JP2003173077A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device having a toner concentration controller that appropriately controls the concentration of toner in two- component developer. <P>SOLUTION: Due to the repetition of an image forming operation, for example, charging carrier used for magnetic brush charging sticks to an image carrier and, as a result, it is mixed into a developing unit and accumulated therein. In this case, an amount of correction is provided in order to set a new reference value relative to a reference T/D ratio according to the sensitivity of an inductance detection sensor being used. <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 a copying machine, a printer, a recorded image display for developing an electrostatic latent image formed on an image carrier by an electrophotographic system, an electrostatic recording system or the like to form a visible image. Image forming apparatus such as a device and a facsimile,
In particular, the present invention relates to an image forming apparatus equipped with a toner concentration control device that appropriately controls the toner concentration of a two-component developer.

[0002]

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

As is well known, the toner concentration of this two-component developer (that is, the ratio of the weight of toner particles to the total weight of carrier particles and toner particles; hereinafter referred to as T / D ratio) stabilizes the image quality. It is a very important factor above. The toner particles of the developer are consumed at the time of development, and T / D
The ratio changes. Therefore, the toner density control device (AT
R) is used to accurately detect the T / D ratio of the developer at appropriate times,
It is necessary to replenish the toner according to the change and to keep the T / D ratio within an appropriate range to maintain the image quality.

As described above, the T / D in the developing device is increased by the development.
In order to correct the change in the ratio, that is, to control the amount of toner replenished to the developing device, the T / D in the developing container is adjusted.
Conventionally, various types of ratio detection and concentration control devices have been proposed and put into practical use. For example, a developer carrying member (which is generally referred to as a “developing sleeve” in the following description since a developing sleeve is generally used) or a developer carried on the developing sleeve in the vicinity of the developer carrying path of the developing container. Alternatively, utilizing the fact that the reflectance when light is applied to the developer in the developing container differs depending on the T / D ratio,
A toner concentration control device for detecting and controlling the T / D ratio, or an inductance head for detecting an apparent magnetic permeability due to the mixing ratio of the magnetic carrier and the non-magnetic toner and converting it into an electric signal is installed on the side wall of the developing device. An inductance detection type toner concentration control device is used in which the actual T / D ratio of the developer is detected by a detection signal from the inductance head and the toner is replenished by comparison with a reference value.

An image carrier (photosensitive drum is generally used in many cases, so in the following description, "photosensitive drum")
Called. ) The patch image density formed above is read by a light source provided at a position facing the surface and a sensor that receives the reflected light, converted into a digital signal by an analog-digital converter, and then sent to the CPU, and initialized by the CPU. Compared with the value, if the density is higher than the initial setting value, toner replenishment is stopped until it returns to the initial setting value, and if the density is lower than the initial setting value, toner is forcibly replenished until it returns to the initial setting value. As a result, the T / D ratio is indirectly maintained at a desired value.

However, in the method of detecting the T / D ratio from the reflectance when the developer carried on the developing sleeve or the developer in the developing container is irradiated with light, the detecting means is contaminated by toner scattering or the like. If it does, there is a problem that the T / D ratio cannot be detected accurately. Further, the method of indirectly controlling the T / D ratio from the patch image density has a problem that a space for forming a patch image and a space for installing a detection unit cannot be secured due to downsizing of a copying machine or an image forming apparatus. is there.

On the other hand, in the case of using the inductance detection sensor, the cost of the sensor alone is low, and the cost of the sensor is low because it is not affected by the problem of space and the problem of dirt due to toner scattering. Optimal T / D for copiers or image forming devices in small spaces
It can be said to be a ratio detection method.

A T / D ratio control device using the above inductance detection sensor (hereinafter referred to as "inductance detection method AT
R ”. ) Is, for example, a reference value corresponding to the apparent magnetic permeability of the prescribed T / D ratio (T / D ratio at the time of initial installation) of the developer is set in advance, and the developer T / D is set by image formation.
When the ratio changes, the detection signal corresponding to the apparent magnetic permeability of the developer detected by the inductance detection sensor is
When it is detected that it is larger than the reference value, it means that the ratio of the carrier particles in the developer in the constant volume is large and the T / D ratio is low, so that the toner replenishment is started,
On the contrary, when the detected value corresponding to the apparent magnetic permeability becomes smaller than the reference value, it means that the ratio of the carrier in the developer in the fixed volume is small and the T / D ratio becomes high. The T / D ratio is controlled based on control such as stopping the supply.

In general, a corona charger is generally used as a charging means for an image carrier such as an electrophotographic photosensitive member or an electrostatic recording dielectric, which is included in an electrophotographic or electrostatic recording type image forming apparatus. Came.

In recent years, because of advantages such as low ozone and low electric power, a contact charging device, that is, a device of a type in which a charging member to which a voltage is applied is brought into contact with a charged member to charge the charged member It has been put to practical use. In particular, a roller charging type device using a conductive roller as a charging member is preferably used from the viewpoint of charging stability.

However, in the roller charging method described above, charging is performed by discharging from the charging member to the member to be charged.
The surface potential of the photoconductor also fluctuates due to changes in the electrical resistance of the charging roller and the electrophotographic photoconductor due to changes in the environment.

Therefore, recently, as a charging method with little environmental change, a voltage is applied to a conductive contact charging member (charging fur brush, charging magnetic brush, charging roller, etc.) in Japanese Patent Application No. 5-66150. , A method of injecting electric charges having the same polarity as the electric potential of the photoconductor to a photoconductor having a charge injection layer on the surface of which a conductive powder (SnO2 or the like) serving as a trap level is dispersed to perform contact charging. This injection charging method is not only environmentally less dependent, but does not use discharge, so that the applied voltage is sufficient at the same level as the potential of the photoconductor, and there is an advantage that ozone that shortens the life of the photoconductor is not generated.

Further, in the contact charging using discharge, in order to obtain a desired charging potential Vs on the body to be charged, the discharge start voltage Vth (DC voltage is applied to the contact charging member is applied to the desired charging potential Vs. DC bias Vs + Vt added with (applied voltage of contact charging member when charging of member starts)
Although it is necessary to apply h to the charging member, the charge injection charging can obtain the charging potential Vs that is almost the same as the DC bias applied to the charging member, so that the cost of the charging power source can be reduced.

As the contact charging member in the case of such a charge injection method, a magnetic brush charging member or a fur brush charging member is preferably used in view of stability of charging and contact.

