JP2003337458A - Image density detecting device and image density controller using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To always stabilize color tone reproduction by detecting image density in real time and simultaneously performing image density control in a tandem system color image forming apparatus composed of a plurality of process units. <P>SOLUTION: This tandem system color image forming apparatus has a carrying belt for carrying transfer material and a development color with a small density control frequency is arranged as the first color. The relation between the position of a density sensor and a part between sheets of paper is determined: (patch area length)+(sensor position to second color transfer position length)+(photoreceptor drum outer circumferential length from second color transfer position to second color development position)≤(each transfer position interval length). This configuration can perform image density gradation control in real time. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrostatic latent image formed on a latent image carrier and visualizes the toner image directly on a transfer material carrier. The present invention relates to an image density detection device in a tandem type image forming apparatus, and particularly to reflect the density detection result by a density detection toner image (hereinafter referred to as a patch) transferred to a transfer material carrier to image formation control in real time. The present invention relates to a suitable image density detection device.

[0002]

2. Description of the Related Art Generally, it is desired that a color copying machine or a color printer of this type can always output a stable image. In particular, in recent years, the network environment for handling various information has expanded, and the number of printers via the network has increased. As a result, various types of image data are being output not only by the same printer but also by unspecified printers connected to the network. At this time, it is necessary to obtain the same color reproduction image regardless of the printer output. However, in reality, the same color reproducibility can be obtained even with the same image data due to differences in the photoconductor sensitivity due to different printer models, different toner color development, different resolution, different image forming processes, etc. I can't.
Even with the same model, the color reproducibility may be deviated due to changes in environmental temperature and humidity and deterioration of the image forming system over time. In particular, in a tandem type color printer having a plurality of photoconductors, the characteristics of each photoconductor change independently, so that it is sensitive to changes in environmental temperature and humidity, and it is not possible to change the image forming system over time. The state of deterioration etc. will be different.

In this respect, generally, for example, Japanese Patent Laid-Open No. 1-
As disclosed in Japanese Laid-Open Patent Publication No. 167769, a density pattern image is transferred onto a transfer belt so that stable color reproduction can be performed even if environmental changes or changes over time occur, and the density of this density pattern image is optically measured. Is used to control 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 used to optically detect a positioning pattern for correcting color misregistration between colors, which is a problem in this type of color printer. By using the detection system that is also used, the space is effectively used.

Further, in recent years, there has been an increasing demand from users for the image quality of color images, and there is a demand for further stable color tone reproducibility in images during continuous output. That is, the first image and the last image of the color images that are continuously output have the same color tone. Here, as the conventional image density adjustment timing, the image density is detected and the image forming process control is performed at a predetermined timing such as when the power is turned on or after several hundreds to thousands of images have been formed.

[0005]

However, at the timing of the above-mentioned conventional image density adjustment, the color tone reproducibility may differ before and after the density adjustment. This is because there may be a difference between the colors in the change of the toner density with time. For example, the black color is generally widely used as a coloring agent having conductive carbon black, and the toner has a relatively low charge amount, but it also has the property of not holding an excessive charge due to its electrical characteristics. Fluctuations due to temperature fluctuations and changes in the charge amount over time are small. Further, when a magnetic toner containing a magnetic material is used, the development is performed by adjusting the amount of Coulomb force and the amount of magnetic force, so that the stability is further stabilized. On the other hand, non-magnetic toners such as magenta, cyan, and yellow have large changes in the amount of charge with respect to the environmental fluctuation range of the amount of charge of the toner and changes over time, and the Coulomb force is dominant, so the amount of charge of the toner changes It appears noticeably in the image density. In particular, in a color image forming apparatus such as a tandem system, when color images can be continuously formed at high speed, each process means is in continuous operation at the time of image formation, so that a rapid change with time progresses in a short period of time. 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 along with this, the image pickup property and the transfer property also change little by little, and stable color tone reproducibility is obtained. It's hard to get. One example will be described in detail. FIG. 11 shows, as an example, a change in the image density of a solid color when the image density control is performed for every 100 prints using black and magenta. In the figure, the solid line represents the density transition of magenta, and the broken line represents the density transition of black. As shown in the figure, the image density change of black color is slight before and after the timing of image density control, whereas the image density of magenta color is largely changed before and after the image density control is executed. The same applies to cyan and yellow as magenta. Therefore, with respect to the color overlapping portion of a plurality of colors, the influence of the density change is increased, and the color tone is largely changed before and after the image density control.

