JP2006276662A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a color image forming apparatus which can correct the positional deviation and a density error, by minimizing the influence of the abnormality of the detection data of mark images detected with some of a plurality of detecting means, even if the abnormality occurs. <P>SOLUTION: The image forming apparatus is provided in which a plurality of image forming engines are arrayed and each color component image formed, by each image forming engine is transferred to a recording material directly or through an intermediate transfer body, and the image forming apparatus is provided with a correction mode control part transferring the mark images of each color to a recording material carrier or the intermediate transfer body; the plurality of detecting means reading the mark images of each color; a detection information extracting part for extracting the detection information of each color toner image from the detection signals of the detecting means; and a condition correcting part for correcting image forming condition according to the detection information. The condition correcting part destroys the detection information of one detecting means that is decided and uses the detection information of the other detecting means, when a judging part for deciding whether the detection information is valid or not decides that the detection information of at least one detecting means is invalid. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a color image that performs positional deviation correction and density error correction based on the detection result of each color mark image for correcting the positional deviation and density error of the image formed on the surface of the intermediate transfer member or the recording material conveyance body. The present invention relates to a method of processing a detection result of a mark image in a forming apparatus.

JP-A-8-248721

  In recent years, in order to achieve high productivity in electrophotographic color image forming apparatuses, those adopting a tandem method have become mainstream. In this tandem system, a toner image formed on each color photoconductor is multiple-transferred onto an intermediate transfer body, and the multiple-transferred recorded image is transferred from the intermediate transfer body to a recording sheet, and a recording material transport body The photosensitive members of the respective colors are arranged at positions opposite to the recording sheet conveyance path of the recording sheet, the recording sheets are conveyed, and the toner images of the respective colors are sequentially transferred onto the recording sheet to form a recorded image. In any case, the toner image of each color is transferred to the recording sheet at a relatively shifted position due to misalignment of the exposure device, the photosensitive member, the intermediate transfer member, and the recording material conveying member. Therefore, it is indispensable to perform such misregistration correction.

  As a method of correcting this misalignment, generally, a plurality of yellow, magenta, cyan, and black color mark images are successively formed on the surface of an intermediate transfer member, a recording material transport member, and the like. Is detected by a sensor to correct the misalignment (Patent Document 1). In this method, by calculating the average value for each color component from the detection result of the sensor, the amount of misregistration of each color is calculated, and based on this misregistration amount, image formation of ROS, LED array, laser array, etc. The timing of drawing an image is fed back to the means.

  In an image forming apparatus that detects a displacement by a sensor, a CCD sensor, a Bi-Cell type, a Single-Cell type photodiode, or the like is applied as the sensor. Among these three types of sensors, the CCD and Bi-Cell types have high detection accuracy, but are somewhat expensive and cause a cost increase. Therefore, the Single-Cell type is commonly used as a concentration error detection sensor. Often applied. However, in such a sensor that shares both misregistration detection and density error detection, a misregistration detection mark image and a density error detection mark image are formed separately, and each mark image needs to be detected sequentially. There is a problem that the waiting time required for the correction mode until image formation by the user is started becomes long.

  In addition, when a misregistration detection mark image and a density error detection mark image are formed, toner is consumed for a purpose other than image formation, which increases the burden on the user. Further, after the detection of these mark images, these mark images need to be collected as waste toner by a cleaner that removes the toner image on the surface of the recording material conveyance body or intermediate transfer body, which increases the load on the image forming apparatus. .

  In order to solve these problems, there has been proposed an image forming apparatus in which the position error detection mark image and the density error detection mark image are combined into one mark image. With such an image forming apparatus, it is possible to reduce the waiting time until the start of image formation and reduce the amount of toner consumed and the amount of waste toner discharged.

  In the correction mode of this image forming apparatus, a mark image based on an image data signal of a predetermined density is formed on each photoconductor corresponding to each color of yellow, magenta, cyan, and black. A pattern block in which these mark images are grouped is formed by being transferred in a line on the recording material conveyance body. In order to accurately correct the density of each color toner image from a low density area to a high density area, the pattern block is repeatedly formed with respect to image data signals of several different densities. The positional deviation of each color toner image is performed, for example, by using cyan as a reference color and calculating the amount of positional deviation relative to cyan of other colors, and the positional deviation of each color of the grouped pattern blocks. By averaging the amount with a plurality of blocks, the displacement amount can be grasped with higher accuracy. Further, the density error of each color toner image is grasped for each pattern block having a different writing density. For example, if a pattern block is formed with respect to five levels of density, five densities from a low density area to a high density area for each color are obtained. Concentration correction data will be created for.

  Here, when extracting the positional deviation information and density error information of the toner image from one mark image transferred to the intermediate transfer body or the recording material conveyance body, a signal corresponding to the reflected light amount of the mark image is output by the sensor, While the density of the mark image is directly grasped from the output signal, the edge or the center of gravity of the mark image is identified from the waveform of the output signal, thereby grasping the position of the mark image. For this reason, there are scratches or dirt on the surface of the intermediate transfer member or the recording material transport member, and noise is included in the output signal of the sensor as a result of the mark image being transferred over these scratches. In this case, the density and position of the mark image grasped from the output signal become incorrect, and both the density correction data and the positional deviation correction data to be created become inappropriate.

  Further, in an image forming apparatus that requires high image quality, a plurality of sensors are arranged in the main scanning direction at positions opposed to the intermediate transfer member or the recording material conveyance member, and a mark image is read at each sensor arrangement position, thereby providing a more precise image. Misalignment correction and density error correction are performed. However, one of these sensors contains noise in the output signal due to scratches or dirt on the surface of the intermediate transfer member or recording material carrier, or misalignment correction and density error when the sensor itself fails. When the correction is performed, the positional deviation correction data and the density correction data become inappropriate due to the influence of the noise, and the correction accuracy is deteriorated or cannot be corrected.

  The present invention has been made in view of such problems, and the object of the present invention is to perform detection even when there is an abnormality in the detection data of the mark image detected by some of the plurality of detection means. It is an object of the present invention to provide a color image forming apparatus that minimizes the influence and enables the positional deviation correction and the density error correction.

  In order to perform misregistration correction of a recorded image in a color image forming apparatus, it is necessary to detect relative misregistration of other colors with respect to a reference color. Therefore, when correcting the misregistration of all colors, it is necessary to form a reference color and a comparative color for comparing the relative misregistration with respect to the reference color, that is, mark images of cyan, yellow, magenta, and black. For this reason, in the present invention, in the correction mode, the mark images of the four colors are used as a single pattern block, and pattern blocks having several types of density are continuously formed. The positional deviation of the mark image is detected by calculating a relative positional deviation amount of the comparative color with respect to the reference color. The density error is detected from a relative value between the reflected light as a reference and the reflected light incident on the light receiving element of the detection unit when the mark image of each color passes through the detection visual field of the detection unit. Yes. In the positional deviation correction and the density error correction, an output value exceeding a predetermined threshold is applied with an output signal corresponding to the amount of reflected light from the mark image. In a color image forming apparatus having a corporate color other than the four colors of cyan, yellow, magenta, and black, a pattern block in which the mark images of all colors including the four colors and the other colors are collected as one unit. Are formed continuously for several concentrations.

  For example, in an image forming apparatus having a plurality of detection means, the mark image formed on the surface of the intermediate transfer body or the recording material transport body for at least one detection means is contaminated or scratched, and an output signal of the mark image If abnormal positional deviation information and / or density error information is extracted for positional deviation correction and density error correction, if positional deviation correction and density error correction are performed as they are, abnormal positional deviation information or / And the image quality may deteriorate due to the influence of density error information. Therefore, in the present invention as the first means, when it is determined that the positional deviation information or / and density error information of at least one of the plurality of detection means is invalid, the positional deviation information and density of the detection means. The error information is not used in the correction mode, but valid positional deviation information and density error information of other detection means are used.

  Therefore, in the image forming apparatus of the present invention according to the first means, an image in which a plurality of image forming engines are arranged and each color component image formed by each image forming engine is transferred to a recording material directly or via an intermediate transfer member. In the forming apparatus, a correction mode control for sending a signal to each image forming means and sequentially transferring a mark image of each color for correcting the positional deviation and density error of the image onto the surface of the recording material carrier or the intermediate transfer body. A plurality of detection means arranged in the main scanning direction so as to oppose the recording material carrier or the intermediate transfer body for optically reading the mark images of the respective colors, and the toner images of the respective colors from the detection signals of these detection means Information extracting unit for extracting both the positional deviation information and the density error information of the toner, and a condition correcting unit for correcting the image forming condition of the toner image in each image forming unit according to the positional deviation information and the density error information The condition correction unit includes a determination unit that determines whether the positional deviation information and the density error information extracted by the detection information extraction unit are valid or invalid, and the determination unit includes positional deviation information of at least one detection unit. Alternatively, when it is determined that the density error information is invalid, the condition correction unit discards the detection information from which the invalid information is extracted without using the positional deviation information and the density error information, and other detection means. The positional deviation and density error are corrected using the effective positional deviation information and density error information.

