JPH11174748A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device which speedily and highly accurately determines a density target value by a density sensor. SOLUTION: The device performs a process (S101-S106) for forming on an image carrier first - third standard patches with the light intensity of a laser exposure unit set at a weak level, an intermediate level, and an intense level, respectively, a process (S107-S109) for measuring the densities of the first - third standard patches by means of a density sensor, and a process (S110 and S111) for transferring and fixing the first - third standard patches onto paper. The device further performs a process for measuring the densities of the patches fixed on the paper, and a process for determining the density target value by the density sensor based on information on the densities of the patches before and after the fixing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine and a printer using an electrophotographic system, and more particularly, to forming a toner image for density control on an image carrier and detecting the density of the toner image. And an image forming apparatus having a function of controlling the density of an image to be formed.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus using an electrophotographic system, a function for controlling an image density is as follows.
2. Description of the Related Art There is widely known a device provided with a density sensor for detecting the density of a density control toner image formed on an image carrier. In the case of this type of image forming apparatus, the density target value of the density sensor for the density control toner image is determined for each apparatus due to individual differences in the detection performance of the density sensor and variations in the attachment of the density sensor to the apparatus. There is a need.

The prior art is disclosed in, for example, Japanese Patent Laid-Open No. 4-2
Japanese Patent No. 46672 discloses a method of controlling the density of a toner image for density control after supplying or consuming the toner so that the density of the toner image for density control is constant on a sheet of paper after the developing potential of the toner image for density control is constant. An image sensor is disclosed in which an image is measured by a density sensor and the measured value is used as a density target value.

In addition to this, for example, Japanese Patent Application Laid-Open No. 9-60
No. 69 discloses a method of measuring a density control toner image with a density sensor, fixing the toner image on a sheet, outputting the image, measuring the image density on the sheet, and determining a predetermined image density on the sheet and the density sensor. There is disclosed a technique in which a measurement value of a density sensor corresponding to an image density on a sheet to be targeted is predicted from a relationship with the measured value, and the predicted value is used as a density target value.

[0005]

However, in the technique disclosed in Japanese Patent Application Laid-Open No. Hei 4-246672, it is possible to determine an accurate density target value, but it is necessary to supply or consume toner to obtain an appropriate density. Takes a long time. Normally, the density target value is determined based on the adjustment process at the time of manufacturing the image forming apparatus or the density target value being used because the density sensor is replaced while the image forming apparatus is being used. This is when it is no longer possible. Therefore,
When supplying or consuming toner to obtain an appropriate density,
For the time spent on it, assembling and adjusting the device,
In addition, the repair time of the equipment in the customer market is lengthened.

On the other hand, in the technique disclosed in Japanese Patent Application Laid-Open No. 9-6069, although the time required for determining the density target value is shortened, the relationship between the image density on paper and the measurement value of the density sensor is as follows. Since it is necessary to make predictions using a relational expression that captures the individual differences of the concentration sensors and the variation in the attachment, the concentration target value cannot be determined with high accuracy.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and forms an electrostatic latent image by exposing an image carrier and developing a toner by developing the electrostatic latent image. An image forming apparatus that forms an image, transfers the toner image to a transfer material, and fixes the toner image on the transfer material. Means for forming a toner image, density detecting means for detecting image densities of the plurality of density control toner images before fixing, density obtaining means for obtaining image densities of the plurality of density control toner images after fixing, A target value determining unit that determines a density target value of the density detection unit for the density control toner image based on the density information detected by the density detection unit and the density information acquired by the density acquisition unit. Has adopted.

In the image forming apparatus having the above-described structure, a plurality of density control toner images having different image forming conditions are formed on an image carrier, and the image densities of the respective density control toner images before fixing are determined. Since the detection unit detects the image density and the image density after fixing is acquired by the density acquisition unit,
Regardless of the individual difference of the concentration detection means and the variation of installation,
It is possible to accurately grasp the characteristics of each concentration detecting means. Also, as compared with the conventional case where the toner supply amount is increased / decreased to keep the image density on the transfer material constant and the image has a density target value determined by the value detected by the density detection means, the density target The time required to determine the value is greatly reduced. As a result, it is possible to quickly and accurately determine the density target value of the density detection means for the density control toner image based on the image density information before and after fixing.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a schematic configuration diagram showing an embodiment of an image forming apparatus according to the present invention. In the illustrated image forming apparatus, a platen (original table) 2 on which an original 1 is set
Below, an image reading system including an exposure lamp 3, a scanning optical system 4, and a CCD sensor 5 is provided. The CCD sensor 5 receives the reflected light from the document 1 by the irradiation light from the exposure lamp 3 through the scanning optical system 4 and photoelectrically converts the reflected light into 256 gradations at a resolution of, for example, 400 dpi, thereby forming a document image. Generate corresponding image data. This image data is stored in the image memory 6.

