JP2009175470A - Optical scanner and image forming apparatus equipped with the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical scanner in which the shift of the scanning magnification of a beam is controlled by using a single beam detection sensor, and to provide an image forming apparatus equipped with the optical scanner. <P>SOLUTION: The beam deflected by the respective reflection faces 31a to 31d of a polygon mirror 31 is detected by the beam detection sensor 33, scanning times between faces are successively measured by a time counting part 34, and the latest four scanning times between faces t1 to t4 are saved by overwriting in a storage part 35. The scanning time Ti per rotation of the mirror is calculated by adding the recorded scanning times between faces t1 to t4. The scanning time Ti is compared with a reference time TO at a comparison control part 36, a reference clock frequency FO generated from a reference clock generation part 37 is multiplied by the ratio Ti/TO of shift quantity of the scanning time Ti with respect to the reference time TO and a write clock Fi used for i-th scanning is generated. Further, obtained write clock Fi is used to perform the i-th scan. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical scanning device used in an image forming apparatus such as a digital copying machine, a laser printer, and a laser facsimile, and an image forming apparatus including the optical scanning device.

  In general, an optical scanning device uses a scanning optical system including a cylinder lens, a polygon mirror, an fθ lens, and the like to emit a beam emitted from a beam light source device (LD unit) having a laser light emitting unit (hereinafter referred to as LD). The image is formed as a spot on the surface to be scanned, and the polygon mirror is rotated so that the surface to be scanned is scanned at a constant speed in the main scanning direction. In particular, a multi-optical scanning device having a plurality of LDs can quickly scan the surface to be scanned and form an electrostatic latent image without increasing the number of rotations of the polygon mirror, compared to scanning using a single beam. Therefore, it is widely used in image forming apparatuses such as copying machines, laser printers, and laser facsimiles.

  The configuration of a conventional optical scanning device is shown in FIG. The optical scanning device 4 emits a beam from the LD unit 30 based on an instruction from an LD driving unit (not shown), and the emitted beam passes through a collimator lens (not shown) or a cylinder lens (not shown). It passes through and enters the polygon mirror 31. The polygon mirror 31 has an accurate polygon shape, rotates in a constant direction at a constant speed, deflects the beam from the LD unit 30, and is on the surface to be scanned 101 in the main scanning direction (the white arrow direction in the figure). Make it scan fast. In this example, it is assumed that the polygon mirror 31 is a regular hexagon and rotates clockwise at a constant speed. The beam deflected by the polygon mirror 31 enters the fθ lens 32. The fθ lens 32 forms an image on the scanned surface 101 using the beam from the LD unit 30 as a beam spot.

  The beam detection sensor 33 detects the beam deflected by the polygon mirror 31, and is provided outside the effective exposure region on the scanning start side of the surface to be scanned 101 in order to set the scanning start timing in the main scanning direction of the beam. ing. Based on the detection result of the beam detection sensor 33, the scanning start timing in the main scanning direction of the beam deflected by the polygon mirror 31 is corrected.

  In such an optical scanning device, the printed image or the like may be caused by uneven rotation of a polygon motor that rotates a polygon mirror, a wavelength difference due to variation in LD characteristics, or a change in characteristics of an fθ lens due to a change in environmental temperature or the like. Image defects such as double (scanning magnification) failure and vertical line fluctuation are likely to occur. However, in the above optical scanning device, since the beam detection sensor 33 is provided only on the scanning start side, information on the entire writing width cannot be obtained, and the deviation of the writing width due to the change in scanning magnification can be corrected. Can not.

  Therefore, a method has been proposed in which the beam detection sensor detects the beam on the scanning start side and the scanning end side and corrects the deviation of the writing width, and Patent Document 1 discloses the beam detection sensor as the scanning start side and the scanning end side. An optical scanning device disposed at two locations is disclosed. Further, Patent Document 2 discloses a beam scanning device that can detect with one sensor by deflecting beams on the scanning start side and scanning end side with an optical path changing unit.

