JP2013215920A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of correcting irregular density without causing lowering of contrast.SOLUTION: An image forming apparatus includes: a print head constituted of a plurality of nozzles; an adjustment chart acquiring part for acquiring an adjustment chart showing maximum ejection performance of the nozzles; a maximum density calculation part for obtaining maximum density characteristic at maximum ejection time for each nozzle by using the adjustment chart; a correction pattern generation part for generating a correction pattern to decrease the irregular density; a parameter setting part for setting a control parameter for correction to control a maximum ejection amount of each nozzle based on the maximum density characteristic and the correction pattern; and a nozzle controller for controlling the maximum ejection amount of each nozzle based on the control parameter for correction.

Description

  Embodiments described herein relate generally to an image forming apparatus that corrects density unevenness caused by a difference in ejection characteristics of a print head.

  An image forming apparatus that forms an image using a print head composed of a plurality of nozzles has a problem that density unevenness occurs on a print screen due to variations in ejection characteristics between nozzles.

  The density unevenness includes a belt-like shape and a stripe shape. The band-shaped unevenness is easily observed when nozzles having a low discharge density are continuously adjacent to each other, and the stripe-shaped unevenness is easily observed when the discharge density difference between adjacent nozzles is large.

  In order to correct such variation in ejection characteristics between nozzles, the amount of edge pixels included in the printed image data is evaluated, and smoothing processing is performed according to the evaluation result to correct the image data. There is a thing (refer patent document 1).

  In addition, there is a technique in which the ejection characteristics of individual nozzles are investigated in advance, the ejection characteristics are stored as a table in a control memory in the apparatus, and the ejection characteristics are corrected (for example, see Patent Document 2).

  In particular, unevenness tends to be noticeable when a density drop occurs in a partial area of the print head with respect to uniform image data having a high density. In order to correct this density unevenness, there is a method in which the print density of all the nozzles is matched with the print density causing the density drop. However, this causes a density drop in the entire image, and an appropriate contrast is obtained. There is a problem that can not be.

JP 2010-16488 A JP 2006-51677 A

  The problem to be solved by the present invention is to solve the above problems and provide an image forming apparatus capable of correcting density unevenness without reducing contrast.

  In order to achieve the above object, an image forming apparatus according to the present embodiment includes a print head including a plurality of nozzles, an adjustment chart acquisition unit that acquires an adjustment chart indicating the maximum discharge performance of the nozzles, and the adjustment chart. A maximum density calculation unit for obtaining a maximum density characteristic at the time of maximum ejection for each nozzle, a correction pattern generation unit for generating a correction pattern for reducing density unevenness, and based on the maximum density characteristic and the correction pattern A parameter setting unit that sets a correction control parameter that controls the maximum discharge amount for each nozzle, and a nozzle control unit that controls the maximum discharge amount for each nozzle based on the correction control parameter.

1 is an overall configuration diagram of an image forming apparatus according to a first embodiment. FIG. 3 is a relationship diagram of a density correction unit, a print head, and a drum in the embodiment. The block diagram of the print head in the embodiment. The block block diagram of the density | concentration correction | amendment part in the embodiment. The block block diagram of the parameter setting part in the embodiment. The flowchart figure for acquiring the gamma curve characteristic of the nozzle unit in the same embodiment. The example of the adjustment chart in the embodiment. 5 is an example of a gamma curve characteristic indicating the discharge performance of the nozzle in the embodiment. The flowchart figure of the density nonuniformity correction process in the same embodiment. The relationship between the nozzle and the maximum density (maximum discharge amount) in the same embodiment. Explanatory drawing of the minimum density setting process of the correction pattern in the embodiment. Explanatory drawing of the scaling process of the correction pattern in the embodiment. Explanatory drawing of the masking process of the correction pattern in the embodiment. 6 is an example of a gamma curve characteristic after density unevenness correction in the embodiment. The flowchart figure of the image quality adjustment mode selection in the embodiment. 6 is a screen example of a UI panel for density unevenness correction processing in the embodiment.

  Hereinafter, embodiments will be described in detail with reference to FIGS. 1 to 16.

(First embodiment)
The image forming apparatus of the present embodiment includes a color copying machine that performs color printing at a high resolution in addition to an image forming apparatus such as a color inkjet printer. Furthermore, a multifunction device (MFP) capable of performing various processes such as a function of scanning and copying a base paper, a function of communicating with an external device, a function of transmitting / receiving and printing as a facsimile machine, and the like are also included.

