JP6372337B2 - Image forming apparatus and image forming method - Google Patents

Description

  The present invention relates to an image forming apparatus and an image forming method.

In an electrophotographic image forming apparatus, an image developed by supplying a color material such as toner onto a photosensitive drum exposed based on original image data is transferred onto a sheet via an intermediate transfer belt. An image is formed on the paper.
Since the density unevenness of the image may occur due to the characteristics of the image forming member, changes with time, etc., the original image data is formed so that the test image is formed, the density unevenness is detected from the density distribution, and the detected density unevenness is offset. Is generally corrected (see, for example, Patent Documents 1 and 2).

Among the density unevenness, there is density unevenness that appears in the rotation period of the rotating body in the sub-scanning direction due to the rotating body such as the photosensitive drum and the intermediate transfer belt. The greater the number of density unevenness cycles that can be detected from the density distribution of the test image, the more accurately the density unevenness can be identified and correction can be made with high accuracy. However, the density obtained from the test image on one sheet of paper Distribution is limited.
For example, when an A3 size sheet having a rotation period of 20 cm / cycle of the photosensitive drum and a length of 42 cm in the sub-scanning direction is used for forming a test image, the photosensitive drum can be obtained from one sheet. The concentration distribution is about two cycles of the rotation cycle.

When the rotation cycle is longer than the length of the paper, the density distribution of the number of one cycle cannot be obtained only by forming the test image on one paper.
For example, when the rotation cycle of the intermediate transfer belt is 100 cm / cycle, a density distribution of about 0.4 cycle of the rotation cycle of the intermediate transfer belt can be obtained from one sheet even if the A3 size paper is used. Only.

  In order to obtain a density distribution with a fixed number of rotation cycles, a test image is formed on a plurality of sheets, and the density distribution obtained from the test image is increased (see, for example, Patent Document 3).

JP 2009-192896 A JP 2010-220182 A JP 2014-116711 A

  However, when a plurality of sheets are used, a test image cannot be formed between sheets, so that the density distribution of all phases of the rotation cycle cannot be obtained. Considering such a lack of density distribution in some phases, in order to obtain a density distribution having a constant number of cycles in all phases of the rotation cycle, it is necessary to form test images on an extra large number of sheets. Since paper, toner, and the like are consumed wastefully, it is inefficient.

  An object of the present invention is to efficiently form a test image.

According to the invention of claim 1,
An image forming unit for forming a plurality of test images for detecting density unevenness on at least one sheet;
A signal output unit that outputs a detection signal of a reference position in the sub-scanning direction of a rotating body used as an image forming member in the image forming unit;
A control unit that detects a phase of a rotation period of the rotating body based on the detection signal output by the signal output unit;
The control unit determines a phase for starting the formation of each test image in the rotation period so that a density distribution having a constant number of periods is obtained from the plurality of test images in all phases of the rotation period of the rotating body. Then, an image forming apparatus is provided in which the formation of each test image is started at a timing at which the phase of the rotation period of the rotating body detected by the detection signal reaches each determined phase.

According to invention of Claim 2,
The control unit determines the minimum number of sheets necessary to obtain a density distribution having a fixed number of periods in all phases of the rotation period from the rotation period and the length of the sheet in the sub-scanning direction. The image forming apparatus according to claim 1, wherein a phase for starting the formation of the test image on each of the number of sheets is determined.

According to invention of Claim 3,
3. The image forming apparatus according to claim 1, wherein the control unit determines a phase for starting the formation of each test image to be the same as a phase for ending the formation of the previous test image. 4. Is done.

According to invention of Claim 4,
The image forming unit circulates a sheet to form a plurality of test images at different positions on one sheet,
The control unit is configured to obtain the plurality of tests on the one sheet in the rotation period so that a density distribution having a fixed number of periods is obtained from the plurality of test images in all phases of the rotation period of the rotating body. The phase at which image formation is started is determined, and each test image at a different position on the one sheet at the timing when the phase of the rotation period of the rotating body detected by the detection signal reaches each determined phase. The image forming apparatus according to any one of claims 1 to 3 is provided.

