JPH11208023A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To adjust image write starting positions to be the same in a simple constitution when images of a plurality of lines are to be formed with the use of a plurality of laser lights. SOLUTION: A first laser light is scanned twice (S1), and a difference Δt1 of output start timings of two pulse signals output from a synchronous detection sensor is obtained as a reference value (S2). A second laser light is scanned after the first laser light is scanned(S3, S4) and a difference Δt2 of output start timings of two pulse signals output from the synchronous detection sensor is obtained as a value to be compared (S5). If scan start positions of the first laser light and second laser light shift in a main scan direction, the value Δt2 to be compared becomes larger by a time corresponding to a shift width than the reference value Δt1. Therefore, a difference of the reference value Δt1 and value Δt2 to be compared is obtained as a correction value Δt (S6). An image write timing is controlled on the basis of the correction value Δt.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a digital copying machine, and more particularly to a method for writing an image for a plurality of lines on a recording medium such as a photosensitive member by a single scan using a plurality of laser beams. And a so-called multi-beam type image forming apparatus.

[0002]

2. Description of the Related Art A multi-beam type digital copying machine is
This is designed to improve the copying speed as compared with a single-beam type digital copying machine in which one line of image is sequentially written on a photosensitive member using one laser beam. Such a multi-beam type digital copying machine is disclosed, for example, in Japanese Patent Application Laid-Open No. 63-124664.

FIG. 8 is a schematic diagram showing a configuration of a laser optical system in a multi-beam digital copying machine disclosed in the above-mentioned publication. This laser optical system writes an image for two lines on the image plane 71 in one scan.
Two semiconductor lasers 72 and 73 are provided. A laser beam 74 emitted from one semiconductor laser 72 is converted into a parallel light beam by a collimator lens 75 and enters a polarization beam splitter 76. At this time, the laser beam 74 is made to be s-polarized with respect to the polarization beam splitter 76. The laser light 77 emitted from the other semiconductor laser 73 is converted into a parallel light beam by the collimator lens 78, and then passes through the half-wave plate 79. As a result, the laser beam 77 becomes p-polarized with respect to the polarization beam splitter 76.

[0004] The two laser beams 74 and 77 are incident on a polarization beam splitter 76 while maintaining the above-mentioned polarization.
As a result, each of the laser beams 74 and 77 is emitted toward the polygon mirror 80. Thereafter, the light is reflected by the reflecting surface 81 of the rotating polygon mirror 80, converged by the fθ lens 82, and guided to the image surface 71. Thus, the image plane 71 is scanned along the main scanning direction 83 by the two laser beams 74 and 77.

When manufacturing the laser optical system, it is extremely difficult to make the scanning start positions of the laser beams 74 and 77 completely the same in terms of the arrangement accuracy of the semiconductor lasers 72 and 73. In this case, since the timing at which the laser beams 74 and 77 reach the image plane 71 is different, if the image writing timing is the same for each of the laser beams 74 and 77, the writing start position of the image will be shifted. Therefore, this laser optical system employs the following configuration in order to set the image writing timing for each of the laser beams 74 and 77 independently.

A folding mirror 84 for reflecting the laser beams 74 and 77 is disposed near the scanning start side of the laser beams 74 and 77. The laser beams 74 and 77 reflected by the folding mirror 84 are made incident on the polarization beam splitter 85 and emitted in directions according to the respective polarizations. After being emitted, the laser beams 74 and 77 are guided to scanning start sensors 86 and 87, respectively. Each of the scanning start sensors 86 and 87 outputs a laser beam 74,
A write start signal is output in response to the input of the signal 77. When the output of the write start signal is shifted in time, the image writing timing by the laser beams 74 and 77 emitted from the semiconductor lasers 72 and 73 is changed in consideration of the time shift. As a result, the image writing start positions can be made substantially the same.

[0007]

However, in the above technique, it is necessary to arrange the same number of scanning start sensors as the number of laser beams in order to make the writing start position of an image the same. Therefore, as the number of laser beams increases, there is a problem that the configuration becomes complicated and the cost increases.

Accordingly, an object of the present invention is to solve the above-mentioned technical problem, and to write an image for a plurality of lines on a recording medium using a plurality of laser beams with a simple configuration. Is to provide an image forming apparatus capable of adjusting the same.

