JP4858637B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that forms a latent image on a latent image carrier by scanning a light beam in a direction orthogonal to a direction in which the latent image carrier is driven.

  This type of image forming apparatus includes a photoreceptor, an exposure unit, and a developing unit, and forms a toner image on the photoreceptor as follows. That is, the light source of the exposure unit is controlled based on the image data indicating the toner image, and the light beam from the light source is deflected by the deflector of the exposure unit so that the spot is scanned on the photosensitive member surface in the main scanning direction. A latent image corresponding to the image data is formed on the surface of the photoreceptor. The latent image is developed with toner to form a toner image.

  Further, in order to reduce the size and speed of the deflector, it has been conventionally proposed to use the deflecting mirror surface as a deflector by vibrating the surface (see Patent Document 1). That is, in this apparatus, the deflection mirror supported by the torsion bar is vibrated, and the light beam emitted from the light source is reflected by the deflection mirror and reciprocated on the surface of the photoreceptor.

JP 2002-182147 A (the third page and FIGS. 9 and 10)

  In such an image forming apparatus, it is possible to selectively switch between a double-sided scanning mode in which a light beam from a light source is scanned on the photoreceptor in both the forward path and the backward path, and a single-sided scanning mode in which only one of them is scanned. Thus, it is possible to form an image according to the printing mode. For example, when high resolution is not required, a spot is scanned on the photoreceptor only in either the forward path or the backward path to form an image with a low resolution, and when high resolution is required, the forward path and the backward path In both cases, the spot can be scanned over the photoreceptor surface. In addition, in such an image forming apparatus, it is possible to print not only line drawings such as characters but also images having shades such as photographs by reproducing gradation.

  However, in the above-described image forming apparatus, there is a large difference in gradation reproduction characteristics between the case where a latent image is formed on the photoconductor in the double-sided scanning mode and the case where the latent image is formed on the photoconductor in the single-sided scanning mode. Exists. The reason for this will be described. First, the first reason is as follows. The scanning pitch in the sub-scanning direction in the double-sided scanning mode is narrower than in the single-sided scanning mode. Therefore, in the double-sided scanning mode, the spots that scan the surface of the photosensitive member overlap more greatly between adjacent scanning lines than in the single-sided scanning mode. Therefore, in the double-sided scanning mode, there are cases where more toner adheres at the spot overlapping portions and the color density becomes darker than in the single-sided scanning mode. The second reason is as follows. In the image forming apparatus described above, the scanning pitch in the sub-scanning direction is constant in the single-sided scanning mode, whereas the scanning pitch in the sub-scanning direction is not constant in the double-sided scanning mode. Therefore, when a latent image is formed by scanning a spot on the surface of the photosensitive member in the double-sided scanning mode, the scanning pitch in the sub-scanning direction is not constant, so that the degree of overlap of spots in the sub-scanning direction varies. It becomes. In other words, the spot overlap in the sub-scanning direction increases at a narrow scanning pitch in the sub-scanning direction, whereas the spot overlap in the sub-scanning direction decreases at a wide scanning pitch in the sub-scanning direction. Therefore, in the double-sided scanning mode, color shading may occur corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction. As described above, there is a great difference in gradation reproduction characteristics between the case where the latent image is formed on the photoconductor in the double-sided scanning mode and the case where the latent image is formed on the photoconductor in the single-sided scanning mode. The difference in the gradation reproduction characteristics realizes good gradation reproduction in any scanning mode in the above-described apparatus that executes image formation by switching between the two-sided scanning mode and the one-sided scanning mode as necessary. It becomes a big obstacle to doing. That is, for example, even if good gradation reproduction can be obtained in the single-sided scanning mode, after switching to the double-sided scanning mode, the shading may become darker or unnecessary patterns may occur due to the above-mentioned reasons. Even if good gradation reproduction can be obtained in the scanning mode, the shade may become lighter after switching to the one-side scanning mode.

  The present invention has been made in view of the above problems, and is an image forming apparatus that forms a latent image by scanning a spot in the main scanning direction by a polarizing mirror that vibrates on a latent image carrier driven in the sub-scanning direction. An object of the present invention is to provide a technology that can selectively switch between the single-sided scanning mode and the double-sided scanning mode and that realizes good gradation reproduction in both the double-sided scanning mode and the single-sided scanning mode. .

In order to achieve the above object, the image forming apparatus according to the present invention reflects the light by the latent image carrier driven in the first direction and the deflecting mirror that oscillates, so that the first image is formed on the latent image carrier. A latent image forming unit that scans light in a second direction orthogonal or substantially orthogonal to the direction of the image, a one-side scanning mode that scans light in one of the second directions on the latent image carrier, and a latent image carrier A scanning mode control unit that selectively switches between one side in the second direction and the other side scanning mode in which light is scanned in the opposite direction to the other, and the latent image forming unit executes the one-side scanning mode to perform the latent image carrier. The one-side characteristic detection unit that obtains the gradation reproduction characteristics when the one-side scanning mode is executed based on the density detection result of the image formed by the developing unit developing the latent image to be formed on the two-side scanning. The development unit develops the latent image formed on the latent image carrier by executing the mode. The double-sided characteristic detection unit that obtains the gradation reproduction characteristics when the double-sided scanning mode is executed based on the density detection result of the image formed in this manner, and the latent image forming unit An image formed by the developing unit developing a latent image formed in the one-side scanning mode is formed based on the one-side threshold matrix in which the threshold value is corrected with the gradation reproduction characteristic obtained by the one-side characteristic detecting unit. When forming an image in the scanning mode, the tone reproduction characteristics obtained by the characteristic detection unit for both sides are formed by the developing unit developing the latent image formed by the latent image forming unit in the double-sided scanning mode. And an image formation control unit that performs based on the threshold matrix for both sides whose thresholds have been corrected in step 1, and one element of the threshold matrix for one side and two elements arranged in the first direction of the threshold matrix for both sides are associated with each other. Given the same threshold It is is characterized by reproducing the gradation of the image in the same tone generation pattern of the threshold matrix and bilateral threshold matrix for one side.

  At this time, the first direction may be the sub-scanning direction, and the second direction may be the main scanning direction.

