JP3113561B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine or a printer of an electrophotographic type or an electrostatic recording type.
The present invention relates to an image forming apparatus including a density control device for controlling a toner density or an image density of a developer.

[0002]

2. Description of the Related Art In general, a developing device provided in an electrophotographic or electrostatic recording type image forming apparatus includes a one-component developer containing a magnetic toner as a main component or a non-magnetic toner and a magnetic carrier as main components. A two-component developer is used. In particular, in a color image forming apparatus that forms a full-color or multi-color image by an electrophotographic method, most developing apparatuses use a two-component developer from the viewpoint of the tint of an image and the like.

As is well known, the toner concentration (the ratio of the weight of the toner to the total weight of the carrier and the toner) of the two-component developer is a very important factor in stabilizing image quality. The developer toner is consumed during development, and the toner concentration of the developer decreases. For this reason, the developer density or the image density is detected at appropriate times using a developer density control device or an image density control device, and toner is replenished in accordance with the change, and the toner density or the image density is always kept constant. It is necessary to control and maintain the image quality.

FIG. 4 shows an overall configuration example of an image forming apparatus having a conventional density control device, in this embodiment, an electrophotographic digital copying machine.

[0005] First, an image of an original 21 is read by the CCD 1, and the obtained analog image signal is amplified to a predetermined level by an amplifier 2, and the analog-to-digital converter (A)
/ D converter) 3 converts the digital image signal into a digital image signal of, for example, 8 bits (0 to 255 gradations). Next, this digital image signal is converted to a γ-converter 5 (256 bytes of R in this example).
AM, which performs density conversion by a look-up table method, and is subjected to γ correction.
It is input to a digital-analog converter (D / A converter) 9.

[0006] The digital image signal is again converted into an analog image signal by the converter 9 and supplied to one input of a comparator 11. The other input of the comparator 11 is supplied with a triangular wave signal of a predetermined cycle generated from the triangular wave generating circuit 10. The analog image signal supplied to one input of the comparator 11 is compared with the triangular wave signal. Pulse width modulation. The pulse width modulated binarized image signal is directly input to the laser drive circuit 12, and the laser diode 13 emits light.
Used as an off control signal. Laser diode 1
The laser light radiated from 3 is a well-known polygon mirror 1
4, is scanned in the main scanning direction, and is radiated through the f / θ lens 15 and the reflection mirror 16 onto the photosensitive drum 40 as an image carrier rotating in the direction of the arrow to form an electrostatic latent image. become.

On the other hand, the photosensitive drum 40 is uniformly discharged by the exposure unit 18 and is uniformly charged, for example, negatively by the primary charger 19. Thereafter, an electrostatic latent image corresponding to the image signal is formed by receiving the above-described laser light irradiation. This electrostatic latent image is converted into a visible image (toner image) by the developing device 20.
Is developed. A toner 29 for replenishment is provided above the developing device 20.
Is supplied, and the toner supply layer 8 containing
A toner conveying screw 30 that conveys the toner 29 and supplies the toner 29 into the developing device 20 by being rotationally driven by a motor 28 is provided in a lower portion of the inside.

The toner image formed on the photosensitive drum 40 is transferred by the transfer charger 22 onto the transfer material P conveyed to the photosensitive drum 40 by the transfer material carrying belt 17. The transfer material carrying belt 17 has two rollers 25.
The transfer material P, which is stretched between a and 25b and is driven endlessly in the direction of the arrow in the figure, is conveyed to the photosensitive drum 40. The transfer residual toner remaining on the photosensitive drum 40 is thereafter scraped off by the cleaner 24.

For simplicity of explanation, only a single image forming station (including a photosensitive drum 40, an exposing unit 18, a primary charging unit 19, a developing unit 20, etc.) is shown, but a color image forming apparatus is shown. , For example, cyan,
The image forming stations for each of the colors magenta, yellow, and black are sequentially arranged on the transfer material carrying belt 17 along the moving direction.

The image forming apparatus controls the supply of toner to the developer 21 in the developing device 20 in which the toner density has been reduced by the above-described development, so as to control the toner density or the image density of the developer to be constant. Various types of concentration control devices (A
TR) is installed.

