JP2008310234A - Detecting method of fog in color image forming apparatus, and color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To automatically detect the presence or absence of fog with ease without wastefully consuming a recording medium such as paper. <P>SOLUTION: The density of the surface of an intermediate transfer belt 18 is detected by a density sensor 20 while a fog toner image is absent on an intermediate transfer belt 18. The operation of forming a fog toner image on a photoreceptor 11 and transferring it to the intermediate transfer belt 18 is performed more than one time so that the fog images are superimposed one on another on the intermediate transfer belt 18. Thereby, fog toner images in two or more colors are formed within the period of one rotation of the intermediate transfer belt 18. Each of the densities of the fog toner images in their respective colors formed one on another on the intermediate transfer belt 18 is detected by the density sensor 20. From the detected density of the fog toner image in each color, the presence or absence of fog is determined for the respective colors. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fog detection method and a color image forming apparatus in a color image forming apparatus provided with an electrophotographic image forming means.

  In an image forming apparatus (color image forming apparatus) using an electrophotographic technique, defective charged toner having a reduced charge amount may increase in the developing device due to various factors such as environmental changes or changes with time. As a result, the poorly charged toner adheres to the white background (background) of the image, and so-called fog occurs.

  Such fog may also occur due to factors such as deterioration in charging performance due to durability deterioration of the photosensitive member and charger, and the surface potential of the photosensitive member not reaching a specified potential.

  The fog is transferred to the recording paper (paper), and is recognized by the user as a stain on the background image. When the fog occurs, not only the image quality is deteriorated but also the inside and outside of the apparatus of the image forming apparatus are contaminated, and the consumption of toner increases, so that there is a possibility that the specified number of sheets cannot be printed.

  By the way, since the amount of toner in the fog itself is very small, it is difficult to detect the presence or absence of fog with an existing optical detector, and it is not easy to detect fog and perform automatic control to eliminate fog. . Therefore, in the market, there is a problem that a user or a serviceman must visually check the fogged image and adjust it manually.

  Conventionally, there are those described in Patent Documents 1 and 2 as prior art for detecting fog. That is, Patent Document 1 discloses a method for visualizing a fogged image on a recording medium by controlling a developing bias and a transfer bias. Also, after forming a fogged image superimposed on the intermediate transfer member a plurality of times, the fogged image is transferred to a recording medium, and the fogging curve plotted with the fogging density on the recording medium and the number of fogging images becomes close to the initial fogging curve. A method of setting a combination in which the surface potential and the developing bias are changed is disclosed.

Patent Document 2 discloses a method of detecting fog with an optical detector when the development bias potential is fixed and the output of the charging grid potential is changed stepwise.
JP 2005-62858 A JP-A-11-160930

  However, in the method disclosed in Patent Document 1, it is necessary to form a fog by changing the charging potential and the developing bias in order to obtain a fogging curve. The fog image forming process is required. In addition, since the fog density is detected on the recording medium, a new device for detecting the fog density is required, and the recording medium such as paper is wasted only for the fog detection. . Furthermore, there is a problem that the user and the serviceman are bothered to detect the fog density.

  The method disclosed in Patent Document 2 is effective when the amount of fog toner is large, but there is a problem that detection is difficult when the amount of fog toner is small.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to easily and automatically detect the presence or absence of fog without wastefully consuming a recording medium such as paper.

  In the method according to the present invention, an electrostatic latent image formed on a photoconductor by exposure is developed by a developing unit, and the obtained toner image is transferred to an image carrier by a transfer unit and further transferred to a recording medium. A method for detecting the presence or absence of fog in a configured color image forming apparatus, wherein a plurality of fog toner images are formed by performing a plurality of operations of forming a fog toner image on the photoreceptor and transferring it to the image carrier. Two or more fog toner images are formed on the image carrier at the time of one rotation period of the image carrier, and each color fog toner image formed on the image carrier is overlapped. Is detected by a density sensor, and the presence or absence of fog for each color is determined from the detected density of the fog toner image of each color.

  Preferably, the density of the surface of the image carrier is detected by the density sensor in a state where there is no fog toner image on the image carrier, and the density of the detected fog toner image of each color and the state in which there is no fog toner image. The presence / absence of fogging for each color is determined from the density of the surface of the image carrier.

  If the difference between the density of the detected fog toner image and the density of the surface of the image carrier in the absence of the fog toner image is equal to or less than a specified value, it may be determined that there is no fog.

  When it is determined that there is fogging, the absolute value of the voltage of the charging means for charging the photoconductor is changed so as to increase, and then the above-described method is executed again, and it is determined that there is no fogging. Sometimes, a value based on the voltage of the charging unit at that time may be set as the voltage value of the charging unit in normal image formation.

  The change of the voltage of the charging unit may be performed in stages until the voltage reaches a limit value determined by using values relating to the environmental state of the color image forming apparatus and the operation time of the photosensitive member as parameters. Good.

  Further, the charging means for each color is changed so as to have two or more different outputs within one rotation period of the image carrier, and the density of each fog toner image is detected by a density sensor. The presence or absence of fog for each color may be determined from the density of the fog toner image at each level.

  According to the present invention, the presence / absence of fog can be easily detected without wastefully consuming a recording medium such as paper.

