JPH08248730A - Image forming device - Google Patents

Abstract

PURPOSE: To obtain a color image free from color slurring by forming plural register marks in a ratio of one mark for every half the rotation of an image carrier and executing a register correction based on the mean value of plural pieces of readable data. CONSTITUTION: In every image forming station, plural register marks 9 and 10 are formed on a carrying belt 7 in a ratio of one mark for every half the rotation of each photoreceptor drum 1C, 1M, 1Y and 1BK. The position of each register mark 9 and 10 is separately detected by each mark detector 11 and 12. In the case an unreadable mark exists in plural register marks, a couple of data is invalidated. Also the mean value of plural remaining valid register data is obtained, then, the register correction is executed by the mean value. Thus, the error of the register data due to the rotating speed fluctuation of the photoreceptor drum is cancelled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, such as a laser beam copying machine or a facsimile, which forms an image by exposing on an image carrier by using an electrophotographic system, and more particularly to an optical scanning means. The present invention relates to an image forming apparatus that forms a multi-color, multi-color or full-color image by arranging a plurality of images.

[0002]

2. Description of the Related Art Conventionally, a plurality of image carriers are arranged side by side, an image is formed on each image carrier, and the obtained images are sequentially transferred onto a recording medium conveyed by a conveyor belt. An image forming apparatus for obtaining images of a plurality of colors has been put into practical use.

FIG. 8 is a cross-sectional view showing the structure of this type of image forming apparatus, which is composed of a reader section R, a printer section P, and the like. The printer section P has a conveyor belt 7 moving in the direction of arrow A. Along, cyan (C), magenta (M),
Four image forming stations corresponding to four colors of yellow (Y) and black (BK) are provided.

A reading section 40 is installed in the reader section R, and the reading section 40 includes an illumination lamp 143, a reflecting mirror 142, an optical element 141, and a light receiving element 14 such as a CCD.
It is composed of 4 etc. The reading unit 40 moves in the arrow direction along the platen glass 43 via the timing belt 48 and the pulley 47 by the stepping motor 49, and scans and reads the document O on the platen glass 43.

The cyan image forming station of the printer section P is provided with a photosensitive drum 1C, a charger 101C, a laser scanning optical system 3C, a developing device 102C, a transfer charger 7C, a cleaner 103C, etc., and is exposed by a well-known electrophotographic process. A cyan toner image is formed on the drum 1C.

To the cyan image forming station, the transfer material S supplied by the resist roller 2 is supported and conveyed on the conveyor belt 7, and is exposed by the transfer charger 7C at the transfer position facing the photosensitive drum 1C. The cyan toner image on the drum 1C is transferred onto the transfer material S.

Similarly, the magenta image forming station of the printer section P similarly includes a photosensitive drum 1M, a charger 101M, a laser scanning optical system 3M, a developing device 102M, and a transfer charger 7.
M, a cleaner 103M, and the like, and a cyan toner image is formed on the photosensitive drum 1M by an electrophotographic process.
The cyan toner image is transferred by the transfer charger 7M onto the transfer material S that has been conveyed to the transfer position of the magenta station.

Similarly, the yellow image forming station includes a photosensitive drum 1Y, a charger 101Y, a laser scanning optical system 3Y, a developing device 102Y, a transfer charger 7Y, and a cleaner 1.
03Y etc., the black image forming station is provided with a photosensitive drum 1BK, a charger 101BK, a laser scanning optical system 3BK, a developing device 102BK, a transfer charger 7BK, a cleaner 103BK, etc., and a yellow toner image is formed on the photosensitive drum 1Y. Black toner images are respectively formed on the photosensitive drums 1BK, and those toner images are transferred onto the same transfer material S that is sequentially conveyed to the transfer position of each station.

The transfer material S on which the four color toner images have been transferred
Are separated at the downstream end of the conveyor belt 7 and sent to a fixing device 80, where they are fixed and then discharged onto a tray 90.

As described above, in an image forming apparatus in which a plurality of image forming stations are arranged in parallel, since toner images of a plurality of colors are sequentially transferred onto the same surface of the same transfer material S, the transfer position in each station is improved. If the color shifts, the color shift of the image occurs and the image quality deteriorates. This color misregistration occurs due to component accuracy and environmental changes, etc., and there is a steady color misregistration (the same deviation occurs in the transfer material as a whole) and a variable color deviation (the deviation in the transfer material partially occurs). However, since it is compounded, it becomes a very troublesome problem.

