Abstract:
Within a printer, compensation is provided for color migration within ink drops. Color compensation is provided by varying firing frequency of each print nozzle so as to fire high frequency bursts of ink drops. Each print nozzle is idle, not being used to eject ink drops, between high frequency bursts of ink drops.

Description:
BACKGROUND 
     Inkjet printing mechanisms use moveable cartridges, also called pens, that use one or more printheads formed with very small nozzles through which drops of liquid ink (e.g., dissolved colorants or pigments dispersed in a solvent) are fired. To print an image, the carriage traverses over the surface of the print medium, and the ink ejection elements associated with the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller. The pattern of pixels on the print media resulting from the firing of ink drops results in the printed image. Certain ink jet inks undergo a process of colorant migration wherein the colorant in the firing chamber is depleted over a short time period. Colorant migration causes print quality defects, especially in documents with fine lines and narrow text characters. 
     For ink jet inks that undergo a process of colorant migration, delay between firing drops from a nozzle, allows the migration to occur. Thus after a 1.5 second period of non-printing, a drop of black ink can have a shading more like gray than black. After about a 3 second period of non-printing, a drop of black ink can lose most of the colorant and appear almost clear. 
     The problem of colorant migration is diminished with larger drop volumes. For example, for print cartridges that eject drops that are 30 nanograms (ng) or larger, the large drop weight makes colorant migration less noticeable. However, writing systems that use large drop volumes have significantly worse image quality than those with lower drop weights. 
     Continuous firing of a print nozzle at high frequency also serves to significantly diminish the effects of colorant migration. For example, the HP Business InkJet 2200 printer, available from Hewlett-Packard Company, having a business address of 3000 Hanover Drive, Palo Alto, Calif. 94304, uses smaller (lower drop weight) 18 ng black drops fired at 36 kilohertz (kHz) from a 600 nozzles per inch (npi) cartridge. However, continuous firing at 36 kHz can cause the printhead to over heat, can cause drop ejection problems and puts constraints on the fluidic architecture design. 
     SUMMARY OF THE INVENTION 
     In accordance with the preferred embodiment of the present invention, within a printer, compensation is provided for color migration within ink drops. Color compensation is provided by varying firing frequency of each print nozzle so as to fire high frequency bursts of ink drops. Each print nozzle is idle, not being used to eject ink drops, between high frequency bursts of ink drops. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of an inkjet printer according to an embodiment of the present invention. 
     FIG. 2 is a simplified block diagram of print electronics within the inkjet printer shown in FIG.  1 . 
     FIG. 3 is a simplified diagram (not to scale) of a printhead used with the inkjet printer shown FIG.  1 . 
     FIG. 4 shows a print mask used within the inkjet printer shown in FIG.  1 . 
     FIG. 5 shows a print mask used within the inkjet printer shown in FIG. 1 in accordance with an embodiment of the present invention. 
     FIG. 6 shows a print mask used within the inkjet printer shown in FIG. 1 in accordance with an embodiment of the present invention. 
     FIG. 7 shows a print mask used within the inkjet printer shown in FIG. 1 in accordance with an embodiment of the present invention. 
     FIG. 8 shows a print mask used within the inkjet printer shown in FIG. 1 in accordance with an embodiment of the present invention. 
     FIG. 9 shows an example of a portion of a printing mask used for four pass printing in accordance with an embodiment of the present invention. 
     FIG. 10 shows an example of a portion of a printing mask used for eight pass printing in accordance with an embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a simplified block diagram of an inkjet printer  10 . Inkjet printer  10  includes, for example, a controller  32  that is connected to a computer system  31  via an interface unit  30 . The interface unit  30  facilitates the transferring of data and command signals to controller  32  for printing purposes. Interface unit  30  also enables inkjet printer  10  to be electrically connected to an input device  33  for the purpose of downloading print image information to be printed on a print medium  35 . Input device  33  can be any type of peripheral device (e.g., a scanner or fax machine) that can be connected to inkjet printer  10 . 
