JP2001194862A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of feeding back density adjusting control to each image forming unit in real time and always forming a color image with stable color tone reproducibility even in the midst of forming the image. SOLUTION: When it is assumed that the length between the density detecting position of a density sensor 20 and an electrophotographic processing part 8Y is L1y, the outer peripheral length of a photoreceptor 9 in the processing part 8Y from a developing position on the photoreceptor 9 to a transfer position in the processing part 8Y is L2y, the length of a patch Py formed by the processing part 8Y is L3 and the length between transfer paper and transfer paper carried on a carrying belt 7 is TrB, they satisfy relation TrB>=L1+L2+L 3, and the patch Py is formed between the transfer paper and the transfer paper.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that detects an image density using a toner image for detecting an image density and controls the density of an image transferred to a transfer material. .

[0002]

2. Description of the Related Art Generally, it is desired that a stable image is always output in a color copying machine, a color printer, or the like, which is an image forming apparatus of this type.

In particular, in recent years, a network environment for handling various types of information has expanded, and printers via a network have increased. As a result, various types of image data are output not only by the same printer but also by unspecified printers connected to a network.

At this time, the same color reproduction image must be obtained regardless of the output from any printer. However, in reality, differences in photoreceptor sensitivity due to differences in printer models, differences in toner coloring, differences in resolution,
The same color reproducibility may not be obtained even with the same image data due to a difference in the image forming process or the like.

[0005] In addition, even in the case of the same model, color reproducibility may be degraded due to changes in environmental temperature and humidity, deterioration of the image forming system over time, and the like.

In particular, in a tandem type color printer having a plurality of photoconductors, the characteristics of each photoconductor independently change, so that the photoconductor is sensitive to a change in environmental temperature and humidity, or may be operated over time. The state such as deterioration of the image system is different.

In this respect, generally, for example, Japanese Patent Application Laid-Open
As disclosed in Japanese Patent No. 67769, a density pattern image is transferred onto a transfer belt so that stable color reproduction can be performed even when an environmental change or a temporal change occurs, and the density of the density pattern image is optically measured. Controls the process conditions of each image forming station.

Further, according to the publication, a detection system for measuring the density of such a density pattern image is provided by optically determining a positioning pattern for correcting a color shift between respective colors, which is a problem in this type of color printer. By using the detection system for detection as well, effective use of space is achieved.

In recent years, the demands of users for the quality of color images have been increasing, and there has been a demand for more stable color tone reproducibility of images during continuous output.

That is, it is required that the first image and the last image of the continuously output color images have the same color tone. Here, as a conventional image density adjustment timing, the image density is detected at a predetermined timing such as when the power is turned on or after hundreds to thousands of images have been formed, and the image forming process control is performed.

Further, the conventional image forming apparatus generally has a look-up table (hereinafter also referred to as LUT) for correcting input / output characteristics of the image forming apparatus, and has a desired gradation characteristic for input image data. Alternatively, these LUTs are set so as to obtain color reproducibility.

Generally, the laser writing output characteristic for image data is changed by setting it in an LUT, so that the gradation and color reproducibility of the final output image match the input image data.

In practice, a tone pattern image is created for each machine, the density of the output image is measured, and an LUT is created so as to obtain desired tone characteristics.

Here, the LUT will be briefly described. In an apparatus for forming a halftone multi-valued image, not only the exposure ON / OFF of the exposure device but also the exposure in accordance with the input multi-valued image signal. It is well known that halftone images are formed by using intensity modulation, exposure time modulation, or dither patterns.

In such a multilevel image forming apparatus,
By changing the density characteristic of the halftone, a desired density gradation can be obtained.

In general, it is desirable that the gradation of the density output with respect to an input signal has a linear relationship, but usually becomes non-linear due to the characteristics of electrophotography.

Therefore, conventionally, linearity has been ensured by converting and correcting an input signal to an exposure apparatus using an LUT.

Hereinafter, a method of creating a typical LUT will be described with reference to FIGS. FIG. 16 shows a schematic graph of LUT creation used in the present invention and the conventional image forming apparatus, and FIG. 17 shows a schematic diagram of a dot matrix used in the present invention and the conventional image forming apparatus.

A curve A in FIG. 16 shows a density gradation characteristic when the image signal level is changed.
In FIG. 16, the horizontal axis indicates, for example, the ratio of black pixels in a predetermined matrix in the input data shown in FIG.
The vertical axis is generally Y (density) = − log ₁₀ (reflected light amount /
(The amount of incident light). The above is the same in this specification.

In this example, a halftone pattern was formed by a dither pattern growth method for growing dots in a 4 × 4 dot matrix shown in FIG.

On the other hand, as can be seen from FIG. 16, the density gradation characteristic, which is the output of the input signal, is not linear.

Therefore, an LUT of a curve B having a characteristic obtained by inverting the curve A of FIG. 16 with respect to Y = X shown in FIG. 16 is created. However, the straight line shown in FIG. 16 does not represent Y = X, but in the present specification, is a straight line extending from the point of density 0, and connects, for example, from the point of density 0 to the point of maximum density. Straight line. Such a straight line is generically referred to as a straight line Y = X. This is the same in the present specification.

Then, by converting the image signal using the created curve B to form an image, a desired input signal and the linearity of the density gradation can be obtained.

Here, in the conventional image forming apparatus, the LUT is created by detecting the image density at power-on or at a predetermined timing of several hundred to several thousand sheets as the LUT creation timing.

[0025]

However, in the above-described conventional image density adjustment timing and LUT creation timing, color tone reproducibility may differ before and after density adjustment.

(Problems Caused by Timing of Image Density Adjustment) For example, in a color image forming apparatus such as a tandem system, a color image can be continuously formed at a high speed. Because of the state, the temporal change may progress rapidly in a short period of time.

In the developing device, since the continuous operation is performed, the charge amount of the toner in the developing device changes drastically, and accordingly the developing property and the transfer property also change little by little, so that a stable color tone reproducibility is obtained. Becomes difficult.

An example will be described. FIG. 18 shows a solid image density change when image density control is performed for every 100 printed sheets using magenta as an example. The image density changes before and after the control is performed. FIG. 18 shows a graph of a change in solid image density when image density control is performed for every 100 printed sheets in the conventional image forming apparatus.

Such a change similarly occurs for other colors before and after the image density control. Therefore, the influence of the density change is increased in the color overlapped portion of a plurality of colors, so that the color tone is largely changed before and after the image density control.

(Problems Caused by LUT Creation Timing) Further, at the above-described conventional image density adjustment timing, color tone reproducibility may differ before and after LUT creation.

Particularly, in a color image forming apparatus such as a tandem type, a color image can be continuously formed at a high speed. On the other hand, when an image is formed, each process means is in a continuous operation state, and a rapid change with time in a short period of time. May progress.

For example, in the developing device, since the continuous operation is performed, the charge amount of the toner in the developing device changes drastically, and accordingly, the developing property and the transfer property also change little by little, so that stable color tone reproducibility is obtained. It will be difficult to obtain.

The above point will be described with reference to FIG. FIG. 19 shows a graph of density gradation characteristics for an arbitrary number of input signals when a continuous image is formed on the order of several thousand sheets from the beginning when magenta color is used in the conventional image forming apparatus.

As shown in FIG. 19, the density gradation characteristics change depending on the progress of image formation. More specifically, in a portion where the contrast potential of an electrostatic latent image formed by an isolated dot latent image is locally changed, the toner charge amount is in an initial operation state of the developing device. In some cases, the toner cannot be sufficiently held. In this case, the Coulomb force is reduced, so that the toner is less likely to be transferred to the photoconductor, and as a result, a density for an input signal in a light halftone area cannot be obtained (FIG. 19). Medium, curve A).

Conversely, if the developing device is frequently operated and the toner has an excessive charge amount, the Coulomb force is large and the electrostatic latent image formed by an isolated dot latent image is formed. The toner is transferred to the photoreceptor too sensitively to the portion where the contrast potential is locally changed, and the toner is further strengthened by the mirroring force generated by the charge amount of the transferred toner. Attempts to adhere to the photoreceptor.

Therefore, there is a case where the density for the input signal in the light halftone area is obtained more than necessary (FIG. 19).
Medium, curve B).

