AU2005338846B2 - Method of modulating printhead peak power requirement using redundant nozzles - Google Patents

Description

WO 2007/065187 PCT/AU2005/001829 METHOD OF MODULATING PRINTHEAD PEAK POWER REQUIREMENT USING REDUNDANT NOZZLES 5 Field of the Invention This invention relates to a method of printing from an inkjet printhead, whilst modulating a peak power requirement for the printhead. It has been developed primarily to reduce the demands on a pagewidth printhead power supply, although other advantages of the methods of printing described herein will be apparent to the person skilled in the art. 10 Cross References to Related Applications Various methods, systems and apparatus relating to the present invention are disclosed in the following US Patents/ Patent Applications filed by the applicant or assignee of the present invention: 09/517539 6566858 09/112762 6331946 6246970 6442525 09/517384 09/505951 6374354 09/517608 6816968 10/203564 6757832 6334190 6745331 09/517541 10/203559 10/203560 10/636263 10/636283 10/866608 10/902889 10/902833 10/940653 10/942858 10/727181 10/727162 10/727163 10/727245 10/727204 10/727233 10/727280 10/727157 10/727178 10/27210 10/727257 10/727238 10/27251 10/727159 10/27180 10/27179 10/727192 10/727274 10/727164 10/727161 10/727198 10/727158 10/754536 10/754938 10/727227 10/727160 10/934720 11/212702 PEA31US 10/296522 6795215 10/296535 09/575109 6805419 6859289 09/607985 6398332 6394573 6622923 6747760 6921144 10/884881 10/943941 10/949294 11/039866 11/123011 11/123010 11/144769 11/148237 11/248435 11/248426 10/922846 10/922845 10/854521 10/854522 10/854488 10/854487 10/854503 10/854504 10/854509 10/854510 10/854496 10/854497 10/854495 10/854498 10/854511 10/854512 10/854525 10/854526 10/854516 10/854508 10/854507 10/854515 10/854506 10/854505 10/854493 10/854494 10/854489 10/854490 10/854492 10/854491 10/854528 10/854523 10/854527 10/854524 10/854520 10/854514 10/854519 10/854513 10/854499 10/854501 10/854500 10/854502 10/854518 10/854517 10/934628 11/212823 10/728804 10/728952 10/28806 10/728834 10/728790 10/728884 10/728970 10/728784 10/728783 10/728925 6962402 10/728803 10/728780 710/28779 10/773189 10/773204 56/73198 10/773199 WO 2007/065187 PCT/AU2005/001829 2 6830318 10/773201 10/773191 10/773183 10/773195 10/773196 10/773186 10/773200 10/773185 10/773192 10/773197 10/773203 10/773187 10/773202 10/773188 10/773194 10/773193 10/773184 11/008118 11/060751 11/060805 11/188017 6623101 6406129 6505916 6457809 6550895 6457812 10/296434 6428133 6746105 10/407212 10/407207 10/683064 10/683041 6750901 6476863 6788336 11/097308 11/097309 11/097335 11/097299 11/097310 11/097213 11/210687 11/097212 11/212637 11/246687 11/246718 11/246685 11/246686 11/246703 11/246691 11/246711 11/246690 11/246712 11/246717 11/246709 11/246700 11/246701 11/246702 11/246668 11/246697 11/246698 11/246699 11/246675 11/246674 11/246667 11/246684 11/246672 11/246673 11/246683 11/246682 10/60272 10/760273 10/760187 10/760182 10/760188 10/760218 10/760217 10/760216 10/760233 10/760246 10/760212 10/760243 10/760201 10/760185 10/760253 10/760255 10/760209 10/60208 10/760194 10/760238 10/760234 10/760235 10/760183 10/760189 10/760262 10/760232 10/760231 10/760200 10/60190 10/760191 10/760227 10/760207 10/760181 10/815625 10/815624 10/815628 10/913375 10/913373 10/913374 10/913372 10/913377 10/913378 10/913380 10/913379 10/913376 10/913381 10/986402 11/172816 11/172815 11/172814 11/003786 11/003354 11/003616 11/003418 11/003334 11/003600 11/003404 11/003419 11/003700 11/003601 11/003618 11/003615 11/003337 11/003698 11/003420 11/003682 11/003699 11/071473 11/003463 11/003701 11/003683 11/003614 11/003702 11/003684 11/003619 11/003617 11/246676 11/246677 11/246678 11/246679 11/246680 11/246681 11/246714 11/246713 11/246689 11/246671 10/922842 10/922848 11/246704 11/246710 11/246688 11/246716 11/246715 11/246707 11/246706 11/246705 11/246708 11/246693 11/246692 11/246696 11/246695 11/246694 10/760254 10/760210 10/760202 10/60197 10/760198 10/760249 10/760263 10/760196 10/760247 10/760223 10/760264 10/760244 10/760245 10/760222 10/760248 10/760236 10/760192 10/760203 10/760204 10/760205 10/760206 10/760267 10/760270 10/760259 10/760271 10/760275 10/760274 10/60268 10/760184 10/760195 10/760186 10/760261 10/760258 11/014764 11/014763 11/014748 11/014747 11/014761 11/014760 11/014757 11/014714 11/014713 11/014762 11/014724 11/014723 11/014756 11/014736 11/014759 11/014758 11/014725 11/014739 11/014738 11/014737 11/014726 11/014745 11/014712 11/014715 11/014751 11/014735 11/014734 11/014719 11/014750 11/014749 WO 2007/065187 PCT/AU2005/001829 3 11/014746 11/014769 11/014729 11/014743 11/014733 11/014754 11/014755 11/014765 11/014766 11/014740 11/014720 11/014753 11/014752 11/014744 11/014741 11/014768 11/014767 11/014718 11/014717 11/014716 11/014732 11/014742 11/097268 11/097185 11/097184 11/124202 11/124163 11/124157 11/124201 11/124167 11/228481 11/228477 11/228485 11/228483 11/228521 11/228517 09/575197 09/575195 09/575159 09/575132 09/575123 09/575148 09/575130 09/575165 09/575153 09/575118 09/575131 09/575116 09/575144 09/575139 09/575186 6681045 6728000 09/575145 09/575192 09/575181 09/575193 09/575156 09/575183 6789194 09/575150 6789191 6644642 6502614 6622999 6669385 6549935 09/575187 6727996 6591884 6439706 6760119 09/575198 6290349 6428155 6785016 09/575174 09/575163 6737591 09/575154 09/575129 09/575124 09/575188 09/575189 09/575162 09/575172 09/575170 09/575171 09/575161 An application has been listed by its docket number. This will be replaced when the application number is known. The disclosures of these applications and patents are incorporated herein by reference. 5 Background to the Invention Inkjet printers are now commonplace in homes and offices. For example, inkjet photographic printers, which print color images generated on digital cameras, are, to an increasing extent, replacing traditional development of photographic negatives. With the increasing use of inkjet printers, the 10 demands of such printers in terms of print quality and speed, continue to increase. All commercially available inkjet printers use a scanning printhead, which traverses across a stationary print medium. After each sweep of the printhead, the print medium incrementally advances ready for the next line(s) of printing. Such printers are inherently slow and are becoming unable to meet the needs of current demands of inkjet printers. 15 The present Applicant has previously described many different types of pagewidth printheads, which are fabricated using MEMS technology. In pagewidth printing, the print medium is continuously fed past a stationary printhead, thereby allowing high-speed printing at, for example, one page per 1-2 seconds. Moreover, MEMS fabrication of the printhead allows a much higher nozzle density than traditional scanning printheads, and print resolutions of 1600 dpi are possible. 20 Some of the Applicant's MEMS pagewidth printheads are described in the patents and patent applications listed in the cross-references section above, the contents of which are herein incorporated by reference.