     In order to store the data, at least temporarily, inkjet printer  10  includes a memory unit  34 . Memory unit  34  is divided into a plurality of storage areas that facilitate printer operations. The storage areas include a data storage area  44 , driver routines storage  46 , and algorithm storage area  48  that holds the algorithms that facilitate the mechanical control implementation of the various mechanical mechanisms of inkjet printer  10 . 
     Data area  44  receives data files that define the individual pixel values that are to be printed to form a desired object or textual image on medium  35 . Driver routines  46  contain printer driver routines. Algorithms  48  include the routines that control a sheet feeding stacking mechanism for moving a medium through the printer from a supply or feed tray to an output tray and the routines that control a carriage mechanism that causes a printhead carriage unit to be moved across a print medium on a guide rod. 
     In operation, inkjet printer  10  responds to commands by printing full color or black print images on print medium  35 . In addition to interacting with memory unit  34 , controller  32  controls a sheet feeding stacking mechanism  36  and a carriage mechanism  38 . Controller  32  also forwards printhead firing data to one or more printheads, represented in FIG. 1 by a printhead  40 . The input data received at interface  30  includes, for example, information describing printed characters and/or images for printing. For example, input data may be in a printer format language such as Postscript, PCL  3 , PCL  5 , HPGL, HPGL  2  or some related version of these. Alternatively, the input data may be formatted as raster data or formatted in some other printer language. The printhead firing data sent to printhead  40  is used to control the ejection elements associated with the nozzles of an ink jet printer, such as for thermal ink jet printer, piezo ink jet printers or other types of ink jet printers. 
     For example, as shown in FIG. 2, printhead firing data is used by a pulser  12  to generate pulses that control an ink ejection element (IEE)  23  associated with a nozzle  13  located on a printhead  40 . Pulser  12  may be located on or off printhead  40 , depending on the particular embodiment of the present invention. In the example shown in FIG. 2, printer electronics  11  provides to pulser  12  printhead firing data on two lines. Information on the first line sets the pulse rate and information on the second line indicates which pulses are to be forwarded to ink ejection element  23 . The pulses forwarded to ink ejection element  23  are forwarded as a current pulse that is applied to a resistor within ink ejection element  23 . The current pulse causes an ink droplet  15 , formed with ink from an ink reservoir  14 , to be emitted from nozzle  13 . 
     Printhead firing data generated by controller  32  is also used by a pulser  16  to generate pulses which control an ink ejection element (IEE)  24  associated with a nozzle  17 . Controller  32  provides to pulser  16  printhead firing data on two lines. Information on the first line sets the pulse rate and information on the second line indicates which pulses are to be forwarded to ink ejection element  24 . The pulses forwarded to ink ejection element  24  are forwarded as a current pulse that is applied to a resistor within ink ejection element  24 . The current pulse causes an ink droplet  19 , formed with ink from an ink reservoir  18 , to be emitted from nozzle  17 . Nozzle  17  can be located on printhead  40  or on another printhead. 
     The printhead firing data is also used by a pulser  26  to generate pulses which control an ink ejection element (IEE)  25  associated with a nozzle  27 . Controller  32  provides to pulser  26  printhead firing data on two lines. Information on the first line sets the pulse rate and information on the second line indicates which pulses are to be forwarded to ink ejection element  25 . The pulses forwarded to ink ejection element  25  are forwarded as a current pulse that is applied to a resistor within ink ejection element  25 . The current pulse causes an ink droplet  29 , formed with ink from an ink reservoir  28 , to be emitted from nozzle  27 . Nozzle  27  can be located on printhead  40  or on another printhead. 
     For more information on inkjet printers, see for example U.S. Pat. No. 6,302,505, issued on Oct. 16, 2001 to Askeland et al. for “Printing System that Utilizes Continuous and Non-continuous Firing Frequencies”, which is commonly assigned and the subject matter of which is herein incorporated by reference. 
