JPH11174756A - Color image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance detection accuracy by widening the dynamic range of an output and to prevent a detection infeasible state from occurring by setting the range of an output value to be within the detection feasible range by executing the setting, in which the case that color toner is used and the case that black toner is used are different, of a voltage value added by an offset voltage addition means. SOLUTION: A toner image for detecting density (patch) is formed on a photoreceptor drum being an image carrier. Then, offset bias is set. The offset bias is set according as the cases, that means, according to whether the patch is formed by using the color toner or the black toner. For example, the offset bias is decided to be -5 V in the case of the color toner and decided to be zero V in the case of the black toner. A value outputted from a density sensor 9 becomes the voltage value obtained by adding the offset bias to a voltage outputted from a reflected light quantity detection part 92 detecting the quantity of light reflected from the patch. Then, it is inputted to the CPU part 50 of a main body and converted to a digital signal. Besides, it is converted to density data. Then, optimum developing bias is selected according to the density of the patch by a developing bias control part 53.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus, such as a copying machine, a printer, or a facsimile machine, having a function of forming a color image on a transfer material such as a sheet.

[0002]

2. Description of the Related Art Conventionally, as an electrophotographic color image forming apparatus, for example, there are a copying machine, a printer, a facsimile machine and the like.

A copying machine generally has a function of reading an image of an original or the like and forms an image on a sheet based on the read image information. Some copiers have a function of inputting image information to be sent.

[0004] A printer is generally an external device,
For example, an image is formed on a sheet based on image information sent from an apparatus such as a computer, and a facsimile apparatus generally has a function of reading an image of a document,
Further, it has a communication function, sends read image information to the outside, and forms an image on a sheet based on information sent from the outside.

Since the basic configuration of the color image forming apparatus is a well-known technique, a detailed description thereof will be omitted and will be briefly described below.

First, an image carrier (for example, a photoconductor) provided in a color image forming apparatus is irradiated with image light (laser light) to form an electrostatic latent image corresponding to each color on the image carrier. .

[0007] Then, an image can be developed (toner image) by electrostatically attaching a developer of each color to the electrostatic latent image by a developing device.

Next, the development is transferred to a primary transfer member.

By repeating this process for all colors, a plurality of colors of development (toner images) are formed.

Thereafter, the development (toner image) is transferred onto the conveyed secondary transfer member (transfer material such as a sheet), and the secondary transfer member is conveyed to a fixing device, where the toner image is transferred. The image formation is completed by fixing the unfixed image formed on the image forming apparatus.

In the above-described color image forming apparatus, the image density fluctuates depending on the use environment and the like.

Therefore, a toner image for density measurement is formed on the image carrier, the density is detected by a density sensor, and an exposure amount, a developing bias, and the like are fed back based on the detected density to obtain an image. The control to adjust the density was performed.

[0013]

However, in the case of the above-described prior art, the following problems have occurred.

Hereinafter, problems of the color image forming apparatus according to the prior art will be described with reference to FIGS.

(1) Generally, color toners such as yellow, cyan, and magenta reflect light, so that the output of the density sensor increases as the toner density increases as shown in FIG.

On the other hand, since the black toner absorbs light,
As the toner density increases, the output from the density sensor decreases.

Here, the sensor output when the density is 0 in FIGS. 10 and 11 represents the output of the reflected light from the image carrier serving as the base.

Since the accuracy of density detection depends on the variation range of the sensor output with respect to the change in toner density, that is, the dynamic range, when detecting the density of the color toner, the reflectance of the color toner as shown in FIG. The use of a low image carrier (dark object) can increase the dynamic range and increase the accuracy of density detection.

On the other hand, when detecting the density of the black toner,
As shown in FIG. 11, a base with high reflectance (whitish image carrier)
It is more advantageous to use.

However, it is very difficult to make the optical reflectivity of an image carrier such as a photoreceptor different between a portion where a color toner patch is applied and a portion where a black toner patch is applied, that is, it is very difficult to apply two colors. Therefore, generally, the optical reflectance of the image carrier (photoreceptor in this case) serving as a base is adjusted to be intermediate between the reflectances of the black toner and the color toner.

