhead based upon the determined first correction. 
         [0012]    In a further embodiment, a method of aligning a continuous web printer includes determining a speed of a media accelerating along a process path, comparing the speed of the accelerating media to a first threshold speed, printing a first test pattern on the accelerating media with a first printhead based upon the comparison to the first threshold speed, detecting the first test pattern, extracting first roll and position data for the first printhead using the detected first test pattern, and adjusting a roll and a position of the first printhead based upon the extracted first roll and position data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  depicts a partial perspective view of a continuous web printing system with four print stations; 
           [0014]      FIG. 2  depicts a schematic of an alignment control system that may be used with the system of  FIG. 1 ; 
           [0015]      FIG. 3  depicts a flow diagram of an alignment procedure that may be performed by the alignment control system of  FIG. 2 ; 
           [0016]      FIG. 4  depicts a top plan schematic view of four test patterns printed on a media by two different printheads wherein the two printheads are initially misaligned; and 
           [0017]      FIG. 5  depicts a top plan schematic view of two test patterns printed on a media by two printheads of  FIG. 1  using selected nozzles to generate a series of dashes from each of the printheads. 
       
    
    
     DESCRIPTION 
       [0018]    With initial reference to  FIG. 1 , a continuous web printer system  100  includes four print stations  102 ,  104 ,  106 , and  108 . The print station  102  includes printheads  110  and  112 , the print station  104  includes printheads  114  and  116 , the print station  106  includes printheads  118  and  120 , and the print station  108  includes printheads  122  and  124 . A web of print media  126  is positioned on a spindle  128  to provide media for the continuous web printer system  100 . The print media  126  is fed along a process path  130  indicated by a series of arrows. 
         [0019]    The process path  130 , which is the actual path along which the media  126  proceeds, includes process path segment  132  which is located adjacent to the print stations  102  and  104 , and process path segment  134  which is located adjacent to the print stations  106  and  108 . A process path segment  136  is located adjacent to a linear array sensor  138 . The process path segment  132  is defined by rollers  140  and  142  while the process path segment  134  is defined by rollers  144  and  146 . A roller  148  defines, in part the process path segment  136 . Alignment of the print stations  102 ,  104 ,  106 , and  108  with the respective process path segment  132  or  134  is controlled by an alignment control system  150  shown in  FIG. 2 . 
         [0020]    The alignment control system  150  includes a processor  152  and a memory  154 . The processor  152  is connected to the linear array sensor  138  and a speed sensor  156  which in this embodiment detects the rotational speed of the roller  140 . The processor  152  is further connected to the print stations  102 ,  104 ,  106 , and  108 . Alternative embodiments may include more or fewer printhead stations. 
         [0021]    The print station  102  includes a cross-process motor  158  and a roll motor  160  for positioning the printhead  110  along with a cross-process motor  162  an a roll motor  164  for positioning the printhead  112 . Likewise, print station  104  includes a cross-process motor  166  and a roll motor  168  for positioning the printhead  114  along with a cross-process motor  170  and a roll motor  172  for positioning the printhead  116 , the print station  106  includes a cross-process motor  174  and a roll motor  176  for positioning the printhead  118  along with a cross-process motor  178  and a roll motor  180  for positioning the printhead  120 , and the print station  108  includes a cross-process motor  182  and a roll motor  184  for positioning the printhead  122  along with a cross-process motor  186  and a roll motor  188  for positioning the printhead  124 . Each of the printheads  110 ,  112 ,  114 ,  116 ,  118 ,  120 ,  122 , and  124 , the cross-process motors  158 ,  162 ,  166 ,  170 ,  174 ,  178 ,  182 , and  186 , and roll motors  160 ,  164 ,  168 ,  172 ,  176 ,  180 ,  184 , and  188  are controlled by the processor  152 . 
         [0022]    The memory  154  is programmed with command instructions which, when executed by the processor  152 , align the printheads  110 ,  112 ,  114 ,  116 ,  118 ,  120 ,  122 , and  124 . In one embodiment shown in  FIG. 3 , an alignment process  200  begins when the printer system  100  is energized (block  202 ) thereby accelerating the media  126  along the process path  130 . The movement of the media  126  may be sensed directly or indirectly. In this embodiment, the speed sensor  156  detects the revolutions of the roller  140 . The speed of revolution of the roller  140  combined with data for the circumference of the roller  140  can be used to determine the speed of the media  126  along the process path  130  (block  204 ). 
