JPH0815945A - Multicolor image forming device - Google Patents

Abstract

PURPOSE:To obtain a multicolor image forming device which adjusts exposure for each color according to its image type by means of a simple device structure. CONSTITUTION:A value obtained as the result of count by an image density counter 110 is outputted to a CPU 112, and a ratio is computed in this CPU 112 to obtain toner density. In the CPU 112, a transmissivity is read based on the toner density obtained by the computation, and is outputted to an exposure controller 116. In the exposure controller 116, a correction value alpha is read based on the transmissivity inputted from the CPU 112, and is outputted to an LD switching circuit 108. This correction value a controls the output current value of an LD 75 which emits light in response to an on-signal from a screen generator 106.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention repeats the steps of charging, exposing, and developing toner on a photosensitive member for each color to form a toner image of a plurality of colors, and then transfers the toner image onto a transfer material to form an image. The present invention relates to a multicolor image forming apparatus for forming a sheet.

[0002]

2. Description of the Related Art Conventionally, a photosensitive member (for example, a photosensitive drum)
In the image forming apparatus, in which the steps of charging, exposing, and developing are repeated for each color, and a plurality of toner images are formed, and then transferred collectively to obtain a multicolor (for example, color) image, each color is individually processed. The exposure is performed with a uniform exposure amount on the basis of the image signal determined in.

However, when the exposure for forming the second color toner image is performed after the first color toner image is present,
Part of it is exposed from above the first color toner image. At this time, due to the influence of this toner image, the irradiated light is absorbed by the toner or scattered by the toner, and so-called shielding occurs.

Due to this shielding, the potential difference between the exposed portion and the non-exposed portion (image portion and non-image portion) cannot be sufficiently obtained, and the color tone may be disturbed or the image may be faint.

Therefore, in order to solve this problem, it has been proposed to increase the exposure amount of each color in the subsequent steps to reduce the influence of shielding (for example, Japanese Unexamined Patent Application Publication No. Sho 6-62).
3-65460 gazette).

The above publication discloses three cases as shown in the following table (Table 1). That is, in case I,
In the case II
In, the light quantity is changed depending on whether or not the spots are superposed, and in Case III, a signal for gradation expression is added to Case II.

[0007]

[Table 1]

However, according to the above-mentioned means, the image formed with a relatively low density and the image formed with a high density are exposed with a constant exposure amount even though the degree of shielding is different. There is a problem in that there is a difference in color tone.

As another means, it has been proposed to store the address of a pixel in which an image has already been written and control the light quantity of the color to be written next based on that information (as an example, (See JP-A-63-298360).

That is, as shown in FIG. 8, with respect to the address of the pixel to be developed in the first color, for example, the driving current of the semiconductor laser applied as a light source is as shown in FIG. It outputs a current of 100% and is 0% in other cases. On the other hand,
As shown in 0, the driving current of the semiconductor laser with respect to the address of the pixel to be developed in the second color is, as shown in FIG. 11, the pixels of the first color which do not overlap (X, Y = 51, 10).
2) is a pixel that overlaps with 100% of the first color (X, Y = 52,
101) controls so as to output a current of 160%.

As a result, it is possible to reduce the influence of the shielding in the portions where the colors are overlapped with each other, and it is possible to prevent the disturbance of the color tone and the blurring of the image.

[0012]

However, since it is necessary to store the past density data or the presence / absence of recording for all pixels as an address, a huge memory capacity is required, and exposure for each pixel is required. Since control is required, control is difficult, and a high-speed and highly stable laser intensity modulation circuit or the like is required, which complicates the apparatus and leads to cost increase.

In consideration of the above facts, the present invention provides a multicolor image forming apparatus capable of adjusting the exposure amount for each color according to the type of image and implementing the adjustment with a simple apparatus configuration. That is the purpose.

[0014]

According to a first aspect of the present invention, the steps of charging, exposing and developing toner are repeated for each color on a photoconductor to form a toner image of a plurality of colors, and then the toner image is formed. A multicolor image forming apparatus for transferring an image onto a transfer material to form an image, wherein in the exposure step, the exposure amount for the exposure of the next color is set based on the toner density already existing on the photoconductor. It is characterized in that it has an exposing means for exposing.