Further, in general, when forming a color image using four colors of YMCK, an achromatic gray color improves the graininess and the continuity of gradation from a chromatic color to an achromatic color improves. Instead, it is formed using the four colors of YMCK. However, if the YMC balance is lost, the gray becomes gray with a reddish or bluish tint. Therefore, an object of the present invention is to make it possible to feed back the density adjustment control in real time to each image forming unit, particularly to a chromatic color image forming unit such as magenta, cyan, and yellow, which is always stable during image formation. It is possible to form a color image having color tone reproducibility.

[0007]

In order to achieve the above object, in the present invention, each has at least a latent image carrier and a developing device, and toner images of different colors are formed on the latent image carrier. A color image forming apparatus for forming a color image by sequentially forming toner images formed by a plurality of process means for forming a toner image on a transfer material carried on an endless carrier which is rotationally driven and conveyed. In the image density detecting device for transferring the image density detecting toner image of the device to the endless carrier and detecting the image density detecting toner image on the endless carrier by an optical sensor, A developing color process means having a small amount of development color is disposed at the most upstream position in the rotation direction of the endless carrier, and the optical sensor is the most downstream in the rotation direction of the endless carrier. The toner image detecting position for detecting the image density of the optical sensor and the transfer position of the process means of the second color from the most upstream position in the rotation direction of the endless carrier are located on the downstream side of the stationary process means. L1 is the length, and L2 is the outer circumferential length of the latent image carrier between the developing position and the transfer position of the process means for the second color process means from the most upstream position. For the second color process means from the most upstream position. When the length of the toner image forming area of the image density detecting toner image is L3 and the length of the endless carrier of the transfer material non-conveying portion in the rotation direction is TrB, TrB ≧ L1 + L2 + L3
And the image density detecting toner image is formed on the transfer material non-conveying portion.

According to another aspect, in the image density detecting device, a developing color process means having a low image density detecting frequency is disposed at the most downstream position in the rotation direction of the endless carrier, and the optical sensor is provided. Is located between the process means between the most downstream positions in the rotation direction of the endless carrier and the process means for the second color from the most downstream position, and the toner image detection position for detecting the image density of the optical sensor and the endless end. The length of the most upstream position in the rotational direction of the sheet-like carrier with the transfer position of the process means is L1, and the outer circumference of the latent image carrier with the developing position in the process means at the most upstream position and the transfer position of the process means. The length is L2, the length of the toner image forming area of the image density detecting toner image for the process means at the most upstream position is L3, and the length of the transfer material in the normal color image formation is not. The rotational direction of the length of the endless carrier of the feeding portion when the TrB, TrB ≧ L1 + L2 + L
3 is satisfied, and the toner image for image density detection is formed on the transfer material non-conveying portion.

According to another aspect, the nth color (n ≧
The length L3 (n) of the toner image forming region of the image density detecting toner image for the process means of 2) is the transfer position of the process means of the nth color and the transfer position of the process means of the (n-1) th color. The length of the endless carrier in the rotation direction is
When it is set to (n), it is achieved by the image density detecting device characterized in that L3 (n) ≦ Dis (n).

According to another aspect, each has at least a latent image carrier and a developing device, and is formed by a plurality of process means for forming a toner image on the latent image carrier by toners of different colors. A toner image for image density detection in a color image forming apparatus is formed by sequentially forming the toner images on a transfer material carried on an endless carrier which is rotationally driven and conveyed in order to form a color image. An image density detecting device for transferring the toner image for detecting the image density on the endless carrier by an optical sensor to detect the image density detected by the image density detecting device. In an image density control method for carrying out density control for a predetermined process means based on the density of a toner image for development, the process means for developing color which has a low image density detection frequency is endlessly supported. Is disposed at the most upstream position in the rotation direction of the endless carrier, the optical sensor is located downstream of the process means at the most downstream position in the rotation direction of the endless carrier, and the toner for image density detection of the optical sensor is provided. The length between the image detection position and the transfer position of the second color process means from the most upstream position in the rotation direction of the endless carrier is L1, and the developing position in the second color process means from the most upstream position and the process means The outer circular station length of the latent image carrier with respect to the transfer position is L2, the length of the toner image forming area of the image density detecting toner image for the second color process means from the most upstream position is L3, and in the normal color image formation When the length of the transfer material non-conveying portion in the rotation direction of the endless carrier is TrB, the relationship of TrB ≧ L1 + L2 + L3 is satisfied and the nth color ( The length L3 (n) of the toner image forming area of the image density detecting toner image for the process means of ≧ 2) is determined by the transfer position of the process means of the nth color and the transfer position of the process means of the (n−1) th color. When the length in the rotational direction of the endless carrier with respect to the position is Dis (n), L3 (n) ≦ Dis (n) is satisfied, and the toner image for image density detection has the transfer material non-conveying portion. And an image density detecting toner image formed by changing a developing bias applied to a developing device, and the image density detecting toner image detected by the image density detecting apparatus. The image density control method is characterized in that the developing bias to be applied to the developing device of the process means is determined based on the density of the image density and the density is controlled.