[0009]

Means for Solving the Problems and Effects of the Invention According to the first aspect of the present invention, a plurality of laser beams are lit by turning on a plurality of laser beams, respectively, and a plurality of laser beams are irradiated by a scanning unit. By scanning along a direction, in an image forming apparatus for writing an image for a plurality of lines by a plurality of laser beams on a recording medium in one scan, a plurality of laser beams scanned by the scanning unit are used. A signal output means for outputting a pulse signal during a period when any one of the laser beams is incident on the incident surface, and a predetermined reference laser among the plurality of lasers; And the output timing of a signal output from the signal output means when only one of the plurality of lasers is turned on. The target laser is referred to as “target laser”), and the difference from the output timing of the signal output from the signal output means is determined as a comparison target value. A correction value calculating means for calculating a difference between the comparison value calculated by the comparison value calculating means and a predetermined reference value as a correction value; and a correction value calculating means. Based on the obtained correction value,
Timing control means for controlling an image writing timing by a plurality of laser beams respectively radiated from the plurality of lasers so as to match an image writing start position on the recording medium. An image forming apparatus.

When the reference laser is turned on, the time required from the start of laser beam scanning to the start of signal output, and when the comparison target laser is turned on, the time required from the start of laser beam scanning to the start of signal output. Differs by a time corresponding to the shift width of each laser beam in the main scanning direction. Therefore, as in the present invention, for example, the same laser is used for two times.
A reference value corresponding to an output timing difference between two signals output when the light is turned on once, and a comparison target which is a signal output timing difference when the reference laser is turned on and the comparison target laser is turned on separately. If a difference from the value is obtained, a time corresponding to the shift width can be obtained.

If the scanning start position of the reference laser is located upstream of the scanning start position of the comparison target laser in the main scanning direction, the image writing timing of the reference laser is changed to the image writing timing of the comparison target laser. If it is later than the timing by the acquired time, the writing start position of the image can be the same. Conversely, when the scanning start position of the comparison target laser is located on the upstream side in the main scanning direction from the scanning start position of the reference laser, the image writing timing by the comparison target laser may be delayed.

As described above, since the image writing start positions are matched using the signal output from one signal output means, a sensor for detecting the number of laser beams equal to the number of laser beams is provided. Can be prevented from increasing in number of parts as compared with the conventional example in which it is necessary to dispose the components. Therefore, the configuration can be simplified and the cost can be reduced.

According to a second aspect of the present invention, there is further provided a reference value calculating means for obtaining, as a reference value, an output timing difference between signals output from the signal output means when the reference laser is turned on twice. Wherein the correction value calculation means obtains a difference between the comparison target value calculated by the comparison value calculation means and the reference value calculated by the reference value calculation means as a correction value. Item 2. The image forming apparatus according to Item 1.

According to the present invention, since the reference value can be obtained, it is possible to cope with a change in the reference value due to aging or the like. Therefore, an accurate correction value can be obtained over a long period. According to a third aspect of the present invention, the scanning means has a polygon mirror having a plurality of reflection surfaces, and the reference laser beam and the comparison laser light are respectively used when the output timing difference is calculated by the comparison target value calculation means. 3. The image forming apparatus according to claim 2, wherein the reflecting surface to be incident is the same as the reflecting surface to which the reference laser light is to be incident twice when the output timing difference is determined by the reference value calculating means. Device.

According to the present invention, since the same reflection surface as that used for obtaining the reference value is used as the reflection surface used for obtaining the comparison value, the surface of the reflection surface is used. Even if the accuracy differs for each reflective surface,
The time corresponding to the deviation width can be accurately obtained.

[0016]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a schematic sectional view showing a part of the internal configuration of a digital copying machine according to a first embodiment of the present invention. This digital copying machine has a first laser L1 and a second laser L2 composed of a semiconductor laser or the like.
First laser light LB1 respectively emitted from laser L1
In addition, by scanning the photoconductor 5 with the second laser beam LB2, an image corresponding to two lines is written on the photoconductor 5 in one scan.