An image processing unit (IPS) 7 includes an image memory 6
Is subjected to various types of image processing, and the image signals subjected to the image processing are supplied to the laser exposure device 8. The laser exposure device 8 includes a light emitting driver 8a and a semiconductor laser 8b, and emits a laser beam when the light emitting driver 2a drives the semiconductor laser 2b according to the image signal processed by the image processing unit 7. The rotating polygon mirror (polygon mirror) 9 scans the laser light emitted from the laser exposure device 8 in the axial direction of the image carrier (photosensitive drum) 10.

The image carrier 10 is provided so as to be rotatable in the direction of the arrow in FIG. Around the image carrier 10, a charger 11, a developing device 12, a transfer device 13, a density sensor 14, a cleaner 15, and a static eliminator 16 are sequentially arranged according to the rotation direction.

The developing unit 12 agitates and mixes the two-component developer charged in the developer stirring chamber to charge the developer.
And a developing roll 12 b rotatably disposed at a position facing the image carrier 10 and transporting the charged developer to a position facing the image carrier 10. For example, a DC of -500 V or a voltage obtained by superimposing AC on DC is applied to the developing roll 12b from the developing bias voltage applying unit 17.

The density sensor 14 is an optical sensor composed of, for example, a combination of a light emitting diode and a photosensor. The photosensor receives reflected light of light emitted from the light emitting diode toward the image carrier 10 and receives the reflected light (reflected amount). Light intensity)
The density detection is performed according to the following.

On the other hand, a toner box 18 connected to the developing device 12 stores toner for development. The weight ratio (TC%) of the toner in the developer is maintained at a predetermined level by driving a dispense motor 20 controlled by a motor driver 19, and the toner is transferred to the developing device 1
2.

The paper feed roll 21 is for transporting the paper P as a material to be transferred, and a transport guide 22, a fixing device 23, a discharge roll 24, and a tray 25 are sequentially arranged downstream in the transport direction. I have. The fixing device 23 permanently fixes the toner image transferred onto the paper P by a heating / pressing action.

The control section 26 controls the processing operation of the entire image forming apparatus. For example, a ROM storing a control program or an RA storing various control data is provided.
M, and a CPU that performs processing operations in accordance with a control program stored in the ROM. The control unit 26 controls the image processing unit 7 and the bias voltage applying unit 1
7, a motor driver 19, a patch data memory 27, a light intensity determination unit 28, and the like. The processing operation of the control unit 26 will be described later in detail with reference to a flowchart.

The patch data memory 27 stores patch data for density control. The patch data is read by the control unit 26 as needed. The light intensity determining unit 28 determines the intensity of the laser light according to the control signal from the control unit 26, and drives the light emission driver 8a.

Next, the basic operation of the image forming apparatus having the above configuration will be described. First, upon receiving an image formation start signal from the control unit 26, the laser exposure unit 8 emits laser light in accordance with the image signal processed by the image processing unit 7. The image signal in this case may correspond to electronic information transferred via a network, or may correspond to a document image read by an attached image reading system (3, 4, 5).

On the other hand, the surface of the image carrier 10 is previously charged at a predetermined voltage level (for example, -700 V) by the charger 11.
And moves to the exposure position by the rotating polygon mirror 9 in this state. At this time, the laser beam emitted from the laser exposure device 8 is irradiated on the image carrier 10 while being scanned by the rotating polygon mirror 9, so that an electrostatic latent image is formed on the image carrier 10.

The electrostatic latent image thus formed on the image carrier 10 is visualized by toner supplied from the developing device 12. On the other hand, the paper P serving as the transfer material is sent by the paper feed roll 21 to the portion where the image carrier 10 and the transfer device 13 are opposed to each other, that is, to the image transfer position 29. At the image transfer position 29, the toner image carried on the image carrier 10 is transferred onto the paper P by the electrostatic force of the transfer device 13. At this time, the residual toner not transferred from the image carrier 10 to the sheet P is removed by the cleaner 15. Unnecessary charges on the surface of the image carrier 10 after cleaning are removed by the charge remover 16.

Thereafter, the paper P on which the toner image has been transferred is
The toner image is transferred to a fixing device 23 while being guided by a transport guide 22, where the toner image is fixed by a heating / pressing action, and then discharged to a tray 25 by a discharge roller 24. Thus, a series of image forming operations is completed.

Next, a procedure for controlling the image density in the image forming apparatus will be described. It goes without saying that the so-called image density such as the density of the toner image formed on the image carrier 10 and the density of the toner image transferred onto the paper P depends on the image signal given from the image processing unit 7. However, even if the image signal is the same, the image density differs due to various factors. Determining factors of the image density for the same image signal include the charger 11
Output, light intensity of the laser exposure device 8, developing bias voltage,
There are TC% and the operation time of the dispense motor 20 for maintaining TC% at a desired level, the rotation speed ratio between the developing roll 12b and the image carrier 10, the transfer electric field, and the like.

