JP5679888B2 - Color image forming apparatus - Google Patents

Description

  The present invention relates to an apparatus having a plurality of image forming units such as a color printer and a color copying machine using an electrophotographic recording method, and in particular, can reduce color misregistration in a recording medium conveyance direction and can output a high-quality image. The present invention relates to a color image forming apparatus.

Various electrophotographic color image forming apparatuses having a plurality of image forming units have been proposed for speeding up. Specifically, an image formed by developers (toners) of yellow Y, magenta M, cyan C, and black Bk, which are different colors, is sequentially transferred onto an intermediate transfer belt or a recording material held on a conveyance belt. Problems of the tandem type color image forming apparatus, which is a tandem type color image forming apparatus having a plurality of image forming units aimed at speeding up, are as follows. Due to mechanical accuracy or the like, speed fluctuations of a plurality of photosensitive drums or intermediate transfer belts, and meandering of the intermediate transfer belt occur. As a result, the difference in the amount of movement between the outer peripheral surface of the photosensitive drum and the intermediate transfer belt at the transfer position of each image forming unit occurs for each color, and does not match when the images are superimposed, and the color of 100 to 150 μm. This is to generate a positional shift that is a shift.

  Therefore, a method for forming a visible image for position detection with a toner image when image formation is not performed in each color image forming unit, and transferring the image onto a moving member such as an intermediate transfer belt to form a position detection mark. Many have been proposed. This method includes a position detection mark and a mark detection unit that detects the position detection mark, and controls each image forming unit to correct a transfer image shift based on a detection signal of the mark detection unit. It is the method that was made.

  In Patent Document 1, in order to perform position correction when image formation is not performed, an electrostatic latent image is transferred onto a moving member so as to suppress toner consumption, and the position of the electrostatic latent image is detected. However, a method for correcting the image shift has been proposed. More specifically, in Patent Document 1, an electrostatic latent image having a predetermined pattern for color misregistration adjustment is formed on a first image carrier, and a charge for forming the electrostatic latent image is formed with a high resistance material. To be transferred to the transfer target. Then, a predetermined electrostatic latent image is formed on the second image carrier, and superimposed on the electrostatic latent image transferred from the first image carrier to the transfer target, The formed electrostatic latent image is transferred onto the transfer target. In this case, the amount of charge moving from the second image carrier to the transfer target is detected, and the presence or absence of color shift or the degree of color shift is determined based on the detected value.

  Japanese Patent Application Laid-Open No. 2003-228561 discloses a method of performing alignment by reading an image pattern created without using toner in order to perform position correction during image formation. The configuration of Patent Document 2 is shown in FIG. The first image forming drum 26 </ b> A is driven by the belt 10, and the downstream image forming drums 26 </ b> B, 26 </ b> C, and 26 </ b> D are driven by respective motors 64. The writer 66 is associated with the first drum 26 </ b> A and writes a code 58 on the belt 10.

  The writing device 66 is synchronized with the pulse signal obtained from the encoder 54 corresponding to the drum 26A. The magnetic recording method is used as the writing method. Reference numeral 58 is read by the detector 60. Each of the detectors 60 sends a signal indicative of the local displacement of the belt 10 to the respective controller 68. The controller 68 further receives a signal from the encoder of the corresponding drum 26, and performs feedback control of the drive motor 64 of this drum so that the displacement of the drums 26B to 26D is controlled by reference numeral 58. An eraser 70 for erasing the code is arranged after the last image forming unit 24D.

  By configuring in this way, it is possible to control all image forming media to the target speed determined from the motion of the image carrier, thereby reducing the difference between all image distortions to zero. .

JP-A-10-39571 Japanese Patent Laid-Open No. 10-293435

  In Patent Document 1, the positional relationship between the electrostatic latent image patch transferred from the first photosensitive drum to a paper conveyance belt and the electrostatic latent image patch formed on the downstream photosensitive drum is grounded from the downstream photosensitive drum. This is detected by the amount of current flowing through the. Therefore, the resolution of the position shift that can be detected is determined by the measurement resolution of the minute current amount. According to Patent Document 1, it is described that a current difference of 2 μA corresponds to a positional deviation of 0.125 mm. That is, in order to detect a positional shift in units of microns, it is necessary to decompose the current measurement up to a unit of 0.01 μA.

  In addition, in order to cope with various recording papers, instead of a direct transfer method in which toner is directly transferred from a photosensitive drum to a recording paper, a toner image of four colors is once transferred and formed on an intermediate transfer belt, and then the recording paper is used. For this, the indirect transfer method that transfers in a batch is more suitable. In Patent Document 1, since a paper conveyance belt in which a high-resistance material polyester is coated on a metal belt at 50 μm is used, the configuration cannot be applied to a recording method using an intermediate transfer belt for secondary transfer of toner at a high voltage. There was a problem of being.

  Further, since the technique disclosed in Patent Document 1 corrects misalignment when image formation is not performed as described above, it cannot cope with image misalignment that occurs during image formation. Met.

  On the other hand, Patent Document 2 corrects an image position while forming an image. First, the image position information transferred from the first photosensitive drum is written in an information writing area provided on the intermediate transfer belt at the image transfer position of the first photosensitive drum. Next, at the transfer position of the second photosensitive drum, the first image position information written on the intermediate transfer belt and the position information recorded or stored corresponding to the image information of the second photosensitive drum are simultaneously read. The rotational position of the photosensitive drum is controlled to be aligned by the photosensitive drum driving means provided independently. In addition, the writing of the position information on the intermediate transfer belt uses a method such as magnetic recording.

  When configured as described above, in the first image forming unit, detection means for detecting information written on the photosensitive drum on the same main scanning line at the image transfer position of the photosensitive drum and the intermediate transfer belt, It is necessary to arrange writing means for writing information on the transfer belt. In that case, mutual relative positioning errors occur, and reading errors and writing errors occur when reading information on the photosensitive drum and writing the information on the intermediate transfer belt.

  Positioning errors generally occur from several μm to several tens of μm, and reading errors and writing errors are several μm depending on the inclination of the detecting means / writing means or the inclination of the intermediate transfer belt, although depending on the method. It is common for this to occur. Further, the relative positional deviation between the detecting means and the writing means is caused by several μm due to the influence of temperature and vibration. Further, in the second image forming unit and thereafter, detection means for detecting information written on the photosensitive drum on the same line at the image transfer position of the photosensitive drum and the intermediate transfer belt, and information written in the intermediate transfer belt A similar error occurs when the detection means for detecting the signal is arranged.

  As described above, the apparatuses disclosed in Patent Document 1 and Patent Document 2 do not disclose any countermeasures against these errors, and it is difficult to actually perform high-precision alignment of toner images. It was.

  Therefore, the present invention can perform image position correction with high accuracy while forming an image, and can reliably form a high-quality image without being affected by the contamination of the developer (toner). An object of the present invention is to provide a color image forming apparatus having a high image quality.

In order to achieve the above object, each of the color image forming apparatuses according to the present invention is rotatable and sequentially arranged in a predetermined direction so as to form an image with a developer corresponding to a color in each effective image area. A plurality of image carriers, a movable transfer medium that faces the plurality of image carriers, and an image formed on the image carrier that faces the image carrier via the transfer medium. A plurality of transfer members that respectively transfer to the transfer medium, and a first electrostatic latent image scale corresponding to the image by the developer are formed in a scale drawing area outside the effective image area provided in each of the plurality of image carriers. An electrostatic latent image on the upstream side of the first electrostatic latent image graduation on the side end portion outside the effective image area in the direction that intersects the moving direction of the first graduation forming means and the transfer medium When the scale is transferred with the developer A second graduation forming means for forming a second electrostatic latent image graduation by copying, and the upstream electrostatic latent image provided on the opposite side of the image carrier with the transfer medium interposed therebetween; A first detection means for detecting the first electrostatic latent image graduation downstream of the graduation; and a first detection means provided on the opposite side of the image carrier from the transfer medium, The position of the transfer medium in the moving direction coincides, and the second static detection scale is detected by the second detection means, the first detection means, and the first detection means. Control means for controlling the rotational speed of the image carrier on the downstream side so as to reduce detection deviation of the electrostatic latent image graduation and the second electrostatic latent image graduation.
In another color image forming apparatus according to the present invention, each of the plurality of color image forming apparatuses is rotatable, and is arranged in order in a predetermined direction so as to form an image with a developer corresponding to the color in each effective image area. An image carrier, a movable transfer medium that faces the plurality of image carriers, and the image carrier that is opposed to the image carrier via the transfer medium, and that forms an image on the image carrier. A color image forming apparatus having a plurality of transfer members, each of which is a scale drawing area provided on each of the plurality of image carriers, and is located on a side edge outside the effective image area in a direction intersecting the moving direction. A first electrostatic latent image graduation corresponding to the image by the developer is formed in a graduation drawing area starting from an area preceding the effective image area in the moving direction and reaching each of the effective image areas. Stillness A latent image graduation forming unit and an electrostatic latent image graduation on the upstream side of the first electrostatic latent image graduation at a side end portion outside the effective image area in the direction that intersects the moving direction in the transfer medium. And a second transfer member for forming a second electrostatic latent image graduation by transferring the image when transferring the image with the developer, and provided on the opposite side of the image carrier with the transfer medium interposed therebetween. A first detection means for detecting the first electrostatic latent image graduation downstream from the upstream electrostatic latent image graduation, and on the opposite side of the image carrier with the transfer medium interposed therebetween. A second detection means for detecting the second electrostatic latent image graduation, the first detection means and a position in the moving direction of the transfer medium coincide with each other, the first detection means, and the first detection means; The first electrostatic latent image graduation and the second electrostatic latent image by two detection means So as to reduce the detected deviation of the prime, and having a control means for controlling the rotational speed of the downstream image bearing member.

