JPH08110700A - Image forming device - Google Patents

Abstract

PURPOSE: To provide an image forming device capable of preventing the staining and the scattering of toner in the case of such as the fluctuation of the potential by electrification transiently caused at the time of the reference image forming, and preventing the image gray level deterioration by surely increasing the toner concn. in the case of reduction of the toner concn. perpetually caused. CONSTITUTION: In the image forming device, correcting the control target value VTref based on the output value of the P sensor 14 for detecting the image gray level of the reference image formed on the photoreceptor drum 7, while driving the toner replenishment roller 92 corresponding to the difference between the T sensor 93 and the control target value VTref, the correction amount ΔVT in the direction of increasing the toner replenishment amount of the control target value VTref is set so that the increasing amount of the toner concn. becomes <= the specified upper limit (0.5wt.%), and >= the specified lower limit (0.2wt.%).

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, a facsimile and a printer, and more specifically, it detects the toner concentration of a two-component developer in a developing device by means of a toner concentration detecting means and outputs it. An image for controlling the toner replenishing means in accordance with the result of comparison between the control value and the control target value, detecting the image density of the reference image formed on the image carrier, and correcting the control target value based on the detected value. The present invention relates to a forming device.

[0002]

2. Description of the Related Art Conventionally, as a device of this type, a latent image is formed on an image bearing member by uniformly charging the image bearing member and then irradiating the image with a light image. It is known that the latent image is visualized with a two-component developer (hereinafter referred to as a developer) and the visualized image is transferred to a transfer material.

Since the weight ratio of the toner to the carrier of the developer in the developing device (hereinafter referred to as the toner density) affects the image density, toner is appropriately replenished in the developing device to keep the toner density constant. Is desirable. As the toner replenishment control means, an output value of the image density detection means for detecting the image density of the reference image formed on the image carrier,
It is known that the toner replenishing means such as a toner replenishing roller is controlled based on the output value of the toner concentration detecting means for detecting the toner concentration in the developing device.

The toner replenishment control is performed as follows, for example. At each image forming operation, the toner density detecting means detects the toner density in the developing device, and outputs the output value V.
The drive time of the toner replenishing means is calculated according to the comparison result of T and the control target value VTref, and the toner replenishing means is controlled to be driven for the calculated drive time. Then, each time a predetermined number of image forming operations are completed, a reference image is formed on the image carrier, the image density is detected by the image density detecting means, and the correction amount ΔVT of the control reference value is based on the output value.
Is calculated. Then, based on the correction amount ΔVT and the output value VTp of the toner density detecting means at the time of creating the reference image, the control target value VTr is expressed as VTref = VTp + ΔVT.
Determine ef. This corrected control target value VTref is
It is used for toner replenishment control until the next reference image formation.

As described above, the toner replenishment control using both the image density detection means and the toner density detection means has a constant toner density as compared with the case where the toner replenishment control is performed using only the toner density detection means. Nevertheless, even if the image density changes due to changes in the characteristics of the developer, the environment such as wet temperature, and the development conditions, the image density during image formation can be kept constant. In the combined toner replenishment control, the toner replenishment control using only the image density detection means, that is, the reference image is formed at each image formation, and the toner replenishment control is performed based on the detected value of the image density. Differently, in a device in which the transfer member that transfers the image to the transfer material is a belt-shaped transfer member and the reference image cannot be formed on the image carrier between the transfer materials, the image density during image formation is constant. It is effective when keeping.

[0006]

In the toner replenishment control, if the image density of the reference image detected by the image density detecting means becomes lower than the predetermined density for some reason, the true toner density in the developing device is reached. Regardless of the above, it is uniformly determined that the density is lower than the predetermined density, and the control target value is corrected to increase the toner replenishment amount during the subsequent toner replenishment operation. There is a possibility that the toner will be replenished with the toner to cause scumming and toner scattering. For example, when the charge potential of the image carrier fluctuates temporarily during the formation of the reference image and the image density of the reference image is reduced, the toner density in the developing device will be the predetermined density unless the control target value is corrected. However, the control target value is corrected even though the image density at the time of subsequent image formation is also maintained at the predetermined density, and therefore there is a possibility that the toner replenishment will be excessive during the subsequent toner replenishment operation. Further, for example, when the reference image is formed immediately after the black solid image is formed, the reference image with a lower image density is obtained as compared with the case where the reference image is formed immediately after the normal image forming operation. Since the toner is formed and it is determined that the toner concentration in the developing device is lower than the actual toner concentration, the control target value may be overcorrected and the toner may be excessively replenished during the subsequent toner replenishing operation.

