JPH08202092A - Digital image forming device - Google Patents

Abstract

PURPOSE: To adjust halftone. CONSTITUTION: The speed of rotation of a rotary sleeve is controlled so as to make a circumferential speed ratio on a proper image forming condition by forming a solid patch image and detecting the density of the solid patch image (step 6), and a halftone patch image is formed (step 8), and the density of the halftone patch image is detected (step 9) and potential by electrification VH is selected based on a detected output value V0 (step 12) and the potential VH by the electrification and writing laser power are fixed so as to obtain an optimum color digital image forming condition (step 13).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital image forming apparatus utilizing an electrophotographic process such as a copying machine and a printer, and more particularly to a technique for properly reproducing halftone density.

[0002]

2. Description of the Related Art In recent years, image forming apparatuses of digital type and reversal development have been put to practical use, and for example, digital image forming apparatuses forming an output section of color electrophotographic copying machines, personal computers, facsimiles, etc. are known. .
In such a digital image forming apparatus, a patch detection type image density control technique is adopted so that a recorded image with good reproducibility can be obtained even during continuous operation, and adjustment work can be performed easily.

As a recent trend of color printing systems, systems for realizing more accurate color matching have been proposed. In view of this situation, in the digital image forming apparatus using the electrophotographic method, it is necessary to accurately control the gradation of the image including the halftone. In Japanese Patent Application Laid-Open No. 63-113568, tone pattern density data representing gradations is stored in advance and a tone density pattern for each color is formed on a photoconductor before forming a digital image. Then, the pattern density is measured by a detection sensor, and the measured pattern density is compared with the stored density pattern image data to variably control the exposure amount of a laser beam or the like to perform density adjustment processing. Has been done.

However, in such a technique, the types and quantities of the measurement data of the gradation density pattern for each color become enormous, the program related to the adjustment processing becomes complicated, and the correction data prepared increases, resulting in an enormous design cost. There is a problem. Therefore, as a digital image forming apparatus of the same patch detection method, by performing feedback control of the rotation speed of the developing sleeve in a variable speed in accordance with the fluctuation of the detected density data value, the toner on the appropriate image carrier during digital image formation is adjusted. A technique has been developed in which the peripheral speed ratio of the developer carrying carrier to the image carrier is set so as to obtain the amount of adhesion (Japanese Patent Application No. 6-189211).

[0005]

However, in such a digital image forming apparatus, since the solid patch image is formed and the density thereof is detected, the image density of the solid patch image is accurately reproduced. Halftone image densities are not necessarily compensated for due to the influence of the environment and the like.

Explaining this point in detail, first, the image is binarized into multi-tone image data by, for example, the conventionally used dither method. This method is shown in FIG.
As shown in, for example, a 4 × 4 threshold matrix that corresponds to the pixels in a one-to-one relationship (the figure shows the type of the fattening type)
In this method, the value corresponding to each element value of the threshold value matrix is used as a threshold value and is binarized. However, when the threshold value is exceeded, the toner is made to fly and adhere to the pixel to be displayed. The gradation of the display image is according to the area of the fixed toner,
The toner area is affected by the surface condition of the image carrier, environment such as temperature and humidity.

When this is measured by a chromaticity meter or the like, as shown in FIG. 10 showing the color space of the Lab color system, yellow (Y), red (R), and magenta of the 10th gradation (outer periphery), respectively. (M),
Blue (B), cyan (C), and green (G) are accurately reproduced, but due to the flying property of the toner, for example, in the halftone near the white of the center O, it is close to white and becomes 10 gradations. The gradation seems to change suddenly.

Therefore, in such a case, the halftone image density does not change uniformly from white to the 10th gradation, and even if the density of the solid patch image is controlled to an appropriate density, the halftone image density is controlled. The image does not always have an appropriate gradation due to the influence of the environment or the like. The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a digital image forming apparatus capable of adjusting even a halftone.

