Abstract:
A two pass print mode method and apparatus limits wind-related print defects produced during printing, utilizing a reciprocating carrier of a printer carrying a printhead having an array of columns of actuator-fired fluid-jetting nozzles along a bi-directional scanning path. Due to instructions from a controller, printing proceeds along an initial partial swath on a print medium during a first pass along the scanning path by firing actuators associated with a first plurality of segments of a given column of nozzles. Then, printing proceeds along a final partial swath on the print medium during a second pass along the scanning path by firing actuators associated with a second plurality of segments of the given column of nozzles. Each segment of nozzles of the first and second pluralities includes more than one consecutive nozzle so that gaps are created in the partial swath printing accommodating wind-related effects without causing wind-related print defects.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     None. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to an inkjet printing and, more particularly, to a two pass print mode method and apparatus for limiting wind-related print defects. 
     2. Description of the Related Art 
     Inkjet printers apply ink to a print medium, such as paper, by ejecting ink droplets from at least one printhead through a column(s) or array(s) of nozzles. The printhead is mounted on a carrier that is movable in a lateral direction across the print medium, commonly termed a unidirectional scan, and ink droplets are selectively ejected from the nozzles at corresponding ink drop placement locations. Specifically, each nozzle is associated with an actuator in the printhead that is “fired” when sufficient current passes through it, the firing causing ink within an associated ink reservoir to be ejected in droplet form from the nozzle. The printhead is moved in a series of unidirectional scans or swaths across the print medium, and between the swaths, the print medium is advanced in a longitudinal or advance direction. Since the printhead moves in a direction that is perpendicular to the advance direction of the print medium, each nozzle passes in a linear manner over the print medium. A printer controller determines which actuators will be “fired” and the proper firing sequence so that a desired image is printed on the print medium. 
     For a given stationary position of the print medium, printing may take place during one or more unidirectional scans of the printhead carrier. As used herein, the term “unidirectional” will refer to scanning in either, but only one, of the two possible scanning directions (left to right, or right to left). Thus, bi-directional scanning refers to two successive unidirectional scans in opposite directions. The term “swath” will refer to a plurality of printing lines traced along imaginary rasters, the imaginary rasters being spaced apart in the sheet feed or advance direction. Ink droplets are deposited along the printing lines on the print medium during a particular scan of the printhead carrier by selective actuation of the individual actuators associated with individual nozzles of the printhead to expel the ink droplets. 
     The quality of printed images produced by an inkjet printer depends in part on the resolution of the printheads. Thus, as the market pull for inkjet printing quality to approach that of silver halide photography continues, one method to achieving that goal is to increase the vertical and horizontal resolution of the printhead. This requires changes that will decrease both the ink droplet size and nozzle-to-nozzle spacing, therefore, necessitating an increase in firing frequency of the heater resistors to achieve the same or greater throughput while maintaining the same or greater color gamut and coverage of larger droplet size printheads. The result of these changes is optimally a decrease in graininess and an increase in sharpness. 
     However, aerodynamic forces and fluidic interactions from neighboring nozzles more adversely affect nozzles that are spaced closer together, and whose actuators are fired at higher frequencies, compared to nozzles producing larger droplets that are spaced farther apart and whose actuators are fired at lower frequencies. The results of these aerodynamic forces and fluidic interactions are severe print quality defects such as swath contraction, non-uniform horizontal intraswath banding, and overspray. 
     Print quality defects associated with aerodynamic and fluidic events, commonly referred to as wind-related defects, are particularly bad in monochromatic or black only printing. This is due to the fact that black only printing modes operate at much higher duty cycles and print speeds. Wind-related defects have also been found to be present at half frequency. Half frequency printing helps to support that the wind-related defects are primarily associated with aerodynamic events, and less contingent on a fluidic event occurring at the same time. Furthermore, wind-related defects have been found to occur at half duty cycle (specifically a typical two pass printing mode that uses a checkerboard pattern). Typical two pass printing is not only half frequency, but it is also half nozzle usage. 
     In summary, therefore, wind-related print defects refer to print quality defects that are caused by a combination of aerodynamic and fluidic events. The main driver is currently thought to be aerodynamic forces that effect satellite formation and placement of the satellites on print media. Wind related print defects are primarily seen in black only print modes, are present in all three of the current easy to implement print methods, and comprise some of the largest hindrances to better text quality. 
     Thus, there is a need for an innovation that will permit continued increase in the resolution of printheads without the accompanying aerodynamic and fluidic events that produce wind-related print defects. 
