JP2007121496A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a high-quality image by reducing the color shift of an image write start position due to multicolor superposition by a simple method. <P>SOLUTION: A mark detecting means detects a first mark which is a write start reference for the first color arranged on an intermediate transfer member along its moving direction and a second mark located apart approximately the same distance as the distance moving from the first mark to the write start of the second color, and is installed in a prescribed position. A control means starts the writing of the first color based on a second main scanning synchronizing signal from the detection of the first mark in the first rotation of the intermediate transfer member, starts the writing of the second color, based on the second main scanning synchronizing signal from the detection of the first mark, starts the writing of the third color in synchronization with the second main scanning synchronizing signal to minimize the amount of the color shift from that of the first color after the detection of the first mark at the second rotation of the intermediate transfer member, and starts writing of the fourth color in synchronization with the main scanning synchronizing signal to minimize the amount of the color shift from that of the second color after the detection of the second mark. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more specifically, an image forming apparatus for transferring a toner image formed on an image carrier such as a photosensitive drum to an intermediate transfer member and transferring the transferred image onto a sheet-like recording medium. About.

  An electrophotographic multicolor image forming apparatus forms four color toner images of yellow, magenta, cyan, and black on an intermediate transfer member such as an intermediate transfer belt, and forms the superimposed toner image on a recording medium. To perform full color printing. In such a multicolor image forming apparatus, if the writing position of each color is shifted when performing full color printing, the toner image formed in an overlapping manner is transferred to a recording medium, resulting in a color shift and lowering the image quality. End up. Therefore, position control is required to match the image writing start position.

  Therefore, in an image forming apparatus that conventionally forms an image by transferring a toner image to an intermediate transfer member, an alignment mark is provided on the intermediate transfer member, this mark is detected by a sensor, and image writing starts from the mark detection. There is known an image forming apparatus that counts up to a hardware counter and starts writing an image when it matches a set value from mark detection to image writing start, and aligns the images of the respective colors.

  In addition, a method has been proposed in which a toner image having an alignment pattern is formed on an intermediate transfer member, the pattern is detected by an optical sensor, and the image position is corrected by obtaining an image displacement amount.

  Further, as an image forming apparatus that forms an image by transferring a toner image to an intermediate transfer member, an image forming apparatus having four sets of image forming units arranged facing each other along the moving surface of the intermediate transfer member is known. . In this image forming apparatus, a mark for alignment provided on the intermediate transfer member is detected, and writing of the first color is performed with reference to the detection timing. Writing from the second color to the fourth color is performed sequentially with the time for the intermediate transfer member to move the distance between the image forming units from the reference of the first color so that the images overlap on the intermediate transfer member. Is done.

  In addition, in an image forming apparatus having two sets of image forming units arranged facing each other along the moving surface of the intermediate transfer member, a mark for alignment provided on the intermediate transfer member is detected, and the detection timing is used as a reference. As a result, the first color is written. After the intermediate transfer member makes one round, the same mark is detected again, and writing of the third color is performed based on the detection timing. Note that the writing of the second color is performed after the time for the intermediate transfer member to move the distance between the image forming units from the reference of the first color. Similarly, after the intermediate transfer member makes one revolution, the fourth color writing is performed after the intermediate transfer member has moved the distance between the image forming units from the reference of the third color.

  Further, in Patent Document 1, the period of the main scanning synchronization signal of the writing means is T, and after time tx1 from time tx1 obtained by adding T / 2 to the detection time of the image forming start signal in the sub-scanning direction at the time of reference image formation. The time (ty1−tx1) that elapses until the time ty1 when the main scanning synchronization signal is detected for the first time is t1, and the main scanning synchronization signal from the detection time tx2 of the image formation start signal of the sub-scanning synchronization signal at the time of image formation outside the reference image. When the time (ty2-tx2) until the detection time ty2 is t2, the writing means starts image formation for one line when (t1-t2) is positive when forming an image outside the reference image. There has been proposed an image forming apparatus that delays.

In Patent Document 2, a light source used for the first line recording is selected from a plurality of light sources in accordance with a time difference between an image formation start signal for determining an image formation start position in the sub-scanning direction and a main scanning synchronization signal. A multi-beam laser recording apparatus that delays the start of image formation for the first line according to the time difference has been proposed.
JP 2003-255656 A Japanese Patent Laid-Open No. 11-212009

  However, in the image forming apparatus of Patent Document 1, on the premise that the writing start timing of the already formed color is accurately formed, the image forming start timing of the subsequent colors is minimized so that the color misregistration amount is minimized. Has been determined. Accordingly, the image forming positions of the first color and the second color, and the third color and the fourth color formed by different image forming means are written due to the fluctuation of the speed of the intermediate transfer belt, the variation of the photosensitive drum diameter, the eccentricity, and the like. It is difficult to accurately adjust as per control at a level below the scanning unit. Therefore, in such a control method, there is a problem that the color misregistration amount may actually increase when the above-described fluctuations or the like are met.

  Further, in the image forming apparatus of Patent Document 2, in addition to performing a scanning delay at the start of image formation according to the timing of the image formation start signal and the main scanning synchronization signal for each color, alignment control is performed because beam selection is performed. There is a problem that it becomes complicated.

  Furthermore, according to the image forming apparatus having four sets of image forming units, it is necessary to adjust the writing start timing of the three colors from the second color to the fourth color with respect to the first reference color. There is a problem that it takes time and is difficult. In addition, when speed fluctuation occurs in the intermediate transfer member, there is a problem that the three colors of the second color to the fourth color are shifted from the reference color of the first color.

  Further, according to the image forming apparatus having two sets of image forming units, the image forming positions of the first color and the third color substantially coincide with each other, but when the speed variation occurs in the intermediate transfer member, the reference of the first color There is a problem that the second color and the fourth color are shifted from each other. At this time, it is necessary to adjust the writing start timing of the second color with respect to the reference color of the first color, and the writing start timing of the fourth color with respect to the reference color of the third color. In addition, the first transfer and the second rotation of the intermediate transfer member have different timings at which the cleaning process and the transfer process to the recording medium are performed, and load fluctuations occur. Therefore, the intermediate transfer member is rotated twice to form a four-color image, the first, second, third, and fourth color image formation positions are detected, the misregistration amount is confirmed, and the writing start position is determined. It needs to be adjusted.

