JP2007062208A - Image forming device and method of controlling the device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a favorable halftone image without a density varition in particular, irrespective of the status of a use and an environment by controlling sole gradation potential characteristics or gradation density characteristics of a light beam to ensure its coincidence with characteristics of one light beam in an image forming device which performs image formation by a plurality of light beams. <P>SOLUTION: The image forming device which emits a plurality of light beams corresponding to image data, exposes and scans the light beams on a photosensitive body and forms an image by an optical scanning means of a plurality of beams to reproduce a resolution higher than the resolution of an optical scanning speed, and controls an independent exposure control means by which the light beams independently form a plurality of gradation patterns on the photosensitive body at preset operation intervals and independently control the amount of exposure independently to ensure a difference between the detection value of one light beam from among the light beams and the detection value of the other light beam is equal to or below a prescribed value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus for forming an image on a recording material and a control method thereof. More specifically, the present invention relates to an image forming apparatus having a plurality of exposure means capable of forming an image with a resolution higher than that of the scanning exposure means.

  Conventionally, as a technique of an image forming apparatus having a plurality of beams, there are techniques described in Patent Document 1 and Patent Document 2, for example.

  An image forming apparatus described in Patent Document 1 is an image information source that supplies image data having a resolution twice that of an optical scanning device, and an image that determines a pattern in the sub-scanning direction of image data input from the image information source. Pattern determining means is provided. Exposure control means is provided for outputting a light beam lighting signal and a light beam intensity signal in order to reproduce the image information by an optical scanning device having a resolution of 1/2 of the image data in accordance with the image pattern. Then, by providing a modulation exposure means for modulating the light beam based on the light beam lighting signal and the light beam intensity signal from the exposure control means, an image is formed at a resolution twice that of a writing apparatus having an optical scanning system. That's it.

The image forming apparatus described in Patent Document 2 provides a more faithful output image with respect to input data by appropriately adjusting the balance of a plurality of light beams.
Japanese Patent Laid-Open No. 10-181091 JP 2003-280297 A

  However, in an image forming apparatus using a plurality of light beams, the balance of the amount of light of each light beam, the laser power at the time of multi-value image formation, and the gradation characteristics of the image density are different, so that high definition and good are achieved. It was difficult to provide images. For example, when two light beams are used, if each of the A laser and the B laser is used, this is caused by the difference in the rise and fall characteristics and spot diameter of each of the A and B lasers. It is difficult to close the difference below a predetermined level.

  In the above-mentioned prior art document 2, reference is made to the light quantity of each beam and the balance adjustment of each beam when a plurality of beams are combined. However, since the difference in the gradation characteristics of each laser remains, the image quality in the halftone area is affected by the difference in gradation characteristics when image formation is performed at a higher definition, for example, 600 dpi. End up. In addition, the characteristics of the laser, particularly the gradation characteristics, tend to change over time and over time, and even if adjustments are made at the initial stage of the image forming apparatus, changes occur, and it is difficult to always obtain a good image. Met.

In view of the above-described conventional problems, the present invention provides a single gradation potential characteristic or gradation density characteristic of each light beam in an image forming apparatus that forms an image with a plurality of light beams. By controlling so as to match the characteristics, it is possible to provide a good image with no density unevenness, particularly in a halftone, regardless of the use situation or environment.
After that, multiple light beams are turned on, a predetermined gradation pattern is created, and total gradation control and LUT are changed to provide a good gradation image without disturbing the halftoning (dither) structure. To do.

  In order to solve such a problem, the image forming apparatus of the present invention emits a plurality of light beams according to image data, performs exposure scanning on the photosensitive member, and reproduces a resolution higher than the resolution of the optical scanning speed. Optical scanning system means, single gradation forming means for forming a plurality of gradation patterns on the photosensitive member independently for each predetermined operation interval, and independent light intensity control for each light beam independently. A detection value corresponding to the gradation of each light beam detected from the image formed by the single gradation forming means is a detection value of any one of the plurality of light beams. The independent exposure control unit is controlled so that a difference from a detection value of another light beam is equal to or less than a predetermined value.

