JP2006349847A - Color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a color image forming apparatus capable of more accurately correcting color slurring or periodic pitch variation and aperiodic pitch variation. <P>SOLUTION: The color image forming apparatus is equipped with: a plurality of image carriers 2; light emitting element array heads 1 for writing different images respectively on the plurality of image carriers 2; developing units 3 for sticking toner to latent images formed on the image carriers 2; transfer means 4 transferring the images on the image carriers 2 to the same transfer material; and a conveying means 7 for conveying the transfer material to which the image has been transferred. It is equipped with: an image passage timing detection means 8 provided on the upstream side of each image carrier 2 and detecting timing when the image on the conveyed transfer material passes through; and a lighting timing prediction means 10 for predicting timing when the light emitting element array head 1 is lit by detecting the passage timing of a reference image. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a color image forming apparatus capable of forming a multicolor image applied to a color copying machine, a color facsimile, a color printer, and the like, and in particular, uses a plurality of image carriers (photosensitive drums) and emits light as writing means. The present invention relates to a color matching and line pitch correction technique in an electrophotographic color image forming apparatus using an element array head.

In recent years, an electrophotographic color image forming apparatus uses image carriers (photosensitive drums) for three to four colors in order to improve the image recording speed, and sequentially prints color on recording paper held on a transfer belt. Various methods for image transfer have been proposed.
On the other hand, a method using a light emitting element array head (for example, an LED (light emitting diode) array head), which is advantageous in terms of downsizing and cost reduction as compared with a laser diode (LD) method, is attracting attention as an electrophotographic writing system. Yes.
By the way, as a problem of the system using three or four photosensitive drums, when the images formed on the photosensitive drums are superimposed on the recording paper on the transfer belt, the images are not accurate. It does not match due to the cause, and as a result, color misregistration occurs.
Another problem with this method is that periodic line pitch fluctuations (which also cause color misregistration) occur due to fluctuations (eccentricity, uneven driving, etc.) existing in the photosensitive drum system and transfer belt (conveyance roller) system. One of the points. In order to cope with this color misregistration by improving the machine accuracy, it is necessary to greatly improve the accuracy of the components (gears, etc.) constituting it, resulting in an increase in the cost of the machine.
Therefore, in order to solve the problem of color misregistration, in a multicolor image forming apparatus having a plurality of image carriers, a stable image without misregistration is detected by detecting misregistration and correcting image formation based on the detection result. Has been proposed (see, for example, Patent Document 1).
That is, the multicolor image forming apparatus described in the above publication includes an image forming unit that forms different images on a plurality of image carriers, a transfer unit that transfers images on the plurality of image carriers onto the same transfer material, Conveying means for conveying the transfer material, control means for controlling the image forming means to form marks for alignment of the images on the plurality of image carriers, and detecting means for detecting the recording positions of the marks And correction means for correcting the positional deviation of each image in the moving direction of the transfer material based on the detection output of the detection means to obtain a high-quality multiplexed image.
In the prior art, a set of resist patterns of each color is repeatedly formed so that the formation range is one turn of the transfer belt. By detecting these resist patterns, cyan, magenta, and yellow color misregistration data for black is obtained for one rotation of the belt. From the color misregistration data, components caused by uneven rotation of the photosensitive drum and the transfer belt are obtained. Extract and save components due to running unevenness.
At the time of image formation, the phases of the photosensitive drum and the transfer belt are detected, the color shift data of the respective components are combined in accordance with the phase, and the photoconductors of the respective colors are eliminated so that the combined color shift is eliminated. A correction pulse in which the writing timing for each scanning line to the drum is corrected is generated, and each LED array is driven in accordance with the correction pulse.
JP 2000-284561 A

In the above prior art, the color shift can be reduced by detecting the phase of the photosensitive drum and the transfer belt and controlling it by feeding back to the light emission timing. Since there is a time delay until the pattern is detected, the unsteady fluctuation generated between writing and detection cannot be corrected.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a color image forming apparatus capable of correcting a color shift or periodic pitch fluctuation and non-periodic pitch fluctuation with higher accuracy. And