  The present invention is for solving these problems. In an image forming apparatus having two sets of image forming units arranged facing each other along a moving surface of an intermediate transfer member, a multicolor superposition is performed by a simple method. An object of the present invention is to provide an image forming apparatus capable of reducing the color misregistration at the image writing start position due to the alignment, and obtaining a high-quality image by adjusting the image writing start position for each image forming unit with high accuracy.

  The image forming apparatus of the present invention has a single image carrier and two colors of latent images formed by scanning type writing means that are arranged opposite to the image carrier and form a latent image on the image carrier. The first and second image forming units composed of a plurality of color-switchable developing means that develop with the developer and the toner images formed by the first and second image forming units are sequentially superimposed and transferred. An intermediate transfer member and transfer means for transferring a color image formed on the intermediate transfer member to a sheet-like recording medium are provided. The image forming apparatus of the present invention includes a mark detection unit and a control unit. The mark detecting means moves on the intermediate transfer member along the moving direction, the first mark serving as a reference for starting writing of the first color, and from the first mark to the start of writing of the second color. A second mark arranged at a distance substantially the same as the distance is detected and provided at a predetermined position. Further, the control means starts writing the first color with reference to the second main scanning synchronization signal from the detection of the first mark in the first rotation of the intermediate transfer member, and 2 from the detection of the second mark. Writing of the second color is started on the basis of the first main scanning synchronization signal, and after the first mark is detected in the second rotation of the intermediate transfer body, the main scanning that minimizes the amount of color deviation from the first color Writing of the third color is started in synchronization with the synchronization signal, and writing of the fourth color is started in synchronization with the main scanning synchronization signal that minimizes the amount of color deviation from the second color after detection of the second mark. Control to do. Therefore, a high-quality image can be obtained by aligning the writing start positions of the first color and the third color, and the second color and the fourth color formed by the same image forming unit with high accuracy.

  Further, the writing start from the first color to the fourth color is delayed by a predetermined delay count number, and the second color and the fourth color are respectively corrected by a predetermined fine adjustment count number from the detection of the second mark. By starting writing, the position of the image formed on the intermediate transfer member can be arbitrarily changed, so that the deterioration of the intermediate transfer member can be prevented.

  Further, a plurality of sets of reference marks are arranged along the moving direction of the intermediate transfer member at substantially the same distance as the distance from the start of writing of the first color to the start of writing of the second color. Therefore, since a plurality of write start positions can be obtained on the intermediate transfer member, when a plurality of images are formed per circumferential length of the intermediate transfer member, the write start positions can be adjusted with high accuracy, respectively. An image can be obtained. Furthermore, when printing a plurality of sheets, a printing interval can be selected depending on the paper size.

  In addition, when the high image quality mode and the standard mode are provided and the mode is set to the high image quality mode, the time from when the first mark is detected until the second mark is detected is measured. If it is out of the set time range, writing of the second color or the fourth color is suspended, and the intermediate transfer member is conveyed once, so that writing is not performed when the intermediate transfer member changes in speed. As a result, a high-quality image can be obtained.

  Further, the image forming apparatus of the present invention has first and second mark detection means and control means. Then, the first mark detection means detects a mark serving as a first color writing start reference arranged on the intermediate transfer member along the moving direction. The second mark detection means is arranged at a distance substantially the same as the distance that the intermediate transfer member moves from the first detection means to the start of writing of the second color. Further, the control means starts writing the first color at the first rotation of the intermediate transfer body with reference to the second main scanning synchronization signal after the mark is detected by the first mark detection means, and the second rotation. The writing of the third color is started at the eye, and the writing of the second color is performed at the first rotation of the intermediate transfer body with reference to the second main scanning synchronization signal after the mark is detected by the second mark detecting means. Start and control to start writing of the fourth color at the second rotation. Therefore, it is possible to obtain a high-quality image by matching the writing start positions of the first and third colors, and the second and fourth colors formed by the same image forming unit with high accuracy.

  Further, the writing start from the first color to the fourth color is delayed by a predetermined delay count number, and the second color and the fourth color are respectively determined after the mark is detected by the second mark detecting means. The fine adjustment count number is corrected and writing is started. Therefore, since the position of the image formed on the intermediate transfer member can be arbitrarily changed, the deterioration of the intermediate transfer member can be prevented.

  Further, by setting the second color to yellow, it is possible to obtain a high-quality image in which the amount of color misregistration that is actually visually recognized is reduced in consideration of colors that are difficult to visually recognize.

  In addition, by setting the fourth color to black, it is possible to obtain a high-quality image that reduces the amount of color misregistration that is actually recognized visually by an image that frequently uses black as a character.

  According to the image forming apparatus of the present invention, it is possible to reduce the color misregistration at the image writing start position by multi-color superposition by a simple method, and in particular, by adjusting the image writing start position for each image forming unit with high accuracy. An image is obtained.

  FIG. 1 is a schematic diagram showing the configuration of an image forming apparatus to which the present invention is applied. The image forming apparatus shown in the figure is a so-called two-station type image forming apparatus, and a toner image is generated by a developing means from a latent image formed on a photosensitive drum as an image carrier by a scanning writing means, and the toner image Is a color image forming apparatus that repeats the process of transferring the toner image onto an intermediate transfer member a plurality of times for each color and forms a color image by sequentially superimposing toner images for each color.

  An image forming apparatus 100 shown in FIG. 1 uses drum-like photoreceptors (hereinafter referred to as photoreceptor drums) 14 and 14 ′ as image carriers, and charging means is provided around the photoreceptor drums 14 and 14 ′. 15 and 15 ′, writing means 16 and 16 ′, developing means 13 and 13 ′, and first and second image forming units 20 and 20 ′ in which means such as a cleaning means (not shown) are arranged. These image forming units 20 and 20 ′ are arranged at regular intervals along the same moving surface of the intermediate transfer belt 1. The intermediate transfer belt 1 is an endless belt, and includes a driving roller 2 and a driven roller 3 attached to respective shafts (not shown) that are rotatably supported in a rotation direction indicated by an arrow a in the drawing. It is stretched. Further, the intermediate transfer belt 1 is provided with marks M1 and M2 serving as reference positions on the intermediate transfer belt 1 in a non-image forming region on one end side in the width direction orthogonal to the rotation direction indicated by an arrow a in the drawing. . There are cases where a single mark is provided or a plurality of marks are provided as shown in FIG. The mark has a line shape, and only this portion reflects light compared to other portions. Mark detection sensors 5 and 5 'including a light emitter and a light receiver for detecting the mark are provided and fixed on a non-illustrated immovable member at a predetermined interval. Further, the driven roller 3 is provided with a transfer roller 6 for applying a bias voltage for transfer so as to be able to contact and separate with the intermediate transfer belt 1 interposed therebetween. Further, the intermediate transfer belt 1 is longer than the length in the moving direction of the maximum size transfer paper used in the image forming apparatus 100 by the non-image forming area.