  Here, it has a potential detecting means for detecting the surface potential of the photoconductor, and detects the gradation potential of each light beam detected by the potential detecting means for the electrostatic latent image formed by the single gradation forming means. The independent exposure control unit is controlled so that a difference between a gradation potential detection value of one of the plurality of light beams and a gradation potential detection value of another light beam is equal to or less than a predetermined value. .

  A developing unit that visualizes the electrostatic latent image formed on the photoreceptor by the optical scanning unit with a toner image; a transfer unit that transfers the visible toner image to another medium; Density detection means for detecting the toner density on the image carrier, and the detected value of the tone density of each light beam detected by the density detection means for the toner image formed by the single tone forming means, The independent exposure control means is controlled so that the difference between the gradation density detection value of any one of the plurality of light beams and the gradation density detection value of the other light beam is not more than a predetermined value. The image carrier is a photoreceptor. The image carrier is an intermediate transfer member. The image carrier is a transfer belt. The image forming apparatus according to claim 3, wherein the image carrier is a transfer drum.

  A developing unit that visualizes the electrostatic latent image formed on the photosensitive member by the optical scanning unit with a toner image; a transfer unit that transfers the visualized toner image to a transfer material; Density detection means for detecting the toner density on the material, and a detection value of the gradation density of each light beam detected by the density detection means on the toner image on the transfer material formed by the single gradation forming means However, the independent exposure control means is controlled so that the difference between the gradation density detection value of any one of the plurality of light beams and the gradation density detection value of the other light beam is not more than a predetermined value.

  Here, the light beam can be multi-level modulated. The modulation method of the light beam is pulse width modulation. The modulation method of the light beam is power modulation.

  Also, the control method of the image forming apparatus of the present invention is a multiple beam optical scanning system that emits a plurality of light beams according to image data, exposes and scans the photosensitive member, and reproduces a resolution higher than the resolution of the optical scanning speed. A method of controlling an image forming apparatus that forms an image by means, wherein a plurality of gradation patterns on a photoconductor are formed independently at each predetermined operation interval and detected from the formed electrostatic latent image The detection value corresponding to the gradation of each light beam is such that the difference between the detection value of any one of the plurality of light beams and the detection value of the other light beam is a predetermined value or less. Independent exposure control means for controlling the exposure amount of each beam independently is controlled.

  As described above, in an image forming apparatus that forms an image with a plurality of light beams, the single gradation potential characteristic or gradation density characteristic of each light beam matches the characteristics of one of the light beams. By controlling, it is possible to provide a good image with no density unevenness, particularly in a halftone, regardless of the use situation or environment. After that, multiple light beams are turned on, a predetermined gradation pattern is created, and total gradation control and LUT are changed to provide a good gradation image without disturbing the halftoning (dither) structure. To do.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the relative arrangement, numerical formulas, numerical values, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified.

<Example of Configuration of Image Forming Apparatus of Present Embodiment>
FIG. 1 shows a configuration example of a full-color printer as an image forming apparatus according to the present embodiment.

  Here, the image forming apparatus 100 is connected to or integrated with the reader unit 200. The reader unit 200 is a device that converts external information into an image signal. That is, it is a document reading device, a personal computer or the like. For example, the luminance signal of the read document image or the image signal transferred from the personal computer or the like is transmitted to the image forming device 100 that performs image formation.

  Image formation in a full-color printer 100 (hereinafter referred to as “printer”) as the image forming apparatus shown in FIG. 1 includes a photosensitive drum 1 as an image carrier. Based on the image information from the reader unit 200, a charging process, a latent image forming process, a developing process, and a transfer from the photosensitive drum 1 until a developed image (toner image) is formed on the photosensitive drum 1 with a developer. A transfer process for transferring the toner image to the material P is performed. Then, it is performed according to an image forming process based on a fixing process for fixing the toner image on the transfer material P.

