Inkjet printhead calibration

Abstract: In various methods, image registration variations among test patterns are used to calibrate misalignment among one or more printheads (60), paper advance distance, different portions of an inkjet printhead, or bidirectional printhead alignment. A set of test patterns (81-87 / 72-78 / 102-108) are printed. Each pattern includes a reference portion (90/71/101) and a varying portion (92/73/103). The varying portion is changed from pattern to pattern in a manner for testing a parameter being calibrated. An optical sensor (58) scans each test pattern. The parameter setting for the test pattern having the highest reflectance (i.e., most blank space) is selected as the calibrated parameter value.

Description: BACKGROUND OF THE INVENTION This invention relates generally to printers, plotters, and marking devices, and more particularly, to inkjet printers, plotters and marking devices having multiple printheads for multi-color printing. Inkjet marking devices typically include one or more inkjet pens mounted on a carriage. Each pen includes a printhead having a plurality of inkjet nozzles. During printing, the carriage moves across a media sheet while the nozzles discharge ink drops. The timing of the ink drop ejection is controlled to precisely place the drops at desired locations. A typical multi-color inkjet marking device includes two or more inkjet pens with respective printheads. One pen stores black ink, while the others store ink of one or more colors, (e.g., cyan, magenta or yellow). The four inks represent four base colors which are applied to a media sheet to derive any of multiple colors. The pens typically are mounted in stalls within the carriage. To achieve desired print quality the ink colors need to be precisely placed at desired locations on the media sheet. To do so the pen printheads are to be maintained in precise alignment. The pens typically are loaded and replaced periodically by the end user. As a result, mechanical misalignment is likely to occur. Mechanical misalignment results in offsets of one or more pens' nozzles relative to the other pens' nozzles. This misalignment manifests as a misregistration of the dots forming a print symbol, image, or graphic representation. Other sources of misalignment also may occur due to the speed of motion of the carriage, the curvature of the platen and the spray of the nozzles. One conventional approach for aligning the pens is to use an optical drop detector. This technique is described in U.S. Patent No. 4,922,270, issued May 1, 1990 to Cobbs et al., entitled "Inter Pen Offset Determination and Compensation in Multi-Pen Thermal Ink Jet Printing Systems." The optical drop detectors detect the position of each ink drop as it leaves the pen. The system then calculates the point of impact of the drop on the print media. Unfortunately the actual impact point often differs substantially from the calculated impact point due to angularity and mechanical tolerances in the system. Angularity results from the movement of the pen in the scan axis as ink is being ejected. There is a delay between the time that the drop of ink is ejected and the time that the drop impacts the media. This flight time delay causes the drop to traverse an angular path toward the media. Inaccurate correction for this delay distorts the image. The closest prior art U.S. Patent No. 5,289,208 issued February 22, 1994 of Robert D. Haselby entitled "Automatic Print Cartridge Alignment Sensor System" discloses a technique in which an optical sensor detects the position of test line segments. Vertical alignment is achieved by printing a plurality of non-overlapping horizontal test line segments. A quad photodiode detector detects the vertical positions of the horizontal test line segments relative to a fixed reference. Horizontal alignment is achieved by printing a plurality of non-overlapping vertical test line segments in a vertical direction. If properly aligned the line segments connect to form a straight line (e.g., for printing 2 line segments the line is 2 line segments long). If misaligned, the line segments form an angled line (e.g., for printing 2 line segments the line is 2 line segments long). A quad photodiode detector detects the horizontal positions of the vertical test line segments to determine if the segments are aligned. U.S. Patent No. 5,451,990 discloses a reference pattern for use in performing image registration for multiple inkjet cartridges. The reference pattern includes four test patterns. One test pattern is generated along the scan axis to exercise the pens. It includes an individual section for each color. A second test pattern is used to test for pen offset due to speed and curvature. The second pattern is a bidirectional test pattern in which the cartridge is moved at differing speeds in each direction. A pattern is generated for each color. A third pattern is generated by causing each pen to print a plurality of horizontally spaced vertical bars. The fourth patten includes five columns of vertically spaced horizontal bars. Each column has three sections. The first section in each column is generated using one color (e.g., cyan). In the second section the same color (cyan) is used in columns one and five. The other colors are used respectively in columns 2-4 (e.g., magenta in column 2; yellow in column 3; black in column 4). In the third section of each column, the first color (cyan) is used. Note that the colors do not overlap in any of the patterns. SUMMARY OF THE INVENTION The invention