JPH05150655A - Image forming device - Google Patents

Abstract

PURPOSE:To accurately control toner concentration for each copy without soiling a transfer material carrying means even during the consecutive copying operation. CONSTITUTION:A first LED 12K and a photodiode 13K both of which are used for detecting the density of a visible image (toner image) 22K formed on a photosensitive drum 1K by an ordinary image-forming process and remaining on the photosensitive drum 1K without being transferred to a transfer material on a transfer belt 10 are arranged on the downstream side of a transfer area. Also, a second LED 14K and photodiode 15K both of which are used for detecting the density of the toner image in the same position and area transferred to the transfer material on the transfer belt 10 are disposed. The detection outputs from the first and second photodiodes 13K and 15K are sent to a CPU 20. The toner concentrations are detected and integrated to be compared with a reference value corresponding to reference concentration stored in the CPU 20. Toner replenishment is started after the length of time that toner replenishment is carried out is determined according to the difference.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming for a copying machine, a printer, etc., such as an electrophotographic system or an electrostatic recording system in which a developer is attached to a latent image formed on an image carrier to make it a visible image. More specifically, the present invention relates to an image forming apparatus including a developer concentration control device that appropriately controls the toner concentration of a two-component developer.

[0002]

2. Description of the Related Art Conventionally, as a recording system for forming a full-color image, an electrophotographic system, an inkjet recording system, a thermal transfer recording system and the like are known. Among them, the electrophotographic method is capable of high speed and can be used for plain paper, and in recent years, product development has been increasingly advanced. Various types of electrophotographic color image forming apparatuses have been proposed. One of them is provided with an image forming station for each developer of each color, and an image carrier is provided in each image forming station. An electrophotographic color image in which a visible image for each color is formed on a photosensitive drum by a well-known image forming process, these visible images are sequentially transferred to a transfer material supplied from the outside, and are collectively fixed to obtain a color image. There is a forming device. In this case, the transfer material is placed on the transfer material transfer means that moves endlessly and is sequentially transferred to each image forming station, and the visible image formed on each photoconductor drum is sequentially transferred to this transfer material. ..

An example of a conventional electrophotographic color laser beam printer having such a configuration is shown in FIG. This color laser beam printer has four image forming stations P _M , namely, first, second, third and fourth image forming stations P _M ,
P _C , P _Y and P _K , and also on one side thereof, ie FIG.
A sheet feeding section (only the sheet feeding cassette 7 is shown in the figure for simplification of description) is provided on the right side of the figure, and a fixing device 8 and a sheet ejection tray 9 are provided on the opposite side, that is, on the left side of FIG. There is. Below the path from the paper feeding section to the fixing device 8 in the printer body, there is an endless transfer belt transporting means for transporting the transfer sheet, for example, an endless transfer belt 10. It is stretched between multiple rollers. The transfer belt 10 is driven in the direction shown by the arrow in the figure, and carries the transfer material fed through the paper feeding section, and the image forming stations P _M , P _C , P _Y and P _{K described above.}
To be sequentially transported to.

Image forming stations P _M , P _C , P _Y
And P _K have substantially the same structure and include photoconductor drums 1M, 1C, 1Y and 1K which are image carriers that are normally driven to rotate in the direction of the arrow in the drawing, and the periphery of each photoconductor drum. Are charging devices (corona charging devices) 2M, 2C, 2Y and 2K for uniformly charging the photosensitive drum, and developing devices 4M, 4C, 4Y for developing the electrostatic latent image formed on the photosensitive drum.
And 4K, transfer chargers 6M, 6C, 6Y and 6K for transferring the developed visible image to a transfer material, and drum cleaners 5M, 5C, 5 for removing the toner remaining on the photosensitive drum.
Y and 5K are sequentially arranged in the drum rotation direction. Image exposure devices 3M, 3C, 3Y and 3K are provided above the respective photosensitive drums 1M, 1C, 1Y and 1K.