The magnetic brush charging member has a magnetic brush of conductive magnetic particles (hereinafter referred to as "charging carrier") magnetically restrained and formed and held on a carrier which also serves as a feeding electrode.
The magnetic brush is brought into contact with the member to be charged to supply power to the carrier. More specifically, the charging carrier is directly held by a magnet or a sleeve containing the magnet as a magnetic brush so as to be magnetically restrained and held, and the magnetic brush charging member is stopped or rotated while the magnetic brush portion is charged. The member to be charged is charged by bringing it into contact with and applying a voltage.

The fur brush charging member has a brush portion (fur brush portion) of a conductive fiber carried on a carrier which also serves as a power feeding electrode, and the conductive fiber brush is brought into contact with a member to be charged to carry the carrier. It supplies power to.

In comparison between the magnetic brush charging member and the fur brush charging member, the charging property of the fur brush charging member is deteriorated when the hair collapses due to long-term use or long-term storage, and the charging uniformity is uniform. However, such a phenomenon does not occur with the magnetic brush charging member, and uniform and stable charging can be performed.

However, a problem with the magnetic brush charging device as described above is that the charge carrier forming the charging magnetic brush may adhere to or flow out from the surface of the image carrier. When it is collected in the developing device, in the developing device using the inductance detection type sensor, even if a small amount of the charged carrier adheres which is not particularly problematic in terms of image, the number of copies increases and the accumulated in the developing device. When the magnetic permeability of the developing carrier is different from that of the charging carrier, the apparent magnetic permeability of the developer as a whole may change, and an error may occur in the T / D ratio control by the inductance detection type sensor.

That is, when the magnetic permeability of the charge carrier is higher than that of the developing carrier, the toner concentration in the developing container is at the same level as the reference toner concentration, but the charge carrier is mixed in the developer, so that the average of the developer is increased. If the magnetic permeability is large, it is detected by the inductance detection sensor, and this is because the ratio of carrier particles in the developer in a certain volume is large, and it is erroneously detected that the T / D ratio is low, and toner supply is started. , The concentration control higher than the proper T / D ratio will be performed.

On the contrary, when the magnetic permeability of the charge carrier is smaller than that of the developing carrier, if the charge carrier is mixed in the developer, the average magnetic permeability of the developer becomes small, and the output value of the inductance detecting sensor becomes smaller than the reference value. It is detected as low, which means that the ratio of the carrier in the developer within a certain volume is small and the T / D ratio is high, so the toner supply is stopped and the T / D ratio is lower than the proper value. Problems such as concentration control occur.

In the former case, there is a problem that the image density becomes dark due to excessive toner supply, a problem that the developer amount increases with an increase in the toner amount and the developer overflows from the developing container, or a toner ratio in the developer. As a result of the decrease in the toner charge amount due to the increase of the toner, problems such as toner scattering and image fog occur.

On the other hand, in the latter case, problems such as image deterioration and low image density due to a decrease in the amount of toner in the developer occur. Further, the above-mentioned problem may increase in influence particularly as the image forming operation, that is, as the number of copies increases.

To solve this problem, a method has been devised in which the amount of charge carriers collected in the developing device is predicted based on the number of image formations and the number of copies, and the reference value for the reference toner density is reset as a new reference value. ing. For example, as shown in FIG. 14B, the optimum T / D ratio of the developer is 8%.
It is assumed that the initial value of the detection signal from the inductance head is 2.5 V and the initial value is set to the reference value so that the T / D ratio can be maintained.

However, when the charge carriers are gradually accumulated in the developing device as the image forming operation is repeated,
When the magnetic permeability of the charging carrier is larger than that of the developing carrier, the apparent magnetic permeability of the developer is shown in FIG.
It gradually increases as shown by the dotted line in (i), and gradually decreases as shown by the dotted line in FIG. 14 (a) (ii) when the magnetic permeability of the charge carrier is smaller than that of the developing carrier. become.

Therefore, assuming that the T / D ratio is controlled to the initial value of 8%, the output is as shown in FIG. 14B when the permeability of the charging carrier is larger than that of the developing carrier.
It gradually increases as shown by the dotted line in (i), and when the magnetic permeability of the charge carrier is smaller than that of the developing carrier, it gradually decreases as shown by the dotted line in FIG. 14 (b) (ii). It will be.

However, in reality, in the inductance detection sensor, the toner is replenished so that the initial reference value is 2.5 V (FIGS. 14 (b) (iii)). As a result, as shown in FIG. 14 (c), the charge carrier is charged. 14 (c) is higher than the magnetic permeability of the developing carrier.
As shown by the dotted line in (i), the T / D ratio gradually increases, and when the magnetic permeability of the charge carrier is smaller than that of the developing carrier, the T / D ratio becomes T as shown by the dotted line in FIG. 14 (c) (ii).
The / D ratio will gradually decrease.

Details will be described below. The amount of charge carriers mixed in the developing device during image formation with 5% duty is about 10
It is assumed that it was about g / 50,000 times. Further, the inductance detection sensor used has a detection sensitivity (a sensor output change amount with respect to the T / D ratio change amount) that changes by 0.5 V with respect to a change of the T / D ratio of 1%.

Here, when the detection signal of the inductance detection sensor is set to 2.5 V with respect to the reference T / D ratio, if 10 g of the charge carrier is forcibly mixed into the developer, the appearance of the developer is apparent. Since the magnetic permeability of No. 2 becomes large, the detection signal of the inductance detection sensor when the initial optimum T / D ratio was maintained increased from the initial reference value of 2.5V to 0.5V by 0.5V.

Therefore, the toner is actually replenished so that the detection signal becomes 2.5V, so that the toner is replenished excessively and the image forming operation is repeated 50,000 times, so that the toner density is finally increased. Is the optimum toner density of 8%
It deviated from 1%, and was controlled to 9%.

In order to solve this problem, the CPU 2 is preset so as to reset the initial reference value of the inductance detecting sensor when the image forming operation is repeated for a certain time or a certain number of times.
5 (FIG. 7) is set in advance, and the CPU 25 sets a new reference signal value at a predetermined timing. Specifically, in the above image forming apparatus, the image forming operation is performed 25
When it is repeated 000 times, if the mixing amount of the charging carrier is proportional to the image forming operation, the amount is 10 g / 500.
Since it is 00 times, it becomes 5g / 25,000 times. The value of the inductance detection sensor changes by 0.5 V when 10 g of the charge carrier is mixed, so that the value changes by 0.25 V when about 5 g of the charge carrier is mixed.

