JPH05323749A - Electrophotographic copying device - Google Patents

Abstract

PURPOSE:To always suitably reproduce the halftone of an image even when the photosensitive characteristic of a photosensitive body is changed in an image recorder provided with plural developing devices. CONSTITUTION:An electrostatic charge device 3, a surface potential sensor 30, the developing devices 6Y, 6C, 6M and 6BK are disposed around the photosensitive surface 2a of a photosensitive drum 2. Since disposing distances between an electrostatic charging position M1 on the surface 2a and the respective developing devices are different, the dark decay quantity of a developing time by the respective developing devices is different. By the sensor 30, the electrostatic charge potential of the surface 2a is detected and the electrostatic charge potential of an identical part after the drum 2 is rotated by prescribed times is detected. Based on difference between the detected electrostatic charge potential and a time required for rotating the drum 2 by prescribed times, the dark decay a characteristic of the drum 2 is detected. Based on the detected dark decay characteristic and the disposing distances of the respective developing devices, the dark decay quantity extending over the respective developing devices is estimated after the surface 2a is electrostatically charged. Based on the estimated quantity, a look-up table or the like is switched by a tone correction means 40 in every developing device and tone is corrected. Then, the excellent halftone is always reproduced even when the photosensitive characteristic is changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic apparatus such as a copying machine or a laser beam printer, and more specifically, a plurality of developing devices are arranged around a photoconductor to adjust the dark decay characteristics of the photoconductor. , An electrophotographic apparatus for controlling image forming conditions for each developing device.

[0002]

2. Description of the Related Art A photoconductor generally used in an electrophotographic apparatus such as a copying machine has a so-called charging device which is charged with a charging device and is left as it is. Causes dark decay. It is known that this dark decay (decrease in potential) increases with time. Therefore, for example, when the photoconductor rotates, the dark attenuation increases according to the distance the photoconductor moves after charging.

On the other hand, there is known an electrophotographic apparatus provided with a plurality of developing devices around a photoconductor. As a concrete example,
A full-color copying machine having four developing devices that respectively store toners of four colors of cyan, magenta, yellow, and black can be given. The four developing devices are arranged around the photoconductor at different positions along the rotation direction of the photoconductor, and toners of respective colors are sequentially attached to the corresponding latent images. Therefore, the disposition distances from the charging position for charging the photosensitive member (position of the charging device) to each developing device measured along the circumferential direction of the photosensitive member are different from each other. For example, the distance from the charging position to the closest yellow developing device, the distance to the next cyan, the distance to the next magenta, and the distance to the farthest black become longer in this order. Based on this, the time from the charging of the photoconductor to the start of development by each developing device is different for each developing device, and the dark attenuation amount of the photoconductor for each developing device is different. .. That is, the dark attenuation amount of the photoconductor at the time of development differs depending on the position of each developing device.

When the dark attenuation amount is different as described above, it is difficult to obtain a high quality image because the density of the toner image after development is different even if the charge amount by the charger is the same.

In order to prevent this problem, the present applicant has made the following proposals. JP-A-61-12
In Japanese Patent Laid-Open No. 0175, there is provided control means for changing and controlling the output of the charging device in accordance with the developing device selected from the plurality of developing devices (difference in dark attenuation amount due to difference in position). In Japanese Unexamined Patent Publication No. 61-117572, image density adjusting means for changing and adjusting the developing bias voltage or the image exposure amount is provided for each developing device in accordance with the dark attenuation amount of the photoconductor. The same image density level is always obtained under constant density adjustment conditions adjusted by the user.

[0006]

However, according to the above-described conventional example, even if the image density is constant, the halftone gradation of the original image may be different, and particularly the halftone reproduction is severe. In a necessary full-color image recording device or the like, there is a possibility that an image defect may occur.

Further, the dark decay characteristics of the photoconductor change with time, and even if density adjustment such as halftone is performed at the beginning of installation of the image recording apparatus, the balance of the halftone is lost with long-term use. It will be.