More specifically, a method of detecting and controlling the toner density of the developer 21 in the developing device 20 by the amount of reflected light by a toner density sensor 23 installed in the developing device 20 (developer reflection ATR), A system in which a patch image 26 is formed on a drum 40 for reference, and the image density is detected and controlled by a sensor 23 such as a potential sensor disposed opposite to the photosensitive drum 40 (patch detection ATR), a video counter 4
From the output level of the digital image signal for each pixel from the CPU and calculates the required toner amount (video count A
TR).

In each case, the rotation of the motor 28 is controlled by the CPU 6 via the motor drive circuit 7 based on the information obtained by the respective methods, whereby the developing container 2
The toner supply to the developer 21 is controlled so that the toner density of the developer or the image density is kept constant.

[0013]

However, the conventional concentration control device has the following problems.

In the developer reflection ATR, since the toner concentration of the developer 21 in the developing device 20 is directly detected, the developer
Can be kept constant. But,
The amount of triboelectric charge (tribo) of the toner changes depending on the quality of the magnetic carrier in the developer and the fluctuation of the environment, and the developing property changes. For this reason, even if the toner concentration of the developer 21 can be kept constant, there is a disadvantage that the image density changes when the quality of the carrier changes or the environment changes.

The video counter ATR converts the density information of the original image into a video count number, predicts the toner consumption from this value, initializes the toner density, and supplies toner to the consumption system in order to supply toner. When the consumption amount or the toner supply amount of the supply system fluctuates, there is a disadvantage that the toner density deviates from the initial set value by the fluctuation amount.

Further, in the patch detection ATR, since the density of the patch image 26 on the photosensitive drum 40 is detected, it is possible to keep the image density constant. When a low patch image density is obtained due to a decrease in the toner density, the ATR determines this and continues the toner supply state, so that the toner is filled in the developing device 20 and the developer overflows or fogs. In addition, a high patch image density is obtained due to an increase in developability due to a change in the triboelectric charge amount of the toner, and when this is determined by the ATR, the toner replenishment stop state is continued. Due to the decrease, the amount of the developer coating on the developing sleeve is reduced, and there is a serious disadvantage that image deterioration is caused.

Accordingly, an object of the present invention is to provide a method for forming an image by maintaining a constant toner concentration of a developer even when a change in toner developability due to environmental fluctuation or a fluctuation in toner consumption of a toner consuming system occurs. An object of the present invention is to provide an image forming apparatus capable of obtaining a high quality color image having a constant image density from the beginning.

[0018]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides a plurality of image carriers, a plurality of latent image forming means for digitally forming an electrostatic latent image corresponding to each color component of the image on the image carrier, A plurality of developing means for developing a toner image by developing the electrostatic latent image formed on the two-component developer with a corresponding color, a plurality of toner replenishing means for replenishing the developing means with toner, A density control device for controlling the toner density of the developer or the image density of the toner image through control of toner supply to the developing means, provided with respect to the developing means, at least as the density control device; A first controller for forming a reference image for density detection on an image carrier and detecting the reference image density to obtain a toner replenishment amount is provided for each developing unit, and a developer concentration of the developing unit is set. Detects toner A second control device for obtaining the supply amount is provided for the developing means using a chromatic developer, and a third control device for obtaining the toner supply amount from the digital image signal for each pixel of the reference image is provided with an achromatic color developer. The second and third control devices are provided with respect to the developing means using the developer, and feed back the toner supply amount from the reference image density of the first control device to the second and third control devices. An image forming apparatus characterized in that the toner supply control is corrected and performed.

According to the present invention, the pattern of the electrostatic latent image for the reference image can be changed for each color developer or between a chromatic developer and an achromatic developer. The change of the pattern can be the line frequency of the pattern or the screen angle of the pattern, and further can be the line frequency of the pattern and the screen angle.

[0020]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

Embodiment 1 An image forming apparatus to which the present invention can be applied includes, for example, a photosensitive member,
A latent image corresponding to an image information signal of each color component of an image is formed on a plurality of image carriers such as a dielectric by an electrophotographic method, an electrostatic recording method, or the like, and the latent image is formed by a developing device using a non-magnetic toner. And a two-component developer containing a magnetic carrier as a main component to form a visible image (toner image) of each color,
Any configuration may be used as long as the toner images of the respective colors are superimposed and transferred onto a transfer material such as paper, and a permanent image of color is formed by a fixing unit.

First, an overall configuration of an embodiment of an image forming apparatus according to the present invention will be described with reference to FIG. In this embodiment, a case where the present invention is applied to an electrophotographic digital copying machine will be described, but it goes without saying that the present invention can be equally applied to various other image forming apparatuses of the electrophotographic type and the electrostatic recording type.