WO 2007/065187 PCT/AU2005/001829 4 To a large extent, pagewidth printing has been made possible by reducing the total energy required to fire each ink droplet and/or efficiently removing heat from the printhead via ejected ink. In these ways, self-cooling of the printhead can be achieved, which enables a pagewidth printhead having 5 a high nozzle density to operate without overheating. However, whilst a total amount of energy to print, say, a full-color photographic page will be approximately constant for any given pagewidth printhead, the power requirement of the printhead may, of course, vary. An average power requirement for printing a page is determined by the total energy required and the total time taken to print the page, assuming an equal distribution of printing 10 over the time period. In addition, the power requirement of the printhead during printing of the page may fluctuate. Due to a particular configuration of the printhead or printer controller, some lines of print may consume more power than other lines of print. Hence, a peak power requirement for each line of printing may be different. In a typical pagewidth printhead, nozzles ejecting the same color of ink are arranged 15 longitudinally in color channels along the length of the printhead. Each color channel may comprise one or more rows of nozzles, all ejecting the same colored ink. In a simple example, there may be one cyan row of nozzles, one magenta row of nozzles and one yellow row of nozzles. Usually, each row of nozzles will be fired sequentially during printing e.g. cyan then magenta then yellow. Furthermore, a typical pagewidth printhead may be comprised of a plurality of printhead 20 modules, which abut each other and cooperate to form a printhead extending across a width of the page to be printed. Each printhead module is typically a printhead integrated circuit comprising nozzles and drive circuitry for firing the nozzles. The rows of nozzles extend over the plurality of printhead modules, with each printhead module including a respective segment of each nozzle row. In previous patent applications, listed below, we described various types of printheads, printer 25 controllers and methods of printing. The contents of these patent applications are herein incorporated by reference: 10/854521 10/854522 10/854488 10/854487 10/854503 10/854504 10/854509 10/854510 10/854496 10/854497 10/854495 10/854498 10/854511 10/854512 10/854525 10/854526 10/854516 10/854508 10/854507 10/854515 10/854506 10/854505 10/854493 10/854494 10/854489 10/854490 10/854492 10/854491 10/854528 10/854523 10/854527 10/854524 10/854520 10/854514 10/854519 10/854513 10/854499 10/854501 10/854500 10/854502 10/854518 10/854517 10/934628 11/212823 WO 2007/065187 PCT/AU2005/001829 5 In our previous patent applications USSN 10/854498 (Docket No. PLT012US), filed May 27, 2004, USSN 10/854516 (Docket No. PLT017US), filed May 27, 2004 and USSN 10/854508 (Docket No. PLT018US), filed May 27, 2004, we described a method of printing a line of dots where not all 5 nozzles in one row or one segment are fired simultaneously. Rather, the nozzles are fired sequentially in firing groups in order to minimize the peak power requirement during printing of one line. As a consequence, each line of printing is typically not a perfectly straight line (unless the physical arrangements of the nozzles directly compensates for the firing order in which case it can be a straight line), although this imperfection is undetectable to the human eye. Each segment on a printhead 10 module may comprise, for example, 10 firing groups of nozzles, in order to minimize, as far as possible within the print speed requirements, the peak power requirement for firing that segment of the nozzle row. In our previous patent applications USSN 10/854512 (Docket No. PLTO14US), filed May 27, 2004 and USSN 10/854491 (Docket No. PLT028US), filed May 27, 2004, we described a means for 15 joining abutting printhead modules such that the effective distance between adjacent nozzles ('nozzle pitch') in the row remains constant. At one end of each printhead module, there is a displaced nozzle row portion, which is not aligned with its corresponding nozzle row. The firing of these displaced nozzles is timed so that they effectively print onto the same line as the row to which they correspond. As such, all references to "rows", "rows of nozzles" or "nozzle rows" herein include nozzle rows 20 comprising one or more displaced row portions, as described in USSN 10/854512 (Docket No. PLT014US), filed May 27, 2004 and USSN 10/854491 (Docket No. PLT028US), filed May 27, 2004. In our previous patent applications USSN 10/854507 (Docket No. PLT019US), filed May 27, 2004 and USSN 10/854523 (Docket No. PLT030US), filed May 27, 2004, we described a means by which the visual effect of defective nozzles is reduced. The printhead described comprises one or more 25 'redundant' color channels, so that for a first row of nozzles ejecting a given color, there is a corresponding second ('redundant') row of nozzles from a different color channel which eject the same color. As described in USSN 10/854507 (Docket No. PLT019US), filed May 27,2004 and USSN 10/854523 (Docket No. PLT030US), filed May 27, 2004, one line may be printed by the first nozzle row and the next line is printed by the second nozzle row so that the first and second nozzle rows print 30 alternate lines on the page. Thus, if there are unknown defective nozzles in a given row, the visual effect on the page is halved, because only every other line is printed using that row of nozzles. Alternatively, if there are known dead nozzles in a given row, the corresponding row of nozzles may be used to print dots in those positions where there is a known dead nozzle. In other words, only a small number of nozzles in the 'redundant' row may be used to print.

