JP2003302817A - Toner concentration controller for image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct a change in a toner concentration sensor output due to the polarity change of a developing bias voltage, and to keep image density constant, as for an image forming apparatus. <P>SOLUTION: The image forming apparatus using two-component developer is provided with a means 10 for changing the size and the polarity of the developing bias voltage to be applied on a developing roll 4, a toner concentration sensor 8 for detecting the toner concentration in a developing tank 2, a storing/ holding means 12 for holding a toner concentration control reference value referred to the toner concentration control, a toner replenishing means 13 which works on condition that the toner concentration detected by the toner concentration sensor becomes lower than the reference value and a control means 11 for inhibiting the toner replenishing operation when the developing bias voltage whose polarity is opposite to that of the developing bias voltage in the midst of an image forming process is applied, irrespective of the detection value of the toner concentration sensor. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a toner density control device in an image forming apparatus such as a copying machine or a laser beam printer, and more particularly to two-component reversal development for manifesting an electrostatic latent image formed on the surface of a photoconductor. The present invention relates to a toner density control device in a system image forming apparatus.

[0002]

2. Description of the Related Art Conventionally, in an electrophotographic copying machine, a developing tank is arranged close to a photoconductor, and the toner contained in the developing tank is electrostatically transferred onto an electrostatic latent image formed on the surface of the photoconductor. A structure is used in which the image is visualized by electroadsorption.
In the developing tank having such a structure, in order to keep the image density constant, it is required to keep the toner density in the developing tank at a proper density. Therefore, a toner concentration sensor for detecting the toner concentration inside the developing tank is provided, and the developer is charged and stirred, and the toner concentration at that time is stored as a control reference value, and the toner concentration is compared with the above reference value. There has been provided a device for performing control such as toner replenishment when the toner concentration is low. (Japanese Utility Model Publication No. 57-74447)

Usually, in an image forming apparatus using a two-component developer, since the carrier of the developer is a ferromagnetic material, the toner is not absorbed in the developer when it is normally adsorbed on the surface of the carrier by frictional charging. A toner density control device of magnetic permeability detection type has been proposed in which the toner density is controlled inversely to the magnetic permeability of the developer, and the magnetic permeability of the developer is detected to control the toner density. (Japanese Patent Publication No. 46-8280) Also, while the surface of the photoreceptor charged by the charger prior to image formation reaches the developing tank located downstream of the charger or after the image formation, When the surface of the photoconductor from which the surface potential has been removed passes through the developing tank, a so-called fogging phenomenon occurs in which weakly charged toner adheres to the surface of the photoconductor even if the developing bias voltage applied to the developing roll is 0V. . In order to maintain a gap between the surface potential and the developing voltage, that is, a so-called fog prevention potential, a control method of applying a bias voltage having a reverse polarity to that at the time of image formation to the developing roll is provided. (JP-A-7-25369
(Gazette No. 3)

[0004]

In a developing tank using a two-component developer, the location of the toner concentration sensor in the developing tank is an important factor for keeping the toner concentration in the developing tank constant. Is. Conventionally, as shown in FIG. 9, the toner concentration sensor 8 is arranged in the vicinity of the point where the toner 14 drops from the replenishing means 13 into the developing tank 2. Then, the toner density is detected in a state where the toner density is not uniform before the toner is uniformly stirred in the developing tank (the toner density is higher than that in other portions), and the image density cannot be stabilized. In particular, in a small-sized developing tank that rotates at high speed, the toner concentration sensor 8 must be arranged near the developing roll 4 in order to accurately detect the toner concentration.

In a developing tank in which such a toner concentration sensor is arranged in the vicinity of the developing roll, the toner concentration change in the vicinity of the toner concentration sensor portion changes due to the polarity change of the voltage applied to the developing roll, and accurate development is performed. Since the toner density inside the tank cannot be detected, the toner density inside the developing tank cannot be kept constant, which causes a problem that the image density changes.

