JP5470342B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that forms an electrostatic latent image by scanning light emitted from a light source in a main scanning direction of an image carrier using a polygon mirror.

  An electrophotographic image forming apparatus forms an electrostatic latent image by reflecting light emitted from a light source to a polygon mirror that is rotationally driven and scanning it in the main scanning direction of the photosensitive drum. Each surface of the polygon mirror is a reflecting mirror, and the surface of the photosensitive drum that rotates by reflecting light is scanned in the main scanning direction. At this time, if the inclination of each surface is equal to the rotation axis, the pitch of lines drawn by each surface (the line pitch in the sub-scanning direction that is the rotation direction of the photosensitive drum) is constant.

  However, there may be variations in the pitch of lines drawn by the surfaces of the polygon mirror due to variations in the tilt of the surfaces with respect to the rotation axis due to variations in processing accuracy (surface tilt) and vibrations caused by high-speed rotation. As a result, for example, even if image data composed of a plurality of horizontal lines at equal intervals is used, the pitch of each horizontal line in the formed image varies, so that there is a problem that an image with good image quality cannot be obtained. .

  Therefore, Patent Document 1 describes a method of correcting the main scanning direction with respect to the surface tilt while specifying the reflection surface of the polygon mirror.

JP-A-2005-96086

  However, although the method of Patent Document 1 is effective for aligning the writing position in the main scanning direction, it cannot correct the line shift in the sub-scanning direction and cannot suppress deterioration in image quality.

  The present invention has been made to solve the above-described problems, and provides an image forming apparatus capable of suppressing deterioration in image quality of a formed image even when a polygon mirror is tilted or vibrated due to high-speed rotation. It is for the purpose.

  The image forming apparatus according to the first aspect of the present invention includes a data conversion unit that converts input image data into dot image data configured by arranging pixel values having gradation information in a matrix, and an image carrier. A light source that forms an electrostatic latent image on the image carrier by emitting light modulated according to the pixel value for each line of the dot image data, a rotating polyhedron having a plurality of reflecting surfaces, and the rotation A driving means for rotationally driving the polyhedron, the driving means rotating the rotating polyhedron, and deflecting means for guiding light emitted from the light source in the main scanning direction of the image carrier; and each reflection of the rotating polyhedron Storage means for storing surface tilt information, assigning means for assigning, for each line of the dot image data, the number of the reflection surface that the light emitted from the light source reflects, and the surface tilt information Determining means for determining the writing timing of each line of the dot image data based on the number of the reflecting surface assigned to each line of the dot image data and the pixel value of the dot image data; Exposure control means for emitting light to the light source in accordance with the writing start timing.

  According to this configuration, since the determining unit determines the writing timing of each line based on the surface tilt information of the rotating polyhedron and the pixel value of the dot image data, image quality deterioration due to the surface tilt can be reduced.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the determining unit may include the light reflected on the reflective surface and the light immediately before the reflective surface at each reflective surface. A reflection surface whose interval in the sub-scanning direction on the image carrier of the light reflected from the reflection surface is wider than a predetermined reference pitch or a reflection surface that is narrower than the reference pitch is specified from the surface tilt information, and the identification The pixel value of the line of the dot image data to which the number of the reflecting surface is assigned is compared with the pixel value of the line immediately above the line to determine the writing start timing of the line.

  According to this configuration, when the line pitch is not normal, the pixel value between the lines is compared and the writing timing is changed, so that the writing timing can be adjusted in consideration of the positional relationship of the dots between the lines. It is possible to reduce image quality deterioration due to this.

  According to a third aspect of the present invention, in the image forming apparatus according to the second aspect, the determining means is a line other than white among the lines of the dot image data to which the number of the specified reflecting surface is assigned. The target pixel is sequentially set for the pixel, and in the data indicated by the line one line above the line, the pixel immediately above and the diagonally upper right pixel when viewed from the target pixel are white, and the diagonally upper left pixel is If the pitch is other than white, the first count is incremented when the pitch between the line and the line immediately above the line is wider than the reference pitch, and the second count is incremented when the pitch is narrower than the reference pitch. When the pixel just above the target pixel and the diagonally upper left pixel are white and the diagonally upper right pixel is other than white, the pitch between the line and the line one above the line is The second count is incremented when wider than the quasi-pitch, the first count is incremented when narrower than the reference pitch, and when the first count is greater than the second count, When the write timing is advanced by a predetermined time from the normal write timing and the second count number is larger than the first count number, the line write timing is determined to be delayed by a predetermined time from the normal write timing.

