Patent Publication Number: US-6338544-B1

Title: Reduction of stitch joint error by alternating print head firing mode

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to stitch errors in printing. 
     2. Description of Related Art 
     Fluid ejecting devices such as, for example, ink jet printers, fire drops of fluid from rows of nozzles of an ejection head. The nozzles are usually fired sequentially in groups beginning at one end of the head and continuing to the other end of the head. While the nozzles are being fired, the head moves at a rate designed to advance it by a resolution distance before the next firing sequence begins. If the nozzles are not fired simultaneously, the rows of nozzle are usually tilted so that drops fired from all nozzles land in a substantially vertical column. The ejection head can have one or more dies, each die having a plurality of nozzles. Some devices have ejection heads with only one die, and some devices have ejection heads with multiple dies. If an ejection head has multiple dies, the dies can be, for example, arranged vertically with respect to one another so that the head can eject more drops in a single swath of the head compared to a head having a single die. 
     The line at which the swaths ejected by adjacent dies, or at which the adjacent swaths, meet is called the stitch joint. Stitch joint error exists when the swaths meeting at the stitch joint meet in such a way that the resulting arrangement of drops at the stitch joint of a printed image is undesirable. Because the spacing of the stitch joint errors is typically ½ to 1 times the printing width of the print head (typically ¼″ to ½″), the stitch joint errors are very noticeable because the human eye is very sensitive to this spatial frequency region. 
     Stitch joint error can be, for example, the result of a gap between the drop of one die adjacent the stitch joint and the drop of an adjoining die adjacent the stitch joint. Such a gap can be the result of the same firing sequence being used for the nozzles of both dies. A similar stitch joint error can be caused when the same nozzle firing sequence is used for each swath of a single die ejection head. 
     SUMMARY OF THE INVENTION 
     The stitch joint error can be reduced by firing the nozzles of adjacent dies in a multi-die ejection head using different firing sequences. Similarly, the nozzles of a single die ejection head can be fired using different sequences in adjacent swaths of the ejection head. By firing the nozzles in different sequences as discussed above, the drops at the stitch joint can be positioned closer to each other than they would be if the same firing sequence was used for each die/swath. By reducing the distance between the drops on either side of the stitch joint, the location of the stitch joint becomes less apparent. 
     When fabricating multi-die ejection heads, it is often difficult to precisely position adjacent dies so that, in the case of vertically positioned dies, the spacing between the lowermost nozzle of the upper die and the uppermost nozzle of the lower die is equal to the nozzle spacing of each die. As a result, it can be cost effective to overlap the dies and then select which nozzles will be used. For example, using the second or third nozzle from the upper edge of the lower die may result in a more proper spacing with relation to the lowermost nozzle of the upper die. Such die overlapping is another factor that must be considered when determining what firing sequence of the lower die results in the least amount of stitch joint error. 
     These and other features and advantages of the invention are described in or are apparent from the following detailed description of the exemplary embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of the invention will be described in relation to the following drawings in which like reference numerals refer to like elements, and wherein: 
     FIG. 1 is a perspective view of an exemplary image recording apparatus in which the systems and methods of the invention can be used; 
     FIG. 2 is one exemplary embodiment of a face of a print head of the invention; 
     FIG. 3 is another exemplary embodiment of a face of a print head of the invention; 
     FIG. 4 is one exemplary embodiment of a print head of the invention having two dies; 
     FIG. 5 shows stitch joint error without using the invention; 
     FIG. 6 shows one example of reduced stitch joint error using the invention; 
     FIG. 7 shows one example of reduced stitch joint error using the invention with overlapping dies; 
     FIG. 8 shows another example of reduced stitch joint error using the invention with overlapping dies; 
     FIG. 9 shows another example of reduced stitch joint error using the invention with overlapping dies; 
     FIG. 10 is a functional block diagram of an exemplary embodiment of the invention; and 
     FIG. 11 is a flowchart showing a process of a controller of the invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     One exemplary embodiment of a fluid ejection device according to this invention is an image recording apparatus having a print head movable in a first direction. Other embodiments of the image recording apparatus can have a print head movable in a first direction and a second direction opposite the first direction. In an image recording apparatus incorporating the systems and methods of this invention, a controller controls the firing and firing sequence of drops of a recording fluid such that a stitch joint error is reduced or eliminated. 
     FIG. 1 shows a portion of an image recording apparatus that incorporates the systems and methods of the invention. As shown in FIG. 1, a print head  10  slides in a first direction A along a guide rod  15 . As the print head  10  moves back and forth, an image is recorded on a recording medium  30  which is supported by a platen  25 . A controller  20  provides print information to the print head  10  to control the image printed by the print head  10 . 
