JPH07199561A - Image forming device - Google Patents

Abstract

PURPOSE:To provide a copying machine capable of obtaining a proper image density even if a developer is deteriorated. CONSTITUTION:The set value of a developing potential at the time of forming a reference image for detecting a P-sensor is switched based on the result of the comparison of target value Vref of a T-sensor 93 with its threshold. The developing potential can be switched when the target value Vref consecutively becomes the threshold or above several times or after the number of copies from the point of time that the target value Vref becomes the threshold or above exceeds a prescribed number. Further, the threshold can be changed over whenever the number of copies is increased by a prescribed number, or the threshold can be set when the developing potential can not be or can be switched by a prescribed umber of times. Moreover, control for correcting the voltage of an exposing lamp 3 and the conditions of the electrification and developing bias of an electrifier 8 can be changed over every time the developing potential is switched over.

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 particularly to controlling toner density in a developing device of the image forming apparatus.

[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 by a two-component developer (hereinafter referred to as a developer) and the visualized image is transferred to a transfer paper.

Since the weight ratio of the toner of the developer in the developing device to the carrier (hereinafter referred to as toner concentration) affects the image density, it is desirable to keep the toner concentration constant. A method is known in which a toner density detecting means for detecting the toner density is provided and the toner replenishment in the developing device is controlled based on the result of comparing the output value of the toner density detecting means with a target value. For example, in Japanese Patent Laid-Open No. 4-356080, a toner as toner concentration detecting means is provided every time when the number of recorded transfer sheets increases by a predetermined number so that the toner concentration is maintained even if the carrier of the developer changes with time. It has been proposed to change the control voltage for controlling the strength of the magnetic field generated by the concentration sensor.

However, even if the toner density is kept constant, the image density is affected by changes in the physical properties of the developer over time, environment such as temperature and humidity, and development conditions. Therefore, a reference image for detecting the image density is formed on the image carrier by a predetermined developing potential (difference between the charging potential of the surface of the image carrier and the developing bias voltage), and the reflection type photo as the image density detecting means. A sensor (hereinafter referred to as P sensor) detects the image density of the reference image and corrects the target value of the toner density based on the detection result, so that a more stable image can be obtained.

[0005]

However, in the above-mentioned conventional image forming apparatus, as shown in FIG. 2, the charging ability is lowered by the use of the developer, and the output value of the P sensor is normal even if the toner concentration is low. Since the value is indicated, the image density is reduced. For example, when the control point of the P sensor is set to the developing potential 300V, the ratio Vsp of the output value of the P sensor is set to Vsp.
/Vsg=0.1 (Vsp: P sensor output value of the reference image,
Vsg: the output value of the P sensor in the non-image portion), the developing potential-ID (image density) curve shown by the curve is obtained when the developer has the charging ability.
Although the target ID can be obtained, if the developer deteriorates and the charging ability decreases, the development potential-ID as shown by the curve
There is a problem that a desired ID cannot be obtained in a region where the development potential is large even if a curve is formed and control by the P sensor is performed.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of obtaining an appropriate image density even if the developer deteriorates. is there.

[0007]

In order to achieve the above object, the invention of claim 1 provides a developing device for supplying a toner to a latent image formed on an image carrier to visualize the latent image. Toner replenishing means for replenishing toner to the developing portion of the developing device; toner concentration detecting means for detecting toner concentration in the developing device;
A toner replenishment control unit that controls the toner replenishment unit based on a result of comparison between an output value of the toner concentration detection unit and a target value, and a reference latent image for image density detection formed on the image carrier. A reference image forming unit that visualizes the reference latent image to form a reference image, an image density detecting unit that detects the image density of the reference image, and the target value is corrected based on the output value of the image density detecting unit. In the image forming apparatus including the target value correcting unit, a developing potential control unit that switches the set value of the developing potential when the reference image is formed based on the result of comparison between the target value and a preset threshold value. Is provided.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the set value of the developing potential is switched when the target value continuously exceeds the threshold value a plurality of times. It is characterized by.

According to a third aspect of the present invention, in the image forming apparatus according to the first aspect, after the number of transfer sheets on which an image is formed exceeds a predetermined number after the target value exceeds the threshold value. The setting value of the developing potential is switched.