  FIG. 1 is a front sectional view showing a schematic configuration of an image forming apparatus 1 according to an embodiment of the present invention, FIG. 2 is a diagram showing an imaging unit U around a development, which is a part of the image forming apparatus 1, and FIG. FIG. 4 is a diagram showing the relationship between the surface potential of the photoconductor 11 and the image, FIG. 4 is a diagram showing the relationship between the toner adhesion amount on the intermediate transfer belt 18 and the output of the density sensor 20, and FIG. 5 is the fog detection in the image forming apparatus 1. 6 is a block diagram showing the configuration of the control device KS, FIG. 6 is a diagram showing a setting state of each part in the fog detection control, FIG. 7 is a diagram showing a change in toner density on the intermediate transfer belt 18 in the fog detection control, and FIG. FIG. 9 is a diagram illustrating a state of the fog toner image KG formed on the transfer belt 18, FIG. 9 is a diagram illustrating an example of an upper limit value table GV storing data on the upper limit value of the change in the output voltage Vg of the charging device 12, and FIG. Is the fog limit Is a diagram illustrating an example of a fog curve CV for finding.

  In FIG. 1, an image forming apparatus 1 has a tandem transfer unit, and sequentially superimposes toners of four colors of yellow (Y), magenta (M), cyan (C), and black (K or Bk). A color image is formed. The image forming apparatus 1 develops the electrostatic latent image formed on the photoconductor 11 by exposure by the developing device 13, and the obtained toner image is transferred to the intermediate transfer belt 18 as an image carrier by the primary transfer roller 16. It is configured to transfer and further transfer to a recording sheet.

  That is, in the image forming apparatus 1, four color imaging units UY, UM, UC, UK of Y, M, C, and K are arranged in a tandem arrangement, and each color formed by these imaging units U is arranged. A toner image (toner image) is superimposed on the intermediate transfer belt 18 to be transferred and synthesized.

  When the toner images of the respective imaging units UY, UM, UC, UK are transferred to the intermediate transfer belt 18, one corresponding primary transfer roller 16Y, 16M, 16C, 16K is pressed against the intermediate transfer belt 18, and the others The primary transfer roller 16 is separated.

  The “primary transfer roller 16” indicates all or part of the “primary transfer rollers 16Y, 16M, 16C, 16K”. The same applies to other elements.

  As shown in FIG. 2, each imaging unit U is configured by arranging a charging device 12, a developing device 13, a cleaner 15, and the like around a drum-shaped photoconductor 11. The surface of the photoconductor 11 is charged to a predetermined voltage (charging potential) V0 by the charging device 12, and an electrostatic latent image is formed by exposure from the exposure unit. The electrostatic latent image is supplied with toner charged from the developing roller 13a to the potential gap ΔV between the electrostatic latent image potential and the developing bias voltage Vdc by the developing roller 13a to which the developing bias voltage Vdc is applied. The toner image is formed by developing the image.

  The toner image visualized on the surface of the photoreceptor 11 is primarily transferred to the intermediate transfer belt 18 by the primary transfer roller 16. The toner remaining on the photoreceptor 11 is scraped off by the cleaner 15. The toner image on the intermediate transfer belt 18 is secondarily transferred onto the recording paper conveyed from the paper supply unit 22 by the secondary transfer roller 23. The recording paper is fixed by the fixing unit 24 and discharged onto the paper discharge tray 25 as an electrophotographic image.

  The toner (transfer residual toner) that is not completely transferred in the secondary transfer and remains on the intermediate transfer belt 18 is removed and collected by the cleaning unit 19. That is, the surface of the intermediate transfer belt 18 is cleaned by the cleaning unit 19 after the secondary transfer. In this embodiment, the cleaning unit 19 is a cleaning blade type that is movable between the pressure contact and the separation with respect to the intermediate transfer belt 18. However, in addition to this, the intermediate transfer belt 18 is used. It is possible to use various types or mechanisms, such as a brush type in which the voltage applied to is controlled on and off.

  Incidentally, in order to set the toner adhesion amount on the surface of the photoconductor 11 to a target amount, the charging potential V0, the exposure amount, and the development bias voltage Vdc of the photoconductor 11 execute an operation mode called stabilization control. It is adjusted by doing. The stabilization control is performed when the output of the temperature / humidity detector (environmental sensor) 21 inside or outside the image forming apparatus 1 changes from a specified value, when the print counter CT (see FIG. 5) reaches a specified value, or This is activated when a change occurs in the image forming apparatus 1, such as when the operating time of the photoconductor 11 reaches a predetermined time, or when the mounting of a new imaging unit U is detected. The value of the print counter CT is a value related to the operation time.

  The image forming apparatus 1 is provided with an optical density sensor (also referred to as an adhesion amount sensor or IDC sensor) 20 in order to detect the toner concentration (toner adhesion amount) on the surface of the intermediate transfer belt 18. The output of the sensor 20 is used for stabilization control. The output of the density sensor 20 is also used in fog detection control described later.

  The control device 101 controls the entire image forming apparatus 1 by performing stabilization control, fog detection control, other control, image processing, or communication with an external device such as a network. The control device 101 includes a CPU, a DSP, a ROM, a RAM, and other peripheral elements. Image formation is performed by the CPU executing an appropriate program, by hardware circuit elements, or a combination thereof. Various functions in the device 1 are realized.

  In the present exemplary embodiment, an example based on a two-component development tandem configuration is described as the image forming apparatus 1, but the configuration is not limited thereto. For example, a density sensor for detecting the toner adhesion amount of a toner image formed on various types of image carriers other than the photoconductor 11 or the intermediate transfer belt 18 and cleaning of residual toner on the image carrier. If a cleaning unit that can be switched on and off is provided, other development methods such as a one-component development method and a liquid development method may be used, and a multi-transfer method and a direct transfer method may be used.