Therefore, according to the present invention, by controlling the gradation correction condition for each developing device in accordance with the dark attenuation characteristic of the photoconductor, the reproduction of the halftone is improved and the image quality is improved. An object of the present invention is to provide a recording device.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and includes a movable photoconductor, a charger for charging the photoconductor, and a moving direction of the photoconductor. And a plurality of developing units arranged at different disposition distances from the charger, and a gradation correcting unit for correcting the gradation of an image. Dark attenuation detecting means for detecting a dark attenuation amount corresponding to the distance is provided, and the gradation correction condition in the gradation correcting means is changed for each of the developing devices according to the detection result of the dark attenuation detecting means. It is characterized by

In this case, the dark decay detecting means can be constructed in the following three configurations.

First of all, a sensor is provided on the surface for detecting the charging potential of the photoconductor, and the surface potential sensor is used to measure the potential of a predetermined portion immediately after charging of the photoconductor and the rotation of the photoconductor after a predetermined rotation. The difference between the potential of the predetermined part is detected, and the dark decay amount of the photoconductor corresponding to the disposition distance of each developing device is determined based on the detection result and the time required for the photoconductor to rotate for a predetermined time. To detect.

The second one has a surface potential sensor for detecting the charging potential of the photoconductor, and the surface potential sensor is used to stop the photoconductor at a predetermined portion immediately after charging the photoconductor and to stop the photoconductor for a predetermined time. After that, the difference between the potential of the predetermined portion and the potential of the predetermined portion is detected, and the dark attenuation amount of the photoconductor corresponding to the arrangement distance of each developing device is detected based on the detection result and the predetermined time.

A third one has a plurality of surface potential sensors arranged at predetermined intervals along the moving direction of the photoconductor, and the potential of a predetermined portion of the photoconductor detected by these surface potential sensors. And the dark decay amount of the photoconductor corresponding to the disposition distance of each of the chargers, based on the time required for the photoconductor to move by the predetermined interval.

[0014]

According to the above structure, since the developing devices are arranged along the moving direction of the photoconductor, the disposition distances from the charging device to the developing devices measured along the surface of the photoconductor are different. This means that after the charging device charges the photoconductor, the time until development is performed by each developing device is different. Therefore, based on the fact that the dark decay of the photoconductor increases with the lapse of time, that is, the charging potential of the dark portion of the photoconductor decreases with time, the developing device located farther from the charger is more likely to develop. The dark area potential is low. Such a decrease in the dark area potential has a greater effect on the halftones than on the image area (black area) of the original image. That is, the desired halftone cannot be obtained, and the quality of the final image deteriorates.

Therefore, the gradation correction condition is controlled for each image developing device according to the dark attenuation characteristic of the photoconductor to improve the reproducibility of the halftone in each developing device.

By performing such control,
It is possible to perform development according to the attenuation characteristic of the photoconductor at the time of development. That is, in addition to the difference in the attenuation amount due to the difference in the arrangement position of the developing device, the image formation can be performed in consideration of the fact that the dark attenuation characteristic changes with the use of the photoconductor with time. it can.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. <First Embodiment> FIG. 1 schematically shows a four-color full-color laser beam printer as an image recording apparatus. This device has a photosensitive drum 2 rotatably supported by a device body 1. A photosensitive surface (photoreceptor) 2a is formed on the outer periphery of the cylindrical photosensitive drum 2, and the photosensitive surface 2a sequentially moves in the arrow R1 direction as the photosensitive drum 2 rotates. Around the photosensitive surface 2a, the charging device 3, the exposure device 5, the developing device 6,
Transfer charging device 7, cleaning device 9, pre-exposure lamp 1
0 and the like are provided. The developing device 6 includes four developing devices,
That is, the developing units 6Y, 6C, 6M and 6BK for yellow, cyan, magenta and black are provided. The disposition distances of these developing devices 6Y, 6C, 6M, and 6BK (distances measured along the photosensitive surface 2a from the charging position M ₁ facing the charging device 3 on the photosensitive drum 2) become longer in this order. ..

Further, a transfer drum 11 is provided which rotates in the direction of arrow R2 while contacting the photosensitive surface 2a of the photosensitive drum 2. The transfer drum 11 is a cylindrical transfer sheet 1.
The transfer sheet 11a is provided with a suction charger 12, a suction roller 13, and a separation charger 15 around the transfer sheet 11a.

Below the transfer drum 11, a paper feed cassette 16 containing a recording material P which is an image recording destination is set. Between the paper feed cassette 16 and the transfer drum 11, roller groups 17, 17 ... Composed of paper feed rollers, transport rollers, registration rollers, etc. are arranged, and a fixing device is provided downstream of the transfer drum 11. 19 are arranged. In the laser beam printer shown in FIG. 1, a re-conveying path 20, a reversing path 21, and an intermediate tray 22 for forming an image again on the recording material P after fixing.
Etc. are provided.

Further, a surface potential sensor 30 for detecting the charging potential of the photosensitive surface 2a is arranged in front of the developing device 6Y slightly downstream of the charging position M1 of the photosensitive drum 2 so as to face the photosensitive surface 2a. I am doing it.

Next, the operation of the laser beam printer will be described. First, the photosensitive drum 2 is rotated in the arrow R1 direction, and the dielectric transfer sheet 11a on the transfer drum 11 that is in contact with the photosensitive drum 2 is also rotated in the arrow R2 direction. After the photosensitive surface 2a of the photosensitive drum 2 is neutralized by the pre-exposure lamp 10, it is uniformly charged by the charging device 3 and modulated by the black image signal.
A laser beam scanned by the polygon scanner 5a of the exposure device 5 forms a black electrostatic latent image on the photosensitive surface 2a. A black developer (toner) is attached to the electrostatic latent image by the developing device 6BK to form a toner image.

On the other hand, the recording material P in the paper feed cassette 16 is
The roller group 17, 17 ... Passes a paper feed path (shown by a dotted line in the figure) to give a charge at a predetermined timing by the attraction charger 12 and a conductive attraction roller 13 which is grounded, and transfers the charge to the transfer sheet 11a. Adsorb. Black developing device 6BK
The developed image (toner image) developed in 1 is transferred to the recording material P by the transfer charging device 7 at the transfer portion. Transfer drum 1
No. 1 continues to rotate as it is to prepare for the transfer of the next color (magenta in FIG. 1).

On the other hand, after the photosensitive surface 2a of the photosensitive drum 2 is cleaned by the cleaning device 9 and the charge is again removed by the irradiation of the pre-exposure lamp 10, the charging device 3 is again charged.
And is exposed by the image signal of the next color. After the development and transfer of the next color are completed and these steps are carried out for each of the other colors, the recording material P is separated from the transfer sheet 11a by a separating means such as a separating charger 15 and the fixing device 19 is heated. After being sent to the roller 19a and the pressure roller 19b, a series of full-color printing sequence is completed,
The required full color print image is formed.

The block diagram of FIG. 7 schematically shows a signal processing system in such a laser beam printer. In the figure, for example, the luminance signal corresponding to the original image is CCD3.
Obtained in 1. The obtained luminance signal passes through a shading circuit 32 that corrects the sensitivity variations of individual CCD elements. To convert the modified luminance signal to a density signal,
Pass through the LOG converter 33. The obtained density signal is converted by the LUT 35 in order to correct the gradation conversion due to the influence of the development characteristics of the electrophotographic system, and then is converted by the pulse width converter 36.
Then, the signal is converted into a signal corresponding to the dot width and sent to the laser driver 37.

With the above digital signal processing, an electrostatic latent image having gradation characteristics due to a change in dot area is formed on the photosensitive surface 2a of the photosensitive drum 2 by laser scanning, and undergoes development, transfer and fixing processes. , A gradation image can be obtained. When reproducing a full-color gradation image, steps such as color separation, undercolor removal, and masking are performed, and multiple color signals are processed in parallel. The gradation reproduction method for each color is Is the same as.

A feature of the present invention is that it has a plurality of LUTs (look-up tables) 35 in FIG. 7, and the dark decay detecting means 39 provided with the surface potential sensor 30 described above provides gradation correction means in response to the detection signal. By changing the instruction signal to the LUT switching means 40, the optimum LUT is selected.

The dark decay detecting means 39 has the following configuration, for example. The operation timing is, for example, when the main power is turned on, and the temperature of the heating roller 19a of the fixing device 19 is lower than the fixing temperature control temperature by 30 ° C. or more, so that it is sufficient to catch the change of the dark attenuation characteristic. The interval is short, and there is an advantage that the loss time for detecting the dark attenuation amount can be included in the wait time. First, the charging device 3 charges the photosensitive surface 2a of the photosensitive drum 2 for about a half turn, and the image exposure by the exposure device 5 is turned off by the surface potential sensor 30.
The charging potential of a predetermined part of is measured (the potential of the first rotation in FIG. 4). At that time, the developing devices 6Y, 6C, 6M, and 6BK are mechanically detached so that they are not developed. The transfer charging device 7 is also turned off. The pre-exposure lamp 10 also does not light. The charging potential of the above-mentioned predetermined portion after the photosensitive drum 2 makes one rotation in the dark state of the apparatus is measured and stored (the second rotation potential in the figure). Both the potentials of the first rotation and the potentials of the second rotation are averaged over the portions of the photosensitive drum 2 which are opposed to each other by 180 degrees to eliminate variations in the charge amount and measurement error due to eccentricity of the photosensitive drum 2. After that, the photosensitive drum 2 is rotated with only the pre-exposure lamp 10 turned on, and the static charge is uniformly eliminated, whereby the dark attenuation amount detection ends.

In FIG. 4, which schematically shows the elapsed time with respect to the charging position M ₁ and the surface potential, linear interpolation is performed from the first rotation potential and the second rotation potential of the photosensitive drum 2 having the dark attenuation characteristic. , Developing units 6Y, 6C, 6M, 6B
After calculating the elapsed time corresponding to the disposition distance of K, the dark attenuation amount corresponding to each developing device 6Y, 6C, 6M, 6BK is estimated. If the dark attenuation amount can be estimated, each developing device 6
It is also possible to estimate the halftone gradation at each of the Y, 6C, 6M, and 6BK positions. Then, the LUT suitable for each halftone is selected by the LUT switching means 40, and the halftone is corrected.

A schematic LUT for the sensitometry of FIG.
The switching of 35 will be described. In the figure, the original density OD is a signal before the LUT 35 in FIG. 7 (a signal corresponding to the original density), and the exposure amount E is a signal after the LUT 35. V is the normalized charge potential of the photosensitive surface 2a of the photosensitive drum 2, and CD is the normalized optical density of the recorded image. It is ideal that the OD and the CD have a linear relationship like DD-1 shown by the solid line in the DD zone.

In the image formation corresponding to the yellow developing device 6Y having the smallest dark attenuation among the four developing devices, the signal conversion is carried out by the LUT-Y as shown by the solid line in FIG. 2 and the EV such as EV-Y is converted. Characteristics, VD characteristics like VD, DD-
A linear gradation characteristic such as 1 is obtained. On the other hand, in the image formation corresponding to the black developing device having the largest dark attenuation among the four developing devices, the EV characteristic normalized to the EV zone due to the dark attenuation is EV-B, and when LUT-Y is applied, it becomes DD-2.
The DD characteristics are not ideal as described above. Therefore, in the present embodiment, EV-Y corresponding to the difference in dark attenuation amount
And EV-B are measured in advance, and by applying LUT-B for B, DD such as DD-1
It guarantees the characteristics.

The dark attenuation is measured at a predetermined interval, and the result shown in FIG.
When the photosensitive drum 2 is changed to the characteristic indicated by the dark attenuation characteristic, the halftone correction described above is performed based on the estimated dark attenuation amount corresponding to each of the developing devices 6Y, 6C, 6M, and 6BK, and it is always stable. Guaranteed halftone characteristics.

