JP4974111B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus using electrophotography, and more particularly to a method for correcting the density and color misregistration of an output image in a tandem color image forming apparatus.

  In a color image forming apparatus using an electrophotographic process, when a mode for appropriately setting image density and registration (hereinafter referred to as calibration mode) is set, the toner is directly transferred onto the toner carrier. Then, a patch image (reference image) is formed, and the toner amount and the deviation amount from the reference position are detected to correct the density and color deviation. For example, in the case of a tandem type full-color image forming apparatus, a reference image for correction of each color is formed on the conveyance belt or the intermediate transfer belt by the image forming units of yellow, cyan, magenta, and black, and the density and position of the reference image are detected by the detection unit. Is detected and density and color misregistration correction is performed.

  Examples of the method for adjusting the image density include a method of adjusting the charging potential of the photosensitive member, the developing bias potential, or the exposure amount by the exposure unit based on the detected image density. The characteristic value of the developing bias is adjusted. For example, in the case of using a developing bias in which an AC bias is superimposed on a DC bias, a DC component voltage (Vdc), an AC component peak-to-peak value (Vpp), and a positive waveform with respect to one cycle of the AC waveform are used. Either the time ratio (Duty ratio) or the frequency (f) is changed.

  However, the conveyance belt or the intermediate transfer belt on which the reference image is formed is formed of a dielectric resin or an elastic body, and the toner density cannot be detected with high sensitivity because the surface light reflectance is generally small. Further, there has been a problem that the smoothness of the belt surface is reduced in whole or in part due to the rubbing of the cleaning member, the transfer roller, and the like, and the light reflectance is further reduced and varies. This decrease in detection accuracy also occurs in the detection of a reference image for color misregistration correction, but it has been a problem in the detection of a reference image for density correction that requires highly accurate detection.

In view of this, a method for improving the accuracy of density and color misregistration correction has been proposed. For example, Patent Document 1 performs image density correction prior to registration misregistration (color misregistration) correction, and feeds back the result of density correction to color An image forming apparatus is disclosed in which detection accuracy of a color misregistration measurement pattern image (reference image) is improved by performing misregistration correction. Further, in Patent Document 2, by accurately matching the measurement position of the light reflectance of the belt surface with the formation position of the density correction image, the influence of noise on the belt surface is removed, and accurate density correction is always performed. An image forming apparatus that can be executed is disclosed.
JP 2003-162123 A JP 2005-338673 A

  However, in the method of Patent Document 1, since density correction and color misregistration correction are continuously performed, there is a problem in that it takes time for calibration and a user's print waiting time becomes long. Further, since the density measurement pattern image is formed on the belt surface, the toner density cannot be detected with high sensitivity, and the reduction in detection accuracy and the variation cannot be eliminated. Further, although the method of Patent Document 2 can reduce the influence of noise on the belt surface to some extent, the accuracy of density correction is not sufficiently satisfactory.

  As a method of performing density correction with high accuracy, a method of measuring density on a photoconductor having excellent surface smoothness before transferring a density correction reference image formed on the surface of the photoconductor onto the belt can be considered. . According to this method, density detection with high accuracy is possible regardless of the surface state of the belt, and density correction can be performed accurately and stably. Further, in the tandem color image forming apparatus in which the belt image forming unit is provided for each color, it is possible to form the reference image and measure the density at the timing of each color, so that the correction time can be shortened.

  In this case, the total correction time can be further shortened by forming the reference image for density correction and the reference image for color misregistration correction at the same timing and performing density correction and color misregistration correction in parallel. Here, in the case of color misregistration correction, it is necessary to detect a misregistration amount by transferring a standard image for color misregistration correction of each color onto a belt. For this reason, the density correction reference image after density detection is also transferred onto the belt together with the color misregistration correction reference image, but the density correction and color misregistration correction reference images are simultaneously removed from the belt. It is desirable to reduce the cleaning time.

  Normally, the reference image for density correction is composed of patch images formed in multiple stages from low density to high density, and in order to accurately detect the density at each stage, the patch image of each density is rotated in the drum rotation direction. It is necessary to form a predetermined size. For this reason, it is inevitable that the reference image for density correction becomes somewhat long, and if it is attempted to transfer to a predetermined range on the belt in order to shorten the cleaning time, the reference for density correction for each color when transferred onto the belt. The images overlap.

  However, since the high density reference image has a larger toner adhesion amount (toner thickness) per unit area than the normal image forming conditions, when the high density reference images overlap, the toner adhesion amount further increases, A belt cleaning device such as a cleaning blade may not be completely removed at once.

  In view of the above problems, the present invention provides a tandem type color image forming apparatus capable of performing density correction at the time of calibration execution with high accuracy and in a short time and easily cleaning a reference image transferred onto a belt. The purpose is to do.