According to another aspect, in the image density control method, a developing color process means having a low image density detection frequency is disposed at the most downstream position in the rotation direction of the endless carrier, and the optical sensor is The endless carrier is located between the process unit at the most downstream position in the rotation direction of the endless carrier and the process unit for the second color from the most downstream position, and the toner image detection position for detecting the image density of the optical sensor and the endless carrier. The length of the transfer position of the process means at the most upstream position in the rotational direction of the carrier is L1, and the outer circular station length of the latent image carrier at the developing position in the process means at the most upstream position and the transfer position of the process means is L2, the length of the toner image forming area of the image density detecting toner image for the process means at the most upstream position is L3, and the transfer material in the normal color image forming is not The rotational direction of the length of the endless carrier of the feeding portion when the TrB, TrB ≧ L1 + L2 + L
3 and the length L3 (n) of the toner image forming area of the image density detecting toner image for the nth color (n ≧ 2) process means is equal to the transfer position of the nth color process means. When the length in the rotation direction of the endless carrier with respect to the transfer position of the (n−1) th color process means is Dis (n), L3 (n) ≦ Dis (n) is satisfied, and the image is The density detecting toner image is an image density detecting device formed on the transfer material non-conveying portion, and the image density detecting toner image is created by changing a developing bias applied to a developing device. The image density control method is characterized in that the developing bias applied to the developing device of the process means is determined and the density is controlled based on the density of the image density detecting toner image detected by the apparatus.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION (First Embodiment) An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic view of a color image forming apparatus to which the present invention is applied. As shown in FIG. 1, a paper feed path 4 for guiding a transfer paper 1 as a recording medium from a paper feed unit 2 to a paper discharge unit 3 is provided. This paper path 4 is
It partially includes a conveyor belt 7 stretched between a belt drive roller 5 which is driven by a drive source (not shown) to rotate and a rotatable belt driven roller 6. On the conveyor belt 7, four electrophotographic process units 8K, 8M, 8C for black, magenta, cyan, and yellow,
8Y is the first color, which is black whose image density control frequency is low, and is arranged in order. In this embodiment, magnetic toner is used as the black developing toner and non-magnetic toner is used as the magenta, cyan, and yellow colors, and a peta-color image is obtained when the image density control is performed every 100 prints. The change in concentration has the form shown in FIG. The electrophotographic process unit 8 mainly includes a photosensitive drum 9 as a photosensitive member that contacts the transport belt 7, and a charger 10, an exposure unit 11, a developing unit 12, a transfer unit 13, and a photosensitive drum 9 around the photosensitive drum 9. The photoconductor cleaners 14 are formed in order. Further, the sheet passing path 4 is provided with a fixing device 15 located at a position where the conveying belt 7 has passed through. In the case of such a structure, the uppermost transfer paper 1 is sent from the paper feed unit 2 to the paper path 4 and is conveyed by the conveyor belt 7. In the process, the electrophotographic process unit 8 for each color forms an image using an electrophotographic process of charging, exposing, developing, and transferring. As a result, a color toner image is transferred to the transfer paper 1, which is heated by the fixing device 15.
By being pressed, it firmly adheres to the transfer paper 1.

