JP2011043569A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus in which detection of density unevenness is stabilized and a process of correcting the density unevenness is simplified. <P>SOLUTION: The image forming apparatus includes: a plurality of photoreceptor drums 12 for a first special color (S1), a second special color (S2), yellow (Y), magenta (M), cyan (C) and black (K); and an exposure device for forming electrostatic latent images on respective photoreceptor drums 12 by irradiating the respective photoreceptor drums 12 with light. Furthermore, on the photoreceptor drums 12 of respective colors, reference sections Z (Z<SB>S1</SB>, Z<SB>S2</SB>, Z<SB>Y</SB>, Z<SB>M</SB>, Z<SB>C</SB>, Z<SB>K</SB>) becoming the references for the rotational positions, are arranged, respectively. Then, among the plurality of photoreceptor drums 12, with respect to one photoreceptor drum 12 to which writing of the electrostatic latent image is performed first, positions of the reference sections Z of the other photoreceptor drums 12, which shift according to the rotations of the photoreceptor drums 12, are deviated on the basis of an interaxial distance between the one photoreceptor drum 12 and the other photoreceptor drums 12. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus.

  In the image forming apparatus, density unevenness correction processing for correcting density unevenness occurring in the output image is performed. For example, an image forming apparatus described in Patent Document 1 is known as a related art relating to density unevenness correction. Here, in Patent Document 1, an electrostatic latent image is formed on a photosensitive member by exposing a cylindrical photosensitive member rotating in a second direction in which a light beam moving in the first direction intersects the first direction. An image is formed when an image is formed by an image forming apparatus that forms an image on the recording medium by forming the image and developing the electrostatic latent image while the photoconductor rotates and transferring the image to the recording medium. Set the correction amount of the physical quantity that affects the density. The correction amount is a plurality of positions in the second direction between two reference position corresponding positions corresponding to the reference position on the recording medium, based on a predetermined reference position serving as a reference for the rotational position of the photosensitive member. Are set individually corresponding to at least one of the positions.

Japanese Patent Laid-Open No. 2005-10568

  An object of the present invention is to provide an image forming apparatus that can stabilize density unevenness detection and simplify density unevenness correction processing.

  According to the first aspect of the present invention, a plurality of image carriers each provided with a reference portion serving as a reference for a rotational position, and the image carrier is irradiated with light, and an electrostatic latent image is applied to the image carrier. A light irradiating unit for writing, and among the plurality of image carriers, one image carrier to which an electrostatic latent image is first written is the other image carrier. The image forming apparatus is characterized in that the position of the reference portion that changes with the rotation of the image carrier is shifted based on the interaxial distance between the image carrier and the other image carrier.

According to a second aspect of the present invention, the light irradiating unit is configured such that when the reference unit provided in the one image holding body where the electrostatic latent image is first written is positioned at a specific position, 2. The image forming apparatus according to claim 1, wherein writing of the electrostatic latent image is started.
According to a third aspect of the present invention, the length of the outer periphery of the one image carrier is such that the length of the outer periphery of the one image carrier is an interaxial distance from another image carrier adjacent to the one image carrier. The image forming apparatus according to claim 1, wherein the image forming apparatus is different from the image forming apparatus.
According to a fourth aspect of the present invention, there is provided a drive unit that collectively drives the plurality of image carriers, and the plurality of image carriers are rotated in synchronization by the drive unit. The image forming apparatus according to any one of Items 1 to 3.

  According to a fifth aspect of the present invention, there is provided a plurality of image carriers each provided with a reference portion serving as a reference for a rotational position, and the image carrier is irradiated with light, and an electrostatic latent image is applied to the image carrier. A light irradiating unit for writing image, and when the light irradiating unit writes an electrostatic latent image of an image used for detecting density unevenness to the plurality of image holding members, each electrostatic latent image on each image holding member The image forming apparatus is characterized in that the positional relationship between the writing start position and the position of each reference portion in each image holding body is uniform among the plurality of image holding bodies.

The invention according to claim 6 is characterized in that the light irradiation unit changes a combination of image carriers to which the electrostatic latent image is written in accordance with an image to be formed. An image forming apparatus.
According to a seventh aspect of the present invention, the plurality of image carriers are configured such that the one image carrier after the electrostatic latent image writing is started on the one image carrier constituting the plurality of image carriers. 7. The writing of an electrostatic latent image on another image holding body adjacent to the one image holding body is started with a time shift corresponding to the inter-axis distance with respect to. An image forming apparatus.
According to an eighth aspect of the present invention, in one image holding member constituting the plurality of image holding members, the length of the outer periphery of the one image holding member is another image holding member adjacent to the one image holding member. The image forming apparatus according to claim 5, wherein the image forming apparatus is different with respect to a distance between axes with respect to the body.

According to the first aspect of the present invention, in the image forming apparatus, it is possible to stabilize density unevenness detection and simplify density unevenness correction processing.
According to the invention of claim 2, in the formed image, the positional relationship between the writing start position of the electrostatic latent image and the position corresponding to the reference portion is set in each image formed by each image carrier. Can be aligned.
According to the third aspect of the present invention, even in the case where a configuration in which the arrangement interval between adjacent image carriers and the outer circumference of the image carrier are different is adopted, each image in each image formed by each image carrier It is possible to align the position corresponding to the reference portion and the writing start position of each electrostatic latent image.
According to the invention of claim 4, a plurality of image holding bodies can be driven using a single drive unit, and the apparatus configuration, drive control, etc. are simplified as compared with the case where this configuration is not provided. be able to.

