Abstract
A print medium having a calibration pattern printed thereon for generating alignment data for a printhead. The calibration pattern contains rows of spaced apart fiducials, each fiducial having a plurality of concentric shapes representing a Barker code.
Claims
1. A print medium having a calibration pattern printed thereon for generating alignment data for a printhead, the calibration pattern comprising one or more rows of spaced apart fiducials, each fiducial comprising a plurality of concentric shapes representing a Barker code.
2. The print medium of claim 1, wherein each fiducial comprises a plurality of concentric annuli.
3. The print medium of claim 2, wherein the code sequence has a sequence of N code values, each value being represented by a presence or absence of an annulus at a predetermined distance from a centre of the fiducial, and where N is an integer from 3 to 20.
4. The print medium of claim 3, wherein the Barker code has the code sequence: [+1, +1, +1, +1, +1, −1, −1, +1, +1, −1, +1, −1, +1].
5. The print medium of claim 4, wherein each code value of +1 is represented by an absence of an annulus and each code value of −1 is represented by a presence of an annulus.
6. The print medium of claim 1, wherein the calibration pattern is printed using a printing system comprising at least one of: a plurality of overlapping printheads extending across a pagewidth; and a plurality of printheads arranged along a media feed direction.
7. The print medium of claim 6, wherein the fiducials are used for generating the alignment data for the printing system.
8. The print medium of claim 1, further comprising a plurality of identification codes, each identification code identifying a respective print chip of a printhead used to print the calibration pattern.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] One or more embodiments of the present invention will now be described with reference to the drawings, in which:
[0040] FIG. 1 shows part of a calibration pattern according to the present invention;
[0041] FIG. 2 shows schematically a print system having an array of printheads;
[0042] FIG. 3 shows part of a printhead having butting print chips;
[0043] FIG. 4 shows schematically a printhead bowed along its length;
[0044] FIG. 5 shows an individual imaged fiducial;
[0045] FIG. 6 shows a template fiducial;
[0046] FIG. 7A shows graphically cross-correlation values for an imaged fiducial;
[0047] FIG. 7B shows graphically a magnified subset of cross-correlation values;
[0048] FIG. 8 shows a fiducial location at a second resolution
[0049] FIG. 9 shows a flow chart for generating alignment data; and
[0050] FIG. 10 shows schematically an optical scanner and processor for generating alignment data.
DETAILED DESCRIPTION
[0051] Referring to FIG. 1, there is shown part of a calibration pattern 1 comprising multiple rows of fiducials 3. The calibration pattern 1 is printed by a modular printing system 100 of the type described in detail in US 2017/0313061, and part of which is shown schematically in FIG. 2. The printing system 100 comprises four monochrome print bars 102a, 102b, 102c and 102d ejecting black, cyan, magenta and yellow inks, respectively. Each print bar comprises at least first and second print modules (“printheads 104”), which are overlapped across a media width in order to achieve pagewide printing by feeding media past the printheads in a direction indicated by arrow M. The overlapping region between the first and second printheads 104 is referred to as a stitch region 106, in which nozzles from one printhead are stitched with nozzles from an adjacent printhead to provide seamless printing across the stitch region. Various methods of stitching overlapping printheads 104 are known in the art. Typically, overlapping printheads are stitched together using butt stitching, feathered stitching or combinations thereof, as described in, for example, US 2018/0126750, the contents of which are incorporated herein by reference. In FIG. 2, one stitch region 106 is shown for a pair of overlapping printheads 104 in each print bar 102; however, it will of course be appreciated that print bars may comprise N printheads with N−1 stitch regions, where N is an integer from 1 to 20 (e.g. 1 to 12). Likewise, although FIG. 2 shows four aligned print bars 102a-d for printing conventional CMYK inks, it will be appreciated that the printing system 100 may comprises M aligned print bars, where M is an integer from 1 to 20 (e.g. 1 to 12) for printing additional inks, such as spot colors, infrared inks, UV inks etc.
[0052] In each printhead 104, multiple print chips are arranged to provide seamless printing along a length of the printhead. For example, a Memjet® A4 printhead (as described in U.S. Pat. No. 9,950,527, the contents of which are incorporated herein by reference) contains eleven print chips 108, which are butted together in a single row to provide seamless pagewide printing. FIG. 3 is a magnified view of three butting print chips 108 in a Memjet® printhead. In other types of pagewide printhead (as described in, for example, U.S. Pat. No. 9,168,739, assigned to HP, Inc.), multiple print chips are positioned in a staggered overlapping arrangement to provide pagewide printing.