That is, depending on the operating state of the developing device,
The relationship between the input signal and the output density draws various curves as shown in FIG.

Therefore, according to the conventional method of detecting an image density at a predetermined timing of several hundreds to several thousand sheets and creating an LUT, a new LUT is created from the density gradation characteristics obtained to create the previous LUT. Therefore, there is a possibility that the obtained density gradation characteristics are shifted, and in this case, the density gradation characteristics of the image differ before and after the LUT is created, and the halftone density of the output image may be shifted.

In the above-described conventional method of detecting an image density at a predetermined timing of several hundreds to several thousand sheets to create an LUT, the shift of the halftone density of the output image changes similarly for other colors. I will show you.

Therefore, if the density gradation characteristics of the output density with respect to the input signal deviate for each color, the effect of the density change of each color appears on the color superimposed portion of a plurality of colors, so that the color tone changes greatly.

The present invention has been made in view of the above circumstances,
It is an object of the present invention to provide an image forming apparatus which enables real-time feedback of density adjustment control and enables formation of an image having stable color tone reproducibility even during image formation.

It is a further object of the present invention to provide an image forming apparatus which enables the density adjustment control to be fed back to the LUT in real time, and which can always form an image with stable color tone reproduction. .

[0043]

In order to achieve the above object, an image forming apparatus according to the present invention comprises a latent image carrier on which a latent image is formed, and a developing device for developing a toner image corresponding to the latent image. And a plurality of process means for forming toner images on the latent image carrier with toners of different colors each having a transfer device for transferring the toner image onto a transfer material, and an endless conveying means for conveying the transfer material. In an image forming apparatus including a carrier and an image density detection device that detects the density of an image density detection toner image formed on the endless carrier, the image density detection device may be configured to move in a sheet conveying direction. It is located downstream of the process means at the most downstream position, and the position of the most upstream position in the conveyance direction of the sheet material with the density detection position of the toner image for image density detection of the image density detection device. The length of the transfer position of Seth means L
1. L2 is the outer peripheral length of the latent image carrier from the developing position on the latent image carrier of the process unit at the most upstream position to the transfer position in the process unit, and the image density detection is performed by the process unit at the most upstream position The length of the toner image forming area of the toner image on the endless carrier is L3, and the endless shape of the transfer material non-conveying portion between the transfer materials conveyed on the endless carrier. When the length of the carrier is TrB, TrB ≧ L1 + L2 + L
And the toner image for image density detection is formed in the transfer material non-conveying portion.

The latent image carrier from the developing position on the latent image carrier of the n-th (n.gtoreq.2) process means counting from the downstream side of the endless carrier to the transfer position in the process means. The outer peripheral length L2 (n) of the latent image carrier from the developing position on the latent image carrier of the (n-1) th process means counting from the downstream of the endless carrier to the transfer position in the process means L2 (n) ≧ L with respect to the outer peripheral length L2 (n−1) of the body
2 (n-1).

The length L3 (n) of the toner image forming area of the image density detecting toner image of the n-th (n ≧ 2) process means counted from the downstream of the endless carrier is the n-th. L3 (n) ≦ Dis (n), where Dis (n) is the length of the endless carrier between the transfer position of the process means and the transfer position of the (n-1) th process means. Satisfy the relationship

Further, a plurality of the image density detecting toner images are formed by making the developing bias applied to the developing device different, and the image density detecting toner images detected by the image density detecting device are formed. Based on the density, a developing bias to be applied to the developing device is determined to control the density.

The image forming apparatus further comprises storage means for storing a value of the developing bias applied to the developing device, wherein the developing device is set to a predetermined variable range based on the value of the developing bias stored in the storing means. The toner image for image density detection is formed using a developing bias.

A latent image carrier on which a latent image is formed by exposure, an exposure means for exposing the latent image carrier to light, and a developing device for developing a toner image corresponding to the latent image. A plurality of process means for forming toner images on the latent image carrier using toners of different colors each having a transfer device for transferring the toner image onto a transfer material; and an endless carrier for transporting the transfer material. And an image density detection device that detects the density of the toner image for image density detection formed on the endless carrier, wherein the image density detection device is the most downstream in the conveyance direction of the sheet material. And a process at the most upstream position in the conveying direction of the sheet material, wherein the image density detecting device is located downstream of the process means. The length from the transfer position of the means to the transfer position in the latent image carrier from the exposure position on the latent image carrier of the process means at the most upstream position to the transfer position in the process means is L1; The length of the toner image forming area of the image density detection toner image on the endless carrier in the process means at the position is L3;
When the length of the endless carrier in the transfer material non-conveying portion between the transfer material conveyed on the endless carrier and the transfer material is TrB, the relationship TrB ≧ L1 + L2 + L3 is satisfied, and The toner image for detecting image density is
It is formed in the transfer material non-conveying section.

The latent image carrier from the exposure position on the latent image carrier of the n-th (n.gtoreq.2) process means counting from the downstream side of the endless carrier to the transfer position in the process means. The outer peripheral length L2 (n) of the latent image carrier from the developing position on the latent image carrier of the (n-1) th process means counting from the downstream of the endless carrier to the transfer position in the process means L2 (n) ≧ L with respect to the outer peripheral length L2 (n−1) of the body
2 (n-1).

The length L3 (n) of the toner image forming area of the image density detecting toner image of the n-th (n ≧ 2) process means counted from the downstream of the endless carrier is the n-th. L3 (n) ≦ Dis (n), where Dis (n) is the length of the endless carrier between the transfer position of the process means and the transfer position of the (n-1) th process means. Satisfy the relationship

The exposure means exposes the latent image carrier to light based on input data, and the image density detection is performed based on the density of the image density detection toner image detected by the image density detection device. And a processing unit that outputs a density graph indicating the density for the input data that has formed the toner image for use, and the density graph output from the calculation processing unit while outputting the input data. A control unit that calculates an inverted graph obtained by inverting a straight line connecting the point at which the density is maximum, and an inverted graph storage unit that stores the inverted graph calculated by the control unit, wherein the control unit includes: The input data to be output to the exposure means is processed and output based on the inverted graph stored in the inverted graph storage means.

The image processing apparatus further includes an image control unit for rasterizing the input data output to the exposure unit processed by the control unit and outputting the rasterized data to the exposure unit.

Further, density group storage means for storing a first density group which is a set of the densities of the plurality of toner images for image density detection detected by the image density detection device, and storage in the density group storage means. A second density, which is a combination of the respective densities constituting the first density group and the density of the image density detection toner image formed next to the image density detection toner image.
And a comparing means for comparing the respective densities of the first and second density groups with each other. The comparing means determines a difference between the density in the first density group and the density in the second density group. When the density of the second density group is stored in the density group storage unit, the difference between the density of the first density group and the density of the first density group is within a predetermined range. Instead of the concentration in the second concentration group determined to be present, the second
The density in the first density group corresponding to the density in the second density group and corresponding to the density in the second density group within a predetermined range is stored in the density group storage means. .

In addition, the arithmetic processing unit is configured to determine the image density based on a set of the densities of the plurality of image density detection toner images detected by the image density detection device stored in the density group storage means. A density graph indicating the density with respect to the input data on which the detection toner image is formed is output.

Further, the control unit may determine that the difference between the density in the first density group and the density in the first density group is within a predetermined range. Stop output of input data corresponding to.

Further, the stop of the output of the input data in the control unit is released when a predetermined number of images are formed in each of the process units.

Therefore, according to the image forming apparatus of the present invention, the position between the density detection position of the image density detection toner image of the image density detection device and the transfer position of the process means at the most upstream position in the sheet material conveyance direction. L1, the outer peripheral length of the latent image carrier from the developing position or exposure position on the latent image carrier of the process unit at the most upstream position to the transfer position of the process unit is L2, and The length of the toner image forming area of the toner image for image density detection on the endless carrier is L3, and the endless shape of the transfer material non-conveying portion between the transfer materials conveyed on the endless carrier. When the length of the carrier is TrB, the relationship of TrB ≧ L1 + L2 + L3 is satisfied, so that the density detection result of the image density detection toner image is immediately formed on the transfer material. Can reflect, it is possible to perform image formation process efficiently.