     FIG. 2 shows the face  1   1  of one exemplary embodiment of the print head  10 . This exemplary embodiment of the print head  10  has one row of nozzles  40  on the face  11 . FIG. 3 shows the face  12  of a second exemplary embodiment of the print head  10 . This exemplary embodiment of the print head  10  has four rows of nozzles  40  on the face  12 . FIG. 4 shows a print head  10  having a first die  50  and a second die  51 . The face  13  of first die  50  and the face  14  of the second die  51  are each shown having one row of nozzles  40 . FIGS. 2-4 are simply examples of many configurations of print heads usable with the systems and methods of the invention. The print head  10  could have any appropriate number of dies and any appropriate number of rows of nozzles or other configurations of nozzles controllable by the controller. 
     FIGS. 5-9 show dots of recording fluid, for example ink, on a recording medium. The horizontal placement of each dot of recording fluid is determined by the firing sequence of the nozzles of the print head. In FIGS. 5-9, the print head moves from left to right while firing the recording fluid. Therefore, the leftmost dot of the upper swath shown in each figure is the first dot fired in the sequence shown. The horizontal dotted line shown in FIGS. 5-9 represents the stitch joint between two swaths of a print head or between two dies of a multi-die print head. 
     In some print heads, the nozzles are fired in groups so that several nozzles in a particular print head will be fired simultaneously. In ink jet printing, simultaneously firing two adjacent nozzles can cause ink drop interactions that result in a degraded image. For the purpose of explaining examples of the systems and methods of this invention, a print head will be used that fires its nozzles in four groups. For example, if the print head has 80 nozzles, there will be four firing events, each containing 20 nozzles fired simultaneously. In this example, if the nozzles are numbered sequentially  1 - 80 , nozzles  1 ,  5 ,  9 ,  13  . . .  77  will be fired simultaneously, nozzles  2 ,  6 ,  10 ,  14  . . .  78  will be fired simultaneously, nozzles  3 ,  7 ,  11 ,  15  . . .  79  will be fired simultaneously, and nozzles  4 ,  8 ,  12 ,  16  . . .  80  will be fired simultaneously. 
     One example of firing sequences of the groups of nozzles is known as “4-ripple”. In the 4-ripple firing mode, there are four sequences in which the group of nozzles can be fired. Each sequence is referred to as a “state”, with the state being determined by the first nozzle fired. State 1 is the sequence  1 - 3 - 2 - 4 , state 2 is the sequence  2 - 4 - 1 - 3 , state 3 is the sequence  3 - 1 - 4 - 2 , and state 4 is the sequence  4 - 2 - 3 - 1 . All of these firing states avoid the nearest neighbor interaction of simultaneously fired adjacent nozzles. 
     In the case of multiple die print heads in which the dies are oriented along a direction perpendicular to the direction of travel of the print head, and in the case of a single die print head that prints in only one direction, a significant systematic stitch error results if the adjacent dies or swaths are fired in the same state. FIG. 5 shows this condition. In FIG. 5, a first dot  511 , a second dot  512 , a third dot  513  and a fourth dot  514  are fired from a first die or during a first swath. A fifth dot  521 , a sixth dot  522 , a seventh dot  523  and an eighth dot  524  are fired from a second die located adjacent to and below the first die or during a second swath. Both the first die and the second die, or the single die in the first and second swaths, are fired in state 1 ( 1 - 3 - 2 - 4 ) as evidenced by the relative horizontal location of the dots in FIG.  5 . The gap between fourth dot  514  and fifth dot  521  is the systematic stitch error caused by firing the first die and the second die, or the single die in the first and second swaths, in the same state. This firing mode stitch error is compounded by any die-die stitch error resulting from die-die x axis misplacement. The misplacement of adjacent dies often results from manufacturing tolerances. 
     The systems and methods of this invention reduce the stitch joint error by selecting different firing states for each adjacent die or each adjacent swath of a single die print head. FIG. 6 shows an example of the invention in which the second die or the second swath is in state 2 ( 2 - 4 - 1 - 3 ). FIG. 6 shows that changing the state of the second die or the second swath can minimize the firing order stitch error so that a significant error is not systematically added to any die-die stitch error that exists at the stitch line between the two dies. In FIG. 6, a first dot  611 , a second dot  612 , a third dot  613  and a fourth dot  614  correspond to the first-fourth dots  511 - 514  of FIG. 5 because the first die in FIG. 6 is in state 1 ( 1 - 3 - 2 - 4 ). However, the second die or the second swath in FIG. 6 is in state 2 ( 2 - 4 - 1 - 3 ), as shown by the horizontal placement of a fifth dot  621 , a sixth dot  622 , a seventh dot  623  and an eighth dot  624 . 
     The appropriate state for the second die or the second swath is determined by the state of the first die or the first swath. The appropriate state for the second die or the second swath for each possible state of the first die or the first swath can be stored, for example, in a look-up table to be referenced by the controller  20  during printing. 