In the case of the reversal developing method, more detailed description will be given below. FIG. 2 shows the surface potential of the photosensitive member, the developing bias voltage, and the toner density output sensor value in the operating state of the image forming apparatus. The dotted line in (a) shows the surface potential of the photoconductor, and the solid line shows the developing bias voltage value of the developing roll. In the pre-rotation step, the photoconductor and developing tank start to start for image formation, and the surface potential of the photoconductor is 0V, but + 150V voltage is applied to the developing roll at the same time or before that to prevent fogging. To be done. The output of the toner density sensor at that time is Vs2. Here, the toner concentration sensor output is in the output range of 0 to 5 V, which means that the toner concentration is low when the output voltage is high and the toner concentration is high when the output voltage is low.

Next, in the image forming process, for image formation,
The surface potential of the photoconductor is -600V, and the developing bias voltage is -5.
00V is applied. The toner density sensor output at that time is Vs1. During this time, toner adheres to the photoreceptor,
Even though the toner is not replenished, the phenomenon that the output value of the toner concentration sensor is different occurs. In the case of the reversal developing method, the toner has a negative charge, and when the developing bias voltage is applied to +150 V, the toner near the developing roll is strongly attracted electrostatically to the developing roll.

On the other hand, when the developing bias voltage is applied to -500V, the developing roll is likely to be electrostatically dissociated. It is considered that due to the electrostatic attraction and separation phenomenon, the toner concentration sensor output fluctuation phenomenon occurs due to the polarity change of the developing bias voltage. The toner movement caused by such a phenomenon is not carried out to the surface of the photoconductor (the fog prevention potential for preventing toner adhesion is maintained in the pre-rotation step, the image forming step, and the post-rotation step), and the development is performed. It is considered that a temporary non-uniform toner concentration distribution is generated inside the tank.

In particular, when a single print is repeatedly performed, the pre-rotation step and the post-rotation step, that is, the steps in which the developing bias polarity is positive, are relatively large as compared with the continuous printing, and in reality, in the developing tank. Although the average toner density of the toner does not change, the toner in the vicinity of the toner density sensor is electrostatically attracted to the developing roll, so it is detected as under toner and the toner is replenished so that the developing tank The toner density inside will increase.

The present invention has been made to solve such a problem, and an image forming apparatus capable of maintaining a constant image density by correcting a change in the toner density sensor output due to a change in the polarity of a developing bias voltage. The present invention provides a toner concentration control device.

[0011]

In order to achieve the above object, a toner concentration control device of an image forming apparatus according to the present invention comprises an electrostatic latent image formed on the surface of a photoconductor, which is composed of a magnetic carrier and toner. In an image forming apparatus that magnetically attracts a component developer to a developing roll and develops it, a means for changing the magnitude and polarity of a developing bias voltage applied to the developing roll, and a toner concentration sensor for detecting the toner concentration inside the developing tank. And storage holding means for holding a toner density control reference value referred to for toner density control, and toner replenishing means operating on condition that the toner density detected by the toner density sensor is lower than the reference value. And a developing bias voltage having a polarity opposite to that of the developing bias voltage during the image forming process is applied regardless of the toner concentration sensor detection value. And control means for prohibiting the toner supply operation when being, those having a.

A toner concentration control device of an image forming apparatus according to the present invention magnetically adsorbs an electrostatic latent image formed on a surface of a photoconductor to a developing roll by a two-component developer composed of a magnetic carrier and a toner. In an image forming apparatus for developing, a means for changing the magnitude and polarity of the developing bias voltage applied to the developing roll, a toner density sensor for detecting the toner density inside the developing tank, and a toner density referenced for toner density control. Storage holding means for holding the control reference value,
The toner replenishing means that operates on condition that the toner concentration detected by the toner concentration sensor is lower than the above reference value, and the toner concentration detection result or the toner concentration reference value according to the change in the polarity of the developing bias voltage. And a correction control means for correcting.

