Image forming apparatus

Abstract

To suppress variation in the exposure start position in a main scanning direction caused by face tilt of a polygon mirror.SOLUTION: A control unit starts exposure of a first photosensitive drum with a first beam BK when a first time has elapsed from detection of the first beam BK by a first sensor 108, and starts exposure of a second photosensitive drum with a second beam BY when a second time has elapsed from detection of the first beam BK by the first sensor 108. A correction value to correct the second time in a case where the second photosensitive drum is exposed with the second beam BY reflected by a specific surface is calculated on the basis of a difference between the detection time being the time from the detection of the second beam BY reflected by the specific surface among a plurality of mirror surfaces 51A by a second sensor 109 to the detection of the first beam BK reflected by the specific surface by the first sensor 108, and a reference time.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus that scans a plurality of beams onto a plurality of different photosensitive drums to form an image in a plurality of colors on a sheet. [Background technology]

[0002] An optical scanning device for a color laser printer is known, which scans multiple beams with a single polygon mirror (see Patent Document 1). The optical scanning device has an optical sensor (hereinafter referred to as a "BD sensor") that detects the beams reflected by the polygon mirror, and starts exposing the photosensitive drum a predetermined time after the BD sensor detects the beams. The predetermined time that defines the exposure start timing is set for each beam corresponding to a color that forms an image. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-180772 Summary of the Invention [Problem to be solved by the invention]

[0004] The mirror surface of a polygon mirror may tilt with respect to the axis of rotation due to manufacturing errors (hereinafter, this tilt will be referred to as "face tilt"), and the amount of face tilt varies among multiple mirror surfaces. When face tilt exists, the beam incident on the mirror surface will not only have an error in the direction of travel in the sub-scanning direction, but also an error in the direction of travel in the main scanning direction.

[0005] Due to an error in the direction of travel in the main scanning direction, if the predetermined time that defines the exposure start timing is set without taking into account the surface tilt, there is a problem that a deviation occurs in the exposure start position on the photosensitive drum depending on the amount of surface tilt.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to suppress variations in the exposure start position in the main scanning direction due to tilt of the polygon mirror surface. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, the image forming apparatus includes a first photosensitive drum, a second photosensitive drum, a first incident optical system, a second incident optical system, a polygon mirror, a first scanning optical system, a second scanning optical system, a first sensor, a second sensor, and a control unit. The first incident optical system emits a first beam that exposes a first photosensitive drum. The second incident optical system emits a second beam that exposes a second photosensitive drum. The polygon mirror rotates about a rotation axis extending in the first direction and deflects the first beam and the second beam in the main scanning direction. The polygon mirror has a plurality of mirror surfaces. The first scanning optical system is disposed on one side of the polygon mirror in a second direction perpendicular to the first direction, and the first beam deflected by the polygon mirror is incident on the first scanning optical system. The second scanning optical system is disposed on the other side of the polygon mirror in the second direction, and the second beam deflected by the polygon mirror is incident on the second scanning optical system. The first sensor detects the first beam reflected by the polygon mirror, and the first sensor detects the first beam upstream of an effective scanning range of the first scanning optical system in the scanning direction of the first beam in the first scanning optical system. The second sensor detects the second beam reflected by the polygon mirror downstream of the effective scanning range of the second scanning optical system in the scanning direction of the second beam in the second scanning optical system. The control unit starts exposing the first photosensitive drum with the first beam when a first time has elapsed since the first sensor detected the first beam, and starts exposing the second photosensitive drum with the second beam when a second time has elapsed since the first sensor detected the first beam. The control unit is configured to calculate a correction value for correcting the second time when exposing the second photosensitive drum with the second beam reflected on the specific surface, based on the difference between a detection time, which is the time from when the second sensor detects the second beam reflected on a specific surface of the multiple mirror surfaces to when the first sensor detects the first beam reflected on the specific surface, and a reference time.

[0008] The difference between the detection time, which is the time from when the second sensor detects the second beam reflected by a specific surface of the multiple mirror surfaces until when the first sensor detects the first beam reflected by the specific surface, and the reference time is a value that reflects an error in the direction in which the first and second beams are reflected in the main scanning direction due to the surface tilt of the specific surface. Therefore, by calculating a correction value for correcting the second time when the second photosensitive drum is exposed by the second beam reflected by the specific surface based on this difference, it is possible to reduce errors in the exposure start timing of the second photosensitive drum. In other words, it is possible to suppress variations in the exposure start position in the main scanning direction due to the surface tilt of the polygon mirror.

[0009] In the image forming apparatus, the first incident optical system may make the first beam incident on the polygon mirror obliquely in the sub-scanning direction, and the second incident optical system may make the second beam incident on the polygon mirror obliquely in the sub-scanning direction. The first incident optical system and the second incident optical system may be arranged so that both the first beam and the second beam are incident on the polygon mirror from one side of a plane that passes through the polygon mirror and is orthogonal to the first direction.

[0010] When the second incident optical system is incident on the polygon mirror from the same side as the first incident optical system with respect to a plane passing through the polygon mirror and perpendicular to the first direction, an error is likely to occur in the exposure start position of the second beam in the main scanning direction, but the error can be reduced by applying a correction value to the second time that determines the exposure start timing.