[0009]

Therefore, in the digital image forming apparatus according to the invention of claim 1, as shown in FIG. 1, a solid patch image output means for outputting a solid patch image before the digital image formation, and an image Detecting means for optically detecting the image on the carrier, and peripheral speed for variably setting the peripheral speed ratio of the developer carrying carrier to the image carrier during digital image formation based on the signal output value of the detecting means. In the digital image forming apparatus provided with the ratio variable setting means, the halftone patch image output means for outputting the halftone patch image at the peripheral speed ratio at the time of the set digital image formation, and the image carrier. At least one of the charging potential on the image carrier and the power of the laser for writing so that the density of the halftone patch image becomes a desired density based on the signal output value of the detection means at the time of detection. And halftone control means for variably controlling,
Equipped with.

In the digital image forming apparatus according to the second aspect of the present invention, the halftone patch image output means may output a halftone patch image by a dither method using a Bayer type threshold matrix. In the digital image forming apparatus according to the third aspect of the present invention, the halftone patch image output means is configured to output a halftone patch image of about 1/2 gradation of the solid patch image.

In the digital image forming apparatus according to a fourth aspect of the present invention, the halftone control means has a signal output value when the detecting means detects the halftone patch image forming area,
At least one of the charging potential and the writing laser power is variably controlled based on a ratio between the detection unit and a signal output value when the peripheral region of the patch image forming region is detected.

In the digital image forming apparatus according to the fifth aspect of the present invention, the halftone patch image output means outputs the halftone patch image for each color of the color image, and the detection means optically outputs the color image for each color. The halftone control means,
At least one of the charging potential and the writing laser power on the image carrier is variably controlled based on the signal output value for each color of the color image.

[0013]

According to the structure of the digital image forming apparatus of the first aspect of the present invention, the petapatch image is output and optically detected before the digital image formation. Then, based on this signal output value, the peripheral speed ratio of the developer transport carrier to the image carrier during digital image formation is set. After that, the halftone patch image is output by the halftone patch image output means at the set peripheral speed ratio at the time of digital image formation, and the detection means is controlled by the detection control means so as to optically detect the image carrier. It Then, the halftone control means sets at least one of the density of the halftone patch image by controlling at least one of the charging potential of the image carrier and the power of the writing laser for writing in the image display area.
Therefore, it is possible to adjust the image density of the entire area from the halftone image to the solid image.

According to the configuration of the digital image forming apparatus of the second aspect, in the Bayer type dither method,
From white to halftone, the density change amount with respect to the gradation change is large, so that it becomes easy to detect the density of the halftone patch image. According to the configuration of the digital image forming apparatus of the third aspect of the present invention, the density of the halftone patch image is about ½ gradation of the solid patch image and changes in the charging potential and the writing laser power on the image carrier. Since it changes most, it becomes easy to adjust the image density of the halftone.

According to the structure of the digital image forming apparatus of the fourth aspect of the present invention, for example, the surface state of the image carrier changes after a long period of use of the image carrier, and the baseline output of the detection signal of the detection means is performed. , The signal output value at the time of detecting the halftone patch image and the signal output value at the time of detecting the periphery of the patch image change in the same manner. It is not affected by the surface condition of. Therefore, the detection accuracy of the halftone patch image density is improved, and an accurate output value of the halftone patch image density can be obtained even if the baseline correction is finished at a certain level.

With the configuration of the digital image forming apparatus according to the fifth aspect of the present invention, it is possible to obtain an appropriate halftone image even for a color image when forming a digital image.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. FIG. 2 is a schematic control block diagram of the digital color image forming apparatus in this embodiment. The operation of the normal image recording mode will be described with reference to FIG.

A charging device 2, a developing device 3, a transfer roller 4, a charge eliminating brush 5 and a cleaning portion 6 are arranged facing the photosensitive drum 1 as an image bearing member which rotates in the direction of the arrow. When the exposure unit 11 irradiates the photosensitive drum 1 charged by the charger 2 with a laser beam corresponding to an image, toner adheres to the irradiated portion of the surface of the photosensitive drum 1 by the developing unit 3 and electrostatically accumulates. The latent image is visualized. The developing device 3 has a primary color of Yellow (Y), Magenta
(M), Cyan (C), and Black (Bk), four developing sleeves 3a and 3b serving as developer carrying carriers.
3C, 3d and a developing sleeve drive motor (not shown) are provided, and when forming an image of a specific color, a direct current (D
C) developing sleeves 3a, 3b, 3 biased
By superimposing an alternating current (AC) bias on a specific developing sleeve from among c and 3d, toner particles on the developing sleeve rotated by the developing sleeve drive motor fly and adhere to the photosensitive drum 1 to form an image of a specific color. Is formed. Regarding the secondary color, a desired color can be formed by combining the developing sleeves 3a, 3b, 3c, 3d.

