JP2003094722A - Image forming device - Google Patents

Abstract

(57) [Summary] (With correction) [PROBLEMS] Color misregistration may occur when a writing position of a black image is misaligned during full-color image formation. A monochrome image improves productivity, and a full-color image does not cause color shift. SOLUTION: When a monochrome image is formed, writing is performed using a light source unit 53 for a black image using two or more beams, thereby increasing the number of output sheets per unit time as compared with the case of forming a full-color image. When forming a full-color image, only one light beam is emitted from the light source unit 53, and the pixel density position of the light spot by the light beam corresponds to the pixel density predetermined according to the high image quality mode or the speed priority mode, respectively. The light spot position control means B controls the pixel density position of the light spot from the black image light source unit 53 by the pixel density switching mechanism A. Color shift can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention writes an image on each of a plurality of image carriers arranged in parallel by an optical writing device, and sets the pixel density when writing the image in a high image quality mode and a speed priority mode. The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a plotter, which is provided with a pixel density switching mechanism that switches between time and time.

[0002]

2. Description of the Related Art Conventionally, light beams emitted from a plurality of light sources are applied to photosensitive drums, which are four image carriers arranged in parallel, and latent images are written on the photosensitive drums. Different latent images above, such as yellow, magenta,
2. Description of the Related Art A color image forming apparatus is known in which a developer is developed with cyan and black toner colors to form a visible image (toner image). In the color image forming apparatus, the toner images of the respective colors formed on the photoconductor drum as described above are sequentially superposed and transferred onto the transfer paper which is carried while being carried on the transfer / conveyor belt, and then fixed. It is fixed by a device to obtain a multicolor image. As such a color image forming apparatus, there is conventionally known one in which a plurality of optical writing devices provided on a plurality of photosensitive drums corresponding to the respective photosensitive drums write a latent image. ing. However, such a configuration requires a large number of optical writing devices that use an optical deflector including a relatively expensive polygon mirror and a driving motor that rotates the polygon mirror, which results in a significant cost reduction. There was a problem that it would be up. Further, if a plurality of optical writing devices each having an optical deflector are provided corresponding to the number of photoconductor drums, a large installation space is required, which causes a problem that the entire image forming apparatus becomes large. It was

Further, even in an image forming apparatus capable of forming a full-color image, a typical office often outputs only a monochrome image rather than a full-color image.
Therefore, in order to increase the productivity in the monochrome image forming mode more than the productivity in the full color image forming mode, the tandem type color image forming apparatus has a monochrome image higher than the linear velocity in the full color image forming mode. In some cases, the linear velocity in the formation mode is increased so that the number of monochrome images output per unit time is greater than the number of full color images output. Further, in an image forming apparatus capable of forming a full-color image, when the full-color image is formed, the number of output sheets per unit time is reduced, but the pixel density is set to 1200 dpi (the linear velocity is slowed to reduce the writing density. Higher image quality mode) and the pixel density at which the image quality drops slightly to 600 dpi (a structure in which the linear velocity is increased and the pixel density is decreased) to emphasize productivity and reduce the number of output sheets per unit time. There is also one that can be switched to the speed priority mode.

In this way, the pixel density is switched so that the number of output sheets per unit time for forming a full-color image is larger than the number of output sheets per unit time (processing speed is high), and the high image quality mode is set. In the image forming apparatus capable of selecting the speed mode and the speed priority mode, for example, there are two light beams for the black (BK) image, and the beam pitches of the two light spots on the photoconductor in the sub-scanning direction. Can be switched, and the light beam of each color of yellow, magenta, and cyan (Y, M, C) has one beam. Then, in the high-quality mode during monochrome image formation, a black (BK) image is formed by two-beam writing with a pixel density of 1200 dpi, and the beam pitch of two light spots on the photoconductor in the sub-scanning direction is linear. The pixel pitch is 1200 dpi, which makes the speed slower.

Further, in the speed priority mode during monochrome image formation, a black (BK) image is formed by two-beam writing with a pixel density of 600 dpi, and a beam pitch of two light spots on the photoconductor in the sub-scanning direction. Is
The beam pitch is set to 600 dpi, which increases the linear velocity. Further, in the high image quality mode during color image formation, the pixel density is set to 1200 dpi, and the light beam for the black (BK) image is written by one beam (only one of the two beams is turned on) to form a black image. , Y, M, and C color images are each written by writing one beam, and the beam pitch in the sub-scanning direction between the light spots on the photoconductor is set to a beam pitch with a pixel density of 1200 dpi at which the linear velocity becomes slow. In addition, in the speed priority mode during color image formation, the pixel density is set to 600 dpi, and a black (BK) image light beam is written by one beam (only one of the two beams is turned on) to form a black image. , Y, M, and C color images are written by writing one beam, and the beam pitch between the light spots on the photoconductor in the sub-scanning direction is a beam pitch with a pixel density of 600 dpi that increases the linear velocity.