The developing device 4M contains magenta toner, the developing device 4C contains cyan toner, the developing device 4Y contains yellow toner, and the developing device 4K contains black toner. There is. The image exposure devices 3M, 3
In this example, C, 3Y, and 3K are composed of a semiconductor laser that is a light source, a rotating polygon mirror that scans a laser beam, an fθ lens that focuses a scanning beam, and the like. The laser beam modulated corresponding to the above-mentioned chargers 2M, 2C, 2Y and 2K.
And the developing units 4M, 4C, 4Y and 4K are scanned in the drum generatrix direction to expose the drum surface to form an electrostatic latent image of a corresponding color. The image exposure device 3M receives a pixel signal corresponding to a magenta component image of a color image, the image exposure device 3C receives a pixel signal corresponding to a cyan component image, and the image exposure device 3Y receives a pixel signal corresponding to a yellow component image. , And the pixel signals corresponding to the black component image are input to the image exposure device 3K. Further, between the first image forming station P _M and the paper feed unit, a pair of adsorption chargers 51 and 52, which constitute a transfer material adsorption unit, are installed opposite to each other with the transfer belt 10 interposed therebetween. Corona discharge is performed in order to surely attract the transfer material supplied from the transfer belt 10 to the transfer belt 10. On the other hand, the fourth image forming station P _K
A static elimination charger 53 is provided between the fixing unit 8 and the fixing unit 8. An AC voltage is applied to the static elimination charger 53 to separate the transfer material adsorbed on the transfer belt 10.

In the color printer having the above construction, when the transfer material is fed from the paper feed cassette 7 onto the transfer belt 10, the transfer charger is surely adsorbed onto the transfer belt 10 by the action of the adsorption chargers 51 and 52. It Along with the movement of the transfer belt 10 in the direction of the arrow in the figure, a magenta image is formed on the photosensitive drum 1M of the first image forming station P _M , and
Cyan image on the photoconductor drum 1C of the image forming station P _C , the yellow image on the photoconductor drum 1Y of the third image forming station P _Y , and the photoconductor drum 1K of the fourth image forming station P _K. , The black image is shared among them. These images are conveyed toward the fixing device 8 by sequentially moving the transfer material under the photosensitive drums 1M to 1K of the first to fourth image forming stations P _{M to} P _K by the movement of the transfer belt 10. In the meantime, the transfer chargers 6M, 6C, 6Y, and 6K of the respective image forming stations sequentially transfer the transfer images onto the transfer material so that a color image is synthesized. After passing through the fourth image forming station P _K , the transfer material is discharged by the discharging charger 53 to which an AC voltage is applied and separated from the transfer belt 10. The transfer material separated from the transfer belt 10 is sent to the fixing device 8, and after the transferred image is fixed in the fixing device 8, it is discharged from the transfer material discharge port to the discharge tray 9, thus making one copy. The cycle ends.

By the way, a color image forming apparatus for forming full-color or multi-color images such as a color laser beam printer or a copying machine, particularly, a developing apparatus of an electrophotographic color image forming apparatus is used from the viewpoint of color tone of an image. Two-component developers are often used. As is well known, the toner concentration (that is, the ratio of the weight of toner particles to the total weight of carrier particles and toner particles) of this two-component developer is a very important factor in stabilizing the image quality. The toner particles of the developer are consumed during development, and the toner concentration changes. Therefore, the developer concentration control device (AT
It is necessary to accurately detect the toner concentration of the developer by using R), to replenish the toner according to the change, to constantly control the toner concentration, and to maintain the image quality.

FIG. 5 shows an example of a developing device equipped with a conventional developer concentration control device used in each image forming station of the above-mentioned electrophotographic color laser beam printer. As a representative example, the developing device 4K of the fourth image forming station for forming a black image is shown, but the developing devices of the same structure are also used for the first to third image forming stations.