  Next, when density error information that is invalid for density error correction is extracted for at least one of the plurality of detection means, the density error information of the detection means is not used and the positional deviation information of the detection means is used. As for the validity, it is preferable to determine the validity / invalidity based on the fluctuation width of the positional deviation information of the detection means and the result of comparing the fluctuation width and the fluctuation width of the positional deviation information of the other detection means. Therefore, in the present invention as the second means, when the density error information of at least one of the plurality of detection means is invalid, the density error information of the detection means is not used in the correction mode, and the detection means The misregistration information is used and corrected when a preset condition is satisfied.

  Therefore, as the second means, in the image forming apparatus in which a plurality of image forming engines are arranged and each color component image formed by each image forming engine is transferred to a recording material directly or via an intermediate transfer member, A correction mode control unit for sending a signal to an image means and sequentially transferring a mark image of each color for correcting a positional deviation and a density error of the image on the surface of the recording material carrier or the intermediate transfer member; and a mark for each color A plurality of detection means arranged in the main scanning direction so as to oppose the recording material carrier or the intermediate transfer body for optically reading an image, and positional deviation information and density of each color toner image from detection signals of these detection means A detection information extraction unit that extracts both error information; and a condition correction unit that corrects an image forming condition of a toner image in each image forming unit according to the positional deviation information and density error information, and the determination unit When it is determined that the density error information of at least one detection unit is invalid, the fluctuation range of the positional deviation information of the detection unit from which invalid density error information is extracted is equal to or less than a preset threshold value and / or Compare the fluctuation width of the positional deviation information of the detection means with the fluctuation width of the positional deviation information of the other detection means, and if the difference is equal to or less than a preset threshold, the positional deviation information of the detection means is valid. The condition correction unit discards the density error information of the detection unit without using it, and corrects the position error and the density error using valid position error information and density error information. To do.

  In addition, when misregistration information that is invalid for misregistration correction is extracted for at least one detection means among a plurality of detection means, the misregistration information of the detection means is not used and the density error information of the detection means is used. The valid / invalid is preferably determined based on the result of comparing the output value of the detection signal related to the density error information of the detection means and the output value of the detection signal related to the density error information of the other detection means. Therefore, in the present invention as the third means, when the positional deviation information of at least one detection means among the plurality of detection means is invalid, the positional deviation information of the detection means is not used in the correction mode. The density error information is corrected when it is used when a preset condition is satisfied.

  Therefore, as a third means, in the image forming apparatus in which a plurality of image forming engines are arranged and each color component image formed by each image forming engine is transferred to a recording material directly or via an intermediate transfer member, A correction mode control unit for sending a signal to an image means and sequentially transferring a mark image of each color for correcting a positional deviation and a density error of the image on the surface of the recording material carrier or the intermediate transfer member; and a mark for each color A plurality of detection means arranged in the main scanning direction so as to oppose the recording material carrier or the intermediate transfer body for optically reading an image, and positional deviation information and density of each color toner image from detection signals of these detection means A detection information extraction unit that extracts both error information; and a condition correction unit that corrects an image forming condition of a toner image in each image forming unit according to the positional deviation information and density error information, and the determination unit When it is determined that the positional deviation information of at least one of the detection means is invalid, the output value of the detection signal related to the density error information of the detection means from which invalid positional deviation information is extracted, and the density of other detection means The output value of the detection signal related to the error information is compared, and if the difference is within a predetermined threshold range, the density error information of the detection means is determined to be valid, and the condition correction unit The positional deviation information of the means is discarded without being used, and the positional deviation and density error are corrected using effective positional deviation information and density error information.

  Furthermore, when the number of detection means whose position deviation information and / or density error information is invalid among a plurality of detection means is large, the accuracy of position deviation correction and density error correction is reduced. For example, when the information of two detection means is invalid among three detection means, or when the information of three detection means is invalid among five detection means, the positional deviation correction is performed. If the density error correction is performed, the image quality may be deteriorated. Therefore, in the present invention, as the fourth means, for the detection means for which the positional deviation information is determined to be invalid among the plurality of detection means, the validity / invalidity of the density error information of the detection means is determined, and the positional deviation information or / In addition, according to the number of detection means whose density error information is invalid, only density error correction is performed or correction mode cancellation is selected.

  Therefore, as the fourth means, in the image forming apparatus of the present invention according to any one of the first means to the third means, the determination unit determines whether the positional deviation information or / and the density error information are invalid. According to the number, only density error correction is performed or cancellation of the correction mode is selected.

  As the image forming means, it is preferable to apply image forming means used in an image forming apparatus using an electrophotographic method. However, if an image forming apparatus forms a full-color image by superimposing images of respective colors, the image forming means is applied to an electrophotographic method. Not exclusively. In addition to the four colors of cyan, yellow, magenta, and black, a special color such as a corporate color may be added.

  The detection means for detecting the mark image formed on the recording material carrier or the intermediate transfer body combines a light emitting element and a light receiving element, and irradiates the mark image with the light emitting element to the light receiving element. There is no problem as long as it is arranged at a position where the reflected light from the mark image or the transmitted light transmitted through the mark image can enter.

  The detection information extraction unit can perform calculation processing based on the interval and output value of the output signal detected by the detection means, and can detect the mark image formed on the surface of the recording material carrier or the intermediate transfer member. It is only necessary to send the detection result to the condition correction unit.

  According to the image forming apparatus of the present invention configured as described above, even if there is an abnormality in the detection data of the mark image detected by some of the plurality of detection means, the influence is minimized. However, it is possible to correct misalignment and density error.

  Hereinafter, an image forming apparatus of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a schematic configuration diagram of a color image forming apparatus using an electrophotographic system to which the present invention is applied. In this configuration diagram, after charging the surface of the photoreceptor with a contact charger, an electrostatic latent image is formed by irradiation with a laser beam, and the xerographic engine that develops the electrostatic latent image with toner is yellow, magenta, cyan, An outline of an IOT (image output terminal: image output unit) of a tandem type color electrophotographic image forming apparatus provided for each color of black is shown. In the drawing, the image reading unit and the image processing unit of the image forming apparatus are omitted.

  The IOT of this image forming apparatus includes four photoconductors 1Y, 1M, 1C, and 1K that rotate in the direction of arrow A in the figure, and contact chargers 2Y, 2M, 2C, and 2K that charge the surface of each photoconductor. Then, the surface of each charged photoconductor is exposed with exposure light modulated based on the image information of each color, and light is emitted from a ROS (laser output unit) or LED array that forms an electrostatic latent image on each photoconductor. The element arrays 3Y, 3M, 3C, and 3K, the developing devices 4Y, 4M, 4C, and 4K that develop the electrostatic latent image on each photoconductor with each color developer to form a toner image on the photoconductor, and the photoconductor The primary transfer devices 5Y, 5M, 5C, and 5K that transfer the toner images on the respective colors to the intermediate transfer belt 6, the secondary transfer device 7 that transfers the toner image on the intermediate transfer belt 6 to the paper P, and the paper P The fixing device 9 for fixing the toner image transferred to the paper and the paper P are stored. A sheet tray T, a cleaner (not shown) for cleaning the surface of each photoconductor, a static eliminator (not shown) for removing residual charges on the surface of each photoconductor, and the surface of the intermediate transfer belt 6. And a belt cleaner 8 that cleans the surface of the intermediate transfer belt 6.

  As an image forming operation in the image forming apparatus shown in the configuration diagram, first, an original image signal read from an original by an image reading unit (not shown) or an external computer (not shown) is used. The original image signal is input to an image processing unit (not shown). The input image signal is decomposed into image information of each color, and then input to light emitting element arrays 3Y, 3M, 3C, and 3K such as a ROS (laser output unit) or an LED array, and the laser beam L is modulated. The modulated laser beam L is applied to the surfaces of the photoreceptors 1Y, 1M, 1C, and 1K that are uniformly charged by the contact chargers 2Y, 2M, 2C, and 2K. When the laser beam L is raster-irradiated on the surface of each photoconductor, an electrostatic latent image corresponding to the input image signal is formed on each photoconductor. Subsequently, the electrostatic latent images on the photoconductors are developed with toner by the color developing devices 4Y, 4M, 4C, and 4K, and toner images are formed on the photoconductors. The toner image formed on each photoconductor is transferred to the intermediate transfer belt 6 by the primary transfer units 5Y, 5M, 5C, and 5K. Each photoreceptor after the transfer of the toner image to the intermediate transfer belt 6 is cleaned of adhering matter such as residual toner adhering to the surface by a cleaner, and the residual charge is removed by a static eliminator.

