JPH10161365A - Correcting method of image density - Google Patents

Abstract

PROBLEM TO BE SOLVED: To automatically correct gradation, without the damage of a function caused by the fluctuation of the density of an image, when the use of an image forming device is started and during its operation. SOLUTION: Just after a power source is applied or a power saving mode is cleared, it is decided whether the gradation is corrected or not (F01A). When it is decided that the correction of the gradation is required, the gradation is corrected for the first time, during a warm-up. After the gradation is corrected for the first time, the gradation is corrected for the second time, together with the formation of the image. When the gradation is corrected, a gradation correcting patch image with a halftone is formed without exposure, to correct the density.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image density correcting method for correcting the gradation of an image forming apparatus.

[0002]

2. Description of the Related Art In a copying apparatus for obtaining a copy of an original image by an electrophotographic system, a copy image is basically obtained by the following process. Corona charging is performed on the rotating electrophotographic photosensitive member, and image exposure of an original image is performed on the photosensitive member having a uniform charging potential by a scanning optical system. The potential of the photoreceptor portion that has been subjected to image exposure decreases, and an electrostatic latent image corresponding to the original image is formed on the photoreceptor. The electrostatic latent image is developed when it passes through, for example, a developing sleeve portion of a developing device that holds a developer obtained by mixing and stirring a toner and a carrier. That is, the toner is charged in the opposite polarity to the photoconductor in the case of normal development by frictional charging, and the potential on the photoconductor is lowered by applying a developing bias having the same polarity as the photoconductor to the developing sleeve. The charged toner does not adhere to the latent image portion on the white background of the original, and the toner adheres to the latent image portion of the original character or image where the potential does not decrease to form a toner image. The toner image is transferred onto the transfer paper, and the heat-fixed transfer paper is discharged out of the apparatus.

In such a copying apparatus, a charging voltage by a corona charger, a light amount of an illumination lamp for irradiating a document by an image exposure means, a developing sleeve in a developing device, and the like are adjusted in accordance with the characteristics of the photosensitive member. About the bias voltage applied to
It is necessary to set an optimum condition and to obtain a copy image having an appropriate density that is always stable and easy to read even for a dark original or a light original.

In general, an image forming apparatus uses a heat fixing method, and includes a fixing device as a heat source and an exposure lamp. Therefore, when the power of the image forming apparatus is turned on to form an image, the temperature inside the image forming apparatus rises due to the heat source or the like, and stably changes at a certain temperature (for example, 50 ° C.). Along with this, the temperature of the developer in the developing device also rises, and the developer stably changes at a certain temperature. Thus, in the image forming apparatus, in addition to the temperature change of the environment,
This causes a change in the temperature of the developer. The toner in the developer is charged by frictionally charging with the carrier, but when the temperature rises or the humidity drops, the moisture held in the toner is released, and the charge that the toner can hold increases. On the other hand, when the temperature decreases or the humidity increases, the toner absorbs moisture and retains moisture, and the charge retained by the toner decreases. The image forming apparatus is not always used, and sometimes is not used for power saving or is not used at night, so the power is turned off. As described above, when the image forming apparatus is used, the temperature in the image forming apparatus increases when the image forming apparatus is used, and the temperature in the image forming apparatus decreases when the image forming apparatus is not used. Since the temperature of the developer in the image forming apparatus changes due to the above circumstances, the developer has a problem that the charge amount (total charge) of the toner, which is a characteristic of the developer, changes.

That is, the temperature of the developer in the image forming apparatus increases during use, and decreases when the developer is not used. In conjunction with this, the toner in the developer has a low temperature at the start of use and absorbs moisture, so that the charge leaks and the charge amount becomes low. Will be shown. As described above, when the toner absorbs moisture and the charge amount (Q / M) of the toner leaks, the image density of the copy is lower than that of the copy before use even if the potential and the light amount on the photosensitive drum surface are the same. There is a problem of darkening. FIG. 8 shows that the original density (O
D) shows the density transition on the copy for three densities of 0.2, 0.4 and Dmax. At the start of the copy, the OD is particularly high for the OD of 0.2 and 0.4. The density (CD) is extremely high as compared with the copy density in use.