Therefore, in order to control the image density, the control section 26 reads out patch data for density control from the patch data memory 27 separately from the normal image formation described above, and based on the read patch data. By driving the laser exposure unit 8, an electrostatic latent image (hereinafter, a patch latent image) corresponding to the patch data is formed on the image carrier 10. The formation of such a patch latent image is performed during a pre-rotation operation of the image carrier 10 before and after the start of image formation, or in a non-image area during image formation.

The patch latent image formed on the image carrier 10 in this manner is a toner image (hereinafter referred to as a
In a state where the image is visualized as a toner patch), it moves to a position facing the density sensor 14. Incidentally, in the apparatus configuration shown in FIG. 1, the density sensor 14 is arranged downstream of the image transfer position 29 in the rotation direction of the image carrier 10.
In such a case, unlike normal image formation, the toner patch is fed to the position facing the density sensor 14 without being transferred to the paper P.

Then, the density sensor 14 measures the density of the toner patch sent to the opposing position, that is, the density measurement area. This measured value is referred to as “VPACH”.
At this time, it is preferable to measure the density in the toner patch a plurality of times, average the density, and obtain a VPACH, so that the variation in density measurement by the density sensor 14 can be reduced. Further, the density sensor 14
In order to correct the contamination of the image carrier 10 and the sensor itself and the fluctuation of the sensor measurement value due to the surrounding environment (temperature, etc.), for example, a clean image carrier 10 on which no toner image is formed at the start of image formation. Measure the surface. This measured value is referred to as “VCLEAN”.

On the other hand, in the control unit 26, the above-mentioned VP
The ratio of ACH to VCLEAN (VPACH / VCLEA
N) is obtained and used as a patch density for subsequent density control. The patch density used for this density control is represented by “R
adc ". After the patch density Radc is calculated in this way, the control unit 26 calculates the Rad calculated by the calculation.
c is compared with a preset target value of Radc, and the image density determining factor is changed according to the comparison result (for example, a difference) to control the image density.

Here, a processing procedure for controlling the image density by changing the operation time of the dispense motor 20, which is one of the image density determining factors, will be described with reference to the flowchart of FIG.

First, the above-mentioned VCLEAN is obtained by measuring the surface of the image carrier (image carrier) 10 immediately after starting the image forming operation with the density sensor 14 (step S1). At this time, the surface of the image carrier is kept in a clean state because all the residual toner has been removed by the cleaner 15 in the preliminary rotation operation at the end of the previous image forming operation and before the new image forming operation is started. I'm dripping.

Next, it is repeatedly determined whether or not it is time to create a toner image (patch) for density control.
At the creation timing, a toner image (patch) for density control is formed on the surface of the image carrier 10 (steps S2 and S3). The timing of the patch creation is instructed, for example, at a rate of once while normal image formation is performed 20 times, and patch creation is performed based on this instruction. After that, the density of the toner patch formed on the image carrier 10 is measured by the density sensor 14 so that the above-mentioned VPACH
Is obtained (step S4).

Subsequently, the ratio of the previously acquired VCLEAN to the subsequently acquired VPACH (VPACH / VC
(LEAN × 200), the patch density Radc is calculated (step S5). Here, for convenience of concentration calculation, the ratio of actual measured concentration values (VPACH / VCLEA)
N) is multiplied by 200 to calculate the patch density Radc.

Next, the patch density Radc determined earlier
And the difference (ΔRad
After calculating c), the dispense motor operating time corresponding to the calculated ΔRadc is obtained with reference to a correspondence table between ΔRadc and the dispense motor operating time prepared in advance (steps S6 and S7).

Incidentally, the above-mentioned correspondence table between ΔRadc and the dispense motor operating time shows the relationship between ΔRadc (density difference from the target value) and the amount of toner excess or deficiency, and the dispense motor necessary to supply the insufficient toner amount. The operation time is set appropriately after experimentally grasping the operation time in advance.

Next, an instruction is given to the motor driver 19 to operate the dispense motor 20 for the dispense motor operation time obtained earlier (step S).
8). Thereafter, it is determined whether or not the image forming operation has been completed. If the image forming operation has not been completed, the process returns to step S2 to repeat the same processing. If completed, the process exits a series of processes (step S9).

In such image density control processing, the patch densities Radc and Rad calculated from the actual measured density values are used.
Since the image density determining factor (such as the operation time of the dispense motor 20) is set by the difference (ΔRadc) from the c target value, if the Radc target value (the density target value of the density sensor 14) is inappropriate, a desired value is obtained. Image density cannot be obtained.

Therefore, in the image forming apparatus of the present embodiment, the Radc target value is determined by the following processing.

