JP2013122549A - Image density control method and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image density control method for predicting a change in a toner charge amount or a change in physical adhesion between toner and a carrier to maintain constant image density during printing, and decreasing costs in an image forming apparatus including a developing device that does not have a toner density sensor for detecting density of toner in a developing unit.SOLUTION: In order to maintain constant image density during printing, a developing potential (developing pot) as an imaging condition is changed in a non-image area in accordance with image output information or operation information of a toner supply device.

Description

  The present invention relates to an image density control method, an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a plotter for carrying out the image density control method, and a multifunction machine including at least one of them.

In recent image forming apparatuses such as copying machines and laser printers, high image quality is demanded, and 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.
As a conventional technique, a two-component developer (hereinafter referred to as “developer”) composed of a non-magnetic toner and a magnetic carrier is held on a developer carrier (hereinafter referred to as “development sleeve”), and is magnetized by a magnetic pole contained therein. 2. Description of the Related Art A two-component development system that performs development by forming a brush and applying a development bias to a developing sleeve at a position facing a latent image carrier (hereinafter referred to as “photosensitive member”) is widely known.

This two-component development method is currently widely used because it can be easily colored. In this system, the developer is transported to the development area by rotating the developing sleeve. As the developer is transported to the development area, a large number of magnetic carriers gather with the toner along the magnetic field lines of the development pole to form a magnetic brush.
In the two-component development method, unlike the one-component development method, it is very important to control the weight ratio (toner concentration) of the toner and the carrier with high accuracy in order to improve the stability. For example, if the toner concentration is too high, background stains will occur on the image and the detail resolution will be reduced. Further, when the toner density is low, problems such as a decrease in density of 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.

Many image forming apparatuses in recent years adopt a technique for reducing stress in a developing apparatus. These are considered to be very effective techniques in order to achieve the contradictory purpose of reducing the amount of developer due to the demand for downsizing of the developing device and extending the life of the developer.
For example, in a two-component color image forming apparatus, an additive such as silica (SiO ₂ ) or titanium oxide (TiO ₂ ) is externally added to many toner surfaces in order to improve toner dispersibility. These additives are very vulnerable to mechanical stress and heat stress. For this reason, a phenomenon occurs that the toner is buried in the toner or detached from the surface by stirring in the developing device.
As a result, the flowability and charging characteristics of the developer (including toner and carrier) and the physical adhesion between the toner and the carrier change. Low stress development makes it possible to suppress the above phenomenon as much as possible.

On the other hand, there is a case where the toner charging ability (ability of the developing device to charge the toner) is lowered due to the low stress of the developing device. Briefly explaining this phenomenon, for example, when the area of the output image is relatively low (the toner replacement amount per unit time or unit number is small), the developing capacity (the toner development amount with respect to the development bias is set to be smaller). While the slope of the plotted graph is kept substantially constant, when a high image area ratio image is output, the developing ability increases.
That is, depending on how much toner is replaced in the developer, the charge amount of the toner existing in the developing device changes, resulting in a difference in developing ability.
For the above reason, it is necessary to perform some toner charge amount adjustment during printing in order to keep the developing ability constant.

In the image forming apparatus having such characteristics, in order to control the toner charge amount, the toner density is accurately detected by a toner density sensor (toner density detecting means), and the toner density is controlled based on the result. It has been essential.
Here, as the toner density control, the output value: Vt of the toner density detecting means (for example, magnetic permeability sensor) is compared with the control reference value of the toner density: Vtref, and the toner replenishment amount is calculated from the calculation formula according to the difference. A general method is to calculate and replenish toner in the developing device by a toner replenishing device.
As a method for detecting the toner density, a method using a magnetic permeability sensor is widely used. In this method, the change in the magnetic permeability of the developer due to the change in the toner concentration is converted into the toner concentration. In a color image forming apparatus, this magnetic permeability sensor is installed in each developing device for K (black), C (cyan), M (magenta), and Y (yellow). It is being done.

