JPH06148992A - Electrophotographic device - Google Patents

Abstract

PURPOSE:To accurately measure the density of a gradation correction pattern even though the gradation correction pattern forming area of an intermediate transfer body is damaged because of secular change by detecting the surface density of the intermediate transfer body at plural positions where the gradation correction pattern is formed and setting the detected result as a white reference for each gradation correction pattern. CONSTITUTION:Both output from a density sensor 32 and output from a 2nd density sensor 42 are inputted in the A/D conversion port of a CPU 43, and the CPU 43 can always refer to the output from the sensors 32 and 42. In the case of detecting the density of the gradation correction pattern, the CPU 43 reads the output from the sensor 32 in timing when the gradation correction pattern formed on the intermediate transfer body 1 reaches the position of the sensor 32. Hereafter, the density of one of the gradation correction patterns is detected in a specified cycle specified times, and the result of read is averaged and stored in a RAM 45 via the CPU 43. Then, it is set as the white reference for each gradation correction pattern.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic apparatus for obtaining a high quality recorded image.

[0002]

2. Description of the Related Art Conventionally, printers of various principles have been proposed as output terminals for personal computers, workstations, etc. In particular, a laser beam printer using an electrophotographic process and laser technology has a high recording speed and a high print quality. In terms of advantages, it is the mainstream printer.

In the market, there is an increasing demand for full-color laser beam printers.
For example, if the image data is 8 bits, 256 units are set for each color.
About 167 in combination of gradation, cyan, magenta and yellow
Since the reproducibility of 0,000 colors is required, the gradation reproducibility is a particularly important factor.

Generally, in this type of device, a gradation correction pattern is formed on an image carrier for forming or holding an image using prescribed image data, and this is detected by a density sensor to which a reflection type sensor or the like is applied. The input / output non-linearity of the device is corrected (γ correction), and the parameter of each process is changed so that the read value becomes a predetermined value.

A conventional electrophotographic apparatus will be described below. An electrostatic latent image formed by a laser beam or the like on a photoconductor is developed by a developing device for each color, and a visualized monochromatic image is temporarily displayed. Transferring and synthesizing on an image forming medium called an intermediate transfer body, the combined image on the intermediate transfer body is collectively transferred to a sheet,
A so-called intermediate transfer body type full-color printer will be mainly described.

FIG. 6 is a block diagram of a conventional electrophotographic apparatus. In FIG. 6, reference numeral 1 denotes a seamless loop belt-shaped intermediate transfer member made of a conductive resin or the like, which is a medium for synthesizing a monochromatic image. Reference numeral 2 denotes an intermediate transfer member position detection mark such as a slit arranged at the end of the intermediate transfer member 1, and is used for detecting the image forming reference position. The photosensitive member 3 is a seamless belt having a photosensitive layer on its surface, is supported by three photosensitive member conveying rollers 4, 5 and 6, and is rotated in the direction of arrow d1 by a drive motor (not shown). Around the belt-shaped photosensitive member 3, a charger 7, along the direction of arrow d1,
Exposure optical system 8, developing units 9K, 9Y for black (K), yellow (Y), magenta (M), and cyan (C)
9M and 9C, an intermediate transfer body unit 10, a photoconductor cleaning device 11, and a static eliminator 12 are provided.

The charger 7 is composed of a charging wire made of a tungsten wire and the like, a shield plate made of a metal plate, a grid plate and the like (not shown). When a high voltage is applied to the charging wire, the charging wire causes corona discharge, The photoconductor 3 is uniformly charged to a negative potential via the grid plate. An exposure light beam 13 is emitted from the exposure optical system 8. The exposure light beam 13 is obtained by subjecting an image signal from a gradation conversion device (not shown) to light intensity modulation or pulse width modulation by a laser drive circuit (not shown), and a static image corresponding to a specific color on the photoconductor 3. Form a latent image. The developing devices 9K, 9Y, 9M and 9C respectively store black, yellow, magenta and cyan toners. The development of each color is performed by the separation cams 14K, 14M, 14 of each color.
Dedicated motors (not shown) corresponding to Y and 14C are driven to move, for example, the selected developing device 9K in the direction of arrow d3 to bring it into contact with the photoconductor 3.