  In the invention (image forming apparatus) configured as described above, the one-side scanning mode in which the spot is scanned only in the first direction (one side) in the main scanning direction (second direction), the first direction (one side), and the It is configured to be able to selectively switch between both-side scanning modes for scanning in both the first direction (one side) and the second direction (the other side) opposite to the first direction. For example, when a line latent image is formed on the latent image carrier as shown in FIG. 7, a line latent image LI (() is generated by a spot scanned in the first direction (one side) in the main scanning direction (second direction). + X) is formed, while the line latent image LI (−X) is formed by the spot scanned in the second direction (the other) opposite to the first direction (the one). Therefore, in the double-sided scanning mode in which the spot used for forming the latent image is scanned in the first direction (one) and the second direction (the other), the line latent images LI (+ X) and LI (−X) are in the sub-scanning direction (first). 1 direction). In contrast, in the one-side scanning mode in which the spot is scanned only in either the first direction (one) or the second direction (the other), only one of the line latent images LI (+ X) and LI (-X) is detected. Are formed in the sub-scanning direction (first direction). Therefore, in the double-sided scanning mode, compared with the single-sided scanning mode, the scanning pitch in the sub-scanning direction (first direction) is narrowed, so that the color shading tends to be dark as described above.

  In the invention configured as described above, the spot is reciprocated in the main scanning direction (second direction) on the surface of the latent image carrier, and the main scanning direction (second direction) on the surface of the latent image carrier. ) In the sub-scanning direction (first direction) that is substantially orthogonal. Therefore, the scanning trajectory of the spot on the latent image carrier in the double-sided scanning mode is as shown by the alternate long and short dash line in FIG. 15, so the scanning pitch in the sub-scanning direction (first direction) is not constant. . Accordingly, in the double-sided scanning mode, when a latent image is formed by scanning a spot on the surface of the photosensitive member, the scanning pitch in the sub-scanning direction (first direction) is not constant, so the sub-scanning direction (first direction) ) Will vary in the degree of spot overlap. That is, when the scanning pitch in the sub-scanning direction (first direction) is narrow, the overlap of spots in the sub-scanning direction (first direction) becomes large, whereas scanning in the sub-scanning direction (first direction). Where the pitch is wide, the overlap of spots in the sub-scanning direction (first direction) becomes small. Therefore, in the double-sided scanning mode, color shading may occur corresponding to the non-uniformity of the scanning pitch in the sub-scanning direction (first direction).

  However, in the present invention, the one-side scanning mode is executed to form a toner image as a gradation patch image, and the gradation reproduction characteristics when the one-side scanning mode is executed are controlled based on the density detection result of the toner image. The two-sided scanning mode is executed to form a toner image as a gradation patch image, and the gradation reproduction characteristics when the two-sided scanning mode is executed are controlled based on the density detection result of the toner image. That is, a toner image is formed as a gradation patch image in each of the one-side scanning mode and the both-side scanning mode, and the gradation reproduction characteristics of the apparatus are optimized in each scanning mode based on the density detection result of the toner image. . When the latent image is formed by executing the one-side scanning mode or the both-side scanning mode, the latent image is formed using the gradation reproduction characteristics optimized for each scanning mode. Therefore, regardless of the difference in the gradation reproduction characteristics between the double-sided scanning mode and the one-sided scanning mode as described above, it is possible to realize good gradation reproduction in any scanning mode.

In the present invention, a toner image is formed as a gradation patch image based on the same gradation generation pattern . With this configuration, it is not necessary to provide a gradation generation pattern for each scanning mode, and the configuration is simplified.

1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus in FIG. 1. FIG. 2 is a main scanning sectional view showing a configuration of an exposure unit in the image forming apparatus of FIG. 1. The figure which shows the scanning area | region of the light beam in the exposure unit of FIG. FIG. 2 is a diagram showing signal processing blocks in the image forming apparatus of FIG. 1. Explanatory drawing about the dither method. FIG. 2 is a diagram showing a line latent image formed by the image forming apparatus of FIG. 1. 6 is a flowchart showing the operation of gradation control processing in the one-side scanning mode. Explanatory drawing of a gradation characteristic. 6 is a flowchart showing the operation of gradation control processing in a double-sided scanning mode. The figure which shows the threshold value matrix at the time of the single-sided scanning mode and the double-sided scanning mode. 6 is a flowchart showing a latent image forming operation of the image forming apparatus. The figure which shows the threshold value matrix and gradation image patch in an Example. The figure which shows the threshold value matrix after the gradation control processing word in an Example. The figure which shows the scanning pitch in the double-sided scanning mode.

    FIG. 1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This image forming apparatus is a so-called tandem type color printer, and yellow (Y), magenta (M), cyan (C), and black (K) four-color photoconductors 2Y, 2M, and 2C as latent image carriers. 2K is provided in the apparatus main body 5. The apparatus forms a full-color image by superimposing the toner images on the photoreceptors 2Y, 2M, 2C, and 2K, or forms a monochrome image using only the black (K) toner image. That is, in this image forming apparatus, when a print command is given to the main controller 11 from an external device such as a host computer in response to an image formation request from the user, an engine controller is responded to the print command from the CPU 111 of the main controller 11. 10 controls each part of the engine unit EG to print an image corresponding to the print command on a sheet (recording medium) S such as a copy sheet, a transfer sheet, a sheet, and an OHP transparent sheet.

  In the engine unit EG, a charging unit, a developing unit, an exposure unit, and a cleaning unit are provided for each of the four photoconductors 2Y, 2M, 2C, and 2K. As described above, for each toner color, an image forming unit is provided that includes a photoreceptor, a charging unit, a developing unit, an exposure unit, and a cleaning unit, and forms a toner image of the toner color. The configuration of these image forming means (photosensitive member, charging unit, developing unit, exposure unit, and cleaning unit) is the same for all color components. Therefore, the configuration relating to yellow will be described here, and the other color components will be described. Are denoted by corresponding reference numerals, and description thereof is omitted.

  The photoreceptor 2Y is rotatably provided in the arrow direction (sub-scanning direction) in FIG. More specifically, the drive motor MT is mechanically connected to one end of the photoreceptor 2Y. The motor control unit 105 electrically connected to the drive motor MT controls the drive motor MT. As a result, the photoreceptor 2Y rotates. Thus, in this embodiment, the photoconductor 2Y is driven by transmitting the driving force from the drive motor MT only to one end side of the photoconductor 2Y. In this embodiment, the arrangement position of the drive motor MT, the horizontal synchronization sensor 60 described later, and the scanning direction of the light beam are set to satisfy a predetermined relationship. This point will be described in detail later.