According to the invention of claim 5, in the image forming apparatus, it is possible to stabilize the density unevenness detection and simplify the process of correcting the density unevenness.
According to the invention of claim 6, even if the combination of the image carriers used for image formation changes, the positions corresponding to the respective reference portions in the image used for detecting the density unevenness are aligned among the plurality of image carriers. Is possible.
According to the seventh aspect of the present invention, the positional relationship between the electrostatic latent image writing start position and the position corresponding to the reference portion can be aligned for each image formed on each image carrier, and this configuration is Compared with the case where it does not have, for example, it becomes easier to detect density unevenness.
According to the eighth aspect of the present invention, even in the case where a configuration in which the arrangement interval between adjacent image carriers and the length of the outer periphery of the image carrier are different is employed, in each image formed by each image carrier. It is possible to align the position corresponding to each reference portion and the writing start position of each electrostatic latent image.

1 is a diagram illustrating an overall configuration of an image forming apparatus according to an exemplary embodiment. It is a figure for demonstrating in detail about several photoconductor drums. It is a figure for demonstrating the phase of a photoconductor drum. 6 is a timing chart when an image is formed using an image forming unit of six colors. It is an example of an image formed on a sheet using an image forming unit of six colors. 4 is a timing chart when an image is formed using a four-color image forming unit. 3 is an example of an image formed on a sheet using a four-color image forming unit. It is a figure for demonstrating the image formation part of a comparative example. It is an example of the image formed on the paper using the image forming unit of the comparative example.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating an overall configuration of the image forming apparatus according to the present exemplary embodiment.
As shown in FIG. 1, the image forming apparatus 1 includes an image forming unit 2 and a scanner unit 3. In addition, the image forming apparatus 1 includes a control unit 100 that comprehensively manages and controls the operation of each part of the image forming unit 2 and the scanner unit 3.
The image forming unit 2 conveys the toner image formed by the image forming unit 11 toward the paper P and the image forming unit 11 (11Y, 11M, 11C, 11K, 11S1, 11S2) that forms the toner image of each color. An intermediate transfer belt 16, a belt driving unit 17 that drives the intermediate transfer belt 16, and a transfer roll 18 that transfers the toner image on the intermediate transfer belt 16 to the paper P are provided. The image forming unit 2 also includes a fixing device 5 that fixes the toner image transferred onto the paper P onto the paper P using pressure and heat.

  The image forming unit 2 includes six image forming units that form yellow (Y), magenta (M), cyan (C), black (K), the first special color (S1), and the second special color (S2). 11Y, 11M, 11C, 11K, 11S1, and 11S2. Each image forming unit 11 exposes a photosensitive drum 12 as an example of an image carrier having a photosensitive layer, a charger 13 for charging the surface of the photosensitive drum 12, and the photosensitive drum 12 to expose the photosensitive drum 12 to static. An exposure device 14 as an example of a light irradiation unit that forms an electrostatic latent image and a developing device 15 that develops the electrostatic latent image on the photosensitive drum 12 are provided.

  The intermediate transfer belt 16 rotates in the direction of arrow X (leftward in the figure). As shown in FIG. 1, the image forming units 11 of the respective colors are arranged in the order of S1, S2, Y, M, C, and K from the upstream side to the downstream side in the rotation direction of the intermediate transfer belt 16. .

The image forming unit 2 according to the present embodiment has a special color (such as Y, M, C, and K) that is frequently used for normal color expression (usual color) that is not used for normal color image formation ( The first special color S1 and the second special color S2) are formed. In this embodiment, a clear (transparent) color is used as the first special color S1, and a low gloss black image material is used as the second special color S2.
As the first special color S1 and the second special color S2 (special drawing material), for example, invisible toner or a corporate color dedicated to a special user (for example, green of a specific film company, red of a specific drink company, etc.) Alternatively, a foaming toner for Braille, a toner for improving fluorescent color and gloss, and the like may be used. In the present embodiment, two types of special color image forming units 11 are provided. However, the present invention is not limited to this. One, three, or more special color image forming units 11 may be provided.

  The exposure apparatus 14 of the present embodiment is of a type that forms an electrostatic latent image on the photosensitive drum 12 by scanning the photosensitive drum 12 with laser light in the main scanning direction. Further, in the image forming unit 2 of the present embodiment, the relative positional relationship between the exposure device 14 and the photosensitive drum 12 is unified in the image forming units 11 of the respective colors. That is, the position on the photosensitive drum 12 that receives the laser beam from the exposure device 14 (hereinafter referred to as the irradiated position) is the same in the image forming units 11 of the respective colors. The exposure device 14 may be of a type in which a plurality of light emitting elements are arranged in an array in the main scanning direction of the photosensitive drum 12 and light is flashed toward the photosensitive drum 12.

  In addition, the “electrostatic latent image” formed in the present embodiment is, for example, an image A shown in FIG. 5 to be described later. An example of forming an image aligned with the above is taken as an example.

  The drum drive unit M as an example of the drive unit rotates the photosensitive drums 12 of each color. The drum driving unit M includes a motor as a power source and a transmission mechanism that transmits the driving force of the motor to the photosensitive drums 12 of the respective colors. In this embodiment, the six photosensitive drums 12 are collectively driven by a single motor. Accordingly, in the present embodiment, the six photosensitive drums 12 provided in the image forming unit 2 rotate in synchronization. Note that various motors such as a stepping motor and a DC motor can be used for the drum driving unit M of the present embodiment.

