JP6261323B2 - Image forming apparatus and control method - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus and a control method for performing image formation by exposing a photoreceptor with a plurality of lasers (hereinafter referred to as multi-beams).

  Recently, an electrophotographic image forming apparatus using a laser is accepted in the light printing market because its resolution, printing speed, image quality, and the like are close to those of offset printing. Considering an electrophotographic image forming apparatus as a substitute for a full-fledged offset printing machine, it is essential to improve the resolution and printing speed of the image forming apparatus (for example, resolution: 2400 dpi, printing speed: 200 PPM (number of sheets / minute)). Absent.

  In an electrophotographic image forming apparatus, image data is expressed by a laser beam when an image is created, and an electrostatic latent image is generated by irradiating the photosensitive member with a laser beam through a rotating polygon mirror (hereinafter referred to as a polygon mirror). An image is formed. In order to improve the resolution and printing speed of the image forming apparatus, it is conceivable to first rotate the polygon mirror at a high speed. However, if the rotation speed of the polygon mirror is high, heat generation, resonance noise, and the like are generated, so that it cannot be increased beyond a certain rotation speed. Therefore, a method of increasing the printing speed by increasing the number of laser beams (multi-beam) is used.

In the case where multi-beam conversion is performed in an image forming apparatus, it is easy to increase the resolution. Therefore, an image forming apparatus using a 32-beam laser with a resolution of 2400 dpi is also on the market. However, in the multi-beam method, the following problems of variation remain, and moire (a striped pattern generated by mutual interference between pixels) is likely to occur due to interference with the screen, which is a pseudo halftone process.
・ Emission point variation in the laser chip of the laser unit ・ Adjustment residue of the laser unit alone ・ Position misalignment due to the assembly of the main body of the image forming apparatus (curvature component and defocus characteristics of the photoconductor) The following techniques have been proposed as techniques for avoiding moire due to variations (see, for example, Patent Document 1 and Patent Document 2). Patent Document 1 discloses a technique for irradiating a photosensitive member with laser scanning light, detecting the surface potential of the photosensitive member with a potential sensor, grasping the amount of interference, and reducing the amount of interference by pulse width modulation. In Patent Document 2, a thin line or a halftone pattern (resolution 300 dpi pattern) is formed while changing the position of each laser beam in the main scanning direction, the reference position is specified by density and moire, and the adjustment value is visually checked. A configuration for determining is disclosed.

JP 2000-85177 A JP 2005-53000 A

  However, as in Patent Document 1, the amount of positional deviation of the laser scanning light that can be grasped from the potential measurement result is a relatively large positional deviation of about 20 microns, and a small positional deviation cannot be detected. The technique described in Patent Document 1 is a rough adjustment method (rough adjustment), and cannot be said to be an adjustment method for maintaining high quality. Even if the positional deviation amount is obtained by the potential difference, the positional deviation is grasped from the potential difference detection and the position is determined by the laser light quantity unless the relationship between the laser light quantity and the sensitivity of the photosensitive member (hereinafter referred to as the EV curve) is a linear relation. It is difficult to correct the deviation.

  In the configuration described in Patent Document 2, a line is output and an adjustment value is determined by visual judgment based on a line density difference. However, in-plane uniformity other than laser (exposure) (transfer, fixing, etc.) There is a fluctuation factor, and this adjustment method is also coarse. In the technique described in Patent Document 2, one line can be reproduced up to a resolution of about 600 dpi, but fine lines with a resolution of 1200 dpi or more are not reproducible and difficult to adjust.

  As another configuration of Patent Document 2, a configuration using a halftone pattern (resolution 300 dpi pattern) is described. However, with a 300-beam pattern of 32 beams, the moire period is 340 μm pitch, which is not a large moire when considering the visual sensitivity characteristics (hereinafter referred to as VTF) of the eyes. Therefore, it is difficult to visually detect moire. Further, in the screen pattern having an angle of 0 degree or 90 degrees, even when moire occurs, a streaky moire that is long in the main scanning direction, so-called horizontal stripes, is generated. The horizontal stripes are called banding, and are also generated by vibration of the image forming apparatus main body, surface tilt of the polygon mirror surface, and the like, and it has been difficult to identify moire.

  Furthermore, moire cannot be detected due to the visibility characteristics of the eyes, and there is a slight deviation. The minute misalignment led to variations in dot center of gravity and dot diameter, which deteriorated the graininess of the image. Granularity is also an important image quality item in order to ensure the quality of offset printing. Therefore, when an electrophotographic image forming apparatus is considered as an alternative to an offset printing press, an image forming apparatus capable of detecting and correcting a minute shift has been desired.

  An object of the present invention is to provide an image forming apparatus and a control method capable of forming an image with good graininess without moire.

In order to achieve the above object, an image forming apparatus of the present invention includes an exposure unit that exposes the photoconductor by scanning the photoconductor with a plurality of lasers, and an exposure unit that exposes the photoconductor to expose the photoconductor. An electrostatic latent image to be formed is developed with toner, and the developed toner image is transferred to a recording material to form an image on the recording material, and transferred to the recording material by the image forming device Control means for controlling at least one of the exposure means and the image forming means so that a pattern image is formed under an image forming condition different from the image forming condition for forming the image to be formed, and based on the formation state of the pattern image and a correcting means for correcting the exposure conditions of the photosensitive member by the exposure means, different from the image forming conditions for forming an image to be transferred onto the recording material image The composition condition is a condition for forming an image in the pseudo halftone process in which the moire is most likely to occur among the plurality of pseudo halftone processes in the image forming apparatus, and the pseudo halftone process in which the moire is most likely to occur is The moiré pattern overlaps with the second adjacent period of the screen, the screen angle is other than 0 degrees, 45 degrees, and 90 degrees, and the moire period is 500 μm or more and 3 mm or less .