  Next, an image forming mechanism on the photoconductor 11 and a fog generation mechanism will be described. In the present embodiment, it is assumed that the photoconductor 11 is an organic photoconductor that is negatively charged.

  Due to the output voltage Vg of the charging device 12, the surface of the photoreceptor 11 is charged to the charging potential V0. When a scorotron is used as the charging device 12, the output voltage Vg is substantially equal to the voltage applied to the grid.

  In FIG. 3, the non-exposed portion (non-image portion) on the surface of the photoconductor 11 has the same voltage V0 as the charging potential V0, and the exposed portion (image portion) decreases to the voltage Vi. The image background portion on the surface of the photoconductor 11 is slightly reduced in voltage from the non-exposed portion to the voltage V0 ′ by the amount of exposure bias light.

  In this state, by applying a negative developing bias voltage Vdc to the developing roller 13a, the negatively charged toner has a total toner charge amount corresponding to the potential of the potential gap ΔV (= | Vdc | − | Vi |). Minutes adhere to the surface of the photoreceptor 11.

  Further, the fog is regulated by the fog margin potential Vm (= | V0 ′ | − | Vdc |). When Vm is increased, the fog amount is decreased, and when Vm is decreased, the fog amount is increased. If the fog margin potential Vm is to be increased, (1) the charging potential V0 is increased, so that the carrier adhesion to the photosensitive member 11 occurs in the two-component development method, and (2) the potential gap ΔV is decreased. There arises a problem that a prescribed toner adhesion amount may not be obtained. Usually, the fog margin potential Vm is determined by a value at which the presence / absence of fog and these two problems (1) and (2) are optimal.

  Further, if the fog margin potential Vm is constant and the toner charge amount increases, the fog amount decreases. If the toner charge amount decreases, the fog amount increases. That is, fogging occurs when the charging potential V0 is not charged to the specified potential due to durability deterioration of the charging device 12 or the photoconductor 11, or when the toner charge amount is decreased due to durability deterioration.

However, by performing the fog detection control described below, it is possible to suppress the fog that occurs when the charging device 12 or the photoreceptor 11 is deteriorated in durability or when the toner charge amount is reduced.
[Fog detection control]
In this embodiment, the fog detection control is performed by detecting the density of the surface of the intermediate transfer belt 18 with the density sensor 20 in a state where there is no fog toner image on the intermediate transfer belt 18 and forming a fog toner image on the photoconductor 11. The operation of transferring to the intermediate transfer belt 18 is performed a plurality of times so that the plurality of fog toner images overlap on the intermediate transfer belt 18, and at this time, the fog toner images of two or more colors within one rotation period of the intermediate transfer belt 18. The density sensor 20 detects the density of each color fog toner image formed on the intermediate transfer belt 18 so as to overlap, and the detected density of each color fog toner image and the intermediate in the absence of the fog toner image. The presence or absence of fog is determined from the density of the surface of the transfer belt 18.

  At this time, if the difference between the density of the detected fog toner image and the density of the surface of the intermediate transfer belt 18 in the absence of the fog toner image is equal to or less than a specified value (allowable value) ε, it is determined that there is no fog. To do.

  When it is determined that fog is present, the absolute value of the voltage of the charging device 12 that charges the photosensitive member 11 is changed so as to increase, and then the above operation is executed again. When it is determined that there is no fog, a value based on the voltage of the charging device 12 at that time is set as the voltage value of the charging device 12 in normal image formation.

  The voltage of the charging device 12 is changed in a stepwise manner until the voltage reaches a limit value determined by using the values relating to the environmental state of the image forming apparatus 1 and the operation time of the photoconductor 11 as parameters.

  Further, the charging device 12 is changed for each color so as to have two or more different outputs within one rotation period of the intermediate transfer belt 18, and the density sensor 20 detects the density of each level of the fog toner image. It is also possible to determine the presence or absence of fogging for each color from the density of the fogging toner image at each level.

  When the surface of the intermediate transfer belt 18 is cleaned by the cleaning unit 19 so that there is no fog toner image on the intermediate transfer belt 18 and a plurality of fog toner images are overlapped on the intermediate transfer belt 18, the cleaning unit Do not perform 19 cleaning.

  That is, in FIG. 5, the fog detection control device KS includes an adhesion amount detection unit 111, a control determination unit 112, a charging potential control unit 113, and the like.

  The adhesion amount detection unit 111 acquires the reference concentration Sbs and the fog detection value Sn based on the output value from the concentration sensor 20.

  The control determination unit 112 controls the imaging unit U, the primary transfer roller 16, the intermediate transfer belt 18, the cleaning unit 19, and the like so that there is no fog toner image on the surface of the intermediate transfer belt 18, and the intermediate transfer Control necessary for fog detection control is performed such that the fog toner image overlaps the surface of the belt 18 and the value of the output voltage Vg of each level of the charging device 12 in the fog detection control is determined. Further, the control determination unit 112 compares the reference concentration Sbs and the fog detection value Sn to determine the presence or absence of fog.

  Further, the control determination unit 112 is provided with an upper limit value table GV for each color, and when determining the value of the output voltage Vg of each level of the charging device 12, the temperature output from the temperature / humidity detector 21 and Using the information such as the humidity, the count value from the print counter CT, and the operation time of the photoconductor 11 from the timer TM as parameters, the upper limit value (limit value) of the offset output potential Vg ′, which is the change in the output voltage Vg. Reading from the upper limit value table GV and controlling so as not to exceed the upper limit value.