The toner concentration control device of the image forming apparatus according to the present invention magnetically adsorbs an electrostatic latent image formed on the surface of a photoconductor to a developing roll by a two-component developer composed of a magnetic carrier and toner. In an image forming apparatus for developing, a means for changing the magnitude and polarity of the developing bias voltage applied to the developing roll, a toner density sensor for detecting the toner density inside the developing tank, and a toner density referenced for toner density control. Storage holding means for holding the control reference value,
The toner replenishing means that operates on condition that the toner concentration detected by the toner concentration sensor is lower than the reference value, and the toner concentration detection result during a period other than a certain period before and after the change in the polarity of the developing bias voltage are displayed. And a control unit for controlling the toner density reference value to be compared.

The toner density control device of the image forming apparatus according to the present invention is further adapted to apply a developing bias voltage having a polarity opposite to the polarity of the developing bias voltage during the image forming process regardless of the toner density sensor detection value. The control means for prohibiting the toner supply operation is provided. The toner concentration control device of the image forming apparatus according to the present invention further comprises a correction control means for correcting the toner concentration detection result or the toner concentration reference value according to the change in the developing bias polarity.

The toner concentration control device of the image forming apparatus according to the present invention further includes a rotation time detecting means for detecting the rotation time of the developing tank, a sheet number counting means for detecting the number of printed sheets, and the number of printed sheets or the rotating time of the developing vessel. According to the detection result and the development bias polarity detection result, a correction control unit for increasing the correction amount for correcting the toner density detection result or the toner density reference value is provided.

[0016]

According to the toner density control device of the image forming apparatus of the present invention, when the polarity of the detected developing bias voltage is positive, the sampling result is ignored and the toner replenishing operation is not performed. Only the sampling result is valid and the data for calculating the average value is used, so that the accurate toner concentration in the developing tank can be detected, and the toner replenishing operation is performed based on this to control the toner concentration. As a result, the image density can be kept constant.

According to the toner concentration control device of the image forming apparatus of the present invention, the toner concentration sampling result is corrected or the toner concentration reference value is corrected according to the change of the polarity of the developing bias voltage, so that the inside of the developing tank is corrected. The toner density can be accurately detected, the image density can be kept constant, the toner replenishment time can be secured, and the image density can be kept constant even in high print printing.

According to the toner concentration control device of the image forming apparatus of the present invention, the toner concentration detection result during the period excluding the period during which the developing bias voltage is gradually changing while changing the polarity is adopted, thereby making it more accurate. The toner density in the developing tank can be detected, and the image density can be kept constant. According to the toner concentration control device of the image forming apparatus of the present invention, it is possible to read the toner concentration sensor output in which the toner concentration sensor output has not changed due to the change in developing polarity, and it is possible to perform more accurate toner concentration control.
According to the toner concentration control device of the image forming apparatus of the present invention, by adopting the toner concentration detection result during the period excluding the period in which the developing bias voltage is gradually changing while changing the polarity, the inside of the developing tank is more accurate. The toner density of can be detected and the image density can be kept constant, and
Since the toner replenishment time can be secured, the image density can be kept constant. According to the toner concentration control device of the image forming apparatus of the present invention, it is possible to more accurately detect the toner concentration in the developing tank in consideration of the deterioration of the developer, and it is possible to keep the image concentration constant,
Since the toner replenishment time can be secured, the image density can be kept constant.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described based on examples with reference to the drawings. FIG. 1 is a system diagram showing positions of a developing tank and a toner concentration sensor of the present invention. The electrostatic latent image formed on the surface of the photoconductor 1 is developed in the developing tank 2, and the toner contained in the developer is attached to the surface of the photoconductor. The developer is composed of a magnetic carrier and a toner. The toner is electrostatically adsorbed on the magnetic carrier, and the magnetic carrier is attracted onto the sleeve (developing roll) 4 by the magnetic force of the magnet roll 3. The magnet roll 3 is fixed, and when the sleeve (developing roll) 4 is rotated in the direction of the arrow, the developer is also rotationally conveyed.