As another toner density detection method, there is a method using an optical sensor. In this method, a reference patch (toner pattern) is created on a photoreceptor or an intermediate transfer belt, and LED light is irradiated. Then, reflected light (regularly reflected light or irregularly reflected light) from this pattern is detected by an optical sensor (with a photodiode, phototransistor, etc.), and toner density (toner adhesion amount) is detected based on the result, and development is performed. The toner density information in the container is obtained.
During printing, a reference toner pattern is created between the transfer sheets (from the end of the previous image formation to the start of the current image formation), and the reflected light from the reference toner pattern is detected by a photosensor, thereby sequentially transmitting. A density control method for controlling the toner density control reference value Vtref of the magnetic sensor is known.
For example, Patent Document 1 and Patent Document 2 have means for creating a toner pattern in a non-image area and detecting the pattern density and the toner density in the developing device. A method of maintaining the image density by changing the toner density control target value in the container is disclosed.

However, creating a toner pattern in a non-image area has a problem of increasing the amount of toner that does not contribute to image formation. Therefore, it is necessary to widen the image formation interval of the toner pattern. If the image forming interval is widened, there is a concern that the control accuracy will naturally decrease.
In the method of creating a toner pattern, in order to create a toner pattern on the intermediate transfer belt, it is necessary to separate the secondary transfer unit from the intermediate transfer belt for each image or to install a cleaning device in the secondary transfer unit. There is a tendency to refrain from the middle- and low-speed planes where cost constraints are large.

Further, a developing device has conventionally been provided with a toner density sensor, but an image forming apparatus that does not have this sensor has been proposed in response to a request for cost reduction.
In such a developing device, since the toner density control reference value Vtref cannot be set as the control target value, new control is required to keep the image density constant during printing.

  The present invention has been made in view of the above-described situation, and in an image forming apparatus having a developing device that does not have a toner concentration sensor for detecting the toner concentration in a developing device, the toner charge amount change or toner The main object of the present invention is to provide an image density control method that can predict a change in physical adhesion force between a carrier and a carrier, maintain a constant image density during printing, and can reduce costs.

  To achieve the above object, according to the present invention, a developer carrier disposed opposite to an image carrier carries a two-component developer composed of toner and a magnetic carrier for holding the toner, A developing device that develops an electrostatic latent image formed on the surface of the image carrier with toner in a development region formed between the image carrier and an image that adjusts the image density by changing imaging conditions In an image forming apparatus, comprising: a density adjusting unit; and a toner replenishing device that replenishes toner to the developing device, wherein the developing device does not include a toner concentration detecting unit that detects a toner concentration in the developer. The image forming condition is changed according to image output information or operation information of the toner replenishing device in order to keep the image density constant during printing.

  According to the present invention, the toner replacement amount information in the developer is acquired based on the image area information within a predetermined period or the operation information of the toner supply device, and the image forming condition is corrected based on the result. The image density during printing can be stably controlled without greatly changing the development γ.

1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment of the present invention. It is a schematic sectional drawing which shows the structure of a process cartridge. It is a control block diagram. It is a top view which shows the pattern for image density adjustment (a detection pattern, a gradation pattern). 6 is a flowchart for explaining an operation of calculating development γ. FIG. 6 is a characteristic diagram illustrating a relationship between a development potential and a toner adhesion amount. It is a graph which shows the difference of development (gamma) by an image area rate. It is another graph which shows the difference of development (gamma) by an image area rate. It is a flowchart which shows control operation. It is a timing chart which shows the change timing of development potential. FIG. 10 is a schematic diagram illustrating a relationship between image output and toner supply in a second embodiment. It is a flowchart which shows the control action in 2nd Embodiment. It is a flowchart which shows the control action in 3rd Embodiment. It is a graph of the comparative experiment in the relationship between the number of prints and image density. 6 is a graph showing a relationship between an image area and a toner supply amount.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram showing a schematic configuration of an image forming apparatus (color copying machine) according to the present embodiment. Below the intermediate transfer unit 12 having the intermediate transfer belt 10, a process cartridge 14 </ b> Y as an image forming unit detachably attached to an apparatus main body (not shown) along the moving direction (arrow direction) of the intermediate transfer belt 10. 14M, 14C, and 14K are provided and have a tandem configuration.
The process cartridges 14Y, 14M, 14C, and 14K have the same configuration. The process cartridge 14Y will be described as a representative. The process cartridge 14Y includes a photosensitive drum 16Y as an image carrier, a charging device 18Y, a cleaning device 22Y, and the like. The units corresponding to the colors of yellow (Y), magenta (M), cyan (C), and black (K) are indicated by adding Y, M, C, and K to the end of the reference numerals.