This digital copying machine reads an original placed on an original glass 1 to generate an image signal, and an image for subjecting the image signal generated by the original reading unit 2 to predetermined image processing. And a processing unit 3. This digital copier modulates a laser beam based on an image signal after image processing is performed, and scans the photoconductor 5 with a laser beam scanning unit 4 that scans the photoconductor 5 with the modulated laser beam. And an image forming unit 6 for transferring a document image to a copy sheet P.

The original reading section 2 includes a light source 11 for irradiating the original placed on the original glass 1 with light, and three reflecting mirrors 12a, 12b, and 12c for reflecting light reflected from the original.
, An imaging lens 13 for converging the light reflected by the reflection mirrors 12a to 12c, and a CCD (charge coupled device) on which the light converged by the imaging lens 13 is incident.
vice) and the like.

When copying is performed, the light source 11 and the reflection mirrors 12a to 12c move rightward in FIG. As a result, the reflected light from the document sequentially enters the image sensor 14. In the image sensor 14, an image signal is generated for each line according to the intensity of the incident reflected light. The created image signal for each line is
The image data is supplied to the image processing unit 3 for each line, and the image processing unit 3 performs image processing such as shading correction. The image signal that has been subjected to the image processing is supplied to the scanning control unit 15 for each line.

The scanning control unit 15 controls the first laser beam LB1 and the second laser beam LB based on image signals for two lines.
2 are modulated, and the modulated laser light LB
1 and LB2 are respectively emitted from the lasers L1 and L2 at a predetermined timing. As a result, each laser L1, L2
After that, a laser beam of an amount of light corresponding to the density of each image signal is emitted. The diameter of the laser beam is, for example, 40 (μm).

The laser beams LB1 and LB2 emitted from the lasers L1 and L2 are respectively
Through the reflecting surface 23 of the polygon mirror 22. In this case, the laser beams LB1 and LB2 are shifted from the reflecting surface 23 of the polygon mirror 22 by a predetermined vertical shift width dv in the vertical direction (sub-scanning direction on the surface of the photoconductor 5) in FIG. Incident. Vertical deviation width dv
Corresponds to a pitch of two lines when an image is formed.
The polygon mirror 22 can be rotated about a rotation axis 25 by a polygon motor 24 composed of a DC motor or the like.

The laser beams LB 1 and LB 2 reflected by the reflection surface 23 of the polygon mirror 22 are
Is reflected by the reflection mirror 27 and guided to the photoconductor 5. Then, the photoconductor 5 is scanned along the main scanning direction as the polygon mirror 22 rotates. As a result, the photosensitive member 5 is scanned simultaneously by two lines. Thus, an image for two lines is written on the photoconductor 5.

The image forming section 6 includes a charger 31 for uniformly charging the surface of the photoconductor 5. Each laser beam L
B1 and LB2 scan the surface of the charged photoconductor 5. As a result, the surface of the photoreceptor 5 is exposed, and an electrostatic latent image corresponding to the light amount of each of the laser beams LB1, LB2 is formed. In the first embodiment, the formation of the electrostatic latent image is defined as image writing. The electrostatic latent image is developed into a toner image by the developing device 32. The toner image is transferred to the copy paper P by the transfer device 33. The copy paper P after the transfer is separated from the photoconductor 5 by the separator 34. Thereby, copying of the image onto the copy paper P is achieved. Photoconductor 5 after copying
Residual toner and the like adhering to the surface of the cleaning device 35
To be cleaned. Photoconductor 5 after cleaning
Is charged again by the charger 31 and is prepared for copying the next original image onto the copy paper P.

FIG. 2 is a plan view showing the configuration of the laser beam scanning section 4. The first laser L1 and the second laser L2 are
In a plan view, the first laser light LB1 and the second laser light LB2 are arranged at positions where they are irradiated in directions substantially orthogonal to each other. First laser L1 and second laser L
The first laser beam LB1 and the second laser beam LB2 respectively emitted from the beam splitter 2 emit a laser beam in different directions based on the incident direction of the laser beam.
Is incident on.

As a result, the first laser beam LB1 emitted from the first laser L1 is emitted from the beam splitter 21 without changing the optical axis. On the other hand, the second laser L2
The second laser light LB2 emitted from the laser beam is bent approximately 90 degrees so that the optical axis is in the same direction as the optical axis of the first laser light LB1, and is emitted from the beam splitter 21. Thereby, the first laser light LB1 and the second laser light LB2
Are guided toward the polygon mirror 22.