  To achieve the above object, the present invention provides a plurality of image forming units including a photoconductor and a developing device, an intermediate transfer belt on which toner images developed on the photoconductor are sequentially laminated, and the photoconductor A transfer means for laminating the toner image developed on the intermediate transfer belt; a secondary transfer means for transferring the toner image laminated on the intermediate transfer belt onto a recording medium at a time; and on the photoreceptor. First detection means for detecting the toner adhesion amount of the formed density correction reference image, and second detection for detecting the color misregistration amount of the color misregistration correction reference image transferred onto the intermediate transfer belt by the transfer means. And a control unit that performs density and color misregistration correction in parallel based on the detection results of the first and second detection units, wherein the reference image for density correction is the photoconductor. Formed in the direction of rotation The density correction for each color is made up of a plurality of patch images having different degrees, and after the toner adhesion amount is detected by the first detection means, the toner amount per unit area on the intermediate transfer belt does not exceed a predetermined amount. The reference image for transfer is superimposed and transferred in the belt traveling direction.

  The present invention also provides a plurality of image forming units including a photosensitive member and a developing device, a conveying belt for conveying a recording medium to the image forming unit, and a toner image developed on the photosensitive member on the recording medium or the recording medium. Transfer means for transferring onto the conveying belt, first detecting means for detecting the toner adhesion amount of the density correction reference image formed on the photosensitive member, and color misregistration transferred onto the conveying belt by the transferring means Image formation comprising: a second detection unit that detects a color misregistration amount of the reference image for correction; and a control unit that performs density and color misregistration correction in parallel based on the detection results of the first and second detection units. In the apparatus, the reference image for density correction is composed of a plurality of patch images having different densities formed in the rotation direction of the photoconductor, and after the toner adhesion amount is detected by the first detection unit, Simple in Toner amount per unit area is characterized by transfer by superimposing the density correction reference images of the respective colors so as not to exceed a predetermined amount in the direction of belt travel.

  According to the present invention, in the image forming apparatus having the above-described configuration, the density correction reference image has a density that gradually decreases from the highest density patch image from the front end to the rear end, or the lowest density patch image. It is characterized in that it is formed so as to increase in concentration stepwise.

  According to the first configuration of the present invention, by forming a reference image for density correction on a photoconductor excellent in surface smoothness, it is possible to detect density with high accuracy regardless of the surface state of the intermediate transfer belt. Thus, the density correction can be performed accurately and stably. Also, the density correction reference image transferred onto the intermediate transfer belt is transferred by superimposing and transferring the detected density correction reference image on the intermediate transfer belt so that the toner amount per unit area does not exceed a predetermined amount. While shortening the cleaning time by minimizing the total length of the image, it is possible to prevent poor cleaning of the intermediate transfer belt.

  Further, according to the second configuration of the present invention, the density correction reference image is formed on the photoreceptor having excellent surface smoothness, so that the density detection can be performed with high accuracy regardless of the surface state of the conveyor belt. Therefore, the density correction can be performed accurately and stably. Also, the density correction reference image transferred onto the conveyance belt is transferred by superimposing and transferring the detected density correction reference image on the intermediate transfer belt so that the toner amount per unit area does not exceed a predetermined amount. The overall length of the belt can be minimized and the cleaning time can be shortened.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, the density gradually decreases from the highest density patch image from the front end to the rear end, or By forming the density correction reference image so that the density gradually increases from the lowest density patch image, it is possible to easily set the toner thickness of the overlapping portion of the reference image. Further, when comparing the toner density calculated from the output value of the first detection means with the standard density, or when correcting the density by changing each parameter value such as the charge amount, the development bias characteristic value, and the exposure amount. Since it is easy to associate, the calibration control can be simplified.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of a tandem color image forming apparatus according to the first embodiment of the present invention. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (yellow, cyan, magenta, and black), and yellow, cyan, magenta, and the like are respectively performed by charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  Photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and are formed on the photosensitive drums 1a to 1d. The toner images thus transferred are sequentially transferred (primary transfer) onto an intermediate transfer belt 8 that moves adjacent to each image forming unit while rotating clockwise in FIG. 1 by a driving means (not shown). The image is transferred onto the paper P at a time (secondary transfer) by the next transfer roller 9 and further fixed on the paper P by the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The paper P onto which the toner image is transferred is housed in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. Further, a cleaning blade 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure unit 4 for exposing the toner, the developing units 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided.

  When the image formation start is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure unit 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. Each of the developing units 3a to 3d includes a developing roller (developer carrying member) disposed so as to face the photosensitive drums 1a to 1d, and each of yellow, cyan, magenta, and black toners is supplied by a replenishing device (not shown). A predetermined amount is filled. This toner is supplied onto the photosensitive drums 1a to 1d by the developing rollers of the developing units 3a to 3d, and electrostatically adheres to the electrostatic latent image formed by exposure from the exposure unit 4. A toner image is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, yellow, cyan, magenta and black toner images on the photosensitive drums 1a to 1d are transferred onto the intermediate transfer belt 8 by the transfer rollers 6a to 6d. Primary transcription. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched over a driven roller 10, a drive roller 11, and a tension roller 20, and the intermediate transfer belt 8 starts to rotate clockwise as the drive roller 11 is rotated by a drive motor (not shown). Then, the sheet P is conveyed from the registration roller 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the sheet P is conveyed at a nip portion (secondary transfer nip portion) with the intermediate transfer belt 8. A full color image is secondarily transferred onto P. The paper P on which the toner image is transferred is conveyed to the fixing unit 7.