First, as the patch data stored in the patch data memory 27, as shown in FIG.
The pattern image of 200 dpi has a smaller change amount of the detection level of the density sensor 14 with respect to the change of the image density than the pattern image of the Pi, and the density can be accurately detected even if the fluctuation width of the density change is large. I understand. In addition, in the case of both 400 dpi and 200 dpi pattern images, in the region where the image density is less than 50% and more than 85% (indicated by the arrow in the figure), the output of the density sensor 14 is saturated and a correct measurement value is obtained. I can't. For this reason, in the present embodiment, it is assumed that patch data having an image density of 60% is used for a 200 dpi pattern image.

FIG. 4 is a flowchart showing an operator operation procedure for determining a density target value (Radc target value). First, the operator executes a "PG copy" program (step S10). The “PG copy” program includes a process of fixing and outputting a test image for density measurement (hereinafter, PG copy) and a process of measuring the density of the test image (toner patch) before fixing by the density sensor 14. It is a control program including:

Next, the operator sets the output PG copy on the platen 2 (step S20),
Execute the “IIT measurement” program (step S3
0). This "ITT measurement" program measures the image density of the PG copy set on the platen 2 with the CCD sensor 5, and compares the measurement value of the CCD sensor 5 with the measurement value of the density sensor 14 in the "PG copy" program. The control program is used to calculate a Radc target value.

In this case, the density after fixing is obtained by measuring the image density of the PG copy using the original reading system (3, 4, 5) attached to the image forming apparatus. The purpose of the density adjustment is to correlate the density of the test image measured by the density sensor 14 before fixing with the density of the test image after fixing. Means are not necessarily limited to those using an attached image reading system. For example, in the case of an image forming apparatus such as a printer that does not have an image reading system, an operator measures the image density after fixing using another density measuring device or scanner, and inputs the measurement result by key input or network or The image density after fixing may be obtained by inputting the image density to the image forming apparatus via the communication interface.

Next, a specific processing procedure based on the "PG copy" program will be described. The “PG copy” program is slightly different depending on whether the density sensor 14 is located upstream or downstream of the image transfer position 29 in the rotation direction of the image carrier 10. Therefore, first, the density sensor 14 is moved to the image transfer position 2.
9 will be described below. After that, the density sensor 14 detects the image transfer position 29.
A processing procedure in the case of being located further downstream than will be described.

FIG. 5 shows that the density sensor 14 is positioned at the image transfer position 29.
9 is a flowchart illustrating a processing procedure when the apparatus is located on the upstream side. First, at the start of image formation, the surface of the image carrier 10 is measured by the density sensor 14 to obtain VCLEAN (step S100). Next, as a first level of patch formation, the light intensity of the laser exposure device 8 is set to a low level (for example, 150 μW) (Step S101). The setting of the light intensity level is performed by giving a control command to the light intensity determination unit 28 to that effect. Next, by controlling the light emission driver 8a based on the patch data read from the patch data memory 27, a first-level patch is formed on the image carrier 10 with the light intensity (weak light intensity) previously set, At the same time, a timer for controlling the position of the patch is reset to 0 (step S10).
2).

Subsequently, as a second level of patch formation, the light intensity of the laser exposure unit 8 is set to an intermediate level (for example, 220 μm).
W) (step S103), and thereafter, when the timer value reaches a predetermined value (Tmsec), a second-level patch is formed on the image carrier 10 (step S10).
4). Next, as a third level of patch formation, the light intensity of the laser exposure device 8 is set to a high level (for example, 290 μW) (Step S105), and thereafter, when the timer value reaches a predetermined value (2 × Tmsec). Then, a third level patch is formed on the image carrier 10 (step S106).

As a result, three toner patches are formed on the image carrier 10 at regular intervals in the rotation direction. The density of each of the patches formed at the first to third levels corresponds to the third level in a form corresponding to the light intensity of the laser exposure unit 8.
The level patches are the darkest, then the second level patches, and the first level patches are the lightest.

Subsequently, the densities of the first to third level patches (toner patches) are sequentially measured by the density sensor 14 in accordance with the rotation operation of the image carrier 10, whereby the density measurement corresponding to the first level is performed. A value VPACH1, a density measurement value VPACH2 corresponding to the second level, and a density measurement value VPACH3 corresponding to the third level are obtained (step S1).
07 to S109).

Thereafter, the patches of the first to third levels, for which the density measurement has been completed, are transferred onto one sheet of paper P, and the transferred patches are fixed on the sheet of paper P by the fixing device 23 (steps S110 and S111). . As a result, on one sheet of paper P, as shown in FIG. 6, the patch 30a corresponding to the first level, the patch 30b corresponding to the second level, and the third level in the sheet feeding direction (the direction of the arrow). Corresponding patch 30c
Are sequentially formed at equal intervals, and the sheet P on which the patch has been formed is thereafter handled as a PG copy.