With such a configuration, the time difference between the timing at which the beam detection sensor detects the beam on the scanning start side and the scanning end side, that is, the scanning time, is measured and compared with the reference time to thereby determine the writing width of the beam. The deviation can be corrected, and the change in the scanning magnification of the beam and the scanning start timing can be controlled due to the difference in wavelength due to the rotation unevenness of the polygon mirror and the variation of the LD, or the characteristic change of the fθ lens due to the fluctuation of the environmental temperature. , Image deterioration can be prevented.
Japanese Patent Laid-Open No. 1-240072 JP 2005-62712 A

  However, in the method of Patent Document 1, it is necessary to arrange the beam detection sensors at two places, and in the method of Patent Document 2, it is necessary to arrange optical path changing means such as a folding mirror. An arrangement space has to be secured, and the number of parts is increased and the manufacturing process is complicated, which is disadvantageous in terms of cost. Further, Patent Document 1 also describes that the effective scanning time can be measured using only one of the sensors on the scanning start side or the scanning end side, but a specific measurement method, measurement timing, etc. Was not described at all.

  SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a simple optical scanning apparatus capable of correcting a change in scanning magnification of a beam and a shift in scanning start timing using a single beam detection sensor, and an image forming apparatus including the same. The purpose is to provide.

  In order to achieve the above object, the present invention provides a regular polyhedral optical deflector having a plurality of reflecting surfaces for deflecting a beam emitted from a light source, and a beam scanned in the main scanning direction by the optical deflector. An optical scanning apparatus comprising: a beam detecting unit that detects outside an effective exposure area of a scanning surface; and a control unit that modulates a writing frequency of a beam emitted from a light source based on a detection timing of the beam detecting unit. The control means scans the inter-surface scanning time from when the beam deflected by the beam deflecting means is detected by the predetermined reflecting surface of the optical deflector until the beam deflected by the next reflecting surface is detected. A scanning time per predetermined rotation of the optical deflector is calculated by adding back an integral multiple of the number of reflection surfaces from immediately before, and the calculated scanning time is compared with a reference time to calculate the writing period in the scanning. It is characterized by modulating the number from the reference frequency.

  According to the present invention, in the optical scanning device having the above-described configuration, the control unit scans per predetermined rotation of the optical deflector calculated by adding the inter-surface scanning time more than twice the number of reflection surfaces. The writing frequency is modulated from the reference frequency by comparing the time with a reference time every time the optical deflector rotates a predetermined number of times more than twice.

  According to the present invention, in the optical scanning device having the above-described configuration, a storage unit capable of storing the inter-surface scanning time by at least the number of reflection surfaces is provided, and the control unit is a surface stored in the storage unit. The scanning time per predetermined rotation of the optical deflector is calculated by adding the inter-scanning time by an integral multiple of the number of the reflecting surfaces in the order from immediately before scanning.

  According to the present invention, in the optical scanning device having the above-described configuration, storage means for storing a scanning time per predetermined rotation of the optical deflector is provided, and the control means stores the scanning stored in the storage means immediately before scanning. It is characterized by comparing time with reference time.

  The present invention is also an image forming apparatus including the optical scanning device having the above-described configuration.

  According to the present invention, in the image forming apparatus configured as described above, the image forming apparatus includes an image detection unit that detects an image printed on a recording medium or an image carrier, and is based on position information of a reference image detected by the image detection unit. The writing frequency is adjusted to correct the scanning magnification, the writing frequency at the time of correction is set to the reference frequency, and the inter-surface scanning time detected by the beam detecting means during printing of the reference image is set to the reflecting surface. The reference time is set by adding an integral multiple of the number of faces.

  According to the first configuration of the present invention, the writing frequency is corrected using the scanning time per predetermined rotation of the optical deflector obtained by adding the inter-plane scanning time by an integral multiple of the number of reflection surfaces. Therefore, regardless of the surface accuracy of the optical deflector, it is possible to control the change in the scanning magnification due to the uneven rotation of the mirror with high accuracy. Further, since the inter-surface scanning time is added retroactively from immediately before scanning, the latest scanning time can always be compared with the reference time, and the correction accuracy of the writing frequency becomes higher.

  According to the second configuration of the present invention, in the optical scanning device of the first configuration, the scanning time is calculated by adding the inter-surface scanning time to twice or more the number of reflection surfaces, and the optical deflector In the optical scanning device in which the optical deflector rotates at a high speed, the write frequency is corrected from the reference frequency by modulating the write frequency from the reference frequency every time the lens is rotated a predetermined number of times more than twice. The processing speed can be made to follow the scanning speed. Also, the load on the control means can be reduced.

