JP2009276559A - Image density control device and image forming apparatus - Google Patents

Abstract

An image density control apparatus capable of maintaining an appropriate and stable image density even when an abrupt change in an image output mode occurs, and an image forming apparatus including the image density control apparatus.
When an inclination of a change curve with respect to time of an image area ratio cumulative moving average value deviates from a predetermined threshold value K range, a differential value of the change curve at the time point X1 to X3 when the value deviates from the threshold value (the curve concerned). The target output value Vtref is set so that the potential control is forcibly performed as indicated by arrows Z1, Z2, and Z3 in accordance with the inclination of the image and the development γ approaches the target development γ (Y value). I am trying to correct it. When the image area ratio cumulative moving average value is within the predetermined threshold value K, the second target output value correction device calculates the target output value correction amount (ΔVtref) for each sheet and calculates the target output value (Vtref). Thus, toner consumption due to potential control is suppressed and the target output value can be corrected with high accuracy.
[Selection] Figure 5

Description

  The present invention relates to an image density control device used in an image forming apparatus such as a copying machine, a printer, and a facsimile, and in particular, an image density control device using a two-component developer composed of toner and a magnetic carrier and the same. The present invention relates to an image forming apparatus including

In recent copying machines and laser printers, high image quality is demanded, and at the same time, high durability and high stability are also desired. In other words, it is necessary to reduce the change in image quality due to changes in the usage environment (including continuous printing and intermittent printing), and to provide a stable image over time. Conventionally, a two-component developer (hereinafter referred to as a developer) composed of a non-magnetic toner and a magnetic carrier is held on a developer carrier (hereinafter referred to as a development sleeve) and is magnetically generated by a magnetic pole contained in the development sleeve. Two-component development in which a brush is formed and development is applied by applying a developing bias at a position facing an image carrier (hereinafter referred to as a photoreceptor) that carries an electrostatic latent image (hereinafter referred to as a latent image) on the developing sleeve. The method is widely known.
This two-component development system is currently widely used because it is easy to colorize and the gradation on the highlight side is relatively linear. In this system, the two-component developer is transported to a developing area close to the photosensitive member as the developing sleeve rotates. As the developer is transported to the developing area, a large number of magnetic carriers in the developer gather together with the toner along the magnetic field lines of the magnetic poles contained in the developing sleeve to form a magnetic brush. The toner is transferred from the magnetic brush to a latent image on the photosensitive member by an electric field action, and the latent image is converted into a toner image and developed.
In such a two-component development method, unlike the one-component development method, it is very important to accurately control the weight ratio (toner concentration) of the toner and the magnetic carrier in order to improve the stability of image formation. ing. For example, if the toner concentration is too high, background stains are generated on the image and the detail resolution is reduced. Further, when the toner density is low, problems such as a decrease in toner density in the solid image portion and carrier adhesion occur. Therefore, it is necessary to control the toner replenishment amount and adjust the toner concentration in the developer to an appropriate range.
Such toner density control is performed by comparing the toner density in the developing device with an output value (Vt) detected by a detecting means such as a magnetic permeability sensor and a preset toner density control reference value (Vtref). The toner replenishing amount is calculated from an arithmetic expression according to the difference, and toner is replenished from the toner replenishing device into the developing device.

In an image forming apparatus, it is generally important to perform image formation so that a constant image density is always obtained. The image density is mainly determined by the developing ability of the developing device. The developing ability is the ability to indicate how much toner can be attached to the latent image during development. In addition to the toner density, the developing sleeve to which the latent image on the surface of the photoreceptor and the developing bias are applied. It also changes depending on development conditions such as development potential, which is a potential difference from the surface, and the charge amount of toner that contributes to development. As an index indicating the developing ability, a slope (development γ) of a relational expression indicating a toner adhesion amount with respect to the developing potential is widely known. As described above, since the image density is determined by the developing ability of the developing device, the image density cannot be made constant only by performing the toner density control that maintains the constant toner density by the magnetic permeability sensor as described above.
Although development conditions such as development potential can be made relatively easily constant, it is difficult to make the toner charge amount contributing to development constant. For this reason, even if the developing conditions are made constant and the toner density is controlled so that the toner density becomes constant, the developing ability cannot be made constant and a constant image density cannot be obtained.
More specifically, for example, when an image with a low image area ratio is output, the amount of toner consumed by the development is relatively small, so the amount of toner replenished to maintain a desired toner density is small. . For this reason, the amount of toner present in the developing device for a relatively long time is large. Since the toner that has been present in the developing device for a relatively long time has been subjected to a stirring action for a long time, the toner in the developing device tends to be excessively charged. Therefore, the developing ability is relatively low.

On the other hand, when an image with a high image area ratio is output, there are many new toners that have not been sufficiently charged and have just been replenished, and are sufficiently charged to the desired charge amount in the toner that contributes to development. A large proportion of toner is not used. As a result, the developing ability is relatively high. In particular, in recent years, there is a tendency to reduce the amount of developer held in the developing device as much as possible in order to meet the demand for downsizing the developing device. For this reason, at the time of image formation performed after outputting an image with a high image area ratio, the proportion of toner that is not sufficiently charged to a desired charge amount contributes to development. Therefore, the developing ability at the time of image formation after outputting an image having a high image area ratio tends to be relatively high.
Further, depending on the configuration, the developing ability may be higher when an image with a low image area ratio is output than when an image with a high image area ratio is output. For example, when using a toner with an external additive attached and using a developing device with a high stress on the toner, the toner existing in the developing device for a relatively long time is subjected to a stirring action for a long time. The agent is buried or the external additive is detached from the toner surface. When such toner is abundant, the fluidity of the developer is deteriorated or the charging ability of the toner itself is lowered, so that the toner contributing to development cannot be sufficiently charged to a desired charge amount.
Therefore, when an image with a low image area ratio is output, the proportion of toner that is not sufficiently charged up to the desired charge amount in the toner that contributes to development increases, so that the development ability is relatively high It becomes. On the other hand, when an image with a high image area ratio is output, the amount of toner replenished is large, so that the amount of toner present in the developing device for a relatively long time is small. Therefore, there are many toners having sufficiently good developer fluidity and sufficiently high charging ability. Therefore, the toner that contributes to development can be sufficiently charged up to a desired charge amount, so that the developing ability is relatively low.
As described above, when an image with a low image area ratio is output and when an image with a high image area ratio is output, the ratio of new toner existing in the developing device differs depending on the subsequent toner supply. There will be a difference in ability. Therefore, even if the developing conditions are made constant and the toner density is controlled so that the toner density becomes constant, the developing ability cannot be made constant and a constant image density cannot be obtained.