In order to achieve the above object, the invention described in claim 1 includes a plurality of image carriers, a light emitting element array head for writing different images on the plurality of image carriers, and a plurality of image carriers on the image carriers. A developing unit for attaching toner to the formed latent image, a transfer unit for transferring the image on the image carrier onto the same transfer material, and a transport unit for transporting the transfer material after the image transfer. In the color image forming apparatus, an image passage timing detecting means provided on each upstream side of the image carrier for detecting the passage timing of the image on the transferred transfer material, and detecting the passage timing of the reference image. And a lighting timing predicting means for predicting the timing of lighting the light emitting element array head.
According to a second aspect of the present invention, a plurality of image carriers, a light emitting element array head for writing different images on the plurality of image carriers, and a toner on a latent image formed on the image carriers. In a color image forming apparatus comprising: a developing unit for attaching; a transfer unit that transfers an image on the image carrier onto the same transfer material; and a transport unit that transports the transfer material after image transfer. Image passing timing detection means for detecting the timing at which the image on the transferred transfer material passes, provided on the upstream side of the image carrier, and the reference timing on the transfer material at that time by detecting the passing timing of the reference image Lighting timing prediction means for predicting the timing of lighting the light emitting element array head from the image position.
According to a third aspect of the present invention, there are provided a plurality of image carriers, a plurality of image carriers for writing different images on the plurality of image carriers, and different images on the plurality of image carriers. A light emitting element array head for writing; a developing unit for attaching toner to the latent image formed on the image carrier; and a transfer means for transferring an image on the image carrier onto the same transfer material; In the color image forming apparatus including a transfer unit that transfers the transfer material after image transfer, an image that is provided on each upstream side of the image carrier and detects a timing at which an image on the transferred transfer material passes. A passage timing detection unit; a lighting timing prediction unit that detects a passage timing of the reference image and predicts a timing at which the light emitting element array head is turned on based on a rotational phase position of the image carrier at that time; Characterized by comprising a.

According to a fourth aspect of the present invention, a plurality of image carriers, a light emitting element array head for writing different images on the plurality of image carriers, and a toner are attached to a latent image formed on the image carrier. In the color image forming apparatus, the image forming apparatus includes a developing unit, a transfer unit that transfers the image on the image carrier onto the same transfer material, and a transport unit that transports the transfer material after the image transfer. An image passage timing detection means that is provided upstream of the carrier and detects the timing at which the image on the transferred transfer material passes, and the reference image position on the transfer material at the time of detection of the reference image passage timing. And a lighting timing prediction means for predicting the timing of lighting the light emitting element array head from the rotational phase position of the image carrier.
According to a fifth aspect of the present invention, in the color image forming apparatus according to any one of the first to fourth aspects, an interval between the image passing timing detecting unit and the immediately downstream downstream image carrier is as follows. It is longer than the circumference of the image carrier from the light emitting element array head to the transfer point.
According to a sixth aspect of the present invention, in the color image forming apparatus according to any one of the first to fifth aspects, the image passing timing detecting means is located at positions corresponding to both ends of the light emitting element array head. It is characterized by providing.
According to a seventh aspect of the present invention, in the color image forming apparatus according to the sixth aspect, the lighting timing of the light emitting element is controlled based on the image passage timing detecting means provided at two locations.
According to an eighth aspect of the present invention, in the color image forming apparatus according to any one of the first to seventh aspects, the image passing timing detecting means has a gap in a direction perpendicular to the image conveying direction. It consists of two sensors.