  The sheet P conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13 to fix the toner image on the surface of the sheet P, and a predetermined full-color image is formed. It is formed. The paper P on which the full-color image is formed is distributed in the transport direction by the branching section 14 that branches in a plurality of directions. When an image is formed on only one side of the paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when forming images on both sides of the paper P, a part of the paper P that has passed through the fixing unit 7 is once projected from the discharge roller 15 to the outside of the apparatus. Thereafter, the paper P is distributed to the paper transport path 18 by the branching section 14 by rotating the discharge roller 15 in the reverse direction, and is transported again to the secondary transfer roller 9 with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the paper P on which the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image, It is discharged to the discharge tray 17.

  FIG. 2 is a side cross-sectional view showing a configuration around the intermediate transfer belt of the tandem type color image forming apparatus according to the first embodiment. Portions common to FIG. 1 are denoted by the same reference numerals and description thereof is omitted. In FIG. 2, the chargers 2a to 2d and the cleaning units 5a to 5d are not shown.

  Density detection sensors 21a to 21d are arranged on the downstream side of the developing units 3a to 3d and the upstream side of the transfer rollers 6a to 6d in the rotation direction of the photosensitive drums 1a to 1d. As the density detection sensor 21, an optical sensor including a light emitting element composed of an LED or the like and a light receiving element composed of a photodiode or the like is generally used. When measuring the toner adhesion amount on the photosensitive drums 1a to 1d, if the reference light formed on the photosensitive drums 1a to 1d is irradiated with measurement light from the light emitting elements, the measurement light is reflected by the toner. And incident on the light receiving element as light reflected by the drum surface.

  When the toner adhesion amount is large, the reflected light from the drum surface is shielded by the toner, so that the light reception amount of the light receiving element decreases. On the other hand, when the toner adhesion amount is small, the amount of reflected light from the drum surface increases, resulting in an increase in the amount of light received by the light receiving element. Therefore, the density of each color is corrected by detecting the density of the reference image of each color based on the output value of the received light signal based on the amount of reflected light received and adjusting the characteristic value of the developing bias in comparison with the predetermined reference density. Is done.

  The density detection sensors 21a to 21d may be arranged at other positions where the reference images on the photosensitive drums 1a to 1d can be detected. For example, when the density detection sensors 21a to 21d are arranged downstream of the transfer rollers 6a to 6d, development is performed. The time from when the reference image is formed by the units 3a to 3d to when the density detection is performed becomes longer. Further, when the reference image comes into contact with the intermediate transfer belt 8, the surface state of the reference image may change. Therefore, as shown in FIG. 2, it is preferable to dispose downstream of the developing units 3a to 3d and upstream of the contact position of the intermediate transfer belt 8.

  The color misregistration detection sensor 23 is disposed on the downstream side of the photosensitive drum 1d disposed on the most downstream side in the traveling direction of the intermediate transfer belt 8 and on the upstream side of the secondary transfer roller 9, and the image forming unit. The position of a reference image for color misregistration correction formed in Pa to Pd and transferred onto the intermediate transfer belt 8 is detected. As the color misregistration detection sensor 23, a reflective optical sensor similar to the density detection sensors 21a to 21d is used, but other sensors can also be used. For example, a line sensor or an area sensor that is composed of a CCD element, a lens, a driver circuit, etc., forms an image of an object on the surface of the CCD element by a lens, converts the amount of light into a video pulse signal, and outputs it. Can be mentioned.

  The color misregistration detection sensor 23 may be disposed at another position where the reference image on the intermediate transfer belt 8 can be detected. For example, when the color misregistration detection sensor 23 is disposed on the downstream side of the secondary transfer roller 9, the intermediate transfer belt. There is a possibility that the time from when the reference image is transferred onto 8 until color misregistration detection is performed becomes longer, and when the reference image comes into contact with the secondary transfer roller 9, the surface state of the reference image may change. For this reason, it is preferable to dispose the image forming unit Pd in the vicinity of the most downstream side as shown in FIG. The density detection sensors 21a to 21d and the color misregistration detection sensor 23 transmit an output signal corresponding to the detection result to the control unit 32 described later.