[0011] The image forming apparatus can further include a third photosensitive drum and a third incident optical system that emits a third beam that is exposed to the third photosensitive drum. In this case, the control unit may be configured to start exposing the third photosensitive drum with the third beam when a third time has elapsed since the first sensor detected the first beam, and to correct the third time using the correction value.

[0012] The control unit may acquire the detection time corresponding to each of the multiple mirror surfaces, and calculate the reference time by averaging the acquired detection times corresponding to each surface. [Effects of the Invention]

[0013] According to the present invention, it is possible to suppress variations in the exposure start position in the main scanning direction due to tilt of the polygon mirror surface. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2 is a perspective view of the scanning optical device as seen from the other side in the first direction. [Figure 2] FIG. 2 is a diagram illustrating an optical system. [Figure 3] FIG. 3 is a cross-sectional view taken along the line III-III in FIG. [Figure 4] FIG. 4 is a cross-sectional view taken along line IV-IV of FIG. [Figure 5] FIG. 2 is a diagram showing the scanning optical device as viewed from above, illustrating the path of a beam emitted from a semiconductor laser until it reaches an optical sensor. [Figure 6] 1A is a diagram illustrating tilt of the polygon mirror surface, and FIG. 1B is a diagram illustrating deviation of the beam traveling direction in the main scanning direction due to tilt of the polygon mirror surface. [Figure 7] 10A and 10B are diagrams illustrating a geometric deviation of the scanning range due to a tilt of the surface in the positive direction. [Figure 8] 8 is a diagram showing the scanning range when the polygon mirror is rotated by θ3 in comparison with FIG. 7 so that the first beam is directed toward the first sensor. [Figure 9] 10 is a timing chart illustrating correction of exposure timing. DETAILED DESCRIPTION OF THE INVENTION

[0015] 1 and 2, the scanning optical device 1 includes a frame F, an incident optical system Li, a deflector 50, a scanning optical system Lo, a circuit board 100, a first sensor 108, and a second sensor 109. In this embodiment, the scanning optical device 1 is applied to an electrophotographic image forming apparatus. The image forming apparatus includes four photosensitive drums 200 (see FIG. 4). In the scanning optical device 1, the timing of exposing the photosensitive drums 200 is controlled by a control unit CR included in the image forming apparatus.

[0016] In the following description, a direction parallel to the rotation axis X1 of the polygon mirror 51 (described later) is referred to as the "first direction." A direction perpendicular to the first direction, in which the polygon mirror 51, the first scanning lens 60CK, and the first scanning lens 60YM (see FIG. 4) are aligned, is referred to as the "second direction." A direction perpendicular to the first and second directions is referred to as the "third direction." The third direction corresponds to the main scanning direction, and the first direction corresponds to the sub-scanning direction of the incident optical system Li. Arrows indicating each direction in the drawings point to one side in each direction.

[0017] As shown in FIG. 4, in this embodiment, the four photosensitive drums 200 include a first photosensitive drum 200K corresponding to black, a second photosensitive drum 200Y corresponding to yellow, a third photosensitive drum 200C corresponding to cyan, and a fourth photosensitive drum 200M corresponding to magenta.

[0018] In this embodiment, the first color is "black (K)," the second color is "yellow (Y)," the third color is "cyan (C)," and the fourth color is "magenta (M)." In the following description, the components corresponding to each color may be distinguished by adding "K," "Y," "C," or "M" to the end of their reference numerals.

[0019] As shown in FIG. 2, the incident optical system Li includes a first incident optical system LiK corresponding to black, a second incident optical system LiY corresponding to yellow, a third incident optical system LiC corresponding to cyan, and a fourth incident optical system LiM corresponding to magenta. The first incident optical system LiK emits a first beam BK that exposes the first photosensitive drum 200K. The second incident optical system LiY emits a second beam BY that exposes the second photosensitive drum 200Y. The third incident optical system LiC emits a third beam BC that exposes the third photosensitive drum 200C. The fourth incident optical system LiM emits a fourth beam BM that exposes the fourth photosensitive drum 200M. The incident optical system Li includes a semiconductor laser 10, a coupling lens 20, an aperture plate 30, and a condenser lens 40.

[0020] The semiconductor laser 10 is a device that emits light. Four semiconductor lasers 10 are provided corresponding to the four photosensitive drums 200 (see FIG. 4) scanned by the scanning optical device 1. A toner image of a different color is formed on each photosensitive drum 200. The toner images formed on each photosensitive drum 200 are transferred onto the same sheet, and a color image is printed on the sheet.

[0021] The semiconductor laser 10 includes a first semiconductor laser 10K, a second semiconductor laser 10Y, a third semiconductor laser 10C, and a fourth semiconductor laser 10M.

[0022] The first semiconductor laser 10K and the third semiconductor laser 10C are arranged side by side in the first direction with a gap between them. The first semiconductor laser 10K is located on one side in the first direction with respect to the third semiconductor laser 10C.

[0023] The second semiconductor laser 10Y and the fourth semiconductor laser 10M are arranged side by side in the first direction with a gap between them. The second semiconductor laser 10Y is located on one side in the first direction with respect to the fourth semiconductor laser 10M.

[0024] The second semiconductor laser 10Y and the fourth semiconductor laser 10M are arranged in the second direction at intervals from the first semiconductor laser 10K and the third semiconductor laser 10C. The second semiconductor laser 10Y and the fourth semiconductor laser 10M are located on the other side in the second direction from the first semiconductor laser 10K and the third semiconductor laser 10C.