  According to the present invention, at least one of the exposure unit and the image forming unit is controlled so that a pattern image is formed under an image forming condition different from the image forming condition for forming an image to be transferred onto the recording material. Further, the exposure condition of the photosensitive member by the exposure unit is corrected based on the pattern image formation state. That is, the exposure condition is corrected based on the pattern image formation state (the degree of moire). This makes it possible to adjust the laser with high accuracy using a pattern image in which moiré is easy to detect, and it is possible to form an image with good graininess without moiré.

1 is a configuration diagram illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention. It is a schematic diagram which shows the structure of the exposure control part of an image forming apparatus. 2 is a block diagram illustrating a configuration of a laser control system of the image forming apparatus. FIG. 5 is a flowchart showing processing related to laser unit assembly and laser unit adjustment of the image forming apparatus. It is a figure which shows the example of irradiation of the laser beam with respect to the photoreceptor of an image forming apparatus. 3 is a flowchart illustrating a flow of assembly, adjustment, and normal output of the image forming apparatus. 3 is a flowchart illustrating a flow of assembly, adjustment, and normal output of the image forming apparatus. 2 is a block diagram illustrating a configuration of an overall control system of the image forming apparatus. FIG. 2 is a block diagram illustrating a configuration of a control system of a printer unit of the image forming apparatus. FIG. 2 is a block diagram illustrating a configuration of a control system of an input image processing unit of the image forming apparatus. FIG. (A) is a diagram showing an A4 page layout of pattern 1, and (b) is an enlarged view of a part of pattern 1. (A) is a figure which shows the example of main scanning magnification, (b) is a figure which shows the example which changes the main scanning magnification of each laser beam. It is a figure which shows the example from which the irradiation position of the laser beam with respect to the photoconductor of an image forming apparatus fluctuates. It is a figure which shows the relationship between the screen line number which consists of a main scanning pixel period, and a subscanning pixel period, and a screen angle. It is a figure which shows typically the mode of the latent image when a subscanning pitch is narrow in 1st scanning and 2nd scanning, and the latent image when a subscanning pitch is wide. It is a figure which shows typically the mode of the latent image when the main scanning magnification in the 1st scanning and the 2nd scanning is insufficiently adjusted, and the latent image when the main scanning magnification has been adjusted. It is a figure which shows the moire of 45 degree | times pseudo | simulation halftone processing. It is a figure which shows when a moire generate | occur | produces at the same angle as the nearest dot. It is a figure which shows when a moire generate | occur | produces in the 2nd proximity | contact dot. It is a figure which shows the example of matching of conditions 1-4. It is a figure which shows the example of a match of conditions 1-5.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<Overall Configuration and Basic Operation of Image Forming Apparatus>
FIG. 1 is a configuration diagram showing the overall configuration of an image forming apparatus according to an embodiment of the present invention.

  In FIG. 1, the image forming apparatus is configured as an electrophotographic multifunction peripheral (MFP) having a plurality of functions such as a copying function, a printing function, and a facsimile function. As will be described later, the image forming apparatus develops an electrostatic latent image formed on a photoconductor with toner, and transfers the developed toner image to a recording material to form an image on the recording material. Do.

  The image forming apparatus includes a document feeding unit 1, a document table glass 2, a scanner unit 4, an image sensor unit 9, an exposure control unit 10, a photoconductor 11, a developing device 13, transfer material stacking units 14 and 15, a transfer unit 16, A fixing unit 17, a paper discharge unit 18, a controller 100, and the like are provided. The scanner unit 4, the image sensor unit 9 and the like constitute a reader unit, and the photoconductor 11, the developing device 13, the transfer unit 16, the fixing unit 17 and the like constitute a printer unit. Here, the exposure control unit 10 corresponds to the exposure unit of the present invention, and the image forming unit (developer 13, transfer unit 16, etc.) corresponds to the image forming unit of the present invention. In FIG. 1, the post-processing unit 110 (FIG. 7) is not shown.

  Documents stacked on the document feeder 1 are sequentially conveyed onto the glass surface of the document table glass 2 one by one. When the original is conveyed, the scanner unit 4 moves while the scanner 3 which is a lamp is lit, and irradiates the original. The reflected light of the original passes through the lens 8 via the mirrors 5, 6, 7 and then is input to the image sensor unit 9. The image signal photoelectrically converted by the image sensor unit 9 is subjected to image processing by an input image processing unit 102, an MFP control unit 101, an output image processing unit 112 (see FIG. 7), etc., which will be described later, and is input to the exposure control unit 10. Is done.