The developing device 4K is arranged so as to face the photosensitive drum 1K, and contains a two-component developer (including a magnetic carrier and black non-magnetic toner) 40K therein, and the two-component developer 40K. Is a magnet (not shown)
Is carried and conveyed as a thin layer of a two-component developer whose layer thickness is regulated by a blade (not shown) by a non-magnetic developing sleeve 41K containing therein, and is supplied to the photosensitive drum 1K in a developing area facing the photosensitive drum 1K, Develop the electrostatic latent image. A power supply (not shown) is provided to the developing sleeve 41K in order to improve the developing efficiency, that is, the rate of applying toner to the latent image.
Is applied with a developing bias voltage in which a DC voltage and an AC voltage are superposed. Further, when the toner in the developing device 4K is consumed by the development and the toner concentration is lowered, the conveying screw 43K of the toner replenishing tank 42K is driven by a driving source (motor).
44K is driven to rotate for a predetermined time, and a proper amount of black toner 45 is discharged from the toner discharge port of the toner supply tank 42K.
K is configured to be supplied.

On the other hand, the developer concentration control device is a so-called reflected light amount detection system, and when operated at a predetermined timing, a predetermined patch-shaped electrostatic latent image formed on the photosensitive drum 1K. Under constant conditions
A reference image 11K of a patch-like standard density formed by developing with K is irradiated with light from a bidirectional light emitting type LED (light emitting diode) 12K, and the reflected light is received by a photodiode 13K, and the reflected light amount Is converted into an electric signal to detect the toner concentration of the developer at the present time, this is compared with a signal of a reference level corresponding to the reference concentration, and the drive source 4 of the toner replenishing tank 42K is timed according to the difference.
4K is rotationally driven and the conveying screw 43K is rotated to replenish the developing device 4K with a toner amount corresponding to the above difference from the toner replenishing tank 42K, and the toner density is appropriately controlled.

[0011]

However, in the case of using the above-mentioned conventional developer concentration control device of the reflected light amount detection type, an endless circular transfer material transfer means such as the transfer belt 10 is used. Since the reference image of the standard density is formed on the photoconductor drum, the electric charge remains on the transfer material conveying means even when the transfer current is cut off. Was transferred to the transfer material conveying means, and the transfer material conveying means was contaminated. This drawback also occurs when the developer concentration control device of the reflected light amount detection type is used only in one image forming station due to the structure of the image forming device or the property of the material of the developer.

Therefore, it is necessary to clean dirt on the transfer material conveying means, and the developer concentration control device cannot be operated during continuous copying. Therefore, for example, even in the case of an apparatus having a maximum of 99 continuous sheets, when copying a plurality of originals and a plurality of copies, a continuous copy of several hundred sheets to nearly 1,000 sheets is made, and in the meantime, the developer is completely removed. Since it is not possible to control the density, if the number of copies exceeds a certain number, the image density gradually decreases, resulting in a copy with deteriorated image quality.

Further, since the developer concentration detecting sensor is arranged below the developing device, the light receiving surface of the sensor is soiled due to the scattering of toner from the developing device, which makes it impossible to accurately control the developer concentration. There was a drawback.

Therefore, it is an object of the present invention to control the developer concentration for each copy without contaminating the transfer material conveying means even during the continuous copying operation, and prevent the light receiving surface of the concentration detection sensor from being soiled. An object of the present invention is to provide an image forming apparatus in which a density detection sensor is arranged at various positions so that the developer density can be accurately controlled.

[0015]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention relates to an image carrier, a developing device having a two-component developer that develops a latent image formed on the image carrier into a visible image, and an image carrier developed by the developing device. An image forming apparatus having a transfer material conveying means for carrying and conveying a transfer material to which the visible image is transferred, the image after the visible image formed on the image carrier is transferred to the transfer material. First density detecting means for detecting the toner density of the visible image remaining on the carrier, second density detecting means for detecting the toner density of the visible image transferred to the transfer material, and these first and first Two
An image forming apparatus comprising: a control unit that calculates the detection output from the density detecting unit and supplies the toner to the developing device based on the calculation result.

In one embodiment of the present invention, the first and second density detecting means, such as a photodiode, having the same position and area in the visible image remaining on the image carrier and transferred to the transfer material. Detected by each of them, and the detected outputs are integrated into a density output value, and the pixel integrated value at the same position and area in the actual image read by the document image reading means is used as a reference density value and compared with the detected density output value. Then, the toner is replenished based on the comparison result.