Further, counting from the downstream side of the endless carrier,
The outer peripheral length L2 (n) of the (n ≧ 2) th process means is
With respect to the outer peripheral length L2 (n-1) of the (n-1) th process means counted from the downstream of the endless carrier, L2 (n)
Since the relationship of ≧ L2 (n−1) is satisfied, even if a toner image for image density detection is continuously formed by the process means, the image density detection device reads the toner image while reading it.
The plurality of developing devices can reflect the detection result of the image density detection device in real time on the density of the toner image formed on the transfer material.

Further, counting from the downstream side of the endless carrier,
The length L3 (n) of the toner image forming area of the image density detection toner image of the (n ≧ 2) th process means is determined by the transfer position of the nth process means and the (n−1) th process means. Is the length of the endless carrier between the transfer position of
When (n) is satisfied, the relationship of L3 (n) ≦ Dis (n) is satisfied, and while the image density detecting device reads this, the plurality of developing devices read the toner image formed on the transfer material. The detection result of the image density detection device can be reflected on the density in real time.

Further, a plurality of image density detecting toner images are formed by making the developing bias applied to the developing device different from each other, and based on the density of the image density detecting toner image detected by the image density detecting device. Since the leverage bias is determined, it becomes easy to make the density of the toner image developed by the developing device appropriate.

Further, since the developing device forms the toner image for image density detection using the developing bias set in a predetermined variable range based on the value of the developing bias stored in the storage means, high accuracy is achieved. Stable concentration control can be performed.

Further, since the controller calculates the inverted graph by inverting the density graph and processes the input data input to the exposure means based on the inverted graph, it is possible to control the exposure operation in the exposure means. Accordingly, the density detection result of the toner image for image density detection can be immediately reflected on the transfer material, and the image forming process can be efficiently performed.

Since the input data input to the exposure means by the image control unit is rasterized, more appropriate input data can be obtained. Here, rasterizing includes changing a graphic object consisting of a vector or a line element represented by input data into a dot format for transfer to a laser printer.

When the density of the image density detecting toner is detected, the detected density is compared with the density of the image density detecting toner detected next, and the difference between these densities is set within a predetermined range. In some cases, since the density of the immediately preceding image density detection toner is used instead of the next density data within the predetermined range, stable and stable density adjustment can be performed.

Further, since the arithmetic processing section outputs the density graph based on the set of densities stored in the density group storage means, it is possible to output a more appropriate density graph.

The control unit stops outputting the input data corresponding to the density of the immediately preceding image density detection toner image and the image density detection toner image following the image density detection toner image within a predetermined range. Therefore, it is possible to prevent waste of toner.

Further, the stop of the output of the input data in the control unit is released for every predetermined number of sheets, for example, 100 sheets, 200 sheets, or 1000 sheets of the transfer material. In this case, the image is formed again.

[0068]

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the dimensions of the components described in this embodiment,
The materials, shapes, relative arrangements, and the like are not intended to limit the scope of the present invention only to them unless otherwise specified.

In the following drawings, the same reference numerals are given to members described in the drawings used in the description of the related art and members similar to the members described in the drawings described above.

(First Embodiment) Hereinafter, a first embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings.

FIG. 1 is a schematic view of a color image forming apparatus as a first embodiment to which the image forming apparatus according to the present invention is applied.

As shown in FIG. 1, a paper passing path 4 from a paper feeding section 2 to a paper discharging section 3 for guiding a transfer paper 1 as a transfer material of the present invention is provided.

The paper passing path 4 is an endless carrier of the present invention which is stretched between a belt driving roller 5 and a rotatable belt driven roller 6 which are rotated by applying a driving force from a driving source (not shown). The conveyor belt 7 as a part is included.

Then, on the conveyor belt 7, yellow,
Four electrophotographic processing units 8Y, 8M, 8C as process means of the present invention for magenta, cyan, and black;
8K are arranged in order.

Each of these electrophotographic processing units is provided with a photosensitive member 9 serving as a latent image carrier of the present invention, which comes into contact with the conveyor belt 7.
A charger 10, an exposure device 11, a developing device 12, a transfer device 13 as a transfer device of the present invention, and a photoconductor cleaner 14 are sequentially arranged around the photoconductor 9.

Further, the paper passing path 4 is provided with a fixing device 15 located at a place where it has passed through the conveyor belt 7.

In such a structure, the uppermost transfer sheet 1 is sent from the sheet feeder 2 to the sheet feed path 4 and is conveyed by the conveying belt 7. In the process, an image is formed using an electrophotographic process of charging, exposure, development, and transfer by an electrophotographic process unit of each color.

As a result, the color toner image is transferred to the transfer paper 1, and is heated and pressed by the fixing device 15, so that the color toner image is firmly adhered to the transfer paper 1.

Next, an image density detecting method will be described.
As shown in FIG. 1, a density sensor 20 as an image density detection device of the present invention is disposed near the transport belt 7 downstream of the transport belt 7 of the electrophotographic process unit 8K for black as a final development color. I have.

As described above, in an electrophotographic color image forming apparatus, if the image density fluctuates due to various conditions such as the use environment and the number of prints, a correct color tone cannot be obtained as an original color image.

Therefore, a patch image is formed on a trial basis with the toner of each color, the density of the patch image is detected by the density sensor 20, and the result is fed back to the developing bias to control the image density so that a stable image is obtained. I have to.

The density sensor 20, as shown in FIG.
A light emitting element 21 such as an LED and a light receiving element 22 such as a photodiode, and irradiating infrared light from the light emitting element 21 onto the patch P as a toner image for image density detection of the present invention on the conveyor belt 7; The patch density is detected by receiving the reflected light from the light receiving element 22 and measuring the amount of reflected light. FIG. 2 shows a structural diagram of a density sensor used in the image forming apparatus shown in FIG.

The light reflected from the patch P includes a specular reflection component and an irregular reflection component. The specular reflection component indicates the state of the surface of the conveyor belt 7 as the base of the patch P and the distance between the density sensor 20 and the patch P. , The amount of light greatly fluctuates, and if the reflected light from the patch P to be measured contains a specular reflection component, the detection accuracy is significantly reduced.

Therefore, in the density sensor 20, the irradiation angle on the patch P is set to 45 ° with respect to the normal I of the surface of the conveying belt 7 so that the regular reflection light from the patch P does not enter the light receiving element 22. The light receiving angle of the reflected light from the patch P is set to 0 °, and only the irregularly reflected light is measured.

Next, a specific image density control method of this embodiment will be described with reference to FIG. FIG. 3 shows a schematic structural diagram of the image forming apparatus shown in FIG.

FIG. 3 shows only the photoreceptor 9, developing unit 12, and transfer unit 13 of the electrophotographic process unit for explanation, and other members are omitted for convenience.

The photosensitive member 9, the developing device 12, the transfer device 13
Are described with subscripts according to the color of the electrophotographic process unit, Y represents yellow, M represents magenta, C represents cyan, and K represents black.

FIG. 3 shows the transfer paper TrM after image formation.
1 and the transfer paper TrM2 for which image formation is being performed is continuously conveyed, and the transfer paper TrM1 and the transfer paper TrM2 are shown.
3 illustrates an image density detection state in a non-image forming area (hereinafter, referred to as a sheet interval TrB) as a transfer material non-conveying portion according to the present invention.

First, after completion of image formation on the transfer paper TrM1,
The image control unit 33 changes the developing bias of the patch latent image image-exposed at a predetermined light amount by an exposure unit (not shown), and the yellow patch P, which is an electrophotographic process unit for the first color.
y1, Py2, Py3, and Py4 are created by changing their densities.

In the present embodiment, the surface of the photosensitive member 9 is uniformly charged to -700 [V] as the dark portion potential VD by the charger 10 and then the exposure device 11 sends O to the dark in accordance with the patch image information.
Scanning exposure is performed with a laser beam that is N / OFF controlled, and a patch latent image with a bright portion potential VL of -100 [V] is formed.

A developing bias voltage that increases at a predetermined stage while corresponding to the patch latent image is output from the image control unit 33, and the patch latent image on the photosensitive member 9 is formed on the photosensitive member 9 as a patch toner image having a different density. It is visualized.