  An electrostatic latent image is formed on the photosensitive member 11 by a laser beam (multi-beam) generated from a semiconductor laser included in the exposure control unit 10 and then developed by the developing unit 13. The recording material (sheet) is conveyed from the transfer material stacking unit 14 or 15 in synchronization with the formation of the electrostatic latent image, and the developed toner image is transferred to the recording material in the transfer unit 16. After the toner image is fixed by the fixing unit 17, the recording material is discharged from the paper discharge unit 18 to the outside of the apparatus. The surface of the photoconductor 11 after the transfer is cleaned with a cleaner 25 and discharged with an auxiliary charger 26. Then, the surface of the photoconductor 11 is charged by the primary charger 28. A plurality of images are formed by repeating the above steps.

  The controller 100 controls the entire operation of the image forming apparatus, and includes a CPU, a memory, various interface circuits, and the like. The controller 100 includes an MFP control unit 101, an input image processing unit 102, a dedicated I / F unit 105 in FIG. USB I / F unit 106 and the like are included.

<Configuration of Exposure Control Unit of Image Forming Apparatus>
FIG. 2 is a schematic diagram illustrating a configuration of the exposure control unit 10 of the image forming apparatus.

  2, the exposure control unit 10 includes a semiconductor laser 120 (corresponding to the semiconductor laser 210 in FIG. 3), a laser unit 121, a polygon mirror 123, an f-θ lens 124, a collimator lens 125, and the like. A PD (photodiode) sensor that detects a part of the laser beam is provided inside the semiconductor laser 120. APC (Auto Power Control) control, which is automatic light quantity control of the semiconductor laser 120, is performed using the detection signal of the PD sensor. The laser unit 121 includes a laser chip (not shown).

  The laser beam emitted from the semiconductor laser 120 becomes substantially parallel light by the collimator lens 125 and the stop 122 and enters the polygon mirror 123 with a predetermined beam diameter. The polygon mirror 123 rotates at an equal angular velocity in the direction of the arrow. As the polygon mirror 123 rotates, the incident laser beam is reflected as a deflected beam that continuously changes its angle. The laser beam that has become a deflected beam is focused by the f-θ lens 124. The f-θ lens 124 simultaneously corrects distortion aberration so as to guarantee the temporal linearity of the laser beam scanning on the surface of the photoconductor 11, so that the laser beam moves on the surface of the photoconductor 11 at a constant speed in the direction of the arrow. Is scanned.

  A beam detect (hereinafter referred to as BD) sensor 126 detects reflected light from the polygon mirror 123. The detection signal of the BD sensor 126 is used as a synchronization signal for synchronizing the rotation of the polygon mirror 123 and the writing of image data on the photoconductor 11.

<Configuration of main part of image forming apparatus>
FIG. 3 is a block diagram illustrating a configuration of a laser control system of the image forming apparatus.

  In FIG. 3, the image forming apparatus includes a laser driver 217, a print control unit 219, semiconductor lasers 210a to 210h (8-beam configuration), a video controller 222, a BD sensor 126, and the like. Hereinafter, the semiconductor lasers 210a to 210h are collectively referred to as a semiconductor laser 210. The semiconductor laser 210 is disposed on the laser scanner. The BD sensor 126 detects the main scanning synchronization signal of the laser beam by detecting the reflected light from the polygon mirror 123 as described above. The laser driver 217 controls the semiconductor laser 210.

  The print control unit 219 includes a laser control circuit 221, an A / D conversion circuit 223, a D / A conversion circuit 224, and a ROM 225. The print control unit 219 performs control related to image formation such as control of the laser control circuit 221 and drive control of the scanner motor that drives the scanner unit 4. The print control unit 219 performs the following control. A control signal is transmitted to the laser driver 217 in synchronization with the main scanning synchronization signal transmitted from the BD sensor 126, the correction value of the laser light amount is read from the ROM 225, and the correction value of the laser light amount is transmitted to the D / A conversion circuit 224. The bias current value is adjusted to a desired value.

  The laser control circuit 221 sends a laser emission instruction to the laser driver 217, transmits an image writing timing signal and magnification correction information to the video controller 222, and receives a laser light amount from the laser driver 217. Further, the laser control circuit 221 transmits an image writing timing signal to the video controller 222 in synchronization with the main scanning synchronization signal.

  The A / D conversion circuit 223 converts an analog value indicating the laser emission amount transmitted from the semiconductor laser 210 into a digital value. The D / A conversion circuit 224 is used to arbitrarily change the bias current value of the laser driver 217. In the ROM 225, as correction data for correcting the exposure conditions, the correction value of the light amount of each semiconductor laser, the write delay amount (write position adjustment result) of each semiconductor laser with respect to the photosensitive member 11, the main scanning magnification of each semiconductor laser. The correction value is stored.

  The video controller 222 transmits image information for forming a latent image on the photoconductor 11 to the laser driver 217. In addition, the video controller 222 adjusts the main scanning magnification of each laser by a 1/16 pixel pixel piece insertion / extraction method based on information stored in the ROM 225 using a method described in, for example, Japanese Patent Application Laid-Open No. 2009-126091. I do. Note that the resolution of the laser unit 121 (FIG. 2) of the image forming apparatus is 1200 dpi.

  FIG. 4 is a flowchart showing processing related to laser unit assembly and laser unit adjustment of the image forming apparatus.

  In FIG. 4, in the laser unit assembling process in step S101, the laser chip is bonded and fixed while adjusting the inclination of the laser chip included in the laser unit. The tilt of the laser chip is adjusted by adjusting the sub-scanning pitch while actually measuring the sub-scanning pitch with a jig equipped with a magnifying lens on the CCD so that the sub-scanning pitch of each laser beam on the surface of the photoconductor 11 becomes 21.2 μm, for example. Is done. The adjusted laser chip is bonded and fixed in the laser unit.