In order to prevent excessive toner replenishment, the toner replenishing amount by the toner replenishing means at each replenishment level set according to the comparison result of the output value VT of the toner concentration detecting means and the control target value VTref is set. A method of uniformly reducing the driving time, for example, a method of uniformly shortening the driving time of the toner replenishing means can be considered. However, in this case, when an image is formed on a document having a large image area ratio, the toner supply may be insufficient and the image density may be reduced, or the carrier may be attached.

The present invention has been made in view of the above problems, and an object of the present invention is to control toner replenishment when a temporary change in charging potential occurs during formation of a reference image or immediately after formation of a solid black image. To provide an image forming apparatus capable of preventing background stains and toner scattering when a reference image for use is formed, and reliably increasing the toner concentration to prevent the image concentration from decreasing when the toner concentration really decreases. Is.

[0009]

In order to achieve the above-mentioned object, the invention of claim 1 is a developing method in which a latent image formed on an image bearing member is visualized by a two-component developer consisting of a carrier and a toner. Device, toner replenishing means for replenishing toner in the developing device, toner concentration detecting means for detecting toner concentration in the developing device, and reference for forming a reference image for image density detection on the image carrier The image forming means, the image density detecting means for detecting the image density of the reference image, and the toner replenishing means are driven according to the result of comparison between the output value of the toner density detecting means and the control target value. In an image forming apparatus including a control unit that corrects the control target value based on the output value of the detection unit, the correction amount in the direction in which the toner replenishment amount of the control target value increases increases the corrected toner density. The amount is less than the specified upper limit And it is characterized in that set to be equal to or greater than the predetermined lower limit amount.

In particular, the invention of claim 2 sets a plurality of replenishment levels for the replenishment amount by the toner replenishing means,
The image according to claim 1, wherein one of the replenishment levels is selected corresponding to the difference between the output value of the toner concentration detection means and the control target value, and the toner replenishment means is controlled in accordance with the selected replenishment level. In the forming apparatus, the absolute value of the maximum correction amount in the direction in which the toner replenishment amount of the control target value increases is determined by the output value of the toner concentration detecting means corresponding to the maximum replenishment level of the toner replenishing means and the corrected control target. It is characterized in that it is equal to the minimum absolute value of the difference from the value.

According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the correction of the control target value is executed at a predetermined interval, based on the output value of the image density detecting means. It is characterized by controlling so as to change the setting of.

[0012]

According to the present invention, the control means controls the toner replenishing means to drive the toner replenishing means to replenish the developing device with toner according to the result of comparison between the output value of the toner density detecting means and the control target value. To do. For example, the drive time of the toner replenishing means is determined according to the difference or ratio between the output value of the toner concentration detecting means and the control target value, and the toner replenishing means is controlled to be driven for the determined drive time. The control target value is corrected based on the output value of the image density detecting means for detecting the image density of the reference image formed on the image carrier.
For example, the correction amount of the control target value is determined according to the output value of the image density detecting means, and the sum of this correction amount and the output value of the toner density detecting means at the time of forming the reference image is used as the control target value for the new toner replenishment control. Control to

Then, the correction amount in the direction in which the toner replenishment amount of the control target value used for the toner replenishment control increases is set so that the increased amount of the corrected toner concentration is equal to or less than a predetermined upper limit amount. When the image density of the reference image detected by the image density detecting means is lower than the predetermined lower limit density, a small amount of toner is replenished during the subsequent toner replenishment operation to prevent excessive toner replenishment. Further, the correction amount in the direction in which the toner replenishment amount of the control target value is increased is set so that the increased amount of the corrected toner density is equal to or larger than a predetermined lower limit amount. When the image density of the image is low, the toner is replenished within the range in which the image density does not fall below the desired density during the subsequent toner replenishment operation.

The upper limit and the lower limit of the toner density increase amount and the corresponding correction amounts of the control target value vary depending on the kind of the developer used in the developing device, so that the experiment is conducted in advance. Etc. For example, the upper limit amount of the toner concentration increase amount is set to 0.5 wt%, the lower limit amount is set to 0.2 wt%, and the correction amount of the control target value is set to correspond to them.