provides a method according to claim 1, and a system according to claim 6. Preferred method embodiments are defined in claims 2 to 5. One advantage of the invention is that by overlaying the inks of differing color a smaller area is used to perform color registration and printhead alignment. As a result, the registration process is faster. Another advantage is that the proper alignment may be identified visually by an operator. Another advantage is that all nozzles of a printhead may participate in the calibration process. This results in a more reliable alignment method which provides effective results even when one or more individual nozzles fail. These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a portion of an inkjet printing apparatus according to an embodiment of this invention; Fig. 2 is a diagram of printheads for the inkjet pens of Fig. 1; Fig. 3 is a diagram of ink drop ejection from the pens of Fig. 1 onto a media sheet; Fig. 4 is a diagram depicting multiple sets of test patterns according to an embodiment of this invention; Fig. 5 is a magnified view of a portion of a set of test patterns of Fig. 4; Fig. 6 is a magnified view of a portion of a test pattern of Fig. 5 having desired registration according to one embodiment of this invention; Fig. 7 is a magnified view of a portion of a test pattern of Fig. 5 having desired registration according to an alternative embodiment of this invention; Fig. 8 is a magnified view of a portion of a test pattern of Fig. 5 having poor registration; Fig. 9 is a diagram of test patterns according to another embodiment of this invention; Fig. 10 is a diagram of test patterns according to another embodiment of this invention; Fig. 11 is a diagram of test patterns for a method of calibrating paper advancement distance; Fig. 12 is a diagram of a printhead nozzle layout having multiple printhead portions to be calibrated; Figs. 13a-13c are diagrams of a set of test patterns for calibrating inter-column printhead alignment; Figs. 14a-14c are diagrams of a set of test patterns for calibrating array length variation; Figs. 15a-15b are diagrams of a set of test patterns for calibrating bidirectional printing alignment. DESCRIPTION OF SPECIFIC EMBODIMENTS Overview Fig. 1 shows a portion of a color inkjet marking apparatus 10. The apparatus 10 includes multiple inkjet pen 14, 16, 18, 20 which print characters, symbols, graphics or other imagery and markings onto a media sheet 12. The pens 14-20 are shuttled along a scanning axis 26, while the media sheet 12 is moved along a media path in a media direction 28. The scanning axis 26 is referred to herein as a horizontal axis 26 given the same part number. The media direction 28 corresponds to a vertical axis 28 given the same part number. The axes 26, 28 may be oppositely named. Other naming conventions also may be used. The media sheet 12 is moved by rollers 30 on an axle 32, which in turn are driven by gears 34 and a motor 36. The pens 14-20 are carried in a carriage 22 which moves along a carriage rod 24. A drive belt 38 coupled to the carriage 22 exerts a drive force which moves the carriage 22. A drive motor 40 generates the drive force. The motor 40 turns a drive pulley 42 on a drive shaft 44. The drive belt 38 runs along the drive pulley 42 and an idler pulley 46 coupled to an idler spring 48. Carriage position and media sheet position are monitored by a processor 52. Carriage position is derived from a signal from a digital encoder 56 indicative of drive belt position. Media sheet position is determined from signals marking the passing of a media sheet at a known point and from a signal from a digital encoder. The motor 36 includes the digital encoder for tracking the roller 30 position. An optical sensor 54 detects the passing edge of the media sheet 12. Another optical sensor 58 moves with the carriage 22 along the carriage rod 24 for use in calibrating image registration. The inkjet pens 14-20 store ink of different colors, e.g., black, cyan, magenta and yellow. As the carriage 22 and media sheet 12 translate relative to each other, the pens 14-20 scan the media sheet along the horizontal axis 26 and vertical axis 28. Referring to Fig. 2, each pen 14-20 includes a printhead 60 having an array 62 of nozzles 64. The nozzles 64 eject ink drops 66 onto the media sheet 12 as shown in Fig. 3. The number of drops, the density of the drops and the ink color of the drops determine the colors perceived by a viewer in a printed image or marking. Accordingly, to achieve accurate printing of desired colors it is important that the ink drops be placed precisely in desired positions. One challenge to such positioning is misalignment of the pens 14-20 in the carriage 22. Once the pens are locked into the carriage 22 there position is generally fixed. However, such position may vary when a pen is removed or replaced. To assure high quality printing, the registration of dots from the various printheads 60 of pens 14-20 are calibrated so that the printheads 60 are in a known position relative to each other. Test Patterns Fig. 4 shows multiple sets 70, 72, 74, 78, 80 of test patterns used for calibrating registration of the printheads 60 of the inkjet pens 14-20. In the embodiment illustrated each set includes 7 test patterns 81-87, although the number may vary. In a preferred embodiment one printhead is taken to be a reference. The other printheads are calibrated relative to the position of such printhead. Registration of each of the non-reference printhead from corresponding pens 14, 16 and 