At this time, the doctor 5 limits the amount of the developer so that it contributes to the development of the electrostatic latent image on the surface of the photoreceptor. The toner concentration sensor 8 is arranged below the stirring roll 7 and detects the toner concentration inside the developing tank. The control means 11 compares the reference value stored in the storage holding means 12 with the detection value of the toner concentration sensor 8, controls the replenishing means 13 based on the result, replenishes the toner 14 from the replenishing container 15, and The stirring rolls 6 and 7 are controlled to stir and circulate the toner in the developing tank 2. Further, the control means 11 controls the magnitude and polarity of the bias power supply 9 voltage by the varying means 10 and applies it to the sleeve 4 as a developing bias voltage.

FIG. 3 is a flow chart showing the flow of toner density control of the first embodiment. The rotation of the developing tank 2 is started by the image formation start signal (S301), and sampling of the toner density sensor is started. Normally, the position of the agitating rolls 6 and 7 before starting (stopped state) causes the density of the toner near the toner concentration sensor 8 to become coarse and dense, so after providing a stable period of several seconds, several msec to several tens msec (S302). ), Sampling is started (S303). Next, the polarity of the developing bias voltage is detected (S304). In the case of the reversal development method, the negative polarity is detected during image formation. Therefore, if the detected polarity is positive (S305), the output of the toner concentration sensor increases due to the reason described above, and the sampling result is ignored, and the negative polarity is detected. In this case, the sampling result is valid and used as the data for calculating the average value. Therefore, when the polarity of the developing bias voltage is positive, the toner replenishing operation is not performed.

Since this period is longer than other periods in the entire image forming process, the output result of the toner concentration sensor 8 is used only during this period to control the toner concentration. . Even after the sensor output is stabilized, ripples are generated in the toner concentration sensor output during the rotation cycle of the stirring rolls. Therefore, if one cycle of the stirring rolls 6 and 7 is, for example, 330 msec, the sampling is performed every 30 msec in order to accurately detect the toner concentration. This is repeated, and the average value is calculated (S306). Then, it is compared with a reference value stored in advance (S307), and the toner replenishing operation is performed under the condition (S308) that the toner concentration detected by the toner concentration sensor is lower than the reference value (S30).
9).

By performing the toner density control as described above, the accurate toner density in the developing tank can be detected and the image density can be kept constant. In the present invention, the polarity of the developing bias is detected and controlled, but the toner density control may be performed only during the period from T1 to T4 according to the time chart.

Depending on the amount of toner dropped by the replenishing means 13, when the sampling of the toner concentration sensor is ignored by detecting the positive polarity of the developing bias voltage, the toner replenishing operation time is high when printing a high print image with a high black coverage ratio. There is a possibility that toner replenishment will not follow due to lack of toner. Such a case will be described with reference to FIG. 4 which is a flowchart showing the flow of toner concentration control of the second embodiment. In this case, until (S404)
This is the same as S304 of 1 (FIG. 3), and then the average value of toner density is calculated (S405) and the polarity of the developing bias voltage is detected (S406). If the detected polarity is negative, it is stored in advance. Compare with the reference value (S40
7) The toner replenishing operation is performed (S411) on condition that the toner concentration detected by the toner concentration sensor is lower than the reference value (S410).

On the other hand, if the detected polarity is positive, S40
As shown in 8, by correcting the toner density sampling result and adding it to the data for calculating the average value, the determination time of the average value calculation result is shortened and the toner replenishing operation time can be secured. If the output of the toner concentration sensor in the start-T1 section of FIG. 2 is ignored, even if the toner is under toner, the toner replenishment starts after T1, and the toner replenishable time becomes short. Therefore, by correcting the toner concentration sensor output from the start up to the T1 section, the toner concentration sensor output is adopted as effective data, so that the toner replenishment operation possible time can be taken longer. Conversely, in the data for calculating the average value, the ratio of the case where the polarity of the developing bias voltage at the time of sampling the toner concentration sensor output is positive and the case where it is negative is calculated, and the ratio for controlling the toner concentration is calculated according to the ratio. It is also possible to correct the reference value. This is because when the development bias polarity is positive 3 times and the remaining 8 times is negative polarity among the 11 times sampled to calculate the average value, the sensor output when the polarity is positive is X times the sensor output when the polarity is negative. Then, the reference value is ((8/11) +
It is possible to make effective use of data that is mixed with (3/8) X) times and mixed with developing bias polarities. The thus-corrected value is compared with the toner concentration reference value (S409), and thereafter, the condition that the toner concentration detected by the toner concentration sensor is lower than the reference value as described above (S41).
0) and the toner replenishing operation is performed (S411).