According to a second aspect of the present invention, the toner density present on the photoconductor is proportional to the toner density, and is an average value of the densities obtained from the image signals of the respective colors for which the developing process has been completed. A conversion unit for converting and an exposure amount set based on an average value of densities obtained from the image signal
And an exposure amount setting means.

According to a third aspect of the present invention, the toner density existing on the photoconductor is converted into a detection signal from a density sensor for detecting the density on the photoconductor, which is proportional to the toner density. It is characterized in that it is constituted by a conversion means and a second exposure amount setting means for setting an exposure amount based on a detection signal from the density sensor.

[0017]

According to the first aspect of the invention, the toner density on the photosensitive member increases each time charging, exposure and toner development are repeated for each color. Here, in the exposure step, the exposure amount is adjusted based on the toner density already existing on the photoconductor. The adjustment of the exposure amount is as follows.

That is, when the toner density is high, the light is largely shielded by the toner. Therefore, in order to prevent the disturbance of the color tone and the blurring of the image due to the shielding, the exposure amount in the exposure process of the next color is set to the exposure of the previous process. Make it significantly larger than the amount.

On the other hand, when the toner density is low, since the light shielding by the toner is small, it is not necessary to increase the exposure amount during the exposure of the next color.

As described above, since the exposure amount in the exposure process of the next color is determined according to the toner density on the photosensitive member, it is possible to prevent the change of the color tone due to the difference of the influence of the shielding.

Further, the exposure amount may be switched for each color process or for each specific image area, and complicated control such as storing the address of each pixel and determining the exposure amount for each pixel is unnecessary. Become. As a result, a huge memory capacity for storing an address is not required, and a high-speed laser intensity modulation circuit or the like is not required when a semiconductor laser is used as a light source, so that the device configuration is simplified.

According to the second aspect of the invention, the toner density is proportional to the average value of the densities obtained from the image signals of the respective colors for which the developing process has already been completed. Then, the appropriate exposure amount for each color can be set by converting the value into the above formula and determining the exposure amount based on this average value.

According to the third aspect of the present invention, the toner density is proportional to the detection signal of the density sensor for detecting the density on the photoconductor, and therefore, the exposure is performed based on the detection signal from the density sensor. By determining the amount, an appropriate exposure amount for each color can be set.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a multicolor image forming apparatus 70 (hereinafter, simply referred to as image forming apparatus 70) according to this embodiment.

In the image forming apparatus 70, the casing 70A
A photosensitive drum 72 is provided in the substantially central portion of the inside.
The photosensitive drum 72 is rotated in the direction of arrow C in FIG. 1 by a driving device (not shown).

Above the photosensitive drum 72, a charger 74 for charging the outer peripheral surface (image forming surface) of the photosensitive drum 72.
Further, an exposure device 76 is provided which exposes the image forming surface with a laser beam from an LD (laser diode) 75 (see FIG. 2) according to the image data. The exposure device 76 is controlled by an exposure control unit 100 described later.

Further, in FIG. 1, on the right side of the photosensitive drum 72, from the upstream side in the rotation direction of the photosensitive drum 72,
A developing device 78Y (for yellow), a developing device 78M (for magenta), a developing device 78C (for cyan), and a developing device 78K (for black) are sequentially provided.

In the image forming apparatus 70, an electrostatic latent image is directly formed on the outer peripheral surface (image forming surface) of the photosensitive drum 72 by the charger 74 and the exposure device 76, and the electrostatic latent image is developed by the developing device 78Y. By developing with 78M, 78C, and 78K, a toner image is formed on the image forming surface.

As described above, in the image forming apparatus 70, the charging device 74, the exposure device 76, and the developing devices 78Y, 78M, 78.
Images of multiple colors are formed by C and 78K.

Below the photoconductor drum 72, a transfer roller 80 is provided so as to contact the image forming surface of the photoconductor drum 72. Further, in FIG. 1, a tray 8 for storing the recording paper P is provided on the lower right side of the casing 72A.
2 is loaded.