Each of the laser beams LB1 and LB2 emitted from the beam splitter 21 is guided to a polygon mirror 22 having m (eight in the first embodiment) reflecting surfaces 23. When scanning is started, the polygon mirror 22
As shown by a two-dot chain line, for example, each laser beam LB1, LB2 is located near the end of the reflection surface 23a on the downstream side in the rotation direction C.
Is in the position where is guided. In this case, each laser beam LB1,
The reflection angle of LB2 is large, and the laser beams LB1 and LB2 are reflected toward the upstream side in the main scanning direction M. As the scanning progresses, the polygon mirror 22 rotates along the rotation direction C,
When one scan is completed, as shown by the solid line, each laser beam L is positioned near the end of the reflection surface 22a on the upstream side in the rotation direction.
It reaches the position where B1 and LB2 are guided. Therefore, the laser beams LB1 and LB2 are scanned along the main scanning direction M as the polygon mirror 22 rotates.

In the scanning range A of each of the laser beams LB1 and LB2, a reflection mirror 27 is disposed.
The scanning range A of B1 and LB2 is set longer than the length of the reflection mirror 27 along the main scanning direction M. The length of the reflection mirror 27 along the main scanning direction M is substantially the same as the length of the photoconductor 5 (see FIG. 1) along the main scanning direction M. Therefore, the scanning range A of each of the laser beams LB1 and LB2 is set longer than the length of the photoconductor 5 along the main scanning direction M.

One synchronization detection sensor 41 is disposed at a position upstream of the reflection mirror 27 in the main scanning direction M.
The synchronization detection sensor 41 has one incident surface 42 on which a plurality of laser beams LB1 and LB2 are incident, and outputs a pulse signal during a period when any one of the laser beams is incident on the incident surface 42. . This pulse signal is used to define the timing of writing an image on the photoconductor 5.

FIG. 3 is a block diagram showing the electrical configuration of the laser beam scanning unit 4. The laser beam scanning unit 4 includes a scanning control unit 15 functioning as a control center. The scanning control unit 15 includes a laser control unit 52 that controls irradiation of the laser beams LB1 and LB2 from the first laser L1 and the second laser L2, and a polygon control unit 53 that controls rotation of the polygon mirror 22. I have. The laser control unit 52 includes:
A pulse signal is provided from the synchronization detection sensor 41.

The laser control unit 52 is provided with an image signal from the image processing unit 3. Further, a pulse signal is provided from the synchronization detection sensor 41. The laser control unit 52 controls the first laser L1 and the second laser L1.
A laser drive signal is given to the laser L1, and the first laser L1 and the second laser L2 are turned on. When a pulse signal is given from the synchronization detection sensor 41, the irradiation of the modulated first laser light LB1 and the modulated second laser light LB2 is started at a predetermined timing, and the image on the photoconductor 5 is started. Start writing.

The polygon controller 53 includes a motor driver 5
5 to rotate the polygon motor 24 at a predetermined number of revolutions. In this case, the polygon mirror 22 rotates around the rotation shaft 25 at a predetermined rotation speed. By the way, in FIG. 2, when the lasers L1 and L2 are arranged at positions indicated by solid lines, the optical axes of the laser beams LB1 and LB2 emitted from the beam splitter 21 substantially overlap in plan view. . In this case, each of the laser beams LB1, LB2
4A, as shown in FIG. 4A, is incident on the same position in the longitudinal direction of the reflection surface 23 of the polygon mirror 22 (in the main scanning direction on the surface of the photoconductor 5). At this time, each laser beam L
The scanning start positions S1 and S2 of B1 and LB2 are the same. Therefore, if the image writing timing by each of the laser beams LB1 and LB2 is made the same, the image writing start position becomes the same.

On the other hand, in FIG. 2, for example, when the second laser L2 is arranged at a position indicated by a two-dot chain line,
Each laser beam LB emitted from the beam splitter 21
1, the optical axis of LB2 is shifted in plan view. In this case, as shown in FIG. 4B, the laser beams LB1 and LB2 are incident on positions shifted by the horizontal shift width dh with respect to the longitudinal direction of the reflection surface 23 of the polygon mirror 22. In this case, the scanning start position S1, of each of the laser beams LB1, LB2,
Since S2 is shifted by the horizontal shift width dh, if the image writing timing is the same, the writing start position of the image is shifted.