  “The pixel directly above the target pixel and the pixel on the diagonally upper right are white and the pixel on the diagonally upper left is other than white” means that the pixel on the diagonally upper left among the three pixels on the target pixel Only means non-white (such as black). At this time, when the line pitch is wider than the reference pitch (the lines are separated from each other), the separation of the lines can be visually filled by bringing the target pixel and the diagonally upper left pixel closer to each other. In addition, when the line pitch is narrower than the reference pitch (the lines overlap), the overlap between the lines can be visually reduced by separating the target pixel and the diagonally upper left pixel.

  Similar determination is also made for the case where “the pixel directly above the target pixel and the pixel on the diagonally upper left are white and the pixel on the diagonally upper right is other than white”. The first count number is a number by which a visual effect can be obtained between the two pixels by advancing the line write timing, and the second count number is determined by delaying the line write timing. The number that can obtain a visual effect between pixels is shown.

  Is it better to advance the writing timing of the line by setting the pixels other than white for one line sequentially as target pixels and finally comparing the first count number and the second count number (majority decision)? Judge whether it is better. As described above, the determining means adjusts the writing start timing, so that it is possible to suppress deterioration in image quality due to trouble.

  According to the present invention, when the line pitch of the image to be formed is not normal due to the tilting of the rotating polyhedron, the pixel timing between the lines is compared and the writing timing is changed. Thus, it is possible to adjust the writing start timing, and it is possible to reduce the deterioration of image quality due to the trouble.

1 is a schematic diagram of an internal structure of an image forming apparatus. The schematic diagram which shows the state which irradiated the light beam to the reflective surface of the polygon mirror. The block diagram which shows the irradiation control system of a light beam. The figure which compared the irradiation position of the light beam when a polygon mirror does not have a plane fall and a plane fall. The figure which showed an example of the dot image. The figure which showed an example of the dot image. The figure which extracted and showed line numbers 1 and 2 among the dot images shown in FIG. The figure which extracted and showed line numbers 2 and 3 among the dot images shown in FIG. The flowchart which showed the flow of the exposure process. The flowchart which showed the flow of the exposure process following FIG.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. FIG. 1 is a diagram schematically showing an internal configuration of an image forming apparatus provided with a fixing device according to the present invention. In this embodiment, a printer is described as an example of an image forming apparatus. However, in addition to this, an image forming apparatus that employs an electrophotographic system, such as a copier, a facsimile machine, or a multifunction machine having a plurality of these functions. I just need it. The image forming apparatus described in this embodiment is a color printer, but may be a monochrome printer.

  The image forming apparatus 1 is a tandem color printer including image forming units 2 (2M, 2C, 2Y, and 2K) for each color of magenta (M), cyan (C), yellow (Y), and black (K). is there. Each of the image forming units 2M, 2C, 2Y, and 2K includes a developing device 3, a charger 5, an exposure device 6, a toner supply unit 7, a cleaner 21, and a primary transfer roller 9.

  The toner supply unit 7 stores magenta, cyan, yellow, and black toners. The developing device 3 supplies the toner supplied from the toner supply unit 7 to the photosensitive drum 4.

  The photosensitive drum 4 is located below a transfer belt 8 described later and is disposed in contact with the outer surface of the transfer belt 8. From the upstream side in the rotation direction B of the transfer belt 8, a magenta photosensitive drum 4, a cyan photosensitive drum 4, a yellow photosensitive drum 4, and a black photosensitive drum 4 are arranged in parallel. The photosensitive drum 4 is made of a-Si (amorphous silicon) or the like and rotates in the clockwise direction (A direction in the figure) in FIG.

  A primary transfer roller 9 is disposed through the transfer belt 8 in a state where the primary transfer roller 9 is in contact with the inner surface of the transfer belt 8 at a position facing the photosensitive drum 4. The primary transfer roller 9 is a roller that is driven to rotate by the rotation of the transfer belt 8, nips the transfer belt 8 with the photosensitive drum 4, and transfers the toner images of the respective colors formed on the photosensitive drum 4 to the transfer belt 8. A primary transfer portion T for primary transfer is configured. In the primary transfer portion T, the toner images of the respective colors are transferred onto the transfer belt 8 in a multiple manner. As a result, a color toner image is formed on the transfer belt 8.