First, the steady color shift will be described with reference to FIG. FIG. 7 is a diagram for explaining the factors of color misregistration in the image forming apparatus of FIG. 8, and schematically shows various states of color misregistration on the photosensitive drum. The direction of arrow A is the conveying direction of the transfer material S, the direction of arrow B is the photosensitive drums 1C, 1M,
It is the laser scanning direction in 1Y and 1BK.

For example, as shown in FIG. 1A, when a misregistration occurs on the photosensitive drum in the direction of arrow A from the regular writing position indicated by the solid line m ₀ to the position indicated by the broken line m ₂ , the top margin is reduced. There is a top margin shift that causes a mismatch.

As shown in FIG. 3B, when a misregistration occurs in the direction of arrow B from the regular writing position indicated by the solid line m ₀ to the position indicated by the broken line m ₄ , the left margin becomes inconsistent. Margin shift occurs.

As shown in FIG. 3C, when the normal writing position indicated by the solid line m ₀ in the drawing is inclined to the position inclined in the oblique direction indicated by the broken line m ₃ , inclination deviation occurs.

Further, as shown in FIG. 3D, a magnification error deviation may occur from the regular writing position indicated by the solid line m ₀ in the figure as indicated by the broken line m ₁ .

With respect to these stationary color shifts, proposals have already been made to solve the problem by electrically adjusting the scanning timing of the optical system and moving and adjusting the mirror of the scanning optical system. There is.

In each of the image forming stations, registration marks which serve as misregistration detection marks are formed at predetermined intervals, the registration marks are transferred onto the conveyor belt 7, and the transferred registration marks are read by a detector. Based on the read result, the above-mentioned four types of steady color shifts are detected and corrected.

Next, the variable color shift will be described. This means that in one sheet of the transfer material S, a slight color misregistration occurs periodically in the conveying direction (direction A). The cause is that the moving speed of the transfer material S varies depending on its driving element, so that a slight expansion and contraction of the image occurs, and the eccentric component of the drive motor of the conveyor belt 7 and the required reduction ratio gear train, drive The eccentricity component of the roller 31 is a direct cause.

Therefore, conventionally, the arrangement interval L of each image forming station is made constant, and the interval L is set to the drive roller 3.
Even if a slight fluctuation occurs in the moving speed of the transfer material at each station, by making the integral multiple of the circumference of 1 and the reduction ratio from the drive motor to the drive roller 31 an integer multiple, Becomes a periodic function in which one rotation of the drive roller 31 is one period, and the phase of each station is the same, so that image expansion and contraction occur, but since they have the same pattern in each station, as a result, The periodic color shift, which appears as a difference in the amount of positional shift due to image expansion / contraction of the station, is set to "0".

Further, a variable color shift also occurs due to a factor different from the above factors. That is, in one sheet of the transfer material S, a slight misalignment occurs periodically in the transport direction, resulting in color misregistration. The reason for this is that the rotational speeds of the four photosensitive drums 1C, 1M, 1Y, 1BK fluctuate depending on the driving elements, so that a slight expansion / contraction of the image occurs at each image forming station, and the phase thereof at each station. They are different, and the eccentricity component of the drive motor and the reduction gear train is a direct factor.

This phenomenon can be reduced by increasing the precision of the drive motor and the like related to the above direct factors, but there is a cost demerit,
There are also limits. Therefore, conventionally, the same drive system is used for the four photosensitive drums 1C to 1BK, the frequencies of fluctuations in the rotational speed thereof are made the same, and further, the phases thereof are matched on the transfer material S, whereby the image expansion and contraction is achieved. Occurs at each image forming station, but it occurs at every station as well, so that a method of solving the variable color shift caused by the difference in image expansion / contraction of each station is adopted.

[0022]

However, when adjusting the top margin, which is one of the above-mentioned steady color deviations, there is a problem of image expansion and contraction due to fluctuations in the rotation speed of the photosensitive drum. This is because the registration mark formed on the conveyor belt 7 for detecting the positional deviation appears as a composite color deviation of a steady color deviation and a variable color deviation, and the detection can separate the two. Because you can't. For example, when the positional deviation between cyan and magenta is ΔLC and ΔLM, they can be expressed as in the following equation (1).