  In step S102, a laser light amount correction value is obtained for the laser unit that has been assembled. Further, the input current value and output light amount of each laser beam are confirmed by a light amount confirmation jig, a correction value (look-up table) of the laser light amount is created so as to have a prescribed input / output relationship (linear), and stored in the ROM 225.

  In step S <b> 103, the writing delay amount of each laser beam is confirmed for the laser unit whose laser light quantity is adjusted, and the writing delay amount of each laser beam is stored in the ROM 225. In many cases, the laser emission points in the main scanning direction do not coincide with each other due to variations in manufacturing laser chips. In addition, since the sub-scanning pitch is adjusted using the inclination of the laser chip in step S101, the main scanning pitch varies.

  Therefore, a slit is prepared at the specified main scanning position, how much time has elapsed from the emission start time of the laser beam that has passed through the slit, and the writing position of the laser beam is obtained. These are carried out for all laser beams (8 beams). The positional deviation is detected by the main scanning direction detection jig using the slit, and the light emission delay amount of each laser beam is controlled so that the main scanning central portion of all the laser beams is aligned.

  In step S104, next, a correction value for the main scanning magnification of each laser beam is determined. The main scanning magnification may change depending on whether the laser beam has passed through the center of the lens. Further, as shown in FIG. 5, the main scanning magnification of each laser beam changes depending on the angle at which the laser beam is applied to the photosensitive member 11. In this embodiment, the basic photosensitive member irradiation position is calculated in advance, and the correction value of the main scanning magnification of each laser beam is stored.

  In FIG. 5, reference numeral 127 denotes a polygon motor that rotationally drives the polygon mirror 123, 128 denotes a folding mirror, and 129 denotes a laser beam. In FIG. 5, the number of laser beams is shown as four for easy understanding, but the same phenomenon occurs with eight laser beams having the configuration of this embodiment.

  In step S105, the laser light amount correction value, the writing delay amount, and the main scanning magnification correction value are stored in the ROM 225. Thereby, the processes related to laser unit assembly and laser unit adjustment are completed.

  In the laser unit assembling process, inspection is performed using various inspection adjustment jigs, and the inspection adjustment results are stored in the ROM 225. The laser unit is not provided with a CPU. Further, the image is not stored in the ROM 225 using the CPU of the image forming apparatus. Therefore, it is necessary to prepare a CPU and an interface configuration for accessing the ROM 225 on the inspection adjustment jig side.

<Control system configuration and body assembly / adjustment / normal output of image forming apparatus>
Next, the configuration of the control system of the image forming apparatus and the flow up to assembly / adjustment / normal output of the image forming apparatus will be described with reference to FIGS.

  FIG. 7 is a block diagram showing the configuration of the overall control system of the image forming apparatus.

  7, the image forming apparatus includes an MFP control unit 101, an input image processing unit 102, a facsimile (FAX) unit 103, a NIC (Network Interface Card) unit 104, and a dedicated interface (I / F) unit 105. Further, the USB I / F unit 106, the compression / decompression unit 107, the document management unit 108, the resource management unit 109, the post-processing unit 110, the printer unit 111, the output image processing unit 112, the RIP (Raster Image Processor) unit 113, and the operation unit 114. It has.

  The MFP control unit 101 controls the entire image forming apparatus (MFP). The MFP control unit 101 includes an exposure unit (exposure control unit 10) and an image forming unit (developing unit 13, so that a pattern image is formed under an image forming condition different from an image forming condition for forming an image to be transferred onto a recording material. At least one of the transfer portions 16) is controlled. Further, the MFP control unit 101 corrects the exposure condition of the photoconductor 11 by the exposure control unit 10 based on the pattern image formation state. The input image processing unit 102 reads an image from a paper document and performs image processing. In addition to the image forming process, the printer unit 111 performs potential control (see FIG. 6) described later. The operation unit 114 is used for various instructions to the image forming apparatus. Since the configuration other than the above is publicly known, the description is omitted.

  FIG. 8 is a block diagram showing the configuration of the control system of the printer unit of the image forming apparatus.

  In FIG. 8, a printer unit 111 includes an exposure control unit 10, a developing unit 13, a transfer unit 16, a fixing unit 17, a charging unit 27 (primary charging unit 28 and auxiliary charging unit 26), a patch detection sensor 21, and a potential measuring device. 40, an engine control unit 500 is provided.

  The engine control unit 500 controls the exposure control unit 10, the developing device 13, the transfer unit 16, the fixing unit 17, the charging unit 27, and the like. The potential measuring device 40 measures a Vgrid potential in potential control described later. The exposure control unit 10 performs exposure control for exposing the photoconductor 11 with a plurality of laser beams (multi-beams) to form a latent image. Since the configuration other than the above is already described or publicly known, description thereof is omitted.

  FIG. 9 is a block diagram illustrating a configuration of a control system of the input image processing unit of the image forming apparatus.

  In FIG. 9, the input image processing unit 102 includes a shading correction unit 131, a filter processing unit 132, a density calculation unit 133, a moire calculation unit 134, and a graininess calculation unit 135.