  The charger 5 uniformly charges the peripheral surface of the photosensitive drum 4. The exposure device 6 has a polygon mirror 28 that guides a light beam based on image data transmitted from an external device to the peripheral surface of the photosensitive drum 4. The polygon mirror 28 is rotated by a motor 32 which will be described later, and scans a light beam on the circumferential surface of each photosensitive drum 4 in the main scanning direction to form an electrostatic latent image on each circumferential surface. The main scanning direction is a direction in which the light beam is scanned in the longitudinal direction of the photosensitive drum 4. The polygon mirror 28 is shared between the plurality of photosensitive drums 4.

  The developing device 3 supplies toner to the photosensitive drum 4. As a result, toner adheres to the electrostatic latent image, and a toner image is formed on the photosensitive drum 4. The cleaner 21 is disposed on the peripheral surface of each photosensitive drum 4 and removes residual toner and the like on the peripheral surface.

  The transfer belt 8 is arranged above the row of the photosensitive drums 4 and is stretched between the driven roller 10 and the driving roller 11 so that the outer surface is in contact with the peripheral surface of the photosensitive drum 4. It is installed. Further, the transfer belt 8 is urged upward by a tension roller 19. The driving roller 11 rotates in response to a driving force from a driving source (not shown) to drive the transfer belt 8 to rotate. The driven roller 10 is driven to rotate by the rotation of the transfer belt 8. As a result, the transfer belt 8 rotates in the B direction (counterclockwise direction).

  Further, the transfer belt 8 is in a state where a portion wound around the driving roller 11 is bent, and the toner image primarily transferred to the transfer belt 8 is secondarily transferred to the paper P in this bent portion. Secondary transfer position P1. At the secondary transfer position P1, a secondary transfer roller 18 facing the driving roller 11 with the transfer belt 8 interposed therebetween is provided. The secondary transfer roller 18 forms a nip with the driving roller 11, and secondarily transfers the toner image on the outer surface of the transfer belt 8 onto the paper P passing through the nip.

  A pair of registration rollers 17 is disposed below the secondary transfer position P1. The registration roller 17 transports the paper P toward the secondary transfer position P1 at an appropriate timing and corrects the oblique feeding of the paper P.

  Above the secondary transfer position P1, there is provided a fixing device 14 that performs a fixing process on the paper P on which the secondary transfer of the toner image has been performed at the secondary transfer position P1. The fixing device 14 includes a heating roller 14a and a pressure roller 14b, and this pair of rollers forms a fixing nip portion NP. When the paper P passes through the fixing nip NP, the heating roller 14a heats the paper P, and the pressure roller 14b presses the paper P, so that the second-transferred toner image is fixed on the paper P. To do.

  A paper feed cassette 12 that stores a bundle of paper is disposed below the exposure device 6. Between the paper feed cassette 12 and the secondary transfer position P1, a paper transport path 13 for guiding the paper P from the paper feed cassette 12 to the secondary transfer position P1 is provided. The registration roller 17 described above is disposed in the sheet conveyance path 13. In addition to the registration rollers 17, a plurality of roller pairs for guiding the paper P are disposed at appropriate positions in the paper transport path 13.

  The heating roller 14 a and the pressure roller 14 b convey the paper P toward the discharge roller pair 151 after passing through the fixing nip NP. On the upper surface of the image forming apparatus 1, a discharge unit 16 that discharges the paper P that has been subjected to the fixing process by the fixing device 14 is formed, and between the discharge unit 16 and the fixing device 14, the paper P Is provided with a paper discharge path 15. The paper P is conveyed to the paper discharge path 15 by driving the discharge roller pair 151 and is discharged to the discharge unit 16.

  FIG. 2 is a schematic diagram showing a state in which the light beam LB is irradiated on the reflection surface 282 of the polygon mirror 28 in the present embodiment. FIG. 3 is a block diagram showing a light beam irradiation control system according to the present embodiment. In FIG. 3, optical components such as a collimator lens and an fθ lens are not shown.

  The light beam generation unit 30 includes a light source 31 that emits the light beam LB, a drive circuit that drives the light source 31, and the like. As the light source 31, for example, a semiconductor laser is used.

  The polygon mirror 28 reflects the light beam LB generated by the light beam generator 30. The polygon mirror 28 rotates in the direction of the arrow R <b> 1 about the rotation shaft 281 by driving the motor 32. The polygon mirror 28 has, for example, a regular pentagonal side surface, and the five side surfaces correspond to the reflecting surface 282.

  The control unit 300 controls the entire image forming apparatus 1, and includes, for example, a CPU (Central Processing Unit), a ROM (Read Only Memory) that stores various programs and data necessary for the execution in advance, a working It is constituted by a microcomputer including a RAM (Random Access Memory) serving as a memory and its peripheral circuits, and has an exposure control unit 303, a write timing determination unit 307, and a face tilt information storage unit 309.