  As an image forming unit that performs these image forming steps, the printer 100 applies a charging bias in the charging step to uniformly charge the surface of the photosensitive drum 1 to a predetermined potential, and a charging roller as a charging unit. 2 The exposure unit 3 is a laser writing unit that sequentially irradiates laser light L in accordance with image information of each color of yellow, magenta, cyan, and black from the reader unit 200 in the latent image forming step. By irradiating the photosensitive drum 1 uniformly charged at a predetermined potential in the charging step with the laser beam L in the latent image forming step, that is, here in the exposure step, the surface potential of the photosensitive drum 1 in the irradiated portion is increased. The part is changed to become an electrostatic latent image.

  The surface potential of the photosensitive drum 1 is detected / measured by the potential sensor 50.

  The printer 100 includes a plurality of developing devices 4 that store, as developing means, a developer in which toner and a carrier are mixed at a predetermined ratio for each developer color. Here, a yellow developing device 4Y containing a yellow developer, a developing device 4M containing a magenta developer, a developing device 4C containing a cyan developer, and a developing device 4Bk containing a black developer are provided. The four developing devices 4 are provided in a rotary developing unit 41. In the developing process, these developing devices 4 sequentially transfer the developer to the latent image portion formed on the photosensitive drum 1 to form a toner image on the photosensitive drum 1.

  When a toner image is formed on the photosensitive drum 1 by one developing device 4, an intermediate transfer belt that is an intermediate transfer body is formed by transferring a toner image that is made visible by a transfer roller 5B that is a transfer means in a transfer process. Transfer to 5A. Thereafter, a toner image formed by another developing device 4 is transferred onto the previous toner image, and the four color toner images formed by the four developing devices 4 are formed on the intermediate transfer belt 5A. Overlaid. In the transfer step, the toner image transferred from the photosensitive drum 1 onto the intermediate transfer belt 5A is further transferred onto the desired transfer material P by the transfer roller 6. This unfixed toner image formed on the transfer material P is fixed by a fixing device 8 as a fixing means in a fixing process.

  In the above image forming process, the toner remaining on the photosensitive drum 1 is cleaned by the cleaner 7A. The toner remaining on the intermediate transfer belt 5A is cleaned by the cleaner 7B, and after the previously formed toner image is removed, the next image formation is performed.

  Here, the photosensitive drum 1, the charging roller 2 as the charging means, the exposure means 3 as the latent image forming means, the developing device 4 as the developing means, and the intermediate as the transfer means used in the above image forming process. Each of the transfer belt 5A and the transfer rollers 5B and 6 will be described in detail.

  (Photosensitive drum 1): In this embodiment, the photosensitive drum 1 uses an OPC photosensitive member having a diameter of 80 mm and a length of 320 mm. This is a negatively charged photoconductor (negative photoconductor) composed of a conductive drum substrate such as aluminum and a photoconductive layer (photoconductive layer) formed on the outer peripheral surface thereof. Each is rotationally driven at a process speed (peripheral speed) of 150 mm / sec in the direction of the arrow.

  (Charging roller 2): The charging roller 2 is a composite layer structure comprising a central core, an elastic conductive layer concentrically formed on the outer periphery thereof, and a resistance layer formed on the outer peripheral surface thereof. The roller. The elastic conductive layer is, for example, a single layer or a composite layer such as a conductive rubber of 104 Ωcm or less, and the resistance layer is a single layer or a composite layer of a conductive rubber of 107 to 1011 Ωcm and a thickness of about 100 μm or less.

  The charging roller 2 has both ends of its core metal supported by a bearing member (not shown) so as to freely rotate, and is pressed against the photosensitive drum 1 by a pressing means (not shown) with a predetermined pressing force. In this case, the photosensitive drum 1 is driven to rotate as the photosensitive drum 1 rotates.

  A charging bias, which is a predetermined bias voltage, is applied to the core of the charging roller 2 by a power source (not shown), and the outer peripheral surface of the photosensitive drum 1 is uniformly charged. In this embodiment, an AC charging method with excellent potential convergence is used as the charging bias application method. The AC charging method is a method in which a DC bias is superimposed on an AC bias. When the AC bias is equal to or greater than a predetermined electric field, the potential of the photosensitive member converges substantially equally to the DC bias.

