JP2000066463A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of surely correcting color slurring while reducing toner consumption accompanying a resist mark formation as much as possible. SOLUTION: A larger resist mark 90 is used when detecting the positional deviation at the first time just after turning-on power source, and a smaller resist mark 91 is used when detecting the positional deviation at the time other than the above. The forming position of the smaller resist mark 91 on a photoreceptor drum is corrected by the deviation of the larger resist mark 90 from the detection line.

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 copying machine, and more particularly to a technique for preventing color misregistration between colors in an image forming apparatus for forming a color image by superimposing a plurality of images formed for each color. About.

[0002]

2. Description of the Related Art As an image forming apparatus for forming a color image by superimposing respective color images, a tandem type has been attracting attention. The tandem method is to provide a color image by forming an image forming unit including a photoreceptor for each color, forming a toner image on the unit, and superimposing and transferring the toner image on a recording sheet conveyed by a transfer belt. Is what you get.

According to the tandem system, a color image can be obtained in one pass of paper so that color copying can be performed at a high speed. On the other hand, a color image for accurately superimposing toner images formed by the respective image forming units can be obtained. Difficult to respond. There are various factors causing the color misregistration.For example, with the temperature change in the copying machine, the optical system including the scanning lens in each image forming unit and the mounting member of the optical system contract or expand, The displacement of the exposure position of the photoreceptor by the laser beam in different directions for each image forming unit may result in color shift.

As one of the countermeasures against the color misregistration, a pattern image for color misregistration detection (hereinafter, referred to as "registration mark") formed by each image forming unit is transferred to a transfer belt, and is transferred onto the transfer belt. To prevent color misregistration by correcting the timing of forming a latent image on each photoreceptor according to the relative misregistration amount between each registration mark obtained by detecting each registration mark with a reflection type photo sensor. The method has been adopted.

Since the above-mentioned registration mark does not contribute to the original image formation at all, it is desirable that the registration mark be as small as possible from the viewpoint of toner consumption.

[0006]

However, the size of the registration mark is determined in consideration of the degree of thermal deformation of the optical system and its mounting member, that is, the displacement width of the registration mark with respect to the sensor detection position. If it is too small, any part of the registration mark will not pass through the sensor detection area,
As a result, color misregistration cannot be corrected.

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide an image forming apparatus capable of reliably performing color misregistration correction while minimizing toner consumption for forming a registration mark.

[0008]

In order to achieve the above object, an image forming apparatus according to the present invention controls an image writing unit to write an image on an image carrier, and performs image misalignment. An image forming apparatus having control means for writing a registration mark when an amount is detected, and transferring the written image and the registration mark to an object to be transferred, wherein the control means includes a first registration mark according to a predetermined condition. And one of the second registration marks smaller than the first registration mark is selected, and the selected registration mark is written to the image writing means.

The control means selects the first registration mark for the first detection of the displacement after the power supply of the image forming apparatus is turned on, and selects the first registration mark for the other detection of the displacement. The second registration mark is selected. In addition, the control means continuously forms a plurality of registration marks for one positional deviation amount detection, selects at least one of the first registration marks from the top, and sets the second registration mark thereafter. It is characterized by selecting.

Further, the image forming apparatus further comprises a detecting means for detecting the registration mark transferred to the transfer object, and an obtaining means for obtaining a shift amount of the registration mark from a reference line based on a detection signal of the detecting means. Means,
The control means corrects the formation position of the second registration mark on the image carrier by a shift amount of the first registration mark.

Further, the second registration mark has a shape in which a part of the first registration mark is deleted.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described with reference to a case where the present invention is applied to a digital color copying machine. FIG. 1 is a front view of the digital color copying machine (hereinafter, simply referred to as "copying machine").

This copying machine is a so-called tandem-type copying machine, and has a discharge cassette attached to a left side wall 13 of a housing in a protruding posture from a paper feed cassette 12 which is set at the bottom of a housing 11 so as to be freely inserted and removed. A transfer belt 15 is installed horizontally in the lower space of the housing up to the paper tray 14.
5 (four in the example shown) along the belt longitudinal direction
The image forming units 40C, 40M, 40Y, and 40K are arranged in a row, and the toner image of each color component is transferred onto the recording sheet S by each image forming unit while the recording sheet S is conveyed by the transfer belt 15, and the respective colors are superimposed. The printer has a printer unit 10 for forming a color image by combination.