The intermediate transfer belt 10 is supported by a plurality of support rollers 24, 26, 27 and a primary transfer roller 28. Reference numeral 30 denotes a tension roller.
A secondary transfer roller 32 as a secondary transfer device is disposed opposite to the support roller 24 as a drive roller. An optical sensor 34 as a pattern detection unit is disposed in a non-contact state with the intermediate transfer belt 10 at a portion facing the support roller 27.
A toner replenishing device 36 that replenishes toner to each developing device 20 is provided above the intermediate transfer unit 12. The toner replenishing device 36 includes a toner tank 38 for each color, a toner transport path (not shown) connecting the toner tanks 38 to the corresponding developing devices 20, a toner replenishing drive motor (see FIG. 3) 40, and the like.
A transfer sheet 42 as a recording medium fed from a paper feeding device (not shown) disposed at a lower portion of the device main body (not shown) is conveyed in the apparatus main body in a substantially vertical direction, and is temporarily stopped by a registration roller pair 44. After correcting the oblique shift and the like, the sheet is conveyed to the secondary transfer portion at a predetermined timing, and the image on the intermediate transfer belt 10 is transferred by the secondary transfer roller 32. Thereafter, the paper passes through the fixing device 46 and is discharged to a paper discharge portion formed on the upper portion of the apparatus main body (not shown).

The surface of the photosensitive drum 16 is uniformly charged by the charging device 18 and then exposed by the exposure device 20 to form an electrostatic latent image. The developing device 20 conveys the two-component developer in the device to a developing area facing the photosensitive drum 16 by a developing roller as a developer carrying member, and develops the developer into an electrostatic latent image formed on the surface of the photosensitive drum. The toner inside is attached and visualized.
The toner image is transferred onto the intermediate transfer belt 10 by a primary transfer roller 28 as primary transfer means in a transfer region where the photosensitive drum 16 and the intermediate transfer belt 10 face each other.
As the intermediate transfer belt 10 moves, the toner image transferred onto the intermediate transfer belt 10 faces the secondary transfer roller 32 in a state where the other color toner is accurately overlaid in the other color transfer region. And is transferred to the transfer paper 42 at that position to form an image on the transfer paper.
The toner remaining on the photosensitive drum 16 is removed by the cleaning blade of the cleaning device 22 and stored in a waste toner bottle (not shown). The surface of the photosensitive drum that has passed through the cleaning device 22 is then uniformly charged again by the charging device 18 and the next image forming process is repeated.

FIG. 2 is an enlarged sectional view around the photosensitive drum 16.
A two-component developer (hereinafter simply referred to as “developer”) is moved from the conveying screw portion 52 in the developing device to the developing roller 50 by a pumping magnetic pole (not shown) of the developing roller (developing sleeve) 50. Thereafter, as the developing roller 50 rotates, the developer is transported to the vicinity of the doctor blade 54 by the magnetic field of the transport pole and the frictional force on the surface of the developing roller 50.
The developer transported to the vicinity of the doctor blade 54 temporarily stays in the upstream portion of the doctor blade, the layer thickness is regulated by the gap between the edge portion (tip) of the doctor blade and the developing roller 50, and the developer is transported to the developing region. A predetermined developing bias is applied to the developing region, and a developing electric field is formed in the direction in which the toner is urged to the electrostatic latent image formed on the photosensitive drum 16, so that the toner is in the photosensitive drum 16. Developed on top.
The developer that has passed through the developing region leaves the developing roller at the developer separating pole position on the developing roller and returns to the transport screw unit 52. Thereafter, the toner moves to the conveying screw unit 56, adjusted to an appropriate toner density by the toner replenishing unit connected to the toner replenishing device 36, and conveyed again to the developing roller 50.

