JP2005189494A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately control image density in accordance with toner image density detected by a density sensor, dispensing with the installation of a new sensor. <P>SOLUTION: A time until a reference toner image on an intermediate transfer belt is detected by the density sensor 21 after exposing photoreceptor drums 12a to 15a and transferring the reference toner image formed by developing from the photoreceptor drums to the intermediate transfer belt 16 is measured as a 1st measuring time, and also, a time until the reference toner image is detected by the density sensor again after the reference toner image formed on the intermediate transfer belt is detected by the density sensor is measured as a 2nd measuring time, and a toner image for detecting the image density is formed after detecting the background optical density based on the 1st and 2nd measuring times, and then, the toner density of the toner image for detecting the image density is detected, and the image density is controlled in accordance with the background optical density and the toner density. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine using an electrophotographic process, and more particularly to image density control in the image forming apparatus.

  In general, in an image forming apparatus, an electrostatic latent image formed on an electrostatic latent image carrier such as a photosensitive drum is developed with a developer (toner) to form a visible image (toner image), and this visible image is intermediately transferred. It is known that after transfer onto an intermediate transfer member such as a belt, a visible image on the intermediate transfer member is transferred to a recording medium to form an image. In this type of image forming apparatus, The image density of the visible image changes due to factors such as the environmental atmosphere and durability fluctuations.

  For example, when the developer charging characteristics change due to changes in the environmental atmosphere and the charging characteristics change, a predetermined amount of toner flies from the developing device side to the photosensitive drum side in a state where the developing bias is kept constant. It becomes difficult to let you. Furthermore, in the photosensitive drum, when the number of image formation increases, the photosensitive layer thickness decreases due to mechanical stress due to the polishing effect by the cleaning blade and the intermediate transfer member, and the surface charging potential of the photosensitive drum is kept constant. Tends to be difficult, and the surface charge potential tends to gradually increase. Then, due to such a change in the surface charge potential, a decrease in image density occurs, which adversely affects image quality. In particular, in a color image forming apparatus having a tandem configuration, if there is a difference in the degree of reduction in the photosensitive layer thickness of the photosensitive drum corresponding to each color, the difference from the standard value of the surface charging potential of each photosensitive drum is different. A phenomenon occurs in which the color of the formed image is different from the desired one.

  In order to cope with the above-described decrease in image density, the surface charging potential of the photosensitive drum is measured by a surface potential meter before and after exposure, and charging by the charger is controlled based on the measurement result. Furthermore, the density sensor is used to detect the density of the toner image formed on the photosensitive drum or intermediate transfer member (hereinafter, the photosensitive drum or intermediate transfer member may be referred to as an image carrier). The image density is adjusted by controlling the process conditions such as the developing bias voltage in accordance with the detection result. In this case, the influence of the background density (background density) on the surface of the image carrier is performed. Need to be considered.

  That is, when image density control is performed by detecting the density of the toner image formed on the image carrier using a density sensor, the background density of the image carrier is measured, and the toner image corresponding to the background density is measured. It is necessary to correct the density, and the corrected toner image density varies greatly depending on the background condition such as the background density (which greatly depends on the background condition). In image density control, a plurality of toner images having different image densities are formed and the density of the toner images is measured. The lower the image density, the more the toner image from the background of the image carrier surface. The amount of reflected light increases, and the influence of the background density on the surface of the image carrier increases.

  In the photosensitive drum, the distance between the density sensor and the surface of the photosensitive drum inevitably varies due to the eccentricity and the accuracy of the outer peripheral diameter. Furthermore, when the surface of the photosensitive drum is soiled or scratched by rubbing with a cleaning member such as a cleaning blade and a transfer roller, the state of the surface of the photosensitive drum (that is, the ground) changes, The toner image density (detected toner image density) detected by the density sensor is affected.

  This is unavoidably generated in an intermediate transfer member such as an intermediate transfer belt. When image density control is performed, the detected toner image density is corrected according to the background state of the image carrier. Otherwise, it is difficult to accurately control the image density. Then, when correcting the detected toner image density, for example, the detected toner image density is the average value of the background density, although the toner image density is affected by the background density immediately under the toner image formed. If this correction is performed, the correction results vary and image density control cannot be performed with high accuracy.