According to the invention of claim 5,
A density detector for detecting the density of the test image formed on the paper;
The density detection unit sequentially detects the density of each test image by moving according to each position of the plurality of test images formed at different positions on the one sheet. An image forming apparatus according to claim 4 is provided.

According to the invention of claim 6,
An image forming method for forming a plurality of test images for detecting density unevenness on at least one sheet by an image forming unit,
(A) outputting a reference position detection signal in the sub-scanning direction of a rotating body used as an image forming member in the image forming unit by a signal output unit;
(B) determining a phase at which formation of each test image is started in the rotation period so that a density distribution having a constant period is obtained in all phases of the rotation period of the rotating body from the plurality of test images. When,
(C) detecting the phase of the rotation period of the rotating body based on the detection signal output by the signal output unit;
(D) starting the formation of each test image at a timing when the phase of the rotation period of the rotating body detected in step (c) reaches each phase determined in step (b);
An image forming method is provided.

  According to the present invention, the timing of starting the formation of each test image is synchronized with the phase of the rotation period of the rotating body so that a density distribution having a constant number of periods can be obtained in all phases of the rotation period of the rotating body. Can do. A test image can be formed without losing the density distribution of the phase between the sheets, and the test image can be formed efficiently.

1 is a front view illustrating a schematic configuration of an image forming apparatus according to an embodiment. 1 is a block diagram illustrating a configuration of an image forming apparatus for each function. It is a perspective view which shows schematic structure of a photosensitive drum and an exposure part. 6 is a flowchart illustrating a processing procedure when the image forming apparatus generates density unevenness correction data. FIG. 5 is a diagram illustrating a sheet on which a test image is formed without adjusting the timing for starting the formation of the test image, and density unevenness that can be detected from the density distribution of the test image. It is a figure which shows the density nonuniformity which can be detected from the density distribution of the test image shown to FIG. 5A for every phase. FIG. 5 is a diagram illustrating a sheet on which a test image is formed by adjusting the timing at which test image formation is started, and density unevenness that can be detected from the density distribution of the test image. It is a figure which shows the density nonuniformity which can be detected from the density distribution of the test image shown to FIG. 6A for every phase. FIG. 5 is a diagram illustrating a sheet on which a test image is formed without adjusting the timing for starting the formation of the test image, and density unevenness that can be detected from the density distribution of the test image. It is a figure which shows the density nonuniformity which can be detected from the density distribution of the test image shown to FIG. 7A for every phase. FIG. 5 is a diagram illustrating a sheet on which a test image is formed by adjusting the timing at which test image formation is started, and density unevenness that can be detected from the density distribution of the test image. It is a figure which shows the density nonuniformity which can be detected from the density distribution of the test image shown to FIG. 8A for every phase. FIG. 6 is a diagram illustrating a single sheet on which a plurality of test images are formed at different positions and density unevenness that can be detected from the density distribution of each test image. FIG. 6 is a top view showing a density detector when detecting the densities of a plurality of test images formed at different positions on one sheet in parallel. FIG. 6 is a top view showing a density detection unit in the case where the density of each of a plurality of test images formed at different positions on one sheet is detected in order by moving the position.

  Embodiments of an image forming apparatus and an image forming method of the present invention will be described below with reference to the drawings.

FIG. 1 shows a schematic configuration of an image forming apparatus 1 according to the present embodiment.
As shown in FIG. 1, the image forming apparatus 1 includes an image forming unit 18 that forms an image on a sheet, and a density detection unit 19 that detects the density of the image formed on the sheet by the image forming unit 18. I have.

FIG. 2 is a block diagram illustrating the configuration of the image forming apparatus 1 for each function.
As shown in FIG. 2, the image forming apparatus 1 includes a control unit 11, a storage unit 12, an operation unit 13, a display unit 14, a communication unit 15, an image generation unit 16, an image processing unit 17, an image forming unit 18, and density detection. A unit 19 and a signal output unit 20 are provided.