The intermediate transfer body unit 10 is an intermediate transfer body 1.
And three intermediate transfer body conveying rollers 15, 16 and 17 that support the intermediate transfer body 1, and the intermediate transfer body 1 for sandwiching the intermediate transfer body 1 for transferring the toner image on the photosensitive body 3 to the intermediate transfer body 1. It has an intermediate transfer roller 18 arranged to face the body 3. Reference numeral 19 denotes an intermediate transfer member position detection sensor that detects the intermediate transfer member position detection mark 2. 20 is an intermediate transfer member 1
This is an intermediate transfer member cleaning device for scraping off the above residual toner, and is separated from the intermediate transfer member 1 while a composite image is being formed on the intermediate transfer member 1, and contacts only during cleaning. Reference numeral 21 is a recording paper cassette that houses recording paper 22. The recording paper 22 is the recording paper cassette 2
Paper feed path 24 from 1 by sheet feed roller 23
Sent to. Reference numeral 25 denotes a registration roller for temporarily stopping and waiting the recording paper 22 in order to match the positions of the recording paper 22 and the composite image on the intermediate transfer body 1, and is in pressure contact with the driven roller 26. Reference numeral 27 denotes a paper transfer roller for transferring the composite image on the intermediate transfer body 1 to the recording paper 22, and rotates in contact with the intermediate transfer body 1 only during transfer. Reference numeral 28 denotes a fixing device composed of a heat roller 29 having a heat source inside and a pressure roller 30, and the combined image transferred onto the recording paper 22 is pressed against the pressure and heat as the heat roller 29 and the pressure roller 30 are nipped and rotated. Then, the image is fixed on the recording paper 22 to form a color image. Reference numeral 31 is a temperature sensor such as a thermistor that detects the temperature of the heat roller 29. Reference numeral 32 is a density sensor configured of a reflection type sensor or the like for detecting the toner density on the intermediate transfer member 1.

The operation of the electrophotographic apparatus configured as described above will be described below. Here, an image is formed with four colors of black, cyan, magenta, and yellow, and the order of formation is black, cyan, magenta, and yellow.

When an image formation start command is issued from the outside, the photosensitive member 3 is driven in the direction of arrow d1 and the intermediate transfer member 1 is driven in the direction of arrow d2 by the same driving source (not shown), respectively. The reduction ratio is set so that the peripheral speed becomes the same constant speed.

When the intermediate transfer member 1 and the photosensitive member 3 reach a constant speed rotation, the charging line in the charging device 7 connected to the high voltage power source is -4.
A high voltage of about 000v to -5000v is applied to cause corona discharge, and the surface of the photoconductor 3 is uniformly charged to about -700v. Next, when the exposed light beam 13 modulated in accordance with, for example, black (K) print data is applied to the surface of the charged photoconductor 3, the irradiated part on the photoconductor 3 loses its charge and an electrostatic latent image is formed. It is formed. At this time, the electrostatic latent image formation start timing is determined by the position of the intermediate transfer member position detection sensor 1
It is determined by the signal from 9.

The developing device 9K is pressed in the direction of the arrow d3 by the rotation of the separation cam 14K and comes into contact with the photoconductor 3 after a lapse of a predetermined time after the mark 2 for detecting the position of the intermediate transfer member is detected. Immediately before this contact, a voltage of about −300 V is applied to the developing sleeve 33K, and after a lapse of a predetermined time after the contact, the developing sleeve 33K starts to rotate. Developing device 9K when rotation starts
The toner is supplied from the developing sleeve 33K to the developing sleeve 33K. Since the electric charge of the toner is controlled to be negative, the toner adheres only to the portion (electrostatic latent image portion) on the photoconductor 3 where the exposure light beam is irradiated and the electric charge disappears, and so-called reversal development is performed. The developing device 9K, which has completed the development, has a contact cam 14K of 18
Rotation by 0 degrees moves from the contact position with the photoconductor 3 to the separated position.

