JP2003177580A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To output, in an electrophotographic system, high density image with sufficient density even in the case that the toner density of developer decreases due to, for example, consecutive high-density image printing. <P>SOLUTION: The toner density TC of developer comprising toner and carrier is monitored by a sensor. A threshold B1 for assuring sufficient output density with respect to the input of 100% density is provided in density that is higher than a threshold A1 for preventing a critical defect such as beads-carry-over. As shown in the graph 100, when toner density decreases due to, for example, consecutive high-density image printing and reaches the threshold B1, a development potential is altered only by a specific amount in order to decrease developability, thereby increasing an amount of toner supply which is feed-back controlled according to an image density detection value. This increases the toner density TC and ensures the amount of toner required to print the high density images with sufficient density. The amount that the development potential is altered is determined so that the output density to the same input density is unperceptible to the eye. <P>COPYRIGHT: (C)2003,JPO

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 an electrophotographic copying machine or printer.

[0002]

2. Description of the Related Art Conventional image forming apparatuses such as electrophotographic copying machines and printers generally use a two-component developer in which a non-magnetic toner and a magnetic carrier are mixed. In such a two-component developing type image forming apparatus, a control toner image for image density control is formed on a photoconductor, and the control toner image or the control toner image is transferred to an intermediate transfer member (IBT).
There is known a device that controls the image density by detecting the density of the image transferred on a belt or the like with a sensor and controlling the toner supply amount to the developing device according to the detected density (hereinafter, this control is performed by an ADC. [Auto Developbility Control]
Call control). In the image forming apparatus of the ADC control system, 2
The density of toner in the component developer (ratio to the total of toner and carrier) changes due to many parameters that determine the image density and changes in the characteristics of the developer itself, so there is a problem that it tends to cause image quality fluctuations. .

In order to deal with this, for example, in Japanese Patent Laid-Open No. 4-238366, the toner density is controlled to be constant, and when the developing property changes and the image density changes, the developing potential other than the toner density is changed. A method of changing is disclosed.

However, in this method, the variation of the developer characteristics is corrected by the control parameters for changing the developing potential, that is, the charging grid voltage, the developing bias, the ROS light amount, etc., so that the gradation reproducibility is deteriorated. Or, the stability of the output of each control parameter is impaired due to a large change range of the development potential.

Therefore, the applicant of the present invention has filed Japanese Patent Application Laid-Open No. 2001-154.
The device disclosed in Japanese Patent No. 427 is proposed. In this device, the concentration of the developer toner in the developing device is monitored by a sensor, and if it falls out of the range between the predetermined upper limit value and the lower limit value,
By offsetting the control target value of the development potential width and toner image density by a predetermined amount, it is possible to prevent the occurrence of image quality defects and the resulting damage to the image forming mechanism such as the photoconductor, and to increase the toner density. While the value is between the lower limit values, the above A
The toner supply to the developing device is controlled by DC control.
According to this control method, the BCO due to the toner density is too low.
(Bead carry-over: A phenomenon in which the carrier is discharged to the outside of the developing device. White areas in the image that should be black occur, and the carrier damages the photoconductor, etc.) and toner layer formation failure due to excessive toner density (development). A phenomenon in which the developer layer cannot be formed well on the magnet roll of the developing device because the amount of toner in the agent increases and the amount of carrier decreases, causing white spots in the output and partial density decrease.)
While avoiding the image quality defect such as, the stable control can be realized by the ADC control while the toner concentration is in the range where such a defect does not occur.

[0006]

However, in this control method, for example, when a large amount of input images having a high image density are continuously printed, the toner density decreases due to the change of the developer characteristics, and the ADC control is appropriately performed. Despite the control, the print image density in the high density image portion may be insufficient. The reason for this will be described below.

When a high-density image is continuously printed, the toner is actively supplied and consumed, so that the toner charge decreases with time. Then, the electrostatic force for separating the toner from the carrier is reduced, so that if the developing potential is the same, more toner will be carried to the photoconductor, and the developing property will be improved. Therefore, since the image density of the control toner image increases, the toner supply to the developing device is gradually reduced by the ADC control, and the toner density in the developer gradually decreases.

In general ADC control, if the control toner image having the highest input image density Cin (Cin = 100%) is used, the control toner image formed on the photoconductor or the intermediate transfer member can be cleaned. For reasons such as becoming insufficient,
The control is performed on the basis of an input image density lower than the maximum density (for example, 60%).

Therefore, in the case of high-density image continuous printing,
Even if the image with the standard density such as Cin 60% is printed with the correct density by the ADC control, the toner density itself in the developer is lowered, so the toner is insufficient for the high density image close to 100% Cin, It may not be possible to print at a certain density. In this case, although there are no image defects such as “white spots” and “fogs” that impair the image itself, the high-density portion of the image becomes thin in the output result,
The density reproducibility of the entire image deteriorates.