The control unit 11 reads out a program stored in the storage unit 12 and controls each unit of the image forming apparatus 1 by executing the program. The control unit 11 can be configured by a CPU (Central Processing Unit), a RAM (Random Access Memory), and the like.
For example, the control unit 11 causes the image processing unit 17 to perform image processing on the original image data generated by the image generation unit 16, and the image forming unit 18 according to the gradation value of each pixel of the original image data after image processing. To form an image on the paper.

The control unit 11 adds correction data for reducing density unevenness appearing in the rotation cycle of the rotating body due to the rotating body used as an image forming member in the image forming unit 18 to the image formed by the image forming unit 18. Can be generated.
Specifically, the control unit 11 causes the image forming unit 18 to form a plurality of test images for detecting density unevenness on at least one sheet, and sets the density of the test image formed on each sheet to the density. Detection is performed by the detection unit 19. The control unit 11 detects density unevenness based on the density distribution of each test image obtained by the density detection unit 19, and generates correction data for the detected density unevenness.

  When forming a test image, the control unit 11 starts forming each test image in the rotation period so that a density distribution having a constant number of periods can be obtained from a plurality of test images in all phases of the rotation period of the rotating body. Determine the phase. The control unit 11 detects the phase of the rotation period of the rotating body based on the detection signal output from the signal output unit 20, and starts the formation of each test image at the timing when the detected phase reaches each determined phase.

The storage unit 12 stores a program that can be read by the control unit 11, a file that is used when the program is executed, and the like. As the storage unit 12, a large-capacity memory such as a hard disk can be used.
The storage unit 12 stores information such as the rotation period of each rotating body used as an image forming member and the number of rotation periods of the density distribution necessary for correcting density unevenness caused by the rotating body. For example, the storage unit 12 has a photosensitive drum rotation cycle of 20 cm / cycle, a density distribution of three cycles is necessary for correcting density unevenness caused by the photosensitive drum, and an intermediate transfer belt rotation cycle of 100 cm / cycle. This is a period, and information such as the fact that one period of density distribution is necessary for correction of density unevenness caused by the intermediate transfer belt is stored.

The operation unit 13 and the display unit 14 are provided on the upper portion of the image forming apparatus 1 as a user interface as shown in FIG.
The operation unit 13 generates an operation signal corresponding to a user operation and outputs the operation signal to the control unit 11. Examples of the operation unit 13 include a key and a touch panel configured integrally with the display unit 14.
The display unit 14 displays an operation screen and the like according to instructions from the control unit 11. As the display unit 14, an LCD (Liquid Crystal Display), an OELD (Organic Electro Luminescence Display), or the like can be used.

The communication unit 15 communicates with external devices on a network such as a user terminal, a server, and other image forming apparatuses.
For example, the communication unit 15 receives data (hereinafter referred to as PDL data) in which an instruction content for forming an image is described in a page description language (PDL) via a network from a user terminal.

  The image generation unit 16 rasterizes the PDL data received by the communication unit 15 and converts the original image data in a bitmap format having a gradation value for each pixel into C (cyan), M (magenta), and Y (yellow). And K (black) for each color. The gradation value is a data value representing the density of an image. For example, an 8-bit data value represents a gradation of 0 to 255 gradations.

  As shown in FIG. 1, the image forming apparatus 1 includes an image reading unit 161 for copying. By reading an image of a document set by a user using the image reading unit 161, R (red), G (green), and It is also possible to generate original image data for each color of B (blue).

  The image processing unit 17 performs image processing such as gradation correction processing and halftone processing on the original image data generated by the image generation unit 16. The gradation correction process is a process for converting the gradation value of each pixel of the original image data into a gradation value corrected so that the density of the image formed on the paper matches the target density. The halftone processing is, for example, error diffusion processing, screen processing using a systematic dither method, or the like.

The image processing unit 17 can also perform color conversion on the R, G, and B original image data generated by the image reading unit 161 to generate original image data of each color of C, M, Y, and K.
Further, the image processing unit 17 can perform density unevenness correction processing on the original image data using the density unevenness correction data generated by the control unit 11.

