JP2008167104A - Device for forming image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form an image having a high image quality by removing an edge enhancement and an edge lack. <P>SOLUTION: The profile of a toner image is detected, and a screen is selected on the basis of the result of a detection. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus, and more particularly to a technique for improving the image quality of an image edge portion.

  An electrophotographic image forming apparatus has been used in various fields such as a color image and a light printing. As the use field expands, the need for high image quality increases, and edge emphasis that increases the density of the edge and edge chipping that decreases the density of the edge become problems, and countermeasures against these phenomena have been studied. ing.

  Japanese Patent Application Laid-Open No. 2004-133867 proposes that the spatial digital filter is corrected when edge density is detected and edge enhancement or edge missing is detected.

In Patent Document 2, it is proposed to reduce the edge enhancement effect by selecting filter conditions of a spatial digital filter having a plurality of filter conditions.
Japanese Patent No. 3373556 JP 2005-238528 A

  Edge enhancement and edge loss often occur due to fluctuations in the charge amount of toner due to environmental changes and fluctuations in appropriate development conditions. Further, edge emphasis or edge missing may occur due to a phenomenon called carrier counter charge in two-component development. Furthermore, edge emphasis and edge missing occur due to the influence of the developing brush (magnetic brush).

  Edge enhancement and edge chipping caused by the influence of the carrier counter charge and the developing brush occur in the process of relative movement between the image carrier and developer carrier, and therefore have directionality. It becomes.

  In the image processing using the spatial filter, a sufficient correction effect cannot be exhibited for density unevenness having such directionality, and edge enhancement and edge defect correction are not sufficient. The correction using the spatial frequency filter corrects the spatial frequency characteristics of the image, but edge enhancement and edge loss cannot be completely removed by correction of the spatial frequency.

  The present invention solves the problem that conventional countermeasures for edge enhancement and edge missing are insufficient, and an image forming apparatus capable of forming a high-quality image in which edge enhancement and edge missing are sufficiently corrected. The purpose is to provide.

The object is achieved by the following invention.
1.
In an image forming apparatus having an image carrier, a screen processing unit that performs screen processing on image data, and a toner image forming unit that forms a toner image on the image carrier based on the screen-processed image data.
A detection unit that detects a profile of a patch toner image of a predetermined pattern and a screen control unit;
The patch toner image is formed on the image carrier, and the screen control unit controls the screen processing in the screen processing unit based on a detection result of the detection unit that detects the patch toner image. An image forming apparatus.
2.
2. The image forming apparatus according to claim 1, wherein the screen control unit selects a screen from a plurality of screens built in the screen processing unit.
3.
3. The image forming apparatus according to 1 or 2, wherein the detection unit includes a toner image height detection unit.
4).
3. The image forming apparatus according to 1 or 2, wherein the toner detection unit includes a density detection unit.
5.
3. The image forming apparatus according to 2, wherein the screen control unit selects a screen having a small number of screen lines when edge enhancement or edge missing is detected by the detection unit.
6).
3. The image forming apparatus according to 2 above, wherein the screen control unit selects a screen having a small screen angle when edge enhancement or edge missing is detected by the detection unit.
7).
The plurality of patch toner images subjected to different screen processing are formed, and the screen control means selects a screen from which edge enhancement or edge missing is not detected from the plurality of patch toner images. The image forming apparatus of any one of -6.
8).
8. The image formation according to any one of 1 to 7, wherein the character mode / non-character mode can be set and the screen processing is not performed when the character mode is set. apparatus.
9.
The image forming apparatus according to any one of 1 to 7, wherein the plurality of screens include screens having different numbers of screen lines.
10.
8. The image forming apparatus according to claim 2, wherein the plurality of screens include screens having different screen angles.

  In the present invention, since edge enhancement and edge missing are corrected by changing the screen, these image quality defects can be sufficiently removed, and high-quality images can be stably formed.

Hereinafter, the present invention will be described with reference to embodiments, but the present invention is not limited to the embodiments.
<Image forming apparatus>
FIG. 1 is a diagram showing an overall configuration of an image forming apparatus according to an embodiment of the present invention.