Further, the invention of claim 4 is based on claims 1 to 3.
In the image forming apparatus, the set value of the threshold value is switched each time the number of transfer sheets on which an image is formed increases by a predetermined number.

The invention of claim 5 relates to claims 1 to 3.
In the image forming apparatus, the setting value of the threshold value is switched every time the setting value of the developing potential is switched.

Further, the invention of claim 6 is the same as claims 1 to 5.
In the image forming apparatus, the setting value of the developing potential is not changed over a predetermined number of times.

The invention of claim 7 is the same as claims 1 to 5.
In the image forming apparatus, the threshold value is not set when the set value of the developing potential is switched a predetermined number of times.

Further, the invention of claim 8 comprises an exposure condition correcting means for correcting an exposure condition at the time of forming a latent image on the image carrier based on an output value of the image density detecting means. In the image forming apparatus, the correction condition in the exposure condition correction means is switched every time the set value of the development potential is switched.

According to a ninth aspect of the invention, based on the output value of the image density detecting means, the charging condition of the image carrier and the developing bias voltage application condition at the time of developing the latent image on the image carrier are corrected. In the image forming apparatus according to the first aspect of the present invention, the correction condition in the correction unit is switched each time the set value of the development potential is switched.

[0016]

As described above, the use of the developer reduces its charging ability, and the output value of the P sensor shows a normal value even with a low toner concentration (see the curve in FIG. 2).
Is decreased, and the desired ID cannot be obtained even if the P sensor is used for control. In view of this, in the invention of claim 1, the developing potential at the time of forming the reference image is based on the result of comparison between the target value of the toner concentration detecting means and the preset threshold value by the developing potential control means. The setting value is being changed. This allows
The relationship between the development potential and the ID is as shown by the curve in FIG. 2, and even if the development potential is large, the target ID
Will be obtained. In addition, the toner can be used with a high toner density.

According to the second aspect of the present invention, when the target value exceeds the threshold value a plurality of times in succession, the set value of the developing potential is switched. Therefore, the variation of the developing potential at the time of forming the reference image is varied. It becomes possible to prevent false detection.

In the invention of claim 3, the set value of the developing potential is switched after the number of transfer sheets on which an image is formed exceeds a predetermined number after the target value becomes equal to or more than the threshold value. Since this is performed, it is possible to prevent variations in development potential and erroneous detection when forming a reference image.

According to the fourth aspect of the present invention, the set value of the threshold value is switched each time the number of transfer sheets on which an image is formed increases by a predetermined number, so that the number of transfer sheets gradually increases and development is performed. Even when the physical properties of the agent change, the toner concentration does not increase more than necessary over time.

According to the fifth aspect of the present invention, the set value of the threshold value is switched every time the set value of the developing potential is switched, so that the toner concentration does not increase more than necessary over time.

According to the sixth aspect of the present invention, since the setting value of the developing potential is not switched a predetermined number of times or more, the developing potential does not fluctuate excessively.

According to the seventh aspect of the invention, the threshold value is not set when the set value of the developing potential is switched a predetermined number of times, so that the developing potential does not fluctuate excessively.

In the invention of claim 8, the correction condition in the exposure condition correction means for correcting the exposure condition when the latent image is formed on the image carrier is switched every time the set value of the developing potential is switched. Even when the development potential is switched and the development gamma becomes large, the exposure condition is not excessively corrected.

According to a ninth aspect of the invention, the correction condition in the correction means for correcting the charging condition of the image carrier and the developing bias voltage application condition at the time of developing the latent image on the image carrier is set to the developing potential. Since the setting value is switched every time it is switched, even when the development potential is switched and the development gamma is increased, the charging condition and the developing bias voltage applying condition are not excessively corrected.

[0025]

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 photoconductor drum 7, a charging device 8 that uniformly charges the surface of the photoconductor drum 7, a developing device 9 that visualizes the electrostatic latent image formed on the photoconductor drum 7, and a visualization device. A belt type transfer device 10 for transferring the formed image to a transfer sheet, a cleaning device 11 for removing transfer residual toner on the photoconductor drum 7, a charge removal device 12 for removing residual charge on the photoconductor drum 7, and the like are provided. ing.