The DC bias voltage of the developing bias voltage output from the image controller 33 is variable.

FIG. 4 is a graph showing the relationship between the DC component voltage value of the developing bias and the image density with respect to the light portion potential VL-100V on the photosensitive member 9 of the image forming apparatus shown in FIG.

In this embodiment, the DC component voltage values of the developing bias for developing the patch latent image are V1 and V1.
2, V3, and V4, and each DC component voltage value is -1.
50 V, -175 V, -200 V, and 225 V.

As shown in FIG. 3, these patches formed as described above are transferred to the transport belt 7 and
The density sensor 20 detects the amount of reflected light from the patch according to an instruction from the sampling control unit 31 in communication with the control unit 30 that controls the apparatus, and the processing unit 32 determines the patches Py1 and Py2 based on the detected amount of reflected light. , Py3, and Py4 are detected.

The control unit 30 calculates a developing bias value that provides an optimum density (in this embodiment, the density is 1.4) from the density value calculated by the calculation processing unit 32, and sends the calculation result to the image control unit 33. The image is output again, and the image control unit 33
By outputting the optimum developing bias value calculated at 0, the result of the patch density measurement is fed back to the developing device 12Y.

Then, an image for the transfer paper TrM2 is formed based on the feedback result. At this time, the patches Py1 and Py along the moving direction of the transport belt 7
2, the area of Py3 and Py4 is L3y, the density sensor 20
The area from the density detection position to the yellow color transfer position, which is the electrophotographic process unit for the first color, is L1y, and the photoconductor 9 from the yellow color development position to the yellow color transfer position
Assuming that the upper peripheral length is L2y, in the present embodiment, as shown in FIG. 3, the sheet interval TrB is Lly + L2y + L.
3Y or more, (TrB ≧ Lly + L2
y + L3y: when yellow is the uppermost stream), the calculation processing result including the patch density result of the yellow final patch Py4 is immediately reflected in the next image and can be reflected in real time on the next transfer paper (here, TrM2). it can.

The length L2'y along the conveyor belt 7 in FIG. 3 is equal to L2y, and is used for illustrative purposes.

Subsequently, as shown in FIG. 5, the same applies to the first color magenta color which is the electrophotographic process unit.
Yellow patches Py1, Py2, Py3, Py of the color eye
After magenta 4, magenta patches Pm1, Pm2,
Pm3 and Pm4 are formed and fed back to the developing device 12M. FIG. 5 shows a schematic structural diagram of the image forming apparatus shown in FIG.

In FIG. 5, the electrophotographic process section for the first color is omitted. Here, if the configuration of each electrophotographic process unit is the same, the outer peripheral length of the photoconductor 9 from the developing position to the transfer position is the same in each electrophotographic process unit, so that L2y = L2m.

However, in the image forming apparatus according to the present invention, as described above, the outer peripheral length of the photosensitive member 9 from the developing position to the transfer position is the same in each electrophotographic processing unit, and L
However, the present invention is not limited to the case where 2y = L2m.
What is necessary is just to be equal to or longer than the outer peripheral length of the photoconductor 9 one step further downstream.

The patch formed on the conveyor belt 7 is substantially equal to the area obtained by subtracting L1m from L1y, that is, the distance Dis (ym) between the transfer position of the first color and the transfer position of the second color. Also magenta patches Pm1 to Pm
When the patch forming area L3m up to 4 is accommodated, the calculation processing result including the patch density result of the magenta final patch Pm4 is similar to the yellow color, and the next image is immediately formed by the control for obtaining the optimum value of the developing bias. Paper TrM
2 can reflect the magenta density result in real time.

Here, the patch forming area L3m is located at the distance Dis (y−y−2) between the transfer position of the first color and the transfer position of the second color.
If m is larger than m), the timing of the subsequent formation of cyan or black patches is delayed, so that the formation of the black image of the next image TrM2 must be started during the detection of the black patch density. In other words, there is a possibility that the result of concentration detection cannot be reflected in real time.

Hereinafter, real-time image density control becomes possible by forming and processing patches for the third color cyan and the fourth color black at similar timing.

FIG. 6 shows the execution result of the image density control by the image forming apparatus of the present embodiment described above. FIG. 6 shows a graph of an execution result of image density control by the image forming apparatus shown in FIG.

FIG. 6 shows the transition of the image density of the solid magenta color with respect to the number of prints. Line A shows the transition of this embodiment.
As a comparative example, a line B indicates the transition of the magenta solid image density when the image density control is performed for every 100 sheets described above.

As can be seen from the figure, in the image forming apparatus of the present embodiment, the image density is controlled to be substantially stable, so that there is almost no change in the color tone of the overlapped portion of a plurality of colors. Was.

As described above, the image forming apparatus according to the present embodiment forms a patch between sheets, and sets the sheet interval to be equal to or longer than the area obtained by adding the patch area and the length from the last patch position to the developing position. As a result, the patch detection result of each color can be immediately fed back to the developing device, real-time image control can be performed, the optimal density state can be always maintained, and stable color tone reproduction can be obtained.

Although the method of forming patch images having different densities has been described in the present embodiment by changing the DC component voltage of the developing bias, the toner image density such as the AC component voltage of the developing bias and the AC frequency is changed. A parameter that changes the value may be used.

(Second Embodiment) Next, an image forming apparatus according to a second embodiment of the present invention will be described with reference to the drawings. FIG. 7 shows a schematic diagram of a color image forming apparatus as a second embodiment to which the image forming apparatus according to the present invention is applied.

The members described in the first embodiment are given the same numbers, and their description is omitted. FIG. 7 is provided with a memory 37 as a storage unit of the present invention in which the control unit 30 is connected to and stores the optimum value data of the developing bias of each color controlled for each image formation. The present embodiment uses the memory 37 to change the bias voltage around the optimum developing bias voltage after the execution of the image density control, thereby enabling more stable image density control.

In this embodiment, similarly to the first embodiment, patches are formed between papers, and the space between papers is set to be equal to or larger than the patch area and the area obtained by adding the length from the last patch position to the development position. It has been reconfigured to enable real-time concentration control.

FIG. 8 is a flow chart for controlling an image according to the present embodiment, and FIG. 7 and FIG. 8 show a second embodiment of the image forming apparatus according to the present invention.
This will be described with reference to FIG. FIG. 8 shows a flowchart of an image control operation of the image forming apparatus according to the second embodiment of the present invention.

First, when a print signal is input to the image forming apparatus (step S1), the developing bias for patch formation described in the first embodiment is -150V, -175.
V, −200 V, and −225 V are output from the image control unit 33 at four levels of developing bias voltages to form patches (step S2).

The density sensor 20 detects the amount of reflected light from the patch, and based on the detected amount of reflected light, the arithmetic processing unit 3
At 2, the density of the patch is detected (step S3).

The control unit 30 calculates a developing bias value m that becomes the optimum density (in the present embodiment, the density is 1.4) from the density value calculated by the calculation processing unit 32 (step S).
4).

Here, the developing bias value m is a value unique to each of the four colors. Originally, the optimum value my for yellow color, the optimum value m for magenta color, the optimum value mc for cyan color, and the optimum value mk for black color are given. However, since the processing contents described later are the same, they are used as representatives for convenience.

Next, the developing bias value m obtained by the control unit 30 is sequentially stored for each color in the memory 37 connected to the control unit 30 (step S5). Then, the developing bias voltage corresponding to the developing bias value m stored in the memory 37 is sequentially output to each developing device to perform a normal image forming operation (step S6).

When the first yellow image forming operation is completed, it is determined whether continuous printing is requested (step S7). If there is no printing request (NO), after the black image forming is completed, The operation directly proceeds to the print end operation (step S12).

When there is a continuous print request (YES)
Performs a density control by forming a patch again between sheets. Here, a variable value of the developing bias at the time of forming the patch is obtained by (m−15) V,
The control unit 30 changes and sets (m-5) V, (m + 5) V, and (m + 15) V (step S8).

Then, the changed developing bias voltages (m-15) V, (m-5) V, (m + 5) for forming the patch are changed.
V and (m + 15) V are output from the image controller 33 to the developing device 12 to form a patch (step S9), a patch density is obtained from the patch reflected light amount (step S10), and a new developing bias value m 'is calculated. It is determined (step S11).

