BACKGROUND

[0001] An inkjet printer is a non-impact printing device for forming characters and other images by ejecting print fluid, such as ink drops, in a controllable way from a printhead onto a print medium.

[0002] The printhead may eject ink through multiple nozzles in the form of droplets which "fly" a small distance and strike a print medium. Different printheads may be used for different colours. Inkjet printers may usually print within a range of 180 to 2400 or more dots per inch. The ink drops dry on the print medium soon after being deposited to form the printed image.

[0003] There are several types of inkjet printheads including, for example, thermal printheads and piezoelectric printheads. Thermal inkjet printing is based on accurate ballistic delivery of small ink droplets to exact locations onto the paper or other print medium. One factor for sharp and high quality images stems from the accuracy of the droplet placement. Droplet placement inaccuracy may result in line discontinuity and roughness, as well as banding and colour inconsistencies.

[0004] A print liquid may be broadly understood to mean a fluid in liquid form that is amenable to controlled ejection from an inkjet printhead. The liquid may be referred to as a printing liquid or a print fluid, which in some cases is an ink and in other cases may be non-marking fluids such as, e.g. fixers or optimizers. The medium may be any type of suitable medium for receiving the ejected liquid, including sheet or roll material, such as paper, card stock, cloth or other fabric, transparencies, mylar, among others.

BRIEF DESCRIPTION OF DRAWINGS

[0005] Non-limiting examples will now be described with reference to the accompanying drawings, in which:

[0006] Figure 1 is a flowchart of a method according to some examples;

[0007] Figure 2 is another flowchart of a method according to some examples; [0008] Figures 3a and 3b are simplified schematics of a device according to some examples; [0009] Figure 4 is a further simplified schematics of a device according to some examples;

[0010] Figure 5 is another flowchart of a method according to some examples; [0011] Figure 6 is a simplified example of print fluid deposition in forward and reverse directions;

[0012] Figure 7 is a simplified example of inaccurate print fluid deposition; [0013] Figure 8 is a graph showing an example of changes in pen to platen distance along a scan axis of a printer carriage;

[0014] Figure 9 is a graph showing an example of bidirectional adjustment values along a scan axis of a printer carriage;

[0015] Figure 10 is a graph showing an example of bidirectional adjustment values for different colours along a scan axis of a printer carriage; and [0016] Figure 11 is a graph showing an example of bidirectional adjustment values for different colours, adjusted according to some examples, along a scan axis of a printer carriage.

DETAILED DESCRIPTION

[0017] Print systems may undergo an alignment calibration in order to reduce dot placement errors that may come from varying printhead position mechanical tolerances and delays, for example caused by printing at different carriage speeds. In most instances it is possible to break down and calibrate different attributes such as pen to pen distance in paper axis, pen to pen distance in scan axis, pen rotations, bidirectional deviations, etc.

[0018] The pen to paper spacing (PPS) or pen to platen spacing, sometimes referred to as pen to platen rib spacing or pen to reference spacing (PRS), may influence bidirectional deviations. One reference point may be a rib. “rib” may refer to the surface of the platen. Bidirectional deviations may be inaccuracies in the actual print fluid deposition location relative to the intended print fluid deposition location, in particular due to the bidirectional movement (forward and reverse) of the carriage. According to some examples, there is provided a way of mitigating such deviations.

[0019] In accordance with some examples, as described herein, and shown in Figure 1, there is provided a printer alignment calibration method. The printer alignment calibration method may be suitable for use with a bidirectional printer. Bidirectional may include a printer having a moveable printer carriage that moves in a forward and a reverse movement over a print medium along a scan axis. The method may comprise measuring S101 first deviations between images printed during a forward and a reverse scanning pass at a plurality of positions along a scan axis, for a first colour. The method may further comprise measuring S102 a second deviation between the images printed during the forward and the reverse scanning pass at a selected position of the plurality of positions along the scan axis, for a second colour. The method may further comprise determining S103 a difference between the first deviation and the second deviation for the selected position. The method may further comprise calculating S104 a representative deviation for the first colour, based on the first deviations. The method may further comprise using S105 the representative deviation for the first colour to calibrate printer alignment for the first colour or, in other words, calibrating the printer alignment for the first colour using the representative deviation. This may comprise adjusting (delaying or advancing) the firing timing of print fluid droplets for the first colour to counteract the deviations between the intended droplet deposit location and the actual droplet deposit location. The method may further comprise using S106 the representative deviation for the first colour and the difference between the first deviation and the second deviation for the selected position to calibrate printer alignment for the second colour. The first deviation, measured in relation to the same position, along the scan axis, as the second deviation may give an appropriate adjustment to the representative deviation for the second colour. In this way, reduction of the droplet deposit location deviations for the second colour may be achieved and image quality of the printed image may be improved.