  The reflected light of the paper document is photoelectrically converted into an image signal by the image sensor unit 9 and input to the shading correction unit 131 after A / D conversion by the A / D conversion unit 29. The shading correction unit 131 performs shading correction on the image signal, and the filter processing unit 132 performs filter processing (normal job). The density calculation unit 133 calculates the density of the image based on the pattern 1. The moire calculating unit 134 calculates moire based on the patterns 2 and 3. The graininess calculator 135 calculates the graininess of the image based on the pattern 3.

  6A and 6B are flowcharts showing the flow of assembly / adjustment / normal output of the image forming apparatus.

  6a and 6b, the process proceeds to the step of attaching the shipped laser unit after the adjustment of the laser alone to the image forming apparatus main body. A printer engine (configuration other than the laser unit in the image forming apparatus) is assembled in advance (step S201), and the laser unit is assembled to the printer engine (step S202).

  When the operator inputs an instruction to “perform laser adjustment” from the operation unit 114 (step S203), the MFP control unit 101 of the image forming apparatus instructs the printer unit 111 to execute potential control. The printer unit 111 refers to the information stored in the ROM 225, refers to the laser conditions used for potential control, and executes potential control (step S204). In the potential control, the relationship between the grid bias (Vgrid) and the potential (Vd) (also referred to as charging ability) and the relationship between the laser light quantity (hereinafter referred to as LPW) and the potential (EV curve) under a specified charging condition are expressed. calculate.

  For example, a Vgrid potential at which Vd becomes −700 V is actually measured by the potential measuring device 40, and a value obtained by subtracting a prescribed Vback (Vd−Vdc (development bias) for fog removal potential) is set as the development bias. From the developing bias (Vdc), an exposed portion potential (Vl) forms a potential difference where the toner called Vcont adheres to the photoreceptor. In the potential control in step S204, the charging ability (Vgrid−Vd) and the EV curve when Vd = −700 are calculated.

  Under the calculated conditions, Vd is set to −700 V, and Vback is set to Vback (100 V) lower than that during normal output (175 V). That is, Vdc is set to -600V. Under this calculated condition, the photoconductor 11 is exposed in accordance with the light intensity adjustment pattern 1 (FIG. 10A) having a certain area with each laser beam alone, and an image of the pattern 1 is formed (step S205).

  Here, FIG. 10A is a diagram showing an A4 page layout of pattern 1, and FIG. 10B is an enlarged view of a part of pattern 1. Since the patterns are arranged so as to be formed by each laser beam alone, one pattern in FIG. 10A is composed of a horizontal line (main scanning line) pattern having a resolution of 1200 dpi 1L7S. Therefore, it is difficult to detect the density with a very thin pattern.

  Further, considering that the instability of the image forming apparatus adversely affects the result, Vback is lowered by 75 V, and the image is output under the pattern 1 specific image forming conditions with good fine line reproducibility. In addition, the arrow of FIG. 10A, the characters A to H, and the comment are not formed when the image is output. Further, the laser A of FIG. 10A shows an example in which only one light amount is low for explanation. The laser light amount correction value is changed so that each laser beam has the same density.

  The image output pattern 1 is read by the reader unit, and the density is calculated by the density calculator 133 (step S206). Considering in-plane uniformity and the like, there are patterns at eight positions of the beam. The average value of 6 points obtained by cutting the upper and lower 2 points out of the 8 points is used. When the densities of 8 beams from A to H are calculated and the deviation from the average value is ΔD> 0.03, the information in the ROM 225 is changed. When changing the information, the relationship between the density difference D and LPW is prepared in advance in the table, and the LPW of the corresponding laser is changed so as to be within ΔD0.03 (step S207). When the amount of deviation from the average value is ΔD <0.03 or the laser after the light amount adjustment, the process proceeds to the main scanning magnification correction process which is the next step.

  The MFP control unit 101 of the image forming apparatus in which the light amount of each laser beam is corrected from the density difference using the pattern 1 performs main scanning magnification correction and outputs the pattern 2 (step S208). As the pattern 2, a screen pattern in which moire, which is a characteristic part of the present invention, easily occurs is used. Then, the main scanning magnification of each laser beam is intentionally changed as shown in FIG. 11B while using, for example, a pixel piece insertion / extraction technique described in Japanese Patent Application Laid-Open No. 2009-126091.

  As shown in FIG. 12, even if the laser irradiation position on the photoconductor 11 changes, the magnification does not change so that the oblique incidence does not occur (the more the laser beam is irradiated toward the front side from the center of the photoconductor 11 (right side in FIG. 12)). . Therefore, the main scanning magnification of the LD 8 is larger than that of the LD 1 that is a laser at the end of the laser chip. An image as shown in FIG. 11A is output while changing the combination of magnifications from 1 to 5 to maintain the relationship.

  In addition, one pattern of FIG. 11A employs 170 lpi of 1200 dpi main 7 pixels and sub-scanning 1 pixel period which is weak against moire. The reason for adopting 170 lpi is a feature of the present invention, and details will be described later.

  The magnification adjustment pattern 2 output in step S208 is read by the reader unit, and the MFP control unit 101 evaluates the moire intensity (step S209). Moire is subjected to frequency analysis by a general FFT (Fast Fourier Transform). In the case of 170 lpi of 1200 dpi main 7 pixels and sub-scanning 1 pixel period employed in the present embodiment, the moiré pitch appears at a pitch of 845 μm. Focusing on the frequency band, the moire intensity may be quantified.