  Next, the image forming process in the image forming apparatus 100 of FIG. 1 will be outlined. If attention is paid to one image forming unit, for example, the first image forming unit 20, the image forming apparatus 100 is in the dark according to the general electrostatic recording method. Then, an electrostatic latent image of a certain color is written on the photosensitive drum 14 uniformly charged by the charging means 15 by the writing means 16, and the electrostatic latent image is visualized by the developing means 13 to be intermediate transferred. Transfer to belt 1. Assuming that the developing means 13 and 13 ′ in the first and second image forming units 20 and 20 ′ have a function of visualizing two different colors of toner, black is added to the three primary colors. Since there are four colors, a full color image can be formed by sharing these colors with each developing means.

  Accordingly, while the same image forming area on the intermediate transfer belt 1 sequentially passes through the first and second image forming units 20 and 20 ′, one color is respectively set by the first and second image forming units 20 and 20 ′. The toner images are transferred one by one, and the two-color superimposed transfer image area on the intermediate transfer belt 1 passes through the first and second image forming units 20 and 20 ′ again in sequence. If the first and second image forming units 20 and 20 ′ transfer and transfer toner images of different colors from the previous ones, the same image forming area will be the first and second image forming units 20 and 20 respectively. The full color toner image is overlaid and transferred when it passes through 'twice. When this full-color toner image is transferred onto a transfer sheet as a recording medium conveyed from a paper supply unit (not shown), a full-color image is obtained on the transfer sheet. Then, the toner image on the transfer paper can be fixed by the fixing means 6 to obtain a full-color final image on the transfer paper.

  The image forming apparatus 100 is capable of obtaining a high-speed print output in synchronization with the rotation of the intermediate transfer belt 1. The image forming apparatus 100 includes photosensitive drums 14 and 14 ′ serving as an image carrier, writing means 16, 16 'is an example using a combination of an LED and a converging light transmitter, but as this modified application, an endless belt-like one may be used as an image carrier, or writing A laser light source may be used as a means. Furthermore, the image carrier is not limited to a photoconductor, but can be a medium that can form a latent image only by an action other than light, and such an image carrier can be electrically and magnetically changed by an action other than light. Any possible writing means can be used. For example, in the case of a type using a magnetic recording drum instead of the photosensitive drum, a magnetic writing unit is used instead of the writing unit.

  FIG. 2 is a schematic plan view showing an example of writing means using a laser scanning optical system. In the figure, the writing means 30 is provided with a light source 31, and this light source 31 is sequentially modulated according to image information of each color of cyan, yellow, magenta and black by a modulation means (not shown), and cyan, yellow, magenta and black. A laser beam is emitted as a light beam that is sequentially modulated with image information of each color. The laser beam from the light source 31 is collimated by the collimating lens 32 and then deflected by the deflecting / reflecting surface by a rotating polygon mirror 33 as scanning means. The rotary polygon mirror 33 is repeatedly driven in the main scanning direction by being rotationally driven by a driving unit (not shown). The laser beam from the rotary polygon mirror 33 is narrowed down by the imaging lens 34 and imaged on the photosensitive drum 35 as a laser spot. Here, the photosensitive drum 35 corresponds to the photosensitive drums 14 and 14 'in FIG. The laser spot is rotated and driven by a driving means (not shown), so that the photosensitive drum 35 is repeatedly scanned in the main scanning direction to form an electrostatic latent image on the photosensitive drum 35.

  Here, the main scanning synchronization signal detection sensor 36 as the main scanning synchronization signal generating means is arranged outside the image range within the laser beam scanning range, receives the laser beam from the rotary polygon mirror 33 and detects it, A main scanning synchronization signal for determining a recording start position (lateral registration) in the main scanning direction is generated. On the other hand, the image forming start signal in the sub-scanning direction for determining the image forming start position in the sub-scanning direction is the mark M1 or mark M2 provided on the intermediate transfer belt 37, the mark detecting means 5 or mark detecting means in FIG. By detecting by 5 ', an image formation start signal in the sub-scanning direction is generated.

  Further, the main scanning synchronization signal from the main scanning synchronization signal detection sensor 36 and the image formation start signal in the sub-scanning direction from the mark detection means 5 or the mark detection means 5 ′ are sent to the CPU 41 shown in FIG. In accordance with the main scanning synchronization signal from the main scanning synchronization signal detection sensor 36 and the image formation start signal in the sub-scanning direction from the mark detection means 5 or the mark detection means 5 ′, writing is performed on the photosensitive drum 35 by the writing means.

  The control of the image forming apparatus described above is performed by the control system shown in FIG. FIG. 2 shows a control system in the image forming apparatus 100 including the first and second image forming units 20 and 20 ′ shown in FIG. The control unit 40 mainly controls the operation of the image forming apparatus 100 during image writing, and also controls the overall operation of the image forming apparatus 100. The control means 40 includes a CPU 41, an I / O (input / output) port (not shown), a RAM 42, a ROM 43, a timer 44, and the like, and a microcomputer having a configuration in which they are connected by a signal bus (not shown). The control unit 40 receives information from an operation panel 45 having operation switches and keys for instructing image forming conditions for the image forming apparatus 100, the mark detection sensors 5 and 5 ', and the main scanning synchronization signal detection sensor 36. It is designed to be entered.

  Further, the control means 40 inputs control information necessary for image formation to the first image forming unit 20, the second image forming unit 20 ′, and other controlled object systems 46. When a drum-shaped intermediate transfer drum is used instead of the intermediate transfer belt 1, a control signal is output to the intermediate transfer drum drive system instead of the intermediate transfer belt drive system 47.

  Further, the timer 44 is a so-called external timer, which is initialized when the first mark M1 or the mark M2 is detected by the mark detection sensors 5 and 5 ′, that is, is reset and starts counting the elapsed time. It has a counter function. The counter 44-1 is not limited to the external timer, and may be provided in a so-called internal timer provided in the CPU 41, or the external timer and the internal timer may be assigned functions as appropriate.

