JP4956300B2 - Image forming apparatus and control method - Google Patents

Description

  The present invention relates to an image forming apparatus that forms an image on a recording medium and a control method for the image forming apparatus. The present invention is suitable for an image forming apparatus such as a copying machine, a printer, and a facsimile machine.

  Conventionally, a tandem type image forming apparatus is known as an apparatus for forming a color image. Such an image forming apparatus includes a plurality of image forming units that are arranged along a conveying direction of a transfer medium such as an intermediate transfer belt and that form toner images of different colors. The image forming unit includes an image carrier (for example, a photosensitive drum) driven at a predetermined rotational speed, a charging unit disposed around the image carrier, an image writing unit (for example, a laser scanner), and a developing unit. The The toner images formed on each of the image carriers are superimposed on a transfer medium to form a color image, and a color image is formed on the recording medium through transfer and fixing processes.

  The tandem type image forming device forms a color image by superimposing a plurality of toner images, so a high-quality color image is formed if the toner images are not accurately superimposed without causing a shift in the overlapping position. Can not do it. However, in the image forming apparatus, the overlapping positions of the toner images may be shifted due to tolerances of members constituting the image forming unit or temperature rise. Therefore, in order to accurately superimpose the toner images, the image forming apparatus forms a calibration pattern on the intermediate transfer belt and reads the pattern with a sensor to adjust the writing position in the main scanning direction and the sub-scanning direction.

The image writing unit scans the laser beam in a direction orthogonal to the conveyance direction of the recording medium to form an image. In the image writing unit, if the scanning line by the laser beam is curved or inclined, the image formed on the recording medium is distorted. In view of this, an image forming apparatus has been proposed in which an adjustment mechanism is provided in an optical system and a scanning system constituting the image writing unit, and the image writing unit is adjusted so that the scanning lines are aligned. Further, it is formed on a recording medium by using an image signal curved in a direction opposite to the curve direction of the scan line without adjusting the scan line (that is, while the scan line is curved). An image forming apparatus that reduces image distortion has also been proposed (see Patent Document 1). Such an image forming apparatus has an advantage that an increase in the cost of the image forming apparatus can be suppressed because there is no need to provide an adjustment mechanism in the optical system and the scanning system constituting the image writing unit.
JP 2006-255958 A

  However, when forming a straight line as an image in an image forming apparatus as disclosed in Patent Document 1, a single straight line is formed by a plurality of curved scanning lines, so that the straight line has a discontinuous shape. End up. In order to make such a discontinuous shape inconspicuous, the image forming apparatus disclosed in Patent Document 1 performs halftone processing in the vicinity of a discontinuous portion (that is, a portion where scanning lines are connected). . However, the image forming apparatus disclosed in Patent Document 1 forms all images including a calibration pattern by using an image signal curved in a direction opposite to the scanning line bending direction. Halftone processing is also applied to such a calibration pattern. Therefore, when detecting the calibration pattern, the detection signal in the vicinity of the portion subjected to the halftone process becomes gentle, and the calibration pattern cannot be detected accurately (with high accuracy). As a result, the writing position in the main scanning direction and the sub-scanning direction is shifted, and the toner images cannot be accurately superimposed, resulting in deterioration of image quality.

  Accordingly, an exemplary object of the present invention is to provide an image forming apparatus capable of detecting a calibration pattern with high accuracy and accurately superimposing toner images and forming a high-quality image. To do.

In order to achieve the above object, an image forming apparatus according to one aspect of the present invention is an image forming apparatus that forms an image on a recording medium, and includes an image carrier that carries a latent image of the image, and the image carrier. A laser scanner that scans the body with a laser beam corresponding to the image and forms the latent image; a correction unit that corrects distortion of the image due to curvature of a scanning line formed by the laser beam; and at least Control means for controlling not to perform correction by the correction means when forming a pattern for adjusting the writing position of the image in a direction orthogonal to the scanning direction of the laser light. And

  Further objects and other features of the present invention will become apparent from the preferred embodiments described below with reference to the accompanying drawings.

  According to the present invention, for example, an image forming apparatus capable of detecting a calibration pattern with high accuracy and accurately superimposing toner images and forming a high-quality image is provided.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings. In addition, in each figure, the same reference number is attached | subjected about the same member and the overlapping description is abbreviate | omitted.

  FIG. 1 is a schematic cross-sectional view showing a configuration of an image forming apparatus 1 as one aspect of the present invention. The image forming apparatus 1 is an image forming apparatus that forms an image (color image) on a recording medium PM using an electrophotographic method. In this embodiment, the image forming apparatus 1 superimposes and transfers a toner image of each color of yellow, magenta, cyan, and black on the recording medium PM, and heats and pressurizes the recording medium PM at a predetermined temperature. Is realized as a color laser printer for fixing the image to the recording medium PM.

  As shown in FIG. 1, the image forming apparatus 1 includes an image forming unit 10Y that forms a yellow image, an image forming unit 10M that forms a magenta image, and an image forming unit 10C that forms a cyan image. And an image forming unit 10Bk that forms a black image. Further, the image forming apparatus 1 includes a paper feeding unit 20, a fixing unit 30, a transport unit 40, a sensor 70, and a control unit 50 shown in FIG.