As a countermeasure, if the apparatus is left for a certain time or longer, the amount of exposure is increased by estimating the amount of leakage of Q / M,
The developing bias was corrected in a direction to make the image lighter. In this method, since estimation is included, complicated and complicated control is required such that parameters such as a standing time, charge-up due to agitation of the developing device, and deterioration of the developer have to be grasped and corrected. .

[0007] When it is detected that the toner has been left unattended, pre-rotation is performed before copying, and the developer is stirred again.
A method of charging up the / M leak again by stirring has been adopted. This method requires the user to return to the original Q / M state within a time that can be tolerated by the user. Has to be greatly reviewed.

[0008]

When the image forming apparatus is started to be used, the inside of the apparatus is at room temperature and the developer used is in a state of absorbing moisture.

When the image forming apparatus is used, the temperature inside the image forming apparatus rises due to a heat source such as a fixing device, and the water held in the developer is released. As described above, the developer is affected by the moisture absorbed in the early stage of operation, and has a low triboelectric charging characteristic. However, as the temperature in the image forming apparatus increases, the temperature of the developer also increases, and the developer is dried. Therefore, the triboelectric charging characteristics become higher. The temperature in the image forming apparatus becomes stable when the temperature rises to some extent, and the frictional charging characteristics of the developer become stable. The image forming apparatus is set in accordance with the stable triboelectric charging characteristics of the developer.

Therefore, in the early stage of operation, the charging characteristics of the developer are low, and the density of the halftone image becomes high, and the gradation is deteriorated. For this reason, it is necessary to correct the gradation at the beginning of operation.

However, there is a problem in necessarily performing the gradation correction at the start of use of the image forming apparatus. For example, when the image forming apparatus is started to be used again only a short time after the previous operation is completed, the developer does not absorb moisture and the high Q / M is maintained while the temperature in the apparatus remains high. As a result, even if the gradation is corrected, the result is that the correction is not actually required. Further, since it takes a little time to execute the gradation correction, always performing the gradation correction at the start of use every time delays the start of image formation.

An object of the present invention is to provide an image density correction method in which gradation correction is performed only when gradation correction is required without delaying the start of image formation. Aim.

[0013]

SUMMARY OF THE INVENTION It is an object of the present invention to determine whether or not to correct the gradation immediately after turning on the power or immediately after clearing the energy saving mode. This is achieved by an image density correction method characterized by performing tonality correction.

In the present invention, the second and subsequent gradation corrections after the first gradation correction are performed in a region before the image forming region on the photosensitive member. In a preferred embodiment, the patch image is formed to correct the cooperativeness.

In the gradation correction performed here, the surface of the photoreceptor is charged to a lower charging potential than during normal image formation, an unexposed latent image is formed on the surface of the photoreceptor, and the latent image is developed with a developer. In a preferred embodiment, a patch image for gradation correction is formed on the photoreceptor by developing the image on the photoconductor, and gradation correction performed based on the patch image for gradation correction is performed.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to the description of the image density correcting method of the present invention, the configuration and operation of an electrophotographic copying apparatus to which the present invention is applied will be described with reference to the block diagram of FIG.

In FIG. 1, a document D placed on a transparent document table 1 on the top of the apparatus is pressed by a document pressing cover 2, and when a copy button (not shown) is pressed, an illumination lamp 3a constituting a scanning optical system is turned on. The first mirror unit 3 composed of the first mirror and the second mirror unit 4 composed of the second mirror and the third mirror positioned in a V shape turn on the illumination lamp 3a and operate at the speed of the first mirror unit 3. Due to the reading operation by V and the moving operation of the second mirror unit 4 at the speed V / 2 in the same direction, the reflected light of the original D image emitted by the illumination lamp 3a passes through the imaging lens 5 as shown by the dotted line. Then, an image is formed on the photosensitive drum 10 as an image carrier via the reflection mirrors 6a, 6b and 6c.
Photoreceptor drum 10 having a small diameter (φ60 mm in this embodiment) having an organic photoconductive compound (OPC) photosensitive layer applied to the peripheral surface
Is rotated clockwise at a peripheral speed V (270 mm / sec in the present embodiment), and the photosensitive drum 10 uniformly charged (−780 V in the present embodiment) by the scorotron charger 11 is a document D by image exposure. And the same latent image is formed. A partial exposure means (CEL) 12 for erasing the potential of the photoconductor is provided between the scorotron charger 11 and the developing device 13 so as to irradiate the outside of the frame of the latent image portion of the original on the photoconductor drum 10. The potential is lost to prevent toner from adhering outside the frame of the latent image.