WO 2007/065187 PCT/AU2005/001829 6 As already mentioned, the redundancy scheme described in USSN 10/854507 (Docket No. PLT019US), filed May 27, 2004 and USSN 10/854523 (Docket No. PLT030US), filed May 27, 2004 has the advantage of reducing the visual impact of dead nozzles, either known or unknown. Moreover, careful choice of redundant colors may be used to further reduce the visual impact of dead nozzles. For 5 example, since yellow makes the lowest contribution (11%) to luminance, the human eye is least sensitive to missing yellow dots and, therefore, yellow would be a poor choice for a redundant color. On the other hand, black, makes a much higher contribution to luminance and would be a good choice for a redundant color. However, while the redundancy scheme described in USSN 10/854507 (Docket No. 10 PLT019US), filed May 27, 2004 and USSN 10/854523 (Docket No. PLT030US), filed May 27, 2004 can compensate for dead nozzles and reduce (e.g. halve) the number of dots fired by some nozzles, it places increased demands on the power supply which is used to power the printhead. The reason is because in the time it takes for the print medium to advance by one line (one 'line-time'), each nozzle row must be allotted a portion of the line-time in which to fire, in order to achieve dot-on-dot printing 15 and provide the desired image. Each nozzle row is allotted a portion of the line-time, since not all nozzle rows can fire simultaneously. (If all nozzle rows were to fire simultaneously, there would be an unacceptable current overload of the printhead). In a simple CMY pagewidth printhead, having three rows of nozzles and no redundant color channels, each nozzle row must fire in one-third of the line-time. If the average power requirement of 20 the printhead is x, then the peak power requirement over the duration of the line-time is as shown in Table 1: Line-time Color Peak Power Channel Requirement 0 C x 0.33 M x 0.67 Y x 0 (new line) C x ... etc. Table 1 In this simple CMY printhead with no redundant nozzles, power is distributed evenly over the 25 duration of the line-time so that the peak power requirement is constant and equal to the average power requirement of the printhead. From the standpoint of the power supply, this situation is optimal, but, on the other hand, there is no means for minimizing the visual effects of dead nozzles.

