BACKGROUND

[0001] Some inkjet printers are printers that sweep a carriage back and forth across the media as prhitheads mounted in the carriage deposited printing fluids onto the media. Many printers use multiple passes of the printhead to print each swath. The media is advanced after each swath of the ima ge is printed onto the media. After all the swaths are printed the media is ejected from the printer. Printing fluid is any fluid deposited onto media to create an image, for example a pre-conditioner, gloss, a curing agent, colored inks, grey ink, black ink, metallic ink and the like.

BRIEF DESCRIPTION OF THE DRA WINGS

[0002] FIG. 1 is a side view of an example printer 100.

[0003] FIG. 2 is a sectional top view of the example printer 100.

[0004] FIG. 3 is an example alignment block printed with the prmtheads traveling in a first direction along the scan axis.

[0005] FIG. 4 is an example image printed with the printhead depositing printing fluid while traveling in both directions,

[0006] FIG. 5 is an electrical block diagram of an example printer 500.

[0007] FIG. 6 is an example flow chart for aligning drops from a printhead while the printhead is traveling in two different directions.

DETAILED DESCRIPTION

I Some inkjet printers sweep a carriage back and forth across the media as pnntheads mounted in the carriage deposit printing fluids onto the media, The carriage sweeps along a scan axis over stationary media, Some printers only deposit printing fluid when the prmtheads are traveling in one direction across the width of the media. Other printers deposit printing fluid while the printheads are traveling in both directions across the media. Depending on the print mode used, scanning Inkjet printers may print only a subset of the total amount of ink for a swath during a single pass of the printh ead over the media, for example ¼ of the total ink in each pass. Therefore it typically takes multiple passes of the printhead across the media to complete each swath in the image to be printed,

[0009] Printers that deposit printing fluids while the printheads are traveling in both directions need to align the drops deposited while the printheads are traveling in the first direction with the drops deposited while the printheads are traveling in the second direction, Printers typically align the drops by printing alignment blocks while the printheads are traveling in the two different directions. The alignment blocks are typically multiple sets or columns of drops printed with a predetermined number of nozzles, The leading and trailing edges of each set of drops in the alignment block are located, typically with a sensor mounted in the carriage. The location of the center of the drops may be calculated, for example, as the average location between the leading and trai ling edges of the drops.

[0010] The printer uses the l ocation of the center of the drops in the two alignment blocks to adjust the location of drops deposited in the two different directions, The printer adjusts the position of the drops by changing the timing for when the nozzles in the printhead are fired. For example the printer may advance or delay the firing of the nozzles while the printhead is moving in one direction compared to the firing of the nozzles if no alignment was used.

[0011] When a printer fires a nozzle in a printhead to deposit a drop of printing fluid from the printhead, a main drop ejected from a nozzle may comprise a tail, The tail may comprise some smaller satellite drops and/or some spray (spray may also be known as aerosol). The main drop, being larger than the drops/spray in the tail typically has a higher drop velocity than the tail. Because the main drop fails faster (i.e. has a higher drop velocity), the main drop reaches the media first followed by the drops or spray in the tail. Because the printhead is traveling across the width of the page as the printhead deposits printing fluid onto the media, the main drop may end up at a different location than the tail (see figure 3). [0012] Because the measurement of the trailing edge of the alignment block includes the tail, the center of the drop is offset from the center of the main drop.

Therefore, when the average location between the leading edge and the trailing edge of the drops are used to align the drops from the two different printhead directions the main drops from the two different printhead directions will not be aligned in the scan axis.

[0013] in one example, an Inkjet printer will align the drops deposited while the printheads are traveling in the first direction with the drops deposited while the printheads are traveling in the second direction using the location of the center of the main drop. The location of the center of the main drop will be determined using the location of the leading edge of the main drop and the diameter of the main drop. The diameter of the main drop may be determined by measuring the height of the alignment block and using the known spacing between the top and bottom nozzles used to print the alignment block. By using the leading edge and the main drop diameter, the effects of the tails in the alignment blocks may be eliminated. Therefore the main drops from the two different printhead directions will be aligned in the scan axis (see figure 4)

[0014] Figure 1 is a side view of an example printer 100. Printer 100 comprises a media source 102, a pair of pinch rollers 104, a pair of take-up rollers 106, a print engine 108, a sensor 1 10 and media 114. A media path runs from the media source 102, between the pair of pinch roll ers 104, underneath the print engine 108 and between the pair of take-up rollers 106, Media 1 14 is shown in the media path , During printing the media 1 14 travels along its length in a printing direction as shown by arrow 122. A print zone 1 16 is underneath the print engine 108, The print zone is defined as the location where printing fluid from the print engine is deposited onto the media 1 14. Printing fluid is any liquid that is deposited by the print engine and can comprise black ink, colored inks, gloss, pre-treatment fluids, finishing fluids and the like.