FIG. 3 shows an outline of the configuration of the control unit in this embodiment. The control unit as the image density adjustment means indicated by the broken line in the figure comprises a CPU 60, a read only memory (ROM) 62, a read / write memory (RAM) 64, and an I / O board 66, and toner replenishment via the I / O board 66 A control signal is transmitted to a toner replenishment drive motor 40 that drives the device 36.
The RAM 64 is provided with a register for storing the toner supply device driving history and the image area ratio, a Vs register for storing the output value Vs from the optical sensor 34 installed in the vicinity of the intermediate transfer belt, and the like.
The ROM 62 stores a toner density control program. A toner replenishment amount reference value is calculated by inputting image area information to this control unit, and after the toner replenishment amount or toner replenishment device drive time is corrected by the toner density control program, toner is replenished to the developing device. .
As shown in FIG. 15, the toner replenishment amount is a value uniquely determined from the image area. 15 means a [cm @ 2] is [cm ^2].

Next, the development γ calculation flow in this embodiment will be described.
FIG. 4 schematically shows an image density adjustment pattern. The image density adjustment pattern is formed in series for each color and detected by one optical sensor 34. In this embodiment, images are formed so as to be within the image forming pitch of the non-image area (between transfer sheets), and each color has five gradations. By using the same sensor, it is possible to eliminate the influence of detection errors due to individual variations of sensors, and therefore it is less likely to be affected by density variations for each color. For this reason, it is very advantageous with respect to the color fluctuation of the halftone portion.
In addition, it is possible to maintain consistency between the solid portion image density and the halftone portion density.

Hereinafter, the image density adjustment method of the present embodiment will be specifically described with reference to the flowchart of FIG.
S-1: [Execution of optical sensor calibration]
Here, the LED current is adjusted so that the regular reflection light from the background portion of the intermediate transfer belt is within the range of 4.0 ± 0.5 [V].
S-2: [Create image density adjustment pattern]
The image density adjustment pattern created here is as shown in FIG. In this pattern, the potential of the writing unit (exposure device) is fixed, the developing bias and the charging bias are changed, and images are sequentially formed from the side having the lower developing potential. As another method, a technique may be used in which the development bias is fixed and the exposure is changed by changing the writing duty or power to generate gradation. However, in the case of this method, it is possible to set the potential of the image density adjustment pattern more accurately by mounting a potential sensor that measures the exposure portion potential.

S-3: [Detection of reflected light from gradation pattern]
Here, the reference toner pattern is irradiated with LED light, and the reflected light is detected by a phototransistor (PTr). In this embodiment, the K pattern detects only regular reflection light, and the color pattern (Y, M, C) detects both regular reflection light and diffuse reflection light.
S-4: [Conversion of sensor detection value to toner adhesion amount]
The reflected light from the reference pattern created in S-3 is converted into the toner adhesion amount. As the conversion algorithm, the method disclosed in Patent Document 3 was used. Detailed explanation is omitted.
S-5: [Calculation of developing ability]
FIG. 6 is a plot of the adhesion amount data calculated in S-4 above against the development potential at the time of image density adjustment pattern creation. The slope of the relational expression obtained by linearly approximating these points by the least square method is development γ. The intersection of this relational expression and the X axis is called V development start voltage: Vk.
In this embodiment, linear approximation is used, but quadratic approximation may be adopted. However, the development γ when the quadratic approximation is adopted is a differential value of the above relational expression in terms of obtaining the target adhesion amount.