In another embodiment of the present invention, the same position and area in the visible image remaining on the image carrier and transferred to the transfer material have the same first and second positions as the CCD.
Detected by the respective density detecting means, and the detected output is subjected to statistical processing and integrated to obtain a density output value,
The pixel integrated value at the same position and area in the actual image read by the document image reading means is compared with the detected density output value as a reference density value, and toner is replenished based on the comparison result.

[0018]

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. To simplify the description, an embodiment in which the present invention is applied to the electrophotographic color laser beam printer shown in FIG. 4 will be described.
For example, an electrophotographic system on an image carrier such as a photoconductor or a dielectric,
A latent image formed by an electrostatic recording method or the like is developed by a developing device to form a visible image, and the visible image is transferred to a transfer material such as paper carried and transferred by a transfer material transfer means, It goes without saying that the present invention is equally applicable to image forming apparatuses of various configurations that are fixed by a fixing unit to form a permanent image. The overall structure of the image forming apparatus of this embodiment is the same as that shown in FIG.

FIG. 1 is a schematic block diagram showing a first embodiment of the present invention. As in FIG. 5, a developing device 4K of a fourth image forming station P _K for forming a black image and its developer. 2 shows a concentration control device. Therefore, elements and components corresponding to those shown in FIG. 5 are designated by the same reference numerals, and the description thereof will be omitted unless necessary. In the present embodiment, the first to third image forming stations irradiate the developer directly with the developing devices 4M, 4C and 4K and receive the reflected light to detect the toner density. A density control device or a developer concentration control device that detects the toner density by detecting the apparent inductance of the developer is used. This is due to the developer characteristics and is used for black toner. This is because carbon absorbs near-infrared light, and even if the developer is directly irradiated with light from the LED, the light is hardly reflected.

In this embodiment, the reference image having the standard density for detecting the toner density is not formed on the photosensitive drum 1K, but instead, the visible image formed on the photosensitive drum 1K by the normal image forming process is formed. The image (toner image) 22K is used to detect the toner density of the developer. Further, in this embodiment, the first LED 12K of the bidirectional light emitting type for irradiating the light for detecting the toner density and the first photodiode 13K for receiving the reflected light are provided on the downstream side of the transfer area (the transfer area is different from the developing device 4K). The light quantity reflected from the toner image remaining on the photosensitive drum 1K after being transferred to the transfer material on the transfer belt 10 is received by the photodiode 13K and converted into an electric signal. Then, the toner density of the toner image on the photosensitive drum 1K after the transfer is detected. Further, in this embodiment, a bidirectional light emitting type second LED 14K for irradiating the toner image transferred to the transfer material on the transfer belt 10 with light for detecting the toner density is provided, and the toner image transferred to the transfer material is provided. Of the photoconductor drum 1
The amount of light reflected from the same position and area as the residual toner image on K is received by the second photodiode 15K, converted into an electric signal, and the toner density of the transferred toner image is detected.

These first and second photodiodes 13
The analog electric signals from K and 15K are converted into digital signals by analog-digital converters 16 and 17, respectively, amplified by amplifiers 18 and 19 respectively, and then sent to a CPU (central processing unit) 20. CPU
Reference numeral 20 denotes the toner concentration of the toner image remaining on the photoconductor drum 1K after transfer detected by the first photodiode 13K and the toner concentration of the toner image transferred to the transfer material detected by the second photodiode 15K. Detected by the calculation of the two input electric signals, integrated and already CP
U20 is compared with the reference value corresponding to the reference density, the toner replenishment time is determined according to the difference, and the drive source 44K of the toner replenishment tank 42K is determined by this toner replenishment time.
It has a function of generating a signal for driving.

Next, the operation mode of this embodiment having the above-mentioned configuration will be described with reference to the flowchart of FIG.