     SUMMARY OF THE INVENTION 
     The present invention meets this need by providing an innovation that, in a two pass mode of printing, segments the utilization of the nozzles in given columns thereof. The nozzle utilization is determined by the selected firing of the actuators associated with those nozzles. Specifically, only about half of the nozzles in each column are utilized at the same time during a given pass. It has been determined that severity of the wind-related print defect is dependent upon the number of consecutive nozzles in given columns of an array of nozzles that are active or utilized (that is, the number of consecutive actuators firing) at the same time. For instance, by centering the nozzles and using the entire swath height (all of the nozzles in the advance direction) a printed swath will have maximum wind-related print defects. Shortening the swath by eliminating the use of end nozzles eventually the printed swath will not show objectionable wind-related effects. Further, by sufficiently reducing the swath height, the severity and amount of wind-related effects will decrease and eventually disappear. However, due to the desire for high print speed, decreasing the swath height is not an appropriate solution. 
     The solution provided by this innovation is to exploit the result of smaller swath height on wind-related print defects without adopting actual swath height reduction and its attendant adverse effect on print speed. By segmenting each array or column of nozzles utilized so that only half of the nozzles in each column are utilized during a given pass, the number of consecutive nozzles jetting ink droplets in a given column is thereby limited so as to simulate the printing of a reduced swath height for that segment of the swath printed on the given pass. The result is that the gaps left in the printing by the nozzles that are dormant or not utilized, during that pass allow air flow to pass more freely through such gaps minimizing the wind-related print defect. Then, the second pass is performed (either with no advancement of paper or an advancement implemented secondarily) and the nozzles that were not utilized, or that were dormant or idle, during the first pass are now active, or utilized, during the second pass, thereby addressing the full grid within a given region in two passes. 
     Accordingly, in an aspect of the present invention, a two pass print mode method for limiting wind-related print defects, produced during printing by an inkjet printer including a reciprocating carrier that carries a printhead having an array of columns of actuator-fired fluid-jetting nozzles along a bi-directional scanning path, includes printing an initial partial swath on a print medium during a first pass along the scanning path by firing actuators associated with a first plurality of segments of a given column of nozzles, and printing a final partial swath on the print medium during a second pass along the scanning path by firing actuators associated with a second plurality of segments of the given column of nozzles such that each of the segments of the nozzles of the first and second pluralities thereof includes more than one consecutive nozzle so that gaps are created in the partial swath printing that accommodate wind-related effects without causing wind-related print defects on the print medium. 
     In another aspect of the present invention, a two pass print mode apparatus for limiting wind-related print defects includes a printer having a reciprocating carrier that carries a printhead having an array of columns of actuator-fired fluid-jetting nozzles along a bi-directional scanning path, and a controller communicatively coupled to the printhead carried by the reciprocating carrier and executing instructions to effect printing an initial partial swath on a print medium during a first pass along the scanning path by firing actuators associated with a first plurality of segments of a given column of nozzles, and printing a final partial swath on the print medium during a second pass along the scanning path by firing actuators associated with a second plurality of segments of the given column of nozzles such that each of the segments of the nozzles of the first and second pluralities thereof includes more than one consecutive nozzle so that gaps are created in the partial swath printing that accommodate wind-related effects without causing wind-related print defects on the print medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale and in some instances portions may be exaggerated in order to emphasize features of the invention, and wherein: 
         FIG. 1  is a block diagram of an inkjet printing apparatus for performing a two pass print mode method for limiting the amount of wind-related print defects in accordance with the present invention. 
         FIG. 2  is a front view of a portion of the printing apparatus of  FIG. 1 . 
         FIG. 3  is a plan view of a printhead nozzle array of the printing apparatus of  FIG. 1  and the relationship between individual nozzles of the printhead nozzle array and a rectilinear grid. 
         FIG. 4  is a diagram of an exemplary pattern of segments of active and idle nozzles in an array of nozzles of a printhead in accordance with the two pass mode method and apparatus of the present invention. 
         FIG. 5  depicts other diagrams of alternative patterns of segments of active and idle nozzles to that of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numerals refer to like elements throughout the views. 
     Referring now to the drawings and particularly to  FIG. 1 , there is shown a schematic view of an inkjet printing apparatus, generally designated  10 , that is operable for performing a two pass print mode method for limiting the amount of wind-related print defects in accordance with the present invention. The printing apparatus  10  includes a host computer  12  and an inkjet printer  14 . The host computer  12  is coupled to the printer  14  via a bi-directional communications link  16 . The communications link  16  can be effected, for example, using point-to-point electrical cable connections between serial or parallel ports of the printer  14  and host computer  12 , using an infrared transceiver unit at each of the printer  14  and host computer  12 , or via a network connection, such as an Ethernet network. The host computer  12  includes application software operated by a user, and provides image data representing an image to be printed, and printing command data, to the printer  14  via the communications link  16 . During bi-directional communications, the printer  14  supplies printer information, such as for example printer status and diagnostics information, to the host computer  12  via the communications link  16 . 