S-6: [Calculation of image forming bias]
As shown in FIG. 6, the development potential [-V] is calculated from the relational expression obtained in S-5. The calculation procedure is as follows.
(1) Acquire relational expression of development γ (approximate expression obtained in S-5)
(2) Obtain the maximum adhesion amount target value (3) Calculate the development potential to obtain the target adhesion amount

Next, a method for converting the development potential into the development bias will be described. In the present embodiment, the exposure portion potential is a fixed value and is calculated using the following formula. In a system having a photoconductor surface potential meter, it is desirable to measure the exposure portion potential each time.
Development bias [-V] = Development potential +50 [-V] ... Formula (1)
Here, the exposure part potential is set to 50 [-V].
Charging bias [-V] = Development bias [-V] +200 [-V] ・ ・ ・ ・ Formula (2)
Here, the background potential is set to 200 [-V].
The background potential is a potential difference that is set offset from the development bias in order to prevent background contamination.
S-7: [Image bias setting]
Set the development bias and charging bias calculated in S-6.

Hereinafter, the developer characteristic value measuring method and the correcting method of this embodiment will be described in detail.
FIG. 7 shows the difference in development γ by the output image area ratio moving average.
This figure shows the development γ when the toner is supplied so that the toner density during printing is kept constant in the standard linear velocity mode (144 mm / sec) and 100 sheets are continuously output. As can be seen from the figure, even with the same toner concentration, the development γ tends to increase as the toner replacement amount within a certain period increases (the image area ratio increases).
This suggests that even if the toner density is controlled to be constant, the toner charge amount changes, so that the image density cannot be maintained constant. The change in the development γ is considered to be influenced by changes in the physical adhesion force and electrostatic adhesion force between the toner and the carrier in addition to the toner charge amount change.
In other words, it is necessary to perform image density correction that takes into account changes in developing ability due to differences in the toner replacement amount within a certain period.

In order to stabilize the image density during printing, the present inventors have made extensive studies on the above problem of developing ability change, and in order to stabilize the image density during printing, control is performed to predict changes in the toner charge amount and to change the image forming conditions. It came to the conclusion that it is effective to do.
When the toner density sensor is mounted, the toner density can be guided in the target direction by changing the toner density control reference value. However, as in the present embodiment in which the toner density sensor is not mounted. In such a configuration, it is necessary to correct the image forming condition, more specifically, the development potential based on the toner replacement amount that is the toner consumption / replenishment history.
As the toner replacement amount within a certain period, it is easiest to use a moving average of the image area ratio [%]. When the moving average of the image area ratio [%] is used as the toner replacement amount within a certain period, the unit is [% / page], and correction is performed accordingly.

Transfer paper: When 10 sheets of 100% solid images (image area ratio is 100%) are output on A4, the moving average of the past 10 sheets is 100 [% / page]. Since the toner consumption per sheet of A4 solid output is about 300 [mg], the average toner consumption for the past 10 sheets is 300 [mg / page].
Usually, since it is necessary to replenish toner in the same amount as the consumption amount, the average value of the replacement amount of the past 10 sheets is similarly 300 [mg / page].
Similarly, when outputting 100% solid images after outputting 5 sheets of 50% solid images (image area ratio is 50%) with A4 transfer paper, the moving average of the past 10 sheets is 75 [% / page]. Become.
Since the toner consumption per sheet of A4 solid output is about 225 [mg], the average toner consumption for the past 10 sheets is 225 [mg / page]. Normally, since it is necessary to replenish toner in an amount equal to the consumption amount, the average value of the replacement amount of the past 10 sheets is similarly 225 [mg / page].

By acquiring the moving average of the image area ratio as described above, the toner replacement amount can be known.
However, since the image area ratio [%] does not include the travel distance information of the developing device, in order to convert it to the toner replacement amount within a certain period, for example, the standard transfer paper is set to A4 landscape paper. Therefore, it is necessary to devise such as converting all transfer papers to this size to obtain an image area ratio [%] (for example, A3 size is counted as 2 sheets of A4 side).
Further, in order to convert the image area [cm ² ] into the toner replacement amount, a method such as accumulating the image area [cm ² ] formed while the developing roller travels for a specific period may be adopted. Good. In this case, the unit of the toner replacement amount is cm ² / mm.

