JP2013228640A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve density correction accuracy by suppressing incorrect detection associating with pressing operation of a back-up roller when a density patch formed on a sheet is read.SOLUTION: After a downstream end in a conveyance direction of a sheet P passes a detection position by a color sensor 301 by a predetermined amount, conveyance of the sheet P by double-sided upstream driving rollers 306a and 306b is stopped. The conveyance of the sheet P by the double-sided upstream driving rollers 306a and 306b is then resumed, while the sheet P is pressed by a back-up roller 302 against the color sensor 301 at a pressing position after operation to move the back-up roller 302 to the pressing position. The sheet P is conveyed while image density detection is performed by the color sensor 301.

Description

  The present invention relates to an image forming apparatus that forms an image on a sheet.

  Network printers as image output devices are rapidly spreading due to advances in network technology centering on computers. In particular, along with the colorization of output images, development of color printers has become active in recent years, and demands for improving the stability and stability of color image quality and making color image quality uniform among multiple color printers have increased. It is coming. Recently, regarding color reproducibility, high stability is required regardless of installation environment, changes with time, and machine differences.

  In an image forming apparatus using an electrophotographic system, feedback control is performed to keep the image density optimal. In general feedback control, the density reproduction status and the environmental conditions in the device are monitored by the density patch, the difference from the target density of the reference paper in each basis weight category is obtained, and this is multiplied by the feedback gain. The set value correction amount for the control actuator is calculated. For example, it is known to obtain a desired image density by measuring a density patch and controlling exposure conditions and development bias conditions.

  As the density patch, an unfixed toner image density patch after the development process or a fixed image density patch after the fixing process formed on a sheet such as paper is used. The toner image density patch is used because the development image can be easily created and erased as compared with a transfer image and a fixed image created on the sheet. However, although the toner image density patch has a high correlation with the fixed image density, it is difficult to detect the influence of variations in the transfer process and the fixing process, which are subsequent processes. The fixed image density patch is a fixed image itself that is finally obtained by the user as an image form, and the image quality can be evaluated including factors of variation in the transfer process and the fixing process.

  As an example of such an image forming apparatus, there is an apparatus that uses an image reading unit incorporated in an apparatus main body to monitor a fixed image density. However, in the fixed image density monitor, in order to detect the image, the user himself has to perform the operation of returning the output image once to the image reading unit and reading it again, which is a problem for daily image quality management. It was annoying. Further, in the case of an image forming apparatus that does not include an image reading unit such as a printer, an image cannot be detected in principle.

  Therefore, there is a color image monitor sensor as means for enabling output image monitoring after the fixing step. This sensor is a blue (B), green (G), or red (R) light emitting diode (visible light LED, hereinafter referred to as a yellow (Y), magenta (M), or cyan (C) toner single color. LED) is used as a light source, and reflected light from an output image is received by a photodiode. In general, the emission spectrum of an LED has a narrower band than that of an RGB filter or the like, and it is said that it is difficult to spectrally divide the entire color gamut with high accuracy. However, as a use condition of the sensor, the monitor output image is formed as a single color patch of toner such as YMC, and is limited to the detection of the density of each single color toner. This is far more advantageous in terms of cost and size than conventional color sensors that identify each color of the full color by splitting the entire color gamut, and also provides a monitor sensor that is necessary and sufficient in terms of performance. Can be provided. Hereinafter, a method of providing this color image monitor sensor in the image forming apparatus main body and performing image density correction is referred to as printer pascal.

  However, when the output image of the image forming apparatus is to be monitored, the vertical movement of the measured surface of the output sheet as the image forming medium that is the object, that is, the fluctuation in the direction perpendicular to the traveling direction of the sheet is a problem. Therefore, there is a possibility that accurate measurement cannot be performed. Therefore, a configuration is used in which the posture of the output sheet is corrected so as to be a certain distance from the sensor (Patent Document 1). Specifically, a backup roller or the like of an elastic member is arranged on the opposite side of the color image monitor sensor, a window portion for measuring the output sheet is provided in the sheet conveyance guide, and the output sheet is pressed in the direction of the sensor. I am doing so.