Particularly, in the second aspect of the present invention, a plurality of replenishment levels are set for the replenishment amount by the toner replenishing means, and the replenishment is made to correspond to the difference between the output value of the toner concentration detecting means and the control target value. One of the levels is selected, and the drive of the toner replenishing means is controlled according to the selected replenishment level.

Then, the absolute value of the maximum correction amount in the direction in which the toner replenishment amount of the control target value is increased is determined by the output value of the toner density detecting means corresponding to the maximum replenishment level of the toner replenishing means and the corrected control target value. By making it equal to the minimum absolute value of the difference, the toner is replenished at the maximum replenishment level during the first image forming operation after the correction of the control target value,
During each image forming operation from the second time to the next correction of the control target value, the toner is replenished at a replenishment level smaller than the maximum replenishment level so that a large amount of toner is not continuously supplied.

Further, according to the third aspect of the invention, when the image density of the reference image is judged to be lower than the predetermined density based on the output value of the image density detecting means, the control target value is corrected. By controlling the interval to be short, even if the control target value is overcorrected, the control target value can be promptly returned to the appropriate value.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to an electrophotographic copying machine (hereinafter referred to as a copying machine) which is an image forming apparatus will be described below. FIG. 1 is a front view showing a schematic configuration of a copying machine according to this embodiment. A document 2 placed on a document table 1 is illuminated by an exposure lamp 3, and its reflected light 4 is projected through a mirror 5 and a lens 6 onto a photoconductor drum 7 serving as an image carrier, whereby a photoconductor is formed. An electrostatic latent image of the original 2 is formed on the drum 7. Around the photosensitive drum 7, a charging device 8 that uniformly charges the surface of the photosensitive drum 7 to a negative polarity, a developing device 9 that visualizes an electrostatic latent image formed on the photosensitive drum 7, A belt type transfer device 10 for transferring the visualized image onto a transfer sheet, a cleaning device 11 for removing transfer residual toner on the photoconductor drum 7, a static eliminator 12 for removing residual charge on the photoconductor drum 7, and the like are provided. It is arranged.

The developing device 9 detects the toner concentration of a toner hopper 91 and a toner replenishing roller 92 as a toner replenishing means for replenishing toner into the developing portion, a carrier in the developing portion, and a two-component developer composed of positively charged toner. A magnetic permeability sensor (hereinafter referred to as a T sensor) 93 as a toner concentration detecting means, a developing sleeve 94 as a developer carrying member for carrying the developer in the developing unit and supplying the toner onto the photosensitive drum 7, and a developing sleeve. A doctor blade 95 for restricting the developer on 94 is provided. A negative developing bias voltage is applied to the developing sleeve 94 from a power source (not shown), and a predetermined developing potential is formed between the developing sleeve 94 and the photosensitive drum 7.

FIG. 3 shows an electric component section for controlling toner replenishment using a T sensor 93 and a reflection type photo sensor (hereinafter referred to as P sensor) 14 as an image density detecting means for detecting the image density on the photosensitive drum. It is a block diagram. The control unit 16 as a control means includes a CPU, RAM, ROM, I
The I / O interface is connected to a main motor 20, a developing clutch 21, and a toner replenishing clutch 22 via drivers 17, 18, and 19, respectively. The developing clutch 21 is for turning on / off the transmission of the rotational driving force from the main motor 20 as a drive source to the developing roller 94 of the developing device 9, and the toner replenishing clutch 22 is for rotating the toner replenishing roller 92. This is for turning on / off the transmission of the driving force.

The I / O interface of the control unit 16 has a PWM controller 23 for supplying a predetermined voltage to the T sensor 93 and the P sensor 14, and the T sensor 93.
Further, A / D converters 24 and 25 for converting an analog signal output from the P sensor 14 into a digital signal, and an operation display unit 26 are connected.

Toner is replenished from the toner hopper 91 into the developing section by the output value VT of the T sensor 93 and the control reference value V.
Tref is compared, and the toner supply roller 92 is driven and controlled according to the comparison result. By this toner replenishment control, the toner density, which is one of the factors that affect the image density, can be kept constant, but the image density depends on changes in the physical properties of the developer over time, environment such as temperature and humidity, and development. It is also affected by the conditions (distance between the developing sleeve 94 and the photosensitive drum 7, distance between the doctor blade 95 and the developing sleeve 94) and the like. Therefore, in the present embodiment, a reference corresponding to a reference pattern 15 having a predetermined reference density is used by using a charging device 8 as a reference image forming unit, an exposure device such as an exposure lamp 3, an erase device 13, a developing device 9 and the like. An image is formed on the photoconductor drum 7, the density of the reference image is detected by the P sensor 14, and the control reference value VTref of the T sensor 93 is controlled to be corrected based on the detection result, and a more stable image is obtained. Is being obtained.