  FIG. 3 is a block diagram illustrating a control path of the tandem color image forming apparatus according to the first embodiment. Portions common to FIGS. 1 and 2 are denoted by the same reference numerals and description thereof is omitted. The image forming apparatus 100 includes image forming units Pa to Pd provided with density detection sensors 21a to 21d, an image input unit 30, an AD conversion unit 31, a control unit 32, a storage unit 33, an operation panel 34, a fixing unit 7, and an intermediate transfer. The configuration includes the belt 8 and the color misregistration detection sensor 23.

  When the image forming apparatus 100 is a color copying machine, the image input unit 30 includes a scanning lamp mounted with a scanner lamp that illuminates the original during copying and a mirror that changes the optical path of reflected light from the original, and reflection from the original. 1 is an image reading unit that includes a condenser lens that collects light to form an image and a CCD that converts the formed image light into an electrical signal. In the case of a printer, the receiving unit receives image data transmitted from a personal computer or the like. The image signal input from the image input unit 30 is converted into a digital signal by the AD conversion unit 31 and then sent to the image memory 40 in the storage unit 33.

  The storage unit 33 includes an image memory 40, a RAM 41, and a ROM 42. The image memory 40 stores an image signal input from the image input unit 30 and digitally converted by the AD conversion unit 31, and is stored in the control unit 32. Send it out. The RAM 41 and the ROM 42 store a processing program, processing content, and the like of the control unit 32.

  Further, the RAM 41 (or ROM 42) stores the relationship between the output values of the density detection sensors 21a to 21d and the toner adhesion amount as toner adhesion amount data. The toner concentration determined from the toner adhesion amount and the charge amount are stored. A density correction table in which each parameter value used for density correction is stored in association with each other, such as a development bias characteristic value or an exposure amount, a color shift amount of each color image, and an exposure start timing or exposure start of the exposure unit 4 A color misregistration correction table stored in association with the position is stored.

  The parameter values used for density and color misregistration correction may be calculated each time using the output values of the density detection sensors 21a to 21d and the color misregistration detection sensor 23 in the control unit 32. In order to reduce the load, it is preferable to store the data in the RAM 41 (or ROM 42) in advance.

  The operation panel 34 includes an operation unit composed of a plurality of operation keys, and a display unit (none of which is shown) that displays setting conditions, the state of the apparatus, and the like, and the user sets printing conditions and the like. In addition, for example, when the image forming apparatus 100 has a facsimile function, it is also used for various settings such as registering a facsimile transmission destination in the storage unit 33 and reading or rewriting the registered transmission destination.

  The control unit 32 is, for example, a central processing unit (CPU), and according to a set program, the image input unit 30, the image forming units Pa to Pd, the fixing unit 7, and the sheet P from the sheet cassette 16 (see FIG. 1). In addition to overall control of conveyance and the like, the image signal input from the image input unit 30 is converted into image data by scaling processing or gradation processing as necessary. The exposure unit 4 irradiates a laser beam based on the processed image data, and forms latent images on the photosensitive drums 1a to 1d.

  Further, when the calibration mode is set by a key operation on the operation panel 34 or the like, the control unit 32 receives output signals from the density detection sensors 21 a to 21 d and the color misregistration detection sensor 23 and stores them in the storage unit 33. A function for calculating the toner adhesion amount and the color misregistration amount based on the toner adhesion amount data and the color misregistration data, and determining the density of the reference image based on the calculated toner adhesion amount and comparing it with a predetermined standard density Then, by adjusting at least one of the image forming conditions of the image forming units Pa to Pd, the function of performing density correction for each color, and the image forming timing of the image forming units Pa to Pd based on the calculated color shift amount It has a function of correcting color misregistration by adjusting. The calibration mode may be automatically set when the apparatus is turned on or when a predetermined number of image forming processes are completed.

  Next, density and color misregistration correction of this embodiment will be described. 4 and 5 are graphs showing examples of output waveforms when the density detection sensor detects a blank state (a state where toner is not placed) on the surface of the intermediate transfer belt and the surface of the photosensitive drum with a density detection sensor. In this example, a rubber layer is laminated on a belt substrate made of a dielectric resin, an intermediate transfer belt having a fluorine coating, and a photosensitive drum having an amorphous silicon photosensitive layer formed on the surface of an aluminum tube. The output value was measured at 1 ms intervals.

  Since the surface of the intermediate transfer belt has poor smoothness and the amount of reflected light is small, the output value of the density detection sensor is as low as 2.25 to 2.55 V as shown in FIG. Further, since the surface state is non-uniform and the reflected light varies depending on the location, a maximum variation of 0.3 V occurs. This variation in output value is considered to increase over time due to contamination or scratches by the toner additive.

  On the other hand, the surface of the photosensitive drum is higher than that of the intermediate transfer belt, and the amount of reflected light is larger. Therefore, as shown in FIG. 5, the output value of the density detection sensor is 3.5 to 3.55 V. It is high. Further, although the output waveform shows periodic fluctuations, the maximum variation is as small as 0.05V.