As what can suppress this malfunction, the image forming apparatus described in patent document 1 or patent document 2 is mentioned, for example. This image forming apparatus has a toner density detecting means for detecting and outputting the toner density in the two-component developer in the developing apparatus, and compares the output value of the toner density detecting means with the toner density control reference value. Based on the comparison result, the toner replenishing device is controlled so that the toner concentration of the two-component developer in the developing device becomes a desired toner concentration. Then, by detecting the density of the reference toner pattern formed in the non-image portion, the image density at the time of forming the reference toner pattern is grasped, and the toner density control reference value is corrected based on the detection result. According to this method, it is possible to form an image with a desired image density for a while after the correction. Therefore, it is possible to obtain a constant image density by periodically forming the reference toner pattern and correcting the toner density control reference value according to the detection result.
However, in these patent documents 1 and 2, in order to correct the toner density control reference value, it is necessary to form a reference toner pattern each time. Therefore, there arises a problem that the amount of toner that is not used for image formation increases.
In order to solve the above problems, the present inventors have proposed an image forming apparatus described in Patent Document 3. More specifically, the image forming apparatus described in Patent Document 3 detects information for grasping the toner replacement amount in the developing device within a predetermined period, for example, a moving average value of the image area ratio of the output image. It has information detection means. Based on the detection result of the information detection means, the ratio of the new toner or the old toner present in the developing device is grasped, and the developing ability of the developing device is grasped.
Then, based on the detection result of the information detection means, the toner density control reference value correction means corrects the toner density control reference value and adjusts the toner density in the developing device, thereby obtaining a constant image density. In the image forming apparatus described in Patent Document 3, the information on the toner replacement amount used for correcting the toner density control reference value does not create a toner pattern such as detecting the image area ratio of the output image, and consumes extra toner. Therefore, it is possible to suppress an increase in consumption of toner that is not used for image formation.
JP-A-57-136667 JP-A-2-34877 JP 2007-133235 A

According to the method described in Patent Document 3, the toner density is adjusted and the developing ability is kept constant without generating a toner pattern and consuming extra toner when correcting the target output value (Vtref), and a constant image density is maintained. Can be obtained. However, this method has a problem that it is difficult to follow the image output mode when it suddenly changes, and it is difficult to stabilize the image density.
The present invention has been made in consideration of the above circumstances, and includes an image density control device capable of maintaining an appropriate and stable image density even when a sudden change in the image output mode occurs, and the same. Another object of the present invention is to provide an image forming apparatus.

  In order to solve the above problems, the invention of claim 1 is directed to an image carrier that carries a latent image, a charging device that charges the image carrier to a predetermined potential, and a surface of the image carrier that is charged to a predetermined potential. A latent image forming apparatus for forming a latent image on the toner carrier, and a developer carrying member carrying a two-component developer containing toner and a magnetic carrier, while applying a developing bias to the developer carrying member, the toner on the developer carrying member is transferred to the image carrier. A developing device that transfers the latent image to the upper latent image and develops the latent image, a replenishing device that replenishes toner in the developing device, and a toner concentration that detects the toner concentration in the two-component developer in the developing device And a toner concentration control device that controls toner supply by controlling toner supply from the supply device in accordance with a toner concentration control reference value that is referred to for controlling the toner concentration in the developing device. Image density with In the control device, a transfer device for transferring the toner image obtained by the development from the image carrier to the transfer member, and a plurality of toners formed on the transfer member under image forming conditions having different adhesion amounts Calculated from a gradation pattern forming device that forms a gradation pattern composed of patches, an optical detection device that detects reflected light from the gradation pattern, and an output of the gradation pattern detected by the optical detection device A developing γ calculating device that calculates a development γ from a toner adhesion amount that is generated, a developing potential calculated from the charging potential of the charging device and the developing bias, and the developing γ calculated from the developing γ calculating device. A first toner density control reference value correcting device that corrects the toner density control reference value by changing a developing potential; and a toner image formed on the image carrier. An image area rate average value change rate calculating device for calculating a change rate of the image area rate average value and a change rate of the image area rate average value of the toner image calculated from the image area rate average value change rate calculating device. The change rate of the average image area ratio calculated from the second toner density control reference value correction apparatus for correcting the toner density control reference value and the image area ratio average value change rate calculation apparatus deviates from a predetermined range. And a forcible operation control device for operating the first toner density control reference value correction device.

According to a second aspect of the present invention, in the image density control apparatus according to the first aspect, the average image area ratio of the toner image is a moving average of the image area ratio of an image formed within a predetermined period. Features.
According to a third aspect of the present invention, in the image density control apparatus according to the second aspect, the moving average value of the image area ratio of the image formed within the predetermined period is:
M (i) = (1 / N) × {M (i−1) × (N−1) + X (i)}
(In the above equation, “N” is the sampling number of the image area ratio, “M (i−1)” is the moving average value calculated last time, and “X (i)” is the image detected this time. Area ratio)
It is calculated based on.
According to a fourth aspect of the present invention, in the image density control device according to any one of the first to third aspects, the forced operation control device includes:
| ΔMn | ≧ K
(Where “| ΔMn |” is the absolute value of the rate of change of the average value of the image area ratio of the predetermined number of sheets, and “K” is 25% / (predetermined number of sheets))
The first toner density control reference value correction device is controlled to operate when the above condition is satisfied.

According to a fifth aspect of the present invention, in the image density control device according to any one of the first to fourth aspects, the humidity change rate received by the image density control device deviates from a predetermined range. It is controlled to operate sometimes.
According to a sixth aspect of the present invention, in the image density control apparatus according to the fifth aspect, the humidity change rate is a humidity change rate per time.
According to a seventh aspect of the present invention, in the image density control device according to any one of the first to sixth aspects, the developing device includes a stirring device that stirs the toner and the magnetic carrier, and the stirring device includes the second stirrer. When the toner density control reference value is corrected by the toner density control reference value correction device, the toner and the magnetic carrier are controlled to be agitated.
According to an eighth aspect of the present invention, there is provided an image forming apparatus comprising the image density control apparatus according to any one of the first to seventh aspects.