     FIG. 3 is a simplified diagram (not to scale) showing the arrangement of a portion of the nozzles of printhead  40 . For example, printhead  40  has four vertical columns of nozzles. These are represented in FIG. 3 by a vertical column  51  of nozzles, a vertical column  52  of nozzles, a vertical column  53  of nozzles and a vertical column  54  of nozzles. For example, vertical column  51  of nozzles is separated from vertical column  52  of nozzles by a distance  55  of approximately 0.2 millimeters. For example, vertical column  52  of nozzles is separated from vertical column  53  of nozzles by a distance 56 of approximately 1.9 millimeters. 
     The vertical columns of nozzles are perpendicular to a scan direction  61  and parallel to a media movement direction  62 . The columnar vertical spacing  57  between adjacent nozzles in a column is, for example, {fraction (1/600)}th inch. By using four columns of nozzles instead of one, and logically treating the nozzles as a single column, the effective vertical spacing (represented in FIG. 3 by a distance  59  and a distance  60 ) is reduced to {fraction (1/2400)} th  inch, thus achieving improved printing resolution in direction of the media advance direction  92 . Distance  58 , equal to {fraction (1/1200)} th  of an inch, represents the columnar vertical spacing  57  between adjacent nozzles in vertical column  53  and vertical column  54 . For example, vertical column  51 , vertical column  52 , vertical column  53  and vertical column  54  each have 528 nozzles, for a total of 2112 nozzles. The number of nozzles may be arbitrarily selected. 
     Printing can be performed in one or multiple passes. Some printers utilize print modes to vary the number of passes used for printing. One pass operation facilitates increased throughput on plain paper. In a one-pass mode, all dots to be fired on a given row of dots are placed on the medium in one swath of the printhead, and then the print medium is advanced into position for the next swath. A two-pass print mode is a print pattern wherein approximately one-half of the dots available for a given row of available dots per swath are printed on each pass of the printhead, so two passes are needed to complete the printing for a given row. Similarly, a four-pass mode is a print pattern wherein approximately one fourth of the dots for a given row are printed on each pass of the printhead. In a print mode of a certain number of passes, each pass should print, of all the ink drops to be printed, a fraction equal roughly to the reciprocal of the number of passes. 
     Print modes are also used to determine specific partial-inking patterns. Print modes also allow the printer to control several factors during printing that influence image quality, including the amount of ink placed on the media per dot location, the speed with which the ink is placed, and the number of passes required to complete the image. Providing different print modes to allow placing ink drops in multiple swaths can help with hiding nozzle defects. Different print modes are also employed depending on the media type. 
     The pattern used in printing each nozzle section is known as a “print mask.” Typically, if more than one pass is used to print, a different print mask is used for each pass. During multipass printing, a print mask is a binary pattern that determines exactly which ink drops are printed in a given pass. In other words, a print mask determines which passes are used to print each pixel. Thus, the print mask defines both the pass and the nozzle which will be used to print each pixel location, i.e., each row number and column number on the media. The print mask can be used to “mix up” the nozzles used in such a way as to reduce undesirable visible printing artifacts. In single pass printing and in multiple pass printing, a print mask can be used to reduce the firing frequency of each nozzle. 
     Print controller  32  (shown in FIG. 1) controls carriage mechanism  38  and media  35  movements and activates the nozzles for ink drop deposition. By combining the relative movement of the carriage mechanism  38  along the scan direction  61  with the relative movement of the print medium  35  along the medium movement direction  62 , each printhead  40  can deposit one or more drops of ink at each individual one of the pixel locations on the print medium  35 . A print mask is used by print controller  32  to govern the deposition of ink drops from the printhead  40 . For example, a separate print mask may exist for each discrete intensity level of color (e.g. light to dark) supported by inkjet printer  10 . For each pixel position in a row during an individual printing pass, the print mask has a print mask pattern which acts both to enable the nozzle positioned adjacent the row to print, or disable that nozzle from printing, on that pixel location, and to define the number of drops to be deposited from enabled nozzles. Whether or not the pixel will actually be printed on by the corresponding enabled nozzle depends on whether the image data to be printed requires a pixel of that ink color in that pixel location. The print mask is typically implemented in firmware in inkjet printer  10 , although it can be alternatively implemented in a software driver in a computing processor (not shown) external to the printer. 