     As shown schematically in  FIG. 1 , the printer  14  includes a data buffer  18 , a controller  20 , a printhead carriage unit  22  and a print media sheet feed unit  24 . The printing command data and image data received by the printer  14  from the host computer  12  are temporarily stored in the data buffer  18 . The controller  20 , which includes a microprocessor with associated random access memory (RAM) and read only memory (ROM), executes program instructions to retrieve the print command data and image data from the data buffer  18 , and processes the printing command data and image data. For the printing command data and image data, the controller  20  executes further instructions to effect the generation of control signals which are supplied to the printhead carriage unit  22  and print media sheet feed unit  23  to effect the printing of an image on a print medium, such as paper. The image data supplied by the host computer  12  to the printer  14  may be in a bit image format, wherein each bit of data corresponds to the placement of an ink droplet at a particular pixel location in a rectilinear grid of possible pixel locations. 
     Referring to  FIG. 2 , the printhead carriage unit  22  includes a printhead carrier  24  for carrying a color printhead  26  and a mono or black printhead  28 . A color ink reservoir  30  is provided in fluid communication with the color printhead  26 , and a mono or black ink reservoir  32  is provided in fluid communication with the mono printhead  28 . The printhead carrier  26  is guided by a pair of guide rods  34  which define a bi-directional scanning path  34   a  for the printhead carrier  24 . The printhead carrier  24  is connected to a carrier transport belt  36  that is driven by a carrier motor (not shown) to transport the printhead carrier  24  in a reciprocating manner along the guide rods  34 . Thus, the reciprocation of the printhead carrier  24  transports the printheads  26 ,  28  across a print medium  38 , such as paper, along bi-directional scanning path  34   a  to define a print zone  40  of the printer  14 . This reciprocation occurs in a main scan direction  42  that is parallel with the bi-directional scanning path  34   a , and is also commonly referred to as the horizontal direction. 
     During each scan of the printhead carrier  24 , the print medium  38  is held stationary by the print media sheet feed unit  23 . The print media sheet feed unit  23  includes an index roller  39  that incrementally advances the print medium  38  in a sheet feed direction  44 , also commonly referred to as a sub-scan direction or vertical direction, through the print zone  40 . As shown in  FIG. 2 , the sheet feed direction  44  is depicted as an X within a circle to indicate that the sheet feed direction  44  is in a direction substantially perpendicular to the plane of  FIG. 2 , toward the reader. The sheet feed direction  44  is substantially perpendicular to the main scan direction  42 , and, in turn, substantially perpendicular to the bi-directional scanning path  34   a . The printhead carriage unit  22  and printheads  26 ,  28  may be configured for unidirectional printing or bi-directional printing. 
     Referring to  FIG. 3 , taking the mono printhead  28  for example, it includes an array  46  of ink jetting orifices, commonly referred to as nozzles  48 . Each nozzle  48  of the nozzle array  46  has an associated actuator (not shown), such as a heater element or a piezoelectric element, which, when energized at the directive of the controller  20 , causes an ink droplet to be expelled from the nozzle  48 . Thus, each ink jetting nozzle  48  of the mono printhead nozzle array  46  can be individually and selectively actuated by the controller  20  to expel an ink droplet to form a corresponding ink dot on the print medium  38 . The ink jetting nozzles  48  in the nozzle array  46  are disposed in a staggered and horizontally adjacent relationship relative to each other. It will be appreciated that the number of ink jetting nozzles  48  within each array  46  may vary from that shown without departing from the scope of the present invention. 
     Still referring to  FIG. 3 , there is also shown the print medium  38  overlaid by an imaginary rectilinear grid  50  of possible pixel locations defined within the printable boundaries of the print medium  38 , those locations being where the ink droplets ideally are to be formed. The rectilinear grid  50  includes a plurality of pixel rows (also commonly called rasters r 1 , r 2 , r 3 , . . . rN)  50   a  and pixel columns  50   b  defining the printable image area on the print medium  38 . The pixel rows  50   a  are arranged to be horizontally parallel, and parallel with the main scan direction  42 . The pixel columns  50   b  are arranged to be vertically parallel, and parallel with the sheet feed direction  46 . Each pixel row  50   a  will correspond to a potential printing line on the print medium  38 . The center-to-center distance between pixels, sometimes referred to as dot pitch, is determined by the resolution of the printer  14 . For example, in a printer capable of printing 1200 dots per inch (dpi), the dot pitch of the array is one twelve-hundredth of an inch. The ink droplets ideally are deposited at the intersections of the lines of the grid  50  defined by the pixel rows and columns  50   a ,  50   b.    