The toner image formed on the photosensitive member 3 by the developing device 9K is transferred to the intermediate transfer member 1 by applying a high voltage of about +800 V to the intermediate transfer roller 18 arranged in contact with the photosensitive member 3. . The residual toner not transferred from the photoconductor 3 to the intermediate transfer body 1 is removed by the photoconductor cleaning device 11. Furthermore, the charge on the photoconductor 3 from which the residual toner has been scraped off by the static eliminator 12 is removed.

Next, when the intermediate transfer member position detection sensor 19 detects the intermediate transfer member position detection mark 2, cyan (C) image formation is performed. Intermediate transfer member position detection mark 2
After a lapse of a predetermined time after the detection, the separation cam 14C rotates, and this time the developing device 9C is pushed in the direction of arrow d3, and the photoconductor 3 is pressed.
And the development of cyan (C) is started. The subsequent process is exactly the same as in the case of black.

Next, when the intermediate transfer member position detection mark 2 is detected, image formation is performed in the order of magenta and then yellow, and the images of the respective colors are sequentially superposed on the intermediate transfer member 1. Finally, a four-color composite image is formed on the intermediate transfer body 1.

On the other hand, the paper feed roller 23 is rotated during the formation of the fourth color (yellow) image, and the recording paper 22 is sent from the recording paper cassette 21 to the paper transport path 24. Recording paper 22
Is temporarily stopped between the registration roller 25 and the driven roller 26. Next, the registration roller 25 starts to rotate at the timing when the leading end position of the composite image formed on the intermediate transfer body 1 reaches the specified position of the recording paper 22.

After the registration roller 25 starts rotating, the sheet transfer roller 27, which has been separated so far, is brought into contact with the intermediate transfer body 1, and a high voltage of about +1000 V is applied to the sheet transfer roller 27. The formed composite image is collectively transferred to the recording paper 22 by Coulomb force and pressure.

The recording paper 22 on which the toner image is transferred is sent to a fixing device 28, where it is fixed by the heat of the heat roller 29 and the nip pressure of the pressure roller 30 and is output as a color image.

On the other hand, the residual toner on the intermediate transfer body 1 which has not been completely transferred to the recording paper 22 is removed by the intermediate transfer body cleaning device 20. The intermediate transfer body cleaning device 20 is at a position apart from the intermediate transfer body 1 during the period in which the combined image is formed, and after the combined image is transferred onto the recording paper 22 by the paper transfer roller 27, the contact state is established. ,
Residual toner is removed. With the above operation, recording of one image is completed, and a color recorded image is obtained.

Next, the conventional gradation correction in the electrophotographic apparatus for forming an image by the above-mentioned operation will be described. The electrophotographic device initializes the device when the power is turned on,
Gradation correction is executed during this initialization. When gradation correction is executed, an electrostatic latent image is formed on the photoconductor 3 based on the built-in data created inside the electrophotographic apparatus, and transferred to the intermediate transfer body 1 after development. However, since the cleaning device 20 for the intermediate transfer body 1 always contacts and cleans the intermediate transfer body 1, the images do not overlap on the intermediate transfer body 1. Therefore, the gradation correction pattern based on the built-in data is formed on the intermediate transfer member 1 for each color and is cleaned each time. Since all process timings are time-controlled with the detection of the intermediate transfer member position detection mark 2 by the intermediate transfer member position detection sensor 19 as a starting point, the density sensor 32
Can periodically detect the formed gradation correction pattern based on a predetermined timing. Further, during gradation correction, each process of paper feeding, image paper transfer, and fixing is naturally not performed.

Next, the process of detecting the gradation correction pattern formed on the intermediate transfer member 1 by the density sensor 32 will be described with reference to FIG. FIG. 7 is an output diagram of the density sensor with respect to the gradation correction pattern of the chromatic components (cyan, magenta, yellow) and the achromatic component (black) in the conventional electrophotographic apparatus. The output from the density sensor 32 is a predetermined reference value 34a for the background of the intermediate transfer body 1 in a stationary state.
Is adjusted so that it becomes a white reference for density detection. White here means a state where the intermediate transfer body 1 is free of toner. Further, the gradation correction pattern for each color is predetermined so that the density increases in order from the beginning.

