Abstract:
Non-black ink jet printheads are arranged in a printer carriage such that the printhead nozzle arrays are non-overlapping in the carriage swath direction. A fixer printhead is positioned such that the print medium is first advanced past the fixer printhead prior to reaching any of the other printheads. The printheads are selectively driven during each direction of a bi-directional carriage movement, and the print medium is incrementally advanced before each change in carriage movement direction. The order of laying down droplets of different colors is the same during the movement in each direction, thereby eliminating bi-directional hue shifting print artifacts.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to ink-jet swath printing devices, and more particularly to techniques for eliminating hue shifting due to bi-directional swath printing. 
     BACKGROUND OF THE INVENTION 
     An ink jet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes “dot locations”, “dot positions”, or “pixels”. Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink. 
     Ink jet printers print dots by ejecting very small drops of ink onto the print medium, and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed. 
     Color ink jet printers commonly employ a plurality of printheads, for example four, mounted in the print carriage to produce different colors. Each printhead contains ink of a different color, with the commonly used colors being cyan, magenta, yellow, and black. These base colors are produced by depositing a drop of the required color onto a dot location. Secondary or shaded colors are formed by depositing drops of different colors on adjacent dot locations; the human eye interprets the color mixing as the secondary or shading, through well known optical principles. 
     Print quality is one of the most important considerations of competition in the color ink jet printer field. Since the image output of a color ink jet printer is formed of millions of individual ink drops, the quality of the image is ultimately dependent upon the quality of each ink drop and the arrangement of the ink drops on the print medium. 
     Print head arrangements employed in the past have typically used linear arrays of print elements, wherein the pens of different color are one next to the other. There are several negative consequences of such an arrangement. One is that, when the carriage is going from left to right, the colors are laid down in one order, say YMC for example. When the carriage goes in the other direction, from left to right, the colors are laid down in the opposite order, CMY in this example. The problem with this is that the blue made by first printing cyan and then magenta is slightly different from the blue made in the reverse order. This is because the final dot will inevitably cover a bit of the first dot. This problem is illustrated in FIGS. 1A and 1B. FIG. 1A shows the case of movement left to right, depositing magenta before cyan, so that some cyan overlaps an adjacent magenta dot. FIG. 1B shows the case of movement in the right to left direction, depositing cyan before magenta, so now some magenta overlaps the adjacent cyan dot. Of course, the blue color shifting is only one example; other colors will also suffer from hue shifting in a similar fashion. 
     A fixer pen can be employed to deposit droplets of a fixer liquid on the print medium at dot locations to subsequently receive droplets of a colored ink. Fixer liquids can be used to cause the subsequently applied colored ink droplets to precipitate very quickly. The fixer liquids can be a cobalt salt, or any other substance by which the colored inks are enhanced. Another problem is that if one wishes to add a fixer ink, which must be printed first, the printer must have two fixer ink pens, one on either side of the colored ink pens, to achieve bi-directional printing. This adds to the expense of the printer. Moreover, the required width of the carriage is increased, due to the need to accommodate two fixer pens. For some printer embodiments, a top coat could also be applied by a top coat pen after applying the colored ink to the print medium, in a post-printing top coat step. An exemplary top coat material is a transparent polymer. Two such top coat pens would be needed for bi-directional printing, further adding to the expense and width of the printer. 