  Further, according to the third configuration of the present invention, in the optical scanning device having the first or second configuration, the inter-surface scanning time is stored in the storage unit at least by the number of reflection surfaces. By calculating the scanning time per predetermined rotation of the optical deflector by adding the inter-surface scanning time by an integer multiple of the number of reflective surfaces in the order from immediately before scanning, the scanning time immediately before each scanning can be quickly and efficiently performed. Can be calculated.

  According to the fourth configuration of the present invention, in the optical scanning device having the first or second configuration, the scanning time per predetermined rotation of the optical deflector calculated by adding the inter-plane scanning time in advance is set. By storing in the storage means and comparing the scan time stored immediately before the scan with the reference time, the storage capacity of the storage means can be reduced.

  Further, according to the fifth configuration of the present invention, by mounting the optical scanning device having any one of the first to fourth configurations, the rotation unevenness of the optical deflector, the wavelength variation of the light source, the lens, etc. An image forming apparatus capable of effectively preventing the expansion and contraction of the image by accurately correcting the change in the scanning magnification caused by the change in the characteristics of the optical member.

  According to the sixth configuration of the invention, in the image forming apparatus having the fifth configuration, the scanning frequency is corrected by adjusting the writing frequency based on the position information of the reference image detected by the image detection means. At the same time, the writing frequency at the time of correction is set to the reference frequency, and the inter-surface scanning time detected during printing of the reference image is added to set the reference time, so that it is appropriate according to the state of the device, environmental conditions, etc. It is possible to set an appropriate reference time and reference frequency.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of a tandem type color image forming apparatus equipped with the optical scanning device of the present invention. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (magenta, cyan, yellow, and black), and magenta, cyan, and yellow are respectively subjected to charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  Photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and are formed on the photosensitive drums 1a to 1d. The toner images thus transferred are sequentially transferred (primary transfer) onto an intermediate transfer belt 8 that moves adjacent to each image forming unit while rotating clockwise in FIG. 1 by a driving means (not shown). The image is transferred onto the paper P at a time (secondary transfer) by the next transfer roller 9 and further fixed on the paper P by the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The paper P onto which the toner image is transferred is housed in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. Further, a cleaning blade 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. Removes the developer (toner) remaining on the photosensitive drums 1a to 1d, the optical scanning device 4 that exposes the toner, the developing units 3a, 3b, 3c, and 3d that form toner images on the photosensitive drums 1a to 1d. Cleaning parts 5a, 5b, 5c and 5d are provided.

  When the image formation start is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then the beam is irradiated by the optical scanning device 4 to each photosensitive drum 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on 1d. Each of the developing units 3a to 3d includes a developing roller (developer carrying member) disposed opposite to the photosensitive drums 1a to 1d, and each of magenta, cyan, yellow, and black toners is supplied by a replenishing device (not shown). A predetermined amount is filled. This toner is supplied onto the photosensitive drums 1a to 1d by the developing rollers of the developing units 3a to 3d and electrostatically adheres to the electrostatic latent image formed by exposure from the optical scanning device 4. A toner image is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, magenta, cyan, yellow, and black toner images on the photosensitive drums 1a to 1d are transferred onto the intermediate transfer belt 8 by the transfer rollers 6a to 6d. The primary transfer. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched over a driven roller 10, a drive roller 11, and a tension roller 20, and the intermediate transfer belt 8 starts to rotate clockwise as the drive roller 11 is rotated by a drive motor (not shown). Then, the sheet P is conveyed from the registration roller 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the sheet P is conveyed at a nip portion (secondary transfer nip portion) with the intermediate transfer belt 8. A full color image is secondarily transferred onto P. The paper P on which the toner image is transferred is conveyed to the fixing unit 7.

  The sheet P conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13 to fix the toner image on the surface of the sheet P, and a predetermined full-color image is formed. It is formed. The paper P on which the full-color image is formed is distributed in the transport direction by the branching section 14 that branches in a plurality of directions. When an image is formed on only one side of the paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when forming images on both sides of the paper P, a part of the paper P that has passed through the fixing unit 7 is once projected from the discharge roller 15 to the outside of the apparatus. Thereafter, the paper P is distributed to the paper transport path 18 by the branching section 14 by rotating the discharge roller 15 in the reverse direction, and is transported again to the secondary transfer roller 9 with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the paper P on which the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image, It is discharged to the discharge tray 17.