The present invention has been made in view of the above problems, and provides an image forming apparatus capable of preventing or reducing insufficient output density of a high density image in the case of continuous printing of high density images. With the goal.

[0011]

SUMMARY OF THE INVENTION An apparatus according to the present invention transfers toner in a developer contained in a developing device to a photoconductor while applying a development potential controlled to a predetermined potential target value. As a result, the electrostatic latent image on the photoconductor is developed to form a toner image on the photoconductor, and the toner image formed on the photoconductor is finally transferred onto a predetermined recording medium. The image forming apparatus forms an image on the recording medium by detecting the image density of the control toner image formed based on the predetermined control toner image data, and the detected image density has the predetermined control toner image. Toner supply control means for controlling the toner supply to the developing device so as to approach the image density target value, toner density detecting means for detecting the toner density of the developer, and image formation for an input image showing a predetermined image density. To ensure the concentration reproducibility of When the toner density detected by the toner density detecting means is less than or equal to the first threshold value, the toner supply control means controls A target value adjusting means for changing a target value of a predetermined image forming condition in a direction in which the toner supply to the developing device is increased, and the target value adjusting means is the toner detected by the toner density detecting means. When the density is equal to or lower than the first threshold value, it is determined that there is no visually significant difference in the image density of the image forming result before and after the change of the target value of the predetermined image forming condition. The target value of the predetermined image forming condition is changed by the predetermined first change amount.

In a further preferred aspect, the target value adjusting means further has a predetermined second threshold value lower than the first threshold value, which corresponds to the toner density causing an image quality defect in an output image, When the toner density detected by the toner density detecting means becomes equal to or lower than the second threshold value, the target value of the predetermined image forming condition is controlled by the toner supply controlling means to supply the toner to the developing device. The supply is increased by a predetermined second change amount larger than the first change amount.

In another preferred aspect, the target value adjusting means further includes an image of image data of an image forming object when the toner density detected by the toner density detecting means becomes equal to or lower than the first threshold value. The density is checked, and the first change amount is changed according to the image density.

In another preferred aspect, the target value adjusting means changes the toner density registered in advance when the toner density detected by the toner density detecting means becomes equal to or lower than the first threshold value. The predetermined image forming condition is changed by an amount of change corresponding to the change rate of the toner density at that time according to the information on the correspondence relationship between the rate and the predetermined image forming condition.

Further, the image forming apparatus according to the present invention detects the image density of the control toner image formed based on the predetermined control toner image data, and the detected image density becomes the predetermined control toner image density target value. The toner supply control means for controlling the toner supply to the developing device so as to approach, the toner concentration detecting means for detecting the toner concentration of the developer of the developing device, and the toner concentration of the developer cause the image quality defect of the output image. A first target value of a predetermined image forming condition is changed in a direction in which the toner concentration returns to the defect prevention range by the control of the toner supply control means when the toner density is out of a predetermined defect prevention range for prevention. And a density ensuring range corresponding to the toner density range necessary to ensure the density reproducibility of the image formation result of the input image in the predetermined image density range, which is set inside the defect prevention range. If the toner density of the developer is out of the density ensuring range, the target value of the predetermined image forming condition is set in the direction in which the toner density returns to the density ensuring range by the control of the toner supply control means. Second control means for changing the target value by a predetermined change amount determined so that no visually significant difference is recognized in the image density of the image formation result before and after the change. And

In a preferred aspect of the present invention, the second control means changes the potential target value of the developing potential as the target value of the predetermined image forming condition.

In another preferred aspect, the image forming apparatus further forms a gradation correction means for performing gradation correction on the input image based on predetermined gradation characteristic information, and a predetermined gradation pattern image. However, in the case where the predetermined image forming condition is changed by the gradation setup means for adjusting the gradation characteristic information of the gradation correction means based on the density detection result of the gradation pattern image and the first control means. And means for causing the gradation characteristic setup means to execute the adjustment processing of the gradation characteristic information.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings.

FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus to which the control according to the present invention is applied.

An image forming apparatus 1 shown in FIG. 1 is an apparatus for forming a color image, and four photosensitive materials corresponding to four colors of Y (yellow), M (magenta), C (cyan) and K (black). It has a body 2. In this device 1, an electrostatic latent image is formed on each of the photoconductors 2, and a carrier and toner contained in a developing device 3 are formed while applying a developing potential controlled to a predetermined control target value. By transferring the toner in the component type developer to each photoconductor 2, the electrostatic latent image on each photoconductor 2 is developed and a toner image corresponding to an image signal is formed on each photoconductor 2. . Then, each of the photoconductors 2
The upper toner image is transferred to the intermediate transfer member 4, and the toner image transferred on the intermediate transfer member 4 is transferred and fixed on a predetermined medium such as paper to form a color image.