  The writing timing determination unit 307 determines the writing timing of each line of dot image data to be described later based on the surface tilt information of each reflecting surface 282 of the polygon mirror 28 stored in the surface tilt information storage unit 309. A method for determining the writing timing will be described in detail later.

  The exposure control unit 303 controls the light beam generation unit 30 to irradiate the light beam LB and draw an electrostatic latent image on the photosensitive drum 4. That is, the exposure control unit 303 sends the image data stored in the storage unit 301 to the light beam generation unit 30 while rotating the polygon mirror 28 by the motor 32, and according to the write timing determined by the write timing determination unit 307. The light beam generation unit 30 emits the light beam LB. The light beam LB emitted from the light source 31 is applied to the rotating polygon mirror 28, deflected by the reflecting surface 282, and draws the scanning line SL on the photosensitive drum 4 in the main scanning direction D1. One scanning line SL is drawn by one reflecting surface 282. By repeating the drawing of one scanning line SL on the photosensitive drum 4 thus rotating, an electrostatic latent image is drawn along the sub-scanning direction. The sub-scanning direction corresponds to the rotation direction R2 of the photosensitive drum 4.

  As illustrated in FIG. 3, the timing signal generation unit 305 includes a BD (Beam Detect) sensor 27 and a BD signal conversion unit 29. The light beam LB is repeatedly scanned in the main scanning direction D1 within a scanning range longer than the dimension of the photosensitive drum 4 in the main scanning direction D1. The BD sensor 27 is installed in a position where the light beam LB receives the light beam LB before the light beam LB starts scanning the photosensitive drum 4 in the scanning range.

  The BD sensor 27 is a photosensor, and receives the light beam LB reflected by the reflecting surface 282 and outputs the received light signal to the BD signal conversion unit 29. The BD signal conversion unit 29 shapes the received light signal into a rectangular wave timing signal BD, and outputs the timing signal BD to the exposure control unit 303.

  The timing signal BD is a reference signal for aligning the writing position in the main scanning direction D1 when an electrostatic latent image is drawn on the photosensitive drum 4. The exposure control unit 303 emits the light beam LB according to the writing timing determined by the writing timing determination unit 307 with the timing signal BD as a reference.

  Next, the surface tilt of the polygon mirror will be described. FIG. 4 is a diagram comparing the irradiation position S1 of the light beam on the photosensitive drum 4 when the polygon mirror is not tilted and the irradiation position S2 of the light beam when the polygon mirror is tilted. A, B, C, D, and E planes indicate the respective reflecting surfaces 282 of the polygon mirror 28. Each straight line in the figure represents an ideal irradiation position on each reflecting surface 282 (a normal line pitch is indicated between lines). In the case of a polygon mirror having no surface tilt, the pitch of lines formed by the respective reflecting surfaces is constant as shown in FIG.

  On the other hand, in the case of a polygon mirror with surface tilt, the irradiation position S2 of the light beam is deviated from the ideal irradiation position depending on the reflection surface as shown in FIG. 4B. According to FIG.4 (b), although A surface, C surface, and E surface do not have surface inclination, it turns out that there exists surface inclination because the irradiation position S has shifted | deviated from the ideal position in B surface and D surface. . Due to such a surface tilt, the pitch of lines formed on the photosensitive drum 4 varies. If the line pitch varies, a good quality image cannot be obtained. It is also known that the same phenomenon occurs when vibration is generated by the polygon mirror 28 rotating at a high speed.

  5 and 6 are diagrams showing examples of dot images. FIG. 5 is a dot image formed using an ideal polygon mirror that does not have surface tilt, and FIG. 6 is a dot image formed using a polygon mirror that has surface tilt. In addition, although the dotted line in the figure is a line that is not originally formed, it is shown for easy understanding that each dot is formed corresponding to pixel data arranged in a matrix.

  The control unit 300 converts image data input from an external device or the like into dot image data configured by arranging pixel values having gradation information in a matrix. The exposure control unit 303 causes the light beam generation unit 30 to generate a light beam LB modulated based on the pixel value (gradation information) of each pixel constituting the dot image data.

  When an ideal polygon mirror is used, as shown in FIG. 5, dots are formed at a regular line pitch without overlapping or separating adjacent dots.

