JP2013208836A - Method of suppressing density unevenness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate effects of a reading error of an image sensor of a reading means in acquiring a relationship between a pattern density and a duty that is used for duty correction to suppress density unevenness of a pattern.SOLUTION: First, by discharging ink from nozzles with different duties, a first inspection pattern 51 and a second inspection pattern 52 are formed on recording paper P, respectively. Next, the first inspection pattern 51 and the second inspection pattern 52 are read by the same image sensor 62 of a scanner 32, to obtain a density of each of the patterns 51, 52. Then, a density difference between the first inspection pattern 51 and the second inspection pattern 52 that are read by the same image sensor 62 is determined, and a relationship between the density difference and a duty difference is calculated.

Description

  The present invention relates to a method of suppressing pattern density unevenness formed on a recording medium by an inkjet head.

  When an ink jet printer having an ink jet head having a plurality of nozzles is used to record various patterns such as characters and images on a recording medium, density unevenness may occur in the pattern formed on the recording medium. .

  The above-described density unevenness is mainly caused by variations in ejection characteristics (amount and speed of ejected droplets) of a plurality of nozzles. Therefore, conventionally, the density unevenness has been suppressed by adjusting the duty of each nozzle (ink discharge amount with respect to the unit area of the recording medium). More specifically, a test pattern is formed on a recording medium by ejecting ink from all nozzles with a predetermined duty, and density unevenness occurring in the test pattern is detected by a scanner or the like, and then the density unevenness is suppressed. As described above, the duty (ink discharge amount) of each nozzle is corrected. However, in the past, since the determination of how much the duty should be corrected based on the density unevenness of the test pattern has been performed by a human sensory evaluation, there has been a problem that the correction value varies depending on the person performing the evaluation.

  Patent Document 1 discloses a method for correcting the above-described density unevenness. First, five types of belt-like patterns having different nozzle specified gradation values (corresponding to the duty) are printed on a recording medium by an inkjet head having a plurality of nozzles. Next, these five types of belt-like patterns are read with a scanner. From the read image data of the belt-like pattern, the density of each of the five types of belt-like patterns having different designated gradation values is acquired for a partial region (row region) of the pattern formed by one nozzle.

  Here, if there is a variation in ejection characteristics among a plurality of nozzles, a density difference is generated between a plurality of row regions corresponding to the plurality of nozzles even in a band-like pattern having the same designated gradation value. This causes density unevenness. Therefore, the gradation value of each nozzle is corrected with respect to the designated gradation value so that the density is uniform at a certain target value in the same strip pattern for all the row regions. Specifically, first, the relationship between the designated gradation value and the density measurement value is acquired for each of the plurality of row regions. When the density measurement value of a certain row area is deviated from a predetermined target value (for example, the density average value of a plurality of row areas), the relationship between the specified gradation value acquired previously and the density measurement value is obtained. The specified gradation value is corrected so that the density of the row region becomes the target value.

JP 2006-305963 A (in particular, FIGS. 11 to 20)

  In Patent Document 1, five types of belt-like patterns formed on a recording medium are read by a scanner, and a relationship between a specified gradation value and a density measurement value is obtained for each of a plurality of row regions (a plurality of nozzles). doing. However, a certain reading error exists in the image pickup device of the scanner. For this reason, it is conceivable that the relationship between the acquired designated gradation value and the measured density value is deviated from the actual relationship due to the reading error of the image sensor.

  In addition, a general scanner has a plurality of image sensors arranged in one direction, and scans the plurality of image sensors in a direction orthogonal (or intersects) with the arrangement direction, whereby an original (here, a target) is scanned. The pattern formed on the recording medium is read. However, it is also conceivable that reading errors vary among a plurality of image sensors due to differences in characteristics of the elements themselves. In this case, there may be a sufficient problem that the density information of the acquired pattern varies depending on which image sensor has read the pattern.

  As described above, if the relationship between the designated gradation value (duty) and the density measurement value cannot be obtained accurately due to the reading error of the scanner, the gradation value correction accuracy deteriorates, and thus the pattern density unevenness. Cannot be sufficiently suppressed.

  An object of the present invention is to eliminate the influence of reading error of an image sensor of a scanner (reading means) in acquiring the relationship between pattern density and duty used for duty correction for suppressing pattern density unevenness. is there.

The density unevenness suppressing method of the first invention is a method for suppressing pattern density unevenness formed on a recording medium by an inkjet head having a plurality of nozzles,
A first pattern forming step of forming a first pattern on the recording medium by ejecting ink from the nozzles of the inkjet head to the recording medium with a predetermined duty; and from the nozzles of the inkjet head, A second pattern forming step of forming a second pattern on the recording medium by ejecting ink to the recording medium at a duty different from that of the first pattern; and the first pattern formed on the recording medium; A reading step of reading the second pattern with a reading unit having a plurality of image sensors and moving relative to the recording medium to obtain the density of each pattern; and the density of the first pattern Based on the density of the second pattern, a pattern formed on the recording medium by the inkjet head. And calculating the relationship between the density of the toner and the duty specified when forming the pattern, and the pattern formed on the recording medium based on the relationship between the density of the pattern and the duty. A correction step of correcting the duty of the plurality of nozzles so as to reduce density unevenness,
In the reading step, the first pattern and the second pattern are read by the same image sensor of the reading means, and in the calculation step, the density of the first pattern and the second pattern read by the same image sensor. The difference is obtained, and the relationship between the density difference and the duty difference is calculated as the relationship between the density and duty of the pattern.