Therefore, the scanning control unit 15 according to the first embodiment obtains a time difference corresponding to the horizontal deviation width dv of each of the laser beams LB1 and LB2 as a correction value Δt, and based on the obtained correction value Δt. The image writing timing by each of the laser beams LB1 and LB2 is controlled to correct the shift of the image writing start position. The following explanation is
It is assumed that the scanning start position S1 of the first laser light LB1 is located downstream of the scanning start position S2 of the second laser light LB2 in the main scanning direction M.

FIG. 5 is a flowchart for explaining the correction value calculation processing in the scanning control unit 15. The scanning control unit 15 uses the first laser beam LB1 as a reference laser beam and scans the first laser beam LB1 twice (step S1). More specifically, the polygon control unit 53 drives the motor driver 55 at a predetermined timing to rotate the polygon motor 24 at a predetermined rotation speed. The laser controller 52 starts supplying a laser drive signal to the first laser L1 at an appropriate timing (t1) (see FIG. 6A). As a result, the first laser L1 outputs the first
Irradiation of the laser beam LB1 is started, and the first laser beam L
B1 is the main scanning direction M as the polygon mirror 22 rotates.
Start to be scanned along. On the way, the first laser beam LB1 passes through the synchronization detection sensor 41. As a result, from the synchronization detection sensor 41, the time Δ Δ during which the first laser beam LB1 passes through the incident surface 42 along the main scanning direction M
A pulse signal is output over t1 (see FIG. 6 (c)).
At this time, the scanning control unit 15 outputs the pulse signal output time Δ
t1 is obtained as a reference time (step S2). The laser control unit 52 stops supplying the laser drive signal to the first laser L1 at a timing (t2) when a predetermined time kt1 has elapsed since the output of the pulse signal. Thereby, one scan by the first laser beam LB1 is achieved.

Next, at a timing (t3) when a predetermined time kt2 has elapsed from the timing when the pulse signal was output in the first scan, the laser control unit 52 performs the first
The second scanning of the laser beam LB1 is started (step S3). Specifically, after starting the supply of the laser drive signal to the first laser L1, the laser control unit 52 elapses a predetermined time kt1 starting from the output timing of the pulse signal output from the synchronization detection sensor 41. Timing (t
In 4), the supply of the laser drive signal is stopped (FIG. 6).
(a)).

As a result, the first laser beam LB1 is scanned in the main scanning direction M while being reflected by the reflection surface adjacent to the reflection surface 23 upstream of the reflection surface 23 reflected in the previous rotation in the rotation direction C. Also in this case, from the synchronization detection sensor 41,
A pulse signal is output during the time when the first laser beam LB1 passes through the incident surface 42 along the main scanning direction M (see FIG. 6C).

When the same laser beam is scanned twice, the time from the start of the laser beam irradiation to the start of the output of the pulse signal from the synchronization detection sensor 41 has the same value in any of the scans. Therefore, the laser control unit 52
Determines the difference Δt1 between the output start timings of the two pulse signals output from the synchronization detection sensor 41 by the two scans with the first laser beam LB1 as a reference value (step S2).

Next, the laser controller 52 scans the first laser beam LB1 once at a timing (t5) when a predetermined time kt2 has elapsed from the output timing of the pulse signal output during the second scanning. (Step S3, FIG. 6
(a)). That is, the laser control unit 52 starts the supply of the laser drive signal to the first laser L1, and then starts the output timing of the pulse signal output from the synchronization detection sensor 41 as a starting point when a predetermined time kt1 has elapsed.
At (t6), the supply of the laser drive signal is stopped (see FIG. 6A). Further, at a timing (t7) when a predetermined time kt2 has elapsed from the output timing of the pulse signal output during the third scan, the second laser beam LB2 is set to 1
The scanning is performed twice (step S4, see FIG. 6B). That is, after starting supply of the laser drive signal to the first laser L1, the laser control unit 52 sets the output timing of the pulse signal output from the synchronization detection sensor 41 to the start point kt1.
At the timing (t8) when the time has elapsed, the supply of the laser drive signal is stopped (see FIG. 6A).