     Print throughput is degraded with wide carriages due to the increased overtravel. This is illustrated in the diagrammatic illustration of FIG. 2, wherein a printer carriage  10  is mounted for swath movement along a scan axis  12  over a print zone  14  of width A—A. The carriage supports six ink jet pens  16 A- 16 F in a lateral arrangement. FIG. 2 shows the carriage  10  at the leftmost position needed to support printing on the width of the print media, i.e. over the print zone width A—A. The carriage overtravel past the left edge of the print zone is relatively large, due to the pen to pen width and the required acceleration distance needed to accelerate the carriage to achieve the carriage printing speed. Six pens are used here, including two pens  16 A,  16 F with fixer ink at each end of the carriage. Disposed intermediate the fixer pens are the color pens, e.g.  16 B (cyan),  16 C (magenta),  16 D (yellow) and  16 E (black). Two fixer pens are needed in this case to support bi-directional printing. Only the leftmost position of the carriage is shown in FIG. 2; the same carriage overtravel distance must be accommodated on the right side of the print zone. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the invention, a technique is described for color inkjet printing to eliminate bi-directional hue shifting. The technique includes: 
     mounting a printer carriage for bidirectional movement along a print swath axis; 
     providing a plurality of ink jet printheads of different colors, each printhead having a nozzle array for emitting droplets of ink of non-black color; 
     supporting the plurality of printheads on the printer carriage such that the respective nozzle arrays do not overlap in a direction along the print swath axis; 
     moving the printer carriage in a first direction along the swath axis from one side of a print area to a second opposite side of the print area while driving one or more of the printheads to emit droplets under computer control onto a print medium; 
     providing relative motion between the print medium and the carriage in a direction transverse to the swath axis;advancing the print media; 
     moving the printer carriage in a second direction along the swath axis from the second side of the print area to the first side while driving one or more of the printheads to emit droplets under computer control onto the print medium; 
     providing relative motion between the print medium and the carriage in a direction transverse to the swath axis, 
     wherein the order of laying down droplets of different colors is the same during the movement in the second direction as during the movement in the first direction, thereby eliminating bi-directional hue shifting print artifacts. 
     Another embodiment of the invention is a color inkjet printer, including a printer carriage supported for bi-directional movement along a print swath axis, and a plurality of ink jet printheads of different non-black colors, each printhead having a nozzle array for emitting droplets of ink of non-black color. The plurality of printheads is supported on the printer carriage such that the respective nozzle arrays do not overlap in a direction along the print swath axis. The printer further includes a carriage drive mechanism for moving the printer carriage in a first direction along the swath axis from one side of a print area to a second opposite side of the print area, and in a second direction along the swath axis from the second side of the print area to the first side. A motordriven media advance system provides relative motion between the print medium and the carriage in a direction transverse to the swath axis. A printer controller is coupled to and controls the plurality of printheads, the carriage drive mechanism and the media advance system to achieve bi-directional swath printing on a print medium. The printheads are selectively driven to emit droplets when the carriage is moving in the first direction and when the carriage is moving in the second direction, and to incrementally advance the print medium between carriage swath movements. The order of laying down droplets of different non-black colors is the same during the movement in the second direction as during the movement in the first direction, thereby eliminating bi-directional hue shifting print artifacts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other features and advantages of the present invention will become more apparent from the following detailed description of an exemplary embodiment thereof, as illustrated in the accompanying drawings, in which: 
     FIGS. 1A and 1B illustrate the problem of hue-shifting due to bi-directional swath printing. FIG. 1A shows the case of movement left to right, depositing magenta before cyan, so that some cyan overlaps an adjacent magenta dot. FIG. 1B shows the case of movement in the right to left direction, depositing cyan before magenta. 
     FIG. 2 is a diagrammatic view of a conventional color printhead arrangement, wherein a printer carriage is mounted for swath movement along a scan axis over a print zone. 
     FIG. 3 illustrates a color printhead arrangement embodying an aspect of this invention. 
     FIG. 4 is a diagrammatic view of a printer carriage and mounting structure for a printer embodying the invention. 
     FIG. 5 is a side diagrammatic view illustrating additional aspects of the printer of FIG.  4 . 
     FIG. 6 is a schematic block diagram of aspects of the printer of FIGS. 4 and 5. 
     FIGS. 7A-7D illustrate respective alternate configurations of a color printhead arrangement embodying the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 illustrates a color printhead arrangement embodying an aspect of this invention. In this diagrammatic view, the printer carriage is depicted by dashed box  20 . The carriage moves along the swath axis  22  over a print zone  24  of width A—A. The carriage supports the five pens  26 A- 26 E in an arrangement wherein the nozzle arrays of each of the non-black-ink pens do not overlap in the scan direction over the print zone. The nozzle arrays include two columns of nozzles in this embodiment, with each column being represented by two lines for each pen, e.g. nozzle array columns  26 A 1  and  26 A 2  for pen  26 A. Assume that a media advance direction is depicted by arrow  28 . Pen  26 A is a fixer pen, emitting droplets of the ink fixer. Pen  26 B is a yellow ink pen. Pen  26 C is a black ink pen, and overlaps the swath coverage of the yellow pen. The black ink pen would not typically be activated in a print swath for color mixing with a yellow dot, or a magenta or cyan dot, in a typical swath, and is typically used for (monochrome) text printing. Of course, the black ink pen could be used for printing during a color print job, e.g., for shading, borders, text, etc. By positioning the black ink pen immediately adjacent the fixer pen, higher speed printing can be achieved for text (black only) printing. Pen  26 D is a cyan ink pen, and pen  26 E is a magenta ink pen. This arrangement allows the use of a single fixer pen  26 A, instead of requiring a second fixer pen as in the arrangement shown in FIG. 2, since the fixer pen will always encounter a given swath position first, in advance of any of the other pens. 
