Determining alignment of a printhead

Abstract

A method of determining a difference in height between a current position and a calibrated position of a printhead of a printer is described. An alignment value for the printhead is then determined based on the difference.

Description

Technical Field

[0001] This disclosure generally relates to determining the alignment of the printhead. Background Technology

[0002] Misalignment of the printer's printhead can affect the quality of the printed image. For example, in bidirectional printing, droplets of printing fluid are excited from the printhead during both the forward and reverse strokes. Therefore, misalignment of the printhead can cause a mismatch between the droplets excited during the forward stroke and those excited during the reverse stroke. Summary of the Invention

[0003] According to a first aspect of this disclosure, a method for printing is provided, comprising: determining a height difference between a current position and a calibrated position of a printhead of a printer; and determining an alignment value of the printhead based on the difference, wherein the alignment value is determined for each printing fluid excited by the printhead based on behavioral differences of each printing fluid.

[0004] According to a second aspect of this disclosure, a printer is provided, comprising: a printhead having nozzles through which droplets of different printing fluids can be aroused; a processor; and a storage medium storing instructions executed by the processor, the instructions, when executed by the processor, causing the processor to: determine an alignment value for each printing fluid aroused by the printhead based on a height difference between a current position and a calibration position of the printhead and behavioral differences of each printing fluid; and generate a signal at a time defined by the alignment value to arouse the nozzles of the printhead.

[0005] According to a third aspect of this disclosure, a non-transitory storage medium is provided, the non-transitory storage medium storing instructions that, when executed by a printer's processor, cause the processor to: determine an alignment value based on the height difference between the current position and a calibration position of the printer's printhead, wherein the alignment value is determined for each type of print fluid excited by the printhead based on the behavioral differences of each type of print fluid; and generate a signal at a time defined by the alignment value to excite the nozzles of the printhead. Attached Figure Description

[0006] Figure 1 A sample printer is shown;

[0007] Figure 2 The trajectories of droplets excited from the nozzles of the printhead during forward and reverse strokes are shown, with the printhead in (a) a calibration position and (b) an elevated position.

[0008] Figure 3 An example method for determining printhead alignment values ​​is shown; and

[0009] Figure 4 The display shows how the printhead alignment value changes with height. Detailed Implementation

[0010] Figure 1 An example printer 10 is shown, which includes a tray assembly 20 and a control unit 30.

[0011] The carriage assembly 20 includes a plurality of printheads 21 carried by a carriage 22. A drive assembly (not shown) moves the carriage assembly 20 along the scan axis in response to a drive signal from a control unit 30.

[0012] Each printhead 21 includes multiple dies 23, each die including multiple nozzles through which droplets of printing fluid are agitated. The printhead 21 is fluidly coupled to a fluid reservoir (not shown), which supplies printing fluid to the printhead. Each printing fluid can be a colorant or a non-colorant, such as a pretreatment (e.g., fixer / optimizer) or post-treatment (e.g., overcoat) fluid. In the illustrated example, printer 10 includes five printheads 21 that provide eight different printing fluids: cyan (C), magenta (M), yellow (Y), black (K), light cyan (c), light magenta (m), optimizer (OP), and overcoat (OC).

[0013] The control unit 30 includes a processor 31, a storage medium 32, and an input / output interface 33. The processor 31 controls the operation of the printer 30 and executes an instruction set 35 stored in the storage medium 32. The instruction set 35 includes instructions that, when executed by the processor 31, implement the printing module 36 and the alignment module 37. In addition to the instruction set 35, the storage medium 32 stores multiple calibration alignment values ​​38.

[0014] In response to print data 40 received at input / output interface 33, print module 36 generates an excitation signal that causes the nozzles of printhead 21 to be activated. When the nozzles of printhead 21 are activated, the trajectory of the resulting print fluid droplets has a velocity component parallel to the scan axis, i.e., the carriage assembly 20 moves back and forth on the print media along this axis. This velocity component occurs because when the nozzles are activated, printhead 21 is not stationary but moves along the scan axis. Therefore, there is a difference between the nozzle activation position and the position where the resulting print fluid droplets impact the print media. In bidirectional printing, if not corrected, this difference can lead to misalignment between droplets activated during the forward stroke and droplets activated during the reverse stroke.

[0015] Therefore, the printing module 36 applies an offset to the position or time at which the nozzles of the printhead 21 are aroused, such that the aroused droplets are aligned during both the forward and reverse strokes. The offset may be applied only to either the forward or reverse stroke. For example, during the forward stroke, the nozzle may be aroused when the printhead is at position x, while during the reverse stroke, the nozzle may be aroused when the printhead is at position x+Dx (where Dx is the offset). Alternatively, the offset may be applied partly during the forward stroke and partly during the reverse stroke. Thus, for example, during the forward stroke, the nozzle may be aroused when the printhead is at position x-Dx / 2, while during the reverse stroke, the nozzle may be aroused when the printhead is at position x+Dx / 2.