  In this embodiment, as the charging bias at the time of image formation, a sinusoidal wave having a frequency of 1200 Hz and Vpp of 1.7 kV is used as an AC bias, and −620 V is applied as a DC bias, so that the surface potential of the photosensitive drum 1 is − 600V could be obtained.

  (Exposure means 3): Whether the image information of each color of yellow, magenta, cyan, and black for emitting (exposing) the laser beam L is obtained by reading an image of an original with an image reading device in the reader unit 200, Or it was transferred from a personal computer or the like. Based on the image information of these four colors, the image data is subjected to predetermined image processing by an image processing unit installed in the reader unit 200. These four color image data are image reading devices in the reader unit 200. In synchronism with this reading operation, it is transferred to the laser writing unit 3 which is an exposure means.

  In the present embodiment, the amount of exposure is adjusted by the laser light L emitted from the laser writing unit 3 so that the surface potential of the photoconductive drum 1 of the image forming unit in each color is -100V for each color. Has been. That is, the surface potential of the latent image portion is lowered by the laser beam L to the surface potential of −600 V on the charging surface by the charging roller 2 (the magnitude in the − direction). A portion where the surface potential of the photosensitive drum 1 is changed becomes a latent image.

  The potential sensor 50 detects the surface potential on the photosensitive drum 1, detects the surface potential at a predetermined timing, and applies the bias applied to the charging roller 2 and the exposure means 3 exposure so as to obtain a desired potential. Control the amount.

  (Developer 4): Each color developer 4M, 4Y, 4C, 4Bk arranged in the rotary developing unit 41 is a two-component developing system, and the developer contains toner and magnetic particles (with a predetermined ratio). A two-component developer mixed with a carrier). In each developing device 4, the developer is restrained on a developing sleeve which is a developer carrying member including a magnet, and the developer moves onto the photosensitive drum 1 by a developing bias (not shown), so that a desired density is obtained. It is set to execute image formation. Further, all the toners of the present embodiment are negative (negative toner).

  In this embodiment, a rectangular bias wave having a frequency of 2400 Hz and Vpp of 2.0 kV is used as an AC bias, and −450 V is superimposed as a DC bias as a developing bias at the time of image formation. The ratio of the developer in each developing device 4 is set so that the maximum density of each color is 1.5 (optical density). In this embodiment, the ratio of toner to carrier (hereinafter referred to as “T / C ratio”) is set to 10% for each color.

  (Transfer means): The intermediate transfer belt 5A is an intermediate transfer member that is held flat over an area including the transfer portion of the photosensitive drum 1, and is formed on the photosensitive drum 1 by the developing unit 4 described above. The toner images thus transferred are sequentially transferred. The circumferential length of the intermediate transfer belt 5A is an integral multiple (for example, 2 to 5 times) of the circumferential length of the photosensitive drum 1. In the present embodiment, the circumference is set to 2 × 80 × π (mm). The intermediate transfer belt 5A is formed of a single layer of conductive rubber and has a thickness of 100 μm and a resistance of 1 × 10 9 Ω.

  In addition, the intermediate transfer belt 5A is synchronized with the rotation of the photosensitive drum 1 by three rollers 5C, 5D, and 5E including a driving roller 5C driven by a driving mechanism (not shown). ) And can be circulated along the direction of the arrow at the same speed.

  As primary transfer means for transferring the toner images of the respective colors formed on the photosensitive drum 1 onto the intermediate transfer belt 5A, a transfer roller 5B is provided on the back side of the surface facing the photosensitive drum 1 of the intermediate transfer belt 5A. That is, it is provided in the transfer portion. The transfer roller 5B includes a central cored bar and an intermediate resistance elastic layer formed concentrically on the outer periphery of the cored bar. The transfer roller 5B in this embodiment is a conductive rubber roller having a resistance of 5 × 10 6 Ω and a diameter of 16 mm. A predetermined bias having a polarity opposite to that of the toner image, which is a primary transfer bias from a power source (not shown), is applied to the metal core portion of the transfer roller 5B, and in the present embodiment, a positive bias is applied to the intermediate transfer belt 5A. Transfer of the toner images of the respective colors formed on the drum 1 is executed.