  Therefore, in this embodiment, the reference image for color misregistration correction is formed on the intermediate transfer belt 8, and the reference image for density correction is formed on the photosensitive drums 1a to 1d, so that each reference image is formed. It was decided that density and color misregistration correction should be performed in parallel. As a result, high-precision density detection is possible regardless of the surface state of the belt, and density correction can be performed accurately and stably over a long period of time. In color misregistration correction, it is only necessary to accurately detect the positional relationship between the reference images of the respective colors on the belt. Strict detection of the density difference such as density correction is unnecessary, so that detection is performed on the belt as usual. There is no particular problem.

  FIG. 6 is an example of a reference image for color misregistration correction. On the intermediate transfer belt 8, a reference image composed of diagonal lines of yellow (Y), cyan (C), magenta (M) and black (B) and horizontal lines Y, C, M and B is formed. The arrow X1 indicates the belt traveling direction. The patterns of the reference images Y to B shown in FIG. 6 are general, and color misregistration in the main scanning direction (belt width direction) is detected using diagonal lines and horizontal lines of each color, and sub-intervals are determined from the intervals between the horizontal lines of each color. Color misregistration in the scanning direction (belt circumferential direction) is detected.

  Further, the reference images Y to B are formed in the same pattern at both ends in the main scanning direction, so that the main scanning equality and the scanning inclination can be detected. The positional relationship between the oblique lines and straight lines of these colors is detected by the color misregistration detection sensor 23 and compared with a predetermined reference position. When correcting the color misregistration in the main scanning direction, the exposure start position of the exposure unit 4 is adjusted. When correcting the color misregistration in the sub-scanning direction, the color misregistration correction is performed for each color by adjusting the exposure start timing of the exposure unit 4.

  FIG. 7 is an example of a reference image for density correction. In the central portion in the longitudinal direction of the photosensitive drum 1a, a reference image y composed of patch images y1 to y10 having 10 levels of density from the lightest image (y1) to the darkest image (y10) is in the drum rotation direction ( Are formed in a line in order from the downstream side in the direction of arrow X2. Adjacent patch images are each formed in a single color so that the density changes at the boundary, and the density detection sensor 21a is disposed opposite to the position where the reference image y is formed.

  Here, the yellow reference image y formed on the photosensitive drum 1a has been described as an example, but the cyan, magenta, and black reference images c, m, and black formed on the photosensitive drums 1b to 1d are described. The configuration for b is exactly the same. In addition, the reference images yb can be formed not only in the central portion in the longitudinal direction of the photosensitive drums 1a-1d, but also at arbitrary positions.

  Next, density correction control in the image forming apparatus according to the first embodiment will be described. FIG. 8 is a flowchart illustrating the density correction procedure of the image forming apparatus according to the first embodiment. A calibration execution procedure will be described in accordance with the steps in FIG. 8 with reference to FIGS.

  When the calibration mode is executed and density and color misregistration correction is started, reference images y and b (see FIG. 7) for density correction are formed on the photosensitive drums 1a to 1d in the image forming units Pa to Pd. (Step S11). Further, reference images Y to B for color misregistration correction (see FIG. 6) are also simultaneously formed on the photosensitive drums 1a to 1d (step S12).

  Next, the toner detection amounts (toner concentrations) of the reference images y and b are detected by the density detection sensors 21a to 21d (step S21). The detected toner density is compared with the standard density in the control unit 32, and an average value of density differences between the toner density and the standard density is calculated. The reference images y and b whose toner density is detected are transferred to predetermined positions on the intermediate transfer belt 8 together with the reference images Y and B for color misregistration correction by the transfer rollers 6a to 6d. Next, the positional relationship between the reference images Y to B is detected by the color misregistration detection sensor 23 (step S22). The detected positional relationship is compared with the reference position in the control unit 32, and the color misregistration amount of each color is calculated.

  Then, the parameter value used for density correction is read from the density correction table in the storage unit 33 according to the average value of the density differences obtained in step S21, and the control unit 32 transmits a control signal for changing the parameter value. Then, density correction is executed (step S31). In addition, parameter values used for color misregistration correction are read out from the color misregistration correction table in the storage unit 33 according to the color misregistration amount of each color obtained in step S22, and the control unit 32 determines the exposure start position of the exposure unit 4 or By adjusting the exposure start timing, color misregistration correction is executed for each color (step S32). Thereafter, the reference image yb and the reference image YB on the intermediate transfer belt 8 are removed by the cleaning blade 19 (step S4), and the calibration is completed.

  By performing density and color misregistration correction according to the above procedure, the photosensitive drums 1a to 1d having excellent surface properties are used as the background of the reference images y to b. This is performed with good sensitivity and stability, and the density correction accuracy can be maintained at a high level. Further, since the density correction and the color misregistration correction are performed in parallel, the calibration time can be shortened.