         [0023]    Once data related to the speed of the media  126  along the process path  130  is obtained, the speed data is compared to minimum velocity data stored in the memory  154  (block  206 ). The minimum velocity data is associated with the minimum speed of the media  126  along the process path  130  for obtaining reliable alignment data. If the determined speed of the media  126  along the process path  130  is too slow, the process  200  waits for a predetermined time (block  208 ) allowing the speed of the media  126  along the process path  130  to increase. After the predetermined amount of time, the speed of the media  126  is again determined (block  204 ) and compared to the threshold speed (block  206 ). 
         [0024]    Once the comparison (block  206 ) reveals that the media  126  is travelling at or above the threshold speed, the processor  152  controls the printhead  110  to generate a test pattern on the media  126  (block  210 ) and the printhead  112  to generate a test pattern on the media  126  (block  212 ). As the portion of the media  126  with the test patterns approaches the linear array sensor  138 , the linear array sensor  138  is energized. Timing of the energization of the linear array sensor  138  may be based upon the sensed speed along with knowledge of the length of the process path  130  between the particular printhead and the linear array sensor  138 . Allowance for the continued acceleration of the media  126  along the process path  130  throughout the procedure  200  is included in determining the energization time. 
         [0025]    As the test patterns pass the linear array sensor  138 , the test patterns are detected by the linear array sensor  138  (blocks  214  and  216 ) and data indicative of the detected test patterns are communicated to the microprocessor  152 . The processor  152  analyzes the data associated with the test patterns to identify the printhead or heads used to generate the particular pattern(s) (block  218 ). The processor  152  further uses the data associated with the test patterns to identify cross-process position and roll of the respective printhead with respect to a desired reference (block  220 ). Comparison of the cross-process position and roll of the respective printhead with the desired cross-process position and roll for the respective printhead (block  222 ) yields correction data for the respective printhead. 
         [0026]    In this embodiment, the correction data for the inner printhead, that is, the printhead closest to the left side of the media  126 , is used by the processor  152  to control the respective cross-process and roll motors to align the inner printhead (block  224 ). The correction data for the outer printhead, along with data associated with the extent of the inner printhead, is used by the processor  152  to control the respective cross-process and roll motors the align the outer printhead with respect to the desired reference (block  226 ). 
         [0027]    The desired reference or references may be defined differently for different systems. Thus, in some systems, the edge of the web media may be used to provide the in-process axis with the cross-process axis perpendicular to the in-process axis. Alternatively, one nozzle of a selected printhead may be designated as the reference and the cross-process position of the other printheads adjusted based upon the location of the designated nozzle. In a further alternative, a sensing member of the linear array sensor may be designated as the reference establishing an in-process axis while the extent of the linear array sensor defines a cross-process axis. In a further alternative, the reference is chosen so that the adjustment of all the heads average to zero. 
         [0028]    The memory  154  may include instructions which, when executed by the processor  152 , determine whether or not an additional alignment is conducted based upon various criteria. By way of example, a device which has not been running may become misaligned even after an initial correction as the temperature of the various components continues to increase. If the criteria for an additional alignment is met (block  228 ), then the value of the monitoring velocity is modified (block  230 ) and the alignment process  200  continues by determining the current speed of the media  126  along the process path  130  (block  204 ). By selectively adjusting the monitoring velocity (block  230 ), the number of alignment iterations may be established for a particular system as the system is brought online. 
         [0029]    If the criteria for an additional alignment is not met (block  228 ), the alignment procedure  200  ends (block  232 ). Thereafter, the media  126  continues to accelerate along the process path  130  until normal operating speed is achieved. The processor  152  then controls the print stations  102 ,  104 ,  106 , and  108  to complete the print job. 
         [0030]    The alignment procedure  200  may be used to correct a variety of alignment issues on a variety of systems as is explained with reference to  FIG. 4 .  FIG. 4  depicts a portion of the media  126  located at the process segment  136  which is adjacent to the linear array sensor  138 . Eight test patterns contained in the regions  240 ,  242 ,  244 ,  246 ,  248 ,  250 ,  252 , and  254  are shown on the media  126 . 
         [0031]    Reference lines  256  and  258  are also shown in  FIG. 4 . The reference lines  256  and  258  show an in-process axis ( 256 ) and cross-process axis ( 258 ) to which the printheads in the system  100  were previously aligned for the process path of a previous print job. In this example, the first nozzle of the first printhead is used to define the desired reference. The in-process axis  256  is thus located directly beneath the first nozzle of the first printhead and perpendicular to the cross-process axis  258  when viewed in plan. The reference line  260  also lies directly beneath the first nozzle of the first printhead and is perpendicular to the reference lines  262 ,  264 ,  266 , and  268  are  260 . 