FIG. 12 shows the output of the density sensor when such a photosensitive member is used.

In the figure, curve A is the output of the color toner, and curve B is
Represents the output of the black toner.

In this case, the dynamic range of the sensor output is determined when the base is painted in two colors (FIGS. 10 and 1).
This is about one-half that of 1), and the accuracy of density detection is reduced.

(2) In general, the photosensitive drum 1 maintains a good image by actively polishing the surface with a cleaning blade and refreshing the surface.
As a result, the thickness of the photosensitive layer decreases as the paper is durable.

A photosensitive member provided with an OCL layer as a protective layer
Since a conductive material such as tin oxide dispersed in the OCL layer absorbs a large amount of infrared light, the reflectance of the photosensitive drum increases as the film thickness decreases.

In this case, as shown in FIG. 13, the output signal level (voltage value) from the density sensor exceeds the A / D capture width of the CPU on the high-density side of the color toner, so that the density cannot be detected. .

In the figure, curve A is the output of the color toner, and curve B is
Represents the output of the black toner, and the straight line H represents the maximum value of 5 V for the A / D capture of the CPU.

On the other hand, it is easy to set the light emission amount of the sensor and the amplification factor of the light receiving portion circuit low in advance in consideration of the fluctuation of the reflectance of the photosensitive drum due to the durable paper passing. , The dynamic range is reduced, and the accuracy of density detection is deteriorated.

It goes without saying that the above-mentioned disadvantages occur not only when the density is detected on the photosensitive member but also when the density is detected on an intermediate transfer or a transfer drum.

The present invention has been made to solve the above-mentioned problems of the prior art, and it is an object of the present invention to improve the accuracy of detection and to excel in quality to prevent undetectability due to durability. To provide a color image forming apparatus.

[0031]

In order to achieve the above object, the present invention provides a density detecting means for detecting the density of a toner image for density measurement formed on an image carrier, and the density detecting means. Means for adding an offset voltage to an output signal detected by the means, and a color image forming apparatus for controlling image forming conditions in accordance with the density calculated by the output signal from the offset voltage adding means. Therefore, depending on whether the toner is a color toner or a black toner,
It is characterized in that the addition voltage value by the offset voltage addition means is set differently.

Therefore, the detected output value is adjusted.

By adding the offset voltage by the offset voltage adding means, the output value range for the color toner and the output value range for the black toner may be substantially the same.

Therefore, it is possible to make the most effective use of the output area that can be detected.

A density detecting means for detecting the density of the toner image for density measurement formed on the image carrier, and an offset voltage adding means for adding an offset voltage to an output signal detected by the density detecting means. Means for controlling image forming conditions in accordance with a density calculated by an output signal from the offset voltage adding means, wherein the offset voltage adding means is controlled in accordance with an optical reflectance of the image carrier. It is possible to set an added voltage value by means.

Therefore, the detected output value is adjusted according to the optical reflectance.

Preferably, the optical reflectance of the image carrier is detected by the density detecting means.

By adding the offset voltage by the offset voltage adding means, the range of the output value can be kept within a detectable range.

Therefore, detection failure can be prevented.

The density detecting means preferably includes a light emitting means for irradiating a light beam to the toner image for density measurement, and a light receiving means for receiving light reflected by the light emitting means.

[0041]

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 material, shape, relative arrangement, and the like are not intended to limit the scope of the present invention only to them unless otherwise specified.

First, the configuration and the like of a color image forming apparatus according to an embodiment of the present invention will be described with reference to FIGS.

FIG. 5 is a schematic configuration diagram of a color image forming apparatus according to the embodiment of the present invention.

The photosensitive drum 1 serving as an image carrier is driven in a direction indicated by an arrow by driving means (not shown), and is uniformly charged by a primary charger 2.

The photosensitive drum 1 will be described in detail with reference to FIG. 6. A photosensitive layer composed of a normal organic photoconductor layer (OPC) is formed on the outer peripheral surface of a grounded drum base 1a made of a conductive material such as aluminum. 1b is formed and applied, and a protective layer (OCL) 1c having excellent wear resistance is applied and formed thereon.