  FIG. 2 is a side cross-sectional view showing a configuration around the intermediate transfer belt in the tandem type color image forming apparatus of FIG. Portions common to FIG. 1 are denoted by the same reference numerals and description thereof is omitted. In FIG. 2, the chargers 2a to 2d and the cleaning units 5a to 5d are not shown.

  Density detection sensors 21a to 21d are arranged on the downstream side of the developing units 3a to 3d and the upstream side of the transfer rollers 6a to 6d in the rotation direction of the photosensitive drums 1a to 1d. As the density detection sensor 21, an optical sensor including a light emitting element composed of an LED or the like and a light receiving element composed of a photodiode or the like is generally used. When measuring the toner adhesion amount on the photosensitive drums 1a to 1d, if the reference light formed on the photosensitive drums 1a to 1d is irradiated with measurement light from the light emitting elements, the measurement light is reflected by the toner. And incident on the light receiving element as light reflected by the drum surface.

  When the toner adhesion amount is large, the reflected light from the drum surface is shielded by the toner, so that the light reception amount of the light receiving element decreases. On the other hand, when the toner adhesion amount is small, the amount of reflected light from the drum surface increases, resulting in an increase in the amount of light received by the light receiving element. Therefore, the density of each color is corrected by detecting the density of the reference image of each color based on the output value of the received light signal based on the amount of reflected light received and adjusting the characteristic value of the developing bias in comparison with the predetermined reference density. Is done. The density detection sensors 21a to 21d may be arranged at positions where the reference image transferred onto the intermediate transfer belt 8 can be detected.

  The color misregistration detection sensor 23 is disposed on the downstream side of the photosensitive drum 1d disposed on the most downstream side in the traveling direction of the intermediate transfer belt 8 and on the upstream side of the secondary transfer roller 9, and the image forming unit. The position of a reference image for color misregistration correction formed in Pa to Pd and transferred onto the intermediate transfer belt 8 is detected. As the color misregistration detection sensor 23, a reflective optical sensor similar to the density detection sensors 21a to 21d is used, but other sensors can also be used. For example, a line sensor or an area sensor that is composed of a CCD element, a lens, a driver circuit, etc., forms an image of an object on the surface of the CCD element by a lens, converts the amount of light into a video pulse signal, and outputs it. Can be mentioned.

  FIG. 3 is an example of a reference image for color misregistration correction. On the intermediate transfer belt 8, a reference image composed of diagonal lines and horizontal lines M, C, Y, and B of magenta (M), cyan (C), yellow (Y), and black (B) is formed. The arrow X1 indicates the belt traveling direction. The patterns of the reference images M to B shown in FIG. 3 are general, and color misregistration in the main scanning direction (belt width direction) is detected using the diagonal lines and horizontal lines of each color, and sub-intervals are determined from the intervals between the horizontal lines of each color. Color misregistration in the scanning direction (belt circumferential direction) is detected.

  Further, the reference images M to B are formed in the same pattern at both ends in the main scanning direction, so that the scanning magnification and the scanning inclination can be detected. Further, in order to reduce uneven detection of color misregistration in the belt circumferential direction, the reference images M to B are repeatedly formed in the sub-scanning direction, and the same pattern is measured a plurality of times to obtain an average value of the misregistration amount. ing. When the positional relationship between the oblique lines and the straight lines of these colors is detected by the color misregistration detection sensor 23 and compared with a predetermined reference position, and the color misregistration in the main scanning direction is corrected, the exposure start position and writing of the optical scanning device 4 are corrected. When adjusting the frequency and correcting the color misregistration in the sub-scanning direction, the color misregistration correction is performed for each color by adjusting the exposure start timing of the optical scanning device 4.