  This image forming apparatus is called a full-color image forming apparatus having a vertical tandem configuration, and includes a plurality of sets of image forming units 10Y, 10M, 10C, and 10K, an intermediate transfer body unit 7, a paper feeding and conveying means 21, and A fixing device 24 is included. A document image reading device SC is disposed on the upper part of the main body A of the image forming apparatus.

  An image forming unit 10Y that forms a yellow image includes a drum-shaped photoreceptor 1Y, a charging device 2Y, an exposure device 3Y, a developing device 4Y, and a primary transfer roller as a primary transfer unit disposed around the photoreceptor 1Y. 5Y and a cleaning device 6Y. The image forming unit 10M that forms a magenta image includes a drum-shaped photoconductor 1M, a charging device 2M disposed around the photoconductor 1M, an exposure device 3M, a developing device 4M, and a primary transfer roller as a primary transfer unit. 5M and a cleaning device 6M. An image forming unit 10C that forms a cyan image includes a drum-shaped photoreceptor 1C, a charging device 2C, an exposure device 3C, a developing device 4C, and a primary transfer roller as a primary transfer unit disposed around the photoreceptor 1C. 5C and a cleaning device 6C. An image forming unit 10K that forms a black image includes a drum-shaped photoconductor 1K, a charging device 2K arranged around the photoconductor 1K, an exposure device 3K, a developing device 4K, a primary transfer roller 5K as a primary transfer unit, It has a cleaning device 6K.

  The intermediate transfer member unit 7 includes an intermediate transfer member 70 as a semiconductive endless belt-shaped image carrier that is wound around a plurality of rollers and is rotatably supported.

  The color images formed by the image forming units 10Y, 10M, 10C, and 10K are sequentially transferred onto the rotating intermediate transfer body 70 by the primary transfer rollers 5Y, 5M, 5C, and 5K, and are combined. Is formed. A transfer material P such as a sheet as a recording medium accommodated in the sheet feeding cassette 20 is fed by a sheet feeding means 21, passes through a plurality of intermediate rollers 22 A, 22 B, 22 C, 22 D, and a registration roller 23, and is subjected to secondary transfer. The image is conveyed to a secondary transfer roller 5A as a means, and the color image is collectively transferred onto the transfer material P. The transfer material P onto which the color image has been transferred is subjected to fixing processing by the fixing device 24, is sandwiched between the paper discharge rollers 25, and is placed on a paper discharge tray 26 outside the apparatus.

  On the other hand, after the color image is transferred to the transfer material P by the secondary transfer roller 5A as the secondary transfer means, the residual toner is removed by the cleaning device 6A from the intermediate transfer body 70 from which the transfer material P is separated by curvature.

  During the image forming process, the primary transfer roller 5K is always in pressure contact with the photoreceptor 1K. The other primary transfer rollers 5Y, 5M, and 5C are pressed against the corresponding photoreceptors 1Y, 1M, and 1C, respectively, only during color image formation.

  The secondary transfer roller 5A is in pressure contact with the intermediate transfer member 70 only when the transfer material P passes through the secondary transfer roller 5A.

  The image forming units 10Y, 10M, 10C, and 10K are arranged in tandem in the vertical direction. An intermediate transfer body unit 7 is disposed on the left side of the photoreceptors 1Y, 1M, 1C, and 1K in the figure. The intermediate transfer body unit 7 includes an intermediate transfer body 70 that can be rotated by winding rollers 71, 72, 73, 74, 76, and 77, primary transfer rollers 5Y, 5M, 5C, and 5K, and a cleaning device 6A.

  A charging device (2Y, 2M, 2C, 2K), an exposure device (3Y, 3M, 3C, 3K), a developing device (4Y, 4M, 4C, 4K) and a primary transfer roller on the photoreceptors 1Y, 1M, 1C, 1K (5Y, 5M, 5C, and 5K) constitute toner image forming means for forming a toner image on the intermediate transfer member 70 as an image carrier. The toner images of the respective colors are primarily transferred and superimposed on the intermediate transfer body 70, and then are collectively transferred to the transfer material P, and fixed and fixed by the fixing device 24 by pressing and heating. The photoreceptors 1Y, 1M, 1C, and 1K after the toner image is transferred to the intermediate transfer body 70 are cleaned by the cleaning devices 6Y, 6M, 6C, and 6K after the residual toner remaining on each photoreceptor is transferred. Then, the charging, exposure and development cycles described above are entered, and the next image formation is performed.