The image forming unit 18 forms an image having a plurality of colors on a sheet in accordance with the gradation value of each pixel of the original image data subjected to image processing by the image processing unit 17.
As shown in FIG. 1, the image forming unit 18 includes four writing units 181, an intermediate transfer belt 182, a secondary transfer roller 183, a fixing device 184, a paper feed tray 185, a circulation path 186, and a reverse path 187. . Each writing unit 181 is arranged in series along the belt surface of the intermediate transfer belt 182. The intermediate transfer belt 182 is wound and rotated by a plurality of rollers. Among the plurality of rollers, a primary transfer roller 18f and a secondary transfer roller 183 are included. The secondary transfer roller 183 and the fixing device 184 are arranged on the conveyance path of the sheet conveyed from the sheet feeding tray 185.

  The four writing units 181 form images of C, M, Y, and K colors, respectively. Each writing unit 181 has the same configuration, and includes an exposure unit 18a, a photosensitive drum 18b, a development unit 18c, a charging unit 18d, a cleaning unit 18e, and a primary transfer roller 18f, as shown in FIG.

At the time of image formation, in each writing unit 181, the charging unit 18 d applies a voltage to the photosensitive drum 18 b to charge it. Since the original image data of the corresponding colors C, M, Y and K are modulated and inputted to each writing unit 181, the laser is exposed by the exposure unit 18 a according to the gradation value of each pixel of the inputted original image data. Beams are irradiated to expose the photosensitive drum 18b.
FIG. 3 shows a schematic configuration of the exposure unit 18a and the photosensitive drum 18b.
As shown in FIG. 3, the laser beam emitted from the light source a1 in the exposure unit 18a is deflected by the polygon mirror a2, and scans the surface of the photosensitive drum 18b. This scanning direction is the main scanning direction x, and the direction orthogonal to the main scanning direction x on the surface of the photosensitive drum 18b is the sub-scanning direction y. Since one line of scanning is performed each time the mirror surface of the polygon mirror a2 is switched, an image for each line can be formed by shifting the scanning position of one line in the sub-scanning direction y by the rotation of the photosensitive drum 18b. .

When a color material such as toner is supplied by the developing unit 18c and the electrostatic latent image formed on the photosensitive drum 18b is developed by exposure, an image of each color is formed on the photosensitive drum 18b of each writing unit 181.
The image on each photosensitive drum 18b is sequentially transferred onto the intermediate transfer belt 182 by each primary transfer roller 18f, and an image having a plurality of colors is formed on the intermediate transfer belt 182. After the image transfer, in each writing unit 181, the color material remaining on the photosensitive drum 18 b is removed by the cleaning unit 18 e.

Thereafter, when the paper is fed by the paper feed tray 185 in accordance with the timing of transferring the image on the intermediate transfer belt 182 to the paper, the image on the intermediate transfer belt 182 is transferred onto the paper by the secondary transfer roller 183. The transferred paper is heated and pressed by the fixing device 184 to fix the image on the paper.
When an image is formed again on the same surface of one sheet, the sheet is conveyed again toward the secondary transfer roller 183 via the circulation path 186. When images are formed on both sides of a single sheet, the sheet is conveyed to the reversing path 187 to invert the sheet surface, and then the sheet is conveyed again to the secondary transfer roller 183 via the circulation path 186.

As shown in FIG. 1, the density detection unit 19 is arranged on the paper conveyance path, and detects the density of the image formed on the paper by the image forming unit 18.
As the concentration detection unit 19, an optical sensor whose detection range is a spot may be used, and a line sensor, an area sensor, or the like may be used when a wider detection range is required.