  FIG. 9 is a plan view showing a state in which the reference images Y to B for color misregistration correction (see FIG. 6) and the reference images y to b for density correction (see FIG. 7) are transferred onto the intermediate transfer belt 8. FIG. 10 is a plan view showing the positional relationship between the reference images y and b of the respective colors in the belt traveling direction, and FIG. 10 is a side view of the reference images y and b shown in FIG. In the side views of the reference images yb in FIG. 10 and subsequent figures, the thickness direction is exaggerated, and the ratio between the length direction and the thickness direction is different from the actual reference image.

  As shown in FIG. 9, the reference images y and b for each color are formed so that the density gradually decreases from the highest density patch image from the downstream side (left side in FIG. 9) to the upstream side in the belt traveling direction. In the adjacent reference images yb, the patch images of a part of the rear end side and a part of the front end side (here, three stages) are transferred so as to overlap each other. That is, the reference images are formed on the photosensitive drums 1a to 1d at the timing when the reference images yb and YB are transferred in this way.

  Here, when the reference images y and b are transferred onto the intermediate transfer belt 8, the toner thickness (the amount of toner per unit area) is maximized as shown in FIG. The third patch image from the density side (patch image b3 in the reference image b) overlaps with the highest density patch image (patch image m10 in the reference image m) of the upstream reference image.

  In the present invention, the toner thickness of the overlapping portion (the sum of the toner thicknesses of the patch images b3 and m10) is set to be smaller than the maximum thickness that can be removed by the cleaning blade 19 (see FIG. 1). The reference image yb is removed at a time by the cleaning blade 19 while the total length L of the reference image yb transferred onto the intermediate transfer belt 8 is kept to a minimum (about 3/4 of the case where the superposition is not performed). It becomes possible. Accordingly, the size (detection length) of each patch image constituting the reference image is ensured to maintain detection accuracy, the time required for calibration can be further shortened, and the occurrence of defective cleaning can be reliably prevented. .

  FIG. 11 is a plan view showing another pattern in which the reference images Y to B for color misregistration correction and the reference images y to b for density correction are transferred onto the intermediate transfer belt 8, and the reference images y to b for the respective colors. FIG. 12 is a side view of each reference image yb shown in FIG. 11. In FIG. 11, the reference images y and b of each color are formed so that the density increases stepwise from the lowest density patch image from the downstream side (left side in FIG. 11) to the upstream side in the belt traveling direction. In the adjacent reference images y and b, the patch images of the rear end side and a part of the front end side (here, three stages) are transferred so as to overlap each other.

  In this case, when the reference images yb are transferred onto the intermediate transfer belt 8, the toner thickness (the amount of toner per unit area) is maximized as shown in FIG. The high-density patch image (patch image b10 for the reference image b) overlaps with the third patch image (patch image m3 for the reference image m) from the low-density side of the upstream reference image.

  The toner thickness of this overlapping portion (the sum of the toner thicknesses of the patch images b10 and m3) is smaller than the maximum thickness that can be removed by the cleaning blade 19. Thus, similarly to the pattern shown in FIG. 9, the total length L of the reference images y and b transferred onto the intermediate transfer belt 8 is suppressed to a minimum (about 3/4 in the case where the overlapping is not performed) The reference images yb can be removed at once by the cleaning blade 19.

  Further, the reference images y to b of the respective colors are not limited to the configuration in which the density changes stepwise from the downstream side to the upstream side in the belt traveling direction, and can be formed in other patterns. For example, as shown in FIG. 13, the most downstream (black) reference image b and the third (cyan) reference image c from the downstream side are formed so that the density gradually decreases from the downstream side to the upstream side. When the second (magenta) reference image m from the downstream side and the most upstream (yellow) reference image y are formed so that the density increases stepwise from the downstream side to the upstream side, the reference image b and the reference image When m is completely overlapped, the sum of the toner thicknesses of the reference image b and m becomes substantially constant, and the maximum toner thickness becomes smaller than those in FIGS. On the other hand, the same applies when the reference image c and the reference image y are superimposed.

  Therefore, the total length L of the reference images y and b is about twice the length of each reference image, and can be shortened as compared with FIGS. When the reference image m and the reference image c are overlapped, the patch images on the high density side overlap each other, so that the toner thickness may exceed the maximum thickness that can be removed by the cleaning blade 19. Therefore, it is preferable to secure a predetermined amount of the transfer interval on the intermediate transfer belt 8 by adjusting the formation timing of the reference image m and the reference image c.

  Alternatively, as shown in FIG. 14, the central portion of each reference image yb may have the highest density, and the density may gradually decrease toward both ends. In this case, even if half (five levels) of each of the reference images yb are overlapped, the sum of the toner thicknesses is substantially constant, and the maximum toner thickness is smaller than in the case of FIGS. Therefore, the total length L of the reference images y and b is about 2.5 times the length of each reference image, and can be shortened as compared with FIGS.