ΔLC = LC + AF (t) ΔLM = LM + AF (t) (1)

In the equation (1), the first term on the right side represents a component corresponding to a steady positional deviation, the second term on the right side represents a variable positional deviation component, and F (t) is -1 to -1. It is a periodic function with a magnitude of 1. At this time, the registration mark is Δ
When formed as LC = LC + A and ΔLM = LM−A, the color shift amount ΔLC−ΔLM is expressed by the following equation (2).

ΔLC−ΔLM = (LC−LM) + 2A (2)

That is, the adjustment of the top margin is performed by (LC
-LM) must be adjusted, but (LC-L
M) + 2A is adjusted and there is a problem that an error occurs.

The object of the present invention is to obtain the AF of the above formula (1).
(T) is canceled so that the error 2A of the equation (2) is not generated, that is, the error of the resist data due to the fluctuation of the rotation speed of the photosensitive drum is canceled, and the top margin can be adjusted accurately, which is variable. An object of the present invention is to provide an image forming apparatus capable of constantly obtaining a high-quality color image without color shift.

[0028]

The above object is achieved by the image forming apparatus according to the present invention. In summary, according to the present invention, a plurality of image carriers are juxtaposed, and images formed on the respective image carriers are sequentially transferred onto a transfer material passing through a transfer position of each image carrier to obtain a color image. In the image forming apparatus for obtaining the above, N (N is an even number) registration marks are formed for each half rotation period of the image carrier to match the resists of the images formed on the image carriers. Based on the read data of N registration marks, K pieces (K is an even number and K ≦
The image forming apparatus is characterized in that N) data is selected as effective data, and the registration correction is executed by an average value of the K effective data.

According to the invention, the K valid data are
In total, two consecutive images formed during one rotation of the image carrier.
Satisfying that it is the registration mark read data
It is supposed to be. Also read N registration marks
Data in time order, D₁ , D₂ , D₃ , D_Four ...
D_N And then Δ₁ = D₁ -D₂ , Δ₂ = D₃ -D
_Four , ....., Δ_{N / 2} = D_N-1 -D_N And Δ
₁ ~ Δ_{N / 2} Of the values below the predetermined threshold
The thing is made to be effective data.

Furthermore, when valid data is selected, if the number of valid data is smaller than a predetermined number, the registration correction from the valid data can be prohibited. When the valid data is selected, if the number of valid data is smaller than a predetermined number, the execution of the registration correction from the valid data is stopped, and the N registration marks are formed again. You can try again.

Furthermore, N pieces of registration mark read data are D ₁ , D ₂ , D ₃ , D ₄ , ...
.., D _N , Δ ₁ = D ₁ −D ₂ , Δ ₂ = D ₃ −
D ₄ , ..., Δ _{N / 2} = D _N-1 −D _N is calculated, and K values are selected from the values of Δ _{1 to} Δ _{N / 2} starting from the smallest value. can do.

[0032]

1 is a perspective view showing the structure of an image forming apparatus according to an embodiment of the present invention. This image forming apparatus has four
1 illustrates a drum full-color image forming apparatus.

In the figure, reference numerals 1C, 1M, 1Y, 1B
K represents the photosensitive drums provided in the cyan, magenta, yellow, and black image forming stations, respectively. These photosensitive drums 1C, 1M, 1Y, 1BK are
They are juxtaposed at a predetermined interval L and rotate in the direction of the arrow in the figure. Around the photosensitive drums 1C, 1M, 1Y, 1BK,
A primary charging device (not shown) for uniformly charging, laser scanning optical systems 3C, 3M, 3Y, 3BK as image writing means (latent image forming means), a developing device for developing the latent image with toner, and a cleaner. And transfer charger (neither is shown)
Are arranged respectively. Reference numeral 2 denotes a registration roller, and the controller 16 controls the formation timing of the registration marks formed on the photosensitive drums 1C, 1M, 1Y, and 1BK based on the driving of the registration roller 2.

The registration mark is a linear toner image having, for example, a "+" shape, and is formed on the photosensitive drums 1C, 1C.
By transferring the linear toner images formed on M, 1Y, and 1BK onto the conveyor belt 7 at their transfer positions, the registration marks 9 and 10 in FIG. "+"-Shaped resist marks for four colors are formed at predetermined intervals along the portion.