  Moreover, the control means 40 is provided in the control board arrangement | positioning part in an apparatus main body. The control means 40 is based on various signals from the operation panel 45, mark detection signals from the mark detection sensors 5 and 5 ′, main scanning synchronization signal from the main scanning synchronization signal detection sensor 36, and an operation program called from the ROM 43. The operations of the first and second image forming units 20, 20 ′, the intermediate transfer belt drive system 47, and other controlled object systems 46 are controlled to control the operation of the entire image forming apparatus 100.

  Further, the ROM 43 stores an operation program for the entire image forming apparatus 100, necessary data, and the like, and the operation program is appropriately read by the CPU 41. The RAM 42 temporarily stores and holds the calculation result of the CPU 41, various signals from the operation panel 45, mark detection signals from the mark detection sensors 5 and 5 ′, and main scanning synchronization from the main scanning synchronization signal detection sensor 36. It has a function of storing signals and on / off signals as needed.

  Here, the first measuring means 44-2 measures the elapsed time from the detection of the mark M1 serving as the writing start reference by the mark detection sensor 5 or the mark detection sensor 5 ′ to the next main scanning synchronization signal. . The elapsed time measured by the first measuring means 44-2 is stored and held in the RAM. The calculating means 41-4 also includes an elapsed time t3 measured by the first measuring means 44-2 for the second rotation of the intermediate transfer belt and an elapsed time t1 measured by the first measuring means 44-2 for the first rotation. The time difference (t3−t1) is calculated. Then, the first determination unit 41-2 determines a main scanning synchronization signal for starting writing of the second rotation based on the magnitude of this time difference. Next, the second measuring unit 44-3 measures the elapsed time from the detection of the mark M2 serving as the writing start reference by the mark detection sensor 5 or the mark detection sensor 5 ′ to the next main scanning synchronization signal. . The calculating means 41-4 also includes an elapsed time t4 measured by the second measuring means 44-3 at the second rotation of the intermediate transfer belt and an elapsed time t2 measured by the second measuring means 44-3 at the first rotation. The time difference (t4−t2) is calculated. Then, the second determination unit 41-3 determines the main scanning synchronization signal for starting the writing of the second rotation based on the magnitude of this time difference.

  Next, a first exemplary embodiment of the present invention applied to the image forming apparatus 100 shown in FIGS. 1 to 3 will be described with reference to FIGS. In FIG. 4 and FIG. 8 to be described later, a single rotation of the intermediate transfer belt 1 is performed by adding a comma “′” to the marks such as marks M1, M2 detected in the second rotation of the intermediate transfer belt 1. Distinguish from eye detection. Since this operation is controlled under the control of the CPU 41 of the control means 40 of FIG. 3, when explaining detailed operations such as activation, operation, and stop of actuators (control target drive means) such as various motors, It is based on a command or a command signal from the CPU 41 of the means 40.

  First, when the control means 40 receives a print instruction signal from a host computer (not shown), the image forming apparatus 100 shown in FIG. 1 starts printing in response to a command signal from the control means 40. First, the photosensitive drums 14, 14 'and the intermediate drum The transfer belt 1 is driven. In FIG. 1, when the intermediate transfer belt 1 is driven, the mark M <b> 1 is detected by the mark detection sensor 5 and a mark detection signal is output to the CPU 41. The image forming apparatus 100 starts an image forming operation with reference to a mark detection signal first detected after the peripheral speed of the intermediate transfer belt 1 reaches a constant speed. A known control method is used in which the peripheral speed of the intermediate transfer belt 1 reaches a constant speed by determining the constant speed arrival time in advance and measuring the driving time of the intermediate transfer belt 1.

  Here, at the timing when the mark M1 provided on the intermediate transfer belt 1 shown in FIG. 1 is detected by the mark detection sensor 5 or the mark detection sensor 5 ′, the first image forming unit 20 performs the first color A color. When the second image forming unit 20 ′ starts forming the second B color image at the timing when the mark M2 is detected by the mark detection sensor 5 or the mark detection sensor 5 ′. As shown in FIG. 5, the mark M1 and the mark M2 are outside the image forming area at an interval L such that the first color A toner image and the second color B toner image overlap on the intermediate transfer belt 1. Is provided. When the mark M1 is detected by the mark detection sensor 5 or the mark detection sensor 5 ′ at the first rotation of the intermediate transfer belt, the mark M1 is detected by the first measuring unit 44-2 shown in FIG. The elapsed time t1 until the main scanning synchronization signal detected is measured and stored in the RAM 42. Further, the counter of the main scanning synchronization signal shown in FIGS. 4 and 6 is reset and starts counting. As shown in FIG. 6, when the timing of the second main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 2, in the first image forming unit 20 of FIG. The writing of the first color A is started by the means 16, and the formation of the latent image on the photosensitive drum 14 is started. The latent image is developed in the first color A by the developing device 12 of the developing means 13 to form a toner image on the photosensitive drum 14. This toner image is transferred to the intermediate transfer belt 1, and a first-color A-color toner image is formed on the intermediate transfer belt 1. As described above, since the mark detection signal and the main scanning synchronization signal are asynchronous, the time from the mark detection signal to the next main scanning synchronization signal detection causes a time difference of one scan at the maximum depending on the detection timing.

  Next, when the mark M2 is detected by the mark detection sensor 5 or the mark detection sensor 5 ′, the second measurement unit 44-3 shown in FIG. 3 detects the mark M2 and then detects the main scanning. The elapsed time t2 until the synchronization signal is measured and stored in the RAM 42. Further, the counter of the main scanning synchronization signal shown in FIGS. 4 and 6 is reset and starts counting. As shown in FIG. 6, when the timing of the second main scanning synchronization signal from the mark detection signal of the mark M2, that is, the count value of the main scanning synchronization signal becomes 2, in the second image forming unit 20 ′ of FIG. Writing of the second B color is started by the writing means 16 ′, and formation of a latent image on the photosensitive drum 14 ′ is started. The latent image is developed in the second color B by the developing device 12 ′ of the developing means 13 ′ to form a toner image on the photosensitive drum 14 ′. The toner image is transferred to the intermediate transfer belt 1 so as to overlap the first color A toner image, and the first color A and the second color B are transferred onto the intermediate transfer belt 1. A synthetic toner image is formed.