In FIG. 1, an image of a document 31 to be copied is projected on an image pickup device 33 such as a CCD by a lens 32. The image sensor 33 decomposes an original image into a number of pixels and generates a photoelectric conversion signal corresponding to the density of each pixel. The analog image signal output from the image sensor 33 is sent to an image signal processing circuit 34, where the analog image signal is converted into a pixel image signal having an output level corresponding to the density of the pixel. Sent.

The pulse width modulation circuit 35 has a width (time length) corresponding to the level of each input pixel image signal.
Is formed and output. That is, as shown in FIG. 3A, a wider drive pulse W is applied to a high-density pixel image signal, and a narrower drive pulse S is applied to a low-density pixel image signal. , A drive pulse I having an intermediate width is formed for a medium-density pixel image signal.

The laser drive pulse output from the pulse width modulation circuit 35 is supplied to the semiconductor laser 36, and causes the semiconductor laser 36 to emit light for a time corresponding to the pulse width. Therefore, the semiconductor laser 36 is driven for a longer period of time for the high-density pixels, and is driven for a shorter period of time for the low-density pixels. Therefore, the photosensitive drum 40
With the optical system described below, a longer range is exposed in the main scanning direction for high density pixels, and a shorter range is exposed in the main scanning direction for low density pixels. That is, the dot size of the electrostatic latent image differs according to the density of the pixel. Therefore, it goes without saying that the toner consumption for the high density pixels is larger than that for the low density pixels. In FIG. 3D, the electrostatic latent images of the low, medium and high density pixels are indicated by L, M and H, respectively.

The laser light 36a emitted from the semiconductor laser 36 is swept by a rotating polygon mirror 37, and a fixed mirror for directing the lens 38 such as an f / θ lens and the laser light 36a toward the photosensitive drum 40 as an image carrier. By 39, a spot image is formed on the photosensitive drum 40. Thus, the laser beam 36a scans the photosensitive drum 40 in a direction substantially parallel to the rotation axis (main scanning direction) to form an electrostatic latent image.

The photoreceptor drum 40 is an electrophotographic photoreceptor drum having a photoreceptor such as amorphous silicon, selenium, or OPC on its surface and rotating in the direction of the arrow. It is uniformly charged by the charger 42. Thereafter, exposure scanning is performed with a laser beam modulated in accordance with the image information signal described above, whereby an electrostatic latent image corresponding to the image information signal is formed. This electrostatic latent image is reversely developed by a developing device 44 using a two-component developer 43 in which toner and carrier are mixed, and is visualized as a toner image. Here, the reversal development is a development method in which a toner charged to the same polarity as the latent image is attached to a region of the photosensitive drum 40 exposed to light, and the latent image is visualized.

This toner image is transferred onto the transfer material P conveyed to the photosensitive drum 40 by the transfer material carrying belt 47 by the action of the transfer charger 49. Transfer material carrying belt 4
Reference numeral 7 is stretched between the two rollers 45a and 45b, and is driven endlessly in the direction of the arrow in the figure to convey the transfer material P held thereon to the photosensitive drum 40. The transfer material P to which the toner image has been transferred is separated from the transfer material carrying belt 47, transported to a fixing device (not shown), and fixed to a permanent image.
Further, the residual toner remaining on the photosensitive drum 40 after the transfer is removed by the cleaner 50 thereafter.

Although only a single image forming station (including a photosensitive drum 40, an exposing unit 41, a primary charging unit 42, a developing unit 44, and the like) is shown in FIG. For example, it is a color image forming apparatus having image forming stations for cyan, magenta, yellow and black, and the image forming stations are sequentially arranged on the transfer material carrying belt 47 along the moving direction, and each image is formed. An electrostatic latent image for each color (each color component of the image) obtained by color separation of the image of the original is sequentially formed on the photosensitive drum of the forming station, and is developed by a developing device using a developer having a toner of a corresponding color. Then, the transfer material P is sequentially superimposed and transferred onto the transfer material P conveyed by the transfer material carrying belt 47.

FIG. 2 shows an example of the developing device 44. As shown in the drawing, the developing device 44 is disposed so as to face the photosensitive drum 40, and the inside thereof has a partition wall 5 extending in the vertical direction.
1, the first chamber (developing chamber) 52 and the second chamber (stirring chamber)
53. A non-magnetic developing sleeve 54 that rotates in the direction of the arrow is disposed in the first chamber 52, and a magnet 55 is fixedly disposed in the developing sleeve 54.