Here, the time from the start of the irradiation of the second laser light LB2 to the start of outputting the pulse signal is longer than that of the first laser light LB1. This is because the scanning start position S2 of the second laser beam LB2 is the first laser beam L
This is because it is assumed that the position B1 is located at a position shifted by the horizontal shift width dh upstream of the scanning start position S1 in the main scanning direction M.

Therefore, the laser control unit 52 determines the difference between the output start timing of the pulse signal output by scanning the first laser beam LB1 and the output start timing of the pulse signal output by scanning the second laser beam LB2. Δt2 is obtained as a comparison target value (step S5). In this case, the comparison value Δt2 corresponds to the horizontal shift width dh. And
The difference (Δt2−Δt1) between the determined comparison value Δt2 and the reference value Δt1 determined in step S2 is determined, and the determined difference is determined as a correction value Δt (step S6).

The laser control unit 52 holds the correction value Δt thus obtained in, for example, a RAM, and every time copying is performed, the correction value Δt stored in the RAM is stored.
t, and based on the read correction value Δt,
The image writing timing is controlled. Specifically, the laser control unit 52 simultaneously turns on the lasers L1 and L2.
In this case, the first laser light LB1 is incident on the incident surface 42 of the synchronization detection sensor 41 before the second laser light, and the synchronization detection sensor 41 outputs a pulse signal. When the pulse signal is output from the synchronization detection sensor 41, the laser control unit 52 responds to the elapse of the predetermined write standby time tw from the start of the output of the pulse signal, and modulates the modulated first laser light LB1. Is started. As a result, image writing by the first laser beam LB1 is started.

Note that the write standby time tw corresponds to the time from when the laser beam being scanned reaches the synchronous detection sensor 41 to when it reaches the reflection mirror 27. Next, in response to the lapse of the time corresponding to the correction value Δt from the image writing timing by the first laser light LB1, the irradiation of the modulated second laser light LB2 is started. As a result, the second
Image writing by the laser beam LB2 is started.

As described above, the image writing timings of the respective laser beams LB1 and LB2 are not made the same, and the second
The image writing timing of the laser beam LB2 is set to the horizontal shift width dh.
The first laser light L for a time corresponding to the correction value Δt corresponding to
Since the image writing timing of B1 is delayed, the image writing start position becomes the same. As described above, this first
According to the embodiment, the correction value Δt corresponding to the horizontal deviation width dh of each of the laser beams LB1 and LB2 is set to one synchronization detection sensor 4
1, the image writing timing by the laser beams LB1, LB2 is controlled based on the obtained correction value Δt to correct the deviation of the writing start position. Therefore, the number of parts can be reduced as compared with the conventional example in which the same number of sensors as the number of lasers need to be provided. Therefore, the configuration can be simplified and the cost can be reduced.

Contrary to the above description, the second laser beam LB2 is used as a reference laser, the second laser beam LB2 is scanned twice, and the output start timing difference between the two pulse signals at that time is determined as a reference value. And then the second laser beam LB
The second and first laser beams LB1 are scanned in this order, the difference between the output start timings of the two pulse signals at that time is set as a comparison target value, and the difference between the reference value and the comparison target value is determined as a correction value. Of course it is good.

FIG. 7 is a flowchart for explaining correction value calculation processing in the digital copying machine according to the second embodiment of the present invention. In the second embodiment, the configuration provided in the digital copying machine is the same as that in the first embodiment. In the first embodiment, the reference value Δt1 and the comparison target value Δt2 are obtained by sequentially scanning the laser beam using the four continuous reflecting surfaces 23, whereas in the second embodiment, 1
The laser beam is scanned using only one of the reflection surfaces 23 to obtain the reference value Δt1 and the comparison value Δt2.
That is, when different reflecting surfaces are used, the scanning state of the laser beam is different due to the influence of the surface accuracy of the reflecting surface, and it may not be possible to obtain accurate values as the reference value Δt1 and the comparison value Δt2. is there.