By performing the toner density control as described above, the accurate toner density in the developing tank can be detected, the image density can be kept constant, and the toner replenishment time can be secured. The image density can be kept constant even in high-print printing.

Further, although the polarity of the developing bias voltage is switched in several tens of msec, the fluctuation of the output of the toner concentration sensor takes a longer time than the polarity switching of the developing bias voltage because the toner moves. Therefore, the transitional period during which the toner is moving (T1-T2 section and T in FIG. 2)
In the section (3-T4), there is a transient region in which the sensor output does not depend on either polarity of the developing bias voltage. If the toner concentration detection result during the period in which the developing bias voltage is gradually changing while changing the polarity is adopted, the accurate toner concentration in the developing tank cannot be detected.

Such a case will be described based on the third and fourth embodiments. FIG. 5 is a flow chart showing the flow of toner density control of the third embodiment, and S50
Up to 5 is the same as S405 of the second embodiment (FIG. 4), but the output of the toner concentration sensor is ignored during a certain period (transition period) before and after the switching of the developing bias voltage (S50
6) adopting the detection result of the toner density in the period excluding the period and comparing with the toner concentration reference value (S507), it is a condition that the toner concentration detected by the toner concentration sensor is lower than the above reference value ( In step S508, the toner supply operation is performed (S509). Note that the output of the toner concentration sensor is ignored during a certain period (transition period) before and after the development bias voltage switching in the third embodiment (FIG. 5) (S50).
6), the detection result of the toner density during the period excluding the period is adopted and compared with the toner concentration reference value (S507), and the condition is that the toner concentration detected by the toner concentration sensor is lower than the reference value ( If the toner replenishing operation is performed in step S508 (S509) and the replenishment is prohibited when the developing bias voltage in Example 1 has a positive polarity, the toner density sensor output change due to the developing polarity change occurs. It is possible to read the output of the toner concentration sensor that is not present, and more accurate toner concentration control can be performed.

FIG. 6 is a flow chart showing the flow of toner density control of the fourth embodiment. Up to S606 is the same as that up to S506 of the third embodiment (FIG. 5), but before and after the switching of the developing bias voltage. The polarity of the developing bias voltage is detected during a period excluding a certain period (transition period) (S60
7), if the detected polarity is negative, Example 3 (Fig. 5)
However, if the detected polarity is positive, the toner concentration sampling result is corrected accordingly and added to the data for calculating the average value, so that the accurate toner concentration can be detected. Yes (S60
9). Alternatively, in the data for calculating the average value, the ratio of the case where the polarity of the developing bias voltage at the time of sampling the toner concentration sensor output is positive and the case where it is negative is calculated, and the reference value for controlling the toner concentration is calculated according to the ratio. Can be corrected (S609). Further, from the time chart of the developing bias voltage, it is possible to perform control so as to ignore the toner density detection result during a certain period (transition period) of switching the developing bias voltage polarity (S606-S60).
3).

By performing the toner concentration control as described above, it is possible to detect the toner concentration in the developing tank more accurately in consideration of the period in which the developing bias voltage is gradually changing while changing the polarity. The image density can be kept constant and the image replenishment time can be secured, so that the image density can be kept constant.

Since the developer repeatedly rotates in the developing tank, its characteristics change with use. Actually, as shown in FIG. 7, the charge amount of the carrier decreases according to the stirring time of the developing tank. Therefore, the toner is likely to be dissociated from the carrier in proportion to the stirring time of the developing tank. It is considered that this is due to peeling of the charge control agent from the carrier surface and adhesion of the toner to the carrier surface.