         [0032]    Comparing the reference line  256  with the reference line  260  reveals that the in-process axis  260  is rotated from the direction of the in-process axis  256 . Thus, while the test pattern  240  is aligned with the reference line  260  in the in-process direction, the test pattern  240  is not aligned with the cross-process axis  262 . Additionally, the test pattern  242  is located too close to the reference line  260 , resulting in an overlap area  270 . The overlap  270  indicates that the printheads  110  and  112 , which were used to generate the test patterns  240  and  242 , respectively, closer together than desired due to some physical disturbance when they were aligned with the reference lines  256  and  258 . Once source of a physical disturbance is a change in temperature. 
         [0033]    The test patterns  244  and  246  depict the location of the test pattern marks generated after a cross-process correction has been effected. The test pattern  244  does not change since in this embodiment, the test pattern  244  is formed in part by the reference for the in-process axis. Application of a cross-process correction to the printhead  112 , however, moves the printhead  112  away from the printhead  110 . Thus, the overlap area  270  has been essentially eliminated. 
         [0034]    Both of the test patterns  244  and  246  are rotated with respect to the cross-process axis  264 . The test pattern  246 , however, is rotated less with respect to the cross-process axis  264  than is the test pattern  244 . Application of roll correction pursuant to the procedure  200  to both of the printheads  110  an  112  produces rotation of the printheads  110  and  112 , effectively rotating the patterns generated by the printheads  110  and  112  about the axes  274  and  276 , respectively, in the direction of the arrows  278  and  280 , respectively. In alternative embodiments, printheads may share a common axis of rotation. 
         [0035]    The test patterns  248  and  250  are generated after the roll correction has been applied to the printheads  110  and  112 . The rotation of the printhead  110  results in the alignment of the test pattern  248  with both the in-process axis  260  and the cross-process axis  266 . The rotation of the printhead  112  results in the alignment of the test pattern  250  with an axis that is parallel to the cross-process axis  266 . 
         [0036]    In the last pair of patterns, the alignment of the test pattern  252  is identical to the test pattern  248 . The test pattern  254 , however, has been further corrected in the in-process direction with respect to the test pattern  252 . Thus, the test patterns  252  and  254  are adjacent to each other. Adjustment along the process path  130  is accomplished by modification of the timing between the jetting of the nozzles on the printhead  110  and the jetting of the nozzles on the printhead  112 . Specifically, increasing the delay between jetting of the nozzles has the effect of moving the test pattern generated by the printhead  110  further along the process path  130 . 
         [0037]    Thus, once the procedure  200  is executed, the width of the images generated by the printheads  110  and  112  are wider than the width of the images formed by the printheads  110  and  112  during the print job using the alignment indicated by the test patterns  240  and  242 . Degradation of the image due to printhead overlap, however, is reduced by incorporating additional cross-process correction based upon the extent of the printheads  110  and  112 . 
         [0038]    Additionally, in the event that the printheads  110  and  112  move closer together due to some physical process, such as perhaps cooling of the print heads, the images formed by the print stations  110  and  112  shrink. Consequently, the cross-process position of the nozzles within the respective printheads is spread more narrowly. This reduction results in a gap area between the patterns formed by the printheads  110  and  112 . The procedure  200  may be used to identify and implement appropriate corrections to eliminate any such gap. An image formed subsequent to gap elimination is smaller than an image formed without the correction, but degradation due to gap formation is reduced. 
         [0039]    Even though an alignment procedure may be fully accomplished with a single test pattern from each printhead, using each of the nozzles in a printhead during any alignment results in increased ink usage. Moreover, detection of overlap errors such as described above with respect to  FIG. 4  is difficult unless the patterns are formed on the media in a staggered fashion. Additionally, care must be taken to ensure that the printed pattern is associated with the proper printhead by incorporating an understanding of the media speed into such association. 
         [0040]    One approach which ameliorates one or more of the foregoing issues is to use different nozzle groupings for each printhead in forming a test pattern. This approach is described with reference to  FIG. 5  wherein the nozzles of the printheads  110  and  112  of the system are shown. The printhead  110  includes eight columns of nozzles  280   1-128 . Each row column includes 16 nozzles  280   x . Likewise, the printhead  112  has eight rows columns of nozzles  282   1-128  with 16 nozzles  282   x  in each column. 