20 is calibrated along both the horizontal axis 26 and the vertical axis 28. There is a set of test patterns for each calibration of each non-reference printhead. Thus, Fig. 4 shows six sets of test patterns. For example, set 70 is for calibrating the black pen 14 printhead relative to the horizontal axis. Set 72 is for calibrating the black pen 14 printhead relative to the vertical axis. Set 74 is for calibrating the cyan pen 16 printhead relative to the horizontal axis. Set 76 is for calibrating the cyan pen 16 printhead relative to the vertical axis. Set 78 is for calibrating the yellow pen 20 printhead relative to the horizontal axis. Set 80 is for calibrating the yellow pen 20 printhead relative to the vertical axis. The ordering of the sets may vary. Each set 70-80 of test patterns is arranged along the horizontal axis 26, although in other embodiments they may be aligned along the vertical axis 28. Further, in some embodiments the horizontal calibration sets 70, 74, 78 may be aligned along one of the axes 26, 28 while the vertical calibration sets 72, 76, 80 are aligned on the other of the axes 26, 28. Each set 70, 74, 78 for horizontal calibration includes a plurality of vertical bars spaced apart along the horizontal axis 26. Conversely, each set 72, 76, 80 for vertical calibration includes a plurality of horizontal bars spaced apart along the vertical axis 28. Although bars are shown and described, circles, diamonds, squares or other shapes may be used. Each test pattern 70-80 includes two portions. Each portion is of the same size and shape. One portion is formed of ink drops from the reference pen 18 printhead, while the other portion is formed of ink drops from the printhead being calibrated. Thus sets 70, 72 include magenta ink drops from the reference pen 18 printhead and black ink drops from the pen 14 printhead. Sets 74, 76 include magenta ink drops from the reference pen 18 printhead and cyan ink drops from the pen 16 printhead. Sets 78, 80 include magenta ink drops from the reference pen 18 printhead and yellow ink drops from the pen 20 printhead. Within each given set 70-80 of test patterns, the registration of the reference pen 18 printhead is the same for each test pattern 81-87, while the registration of the pen printhead under test varies for each test pattern 81-87. Fig. 5 shows 4 test patterns 83-86 of a given set of test patterns for a sample process for calibrating cyan ink pen 16 printhead for the horizontal axis 26. As described above each test pattern includes a plurality of vertical bars horizontally spaced. For purposes of illustration the reference ink bars 90 are drawn as solid lines and the cyan bars 92 are drawn as dashed lines. From test pattern 83 to test pattern 86 the registration of the cyan bars 92 is shifting left on the page of the drawing. In test pattern 85 the cyan bars 92 and reference bars 90 overlap. Fig. 6 shows a magnified view of a portion of the sample test pattern 85 of Fig. 5 having the desired registration along axis 27 (e.g., one of axes 26, 28). Three bars 94, 96, 98 are shown. Each bar is shown as a plurality of ink drops. For purposes of illustration a reference color ink drop is depicted with an 'x', while the printhead under test ink drops (e.g., cyan) are depicted with an 'o'. In one embodiment, the dots of the respective printhead also overlap for the desired registration. In another embodiment, every other nozzle is used in the test pattern to draw the bars 90, 92. Fig. 7 shows the example where the bars 90, 92 overlap to define the respective bars 94', 96', 98' of a test pattern 85' having the desired registration along the axis 27 being calibrated. For the desired registration the bars from the reference pen 18 printhead and the bars from the pen under test overly each other. In the Fig. 6 embodiment the dots of the two colors also overly each other. In the Fig. 7 embodiment the dots align along the non-calibrating axis irrespective of whether the dots themselves overlap, (e.g., for horizontal calibration, the dots align vertically even if they do not overlap). Fig. 8 shows a magnified view of a portion of a sample test pattern 83 having poor registration. In a poor registration the bars from the reference pen 18 printhead and the bars from the printhead under test do not overlap. Each bar 90, 92 is shown as a plurality of ink drops. For purposes of illustration a reference color ink drop is depicted with an 'x', while each printhead under test ink drop (e.g., cyan) is depicted with an 'o'. Note that test pattern 85 is shown to have desired registration and test pattern 83 is shown to have poor registration. The test pattern of a given set of test patterns having the best registration need not be pattern 85 and may differ from one set to the next. Similarly, test pattern 83 need not be a poor test pattern. These were represented as having desired registration and poor registration merely for purposes of illustration and description. Although Figs. 5-8 depict dots for a test pattern having vertical lines, similar alignment and misalignment occurs for the test patterns formed by horizontal lines. It is significant that the bars of differing color ink drops overlap in the desired registration and do not overlap in the poor registration. As the registration varies from desired to poor the degree of overlap decreases. As the amount of overlap decreases, the amount of background media sheet covered with ink increases. According to an aspect of this invention, the reflectance of the media sheet is sensed for each