  According to the present invention, a transfer device that transfers a toner image obtained by development from an image carrier to a transfer body, and a plurality of toners that are formed on the transfer body under image forming conditions with different adhesion amounts. Calculated from a gradation pattern forming device that forms a gradation pattern composed of patches, an optical detection device that detects reflected light from the gradation pattern, and an output of the gradation pattern detected by the optical detection device A developing γ calculating device that calculates a development γ from a toner adhesion amount that is generated, a developing potential calculated from the charging potential of the charging device and the developing bias, and the developing γ calculated from the developing γ calculating device. A first toner density control reference value correction device that corrects the toner density control reference value by changing a development potential; and an image surface of a toner image formed on the image carrier. An image area rate average value change rate calculating device that calculates a rate average value change rate, and the toner according to a change rate of an image area rate average value of a toner image calculated from the image area rate average value change rate calculating device When the rate of change of the image area rate average value calculated from the second toner density control reference value correction device for correcting the density control reference value and the image area rate average value change rate calculating device deviates from a predetermined range, By providing a forcible operation control device that operates the first toner concentration control reference value correction device, it is possible to maintain an appropriate and stable image density even when an abrupt change in the image output mode occurs. Image density control apparatus and an image forming apparatus including the same can be provided.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus includes four image forming units 1Y, 1C, 1M, 1Bk (hereinafter referred to as “image forming units”) for forming images of magenta (M), cyan (C), yellow (Y), and black (Bk) colors. The subscripts Y, C, M, and Bk of the respective symbols indicate that they are members for yellow, cyan, magenta, and black, respectively), but are endlessly stretched around a plurality of rollers 12 as intermediate transfer members. The intermediate transfer belt 6 is disposed in order from the upstream side in the surface movement direction (the direction of arrow A in FIG. 1). The image forming units 1Y, 1C, 1M, and 1Bk respectively include photoreceptor units 10Y, 10C, 10M, and 10Bk having drum-like photoreceptors 11Y, 11C, 11M, and 11Bk as image carriers, and a developing device 20Y. 20C, 20M, and 20Bk. The image forming units 1Y, 1C, 1M, and 1Bk are arranged such that the rotation axes of the photoconductors 11Y, 11C, 11M, and 11Bk in each photoconductor unit are parallel to each other and the surface movement direction of the intermediate transfer belt 6 is set. Are arranged at a predetermined pitch.

The toner images on the photoconductors 11Y, 11C, 11M, and 11Bk formed by the image forming units 1Y, 1C, 1M, and 1Bk are sequentially superimposed and primarily transferred onto the intermediate transfer belt 6 by the primary transfer roller 13. The The color image obtained by overlapping is conveyed to a secondary transfer portion between the intermediate transfer belt 6 and the secondary transfer roller 3 as the surface of the intermediate transfer belt 6 moves.
In the image forming apparatus, in addition to the image forming units 1Y, 1C, 1M, and 1Bk, an optical writing unit (not shown) is disposed below the image forming unit 1Y, 1C, 1M, and 1Bk. ing. A one-dot chain line in FIG. 1 indicates a conveyance path of the transfer paper P. The transfer paper P fed from a paper feed cassette (not shown) is transported by a transport roller while being guided by a transport guide (not shown), and is sent to a temporary stop position where a registration roller 5 is provided. The transfer paper P is supplied to the secondary transfer roller 3 by the registration roller 5 at a predetermined timing. The color image formed on the intermediate transfer belt 6 is secondarily transferred onto the transfer paper P, and a color image is formed on the transfer paper P. The transfer paper P on which the color image is formed is discharged onto a paper discharge tray (not shown) after the toner image is fixed by the fixing unit 7.

FIG. 2 is an enlarged sectional view showing a schematic configuration of the yellow image forming means 1Y among the image forming means 1Y, 1C, 1M, and 1Bk shown in FIG. Since the other image forming means 1M, 1C, and 1Bk have the same configuration, their descriptions are omitted. In FIG. 2, the image forming unit 1Y includes the photoreceptor unit 10Y and the developing device 20Y as described above. In addition to the photoconductor 11Y, the photoconductor unit 10Y includes a cleaning blade 13Y that cleans the surface of the photoconductor, a charging roller 15Y that uniformly charges the surface of the photoconductor, and the like.
Also provided is a lubricant application and static elimination brush roller 14Y having a function of applying a lubricant to the surface of the photoreceptor and removing the charge on the surface of the photoreceptor. The lubricant application and static elimination brush roller 14Y has a brush portion made of conductive fibers, and a metal base power source (not shown) for applying a static elimination bias is connected to the core portion.
In the photoreceptor unit 10Y having the above configuration, the surface of the photoreceptor 11Y is uniformly charged by the charging roller 15Y to which a voltage is applied. When the surface of the photoconductor 11Y is irradiated with laser light L _Y (see FIG. 1) modulated and deflected according to image information by an optical writing unit (not shown) while being scanned, the surface of the photoconductor 11Y is electrostatically charged. A latent image is formed. The electrostatic latent image on the photoreceptor 11Y is developed by a developing device 20Y described later to become a yellow toner image. The toner image on the photoreceptor 11Y is transferred onto the intermediate transfer belt 6 by the primary transfer roller 13 (see FIG. 1) of the primary transfer portion where the photoreceptor 11Y and the intermediate transfer belt 6 face each other.

The surface of the photoconductor 11Y after the toner image is transferred is cleaned by a cleaning blade 13Y as photoconductor cleaning means, and then a predetermined amount of lubricant is applied by a lubricant application / static elimination brush roller 12Y and static elimination is performed. To prepare for the next electrostatic latent image.
The developing device 20Y uses, as a developer for developing the electrostatic latent image, a two-component developer (hereinafter simply referred to as “developer”) containing a magnetic carrier and, for example, negatively charged toner. ing. Further, the developing device 20Y includes a developing sleeve 22Y made of a nonmagnetic material as a developer carrying member disposed so as to be partially exposed from the opening 20Yb on the photosensitive member side of the developing case 20Ya, and a developing sleeve 22Y. A magnet roller (not shown) as a magnetic field generating means arranged in a fixed manner, stirring and conveying screws 23Y and 24Y as a stirring and conveying member, a developing doctor 25Y, a magnetic permeability sensor 26Y as a toner concentration detecting means, and a powder pump as a toner replenishing device 27Y etc. are provided.
A developing bias voltage in which an AC voltage AC (AC component) is superimposed on a negative DC voltage DC (DC component) is applied to the developing sleeve 22Y by a developing bias power supply (not shown) as a developing electric field forming unit, and the developing sleeve 22Y is developed. Is biased to a predetermined voltage with respect to the metal substrate layer of the photoreceptor 11Y. Note that only a negative DC voltage DC (DC component) may be applied as the developing bias voltage.

In FIG. 2, the developer contained in the developing case 20Ya is agitated and conveyed by the agitating and conveying screws 23Y and 24Y, whereby the toner is frictionally charged. Then, a part of the developer in the first agitation conveyance path where the first agitation conveyance screw 23Y is arranged is carried on the surface of the development sleeve 22Y, and the layer thickness is regulated by the development doctor 25Y, and then the photoreceptor 11Y It is conveyed to the opposite development area. In the developing area, the toner in the developer on the developing sleeve 22Y adheres to the electrostatic latent image on the photoconductor 11Y by the developing electric field and becomes a toner image.
Thereafter, the developer that has passed through the developing region leaves the developing sleeve 22Y at the developer separating pole position on the developing sleeve 22Y and returns to the first agitation transport path. The developer conveyed to the downstream end of the first agitation conveyance path moves to the upstream end of the second agitation conveyance path where the second agitation conveyance screw 24Y is arranged, and receives toner supply in the second agitation conveyance path. . Thereafter, the developer transported to the downstream end of the second stirring transport path moves to the upstream end of the first stirring transport path. A magnetic permeability sensor 26Y is installed in the developing case 20Ya that forms the bottom of the second agitation transport path.
Since the toner density of the developer in the developing case 20Ya is lowered due to toner consumption accompanying image formation, the toner pump 30Y from the toner cartridge 30Y shown in FIG. 1 may be used as needed based on the output value Vt of the magnetic permeability sensor 26Y. Thus, the toner is replenished so as to be controlled within an appropriate range.