  On the other hand, since the backup roller rotates while being in contact with the window portion of the sheet conveyance guide, if the output sheet is in a pressurized state when the sheet is not passed, the surface layer of the roller is scraped and the image is removed. Defects occur. For this reason, the backup roller is separated by the attaching / detaching mechanism while the sheet is not being conveyed and is not in contact with the sheet conveying guide, and is pressed by the attaching / detaching mechanism only when the sheet is conveyed.

  Also, when measuring the measured surface of the output sheet that fluctuates up and down using an optical system that combines a light source, a lens, and a light receiving element (photoelectric conversion element), for example, even when the paper surface fluctuates slightly by about 1 mm, It has been found that the output of the light receiving element changes by about 15%.

Japanese Patent Laid-Open No. 8-15943

  However, the conventional configuration has the following problems.

  Here, a configuration (printer pascal) in which image density is detected by a color image monitor sensor and image density correction is performed will be described as an example. First, at the time of printer pascal, a color patch, which is a density patch for image density detection for detection by a color image monitor sensor, is formed and transferred and fixed on a sheet. Then, the backup roller is moved to the pressing position by the driving means at the timing when it passes through the color image monitor sensor while conveying the output sheet on which the color patch is fixed, and the output sheet is pressed against the color image monitor sensor.

  However, for example, when pressing is performed by the backup roller using a solenoid as drive means, the backup roller vibrates for several hundreds of milliseconds after pressing, and the measured surface of the output sheet is not stabilized in the vertical direction accordingly. Also, for example, when pressing is performed by a backup roller using a motor as a driving means, if the backup roller is an elastic member, it takes time to stabilize the shape, and the output sheet follows the shape of the backup roller. The surface to be measured is not stable in the vertical direction. In any case, since the measured surface of the output sheet is not stable in the vertical direction for a certain period of time, the detection error of the light receiving element becomes large, so that it is difficult to detect with the color image monitor sensor.

  For this reason, the area where detection by the color image monitor sensor can be performed is an area where the detection error of the light receiving element is reduced. As a result, in order to perform density detection with the color image monitor sensor, it is necessary to reduce the area in which the color patch is used.

  However, if the area in which the color patch is used is reduced, if a minute horizontal streak image or the like is generated at the time of forming the color patch, the reading of the patch is likely to be erroneously detected, and the density correction accuracy is lowered. The horizontal streak image described here is, for example, a phenomenon caused by the sheet transport posture generated in the image transfer unit, and may be a horizontal streak of about 5 mm in the transport direction. For this reason, there is a need for density detection that can increase the size of each patch and that is difficult to detect erroneously.

  Accordingly, an object of the present invention is to suppress erroneous detection associated with the wearing operation of a backup roller when reading a density patch formed on a sheet, and to improve density correction accuracy.

  To achieve the above object, the present invention provides an image forming unit that forms an image on a sheet, a transport unit that transports an image-formed sheet, a density detection unit that detects an image density of the transported sheet, Between the pressing position for pressing the sheet on which the density patch for detecting the image density is formed by the image forming unit and pressing the sheet against the density detecting means and the separated position spaced apart from the density detecting means. A pressing member that moves, a conveying unit, and a control unit that controls the operation of the pressing member, the control unit after the downstream end in the sheet conveying direction has passed a predetermined amount of the detection position by the density detecting unit. In the state in which the conveyance of the sheet by the conveying unit is stopped and the pressing member is moved to the pressing position to press the sheet against the density detecting unit, the sheet by the conveying unit is moved. It resumes the conveyance, characterized in that for conveying the sheet while performing the image density detection by the density detecting means.

  According to the present invention, the sheet is temporarily stopped before the image density detection using the density detection means, and the operation of moving the pressing member to the pressing position is performed to press the sheet against the density detection means, and then the sheet is conveyed again. This makes it possible to shorten the sheet transport distance necessary for the surface to be measured to be stabilized in the vertical direction at the detection position by the density detection means, and to secure a sufficient density patch formation region. For this reason, even when there is a local horizontal streak image or the like in the density patch, it is possible to reduce a decrease in reading accuracy in the density detection means. As a result, erroneous detection associated with the pressing operation of the pressing member when reading the density patch formed on the sheet can be suppressed, and the density correction accuracy can be improved.