     The term “printing pass”, as used herein, refers to those passes in which printhead  40  is enabled for printing as the nozzle arrangement moves relative to the medium  35  in the scan direction  61 . In bi-directional printing, each forward and rearward pass along the scan direction  61  can be a printing pass. In unidirectional printing, printing passes can occur in only one of the directions of movement. In a given printing pass of the carriage mechanism  38  over the print medium  35  in a multi-pass printer, only the certain pixel locations enabled by the print mask can be printed, and inkjet printer  10  deposits the number of drops specified by the print mask for the corresponding pixel locations if the image data so requires. The print mask pattern is such that additional drops for the certain pixel locations, as well as drops for other pixel locations in the swath, are filled in during other printing passes. 
     FIG. 4 shows an example of a portion of a printing mask  64  used for single pass printing with a 2400 nozzles per inch (npi), 18 nanogram (ng) print cartridge. The print mask can be used for any color (including black) ink utilized by a printer. Each small rectangle represents a {fraction (1/1200)} inch wide by {fraction (1/2400)} inch tall pixel. An “X” indicates where a drop is made on the media. 
     Each row represents the firing pattern of a single nozzle. The printing mask shown in FIG. 4 spreads out the firing of nozzles to give the lowest firing frequency. In any 4×4 matrix of printing mask  64 , each nozzle can fire at most 1 time. For example, printing mask  64  in FIG. 4 would have a maximum firing frequency of 9 kHz at a 30 inch per second (ips) scan rate. 
     In order to print a {fraction (1/300)} inch wide line indicated by width  65 , drops in a column  66 , a column  67 , a column  68  and a column  69  are used. Within width  65 , each nozzle is fired at most one time per pass. If the nozzles used to print the line delineated by width  65  are idle for a significant length of time (e.g., more than a one second delay) before beginning to print the line delineated by width  65 , color depletion of ink drops can have a significant impact on the print quality of the line. 
     A solution to the print quality problem caused by color depletion of ink drops is to use a mask that allows for the firing of a 2 drop burst at high frequency. This is illustrated in FIG.  5 . 
     Throughout the descriptions of the Figures, vertical lines are used as printing examples. As will be understood by persons of ordinary skill in the art, the print quality issues that arise printing vertical lines arise when printing other shapes and so the use of vertical lines is merely exemplary. 
     FIG. 5 shows an example of a portion of a printing mask  70  used for single pass printing with a 2400 nozzles per inch (npi), 18 nanogram (ng) print cartridge. Each small rectangle represents a {fraction (1/1200)} inch wide by {fraction (1/2400)} inch tall pixel. The printing mask shown in FIG. 5 includes two drop burst firing of nozzles. In any 4×4 matrix of printing mask  70 , at least one nozzle fires twice in succession. 
     In order to print a {fraction (1/300)} inch wide line indicated by width  75 , drops in a column  71 , a column  72 , a column  73  and a column  74  are used. Within width  75 , half the nozzles are fired twice per pass. If the nozzles used to print the line delineated by width  75  are idle for a significant length of time (e.g., more than a one second delay) before beginning to print the line delineated by width  75 , color depletion of ink drops can result in color depletion of the first drop fired by the nozzle. However the second drop in the two drop burst fired by the nozzle will not be color depleted. For example, the colorant in the ink can be any color, including black, used by a printer. 
     For example, the nozzle represented by a row  76  may fire a color depleted drop in column  71 , but will fire a full colorant drop in column  72 . Likewise, the nozzle represented by a row  77  may fire a color depleted drop in column  73 , but will fire a full colorant drop in column  74 . The nozzle represented by a row  78  may fire a color depleted drop in column  71 , but will fire a full colorant drop in column  72 . The nozzle represented by a row  79  may fire a color depleted drop in column  73 , but will fire a full colorant drop in column  74 . This will result in a vertical line that is composed of 50% potentially depleted and 50% full colorant drops. 