As shown in FIG. 1, a toner replenishing layer 60 containing a replenishing toner 63 is mounted on the upper portion of the developing device 44, and a toner conveying screw 62 is installed below the toner replenishing layer 60. Have been. By rotating the toner conveying screw 62 by a motor 70 connected via a gear train 71, the toner 63 in the replenishing layer 60 is conveyed and supplied into the developing device 44. The supply of toner by the transport screw 62 is controlled by the CPU 67 controlling the rotation of a motor 70 via a motor drive circuit 69. The RAM 68 connected to the CPU 67 stores control data supplied to the motor drive circuit 69 and the like.

The first chamber 52 and the second chamber 53 are provided with developer stirring screws 58 and 59, respectively. The screw 58 stirs and conveys the developer in the first chamber 52, and the screw 59 and the toner 63 supplied by the rotation of the conveying screw 62 from the toner replenishing tank 60 in FIG. The developer 43 is agitated and conveyed to make the toner concentration of the developer 43 uniform. Partition wall 51
2, a developer passage (not shown) for communicating the first chamber 52 and the second chamber 53 with each other is formed at the front and rear ends in FIG.
The toner in the first chamber 52 whose toner concentration has been reduced due to the consumption of the toner due to the conveyance force moves from one of the passages into the second chamber 53, and the toner concentration has recovered in the second chamber 53. The developer is configured to move from the other passage into the first chamber 52.

The two-component developer 43 in the developing device 44 is carried on the developing sleeve 54 by the magnetic force of the magnet 55, and the thickness of the layer is regulated by the blade 56. Is transported to the developing area opposite to. Then, in the development area, the developer 43 is used for developing the latent image on the photosensitive drum 40. In order to improve the development efficiency, that is, the toner application rate to the latent image, a development bias voltage in which a DC voltage is superimposed on an AC voltage is applied to the development sleeve 54 from a power supply 57.

Since the toner density of the developer 43 in the developing device 44 is reduced by the development of the electrostatic latent image, the density controller controls supply of the toner 63 from the toner supply layer 60 to the developing device 44. Then, the toner density of the developer 43 is controlled to be constant, or the image density is controlled to be constant.

In the image forming apparatus of the present invention, as a density control device, a patch image is formed on the photosensitive drum 40 for reference, and the image density is set to a light emitting unit 73a and a light receiving unit which are installed opposite to the photosensitive drum 40. Concentration sensor 7 having 73b
(3) A method of detecting and controlling according to (Patch detection ATR)
A light emitting unit 77a and a light receiving unit 77b installed in the developing unit 44
A method of detecting and controlling the toner density of the developer 43 in the developing device 44 by using the density sensor 77 having the above (developer reflection ATR), and the required toner amount based on the output level of the digital image signal for each pixel from the video counter 66 (Video count ATR).

In the present invention, the output signal of the toner replenishment control by the patch detection ATR as the first density control device is changed to the toner replenishment control by the developer reflection ATR and the video count ATR as the second and third density control devices. It is one feature to use to correct for. The details will be described below.

First, the patch detection ATR is activated at a predetermined timing to form a patch image on the photosensitive drum 40 as a reference image for density detection. That is, as shown in FIG. 1, a patch image signal generating circuit 72 for generating a patch image signal having a signal level corresponding to a predetermined density is provided, and the patch image signal from the generating circuit 72 is subjected to the pulse width modulation. The laser driving pulse is supplied to a circuit 35 and has a pulse width corresponding to the predetermined density. This laser drive pulse is applied to the semiconductor laser 3
6 and emits the laser 36 for a time corresponding to the pulse width, and scans the photosensitive drum 40. At this time, the counter 66 is not operated. As a result, a patch electrostatic latent image corresponding to the above-mentioned predetermined density is formed on the photosensitive drum 40, and this patch electrostatic latent image is developed by the developing unit 44.

Next, the patch image (toner image) thus obtained is added to the density sensor 73 of the patch detection ATR.
Light is emitted from a light emitting unit 73a such as an LED, and the reflected light is received by a light receiving unit 73b such as a photoelectric conversion element to detect the actual patch density of the patch image. The detected patch density corresponds to the actual density of the developer 43 in the developing device 44.