  Next, the toner image on the intermediate transfer belt 6 is transferred onto the paper P sent from the paper tray by the secondary transfer device 7 and then transferred onto the paper P by the fixing device 9. Is fixed and a desired image is obtained. The intermediate transfer belt 6 on which the transfer of the toner image onto the paper P has been completed is cleaned of adhering matters such as residual toner adhering to the surface by the belt cleaner 8, and one image forming operation is completed.

  In an electrophotographic color image forming apparatus, image density fluctuations and misregistration of toner images of various colors occur due to environmental conditions such as temperature and humidity and deterioration with time. For this reason, it is necessary to perform positional deviation correction and density error correction before image output to the paper P or during output standby. As the method, first, a mark image is formed on the intermediate transfer belt 6. Then, the mark image is detected by the photosensor 10 and an output signal is sent to the control unit. Further, based on the position shift amount and density error results obtained from the output signal, position shift correction and density error correction are performed as necessary.

  FIG. 2 is a block diagram showing the flow of positional deviation correction and density error correction in the color image forming apparatus shown in FIG. The photosensitive member 1 is charged by the contact charger 2, and an electrostatic latent image is formed by exposing the photosensitive member 1 with the ROS 3 according to the signal of the mark image output from the correction mode control unit 11. After development, the mark image is transferred to the intermediate transfer belt 6. Then, the mark image transferred onto the intermediate transfer belt 6 is read by a plurality of photosensors 10 (shown as one in the figure).

  The detection information extraction unit 12 detects positional deviation and density error from the signals output from the plurality of photosensors 10, and the determination unit 13 is extracted from the plurality of photosensors 10 sent from the detection information extraction unit 12. It is determined whether the positional deviation information and density error information are valid or invalid for correction. When the determination unit 13 determines that the positional deviation information and / or density error information of at least one photosensor 10 among the plurality of photosensors 10 is invalid, the condition correction unit 14 determines that invalid information The positional deviation information and / or density error information of the photosensor 10 from which the image data is extracted are discarded, and the positional deviation correction and density error correction are performed based on the positional deviation information and density error information effective for correction. Specifically, according to the output signal output from the photosensor 10, the laser power of the ROS 3 is controlled to correct the image density, the writing timing of the ROS 3 is controlled, and the image forming position is corrected. In addition, the condition correction unit 14 includes a storage unit 15 that stores positional deviation information and density error information detected by each photosensor.

  FIG. 3 shows an example of the arrangement of mark images applied to the present invention. FIG. 2A shows an example of an arrangement of mark images formed on the intermediate transfer belt in the correction mode. In the partial drawing (a), cyan is used as a reference color, a mark image (Mc) of the reference color cyan and other comparative colors are combined, and a combination of cyan (Mc) and yellow (My), cyan (Mc). And magenta (Mm), cyan (Mc) and black (Mb). These mark images move in the direction of the arrow in the figure, and sequentially pass through the detection field of view (not shown) of the photosensor 10 from the mark image Mc to the mark image Mb, thereby detecting misalignment and density error. It has a configuration. In describing the mark image, all combinations are collectively referred to as a pattern block M.

FIG. 2B shows an example of the arrangement of the pattern blocks M formed on the intermediate transfer belt (not shown) in the correction mode of the image forming apparatus having three photosensors. In the partial diagram (b), three photosensors are arranged in the main scanning direction at positions facing the intermediate transfer belt, and detection visual fields R _In , R _Cen , and R _{Out of} each photo sensor are shown. . A pattern block M ₁ having a density of halftone dot coverage: Cin 80% and a pattern block M having a density of halftone dot coverage: Cin 50% at positions passing through the detection visual fields R _In , R _Cen , R _Out of the photosensor. _{As the} intermediate transfer belt moves in the direction of the arrow in the figure, it sequentially passes through the detection visual fields R _In , R _Cen , R _Out in the order of the pattern block M ₁ and pattern block M _2. It is supposed to be. In this figure, halftone dot coverage: pattern blocks M ₁ and M ₂ having densities of Cin 80% and Cin 50% are shown. Following pattern blocks M ₁ and M ₂ , halftone dot coverage: density of Cin 20% By forming a pattern block having a density of halftone dot coverage: Cin 80%, Cin 50%, Cin 20% by forming a pattern block having a density at a position passing through the detection visual fields R _In , R _Cen , R _Out Two sets of pattern blocks having different densities may be formed. The density type and number of sets of pattern blocks formed on the intermediate transfer belt in the correction mode can be set as appropriate. The image forming apparatus according to the description of the present invention will be described using an example in which three photosensors are provided.

FIG. 4 shows image characteristics that are generally corrected in the color image forming apparatus. From the partial diagrams (a) to (e), the positional deviation correction is performed, and the partial diagram (f) is illustrated. It shows characteristics that require density error correction. In this figure, alternate long and _short dash lines r _In , r _Cen , and r _Out indicate the center positions of the detection visual fields R _In , R _Cen , and R _Out of three photosensors arranged in the main scanning direction. In the partial diagram (a), the reference line S indicates the reference line in the main scanning direction, and the center line S _Cen indicates the center position of the reference line S in the main scanning direction. From the dashed line r _{an In} reference line S to the dashed line r _Cen, and each length of the dashed line r _Cen to the dashed line r _Out l _{an In,} a l _Out, a relationship that l _In = l _Out. The straight line E is parallel to the main scanning direction, and the center line E _Cen indicates the center position of the straight line E in the main scanning direction. From the center line E _Cen to the dashed line r _{an In,} and each length of the center line E _Cen to the dashed line r _Out is e _{an In,} e _Out, in this diagram, a relation that e _In = e _Out . Furthermore, since the lengths of the reference line S and the straight line E have a relationship of l _In = e _In and l _Out = e _Out , the lengths of the reference line S and the straight line E in the main scanning direction are equal. It has become a thing.

In the partial diagram (a), the straight line E has a shift amount ω with respect to the reference line S in the main scanning direction toward the alternate long and short dash line r _In (hereinafter simply referred to as “In side”). The straight line E has a shift amount v in the sub-scanning direction with respect to the reference line S. The shift amount ω is the main scanning margin, and the shift amount v is the sub-scanning margin. In this figure, the straight line E is shown as if the reference line S has the main scanning margin and the sub-scanning margin. The directions of the shift amount ω and the shift amount v are opposite to each other in the figure, that is, the straight line E moves to the alternate long and short dash line r _Out side (hereinafter simply referred to as “Out side”) in the main scanning direction. Even when there is a shift amount v in the scanning direction downward in the figure, the shift amount ω and the shift amount v are a main scanning margin and a sub-scanning margin, respectively. As described above, when the straight line E has a deviation amount with respect to the reference line S in the main scanning direction and the sub-scanning direction, it is necessary to correct the main scanning margin and the sub-scanning margin. In addition, description of the reference line S is abbreviate | omitted from the partial drawing (b) to the partial drawing (e).

In the partial diagram (b), the straight line E is parallel to the main scanning direction, and the center line E _Cen indicates the center position of the straight line E in the main scanning direction. From the center line E _Cen to the end portion of the In-side, and each length of the center line E _Cen to the end portion of the Out-side e _In, an e _Out, in this view, relationship e _In = e _Out The center line E _Cen to the In side and the center line E _Cen to the Out side have the same magnification. In addition, although the end on the In side of the straight line E and the reference line S matches the one-dot chain line r _In , the center line E _Cen and the end on the Out side of the straight line E are each one-dot chain line r _Cen. When, not suited to the dashed line r _Out, linear E has become as having a magnification in the main scanning direction relative to the reference line S. As described above, when the straight line E is reduced or enlarged with respect to the reference line S in a state having a magnification in the main scanning direction, it is necessary to correct the main scanning magnification.

In the partial diagram (c), the straight line E is parallel to the main scanning direction, and the center line E _Cen indicates the apparent center position of the straight line E in the main scanning direction. The length of the straight line E from the end on the In side to the end on the Out side is equal to that of the reference line S. Further, the center line E _Cen to the end portion of the In-side, and the length from the center line E _Cen to the end portion of the Out-side is e _In, e _Out respectively, in the figure, that e _In <e _Out There is a relationship. Therefore, the straight line E is the reference line S although the length of the main scanning direction are equal, and the end portions of the In-side from the center line E _Cen, and the magnification of the center line E _Cen with the end of the Out-side different It has become. Thus, the straight line E is equal in length to the reference line S in the main scanning direction, the length e _In from the center line E _Cen to the end on the In side, and from the center line E _Cen to the end on the Out side. E _In <e _Out or e _In > e _Out between the length e _Out of the main line and the magnification on the In side and the Out side of the center line E _Cen are different. Magnification balance correction is required.