  In the present invention, the first pattern is formed on the recording medium by ejecting ink from the nozzles of the inkjet head at a predetermined duty, and the second pattern is formed by ejecting ink from the nozzles at a duty different from this. To do. Then, the first pattern and the second pattern are read by the same image pickup device of the reading unit to acquire the density of each pattern. Here, in the first pattern and the second pattern read by the same image sensor, it is considered that the read errors of the image sensor included in the respective density information are almost equal. Therefore, by taking the density difference between the first pattern and the second pattern, the reading error of the image sensor included in the density information of each pattern is canceled, and the relationship between the obtained density difference and the duty difference is The reading error of the image sensor is not included. Therefore, by correcting the nozzle duty based on the relationship between the density difference and the duty difference calculated in this way, it is possible to perform duty correction that eliminates the reading error of the image sensor of the scanner, and is formed on the recording medium. The density unevenness of various patterns can be reliably suppressed.

  In the density unevenness suppressing method of the second invention, in the first invention, the plurality of second patterns having different duties are formed in the second pattern forming step, and the plurality of second patterns are formed in the reading step. Each of the patterns and the first pattern are read by the same image pickup device of the reading unit, and in the calculation step, each of the plurality of second patterns is read by the same image pickup device. The density difference is obtained, and the relationship between the density difference and the duty difference is calculated.

  In the present invention, a plurality of second patterns having different duties are formed in the second pattern forming step. Then, by obtaining a density difference between each of the plurality of second patterns and the first pattern read by the same image sensor, a measurement point for calculating the relationship between the density difference and the duty difference is obtained in the calculation step. Increase. Therefore, the relationship between the density difference and the duty difference can be calculated with higher accuracy.

According to a third aspect of the present invention, there is provided the density unevenness suppressing method according to the first or second aspect, wherein the first pattern is formed on the recording medium by two or more nozzles included in the plurality of nozzles. Forming a pattern, and in the second pattern forming step, the second pattern is formed on the recording medium by two or more nozzles included in the plurality of nozzles;
In the calculating step, the density of the first pattern is set to a density average value of a portion of the first pattern formed by the two or more nozzles, and the density of the second pattern is set to the second pattern. The density average value of the portion formed by each of the two or more nozzles is used.

  In the present invention, the values obtained by averaging the densities of the portions formed by two or more nozzles of the first pattern and the second pattern are adopted as the densities of the respective patterns. As a result, even if the discharge characteristics of some nozzles deviate significantly compared to other nozzles, and there are locations where the density is locally unique, the density at multiple locations in the pattern is averaged. Therefore, the influence on the relationship between the density difference and the duty difference can be suppressed.

  According to a fourth aspect of the present invention, there is provided the density unevenness suppressing method according to any one of the first to third aspects, wherein the first pattern is formed on the recording medium in the first pattern forming step and the second pattern forming step. The second pattern is formed side by side in a predetermined direction, and in the reading step, the reading unit having the plurality of image pickup devices arranged in a direction intersecting the predetermined direction is provided on the recording medium. The first pattern and the second pattern are read by the same image sensor by being relatively moved in a predetermined direction.

  According to the present invention, it is possible to easily read two patterns with the same image sensor by moving a reading unit having a plurality of image sensors in a direction in which the first pattern and the second pattern are arranged.

  According to a fifth aspect of the present invention, there is provided the density unevenness suppressing method according to the fourth aspect, wherein the inkjet head includes the plurality of nozzles arranged along a predetermined arrangement direction, and the recording medium is provided. Ink is ejected from the plurality of nozzles to the recording medium while moving in the head scanning direction intersecting the arrangement direction, and the ink jet head is moved during the single movement in the head scanning direction. By changing the duty, the first pattern and the second pattern are formed side by side in the head scanning direction.

  In the present invention, the first pattern and the second pattern are formed side by side in the head scanning direction of the inkjet head. Further, by changing the duty during the single movement of the inkjet head, the first pattern and the second pattern can be formed on the recording medium at a time by the single movement of the inkjet head.

  According to a sixth aspect of the present invention, there is provided the density unevenness suppressing method according to the fourth aspect, wherein the inkjet head includes the plurality of nozzles arranged along a predetermined arrangement direction, and the recording medium is provided. Ink is discharged from the plurality of nozzles onto the recording medium while moving in the head scanning direction that intersects the arrangement direction, and the first pattern and the second pattern are arranged in the arrangement direction. It is characterized by doing.

  In the present invention, the first pattern and the second pattern are formed side by side in the nozzle arrangement direction.

  According to a seventh aspect of the present invention, there is provided the density unevenness suppressing method according to the sixth aspect, wherein the first nozzle group on the one side in the arrangement direction of the plurality of nozzles and the first nozzle group while moving the inkjet head in the head scanning direction. By ejecting ink with different duty from the second nozzle group on the other side in the transport direction, the first pattern and the first pattern arranged in the arrangement direction can be moved once in the head scanning direction of the inkjet head. Two patterns are formed.

  In the present invention, the first pattern and the second pattern arranged in the nozzle arrangement direction can be formed by one movement of the inkjet head in the head scanning direction.

  According to an eighth aspect of the present invention, in any one of the fourth to seventh aspects, the direction identification mark indicating a direction in which the first pattern and the second pattern are arranged is formed on the recording medium. And a mark forming step.

  The direction in which the first pattern and the second pattern are arranged is a direction in which the reading unit is moved relative to the recording medium in order to read the two patterns with the same image sensor. By attaching a mark indicating this direction to the recording paper, the reading direction is less likely to be mistaken in the subsequent reading process.

  According to a ninth aspect of the present invention, in any one of the first to eighth aspects of the invention, in the calculation step, in the calculation step, the density information of a portion excluding the edge of the first pattern, and the second pattern The relationship between the density difference and the duty is calculated using density information of a portion excluding an edge portion.

  The edge of each pattern is a part adjacent to another other pattern or a region where no pattern is formed, and the conditions such as light reflection are significantly different from the central part of each pattern. Device reading errors are likely to occur. Therefore, it is preferable not to use such edge density information of the pattern in the calculation process.