  Thus, when the development of the first color A electrostatic latent image is completed by the developing device 12, the first color A developing device 12 is switched to the third color C developing device 11. . Similarly, when the development of the B-color electrostatic latent image of the second color is completed by the developing device 12 ′, the D-color developing device 11 ′ of the fourth color is changed from the B-color developing device 12 ′ of the second color. Switch to.

  When the mark M1 is detected again by the mark detection sensor 5 or the mark detection sensor 5 ′ in the second rotation of the intermediate transfer belt after the rotation of the intermediate transfer belt, the first measuring unit 44-2 shown in FIG. The elapsed time t3 from the detection of the mark M1 to the next main scanning synchronization signal detected is measured and stored in the RAM 42. Further, the counter of the main scanning synchronization signal shown in FIGS. 4 and 7 is reset and starts counting. 3 calculates a time Δt1 obtained by subtracting the elapsed time t1 from the elapsed time t3 stored and held in the RAM 42. The first determination means 41-2 compares Δt1 with the first reference value −T / 2 and the second reference value T / 2.

  When the comparison result is Δt1 <−T / 2, when the timing of the third main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 3, the first image In the forming unit 20, writing of the third color C is started by the writing unit 16, and formation of a latent image on the photosensitive drum 14 is started. In the case of −T / 2 ≦ Δt1 ≦ T / 2, when the timing of the second main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 2, the first image In the forming unit 20, writing of the third color C is started by the writing unit 16, and formation of a latent image on the photosensitive drum 14 is started. Further, when Δt1> T / 2, when the timing of the first main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 1, in the first image forming unit 20, Writing of the third color C is started by the writing means 16 and formation of a latent image on the photosensitive drum 14 is started. The latent image is developed by the developing unit 11 of the developing unit 13 for the third color, C, to form a toner image on the photosensitive drum 14. This toner image is transferred to the intermediate transfer belt 1, and a composite toner image of the first color A, the second color B, and the third color C is formed on the intermediate transfer belt 1. .

  Next, when the mark M2 is detected by the mark detection sensor 5 or the mark detection sensor 5 ′, the second measurement unit 44-3 shown in FIG. 3 detects the mark M2 and then detects the main scanning. The elapsed time t4 until the synchronization signal is measured and stored in the RAM 42. Further, the counter of the main scanning synchronization signal shown in FIGS. 4 and 7 is reset and starts counting. 3 calculates a time Δt2 obtained by subtracting the elapsed time t2 from the elapsed time t4 stored and held in the RAM 42. The second determination means 41-3 compares Δt2 with the first reference value −T / 2 and the second reference value T / 2.

  When Δt2 <−T / 2, when the timing of the third main scanning synchronization signal from the mark detection signal of the mark M2, that is, the count value of the main scanning synchronization signal becomes 3, the second image forming unit 20 ′. Then, writing of the fourth color D is started by the writing means 16 ', and formation of a latent image on the photosensitive drum 14' is started. In the case of −T / 2 ≦ Δt2 ≦ T / 2, when the timing of the second main scanning synchronization signal from the mark detection signal of the mark M2, that is, the count value of the main scanning synchronization signal becomes 2, the second image In the forming unit 20 ′, writing of the fourth color D is started by the writing means 16 ′, and formation of a latent image on the photosensitive drum 14 ′ is started. Further, when Δt2> T / 2, when the timing of the first main scanning synchronization signal from the mark detection signal of the mark M2, that is, the count value of the main scanning synchronization signal becomes 1, in the second image forming unit 20 ′. Then, writing of the fourth color, D, is started by the writing means 16 ', and formation of a latent image on the photosensitive drum 14' is started. The latent image is developed by the developing unit 11 ′ of the developing means 13 ′ for the fourth color, D, to form a toner image on the photosensitive drum 14 ′. This toner image is transferred to the intermediate transfer belt 1 so as to overlap the first color A, the second color B, and the third color C composite toner image. In addition, a full color toner image of the first color A, the second color B, the third color C, and the fourth color D is formed.

  The image formation for each color is started after a predetermined count of the main scanning synchronization signal has elapsed from detection of the mark M1 on the intermediate transfer belt 1 or after a predetermined count of the main scanning synchronization signal has elapsed from detection of the mark M2. When the writing means 16 or 16 'in FIG. 1 is a scanning type using a laser scanning optical system, detection of the mark M1 or M2 on the intermediate transfer belt 1 and main scanning synchronization as a writing reference for the writing means 16 or 16' Since the signals are asynchronous, even if image formation is started with the mark M1 or M2 on the intermediate transfer belt 1 as a reference, color misregistration for one scan at most occurs in the superimposed images of the respective colors.

  By performing the above control, writing of the first color A and the third color C is started within a time difference T / 2 or less after the writing start reference mark M1 is detected. . Similarly, writing of the second color B and the fourth color D is started within a time difference T / 2 or less after the writing start reference mark M2 is detected.

  Therefore, even when the speed of the intermediate transfer belt 1 varies between the first and second rotations of the intermediate transfer belt 1, writing of the first color and third color images and the second and fourth color images is started. The amount of displacement can be suppressed to 1/2 scan or less.

Next, a second embodiment of the present invention applied to the image forming apparatus 100 shown in FIGS. 1 to 3 will be described with reference to FIGS.
At the timing when the mark M1 provided on the intermediate transfer belt 1 shown in FIG. 1 is detected by the mark detection sensor 5 ′, the first image forming unit 20 starts image formation of the first color A, and the mark When the second image forming unit 20 ′ starts to form the second color B image at the timing when the mark detection sensor 5 detects M1, the first color A toner image on the intermediate transfer belt 1. Detection sensor that detects a mark M1 provided on the intermediate transfer belt 1 facing the moving surface of the intermediate transfer belt 1 with an interval such that the second color B toner image and the second color toner image overlap. 5 and a mark detection sensor 5 '.

  When the mark M1 is detected by the mark detection sensor 5 ′ at the first rotation of the intermediate transfer belt, the first measurement unit 44-2 shown in FIG. 3 detects the mark M1 and then detects the next. The elapsed time t1 until the scanning synchronization signal is measured and stored in the RAM 42. Also, the counter of the main scanning synchronization signal shown in FIGS. 8 and 9 is reset and starts counting. As shown in FIG. 9, when the timing of the second main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 2, the writing means 16 in the first image forming unit 20 Writing of the first color A is started, and formation of a latent image on the photosensitive drum 14 is started. The latent image is developed in the first color A by the developing device 12 of the developing means 13 to form a toner image on the photosensitive drum 14. This toner image is transferred to the intermediate transfer belt 1, and a first-color A-color toner image is formed on the intermediate transfer belt 1.