Finally, each of the measured concentration values VPACH1,
VPACH2, VPACH3 and step S100 in advance
Then, Radc1, Radc2, and Radc3 are calculated by multiplying each of the ratios to VCLEAN obtained in (1) by 200, and the calculation results are stored in the RAM or the like as the measured values of the density sensor 14 of each patch (step S112).

In the case of the density sensor 14 using a combination of a light emitting diode and a photo sensor, the amount of light reflected from the image carrier 10 is received and the image density (VCLEA
N, VPACH). Therefore, the density sensor 1
The measurement value of 4 has a relationship of Radc1>Radc2> Radc3. From this, Radc1, Radc2,
Radc3 is a numerical value that means that the larger the value is, the thinner the image is.

FIG. 7 shows that the density sensor 14 is positioned at the image transfer position 29.
It is a flowchart which shows the processing procedure in the case of being located further downstream than it. Note that the processing of steps S100 to S106 in FIG.
Since the same is performed even if the position of No. 4 is different, the processing thereafter (step S106 and subsequent steps in FIG. 5) will be described.

That is, steps S100 to S106 in FIG.
After the VCLEAN is acquired in accordance with the processing of the above, after forming the first to third level toner patches in which the light intensity of the laser exposure device 8 is changed on the image carrier, the first to third level patches are changed to one. The transferred patches are transferred onto a sheet of paper P, and the transferred patches are fixed on the sheet P by the fixing device 23 (step S12)
0, S121). As a result, patches 30a, 30b, and 30c are formed on one sheet of paper P as shown in FIG.
The sheet P is discharged as a PG copy. However, since the patch on the image carrier 10 is transferred to the sheet P before reaching the position facing the density sensor 14, the density measurement of the patch by the density sensor 14 cannot be performed.

Therefore, in order to measure the density by the density sensor 14, the residual toner on the image carrier 10 is removed by the cleaner 15, and a patch is again formed on the image carrier 10 according to the following procedure.

First, as the first level of patch formation, after setting the light intensity of the laser exposure unit 8 to the first level (weak) (step S122), the timer sets the first level of the image carrier 10.
It is repeatedly determined whether or not the division has been counted (step S1).
23). Next, when the timer measures one rotation of the image carrier 10, a first-level patch is formed on the image carrier 10, and at the same time, a timer for controlling the position of the patch is set to 0.
(Step S124). As a result, in the rotation direction of the image carrier 10, step S
At step 120, the first-level patch is formed again at the same position as the first-level patch transferred to the sheet P under the same image forming conditions (light intensity).

Thereafter, while changing the light intensity of the laser exposure unit 8 to the second and third levels, the second and third level patches are formed on the image carrier 10 while referring to the timer value (step S125). To S128). Thereby, the second and third
The level patches are also formed at the same positions as the second and third level patches already transferred to the paper P in the rotation direction of the image carrier 10 under the same image forming conditions (light intensity).

After the first to third level patches (toner patches) are formed on the image carrier 10 in this manner, the first to third level patches (toner patches) are formed in accordance with the rotation of the image carrier 10 in the same manner as the processing flow of FIG. The densities of the patches (toner patches) from the first level to the third level are sequentially measured by the density sensor 14, thereby obtaining a density measurement value VPACH1 corresponding to the first level, a density measurement value VPACH2 corresponding to the second level, and a third level. The corresponding measured concentration values VPACH3 are obtained (step S12).
9-S131).

Further, each of the measured concentration values VPACH1,
VPACH2, VPACH3 and step S100 in advance
Then, Radc1, Radc2, and Radc3 are calculated by multiplying the ratio with VCLEAN obtained in (1) by 200, respectively, and the calculation result is stored in a RAM or the like as a measured value of the density sensor 14 of each patch (step S132). By the way,
In this case, since the patches of the first to third levels have already been transferred and fixed on the paper P, the image carrier 1 having completed the density measurement
The patch on 0 is removed by the cleaner 15 as it is.

Here, the electrical characteristics of the image carrier 10 slightly change in the outer peripheral surface thereof. Therefore, even if the surface of the image carrier 10 is charged at the same voltage level and then exposed at the same light intensity, the resulting surface potential remains within one round of the image carrier 10 as shown in FIG. Will fluctuate. Therefore, even if the patches are formed under the same image forming conditions, if the formation positions in the rotation direction of the image carrier 10 are different, they appear as a density difference, which may adversely affect the density measurement. Is done.

On the other hand, in the case of the present embodiment, in the series of processes described above, the first to first transfer and fixation on the paper P are performed.
Since the third-level patch and the first to third-level patches whose density is measured by the density sensor 14 are formed at the same position in the rotation direction of the image carrier 10, the surface of the image carrier 10 is formed. Can reliably be prevented from being affected by changes in the electrical characteristics of the device.