[0016] Storage medium 32 stores a calibration alignment value 38 for each printhead. The printing module then uses the calibration alignment value to define the offset of the corresponding printhead. In one example, the alignment value may be between 0 and 20. The default value for each calibration alignment value may be 10, which, for a nominal printer, allows for alignment during bidirectional printing. However, tolerances in the printer (such as the speed tolerance of the carriage assembly 20, or the positional tolerance of the printhead 21 within the carriage 22) may mean that the printhead is not perfectly aligned when using an alignment value of 10.

[0017] Therefore, printer 10 can be calibrated to determine a calibration alignment value 38 for each printhead 21. Calibration may include printing a test pattern onto a printing medium and then determining the calibration alignment value 38 for the printhead 21 based on the alignment of features within the test pattern. In one example, the test pattern may include pairs of lines for each printhead. Each pair of lines is printed using a different alignment value. Thus, for example, the test pattern may include 21 pairs of lines for each printhead, each pair corresponding to an alignment value between 0 and 20. For each pair of lines, one line is printed during the forward stroke and the other line is printed during the reverse stroke of the printhead. The calibration alignment value for the printhead can then be determined by identifying the pair of lines that appear to be most closely aligned.

[0018] During subsequent use of printer 10, the height of printhead 21 (i.e., the gap or distance between printhead and printing media) can be changed. For example, printhead 21 can be raised to avoid collisions with relatively thick or potentially deformable printing media during printing. In another example, printhead 21 can be raised to prevent collisions with printer accessories (e.g., edge supports that hold the edges of the printing media in place to prevent the edges from rising during printing).

[0019] Figure 2 The trajectory 50 of droplets ejected from nozzles at different heights of printhead 21 is shown. Figure 2In (a), printhead 21 is in the calibration position, i.e., at the calibrated height. The nozzles of printhead 21 are activated at a time defined by the calibration alignment value. Therefore, droplets activated during the forward stroke of printhead 21 are aligned with droplets activated during the reverse stroke, i.e., droplets impact the printing medium 51 at the same location. Figure 2 In (b), the position of printhead 21 has been raised. Again, the nozzles of printhead 21 are excited at the time defined by the calibration alignment value. However, the droplets now travel further, resulting in a longer flight time. As previously mentioned, the droplets have a velocity component parallel to the scan axis. Due to the longer flight time, the difference between the position where the droplets are excited and the position where they impact the printing medium increases. Therefore, the droplets excited during the forward stroke of printhead 21 are no longer aligned with the droplets excited during the reverse stroke.

[0020] from Figure 2 It is evident that when the height of printhead 21 differs from the calibrated height, printhead 21 may become misaligned, potentially compromising the quality of the printed image. To mitigate this, alignment module 37 determines the alignment value for each printhead 21.

[0021] Figure 3 An example method that can be implemented by alignment module 37 is shown.

[0022] Method 100 includes determining the height difference between the current position of the printhead 110 and its calibrated position (i.e., the position where the printhead is calibrated). In one example, printer 10 may include a sensor for detecting the printhead position. In another example, adjustment means for adjusting the printhead position may include an instrument or other means to indicate the current position or a change in position of the printhead. In a further example, the height of the printhead may be set to one of a discrete number of positions, such as low, normal, and high. The user can then input the current position or the change in position through a user interface (not shown).

[0023] Calibration may involve calibrating printhead 21 at a specific location. Alternatively, printhead 21 may be calibrated at any location. In this case, the calibration location of printhead 21 may be stored in storage medium 32 along with calibration alignment value 38. Method 100 can then use the stored calibration location to determine height differences.

[0024] Method 100 also includes determining an alignment value for each printhead 21 based on the difference. The alignment value is then used by the printing module 36 to define the timing at which the nozzles of the printhead 21 are aroused during subsequent printing.

[0025] Determining the alignment value for each printhead 21 may include calculating a correction, and then applying the correction to the calibration alignment value 38 of that printhead.

[0026] The correction varies as a function of the height difference between the current printhead position and the calibration position. When the difference is zero, the correction is zero. Therefore, when printhead 21 is in the calibration position, the alignment value corresponds to the calibration alignment value 38 of that printhead. The correction may have the same or opposite sign as the height difference, depending on how the alignment value is used to define the time when the nozzle is aroused. For example, when the alignment value is zero, print module 36 may apply an offset to the position or time when the nozzle is aroused. The alignment value can then be used to increase or decrease the offset. Thus, for example, when the alignment value is zero, the offset may have a minimum value, and the alignment value can be used to increase the offset. Alternatively, when the alignment value is zero, the offset may have a maximum value, and the alignment value can be used to decrease the offset. Regardless of how print module 36 uses the alignment value, the magnitude of the correction increases with the increase of the difference. Therefore, in response to a larger height difference between the current position and the calibration position, a larger correction is applied to the calibration alignment value.