  For this reason, when performing image density control, the marking provided on the photosensitive drum is detected, and based on the result of the marking detection, the same position of the photosensitive drum is set as a sensor detection position and a toner patch formation position. And the density of the bare surface (background) of the photosensitive drum is detected as the first detection result, and the density of the visible image formed on the image carrier is detected as the second detection result. In some cases, the image density control is performed so that the ratio of the detection results is obtained and the ratio becomes a predetermined value (see Patent Document 1).

  Further, in the color image forming apparatus, a gradation patch for each color is created on the image carrier, the density of these patches is measured, and image density control is performed. The intensity of the regular reflection component and diffuse reflection component of the reflected light is obtained, and the density of the background portion (background) of the image carrier to which the toner does not adhere is detected from the diffuse reflection component of the reflected light. In some cases, the density of the obtained toner image is compared with the density of the background portion so as to reflect the density of the background portion, and the image forming conditions are determined based on the calculated toner image density. (See Patent Document 2).

Japanese Patent No. 3032649 (pages 4 to 6, FIGS. 4 to 10) JP 2003-156888 A (pages 7 to 9, FIGS. 2 to 7)

  However, in the image density control of Patent Document 1, a position detection marking is formed on the photosensitive drum, and the same position of the photosensitive drum is set as a sensor detection position and a toner patch formation position based on the marking detection result. Although the background density and toner image density are detected at the same position, a sensor (position detection sensor) separate from the density sensor must be provided for marking detection, which increases the cost. is there. In addition, in a small-sized image forming apparatus, when a new sensor is provided, the installation space is extremely limited, and in consideration of the component layout, it may not be possible to install a new sensor.

  In the image density control of Patent Document 2, the density of the background portion of the image carrier to which toner does not adhere is detected from at least one reflected light, and is obtained from at least two reflected lights so as to reflect the density of the background portion. Although the image formation conditions are determined by comparing and calculating the density of the obtained toner image and the density of the background portion, the toner image and the background portion do not have a one-to-one correspondence. If the toner image density cannot be corrected with the background density located immediately below and the background density of the image carrier changes rapidly, the toner image density cannot be corrected with high accuracy. For this reason, there is a problem that it is difficult to accurately control the image density.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus that can accurately control image density without attaching a new sensor.

  Therefore, in order to solve such a problem, the present invention detects an intermediate transfer body on which a toner image is formed, and a background density of the intermediate transfer body, and determines the density of the toner image formed on the intermediate transfer body. A reference toner image formed by development after the photosensitive drum is exposed in an image forming apparatus having density detection means for detecting and performing image density control according to the background density and the toner image density Is measured as a first measurement time from when the toner is transferred from the photosensitive drum to the intermediate transfer member and when the reference toner image is detected on the intermediate transfer member by the density detection means, and the density detection is performed. The time from when the reference toner image formed on the intermediate transfer member is detected by the means until the reference toner image is detected again by the density detection means is a second measurement time. It is measured by, and having a control means for forming an image density detecting toner image after the background density detection based on the first and the second measurement time.

  In the present invention, the control means measures the first and second measurement times when the environmental atmosphere changes or when a predetermined number of printed sheets is reached. An intermediate transfer belt or an intermediate transfer drum.

  According to the present invention, there is provided cleaning means for cleaning the intermediate transfer body in contact with the intermediate transfer body, and the control means performs the cleaning when measuring the first and second measurement times. The means is separated from the intermediate transfer member.

  As described above, in the image forming apparatus of the present invention, after the photosensitive drum is exposed, the reference toner image formed by development is transferred from the photosensitive drum to the intermediate transfer member, and the reference toner image is formed on the intermediate transfer member. The first measurement time is measured as the first measurement time, and the time until the reference toner image is detected again is measured as the second measurement time. As a result, it is possible to form a toner image for image density control at the position of the intermediate transfer body where the background density has been detected based on the variation in the circumference of the intermediate transfer body, so that the background density can be obtained without adding a new sensor. In addition, there is an effect that the image density control can be performed with high accuracy according to the toner image density.