Next, a specific processing procedure based on the "IIT measurement" program will be described with reference to the flowchart of FIG. First, a PG copy (paper P shown in FIG. 6) placed on a platen 2 by an operator is scanned by an exposure lamp 3 and a scanning optical system 4 while a CCD sensor 5 is scanned.
The photoelectric conversion is performed on the light received in step (1) to obtain image data corresponding to the PG copy image (step S300).

Next, the densities of the first to third level patches 30a to 30c shown in FIG. 6 are measured from the image data corresponding to the patch formation positions of the PG copy. The corresponding concentration measurement value "II
T1 ", a density measurement value" IIT2 "corresponding to the second level patch 30b, and a density measurement value" IIT3 "corresponding to the third level patch 30c (step S30).
1).

In this case, the concentration is measured, for example, at 400 d.
The image data read by the CCD sensor 5 at the resolution of pi is obtained by calculating the average value of the density levels of continuous 256 × 256 pixels. Therefore,
IIT1 to IIT3 are numerical values meaning that the larger the value is, the deeper the image is.

Then, the previously measured measured density value IIT2 is compared with a preset IIT target value (step S).
302). The IIT target value is a standard image density desired in an output image transferred and fixed on a sheet P as a transfer material.

If the measured concentration value IIT2 is larger than the IIT target value (Yes in step S302), as shown in FIG. 10A, the IIT target value exists between IIT1 and IIT2. Therefore, the density target value (Radc) of the density sensor 14 corresponding to the IIT target value
The target value) exists between Radc1 and Radc2 measured during the processing of the “PG copy” program.

Therefore, in step S303, Radc
"ΔRadc target value" which is the difference between the target value and Radc2
Is obtained by the following equation (1). ΔRadc target value = (Radc1−Radc2) × (IIT target value−IIT2) ÷ (IIT1−IIT2) (1) Incidentally, ΔRadc obtained by the equation (1)
The target value is (Radc1-Radc2) is a plus value,
(IIT target value-IIT2) is a negative value, (IIT1
Since −IIT2) is a negative value, it becomes a positive value.

On the other hand, the measured concentration value IIT2 is II
If it is smaller than the T target value (N in step S302
In the case of o), as shown in FIG. 10B, since the IIT target value exists between IIT2 and IIT3,
The target density value (Radc target value) of the density sensor 14 corresponding to the IIT target value exists between Radc2 and Radc3.

Therefore, in step S304, Radc
The difference “ΔRadc target value” between the target value and Radc2 is obtained by the following equation (2). ΔRadc target value = (Radc3−Radc2) × (IIT target value−IIT2) ÷ (IIT3−IIT2) (2) Incidentally, ΔRadc obtained by the equation (2)
For the target value, (Radc3-Radc2) is a negative value,
(IIT target value-IIT2) is a plus value, (IIT3-
Since IIT2) is a plus value, it becomes a minus value.

Subsequently, in step S305, using the ΔRadc target value obtained in step S303 or S304, a density target value (Radc target value) of the density sensor 14 is obtained by the following equation (3). Radc target value = Radc2 + ΔRadc target value (3)

Finally, the Rad obtained in step S305
The target value c is stored in a memory such as a RAM (step S3).
06). Thus, the “IIT measurement” program is completed, and thereafter, the image density control processing (the flowchart of FIG. 2) is performed using the Radc target value stored in step S306.

Incidentally, the above-mentioned step S303 or S3
In 04, when the difference between the IIT target value and IIT2 is calculated, if the calculated value exceeds the allowable range in which it is determined that the density can be adjusted, that is, in FIG. 10A, the IIT target value is lower than IIT1. Or if the IIT target value is higher than IIT3 in FIG.
To that effect, an operator is warned, or a separately prepared toner density adjustment program (a program for forcibly replenishing or consuming toner until the difference between the IIT target value and IIT2 is within an allowable range in which it is determined that density adjustment is possible) Should be executed.

As described above, in the image forming apparatus of this embodiment, the first to third level patches in which the light intensity of the laser exposure device 8 is changed are formed on the image carrier 10 and the respective patch densities are determined. The patch densities measured by the sensor 14 and transferred / transferred onto the paper P are read by an image reading system (3, 4,
5), the characteristics of each density sensor (the relationship between the sensor measured value for the image before fixing and the image density after fixing) irrespective of the individual difference of the density sensor and the dispersion of the mounting. Can be accurately grasped. Also, as compared with the conventional case where the toner supply amount is increased or decreased to make the image density on the paper constant and the detection value when the image is detected by the density sensor is determined as the density target value, the density target The time required for value determination can be greatly reduced. In particular, in the case of the present embodiment, the image forming conditions are changed by the image density determining factor (light intensity) having a fast response, so that the patch density information before and after fixing can be obtained in a very short time. it can. As a result, it is possible to quickly and accurately determine the density target value of the density sensor 14 that affects the control performance of the image density based on the density information of the patches before and after the fixing.