Here, in the case of the reversal developing method, when the developing bias voltage of the toner is applied to + 150V, the toner near the developing roll is strongly attracted to the developing roll strongly, and the bias voltage is applied to -500V. If it is, the electrostatic dissociation from the developing roll becomes easier, but this electrostatic adsorption separation phenomenon is caused against the electrostatic adsorption force of the carrier and the toner. . That is, in the initial stage when the charge amount of the carrier is high, the toner is more strongly adsorbed to the carrier, so that the toner does not move much due to the polarity change of the developing roll, and therefore the toner concentration sensor output value changes little. On the other hand, when the developer is agitated and the carrier charge amount is low, the toner migration is large due to the polarity change of the developing roll, and thus the change in the toner concentration sensor output value is large.

With respect to the change in the toner concentration sensor output value due to such deterioration of the developer, if the correction amount of the toner concentration detection result is kept constant, the accurate toner concentration in the developing tank can be detected. I can't. Therefore, such a case will be described based on the fifth embodiment. FIG. 8 is a flowchart showing the flow of the toner density control, and the steps up to S807 are the same as the steps up to S607 of the fourth embodiment (FIG. 6). As a result of the polarity detection of the developing bias voltage in S807, when the detected polarity is negative, the sixth embodiment is performed.
However, if the detected polarity is positive, the number of printed sheets or the rotation time of the developing tank is detected (S809), and the toner concentration sampling result corresponding to the polarity of the developing bias voltage is detected. The correction amount is increased (S810).

Alternatively, in the data for calculating the average value, the ratio of the case where the polarity of the developing bias voltage at the time of sampling of the toner concentration sensor is positive and the case where it is negative is calculated, and the toner concentration is controlled according to the ratio. It is also possible to increase the correction amount when correcting the reference value of (S8).
10). By performing the toner density control as described above, it is possible to detect the toner density in the developing tank more accurately in consideration of the deterioration of the developer, and it is possible to keep the image density constant and the toner replenishment time. It is possible to keep the image density constant also by being able to secure the above.

As a result of the polarity detection of the developing bias voltage in S807 of the fifth embodiment (FIG. 8), if the detected polarity is positive, the number of printed sheets or the rotation time of the developing tank is detected (S809), and according to it. Thus, in S406 in the second embodiment (FIG. 4), the correction amount of the toner density sampling result or the correction amount for correcting the toner density reference value is increased according to the polarity of the developing bias voltage (S810). The present invention can also be applied to the case where the polarity of the detected developing bias voltage is positive, and the same effect can be obtained.

[0036]

As described above, according to the present invention, the image density can be kept constant by controlling the toner density according to the polarity change of the developing bias voltage.

[Brief description of drawings]

FIG. 1 is a system diagram showing positions of a developing tank of the present application and a toner concentration sensor.

FIG. 2 is a diagram showing a relationship between a state of applying a developing bias of the present application and a toner concentration sensor output.

FIG. 3 is a flowchart showing a flow of toner density control according to the first embodiment.

FIG. 4 is a flowchart showing a flow of toner density control according to the second embodiment.

FIG. 5 is a flowchart showing a flow of toner density control according to the third embodiment.

FIG. 6 is a flowchart showing a flow of toner density control according to the fourth embodiment.

FIG. 7 is a diagram showing a relationship between a stirring time and a carrier charge amount.

FIG. 8 is a flowchart showing a flow of toner concentration control according to the fifth embodiment.

FIG. 9 is a system diagram showing conventional developing tank and toner concentration sensor positions.