[0006]

However, in the case of such a configuration that the pixel density is switched according to the image forming mode at the time of forming a full-color image, the black (B
K) If the switching position of the pixel density of the light beam for the image is not managed, the black image writing position will shift during full-color image formation, resulting in color shift. This point will be described with reference to FIGS. 13 to 15. In the high image quality mode during the color image formation described above, the pixel density is set to 1200 dpi and the black (B
K) Since the light beam for the image forms a black image by writing one beam, the light beam used for the black image formation is the first beam (lower light spot) as shown in FIG. , The light beam of each color of C, M, and Y is formed on the photoconductor with a pixel density of 1200d.
Since writing is performed with a beam pitch of pi, each color image of BK, C, M, and Y (FIG. 14 shows an example of a line image) in the sub-scanning direction (vertical direction in the drawing) as shown in FIG. ) Is written with an evenly spaced beam pitch.

Here, the light beam for the black image (first
If only one of the beams is turned on), but the pixel density is at a position of 600 dpi as shown in FIG. 13B, the light beams of other C, M, and Y colors are on the photoconductor. Since the writing is performed at a beam pitch of 1200 dpi, the distance between the BK image and the C image is narrowed by the beam shift δ shown in FIG. 13 as shown in FIG. 15, and the BK image and other color images are separated from each other. Color shift will occur between them. The present invention has been made in view of the above problems, and when a full-color image is formed by selecting either the high-quality mode or the speed priority mode, a black image and another color image are generated. The purpose is to prevent color misregistration between them.

[0008]

In order to achieve the above object, the present invention provides a plurality of image carriers arranged in parallel and a light spot by a light beam on each surface to be scanned of the plurality of image carriers. An optical writing device that writes a latent image by forming the image forming device and a pixel density switching mechanism that switches the pixel density between a high image quality mode and a speed priority mode are provided, and the optical writing device has two beams for a black image. In the image forming apparatus including the light source unit and the one-beam light source unit for each color for color images other than black, only one light beam is emitted from the light source unit for the black image when outputting a full-color image. Then, the pixel density position of the light spot formed on the surface to be scanned of the image carrier is set to a pixel density predetermined according to the high image quality mode or the speed priority mode. It is provided with a light spot position control means for controlling the position corresponding to the. The light source unit for the black image of the optical writing device may have two beams. Further, the above-mentioned optical writing device divides the light beams from a plurality of light source units into two symmetrical directions by one optical deflector and deflects and scans the light beams.
It is advisable to adopt a configuration in which the light beams are guided and image-formed on the surfaces to be scanned of the plurality of image carriers via the optical systems symmetrically arranged in the direction.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. 1 is a functional block diagram showing a control system for switching the pixel density of an image forming apparatus according to an embodiment of the present invention, FIG. 2 is a schematic configuration diagram showing the entire image forming apparatus, and FIG. 3 is a plan view showing the structure of the optical writing device of the forming apparatus on the upper surface side of the substrate, FIG. 4 is a cross-sectional view showing a cross section taken along the line AA ′ of the optical writing device of FIG. 3, and FIG. FIG. 6 is a schematic diagram showing a light source unit, an optical deflector and an optical system of the writing device, and FIG. 6 is a schematic diagram showing an arrangement configuration of the optical deflector and the optical system of the optical writing device of FIG.
The image forming apparatus shown in FIG. 2 includes a plurality of (four in this example) photoconductive photoconductor drums 1 which are image carriers arranged in parallel.
2, 3 and 4, an optical writing device 5 for writing a latent image by forming a light spot by a light beam on the surface to be scanned of each photoconductor drum, and a pixel density in high image quality mode and speed priority This is a full-color image forming apparatus provided with a pixel density switching mechanism A (see FIG. 1) which will be described later and which is switched between modes.

As will be described in detail later, the optical writing device 5 includes a light source unit having two or more beams for a black image, and cyan (C), magenta (M), and yellow (Y) other than black (BK). For each color image, a light source unit of one beam is provided for each color.
The four photoconductor drums 1, 2, 3, 4 are juxtaposed along the transfer paper conveyance direction.
2, 3, 4 are, for example, black (B
K), cyan (C), magenta (M), yellow (Y)
The image of each color is formed. The order of forming the images of the respective colors is not limited to this, and the order can be set arbitrarily.