  In the present invention, when the first and second measurement times are measured, the cleaning unit for cleaning the intermediate transfer member is separated from the intermediate transfer member, so that the reference toner image formed on the intermediate transfer member is It will not be removed, and there will be no hindrance to the circumference variation measurement of the intermediate transfer member.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, but are merely illustrative examples. Not too much.

  Referring to FIG. 1, FIG. 1 is a diagram schematically showing a tandem type color image forming apparatus showing an example of an image forming apparatus according to the present invention, and the illustrated image forming apparatus 11 has each color (for example, yellow, cyan, Image forming units 12 to 15 for each of magenta and black), and the image forming units 12 to 15 are provided with photosensitive drums 12a to 15a, respectively. Although not shown, the image forming units 12 to 15 are provided with a charger, an exposure unit, a developing device, a cleaning device, and the like around the photosensitive drums 12a to 15a, respectively.

  Referring also to FIG. 2, image forming units 12 to 15 are sequentially arranged from the upstream side in the rotation direction of the intermediate transfer belt 16 along the intermediate transfer belt 16 that is an intermediate transfer member. These are stretched around the driven roller 18 and the backup roller 19, and are driven to rotate in the direction indicated by the solid arrow by the rotation of the driving roller 17. Further, a pressing roller body 20 (not shown in FIG. 2) is disposed downstream of the driving roller 17 in the rotation direction of the intermediate transfer belt 16, and the intermediate transfer belt 16 is pressed outward by the pressing roller body 20. Yes. A density sensor 21 is arranged on the opposite side of the pressing roller body 20 so as to face the intermediate transfer belt 16. By this density sensor 21, as described later, the surface of the intermediate transfer belt 16 (background: background) is provided. While the state (density) is detected, the density of the toner image (patch) formed on the intermediate transfer belt 16 is detected, and image density control is performed.

  When image formation is performed, the surfaces of the photosensitive drums 12a to 15a are uniformly charged, and the photosensitive drums 12a to 15a are exposed according to the document image data to form electrostatic latent images. Then, the electrostatic latent image is developed by a developing device (not shown), and toner images of respective colors are formed on the photosensitive drums 12a to 15a. Primary transfer rollers 22a to 22d (not shown in FIG. 1) are opposed to the photosensitive drums 12a to 15a with the intermediate transfer belt 16 therebetween, and each color toner image on the photosensitive drums 12a to 15a is 1 The toner images are sequentially transferred to the intermediate transfer belt 16 by the next transfer rollers 22 a to 22 d, and a color toner image is formed on the intermediate transfer belt 16. The residual toner remaining on each of the photosensitive drums 12a to 15a is cleaned by a cleaning device.

  A recording sheet (hereinafter simply referred to as a sheet) is conveyed from the sheet feeding device 23 to the registration roller pair 25 via the sheet conveyance path 24, and the sheet is detected by a registration sensor (not shown) disposed on the upstream side of the registration roller pair 25. Is detected, the registration roller pair 25 is driven in synchronization with the color toner image on the intermediate transfer belt 16, and the sheet is conveyed to the secondary transfer position. A secondary transfer roller 26 is disposed at the secondary transfer position, and the secondary transfer roller 26 faces the backup roller 19 with the intermediate transfer belt 16 in between.

  After the color toner image on the intermediate transfer belt 16 is transferred to the sheet at the secondary transfer position (secondary transfer), the sheet is sent to a fixing device 27 including a heating roller 27a and a pressure roller 27b. The color toner image on the sheet is fixed and discharged onto the discharge tray 29 via the discharge path 28. After the secondary transfer, the intermediate transfer belt 16 is cleaned by the transfer belt cleaning unit 61. The transfer belt cleaning means 61 is, for example, a fur brush or blade 60 that is in contact with the intermediate transfer belt 16 on the driven roller 18. (Reference numeral 61 is not shown in the drawing, but reference numeral 60 is shown in FIG. 1)

  In the illustrated example, each of the photoconductor drums 12a to 15a is a so-called hybrid development using a single-layer positively charged organic photoconductor (positive OPC) having high durability as a photoconductor material, and using a two-component developer as a developing method. The method is adopted. Although the image forming unit 12 shown in FIG. 1 will be described here, the other image forming units 13 to 15 are the same as the image forming unit 12.