Next, FIG. 8 shows a graph in which the horizontal axis represents the image area ratio (%) and the vertical axis represents development γ (mg / cm ² / kV). In the same manner as described above, the experiment method is to perform continuous printing for 100 sheets for each image area ratio while keeping the toner density constant in the standard linear velocity mode (144 mm / sec).
As can be seen from this figure, when the image area ratio exceeds the reference value: 5%, the development γ tends to increase. For this reason, when the image area ratio is higher than 5%, it is necessary to set the developing potential low and induce the image density to be low.
Conversely, when the image area ratio is less than 5%, the development γ tends to be low. Therefore, it is necessary to set the development potential to be high and induce the image density to be high.

This correction control flow will be described with reference to FIG. This correction is performed after obtaining the image area for each printing. First, in STEP 10, the image area of the output image is acquired. Based on this value, the development potential is acquired in STEP20. This development potential is a value obtained at S-6 in the flow of FIG.
Next, in STEP 30, an image area ratio moving average is calculated. In order to calculate the image area ratio moving average [%], the image area ratio [%] is calculated for each printing sheet. When performing this correction, the image area ratio moving average [%] may be a total average from a certain point of time, for example, the average point of time when the potential control is performed as zero, and a total average from that point. preferable.
By doing so, the past several to several tens of toner replacement histories suitable for knowing the current developer characteristics can be known.
In this moving average, image area ratio information for several past images may be stored in a memory and averaged for each predetermined number of images. However, in this embodiment, for the sake of simplicity, the following equation (3) is used. It was decided to calculate.
By using such a calculation formula, it is not necessary to store image area ratios of several to several tens of images in the NV-RAM, which is very effective.
M (i) = (M (i-1) x (N-1) + X (i)) / N ... Equation (3)
Here, M (i) is the current value of the image area rate moving average, M (i−1) is the previous value of the image area rate moving average, and N is the cumulative number.

In the present embodiment, the set value is 10 sheets. X (i) is the current image area ratio [%]. M (i) and X (i) are calculated separately for each color.
As in this embodiment, using the moving average of the image area ratios up to the previous time to obtain the moving average current value can significantly reduce the NV-RAM usage area.
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 STEP 40, it is determined whether or not this correction is executed. The determination condition of this step is whether or not the previous image density adjustment was successful.
If it is determined in STEP 40 that the main correction is to be executed, in STEP 50, the LUT (reference table) stored in advance in the memory is referred to, and the development potential correction coefficient is acquired. Table 1 shows an example of the LUT.

In the present LUT example, when it is less than 10%, the correction step is set every 1%, and when the image area ratio is 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. That is, a finer table may be used.
Next, in STEP 60, the development potential [−V] is corrected by the expression (4) using the development potential correction coefficient: β acquired in STEP 50.
Corrected development potential Vpot '[-V] = Development potential [-V] x β
.... Formula (4)
The adjustment of the maximum correction amount for each color is corrected using, for example, the following equation (5).
Development potential after correction Vpot [-V] = Development potential [-V] x β x Color correction coefficient (5)
As shown in Equation 5, when the environment and time are taken into account, the accuracy can be further improved by multiplying the environment correction coefficient and the time correction coefficient. Although control using LUT was presented this time, it may be calculated using an approximate expression.

Next, in STEP 70, development bias: Vb is calculated according to the following equation.
Development bias: Vb [-V] = Vpot '+ 50 [-V] (6)
Here, the exposure portion potential is set to 50 [-V].
After calculating the development bias in STEP 70, upper and lower limit processing of the development bias is performed in STEP80. If the corrected developing bias exceeds the preset upper limit value, the developing bias is set as the upper limit value.
Next, in STEP80, Vb after the upper and lower limit processing is stored in the nonvolatile memory.
If it is determined in STEP 40 that the correction is not to be executed, the correction is not executed and the process is terminated. The above is the basic development potential correction control.