[0025] The coupling lens 20 includes a first coupling lens 20K, a second coupling lens 20Y, a third coupling lens 20C, and a fourth coupling lens 20M. Each coupling lens 20 is disposed in a position facing the corresponding semiconductor laser 10. The first coupling lens 20K converts the light from the first semiconductor laser 10K into a first beam BK. The second coupling lens 20Y converts the light from the second semiconductor laser 10Y into a second beam BY. The third coupling lens 20C converts the light from the third semiconductor laser 10C into a third beam BC. The fourth coupling lens 20M converts the light from the fourth semiconductor laser 10M into a fourth beam BM.

[0026] The diaphragm plate 30 has an aperture stop 31 through which each of the beams BK, BY, BC, and BM from the coupling lens 20 passes. In this embodiment, the diaphragm plate 30 is formed integrally with the frame F. The diaphragm plate 30 is located between the coupling lens 20 and the condenser lens 40. Four aperture stops 31 are provided corresponding to the four semiconductor lasers 10 and the coupling lens 20.

[0027] The condenser lens 40 is a lens that condenses the beams from the coupling lens 20 onto the mirror surface 51A of the polygon mirror 51 in the sub-scanning direction. The condenser lens 40 is located on the opposite side of the diaphragm plate 30 from the coupling lens 20. The condenser lens 40 is a single cylindrical lens through which the four beams BK, BY, BC, and BM pass. The condenser lens 40 has a cylindrical entrance surface and a flat exit surface.

[0028] As shown in FIGS. 2 and 3 , the first incident optical system LiK makes the first beam incident on the polygon mirror 51 obliquely in the sub-scanning direction. The second incident optical system LiY makes the second beam incident on the polygon mirror 51 obliquely in the sub-scanning direction. The third incident optical system LiC makes the third beam incident on the polygon mirror 51 obliquely in the sub-scanning direction. The fourth incident optical system LiM makes the fourth beam BM incident on the polygon mirror 51 obliquely in the sub-scanning direction. The first incident optical system LiK and the second incident optical system LiY are arranged so that both the first beam BK and the second beam BY are incident on the polygon mirror 51 from one side in the first direction with respect to a plane RP passing through the polygon mirror 51 and perpendicular to the first direction. The third incident optical system LiC and the fourth incident optical system LiM are arranged so that both the third beam BC and the fourth beam BM are incident on the polygon mirror 51 from the other side in the first direction with respect to the plane RP. In this embodiment, the angles of the beams BK, BY, BC, and BM incident on the polygon mirror 51 with respect to the plane RP are the same. That is, the angle that the first beam BK makes with the plane RP and the angle that the second beam BY makes with the plane RP are the same. Also, the angle that the first beam BK makes with the plane RP and the angle that the third beam BC or the fourth beam BM makes with the plane RP are the same.

[0029] As shown in FIG. 3, the deflector 50 is a device that deflects the beam from the coupling lens 20 in the main scanning direction (third direction) and includes a polygon mirror 51, a polygon motor 52, and a motor substrate 53. The polygon mirror 51 rotates about a rotation axis X1 extending in the first direction, thereby deflecting the beams BK, BY, BC, and BM that have passed through the coupling lens 20 in the main scanning direction. The polygon mirror 51 has multiple mirror surfaces 51A that are equidistant from the rotation axis X1. In this embodiment, the polygon mirror 51 has five mirror surfaces 51A (see also FIG. 1). The polygon mirror 51 deflects the beam from the coupling lens 20 in the main scanning direction. The polygon motor 52 is a motor that rotates the polygon mirror 51. The motor substrate 53 supports the polygon motor 52 and is fixed to a frame F. That is, the deflector 50 is fixed to the frame F.

[0030] As shown in FIGS. 2 and 4, the scanning optical system Lo is an optical system that focuses the beam deflected by the deflector 50 onto the surface of the photosensitive drum 200, which serves as an image plane. Note that mirrors in the scanning optical system Lo are omitted in FIG. 2. The components constituting the scanning optical system Lo are fixed to a frame F. The scanning optical system Lo includes a first scanning optical system LoK corresponding to black, a second scanning optical system LoY corresponding to yellow, a third scanning optical system LoC corresponding to cyan, and a fourth scanning optical system LoM corresponding to magenta. The first scanning optical system LoK is a scanning optical system that receives a first beam BK deflected by a polygon mirror 51 and focuses the first beam BK on an image plane. The second scanning optical system LoY is a scanning optical system that receives a second beam BY deflected by the polygon mirror 51 and focuses the second beam BY on an image plane. The third scanning optical system LoC is a scanning optical system that receives a third beam BC deflected by the polygon mirror 51 and focuses the third beam BC on an image plane. The fourth scanning optical system LoM is a scanning optical system on which the fourth beam BM deflected by the polygon mirror 51 is incident and which forms an image of the fourth beam BM on an image plane.

[0031] The first scanning optical system LoK and the third scanning optical system LoC are disposed on one side of the polygon mirror 51 in the second direction. The second scanning optical system LoY and the fourth scanning optical system LoM are disposed on the other side of the polygon mirror 51 in the second direction. That is, the deflector 50 is located between the first scanning optical system LoK and the third scanning optical system LoC and the second scanning optical system LoY and the fourth scanning optical system LoM. The beam from the deflector 50 is incident on each of the scanning optical systems LoK, LoY, LoC, and LoM.