FIG. 1 shows the T sensor 93 and the P sensor 14 described above.
6 is a flowchart of toner replenishment control using the. When a predetermined number of continuous copy operations (hereinafter referred to as 1Job) set on the operation display unit 26 are started, the toner density of the developer in the developing device is detected by the T sensor 93 at each copy (steps 1 and 2). . Photoconductor drum 7
Upper toner adhesion amount m / a, toner concentration TC, and T sensor 9
Since there is a relationship as shown in FIG. 4A with the output value VT of 3, the predetermined image density (toner adhesion amount m ₀ /
To obtain the toner control level TC ₀ corresponding to a),
According to the difference VT-VTref between the output value VT of the T sensor 93 and the control target value VTref, the driving time t of the toner replenishing roller 92, that is, the ON time t (msec) of the toner replenishing clutch 21 is calculated by the formula shown in Formula 1. The toner supply clutch 21 is turned on for the determined drive time t, and toner is supplied to the developing device 9 (steps 3 and 4).
Here, the unit of the transfer paper size is cm ² , and the value of the toner supply amount per unit time is the toner supply roller 92 to be used.
In this example, 0.183 mg / nsec was used, although it varies depending on the structure of the above, its rotation speed, and the like. Further, the value of the matrix shown in Table 1 was used as the coefficient α.

[Equation 1]

[0024]

[Table 1]

The toner replenishment control based on the output value VT of the T sensor 93 is executed for each copy operation during one job (step 5). When a series of copy operations for 1 job is completed, it is judged whether or not the cumulative number of sheets for that 1 job is 10 or more (step 6), and if it is less than 10, the standby state for the next copy operation is entered. On the other hand, 1J above
When the integrated number is 10 or more in ob, the reference image is formed on the photosensitive drum 7, and the P sensor 14 detects the image densities of the reference image (Vsp) and the non-image portion (Vsg). At the same time, the T sensor 93 detects the toner concentration of the developer in the developing device 9 (step 7,
8). Then, based on the ratio Vsp / Vsg of the output values of the P sensor 14, based on the conversion matrix of Table 2 stored in the control unit 16, the correction amount ΔVT of the control target value VTref.
Is determined, and the sum of the correction amount ΔVT and the output value VTp of the T sensor 93 is stored in the control unit 16 as a new control target value VTref (steps 9 to 11).

[0026]

[Table 2]

Here, the maximum correction amount ΔVTmax (ΔVTmax × 5/255 = −8V) of the control target value VTref is set as follows. As shown in FIG. 5, as the number of copies increases from a new state, the charging capacity of the developer in the developing device, that is, the amount of charge Q / of the toner at the same toner density
When M changes, the target toner concentration TC for keeping the image density constant changes from about 2.8 wt% at the initial stage to about 1.5 wt% over time. Then, as shown in the experimental data A (initial) and B (elapsed time) of FIG. 6, the toner concentration of each target (TCa = 2.8 wt%, TCb
= 1.5 wt%), ΔTCa = ΔTCb = 0.5 wt%
When the toner density control level rises, the background stain rank deteriorates from "○" to a rank between "○" and "△".

In the conventional correction of the control target value VTref, the ratio Vsp / Vsg of the toner adhesion amount m / a to the output value of the P sensor 14 in FIG. 4B and the correction amount Δ of the control target value VTref.
Based on the relationship with VT, the correction amount ΔVT is determined from the ratio Vsp / Vsg of the output values of the P sensor 14 using the conversion matrix of Table 3 stored in the control unit 16. For example, even if the toner concentration in the developing device 9 is 2.8 wt% which is normal in the initial state, the toner adhesion amount m / a is lowered due to a temporary charging potential defect during the formation of the reference image, and the P sensor 14 has a small amount. When the output value Vsp / Vsg becomes 0.92 / 4.00, the correction amount ΔVT becomes −15 × 2/255 = −0.29 (V). Then, as shown in FIG. 7, since the sensitivity of the T sensor 93 to the toner concentration TC is −0.5 V / wt%,
The correction is performed so that 0.29 ÷ 0.5 = 0.59 wt% is added to 2.8 wt%, and the next job control target value VTref becomes 3.4 wt%, and copying is continuously performed. If it is taken, the toner concentration in the developing device 9 rises from 2.8 wt% to 3.4 wt%, and scumming and toner scattering easily occur. (Hereinafter, margin)