  1 and 3, first, the surface of the photosensitive drum 12a is charged to 400V by a charger. Thereafter, for example, when exposure is performed with an LED having a wavelength of 770 nm, the post-exposure surface potential is 70V. The developing unit 31 provided in the image forming unit 12 carries a magnetic roll 35 for generating a magnetic brush 34 by a carrier 33 contained in the toner 32, and a toner thin layer 36 by the toner 32 supplied from the magnetic brush 34. A developing roller 37 for developing the electrostatic latent image on the photosensitive drum 12 a is provided, a magnetic brush 34 is formed on the magnetic roller 35, the toner 32 is charged by stirring, and the magnetic brush on the magnetic roller 35 is charged. The layer 34 is regulated by a regulating blade 38, and a toner thin layer 36 is formed on the developing roll 37 by a potential difference between the magnetic roll 35 and the developing roll 37.

  Here, the photosensitive drum 12 a is arranged with a gap of about 250 μm with respect to the developing roll 37. A developing bias voltage composed of a DC voltage (DC: Vdc1) 39a and an AC voltage (AC) 39b is applied to a shaft (not shown) of the developing roll 37, whereby an electrostatic latent image on the photosensitive drum 12a is formed. develop. A DC voltage (DC: Vdc2) 40 is applied to the magnetic roll 35. When the toner thin layer 36 on the developing roll 37 is replaced, the DC voltage 39a is changed while the AC voltage 39b is applied at the end of development, and the toner thin layer (toner) 36 of the developing sleeve is changed to the magnetic brush 34. Recover by.

  FIG. 5 is a diagram for explaining an example of the density sensor 21. In the density sensor 21, the light emitted from the LED 51 is branched when passing through the BS (beam splitter) 52, and one light is on the toner surface (toner The patch 51 irradiates the other light to a PD (photodiode) 54, and the LED 51 is controlled so that the amount of light is always constant according to the output voltage from the photodiode 54.

  The light reflected from the toner surface 53 becomes scattered light, is branched by the BS 55, and enters the PDs 56 and 57, respectively. Then, based on the ratio of the output voltages from the PDs 56 and 57, the controller 43 detects the toner density. In addition, when detecting the background density, the background density detection is performed in the same manner.

  FIG. 6 is a diagram showing the relationship between the printing rate and the output voltage of the density sensor. Here, for convenience of explanation, a case where a toner patch is formed using black toner is shown. The density sensor 21 outputs a voltage value corresponding to the reflected light from the intermediate transfer belt 16, and the output voltage value becomes maximum when there is no toner patch on the intermediate transfer belt 16 (that is, reflection from the ground). As the amount of toner patches on the intermediate transfer belt increases, light is absorbed. As a result, the amount of reflected light decreases and the output voltage value also decreases.

  As shown in FIG. 6, it is desirable to detect the toner patch density in a range where the output voltage value changes rapidly, that is, in a range where the printing rate is 10 to 40%, and the printing rates are 0 to 10% and 40 to 40%. In the range of 100%, the change amount of the output voltage value is small and it is difficult to accurately detect the printing rate.

  2 and 4, the drive roller 17 is driven by a stepping motor 41 (that is, the intermediate transfer belt 16 is driven to rotate by the stepping motor 41). Is coupled to the drive roller 17 via a gear train 42, and the stepping motor 41 is driven and controlled by a controller 43. That is, the controller 43 gives a pulse signal to the stepping motor 41, and the stepping motor 41 is driven to rotate according to the pulse signal.

  Incidentally, the peripheral length of the intermediate transfer belt 16 may slightly change over time. That is, the length of the intermediate transfer belt 16 may be extended due to the stretched relationship between the driving roller 17, the driven roller 18, the backup roller 19, and the like, and the length slightly changes depending on the number of printed sheets. There are things to do. Furthermore, the length of the intermediate transfer belt 16 also changes depending on the environmental atmosphere (for example, temperature). Further, the peripheral length of the intermediate transfer belt 16 may vary in the manufacturing process. For this reason, the circumferential length of the intermediate transfer belt 16 is periodically measured here.