     As illustrated by FIG. 6, a {fraction (1/600)} inch wide line can also be printed with 50% depleted and 50% full colorant drops. FIG. 6 shows an example of a portion of a printing mask  80  used for single pass printing with a 2400 nozzles per inch (npi), 18 nanogram (ng) print cartridge. Each small rectangle represents a {fraction (1/1200)} inch wide by {fraction (1/2400)} inch tall pixel. The printing mask shown in FIG. 6 includes two drop burst firing of nozzles. Any {fraction (1/600)} inch wide line of printing mask  80  will be composed of 50% potentially depleted and 50% full colorant drops. In FIG. 6, the horizontal resolution of printing is 600 dpi, so each nozzle will fire a two drop burst for every printed pixel. 
     In order to print a {fraction (1/600)} inch wide line indicated by width  83 , drops in a column  81  and a column  82  are used. Within width  83 , one fourth of the nozzles are fired twice per pass. If the nozzles used to print the line delineated by width  83  are idle for a significant length of time (e.g., more than a one second delay) before beginning to print the line delineated by width  83 , color depletion of ink drops can result in color depletion of the first drop fired by the nozzle. However, the second drop in the two drop burst fired by the nozzle will not be color depleted. 
     For example, the nozzle represented by a row  84  may fire a color depleted drop in column  81 , but will fire a full colorant drop in column  82 . Likewise, the nozzle represented by a row  85  may fire a color depleted drop in column  81 , but will fire a full colorant drop in column  82 . The nozzle represented by a row  86  may fire a color depleted drop in column  81 , but will fire a full colorant drop in column  82 . This will result in a vertical line that is composed of 50% potentially depleted and 50% full colorant drops. 
     In FIG. 6, higher frequency bursts or ink drops are separated by a period of time approximately equal to seven times a length of duration of each higher frequency burst of ink drops. This is illustrated in each row of FIG. 6 by two X&#39;s in immediately adjacent rectangles (representing, the higher frequency bursts of ink drops), followed by six rectangles without an X, before another X occurs in a seventh rectangle (representing the period of time approximately equal to seven times a length of duration of each higher frequency burst of ink drops). For example, the period (duration) of time for a higher frequency burst of ink drops to occur, as represented in FIG. 6, is the time from the beginning of when the first ink drop in the burst is fired, until the time the last ink drop in the burst has been fired. For a two-drop burst this is equal to the time it takes to fire the first ink drop, plus the time between firing the first ink drop and the second ink drop, plus the time it takes to fire the second ink drop. When the firing of an ink drop is considered to be almost instantaneous, then the period of time for a higher frequency burst of ink drops to occur is approximately equal to the time between firing the first ink drop and the second ink drop. This is represented in FIG. 6 by a duration  87 . For example, duration  87  is shown based on an assumption that each ink drop is fired at a time location equivalent to the center of a rectangle. As can be seen from duration  87 , the duration of time for a higher frequency burst of two ink drops to occur is approximately equal to the duration of time represented by one rectangle. 
     In FIG.  5  and FIG. 6, good line quality is achieved in both cases. This approach requires the printing of a 2 drop burst at 36 kHz which is much easier to accomplish than continuous firing at 36 kHz and typically does not cause the overheating and drop ejection problems of continuous printing. 
     FIG. 7 shows how good line quality can be achieved using a 2 drop burst with a 9 ng, 2400 npi writing system. (50% depleted drops and 50% full colorant drops). In order to print a {fraction (1/600)} inch wide line indicated by width  93 , drops in a column  91  and a column  92  are used. Within width  93 , half the nozzles are fired twice per pass. If the nozzles used to print the line delineated by width  93  are idle for a significant length of time (e.g., more than a one second delay) before beginning to print the line delineated by width  93 , color depletion of ink drops can result in color depletion of the first drop fired by the nozzle. However, the second drop in the two drop burst fired by the nozzle will not be color depleted. 