  The four image forming units 10Y, 10M, 10C, and 10Bk are arranged in a line at regular intervals along the conveyance direction (movement direction) of the intermediate transfer belt 17. The image forming units 10Y, 10M, 10C, and 10Bk include drum-type electrophotographic photosensitive members (hereinafter referred to as “photosensitive drums”) 11a, 11b, 11c, and 11d as image carriers. Further, around the photosensitive drums 11a, 11b, 11c, and 11d, primary chargers 12a, 12b, 12c, and 12d and developing units 13a, 13b, 13c, and 13d are arranged. Similarly, transfer rollers 14a, 14b, 14c and 14d and cleaners 15a, 15b, 15c and 15d are arranged around the photosensitive drums 11a, 11b, 11c and 11d. Further, a laser scanner unit 16 is disposed below the primary chargers 12a, 12b, 12c and 12d and the developing units 13a, 13b, 13c and 13d.

  The photosensitive drums 11a to 11d are made of, for example, a negatively charged OPC photosensitive member, and have a photoconductive layer on an aluminum drum base. The photosensitive drums 11a to 11d are rotationally driven clockwise at a predetermined process speed by a driving unit (not shown).

  The primary chargers 12a to 12d uniformly charge the surfaces of the photosensitive drums 11a to 11d to a predetermined negative or positive potential by a charging bias applied from a charging bias power source (not shown).

  The laser scanner unit 16 includes a laser light emitting unit that emits light corresponding to a time-series electric digital pixel signal of an image signal given from an image reading device or a computer, a polygon mirror, a lens, and a reflection mirror. The laser scanner unit 16 irradiates the photosensitive drums 11a to 11d with laser light (that is, exposes the photosensitive drums 11a to 11d), and images are formed on the surfaces of the photosensitive drums 11a to 11d charged by the primary chargers 12a to 12d. An electrostatic latent image of each color corresponding to the information is formed.

  The developing units 13a to 13d store yellow toner, cyan toner, magenta toner, and black toner, respectively, and attach the toner of each color to the electrostatic latent images formed on the photosensitive drums 11a to 11d, thereby forming the electrostatic latent image. Development (visualization) as a toner image.

  The transfer rollers 14a to 14d are disposed so as to be in contact with the photosensitive drums 11a to 11d via the intermediate transfer belt 17. The transfer rollers 14a to 14d transfer the toner images on the photosensitive drums 11a to 11d to the intermediate transfer belt 17 and superimpose them at the primary transfer portions PTPa to PTPd.

  The cleaner portions 15a to 15d are configured by a cleaning blade or the like, and scrape off toner remaining on the photosensitive drums 11a to 11d to clean the surfaces of the photosensitive drums 11a to 11d.

  The intermediate transfer belt 17 is disposed on the upper surface side of the photosensitive drums 11 a to 11 d and is stretched between the secondary transfer counter roller 18 a and the tension roller 19. The intermediate transfer belt 17 is made of a dielectric resin such as polycarbonate, polyethylene terephthalate resin film, polyvinylidene fluoride resin film, or the like.

  The secondary transfer counter roller 18a is disposed so as to be in contact with the secondary transfer roller 18b via the intermediate transfer belt 17 at the secondary transfer portion STP.

  The toner image transferred to the intermediate transfer belt 17 is transferred to the recording medium PM by the secondary transfer counter roller 18a and the secondary transfer roller 18b at the secondary transfer portion STP.

  A belt cleaning unit that removes and collects toner remaining on the surface of the intermediate transfer belt 17 is disposed outside the intermediate transfer belt 17 and in the vicinity of the tension roller 19.

  The paper feeding unit 20 has a function of supplying the recording medium PM to the image forming units 10Y, 10M, 10C, and 10Bk (that is, the secondary transfer site STP). The paper feeding unit 20 includes a cassette 21 for storing the recording medium PM, a manual feed tray 22 for manually feeding the recording medium PM, and a pickup roller for feeding the recording medium PM from the cassette 21 or the manual feed tray 22 one by one. Have. In addition, the paper feed unit 20 includes a paper feed roller and a paper feed guide 23 for transporting the recording medium PM fed from the pickup roller to the registration roller 24. The registration roller 24 sends the recording medium PM to the secondary transfer portion STP in accordance with the timing of image formation by the image forming units 10Y, 10M, 10C, and 10Bk.

  The fixing unit 30 heats and pressurizes the recording medium PM to which the toner image is transferred, and fixes the toner image to the recording medium PM. In the present embodiment, the fixing unit 30 includes a fixing film 31 having a heat source such as a ceramic heater substrate therein, and a pressure roller 32 that presses the recording medium PM together with the fixing film 31. Note that the pressure roller 32 may include a heat source.