  When measuring the circumferential length of the intermediate transfer belt 16, the photosensitive drums 12 a to 15 a are simultaneously exposed to start, and a toner image for detecting the circumferential length is transferred to the intermediate transfer belt 16. At this time, a solid image is used to capture the tip of the toner image. Further, the detection accuracy is increased by increasing the image density so that the output voltage from the density sensor 21 rises sharply.

  The controller 43 counts the time from the start of exposure until the leading edge of the circumference detection toner image (reference toner image) is detected by the density sensor 21 with a built-in first counter. To do. Further, when the controller 43 detects the leading edge of the circumferential length detection toner image, the controller 43 starts the time counting by the built-in second counter and determines the time until the leading edge of the circumferential length detection toner image is detected again. The second measurement time is assumed. That is, the controller 43 counts the time from the density sensor 21 as a starting point until the circumference detection toner image makes a full turn on the transfer belt as the second measurement time. Note that when measuring the first and second measurement times (that is, only when the circumference detection toner image makes a round on the intermediate transfer belt 16), the controller 43 causes the transfer belt cleaning means 61 to perform the intermediate transfer. Separate from the belt 16. When the timing is completed, the controller 43 causes the transfer belt cleaning unit 61 to contact the intermediate transfer belt 16 and removes the circumferential length detection toner image on the intermediate transfer belt 16 from the intermediate transfer belt 16.

  When image density control is performed, the controller 43 sets the intermediate transfer belt to a predetermined length (for example, about several centimeters) by the density sensor 21 with no toner image transferred to the intermediate transfer belt 16. 16 background density (background density) is read. Then, after the time indicated by the following Equation 1 has elapsed from the time when the background density is detected, the controller 43 starts exposure for forming a toner patch row on each of the photoconductive drums 12a to 15a. Thus, each color toner patch is transferred to the intermediate transfer belt 16. As a result, each color toner patch is transferred to the position on the intermediate transfer belt 16 where the background density is detected.

[Formula 1]
Second measurement time−first measurement time + time required for the intermediate transfer belt 16 to move from the head of the toner patch row to the position where the formation of the toner patch of the color is started.

  Thereafter, the controller 43 performs image density control according to the toner patch density detected by the density sensor 21. In the image density control, in order to obtain a desired output density, the image forming conditions such as the charging potential of the photosensitive drum, the exposure intensity, and the developing bias condition are controlled. The toner patch density is corrected with the background density obtained from the detection sensor 21, and the correction value of the image data necessary to accurately reproduce the image density is calculated according to the corrected toner patch density, thereby forming an image. At this time, the image data is corrected using this correction value.

  In this manner, the time until the reference toner image (peripheral length detection toner image) is detected on the intermediate transfer belt 16 by the density sensor 21 is measured as the first measurement time. When the reference toner image is detected, the time until the reference toner image is detected again by the density sensor 21 is measured as the second measurement time. Based on the first and second measurement times after the background density is detected. Since a toner image for image density detection is formed, even if the circumference of the intermediate transfer belt 16 changes, a new sensor is not used in addition to the density sensor, and the background of the ground can be accurately controlled. An image density detection toner image can be formed at the reading position (background reading position).

  The circumferential length of the intermediate transfer belt 16 is measured according to the environmental atmosphere or the number of printed sheets (for example, when the temperature of the 10,000 sheets of printed sheets or the temperature near the intermediate transfer belt in the image forming apparatus changes greatly (10 degrees or more). ) Done. If the peripheral length of the intermediate transfer belt 16 is detected periodically, even if there is a variation in the peripheral length of the intermediate transfer belt 16 in each image forming apparatus, the circumference of the intermediate transfer belt 16 is different for each image forming apparatus. The peripheral length of the intermediate transfer belt 16 detected by the controller 43 is stored in a storage device (not shown), and the peripheral length is used for image density control thereafter. .