Next, the development potential change timing in this embodiment will be described.
As shown in FIG. 10, it is desirable to change the image forming condition between non-image areas (non-image forming areas), that is, between transfer sheets. This is because when it is carried out in the image area, there is a possibility of causing a color variation within the same transfer paper.
Even when correction is performed in a non-image area, if the step of changing the image forming bias is reduced and correction is performed in several stages, the color can be corrected satisfactorily without color variation.

As described above, by correcting the development potential as an image forming condition, it is possible to stably control the image density during printing without side effects.
When correcting the development potential by changing the development bias, it is possible to stably control the image density during printing relatively easily without being restricted by the image forming system. Further, since the response to the image density is high, the adjustment can be performed in a short time.
By taking in the moving average of the image area or the image area ratio, it is possible to control in consideration of the history of the developer. By adopting such parameters, the developer mixing state can be predicted to some extent from the past history, so that the image density can be guided in a more stable direction.
If the development capability is predicted to increase, guiding the image density in the direction of lowering the image density makes it possible to stably maintain the image density during printing.
When the developing ability is predicted to be low, if the image density is guided in the direction of increasing, the image density during printing can be stably maintained.
In addition, by changing the development potential stepwise in the non-image area, it is possible to suppress color variation between successive transfer sheets.
According to this embodiment, even in an image forming apparatus that does not have a toner density sensor, it is possible to stably maintain the image density during printing.

A second embodiment will be described with reference to FIGS. 11 and 12. In addition, the same part as the said embodiment is shown with the same code | symbol, The description on the structure and function which were already demonstrated is abbreviate | omitted as long as there is no special need, and only the principal part is demonstrated (same in other following embodiment).
In the first embodiment, the moving average of the image area ratio is set as the toner replacement amount within a certain period, but similar information can be obtained from the accumulated value of the toner replenishment drive motor rotation time during the certain period.
Since this value has already been acquired to calculate the toner consumption, it is not necessary to newly calculate an image area ratio or the like, which leads to a reduction in software load.
FIG. 11 shows the relationship between image output and toner supply. By storing toner replenishment times for the past several to several tens of sheets, the replenishment time moving average [msec / page] can be grasped using the following equation.
Replenishment time moving average = Cumulative replenishment time / Number of printed sheets [msec / page] ... Formula (7)

The toner replenishment drive time correction is performed using the replenishment time moving average.
Next, the control flow of this correction is shown in FIG. Basically, the flow is the same as that shown in FIG. 9, but the toner supply driving time moving average is acquired in STEP 30, and the VUT correction coefficient is acquired in STEP 50 by referring to the LUT. The LUT may be created as shown in Table 2, for example.

By taking the moving average of the rotation time of the toner replenishment drive motor, it is possible to control in consideration of the developer history. By adopting such parameters, the developer mixing state can be predicted to some extent from the past history, so that the image density can be guided in a more stable direction. Since the parameters are also used for calculating the toner consumption and the like, the load on the software design section can be reduced.
If the development capability is predicted to increase, guiding the image density in the direction of lowering the image density makes it possible to stably maintain the image density during printing.
When the developing ability is predicted to be low, if the image density is guided in the direction of increasing, the image density during printing can be stably maintained.
A third embodiment will be described with reference to FIG.
In each of the above embodiments, the development potential is corrected by correcting the development bias. However, in this embodiment, the exposure potential is changed by changing the LD power or the LD duty.
The flow of this control is shown in FIG. Here, parts different from those of the first embodiment will be mainly described.
First, in STEP 35, the development bias: Vb is acquired. This Vb is a value obtained in S-6 in the flow of FIG. STEP40 to STEP60 are the same as those in the first embodiment.
Next, in STEP 70, the target Vl [-V] is calculated using the following equation.
Target exposure area potential: Vl [-V] = Vb [-V] -Vpot '[-V]... Equation (8)
Here, Vb is a value acquired in STEP35.