  The charging potential control unit 113 outputs a charging potential V0 to the charging device 12 of the imaging unit U.

  With such a configuration, the fog detection control device KS first detects the surface of the intermediate transfer belt 18 by the cleaning unit 19, and detects the toner concentration on the surface of the cleaned intermediate transfer belt 18 by the density sensor 20. Then, the second step of acquiring the reference density Sbs, the third step of setting the voltage of the charging device 12 for two or more colors to a predetermined value, and forming a fog toner image of two or more colors on the photoconductor 11. The fourth step of performing the transfer operation to the intermediate transfer belt 18 a plurality of times and superimposing a plurality of fog toner images for the respective colors on the intermediate transfer belt 18, and the fog toner of each color formed overlapping on the intermediate transfer belt 18 Fifth step of detecting the density of the image by the density sensor 20 and obtaining the fog detection value Sn for each color, each color A sixth step for comparing the fog detection value Sn and the reference density Sbs, so that the absolute value of the voltage of the charging device 12 is increased for a color in which the difference between the fog detection value Sn and the reference density Sbs is equal to or greater than the specified value ε. In the seventh step of repeating the first step to the sixth step, it is determined that there is no fog for a color whose difference between the fog detection value Sn and the reference density Sbs is equal to or less than a predetermined value ε. An eighth step for ending the detection is executed.

As described above, the fog detection control (fogging detection control mode) is performed by direct instruction from the user or service person to the image forming apparatus 1, an instruction by a service center connected by a network protocol, or when an installation environment of the image forming apparatus 1 is changed. Triggered when one of the specified number of prints has elapsed. As described above, the fog detection control is performed for all the colors Y, M, C, and K, and is not limited to a specific system speed. This will be described in more detail below.
[Acquisition of Reference Concentration Sbs]
The density sensor 20 is a reflection type density sensor, and as shown in FIG. 4, the amount of reflected light decreases and the output value decreases as the toner adhesion amount increases. That is, a decrease in the output value of the density sensor 20 indicates an increase in the toner adhesion amount. In the fog detection control, the output value of the density sensor 20 is used for determination of fog detection, but instead of the output value, a converted toner adhesion amount as shown in FIG. 4 may be used.

  First, in the TB cycle of FIG. 7, the density (reference density) Sbs of the surface (base surface) to which the toner of the intermediate transfer belt 18 is not attached is acquired. That is, in a state where the surface of the intermediate transfer belt 18 is cleaned, the output value of the density sensor 20 is acquired as the reference density Sbs and stored in the memory.

  By acquiring the reference density Sbs, whether the change in the output value of the density sensor 20 during fog detection control is due to the running of the intermediate transfer belt 18, or a slight fog is detected with respect to the base surface. It is possible to accurately discriminate whether it is due to the fact. Note that the amount of change (increase) from the reference concentration Sbs corresponds to the fog amount.

  One reference density Sbs may be acquired for the entire intermediate transfer belt 18, but each reference color Sbs corresponds to a position or region where fog toner images KGK, KGC, KGM, and KGY for each color described later are formed. Four reference densities Sbs may be acquired.

  Upon detection of the base surface (acquisition of the reference density Sbs), as shown in FIG. 6, the cleaning unit 19 is brought into pressure contact with the intermediate transfer belt 18, and the primary transfer roller 16 and the secondary transfer roller 23 are separated from each other. To do. In this state, the intermediate transfer belt 18 is run and the output value of the density sensor 20 is acquired.

Note that the primary transfer roller 16 and the secondary transfer roller 23 do not necessarily need to be separated from each other as long as the base surface is not contaminated. For example, the transfer current, the charging potential, or the developing bias potential may not be driven or a reverse voltage may be applied. The acquisition of the reference concentration Sbs is not necessarily performed every time the fog detection control is activated, and the reference concentration Sbs acquired before the activation of the fog detection control, the reference concentration Sbs acquired in the previous fog detection control, or the like is used. May be.
[Acquisition of fog detection value Sn]
Next, in order to perform fog detection, as indicated by “on” in FIG. 6, in each rotation cycle, the cleaning unit 19 is in a separated state in order of K, C, M, and Y, and the primary transfer roller 16 is in a pressure contact state. The image forming unit U performs image formation with an exposure light amount of 0 gradation data similar to that of the image background portion, and is set so as to use the developing bias voltage Vdc during normal image formation and the output voltage Vg of the charging device 12. Then, the toner image (fogging toner image) is transferred to the intermediate transfer belt 18.

  Since the amount of fog is very small, image formation in the imaging unit U and transfer to the intermediate transfer belt 18 are repeated a plurality of times in each cycle of T1 to TN in FIG. 18 is overlapped several times (N times). Then, the density of the fog toner image superimposed on the intermediate transfer belt 18 is detected by the density sensor 20, and the output value at that time is set as the fog detection value Sn for the color.

  In other words, the amount of fogging at the first time cannot be detected by the concentration sensor 20 because the amount of fogging is small, but the amount of fogging increases as the second and third times are repeated, and detection by the concentration sensor 20 becomes possible.

  Here, as shown in FIGS. 7 and 8A, within one rotation cycle of the intermediate transfer belt 18, black (1/4 turn), cyan (2/4 turn), magenta (3/4). The four-color fog toner images KGK, KGC, KGM, and KGY are formed. The development bias voltage Vdc and the output voltage Vg are applied so that only a desired fog toner image KG is formed at a specific position (region) of the intermediate transfer belt 18.