  In the present invention, the first pattern and the second pattern formed with different duties are read by the same image sensor, and the density difference between the first pattern and the second pattern is taken. As a result, the reading error of the image sensor included in the density information of each pattern is canceled, and the read error of the image sensor is not included in the relationship between the obtained density difference and duty difference. By correcting the duty of the nozzle based on this relationship, it is possible to perform correction without the reading error of the scanner, and density unevenness can be reliably suppressed.

1 is a schematic plan view of an ink jet printer according to an embodiment. FIG. 2 is a schematic block diagram of a printer inspection system including the ink jet printer of FIG. 1 and an inspection apparatus connected thereto. It is process drawing which shows the procedure of the density nonuniformity suppression method. It is explanatory drawing of a test | inspection pattern formation process. It is a top view of a scanner. The density information of the first inspection pattern (duty 100%) and the second inspection pattern (duty 40%) is shown. It is the graph which plotted the data of the density | concentration difference acquired about six 2nd test | inspection patterns, taking the duty difference on the horizontal axis. It is a figure which shows a density nonuniformity detection pattern. (A) is an example of a density unevenness detection pattern in which density unevenness occurs, and (b) is a density distribution of the density unevenness detection pattern of (a). It is explanatory drawing of the test pattern formation process of a change form. It is explanatory drawing of the test | inspection pattern formation process of another modified form. It is explanatory drawing of the test | inspection pattern formation process of another modified form.

  Next, an embodiment of the present invention will be described. FIG. 1 is a schematic plan view of the ink jet printer of the present embodiment. FIG. 2 is a schematic block diagram of a printer inspection system including the ink jet printer of FIG. 1 and an inspection apparatus connected thereto.

(Schematic configuration of inkjet printer)
First, the schematic configuration of the ink jet printer according to the present embodiment will be described with reference to FIGS. In the following, the front side in FIG. 1 is defined as the upper side, and the other side of the page is defined as the lower side, and the explanation will be made using direction words “up” and “down” as appropriate. As shown in FIG. 1, the inkjet printer 1 includes a platen 2, a carriage 3, an inkjet head 4, a transport mechanism 5, a control device 8, and the like.

  On the upper surface of the platen 2, a recording paper P that is a recording medium is placed. The carriage 3 is configured to be capable of reciprocating in the head scanning direction (left-right direction in FIG. 1) along the two guide rails 10 and 11 in a region facing the platen 2. An endless belt 12 is connected to the carriage 3, and when the endless belt 12 is driven to travel by the carriage drive motor 13, the carriage 3 moves in the head scanning direction as the endless belt 12 travels. .

  The inkjet head 4 is attached to the carriage 3 and moves in the head scanning direction together with the carriage 3. Four types of nozzles 14 are formed on the lower surface of the ink-jet head 4 (the surface on the other side of the paper in FIG. 1) for ejecting four colors of ink of black, yellow, cyan, and magenta, respectively. The four types of nozzles 14 are each arranged in the conveyance direction of the recording paper P, which is orthogonal to the head scanning direction. Further, the printer 1 is provided with a holder 9 in which four ink cartridges 15 each storing four colors of ink of black, yellow, cyan and magenta are mounted. Although not shown, the inkjet head 4 mounted on the carriage 3 and the holder 9 are connected by four tubes (not shown), and the four inks of the four ink cartridges 15 are connected to the four tubes. To the inkjet head 4.

  The transport mechanism 5 includes two transport rollers 16 and 17 disposed so as to sandwich the platen 2 in the transport direction. The two transport rollers 16 and 17 are rotationally driven by a transport motor 18 (see FIG. 2). The transport mechanism 5 transports the recording paper P placed on the platen 2 in the transport direction by two transport rollers 16 and 17. In FIG. 1, the head scanning direction and the conveying direction are orthogonal to each other, but these two directions may intersect at an angle different from 90 degrees.

  As shown in FIG. 2, the control device 8 includes a central processing unit (CPU) 20 and a ROM (Read Only Memory) in which various programs and data for controlling the overall operation of the printer 1 are stored. ) 21, a microcomputer including a RAM (Random Access Memory) 22 that temporarily stores data processed by the CPU 20, and various control circuits for controlling each configuration of the printer 1.

  In particular, the control device 8 includes a print control circuit 23 that controls the inkjet head 4, the carriage drive motor 13, the transport motor 18, and the like to perform a printing operation on the recording paper P. The print control circuit 23 controls each part of the ink jet printer 1 based on print data input from an external device connected to the ink jet printer 1, thereby causing the ink jet printer 1 to perform the following printing operation. In other words, the ink jet printer 1 ejects ink from the nozzles 14 of the ink jet head 4 while moving the carriage 3 in the head scanning direction with respect to the recording paper P placed on the platen 2. At the same time, the recording paper P is transported in the transport direction by the two transport rollers 16 and 17. With the above operation, a desired pattern such as an image or a character is printed on the recording paper P.

  The print control circuit 23 controls the ejection timing and ejection amount of each nozzle 14 of the inkjet head 4, and the duty setting that sets the duty of each nozzle 14 based on the print data input from the external device. And a duty correction unit 26 that corrects the duty set by the duty setting unit 25.

  The “duty” is set for each nozzle 14 and is the ratio (%) of the amount of ink ejected from each nozzle 14 toward the unit area of the recording paper P to a certain maximum value. is there. The maximum duty (hereinafter also referred to as 100% duty) is a case where the amount of ink ejected from each nozzle 14 is the maximum. More specifically, the duty is one time at all ejection timings of each nozzle 14. This is a case where the maximum volume of liquid droplets that can be ejected is ejected. When the pattern is formed on the recording paper P with the duty of each nozzle 14 set to the maximum (100%), the pattern density becomes the highest, and the pattern density becomes lower as the duty of each nozzle 14 is set lower. Become. The duty can be lowered by thinning out the discharge of droplets at an appropriate interval to reduce the number of discharges with respect to the 100% duty. That is, in this embodiment, the duty is adjusted to a value lower than the maximum duty by adjusting the discharge thinning rate with respect to the maximum duty.