[0053] As foreshadowed above, good nozzle alignment is a key requirement for achieving high print quality in pagewide printing systems. However, in the modular printer 100 shown in FIG. 2, it will be appreciated that there are potentially multiple sources of nozzle misalignments: from within each printhead 104; between overlapping printheads in stitch regions 106; and between aligned printheads of different print bars 102. For example, relatively long printheads have a tendency to warp or bow, which may result in significant nozzle misalignments between each end of the printhead 104. FIG. 4 shows schematically the exaggerated effects of printhead warpage, resulting in nozzle misalignments along a nominal x-axis. By way of example only, a warp angle of only 0.26 degrees results in a nozzle misalignment of as much as 1.0 mm in the y-axis for a printhead having a length of 222.2 mm Due to the multiple sources of misalignments, including skewed print chip placement at the factory, the precise misalignment of each nozzle in each printhead 104 (containing thousands of nozzles in one row) cannot be easily predicted. Nevertheless, with precise data on the misalignment of each nozzle, or group of nozzles, relative to a nominal reference point then any nozzle misalignments may be compensated for by adjusting a timing of nozzle firing (e.g. by delaying or advancing the firing of a group of nozzles by a predetermined number or row times). Thus, the actual source of misalignment is immaterial to the compensation method employed, provided that the control electronics has sufficient alignment data for each printhead 104.
[0054] Returning to FIG. 1, the calibration pattern 1 is designed to provide alignment data for predetermined groups of nozzles in each printhead 104 of the printing system 100 in order to enable electronic compensation and, ultimately, optimization of print quality. Providing alignment data at high resolution is necessary, because neighboring nozzles in each print chip 108 are spaced apart by, for example, 15.875 microns in a 1600 dpi printhead. Conversely, a typical optical resolution of an off-the-shelf imaging system (e.g. flatbed scanner) may be about 300 dpi (resolving only an 85 micron pixel separation), which presents a significant challenge for calibrating the printing system 100 using a printed calibration pattern.
[0055] As shown in FIG. 1, the fiducials 3 are arranged into multiple rows 5, each row being printed by nozzles of a respective print bar 102. The first four fiducial rows in FIG. 1 are labelled as rows 5a, 5b, 5c and 5d, although it will be appreciated that each calibration pattern 1 contains dozens of fiducial rows 5 down the page.
[0056] A header portion of the calibration pattern comprises a row of identification codes in the form of 2D barcodes 7 (e.g. QR codes as shown in FIG. 1). Each barcode 7 identifies a respective print chip 108 of a reference printhead 104, together with other information relating to the printing system configuration and the calibration pattern 1.
[0057] Each print chip 108 of each printhead 104 prints four fiducials 3, grouped in fiducial sets 9 of the calibration pattern 1, with the exception of those print chips in the stitch region 106, which print only three fiducials each. The black print bar 102a serves as a reference print bar and prints the first two rows of fiducials 5a and 5b, followed by the cyan print bar 102b printing the next two rows of fiducials 5c and 5d. In summary, the fiducial rows 5 follow the sequence: black-black-cyan-cyan-black-black-magenta-magenta-black-black-yellow-yellow and is repeated down the page. In other words, the black fiducial printed by the reference print bar 102a interleave each of the colored (CMY) fiducials, enabling alignment of each print bar relative to the reference print bar.
[0058] Advantageously, each individual fiducial configuration enables accurate fiducial locations to be determined via optical imaging and decoding, despite the fiducials themselves being relatively large. Referring to FIG. 5, there is shown a captured image of an individual fiducial 3 of the calibration pattern 1 shown in FIG. 1. The fiducial 3 comprises a series of concentric annuli having predetermined ring-widths. Each printed annulus represents one or more code values of the Barker code: [+1, +1, +1, +1, +1, −1, −1, +1, +1, −1, +1, −1, +1]. Thus, the central blank portion 30 of the fiducial 3 represents the first five code values: +1, +1, +1, +1, +1; the innermost printed annulus 31 represents the next two code values: −1, −1; the next outer blank annulus 32 represents next two code values: +1, +1; the next outer printed annulus 33 represents the next code value −1; the next outer blank annulus 34 represents the next code value: +1; the outermost printed annulus 35 represents the penultimate code value: −1; and the outermost blank annulus 36 represents the final code value: +1.
[0059] As seen in FIG. 5, the imaged fiducial 3 has a large amount of noise in the form of blurred edges, from both the printing and imaging processes. However, Barker codes have characteristically low cross-correlation properties, such that cross-correlation of an electronically-generated template fiducial (“kernel”) 40 with each imaged fiducial 3 at a plurality of different displacements yields a centerpoint of each imaged fiducial at the imaging resolution. FIG. 6 shows the template fiducial 40 used for the cross-correlation. The template fiducial 40 is low-pass filtered to simulate the blurred edges of the imaged fiducial 3 so as to optimize the cross-correlation process.
[0060] Thus, the use of concentric Barker codes and cross-correlation with a template fiducial 40 means that processing of the calibration pattern 1 is relatively unaffected by noise, as well as being rotationally invariant for the purposes of imaging. In practice, cross-correlation is performed in the frequency domain in order to simplify the required computational analysis and provide a large set of cross-correlation values for each imaged fiducial 3.