  FIG. 2 is a circuit diagram of the image forming apparatus according to the embodiment of the present invention.

  The image forming apparatus includes an image memory 101, an image processing unit 102 that performs image processing such as scaling and image editing on the image data read from the image memory 101, a gamma correction unit 103, a gradation correction correction unit 104, and MTF processing. The image data processed by these processing units is supplied to the image forming unit 106 to form an image.

  The CPU 100 executes such image formation control according to a program stored in the ROM 110. The CPU 100 also executes an image quality adjustment process described below according to a program stored in the ROM 110.

  A RAM 111 expands data when the CPU 100 executes control.

The image forming apparatus shown in FIGS. 1 and 2 is a color image forming apparatus using an intermediate transfer member. However, in the present invention, a photosensitive member is used as an image carrier, and a toner image is formed on the photosensitive member by screen processing. Of course, it can also be used for the image forming apparatus to be formed.
<Screen processing>
The setting of the image forming conditions including the setting of the screen will be described below. The setting of the screen is performed independently in each of the image forming units 10Y, 10M, 10C, and 10K. In the following description, for example, the image forming units 10Y, 10M, 10C, and 10K are referred to as the image forming unit 10, and the reference numerals Y, M, C, and K are omitted.

  As shown in FIG. 3, the digital image is composed of pixels px1 to pxn. The pixels px1 to pxn are formed by dot exposure with a laser or LED.

  In digital image formation, a plurality of pixels are used as a unit (hereinafter referred to as a unit matrix), and density is expressed by controlling the black area ratio in the unit matrix M. That is, the gradation of the image is expressed by the area gradation method.

  The screen processing in the present invention refers to processing for expressing a density by configuring a unit matrix composed of a plurality of pixels and changing the black area ratio in the unit matrix. Dithering, error diffusion, etc. are also examples of screen processing.

  The blackened area in the unit matrix can be changed by changing the number of black pixels and changing the area of the blackened pixels.

  FIG. 4 shows various screens.

  4 (a) and 4 (b) are halftone dot screens having different numbers of lines, FIGS. 4 (c) and 4 (d) are line screens having different numbers of lines, and FIGS. 4 (e) and 4 (f) are screens having different screen angles θ. Respectively.

  These screens are appropriately selected according to the type of image to be formed.

  The screen processing unit 105 processes the image data to perform a screen process for creating a unit matrix having a blackened area ratio corresponding to the density in the unit matrix. For example, various screens as shown in FIG. The screen is selected based on the command from the CPU 100, and the screen processing is performed according to the command.

By the screen processing, the density control can be stably performed. As a result of the density control being performed with high accuracy, the gradation control is stabilized and a high-quality gradation image can be formed.
<Detection means>
FIG. 5 shows an example of detection means for detecting the profile of the toner image.

  The detection means includes a laser displacement sensor 200 as a toner image height detection means, and the laser displacement sensor 200 measures the toner image formed on the intermediate transfer body 70 that has passed the transfer position, as shown in FIG. And includes a semiconductor laser 201 as a light source, a PSD (Psition Sensing Device) 202 as a light receiving element, an arithmetic control circuit 205, and the like. The detection means will be described with reference to FIGS.

  201 is a semiconductor laser as a light source, 202 is a PSD, and the PSD 202 is arranged on the outgoing optical axis inclined with respect to the incident optical axis as shown. The semiconductor laser 201 generates a light beam, which is shaped into parallel light by a collimator lens 203 and reaches a toner image 210 that is an object to be measured.