The developing device 9 includes a toner hopper 91 and a toner replenishing roller 92 as toner replenishing means for replenishing the toner in the developing portion, and a toner concentration sensor as a toner concentration detecting means for detecting the toner concentration of the developer in the developing portion. (Hereinafter referred to as T sensor) 93, a developing sleeve 94 as a developer carrying member that carries the developer in the developing unit and supplies the toner onto the photosensitive drum 7, and the developer on the developing sleeve 94 is regulated. It is equipped with a doctor blade 95 and the like. To replenish the toner from the toner hopper 91 into the developing unit, the output value VT of the T sensor 93 is compared with the target value Vref, and the toner replenishing roller 9 is based on the comparison result.
2 is driven and controlled. Toner replenishment control means for performing this control include CPU, RAM, ROM, and
A main control unit (not shown) including an I / O interface or the like can be used.

By the toner replenishment control means, the toner density, which is one of the factors affecting the image density, can be kept constant. However, the image density depends on changes in the physical properties of the developer over time, temperature and humidity, etc. And the developing 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 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 are used to set a predetermined reference density on the photosensitive drum 7. A reference image is formed, and the density of the reference image is reflected by a reflection type photo sensor (hereinafter,
The target value Vref of the T sensor 93 is controlled to be corrected based on the detection result, so that a more stable image can be obtained.

FIG. 3 is a characteristic diagram showing the relationship between the toner concentration TC and the output value (V) of the T sensor 93. When the initial developer is charged, the developing device 9 is idled for a predetermined time, the output value of the T sensor 93 is set to a predetermined value, and then the copying machine is used. In this embodiment, A in FIG.
As shown by the dot, the toner concentration TC of the initial developer is 2.5.
wt%, and the initial output value of the T sensor 93 is set to 2.5V. The toner replenishment control is performed according to the rules shown in the matrices of Tables 1 and 2. For example, when the output value VT of the T sensor 93 is 2.6 V, the output value Vsp of the P sensor 14 is
If /Vsg=0.13, the next target value Vref of the T sensor 93 is Vref = VT + correction value = 2.6−0.1 = 2.5V, and toner is not replenished until the next detection by the P sensor 14. , And the toner replenishment time corresponding to VT-Vref (Vref = 2.5V) in Table 2. In this embodiment, the toner density TC is detected by the T sensor 93 for each transfer sheet, but the image density of the reference image is detected by the P sensor 14 by the transfer belt method. Since it is adopted, it is performed every 1 job.

[0029]

[Table 1] (Hereafter, margin)

[0030]

[Table 2] (Hereafter, margin)

By the way, in the above toner density control, as described above, the use of the developer lowers the charging ability, and the output value of the P sensor shows a normal value even at a low toner density, so that the image density decreases. Will end up. For example, as shown by the solid line in FIG. 4, as the number of copies increases and the usage time of the developer increases, the coating film of the carrier of the developer becomes severely scraped, and the charging ability of the developer decreases. Then
The toner density of the developer decreases, and the image density ID decreases.

Therefore, in the present embodiment, a developing potential control means for switching the setting value of the developing potential when forming the reference image based on the result of comparing the target value Vref of the T sensor 93 with a preset threshold value. It is provided. The main control section of the apparatus main body can be used as the developing potential control means.

FIG. 5 shows the relationship between the toner density TC and the image density ID at the control start point (point C in FIG. 4) of the development potential control. As can be seen from FIG. 5, the toner density TC = 1.5 wt% and the image density ID =
Since 1.3 (target image density), TC =
The output value of the T sensor 93 corresponding to 1.5 wt% = 3 V,
It was set as a threshold value of the target value Vref of the T sensor 93 for switching the developing potential.

Then, the target value Vref of the T sensor 93 is 3
When the voltage becomes V or more, the set value of the developing potential at the time of forming the reference image for P sensor detection is changed from 300V to 240V.
Switch to. FIG. 6 shows a flowchart of this development potential control. If P sensor is not detected, T
When the P sensor is detected while leaving the target value Vref of the sensor 93 unchanged, a new target value Vref is determined (steps 1 to 3). When the threshold value is 3 V or less, the toner density control is performed using the determined Vref value (steps 4 and 5). On the other hand, if the threshold value is larger than 3V, Vre
Reset f to the threshold value of 3V, and set P at the next P sensor detection.
The setting value of the developing bias voltage at the time of forming the reference image for sensor detection is increased by 60V (steps 6 to 9). Specifically, when the surface potential of the photosensitive drum 7 is -600V, the developing bias voltage is changed from -300V to -360V. Then, the above control is repeated.