According to the present invention, color misregistration in the recording medium conveyance direction can be extremely reduced,
It is possible to prevent the detection signal of the detection means from being deteriorated due to developer (toner) contamination.

1 is a schematic front sectional view of a color image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a schematic perspective view of a detection system that detects electrostatic latent image graduations on a drum and a belt in the first embodiment. FIG. 3 is a side view illustrating an uppermost stream side image forming unit in the first embodiment. FIG. 3 is a diagram illustrating a configuration and a positional relationship between a primary transfer roller and an electrostatic belt scale transfer roller on the most upstream image forming unit in the first embodiment. FIG. 2 is a side view illustrating an image forming unit on the downstream side of the first embodiment. It is a figure explaining arrangement | positioning of the electric potential sensor which detects the electrostatic latent image scale on the drum of the downstream image formation part of 1st Embodiment. FIG. 3 is a diagram illustrating an arrangement of a potential sensor that detects an electrostatic latent image graduation on a belt of an image forming unit on the downstream side of the first embodiment. It is a top view explaining the structure of the electric potential sensor in 1st Embodiment. It is a side view explaining the structure of the electric potential sensor in 1st Embodiment. It is a top view which shows the positional relationship when an electric potential sensor reads the scale line by the electric charge on a belt. It is a side view which shows the positional relationship when an electric potential sensor reads the scale line by the electric charge on a belt. (A) is a figure which shows a mode that the scale of 0.3384 mm pitch by the electric charge was formed in the to-be-transferred part, electric potential distribution, and a scale detection output voltage. FIG. 6 is a diagram showing a state in which a scale having a pitch of 0.1692 mm is formed on the transferred portion of the present invention, (b) is a diagram showing a potential distribution, and (c) is a diagram showing a scale detection output voltage. It is a figure which shows the magnitude | size of a detection part and a scale when an electric potential sensor detects the scale of 0.0423 mm pitch by the electrostatic latent image on a belt. It is a figure which shows the magnitude | size of a detection part and a scale when an electric potential sensor detects the scale of 0.0423 mm pitch by the electrostatic latent image on a drum. It is a figure which shows a state in case a potential sensor has two detection parts and the phase has shifted | deviated 90 degrees. It is a figure which shows the magnitude | size at the time of detecting the scale of 0.0423 mm pitch by an electrostatic latent image, and its output waveform, when an electric potential sensor has two detection parts and a phase has shifted | deviated 90 degree | times. FIG. 6 is a diagram illustrating an arrangement when a toner image and an electrostatic belt scale are transferred onto an intermediate transfer belt. FIG. 4 is a diagram illustrating a configuration of a top electrostatic belt scale of a page in an arrangement when a toner image and an electrostatic belt scale are transferred onto an intermediate transfer belt. It is a figure explaining the scale alignment operation | movement in 1st Embodiment. It is a block diagram of control in a 1st embodiment. It is a flowchart explaining the operation | movement in 1st Embodiment. It is a top view explaining the structure of two electric potential sensors on the same board | substrate in 2nd Embodiment. It is a top view explaining the structure which made two potential sensors on the same board | substrate approach in the 2nd Embodiment more. . It is a perspective view explaining 2nd Embodiment. FIG. 10 is a side view illustrating an image forming unit on the downstream side of the second embodiment. It is a perspective view explaining a 3rd embodiment. FIG. 10 is a side view illustrating an uppermost stream side image forming unit according to a third embodiment. FIG. 10 is a side view illustrating an image forming unit on the downstream side of a third embodiment. It is a figure explaining the output of the electric potential sensor of 3rd Embodiment. It is a figure which shows the relative position of the electrostatic latent image scale of the head of a page, and an electrostatic belt scale regarding 3rd Embodiment. It is a figure explaining a prior art example.

<< First Embodiment >>
(Image forming device)
In the color image forming apparatus according to the present embodiment described below, it is possible to form an image using only a single color, for example, only red, in addition to black, in addition to a full color. 1 to 7 are diagrams for explaining the configuration of the first embodiment. 1 is a front view, FIG. 2 is a perspective view, FIGS. 3 and 5 are side views as seen from the upstream side in the belt conveyance direction, and FIG. 4 is a partially enlarged view for explaining the configuration around the primary transfer roller in FIG. 6 and 7 are detailed illustrations of the arrangement of the photosensitive drum, the intermediate transfer belt, and the sensor.

  In general, in a tandem type image forming apparatus, as shown in FIGS. 1 and 2, four or more photosensitive drums 12a, 12b, 12c, and 12d are provided on one intermediate transfer belt 24. Is placed. The photosensitive drums 12a, 12b, 12c, and 12d are rotatable in order, and are sequentially arranged in a predetermined direction so as to form images of different colors of yellow Y, magenta M, cyan C, and black Bk.

  A configuration for transferring a color toner image onto the intermediate transfer belt 24 by the primary transfer rollers 4a, 4b, 4c and 4d, which are transfer members, and further transferring the image onto paper as a recording medium is shown in FIGS. Will be described. The intermediate transfer belt 24 is wound around at least three rollers: a belt driving roller 36 that gives a rotational driving force, a belt driven roller 37, and a secondary transfer roller 38.

  A constant tension is applied by the belt driven roller 37 or the secondary transfer roller 38. In the vicinity of the belt driving roller 36, the secondary transfer section 44 scrapes off the remaining toner that is not transferred onto the paper as the recording medium but remains attached to the surface of the intermediate transfer belt 24, and cleans the surface of the intermediate transfer belt. The belt cleaning unit 45 is provided. A grounded static eliminating brush (not shown) is disposed at both ends of the belt cleaning unit 45 and opposed to the transfer target 25 provided on the surface of the intermediate transfer belt 24, and is in contact with the transfer target 25. Thus, the electrostatic belt scale 32 transferred to the transferred portion 25 can be erased.

  Alternatively, as shown in FIG. 1, the transfer portion 25 on the intermediate transfer belt 24 is sandwiched between the belt drive roller 36 and the first photosensitive drum 12a with the upper corona charger 46a and the lower corona charger 46b. Deploy. The latent image belt scale 32 of the transferred portion 25 may be surely erased by applying an AC voltage having an opposite phase.

(First image forming part on the most upstream side)
Next, the configuration of the image forming units 43a, 43b, 43c, and 43d will be described with reference to FIGS. Reference numeral 12a in the first image forming unit 43a on the most upstream side is a first photosensitive drum. The driving force is transmitted from the drum driving motor 6a to the drum rotating shaft 5a via a driving system that transmits the driving force to the rear side of FIG. On the other side (front side), a drum encoder 8a composed of a rotary encoder is connected to the drum rotating shaft 5a via a coupling (not shown).

  In the first image forming unit 43a, the drum driving motor 6a is always rotated based on the output signal from the drum encoder 8a, so that the rotation of the photosensitive drum 12a rotates at an equal angular speed counterclockwise in the direction of the arrow. Drive control. In this embodiment, a photosensitive drum 12a made of an OPC photosensitive member having a photosensitive layer thickness of 30 μm is used.

(Scale of electrostatic latent image on the drum in the first image forming section on the most upstream side)
When a toner image is formed on the surface of the photosensitive drum 12a, first, the photosensitive member on the surface of the photosensitive drum is uniformly charged to about minus -600V by the charging unit 14a. Next, the first exposure means 16a scans the laser beam in accordance with the image signal to change the surface potential of the laser beam irradiated portion on the surface of the first photosensitive drum 12a to about -100V to form an electrostatic latent image. At this time, as shown in FIG. 2, electrostatic latent image graduation lines are formed by irradiating laser light before and after writing an effective image on both ends outside the effective image area at positions where the exposure position 42a of the first photosensitive drum 12a is extended. 31a is written.

  That is, development is performed on a scale drawing area that starts from an area that is outside the effective image area in the direction intersecting the moving direction and that precedes the effective image area in the moving direction and reaches each effective image area. A first electrostatic latent image index 31a corresponding to the image formed by the agent is formed. The electrostatic latent image graduation line 31a, which is the first electrostatic latent image graduation, is formed immediately after the photosensitive drum 12a before the image is written on the photosensitive drum 12a, that is, from the front end of the effective image area. The formation is started from the preceding blank portion or the inter-paper portion. The electrostatic latent image graduation line 31a is continuously written on the photosensitive drum 12a until image formation on the first photosensitive drum 12a is completed. When the electrostatic latent image index 31a is about 5 mm in the axial direction of the photosensitive drum 12a and the resolution of the image in the sub-scanning direction is 1200 dpi, 25.4 ÷ 1200 × 2 = 0.0423333. ... formed at a pitch of 42.3 μm from mm.