Therefore, when the image formation is repeated 25000 times, the reference signal value for the T / D ratio control is set to 2.75V.
An instruction is set in advance in the CPU 25 so that it is set again. When reset, the detection signal is 2.75.
Since the toner replenishment is stopped until the voltage reaches V, the error in the toner density that has occurred up to that point is eliminated, and the initial T / D ratio is controlled again to 8%.

As a result, when the image forming operation is repeated 50,000 times, the reference value is changed in the same manner, so that the image forming 5
It is possible to reduce the error in toner density control as compared with the case where the detection signal is not corrected for the T / D ratio after 0000 times.

Further, since the inductance detection sensor determines the T / D ratio based on the ratio of the carrier particles in the developer within a constant volume, the T / D ratio can be kept constant due to some influence other than mixing of the charged carriers. Regardless, if the bulk density of the developer itself changes, the ratio of the carrier particles in the developer within a certain volume changes due to the change in bulk density, which causes an error in the T / D ratio control by the inductance detection sensor. May occur.

This variation in the bulk density of the developer is mainly caused by the variation in the toner charge amount with respect to the number of image formed sheets. This is because the number of contact between the carrier particles and the toner particles increases as the number of image forming sheets increases, the toner charge amount increases, the repulsion between the developers becomes stronger, and the bulk density of the developer decreases, or the number of image forming sheets further increases. In this case, the charging capacity of the carrier particles is reduced, so that the toner charge amount is reduced and the developer bulk density is increased.

To solve this problem, a method similar to the above is used to predict a change in developer bulk density due to a change in toner charge amount depending on the number of image formations or the number of copies, and a new reference value for the reference toner density is set. A method of resetting to a standard value has been devised.

[0036]

In the method of resetting the reference value for the reference T / D ratio as a new reference value according to the number of passed sheets as described above, the amount of charge carriers collected in the developing device is predicted. Therefore, it is essential to determine the correction amount in advance. For example, in the case of the conventional technique, FIG.
5, the reference signal value is 2.5 when 25,000 sheets of image are formed.
It is necessary to store in the CPU 25 (see FIG. 7) a table for changing from V to 2.75V and further changing the reference signal value from 2.75V to 3.0V for 500,000 sheets of image formation.

However, the inductance type T for detecting the apparent magnetic permeability change of the developer within the constant volume as described above.
The / D ratio detection sensor has a T / D ratio variation amount due to variations in the manufacturing process of the sensor itself, such as transformer variations, case / bobbin variations, and assembly variations.
There may be individual differences in the output change amount of the D ratio detection sensor, that is, the sensitivity of the so-called inductance detection sensor. For example, as shown in FIG. 9, when ± 1% is swung around the T / D ratio of 8%, the sensor sensitivity of the inductance detection sensor A is 0.5 V for a change in the T / D ratio of 1% and the inductance is It is assumed that the detection sensor B has a voltage of 0.25V.

In such a case, with reference to the sensor A, the reference signal value is changed from 2.5V to 2.75V for 25,000 sheets of image formation, as in the case of the conventional technique.
The T / D ratio can be maintained at the initial 8% by determining the correction amount of the reference value with respect to the reference T / D ratio such that the reference signal value is changed from 2.75V to 3.0V for 00000 sheets. When the same correction is performed by the sensor B, the image formation 50
Where you want to set the standard value to 2.75V when 000 sheets,
Since it becomes 3.0V, it is corrected by 0.25V, that is, 1% extra.

On the contrary, if the correction table of the reference value with respect to the reference toner density is determined based on the sensor B, the sensor A cannot correct the T / D ratio by 0.5%. In the former case, there is a problem that the image density becomes dark due to excessive toner supply, a problem that the developer amount increases as the toner amount increases and the developer overflows from the developer container, or a toner ratio in the developer increases. As a result, the toner charge amount decreases, causing problems such as toner scattering.

On the other hand, in the latter case, problems such as image deterioration and low image density due to a decrease in the amount of toner in the developer are caused as described above.

[0041]

The above object can be achieved by the image forming apparatus according to the present invention. In summary, the present invention relates to a charging device that faces an image carrier and charges the image carrier, an exposure device that forms an electrostatic latent image on the charged image carrier, and a toner that charges the electrostatic latent image. In an image forming apparatus having a plurality of developing devices for forming a visible image by using a two-component developer composed of particles and carrier particles, a toner concentration for detecting an apparent magnetic permeability of the two-component developer and transmitting a detection signal. A toner concentration control device having a sensor, which controls the toner concentration of the developer based on the comparison result of the detection signal value and a predetermined reference signal value, has an operating time of each of the plurality of developing devices, or A toner density sensor provided with a reference signal value correction unit that resets the reference signal value as a new reference signal value based on the number of operations, and a correction amount corrected by the reference signal value correction unit is attached to the image forming apparatus. of An image forming apparatus, characterized in that different by degrees.

[0042]

According to the present invention, a charging container used for magnetic brush charging is provided with an inductance detection sensor for detecting an apparent magnetic permeability of a two-component developer in a developing container to control a T / D ratio. When the carrier repeatedly accumulates the charge carrier in the developing device due to the adhesion of the charge carrier to the image carrier due to the repeated image forming operation, the reference value is adjusted according to the sensitivity of the inductance detection sensor used. By having a correction amount that newly sets the reference value for the T / D ratio, even if the inductance detection sensor with a different sensitivity is used, the correction is performed according to the sensor sensitivity, so T / D with less error D ratio control becomes possible.

(First Embodiment) FIG. 1 is a schematic diagram of an electrophotographic image forming apparatus to which the present invention can be applied. In the image forming apparatus, the first, second, third, and fourth image forming units (stations) 4Y, 4M, 4C, and 4K are arranged in a line along the upper track of the transfer belt 30. It is capable of forming full-color images at high speed.

The image forming units 4Y, 4M, 4C and 4K form images of yellow, magenta, cyan and black, respectively. As shown in FIG. 2, each image forming unit includes a photoconductor drum 1 that rotates in the direction of the arrow, around which a charger 2, a transfer discharger 3, and a laser disposed above the photoconductor drum in the figure. The image forming unit is composed of a beam scanner or the like.

An original reading device having a photoelectric conversion element such as a CCD outputs an image signal corresponding to monochrome image information of the original. The semiconductor laser built in the laser beam scanner is controlled according to this image signal and emits the laser beam 5.