  The control unit 100 is predetermined for receiving image data from an external device such as a personal computer (PC) via a network such as a LAN (Local Area Network), image data sent from the scanner unit 3, and the like. Perform image processing and other processing. The above functions in the control unit 100 are realized by a CPU or the like controlled by a program. The control unit 100 also performs processing for detecting density unevenness generated in the image forming apparatus 1 and correcting the density unevenness.

Here, an example of density unevenness correction processing in the image forming apparatus 1 of the present embodiment will be described.
First, when the control unit 100 receives an instruction to correct density unevenness, the control unit 100 causes the image forming unit 2 to form a test image (for example, an image A in FIG. 5 described later) for detecting density unevenness. . The test image is formed extending in the main scanning direction or the sub-scanning direction according to the direction in which the density unevenness is to be detected. In this embodiment, detection of density unevenness in the sub-scanning direction and correction of density unevenness are taken as an example.
Then, using the scanner unit 3, the test image is read. The scanner unit 3 sends image data obtained by reading the test image to the control unit 100. The control unit 100 creates correction data for suppressing density unevenness based on the image data read from the test image. In the subsequent image forming operation, image formation reflecting the correction data is performed. In the present embodiment, density unevenness is corrected by adjusting the intensity (drive current) of light irradiated to the photosensitive drum 12 by the exposure device 14.

  The scanner unit 3 irradiates light to a reading target such as paper, for example, and reads the reflected light by an image sensor, thereby reading the image. The scanner unit 3 transmits the read data to the control unit 100 for image formation or to a PC or the like connected to the scanner unit 3. The scanner unit 3 is also used when reading a test image for correcting density unevenness formed by the image forming unit 2 as described above.

FIG. 2 is a diagram for explaining the plurality of photosensitive drums 12 in detail.
In the image forming unit 2, yellow (Y), magenta (M), cyan (C), black (K), the first special color (S1), and the second special color corresponding to the image forming unit 11 of each color. Six photosensitive drums 12 of (S2) are provided. In the present embodiment, the diameters of the photosensitive drums 12 for the respective colors are the same. Further, the distance between the axes of adjacent photosensitive drums 12 (hereinafter referred to as the distance between the drums) is equal. In the present embodiment, as shown in FIG. 2, the distance between the axes of the adjacent photosensitive drums 12 is a length L.
From the viewpoint of extending the life of the photosensitive drum 12, it is preferable that the outer peripheral length of the photosensitive drum 12 (hereinafter referred to as the peripheral length) is larger. On the other hand, from the viewpoint of downsizing the image forming apparatus 1 (image forming unit 2), it is preferable that the distance between the drums is small. Because of such demands, in general, in the image forming apparatus, the circumferential length of the photosensitive drum 12 and the distance between the drums are different. Also in this embodiment, an example in which the circumferential length of the photosensitive drum 12 and the distance between the drums are different is used.

  Usually, since the electrostatic latent images of the respective colors formed on the photosensitive drums 12 of the respective colors are aligned on the intermediate transfer belt 16, the timing of starting the writing of the electrostatic latent image on the photosensitive drum 12 is shifted between the respective colors. ing. For example, in the two adjacent photosensitive drums 12, the writing of the electrostatic latent image on the upstream photosensitive drum 12 is delayed by a time Δt from the start of writing of the electrostatic latent image on the upstream photosensitive drum 12. Start writing. This time Δt is determined by the distance between the drums. Specifically, the time Δt has a relationship of inter-drum distance (L) / peripheral speed (v) of the photosensitive drum 12.

For example, when the start of writing of the electrostatic latent image on the first special color photosensitive drum 12 is started, the electrostatic latent image is delayed by the time Δt in the second special color photosensitive drum 12. Writing starts. In the yellow photosensitive drum 12, since the distance between the first special color photosensitive drum 12 and the photosensitive drum 12 is 2L, writing of the electrostatic latent image is started with a delay of time 2Δt. In the magenta photosensitive drum 12, the distance between the first special color photosensitive drum 12 and the photosensitive drum 12 is 3L, and thus writing of the electrostatic latent image is started with a delay of time 3Δt. In the cyan photosensitive drum 12, since the distance between the first special color photosensitive drum 12 and the photosensitive drum 12 is 4L, writing of the electrostatic latent image is started with a delay of time 4Δt. Then, since the inter-drum distance between the black photosensitive drum 12 and the first special color photosensitive drum 12 is 5 L, writing of the electrostatic latent image is started with a delay of time 5Δt.
As described above, the writing start timing of the electrostatic latent image on a certain photosensitive drum 12 is determined by the inter-drum distance from the photosensitive drum 12 that is the starting point.

  The image forming apparatus 1 includes six image forming units 11 of Y, M, C, K, S1, and S2. Depending on the contents of the image to be formed, the image forming apparatus 1 actually uses the image forming unit 11. The image forming unit 11 for forming is used properly. For example, if the image to be formed includes all the colors Y, M, C, K, S1, and S2, the image forming units 11 for Y, M, C, K, S1, and S2 are all set. Use. For example, when it is not necessary to use a special color for an image to be formed, four image forming units 11 of Y, M, C, and K are used. Furthermore, when printing a monochrome image, only the black image forming unit 11K is used.

  In the present embodiment, among the image forming units 11 that actually perform image formation, an image forming unit 11 (hereinafter referred to as the leading image forming unit 11) that is positioned at the most upstream position in the rotation direction of the intermediate transfer belt 16 is used. At the starting point, the writing start timing of the electrostatic latent image in all the image forming units 11 is specified.