(Image formation in the first image forming unit on the most upstream side)
The electrostatic latent image in the effective image area portion whose surface potential has been changed to about −100 V by the laser light irradiation is developed by the negatively charged yellow Y toner by the developing means 18a to form the first image yellow Y. At this time, the development area of the developing device 18 is an area of the electrostatic latent image graduation line 31a as shown in FIG. 3 so that the toner is not developed on the electrostatic latent image graduation lines 31a at both ends of the photosensitive drum 12a. The length is determined to be narrower than the width of the primary transfer roller 4a.

  Next, in the first image forming unit on the most upstream side, the Y toner image forming the first image is transferred to the intermediate transfer belt 24 in the first transfer unit where the first photosensitive drum 12a and the intermediate transfer belt 24 are in contact with each other. . In the first transfer portion, as shown in FIG. 4, a primary transfer roller 4a formed of a conductive sponge 601a having an outer diameter of 16 mm and a metal core metal 602a having a diameter of 8 mm is disposed. The toner is transferred by a positive electric field of about +1000 V applied by the primary transfer roller 4a. At this time, as shown in FIG. 3, electrostatic latent image graduation lines 31a formed on the photosensitive drums 12a at both ends of the surface of the intermediate transfer belt 24 are also provided on the surface side (photosensitive drum side) of the intermediate transfer belt 24. It is transferred to the transferred portion 25. This will be described in detail later.

  As shown in FIG. 4, the primary transfer roller 4 a is supported by conductive bearings 603 at the shafts protruding from both ends. The bearing 603 is guided by a bearing support portion 604 fixed to a frame (not shown) so as to be movable in the vertical direction, and a high pressure supply spring 605 of a compression coil spring is incorporated in a lower portion of the bearing 603. The high-pressure supply spring 605 is connected to a high-pressure source at a lower end portion by a wiring (not shown), and is configured to be able to apply a high pressure to the primary transfer roller 4a via a bearing 603. At the same time, the primary transfer roller 4a is pressed against the photosensitive drum 12a from both sides via the intermediate transfer belt 24 by a pressing mechanism using a high-pressure supply spring 605.

(Transfer of the most upstream image and electrostatic latent image scale to the intermediate transfer belt)
The image of the developer on the uppermost photosensitive drum 12a is transferred to the intermediate transfer belt 24 using the primary transfer roller 4a. At the time of transferring the image, the electrostatic latent image graduation line 31 a on the uppermost photosensitive drum is transferred to the intermediate transfer belt 24.

  Here, the electrostatic belt which is a second transfer member for transferring the electrostatic latent image graduation line 31a on the photosensitive drum to the transferred portion 25 on the intermediate transfer belt as a second electrostatic latent image graduation. The scale transfer roller 47 will be described. As shown in FIG. 4, the electrostatic transfer is performed so that the rotation center of the primary transfer roller 4a is in a straight line on the opposite side of the intermediate transfer belt 24 with respect to the transferred portion 25 (photosensitive drum side) and on both sides of the primary transfer roller 4a. A belt scale transfer roller 47 is disposed. As with the primary transfer roller 4a, the electrostatic belt scale transfer roller 47 is composed of a conductive sponge 601a having a diameter of 16 mm and a metal core 602a.

  As with the primary transfer roller 4a, the electrostatic belt scale transfer roller 47 is supported by conductive bearings 603 at the shafts protruding at both ends. A high-pressure supply spring 605 of a compression coil spring is incorporated in a lower portion of the bearing 603 so that the bearing 603 is guided so as to be movable in the vertical direction with respect to a bearing support portion 604 fixed to a frame (not shown). ing. The high voltage supply spring 605 is connected to a high voltage source (not shown) different from that of the primary transfer roller 4a by a wiring (not shown) at the lower end, and applies a high voltage of about +500 V to the electrostatic belt scale transfer roller 47 via the bearing 603. It can be applied. At the same time, the electrostatic belt scale transfer roller 47 is pressed against the photosensitive drum 12a from both sides via the intermediate transfer belt 24 by a pressing mechanism using a high-pressure supply spring 605.

(Restrictions on two types of transfer rollers for transferring the image on the most upstream side and the electrostatic latent image scale / securing creepage distance and spatial distance)
Although about + 1000V is applied to the primary transfer roller 4a, the optimum condition for toner transfer may be about + 1500V depending on environmental conditions. Further, since a high voltage of about + 500V is applied to the electrostatic belt scale transfer roller 47, the potential difference may be about 1000V. Since any roller contacts the back surface of the intermediate transfer belt 24 and applies a high voltage, it is necessary to ensure a creeping distance of a certain distance or more. In this embodiment, since there is a maximum potential difference of 1000 V, it is necessary to ensure 8 mm as the creeping distance (the distance between the right end of the primary transfer roller 4a and the left end of the electrostatic belt scale transfer roller 47 in FIG. 4).

  Similarly, a high voltage is applied to the conductive bearing 603 and the cored bar 602 that support the two rollers. Therefore, the space between the bearings 603 or the cored bar 602 must also have a spatial distance corresponding to a potential difference of 1000 V (the distance between the right end of the left bearing 603 and the left end of the center bearing 603 in FIG. 4). It is necessary to leave an interval of 5 mm or more. In order to achieve both of them, in the present embodiment, the core metal 602 protrudes from the bearing 603 only slightly, and the core metal 602a and the core metal 602b are arranged so that the distance between them is 6 mm. As a result, since the dimension in the width direction of the bearing 603 is 2 mm, the gap between the conductive sponges 601a and 601b can be ensured to be 10 mm or more even if the sponge portion is somewhat crushed.

  Here, as will be described later in detail as an image forming unit on the downstream side, electrostatic latent image scale reading sensors 34b to 34d (FIG. 2) as first detection means for reading the electrostatic latent image scale formed on the photosensitive drum 12. Consider a configuration in which FIG. 5) directly contacts the photosensitive drum 12. Since the sensor 34 wants to read at a position where the toner image is transferred to the intermediate transfer belt 24 and extended in the main scanning direction, the sensor 34 is long enough to protrude from the end of the intermediate transfer belt 24. It is necessary to do it.

  That is, the electrostatic latent image for detecting the electrostatic latent image on the photosensitive drum 12 outside the end of the belt scale reading sensor 33 as the second detecting means outside the range where the intermediate transfer belt 24 exists. The scale reading sensor 34 needs to be arranged. Here, as described above, in order to secure a creepage distance and a spatial distance between the primary transfer roller 4a and the electrostatic belt scale transfer roller 47, the distance between the two rollers is arranged closer to the distance described above. I can't. For this reason, an extra space is required for arranging the electrostatic latent image scale reading sensor 34 that simply detects the electrostatic latent image on the photosensitive drum 12.

  On the other hand, when the electrostatic latent image graduation reading sensor 34 is configured as in this embodiment such that the electrostatic latent image graduation line 31 is read via the intermediate transfer belt 24 without directly contacting the photosensitive drum 12. This problem does not occur. That is, the electrostatic latent image scale reading sensor 34 may be disposed between the primary transfer roller 4 and the electrostatic belt scale 32 inside the electrostatic belt scale 32 within the range where the intermediate transfer belt 24 exists. It becomes possible. In this embodiment, as will be described later, since the electrostatic latent image scale reading sensor 34 has a width of 4 mm, it is disposed in a space of about 8 mm between the primary transfer roller 4 a and the electrostatic belt scale transfer roller 47. Is possible.

  Therefore, a configuration in which the electrostatic latent image graduation line 31 is read via the intermediate transfer belt 24 as in the present embodiment, as compared with a configuration in which the electrostatic latent image graduation reading sensor 34 is in direct contact with the photosensitive drum 12. It is preferable that In this case, it is possible to reduce the width of the apparatus in the main scanning direction while ensuring the creepage distance and the spatial distance.

(Transfer of electrostatic latent image)
With the above configuration, by applying the following potential difference, a part of the charge forming the electrostatic latent image graduation 31a on the surface of the photosensitive drum 12 facing the transferred portion 25 is transferred to the transferred portion 25. Transcribed. Thereby, as shown in FIG. 2, the electrostatic belt scale 32 having the same pitch as the electrostatic latent image scale line 31a is formed as the second electrostatic latent image scale. At this time, the potential difference between the exposed portion where the electrostatic latent image graduation line 31a is formed is about −100V and the electrostatic belt scale transfer roller 47 + 500V is about 600V.

  On the other hand, in the portion between the electrostatic latent image graduation lines 31a, the potential difference between the non-exposed portion of about -600V and the electrostatic belt scale transfer roller 47 + 500V is about 1100V. Due to the difference between the two potential differences, the state of discharge between the photosensitive drum 12 and the intermediate transfer belt 24 is different. That is, as is apparent from Paschen's discharge conditions, discharge occurs in a portion where the potential difference is large, and discharge does not occur in a portion where the potential difference is small. As a result, the electrostatic latent image index 32 is transferred to the intermediate transfer belt 24.

As in this embodiment, the intermediate transfer belt 24 has a volume resistivity of about 10 ¹⁰ Ω · cm, and as will be described later, the transferred portion 25 is made of a material having a volume resistivity of 10 ¹⁴ Ω · cm or more and a thickness of about 30 μm. In this case, it was found from the experiment that the surface potential of the transferred portion is as follows. That is, the latent image forming portion irradiated with the laser light had a small potential difference and no discharge was generated, so that the discharge was generated because the potential difference was large in the portion not irradiated with the laser light. In this way, the scale due to the difference in surface potential between −600 V and −100 V on the photosensitive drum is transferred as the scale due to the difference in surface potential between −200 V and 0 V on the intermediate transfer belt.