In the sequence of the entire image forming apparatus, the photosensitive drum is first uniformly charged by the magnetic brush charger. The photoconductor is 150 mm / s in the clockwise direction indicated by the arrow.
ec. It rotates at the process speed (peripheral speed) of.

Next, scanning exposure is performed by the laser light 5 modulated by the image signal, an electrostatic latent image is formed on the photosensitive drum 1, and the electrostatic latent image is reversely developed by the developing device 6.

In this embodiment, a two-component contact developing system is used in which a developer in which a non-magnetic toner and a magnetic carrier are mixed is used as the developer, so that the recoverability of the toner discharged from the magnetic brush charging device is improved. .

A full-color image can be obtained by carrying out the above-mentioned steps for four-color images of yellow, magenta, cyan and black. The toner image on the photosensitive drum is transferred to a transfer material such as paper that has advanced through a paper feed roller and a paper feed guide, and a transfer charger (corona charger)
3 is transferred. The toner remaining on the surface of the photosensitive drum 1 without being transferred is temporarily collected by the magnetic brush charger 2. Then, the conductive brush 70 is brought into contact with the photoconductor drum, and the charge is removed by the conductive brush 70 to which an AC bias, a DC bias having a reverse polarity to the charging, or a DC bias having a reverse polarity to the charging with AC is applied.

On the other hand, the transfer material on which the toner has been transferred is sent to the fixing device (heat roller fixing device) 9 by the transfer belt 30 and the image is fixed. Although the photoconductor drum is used in this embodiment, the photoconductor drum is not particularly limited thereto, and may be, for example, a photoconductor belt.

Next, the photosensitive drum used in the present invention will be described. The photoconductor used in this example is a negatively charged OPC photoconductor, and is one in which the following first to fifth functional layers are provided in order from the bottom on a drum base made of aluminum having a diameter of 30 mm. .

The first layer is an undercoating layer and has a thickness provided to smooth defects such as an aluminum drum substrate (hereinafter referred to as an aluminum substrate) and to prevent moire due to reflection of laser exposure. The conductive layer has a thickness of about 20 μm. The second layer is a positive charge injection preventing layer, which plays a role of preventing the positive charge injected from the aluminum substrate from canceling out the negative charge charged on the surface of the photoconductor, and the amylan resin and the methoxymethylated nylon make it 10 ^. It is a medium resistance layer having a thickness adjusted to about 6 Ω · cm and a thickness of about 1 μm. The third layer is a charge generation layer, which is a layer having a thickness of about 0.3 μm in which a disazo pigment is dispersed in a resin, and generates positive and negative charge pairs by being exposed to a laser. The fourth layer is a charge transport layer, which is a P-type semiconductor in which hydrazone is dispersed in a polycarbonate resin.

Therefore, the negative charges charged on the surface of the photoconductor cannot move in this layer, and only the positive charges generated in the charge generation layer can be transported to the surface of the photoconductor. Fifth
The layer is a charge injection layer, and has a particle size of 0.0, which is obtained by doping a photo-curable acrylic resin as a binder with antimony as a light-transmissive conductive filler to reduce the resistance (conductivity).
It is a coating layer of about 3 μm of a material in which ultrafine particles of 3 μm tin oxide are dispersed in 70% by weight of a resin.

The electric resistance value of this charge injection layer is 1 × 10 ^ 10, which is a condition that does not cause sufficient chargeability and image deletion.
It is necessary to be 1 × 10 ^ 14 Ω · cm. In this example, a photosensitive drum having a surface resistance of 1 × 10 11 Ω · cm was used.

Next, the charging device used in the present invention will be described with reference to FIG. The charging device 2 includes a container 10, a sleeve 12 made of a non-magnetic material having a fixed magnet 11 therein, and magnetic particles 1 for injecting an electric charge in contact with a photoconductor.
3. The regulating blade 14 coats the surface of the sleeve with magnetic particles to a uniform thickness. The sleeve 12 made of non-magnetic stainless steel has a 225 mm / sec. It is rotating at the peripheral speed of. The regulation blade 14 made of non-magnetic stainless steel is arranged so that the gap with the sleeve surface is 900 μm.

Magnet 11 fixedly arranged in the sleeve
Has a magnetic pole (main pole) of about 900 G arranged 10 ° upstream from the closest position of the sleeve and the photoconductor drum in the rotation direction of the photoconductor drum. This main pole has an angle (θ in the figure) with the closest position from 20 ° upstream to 10 ° downstream in the rotation direction of the photosensitive drum.
It is desirable to be within the range of 15 °, and 15 ° upstream
It is even better if it is ~ 0 °. If it is downstream from that, magnetic particles are attracted to the main pole position, and retention of magnetic particles easily occurs on the downstream side of the charging nip in the rotation direction of the photosensitive drum.
On the other hand, if it is too upstream, the transportability of the magnetic particles that have passed through the charging nip deteriorates, and retention tends to occur.

If there is no magnetic pole in the charging nip,
Obviously, the binding force of the magnetic particles on the sleeve is weakened, and the magnetic particles tend to adhere to the photosensitive drum. The charging nip described here indicates a region where the magnetic particles are in contact with the photosensitive drum during charging.

The charging bias is applied to the sleeve and the regulating blade by the power supply 15. DC has the same value as the required surface potential of the photosensitive drum (-70 in this embodiment).
0V). AC peak-to-peak voltage (hereinafter Vpp)
Is 100 V or more and 2000 V or less, particularly 300 V or more 1
200V or less is preferable. If Vpp is lower than that, the effect of improving the charging uniformity and the rising property of the potential is small, and if Vpp is higher than this, retention of magnetic particles and adhesion to the photosensitive drum are deteriorated. The frequency is preferably 100 Hz or more and 5000 Hz or less, and particularly preferably 500 Hz or more and 2000 Hz or less. If it is less than that, the effect of deterioration of adhesion of the charging carrier to the photosensitive drum, charging uniformity, and improvement of potential rising property becomes small,
If it is more than that, it becomes difficult to obtain the effect of improving the charging uniformity and the rising property of the potential. AC waveforms are rectangular, triangular,
A sin wave or the like is preferable.

In this embodiment, as the charging carrier, the one obtained by reducing the sintered ferromagnetic material (ferrite) was used, but in addition, the one obtained by kneading the resin and the ferromagnetic powder to form a particle, Alternatively, a mixture of conductive carbon or the like for adjusting the resistance value and a surface-treated one may be used in the same manner.

This charge carrier plays a role of favorably injecting charges into the trap level on the surface of the photoconductor and a charge caused by the concentration of the charge current on defects such as pinholes formed on the photoconductor. It must also have the role of preventing electrical breakdown of the member and the photoconductor.