  Further, as a secondary transfer means for transferring the toner image transferred and carried on the intermediate transfer belt 5A to a desired transfer material P, the transfer roller 6 has a central core and a roller shape that is concentrically integrated with the outer periphery thereof. And a medium-resistance elastic layer. The transfer roller 6 in this embodiment is a conductive rubber roller having a resistance of 5 × 10 8 Ω and a diameter of 16 mm. A predetermined secondary transfer bias having a polarity opposite to that of the toner image is applied from a power source (not shown) to the core portion of the transfer roller 6, and in this embodiment, a positive bias is applied to each color formed on the intermediate transfer belt 5 </ b> A. Transfer of the toner image onto the transfer material P is executed. The transfer material P onto which the toner image has been transferred is conveyed to the fixing device 8 and fixed on the transfer material P, and a series of image formation is completed.

  FIG. 2 is a detailed view showing a configuration example of the exposure means 3 used in this embodiment.

  In the present embodiment, a so-called twin beam laser in which two beams are emitted from one semiconductor laser element 3A is used. The beam is incident on the polygon mirror 3C through the lens element group 3B, then reflected by the polygon mirror 3C, and scanned and exposed on the photosensitive drum 1 through a predetermined mirror.

  The polygon mirror 3B of this embodiment is an 8-sided polygon. In this embodiment, since the writing density on the photosensitive drum 1 is 600 dpi for both main scanning and sub-scanning, the polygon mirror 3B rotates at about 13300 rpm. Laser beams emitted from the semiconductor laser element 3A are referred to as A laser and B laser, respectively. Further, both A and B lasers can form a 16-value multi-value image in one pixel of 600 dpi, and the pulse width modulation method as shown in FIG. 3 is used. Further, the pulse width modulation method shown in FIG. 3 has a configuration having a laser power adjustment table capable of selecting arbitrary 16 values (4 bits) from a level of 255 values (8 bits) as shown in FIG.

<Operation Example of Image Forming Apparatus of this Embodiment>
With the above configuration, in the present embodiment, an attempt is made to reproduce a good image, particularly a halftone gradation characteristic, using 16-value multi-value error diffusion, multi-value dither processing, or the like.

  The image forming apparatus according to the present embodiment forms an image according to the image forming process using the image forming unit described above. However, an electrophotographic image forming apparatus such as the printer 100 has a surrounding environment, usage status, and the like. Therefore, it is difficult to output an image with always stable color under fixed image forming conditions.

  In particular, when the semiconductor laser 3A having a plurality of beams is used as the exposure means as in this embodiment, the characteristics of the A laser and B laser change, so even if the maximum exposure is adjusted, the The tone characteristics will change. For this reason, when image formation is performed in halftone with each laser in one line and one space in the sub-scanning direction as shown in FIG. 5A, the density of the two is different or the sub-scanning as shown in FIG. 5A. In the pattern in which image formation is repeatedly performed for 3 lines and 2 spaces in the direction, if the number of times of image formation of the A and B lasers in the line portion is alternately different, the density of the line portion varies alternately, resulting in a good halftone. It cannot be reproduced.

(Example of correction using the surface potential of the photosensitive drum)
Therefore, in the present embodiment, the gradation characteristics of the A and B lasers are made to coincide with each other at a predetermined timing and when the image forming apparatus is turned on by a flow as shown in FIG.

  First, with the A laser alone, the multi-value gradation at the time of image formation as shown in FIG. 7 is exposed on the photosensitive drum 1 in a size of 30 × 30 mm for 16 gradations (S1). The surface potential of the photosensitive drum at that time is measured by the potential sensor 50 (S2) and stored in a storage means (not shown) (S3). The laser power adjustment table shown in FIG. 4 was used for the pulse width of each gradation at this time.