  Next, when the mark M1 is detected by the mark detection sensor 5, the elapsed time from the detection of the mark M2 to the next main scanning synchronization signal detected by the second measuring means 44-3 shown in FIG. t2 is measured and stored in the RAM 42. Also, the counter of the main scanning synchronization signal shown in FIGS. 8 and 9 is reset and starts counting. As shown in FIG. 9, when the timing of the second main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 2, the writing means 16 in the second image forming unit 20 ′. By this, writing of the second color B is started, and formation of a latent image on the photosensitive drum 14 'is started. The latent image is developed in the second color B by the developing device 12 ′ of the developing means 13 ′ to form a toner image on the photosensitive drum 14 ′. The toner image is transferred to the intermediate transfer belt 1 so as to overlap the first color A toner image, and the first color A and the second color B are transferred onto the intermediate transfer belt 1. A synthetic toner image is formed.

  Thus, when the development of the first color A electrostatic latent image is completed by the developing device 12, the first color A developing device 12 is switched to the third color C developing device 11. . Similarly, when the development of the B-color electrostatic latent image of the second color is completed by the developing device 12 ′, the D-color developing device 11 ′ of the fourth color is changed from the B-color developing device 12 ′ of the second color. Switch to.

  When the mark M1 is detected again by the detecting means 5 ′ after the intermediate transfer belt makes one round and the intermediate transfer belt rotates twice, the mark M1 is detected by the first measuring means 44-2 shown in FIG. The elapsed time t3 from the first to the next detected main scanning synchronization signal is measured and stored in the RAM 42. Further, the counter of the main scanning synchronization signal shown in FIGS. 8 and 10 is reset and starts counting. 3 calculates a time Δt1 obtained by subtracting the elapsed time t1 from the elapsed time t3 stored and held in the RAM 42. The first determination means compares Δt1 with the first reference value −T / 2 and the second reference value T / 2.

  When Δt1 <−T / 2, when the timing of the third main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 3, in the first image forming unit 20 Then, writing of the third color C is started by the writing means 16 and formation of a latent image on the photosensitive drum 14 is started. In the case of −T / 2 ≦ Δt1 ≦ T / 2, when the timing of the second main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 2, the first image In the forming unit 20, writing of the third color C is started by the writing unit 16, and formation of a latent image on the photosensitive drum 14 is started. Further, when Δt1> T / 2, when the timing of the first main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 1, in the first image forming unit 20, Writing of the third color C is started by the writing means 16 and formation of a latent image on the photosensitive drum 14 is started. The latent image is developed by the developing unit 11 of the developing unit 13 for the third color, C, to form a toner image on the photosensitive drum 14. This toner image is transferred to the intermediate transfer belt 1, and a composite toner image of the first color A, the second color B, and the third color C is formed on the intermediate transfer belt 1. .

  Next, when the mark M1 is detected by the mark detection sensor 5, the elapsed time from the detection of the mark M1 to the next main scanning synchronization signal detected by the second measuring means 44-3 shown in FIG. t4 is measured and stored in the RAM 42. Further, the counter of the main scanning synchronization signal shown in FIGS. 8 and 10 is reset and starts counting. 3 calculates a time Δt2 obtained by subtracting the elapsed time t2 from the elapsed time t4 stored and held in the RAM 42. The second determination means 41-3 compares Δt2 with the first reference value −T / 2 and the second reference value T / 2.

  When Δt2 <−T / 2, when the timing of the third main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 3, the second image forming unit 20 ′. Then, writing of the fourth color D is started by the writing means 16 ', and formation of a latent image on the photosensitive drum 14' is started. In the case of −T / 2 ≦ Δt2 ≦ T / 2, when the timing of the second main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 2, the second image In the forming unit 10 ′, writing of the fourth color D is started by the writing unit 16 ′, and formation of a latent image on the photosensitive drum 14 ′ is started. Further, when Δt2> T / 2, when the timing of the first main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 1, in the second image forming unit 20 ′. Then, writing of the fourth color, D, is started by the writing means 16 ', and formation of a latent image on the photosensitive drum 14' is started. The latent image is developed by the developing unit 11 ′ of the developing means 13 ′ for the fourth color, D, to form a toner image on the photosensitive drum 14 ′. This toner image is transferred to the intermediate transfer belt 1 so as to overlap the first color A, the second color B, and the third color C composite toner image. In addition, a full color toner image of the first color A, the second color B, the third color C, and the fourth color D is formed.

  By performing the control as described above, the first color A and the third color C can be written within a time difference T / 2 or less after the write start reference mark M1 is detected by the mark detection sensor 5 ′. Will be started. Similarly, the writing of the second color B and the fourth color D is started within a time difference T / 2 or less after the writing start reference mark M1 is detected by the mark detection sensor 5.

  Therefore, according to the second embodiment, as in the first embodiment, when the intermediate transfer belt 1 fluctuates in speed during the first and second rotations of the intermediate transfer belt 1, In addition, it is possible to suppress the writing start position shift amount of the first color and the third color, and the second color and the fourth color images to 1/2 scan or less.

Next, a third embodiment of the present invention applied to the image forming apparatus 100 shown in FIGS. 1 to 3 will be described with reference to FIGS. 3, 6 and 7.
Compared with the first embodiment, the third embodiment has a function as a storage unit that stores and holds a preset delay count value that delays the timing for starting image formation for each color. The control unit 40 functions as a storage unit that stores and holds a preset fine adjustment count value that adjusts the timing of starting image formation for the second and fourth colors, the points added to the RAM 42 of the third control unit 40. The main difference is that the CPU 41 is added with a function of controlling the writing of each color to start after the count of the main scanning synchronization signal from the writing start timing of each color.

  An example in which control is performed using the main scanning synchronization signal shown in FIGS. 6 and 7 will be described. When the count value to be corrected is 0, writing of the first color and the second color is started at the timing shown in FIG. 6 as in the first embodiment. Here, when the delay count value to be corrected is “10”, the writing of the first color and the second color is started at the timing of the main scanning synchronization signal which is delayed by 10 from the timing shown in FIG. . Similarly, the writing of the third color and the fourth color is started at the timing of the main scanning synchronization signal which is delayed by 10 from the timing shown in FIG. Therefore, the entire image can be formed at a position delayed by 10 lines. That is, by starting the writing by delaying the timing for starting the writing of each determined color by a preset count value, the intermediate transfer belt 1 is used regardless of the positions where the marks M1 and M2 are formed. The position of the image formed above can be arbitrarily changed and moved. Further, since the image forming position on the intermediate transfer belt 1 can be arbitrarily changed, the deterioration of the intermediate transfer belt 1 can be prevented. Further, by finely adjusting the timing of starting writing of the second color and the fourth color with the fine adjustment count value, it is possible to obtain a high-quality image with a small amount of color misregistration.