         [0041]    Formation of a test pattern with the printhead  110  is accomplished, in this example, by commanding nozzles  280   4 ,  280   23 ,  280   48 ,  280   72 ,  280   83 , and  280   97  to fire thereby forming a pattern of lines  284   x  on the media  126  wherein each line  284   x  is formed by an associated nozzle  280   x . Likewise, formation of a test pattern with the printhead  112  is accomplished, in this example, by commanding nozzles  282   9 ,  282   30 ,  282   41 ,  282   64 ,  282   91 , and  282   110  to fire thereby forming a pattern of lines  286   x  on the media  126 . 
         [0042]    In this embodiment, the printheads  110  and  112  are controlled such that the respective test patterns are formed on the media  126  substantially adjacent to each other. The patterns formed may be distinguished from each other in a number of ways. By way of example, the last nozzle used on the printhead  110  (farthest to the right as viewed in  FIG. 5 ) and the first nozzle used on the printhead  112  (farthest to the left as viewed in  FIG. 5 ) may be selected to ensure that the two patterns cannot overlap along a cross-process axis. Thus, for example, the spacing between the nozzles  280   97  and  282   9  is greater than the total possible misalignment of both of the printheads  110  and  112  with respect to the media  126 . 
         [0043]    When the patterns  284   x  and  286   x  are detected by the linear array sensor  138 , the spacing between the individual marks (e.g.,  286   9  and  286   30 ) may be used to specifically identify the printhead used to form the marks in a manner similar to a barcode. Once the pattern is associated with the proper printhead, the spacing of the marks and data regarding the particular nozzles fired to generate the marks may be used to extrapolate the cross-process position of each of the nozzles for the particular printhead. 
         [0044]    By selectively firing specific nozzles, a roll correction for a particular printhead may be established. Specifically, the distance and orientation between the particular nozzles on a printhead is known. Accordingly, the cross-process spacing between the marks formed by two nozzles may be used to identify the roll of the printhead with respect to the media. By way of example, if the printhead  110  is rotated in a counter clockwise direction to the position of printhead  110 ′, the resultant marks  284   48 ′ and  284   97 ′ are spaced farther apart than the marks  284   48  and  284   97 . Rotation of the printhead  110  in a clockwise direction to the position of printhead  110 ″ results in the marks  284   48 ″ and  284   97 ″ which are spaced closer together than the marks  284   48  and  284   97 . 
         [0045]    Additionally, the time between generation of the patterns  284   x  and  286   x  and the time at which the patterns  284   x  and  286   x  pass the linear sensor array  138  may be used to determine the speed of the media  126  since the distance between the printheads  110  and  112  and the linear array sensor  138  along the process path  130  is known, albeit the actual speed is constantly changing as the speed of the media  126  along the process path  130  is accelerating. Thus, in embodiments which do not include a speed sensor, so long as the linear array sensor is energized prior to the arrival of a test pattern at the linear array sensor, the speed of the media may be determined. 
         [0046]    Once the media speed is known using either a linear array sensor or a speed sensor, jetting of the nozzles may be modified to reduce the amount of ink expended while ensuring a good contrast ratio is presented to the linear array sensor  138 . Specifically, the nozzles within the printheads  110 ,  112 ,  114 ,  116 ,  118 ,  120 ,  122 , and  124  are configured to provide a desired contrast when the system  100  is operating at normal or target speed. The contrast is achieved by depositing a particular concentration of ink on the media which is established by a designed flow rate of ink. In the event the speed of the media  126  along the process path  130  is less than the normal operating speed, the same concentration of ink may be deposited on the media  126  by selectively de-energizing the nozzle. 
         [0047]    One illustration of the foregoing approach is if the normal operating speed of the media  126  along the process path  130  is 100 inches/second, and the instantaneous speed of the accelerating media  126  during an alignment procedure is 25 inches/second. In this situation, the same amount of ink may be deposited on the media  126  during the alignment procedure by jetting the nozzles for ¼ of the time that the nozzles would be jetted if the media  126  was moving at full speed. Thus, a nozzle jetting pattern of 1-on 3-off while forming the test pattern may be used. Of course, the actual speed of the media  126  along the process path  130  during the alignment procedure  200  is constantly increasing. The change in speed during formation of a test pattern, however, will not significantly alter the concentration of ink achieved. 
         [0048]    The various steps performed in the procedure  200  may be performed in different order and modified for particular applications in various ways in addition to the variations described above. By way of example, all of the printheads in a system may be controlled to simultaneously print test patterns. 
         [0049]    It will be appreciate that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.