  For example, at the ¼ turn of the intermediate transfer belt 18 that forms the black fogged toner image KG, the primary transfer roller 16 is pressed against only black and the development bias voltage Vdc and the output voltage Vg are applied. At the same time, for the other colors, the primary transfer roller 16 is separated and the development bias voltage Vdc and the output voltage Vg are applied, or the primary transfer roller 16 is pressed and the development bias voltage Vdc and the output voltage Vg are not applied. Then, the primary transfer roller 16 is pressed and a development bias voltage Vdc and an output voltage Vg are applied, and a bias having a polarity opposite to that at the time of image formation may be applied to the primary transfer roller 16.

In this way, the four-color fog toner images KG are each formed in a quarter cycle within one rotation cycle of the intermediate transfer belt 18, and the same color fog toner images KG of each color are superimposed N times. The fog detection values Snk, Snc, Snm, and Sny are obtained for the fogged toner images KGK, KGC, KGM, and KGY that are superimposed N times.
[Formation of fog toner image KG with output voltage Vg of two or more levels]
By the way, the fog toner image KG of each color is not formed with one (one level) output voltage Vg in one rotation cycle of the intermediate transfer belt 18, but two or more (two levels) in one rotation cycle. The fog toner image KG may be formed with the output voltage Vg described above.

  For example, as shown in FIG. 8B, fog toner images KG corresponding to three levels of output voltages Vg1, Vg2, and Vg3 may be formed at a time for each color.

  In this case, the developing bias voltage Vdc is the same as that during normal image formation, and the offset output potential Vg ′, which is the amount of change in the output voltage Vg of the charging device 12, is plural types (plural levels), here three types (3 Standard) Prepare.

  That is, for example, three levels of 0V, 10V, and 20V are prepared as the offset output potentials Vg′1, Vg′2, and Vg′3, and these are added to the output voltage Vg during normal image formation. That is, the three levels of offset output potentials Vg′1, Vg′2, and Vg′3 are added to the output voltage Vg, respectively, to obtain the three levels of output voltages Vg1, Vg2, and Vg3.

Vg1 = | Vg | + | Vg′1 |
Vg2 = | Vg | + | Vg′2 |
Vg3 = | Vg | + | Vg′3 |
The offset output potential Vg ′ is a potential that acts to suppress fogging, that is, a potential for expanding the fog margin. The offset output potential Vg ′ has a value read from the upper limit value table GV shown in FIG. 9 as its upper limit value according to the environmental state such as temperature or humidity, the operation time of the photoconductor 11 or the like.

  Thus, for each level of the offset output potential Vg ', 0 is the minimum value, and the upper limit value read from the upper limit value table GV is the maximum value. Then, the minimum value and the maximum value are divided according to the number of levels so as to change stepwise. When the number of levels is “3”, it is divided into two. The division may be equal division or may be given an appropriate bias.

  Then, the three levels of the output voltages Vg1, Vg2, and Vg3 for these colors are sequentially switched during the formation of the fog toner image KG for each color in one rotation cycle of the intermediate transfer belt 18, and set to the charging device 12. . In this state, the photoconductor 11 is exposed with an exposure light amount of 0 gradation data similar to that of the image background portion, an image is formed by the imaging unit U, and the toner image (fogging toner image) is transferred to the intermediate transfer belt 18. .

  FIG. 8B shows three-level fog toner images KG1, KG2, and KG3 formed at three levels of output voltages Vg1, Vg2, and Vg3 for each color. The intermediate transfer belt 18 is shown in a state where the entire length is developed.

  Further, for each color, instead of the three-level output voltage Vg, the output voltage Vg of the second level, the fourth level, or the sixth level may be set to form each fog toner image KG in one rotation cycle. .

  In this way, by simultaneously forming the fog toner image KG with the output voltage Vg of two or more levels for each color within one cycle of the intermediate transfer belt 18, the time required for fog detection control is further shortened.

  In the above example, the fog toner images KGK, KGC, KGM, and KGY of four colors K, C, M, and Y are formed within one rotation period of the intermediate transfer belt 18, and the fog detection values Snk, Although Snc, Snm, and Sny were acquired at the same time, as shown in FIG. 8C, two colors may be divided twice. That is, in the example shown in FIG. 8C, the K and C fog toner images KGK and KGC are formed at the first time to obtain the fog detection values Snk and Snc, and the M and Y fog toner images KGM at the second time. , KGY are formed to obtain the fog detection values Snm and Sny. In these cases, the order and combination of colors may be variously changed.

As shown in FIG. 8A, the density sensor 20 is arranged at the end or center of the intermediate transfer belt 18 in the width direction. In some cases, a plurality of density sensors 20 are arranged and used.
[Fog judgment]
The reference density Sbs and the fog detection values Snk, Snc, Snm, and Sny acquired as described above are compared to determine the presence or absence of fog. A predetermined value ε is used to determine whether fog is present. If the difference Snk ′, Snc ′, Snm ′, Sny ′ between the reference concentration Sbs and the fog detection values Snk, Snc, Snm, Sny is equal to or less than the specified value ε, it is determined that there is no fog. That is, for each color
Snk ′ = Sbs−Snk <ε
Snc ′ = Sbs−Snc <ε
Snm ′ = Sbs−Snm <ε
Sny ′ = Sbs−Sny <ε
Is satisfied, it is determined that there is no fog for the color. For a color determined to have no fog, the fog detection control for that color is terminated. When it is determined that there is no fog for all colors, the fog detection control ends and the normal image forming mode is restored. At this time, the output voltage Vg used for the fog detection control may be used as it is as the output voltage Vg in the image forming mode, or a voltage slightly higher than that may be used as the output voltage Vg in the image forming mode. Good. In the fog detection control, when the output voltage Vg in the image forming mode is not changed, the output voltage Vg used in the image forming mode may be used as it is.