As described above, according to this embodiment, the set value of the developing potential for forming the reference image is switched based on the result of comparison between the target value Vref of the T sensor 93 and the preset threshold value. Development potential and ID
2 becomes a curve shown in FIG. 2, and the target ID can be obtained even in a region where the development potential is large. Further, the toner density T after the control of FIG.
Changes in C, image density ID, developer charging ability, and scraping of the developer carrier coat film are as shown by the broken lines in FIG. 4, and the target ID can be maintained and the developer charging ability is reduced. The amount of abrasion of the carrier coat film is reduced, and the life of the developer is extended.

In the above embodiment, when the target value Vref of the T sensor 93 exceeds the threshold value 3V even once, the set value of the developing potential at the time of forming the reference image is switched, but Vref is set a plurality of times. For example, when the threshold value 3V is exceeded five times in a row, the set value of the developing potential at the time of forming the reference image may be switched.

FIG. 7 shows a flow chart of control in the case of this modification. In steps 1 to 8, the same control as in FIG. 6 is performed. Then, the newly determined target value Vref
Is larger than the threshold value 3V and Vref is set to the threshold value 3V again, it is judged whether or not the target value Vref becomes larger than the threshold value 3V for five consecutive times. If NO, the developing bias voltage is not changed. Control is continued, and if YES, P
The set value of the developing bias voltage at the time of forming the reference image for sensor detection is increased by 60V (steps 9 to 11).

As in this modification, the setting value of the developing potential at the time of forming the reference image is switched by Vref a plurality of times.
For example, by performing the operation 5 times consecutively when the threshold value exceeds 3V, it becomes possible to prevent the development potential variation and erroneous detection at the time of forming the reference image. Therefore, the toner density increases due to these variation and erroneous detection. Therefore, there is no possibility of background stains and toner scattering. In particular, when the charging device is of the charging roller type, the variation of the charging potential due to environmental changes is larger than that of the wire type and erroneous detection is more likely to occur, so the control of this modification is effective.

Further, when the target value Vref of the T sensor 93 exceeds the threshold value 3V and the number of copies on the transfer paper reaches a predetermined number, control is performed so as to switch the set value of the developing potential at the time of forming the reference image. You may. FIG. 8 shows a flowchart of a control example in this case. In steps 1 to 8, the same control as in FIG. 6 is performed. Then, the newly determined target value Vref is larger than the threshold value 3V and Vref is set to the threshold value 3
After resetting to V, it is judged whether or not the number of copies on the transfer paper has reached a predetermined number (for example, 3000 in this example), and if NO, the developing bias voltage is not changed without changing. If the control is continued and if YES, the set value of the developing bias voltage at the time of forming the reference image for P sensor detection is increased by 60V (steps 9 to 11).

As in this modification, the setting value of the developing potential at the time of forming the reference image is switched, and after the target value Vref of the T sensor 93 exceeds the threshold value 3V, the number of copies on the transfer sheet becomes a predetermined number. As in the case of the modification shown in FIG. 7, the toner density increases due to variations in the development potential at the time of forming the reference image, erroneous detection, and the like, causing background stains and toner scattering. Nor.

By the way, in the above embodiment, when the physical properties of the developer are changed, the toner concentrations TC and T
The relationship with the output value VT of the sensor 93 changes over time. For example, the straight line in FIG. 9 is the initial developer, and the straight line is 60 k.
The straight line of the developer after copying one sheet is 120k.
The case of the developer after copying one sheet is shown. As can be seen from FIG. 9, when the threshold value of the target value Vref of the T sensor 93 is set to 3 V, the lower limit of the target toner density TC is 1.5 after copying 60 k sheets.
Not at wt%, but at about 2 wt% on the straight line, that is, even though the image density ID is sufficiently obtained, the set value of the development potential at the time of forming the reference image is switched, so that the toner is unnecessarily excessive. The density TC rises, causing problems such as background stains and toner scattering.