The recording paper P stored in the tray 82 is
The paper is taken out by a take-out roller 84 provided above the tray 82 at the loading position, sandwiched by a pair of rollers 86, guided by a guide 88, and conveyed between the transfer roller 80 and the photosensitive drum 72. The image formed on the image forming surface of the photosensitive drum 72 is transferred.

The recording paper P on which the image has been transferred is conveyed by an endless conveyor belt 92 that is wound around a pair of rollers 90, ejected to the outside of the casing 72A via a fixing device 94, and is ejected onto a paper ejection tray 96. It is supposed to be arranged. The image forming surface of the photoconductor drum 72 is cleaned by the cleaning device 98 arranged on the left side of the photoconductor drum 72 in FIG. 1, and waits for the next image forming process.

FIG. 2 shows the details of the exposure controller 100. The exposure control unit 100 includes an image reading device 102.
Are connected via the image processing apparatus 104, and in the image reading apparatus 102, an image should be formed in this apparatus,
Image data (for example, data obtained by scanning a base document or data stored in advance as an image signal) is read. The image data at this time is analog data.

The image data read by the image reading device 102 is output to the image processing device 104 for color processing, editing,
After processing such as processing, it is output to the screen generator 106 of the exposure control unit 100.
In the screen generator 106, the sent multi-bit (for example, 8-bit) image signal is binarized in the lighting time of the LD 75 (lighting DUTY or lighting position for each pixel), and is binarized to the LD switching circuit 108 of the exposure device 76. Is output. The LD switching circuit 108 includes the LD 7
5 is connected, and the LD 75 is controlled to be turned on and off based on the signal for each color sent from the screen generator 106. As a result, the photosensitive drum 72 can be exposed.

The image data output from the image processing device 104 is also output to the image density counter 110. In the image density counter 110,
It has a role of detecting the gradation (that is, density) of the image of each color, and as shown in FIG. 3, is represented by a numerical value of 8 levels from 0 to 7 for each predetermined image area. ing. Since this gradation distribution is substantially proportional to the amount of toner attached to the photosensitive drum 72, the toner density on the photosensitive drum 72 can be recognized from this gradation distribution.
That is, the sum of the actual gradations of each area (sum of numerical values) with respect to the case where all the image areas shown in FIG. 3 have gradation 7.
Is the average value of the image data, the toner density can be obtained by outputting the numerical value counted by the image density counter 110 to the CPU 112 and calculating the ratio by the CPU 112.

For example, when the average density D _A is calculated with the image density count value shown in FIG. 3, the density at the gradation 7 is 1.0
Then, D _A = 181 / (6 × 20 × 7) ≈0.22.

A memory 114 in which toner density-transmittance characteristics are tabulated and stored is connected to the CPU 112, and the transmittance is calculated based on the toner density (average density D _A ) obtained by the above calculation. Is read out and output to the light amount control device 116. In the light quantity control device 116, the current flowing through the LD 75, that is, the LD
The current value when the switch circuit 108 is turned on is controlled, the current value based on the transmittance is preset, and the current value based on the transmittance obtained from the toner density input from the CPU 112 is set. The data is read out and output to the LD switching circuit 108. As a result, the output current value of the LD 75 that emits light in response to an ON signal from the screen generator 106 is controlled.

The operation of this embodiment will be described below with reference to the flowchart of FIG. In step 200, initial settings are made. That is, each part such as the position of the photosensitive drum 72 is set to the initial position, the variable I is set to 1, and the process proceeds to step 202.

In step 202, the rotation of the photosensitive drum 72 is started, and in step 204, it is judged whether or not the photosensitive drum 72 is in the fixed position.
In step 206, the peripheral surface of the photosensitive drum 72 is charged by the charger 74.

In the next step 208, it is judged whether or not the variable I is 1, and if the judgment is affirmative, it is judged that the processing is for the first color (for example, C color), and the routine proceeds to step 210. , C image data is read out, and then in step 212, the exposure amount L _C of this C image is calculated. At this time, since the C color is the first color to be exposed on the photoconductor drum 72, toner is not attached to the photoconductor drum 72. Therefore, the calculation sent from the screen generator 106 is performed. The exposure amount L _C thus applied is applied as it is.