  FIG. 4 is a block diagram showing a schematic configuration diagram and a control path of the optical scanning device of the present invention. Portions common to FIG. 10 of the conventional example are denoted by the same reference numerals and description thereof is omitted. Here, a case where any one of the photosensitive drums 1a to 1d shown in FIG. In the optical scanning device of the present invention, a beam emitted from the LD unit 30 and deflected and scanned by the polygon mirror 31 is detected by one beam detection sensor 33, and the scanning time per rotation of the polygon mirror 31 is determined based on the detection timing. calculate. Then, the change in the scanning magnification is corrected by comparing the calculated scanning time with the reference time.

  A method of correcting the scanning magnification in the optical scanning device of the present invention will be described. In FIG. 4, the beam emitted from the LD unit 30 enters the polygon mirror 31. Here, the polygon mirror 31 is a square having four reflecting surfaces 31a to 31d and rotates at a constant speed in the clockwise direction. However, the polygon mirror 31 is not limited to a square, and any other shape can be used as long as it is a regular polygon. Can also be used. Further, the rotation direction and rotation speed of the polygon mirror 31 can be set as appropriate in accordance with the specifications of the optical scanning device 4.

  The beam deflected by the polygon mirror 31 enters the fθ lens 32, and then forms an image on the scanned surface 101 as a beam spot, and is scanned in the main scanning direction (the white arrow direction in the figure). At this time, the beam detection sensor 33 is used to detect the beam on the scanning start side outside the effective exposure area of the surface to be scanned 101.

  The beam detection sensor 33 outputs a signal to the time counting unit 34 according to the timing at which the beam is detected. As the beam detection sensor 33, various optical sensors such as a photodiode, a phototransistor, and a photo IC can be used. In FIG. 4, the beam detection sensor 33 is arranged on the scanning start side outside the effective exposure region of the scanned surface 101, but the beam detecting sensor 33 does not affect the scanning end side or scanning of the scanned surface. It is also possible to arrange in other places. Further, the beam optical path from the LD unit 30 to the beam detection sensor 33 may be changed by providing a mirror, a prism, or the like.

  The time counting unit 34 measures the scanning time (inter-surface scanning time) from each reflective surface of the polygon mirror 31 to the next reflective surface based on the interval of the detection signals output from the beam detection sensor 33. The relationship between the output of the beam detection sensor 33 and the rotation period of the polygon mirror 31 is shown in FIG. Since the surface accuracy (angle accuracy of adjacent surfaces) of the reflection surfaces 31a to 31d of the polygon mirror 31 is different in each surface, the inter-surface scanning times t1 to t4 from the first surface to the fourth surface are also different as shown in FIG. Yes. The inter-surface scanning time measured by the time counting unit 34 is transferred to the storage unit 35.

  The storage unit 35 can store an inter-surface scanning time that is at least the number of reflection surfaces (four in this case) of the polygon mirror 31. FIG. 6 shows the relationship between the inter-plane scanning time and the scanning time per one rotation of the mirror. As shown in FIG. 6, the inter-surface scanning times t1 to t4 between the reflecting surfaces 31a to 31d are different, but the rotation per mirror rotation obtained by adding the inter-surface scanning times t1 to t4 so as to include one by one. In the scanning times T1, T2,... (Hereinafter referred to as Ti), the influence of the surface accuracy of the reflecting surfaces 31a to 31d is eliminated. Therefore, by comparing the scanning time Ti with a preset reference time, only the rotation unevenness of the polygon mirror 31 can be accurately detected.

  The scanning time Ti per one rotation of the mirror obtained by adding the inter-surface scanning times t1 to t4 measured by the time counting unit 34 in advance may be stored in the storage unit 35. In this way, the amount of data stored in the storage unit 35 decreases, so that the storage capacity can be reduced. If the scanning time Ti obtained by adding the inter-surface scanning times t1 to t4 immediately before scanning is overwritten and stored, only the latest scanning time Ti is always stored in the storage unit 35, so that the storage capacity is minimized. Can be limited.

  The calculated scanning time Ti is transferred to the comparison control unit 36. The comparison control unit 36 includes data of the reference scanning time, compares the transferred scanning time Ti with the reference time T0, and calculates the ratio of deviation from the reference time T0. The write clock generator 38 corrects the frequency of the reference clock (reference frequency F0) generated from the reference clock generator 37 in accordance with the ratio of the deviation. For example, when the measured scanning time Ti is longer than the reference time T0, the beam position is in front of the swing angle of the standard polygon mirror 31, so the image signal is output later. The image needs to be stretched. Therefore, the video clock is delayed in accordance with the ratio of the scanning time deviation, that is, the frequency of the video clock is set lower than the reference frequency F0. On the contrary, when the measured scanning time Ti is shorter than the reference time T0, the video clock is set earlier in accordance with the ratio of the scanning time deviation, that is, the video clock frequency is set higher than the reference frequency F0.