The signal output unit 20 generates and outputs a detection signal of a reference position in the sub-scanning direction of a rotating body used as an image forming member in the image forming unit 18.
The rotator that detects the reference position by the signal output unit 20 is a rotator that can cause density unevenness in the sub-scanning direction of the image to be formed. As such a rotating body, for example, a polygon mirror used in the exposure unit 18a, a photoconductor such as a photosensitive drum 18b, a developing sleeve used in the developing unit 18c, an intermediate transfer belt 182, a primary transfer roller 18f, and a secondary transfer. Examples thereof include a transfer body such as a roller 183, and a roller that performs heating and pressurization in the fixing device 184.

For example, when detecting the reference position of the photosensitive drum 18b, the signal output unit 20 can be arranged as shown in FIG.
As shown in FIG. 3, on the circumference of the rotation axis of the photosensitive drum 18b, two marks M indicating the reference position of the photosensitive drum 18b are provided every half circle, and correspond to the two marks M above the rotation axis. The signal output unit 20 is disposed at the position.
The signal output unit 20 generates and outputs a detection signal of this reference position. The detection signal may be a signal that rises when one mark M is detected by the rotation of the photosensitive drum 18b, and a signal that falls when the other mark M is detected after half rotation, or a signal that rises every time the mark M is detected. May be. According to such a detection signal, the phase of the rotation period in the sub-scanning direction y can be detected every 0.5 period.

As described above, the image forming apparatus 1 can detect density unevenness appearing in the rotation cycle of a rotating body used as an image forming member by the control unit 11 and generate correction data thereof.
FIG. 4 shows a processing procedure when the control unit 11 generates correction data for density unevenness.

As shown in FIG. 4, the control unit 11 obtains a density distribution having a fixed number of cycles in all phases of the rotation cycle from the rotation cycle of the rotating body and the length of the paper used for forming the test image in the sub-scanning direction. Therefore, the minimum required number of sheets is determined (step S1).
As the test image, for example, an image having a halftone value that is likely to cause density unevenness can be used. Since it is only necessary to obtain a density distribution in the sub-scanning direction, it is possible to use a strip-like image that is long in the sub-scanning direction as a test image, instead of forming a test image over the entire area of the paper. In this case, it is preferable because the consumption of toner and the like can be suppressed and the cost can be reduced.
The fixed number of periods of the density distribution may be a number of periods set in advance for each rotating body and stored in the storage unit 12 as a number of periods necessary for accurately correcting density unevenness, May be the number of periods specified for each rotating body.

For example, the rotation cycle of the photosensitive drum 18b is 20 cm / cycle, and in order to obtain a density distribution of 3.0 cycles of this rotation cycle, the length in the sub-scanning direction is 60 cm (20 cm / cycle × 3.0 cycles). Several) test images need to be formed. When a sheet having a length of 42 cm in the sub-scanning direction is used, the minimum number of sheets required for forming a test image having a length of 60 cm is two.
Further, when the rotating body is the intermediate transfer belt 182 having a rotation cycle of 100 cm / cycle, the length in the sub-scanning direction is 100 cm (100 cm / cycle × 1. It is necessary to form a test image of 0 cycle number). In order to form a test image of this length, a minimum of three sheets are required.

Next, the control unit 11 determines a phase for starting the formation of a test image on each of the determined number of sheets in the rotation period so that a density distribution having a fixed number of periods is obtained in all phases of the rotation period. (Step S2).
Specifically, the control unit 11 determines the phase for starting the formation of each test image as the same phase as the phase for ending the formation of the previous test image.

FIG. 5A shows two sheets on which a test image is formed without adjusting the timing for starting the formation of the test image, and density unevenness that can be detected from the density distribution of these test images.
FIG. 5B shows, for each phase, density unevenness that can be detected from the density distribution of the test images on the two sheets shown in FIG. 5A.
When the length of the paper in the sub-scanning direction corresponds to the number of rotation cycles of the rotator, the density distribution of 3.0 cycles can be obtained by using two sheets.
However, in the density distribution of the test image formed without adjusting the timing as shown in FIG. 5A, the phase ranges Y1, Y2, and Y3 in which density unevenness of 3.0 cycles or more can be detected as shown in FIG. 5B. On the other hand, there is also a phase range Y4 in which only 2.0 cycles can be detected. When a density distribution of at least 3.0 cycles is necessary to perform highly accurate correction, the correction accuracy of the phase range Y4 is lowered.