[0027] For a specific type of printer, the applicant has studied the behavior of alignment values ​​in relation to printhead height. This study involved calibrating printheads at various heights, i.e., printing test patterns at each height, and then determining the alignment value for each printhead. This process was then repeated for three different sample printers.

[0028] Figure 4 The alignment value of one of the printheads studied is shown as a function of printhead height. The alignment value variations for each of the three sample printers are shown, along with the best-fit line. From... Figure 4 It is clear that for this specific type of printer, the alignment value changes linearly with the printhead height. Although Figure 4 This only describes the behavior of one printhead, but other printheads have been found to have similar behavior.

[0029] Therefore, the alignment value A for each printhead can be defined as: A = A cal +m*Δh, where A cal Here, m is the calibration alignment value, m is the scaling factor, and Δh is the height difference between the current position and the calibration position. Therefore, the correction (m*Δh) is the product of the difference (Δh) and the scaling factor (m). The scaling factor corresponds to the gradient of the best-fit line (i.e., linear interpolation). Figure 4 In this example, it is -7.10 units / mm. The negative scaling factor occurs because, in this particular example, the alignment value is used to reduce the offset applied to the excitation nozzle. Therefore, as the printhead height increases, the alignment value decreases, and a larger offset is applied to the excitation signal.

[0030] During the use of printer 10, the speed of carriage assembly 20 and the speed at which printhead 21 moves on the printing medium can be varied. For example, printer 10 can have different printing modes with different carriage speeds. As an example, printer 10 can have "fast," "normal," and "optimal" printing modes with carriage speeds of 60, 50, and 40 ips (inches per second), respectively.

[0031] The trajectory of droplets ejected from the nozzle depends not only on the printhead height but also on the printhead speed, i.e., the carriage speed. For example, as the printhead speed increases, the velocity component of the droplets in the direction parallel to the scan axis increases. As a result, the difference between the location where the droplets are ejected and the location where they impact the printing medium increases. The printing module 36 therefore applies an offset to the nozzle ejection, which is defined by the alignment value and the carriage speed. Furthermore, the applicant has found that, at least for the printer under investigation, the alignment value depends not only on the printhead height but also on the printhead speed. Therefore, method 100 can determine the alignment value for each of the 120 printheads, which depends on the difference between the printhead height and speed.

[0032] As referenced above Figure 4 As pointed out, at least for the printers studied, the alignment value was found to vary linearly with the printhead height. Therefore, the alignment value A can be defined as: A = A cal +m*Δh, where A cal Here, is the calibration value, m is the scaling factor, and Δh is the height difference between the current position and the calibration position. The applicant found that at different carriage speeds, the alignment value continues to change linearly with the printhead height. However, the scaling factor (i.e., the gradient of the linear interpolation line) is different for different carriage speeds. Therefore, the alignment value can be defined as: A = A ref +m(s)*Δh, where m is a scaling factor that depends on the printhead speed s. Therefore, it is applied to the calibration alignment value (A cal The correction (m(s)*Δh) varies as a function of the printhead height difference (Δh) and speed (s). Furthermore, at least for the printer studied, the correction can be defined as the product of the height difference (Δh) and the scaling factor (m), where the scaling factor (m) depends on the printhead speed (s).

[0033] Storage medium 32 can store scaling factors for each different printhead speed (i.e., each different carriage speed), and alignment module 37 can select a scaling factor based on the current speed of printhead 21. This provides a relatively simple method for calculating the scaling factor as a function of speed, especially for printers where the carriage speed is one of a set of discrete values ​​(e.g., 40, 50, and 60 ips). However, alignment module 37 can determine the scaling factor in other ways. For example, the alignment module can calculate the scaling factor based on speed using mathematical functions or equations.

[0034] Using the method described above, printhead alignment can be maintained at different heights without the need for printhead recalibration. Therefore, the printhead can be moved while maintaining image quality without any downtime during printing. Furthermore, it saves on printing materials (such as printing media and printing fluid) used for recalibration.

[0035] For the printer under study, the observed alignment value varies linearly with printhead height. Therefore, the correction applied to calibrate the alignment value can be defined as the product of the height difference and a scaling factor. For other types of printers, the function describing the relationship between the alignment value and printhead height may be non-linear. Therefore, in a more general sense, the correction can be considered a function of the height difference, which can be expressed as an arbitrary non-zero order polynomial.