According to the first aspect of the present invention, there is provided image passing timing detecting means for detecting the timing at which the image on the transfer material conveyed upstream of each image carrier passes, and the lighting timing predicting means detects the passing timing of the reference image. By predicting the timing of lighting the light emitting element array head, writing with a small non-periodic variation can be performed by a short time process from detection to transfer.
According to a second aspect of the present invention, there is provided an image passage timing detection means for detecting a timing at which an image on a transfer material conveyed upstream of each of the image carriers is passed, and the lighting timing prediction means detects the passage timing of the reference image. In addition, by predicting the timing at which the light emitting element array head is lit from the reference image position on the transfer material at that time, detection of small non-periodic fluctuations and detection of the influence of periodic fluctuations in the transfer belt conveyance speed are performed. Small writing can be performed.
According to a third aspect of the present invention, there is provided an image passage timing detecting means for detecting a timing at which an image on a transfer material conveyed upstream of each of the image carriers is passed, and the lighting timing predicting means detects the timing of the image carrier at that time. By predicting the lighting timing of the light emitting element array head from the rotational phase position, it is possible to perform detection with small non-periodic fluctuations and writing with little influence of periodic fluctuations of the photosensitive member rotation speed.
According to a fourth aspect of the present invention, there is provided an image passage timing detecting means for detecting a timing at which an image on the transfer material conveyed upstream of each image carrier passes, and the passage timing of the reference image is detected by the lighting timing prediction means. Then, by predicting the timing of lighting the light emitting element array head from the reference image position on the transfer material and the rotational phase position of the image carrier at that time, it is possible to detect a non-periodic fluctuation and detect the transfer belt. Writing can be performed with little influence from periodic fluctuations in the conveyance speed and periodic fluctuations in the photosensitive member rotation speed.
According to the fifth aspect of the present invention, the interval between the image passage timing detection means and the nearest downstream image carrier is made longer than the circumference of the image carrier from the light emitting element array head to the transfer point, so that the light emission timing can be adjusted. Control can be facilitated.
According to the sixth aspect of the present invention, it is possible to detect the mounting position shift of the light emitting element array head by providing the image passage timing detecting means at positions corresponding to both ends of the light emitting element array head.
The invention according to claim 7 corrects image shift and color shift caused by mounting position shift of the light emitting element array head by controlling the lighting timing of the light emitting element based on the image passage timing detecting means provided at two places. be able to.
According to the eighth aspect of the present invention, the image passage timing detecting means is composed of two sensors having a gap in a direction perpendicular to the image conveying direction, so that it is less likely to be influenced by environmental fluctuations at the time of detection.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic configuration diagram of a color image forming apparatus according to an embodiment of the present invention. In FIG. 1, reference numerals 1a to 1d denote light emitting element arrays (LED array heads) for writing different images (images for each color separation) on the respective photosensitive drums, and 2a to 2d are for forming color images, for example. Photosensitive drums 3a to 3d corresponding to the respective colors of yellow, cyan, magenta, and black are development units for attaching toners corresponding to the respective colors to the latent images formed on the photosensitive drums 2a to 2d. 4a to 4d are transfer rollers for transferring the images on the photosensitive drums 2a to 2d onto the same transfer material, and 5a to 5d are the photosensitive drums 2a to 2d after transfer. A cleaning unit for cleaning, 6a to 6d are charging rollers for charging the photosensitive drums 2a to 2d, and 7 is a transfer to which images on the photosensitive drums 2a to 2d are transferred. 8a to 8d are optical sensor units for detecting the position of the image on the transfer belt 7, and 9a and 9b are transport rollers for driving the transfer belt 7. .
In the color image forming apparatus shown in FIG. 1, the photosensitive drums 2a to 2d are uniformly charged by the charging rollers 6a to 6d, and then images for each color separation are written by the LED array heads 1a to 1d, respectively. An image is formed, the latent image is developed by developing units 3a to 3d that hold toner of each color, and the developed color toner images are sequentially superimposed and transferred onto the same transfer material held on the transfer belt 7. Then, the transferred transfer material is conveyed to a fixing device (not shown), and the image is fixed on the transfer material to form a color image (or multicolor image).
By the way, the color image forming apparatus that uses a plurality of photosensitive drums 2a to 2d and sequentially transfers images on the same transfer material to form a multicolor image has the above-described problems. The present invention solves the above-described problems and provides a technique for correcting color shift or periodic pitch fluctuation with higher accuracy. Hereinafter, the correction operation according to the present invention will be described.

FIG. 2 is an explanatory diagram illustrating detection correction at the image writing position.
For simplicity of explanation, only the two-color image forming unit is written. The marker a at the image writing position is first written as a latent image on the photosensitive drum 2a when the right LED array head 1a is turned on, developed, and transferred to the transfer belt 7. The time at which the marker a passes right below is detected by the sensor 8a, and the left LED array head 1b emits light at a predetermined timing based on the detection signal, and an image is formed by the photosensitive drum 2b and a developing unit (not shown). Then, it is transferred to the transfer belt 7.
FIG. 3 is an explanatory diagram showing the relationship between the distance D between the sensor 8 and the transfer position of the photosensitive drum 2 immediately downstream thereof and the peripheral length D ′ of the photosensitive drum 2 from the LED array head 1 to the transfer position. .
Since the linear velocity on the outer periphery of the photosensitive drum 2 and the moving speed of the transfer belt 7 are substantially equal, the LED array head 1 can perform light emission after detecting the marker a by setting D> D ′. It is easy to correct the light emission timing.
FIG. 4 is a block diagram of an LED lighting timing control circuit in the color image forming apparatus according to the embodiment of the present invention.
The marker detection signals 12a to 12d from the sensors 8a to 8d, the photosensitive member rotation position detection signal 15 from the photosensitive drum 2, and the conveyance position detection signal from the transfer belt 7 are input to the light emission timing calculation circuit 10 and adjacent to each other. The light emission timing of each of the LED array heads 1a to 1d is calculated from the marker detection signal of the sensor and the photosensitive member rotation position detection signal 15, and the light emission timing signals are input to the LED drive circuits 11a to 11d.
In the LED drive circuits 11a to 11d, a line synchronization signal (hsync) and a data transfer clock (clock) which are signals for driving the LED array head 1 from the image data 13a to 13d corresponding to each recording color and the light emission timing signal. , A data latch signal (set) and an LED light emission signal (strobe) are generated.