Schematic sectional view showing the entire image forming apparatus (A) and (b) are perspective views of the color sensor unit 194. (A) is a figure explaining the color detection of the color sensor 301, (b) is a figure explaining the structure of the color sensor 301 and its periphery. Block diagram showing a partial configuration of a color image forming apparatus Printer Pascal flowchart (A) is a schematic diagram showing a patch readable region of the present invention, (b) is a schematic diagram showing a conventional patch readable region. The perspective view which shows the relationship between the color sensor unit which concerns on this embodiment, and the image patch on a sheet | seat.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in the following embodiments should be appropriately changed according to the configuration of the apparatus to which the present invention is applied and various conditions. Therefore, unless specifically stated otherwise, the scope of the present invention is not intended to be limited thereto.

(Configuration of image forming device and color sensor unit)
A schematic configuration of the image forming apparatus will be described with reference to FIG. FIG. 1 is a schematic cross-sectional view of the image forming apparatus of the present embodiment. The image forming apparatus according to the present embodiment exemplifies a color image forming apparatus using an electrophotographic system. In recent years, an intermediate transfer tandem system in which four color image forming units are arranged side by side on an intermediate transfer belt has become the mainstream because of the advantage of being adaptable to various types of sheets P and excellent print productivity. This embodiment also employs an intermediate transfer tandem method.

  The sheets P are stored in the form of being stacked in the sheet storages 61 to 65, and the sheets P are fed by the feeding units 61a to 65a in accordance with the image forming timing. The sheet P sent out by the feeding units 61a to 65a passes through a conveyance path 81 and the like, and is conveyed to a registration roller pair 76 that is a conveyance unit before transfer.

  The registration roller pair 76 has a function of correcting skew by following the leading edge of the sheet P by creating a loop by abutting the sheet P conveyed from the sheet storage 61-65. In addition, the registration roller pair 76 applies the secondary sheet P at a predetermined timing in accordance with the timing of image formation on the sheet P, that is, the toner image carried on the image carrier (here, the intermediate transfer belt). It has a function of conveying to the transfer unit. The registration roller pair 76 feeds the sheet P to the secondary transfer portion at a desired timing after correcting the skew feeding.

  The secondary transfer portion is a toner image transfer nip portion to the sheet P formed by the opposing secondary transfer inner roller 32 and secondary transfer outer roller 41, and applies a predetermined pressure and electrostatic load bias. Thus, the toner image is transferred onto the sheet P.

  A process for forming an image sent to the secondary transfer unit at the same timing as the conveyance process of the sheet P to the secondary transfer unit described above will be described. An image forming unit that forms an image on a sheet includes a photoconductor and a process unit that acts on the photoconductor. Here, each image forming unit mainly includes a photoreceptor 11 (11Y, 11M, 11C, 11K), a charging device 12 (12Y, 12M, 12C, 12K), an exposure device 13 (13Y, 13M, 13C, 13K), The developing device 14 (14Y, 14M, 14C, 14K), the primary transfer device 35 (35Y, 35M, 35C, 35K), and the photoreceptor cleaner 15 (15Y, 15M, 15C, 15K) are configured.

  The surface of the photoconductor 11 is uniformly charged in advance by the charging means 12 and the exposure device 13 is driven on the rotating photoconductor 11 based on the image information signal sent to form a latent image. The electrostatic latent image formed on the photoconductor 11 is manifested as a toner image on the photoconductor 11 through toner development by the developing device 14.

  Thereafter, a predetermined pressure and an electrostatic load bias are applied by the primary transfer device 35, and the toner image is transferred onto the intermediate transfer belt 31. Thereafter, the transfer residual toner slightly remaining on the photoconductor 11 is collected by the photoconductor cleaner 15 to prepare for the next image formation again.

  In the case of FIG. 1, there are four sets of image forming units described above: yellow (Y), magenta (M), cyan (C), and black (Bk). Of course, the number of colors is not limited to four, and the color arrangement order is not limited to this.