Further, the photoreceptor layer 1b comprises a subbing layer (CPL)
1b-1, an injection blocking layer (UCL) 1b-2, a charge generation layer (CGL) 1b-3, and a charge transport layer (CTL) 1b-4.

Next, the exposure device 3 irradiates the photosensitive drum 1 with a laser beam L according to a yellow image pattern, so that a latent image is formed on the photosensitive drum 1.

When the photosensitive drum 1 further moves in the direction of the arrow, the developing devices 4a, 4b, 4c and 4d supported by the rotary support 11
Of the developing devices 4a, the developing device 4a containing the yellow toner rotates so as to face the photosensitive drum 1, and the selected developing device 4a
Visualized by a.

The intermediate transfer belt 5 rotates in the direction of the arrow at substantially the same speed as the photosensitive drum 1, and applies the toner image formed on the photosensitive drum 1 to the primary transfer roller 8a.
The primary transfer is performed on the outer peripheral surface of the intermediate transfer belt 5 by the secondary transfer bias.

By performing the above steps for yellow, magenta, cyan, and black, a plurality of color toner images are formed on the intermediate transfer belt 5.

Next, the transfer material is fed from the transfer material cassette 12 by the pickup roller 13 at a predetermined timing.

At the same time, a secondary transfer bias is applied to the secondary transfer roller 8b, and a toner image is transferred from the intermediate transfer belt 5 to a transfer material.

Further, the transfer material is conveyed to the fixing device 6 by the conveyance belt 14 and is fused and fixed to obtain a color image.

Further, the transfer residual toner on the intermediate transfer belt 5 is cleaned by the intermediate transfer belt cleaner 15.

On the other hand, the transfer residual toner on the photosensitive drum 1 is cleaned by a cleaning device 7 of a known blade means.

Further, a density sensor 9 as density detecting means is provided in the main body.

Generally, in an electrophotographic color image forming apparatus, if the image density fluctuates due to changes in the environment in which the apparatus is used, the number of prints, and the like, the original correct color tone cannot be obtained.

Therefore, a toner image (patch) for density detection (measurement) is formed on the photosensitive drum 1 on a trial basis with each color toner, and the density is detected by the density sensor 9.
Based on the detection result, feedback is applied to image forming conditions such as an exposure amount and a developing bias, and image density control is performed to obtain a stable image.

As shown in FIG. 7, the density sensor 9
A light-emitting element 91 as light-emitting means such as D; a light-receiving element 92 as light-receiving means such as a photodiode;
The light reflected from the patch P is measured by irradiating the patch P on the photosensitive drum with infrared light from the light emitting element 91 and measuring the reflected light therefrom with the light receiving element 92.

The light reflected from the patch P contains a specular reflection component and a diffuse reflection component, and the specular reflection component varies depending on the state of the surface of the photosensitive drum which is the base of the patch and the variation in the distance between the sensor and the patch. When the specular reflection component is included in the reflected light from the patch to be measured due to large fluctuations in detection, the detection accuracy is significantly reduced.

Therefore, in the density sensor 9, regular reflection light from the patch P is prevented from entering the light receiving element 92.
With reference to the normal I, the irradiation angle on the patch P is 45
And the light receiving angle of the reflected light from the patch P is 0 °, and only the irregularly reflected light is measured.

The signal value detected by the density sensor 9 is converted into density data by a main body CPU (not shown).

In the present invention, the quality is improved by adjusting the sensor output value of the density sensor 9 as shown in the following embodiments.
This will be described in detail below.

(First Embodiment) In this embodiment, an offset voltage adding circuit as an offset voltage adding means for adding a DC offset voltage to an output signal of a density sensor is provided. By adding a negative offset bias, the sensor output value is adjusted to change within the same range when detecting the density of the color toner and when detecting the density of the black toner, and the dynamic range of the sensor output is expanded to detect the density. The accuracy of the has been improved.

FIG. 1 is a block diagram showing a configuration for performing image density control according to the present embodiment.

In FIG. 1, reference numeral 9 denotes a density sensor section, which is surrounded by a broken line. The density sensor section irradiates the toner image P formed on the photosensitive drum 1 with infrared light, and outputs the reflected light amount to the CPU section 50 as an output signal. Send out.