When the latent image (in this embodiment, negatively charged) on the photoreceptor drum 10 passes through the developing device 13, the toner in the developer (in this embodiment, positively charged) is charged with characters such as characters in the latent image. It adheres to a portion where the potential has not dropped and is developed. At this time, the negative bias voltage applied to the developing sleeve 131 is developed deeper as the absolute value is lower, and is developed lighter as the absolute value is higher. Then, after the charge of the photosensitive drum 10 is discharged by the discharge lamp before transfer, if necessary, the toner image is transferred by the transfer unit 16 onto the transfer paper conveyed by the conveyance roller 15 from the paper supply cassette 14. The transfer paper holding the transferred toner image is separated from the photosensitive drum 10 by the separating unit 17 and then sent to the fixing unit 19 by the transport belt 18.
Pressed by the fixing roller 19a and the pressure roller 19b,
Upon heating, the toner image is fixed on the transfer paper. And
The transfer paper after fixing is discharged by a discharge roller 20 onto a discharge tray outside the apparatus.

On the other hand, the photosensitive drum 10 from which the transfer paper is separated
After the residual charge is removed by the removing electrode 21, the residual toner is removed by the cleaning device 22 or the cleaning blade 22 a made of a rubber material in contact with the photosensitive drum 10. The peripheral surface of the photoreceptor drum 10 from which the residual toner has been removed is exposed by the pre-transfer exposure unit 23 to prepare for the next copy.

A method for correcting image density according to the present invention using the above-described copying apparatus will be described.

FIG. 2 is a circuit diagram for gradation correction, and FIG. 3 is a flowchart according to the present invention.

In the present invention, the warm-up is started when the power is turned on or when the energy saving mode is released (for example, by pressing the operation button), and the control unit CPU
Checks whether or not the gradation correction is necessary (F01A). As a method of checking whether or not the gradation correction is necessary, since the temperature of the fixing device drops when the pause time is long, for example, when the fixing roller temperature of the fixing device 19 is equal to or lower than a predetermined temperature (50 ° C.). A method of checking whether or not there is can be used.

At the time of image formation, the fixing device 19 is controlled at a temperature close to 200 ° C. by a heater provided inside the fixing roller 19a, and the recording paper holding the toner image between the rotating fixing roller 19a and the pressure roller 19b. Is established. In order to improve the thermal efficiency, the fixing device 19 is sealed except for the recording paper carry-in portion and the carry-out portion, so that a relatively good heat retaining property is maintained. The temperature inside the fixing unit 19 is determined by the power supply O.
Since the temperature falls very slowly from the temperature at the time of FF, the pause time of the image forming apparatus can be estimated by a temperature sensor that detects the temperature of the fixing roller in the fixing device 19. For example, the fixing roller 19a by the temperature sensor SH1
When the detected temperature of the portion exceeds a predetermined temperature, for example, 50 ° C., the pause time after the previous operation is not so long, and the current gradation standard value is set without performing the gradation correction (F02).
A), an image is formed based on the current gradation standard value (F)
03A). When the detected temperature is 50 ° C. or lower, the pause time after the previous operation is considerably long, so that it is determined that the gradation correction is necessary. In addition, as a detection unit for determining whether or not the gradation correction is necessary, the detection can be performed by detecting the in-machine temperature of the image forming apparatus in addition to the fixing device temperature by the in-frame temperature sensor SH2. This is because the temperature inside the apparatus increases during operation due to the fixing device, the exposure lamp, and other heating elements.

If it is determined that gradation correction is necessary, the next gradation correction (first time) is performed.

In the gradation correction performed here, a patch latent image is formed by non-exposure without being affected by the amount of light, and developed with a developer to form a developed patch image of halftone (optical density 0.4). The density of the developed patch image formed on the photosensitive drum 10 is measured, and if it is determined that the density is higher than a predetermined density, a gradation correction mode (γ mode)
And the gradation is corrected.