  The transport unit 40 has a function of transporting the recording medium PM supplied from the paper feed unit 20. The conveyance unit 40 includes, for example, a guide 41 for guiding the recording medium PM to the fixing unit 30 (that is, the nip NP between the fixing film 31 and the pressure roller 32), and the recording medium PM that has passed through the fixing unit 30. And an outer discharge roller 42 for discharging the image to the outside of the image forming apparatus 1.

  The control unit 50 controls the operation of the image forming apparatus 1. As shown in FIG. 2, the control unit 50 includes a CPU 51, a read only memory (ROM) 52, a random access memory (RAM) 53, an I / O interface 54, an image processing unit 55, a bus driver address. And a decoder circuit 56. The bus driver / address decoder circuit 56 connects the CPU 51 with the ROM 52, RAM 53, I / O interface 54, and image processing unit 55. Here, FIG. 2 is a schematic block diagram showing the configuration of the control unit 50.

  The CPU 51 has a function of controlling the entire image forming apparatus 1, and sequentially reads out and executes the control program from the ROM 52 storing the control program (control procedure) of the image forming apparatus 1. The RAM 53 is used as a working storage area for the CPU 51.

  The I / O interface 54 is connected to an operation panel 541 that receives instructions from the user and displays the state of the image forming apparatus 1 using liquid crystal or LEDs. The I / O interface 54 is connected to an optical system constituting the laser scanner unit 16, a motor 542 that drives the paper feed unit 20, the transport unit 40, a clutch 543, and a solenoid 544. The I / O interface 54 is connected to a detection sensor 545 for detecting the conveyed recording medium PM and a toner sensor 546 for detecting the amount of toner stored in the developing units 13a to 13d. Further, the I / O interface 54 is connected to switches 547 and a high voltage output system 548 for detecting the home position of each member constituting the image forming apparatus 1 and the open / closed state of the door. The high voltage output system 548 is controlled by the CPU 51 to output (apply) a high voltage to the primary chargers 12a to 12d, the developing units 13a to 13d, the transfer rollers 14a to 14d, and the secondary transfer roller 18b.

  An image signal (image data) is input to the image processing unit 55 from the computer CP connected to the image forming apparatus 1. The image processing unit 55 controls the laser scanner unit 16 (specifically, emission of laser light from the laser scanner unit 16) based on an image signal from the computer CP. When the laser light emitted from the laser scanner unit 16 irradiates (exposes) the photosensitive drums 11a to 11d, the laser light detection state is detected by the laser light detection sensor 549 in the non-image region, and the I / O interface 54.

  In the present embodiment, the image processing unit 55 executes a process of correcting a shift in the main scanning direction and the sub-scanning direction of the scanning line formed by the laser light. Specifically, the image processing unit 55 changes the image signal based on the deviation of the scanning lines in the main scanning direction and the sub-scanning direction (that is, the position (timing) at which the laser beam is irradiated as described later. change). As a result, it is possible to accurately superimpose the toner images of the respective colors (that is, without deviation). The image processing unit 55 also executes processing for correcting image distortion caused by the curvature of the scanning line formed by the laser light from the laser scanner unit 16. Specifically, the image processing unit 55 changes the image signal based on the curvature of the scanning line (that is, changes the position (timing) at which the laser beam is irradiated as described later). The image processing unit 55 also has a function of performing halftone processing on an image. However, the image processing unit 55 does not correct the image distortion caused by the curvature of the scanning line when the calibration pattern for adjusting the writing position of the image in the main scanning direction and the sub-scanning direction is formed. Be controlled.

  Here, a description will be given of a process for correcting the shift of the scanning line in the main scanning direction and the sub-scanning direction and a process for correcting the distortion of the image due to the curvature of the scanning line. First, image distortion caused by the curvature of the scanning line formed by the laser light from the laser scanner unit 16 will be described. Before the laser scanner unit 16 is mounted on the image forming apparatus 1, various adjustments (for example, adjustment of the curvature of the scanning line) are performed for each optical system and scanning system constituting the laser scanner unit 16. The scanning line formed by the laser beam is designed and adjusted so as to be in a straight line on the photosensitive drum. However, in practice, the scanning line on the photosensitive drum is curved due to various variations such as variations in the members constituting the optical system and the scanning system and variations in mounting.

  FIGS. 3 and 4 are diagrams for explaining the scanning lines SL formed on the photosensitive drum 11 by the laser light from the laser scanner unit 16. In FIGS. 3 and 4, the photosensitive drum 11 is a generic term for the photosensitive drums 11a to 11d. The laser scanner unit 16 includes various optical members such as a lens and a mirror. In FIGS. 3 and 4, the laser element 161 that emits laser light and the laser light from the laser element 161 are deflected and scanned. Only the polygon mirror 162 is shown.

  Referring to FIGS. 3 and 4, the laser light emitted from the laser element 161 is deflected and scanned by the polygon mirror 162 to form a scanning line SL on the photosensitive drum 11. The scanning line SL is ideally straight on the photosensitive drum 11 as shown in FIG. 3, but actually, the scanning line SL is curved on the photosensitive drum 11 as shown in FIG.