Reference numeral 35 denotes a density sensor 32 which is a gradation correction pattern of a color component having gradation that is formed on the intermediate transfer member 1.
This is a typical characteristic when detected in. Strictly speaking, the characteristics are different for each color, but there is no difference in that the characteristics increase monotonically as the density of the gradation correction pattern increases.

Reference numeral 36 is a typical characteristic when a grayscale correction pattern of an achromatic color component is detected under the same condition, and it monotonically decreases as the density of the grayscale correction pattern increases. Reference value 3 for chromatic and achromatic components as the density of the gradation correction pattern increases
A major feature is that the value changes in different directions across 4a.

The intermediate transfer body 1 as a dielectric is black because carbon is dispersed therein. When detecting a color component, the reflectance of the coloring material and the scattering of light on the toner surface are both increased, and the density sensor output is obtained. Increases monotonically. On the other hand, when the achromatic component is detected, the irradiation light from the density sensor 32 is absorbed by the toner surface according to the density of the gradation correction pattern, so that the output of the density sensor 32 monotonously decreases.

Next, using predetermined image data,
Gradation correction patterns having different gradations are formed on the intermediate transfer body 1, and the density of the gradation correction pattern is measured by the density sensor 32.
The process of detection will be described.

In the gradation correction pattern of the color component, the difference between the representative characteristic 35 and the reference value 34a in FIG. 7 is defined as the density level 37 of the color component, and the gradation correction pattern of the achromatic component has the reference value 34a and the characteristic. The difference of 36 may be defined as the density level 38 of the achromatic color component. If the characteristic 35-reference value 34a is the density level in the case of the chromatic component and the reference value 34a-characteristic 36 is the density level in the case of the achromatic component, it is convenient because the signs need not be considered in the subsequent processing.

Next, the density level 37 of the chromatic component is normalized with respect to the dynamic range 39 of the chromatic component, and the density level 38 of the achromatic component is normalized with respect to the dynamic range 40 of the achromatic component. When represented by 8 bits, it can be represented by 0 to 255.

Since the input image data has a predetermined value with respect to the normalized density level determined as described above, the relationship of the input image data with respect to the density level is tabulated, and the relationship is set for the image data. If the image is formed through the table characteristics, the gradation of the image is secured. That is, this table serves as a γ correction table for correcting the γ characteristic of the electrophotographic apparatus.

[0029]

However, the electrophotographic apparatus described as the conventional example has the following problems to be solved. That is, in FIG. 7, when the intermediate transfer member 1 is actually rotated to detect the background density thereof, the output of the density sensor 32 changes as indicated by a wavy line 34b. As the cause, for example, a minute foreign substance is caught by the intermediate transfer member cleaning device 20, which rubs the intermediate transfer member 1 for a long period of time, or the intermediate transfer member conveying rollers 15, 1.
6 and 17, and scratches and the like caused by cyclic pressure contact from the back side of the intermediate transfer body 1. Although some of these scratches are sudden, most of them gradually increase as the intermediate transfer member 1 is rotated for a long period of time. That is, the fluctuation of the background density is hardly observed when the intermediate transfer body 1 is new, but increases as the intermediate transfer body 1 is used.

Further, the density sensor 32 is installed at a position facing the intermediate transfer body conveying roller 17 and is designed not to be affected by the positional variation of the intermediate transfer body 1, but the tension applied to the intermediate transfer body 1 may be different. The output of the density sensor 32 also fluctuates due to subtle changes, eccentricity of the intermediate transfer member transport rollers 15, 16 and 17.

The problem when the background density of the intermediate transfer member 1 varies will be discussed below. Since the intermediate transfer body 1 holds the single-color images in a superposed manner, if scratches are locally generated in the image area, for example, adverse effects such as transfer unevenness will occur. If the scratch is a normal image area, a defect occurs in a part of the image, but at least the other part is not affected. However, if this flaw occurs in the area where the gradation correction pattern is formed, the detection accuracy of the gradation correction pattern particularly in the low gradation portion (bright portion of the image) is significantly reduced. For example, if the background density of the low-gradation gradation correction pattern formation region is higher than the predetermined reference value 34a due to scratches, the gradation correction pattern is formed on the high background density.
The density of the gradation correction pattern is detected higher than that when there is no scratch. As a result, a γ correction table that suppresses the low gradation portion to a low level is generated, and gradation cannot be ensured.