test pattern 81-87 in a set of test patterns. The test pattern having the highest reflectance is the test patten having the best degree of overlapping, and thus the best registration. In a preferred embodiment each bar of a given color ink in a test pattern has the same number of dots in width. The bars from a given pen are spaced apart by at least two dot widths. In an exemplary embodiment, each bar of a given color ink is one dot wide and spaced five dots apart. The width of a bar is the same for each printhead. The spacing between bars of a given color is the same for each color ink. What varies is the registration of the bars of one color ink relative to the bars of the reference color ink. A best registration is selected for each set of test patterns, and thus, for each printhead under test in each of the axes 26, 28. The best registration is that corresponding to the test pattern having the highest reflectance within a set of test patterns. Fig. 9 shows a plurality 70 of test patterns 72-78 according to another embodiment of this invention. Each test pattern includes two portions 71, 73. For each test pattern, one portion 71 is printed from one inkjet printhead, while the other portion 73 is printed from another inkjet printhead. One inkjet printhead serves as a reference printhead. The other inkjet printhead is a printhead being calibrated. The image registration for the calibrated printhead is varied for each test pattern 72-78. Such image registration may be varied among one two printing axes. In another embodiment each test pattern may include more than two portions with each portion being printed by a different printhead. Image registration of at least one printhead is changed for each test pattern. At least one other pen serves as a reference printhead in which its image registration is the same for each test pattern. Each portion 71, 73 has the same size and shape. Each test pattern 72-78 is generally circular. Fig. 10 shows another plurality 100 of test patterns 102-108 in which each portion 101, 103 is diamond shaped. Although, test patterns have been described and illustrated to include one or more lines, circles or diamonds, other shapes also may be used. Method of Calibrating Registration To calibrate registration of a given printhead relative to a given axis 26, 28 a set of test patterns are printed. The set includes a plurality of test patterns. Each test patten includes one or more bars, circles, diamonds or other shapes. For a test pattern formed by a plurality of bars, the bars are spaced apart along an axis of calibration. The bars are elongated in the direction perpendicular to the axis under calibration. For circles or diamonds having symmetry along either calibration axis, the orientation of the shape may be the same regardless of the calibration axis. With regard to the test pattern of bars, to calibrate the black ink pen 14 printhead for the horizontal axis a plurality of vertical black bars are printed with horizontal spacing in each test pattern of the set. A similar plurality of bars are printed with the reference printhead for each given test pattern. Thus, each test pattern includes a plurality of bars of the printhead under test and the reference printhead. Optical sensor 58 then scans the set of test patterns to sample the reflectance of each test pattern. The processor 52 receives the sensor samples are derives a value indicative of reflectance for a scanned test pattern. A value is derived for each test pattern in the set. The processor identifies which test pattern has the highest reflectance. Such test pattern has the most overlapping of bars of the two colors (i.e., the ink colors of the printhead under test and of the reference printhead), and thus corresponds to the best registration for the axis calibrated. The registration used for such selected test pattern is the registration selected for the printhead under test for the axis calibrated. Another set of test patterns then is printed for calibrating relative to the other axis 26, 28. The process is repeated to calibrate registration of each printhead relative to a reference printhead. One of the four pens 14-10 is selected as the reference printhead as described above. To vary the registration of the printhead under test from one test pattern to the next the timing or assignment of nozzles to eject ink drops is changed. According to one embodiment the registration is changed by one nozzle width from one test pattern to the next test pattern in a given set of test patterns. The unit of change among test patterns however may vary and need not be of constant increments. There are many variables which may change from embodiment to embodiment, such as the number of bars of a given color ink per test pattern, the spacing between bars, the thickness of each bar, the dot density of each bar, and the change in registration from one pattern to the next. Alignment is achievable for spatial resolutions as fine as 0.25 dot rows. In an alternative embodiment, more than two colors of ink may be used in one or more sets of test patterns of test patterns so that fewer sets of test patterns are printed to calibrate registration. In such an embodiment the registration of one or more printheads is varied while the registration of at least one printhead is held constant for a given set of test patterns. Consider an example in which four sets 70, 72, 74, 76 are printed. Sets 70 and 74 are for calibrating along one axis and sets 72, 76 are for calibrating along the orthogonal axis. In sets 70 and 72 three