  On the contrary, if the difference between the reference concentration Sbs and the fog detection values Snk, Snc, Snm, and Sny is equal to or greater than the specified value ε, it is determined that there is fog. If it is determined that there is fogging, the output voltage Vg is changed by performing charging potential control described later.

  In the example shown in FIG. 7, since Snc ′, Snm ′ <ε <Snk ′, and Sny ′, it is determined that the fog is generated in black and yellow, and the output described later is performed for black and yellow. The voltage Vg is changed. For cyan and magenta in which no fog occurs, the fog detection control ends.

  As the specified value ε, a common value may be used for all colors, but an optimal value may be set for each color. For example, the specified value ε may be set to a small value for a color that is easily visible to human eyes, such as black, and the specified value ε may be set to a large value for a color that is difficult to be seen as dirty, such as yellow.

  Further, in the fog determination, if the fog amount is obtained by using the reference density Sbs acquired corresponding to the position or area where the fog toner images KGK, KGC, KGM, and KGY of the respective colors are formed as the reference density Sbs, A more accurate fog amount is required.

  Further, fog detection and fog determination may be performed every time the fog toner image is superimposed on the intermediate transfer belt 18 N times. In this case, if it is determined that fogging has occurred for all colors before reaching N times, the subsequent fogging toner image formation and transfer are immediately interrupted, and charging potential control described later may be performed. .

In the fog determination, if the difference between the reference density Sbs and the fog detection value Sn is equal to the specified value ε, the fog may or may not be fogged. Note that the prescribed value ε may be set to a value that does not reveal fog on the recording paper. Actually, the value varies depending on the number of times the fog toner image is superimposed.
[Fog judgment when using output voltage Vg of 2 levels or more]
When fog detection is performed using an output voltage Vg of two or more levels for each color, the fog determination may be performed as follows.

That is, for each color, when it is determined that there is fogging at any level, the output voltage Vg used for normal image formation is changed. For this purpose, an offset output potential Vg ′ to be added in normal image formation is obtained. There are two methods for this.
(1) In the first method, the offset output potential Vg ′ corresponding to the fogged toner image KG determined to have no fogging, is the offset output to which the voltage having the minimum absolute value should be applied to normal image formation. The potential is Vg ′.

  For example, in the case of three levels, it is assumed that three fog amounts for a certain color have a relationship of Sn1 '> Sn2'> ε> Sn3 'as shown in FIG. In this case, the offset output potential Vg′3 corresponding to the fog toner image KG3 determined to be free of fog is set to 20 V, which is the offset output potential Vg ′ to be applied to normal image formation. Therefore, in this case, a voltage obtained by adding 20 V to the output voltage Vg used in the image forming mode is used as the output voltage Vg.

Further, if the three fog amounts have a relationship of Sn1 ′>ε> Sn2 ′> Sn3 ′, the offset output potential Vg ′ corresponding to the fog toner images KG2 and KG3 determined as having no fog. Of 2 and Vg′3, Vg′2 having the minimum absolute value is set as an offset output potential Vg ′ to be applied to image formation.
(2) In the second method, based on the offset output potentials Vg ′ 1 to 3 at each level, a limit offset output potential Vg ′ that is determined to be free of fog is obtained.

  That is, in the same example of FIG. 10, the obtained Sn1 ', Sn2', and Sn3 'are plotted with the horizontal axis as the offset output potential Vg' and the vertical axis as the fogging amount. The fogging curve CV1 is obtained by connecting the plotted points. From the intersection of the fog curve CV1 and the specified value ε, a limit offset output potential (fog limit offset output potential) Vg ′ at which it is determined that there is no fog is obtained, and this offset output potential Vg to be applied to normal image formation. 'And. In the example of FIG. 10, the fog limit offset output potential Vg ′ is about 12V.

The fog curve CV1 is a curve passing through three points, but it may be an approximate curve. The greater the number of levels, the better the accuracy. The fog curve CV1 or the fog limit offset output potential Vg ′ may be obtained by the method of least squares. In the case of two points, the fog curve CV is a straight line.
[Charging potential control]
When it is determined that fog is present for each color, the offset output potential Vg ′ is added to the output voltage Vg set for fog detection. That is, the output voltage Vg for the next fog detection is changed to | Vg | + | Vg ′ |.

  The value of the offset output potential Vg ′ is set to “10” in this embodiment, and the fog determination is performed in 10V steps. However, other appropriate voltage values can be used as the offset output potential Vg ′.

  In the repetition of the operation described above, the upper limit of the total value of the offset output potential Vg ′, that is, the upper limit of the change in the output voltage Vg is determined. The upper limit is recorded in the upper limit value table GV for each color as described above.

  In the upper limit table GV shown in FIG. 9, for example, when the absolute humidity is “0” and the print counter CT is “0”, the upper limit value of the offset output potential Vg ′ is “20”. When the absolute humidity or the count value is not recorded in the upper limit value table GV, the upper limit value may be obtained by interpolation using the value in the vicinity thereof, or the upper limit value in the vicinity may be used.