When the toner image formed on the photosensitive drum 1 by the developing device 3 reaches the transfer position of the transfer roller 4,
A recording paper (not shown) whose timing is controlled is also conveyed to the transfer position, a transfer bias having a polarity opposite to that of the toner is applied from the back surface of the recording paper, and electrostatic attraction acts on the photosensitive drum 1. The toner image is transferred to the surface of the recording paper, and the charge eliminating brush 5 removes the unnecessary potential.

When a color image is formed, the charging, exposing and developing steps are carried out for the Y color, and the series of steps are sequentially repeated for the M, C and Bk colors, and the transfer step is finally carried out at the end. The recording paper is separated from the photosensitive drum 1 by a known separation means such as a separation claw or curvature separation, and is thermally fixed by a fixing device (not shown), and then discharged from the apparatus. Then, the cleaning unit 6 cleans the residual toner on the photosensitive drum 1 to prepare for the next recording.

The developing conditions are as follows. (1) Charge potential of photosensitive drum 1 (V _H ): −750 V ± 50
V (2) DC bias voltage (V _DC ) of developing sleeve: -650
V (3) AC bias voltage (V _AC) of developing sleeve: 1.7kV
_PP (4) Development sleeve frequency: 8kHz (5) Developer transport amount (D _WS ): 20-30mg / cm ² (6) Development gap: 0.5mm (between 60) (7) Image writing potential ( _VL ): -40 to -50V (8) Line speed of photoconductor drum 1: 100mm / sec (9) Line speed of developing sleeve for digital image formation: 70mm / sec (10) Line of developing sleeve for test patch formation Speed: 70 mm / sec (11) Developer used: Two-component developer (Bk) (12) Change in charge amount: ± 3 μC / g (humidity 20-80%) (13) Writing laser power: 1.4 ± 0.3 mV Next First, a configuration for performing image adjustment using a color patch image will be described.

Image data for color patch is previously stored in the memory 7.
In the image adjustment mode, the control unit 8 sequentially reads the color patch image data of each color from the memory 7, and outputs the color patch image data to the exposure unit 11, thereby charging the photoreceptor. The drum 1 is exposed, and the electrostatic latent image is visualized by the developing device 3. On the other hand, as shown in the schematic configuration of FIGS. 3 and 4, the detection sensor 9 provided as a detection means between the charge removal brush 5 and the cleaning unit 6 is provided with a pair of scissor angles of 40 °. And a phototransistor (PtTr) for receiving reflected light according to the amount of toner forming a color patch image. In this embodiment, the distance between each element of the detection sensor 9 and the photosensitive drum 1 is set to 6 mm.

The control unit 8 inputs a detection signal from the detection sensor 9 according to the flow chart of FIG. 5, and the peripheral speed ratio of the developing sleeve to the photosensitive drum 1 and the photosensitive drum 1 so that an appropriate digital image is formed. Control the charging potential, writing laser power, etc. Next, the operation of the control unit 8 will be described based on the flowchart of FIG.

Steps (indicated as "S" in the figure,
The same applies hereinafter) 1. By detecting the magnetic permeability of the magnetic developer (L test), the toner density is set to a predetermined density in accordance with the development environment such as temperature, humidity, and the number of developed sheets. . In step 2, the grid voltage of the charger 2 is adjusted according to the developing environment, and the charging potential V _H of the photoconductor 1 is initialized.

In step 3, the writing laser power of the exposure section 11 is initialized. In step 4, baseline correction is performed. That is, the photosensitive drum 1 is spun,
The detection signal value of the detection sensor 9 is ± of the specified baseline output.
The light amount of the light emitting element (LED) of the detection sensor 9 is adjusted to be 2%. In step 5, a solid patch image is formed on the photoconductor 1, and the density of the solid patch image is detected, so that the developing sleeves 3a, 3b, 3c, 3d can be rotated around the photoconductor drum 1 so as to obtain an appropriate digital image forming condition. Calculate the speed ratio.