  The duty correction unit 26 suppresses density unevenness generated in the pattern when a pattern is formed on the recording paper P by ejecting droplets from each nozzle 14 at the duty set by the duty setting unit 25. In addition, the set value of the duty is corrected. This duty correction will be described in detail later.

(Inkjet printer inspection)
As shown in FIG. 2, the inkjet printer 1 is connected to an inspection device 30 before, for example, factory shipment, and various settings and inspections regarding various performances of the printer 1 are performed. The inspection device 30 includes at least a PC 31 and a scanner 32. Various programs including a program for determining a duty correction value, which will be described later, are installed in the PC 31, and various settings and inspections of the printer 1 are performed by executing these programs.

  Hereinafter, among the inspections and settings performed by the inspection apparatus 30, the duty correction of each nozzle 14 for suppressing density unevenness of the pattern formed on the recording paper P by the inkjet head 4 will be described.

  When ink is ejected at the same duty for each of the plurality of nozzles 14 of the inkjet head 4, if there is no variation in the ejection characteristics (the amount of ejected droplets and the velocity of the droplets), recording is performed. The density of the pattern formed on the paper P is uniform and density unevenness does not occur. However, in reality, due to manufacturing tolerances and the like, there are variations in the shape of the nozzle 14 and the ink flow path connected to the nozzle, or variations in the characteristics of the drive elements that apply pressure to the ink in the nozzle 14, and therefore there are a plurality of variations. It is inevitable that the discharge characteristics vary between the nozzles 14. When the discharge amount of the liquid droplets varies between the plurality of nozzles 14, the density of dots formed by the nozzles 14 varies, resulting in density unevenness in the formed pattern. Further, when the droplet speed varies among the plurality of nozzles 14, the landing positions of the droplets vary, so the density of dots on the recording paper P does not become uniform, and density unevenness also occurs.

  Therefore, in the present embodiment, the density unevenness detection pattern is formed on the recording paper P by ejecting each of the plurality of nozzles 14 with the same duty, and the density unevenness of the density unevenness detection pattern is detected to reduce the density unevenness. The duty of each nozzle 14 is corrected. FIG. 3 is a process diagram showing the procedure of the density unevenness suppressing method.

  As shown in FIG. 3, the density unevenness suppression method of the present embodiment is roughly divided into a density-duty relationship acquisition process and a correction process. In the density-duty relationship acquisition step, a density-duty relationship indicating the relationship between the density of the pattern formed on the recording paper P and the duty designated when forming this pattern is acquired. That is, the “density-duty relationship” is information indicating what density the pattern has when the pattern is formed by setting the duty of each nozzle 14 to a predetermined value. In the correction step, the density unevenness detection pattern 70 (see FIG. 8) is formed on the recording paper P, the density unevenness is detected, and the density unevenness occurring in the density unevenness detection pattern 70 is detected using the density-duty relationship. The duty of each nozzle 14 is corrected so that becomes smaller.

[Density-duty relationship acquisition process]
The density-duty relationship acquisition process is further divided into an inspection pattern formation process, a reading process, and a calculation process.

(Inspection pattern formation process)
In the inspection pattern forming step, an inspection pattern 50 for obtaining a density-duty relationship is formed. FIG. 4 is an explanatory diagram of the inspection pattern forming process. In the following description, the inspection pattern 50 is formed by one nozzle row 40 parallel to the transport direction of the inkjet head 4 shown in FIG.

  As shown in FIG. 4, the plurality of nozzles 14 constituting the nozzle row 40 of the inkjet head 4 are divided into a first nozzle group 41a on the upstream side in the transport direction and a second nozzle group 41b on the downstream side in the transport direction. . Then, while moving the inkjet head 4 to the right in FIG. 4, ink is ejected from the first nozzle group 41 a and the second nozzle group 41 b, respectively, so as to extend to the recording paper P in the paper width direction (head scanning direction). The inspection pattern 50 is formed.

  Here, with respect to the plurality of nozzles 14 belonging to the first nozzle group 41a, ink is ejected from all the nozzles 14 with the maximum duty (100%). On the other hand, for the plurality of nozzles 14 belonging to the second nozzle group 41b, the duty is changed stepwise from 100% to 0% while the inkjet head 4 is moving. As a result, the first inspection pattern 51 having a duty of 100% and the duty of 100%, 80%, 60%, 40%, 20%, 0% (ink) by the rightward movement of the inkjet head 4 once. 6 types of second inspection patterns 52a to 52f are formed at a time. The first inspection pattern 51 and the six types of second inspection patterns 52a to 52f are formed side by side in the transport direction (arrangement direction of the nozzles 14).

  The inspection pattern forming process described above corresponds to the first pattern forming process and the second pattern forming process of the present invention. However, among the six second inspection patterns 52a to 52f of the present embodiment, the second inspection pattern 52a at the left end in FIG. 4 has the same duty (100%) as the first inspection pattern 51. This does not correspond to the “second pattern” in the claims of the present application. In addition, since the second inspection pattern 52f at the right end is not actually ejected, this is not strictly a “second pattern” in the claims of the present application. That is, in the present embodiment, the remaining four second inspection patterns 52b to 52e correspond to the “second pattern” in the claims of the present application.

  Further, a direction identification mark indicating a direction in which the first inspection pattern 51 and the second inspection pattern 52 are arranged in a predetermined region of the recording paper P before or after the formation of the first inspection pattern 51 and the second inspection pattern 52. 53 is printed (mark formation step).

(Reading process)
In the reading process, the first inspection pattern 51 and the second inspection pattern 52 formed on the recording paper P are read by the scanner 32 of the inspection apparatus 30 to acquire respective density information.