First, when the start button is pressed to copy a document, it is determined in step S101 whether the image formed is a monochrome black color. If the color is black (YES), step S102.
In the image forming operation is started, first, a voltage of -4 to -9 KV is applied to the corona charger 2K, and the charging current -3 is applied.
The photosensitive drum 1K is uniformly charged at 00 to -900 μA. Thereafter, the photosensitive drum 1 is driven by the image exposure device 3K based on a black image signal from an image input system (not shown).
An electrostatic latent image is formed on K. This electrostatic latent image is developed into a black visible image (toner image) by the developing device 4K to which a predetermined bias is applied. At this time, the reversal developing method is used as the developing method, and the toner of minus (-) polarity is used. When the image forming operation is completed, the toner image on the photosensitive drum 1K is transferred to the transfer belt 1 in step S103.
It is transferred onto a transfer material that is adsorbed and carried on the surface of 0 and conveyed to the transfer area. Next, in step S104, the toner density of the toner image remaining on the photosensitive drum 1K without being transferred is detected. For example, if the transfer efficiency is 80%, the toner density of the remaining 20% of the toner image on the photosensitive drum 1K is detected. Next, in step S105, the toner density of the toner image transferred on the transfer material is detected. In this case, the position and area of the toner image on the transfer material irradiated with the light from the LED 14K should be the same as the position and area of the toner image remaining on the photosensitive drum 1K from the LED 12K. Set.

Next, in step S106, the CPU 20 uses the electric signal corresponding to the toner density detected in steps S104 and S105 to determine the toner density of the toner image remaining on the photosensitive drum 1K after transfer and the transfer material. The toner density of the transferred toner image is calculated and integrated to determine the toner density. This determined toner density value is compared with the reference density value already stored in the CPU 20 in step S107, and the toner replenishment time is determined according to the difference, and in the present embodiment, the drive screw 43K of the toner replenishing tank 42K is driven to rotate. The drive time of the source (motor) 44K, and therefore the rotation speed of the conveying screw 43K, is determined. In this embodiment, the reference density value is the photosensitive drum 1
The toner image remaining on K is determined from the integrated value (video count number) of the density signal for each pixel of the image portion of the original having the same position and area as the light emitted from the first LED 12K. That is, as shown in FIG. 1, the position of the toner image on the photosensitive drum 1K irradiated with light from the first LED 12K,
An image portion of the original 21 having the same area is read by the CCD 23, the obtained analog image signal is amplified by an amplifier 24 to a predetermined level, and an analog-digital converter (A /
D) 25 converts the digital image signal into 8-bit (0 to 255 gradations) digital image signal. The output level of the image signal converted into this digital signal is integrated for each pixel,
This is converted into a video count number by the video counter 26 and sent to the CPU 20. In this case, the entire document 21 is CC
The image signal of the image portion of the original 21 having the same position and area of the toner image on the photosensitive drum 1K that is read by D23 and is irradiated with light from the first LED 12K may be extracted and converted into a video count number. The CPU 20 has a conversion table of the video count number and the toner density, converts the input video count number to the toner density by referring to the conversion table, and stores this toner density as the reference density value. As described above, since the reference image of the standard density is not formed in the present embodiment, the density of the original image portion corresponding to the portion (position, area) of the developed toner image where the toner density is detected is detected to detect the toner density. The reference density is set for each part. Of course, the reference toner density may be determined by another method.

Next, in step S108, the drive source 44K of the toner replenishing tank 42K is driven for the toner replenishing time determined in step S107, and the conveying screw 43K is rotated by the number of revolutions determined as described above. An appropriate amount of toner corresponding to the difference between the two density values is replenished into the developing device 4K.

On the other hand, when it is judged in the judgment step S101 that the image to be formed is a full-color image or a multi-color image, that is, it is not a black monochromatic image (NO), the video count number of the original image as described above. In step S109, the image of the original 21 is read by the CCD 23, the obtained analog image signal is amplified to a predetermined level by the amplifier 24, and the analog-digital converter 25 outputs, for example, 8 bits (0 to 0).
It is converted into a digital image signal of 255 gradations. Next, in step S110, the output level of the image signal converted into the digital signal is integrated pixel by pixel, and the video counter 26 converts the output level into a video count number to determine the video count number. This video count is CP
It is sent to U20 and the CPU 20 converts the input video count number into the toner replenishment time (or replenishment amount) (step S111). Next, in step S112, the driving source of the magenta, cyan, or yellow toner replenishing tank corresponding to the toner replenishment time determined in step S107 is driven, and the conveying screw is rotated by the number of revolutions determined as described above. Then, an appropriate amount of toner corresponding to the video count number is replenished into the developing device.