[0029]

[Table 3]

Therefore, in this embodiment, the correction amount ΔVT in the direction in which the toner concentration of the control target value VTref is increased is set so that the increase amount of the toner concentration control level becomes 0.5 wt% or less. Specifically, as shown in Table 1, the correction amount Δ
The absolute value of VT is set to 8 or less. Thus, P
When the output value Vsp / Vsg × 4.00 of the sensor 14 is larger than 0.82, a small amount of toner is replenished during the subsequent toner replenishment operation to prevent excessive toner replenishment. Therefore, even if the control target value VTref is corrected when a temporary change of the charging potential occurs during the formation of the reference image or when the reference image is formed immediately after the formation of the solid black image, a rapid change in the toner density occurs. Therefore, it is possible to prevent scumming and toner scattering.

Further, as shown in the experimental data C of FIG.
When the toner density decreases by ΔTCc = 0.2 wt% or more over time, the image density ID sharply decreases. Therefore, it is not the abnormality of the charging potential at the time of forming the reference image but the toner density really decreases and the image density decreases. In addition, the minimum absolute value of the correction amount is set so that the toner density reaches the toner density control level even when copying is continuously performed. Specifically, the absolute value of the correction amount ΔVT is set to 2 or more so that the increase amount of the toner concentration control level becomes 0.2 wt% or more.

Table 2 shows the toner replenishment control of this embodiment.
Output value range of P sensor 14 of 0.82 <Vsp / Vsg ×
For 4.00 ≦ 2.00, absolute value is smaller than the minimum value of the difference between the output value VT of the T sensor corresponding to the maximum replenishment level of the toner replenishing roller 92 (coefficient α = 60) and the corrected control target value VTref. Large correction amount ΔVT × 5/255 = -1
Compare with the case of setting 2. In this case, the correction amount ΔVT × 5/2 due to the charging potential abnormality or the like during the formation of the reference image.
When −12 is set as 55, the maximum replenishment amount (coefficient α = 60) is reached until the coefficient α of the toner replenishment amount in Table 1 changes from 60 to 40, even though the toner concentration is an appropriate concentration. Since a large amount of toner is replenished continuously, the charging amount Q / M of the replenished toner is poorly raised, and scumming and toner scattering easily occur.

Therefore, in the present embodiment, the control target value VTre
Maximum correction amount ΔVTmax in the direction in which the toner supply amount of f increases
Is made equal to the minimum value of the difference between the output value VT of the T sensor corresponding to the maximum replenishment level (coefficient α = 60) of the toner replenishing roller 92 and the corrected control target value VTref. Specifically, the absolute value (| -8 | = 8) of the maximum correction amount ΔVTmax in the direction in which the toner supply amount of the control target value VTref in Table 2 increases is set to the maximum supply level of the toner supply roller 92 of Table 1. The difference VT-VTref between the output value VT of the corresponding T sensor and the corrected control target value VTref is made equal. As a result, toner is replenished at the maximum replenishment level (coefficient α = 60) at the time of copying the first sheet after the correction of the control target value, and at the time of each copy operation from the second sheet to the time of correcting the next control target value. Since toner replenishment is performed at a replenishment level smaller than the maximum replenishment level (coefficient α = 45 or less) and a large amount of toner replenishment is prevented from being continuously performed, scumming and toner scattering can be further prevented.

Further, since the maximum replenishment level of toner is replenished during the first image forming operation after the correction of the control target value, the toner density is reduced even if the image forming operation is performed on a document that consumes a lot of toner. Toner replenishment follows up, and it is possible to reliably increase the toner density and prevent the image density from decreasing.

In the above embodiment, the detection timing of the image density of the reference image by the P sensor 14 is set 1 job after the cumulative number of copies is 10 or more. However, as shown in the flowchart of FIG. Output value Vsp
When / Vsg is equal to or more than a predetermined value, the detection timing of the next P sensor 14 may be advanced.