     The procedure described with reference to FIG. 6 is sufficient for a single die print head or if the first and second dies are precisely aligned such that the lowermost nozzle of the first die and the uppermost nozzle of the second die are spaced correctly with relation to the spacing of the other nozzles within each of the first and second dies. However, due to, for example, manufacturing expense and limitations, the first and second dies can be overlapped and a nozzle other than the uppermost nozzle of the second die selected as the uppermost firing nozzle of the second die. In other words, the uppermost one or more nozzles of the second die may not be used. Such overlapping avoids the requirement for precision assembly because misalignment between the two dies can be limited to one-half of the center-to-center nozzle spacing by selecting the optimum uppermost firing nozzle. In addition, nozzle selection can be made to result in a sub-pixel error, i.e., a paper under-advance error, rather than an error greater than a pixel, i.e., a paper over-advance error. This is desirable because paper under-advance of a given magnitude is much less noticeable than paper overadvance of the same magnitude. 
     However, the combination of the 4-ripple firing scheme and die overlapping can result in an additional source of stitch joint error if not compensated for. 
     FIG. 7 shows an example in which the first die is in state 1 ( 1 - 3 - 2 - 4 ) as evidenced by the horizontal location of a first dot  711 , a second dot  712 , a third dot  713  and a fourth dot  714 . The second die in FIG. 7 overlaps the first die such that the second nozzle is selected as the uppermost firing nozzle of the second die. This overlap and first-firing nozzle selection is indicated by the number 2 to the left of a fifth dot  721  in FIG.  7 . Similarly, a sixth dot  722  is fired from the third nozzle of the second die, a seventh dot  723  is fired from the fourth nozzle of the second die and an eighth dot  724  is fired from the fifth nozzle of the second die. As discussed earlier, in this example, the nozzles of each die are fired in four groups, the first group containing nozzles  1 ,  5 ,  9  . . .  77 . As a result, if a die is fired in state 1 all of the nozzles in the first group are the first nozzles fired in that die. Because FIG. 7 shows only the four uppermost fired nozzles of the second die and because the uppermost nozzle of the second die is not fired in this overlap situation, the eighth dot  724  is shown as being fired from the fifth nozzle. Because the fifth nozzle belongs to the first group of nozzles fired in state 1, it is the leftmost dot of the dots fired from the second die in FIG.  7 . Although the second die in FIG. 7 is fired in state 1, similarly to the second die in FIG. 5, the fifth-eighth dots  721 - 724  in FIG. 7 appear in a different pattern than fifth-eighth dots  521 - 524  in FIG.  5 . This is because the uppermost fired nozzle of the first group of nozzles in FIG. 7 ( 1 ,  5 ,  9 ,  13  . . .  77 ) is the fifth nozzle, whereas it is the uppermost, or first, nozzle in FIG.  5 . 
     As can be seen from the preceding discussion, die overlapping, and the resulting selection of the uppermost fired nozzle, can change which state of the second die is most appropriate for reducing stitch joint error. 
     FIGS. 8 and 9 are other examples, similar to FIG. 7, of overlapped dies in which the third uppermost nozzle and fourth uppermost nozzle, respectively, are chosen as the uppermost fired nozzle. FIG. 8 shows a first dot  811 , a second dot  812 , a third dot  813  and a fourth dot  814  fired from the first die in state 1 ( 1 - 3 - 2 - 4 ). In FIG. 8, the third nozzle of the second die has been chosen as the uppermost firing nozzle. As a result, a fifth dot  821  is fired from the third nozzle, a sixth dot  822  is fired from the fourth nozzle, a seventh dot  823  is fired from the fifth nozzle and an eighth dot  824  is fired from the sixth nozzle of the second die. The second die in FIG. 8 is fired in state 1 as evidenced by the fifth nozzle (the uppermost fired nozzle in the first group of nozzles) being the first nozzle fired. FIG. 9 is similar to FIGS. 7 and 8 except that the fourth nozzle of the second die is the uppermost fired nozzle of the second die and the second die is in state 2 ( 2 - 4 - 1 - 3 ). In FIG. 9, a first dot  911 , a second dot  912 , a third dot  913  and a fourth dot  914  correspond to the first-fourth dots  811 - 814  of FIG.  8 . The firing state (state 2) of the second die in FIG. 9 is indicated by the relative horizontal position of a fifth dot  921 , a sixth dot  922 , a seventh dot  923  and an eighth dot  924 . Specifically, state 2 is indicated because the seventh dot  923  fired from the sixth nozzle, i.e., the uppermost fired nozzle of the second group of nozzles ( 2 ,  6 ,  10 ,  14  . . .  78 ) of the second die, is fired first. 