In the density sensor 9, reference numeral 91 denotes the infrared light L.
A light-emitting light source 92 composed of an ED is a reflected light detection unit that detects the amount of reflected light from the toner image P, and is composed of a photodiode and an amplifier circuit.

Here, the light quantity of the light emitting light source 91 and the amplification factor of the reflected light detecting section 92 are such that the reflected output from the photosensitive drum in which no toner image is formed (hereinafter referred to as background output) is 5
V so that the reflection output of the color toner is set to 10 V at the maximum density of 2.0 at which detection is performed (FIG. 2).

An offset voltage generator 94 adds the offset voltage to the detection voltage from the reflected light amount detector 92 and shifts the level.

Reference numeral 50 denotes a CPU for controlling the setting of image forming conditions by receiving signals from the density sensor 9 and performing arithmetic processing.

In this embodiment, the controlled image forming condition is a developing bias voltage applied to the developing device.

In the CPU section 50, reference numeral 51 denotes an A / D capturing section, and a reflected light amount detecting section 92 of the density sensor section 9;
The analog reflected light quantity voltage output from the device is taken in the range of 0 to 5 V and converted into a digital signal.

Reference numeral 52 denotes a density converter for calculating the density value of the toner image from the digital value of the reflected light amount signal.

Reference numeral 53 denotes a developing bias controller, which determines a developing bias voltage value according to the density value output from the density converter 52.

The offset voltage controller 54 controls the offset bias generated by the offset voltage generator 94 of the density sensor 9.

The image density control according to the present embodiment is performed by the above-described configuration.
This will be described in detail with reference to the flowchart of FIG.

First, when a request for image density control is input to the CPU of the apparatus main body, an image density control sequence starts (step S1).

The image density control is performed in accordance with the durability of the developing device and the photosensitive drum, the stoppage time of the main body, and environmental fluctuation.

Next, a toner image (patch) for density detection is formed on a photosensitive drum as an image carrier (S2).

It is preferable that a plurality of patches are formed in an appropriate pattern for determining image forming conditions while changing image forming conditions such as a developing bias and an exposure amount.

In this embodiment, a halftone dither pattern is selected as a patch pattern, and a plurality of (3 to 8) patches are formed while changing the developing bias voltage.

Next, an offset bias, which is a point of the present embodiment, is set.

The setting of the offset bias is classified according to whether the formed patch is for the color toner or for the black toner (S3). In the case of the color toner, the offset bias is set to -5V (S4) and the black toner is set. In the case of (1), it is determined to be 0 V (S5).

Next, the amount of reflected light from the patch is detected (S6). The output value from the density sensor 9 is a voltage value obtained by adding an offset bias to the output voltage of the reflected light detection unit. Input to the A / D capture unit,
It is converted to a digital signal.

Thereafter, the digital signal value is converted into density data by the density conversion unit of the CPU (S7), and an optimum developing bias is selected by the developing bias determination unit according to the density of the patch (S8).

By repeating the above process for the four colors of yellow, cyan, magenta, and black, the optimum developing bias for each color is determined.

The image density control according to the present embodiment has been described above, and the sensor output at that time is shown in FIG.

In the figure, the curve A is the output of the color toner, and the curve B is
Represents the output of the black toner, and the straight line H represents the maximum value of 5 V for the A / D capture of the CPU.

In the image forming apparatus used in the embodiment of the present invention, the density output characteristics of yellow, cyan, and magenta are almost the same, and are shown together on a curve A.

As can be seen from FIG. 3, according to the present embodiment, the dynamic range of the sensor output is approximately twice as large as that of the conventional technology (FIG. 12) for both the color toner and the black toner. Accordingly, the accuracy of image density control can be improved accordingly.

As described above, the offset voltage adding circuit for adding the DC offset voltage to the output signal of the density sensor is provided. When the density of the color toner is detected, a negative offset bias is added to detect the density of the color toner. The sensor output value is adjusted to change within the same range when detecting the density of black toner and the density of black toner, making it possible to expand the dynamic range of the sensor output, improving the accuracy of density detection, and the accuracy of image density control. Could be improved.