Next, the image density detecting method will be described.
As shown in FIG. 1, the conveyor belt 7 is provided downstream of the conveyor belt 7 of the electrophotographic process section 8Y for yellow, which is the final development color.
The density sensor 20 is arranged in close proximity to. As described above, in the electrophotographic color image forming apparatus, if the image density changes due to various conditions such as the use environment and the number of prints, the proper color tone of the color image cannot be obtained. Therefore, patch images are formed on a trial basis with toner of each color, their densities are detected by the density sensor 20, and from the results, a developing pack is feed-packed to perform image density control.
I try to get a stable image. Concentration sensor 20
As shown in FIG. 2, it comprises a light emitting element 21 such as an LED, a light receiving element 22 such as a photodiode, and a holder 23. The infrared light from the light emitting element 21 is conveyed by the conveyor belt 7.
The patch P is irradiated by irradiating the upper patch P, the reflected light from the patch P is received by the light receiving element 22, and the patch density is detected by measuring the reflected light amount. The reflected light from the patch P includes a specular reflection component and a diffuse reflection component. The specular reflection component depends on the condition of the surface of the conveyor belt 7 which is the base of the patch P and the variation in the distance between the sensor 20 and the patch P. Since the amount of light fluctuates greatly, if the reflected light from the patch P to be measured contains a regular reflection component, the detection accuracy will be significantly reduced. Therefore, in this density sensor 20, the irradiation angle to the patch P is set to 450 with reference to the normal I of the surface of the conveyor belt 7 so that the specularly reflected light from the patch P does not enter the light receiving element 22.
And the light receiving angle of the reflected light from the patch P is 0 °, and only the diffusely reflected light is measured.

A specific image density control method of this embodiment will be described with reference to FIG. The black process means, which has a low density control frequency, is arranged on the most upstream side in the rotation direction of the conveyor belt 7, and the image density control is performed once every 100 sheets during continuous operation. In the present embodiment, the black image density control frequency is once for every 100 continuous prints, but the control frequency may be changed depending on the stability of the density of the toner used. In FIG. 3, only the photoconductor 9, the developing device 12, and the transfer device 13 in the magenta, cyan, and yellow electrophotographic process sections are described for explaining the image density control method when the black image density control is not performed. The black electrophotographic process section and other components are omitted in the figure for convenience. In addition, each member of the photoconductor 9, the developing device 12, and the transfer device 13 will be described with a subscript corresponding to the color of the electrophotographic process section. M represents magenta color, C represents cyan color, and Y represents yellow color. ing.

FIG. 3 shows a transfer material TrM on which image formation has been completed.
1 and the transfer material TrM2 which has completed the image formation of the black color which is the first development color and has refrained from forming the image of the second magenta color.
The image density detection state in a non-image forming area (hereinafter, referred to as a sheet interval TrB) between the rM1 and the transfer material TrM2 is shown.

First, after the image formation of the transfer material TrM1 is completed,
A patch latent image that has been image-exposed with a predetermined amount of light by an exposure device (not shown) is used by the image controller 33 while changing the developing bias, and the magenta patch P, which is the electrophotographic process unit for the second color.
m1, Pm2, Pm3, and Pm4 are created by changing the density. In the present embodiment, the surface of the photoconductor 9 is uniformly charged to 1700 (V) as the dark portion potential VD by the charger 10, and then 0N / 0F is supplied from the exposure device 11 according to the patch formation image information.
Scan exposure is performed by an F-controlled laser beam, and a patch latent image of -100 (V) is formed as the bright portion potential VL. A development bias voltage that increases at a predetermined stage while corresponding to the patch latent image is output from the image control unit 33,
The patch latent image on the photoconductor 9 is visualized on the photoconductor 9 as a patch toner image having a different density. The developing bias voltage output from the image control unit 33 has a variable DC component voltage value, and the relationship between the DC component voltage value of the developing bias and the image density with respect to the bright portion potential VL-100V on the photoconductor 9 is shown in FIG. However, in the present embodiment, as the DC component voltage value of the developing bias for developing the patch latent image, V1,
Using V2, V3 and V4, each DC voltage value is -1
It is 50V, -175V, -200V, and -225V.

The patches formed as described above are transferred to the conveyor belt 7, and the density sensor 20 is instructed by the sampling controller 31 in communication with the apparatus controller 30.
The amount of reflected light from the patch is detected at, and the patches Pm1, Pm2, and Pm2 at the arithmetic processing unit 32 based on the detected amount of reflected light.
The respective concentrations of Pm3 and Pm4 are detected. Based on the density value calculated by the calculation processing unit 32, the control unit 30 calculates the developing bias value that provides the optimum density (the density is 1.4 in this embodiment), and outputs the calculation result to the image control unit 33 again. The image controller 33 outputs the optimum developing bias value calculated by the controller 30 to feed-pack the result of the patch density measurement to the developing device 12M. Then, based on the result of the feed pack, image formation for the transfer material TrM2 is executed. At this time, the areas of the patches Pm1, Pm2, Pm3, and Pm4 along the moving direction of the transport belt 7 are L3m, and the area from the density detection position of the density sensor 20 to the magenta transfer position of the second color electrophotographic process unit. L1m, when the outer peripheral length of the photoconductor 9 from the magenta developing position to the magenta transferring position is L2m,
In this embodiment, as shown in FIG.
Is equal to or larger than the area of L1m + L2m + L3m, the calculation processing result including the patch density result of the final patch Pm4 of magenta color immediately follows the image (here, Tr
It can be reflected in M2) in real time. In FIG. 3, L2'm has the same length as L2m and is used for the purpose of illustration.