In the partial diagram (d), the center line E _Cen of the straight line E indicates the center position in the longitudinal direction of the straight line E, from the center line E _Cen to the end on the In side, and from the center line E _Cen to the Out side. The lengths to the ends are e _In and e _Out , respectively, and in this figure, e _In = e _Out . The lengths of the straight line E and the reference line S are e _In = l _In and e _Out = l _Out , and the lengths of the straight line E and the reference line S in the longitudinal direction are equal. It has become. Further, the straight line E has an angle θ with respect to the reference line S around the end S ₀ on the In side of the reference line S. As described above, when the length of the straight line E is equal to the length of the reference line S and the straight line E is at an angle with respect to the reference line S, it is necessary to correct the sub-scanning skew.

In the partial diagram (e), the center line E _Cen of the curve E is on the one-dot chain line r _Cen and indicates the center position of the curve E in the main scanning direction. The ends on the In side and Out side of the curve E are aligned with the ends on the In side and Out side of the reference line S, respectively, but at the position of the center line E _Cen (dashed line r _Cen ), sub-scanning is performed. There is a deviation amount v in the direction, and the curve E is bent like a bow. As described above, although the In side and Out side ends of the curve E and the reference line S are combined, the sub-scan with respect to the reference line S at the center position (center line E _Cen ) of the curve E in the main scanning direction. When there is a deviation amount in the direction, it is necessary to correct the bow.

In the fractional diagram (f), the horizontal axis of the graph at the bottom of the diagram indicates the position in the main scanning direction in the image region, and the detection field of view R _In of the photosensor from the fractional diagrams (a) to (e), This corresponds to R _Cen and R _Out and the alternate long and _short dash lines r _In , r _Cen and r _Out indicating the center position. The vertical axis indicates the image density in the image area and the amount of laser light that is raster-irradiated on the photosensitive member by the ROS. As shown in this graph, in general, in an electrophotographic image forming apparatus, the amount of light (dotted line) of the laser beam raster-irradiated to the photosensitive member by the ROS is low in the central portion of the image area. A high light amount distribution is set on the In side and the Out side of the image area. On the other hand, the image density (solid line) is high in the center of the image area, but low on the In side and Out side of the image area, and the density distribution is set to be symmetrical. As described above, when there is an error in the density at the detection field of view R _In , R _Cen , R _Out of the photosensor with respect to the preset density distribution, the In / Center / Out density distribution is corrected. Necessary. Further, if there is a difference in density between the In side and the Out side, correction of the In / Out density difference is required.

The image characteristics that require the positional deviation correction and density error correction described in the partial diagrams (a) to (f) are the detection fields R _In , R _Cen , and the like of the photosensor on the intermediate transfer belt in the correction mode. It is detected by reading a pattern block (see FIG. 3 (b)) formed at a position passing through R _Out . For example, in the case of the image characteristics shown in the partial diagrams (a) to (e), the reference of each mark image of the same color arranged in the main scanning direction (the leading cyan in the moving direction in FIG. 3 (b)). By detecting the amount of deviation from the position (the reference line S in the partial diagram (f) from the partial diagram (a)), it is specified which characteristic is possessed. Further, in the case of the image characteristics of the partial diagram (f), the In / Out density difference and the In / Center / Out density distribution are grasped by detecting the density of the mark images of the same color arranged in the main scanning direction. The Further, the entire image is corrected by detecting the positional shift and density error of the comparison colors (yellow, magenta, black) with respect to the reference color (cyan).

  FIG. 5 shows a schematic configuration diagram of the photosensor 10 in the color image forming apparatus to which the present invention is applied. The photosensor 10 includes an illumination unit, a light receiving optical system, and a light receiving element. This illumination means consists of two LEDs 10a, 10b. The light receiving optical system includes a lens 10c and a mask 10e. In this figure, the left-right direction is the main scanning direction. FIG. 6 is a block diagram showing the flow of processing the output signal from the photodiode 10d by the detection information extraction unit 12, and shows AMP, a peak detection circuit, an under-peak detection circuit, and two sample-and-holds. The output signal from each circuit is sent to the determination unit 13 in FIG.

  In order for the photosensor 10 to detect a positional shift and a density error, it is necessary to irradiate the pattern block shown in FIG. Accordingly, the illumination unit irradiates the pattern block M composed of mark images arranged in order of cyan, yellow, cyan, magenta, cyan, and black.

  When irradiating the pattern block, one of two types of light sources, LEDs 10a and 10b, is selected and used, and the lens 10c of the light receiving optical system receives reflected light from the mark image constituting the pattern block as a photodiode 10d. It is arranged so that an image is formed on the light receiving surface. Immediately before the photodiode 10d, a mask 10e for restricting the detection visual field of the photodiode 10d light receiving surface is provided. The mask 10e is black to prevent stray light.

  When the reflected light from the mark image is projected onto the light receiving surface of the photodiode 10d, the photodiode 10d outputs a current corresponding to the amount of reflected light, that is, the density of the mark image. As shown in FIG. 6, the current output from the photodiode 10 d is subjected to current-voltage conversion / amplification by the AMP 20, and then as a sensor output signal, a determination unit (not shown), a peak detection circuit 21, an under-peak detection circuit 23. , And two sample and hold circuits 22 and 24.

  The peak detection circuit 21 detects the maximum position of the sensor output signal, supplies it to the sample & hold circuit 22 as a peak detection signal, and outputs it to the determination unit. By using this peak detection circuit 21 to detect the maximum position of the sensor output signal, the center position in the thickness direction of the mark image can be detected. Then, the determination unit determines whether the peak detection signal is valid or invalid for misregistration correction as the misregistration information of each color mark image, and specifies a photosensor in which invalid misregistration information is detected for misregistration correction. Yes.

  The sample & hold circuit 22 holds the sensor output signal output from the AMP 20 with the peak detection signal output from the peak detection circuit 21 as a trigger. As a result, the maximum value of the sensor output signal is held and output as a peak hold signal to the determination unit. Then, the determination unit determines whether the hold signal of the maximum value of the output signal is valid or invalid for density error correction as density error information of cyan, yellow, and magenta color mark images, and density error information invalid for density error correction is obtained. The detected photo sensor is specified.

  The under peak detection circuit 23 detects the minimum position of the sensor output signal and supplies it to the sample and hold circuit 24 as an under peak detection signal. The sample and hold circuit 24 holds the sensor output signal output from the AMP 20 with the under peak detection signal output from the under peak detection circuit 23 as a trigger. As a result, the minimum value of the sensor output signal is held and output to the determination unit as an under-peak hold signal. Then, the determination unit determines whether the hold signal having the minimum value is valid or invalid for the density error as the density error information of the black mark image, and identifies the photosensor in which the density error information invalid for the density error correction is detected. is doing. Note that a general electric circuit may be applied to the AMP 20, the peak detection circuit 21, the under peak detection circuit 23, and the sample and hold circuits 22 and 24, and the description thereof is omitted.

  In order to detect the density of the pattern block based on the output signal from the photodiode 10d, it is necessary to compare the reference output signal with the output signal detected from the pattern block. Therefore, it is necessary to have means capable of switching between the case where the reference light is incident on the photodiode 10d and the case where the reflected light from the pattern block is incident. Therefore, the shutter 10f as shown in FIG. 7 is attached to the photosensor 10 in a slidable state on the housing of the photosensor 10 facing the intermediate transfer belt 6 (FIG. 5). . This figure is a plan view of the shutter 10f as seen from the LED side. The shutter 10f is provided with a measurement window 10g and a reference plate 10h for obtaining a reference for the output voltage of the sensor. A mechanism is provided that moves by a drive device (not shown) in the direction of the arrow in the figure in accordance with the reflected light incident on the photodiode 10d. The shutter 10f is in such a position that the reference plate 10h is disposed on the light receiving system optical axis in the normally closed state, and the shutter 10f is opened only during pattern block measurement and the measurement window 10g is disposed on the light receiving system optical axis. To move.

FIG. 8 shows the positional relationship between the mark image m formed on the intermediate transfer body belt 6 and the detection visual field R of the photosensor 10 on the intermediate transfer body belt 6 with the passage of time. Graph (a) shows the waveform of the sensor output signal corresponding to the position of the detection visual field R of the photosensor 10. The lowermost graph (b) shows the peak detection signal of the mark image m output from the peak detection circuit in correspondence with the passage of time. Here, the mark image is formed such that the thickness t of each of the sides m ₁ and m ₂ is slightly smaller than the diameter d (1 mm) of the detection visual field R.

The mark image m primarily transferred onto the intermediate transfer belt 6 passes through the front surface of the photosensor 10 as the intermediate transfer belt 6 rotates, and crosses the detection visual field R of the photosensor 10. When the mark image m moves with the intermediate transfer body belt 6 and the detection visual field R of the photosensor 10 reaches a point A on the intermediate transfer body belt shown in FIG. As m ₁ enters, the sensor output signal starts changing. When the mark image m further moves, the area of the mark image m included in the detection visual field R, that is, the overlapping area between the detection visual field R and one side m ₁ of the mark image m increases, so that the sensor output signal gradually increases. The sensor output signal becomes maximum at the point B where the detection visual field R is substantially covered with the mark image m.