[0032] The first scanning optical system LoK includes a first scanning lens 60CK, a second scanning lens 70K, and a reflecting mirror 81K. Of the optical components constituting the first scanning optical system LoK, the first scanning lens 60CK is the optical component closest to the deflector 50. The first scanning lens 60CK refracts the first beam BK and the third beam BC deflected by the deflector 50 in the main scanning direction to form images on the photosensitive drums 200K and 200C. The first scanning lens 60CK also has an fθ characteristic that causes the first beam BK and the third beam BC, which have been scanned at a constant angular velocity by the deflector 50, to move at a constant velocity on the photosensitive drums 200K and 200C.

[0033] The reflecting mirror 81K is a mirror that reflects the first beam BK from the first scanning lens 60CK toward the first photosensitive drum 200K.

[0034] The second scanning lens 70K is a lens that refracts the first beam BK reflected by the reflecting mirror 81K in the sub-scanning direction to form an image on the first photosensitive drum 200K. In the scanning optical system Lo, the sub-scanning direction corresponds to a direction perpendicular to the main scanning direction and the beam traveling direction. The second scanning lens 70K is disposed on one side of the polygon mirror 51 in the first direction.

[0035] The second scanning optical system LoY has a structure that is generally symmetrical to the first scanning optical system LoK with respect to a plane MP that passes through the rotation axis X1 of the polygon mirror 51 and is perpendicular to the second direction. Specifically, the second scanning optical system LoY includes a first scanning lens 60YM, a second scanning lens 70Y, and a reflecting mirror 81Y, which have the same functions as the components of the first scanning optical system LoK. Of the optical components that make up the second scanning optical system LoY, the first scanning lens 60YM is the optical component closest to the deflector 50. The reflecting mirror 81Y reflects the second beam BY from the first scanning lens 60YM toward the second photosensitive drum 200Y. The second scanning lens 70Y refracts the second beam BY reflected by the reflecting mirror 81Y in the sub-scanning direction to form an image on the second photosensitive drum 200Y.

[0036] The third scanning optical system LoC has a first scanning lens 60CK, a second scanning lens 70C, a reflecting mirror 81C, and a mirror 82C. The first scanning lens 60CK is the optical component closest to the deflector 50 among the optical components constituting the third scanning optical system LoC.

[0037] The first scanning lens 60CK is shared with the first scanning optical system LoK. The mirror 82C reflects the third beam BC from the first scanning lens 60CK to a reflecting mirror 81C. The reflecting mirror 81C reflects the third beam BC reflected by the mirror 82C toward the third photosensitive drum 200C. The second scanning lens 70C refracts the third beam BC reflected by the reflecting mirror 81C in the sub-scanning direction to form an image on the third photosensitive drum 200C.

[0038] The fourth scanning optical system LoM has a structure that is generally symmetrical to the third scanning optical system LoC with respect to a plane MP that passes through the rotation axis X1 of the polygon mirror 51 and is perpendicular to the second direction. Specifically, the fourth scanning optical system LoM includes a first scanning lens 60YM, a second scanning lens 70M, a reflecting mirror 81M, and a mirror 82M, each of which has the same functions as the components of the third scanning optical system LoC. Of the optical components constituting the fourth scanning optical system LoM, the first scanning lens 60YM is the optical component closest to the deflector 50. The first scanning lens 60YM is shared with the second scanning optical system LoY. The mirror 82M reflects the fourth beam BM from the first scanning lens 60YM to the reflecting mirror 81M. The reflecting mirror 81M reflects the fourth beam BM reflected by the mirror 82M toward the fourth photosensitive drum 200M. The second scanning lens 70M refracts the fourth beam BM reflected by the reflecting mirror 81M in the sub-scanning direction to form an image on the fourth photosensitive drum 200M.

[0039] 1, the first sensor 108 and the second sensor 109 are arranged on the circuit board 100. The first semiconductor laser 10K, the second semiconductor laser 10Y, the third semiconductor laser 10C, and the fourth semiconductor laser 10M are also arranged on the circuit board 100.

[0040] The first sensor 108 is a sensor that detects the first beam BK deflected by the deflector 50. The first sensor 108 detects the first beam BK corresponding to black. More specifically, the first sensor 108 is a sensor that determines the write timing of the beams BK, BY, BC, and BM based on a signal detected from the first beam BK.

[0041] The first sensor 108 is disposed at one end of the circuit board 100 in the second direction. As shown in FIG. 5, a detection surface 108A of the first sensor 108 detects the first beam BK reflected by the polygon mirror 51 and then reflected by the first mirror 111.

[0042] The control unit CR starts exposure of each photosensitive drum 200 when a predetermined time has elapsed since the first sensor 108 detected the first beam BK. That is, the control unit CR starts exposure of the first photosensitive drum 200K with the first beam BK when a first time T1 has elapsed since the first sensor 108 detected the first beam BK, starts exposure of the second photosensitive drum 200Y with the second beam BY when a second time T2 has elapsed since the first sensor 108 detected the first beam BK, starts exposure of the third photosensitive drum 200C with the third beam BC when a third time T3 has elapsed since the first sensor 108 detected the first beam BK, and starts exposure of the fourth photosensitive drum 200M with the fourth beam BM when a fourth time T4 has elapsed since the first sensor 108 detected the first beam BK. In this embodiment, T3=T1 and T4=T2. Note that T1 to T4 are values ​​that do not take into account errors due to surface tilt, etc. As will be described later, T1 to T4 are reference values ​​that are corrected taking into account errors due to surface tilt.