Subsequently, as shown in FIG. 5, similarly for the cyan color, which is the electrophotographic process section of the third color, similarly, the magenta patches Pm1, Pm2, Pm3, Pm4 of the second color are formed.
After, the cyan patches Pc1, Pc2, Pc
3, Pc4 is formed, and the developer 12C is feed-packed. In FIG. 5, the electrophotographic process section up to the second color is omitted. Here, if the electrophotographic process units have the same shape and configuration, the outer peripheral length of the photoconductor 9 from the developing position to the transfer position is the same in each electrophotographic process unit, and thus L2m = L2c. For patches formed on the conveyor belt 7, L1m to L
By inserting the patch forming area L3c of the cyan patches Pc1 to Pc4 into the area obtained by subtracting lc, that is, the interval Dis (mc) between the transfer position of the second color and the transfer position of the third color, As with the magenta color, the calculation processing result including the patch density result of the final patch Pc4 can immediately reflect the cyan color density result in the next image TrM2 by the control for obtaining the optimum value of the developing bias. .

Hereinafter, by forming and processing the patch for the fourth yellow color at the same timing, it is possible to control the image density in real time. Also for the first color, black, when the density detection timing occurs, the paper interval TrB is set to L1k + L2k + L3k.
The image density control is performed based on the density detection method described above. However, L3k is a black patch area, L1k is an area from a density detection position of the density sensor 20 to a black transfer position, and L2k is a photosensitive member 9 from a black development position to a black transfer position. Indicates the perimeter.

FIG. 6 shows the execution result of the image density control by the image density detecting apparatus of the present embodiment described above. Figure 6
Is a transition of the image density of a magenta solid image with respect to the number of printed sheets. This is shown by a solid line, and the image density transition of a magenta peta image when the image density control is performed every 100 sheets described above as a comparative example. Is indicated by a broken line. As can be seen from FIG. 6, in the image density detecting apparatus of the present embodiment, the image density is controlled in a substantially stable state, and as a result, there is almost no change in the color tone of the overlapping image portions of a plurality of colors.

As described above, the patch detection result of each color is obtained by forming the patch between the papers and by making the space between the papers more than the area including the patch area and the length from the final patch position to the developing position. Can be immediately fed back to the developing device, real-time image control can be performed, and the optimum density condition can always be maintained, and stable color tone reproduction can be obtained. In addition, by placing the developing color with low frequency of image density control on the first color and controlling the image density, the optimum state of density is always maintained without the need for a longer paper interval during normal continuous image formation. It is possible to obtain stable color tone reproduction.

In this embodiment, the black process means for controlling the image density is arranged on the most upstream side with respect to the rotating direction of the conveyor belt 7. However, as shown in FIG. Even if the density sensor 20 is provided on the downstream side and the image density control is performed between the black color process means and the yellow color process means, it is possible to stabilize the continuous image formation without the need for an unnecessary paper interval. Color reproduction is obtained. In the density control operation at this time, during normal image formation, the image density control is performed in the paper interval TrB in the L1m + L2m + L3m region or more as in the above description, and when the black color density control timing occurs, the paper interval TrB is changed. The black patch area L3k is widened, and the black patch toner image is rotated once together with the conveyor belt 7, and then density detection and density control are performed.

Further, in the present embodiment, the magnetic toner is used as the achromatic black toner, but stable color tone reproduction can be obtained even if the above image density control is performed using the non-magnetic toner.

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

(Second Embodiment) FIG. 8 shows a schematic diagram of a color image forming apparatus to which the present invention is applied as a second embodiment. In the figure, the members described in the first embodiment are denoted by the same reference numerals, and their deviance is omitted. Figure 8
Is provided with a memory 34 that communicates with the control unit 30 and stores optimum value data of the magenta, cyan, and yellow developing biases that are controlled for each image formation. In the present embodiment, this memory 34 is provided. By making the optimum developing bias voltage after the execution of the image density control the center, the image density can be controlled more stably. still,
Also in the present embodiment, as in the first embodiment, the black color, which has a lower image density control frequency than other colors, is developed.
The patches are arranged between the colors to form a patch between the papers, and the space between the papers is set to be equal to or more than the area obtained by adding the patch area and the length from the final patch position to the developing position, which enables real-time density control. The image density control for the black color is performed once every 100 sheets during continuous image formation. In addition, as described in the first embodiment, the black color having a low image density control frequency is arranged at the most downstream position in the rotation direction of the conveyor belt 7, and the density sensor 20 is arranged between the yellow color and the black color. The concentration may be controlled by controlling.