  FIG. 5 shows an example in which an image (in this embodiment, the letter “A”) is formed by an ideal (ie, straight) scanning line SL as shown in FIG. FIG. 6 shows an example in which an image is formed with a curved scanning line SL as shown in FIG. Referring to FIGS. 5 and 6, when an image is formed with a curved scanning line, the image (FIG. 5) formed with a straight scanning line becomes a distorted image (FIG. 6).

  Therefore, the laser scanner unit 16 is adjusted and mounted on the image forming apparatus 1 so that the scanning lines are aligned on the photosensitive drum. Note that the writing position or timing of the image in the main scanning direction and sub-scanning direction main scanning direction (that is, the irradiation position of the laser beam) is different from that assumed before the laser scanner unit 16 is mounted on the image forming apparatus 1. Therefore, readjustment of the laser scanner unit 16 is necessary. Further, if the image forming apparatus 1 is continuously operated for a long time, the mirror, the lens, the housing, and the like constituting the laser scanner unit 16 are slightly deformed due to the temperature rise, and the writing position in the main scanning direction or the sub scanning direction is shifted. Sometimes.

  FIG. 7 is a diagram showing scanning lines SLa to SLd formed by laser light on the intermediate transfer belt 17. In FIG. 7, the vertical direction is the scanning direction of the scanning lines, and the horizontal direction is the driving direction of the intermediate transfer belt 17. In FIG. 7, each of the scanning lines ISLa to ISLd indicated by solid arrows is an ideal scanning line for forming yellow, magenta, cyan, and black images (toner images).

  Referring to FIG. 7, in practice, there is little coincidence with ideal scanning lines ISLa to ISLd, such as scanning lines SLa to SLd indicated by dotted arrows. In such a case, as shown in FIG. 8, the four toner images of yellow, magenta, cyan, and black do not accurately overlap, so that the image quality deteriorates. Here, FIG. 8 is a diagram showing an example of a toner image (image) formed on the intermediate transfer belt 17 by the scanning lines SLa to SLd shown in FIG.

  In order to prevent such deterioration of image quality, the control unit 50 detects a shift in the main scanning direction and the sub-scanning direction of the image (scanning line), and executes a process for correcting the shift. For example, a calibration pattern for detecting a shift in the main scanning direction and the sub-scanning direction of the image (that is, adjusting the writing position in the main scanning direction and the sub-scanning direction of the image) is formed on the intermediate transfer belt 17. Such a calibration pattern is read.

  FIG. 9 is a diagram illustrating an example of the calibration pattern CP formed on the intermediate transfer belt 17. In FIG. 9, the vertical direction is the sub-scanning direction, and the horizontal direction is the main scanning direction. Patterns CP1, CP2, CP3, and CP4 are patterns for adjusting the writing position in the sub-scanning direction of yellow, magenta, cyan, and black images. Patterns CP5, CP6, CP7, and CP8 are patterns for adjusting the writing position in the main scanning direction of yellow, magenta, cyan, and black images.

  The detection line 70a indicates the reading position of the patterns CP1 to CP8 (on the intermediate transfer belt 17) by the sensor 70 disposed in the image forming apparatus 1 (specifically, in the vicinity of the intermediate transfer belt 17). FIG. 10 is a schematic cross-sectional view showing the configuration of the sensor 70 that reads (detects) the calibration pattern CP. As shown in FIG. 10, the sensor 70 includes an irradiation unit 71 that irradiates light to the intermediate transfer belt 17 and a light receiving unit 72 that receives light reflected by the intermediate transfer belt 17. 10A shows a case where the light from the irradiation unit 71 is applied to the calibration pattern CP, and FIG. 10B shows the case where the light from the irradiation unit 71 is applied to the intermediate transfer belt 17. Shows the case.

  Referring to FIGS. 10A and 10B, the light irradiated from the irradiation unit 71 is reflected by the intermediate transfer belt 17 but absorbed by the calibration pattern CP (toner image). Therefore, when the light receiving unit 72 does not receive light, it indicates that the calibration pattern CP exists on the intermediate transfer belt 17. When the light receiving unit 72 receives light, calibration is performed on the intermediate transfer belt 17. This indicates that the action pattern CP does not exist. As described above, the light receiving unit 72 can detect the calibration pattern CP by monitoring the light reflected from the intermediate transfer belt 17. In the present embodiment, the sensor 70 outputs a level “0” when the calibration pattern CP is detected, and outputs a level “+” when the calibration pattern CP is not detected. Therefore, when the sensor 70 reads the calibration pattern CP shown in FIG. 9, a detection result DR (see FIG. 9) is obtained.

  Hereinafter, a process of correcting the shift of the scanning line in the main scanning direction and the sub scanning direction based on the detection result by the sensor 70 will be described.