  In the above example, the example in which the image density control is performed in consideration of the background density of the intermediate transfer belt 16 has been described. However, in the image forming apparatus using the intermediate transfer drum as the intermediate transfer body, the background density of the intermediate transfer drum is detected. The present invention can also be applied to image density control. In this case, the intermediate transfer drum may be driven by a stepping motor. Further, the present invention can be applied to the case where the background density of the photosensitive drum is detected and image density control is performed. In this case, the photosensitive drum may be driven by the stepping motor 41. In any case, it is used when image density control is performed in consideration of the background density of the image carrier by driving an image carrier such as an intermediate transfer belt, an intermediate transfer drum, or a photosensitive drum by the stepping motor 41. Can do.

  The intermediate transfer belt 16 has an elastic layer formed on the substrate, and, for example, a fluorine layer is formed on the elastic layer. When the film thickness of the fluorine layer is 4 μm or less, the intermediate transfer belt 16 is formed on the intermediate transfer belt 16. When the density correction toner image (toner patch) is read by the density sensor 21, the density data of the toner patch for one rotation of the intermediate transfer belt 16 is averaged, and image density control is performed according to this average value, an error occurs. It was confirmed that the image density became extremely large and the image density unevenness became remarkable. According to the inventors' experiment, when the density data (voltage value) for each toner patch is equal to or higher than the average value (voltage value) ± 0.25 V of the density data, the image density control is performed as described above. It was confirmed that this was particularly effective.

  The time from when the photosensitive drum is exposed to when the reference toner image formed by development is transferred from the photosensitive drum to the intermediate transfer member and the reference toner image is detected on the intermediate transfer member is a first measurement time. Then, the time until the reference toner image is detected again is measured as the second measurement time, and the fluctuation of the circumference of the transfer body during the first and second measurement is grasped. As a result of forming an image density control toner image at the position of the intermediate transfer body where the density is detected, it is possible to accurately control the image density according to the background density and the toner image density without adding a new sensor. It can be applied to image density control in an image forming apparatus such as a copying machine, a printer, or a facsimile machine.

1 is a diagram schematically showing Embodiment 1 of an image forming apparatus according to the present invention. FIG. 2 is a diagram illustrating an intermediate transfer belt and a photosensitive drum in the image forming apparatus illustrated in FIG. 1. FIG. 2 is a view showing an example of a developing device used in the image forming apparatus shown in FIG. 1 together with a photosensitive drum. FIG. 2 is a block diagram showing a drive control system for an intermediate transfer belt used in the image forming apparatus shown in FIG. 1. It is a figure which shows an example of the density sensor used with the image forming apparatus shown in FIG. It is a figure which shows the relationship between a printing rate and a density sensor output.

Explanation of symbols

11 Image forming devices 12 to 15 Image forming units 12a to 15a Photosensitive drum 16 Intermediate transfer belt 17 Driving roller 18 Driven roller 19 Backup roller 20 Pressing roller body 21 Density sensors 22a to 22d Primary transfer roller 23 Paper feeding device 26 Secondary Transfer roller 27 fixing device 31 developing device 32 toner 33 carrier 34 magnetic brush 35 magnetic roll 36 toner thin layer 37 developing roll 38 regulating blade 41 stepping motor 42 gear train 43 controller 61 transfer belt cleaning means

Claims (4)

An intermediate transfer body on which a toner image is formed; and a density detection means for detecting the density of the background of the intermediate transfer body and detecting the density of the toner image formed on the intermediate transfer body. And an image forming apparatus that performs image density control according to the density of the toner image,
A reference toner image formed by development after the photosensitive drum is exposed is transferred from the photosensitive drum to the intermediate transfer member until the reference toner image is detected on the intermediate transfer member by the density detecting means. Until the reference toner image is again detected by the density detection means after the reference toner image formed on the intermediate transfer body is detected by the density detection means. And a control means for forming a toner image for detecting the image density after detecting the background density based on the first and second measurement times. apparatus.   The image forming apparatus according to claim 1, wherein the control unit measures the first and second measurement times when the environmental atmosphere changes or when a predetermined number of printed sheets is reached.   The image forming apparatus according to claim 1, wherein the intermediate transfer member is an intermediate transfer belt or an intermediate transfer drum. Cleaning means for cleaning the intermediate transfer member in contact with the intermediate transfer member;
3. The image forming apparatus according to claim 1, wherein the control unit separates the cleaning unit from the intermediate transfer member when measuring the first and second measurement times. apparatus.

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