For specific description, an image control flowchart for magenta, cyan, and yellow colors in this embodiment is shown in FIG. 9, and the image control process will be described with reference to FIG. First, when a print signal is input to the image forming apparatus (S1), the developing bias for patch formation described in the first embodiment is -150V, -175V, -200.
The image control unit 33 outputs the four-stage development bias voltages of V and -225V to form a patch (S2). The density sensor 20 detects the amount of reflected light from the patch, and the arithmetic processing unit 32 detects the density of the patch based on the detected amount of reflected light (S3). Based on the density value calculated by the calculation processing unit 32, the control unit 30 determines the optimum density (density 1.4 in this embodiment).
Then, the developing bias value m is calculated (S4).
The developing bias value m is a value peculiar to each color, and is originally the optimum value mm for magenta color, the optimum value mc for cyan color, and the optimum value my for yellow color, but the processing contents described later are the same. Therefore, it is used as a representative for convenience. Development bias value m determined by the control unit 30
Are sequentially stored for each color in the memory 34 in communication with the control unit 30 (S5), and the image control unit 33 outputs the developing bias voltage corresponding to the developing bias value m stored in the memory 34 via the control unit 30. The images are sequentially output to the individual developing devices and a normal image forming operation is performed (S6). When the first black image forming operation is completed, it is determined whether or not continuous printing is requested (S7). If there is no print request, the yellow image forming operation is completed and then the print terminating operation is performed. (S12). When there is a continuous print request, a patch is formed again between sheets and density control is executed. Here, a variable value of the developing bias at the time of patch formation is obtained with respect to the developing bias value m (m-15). V,
The controller 30 changes and sets the values so as to be (m-5) V, (m + 5) V, and (m + 15) V (S8). And changed developing bias voltage for patch formation: (m-15)
V, (m-5) V, (m + 5) V, and (m + 15) V are output from the image controller 33 to the developing device 12 to form a patch (S9), and the patch density is obtained from the patch reflection light amount (S10). ), A new developing bias value m'is obtained (S1
1). The obtained developing bias value m'is again stored in the memory 34.
By storing in the memory 34 (S5), the content of the developing bias value m in the memory 34 is changed to the content of the new developing bias value m'and applied to the next normal image formation (S5).
6). After that, the steps S5 to S11 are repeated until the continuous print request is exhausted, and the image density control and the image formation are performed.

According to the density control method described above, for the magenta, cyan, and yellow colors, the patch density is detected and the developing bias can be controlled for each image formation. Therefore, the fluctuation of the controlled developing bias value for each density control is small. So
In the density control that determines the developing bias for the second and subsequent sheets, by changing the developing bias that is changed during patch formation centering on the value calculated as the most current actual bias performed immediately before, The minute prediction can be reliably executed, and the control accuracy with respect to the target concentration can be improved. Further, by performing the processing according to the above-described density control method for the black color as well, it is possible to improve the control accuracy with respect to the target density for each density control, like the magenta, cyan, and yellow colors.

FIG. 10 shows the execution result of the image density control of this embodiment described above. FIG. 10 is a transition of the image density of the magenta and peta colors with respect to the number of printed sheets, which is shown by a solid line. As a comparative example, the image density transition of the magenta and solid colors when the image density control is performed every 100 sheets. Is indicated by a broken line. As can be seen from the figure, in the image density detecting apparatus of the present embodiment, the image density is controlled in a more stable state, and as a result, there is almost no change in the color tone of the overlapping image portions of a plurality of colors.

As described above, a memory for storing the optimum value data of the developing bias for each color controlled for each image formation is provided, and the value calculated as the optimum developing bias immediately before is stored in this memory. By storing and forming a patch in a variable range centered on this developing bias value, it is possible to perform highly accurate and stable image density control, and always maintain the optimum density condition. Therefore, stable color tone reproduction can be obtained.

In this embodiment, the variable range of the developing bias for patch formation is 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 changed as appropriate depending on the power supply performance.