Therefore, in the above embodiment, instead of using a constant value as the threshold value of the target value Vref of the T sensor 93, this threshold value is set every time the number of copies on the transfer sheet increases as shown in Table 3. You may control so that it may switch. As a result, even when the number of copies is gradually increased and the physical properties of the developer are changed, the toner density does not increase more than necessary over time, and it is possible to prevent the occurrence of background stains and toner scattering. Like

[0043]

[Table 3]

In the above embodiment, as shown in Table 4, the set value of the threshold value of the target value Vref of the T sensor 93 is
Control may be performed such that the development bias voltage at the time of forming the reference image, that is, the development potential setting value is switched each time the set value is switched.

[0045]

[Table 4]

FIG. 10 shows a flowchart of control in the case of this modification. In steps 1 to 8, the same control as in FIG. 6 is performed. Then, the newly determined target value Vre
f is larger than the threshold value 3V, and this target value Vref is set to the threshold value 3V.
Then, the set value of the developing bias voltage at the time of forming the reference image for P sensor detection is increased by 60 V (step 9). Here, the threshold value of Vref after that is set to 3.
Set to 2V.

At the next P sensor detection, a new developing bias voltage 360V is used to form a reference image, and the P sensor 14 detects the image density of the reference image.
The target value Vref is corrected and newly determined based on the detection result of (step 10). Next, it is determined whether or not the newly determined target value Vref is larger than the threshold value 3.2V (step 11). When the threshold value is 3.2 V or less, the toner density control is performed using the determined Vref value (steps 12 and 13). On the other hand, if it is larger than the threshold value 3.2V, Vref is reset to the threshold value 3.2V, and in the next P sensor detection, the set value of the developing bias voltage at the time of forming the reference image for P sensor detection is further increased by 40V. (It means 100V increase from the initial value) (Step 14
~ 17). Here, the threshold value of Vref after that is set to 3.4V.

At the next P sensor detection, a new developing bias voltage of 400 V is used to form a reference image, the image density of the reference image is detected by the P sensor 14, and the target value is detected based on the detection result of the P sensor 14. Vref is corrected and newly determined (step 18). Next, it is determined whether or not the newly determined target value Vref is larger than the threshold value 3.4V (step 19). If the threshold value is 3.4 V or less, toner density control is performed using the determined Vref value (steps 20 and 21). On the other hand, when it is larger than the threshold value 3.4V, Vref is reset to the threshold value 3.4V to prepare for the next P sensor detection.

As in this modification, the threshold setting value of the target value Vref of the T sensor 93 is changed every time the setting value of the developing bias voltage, that is, the setting value of the developing potential at the time of forming the reference image is changed, so that the time elapses. The toner density does not increase more than necessary, and it becomes possible to prevent the occurrence of background stains and toner scattering.

Further, in the above embodiment, the setting value of the developing potential may be controlled not to be switched a predetermined number of times when the reference image is formed. For example, as shown in Table 5, the setting value of the developing bias voltage is switched, that is, the setting value of the developing potential is switched up to 3 times and is controlled not to be performed 4 times or more. As a result, it becomes possible to prevent problems caused by the development potential becoming excessively large.

Further, when the set value of the developing potential at the time of forming the reference image is switched a predetermined number of times,
It is also possible to control so as not to set the threshold value of the target value Vref of the T sensor 93. For example, as shown in Table 5,
When the setting value of the developing bias voltage is changed for the third time, that is, the setting value of the developing potential is changed, control is performed so as to eliminate the threshold value of Vref. This also makes it possible to prevent problems caused by the development potential becoming excessively large.

[0052]

[Table 5]

Further, in the above embodiment, in order to suppress the change of the image due to the change of the photoconductor with the passage of time and the contamination of the optical system, it is based on the result of the reference image by the P sensor 14 as is conventionally done. The exposure lamp voltage correction control for correcting the voltage applied to the exposure lamp 3 may be performed.
Conventionally, in this type of control, the correction voltage of the exposure lamp voltage is determined as shown in Table 6 on the assumption that the development potential-image density ID characteristic (development gamma) is maintained constant. That is, as shown by the curve in FIG. 11, the reference image is formed by using the reference document that is attached in advance at the initial setting, and the output value Vp of the P sensor 14 at this time is formed.
The developing bias voltage VBR is set so that / Vg becomes 1/2. Vp / Vg at this time is VRP / VRG = VLref
This value is used for the exposure lamp voltage correction control thereafter. Then, a reference image is formed with the developing bias voltage VBR for each 1k of the number of copies, and the image density is detected by the P sensor 14, and the ratio Vdat / VLref of the output value Vdat = Vdp / Vdg and VLref at the time of initial setting. Accordingly, the voltage applied to the exposure lamp 3 is corrected as shown in Table 6.