  Next, using only the B laser, similarly, using the laser power adjustment table of FIG. 4, the multi-value gradation at the time of image formation is 30 × 30 mm per gradation on the photosensitive drum 1 on the 16th floor. The exposure is carried out (S4), and the surface potential of the photosensitive drum at that time is measured by the potential sensor 50 (S5) and stored in a storage means (not shown) (S6).

  Next, when the gradation potential characteristics of the A and B lasers as shown in FIG. 8 are obtained, the potential values of each gradation and 16 gradations are respectively compared (S7). In the embodiment, when the voltage is 8 V or more (S8), the laser power adjustment table of the B laser is changed (S9), and the adjustment is performed so that the gradation potential characteristics coincide with those of the A laser.

  By the way, in the present embodiment, the gradation is uniformly 8V, but the difference value may be changed for each gradation. For example, the gradation area of the low density portion may be smaller and the gradation area of the high density portion may be larger. Of course. Alternatively, the gradation potential of the B laser may be formed again, detected by the potential sensor 50, and repeated until convergence to a predetermined value or less.

Thereafter, the process proceeds to other image adjustment and image formation. (S10)
As an adjustment method, as shown in FIG. 9, the pulse width of the corresponding gradation is obtained by linearly approximating between each potential result and the potential in the laser power adjustment table at that time to obtain a desired potential. Is changed so that the gradation potential characteristics of both the A and B lasers are made coincident, it is possible to always realize a good image, particularly a halftone gradation expression regardless of the use environment and the use situation.

  That is, Va (N) is the absolute value of the potential of the gradation N portion of the A laser, Vb (N) is the absolute value of the potential of the gradation N portion of the B laser, and the pulse width level of the gradation N portion of the A laser. Assuming that the pulse width level of the gradation N portion of the B laser is Pb (N), it branches depending on whether or not the potential value of the B laser is larger than the potential value of the A laser (S91). The pulse width level is linearly interpolated with the gradation N + 1 portion to calculate a new pulse width level Pb ′ (N) (S92). The linear interpolation is performed to calculate a new pulse width level Pb ′ (N) (S93), and the laser power adjustment table for the B laser is corrected as shown in FIG. 10 (S94).

(Example of correction based on the density of the toner image on the photosensitive drum)
In the above embodiment, the gradation characteristics of the A and B lasers are matched by detecting the gradation potential characteristic of each laser by the potential sensor 50. However, in this embodiment, the density detection means 10 shown in FIG. Using.

  The density detecting means 10 irradiates the toner image formed on the photosensitive drum 1 with the LED 10a, receives the reflected light with the PD (10b) of the light receiving element, and converts it into density at the output level. FIG. 12 shows an example of the relationship between the black toner density and the output value (V) of the density detecting means 10.

  In this embodiment, as shown in the flowchart of FIG. 13, first, only the A laser is used, and the multi-value gradation at the time of image formation shown in FIG. 7 is 30 × 30 mm per gradation on the photosensitive drum 1. (S11), the electrostatic latent image is developed with black toner (S12), and the toner density on the photosensitive drum after development is detected by the density detector 10 (S13). (S14). The laser power adjustment table shown in FIG. 4 was used for the pulse width of each gradation at this time.

  Next, with the B laser alone, similarly, using the laser power adjustment table of FIG. 4, the multi-value gradation at the time of image formation is 30th per 30 gradation on the photosensitive drum 1 on the 16th floor. The exposure is divided (S15), the electrostatic latent image is developed with black toner (S16), and the toner density on the photosensitive drum after development is detected by the density detector 10 (S17). (S18).

  Next, since the gradation density characteristics of the A and B lasers as shown in FIG. 14 can be obtained, the density values of each gradation and 16 gradations are respectively compared (S19). In the case of 0.05 (S20), the laser power adjustment table of the B laser was changed (S21), and adjustment was performed so that the gradation density characteristics coincided with those of the A laser.

  By the way, in the present embodiment, the density difference is uniformly 0.05 for each gradation, but the difference value is changed for each gradation. For example, the gradation area in the low density part is smaller and the gradation area in the high density part is larger. Of course, it may be. Alternatively, the gradation density of the B laser may be formed again, detected by the density detecting means 10, and repeated until convergence to a predetermined value or less.