First, a process for correcting the shift of the scanning line in the sub-scanning direction will be described. First, in the output (detection result) from the sensor 70 corresponding to the patterns CP1 to CP4 for adjusting the writing position in the sub-scanning direction, as shown in FIG. 11B, it corresponds to the adjacent patterns CP1 to CP4. Find the distance between the centers of the pulses. Specifically, the output from the sensor 70 (detection results of the patterns CP1 to CP4) is binarized (in this embodiment, “0” and “+”), which is sufficiently larger than changes in the patterns CP1 to CP4. Count using a fast clock. For example, let D ′ _MY be the center-to-center distance between the pulse corresponding to the pattern CP1 (yellow) and the pulse corresponding to the pattern CP2 (magenta). Similarly, the distance between the centers of the pulse corresponding to the pattern CP2 (magenta) and the pulse corresponding to the pattern CP3 (cyan) is D' _CM, and the pulse corresponding to the pattern CP3 (cyan) and the pattern CP4 (black) are supported. _Let D ′ _KC be the center-to-center distance from the pulse to be _transmitted . In order to prevent the toner image (image) formed on the intermediate transfer belt 17 from shifting, the distances D ′ _MY , D ′ _CM and D ′ _KC between the centers are set to the predetermined centers shown in FIG. during the distance _{D MY,} corrected so that the _{D CM} and _{D KC.} For example, when D = D _MY = D _CM = D _KC and yellow is used as a reference, the position (timing) of laser light irradiation in the sub-scanning direction is advanced for magenta by D _MY -D. Similarly, the irradiation position (timing) of the laser beam in the sub-scanning direction is advanced by D _MY + D _CM −2D for cyan and D _MY + D _CM + D _KC −3D for black. Thereby, the distance in the sub-scanning direction of yellow, magenta, cyan, and black becomes a desired distance, and four toner images of yellow, magenta, cyan, and black can be accurately superimposed. Here, FIG. 11 is a diagram for explaining a process of correcting the shift of the scanning line in the sub-scanning direction.

Next, a process for correcting the deviation of the scanning lines in the main scanning direction will be described. When the patterns CP5 to CP8 for adjusting the writing position in the main scanning direction are read using a two-dimensional sensor, it is possible to detect a deviation in the main scanning direction in addition to a deviation in the sub scanning direction. However, when the shapes of the patterns CP5 to CP8 are known in advance, the shift in the sub-scanning direction can be converted into the shift in the main scanning direction. For example, as shown in FIG. 9, when all of the “<” shaped patterns CP5 to CP8 change linearly, the distance between the centers of adjacent pulses (output of the sensor 70) and the main scanning direction from the reference position A correlation occurs in the deviation. Specifically, as shown in FIG. 12, the distance in the main scanning direction from the reference position SP to the detection line 70a of the sensor 70 is equal to A times the distance D between the centers of adjacent pulses in the calibration pattern CP. The distances between the centers of adjacent pulses in each of the pattern CP5 (yellow), the pattern CP6 (magenta), the pattern CP7 (cyan), and the pattern CP8 (black) are D _Y , D _M , D _C , and D _K. When the desired distance is D, _D Y -D for yellow, _D M -D for magenta, _D C -D for cyan, the position of irradiating the _D K -D only the laser beam in the main scanning direction for the black Adjust (timing). Specifically, when the scanning direction of the laser beam is α direction, _D Y -D for yellow, for magenta _D M -D, _D C -D for cyan, for black is _D K -D Only the position (timing) of irradiation of the laser beam in the main scanning direction is advanced. On the other hand, when the scanning direction of the laser beam is β direction, for yellow _D Y -D, for magenta _D M -D, _D C -D, for black _D K -D only the main scanning for cyan The position (timing) to which the laser beam in the direction is irradiated is delayed. As a result, the writing positions of the respective colors in the main scanning direction can be matched, and four toner images of yellow, magenta, cyan, and black can be accurately superimposed. As shown in FIG. 13, even when the distance in the main scanning direction from the reference position SP to the detection line 70a of the sensor 70 changes, the distance between the centers of adjacent pulses (output of the sensor 70) and the reference position There is a correlation in the deviation in the main scanning direction. For example, in FIG. 13, the distance in the main scanning direction from the reference position SP to the detection line 70a of the sensor 70 is equal to A times the center distance DD between adjacent pulses in the calibration pattern CP. Here, FIG. 12 and FIG. 13 are diagrams for explaining the process of correcting the deviation of the scanning line in the main scanning direction.

  In this manner, the calibration pattern CP formed on the intermediate transfer belt 17 is detected by the sensor 70 to obtain a deviation from the ideal value, and the position (timing) to which the laser beam is irradiated is moved back and forth. Thereby, the toner images of the respective colors can be accurately superimposed (that is, without being shifted).

  Hereinafter, processing for correcting image distortion caused by the curvature of the scanning line will be described. As described above, the curvature of the scanning line is corrected by providing an adjustment mechanism in the optical system and the scanning system constituting the laser scanner unit 16 and adjusting the laser scanner unit 16 via the adjustment mechanism (that is, scanning). Line can be made straight on the photosensitive drums 11a to 11d). However, if the laser scanner unit 16 is provided with an adjustment mechanism, the cost of the image forming apparatus 1 is increased. Further, the adjustment for making the scanning lines straight on the photosensitive drums 11a to 11d is very complicated and takes a long time. Therefore, in the present embodiment, the laser scanner unit 16 is not provided with an adjustment mechanism, and the scanning line is curved and the position (timing) for irradiating the laser beam is changed (that is, the image signal is changed). The image distortion caused by the curvature of the scanning line is corrected.