As described above, in this embodiment, the toner density of the toner image remaining on the photosensitive drum 1K and the toner density of the toner image transferred to the transfer material on the transfer belt 10 are detected at the time of forming a black monochromatic image. However, since these detection results are integrated and the toner density is controlled by comparing with a preset reference value, it is possible to control the toner density for each sheet without soiling the transfer belt even during continuous copying operation. Therefore, a high-quality image with stable image density can be obtained. Further, since the toner concentration detection sensor is on the opposite side of the developing device across the transfer area and is isolated from the developing device by the photoconductor drum, the light receiving surface of the detection sensor is hardly contaminated, and the highly accurate density is obtained. There is an advantage that detection can be performed.

The black fourth image forming station P _K is arranged at the position of the magenta first image forming station P _M in FIG. 4, and the first to third image forming stations P _M , P _C , A configuration in which P _Y is shifted backward one by one (the third yellow image forming station P _Y
Is placed at the position of the black image station), even when forming full-color or multi-color images,
When black toner is used, the black toner concentration can be controlled for each sheet without soiling the transfer belt during the continuous copying operation. In short, the present invention can be effectively applied to at least an image forming station for forming a toner image first transferred to a transfer material, not only when forming a single color image but also when forming a full-color or multi-color image, The same effect can be obtained. (It is needless to say that if there is an image portion that does not mix colors, it can be applied to the image forming station.) In the above embodiment, the first and second photodiodes 13K and 15 are used.
Although the position and area of the toner image on the photosensitive drum and the transfer material, which detect the toner density by K, are the same position and area, they are not specific positions but arbitrary positions. It is preferable that the density level to be performed is higher because errors are less likely to enter.Therefore, in the image that passes through the light-receiving surface of the photodiode, the highest density area of the area equal to the light-receiving area is the image area for toner density detection. If so, the detection accuracy is increased, and the toner concentration control can be performed with higher accuracy. To this end, the position and area showing the largest value among the integrated values of the output level for each pixel of the original image integrated by the video counter 26 are determined as the image area for toner concentration detection, and from this image area The toner concentration may be detected by the detection signal of

In the above embodiment, the photodiode 13 is used.
Based on the toner density values detected by K and 15K, the CPU 20 determines only the toner replenishment time by comparison with the reference density value, and drives the drive source 44K of the toner replenishment tank 42K for this toner replenishment time, 43K
The toner is supplied to the inside of the developing device 4K by rotating the. However, not only the toner density is controlled based on the detected toner density value, but also the latent image forming means, the developing device from the CPU 20 If a control signal is sent to an image forming means such as a transfer device to stabilize the image quality, it is possible to form a stable image of higher image quality. For example, the control signal from the CPU 20 controls the potential of the corona charger 2K or the development contrast potential, which is the difference between the development bias potential and the bright potential, to keep the maximum density D _MAX constant. Alternatively, the LUT (look-up table) which is the gradation correction means is changed to perform control for stabilizing the image quality such as keeping the gradation linearity constant.

Further, although the photodiode is used as the concentration detecting sensor in the above embodiment, the same operational effect can be obtained by using the CCD. In this case, for example, the CCD pitch is 1200 dpi (dot / inch) and the CCD window is 1
When a CCD having a size of 3 μ × 18 μ and an output pixel density of 200 dpi is used, as shown in FIG. 3, variations occur even if the line density levels of the output pixels are the same density level. It is preferable that the output value is obtained by statistically processing by using, and the error due to the variation is eliminated.