In the toner replenishment control, based on the difference value Tn (= Vtref−Vt) between the output value Vt and the target output value Vtref which is the toner density control reference value, the toner density is sufficient when the difference value Tn is + (plus). When the difference value Tn is-(minus) when the toner value is determined to be high and the difference value Tn is-(minus), the toner supply amount is increased as the absolute value of the difference value Tn increases, and the output value Vt becomes equal to the target output value Vtref. Do as close to the value.
This target output value Vtref needs to be corrected according to the image forming conditions in order to obtain a stable image density. As will be described later, in the present invention, the first target output value Vtref is based on the potential control. Correction is performed by a correction device and a second target output value correction device based on an image area ratio average value change rate described later.
Of the four photoconductors 11Y, 11C, 11M, and 11Bk, only the black photoconductor 11Bk on the most downstream side is always in contact with the intermediate transfer belt 6, and the remaining photoconductors are in contact. 11M, 11C, and 11Y can contact and separate from the intermediate transfer belt. When forming a color image on the transfer paper, the four photoconductors 11Y, 11C, 11M, and 11Bk are in contact with the intermediate transfer belt 6, respectively. On the other hand, when a black monochrome image is formed on the transfer paper, only the black photoconductor 11Bk on which the toner images with black toner are formed by separating the photoconductors 11Y, 11C, and 11M for each color from the intermediate transfer belt 6. Is brought into contact with the intermediate transfer belt 6.

Next, a control unit as a control unit that performs toner density control will be described. FIG. 3 is an explanatory diagram illustrating the configuration of a control unit according to an embodiment of the present invention that performs toner density control. The control unit 100 is provided for each developing device, but since the basic configuration is the same, the following description will be made with the color code (Y, C, M, Bk) omitted. A part of the control unit 100 (CPU 101, ROM 102, RAM 103, etc.) of each developing device is shared among the developing devices.
The control unit 100 according to this embodiment includes a CPU 101, a ROM 102, a RAM 103, an I / O unit 104, and the like. The magnetic permeability sensor 26 and the reflection density sensor 62 are connected to the I / O unit 104 via A / D converters (not shown). When the CPU 101 executes a predetermined toner concentration control program, the control unit 100 transmits a control signal to the toner replenishment drive motor 31 that drives the powder pump 27 via the I / O unit 104 to perform the toner replenishment operation. Control.
In addition, by executing a predetermined target output value correction program, the target output value Vtref is corrected for each image formation so that a constant image density can be obtained. The ROM 102 stores a toner density control program executed by the CPU, an image density control parameter correction program, and the like. A RAM (NV-RAM) 103 has a Vt register for temporarily storing the output value Vt of the magnetic permeability sensor 26 acquired via the I / O unit 104, and the toner concentration of the developer in the developing device 20 is a target toner concentration. A Vtref register for storing a target output value (toner density control reference value) Vtref to be output by the magnetic permeability sensor 26 at a certain time, a Vs register for storing an output value Vs from the reflection density sensor 62, and the like are provided.
In this embodiment, the control unit 100 also functions as a first target output value correction device based on potential control described later and a second target output value correction device based on image area ratio average value change rate described later. However, in order to easily understand the present invention, the expressions of a first target output value correction device based on potential control and a second target output value correction device based on an image area ratio average value change rate described later are used as they are. .

  FIG. 4 is a graph showing the characteristics of the magnetic permeability sensor 26 in which the vertical axis represents the output value of the magnetic permeability sensor 26 and the horizontal axis represents the toner concentration of the developer to be detected. As shown in FIG. 4, within the practical toner concentration range, the relationship between the output value of the magnetic permeability sensor 26 and the toner concentration of the developer can be linearly approximated. The higher the toner concentration in the developer, the smaller the output value of the magnetic permeability sensor 26 is. Using this characteristic, when the output value Vt of the magnetic permeability sensor 26 is larger than the target output value Vtref, the powder pump 27 is driven to supply toner. On the contrary, when the output value Vt is smaller than the target output value Vtref, the powder pump 27 is stopped and the toner is not supplied. In the present embodiment, toner replenishment control is performed based on the output value Vt of the magnetic permeability sensor 26 for each image formation.

(First embodiment)
Next, an overall outline of the control according to the first embodiment relating to the target output value correction process which is a characteristic part of the present invention will be described with reference to FIG. 5A and 5B are diagrams showing the relationship of control according to the present embodiment, in which FIG. 5A is a graph showing the temporal variation of development γ by the control method according to the present embodiment, and FIG. 5B is an image area ratio described later. The graph which shows the time fluctuation | variation of the correction amount ((DELTA) Vtref) of the target output value Vtref by the 2nd target output value correction | amendment apparatus based on a cumulative moving average value, (c) is a time fluctuation | variation of an image area ratio cumulative moving average value. FIG.
In the present embodiment, when the slope of the change curve with respect to time of the image area ratio cumulative moving average value shown in FIG. 5C (the differential value of the change curve) deviates from a predetermined threshold K range described later, As shown by arrows Z1, Z2, and Z3 according to the differential values of the change curve at the time points X1 to X3 that deviate from the threshold value (inclination of the curve), the potential control is forcibly performed, and FIG. ), The value of the target output value Vtref is corrected so that the development γ approaches the target development γ (Y value). Further, when the image area ratio cumulative moving average value is within the predetermined threshold value K, as shown in FIG. 5B, the second target output value correction device sets the target output value for each sheet. By correcting the target output value (Vtref) by calculating a correction amount (ΔVtref), toner consumption due to potential control is suppressed and the target output value can be corrected with high accuracy.

The control unit 100 shown in FIG. 3 includes a development γ calculation device described later, a first target output value correction device based on potential control, an image area rate average value change rate calculation device described later, and a change in the image area rate average value described later. It also functions as a second target output value correction device based on the rate and a forced operation control device. Therefore, in order to easily understand the present invention, the development γ calculation device, the first target output value correction device based on the potential control, the image area rate average value change rate calculation device, the image area are not described as the control unit 100. Description will be made using the expressions of the second target output value correction device and the forced operation control device as they are based on the rate average value change rate.
As will be described later, the first target output value correction apparatus based on the potential control according to the present embodiment uses the second target output value correction apparatus to obtain the image area ratio obtained when calculating the correction amount of the target output value. When the change in the average value change rate deviates from a predetermined threshold value, the forcible operation control device is activated and the target output value is forcibly corrected by potential control.