  The reflected light from the toner image 210 is imaged on the PSD 202 by the imaging lens 204. Since the PSD 202 is arranged on the outgoing optical axis inclined with respect to the incident optical axis, when the height of the surface of the toner image 210 is h1, the image is projected at the position p1 on the PSD 202, When the surface height of the toner image 210 is h2, the image is projected at the position p2, and when the surface height of the toner image is h3, the image is projected at the position p3. As described above, the projection position on the PSD 202 varies depending on the height of the toner image 210. Since the PSD 202 outputs a signal of a different level depending on the light incident position, the height of the toner image 210 is measured by reading the output of the PSD 202.

  In addition, as a light receiving element, it can replace with PSD and can also use image pick-up elements, such as CCD.

  205 controls the output of the semiconductor laser 201, creates a profile of a toner image, which will be described later, from the output of the PSD 202, calculates the height of the toner image and the line width of the line formed by the toner image, and An arithmetic control circuit for outputting measurement data, 206 a drive circuit for driving the semiconductor laser, and 207 an amplifier circuit for amplifying the output of the PSD.

  FIG. 6 is a diagram showing a cross-sectional shape, that is, a profile of the measured toner image, and the height and line width of the toner image are obtained from this profile.

  For example, as the height H of the toner image, an average value of the toner image heights in the section of the highest point hp1 ± 100 μm with the highest point (vertex) hp1 of the toner image as the center is used.

  Further, as the line width W of the toner image, the distance between the outermost points hp2 and hp3 having a height that is a half of the height at the highest point hp1 is used.

  Furthermore, parameters other than the height or width obtained from the cross-sectional shape of the high toner, such as the volume of the toner image, can be used for controlling the image forming conditions.

  FIG. 7 shows an example of a density sensor 220 as detection means for detecting the density of the toner image.

  The parallel light emitted from the light emitting element 221 using the LED as the light source irradiates the toner image 223, and the light receiving element 222 made of photo diode receives the reflected light and detects the density of the toner image 223.

  Even using the density sensor 220 of FIG. 7, it is possible to detect a density distribution corresponding to the cross-sectional shape shown in FIG. 6, in other words, a density profile, and edge enhancement and edge missing are detected.

  As described above, as the detection means, a laser displacement sensor as a toner image height detection means for detecting the height of the toner image or a density sensor for detecting the image density of the toner image can be used, but the image density is detected. The density sensor tends to have low detection sensitivity in the high density portion, whereas the toner image height detection means is more excellent because it has sufficient sensitivity to edge enhancement and edge loss.

  As shown in FIG. 1, the laser displacement sensor 200 or the density sensor 220 is disposed between the secondary transfer unit and the cleaning device 6A, and detects a toner image formed on the intermediate transfer body 70.

As will be described later, detection of the toner image is performed in correction control for edge enhancement and edge loss. In the correction control, the secondary transfer unit does not operate, and the toner image on the intermediate transfer body 70 is transferred to the secondary transfer unit. Instead, it is carried on the intermediate transfer member 70, cleaned by the cleaning device 6 </ b> A, and removed from the intermediate transfer member 70.
<Correction control for edge enhancement and edge loss>
Edge emphasis or edge loss occurs when the environment, that is, temperature or humidity changes, or when the characteristics of components constituting the image forming unit of the image forming apparatus such as a photoconductor change.

  Therefore, prior to image formation after a long pause of the image forming apparatus, an image quality adjustment process for selecting a screen is performed when the environment changes.

  FIG. 8 is a flowchart of the image quality adjustment process.

  In STEP1, the maximum density, Dmax, is adjusted.

  In this highest density adjustment, the exposure device 3 is driven with image data having the highest density pixel value, and the density of the toner image formed by development is detected by the density sensor 220, and the detected density of the density sensor 220 reaches a predetermined value. As described above, the image forming condition such as the developing condition is controlled.

  In STEP2, it is determined whether the character mode or the non-character mode. If the character mode is selected (No in STEP2), the process ends. Therefore, in the character mode, image formation is performed without screen processing.

  In the non-character mode, that is, the halftone mode (STEP2 Yes), a profile detection pattern is created in STEP3.

  As shown in FIG. 9A, the profile detection pattern includes a predetermined pattern of patch toner images PT formed in edge determination.