  Next, the intermediate transfer belt 31 will be described. The intermediate transfer belt 31 is stretched by stretching rollers such as a driving roller 33, a tension roller 34, and a secondary transfer inner roller 32, and is conveyed and driven in the direction of arrow B in the drawing. The image forming process of each color processed in parallel by each of the Y, M, C, and Bk image forming units described above is performed at the timing of superimposing on the upstream color toner image that is primarily transferred onto the intermediate transfer belt 31. . As a result, a full-color superimposed toner image is finally formed on the intermediate transfer belt 31 and conveyed to the secondary transfer unit. As described above, the full-color toner image is secondarily transferred onto the sheet P in the secondary transfer portion by the sheet P conveying process and the image forming process described above.

  Thereafter, the sheet P is transported to the fixing device 5 by the suction transport means 42. The suction conveyance means 42 conveys the sheet P by air suction using a fan or the like. The fixing device 5 melts and fixes the toner image on the sheet P by applying a predetermined pressing force by an opposing roller or belt or the like and generally a heating effect by a heat source such as a heater.

  The sheet P having a fixed image obtained in this way is to be conveyed to either the discharge conveyance path 82 for discharging it onto the discharge tray 66 as it is, or to the reversal induction path 83 when double-sided image formation is required. Route selection is performed.

  When double-sided image formation is required, the sheet P is drawn from the reversal guide path 83 to the switchback path 84, and the rotation direction of the second reversing roller pair 79 is rotated forward and backward (switchback operation), thereby leading and trailing edges. Are transferred to the double-sided conveyance path 85.

  In order to improve productivity, the sheet P conveyed to the double-sided conveyance path 85 is increased in sheet conveyance speed based on a signal from the sheet position detection sensor 207. Thereafter, the sheets are rejoined at the same timing as the sheet P of the succeeding job conveyed from each of the feeding units, and are similarly fed to the secondary transfer unit via the registration roller pair 76.

  The image forming process on the back surface (second surface) is the same as that of the above-described front surface (first surface), and thus description thereof is omitted.

  Further, when the sheet P is reversed and discharged, after the sheet P passes through the fixing device 5, the sheet P is drawn from the reverse guiding path 83 to the switchback path 84, and then the first reverse roller pair 78 and the second reverse roller pair. Due to the reverse rotation of 79, the rear end at the time of feeding is set at the top, the sheet is withdrawn in the direction opposite to the direction of feeding, and is discharged to the discharge tray 66.

  Here, reference numeral 194 denotes a color sensor unit in the image forming apparatus according to the present embodiment. 2A and 2B are perspective views of the color sensor unit 194 according to the present embodiment. In the color sensor unit 194, a color sensor 301 serving as a density detecting unit is disposed on the upper side in the vertical direction with respect to the conveyance path, and a backup roller 302 serving as a pressing member that presses the sheet against the color sensor 301 is disposed on the lower side. The backup roller 302 is rotationally driven by a drive motor 303. Note that the backup roller 302 is a urethane dumpling roller having a diameter of 24 mm, and at least more than the number of color sensors 301 are arranged.

  Further, the backup roller 302 is provided with a pressing function with respect to the double-sided conveyance path upper guide 85a, and is driven by an operation of pressing or separating by a solenoid 304 as a driving unit. The backup roller 302 is provided facing the color sensor 301, and is separated from the color sensor 301 and a pressing position for pressing the sheet on which the density patch for detecting the image density is formed by the image forming unit to the color sensor 301. Move between the separated positions. The backup roller 302 is set to a separation position during normal paper passing, and a pressing position is set only in a printer pascal mode described later.

(Basic explanation of image correction operation by color sensor)
Next, the image correction operation by the color sensor will be described. FIG. 3A is a diagram for explaining color detection of the color sensor 301 as the density detection unit according to this embodiment, and FIG. 3B is a configuration of the color sensor 301 according to this embodiment and its surroundings. FIG.

  As shown in FIG. 3A, the color sensor 301 includes a white LED as the light emitting element 401, and uses a charge storage type sensor with an on-chip filter of three or more colors such as RGB as the light receiving element 403. The light from the light emitting element 401 is incident at an angle of 45 degrees with respect to the sheet P on which the patch T after fixing is formed, and the irregularly reflected light intensity in the vertical direction with respect to the sheet P is detected by the light receiving element 403. The light receiving unit 406 of the charge storage sensor with an RGB on-chip filter as the light receiving element 403 is a pixel in which RGB is independent. The light receiving element 403 is not limited to the above-described sensor, and may be a photodiode, for example. Further, a plurality of sets of three pixels of RGB may be arranged. Further, a configuration in which the incident angle is 0 degree and the reflection angle is 45 degrees may be employed. Furthermore, the color sensor as the density detecting means may be configured by LEDs (light emitting elements) that emit light of three or more colors such as RGB and sensors without filters (light receiving elements).