  On the other hand, when a polygon mirror with surface tilt is used, the line pitch is disturbed, so that adjacent dots are overlapped or separated. For example, when the B surface and D surface of the polygon mirror 28 are tilted, the dots of line numbers 2 and 4 are formed as shifted as shown in FIG.

  Specifically, a dot (line number 1) formed by the light beam LB reflected by the A surface which is one of the reflecting surfaces 282 and a dot (line number) formed by the light beam LB reflected by the B surface. In 2), adjacent dots overlap each other. Hereinafter, the dot position in the main scanning direction is indicated by a column number, the dot position in the sub-scanning direction is indicated by a line number, and the dot of line number X and column number Y is expressed as “dot (XY)”.

  In addition, the diagonal dots (for example, the dot 1-3 and the dot 2-2) should originally be formed at positions where the corners face each other, but a part of each side is in contact with each other. It has become. Further, the dot (line number 2) formed by the light beam LB reflected by the B surface is separated from the dot (line number 3) formed by the light beam LB reflected by the C surface.

  Similarly, the dot (line number 3) formed by the light beam LB reflected on the C surface is separated from the dot (line number 4) formed by the light beam LB reflected on the D surface. Further, the dots formed by the light beam LB reflected by the D surface (line number 4) and the dots formed by the light beam LB reflected by the E surface (line number 5) are adjacent to each other. . In addition, the dots in a diagonal relationship (for example, the dots 4-5 and 5-4) are in a state where a part of each side is in contact. In this way, when contact or separation of edges between diagonally related dots occurs, the contacted portion has a high density, and the separated portion appears to have a low density. I can't.

  Therefore, the writing timing determination unit 307 determines the writing timing of each line based on the surface tilt information of each reflecting surface 282. As a result, it is possible to suppress deterioration in image quality caused by variations in line pitch due to surface tilt. Hereinafter, a method for determining the writing timing will be described in detail.

  The surface tilt of each reflecting surface 282 of the polygon mirror 28 is measured in advance before the image forming apparatus 1 is shipped from the factory. The surface tilt information storage unit 309 stores the information in advance in association with each reflective surface 282. The writing start timing determination unit 307 calculates which reflection surface 282 the light beam LB emitted for each line of the dot image data is reflected, and assigns a reflection surface number to each line. Then, based on the reflecting surface number assigned to each line and the surface tilt information of each reflecting surface 282 stored in the surface tilt information storage unit 309, the writing timing determination unit 307 determines the writing timing of each line.

  FIG. 7 is a diagram showing line numbers 1 and 2 extracted from the dot image shown in FIG. The dot image data used when the writing timing determination unit 307 determines the writing timing is data in which pixel values are arranged in a matrix without overlapping between lines as shown in FIG. Then, in order to make it easy to understand visually, the dot timing data used for the processing by the writing timing determination unit 307 is illustrated and explained with a shift of the line pitch.

  First, a target pixel is determined for line number 2. The target pixel is a pixel having a pixel value other than white. Then, when the target pixel is the dot 91B (dot 2-2), the pixel values of the peripheral pixels of the dot 91B on the line of line number 1 that is the line one above are viewed. The peripheral pixels of the dot 91B in the line number 1 are the pixel 1-1, the pixel 1-2, and the pixel 1-3, and the pixel 1-3 (dot 91A) has a pixel value other than white.

  When the polygon mirror 28 is not tilted, the corners of the dots 91A and 91B face each other. However, if the surface is tilted, the line pitch is disturbed, resulting in a positional relationship in which a part of each side of the two dots is in contact. Therefore, the writing timing is shifted in a direction to cancel the contact relationship. That is, since the contact with the dot 91A can be eliminated by moving the dot 91B in the direction of the arrow shown on the dot 91B (by advancing the writing start timing), the writing timing determining unit 307 can Increment the count in the direction to advance the timing (early count).

  Next, when the target pixel is the dot 92B (dot 2-6), the pixels other than white among the peripheral pixels of the dot 92B in the line number 1 are the dot 92A (dot 1-5) and the dot 93A (dot 1-). 6). Of the peripheral pixels of the target pixel, the pixel immediately above is white and the target pixel is moved only when one of the diagonal pixels is other than white. The count in the direction to advance the write timing (early count) and the count to delay the write timing (late count) are not incremented.

  Subsequently, when the target pixel is the dot 93B (dot 2-8), the pixel other than white among the peripheral pixels of the dot 93B in the line number 1 is the dot 94A (dot 1-9). Therefore, the dot 93B is moved in the direction of the arrow shown on the dot 93B. In other words, since the dot 93B can be moved in the direction of the arrow shown on the dot 93B by advancing the write timing, the write timing determining unit 307 counts in the direction to advance the write timing (early count). Increment.