  As shown in FIG. 3, the transferred portion 25, which is a portion attached to the surface side (photosensitive drum side) of the intermediate transfer belt 24, is thicker than the other portions of the intermediate transfer belt 24. . Specifically, the thickness of the transferred portion 25 is about 30 μm, and the thickness of the intermediate transfer belt 24 serving as a base is about 90 μm. Even if the electrostatic belt scale transfer roller 47 is a sponge made of the same material as the primary transfer roller 4a, has the same outer diameter, and the cored bar is arranged in the same straight line, the thickness of the transferred portion 25 is reduced by the sponge. It is a level that can be absorbed sufficiently. The conveyance of the intermediate transfer belt 24 is not particularly affected.

  The toner that cannot be transferred onto the intermediate transfer belt 24 and remains on the surface of the photosensitive drum 12a is scraped off by the drum cleaning unit 22a and collected in a waste toner box (not shown).

  In the present embodiment, the latent image belt scale transfer roller 47 formed of a conductive sponge roller is used as the second transfer member. However, the present invention is not limited to this, and as a means for applying a charge when transferring a latent image, a corona charger using a wire, a charger using a neutralizing core used for a static eliminator, a blade charger, or the like is used. May be.

(Specific configuration of the transferred portion to which the electrostatic latent image scale is transferred)
The transferred portion 25 is made of a material having a volume resistivity of 10 ¹⁴ Ω · cm or more, and in this embodiment, a 30 μm-thick PET film formed in a tape shape having a width of 5 mm is attached to both ends of the intermediate transfer belt 24. It is a thing. In the present embodiment, the transferred portion 25 is provided on the surface side (photosensitive drum side) where the photosensitive drum 12a of the intermediate transfer belt 24 is present. Since the transferred portion 25 is made of a high-resistance material having a volume resistivity of 10 ¹⁴ Ω · cm or more, the transferred charge is held without moving and functions as an electrostatic belt scale 32.

On the other hand, the volume resistivity of the intermediate transfer belt 24 is made of a medium resistance material of 10 ⁹ to 10 ¹⁰ Ω · cm in order to maintain toner transfer performance. When the electrostatic latent image graduation 31a is brought into direct contact with the intermediate transfer belt 24, the electric charge is once transferred, but the electric charge moves because the resistance value is low, and the electrostatic belt graduation 32 can be formed as it is. Can not.

In the configuration in which a color image is secondarily transferred to a transfer medium such as recording paper after a color image is formed on the intermediate transfer belt as in this embodiment, the volume resistance value is different from that of the intermediate transfer belt 24. It is necessary to paste the material. Alternatively, it is necessary to form a region having a high volume resistivity by painting or coating with a spray or the like. As for the material of the transferred portion 25, the material is limited to Teflon (registered trademark) such as PET and PTFE, or polyimide as long as it can be formed on the surface of the intermediate transfer belt 24 with a volume resistivity of 10 ¹⁴ Ω · cm or more. Not what you want.

(Downstream image forming unit)
Next, the configuration of the second image forming unit 43b, the third image forming unit 43c, and the fourth image forming unit 43d on the downstream side shown in FIG. 1 will be described. Since the second image forming unit 43b, the third image forming unit 43c, and the fourth image forming unit 43d all have the same configuration, the configuration of the second image forming unit 43b will be described representatively. FIG. 5 shows the second image forming unit 43b as viewed from the upstream side in the conveyance direction. FIG. 6 shows a cross section AA of the second image forming unit 43b shown in FIG. FIG. 7 shows a cross section BB of the second image forming unit 43b shown in FIG. In FIGS. 6 and 7, the primary transfer roller 4b is omitted.

  The second image forming unit 43b uses the photosensitive drum 12b having the same shape as the first image forming unit 43a.

(Electrostatic latent image scale on the downstream drum)
Here, on the downstream side, as shown in FIG. 5, electrostatic latent image graduation lines 31b corresponding to images are formed in the vicinity of both ends of the second photosensitive drum 12b. In other words, the electrostatic latent image graduation lines 31a formed on both ends of the first photosensitive drum 12a shown in FIG. 3 are electrostatically placed inside the area outside the effective image area (toner transfer range) shown in FIG. A latent image graduation line 31b is formed. Specifically, at the same time as image formation is performed by the exposure unit 16b, electrostatic latent image graduation lines 31b corresponding to the image are formed at both ends thereof.

(Detection means for electrostatic latent image scale on downstream drum)
Further, as shown in FIG. 6, at the position where the transfer position transfer line where the second photosensitive drum 12b and the intermediate transfer belt 24 come into contact and the toner image is transferred is extended in the photosensitive drum axial direction, the intermediate transfer belt 24 An electrostatic latent image scale reading sensor 34b is arranged on the back side. The reason for providing it on the back side of the intermediate transfer belt 24 is as follows.

When the electrostatic latent image scale reading sensor 34b is brought into direct contact with the photosensitive drum 12b, the scale portion and the detection electrode portion of the sensor can be brought close to each other, the sensor output can be increased, and the SN ratio can be increased. Can improve detection accuracy. But,
Although there are very few toners that are not developed and are floating around the photosensitive drum 12, these toners are likely to adhere to the electrostatic latent image graduation line 31 that is the exposed portion of the photosensitive drum 12. .

  For this reason, if the electrostatic latent image graduation reading sensor 34b is in direct contact with the photosensitive drum 12b, the electrostatic latent image graduation line 31 adheres to the contact portion when the continuous operation is performed for a long time. The present inventors have confirmed that toner accumulates. When a certain amount or more is accumulated, it is confirmed that there is toner that passes through the nip formed by the photosensitive drum 12 and the electrostatic latent image graduation reading sensor 34 as a group.

  At that time, the gap between the photosensitive drum 12 and the electrostatic latent image graduation reading sensor 34 is changed, causing the output signal from the electrostatic latent image graduation reading sensor 34 to be greatly disturbed. Therefore, in the present embodiment, although the signal output is slightly reduced, the scale is read via the intermediate transfer belt 24 without directly contacting the photosensitive drum 12 and the electrostatic latent image scale reading sensor 34.

(Detection means for detecting the electrostatic latent image scale transferred on the belt on the most upstream side on the downstream side)
In the second image forming unit 43b, as shown in FIGS. 5 and 7, the belt scale reading sensor 33b is placed on the back side of the intermediate transfer belt 24, and the electrostatic belt scale 32 based on the electrostatic latent image transferred to the transferred portion 25. Is arranged so that it can be detected.

  The space inside the intermediate transfer belt 24 is substantially closed by a rear side plate and a front side plate for supporting the rollers around which the intermediate transfer belt 24 is wound, and the floating toner relatively enters. It has a difficult structure. Therefore, even when continuous operation is performed for a long time, toner or the like does not accumulate in the nip formed by the intermediate transfer belt 24 and the electrostatic latent image scale reading sensor 34, and a stable output can be obtained. A highly reliable device can be provided.

(Positional relationship between two detection means)
As described above, in the second image forming unit 43b, the belt scale reading sensor 33b and the electrostatic latent image reading sensor 34b are disposed on the back side of the intermediate transfer belt 24. These sensors can read both the electrostatic latent image graduation line 31b on the photosensitive drum 12b and the electrostatic belt graduation 32 transferred to the transferred portion 25 of the intermediate transfer belt 24 on the same line as the transfer line. It is arranged as follows. That is, in this embodiment, the electrostatic latent image scale reading sensor 34 as the first detection unit and the belt scale reading sensor 33 as the second detection unit coincide with each other in the movement direction of the intermediate transfer belt. Yes.

(Configuration of two detection means)
A specific configuration of the sensor, which is the two detection means, will be described with reference to FIGS. The electrostatic latent image graduation reading sensor 34 and the belt graduation reading sensor 33 are potential change detection sensors capable of detecting a change in potential, and the basic configuration thereof is described in detail in Japanese Patent Application Laid-Open No. 11-183542. ing. Therefore, only the parts specific to this embodiment will be described.

  FIG. 8 shows the configuration of the potential sensor 330 used in this embodiment. FIG. 9 shows a cross section BB of FIG. As shown in FIG. 8, a conducting wire 331 made of a metal wire having a diameter of 10 μm is bent into an L shape, and the tip thereof becomes a detection unit 334, and the length of the detection unit 334 is about 2 mm. When the potential sensor 330 is used as the electrostatic latent image scale reading sensor 34 or the belt scale reading sensor 33. As shown in FIG. 10, the potential sensor 330 is fixed so that the detection unit 334 and the graduation line composed of electric charges are parallel to each other.

  As shown in FIGS. 8 and 9, the potential sensor 330 is formed by bonding a lead wire 331 bent in an L shape onto a base film 332 made of a polyimide film having a width of 4 mm, a length of 15 mm, and a thickness of 25 μm. Place after applying the agent. A protective film 333 made of a polyimide film having the same size and thickness as the base film 332 is adhered thereto. Although not shown in FIG. 9, the adhesive mainly exists between the base film 332 and the protective film 333, and does not exist between the conductive wire 331 and the base film 332, or between the protective film 333. .