In the next step 214, the photosensitive drum 7
It is determined whether or not 2 has reached the exposure position. If it is determined that the exposure position has reached 2, the process proceeds to step 216 and the exposure device 76
The LD 75 irradiates a laser beam to record a latent image on the surface of the photosensitive drum 72. Then step 218
Then, it is determined whether or not it is the developing position, and when it is determined to be the developing position, the process proceeds to step 220, the developing device 78C is operated, and the C color toner is sent to the peripheral surface of the photosensitive drum 72, Toner adheres to the exposed portion to form a toner image.

At the next step 222, the variable I is incremented and the routine proceeds to step 224. Step 22
In No. 4, it is judged whether or not the variable I becomes 5 or more, and in the case of a negative judgment, that is, when the development of all colors (each color of CMYK) is not completed, the process proceeds to Step 204 and the charging is performed. The charging process by the container 74 is executed (steps 204, 206, 208).

In step 208, when the development of only C color has been completed, a negative determination is made, and the routine proceeds to step 26. In step 226, when the development of C and M colors is completed, step 226
Is determined negatively, and the routine moves to Step 228. Also,
If an affirmative decision is made, then the processing advances to step 230 in order to execute the M color development processing.

In step 230, the average density is calculated based on the C image data on which the toner image has already been formed.
As shown in FIG.
Are expressed by the gradations up to and calculated based on the numerical values expressed by the gradations. The transmittance on the photosensitive drum 72 is obtained based on the calculated average density, and in step 232, the correction coefficient α is read based on this transmittance.

In the next step 234, the data of the M image is read out, and the screen generator 106 calculates the reference exposure amount L _M (step 23).
6).

Here, when exposure is performed with the calculated exposure amount L _M , light may be reflected and absorbed at the portion where toner has already adhered, and proper exposure may not be possible.
Therefore, in step 238, the exposure amount L _M is corrected (L _M ← L _M · α), and the optimum exposure amount L _M when the toner of the C image is already attached is obtained.

At the next step 240, it is judged whether or not it is at the exposure position, and if a positive judgment is made, the routine proceeds to step 242, where the laser beam is irradiated by the LD 75 of the exposure device 76 and the latent image is formed on the surface of the photosensitive drum 72. The image is recorded. Next, at step 244, it is judged whether or not it is the developing position,
When it is determined that the developing position is reached, the process proceeds to step 246, the developing device 78M is operated, the M color toner is sent to the peripheral surface of the photosensitive drum 72, and the toner adheres to the exposed portion, A toner image is formed.

When the development of this M color is completed, step 2
The process moves from 46 to step 222. Therefore, C color and M
When the color development is completed, the variable I becomes 3 (step 222) and the next steps 224, 204 and 2 are executed.
After 06, 208, 226, an affirmative decision is made in step 228, and the routine proceeds to step 248, where the third color, that is, Y color developing process step is executed.

In step 248, the average density is calculated based on the C image and M image data on which the toner image has already been formed. The transmittance on the photosensitive drum 72 is obtained based on the calculated average density, and in step 250, the correction coefficient α is read based on this transmittance.

In the next step 252, the Y image data is read out, and the screen generator 106 calculates the reference exposure amount L _Y (step 25).
4).

Here, when exposure is performed with the calculated exposure amount L _Y , light may be reflected and absorbed at the portion where the toner has already adhered, and proper exposure may not be possible.
Therefore, in step 256, the exposure amount L _Y is corrected (L _Y ← L _Y · α), and the optimum exposure amount L _Y when the toners of the C image and the M image are already attached is obtained.