By storing the determined developing bias value m 'in the memory 37 again (step S6), the content of the developing bias value m in the memory 37 is changed to the content of the new developing bias value m'. This is applied to the next normal image formation (step S6).

Thereafter, steps S6 to S11 are repeated until image density control and image formation are performed until there is no continuous print request.

According to the above-described density control method, the patch density is detected for each image formation, and the development bias control can be performed. Therefore, the variation of the controlled development bias value for each density control is small. In the density control for determining the developing bias, the variable developing bias at the time of patch formation is changed around the value calculated as the optimal developing bias performed immediately before, so that the internal portion prediction is reliably executed. This makes it possible to improve the control accuracy with respect to the target density.

FIG. 9 shows an execution result of the image density control of the present embodiment described above. FIG. 9 is a graph showing the transition of the image density of the magenta solid image with respect to the number of prints in the image forming apparatus shown in FIG.

In FIG. 9, a line A indicates the change in image density of the image forming apparatus according to the present embodiment. As a comparative example, the change in image density of magenta solid color when image density control is performed every 100 sheets. Shown by line B.

As can be seen from the figure, in the image forming apparatus of the present embodiment, the image density is controlled to be more stable, so that there is almost no change in the color tone of the overlapping image portion of a plurality of colors. Was.

As described above, in this embodiment, the memory 37 for storing the optimum value data of the developing bias of each color controlled for each image formation is provided. By storing a value calculated as a bias and forming a patch in a variable range centered on the current bias value, it is possible to perform high-accuracy and stable image density control. Since the state can be maintained, stable color tone reproduction can be obtained.

In this embodiment, the variable range of the developing bias for forming the patch is specifically described as a variable range of ± 15 V centering on the developing bias value obtained by the density control. The purpose is to reduce the load, and may be appropriately changed depending on the power supply performance.

(Third Embodiment) Next, a third embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. FIG. 10 is a schematic diagram of a color image forming apparatus as a third embodiment to which the image forming apparatus according to the present invention is applied.

In the third embodiment, as shown in FIG. 10, similarly to the above-described first embodiment, a sheet is discharged from a sheet feeding unit 2 for guiding a transfer sheet 1 as a transfer material of the present invention. A paper passing path 4 leading to the unit 3 is provided.

The paper passing path 4 is an endless carrier of the present invention which is stretched between a belt driving roller 5 which is rotated by applying a driving force from a driving source (not shown) and a rotatable belt driven roller 6. The conveyor belt 7 as a part is included.

Then, on the conveyor belt 7, yellow,
Four electrophotographic processing units 8Y, 8M, 8C as process means of the present invention for magenta, cyan, and black;
8K are arranged in order.

The electrophotographic process section mainly includes a photosensitive member 9 as a latent image carrier of the present invention in contact with the conveyor belt 7, a charger 10 around the photosensitive drum 9, and an exposure means of the present invention. Exposure unit 11, developing unit 12, transfer unit 13 as a transfer device of the present invention, and photoreceptor cleaner 14 are sequentially arranged and formed.

Further, the paper passing path 4 is provided with a fixing device 15 located at a place where it has passed through the conveyor belt 7.

In such a structure, the uppermost transfer paper 1 is sent from the paper supply unit 2 to the paper path 4 and is transported by the transport belt 7.

In the process, an image is formed by an electrophotographic process of charging, exposure, development and transfer by an electrophotographic process unit of each color.

As a result, the color toner image is transferred to the transfer paper 1, and is heated and pressed by the fixing device 15, so that the color toner image is firmly adhered to the transfer paper 1.

Next, an image density detecting method will be described.
As shown in FIG. 10, a density sensor 20 as an image density detection device of the present invention is disposed near the conveyance belt 7 downstream of the conveyance belt 7 of the electrophotographic process unit 8K for black as the final development color. I have. As described above, in an electrophotographic color image forming apparatus, if the image density fluctuates due to various conditions such as a use environment and the number of prints, a correct color tone cannot be obtained as an original color image.

Therefore, a patch image is formed on a trial basis with toner of each color, their densities are detected by the density sensor 20, and a LUT is created from the results, so that the desired input signal and the linearity of the density gradation are obtained. As shown in FIG.
The conversion is performed based on the UT, and the converted signal is output (feedback) to the exposure device 11 to form an image, thereby obtaining a stable image.

As shown in FIG. 2, the density sensor 20 is composed of a light emitting element 21 such as an LED and a light receiving element 22 such as a photodiode. The patch density is detected by irradiating the patch P as the image density detection toner image of the present invention, receiving the reflected light from the patch P, and measuring the amount of reflected light.

The light reflected from the patch P contains a specular reflection component and an irregular reflection component. The specular reflection component indicates the state of the surface of the conveying belt 7 as the base of the patch P and the distance between the density sensor 20 and the patch P. , The amount of light greatly fluctuates, and if the reflected light from the patch P to be measured contains a specular reflection component, the detection accuracy is significantly reduced.

Therefore, in the present embodiment, as described above, in the density sensor 20, the patch P based on the normal I of the surface of the conveying belt 7 is used so that the regular reflection light from the patch P does not enter the light receiving element 22. Assuming that the irradiation angle to the light is 45 ° and the light receiving angle of the reflected light from the patch P is 0 °, only the irregularly reflected light is measured.

A specific image density control method of this embodiment will be described with reference to FIG. In FIG. 11, only the exposing unit 11, the photosensitive member 9, the developing unit 12, and the transferring unit 13 of the electrophotographic process unit are illustrated for explanation, and other members are omitted for convenience. FIG. 11 shows a schematic structural diagram of the image forming apparatus shown in FIG.

The exposure unit 11, the photosensitive member 9, the developing unit 12,
Each member of the transfer unit 13 will be described with a suffix corresponding to the color of the electrophotographic process unit. Y represents yellow, M represents magenta, C represents cyan, and K represents black.

In this embodiment, each of the exposure units 11 is set in advance at the time of starting the main body or at a predetermined image forming interval.
The patch density of each of the electrophotographic process units for the first to fourth colors is varied while varying the developing bias of the patch latent image image-exposed with a predetermined amount of light, and these patches are detected by the density sensor 20. A developing bias for obtaining the maximum image density (in this embodiment, the density is set to 1.4) is determined for each color.

FIG. 11 shows the transfer paper Tr on which image formation has been completed.
M1 and the transfer paper TrM2 for which image formation is being performed are continuously transported, and the transfer paper TrM1 and the transfer paper TrM are shown.
FIG. 9 illustrates an image density detection state in a non-image forming area (hereinafter, referred to as a sheet interval TrB) as a transfer material non-conveying portion of the present invention between M2 and M2.

First, after completion of image formation on the transfer paper TrM1,
The plurality of dither patterns changed by the image control unit 33 are exposed to a halftone image from the exposure unit 11Y, and are exposed to a halftone image at a predetermined developing bias so that the maximum image density is constant. Yellow patch Py
1, Py2, Py3, Py4, and Py5 are created with variable densities.

Here, in this embodiment, the surface of the photosensitive member 9 is uniformly charged to -700 [V] as the dark portion potential VD by the charger 10, and then the exposure unit 11 is charged in accordance with the patch image information.
Scan exposure is performed by a laser beam that is more ON / OFF controlled, and a patch latent image is formed.

By the way, the solid light potential VL on the surface of the photoreceptor 9
Becomes -100 [V]. In this embodiment, LU
A 4 × 4 dot matrix shown in FIG. 17 is used as a patch for creating T, Py1 is a dot that has grown 2 dots, Py2 is a dot that has grown 5 dots, Py3 is a dot that has grown 8 dots, and Py4 is an 11 dot dot. What grew, P
y6 is obtained by growing 14 dots.

The patches formed as described above are transferred to the transport belt 7 and are controlled by the control unit 3 for controlling the apparatus.
The density sensor 20 detects the amount of reflected light from the patch in accordance with an instruction from the sampling control unit 31 that has communicated with 0, and based on the detected amount of reflected light, the arithmetic processing unit 32 detects the patch Py.
The concentrations of 1, Py2, Py3, Py4, and Py5 are detected.