As described above, since the thickness t of each side m ₁ and m ₂ of the mark image m is formed slightly smaller than the diameter d of the detection field R of the photosensor 10, the mark image m passes the B point. Then, since the area of the mark image m included in the detection visual field R, that is, the overlapping area of the detection visual field R and the mark image m decreases, the sensor output signal gradually decreases and the mark image m becomes a photosensor. The sensor output signal becomes the minimum (point C) when it completely leaves the 10 detection visual fields R.

In this way, in the example shown in FIG. 8, when one side m ₁ of the mark image m passes through the detection field R of the photosensor 10 (between points A and B), the detection field R and the mark image m Are overlapped with the progress of the intermediate transfer belt 6, and the sensor output signal having the same intensity is not continuously output from the photosensor 10. That is, the maximum value is instantaneously generated in the sensor output signal. Such a waveform of the sensor output signal can be obtained by forming the detection field R of the photosensor 10 in a circular shape and making the thickness of the mark image m slightly smaller than the diameter of the detection field R. .

As shown in FIG. 8, when an instantaneous maximum value occurs in the waveform of the sensor output signal, the maximum value is detected by the photosensor 10 at the center position (center of gravity position) in the thickness direction of _one side m ₁ of the mark image m. Occurs when it matches the center position of the field of view R. Therefore, if the maximum value (peak) of the sensor output signal is detected by the peak detection circuit and a pulse-shaped peak detection signal is output in accordance with the maximum value as shown in FIG. The rising edge portion of the peak detection signal indicates the center position (center of gravity position) of one side m ₁ of the mark image m, and the position of m ₁ can be accurately detected.

Further, the mark image m shown in FIG. 8 is formed in a V shape having two sides m ₁ and m ₂ inclined at approximately 45 degrees in different directions with respect to the moving direction of the intermediate transfer belt 6. Therefore, by detecting one of the mark images m with the photosensor 10 of the present embodiment, it is possible to grasp the amount of positional deviation in the main scanning direction and the sub-scanning direction at a time. Specifically, the position deviation amount in the main scanning direction from the interval of m ₁ and m _2, the positional deviation amount in the sub-scanning direction from the relationship with the other colors of the center position of m ₁ and m ₂ can be seen.

When the V-shaped mark image m is read by the photosensor 10, as shown in FIG. 8B, the center position (center of gravity position) in the thickness direction of each side m ₁ and m ₂ of the mark image m is the detection field R. As a result, a pair of pulsed peak detection signals are output from the peak detection circuit corresponding to the points B and E that overlap with the center position.

  Next, the flow for determining the validity / invalidity of the positional deviation information and the density error information in the correction mode of the present invention will be described with reference to the flowchart of FIG. As described above, the description will proceed as an example of an image forming apparatus including three photosensors. First, the pattern block M shown in FIG. 3 is formed on the intermediate transfer belt (step S1). Then, the pattern block is measured by the photosensor (step S2), and then, for all the photosensors, the output signal from each color mark image is converted into positional deviation information and density error information by the determination unit 13 based on the output signal of the photodiode. It is determined whether the correction is valid or invalid (step S3). Here, if the positional deviation information and density error information are valid for correction for all the photosensors, positional deviation correction and density error correction are performed (step S4). On the other hand, when the positional deviation information and / or density error information of the photosensor is invalid for correction, the case is classified according to the number of photosensors determined to be invalid (step S11).

  If the number of photosensors determined to be invalid is one, it is determined which of the positional deviation information and the density error information is invalid (step S12). When it is determined that the density error information is invalid, the density error information of the photosensor determined to be invalid by the condition correction unit 14 is immediately discarded (step S13a), and the determination unit 13 determines that the density error information of the photosensor is invalid. It is determined whether the misregistration information is valid or invalid (step S14a). On the other hand, when it is determined that the positional deviation information is invalid, the positional deviation information of the photosensor determined to be invalid by the condition correction unit 14 is immediately discarded (step S13b), and the determination unit 13 determines the position of the photosensor. Whether the density deviation information is valid or invalid is determined (step S14b).

  Here, the criteria for determining the validity / invalidity of the positional deviation and density error information will be described. FIG. 10 shows a criterion for determining whether the positional deviation information and density error information are valid or invalid. In the diagram (a), the magenta mark image with respect to the cyan mark image Mc when the mark images Mc and Mm of the pattern blocks pass through the center position (one-dot chain line r) of the plurality of pattern blocks. The position variation of Mm in the main scanning direction is shown. In the upper part of the partial diagram (a), a state in which mark images Mc and Mm of one pattern block among the plurality of pattern blocks passes through the detection visual field R is shown. The lower graph of the partial diagram (a) shows the position variation in the main scanning direction of the mark image Mm with respect to the mark image Mc in each pattern block among the plurality of pattern blocks (six in the present diagram). For convenience of explanation, the position in the main scanning direction of the mark image Mm with respect to the position of the reference mark image Mc in each pattern block (the one-dot chain line in the graph) is left in the order of the pattern blocks passing through the detection visual field R. It is shown as six black dots, shifted to the right. As shown in the partial diagram (a), the position of the mark image Mm in the main scanning direction with respect to the mark image Mc of each pattern block varies, and the amplitude becomes the variation width h.

  Similarly to FIG. 10 (a), FIG. 10 (b) shows the case where the mark images Mc and Mm of each pattern block among the plurality of pattern blocks pass through the center position (dashed line r) of the detection visual field R. The position change in the main scanning direction of the magenta mark image Mm with respect to the cyan mark image Mc is shown. In the upper part of the partial diagram (b), a state in which mark images Mc and Mm of one pattern block among the plurality of pattern blocks passes through the detection visual field R is shown. The graph below the partial diagram (b) shows the position variation in the main scanning direction of the mark image Mm with respect to the mark image Mc in each pattern block among the plurality of pattern blocks (six in the present diagram). For convenience of explanation, the position in the main scanning direction of the mark image Mm with respect to the position of the reference mark image Mc in each pattern block (the one-dot chain line in the graph) is left in the order of the pattern blocks passing through the detection visual field R. It is shown as six black dots, shifted to the right. Also in the diagram (b), the position of the mark image Mm in the main scanning direction with respect to the mark image Mc of each pattern block varies greatly, and the mark for the mark image Mc farthest in the opposite direction from the one-dot chain line in the graph. The sum of the positions of the image Mm in the main scanning direction is the fluctuation range h.

When it is determined that the position error information of the photosensor whose density error information is determined to be invalid by the determination unit 13 (FIG. 9, step S14a), it is shown in the partial diagrams (a) and (b). The fluctuation range h that is set is the reference. In the determination, the fluctuation width of the positional deviation information of the photosensor determined to be invalid is h ₀ , and the fluctuation width of the positional deviation information of the other photosensors (two) is h ₁ and h _2. The determined fluctuation width h ₀ of the photosensor is equal to or less than a preset threshold value, that is, has a large fluctuation width that is invalid for positional deviation correction, and (2) the fluctuation width h of the photosensor determined to be invalid Δh ₁ = h ₀ −h ₁ , Δh ₂ = h ₀ −h ₂ , and the difference between ₀ and the fluctuation widths h ₁ and h ₂ of the other photosensors are determined, and the values of Δh ₁ and Δh ₂ are set in advance. It is judged whether it is below the threshold value. Then, when it corresponds to either (1) or (2) and when it corresponds to both (1) and (2), the positional deviation information of the photosensor determined that the density error information is invalid. Is determined to be suitable for misalignment correction.

  FIG. 10 (c) shows one pattern block M and a sensor output signal. A state in which one pattern block M passes through the detection visual field R is shown in the upper part of the partial diagram (c). Below the partial diagram (c), sensor output signals of the mark images when the mark images Mc, My, Mc, Mm, Mc, and Mb constituting the pattern block M pass through the detection visual field R are shown. And the average value of the output values of each side of the mark image My among the output values of the sensor output signals of the mark images Mc, My, Mc, Mm, Mc, and Mb is shown as the output value u.

When the determination unit 13 determines the validity / invalidity of the density error information of the photosensor whose position deviation information is determined to be invalid (FIG. 9, step S14b), the output value of the sensor output signal of each mark image is used as a reference. Become. In the determination, the difference between the output value of the sensor output signal of each mark image of the photosensor determined to be invalid and the output value of the sensor output signal of each mark image of the other photosensors (two) is set in advance. As long as it is within the range of the threshold value, it is determined that the density error information of the photosensor for which the positional deviation information is invalid is suitable for the density error correction. For example, in the case of the mark image My in FIG. 10C (c), the output value of the mark image My in the photo sensor determined to be invalid is u ₀ , and the output value of the mark image My in the other photo sensors is u _1. , U _2, and their differences Δu ₁ = u ₀ −u ₁ and Δu ₂ = u ₀ −u ₂ are obtained, and it is determined whether the values of Δu ₁ and Δu ₂ are within a preset threshold range. Is done. Similarly, with respect to other mark images constituting the pattern block M, a difference in output value is obtained for each mark image, and valid / invalid is determined.