An image reader section 20 is disposed above the printer section 10. The original image optically read here is subjected to required image processing by a control section 30, and Y (yellow), M (magenta) ), C (cyan) and K (black) color components,
The laser diodes 51C to 51K of the optical units 50C to 50K disposed above the .about.40K are optically modulated based on the respective color component signals. The light-modulated laser light is scanned by the polygon mirrors 52C to 52K in the main scanning direction and is introduced into the corresponding color component image forming units 40C to 40K. Further, as shown in FIG. 2, the optical units 50C to 50K include an SOS (Start of Sca) including a photo sensor.
n) It has sensors 53C to 53K. Note that FIG.
FIG. 3 is a diagram of the optical unit as viewed from the upstream side in the transport direction of the recording sheet. The SOS sensors 53C to 53K include a polygon mirror 52C having a regular polygon (in the illustrated example, a regular hexagon).
To photosensitive drums 41C to 41K one by one on one surface
It is provided for aligning the write start position of each scan line written to K. That is, the laser diode 5
Light is emitted from 1C-51K, and polygon mirrors 52C-5
The laser light polarized at 2K is applied to the photosensitive drums 41C to 41C.
SOS sensor 53 at a position on the upstream side of scanning K
C to 53K, and the SOS sensors 53C to 53K
Receives the laser beam, and starts modulating the laser beam in synchronization with the SOS signal generated thereby.
Therefore, the position of the electrostatic latent image in the main scanning direction is determined by the time from when the SOS signal is received to when the laser diode emits light (hereinafter, referred to as “exposure waiting time”).

Returning to FIG. 1, the image forming units 40C to 40K
Are charged photosensitive drums 41C to 41K, and a charger, a developing device and the like are arranged around the photosensitive drums 41C to 41K.
This is a unit structure that forms an image by a so-called electrostatic copying method in which an electrostatic latent image formed by exposure is exposed as toner by a developing device while exposing 1K. The developing units of the respective units supply C, M, Y, and K toners as developing agents to the photosensitive drums corresponding to the light modulation color components of the optical units 50C to 50K.

Transfer chargers 16C to 16K are disposed immediately below the photosensitive drums of the respective image forming units 40C to 40K via a transfer belt 15, and transfer toner images on the photosensitive drum surfaces to recording sheets on the transfer belt 15. S is transferred. The transfer belt 15 includes a driving roller 17
And the driven roller 18
However, when the transfer belt 15 is driven to rotate in the direction of arrow b by a motor (not shown), the transfer belt 15 travels in the direction of arrow c.
At this time, the rotation speed of the motor is controlled so that the traveling speed of the transfer belt 15 and the peripheral speed of the photosensitive drum during image formation (hereinafter, referred to as “system speed”) match. Further, below the driven roller 18, a toner or other transfer belt 1 for forming a registration mark described later
Belt cleaner 1 for removing extraneous matter (dust etc.) on 5
9 are provided.

A mark detector 60 for detecting a registration mark formed on the transfer belt 15 is provided above the transfer belt 15 between the driving roller 17 and the image forming unit 40K. As shown in FIG. 3, the mark detector 60 includes a reflection type photo sensor 61 including an LED 611 and a photodiode 612.
The LED driving element 62 turns on the LED 611 in response to a control signal from the CPU 31 described later. LED61
The light emitted from 1 is condensed by a condenser lens (not shown) and irradiates the transfer belt 15. Light reflected from the transfer belt 15 is received by the photodiode 612 and converted into an electric signal. The electric signal is amplified by the amplifier 63. The amplified electric signal is further multi-valued digital signal (hereinafter, referred to as “detection signal”) by the AD converter 64.
And output to the CPU 31.

Returning to FIG. 1, an operation panel 70 is provided at an easy-to-operate position on the front of the image reader unit 20, and includes a numeric keypad for inputting the number of copies, a copy start key for instructing the start of copying, and various types of copying. There are provided a setting key for setting a mode, a display unit for displaying a mode set by the setting key or the like by a message, and the like.

In the copying machine configured as described above,
The recording sheet S taken out of the sheet cassette 12 by the pickup roller 81 and the separating roller 82 is further conveyed to the downstream side by the intermediate roller 83, and after the leading end thereof abuts on the timing roller 84 in the rotation stopped state and waits. The sheet is fed onto the transfer belt 15 by a timing roller 84 that is driven to rotate at a predetermined timing.
The recording sheet S conveyed by the transfer belt 15 includes C,
The recording sheet S on which the toner images of the respective colors M, Y, and K are sequentially transferred and the toner image is transferred is conveyed to the fixing unit 85 by the transfer belt 15, where the image is fixed, and then the discharge roller 86 is fixed. Is discharged to the paper discharge tray 14. When copying on the back side of the recording sheet (in a so-called “double-sided copy mode”), the switching claw 87 is operated to transfer the recording sheet discharged from the fixing unit 85 to the intermediate position from the discharge side of the fixing unit 85. The sheet is guided to a re-feeding path 88 that reaches the roller 83, is reversed by a known reversing mechanism 89 provided in the middle of the re-feeding path 88, and is fed again.

FIG. 4 is a diagram showing the configuration of the control unit 30. As shown in the figure, the control unit 30 includes a CPU 31, an image processing unit 32, an image memory 33, a displacement correction unit 34,
Laser diode drive unit 35, RAM 36 and ROM
37. The image processing unit 32 converts the R, G, and B electrical signals obtained by scanning the original to generate image data composed of multi-valued digital signals, and further performs correction such as shading correction and edge enhancement processing. After that, image data of C, M, Y, and K reproduction colors is generated and output to the image memory 33, and the image data is stored for each reproduction color. At this time, the number of pages of the read document and the storage position (address) in the image memory 33 are stored in a management table (not shown) in the RAM 36 in association with each other.