In the flow chart of FIG. 8, when one job, which is a continuous copying operation for a predetermined number of sheets set from the operation display unit 26, starts, the T sensor 9 is used at each copying.
3 detects the toner concentration of the developer in the developing device, and the driving time t of the toner replenishing roller 92, that is, the toner replenishing clutch 21 according to the difference VT-VTref between the output value VT of the T sensor 93 and the control target value VTref. ON time t (msec) is determined, and the toner replenishing clutch 21 is turned on for the determined driving time t to replenish the toner in the developing device 9 (steps 1 to 4). Then, the ratio Vsp of the output values of the P sensor 14 at the time of correction of the immediately preceding control target value
It is judged whether the value of / Vsg is 0.75 / 4.00 or more, that is, whether the reference image formed immediately before is lighter than a predetermined density (step 5). Where Vsp / V
When the value of sg is 0.75 / 4.00 or more, it is judged whether or not a total of five or more copies have been counted since the last time the control target value was corrected (step 6). When it is determined that the total number of sheets has reached 5, the new control target value VTref is calculated and stored in the control unit 16 as in steps 7 to 11 of the flowchart of FIG. 1 of the above embodiment (steps 8 to 12). ). In addition, 5 is added in step 6 above.
Even when the number of sheets has not reached the number of sheets, when one job at that time ends, the above steps 8 to 12 are executed to store a new control target value VTref. If 1 job has not been completed, the process returns to step 2 (step 1
3).

On the other hand, when it is determined in step 5 that the value of Vsp / Vsg has not reached 0.75 / 4.00, steps 5 to 5 in the flow chart of FIG.
Similar to 11, at the time when one job with 10 or more integrated sheets is completed, a new control target value VTref is calculated and the control unit 16
(Steps 14 to 20).

When it is determined that the image density of the reference image is lower than the predetermined density based on the output value of the P sensor 14 as in the toner replenishment control in the flowchart of FIG. 8, the control target value VTref is corrected. By controlling the interval to be shortened from 10 or more in total to 5 or less in total, even if overcorrection of the control target value VTref is performed, the control target value VTref is promptly returned to the proper control target value VTref, so that further scumming may occur. Toner scattering can be prevented. Further, when it is determined that the toner density really decreases and the image density of the reference image is lower than the predetermined density, it is possible to quickly and surely increase the toner density and prevent the image density from decreasing.

Further, in the above embodiment, the P sensor 14 whose output value decreases as the image density of the reference image increases (the toner adhesion amount increases) is used as the image density detecting means. Developing device 9
Although the T sensor 93 whose output value becomes smaller as the toner density inside becomes higher, the present invention increases the image density detecting means and the toner density at which the output value becomes larger as the image density becomes higher. Accordingly, the present invention can be applied to the case where the toner density detecting means whose output value increases according to the above is provided, and the same effect can be obtained.

In the above embodiment, the copying machine in which the photosensitive drum 7 is negatively charged and the regular developing method using the positively charged toner is adopted has been described. However, the present invention does not depend on whether the charging polarity is positive or negative. The present invention can be applied without any modification, and can also be applied to an image forming apparatus employing a reversal development method.

[0041]

According to the invention of claim 1, the correction amount in the direction in which the toner replenishment amount of the control target value used for the toner replenishment control is increased so that the corrected toner concentration increase amount is equal to or less than the predetermined upper limit amount. By setting so that the image density of the reference image detected by the image density detecting means is lower than the predetermined lower limit density, a small amount of toner is replenished during the subsequent toner replenishment operation to prevent excessive toner replenishment. I have to. Therefore, even if the control target value is corrected when a temporary fluctuation of the charging potential occurs during the formation of the reference image or when the reference image is formed immediately after the solid black image is formed,
Since the toner density does not change suddenly, there is an effect that it is possible to prevent background stains and toner scattering. Further, the correction amount in the direction in which the toner replenishment amount of the control target value is increased is set so that the increased amount of the corrected toner density is equal to or larger than a predetermined lower limit amount. When the image density of the image is low, the toner is replenished within the range where the image density does not fall below the desired density in the subsequent toner replenishment operation. Therefore, there is an effect that when the toner density really decreases, the toner density can be surely increased to prevent the image density from decreasing.