  As shown in FIG. 3B, the color sensor 301 and its peripheral devices include a light emitting element 401, a light receiving element 403, an A / D converter 404, and a computer 405. The light emitting element 401 is a light source of the color sensor 301 and makes light incident on the measuring object 402 (here, a sheet). This light is reflected by the measurement object, and the reflected light enters the light receiving element 403 that converts the light into an analog electric signal. Further, the analog electric signal is converted into a digital electric signal by the A / D converter 404. This digital electrical signal is captured by the computer 405, and calculations for color detection and correction are performed.

(Detailed explanation of color sensor unit and density correction patch)
Normally, image correction is not necessary if an expected image is formed. However, due to changes in environmental conditions such as temperature and humidity, the density on the sheet may change even when images are formed with the same image data. At this time, it is possible to output a more stable image by detecting the density of the image formed on the sheet by the color sensor and correcting the image data. FIG. 7 is a perspective view showing the relationship between the color sensor unit according to this embodiment and the image patch on the sheet.

  On the sheet P, the density detection patches S1 to S8 created by the image forming unit are transferred by the secondary transfer unit and then fixed by the fixing unit. The density detection patches S1 to S8 are arranged at a constant interval Ls in the sheet conveyance direction, and each patch has a different color or density. The color sensor 301 detects irregularly reflected light from the patches S1 to S8 formed on the sheet P. A stable image can be output by correcting the image data.

(Explanation of printer pascal sequence using color sensor)
FIG. 4 is a block diagram showing a partial configuration of the electrophotographic color image forming apparatus according to the present embodiment. This color image forming apparatus includes an image processing unit 200 and a control unit 211.

  First, processing in the image processing unit 200 will be described. The color matching table 201 converts an RGB signal representing an image color sent from a host such as a personal computer into a device RGB signal (hereinafter referred to as DevRGB) that matches the color reproduction range of the color image forming apparatus. The color separation table 202 converts the converted DevRGB signal into a CMYK signal that is a color material color such as toner in the control unit 211. The correction table 203 is a table for correcting density-gradation characteristics specific to the control unit 211, and converts the CMYK signal into a C′M′Y′K ′ signal obtained by correcting the density-gradation characteristics. Furthermore, a PWM (Pulse Width Modulation) table 204 converts the C′M′Y′K ′ signal into exposure times Tc, Tm, Ty, Tk by the exposure unit 205 in the control unit 211.

  Next, the control unit 211 will be described. The main parts relating to image formation here include a backup roller drive motor 303, a sheet position detection sensor 207, a color sensor 301, an exposure unit 205, and an operation unit 206. Further, including the double-sided upstream drive motor 305 and the computer 405, these are controlled and executed by the control means 210.

  Next, the printer Pascal sequence using the color sensor 301 will be described in detail. FIG. 5 is a flowchart of printer Pascal using the color sensor 301 according to the present embodiment. Image density correction is performed by performing the following printer pascal at the timing when the user needs to perform density adjustment in the market and changing the image forming conditions. Here, the printer Pascal is a method for performing image density correction using the color sensor 301.

  First, the printer Pascal is started from the operation unit 206 of the image forming apparatus or from the computer 405 connected to the image forming apparatus directly or via a network (S601). At this time, the sheet P to be used is plain paper or coated paper having a basis weight of 64 g to 300 g of A3 size or more, and is an arbitrary sheet that the user wants to correct. Each sheet P is fed from the feeding unit 65.

  After the feeding operation of the sheet P is started, the sheet P passes through the secondary transfer unit, the fixing device 5 where the secondary transfer inner roller 32 and the secondary transfer outer roller 41 face each other, and the switchback path 84, both sides Guided to the transport path 85. At this time, when the sheet P passes through the secondary transfer portion, a color gradation patch which is a density patch for image density detection formed in the image forming portion is transferred. The sheet P transported to the duplex transport path 85 is transported by the duplex upstream drive rollers 306a and 306b serving as transport means, and is detected by the sheet position detection sensor 207 (S602). The double-sided upstream drive rollers 306a and 306b are driven by a double-sided upstream drive motor 305.