[0015] Printer 100 is shown with media fed from a roll, in other examples the printer may have sheets of media fed from an input tray. In yet another example, the printer may be a 3D printer and the media may be a support platform on which a layer of a powdered build material has been formed. [0016] In this example, sensor 110 is located on the print engine 110. In other examples the sensor may be located in a different position, for example adjacent to the print engine. Sensor 1 10 can be any type of sensor that detects light intensity, for example a charged coupled device (CCD). In this example, sensor 110 is a spot sensor. In other examples the sensor may be a line sensor that can image the full width of a swath with one pass. Media 114 has a first side 118 and a second side 120. The first side 118 of the media is facing the print engine 108. Sensor is located above the first side 1 18 of the media 114. In this example sensor includes a light source. In other examples, the light source may be separate from sensor 110. In this example, sensor 110 is mounted on the downstream side of print engine 108, Therefore sensor 110 can detect printing fluid deposited on the media as the media is advanced after each swath is printed.

[0017] Figure 2 is a sectional top view of the example printer 100. In this example print engine 108 is configured as a carriage mounted on guide rail 232. The carriage travels back and forth across the width W of the media 114 along a scan axis as shown by arro w 234. In some examples the width of the media may be between 60 and 180 inches wide (1524 to 4572 mm wide), for example 130 inches (3,302 mm) wide. In other examples the width of the media may be smaller or larger. The print engine 108 may also comprise motors, drive belts or gears, additional guide rails, linear or angular position sensors and the like, but these items are not shown for clarity.

[0018] Priniheads (230 A - D) mounted in the carriage deposit printing fluids onto the first side 1 18 (see figure 1) of media 1 14 as the carriage travels across the width of the media 1 14, In this example 4 priniheads (230 A - D) are shown mounted in the carriage. The carriage has a mounting system that allows the priniheads (230 A - D) to be removably mounted onto the print engine. The priniheads (230 A - D) are typically user loadable/replaceable. In some examples, the printheads may be shipped to the end user in a separate package than the printer.

[001 ] Each printhead may deposit one or more different printing fluids, for example printhead 230 A. may deposit yellow ink and magenta ink. In one example the printer may use a 4 color ink system, for example cyan ink, yellow ink, magenta ink and black ink (CYMK). In other examples the printer may use a higher or lower number of ink colors, for example 6 different ink color. In other examples there may be more or fewer prmtheads mounted in the carriage. When printing an image the media 114 is advanced in the printing direction 122 after each swath of the image is printed.

[0020] Sensor 1 10 is mounted in the carriage and sweeps back and forth across the media with the carriage. The media 114 travels underneath the sensor 110. in some examples the sensor 1 10 can he used when the prmtheads are depositing printing fluids onto the media 1 14 and when the carriage is sweeping across the media when the printheads are not depositing printing fluids onto the media 114. By sweeping the carriage across the full width of the media and incrementally advancing the media 114 in the printing direction, the sensor 110 can scan all areas of the media.

[0021] Printer 100 deposits printing fluid onto the media while the carriage is moving in both directions. Therefore printer 100 aligns the printheads by executing an alignment procedure (the alignment procedure may also be known as a calibration procedure or step). The alignment procedure causes the printer to print an alignment block while the printheads are traveling in the two different directions. The alignment blocks are typically multiple sets or columns of drops printed with a predetermined number of nozzles. The leading edge and the height of each set of drops in the alignment block are located using sensor 110. In some examples the leading edge is located at multiple positions along the column of drops, for example 5 locations.

[0022] Figure 3 is an example alignment block printed with the printheads traveling in a first direction along the scan axis, in this example the printhead direction is show by arrow 338 (the printhead is traveling horizontally from the left side of the figure towards the right side of the figure while the alignment block is printed).

[0023] The alignment block comprises N columns of drops, with each column comprising M number of drops. Each drop may comprise a main drop 332 and a tail 334. The tail may comprise one or more smaller satellite drops and/or some spray. Because the main drop is larger in size than the tail, it has a faster drop rate and reaches the media more quickly than the tail, This causes the tail to be displaced from the main drop in the direction of travel of the printhead. When an alignment block is printed with the printhead traveling in the opposite direction from arrow 338, the tails will be located on the opposite side of the main drop along the scan axis. [0024] The location of the leading edge of the drops in column N is indicated by line 336B. The trailing edge of the drops in column N is indicated by line 340. The average location between the leading edge and the trailing edge of the drops in column N in the scan axis is indicated by line 342, When the average location between the leading edge and the trailing edge of the drops are used to align the drops from the two different printhead directions (i.e. line 342) the main drops from the two different printhead directions will not be aligned in the scan axis.

[0025] The location of the leading edge of the drops in column I is indicated by line 336A. The spacing between nozzles in a printhead is tightly controlled. The spacing between the nozzles used to print column I is S. The spacing between the nozzle that printed the top drop in column 1 (drop 1) and the nozzle that printed the bottom drop in column 1 (drop M) is distance H2, Distance H2 will be known as the total nozzle distance for a column of drops in an alignment block. The total nozzle distance H2 is equal to the spacing between nozzles S and the number of nozzles M used to print a column of drops (H2 = S * M).