As shown in FIG. 1, the image forming apparatus 1
Is an ADC sensor (control toner image sensor) that detects the density of the control toner image 20 transferred onto the intermediate transfer member 4.
5 and the toner supply amount to each developing device 3 so that the density of the control toner image 20 of each color detected by the ADC sensor 5 approaches a predetermined control toner image density target value (ADC target value) 6a. A toner supply control section 6 for controlling the toner concentration sensor 7, a toner concentration sensor 7 for detecting the toner concentration TC of the developer in the developing device 3, and a toner concentration of the developing device 3 based on the toner concentration TC detected by the toner concentration sensor 7. And a toner density limit control unit 8 for controlling the toner density within an appropriate range. The toner concentration limit control unit 8 controls the developing potential and the ADC control target value 6a in order to control the toner concentration within an appropriate range.
The target value adjusting unit 17 for adjusting the target value and the reset processing unit 18 for resetting the target value adjusted by the target value adjusting unit 17 are provided.

The ADC sensor 5 is provided for each color of CMYK and detects the density of the control toner image of each color in charge.

The image forming apparatus 1 also includes an image processing unit 11 for performing image processing such as gradation correction on an image signal 10 representing an image to be printed, and a control toner image pattern generator for generating a control toner image pattern. 12, an image signal processed by the image processing unit 11 and a control toner image pattern generator 12
PWM by modulating the control toner image pattern generated in
A pulse width modulation circuit 13 that generates a signal, and a laser driver 1 that drives each exposure device 15 based on the PWM signal
4. An exposure device 15 that is driven by a laser driver 14 to irradiate each photoconductor 2 with a laser beam to form an electrostatic latent image on the surface of each photoconductor 2, and to uniformly charge the surface of each photoconductor 2 before exposure. The charging device 16 and the like are provided. Image processing unit 1
The gradation correction in 1 is performed by using the gradation table 11a in which the TRC (tone reproduction curve) of each color of CMYK is converted into a look-up table. This gradation table 11a
Is corrected by the gradation table correction unit 9 at a predetermined timing (for example, when the power is turned on, every 1000 pages of output, or when shifting to the power saving mode) in order to correct the image quality change of image formation.

A recording medium to which the toner image primarily transferred on the intermediate transfer member 4 is transferred, a transfer device for secondarily transferring the toner image on the intermediate transfer member 4 to the recording medium, and a toner image transferred to the recording medium. Illustration of a fixing device for fixing the toner image is omitted.

In the image forming apparatus 1 described above, the control toner image density detected by the ADC sensor 5 is the target value 6a.
In the basic control framework of controlling the toner supply so that the above, the development potential and ADC are further calculated based on the toner concentration detection value TC detected by each toner concentration sensor 7.
By adjusting the target value 6a by the target value adjusting unit 17, the developer toner density is stabilized and the overall image quality control is stabilized.

Next, the principle of control by the target value adjusting unit 17 when the toner density is lowered will be described with reference to FIG. FIG. 2 is a diagram schematically showing a graph 100 of the transition of the developer toner concentration TC when continuously printing images with high image density under the control of the target value adjusting unit 17 of the present embodiment. In this graph, the vertical axis represents the toner concentration sensor 7
Indicates the toner concentration TC detected by, and the horizontal axis indicates the passage of time. In this control, three threshold values A1, A2 and B1 are set for the toner density TC. The thresholds A1 and A2 are for preventing image quality defects due to BCO (bead carry over), defective layer formation, and the like (and accompanying damage to mechanical mechanisms such as the photoconductor and fixing unit). Is a threshold value set in. That is, if the toner concentration TC is lower than the lower limit value A1, the possibility of image quality defects such as BCO is greatly increased, and if the toner concentration TC is higher than the upper limit value A2, image quality defects due to defective layer formation or the like is significantly generated. Increased (strictly speaking, in consideration of the control delay, the threshold values A1 and A2 are set to allow a little margin on the normal side (center side) from the toner density at which the possibility of the image quality defect cannot be ignored. Exist). These thresholds A1 and A2 are the same as those described in the above-mentioned JP 2001-15.
The upper limit and the lower limit used in the apparatus of Japanese Patent No. 4427 (hereinafter referred to as a conventional apparatus) may be the same. On the other hand, the threshold value B1 is provided in order to secure an appropriate output density (density of an image formed on a medium) according to the density of a high density image even when high density images are continuously printed. It is a threshold. This threshold value B1 may be obtained by experiments or the like. In the experiment, for example, the image forming apparatus 1
In the above, the toner density TC is variously changed, a predetermined inspection image is formed on the recording medium at each of the toner density TC, and the maximum density (Cin1
(00%) is the limit T at which the output density of the image is sufficiently secured.
The C value may be calculated. The limit TC obtained in this way
The value becomes the threshold value B1 (this is also strictly determined in consideration of the control delay).