  Therefore, the distance between the surface of the conducting wire 331 and the surface of the base film 332 or the protective film 333 is 25 μm. Further, the end of the L-shaped conducting wire 331 opposite to the detection unit 334 is a signal output unit 335.

  FIG. 10 and FIG. 11 show layout diagrams when this potential sensor 330 is used as the belt scale reading sensor 33. In order for the base film 332 side of the conducting wire 331 to come into contact with the surface (belt back surface) opposite to the position where the transferred portion 25 is attached to the intermediate transfer belt 24, the potential sensor 330 is curved as shown in FIG. It arrange | positions by the support part which is not shown in figure. At this time, the conductive wire 331 may be pressed from above the protective film 333 with a spring so that the distance between the conductive wire serving as the detection unit 334 and the transferred portion 25 is always constant. 10 and 11, the region of the high potential portion 341 having a relatively high potential transferred to the transferred portion 25 is represented by black, and the region of the relatively low potential portion 342 is represented by white.

(Output from the potential sensor that detects the electrostatic latent image scale)
Next, the output from the potential sensor 330 will be described with reference to FIGS. FIG. 12A is a diagram showing the distribution of the high potential portion 341 and the low potential portion 342 of the electrostatic belt scale 32 due to the charges transferred to the transfer target portion 25. As described above, in this embodiment, the surface potential of the portion where the exposed portion on the photosensitive drum is transferred is about 0 V, and the surface potential of the portion where the non-exposed portion is transferred is about -200 V. In the case of FIG. 12A, an exposed portion for 8 lines / 8 spaces and an unexposed portion for 8 lines are repeated at an image resolution of 1200 dpi. This is a case of a latent image graduation having a pitch of 0.3384 mm which is 16 times the pixel pitch of 0.02115 mm with a spacing of 1200 dpi.

  The actual potential distribution does not become a clean rectangular wave because the amount of exposure light by the laser has a distribution and decreases at the peripheral portion, and is a distribution as shown in FIG. When the potential sensor 330 is moved in the direction in which the potential changes in a region where the potential distribution as shown in FIG. 12B exists, an output signal can be obtained from the potential sensor 330. That is, an induced current is generated by a change in the electric field in the vicinity of the detection unit 334 of the potential sensor, and the output voltage of the output unit 335 of the potential sensor 330 is as shown in FIG. A signal having a waveform obtained by differentiating the potential distribution is output. The point of the peak slope 0 of the potential distribution in FIG. 12B is the center of the scale, and the time when the output voltage becomes 0 in FIG. 12C can be specified as the time when the scale line due to the charge is detected.

  In FIG. 12 (c), since the pitch of the latent image graduation is coarse, there is a certain time interval between the occurrence of the potential change and the next potential change, so the output signal from the potential sensor is different from the sine wave. It was a shape. As shown in FIG. 13A, when the pitch of the latent image graduation is half 0.1692 mm, that is, 4 lines / 4 spaces, the potential distribution is as shown in FIG. 13B, and the potential sensor output is It becomes a sine wave as shown in FIG.

(High resolution detection of electrostatic latent image scale)
Further, in the case of a latent image graduation with a minimum pitch of 42.3 μm and a minimum one line / space that can be realized at an image resolution of 1200 dpi by reducing the pitch of the latent image graduation, the size of the detection unit 334 and the size of the latent image graduation The relationship is as shown in FIG. Since the width of 10 μm of the detection unit 334 is less than half the width of 21.15 μm for one line of the latent image graduation, the potential sensor 330 according to the present embodiment extends to the latent image graduation with the minimum pitch that can be realized at 1200 dpi. Can be detected, and its signal output is also a sine wave.

  Similarly to the description in FIG. 12C, the time when the output voltage of the potential sensor 330 becomes 0 can be specified as the time when the graduation line due to the charge is detected. That is, the potential sensor according to the present embodiment can detect a latent image graduation having a pitch of 42.3 μm.

  The above is the description of the case where the belt scale reading sensor 33 reads the electrostatic belt scale 32 transferred to the transferred portion 25 provided on the intermediate transfer belt 24. However, the same applies to the case where the electrostatic latent image graduation line 31 on the photosensitive drum 12 is read via the intermediate transfer belt 24 and the transfer belt 24 as shown in FIG. However, as shown in FIG. 15, the electrostatic latent image scale reading sensor 34 including the potential sensor 330 needs to be disposed directly below the nip portion formed by the photosensitive drum 12 and the intermediate transfer belt 24.

  That is, this position is a position where the toner image formed on the photosensitive drum 12 is transferred to the intermediate transfer belt 24, and the distance between the electrostatic latent image graduation line 31 and the detection portion of the electrostatic latent image graduation reading sensor 34 is as follows. It is the position where the sensor output becomes the smallest and the sensor output becomes the maximum.

  Furthermore, in order to read the latent image graduation at a high resolution, a potential sensor b330b in which conductors a331a and b331b as shown in FIG. You may make it use. In FIG. 16, when the detection unit is arranged with a quarter of the minimum latent image graduation pitch of 42.3 μm, that is, with the phase shifted by 90 °, the output signal is also shifted by 90 ° as shown in FIG. Two outputs can be obtained.

  If a sine wave whose phase is shifted by 90 ° as shown in FIG. 17 is used, the signal can be electrically divided. As for the electric division method, no new method is used, and the division into 16 can be easily realized by using the method described in Japanese Patent Application Laid-Open No. 2003-161645. Division can also be easily realized. As a result, a scale signal with a pitch of 42.3 ÷ 64 = 0.66 μm can be obtained, and a signal having a resolution sufficient to adjust the position in units of μm can be obtained.

  As described above, by using the potential sensor 330 that detects a potential change, the latent image graduation formed of the potential distribution can be measured with sufficiently high accuracy. The case where the potential distribution of the transferred portion 25 is measured by the potential sensor 330 has been described above, but the electrostatic latent image graduation line 31 formed on the photosensitive drum 12 is read from the electrostatic latent image graduation formed by the potential sensor 330. Similarly, when reading with the sensor 34, reading with high resolution is possible.

(Explanation of image alignment operation)
Next, with reference to FIG. 18 to FIG. 20, an operation for actually performing image alignment, that is, performing scale alignment after the second image forming unit 43b will be described. FIG. 18 shows the positional relationship between the toner image transferred onto the A4 horizontal recording paper transferred onto the intermediate transfer belt by the first image forming unit 43a and the electrostatic belt scale 32 transferred to the transfer target 25. It is a figure explaining the structure. FIG. 19 is a partially enlarged view for explaining the configuration of the electrostatic belt scale at the front end of the image shown by part A in FIG.

  In FIG. 18, the toner image of the image formed on the A4 horizontal paper by the first image forming unit 43a and the electrostatic belt scale 32 are transferred onto the intermediate transfer belt 24 for two continuous pages by continuous rotation of the photosensitive drum 12a. Shows the state. In FIG. 18, the electrostatic belt scale 32 is formed only in the leading edge margin, the effective image area, and the trailing edge margin as one page, and the electrostatic belt scale is formed in the inter-paper portion up to the leading margin for the next second page. Although 32 is not formed, it may be formed also in this inter-paper portion.

  When transferring a toner image from the photosensitive drum to the intermediate transfer belt, and further from the intermediate transfer belt to the recording paper, the transfer operation is generally performed while sliding each other with a speed difference of about 0.5%. is there. However, in this embodiment, in order to simplify the description, a toner image having a slip amount in the conveyance direction of zero and having the same size as the toner image after transfer onto the recording paper is formed on the photosensitive drum and the intermediate transfer belt. To do.

  It is not possible to form an image on the entire surface of A4 horizontal recording paper, and image formation is performed with margins on the front, back, left and right of the recording paper. In the case of this embodiment, as shown in FIG. 18, the front and rear margins are 2.5 mm, and the left and right margins are 2 mm. When image formation for one page is performed on the photosensitive drum 12a of the first image forming unit 43a, an exposure operation is started from a portion corresponding to the leading margin of the recording paper, and a toner image forming area of 2.5 mm is formed. The formation of electrostatic latent image graduations 31a is started at both ends of the photosensitive drum 12a from the front.

  In this embodiment, the image forming apparatus has an image resolution of 1200 dpi, and the pitch of the laser light to be exposed is set to 0.02115 mm from 25.4 mm / 1200 = 0.021166666. In order to form the electrostatic latent image graduation line 31a, the graduation with the minimum pitch is one line / one space in which exposure / non-exposure is repeated every other line. In the present embodiment, the smallest graduation pitch is 0. 02115 × 2 = 0.0423 mm.

  Therefore, the electrostatic latent image graduation line 31a in the area where the toner image is formed forms a graduation with a pitch of 0.0423 mm, which is the minimum pitch that can be formed in 1 line / 1 space. As described above, in this embodiment, the scale of 0.0423 mm is read by the potential sensor 330b whose phase is shifted by 90 °, and can be used as a scale of 0.66 μm pitch.