Charging portions (charging roller, charging brush, charging charger, etc.) 6, 7 for forming an image by an electrophotographic process are provided around each of the four photosensitive drums 1, 2, 3, 4. , 8, 9 and the light beams L1, L2, L emitted from the optical writing device 5 arranged above them.
3, L4 exposure units, developing units (developing devices for each color of BK, C, M, and Y) 10, 11, 12, and 13, transfer conveying belt 22a, and transfer means (transfer roller, which is arranged on the back surface thereof). (Transfer brush, etc.) 14, 15, 16, 17 and a transfer transport device 22, and a cleaning unit (cleaning blade, cleaning brush, etc.) 18, 19, 20, 21
Etc. are arranged, and each photosensitive drum 1,
Images of respective colors can be formed on 2, 3, and 4. Here, in FIG. 2, when the Z direction in the drawing is a vertical upward direction and the X and Y directions are horizontal directions, the four photoconductor drums 1, 2, 3, 4 are arranged in a horizontal plane (a plane in the X direction). ) Is inclined to.

The transfer / transport device 22 has a transfer / transport belt 22a stretched between a drive roller and a plurality of driven rollers, and is rotated in the direction of the arrow in FIG. 2 by the rotation of the drive roller. The surface of the transfer / conveying belt 22a facing the photoconductor drums 1, 2, 3, 4 is substantially parallel to the direction in which the four photoconductor drums 1, 2, 3, 4 are arranged side by side. , Is inclined with respect to the horizontal plane. Below the transfer / transport device 22, there are installed paper feed units 23 and 24 for accommodating a transfer paper P which is a transfer material such as a recording paper. Paper is fed to the transfer / transport belt 22a via the paper feed roller, the transport roller, and the registration roller 25, and the transfer paper P is carried on the transfer / transport belt 22a. The transfer / transport device 22
A fixing device 26 is disposed on the downstream side of the transfer paper conveyance direction. The optical writing device 5 includes four photosensitive drums 1, 2,
A housing 50 of the optical writing device 5 is arranged diagonally above an image forming unit in which 3 and 4 are juxtaposed, and is substantially parallel to the juxtaposed direction of the four photoconductor drums 1, 2, 3 and 4. And the inclined frames 29 and 30 of the image forming apparatus main body, which are inclined with respect to the horizontal plane (the surface in the X direction in FIG. 2).
It is fixed to.

The optical writing device 5 includes four light source units 52, 53, 5 as shown in FIGS. 3 and 4, for example.
4, 55, an optical deflector 62 for deflecting and scanning the light beams L1, L2, L3, L4 shown in FIG. 5 from each light source unit in two symmetrical directions, and with the optical deflector 62 as the center, An optical system that guides a plurality of light beams L1, L2, L3, L4 deflectively scanned by the optical deflector 62 symmetrically in two directions onto the surfaces to be scanned of the corresponding photoconductor drums 1, 2, 3, 4 to form an image. It has a system. As shown in FIG. 4, the optical system includes imaging lenses 63, 64, 69,
70, 71, 72 and mirrors 65, 6 for folding the optical path
6,67,68,73,74,75,76,77,7
It is composed of optical members such as 8, 79 and 80. The members forming these optical systems are housed in a single housing 50. The housing 50 is shown in FIG.
As will be clearly shown in FIG. 5, it has a base 50A on which the optical deflector 62 and the optical system are arranged, and a frame-shaped side wall 50B surrounding the periphery of the base 50A. The base 50A is the side wall 50.
The light source unit 52, 53,
54 and 55 are arranged on the side wall 50B of the housing 50 and arranged side by side in substantially the same direction as the direction in which the photosensitive drums 1 to 4 (FIG. 2) are arranged.

Further, the optical deflector 62 is arranged at a substantially central portion of the base 50A of the housing 50, and forms image forming lenses 63, 64, 69, 70, 71, 72.
And the optical path folding mirrors 65, 66, 67, 68, 7
Optical members such as 3, 74, 75, 76, 77, 78, 79, 80 are separately arranged on both upper and lower surfaces of the base 50A. Further, as shown in FIG. 4, covers 88 and 87 are provided on the upper and lower parts of the housing 50, respectively, and an opening for passing a light beam is formed in the lower cover 87, and the opening is formed in the opening. Dustproof glass 8
3, 84, 85 and 86 are attached respectively.
The optical writing device 5 converts color-separated image data input from a document reading device (scanner) or an image data output device (personal computer, word processor, facsimile receiving unit, etc.) into a signal for driving a light source. The light source (semiconductor laser (LD)) in each of the light source units 52, 53, 54, 55 shown in FIG. 5 is driven according to the converted signal and the light beam is emitted.

Each of the light source units 52, 53, 54, 5
The light beam emitted from 5 reaches the optical deflector 62 either directly through the cylindrical lenses 56, 57, 58 and 59 for plane tilt correction or via the mirrors 60 and 61. Then, the light beam is the polygon motor 62 shown in FIG.
a two-stage polygon mirror 62a which is rotated at a constant speed by c,
Deflection scanning is performed in two symmetrical directions at 62b. In the optical writing device 5, the optical deflector 62 has a structure in which the polygon mirror 62b used for the L1 and L4 light beams and the polygon mirror 62a used for the L2 and L3 light beams are divided into two stages, upper and lower. Although an optical deflector is used, the optical deflector may be configured to deflect and scan all of the light beams L1 to L4 with one thick polygon mirror.

The polygon mirror 62 of the optical deflector 62
The light beams, which are deflected and scanned in two directions by two beams by a and 62b, respectively pass through image forming lenses 63 and 64 formed of, for example, an upper and lower two-layered fθ lens, and the first folding mirrors 65, 66, and 67. , 68, and after passing through the opening of the base 50A, the second imaging lenses 69, 70, 7 made of, for example, a long toroidal lens or the like.
1, 72, and the second folding mirrors 73, 75, 7
7, 79, third folding mirror 74, 76, 78, 8
0, through the dust-proof glass 83, 84, 85, 86 is irradiated onto the surface to be scanned of the photoconductor drums 1, 2, 3, 4 for each color, and an electrostatic latent image is written there. The optical writing device 5 is provided with a synchronization detection mirror (not shown) for extracting the light beam at the scanning start position in the main scanning direction in the optical path of each of the light beams L1, L2, L3, L4, and the synchronization detection is performed. The light flux reflected by the mirror for use is received by the synchronization detectors 81 and 82 as shown by the broken line in FIG. 6, and the synchronization signal for starting scanning is output.

The third folding mirrors 74, 76, 78 arranged in the optical paths of the light beams L1, L2, L3 have stepping motors 92, 93, 93 for skew adjustment.
By providing 94, the deviation of the scanning line positions of the light beams of L1, L2, and L3 can be corrected with reference to the scanning line position of the light beam of L1. The scanning direction of the light beam deflected and scanned by the optical deflector 62 is the main scanning direction, which is the axial direction of each of the photosensitive drums 1 to 4.
Further, the direction orthogonal to the main scanning direction is the sub-scanning direction, which is the rotation direction of the photosensitive drums 1 to 4 (movement direction of the surface of the photosensitive drum), and further the transfer / conveying belt 2.
It is also the conveying direction of 2a. That is, the transfer / transport belt 2
The width direction of 2a is the main scanning direction, and the conveyance direction (movement direction) of the transfer conveyance belt 22a is the sub scanning direction.

In the image forming apparatus shown in FIG. 2, the latent images on the photosensitive drums 1, 2, 3, 4 are transferred to the developing units 10, 1 respectively.
1, 12, and 13 are developed with toners of BK, C, M, and Y colors and visualized, and the visualized BK, C, M, and
The toner image of each color of Y is transferred by the transfer means 14, 15, 16, 17 of the transfer / transport device 22 to the transfer / transport belt 22.
The images are sequentially superposed and transferred onto the transfer paper carried on a. Then, the transfer paper on which the four-color superimposed image is transferred is conveyed to the fixing device 26, where the image is fixed, and then the paper is discharged onto the paper discharge tray 28 by the paper discharge roller 27. In the black-and-white image forming mode, the above-described image forming operation is performed only on the black photosensitive drum 1.

The four light source units 52, 53, 54, 55 of the optical writing device 5 shown in FIGS. 3 and 5 respectively collimate the semiconductor laser (LD) which is the light source and the luminous flux emitted by the semiconductor laser. And a collimating lens, which are integrally incorporated in a holding member such as a holder. And these four light source units 52, 53, 5
Of the light sources 4, 55, a light source unit 53 for black (used for the other light source units 52,
54 or 55), two or more light sources (LD) and a collimator lens corresponding to each light source are held in order to improve productivity during high speed writing by performing high speed writing. It has a multi-beam structure in which members are integrally held. The light source unit 53 has semiconductor lasers 111 and 112, which are light sources, fixed to supports 113 and 114, respectively, as shown in FIGS. Then, the semiconductor lasers 111, 1
Screws 118, 119, etc. are provided on the back surface of the base body 115, which is a holding member and serves as a collimator lens holder, with the support bodies 113, 114 to which 12 is fixed, respectively aligned with the optical axes of collimator lenses 116, 117 described later. It is fixed by using.

The collimating lenses 116 and 117 are for converting a light beam into a parallel light beam, each of which is housed in a lens barrel, and the fitting hole 11 formed in the base 115.
The semiconductor lasers 111 and 112 are engaged with the semiconductor lasers 111 and 112 in alignment with the semiconductor lasers 5a and 115b, respectively, and are bonded by an adhesive in that state. The collimating lenses 116, 1
The exit sides of 17 are provided with diaphragm plates 120, 120 for obtaining a predetermined beam diameter for the outgoing light, respectively, and a beam combining means 121 composed of a prism or the like is provided in front of them. There is. Two semiconductor lasers 11
1, 112 are arranged on the same plane with their pn junction surfaces aligned, and one of the beams (the beam on the semiconductor laser 111 side in this example) is the beam combining means 12.
The half-wave plate 122 attached to the first incident surface rotates the polarization plane by 90 °, and the beam combining means 12
It passes through the first polarization beam splitter surface 121b. The beam of the semiconductor laser 112 is internally reflected by the inclined surface 121 a of the beam combining means 121, and the polarization beam splitter surface 121 of the beam combining means 121.
It is reflected by b and is combined with the beam of the semiconductor laser 111 in the vicinity of the optical axis of the reference semiconductor laser 111.

Incidentally, the respective optical axes of the semiconductor lasers 111 and 112 are mutually offset by an angle θ (FIG. 7) shown on the output side of the beam synthesizing means 121 so as to correspond to positions slightly shifted from each other in the main scanning direction. It is designed to be displaced.
The beam synthesizing means 121 and the diaphragm plate 120 are supported at predetermined positions on the back surface of the flange member 123, and the flange member 123 is fixed to the base 115 by screws 124 and 125. Then, the flange member 123 and the base body 1
15 is fixed to a substrate 126 provided with a drive circuit for the semiconductor lasers 111 and 112 with screws or the like (not shown).
Each member of the optical path from 1,112 to the flange member 123 constitutes a light source unit integrally fixed to the substrate 126.

The flange member 12 of the light source unit 53
The cylindrical portion 123a standing on the emitting side of 3 is inserted into the hole 132a of the optical frame 132 provided in the side wall 50B of the housing 50 (FIG. 3) of the optical writing device 5 described above, and is inserted into the spring 130. Through the hole 131a of the spring pressing plate 131. In this state, the semiconductor laser 111,
The light source unit 53 in which the members from 112 to the flange member 123 are assembled to the substrate 126 is pulled in the direction of the arrow α in FIG. 7, and the spring pressing plate 131 is rotated 90 ° to project the protrusions of the spring pressing plate 131. 131b is hooked on the protrusion 123b of the cylindrical portion 123a. Accordingly, the light source unit 53 is rotatably attached to the optical frame 132 with the center of the cylindrical portion 123a of the flange member 123 (corresponding to the optical axis) as the rotation center.

Next, it corresponds to the interval (beam pitch) in the sub-scanning direction between the two light spots emitted from the two semiconductor lasers 111 and 112 of the light source unit 52 and imaged on the photosensitive drum 1. An example of a pixel density switching mechanism that switches the pixel density by rotating the light source unit 53 around the optical axis in the high image quality mode and the speed priority mode will be described. In FIG. 7, reference numeral 127 is a sliding member, and 128 is a feed screw. A male screw having a nominal diameter M3 is cut on the feed screw 128, and a female screw M3 is cut inside the sliding member 127. Also,
The outer shape of the sliding member 127 is substantially D-shaped.

The male screw portion of the feed screw 128 is previously rotationally inserted into the female screw portion of the sliding member 127 to form a D-shaped cylinder 132b provided on the optical frame 132 side of the housing 50 (FIG. 3). The sliding member 127 is slidably inserted into the hole. At that time, the column 132b of the optical frame 132
The rotary shaft 129a of the pitch varying stepping motor 129 is inserted into the hole of (1), and the lower end side of the feed screw 128 is press-fitted and fixed to the tip of the rotary shaft 129a. The pitch changing stepping motor 129 is
The main body is fixed to the optical frame 132, and by rotating the pitch varying stepping motor 129, the feed screw 128 press-fitted into the rotating shaft 129a of the motor is rotated. Since the hole of the column 132b is D-shaped, the sliding member 1
27 does not rotate but slides only in the vertical direction.

On the other hand, the flange member 1 of the light source unit 53
23, an arm 123c is formed that extends in the direction of the sliding member 127 and has a tip end portion that abuts the upper end of the sliding member 127. A spring 135 is provided between the arm 123c and the optical frame 132 so as to exert a biasing force in a direction in which the arm 123c and the optical frame 132 are attracted to each other, and the lower surface of the tip of the arm 123c is formed in a planar shape of the sliding member 127 by the spring 135. It can be pressed against the upper surface. Accordingly, when the pitch varying stepping motor 129 is rotated, the sliding member 127 slides in the vertical direction, so that the arm 123c of the flange member 123 moves in the vertical direction. As a result, the light source unit 53 rotates about the center of the cylindrical portion 123a of the flange member 123 as the center of rotation.

As means for controlling the rotation angle of the light source unit 53, the optical frame 132 has a light emitting section 133a.
An optical home position sensor 133 including a photo interrupter including a light receiving portion 133b and a light receiving portion 133b is fixed by a screw or the like (not shown). In addition, the flange member 12
The filler 123 having an edge portion 123e capable of shielding between the light emitting portion 133a and the light receiving portion 133b of the home position sensor 133 on a part of the third side opposite to the arm 123c.
d is provided. Then, the edge portion 123e of the filler 123d of the flange member 123 is set so that the position at the moment when the edge portion 123e of the filler 123d shields the light emitting portion 133a and the light receiving portion 133b of the home position sensor 133 becomes the home position (HP), The home position is used as a reference for rotation adjustment of the light source unit 53. As described above, in this embodiment, the sliding member 127, the feed screw 128,
The optical frame 132 having the cylinder 132b, the pitch changing stepping motor 129, the flange member 123 having the arm 123c and the filler 123d, and the home position sensor 133 function as a pixel density switching mechanism A (see FIG. 1).

Next, a method of changing the beam pitches of the two light spots in the sub-scanning direction by the above-described pixel density switching mechanism A will be described with reference to FIG. In FIG. 9, the broken line indicates the light emitting unit 13 of the home position sensor 133.
3a and the light receiving portion 133b are shown, and the shielding filler 12 provided on the flange member 123 as described above.
The edge portion 123e of 3d has a light emitting portion 133a and a light receiving portion 13.
The home position (HP) is the position at the moment when the space between 3b is blocked. Further, position B in the figure shows the position where the light source unit 53 is rotated from the home position (HP) about the optical axis by the angle θ _1, but the pitch is variable to obtain this rotation angle. The stepping motor 129 (FIG. 7) rotates by a predetermined number of pulses to move the sliding member 127 upward by a predetermined amount, and the light source unit 5 is moved.
Rotate 3 by an angle θ ₁ . Similarly, at position A in the figure, the light source unit 53 is at the home position (H.
P. ) Indicates the position rotated by an angle θ ₂ with the optical axis as the center of rotation.

Here, the light source unit 53 is set to the angles θ ₁ and θ.
_{In the} case of rotating it to ₂ , the rotation center is adjusted with one of the light spots from the two semiconductor lasers as the rotation center as shown in FIG.
In some cases, as shown in FIG. 1, the center position between the two light spots is used as the center of rotation for rotation adjustment. In the case of FIG. 10, when the light source unit 53 is rotated by an angle θ ₁ , it becomes 2
The beam pitch between the light spots emitted from the two semiconductor lasers on the photoconductor drum is P ₁ , and when rotated by an angle θ ₂ , the beam pitch is P ₂ . In the case of FIG. 11, when the light source unit 53 is rotated by the angle θ ₁ , the beam pitch of the light spots emitted from the two semiconductor lasers on the photoconductor drum becomes P ₃ , and when the angle θ _{2 is} rotated, the beam pitch is changed. The pitch is P ₄ , and the beam pitch between the two light spots is larger than that in the case of FIG.

In this way, the light source unit 53 is rotated about one of the two light spots as the center of rotation, or the light source unit 53 is rotated about the central position between the two light spots. Either of these methods can adjust the beam pitch (distance in the sub-scanning direction) of the two light spots on the photosensitive drum. Then, the rotation angle of the light source unit 53 is the same as that of the stepping motor 129 described with reference to FIG. P. From the H.V. corresponding to the above angles θ ₁ and θ ₂ . P. It is possible to easily control by obtaining the number of drive pulses from the above and rotating the stepping motor 129 by the number of drive pulses. By the way, in an image forming apparatus in which the beam pitches of a plurality of light spots on the photosensitive drum in the sub-scanning direction are changed according to the recording density, for example, two light beams on the photosensitive drum at a recording density of 600 dpi are used. When the beam pitch P of the spot in the sub-scanning direction is set to P = 42 μm, P. The number of pulses Pa of the stepping motor 129 from the above, and the beam pitch P = 21 μm when the recording density is 1200 dpi.
H. when setting to H. P. If the number of pulses Pb from P is stored in the memory of the control unit in the image forming apparatus or the like, the stepping motor 129 is rotated according to the required recording density in the high image quality mode and the speed priority mode. Thus, the beam pitch of the light spot on the photosensitive drum in the sub-scanning direction can be easily switched.

In this image forming apparatus, when the power is turned on, the light source unit 53 (for black image) is at a predetermined position, for example, the rotation angle at the recording density of 600 dpi (see FIG. 1).
The rotation is controlled to 0, position B) in FIG. This is because once the power is turned on, the H. P. After homing, the stepping motor 129 is driven in the predetermined direction by the number of pulses Pa to set the beam pitch when the pitch of the two light spots for the black image on the photoconductor drum in the sub-scanning direction is 600 dpi. This is to rotate the light source unit 53 so that Thereafter, the rotation angle for rotating the light source unit 53 is stored in the RAM which is the data memory of the control device 40 (FIG. 12) described later in association with the above-described recording density 1200 dpi in the high image quality mode and recording density 600 dpi in the speed priority mode. Since it is stored, when a recording density of 1200 dpi is requested, 60
Stepping motor 129 from the position at 0 dpi [P
b-Pa] to rotate the light source unit 53 so as to change the beam pitch in the sub-scanning direction of the two light spots for the black image on the photosensitive drum to the beam pitch when the recording density is 1200 dpi. Rotate.

By the way, as described with reference to FIGS. 13 to 15, when the pixel density is switched according to the image forming mode, a black (BK) image is formed at the time of full color image formation. If the switching position of the pixel density of the light beam is not managed, the shift amount δ shown in FIG.
However, as shown in FIG. 15, the distance between the BK image and the C image is narrowed, and a color shift may occur between the BK image and another color image. Therefore, in the image forming apparatus according to this embodiment, as shown in FIG. 1, the light spot position control means B for driving and controlling the pixel density switching mechanism A is provided. Then, the light spot position control means B emits only one light beam located closer to the cyan beam from the light source unit 53 for the black (BK) image at the time of outputting the full-color image, and the photoconductor drum 1 The pixel density position of the light spot formed on the surface to be scanned of is set to the high image quality mode (1200 dp
i) or the speed priority mode (600 dpi) is controlled to the position corresponding to a predetermined pixel density.

The light spot position control means B is shown in FIG.
Is a control device 40 as shown in FIG.
Is a central processing unit (CP) that has various judgment and processing functions.
U) 41, a ROM 42 that stores each processing program and fixed data, a RAM 43 that is a data memory that stores the processing data, and an input / output circuit (I / O) 44. This control device 40
Inputs signals such as the number of images to be formed and image forming modes such as a high image quality mode and a speed priority mode from an operation panel 45 provided in the image forming apparatus. Further, various sensor signals are input at predetermined timings from sensors provided in various places of the image forming apparatus. Then, it outputs signals for driving various loads such as various drive systems and display systems for image formation at predetermined timings. In addition, the stepping motor 129 of the pixel density switching mechanism A described above is driven to drive the signal to switch the beam pitch to a pitch corresponding to the pixel density 1200 dpi in the high image quality mode and the pixel density 600 dpi in the speed priority mode. Output via 47.

The control device 40 uses the operation panel 45 to display a full-color image in a high-quality mode (pixel density 1200
When (dpi) is selected, the light beam for the black image is emitted from only one beam (semiconductor laser) located closer to the cyan beam from the light source unit 53, and the surface to be scanned of the photosensitive drum 1 is emitted. Form a light spot on top. At this time, in this image forming apparatus, as described above, the light source unit 53 (for BK image) is set to 60 when the power is turned on.
It is rotated to the rotation angle at the pixel density of 0 dpi (the rotation angle at the position B described in FIGS. 10 and 11). Therefore, if writing is started with the light spot position for the BK image kept at the same pixel density of 600 dpi, C, M, and Y other than the BK image have a pixel density of 1
Since the writing is performed at the light spot position of 200 dpi, color misregistration occurs between the BK image and another color image as described in FIG.

However, since this image forming apparatus is provided with the light spot position control means B as described above, when the light spot position control means B selects the high image quality mode (pixel density 1200 dpi) for a full color image. , BK image light source unit 53 with a pixel density of 600d
By rotating the stepping motor 129 from the position of pi, it is controlled to move to the position of the pixel density of 1200 dpi. Therefore, from the light source unit 53 for the BK image, one of the two semiconductor lasers moved to the position of the pixel density of 1200 dpi, which is located closer to the cyan beam (the lower side in FIG. 11). (A semiconductor laser on the side corresponding to the first beam light spot), a light beam is emitted to form a light spot for a BK image on the surface to be scanned of the photosensitive drum 1. As a result, as shown in FIG. 14, images of colors BK, C, M, and Y are formed at beam pitches of equal intervals corresponding to a pixel density of 1200 dpi, so that a full-color image without color misregistration can be obtained. .

In the case of an image forming apparatus in which the speed priority mode (pixel density 600 dpi) can be selected for a full-color image, the light source units for C, M, and Y are also located at pixel densities 1200 dpi and 600 dpi, respectively. Is required, and when the speed priority mode is selected for full color images, C, M, Y
The light source unit for each color is set to the position of the pixel density of 600 dpi, and the light source unit for the BK image is set to 6 when the power is turned on.
Since it is rotated to the pixel density position of 00 dpi, control is performed so as to start writing the BK image at that position. As a result, a full-color image with no color shift can be obtained.

[0036]

As described above, according to the present invention, the following effects can be obtained. According to the image forming apparatus of claims 1 and 2, when a monochrome image is formed, a light source unit for a black image is used to write by using two or more beams, and thus a unit of time per unit time is longer than that when a full color image is formed. It is possible to improve the productivity by increasing the number of output sheets of. Then, when outputting a full-color image, only one light beam is emitted from the light source unit for the black image, and the pixel density position of the light spot by the light beam is determined in advance according to the high image quality mode or the speed priority mode. Since the position is controlled to the position corresponding to the pixel density, even if the pixel density switching mechanism switches the pixel density position of the light spot for the black image, the black image causes color misregistration with respect to images of other colors. You can avoid it. Further, according to the image forming apparatus of the third aspect, since the optical writing device is used in which the light beams from the plurality of light source units are deflected and scanned by being divided into two symmetrical directions by one optical deflector, a plurality of images are formed. Since it is not necessary to provide a relatively expensive optical deflector for each carrier, the cost can be reduced and the entire apparatus can be downsized.

[Brief description of drawings]

FIG. 1 is a functional block diagram showing a control system for switching a pixel density of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram showing the entire image forming apparatus.

FIG. 3 is a plan view showing the configuration of the optical writing device of the image forming apparatus on the upper surface side of the base.

FIG. 4 is a cross-sectional view showing a cross section taken along line AA ′ of the optical writing device of FIG.

5 is a schematic diagram showing a light source unit, an optical deflector, and an optical system of the optical writing device of FIG.

FIG. 6 is a schematic diagram showing an arrangement configuration of an optical deflector and an optical system of the optical writing device of FIG.

FIG. 7 is an exploded perspective view showing a configuration of a multi-beam light source unit.

FIG. 8 is a sectional view of the main part of the same multi-beam light source unit.

FIG. 9 is an explanatory diagram for explaining a method of varying a beam pitch of two light spots in a sub scanning direction by a light spot position control unit.

FIG. 10 is an explanatory diagram showing an example in which one of the light spots from the two semiconductor lasers is rotated about the center of rotation when the light source unit is rotated.

FIG. 11 is an explanatory diagram showing an example in the case of rotating the light source unit with the center position between the two light spots as the center of rotation.

FIG. 12 is a block diagram showing a specific configuration of a light spot position control unit and its related configuration.

FIG. 13 shows a light beam for a black image with a pixel density of 120.
It is an explanatory view for explaining a gap of a light spot position when it is set to 0 dpi and 600 dpi.

FIG. 14 is an explanatory diagram showing a line image when the light beam for the black image is written at a writing position with a pixel density of 1200 dpi together with other color line images.

FIG. 15 is an explanatory diagram showing a line image when the light beam for the black image is written at a writing position with a pixel density of 600 dpi together with other color line images.

[Explanation of symbols]

1, 2, 3, 4: Photoreceptor drum (image carrier) 5: Optical writing device 40: Control device 52, 53, 54, 55: Light source unit 62: Optical deflector 123: Flange member 123c: Arm 123d: Filler 127: Sliding member 128: Feed screw 129: Stepping motor for variable pitch 132: Optical frame 132b: Cylinder 133: Home position sensor A: Pixel density switching mechanism

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2C362 BA51 BA52 BA53 BA57 BA66                       BA68 BA71 CA17 CA18 CA22                       CA39 CB02 CB06 CB08 CB62                       CB63 CB64                 2H030 AB02 AD06 AD17 BB02 BB13                       BB23 BB63                 5C074 AA10 BB17 CC22 DD04 DD12                       DD24 DD28 EE02 FF15                 5C076 AA21 AA22 BA02

Claims (3)

[Claims] 1. A plurality of image carriers arranged in parallel, an optical writing device for writing a latent image by forming a light spot by a light beam on a surface to be scanned of the plurality of image carriers, and a pixel. A pixel density switching mechanism for switching the density between a high image quality mode and a speed priority mode is provided, and the optical writing device has a light source unit of two beams or more for a black image and each color for a color image other than black. In an image forming apparatus having a 1-beam light source unit, only a 1-beam light beam is emitted from the light source unit for the black image when a full-color image is output and is formed on a surface to be scanned of the image carrier. The pixel density position of the light spot to be controlled is set to a position corresponding to a predetermined pixel density according to the high image quality mode or the speed priority mode. An image forming apparatus characterized in that a spot position control means. 2. The image forming apparatus according to claim 1, wherein the light source unit for the black image of the optical writing device has two beams. 3. The optical writing device divides the light beams from the plurality of light source units into two symmetrical directions by a single optical deflector and deflects and scans the beams, and the optical deflector is used as a center for the two directions. 3. The image forming apparatus according to claim 1, wherein the image forming apparatus has a configuration in which a light beam is guided to form an image on a surface to be scanned of the plurality of image carriers via optical systems arranged symmetrically with respect to each other.