First, correction of the target output value by this potential control will be described.
In order to measure development γ (development ability) as the first step, the development potential is changed, and a gradation pattern composed of toner patches for density measurement of 10 gradations on the respective photoreceptors 11Y, 11M, 11C, and 11Bk. Create This gradation pattern is formed by fixing the potential of the writing portion and changing the developing bias and the charging bias. An image is formed so that the background potential is fixed at 100 V by the difference between the charging bias and the developing bias. In addition, although it is a toner patch, it images one by one from the side with a low developing potential.
Next, a gradation pattern composed of toner patches developed on the photoconductors 11Y, 11M, 11C, and 11Bk of each station is transferred onto the intermediate transfer belt 6. (In this embodiment, ten density measurement patches are prepared on each of the photoreceptors 11Y, 11M, 11C, and 11Bk, but the development γ can be measured with fewer patches. Desirably, the density is changed. The density measurement patches transferred in parallel to the respective colors on the intermediate transfer belt 6 are reflected by a reflection density sensor 62 including a photosensor installed downstream in the rotation direction of the intermediate transfer belt 6. The output corresponding to the toner density of each patch is measured sequentially.
Thereafter, this output is converted into a toner adhesion amount (mg / cm ² ), and a relational expression between the adhesion amount (mg / cm ² ) and the development potential (−KV) is obtained. The slope of the above relational expression is development γ indicating development ability. Further, the development bias value for obtaining the target toner adhesion amount can be calculated from the above relational expression. At the same time, the toner density control reference value (Vtref) is corrected in accordance with the predetermined development γ target value and the deviation of development γ obtained by measurement.

In the control by the first target output value correction device based on the potential control in the present embodiment, a predetermined development γ target value (in accordance with each environment, the rotation distance (m) of the developing sleeve, the rotation time (second) of the photosensitive member, and the like. The target development γ) is set. The development γ target value is compared with the current value of development γ calculated earlier. If the current value is significantly larger than the target value, Vtref is increased to induce a lower toner density. When the current value is smaller than the target value, control is performed such that Vtref is set low and the toner density is guided higher.
Conventionally, the execution frequency of this potential control is fixed, for example, every 200 color images output. However, since it is difficult to obtain a stable image density by the potential control using such a fixed method, in the present invention, the change in the average value of the image area ratio between the predetermined number of image formations (that is, the image area ratio shift). The potential control is intervened when the change rate of the average value changes significantly. As a result, even when the rate of change in the image area ratio moving average value is large, it is possible to create a toner patch with the minimum necessary level and appropriately control the development γ to obtain a stable image density. .

The change rate (ΔMn) of the image area ratio average value is expressed by the following equation, where the current value (Mn) of the moving average of the image area ratio and m (n−m) is the value of the moving average of the image area ratio. It can be expressed as (1).
ΔMn = (Mn−M (n−m)) / m (% / (m sheets)) (1)
(However, in this embodiment, m = 5.)
In the present embodiment, for simplicity, the moving average value M (nm) of the image area ratio before a predetermined number of sheets and the moving average value Mn of the current image area ratio are calculated from the above equation (1). For example, the rate of change may be calculated from a linear approximation of Mn, M (n−1),... M (n−10). Similarly, a method of calculating the inclination by approximating Mn, M (n−1),... M (n−10) to a quadratic curve may be used.
In the present embodiment, the potential control is performed when the change rate of the moving average value of the image area ratio deviates from a predetermined threshold range, particularly when the change rate of the moving average value exceeds a predetermined threshold value. The direct potential control execution flag is set to force execution. As an execution condition in this case, the following equation (1-A) can be used.
| ΔMn | ≧ K (1-A)
(Where | ΔMn | is the absolute value of the rate of change of the average image area ratio, and K is a value determined experimentally by each machine, but in this embodiment it is 25% / (m sheets). is there.)
Further, as shown in FIG. 6, by using a potential control correction coefficient that fluctuates according to the change rate ΔMn of the average image area ratio, the number of executions that is different from the normal potential correction control from the following equation (1-B) May be calculated, and the potential control may be executed based on the number of executions.
Number of executions = normal number of executions × potential control correction coefficient (1-B)
In this embodiment, the intervention time of the adjustment operation of the target output value Vtref by potential control is changed from the change rate ΔMn of the average image area ratio, but the intervention time is determined by incorporating information such as the leaving time. May be. In the present embodiment, the unit of change rate is (% / (m sheets)) for each number of printed sheets, but it may be converted to the travel distance (mm) of the developing device (% / mm).

Next, correction of the target output value Vtref using the second target output value correction device when the change rate of the image area ratio average value is within a predetermined threshold will be described.
The control means by the second target output value correcting device in the present embodiment changes Vtref according to the toner replacement amount in the developing device. The developer characteristic value measurement method and correction method used for this control are as follows.
FIG. 7 is a graph showing the difference between the development potential and the toner adhesion amount when the output image area ratio is changed, that is, the difference in development γ (the slope of the toner adhesion amount relational expression with respect to the development potential). In FIG. 7, a straight line 1 indicates the target development γ when the cumulative average value of the image area ratio is 5%, and a straight line 2 indicates a case where the cumulative average value of the image area ratio is 80%. This is the value when 200 images of the same image area ratio image are continuously output in the standard linear velocity mode (100 mm / sec). As can be seen from FIG. The larger the toner replacement amount (the higher the image area ratio), the higher the development γ. This suggests that the physical adhesion force and electrostatic adhesion force between the toner and the carrier are changed. That is, it is suggested that a correction that takes into account the difference in developing ability due to the difference in the toner replacement amount within a certain period is necessary.

In order to solve these problems, the inventors of the present invention have made extensive studies and have come to the conclusion that it is most effective to control the toner density so as to stabilize the developer. In principle, the toner density control reference value is changed so that the development γ is constant, in other words, the toner charge amount is constant. As the toner replacement amount in the developing device within a certain period, an image area (cm ² ), an image area ratio (%), and the like can be considered, but using the image area ratio (%) is the simplest and understandable. Cheap. When the image area ratio (%) is used as the toner replacement amount within a certain period, the unit is (mg / page), and correction is performed accordingly. When the transfer paper is A4 size and outputs a 100% solid image, 300 mg of toner is consumed, and 300 mg of toner is supplied, so the replacement amount is 300 (mg / page). However, in order to set the image area ratio (%) as the toner replacement amount, for example, the standard transfer paper is set to A4 size horizontal paper, and all the transfer papers are converted into this size, and the image area ratio (% ) Is necessary. Incidentally, the developer capacity of the developing device used in this experiment is 225 g.