That is, when the intermediate transfer body 1 is new and has no scratches, even if the white reference of the intermediate transfer body 1 is uniquely set like the reference value 34a, the density of the gradation correction pattern is detected relatively accurately. However, if the intermediate transfer body 1 is damaged due to aging or the like, the detection accuracy of the gradation correction pattern is significantly reduced.

Therefore, an object of the present invention is to provide an electrophotographic apparatus capable of accurately measuring the density of a gradation correction pattern even if the gradation correction pattern forming region of the intermediate transfer member 1 is damaged due to aging or the like. And

[0034]

In order to solve the above-mentioned problems, the electrophotographic apparatus of the present invention detects the background density of an intermediate transfer member at a plurality of positions where a gradation correction pattern is formed, and determines this for each gradation. The white reference is used for the correction pattern.

[0035]

By the above means, the white reference is independently determined for each position where the gradation correction pattern is formed. Therefore, even if the gradation correction pattern forming region of the intermediate transfer member is scratched, the gradation correction is accurately performed. The density of the pattern can be detected.

[0036]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an electrophotographic apparatus according to an embodiment of the present invention. FIG. 1 is the same as the conventional example except that a calibration plate 41 and a second concentration sensor 42 for detecting the concentration of the calibration plate 41 are added, and therefore the description other than these is omitted.

The calibration plate 41 is a white plate having a predetermined reflectance, and the second density sensor 42 is the calibration plate 4
The density of 1 is detected and used as the black reference. The black reference is used when normalizing the density of the color component toner. The black reference for the achromatic toner uses a predetermined value and does not use the detection value of the second density sensor 42.

The white plate is used as the black reference for the color components for the following reasons. When detecting the toner density with a reflective sensor, the actual change in reflectance is detected.
When the toner density of the color component increases, the reflectance also increases.
Since the black reference defines the upper limit of the color toner density, high reflectance, that is, white is suitable.

On the other hand, strictly speaking, the black reference for achromatic toner should be detected using a black calibration plate having a low reflectance, but black in a full-color image is basically a chromatic component (cyan, magenta, yellow). ), And is mainly used for image contrast enhancement, etc.,
Gradation as much as the coloring component is not required. Therefore, in this embodiment, the state in which the reflectance is 0 is given as a numerical value in advance as the black reference of the achromatic component.

Next, referring to FIG. 2, the control for the density sensor 32 and the density sensor 32 and the second density sensor 42 will be described.
The concentration detection process by will be described. The density sensor 32 and the second density sensor 42 are both the same reflection type sensor, and the CPU 43 sets a numerical value in the D / A converter 44,
It is configured to control the drive current on the light emitting side of the reflective sensor. In the embodiment, the value that can be set in the D / A converter 44 is 6 bits, and a value of 0 to 63 can be set. Since the light emitting sides of the density sensor 32 and the second density sensor 42 are electrically connected in series and driven by the same current, the light emission amounts of the two sensors are substantially the same.

On the other hand, the output of the density sensor 32 and the output of the second density sensor 42 are both input to the A / D conversion port of the CPU 43, and the CPU 43 always refers to the outputs of the density sensor 32 and the second density sensor 42. It can be configured. For example, when detecting the density of the gradation correction pattern, the CPU 43 reads the output of the density sensor 32 at the timing when the gradation correction pattern formed on the intermediate transfer body 1 reaches the position of the density sensor 32. After that, the density detection for one of the gradation correction patterns is 1
The reading is averaged 8 times in total in the 0 ms period, and the reading results are averaged and stored in the RAM 45 via the CPU 43.