  Further, as the reference time T0 and the reference frequency F0, it is preferable to use values at the time of correcting the scanning magnification (at the time of calibration) based on the detection results of the reference images M to K (see FIG. 3) by the color misregistration detection sensor 23. . Thereby, it is possible to set appropriate T0 and F0 according to the state of the apparatus, environmental conditions, and the like. In addition, when the optical scanning device 4 is replaced, when the image forming apparatus 100 is activated, or when calibration is automatically executed every predetermined number of printed sheets, T0 and F0 can be updated each time calibration is executed. By doing so, the frequency of the video clock can always be corrected using the latest T0 and F0.

  In this way, the write clock is generated by correcting the frequency of the reference video clock and transmitted to the LD drive unit 39. The LD drive unit 39 can correct the scanning magnification to prevent image expansion and contraction by converting the frequency of the image signal transmitted from the image signal generation unit (not shown) to the LD drive unit 39 to the write clock frequency. it can. Here, the setting of the scanning start position of the LD unit 30 is not described in detail, but the beam detection on the scanning start side detected by the beam detection sensor 33 based on the detection result of the color misregistration detection sensor 23 (see FIG. 2). The start position can also be set by correcting the timing.

  FIG. 7 is a flowchart showing a procedure for correcting the scanning magnification during printing in the image forming apparatus including the optical scanning device of the present invention. The procedure for correcting the scanning magnification according to the steps of FIG. 7 will be described in more detail with reference to FIGS. 4 to 6 as necessary. First, rotation of the polygon mirror 31 is started prior to printing (step S1).

  Then, when the rotation of the polygon mirror 31 is stabilized, the LD in the LD unit 30 is turned on, the beam deflected by each of the reflection surfaces 31a to 31d of the polygon mirror 31 is detected by the beam detection sensor 33, and the time counting unit 34 The inter-plane scanning time tn (here, t1 to t4) is sequentially measured and overwritten and recorded in the storage unit 35 (step S2). As a result, the latest four inter-surface scanning times t1 to t4 are always stored in the storage unit 35. Then, when printing is started (step S3), the inter-surface scanning times t1 to t4 recorded in the storage unit 35 are added to calculate the scanning time Ti per one rotation of the mirror immediately before the i-th scanning ( Step S4). The calculated scanning time Ti is transmitted to the comparison control unit 36.

  Next, the comparison control unit 36 compares the scanning time Ti with the reference time T0 (step S5). As shown in FIG. 8, since the motor that drives the polygon mirror 31 generally has long-period rotation unevenness, the scanning time Ti also changes from the reference time T0 at the time of scanning magnification correction as time elapses. Therefore, the write clock Fi used for the i-th scanning is generated by multiplying the ratio Ti / T0 of the deviation amount of the scanning time Ti with respect to the reference time T0 by the frequency F0 of the reference clock generated from the reference clock generator 37 (step S6). ).

  For example, when Ti is smaller than T0, the image is enlarged in the main scanning direction. Therefore, the write clock generation unit 38 sets the frequency of the reference clock F0 by the ratio of the deviation amount with respect to the reference time T0 of Ti. The raised write clock Fi is generated. On the other hand, when Ti is larger than T0, the image is reduced in the main scanning direction. Therefore, in the write clock generation unit 38, the frequency of the reference clock is equal to the deviation amount ratio of T1 to the reference time T0. A write clock Fi with F0 lowered is generated.

  Then, using the obtained write clock Fi, the i-th (first, second,...) Scan is executed as shown in FIG. 6 (step S7). Then, it is determined whether or not the scanning is finished (step S8). If the scanning is finished, the process is finished. On the other hand, if scanning continues, 1 is added to i (step S9), and the process returns to step S4 and the same processing is repeated thereafter (steps S4 to S8).