printheads are used. The registration of two printheads are varied from test pattern to test pattern, while that of the third is held constant. The registrations corresponding to the test pattern with the highest reflectance then are selected for each axis. The remaining sets 74, 76 then are used to calibrate the desired registration along the respective axes for the remaining printhead. Thus, sets 74, 76 include bars printed from the remaining printhead not included in sets 70, 72 and at least one other printhead. Sets 74, 76 may use ink from 2, 3 or 4 printheads. Only the registration of the remaining printhead is changed from pattern to pattern in sets 74, 76. The patterns of set 74 are scanned. The registration corresponding to the pattern with the highest reflectance is used for such remaining printhead. Such registration is the calibrated registration along the axis calibrated using set 74. Similarly, the patterns of set 76 are scanned. The registration corresponding to the pattern with the highest reflectance is used for such remaining pen. Such registration is the calibrated registration along the axis calibrated using set 76. In another embodiment an operator inspects the test patterns, rather than an optical sensor. In one example, an operator enters a pattern number to identify which pattern has the best alignment. In a best mode multi-color printing embodiment, the reference printhead ejects black ink. Method for Calibrating Paper Advancement Distance Referring to Fig. 11, a method for calibrating a most desirable paper advancement distance is implemented by printing a set of test patterns 105, 107, 109, 111 onto a media sheet. Each test pattern includes a first portion and a second portion which have a common pattern, size and shape. The portions are generally overlapping. At one step a first portion 102 of test pattern 105 is printed using a first subset of nozzles of an inkjet printhead 60. At another step, the media sheet is advanced by a media advancement distance. Then a second portion 104 of test pattern 105 is printed with a second subset of nozzles differing from the first subset. The second subset is picked so as to be spaced from the first subset approximately by media advancement distance. Accordingly, the second portion 104 overlaps the first portion 102. Next, the media is advanced to a clean area of the media sheet and the steps then are repeated to achieve test pattern 107 with generally overlapping test pattern portions 106 and 108. The portion 106 is printed using the first subset of nozzles while portion 108 is printed using the second subset of nozzles. For test patterns 107, the media advancement distance is different than the distance moved between printing the portions 102 and 104 of test pattern 105. Thus, the overlapping of test pattern 107 portions 106 and 108 differs from the overlapping of test pattern 105 portions 102 and 104. More particularly the amount of blank space, and thus the reflectance among the test patterns 105, 107 varies. The media sheet then is advanced again to a clean area of the media sheet and the steps then are repeated to achieve test pattern 109 with generally overlapping test pattern portions 110 and 112. The portion 110 is printed using the first subset of nozzles while portion 112 is printed using the second subset of nozzles. For test pattern 109, the media advancement distance is different than the distance moved between printing the portions 102 and 104 of test pattern 105, and between printing the portions 106 and 108 of test pattern 107. Thus, the overlapping of test pattern 109 portions 110 and 112 differs from the overlapping in test pattern 105 and in test pattern 107. More particularly the amount of blank space, and thus the reflectance among the test patterns 105, 107 and 109 varies. The media sheet then is advanced again to a clean area of the media sheet and the steps then are repeated to achieve test pattern 111 with generally overlapping test pattern portions 114 and 116. The portion 114 is printed using the first subset of nozzles while portion 116 is printed using the second subset of nozzles. For test pattern 111, the media advancement distance is different than the distance moved between printing the portions of test patterns 105, 107 and 109. Thus, the overlapping of test pattern 111 portions 114 and 116 differs from the overlapping in test patterns 105, 107 and 109. More particularly the amount of blank space, and thus the reflectance among the test patterns 105, 107 and 109 varies. Note that the spacing between test pattern portions in a given test pattern is exaggerated for purposes of illustration. Further, dotted lines are used for one portion of a test pattern while solid lines are used for the other portion for purposes of illustration. Preferably each test pattern portion within a given test pattern is the same. The various test patterns 105, 107, 109 and 111, however, may vary in a given embodiment. Optical sensor 58 then scans the set of test patterns 105, 107, 109, 111 to sample the reflectance of each test pattern. The processor 52 receives the sensor samples are derives a value indicative of reflectance for a scanned test pattern. A value is derived for each test pattern. The processor identifies which test pattern has the highest reflectance. Such test pattern has the most closely aligned overlapping portions, and corresponds to a best paper advancement distance. The paper advancement distance used for such selected test pattern is the paper advancement distance selected. For