  In the configuration shown in FIG. 13, the reference images y to b for each color are not arranged in the same density order. In the configuration shown in FIG. 14, the patch images constituting the reference images y to b for each color are arranged in stages. Therefore, when comparing the toner density calculated from the output values of the density detection sensors 21a to 21d with the standard density, or changing the parameter values such as the charge amount, the development bias characteristic value, and the exposure amount, the density correction is performed. The correspondence when doing is complicated. Therefore, from the viewpoint of simplification of calibration control, as shown in FIGS. 9 to 12, the density is gradually decreased from the highest density patch image from the front end to the rear end, or the lowest density. It is preferable to form the reference images yb so that the density increases stepwise from the patch image.

  The area where the reference images y and b are overlapped may be appropriately set based on the toner amount (toner thickness) of each patch image and the cleaning performance of the cleaning blade 19. Since the reference images Y to B are transferred at a predetermined interval necessary for color misregistration correction, the transfer length L ′ (see FIG. 9) of the reference images Y to B cannot be shortened, and the reference images y to Even if the total length L of b is shorter than L ′, the time required for cleaning does not change.

  In the first embodiment, an intermediate transfer tandem that transfers a full-color image formed by sequentially laminating toner images of each color onto an intermediate transfer belt 8, which is an example of a toner carrier, onto a sheet P at a time. Although the present invention has been described with reference to the type color image forming apparatus, the present invention also applies to the direct transfer type tandem type color image forming apparatus that sequentially transfers the toner images of the respective colors onto the paper P carried and conveyed on the conveyance belt. Applicable.

  FIG. 15 is a schematic diagram showing the configuration of the image forming apparatus according to the second embodiment of the present invention, and FIG. 16 is a side view showing the configuration around the conveyance belt of the tandem color image forming apparatus according to the second embodiment. It is sectional drawing. In the present embodiment, instead of the intermediate transfer belt 8, a conveyance belt 50 that sequentially conveys the paper P to the image forming portions Pa to Pb is disposed so as to abut on the lower portions of the photosensitive drums 1a to 1d. In place of the unit 4, LED heads 4a to 4d for individually exposing the photosensitive drums 1a to 1d are provided. Since the configuration of other parts is exactly the same as that of the first embodiment, description thereof is omitted.

  Also in the present embodiment, the density correction reference images yb formed on the photosensitive drums 1a to 1d are detected using the density detection sensors 21a to 21d provided on the downstream side of the developing units 3a to 3d. As a result, the density correction can be executed accurately and stably over a long period of time. Further, by detecting the color misregistration correction reference images Y to B formed on the transport belt 50 using the color misregistration detection sensor 23, color misregistration correction can be performed in parallel with the density correction, and calibration. Execution time is reduced. Since the density and color misregistration correction method, the calibration execution procedure, and the transfer timing of the reference images y to b and Y to B on the transport belt 50 are exactly the same as those in the first embodiment, description thereof will be omitted.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the reference image pattern shown in the above embodiment is merely an example, and other patterns may be used.

  The present invention provides a plurality of image forming units including a photosensitive member and a developing device, an intermediate transfer belt on which toner images developed on the photosensitive member are sequentially stacked, a conveying belt for conveying a recording medium, and a photosensitive member on the photosensitive member. A transfer unit that transfers the developed toner image onto an intermediate transfer belt or a conveyance belt; and a first detection unit that detects a toner adhesion amount of a reference image formed on the photoreceptor; Second detection means for detecting the color misregistration amount of the reference image transferred onto the intermediate transfer belt or the conveyance belt by the transfer means, and density and color misregistration correction in parallel based on the detection results of the first and second detection means. In this case, the density correction reference image is composed of a plurality of patch images having different densities formed in the rotation direction of the photosensitive member. After the arrival amount is detected, the density correction reference image of each color is superimposed and transferred in the belt traveling direction so that the toner amount per unit area on the intermediate transfer belt or the conveyance belt does not exceed a predetermined amount. .

  This makes it possible to detect the density with high accuracy regardless of the surface condition of the belt, and furthermore, the density correction and the color misregistration correction can be performed in parallel. Thus, it is possible to provide a tandem type color image forming apparatus that can shorten the calibration time as much as possible and prevent the belt surface from being poorly cleaned.

  In addition, since the density correction reference image is formed so that the density gradually decreases from the highest density patch image from the leading edge to the trailing edge, or the density gradually increases from the lowest density patch image. Thus, the toner thickness of the overlapping portion of the reference image can be easily set, and the image forming apparatus can also simplify the calibration control.