Next, in STEP80, the LD power is changed. When the absolute value of the exposed portion potential is reduced, the LD power is increased. Conversely, when increasing the absolute value, the LD power is decreased. Also in the case of Duty, if the absolute value is similarly reduced, Duty is increased. When increasing the absolute value, decrease the duty. In STEP90, measure Vl with a photoreceptor surface potential meter and confirm that Vl has reached the target value.
In step 100, the upper and lower limits of Vl are processed, and in step 110, Vl is saved. In a system that does not have a photoconductor surface potential meter, Vl may be obtained as a table from the relationship between photoconductor potential: Vc and Vd power.
By acquiring the toner replacement amount information, correcting the LD power or Duty based on the result, and measuring the exposure portion potential with the potential sensor, the correction can be performed more accurately.

Next, in order to comprehensively evaluate the control performance of the embodiment according to the present invention, a short running test was performed, and the conventional toner supply control and the control according to the embodiment of the present invention were compared. The results are shown in FIG.
The test conditions were that the standard linear velocity mode (144 mm / s) changed the image area ratio and passed 100 sheets continuously. The paper passing pattern is as shown in Table 3 below.

  In the comparative example, the image density increased as the image area ratio increased. On the other hand, the control of the present example (embodiment according to the present invention) was able to control the image density almost constant even when the image area ratio increased.

16 Photosensitive drum as image carrier 20 Developing device 34 Optical sensor as pattern detection means 36 Toner replenishing device 50 Developing roller as developer carrier

JP 2000-181155 A JP-A-2-34877 JP 2006-139180 A

Claims (13)

A developer carrier disposed opposite to the image carrier carries a two-component developer composed of toner and a magnetic carrier for holding the toner, and a development area formed between the image carrier and the image carrier A developing device for developing the electrostatic latent image formed on the surface of the image carrier with toner,
Image density adjusting means for adjusting the image density by changing image forming conditions;
A toner supply device for supplying toner to the developing device;
And the developing device is not provided with a toner concentration detecting means for detecting the toner concentration in the developer.
An image density control method, wherein the image forming condition is changed according to image output information or operation information of the toner replenishing device in order to keep the image density constant during printing. The image density control method according to claim 1,
An image density control method, wherein the developing potential is corrected as the image forming condition. The image density control method according to claim 2,
An image density control method, wherein a development potential is corrected by changing a development bias. The image density control method according to claim 2,
An image density control method, wherein the development potential is corrected by changing LD power or LD light emission duty which is a writing condition. In the image density control method according to any one of claims 1 to 4,
An image density control method, wherein the image output information is an image area within a predetermined section or a moving average of image area ratios. The image density control method according to claim 5,
An image density control method, comprising: correcting a development potential so that a development potential is reduced when an image area or an image area ratio in the predetermined section is higher than a reference value. The image density control method according to claim 5,
An image density control method, comprising: correcting a developing potential so that a developing potential becomes large when an image area or an image area ratio in the predetermined section is lower than a reference value. In the image density control method according to any one of claims 1 to 4,
An image density control method, wherein the image output information is an average of driving times of the toner replenishing device within a predetermined period. The image density control method according to claim 8.
An image density control method, comprising: correcting a developing potential so that the developing potential is reduced when the driving time of the toner replenishing device is longer than a reference value. The image density control method according to claim 8.
An image density control method, comprising: correcting a developing potential so that a developing potential is increased when a driving time of the toner replenishing device is shorter than a reference value. In the image density control method according to any one of claims 1 to 10,
An image density control method, wherein the image forming condition is changed in a non-image forming area. The image density control method according to any one of claims 1 to 11,
An image density control method, wherein the image forming condition is changed in several steps.   An image forming apparatus that implements the image density control method according to claim 1.

Images