  A charging unit 3Y, a developing unit 4Y, and a cleaning unit (not shown) are arranged around the photosensitive member 2Y driven in this way along the rotation direction. The charging unit 3Y is composed of, for example, a scorotron charger, and uniformly charges the outer peripheral surface of the photoreceptor 2Y to a predetermined surface potential by applying a charging bias from the charging control unit 103. Then, the scanning light beam Ly is irradiated from the exposure unit 6Y toward the outer peripheral surface of the photoreceptor 2Y charged by the charging unit 3Y. As a result, an electrostatic latent image corresponding to the yellow image data included in the print command is formed on the photoreceptor 2Y. Thus, the exposure unit 6Y corresponds to the “latent image forming unit” of the present invention, and operates in accordance with a control command from the exposure control unit 102Y. The configuration and operation of the exposure unit 6 (6Y, 6M, 6C, 6K) and the exposure control unit 102 (102Y, 102M, 102C, 102K) will be described in detail later.

  The electrostatic latent image formed in this way is developed with toner by a developing unit (developing means) 4Y. The developing unit 4Y contains yellow toner. When a developing bias is applied from the developing device controller 104 to the developing roller 41Y, the toner carried on the developing roller 41Y partially adheres to each surface portion of the photoreceptor 2Y according to the surface potential. As a result, the electrostatic latent image on the photoconductor 2Y becomes visible as a yellow toner image. As the developing bias applied to the developing roller 41Y, a DC voltage or a voltage obtained by superimposing an AC voltage on the DC voltage can be used. In particular, the photosensitive member 2Y and the developing roller 41Y are spaced apart from each other. In a non-contact development type image forming apparatus that develops toner by flying toner with a voltage waveform in which an alternating voltage such as a sine wave, a triangular wave, or a rectangular wave is superimposed on a direct current voltage in order to efficiently fly the toner Is desirable.

  The yellow toner image developed by the developing unit 4Y is primarily transferred onto the intermediate transfer belt 71 of the transfer unit 7 in the primary transfer region TRy1. The color components other than yellow are configured in the same manner as yellow, and magenta toner images, cyan toner images, and black toner images are formed on the photoreceptors 2M, 2C, and 2K, respectively, and primary transfer is performed. Primary transfer is performed on the intermediate transfer belt 71 in the regions TRm1, TRc1, and TRk1, respectively.

  The transfer unit 7 includes an intermediate transfer belt 71 stretched between two rollers 72 and 73, and a belt driving unit (not shown) that rotates the intermediate transfer belt 71 in a predetermined rotation direction R2 by rotating the roller 72. ). In addition, a secondary transfer roller 74 is provided at a position facing the roller 73 with the intermediate transfer belt 71 interposed therebetween. The secondary transfer roller 74 is configured to be brought into contact with and separated from the surface of the belt 71 by an electromagnetic clutch (not shown). Yes. When a color image is transferred to the sheet S, the primary transfer timing is controlled to superimpose the toner images to form a color image on the intermediate transfer belt 71, and the color image is taken out from the cassette 8 to be intermediate. The color image is secondarily transferred onto the sheet S conveyed to the secondary transfer region TR2 between the transfer belt 71 and the secondary transfer roller 74. On the other hand, when a monochrome image is transferred to the sheet S, only the black toner image is formed on the photoreceptor 2K, and the monochrome image is secondarily transferred onto the sheet S conveyed to the secondary transfer region TR2. In addition, the sheet S that has received the secondary transfer of the image in this way is conveyed toward the discharge tray portion provided on the upper surface portion of the apparatus main body via the fixing unit 9.

  The surface potential of each of the photoreceptors 2Y, 2M, 2C, and 2K after primary transfer of the toner image to the intermediate transfer belt 71 is reset by a neutralizing unit (not shown), and the toner remaining on the surface is further removed. Is removed by the cleaning unit, and then charged by the charging units 3Y, 3M, 3C, and 3K.

  In the vicinity of the roller 72, a transfer belt cleaner 75, a density sensor 76 (FIG. 2), and a vertical synchronization sensor 77 (FIG. 2) are arranged. Among these, the cleaner 75 can be moved toward and away from the roller 72 by an electromagnetic clutch (not shown). Then, the blade of the cleaner 75 is in contact with the surface of the intermediate transfer belt 71 that is stretched over the roller 72 while moving to the roller 72 side, and the toner that remains on the outer peripheral surface of the intermediate transfer belt 71 after the secondary transfer. Remove. The density sensor 76 is provided to face the surface of the intermediate transfer belt 71 and measures the optical density of the gradation patch image formed on the outer peripheral surface of the intermediate transfer belt 71. Further, the vertical synchronization sensor 77 is a sensor for detecting the reference position of the intermediate transfer belt 71, and is a synchronization signal output in association with the rotational drive of the intermediate transfer belt 71 in the sub-scanning direction, that is, a vertical synchronization signal. It functions as a vertical sync sensor for obtaining Vsync. In this apparatus, the operation of each part of the apparatus is controlled based on the vertical synchronization signal Vsync in order to align the operation timing of each part and to superimpose toner images of each color accurately. Further, a color misregistration sensor 78 is disposed between the rollers 72 and 73, and detects the color misregistration amount of each color toner image.

  In FIG. 2, reference numeral 113 denotes an image memory provided in the main controller 11 for storing image data given from an external device such as a host computer via the interface 112, and reference numeral 106 is executed by the CPU 101. A ROM for storing calculation data, control data for controlling the engine unit EG, and the like, and a reference numeral 107 are RAMs for temporarily storing calculation results in the CPU 101 and other data. Reference numeral 108 denotes an FRAM (ferroelectric memory) for storing information on the usage status of each part of the engine.

  3 is a main scanning sectional view showing the structure of the exposure unit equipped in the image forming apparatus of FIG. 1, FIG. 4 is a view showing the scanning region of the light beam in the exposure unit of FIG. 3, and FIG. FIG. 3 is a diagram illustrating a signal processing block in one image forming apparatus. Hereinafter, the configurations and operations of the exposure unit 6 and the exposure control unit 102 will be described in detail with reference to these drawings. The configuration of the exposure unit 6 and the exposure control unit 102 is the same for all color components, so the configuration relating to yellow will be described here, and the other color components will be denoted by corresponding reference numerals and description thereof will be omitted.