  FIG. 14 is a diagram illustrating an image (in this embodiment, a letter “A”) formed by changing the position (timing) of laser light irradiation while the scanning line is curved. Referring to FIG. 14, in the present embodiment, the main scanning direction is divided into fine regions as indicated by a one-dot chain line OL, and the position (timing) for irradiating the laser light to such regions is changed, which is ideal. It is close to the image (FIG. 5) when formed with a simple scanning line. Accordingly, as shown in FIG. 15 (a), discontinuous portions are generated in each portion in order to correct a straight line inclined due to the curvature of the scanning line (to make it appear as a straight line). In order to correct such discontinuous portions, in this embodiment, halftone processing is performed on surrounding images (pixels) as shown in FIG. Here, FIG. 15 is a partially enlarged view of the image shown in FIG.

  Here, consider a case where a calibration pattern CP for detecting a shift in the main scanning direction and sub-scanning direction of an image (scanning line) is formed. For example, when forming the pattern CP1 for detecting the shift in the sub-scanning direction, if the scanning line is not curved, the pattern CP1 shown in FIG. 16A is formed. However, if the scanning line is curved, a process for correcting the distortion of the image due to the curvature of the scanning line is performed to form a pattern CP1 ′ shown in FIG. A correction halftone process is performed to form a pattern CP1 ″ shown in FIG. Similarly, when forming the pattern CP5 for detecting the deviation in the main scanning direction, if the scanning line is not curved, the pattern CP5 shown in FIG. 17A is formed. However, if the scanning line is curved, a process for correcting image distortion caused by the scanning line curvature is performed to form a pattern CP5 ′ shown in FIG. A halftone process to be corrected is performed to form a pattern CP5 ″ shown in FIG. Here, FIG. 16 is a diagram showing a calibration pattern CP (pattern for detecting a deviation in the sub-scanning direction) formed when the scanning line is not curved and when the scanning line is curved. . FIG. 17 is a diagram showing a calibration pattern CP (pattern for detecting a deviation in the main scanning direction) formed when the scanning line is not curved and when the scanning line is curved.

  FIG. 18 shows an output result from the sensor 70 when the pattern CP5 shown in FIG. On the other hand, FIG. 19 shows an output result from the sensor 70 when the pattern CP5 ″ shown in FIG. Referring to FIG. 19, when the sensor 70 reads the detection line 70 a, even if the pattern CP <b> 5 ″ reaches the sensor 70, a halftone exists, so the output result from the sensor 70 is the output shown in FIG. 18. Compared with the result, the change becomes gradual. FIG. 20A is an enlarged view of a main part of the output result from the sensor 70 when the pattern CP5 shown in FIG. FIG. 20B is an enlarged view of a main part of the output result from the sensor 70 when the pattern CP5 ″ shown in FIG.

When the sensor 70 reads the detection line 70a ′ (that is, the scanning line and the conveyance direction of the intermediate transfer belt 17 are not orthogonal to each other) (see FIG. 19), halftones are formed at both ends of the pattern CP5 ″. Therefore, the detection accuracy in the width direction is lowered. Specifically, when reading the pattern CP5 halftone is not present in the sensor 70, the output result from the sensor 70 is as shown in FIG. 21 (a), the distance between the centers of adjacent pulses detected as D _I Is done. However, when a pattern CP5 ″ having a halftone is read by the detection line 70a shown in FIG. 19, the output result from the sensor 70 is as shown in FIG. 21B, and the distance between the centers of adjacent pulses is It is detected as D _S. Although the center distance D _I and the center distance D _S must be the same, the pattern CP5 ″ has a halftone, so that the detected center distance D _I and the center distance D _S are Will be different. Further, when a pattern CP5 ″ having a halftone is read by the detection line 70a ′ shown in FIG. 19, the output result from the sensor 70 is as shown in FIG. 21C, and the distance between the centers of adjacent pulses is shown. Is detected as _DS '. Thus, the detected center-to-center distance D _S ′ is different from the center-to-center distance D _I and is different from the center-to-center distance D _S. Here, FIG. 21A is an enlarged view of a main part of the output result from the sensor 70 when the pattern CP5 shown in FIG. FIGS. 21B and 22B are enlarged views of the main part of the output result from the sensor 70 when the pattern CP5 ″ shown in FIG.

  The pattern for detecting the shift in the main scanning direction has been described so far, but the same applies to the pattern for detecting the shift in the sub scanning direction.

  From the above, it can be seen that the sensor 70 detects the length of the calibration pattern in the sub-scanning direction. Even when a calibration pattern (image) is formed with a curved scanning line, the length of the calibration pattern (image) in the sub-scanning direction does not change. From this, the present inventor has found that even if the sensor 70 reads a calibration pattern formed by curved scanning lines, the output result from the sensor 70 is not affected. In other words, the output result from the sensor 70 when the calibration pattern formed with an ideal (straight line) scanning line is read and the output from the sensor 70 when the calibration pattern formed with a curved scanning line is read. The output result is the same.