  The MFP control unit 101 adopts the magnification setting value at which the most moire has not occurred, recalculates the main scanning magnification correction value (step S210), and stores the main scanning magnification correction value in the ROM 225 (step S214). This completes the adjustment up to the main scanning magnification correction that can also correct the mounting position accuracy of the image forming apparatus main body.

  Next, the process proceeds to a process of correcting the sub-scanning pitch residue by using lasers (LD1 and LD8) at the end of the laser chip of the laser unit 121. Even in an image forming apparatus adjusted to a magnification, moire may not be lost. As described above, the laser unit is assembled while adjusting the inclination of the laser chip in the laser unit assembling process of step S101 in FIG. 4, but it is very difficult to make the adjustment residue between the pitches of the laser chips zero.

  From the conventional method, a correction method using a laser chip end and one (1), (2), (7), (8) in case of 8 beams is disclosed. Since the laser chip is already fixed, further hardware adjustment is difficult. Therefore, also in this embodiment, the sub-scanning pitch adjustment residue is adjusted by the light quantity at the end of the laser chip.

  FIG. 13 is a diagram showing the relationship between the number of screen lines composed of the main scanning pixel period and the sub-scanning pixel period and the screen angle.

  FIG. 14 is a diagram schematically illustrating a latent image when the sub-scanning pitch is narrow in the first scanning and the second scanning and a latent image when the sub-scanning pitch is wide. FIG. 14 shows a first scan and a second scan in which the surface of the polygon mirror is switched to the upper and lower stages. Further, the left side of FIG. 14 shows the time when the sub-scanning pitch is narrow, and the right side of FIG. 14 shows the time when the sub-scanning pitch is wide.

  The dot density between scans varies depending on the sub-scanning pitch. As shown on the right side of FIG. 14, when the sub-scanning pitch is wide, LD8 and LD1 are close and emphasized, and when the sub-scanning pitch is narrow as shown on the left of FIG. 14, the dots are weakened. That is, the adjustment residue does not know whether the sub-scanning pitch is close or far. Therefore, the laser light quantity at the end of the laser chip is changed up and down when the adjustment pattern is output.

  FIG. 15 is a diagram schematically showing a latent image when the main scanning magnification is insufficiently adjusted in the first scanning and the second scanning and a latent image when the main scanning magnification is already adjusted.

  In the same layout as in FIG. 11A, the light quantity of the laser beam is changed at the portion where the magnification has been changed. The degree of change is 5 steps, (LD1-LPW, LD8-LPW) = 1: (REF-10%, REF-10%), 2: (REF-5%, REF-5%), 3: (REF , REF), 4: (REF + 5%, REF + 5%), 5: (REF + 10%, REF + 10%). The reference is abbreviated as REF.

  With the above configuration, the MFP control unit 101 outputs the pattern 3 while changing the light quantity of the LD1 and LD8 with the same layout and screen as in FIG. 11A (step S211). Next, the image is read by the reader unit in the same manner as in step S209, and the MFP control unit 101 quantifies moire (step S212), and changes the light amount correction values of LD1 and LD8 (step S213). Further, the changed light amount correction value is stored in the ROM 225 (step S214).

  In subsequent steps S215 to S220, since the same processing as that of the conventional product is executed, the description thereof is omitted. In automatic gradation correction (step S216) and normal image formation (step S219), the following output is performed. In the pattern output of FIG. 11A, the output is performed using 166 lpi main 6 pixels, which is resistant to moire, and dithering with a sub 4 pixel period, instead of the 170 dpi main 7 pixels and the sub scanning 1 pixel period 170 lpi.

  This is for the purpose of facilitating the detection of moiré only when outputting at 170 lpi, which is 1200 dpi main pixels that are weak to moiré and one pixel period of sub-scanning. The dither employed in FIG. 11A is a 170 lpi dither with a period of 1200 dpi main 7 pixels and sub-scanning 1 pixel which is weak against moire.

  In this embodiment, the main scanning magnification and the sub scanning pitch are adjusted. In both cases, as shown in FIG. 14 and FIG. 15, since the latent image strength is generated at the scanning pitch for each surface of the polygon mirror, the portion where the dot strength is estimated to be generated in the sub-scanning 8 beam period is analyzed. did.

  Next, a process for outputting a screen pattern under predetermined conditions (image formation conditions different from normal: condition 1, condition 2, condition 3, condition 4, condition 5), which is a feature of the image forming apparatus of the present embodiment, will be described. .

  The screen pattern is a square screen dot growth. When the specified number of sub-scanning pixels is moved, the scanning surface of the polygon mirror 123 is switched. Therefore, depending on the screen pattern, the intensity of the latent image is generated at the scanning surface switching portion of the polygon mirror 123 as described with reference to FIGS. Although a moire is generated depending on the strength of the latent image, a screen pattern in which the moire easily occurs is used as an adjustment pattern in FIG. In FIG. 11A, an adjustment screen that conforms to the following rules is adopted.

Condition 1: Number of screen lines is 133 lines or more and 212 lines or less.
If the number of screen lines is different, the dot area for the signal value is different.
The higher the number of screen lines, the smaller the dot area. Actually, since the dot area also affects the moire degree, the same number of screen lines (133 to 212 lines) as the normal image is used even for the adjustment pattern.
In FIG. 19, it is a gray filling part.