  The image forming apparatus 1 of the present embodiment shown in FIG. 1 is roughly divided into a scanner unit 11 that reads a document, a main control unit 12 that controls the image forming apparatus 1 in an integrated manner, and a printer unit that prints and outputs on paper P. 13.

  The scanner unit 11 includes a document table 111 that reads a document, a light source 112 that illuminates the document, mirrors 113a, 113b, and 113c that guide reflected light of the document, a lens 114 that collects reflected light, and a line sensor unit 115. . As indicated by the arrows, the document pattern read from the document table 111 of the scanner unit 11 is irradiated by the light source 112, and the reflected light is incident on the line sensor unit 115 through the lenses 114 from the mirrors 113a, 113b, and 113c. The line sensor unit 115 photoelectrically converts this by a line sensor arranged for each color and outputs an image signal.

  The main control unit 12 includes a processor, an internal memory, and the like, and includes an input / output IF (interface) 14 that reads print data from an external storage device such as a flash memory and a USB memory, a network or a directly connected PC (personal computer) A communication IF 15 that receives print data from a user, a user interface panel (UI panel) 16 that displays various setting values such as maintenance and adjustment, a print status, and the like, and a scanner that performs signal processing of image data acquired by the scanner unit 11 A signal processing unit 17, a storage unit 18 that stores program information and maintenance / adjustment setting information of the main control unit, an image processing unit 19 that performs image processing of image data to be printed, and a density correction unit 20 that corrects density unevenness. Etc. are integrated and controlled.

  The printer unit 13 includes a paper feed cassette 130, a pickup roller 131, a registration sensor 132, a registration roller 133, a drum 134, a print head unit 135, a plurality of transport rollers 136a, 136b, and 136c, a paper discharge port 137, a peeling claw 138, and charging. It has a roller 139, a static elimination charger 140, and the like.

  The leading edge of the paper P is detected by the registration sensor 132 and the registration roller 133 of the paper P taken out from the paper feed cassette 130 by the pickup roller 131. Thereafter, the charge is applied by the charging roller 139, and the sheet is conveyed to the print head unit 135 while being in close contact with the drum 134. After the image formation by the print head unit 135, the charge on the paper P is removed by the charge removal charger 140. After the paper P is peeled from the drum 134 by the peeling claw 138, the paper P is transported by the transport rollers 136 a, 136 b, and 136 c and discharged from the paper discharge port 137.

  FIG. 2 shows the relationship among the density correction unit 20, the print head unit 135, and the drum 134. The print head unit 135 is configured by juxtaposing a plurality of print heads (here, five of 31a to 31e) in the lateral direction (axial direction of the drum). In order to avoid overlapping of the end portions of the print heads 31a to 31e, the print heads 31a to 31e are arranged in a staggered manner. A plurality of nozzle rows that eject ink are formed by the staggered arrangement of the print heads 31.

  FIG. 3 shows the configuration of the print head 31. In this example, ten nozzles 32 from nozzle number 0 to nozzle number a are shown. In order to increase the resolution, adjacent nozzles are staggered. Note that the number of print heads 31 and the number of nozzles shown here are numbers for convenience of explanation, and are not limited thereto.

  The image data processed by the image processing unit 19 is controlled by the density correction unit 20 in accordance with the image signal on the paper P by rotating the drum 34 in which the paper P is in close contact. An image of density distribution is formed.

  FIG. 4 is a block configuration diagram of the density correction unit 20. The density correction unit 20 includes an adjustment chart acquisition unit 41, a maximum density calculation unit 42, a correction pattern generation unit 43, a parameter setting unit 44, and a nozzle control unit 45.

  FIG. 5 is a block diagram of the parameter setting unit 44. The parameter setting unit 44 includes a minimum density setting unit 51, a scaling processing unit 52, a masking processing unit 53, and a gamma curve setting unit 54.

  In the present embodiment, in the image forming apparatus 1 configured as described above, the variation in the ejection performance of the nozzles is corrected with a repetitive pattern having a short period in the nozzle row direction, so that the high density region does not occur without causing a decrease in contrast. The correction is performed so that the density unevenness of the image is not noticeable.

  First, the maximum ejection performance of each nozzle 32 of the print head unit 135 is obtained. FIG. 6 is a flowchart for obtaining the gamma curve characteristics of each nozzle.