WO 2007/065187 PCT/AU2005/001829 7 In a CMY printhead having redundant cyan and magenta color channels (i.e. Cl, C2, Ml, M2 and Y color channels) and a pair of nozzle rows in each color channel (for even and odd dots), each nozzle row is allotted one-tenth of the line-time, since there are now ten nozzle rows. Now if the average power requirement of the printhead is x, with the redundancy scheme and firing sequence 5 described in USSN 10/854507 (Docket No. PLTO19US), filed May 27, 2004 and USSN. 10/854523 (Docket No. PLT030US), filed May 27, 2004, the peak power requirement over the duration of two line-times is as shown in Table 2: Line-time Color Peak Power Channel Requirement 0 C1 (even) 1.67x 0.1 C2 (even) 0 0.2 M1 1.67x (even) 0.3 M2 0 (even) 0.4 Y (even) 1.67x 0.5 C1 (odd) 1.67x 0.6 C2 (odd) 0 0.7 M1 (odd) 1.67x 0.8 M2 (odd) 0 0.9 Y (odd) 1.67x 0 (new line) C1 (even) 0 0.1 C2 (even) 1.67x 0.2 M1 0 (even) 0.3 M2 1.67x (even) 0.4 Y (even) 1.67x 0.5 C1 (odd) 0 0.6 C2 (odd) 1.67x 0.7 M1 (odd) 0 0.8 M2 (odd) 1.67x WO 2007/065187 PCT/AU2005/001829 8 0.9 Y (odd) 1.67x 0 (new line) Cl (even) 1.67x ... etc Table 2 It is evident from the above table that the peak power requirement of the printhead fluctuates severely between 1.67x and 0 within the period of a line-time, even though the average power consumed over the whole line-time is still x. In practical terms, it is difficult to manufacture a power 5 supply which is able to deliver severely fluctuating amounts of power within each line-time. Hence, the redundancy described in USSN 10/854507 (Docket No. PLTO19US), filed May 27, 2004 and USSN 10/854523 (Docket No. PLT030US), filed May 27, 2 004is difficult to implement in practice, even though it offers considerable advantages in terms of reducing the visual effects of known dead nozzles. Of course, a printhead could be configured not to fire redundant color channels in a given line 10 time, resulting in an average of x peak power for each nozzle row. Such a configuration is effectively the same as that described in Table 1. While this configuration would address peak power and misdirectionality issues, it would not address the problem of known dead nozzles, since only one of each redundant color channel would be able to be fired in a given line-time, thereby losing one of the major advantages of redundancy. 15 It would be desirable to provide a method of printing whereby fluctuations in a peak power requirement are minimized. It would be further desirable to provide a method of printing whereby the average power requirement of the printhead is substantially equal to the peak power requirement at any given time during printing. It would be further desirable to provide a method of printing, whereby, in addition minimizing fluctuating peak power requirements, the visual effects of dead or malfunctioning 20 nozzles are reduced. It would be further desirable to provide a method of printing, whereby, in addition to minimizing fluctuating peak power requirements, the visual effects of misdirected ink droplets is reduced. Summary of the Invention 25 In a first aspect, there is provided a method of modulating a peak power requirement of an inkjet printhead, said printhead comprising a plurality of first nozzles and a plurality of second nozzles supplied with a same colored ink, said first nozzles and second nozzles being configured in a plurality of sets, wherein each set of nozzles comprises one first nozzle and one corresponding second nozzle, each nozzle in a set being configurable to print a dot of said ink onto a substantially same position on a 30 print medium, said method comprising: (a) selecting a firing nozzle from at least one set of nozzles, said selection being on the basis of modulating said peak power requirement; and WO 2007/065187 PCT/AU2005/001829 9 (b) printing dots onto said print medium using said firing nozzle. In a second aspect, there is provided a method of printing a line of dots from an inkjet printhead, said printhead comprising a plurality of first nozzles and a plurality of second nozzles supplied with a same colored ink, said first nozzles and second nozzles being configured in a plurality 5 of sets, wherein each set of nozzles comprises one first nozzle and one corresponding second nozzle, each nozzle in a set being configurable to print a dot of said ink onto a substantially same position on a print medium, said method comprising printing a line of dots across said print medium such that said first nozzles and said second nozzles each contribute dots to said line. 10 In a third aspect, there is provided a method of modulating a peak power requirement of an inkjet printhead, said printhead comprising a plurality of transversely aligned color channels, each color channel comprising at least one nozzle row extending longitudinally along said printhead, each nozzle in a color channel ejecting the same colored ink, wherein said printhead is comprised of a plurality of printhead modules, each printhead module comprising a respective segment of each nozzle 15 row, said method comprising each of said printhead modules firing a respective segment within a predetermined segment-time, wherein at least one of said fired segments is contained in a different color channel from at least one other of said fired segments. In a fourth aspect, there is provided an inkjet printhead comprising a plurality of transversely 20 aligned color channels, each color channel comprising at least one nozzle row extending longitudinally along said printhead, each nozzle in a row ejecting the same colored ink, wherein said printhead is comprised of a plurality of printhead modules, and the number of color channels is equal to the number of printhead modules. In a fifth aspect, there is provided a printer controller for supplying dot data to an inkjet 25 printhead, said printhead comprising a plurality of first nozzles and a plurality of second nozzles supplied with a same colored ink, said first nozzles and second nozzles being configured in a plurality of sets, wherein each set of nozzles comprises one first nozzle and one corresponding second nozzle, each nozzle in a set being configurable by said printer controller to print a dot of said ink onto a substantially same position on a print medium, said printer controller being programmed to supply dot 30 data such that said first nozzles and said second nozzles each contribute dots to a line of printing. In a sixth aspect, there is provided a printer controller for supplying dot data to a printhead, said printhead comprising a plurality of transversely aligned color channels, each color channel comprising at least one nozzle row extending longitudinally along said printhead, each nozzle in a color channel ejecting the same colored ink, wherein said printhead is comprised of a plurality of WO 2007/065187 PCT/AU2005/001829 10 printhead modules, each printhead module comprising a respective segment of each nozzle row, said printer controller being programmed to supply dot data such that each of said printhead modules fires a respective segment within a predetermined segment-time, wherein at least one of said fired segments is contained in a different color channel from at least one other of said fired segments. 5 In a seventh aspect of the invention, there is provided a printhead system comprising an inkjet printhead and a printer controller for supplying dot data to said printhead, said printhead comprising a plurality of first nozzles and a plurality of second nozzles supplied with a same colored ink, said first nozzles and second nozzles being configured in a plurality of sets, wherein each set of nozzles comprises one first nozzle and one corresponding second nozzle, each 10 nozzle in a set being configurable by said printer controller to print a dot of said ink onto a substantially same position on a print medium, said printer controller being programmed to supply dot data such that said first nozzles and said second nozzles each contribute dots to a line of printing. In an eighth aspect of the invention, there is provided a printhead system comprising an inkjet 15 printhead and a printer controller for supplying dot data to said printhead, said printhead comprising a plurality of transversely aligned color channels, each color channel comprising at least one nozzle row extending longitudinally along said printhead, each nozzle in a color channel ejecting the same colored ink, wherein said printhead is comprised of a plurality of printhead modules, each printhead module comprising a respective segment of each nozzle row, 20 said printer controller being programmed to supply dot data such that each of said printhead modules fires a respective segment within a predetermined segment-time, wherein at least one of said fired segments is contained in a different color channel from at least one other of said fired segments. All aspects of the invention provide the advantage of modulating a peak power requirement of 25 the inkjet printhead. The corollary is that a power supply, which supplies power to the printhead, need not be specially adapted to supply severely fluctuating amounts of power throughout each print cycle. In the present invention, the degree of peak power fluctuations within each line-time are substantially reduced. Hence, the design and manufacture of the printhead power supply may be simplified and the power supply is made more robust by virtue of not having to deliver severely fluctuating amounts of 30 power to the printhead. In addition to modulating the peak power requirement of the printhead, the present invention allows print quality to be improved by using redundant nozzle rows, and without compromising the above-mentioned improvements in peak power requirement. Print quality may be improved by, for WO 2007/065187 PCT/AU2005/001829 11 example, reducing the visual effects of unknown dead nozzles in the printhead, and reducing the visual effects of misdirected ink droplets. As used herein, the terms "row", "rows of nozzles", "nozzle row" etc. may include nozzle rows 5 comprising one or more displaced row portions. As used herein, the term "ink" includes any type of ejectable fluid, including, for example, IR inks and fixatives, as well as standard CMYK inks. Likewise, references to "same colored ink" include inks of a same color or type e.g. same cyan ink, same IR ink or same fixative. As used herein, the term "substantially the same position on a print medium" is used to mean 10 that a droplet of ink has an intended trajectory to print at a same position on the print medium (as another droplet of ink). However, due to inherent error margins in firing droplets of ink, random misdirects or persistent misdirects, a droplet of ink may not be printed exactly on its intended position on the print medium. Hence, the term "substantially the same position on a print medium" includes misplaced droplets, which are intended to print at the same position, but may not necessarily print at 15 that position. In accordance with some forms of the invention, -the first nozzles and second nozzles are configured in a plurality of sets, wherein each set of nozzles comprises one first nozzle and one corresponding second nozzle. Further, each nozzle in a set is configurable to print a dot of ink onto a 20 substantially same position on a print medium, so that the nozzles can be used interchangeably. Optionally, a set is a pair of nozzles consisting of one first nozzle and one second nozzle. However, a set may alternatively comprise further (e.g. third and fourth) nozzles, with each nozzle in the set being configurable to print a dot of ink onto a substantially same position on a print medium. In other words, the present invention is not limited to two rows of redundant nozzles and may include, for 25 example, three or more rows of redundant nozzles. Preferably, the printhead is a stationary pagewidth printhead and the print medium is fed transversely past the printhead. The present invention has been developed primarily for use with such pagewidth printheads. Optionally, the printhead comprises a plurality of transversely aligned color channels, each 30 color channel comprising at least one nozzle row extending longitudinally along the printhead, each nozzle in a color channel ejecting the same colored ink. As described in more detail below, each transversely aligned color channel is allotted a portion of a line-time for firing. In this way, dot-on-dot printing can be achieved, which is optimal for dithering.