[0061] FIG. 7A shows the results of cross-correlation for an imaged fiducial. The central dark patch 50 graphically represents cross-correlation maxima and indicates a centerpoint location of the fiducial 3 at the imaging resolution (300 dpi). FIG. 7B graphically shows the subset 50 of cross-correlation values in magnified view. Although the cross-correlation process has minimized the effects of noise, the fiducial location has still only been determined to within an accuracy of about 85 microns. In order to further improve the accuracy of the centerpoint location, the subset 50 of cross-correlation values for each imaged fiducial (graphically represented in FIG. 7B) are interpolated using a suitable interpolation technique (e.g. bicubic, nearest neighbour, cubic spline, shape-preserving, biharmonic, thin-plate spline etc.) to determine the centerpoint location of each fiducial 3 at a higher resolution. FIG. 8 shows the centerpoint of an imaged fiducial after interpolation of the subset 50 of cross-correlation values graphically represented in FIG. 7B. After interpolation, each fiducial location is determined to within an accuracy of about 8 microns, which is effectively an imaging resolution of 3175 dpi—more than ten times the original imaging resolution and at a fraction of the cost of an equivalent optical imaging apparatus. Crucially, the fiducial location accuracy is greater than the nozzle pitch of the printhead 104 (about 16 microns), such that the alignment data generated by the calibration pattern 1 and image processing has sufficient accuracy for compensating nozzle misalignments in the printheads 104, notwithstanding the effects of noise in the imaged calibration pattern and a relatively low imaging resolution.
[0062] As shown in FIG. 1, each print chip 108 of each printhead 104 prints four fiducials 3 (with the exception of print chips in the stitch region 106). The maximum number of printable fiducials per print chip is determined, to some extent, by the ring-width of the thinnest annuli (i.e. annuli 33 and 35) resolvable by the optical imaging apparatus. For an A4 printhead 104, this provides 43 alignment values per printhead for use in subsequent nozzle misalignment compensation. Further optimization of the calibration process is achievable by interpolating the locations along each fiducial row 5 to generate a continuous smooth curve representing the varying misalignments along the length of an entire print bar 102, which may include multiple printheads 104 and multiple stitch regions 106. Any suitable interpolation technique may be used for this second interpolation step (e.g. bicubic, nearest neighbour, cubic spline, shape-preserving, biharmonic, thin-plate spline etc.), which may be the same or different than the first interpolation technique used on each subset 50 of cross-correlation values.
[0063] An advantage of interpolating the fiducial locations along each row 5 in the calibration pattern 1 is that a greater number of alignment values can be generated by sampling the resultant smooth interpolated curve at predetermined intervals in order to improve further the accuracy of misalignment compensation. For example, in a Memjet® print chip of length 20.2 mm containing 1280 nozzles per row, each nozzle row may be divided into 40 sections with each section containing 32 pixels (nozzles). Thus, an alignment value is assigned to each of the 40 sections per print chip (i.e. about 50 sections per inch of printhead), with each alignment value being extracted from the interpolated curve representing the overall warpage of a printhead 104 and/or a print bar 102. A further advantage of assigning an alignment value to a relatively small group of nozzles in each print chip 108 is that large step changes in firing timings are avoided during nozzle misalignment compensation. For example, changes in firing timings may be limited to +1 or −1 timing units between neighboring sections (e.g. 1 timing unit=1 row firing time). By avoiding large step changes in firing timings along a length of the printhead 104 or print bar 102, further optimization of print quality may be achieved.
[0064] Returning to FIG. 1, it will be appreciated that accurate locations for each fiducial 3 may be used not only for electronic alignment along a nominal x-axis (i.e. row-wise fiducial analysis across a media width), but also color-to-color alignment of print bars 102b, 102c and 102d relative to a reference (black) print bar 102a (i.e. column-wise fiducial analysis along the media feed direction M). Alignment of print bars 102a-d for dot-on-dot printing is achieved by using a timing signal from a media encoder and comparing printed fiducial locations down each fiducial column with an expected fiducial location, relative to the reference print bar 102a.
[0065] In addition, column-wise analysis of fiducials 3 printed from the same print bar (e.g. reference print bar 102a) may be used to provide additional alignment data for subsequent processing and compensation.
[0066] In summary, it will be appreciated that the calibration pattern 1 and the methods described herein may be used to generate a large amount of alignment data, which can be manipulated to enable compensation of nozzle misalignments in a modular two-dimensional array of printheads 104, such as the modular printing system 100 shown in FIG. 2.
[0067] FIG. 9 outlines a basic sequence of steps for generating alignment data in accordance with the method described herein, while FIG. 10 shows schematically an apparatus comprising a flatbed scanner 60 connected to a processor 62 suitable for generating alignment data in accordance with the methods described herein.
[0068] The foregoing describes only some embodiments of the present invention, and modifications of detail may be made thereto without departing from the scope of the invention, the embodiments being illustrative and not restrictive.