  For example, when image formation is performed using the six image forming units 11 of Y, M, C, K, S1, and S2, the image forming unit 11S1 of the first special color located at the most upstream position This is the starting point for starting to write the electrostatic latent image on the body drum 12. Further, when image formation is performed using the Y, M, C, and K image forming units 11, the yellow image forming unit 11 </ b> Y located at the most upstream position of the electrostatic latent images on all the photosensitive drums 12. This is the starting point for writing.

The photosensitive drum 12 of the present embodiment is provided with a reference portion Z that serves as a reference for specifying the rotational position of the photosensitive drum 12. In the present embodiment, in order to read the rotation direction and rotation angle of the photoconductive drum 12, an encoder is provided for each photoconductive drum 12. As the encoder of this embodiment, an incremental rotary encoder is used. In this embodiment, one slit constituting the z phase is used as the reference portion Z in the slit disk provided in the encoder.
In the following description, with respect to each reference portion Z provided on each photoconductor drum 12, those corresponding to Y, M, C, K, S1, and S2 are respectively referred to as yellow reference portion Z _Y and magenta reference portion Z _M. , A cyan reference portion Z _C , a black reference portion Z _K , a first special color reference portion Z _S1 , and a second special color reference portion Z _S2 .
In this embodiment, a line extending in the radial direction from the rotation center of the photosensitive drum 12 and passing through the slit as the reference portion Z passes through a position where the outer periphery of the photosensitive drum 12 intersects. Is referred to as “reference line ZL”.

  In this embodiment, since the image to be formed (image A and image B described later) is determined, the electrostatic latent image writing start position on the photosensitive drum 12 of each color is specified. Then, the electrostatic latent image writing start position on each color photoconductor drum 12 and each reference line ZL are configured to have a fixed positional relationship. In particular, in the present embodiment, the writing start position of each electrostatic latent image on the photosensitive drum 12 of each color is matched with the position of each reference line ZL.

In the photosensitive drum 12 in the leading image forming unit 11 (hereinafter referred to as the leading photosensitive drum 12), the electrostatic latent image is written from when the reference portion Z of the leading photosensitive drum 12 is located at a specific position. Is started. In the present embodiment, the writing of the electrostatic latent image is started from the moment when the position of the reference line ZL of the leading photosensitive drum 12 is positioned at the irradiation position of the laser beam.
For example, when image formation is performed using all the colors Y, M, C, K, S1, and S2, the top photosensitive drum 12 is the photosensitive drum 12 of the first special color. Accordingly, in this case, writing of the electrostatic latent image on the first special color photosensitive drum 12 is started from the moment when the reference line ZL of the first special color photosensitive drum 12 reaches the irradiation position of the laser beam. The When image formation is performed using four colors Y, M, C, and K without using S1 and S2, the leading photosensitive drum 12 is the yellow photosensitive drum 12. In this case, the writing of the electrostatic latent image on the yellow photosensitive drum 12 is started from the moment when the reference line ZL of the yellow photosensitive drum 12 reaches the irradiation position of the laser beam.

  In addition, the timing of starting writing of the electrostatic latent image on the photosensitive drums 12 other than the head is determined by the inter-drum distance with respect to the head photosensitive drum 12. The timing of starting the electrostatic latent image writing on the leading photosensitive drum 12 is determined by the position of the reference portion Z of the leading photosensitive drum 12 as described above. Thereby, the timing of starting writing of each electrostatic latent image on the photosensitive drum 12 of each color when forming one image is specified.

FIG. 3 is a diagram for explaining the phase of the photosensitive drum 12.
In the present embodiment, the phase of each photosensitive drum 12 is specified by the position of the reference portion Z. For example, as shown in FIG. 3A, when the reference portions Z of the two left and right photosensitive drums 12 are at the same position, the phases of these photosensitive drums 12 are the same.
On the other hand, as shown in FIG. 3B, when the position of the reference portion Z of the right photosensitive drum 12 and the position of the reference portion Z of the left photosensitive drum 12 are at different positions, these photosensitive drums are detected. The body drum 12 is out of phase. Further, as shown in FIG. 3B, the position of the reference portion Z of the other photosensitive drum 12 is shifted by an angle θ relative to the position of the reference portion Z of one of the photosensitive drums 12 (one photosensitive drum 12 are rotated by an angle θ), the photosensitive drums 12 are in a state in which the phase is shifted by the angle θ.

In the present embodiment, in the image forming units 11 of the respective colors, the phases of the photosensitive drums 12 of the respective colors when writing of the electrostatic latent images on the photosensitive drums 12 of the respective colors are started are the same. It is configured so that everything is in line with each other. That is, when writing of the electrostatic latent image on the photosensitive drums 12 of each color is started, the positions of the reference portions Z on the photosensitive drums 12 of the respective colors are all aligned due to the relationship between the photosensitive drums 12 of the respective colors. .
For this reason, in the present embodiment, the photosensitive drums 12 of the respective colors are attached in a state where the phases of the photosensitive drums 12 of the respective colors are shifted in advance. At this time, as shown in FIG. 2, the position of the reference portion Z of the photoconductive drum 12 in one photoconductive drum 12 relative to the other photoconductive drum 12 arranged adjacent to the upstream side is the image. It is attached in a state in which a predetermined angle (an angle α described later) is shifted in a direction opposite to the rotation direction at the time of formation.

  As described above, in order to align the electrostatic latent images (images) formed on the six-color photosensitive drums 12 on the intermediate transfer belt 16, for example, the timing between the adjacent photosensitive drums 12 is the time Δt. Is shifted and the electrostatic latent image is written. Therefore, as in the present embodiment, in the photosensitive drums 12 of the respective colors, in order to start writing of the electrostatic latent image from the respective reference lines ZL, between the adjacent photosensitive drums 12, The phase of the photosensitive drum 12 may be shifted so that the reference portion Z is shifted by the angle α at which the photosensitive drum 12 rotates during the time Δt.

  Here, the angular velocity ω [rad / s] of the photosensitive drum 12 is represented by v / r (v: peripheral speed of the photosensitive drum 12, r: radius of the photosensitive drum 12). The time Δt [s] has a relationship of L / v (L: distance between drums). Therefore, the angle α [rad] at which the photosensitive drum 12 rotates during the time Δt is L / r. This angle α corresponds to an arc of length L (distance between the drums) on the outer periphery of the photosensitive drum 12. Conversely, the angle α is the angle of the central angle from the rotation center of the photosensitive drum 12 with respect to the arc of the length L on the outer periphery of the photosensitive drum 12 having the radius r.

  Specifically, the phase shift between the adjacent photosensitive drums 12 will be described. The phase of the second special color photosensitive drum 12 is shifted by an angle α with respect to the adjacent first special color photosensitive drum 12. Attached. The yellow photosensitive drum 12 is attached to the adjacent second special color photosensitive drum 12 with a phase shifted by an angle α. Similarly, the magenta photosensitive drum 12, the cyan photosensitive drum 12, and the black photosensitive drum 12 are attached to the adjacent photosensitive drums 12 with phases shifted by an angle α.

  The phase shift between the photosensitive drums 12 will be specifically described with reference to a certain photosensitive drum 12. As described above, the angle α is specified according to the inter-drum distance (L) with respect to the reference photosensitive drum 12. For example, when the photosensitive drum 12 of the first special color is used as a reference, the photosensitive drum 12 of the second special color is attached with the phase shifted by an angle α. Similarly, when the photosensitive drum 12 for yellow is based on the photosensitive drum 12 for the first special color, the phase distance between the photosensitive drum 12 for the first special color and the photosensitive drum 12 for the first special color is 2L. It is shifted and attached. Similarly, when the photoconductor drum 12 of the first special color is used as a reference, the phase of the magenta photoconductor drum 12 is shifted by an angle 3α, and the phase of the cyan photoconductor drum 12 is shifted by an angle 4α. The photosensitive drum 12 is mounted with a phase shifted by an angle 5α. In other words, the photosensitive drums 12 are mounted with their angles shifted based on the inter-drum distance with respect to the reference photosensitive drum 12.

Next, the exposure timing when image formation is performed using the image forming units 11 of all colors Y, M, C, K, S1, and S2 will be described.
FIG. 4 is a timing chart when an image is formed using the six-color image forming unit 11.
FIG. 4 shows the on / off timing of light irradiation by the exposure device 14 in each color image forming unit 11. Further, in FIG. 4, a timing at which each reference line ZL of each photosensitive drum 12 is positioned at an irradiation position of each laser beam is displayed by a triangular mark.

  In this example, the leading image forming unit 11 is the first special color image forming unit 11S1. That is, the first photosensitive drum 12 is the first special color photosensitive drum 12. Then, as shown in FIG. 4, from the moment when the image formation is started and the reference line ZL of the photosensitive drum 12 of the first special color reaches the irradiated position of the laser light, the photosensitive drum of the first special color is used. The writing of the electrostatic latent image to 12 is started.

Then, when the time Δt elapses after the exposure in the first special color image forming unit 11 is started, the exposure in the second special color image forming unit 11S2 is started. The photosensitive drum 12 of the second special color is rotated by an angle α while the time Δt has elapsed since the writing of the electrostatic latent image on the photosensitive drum 12 of the first special color is started. Here, the photosensitive drum 12 of the second special color is attached in advance so that the position of the second special color reference portion Z _S2 is shifted by an angle α with respect to the position of the first special color reference portion Z _S1 . Accordingly, the writing of the electrostatic latent image for the second special color is started from the moment when the reference line ZL of the photosensitive drum 12 of the second special color reaches the irradiation position of the laser beam.

Further, when the time Δt (time 2Δt from the first special color) has elapsed since the writing of the electrostatic latent image on the second special color photoconductor drum 12 is started, similarly, the yellow photoconductor drum 12 is used. Writing of the electrostatic latent image to is started. At this time, the yellow photosensitive drum 12 has the reference line ZL positioned at the irradiated position of the laser beam. The yellow photosensitive drum 12 starts to write an electrostatic latent image at the moment when the reference line ZL reaches the position irradiated with the laser beam.
Similarly, writing of an electrostatic latent image is also started in order on the magenta photosensitive drum 12, the cyan photosensitive drum 12, and the black photosensitive drum 12. At this time, in each photoconductor drum 12, writing of the electrostatic latent image is started from the moment when each reference line ZL in each photoconductor drum 12 is positioned at the irradiation position of the laser beam.

FIG. 5 is an example of an image (hereinafter, image A) formed on a sheet using the six-color image forming unit 11.
An image A shown in FIG. 5 is an image for detecting correction of density unevenness. In the image A, the first special color, the second special color, yellow, magenta, cyan, and black belt-like patterns are displayed in order from the top. The belt-like pattern of each color is formed so as to exceed at least one round of the photosensitive drum 12 in the sub-scanning direction.
Further, in FIG. 5, positions corresponding to the respective reference lines ZL of the photosensitive drums 12 of the respective colors on the paper (hereinafter referred to as “paper on-reference positions”) are indicated by broken lines. It should be noted that the on-paper reference position for each color on the paper is the first special color on-paper reference position S1 _ZL , the second special color on-paper reference position S2 _ZL , the yellow on-paper reference position Y _ZL , and the magenta paper. The upper reference position M _ZL , the cyan paper upper reference position C _ZL , and the black paper upper reference position K _ZL .

As shown in FIG. 5, in the first special color belt-like pattern, the writing start position (end of the belt-like pattern) of the first special color electrostatic latent image and the first special color on-paper reference position S1 _ZL are as _follows. Match. In the second special color belt-like pattern, the writing start position of the second special color electrostatic latent image coincides with the on-paper reference position S2 _{ZL of} the second special color. In the yellow belt-like pattern, the yellow electrostatic latent image writing start position coincides with the yellow on-paper reference position Y _ZL . In the magenta belt-like pattern, the writing start position of the magenta electrostatic latent image coincides with the magenta on-paper reference position M _ZL . In the cyan belt-like pattern, the cyan electrostatic latent image writing start position coincides with the cyan on-paper reference position C _ZL . In the black belt-like pattern, the writing start position of the black electrostatic latent image coincides with the black on-paper reference position K _ZL .

In the present embodiment, the phases of the photosensitive drums 12 of the respective colors when writing of the electrostatic latent image on the photosensitive drums 12 of the respective colors are made to coincide with each other. In particular, in the present embodiment, writing of each electrostatic latent image is started from the moment when the position of each reference line ZL of the photosensitive drum 12 of each color reaches the irradiation position of the laser beam. Therefore, as shown in FIG. 5, in the strip pattern of each color, the writing start position of the electrostatic latent image of each color coincides with the reference position on the paper of each color.
As a result, the first special color on-paper reference position S1 _ZL , the second special color on-paper reference position S2 _ZL , the yellow on-paper reference position Y _ZL , the magenta on-paper reference position M _ZL , on the cyan paper The reference position C _ZL and the black on-paper reference position K _ZL coincide with the sub-scanning direction on the sheet. As described above, in the image formed by the image forming unit 2 of the present embodiment, the position of the reference position on the sheet on the sheet is aligned in all colors.

Next, the exposure timing when image formation is performed using the four-color image forming units 11 of Y, M, C, and K will be described.
FIG. 6 is a timing chart when an image is formed using the four-color image forming unit 11.
In this example, since S1 and S2 are not used, the top image forming unit 11 is the yellow image forming unit 11Y. That is, the leading photosensitive drum 12 is the yellow photosensitive drum 12. Then, as shown in FIG. 6, from the moment when the image formation is started and the reference line ZL of the yellow photosensitive drum 12 reaches the irradiation position of the laser beam, the electrostatic latent image on the yellow photosensitive drum 12 is displayed. Writing starts.

Then, when the time Δt elapses after the exposure in the yellow image forming unit 11Y is started, the exposure in the magenta image forming unit 11M is started. The magenta photosensitive drum 12 is rotated by an angle α during the time Δt has elapsed since the writing of the electrostatic latent image on the yellow photosensitive drum 12 is started. Here, the magenta photosensitive drum 12 is mounted in advance so that the position of the magenta reference portion Z _M is shifted in phase by the angle α with respect to the position of the yellow reference portion Z _Y. Accordingly, the writing of the electrostatic latent image for magenta is started from the moment when the reference line ZL of the photoconductor drum 12 of magenta reaches the irradiation position of the laser beam.

  Similarly, writing of the electrostatic latent image is sequentially started on the cyan photosensitive drum 12 and the black photosensitive drum 12. At this time, in each photosensitive drum 12, writing of the electrostatic latent image is started from the moment when each reference line ZL in each photosensitive drum 12 reaches the irradiation position of the laser beam.

FIG. 7 is an example of an image (hereinafter, image B) formed on a sheet using the four-color image forming unit 11.
As shown in FIG. 7, in the yellow belt-like pattern, the yellow electrostatic latent image writing start position coincides with the yellow on-paper reference position Y _ZL . In the magenta belt-like pattern, the writing start position of the magenta electrostatic latent image coincides with the magenta on-paper reference position M _ZL . In the cyan belt-like pattern, the cyan electrostatic latent image writing start position coincides with the cyan on-paper reference position C _ZL . In the black belt-like pattern, the writing start position of the black electrostatic latent image coincides with the black on-paper reference position K _ZL .
As described above, even when the image is formed on the paper using the four-color image forming unit 11, the yellow paper upper reference position Y _ZL , the magenta paper upper reference position M _ZL , the cyan paper upper reference position C _ZL , The black on-paper reference position K _ZL coincides with the sub-scanning direction on the paper. As described above, in the image formed by the image forming unit 2 of the present embodiment, the positions of the on-paper reference positions of the respective colors on the paper are all aligned in the sub-scanning direction.

Then, the image A shown in FIG. 5 is compared with the image B shown in FIG. Of the image A and the image B, when the present in both example, yellow belt-shaped pattern as an example, the sheet on the reference position Y _ZL yellow in the image A, the position of the sheet on the reference position Y _ZL yellow in the image B Match. The same applies to the magenta, cyan, and black belt-like patterns formed in the images A and B. The same applies to the first special color and the second special color that are not formed in the image B shown in FIG.

  As in this embodiment, the set of image forming units 11 that are actually used for image formation changes according to the content of the image to be formed, and the photosensitive drum 12 ( Even when a configuration in which the leading photosensitive drum 12) is changed is adopted, the reference position on each sheet is formed at the same position for each color.

Here, density unevenness correction data for correcting density unevenness will be considered. Data for correcting the density unevenness of yellow is taken as an example. In forming the yellow belt-like pattern when forming the image A, in this embodiment, the reference line ZL of the yellow photosensitive drum 12 and the electrostatic latent image writing start position match. Therefore, correction data for uneven density necessary for forming a yellow image is prepared starting from the reference line ZL.
On the other hand, when forming the image B, in forming the yellow belt-like pattern, in this embodiment, writing of the electrostatic latent image is started from the reference line ZL of the yellow photosensitive drum 12. Even in this case, correction data for uneven density necessary for forming a yellow image is prepared starting from the reference line ZL. This also applies to other colors. The same applies to the case where the set of image forming units 11 actually used differs depending on the contents of the image to be formed.

  As described above, even if the set of image forming units 11 to be actually used is changed, writing of each reference line ZL of each color photosensitive drum 12 and each electrostatic latent image is performed for each color before and after the change. The positional relationship with the start position matches. Therefore, for example, the correction pattern for uneven density created in the sub-scanning direction using the reference line ZL as a base point used when creating the image A can be used for the image B as it is. That is, in this embodiment, by preparing at least one density unevenness correction pattern for each color image forming unit 11 (photosensitive drum 12), regardless of the combination of image forming units 11 actually used. The density unevenness can be corrected.

Here, a comparative example will be described with reference to FIGS.
FIG. 8 is a diagram for explaining an image forming unit of a comparative example.
The basic apparatus configuration of the image forming unit of the comparative example shown in FIG. 8 is the same as that of this embodiment. However, the image forming unit of the comparative example is different from the above-described embodiment in the phase relationship of the plurality of photosensitive drums 92 attached to the image forming unit.

Although not shown, the first special color (S1), the second special color (S2), yellow (Y), magenta (M), cyan (C), and black are also shown in the comparative image forming unit. An image forming unit (K) is provided. Each unit is provided with a photosensitive drum 92, a charger, an exposure device, a developing device, and the like.
As shown in FIG. 8, the first special color photoconductor drum 92S1, the second special color photoconductor drum 92S2, the yellow photoconductor drum 92Y, the magenta photoconductor drum 92M, the cyan photoconductor drum 92C, And a black photosensitive drum 92K. Further, each photosensitive drum 92 includes a first special color reference portion Z _S1 , a second special color reference portion Z _S2 , a yellow reference portion Z _Y , a magenta reference portion Z _M , a cyan reference portion Z _C , and a black reference portion. Z _K is provided. Here, as shown in FIG. 8, in the image forming unit of the comparative example, the positions of the reference portions provided on the photosensitive drums 92 are attached so that all the photosensitive drums 92 of the respective colors coincide with each other. Yes. In other words, the photoconductor drums 92 of the respective colors are mounted in a state where the phases match.

  In the image forming unit of the comparative example, the combination of the image forming units (photosensitive drums 92) that are actually used varies depending on the image to be formed. The reference line ZL of the photosensitive drum 92 in the first image forming unit among the image forming units used for image formation is set to coincide with the image writing position. On the other photosensitive drum 92, writing of the electrostatic latent image is started with a delay of a predetermined time (Δt) in accordance with the inter-drum distance from the leading photosensitive drum 92.

  FIG. 9 is an example of an image formed on a sheet using the image forming unit of the comparative example. Note that an image 9A shown in FIG. 9A is a case where six colors of S1, S2, Y, M, C, and K are formed. On the other hand, an image 9B shown in FIG. 9B is a case where four colors of Y, M, C, and K are formed. Further, similarly to the image A described with reference to FIG. 5, an image in which a band-shaped pattern of each color is formed is shown as an example. Further, as in FIG. 5, the reference position on the paper for each color is also displayed.

When image formation is performed using the photosensitive drums 92 of all six colors, the leading photosensitive drum 92 becomes the photosensitive drum 92 of the first special color. Then, as in the image 9A shown in FIG. 9A, the writing start position of the first special color electrostatic latent image coincides with the on-paper reference position S1 _{ZL of} the first special color. However, the writing start position of the second special color electrostatic latent image deviates from the second special color on-paper reference position S2 _ZL . Also in yellow, magenta, cyan, and black, the electrostatic latent image writing start position and the reference position on the paper are deviated. This is determined in advance according to the inter-drum distance between the first photosensitive drum 92 and the photosensitive drum 92 other than the first photosensitive drum 92 in each color photosensitive drum 92 mounted in phase. This is because the writing of the electrostatic latent image is started with a delay of time (Δt).

Further, when image formation is performed using the Y, M, C, and K photosensitive drums 92 without using S1 and S2, the leading photosensitive drum 92 becomes the yellow photosensitive drum 92. As shown in an image 9B shown in FIG. 9B, the yellow electrostatic latent image writing start position coincides with the yellow on-paper reference position Y _ZL . However, the writing start position of the magenta electrostatic latent image deviates from the magenta on-paper reference position M _ZL . Also in cyan and black, the respective electrostatic latent image writing start positions deviate from the reference position on the paper.

When the yellow belt-like patterns formed in the images 9A and 9B are compared, the position of the on-paper reference position Y _ZL in the image is shifted in the sub-scanning direction. The same applies to other colors. In addition to the two comparisons in FIG. 9, when the combination of the image forming units 11 (photosensitive drums 12) used for image formation is changed, the same colors when the same belt-like pattern is formed However, the location where the on-paper reference position Y _ZL is located changes each time.
In such a case, it is necessary to prepare a pattern of correction data for uneven density according to the number of combinations of image forming units 11 used for actual image formation. As the number of image forming units 11 provided in the image forming apparatus 1 is increased, the number of combinations of image forming units 11 used for actual image formation is further increased.

  On the other hand, in the image forming apparatus 1 of the present embodiment, for example, it is only necessary to provide at least the number of correction patterns for correcting density unevenness, and more correction patterns are prepared in advance. It is not an essential requirement. In this way, in this embodiment, regardless of the combination of the image forming units 11 corresponding to the image to be formed, the density unevenness correction pattern prepared for each color is shared, and the density unevenness correction process is simplified. It has become.

  In addition, as in the present embodiment, by starting writing of each color image from the reference line ZL, the end of the image formed on the paper always corresponds to the position of the reference portion Z on the photosensitive drum 12. It will be. Also, the on-paper reference position for each color is formed at the same position for all colors in the sub-scanning direction. In this way, in this embodiment, when detecting density unevenness, the position of the photosensitive drum 12 on the sheet corresponding to the reference portion Z in the photosensitive drum 12 (reference position on the sheet) can be easily determined. The detection of uneven density is stabilized. In the image forming apparatus 1 of the present embodiment, the test image for detecting density unevenness has a simple configuration as exemplified by the image A, for example.

In the present embodiment, the example in which writing of the electrostatic latent image is started from the reference line ZL of the photosensitive drum 12 in the image forming unit 11 of each color is described, but the present invention is not limited to this. For example, all the photosensitive drums 12 may be configured to start writing an electrostatic latent image from a position away from the reference line ZL by a specific distance.
The detection of density unevenness is not limited to the configuration in which the scanner unit 3 reads the test image formed on the paper as in the above-described example. For example, a density detection sensor is provided on the intermediate transfer belt 16 and is held on the intermediate transfer belt 16. The toner image may be configured to detect uneven density of the toner image. Further, density unevenness detection may be performed using an external measuring machine.

In this embodiment, a configuration in which the toner image is transferred to the intermediate transfer belt 16 and then transferred to the paper P is used as an example. On the other hand, for example, a configuration in which the toner image of each color is directly transferred to the conveyed paper P may be used. In this case, a sheet conveying belt for conveying the sheet P may be provided instead of the intermediate transfer belt 16.
Further, a drive motor may be provided for each color photoconductor drum 12. In this case, before image formation, the phase is adjusted in advance so that each photosensitive drum 12 satisfies the phase relationship described above. When an image is actually formed by using the image forming unit 11, a plurality of photosensitive drums 12 are collectively driven by a single drive motor by rotating the respective drive motors synchronously. A similar operation is realized.

  Furthermore, in this embodiment, the phase of each photoconductive drum 12 is adjusted by shifting the rotational position of the photoconductive drum 12. However, the phase may be adjusted by changing the detection position of the rotary encoder.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 2 ... Image forming part, 3 ... Scanner part, 11 ... Image forming unit, 12 ... Photosensitive drum, 14 ... Exposure apparatus, Z ... Reference | standard part, ZL ... Reference | standard line

Claims (8)

A plurality of image carriers each provided with a reference portion serving as a reference for the rotational position;
A light irradiation unit for irradiating the image carrier with light and writing an electrostatic latent image on the image carrier;
Of the plurality of image carriers, one image carrier to which an electrostatic latent image is first written is compared with another image carrier between the one image carrier and the other image carrier. An image forming apparatus according to claim 1, wherein a position of the reference portion that shifts with rotation of the image holding member is shifted based on an inter-axis distance.   The light irradiation unit starts writing of the electrostatic latent image when the reference unit provided in the one image holding body where the writing of the electrostatic latent image is performed first is located at a specific position. The image forming apparatus according to claim 1.   The one image carrier is characterized in that the length of the outer periphery of the one image carrier is different with respect to the inter-axis distance from another image carrier adjacent to the one image carrier. Item 3. The image forming apparatus according to Item 1 or 2. A drive unit that collectively drives the plurality of image carriers;
The image forming apparatus according to claim 1, wherein the plurality of image holding members are rotated in synchronization by the driving unit. A plurality of image carriers each provided with a reference portion serving as a reference for the rotational position;
A light irradiation unit that irradiates the image carrier with light and writes an electrostatic latent image on the image carrier;
When the light irradiating unit writes an electrostatic latent image of an image used to detect density unevenness on the plurality of image carriers, each electrostatic latent image write start position on each image carrier and each image An image forming apparatus characterized in that a positional relationship with positions of the respective reference portions on the holding body is uniform among the plurality of image holding bodies.   The image forming apparatus according to claim 5, wherein the light irradiation unit changes a combination of image holders that write the electrostatic latent image in accordance with an image to be formed.   The plurality of image holders are provided for a time corresponding to an interaxial distance with respect to the one image carrier after writing of an electrostatic latent image is started in the one image carrier constituting the plurality of image carriers. 7. The image forming apparatus according to claim 5, wherein writing of an electrostatic latent image on another image carrier adjacent to the one image carrier starts.   One image carrier constituting the plurality of image carriers has a length of an outer periphery of the one image carrier with respect to an inter-axis distance with another image carrier adjacent to the one image carrier. The image forming apparatus according to claim 5, wherein the image forming apparatuses are different from each other.

Images