  Thereafter, the sheet P is conveyed by a predetermined amount after the downstream end in the conveyance direction is detected by the sheet position detection sensor 207. Here, the double-sided upstream drive motor 305 stops at the position where the leading end position (downstream end in the transport direction) of the sheet P has passed a predetermined amount from the detection position of the color sensor 301 (for example, the position where 10 mm has passed), and the double-sided upstream drive roller 306a, 306b stops (S603). Simultaneously with the temporary stop timing of the conveyance of the sheet P, the solenoid 304 for driving the backup roller 302 is turned on, and the backup roller 302 is urged against the sheet P (S604). As a result, the backup roller 302 moves to the pressing position and presses the sheet P against the color sensor 301. Then, after the solenoid 304 is turned on, the backup roller drive motor 303 and the duplex upstream drive motor 305 are driven in 500 ms, and the backup roller 302 and the duplex upstream drive rollers 306a and 306b resume conveyance (S605).

  Thereafter, the sheet P is nipped and conveyed also by double-sided downstream drive rollers 308a and 308b as downstream conveying means. The double-sided downstream drive rollers 308 a and 308 b are driven by a double-sided downstream drive motor 307. The sheet conveyance speed at this time is 348 mm / s, and this speed is determined by the length of the color gradation patch, the time required for one reading of the color sensor 301, and the number of times the patch is read.

  For example, if the length of one patch in the conveyance direction is 13.5 mm, the time required for one reading of the color sensor 301 is 3.88 ms, and the number of times of patch reading is 10, the conveyance speed is 348 mm / s. Is the upper limit. If the speed is higher than this, patch reading cannot be performed 10 times, and the density correction accuracy is lowered.

  The density detection by the color sensor 301 is started after a predetermined time has elapsed after the backup roller drive motor 303 starts driving (S606). After the density detection by the color sensor 301 has been completed, the sheet P passes again through the registration roller pair 76, the secondary transfer inner roller 32 and the secondary transfer outer roller 41 corresponding to the corresponding secondary transfer unit, and the fixing device 5, and is discharged to the discharge tray 66. It is discharged (S607).

  If all the colors and image patterns necessary for density correction have been completed, the printer pascal is completed (S610). On the other hand, if it has not been completed yet, in order to detect the density of the next color or image pattern, the passing of the next sheet P is started (S609), and the density detection process from (S602) is repeated again. .

  As described above, the difference between the RGB signal representing the detected image density and the correct RGB signal measured by a densitometer or the like is compared in the image processing unit 200, and finally the exposure time Tc, By changing Tm, Ty, and Tk, the image forming conditions are changed so that the correct density value is obtained.

  Finally, the effect of using the proposed printer Pascal sequence is shown. FIG. 6 is a diagram comparing the conventional sequence and the sequence of the present embodiment with respect to the patch readable area. FIG. 6A is a schematic diagram showing a patch readable area of the present embodiment, and FIG. 6B is a schematic diagram showing a conventional patch readable area.

  In FIG. 6, the readable area (B) is a state where the sheet maintains a stable posture and indicates an area where there is almost no density detection error by the color sensor 301. The unreadable area (A) indicates an area that is not suitable for performing density detection because the sheet is not in a stable posture and a density detection error is generated by the color sensor 301.

  FIG. 6A shows the sequence of this embodiment. When the backup roller 302 is pressed against the conveyed sheet P, the conveyance of the sheet P is temporarily stopped to allow time for vibrations associated with the pressing operation to be sufficiently reduced. Therefore, the sheet after the pressing operation becomes a readable area (B). On the other hand, the area from the leading edge of the sheet P to the detection position of the color sensor 301 is an unreadable area (A).

  FIG. 6B shows a conventional sequence. The pressing operation of the backup roller 302 is performed on the conveyed sheet P without stopping. For this reason, after the pressing operation is performed, the vibration associated with the pressing operation remains for several hundreds of milliseconds, so that the unreadable area (A) becomes large. After the vibration is reduced and the posture of the sheet is stabilized, it becomes the readable area (B).