  By performing the above control, writing can be performed at a frequency corrected in consideration of motor rotation unevenness, LD wavelength variation, optical member characteristic change, etc., and the scanning magnification can be corrected with high accuracy. it can. For example, considering the case where 5 dots are uniformly written in the effective print width in the i-th scan, when the polygon mirror scan time is the reference time T0, the reference image written at the reference frequency F0 is shown in FIG. And As shown in FIG. 8, since the scanning time Ti at the i-th scanning is smaller than T0, when the write frequency F0 is not modulated, the image is shifted in the main scanning direction from the reference image as shown in FIG. 9B. Enlarged and written. Therefore, by writing at a frequency Fi modulated by multiplying F0 by Ti / T0, it is possible to fit within the effective print width as in the reference image as shown in FIG. 9C.

  In step S5 in FIG. 7, it is determined as normal if T1 / T0 is an allowable magnification error range, and the writing frequency is corrected when the time difference is larger than that, which is practically necessary. Correction may be performed only in such a case. In this way, it is possible to reduce the load on the comparison control unit 36 or the LD drive unit 39 due to the correction processing being performed in all the scans, thereby speeding up the processing.

  Further, here, the scanning time Ti per rotation of the polygon mirror 31 obtained by adding the inter-surface scanning times t1 to t4 corresponding to the number of reflection surfaces (four surfaces) of the polygon mirror 31 is compared with the reference time T0. The writing frequency is controlled to be corrected. For example, the scanning time per multiple rotations of the polygon mirror 31 is obtained by adding the inter-plane scanning time that is twice or more (8, 12,...) The number of reflecting surfaces. Control may be performed to correct the writing frequency by comparing with the reference time every time the polygon mirror 31 rotates a predetermined number of times. With this control, the correction frequency of the writing frequency is reduced, so that the correction processing speed can be made to follow the writing speed even in an image forming apparatus that has a high printing speed and needs to rotate the polygon mirror 31 at a high speed. Also, the load on the comparison control unit 36 can be reduced.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, in the above-described embodiment, the single beam scanning apparatus including one LD has been described, but a multi-beam scanning apparatus including a plurality of LDs can be described in exactly the same manner. In the case of a multi-beam scanning device, the beam detection sensor 33 is shifted in time so that beams are incident from a plurality of LDs, thereby generating detection signals for each beam individually, and based on this, the write clock frequency is modulated. Just do it.

  The optical scanning device of the present invention is not limited to the tandem type color printer as shown in FIG. 1, but is also applicable to other image forming apparatuses such as monochrome and color copiers, digital multifunction peripherals, monochrome printers, and facsimiles. Of course you can.

  The present invention provides a regular polyhedral optical deflector having a plurality of reflecting surfaces for deflecting a beam emitted from a light source, and a beam scanned in the main scanning direction by the optical deflector outside the effective exposure area of the scanned surface. In the optical scanning device comprising: a beam detecting means for detecting the light beam; and a control means for modulating the writing frequency of the beam emitted from the light source based on the detection timing of the beam detecting means. The inter-plane scanning time from the detection of the beam deflected by the predetermined reflecting surface of the optical deflector until the detection of the beam deflected by the next reflecting surface is an integral multiple of the number of reflecting surfaces immediately before scanning. The scanning time per predetermined rotation of the optical deflector is calculated by going back and forth, the calculated scanning time is compared with the reference time, and the writing frequency in the scanning is modulated from the reference frequency.

  Thereby, it is possible to provide an optical scanning device capable of controlling the change in scanning magnification due to the rotation unevenness of the optical deflector with high accuracy regardless of the surface accuracy of the reflecting surface. Further, since the latest scanning time obtained by adding the inter-surface scanning time in the order from immediately before the scanning is compared with the reference time, the writing frequency correction accuracy is further improved.

  Further, in an optical scanning apparatus that performs scanning at high speed, it is difficult to follow the scanning speed if the writing frequency correction processing is performed for all scanning, but the inter-surface scanning time is twice the number of reflection surfaces. If the scanning frequency is calculated by adding the above and the write frequency is modulated from the reference frequency in comparison with the reference time every time the optical deflector rotates two or more times, the correction frequency decreases. Even in an optical scanning device in which the optical deflector rotates at high speed, the processing speed can be made to follow the scanning speed. Also, the load on the control means can be reduced.