     For example, the nozzle represented by a row  94  may fire a color depleted drop in column  91 , but will fire a full colorant drop in column  92 . Likewise, the nozzle represented by a row  95  may fire a color depleted drop in column  91 , but will fire a full colorant drop in column  92 . The nozzle represented by a row  96  may fire a color depleted drop in column  91 , but will fire a full colorant drop in column  92 . The nozzle represented by a row  97  may fire a color depleted drop in column  91 , but will fire a full colorant drop in column  92 . This will result in a vertical line that is composed of 50% potentially depleted and 50% full colorant drops. 
     In FIG. 7, higher frequency bursts of ink drops are separated by a period of time equal to approximately three times a length of duration of each higher frequency burst of ink drops. In FIG. 7, in order to avoid depleted pixels when firing a thin horizontal line, it is necessary to horizontally position lines so that nozzles fire a two drop burst for every printed pixel. 
     Burst lengths greater than two may also be used. For example, FIG. 8 shows how good line quality (⅓ potentially depleted drops and ⅔ full colorant drops) can be achieved using a 3 drop burst with a 12 ng, 2400 npi writing system. In order to print a {fraction (1/300)} inch wide line indicated by width  105 , drops in a column  101 , a column  102 , a column  103  and a column  104  are used. Within width  105 , half the nozzles are fired three times per pass. If the nozzles used to print the line delineated by width  105  are idle for a significant length of time (e.g., more than a one second delay) before beginning to print the line delineated by width  105 , color depletion of ink drops can result in color depletion of the first drop fired by the nozzle. However, the second drop and the third drop in the three drop burst fired by the nozzle will not be color depleted. 
     For example, the nozzle represented by a row  106  may fire a color depleted drop in column  101 , but will fire a full colorant drop in column  102  and in column  103 . Likewise, the nozzle represented by a row  107  may fire a color depleted drop in column  102 , but will fire a full colorant drop in column  103  and column  104 . The nozzle represented by a row  108  may fire a color depleted drop in column  101 , but will fire a full colorant drop in column  102  and column  103 . The nozzle represented by a row  109  may fire a color depleted drop in column  102 , but will fire a full colorant drop in column  103  and column  104 . This will result in a vertical line that is composed of ⅓ potentially depleted and ⅔ full colorant drops. 
     Various embodiments of the present invention can also implemented with multiple pass systems. For example, FIG. 9 shows an example of a portion of a printing mask  110  used for four pass printing with a 2400 nozzles per inch (npi), 18 nanogram (ng) print cartridge. Each small rectangle represents a {fraction (1/1200)} inch wide by {fraction (1/2400)} inch tall pixel. A number in a rectangle indicates the number of the pass in which a drop is made on the media. The printing mask shown in FIG. 9 includes two drop burst firing of nozzles. Any {fraction (1/600)} inch wide line of printing mask  110  will be composed of 50% potentially depleted and 50% full colorant drops. In FIG. 9, the horizontal resolution of printing is 600 dpi, so each nozzle will fire a two drop burst for every printed pixel. 
     Likewise, FIG. 10 shows an example of a portion of a printing mask  120  used for eight pass printing with a 2400 nozzles per inch (npi), 18 nanogram (ng) print cartridge. Each small rectangle represents a {fraction (1/1200)} inch wide by {fraction (1/2400)} inch tall pixel. A number in a rectangle indicates the number of the pass in which a drop is made on the media. The printing mask shown in FIG. 10 includes two drop burst firing of nozzles. Any {fraction (1/600)} inch wide line of printing mask  110  will be composed of 50% potentially depleted and 50% full colorant drops. In FIG. 10, the horizontal resolution of printing is 600 dpi, so each nozzle will fire a two drop burst for every printed pixel. 
     While the present invention was described as used within ink jet printer  10 , the present invention can be embodied in other printing systems, for example, such as those that utilize a drum printer or a stationary page wide array. The disclosed embodiments of the present invention can be used to overcome the text, line and image quality problems, associated with colorant migration in low drop weight, high npi writing systems. 
     The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.