  FIG. 22A shows a pattern CP1 formed by an ideal (straight line) scanning line and an output result from the sensor 70 when the pattern CP1 is read. Referring to FIG. 22A, the distance between the centers of adjacent pulses is A. On the other hand, FIG. 22B is a diagram showing a pattern CP1 "" formed by a curved scanning line and an output result from the sensor 70 when the pattern CP1 "" is read. Referring to FIG. 22B, the distance between the centers of adjacent pulses is AA. It should be noted that the pattern CP <b> 1 ″ ″ is not subjected to a process for correcting image distortion due to the curvature of the scanning line or a halftone process. Since the conveyance speed of the intermediate transfer belt 17 and the irradiation time of the laser beam are irrelevant to the curve of the scanning line, A = AA is established.

  The same applies to the pattern CP5 for detecting the deviation in the sub-scanning direction. FIG. 23A is a diagram showing a pattern CP5 formed by an ideal (straight) scanning line and an output result from the sensor 70 when the pattern CP5 is read. Referring to FIG. 23A, the center-to-center distance between adjacent pulses is B. On the other hand, FIG. 23B is a diagram showing a pattern CP5 "" formed by a curved scanning line and an output result from the sensor 70 when the pattern CP5 "" is read. Referring to FIG. 23B, the center-to-center distance between adjacent pulses is BB. Note that the pattern CP <b> 5 ″ ″ is not subjected to processing for correcting image distortion due to the curvature of the scanning line or halftone processing. Since the conveyance speed of the intermediate transfer belt 17 and the irradiation time of the laser beam are irrelevant to the curve of the scanning line, B = BB is established.

  Therefore, when forming a calibration pattern, the detection accuracy of the sensor 70 is improved without correcting the curvature of the scanning line formed by the laser light, and the toner images can be accurately superimposed. In this embodiment, the control unit 50 does not correct image distortion caused by the curvature of the scanning line when forming a calibration pattern for adjusting the writing position of the image in the main scanning direction and the sub-scanning direction. In this manner, the image processing unit 55 is controlled. As a result, the sensor 70 can detect the calibration pattern with high accuracy, so that the toner images can be accurately superimposed. Therefore, the image forming apparatus 1 can form a high-quality image.

  Hereinafter, the operation of the image forming apparatus 1 (that is, the control method of the image forming apparatus 1) will be described with reference to FIG. Here, FIG. 24 is a flowchart for explaining a control method of the image forming apparatus 1 as one aspect of the present invention.

  First, in step S <b> 1002, the control unit 50 determines whether to form a calibration pattern CP on the intermediate transfer belt 17. Specifically, the control unit 50 determines whether to form an image on the recording medium PM or to form a calibration pattern CP on the intermediate transfer belt 17.

  If it is determined that an image is to be formed on the recording medium PM, the control unit 50 performs control so that the image processing unit 55 executes processing for correcting image distortion caused by the curvature of the scanning line in step S1004. Next, in step S1006, an image is formed on the recording medium PM while executing processing for correcting image distortion caused by the curvature of the scanning line.

  On the other hand, if it is determined that the calibration pattern CP is to be formed on the intermediate transfer belt 17, the control unit 50 performs processing in step S1008 for the image processing unit 55 to correct image distortion caused by the curvature of the scanning line. Control not to. Next, in step S1010, the calibration pattern CP is formed on the intermediate transfer belt 17 without executing processing for correcting image distortion due to the curvature of the scanning line.

  In step S1012, the sensor 70 detects (reads) the calibration pattern CP formed in step S1010. In step S1014, the control unit 50 (image processing unit 55), based on the detection result of the calibration pattern CP by the sensor 70 (output result from the sensor 70), the main scanning direction when forming an image on the recording medium PM. And the writing position in the sub-scanning direction is adjusted. Note that the adjustment in step S1014 is performed by changing the position (timing) at which the laser beam is irradiated, as described above. When the adjustment in step S1014 is completed, the process returns to step S1002.

  As described above, when the calibration pattern CP is formed, the calibration pattern CP can be detected with high accuracy by not correcting the distortion of the image due to the curvature of the scanning line. As a result, it is possible to adjust the writing position in the main scanning direction and the sub-scanning direction when forming an image on the recording medium PM with high accuracy. On the other hand, when an image is formed on the recording medium PM, the image can be brought closer to an image formed with an ideal (straight line) scanning line by correcting the distortion of the image due to the curvature of the scanning line. . Therefore, the image forming apparatus 1 can form a high-quality image.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the gist.