     FIGS. 6-9 show examples of appropriate states of the first and second dies when the uppermost fired nozzle of the second die is the first, second, third or fourth nozzle, respectively, of the second die when the first die is in state 1. It will be apparent that other combinations of the first die state and the uppermost fired nozzle of the second die will result in different optimum states for the second die. As discussed above, the optimum state of the second die for each possible condition can be stored, for example, in a look-up table in the controller. 
     FIG. 10 is a functional block diagram of one exemplary embodiment of a printing device  200  incorporating the systems and methods of the invention. The printing device  200  has an input/output device  110  that connects the printing device  200  to an input device  300 , such as, for example, a keyboard or interactive display, and an image data source  400  such as, for example, a computer. In general, the image data source  400  can be any one of a number of different sources, such as a scanner, a digital copier, a facsimile device that is suitable for generating electronic image data, or a device suitable for storing and/or transmitting electronic image data, such as a client or server of a network, or the Internet, and especially the World Wide Web. For example, the image data source  400  may be a scanner, or a data carrier such as a magnetic storage disk, CD-ROM or the like, or a host computer, that contains image data. Thus, the image data source  400  can be any known or later developed source that is capable of providing image data to the printing device  200  of this invention. 
     When the image data source  400  is a personal computer, the data line connecting the image data source  400  to the printing device  200  can be a direct link between the personal computer and the printing device  200 . The data line can also be a local area network, a wide area network, the Internet, an intranet, or any other distributed processing and storage network. Moreover, the data line can also be a wireless link to the image data source  400 . Accordingly, it should be appreciated that the image data source  400  can be connected using any known or later developed system that is capable of transmitting data from the image data source  400  to the printing device  200 . 
     The input/output device  110 , a memory  130 , an overlap determining circuit  140 , and a state determining circuit  150  communicate over a data/control bus with a controller  120 . The overlap determining circuit  140  determines a degree of overlap of the second print head in order to select the most appropriate uppermost fired nozzle of the second print head. The state determining circuit  150  determines which state is most appropriate to produce the minimum stitch joint error. The appropriate state is then supplied to a printing apparatus  160 . The printing apparatus  160  can include, for example, the print head. 
     It should be understood that each of the circuits shown in FIG. 10 can be implemented as portions of a suitably programmed general purpose computer. Alternatively, each of the circuits shown in FIG. 10 can be implemented as physically distinct hardware circuits within an ASIC, or using a FPGA, a PDL, a PLA or a PAL, or using discrete logic elements or discrete circuit elements. The particular form each of the circuits shown in FIG. 10 will take is a design choice and will be obvious and predicable to those skilled in the art. 
     FIG. 11 is a flow chart showing one example of a process of the invention. In step S 100 , a state of the first die (or first swath if a single die print head is used) is determined. If it is determined in step S 200  that a second die is present and that the second die overlaps the first die, processing proceeds to step S 300 . If not, processing jumps directly to step S 400 . In step S 300 , the first nozzle of the second die is determined based on which nozzle of the second die provides the proper spacing relative to the lowermost nozzle of the first die. In step S 400 , the state of the second die that produces the smallest stitch joint error is determined based on the state of the first die and possibly on the determined uppermost fired nozzle of the second die. 
     As shown in FIG. 10, the printing device  200  is preferably implemented on a programmed general purpose computer. However, the printing device  200  can also be implemented on a special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit elements, an ASIC or other integrated circuit, a digital signal processor, a hardwired electronic or logic circuit such a discrete element circuit, a programmable logic device such as a PLD, PLA, FPGA or PAL, or the like. In general, any device, capable of implementing a finite state machine that is in turn capable of implementing the flow chart shown in FIG. 11, can be used to implement the printing device  200 . 
     While the systems and methods of the invention have been explained with relation to a print head having a row of nozzles that are sequentially fired in groups, the nozzles of each particular group being fired simultaneously, the systems and methods of the invention are also applicable to other types of printing systems. For example, printing systems as shown in U.S. Pat. No. 5,675,365, incorporated herein by reference, can benefit from the invention by scheduling the activation of specific ejectors such that the stitch joint error is reduced or eliminated. 
     Further, while the systems and methods of the invention have been explained using four groups of 20 nozzles each, the systems and methods of the invention are also applicable to image forming systems and methods using any number of nozzles and any number of groups. In addition, while one skilled in the art of printing will apply the systems and methods of the invention to printing with ink, it is noted that the systems and methods of the invention apply to fluids other than ink. 
     In some exemplary embodiments of the invention, an alignment procedure where the user is allowed to choose from the best aligned of a series of vertical lines can be performed to determine the best print head states for a particular print head. 
     While the invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiments of the invention as set forth above are intended to be illustrative and not limiting. Various changes may be made without departing from the spirit and scope of the invention as described herein.