The control unit 30 performs an operation based on the density value calculated by the calculation processing unit 32 so that the density gradation characteristic of the output image density becomes linear, and stores the calculation result as an inverted graph storage unit of the present invention. The data is sent to the LUT storage unit 34 to create the LUT-y.

As shown in FIG. 12, LUT-y is composed of patches Py1 and Py obtained by growing dots.
By detecting the patch densities of 2, Py3, Py4, and Py5 with the density sensor 20, the arithmetic processing unit 32 detects the patch densities Dy1, Dy2, Dy3, and Dy3 corresponding to the respective input data.
Dy4 and Dy5 are obtained. FIG. 12 shows a graph of the patch density created in the image forming apparatus shown in FIG.

In order to perform density interpolation between the patches based on the obtained patch densities, the control unit 30 interpolates between the respective patterns by linear approximation or curve approximation by a third to fifth order function. In the present embodiment, the density curve A is obtained by interpolating between patterns by linear approximation.

The end points of the density curve A are defined as a point where the image density is 0 corresponding to the input image data 0% and a point where the maximum density is obtained corresponding to the input image data 100%.

Based on the density curve A obtained in this manner, the control section 30 inverts Y = X for the characteristic curve B.
Is generated, and the curve B is output to the LUT storage unit 34, and the LUT-
Create y.

When the subsequent image formation for the transfer paper TrM2 is executed, the input image signal for the transfer paper TrM2 is stored in the LUT storage unit 34 based on the LUT-y stored in the LUT storage unit 34. After the conversion, the image control unit 33 rasterizes the converted image signal,
The rasterized data is output to the exposure device 11Y and the transfer paper Tr
Image formation for M2 is performed.

As described above, in the present embodiment, the patch density is detected from the halftone patch, an LUT is created based on the patch density, and the LUT is immediately output to the exposure unit 11 for the input image signal of the next image. LUT rasterization signal
The feedback control of conversion based on
It is possible to form an image in which the density gradation characteristic of the output image is always low.

By the way, at this time, the patches Py1, Py2, Py3, Py4,
The area y5 is L3y, the area from the density detection position of the density sensor 20 to the yellow transfer position, which is the electrophotographic process unit for the first color, is L1y, and the exposure from the yellow exposure irradiation position to the yellow transfer position. Assuming that the outer peripheral length on the body 9 is L2y, in this embodiment, as shown in FIG. 11, the sheet interval TrB is equal to or larger than the area of L1y + L2y + L3y (TrB ≧ Lly + L2y + L3).
y: when yellow is the uppermost stream), the calculation processing result including the patch density result of the yellow final patch Py4 can be immediately reflected in the next image (here, TrM2) in real time.

The length L2'y along the conveyor belt 7 in FIG. 11 is equal to the length L2y, and is used for the illustration.

Subsequently, as shown in FIG. 13, similarly for the magenta color, which is the electrophotographic process unit for the second color, similarly, the yellow patches Py1, Py2, Py3, and P of the first color yellow
After y4 and Py5, the magenta patch Pm
1, Pm2, Pm3, Pm4, and Pm5 are formed and fed back to the developing device 12M. FIG. 13 is a schematic operation diagram of the image forming apparatus shown in FIG.

In FIG. 13, the electrophotographic processing section for the first color is omitted. Here, if the configuration of each electrophotographic process unit is the same, the outer peripheral length of the photoconductor 9 from the exposure irradiation position to the transfer position is the same in each electrophotographic process unit, so that L2y = L2m.

However, in the image forming apparatus according to the present invention, as described above, the outer peripheral length of the photosensitive member 9 from the developing position to the transfer position is the same in each electrophotographic process unit, and
The present invention is not limited to the case where 2y = L2m, but it is sufficient that the outer peripheral length of the photoconductor 9 upstream of the transfer material 1 in the transport direction is equal to or longer than the outer peripheral length of the photoconductor 9 one stage downstream from the photoconductor 9.

The patch formed on the conveyor belt 7 is substantially equal to the area obtained by subtracting L1m from L1y, that is, the distance Dis (ym) between the transfer position of the first color and the transfer position of the second color. Also magenta patches Pm1 to Pm
5, the processing result including the patch density result of the magenta final patch Pm5, like the yellow color, is immediately transferred to the next image TrM2 by the control for obtaining the optimum value of the developing bias. It is possible to reflect the density result of M color in real time.

Here, the patch forming area L3m is located at the distance Dis (y−y) between the transfer position of the first color and the transfer position of the second color.
If m is larger than m), the timing of the subsequent formation of cyan or black patches is delayed, so that the formation of the black image of the next image TrM2 must be started during the detection of the black patch density. In other words, there is a possibility that the result of concentration detection cannot be reflected in real time.

Hereinafter, patches are formed and processed at the same timing for the third color cyan and the fourth color black, so that real-time image density control becomes possible.

FIG. 14 shows the execution result of the image density control by the image forming apparatus of the present embodiment described above. FIG.
FIG. 10 is a graph showing an execution result of image density control by the image forming apparatus shown in FIG.

FIG. 14 is a solid line showing density gradation characteristics for an arbitrary number of input signals when several thousand continuous images are formed from the beginning when magenta is used.

As can be seen from the figure, in the image forming apparatus of the present embodiment, the density gradation characteristics are controlled to be almost stable, so that the color tone change of the overlapping image portion of a plurality of colors is almost observed. Did not.

As described above, in the image forming apparatus of the present embodiment, a patch is formed between sheets,
By setting the sheet interval to be equal to or longer than the area obtained by adding the length from the patch area and the last patch position to the exposure irradiation position, the patch detection result of each color can be immediately fed back to the LUT,
Real-time image control is possible, and the optimal state of the density gradation characteristic can be always maintained, and stable color tone reproduction can be obtained.

In the present embodiment, a dither pattern consisting of a 4 × 4 dot matrix has been described as a patch for creating an LUT of a halftone multi-valued image. , A dither pattern of a different dot matrix from the present example or a dither pattern of a different dot growth method, intensity modulation of exposure, exposure time modulation or dither pattern or intensity modulation of these exposures,
Similar effects can be obtained by using a combination with the exposure time modulation.

In the present embodiment, the description has been made using five patches as the patches for determining the LUT. However, the present invention is not limited to this number.

(Fourth Embodiment) Next, a fourth embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings.

FIG. 15 is a schematic structural view of a color image forming apparatus as a fourth embodiment to which the image forming apparatus according to the present invention is applied.

In the figure, the same reference numerals are given to the members described in the third embodiment, and the description is omitted.

FIG. 15 shows the memory 3 as density group storage means of the present invention in which the halftone patch density data of each color detected for each image formation is stored in contact with the control section 30.
5 and a comparator 36 as comparison means of the present invention. In this embodiment, the memory 35
By using the patch density data in which a part of the patch density data required for creating the LUT is stored, stable image density control can be maintained and toner consumption can be reduced.

In this embodiment, similarly to the third embodiment, a patch is formed between papers, and the space between the papers is equal to or greater than the patch area plus the length from the last patch position to the exposure position. To enable real-time concentration control.

Hereinafter, the present embodiment will be described in detail with the yellow color as a representative. First, as described in the third embodiment, a 4 × 4 patch shown in FIG.
In the dot matrix of Py1, Py2 has grown 2 dots, Py2 has grown 5 dots, Py3 has grown 8 dots, Py4 has grown 11 dots, Py
A patch image 6 is formed by using each of 14 dots.

Subsequently, these patch densities are detected by the density sensor 20, and the patch density Dy 1, Dy 2, Dy 3, Dy 4, Dy corresponding to each input data is calculated by the arithmetic processing unit 32.
Get 5.

Based on the obtained patch densities, the control unit 30 performs density interpolation between the patches, and based on the obtained density curves, the control unit 30 inverts Y = X for the characteristic curve (LUT). ) Is created, and this LUT is stored in the LUT storage unit 3.
4 to create LUT-y0.

At this time, the patch densities Dy1, Dy2, Dy corresponding to the respective input data obtained by the arithmetic processing section 32
3, Dy4 and Dy5 are stored in the memory 35.