An output signal from the light receiving section 73b which detects the actual patch density is supplied to one input of a comparator 75. A reference signal corresponding to a specified density (initial density) of the patch image is input from the reference voltage signal source 76 to the other input of the comparator 75. The comparator 75 compares the actual density of the patch image with the initial density to obtain a density difference, and supplies an output signal of the density difference to the CPU 67. The output signal of the density difference is used for toner supply control to the developer 43 in the developing device 44 by the developer reflection ATR and the video count ATR.

The image forming apparatus of the present invention includes image forming stations for four colors, for example, yellow, magenta, cyan and black. Is detected and compared with the initial density, the density difference between the actual density of the patch image of each color and the initial density is obtained, and an output signal of the density difference is supplied to the CPU 67.

Another feature of the present invention is that the developer reflection A
The toner supply control by the TR and the video count ATR is to be performed separately for yellow, magenta, and cyan chromatic developers (color developers) and achromatic black developers.

That is, the yellow, magenta, and cyan color developers can detect a change in tint based on the amount of reflected light.
It is possible to maintain the developer concentration at a certain level by using the developer reflection ATR by the density sensor 77 installed in the developing device 44. However, when there is a change in the amount of frictional charge of the toner due to a change in the humidity of the environment or the like, the image cannot follow the change in image density due to the change in the amount of charge of the toner.

Therefore, for the color developer, the developer reflection AT
R, the toner supply control is performed.
In U67, from the output signal of the density difference between the patch images (yellow, magenta, cyan), the amount of toner excess / deficiency (toner replenishment amount) of the color developer required for returning the patch image density to the initial density is calculated. The toner supply amount is converted to a signal level corresponding to the toner correction amount and fed back to each developing unit 44. Then, the signal of the toner correction amount is added to or subtracted from the signal level of the toner replenishment amount with respect to the target density set in the developer reflection ATR for correction.
The toner supply control based on the developer reflection ATR method is performed based on the corrected toner supply amount.

For black developer, developer reflection AT
Since the application of R is impossible, in the present invention, the video counter 66 accumulates the required toner amount from the output level of the digital image signal for each pixel to obtain a video count AT.
R controls toner supply to the developer. At this time, the above-mentioned patch image (black) is generated by the CPU 67.
From the density difference output signal, the amount of toner excess or deficiency (toner replenishment amount) of the black developer required to return the patch image density to the initial density is calculated, and the toner replenishment amount is used as a corrected toner amount. The signal level is converted to a signal level and fed back to the counter 66. Then, the signal of the corrected toner amount is added to or subtracted from the signal of the video count value obtained by integrating the required toner amount in the control by the count ATR, and the correction is performed. Let them do it.

As described above, in the present invention, when the toner supply control of the two-component developer of yellow, magenta, cyan, and black is performed, the toner supply amount in the toner supply control by the patch detection ATR is determined. , Magenta, and cyan color developers, the toner supply amount is used as a corrected toner amount, and toner supply control is corrected by the developer reflection ATR. For black developer, the video counter ATR is calculated based on the corrected toner amount.
The toner replenishment control is performed after the correction, so that the change in toner developability due to environmental fluctuations and the fluctuations in the toner consumption of the toner consuming system, etc. are taken into account, and the toner concentration of any developer is suitable for the toner image. It is possible to control the density to obtain the density, and it is possible to form a high-quality color image in which the image density of each color toner image is appropriately controlled.

Embodiment 2 In this embodiment, when correcting the toner supply amount to the target density of the developer reflection ATR used for the color developer by adding or subtracting the toner correction amount in the first embodiment,
The upper limit and the lower limit are set for the signal level of the toner supply amount with respect to the target density. That is, an upper limit and a lower limit are set for the signal level of the corrected toner supply amount.

Therefore, when the correction of the toner replenishment amount becomes excessive for some reason, the toner concentration of the developer 43 in the developing device 44 becomes excessive or the developer overflows from the developing device 44. When the correction of the toner replenishment amount becomes too small, the toner concentration of the developer 43 in the developing device 44 becomes insufficient or the developer 4
The coating amount of No. 3 can be reliably prevented from decreasing.

Since the present embodiment is configured as described above,
By properly controlling the toner density of the developer of each color, it is possible to obtain a high quality color image in which the image density of the toner image of each color is appropriately controlled. Sex can be achieved.