The target value adjusting unit 17 constantly monitors the detection value of the toner density sensor 7 of each color developing device 3. In this monitoring, for example, a predetermined sampling interval (for example, 0.1
The detection value is sampled for each second, and the toner density TC in which the detection noise is suppressed can be obtained by ignoring the abnormal value and moving average processing similar to the above-described conventional apparatus.

At the time of high-density image continuous printing or the like, as shown by the graph 100 in FIG. 2, the detected toner concentration TC decreases as time passes as described above. For example, when the target value adjusting unit 17 detects that the toner concentration TC becomes smaller than the threshold value B1 at time t1, the target value adjusting unit 17 influences the developability (that is, the ease with which the toner is transferred from the carrier to the photoconductor). The control target value of the developing potential is changed by a predetermined amount .DELTA.E1 in the direction of decreasing the developing potential width (the larger the width, the higher the developing property). As a result, the developability tends to decrease, that is, the density of an image formed with the same toner density tends to decrease.
By the control, the supply of toner is increased, that is, the toner concentration TC is increased. Therefore, even for an input image having a higher density than the ADC control reference density (for example, Cin = 60%), a toner density sufficient to obtain an output density corresponding to the input density is secured.

Incidentally, the adjustment amount ΔE1 of the developing potential at this time
Is a relatively small value (for example, about 5V) that is set so that the change in the density of the output image on the recording medium before and after the adjustment with respect to the same input density falls within the range that cannot be seen by human eyes. The value of this adjustment amount ΔE1 may be obtained in advance by experiments or the like.

Now, when the developing potential is adjusted in this way,
The toner concentration TC once rises to become the threshold value B1 or more, but when the printing is further continued, the toner concentration TC decreases again and exceeds the threshold value B1. Then, the target value adjusting unit 17 tries to increase the toner density by changing the target value of the developing potential as described above.
If printing is further continued and such a development potential change is repeated a number of times, a situation in which a decrease in toner concentration TC cannot be suppressed by such a minute amount (ΔE1) potential change eventually occurs (time t2). . In this case, the toner concentration TC
Becomes smaller than the threshold value B1 and the output density of the high density input image is reduced (density reproducibility is deteriorated), but image quality defects such as BCO (and damage to the machine that may occur with it) are still generated. The toner concentration TC does not decrease until it occurs.

However, if printing continues from this state,
The toner concentration TC reaches the threshold value A1 (time t3),
The likelihood of causing such a serious defect increases. In such a case, the target value adjusting unit 17 first determines the ADC.
The control target value 6a is changed by a predetermined amount ΔA in the direction of increasing the formation target density of the control toner image 20. As a result, the ADC control increases the toner supply in order to increase the density of the control toner image, so that the toner density TC increases and the occurrence of a serious defect can be prevented. At this time, by changing the ADC control target value 6a, the toner density is sufficiently restored to the normal side (in this case, the toner density increasing side), as in the case where the toner density is equal to or lower than the predetermined lower limit value in the conventional device. The amount is changed by a predetermined amount ΔA that is determined so that it can be changed.

By changing the ADC target value 6a, the occurrence of image quality defects and the like is once avoided, but if the printing is further continued, the toner density TC will decrease again and the threshold value A1 will be reached again. In this case, the ADC target value 6a is further changed in the same direction as the previous time by a predetermined amount. However, while repeating this, due to the accumulation of changes, the ADC target value 6
a is outside the proper range, or the ADC target value is 6
By changing the predetermined amount of “a”, the decrease of the toner concentration TC can no longer be suppressed. In this case, in this embodiment, the toner concentration is changed to the normal side (in this case, the toner concentration is increased) by changing the control target value of the developing potential by the predetermined amount ΔE2. In this case, the changing direction of the developing potential is a direction in which the developing potential width is reduced as in the case of the threshold value B1. However, the amount of change in development potential ΔE2
In order to avoid occurrence of serious problems such as image quality defects with high certainty, the change amount ΔE1 at the threshold value B1 is
Is larger than (for example, ΔE1 = 5V, Δ
E2 = 15V). That is, in the above-described control when the threshold value B1 is reached, the potential change amount ΔE is set in order to suppress the change in image quality.
Although 2 is set to a small value, the change amount ΔE2 is set to a large value here in order to prevent image quality defects such as BCO. Therefore,
This change in ΔE2 causes a large change in the output image quality to some extent, but it is possible to deal with such a change by setting up the gradation table 11a, which is preferable (details will be described later). This potential change amount .DELTA.E2 is set in advance by experiments or the like.