  The difference about the above readable area (B) is specifically shown by a numerical value. The conditions are a conveyance speed of 348 mm / s at the time of reading by the color sensor 301, a length of 420 mm in the sheet conveyance direction, a non-patch area of 20 mm at the rear end of the sheet, 26 patches, and a stop position of 5 mm at the front end of the sheet. Furthermore, in the conventional sequence, it takes about 0.5 s from the pressing operation of the backup roller 302 until the posture of the sheet is stabilized.

  Based on the above, the readable area (B) is calculated to be 226 mm under the conventional conditions, and 395 mm under the configuration of the present embodiment. When this is converted into a sub-scanning direction distance of one patch, it is 8.7 mm under the conventional conditions and 15.2 mm according to the configuration of the present embodiment. Here, for example, when a thin horizontal streak of about 5 mm is included on the patch, the horizontal streak area for a certain patch is about 57% in the conventional configuration and 33% in the configuration of the present embodiment, which is halved. Can do. As a result, errors during density correction can be reduced.

  In the present embodiment, the means for temporarily stopping the conveyance of the sheet P conveyed to the color sensor 301 is exemplified, but the present invention is not limited to this. For example, the conveyance speed of the sheet P conveyed before the leading edge of the sheet P reaches the detection position of the color sensor 301 is decelerated, and the backup roller 302 is reached at the timing when the leading edge of the sheet P reaches the detection position of the color sensor 301. The pressing operation may be performed. Thereafter, by increasing the conveyance speed of the sheet P and performing the reading operation, the readable area (B) can be increased similarly to the above-described form, and the same effect can be obtained. For example, the conveyance speed is reduced from 348 mm / s to 100 mm / s when the leading edge of the sheet P reaches a position 10 mm before the detection position of the color sensor 301. The pressing operation of the backup roller 302 is performed, and after the vibration of the sheet P is settled, the conveyance speed is accelerated to 308 mm / s. By setting the speed to 308 mm / s without accelerating to 348 mm / s, the predetermined number of times of reading the patch can be secured. After performing the pressing operation in this way, vibrations accompanying the pressing operation remain for several hundreds of milliseconds. However, since the pressing operation is performed with the conveyance speed of the sheet P being reduced, a sufficiently readable area (B) is secured. It becomes possible to do. On the other hand, production is not reduced by increasing the conveyance speed of the sheet P and performing the reading operation.

  As described above, according to the present embodiment, the sheet is temporarily stopped before the image density detection using the color sensor 301, or the sheet conveyance speed is reduced to a certain speed, and the backup roller 302 is moved to the pressing position. After the moving operation is performed and the sheet is pressed against the color sensor 301, the conveyance is started again. Accordingly, the sheet conveyance distance necessary for the surface to be measured of the sheet P to be stabilized in the vertical direction at the detection position by the color sensor 301 can be shortened, and a sufficient color gradation patch forming area can be secured. For this reason, even when there is a local horizontal streak image or the like in the patch, it is possible to reduce the reading accuracy of the color sensor 301. As a result, erroneous detection associated with the pressing operation of the backup roller 302 when reading a patch formed on the sheet can be suppressed, and density correction accuracy can be improved.

  In the above-described embodiment, the configuration in which the double-sided upstream drive rollers 306a and 306b are temporarily stopped after the leading end position (downstream end in the conveyance direction) of the sheet P has passed a predetermined amount from the detection position of the color sensor 301 is exemplified. It is not limited to this. For example, a configuration using double-sided downstream driving rollers 308a and 308b as the downstream conveying unit provided downstream of the color sensor unit 194 in the sheet conveying direction may be used. Specifically, after the leading end position (downstream end in the conveyance direction) of the sheet P passes a predetermined amount of the detection position of the color sensor 301 and reaches the duplex downstream drive rollers 308a and 308b, the duplex upstream drive rollers 306a and 306b are temporarily moved. It may be configured to stop. Then, after the operation of moving the backup roller 302 to the pressing position is performed and the sheet P is pressed against the color sensor 301, the conveyance of the sheet by the duplex downstream driving rollers 308 a and 308 b is started, and the image density by the color sensor 301 is started. Perform detection. Also with this configuration, the same effects as those of the above-described embodiment can be obtained.