The electrophotographic apparatus in this embodiment performs gradation correction at the initialization stage as in the conventional example. After the intermediate transfer body 1 and the photoconductor 3 reach the constant speed, these image forming media are rotated for a predetermined time. During this period, the intermediate transfer body cleaning device 20 enters into the cleaning operation, scrapes off the toner on the intermediate transfer body 1, and is in a state in which the background density of the intermediate transfer body 1 can be detected.

As the first step of gradation correction, the amount of light emission on the light emission side of the density sensor 32 is determined for each of the chromatic components (cyan, magenta, yellow) and the achromatic component (black). A description will be given below with reference to FIG. 3 which is a diagram showing the adjustment of the light emission amount of the density sensor of the electrophotographic apparatus in one embodiment of the present invention. Figure 3
The horizontal axis represents the number of rotation cycles of the intermediate transfer member 1, and the vertical axis represents the density sensor output after A / D conversion, that is, the density data detected by the CPU 43.

In the state where the intermediate transfer member 1 has been cleaned, the adjustment target value 46 of the background density of the color component is, for example, 1.25 v at the analog level, that is, "64" (= as data after A / D conversion). 1.25v / 5.00v x 2
55). The CPU 43 sets the median value (= “32”) of the 6-bit amount in the D / A converter 44 (coloring component first cycle in FIG. 3) and sets the light emission amount of the density sensor 32. The intermediate transfer body 1 is rotated once and the background density of the intermediate transfer body 1 is detected at a specified sampling cycle (for example, 10 ms cycle), and the minimum value is updated and held.
When one rotation of the intermediate transfer member 1 is completed, the minimum background density value held and the background density adjustment target value 46 (= “6
4 ”).

In the case of the first cycle of the color component in which the D / A converter 44 is set to "32" in FIG. 3, the minimum background density 47 during one cycle of the intermediate transfer body 1 exceeds the adjustment target value 46. Therefore, it is determined that the light amount needs to be reset.

In the next color component second cycle, "16"
(= 32-16) is set in the D / A converter 44. The change width at this time is "16". As shown in the figure, in the second cycle of the color component, the background density is lower than the adjustment target value 46, so that the light amount also needs to be reset. The previous change width "16" is halved, and the current change width is "8". Since the minimum value of the background density is smaller than the adjustment target value 46, it is determined that the amount of light emitted from the density sensor 32 should be increased.

In the third cycle of the coloring component, "24" (= 1
6 + 8) is set in the D / A converter 44, and the above-described operation is repeated. Actually, the minimum background density and the adjustment target value 4
If the difference from 6 is less than the specified value, the current D / A converter 4
The setting value of 4 is held in the memory, and the setting of the emitted light amount of the density sensor 32 at the time of measuring the color component is completed, but the change width is halved in cycle units, and light is emitted when the change width becomes 0. Since the amount adjustment operation is terminated, the above operation does not become an infinite loop. As the measurement cycle progresses,
The width of change to the setting of the D / A converter 44 becomes small,
The set value converges.

Next, the density sensor 32 for the achromatic component
Determines the amount of light emitted. This process is almost the same as the case of the coloring component, but the background density adjustment target value 48 is, for example, 3.0 v at the analog level, that is, “153” (= 3.00 v / 5.00 v as the data after A / D conversion). × 2
55), and a value higher than the target value of the color component is set.

The CPU 43 sets the median value (= “32”) of the 6-bit amount in the D / A converter 44 (first cycle of the achromatic color component in FIG. 3) and sets the light emission amount of the density sensor 32. The intermediate transfer body 1 is rotated once to detect the background density of the intermediate transfer body 1 at a specified sampling cycle (for example, 10 ms cycle), and the maximum value is updated and held.

When one revolution of the intermediate transfer member 1 is completed, the maximum background density value held is compared with the background density adjustment target value 48 (= “153”). In the case of the achromatic color component first cycle in which “32” is set in the D / A converter 44 in FIG. 3, the background density maximum value 4 during one round of the intermediate transfer member 1 is 4
In No. 9, since it falls below the adjustment target value 48, it is judged that it is necessary to reset the light amount.