The basic concept of the control in the case where the toner concentration TC of the developer in the developing device 3 is decreased by the high-density image continuous printing or the like has been described above. On the contrary, the toner concentration TC continues to increase. In such a case, the control method similar to the control of the apparatus disclosed in JP 2001-154427 A may be used.

Next, with reference to FIGS. 3 and 4, an example of the overall control by the target value adjusting unit 17 including the case where the toner density is increased will be described.

FIG. 3 is a flow chart showing the procedure of this control processing. As shown in FIG. 4, this process is performed in addition to the threshold value B1 shown in the example of FIG. 2 above, and further before the toner density upper limit value A2 for preventing image quality defects and the like (on the normal toner density side). The control when the preventive threshold value B2 for ensuring the density reproducibility is provided is shown. The provision of this threshold value B2 is basically the same as the threshold value B1 on the lower limit value A1 side, and is based on the following consideration.

That is, the upper limit value A2 is for preventing image quality defects (and mechanical damage that may occur with it) such as layer formation defects, and is intended to ensure good density reproducibility. Toner density T
When C reaches near A2, for example, the input image density C
There is a possibility that the density reproducibility of the output for low values of in, etc. may be poor. Further, the toner concentration TC is the upper limit value A2.
As in the case where the lower limit value A1 is reached, the control in the case of reaching the target value is to greatly change the ADC target value 6a or the target value of the developing potential for the purpose of avoiding the image quality defect with high certainty. Change the image quality of. Therefore, here, the upper limit threshold value B2 of the toner density TC that provides a good output density over almost the entire input image density Cin is provided on the normal side (lower toner density side) than the upper limit value A2. In the range between these threshold values, an output image with good density reproducibility can be obtained over almost the entire area of the input image density. When the toner concentration TC reaches the threshold value B2, the control for returning the toner concentration TC to the normal side is similar to the case of the above-mentioned threshold value B1 so that the image quality hardly changes visually. A small change in development potential is sufficient. Here, the range between the threshold value B2 thus determined and the above-mentioned B1 is referred to as the density ensuring range R1, and the toner density lower limit value A1 for preventing image quality defects such as BCO and layer formation failure. The range between the upper limit value A2 and the upper limit value A2 is called a defect prevention range R2.

The control here will be described below with reference to FIG. When the driving of the developing device 3 is started, the target value adjusting unit 17 acquires the toner concentration TC detected by the toner concentration sensor 7 of the developing device 3 at predetermined detection intervals (S
10), it is determined whether or not this has deviated from the density ensuring range R1 shown in FIG. 4 (S12). If the toner density TC is within the density ensuring range R1 (the determination result in S12 is N (negative)), the target value adjusting unit 17 does nothing and waits until the next toner density TC detection (S10). The toner concentration TC changes due to continuous printing or the influence of environmental conditions (temperature, humidity, etc.), and step S1
When it is detected that the boundary B1 or B2 of the density ensuring range R1 is reached in 2 (determination result of S12 is Y (affirmative)), the toner density TC is in the center side (that is, in a more normal direction) in FIG. The developing potential is changed by the above-mentioned predetermined amount .DELTA.E1 so as to change (S14). The direction of this change is to decrease the developing potential width when the low density threshold B1 is reached, and to increase the developing potential width when the high density threshold B2 is reached. is there. Here, the amount of potential to be changed is set to the same amount ΔE1 when TC reaches the threshold value B1 on the low concentration side and when it reaches the threshold value B2 on the high concentration side. Of course, it is also possible to individually set the change amount in consideration of the characteristics.

The control in step S14 is intended to secure an appropriate output image density over a wide range of the input image density Cin, but if this control is repeated many times, its effect is diminished. Sometimes. Therefore, after step S14, it is determined whether the toner density TC has been improved by the control, that is, whether the TC has returned to within the density ensuring range R1 (S16).
In practice, it takes some time after the control in step S14 is performed until the effect appears as a change in the toner concentration TC, so the determination in step S16 is performed in a predetermined required time (experiment). Etc.) and then execute. When the toner density TC is high, the toner density TC does not decrease unless the toner is actually consumed even if the toner supply is reduced. Therefore, more preferably, the cumulative toner consumption after step S14 is determined by image counting or the like. The amount is calculated, and after confirming that it exceeds a predetermined value, the confirmation in step S16 is performed.

In step S16, when the effect of the control in step S14 is confirmed (the determination result in S16 is Y),
Returning to step S10, the above operation is repeated.