Next, FIG. 8 shows the relationship between the cumulative average value (%) of the image area ratio and the development γ (mg / cm ² / kV). In the same manner as described above, the experimental method is to perform 100 continuous printing for each image area ratio while keeping the toner density constant in the standard linear velocity mode (138 mm / sec). As can be seen from this figure, when the cumulative average value of the image area ratio exceeds 5%, the development γ tends to be higher than the target reference value 0.85. Therefore, when the cumulative average value of the image area ratio is higher than 5%, it is necessary to induce the toner density to be lower by increasing the target output value (toner control reference value: Vtref). Conversely, when the image area ratio is less than 5%, the development γ tends to be low. Therefore, it is necessary to induce a higher toner density by lowering the target output value (Vtref).

Such correction of the target output value (Vtref) will be described with reference to the flowchart shown in FIG. This correction process is started every time one JOB of each printing is completed. First, in STEP 1 (S1), the cumulative moving average value of the image area ratio (%) of the output image is calculated by a method described later. In calculating the cumulative moving average value of the image area ratio (%), the image area ratio (%) is calculated for each printing sheet. When this correction is executed, the image area ratio (%) may be the total average from a certain point in time (for example, the time point when the potential control is performed is set to zero, and the total average is performed from that point). The average should be used. By using this moving average, it is possible to know past toner replacement histories of several sheets suitable for knowing the current developer characteristics.
The cumulative moving average may be simply an average of several past images, but in the present embodiment, it is calculated according to the following equation (2) for simplicity. By using such a calculation formula, it is not necessary to store the image area ratio of the past several to several tens of images in NV-RAM (Non Volatile Random Access Memory), which is very effective.
M (i) = (1 / N) {M (i−1) × (N−1) + X (i)} (2)
Here, M (i) is the current value of the image area rate cumulative moving average, M (i−1) is the previous value of the image area rate cumulative moving average, and N is the cumulative number. X (i) is the current image area ratio (%).
Note that M (i) and X (i) are calculated individually for each color. As in this embodiment, the use area of the NV-RAM can be significantly reduced by obtaining the moving average current value using the cumulative moving average value of the image area ratio up to the previous time. Also, it is possible to change the control response by changing the cumulative number. For example, it is possible to control more effectively if the value is changed in accordance with environmental fluctuations or time.

Next, in STEP2 (S2), the Vtref current value (the Vtref current value calculated in S8 described later by the control of the second target output value correction device) and the Vtref initial value (Vtref corrected by the previous potential control). Value). Here, the relationship between the Vtref initial value and the Vtref current value is defined as in the following equation (3).
Vtref current value = Vtref initial value + ΔVtref (3)
In this case, the current Vtref and the initial value of Vtref are calculated individually for each color of black, magenta, cyan, and yellow.
ΔVtref is a correction amount of Vtref calculated from an LUT (Look Up Table) shown in FIG. 10 described later, and is obtained from the following equation (4). Details will be described later.
Next, in STEP 3 (S3), sensitivity information of the magnetic permeability sensor (T sensor) 26 is acquired. The sensitivity of the magnetic permeability sensor 26 is expressed in units of (V / wt%), and is a value unique to the sensor. The absolute value of the slope of the relationship line between the toner density of the sensor and the Vt output value is the sensitivity. Yes (in this embodiment, the absolute value of the slope of the relationship line shown in FIG. 4 is sensitivity).
Next, in STEP 4 (S4), the output value (Vt) of the immediately preceding magnetic permeability sensor is acquired. Subsequently, in STEP 5 (S5), Vt−Vtref is calculated from the Vt value acquired in S4 and the Vtref acquired in S2. Thereafter, it is determined in STEP 6 (S6) whether or not the target output value (Vtref) needs to be corrected based on the result of Vt−Vtref calculated in S5. The determination criteria in this case are, for example, whether or not the previous potential control is “successful”, whether or not Vt−Vtref is within a predetermined value (whether toner density control is executed normally), and the like. good. If this correction is not executed (N), the process is terminated as it is.

When this correction is executed (Y), the LUT is referred to in STEP 7 (S7). An example of the LUT is shown in FIG. First, ΔTC (amount for changing the toner density) to be changed is determined according to the cumulative moving average value Mi of the image area ratio indicated by the LUT. Subsequently, after determining ΔTC, the correction amount ΔVtref of the target output value is calculated by the following equation (4) using the sensitivity of the magnetic permeability sensor calculated in S3. After calculating ΔVtref, it is stored in NV-RAM.
ΔVtref = (− 1) × ΔTC × permeability sensor sensitivity (4)
In this case, ΔVtref is calculated individually for each color of black, magenta, cyan, and yellow.
The LUT used in this embodiment was created using the following method.
FIG. 11 shows a correction amount (ΔTC) (wt%) for changing the toner density for making the development γ constant with respect to a certain reference TC (toner density) and the cumulative moving average value Mi (%) of the image area ratio. It is a graph which shows the relationship. For example, when the cumulative moving average value Mi of the image area ratio is 80%, ΔTC is set to 1 wt%, and when the image is output, the development γ is kept constant. The ΔTC correction amount for the cumulative moving average value Mi of the image area ratio can be approximated most accurately by logarithmic approximation. Therefore, the ΔTC amount with respect to the cumulative moving average value Mi of the image area ratio used for the LUT is determined using this method.

Further, in this example, when the cumulative moving average value of the image area ratio is less than 10%, the correction step sets the cumulative moving average value of the image area ratio every 1%, and the cumulative average value of the image area ratio is In the case of 10% or more, the correction step is set every 10%. This correction step can be arbitrarily changed according to the characteristics of the developer and the developing device. Adjustment of the maximum correction amount for each color can be performed using, for example, the following equation (5).
ΔVtref = (− 1) × ΔTC × permeability sensor sensitivity × color correction coefficient (5)
After calculating ΔVtref in STEP 7 (S7), a new Vtref current value is calculated in STEP 8 (S8). The current Vtref value is calculated according to the following equation (6) of ΔVtref and Vtref initial value calculated above.
Vtref current value = Vtref initial value + ΔVtref (6)
Even in this case, the Vtref current value is calculated individually for each color of black, magenta, cyan, and yellow.
Next, in STEP 9 (S9), upper and lower limit processing of the current Vtref value is performed. If the corrected Vtref current value is greater than or equal to a preset upper limit value, the Vtref is not changed and is used as the previous Vtref current value. When the corrected Vtref exceeds the lower limit value, the Vtref current value is set to a preset lower limit value. After the upper / lower limit processing is completed, the current Vtref value is stored in the NV-RAM in STEP 10 (S10).
Note that this correction is preferably executed by calculating a correction value between the transfer sheets (time or distance between the end of the previous image formation and the start of the current image formation) during printing. By executing at such a frequency, the toner density control reference value: Vtref can be calculated appropriately for each output image, so that the image density can be further stabilized.