In the above description, the case where the sensor output values for the color toner and the black toner are set to be in the same range has been described.
By increasing the overlapping range, it is possible to widen the dynamic range by that much, and a predetermined object can be achieved.

(Second Embodiment) In this embodiment, an offset voltage adding circuit for adding a DC offset voltage to the output signal of the density sensor is provided, and the reflection from the photosensitive drum as an image carrier is performed by the density sensor. Measure the light,
By adding the DC offset voltage value according to the amount of reflected light, when the reflectivity of the photosensitive drum increases, the output signal level (voltage value) from the density sensor becomes higher than the A / D of the CPU on the high density side of the color toner. It is possible to prevent the density detection from becoming impossible due to exceeding the capture width.

The configuration for performing image density control in this embodiment is the same as that in the above-described first embodiment, and a detailed description thereof will be omitted.

The amount of light emitted from the light source 91 and the amplification factor of the reflected light detector 92 are such that the reflected output from the photosensitive drum in the initial state (hereinafter referred to as background output) is 2.5 V, and the reflected output of the color toner is detected. Is set to 5 V at the maximum density of 2.0.

Next, the operation of the image density control in the present embodiment will be described in detail with reference to the flowchart of FIG.

First, when an image density control request is input to the CPU of the apparatus main body, an image density control sequence starts (S1).

The image density control is performed in accordance with the durability of the developing device and the photosensitive drum, the stopping time of the main body, environmental fluctuation, and the like.

Next, the density sensor 9 measures the amount of reflected light from a state in which no toner is on the photosensitive drum as the image carrier (base) (S2).

Subsequently, the main body CPU calculates an offset bias according to the background output detected in S2 (S3).

The calculation of the offset bias is as follows.

Offset voltage = initial background output−background output during image density control For example, the background reflectance is 3 V due to durability.
The offset voltage when the photosensitive drum is used is increased to 2.5V (initial output) -3V = -0.5V.

Next, a toner image (patch) for density detection is formed on a photosensitive drum as an image carrier (S4).

It is preferable that a plurality of patches are formed in an appropriate pattern for determining image forming conditions while changing image forming conditions such as a developing bias and an exposure amount.

In this embodiment, a halftone dither pattern is selected as the patch pattern, and a plurality of (3 to 8) patches are formed while changing the developing bias voltage.

Next, the amount of reflected light from the patch is detected (S5). The output value from the density sensor 9 is a voltage value obtained by adding an offset bias to the output voltage of the reflected light detection unit. Input to the A / D capture unit,
It is converted to a digital signal.

Thereafter, the digital signal value is converted into density data by the density conversion unit of the CPU (S6), and an optimum developing bias is selected by the developing bias determining unit according to the patch density (S7).

By repeating the above process for the four colors of yellow, cyan, magenta, and black in order, the optimum developing bias for each color is determined.

The image density control according to the present embodiment has been described above, and the sensor output at that time is shown in FIG.

In the figure, curve A is the output of the color toner, and curve B is
Represents the output of the black toner, and the straight line H represents the maximum value of 5 V for the A / D capture of the CPU.

In the image forming apparatus used in the embodiment of the present invention, the density output characteristics of yellow, cyan, and magenta are almost the same, and are shown together on the curve A.

FIG. 9 shows the case where the reflection output of the base of the photosensitive drum is 3 V, and the sensor output on the high-density side of the color toner by the offset bias of -0.5 V is also A
The / D capture maximum value is 5 V or less, and detection is possible.

The background reflection output 3V is an output value when the photosensitive drum used in the embodiment of the present invention has reached the end of its service life. Accordingly, the calculation method of the light amount of the light emitting light source 91 and the amplification factor and the offset bias of the reflected light detecting unit 92 are set to be optimal.

In this embodiment, the background reflection output during image density control is measured by using a density sensor. However, for example, the background reflection output is determined based on the usage of the photosensitive drum such as the cumulative number of sheets passed, the number of rotations, and the charging time. If the reflectance can be inferred, that value may be used.

By doing so, the procedure for measuring the reflectance of the background can be omitted, and the time required for image density control can be reduced.