Scanning mirrors 24C, 24M, 24Y and 24BK are provided in the respective image forming stations, and the respective laser scanning optical systems 3C, 3M and 3B are provided.
Light emitted from Y and 3BK is applied to each photosensitive drum 1C, 1
Images are formed on M, 1Y, and 1BK. Scanning mirror 24C, 2
Each of the 4M, 24Y, and 24BK has an actuator 2 including, for example, a linear stepping motor or the like.
Scanning mirrors 24C, 24M, 24Y, 24BK are provided according to the detection data of the registration marks 9, 10 detected by the mark detectors 11, 12.
Can be moved back and forth in the horizontal direction to adjust the inclination of the scanning line. Further, actuators 6 each composed of, for example, a linear stepping motor are also provided, and according to the detection data of the registration marks 9 and 10 detected by the mark detectors 11 and 12,
The scanning mirrors 24C, 24M, 24Y, and 24BK can be vertically moved up and down to adjust the magnification error of the scanning line.

The conveyor belt 7 extends over each of the image forming stations, and the photosensitive drums 1C, 1M, 1Y, 1BK.
Is disposed underneath. Since the conveyor belt 7 is rotationally driven in the direction of arrow A at a constant speed P (mm / sec), the passage times Δt ₁ , Δt ₂ , and Δt ₃ between the photosensitive drums 1C, 1M, 1Y, and 1BK are: , L / P seconds (where L is the photosensitive drum arrangement interval).

A cleaner member 8 is installed near the lower track on the downstream side in the moving direction of the conveyor belt 7.
The resist marks 9 and 10 formed above are removed by cleaning.

The mark detectors 11 and 12 are provided at positions downstream of the final image forming station, and are arranged above both ends of the conveyor belt 7 in the vicinity thereof. The detectors 11 and 12 are similar to image reading sensors generally used in facsimiles and the like, and are composed of charge coupled devices such as CCDs. The mark detector 11 uses the lens 14 to reflect the reflected light of the light emitted from the lamp 13b onto the conveyor belt 7.
The light is received via b and the position of the registration mark 9 is detected. Similarly, the mark detector 12 receives the reflected light of the light emitted from the lamp 13a onto the conveyor belt 7 via the lens 14a, and detects the position of the registration mark 10.

Scanning mirrors 24C, 24M, 24 are provided on one side of each of the photosensitive drums 1C, 1M, 1Y, 1BK.
BD sensor 15C, 15M, 15 between Y and 24BK
Y, 15BK are installed, and laser scanning optical systems 3C, 3
The laser beam LB emitted from M, 3Y, and 3BK is detected before image writing, and a BD signal as a detection output is sent to the controller 16.

FIG. 2 is a perspective view showing the arrangement of the scanning mirror and the optical scanning system shown in FIG. The same configuration is provided for each image forming station.

As shown in FIG. 2, an optical box 23 is arranged substantially above the photosensitive drum 1, and fθ is contained in the optical box 23.
Lens 20, polygon mirror 21 and laser light source 22
Are housed together. The fθ lens 20 forms an image of the laser beam LB generated from the laser light source 22 and deflected by the polygon mirror 21 rotating at a constant speed on the photosensitive drum 1 at a constant speed, for example. The laser beam LB from the polygon mirror 21 is emitted from the opening 23 a of the optical box 23 via the fθ lens 20.

The scanning mirror 24 has both ends of the frame 4 bent downward, and the first reflecting mirror 24a is provided in the both ends.
And a second reflection mirror 24b are housed therein, and the first and second reflection mirrors 24a and 24b are opposed to each other at a substantially right angle in cross section. The laser beam LB emitted from the optical box 23 is imaged on the photosensitive drum 1 via the first reflection mirror 24a and the second reflection mirror 24b.

The linear step actuator 25 described above.
Is attached to the upper surface of the frame 4 of the scanning mirror 24,
Depending on the step amount output from the controller 16,
The scanning mirror 24, which integrally supports the first reflection mirror 24a and the second reflection mirror 24b, is moved up and down stepwise in the direction a in the figure. Linear step actuator 26, 27
Are attached to both ends of the frame 4 of the scanning mirror 24, and similarly, depending on the step amount output from the controller 16, the first reflecting mirror 24a and the second reflecting mirror 2
The scanning mirrors 24 that integrally support 4b are horizontally moved independently in the direction of b in the figure.