     The arrangement of FIG. 3 supports bi-directional swath printing without resulting in undesirable hue-shifting from a swath in a first direction and a swath in the opposite direction. This is because the hue-laying order is the same for each printing direction. This results from the following printing sequence. 
     As a print medium is advanced along the media path in the printer from an input location to the print zone, arriving at the print zone along direction  28 , the leading edge first encounters the fixer pen  26 A. A first pass of the carriage on a given print medium in a first direction, say left-to-right, will use only the fixer pen  26 A to lay down a pattern of fixer ink dots along the coverage area of its nozzle array. After the first pass, the medium is incrementally advanced by an advance distance. The advance distance will depend on the print mode, and is typically equal to the length of the fixer pen nozzle array, although a smaller distance may sometimes be used to prevent print defects due to such factors as line feed error, misdirected nozzles, weak nozzles and the like. A fresh area of the medium is now positioned below the fixer pen, and the area to which the fixer ink drops was applied is now below the yellow pen  26 B and the black pen  26 C. 
     For the second pass of the carriage in the reverse direction, i.e. from right-to-left, the fixer pen  26 A and the yellow pen  26 B are driven to apply drops of the corresponding liquid. Upon completion of the second pass, the medium is advanced by the same incremental distance, such that a fresh medium area is again below the fixer pen, the second area just traversed by the fixer pen during the second pass is below the yellow pen, and the area to which both fixer and yellow drops have been applied is now below the cyan pen  26 D. Now the carriage traverses the print zone on the third pass from left-to-right, with the fixer, yellow and cyan pens driven to apply drops of the corresponding liquid. After completion of the third pass, the medium is again incrementally advanced, and on the fourth pass from right-to-left, all pens are driven to apply droplets of the corresponding liquid. 
     For the subsequent passes over the body of the medium until the end of the page or print job is approached, all four pens  26 A,  26 B,  26 D,  26 E will be driven under computer control to achieve the desired color image, and the order in which drops are applied to a given pen area is the same for both scan directions. Once the bottom of the image is approached, for the last four scans, the operation will be reversed from that described to start the print job. 
     FIGS. 4-6 illustrate an exemplary printer system employing the printhead arrangement shown in FIG.  3 . The carriage  20  is supported for reciprocating movement over the print zone  24  on guides  30 ,  32 , in turn supported by a frame  40 . A carriage drive system (not shown in FIG. 4) is connected to the carriage for accurately positioning and driving the carriage back and forth along the swath axis. Carriage drive systems suitable for the purpose are well known in the art, and can include, for example, an endless belt connected to a motor drive with encoder feedback. The pens  26 A- 26 E are supported in the carriage, and can be replaceable pens such as self-contained cartridges including a printhead with nozzle array and an internal reservoir, or with a tube connecting to an off-carriage reservoir for replenishing an internal ink supply. 
     One exemplary form of media advance apparatus is illustrated in the simplified diagrammatic side view of FIG. 5. A motor driven pick roller  52  is activated to pick a sheet of the print media from an input source  54 , and pass it into the nip between drive roller set  56 . The print media may be any type of suitable material, such as paper, card-stock, transparencies, photographic paper, fabric, mylar, metalized media, and the like, but for convenience, the illustrated embodiment is described using paper as the print medium. The invention is also applicable to roll-fed media as well. The sheet is advanced onto an endless perforated belt  58 , mounted for rotation on belt pulleys  60 ,  62 . The pulleys are driven to advance the sheet to the print zone  24  under the pens  26 A- 26 E. A vacuum plenum  62  holds the sheet tightly against the belt surface at the print zone. The exiting sheet is passed through the nip formed by output roller set  64  to an output tray (not shown in FIG.  5 ). 