When there is a continuous printing request, a yellow patch is similarly formed on the conveyor belt 7 and new patch densities Dy1 ', Dy2', Dy3 ', Dy
4 ′ and Dy5 ′ are obtained, and LUT-y1 is created. At this time, since the printing is in the continuous printing state, if the change in the density gradation characteristic of the electrophotographic process unit is small, the patch density Dy (n) ′
(N is an integer satisfying 1 ≦ n ≦ 5) and the patch density Dy
In some parts of (n) (n is an integer satisfying 1 ≦ n ≦ 5), the patch densities may almost coincide.

Therefore, in this embodiment, in such a case,
If there is no change in the patch density when creating the LUT,
Alternatively, when the change in patch density is within a predetermined range (hereinafter, also referred to as coincidence), the LUT
Referencing the previous patch density data as a part of the created density data, and omitting the formation of a new patch corresponding to a portion where there is no change in patch density, wastes toner. prevent.

That is, each of the patch densities Dy (n) stored in the memory 35 and each of the patch densities Dy (n) ′ obtained by the arithmetic processing section 32 are compared by the comparator 36, and the respective input data are compared. If a part of the corresponding patch densities match, the patch densities determined to be coincident are determined as Deq-y
(M) (m is an integer satisfying 1 ≦ m ≦ 5), and the patch Peq-y (m) corresponding to the patch density Deq-y (m) determined to be coincident in the next patch formation
(M is an integer satisfying 1 ≦ m ≦ 5) is not formed.

At this time, the patch density data D in the memory 35
y (n) is updated to the patch density Dy (n) ′ obtained by the arithmetic processing unit 32 and compared by the comparator 36.

Usually, Py1 of a 4 × 4 dot matrix is obtained by growing 2 dots, Py2 is obtained by growing 5 dots, Py3 is obtained by growing 8 dots, and Py4 is obtained.
A patch image is formed by using an image that has grown by 11 dots and Py5 is formed by using an image that has grown by 14 dots. Based on the remaining patch images from which Peq-y (m), which is a part of the patch images, is omitted. In creating Lm from the patch densities obtained in (1), Lm-y2 is created by using together the density Deq-y (m) corresponding to the omitted patch Peq-y (m) stored in the memory 35. .

Here, in the present embodiment, as a criterion for the comparator 36, a case where the compared density data is within a deviation of 0.02 is regarded as a match.

However, the stop of the formation of the patches regarded as coincident with the above is performed by the image forming apparatus according to the present embodiment on a predetermined number (for example, 100 sheets, 200 sheets, or 1000 sheets) of the transfer material. It may be canceled when an image is formed.

As described above, a stable density gradation characteristic was always obtained without deteriorating the linearity of the density gradation characteristic even if the LUT made by reducing the number of patches to be formed next time was used. .

As described above, the method of controlling the density of yellow color has been described in chronological order. Of course, the patch density is similarly detected for the magenta, cyan and black colors following the yellow color between each sheet, and the yellow and black colors are detected. It goes without saying that similar control is performed.

As described above, the memory 35 for storing the patch density of each color obtained for each image formation is provided, and the new patch density and the memory 3 which are continuously detected are stored.
5 is compared with the patch density stored in No. 5, and in a portion where there is no change in the patch density, the previous patch density data is referred to as a part of the density data for creating the LUT, and a portion where the patch density does not change is determined. By omitting the new formation of the corresponding patch, stable density gradation characteristics can always be obtained without impairing the linearity of the density gradation characteristics, and unnecessary toner consumption is prevented. It became possible to do.

In the present embodiment, as a judgment criterion in the comparator 36, a case where the compared density data is within a deviation of 0.02 is regarded as a match. Since the density deviation in a light area is more strictly expressed in terms of color tone reproducibility than the deviation, the comparison criterion is set to 0.02 for the density in a light area.
To reduce the number of newly formed patches by allowing a density deviation of about 0.1 for dark areas according to the color reproduction range of the toner used and the optical characteristics of the photoconductor, etc. And the toner consumption can be further reduced.

In the present embodiment, for the sake of convenience of explanation, an arithmetic processing unit 32 for converting a detection value from the density sensor 20 into a density, and an LUT storage unit 3 for storing an LUT.
4. an image control unit 33 that outputs rasterized data to the exposure unit 11; density data in a memory 35;
Although the comparator 36 for comparing the density data obtained in step 2 has been described separately from the control unit 30, as long as the content is in accordance with the gist of the present invention, a unit having the above-described function is provided inside the control unit 30. Needless to say, there is no problem even if the above-described density control is performed.

[0194]

As described above, according to the image forming apparatus of the present invention, the toner image for detecting the image density is determined by the distance TrB between the transfer materials conveyed on the endless carrier. And transfer of the process means at the density detection position of the image density detection toner image of the image density detection device and the most upstream position in the sheet material conveyance direction so that the distance TrB can be continuously performed between the transfer materials. L1 is the length between the developing position and the exposure position of the latent image carrier of the process unit at the most upstream position from the transfer position in the latent image carrier, and L2 is the outer peripheral length of the latent image carrier.
The length of the toner image forming region on the endless carrier of the image density detecting toner image in the process unit at the most upstream position is L
3. The length of the endless carrier of the transfer material non-conveying portion between the transfer material conveyed on the endless carrier and the transfer material is T
Since rB satisfies the relationship TrB ≧ L1 + L2 + L3, the density detection result of each color is
It is possible to provide an image forming apparatus which can immediately feed back to the UT, enables real-time image control, and can always maintain an optimal state of density, thereby performing stable color tone reproduction and density gradation control. it can.

Further, the image forming apparatus is provided with storage means for storing the optimum value data of the developing bias of each color controlled for each image formation by using the image density detecting device. The calculated value is stored, and the developing bias value used in the developing device is controlled based on the stored developing bias value to form a toner image for image density detection. Therefore, it is possible to provide an image forming apparatus capable of stably reproducing color tones because it is possible to always maintain an optimum density state.

Further, using an image density detecting device, there is provided a density group storing means for storing density data of the toner image for detecting the image density of each color obtained every time an image is formed. Stores density data that is the result of image density detection, and refers to the previous density data as part of the density data for LUT creation in areas where there is no change in new density, and responds to areas where there is no change in density By eliminating the need to form new patches, toner can be prevented from being unnecessarily consumed and, at the same time, the optimal state of the density gradation characteristics can be maintained at all times. And an image forming apparatus capable of performing the above.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a color image forming apparatus as a first embodiment to which an image forming apparatus according to the present invention is applied.

FIG. 2 is a structural diagram of a density sensor used in the image forming apparatus shown in FIG.

FIG. 3 is a schematic structural view of the image forming apparatus shown in FIG.

4 is a graph showing a relationship between a DC component voltage value of a developing bias and an image density with respect to a light portion potential VL-100V on a photosensitive member of the image forming apparatus shown in FIG.

FIG. 5 is a schematic structural view of the image forming apparatus shown in FIG.

FIG. 6 is a graph showing an execution result of image density control by the image forming apparatus shown in FIG. 1;

FIG. 7 is a schematic diagram of a color image forming apparatus as a second embodiment to which the image forming apparatus according to the invention is applied.

FIG. 8 is a flowchart of an image control operation of the image forming apparatus according to the second embodiment of the present invention.

9 is a graph showing a change in the image density of a solid magenta color with respect to the number of prints in the image forming apparatus shown in FIG. 7;

FIG. 10 illustrates a third example to which the image forming apparatus according to the invention is applied.
1 is a schematic diagram of a color image forming apparatus as an embodiment.

11 is a schematic structural view of the image forming apparatus shown in FIG.

12 is a graph of a patch density created in the image forming apparatus shown in FIG.

13 is an operation schematic diagram of the image forming apparatus shown in FIG.

14 is a graph showing an execution result of image density control by the image forming apparatus shown in FIG.

FIG. 15 illustrates a fourth example to which the image forming apparatus according to the present invention is applied.
FIG. 1 is a schematic structural diagram of a color image forming apparatus as an embodiment.

FIG. 16 is a schematic graph of LUT creation used in the present invention and a conventional image forming apparatus.

FIG. 17 is a schematic diagram of a dot matrix used in the present invention and a conventional image forming apparatus.