Therefore, the resistance value of the charging member is 1 × 10 ^ 4.
Ω to 1 × 10 ^ 9Ω is preferable, and particularly 1 ×
It is preferably 10 ^ 4Ω to 1 × 10 ^ 7Ω. If the resistance value of the charging member is less than 1 × 10 4 Ω, pinhole leakage tends to occur, and if it exceeds 1 × 109 Ω, good charge injection tends to be difficult.

In order to control the resistance value within the above range, the volume resistance value of the magnetic particles of the present invention is 1 × 10 ^ 4Ω.
-Cm to 1 x 10 ^ 9 Ω-cm is preferable,
Particularly, it is preferably 1 × 10 ^ 4 Ω · cm to 1 × 10 ^ 7 Ω · cm.

The peak in the average particle size and particle size distribution measurement of the magnetic particles of the present invention is preferably in the range of 5 to 100 μm from the viewpoint of preventing charge deterioration due to contamination of the particle surface. The magnetic permeability of the magnetic particles is 270 emu / cm2 (the measuring method will be described later).

The resistance value of the charging member used in this example is 1
It was × 10 6 Ω · cm, and by applying −700V as the DC component of the charging bias, the surface potential of the photosensitive drum also became −700V.

Next, the developing device described in this embodiment will be described with reference to FIG.

The developing device 44 is arranged so as to face the image carrier 1, and the inside thereof is divided into a first chamber (developing chamber) 52 and a second chamber (stirring chamber) 53 by a partition wall 51 extending in the vertical direction. It is divided into A non-magnetic developing sleeve 54 that rotates in the arrow direction is arranged in the first chamber 52, and a magnet 55 is fixedly arranged in the developing sleeve 54.
The developing sleeve 54 is a two-component developer whose layer thickness is regulated by a blade 56 (including a magnetic carrier and a non-magnetic toner).
Is carried and conveyed, and a developer is supplied to the image bearing member 1 in the developing area facing the image bearing member 1 to develop the electrostatic latent image.

In order to improve the developing efficiency, that is, the rate of applying toner to the latent image, a developing bias voltage in which a DC voltage is superimposed on an AC voltage is applied from a power source 57 to the developing sleeve 54. Developer stirring screws 58 and 59 are arranged in the first chamber 52 and the second chamber 53, respectively.

The screw 58 stirs and conveys the developer in the first chamber 52, and the screw 59 conveys the toner from a toner outlet 61 of a toner replenishing tank 60, which will be described later, with a conveying screw 62.
The toner 63 supplied by the rotation of and the developer 43 already in the developing device are agitated and conveyed to make the toner concentration uniform. The partition wall 51 is provided with a developer passage (not shown) for communicating the first chamber 52 and the second chamber 53 with each other at the front and rear ends in FIG. The toner in the first chamber 52, whose toner concentration has been lowered due to the conveyance force of the toner, is moved from one passage into the second chamber 53, and the toner concentration is recovered in the second chamber 53. The developer flows from the other passage to the first chamber 52.
It is configured to move in.

In this embodiment, in order to correct the change in the developer density in the developing device 44 due to the development of the electrostatic latent image,
That is, in order to control the amount of toner supplied to the developing device 44, the inductance head 20 is installed on the bottom wall of the first chamber (developing chamber) 52 of the developing device 44, and the developing device is detected by the detection signal from the inductance head 20. The actual T / D of the developer 43 in 44, specifically in the first developing chamber 52.
An inductance detection system ATR is provided which detects the ratio and supplies toner by comparison with a reference value.

The toner particles used in the present invention are not particularly limited, and are, for example, spherical polymerized toners, and the production method thereof is a water-based monomer composition prepared by adding a colorant and a charge control agent to the monomers of the polymerization method. Spherical toner particles were obtained by suspending and polymerizing in a medium.

This method is suitable for inexpensively producing spherical toner. Further, a toner produced by a pulverization method which has been widely used in the past may be used. A low-magnetization carrier is used as the developing carrier used in the present invention, and high image quality can be achieved in combination with the spherical polymerization toner.

According to the experiments by the present inventors, the distance between the developer bearing member and the image bearing member (hereinafter referred to as S-Dgap) is 30.
0 to 1000 um, the amount of developer on the developer carrier per unit area (hereinafter referred to as M / S) is 15 to 50 mg / cm
2. When the toner concentration is within the range of 5 to 12%, the strength of magnetization of the developing carrier is 230 emu / cm3 or less when the strength of magnetization (σ1000) in a magnetic field of 1 kilo Oersted,
When it is preferably 140 emu / cm3 or less, the magnetic interaction between the adjacent magnetic brushes is small due to the low amount of magnetization, and as a result, the magnetic brush complement becomes minute and short, which causes the magnetic brush to appear on the latent image. Since the toner-adhering surface of (1) is softly peeled off, so-called scavenging, in which the developing toner is scraped off, can be prevented, and a high-resolution image can be provided.

In this embodiment, the strength of the magnetization of the developing carrier (σ1000) is 135 emu / cm 2. The above-mentioned magnetization characteristics were measured using an oscillating magnetic field type automatic recording apparatus for magnetic characteristics BHV-30 manufactured by Riken Denshi Co., Ltd.

The magnetic property value of the carrier powder is determined by creating an external magnetic field of 1 kilo Oersted and determining the strength of magnetization at that time. The carrier is packed in a cylindrical plastic container so that it is sufficiently tight. In this state, the magnetization moment is measured, and the actual weight when the sample is put in is measured to obtain the magnetization strength (emu / g). Then,
True specific gravity of carrier particles Dry automatic densitometer Acupic 1
330 (manufactured by Shimadzu Corporation), and the magnetic intensity per unit volume (emu / cm3) of the present invention was determined by multiplying the magnetic intensity (emu / g) by the true specific gravity.

Both the charging carrier and the developing carrier used in the present invention are soft magnetic materials, and the strength of the magnetization increases linearly with the increase of the magnetic field up to a magnetic field of about 1 kilo Oersted.

Therefore, the magnetic permeability has a slope tan shown in FIG.
It is proportional to α and tan β. Since the charging carrier has almost twice the magnetization intensity (σ1000) as the developing carrier, the magnetic permeability also doubles. From this, it can be understood that the detection output signals of the inductance detection sensor are different when the magnetic carriers have the same toner concentration but different magnetic permeability.