FIG. 1 is a schematic diagram illustrating an overall configuration of a tandem color image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a side cross-sectional view showing a configuration around an intermediate transfer belt of the tandem type color image forming apparatus according to the first embodiment. FIG. 3 is a block diagram illustrating a control path of the tandem color image forming apparatus according to the first embodiment. FIG. 5 is a graph showing an output waveform when a blank state (a state where no toner is placed) on the surface of the intermediate transfer belt is detected by a density detection sensor for a predetermined time. These are graphs showing output waveforms when a blank state (a state where no toner is placed) on the surface of the photosensitive drum is detected for a predetermined time by the density detection sensor. FIG. 4 is a schematic diagram illustrating an example of a color misregistration correction reference image. FIG. 4 is a schematic diagram illustrating an example of a density correction reference image. FIG. 5 is a flowchart for explaining density and color misregistration correction control procedures in the image forming apparatus of the first embodiment. FIG. 6 is a plan view showing a state in which a color misregistration correction reference image and a density correction reference image are transferred onto an intermediate transfer belt, and a plan view showing a positional relationship of the density correction reference image in the belt traveling direction. FIG. 10 is a side view of the density correction reference image shown in FIG. 9. FIG. 6 is a plan view showing another pattern in which the color misregistration correction reference image and the density correction reference image are transferred onto the intermediate transfer belt, and a plan view showing the positional relationship of the density correction reference image in the belt traveling direction. . FIG. 12 is a side view of the density correction reference image shown in FIG. 11. FIG. 10 is a side view showing another overlay method of the density correction reference image. FIG. 10 is a side view showing another overlay method of the density correction reference image. These are the schematic diagrams which show the whole structure of the tandem type color image forming apparatus which concerns on 2nd Embodiment of this invention. These are side surface sectional drawings which show the structure around the conveyance belt of the tandem type color image forming apparatus according to the second embodiment.

Explanation of symbols

Pa to Pd Image forming section 1a to 1d Photosensitive drum 2a to 2d Charger 3a to 3d Developing unit (developing device)
4 Exposure unit 4a-4d LED head 6a-6d Transfer roller (transfer means)
7 Fixing Section 8 Intermediate Transfer Belt 9 Secondary Transfer Roller (Secondary Transfer Unit)
21a to 21d Concentration detection sensor (first detection means)
23 Color shift detection sensor (second detection means)
32 Control unit (control means)
33 Storage Unit 34 Operation Panel 50 Conveyor Belt 100 Image Forming Apparatus Y to B (for Color Misregistration Correction) Reference Image y to b (Density Correction) Reference Image

Claims (3)

A plurality of image forming units including a photoreceptor and a developing device;
An intermediate transfer belt on which the developed toner images are sequentially laminated on the photoreceptor;
Transfer means for laminating the toner image developed on the photoreceptor on the intermediate transfer belt;
Secondary transfer means for transferring the toner images laminated on the intermediate transfer belt onto a recording medium at a time;
First detection means for detecting a toner adhesion amount of a density correction reference image formed on the photoreceptor;
Second detection means for detecting a color misregistration amount of a color misregistration correction reference image transferred onto the intermediate transfer belt by the transfer means;
Control means for performing density and color misregistration correction in parallel based on the detection results of the first and second detection means;
Cleaning means for removing toner remaining on the intermediate transfer belt;
In an image forming apparatus comprising:
The density correction reference image is composed of a plurality of patch images having different densities formed in the rotation direction of the photoconductor, and after the toner adhesion amount is detected by the first detection unit, the unit on the intermediate transfer belt The density correction corresponding to the high density side patch image of the density correction reference image corresponding to a predetermined color and the other color so that the toner amount per area does not exceed the predetermined amount removable by the cleaning means. The density correction reference image transferred onto the intermediate transfer belt after the density correction reference image of each color is superimposed and transferred in the belt advancing direction so that the patch image on the low density side of the reference image overlaps. Is removed by the cleaning means . A plurality of image forming units including a photoreceptor and a developing device;
A conveying belt for conveying a recording medium to the image forming unit;
Transfer means for transferring the toner image developed on the photoreceptor onto a recording medium or onto the transport belt;
First detection means for detecting a toner adhesion amount of a density correction reference image formed on the photoreceptor;
Second detection means for detecting a color misregistration amount of a color misregistration correction reference image transferred onto the transport belt by the transfer means;
Control means for performing density and color misregistration correction in parallel based on the detection results of the first and second detection means;
Cleaning means for removing toner remaining on the conveyor belt;
In an image forming apparatus comprising:
The density correction reference image is composed of a plurality of patch images having different densities formed in the rotation direction of the photoconductor, and after the toner adhesion amount is detected by the first detection unit, a unit area on the conveyance belt The high density side patch image of the density correction reference image corresponding to a predetermined color and the density correction corresponding to another color so that the amount of toner per hit does not exceed the predetermined amount removable by the cleaning unit The density correction reference image of each color is superimposed and transferred in the belt traveling direction so that the patch image on the low density side of the reference image overlaps, and then the density correction reference image transferred onto the transport belt is An image forming apparatus which is removed by a cleaning unit .   The density correction reference image is formed so that the density gradually decreases from the highest density patch image from the leading edge to the trailing edge, or the density gradually increases from the lowest density patch image. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

Images