The actuators 25 and 26, 27 are
It is a linear step actuator that linearly moves the output shaft of a stepping motor, and has a structure in which trapezoidal screws are formed inside the motor roller and the output shaft. This linear step actuator is usually used mainly for feeding a head of a floppy disk or the like. The actuators 26, 27, and 6 were replaced with linear step actuators, and a lead screw (having a thread on the shaft) was fixed to the shaft of a normal stepping motor, and a screw was formed corresponding to the lead screw. The same function can be achieved by using an actuator in which movable members are combined.

Specifically, the screw formed on the lead screw is 4P0.5 (nominal diameter 4 mm, pitch 0.5 m).
m), when the step angle of the stepping motor is 48 steps / one round, the advance amount SS of the output unit is SS =
0.5 / 48 = 10.42 μm / step, and the scanning mirror 24 can be driven and controlled by the feed amount for each 10.42 μm / step.

Hereinafter, with reference to FIGS. 7A to 7D, the actuators 25 to 27 shown in FIGS. 1 and 2 will be described.
The driving operation will be described. 7 (a)-(d)
The various states of color misregistration on the photosensitive drum are shown, S is a transfer material, and this transfer material S is conveyed in the direction of arrow A (moving direction of the conveyor belt 7). Here, when the actuator 25 is driven in the emission direction of the laser beam LB from the laser scanning optical system in the a ₁ direction in FIG. 2, the scanning mirror 24 is moved substantially in parallel with the a ₁ direction to reach the photosensitive drum 1. When the optical path length is shortened and the actuator 25 is driven in the opposite a ₂ direction, the scanning mirror 24 is moved substantially parallel to the a ₂ direction, the optical path length to the photosensitive drum 1 is lengthened, and the optical path length can be adjusted. . By adjusting the optical path length in this manner, the length of the scanning line of the laser beam LB having a predetermined spread angle on the photosensitive drum 1 is m ₀ (solid line) as shown in FIG. 7D, for example. To m ₁ (dashed line).

Further, by simultaneously driving the actuators 26 and 27 in the same direction, for example, in the b ₁ direction, the scanning mirror 24 is moved in parallel in the b direction which is a direction substantially perpendicular to the a direction described above. 7 (a) scan line m ₀
Can be translated to the position of the scanning line m ₂ (broken line). Further, when either one of the actuators 26 and 27 is driven, or when the actuators 26 are driven in the b ₁ direction and the actuators 27 are driven in the b ₂ direction, driving in the opposite directions is performed. The inclination of the scanning line m ₀ in () can be changed like the scanning line m ₃ (broken line).

As described above, the pair of first and second reflecting mirrors 24a and 24b have a cross section of a substantially right angle, that is, ("C").
The scanning mirror 24 incorporated so as to have the shape of () is disposed in the optical path of the laser beam LB emitted from the laser scanning optical system 3 to the photosensitive drum 1, and the position of the scanning mirror 24 is set to the actuator 25 or 26. , 27, the optical path length or the laser beam scanning position can be adjusted independently. That is, the scanning mirror 2
By moving 4 in the a direction, it is possible to correct only the optical path length of the laser beam LB without changing the position of the scanning line imaged on the photosensitive drum 1, and the scanning mirror 24 in the b direction. By moving, it is possible to correct the image forming position and angle on the photosensitive drum 1 without changing the optical path length of the laser beam LB.

In the four-drum type image forming apparatus (full color printer) of the present embodiment shown in FIG. 1, the scanning mirror 24 and the actuator mechanisms 25, 26 and 27 for adjusting the position of the scanning mirror 24 are provided. , Each image forming station is individually provided, and in each of the photosensitive drums 1, 1M, 1Y, 1BK, the magnification error, the top margin, and the left margin are individually corrected based on the inclination of the scanning line and the optical path length difference, and the transfer is performed. The color misregistration between the toner images of the respective colors sequentially transferred to the material S is removed.

Next, the drive mechanism of the photosensitive drums 1, 1M, 1Y, 1BK shown in FIG. 1 will be described with reference to FIG.

As shown in FIG. 3, each photosensitive drum 1,
1M, 1Y, and 1BK are rotationally driven in the direction of the arrow in the figure by a drive motor 34a that is a common drive source. That is, the worm gears 32C, 32M, 32Y, 32BK are fixedly mounted on the drive shaft 35a connected to the drive motor 34a. The worm gears 32C, 32M, 32Y,
32BK is the photosensitive drum 1, 1M, 1Y, 1B, respectively.
Worm wheels 31C, 31M, 31 attached to the ends of the K drum shafts 33C, 33M, 33Y, 33BK
It meshes with Y and 31BK. Therefore, the drive motor 34
When the drive shaft 35a is rotationally driven by a, the photosensitive drums 1 supported by the drum shafts 33C, 33M, 33Y, and 33BK via the worm gears 32C, 32M, 32Y, and 32BK and the worm wheels 31C, 31M, 31Y, and 31BK. 1M, 1Y, and 1BK are rotationally driven.