WO 2007/065187 PCT/AU2005/001829 12 Color channels in the printhead may eject the same or different colored inks. However, all nozzles in the same color channel are typically supplied with and eject the same colored ink. Color channels ejecting the same colored ink are sometimes termed 'redundant' color channels. Typically, the printhead comprises at least one redundant color channel so that at least one color channel ejects 5 the same colored ink as at least one other color channel. Each color channel may comprise a plurality of nozzle rows. Optionally, each color channel comprises a pair of nozzle rows. Typically, nozzle rows in the same color channel are transversely offset from each other. For example, one nozzle row in a pair may be configured to print even dots on a line, while the other nozzle row in the pair may be configured to print odd dots on the same line. The 10 nozzle rows in a pair are usually spaced apart in a transverse direction to allow convenient timing of nozzle firings. For example, the even and odd nozzle rows in one color channel may be spaced apart by two lines of printing. Optionally, each set of nozzles comprises one first nozzle from a first color channel and one second nozzle from a second color channel. The first and second nozzles in the set are aligned 15 transversely so that each can print onto the substantially same position on a print medium. Optionally, one set of nozzles prints a column of same-colored dots down a print medium, with each nozzle in the set contributing dots to the column. As used herein, a "column" refers to a line of dots printed substantially perpendicular to the printhead and substantially parallel with a feed direction of the print medium. Optionally, one first nozzle in the set prints about half of the column and one 20 second nozzle in the set prints about half of the column, so that the first and second nozzles in the set share printing of the column equally between them. Optionally, a visual effect of misdirected ink droplets is reduced. An advantage of using a plurality (e.g. two) nozzles for printing the same column is that misdirected ink droplets may be averaged out between those nozzles. 25 Optionally, when printing a line of same-colored dots across the print medium, the first nozzles and second nozzles contribute dots to the line. As used herein, a "line" refers to a line of dots printed substantially parallel with the printhead and substantially perpendicular to a feed direction of the print medium. Optionally, the first nozzles print about half of the line and the second nozzles print about half of the line, so that the first and second nozzles share printing of the line equally between them. 30 Accordingly, the peak power requirement for printing the line is reduced by about 50%, as compared to printing the line using only first nozzles or only second nozzles. Optionally, alternate first nozzles in a first nozzle row are used to print about half of the line and alternate second nozzles in a second nozzle row are used to print about half of the line. However, other patterns for sharing printing between the first and second nozzles may also be used.

WO 2007/065187 PCT/AU2005/001829 13 Optionally, a visual effect of malfunctioning or dead nozzles is reduced. The nozzles may be known dead nozzles or unknown dead nozzles. The visual effect of an unknown dead nozzle is reduced by virtue of the fact that the nozzle is only required to print about half of the time. For example, with an unknown dead magenta nozzle, a column of magenta dots would be missing completely with no 5 redundancy, whereas half of the column is still printed using redundancy. The latter is, of course, far more visually acceptable than the former. Optionally, the color (which is the same color printed by the first and second nozzles) is magenta, cyan or black. The human eye is most sensitive to magenta, cyan and black, and these colors are consequently the preferred candidates for redundancy. A printhead may contain more than one 10 redundant color channels. For example, the printhead may comprise first and second magenta nozzles, and first and second cyan nozzles. In accordance with some forms of the invention, there is provided a method of out-of-phase printing so as to modulate a peak power requirement of the printhead. Typically, the printhead comprises a plurality of transversely aligned color channels with each color channel comprising at least 15 one nozzle row extending longitiudinally along the printhead. Each nozzle in a color channel is supplied with and ejects the same colored ink. Typically, the printhead is comprised of a plurality of printhead modules, with each module comprising a respect segment of each nozzle row. Out-of-phase printing is provided by a method in which each of the printhead modules fires a respective segment within a predetermined segment-time, wherein at least one of the fired segments is contained in a 20 different color channel from at least one other of the fired segments. A segment-time may be defined as a predetermined fraction of one line-time. A line-time is defined as the time taken for the print medium to advance past the printhead by one line. Typically, all segments in a nozzle row are fired within one line-time. Optionally, a segment-time is equal to one line-time divided by the number of nozzle rows. However, a period of each line-time may be dedicated 25 to a line-based overhead, in which case the segment-time will be less than one line-time divided by the number of nozzle rows. Generally, all segment-times are equal. Optionally, at least one nozzle row has a different peak power requirement from other nozzle rows. For example, a redundant nozzle row would normally have half the peak power requirement of a non-redundant nozzle row. Optionally, a predetermined firing sequence modulates the peak power 30 requirement during each segment-time so that the peak power requirement is within about 10%, optionally within 5%, of the average power requirement of the printhead. In some embodiments of the invention, the peak power requirement of the printhead is equal to the average power requirement of the printhead. Typically, all segments on the printhead are fired within one-line time.