As a conventional gradation correction method, a halftone patch image is created by exposing a halftone patch document. However, since the intensity of the exposure lamp is unstable and fluctuates, when a halftone patch image is created by exposure, deviation from the original characteristics occurs due to variation. In particular, the halftone density is an area where the fluctuation of the image density is large with respect to the fluctuation of the exposure intensity. On the contrary, the gradation correction method used here is:
In order to form a patch image without relying on an exposure lamp,
This is a particularly excellent correction method as a correction method of the gradation property performed immediately after the power is turned on.

First, a patch image (γ patch) for gradation correction
Will be described with reference to the explanatory diagram of FIG. The scorotron charger 11 applies a charging potential of the image forming portion of −780 V to the γ patch portion on the photosensitive drum 10.
In order to have a lower charging potential of -400 V in absolute value,
Charging is performed by lowering the grid voltage (see FIG. 4A).

The peripheral edge of the γ patch portion on the photosensitive drum 10 is irradiated with the CEL 12 to lose the potential (see FIG. 4B).

The exposure lamp is not turned on, and the γ patch portion is not exposed (see FIG. 4C).

A developing bias of -170 V is applied to the developing sleeve 131 of the developing device 13 for the image forming portion, whereas -28 V is applied to the γ patch portion for the image forming portion.
By applying a developing bias of 0 V, a developing patch for the γ patch portion is formed (see FIG. 4D). The developing electric field in the image forming portion is 610 V, while the developing electric field in the γ patch portion is 120 V, and the Q / M
Is in the normal charge amount, the maximum density is CD 1.3 for the image forming portion, and the CD is
A patch image having a density of 0.4 is created.

In the formation of a patch image (F1), in this embodiment, the photosensitive drum 10 is rotated twice, and a patch image is formed twice during that time, so that the characteristics of the photosensitive member are stable and stable image data is obtained. The density (CD) of the resulting second formed patch image is detected by a density sensor SD including a light emitting element and a light receiving element provided on the peripheral portion of the photosensitive drum 10 (F2).

The output of the density sensor SD is displayed and processed with Lsb. Lsb is divided into 63 bits between the outputs 0 to 5 V of the density sensor SD and divided into 6 bits.
Indicate with 3. FIG. 5 shows the relationship between the copy density CD received by the density sensor SD and the output (Lsb), for example, the copy density CD0.4 to be measured.
Is Lsb15. Density detection values at 40 points are read in Lsb units from the patch image for gradation correction by the density sensor SD (F2). A histogram is created from the density detection data at 40 points of the read patch image (F3), and a read value x is determined using the lowest Lsb value bin having three or more points (F4).

Density CD0.4 of patch image for gradation correction
Is used as a threshold value for gradation correction during copying, and the threshold value (Lsb15) is compared with the read value x (F5). When the read value x is larger than Lsb15, since the charge amount of the toner does not decrease and the density of the patch image for gradation correction is lighter than CD0.4, the mode is intermittently repeated (γ mode). No correction is made. That is, in the flowchart of FIG. 3, the gradation standard value is set (F02A), and the operation is performed in the normal image forming mode (F03A) based on the gradation standard value.
When the read value x is smaller than Lsb15, that is, since the density of the patch image for gradation correction is higher than CD0.4, gradation correction is performed. The correction of the gradation is performed by changing at least one of the grid voltage, the developing bias, and the exposure intensity. The flow of FIG. 3 shows an example in which the gradation is corrected by changing the grid voltage.

When the read value x is smaller than Lsb15, the calculation of 15-x is performed (F6B), and
When the number is 3, the change amount is −3 (Lsb), when the number is 4 to 6, the change amount is −6 (Lsb), and when the number is 7 or more, the change amount is −9.
The grid voltage is changed as (Lsb). FIG. 6 shows L
It is a graph which shows the relationship between sb and grid potential. The control unit uses Lsb51 corresponding to the grid potential of 780 V as a reference, and when the change amount is, for example, −3 (Lsb), Lsb51 is used.
By changing to 48, the gradation correction is performed such that the grid potential decreases (the image becomes lighter) (F6).
A).

The calculated gradation correction value α is set (F04), and after completion of warm-up, the document D placed on the document table 1 in the intermittent gradation correction mode is set. An image is formed.

Prior to image formation, it is determined whether or not gradation correction is to be performed by copy count. In this embodiment, 5
The gradation is corrected for each copy.