In the next step 258, it is judged whether or not it is the exposure position, and if the judgment is affirmative, the process proceeds to step 260, in which the laser beam is irradiated by the LD 75 of the exposure device 76 and the latent image is formed on the surface of the photosensitive drum 72. The image is recorded. Next, at step 262, it is judged whether or not it is the developing position,
When it is determined that the developing position is reached, the process proceeds to step 264, the developing device 78M is operated, the M color toner is sent to the peripheral surface of the photosensitive drum 72, and the toner adheres to the exposed portion, A toner image is formed.

When the development of Y color is completed, step 2
The processing moves from 64 to step 222. Therefore, C color, M
When the development of the colors Y and Y is completed, the variable I becomes 4
Next (step 222), the next steps 224, 20
After 4, 206, 208, and 226, a negative determination is made in step 228, the flow proceeds to step 266, and the development processing step of the fourth color, that is, K (black) color is executed.

In step 266, the average density is calculated based on the C image, M image and Y image data on which the toner image has already been formed. The transmittance on the photosensitive drum 72 is obtained based on the calculated average density, and in step 268, the correction coefficient α is read based on this transmittance.

In the next step 270, the K image data is read out, and the screen generator 106 calculates the reference exposure amount L _K (step 27).
2).

Here, when exposure is performed with the calculated exposure amount L _K , light may be reflected and absorbed at the portion where toner has already adhered, and proper exposure may not be possible.
Therefore, in step 274, the exposure amount L _K is corrected (L _K ← L _K · α), and the optimum exposure amount L _K when the toners of the C image, the M image, and the Y image are already attached is obtained. .

In the next step 276, it is judged whether or not it is the exposure position, and if the judgment is affirmative, the process proceeds to step 278, in which the laser beam is irradiated by the LD 75 of the exposure device 76 and the surface of the photosensitive drum 72 is hidden. The image is recorded. Next, at step 280, it is judged whether or not it is at the developing position,
If it is determined that the developing position is reached, the process proceeds to step 282, the developing device 78M is operated, the M color toner is sent to the peripheral surface of the photosensitive drum 72, and the toner adheres to the exposed portion, A toner image is formed.

When the development of K color is completed, step 2
The process proceeds from step 82 to step 222. Therefore, C color, M
When the development of all the colors, Y, and K, is completed, the variable I becomes 5 (step 222), and an affirmative decision is made in the next step 224, whereby step 28
The transfer process after 4 is executed.

At step 284, it is judged whether or not the photoconductor drum 72 is located at the transfer preparation position (a little before the position where the recording paper P is overlaid), and if an affirmative judgment is made,
The process proceeds to step 286 and the conveyance of the recording paper P is started (the uppermost recording paper P is taken out). As a result, the recording paper P moves in the tangential direction of the photoconductor drum 72, and the toner images are sequentially transferred, conveyed to the endless belt 92, fixed by the fixing device 94, and fixed on the paper discharge tray 96. Will be placed.

On the other hand, in step 28
By operating the cleaning device 98 at 8, the residual toner is wiped off and the initial state can be restored.

At the next step 290, it is judged whether or not the processing is continued, and if an affirmative judgment is made, step 200
Then, the processing steps described above are repeated. Also, step 29
When a negative determination is made at 0, the rotation of the photosensitive drum 72 is stopped at step 292, and the processing ends.

As described above, in this embodiment, based on the density of the toner that has already adhered to the photosensitive drum 72, it is appropriate to consider the reflection and absorption that occur when the exposure process is performed on the toner image. Since the exposure process can be performed with the exposure amount, it is possible to reduce the influence of the shielding in the portion where the colors overlap, and to prevent the disturbance of the color tone and the blurring of the image.

Further, since the toner density is obtained from the average value of the gradation (image density) of the image for each color from the read image data, a huge capacity of memory is not required, and the device structure is simplified.

In this embodiment, the image reading device 102
The image data is read by and the image processing device 104 creates the image data conforming to the format of this device. However, as shown in FIG.
It is also possible to take in the signal from 8 via the interface 120. At this time, the image data is a digital signal that has been binarized in advance, and if the resolutions are different,
The screen generator 106 may perform thinning or interpolation.

Further, in the present embodiment, when the average density D _A is obtained, the density of the image in the predetermined area is classified into eight gradations of 0 to 7, but as shown in FIG. , May be obtained by the error diffusion method. The image data of FIG. 6 is the same as the image data of FIG.