WO 2007/065187 PCT/AU2005/001829 14 In some forms of the invention, the number of color channels is equal to the number of printhead modules. This is the optimum number of color channels and modules to achieve perfect out of-phase firing. However, as will be explained in more detail below, the advantages of out-of-phase firing may still be achieved using any number of printhead modules and color channels. 5 Optionally, with equal numbers of modules and color channels, each of the printhead modules fires a segment from a different color channel within the predetermined segment-time. Further, each segment in a nozzle row may be fired sequentially. However, as will be explained in more detail below, each segment in a nozzle row need not be fired sequentially, whilst still enjoying the advantages of out-of-phase firing. 10 Brief Description of the Drawings Specific forms of the present invention will be now be described in detail, with reference to the following drawings, in which: Figure 1 is a plan view of a pagewidth printhead according to the invention; 15 Figure 2 is a plan view of a printhead module, which is a part of the printhead shown in Figure 1; Figure 3 is a schematic representation of a portion of each color channel of the printhead shown in Figure 1; Figure 4A shows which even nozzles fire in one line-time using dot-at-a-time redundancy 20 according to the invention; Figure 4B shows which odd nozzles fire in the next line-time from Figure 4A; and Figure 5 shows a printhead system according to the invention. Detailed Description of the Invention 25 The invention will be described with reference to a CMY pagewidth inkjet printhead 1, as shown in Figure 1. The printhead 1 has five color channels 2, 3, 4, 5 and 6, which are C1, C2, Ml, M2 and Y respectively. In other words cyan and magenta have 'redundant' color channels. The reason for making C and M redundant is that Y only contributes 11% of luminance, while C contributes 30% and M contributes 59%. Since the human eye is least sensitive to yellow, it is more visually acceptable to 30 have missing yellow dots than missing cyan or magenta dots. In this printhead, black (K) printing is achieved via process-black (CMY). The printhead 1 is comprised of five abutting printhead modules 7, which are referred to from left to right as A, B, C, D and E. The five modules 7 cooperate to form the printhead 1, which extends across the width of a page (not shown) to be printed. In this example, each module 7 has a length of WO 2007/065187 PCT/AU2005/001829 15 about 20 mm so that the five abutting modules form a 4" printhead, suitable for pagewidth 4" x 6" color photo printing. During printing, paper is fed transversely past the printhead 1 and Figure 1 shows this paper direction. Each of the five color channels on the printhead 1 comprises a pair of nozzle rows. For 5 example, the Cl color channel 2 comprises nozzle rows 2a and 2b. These nozzle rows 2a.and 2b extend longitudinally along the whole length of the printhead 1. Where abutting printhead modules 7 are joined, there is a displaced (or dropped) triangle 8 of nozzle rows. These dropped triangles 8 allow printhead modules 7 to be joined, whilst effectively maintaining a constant nozzle pitch along each row. A timing device (not shown) is used to delay firing nozzles in the dropped triangles 8, as 10 appropriate. A more detailed explanation of the operation of the dropped triangle 8 is provided in the Applicant's patent applications USSN 10/854512 (Docket No. PLT014US), filed May 27, 2004 and USSN 10/854491 (Docket No. PLT028US), filed May 27, 2004. Each of the printhead modules 7 contains a segment from each of the nozzle rows. For example, printhead module A contains segments 2aA, A, 3aA, 3bA, 4aA etc. Segments from the same 15 nozzle row cooperate to form a complete nozzle row. For example, segments 2aA, 2a, 2ac, 2a D and 2aE cooperate to form nozzle row 2a. Figure 2 shows the printhead module A with its respect segments from each nozzle row. Referring to Figure 3, there is shown a detailed schematic view of a portion of the five color channels 2, 3, 4, 5 and 6. From Figure 3, it can be seen that the pair of nozzle rows (e.g. 2a and 2b) in 20 each color channel (e.g. 2) are transversely offset from each other. In color channel 2, for example, nozzle row 2a prints even dots in a line, while nozzle row 2b prints interstitial odd dots in a line. Furthermore, the even rows of nozzles 2a, 3a, 4a, 5a and 6a are transversely aligned, as are the odd rows of nozzles 2b, 3b, 4b, 5b and 6b. This transverse alignment of the five color channels allows dot-on-dot printing, which is optimal in terms of dithering. Within a period of one line-time, all even 25 nozzles and all odd nozzles must be fired so that dot-on-dot printing is achieved. The even and odd nozzles (e.g. 2a and 2b) in the same color channel (e.g. 2) may be separated by, for example, two lines. Adjacent color channels (e.g. 2 and 3) may be separated by, for example, ten lines. However, it will be appreciated that the exact spacing between even/odd nozzle rows and adjacent color channels may be varied, whilst still achieving dot-on-dot printing. 30 Dot-At-A-Time Redundancy In the printhead 1 described above, there are two cyan (Cl, C2) and two magenta (Ml, M2) color channels. In the Applicant's terminology, the C1/C2 and M1/M2 color channels are described as 'redundant' color channels.