[0043] The first mirror 111 is a mirror that reflects the first beam BK deflected by the deflector 50 and guides it to the first sensor 108. The first mirror 111 is located between the polygon mirror 51 and the first sensor 108 in the third direction. The first mirror 111 is disposed at a position that overlaps with the first sensor 108 when viewed along the third direction.

[0044] 5 shows the scanning range used for image formation by the first beam BK as a first effective scanning range RA1. The first effective scanning range RA1 shown in the figure is the scanning range of the first beam BK when it does not pass through the first scanning lens 60CK, and is the range from the upstream end RA11 to the downstream end RA12. The first beam BK traveling from the deflector 50 to the first mirror 111 is shifted upstream in the rotation direction of the polygon mirror 51 (hereinafter simply referred to as the "rotation direction") from the upstream end RA11 of the first effective scanning range RA1 by a first angle θ1.

[0045] Here, the polygon mirror 51 rotates clockwise when viewed from the other side of the first direction (the direction of the arrow in FIG. 5). The first sensor 108 can detect the first beam BK upstream of the first effective scanning range RA1 of the first scanning optical system LoK in the scanning direction of the first beam BK scanned by the first scanning optical system LoK.

[0046] The second sensor 109 is a sensor that detects the second beam BY deflected by the deflector 50. The second sensor 109 detects the second beam BY corresponding to yellow. More specifically, the second sensor 109 is a sensor that detects the timing after the writing of the second beam BY onto the photosensitive drum 200 is completed.

[0047] The second sensor 109 is disposed at the other end of the circuit board 100 in the second direction. A detection surface 109A of the second sensor 109 detects the second beam BY reflected by the polygon mirror 51 and then reflected by the second mirror 112.

[0048] The second mirror 112 is provided on the frame F and is a mirror that reflects the second beam BY deflected by the deflector 50 and guides it to the second sensor 109. The second mirror 112 is located between the polygon mirror 51 and the second sensor 109 in the third direction. The second mirror 112 is located at a position that overlaps with the second sensor 109 when viewed along the third direction.

[0049] 5 shows the scanning range used for image formation by the second beam BY as a second effective scanning range RA2. The second effective scanning range RA2 shown in the figure is the scanning range of the second beam BY when it does not pass through the first scanning lens 60YM, and ranges from the upstream end RA21 to the downstream end RA22. The second beam BY traveling from the deflector 50 to the second mirror 112 is shifted downstream in the rotation direction of the polygon mirror 51 from the downstream end RA22 of the second effective scanning range RA2 by a second angle θ2.

[0050] The second sensor 109 can detect the second beam BY downstream of the second effective scanning range RA2 of the second scanning optical system LoY in the scanning direction of the second beam BY scanned by the second scanning optical system LoY.

[0051] Next, the correction of the exposure timing for each color by the control unit CR will be described. First, the deviation of reflected light due to tilt of the mirror surface 51A in the sub-scanning direction will be described. The mirror surface 51A of the polygon mirror 51 is designed to be parallel to the first direction along the rotation axis X1 shown in FIG. 6(a). However, if the mirror surface 51A is tilted relative to the first direction so as to face one side of the first direction due to a manufacturing error (hereinafter, for convenience, referred to as "positive tilt"), for example, the first beam BK and the second beam BY incident obliquely from one side of the first direction will be reflected in a direction shifted to one side of the first direction relative to the designed reflection direction indicated by the dashed-dotted line. In this case, as shown in FIG. 6(b), the first beam BK incident obliquely from one side of the first direction will be reflected in a direction shifted counterclockwise in the figure relative to the designed reflection direction indicated by the dashed-dotted line. Furthermore, the second beam BY is reflected with a deviation in the clockwise direction in the figure, the third beam BC is reflected with a deviation in the clockwise direction in the figure, and the fourth beam BM is reflected with a deviation in the counterclockwise direction in the figure. In other words, due to the surface tilt of the mirror surface 51A in the sub-scanning direction, each of the beams BK, BY, BC, and BM is reflected in a direction that is also deviated from the design direction in the main scanning direction. If the mirror surface 51A were tilted with respect to the first direction so as to face the other side of the sub-scanning direction (hereinafter, for convenience, referred to as "opposite surface tilt"), the deviation in these reflection directions would be in the opposite direction. In other words, with respect to the designed reflection direction, the first beam BK is reflected with a deviation in the clockwise direction in the figure, the second beam BY is reflected with a deviation in the counterclockwise direction in the figure, the third beam BC is reflected with a deviation in the counterclockwise direction in the figure, and the fourth beam BM is reflected with a deviation in the clockwise direction in the figure.