[0020] The images may be any suitable images, such as alignment test patterns or simple vertical lines (lines along the page axis, perpendicular to the scan axis) spaced at a predetermined distance apart. First deviations may be measured at a number of positions along the scan axis. Measuring the first deviations may for example involve printing a test pattern, including parts of the pattern printed on the forward printing pass and parts of the pattern printed on the reverse printing pass. The first colour may for example be black, but can be any colour available on the printer. The test pattern, once printed, is then scanned to determine whether the pattern is accurate and, in particular, to establish whether there are any bidirectional deviations present. In an example, bidirectional deviations may be detected by measuring distances between the print fluid droplet deposit locations, of droplets deposited in the forward and the reverse directions, and comparing these distances to expected distances based on the intended droplet deposit locations. Any inaccuracies in the scan axis direction may be deemed to be bidirectional deviations. Such inaccuracies are then measured at various points along the scan axis to give the values for the first deviations.

[0021] The second deviation may be measured in the same manner as the first deviations, but may relate to a different colour to the first colour. In some examples, first deviations may relate to a first group of nozzles and the second deviation may relate to a second group of nozzles, different to the first group of nozzles. The difference between first deviations and second deviations may result from the location of the relevant nozzle groups within the carriage. Different nozzle groups may have different pen to platen spacing or different locations within the carriage relative to the scan axis of the page axis (the direction of movement of the print medium). However, once the difference in the relative locations of the nozzle groups is accounted for, the deviations along the scan axis may be very similar. Therefore, one second deviation may be used to determine the differences between the nozzle groups or colours for the purpose of printer alignment calibration.

[0022] The plurality of positions may be any number of two or more and may in particular be approximately 1 - 10 positions per inch along the scan axis. The representative deviation may be a single value to simplify deviation correction. Many printers do not have the ability to adjust for different deviation amounts along the scan axis. Therefore, taking a representative deviation value presents a suitable option to improve printed image quality.

[0023] In some examples, the representative deviation may be an average or a median of the first deviations. Further representative values, based on the measured first deviations, such as statistically representative values, may also be used as the representative deviation.

[0024] Using a representative deviation, such as the average or the median value of the first deviation values allows the effects of bidirectional deviation to be reduced and extrapolating that representative deviation to other colours avoids the need for multiple deviation values to be measure along the scan axis for each colour. This, in turn, save print fluid, print medium, processing capacity and time. [0025] Calibrating the printer alignment may comprise adjusting a firing timing on the reverse pass of the printer carriage. For example, the representative deviation may be determined and applied to the reverse printing pass, so that the droplet deposit locations for the droplets deposited during the reverse pass are either delayed or advanced by a time difference corresponding to the representative deviation. In another example, the firing timing may be adjusted for the forward pass or both the forward and the reverse pass, to achieve an improved image quality.

[0026] When calculating the representative deviation for the first colour, outlier first deviations may be excluded. Outliers may be calculated in a number of ways. In the simplest options, approximately 10% of first deviation values with the largest deviation in either direction along the scan axis may be excluded from the calculation. Smaller or larger amounts than 10% may be used. In an alternative option, the standard deviation of the first deviation values may be assessed and any individual values significantly affecting the standard deviation may be excluded.

[0027] In some examples, as shown in Figure 2, the method may further comprise storing S207 the difference between the first deviation and the second deviation for the selected position in a memory. Features S201 - S206 correspond to features S101 - S106, respectively, of Figure 1.

[0028] To simplify subsequent printer alignment calibration, difference between deviations for different colours may be stored for later use, particularly when the same type of set up is used, as the differences between the deviations may not be expected to change significantly. [0029] The method may further comprise recalibrating S208 the printer alignment using measured first deviations and the stored differences, as shown in Figure 2.