Photosensitive drums 1, 1M, 1 having such a mechanism
According to the rotational drive of Y and 1BK, the drum shafts 33C and 33C
Speed fluctuations of M, 33Y, 33BK Δv _M , Δv _C , Δv
_Y and Δv _K are mainly worm wheels 31C and 31M,
Each drum shaft 33 is caused by the eccentricity of 31Y and 31BK.
The cycle of speed fluctuation of C to 33BK is one rotation cycle of the drum shaft. Therefore, the image formed at each image forming station is the speed variation Δv _M of each drum shaft 33C to 33BK.
Misaligned ΔL affected by ~Δv _K, since the positional deviation ΔL is the integral of the velocity fluctuation Delta] v, the period to be its main becomes drum shaft 1 rotation period.

Even when the registration mark is formed, the positional deviation of the registration mark is affected by the speed fluctuation of the drum shaft, and the main cycle thereof is one rotation cycle of the drum shaft. The error at the time of forming the registration mark caused by the speed fluctuation of the drum shaft corresponds to the AF (t) in the above-mentioned formula (1). Therefore, in the present invention, in each image forming station, an even number of registration marks are formed at a rate of one for each half cycle of the photosensitive drum, and an average value thereof is calculated, so that an error during registration mark formation is reduced. Cancel it.

FIG. 4 shows the principle. In FIG. 4, ΔL on the vertical axis represents a positional deviation of the registration mark due to fluctuations in the rotation speed of the photosensitive drum, that is, the drum shaft, and as described above, ΔL mainly represents one rotation cycle of the photosensitive drum. It is a function with a period. T _{1 to} t with respect to the time axis t
Reference numeral ₈ indicates the timing of forming the registration mark. In the present embodiment, ₈ marks are formed at a rate of one for each half cycle of the photosensitive drum.
Individual resist marks are formed.

By reading the positions of the eight registration marks and calculating the average value of the eight registration marks, the error in the position of the registration marks due to the speed fluctuation of the photosensitive drum is offset. Therefore, the eight average values become the registration data in each image forming station from which the error due to the speed fluctuation of the photosensitive drum is removed.

In this embodiment, eight resist marks are formed at a rate of one in a half cycle of the photosensitive drum, but the number of resist marks is not limited to eight and basically an even number is acceptable.
According to a study made by the present inventor, the accuracy is improved when there is a certain number of data, so about 6 to 8 is an appropriate number.

In the present invention, the resist data from which the error is removed is obtained as described above, but a measure is taken to further improve the accuracy, and the resist data selected by this is used as effective data. Perform registration correction. In this embodiment, the following two countermeasures are implemented, and in each case, "there are two data in one cycle of the photosensitive drum" is the valid data.

(1) When there is an unreadable one among the eight registration marks, not only the data is invalidated, but also the data paired with the data is invalidated, and the data is always valid. Let the number be an even number.

(2) Even if all of the eight registration marks can be read, the read data considered to be abnormal is invalidated. In this case, the data paired with the data determined to be abnormal is also invalidated, and the number of valid data is set to an even number.

The above (1) will be specifically described.
For example, in FIG. 4, when the registration mark formed at the timing t ₃ cannot be read (for example, scratches or dust adhered on the conveyor belt), the average value is obtained from 7 readable data other than t _3. Instead of t ₃
The read data of the resist mark formed at the timing t ₄ forming a pair with is also invalidated, and the average value is obtained from the remaining 6 data. That is, the data for each half cycle of the photosensitive drum is not treated as independent eight data, but two data in one rotation of the photosensitive drum is treated as one pair.

Since the pair of two data cancel each other due to the speed fluctuation of the photosensitive drum cycle, the valid resist data always cancels out the error due to the speed fluctuation of the photosensitive drum cycle. It becomes possible to do.

Next, (2) will be described. First,
When the abnormal data is classified, the following examples can be considered.