  In step ST601, an adjustment chart capable of printing the discharge amount from 0 to the maximum of each nozzle is printed. For example, as shown in FIG. 7, for all n (n is an arbitrary natural number) 32 nozzles forming the print head unit 135, the input control value for controlling the discharge amount is sequentially set from the minimum value to the maximum value. Enter and print the chart. Then, a chart is obtained in which the horizontal direction is the nozzle row direction and the vertical direction is the print density. Here, this chart is called an adjustment chart.

  In FIG. 7, for the sake of explanation, seven levels of print density from A to G are shown, but in actuality, the density gradually increases according to the input control value. At this time, the printing density G is the maximum density printed by the nozzle 32 upon receiving the maximum value of the input control value. This maximum density can be considered as the maximum discharge performance of each nozzle 32.

  Next, in step ST602, the printed adjustment chart is read by the scanner unit 11 and converted into digital data.

  In step ST603, as shown in FIG. 7, for example, the print density is recorded in the nozzle row direction at the input control values corresponding to the seven levels of print density from A to G. Thereby, the measurement of density unevenness is completed (step ST604). The processing up to this point is performed by the adjustment chart acquisition unit 41.

  In step ST605, the correspondence relationship between the printed nozzle 32 and the print density is obtained based on the measurement of density unevenness in step ST604. That is, matching is performed as to which nozzle 32 has printed at which printing density. Thus, for example, if matching is performed on the print density G, the maximum discharge performance of each nozzle 32 can be obtained.

  In step ST606, the gamma curve characteristics of each nozzle 32 are calculated from the correspondence in step ST605. FIG. 8 is an example of gamma curve characteristics indicating the ejection performance of each nozzle 32. The horizontal axis is the input control value, and the vertical axis is the print density. Here, the input control value region is divided by the input control values L0, L1, L2, and L3 on the horizontal axis. The input control values L0 to L1 are defined as a low concentration region R1, the input control values L1 to L2 are defined as a medium concentration region R2, and the input control values L2 to L3 are defined as a high concentration region R3.

  By plotting the print density with respect to each input control value, the gamma curve characteristic 81 of each nozzle 32 is obtained (here, four of the gamma curve characteristics 81a to 81n for n nozzles are shown as representatives). ). The gamma curve characteristic of the nozzle 32 printed with the maximum density is 81a, and the gamma curve characteristic of the nozzle 32 printed with the lowest density is 81n.

  Further, depending on the discharge performance of the nozzle, the print density may be different even in the middle density region R2. In this case, it is preferable to correct the input control value so that the gamma curve characteristics of the nozzles 32 in the low density region R1 to the medium density region R2 are substantially matched.

  Through the above steps, the maximum discharge performance of each nozzle 32 is obtained. The processing of steps ST605 and ST606 up to this point is performed by the maximum density calculation unit 42.

  Next, density unevenness correction is performed using the maximum discharge performance of each nozzle 32. FIG. 9 is a flowchart of density unevenness correction processing.

  In step ST901, the lowest density nozzle is extracted from the maximum density characteristic at the time of maximum ejection. FIG. 10 shows the relationship between the nozzle and the maximum discharge amount. FIG. 10 shows the matching of each nozzle 32 with the print density G of FIG. More precisely, the print densities at the input control value L3 in FIG. 8 are arranged in the order of nozzle numbers. This graph will be referred to as the maximum density characteristic M. The nozzle having the lowest density is extracted from the maximum density characteristic M. In the example of FIG. 10, the nozzle 32L corresponding to the nozzle number L is the lowest density nozzle.

  In step ST902, the lowest value of the correction pattern is set to the lowest density nozzle. FIG. 11 is an explanatory diagram of the correction pattern minimum density setting process. For example, in the case of a print head having a resolution of 100 DPI (Dot Per Inch), the correction pattern H1 is a pattern in which a pattern with a short period of 4 to 5 dots (number of nozzles) is repeated a predetermined number of times. Here, this short cycle is called a nozzle cycle. This correction pattern H1 can correct strip-like or stripe-like density unevenness, but leaves a slight roughness. For this reason, the conditions such as the nozzle cycle, the number of repetitions, and the amplitude of the correction pattern H1 are reduced by the human visual characteristics, the resolution of the print head unit 135, the ejection amount, the amount of ink bleeding, and the like. Find the optimal conditions. The correction pattern H1 is generated by the correction pattern generation unit 43.