To form the solid patch image and set the peripheral speed, for example, the technique described in Japanese Patent Application No. 6-280719 may be used. In this case, the peripheral speed ratio of the developing sleeve to the photosensitive drum 1 is set to 0.7 to form a solid patch image on the photosensitive drum 1, and the detection sensor 9
Inputs a detection signal when a solid patch image is detected by. Then, based on the input detection signal, the toner adhesion amount of the solid patch image is calculated, the toner adhesion amount according to the peripheral speed ratio is estimated, and the toner adhesion amount calculated by generating a series and the peripheral speed ratio are calculated. The peripheral speed ratio at the time of digital image formation is obtained based on the relationship.

In step 6, the rotation speeds of the developing sleeves 3a, 3b, 3c and 3d are controlled so that the calculated peripheral speed ratio is achieved. The above is the contents of the process control for obtaining an appropriate solid patch image. In step 7 and subsequent steps, process control is performed so as to obtain an appropriate halftone image density.

That is, in step 7, the charging potential V _H is set to the initial value −700 V. This charging potential V _H is set by controlling the grid V _{G of the} charger. In step 8, a halftone patch image is formed for each color. A Bayer type threshold matrix as shown in FIG. 6 is used for the halftone patch image, and an eighth tone halftone patch image is formed. As shown in FIG. 7, the threshold matrix of the Bayer type has an inflection point at about 10 gradations, but from 0 to 8 gradations, the gradation changes greatly, and the gradation of the halftone patch image is changed. It is convenient to match. Incidentally, when forming a digital image, a dithering method of a fattening type which does not have an inflection point unlike Bayer type and has a natural gradation is used.

This step corresponds to a halftone patch image output means. In step 9, the formed halftone patch image is detected and the detection output value is input. The detection output value V ₀ is a value based on the following equation. Detection output value V ₀ = V ₁ / V ₂ where V _{1 is} the output value of the detection sensor 9 when the halftone patch image is detected V _{2 is} the detection sensor 9 when the peripheral portion of the halftone patch image is detected Output value (baseline output value) (1) Next, the process proceeds to step 10 → 11, the charging potential V _H is lowered by a predetermined potential, for example, 10 V, and the step 8 is repeated. 9 is executed. This is repeated until the charging potential V _H reaches the final value −800 V.

When the charging potential V _H reaches the final value −800 V, the process proceeds to steps 10 → 12 to select the charging potential V _H when the detection output value V ₀ has a desired image density. In order to select the charging potential V _H , the detection output value V ₀ of the detection sensor 9 changes depending on the charging voltage V _H. Therefore, first, based on FIG. 7 showing the relationship between the gradation and the detection output value, the eighth floor The detection output value V ₀ of the detection sensor 9 when the halftone patch image of the tone is detected is obtained.

Next, since the detection output value V ₀ of the detection sensor 9 and the image density do not have a one-to-one correspondence, based on FIG. 8 for making these values have a one-to-one correspondence, the detection output value V ₀
Then, the density of the halftone patch image of the eighth gradation is obtained. The detection output value V ₀ of the detection sensor 9 and the image density do not have a one-to-one correspondence because the detection output value V ₀ is a value based on the amount of received reflected light, and the image density is detected by a color difference meter or the like. This is because the value depends on the image density.

The charging potential V _H is selected based on this image density. The data shown in FIGS. 7 and 8 are stored in the memory 7 for each color in advance. Then, in step 13, the charging potential V _H of the photosensitive drum 1 is fixed to this selected potential. When it is impossible to control only the charging potential V _H due to fogging, carrier adhesion, etc., the writing laser power is variably controlled to change the exposure amount of the laser beam or the like.

The steps 12 and 13 correspond to the halftone control means. With this configuration, the peripheral speed ratio of the developing sleeve to the photosensitive drum 1 is set so that an appropriate solid patch image is formed, and then based on the detection output value of the formed 8-tone halftone patch image. By controlling the charging potential of the photosensitive drum 1 and the writing laser power, it is possible to properly adjust the entire area of the color image including black, from the halftone to the solid image.