[0052] FIG. 7 shows the propagation directions of the beams BK, BY, BC, and BM when the mirror surface 51A is tilted in the positive direction, as shown in FIG. 6(a). In FIG. 7, the dashed lines indicate the propagation directions of the beams BK, BY, BC, and BM when the mirror surface 51A is not tilted in the positive direction. When the mirror surface 51A is tilted in the positive direction, the first black beam BK is shifted counterclockwise by θ3 relative to the designed reflection direction. Therefore, if the detection timing of the first sensor 108 is not taken into consideration, the scanning range in the main scanning direction, as shown by the first scanning range SLK, is shifted counterclockwise by θ3 from the designed scanning exposure range (effective scanning range RA1) (see the white arrow; the same applies below). Similarly, the second yellow beam BY is shifted clockwise by θ3, and the second scanning range SLY is also shifted clockwise by θ3. The third cyan beam BC is shifted clockwise by θ3, and the third scanning range SLC is also shifted clockwise by θ3. The fourth magenta beam BM is also shifted counterclockwise by θ3, and the fourth scanning range SLM is also shifted counterclockwise by θ3. Furthermore, the first beam BK reflected by the mirror surface 51A and directed toward the first sensor 108 is also shifted counterclockwise by a third angle θ3.

[0053] However, the control unit CR starts exposure of each beam BK, BY, BC, and BM when a predetermined time has elapsed since the first sensor 108 detected the first beam BK. Therefore, in reality, as shown by the black arrow in FIG. 8, when the first beam BK incident on the first sensor 108 is rotated clockwise by a third angle θ3 compared to FIG. 7, the scanning start position of the first scanning range SLK also rotates clockwise by the third angle θ3, and the other beams BY, BC, and BM also rotate clockwise by the third angle θ3. Therefore, the first scanning range SLK of the first black beam BK is the same as the designed scanning exposure range (effective scanning range RA1), and the fourth scanning range SLM of the fourth magenta beam BM is also the same as the designed scanning exposure range (effective scanning range RA2). Note that only the effect of errors due to surface tilt is explained here; if other errors such as the flatness of the mirror surface 51A, rotational irregularities of the polygon motor 52, and errors in the tilt angle of the fourth beam BM in the sub-scanning direction are taken into consideration, the scanning range of the fourth beam BM will not be the same as the designed scanning exposure range.

[0054] On the other hand, for the second yellow beam BY and the third cyan beam BC, the direction of deviation due to surface tilt (white arrow) is the same as the direction of deviation of the first beam BK incident on the first sensor 108 rotated clockwise (black arrow), so the second scanning range SLY will have an error from the designed scanning exposure range (effective scanning range RA2), and the third scanning range SLC will have an error from the designed scanning exposure range (effective scanning range RA1).

[0055] Therefore, in this embodiment, the exposure start timing of the second beam BY and the third beam BC is corrected so that the scanning ranges of the second beam BY and the third beam BC are aligned with those of the first beam BK and the fourth beam BM. It is not necessary for each beam BK, BY, BC, and BM to perfectly match the designed scanning exposure range; it is sufficient that the scanning ranges of each beam BK, BY, BC, and BM are aligned in the main scanning direction. Even if the scanning ranges of each beam BK, BY, BC, and BM are aligned and shifted in the main scanning direction (third direction) from the designed scanning exposure range, the position of the image on the printed sheet will only be slightly shifted in the main scanning direction, and the image itself will not be affected. If the scanning ranges of each beam BK, BY, BC, and BM are shifted in the main scanning direction, color shift will occur in the main scanning direction, causing image distortion. Therefore, it is necessary to align the scanning ranges of each beam BK, BY, BC, and BM.

[0056] To correct the exposure start timing, the control unit CR calculates a correction value ΔT2 for correcting the second time T2 when the second photosensitive drum 200Y is exposed to the second beam BY reflected by a specific surface based on the difference between the reference time TR and the detection time TS, which is the time from when the second sensor 109 detects the second beam BY reflected by a specific surface of the multiple mirror surfaces 51A to when the first sensor 108 detects the first beam reflected by the specific surface.The control unit CR then corrects the second time T2 using the correction value ΔT2. Similarly, the control unit CR calculates a correction value ΔT3 and corrects the third time T3 using the correction value ΔT3.

[0057] Then, the control unit CR acquires the detection time TS corresponding to each of the multiple mirror surfaces 51A, and calculates the reference time TR by averaging the acquired detection times TS corresponding to each surface.

[0058] The reference time TR is, for example, the average of the detection times TS. More specifically, at least one detection time TS is acquired for each of the multiple mirror surfaces 51A, and the reference time TR can be calculated by averaging the acquired detection times TS corresponding to each surface.

[0059] 9 is a timing chart showing the detection time TS and the timing of exposure, in which the horizontal axis represents time. The "BD1 signal" is a pulse signal obtained when the first beam BK is detected by the first sensor 108. The "BD4 signal" is a pulse signal obtained when the second beam BY is detected by the second sensor 109. In Fig. 9, the pulse signal with the designed timing is indicated by a solid line, and as an example, a pulse signal with a timing that deviates from the design is indicated by a dashed line.

[0060] "BD1 mirror surface" is the number of the mirror surface 51A that reflects the first beam BK. "BD4 mirror surface" is the number of the mirror surface 51A that reflects the second beam BY.

[0061] In this embodiment, there are five mirror surfaces 51A, and therefore five numbers are shown, 0 to 4. In the drawing, due to limitations on the size of the paper, it is not possible to show all of the mirror surfaces 51A, 0 to 4, so reflections from some of the mirror surfaces 51A are shown.