FIGS. 5 to 9 illustrate a correction method in consideration of fluctuations caused by unevenness in the conveyance of the image on the transfer belt 7, uneven rotation of the photosensitive drum 2, and the like.
FIG. 5 is a graph showing a change in time for moving the minute distance of the transfer belt. The conveyance speed fluctuates due to a plurality of factors such as rotational fluctuation of the transfer roller 9, deviation from the center of the shaft, and uneven thickness of the transfer belt 7. In order to measure this variation, a line with an equal pitch is scribed on the end of the transfer belt 7 in a direction perpendicular to the conveying direction, and the line is detected by an optical sensor. Alternatively, there is a method in which magnetic powder is applied to the end of the transfer belt 7, the magnetic powder is magnetized on the line at an equal pitch in a direction perpendicular to the conveying direction, and this is detected by a magnetic head.
The time interval in FIG. 5 is created by integrating the time intervals from a certain reference position (0) over two revolutions of the transfer belt (2L) in the graph of FIG. Further, FIG. 6 is a graph showing the variation in time passing through the minute rotation angle of the photosensitive drum 2 as a time interval with respect to the rotation phase.
The rotational phase of the photosensitive drum 2 can be measured by a method of detecting an equal pitch line drawn on the surface of the rotary encoder or the photosensitive drum 2 with an optical sensor, or by applying magnetic powder on the surface of the photosensitive drum 2 There is a method in which magnetic powder is magnetized on a line at a constant pitch in a direction perpendicular to the conveying direction and detected by a magnetic head. FIG. 8 is a graph obtained by integrating the time intervals of FIG. 6 over a two-round (720 °) from a certain reference angle (0 °) of the photosensitive drum 2.
Here, a method for predicting the light emission timing will be described. Assume that the position of the transfer belt 7 is p2 when the sensor 8 detects the marker (FIG. 7). The time at this time is t2 from the graph. The transfer position p0 is obtained by p0 = p2 + D, and the time at this time is t0. Therefore, if the time from marker detection to transfer is t3, t3 = t0-t2.
Next, when the sensor 8 detects the marker, the rotational phase of the photosensitive drum 2 is set to θ2. The time at this time is t2 from the graph of FIG. The time t0 to the transfer position is t0 = t2 + t3. The photosensitive drum rotation phase at this transfer position is θ0 from the graph.
When the photosensitive drum rotation phase from the LED array head 1 to the transfer position is Θ, the photosensitive drum rotation phase θ1 at the position of the LED array head 1 can be obtained as θ1 = θ0 + Θ, and the time at that time is obtained from the graph. t1. Therefore, the time t from when the sensor 8 detects the marker to when the LED array head 1 starts to emit light can be obtained as t = t1−t2.
FIG. 9 is an operation flowchart in the color image forming apparatus according to the embodiment of the present invention. As described above, marker detection (S1), photosensitive drum phase, transfer belt position detection (S2), LED light emission start time prediction (S3), LED light emission start time determination (S4), and LED light emission start (S5). Is executed.

The signals in FIGS. 10 and 11 will be described.
hsync is a line synchronization signal, and assertion of this signal indicates the start of data transfer of one line.
The clock is a data transfer clock to the register of the LED array head 1.
Data is LED lighting / non-lighting data, and set is a data transfer end signal to the shift register of the LED array head 1 and is normally used as a latch signal for a register connected to the driver from the shift register.
“Strobe” is a lighting signal of the LED array head 1, and an LED set to be lit by data is lit during the signal assertion period.
FIG. 10 is a timing chart (part 1) for explaining fluctuations in the light emission timing.
One line of data is transferred between the hsync signal and the next hsync signal, the data is set in a register connected to the driver by the set signal, and the lighting data is set by the strobe signal to the LED driver. Current is passed to turn on the LED.
In the figure, after one line of data transfer, the LED is lit based on the previous data during the next data transfer. The LED lighting timing (strobe signal pulse cycle) by the strobe signal is changed to t, t-t1, t-2,... according to the transfer position of the transfer belt 7 and the rotation phase of the photosensitive drum 2.
Further, instead of changing the pulse period of the strobe signal, the pulse interval between the hsync signal and the strobe signal may be changed. In the lighting method shown in the figure, when the number of lit LEDs increases, a large amount of current flows in a short time and becomes a source of noise, and as a result, problems such as abnormal light emission of the LEDs easily occur.
Therefore, it is conceivable to divide one line into a plurality of sections and perform data transfer and light emission (see FIG. 11). In this case, the pulse interval between the hsync signal of the next line and the first strobe signal of the line may be changed.