When the effect of the control is not recognized in step S16 (the judgment result in S16 is N), the target value adjusting unit 1
In step 7, the control for ensuring the density reproducibility is canceled, and the control is shifted to the control for preventing the image quality defect such as BCO. That is, first, the toner concentration TC detected by the toner concentration sensor 7
Is acquired at predetermined intervals (S18), and it is determined whether or not this density TC is within the defect prevention range R2 (S18).
20). If it is within the range (the determination result of S20 is N), the target value adjusting unit 17 does nothing and waits until the next toner concentration TC detection (S18). When it is detected that the toner concentration TC has changed due to continuous printing or the like and the defect prevention range R2 has reached the lower limit value A1 or the upper limit value A2 in step S20 (the determination result in S20 is Y (affirmative)), the ADC is first detected. The target value 6a is changed by the above-mentioned predetermined amount ΔA (S24). AD
The direction in which the C target value 6a is changed is to return the toner density TC to the center (normal) side. More directly, when the lower limit value A1 is reached, the density of the control toner image 20 is increased. When the upper limit value A2 is reached, the density of the control toner image 20 is lowered. Note that the change amount ΔA at this time may use different values depending on the characteristics in each case, when the lower limit value A1 is reached and when the upper limit value A2 is reached.

The toner concentration TC returns to the normal side by the control in step S24, and the control of the target value adjusting unit 17 returns to step S18. Here, when the toner density control by changing the ADC target value 6a is repeated, the ADC target value 6a is out of the predetermined appropriate range, and the ADC is further exceeded.
If the target value 6a cannot be changed, or if the target value 6a is changed more, the image quality or the like will be adversely affected. Therefore, the target value adjustment unit 17 monitors the cumulative value of the change amount of the ADC target value 6a and monitors whether it is within a predetermined amount (S22). When it is detected that the toner concentration TC is out of the defect prevention range R2 (S20)
When it is determined that the cumulative change amount exceeds the predetermined amount when the ADC target value 6a is changed (the determination result in S22 is N), the developing potential is changed by the predetermined amount ΔE2 (S26). The direction of change is to decrease the developing potential width when the toner concentration TC reaches the lower limit A1, and to increase the developing potential width when the toner concentration TC reaches the upper limit A2. The change amount .DELTA.E2 at this time may be a different value depending on the characteristics in each case depending on whether the lower limit value A1 is reached or the upper limit value A2 is reached.

Change amount ΔE of developing potential in step S26
Since 2 is large enough to cause a change in image quality, this control is to prevent or reduce such a change in image quality.
After the execution of step S26, the gradation table correction unit 9 sets up (regenerates) the gradation table 11a (S28). This setup process may be performed by a conventional general image forming apparatus. That is, for example, under the changed developing potential, a patch pattern image of each color multi-gradation for gradation setup is provided on the photoconductor 2 or the intermediate transfer member 4.
It suffices to obtain the relationship between the input density and the output density for realizing a predetermined gradation reproduction characteristic from the read result, which is formed above and read by the ADC sensor 5. This step S2
After steps 4 and S26, the toner concentration TC has returned to within the defect prevention range R2, and thereafter, for example, step S1.
It is sufficient to return to 0 and repeat the above control steps. Even if the developing potential is changed in step S26, the toner density T
Since there is a possibility that the change in C will not be improved, the toner concentration TC is monitored after execution of step S26 in consideration of the improvement effect, and TC is not returned to the defect prevention range R2 even if the potential is changed. In this case, it is also preferable to perform control such as stopping driving of the developing device 3 and the like.

The target value adjusting unit 17 causes the ADC
If the adjustment of the target value 6a or the control target value of the developing potential is left as it is, a problem occurs, so the reset processing unit 18
At a predetermined timing (when the image forming apparatus 1 is powered on,
When the predetermined number of images are formed after the power is turned on, when the developer is replaced after the image forming job has continued for 4 hours, the target values are set to the predetermined initial values at predetermined timings. Reset to. The reset process itself may be the same as that used in the conventional image forming apparatus.

Heretofore, an example of the control processing of the target value adjusting section 17 has been described. According to this control, the threshold value for ensuring the density reproducibility inside the threshold values A1 and A2 for preventing the image quality defect such as BCO (and the mechanical damage that may occur accompanying it) By providing B1 and B2 and finely controlling the toner density TC based on these threshold values B1 and B2, density gradation reproducibility can be maintained over a wide range of the input image density. Also,
The adjustment amount of the control target value (development potential) at the threshold values B1 and B2 is smaller than the adjustment amount at the threshold values A1 and A2, and the change in the output image quality due to the adjustment is visually negligible. Further, before the toner density TC reaches the threshold value A1 or A2, the threshold value B1 or B2
Since the toner density TC can be improved by the fine adjustment in step 1, it is possible to reduce the frequency of drastic control target changes that affect the image quality at the threshold value A1 or A2. Further, since the gradation table 11a is set up after the development potential at the threshold value A1 or A2 is changed, it takes some time for the setup, but before and after the setup (that is, before and after the large change in the development potential). It is possible to prevent or reduce the change in the image quality of (1).