[0062] "K exposure", "C exposure", "M exposure", and "Y exposure" indicate that the photosensitive drum 200 is exposed with beams corresponding to black, cyan, magenta, and yellow, respectively.

[0063] TS1 is the detection time TS measured based on the beam reflected by the mirror surface 51A with mirror number 1 (hereinafter referred to as "mirror surface 51A1"), within the detection time TS. In other words, TS1 is the time from when the second sensor 109 detects the second beam BY reflected by the mirror surface 51A1 to when the first sensor 108 detects the first beam BK reflected by the mirror surface 51A1.

[0064] 7, when there is a plane tilt in the positive direction, the detection time TS1 from when the second beam BY is reflected by the mirror surface 51A1 and detected by the second sensor 109 until when the first beam BK is reflected by the mirror surface 51A1 and detected by the first sensor 108 is longer than when there is no plane tilt. More specifically, in FIG. 7, the third beam BY heading toward the second sensor 109 is traveling in the rotation direction (clockwise) more than designed, so the timing at which the third beam BY is detected by the second sensor 109 is earlier. On the other hand, the first beam BK heading toward the first sensor 108 is delayed in the opposite rotation direction (counterclockwise) more than designed, so the timing at which the first beam BK is detected by the first sensor 108 is later. Therefore, in the case of plane tilt in the positive direction, a time that is longer than the designed detection time is detected, as shown by TS1 in FIG.

[0065] In such a case, if no correction is made, the third scanning range SLC for cyan will be shifted to the right in FIG. 8 relative to the designed scanning exposure range, as shown in FIG. 8, so exposure must be started a little earlier through correction. FIG. 9 shows the correction required for this purpose. For example, after the BD1 signal is detected by the mirror surface 51A1 (t1), exposure of the third photosensitive drum 200C starts at a timing earlier than the third time T3 (=T1) by the absolute value of the correction value ΔT31. Similarly, after the BD1 signal is detected by the mirror surface 51A1 (t1), exposure of the second photosensitive drum 200Y starts at a timing earlier than the second time T2 (=T4) by the absolute value of the correction value ΔT21.

[0066] Contrary to Figure 7, when there is a surface tilt in the opposite direction, the detection time TS is shorter, as shown by the detection time TS2 in Figure 9. In this case, after the BD1 signal is detected by the mirror surface 51A2, exposure of the third photosensitive drum 200C starts at a timing that is later than the third time T3 (= T1) by the correction value ΔT32. Also, after the BD1 signal is detected by the mirror surface 51A2, exposure of the second photosensitive drum 200Y starts at a timing that is later than the second time T2 (= T4) by the correction value ΔT21.

[0067] A specific method for calculating the reference time TR can be, for example, as follows. Twenty detection times TS0, TS1, TS2, TS3, and TS4 are acquired. That is, the detection times TS are acquired while the polygon mirror 51 rotates 20 times. 20×5=100 detection times TS are acquired. Then, the arithmetic mean of the 100 detection times TS is calculated and set as the reference time TR. 0-p , the sum for p=1~20 is ΣTS 0-p given that, TR=(ΣTS 0-p +ΣTS 1-p +ΣTS 2-p +ΣTS 3-p +ΣTS 4-p ) / 100

[0068] In addition, 20 TS n Calculate the arithmetic mean of the average ATS n The first detected value of TS0 is TS 0-1 given that, ATS0=(TS 0-1 +TS 0-2 +···+TS 0-20 ) / 20 Similarly, calculate ATS1, ATS2, ATS3, and ATS4.

[0069] Then, the correction value ΔTm for the m-th time Tm of the mirror number n is n teeth, ΔTm n =(ATS n -TR)×km where km is a coefficient, and can be, for example, -0.2 for black, -0.9 for cyan, 0.2 for magenta, and -0.9 for yellow. Note that the coefficients here include values ​​that take into account factors other than surface tilt, and if only surface tilt is taken into consideration, the black coefficient k1 and the magenta coefficient k4 can be 0.

[0070] For example, the correction value ΔT20 for the second time T2 of mirror number 0 is ΔT20=(ATS0-TR)×(-0.9) It can be calculated as follows. This allows, for example, the position of the second scanning range SLY due to surface tilt to be corrected by starting exposure of the second beam BY when T2+ΔT20 has elapsed since the first beam BK was detected at mirror number 0.

[0071] In this embodiment, due to the symmetry of the second incident optical system LiY and the third incident optical system LiC with respect to the reference plane RP and the plane MP, the correction value ΔT2 n and cyan correction value ΔT3 n Therefore, the control unit can set the correction value ΔT2 n and correction value ΔT3 n Alternatively, the calculation may be configured to calculate only one of the above.

[0072] 9, when exposure of the second beam BY starts when T2+ΔT21 has elapsed since the BD1 signal was detected by mirror surface 51A1, the second beam BY is reflected by mirror surface 51A3. In this respect, although an error may occur due to differences in the mirror numbers of mirror surface 51A, since the error in the timing of receiving the BD1 signal is eliminated, the scanning ranges of each color can be aligned compared to the case where no correction is made.