  Further, in the present embodiment, in order to surely perform the first scale alignment after the second image forming section on the downstream side, the pitch is larger than the effective image area at the leading edge margin when forming an image for one page. An exposure operation is performed so as to form a scale. FIG. 19 is an enlarged view of a portion A in FIG. 18 and shows a configuration of an electrostatic latent image scale formed in a blank portion at the leading end of the image. In FIG. 19, first, a graduation line is formed at a portion corresponding to the head of the margin, and four graduations with a pitch of 0.3384 mm corresponding to 8 times 0.0423 mm which is the scale pitch of the effective image portion are formed.

  Subsequently, three scale lines having a pitch of 0.1692 mm with half the pitch are formed. Next, three graduations with a pitch of 0.08846 mm with a half pitch are formed, and then a 0.0423 mm graduation with the same pitch as that formed in the effective image area is formed. An electrostatic latent image scale having a pitch of 0423 mm is formed.

As shown in FIG. 19, the area for forming a scale pitch larger than the scale pitch of the image forming unit is
0.3384 × 3 + 0.1692 × 3 + 0.0846 × 3 = 1.0152 + 0.5076 + 0.2538 = 1.7766 mm, which is an area shorter than the leading edge margin. Similarly, after the second image forming unit 43b, the leading edge margin scale pitch starts from coarse to fine, starting from 8 times the scale pitch of the effective image portion, and gradually increases from 4 to 2 times. To make the minimum pitch scale.

  In the conventional electrophotographic apparatus, since the image position deviation is about 100 to 150 μm, the drum at the transfer position of the second image forming unit is compared with the electrostatic belt scale transferred by the first image forming unit. The position of the latent image graduation was a deviation of about 150 μm at the maximum. Therefore, after detecting the latent image graduation of either the drum or the belt, the other latent image graduation is always detected, and the corresponding graduations are alternately detected. Therefore, the rotational speed of the photosensitive drum may be adjusted so that the drum-side latent image scale is aligned with the position of the belt-side latent image scale every time the drum-side latent image scale is detected.

  By gradually reducing the scale pitch at the leading edge margin, alignment can be continued without losing sight of the corresponding scale up to the effective image area. FIG. 20 shows an image of scale alignment when the head of the drum latent image scale is shifted by 0.150 mm with respect to the belt latent image scale. Since the leading scale is displaced by about 150 μm at most, it is assumed in FIG. 20 that the leading scale m0 and M0 are shifted by 0.150 mm.

(Control of rotational speed of photosensitive drum for correcting displacement of two detected electrostatic latent image graduations)
In this embodiment, the rotation of the photosensitive drum of the second image forming unit is reduced so as to reduce the detection time difference based on the detection time difference when the electrostatic latent image graduations of the photosensitive drum and the intermediate transfer belt are detected by the detection unit. Control the speed. This detection and control is performed for the entire period including the effective image area and the trailing edge margin, starting from the leading edge margin or the inter-paper area preceding the effective image area.

  Specifically, it is as follows. In the state of the drum scale m0 and the belt latent image scale M0 in FIG. 20, the rotation speed of the photosensitive drum driving motor is changed from the result of reading the position of each scale so that the next scales m1 and M1 are aligned. Make it work. However, since the position error is too large to fit, rotation control is further performed so that m2 and M2, and m3 and M3 are matched. Then, the scale position can be almost adjusted. From this point onward, the position of the drum latent image graduation and the belt latent image graduation can continue to be aligned even if the graduation pitch is reduced, and the same is true even when the graduation pitch is 0.0423 mm which is the minimum.

  Thereby, the drum latent image scale can be aligned with the belt latent image scale from the head of the effective image. That is, it is possible to continue transferring the toner image with a small color shift from the second image forming unit to the toner image transferred from the drum to the belt in the first image forming unit.

  Next, FIG. 21 is a control block diagram for explaining the control of the electrophotographic apparatus of the present invention, and FIG. 22 is an operation flowchart for explaining the control contents. In FIG. 21, since the second image forming unit and the subsequent parts have the same configuration, only the second image forming unit is shown. Image forming and image alignment operations in the present embodiment will be described with reference to the flowchart of FIG.

  First, when the control unit 48 receives a print start signal in step S1, the control unit 48 gives a rotation start instruction to the drum drive motors 6a and 6b and a belt drive motor (not shown). Then, while reading the signals of the drum encoders 8a and 8b directly connected to the drum drive shaft, the drum drive motors 6a and 6b are controlled to rotate at a constant speed to rotate the photosensitive drums 12a and 12b at a constant speed in the direction of the arrow R1. Similarly, a belt drive motor (not shown) is driven to rotate at a constant speed by a signal from a belt drive roller encoder attached on a belt drive roller shaft (not shown). Then, the intermediate transfer belt 24 wound around the belt driving roller 36 is rotated in the direction of arrow R2 at a constant speed (step S2).

  In step S3, application of a predetermined high voltage to the charging units 14a and 14b, the primary transfer rollers 4a and 4b, and the electrostatic belt scale transfer roller 47 is started, and the surfaces of the photosensitive drums 12a and 12b are the same as those in the present embodiment. In this case, it is charged to -600V.

  Next, when the control unit 48 receives an image signal in step S4, the first exposure means 16a starts an exposure operation, and the electrostatic latent image graduation starts from the portion corresponding to the leading edge margin as described with reference to FIGS. 31a is formed at a predetermined pitch. After that, when the image data exposure operation is started, the exposure operation is continued until the image data for one page is completed together with the electrostatic latent image index 31a.

  Next, in step S5, it is determined whether 0.8333333 seconds have elapsed since the first exposure unit 16a started the exposure operation, and then in step S6, the second exposure unit 16b starts the exposure operation. In this embodiment, the photosensitive drum diameter is set to 84 mm, and the pitch between the pitch stations between the first image forming unit 43 a and the second image forming unit 43 b is set to 250 mm. Also, the exposure-transfer distance from the exposure position on the surface of the photosensitive drum to the position where the toner image is transferred to the intermediate transfer belt is set to 125 mm, and the belt conveyance speed and the peripheral speed of the photosensitive drum are set to 300 mm / s. ing.

  The timing of writing the latent image on the photosensitive drum 12 is delayed by the following time from the position where the toner image is transferred from the photosensitive drum 12 to the intermediate transfer belt 24 in each upstream image forming unit 43. That is, from the upstream side the writing is performed for the time during which the intermediate transfer belt 24 is conveyed from the position where the toner image is transferred at the upstream image forming unit to the position where the toner image is transferred at the next image forming unit 43. Control may be performed so that writing is delayed. The time interval from the start of image formation at the first image forming unit 43a to the start of image formation at the second image forming unit is calculated as 250 mm / 300 mm / s, which is 0.8333333 seconds.

  Next, in step S7, i = 0 is set. When the photosensitive drums 12a and 12b and the intermediate transfer belt 24 are not moved at a constant speed and are always transported between the transfer positions at a certain time interval, the toner image formed on the intermediate transfer belt is displaced. do not do. In addition, the image position is detected when the speed of the intermediate transfer belt is uneven due to the eccentricity of the belt drive roller or the thickness of the intermediate transfer belt, or when the speed of the photosensitive drum drive motor or the belt drive roller drive motor fluctuates. Deviation occurs.

  It is possible to correct the speed unevenness by measuring the eccentricity of the belt driving roller and the thickness unevenness of the intermediate transfer belt in advance, and the motor speed fluctuation is an encoder mounted on the same shaft. It is possible to correct the speed. However, due to a difference in the amount of toner transferred in each image forming unit, tension variation of the intermediate transfer belt 24 occurs in each image forming unit, and thereby the intermediate transfer belt 24 expands and contracts.

  The amount of expansion / contraction varies depending on the image and varies depending on the transfer toner amount determined by the process conditions, the value of the primary transfer voltage, etc., and thus cannot be predicted, and it is very difficult to correct. Due to this tension variation, the time until the toner image on the intermediate transfer belt 24 transferred by the upstream image forming unit reaches the downstream image forming unit varies, and color misregistration occurs by this variation time. It will be.

  In the present embodiment, even when the speed fluctuation of the intermediate transfer belt 24 that cannot be predicted as described above occurs, the electrostatic latent image graduation line 31b is reliably read at the transfer position. Then, the rotation of the drum drive motor 6 connected to the photosensitive drum 12 is controlled so as to match the corresponding electrostatic belt scale 32, thereby preventing color misregistration.

  Next, in Steps 8a and 8b, one of the i-th i = 0 latent image scales is detected first by the belt scale reading sensor 33b or the electrostatic latent image scale reading sensor 34b. As shown in FIGS. 18 and 19, since the scale pitch of the leading margin is expanded to 0.3384 mm, which is 8 times, at least before detecting the next latent image scale, as shown in FIG. The other latent image scale should be detected. In step S9, a detection time difference Δi at which the latent image graduations at the heads of the drum and the belt are detected is calculated. In step S10, a value obtained by dividing Δi and the graduation pitch Pi by the conveyance speed of 300 mm / s is compared.

  When Δi is smaller than Pi / 300, it means that the other latent image scale is detected before detecting the second latent image scale, and it is clear which scale should be matched. Become. On the other hand, when Δi is larger than Pi / 300, the other latent image graduation cannot be detected until the second latent image graduation is detected. It will not be possible to determine if it should be done.