     FIG. 6 is a schematic block diagram of the control system for the printer of FIGS. 4-5. A controller  70  such as a microcomputer receives print job commands and data from a print job source  72 , which can be a personal computer, digital camera or other known source of print jobs. The controller acts on the received commands to activate the pick roller motor  74  to pick a sheet from the input tray  54 , advance the sheet to the nip between the drive roller and pinch roller set  56 , and activate the drive motor system  76  to advance the sheet onto the belt, and move the belt to advance the sheet to the print zone. The carriage drive  78  is driven by the controller to position the carriage  20  for commencement of a print job, and to scan the carriage along the slider rods. As this is done firing pulses are sent to the printheads comprising the pens  26 A- 26 E. The controller receives encoder signals from the carriage encoder  80  to provide position data for the carriage. The controller is programmed to advance incrementally the sheet to position the sheet for successive swaths, and to eject the completed sheet into the output tray. 
     Other printhead arrangements can alternatively be employed in accordance with this invention. For example, a printer employing more than four ink colors can employ a printhead arrangement to eliminate bi-directional hue shifts. Four exemplary additional embodiments are illustrated in FIGS. 7A-7D. In the arrangement of FIG. 7A, eight pens  102 A- 102 H are arranged on a diagonal  104 , for ease of construction. In this embodiment pen  102 A is a fixer ink pen, pen  102 B a black ink pen, pen  102 C a yellow ink pen, pen  102 D a cyan ink pen, pen  102 E a magenta ink pen, pen  102 F a red ink pen, pen  102 G a green ink pen, and pen  102 H a blue ink pen. 
     In the arrangement  110  of FIG. 7B, the pens  112 A- 112 H are arranged in a staggered arrangement along an axis  114  parallel to the media advance direction  116 , to reduce the width of the carriage in the carriage scan direction, perpendicular to direction  116 . In this embodiment, pen  112 A is a fixer ink pen, pen  112 B a black ink pen, pen  112 C a yellow ink pen, pen  112 D a cyan ink pen, pen  112 E a magenta ink pen, pen  112 F a red ink pen, pen  112 G a green ink pen, and pen  112 H a blue ink pen. 
     In the arrangement  120  of FIG. 7C, the pens  122 A- 122 M are arranged on a diagonal  124 . In this embodiment pen  122 A is a fixer ink pen, pen  122 B a black ink pen, pen  122 C a yellow ink pen, with the nozzle array of the black ink pen overlapping the nozzle array of the yellow ink pen. The remaining pens are in overlapping pairs of light and dark corresponding ink colors. Pen  122 D is a light cyan ink pen, and pen  122 E is a dark cyan ink pen. Pen  122 F is a light magenta ink pen, and pen  122 G is a dark magenta ink pen. Pen  122 H is a light red ink pen, and pen  122 I is a dark red ink pen. Pen  122 J is a light green ink pen, and pen  122 K is a dark green ink pen. Pen  122 L is a light blue ink pen, and pen  122 M is a dark blue ink pen. 
     In the arrangement  130  of FIG. 7D, the pens  132 A- 122 M are arranged on an axis  134  parallel to the media advance direction  136 . In this embodiment pen  132 A is a fixer ink pen, pen  132 B a black ink pen, pen  132 C a yellow ink pen, with the nozzle array of the black ink pen overlapping the nozzle array of the yellow ink pen. The remaining pens are in overlapping pairs of light and dark corresponding ink colors. Pen  132 D is a light cyan ink pen, and pen  132 E is a dark cyan ink pen. Pen  132 F is a light magenta ink pen, and pen  132 G is a dark magenta ink pen. Pen  132 H is a light red ink pen, and pen  132 I is a dark red ink pen. Pen  132 J is a light green ink pen, and pen  132 K is a dark green ink pen. Pen  132 L is a light blue ink pen, and pen  132 M is a dark blue ink pen. 
     In each of the arrangements illustrated in FIGS. 7A-7D, colors which may be used for color mixing to achieve a desired secondary color are positioned in non-overlapping positions along the scan axis. Colors which will not be applied in adjacent dot locations for the purpose of color mixing can be arranged in overlapping relationship, and this is illustrated in FIGS. 7C-7D, where black and yellow overlap, as do the respective light and dark colored ink pens. 
     It is understood that the above-described embodiments are merely illustrative of the possible specific embodiments which may represent principles of the present invention. Other arrangements may readily be devised in accordance with these principles by those skilled in the art without departing from the scope and spirit of the invention.