[0032]

As described above, according to the image density detecting apparatus of the present invention, a patch is formed between sheets and
The space between the sheets is made larger than the patch area and the area from the last patch position to the development position is added, and the patch detection results for each color can be immediately feed-packed to the developing device, enabling real-time image control and always ensuring optimal density. Since the state can be maintained, stable color tone reproduction can be obtained. Also,
By arranging the developing colors having a low image density control frequency at the positions of the first color or the fourth color, it is possible to obtain stable color tone reproduction without taking a longer paper interval than necessary during normal continuous image formation. Further, by using the image density detecting device, a memory for storing the optimum value data of the developing bias of each color controlled for each image formation is provided, and the value calculated as the optimum developing bias immediately before is stored in this memory. By storing and forming a patch in a variable range centered on this developing bias value, it is possible to perform highly accurate and stable image density control, and always maintain the optimum density condition. Therefore, stable color tone reproduction can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a color image forming apparatus according to a first embodiment.

FIG. 2 is a schematic sectional view of a concentration sensor.

FIG. 3 is an explanatory diagram of a concentration detection method according to the first embodiment.

FIG. 4 is an explanatory diagram of a density control method according to the first embodiment.

FIG. 5 is an explanatory diagram of a concentration detection method according to the first embodiment.

FIG. 6 is an explanatory diagram of a transition of image density according to the first embodiment.

FIG. 7 is a schematic diagram of the color image forming apparatus according to the first embodiment.

FIG. 8 is a schematic diagram of a color image forming apparatus according to a second embodiment.

FIG. 9 is an explanatory diagram of image density transition according to the second embodiment.

FIG. 10 is an explanatory diagram of a transition of image density according to the second embodiment.

FIG. 11 is an explanatory diagram of transition of image density in a conventional example.

[Explanation of symbols]

1 Transfer material 7 Conveyor belt 8 Electrophotographic process department 9 photoconductor 12 Developer 13 Transfer device 20 concentration sensor 21 Light emitting element 22 Light receiving element

Continued front page    F-term (reference) 2H027 DA09 DE02 DE10 EA05 EB06                       EC03 EC07 EC20 EG04                 2H300 EA05 EB04 EB07 EB12 EF02                       EF06 EF08 EG02 EH15 EJ09                       EJ34 EJ39 EJ44 EJ45 EJ47                       EL05 FF02 FF05 GG01 GG02                       GG07 GG12 GG32 GG37 HH15                       HH16 HH30 QQ25 QQ26 QQ29                       RR21 RR31 RR37 RR39 RR40                       RR50 SS07 TT04 TT05                 5C074 AA20 BB26 DD01 DD24 EE04                       EE11 GG13

Claims (7)