Further, in the above embodiment, the toner density of an arbitrary portion (position, area) of the developed toner image or a high toner density portion (position, area) of the specific portion is detected. If the original image has a uniform solid portion or a uniform halftone dot portion, the solid area or halftone dot portion is subjected to image area separation using the original reading means, and the density of this portion is determined. While detecting and setting the reference density, the solid portion or halftone dot portion is determined as the density reading position, and the corresponding portion (position, area) of the developed toner image and the corresponding portion of the transferred toner image are determined. You may make it detect the density of (position, area). If such a uniform solid portion or a uniform halftone dot portion is used as the density reading position, an error during density detection is less likely to occur, so high-precision control can be performed even with an inexpensive low-sensitivity sensor. There is an advantage that. When there is no uniform solid part or uniform halftone dot part in the original image, the part with the highest density is set as the density reading position.

In the above embodiment, the case where the present invention is applied to the electrophotographic color laser beam printer has been described. However, the present invention is applied to an electrophotographic system on an image carrier such as a photoconductor or a dielectric. A latent image formed by an electrostatic recording method or the like is developed by a developing device to form a visible image, and the visible image is carried by a transfer material conveying unit.
The present invention can be equally applied to image forming apparatuses having various configurations, which are transferred to a transfer material such as a conveyed paper and fixed by a fixing unit to form a permanent image.

[0033]

As described above, according to the image forming apparatus of the present invention, the first density detecting sensor for detecting the density of the visible image formed on the image carrier in the normal image forming process is developed. A second density detection sensor, which is arranged on the downstream side of the transfer area on the opposite side of the apparatus and which detects the density of the visible image transferred to the transfer material on the transfer material conveying means, is arranged on the actual image. The density at the same position and area is detected by these first and second density detecting sensors, and the toner is replenished based on the calculation result, so that the transfer material conveying means is not polluted even during the continuous copying operation. The toner density can be controlled for each sheet, and a high-quality image with stable image density can be obtained. Further, since the density detection sensor is located on the downstream side of the transfer area and is separated from the developing device by the image carrier, the sensing surface of the density detection sensor is hardly soiled, and high-precision density detection can be performed. It has a remarkable effect.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a main part of a first embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a flowchart illustrating an operation mode of the first exemplary embodiment of the present invention.

FIG. 3 is a diagram showing output characteristics of a CCD.

FIG. 4 is a schematic configuration diagram showing an example of an electrophotographic color laser beam printer to which the present invention can be applied.

5 is a schematic configuration diagram showing an example of a developing device having a conventional developer concentration control device used in each image forming station of the electrophotographic color laser beam printer shown in FIG.

[Explanation of symbols]

 1M to 1K Photoconductor drum 4M to 4K Developing device 6M to 6K Transfer charger 10 Transfer belt 12K, 14K LED 13K, 15K Photodiode 20 CPU 22 Visible image 26 Video counter 42K Toner supply tank 44K Drive source

Claims (8)

[Claims] 1. A developing device having an image carrier, a two-component developer for developing a latent image formed on the image carrier into a visible image, and a visible image on the image carrier developed by the developing device. An image forming apparatus having a transfer material transporting means for carrying and transporting a transfer material on which the visible image formed on the image carrier is transferred onto the transfer material. To detect the toner density of visible image remaining in the first
Density detecting means, second density detecting means for detecting the toner density of the visible image transferred to the transfer material, and detection outputs from the first and second density detecting means, and the calculation result And a control unit for replenishing the developing device with toner based on the above. 2. The image forming apparatus according to claim 1, wherein the positions and areas of the visible image detected by the first and second density detecting means are the same positions and areas in the actual image. 3. The image forming apparatus according to claim 2, wherein the same position and area in the actual image are determined based on the pixel integrated value read by the document image reading means. 4. The image forming apparatus according to claim 2, wherein the same position and area in the actual image are the highest pixel integrated values read by the original image reading means. 5. The pixel integrated value read by the original image reading means is used as a reference density value and compared with the calculation result of the detection output from the first and second density detecting means. Image forming apparatus. 6. The image forming apparatus according to claim 1, wherein the detection outputs from the first and second density detecting means are integrated to obtain a detected density output value. 7. The image forming apparatus according to claim 1, wherein the detected output from the first and second density detecting means is statistically processed and calculated to obtain a detected density output value. 8. The image forming apparatus according to claim 1, wherein the original image reading means is used to separate an image area of a specific portion of the original image, and the density of the separated image area is detected.