By using only the second toner density control device as described above, the stability of the output image is greatly improved. However, there are some points to be improved in the control.
[1] Since it was necessary to make the correction amount small so as not to over-correct, the image density could not be completely corrected.
[2] The image density could not be corrected due to sudden environmental changes or sudden changes in the image output mode.
These problems are considered to be caused by the fact that the first target output value correction device based on the potential control and the toner density control means by the second target output value correction device do not perform the coordinated control. It is done.
Therefore, the present inventors did not execute the control by the first target output value correction device based on the potential control and the control by the second target output value correction device at a single timing, but took advantage of each advantage. I devised a way to merge in form.

In the toner density control by the second target output value correction device, the parameter tends to change depending on the operation start point. Therefore, errors may occur due to parameter measurement and machine tolerances. Therefore, with only the toner density control means by the second target output value correction device, the error caused by these becomes the control error as it is. Even if there are disturbances (environmental fluctuations, leaving time), there is no way to deal with these disturbances. For this reason, if only this control is used, the amount of parameter movement to be changed to suppress overcorrection has to be reduced. In this sense, it can be said that it is difficult to completely control the process only with this control.
As in the present embodiment, the image area ratio average is used as a reference to be referred to for execution determination of control by the first target output value correction device based on potential control and toner density control by the second target output value correction device. By using the rate of change of values to match, the execution timing of the potential control of the first target output value correction device can be optimized. For this reason, the image density can be controlled with higher accuracy, and generation of waiting time (preparation time until the start of printing) can be suppressed as much as possible, which is a great sales advantage.
In the present embodiment, the first target output value correction device and the second target output value correction device based on the potential control are used. However, even when the control is performed only by the potential control, As described above, a sufficient effect can be obtained by intervening the first target output value correction device based on the potential control.

(Second Embodiment)
In order to minimize the waiting time, it is possible to change the adjustment operation performed as follows from the change rate of the image area ratio and the average value of the image area ratio. For example, when the rate of change (ΔMn) of the average image area ratio is large, but the image area ratio (Xi) itself of the image is small, the potential control is performed by the first target output value correction device based on the potential control. First, it is possible to obtain a stable image density by increasing the charge amount of the toner only by stirring the toner and the carrier in the developing device.
In the configuration of the second embodiment, a cleaning member that cleans the residual toner remaining on the intermediate transfer belt 6 during the secondary transfer of the toner image from the intermediate transfer belt 6 to the transfer paper P is provided for cost reduction. Not. In such a case, the secondary transfer roller 3 needs to be separated to prevent the gradation patch from adhering to the secondary transfer roller 3.
This separation operation is performed by adopting a mechanical cam mechanism. However, since it takes about 1 second each to determine separation and contact, this alone causes a waiting time. . As described above, when only the agitation operation of the toner and the carrier is executed for correcting the target output value Vtref, the separation contact operation of the secondary transfer roller 3 is not necessary, so the waiting time is reduced. It is possible to stabilize the image density after shortening.

Further, when the rate of change (ΔMn) of the average image area ratio is large and the image area ratio (Xi) is also large, along with the potential control by the first target output value correction device based on the potential control described above, The toner and the carrier are agitated by the agitating and conveying screws 23 and 24 in FIG. By setting as described above, it is possible to appropriately adjust the image density while minimizing the waiting time. Specifically, as shown in FIG. 12, the execution adjustment operation in the case where the image area ratio cumulative moving average value (Mi) and the area ratio (Xi) of the image are within the respective predetermined ranges is shown. In FIG. 12, “large”, “medium”, and “small” of the image area ratio are respectively “large” for an image area ratio Xi> 30%, and “medium” for 10% ≦ image area ratio Xi. ≦ 30%, “Small” indicates an image area ratio Xi <10%.
Further, “stirring” in FIG. 12 indicates the stirring operation of the toner and the carrier by the stirring and conveying screws 23 and 24 in the developing device, and “potential control” is performed by the first target output value correcting device based on the potential control. The potential control, and “stirring + potential control” indicates the potential control by the first target output value correction device based on the potential control and the stirring operation of the toner and the carrier by the stirring and conveying screws 23 and 24 in the developing device. ing.
Thus, by selecting the adjustment operation according to the change rate (ΔMn) of the average image area rate (ΔMn) and the image area rate (Xi) of the image, it is possible to control the image density to an appropriate value. In addition, the necessary “potential control” of the toner patch can be suppressed to the minimum necessary limit, and toner consumption can be suppressed.

(Third embodiment)
In the present embodiment, the same target output value correction operation as that in the first and second embodiments is executed in a state where changes in the environment are taken into account. In a low-temperature and low-humidity environment, the toner charge amount tends to increase, so it is necessary to perform some image quality adjustment operation. Conversely, in a high temperature and high humidity environment, the toner charge amount tends to decrease, so it is necessary to perform some image quality adjustment operation. Usually, the execution of the adjustment operation is determined simply by looking only at the humidity (for example, the adjustment operation is executed under the conditions of absolute humidity <3 g / m ³ and absolute humidity> 16 g / m ³ ).
However, when printing is performed constantly in a low-temperature and low-humidity environment or a high-temperature and high-humidity environment, the developer is stable under the environmental conditions. Therefore, it is not necessary to intervene a lot of adjustment operations. Conversely, even in an environmental condition that normally does not require intervention of the adjustment operation, the adjustment operation needs to be executed if the environmental condition suddenly changes. Therefore, in the present embodiment, the adjustment operation is executed according to the rate of change of absolute humidity per predetermined time, not the absolute humidity itself.

That is, in consideration of the change in absolute humidity at the time of the previous adjustment operation and the current absolute humidity per time, the adjustment operation is performed when the amount of change deviates from a predetermined range.
Specifically, when the previous absolute humidity is (A (n-1)) and the current absolute humidity is (An), the average rate of change per hour (ΔMAn) is expressed by the following equation (7). Given in.
ΔMAn = (An−A (n−1)) / hour (7)
In this case, the absolute humidity may be measured for each sheet or may be measured for several to several tens of sheets (however, it is necessary to store time for each printing). Moreover, it is good also as a system which measures at every fixed time. The execution condition follows formula (7-A) below.
| ΔMAn | ≧ R (7-A)
Here, R (g / m ³ / sheet) is used. In the present embodiment, only absolute humidity is used as an index, but relative humidity or temperature may be used as an index.
In this way, by considering the humidity change, it is possible to reduce the number of interventions of the adjustment operation. Furthermore, since it is possible to determine the presence or absence of intervention based on the change rate of the environment, the image density can be further stabilized.

FIG. 13 shows a case where the target output value Vtref is corrected by using both the first target output value correcting device and the second target output value correcting device based on the potential control in the first embodiment (curve 3) and as a comparative example. It is a graph which shows the control state of the image density in the case of using only the second target output value correction device and performing a fixed potential control (curve 4). In FIG. 13, symbols A, B, and C indicate that the potential control is forcibly performed by changing the image forming bias.
In this case, in the standard linear velocity mode (120 mm / second), 100 sheets of 70% solid images are continuously passed. In the comparative example shown by the curve 4, the image density increases as the print job progresses. Although the number of printed sheets is 80 and the potential control is intervened, the difference in image density is large before and after the execution of the potential control. This indicates that the potential control intervention is slow.
On the other hand, in the embodiment according to the present embodiment indicated by the curve 3, an increase in the image density accompanying the progress of the print job is appropriately suppressed. Further, since the intervention timing of the potential control is also appropriate, the difference in image density before and after the potential control is kept small. By adopting the control as in the present embodiment, it is possible to greatly improve the image density stability when outputting an image with many toner replacements, that is, an image with a high image area ratio.