a printing apparatus in which the paper advancement has a consistent paper advance error, the paper advance can be calibrated according to the method described above or by an alternative method. In the alternative method, the paper advance can be measured in terms of the number of nozzles moved and by which nozzles line up. The paper advance is altered by changing the paper advance distance in proportion to the measured nozzle distance. According to the alternative method, the paper advance is calibrated together with printhead nozzle array length as described below with regard to Figs. 14A-C. Method for Calibrating Different Parts of a Printhead Referring to Fig. 12, a printhead 60 includes nozzles 64 allocated among four different portions 122, 124, 126, 128 to be calibrated. Such portions are referred to as portions a, b, c and d. In one method inter-column alignment is calibrated. In another method printhead array length is calibrated. For each method multiple test patterns are printed using different parts of the printhead. Deviation from nominal offsets in the nozzle positions of a given portion a, b, c or d are determined from the degree of overlap among portions of a test pattern. The test pattern preferably is a set of regularly spaced lines. However, other shaped may be used such as diamond patterns, circular patterns, square pattern and other regular or irregular shaped patterns. For the inter-column alignment calibration method, a first portion of each of multiple test patterns 130, 132, 134, 136 is printed using nozzles from printhead portion a as shown in Fig. 13a. Each portion is spaced from the next portion by a distance x. Referring to Fig. 13b, at another step a second portion of each of the multiple test patterns is printed using nozzles from portion b. Note that the first portion and second portion of each test pattern may be printed on the same scan of the inkjet pen across the media sheet. The first portion and the second portion of each test pattern are identical. The second portions, however, are printed at a spacing y between each test portion. For the second portions to precisely overlay the respective first portions, the nominal inter-column distance between the nozzles in printhead portion a and printhead portion b are to be accounted for in determining the starting position of pattern 130. By using a spacing y distinct from x, the different patterns will be printed with offsets that are multiples of the x-y distance. Referring to Fig. 13c, each test pattern 130-136 will have a different reflectance as some dots are superimposed. The inter-column offset distance used to achieve the test pattern having the highest reflectance (i.e., most overlay) is added to the nominal inter-column distance to determine the actual inter-column distance. For the printhead array length calibration method, a first portion of each of multiple test patterns 140, 142, 144, 146 is printed using nozzles from printhead portion a as shown in Fig. 14a. Next, the media sheet is advanced by a nominal distance equivalent to the distance between the centroids of the nozzle groups to be aligned. Referring to Fig. 14b, at another step a second portion of each of the multiple test patterns is printed using nozzles from portion c. The first portion and the second portion of each test pattern are identical. For each test pattern 140-146 the second portions are offset vertically by a small amount (e.g., one nozzle spacing). For the second portions to precisely overlay the respective first portions, the printhead array length variation between printhead portions a and c is compensated for, as is the paper advance distance. Referring to Fig. 14c, each test pattern 140-146 will have a different reflectance as some dots are superimposed. The test pattern having the highest reflectance corresponds to the array length variation between portions a and c for a given paper advance increment. In effect array length and paper advance are calibrated together. Method for Calibrating Bidirectional Printhead Alignment To calibrate image registration variations for printing while scanning in one direction across a media sheet versus printing while scanning in the opposite direction across the media sheet, a calibration process is performed. As with the other calibration methods described above, multiple test patterns are printed. Referring to Fig. 15a a first portion of each one of multiple test patterns 150, 152, 154, 156 is printed while scanning the inkjet printhead 60 across the media sheet in a first direction 148. The spacing between each test pattern is the same. At another step a second portion of each of the test patterns is printed while scanning back across the media sheet in the opposite direction 149. The spacing between each second portion, however varies. Thus, the registration of the first portion and second portion pf each test pattern 150-156 varies. The test pattern of 150-156 having the highest reflectance (e.g., most background space) corresponds to a calibration spacing to be used to achieve bidirectional printhead alignment. The method may be implemented conversely in which printheads attempt to print completely out of phase. The test pattern with the minimal reflectance would then corresponds to the best alignment. For some alternate test patterns, such as concentric circles or diamonds, the test patterns can alternately be evaluated for the consistency of the reflectance across the pattern, where the most consistent reflectance across the pattern indicates the best alignment.