  In the present embodiment, the pitch of the latent image graduation to be formed is increased in the blank area at the leading edge of the image, and the leading latent image graduation is set so that it can be detected alternately even in a normal state. If the load acting on the intermediate transfer belt increases due to some abnormality and a large slip occurs between the belt driving roller and the intermediate transfer belt, the leading latent image graduation cannot be detected alternately. In that case, an error is determined in step S11, and the operation of the apparatus is stopped.

  Next, in step S12, based on Δi calculated in step S9, the correction amount of the speed of the drum drive motor 6b of the second image forming unit 43b so that the positional deviation of the latent image graduations between the photosensitive drum and the intermediate transfer belt is eliminated. Is calculated. In step S13, the rotational speed of the drum drive motor 6b is corrected, the scale pitch is converged to the minimum pitch before reaching the effective image area, and the rotational speed of the drum drive motor is corrected so that the positional deviation between the scales is reduced. To control. This is repeated until the image data for one page is completed (step S15). When the image data is completed, the exposure operation is stopped (step S16).

(When there are multiple pages of print data)
If there is print data for the next page (step S17), the process returns to step S4, and image formation is performed while the same operation is repeated and image alignment is performed. That is, when there are print data for a plurality of pages, the electrostatic belt scales are sequentially formed on the intermediate transfer belt over a plurality of pages. Therefore, the electrostatic belt scales are almost all of the intermediate transfer belt as shown in FIG. It will be formed over the circumference.

(In case of single page print data)
If the print data has been completed for one page, the high voltage of the charging unit, the primary transfer roller high voltage unit, the latent image scale transfer high voltage unit, etc. is stopped (step S18). Then, the rotation of the photosensitive drum and the intermediate transfer roller continues until the secondary transfer onto the recording paper is completed (step 19). If it is determined that the secondary transfer of all the image data has been completed, all the nine-degree motors of the photosensitive drum and the intermediate transfer belt are stopped (step S20), and the printing operation is ended (step S21).

(Effect of this embodiment)
As described above, with respect to the electrostatic belt scale 32 corresponding to the toner image transferred by the first image forming unit 43a, the electrostatic latent image scale line 31 corresponding to the toner image in the second image forming unit 43b and later is used. Adjust the position. As a result, the toner image formed on the intermediate transfer belt 24 can be superimposed and transferred with high accuracy at the second image forming unit 43b and thereafter, and a color toner image free from color misregistration can be obtained. Can do.

  In the above description, it has been described that the scales on one side of the photosensitive drum 12 and the intermediate transfer belt 24 are combined, but in this embodiment, two sets of scales on the front side and the back side of the apparatus are used. It is also possible to perform alignment of toner images. In other words, if the scale position detected on the near side and the far side is shifted, the scale alignment position is set to an intermediate position between the near side and the far side, so that a more accurate toner image can be obtained. It is possible to perform alignment.

  The color toner image formed on the intermediate transfer belt 24 is conveyed to the second transfer unit 44 shown in FIG. 1, and is applied to the secondary transfer roller 38 with respect to the recording paper conveyed from a paper feeding device (not shown). Secondary transfer is performed by the applied electric field. Further, the toner image is conveyed to a fixing device (not shown), and after the toner image is fixed on the recording paper, it is discharged out of the apparatus. The toner that cannot be transferred onto the recording paper from the intermediate transfer belt 24 but remains on the surface of the intermediate transfer belt 24 is scraped off by the belt cleaning unit 45 and collected in a waste toner box (not shown).

  As in this embodiment, the latent image graduation corresponding to the toner image is read by a potential sensor that changes the potential of the latent image and changes to a pulse signal, and the corresponding graduations are always aligned. As a result, it is possible to correct the image position deviation caused by the expansion and contraction of the intermediate transfer belt caused by the formation of the toner image on the intermediate transfer belt with high accuracy, and an image forming apparatus with less color misregistration can be achieved. Can be provided.

  In addition, the potential sensor used in the present embodiment is simply a conductor pattern arranged on a flexible substrate, and can be configured not only at a very low cost but also for reading a latent image itself. Therefore, since no other writing / reading means is required, errors can be reduced, and an image forming apparatus with higher accuracy and lower cost can be provided.

  Further, the scale is read via the intermediate transfer belt 24 without directly contacting the photosensitive drum 12 and the electrostatic latent image scale reading sensor 34. Therefore, even when continuous operation is performed for a long time, toner or the like does not accumulate in the nip formed by the intermediate transfer belt 24 and the electrostatic latent image scale reading sensor 34, and a stable output is obtained. It becomes possible to provide a more reliable device.

  Further, a belt scale reading sensor 33 for reading the electrostatic belt scale 32 transferred to the transferred portion 25 is also arranged inside the intermediate transfer belt. For this reason, toner or the like does not accumulate in the nip formed by the intermediate transfer belt 24 and the belt scale reading sensor 34, and a stable output can be obtained and a more reliable device can be provided.

  Further, when the photosensitive drum 12 and the electrostatic latent image scale reading sensor 34 are brought into direct contact, the electrostatic latent image scale reading sensor 34 must be disposed outside both ends of the intermediate transfer belt 24. In order to minimize the width of the apparatus, it is necessary to place the electrostatic latent image scale reading sensor 34 as close to the belt end as possible. However, in order to prevent the moving belt meandering in which the position of the intermediate transfer belt 24 in the main scanning direction fluctuates, a belt end position detection sensor for detecting the position of the intermediate transfer belt 24 in the main scanning direction is provided. There is a device to change the position.

  Alternatively, there is an apparatus in which rib members are attached to both end portions of the intermediate transfer belt 24 so as not to cause belt meandering. In these devices, an abnormality of the end position detection sensor, an abnormality of the roller position changing means, or a belt meandering over the rib member may occur. In these cases, the intermediate transfer belt 24 approaches the electrostatic latent image scale reading sensor 34 disposed in the immediate vicinity thereof, and the belt end and the latent image sensor end come into contact with each other. Further, if the belt is moved too far, the intermediate transfer belt 24 or the electrostatic latent image scale reading sensor 34 may be damaged.

  However, as in this embodiment, the electrostatic latent image graduation reading sensor 34 is disposed so as to contact the back surface side of the intermediate transfer belt 24, so that at least the static latent image graduation reading sensor 34 is at least static even when the belt 24 is moved all the way. The electrostatic latent image scale reading sensor 34 is not destroyed. Further, since the electrostatic latent image graduation reading sensor 34 does not directly contact the photosensitive drum 12 but reads the electrostatic latent image graduation line 31 via the intermediate transfer belt 24, the following effects are obtained.

  That is, as described above, the creeping distance and the spatial distance between the primary transfer roller and the electrostatic belt scale transfer roller 47 are spaced apart from each other, and the area between them is used as the area of the electrostatic latent image scale line 31. An electrostatic latent image scale reading sensor 34 can be arranged. For this reason, the apparatus width in the main scanning direction can be further reduced.

  The apparatus according to the present embodiment configured as described above is a tandem type color image forming apparatus having a plurality of image forming units aimed at speeding-up, and is arranged on the photosensitive drum by the first image forming unit. A latent image graduation is formed by exposure light. As a result, it is possible to form a scale on the photosensitive drum with no error in positional deviation from the image. Further, at the same time when the developed toner image is transferred to the intermediate transfer belt, the latent image graduation formed on the photosensitive drum is transferred to the transferred portion of the intermediate transfer belt to form an electrostatic belt graduation due to electric charge. ing.

  This eliminates all errors during scale writing and reading, and is not affected by temperature fluctuations. Therefore, the electrostatic belt scale has an error in the sub-scanning direction with respect to the toner image transferred to the intermediate transfer belt. It becomes possible to form without.

  In the second image forming unit and thereafter, at each transfer position, the electrostatic belt scale formed with respect to the toner image on the intermediate transfer belt without any positional error, and the positional error with respect to the toner image developed on the photosensitive drum. Read the scale of the latent image formed. Then, the transfer positions of all the photosensitive drums after the second image forming unit are changed with respect to the intermediate transfer belt during image formation so that these correspond. As a result, the toner image can be transferred with a small positional shift at each transfer position after the second image forming unit with respect to the toner image transferred by the first image forming unit.

  As a result, it is possible to provide a high-speed color electrophotographic apparatus capable of reducing color misregistration to about 20 μm or less and outputting a high-quality image.

  In addition, since the graduation on the surface of the photosensitive drum is formed by the electrostatic latent image, it is not necessary to provide a dedicated graduation writing means, so that the configuration is simpler and the number of adjustment points can be reduced. It becomes possible to provide.

<< Second Embodiment >>
23, 24, 25, and 26 are diagrams for explaining a second embodiment according to the present invention. Only the parts different from the first embodiment will be described. In this embodiment, as shown in FIGS. 23 and 24, the belt scale reading sensor 33 and the electrostatic latent image scale reading sensor 34 are configured on the same flexible printed circuit board. Are arranged in a straight line. In the case of FIG. 23, it has a shape in which a cut is made between two detection units.

  In FIG. 23, even if there is a step on the back surface of the intermediate transfer belt 24 because the transfer target portion 25 is on the front surface side of the intermediate transfer belt 24, none of the detection portions come into contact with the back surface of the belt. It has a configuration that can be done. In the case of FIG. 24, two detectors can be arranged closer to each other on the rectangular flexible printed circuit board as compared with the case of FIG.