1) Read the conveyor belt for scratches and dust,
When judging it as registration data; 2) When reading the registration mark, noise enters the reading counter and changes to incorrect data; 3) Instantaneous rotation of the photosensitive drum due to external vibration or shock. When there is a dynamic fluctuation.

The resist data under such a condition naturally includes unnecessary error and is so-called abnormal data.

Next, a method of discriminating this abnormal data will be described. In FIG. 4, among the read data of the registration marks formed at the timings t _{1 to} t ₈ , the data in the sub-scanning direction are D ₁ , D ₂ , D ₃ , D ₄ , and D, respectively.
₅ , D ₆ , D ₇ , and D ₈ . Data D ₁ is data D ₂
The data D ₃ is D ₄ , the data D ₅ is D _6, and the data D ₇ is D ₈ in pairs within one cycle of the photosensitive drum.

In this embodiment, the difference between two paired data values is obtained, and when the difference exceeds a certain value BK, it is judged as abnormal data. The threshold value K should be selected in consideration of the amount of variation in the registration mark that may occur due to the speed fluctuation of the photosensitive drum. In the present embodiment, the threshold value K is set to a counter value equivalent to 50 μm. . Therefore, if the threshold value K is exceeded, it can be determined that there is another abnormal value that exceeds the range of variations in the registration data that can occur due to speed fluctuations of the photosensitive drum.

This one set of data judged to be abnormal is also invalidated in pairs, and the average value is obtained from the remaining valid even-numbered resist data.

FIG. 5 is a flowchart showing the processing described above when there is unreadable data (1) and when there is abnormality in the read data (2).

First, in the flow 1001, a resist mark is formed at timings t _{1 to} t ₈ , and the flow 100
In 2, the registration marks are read by the detectors 11 and 12. Next, it is determined whether there is unreadable data (flow 1003). If there is no unreadable data, abnormal data determination processing is performed to see if there is any abnormality in each resist data (flow 100).
6). If there is no abnormal data, the average value is calculated using all data as valid data (Flow 10).
10).

If there is unreadable data, as described above, the data paired with the unreadable data is also treated as invalid data, and valid data is selected (flow 1004). If the number of valid data is N or more, the process returns to the abnormal data semi-division processing of flow 1006, but if the number of valid data is less than N, the error post-processing is executed (flow 1011). The error post-processing is a retry mode in the case of "the number of valid data is small", and the process returns to the flow 1001 again to execute the registration mark formation again.

Flow 101 by executing the retry mode
When the number reaches 0, the registration data is completed, but when the process returns to the flow 1011 again, it is determined that the "registration mark reading error" occurs, the machine is stopped, and the error code is displayed. This error post-processing is executed even when the number of valid data after the abnormal data discrimination processing is small (flows 1007 to 1009, 1011).

The above-mentioned abnormal data discrimination method is one example,
Other methods may be used.

Another embodiment of the present invention will be described. FIG. 6 is a flow chart showing the flow of processing in the present embodiment, which is different from the case of FIG. 5 in that instead of performing the abnormal data discrimination processing, a method of squeezing effective data is adopted. is there. The details will be described below along the flow.

First, the flow 1001 to flow 1005 are the same as in FIG. That is, an even number of registration marks are formed at a rate of one every half cycle of the photosensitive drum (flow 100), and each registration mark is detected by the detector 1.
It is read by the numbers 1 and 12 (flow 1002). If there is an unreadable registration mark, the data of the registration mark paired with the registration mark is also invalidated (flow 100).
3-1004). Then, if the number of readable and valid data is N ₁ or more, the valid data is narrowed down (flow 1021).

For example, if the valid resist data is D ₁ ~
It is assumed that there are (k + 1) pieces up to D _k (k + 1 is an even number).
These data show that D ₁ is D ₂ , D ₃ is D ₄ , ...
.., D _k-1 is paired with D _k within one cycle of the photosensitive drum. . Here, the difference between two data values forming a pair is obtained, and the data of the pair having the maximum difference is invalidated (that is, the resist data having the largest variation as the drum cycle interval registration data is excluded). If the number of remaining resist data is N ₂ or more, it is determined that the number of valid data is the lower limit limiter or more (flow 102).
2) The average value is calculated (flow 1023).

Post-error processing in FIG. 6 (flow 1011)
Is the same as that in FIG. 5, and the description is omitted.