  In FIG. 11, the lowest value for the correction pattern H1 is set to be equal to the printing density of the lowest density nozzle 32L. However, it is not always necessary to set them equal, and the minimum value of the correction pattern H1 may be set to a predetermined value in association with the minimum density of the nozzle having the minimum maximum density characteristic M. As a result, it is possible to adjust the correction effect for the roughness and density unevenness. The operation of step ST902 is performed by the minimum density setting unit 51.

  In step ST903, the correction pattern H1 is scaled, and this correction pattern is repeatedly applied. FIG. 12 is an explanatory diagram of the scaling process. First, in FIG. 11, an average value H1A of the correction pattern H1 and an average value MA of the maximum density characteristic M are obtained.

  In FIG. 12, scaling is performed in the amplitude direction while the minimum value of the correction pattern H1 set in step ST902 is fixed, and the correction pattern is further repeated in the nozzle row direction (the correction pattern after scaling is referred to as H2). .

  At this time, in FIG. 12, scaling is performed so that the average value H2A of the correction pattern H2 and the average value MA of the maximum density characteristic M coincide. However, it is not always necessary to make them coincide, and the average value H2A of the correction pattern H2 and the average value MA of the maximum density characteristic may be set to a predetermined value. As a result, it is possible to adjust the correction effect for the feeling of roughness and contrast. The operation in step ST903 is performed by the scaling processing unit 52.

  In step ST904, the maximum discharge amount of each nozzle 32 by masking processing is set. FIG. 13 is an explanatory diagram of a masking method for the correction pattern H2. As shown in FIG. 13, the correction pattern H2 is repeated in the nozzle row direction of the print head unit 135 by the scaling process performed in step ST903, and masking with the maximum density characteristic M is performed.

  At this time, if the maximum density characteristic M is greater than or equal to the density of the correction pattern H2, the maximum discharge amount of the nozzle 32 is adjusted to be the density of the correction pattern H2, and the maximum density characteristic M is greater than the density of the correction pattern H2. If it is too low, the corresponding nozzle 32 cannot be printed at a density higher than that, so that the maximum density characteristic M is adjusted. By this masking process, the maximum discharge amount for all the nozzles 32 is set. The correction pattern in which the maximum discharge amount is set is H3. This step ST903 is performed by the masking processing unit 53.

  In step ST905, the gamma curve characteristics of each nozzle 32 from the low density area to the medium density area are set equal. FIG. 14 shows the gamma curve characteristics after density unevenness correction. In steps ST901 to ST904, the print density setting value for the input control value L3 shown in FIG. 14 is determined. Here, in the low density region R1 and the medium density region R2, the gamma curve characteristics 141 of each nozzle are all set equal.

  Next, in step ST906, the gamma curve of the high density region R3 is interpolated from the maximum discharge amount of each nozzle 32 set by the correction pattern H3 and the gamma curve point E of the medium density region R2, thereby obtaining the gamma curve of each nozzle 32. Curve characteristics 141a to 141n are set. Steps ST905 and ST906 are performed by the gamma curve setting unit 54. The gamma curve characteristic 141 set by the gamma curve setting unit 54 is input to the parameter setting unit 44 as a correction control parameter, and each nozzle 32 is controlled by the nozzle control unit 45.

  Through the above steps, the corrected ejection amount corresponding to the input control values from the low density region R1 to the high density region R3 is set for all the nozzles 32. From the low density region R1 to the medium density region R2, printing is performed with the same gamma curve characteristics, so density unevenness does not occur, and in the high density region R3, printing is performed with light and shade with a short cycle of the correction pattern H3. However, the shading of this short cycle is perceived as roughness by human vision, and is not recognized as uneven density. In addition, as shown in step ST903 (and FIG. 12), since the average value of the original maximum density characteristic M and the correction pattern H2 are set to substantially coincide with each other, density unevenness correction is performed without causing contrast reduction. Is possible.

  Finally, a procedure for setting the density unevenness correction of the present embodiment from the UI panel 16 will be described with reference to the flowchart of FIG. FIG. 16 is a screen example of the UI panel during density unevenness correction processing.

  In step ST151, the operator selects an image quality adjustment mode from the UI panel 16. If the image quality adjustment mode for correcting density unevenness is selected (step ST151: Yes), the process proceeds to step ST152.

  In step ST152, the operator presses an “adjustment chart printing” execution button 162 in FIG. 16 to print the adjustment chart in FIG. In order to print the adjustment chart of step ST601, the main control unit 12 sets an adjustment chart parameter for obtaining the maximum discharge amount from 0 of each nozzle 32 in the parameter setting unit 44.