  In addition, by mounting the optical scanning device having the above configuration on the image forming apparatus, it is possible to prevent image expansion and contraction due to uneven rotation of the optical deflector, variation in the wavelength of the LD, and changes in the characteristics of the optical member. Image formation can be realized. Furthermore, if the scanning time and the writing frequency at the time of calibration for correcting the scanning magnification from the position information of the reference image using the image detection means are set to the reference time and the reference frequency, the accuracy according to the state of the apparatus and the environmental conditions, etc. It is possible to set an appropriate reference time and reference frequency.

These are schematic sectional views of an image forming apparatus on which the optical scanning device of the present invention is mounted. FIG. 2 is a side cross-sectional view illustrating a configuration around an intermediate transfer belt of the image forming apparatus illustrated in FIG. 1. FIG. 4 is a schematic diagram illustrating an example of a color misregistration correction reference image. These are the block diagram which shows the schematic block diagram and control path | route of the optical scanning apparatus of this invention. These are figures which show the relationship between the output of a beam detection sensor, and the rotation period of a polygon mirror. These are figures which show the relationship between the scanning time between surfaces, and the scanning time per rotation of a mirror. These are the flowcharts which show the correction procedure of the scanning magnification in the optical scanning device of the present invention. These are graphs showing changes in the scanning time Ti with time. FIG. 6 is a schematic diagram illustrating an example of correction of scanning magnification. These are the schematic diagrams which show the structure of the conventional optical scanning device.

Explanation of symbols

1a to 1d Photosensitive drum 4 Optical scanning device 8 Intermediate transfer belt 23 Color misregistration detection sensor (image detection means)
30 LD unit (light source)
31 Polygon mirror (optical deflector)
31a to 31d Reflecting surface 32 fθ lens 33 Beam detection sensor (beam detection means)
34 Time counting section 35 Storage section (storage means)
36 Comparison control unit (control means)
37 Reference clock generation unit 38 Write clock generation unit 39 LD drive unit 100 Image forming apparatus 101 Scanned surface

Claims (6)

A regular polyhedral light deflector having a plurality of reflecting surfaces for deflecting a beam emitted from a light source;
Beam detecting means for detecting a beam scanned in the main scanning direction by the optical deflector outside the effective exposure area of the surface to be scanned;
An optical scanning device comprising: control means for modulating the writing frequency of the beam emitted from the light source based on the detection timing of the beam detection means;
The controller is configured to determine an inter-surface scanning time from when the beam deflected by a predetermined reflecting surface of the optical deflector is detected by the beam detecting unit to when the beam deflected by the next reflecting surface is detected. A scanning time per predetermined rotation of the optical deflector is calculated by adding back an integer multiple of the number of reflection surfaces from immediately before scanning, and the writing frequency in the scanning is compared with the reference scanning time. An optical scanning device characterized by modulating the frequency from a reference frequency.   The control means sets the scanning time per predetermined rotation of the optical deflector calculated by adding the inter-surface scanning time to twice or more the number of the reflecting surfaces, and the optical deflector performs a predetermined number of times twice or more. The optical scanning device according to claim 1, wherein the write frequency is modulated from the reference frequency in comparison with the reference time every rotation.   Storage means capable of storing the inter-surface scanning time by at least the number of the reflective surfaces is provided, and the control means sets the inter-surface scanning times stored in the storage means in the order of the reflection surfaces from immediately before scanning. 3. The optical scanning device according to claim 1, wherein a scanning time per predetermined rotation of the optical deflector is calculated by adding an integral multiple of the number of surfaces.   Storage means for storing a scanning time per predetermined rotation of the optical deflector is provided, and the control means compares the scanning time stored in the storage means immediately before scanning with a reference time. Item 3. The optical scanning device according to Item 1 or Item 2.   An image forming apparatus comprising the optical scanning device according to claim 1. Comprising image detection means for detecting an image printed on a recording medium or an image carrier,
The writing frequency is adjusted based on the position information of the reference image detected by the image detecting means to correct the scanning magnification, the writing frequency at the time of correction is set to the reference frequency, and the reference image is printed during printing. 6. The image forming apparatus according to claim 5, wherein the inter-surface scanning time detected by the beam detecting unit is set to the reference time by adding an integral multiple of the number of reflection surfaces.

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