Thereafter, the process proceeds to other image adjustment and image formation. (S22)
As an adjustment method, as shown in FIG. 15, a linear approximation is performed between adjacent densities based on the density difference result of each gradation and the value of the laser power adjustment table at that time, and a pulse of the corresponding gradation is obtained so as to obtain a desired density. By changing the width and matching the gradation density characteristics of both the A and B lasers, it is possible to always realize a good image, particularly a halftone gradation expression, regardless of the use environment or the use situation.

  That is, the density of the gradation N part of the A laser is Da (N), the density of the gradation N part of the B laser is Db (N), and the pulse width level of the gradation N part of the A laser is Pa (N), B If the pulse width level of the gradation N portion of the laser is Pb (N), it branches depending on whether or not the density of the B laser is greater than the concentration of the A laser (S211). And a new pulse width level Pb ′ (N) is calculated (S212). If the pulse width level is smaller, the pulse width level is linearly interpolated with the gradation N-1 part to obtain a new pulse width level Pb ′ (N). The pulse width level Pb ′ (N) is calculated (S213), and the laser power adjustment table of the B laser is corrected as shown in FIG. 16 (S214).

  By the way, in the density detection means 10, as shown in FIG. 12, the detection accuracy / resolution in the low density portion is high, and from the viewpoint of controlling the gradation characteristics of the A and B lasers, the potential sensor 50 is also more accurate / Control with high resolution is possible. More specifically, since the resolution of the potential sensor 50 is around 3 to 5 V, the detection accuracy is about ± 0.03 in the concentration range corresponding to the concentration 0.5, but the concentration detection means 10 has detection accuracy / repetition up to ± 0.02. The accuracy can be improved, and the gradation characteristics of the A and B lasers can be controlled in detail.

  Further, although the density detecting means 10 is the toner density detecting means on the photosensitive drum 1, for example, the toner density on the intermediate transfer belt 5A may be detected or the unfixed state transferred to the transfer material P The toner image density may be detected, or the toner image density transferred to the transfer material P and fixed may be detected. In this case, it is needless to say that density detecting means suitable for the form may be used as appropriate.

  In the above embodiment, multi-level gradation expression is described by the pulse width modulation method, but the same result can be obtained even by the light amount (laser power) modulation method.

  In the above embodiment, the two light beam embodiment of the so-called twin laser has been described. However, the present invention is not limited to this, and can be applied to a system of many light beams such as four beams and eight beams. Of course.

  Needless to say, the potential detection and density detection of the photosensitive drum may be combined as appropriate.

  Although the above embodiment is an embodiment of one drum, the present invention can be applied to a configuration of a plurality of photosensitive drums, for example, four drums. In this case, the above embodiment is applied to each laser of each optical scanning system means. Should be applied.

  In this embodiment, only one color has been described, but it is needless to say that it may be executed for a plurality of colors as necessary.

1 is a diagram illustrating a configuration example of an image forming apparatus according to an exemplary embodiment. It is a figure which shows the detailed structural example of the exposure means 3 of this embodiment. It is a figure which illustrates roughly the relationship between the gradation N and a pulse width. It is a figure which shows the example of the laser power adjustment table of this embodiment. It is a figure which shows the image formation example of 1 line 1 space each independently of A and B laser. It is a figure which shows the image formation example of 3 lines 2 spaces each independently of A and B laser. It is a flowchart which shows the correction | amendment procedure of the laser power adjustment table by the surface potential of a photosensitive drum. It is the schematic of a gradation patch. It is a figure which shows the gradation potential characteristic of A and B laser by the surface potential of a photosensitive drum, and its difference. It is a flowchart which shows the example of a procedure of FIG.6 S9. It is a figure which shows the power adjustment table of B laser after correction | amendment by the surface potential of a photosensitive drum. 3 is a schematic cross-sectional view of the concentration detection means 10. 4 is a diagram showing the relationship between the reflection density (black) and the output of the density detection means 10. FIG. It is a flowchart which shows the correction | amendment procedure of the laser power adjustment table by the surface density of a photosensitive drum. The figure which shows the gradation density characteristic of A and B laser by the surface density of a photosensitive drum, and its difference It is a flowchart which shows the example of a procedure of step S21 of FIG. It is a figure which shows the power adjustment table of B laser after correction | amendment by the surface density of a photosensitive drum.