WO 2007/065187 PCT/AU2005/001829 16 As explained above, with five color channels and a pair of nozzle rows in each color channel, each nozzle row must print in one-tenth of the line-time in order to achieve all the advantages of redundancy and compensate for any known dead nozzles using a redundant color channel. The inherent power supply problems in relation to the redundancy scheme described in USSN 10/854507 (Docket 5 No. PLT019US), filed May 27, 2004 and USSN 10/854523 (Docket No. PLT030US), filed May 27, 2004have also been described above. Dot-at-a-time redundancy is where redundant rows of nozzles are used such that there is never more than one out of every two adjacent nozzles firing within a single nozzle row. In other words, the even dots for a color are produced by two nozzle rows (each printing half of the even dots), and the odd 10 dots for a color are produced by two nozzle rows (each printing half of the dots). For example, nozzle rows 2a and 3a may both contribute even dots to a line of printing, and nozzle rows 2b and 3b may both contribute odd dots to a line of printing. Figures 4A and 4B show a firing sequence for two lines of printing using dot-at-a-time redundancy. The nozzles indicated in Figures 4A and 4B are not fired simultaneously; each nozzle row 15 is allotted one-tenth of the line-time in which to fire its nozzles, with even nozzles rows firing sequentially followed by odd nozzle rows firing sequentially. Referring to Figure 4A, in the first line-time alternate nozzles are fired in each nozzle row from the Cl, C2, M1 and M2 color channels. Nozzles fired from C2 and M2 complement those fired from C1 and Ml. For example, alternate even nozzles are fired from nozzle row 2a and complementary 20 alternate even nozzles are fired from nozzle row 3a. Nozzle rows 6a and 6b in the Y channel have no redundancy and each of these nozzle rows must therefore fire all its nozzles in one-tenth of the line time. Referring to Figure 4B, in the second line-time the alternate nozzles fired in the first line-time are inversed. 25 By using this dot-at-a-time redundancy scheme, print quality is improved by reducing misdirection artifacts (thereby maximizing dot-on-dot placement) and reducing the visual effect of unknown dead nozzles. For example, if half of the dots in a column are from an operational nozzle and half are from a dead nozzle, the visual effect of the dead nozzle will be reduced and the effective print quality is greater than if the entire column came from the dead nozzle. In other words, the present 30 invention achieves at least as good print quality as the line-at-a-time redundancy described in USSN 10/854507 (Docket No. PLT019US), filed May 27, 2004 and USSN 10/854523 (Docket No. PLT030US), filed May 27, 2004. Moreover, the peak power requirements of the printhead are modulated during printing of each line, so that the peak power requirements do not fluctuate as severely as in Table 2. Table 3 shows how WO 2007/065187 PCT/AU2005/001829 17 the peak power requirement of the printhead (having an average power requirement of x) varies over two lines of printing using dot-at-a-time redundancy according to the present invention: Line-time Color Nozzle Peak Power Channel Row Requirement 0 2 (C1) 2a (even) 0.83x 0.1 3 (C2) 3a (even) 0.83x 0.2 4 (M1) 4a (even) 0.83x 0.3 5 (M2) 5a (even) 0.83x 0.4 6 (Y) 6a (even) 1.67x 0.5 2 (C1) 2b (odd) 0.83x 0.6 3 (C2) 3b (odd) 0.83x 0.7 4 (Ml) 4b (odd) 0.83x 0.8 5 (M2) 5b (odd) 0.83x 0.9 6 (Y) 6b (odd) 1.67x 0 (new line) 2 (Cl) 2a (even) 0.83x 0.1 3 (C2) 3a (even) 0.83x 0.2 4 (M1) 4a (even) 0.83x 0.3 5 (M2) 5a (even) 0.83x 0.4 6 (Y) 6a (even) 1.67x 0.5 2 (C1) 2b (odd) 0.83x 0.6 3 (C2) 3b (odd) 0.83x 0.7 4 (Ml) 4b (odd) 0.83x 0.8 5 (M2) 5b (odd) 0.83x 0.9 6 (Y) 6b (odd) 1.67x 0 (new line) 2 (Cl) 2a (even) 0.83x ... etc Table 3 5 It is evident from Table 3 that the fluctuations in peak power requirement are fewer and less severe compared to line-at-a-time redundancy, described in Table 2. In terms of the design of the printhead power supply, dot-at-a-time redundancy according to the present invention offers significant advantages over line-at-a-time redundancy, whilst maintaining the same improvements in print quality. 10 Out-of-Phase Firing WO 2007/065187 PCT/AU2005/001829 18 In all the firing sequences described so far, each color channel is fired in-phase - that is, a whole row of, say, even nozzles from one color channel is fired within its allotted portion of the line time. In-phase firing provides simpler programming of the printer controller, which controls the firing sequence via dot data sent to the printhead 1. 5 However, according to another form of the present invention, the firing may be out-of-phase that is, within the same allotted portion of the line-time (termed the 'segment-time'), at least one segment of nozzles is fired from a color channel that is different from at least one other segment of nozzles. With appropriate sequencing of segment firings, a whole nozzle row can be fired within one line-time, such that the net result is effectively the same as in-phase firing. 10 In the case of the printhead 1, having five color channels and five segments in each nozzle row, it possible to fire segments from all different color channels within one segment time (i.e. one-tenth of a line-time). Segments contained in the same nozzle row are, therefore, fired sequentially during one line-time. A major advantage of out-of-phase firing is that if one or more color channels (e.g. Y) has a 15 different peak power requirement to the other color channels, this difference is averaged into the power requirements of the other color channels within each segment-time. Hence, the spike in power (corresponding to the Y channel) in Table 3 is effectively merged into rest of the line-time. The result is that the peak power requirement during each segment-time is always equal to the average power requirement for the printhead. This situation is optimal for supplying power to the printhead. 20 Table 4 illustrates a sequence of out-of-phase firing for one line of printing from the printhead 1, using dot-at-a-time redundancy. Line- Module A Module B Module C Module D Module E Peak Power time (CC, S, P) (CC, S, P) (CC, S, P) (CC, S, P) (CC, S, P) Requirement 0 C2a, C2, 3a',- M1,4ac, M2, 5 a , 6a, x 0.83x 0.83x 0.83x 0.83x 1.67x 0.1 C, 3a, M14a, M S Y6C1,a 2a, x 0.83x 0.83x 0.83x 1.67x 0.83x 0.2 a1,4a^, M92, 5a7, Y, 6ac, C1,2a, C2,3a, x 0.83x 0.83x 1.67x 0.83x 0.83x 0.3 M2, Y5aA C , 1 4a7 , x 0.83x 1.67x 0.83x 0.83x 0.83x 0.4 Y,6a^, C1, 2a, C , M, 5a', x 1.67x 0.83x 0.83x 0.83x 0.83x WO 2007/065187 PCT/AU2005/001829 19 0.5 C1, 2bA C2,3b, Wh7, 4i M 5b Y, W, x 0.83x 0.83x 0.83x 0.83x 1.67x 0.6 C2, 3bA, Z2, 6 0.83x 0.83x 0.83x 1.67x 0.83x 0.7 M1 i 74bA Th_, C X 0.83x 0.83x 1.67x 0.83x 0.83x 0.8 M2, 5b^,~7~77 Ki7~ 0.83x 1.67x 0.83x 0.83x 0.83x 0.9 Y, 6, x 1.67x 0.83x 0.83x 0.83x 0.83x 0 (new C'U1, ,2a Aii7a7 Y75 V x line) 0.83x 0.83x 0.83x 0.83x 1.67x ... etc CC = Color Channel; S = Segment; P = Peak Power Requirement Table 4 It should be remembered that, even within one segment, not all nozzles fire simultaneously. The nozzles in one segment are arranged in firing groups, which fire sequentially over the course of 5 their allotted segment-time. However, the important point is that at any given instant, some Cl, C2, Ml, M2 and Y nozzles will fire simultaneously, thereby averaging out the higher peak power requirement of the yellow nozzle row. In the case of five printhead modules and five color channels, it can be seen that out-of-phase firing works out well. Segments from each color channel can be rotated so that all different segments 10 are fired in one segment-time. However, it will be appreciated that out-of-phase firing also works well with any number of printhead modules or color channels. For example, using 20 mm printhead modules 7, an A4 pagewidth printhead is comprised of eleven abutting modules [(i) to (xi)]. With five color channels and eleven printhead modules, it is impossible to ensure that each printhead module fires a different color 15 channel within a segment-time (i.e. one-tenth of a line-time). Regardless, out-of-phase firing can still be used to optimize the peak power requirement of the printhead. For example, the A4 pagewidth printhead may have C, M, Y, K1 and K2 color channels. Since there are redundant K channels, these nozzle rows will have a lower peak power requirement than the C, M and Y channels using dot-at-a-time redundancy. Using in-phase firing, there would be 20 appreciable peak power fluctuations during each line-time (C = 1 .25x, M = 1.

2 5x, Y 1.25x, K1 0.625x, K2 = 0.625x).