In the above embodiment, when forming a patch as a density control toner image on the image carrier 10, the light intensity of the laser exposure device 8 is changed as one of the image forming conditions. However, the reason is that one sheet P
Since high-speed responsiveness is required when forming a plurality of (three in this embodiment) patches having different image forming conditions, one factor that satisfies the requirement is the light intensity of the laser exposure device 8. Because. Therefore, if it is possible to meet the demand for high-speed response, the first to third level patches are changed by changing factors other than the light intensity of the laser exposure unit 8, for example, the developing bias voltage and the image density. May be formed. However, regarding the operation time of the dispense motor 20, since the response is very slow as compared with the above-described factors, it is not suitable to be used as a changing factor of the image forming conditions.

By the way, the toner amount (TC
%) And the case where the light intensity of the laser exposure device 8, the developing bias voltage or the image density is adjusted, even if the image density fixed and output on the paper P is equal, the measurement of the density sensor 14 is performed. Values may be different. More specifically, the density sensor 14 measures the amount of reflected light when light from the light emitting diode is reflected on the surface of the image carrier 10 by a photo sensor. When the adhesion state changes, the amount of reflected light also changes.

When the light intensity of the laser exposure device 8 is adjusted, the potential distribution of the electrostatic latent image for the same density information changes, and the state of toner adhesion changes in accordance with the change in the potential distribution. . This is shown in FIG. In FIG. 11, “31a” indicates the potential distribution of the electrostatic latent image when the light intensity of the laser exposure device 8 is high, and “31b” indicates the potential of the electrostatic latent image when the light intensity of the laser exposure device 8 is low. The distribution is shown. “32a” is a cross-sectional view of the toner image when the electrostatic latent image of the potential distribution 31a is developed, and “32b” is a cross-sectional view of the toner image when the electrostatic latent image of the potential distribution 31b is developed. Represents.

On the other hand, the amount of toner in the developer (TC%)
Is adjusted, the potential distribution of the electrostatic latent image does not change,
The amount of toner attached changes. This is shown in FIG.
In FIG. 12, “33” indicates a potential distribution of the electrostatic latent image. “34a” indicates a cross-sectional view of the toner image when the toner density is high, and “34b” indicates a cross-sectional view of the toner image when the toner density is low.

Here, the image density after fixing depends on the total amount of toner on the paper P, while the value measured by the density sensor 14 depends on the coverage of the toner on the image carrier 10.
For this reason, even if the total amount of toner fixed on the paper P is the same, if the state of toner adhesion on the image carrier 10 is different,
The measured value of the density sensor 14 may be different.

FIG. 13 is a diagram showing two types of combination patterns in which the light intensity of the laser exposure unit 8 and the toner amount (TC%) in the developer are adjusted under the condition that the total amount of toner on the paper P is equal. It is. In FIG. 13, "35a"
Indicates the potential distribution of the electrostatic latent image when the light intensity of the laser exposure device 8 is high, and “35b” indicates the potential distribution of the electrostatic latent image when the light intensity of the laser exposure device 8 is low. Also, "3
6a is a sectional view of the toner image when the light intensity of the laser exposure device 8 is large and the toner density is low, and "36b" is a sectional view of the toner image when the light intensity of the laser exposure device 8 is small and the toner concentration is high. Is represented.

In such a case, "36a" and "36b"
Are equal, the image density in a state where the toner is transferred and fixed on the paper P is equal.
Since the measured value of 4 depends on the coverage of the toner on the image carrier 10, the toner image of “36a” having a higher coverage is “3”.
The measured value of the density sensor 14 is different, for example, the image density is measured higher than the toner image of “6b”.

FIG. 14 shows a case where the light intensity of the laser exposure unit 8 and the toner amount (TC%) in the developer are adjusted under the condition that the total amount of the toner on the sheet P is equal as described above. The relationship between the image density on P (image density after fixing) and the measurement value of the density sensor 14 (image density before fixing) is shown. From FIG. 14, it can be seen that the light intensity is changed while keeping the toner amount (TC%) in the developer constant and the toner amount (TC
%), The relationship between the image density on the paper and the value measured by the density sensor 14 is different.

The difference in the characteristics of the density sensor 14 with respect to the light intensity and TC% is due to the fact that a halftone image widely used in a recent electrophotographic image forming apparatus handling digital images is converted into a minute image as shown in FIG. This is particularly noticeable when the image is expressed by a screen having a dark and light cycle.

Therefore, the image density determining factor to be subjected to the density control during the image forming operation is determined by the dispense motor 20.
If the image density determinant when changing the level by the “PG copy” program is a different factor (light intensity of the laser exposure device 8), the operation time of The density target value (Radc) of the density sensor 14 depends on the characteristic difference of the density sensor 14.
There is some concern that the target value is not determined under optimal conditions.