  The exposure unit 6Y (6M, 6C, 6K) has an exposure housing 61. A single laser light source 62Y is fixed to the exposure housing 61, and a light beam can be emitted from the laser light source 62Y. As shown in FIG. 4, the laser light source 62Y is electrically connected to the light source driving unit 1021 of the exposure control unit 102Y. Then, the light source driving unit controls ON / OFF of the laser light source 62Y according to the image signal as follows, and a light beam modulated in accordance with the image data is emitted from the laser light source 62Y. Hereinafter, a description will be given with reference to FIG.

  In this image forming apparatus, when an image signal is input from an external device such as the host computer 100, the main controller 11 performs predetermined signal processing on the image signal. The main controller 11 includes functional blocks such as a color conversion unit 114, an image processing unit 115, two types of line buffers 116A and 116B, a scanning mode switching unit 116C, a pulse modulation unit 117, a gradation correction table 118, and a correction table calculation unit 119. It has.

  In addition to the CPU 101, the ROM 106, the RAM 107, and the exposure control unit 102 shown in FIG. 2, the engine controller 10 detects a gradation characteristic that detects a gradation characteristic indicating the gamma characteristic of the engine unit EG based on the detection result of the density sensor 76. Part 123 is provided. In the main controller 11 and the engine controller 10, these functional blocks may be configured by hardware, or may be realized by software executed by the CPUs 111 and 101. Then, the main controller 11 and the engine controller 10 obtain the gradation reproduction characteristics of the apparatus when executing each scanning mode while controlling each part of the apparatus as will be described later. Thus, the main controller 11 and the engine controller 10 function as “one-side characteristic detecting means” and “both-side characteristic detecting means” of the present invention.

  In the main controller 11 to which the image signal is given from the host computer 100, the color conversion unit 114 converts the RGB gradation data indicating the gradation level of the RGB component of each pixel in the image corresponding to the image signal into the corresponding CMYK. Conversion into CMYK gradation data indicating the gradation level of the component. In this color conversion unit 114, the input RGB gradation data is, for example, 8 bits per pixel per color component (that is, representing 256 gradations), and the output CMYK gradation data is similarly 8 bits per pixel per color component ( That is, it represents 256 gradations). The CMYK gradation data output from the color conversion unit 114 is input to the image processing unit 115.

  The image processing unit 115 performs a halftoning process on the gradation data of each pixel input from the color conversion unit 114 for each color component. As the halftoning process, a method is used in which one halftone dot is formed using a plurality of pixels, and the size of the halftone dot is grown according to the gradation level of the gradation data to realize gradation reproduction. be able to. Further, as a method of generating a halftone dot that grows according to the gradation level of such gradation data, a dither method, an error diffusion method, or the like can be used. In this embodiment, halftoning processing is performed by the dither method. To do. FIG. 6 is an explanatory diagram of the dither method. In the dither method, the gradation level of the inputted gradation data is compared with the threshold value of each element of the threshold value matrix MTX. Here, one element of the threshold matrix corresponds to one pixel. When the gradation level of the gradation data is larger than the threshold value of each element, the value of the position corresponding to that element is “1”, and a latent image is formed at the photosensitive member surface position corresponding to the position. Become. On the contrary, when the gradation level of the gradation data is smaller than the threshold value of each element, the value of the position corresponding to that element becomes “0”, and the latent image is formed on the photosensitive member surface position corresponding to the position. Is not formed. Thus, in the dither method, halftone gradation data is obtained by comparing the gradation level of gradation data with the threshold value of each element of the threshold matrix. In the example of FIG. 6, a threshold value matrix MTX capable of reproducing 16 gradations is used, and the gradation level of gradation data is 4. However, the threshold value matrix MTX is not limited to the example of FIG. 6, and a threshold matrix having higher gradation reproducibility may be used. Further, the method of arranging the threshold values of the threshold value matrix MTX is not limited to the example of FIG. 6, and can be changed as appropriate.

  As described above, in the present embodiment, the halftoning process is performed by the dither method. In other words, the image processing unit 115 compares the threshold value matrix stored in the threshold value matrix storage unit 110A, which is a non-volatile memory, with the gradation data sent from the color conversion unit 114, thereby converting the gradation data into half. Convert to tone gradation data. Further, in this embodiment, in order to always maintain an ideal gamma characteristic of the image forming apparatus, the contents of the threshold value matrix stored in the threshold value matrix storage unit 110A at a predetermined timing are based on the actual measurement result of the image density. The gradation control process to be updated is executed.

  In this gradation control process, for each toner color, a gradation patch gradation image prepared in advance for measuring the gamma characteristic is formed on the intermediate transfer belt 71 by the engine unit EG, and each gradation patch is obtained. The image density of the image is read by the density sensor 76, and based on a signal from the density sensor 76, the gradation characteristic detection unit 123 associates the gradation level of each gradation patch image with the detected image density ( Gamma characteristic of the engine unit EG) is generated and output to the threshold conversion table calculation unit 110B of the main controller 11. Then, the threshold conversion table calculation unit 110B compensates the actually measured gradation characteristic of the engine unit EG based on the gradation characteristic given from the gradation characteristic detection unit 123 to obtain an ideal gradation characteristic. A threshold conversion table is obtained by calculation, and the contents of the threshold matrix stored in the threshold matrix storage unit 110A are updated based on the calculation result. By doing so, this image forming apparatus can form an image with stable quality regardless of variations in gamma characteristics of the apparatus and changes over time.

  The halftone gradation data obtained as described above is input to two types of line buffers 116A and 116B. These line buffers 116A and 116B are common in that they store halftone gradation data (image information) constituting one line image data output from the image processing unit 15, but the readout order of gradation data is the same. Is different. That is, the forward line buffer 116A outputs halftone gradation data constituting one line image data in the forward direction from the head, whereas the reverse line buffer 116B outputs in the reverse direction from the end. is there.