A method of forming an image and forming a patch image in the second and subsequent γ-mode gradation corrections will be described. The scorotron charger 11 applies a potential of −400 V to the γ patch portion on the photosensitive drum 10, and adds a gradation correction value α to the image forming portion to − (780 V−α). (See FIG. 7A).

Irradiation by the CEL 12 is applied to the periphery of the γ patch portion on the photosensitive drum 10 and the outside of the frame of the image forming portion to cause the potential to be lost (see FIG. 7B).

The image forming portion is exposed to the image of the original image, and the γ patch portion is not exposed (see FIG. 7C).

However, in order to stabilize the intensity of the exposure lamp, the exposure lamp may be turned on before the image forming area, and a black patch PD image for image density measurement may be exposed at a position corresponding to the γ patch portion. . In this case, the black patch image portion has an OD of 1.7 and is substantially not exposed. As described above, the efficiency of image formation can be increased by continuously performing the γ patch image and the normal image formation.

A developing bias of -170 V is applied to the image forming portion on the developing sleeve 131 of the developing device 13, whereas -28 V is applied on the γ patch portion.
A development bias of 0 V is applied to form a development patch for the γ patch portion (see FIG. 7D). If an appropriate gradation correction value is obtained by the gradation correction (first time), the maximum density is CD1.
In step 3, a patch image having a density of CD 0.4 is created for the γ patch portion.

In this embodiment, the gradation is corrected for the second, seventh, twelfth,... Sheets every five copies. In this embodiment, a toner density detection patch is created on the first, sixth, and eleventh sheets to control the toner density.
In the second and subsequent tone corrections, a patch image is formed in an area before the image forming area (F12), and the patch image thus formed together with the image formation is copied by the density sensor SD to the copy density (CD). Is performed, and the gradation correction is performed in the same manner as in the first correction of the gradation.

[0043]

According to the present invention, when the image forming apparatus is restarted after the stop, it is determined whether or not the gradation correction is necessary, and when it is determined that the gradation correction is necessary. The first correction of gradation is performed during warm-up, and a high-density image having remarkable fog in the early stage of operation, which has been conventionally recognized, has been revised to improve the function (processing speed). An image of the same density was always obtained stably without any harm. (Invention of Claim 1) Further, according to the invention of Claim 2, a higher degree of density adjustment is performed without deteriorating the functionality.

Further, according to the third aspect of the present invention, since there is no influence from the fluctuation of the exposure, accurate density detection and adjustment based thereon are performed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an electrophotographic copying apparatus to which the present invention is applied.

FIG. 2 is a circuit diagram of gradation correction according to the present invention.

FIG. 3 is a flowchart of the present invention.

FIG. 4 is an explanatory diagram of patch creation according to the present invention.

FIG. 5 is a graph showing a relationship between density and output by a density sensor.

FIG. 6 is a graph showing a relationship between Lsb and grid potential.

FIG. 7 is an explanatory diagram of patch and image creation according to the present invention.

FIG. 8 is a graph showing the relationship between the number of copies and the image density.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Original mounting table 3 1st mirror unit 3a Illumination lamp 10 Photoconductor drum 11 Scorotron charger 12 Partial exposure means (CEL) 13 Developing device 131 Developing sleeve 16 Transfer device 17 Separation means 19 Fixing device 19a Fixing roller SD Density sensor SH1 Temperature Sensor

Claims (3)

[Claims] It is determined whether or not to correct the gradation immediately after turning on the power or immediately after clearing the energy saving mode. When the correction of the gradation is performed, the correction of the gradation during the warm-up is performed. A characteristic image density correction method. 2. The second and subsequent gradation corrections after the first correction of the gradation are performed by applying a gradation correction patch to an area before the image forming area on the photosensitive member. 2. The method according to claim 1, wherein the gradation is corrected by forming an image. 3. The method of claim 1, wherein the correction of the gradation is performed by charging the surface of the photoconductor to a charging potential lower than that during normal image formation, forming an unexposed latent image on the surface of the photoconductor, and applying the latent image to a developer. 3. A patch image for gradation correction is formed on the photoconductor by developing the toner image on the photoconductor, and gradation correction is performed based on the patch image for gradation correction. Correction method for image density.