FIG. 6A shows two sheets on which test images are formed by adjusting the timing at which test image formation is started by the control unit 11, and density unevenness that can be detected from the density distribution of these test images.
FIG. 6B shows the density unevenness that can be detected from the density distribution of the test image on the two sheets shown in FIG. 6A for each phase.
As shown in FIG. 6A, since a density distribution of 1.5 cycles is obtained with one sheet, if the phase for starting the formation of the test image on the first sheet is determined as 0.0 period, A density distribution in the phase range of 0.0 to 1.0 and 0.0 to 0.5 cycle can be obtained from the test image of the first sheet. The test of the second sheet is performed by determining the phase of 0.5 period at which the formation of the test image on the first sheet is completed as the phase at which the formation of the test image on the second sheet is started. A density distribution in the phase range of 0.5 to 1.0 and 0.0 to 1.0 period is obtained from the image. As a result, as shown in FIG. 6B, from the density distribution of each test image, density unevenness of 3.0 cycles can be detected in any phase range Y1 to Y4.

  In this way, by making the phase of the density distribution of each test image continuous, it is possible to obtain a density distribution with a fixed number of cycles in all phases of the rotation cycle with the minimum necessary number of sheets. Becomes efficient.

When the rotation period of the rotator is longer than the length of the paper in the sub-scanning direction, it is necessary to form test images on a plurality of sheets in order to obtain a density distribution of at least 1.0 period.
Also in this case, if a test image is formed without adjusting the timing, a density distribution having a necessary number of cycles may not be obtained in all phases of the rotation cycle.

For example, when the length of the paper in the sub-scanning direction corresponds to the number of rotations of 0.75 of the rotating body, if two sheets are used, density unevenness of 1.5 cycles can be detected by calculation. Is possible.
However, when the test images are formed on the two sheets without adjusting the timing as shown in FIG. 7A, the density distribution obtained from each test image has 2.0 cycles as shown in FIG. 7B. There are phase ranges Y1 to Y3 in which the density unevenness can be detected, and there are phase ranges Y4 in which the density unevenness cannot be detected.

FIG. 8A shows two sheets on which a test image is formed by adjusting the timing at which the control unit 11 starts to form a test image, and density unevenness that can be detected from the density distribution of these test images.
FIG. 8B shows density unevenness that can be detected from the density distribution of the test image on the two sheets of paper shown in FIG. 8A for each phase.
As shown in FIG. 8A, the timing for forming the test image on the first and second sheets is adjusted to the phase of the rotation period of 0.0 and 0.75 period, respectively, as shown in FIG. 8B. Thus, density unevenness of at least 1.0 period can be detected in any phase range Y1 to Y2 from the density distribution of each test image.

As long as density unevenness with a certain number of cycles can be detected, the phases of the density distribution of each test image may be partially overlapped instead of being continuous.
In the case of the adjustment example shown in FIG. 8A, in order to obtain a density distribution of 1.0 cycle number, a density distribution in a phase range of 0.75 to 1.0 cycle that was not obtained from the test image of the first sheet is obtained. What is necessary is just to obtain from the 2nd test image. Therefore, the phase for starting the formation of the test image on the second sheet is set to 0.25 period, and the timing is set so that the density distribution in the phase range of 0.25 to 1.0 period can be obtained from the second test image. It can also be adjusted. In this case, a density distribution capable of detecting density unevenness of 2.0 periods in the phase ranges Y2 and Y3 is obtained.

After determining the phase, the control unit 11 detects the phase in the rotation period of the rotating body based on the detection signal output from the signal output unit 20 (step S3).
For example, as shown in FIGS. 6A, 8A, etc., the detection signal output by detecting the two marks M shown in FIG. Therefore, the control unit 11 can detect which phase in the 1.0 cycle the photosensitive drum 18b is by counting how many lines of the image are formed from the phase at the time of rising or falling. . For the counting, a counter that counts up according to a synchronization signal indicating the start position of each line and resets the count value every time the reference position is detected can be used.