As described above, in this embodiment, the offset voltage adding circuit for adding the DC offset voltage to the output signal of the density sensor is provided, and the reflected light from the photosensitive drum as the image carrier is measured by the density sensor. By adding a DC offset voltage value according to the amount of reflected light, the output signal level (voltage value) from the density sensor when the reflectance of the photosensitive drum increases increases the A level of the CPU on the high density side of the color toner. / D capture width is exceeded,
It was possible to prevent the density detection from being disabled.

Further, by combining this embodiment with the first embodiment, even when the base reflectance of the photosensitive drum increases, the output signal level (voltage value) from the density sensor becomes the color toner. On the high concentration side
It is possible to prevent the A / D capture width of the PU from being exceeded and prevent the density detection from being performed, and also to widen the dynamic range of the sensor output and improve the accuracy of the density detection.

[0118]

As described above, according to the present invention, the output value detected by setting the added voltage value by the offset voltage adding means to be different depending on whether the toner is a color toner or a black toner. If the output value range for the color toner and the output value range for the black toner are made substantially the same range, the output area that can be detected can be used as effectively as possible. The dynamic range of the output can be expanded, the detection accuracy is improved, and the quality is improved.

Further, by setting the added voltage value by the offset voltage adding means according to the optical reflectance of the image carrier, the detected output value can be adjusted according to the optical reflectance, and the offset voltage can be added. By doing so, if the output value range falls within the detectable range, undetection can be prevented, and the quality is improved.

[Brief description of the drawings]

FIG. 1 is a block diagram of a configuration for performing image density control according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an output of a density sensor according to the first embodiment of the present invention.

FIG. 3 is a diagram illustrating an output of a density sensor according to the first embodiment of the present invention.

FIG. 4 is a flowchart according to the first embodiment of the present invention.

FIG. 5 is a schematic configuration diagram of a color image forming apparatus.

FIG. 6 is a partial schematic configuration diagram of a photosensitive drum.

FIG. 7 is a schematic configuration diagram of a density sensor.

FIG. 8 is a flowchart according to the second embodiment of the present invention.

FIG. 9 is a diagram illustrating an output of a density sensor according to the second embodiment of the present invention.

FIG. 10 is a diagram illustrating an output of a density sensor of a color toner.

FIG. 11 is a diagram illustrating the output of a density sensor for black toner.

FIG. 12 is a diagram illustrating a density sensor output of a conventional color image forming apparatus.

FIG. 13 is a diagram illustrating a density sensor output when the background output changes.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Primary charger 3 Exposure device 4 Developing device 5 Intermediate transfer body 6 Fixing device 7 Cleaning device 8 Secondary transfer device 9 Density sensor 50 Body CPU

Claims (6)

[Claims] A density detecting means for detecting the density of a toner image for density measurement formed on an image carrier; and an offset voltage adding means for adding an offset voltage to an output signal detected by the density detecting means. Means for controlling image forming conditions in accordance with a density calculated by an output signal from the offset voltage adding means, wherein the offset is determined depending on whether the toner is a color toner or a black toner. A color image forming apparatus, wherein the added voltage value by the voltage adding means is set differently. 2. An output voltage range for color toner and an output value range for black toner are made substantially the same by adding offset voltages by offset voltage adding means. Item 2. A color image forming apparatus according to Item 1. 3. A density detecting means for detecting the density of a toner image for density measurement formed on an image carrier, and an offset voltage adding means for adding an offset voltage to an output signal detected by the density detecting means. Means for controlling image forming conditions in accordance with a density calculated by an output signal from the offset voltage adding means, wherein the offset voltage adding means is controlled in accordance with an optical reflectance of the image carrier. A color image forming apparatus for setting an added voltage value by means. 4. The color image forming apparatus according to claim 3, wherein an optical reflectance of the image carrier is detected by a density detecting means. 5. The output value range is set within a detectable range by adding an offset voltage by an offset voltage adding means.
3. The color image forming apparatus according to 1. 6. A density detecting means comprising: a light emitting means for irradiating a light beam to a toner image for density measurement; and a light receiving means for receiving light reflected by irradiation of the light emitting means. Item 6. A color image forming apparatus according to any one of Items 1 to 5.