In particular, according to the second aspect of the invention, the absolute value of the maximum correction amount in the direction in which the toner replenishment amount of the control target value increases is determined by the toner concentration detecting means corresponding to the maximum replenishing level of the toner replenishing means. By making the difference between the output value and the corrected control target value equal, the maximum replenishment level of toner is replenished during the first image forming operation after the correction of the control target value, and the second control target value During each image forming operation up to the time of correction, toner replenishment is carried out at a replenishment level smaller than the maximum replenishment level to prevent a large amount of toner replenishment from being performed continuously, thus further preventing scumming and toner scattering. There is an effect. Further, since the toner is replenished at the maximum replenishment level during the first image forming operation after the correction of the control target value, even if the image forming operation is performed on a document that consumes a large amount of toner, the toner is not replenished due to the decrease in toner density. There is an effect that the toner density can be reliably followed up and the decrease in the image density can be prevented.

Further, according to the third aspect of the invention, based on the output value of the image density detecting means, when a temporary change of the charging potential occurs during the formation of the reference image or immediately after the formation of the solid black image, the reference image is formed. When it is determined that the image density of the reference image is lower than the predetermined density, such as when the control target value is formed, the control target value is controlled so as to be shorter so that the control target value is overcorrected. Even if is performed, the target control value is quickly returned to an appropriate control target value, so that it is possible to further prevent the background stain and the toner scattering. In addition, when it is determined that the toner density really decreases and the image density of the reference image is lower than the predetermined density, there is an effect that the toner density can be quickly and surely increased to prevent the decrease of the image density.

[Brief description of drawings]

FIG. 1 is a flowchart of toner supply control of a copying machine according to an embodiment.

FIG. 2 is a schematic configuration diagram of the copying machine.

FIG. 3 is a block diagram of an electric component section of the copying machine.

FIG. 4A is a characteristic diagram showing a relationship between a toner concentration TC, a toner adhesion amount m / a on a photosensitive drum, and an output value VT of a T sensor. FIG. 6B is a characteristic diagram showing the relationship between the output value Vsp / Vsg of the P sensor, the toner adhesion amount m / a on the photosensitive drum, and the correction amount ΔVT of the control target value.

FIG. 5 is a characteristic diagram showing the relationship between the number of copies and the toner density TC.

FIG. 6 shows toner density TC, background stain rank, and image density I.
The characteristic view which shows the relationship with D.

FIG. 7 is a characteristic diagram showing a relationship between toner concentration TC and an output value VT of a T sensor.

FIG. 8 is a flowchart of toner supply control according to a modification.

[Explanation of symbols]

 7 Photoreceptor Drum 8 Charging Device 9 Developing Device 14 P-sensor 15 Reference Pattern 16 Control Part 22 Toner Replenishing Clutch 91 Toner Hopper 92 Toner Replenishing Roller 93 T Sensor 94 Developing Sleeve

Claims (3)

[Claims] 1. A developing device for visualizing a latent image formed on an image bearing member by a two-component developer comprising a carrier and toner, a toner replenishing means for replenishing the toner in the developing device, and the developing device. Toner density detecting means for detecting the toner density in the apparatus, reference image forming means for forming a reference image for image density detection on the image carrier, and image density detecting means for detecting the image density of the reference image A control means for driving the toner replenishing means in accordance with the result of comparison between the output value of the toner density detecting means and the control target value, and correcting the control target value based on the output value of the image density detecting means. In the provided image forming apparatus, the correction amount in the direction in which the toner replenishment amount of the control target value increases is set such that the corrected toner density increase amount is equal to or less than a predetermined upper limit amount.
An image forming apparatus, wherein the image forming apparatus is set to be equal to or more than a predetermined lower limit amount. 2. A plurality of replenishment levels are set for the replenishment amount by the toner replenishing means, and one of the replenishment levels is set in correspondence with the difference between the output value of the toner density detecting means and the control target value. The image forming apparatus according to claim 1, wherein the toner replenishing means is selected and controlled according to the selected replenishment level, the absolute value of the maximum correction amount in the direction in which the toner replenishment amount of the control target value increases An image forming apparatus characterized in that the absolute value of the difference between the output value of the toner density detecting means corresponding to the maximum replenishing level of the replenishing means and the corrected control target value is made equal. 3. The image forming apparatus according to claim 1, wherein the correction of the control target value is executed at a predetermined interval, and the setting of the interval is changed based on the output value of the image density detecting means. An image forming apparatus comprising: