JP4526413B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to an image forming apparatus that appropriately adjusts the excess or deficiency of a post-exposure potential caused by uneven exposure sensitivity or uneven charging on the surface of a photoreceptor.

In electrophotographic image forming apparatuses (copiers, printers, facsimile machines, etc.), the surface of the photoconductor is uniformly charged to a predetermined initial potential by a charging device (charging means), and the charged photoconductor surface is applied. The electrostatic latent image is written by scanning the light beam or by exposing with an LED array light source arranged in the direction of the rotation axis of the drum-shaped photoconductor.
A polygon mirror (polygon scanner) having a polygon mirror that rotates at high speed is generally used as a means for scanning the surface of the photosensitive member with beam light, but the mirror surface that reflects the beam light is reciprocated at high speed within a predetermined rotation range. Reciprocating scanning type such as a resonant scanner (resonance type optical scanner that resonates the mirror at a constant frequency) and a balgano scanner (one that controls the mirror with an external magnetic field) There is also a scanner.
Here, the polygon mirror scans the beam light continuously in one direction with a plurality of mirror surfaces that rotate at high speed in one direction. Therefore, the polygon mirror has the merit that it is efficient with no waste of time. It has a demerit that it takes a long time to reach a steady rotational speed and generates an annoying wind noise.
In addition, the LED array light source has an advantage that the configuration is relatively simple because an optical device for scanning light can be omitted. However, since the light emission intensity is weak, the LED array light source must be provided close to the photoconductor. However, there is a demerit that the layout design of the peripheral device of the photosensitive member is largely limited. In particular, in a tandem type color image forming apparatus that forms a color image by providing a plurality of photoconductors each having a different development color, it is necessary to provide an LED array light source for each photoconductor, and the demerits are remarkable.

In contrast, a reciprocating scanner is advantageous in that it is compact and has a short rise time, quiet operation noise, low power consumption, and less restrictions on layout design of peripheral devices. The drum-shaped photosensitive member can be scanned with the light beam parallel to the rotation axis only in one scanning direction (forward direction or backward direction) in the reciprocating scanning, so that scanning is performed in the other scanning direction. There is a demerit that the exposure for writing the electrostatic latent image cannot be performed and the efficiency is low.
FIG. 3 is a diagram schematically showing the scanning path of the beam light by the reciprocating scanner when the surface of the drum-shaped photoconductor is represented as a plane. R1 and R2 are the forward direction and the backward path, respectively. Represents the direction. That is, a lateral direction R11 parallel to the forward direction R1 is a so-called main scanning direction parallel to the rotation axis of the photosensitive member, and a longitudinal direction R22 orthogonal thereto is a so-called sub-scanning direction.
As shown in FIG. 3, when the scanning direction (scanning angle) of the reciprocating scanner is set so that the forward scanning path is parallel to the rotational axis of the photoreceptor, the backward scanning path R is the photosensitive body rotational axis. It becomes a diagonal direction. Since the light beam scanned in the oblique direction is not suitable for image formation (electrostatic latent image writing), the emission of the light beam is generally stopped.

On the other hand, when image formation is performed, first, a pixel gradation representing a light and shade level for each pixel is determined by a predetermined image processing unit based on image data to be image formed, and is charged in advance by a charging device. The pixel gradation determined by the image processing means is converted into an exposure amount based on predetermined conversion information (usually linear conversion), and the surface of the photoreceptor is exposed by the exposure means according to the exposure amount obtained thereby. .
By the way, there are individual differences due to variations in film thickness and material characteristics of the surface layer of the photoconductor. Even if the surface is uniformly charged by a charging device under a certain condition, a unique potential is generated for each photoconductor. Distribution occurs. This is so-called charging unevenness. Further, even if each region having the same initial potential is exposed with the same exposure amount, it does not necessarily decrease to the same potential, but causes variations. That is, there is a distribution (unevenness) in the ratio (slope) of the difference in potential drop amount to the difference in exposure amount, which is so-called sensitivity unevenness.
When the conversion from the pixel gradation to the exposure amount is performed based on the same (common) conversion information for each region of the surface of the photoconductor having such inherent charging unevenness and sensitivity unevenness, the same exposure is performed. Even if the exposure is performed in an amount, the potential after exposure varies from region to region, and the density developed by the toner (development density) becomes excessive or insufficient with respect to the original density, resulting in development unevenness (density unevenness). appear.
In general, in the case of an apparatus (so-called digital machine) that performs gradation expression by an area gradation method that expresses the lightness and darkness of an image by the array of pixel gradations of a plurality of pixels, an apparatus that expresses the lightness and darkness of an image only by the lightness and darkness of a pixel unit. Compared to (so-called analog machines), even though minute sensitivity unevenness and charging unevenness are less likely to appear as image density unevenness, if there is charging unevenness with a relatively large spatial period, a digital machine that expresses gradation using the area gradation method In this case, uneven density cannot be prevented.
In particular, in a color image forming apparatus that superimposes four color toner images of CMYK (cyan, magenta, yellow, and black), a mixed color gray image is formed by superimposing the three color toner images of CMY. If there is uneven charging on the surface, the CMY balance is lost and a uniform mixed color gray image is not formed (density unevenness occurs).

For example, according to Patent Document 1, if there is a potential unevenness of 5 V or more in the potential after exposure, the density unevenness appears remarkably. Such a phenomenon is particularly remarkable in a so-called tandem color image forming apparatus. In addition, since an a-Si photosensitive member (photosensitive member whose photosensitive layer is made of amorphous silicon) generally has larger charging unevenness than an OPC photosensitive member, unevenness in image density becomes more remarkable. However, in the a-Si photosensitive member, if the charging irregularity is 5 V or less as the quality standard (acceptable level), the yield is remarkably deteriorated, which is not realistic.
On the other hand, Patent Document 1 discloses a technique of providing auxiliary exposure means for correcting the distribution of the initial potential before exposure for writing an electrostatic latent image.
Patent Document 2 discloses a technique for correcting the exposure amount based on the sensitivity information of the photoconductor. Patent Document 3 discloses a technique for correcting sensitivity unevenness for each rotational position of the photoconductor. A technique for correcting the sensitivity unevenness for each exposure position of the photosensitive member is disclosed in Patent Document 5, and a technique for correcting the sensitivity unevenness according to the sensitivity distribution data of the photosensitive member is disclosed.
In the apparatuses shown in Patent Documents 1 to 5, as the exposure means, an exposure unit that uses a polygon mirror to scan the light beam on the photosensitive member and an exposure unit that uses an LED array are employed.
JP 2003-154706 A JP 10-31332 A JP 2000-162834 A JP 2004-61860 A JP 2004-233694 A

However, as disclosed in Patent Document 1, providing an exposure unit that is independent from the exposure unit for writing an electrostatic latent image leads to an increase in the size and cost of the apparatus, and is difficult to apply. There was a problem that there were many. In particular, in the case of a tandem type color image forming apparatus, it is necessary to provide a new exposure unit for each of a plurality of (usually four) photoconductors, and space and cost problems become more prominent.
The techniques disclosed in Patent Documents 2 to 5 all correct the sensitivity unevenness of the photoconductor, that is, the exposure characteristics (relationship between the exposure amount and the potential decrease amount) of the photoconductor as a reference, and the control target. This is to correct the difference in slope (ratio of the difference in potential drop to the difference in exposure amount) from the exposure characteristics of the photosensitive member, so that there is a distribution in the initial potential of the charged photoreceptor before exposure (charging) In the case of unevenness), the potential distribution remains as an offset as it is, and there is a problem that the density unevenness of the image cannot be resolved.

FIG. 9 shows a relationship between the pixel gradation in the a-Si photosensitive member in which charging unevenness and sensitivity unevenness coexist and the potential of the photosensitive member after exposure with an exposure amount corresponding to the pixel gradation (FIG. 9). 9A shows a case where exposure amount correction is not performed (shown by a thick broken line g01), and FIG. 9B shows a case where exposure amount sensitivity unevenness correction is performed (shown by a thick solid line g02). Represent each characteristic. In the drawing, the characteristic represented by the thick solid line (g0) represents the characteristic of the standard (standard) photoconductor (hereinafter referred to as the standard characteristic).
Here, the graph shown in FIG. 9A has the pixel gradation as the horizontal axis, but the conversion from the pixel gradation to the exposure amount is converted into a certain conversion formula (coefficient is fixed) or a conversion table. As long as it is performed based on this, it is equivalent even if the horizontal axis is regarded as the exposure amount. That is, in FIG. 9A, the graph line g0 representing the characteristics of the photoconductor as the reference and the graph line g01 representing the characteristics of the photoconductor as the measurement target to be controlled are both the same conversion formula ( In other words, since the conversion from the pixel gradation to the exposure amount is performed in accordance with no correction), the horizontal axis is replaced with the exposure amount, and exposure characteristics (characteristic of potential with respect to exposure amount (exposure potential characteristics)) are used. Even if it is seen, it is equivalent.

As shown in FIG. 9A, in general, in the exposure potential characteristic representing the correspondence between the exposure amount and the potential on the photosensitive member (particularly, the a-Si photosensitive member), the exposure is almost linear as the exposure amount increases. The potential later decreases and is almost linear in the area excluding the convergence area (the range in which the slope at which the potential decreases with increasing exposure) becomes a residual potential (the potential remaining after exposure at the maximum exposure). The exposure characteristics are shown. For example, the exposure characteristic g01 of the photoconductor to be measured in FIG. 9A shows a substantially linear exposure characteristic in the range of the charge amount E2 or less when the pixel gradation is I2, and serves as a reference photoconductor. The exposure characteristic g0 shows a substantially linear exposure characteristic in a range equal to or less than the charge amount Es2 when the pixel gradation is Is2.
Further, in the case where charging unevenness and sensitivity unevenness coexist on the photoconductor to be measured, as shown in FIG. 9A, an initial potential (charging potential before exposure, that is, a charging potential before exposure, that is, the reference exposure characteristic g0). A difference in y intercept (corresponding to charging unevenness) and a difference in inclination of exposure characteristics (corresponding to sensitivity unevenness) occur. When exposure sensitivity unevenness correction (correction for matching inclinations) is performed on such a photoconductor, as shown in FIG. 9B, a potential difference (initial potential difference) corresponding to charging unevenness is used as an offset. This will cause uneven density in the image.
On the other hand, if the above-mentioned reciprocating scanner is used as the beam scanning means and the beam light in the scanning direction that is not parallel to the rotation axis of the photosensitive member can be used effectively, miniaturization, power saving, noise reduction, and start-up time An image forming apparatus utilizing the advantage of shortening can be configured.
Therefore, the present invention has been made in view of the above circumstances, and the object of the present invention is to make effective use of light beams scanned in both forward and backward directions by a reciprocating scanning scanner, An object of the present invention is to provide an image forming apparatus capable of preventing the occurrence of density unevenness of an image as much as possible even with respect to a photoreceptor having uneven sensitivity.

In order to achieve the above object, the present invention provides an exposure of the photoconductor based on the pixel gradation determined by the image processing means by scanning the surface of the photoconductor previously charged by the charging means with a beam of light. And an optical reciprocating scanning means for reciprocatingly scanning the light beam with respect to the surface of the photosensitive member, so that either the forward path or the backward path of the beam light that is reciprocally scanned is provided. The exposure amount by the first beam light scanned in one direction (beam light scanned parallel to the rotation axis of the drum-shaped photoconductor) is controlled based on the pixel gradation determined by the image processing means. On the other hand (hereinafter referred to as first exposure amount control), the second beam light scanned in the direction opposite to the scanning direction of the first beam light for each of the divided areas obtained by dividing the surface of the photosensitive member into a plurality of areas. The exposure quantity is controlled by and is characterized in (hereinafter, second of exposure control) that. Here, the exposure amount control by the second beam light is performed for each divided region according to the difference between the potential characteristic in the divided region and a predetermined reference potential characteristic common to all the divided regions. Do.
Thus, one of the light beams scanned in the forward and backward directions by the optical reciprocating scanning means (reciprocating scanner) (outward or backward beam light) is converted into an electrostatic latent image based on the pixel gradation. If it is used for writing and the other is effectively used to correct the distribution of potential characteristics (difference from the reference characteristics) for each area of the photoconductor, the advantage of the reciprocating scanner (miniaturization and power saving) In addition, it is possible to configure an image forming apparatus that takes advantage of noise reduction and start-up time reduction, and it is possible to prevent image density unevenness as much as possible even with respect to a photoconductor in which charging unevenness and sensitivity unevenness coexist.
In addition, since it is not necessary to add new exposure means or the like, the apparatus is not increased in size and cost.

More specifically, as has been conventionally performed, in the first exposure amount control, the pixel gradation representing the gray level for each pixel is proportionally converted using a coefficient common to all the divided regions. It is conceivable to set the exposure amount obtained as the exposure amount by the first beam light (hereinafter referred to as common first exposure amount control). The pixel gradation is determined by predetermined image processing means based on predetermined image data, for example, image data read from an original in a copying machine or image data such as a print job in a printer.
In addition, inclination information when the pixel gradation is substantially proportionally converted to the exposure amount for each divided region is stored in advance in a storage unit, and is determined by the image processing unit for each pixel in the first exposure amount control. If the exposure amount obtained by approximately proportionally converting the pixel gradation according to the inclination information for each divided region is set as the exposure amount by the first beam light, the exposure amount correction for the sensitivity unevenness in each of the divided regions is performed. (Hereinafter referred to as individual first exposure amount control).
In the individual first exposure amount control, the exposure time for each pixel of the first beam light is adjusted according to the pixel gradation, and the first beam light for each divided area according to the tilt information. It is conceivable to control the exposure amount by adjusting the exposure intensity.

On the other hand, in the second exposure amount control, a predetermined reference exposure amount common to all the divided regions and a predetermined reference potential common to all the divided regions are applied to the exposure potential characteristics in each of the divided regions. The exposure amount information for specifying the difference exposure amount from the exposure amount obtained in this manner is stored in advance in the storage means, and the exposure amount by the second beam light is set based on the increase exposure amount information ( Control) (hereinafter referred to as raising exposure amount setting).
Here, the exposure potential characteristic in each of the divided areas means a characteristic representing a correspondence relationship between an exposure amount (including exposure amount = 0) and a potential (initial potential and post-exposure potential) in each of the divided areas. Usually, the reference exposure amount and the reference potential are a range showing a substantially linear characteristic of the exposure potential characteristics, that is, a substantially linear exposure potential that is a portion excluding a convergence area to a residual potential in the exposure potential characteristics. It is defined in the range of characteristics.
For example, as the reference potential and the reference exposure amount, for example, a potential at a substantially middle point between the initial potential and the residual potential in the exposure potential characteristic that is a reference common to all the divided regions, and a potential at the substantially middle point It is preferable to set the exposure amount corresponding to (so-called half exposure amount).
As a result, as described later, the post-exposure potential in each of the divided regions is compared with the reference exposure potential characteristic as a whole over the entire exposure amount range or in a specific exposure amount range with a high setting frequency. It is possible to perform exposure so as to approach each other, and it is possible to prevent image density unevenness as much as possible even with respect to a photoconductor in which uneven charging and uneven sensitivity exist. In particular, when performed together with the first exposure amount control based on the tilt information, the post-exposure potential in each of the divided regions can be made uniform at substantially the same level.
When the common exposure amount control is employed as the first exposure amount control, a substantially linear exposure potential characteristic that is a portion excluding a convergence region to a residual potential in the exposure potential characteristics for each of the divided regions. Difference inclination information regarding the difference between the inclination at, and the reference inclination common to all the divided areas is stored in advance in the storage means, and is set by the raised exposure amount setting in the second exposure amount control. The exposure amount of the second beam light to be corrected may be corrected by the exposure amount obtained by proportionally converting the pixel gradation determined by the image processing unit based on the difference inclination information (exposure sensitivity correction). Conceivable.
This also makes it possible to correct the exposure amount for the sensitivity unevenness in each of the divided areas.
In the second exposure amount control, for example, it is conceivable to control the exposure amount by adjusting the exposure time of the second beam light.

The divided area includes an area obtained by dividing the surface of the drum-shaped photosensitive member into a plurality of parts in the axial direction or the circumferential direction (one-dimensional division), or a plurality of areas divided in both directions (two-dimensional division). Conceivable. For example, it is conceivable to divide in units of width or height of one pixel, or to divide in units of width or height for a plurality of pixels.
Here, it is needless to say that the exposure by the exposure means needs to be performed by recognizing each position of the divided area. In general, with respect to the exposure position in the axial direction (that is, the main scanning direction) of the surface of the photoreceptor, the writing position is recognized (detected) at least in pixel units in the exposure means (or its control means). On the other hand, since the absolute position in the circumferential direction (sub-scanning direction) of the surface of the photoconductor is not information directly necessary for image formation, it is necessary to provide means for detecting the rotational position of the photoconductor.
In addition, when the photoconductor is an a-Si photoconductor, charging unevenness is particularly noticeable in many cases, which is suitable for application of the present invention.
In addition, the present invention provides a charging method having a relatively large spatial period when the image processing based on the image data is a processing that performs gradation expression by an area gradation method that determines an array of pixel gradations of a plurality of pixels. Even if unevenness exists, it is preferable in that it can be prevented from appearing as uneven density in the image.
In addition, an image forming apparatus that forms a color image with a plurality of color toners, particularly a tandem type image forming apparatus that forms an image by superimposing a plurality of color toner images with a plurality of photoconductors, has an image caused by uneven charging or uneven sensitivity. This is preferable as an application target of the present invention.

According to the present invention, one of the light beams scanned in both directions of the forward path and the backward path by the reciprocating scanner such as a resonance type optical scanner (outward path or backward path beam light) is determined by the image processing means. A reciprocating scanner is used for writing an electrostatic latent image based on pixel gradation and effectively using the other for correcting the distribution of potential characteristics (difference from the reference characteristics) for each area of the photoconductor. Image forming devices that take advantage of the advantages (miniaturization, power saving, noise reduction and shortening of start-up time), as well as image density unevenness on photoconductors with uneven charging and sensitivity unevenness. Can be prevented as much as possible.
In addition, since it is not necessary to add new exposure means or the like, the apparatus is not increased in size and cost.
In particular, to an image forming apparatus using an a-Si photosensitive member in which charging unevenness often appears remarkably, and a color image forming apparatus in which density unevenness of an image caused by charging unevenness or sensitivity unevenness is more noticeable in a formed image. It is suitable for application.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings so that the present invention can be understood. The following embodiment is an example embodying the present invention, and does not limit the technical scope of the present invention.
FIG. 1 is a schematic sectional view of an image forming apparatus X according to an embodiment of the present invention, FIG. 2 is a perspective view showing a scanning state of beam light by a reciprocating scanner in the image forming apparatus X, and FIG. FIG. 4 is a block diagram showing a schematic configuration of a main part of the image forming apparatus X, and FIG. 5 is a first embodiment. FIG. 6 is a graph showing the relationship between the pixel gradation and the light amount of the light beam and the graph showing the relationship between the pixel gradation and the potential at that time. FIG. FIG. 7 is a graph showing the relationship between the pixel gradation and the potential of the beam light according to the third embodiment. FIG. 8 is a graph showing the relationship between tone and potential. FIG. 9 is a graph showing the relationship between the pixel gradation and the light amount of the beam light according to the example, and the graph showing the relationship between the pixel gradation and the potential at that time. FIG. It is a graph showing an example of the relationship between the pixel gradation of this and the electric potential after exposure.

First, the overall configuration of the image forming apparatus X according to the embodiment of the present invention will be described using the cross-sectional view shown in FIG.
The image forming apparatus X is a printer that is an example of a tandem color image forming apparatus that forms an image using toners of four colors of black (BK), magenta (M), yellow (Y), and cyan (C). It is.
The image forming apparatus X includes an image forming unit α1, which forms a toner image and forms an image on a recording sheet, a paper feeding unit α2, which supplies the recording sheet to the image forming unit α1, and a recording sheet on which image formation has been performed. Is discharged.
Image information (print job) received from a communication unit (not shown) from an external device such as a personal computer is black (BK), magenta (M), yellow (Y), cyan (C) by an image processing unit 52 described later. Are converted into pixel gradations which are grayscale information for each pixel for each of the four colors.

The image forming unit α1 has four photosensitive drums 1 (1BK for black, 1M for magenta, 1Y for yellow, 1C for cyan) carrying the images of the above four colors, and the surface of each of the photosensitive drums 1 is integrated. Charging device 3 (3BK, 3M, 3Y, 3C) for charging in such a manner, an exposure source Z (not shown in FIG. 1) for outputting a plurality of light beams corresponding to each of the photosensitive drums 1, from the exposure source Z A reciprocating scanner 9 such as a resonance type optical scanner that reciprocally scans the output light beam in the direction of the rotation axis of the photosensitive drum 1, and the light beam scanned by the reciprocating scanner 9 is reflected to reflect each of the photosensitive drums 1. A reflection mirror 8 that leads to the surface is provided.
The surface of each of the photosensitive drums 1 that has been charged in advance by the charging device 3 is determined by an image processing unit 12 to be described later by the beam light scanned on the surface of each of the photosensitive drums 1 through the reflection mirror 8. Exposure based on the pixel gradation is performed, and an electrostatic latent image is written.
Further, the image forming unit α supplies a developing device 2 (2BK, 2M, 2Y, 2C) that develops a toner image by supplying toner to the electrostatic latent image written on each of the photosensitive drums 1, and the photosensitive drum. The toner image formed on the surface of each of the body drums 1 is sequentially transferred, and the toner image is transferred onto the recording paper in the transfer unit 40, the intermediate transfer belt 7 that transfers the recording paper, the transport roller 4 that transports the recording paper, and the transfer onto the recording paper. A fixing device 13 for heating and fixing the toner image, a neutralizing device 5 (5BK, 5M, 5Y, 5C) for neutralizing the surface of the photosensitive drum 1 after the toner image is transferred to the recording paper are schematically configured.

The photosensitive drum 1 is, for example, an a-Si photosensitive member that is excellent in durability because of its high hardness and stable properties, while charging unevenness is likely to appear relatively remarkably in addition to sensitivity unevenness.
The charging device 3 uniformly charges the surface of the photosensitive drum 1 along its axial direction. If the photosensitive drum 1 is unevenly charged, Distribution occurs in the potential (initial potential) before exposure.
The developing device 2 includes a developing roller that supplies toner to the photosensitive drum 1, and corresponds to a potential gap between a potential (developing bias potential) applied to the developing roller and a potential on the surface of the photosensitive drum 1. The toner on the developing roller is attracted onto the surface of the photosensitive drum 1, and the electrostatic latent image is visualized as a toner image.
The paper feed unit α2 is schematically configured to have a paper feed cassette 11, a paper feed roller 12, and the like. The recording paper previously stored in the paper feed cassette 11 is conveyed to the image forming unit α1 when the paper feed roller 12 is driven to rotate.
The recording paper delivered from the paper feeding unit α2 is transferred by the transfer roller 40 while the toner image is transferred from the intermediate transfer belt 7 while being transported by the transport roller 4, and passes through the fixing device 13 to the paper discharge unit α3. It is transported and discharged to a discharge tray outside the device.

Next, scanning of the beam light by the reciprocating scanner 9 will be described using the perspective view shown in FIG. In FIG. 2, the broken line represents the optical path of the beam light.
A plurality of light beams for writing an electrostatic latent image are supplied from an exposure source Z that is unitized by incorporating a plurality of light sources (laser diodes or the like) for outputting them.
The exposure source Z includes a plurality of (four in FIG. 2) single light source units 22 (22BK, 22M, 22Y, 22C) each provided with an electronic substrate on which one light source is mounted, and these single light source units. 22 is a unit in which a plurality of light beams (laser beams) output from each of the optical devices that determine the mutual positional relationship (optical path) are integrated into an integrally formed mounting base (not shown).
The four light beams emitted from each single light source unit 22 pass through the collimator lens mounted inside the collimator lens tube 24, are reflected by the reflection mirrors 25 to 27, and reach the common reflection mirror 27. At the stage, the four light beams are arranged in a line in the substantially vertical direction (sub-scanning direction).
A bundle of a plurality of light beams reflected by the common reflection mirror 27 passes through a cylindrical lens 28, and a plurality of light beams are collectively reciprocated by a reflection mirror 29 that collectively reflects each of the light beams. The light is reflected toward the reflection mirror 9 a of the reciprocating scanner 9.
The reciprocating scanner 9 includes a reflecting mirror 9a that reflects the beam light and a drive unit 9b that reciprocally rotates the reflecting mirror at a high speed within a predetermined rotation range.
Each of the plurality of beam lights reciprocally scanned by the reciprocating scanner 9 is guided to the corresponding photosensitive drum 1 by the reflecting mirror 8 shown in FIG. 1 and reciprocates with respect to the surface of the photosensitive drum 1. Scanned (an example of optical reciprocating scanning means).
The beam light scanned on the surface of each of the photosensitive drums 1 in this way is scanned in parallel with the rotation axis of the photosensitive drum 1 in the forward direction R1 as described above with reference to the schematic diagram of FIG. In the backward direction R2, scanning is performed obliquely with respect to the rotation axis.
Note that scanning may be performed in parallel with the rotation axis of the photosensitive drum 1 in the forward path direction, and scanning in an oblique direction with respect to the rotation axis in the backward path direction.

FIG. 4 is a block diagram illustrating a schematic configuration of a main part of the image forming apparatus X.
The image forming apparatus X includes an MPU and its peripheral devices such as ROM and RAM in addition to the charging device 3, the exposure source Z, the developing device 2 and the charge eliminating device 5. A control unit 50 that controls components, a display operation unit 51 such as a liquid crystal touch panel that is a unit for inputting information according to a user operation, and an image processing unit 52 that performs various types of image processing. , An EEPROM or the like, which is a readable / writable storage means, and includes a data storage unit 53 for storing various data, a rotation position detection unit 54 for detecting the position of each of the photosensitive drums 1 in the rotation direction, and the like.
The image processing unit 52 calculates a pixel gradation representing a gray level for each pixel for each color of toner based on predetermined image data (print job or the like) input from an external device via a communication control unit (not shown). The process determined by the digital method is executed.
Here, the image processing unit 52, based on the image data, arranges pixels to be drawn (printed) in units of a pixel group composed of a plurality of pixels (hereinafter referred to as a unit pixel group), and the pixel to be drawn. Density gradation expression of an image is performed by an area gradation method such as an error diffusion method or a screen method for determining pixel gradation.
In the data storage unit 53, for each of the photosensitive drums 1, the surface of each of the photosensitive drums 1 is divided into a plurality of divided areas, and the potential characteristics in the divided areas and predetermined reference potential characteristics common to all the divided areas are stored in advance. The exposure amount adjustment information corresponding to the difference is stored separately. For example, as identification information of each of the divided areas, a beam light scanning position (irradiation position) in the main scanning direction (rotational axis direction of the photosensitive drum 1) detected by the reciprocating scanner 9 and the rotational position detection unit 54 A combination with the detected value is stored, and the exposure amount adjustment information is stored in association with each combination (identification information of each divided region). Specific contents of the information will be described later.
Here, the divided region is, for example, a region corresponding to each pixel (a width of one pixel (axial direction) × a height of one line (circumferential direction)) or an area floor in the image processing unit 52. A region corresponding to the unit pixel group employed in the image processing in the gradation method may be considered.
As the configuration of the rotational position detector 54, for example, a rotational potentiometer is provided on the rotational axis of the photosensitive drum 1 to detect the rotational position, or a protrusion or the like is disposed on the rotational shaft of the photosensitive drum 1. A configuration in which a reference portion is provided, a passing position of the reference portion is detected by a contact-type switch, a photocoupler, or the like, and an elapsed time from the detection time point is measured.

  The control unit 50 outputs a scanning direction signal indicating whether the beam light is being scanned forward or backward from the reciprocating scanner 9 and a scanning position signal indicating the scanning position to the image processing unit 52. The pixel gradation determined by image processing is acquired from the rotation position, and the rotation position signal of the photosensitive drum 1 is acquired from the rotation position detector 54, and the exposure light source Z is controlled to control the light amount of the beam light. I do. At this time, the control unit 50 determines the irradiation position of the beam light in the main scanning direction (rotational axis direction of the photosensitive drum 1) and the sub-scanning direction on the photosensitive drum 1, the scanning position signal, and the scanning position signal. Recognizing based on the rotational position signal of the photosensitive drum 1 and extracting (searching) the exposure amount adjustment information corresponding to the position (pixel or the divided region) from which the light beam is to be irradiated from the data storage unit 53. Read out.

The image forming apparatus X is subjected to a characteristic evaluation test for obtaining the exposure characteristics of each of the photosensitive drums 1 incorporated therein in the manufacturing stage. More specifically, the characteristic evaluation test (preliminarily measured) is performed on the photosensitive drum 1 charged (charged) by the charging device 3 under a condition of a plurality of exposure amounts for each divided region. The exposure source 2 performs exposure and the initial potential before exposure and the potential after exposure for each divided region are measured, and the exposure characteristics of each divided region, that is, the exposure amount and the potential after exposure, The characteristics representing the relationship (hereinafter referred to as measured exposure characteristics) are clarified. A thick broken line graph g0 shown in FIG. 9A is an example of the exposure characteristic thus clarified.
Here, as a method for measuring the exposure characteristics of each of the divided areas, for example, the exposure characteristics of each of the divided areas are densely switched and exposed, and the potential after exposure is measured to measure accurate exposure characteristics. it can. In addition, as shown in FIG. 9 (a), the tendency of exposure characteristics (curve shape) is determined to some extent, and it is general that it can be formulated by a common equation if only the coefficient is changed. The exposure characteristics may be estimated based on the result of measuring the potential after exposure with a plurality of representative exposure amounts.
For example, in the case of an a-Si photosensitive drum, the residual potential is almost constant regardless of the position of the surface of the photosensitive drum 1, so that the exposure is performed with the initial potential and one exposure amount within the range of the substantially linear characteristic. By measuring the potential after the exposure, the exposure characteristics can be estimated with sufficient accuracy.

Hereinafter, the exposure amount control of the beam light by the control unit 50 will be described with reference to FIGS. 9 and 5 to 8 described above.
Here, among the light beams scanned reciprocally by the reciprocating scanner 9, the control unit 50 scans light beams scanned in the forward direction parallel to the rotation axis of the photosensitive drum 1 (hereinafter referred to as first beam light). ) And the beam light scanned in the opposite direction (hereinafter referred to as second beam light).
Further, in the examples shown in FIGS. 5 to 8, the above-described divided area on the surface of the a-Si photosensitive drum 1 has an exposure in which the characteristics shown in FIG. 9A, that is, charging unevenness and sensitivity unevenness coexist. This is an example of how to set (control) the respective light amounts of the first beam light and the second beam light in the case of having the characteristic (g0).

<First embodiment>
FIG. 5B is a graph showing the relationship between the pixel gradation and the amount of beam light according to the first embodiment for the divided region having the exposure characteristics shown in the graph g01 in FIG. 9A. FIG. 5A is a graph showing the relationship between the pixel gradation and the potential (initial potential and post-exposure potential) when exposure is performed according to the relationship of FIG.
The control unit 50 controls the exposure source Z so that the exposure amount by the first beam light becomes an exposure amount corresponding to the pixel gradation (an example of first exposure amount control means).
Specifically, as shown by a one-dot chain line in FIG. 5B, the exposure amount for each pixel by the first light beam is set to an exposure amount proportional to the pixel gradation value (exposure amount converted proportionally). Adjust so that At that time, the proportionality coefficient k0 (slope) in the relationship between the pixel gradation and the exposure amount is constant (common) for all the divided regions. This linear characteristic (Ei = k0 · I, Ei is the exposure amount, I is the pixel gradation, and k0 is the slope) is hereinafter referred to as a first beam reference light amount setting characteristic.
The characteristics shown in the graphs g0 and g01 in FIG. 9A are characteristics when the exposure amount of the beam light is set according to the first beam reference light amount setting characteristic (slope = k0). This also applies to FIGS. 6 to 9 described later.
Here, the control unit 50 adjusts the exposure amount of the first beam light by adjusting the beam light output time (hereinafter referred to as exposure time) from the exposure source Z. That is, the exposure time (light emission time) is set to a time proportional to the pixel gradation while the intensity level of the beam light at the exposure source Z is kept constant.
However, if there is a rise loss at the start of beam light output at the exposure source Z, an exposure time sufficient to compensate for the rise loss is added separately.

Further, the control unit 50 controls the exposure source Z so that the exposure amount by the second beam light becomes an exposure amount according to the exposure amount adjustment information stored in the data storage unit 53 for each divided region. (An example of second exposure amount control means). Thus, the second beam light is controlled so as to have an exposure amount corresponding to the difference between the potential characteristic in each of the divided areas and a predetermined reference potential characteristic common to all the divided areas.
Specifically, the following control is performed.
First, a reference potential Vs1 common to all the divided areas and a reference exposure amount Es1 corresponding to the reference potential Vs1 are determined in advance.
For example, in a reference exposure potential characteristic common to all the divided regions (graph g0 in FIG. 9A), the potential Vs1 (= (V0 + VL) / 2) at the midpoint between the initial potential V0 and the residual potential VL is obtained. The reference potential Vs1 is set, and the exposure amount corresponding to the reference potential is set as the reference exposure amount Es1. This reference exposure amount Es1 is a so-called half exposure amount in the reference exposure potential characteristic.
Further, as the exposure amount adjustment information, the reference exposure amount Es1 and the exposure potential characteristic in each of the divided regions (g01 in FIG. 5A: a characteristic indicating the correspondence between the exposure amount and the post-exposure potential). Is stored in the data for each of the divided areas as a raised exposure amount ΔE1 (= E1-Es1, an example of raised exposure amount information), which is a difference exposure amount from the exposure amount E1 obtained by applying the reference potential Vs1. This is stored in the unit 53 (an example of a raised exposure amount information storage unit).
In addition, the control unit 50 controls the exposure source Z, so that the second beam light (beam light during the backward scanning) is divided into each divided area for each pixel included in the divided area. Exposure is performed with the exposure amount set to the raised exposure amount ΔE1 (an example of a raised exposure amount setting unit).
As a result, the total exposure amount E of each pixel is increased by the increase exposure amount ΔE1 (E = k0 · I + ΔE1), and as shown in FIG. 5A, the post-exposure potential in each of the divided regions is increased. The exposure can be performed so that the characteristic gx1 approaches the entire exposure amount over the entire range (that is, the entire range of the pixel gradation). As a result, it is possible to prevent the occurrence of uneven image density as much as possible for the photosensitive drum 1 having uneven charging and uneven sensitivity.

Here, by setting the reference exposure amount Es1 to a half exposure amount, the exposure potential characteristic g0 as the reference can be approximated over the entire exposure amount range. On the other hand, when it is known in advance that the frequency of exposure within the specific exposure amount range is high, the exposure amount within the specific exposure amount range (for example, in the reference exposure potential characteristic g0, for example, It is also conceivable that the center exposure amount in that range is the reference exposure amount Es1, and the corresponding potential is the reference potential Vs1. Thus, each of the divided areas can be exposed so as to be close to the reference exposure potential characteristic g0 in a specific exposure amount range that is frequently set, and the occurrence of uneven density in the image is prevented as much as possible. it can.
Here, the controller 50 adjusts the exposure amount of the second beam light by adjusting the exposure time or exposure intensity by the exposure source Z. That is, the intensity level of the beam light in the exposure source Z is kept constant, and the exposure time (light emission time) is set to a time proportional to the raised exposure amount ΔE1, or the exposure time is kept constant. , The exposure intensity (emission intensity) is set to an intensity proportional to ΔE1. In this case as well, if there is a rise loss at the start of beam light output, the exposure time or exposure intensity sufficient to compensate for this is added separately.

<Second embodiment>
FIG. 6B is a graph showing the relationship between the pixel gradation and the amount of beam light according to the second embodiment for the divided region having the exposure characteristics shown in the graph g01 in FIG. 9A. FIG. 6A is a graph showing the relationship between the pixel gradation and the potential (initial potential and post-exposure potential) when exposure is performed according to the relationship of FIG. 6B.
In the second embodiment, the control of the exposure amount by the first beam light is the same as that in the first embodiment. That is, the control unit 50 sets the exposure amount of the first light beam so that the exposure amount (Ei = k0 · I) is proportional to the pixel gradation in accordance with the proportional coefficient k0 common to all the divided regions. (An example of common first exposure amount control means).
On the other hand, the exposure amount control of the second beam light is different from that of the first embodiment.
In the second embodiment, as the exposure amount adjustment information, in addition to the raised exposure amount ΔE1, the residual potential VL in the exposure potential characteristic (g01 in FIG. 9A) for each of the divided regions is previously provided. A difference slope k2 representing a difference between a slope in a substantially linear exposure potential characteristic that is a portion excluding a convergence area and a reference slope common to all the divided areas is previously stored for each divided area. 53 (an example of difference inclination information storage means). The difference gradient k2 is a gradient k2 (hereinafter referred to as a sensitivity correction coefficient k2) when the pixel gradation is proportionally converted to the corrected exposure amount.
That is, this sensitivity correction coefficient k2 is obtained as k2 = k1−k0 when the exposure sensitivity of the divided area is represented by k1 (gradient representing potential change (absolute value) with respect to exposure amount change). If the exposure amount ΔE2 (= k2 · I) obtained by proportionally converting the pixel gradation by the sensitivity correction coefficient k2 is added to the exposure amount Ei (= k0 · I) of one beam light for each pixel, Sensitivity unevenness can be corrected.
Then, the control unit 50 controls the exposure source Z, thereby increasing the exposure amount of each pixel included in the divided region for the second beam light to the raised exposure amount ΔE1. Further, the exposure amount corresponding to the sensitivity correction obtained by proportionally converting the difference (Is1-I) of the pixel gradation I with respect to the reference pixel gradation Is1 (= Es1 / k0) corresponding to the reference exposure amount Es1 by the sensitivity correction coefficient k2. Exposure is performed by setting an exposure amount (ΔE1 + ΔE2) obtained by adding ΔE2 (= k2 · (Is1−I)). That is, for the second beam light, correction is performed for an exposure amount ΔE2 obtained by proportionally converting the pixel gradation based on the difference slope k2 with respect to the raised exposure amount ΔE1 (an example of an exposure sensitivity correction unit).
Also in this case, the controller 50 adjusts the exposure amount of the second beam light by adjusting the exposure time or exposure intensity by the exposure source Z.
As a result, as shown in FIG. 6A, the post-exposure potential characteristic gx2 in each of the divided regions can be made to substantially match the reference exposure potential characteristic g0 common to all the divided regions. Both sensitivity unevenness and charging unevenness can be corrected almost completely.

<Third embodiment>
FIG. 7B is a graph showing the relationship between the pixel gradation and the amount of beam light according to the third embodiment for the divided region having the exposure characteristics shown in the graph g01 in FIG. 9A. FIG. 7A is a graph showing the relationship between the pixel gradation and the potential (initial potential and post-exposure potential) when exposure is performed according to the relationship of FIG. 7B.
In the third embodiment, the exposure amount control is performed in substantially the same manner as in the second embodiment, but the reference exposure amount is set to 0 (that is, no exposure), and the reference potential is set to the reference exposure. This is an example in which the initial potential V0 in the potential characteristic g0 is used. In this case, the raised exposure amount ΔE1 is the difference between the reference exposure amount (= 0) and the exposure amount E1 ′ obtained by applying the reference potential V0 to the exposure potential characteristic in each of the divided regions. The exposure amount (= E1′−0 = E1 ′).
Further, since the reference pixel gradation Is1 = 0, as shown in FIG. 7B, the exposure amount ΔE2 = k2 · I corresponding to the correction of the exposure sensitivity.
Also in this case, the controller 50 adjusts the exposure amount of the second beam light by adjusting the exposure time or exposure intensity by the exposure source Z.
Even with such exposure amount control, as shown in FIG. 7A, the exposure potential characteristic gx3 can be made substantially coincident with the reference characteristic g0, and the same effect as the second embodiment can be obtained.

<Fourth embodiment>
FIG. 8 is a graph showing the relationship between the pixel gradation and the amount of beam light according to the fourth embodiment for the divided region having the exposure characteristics shown in the graph g01 of FIG. 9A.
In the fourth embodiment, the exposure amount by the second beam light is set to the raised exposure amount ΔE1 as in the first embodiment. However, in this example, the reference exposure amount Es1 is set to 0, and the reference potential is set to the initial potential V0 in the reference characteristic g0.
On the other hand, the exposure amount control of the first beam light is different from that of the first embodiment.
That is, in the data storage unit 53, as information representing the exposure sensitivity of each of the divided areas, a gradient k1 when the pixel gradation is proportionally converted to the exposure amount is stored in advance for each divided area (inclination information). An example of storage means), the exposure amount obtained by proportionally converting the pixel gradation determined by the image processing unit 52 for each pixel with respect to the first light beam according to the inclination k1 for each divided region by the control unit 50 (Ei = k1 · I) is set as the exposure amount by the first light beam (an example of the individual first exposure amount control means).
In this case, the control unit 50 controls the exposure source Z, for example, to adjust the exposure time for each pixel of the first beam light according to the pixel gradation, and to adjust the exposure time according to the inclination k1. The exposure amount may be controlled by adjusting the exposure intensity of each divided region of the first beam light (set to (k1 / k0) times the reference intensity).
By such control, the sensitivity unevenness in each of the divided regions is corrected by the first beam light (outward path), and the charging unevenness is corrected by the second beam light (return path). The same effects as the example and the third embodiment are achieved.

As described above, according to the present invention, one of the light beams scanned in the forward and backward directions by the reciprocating scanner 9 (outward or backward beam light) is based on the pixel gradation. Since it is used for electrostatic latent image writing and the other is used effectively for correcting the distribution of potential characteristics (difference from the reference characteristics), the advantages of a reciprocating scanner (miniaturization, power saving, noise reduction) In addition, it is possible to configure an image forming apparatus that takes advantage of (and shortening of startup time), and it is possible to prevent the occurrence of uneven image density as much as possible even with respect to a photoconductor having uneven charging and uneven sensitivity.
In addition, since it is not necessary to add a new exposure source, the size and cost of the apparatus are not increased.
Incidentally, as shown in FIG. 3, the scanning position of the first beam light (outward path) and the scanning position of the second beam light (return path) in the reciprocatingly scanned beam light are maximum in the sub-scanning direction R22. There is a shift of half a pixel. That is, a deviation occurs between the position where the electrostatic latent image is written (outward path) and the exposure position (return path) for correcting uneven charging.
However, in general, the charging unevenness and the sensitivity unevenness show a distribution with a relatively large spatial period, and the exposure amount by the second beam light for correcting the charging unevenness or the like is the first beam for writing the electrostatic latent image. Since the exposure amount is relatively lower than the exposure amount by light, there is no particular problem even if the deviation in the sub-scanning direction R22 is not taken into consideration.
The present invention is particularly suitable when an a-Si photosensitive member, in which charging unevenness often appears remarkably, and when applied to a color image forming apparatus in which image density unevenness is remarkably visually recognized. , Application to other image forming apparatuses is also possible.
Further, the divided area is not limited to the case where the surface of the photosensitive drum 1 is divided into a plurality of areas both in the axial direction and in the circumferential direction.
For example, when charging unevenness or sensitivity unevenness mainly in the axial direction or circumferential direction of the photosensitive drum 1 is a problem, the divided region is divided into a plurality of parts only on the surface of the photosensitive drum 1 in the axial direction. It is also conceivable to use a region obtained by dividing the photosensitive drum 1 in a ring shape or a region obtained by dividing the photosensitive drum 1 in the circumferential direction.

  The present invention can be used for an image forming apparatus.

1 is a schematic sectional view of an image forming apparatus X according to an embodiment of the present invention. 3 is a perspective view illustrating a scanning state of beam light by a reciprocating scanner in the image forming apparatus X. FIG. FIG. 3 is a schematic diagram showing a beam scanning path by a reciprocating scanning scanner when the surface of a drum-shaped photoconductor is represented as a plane. 2 is a block diagram illustrating a schematic configuration of a main part of the image forming apparatus X. FIG. The graph showing the relationship between the pixel gradation which concerns on 1st Example, and the light quantity of beam light, and the graph showing the relationship between the pixel gradation at that time, and an electric potential. The graph showing the relationship between the pixel gradation which concerns on 2nd Example, and the light quantity of beam light, and the graph showing the relationship between the pixel gradation at that time, and an electric potential. The graph showing the relationship between the pixel gradation which concerns on 3rd Example, and the light quantity of beam light, and the graph showing the relationship between the pixel gradation at that time, and an electric potential. The graph showing the relationship between the pixel gradation which concerns on 4th Example, and the light quantity of beam light, and the graph showing the relationship between the pixel gradation at that time, and an electric potential. The graph showing an example of the relationship between the conventional pixel gradation and the potential after exposure on the surface of the photoreceptor where charging unevenness and sensitivity unevenness coexist.

Explanation of symbols

X: Image forming apparatus Z according to an embodiment of the present invention: Exposure sources 1BK, 1M, 1Y, 1C ... Photoconductor drums 2BK, 2M, 2Y, 2C ... Developing devices 3BK, 3M, 3Y, 3C ... Charging devices 5BK, 5M , 5Y, 5C ... neutralization device 7 ... intermediate transfer belt 9 ... reciprocating scanner 50 ... control unit 51 ... display operation unit 52 ... image processing unit 53 ... data storage unit 54 ... rotational position detection unit

Claims (11)

An image forming apparatus for writing an electrostatic latent image by scanning a light beam on the surface of a pre-charged photoconductor,
Optical reciprocating scanning means for reciprocatingly scanning the beam light with respect to the surface of the photosensitive member;
The exposure control for writing the electrostatic latent image is performed by controlling the exposure amount of the first light beam scanned in either the forward or backward direction among the light beams scanned back and forth by the optical reciprocating scanning means. First exposure amount control means for performing;
Second exposure amount control means for performing exposure control for correcting a characteristic distribution on the surface of the photosensitive member by controlling an exposure amount by a second beam light scanned in a direction opposite to the first beam light;
In images forming apparatus ing comprises a,
The second exposure amount control means, for each divided region obtained by dividing the surface of the photosensitive member into a plurality of regions, according to a difference between a potential characteristic in the divided region and a predetermined reference potential characteristic common to all the divided regions. The exposure amount is controlled by the second beam light.
The first exposure amount control means sets the exposure amount obtained by proportionally converting the pixel gradation representing the gray level for each pixel using a common coefficient for all the divided regions as the exposure amount by the first beam light. An image forming apparatus which is a first exposure amount control means . Inclination information storage means for storing, for each of the divided areas, inclination information when the pixel gradation is proportionally converted to an exposure amount;
Individual first exposure in which the first exposure amount control means sets, as the exposure amount by the first beam light, an exposure amount obtained by proportionally converting a pixel gradation representing a gray level for each pixel in accordance with the inclination information for each divided region. The image forming apparatus according to claim 1, wherein the image forming apparatus is an amount control unit. The first exposure amount control means adjusts an exposure time for each pixel of the first light beam according to the pixel gradation, and exposes the first light beam for each divided region according to the tilt information. The image forming apparatus according to claim 2 , wherein the exposure amount is controlled by adjusting the intensity. A predetermined reference potential common to all the divided areas is applied to a predetermined reference exposure amount common to all the divided areas and an exposure potential characteristic representing a correspondence relationship between the exposure amount and the potential in each of the divided areas. A raised exposure amount information storage means for storing, for each of the divided areas, raised exposure amount information for specifying a difference exposure amount between the obtained exposure amount and
The second exposure control means, formed by including the exposure amount setting means raise the to set an exposure amount of said second light beam on the basis of the raising exposure information for each of the divided regions according to claim 1 to 3 The image forming apparatus according to any one of the above. The reference potential and the reference exposure amount are approximately the midpoint potential between the initial potential and the residual potential in the exposure potential characteristics and the exposure amount corresponding to the approximately midpoint potential in the exposure potential characteristics that are common to all the divided regions. The image forming apparatus according to claim 4 . When the first exposure amount control means is the common first exposure amount control means,
The difference regarding the difference between the inclination in the substantially linear exposure potential characteristic that is the portion excluding the convergence area to the residual potential in the exposure potential characteristic for each of the divided areas and the reference inclination common to all the divided areas. Differential inclination information storage means for storing inclination information for each of the divided areas;
The second exposure amount control means determines the pixel gradation determined by the image processing means with respect to the exposure amount of the second beam light set by the raised exposure amount setting means based on the difference inclination information. the image forming apparatus according to any one of formed by further comprising an exposure sensitivity correction means for performing a proportional-converted exposure amount of correction according to claim 4 or 5 Te. The second exposure control means, the image forming apparatus according to any one of claims 1 to 6 formed by controlling the exposure amount by adjusting the exposure time or the exposure intensity of the second light beam. The divided region, the image forming apparatus according to any one of claims 1 to 7, which is a region divided into a plurality of one or both of the the circumferential direction the axial direction of the surface of the drum-like of the photosensitive member. The image forming apparatus according to any one of claims 1-8 wherein the photoconductor is an a-Si photosensitive member. Wherein the image processing means, image formation according to any one of the image according to claim 1-9 in an area gray scale method is intended to perform gradation expression to determine the sequence of the pixel gray levels of a plurality of pixels based on the data apparatus. The image forming apparatus according to any one of claims 1 to 1 0 by a plurality of colors of toner and performs color image formation.

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