Condition 2: Screen angle is other than 0 degrees, 45 degrees, and 90 degrees.
When the screen angle is set to 0 degrees, 45 degrees, and 90 degrees, a horizontal stripe-like moire is generated as shown in FIG. However, since it is a horizontal streak, it cannot be distinguished whether it is moire or banding due to vibrations, etc., so adjustment screens other than the above 0 degrees, 45 degrees, and 90 degrees were adopted. FIG. 16 shows 170 lines with a period of 5 pixels in the main scanning and a period of 5 pixels in the sub-scanning up to the nearest dot, and a portion having an 8 beam period is painted in gray. In addition, in FIG. 19 mentioned later, it has shown by "number".

Condition 3: Adjustment is made so that the moire is not the closest dot angle (different from the screen angle).
FIG. 17 is a diagram showing when moire occurs at the same angle as the closest dot. Even if moiré occurs, the number of screen lines and the screen angle are easily mistaken for the structure of the screen and are not suitable for quantitative evaluation. Therefore, the same pattern as the number of screen lines is not used. In FIG. 19 described later, this is an area indicated by “(1)”.

Condition 4: Quantified by the moire of the second adjacent period of the screen so that moire is easily generated in any density range.
FIG. 18 is a diagram illustrating when moire occurs at the same angle as the closest dot. A pattern is selected so that moire is generated in the screen structure of the second proximity period of the screen, that is, the second closest period. In FIG. 19, the area is indicated by “(2)”.

Condition 5: The moire cycle is set to a pitch of 500 μm or more and 3 mm or less so that the moire frequency can be easily detected by human eyes or a reader unit.
Considering the visibility characteristics (VTF) of the eye, it is difficult to determine the moire of 500 μm or less. Even if the moire is detected by the reader unit, some blurring occurs due to the MTF characteristic including the optical characteristic of the reader unit. I want to evaluate moire even if it is blurred by the VTF of my eyes or the MTF of my reader. Therefore, the moire cycle is set to a pitch of 500 μm or more and 3 mm or less.

  FIG. 19 shows a summary of the above conditions 1 to 4 and FIG. 20 shows a result of adding conditions 5 to conditions 1 to 4. 19 and 20, the horizontal numbers (0 to 10) indicate the main scanning period, and the vertical numbers (0 to 10) indicate the sub-scanning period.

  As can be seen from FIG. 19, (1) (the same moiré as the number of screen angle lines) shown in FIG. 19 is (1) when the main scanning cycle or the sub-scanning cycle is the same as the beam cycle.

  The illustrated (2) (second approach period of the screen) is basically (2) when the number of beams is obtained by adding the main scanning period and the sub-scanning period. However, if the beam period is exceeded, 9-1 = 8 is added and subtracted from 9-1 as in main 9 and sub1, and 7 + 9 = 16 as main 7 and sub9 is twice the number of laser beams (an integer number of laser beams). Note that this is also the second proximity period.

FIG. 20 shows the result of extracting whether or not the condition 1 to the condition 4 are extracted and whether or not the condition 5 is satisfied. The numbers are moire pitch intervals. For example, if the main 7 and the sub 1 are used, it means that a moire of 845 μm pitch occurs. Therefore, the condition 5 is met in the black-filled area (main-sub) = 7-1, 9-1, 5-3, 3-5, 1-9, 1-7
It becomes.

  By performing the above calculation, the magnification adjustment pattern and the laser chip end light amount adjustment pattern have been determined to be 170 lpi which is the main 7-sub 1. If there are 8 beams, (main-sub) = 7-1, 9-1, 5-3, 3-5, 1-9, 1-7 may be used. However, this combination means that it is very vulnerable to misalignment moire. Therefore, this pattern should not be adopted during normal image formation. The dot area is 1200 dpi 2 × 2 dots. From the principle of moire generation between scans, a pattern of two or more sub-scan pixels is desirable.

  Here, features of the present embodiment will be described.

  In this image forming apparatus, the exposure unit (exposure control unit 10) and the image formation are performed so that a pattern image is formed under image forming conditions (conditions 1 to 5) different from the image forming conditions for forming an image to be transferred onto the recording material. At least one of the means (developer 13 and transfer unit 16) is controlled. Further, the exposure condition of the photoconductor 11 by the exposure controller 10 is corrected based on the pattern image formation state (degree of moire).

  In addition to the operation unit 114 of the image forming apparatus, an information processing apparatus (such as a PC) that can communicate with the image forming apparatus is used as input means for inputting correction data for correcting the exposure conditions of the exposure control unit 10. is there.

  The image forming condition different from the image forming condition for forming the image to be transferred onto the recording material is to output an image by a pseudo halftone process that is most likely to cause moire among a plurality of pseudo halftone processes of the image forming apparatus. It is a condition.

  The pseudo halftone process in which moiré is most likely to occur is that the moiré pattern overlaps with the second adjacent period (condition 4), the screen angle is other than 0 degrees, 45 degrees, and 90 degrees (condition 2), and moiré is performed. The pitch is 500 μm or more and 3 mm or less (condition 5).

  Further, the pseudo halftone process in which moiré is most likely to occur further includes a process (condition 3) for adjusting the moiré so that it does not have the closest dot angle.