In FIG. 7, the “* scan” indicates a scan performed when it is assumed that the laser beam is continuously irradiated. In the case of having a surface, scanning using the same reflecting surface is, for example, “first scanning”, “ninth scanning”, “17th scanning”, “25th scanning”, and the like. The laser control unit 52 performs the first first
The scanning of the laser beam LB1 is started (step T1). Next, the laser control unit 52 starts the second scan of the first laser beam LB1 in response to the elapse of the time required for the polygon mirror 22 to make one round (Step T).
2). In this case, since the polygon mirror 22 makes one rotation from the start of scanning, the second scanning of the first laser beam LB1 is achieved by the reflection surface used for the first scanning of the first laser beam LB1. Then, the output start timing difference Δt1 between the two pulse signals output from the synchronization detection sensor 41 by the two scans is obtained as a reference value (step T3).

Further, the laser control section 52 starts the third scanning with the first laser beam LB1 in response to the elapse of the time required for the polygon mirror 22 to make one revolution (step T4). Also in this case, since the polygon mirror 22 makes just one revolution from the start of the second scanning of the first laser beam LB1, the polygon mirror 22 performs the third scanning by the reflecting surface used for the first and second scanning of the first laser beam LB1. Of the first laser beam LB1 is achieved. Further, at the timing when the polygon mirror 22 makes just one revolution from the third scanning with the first laser beam LB1, the first scanning with the second laser beam LB2 is started (step T5). Each output start timing difference Δt2 between the two pulse signals output from the synchronization detection sensor 41 by the two scans
Is determined as a comparison target value (step T6). And
The difference between the reference value Δt1 and the comparison value Δt2 is obtained as a correction value Δt (step T7).

As described above, the scanning of the laser beam for obtaining the correction value Δt is realized by using the same reflecting surface among the reflecting surfaces 23 of the polygon mirror 22.
Even if the surface accuracy of No. 3 is different, an accurate reference value Δt1 and a comparison target value Δt2 can be obtained. Therefore, a more accurate correction value Δt can be obtained as compared with the first embodiment, and the same image writing start position can be achieved with high accuracy.

In the second embodiment, the same reflection surface is used not only for all four scans, but also when the first laser beam LB1 is used for obtaining the reference value Δt1, for example.
The "first reflecting surface" and the "second reflecting surface" are used as the reflecting surfaces to be incident twice, and the first and second laser beams LB1 and LB2 are used as the reflecting surfaces to which the first laser beam LB1 and the second laser beam LB2 are incident when the comparison value Δt2 is obtained. Alternatively, the same “first reflection surface” and “second reflection surface” as the reflection surface used when obtaining the reference value Δt1 may be used. In addition, the “second
The “reflecting surface” may be changed to any of the “third reflecting surface” to the “eighth reflecting surface”.

According to this configuration, the same reflection surface is used for obtaining the reference value Δt1 and the comparison value Δt2. Therefore, even if the surface accuracy of the reflection surface is different, the accurate reference value Δt1 and the comparison value The target value Δt2 can be obtained. The two embodiments of the present invention have been described above, but it goes without saying that the present invention can take embodiments other than the above-described embodiments. For example, in each of the above embodiments, a case is described in which the present invention is applied to a digital copying machine in which an image for two lines is written on the photoconductor 5 in one scan, but the present invention is applied to n (n ≧
3) The present invention can be easily applied to a digital copying machine having three lasers and writing an image for n lines on the photosensitive member 5 in one scan.

In this case, a laser beam emitted from any one of the n lasers is set as a reference laser beam,
In the second and subsequent scans, after scanning the reference laser beam, a process of scanning any of the remaining laser beams is sequentially performed, and the horizontal deviation width dh between the reference laser beam and each of the remaining laser beams is calculated. The corresponding correction value Δt may be obtained.

In each of the above embodiments, for example, the first
After scanning the laser beam LB1 twice to determine the reference value Δt1, the first laser beam LB1 and the second laser beam LB2 are scanned to determine the comparison value Δt2. Scanning of the laser beam LB1 is omitted and 2
After the second scanning of the first laser beam LB1, the second laser beam LB
2 may be performed to obtain the comparison target value Δt2. According to this configuration, since the scanning of the laser beam is performed only three times, the processing is simplified as compared with the above embodiments.