[0026] The height of the drops in column 1 is H I . Height HI is equal to the total nozzle distance H2 plus the diameter of the main drop D (HI = H2 + D). Therefore the diameter of the main drop D is equal to the height of the drops HI minus the total nozzle distance H2 (D = HI - H2), The location of the center of the main drops (indicated by line 339) in the carriage axis of motion can be determined using the location of the leading edge of the drops (indicated by line 336A) and one half the diameter of the main drop D. When the locations of the center of the main drops in the carriage axis of motion are used to align the drops from the two different printhead directions (i.e. line 339) the main drops from the two different printhead directions will be aligned in the scan axis (see figure 4).

[0027] n this example, the printer has a resolution of 600 dots per inch (DPI).

Therefore the spacing between nozzles is 1/600 of an inch in each row of nozzles on the printhead . In some examples, a single die in the printhead may ha ve 2 rows of nozzles for each color. The spacing between nozzles in each row may be 1/600 of an inch. But the first row may be offset from the second row by ½ the nozzle spacing, for example 1/1200 of an inch. Therefore the spacing between the nozzles on the two rows is double the spacing on a single row. This allows a printer to have a resolution of 1200 DPI using two rows of nozzles with a spacing of 1/600 of an inch.

[0028] To align the printheads for printing with the printheads traveling in both directions, an alignment procedure may be periodically executed by the printer. The alignment procedure prints an alignment block while the printhead is traveling in each direction. The printer uses the location of the center of the main drops in the two alignment blocks to adjust the location of drop deposited in the two different directions. The printer adjusts the position of the drops by changing the timing for when the nozzles in the printhead are fired.

[0029] Figure 4 is an example image printed with the printhead depositing printing fluid while traveling in both directions. In this example the printheads were aligned using the location of the center of the main drop in the alignment blocks. The solid or black drops were printed with the printhead traveling along the scan direction in the direction shown by arrow 338. The lighter shade drops were printed with the printhead traveling along the scan axis in the direction shown by arrow 450. Main drop 332B was printed with the printhead traveling to the right and main drop 452B was printed with the printhead traveling to the left. As can be seen the center of main drop 332B and 452B are aligned in the carriage axis of motion fi. e. the scan axis).

[0030] In one example, the alignment procedure may be executed for each printhead mounted in the carriage, in other examples, the alignment procedure may be executed for only one of the printheads mounted in the carriage, for example the printhead depositing black ink. The offset determined for the one printhead would be used to adjust all the printheads in the carriage.

[0031] The size of the drop formed on media may vary dependent on the type of media. The drop size may vary due to the difference in the absorption rate of the printing fluid into the media, the surface tension between the printing fluid and the media and the like. In some examples, the alignment procedure may be executed for different types of media. The offsets for that media type may be stored in printer memory and re-used each time that type of media is loaded. [0032] Figure 5 is an electrical block diagram of an example printer 500. Printer comprises a controller 560, memory 562, input/output (I/O) module 564 and a print engine 566 ail coupled together on bus 568. In some examples printer may also have a user interface module, an input device, and the like, but these objects are not shown for clarity. Controller 560 comprises at least one processor 570. The processor 570 may comprise a central processing unit (CPU), a micro-processor, an application specific integrated circuit (ASIC), or a combination of these devices. Memory 562 may comprise volatile memory, non-volatile memory, and a storage device. Memory 562 is a non- transitory computer readable medium. Examples of non-volatile memory include, but are not limited to, electrically erasable programmable read only memory (EEPROM) and read only memory (ROM), Examples of volatile memory include, but are not limited to, static random access memory (SRAM), and dynamic random access memory (DRAM). Examples of storage devices include, but are not limited to, hard disk drives, compact disc drives, digital versatile disc drives, optical drives, and flash memory devices.

[0033] I/O module 564 is used to couple printer to other devices, for example the

Internet or a computer. Printer has computer executable code, typically called firmware 572, stored in the memory. The firmware 572 is stored as computer readable instructions in the non-transitory computer readable medium (i.e. the memory 562). The processor generally retrieves and executes the instructions stored in the non-transitory computer- readable medium to operate the printer and to execute functions. In one example, processor executes code that aligns the drops from a prmthead when traveling in two different directions.

[0034] Firmware 572 contains an alignment module 574. The processor executes the code in the alignment module 574 to align drops from a printhead when traveling in two different directions. The alignment module may use the method shown in figure 6 to align the drops from a printhead when traveling in two different directions.

[0035] Figure 6 is an example flow chart for aligning drops from a printhead while the printhead is traveling in two different directions. At box 682 the printer prints a first set of drops of printing fluid from a printhead mounted in a carriage as the carriage is traveling in a first direction along a scan axis. At bo 684 the printer prints a second set of drops of printing fluid from the printhead as the carriage is traveling in a second, opposite direction along the scan axis. At box 686 the location of a leading edge and a height H I of the first and second set of drops is detected, At 688 the location of the center of a main drop in the first and second set of drops is determined. At box 690 the printer aligns the drops of printing fluid deposited when the carriage is traveling in the first direction along the scan axis with the drops of printing fluid deposited when the carriage is traveling in the second direction along the scan axis based on the locations of the center of the main drops.