In the second cycle of the achromatic color component, "4
8 ”(= 32 + 16) is set in the D / A converter 44.
The change width at this time is "16". In the second cycle of the achromatic component, the background density is below the adjustment target value 48, so
After all, it is necessary to reset the light amount. Previous change range "1
6 ”is halved, and the range of change this time is“ 8 ”. Further, since the maximum value of the background density is smaller than the adjustment target value 46, it is determined that the emitted light amount of the density sensor 32 should be increased.

In the third cycle of the achromatic color component, "56" (=
48 + 8) is set in the D / A converter 44, and the above-described operation is repeated. In practice, if the difference between the minimum background density and the adjustment target value 46 is less than or equal to the specified value, the current setting value of the D / A converter 44 is held in the memory, and the density sensor 32 emits light when measuring the achromatic component. Although the light amount setting is completed, the change width is halved in cycle units, and the light emission amount adjustment operation is terminated when the change width becomes 0, so the above operation does not become an infinite loop. As the measurement cycle progresses, the change range for the setting of the D / A converter 44 becomes smaller and the set value converges.

By the above-mentioned operation, the light emission amounts of the density sensor 32 for the chromatic component and the achromatic component are determined.
This operation means that the white reference baseline is changed by the color component.

When different light emission amounts are determined depending on the color components, the second step of gradation correction starts next. In the second stage,
The light amount of the density sensor 32 is switched to two settings for measuring the chromatic component and for measuring the achromatic component, and the background density of the intermediate transfer body 1 at a plurality of positions where the gradation correction pattern is formed is measured.

First, the light emission amount of the density sensor 32 is set at the time of measuring the color component, and the detection of the intermediate transfer member position detection mark 2 is started. When the mark 2 for detecting the position of the intermediate transfer member is detected, the CPU 43 measures the output of the density sensor 32 after a predetermined time has elapsed.

The measurement is carried out over a plurality of areas corresponding to a plurality of pattern positions, but the detection timing is exactly the same as when detecting the density of the gradation correction pattern.

Next, when the mark 2 for detecting the position of the intermediate transfer member is detected, the CPU 43 sets the density sensor 32 to the light amount at the time of measuring the achromatic component after a lapse of a predetermined time, as in the case of the chromatic component. The background density is measured at each position where each pattern of the achromatic component is formed. Hereinafter, the background density for each pattern position of the chromatic component is WL-CMY [n], and the background density for each pattern position of the achromatic component is WL-CMY [n].
Let BK [n]. Where n is the number of patterns,
In the case of this embodiment, the number of patterns is 10, and n = 0.
It takes a range of ~ 9.

When the second step is completed, the third step of detecting the density of the gradation correction pattern formed on the intermediate transfer member 1 is started. First, the gradation correction pattern will be described. Figure 4
FIG. 6 is a diagram showing a gradation correction pattern of the electrophotographic apparatus in one embodiment of the present invention. Since the gradation correction pattern is always formed at the same position on the photoconductor 3, even if the gradation correction pattern forming region is deteriorated by forming the same pattern many times, the deterioration of the image quality is psychologically less noticeable. It is formed at the edge of the image area. There are a total of 10 gradation correction patterns, and the image data is set in advance so as to form patterns of different densities. For example, the first pattern is 10H in hexadecimal notation, the next pattern is 20H, and so on.
The density is set to increase from the beginning of the image.

Further, the formation position of the gradation correction pattern is common to each color and the image data is also common, but each color image is formed using a different screen angle for each color, and for example, image data transferred from a host computer or the like is used. The screen angle when printing and the screen angle when executing gradation correction are common.

When the intermediate transfer member position detection mark 2 is detected after the completion of the second step of gradation correction, the gradation correction pattern is statically set on the photoconductor 3 based on the density data contained in the electrophotographic apparatus. A latent image is formed. After a lapse of a predetermined time, the developing device 9K comes into contact with the photoconductor 3 to perform development. The visualized black gradation correction pattern is transferred to the intermediate transfer member 1,
The density of each pattern is detected at the timing when the position of the density sensor 32 is reached.

The density sensor 32 measures eight points for one gradation correction pattern, and the average value of these is used as the density value of the gradation correction pattern.