1 is a schematic cross-sectional view illustrating a configuration of an image forming apparatus as one aspect of the present invention. FIG. 2 is a schematic block diagram illustrating a configuration of a control unit of the image forming apparatus illustrated in FIG. 1. FIG. 2 is a diagram for explaining scanning lines formed on a photosensitive drum by laser light from a laser scanner unit in the image forming apparatus shown in FIG. 1. FIG. 2 is a diagram for explaining scanning lines formed on a photosensitive drum by laser light from a laser scanner unit in the image forming apparatus shown in FIG. 1. It is a figure which shows the example at the time of forming an image with an ideal scanning line as shown in FIG. It is a figure which shows the example at the time of forming an image with the curved scanning line as shown in FIG. FIG. 2 is a diagram showing scanning lines formed by laser light in the intermediate transfer belt of the image forming apparatus shown in FIG. 1. FIG. 8 is a diagram illustrating an example of a toner image (image) formed on the intermediate transfer belt by the scanning line illustrated in FIG. 7. FIG. 2 is a diagram illustrating an example of a calibration pattern formed on an intermediate transfer belt of the image forming apparatus illustrated in FIG. 1. It is a schematic sectional drawing which shows the structure of the sensor of the image forming apparatus shown in FIG. It is a figure for demonstrating the process which correct | amends the shift | offset | difference of the scanning line of the full scan direction. It is a figure for demonstrating the process which correct | amends the shift | offset | difference of the scanning line of the main scanning direction. It is a figure for demonstrating the process which correct | amends the shift | offset | difference of the scanning line of the main scanning direction. It is a figure which shows the image formed by changing the position (timing) which irradiates a laser beam, with a scanning line curved. It is the elements on larger scale of the image shown in FIG. It is a figure which shows the calibration pattern (pattern for detecting the shift | offset | difference of a subscanning direction) formed when the scanning line is not curving and when the scanning line is curving. It is a figure which shows the calibration pattern (pattern for detecting the shift | offset | difference of the main scanning direction) formed when the scanning line is not curved and when the scanning line is curved. It is a figure which shows the output result from a sensor at the time of reading the pattern shown to Fig.17 (a) with a sensor. It is a figure which shows the output result from a sensor at the time of reading the pattern shown in FIG.17 (c) with a sensor. 20A is an enlarged view of a main part of the output result from the sensor when the pattern CP shown in FIG. 17A is read by the sensor, and FIG. 20B is shown in FIG. It is a principal part enlarged view of the output result from a sensor at the time of reading a pattern with a sensor. FIG. 21A is an enlarged view of a main part of the output result from the sensor when the pattern shown in FIG. 17A is read by the sensor, and FIG. 21B and FIG. It is a principal part enlarged view of the output result from a sensor at the time of reading the pattern shown in (c) with a sensor. It is a figure which shows the output result from the sensor at the time of reading the calibration pattern formed with the ideal (straight line) scanning line or the curved scanning line, and such a calibration pattern. It is a figure which shows the output result from the sensor at the time of reading the calibration pattern formed with the ideal (straight line) scanning line or the curved scanning line, and such a calibration pattern. 3 is a flowchart for explaining a control method of the image forming apparatus according to one aspect of the present invention.

Explanation of symbols

1 Image forming apparatuses 10Y, 10M, 10C, and 10Bk Image forming units 11a to 11d Photosensitive drums 12a to 12d Primary chargers 13a to 13d Developing units 14a to 14d Transfer rollers 15a to 15d Cleaner unit 16 Laser scanner unit 17 Intermediate transfer belt 18a Transfer opposing roller 18b Transfer roller 19 Tension roller 20 Paper feed unit 21 Cassette 22 Manual feed tray 23 Paper feed guide 24 Registration roller 30 Fixing unit 31 Fixing film 32 Pressure roller 40 Transport unit 41 Guide 42 Outer discharge roller 50 Control unit 51 CPU
52 ROM
53 RAM
54 I / O interface 55 Image processing unit 70 Sensor 71 Irradiation unit 72 Light receiving unit PM Recording medium CP Calibration pattern

Claims (4)

An image forming apparatus for forming an image on a recording medium,
An image carrier for carrying a latent image of the image;
A laser scanner that scans the image carrier with a laser beam corresponding to the image and forms the latent image;
Correction means for correcting image distortion caused by the curvature of the scanning line formed by the laser beam;
Control means for controlling not to perform correction by the correction means when forming a pattern for adjusting the writing position of the image at least in a direction orthogonal to the scanning direction of the laser beam ;
An image forming apparatus comprising: The correction means changes the image signal of the image based on the curvature of the scanning line so that the image signal is curved in a direction opposite to the bending direction of the scanning line. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 2, wherein the correction unit performs halftone processing on a portion where the image becomes discontinuous by changing an image signal of the image. An image carrier that carries a latent image of an image, a laser scanner that scans the image carrier with a laser beam corresponding to the image and forms the latent image, and a scanning line formed by the laser beam. A control method for an image forming apparatus, comprising: a correction unit that corrects distortion of an image due to curvature,
A formation step of forming a pattern for adjusting the writing start position in the direction of the image perpendicular to the scanning direction of at least said laser beam,
A reading step of reading the pattern formed in the forming step;
An adjustment step of adjusting a writing position in a direction orthogonal to the scanning direction of the laser beam based on the pattern read in the reading step;
In the forming step, the pattern is formed without performing correction by the correction means .