However, in the above-described embodiment, the development gamma is increased as shown by the curve in FIG. 11 by switching the development potential when the reference image for detecting the P sensor is changed. Therefore, the exposure lamp voltage correction control is performed. However, the correction becomes excessive, resulting in a bright image. Therefore, when the exposure lamp voltage correction control is performed in the above embodiment, as shown in Table 7, the development potential at the time of forming the reference image is switched and
It is preferable to control so as to correct and change the developing bias voltage VBR when creating a reference image for exposure lamp voltage correction control. As a result, an image with proper brightness can always be obtained. In the example of the exposure lamp voltage correction control, the developing bias voltage VBR is corrected and changed. However, instead of this, the control may be performed to change the value of Vdat / VLref.

[0055]

[Table 6]

[0056]

[Table 7]

Further, in the above-described embodiment, in order to suppress the fluctuation of the image due to the change of the photosensitive member with time, etc., the charging device 8 is based on the result of the reference image by the P sensor 14, as is conventionally done. The residual potential correction control for correcting the charging potential and the developing bias voltage may be performed. Conventionally, also in this type of control, each correction voltage is determined as shown in Table 8 on the assumption that the development gamma is maintained constant, as in the above-described exposure lamp voltage correction control. For example, as shown by the curve in FIG. 12, when the residual potential of the photoconductor is 80 V, the P sensor output VRP / V
RG becomes approximately 0.46, and based on Table 8, correction is performed by an appropriate amount of -80V.

[0058]

[Table 8]

However, in the above-described embodiment, the development gamma is increased as shown by the curve in FIG. 12 by switching the development potential when the reference image for detecting the P sensor is changed. Therefore, the residual potential correction control is performed. If you try to do that, the correction will be over 160V.
If such overcorrection is performed, an abnormal image such as white spots at the rear end of the halftone image portion is likely to occur.

Therefore, when the residual potential correction control is performed in the above-described embodiment, as shown in Table 9, the developing potential at the time of forming the reference image is switched, and the charging potential and the developing bias voltage at the residual potential correction control are changed. It is preferable to control so as to change the correction amount. As a result, an appropriate correction amount can always be obtained, and the occurrence of abnormal images can be suppressed.

[0061]

[Table 9] (Hereafter, margin)

[0062]

According to the first aspect of the invention, when the reference image is formed on the basis of the result of the comparison between the target value of the toner density detecting means and the preset threshold value by the developing potential control means. By changing the setting value of the developing potential of No. 1, the target ID can be obtained, and there is an effect that an appropriate image density can be maintained even over time. Further, since the toner can be always used with a high toner concentration, there is an effect that it is possible to prevent the deterioration of the developer such as the abrasion of the coat film of the carrier of the developer and the deterioration of the charging ability.

Particularly, according to the second or third aspect of the present invention, it becomes possible to prevent variations in development potential and erroneous detection during the formation of the reference image, so that the toner density increases due to these variations and erroneous detection. There is an effect that it is possible to prevent the background stain, the toner scattering and the like caused by the above.

Further, according to the invention of claim 4 or 5, in particular, since the toner concentration does not increase more than necessary over time, there is an effect that scumming, toner scattering and the like can be prevented.

Further, according to the invention of claim 6 or 7, in particular, since the developing potential does not excessively fluctuate, it is possible to prevent a problem due to the temporal fluctuation of the output of the toner concentration detecting means, and to develop the toner. This has the effect of preventing problems caused by excessive fluctuations in the potential.

According to the eighth aspect of the invention, the correction condition in the exposure condition correction means for correcting the exposure condition when the latent image is formed on the image carrier is changed every time the set value of the developing potential is switched. Even if the development potential is switched and the development gamma is increased by switching to, the exposure condition is not excessively corrected, so that an image with appropriate brightness can be always obtained. effective.