  When the target pixel is the dot 94B (dot 2-10), the pixels other than white among the peripheral pixels of the dot 94B in the line number 1 are the dots 94A (dots 1-9). Accordingly, the dot 94B is moved in the direction of the arrow shown on the dot 94B. That is, since the dot 94B can be moved in the direction of the arrow shown on the dot 94B by delaying the writing timing, the writing timing determination unit 307 counts the direction in which the writing timing is delayed (late count). Increment.

  In this way, the write timing determination unit 307 sequentially sets the pixel other than white with line number 2 as the target pixel, and only the pixel on the upper right side of the pixel on the line one pixel above the target pixel is white. Otherwise, the count in the direction to advance the write timing (early count) is incremented, and when only the upper left pixel is other than white, the count in the direction to delay the write timing (late count) is incremented. Then, the write timing determination unit 307 finally compares the numbers of the early count and the late count. When “early count> late count”, the write timing is advanced, and when “early count <late count”, the write timing is determined. When the delay is “early count = late count”, the writing timing is not changed (the normal writing timing set in advance is used). In the case of FIG. 7, since the early count = 2 and the late count = 1, the write timing determination unit 307 sets the write timing of the line number 2 earlier by a predetermined time. The change width of the writing timing is set to a predetermined time, for example, 1/4 pixel.

  Next, a case where the lines are separated from each other will be described. In FIG. 6, since the dot of line number 2 is closer to the line number 1 side, there is a gap between the dots of line numbers 2 and 3. Specifically, for the dots 2-2 and 3-3, the dots 2-6 and 3-5, the dots 3-9 and 2-8, and the dots 2-10, the corner vertices are in contact with each other. Should have been separated. Therefore, in line number 3, the writing timing is changed in a direction in which the distance (inter-center distance) connecting the centers of the diagonal dots is reduced. By reducing the distance between the centers of the diagonal dots, it is possible to draw out the visual illusion that the dots appear to be connected even if the dots are actually spaced apart.

  A method for determining the write timing by the write timing determining unit 307 for line number 3 will be described. FIG. 8 is a diagram showing line numbers 2 and 3 extracted from the dot image shown in FIG. First, when the target pixel is the dot 91C (dot 3-3), the pixel other than white among the peripheral pixels of the dot 91C in the line number 2 is the dot 91B (dot 2-2). Accordingly, the dot 91C is moved in the direction of the arrow shown on the dot 91C. That is, since the dot 91C can be moved in the direction of the arrow shown on the dot 91C by advancing the write-out timing, the write-out timing determination unit 307 increments the early count.

  Next, when the target pixel is the dot 92C (dot 3-5), the pixels other than white among the peripheral pixels of the dot 92C in the line number 2 are the dots 92B (dots 2-6). Therefore, a determination is made to move the dot 92C in the direction of the arrow shown on the dot 92C. That is, by delaying the write start timing, the dot 92C can be moved in the direction of the arrow shown on the dot 92C, so the write start timing determination unit 307 increments the late count.

  When the target pixel is the dot 93C (dot 3-9), the pixels other than white among the peripheral pixels of the dot 93C on the line number 2 are the dot 93B (dot 2-8) and the dot 94B (dot 2-10). ). Accordingly, the write timing determination unit 307 does not increment the early count and the late count.

  As described above, in the case of FIG. 8, since the early count = 1 and the late count = 1, which are the same numbers, the write timing determination unit 307 does not change the write timing of the line number 3 and is set in advance. Use normal export timing.

  9 and 10 are flowcharts showing the flow of the exposure process in the present embodiment. First, the writing timing determination unit 307 calculates which reflection surface 282 the light beam LB of each line of the dot image data reflects, and assigns a reflection surface number to each line (step S11). For the polygon mirror 28, the exposure control unit 303 controls the motor 32 before image formation in order to perform image formation from the home position. The home position refers to a state in which, for example, a reflection surface irradiated with the light beam LB of line number 1 is set in advance and the reflection surface is at a position facing the light source 31. Since the reflection surface irradiated with the light beam LB with the line number 1 is determined in advance, the writing timing determination unit 307 may assign the reflection surface number to each line sequentially.