[0030] In accordance with some examples, as described herein, and shown in Figures 3a and 3b, there is provided a printer alignment calibration device 10. The printer alignment calibration device 10 may comprise a sensor 100 to measure first deviations between images printed during a forward and a reverse scanning pass at a plurality of positions along a scan axis for a first colour and to measure a second deviation between the images printed during a forward and a reverse scanning pass at a selected position of the plurality of positions along the scan axis for a second colour. The printer alignment calibration device 10 may further comprise a processor 110 to determine a difference between the first deviation and the second deviation for the selected position and to calculate a representative deviation for the first colour, based on the first deviations. The printer alignment calibration device may further include a calibration module 120a, 120b to calibrate printer alignment settings for the first colour, using the representative deviation, and for the second colour, using the representative deviation and the difference between the first deviation and the second deviation for the selected position.

[0031] Figure 3a shows a first arrangement in which the calibration module 120a is separate from the processor 110. In another arrangement, shown in Figure 3b, the calibration module 120b may be part of or located in the processor 110. [0032] The printer alignment calibration device 10 may be a single piece of hardware or may include multiple units, and/or may be arranged as part of a printer or as a peripheral device either attached to or in communication with a printer. In some examples, the printer alignment calibration device 10 may be arranged on the printer carriage. In another example, the sensor 100 may be arranged on the printer carriage and the processor 110 and calibration module 120a, 120b may be located separately to the sensor 100.

[0033] In some examples, the processor 110 may exclude outlier first deviations, when calculating the representative deviation for the first colour. [0034] In some examples, as shown in Figure 4, the printer alignment calibration device may further comprise a memory 130 to store the difference between the first deviation and the second deviation for the selected position. The memory 130 may be any suitable storage device. The calibration module may further recalibrate the printer alignment, during a subsequent calibration, using re-measured first deviations and the stored differences between the first deviation and the second deviation for the selected position.

[0035] The representative deviation may for example be the average or the median of the first deviations. The calibration module may further adjust a firing timing on the reverse pass of the printer carriage, when calibrating printer alignment. In another example, the firing timing may be adjusted for the forward pass or both the forward and the reverse pass, to achieve an improved image quality.

[0036] In accordance with some examples, as described herein, and shown in Figure 5, there is provided a printer alignment calibration method. The printer alignment calibration method may comprise measuring S501 first deviations between images printed during a forward and a reverse scanning pass at a plurality of positions along a scan axis for a first colour. The method may further comprise determining S502 a representative deviation for the first colour, based on the first deviations. The method may further comprise accessing S503 stored deviation differences between the first colour and a second colour for a selected position of the plurality of positions. The method may further comprise calibrating S504 printer alignment for the first colour using the representative deviation. The method may further comprise calibrating S505 printer alignment for the second colour using the representative deviation and an accessed deviation difference. [0037] The representative deviation may be the average or the median of the first deviations. In some examples, when determining the representative deviation, the method may comprise excluding outlier first deviations.

[0038] In further detail, as shown in Figure 6, drop trajectories may be controlled by factors including printhead carriage speed and drop velocity. Due to the factor of carriage speed, the landing position of a drop fired with the same nozzle in the forward and reverse direction is going to be different. In order to ensure that a drop fired in the forward direction falls in the same place as a drop fired in the reverse direction, the firing time in the reverse direction is different to the firing time in the forward direction. In Figure 6, the distance X corresponds to either the pen to platen or pen to paper distance. Figure 6 shows a simplification of the drop trajectory as a straight line. In particular, through simple calculation, the deviations measured, relating to forward and reverse printing passes for an image, may be corrected by adjusting the firing time for the print fluid deposition. To improve accuracy, factors such as carriage movement speed, drop velocity and pen to platen spacing or pen to paper spacing may be determined or measured. Flowever, appropriate adjustment may be achieved based on the measured deviation without further factors being taken into account.

[0039] Figure 7 shows an example of a deviation occurring between where the print fluid droplet, fired by a forward moving carriage, and a droplet, fired by a reverse moving carriage, are deposited on the print medium. It should be noted that the droplets in respective directions may not be intended to be deposited at exactly the same location, but this example is used for demonstration purposes here. For example, if a printer alignment calibration is set up as shown in Figure 6, accurate drop deposition may be achieved. If then the pen to platen spacing, distance X, is increased, as shown in Figure 7, the same printer may no longer be appropriately aligned and may need recalibration as set out in the examples. [0040] Figure 8 shows a graph with an example of a pen to platen distance, labelled X, profile along the scan axis in inches. The pen to platen distance profile may correspond to the measured deviations, in that the pen to platen distance may have an effect on print fluid droplet placement. As shown in Figure 8, the pen to platen distance may vary over the length of the scan axis.