[Claims] 1. A toner image formed by a plurality of process means, each of which has at least a latent image carrier, a developing device, and a transfer device, and which forms a toner image on the latent image carrier with toners of different colors. In a color image forming apparatus for forming a color image by sequentially forming a color image on a transfer material carried on an endless carrier rotatably driven, the toner image for image density detection of the apparatus is described above. In an image density detecting device for transferring to an endless carrier and detecting the image density detecting toner image on the endless carrier by an optical sensor, a developing color process means having a low image density detection frequency is used as the endless carrier. Arranged at the most upstream position in the direction of body rotation,
The optical sensor is located downstream of the process means at the most downstream position in the rotation direction of the endless carrier, and the toner image detection position for detecting the image density of the optical sensor and the rotation direction of the endless carrier. L1 is the length of the transfer position of the process means located in the second color from the most upstream position in the above, and a latent image carrier between the developing position in the process means located in the second color from the amount upstream position and the transfer position of the process means. The circumference length above is L2, the length of the toner image forming area of the image density detecting toner image for the process means of the second color from the most upstream position is L3, and the length of the transfer material non-conveying portion in normal color image formation is the above. When the length of the endless carrier in the rotation direction is TrB, TrB ≧ L1 + L2
An image density detecting apparatus, characterized in that a relationship of + L3 is satisfied, and the image density detecting toner image is formed on the transfer material non-conveying portion. 2. In the image density detection device, a developing color process means having a low image density detection frequency is disposed at the most downstream position in the rotation direction of the endless carrier, and the optical sensor is the endless carrier. Is located between the process means at the most downstream position and the process means for the second color from the most downstream position in the rotation direction of, and the toner image detection position for detecting the image density of the optical sensor and the rotation direction of the endless carrier. L1 is the length of the transfer position of the process means located at the most upstream position, and the circumferential length on the latent image carrier between the developing position of the process means located at the upstream position and the transfer position of the process means. L2, the length of the toner image forming region of the image density detecting toner image for the process means at the most upstream position is L3, and the transfer material is not conveyed in normal color image formation. TrB ≧ L1 + L2 + L3, where TrB is the length of the endless carrier in the rotation direction.
The image density detecting apparatus is characterized in that the following relationship is satisfied, and the image density detecting toner image is formed on the transfer material non-conveying portion. 3. The length L3 of the toner image forming area of the image density detecting toner image for the nth color (n ≧ 2) process means.
(N) is the transfer position of the process means for the nth color and (n)
-1) When the length in the rotational direction of the endless carrier with respect to the transfer position of the color process means is Dis (n), L3
The image density detecting device according to claim 1, wherein (n) ≦ Dis (n). 4. A toner image formed by a plurality of process means, each of which has at least a latent image carrier, a developing device, and a transfer device, and which forms a toner image on the latent image carrier with toners of different colors. In the color image forming apparatus for forming a color image by sequentially forming the toner images on a transfer material carried on an endless carrier which is rotationally driven and conveyed, the toner image for image density detection of the apparatus is A toner image for detecting an image density detected by the image density detecting device, which has an image density detecting device for detecting the image density detecting toner image on the endless carrier by an optical sensor. In the image density control method for carrying out density control for a predetermined process means based on the density of, the process means for developing color which has a low image density detection frequency is rotated by the endless carrier. Disposed at the most upstream position in,
The optical sensor is located downstream of the process means at the most downstream position in the rotation direction of the endless carrier, and the toner image detection position for detecting the image density of the optical sensor and the rotation direction of the endless carrier. The length from the most upstream position to the transfer position of the second color process means is L
1. L2 is the outer circumferential length of the latent image carrier between the developing position and the transfer position of the second color process means from the most upstream position, and the image density for the second color process means from the most upstream position. The length of the toner image forming region of the detection toner image is L3, and the length of the transfer material non-conveying portion in the rotation direction of the endless carrier in the normal color image formation is T.
When rB is satisfied, the relationship of TrB ≧ L1 + L2 + L3 is satisfied, and the length L3 (n) of the toner image forming area of the image density detecting toner image for the process means of the nth color (n ≧ 2) is When the length in the rotational direction of the endless carrier between the transfer position of the process means for the nth color and the transfer position of the process means for the (n-1) th color is Dis (n),
L3 (n) ≦ Dis (n) is satisfied, and the image density detecting toner image is an image density detecting device which is formed on the transfer material non-conveying portion, and the image density detecting toner image is applied to a developing device. The density of the developing bias applied to the developing device of the process means is determined based on the density of the image density detecting toner image detected by the image density detecting device, and the density is controlled. An image density control method characterized by: 5. In the image density control method, a developing color process means having a low image density detection frequency is arranged at the most downstream position in the rotation direction of the endless carrier, and the optical sensor is the endless carrier. Is located between the process means at the amount downstream position and the process means for the second color from the most downstream position in the rotation direction, and the toner image detection position for detecting the image density of the optical sensor and the rotation direction of the endless carrier. Is L1 and the outer circumferential length of the latent image carrier between the developing position in the process means at the most upstream position and the transfer position in the process means is L2. The length of the toner image forming area of the image density detecting toner image for the process means at the upstream position is L3, and the endless transfer of the transfer material non-conveying portion in normal color image formation is carried out. When the length of the holder in the rotation direction is TrB, the relationship of TrB ≧ L1 + L2 + L3 is satisfied, and at the same time, the toner image forming area of the toner image for image density detection for the process means of the nth color (n ≧ 2) Length L3
(N) is the transfer position of the process means for the nth color and (n)
-1) When the length in the rotational direction of the endless carrier with respect to the transfer position of the color process means is Dis (n), L3
(N) ≦ Dis (n) is satisfied, and the one image density detecting toner image is an image density detecting device which is formed on the transfer material non-conveying portion, and the image density detecting toner image is applied to a developing device. It is created by changing the development bias.
An image density control method, wherein the density is controlled by determining a developing bias applied to a developing device of the process means based on the density of the image density detecting toner image detected by the image density detecting device. 6. A storage means for temporarily storing the value of the developing bias applied to the developing device determined immediately before, and the developing bias stored in the storage means when the next toner image for image density detection is formed. 6. The image density control method according to claim 4, wherein the image is formed by using a developing bias set in a predetermined variable range corresponding to the value of. 7. The process means having a low image density detection frequency is a plaque color.
Image density detection device and image density control method.