  Then, the writing timing determining unit 307 substitutes 2 for a variable i (i is an integer of 2 or more), and whether the line pitch in the sub-scanning direction of the line number i-1 and the line number i is a normal line pitch. It is judged using the face-down information stored in the face-down information storage unit 309 (step S13). As to whether the line pitch is normal, the write timing determination unit 307 reads out the face-down information from the face-down information storage unit 309 based on the line number i-1 and the reflective surface number assigned to the line number i. Judgment is made by calculating the pitch of the line.

  When the line pitch between the line number i-1 and the line number i is not normal (step S13; NO), the writing timing determination unit 307 substitutes 1 for a variable c (c is an integer of 1 or more), and performs early count and late 0 is substituted into the count (both are variable names and are integers greater than or equal to 0) (step S14).

  Next, the writing timing determination unit 307 determines whether the pixel (i, c) is other than white (step S15). When the pixel is other than white (step S15; YES), the writing timing determination unit 307 performs the following processing with the pixel (i, c) as the target pixel.

  Subsequently, it is determined whether the pixel (i-1, c) is white and only one of the pixel (i-1, c-1) and the pixel (i-1, c + 1) is other than white (step) S16). In this step, it is inquired whether the pixel above the target pixel is white and either one of the diagonally upper right or diagonally upper left is a dot other than white.

  Pixel (i-1, c) is white, and only one of pixel (i-1, c-1) and pixel (i-1, c + 1) is other than white (the pixel above the target pixel is It is white and either one of the diagonally upper right or diagonally upper left is other than white (step S16; YES), and the pixel (i-1, c-1) is other than white (the diagonal upper left of the target pixel is If the line pitch of the line numbers i-1 and i is wider than the normal line pitch (step S18; YES), the write timing determination unit 307 increments the early count. (Step S20). On the other hand, when the line pitch of line number i-1 and line number i is narrower than a normal line pitch (step S18; NO), write-out timing determination part 307 increments a late count (step S21).

  Further, the pixel (i-1, c-1) is white (that is, the pixel (i-1, c + 1) is other than white, that is, the diagonally upper right of the target pixel is other than white) (step S17; NO), and the line When the line pitch of the number i-1 and the line number i is wider than the normal line pitch (step S19; YES), the writing timing determination unit 307 increments the late count (step S21). On the other hand, when the line pitch of line number i-1 and line number i is narrower than a normal line pitch (step S19; NO), write-out timing determination part 307 increments an early count (step S21).

  If the pixel (i, c) is not the last pixel of the line (step S22; NO), the writing timing determination unit 307 increments the variable c (step S23) and shifts the processing to step S15.

  When the pixel (i, c) is the last pixel of the line (step S22; YES), the writing timing determination unit 307 compares the early count value and the late count value (step S24). If early count> late count, the write timing determination unit 307 advances the write timing of the line number i by a predetermined time (step S25). On the other hand, if early count <late count, the write timing determination unit 307 delays the write timing of the line number i by a predetermined time (step S26).

  If the line number i is not the last line number of the image data (step S27; NO), the writing timing determination unit 307 increments the variable i (step S28), and the process proceeds to step S13.

  When the line number i is the last line number of the image data (step S27; YES), the exposure control unit 303 applies light to the light beam generation unit 30 based on the writing timing of each line determined by the writing timing determination unit 307. A beam is generated and irradiated (step S29). The emitted light beam LB is reflected on each reflecting surface 282 of the polygon mirror 28 and irradiates the photosensitive drum 4, and an electrostatic latent image is formed on the peripheral surface of the photosensitive drum 4.

  In addition, when the line pitch between the line number i-1 and the line number i is normal (step S13; YES), when the pixel (i, c) is white (step S15; NO), the pixel (i-1, When c) is white and both the pixel (i-1, c-1) and the pixel (i-1, c + 1) are other than white (step S16; NO), the writing timing determination unit 307 proceeds to step S27. Migrate processing.

  Note that the write timing determination method by the write timing determination unit 307 described above is for the line with the target pixel (pixel (i, c)) when determining which of the early count and late count is incremented. In the above description, the determination is made using the pixels (pixel (i−1, c−1), pixel (i−1, c + 1)) in the line one above. In addition to this, the pixel in the line immediately below the line where the target pixel is located (that is, the pixel (i + 1, c−1) or the pixel (i + 1, c + 1)) is also considered in the stage of determining the write timing. Also good.

  When the write timing determination unit 307 determines to advance the write timing of the line number i by a predetermined time (for example, time t), the write timing of the line number i may be advanced as it is by the time t. May be advanced by time t / 2, and the write timing of line number i-1 may be delayed by time t / 2, whereby the write timing of line number i and line number i-1 is relatively shifted by time t. become. Since the amount of change in the write timing can be reduced as compared with the case where only the write timing of one line is changed, the amount of deviation from the surrounding lines caused by changing the write timing can be reduced.