  The halftone gradation data thus output is input to the scanning mode switching unit 116C, and only the halftone gradation data output from one line buffer based on the scanning mode switching signal is scanned at an appropriate timing. 116C is output to pulse modulation section 117. The main reason why the two types of line buffers 116A and 116B are provided in this manner is to cope with the difference in the scanning mode of the latent image forming light beam depending on the printing mode, as will be described later. In addition, the gradation data is input to the pulse modulation unit 117 at the timing and order corresponding to each color component by the scanning mode switching unit 116C. Thus, in this embodiment, the line buffers 116A and 116B and the scanning mode switching unit 116C correspond to the “scanning mode control means” of the present invention. In this embodiment, the threshold matrix storage unit 110A stores the one-side threshold matrix 1101 and the two-side threshold matrix 1102 corresponding to each scanning mode, and executes the above-described gradation control processing in each scanning mode. The one-sided threshold matrix 1101 and the two-sided threshold matrix 1102 are updated, which will be described in detail later.

  The gradation data after halftoning input to the pulse modulation unit 117 is a multilevel signal indicating the size and arrangement of toner dots of each color to be attached to each pixel, and the pulse modulation unit 117 that has received such data receives the data. Using the halftone gradation data, a video signal for pulse width modulating the exposure laser pulse of each color image of the engine unit EG is generated and output to the engine controller 10 via a video interface (not shown). Upon receiving this video signal, the light source driving unit 1021 of the exposure control unit 102Y performs ON / OFF control of the laser light source 62Y of the exposure unit 6. The same applies to the other color components.

  Next, returning to FIGS. 3 and 4, the description will be continued. In the exposure housing 61, a collimator lens 631, a cylindrical lens 632, a deflector 65, and a scanning lens 66 are provided to scan and expose the light beam from the laser light source 62Y onto the surface (not shown) of the photoreceptor 2Y. It has been. That is, the light beam from the laser light source 62Y is shaped into collimated light of an appropriate size by the collimator lens 631, and then incident on the cylindrical lens 632 having power only in the sub-scanning direction Y. Then, by adjusting the cylindrical lens 632, the collimated light is imaged in the vicinity of the deflection mirror surface 651 of the deflector 65 in the sub-scanning direction Y. Thus, in this embodiment, the collimator lens 631 and the cylindrical lens 632 function as the beam shaping system 63 that shapes the light beam from the laser light source 62Y.

  The deflector 65 is formed by using a micromachining technique in which a micromachine is integrally formed on a semiconductor substrate by applying a semiconductor manufacturing technique, and includes a vibrating mirror that resonates and oscillates. That is, in the deflector 65, the light beam can be deflected in the main scanning direction X by the deflecting mirror surface 651 that resonates and vibrates. More specifically, the deflecting mirror surface 651 is pivotally supported around a swing shaft (torsion spring) that is substantially orthogonal to the main scanning direction X, and responds to an external force applied from an operating portion (not shown). Swings sine around the swing axis. This actuating unit applies an electrostatic, electromagnetic or mechanical external force to the deflection mirror surface 651 based on a mirror drive signal from a mirror drive unit (not shown) of the exposure control unit 102 to mirror the deflection mirror surface 651. Swing at the frequency of the drive signal. Note that any driving method such as electrostatic adsorption, electromagnetic force, or mechanical force may be adopted as the driving method by the operating unit, and since these driving methods are well known, description thereof is omitted here.

  The light beam deflected by the deflecting mirror surface 651 of the deflector 65 is deflected toward the scanning lens 66 at the maximum amplitude angle θmax as shown in FIG. In this embodiment, the scanning lens 66 is configured so that the F values are substantially the same in the entire effective image region IR of the photoreceptor 2. Accordingly, the light beam deflected toward the scanning lens 66 is focused on the effective image area IR on the surface of the photosensitive member 2 through the scanning lens 66 with substantially the same spot diameter. As a result, a line-shaped latent image extending in the main scanning direction X is formed on the effective image area IR of the photosensitive member 2 by scanning the light beam in parallel with the main scanning direction X. In this embodiment, the scanning area SR that can be scanned by the deflector 65 is set wider than the effective image area, as shown in FIG. The effective image area IR is located substantially at the center of the scanning area SR and is substantially symmetric with respect to the optical axis. Further, the symbol θir in the figure indicates the amplitude angle of the deflection mirror surface 651 corresponding to the end of the effective image region IR, and the symbol θs indicates the amplitude angle of the deflection mirror surface 651 corresponding to the horizontal synchronization sensor 60 described below. Is shown.

  Further, in the apparatus configured as described above, the light beam can be reciprocated in the main scanning direction, that is, the light beam can be scanned in both the (+ X) direction and the (−X) direction. . As described above, the gradation data constituting one line image data is temporarily stored in the storage unit (line buffers 116A and 116B), and the scanning mode switching unit 116C performs gradation data at an appropriate timing and order. Is supplied to the pulse modulation unit 117. For example, in the case of switching to the (+ X) direction, as shown in FIG. 7A, the beam spot is read from the line buffer 116A in the order of gradation data DT1, DT2,. Is irradiated onto the photoconductor 2 in the first direction (+ X) to form a line latent image LI (+ X). On the other hand, when switching to the (−X) direction, as shown in FIG. 7B, the grayscale data DTn, DT (n−1),. A beam spot is irradiated onto the photoconductor 2 in the second direction (−X) based on each gradation data, and a line latent image LI (−X) is formed. For this reason, the spot which forms a latent image on the sightseeing body surface as follows can be made to differ for every printing mode or for every line. More specifically, in this embodiment, information about the resolution (resolution information) included in the print command is temporarily stored in the RAM 107. When high resolution printing is instructed, the spot is scanned in the effective image area IR in the (+ X) direction to form a latent image in the effective image area IR, and the spot is effective in the (−X) direction. A latent image is formed by executing a so-called double-sided scanning mode in which the operation of scanning the image region IR and alternately forming the latent image in the effective image region IR is repeated. On the other hand, when low resolution printing is commanded, a latent image is formed by executing a so-called one-side scanning mode in which the spot is scanned only in the (+ X) direction. Thus, in this embodiment, the spot scanning mode is switched between high resolution printing and low resolution printing based on the resolution information. This point will be described in detail later.