  In the case of FIG. 24, if the thickness of the transferred portion 25 is about 30 μm as described in the first embodiment, even if there is a step, the amount is very small. Therefore, if it is a flexible printed circuit board, even if there is no notch as shown in FIG. By adopting a configuration without cuts, it is possible to reduce the size of the entire sensor flexible, and there is an advantage that the sensor flexible can be manufactured at a lower cost.

  FIGS. 25 and 26 show a state where the sensor as shown in FIG. 23 is incorporated in the belt unit. In FIG. 25, the sensors shown in FIG. 23 are arranged on the back side of the intermediate transfer belt 24 from the second image forming unit 43b to the fourth image forming unit 43d. The electrostatic belt scale 32 transferred to the transferred portion 25 of the intermediate transfer belt 24 and the electrostatic latent image scale 31 formed on the end of the photosensitive drum 12 are applied to the detection portion via the intermediate transfer belt 24. The configuration is such that it can be detected as a scale. The rest of the configuration is the same as that of the first embodiment.

  FIG. 26 shows a side view from the second image forming unit 43b to the fourth image forming unit 43d. By adopting a configuration in which the scale on the belt side and the scale on the drum side are read by an integrated sensor as in this embodiment, the number of parts can be reduced and the space for arranging the sensors can be reduced. The device itself can be downsized. Furthermore, by forming the antenna portion on the same flexible board, the drum latent image reading position and the belt latent image reading position can be made the same position in the sub-scanning direction, and alignment control with less error can be performed. Become.

  Also, even when the sensor position fluctuates due to vibration or the like, the relative position in the transport direction is shifted as a result of the antenna unit being formed on the same flex as compared to the case where it is configured separately. rare. Therefore, there is an effect that it is possible to realize alignment with higher accuracy.

  Furthermore, the electrostatic latent image graduation reading sensor 34 can be positioned at the toner transfer position by positioning so that the value is maximized based on the output amplitude from the electrostatic latent image graduation reading sensor 34 for the drum. is there. Further, the position of the integrated belt scale reading sensor 33 can be determined at the same time.

<< Third Embodiment >>
27, 28, 29, 30 and 31 show a third embodiment according to the present invention. FIG. 31 is a diagram showing the relative positions of the electrostatic latent image graduation at the head of the page and the electrostatic belt graduation in the present embodiment. In this embodiment, the area of the electrostatic belt scale 32 transferred and formed on the intermediate transfer belt 24 and the area of the electrostatic latent image scale line 31 formed on the photosensitive drum 12 are the same in the main scanning direction. That is, in the direction crossing the moving direction, the electrostatic belt scale 32 that is the second electrostatic latent image scale is the electrostatic latent image scale line 31 formed on the photosensitive drum 12 that is the first electrostatic latent image scale. It is arranged at a position so as to overlap with the area.

  The sensor for reading the two scales is configured as a common detection means (only one of the belt scale reading sensors 33). Here, the method for adjusting the scale in the present embodiment will be described with reference to FIG. In this embodiment, when forming the scale of the latent image corresponding to the image, as shown in FIGS. 30 and 31, the latent image scale of the belt is formed on the drum after the second image forming unit 43b. Are formed by the first image forming unit 43a so as to be at an intermediate timing. That is, with respect to the latent image graduation lines formed in the first and second embodiments, the belt latent image graduations should not be odd-numbered (only the broken lines in FIG. 31), and the drum latent image graduations should be even-numbered. It should not be formed (only the solid line in FIG. 31).

  Then, both the latent image graduations are detected by the belt graduation reading sensor 33 at the transfer position, and the photosensitive drum 12 is positioned so that the latent image graduations on the belt are positioned between the graduation intervals of the electrostatic latent image graduations on the drum. Rotation control is performed. That is, the interval between the electrostatic latent image graduations on the belt adjacent to both sides is made equal to each graduation of the electrostatic latent image graduations on the drum.

  In this embodiment, it is possible to realize image alignment equivalent to that in which the rotation control of the photosensitive drum 12 is performed in the first and second embodiments. Since only one type of sensor is required, a lower cost device can be provided. Furthermore, in the first and second embodiments, since the latent image graduation of the drum and the latent image graduation of the belt are on separate lines, more space for forming the latent image graduation is required. Then, the length of the photosensitive drum 12 in the main scanning direction can be further shortened. For this reason, a smaller apparatus can be provided.

  Furthermore, the positional relationship is such that the transferred portion 25 to which the electrostatic latent image graduation 31a is transferred and the electrostatic latent image graduation 31b area of the second photosensitive drum 12b overlap, and the following effects are obtained. That is, by using a structure in which the detection portions of the electrostatic latent image scale reading sensor 34b and the belt scale reading sensor 33b are the same, there is no need to separately provide a drum scale area and a belt scale area. As a result, the drum length can be shortened, and accordingly, the charging area and the exposure area can be shortened.

12a to 12d ... photosensitive drum, 14a to 14d ... charging means, 16a to 16d ... exposure means, 18a to 18d ... developing means, 24 ... intermediate transfer belt, 25 ... transfer portion, 26 ... scale line drawing region, 31 ... static Electrostatic latent image graduation line, 32 ... electrostatic belt scale, 33b to 33d ... belt scale reading sensor, 34b to 34d ... electrostatic latent image scale reading sensor, 47 ... electrostatic belt scale transfer roller, 48 ... control unit

Claims (7)

  A plurality of image carriers, each of which is rotatable and arranged sequentially in a predetermined direction so as to form an image with a developer corresponding to a color in each effective image area;
  A movable transfer medium facing the plurality of image carriers;
  A plurality of transfer members that face the image carrier via the transfer medium and respectively transfer images formed on the image carrier to the transfer medium;
  First scale forming means for forming a first electrostatic latent image scale corresponding to the image by the developer in a scale drawing area outside the effective image area provided in each of the plurality of image carriers;
  An electrostatic latent image graduation on the upstream side of the first electrostatic latent image graduation is formed on the side edge outside the effective image area in the direction intersecting the moving direction in the transfer medium. Second scale forming means for forming a second electrostatic latent image scale by transferring at the time of transfer;
  A first detection means provided on the opposite side of the image carrier from the transfer medium, for detecting the first electrostatic latent image graduation downstream from the upstream electrostatic latent image graduation; ,
  It is provided on the opposite side of the image carrier with the transfer medium in between, and the position of the first detection means and the transfer medium in the moving direction coincides to detect the second electrostatic latent image scale. A second detection means;
  In order to reduce detection deviation of the first electrostatic latent image graduation and the second electrostatic latent image graduation by the first detection means and the second detection means, the downstream image carrier A color image forming apparatus comprising: control means for controlling the rotation speed. A plurality of image carriers, each of which is rotatable and arranged sequentially in a predetermined direction so as to form an image with a developer corresponding to a color in each effective image area;
A movable transfer medium facing the plurality of image carriers;
A plurality of transfer members that face the image carrier via the transfer medium and respectively transfer images formed on the image carrier to the transfer medium;
A color image forming apparatus comprising:
A scale drawing area provided in each of the plurality of image carriers, starting from an area that is outside the effective image area in a direction crossing the moving direction and precedes the effective image area in the moving direction An electrostatic latent image graduation forming unit for forming a first electrostatic latent image graduation corresponding to the image by the developer in a graduation drawing area reaching each effective image area;
An electrostatic latent image graduation on the upstream side of the first electrostatic latent image graduation is formed on the side edge outside the effective image area in the direction intersecting the moving direction in the transfer medium. A second transfer member for forming a second electrostatic latent image graduation by transferring at the time of transfer;
A first detection means provided on the opposite side of the image carrier from the transfer medium, for detecting the first electrostatic latent image graduation downstream from the upstream electrostatic latent image graduation; ,
It is provided on the opposite side of the image carrier with the transfer medium in between, and the position of the first detection means and the transfer medium in the moving direction coincides to detect the second electrostatic latent image scale. A second detection means;
In order to reduce detection deviation of the first electrostatic latent image graduation and the second electrostatic latent image graduation by the first detection means and the second detection means, the downstream image carrier Control means for controlling the rotational speed;
A color image forming apparatus comprising: The first detection means and the second detecting means, a color image forming apparatus according to claim 1 or 2, characterized in that it is constructed integrally on the same substrate. In a direction intersecting the moving direction, wherein the second detecting means color image forming apparatus according to any one of claims 1 to 3, characterized in that arranged outside with respect to the first detection means . The second electrostatic latent image graduation is arranged at a position overlapping the first electrostatic latent image graduation in a direction intersecting the moving direction;
The first detection means and the second detection means are configured as a common detection means, and the second electrostatic latent image graduation is arranged in the middle of each graduation interval of the first electrostatic latent image graduation. 2. A scale is arranged, and each of the scales of the first electrostatic latent image scale is controlled so as to have an equal interval with the second electrostatic latent image scale adjacent on both sides. The color image forming apparatus according to any one of 1 to 3 . Said second electrostatic latent image scale, the continuous rotation of the image bearing member, according to claim 1 to 5, characterized in that it is transferred to the transfer medium over a plurality of pages in the moving direction of the transfer medium The color image forming apparatus according to any one of the above. The scale drawing area, top margin, the effective image area of another color image forming apparatus according to any one of claims 1 to 6, characterized in that it comprises a trailing edge margin.