Claims: A method for calibrating image registration for two inkjet printheads (60), each printhead including a plurality of inkjet nozzles (64), the method comprising the steps of: printing a first plurality of test patterns (81-87 / 72-78 / 102-108) onto a media sheet, each one test pattern of the plurality of test patterns including a first portion (90/71/101) printed with a first inkjet printhead and a second portion (92/73/103) printed with a second inkjet printhead, wherein the first portion and second portion are of the same shape, wherein image registration of one of said two inkjet printheads is varied among each one of the plurality of test patterns; and inspecting the plurality of test patterns; selecting the image registration corresponding to one of the plurality of test pattern having the most unprinted background area. The method of claim 1, wherein during the step of printing the plurality of test patterns, image registration of said one printhead is varied for each one test pattern by changing a selection of nozzles of said one printhead used in printing said one test pattern. The method of claims 1 or 2, in which the step of inspecting comprises the step of sensing the reflectance of each one of the plurality of test patterns, and wherein the step of selecting comprises selecting the image registration corresponding to the test pattern having the highest reflectance. The method of claims 1, 2, 3 or 4 in which the first inkjet printhead prints ink of a differing color than the second inkjet printhead. The method of claim 4, wherein each one of the first plurality of test patterns includes a plurality of horizontally-spaced vertical bars (90, 92) and is formed by printing ink drops from at least two of the plurality of inkjet printheads, wherein horizontal registration of at least one of said two of the plurality of inkjet printheads is varied among each one of the first plurality of test patterns, the method further comprising: printing a second plurality (72) of test patterns (81-87), wherein each one of the second plurality of test patterns includes a plurality of vertically-spaced horizontal bars and is formed by printing ink drops from at least two of the plurality of inkjet printheads, wherein vertical registration of at least one of said two of the plurality of inkjet printheads is varied among each one of the second plurality of test patterns; and wherein inspecting the plurality of test patterns comprises inspecting the first and second plurality of test patterns. An image registration system for a multi-color inkjet marking apparatus (10), comprising: a plurality of inkjet printheads (60), each printhead of the plurality of printheads having a plurality of nozzles (64) adapted to discharge ink in response to a corresponding electrical signal; a carriage (22) retaining the plurality of inkjet printheads, the carriage moving along a first axis (26); rollers (30) for moving a media sheet along a second axis (28) perpendicular to the first axis; a controller (52) providing electrical signals which cause said nozzles to eject ink onto the media sheet and create a first plurality (70/100) of test patterns (72-78 / 102-108), each one test pattern of the plurality of test patterns including a first portion (71/101) printed with a first inkjet printhead and a second portion (73/103) printed with a second inkjet printhead, wherein the first portion and second portion are of the same shape, wherein image registration of one of said two inkjet printheads is varied among each one of the plurality of test patterns; a sensor (58) for optically sensing reflectance of each one of the first plurality of test patterns; and a processor (52) for sampling the sensor, for determining which one of the first plurality of test patterns has the highest reflectance, and for setting image registration of at least one of the first inkjet printhead and the second inkjet printhead.

Inventors: DONOVAN DAVID H, GAST PAUL DAVID, GIL MIQUEL ANTONI, GROS XAVIER, HICKMAN MARK S, DONOVAN, DAVID H., GAST, PAUL DAVID, GIL MIQUEL, ANTONI, GROS, XAVIER, HICKMAN, MARK S.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY, A DELAWARE CORPORATION
Application year: 1999
Main classification: B41J29
IPC codes: B41J2, B41J29