Next, another example of control by the target value adjusting unit 17 will be described with reference to FIG. 5, steps having the same processing contents as those in the procedure of FIG. 3 are denoted by the same reference numerals, and the description thereof will be omitted.

In the control of FIG. 5, when it is determined in step S12 that the toner density TC is out of the density ensuring range R1, the image density of the image to be image-formed (area coverage. All pixels in the image-forming area). (S30), and the change amount of the developing potential is switched according to the image density.

That is, when the toner concentration TC decreases and reaches the threshold value B1, the amount of toner is small, but the higher the density of the image to be image-formed, the faster the toner consumption. Therefore, there is a possibility that the toner decrease (that is, the toner concentration TC decrease) cannot be effectively suppressed by adjusting the developing potential of a small amount (ΔE1) in step S14. In some cases, the threshold value A may be rapidly changed as it is.
The situation may exceed 1. Therefore, step S
If it is determined in 12 that the threshold value B1 is reached, in step S30 the image density of the formation target is set to a predetermined threshold value α.
(For example, the density is 50%), and if the image density is higher than the threshold value, the process proceeds to step S32, and the developing potential is changed by the change amount ΔE3 larger than ΔE1 (the direction of change is the same as in step S14). . Change amount ΔE3 at this time
May be about the same as .DELTA.E2. Since the amount of change in the potential change in step S32 is large, the image quality of the output image is likely to change before and after the change. Therefore, preferably, after step S32, the same gradation characteristic setup processing as in step S28 is performed (S34). If the image density is equal to or lower than the predetermined threshold value α when the toner density TC reaches the threshold value B1, the process may proceed to step S14.

Although the case where the toner density TC is low has been described above, when the TC rises and reaches the threshold value B2,
You can think in the opposite way. That is, in this case, the toner concentration TC is getting higher and higher, and if the image to be formed has a low density, the toner consumption is small. Therefore, even if the developing potential is changed by a small amount, the increase of the toner concentration TC is effectively suppressed. Can not. Therefore, in this case, when the image density is smaller than the predetermined threshold value β, the process proceeds to step 32,
The developing potential is adjusted to a greater extent than in step S14.

As described above, in the control of FIG. 5, the toner concentration TC can be controlled according to the density of the image to be image-formed so that the occurrence of defects can be prevented more effectively.

The preferred embodiment of the present invention has been described above. In the above example, the control (controlling the threshold values B1 and B2) for suppressing the output density insufficiency of the high density image during continuous printing of the high density image is performed by adjusting the developing potential. Instead of this, the ADC target value (controlling toner image density target value) may be adjusted. However, since the change of the development potential has a more direct effect on the developability than the change of the ADC target value, the control response becomes better when the development potential is adjusted.

In the above example, the threshold values A1, A
Although the developing potential of the developing device 3 was changed in the control of No. 2, B1 and B2, the developing potential is defined by the developing potential width (generally speaking, the potential difference between the developing device 3 and the photoconductor 2 is defined as described above. Therefore, instead of the developing potential of the developing device 3, the potential of uniform charging of the photoconductor 2 may be changed.

In the above example, the threshold values B1 and B2
Although the change amount of the developing potential in the control relating to the above is set to the predetermined value ΔE1, it is also preferable to change the change amount according to the changing speed of the toner concentration TC instead. That is, for example, when the toner density TC reaches the threshold value B1 or B2, T
By increasing the change amount of the development potential as the changing speed of C is higher, the change of the toner concentration TC can be effectively suppressed.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an image forming apparatus according to an embodiment.

FIG. 2 is a diagram for explaining the concept of control when the toner concentration is increased.

FIG. 3 is a flowchart illustrating an example of toner density control according to the exemplary embodiment.

FIG. 4 is a diagram for explaining the content of control in FIG.

FIG. 5 is a flowchart showing a modified example of toner density control in the embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 image forming apparatus, 2 photoconductor, 3 developing device, 4 intermediate transfer body, 5 ADC sensor, 6 toner supply control section, 6
a control toner image density target value (ADC target value), 7
Toner concentration sensor, 8 Toner concentration limit control unit, 9
Tone table correction unit, 10 image signal, 11 image processing unit, 11a tone table, 12 control toner image pattern generator, 13 pulse width modulation circuit, 14 laser driver, 15 exposure device, 16 charger, 17 target value Adjusting unit, 18 Reset processing unit.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Satoshi Tanaka             Fuji Zero, 2274 Hongo, Ebina City, Kanagawa Prefecture             Co., Ltd. Ebina Office F-term (reference) 2H027 DA09 DD07 DE02 EA01 EA02                       EA05 EA06 EA18 EB04 EC03                       EC06 EC10                 2H077 AA00 DA02 DA05 DA10 DA42                       DA47 DA63 DB02 DB08 DB12                       DB13 EA03 GA13