  FIG. 5 is a plan view of the scanner 32. As shown in FIG. 5, the scanner 32 includes a rectangular document placing table 60 made of transparent glass, and a line sensor 61 (reading unit) disposed on the back side of the document placing table 60 (the other side in FIG. 5). Have The front side of the document table 60 (the front side in FIG. 5) is covered with a document cover (not shown). The line sensor 61 has a plurality of image sensors 62 (CCD or CIS) arranged in the short direction (hereinafter referred to as main scanning direction) of the document placing table 60, and the document placing table 60 orthogonal to the main scanning direction. Can be moved in the longitudinal direction (hereinafter referred to as sub-scanning direction). The scanner 32 moves the line sensor 61 in the sub-scanning direction while irradiating light from a light source (not shown) onto a document (here, recording paper P) placed on the document placing table 60. At this time, the light reflected by the document placed on the document placement table 60 is received by the plurality of image pickup devices 62, thereby reading an inspection pattern such as an image or a character on the document. In FIG. 5, the main scanning direction of the scanner 32 (the arrangement direction of the image sensor 62) and the sub-scanning direction (the moving direction of the line sensor 61) are orthogonal to each other, but these two directions are angles different from 90 degrees. You may cross at.

  In the reading process, the recording paper P on which the test pattern 50 is printed in the test pattern forming process is placed on the document placing table 60. At this time, the direction in which the first inspection pattern 51 and the second inspection pattern 52 are arranged as indicated by the direction identification mark 53 is set to be parallel to the sub-scanning direction. In this state, the line sensor 61 is moved in the sub-scanning direction to perform reading. As a result, each of the six second inspection patterns 52a to 52f and the portions 51a to 51f of the first inspection pattern 51 aligned with the second inspection pattern 52 in the sub-scanning direction are read by the same image sensor 62 to obtain density information. To get. For example, as indicated by a broken line in FIG. 5, a second inspection pattern 52d having a duty of 40% and a portion 51d of the first inspection pattern 51 arranged in the sub-scanning direction are read by the same image sensor 62.

  Then, the image data of the inspection pattern 50 read by the scanner 32 is processed by the PC 31 so that the density information of the first inspection pattern 51 and the second inspection pattern 52 read by the same image sensor 62 with respect to the position in the sub-scanning direction. To get. FIG. 6 is a graph showing the densities of the first inspection pattern 51 and the second inspection pattern 52d (duty 40%). As apparent from FIG. 6, the density of the second inspection pattern 52d having a duty of 40% is lower than that of the first inspection pattern 51 having a duty of 100%. The density information as shown in FIG. 6 is acquired for each of the six second inspection patterns 52a to 52f. Of course, among the six second inspection patterns 52a to 52f, the higher the duty, the smaller the density difference from the first inspection pattern 51, and the lower the duty, the larger the density difference from the first inspection pattern 51. .

(Calculation process)
In the calculation step, the PC 31 calculates a general relationship between the density and the duty for the pattern formed on the recording paper P based on the density information of the first inspection pattern 51 and the density information of the second inspection pattern 52. To do. More specifically, for each of the six second inspection patterns 52a to 52f, the density difference between the portions 51a to 51f of the first inspection pattern 51 read by the same image sensor 62 is obtained, and the density difference and the duty are determined. Calculate the relationship with the difference.

  In obtaining the density difference, first, the six second inspection patterns 52a to 52f and the first inspection pattern 51 read by the same image sensor 62 as each of the six second inspection patterns 52a to 52f are used. For the six portions 51a to 51f, representative values of the respective densities are obtained. For example, referring to the example of FIG. 6, the average value C1d in the sub-scanning direction of the density of the first inspection pattern 51 and the average value C2d in the sub-scanning direction of the density of the second inspection pattern 52d are obtained. The “average density value in the sub-scanning direction” is expressed as a density average value for a plurality of strip portions respectively formed by the plurality of nozzles 14 constituting the first nozzle group 41a or the second nozzle group 41b. You can also Even if the ejection characteristics of some of the nozzles 14 are significantly different from those of the other nozzles 14 and there are local locations where the density is unusual, a plurality of locations in the test pattern 51 (52). By averaging the density of each, the influence on the relationship between the density difference and the duty difference can be suppressed.

  Then, for each of the six second inspection patterns 52a to 52f, a density difference (C1d−C2d in FIG. 6) from the first inspection pattern 51 is calculated. FIG. 7 shows a graph obtained by linearly interpolating the obtained density difference data for the six second inspection patterns 52a to 52f by plotting the duty difference on the horizontal axis. 7A to 7F show density differences (measurement points) with respect to the first inspection pattern 51 for each of the six second inspection patterns 52a to 52f. Further, the “duty difference” in FIG. 7 is how much the duty is reduced with respect to the duty (100%) of the first inspection pattern 51 to form the second inspection pattern 52. Further, as described above, in this embodiment, the duty is adjusted by thinning out the number of ejections. Therefore, after all, the “duty difference” can also be referred to as a discharge (dot) thinning rate with respect to 100% duty.

  Here, even if the density difference is not taken as described above, it is possible to derive the relationship between the pattern density and the duty from only the density information of the six second inspection patterns 52a to 52f having different duties. However, the density information acquired by the scanner 32 includes a reading error of the image sensor 62. Furthermore, the reading error differs for each image sensor 62 due to the difference in characteristics of the plurality of image sensors 62. Accordingly, the density information of the six second inspection patterns 52a to 52f read by the different image pickup devices 62 includes different reading errors, and it can be said that the relationship between the density and the duty can be accurately grasped. hard.

  In this regard, in the present embodiment, each of the six second inspection patterns 52a to 52f and the six portions 51a to 51f of the first inspection pattern 51 are read by the same image sensor 62 of the scanner 32. Then, for each of the six second inspection patterns 52a to 52f, the density information of the respective patterns 51 and 52 is obtained by taking the density difference from the portion of the first inspection pattern 51 read by the same image sensor 62. The read error of the included image sensor 62 is canceled out. For this reason, the relationship between the obtained density difference and duty difference does not include the reading error of the image sensor 62.