Explanation of symbols

1 Photosensitive drum
3A semiconductor laser
3B lens element group
3C polygon mirror
4 Development unit
50 Potential sensor
10 Concentration detection sensor
0a LED (light emitting part)
0b PD (receiver)

Claims (12)

A plurality of optical scanning system means for emitting a plurality of light beams according to image data, exposing and scanning the photosensitive member, and reproducing a resolution higher than the resolution of the optical scanning speed;
A single gradation forming means for independently forming the plurality of gradation patterns on the photosensitive member for each predetermined operation interval;
An independent exposure control means for controlling the exposure amount independently for each of the light beams,
The detection value corresponding to the gradation of each light beam detected from the image formed by the single gradation forming means is the detection value of one of the plurality of light beams and the detection value of another light beam. The image forming apparatus is characterized in that the independent exposure control means is controlled so that the difference between the two is equal to or less than a predetermined value. Having a potential detecting means for detecting a surface potential of the photoreceptor;
The detection value of the gradation potential of each light beam detected by the potential detection means of the electrostatic latent image formed by the single gradation forming means is the gradation potential detection of any one of the plurality of light beams. The image forming apparatus according to claim 1, wherein the independent exposure control unit is controlled so that a difference between a value and a gradation potential detection value of another light beam is equal to or less than a predetermined value. Developing means for visualizing the electrostatic latent image formed on the photoreceptor by the optical scanning means with a toner image;
Transfer means for transferring the visualized toner image to another medium;
Density detecting means for detecting the toner density on the image carrier,
The detection value of the gradation density of each light beam detected by the density detection means of the toner image formed by the single gradation forming means is the gradation density detection value of any one of the plurality of light beams. The image forming apparatus according to claim 1, wherein the independent exposure control unit is controlled so that a difference between gradation density detection values of other light beams is a predetermined value or less.   The image forming apparatus according to claim 3, wherein the image carrier is a photoconductor.   The image forming apparatus according to claim 3, wherein the image carrier is an intermediate transfer member.   The image forming apparatus according to claim 3, wherein the image carrier is a transfer belt.   The image forming apparatus according to claim 3, wherein the image carrier is a transfer drum. Developing means for visualizing the electrostatic latent image formed on the photoreceptor by the optical scanning means with a toner image;
Transfer means for transferring the visualized toner image to a transfer material;
Density detecting means for detecting the toner density on the transfer material;
The detection value of the gradation density of each light beam detected by the density detection means for the toner image on the transfer material formed by the single gradation forming means is the gradation of any one of the plurality of light beams. The image forming apparatus according to claim 1, wherein the independent exposure control unit is controlled so that a difference between a density detection value and a gradation density detection value of another light beam is a predetermined value or less.   The image forming apparatus according to claim 2, wherein the light beam is capable of multilevel modulation.   9. The image forming apparatus according to claim 2, wherein the modulation method of the light beam is pulse width modulation.   The image forming apparatus according to claim 2, wherein the light beam is modulated by power modulation. This is a method for controlling an image forming apparatus that emits a plurality of light beams according to image data, exposes and scans the photosensitive member, and forms an image by a multi-beam optical scanning system means that reproduces a resolution higher than the resolution of the optical scanning speed. And
A plurality of gradation patterns on the photoconductor are formed independently for each light beam at predetermined operation intervals,
The detection value corresponding to the gradation of each light beam detected from the formed electrostatic latent image is the difference between the detection value of one of the plurality of light beams and the detection value of the other light beam. A control method for an image forming apparatus, comprising: controlling an independent exposure control unit that controls an exposure amount of each of the light beams independently so as to be equal to or less than a predetermined value.

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