1 is a cross-sectional view illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention. It is an expanded sectional view which shows schematic structure of the yellow image formation means of FIG. It is explanatory drawing which shows the structure of the control part of one Embodiment by this invention which performs toner density control. It is a graph which shows the characteristic of the magnetic permeability sensor used with the image forming apparatus which concerns on one Embodiment by this invention. FIG. 5 is a graph showing a control relationship according to the first embodiment of the present invention, where (a) is a graph showing temporal variation of development γ by the control method according to the embodiment, and (b) is an image area ratio. The graph which shows the time fluctuation | variation of the correction amount ((DELTA) Vtref) of the target output value Vtref by the 2nd target output value correction | amendment apparatus based on a cumulative moving average value, (c) is a time fluctuation | variation of an image area ratio cumulative moving average value. 6 is a graph showing the relationship between the upper and lower threshold values of the image area ratio cumulative moving average value. It is a figure which shows the relationship between the change rate of the image area rate average value used in control which concerns on 1st Embodiment by this invention, and an electric potential control coefficient with a table | surface. It is a graph which shows the relationship between the development potential and the adhesion amount when the change rate of the image area ratio average value according to the control according to the first embodiment of the present invention is changed. It is a graph which shows the relationship between the image area ratio cumulative moving average value and development γ related to the control according to the first embodiment of the present invention. 7 is a flowchart showing a correction process of Vtref by the second toner density control reference value correction apparatus according to the first embodiment of the present invention. It is a figure which shows the relationship between the image area rate cumulative moving average value stored in the LUT related to the control according to the first embodiment of the present invention, and ΔTC and ΔVtref in a table. It is a graph which shows the relationship between the image area rate cumulative moving average value and ΔTC according to the control according to the first embodiment of the present invention. It is a figure which shows the relationship between the absolute value of the change rate of the image area rate average value which concerns on control which concerns on 2nd Embodiment by this invention, an image area rate, and adjustment operation to perform with a table | surface. It is a graph which shows the relationship between the number of printed sheets and image density at the time of using the control which concerns on 1st Embodiment by this invention, and the control which concerns on a comparative example.

Explanation of symbols

  1Y, 1C, 1M, 1Bk Image forming means, 2 charging device, 3 secondary transfer roller, 5 registration roller, 6 intermediate transfer belt, 7 fixing unit, 10Y, 10C, 10M, 10Bk photoconductor unit, 11Y, 11C, 11M , 11Bk photoconductor, 13Y cleaning blade, 14Y lubricant application and static elimination brush roller, 15Y charging roller, 20Y, 20C, 20M, 20Bk developing device, 20Ya developing case, 20Yb opening, 22Y developing sleeve, 23Y, 24Y stirring, conveying screw , 25Y Developing doctor, 26, 26Y, 26C, 26M, 26Bk Permeability sensor, 27Y Powder pump, 62 Reflection density sensor, 100 Control unit, 101 CPU, 102 ROM, 103 RAM, 104 I / O unit

Claims (8)

An image carrier for carrying a latent image;
A charging device for charging the image carrier to a predetermined potential;
A latent image forming apparatus that forms a latent image on the surface of the image carrier charged to a predetermined potential;
While applying a developing bias to a developer carrying member carrying a two-component developer containing toner and a magnetic carrier, the toner on the developer carrying member is transferred to a latent image on the image carrying member, and the latent image is transferred. A developing device for developing;
A replenishing device for replenishing toner in the developing device;
A toner concentration detecting means for detecting a toner concentration in the two-component developer in the developing device;
An image provided with a toner concentration control device for controlling the toner concentration by controlling the replenishment of toner from the replenishing device in accordance with a toner concentration control reference value referred to for controlling the toner concentration in the developing device; In the concentration control device,
A transfer device for transferring the toner image obtained by the development from the image carrier to a transfer member;
A gradation pattern forming apparatus for forming a gradation pattern composed of a plurality of toner patches formed on the transfer body under image forming conditions with different adhesion amounts;
An optical detection device for detecting reflected light from the gradation pattern;
Development γ for calculating development γ from the toner adhesion amount calculated from the output of the gradation pattern detected by the optical detection device and the development potential calculated from the charging potential of the charging device and the development bias A calculation device;
A first toner density control reference value correction device that corrects the toner density control reference value by varying the development potential from the development γ calculated by the development γ calculation device;
An image area rate average value change rate calculating device for calculating a change rate of an image area rate average value of a toner image formed on the image carrier;
A second toner density control reference value correction device that corrects the toner density control reference value according to the change rate of the image area ratio average value of the toner image calculated from the image area ratio average value change rate calculation device;
A forcible operation control device that activates the first toner density control reference value correction device when the change rate of the average image area rate value calculated from the average image area rate change rate calculation device deviates from a predetermined range; An image density control apparatus comprising: The image density control apparatus according to claim 1,
2. The image density control apparatus according to claim 1, wherein the average image area ratio of the toner image is a moving average value of the image area ratio of an image formed within a predetermined period. The image density control device according to claim 2.
The moving average value of the image area ratio of the image formed within the predetermined period is
M (i) = (1 / N) × {M (i−1) × (N−1) + X (i)}
(In the above equation, “N” is the sampling number of the image area ratio, “M (i−1)” is the moving average value calculated last time, and “X (i)” is the image detected this time. Area ratio)
The image density control device is calculated based on the above. The image density control device according to any one of claims 1 to 3,
The forced operation control device is:
| ΔMn | ≧ K
(Where “| ΔMn |” is the absolute value of the rate of change of the average value of the image area ratio of the predetermined number of sheets, and “K” is 25% / (predetermined number of sheets))
And controlling the first toner density control reference value correcting device to satisfy the above condition. The image density control apparatus according to any one of claims 1 to 4,
The forced operation control device is controlled to operate when a humidity change rate received by the image density control device deviates from a predetermined range. The image density control device according to claim 5.
2. The image density control apparatus according to claim 1, wherein the humidity change rate is a humidity change rate per hour. The image density control device according to any one of claims 1 to 6,
The developing device includes a stirring device that stirs the toner and the magnetic carrier, and the stirring device corrects the toner concentration control reference value when the toner concentration control reference value correction device corrects the toner concentration control reference value. An image density control device controlled to agitate the magnetic carrier and the magnetic carrier.   An image forming apparatus comprising the image density control device according to claim 1.

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