  In step ST153, the nozzle control unit 45 is controlled to print the adjustment chart.

  In the next step ST154, the operator sets the printed adjustment chart on the document table 111 of the scanner unit 11, and presses the “read adjustment chart” execution button 163. During execution, a progress status and instruction message are displayed, and as shown by a halftone dot, the process is being executed by reverse display or flashing display.

  In step ST155, subsequent to “adjustment chart reading” in step ST154, steps ST605 to ST606 and steps ST901 to ST906 are performed to calculate and store correction control parameters.

  In step ST155, this correction control parameter is set in the parameter setting unit 44. In addition, you may perform the test printing which confirms whether correction | amendment was correct here.

  In step ST157 (step ST151: No), the image quality adjustment mode returns to the normal printing mode, and normal printing is performed using the correction control parameters set in step ST155.

  As described above, according to the first embodiment, high-density image data is printed with a light and shade of a short cycle that is a human visual limit. It is possible to reduce. In addition, density unevenness can be corrected without causing a decrease in contrast.

  In the embodiment, for example, correction of uneven density of K (black) color has been described. However, in the case of color, this is represented by the number of printer head colors, for example, Y (yellow), M (magenta), C (cyan), K (black). Similar density unevenness correction may be performed for all of the above.

  Further, the operation from the UI panel shown in FIG. 15 may be set from a PC or the like connected via the communication IF 15.

  In the embodiment of the present invention, the case has been described where the image processing function for implementing the invention is recorded in advance in the main control unit 12 and the storage unit 18 as a program. You may download to this image forming apparatus. Moreover, you may install in the apparatus what recorded the same function on the recording medium.

  The recording medium may be in any form as long as it can record a program such as a CD-ROM and can be read by the apparatus. In addition, the function obtained by installing or downloading in advance may be realized in cooperation with an OS (operating system) or the like inside the apparatus.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

1 Image forming apparatus,
41 ... Adjustment chart acquisition unit,
42 ... maximum density calculation unit,
43 ... control pattern generation unit,
44 ... parameter setting section,
45: Nozzle control unit.

Claims (6)

A print head composed of a plurality of nozzles;
An adjustment chart acquisition unit for acquiring an adjustment chart indicating the maximum discharge performance of the nozzle;
Using the adjustment chart, a maximum density calculation unit for obtaining a maximum density characteristic at the time of maximum discharge for each nozzle,
A correction pattern generation unit for generating a correction pattern for reducing density unevenness;
A parameter setting unit for setting a correction control parameter for controlling the maximum discharge amount for each nozzle based on the maximum density characteristic and the correction pattern;
A nozzle controller that controls the maximum discharge amount for each nozzle based on the correction control parameter;
An image forming apparatus.   The image forming apparatus according to claim 1, wherein the correction pattern is a pattern having a predetermined nozzle cycle, the number of repetitions, and an amplitude according to the resolution of the print head. The parameter setting unit, a minimum density setting unit that sets a minimum value of the correction pattern in association with a minimum density of a nozzle having the minimum maximum density characteristic;
The correction pattern processed by the minimum density setting unit is repeated in the nozzle row direction, the amplitude of the correction pattern is scaled, and the average value of the corrected correction pattern is set in association with the average value of the maximum density characteristic. A scaling processing unit to
A masking processor that masks the correction pattern processed by the scaling processor with the maximum density characteristic;
A correction pattern processed by the masking processing unit is set as a maximum discharge amount for each nozzle, and further a gamma curve setting unit for setting a gamma curve characteristic for a discharge amount in a low density to high density region;
The image forming apparatus according to claim 2.   The image forming apparatus according to claim 3, wherein the gamma curve setting unit sets the gamma curve characteristics with respect to the discharge amount from the low density region to the medium density region to be substantially equal for the plurality of nozzles.   5. The gamma curve setting unit interpolates a gamma curve characteristic for each nozzle with respect to a discharge amount in a high density region so that a maximum discharge amount for each nozzle and a discharge amount in the medium density region are connected. Image forming apparatus.   The minimum density setting unit sets the minimum value of the correction pattern so as to substantially match the minimum density of the nozzle having the minimum maximum density characteristic, and the scaling processing unit sets the correction pattern after scaling. The image forming apparatus according to claim 5, wherein the average value is set so as to substantially match the average value of the maximum density characteristic.