The operation timing is not limited to the above. For example, a timer may be provided so as to be activated at every predetermined interval, or may be activated for every predetermined number of prints. In some cases, these may be more suitable for the degree of deterioration of the photosensitive drum 2. Further, the charging area is not limited to the above. The photosensitive surface 2a of the photosensitive drum 2 may be charged over the entire circumference and the average potential for the entire circumference may be taken. This has the effect of canceling unevenness in the charging characteristics of the photosensitive drum 2. Further, the rotation speed of the photosensitive drum 2 may be increased in the dark attenuation measurement mode, and the rotation of the photosensitive member is not limited to one rotation, and may be further rotated within a range in which the linearity of the dark attenuation characteristic is obtained. <Second Embodiment> In the first embodiment, an LU is used as the gradation correction means.
Although the LUT switching means 40 for switching T35 is used,
The present invention is not limited to this. Each developing device 6Y,
The latent image forming condition and / or the developing condition corresponding to 6C, 6M, and 6BK are previously changed and adjusted according to the dark attenuation amount, and then the gradation is corrected by the LUT 35 to simultaneously satisfy the maximum density and the gradation. You can <Third Embodiment> In the first embodiment, the photosensitive surface 2a is formed on the surface of the cylindrical photosensitive drum 2, and a plurality of developing devices 6 are arranged in the rotational direction.
An example in which Y, 6C, 6M, and 6BK are arranged has been shown, but the present invention is not limited to this. For example, as shown in FIG. 3, a photosensitive belt 41 may be used instead of the photosensitive drum 2. In this case, the outer surface of the photosensitive belt 41 becomes the photosensitive surface 41a. In this case, four developing devices 6Y, 6C, 6M, 6BK can be arranged in the linear portion of the photoconductor belt 41, and the developing devices 6Y, 6C, 6M, 6BK can have the same configuration and shape. Therefore, there is an advantage that the manufacturing is simple and there is an advantage that only the correction of the image difference due to the dark attenuation difference corresponding to the position of the developing device needs to be considered. The correction algorithm is simplified because it is not necessary to perform the charge correction including the difference in the developing device configuration. <Fourth Embodiment> The dark decay detection means 39 described above is not limited to that of the first embodiment. The dark decay detecting means 39 has a surface potential sensor 30, the surface potential sensor 30 is made to face the photosensitive surfaces 2a, 41a of the photosensitive drum 2 or the photosensitive belt 41, and the photosensitive surfaces 2a, 41a are charged by a certain amount. And
When the charged portion reaches the surface potential sensor 3, the movement of the photosensitive surfaces 2a and 41a is stopped for a predetermined time, and then the potential of the charged portion is measured again. As a result, the photosensitive drum 2
Alternatively, the dark attenuation characteristic of the photosensitive belt 41 may be detected. Elapsed time left in the dark and photosensitive surface 2
The dark attenuation characteristic as shown in FIG. 5 showing the relationship between the surface potentials of a and 41a is measured, and the halftone correction as described above is performed. <Fifth Embodiment> Another example of detecting the dark attenuation amount will be described. As shown in FIG. 8, a plurality of surface potential sensors of the dark decay detecting means 39 may be arranged. A plurality of surface potential sensors are arranged around the photosensitive drum 2 at predetermined intervals along the moving direction of the photosensitive surface 2a, and the photosensitive drum is detected from the difference between the plurality of charging potential values and the time corresponding to the predetermined intervals. The dark decay characteristic of 2 is detected. For example, the surface potential sensor 50 is arranged on the upstream side of the developing device 6Y, and the surface potential sensor 51 is arranged on the developing devices 6M and 6BK, and the dark decay characteristic of the photosensitive drum 2 as shown in FIG. May be estimated. The present embodiment is characterized in that it has a plurality of surface potential sensors, and the location and the number of locations are not limited to those described above. This has an advantage over the first to fourth embodiments described above in that it can be detected without loss time during the image forming sequence, and has the advantage of increasing the arbitrariness of the operation timing.