  In this way, the output voltage Vg is changed stepwise up to an upper limit value corresponding to the environmental state such as temperature or humidity, the operation time of the photoconductor 11, etc., so that the output voltage Vg corresponds to the current state of the image forming apparatus 1. It is possible to easily find and adjust the optimum output voltage Vg without fog.

  The fog detection value Sn immediately before changing the output voltage Vg can be used as the reference density Sbs without performing the operation for obtaining the reference density Sbs as described above. From the point of accuracy, it is desirable to detect again by pressing the cleaning unit 19.

When an output voltage Vg of two or more levels is used, an offset output potential Vg ′ or a fog limit offset output potential Vg ′ to be applied to image formation for the target color is obtained by one fog determination. There is no need to perform charging potential control.
[Charging potential setting]
The value of the offset output potential Vg ′ determined for each color is added to the output voltage Vg set in normal image formation for each color, that is, | Vg | = | Vg | + | Vg ′ | An output voltage Vg for image formation is set.

  If it is determined that there is fogging even if the upper limit value is set by the upper limit table GV, if (Sbs−Sn) tends to decrease as the output voltage Vg approaches the upper limit value, Set the upper limit. If (Sbs−Sn) is not changed, the output voltage Vg immediately before the fog detection control is not changed, and a fog warning is displayed on the display unit of the image forming apparatus 1 or fog occurs. The imaging unit U is instructed to be replaced.

  Next, fog detection control will be described based on a flowchart.

  FIG. 11 is a flowchart showing an example of a procedure for fog detection control in the fog detection control device KS, and FIG. 12 is a flowchart showing an example of a procedure for fog detection control when an output voltage Vg of two or more levels is used.

  In FIG. 11, the cleaning unit 19 is brought into a pressure contact state, and the primary transfer roller 16 and the secondary transfer roller 23 are brought into a separated state (# 11). As a result, the fogging toner image is not present on the base surface of the intermediate transfer belt 18, and the reference density Sbs is acquired by the density sensor 20 (# 12). The cleaning unit 19 is separated (# 13), and the development bias voltage Vdc and the output voltage Vg of the target color are set (# 14) in order to detect fog.

  A fog toner image is sequentially formed on the photoconductor 11 for each color, transferred onto the intermediate transfer belt 18 N times (# 15), and a fog detection value Sn for each color is acquired by the density sensor 20 (# 16). The presence or absence of fog is determined for each color depending on whether or not the difference between the reference density Sbs and the fog detection value Sn of each color exceeds a specified value ε (# 17).

  For a color determined to have fogging, a value obtained by adding the offset output potential Vg ′ to the output voltage Vg is set as a new output voltage Vg (# 19), and fogging detection after step # 11 is executed.

  For the color determined to have no fog, the output voltage Vg at that time is set to the output voltage Vg for normal image formation (# 18).

  In FIG. 12, steps # 21 to 24 are the same as steps # 11 to 14 of FIG. In step # 25, for each color, a fog toner image is formed on the photoconductor 11 using the output voltage Vg of each level, and transferred onto the intermediate transfer belt 18 by being overlapped N times. The density sensor 20 acquires the fog detection value Sn of each level for each color (# 26). For each color, the presence / absence of fog is determined. If it is determined that fog is present, an offset output potential Vg ′ to be applied to normal image formation is obtained (# 27). At this time, the fog curve CV1 may be obtained or the fog limit offset output potential Vg 'may be obtained. If it is determined that there is no fogging, the offset output potential Vg ′ to be applied to normal image formation may be set to zero.

  A value obtained by adding the offset output potential Vg ′ to the output voltage Vg is set as a new output voltage Vg (# 28), and the process returns to the image forming mode.

  According to the embodiment described above, the reference density Sbs and the fog detection value Sn are obtained using the existing density sensor 20 by overlapping the fog on the intermediate transfer belt 18 a plurality of times, and the charging potential of the photoconductor 11 is obtained. By controlling V0, it is possible to form an image without fog. In addition, since the fog detection control is easily and automatically performed in the image forming apparatus 1, it is possible to reduce the consumption of recording paper, which has conventionally been necessary for checking the adjustment time and the degree of fogging performed by a service person. The cost can be reduced.

  In the case of this embodiment, the fog toner image KG of two or more colors is simultaneously formed within one cycle of the intermediate transfer belt 18, so that the time required for fog detection control is shortened. Further, when the fog toner image KG is formed simultaneously with a plurality of levels of output voltage Vg, the time required for fog detection control is further shortened.

  In the above-described embodiment, the fog detection control device KS, the control device 101, or the entire or each part of the image forming apparatus 1, the structure, the shape, the number, the content or order of processing or operation, various voltage values, the upper limit The contents of the value table GV can be appropriately changed in accordance with the spirit of the present invention.

1 is a front sectional view showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention. It is a figure which shows the imaging unit of an image forming apparatus. It is a figure which shows the relationship between the surface potential of a photoconductor and an image. It is a figure which shows the relationship between a toner density and the output of a density sensor. It is a block diagram which shows the structure of a fog detection control apparatus. It is a figure which shows the setting state of each part in fog detection control. FIG. 6 is a diagram illustrating a change in toner density on an intermediate transfer belt. FIG. 4 is a diagram illustrating a fog toner image formed on an intermediate transfer belt. It is a figure which shows the example of an upper limit table. It is a figure which shows the example of the fog curve for calculating | requiring a fog limit. It is a flowchart which shows the example of the procedure of fog detection control. It is a flowchart which shows the other example of the procedure of fog detection control.