     Referring now to  FIG. 4 , there is a diagram showing the patterns of active (designated by squares) and idle (designated by circles) segments  48   a ,  48   b  of nozzles  48  in column pairs K 1 , K 2  for left-to-right (L-to-R) and right-to-left (R-to-L) print directions. In the example shown, half of the nozzles  48  in each array  46  are active during each pass and printed at full frequency, the other half being idle. Experimentation has shown that a five-on, five-off pattern of segments  48   a ,  48   b  for each array  46  results in enhanced print quality. This pattern of active and idle segments  48   a ,  48   b  of nozzles  48  substantially limits (if not entirely eliminates) the amount of wind-related print defects in the image printed on the print medium  38  during L-to-R and R-to-L printing. For each segment  48   a ,  48   b , the opposite one of the two sides of nozzle segments  48   a ,  48   b  in column pairs K 2  is active versus a given one of the two sides of nozzle segments  48   a ,  48   b  in column pairs K 1 . For example, in the first row of nozzle segments  48   a ,  48   b  of column pairs K 1  and K 2  in L-to-R printing the right side of nozzle segments  48   a ,  48   b  of column pairs K 1  (high nozzles) and the left side of nozzle segments  48   a ,  48   b  of column pairs K 2  (low nozzles) are active. This helps to minimize alignment sensitivity due to via-to-via and x-array offsets and equalizes the dot shape when considering main drop and satellite trajectories. In other words, nozzles  48  are laid out in a pattern so that the sides of pairs of segments  48   a ,  48   b  of the column pairs K 2  that are active will always be a mirror image of the sides of the pairs of segments  48   a ,  48   b  of column pairs K 1  that are active resulting in decreased sensitivity to alignments and dot shape differences. Additionally, in any given pass substantially 50% of the ink is deposited for any local area. This minimizes bi-directional banding effects, which often result due to dry time differences. 
     The above-described two pass mode method of the present invention is implemented by printing the two passes without a paperfeed such that the printhead  28  passes over a given swath twice before advancing the paper sheet  38 . However, this printing method can also be implemented using traditional bi-directional printing where the printhead  28  advances a distance half of the printhead height each pass or using a small step-big step method to minimize bi-directional dry time banding. The main limitation is sizing the feed step such that the polarity of the pattern switches from pass to pass. 
     The printer controller  20  executes instructions to carry out the two pass mode method of the present invention. As mentioned, the method uses only half of the nozzles  48  in a given pass (swath), but uses those nozzles  48  during every fire opportunity. The arrangement of the nozzle usage in segments  48   a ,  48   b  of nozzles  48  reduces the wind-related, print defects. The reduced wind-related effect is the result of the segments  48   a ,  48   b  of nozzles  48  being small enough (in number of consecutive nozzles  48  active) to not allow low pressure regions to develop and the voids or breaks being large enough (in consecutive nozzles  48  idle) to allow air flow to pass with less resistance. The number of consecutive nozzles  48  in a given segment  48   a ,  48   b  ranges from a minimum of two to an optimum value determined experimentally (equal to five for the hardware tested) after which the benefit decreases as the number of nozzles increases. The performance improvement can be observed for any nozzle density with the greatest benefit as the dpi increases to 600 dpi and beyond. The preferred number is five nozzles  48  per segment  48   a ,  48   b  for a nozzle density of 1200 dpi. By contrast, a traditional two-pass shingle using a checker pattern in which every other nozzle of a different one half of the nozzles is active during each pass (swath) is subject to wind-related defects which result from increased resistance to air flow such that a low pressure region results on the trailing side of the sheet of jetting nozzles which suspends small ink droplets and eventually releases them onto the sheet resulting in a print quality defect. 
     Turning now to  FIG. 5 , there are depicted diagrams of other potential patterns of segments of active and idle nozzles to address the wind-related print defect problem. The most effective, and relatively defect free, pattern of segments is the one described above and illustrated in  FIG. 4 . The patterns in the diagrams of  FIG. 5  are of lesser effectiveness. 
     To recap, in the employment of the two pass mode method and apparatus of the present invention a strategy is provided for choosing which dots to lay down in a given pass in a way that reduces the aerodynamic effects of a wall of ink being printed at the same time. It breaks, for example, four columns of mono data into segments, and it prints the segments in such a way that there is space left for air to flow around and out. What is involved is a simple change to what nozzles are used or active on a given pass that doesn&#39;t slow printing down like using a nozzle subset or slower carrier speed would. Light areas and non-uniform horizontal bands in mono printing are fixed without the negatives of slowing down or using a smaller subset of the nozzles. Also, this simple change fixes adverse effects that occur on printheads made of larger size and having their nozzles brought closer together or packed at greater density. These adverse effects have not been seen on prior printheads of smaller size. In view of the potential for these adverse effects to occur with increase of the printhead size, the present invention will become more advantageous as printhead size increases to fulfill market demands. 
     The foregoing description of several embodiments of the invention has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.