According to the ninth aspect of the invention, the correction conditions in the correction means for correcting the charging condition of the image carrier and the developing bias voltage application condition at the time of developing the latent image on the image carrier are set as follows. By switching each time the set value of the developing potential is switched, even if the developing potential is switched and the development gamma becomes large, the charging condition and the developing bias voltage applying condition are not excessively corrected. Therefore, there is an effect that it is possible to prevent the deterioration of the image quality due to the occurrence of the trailing edge blank.

[Brief description of drawings]

FIG. 1 is a front view showing a schematic configuration of a copying machine according to an embodiment.

FIG. 2 is a characteristic diagram showing a relationship between developing potential, image density ID, and P sensor output Vsp / Vsg.

FIG. 3 is a characteristic diagram showing a relationship between toner concentration TC and T sensor output.

FIG. 4 is a characteristic diagram showing changes in the coat film of the carrier of the developer, the charging ability of the developer, the toner density TC, and the image density ID with respect to the number of copies.

5 is a characteristic diagram showing the relationship between toner density TC and image density ID at point C in FIG.

FIG. 6 is a flowchart of developing potential control according to the embodiment.

FIG. 7 is a flowchart of development potential control according to a modified example.

FIG. 8 is a flowchart of developing potential control according to another modification.

FIG. 9: Toner concentration TC when the physical properties of the developer change
FIG. 6 is a characteristic diagram showing a relationship between T sensor output and T sensor output.

FIG. 10 is a flowchart of development potential control according to still another modification.

FIG. 11 is a characteristic diagram showing a relationship among a developing potential, an image density ID, and a P sensor output Vsp / Vsg during exposure lamp voltage correction control.

FIG. 12 is a characteristic diagram showing a relationship between a developing potential, an image density ID, and a P sensor output Vsp / Vsg during residual potential correction control.

[Explanation of symbols]

 2 original 3 exposure lamp 7 photoconductor drum 8 charging device 9 developing device 10 transfer device 11 cleaning device 12 static eliminator 13 erase 14 P sensor 91 toner hopper 92 toner supply roller 93 T sensor 94 developing sleeve

Claims (9)

[Claims] 1. A developing device for supplying toner to a latent image formed on an image carrier to visualize the latent image, a toner replenishing means for replenishing toner to a developing portion of the developing device, and a developing device in the developing device. Toner concentration detecting means for detecting the toner concentration, toner replenishing control means for controlling the toner replenishing means based on the result of comparison between the output value of the toner concentration detecting means and the target value, and an image on the image carrier. Reference image forming means for forming a reference latent image for density detection and visualizing the reference latent image to form a reference image, image density detecting means for detecting image density of the reference image, and image density detecting means In an image forming apparatus including a target value correction unit that corrects the target value based on the output value of the reference value, when the reference image is formed based on the result of comparison between the target value and a preset threshold value. Switch the setting value of development potential An image forming apparatus, comprising: a developing potential control unit. 2. The image forming apparatus according to claim 1, wherein the set value of the developing potential is switched when the target value continuously exceeds the threshold value a plurality of times. 3. The set value of the developing potential is switched after the number of transfer sheets on which an image has been formed exceeds a predetermined number from the time when the target value exceeds the threshold value. The image forming apparatus according to claim 1. 4. The image forming apparatus according to claim 1, wherein the set value of the threshold value is switched each time the number of transfer sheets on which an image is formed increases by a predetermined number. 5. The image forming apparatus according to claim 1, wherein the set value of the threshold value is switched each time the set value of the developing potential is switched. 6. The switching of the set value of the developing potential is not performed a predetermined number of times or more.
5 to the image forming apparatus. 7. The image forming apparatus according to claim 1, wherein the threshold value is not set when the set value of the developing potential is switched a predetermined number of times. 8. Based on the output value of the image density detecting means,
2. The image forming apparatus according to claim 1, further comprising an exposure condition correction means for correcting an exposure condition when forming a latent image on the image carrier, wherein the correction condition in the exposure condition correction means is set by a set value of the developing potential. An image forming apparatus that is switched every time it is changed. 9. Based on the output value of the image density detecting means,
The image forming apparatus according to claim 1, further comprising a correction unit that corrects a charging condition of the image carrier and a developing bias voltage application condition during development of a latent image on the image carrier. An image forming apparatus, which switches each time the set value of the developing potential switches.