  The patch toner image having a predetermined pattern is a patch toner image having a predetermined density such as a quadrangle and a circle and having a uniform density. In one example, a plurality of patch toner images PT shown in FIG. Formed by different screen treatments.

  In an example, in an image written at 1200 dpi (number of dots per 25.4 mm), a patch toner image is formed by performing screen processing with a screen line number of 600 lpi, 300 dpi, and 200 lpi (number of lines per 25.4 mm).

  The patch toner image is an image having a uniform density as described above, and is formed with a uniform density of pixel value 180, for example, with 256 being the highest pixel value.

  In STEP 4, the profile of the patch toner image is detected by the laser displacement sensor 200.

  An example of edge enhancement is indicated by ED1 in FIG. 9B, and an example of edge loss is indicated by ED2 in FIG. 9C. An arrow Y is the moving direction of the image carrier.

  In STEP 5, the CPU 100 as the screen control unit performs correction for selecting an appropriate screen based on the detection result of the laser displacement sensor 200. This selection is to select a screen that is formed in STEP 3 and has no edge emphasis or edge missing from a plurality of patch toner images.

  Specifically, when edge emphasis or edge missing occurs, the number of screen lines is reduced or the screen angle is reduced. With these corrections, edge enhancement and edge missing are removed.

  By quantitatively detecting edge emphasis or edge missing and selecting a screen according to the degree, these phenomena can be suppressed more satisfactorily.

  In STEP 6, the γ correction unit 103 corrects γ to an appropriate value and ends the image quality adjustment process.

(1) Example 1
An image formation test was performed by selecting a screen based on the detection of the profile of the patch toner image and the detection under the following common conditions described below.
Image forming apparatus: color image forming apparatus shown in FIG. 1, A4, 50 sheets / minute (digital color machine) in the vertical direction (the conveyance direction of the recording material is the short side)
Recording density: 600 dpi (600 dots per 25.4 mm)
・ Process speed: 220mm / sec
Image area to non-image area ratio: 62.5 to 37.5
(Ratio of image area length to non-image area length in the moving direction of the photoreceptor)
-Photosensitive drum diameter: 60 mm (Y, M, C, K)
・ Reference value of photosensitive member charging voltage: -700V
-Photoconductor potential at the exposed portion (solid image portion potential): -50V
-Reference value of developer carrier potential: -500V
・ AC component of development bias: 1.0 kVpp (rectangular wave, 5 kHz)
Image exposure light source: Semiconductor laser (wavelength 780 nm)
・ Developer carrier: cylindrical body with a diameter of 30 mm (built-in magnet)
・ Axial length of magnetic field part of developer carrier: 330 mm
-Distance between photoconductor / developer carrier: 0.3 mm
・ Developer transport amount on developer carrier: about 300 ± 40 g / m ²
Ratio of moving speed of developer carrier surface / moving speed of photoreceptor surface: 2.0
Rotation direction of developer carrier: reverse (in the development region where the photoconductor and developer face each other, the photoconductor and developer carrier move in opposite directions)
-Developer layer thickness regulating means: magnetic blade-Gap between developer carrier and developer layer thickness regulating means: 0.6 mm
Toner: Polymerized toner having a volume average particle diameter of 6.5 μm for all of Y, M, C, and K. Carrier:
Volume average particle diameter: 38 μm
Magnetization strength: 60 emu / g
-Developer toner concentration: 6.5% by mass
・ Developer mass: 800g for Y, M, C and K
As shown in FIG. 1, the laser displacement sensor 200 is disposed opposite to the intermediate transfer member 70, and the profile of the toner image on the photosensitive member is measured.

  The laser spot diameter was 10 μm, and the distance resolution of the CCD as the light receiving element was 0.1 μm.