  In this embodiment, the scanning direction and the arrangement position of the drive motor MT are set in advance so as to satisfy the following relationship. That is, the drive motor MT is disposed on the downstream side in the scanning direction (+ X). Also, as shown in FIG. 3, the end of the scanning path of the scanning light beam is guided to the horizontal synchronization sensor 60 by the folding mirror 69 on the upstream side in the scanning direction (+ X). The folding mirror 69 is disposed at the end of the scanning region SR on the upstream side in the scanning direction (+ X), and moves on the upstream side in the scanning direction (+ X) within the scanning region SR and outside the effective image region IR. The scanning light beam to be guided is guided to the horizontal synchronization sensor 60. A signal is output from the horizontal synchronization sensor 60 at a timing when the scanning light beam is received by the horizontal synchronization sensor 60 and passes through the sensor position (amplitude angle θs). Thus, in this embodiment, the horizontal synchronization sensor 60 is used as a horizontal synchronization reading sensor for obtaining a synchronization signal when the light beam scans the effective image area IR in the main scanning direction X, that is, the horizontal synchronization signal Hsync. The latent image forming operation is controlled based on the horizontal synchronization signal Hsync.

  FIG. 8 is a flowchart showing the gradation control processing in the one-sided scanning mode according to this embodiment. First, the one-side scanning mode is set (step S11). Next, a scanning mode switching signal corresponding to the scanning mode determined as described above is given to threshold matrix storage section 110A. (Step S12). Thus, the threshold matrix storage unit 110A outputs the one-side threshold matrix 1101 to the image processing unit 115. Next, a gradation patch image is formed on the intermediate transfer belt 71 (step S13). Such a gradation patch image is formed on the intermediate transfer belt in accordance with a gradation generation pattern in which a plurality of toner images having different predetermined gradation levels are arranged in the driving direction of the intermediate transfer belt 71. A plurality of toner images are arranged so that the gradation levels of the respective toner images gradually become lower with respect to the belt driving direction. Further, although toner images can be formed for all gradations from the maximum gradation level to the minimum gradation level, in the present embodiment, toner images are formed only for predetermined gradation levels. In step 12, the threshold matrix storage unit 110A is set to output the one-sided threshold matrix 1101 to the image processing unit 115. Therefore, when forming a gradation patch image, the one-side threshold matrix 1101 stored in the threshold matrix storage unit 110A is used. Next, the density sensor 76 detects the density of a plurality of toner images having different gradation levels in the patch image formed in this way (step S14). Next, a gradation characteristic as shown by a solid line in FIG. 9 in which the gradation level and the detected image density are associated is created in the gradation characteristic detection unit 123 (step S15). Then, in the threshold conversion table calculation unit 110B, a one-side threshold conversion table is created from the gradation characteristics created in this way so that the gradation characteristics linearly change in image density as the gradation level changes, Based on this, the threshold of the one-side threshold matrix is corrected (step S16). That is, the threshold values of the threshold value matrix are corrected so that the gradation characteristic as shown by the solid line in FIG. 9 becomes a straight line as shown by the broken line in FIG. Here, the threshold value corresponding to the gradation level where the toner image is not formed in the gradation patch image is obtained by linearly complementing the one-side threshold conversion table. Then, the contents of the one-side threshold matrix 1101 stored in the threshold matrix storage unit 110A are updated to the corrected one-side threshold matrix contents (step S17). As described above, the gradation control processing in the one-side scanning mode corresponds to the “first step” of the present invention, and the updated one-side threshold matrix corresponds to the “tone reproduction characteristics” in the one-side scanning mode of the present invention. To do.

  FIG. 10 is a flowchart showing the gradation control process in the double-sided scanning mode in the present embodiment. First, the both-side scanning mode is set (step S21). Next, a scanning mode switching signal corresponding to the scanning mode determined as described above is given to threshold matrix storage section 110A. (Step S22). Thus, the threshold matrix storage unit 110A outputs the both-side threshold matrix 1102 to the image processing unit 115. Next, a gradation patch image is formed on the intermediate transfer belt 71 (step S23). Such a gradation patch image is formed on the intermediate transfer belt in accordance with a gradation generation pattern in which a plurality of toner images having different predetermined gradation levels are arranged in the driving direction of the intermediate transfer belt 71. A plurality of toner images are arranged so that the gradation levels of the respective toner images gradually become lower with respect to the belt driving direction. Further, although toner images can be formed for all gradations from the maximum gradation level to the minimum gradation level, in the present embodiment, toner images are formed only for predetermined gradation levels. In step S <b> 22, the threshold matrix storage unit 110 </ b> A is set to output the two-sided threshold matrix 1102 to the image processing unit 115. Therefore, when forming a gradation patch image, the two-sided threshold matrix 1102 stored in the threshold matrix storage unit 110A is used. Furthermore, in this embodiment, the two-sided threshold matrix and the one-sided threshold matrix are configured to satisfy the relationship as shown in FIG. That is, in the double-sided scanning mode, the scanning pitch in the sub-scanning direction is approximately half that in the single-sided scanning mode, and accordingly, in the sub-scanning direction Y of one element of the single-side threshold matrix and the double-sided threshold matrix. Two elements arranged side by side are associated with each other and have the same threshold value. As described above, in this embodiment, the tone generation pattern of the tone patch image in the double-sided scanning mode and the tone generation pattern of the tone patch image in the side-side scanning mode are made common.

  The density sensor 76 detects the densities of a plurality of toner images having different gradation levels in the patch image formed in this way (step S24). Next, a gradation characteristic as shown by the solid line in FIG. 9 in which the gradation level is associated with the detected image density is created in the gradation characteristic detection unit 123 (step S25). Then, the threshold value conversion table calculation unit 110B creates a double-sided threshold value conversion table from the tone characteristics thus created so that the tone characteristics change linearly as the tone level changes, and Based on this, the threshold values of the both-side threshold matrix are corrected (step S26). That is, the threshold values of the both-side threshold matrix are corrected so that the gradation characteristics as shown by the solid line in FIG. 9 become a straight line as shown by the broken line in FIG. Here, the threshold value corresponding to the gradation level in which the toner image is not formed in the gradation patch image is obtained by linearly complementing the both-side threshold conversion table. Then, the contents of the both-side threshold matrix 1101 stored in the threshold matrix storage unit 110A are updated to the contents of the corrected both-side threshold matrix (step S27). As described above, the gradation control process in the double-sided scanning mode corresponds to the “second step” of the present invention, and the double-sided threshold matrix 1101 after the update has the “gradation reproduction characteristics” in the double-sided scanning mode of the present invention. Equivalent to.