FIG. 18 illustrates a conventional image forming apparatus in which the number of printed sheets is 100;
6 is a graph of a solid image density change when image density control is performed for each sheet.

FIG. 19 is a graph of a density gradation characteristic with respect to an input signal for an arbitrary number of sheets when a continuous image is formed on the order of several thousand sheets from the beginning when magenta color is used in a conventional image forming apparatus.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 transfer paper 2 paper feed section 3 paper discharge section 4 paper passing path 5 belt drive roller 6 driven roller 7 transport belt 8Y, 8M, 8C, 8K electrophotographic process section 9, 9Y, 9M, 9C, 9K photoreceptor 10 charger 11, 11Y Exposure device 12, 12Y, 12M, 12C, 12K Developing device 13, 13Y, 13M, 13C, 13K Transfer device 14 Photoconductor cleaner 15 Fixing device 20 Density sensor 21 Light emitting element 22 Light receiving element 30 Control unit 31 Sampling control unit 32 arithmetic processing unit 33 image control unit 34 LUT storage unit 35 memory 36 comparator 37 memory P patch TrM1, TrM2 transfer paper

Continued on front page F term (reference) 2H027 DA09 DC10 DE02 EA05 EB04 EC03 EC06 EC20 ED09 ED16 ED24 EE07 EE08 EF09 GA43 GB02 2H030 AA02 AA03 AA06 AB02 AD05 AD12 AD16 BB02 BB23 BB34 BB36 BB44 BB53

Claims (14)

[Claims] An image forming apparatus comprising: a latent image carrier on which a latent image is formed; a developing device for developing a toner image corresponding to the latent image; and a transfer device for transferring the toner image onto a transfer material. A plurality of process means for forming a toner image on the latent image carrier with the toner, an endless carrier for conveying the transfer material, and an image density detection toner image formed on the endless carrier. An image density detecting device for detecting a density, wherein the image density detecting device is located downstream of the process unit at the most downstream position in the conveying direction of the sheet material, and the image density detecting device L1 is the length between the density detection position of the image density detection toner image and the transfer position of the process unit at the most upstream position in the sheet conveying direction. The outer peripheral length of the latent image carrier from the developing position of the latent image carrier to the transfer position of the process means is L2, and the endless carrier of the image density detection toner image in the process means at the most upstream position. When the length of the toner image forming area is L3, and the length of the endless carrier of the transfer material non-conveying portion between the transfer materials conveyed on the endless carrier is TrB. An image forming apparatus that satisfies the following relationship: TrB ≧ L1 + L2 + L3, and wherein the toner image for image density detection is formed in the transfer material non-conveying section. 2. An n-th counting from a downstream side of the endless carrier.
The outer peripheral length L2 (n) of the latent image carrier from the developing position on the latent image carrier of the (n ≧ 2) process means to the transfer position in the process means is counted from the downstream of the endless carrier. And the outer peripheral length L2 (n-1) of the latent image carrier from the developing position on the latent image carrier of the (n-1) th process means to the transfer position in the process means, L2 (n 2. The image forming apparatus according to claim 1, wherein the following relationship is satisfied: L2 (n-1). 3. An n-th counter counting from a downstream side of the endless carrier.
The length L3 (n) of the toner image forming area of the image density detection toner image of the (n ≧ 2) th process means is determined by the transfer position of the nth process means and the (n−1) th process means. The image forming apparatus according to claim 1, wherein when the length of the endless carrier between the transfer position and the transfer position is Dis (n), the following relationship is satisfied: L3 (n) ≦ Dis (n). 4. A plurality of image density detection toner images are formed by making developing biases applied to the developing devices different from each other, and the image density detection toner images detected by the image density detection device are formed. 4. The image forming apparatus according to claim 1, wherein a density control is performed by determining a developing bias applied to the developing device based on the density. 5. 5. A storage device for storing a value of a developing bias applied to the developing device, wherein the developing device is set to a predetermined variable range based on the value of the developing bias stored in the storing device. The image forming apparatus according to claim 4, wherein the image density detection toner image is formed using a developing bias. 6. A latent image carrier on which a latent image is formed by exposure, an exposure unit for exposing the latent image carrier to light, and a developing device for developing a toner image corresponding to the latent image. ,
A plurality of process means for forming toner images on the latent image carrier with toners of different colors each having a transfer device for transferring the toner image onto a transfer material; and an endless carrier for conveying the transfer material. An image density detection device that detects the density of the toner image for image density detection formed on the endless carrier, wherein the image density detection device is located at a most downstream position in the sheet conveying direction. And the length of the density detection position of the toner image for image density detection of the image density detection device and the transfer position of the process means at the most upstream position in the conveying direction of the sheet material. L1 to the latent image carrier from the exposure position on the latent image carrier of the process means at the most upstream position to the transfer position in the process means. L2, the length of the toner image forming area on the endless carrier of the image density detection toner image in the process unit at the most upstream position is L3, and the transfer carried on the endless carrier. When the length of the endless carrier of the transfer material non-transporting portion between the material and the transfer material is TrB, the relationship TrB ≧ L1 + L2 + L3 is satisfied, and the toner image for image density detection is An image forming apparatus formed in a transfer material non-conveying section. 7. An n-th counting member from a downstream side of the endless carrier.
The outer peripheral length L2 (n) of the latent image carrier from the exposure position of the latent image carrier of the (n ≧ 2) process means to the transfer position of the process means is counted from downstream of the endless carrier. And the outer peripheral length L2 (n-1) of the latent image carrier from the developing position of the (n-1) th process means on the latent image carrier to the transfer position of the process means, L2 (n 7. The image forming apparatus according to claim 6, wherein the following relationship is satisfied: L2 (n-1). 8. An n-th counting from a downstream side of the endless carrier.
The length L3 (n) of the toner image forming area of the image density detection toner image of the (n ≧ 2) th process means is determined by the transfer position of the nth process means and the (n−1) th process means. 8. The image forming apparatus according to claim 6, wherein a relationship L3 (n) ≦ Dis (n) is satisfied when a length of the endless carrier between the transfer position and the transfer position is Dis (n). 9. 9. The image density detecting device according to claim 1, wherein the exposure unit exposes the latent image carrier to light based on input data, and detects the image density based on the density of the image density detecting toner image detected by the image density detecting device. A processing unit that outputs a density graph indicating the density with respect to the input data that has formed the toner image for use, and a point in which the density graph output from the calculation processing unit and the density graph become 0 when the input data is output. A control unit that calculates an inverted graph obtained by inverting a straight line connecting the point at which the concentration is maximum; and an inverted graph storage unit that stores the inverted graph calculated by the control unit. 9. The method according to claim 6, wherein input data to be output to the exposure unit is processed and output based on the inverted graph stored in the inverted graph storage unit. The image forming apparatus. 10. The image forming apparatus according to claim 9, further comprising: an image control unit configured to rasterize input data output to the exposure unit processed by the control unit and output the rasterized data to the exposure unit. 11. A density group storage means for storing a first density group, which is a set of densities of the plurality of image density detection toner images detected by the image density detection device, and stored in the density group storage means. And a second density group, which is a set of the respective densities of the first density group and the densities of the image density detection toner image formed next to the image density detection toner image. Comparing means for comparing the density in the first density group with the density in the second density group within a predetermined range. When the density of the second density group is stored in the density group storage means, the difference from the density in the first density group is determined to be within a predetermined range. The concentration in the second concentration group instead of the concentration in the second concentration group The density in the first density group, the density of which is different from the density in the second density group within a predetermined range, is stored in the density group storage means. Image forming apparatus. 12. The image processing apparatus according to claim 1, wherein the arithmetic processing unit is configured to determine the image density based on a set of the densities of the plurality of image density detection toner images detected by the image density detection device stored in the density group storage unit. The image forming apparatus according to claim 11, wherein a density graph indicating a density with respect to the input data on which the detection toner image is formed is output. 13. The density in the second density group, wherein the control unit determines that the difference from the density in the first density group is within a predetermined range. The image forming apparatus according to claim 11, wherein the output of the input data corresponding to (i) is stopped. 14. The image forming apparatus according to claim 13, wherein the stop of the output of the input data in the control unit is released when a predetermined number of images are formed in each of the process units.