[0041] Figure 9 shows example bidirectional corrections at different points along the scan axis. Flowever, most printers cannot correct bidirectional deviations by applying different values at different points of the scan axis. Instead, a single value may be measured at one position and used. This value may be taken at a deviation peak or at a point with no deviation. Therefore, the correction may not be appropriate at all points along the scan axis. If one colour is adjusted at a peak deviation point, the misalignment in some parts of the platen could exceed I OOmGP. A threshold used in high quality printers may be for example around 40pm.

[0042] In some examples, there is provided a method that allows an average correction value to be used for a reference colour, for example black, and each other colour may then be corrected without having to repeat the deviation measurements along the whole scan axis for all colours. Further calibration factors may be taken into account when measuring the first deviations, such as different carriage speeds, pen to platen or pen to paper spacing, and/or different drop weights. Therefore, measurements for a single colour are taken and the appropriate adjustments may be extrapolated to the other colours.

[0043] As shown in Figure 10, a graph is presented showing example measured deviation correction values that should be applied at a number of positions along the scan axis, with each separate line representing a different colour or printhead nozzle. A deviation may simply be a mismatch between the intended or programmed print fluid deposit location and the actual print fluid deposit location. In an example, when printing an image to measure deviation, the first colour may be used in a test pattern extending along the length of the scan axis. The second colour, and any further colours, may include a test pattern at one point or a portion of the length of the scan axis, in order to measure the second deviation value. As shown in Figure 10, the correction value profiles, which correspond to the measured deviations, correlate closely for each colour, and also correlate closely with the pen to platen or pen to paper profiles. Therefore, a suitable approximate or representative single value to use for each colour may be to use the average value or the median value of the bidirectional corrections measured. In Figure 10, example selected locations are marked (by larger points) along the profiles for each colour. As mentioned above, the selected position may be any position along the scan axis, among the positions where first deviations are measured.

[0044] In Figure 11 , the lines representing respective colours have been adjusted based on the difference between the first deviation and the second deviation for a selected position along the scan axis. As shown, the profiles of all colours are relatively similar, meaning that the first deviations measured for the first colour may be used for all other colours, when adjusted in this way. Therefore, accurate correction of bidirectional deviations may be achieved with reduced processing, reduced print fluid use and reduced print medium waste. [0045] According to the examples described above, it is possible to improve image quality of a printer image, in particular by reducing grain in the image and improving line quality. Further, the examples proposed above represent significant time saving over calibration of all colours across the full scan axis length.

[0046] Examples in the present disclosure can be provided as methods, systems or machine readable instructions, such as any combination of software, hardware, firmware or the like. Such machine readable instructions may be included on a computer readable storage medium (including but is not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon.

[0047] The present disclosure is described with reference to flow charts and/or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. It shall be understood that each flow and/or block in the flow charts and/or block diagrams, as well as combinations of the flows and/or diagrams in the flow charts and/or block diagrams can be realized by machine readable instructions.

[0048] The machine readable instructions may, for example, be executed by a general purpose computer, a special purpose computer, an embedded processor or processors of other programmable data processing devices to realize the functions described in the description and diagrams. In particular, a processor or processing apparatus may execute the machine readable instructions. Thus, functional modules of the apparatus and devices may be implemented by a processor executing machine readable instructions stored in a memory, or a processor operating in accordance with instructions embedded in logic circuitry. The term ‘processor’ is to be interpreted broadly to include a CPU, processing unit, processing module ASIC, logic unit, or programmable gate array etc. The methods and functional modules may all be performed by a single processor or divided amongst several processors.

[0049] Such machine readable instructions may also be stored in a computer readable storage that can guide the computer or other programmable data processing devices to operate in a specific mode.

[0050] Such machine readable instructions may also be loaded onto a computer or other programmable data processing devices, so that the computer or other programmable data processing devices perform a series of operations to produce computer-implemented processing, thus the instructions executed on the computer or other programmable devices realize functions specified by flow(s) in the flow charts and/or block(s) in the block diagrams.

[0051] Further, the teachings herein may be implemented in the form of a computer software product, the computer software product being stored in a storage medium and comprising a plurality of instructions for making a computer device implement the methods recited in the examples of the present disclosure. [0052] While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions may be made without departing from the scope of the present disclosure. It is intended, therefore, that the methods, devices and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above-mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims.

[0053] The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single unit may fulfil the functions of several units recited in the claims.

[0054] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.