In all the above-mentioned embodiments, so-called reversal development is employed in which the laser beam is used to expose the photosensitive drum 2 and the photosensitive belt 41, and toner is attached to the exposed area by the developing device 6. However, it goes without saying that the present invention can also be applied to an image recording apparatus that employs regular development in which toner is adhered to an unexposed photoconductor region, that is, a dark region.

[0034]

As described above, in the image recording apparatus having a plurality of developing devices along the moving direction of the photoconductor,
A dark decay detecting means for detecting the dark decay amount of the photoconductor corresponding to the disposition distance from the charging device is provided, and by performing gradation correction for each developing device according to the detection result of the dark decay detecting means, For example, even if the photosensitivity of the photoconductor changes due to long-term use, it always reproduces good halftones.
The image quality can be kept high.

[Brief description of drawings]

FIG. 1 is a sectional view showing an outline of an image recording apparatus of a first embodiment.

FIG. 2 is a diagram showing sensitometry.

FIG. 3 is a sectional view showing the outline of an image recording apparatus of a third embodiment.

FIG. 4 is a diagram showing the relationship between the elapsed time after charging and the surface potential in the first embodiment.

FIG. 5 is a diagram showing the relationship between the elapsed time after charging and the surface potential in the fourth embodiment.

FIG. 6 is a diagram showing the relationship between the elapsed time after charging and the surface potential in the fifth embodiment.

FIG. 7 is a block diagram showing the operation of dark decay detection means.

FIG. 8 is a sectional view showing the outline of an image recording apparatus according to a fifth embodiment.

[Explanation of symbols]

 2a Photoconductor (photosensitive surface) 3 Charging device (charging device) 6Y, 6C, 6M, 6BK Developing device 30 Surface potential sensor 39 Dark decay detecting means 40 Gradation correcting means (LUT switching means)

Front page continuation (51) Int.Cl. ⁵ Identification code Office reference number FI Technical display location G03G 15/08 7810-2H H04N 1/23 103 B 9186-5C 1/29 G 9186-5C

Claims (4)

[Claims] 1. A movable photoconductor, a charger for charging the photoconductor, a plurality of developing devices arranged at different distances from the charger along the moving direction of the photoconductor, and an image forming device. An electrophotographic apparatus, comprising: a gradation correction unit for correcting gradation; dark attenuation detection unit for detecting a dark attenuation amount corresponding to an arrangement distance of each of the developing devices in the photoconductor; An electrophotographic apparatus, wherein the gradation correction condition in the gradation correction unit is changed for each of the developing devices according to the detection result of the detection unit. 2. The dark decay detection means includes a surface potential sensor for detecting a charging potential of the photoconductor, and the surface potential sensor is used to determine the potential of a predetermined portion immediately after the photoconductor is charged and the predetermined potential of the photoconductor. The difference between the potential of the predetermined portion after rotation is detected, and the darkness of the photoconductor corresponding to the disposition distance of each developing device is determined based on the detection result and the time required for the photoconductor to rotate for a predetermined time. The electrophotographic apparatus according to claim 1, wherein an attenuation amount is detected. 3. The dark decay detecting means has a surface potential sensor for detecting a charging potential of the photoconductor, and the surface potential sensor stops the potential of a predetermined portion immediately after charging of the photoconductor and the photoconductor. Then, the difference between the potential of the predetermined portion after a predetermined time is detected, and the dark attenuation amount of the photoconductor corresponding to the arrangement distance of each developing device is detected based on the detection result and the predetermined time. The electrophotographic apparatus according to claim 1, wherein: 4. The dark decay detecting means has a plurality of surface potential sensors arranged at predetermined intervals along a moving direction of the photoconductor, and a predetermined portion of the photoconductor detected by these surface potential sensors. The dark decay amount of the photoconductor corresponding to the disposition distance of each of the chargers is detected based on the potential of the photoconductor and the time required for the photoconductor to move through the predetermined interval. Item 1. The electrophotographic apparatus according to Item 1.