That is, FIG. 6 shows an image binarized by the error diffusion method, which is the simplest method of adding the error of the pixel of interest only to the next pixel. The average density D _{B of the} image can be obtained by calculating the ratio of the value of this calculation result within the predetermined area. For example, the average density D _{B in} the case of FIG. 6 is D _B = 26 / (6 × 20) ≈0.22, and the same calculation result can be obtained.

By performing such calculation, the memory capacity is smaller than that of counting the density data, but when the analog screen generator 106 is used, it is separate from the screen generator 106 for image formation. A circuit for binarization is required.
However, since the binarization circuit does not need to consider a multi-value expression method within one pixel and correction of a defect in image quality,
The desired performance can be achieved with a relatively small circuit.

Further, in the present embodiment, the image data is used to obtain the average density of the image, and the calculation is performed. However, as shown in FIG. 7, the density sensor is formed on a part of the peripheral surface of the photosensitive drum 72. 122 is provided, and the concentration sensor 122
The detection signal of 1 may be taken in the exposure control apparatus 100.

Further, in the present embodiment, each color was exposed in the order of CMYK, but the order of exposure is not particularly limited in the present invention. However, for example, when an IR light source is used as the light source, the exposure order of YMCK is preferable.

[0070]

As described above, the multicolor image forming apparatus according to the present invention can adjust the exposure amount for each color according to the type of the image, and this can be implemented with a simple apparatus configuration. It has an excellent effect that it can be done.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment.

FIG. 2 is a block diagram of an exposure device and an exposure control unit.

FIG. 3 is a distribution diagram in which gradations of an image in a predetermined area are classified into 8 levels of 0 to 7.

FIG. 4 is a control flowchart for image formation according to the present embodiment.

FIG. 5 is a block diagram of an exposure apparatus and an exposure control unit when image data is obtained from a host computer according to a modification of the present invention.

FIG. 6 is a distribution chart for obtaining an average density of an image by an error diffusion method.

FIG. 7 is a schematic configuration diagram of an image forming apparatus according to a modification of the present invention when the average density of an image is directly detected by a density sensor.

FIG. 8 is a distribution diagram showing addresses of a first color in a conventional multicolor image forming procedure.

9 is a characteristic diagram showing a current value of each address according to the image distribution of FIG.

FIG. 10 is a distribution diagram showing addresses of a second color in a conventional multicolor image forming procedure.

11 is a characteristic diagram showing a current value of each address according to the image distribution of FIG.

[Explanation of symbols]

 70 Image Forming Apparatus (Multicolor Image Forming Apparatus) 72 Photosensitive Drum 74 Charging Device 76 Exposure Device 78C, 78M, 78Y, 78K Developing Device 80 Transfer Roller 100 Exposure Control Device 110 Image Density Counter 122 Density Sensor

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuhiko Nishizawa 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd.

Claims (3)

[Claims] 1. A multicolor image in which the steps of charging, exposing, and toner developing are repeated on a photosensitive member for each color to form toner images of a plurality of colors, and then the toner images are transferred to a transfer material to form an image. The forming apparatus is characterized in that in the exposing step, it has an exposing means for exposing the photoconductor by setting an exposure amount at the time of exposure of a next color based on a toner density already existing on the photoconductor. Color image forming apparatus. 2. The toner density present on the photoconductor is
A conversion unit that is proportional to the toner density and converts to an average value of the densities obtained from the image signals of the respective colors for which the development process has already been completed, and the exposure amount is set based on the average value of the densities obtained from the image signals. 2. A multicolor image forming apparatus according to claim 1, wherein the multi-color image forming apparatus comprises: 3. The toner density present on the photoconductor is
A conversion unit that is in proportion to the toner density and that is converted into a detection signal from a density sensor that detects the density on the photoconductor, and a second exposure that sets the exposure amount based on the detection signal from the density sensor. The multicolor image forming apparatus according to claim 1, wherein the multicolor image forming apparatus comprises an amount setting unit.