  As described above, in the present embodiment, the one-side scanning mode and the both-side scanning mode can be switched. In such an image forming apparatus, the latent image forming operation can be performed by switching the scanning mode according to the printing mode. Therefore, in this embodiment, the scanning mode is switched according to the resolution. In other words, when the resolution is not required, the latent image is formed in the one-side scanning mode with a wide scanning pitch in the sub-scanning direction. The formation is going to be done. Next, the latent image forming operation in this embodiment will be described.

  When a print command is input from an external device such as the host computer 100, a latent image is formed on each photoconductor according to the flowchart shown in FIG. 12, and a color image is formed based on each latent image. That is, in step S30, resolution information included in the print command is acquired. Then, based on the resolution information, it is determined whether the print command requests high-resolution printing or low-resolution printing (step S31).

  If “YES” is determined in the step S31, that is, if it is determined that the low-resolution printing is performed, steps S36 to S39 are executed to form an image with a low resolution, and the image is transferred to the sheet S and the printing process is ended. First, in step S36, the one-side scanning mode is set. Next, a scanning mode switching signal corresponding to the scanning mode determined as described above is given to the threshold value matrix storage unit 110A. (Step S37). Thus, the threshold matrix storage unit 110A outputs the one-side threshold matrix 1101 to the image processing unit 115. Therefore, the image processing unit 115 generates halftone gradation data using the one-side threshold matrix 1101 and outputs it to the line buffer. Further, a scanning mode switching signal corresponding to the scanning mode determined as described above is given to the scanning mode switching unit 116C of the main controller 11 (step S38). On the other hand, the scanning mode switching unit 116C that has received these instructions forms a latent image line by line by fixing the read timing and order of gradation data from the line buffer. That is, only the spot scanned in the first direction while being read out from the forward direction line buffer 116A at an appropriate timing and forward direction (that is, in the order of gradation data DT1, DT2,... DTn) and optically modulated based on each gradation data. Is scanned on the photosensitive member 2 to form a latent image (step S39). In this way, a so-called one-side scanning mode is executed, and a latent image is formed at a low resolution. Further, the latent image formed in this way is developed with toner to form a four-color toner image, and is superimposed on the intermediate transfer belt 71 to form a color image, and then the color image is formed on the sheet S. The image is transferred and the low resolution printing is completed.

  If “NO” is determined in the step S31, that is, if it is determined that high resolution printing is performed, the steps S32 to S35 are executed to form an image with a high resolution, transferred to the sheet S, and the printing process is ended. First, in step S32, the double-sided scanning mode is set. Next, a scanning mode switching signal corresponding to the scanning mode determined as described above is given to the threshold value matrix storage unit 110A. (Step S33). Thus, the threshold matrix storage unit 110A outputs the both-side threshold matrix 1102 to the image processing unit 115. Therefore, the image processing unit 115 generates halftone gradation data using the both-side threshold matrix 1102 and outputs it to the line buffer. Further, a scanning mode switching signal corresponding to the scanning mode determined as described above is given to the scanning mode switching unit 116C of the main controller 11 (step S34). On the other hand, upon receiving these instructions, the scanning mode switching unit 116C alternately switches the timing and order of reading out the gradation data from the line buffer for each line. Thereby, a high-resolution latent image is formed as follows. That is, the spot is scanned in the effective image area IR in the (+ X) direction to form a latent image in the effective image area IR, and the spot is scanned in the effective image area IR in the (−X) direction. The operation of forming a latent image is alternately repeated (step S35). Thus, a so-called double-sided scanning mode is executed to form a latent image with high resolution. Further, the latent image formed in this way is developed with toner to form a four-color toner image, and is superimposed on the intermediate transfer belt 71 to form a color image, and then the color image is formed on the sheet S. The image is transferred and high-resolution printing ends.

  As described above, in this embodiment, the one-side scanning mode is executed to form a toner image as a gradation patch image, and the gradation reproduction characteristics when the one-side scanning mode is executed are controlled based on the density detection result of the toner image. On the other hand, the two-sided scanning mode is executed to form a toner image as a gradation patch image, and the gradation reproduction characteristics when the two-sided scanning mode is executed are controlled based on the density detection result of the toner image. Yes. That is, a toner image is formed as a gradation patch image in each of the one-side scanning mode and the both-side scanning mode, and the gradation reproduction characteristics of the apparatus are optimized in each scanning mode based on the density detection result of the toner image. . Therefore, regardless of the difference in gradation reproduction characteristics between the double-sided scanning mode and the single-sided scanning mode, it is possible to realize good gradation reproduction in any scanning mode.

  Further, in the present embodiment, when performing the gradation control process, a toner image is formed as a gradation patch image based on the same gradation generation pattern in each of the one-side scanning mode and the both-side scanning mode. ing. With this configuration, it is not necessary to provide a gradation generation pattern for each scanning mode, and the configuration is simplified.

  The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the above embodiment, the double-sided scanning mode and the single-sided scanning mode are switched based on the required resolution, but the criteria for switching the scanning mode are not limited to these, and the double-sided scanning mode and the single-sided scanning mode The present invention is applicable to all image forming apparatuses configured to be switchable.

  In the present invention, the halftoning process is performed by the dither method. However, the halftoning process is not limited to the dither method, and for example, an error diffusion method may be used.

  In the above embodiment, the spot is scanned only in the (+ X) direction in the one-side scanning mode, but the spot may be scanned only in the (−X) direction. In short, the spot may be configured to be unidirectionally scanned in the first direction (+ X) or the second direction (−X) in the main scanning direction X.

  In the above-described embodiment, the present invention is applied to a so-called tandem color printer, but the scope of the present invention is not limited to this. For example, only a so-called four-cycle printer or single color printing is used. The present invention can also be applied to a monochrome printer.

  In the above embodiment, the present invention is applied to an image forming apparatus that temporarily forms a color image on an intermediate transfer medium such as an intermediate transfer belt and then transfers the color image to the sheet S. The present invention is also applicable to an apparatus that forms a color image by directly superimposing toner images on a sheet.

  In the above embodiment, the vibrating deflection mirror surface 651 is formed by using a micromachining technique. However, the method of manufacturing the deflection mirror surface is not limited to this, and the vibrating deflection mirror surface is used. The present invention can be applied to all so-called image forming apparatuses in which a light beam is deflected to scan the latent image carrier.