Further, since the threshold matrix according to the dither method is of the Bayer type, which has a large amount of density change with respect to gradation change, from white to halftone, the density detection of the halftone patch image becomes easy. Further, an image of the eighth gradation, which is a half gradation of the solid patch image for each color, is formed, and the density of the patch image of this gradation is the charging potential V _H on the photosensitive drum 1 and the writing laser. Since it changes most according to the change in power, it becomes easy to adjust the image density of the halftone.

Since the detection output value V ₀ of the detection sensor 9 is defined as the ratio of the output value when the peripheral portion of the halftone patch image is detected and the baseline output value, the surface condition of the photosensitive drum 1 and the baseline Not affected by the fluctuation of the output value,
An accurate output value of the halftone patch image density can be obtained.

[0036]

As described above, in the digital image forming apparatus according to the first aspect of the invention, it is possible to adjust the image density of the entire area from the halftone image to the solid image. In the digital image forming apparatus according to the second aspect of the present invention, it is easy to detect the density of the halftone patch image.

In the digital image forming apparatus according to the third aspect of the present invention, it becomes easy to adjust the image density of the halftone.
In the digital image forming apparatus according to the invention of claim 4, an accurate output value of the halftone patch image density can be obtained. With the digital image forming apparatus according to the fifth aspect of the present invention, it is possible to obtain an appropriate halftone image density even when forming a digital image for a color image.

[Brief description of drawings]

FIG. 1 is a claim correspondence diagram showing a configuration of the present invention.

FIG. 2 is a schematic control block diagram of a digital color image forming apparatus according to an embodiment of the present invention.

FIG. 3 is a schematic configuration diagram of the detection sensor of FIG.

FIG. 4 is a flowchart showing the operation of the control unit in FIG.

FIG. 5 is a diagram showing a relationship between a detection signal value and a toner adhesion amount.

FIG. 6 is a diagram showing a Bayer type threshold matrix of the dither method.

FIG. 7 is a diagram showing a relationship between gradation and detection output.

FIG. 8 is a diagram showing a relationship between detection output and image density.

FIG. 9 is a diagram showing a threshold matrix of a fattening type of the dither method.

FIG. 10 is a diagram showing a color space of the Lab color system.

[Explanation of symbols]

 1 Photosensitive Drum 3 Developing Device 7 Memory 8 Control Section 9 Detection Sensor 10 Color Patch Image Detection Circuit

Claims (5)

[Claims] 1. Before forming a digital image, a solid patch image output means for outputting a solid patch image, a detection means for optically detecting an image on an image carrier, and a digital output based on a signal output value of the detection means. In a digital image forming apparatus comprising: a peripheral speed ratio variable setting means for variably setting a peripheral speed ratio of a developer carrying carrier to an image carrier during image formation, the peripheral speed during digital image formation being set. A halftone patch image output means for outputting a halftone patch image in a ratio, and a density of the halftone patch image to a desired density based on a signal output value of the detecting means when the image carrier is detected. A digital image forming apparatus, further comprising: a halftone control unit that variably controls at least one of a charging potential on the image carrier and a power of a laser for writing. 2. The digital image forming apparatus according to claim 1, wherein the halftone patch image output means outputs a halftone patch image by a dither method using a Bayer type threshold matrix. . 3. The halftone patch image output means is configured to output a halftone patch image of about half gradation of a solid patch image. Digital image forming apparatus. 4. The halftone control means includes a signal output value when the detecting means detects a halftone patch image forming area and a signal output value when the detecting means detects a peripheral area of the patch image forming area. And the charging potential based on the ratio of
The digital image forming apparatus according to claim 1, wherein at least one of the writing laser powers is variably controlled. 5. A halftone patch image output means outputs a halftone patch image for each color of a color image, a detection means optically detects the color image for each color, and a halftone control means for the color tone. 5. The apparatus according to claim 1, wherein at least one of a charging potential on the image carrier and a writing laser power is variably controlled based on a signal output value for each color of the image. 1. The digital image forming apparatus described in 1.