  As described above, it is determined whether the positional deviation information and the density error information of the photo sensor determined to be invalid by the determination unit 13 are valid or invalid for the positional deviation correction and the density error correction, and the positional deviation in the photo sensor determined to be invalid. The positional deviation correction and density error correction are performed from the positional deviation information and density error information suitable for correction and density error correction, and the positional deviation information and density error information in the other photosensors (FIG. 9, step S4).

  Next, when the number of photosensors determined to be invalid among the three photosensors is two, the positional deviation information and density error information of the remaining one photosensor, and two invalidity It is determined whether to perform the positional deviation correction and the density error correction from the positional deviation information and the density error information suitable for the positional deviation correction and the density error correction of the photosensor determined as (step S21 in FIG. 9). The determination of the positional deviation information and the density error information in the photosensor determined to be invalid is performed in the same manner as the method described in FIG. When it is determined that the position deviation correction and / or the density error correction are to be performed, the correction is performed (FIG. 9, step S4). When it is determined that the position deviation correction and the density error correction are not to be performed, the correction mode is The process ends (FIG. 9, step S22).

  FIG. 11 shows a table showing the number of usable photosensors and image characteristics to be corrected in the positional deviation correction and the density error correction. In the left column of this table, the image characteristics (main scanning margin, sub-scan margin, main scanning magnification, main scanning left / right magnification balance, sub-scanning skew, BOW) belonging to the misregistration correction and the image characteristics belonging to the density error correction ( In / Out density difference, In / Center / Out density distribution, density correction) are listed. In the three columns on the right side, the image characteristics are “correctable” and “condition” in the case of three, two, and one usable photosensors among the three photosensors. Any of “can be corrected with” and “not corrected” is indicated by “◯”, “Δ”, and “×”, respectively. The “usable sensor” in this table means “a photosensor whose positional deviation information and density error information are effective for correction”.

  As shown in FIG. 11, when three usable photosensors among the three photosensors are used, all image characteristics can be corrected. When two photosensors can be used, the main scanning margin, the sub scanning margin, and the main scanning magnification can be corrected, and the sub scanning skew and the In / Out density difference can be corrected with conditions. Since the main scanning left / right magnification balance, BOW, and In / Center / Out density distribution cannot be detected by the two photosensors, no correction is performed. Further, if the correction mode is executed when the number of usable photosensors is one, the density correction at the arrangement position of the usable photosensors can be performed. It can be corrected. On the other hand, if the correction mode is not executed, naturally all image characteristics are not corrected. As described above, the image characteristics to be corrected in the positional deviation correction and the density error correction (FIG. 9, step S4) are determined according to the number of photosensors from which the positional deviation information and density error information suitable for the correction are extracted.

  FIG. 9 is a flowchart regarding the positional deviation correction and the density error correction regarding the positional deviation correction and the density error correction performed based on the positional deviation information and the density error information determined to be suitable for correction by the determination unit 13 in step S4 of FIG. 12 and FIG. 13 showing a timing chart at the time of mark image measurement will be described. First, in the positional deviation correction, the absolute value of the reference color cyan with respect to the target value in the main scanning direction based on the peak detection signal output from the peak detection circuit 21 by the detection information extraction unit 12 based on the output signal of the photodiode 10d. The condition correction unit 14 measures and calculates the positional deviation amounts of yellow and magenta relative to the positional deviation amount and the reference color cyan (FIG. 12, step S31).

  The positional deviation amount of the mark image is obtained by calculation from the timing chart at the time of mark image measurement shown in FIG. In FIG. 13, from the top, waveforms of the operation signal of the shutter 10f of the photosensor 10, the lighting signals of the LEDs 10a and 10b of the photosensor 10, the sensor output signal, the peak detection signal, the peak hold signal, the under peak detection signal, and the under peak hold signal. It is shown.

  As shown in FIG. 13, in the correction mode, first, the LED 10b is turned on with the shutter 10f closed, and the reflected light of the reference plate 10h is measured for density error correction. Thereafter, before the mark image arranged at the head with respect to the moving direction of the intermediate transfer belt 6 passes through the detection visual field of the photosensor 10, the shutter 10f is opened, and the reflected light from the mark image can be incident on the photodiode 10d. Put it in a state. At this time, the sensor output signal is 0V. This is because the intermediate transfer belt 6 used in this embodiment has a black surface and has a mirror surface or gloss, and the LED illumination light is almost diffused in the non-image portion on the surface of the intermediate transfer belt 6. Therefore, the sensor output signal is 0V.

  The sensor output signal has a pulse-like waveform corresponding to the amount of cyan toner by passing one side of the cyan mark image while the shutter 10f is open. At this time, as shown in FIG. 6, the peak detection circuit 21 detects the maximum value of the sensor output signal and outputs a peak detection signal. Here, the time from the start of misalignment measurement until the peak detection signal is output is tA1. The time until the peak detection signal detected by the peak detection circuit 21 is output with the passage of the remaining one side of the mark image is defined as tA2. Thereafter, in the same manner, the times tT1, tT2, tB1, and tB2 until the peak detection signal is output as the yellow, cyan, magenta, cyan, and black mark images pass are sequentially measured.

  As described above, since the reflected light of the black mark image is different from the reflected light of the cyan, yellow, and magenta mark images, the LED 10b is turned off and the LED 10a is turned on simultaneously with the passage of the black mark image. At this time, the sensor output signal is output at a voltage value corresponding to the reflected light from the intermediate transfer belt 6. As the black mark image passes, the output voltage of the sensor has a pulse-like waveform that decreases according to the amount of black toner. Here, as shown in FIG. 6, the minimum value of the sensor output signal is detected by the under peak detection circuit 23, and the times tU1 and tU2 until the under peak detection signal is output are measured. In FIG. 13, cyan, yellow, cyan, and some of them are omitted, and a black mark image, that is, a state until one pattern block M passes through the detection field of the photosensor 10 is shown. However, in the normal correction mode, a plurality of pattern blocks M having different densities are successively formed on the intermediate transfer belt 6 at positions facing the detection visual fields of the three photosensors 10.

The calculation of the positional deviation amount is performed by obtaining the absolute positional deviation amount with respect to the target value of the reference color cyan in the main scanning direction and the relative positional deviation amounts of yellow and magenta with respect to the reference color cyan. First, the absolute value position shift amount of the reference color cyan in the main scanning direction is
Absolute value position deviation amount in main scanning direction = {(tA2−tA1) −target value} / 2
The relative positional deviation of yellow relative to the reference color cyan is
Sub-scanning direction positional deviation = [(tT2 + tT1) / 2 − ((tA2 + tA1) / 2 + (tB2 + tB1) / 2) / 2] × PS
= [(tT2 + tT1) / 2- (tA2 + tA1) / 4- (tB2 + tB1) / 4] × PS
Position deviation in main scanning direction = [((tB1 + tA1) / 2−tT1 + sub scanning direction error
+ TT2− (tB2 + tA2) / 2−sub-scanning direction error) / 2] × PS
= [((tB1 + tA1) / 2-tT1 + tT2- (tB2 + tA2) / 2) / 2] × PS
Can be obtained. Here, tA1, tA2, tT1, tT2, tB1, and tB2 are times (μs) from the start of positional deviation measurement until the peak detection signal is output, and PS is a process speed (mm / s). The relative misregistration amounts of magenta and black with respect to the reference color cyan are similarly calculated.

  This calculation corresponds to step S31 of FIG. 12, and after the measurement and calculation of the positional deviation amount is completed by the condition correction unit 14, the image formation position at the time of output image formation, that is, the main scanning direction by ROS and the sub scanning direction. An exposure timing is set (FIG. 12, step S32). By performing a series of these operations, the positional deviation correction is performed once in the correction mode.

  Next, in the density error correction, the condition correction unit 14 calculates the density error of the mark image from the hold signal output from the sample and hold circuit in the detection information extraction unit 12 based on the detection signal of the photodiode 10d. (FIG. 12, step S41).

  The density error of the mark image is obtained by calculation from the timing chart at the time of mark image measurement shown in FIG. As described above, in the correction mode, first, the LED 10b is turned on with the shutter 10f closed. As a result, a voltage value corresponding to the reflected light from the reference plate 10h of the shutter 10f is output as the sensor output signal, and this is measured as the reference plate output voltage (Vref) of the sensor. Then, before the mark image arranged at the head with respect to the moving direction of the intermediate transfer belt 6 passes through the measurement position of the sensor, the shutter 10f is opened so that the reflected light from the mark image can enter the photodiode 10d. To do.