As a screen,
300 lpi, 200 lpi, and 150 lpi were prepared, and when edge emphasis or edge missing was detected, a screen that did not generate these image defects was set to be used.
(2) Example 2
The rotation direction of the developer carrying member is set to normal rotation (the photosensitive member and the developer carrying member move in the same direction in the developing region where the photosensitive member and the developer face each other), and as shown in FIG. The displacement sensor 200 was disposed opposite the intermediate transfer member 70, and the profile of the patch toner image on the intermediate transfer member was measured.
(3) Comparative Example As Comparative Examples 1 and 2, the filter conditions of the MTF filter (digital filter) in FIG. 2 were adjusted when edge enhancement or edge loss was detected under the same apparatus conditions as in Examples 1 and 2. .

  The test results are shown in Table 1.

  In Table 1, ◯ indicates that no edge enhancement or edge loss has occurred, Δ indicates that edge enhancement or edge loss has occurred but is at an acceptable level, and × indicates an unacceptable level of edge enhancement. Alternatively, it indicates that an edge defect has occurred.

  Further, in Table 1, as shown in (i)-> (ro)-> (c)-> (d), the image formation test was performed in the order of the NN environment, the LL environment, the HH environment, and finally the NN environment.

  As shown in Table 1, neither edge enhancement nor edge loss occurred in Examples 1 and 2. In Comparative Example 1, edge enhancement at the leading edge of the image occurred in C (HH environment) and D (NN environment). In (b) (LL environment), edge missing at the rear end of the image occurred. In (c) (HH environment), edge enhancement at the rear end of the image occurred.

  Further, in Comparative Example 2, edge missing at the leading edge of the image occurred in B (LL environment), and edge enhancement occurred at the trailing edge of the image in C (HH environment).

  Further, in Comparative Example 2, edge missing occurred in D (NN environment), but it is presumed that it was caused by excessive correction of edge enhancement as indicated by (Δ).

1 is a diagram illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a circuit diagram of an image forming apparatus according to an embodiment of the present invention. It is a figure for demonstrating a screen process. It is a figure which shows various screens. FIG. 5 is a diagram showing an example of detection means for detecting the profile of the toner image. FIG. 6 is a diagram illustrating a profile of a toner image. It is a figure which shows a density | concentration sensor. It is a flowchart of an image quality adjustment process. It is a figure which shows edge emphasis and edge missing.

Explanation of symbols

1, 1Y, 1M, 1C, 1K photoconductor 2Y, 2M, 2C, 2K charging device 3, 3Y, 3M, 3C, 3K exposure device 4Y, 4M, 4C, 4K developing device 5Y, 5M, 5C, 5K primary transfer device 70 Intermediate transfer body 100 CPU
105 Screen processing unit 200 Laser displacement sensor 220 Concentration sensor 0 Laser displacement sensor

Claims (10)

In an image forming apparatus having an image carrier, a screen processing unit that performs screen processing on image data, and a toner image forming unit that forms a toner image on the image carrier based on the screen-processed image data.
A detection unit that detects a profile of a patch toner image of a predetermined pattern and a screen control unit;
The patch toner image is formed on the image carrier, and the screen control unit controls the screen processing in the screen processing unit based on a detection result of the detection unit that detects the patch toner image. An image forming apparatus. The image forming apparatus according to claim 1, wherein the screen control unit selects a screen from a plurality of screens built in the screen processing unit. The image forming apparatus according to claim 1, wherein the detection unit includes a toner image height detection unit. The image forming apparatus according to claim 1, wherein the toner detection unit includes a density detection unit. The image forming apparatus according to claim 2, wherein the screen control unit selects a screen having a small number of screen lines when edge enhancement or edge missing is detected by the detection unit. The image forming apparatus according to claim 2, wherein the screen control unit selects a screen having a small screen angle when edge detection or edge missing is detected by the detection unit. The plurality of patch toner images subjected to different screen processing are formed, and the screen control unit selects a screen from which edge enhancement or edge missing is not detected from the plurality of patch toner images. The image forming apparatus according to any one of 2 to 6. The image according to any one of claims 1 to 7, wherein the character mode / non-character mode can be set, and the screen processing is not performed when the character mode is set. Forming equipment. The image forming apparatus according to claim 1, wherein the plurality of screens include screens having different numbers of screen lines. The image forming apparatus according to claim 2, wherein the plurality of screens include screens having different screen angles.

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