As described above, the two-component developer contains the magnetic carrier and the non-magnetic toner as the main components, and the developer 4
When the T / D ratio of 3 (the ratio of the weight of the toner particles to the total weight of the carrier particles and the toner particles) changes, the apparent magnetic permeability changes depending on the mixing ratio of the magnetic carrier and the non-magnetic toner. Inductance head 2
When detected by 0 and converted to an electric signal, the electric signal changes substantially linearly according to the toner concentration, as shown in FIG. That is, it is possible to detect the actual T / D ratio of the two-component developer in the developing device 44 by the output electric signal from the inductance head 20.

The processing of the output electric signal from the inductance head 20 will be described with reference to FIG. The output electric signal from the inductance head 20 is supplied to one input of the comparator 21. At the other input of the comparator 21, the reference voltage signal source 22 receives a reference electric signal corresponding to the apparent magnetic permeability of the developer 43 at the specified T / D ratio (T / D ratio at the initial setting value). It has been entered.

Therefore, since the comparator 21 compares the specified T / D ratio with the actual T / D ratio in the developing device, the detection signal of the comparator 21 is C as the comparison result of both input signals.
It is supplied to the PU 67. The CPU 67 controls so as to correct the next toner supply time based on the detection signal from the comparator 21. For example, the inductance head 20
When the actual T / D ratio of the developer 43 detected by is smaller than the specified value, that is, when the toner is insufficiently supplied, the CPU 67 supplies the insufficient toner to the developing device 44. The conveying screw 62 of the toner replenishing tank 60 is operated so as to do so. That is, based on the detection signal from the comparator 21, the screw rotation time required to replenish the developing device 44 with the insufficient toner is calculated, and the motor drive circuit 6
9 is controlled and the motor 70 is rotationally driven only for that time to supply the insufficient toner to the developing device 44.

When the actual T / D ratio of the developer 43 detected by the inductance head 20 is larger than the specified value, that is, when the toner is excessively replenished, the CPU 67 causes the comparator 67 to operate. The excess toner amount in the developer is calculated based on the detection signal from 21. Then, in the subsequent image formation by the original, the toner is replenished so as to eliminate the excess toner amount, or the image is formed without replenishing the toner until the excess toner amount is consumed, that is, there is no toner. Replenishment forms an image to consume the excess toner amount, and when the excess toner amount is consumed, the toner replenishing operation is performed as described above.

Next, the above operation will be further described with reference to the flow chart of FIG.

First, when the image forming apparatus is started, toner density detection is started in block S502. The detected voltage signal a from the inductance head is input to the comparator 21 in block S503, is compared with the reference voltage signal b from the reference voltage signal source 22 in the comparator 21 in block S504, and the detected signal difference is detected in block S505. (A-
b) is sent to the CPU 67.

In block S506, the CPU 67 determines whether (ab)> 0. If the T / D ratio is lower than the reference value (YES), the toner replenishment time is determined in block S507. After the copy operation is started in block S508, toner is supplied for the toner supply time determined in block S507 in block S509, and the process returns to the start. If the T / D ratio is higher than the reference value in block S506 (NO), the copy operation in block S510 is started, and the process returns to the start without supplying toner.

The timing for detecting the T / D ratio may be immediately before the restart of the copy operation or during the copy operation. For example,
The operation of the first sheet of the image forming apparatus may be detected immediately before the copy operation is resumed, and thereafter, during the copy operation.

Further, in the inductance detection type ATR used in this embodiment, the optimum T / D ratio (8% in this embodiment. If it is higher than this value, toner fogging, scattering, etc. occur, and it is low. If it is too much, a problem such as a decrease in image density may occur.) The reference value of the detection signal is adjusted to 2.5 V, and if the detection signal of the sensor is larger than the reference value (for example, 3. 0V) toner is replenished, and if the detection signal of the sensor is small (for example, 2.0V), toner replenishment is to be stopped.
The present invention is not of course limited to the above signal processing, and the circuit configuration may be changed so that the reference value is a value other than 2.5 V, and the sensor is used when the T / D ratio is lower than the optimum value. The detection signal of the sensor may be made smaller than the reference value of, and the detection signal of the sensor may be made larger when the T / D ratio is higher than the optimum value.

In the above-mentioned structure, as described in the prior art, as a problem in the magnetic brush charging device, the magnetic particles forming the charging magnetic brush are the magnetic carriers on the surface of the image carrier. Once collected and collected in the developing device due to adhesion and spillage, even if a small amount of charged carrier adheres to the developing device that uses an inductance detection type sensor, there is no problem in terms of image quality. When the magnetic permeability of the developing carrier and that of the charging carrier are different from each other, the apparent magnetic permeability of the developer as a whole changes, and an error may occur in the T / D ratio control by the inductance detection type sensor.

In addition, as described in the problem to be solved by the invention, the inductance head causes variations in transformer,
Variation in the detection value of the inductance detection sensor with respect to the variation in the T / D ratio, due to variations in the manufacturing method of the sensor itself, such as variations in case / bobbin and variations in assembly.
There may be individual differences in the sensitivity of so-called toner concentration sensors.

In response to this problem, the feature of the present invention is that the correction table for resetting the reference value of the detection signal of the inductance detection ATR to a new reference value is different for each sensor sensitivity of the inductance detection sensor. For example, as shown in FIG. 9, when the optimum toner concentration of the developer is 8%, the initial value of the detection signal from the inductance head 20 is 2.
When the toner sensitivity is 0.5V / ± 1% and the sensitivity is 0.5V / wt% and 0.25V / wt%, the inductance detection sensors A and B are set to 5V. Approximately 10g of charge carrier mixed into the device
It is installed in the developing device of the image forming apparatus which is about 50,000 times.

Therefore, when the detection signal of the inductance detection sensor is set to 2.5 V with respect to the reference toner concentration and the charge carrier 10 g is forcibly mixed into the developer, the apparent magnetic permeability of the developer is obtained. Becomes larger,
In the inductance detection sensor A, the initial optimum T / D
When the ratio is maintained, the detection signal of the inductance detection sensor rises from the initial reference value of 2.5V to 0.5V by 0.5V.

Therefore, the toner is actually replenished so that the detection signal becomes 2.5V, so that the toner is replenished excessively, and the sensitivity of the inductance detection sensor A used in the image forming apparatus is 0. Since it is 5 V /% (FIG. 9), as a result of repeating the image forming operation 50,000 times, the toner density finally deviates from the optimum toner density of 8% by 1% and is controlled to 9%. For this malfunction, the initial reference value of the inductance detection sensor is 2.
When the image forming operation is repeated 25,000 times for a voltage of 5 V, if the mixing amount of the charging carrier is proportional to the image forming operation, the amount is 10 g / 50000 times, which is 5 g / 25000 times.