  Next, a fourth embodiment of the present invention applied to the image forming apparatus 100 shown in FIGS. 1 to 3 will be described with reference to FIGS. 3, 9 and 10. Compared with the first embodiment, the fourth embodiment controls a function as a storage unit that stores and holds a preset delay count value that delays the timing of starting image formation for each color. The RAM 42 of the control means 40 has a function as a storage means for storing and holding a preset fine adjustment count value for adjusting the timing for starting image formation of the second color and the fourth color, the points added to the RAM 42 of the means 40. The main difference is that the CPU 41 is added with a function for controlling the writing of each color to start after the count of the main scanning synchronization signal from the writing start timing of each color.

  An example in which control is performed using the main scanning synchronization signal shown in FIGS. 9 and 10 will be described. When the count value to be corrected is 0, writing of the first color and the second color is started at the timing shown in FIG. 9 as in the first embodiment. Here, when the delay count value to be corrected is “10”, the writing of the first color and the second color is started at the timing of the main scanning synchronization signal which is delayed by 10 from the timing shown in FIG. . Similarly, the writing of the third color and the fourth color is started at the timing of the main scanning synchronization signal which is delayed by 10 from the timing shown in FIG. Therefore, the entire image can be formed at a position delayed by 10 lines. That is, the writing start timing of each determined color is delayed by the preset count value, and the writing is started, thereby forming on the intermediate transfer belt 1 regardless of the position where the mark M1 is formed. The position of the image to be changed can be arbitrarily changed and moved. Further, since the image forming position on the intermediate transfer belt 1 can be arbitrarily changed, the deterioration of the intermediate transfer belt 1 can be prevented. Further, by finely adjusting the timing of starting writing of the second color and the fourth color with the fine adjustment count value, it is possible to obtain a high-quality image with a small amount of color misregistration.

  Next, a fifth embodiment of the present invention applied to the image forming apparatus 100 shown in FIGS. 1 to 3 will be described with reference to FIGS. 3 and 11. Compared with the first embodiment, the fifth embodiment moves from the mark M1 as the first color writing start reference to the start of writing the second color, as shown in FIG. The main difference is that a plurality of sets of two marks M1 and M2 of the mark M2 arranged at substantially the same distance as the distance to be arranged are arranged on the intermediate transfer belt 1, and the others are the first embodiment. Similar to the example. The timer 44 shown in FIG. 3 stores and holds the detected time every time the mark detection sensor 5 detects the marks M1 and M2. Here, since the time interval tm between the mark M1 and the mark M2 is determined in advance, the mark M1 and the mark M2 can be specified by obtaining the stored time interval and comparing it with the time interval tm. A mark M1 as a writing start reference is determined from the plurality of marks M1, and a mark M2 detected next to the mark M1 is specified. Hereinafter, as in the first embodiment, writing is started using the mark M1 as the writing start reference for the first and third colors, and the mark M2 as the writing start reference for the second and fourth colors. To do. In the case of printing a plurality of sheets, writing may be performed using different sets of marks M1 and M2.

  Next, in the image forming apparatus 100 shown in FIG. 1, intermediate transfer is performed between the first color formed by the first image forming unit 20 and the second color formed by the second image forming unit 20 ′. Color misregistration is likely to occur due to the influence of speed fluctuation of the belt 1. The same applies between the third color and the fourth color. Further, depending on the image size and the number of printed sheets, there may be a step of cleaning the remaining toner on the intermediate transfer belt 1 from the middle of the fourth color writing depending on the image size and the number of printed sheets. It tends to happen. Accordingly, when the second color forming unit 20 ′, which is likely to cause color misregistration, is yellow, the color misregistration visually recognized in the image formation including many light-colored portions such as yellow. Image formation with a reduced amount can be performed. In addition, by setting the fourth color that is most susceptible to the speed fluctuation of the intermediate transfer belt to black, the color misregistration amount of the color image portion that is visually recognized in the image formation including many characters using black. Can be formed with reduced image quality.

Next, a standard mode and a high image quality mode are provided as image quality modes in the image forming apparatus of the present invention. Hereinafter, a case where the example when the high image quality mode is selected is applied to the first embodiment will be described.
First, when the mark M1 is detected by the mark detection sensor 5 at the first rotation of the intermediate transfer belt, the first scanning unit 44-2 shown in FIG. The elapsed time t1 until the synchronization signal is measured and stored in the RAM 42. Further, the timer 44 starts measuring the elapsed time after detecting the mark M1. Further, the counter of the main scanning synchronization signal shown in FIGS. 4 and 6 is reset and starts counting. As shown in FIG. 6, when the timing of the second main scanning synchronization signal from the mark detection signal of the mark M1, that is, the count value of the main scanning synchronization signal becomes 2, the writing means 16 in the first image forming unit 20 Writing of the first color A is started, and formation of a latent image on the photosensitive drum 14 is started. The latent image is developed in the first color A by the developing device 12 of the developing means 13 to form a toner image on the photosensitive drum 14. This toner image is transferred to the intermediate transfer belt 1, and a first-color A-color toner image is formed on the intermediate transfer belt 1.