Thereafter, each time the intermediate transfer member position detection mark 2 is detected, a cyan, magenta, and yellow gradation correction pattern is formed, the density is detected by the density sensor 32 for each gradation correction pattern, and the CPU 43 RA
Stored in M45.

On the other hand, the output of the second density sensor 42, which detects the density of the calibration plate 41 and determines the black reference, is fixed by the calibration plate 41 and is not easily influenced by the outside. Is read only once after the above, and is stored in the RAM 45 by the CPU 43 as in the case of the gradation correction pattern.

Hereinafter, the toner density at each pattern position of the chromatic component is D-CMY [n], and the toner density at each pattern position of the achromatic component is D-BK [n].

When the tone correction pattern densities are measured for all the colors, the CPU 43 collects the toner density data D-CMY [n] and D-BK [n] for each pattern and the white reference WL- for each pattern. Data processing is started using CMY [n] and WL-BK [n].

Details of the data processing will be described with reference to FIG. FIG. 5 is a diagram showing the relationship between the density measurement result of each pattern and the white reference and the data processing of the electrophotographic apparatus in one embodiment of the present invention. Here, only the coloring component will be described. The data processing of this coloring component is D-CMY.
[N] and WL-CMY [n] are used.

First, DIF [n] = D- for all n.
CMY [n] -WL-CMY [n] is calculated, and DIF
Define [n] as the true density level. Next, the minimum value 50 of the background density and the black reference 51 are set as the dynamic range DR,
DIF [n] is normalized to 8 bits with respect to the dynamic range DR. Further, the normalized data is converted into the density on the recording paper. Density conversion is a conversion table in which a specific pattern is formed on the intermediate transfer member 1 in advance, the data obtained through the process of normalization and the image density of the same pattern formed on the recording paper are measured, and the relationship is tabulated. Using. By converting to the density on the recording paper, it is possible to include the paper transfer / fixing characteristics in the density conversion table. For example, when the paper transfer characteristics fluctuate due to the environment or the like and the gradation is deteriorated, the conversion characteristics of the density conversion table can be changed according to the environment parameters or the like to absorb this influence.

The data of the gradation correction pattern has a predetermined value and is known. The relationship between the gradation correction pattern data and the density on the recording paper obtained as described above is nothing but the γ characteristic of the electrophotographic apparatus. Therefore, if the relationship between the density on the recording paper and the gradation correction pattern data is obtained, the inverse function of the γ characteristic (γ correction table) can be obtained.

[0070]

As described above, according to the present invention,
Conventionally, if the surface of the intermediate transfer member was scratched due to aging, etc., the gradation correction pattern could not be measured normally, but by setting the white reference for each pattern position, the density of the gradation correction pattern Can be measured accurately.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an electrophotographic apparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram showing the control of the density sensor of the electrophotographic apparatus and the flow of density information of the density sensor and the second density sensor in one embodiment of the present invention.

FIG. 3 is a diagram showing light emission amount adjustment of a density sensor of an electrophotographic apparatus according to an embodiment of the present invention.

FIG. 4 is a diagram showing a gradation correction pattern of an electrophotographic apparatus according to an embodiment of the present invention.

FIG. 5 is a diagram showing the relationship between the density measurement result of each pattern and the white reference and the data processing of the electrophotographic apparatus in one embodiment of the present invention.

FIG. 6 is a configuration diagram of a conventional electrophotographic apparatus.

FIG. 7 is an output diagram of a density sensor for gradation correction patterns of chromatic components (cyan, magenta, yellow) and achromatic components (black) of an electrophotographic apparatus of a conventional example.

[Explanation of symbols]

 1 Intermediate Transfer Body 3 Photosensitive Body 32 Density Sensor 41 Calibration Plate 42 Second Density Sensor

Claims (2)

[Claims] 1. An image carrier for holding a visualized toner image, an image forming means for forming a predetermined pattern at a predetermined position on the image carrier, and a density of the pattern. An electrophotographic apparatus having a density sensor for detecting, wherein the background density of the image carrier at a position where the pattern is formed is used as a white reference of image density. 2. The electrophotographic apparatus according to claim 1, wherein the pattern has a plurality of patterns, and the white reference is detected for each pattern.