In the inductance detection sensor, the value changes by 0.5 V when 10 g of the charge carrier is mixed.
When about 5g of charge carrier is mixed, the value is 0.25
V will change. Therefore, an instruction is set in advance in the CPU 25 in FIG. 7 so that the reference value b of the detection signal in S504 of the flowchart in FIG. 8 is reset to 2.75V. At that timing, the CPU 25 sets the reference value b output from the reference voltage signal source 22 to 2.7.
Reset to 5V. At that time, the toner replenishment is stopped until the detection signal becomes 2.75 V, so that the error in the toner density that has occurred up to that point is eliminated and the initial toner density is 8%.
Will be controlled again.

Further, when the image forming operation is repeated 50,000 times, by changing the voltage by 0.25 V to 3.0 V, the toner density after 50,000 times of the image forming operation is about 8% which is the reference toner density and the detection signal It is possible to eliminate an error in toner concentration control (see FIG. 10 (ii)), as compared with the case where no correction is performed (see FIG. 10 (ii)). The correction table at this time is referred to as correction table I.

Next, the sensor B will be described. Similar to the above, when the detection signal of the inductance detection sensor is set to 2.5 V with respect to the reference toner concentration and the charge carrier 10 g is forcibly mixed into the developer, the apparent magnetic permeability of the developer is obtained. Becomes larger and the detection signal of the sensor rises. At this time, the sensor sensitivity of sensor B is 0.25V / wt%, which is half that of sensor A (Fig. 7), so the detection signal of the inductance detection sensor when the initial optimum T / D ratio is maintained is the initial reference. Value 2.5
It rises 0.25V from V to 2.75V.

Therefore, since toner is actually supplied so that the detection signal becomes 2.5 V, the image forming operation is performed 50 times.
As a result of repeating 000 times, finally, like the sensor A, the toner concentration deviates from the optimum toner concentration of 8% by 1%, and is controlled to 9%. Against this malfunction, in the developing device equipped with the inductance detection sensor B, when the initial reference value of the inductance detection sensor was 2.5 V, when the image forming operation was repeated 25,000 times, the flowchart of FIG. An instruction is set in advance in the CPU 25 so as to reset the reference value b of the detection signal in S504 of 2.675V.

At that timing, the CPU 25 sets the reference value b output from the reference voltage signal source 22 to 2.675V.
Set again. At that time, the toner replenishment is stopped until the detection signal becomes 2.675 V, so the error in the T / D ratio that has occurred up to that point is eliminated, and the initial T / D ratio is re-controlled to 8%. Become.

Further, when the image forming operation is repeated 50,000 times, the voltage is further changed by 0.125 V to 2.75 V so that the T / D ratio after the image forming operation is 50,000 times is about 8% which is the reference T / D ratio. As compared with the case where the detection signal is not corrected, the toner concentration control error can be eliminated. The correction table at this time is referred to as correction table II. Therefore, by specifying the above correction table at the time of initial installation of the image forming apparatus or the initial installation of the developing device,
Even if the inductance detection sensors having different sensor sensitivities are used, it is possible to prevent an error in the T / D ratio control caused by the magnetic particles used in the magnetic brush charging device being collected by the developing device.

The correction table is selected at the factory production stage before the inductance detection sensor is installed in the developing device. For example, a cylinder having a hole with the same diameter as the sensor surface of the toner concentration sensor on the bottom surface as shown in FIG. Prepare a jig 80 for the mold
/ D ratio of 7%, 8%, 9% different developer is put so that the developer surface is the same (so that the developer thickness is constant with respect to the toner concentration sensor), and put it in the jig. It suffices to mount the sensors one by one, check the sensor sensitivity from the output value corresponding to the T / D ratio of each sensor, and select the correction table.

A plurality of correction tables are prepared in advance and CP
The inductance detection sensor 2 stored in U25 and manually selected when the developing device is installed in the image forming apparatus or installed in the ROM installed in the developing device.
A sensor sensitivity of 0 may be stored and automatically selected at the time of installation.

According to the present embodiment, an inductance detection sensor for detecting the apparent magnetic permeability of the two-component developer is installed in the developing container to control the T / D ratio, and the charge carrier used for charging the magnetic brush forms an image. By repeating the operation, when the mixture of the charge carriers in the developing device due to the adhesion of the charge carriers to the image carrier is accumulated,
Even if there are variations in the sensitivity of the inductance detection sensor, the T / D ratio can be accurately controlled by having a correction amount that newly sets the reference value for the reference T / D ratio according to the sensitivity of the inductance detection sensor used. In addition, it is not necessary to select the sensitivity of the inductance detection sensor (whether or not it can be used), and the yield of the sensor can be increased.

The timing at which the reference value b newly set by the reference signal value correcting means is reset stepwise is based on the information on the number of formed images, or the video count of the image density signal of the image information signal. You may perform based on a number.

Furthermore, in the present embodiment, the case where the magnetic brush charging is used and the mixing of the charge carrier into the developing device is accumulated is described as an example, but it is needless to say that the charging device does not use the magnetic brush charging. A method of resetting the reference value of the T / D ratio control with respect to the reference toner density to a new reference value due to an error in the T / D ratio control by the inductance detection sensor due to the change in the toner charge amount with the increase in the number of image forming sheets. When the above is adopted, the same effect can be obtained by implementing the present invention.

(Second Embodiment) The reference value b newly set by the reference signal value correcting means in the above first embodiment.
As shown in FIG. 12B, a plurality of correction tables that are newly set linearly are provided depending on the sensitivity of the inductance detection sensor, which enables more accurate T / D ratio control (see FIG. c)).

(Third Embodiment) The charge carrier has a certain degree of spread in the particle size distribution, and it has been found by the study by the present inventors that the charge carrier is likely to adhere to even minute particles. It is considered that such a minutely charged carrier adheres to the carrier especially when the time or the number of times of image forming operation is early, and the change in the apparent magnetic permeability of the developer at the initial stage becomes large.

After that, when the minute charged carriers are reduced in the charging device, the change in the apparent magnetic permeability of the developer is gradually reduced and may be changed in a non-linear manner (FIG. 13A).