The narrowing down of the effective data in FIG. 6 means that the average data of the error due to the speed unevenness of the peripheral speed of the photosensitive drum is used as the effective data while securing the minimum number of data before averaging. is there. Here, the data narrowing method of "invalidate the data of the pair with the maximum difference in the data value to be paired" is used, but conversely, the number of pairs with a small difference in the data value to be paired is calculated. A squeezing method of "selecting pairs, for example, 3 pairs" is also possible.

[0078]

As described above, according to the present invention, an even number of resist marks are formed every half cycle of the photosensitive drum, and when each resist mark is read, the resist data is calculated. , "There are two data in one cycle of the photosensitive drum" are averaged as effective data, so that the error due to the speed fluctuation of the photosensitive drum cycle is always canceled. Further, since the read registration data is paired and the abnormality determination processing is performed, the abnormal data generated by noise, vibration, etc. can be removed and processed, and the reliability of the registration data is enhanced. Further, when the limiter is provided at the lower limit of the number of valid data, the accuracy of the resist data can be guaranteed.

As a result, it is possible to accurately correct the registration deviation such as the top margin caused by the fluctuation of the rotation speed of the photosensitive drum, and always form a high-quality color image without color deviation.

[Brief description of drawings]

FIG. 1 is a perspective view illustrating a configuration of an image forming apparatus according to an exemplary embodiment of the present invention.

FIG. 2 is a perspective view showing an arrangement configuration of a scanning mirror and an optical scanning system installed in each image forming station of the image forming apparatus of FIG.

FIG. 3 shows photosensitive drums 1C and 1 of the image forming station.
It is a perspective view which shows the drive mechanism of M, 1Y, and 1BK.

FIG. 4 is an explanatory diagram showing the principle of an error canceling method at the time of forming a resist mark in the present invention.

FIG. 5 is a flow chart showing a process when there is unreadable data of a registration mark or abnormal read data.

FIG. 6 is a flowchart showing a process in another embodiment of the present invention.

FIG. 7 is an explanatory diagram showing a factor of steady color shift of an image.

FIG. 8 is a sectional view showing a configuration of a conventional image forming apparatus.

[Explanation of symbols]

 1 Photosensitive Drum 3 Scanning Engineering System 9, 10 Registration Mark 11, 12 Detector 16 Controller 24 Scanning Mirror 24a, 24b Reflecting Mirror 25, 26, 27 Linear Step Actuator

Claims (6)

[Claims] 1. An image in which a plurality of image bearing members are juxtaposed and images formed on the respective image bearing members are sequentially transferred onto a transfer material passing through a transfer position of each image bearing member to obtain a color image. In the forming apparatus, N registration marks (N is an even number) are formed at a rate of 1 for each half rotation cycle of the image carrier, and N registration marks for aligning the resists of the images formed on the respective image carriers are formed. An image forming apparatus characterized in that K pieces of data (K is an even number and K ≦ N) are selected as valid data according to the read data of the registration mark, and the registration correction is executed by an average value of the K pieces of valid data. . 2. The image forming apparatus according to claim 1, wherein the K valid data are all two continuous registration mark reading data formed during one rotation of the image carrier. To 3. N pieces of registration mark reading data _{chronologically, D 1, D 2, D} 3, D 4, ···, when a _{D N, Δ 1 = D 1} -D 2 Δ 2 = D ₃ −D ₄・ ・ ・ ・ ・ ・ Δ _{N / 2} = D _N-1 −D _N is obtained, and among the values of Δ _{1 to} Δ _{N / 2} , those which are less than or equal to a predetermined threshold value. The image forming apparatus according to claim 2, wherein the effective data is. 4. The image forming apparatus according to claim 1, wherein when the valid data is selected and the number of valid data is smaller than a predetermined number, registration correction from the valid data is prohibited. 5. When valid data is selected, if the number of valid data is smaller than a predetermined number, the registration correction from the valid data is stopped and the
The image forming apparatus according to claim 1, wherein the process is repeated from the formation of N registration marks. 6. A N number of registration mark reading data _{chronologically, D 1, D 2, D} 3, D 4, ···, when a _{D N, Δ 1 = D 1} -D 2 Δ 2 = D ₃₋ D ₄・ ・ ・ ・ ・ ・ ΔN _{/ 2} = D _N-1 −D _N is calculated, and K pieces are selected starting from the smallest value among Δ _{1 to} Δ _{N / 2.} 2 image forming apparatus.