  Note that each of the first inspection pattern 51 and the six second inspection patterns 52a to 52f shown in FIG. 4 (edges in the main scanning direction and the sub-scanning direction, that is, the patterns 51, 52a to 52f). The outer periphery) is a portion adjacent to a region where no pattern is formed or another pattern. At such an edge, the reading conditions of the image sensor 62 (for example, the conditions for reflecting light from the light source) are significantly different from those at the center of each pattern, and the reading error of the image sensor 62 tends to increase. Therefore, in the calculation step, the relationship between the density difference and the duty is calculated using the density information of the first inspection pattern 51 and the portions other than the respective edges of the six second inspection patterns 52a to 52f. Is preferred.

[Correction process]
Next, using the relationship between the density difference and the duty difference acquired as described above, the duty of each nozzle 14 is corrected so that the density unevenness of the pattern actually formed on the recording paper P is reduced. . As shown in FIG. 3, the correction process is divided into a density unevenness detection pattern forming process, a density unevenness detection process, and a duty correction process.

(Density unevenness detection pattern formation process)
FIG. 8 is a diagram for explaining a density unevenness detection pattern. In the density unevenness detection pattern forming step, ink is ejected from the plurality of nozzles 14 at a predetermined reference duty (here, 100% duty) while moving the inkjet head 4 in the scanning direction, and the density on the recording paper P is determined. The unevenness detection pattern 70 is formed.

(Density unevenness detection process)
In the density unevenness detection step, the density unevenness detection pattern 70 formed on the recording paper P as described above is read by the scanner 32. Then, the density of the plurality of strip-shaped pattern portions 71 formed by each of the plurality of nozzles 14 of the density unevenness detection pattern 70 is individually detected. 8 is placed on the document placing table 60 of the scanner 32 so that the vertical direction of FIG. 8 is the sub-scanning direction of the scanner 32 of FIG. 5 (the moving direction of the line sensor 61). The pattern portion 71 is read by the same image sensor 62.

  If the discharge characteristics of the plurality of nozzles 14 are uniform, the density unevenness detection pattern 70 has a uniform density as shown in FIG. 8, but the discharge characteristics of the plurality of nozzles 14 vary. In the density unevenness detection pattern 70, density unevenness as shown in FIG. In the example of FIG. 9A, since the discharge characteristics are different between the nozzle 14 located on the center side of the nozzle row 40 and the nozzle 14 located on the end side, the density of the pattern portion 71 at the center in the transport direction is It is lower than the density of the pattern portion 71 on both ends. Therefore, the density of the plurality of pattern portions of the density unevenness detection pattern 70 in FIG. 9A has a distribution as shown in FIG.

(Duty correction process)
In order not to cause density unevenness as shown in FIG. 9A, the duty of each nozzle 14 is set to a reference duty (here, 100% duty) so that the density difference in FIG. Adjust it. In the duty correction step, the density distribution of the plurality of pattern portions 71 shown in FIG. 9B and the relationship between the density difference and the duty difference shown in FIG. Determine the duty correction value.

  Specifically, in FIG. 9B, the duty of each nozzle 14 is set to a reference duty (here, the density of all the pattern portions 71 approaches the density of the pattern portion 71 located at the central portion where the density is the lowest). Then lower than 100%). For example, the density difference shown in FIG. 9B exists between the lower pattern portion 71 in FIG. 9A and the central pattern portion 71 having the lowest density. In the previous density unevenness detection step, since the plurality of pattern portions 71 are read by the same image sensor 62, even if each reading error is included in the density information of each pattern portion 71, By taking the density difference between the pattern portions 71, reading errors are eliminated.

  Then, processing is performed by the PC 31 with reference to the relationship of FIG. 7 so as to eliminate the density difference, and it is determined how much the duty of the nozzle 14 corresponding to the pattern portion 71 at the lower end should be reduced. In other words, the discharge thinning rate for the 100% duty of the nozzle 14 corresponding to the pattern portion 71 at the lower end is determined. By performing this for all the nozzles 14, the duty correction value of each nozzle 14 is determined when 100% duty is set during printing.

  Although the above description is about duty correction value determination in the case of 100% duty, various values of duty are set for each nozzle 14 in accordance with print data during actual printing. Therefore, the duty correction of each nozzle 14 when a duty other than 100% is set can be performed as follows. For example, by performing the above correction process for other duty other than 100%, a plurality of reference duties (for example, 5 of 20%, 40%, 60%, 80%, 100%) are obtained for each nozzle 14. A correction value for each type may be acquired. Or you may calculate the correction value with respect to another duty based on the correction value of 100% duty.

  The duty correction amount of each nozzle 14 determined in this duty correction step is set in the duty correction unit 26 in the control device 8 shown in FIG. Then, the control device 8 (printing control circuit 23) determines the final duty by correcting the duty of each nozzle 14 determined based on the print data in the duty setting unit 25 by the duty correction unit 26. To do. The head controller 24 controls the inkjet head 4 according to the determined duty, and prints a desired pattern according to the print data on the recording paper P.

  In the present embodiment described above, each of the six second inspection patterns 52 a to 52 f and the first inspection pattern 51 are read by the same image sensor 62 of the scanner 32. Then, for each of the six second inspection patterns 52a to 52f, by taking the density difference from the portions 51a to 51f of the first inspection pattern 51 read by the same image sensor 62, each of the patterns 51 and 52 is obtained. The reading error of the image sensor 62 included in the density information is canceled out. For this reason, the relationship between the obtained density difference and duty difference does not include the reading error of the image sensor 62. Therefore, by correcting the duty so as to reduce the density unevenness based on this relationship, the correction that eliminates the reading error of the scanner 32 can be performed, and the density unevenness can be reliably suppressed.