Claims (7)

[Claims] 1. An electrostatic latent image on a photoconductor is obtained by transferring a toner in a developer contained in a developing device to a photoconductor while applying a development potential controlled to a predetermined potential target value. An image is developed to form a toner image on the photoconductor, and the toner image formed on the photoconductor is finally transferred onto a predetermined recording medium to form an image on the recording medium. An image forming apparatus, wherein the developing device detects the image density of a control toner image formed based on predetermined control toner image data and makes the detected image density approach a predetermined control toner image density target value. A toner supply control means for controlling the toner supply to the toner, a toner density detection means for detecting the toner density of the developer, and a toner density control means necessary for ensuring density reproducibility of image formation for an input image showing a predetermined image density. 1st corresponding to toner density
When the toner density has a threshold value and the toner density detected by the toner density detecting means becomes less than or equal to the first threshold value, the toner supply to the developing device is increased by the control of the toner supply controlling means. A target value adjusting means for changing a target value of a predetermined image forming condition in a direction, wherein the target value adjusting means is such that the toner density detected by the toner density detecting means is equal to or less than the first threshold value. In this case, the predetermined first change amount determined so that no visually significant difference is recognized in the image density of the image forming result before and after the change of the target value of the predetermined image forming condition, An image forming apparatus characterized by changing a target value of a predetermined image forming condition. 2. The image forming apparatus according to claim 1, wherein the target value adjusting unit corresponds to a toner density that causes an image quality defect in an output image and has a predetermined second value lower than the first threshold value. A threshold value is further provided, and when the toner density detected by the toner density detecting means becomes equal to or lower than the second threshold value, the target value of the predetermined image forming condition is set by the toner supply control means. An image forming apparatus, characterized in that a predetermined second change amount larger than the first change amount is changed in a direction in which the toner supply to the developing device is increased by the control. 3. The image forming apparatus according to claim 1, wherein the target value adjusting unit is configured such that the toner concentration detected by the toner concentration detecting unit is equal to or lower than the first threshold value. Further, the image forming apparatus is characterized in that the image density of the image data of the image forming object is examined, and the first change amount is made variable according to the image density. 4. The image forming apparatus according to claim 1, wherein the target value adjusting unit is configured to, in advance, advance if the toner concentration detected by the toner concentration detecting unit is less than or equal to the first threshold value. The predetermined image forming condition is changed by an amount corresponding to the registered change rate of the toner density and the predetermined image forming condition in accordance with the information on the corresponding relationship. Image forming apparatus. 5. The electrostatic charge on the photoconductor is transferred by transferring the toner in the developer housed in the developing device to the photoconductor while applying the development potential controlled to a predetermined potential target value. The latent image is developed to form a toner image on the photoconductor, and the toner image formed on the photoconductor is finally transferred onto a predetermined recording medium to form an image on the recording medium. An image forming apparatus for detecting the image density of a control toner image formed based on predetermined control toner image data, and developing the image density so that the detected image density approaches a predetermined control toner image density target value. Toner supply control means for controlling toner supply to the container, toner concentration detecting means for detecting toner concentration of the developer, and toner concentration of the developer are predetermined defects for preventing image quality defects of output images. If you are out of the prevention range, A first control unit that changes a target value of a predetermined image forming condition in a direction in which the toner concentration returns to the defect prevention range under the control of the toner supply control unit; and is set inside the defect prevention range, It has a density ensuring range corresponding to the toner density range necessary to ensure the density reproducibility of the image formation result of the input image in the predetermined image density range,
When the toner density of the developer is out of the density ensuring range, the target value of the predetermined image forming condition is changed by the control of the toner supply control unit so that the toner density returns to the density ensuring range. A second control means for changing the target value by a predetermined change amount determined so that no visually significant difference in image density of the image formation result is recognized before and after the change. An image forming apparatus provided. 6. The image forming apparatus according to claim 5, wherein the second control unit changes a potential target value of the developing potential as a target value of the predetermined image forming condition. Image forming apparatus. 7. The image forming apparatus according to claim 5, wherein a gradation correction unit that performs gradation correction on the input image based on predetermined gradation characteristic information, and a predetermined gradation pattern image are formed. However, when the predetermined image forming condition is changed by the gradation setting means for adjusting the gradation characteristic information of the gradation correction means based on the density detection result of the gradation pattern image, and the predetermined image forming condition is changed by the first control means. An image forming apparatus comprising: a means for causing the gradation characteristic setup means to execute adjustment processing of the gradation characteristic information.