Therefore, the reference value b newly set by the reference signal value correcting means in the above-described first embodiment is set as shown in FIG.
As shown in FIG. 13B, by providing a plurality of correction tables to be newly set depending on the sensitivity of the inductance detection sensor, more accurate T / D ratio control becomes possible (FIG. 13).
(C)).

In each of the embodiments described above,
Although the present invention is applied to an electrophotographic digital copying machine, the present invention is equally applied to various copying machines such as electrophotographic and electrostatic recording methods other than the embodiments, and image forming apparatuses such as printers. It is possible. For example, the present invention can be applied to an image forming apparatus that performs grayscale representation of an image by a dither method, and also to an image forming apparatus that forms a toner image based on an image information signal output from a computer or the like, instead of copying a document. The present invention can also be applied.

Furthermore, it goes without saying that various modifications and changes can be made to the configuration of the image forming apparatus and the control system, etc., as required.

[0108]

As described above, according to the present invention,
A charging device facing the image carrier and charging the image carrier;
An exposure device for forming an electrostatic latent image on the charged image carrier,
In an image forming apparatus having a plurality of developing devices for forming a visible image by using a two-component developer composed of toner particles and carrier particles for the electrostatic latent image, the apparent magnetic permeability of the two-component developer is detected, A toner concentration control device that has a toner concentration sensor that transmits a detection signal, and that controls the toner concentration of the developer based on the comparison result of the detection signal value and a predetermined reference signal value, A reference signal value correcting unit for resetting the reference signal value as a new reference signal value based on the operating time or the number of operations of each developing device is provided, and the correction amount corrected by the reference signal value correcting unit is an image. Since the sensitivity of the toner concentration sensor installed in the forming device varies, even if the sensor sensitivity of the toner concentration sensor is different, the charge carrier is mixed in the developer. Even if you were being, it has enabled less toner concentration control of error.

[Brief description of drawings]

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

FIG. 2 is a detailed view of an image forming unit in the image forming apparatus according to the present invention.

FIG. 3 is a schematic cross-sectional view showing a schematic configuration of a charger included in the image forming unit of FIG.

FIG. 4 is a schematic cross-sectional view showing a schematic configuration of a developing device included in the image forming unit of FIG.

FIG. 5 is a diagram illustrating a difference in magnetic permeability between the charging carrier and the developing carrier in the present invention.

FIG. 6 is a characteristic diagram showing a state in which the detection signal from the inductance head changes due to a change in toner concentration of the developer.

FIG. 7 is a diagram illustrating toner replenishment control by the inductance detection sensor of the present invention.

FIG. 8 is a flow chart for explaining the basic operation of the embodiment of the present invention.

FIG. 9 shows the sensor sensitivity of the inductance detection sensor of the present invention.

FIG. 10 is a diagram simply showing the relationship between the operating time of the image forming apparatus and the toner concentration of the developer in the first embodiment.

FIG. 11 shows a jig for confirming the sensitivity of the inductance detection sensor in the first embodiment.

FIG. 12 is a diagram simply showing the relationship between the operating time of the image forming apparatus, the reference signal value of the inductance detection sensor, and the toner concentration of the developer in the second embodiment.

FIG. 13 is a diagram simply showing the relationship between the operating time of the image forming apparatus, the apparent magnetic permeability of the developer, the reference signal value of the inductance detection sensor, and the toner concentration of the developer in the third embodiment.

FIG. 14 is a simplified representation of the relationship between the apparent magnetic permeability of the developer, the detection signal of the inductance detection sensor, and the toner concentration of the developer when charge carriers are mixed and accumulated in the developer. Is. FIG. 6 is a diagram showing that the mixed amount of charge carriers varies depending on the color of the developer.

FIG. 15 shows a table in which a reference signal value is newly set for the number of image formations.

[Explanation of symbols]

3Y, 3M, 3C, 3K charger 4Y, 4M, 4C, 4K stations 30 transfer belt

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 2H027 DA39 DA45 DA46 DB01 DD07                       DE07 EA06 EB04 EC06 EC09                       EC15 EC17 EC18 EC20 EF10                       EF12                 2H030 AB02 AD02 BB23 BB36 BB63                 2H077 AA25 AB02 AB14 AB15 AB18                       AC02 AD06 AD13 AD18 AD36                       DA10 DA42 DA52 DA80 DB02                       DB22 EA03 GA13                 2H200 GA12 GA14 GA16 GA23 GA34                       GA44 GA47 GA53 GA57 GA60                       GB25 GB36 GB37 HA02 HA12                       HA21 HA28 HB03 HB12 HB17                       HB22 HB45 HB46 HB47 HB48                       JA02 JB06 LB03 LB18 MA01                       MA02 MA14 MA20 MB04 MB06                       MC13 MC15 NA04 NA05 NA06                       NA09 NA10 PB01 PB29 PB32                       PB34 PB35 PB38

Claims (7)

[Claims] 1. A charging device facing the image carrier to charge the image carrier, an exposure device to form an electrostatic latent image on the charged image carrier, and the electrostatic latent image to toner particles. In an image forming apparatus having a plurality of developing devices for forming a visible image using a two-component developer composed of carrier particles, a toner concentration sensor for detecting an apparent magnetic permeability of the two-component developer and transmitting a detection signal is provided. A toner concentration control device that controls the toner concentration of the developer based on a comparison result between the detection signal value and a predetermined reference signal value has an operating time of each of the plurality of developing devices, or an operation. A toner density sensor provided with a reference signal value correction unit that resets the reference signal value as a new reference signal value based on the number of times, and the correction amount corrected by the reference signal value correction unit is installed in the image forming apparatus. Feeling of An image forming apparatus comprising differs depending. 2. The charging device is a charging device for charging the image bearing member by bringing a charging member using a magnetic brush into contact with the image bearing member and applying a charging bias to the charging member. The image forming apparatus according to claim 1. 3. The image forming apparatus according to claim 1, wherein the new reference signal value is stepwise changed and set according to the operation time or the number of operations of each developing device. 4. The image forming apparatus according to claim 1, wherein the new reference signal value is linearly changed and set according to the operation time or the number of operations of each developing device. 5. The image forming apparatus according to claim 1, wherein the new reference signal value is non-linearly changed and set according to the operation time or the number of operations of each developing device. 6. The image forming apparatus according to claim 1, wherein the time or the number of times of the image forming operation for each of the developing devices is determined based on copy number information. .. 7. The image forming operation time or number of times for each developing device is determined from the video count number of the image density signal of the image information signal.
The image forming apparatus according to claim 1.