FIG. 11 is a timing chart (part 2) for explaining fluctuations in the light emission timing.
The period of one line is divided into a plurality (N), 1 / N data is transferred in one period, and the light is turned on in the next period. By repeating this N times and writing for one line, the number of LEDs to be turned on at one time is reduced, and the generation of noise is suppressed.
In the figure, the pulse interval of the strobe signal is changed from T to T-T1, so that the light emission of the LED is sequentially advanced as it goes to the end of the line, and finally it is advanced by T1. A deviation on the photosensitive drum 2 can be corrected.
Although the transport position detecting means is not shown, for example, a marker indicating the reference position of the transfer belt 7 and a sensor for detecting the marker, as well as the transfer belt 7 for detecting the position of the transfer belt 7 are provided. It consists of markers engraved at equal intervals and a sensor for detecting it. In order to detect optically, the marker may be printed with stripes at equal intervals on the transfer belt or engraved grooves.

Next, another embodiment of the present invention is shown in FIGS. These objects are intended to correct the color misalignment caused by the linearity inside the LED array head or the head mounting accuracy. As shown in FIG. 8, the photosensors 8a and 8b are provided at both ends of the LED array heads 2a to 2d. Corresponding positions, that is, two positions on both the left and right sides of the conveyor belt 7 are provided.
Initial adjustment is performed so that the output timings of the two photosensors 8a and 8b are the same. If the marker detection timing of the sensor at the beginning of the line is T1 earlier than the marker detection timing of the sensor at the end of the line during actual operation, the interval between the strobe signals for one line is controlled to be T-T1. Color misregistration due to misalignment of the LED array head 1 due to environmental movement after operation or the like can be reduced.
FIG. 12 is a diagram illustrating a configuration example of the sensor 8. The sensor 8 includes two sensors, a sensor 81 and a sensor 82, having a gap parallel to a direction perpendicular to the image conveying direction. The gap between the two sensors 81 and 82 is set to such a distance that the passage detection signals of the reference image are obtained in an overlapping manner. Generally, when a magnification changing element such as a lens is not interposed therebetween, the width is set narrower than the width of the reference image in the image conveying direction.

FIG. 13 is a circuit diagram for generating a passage detection signal for a reference image from the two sensors, and FIG. 14 is a timing chart for the passage detection signal for the reference image generated by the circuit of FIG.
Detection signals from the sensors 81 and 82 are input to the comparator 15 shown in FIG. For example, the comparator 15 is set to output a high signal (voltage) level signal when the signal (voltage) level of the detection signal of the sensor 82 is higher than the signal (voltage) level of the detection signal of the sensor 81. .
As shown in FIG. 14, by setting the signal level at the time of non-detection of the sensor 82 on the upstream side in the transport direction of the reference image to be lower than the signal level at the time of non-detection of the other sensor 81, circuit malfunction due to noise or the like. You can make it difficult to get up. By such a detection method, it is possible to suppress fluctuations in detection timing due to the influence of fluctuations in the signal level of the detection signal by the sensor (due to vibration of the transfer belt, intensity of illumination on the reference word image, etc.).
According to the present invention, color misregistration due to periodic fluctuations or periodic pitch fluctuations can be corrected with high accuracy and color misregistration due to non-periodic fluctuations. (Total Cost Ownership) can be reduced.