  The sensor output signal has a pulse-like waveform corresponding to the cyan toner amount due to the passage of the cyan mark image after the shutter 10f is opened. At this time, the peak detection circuit 21 detects the maximum value of the sensor output signal and outputs a peak detection signal. The sample and hold circuit 22 uses the rising pulse of the peak detection signal output from the peak detection circuit 21 as a trigger to hold the maximum value of the sensor output signal corresponding to the amount of cyan toner, whereby the cyan density voltage (Vc) is held. ) Is measured. Thereafter, in the same manner, yellow density voltage (Vy) and magenta density voltage (Vm) are measured by passing mark images of yellow, cyan, magenta (not shown) and cyan (not shown).

  Next, the LED 10b is turned off and the LED 10a is turned on simultaneously with the passage of the black mark image. At this time, the sensor output signal is output at a voltage value corresponding to the reflected light from the intermediate transfer belt 6. As the black mark image passes, the output voltage of the sensor has a pulse-like waveform that decreases according to the amount of black toner. Here, as shown in FIG. 6, the under peak detection circuit 23 detects the minimum value of the sensor output signal and outputs an under peak detection signal. The sample & hold circuit 24 holds the minimum value of the sensor output signal corresponding to the amount of black toner by using the rising pulse of the under peak detection signal output from the under peak detection circuit 23 as a trigger, thereby causing the black density voltage to be held. (Vk) is measured. Next, after passing the black mark image, the sensor output signal again indicates a voltage value corresponding to the specularly reflected light from the intermediate transfer body belt 6, and this value is measured as a belt surface voltage (Vbelt). And after this belt surface voltage measurement, while turning off LED10a, turning on LED10b, a sensor output signal will be set to 0V.

The image density is calculated differently between black and color (CYM). The image density of black is a value relative to the non-image surface of the intermediate transfer belt 6.
Image density: Dk = Vk / Vbelt
And calculate. On the other hand, the color (CYM) image density is a relative value to the output of the reference plate 10h.
Image density: Dn = ((Vn average value) / Vref)
Where n = toner color (c, y, m)
And calculate.

  As described above, the reason why the relative value for the output of the intermediate transfer belt 6 or the reference plate 10h is used as the image density is that the sensor light is fouled, changes with time, changes in temperature, changes in LED light quantity, PD sensitivity, and the like occur. This is because the density of the pattern block is measured with high accuracy. In this way, the image density of the pattern block is calculated (FIG. 12, step S41), and the error between the predetermined density target value and the calculated image density is calculated (FIG. 12, step S42). .

Next, the correction amount of the laser power is calculated (FIG. 12, step S43). In the calculation, the correction amount of the ROS laser power: ΔLP is
Laser power correction amount: ΔLP = ΔDn / An
Where n = toner color (k, c, y, m)
Is required. Here, ΔDn is the density error of the pattern block obtained in the density error calculation (FIG. 12, step S42), and An is a coefficient indicating the correspondence between the laser power and the image density of the pattern block. This coefficient is obtained in advance by experiments or the like.

  Thereafter, the laser power set value is corrected by subtracting the laser power correction amount ΔLP obtained in the calculation of the laser power correction amount (FIG. 12, step S43) from the laser power at the time of pattern block formation (FIG. 12). 12, Step S44). The laser power setting value obtained at this time is supplied to the ROS 3 as the laser power at the time of output image formation.

  As described above, in the correction mode, mark images of the same color arranged in the main scanning direction in a plurality of pattern blocks formed on the intermediate transfer belt 6 are measured by the three photo sensors, and each photo sensor is measured. By grasping the relative positional deviation amount, the image characteristics relating to the positional deviation are corrected. Similarly, the image characteristics relating to the density error are corrected by grasping the relative density error of each photosensor. By periodically repeating these corrections, the image formation position and the output image density are kept constant.

FIG. 14 shows an example of error distribution of the main scanning left / right magnification balance deviation when two usable photosensors are used. In this figure, alternate long and _short dash lines r _In , r _Cen , and r _Out indicate the center positions of the detection visual fields R _In , R _Cen , and R _Out of three photosensors arranged in the main scanning direction. Reference lines S _In , S _Cen , and S _Out parallel to the sub-scanning direction are shown on the alternate long and _short dash lines r _In , r _Cen , and r _Out . In addition, the photosensor arranged in the center is a photosensor in which the positional deviation information is determined to be invalid or a faulty photosensor, and the state is indicated by “x” in the detection visual field R _Cen. (Hereinafter simply referred to as “abnormal photosensor”). Further, sub-scanning line F _In, F _Cen, F _Out is parallel to the sub scanning direction, and a distance f _{an In} the sub-scanning line F _Cen until subscanning line F _{an In} sub scanning line from the sub-scanning line F _Cen The distance f _Out to F _Out indicates the main scanning magnification (part (a)). For convenience of explanation, the main scanning magnification of the previous In side and Out side the photo sensor disposed at the center becomes abnormal photosensor f _In, and f _Out.

In the partial diagram (a), since the sub-scan lines F _In and F _Out are corrected for misalignment by a usable photosensor, they are on the alternate long and short dash lines r _In and r _Out at the positions of the detection visual fields R _In and R _Out. It is formed and no misalignment has occurred. Therefore, the sub scanning lines F _In and F _Out are aligned with the reference lines S _In and S _Out in the main scanning direction. However, since no misregistration correction is performed at the position of the detection field R _Cen of the abnormal photosensor, the sub-scanning line F _Cen still has a misalignment amount ω ₀ toward the In side in the main scanning direction. The amount of misalignment at the center is increased. Therefore, the main scanning magnifications f _In and f _Out before the photosensor arranged at the center becomes an abnormal photosensor are stored in advance in the storage unit 15 provided in the condition correction unit 14 in the correction mode, and the storage unit 15 It is preferable to perform error distribution of the main scanning left / right magnification balance deviation based on the main scanning magnifications f _In and f _Out stored in. Thereby, the shift amount ω ₀ can be distributed to the In side and the Out side, and the shift in the entire image area can be suppressed to the shift amount ω ₁ (see the fractional diagram (b)). It should be noted that the main scanning magnifications f _n and fOut on the In side and Out side before and after the distribution of the deviation amount (fraction (a) and (b)) are not changed. This is effective only when the temporal fluctuation of the left / right magnification balance is small.

FIG. 15 shows an example of BOW shift error distribution in the case where there are two usable photosensors. In this figure, alternate long and _short dash lines r _In , r _Cen , and r _Out indicate the center positions of the detection visual fields R _In , R _Cen , and R _Out of three photosensors arranged in the main scanning direction. A reference line S indicates a reference line in the main scanning direction. The photosensor arranged at the center is an abnormal photosensor, and the state thereof is indicated by “x” in the detection visual field R _Cen . Further, in the partial diagram (a), the curve B has a BOW with respect to the reference line S. Here, an error in the sub-scanning direction on the alternate long and _short dash line r _Cen before the photosensor arranged at the center becomes an abnormal photosensor is denoted by v ₀ (part (a)).

In the case of an image forming apparatus with little change in BOW, the condition correction unit 14 determines the error v ₀ in the sub-scanning direction on the alternate long and _short dash line r _Cen before the center photosensor shown in the partial diagram (a) becomes an abnormal photosensor. It is good to store beforehand in the memory | storage part 15 with which is equipped. Then, based on the stored error v ₀ , the error may be distributed to the In side and the Out side where usable photosensors are arranged. Thereby, even if the number of usable photosensors is two, the error in the sub-scanning direction can be suppressed to v ₁ and correction can be made (see fractional drawing (b)). This is effective only when the variation with time of the BOW deviation is small.

FIG. 16 shows an example of correction of a scanning line having a skew in the case where two photosensors can be used. In this figure, alternate long and _short dash lines r _In , r _Cen , and r _Out indicate the center positions of the detection visual fields R _In , R _Cen , and R _Out of three photosensors arranged in the main scanning direction. The photosensor disposed on the In side is an abnormal photosensor, and the state thereof is indicated by “x” in the detection visual field _RIn . A reference line S indicates a reference line in the main scanning direction. Further, a curved line K is a curved scanning line and has both BOW and sub-scanning skew image characteristics.

As shown in this figure, the sub-scanning skew amount of the curve K is k ₀ . However, when the sub-scanning skew amount of the curve K is detected by the usable center and out-side photosensors, it is detected from the deviation amount of the curve K with respect to the reference line S at the positions of the one-dot chain line r _Cen and the one-dot chain line r _Out. Therefore, the sub-scanning skew amount is detected as an incorrect sub-scanning skew amount k ₁ . If you do this incorrectly detected subscanning skew amount k ₁ in based positional deviation correction, positional deviation is not eliminated. Therefore, if the sub-scanning bow amount of the curve K before the In photosensor becomes an abnormal photosensor is stored in advance in the storage unit 15 provided in the condition correction unit 14, the tendency of the curve K is grasped. Good. Thereby, in the image forming apparatus in which the sub-scanning BOW is relatively difficult to change, the sub-scanning skew amount k ₁ detected in error can be corrected to the true sub-scanning skew amount k ₀ .