Explanation of symbols

1 Image forming device (color image forming device)
11 Photoconductor 12 Charging device (charging means)
13 Developing device (Developing means)
16 Primary transfer roller (transfer means)
18 Intermediate transfer belt (image carrier)
19 Cleaning unit (toner cleaning means)
20 Concentration sensor 101 Control device 111 Adhesion amount detection unit (means for acquiring fog density)
112 Control determination unit (control means, determination means, voltage change means)
113 Charging Potential Control Unit KS Fog Detection Control Device Sn Fog Detection Value Sbs Reference Density GV Upper Value Table KG Fog Toner Image

Claims (11)

A color image forming apparatus configured to develop an electrostatic latent image formed on a photosensitive member by exposure by a developing unit, and transfer the obtained toner image to an image carrier by a transfer unit and further to a recording medium. A method for detecting the presence or absence of fog in
An operation of forming a fog toner image on the photosensitive member and transferring it to the image carrier is performed a plurality of times so that the plurality of fog toner images overlap on the image carrier. A fog toner image of two or more colors is formed within the rotation period,
The density sensor detects the density of each color fogged toner image formed on the image carrier,
The presence or absence of fog for each color is determined from the density of the detected fog toner image for each color.
A method for detecting fog in a color image forming apparatus. The density of the surface of the image carrier is detected by the density sensor in a state where there is no fog toner image on the image carrier,
Determining the presence or absence of fog for each color from the density of the detected fog toner image of each color and the density of the surface of the image carrier in the absence of the fog toner image;
2. A method for detecting fog in a color image forming apparatus according to claim 1. Determining that there is no fog when the difference between the density of the detected fog toner image and the density of the surface of the image carrier in the absence of the fog toner image is a specified value or less;
A method for detecting fog in a color image forming apparatus according to claim 2. When it is determined that fog is present, the absolute value of the voltage of the charging unit that charges the photosensitive member is changed to be large, and then the method according to claim 2 is performed again,
When it is determined that there is no fogging, a value based on the voltage of the charging unit at that time is set as a voltage value of the charging unit in normal image formation.
A method for detecting fog in a color image forming apparatus according to claim 3. The change of the voltage of the charging unit is performed in stages until the voltage reaches a limit value determined by using the values relating to the environmental state of the color image forming apparatus and the operation time of the photoconductor as parameters.
5. A method for detecting fog in a color image forming apparatus according to claim 4. The charging means for each color is changed so as to have two or more different outputs within one rotation period of the image carrier, and the density of the fog toner image at each level is detected by a density sensor. The presence or absence of fog for each color is determined from the density of the fog toner image of
4. A method for detecting fog in a color image forming apparatus according to claim 2. By cleaning the surface of the image carrier with a toner cleaning means, the image carrier is in a state where there is no fog toner image,
When the plurality of fog toner images are overlapped on the image carrier, the cleaning by the toner cleaning unit is not performed.
The method for detecting fog in the image forming apparatus according to claim 1. A color image forming apparatus configured to develop an electrostatic latent image formed on a photosensitive member by exposure by a developing unit, and transfer the obtained toner image to an image carrier by a transfer unit and further to a recording medium. Because
A density sensor for detecting the density of the surface of the image carrier;
Means for detecting a density in a state where there is no fog toner image on the image carrier by the density sensor to obtain a reference density;
The operation of forming a fog toner image on the photoconductor and transferring it to the image carrier is performed a plurality of times to control the plurality of fog toner images to overlap on the image carrier, and at that time the image carrier Means for controlling to form a fog toner image of two or more colors within one rotation period of the body;
Means for detecting a density of a fog toner image of each color formed on the image bearing member by a density sensor to obtain a fog density for each color;
Means for determining the presence or absence of fogging for each color from the density of the detected fogging toner image of each color and the reference density;
A color image forming apparatus comprising: Voltage determining means for changing the absolute value of the voltage of the charging means for charging the photoconductor corresponding to the color when it is determined that fog is present;
The image forming apparatus according to claim 8. The voltage changing unit performs stepwise until the voltage reaches a limit value determined by using a value relating to an environmental state of the color image forming apparatus and an operation time of the photoconductor as parameters.
The image forming apparatus according to claim 9. The electrostatic latent image formed by exposure on the photosensitive member charged by the charging unit is developed by the developing unit, and the obtained toner image is transferred to the image carrier by the transfer unit and further transferred to the recording medium. A color image forming apparatus,
Toner cleaning means for removing toner adhering to the surface of the image carrier;
A density sensor for detecting the density of the surface of the image carrier;
A fog detection control device for detection of fog on the photoreceptor;
With
The fog detection control device includes:
A first step of cleaning the surface of the image carrier by the toner cleaning means;
A second step of obtaining a reference density by detecting the density of toner on the surface of the cleaned image carrier with the density sensor;
A third step of setting the voltage of the charging means for two or more colors to a predetermined value;
A fourth step of forming a fog toner image of two or more colors on the photoconductor and transferring the image to the image carrier a plurality of times, and superimposing a plurality of fog toner images for each color on the image carrier; ,
A fifth step of acquiring the fog density for each color by detecting the density of the fog toner image of each color formed on the image carrier by the density sensor;
A sixth step of comparing the fog density of each color with the reference density;
For a color in which the difference between the fog density and the reference density is not less than a specified value, the absolute value of the voltage of the charging unit is changed so as to increase, and the seventh step is repeated from the first step to the sixth step. And the steps
An eighth step of determining that there is no fog and ending the detection of fog for a color in which the difference between the fog density and the reference density is a specified value or less,
A color image forming apparatus.

Images