  The correction data for correcting the exposure conditions of the exposure control unit 10 includes a correction value of the light amounts of a plurality of lasers, a write delay amount of the plurality of lasers with respect to the photosensitive member 11, and a correction value of a main scanning magnification of the plurality of lasers. It is.

  Also, evaluation of the screen pattern (pattern image) output from the image forming apparatus and calculation of correction data for correcting the exposure conditions of the exposure control unit 10 are performed outside the image forming apparatus.

  As described above, by deriving a screen pattern that tends to cause moiré from the relationship with the number of laser beams and using it to adjust the main scanning magnification and the laser light quantity at the end of the laser chip, it is possible to adjust the positional deviation of the dots with extremely high accuracy. It was. Dot misalignment affects not only moire but also graininess (feeling of roughness). This is because the deviation of the center of gravity of the dots can be avoided. Since the degree of moire and graininess are very important for the print image quality, it was possible to adjust with higher accuracy than the conventional adjustment method using a normal screen.

  As described above, according to the present embodiment, the following features and effects are obtained. In order to finely adjust the laser printing conditions (exposure conditions) with the output of the image forming apparatus, moire can be clearly recognized (or detected) and adjusted for special image forming conditions not used during normal image formation. Output screen pattern. The laser printing conditions are determined from the degree of moire of the output adjustment screen pattern. The screen pattern for adjustment is calculated from the number of laser beams, the number and angle of halftone pattern lines, and the presence or absence of multiple exposure. Further, image forming conditions other than laser printing conditions and screen patterns are also output under conditions that are different from those during normal image formation in order to easily cause moiré as a detection target.

  That is, in this image forming apparatus, the exposure means (exposure control unit 10) and the image are formed so that the pattern image is formed under image forming conditions (conditions 1 to 5) different from the image forming conditions for forming the image to be transferred onto the recording material. At least one of the forming means (developing device 13 and transfer unit 16) is controlled. Further, the exposure condition of the photoconductor 11 by the exposure controller 10 is corrected based on the pattern image formation state (degree of moire). Accordingly, it is possible to provide an image forming apparatus that can adjust a laser with high accuracy using a screen pattern that easily detects moire, and that can form an image with good graininess without moire.

DESCRIPTION OF SYMBOLS 10 Exposure control part 11 Photoconductor 101 MFP control part 114 Operation part 121 Laser unit 225 ROM

Claims (6)

Exposure means for exposing the photoreceptor by scanning the photoreceptor with a plurality of lasers;
Image formation in which an electrostatic latent image formed on the photosensitive member by exposure by the exposure unit is developed with toner, and the developed toner image is transferred to the recording material to form an image on the recording material. Means,
Control means for controlling at least one of the exposure means and the image forming means so that a pattern image is formed under an image forming condition different from an image forming condition for forming an image to be transferred to the recording material by the image forming means; ,
Correction means for correcting exposure conditions of the photosensitive member by the exposure means based on the formation state of the pattern image;
Equipped with a,
The image forming condition different from the image forming condition for forming the image to be transferred onto the recording material is to form an image by a pseudo halftone process in which moiré is most likely to occur among a plurality of pseudo halftone processes of the image forming apparatus. Condition,
The pseudo halftone process that is most likely to cause moiré is that the moiré pattern overlaps the second adjacent period of the screen, the screen angle is other than 0 degrees, 45 degrees, and 90 degrees, and the moiré period is 500 μm or more and 3 mm or less. an image forming apparatus wherein there.   The image forming apparatus according to claim 1, further comprising an input unit configured to input correction data for correcting an exposure condition of the exposure unit by the correction unit.   The correction data for correcting the exposure conditions of the exposure means include a correction value of the light quantity of the plurality of lasers, a write delay amount of the plurality of lasers with respect to the photosensitive member, and a correction value of the main scanning magnification of the plurality of lasers. The image forming apparatus according to claim 2, wherein the image forming apparatus is an image forming apparatus. The most moire prone pseudo-halftone processing, further, according to any one of claims 1 to 3, characterized in that includes processing of adjusting to be the ones moire is not nearest dot angle Image forming apparatus.   The evaluation of the pattern image formed by the image forming unit and the calculation of correction data for correcting the exposure condition of the exposure unit are performed outside the image forming apparatus. The image forming apparatus described in 1. An image forming apparatus control method comprising:
An exposure step of exposing the photoreceptor by scanning the photoreceptor with a plurality of lasers;
Image formation in which an electrostatic latent image formed on the photoreceptor by exposure in the exposure step is developed with toner, and the developed toner image is transferred to the recording material to form an image on the recording material Process,
A control step for controlling at least one of the exposure step and the image forming step so that a pattern image is formed under an image forming condition different from an image forming condition for forming an image to be transferred to the recording material by the image forming step; ,
A correction step of correcting the exposure conditions of the photoconductor by the exposure step based on the formation state of the pattern image;
Have,
The image forming condition different from the image forming condition for forming the image to be transferred onto the recording material is to form an image by a pseudo halftone process in which moiré is most likely to occur among a plurality of pseudo halftone processes of the image forming apparatus. Condition,
The pseudo halftone process that is most likely to cause moiré is that the moiré pattern overlaps the second adjacent period of the screen, the screen angle is other than 0 degrees, 45 degrees, and 90 degrees, and the moiré period is 500 μm or more and 3 mm or less. method of controlling an image forming apparatus wherein there.