  When the mark M2 is detected by the mark detection sensor 5, the elapsed time t2 from the detection of the mark M2 to the next main scanning synchronization signal detected by the second measuring means 44-3 shown in FIG. Is measured and stored in the RAM 42. Further, the timer 44 acquires the elapsed time since the mark M1 was detected. Here, since this elapsed time is determined in advance, an allowable range of fluctuation of the elapsed time when the speed of the intermediate transfer belt 1 is changed is set. When the elapsed time is within this range, the timing of the second main scanning synchronization signal from the mark detection signal of the mark M2, that is, the count of the main scanning synchronization signal, as in the first embodiment. When the value becomes 2, in the second image forming unit 20 ′, writing of the second B color is started by the writing means 16 ′, and formation of a latent image on the photosensitive drum 14 ′ is started. The latent image is developed in the second color B by the developing device 12 ′ of the developing means 13 ′ to form a toner image on the photosensitive drum 14 ′. The toner image is transferred to the intermediate transfer belt 1 so as to overlap the first color A toner image, and the first color A and the second color B are transferred onto the intermediate transfer belt 1. A synthetic toner image is formed. If the elapsed time is out of this allowable range, it is determined that the intermediate transfer belt 1 has a speed variation and the color misregistration becomes large. In the second image forming unit 20 ′, the writing means 16 ′ causes the second color. B-color writing is not started, and the intermediate transfer belt 1 is fed by one revolution. Further, the feeding is performed until the elapsed time falls within the allowable range. At this time, the image formation of the first color is in progress, but the image formation of the first color is continuously performed. In addition, a value stored in the RAM 42 is stored for the elapsed time t1 from the detection of the first color mark M1 to the next main scanning synchronization signal, and the intermediate transfer belt 1 is fed by one revolution. This is used to calculate the write start timing for the second color later. Also, in the second rotation of the intermediate transfer belt 1, the image formation of the third color and the fourth color is controlled by the same method as the image formation of the first color and the second color.

  The present invention is not limited to the above-described embodiments, and it goes without saying that various modifications and substitutions are possible as long as they are described within the scope of the claims.

1 is a schematic diagram illustrating a configuration of an image forming apparatus to which the present invention is applied. It is a schematic plan view which shows an example of the writing means using a laser scanning optical system. 1 is a block diagram illustrating a configuration of a control system of an image forming apparatus to which the present invention is applied. It is a time chart which shows the mark detection in the 1st Embodiment of this invention, and the main scanning synchronizing signal in each color. FIG. 3 is a schematic diagram showing marks on an intermediate transfer belt. 6 is a time chart showing mark detection and respective synchronization signals at the first rotation of the intermediate transfer belt in the first and third embodiments. 6 is a time chart showing mark detection and respective synchronization signals at the second rotation of the intermediate transfer belt in the first and third embodiments. It is a time chart which shows the mark detection in the 2nd Example of this invention, and the main scanning synchronizing signal in each color. 10 is a time chart showing mark detection and respective synchronization signals at the first rotation of the intermediate transfer belt in the second and fourth embodiments. 10 is a time chart showing mark detection and synchronization signals at the second rotation of the intermediate transfer belt in the second and fourth embodiments. It is the schematic which shows the mark on the intermediate transfer belt in the 5th Example of this invention.

Explanation of symbols

5, 5 '; mark detection sensor, 20; first image forming unit,
20 '; second image forming unit; 40; control means;
41; CPU, 41-1; count acquisition means,
41-2; first determination means; 41-3; second determination means;
41-4; calculation means, 42; RAM, 43; ROM, 44; timer,
44-1; counter; 44-2; first measuring means;
44-3; Second measuring means.

Claims (8)

A color in which a single image carrier and a latent image formed by a scanning-type writing means that is disposed opposite to the image carrier and forms a latent image on the image carrier are developed with two color developers. A first and second image forming unit comprising a plurality of switchable developing means; an intermediate transfer member that sequentially superimposes and transfers toner images formed by the first and second image forming units; In an image forming apparatus comprising transfer means for transferring a color image formed on a transfer body to a sheet-like recording medium,
The first mark, which is the first color writing start reference arranged along the moving direction on the intermediate transfer member, and the distance moved from the first mark to the start of writing the second color are substantially the same. Detecting a second mark arranged at a distance of, and a mark detection means provided at a predetermined position;
In the first rotation of the intermediate transfer member, writing of the first color is started with reference to the second main scanning synchronization signal from the detection of the first mark, and the second color from the detection of the second mark. Writing of the second color is started on the basis of the main scanning synchronization signal, and main scanning synchronization in which the amount of color misregistration with the first color is minimized after the detection of the first mark in the second rotation of the intermediate transfer body. Writing of the third color is started in synchronization with the signal, and writing of the fourth color is started in synchronization with the main scanning synchronization signal that minimizes the amount of color deviation from the second color after the detection of the second mark. And an image forming apparatus comprising: a control unit configured to perform control.   The start of writing from the first color to the fourth color is delayed by a predetermined delay count number, and the second color and the fourth color are respectively corrected by a predetermined fine adjustment count number from the detection of the second mark. The image forming apparatus according to claim 1, wherein writing is started.   The plurality of sets of reference marks are arranged at substantially the same distance as the distance from the start of writing of the first color to the start of writing of the second color along the moving direction of the intermediate transfer member. The image forming apparatus according to 2.   When the high image quality mode and the standard mode are provided and the mode is set to the high image quality mode, the time from when the first mark is detected until the second mark is detected is measured, and the measured time The image forming apparatus according to any one of claims 1 to 3, wherein the start of writing of the second color or the fourth color is suspended and the intermediate transfer member is conveyed by one rotation when the time is outside a preset time range. A color in which a single image carrier and a latent image formed by a scanning-type writing means that is disposed opposite to the image carrier and forms a latent image on the image carrier are developed with two color developers. A first and second image forming unit comprising a plurality of switchable developing means; an intermediate transfer member that sequentially superimposes and transfers toner images formed by the first and second image forming units; In an image forming apparatus comprising transfer means for transferring a color image formed on a transfer body to a sheet-like recording medium,
First mark detecting means for detecting a mark serving as a first color writing start reference disposed on the intermediate transfer member along the moving direction;
A second mark detection unit disposed at a distance substantially the same as the distance to which the intermediate transfer member moves from the first detection unit to the start of writing of the second color;
After the mark is detected by the first mark detection means, writing of the first color is started at the first rotation of the intermediate transfer body with reference to the second main scanning synchronization signal, and at the second rotation. Writing of the third color is started, and writing of the second color is performed at the first rotation of the intermediate transfer body with reference to the second main scanning synchronization signal after the mark is detected by the second mark detecting means. An image forming apparatus comprising: control means for starting and controlling writing of the fourth color at the second rotation.   The start of writing from the first color to the fourth color is delayed by a predetermined delay count number, and the second color and the fourth color are respectively predetermined after the mark is detected by the second mark detection means. The image forming apparatus according to claim 5, wherein writing is started after correcting the fine adjustment count number.   The image forming apparatus according to claim 1, wherein the second color is yellow.   The image forming apparatus according to claim 1, wherein the fourth color is black.

Images