Embodiment 3 The detection sensitivity of a patch image by the density sensor 73 of the patch detection ATR differs for each color due to a difference in physical properties of the toner. Therefore, in this embodiment, the latent image pattern of the patch image is changed for each color of yellow, magenta, cyan, and black, or for yellow, magenta, cyan, and black, and the detection sensitivity of the patch image is good for each color. I tried to be.

More specifically, in this embodiment, the number of latent image lines of the yellow, magenta, and cyan patch images, which are inferior in detection sensitivity, is increased by fine lines compared to the black patch image, so that these yellow and magenta patch images are reduced. The detection sensitivity of the cyan patch image was increased to be substantially the same as the detection sensitivity of the black patch image.

Thus, in this embodiment, the developer reflection ATR for the yellow, magenta, and cyan color developers
, The density difference data obtained by the patch detection ATR used for correcting the toner supply amount can always be obtained and supplied appropriately. As a result, the toner density of not only the black developer but also the color developer can be controlled well, and a high-quality color image in which the image density of each color toner image is appropriately controlled can be obtained more stably. Became.

In the present invention, the formation of the patch image in which the latent image pattern is changed is not limited to the above, and the patch image is formed by providing a screen angle difference in the dot pattern of the latent image for each color. According to this, even better results were obtained. Of course, a patch image can be formed by optimizing the number of lines and the screen angle for each color, and the effect of the present invention can be further improved.

[0053]

As described above, in the image forming apparatus of the present invention, when performing the toner supply control of the two-component developer of a plurality of colors, the toner supply amount in the toner supply control by the patch detection ATR is obtained. For a developer such as chromatic yellow, the toner replenishment control by the developer reflection ATR is performed using the toner replenishment amount as a correction toner amount, and for an achromatic black developer,
The correction of the toner replenishment control by the video counter ATR is performed based on the corrected toner amount. Therefore, even if a change occurs in the developing property of the toner due to an environmental change or a change in the toner consumption of the toner consuming system, the toner density of any developer is changed. It is also possible to control the density at which an appropriate image density can be obtained for the toner image, and it is possible to stably obtain a high-quality color image in which the image density of each color toner image is appropriately controlled from the beginning of image formation. . Of course, there is no overflow of the developer, no fog, and no defective coating of the developer on the developer carrying member.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a schematic sectional view illustrating a schematic configuration of a developing device included in the image forming apparatus of FIG.

3 is a waveform diagram illustrating a method of counting density information of an image information signal in the image forming apparatus of FIG.

FIG. 4 is an explanatory diagram illustrating an overall configuration of an example of a conventional image forming apparatus.

[Explanation of symbols]

 Reference Signs List 40 photosensitive drum 43 two-component developer 44 developing device 60 toner replenishing tank 62 toner transport screw 63 replenishing toner 66 counter 67 CPU 69 motor driving circuit 70 motor 72 patch image signal generating circuit 73 density sensor 75 comparator 76 reference voltage signal Source 77 concentration sensor

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G03G 13/01 G03G 15/01-15/01 117

Claims (5)

(57) [Claims] A plurality of image bearing members; a plurality of latent image forming means for digitally forming an electrostatic latent image corresponding to each color component of the image on the image bearing member; A plurality of developing means for developing the electrostatic latent image using a two-component developer of a corresponding color to form a toner image, a plurality of toner replenishing means for replenishing toner to the developing means, and each developing means And a density control device for controlling the toner density of the developer or the image density of the toner image via the toner supply control to the developing means, wherein at least the image carrier is used as the density control device. A first control device for forming a reference image for density detection on the upper portion and obtaining a toner supply amount by detecting the reference image density is installed for each developing device, and the developer concentration of the developing device is detected. The toner supply amount And a third control device for determining the amount of toner supply from a digital image signal for each pixel of the reference image is provided with a developing device using a chromatic developer. The first and second control units feed back the amount of toner replenishment from the reference image density to the second and third control units. An image forming apparatus characterized in that a supply control is corrected and performed. 2. The image forming apparatus according to claim 1, wherein the pattern of the electrostatic latent image for the reference image is changed for each color developer or between a chromatic developer and an achromatic developer. 3. The image forming apparatus according to claim 2, wherein the change of the pattern is the number of lines of the pattern. 4. The image forming apparatus according to claim 2, wherein the change of the pattern is a screen angle of the pattern. 5. The image forming apparatus according to claim 2, wherein the change of the pattern is a line frequency and a screen angle of the pattern.