WO 2007/065187 PCT/AU2005/001829 20 However, it can be seen from Table 5 that out-of-phase firing accommodates the eleven printhead modules and provides a peak power requirement that is always within 10% of the average power requirement x of the printhead. Indeed, the peak power requirement is always within 5% of the average power requirement x in this example. For the purposes of providing a power supply for the 5 printhead, such small variations in peak power requirement during each line-time are not significant and would not affect the design of the power supply. t (i) (ii) (iii) (iv) (v) (vi) (vii) (vii) (ix) (x) (xi) P 0 C(e) M(e) Y(e) Kl(e) K2(e) C(e) M(e) Y(e) Kl(e) K2(e) C(e) 1 .023x 0.1 M(e) Y(e) K1(e) K2(e) C(e) M(e) Y(e) Kl(e) K2(e) C(e) M(e) 1.023x 0.2 Y(e) K1(e) K2(e) C(e) M(e) Y(e) K1(e) K2(e) C(e) M(e) Y(e) 1.023x 0.3 Kl(e) K2(e) C(e) M(e) Y(e) K1(e) K2(e) C(e) M(e) Y(e) Kl(e) 0.966x 0.4 K2(e) C(e) M(e) Y(e) Kl(e) K2(e) C(e) M(e) Y(e) Kl(e) K2(e) 0.966x 0.5 C(o) M(o) Y(o) Kl(o) K2(o) C(o) M(o) Y(o) K1(o) K2(o) C(o) 1.023x 0.6 M(o) Y(o) Kl(o) K2(o) C(o) M(o) Y(o) Kl(o) K2(o) C(o) M(o) 1.023x 0.7 Y(o) Kl(o) K2(o) C(o) M(o) Y(o) Kl(o) K2(o) C(o) M(o) Y(o) 1.023x 0.8 K1(o) K2(o) C(o) M(o) Y(o) K1(o) K2(o) C(o) M(o) Y(o) Kl(o) 0.966x 0.9 K2(o) C(o) M(o) Y(o) Kl(o) K2(o) C(o) M(o) Y(o) K1(o) :K2(o) 0.966x 0 C(o) M(o) Y(o) Kl(o) K2(o) C(o) M(o) Y(o) K1(o) K2(o) C(o) 1.023x t = line-time; P = Peak Power Requirement (e) = even rows of nozzles; (o) = odd rows of nozzles 10 Table 5 From the foregoing it will be appreciated that the combination of out-of-phase firing together with dot-at-a-time redundancy is optimal for achieving excellent print quality and an acceptable power requirement for the printhead during printing. However, these methods of printing may equally be used individually, providing their inherent 15 advantages, or in combination with other methods of printing. For example, out-of-phase firing or dot at-a-time redundancy may be used in combination with printhead module misplacement correction and/or dead nozzle compensation, as described in our earlier patent applications USSN 10/854521(Docket No. PLTOO1US) filed May 27, 2004 and USSN 10/854515 (Docket No. PLT020US), filed May 27, 2004.. 20 Printer Controller WO 2007/065187 PCT/AU2005/001829 21 It will also be appreciated by the skilled person that a printer controller 10, shown schematically in Figure 5, may be suitably programmed to provide dot data to the printhead 1, so as to print in accordance with the methods described above. A printhead system 20 comprises the printer controller 10 and the printhead 1, which is controlled by the controller. The printer controller 10 5 communicates dot data to the printhead 1 for printing. A suitable type of printer controller, which may be programmed accordingly, was described in our earlier patent application USSN 10/854521(Docket No. PLTO01US) filed May 27, 2004. It will, of course, be appreciated that the present invention has been described purely by way of example and that modifications of detail may be made within the scope of the invention, which is 10 defined by the accompanying claims.

Claims (18)

1. A method of modulating a peak power requirement of an inkjet printhead, said printhead comprising a plurality of first nozzles and a plurality of second nozzles supplied with a same colored 5 ink, said first nozzles and second nozzles being configured in a plurality of sets, wherein each set of nozzles comprises one first nozzle and one corresponding second nozzle, each nozzle in a set being configurable to print a dot of said ink onto a substantially same position on a print medium, said method comprising: (a) selecting a firing nozzle from at least one set of nozzles, said selection being on the basis of 10 modulating said peak power requirement; and (b) printing a dot onto said print medium using said firing nozzle.

2. The method of claim 1, wherein each set is a pair of nozzles, said pair consisting of one first nozzle and one corresponding second nozzle. 15

3. The method of claim 1, wherein said printhead is a stationary pagewidth printhead and said print medium is fed transversely past said printhead

4. The method of claim 3, wherein said printhead comprises a plurality of transversely aligned 20 color channels, each color channel comprising at least one nozzle row extending longitudinally along said printhead, each nozzle in a color channel ejecting the same colored ink.

5. The method of claim 4, wherein each color channel comprises a pair of nozzle rows. 25

6. The method of claim 5, wherein said pairs of nozzle rows are transversely offset from each other.

7. The method of claim 4, wherein said first nozzles are contained in a first color channel and said second nozzles are contained in a second channel. 30

8. The method of claim 7, wherein each set of nozzles comprises one first nozzle from a first color channel aligned transversely with one corresponding second nozzle from a second color channel, thereby allowing either of said first or second nozzles to print at the substantially same position on said print medium. WO 2007/065187 PCT/AU2005/001829 23

9. The method of claim 1, wherein one set of nozzles prints a column of same-colored dots down said print medium, each nozzle in said set contributing dots to said column. 5

10. The method of claim 9, wherein one first nozzle in said set prints about half of said column and one second nozzle in said set prints about half of said column.

11. The method of claim 1, wherein, in printing a line of same-colored dots across said print medium, said first nozzles and said second nozzles each contribute dots to said line. 10

12. The method of claim 11, wherein said first nozzles print about half of said line and said second nozzles print about half of said line.

13. The method of claim 12, wherein alternate first nozzles are used to print about half of said line 15 and alternate second nozzles are used to print about half of said line.

14. The method of claim 11, wherein said peak power requirement is reduced by about 50% for printing said line, compared to printing said line using only first nozzles or only second nozzles. 20

15. The method of claim 1, wherein said method reduces a visual effect of misdirected ink droplets.

16. The method of claim 1, wherein said method reduces a visual effect of unknown malfunctioning nozzles. 25

17. The method of claim 1, wherein said color is magenta, cyan or black.

18. The method of claim 1, wherein said printhead comprises first and second magenta nozzles and first and second cyan nozzles.