1 is a schematic configuration diagram of a color image forming apparatus according to an embodiment of the present invention. It is explanatory drawing explaining the detection correction | amendment in the writing start position of an image. It is explanatory drawing which shows the relationship between the space | interval D of a sensor and the transfer position of the photosensitive drum of the immediate downstream, and the circumference D 'of the photosensitive drum from an LED array head to a transfer position. FIG. 2 is a block diagram of an LED lighting timing control circuit in the color image forming apparatus according to the embodiment of the present invention. FIG. 6 is a characteristic diagram illustrating a relationship between a time interval and a transfer belt position. FIG. 6 is a characteristic diagram illustrating a relationship between a time interval and a photosensitive drum rotation phase. FIG. 6 is a characteristic diagram illustrating a relationship between time and a transfer belt position. FIG. 6 is a characteristic diagram showing a relationship between time and a photosensitive drum rotation phase. 3 is an operation flowchart in the color image forming apparatus according to the embodiment of the present invention. It is a timing chart (the 1) explaining the fluctuation | variation of light emission timing. It is a timing chart (the 2) explaining the fluctuation | variation of the light emission timing. It is a figure which shows the example of 1 structure of a sensor. It is a circuit diagram which produces | generates the passage detection signal of a reference | standard image from two sensors shown in FIG. It is a timing chart of the passage detection signal of the standard image generated by the circuit of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Photosensitive drum (image carrier), 2 LED array head (light emitting element array head), 3 developing unit, 4 transfer roller (transfer means), 7 transfer belt (conveyance means), 8 sensor (image passage timing detection means) 10 Light emission timing calculation circuit (lighting timing prediction means)

Claims (8)

A plurality of image carriers, a light emitting element array head for writing different images on the plurality of image carriers, a developing unit for attaching toner to a latent image formed on the image carrier, In a color image forming apparatus comprising: a transfer unit that transfers an image on the image carrier onto the same transfer material; and a transfer unit that transfers the transfer material after image transfer.
Image passing timing detecting means provided on the upstream side of each of the image carriers for detecting the timing at which the image on the transferred transfer material passes, and the light emitting element array head is turned on by detecting the passing timing of the reference image. A color image forming apparatus comprising: lighting timing prediction means for predicting timing. A plurality of image carriers, a light emitting element array head for writing different images on the plurality of image carriers, a developing unit for attaching toner to a latent image formed on the image carrier, In a color image forming apparatus comprising: a transfer unit that transfers an image on the image carrier onto the same transfer material; and a transfer unit that transfers the transfer material after image transfer.
Image passing timing detection means for detecting the timing at which the image on the transferred transfer material passes, which is provided on the upstream side of each of the image carriers, detects the passing timing of the reference image, and on the transfer material at that time A color image forming apparatus comprising: a lighting timing prediction unit that predicts a timing of lighting the light emitting element array head from a reference image position. A plurality of image carriers, a light emitting element array head for writing different images on the plurality of image carriers, a developing unit for attaching toner to a latent image formed on the image carrier, In a color image forming apparatus comprising: a transfer unit that transfers an image on the image carrier onto the same transfer material; and a transfer unit that transfers the transfer material after image transfer.
Image passing timing detection means for detecting the timing at which the image on the transferred transfer material passes, which is provided on the upstream side of each of the image carriers, detects the passage timing of the reference image, and the image carrier at that time A color image forming apparatus comprising: a lighting timing prediction unit that predicts a timing of lighting the light emitting element array head from a rotational phase position. A plurality of image carriers, a light emitting element array head for writing different images on the plurality of image carriers, a developing unit for attaching toner to a latent image formed on the image carrier, In a color image forming apparatus comprising: a transfer unit that transfers an image on the image carrier onto the same transfer material; and a transfer unit that transfers the transfer material after image transfer.
Image passing timing detection means for detecting the timing at which the image on the transferred transfer material passes, which is provided on the upstream side of each of the image carriers, detects the passing timing of the reference image, and on the transfer material at that time A color image forming apparatus comprising: a lighting timing prediction unit that predicts a timing of lighting a light emitting element array head from a reference image position and a rotational phase position of an image carrier.   5. The color image forming apparatus according to claim 1, wherein an interval between the image passage timing detection unit and the nearest downstream image carrier is determined from a transfer point from the light emitting element array head. A color image forming apparatus characterized by being longer than the peripheral length of the image carrier.   6. The color image forming apparatus according to claim 1, wherein the image passage timing detecting means is provided at a position corresponding to both ends of the light emitting element array head. Forming equipment.   7. The color image forming apparatus according to claim 6, wherein the lighting timing of the light emitting element is controlled based on the image passage timing detecting means provided at two places.   8. The color image forming apparatus according to claim 1, wherein the image passage timing detection unit includes two sensors having a gap in a direction perpendicular to an image conveyance direction. A color image forming apparatus.

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