[Explanation of symbols]

1 photoconductor 2 developing tank 3 magnet roll 4 Development sleeve (Development roll) 5 Doctor 6,7 stirring roll 8 Toner density sensor 9 Bias power supply 10 Variable means 11 Control means 12 Memory holding means 13 Supplying means 14 Toner 15 Supply container

Front page continuation (51) Int.Cl. ⁷ identification code FI theme code (reference) G03G 21/14 G03G 21/00 372 (72) Inventor Koichi Takenouchi 22-22 Nagaikecho, Abeno-ku, Osaka, Osaka In-house (72) Inventor Shoichi Fujita, 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Prefecture Sharp Corporation (72) Inventor Hiroshi Sakita 22-22, Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Prefecture 72) Inventor Osamu Morikuni 22-22 Nagaike-cho, Abeno-ku, Osaka, Osaka Prefecture 2H073 AA02 BA02 BA13 BA21 BA28 BA41 BA45 CA03 2H077 AA11 AA15 AA33 AB02 AC02 AD02 AD06 AD37 BA01 DA03 DA10 DA42 DA51 DB02 DB08 DB14 DB18 DB21 EA01

Claims (6)

[Claims] 1. An image forming apparatus for developing an electrostatic latent image formed on a surface of a photoconductor by magnetically adsorbing a two-component developer comprising a magnetic carrier and a toner on a developing roll.
Means for changing the magnitude and polarity of the developing bias voltage applied to the developing roll, a toner density sensor for detecting the toner density inside the developing tank, and a memory for holding the toner density control reference value referred to for toner density control. A holding unit, a toner replenishing unit that operates on the condition that the toner concentration detected by the toner concentration sensor is lower than the reference value, and a developing bias voltage during the image forming process regardless of the toner concentration sensor detection value. A toner concentration control device for an image forming apparatus, comprising: a control unit that prohibits a toner replenishing operation when a developing bias voltage having a polarity opposite to that of the polarity is applied. 2. An image forming apparatus for developing an electrostatic latent image formed on the surface of a photoconductor by magnetically adsorbing a two-component developer comprising a magnetic carrier and a toner onto a developing roll.
Means for changing the magnitude and polarity of the developing bias voltage applied to the developing roll, a toner density sensor for detecting the toner density inside the developing tank, and a memory for holding the toner density control reference value referred to for toner density control. Holding means, toner replenishing means that operates on the condition that the toner concentration detected by the toner concentration sensor is lower than the reference value, and the toner concentration detection result or toner depending on the change in the polarity of the developing bias voltage. A toner density control apparatus for an image forming apparatus, comprising: a correction control unit that corrects a density reference value. 3. An image forming apparatus for developing an electrostatic latent image formed on the surface of a photoreceptor by magnetically adsorbing a two-component developer comprising a magnetic carrier and a toner on a developing roll.
Means for changing the magnitude and polarity of the developing bias voltage applied to the developing roll, a toner density sensor for detecting the toner density inside the developing tank, and a memory for holding the toner density control reference value referred to for toner density control. Holding means, toner replenishing means that operates on the condition that the toner concentration detected by the toner concentration sensor falls below the reference value, and toner for a period other than a certain period before and after the change in the polarity of the developing bias voltage. A toner concentration control device for an image forming apparatus, comprising: a control unit that employs a density detection result and controls so as to compare with a toner concentration reference value. 4. The toner concentration control device for an image forming apparatus according to claim 3, wherein a developing bias voltage having a polarity opposite to the polarity of the developing bias voltage during the image forming process is applied regardless of the toner concentration sensor detection value. A toner concentration control device for an image forming apparatus, the control device prohibiting a toner replenishing operation while the toner is being charged. 5. The toner density control device for an image forming apparatus according to claim 3, further comprising a correction control unit that corrects the toner density detection result or the toner density reference value according to a change in the developing bias polarity. A toner concentration control device for an image forming apparatus, characterized in that 6. The toner density control device for an image forming apparatus according to claim 2 or 5, further comprising a rotation time detecting means for detecting a rotation time of the developing tank, and a sheet number counting means for detecting the number of printed sheets. An image including: a correction control unit that increases a correction amount for correcting the toner density detection result or the toner density reference value according to the number of printed sheets or the detection result of the developing tank rotation time and the development bias polarity. Toner density control device for forming device.