[0036] In the example method described above, a different correction is applied to the calibration alignment value for each printhead. While the calibration alignment value of one printhead may differ significantly from that of another, the variation in alignment value with printhead height may be small between different printheads. Therefore, a single, universal correction can be applied to each calibration alignment value instead of calculating a correction unique to each printhead. For example, for the printer under study, when printing at a carriage speed of 55 ips, the printhead scaling factor (i.e., ...) is... Figure 4 The gradient of the interpolation line was found to be in the range of -6.24 to -7.90 units / mm. Therefore, instead of calculating a correction unique to each printhead, it is better to calculate a single, universal correction based on a scaling factor (e.g., -7.07 units / mm).

[0037] Figure 1 Printers 10 have printheads 21 that are responsible for arousing different types of printing fluids. For example, each printhead responsible for arousing the toner includes a left wick that arouses a first printing fluid (e.g., light magenta, yellow, and cyan) and a right wick that arouses a second printing fluid (e.g., light cyan, magenta, and black). Because different printing fluids behave differently, each printing fluid may use different calibration alignment values ​​and / or corrections. Therefore, the alignment module 37 can determine multiple alignment values ​​for a single printhead.

[0038] Tolerances in the die 23 of printhead 21 can cause slight misalignment of the droplets. Therefore, each die can have a different calibration alignment value. For dies of printhead 21 that excite the same printing fluid, the same correction can be applied to each calibration alignment value.

[0039] Although the printer 10 described above includes multiple printheads 21, the example method described above can also be applied to printers with a single printhead.

[0040] The foregoing description is for illustrative and descriptive purposes of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in accordance with the foregoing teachings. It should be understood that any feature described in any example may be used alone, in combination with other described features, in combination with any feature of any other example, or in any combination of any other example.

Claims

1. A method for printing, comprising: determining a difference in height between a current position and a calibration position of a printhead of a printer; and determining an alignment value for the printhead based on the difference, wherein the alignment value is determined for each print fluid ejected by the printhead based on a behavior difference of each print fluid and a tolerance in a die of the printhead.

2. The method of claim 1, wherein, Determining the alignment value comprises calculating a correction based on the difference and applying the correction to a calibration alignment value.

3. The method of claim 2, wherein, The correction is calculated as a function of the difference.

4. The method of claim 2, wherein, The correction is calculated as a function of a speed of the printhead.

5. The method of claim 2, wherein, The correction is calculated as a product of the difference and a scaling factor, and the scaling factor depends on a speed of the printhead.

6. The method of claim 5, wherein, The method comprises storing a plurality of scaling factors for a plurality of different speeds, and selecting a scaling factor based on the speed of the printhead.

7. The method of claim 1, wherein, The method comprises determining alignment values for a plurality of printheads based on the difference.

8. The method of claim 7, wherein, The method comprises storing a calibration alignment value for each of the printheads, and determining the alignment value comprises calculating a correction for each of the printheads based on the difference and applying the correction to the respective calibration alignment value.

9. The method of claim 1, wherein, The method comprises ejecting a nozzle of the printhead at a time defined by the alignment value.

10. The method of claim 2, wherein, The method comprises calibrating the printhead at the calibration position, and calibrating the printhead comprises printing a test pattern onto a print medium and determining the calibration alignment value based on an alignment of features within the test pattern.

11. A printer, comprising: a printhead having nozzles through which droplets of different print fluids can be ejected; a processor; and a storage medium storing instructions for execution by the processor, which when executed by the processor cause the processor to: determine an alignment value for each print fluid ejected by the printhead based on a difference in height between a current position and a calibration position of the printhead, a behavior difference of each print fluid, and a tolerance in a die of the printhead; and generate a signal to eject a nozzle of the printhead at a time defined by the alignment value. The storage medium stores a calibration alignment value, and the instructions, when executed by the processor, cause the processor to calculate a correction based on the difference and apply the correction to the calibration alignment value to determine the alignment value.

12. The printer of claim 11, wherein, The printer comprises a plurality of printheads, and the instructions, when executed by the processor, cause the processor to determine an alignment value for each of the printheads based on the difference and generate a signal to eject a nozzle of each of the printheads at a time defined by the respective alignment value.

13. The printer of claim 11, wherein, 14. A non-transitory storage medium storing instructions which, when executed by a processor of a printer, cause the processor to: determine an alignment value for each print fluid ejected by the printhead based on a behavior difference of each print fluid and a tolerance in a die of the printhead; and determining an alignment value based on a difference in height between a current position of a printhead of the printer and a calibrated position, wherein generate a signal to eject a nozzle of the printhead at a time defined by the alignment value. ​

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