  Further, when only one photosensor can be used, the storage unit 15 provided in the condition correction unit 14 stores in advance the sub-scanning skew amount and the BOW shift amount of the sub-scanning skew. It is good to keep track of trends. Accordingly, in an image forming apparatus in which the sub-scanning skew and the BOW are relatively difficult to change, the deviation of the sub-scanning skew amount and the BOW at the position where one usable photosensor is arranged in consideration of the tendency of the image characteristics. From the amount, it is possible to correct the sub-scan margin by distributing these deviations as errors.

  Further, in the case of an image forming apparatus in which main scanning magnification, main scanning left / right magnification balance, sub-scanning skew, and BOW are unlikely to fluctuate, even if only one photosensor can be used, the above-described deviation can be effectively corrected. It is possible to distribute and correct the optimum main scanning margin and sub-scanning margin.

  As for the density error correction, as shown in FIG. 11, the In / Out density difference can be corrected when two photosensors can be used. Also in the density error correction, the storage unit 15 provided in the condition correction unit 14 stores the density at the arrangement position of each photosensor in advance, thereby grasping the tendency of the In / Out density difference and the In / Center / Out density distribution. It is good to leave it. Thereby, even if the number of usable photosensors is two, it is possible to perform the prospective correction for the In / Out density difference. Further, when there is one usable photosensor, the density at the arrangement position of each photosensor, that is, the In / Out density difference stored in advance in the storage unit 15 is the density at the arrangement position of the usable photosensor. Alternatively, correction can be made from the tendency of In / Center / Out concentration distribution, and the correction accuracy can be improved.

  With respect to the tendency of each image characteristic stored in the storage unit 15, a plurality of kinds of tendencies may be detected. Therefore, it is preferable to classify the types of trends for each image characteristic and store them in the storage unit 15. As a result, even when the number of usable photosensors is reduced, it is possible to correct image characteristics while ensuring minimum accuracy.

  Finally, the image forming apparatus according to the description of the present invention is an example having three photosensors, but the image forming apparatus having five photosensors is also corrected according to the number of usable photosensors. The image characteristics to be performed can be set as appropriate. For this setting, when the number of usable photosensors is two, that is, when the number of abnormal photosensors is three, only specific image characteristics are corrected, or the correction mode is not performed. It is good to do. Then, the storage unit 15 provided in the condition correction unit 14 stores the shift amount and the sub-scanning skew amount at the arrangement position of each photosensor, and stores the tendency of each image characteristic, thereby enabling the use of a usable photosensor. Even when the number is reduced to three, all the image characteristics can be corrected.

1 is a schematic configuration diagram of a color image forming apparatus using an electrophotographic system to which the present invention is applied. FIG. 2 is a block diagram showing a flow of positional deviation correction and density error correction in the color image forming apparatus shown in FIG. 1. It is a figure which shows the example of an arrangement | sequence of the mark image applied to this invention. It is a figure which shows the image characteristic generally correct | amended in a color image forming apparatus. It is a figure which shows the image characteristic generally correct | amended in a color image forming apparatus. 1 is a schematic configuration diagram of a photosensor in a color image forming apparatus to which the present invention is applied. It is a block diagram which shows the flow by which the output signal from a photodiode is processed by the detection information extraction part. It is the plain view which looked at the shutter attached to the photo sensor from the LED side. FIG. 4 is a diagram showing a positional relationship between a mark image formed on an intermediate transfer belt and a detection visual field of the photosensor on the intermediate transfer belt along the time course. It is a flowchart which shows the flow in which the valid / invalid of misregistration information and density error information is judged in the correction mode of the present invention. It is a figure which shows the reference | standard which judges the validity / invalidity of position shift information and density error information. FIG. 5 is a table showing the number of usable photosensors and image characteristics to be corrected in positional deviation correction and density error correction. It is a flowchart regarding position shift correction and density error correction. 6 is a timing chart showing waveforms of a shutter operation signal, a photosensor lighting signal, a sensor output signal, a peak detection signal, a peak hold signal, an under-peak detection signal, and an under-peak hold signal when measuring a mark image. It is a figure which shows an example of the main scanning right-and-left magnification balance correction | amendment in case the photosensor which can be used is two. It is a figure which shows an example of BOW correction | amendment in case the photosensor which can be used is two. It is a figure which shows an example of the correction | amendment of the scanning line which has a curve in the case where the usable photosensor is two.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Photoconductor, 2 ... Charger, 3 ... Light emitting element array, such as ROS (laser output part) or LED array, 4 ... Developing device, 6 ... Intermediate transfer body belt, 10 ... Photosensor, 12 ... Detection information extraction unit, 13 ... Judgment unit, 14 ... Condition correction unit, 15 ... Storage unit

Claims (4)

In an image forming apparatus in which a plurality of image forming engines are arranged and each color component image formed by each image forming engine is transferred to a recording material directly or via an intermediate transfer member,
A correction mode control unit that sends a signal to each image forming means and sequentially transfers a mark image of each color for correcting the positional deviation and density error of the image on the surface of the recording material carrier or the intermediate transfer member; A plurality of detection means arranged in the main scanning direction so as to oppose the recording material carrier or the intermediate transfer body for optically reading the mark image, and positional deviation information of each color toner image from the detection signals of these detection means And a detection information extraction unit that extracts both density error information and a condition correction unit that corrects the image forming condition of the toner image in each image forming unit according to the positional deviation information and the density error information,
The condition correction unit includes a determination unit that determines validity / invalidity of the positional deviation information and density error information extracted by the detection information extraction unit, and the determination unit includes positional deviation information of at least one detection unit or / and / or When it is determined that the density error information is invalid,
The condition correction unit discards invalid position information and density error information of the detection means from which invalid information has been extracted without using the valid position deviation information and density error information of other detection means. An image forming apparatus that corrects misregistration and density error. In an image forming apparatus in which a plurality of image forming engines are arranged and each color component image formed by each image forming engine is transferred to a recording material directly or via an intermediate transfer member,
A correction mode control unit that sends a signal to each image forming means and sequentially transfers a mark image of each color for correcting the positional deviation and density error of the image on the surface of the recording material carrier or the intermediate transfer member; A plurality of detection means arranged in the main scanning direction so as to oppose the recording material carrier or the intermediate transfer body for optically reading the mark image, and positional deviation information of each color toner image from the detection signals of these detection means And a detection information extraction unit that extracts both density error information and a condition correction unit that corrects the image forming condition of the toner image in each image forming unit according to the positional deviation information and the density error information,
When the determination unit determines that the density error information of at least one detection unit is invalid,
When the fluctuation range of the positional deviation information of the detection unit from which invalid density error information is extracted is equal to or less than a preset threshold value and / or the fluctuation range of the positional deviation information of the detection unit, and other detection units The fluctuation range of the positional deviation information is compared, and when the difference is equal to or smaller than a preset threshold value, the positional deviation information of the detection unit is determined to be valid, and the condition correction unit calculates the density error information of the detection unit. An image forming apparatus, wherein the image forming apparatus is discarded without being used, and the positional deviation and density error are corrected using valid positional deviation information and density error information. In an image forming apparatus in which a plurality of image forming engines are arranged and each color component image formed by each image forming engine is transferred to a recording material directly or via an intermediate transfer member,
A correction mode control unit that sends a signal to each image forming means and sequentially transfers a mark image of each color for correcting the positional deviation and density error of the image on the surface of the recording material carrier or the intermediate transfer member; A plurality of detection means arranged in the main scanning direction so as to oppose the recording material carrier or the intermediate transfer body for optically reading the mark image, and positional deviation information of each color toner image from the detection signals of these detection means And a detection information extraction unit that extracts both density error information and a condition correction unit that corrects the image forming condition of the toner image in each image forming unit according to the positional deviation information and the density error information,
When the determination unit determines that the positional deviation information of at least one of the detection units is invalid,
The output value of the detection signal related to the density error information of the detection means from which the invalid positional deviation information is extracted is compared with the output value of the detection signal related to the density error information of the other detection means, and the difference is set in advance. If it is within the range of the threshold value, it is determined that the density error information of the detection unit is valid, and the condition correction unit discards the detection unit without using the positional deviation information. An image forming apparatus that corrects misregistration and density error using density error information. The determination unit selects whether to perform only density error correction or cancel the correction mode according to the number of the detection units that have determined that positional deviation information and / or density error information is invalid. The image forming apparatus according to claim 3.

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