  In the above-described embodiment, four image forming units are used for color image formation. However, the number used is not limited, and may be set as necessary.

  In the above-described embodiment, as a process cartridge that is detachable from the image forming apparatus, a process cartridge that integrally includes a photosensitive drum and a charging unit, a developing unit, and a cleaning unit as process units that act on the photosensitive drum. Was illustrated. However, the present invention is not limited to this. For example, in addition to the photosensitive drum, a process cartridge that integrally includes any one of a charging unit, a developing unit, and a cleaning unit may be used.

  Further, in the above-described embodiment, the configuration in which the process cartridge including the photosensitive drum is detachable from the image forming apparatus is illustrated. However, the present invention is not limited to this. For example, image formation in which each component is incorporated. It is also possible to use an image forming apparatus in which the apparatus or each component is detachable.

  In the above-described embodiment, the printer is exemplified as the image forming apparatus, but the present invention is not limited to this. For example, the image forming apparatus may be another image forming apparatus such as a copying machine or a facsimile machine, or another image forming apparatus such as a multi-function machine combining these functions. Further, the present invention is not limited to the above-described image forming apparatus using the intermediate transfer member, and a transfer material carrier is used, and toner images of respective colors are sequentially superimposed and transferred onto the transfer material carried on the transfer material carrier. An image forming apparatus may be used. The same effect can be obtained by applying the present invention to an image density detection unit used in these image forming apparatuses.

P ... Sheets S1 to S8 ... Density detection patch T ... Patch 5 ... Fixing device 81 ... Conveyance path 82 ... Discharge conveyance path 83 ... Reverse guidance path 84 ... Switchback path 85 ... Double-sided conveyance path 194 ... Color sensor unit 207 ... Sheet Position detection sensor 210 ... Control means 211 ... Control unit 301 ... Color sensor 302 ... Backup roller 303 ... Drive motor 304 ... Solenoids 306a, 306b ... Double-sided upstream drive roller 307 ... Double-sided downstream drive motor 308a, 308b ...

Claims (3)

An image forming unit for forming an image on a sheet;
Conveying means for conveying an image-formed sheet;
Density detecting means for detecting the image density of the conveyed sheet;
Between a pressing position for pressing the sheet on which the density patch for detecting the image density is formed by the image forming unit and pressing the sheet on the density detecting unit, and a separated position separated from the density detecting unit. A pressing member that moves,
Control means for controlling the operation of the conveying means and the pressing member;
With
The control unit performs an operation of temporarily stopping the conveyance of the sheet by the conveyance unit and moving the pressing member to the pressing position after the downstream end in the conveyance direction of the sheet has passed a predetermined amount of the detection position by the density detection unit. An image forming apparatus, wherein the sheet is transported by the transport unit while the sheet is pressed against the density detecting unit, and the sheet is transported while the image density is detected by the density detecting unit. Provided downstream of the density detection means in the sheet conveyance direction, and having a downstream conveyance means for conveying the image-formed sheet;
The control unit temporarily stops the conveyance of the sheet by the conveyance unit after the downstream end in the conveyance direction of the sheet has passed the detection position by the density detection unit by a predetermined amount and reaches the downstream conveyance unit. In a state where the sheet is moved to the pressing position and the sheet is pressed against the density detection unit, the sheet conveyance by the downstream conveyance unit is started, and the sheet is conveyed while performing the image density detection by the density detection unit. The image forming apparatus according to claim 1, wherein: An image forming unit for forming an image on a sheet;
Conveying means for conveying an image-formed sheet;
Density detecting means for detecting the image density of the conveyed sheet;
A pressing member that is provided opposite to the density detection unit and moves between a pressing position that presses the sheet against the density detection unit and a separation position that is separated from the density detection unit;
Control means for controlling the operation of the conveying means and the pressing member;
With
The control unit decelerates the sheet conveyance speed by the conveyance unit before the downstream end of the sheet conveyance direction reaches the detection position by the density detection unit, and the downstream end of the sheet conveyance direction is detected by the density detection unit. After the operation of moving the pressing member to the pressing position at the timing of reaching the position and pressing the sheet against the density detecting means, the sheet conveying speed by the conveying means is increased, and the image density by the density detecting means is increased. An image forming apparatus that conveys a sheet while performing detection.

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