The control unit 11 causes the image forming unit 18 to start forming a test image on the paper at the timing when the phase detected in step S3 reaches the phase determined in step S2. Further, the control unit 11 causes the density detection unit 19 to detect the density of the test image.
The image forming unit 18 conveys the sheet at a timing instructed by the control unit 11 and forms a test image (step S4).
The density detection unit 19 detects the density of the test image formed on the paper by the image forming unit 18 (step S5).

The control unit 11 analyzes the density distribution of each test image detected by the density detection unit 19 and detects density unevenness appearing in the rotation cycle of the rotating body (step S6). Next, the control unit 11 generates correction data for the detected density unevenness (step S7).
For example, when the density unevenness correction data shown in FIG. 6B is generated, the control unit 11 calculates the average density value for each of the phase ranges Y1 to Y4 using the density distribution of three periods. The control unit 11 normalizes the calculated average density value and converts it into a gradation value, and a value obtained by subtracting the converted gradation value from the gradation value of the test image is used as a correction value for each of the phase ranges Y1 to Y4. Generate correction data.

  At the time of density unevenness correction processing, the image processing unit 17 specifies the phase of the rotation period of the rotating body corresponding to the position in the sub-scanning direction of each line of the original image data, and the phase range Y1 to Y4 corresponding to the specified phase. Are obtained from the correction data and added to the gradation values of the pixels of each line of the original image data. It is possible to reduce the density unevenness by adding the correction value and correct the target density.

  As described above, the image forming apparatus 1 according to the present embodiment has the image forming unit 18 that forms a plurality of test images for detecting density unevenness on at least one sheet, and the image forming unit 18 performs image processing. Based on the signal output unit 20 that outputs a detection signal of the reference position in the sub-scanning direction of the rotating body used as the forming member, and the detection signal output by the signal output unit 20, the phase of the rotation period of the rotating body is detected. And a control unit 11. The control unit 11 determines and detects a phase at which each test image starts to be formed in the rotation period so that a density distribution having a fixed number of periods can be obtained from a plurality of test images in all phases of the rotation period of the rotating body. The formation of the test image is started at the timing when the phase of the rotation period of the rotating body detected by the signal reaches each determined phase.

  Thereby, the timing for starting the formation of each test image can be synchronized with the phase of the rotation period of the rotating body so that a density distribution having a constant number of periods can be obtained in all phases of the rotation period of the rotating body. A test image can be formed without losing the density distribution of the phase between the sheets, and the test image can be formed efficiently.

  The above embodiment is a preferred example of the present invention, and the present invention is not limited to this. Modifications can be made as appropriate without departing from the spirit of the present invention.

[Modification 1]
In the above embodiment, one test image is formed on one sheet of paper. However, when a strip-shaped image shown in FIG. 6A or the like is used as the test image, the image forming unit 18 circulates the sheet to 1 A plurality of test images can be formed at different positions on a sheet of paper. The control unit 11 starts forming a plurality of test images on one sheet in the rotation period so that a density distribution having a fixed number of periods can be obtained from the plurality of test images in all phases of the rotation period of the rotating body. It is also possible to determine the phase to be performed, and to start the formation of the test image at the timing when the phase of the rotation period of the rotating body detected by the detection signal reaches each determined phase.

FIG. 9 shows an example in which two test images are formed on the same sheet by circulating one sheet.
As shown in FIG. 9, by adjusting the timing for forming each test image to a phase of 0.0 cycle and 0.5 cycle, each test image is similar to the adjustment example shown in FIG. 6A. As shown in FIG. 6B, a density distribution capable of detecting density unevenness of 3.0 cycles in each phase range Y1 to Y4 is obtained.

  According to such a test image forming method, it is possible to detect density unevenness of a certain number of cycles from one sheet, so that it is possible to reduce the number of sheets used for the test image and to correct density unevenness. Cost can be reduced.