  In the inspection pattern forming step, the first inspection pattern 51 and the second inspection pattern 52 are formed side by side in the transport direction (arrangement direction of the nozzles 14), and the line sensor 61 of the scanner 32 is arranged in the direction in which the two patterns 51 and 52 are aligned. By moving the first inspection pattern 51 and the second inspection pattern 52, the image sensor 62 can easily read the first inspection pattern 51 and the second inspection pattern 52. In addition, the direction identification mark 53 indicating the direction in which the first inspection pattern 51 and the second inspection pattern 52 are arranged is printed on the recording paper P, so that the recording placed on the document placement table 60 in a later reading step. The direction of the paper P, that is, the reading direction of the scanner 32 is not easily mistaken. Accordingly, the first inspection pattern 51 and the second inspection pattern 52 can be reliably read by the same image sensor 62.

  Next, modified embodiments in which various modifications are made to the embodiment will be described. However, components having the same configuration as in the above embodiment are given the same reference numerals and description thereof is omitted as appropriate.

1] For the adjustment (decrease) of the duty of each nozzle 14, in addition to the method of thinning out the number of discharges described in the above embodiment, a method of reducing the amount of liquid droplets discharged in one discharge can be adopted. It is also possible to adjust the duty by using both methods together, such as reducing the droplet volume while thinning out the number of ejections.

2] In the above embodiment, as shown in FIG. 4, one nozzle row 40 is divided into two nozzle groups 41 a and 41 b, and two nozzle groups 41 a and 41 b are moved during one movement of the inkjet head 4. Thus, the first inspection pattern 51 and the second inspection pattern 52 are formed at a time. On the other hand, the first inspection pattern 51 and the second inspection pattern 52 may be formed by moving the inkjet head 4 twice. That is, as shown in FIG. 10, when the inkjet head 4 moves to the right in the drawing, the first test pattern 51 is formed by one nozzle row 40, and the inkjet head 4 moves to the left in the drawing. At this time, the six second test patterns 52a to 52f may be formed while changing the duty in the middle by the nozzle row 40 of one row.

3] It is not always necessary to form a plurality of second inspection patterns 52, and only one second inspection pattern 52 may be formed with a duty different from that of the first inspection pattern 51, as shown in FIG. In this case, it seems that only one density difference data for calculating the relationship between the density difference and the duty difference (see FIG. 7) is acquired. However, in practice, when the density difference is 0, the duty difference should be 0. Therefore, the function indicating the relationship between the density difference and the duty difference always passes through the origin of FIG. Therefore, even if the measurement data is one point, the relationship between the density difference and the duty difference (see FIG. 7) can be calculated by drawing a straight line connecting this point and the origin. However, it goes without saying that it is better to form a plurality of second inspection patterns 52 having different duties from the viewpoint of accurately calculating the relationship between the density difference and the duty difference.

4] The direction in which the first inspection pattern 51 and the second inspection pattern 52 having different duties are arranged is not limited to a specific direction. For example, as shown in FIG. 12, the first inspection pattern 51 (duty: 100%) is obtained by changing the duty of each nozzle 14 during the single movement of the inkjet head 4 to the right in the drawing. ) And five second inspection patterns 52b to 52f (duty: 80%, 60%, 40%, 20%, 0%) having different duties from the first inspection pattern 51 may be formed side by side in the scanning direction. it can. In this case, as indicated by the direction identification mark 53 in FIG. 12, the first inspection pattern is obtained by setting the sub-scanning direction (movement direction of the line sensor 61) of the scanner 32 in the reading process to the left-right direction in FIG. 51 and five second inspection patterns 52b to 52f are read by the same image sensor 62.

  As can be seen from the form of FIG. 12, in the present invention, the direction in which the first inspection pattern 51 and the second inspection pattern 52 are arranged is not particularly a problem, and two patterns 51 and 52 having different duties are included. A major feature of the present invention is that the line sensor 61 is moved in the direction of alignment and the first inspection pattern 51 and the second inspection pattern 52 are read by the same image sensor 62.

5] In the embodiment, as shown in FIG. 6, after acquiring density information for the positions of the first inspection pattern 51 and the second inspection pattern 52 in the sub-scanning direction (movement direction of the line sensor 61), the first inspection pattern 51 is obtained. One representative value (average value) is calculated for each of the second and second inspection patterns 52, and one relationship between the density difference and the duty difference shown in FIG. 7 is calculated from the representative density value.

  On the other hand, the relationship between the density difference and the duty difference as shown in FIG. 7 may be calculated for each of the plurality of nozzles 14. For example, in the form of FIG. 10 and FIG. 12 described above, both the first inspection pattern 51 and the second inspection pattern 52 are formed by all the nozzles 14 in one nozzle row 40. That is, one nozzle 14 forms a part of the first inspection pattern 51 and a part of the second inspection pattern 52. Therefore, a part of the first inspection pattern 51 and a part of the second inspection pattern 52 formed by a certain nozzle 14 are respectively read by the same image sensor 62 and the density difference between the two is obtained for each nozzle 14. It is possible to calculate the relationship with the duty difference. In this case, when correcting the duty of each nozzle 14 so as to reduce the density difference in FIG. 9B in the density unevenness detection pattern 70 in the subsequent correction step, the density difference−duty calculated for each nozzle 14. A difference relationship can be used, and correction can be performed with higher accuracy than using the relationship between the density difference and the duty difference calculated from the density representative value.

6] The first inspection pattern 51 and the second inspection pattern 52 do not need to be formed on the same recording paper P, and may be formed separately on the two recording papers P. In this case, in the reading process, in order to read the first inspection pattern 51 and the second inspection pattern 52 by the same image sensor 62 of the scanner 32, the following may be performed. For example, when two sheets of recording paper P are read at a time, two sheets on the document placing table 60 are arranged so that the first inspection pattern 51 and the second inspection pattern 52 are aligned in the sub-scanning direction of the scanner 32. The recording sheets P may be placed side by side. Further, when reading the two recording papers P separately, the first inspection pattern 51 and the second inspection pattern 52 are read at a predetermined position on the document placing table 60 so that they can be read by the same image sensor 62. Reading is performed after the recording paper P is accurately positioned.