[0073] As described above, according to this embodiment, the following effects can be obtained. The second sensor 109 detects a specific surface (mirror surface 51A) of the plurality of mirror surfaces 51A.n The difference between the reference time TR and the detection time TS, which is the time from when the first sensor 108 detects the second beam BY reflected by the specific surface until when the first sensor 108 detects the first beam BK reflected by the specific surface, is a value that reflects an error in the direction in which the first beam BK and the second beam BY are reflected in the main scanning direction due to the surface tilt of the specific surface. Therefore, by calculating a correction value ΔT2n that corrects the second time T2 when the second photosensitive drum 200Y is exposed by the second beam BY reflected by the specific surface based on this difference, it is possible to reduce the error in the exposure start timing of the second photosensitive drum 200Y. In other words, it is possible to suppress variation in the exposure start position in the main scanning direction due to the surface tilt of the polygon mirror 51.

[0074] When the second incident optical system LiY is incident on the polygon mirror 51 from one side of the plane RP that is perpendicular to the first direction and passes through the polygon mirror 51, as with the first incident optical system LiK, the exposure start position of the second beam BY in the main scanning direction is prone to error. However, in this embodiment, a correction value ΔT2 is added to the second time T2 that determines the exposure start timing. n By applying this, the error can be reduced.

[0075] The present invention is not limited to the above-described embodiment, but can be used in various forms as exemplified below. In the above embodiment, the angles that the beams BK, BY, BC, and BM make with the plane RP are the same, but these angles may be different from one another. In this case, the first time T1 before correction and the third time T3 before correction are not the same, and the second time T2 before correction and the fourth time T4 before correction are not the same. In addition, the yellow correction value ΔT2 n and cyan correction value ΔT3 n will have different values.

[0076] In the above embodiment, each mirror surface 51A n The detection time TS for each of 20 n Average ATS obtained by averaging n was calculated, but the detection time TS n The number of mirror surfaces 51A is arbitrary.n There may be one per

[0077] The reference time TR may be a fixed value. For example, the designed detection time TS may be set as the reference time TR.

[0078] In the above embodiment, the image forming apparatus that forms an image using toner of four colors has been described, but the image forming apparatus may also form an image using toner of three or five colors.

[0079] In the above embodiment, the condenser lens 40 and the coupling lens 20 are separate bodies, but the condenser lens may be integrated with the coupling lens.

[0080] The elements described in the above-described embodiment and modified examples may be implemented in any combination. [Explanation of symbols]

[0081] 1. Scanning optical device 10 Semiconductor laser 20 Coupling Lens 50 Deflector 51 Polygon Mirror 51A Mirror surface 108 First Sensor 109 Second Sensor 200 Photosensitive drum 51A Mirror surface BK 1st beam BY 2nd Beam CR control unit LiK 1st incidence optical system LiY 2nd incidence optical system LoK 1st scanning optical system LoY Second scanning optical system RA1 First effective scanning range RA2 Second effective scanning range

Claims

1. a first photosensitive drum; A second photosensitive drum; a first incident optical system that emits a first beam that exposes the first photosensitive drum; a second incident optical system that emits a second beam that exposes the second photosensitive drum; a polygon mirror that rotates around a rotation axis extending in a first direction and deflects the first beam and the second beam in a main scanning direction, the polygon mirror having a plurality of mirror surfaces; a first scanning optical system disposed on one side of the polygon mirror in a second direction orthogonal to the first direction, the first scanning optical system onto which the first beam deflected by the polygon mirror is incident; a second scanning optical system disposed on the other side of the polygon mirror in the second direction, on which the second beam deflected by the polygon mirror is incident; a first sensor that detects the first beam reflected by the polygon mirror, the first sensor detecting the first beam upstream of an effective scanning range of the first scanning optical system in a scanning direction of the first beam in the first scanning optical system; a second sensor that detects the second beam reflected by the polygon mirror, the second sensor detecting the second beam downstream of an effective scanning range of the second scanning optical system in a scanning direction of the second beam in the second scanning optical system; a control unit, The control unit starting exposure of the first photosensitive drum with the first beam when a first time has elapsed since the first sensor detected the first beam; when a second time has elapsed since the first sensor detected the first beam, exposure of the second photosensitive drum with the second beam is started; a correction value for correcting the second time when the second photosensitive drum is exposed to the second beam reflected by the specific surface is calculated based on a difference between a detection time, which is a time from when the second sensor detects the second beam reflected by the specific surface of the plurality of mirror surfaces until when the first sensor detects the first beam reflected by the specific surface, and a reference time; An image forming apparatus comprising:

2. the first incident optical system makes the first beam incident obliquely onto the polygon mirror in the sub-scanning direction; the second incident optical system makes the second beam incident obliquely onto the polygon mirror in the sub-scanning direction; 2. The image forming apparatus according to claim 1, wherein the first incident optical system and the second incident optical system are arranged so that both the first beam and the second beam are incident on the polygon mirror from one side of a plane passing through the polygon mirror, which is perpendicular to the first direction.

3. A third photosensitive drum; a third incident optical system that emits a third beam that is to be exposed onto the third photosensitive drum, The control unit when a third time has elapsed since the first sensor detected the first beam, exposure of the third photosensitive drum with the third beam is started; 3. The image forming apparatus according to claim 2, wherein the third time is corrected based on the correction value.

4. 2. The image forming apparatus according to claim 1, wherein the control unit acquires the detection time corresponding to each of the plurality of mirror surfaces, and calculates the reference time by averaging the acquired detection times corresponding to each of the surfaces.

Images