Further, in the above embodiment, the reference value Δt1 is also obtained each time the correction value Δt is obtained. However, for example, only the reference value Δt1 is separately obtained, and the obtained reference value Δt1 is used each time. You may make it. According to this configuration, the correction value calculation processing can be simplified. However, since the reference value Δt1 itself may change due to aging or the like, it is needless to say that the reference value Δt1 is preferably obtained every time the correction value Δt is obtained as described above.

Further, in each of the above-described embodiments, the correction value Δt is obtained based on the pulse signal output from the synchronization detection sensor 41 for defining the image writing timing. A dedicated sensor for obtaining the correction value Δt may be provided separately from the sensor 41. Further, in each of the above embodiments, the case where the present invention is applied to a digital copying machine has been described. It is needless to say that the present invention can be applied to other multi-beam type image forming apparatuses in which writing is performed on a photoconductor.

In addition, various design changes can be made within the scope described in the claims.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a part of an internal configuration of a digital copying machine according to a first embodiment of the present invention.

FIG. 2 is a plan view illustrating a configuration of a laser beam scanning unit.

FIG. 3 is a block diagram illustrating an electrical configuration of a laser beam scanning unit.

FIG. 4 is a diagram for explaining a difference in a position on a reflection surface of a polygon mirror on which two laser beams are incident according to a positional relationship between two lasers.

FIG. 5 is a flowchart illustrating a process for obtaining a correction value required to correct an image writing start position.

FIG. 6 is a timing chart for explaining a correction value calculation process.

FIG. 7 is a flowchart illustrating a correction value calculation process in a digital copying machine according to a second embodiment of the present invention.

FIG. 8 is a diagram showing a configuration of a laser optical system in a conventional digital copying machine.

[Explanation of symbols]

5 Photoconductor (Recording Medium) 22 Polygon Mirror (Scanning Means) 24 Polygon Motor (Scanning Means) 41 Synchronous Detection Sensor (Signal Output Means) 42 Incident Surface 52 Laser Control Unit (Correction Value Calculation Means, Timing Control Means) 53 Polygon Control Part (scanning means) 55 Motor driver (scanning means) L1 First laser (reference laser) L2 Second laser (comparative laser) LB1 First laser light LB2 Second laser light M Main scanning direction

Claims (3)

[Claims] A plurality of laser beams emitted by turning on a plurality of lasers are scanned in a main scanning direction by a scanning unit, so that a plurality of lines of laser light are applied to a recording medium by a plurality of laser beams. An image forming apparatus for writing an image in one scan, having one incident surface on which a plurality of laser beams scanned by the scanning unit are incident, and any one of the laser beams incident on this incident surface 1 to output a pulse signal during a period
One signal output unit, an output timing of a signal output from the signal output unit when only a predetermined reference laser of the plurality of lasers is turned on, and one of the plurality of lasers other than the reference laser. (Hereinafter referred to as “comparison target laser”), the difference between the output timing of the signal output from the signal output means and the output timing of the signal is calculated as a comparison target value. A comparison value calculating means for performing the comparison with the comparison target laser, and a correction value calculating means for calculating a difference between the comparison value obtained by the comparison value calculation means and a predetermined reference value as a correction value; On the basis of the correction value obtained by the correction value calculating means, each of the plurality of lasers is adjusted so that the image writing start position on the recording medium coincides. An image forming apparatus comprising a timing control means for controlling the image writing timing by a plurality of laser beam irradiated. 2. The apparatus according to claim 1, further comprising: a reference value calculating means for obtaining, as a reference value, an output timing difference between signals output from the signal output means when the reference laser is turned on twice. 2. The image forming apparatus according to claim 1, wherein a difference between the comparison target value obtained by the comparison target value calculating means and the reference value obtained by the reference value calculating means is obtained as a correction value. apparatus. 3. The scanning means has a polygonal mirror having a plurality of reflecting surfaces, and the reference laser and the laser light to be compared should be respectively incident upon obtaining the output timing difference in the comparison value calculating means. 3. The image forming apparatus according to claim 2, wherein the reflection surface is the same as the reflection surface on which the reference laser light is to be incident twice when the output timing difference is determined by the reference value calculation means.