  Next, examples of the present invention will be shown. However, the present invention is not limited by the following examples as a matter of course, and it is of course possible to implement the present invention with appropriate modifications within a range that can meet the gist of the preceding and following descriptions. They are all included in the technical scope of the present invention.

  FIG. 13A is a one-sided threshold matrix used in this embodiment. In this embodiment, a one-sided scanning is performed on a gradation patch image having 2, 4, 6, 8, 10, 12, 14, 16 gradation levels as shown in FIG. The mode was executed to form on the intermediate transfer belt. Table 1 shows a one-side threshold conversion table obtained from the density detection result of the gradation patch image by the same procedure as the gradation control process described in the embodiment. Then, the gradation levels 1, 3, 5, 7, 9, 11, 13, and 15 that are not formed in the gradation patch image are supplemented by linearly complementing Table 1 to obtain FIG. 14A. The one-sided threshold matrix shown was obtained. In forming a latent image in the one-side scanning mode, halftoning processing is executed using the one-side threshold matrix shown in FIG.

  FIG. 13B is a two-sided threshold matrix used in this embodiment. As described above, in this embodiment, the scanning pitch in the sub-scanning direction is substantially half in the double-sided scanning mode compared to that in the single-sided scanning mode. The two elements arranged in the sub-scanning direction Y are associated with each other and have the same threshold value. Accordingly, when the gradation patch image having the gradation levels of 2, 4, 6, 8, 10, 12, 14, 16 using the double side threshold matrix is formed on the intermediate transfer belt by executing the double side scanning mode. As shown in FIG. 13C, a gradation patch having the same gradation generation pattern as the gradation patch image formed by executing the one-side scanning mode is obtained. Table 2 shows a double-sided threshold conversion table obtained from the density detection result of the gradation patch image by the same procedure as the gradation control process described in the embodiment. Then, the gradation levels 1, 3, 5, 7, 9, 11, 13, and 15 that are not formed in the gradation patch image are supplemented by linearly complementing Table 1 to obtain FIG. 14B. The one-sided threshold matrix shown was obtained. Then, when forming a latent image in the double-sided scanning mode, halftoning processing is executed using the one-side threshold matrix shown in FIG.

  As described above, in this embodiment, the one-side scanning mode is executed to form a toner image as a gradation patch image, and the tone reproduction characteristics when the one-side scanning mode is executed are controlled based on the density detection result of the toner image. On the other hand, the two-sided scanning mode is executed to form a toner image as a gradation patch image, and the gradation reproduction characteristics when the two-sided scanning mode is executed are controlled based on the density detection result of the toner image. Yes. That is, a toner image is formed as a gradation patch image in each of the one-side scanning mode and the both-side scanning mode, and the gradation reproduction characteristics of the apparatus are optimized in each scanning mode based on the density detection result of the toner image. . When the latent image is formed by executing the one-side scanning mode or the both-side scanning mode, the latent image can be formed using the gradation reproduction characteristics optimized for each scanning mode. Therefore, regardless of the difference in gradation reproduction characteristics between the double-sided scanning mode and the single-sided scanning mode, it is possible to realize good gradation reproduction in any scanning mode.

  Further, in the present embodiment, when performing the gradation control process, a toner image is formed as a gradation patch image based on the same gradation generation pattern in each of the one-side scanning mode and the both-side scanning mode. ing. With this configuration, it is not necessary to provide a gradation generation pattern for each scanning mode, and the configuration is simplified.

  2,2Y, 2M, 2C, 2K ... photosensitive body (latent image carrier), 4,4Y, 4M, 4C, 4K ... developing unit (developing means) 6,6Y, 6M, 6C, 6K ... exposure unit (latent (Image forming unit), 60 ... horizontal synchronization sensor (detector), 62, 62Y, 62M, 62C, 62K ... laser light source, 71 ... intermediate transfer belt (transfer medium), 651 ... deflection mirror surface, DT1, DT2, DT ( n-1), DTn ... gradation data (image information), IR ... effective image area, Ly, Lm, Lc, Lk ... scanning light beam, LI (+ X), LI (-X) ... line latent image, MT ... Drive motor (drive means), PT, PT1, PT2, PT3 ... scanning pitch, SR ... scanning area, X ... main scanning direction, Y ... sub-scanning direction

Claims (2)

A latent image carrier driven in a first direction;
A latent image forming unit that scans light in a second direction orthogonal or substantially orthogonal to the first direction on the latent image carrier by reflecting light by a vibrating deflection mirror;
A one-sided scanning mode in which the latent image carrier is scanned with light in one of the second directions, and both sides of the latent image carrier that are scanned with light in the second direction and the other opposite to the one. A scanning mode controller that selectively switches between scanning modes;
When the one-side scanning mode is executed based on the density detection result of the image formed by the developing unit developing the latent image formed on the latent image carrier by executing the one-side scanning mode by the latent image forming unit. A one-side characteristic detection unit for obtaining the tone reproduction characteristics of
When executing the double-sided scanning mode based on the density detection result of the image formed by the developing unit developing the latent image formed on the latent image carrier by executing the double-sided scanning mode by the latent image forming unit. A double-sided characteristic detector for obtaining the tone reproduction characteristics of
When the image is formed in the one-side scanning mode , the one-side characteristic detecting unit forms an image formed by the developing unit developing the latent image formed by the latent image forming unit in the one-side scanning mode. When forming an image in the double-sided scanning mode based on the one-sided threshold matrix in which the threshold value is corrected with the obtained tone reproduction characteristics, the latent image formed by the latent image forming unit in the double-sided scanning mode An image forming control unit configured to perform image formation performed by the developing unit based on a threshold matrix for both sides in which thresholds are corrected with the gradation reproduction characteristics obtained by the both-side property detecting unit ;
With
One element of the threshold matrix for one side and two elements arranged in the first direction of the threshold matrix for both sides are associated with each other and given the same threshold value, and the threshold matrix for one side and the threshold value for both sides are given An image forming apparatus that reproduces the gradation of an image with the same gradation generation pattern as a matrix .   The image forming apparatus according to claim 1, wherein the first direction is a sub-scanning direction, and the second direction is a main scanning direction.

Images