[Modification 2]
As described above, when a plurality of test images are formed on one sheet, as shown in FIG. 10A, each density detector 19 uses a line sensor or the like whose detection range is the entire width in the main scanning direction. The test image density may be detected in parallel.
When an optical sensor having a small detection range is used as the density detection unit 19, a plurality of density detection units 19 are formed at different positions in the main scanning direction on one sheet as shown in FIG. 10B. By moving in the main scanning direction according to the respective positions of the test images, the density of each test image can be detected in order.

[Modification 3]
The control unit 11 may adjust the length of the test image formed on each sheet in accordance with the length in the sub-scanning direction of the density distribution necessary for detecting density unevenness with a fixed number of cycles.
For example, in the example shown in FIG. 8A, the test image is formed fully in the sub-scanning direction of the second sheet, but the length of the test image formed on the second sheet in the sub-scanning direction is 0. Even if the length corresponds to a phase range of .75 to 1.0 cycle, a necessary concentration distribution of 1.0 cycle can be obtained.
By setting the length of the test image to the minimum necessary length, the amount of toner consumed by forming the test image can be reduced, and the cost required for forming the test image can be reduced.

  In addition, as a computer-readable medium for a program for causing the control unit 11 to execute the above processing procedure, a non-volatile memory such as a ROM and a flash memory, and a portable recording medium such as a CD-ROM can be applied. is there. A carrier wave is also used as a medium for providing program data via a communication line.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 11 Control part 17 Image processing part 18 Image forming part 19 Density detection part 20 Signal output part

Claims (6)

An image forming unit for forming a plurality of test images for detecting density unevenness on at least one sheet;
A signal output unit that outputs a detection signal of a reference position in the sub-scanning direction of a rotating body used as an image forming member in the image forming unit;
A control unit that detects a phase of a rotation period of the rotating body based on the detection signal output by the signal output unit;
The control unit determines a phase for starting the formation of each test image in the rotation period so that a density distribution having a constant number of periods is obtained from the plurality of test images in all phases of the rotation period of the rotating body. Then, the image forming apparatus starts formation of each test image at a timing when the phase of the rotation period of the rotating body detected by the detection signal reaches each determined phase.   The control unit determines the minimum number of sheets necessary to obtain a density distribution having a fixed number of periods in all phases of the rotation period from the rotation period and the length of the sheet in the sub-scanning direction. The image forming apparatus according to claim 1, wherein a phase for starting the formation of the test image on each of the determined number of sheets is determined.   The image forming apparatus according to claim 1, wherein the control unit determines a phase for starting the formation of each test image to be the same as a phase for ending the formation of the previous test image. The image forming unit circulates a sheet to form a plurality of test images at different positions on one sheet,
The control unit is configured to obtain the plurality of tests on the one sheet in the rotation period so that a density distribution having a fixed number of periods is obtained from the plurality of test images in all phases of the rotation period of the rotating body. The phase at which image formation is started is determined, and each test image at a different position on the one sheet at the timing when the phase of the rotation period of the rotating body detected by the detection signal reaches each determined phase. The image forming apparatus according to claim 1, wherein the image forming apparatus is started. A density detector for detecting the density of the test image formed on the paper;
The density detection unit sequentially detects the density of each test image by moving according to each position of the plurality of test images formed at different positions on the one sheet. The image forming apparatus according to claim 4. An image forming method for forming a plurality of test images for detecting density unevenness on at least one sheet by an image forming unit,
(A) outputting a reference position detection signal in the sub-scanning direction of a rotating body used as an image forming member in the image forming unit by a signal output unit;
(B) determining a phase at which formation of each test image is started in the rotation period so that a density distribution having a constant period is obtained in all phases of the rotation period of the rotating body from the plurality of test images. When,
(C) detecting the phase of the rotation period of the rotating body based on the detection signal output by the signal output unit;
(D) starting the formation of each test image at a timing when the phase of the rotation period of the rotating body detected in step (c) reaches each phase determined in step (b);
An image forming method comprising:

Images