Therefore, in the present embodiment, the following correction means is adopted. First, sensor characteristics (sensor characteristics in FIG. 14) when the TC% is fixed and when the light intensity is fixed are experimentally obtained in advance, and I
Table data indicating the correspondence between the IT target value and the Radc correction coefficient is stored in a memory or the like. The Radc correction coefficient is a coefficient for making the sensor measured value when the light intensity is changed while keeping TC% constant to match the sensor measured value when the TC% is changed while keeping the light intensity constant. In the case of the sensor characteristics shown, 100% is set as a default, for example 50
% To 150%.

In the processing of the actual “IIT measurement” program (flow chart in FIG. 9), the ΔRadc target value obtained in step S303 or S304 is replaced with the II value at that time.
The target value is multiplied by the Radc correction coefficient corresponding to the T target value, and the calculation result is set as the ΔRadc target value again. When this is represented by a calculation formula, it becomes like a formula (4). ΔRadc target value = ΔRadc target value × Radc correction coefficient (4) Subsequently, the Radc target value is calculated by applying the ΔRadc target value obtained by the equation (4) to step S305, and this is calculated in step S306. Just remember.

As described above, the density target value (R) of the density sensor 14 determined by the processing of the “IIT measurement” program
adc target value), an image density determining factor (operating time of the dispense motor 20) to be subjected to density control in a normal image forming operation, and an image density when a level is changed by a “PG copy” program Even when the determinant factor (light intensity of the laser exposure device 8) is different, the density target value of the density sensor 14 can be set under the optimum condition without being affected by the difference in the characteristics of the density sensor 14 for each factor. It becomes possible.

In the above embodiment, three patches (density control toner images) having different image forming conditions are formed on the image carrier 10. However, the present invention is not limited to this. Even when the number of formed patches is two, the characteristics of the density sensor can be grasped. When the number of formed patches is four or more, the properties of the density sensor can be grasped more finely.

[0084]

As described above, according to the image forming apparatus of the present invention, a plurality of density control toner images having different image forming conditions are formed on an image carrier, and the respective density control toner images are formed. The density control is performed by detecting the image density before fixing by the density detecting means, further obtaining the image density after fixing by the density obtaining means, and determining the density target value of the density detecting means based on the density information. It is possible to quickly and accurately determine the density target value of the density detection means for the toner image for use. As a result, the time for assembling and adjusting the image forming apparatus can be reduced, and the density control can be stabilized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating an embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a flowchart illustrating a control procedure of an image density.

FIG. 3 is a diagram showing a relationship between a density sensor detection level and an image density.

FIG. 4 is a flowchart showing an operator operation procedure when determining a concentration target value.

FIG. 5 is a flowchart illustrating a processing procedure based on a “PG copy” program in the embodiment.

FIG. 6 is a diagram illustrating an output state of a density control toner image.

FIG. 7 is a flowchart illustrating another processing procedure based on a “PG copy” program in the embodiment.

FIG. 8 is a diagram showing how the surface potential of the image carrier fluctuates.

FIG. 9 is a flowchart illustrating a processing procedure based on a “IIT measurement” program in the embodiment.

FIG. 10 is a diagram illustrating a specific method of determining a density target value.

FIG. 11 is a diagram illustrating a toner attached state when the light intensity is adjusted.

FIG. 12 is a diagram illustrating a toner adhesion state when TC% is adjusted.

FIG. 13 is a diagram illustrating a toner adhesion state when light intensity and TC% are adjusted under the condition that the total amount of toner on paper is equal.

FIG. 14 is a diagram illustrating a relationship between a measurement value of a density sensor and an image density on a sheet.

FIG. 15 is a diagram showing an example of a gray screen.

FIG. 16 is a diagram illustrating a specific method of correcting a density target value.

[Explanation of symbols]

3 exposure lamp 4 scanning optical system 5 CCD sensor
8 laser exposure device, 10 image carrier, 12 developing device, 1
3 ... Transfer unit, 14 ... Density sensor, 20 ... Dispense motor, 23 ... Fixing unit, 26 ... Control unit, 27 ... Patch data memory, 28 ... Light intensity determination unit, 30a, 30b, 30
c: Patch, P: Paper

Claims (2)

[Claims] An image carrier that is exposed to form an electrostatic latent image, and the electrostatic latent image is developed to form a toner image;
In an image forming apparatus that forms an image on the transfer material by transferring and fixing the toner image to a transfer material, a plurality of density control toner images having different image forming conditions are formed on the image carrier. Forming means; density detecting means for detecting image densities of the plurality of density control toner images before fixing; density obtaining means for obtaining image densities of the plurality of density control toner images after fixing; A target value determination unit that determines a density target value of the density detection unit for the density control toner image based on the density information detected by the density detection unit and the density information acquired by the density acquisition unit. An image forming apparatus characterized in that: 2. The image forming apparatus according to claim 1, further comprising a correcting unit that corrects the density target value according to a detection characteristic of the density detecting unit with respect to the image forming condition.