  As described above, the writing start timing determination unit 307 allocates, for each line, the reflection surface 282 to which the light beam LB emitted for each line of the dot image data is reflected, and the reflection allocated for each line. The writing timing of each line is determined based on the number of the surface 282 and the surface tilt information of each reflecting surface 282 stored in the surface tilt information storage unit 309. By doing so, it is possible to suppress deterioration in image quality due to the tilting of the polygon mirror 28.

  In the present embodiment, it has been described that the writing timing determining unit 307 determines the writing timing of each line when each reflecting surface 282 of the polygon mirror 28 is tilted. In addition to this surface tilt, when the polygon mirror 28 rotates at high speed, the rotation shaft 281 may be tilted by vibration, and the same phenomenon as the surface tilt may occur. Therefore, the vibration at the time of high-speed rotation of the polygon mirror 28 is measured before the image forming apparatus 1 is shipped from the factory. The amount of direction deviation and the direction of deviation are measured and stored in the face tilt information storage unit 309. When image formation is performed by rotating the polygon mirror 28 at a high speed, the writing timing determination unit 307 reads the surface tilt information stored in the surface tilt information storage unit 309 and performs the exposure process shown in FIGS. Thereby, even if vibration occurs during high-speed rotation of the polygon mirror 28, it is possible to suppress deterioration in image quality.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Image forming part 4 Photosensitive drum (Image carrier)
6 Exposure device 28 Polygon mirror (rotating polyhedron, deflection means)
281 Rotating shaft 282 Reflecting surface 27 BD sensor 29 BD signal converter 30 Light beam generator 31 Light source 32 Motor (driving means)
300 Control unit (data conversion means)
301 Storage Unit 303 Exposure Control Unit (Exposure Control Unit)
305 Timing signal generation unit 307 Write timing determination unit (allocation unit, determination unit)
309 Face-down information storage unit (storage means)

Claims (3)

Data conversion means for converting input image data into dot image data configured by arranging pixel values having gradation information in a matrix;
An image carrier;
A light source that forms an electrostatic latent image on the image carrier by emitting light modulated according to the pixel value for each line of the dot image data;
A rotating polyhedron having a plurality of reflecting surfaces; and a driving means for rotationally driving the rotating polyhedron, and the light emitted from the light source when the driving means rotates the rotating polyhedron rotates the main scanning direction of the image carrier. Deflection means leading to
Storage means for storing surface tilt information of each reflecting surface of the rotating polyhedron;
Assigning means for assigning the number of the reflecting surface to which the light emitted from the light source reflects for each line of the dot image data;
Determining means for determining the writing timing of each line of the dot image data based on the surface tilt information, the number of the reflecting surface assigned to each line of the dot image data, and the pixel value of the dot image data When,
Exposure control means for emitting light to the light source in accordance with the determined writing timing;
An image forming apparatus.   The determining means has a predetermined interval in the sub-scanning direction on the image carrier between the light reflected from the reflective surface and the light reflected from the reflective surface immediately before the reflective surface. A reflection surface wider than the reference pitch or a reflection surface narrower than the reference pitch is identified from the surface tilt information, and the pixel value of the line of the dot image data assigned the number of the specified reflection surface, The image forming apparatus according to claim 1, wherein the pixel value of a line immediately above the line is compared to determine the writing start timing of the line.   The determination unit sequentially sets target pixels for pixels other than white among the lines of the dot image data to which the identified reflecting surface number is assigned, and data indicated by a line immediately above the line , When the pixel directly above and the diagonally upper right pixel when viewed from the target pixel is white and the diagonally upper left pixel is other than white, the pitch between the line and the line immediately above the line is When it is wider than the reference pitch, the first count number is incremented. When it is narrower than the reference pitch, the second count number is incremented. The pixel immediately above the target pixel and the diagonally upper left are white, and the diagonally upper right. If the pixel is other than white, the second count is incremented when the pitch between the line and the line immediately above the line is wider than the reference pitch, When the pitch is narrower than the reference pitch, the first count number is incremented. When the first count number is larger than the second count number, the write timing of the line is advanced by a predetermined time from the normal write timing, and the second count number is increased. 3. The image forming apparatus according to claim 2, wherein when the count number is larger than the first count number, the line forming timing is determined to be delayed by a predetermined time from the normal writing timing.

Images