7] In the above embodiment, after calculating the relationship between the density difference and the duty difference in the calculation step, the density unevenness detection pattern 70 is printed again on another recording sheet P. However, the density unevenness detection pattern 70 is printed. Even without this, it is possible to detect density unevenness and correct the duty. For example, in FIG. 4 of the embodiment, one second inspection pattern 52a among the six second inspection patterns 52a to 52f is formed with the same duty (100%) as that of the first inspection pattern 51. As a result, the left end portion of the inspection pattern 50 in the drawing is a pattern formed by a single nozzle row 40 with 100% duty. If this pattern is used in place of the density unevenness detection pattern 70 of FIG. 8, it is not necessary to print the density unevenness detection pattern 70 of FIG.

8] The scanner 32 of the above-described embodiment is a so-called flatbed type scanner that moves the line sensor 61 and reads a document with respect to the document placed on the document placing table 60. And a scanner of a type that moves the document relative to the line sensor whose position is fixed.

9] The ink jet head 4 of the above embodiment is a so-called serial type ink jet head that is mounted on the carriage 3 and reciprocates in the scanning direction. It may be a so-called line type ink jet head that ejects ink in a wet state.

DESCRIPTION OF SYMBOLS 1 Inkjet printer 4 Inkjet head 14 Nozzle 32 Scanner 41a 1st nozzle group 41b 2nd nozzle group 50 1st test pattern 52 2nd test pattern 53 Direction identification mark 61 Line sensor 62 Image sensor

Claims (9)

A method for suppressing pattern density unevenness formed on a recording medium by an inkjet head having a plurality of nozzles,
A first pattern forming step of forming a first pattern on the recording medium by ejecting ink from the nozzles of the inkjet head to the recording medium with a predetermined duty;
A second pattern forming step of forming a second pattern on the recording medium by ejecting ink from the nozzles of the inkjet head to the recording medium at a duty different from that of the first pattern;
The first pattern and the second pattern formed on the recording medium are read by reading means that has a plurality of image sensors and moves relative to the recording medium, and determines the density of each pattern. A reading process to obtain;
Based on the density of the first pattern and the density of the second pattern, the relationship between the density of the pattern formed on the recording medium by the inkjet head and the duty specified when forming the pattern is calculated. A calculation process to perform,
A correction step of correcting the duty of the plurality of nozzles so as to reduce the density unevenness of the pattern formed on the recording medium based on the relationship between the density of the pattern and the duty;
In the reading step, the first pattern and the second pattern are read by the same image sensor of the reading unit,
In the calculation step, a density difference between the first pattern and the second pattern read by the same image sensor is obtained, and a relationship between the density difference and the duty difference is calculated as a relationship between the density and duty of the pattern. A density unevenness suppressing method characterized by the above. In the second pattern forming step, a plurality of the second patterns having different duties are formed,
In the reading step, each of the plurality of second patterns and the first pattern are read by the same image sensor of the reading unit,
In the calculating step, for each of the plurality of second patterns, obtaining a density difference from the first pattern read by the same image sensor, and calculating a relationship between the density difference and the duty difference. The density unevenness suppressing method according to claim 1, wherein the density unevenness is suppressed. In the first pattern forming step, the first pattern is formed on the recording medium by two or more nozzles included in the plurality of nozzles,
In the second pattern forming step, the second pattern is formed on the recording medium by two or more nozzles included in the plurality of nozzles,
In the calculation step,
The density of the first pattern is an average density value of portions of the first pattern formed by the two or more nozzles,
3. The density unevenness suppression method according to claim 1, wherein the density of the second pattern is an average density value of portions of the second pattern formed by the two or more nozzles. 4. In the first pattern forming step and the second pattern forming step, the first pattern and the second pattern are arranged in a predetermined direction on the recording medium,
In the reading step, the first reading unit having the plurality of image pickup devices arranged in a direction intersecting the predetermined direction is moved relative to the recording medium in the predetermined direction, whereby the first The density unevenness suppressing method according to claim 1, wherein the pattern and the second pattern are read by the same image sensor. The inkjet head includes the plurality of nozzles arranged along a predetermined arrangement direction, and the plurality of nozzles while moving in a head scanning direction intersecting the arrangement direction with respect to the recording medium. Each ejects ink onto the recording medium from
The first pattern and the second pattern are formed side by side in the head scanning direction by changing the duty during one movement of the inkjet head in the head scanning direction. 4. The method for suppressing density unevenness according to 4. The inkjet head includes the plurality of nozzles arranged along a predetermined arrangement direction, and the plurality of nozzles while moving in a head scanning direction intersecting the arrangement direction with respect to the recording medium. Each ejects ink onto the recording medium from
The density unevenness suppressing method according to claim 4, wherein the first pattern and the second pattern are formed side by side in the arrangement direction.   While moving the inkjet head in the head scanning direction, ink is discharged from the first nozzle group on one side in the arrangement direction and the second nozzle group on the other side in the transport direction among the plurality of nozzles at different duties. The density unevenness according to claim 6, wherein the first pattern and the second pattern arranged in the arrangement direction are formed by ejecting the ink jet head in a single movement in the head scanning direction. Suppression method.   8. The density according to claim 4, further comprising a mark forming step of forming a direction identification mark indicating a direction in which the first pattern and the second pattern are arranged on the recording medium. Unevenness suppression method.   In the calculation step, the relationship between the density difference and the duty difference is calculated using density information of a portion excluding an edge of the first pattern and density information of a portion excluding an edge of the second pattern. The density unevenness suppression method according to claim 1, wherein: