JP4425505B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
This invention writes pixel images on a plurality of image carriers arranged in parallel by an optical writing device, and switches the pixel density at the time of writing the image between 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 having a mechanism.
[0002]
[Prior art]
Conventionally, light beams emitted from a plurality of light sources are respectively applied to the photosensitive drums, which are four image carriers arranged side by side, and latent images are written there, and the latent images on the image carriers are written. For example, there are known color image forming apparatuses that are developed with different developer colors of yellow, magenta, cyan, and black to form visible images (toner images).
In the color image forming apparatus, the toner images of the respective colors formed on the photosensitive drum as described above are sequentially superimposed and transferred onto a transfer sheet carried on a transfer conveyance belt or the like, and then fixed. A multicolor image is obtained by fixing with an apparatus.
As such a color image forming apparatus, conventionally, a latent image is written on a plurality of photosensitive drums by a plurality of optical writing devices provided corresponding to the respective photosensitive drums. ing.
However, with such a configuration, the optical writing device using the optical deflector composed of a relatively expensive polygon mirror and a driving motor for rotating the polygon mirror is required as many as the number of photosensitive drums. There was a problem of getting up.
In addition, if a plurality of optical writing devices having optical deflectors are provided in the number corresponding to the number of photosensitive drums, a large installation space is required, so that the entire image forming apparatus is increased in size. It was.
[0003]
Further, even in an image forming apparatus capable of forming a full-color image, a general office often outputs only a monochrome image rather than a full-color image. Therefore, in the tandem color image forming apparatus, in order to increase the productivity in the monochrome image forming mode more than the productivity in the full color image forming mode, the monochrome image is higher than the linear velocity in the full color image forming mode. In some cases, the number of output images per unit time of a monochrome image is made larger than the number of output images of a full-color image so as to increase the linear velocity in the formation mode.
In addition, in an image forming apparatus capable of forming a full-color image, the number of outputs per unit time is reduced during full-color image formation, but the pixel density is set to 1200 dpi with a focus on image quality (the writing speed is reduced by reducing the linear velocity). The high image quality mode and the pixel density that slightly lowers the image quality are 600 dpi (configuration in which the linear velocity is increased and the pixel density is lowered), thereby placing importance on productivity and the number of output per unit time. Some models can be switched to the speed priority mode.
[0004]
In this way, high-quality mode and speed priority are achieved by switching the pixel density so that the number of output sheets when forming a monochrome image is higher than the number of output sheets per unit time when forming a full-color image (the processing speed is fast). In the image forming apparatus in which the mode can be selected, for example, there are two light beams for black (BK) image, and the beam pitch in the sub-scanning direction of the two light spots on the photosensitive member of the light beam is switched. The light beams of each color of yellow, magenta, and cyan (Y, M, C) are configured as one beam.
When a monochrome image is formed and in the high quality mode, a black (BK) image is formed by two-beam writing with a pixel density of 1200 dpi, and the beam pitch in the sub-scanning direction of the two light spots on the photoconductor is linear. The beam pitch is a pixel density of 1200 dpi at which the speed decreases.
[0005]
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 the beam pitch in the sub-scanning direction of the two light spots on the photoconductor is linear. The beam pitch is a pixel density of 600 dpi that increases the speed.
Further, when the color image is formed and in the high image quality mode, the pixel density is set to 1200 dpi, and the black (BK) image light beam is formed by writing one beam (only one of the two beams is turned on) to form a black image. Each of the color images of Y, M, and C is written by writing one beam, and the beam pitch in the sub-scanning direction between the respective light spots on the photosensitive member is a beam pitch of 1200 dpi at which the linear velocity becomes slow.
In the speed priority mode at the time of color image formation, the pixel density is set to 600 dpi, and the black (BK) image light beam is formed by writing one beam (only one of the two beams is turned on) to form a black image. , Y, M, and C are written by writing one beam, and the beam pitch in the sub-scanning direction between the light spots on the photosensitive member is set to a beam pitch of a pixel density of 600 dpi at which the linear velocity is increased.
[0006]
[Problems to be solved by the invention]
However, when the pixel density is switched according to the image forming mode at the time of forming a full-color image as described above, the full-color image must be managed unless the pixel density switching position of the light beam for black (BK) image is managed. When the image is formed, the black image writing position shifts, resulting in a color shift.
This point will be described with reference to FIGS.
At the time of color image formation described above and in the high image quality mode, the pixel density is 1200 dpi, and the black (BK) image light beam forms a black image by writing one beam. Therefore, the light beam used for the black image formation Is the first beam (lower light spot) as shown in FIG. 13A, the light beams of C, M, and Y colors are also formed on the photosensitive member at a beam density of 1200 dpi. As shown in FIG. 14, BK, C, M, and Y color images (FIG. 14 shows an example of line images) are equally spaced in the sub-scanning direction (vertical direction in the figure). Written at beam pitch.
[0007]
Here, assuming that the light beam for black image (only one beam of the first beam is lit) has a pixel density of 600 dpi as shown in FIG. Since the light beams of each color of C, M, and Y are written with a beam density of a pixel density of 1200 dpi, as shown in FIG. 15, the interval between the BK image and the C image is narrowed by the beam deviation δ shown in FIG. As a result, a color shift occurs between the BK image and another color image.
The present invention has been made in view of the above-described problems, and even if a full color image is formed by selecting either the high image quality mode or the speed priority mode, the black image and the other color image are formed. The purpose is to prevent color misregistration between them.
[0008]
[Means for Solving the Problems]
  In order to achieve the above-described object, the present invention provides a plurality of image carriers arranged side by side, and light for writing a latent image by forming a light spot by a light beam on the scanned surface of each of the plurality of image carriers. A writing device and a pixel density switching mechanism for switching a pixel density between a high image quality mode and a speed priority mode. The optical writing device includes a light source unit of two or more beams and a color other than black for a black image. In an image forming apparatus having a light source unit of one beam for each color for an image,
  The pixel density position of the light spot of one beam for each color for the color image can be switched between the high image quality mode and the speed priority mode.
  When a full color image is output, a light beam is emitted from only one beam located on the side close to the color image beam from the light source unit for the black image to form a light spot on the surface to be scanned of the image carrier. The pixel density position of the spot for each color for the color image according to the high image quality mode or speed priority modeofLight spot position control means is provided for controlling the pixel density positions of the light spots of the respective colors to be formed at equal intervals of the beam pitch according to the pixel density position of the light spot of one beam.
  The light source unit for black image of the optical writing device may be two beams.
Further, the optical writing device distributes and scans light beams from a plurality of light source units in two symmetrical directions by one optical deflector, and is arranged symmetrically in two directions around the optical deflector. It is preferable that the light beam is guided and imaged on the scanned surfaces of the plurality of image carriers via the optical system.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 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. 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. 5 is a cross sectional view of the optical writing device of FIG. 6 is a schematic diagram showing a light source unit, an optical deflector, and an optical system of the writing apparatus, and FIG. 6 is a schematic diagram showing an arrangement configuration of the optical deflector and the optical system of the optical writing apparatus of FIG.
The image forming apparatus shown in FIG. 2 includes a plurality of (four in this example) photoconductive photosensitive drums 1, 2, 3, 4 that are arranged side by side, and the photosensitive drums of the photosensitive drums. An optical writing device 5 for writing a latent image by forming a light spot by a light beam on the scanning surface, and a pixel density switching mechanism A (described later) for switching the pixel density between the high image quality mode and the speed priority mode. 1)).
[0010]
As will be described in detail later, the optical writing device 5 includes a light source unit having two or more beams for black images, and color images of cyan (C), magenta (M), and yellow (Y) other than black (BK). For each color, a light source unit of one beam is provided for each color.
The four photosensitive drums 1, 2, 3, and 4 are juxtaposed along the transfer paper conveyance direction, and the four photosensitive drums 1, 2, 3, and 4 are, for example, sequentially from the right in FIG. An image of each color of black (BK), cyan (C), magenta (M), and yellow (Y) is formed.
Note that the order in which the images of the respective colors are formed is not limited to this, and the order can be arbitrarily set.
[0011]
Around each of the four photosensitive drums 1, 2, 3, 4 is a charging unit (charging roller, charging brush, charging charger, etc.) 6, 7, 8,. 9, an exposure unit for the light beams L 1, L 2, L 3, and L 4 emitted from the optical writing device 5 disposed on the upper side, and a development unit (development unit for each color of BK, C, M, and Y) 10. 11, 11, 12, 13, a transfer conveyance device 22 including a transfer conveyance belt 22 a and transfer means (transfer rollers, transfer brushes, etc.) 14, 15, 16, 17 disposed on the back surface thereof, and a cleaning unit (cleaning blade) Cleaning brushes 18, 19, 20, 21, etc. are arranged so that respective color image formations can be performed on the photosensitive drums 1, 2, 3, 4.
Here, in FIG. 2, when the Z direction in the drawing is a vertically upward direction and the X and Y directions are horizontal directions, the parallel arrangement direction of the four photosensitive drums 1, 2, 3, 4 is a horizontal plane (surface in the X direction). ).
[0012]
The transfer / conveying device 22 has a transfer / conveying belt 22a stretched between a driving roller and a plurality of driven rollers, and is rotated in the direction of the arrow in FIG. 2 by the rotation of the driving roller. The surface of the transfer / conveying belt 22a that faces the photosensitive drums 1, 2, 3, and 4 is substantially parallel to the juxtaposed direction of the four photosensitive drums 1, 2, 3, and 4. Inclined with respect to the horizontal plane.
Below the transfer conveyance device 22, paper feed units 23 and 24 that store transfer paper P serving as a transfer material such as recording paper are installed, and the transfer paper P stored in the paper feed units 23 and 24 is placed on the transfer paper P. Then, the sheet is fed to the transfer / conveying belt 22a via the sheet feeding roller, the conveying roller, and the registration roller 25, and the transfer paper P is carried on the transfer / conveying belt 22a.
A fixing device 26 is disposed downstream of the transfer conveyance device 22 in the transfer paper conveyance direction.
The optical writing device 5 is disposed obliquely above an image forming unit in which four photosensitive drums 1, 2, 3, and 4 are arranged in parallel, and a housing 50 of the optical writing device 5 includes four photosensitive drums. Inclined frames 29, which are arranged to be inclined with respect to a horizontal plane (surface in the X direction in FIG. 2) so as to be substantially parallel to the direction in which the body drums 1, 2, 3, 4 are arranged in parallel. 30 is fixed.
[0013]
As shown in FIG. 3 and FIG. 4, the optical writing device 5 includes four light source units 52, 53, 54, and 55, and light beams L1, L2, and L3 shown in FIG. , L4 are distributed in two symmetrical directions and deflected and scanned, and a plurality of light beams L1 and L2 deflected and scanned by the optical deflector 62 symmetrically in the two directions around the optical deflector 62. , L3, and L4 are respectively provided on the scanned surfaces of the corresponding photosensitive drums 1, 2, 3, and 4, and an optical system is formed. As shown in FIG. 4, the optical system includes imaging lenses 63, 64, 69, 70, 71, 72 and optical path folding mirrors 65, 66, 67, 68, 73, 74, 75, 76, It consists of optical members such as 77, 78, 79 and 80. The members constituting these optical systems are housed in one housing 50, respectively.
As clearly shown in FIG. 3, the housing 50 includes a base 50A on which the optical deflector 62 and the optical system are disposed, and a frame-like side wall 50B surrounding the base 50A. The base 50A is provided at a substantially central portion of the side wall 50B so as to partition the housing 50 up and down. The four light source units 52, 53, 54, and 55 are arranged on the side wall 50B of the housing 50 and are photosensitive. The body drums 1 to 4 (FIG. 2) are arranged side by side in substantially the same direction.
[0014]
The optical deflector 62 is disposed at a substantially central portion of the base 50A of the housing 50, and includes imaging lenses 63, 64, 69, 70, 71, 72 that constitute an optical system, and an optical path folding mirror 65. , 66, 67, 68, 73, 74, 75, 76, 77, 78, 79, 80, etc. are arranged separately on both the upper surface side and the lower surface side of the base 50A.
Further, as shown in FIG. 4, covers 88 and 87 are provided at the upper and lower portions of the housing 50, respectively, and an opening through which the light beam passes is formed in the lower cover 87. Dustproof glass 83, 84, 85, 86 is attached, respectively.
The optical writing device 5 uses color-separated image data input from an unillustrated document reading device (scanner) or image data output device (personal computer, word processor, facsimile receiver, etc.) as a light source driving signal. According to the signal, the light source (semiconductor laser (LD)) in each of the light source units 52, 53, 54, and 55 shown in FIG. 5 is driven to emit a light beam.
[0015]
The light beams emitted from the light source units 52, 53, 54, and 55 pass through cylindrical lenses 56, 57, 58, and 59 for correcting surface tilt and reach the optical deflector 62 directly or via the mirrors 60 and 61. . The light beam is deflected and scanned in two symmetrical directions by two-stage polygon mirrors 62a and 62b rotated at a constant speed by a polygon motor 62c shown in FIG.
In this optical writing device 5, the optical deflector 62 has a configuration 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 upper and lower stages. Although an optical deflector is used, the light deflector may be configured to deflect and scan all the light beams L1 to L4 with a thick polygon mirror.
[0016]
The light beams deflected and scanned in two directions by two beams by the polygon mirrors 62a and 62b of the optical deflector 62 pass through image forming lenses 63 and 64 each composed of, for example, an upper and lower two-layered fθ lens. After being folded back by one folding mirror 65, 66, 67, 68 and passing through the opening of the base 50A, it passes through second imaging lenses 69, 70, 71, 72 made of, for example, a long toroidal lens. The photosensitive drums 1, 2, 3, and 4 for each color are passed through second folding mirrors 73, 75, 77, and 79, third folding mirrors 74, 76, 78, and 80, and dustproof glasses 83, 84, 85, and 86. The surface to be scanned is irradiated and an electrostatic latent image is written thereon.
The optical writing device 5 is provided with a synchronization detection mirror (not shown) for taking out 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, and L4. As shown by the broken line in FIG. 6, the light beam reflected by the mirror for use is received by the synchronization detectors 81 and 82, and a scanning start synchronization signal is output.
[0017]
The third folding mirrors 74, 76, 78 arranged in the optical path of the L1, L2, L3 light beams are provided with stepping motors 92, 93, 94 for skew adjustment, so that the scanning line of the L1 light beam is provided. The shift of the scanning line position of each of the light beams L1, L2, and L3 can be corrected based on the position.
Note that 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 the photosensitive drums 1 to 4. The direction perpendicular to the main scanning direction is the sub-scanning direction, which is the rotation direction of the photosensitive drums 1 to 4 (moving direction of the surface of the photosensitive drum), and also the conveyance direction of the transfer conveyance belt 22a. Become. That is, the width direction of the transfer conveyance belt 22a is the main scanning direction, and the conveyance direction (movement direction) of the transfer conveyance belt 22a is the sub-scanning direction.
[0018]
In the image forming apparatus shown in FIG. 2, the latent images on the photosensitive drums 1, 2, 3, and 4 are made of toners of the colors BK, C, M, and Y of the developing units 10, 11, 12, and 13. The developed and visualized toner images of the colors BK, C, M, and Y are transferred onto the transfer conveyance belt 22 a by the transfer units 14, 15, 16, and 17 of the transfer conveyance device 22. Are sequentially superimposed and transferred onto the transfer paper carried on the sheet.
The transfer paper onto which the four color superimposed images have been transferred is conveyed to the fixing device 26 where the image is fixed and then discharged onto the paper discharge tray 28 by the paper discharge roller 27. In the monochrome image forming mode, the above-described image forming operation is performed only on the black photosensitive drum 1.
[0019]
The four light source units 52, 53, 54, and 55 of the optical writing device 5 shown in FIGS. 3 and 5 are each a semiconductor laser (LD) that is a light source and a collimating lens that collimates a light beam emitted from the semiconductor laser. They are integrated into a holding member such as a holder.
Of the four light source units 52, 53, 54, and 55, the black light source unit 53 (which can be used as the other light source units 52, 54, and 55) that is frequently used when forming a black and white image is high-speed. In order to improve productivity when writing and forming a black and white image, a multi-beam configuration is adopted in which two or more light sources (LD) and collimating lenses corresponding to the respective light sources are integrally held by a holding member.
In the light source unit 53, semiconductor lasers 111 and 112, which are light sources, are fixed to supports 113 and 114, respectively, as shown in FIGS. And the support bodies 113 and 114 to which the semiconductor lasers 111 and 112 are respectively fixed are made to coincide with the optical axes of the collimating lenses 116 and 117, which will be described later, on the back surface of the base body 115 which is a holding member and a collimating lens holder. And are fixed using screws 118, 119 or the like.
[0020]
The collimating lenses 116 and 117 convert light beams into parallel light beams. Each of the collimator lenses 116 and 117 is housed in a lens barrel, and the semiconductor lasers 111 and 112 are fitted in fitting holes 115a and 115b formed in the base 115, respectively. Are engaged with each other, and in this state, they are bonded by an adhesive.
Diaphragm plates 120 and 120 for obtaining a predetermined beam diameter for the emitted light are provided on the exit side of the collimator lenses 116 and 117, respectively. 121 is provided.
The two semiconductor lasers 111 and 112 are arranged on the same plane with their pn junction surfaces coincided, and one of the beams (in this example, the beam on the semiconductor laser 111 side) The polarization plane is rotated by 90 ° by the half-wave plate 122 affixed to the incident surface, and passes through the polarization beam splitter surface 121 b of the beam combining unit 121.
The beam of the semiconductor laser 112 is internally reflected by the inclined surface 121a of the beam combining means 121, reflected by the polarization beam splitter surface 121b of the beam combining means 121, and near the optical axis of the reference semiconductor laser 111. It is combined with the beam of the semiconductor laser 111.
[0021]
The optical axes of the semiconductor lasers 111 and 112 are shifted from each other by an angle θ (FIG. 7) shown on the output side of the beam combining means 121 so as to correspond to a position slightly shifted in the main scanning direction. It is.
The beam combining 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.
The flange member 123 and the base body 115 are fixed to a substrate 126 on which a drive circuit for the semiconductor lasers 111 and 112 is provided by screws or the like (not shown). Each member of the optical path that reaches the member 123 constitutes a light source unit that is integrally fixed to the substrate 126.
[0022]
The cylindrical portion 123a erected on the emission side of the flange member 123 of the light source unit 53 is in the hole 132a of the optical frame 132 provided on the side wall 50B of the housing 50 (FIG. 3) of the optical writing device 5 described above. It is inserted, passes through the spring 130, and passes through the hole 131 a of the spring pressing plate 131.
In this state, the light source unit 53 in which the members from the semiconductor lasers 111 and 112 to the flange member 123 are assembled to the substrate 126 is pulled in the direction of the arrow α in FIG. 7 to rotate the spring pressing plate 131 by 90 °. Thus, the protrusion 131b of the spring pressing plate 131 is hooked on the protrusion 123b of the cylindrical portion 123a.
Accordingly, the light source unit 53 is attached to the optical frame 132 so as to be rotatable about the center of the cylindrical portion 123a of the flange member 123 (coincident with the optical axis) as the rotation center.
[0023]
Next, the pixel density corresponding to the interval (beam pitch) in the sub-scanning direction of 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 is set. An example of a pixel density switching mechanism that switches by rotating the light source unit 53 around the optical axis between 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. The feed screw 128 is cut with a male screw having a nominal diameter M3, and the sliding member 127 has a female screw M3 cut inside. Further, the outer shape of the sliding member 127 has a substantially D shape.
[0024]
A male screw portion of a feed screw 128 is rotationally inserted in advance into the female screw portion of the sliding member 127, and the D-shaped hole portion of the column 132 b provided on the optical frame 132 side of the housing 50 (FIG. 3). The sliding member 127 is slidably inserted. At that time, the rotary shaft 129a of the pitch varying stepping motor 129 is inserted into the hole of the column 132b of the optical frame 132, and the lower end side of the feed screw 128 is press-fitted and fixed to the tip of the rotary shaft 129a. Like that.
The pitch varying stepping motor 129 has a main body 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. However, since the hole of the cylinder 132b is D-shaped at that time, the sliding member 127 does not rotate but slides only in the vertical direction.
[0025]
On the other hand, the flange member 123 of the light source unit 53 is formed with an arm 123 c that extends in the direction of the sliding member 127 and has a tip that abuts against the upper end of the sliding member 127. Then, a spring 135 is provided between the arm 123c and the optical frame 132 so that a biasing force is exerted in a direction in which they are attracted to each other, and the lower surface of the distal end portion of the arm 123c is formed in a planar shape by the spring 135. It is designed to be pressed against the upper surface.
Accordingly, when the stepping motor 129 for varying the pitch 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. Thereby, the light source unit 53 rotates around the center of the cylindrical portion 123a of the flange member 123 as a rotation center.
[0026]
Further, as a means for controlling the rotation angle of the light source unit 53, an optical home position sensor 133 comprising a photo interrupter provided with a light emitting portion 133a and a light receiving portion 133b is fixed to the optical frame 132 with a screw or the like (not shown). ing. In addition, a filler 123d 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 is provided on a part of the flange member 123 opposite to the arm 123c.
Then, the edge position 123e of the filler 123d of the flange member 123 is set so that the position at the moment when the space between the light emitting part 133a and the light receiving part 133b of the home position sensor 133 is shielded becomes the home position (HP). The home position is used as a reference for rotation adjustment of the light source unit 53.
Thus, in this embodiment, the sliding member 127, the feed screw 128, the optical frame 132 having the column 132b, the pitch variable stepping motor 129, the flange member 123 having the arm 123c and the filler 123d, The home position sensor 133 functions as a pixel density switching mechanism A (see FIG. 1).
[0027]
Next, a method for changing the beam pitch in the sub-scanning direction of two light spots by the above-described pixel density switching mechanism A will be described with reference to FIG.
In FIG. 9, the broken lines indicate the positions of the light emitting portion 133a and the light receiving portion 133b of the home position sensor 133. As described above, the edge portion 123e of the shielding filler 123d provided on the flange member 123 is received by the light emitting portion 133a. The position at the moment when the space between the portions 133b is blocked is the home position (HP).
Further, in the position B in the figure, the light source unit 53 has an angle θ from the home position (HP) with the optical axis as the rotation center.₁In order to obtain this rotation angle, the pitch variable stepping motor 129 (FIG. 7) rotates a predetermined number of pulses and moves the sliding member 127 upward by a predetermined amount to obtain a light source unit. 53 to angle θ₁Just rotate.
Similarly, the position A in the figure is an angle θ from the home position (HP) where the light source unit 53 is centered on the optical axis.₂Only the rotated position is shown.
[0028]
Here, the light source unit 53 is angled θ₁And θ₂, When the rotation center is adjusted with one of the light spots from the two semiconductor lasers as the rotation center as shown in FIG. 10, and the two rotation centers as shown in FIG. In some cases, rotation adjustment is performed with the center position between the light spots as the center of rotation.
In the case of FIG. 10, the light source unit 53 is moved at an angle θ₁When rotated, the beam pitch between the light spots on the photosensitive drum emitted from the two semiconductor lasers is P₁And the angle θ₂When rotated, the beam pitch is P₂It becomes.
Further, in the case of FIG.₁When rotated, the beam pitch of the light spot on the photosensitive drum emitted from the two semiconductor lasers is P₃And the angle θ₂When rotated, the beam pitch is P₄Thus, even if the same angle is rotated, the beam pitch between the two light spots becomes larger than in the case of FIG.
[0029]
As described above, either the light source unit 53 is rotated with one of the two light spots as the rotation center, or the light source unit 53 is rotated with the center position between the two light spots as the rotation center. By this method, the beam pitch (interval in the sub-scanning direction) of the two light spots on the photosensitive drum can be adjusted.
The rotation angle of the light source unit 53 is the same as that of the stepping motor 129 described in FIG. P. Drive pulse number, ie, the angle θ₁, Θ₂Corresponding to. P. The number of drive pulses can be determined in advance, and can be easily controlled by rotating the stepping motor 129 by the number of drive pulses.
By the way, in an image forming apparatus in which the beam pitch in the sub-scanning direction of a plurality of light spots on the photosensitive drum is changed according to the recording density, for example, two lights on the photosensitive drum at a recording density of 600 dpi. H. when the sub-scanning beam pitch P of the spot is set to 42 μm. P. The pulse number Pa of the stepping motor 129 from H. and the beam pitch P at a recording density of 1200 dpi are set to 21 μm. P. If the pulse number Pb is stored in the memory of the control unit in the image forming apparatus, the stepping motor 129 is rotated in accordance with the required recording density in the high image quality mode and the speed priority mode. Thus, the beam pitch in the sub-scanning direction of the light spot on the photosensitive drum can be easily switched.
[0030]
In this image forming apparatus, when the power is turned on, the light source unit 53 (for black image) is rotationally controlled to a predetermined position, for example, a rotation angle at a recording density of 600 dpi (position B in FIGS. 10 and 11). It is like that. This is because the H.P. P. Then, the stepping motor 129 is driven in the predetermined direction by the number of pulses Pa, and the beam pitch when the pitch in the sub-scanning direction of the two light spots for the black image on the photosensitive drum is 600 dpi is obtained. This is because the light source unit 53 is rotated as described above.
Thereafter, the rotation angle at which the light source unit 53 is rotated corresponds to the recording memory 1200 dpi in the high image quality mode and the recording density 600 dpi in the speed priority mode described above in the RAM which is the data memory of the control device 40 (FIG. 12) described later. Therefore, when a recording density of 1200 dpi is requested, the stepping motor 129 is rotated from the position of 600 dpi by the number of pulses of [Pb-Pa], and two lights for black image on the photosensitive drum are used. The light source unit 53 is rotated so that the beam pitch in the sub-scanning direction of the spot is changed to a beam pitch at a recording density of 1200 dpi.
[0031]
As described with reference to FIGS. 13 to 15, when the pixel density is switched according to the image forming mode, the light beam for the black (BK) image is used during full color image formation. If the switching position of the pixel density is not managed, the interval between the BK image and the C image is narrowed by the deviation δ shown in FIG. 13 as shown in FIG. In some cases, color misregistration occurs.
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 described above is provided.
The light spot position control means B emits a light beam from the light source unit 53 for the black (BK) image to only one beam located on the side closer to the cyan beam when outputting a full-color image, and the photosensitive drum 1. The pixel density position of the light spot formed on the surface to be scanned is controlled to be a position corresponding to a predetermined pixel density according to the high image quality mode (1200 dpi) or the speed priority mode (600 dpi).
[0032]
The light spot position control means B is a control device 40 as shown in FIG. 12, and the control device 40 stores a central processing unit (CPU) 41 having various determination and processing functions, and each processing program and fixed data. A microcomputer comprising a stored ROM 42, a RAM 43 as a data memory for storing processing data, and an input / output circuit (I / O) 44 is provided.
The control device 40 inputs signals such as the number of images formed and an image forming mode such as a high image quality mode and a speed priority mode from an operation panel 45 provided in the image forming apparatus. Various sensor signals are input at predetermined timings from sensors provided at various locations of the image forming apparatus.
Then, signals for driving them at a predetermined timing are output to various loads such as various drive systems and display systems for image formation. Further, the driver drives the stepping motor 129 of the above-described pixel density switching mechanism A 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. 47 for output.
[0033]
When a high-quality mode (pixel density of 1200 dpi) is selected for a full-color image by the operation panel 45, the control device 40 has a black image light beam positioned closer to the cyan beam than the light source unit 53. A light beam is emitted only on the beam (semiconductor laser) to form a light spot on the surface to be scanned of the photosensitive drum 1.
At this time, in this image forming apparatus, as described above, when the power is turned on, the light source unit 53 (for BK image) rotates at a pixel density of 600 dpi (rotation angle at the position B described in FIGS. 10 and 11). Has been rotated. Therefore, if writing is started with the light spot position for the BK image as it is at the pixel density of 600 dpi, writing is performed at the light spot position of the pixel density of 1200 dpi for C, M, and Y other than the BK image. Therefore, color misregistration occurs between the BK image as described in FIG. 15 and another color image.
[0034]
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 is a full color image and the high image quality mode (pixel density 1200 dpi) is selected, the BK image The light source unit 53 is controlled to move to a pixel density of 1200 dpi by rotating a stepping motor 129 from a pixel density of 600 dpi.
Therefore, from the light source unit 53 for the BK image, one beam located on the side closer to the cyan beam among the two semiconductor lasers moved to the position of the pixel density of 1200 dpi (the lower side as described in FIG. 11) The light beam is emitted only on the side corresponding to the first beam light spot) to form a light spot for the BK image on the scanned surface of the photosensitive drum 1. As a result, as shown in FIG. 14, images of each color of BK, C, M, and Y are formed at equal beam pitches corresponding to a pixel density of 1200 dpi, thereby obtaining a full-color image without color misregistration. .
[0035]
In the case of an image forming apparatus configured to select a speed priority mode (pixel density 600 dpi) for a full-color image, a mechanism for switching the position of the light source unit for each color of C, M, and Y to a pixel density of 1200 dpi and 600 dpi, respectively. When the speed priority mode is selected for a full-color image, the light source unit for each color of C, M, and Y is set at a pixel density of 600 dpi, and the light source unit for the BK image is turned on when the power is turned on. Since it has been rotated to a pixel density position of 600 dpi, control is performed so that writing of a BK image is started at that position. Thereby, a full color image without color misregistration is obtained.
[0036]
【The invention's effect】
As described above, according to the present invention, the following effects can be obtained.
According to the first and second image forming apparatuses, when a monochrome image is formed, the light source unit for black image uses two or more beams and writing is performed per unit time as compared with the case where a full color image is formed. The output number can be increased to improve productivity.
When a full color image is output, 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 predetermined according to the high image quality mode or the speed priority mode, respectively. Since the pixel density is controlled to a position corresponding to the pixel density, even if the pixel density position of the black image light spot is switched by the pixel density switching mechanism, the black image causes a color shift with respect to other color images. Can not be.
According to the image forming apparatus of the third aspect of the present invention, since the optical writing apparatus that distributes and scans the light beams from the plurality of light source units in two symmetrical directions by one optical deflector is used, a plurality of images are used. 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 the drawings]
FIG. 1 is a functional block diagram showing a control system for switching pixel density of an image forming apparatus according to an embodiment of the present invention.
FIG. 2 is a schematic configuration diagram illustrating the entire image forming apparatus.
FIG. 3 is a plan view showing the configuration of the upper surface side of the optical writing device of the image forming apparatus.
4 is a cross-sectional view showing a cross section taken along the line AA ′ of the optical writing device of FIG. 3;
5 is a schematic view showing a light source unit, an optical deflector, and an optical system of the optical writing device of FIG. 3. FIG.
6 is a schematic diagram showing an arrangement configuration of an optical deflector and an optical system of the optical writing device in FIG. 4;
FIG. 7 is an exploded perspective view showing a configuration of a multi-beam light source unit.
FIG. 8 is a cross-sectional view of an essential part of the multi-beam light source unit.
FIG. 9 is an explanatory diagram for explaining a method of changing the beam pitch in the sub-scanning direction of two light spots by the light spot position control means.
FIG. 10 is an explanatory diagram showing an example of rotating a light source unit around one of the light spots from two semiconductor lasers as a rotation center.
FIG. 11 is an explanatory view showing an example in which the light source unit is rotated with the center position between the two light spots as the rotation center.
FIG. 12 is a block diagram showing a specific configuration of the light spot position control means and its related configuration.
FIG. 13 is an explanatory diagram for explaining a deviation of a light spot position in a case where a black image light beam has a pixel density of 1200 dpi and 600 dpi.
FIG. 14 is an explanatory diagram showing a line image when the black image light beam is written at a writing position with a pixel density of 1200 dpi together with other color line images.
FIG. 15 is an explanatory view showing a line image together with other color line images when the black image light beam is similarly written at a writing position with a pixel density of 600 dpi.
[Explanation of symbols]
1, 2, 3, 4: Photosensitive 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

Claims (3)

A plurality of image carriers arranged side by side, an optical writing device for writing a latent image by forming a light spot by a light beam on the scanned surface of each of the plurality of image carriers, and a pixel density in a high image quality mode A pixel density switching mechanism for switching between the time and the speed priority mode, and the optical writing device has a light source unit of 2 beams or more for a black image and a light source unit of 1 beam for each color for a color image other than black In each of the image forming apparatuses having
The pixel density position of the light spot of one beam for each color for a color image can be switched between the high image quality mode and the speed priority mode.
At the time of outputting a full color image, a light beam is emitted from the light source unit for the black image by only one beam located on the side close to the beam for the color image to form a light spot on the scanned surface of the image carrier. the pixel density position of the light spot, one beam in accordance with the pixel density position of the light spot, pixel density position of the light spot of each color for each color for the high-quality mode or speed priority mode the color image in accordance with the An image forming apparatus, comprising: a light spot position control means for controlling the light beam to be formed at equal beam pitches.   2. The image forming apparatus according to claim 1, wherein the light source unit for black image of the optical writing device has two beams.   The optical writing device distributes and scans light beams from the plurality of light source units in two symmetric directions by one optical deflector, and is arranged symmetrically in the two directions around the optical deflector. 3. The image forming apparatus according to claim 1, wherein the image forming apparatus has a configuration in which a light beam is guided and imaged on the scanned surfaces of the plurality of image carriers via an optical system.

Images