In accordance with an instruction from the CPU 31, the displacement correcting unit 34 changes the storage position (address) of the image data for each pixel as necessary to generate correction data of the image writing position. The laser diode driving unit 35 drives each laser diode based on the corrected image data.

The RAM 36 stores various control variables, the number of copies and the copy mode set from the operation panel 70, the amount of positional deviation and the amount of mark deviation obtained by a positional deviation detecting process described later, and the like. The ROM 37 stores a control program relating to a scanning operation in the image reader unit 20 and an image forming operation in the printer unit 10, a control program relating to detection of positional deviation described later, a program for correcting positional deviation, shape data of registration marks, and the like. I have.

The shape data of the registration mark is of type A
And type B are stored. FIG. 5 shows the shape. The type A registration mark 90 shown in FIG. 9A is a first straight portion 9 which is one side of the reference square SQ.
01 and a second diagonal line drawn from the end point of the first linear portion.
And a straight line portion 902. That is, the registration mark 9
0 indicates that the first straight portion 901 and the second straight portion 902 are at 45 °
V-shaped. The registration mark 90 is formed on the transfer belt 15 in a direction perpendicular to the traveling direction of the transfer belt 15 (that is, in the main scanning direction).
01 is formed, and a latent image is formed on the photosensitive drum such that a second linear portion is formed following the first linear portion 901.

The type B registration mark 91 shown in FIG. 5B has a similar shape to the type A registration mark 90 which is reduced, and like the type A, the first linear portion 911 parallel to the main scanning direction. And the first linear portion 911 and a second linear portion forming an angle of 45 °. Type B is
The orientation when formed on the transfer belt 15 is the same as that of the type A.

Here, a line that bisects the registration mark in the middle (in the main scanning direction) perpendicular to the running direction of the transfer belt, as indicated by the dashed line in FIG. 5, is called the mark center. Exposure waiting times TA and TB for all the marks are stored in the ROM 37 in advance so that both types of registration marks are formed on the transfer belt in such a manner that the mark centers coincide with the detection lines (reference lines). ing.

The purpose of preparing the two large and small registration marks and their proper use will be described later. Returning to FIG. 4, while receiving inputs from the mark detector 60 and various sensors, the CPU 31 reads a necessary program from the ROM 37, performs data processing in the image processing unit 32, and writes / reads image data in the image memory 33. The position shift amount of each color is calculated by a position shift detection process described later, and these data are stored in the RAM 36. Further, it controls the content of correction of the image data in the misregistration correction unit 34, or controls the operations of the printer unit 10 and the image reader unit 20 in a unified manner while taking the timing so as to execute a smooth copying operation.

When detecting the amount of positional deviation, the CPU 31
The laser diode driving unit 35 is controlled based on the registration mark shape data stored in M37 to cause the laser diodes 51C to 51K to emit light. By this light emission, an electrostatic latent image is formed on each of the photoconductor drums 41C to 41K, and a registration mark is formed on the transfer belt 15 through a process similar to the normal image formation.

FIG. 6 shows a case where a type A resist mark is formed. Registration marks 90C to 90K for each color
Are formed in a line on the front side in the width direction of the transfer belt 15. These registration marks 90C to 90K are transferred to the transfer belt when the image writing position (electrostatic latent image forming position) and the transfer position on each of the photosensitive drums 41C to 41K are correctly set, that is, when no color shift occurs. A direction (sub-scanning direction) in which the position in the direction (main scanning direction) orthogonal to the traveling direction c is the same and parallel to the traveling direction c
Are formed on the transfer belt 15 with a distance D from each other.

The registration marks 90C to 90C formed on the transfer belt 15 by the photosensitive drums 41C to 41K
Each linear portion of K is detected by a mark detector 60 on a detection line (reference line) indicated by a broken line in the figure as the transfer belt 15 travels, and a detection signal is sent to the CPU 31. FIG. 7 is a diagram illustrating a waveform of the detection signal. In addition,
Since the detection signal is a multi-valued digital signal as described above,
Actually, the waveform has a step shape, but here, it is represented by a curve for convenience.

The detection signals 191 to 198 are waveforms obtained when the respective linear portions of the registration marks 90K to 90C are sequentially detected from the downstream side in the running direction of the transfer belt 15 in FIG. Photodiode 612 (FIG. 3)
Has a constant sensing width, the waveform of the detection signal has a peak, and therefore, it is difficult to determine the exact position of each linear portion.

Then, the CPU 31 determines the center position (or peak position) of the detected value as a reference position from the detected signal value by a method of calculating the center of gravity or the like, and determines the position as the first straight line portion and the second straight line portion of each registration mark. (Ky to Mn in the lower part of the waveform of the detection signal indicate the reference position of each linear portion. In the drawing, for example, “Ky” is a black color. The first linear portion of the registration mark, “Kn” indicates the second linear portion of the black registration mark, and the same applies to other colors.)

The CPU 31 has a clock generation circuit therein, and includes a first linear portion, a second linear portion,
The number of clocks at the time of detecting each reference position of the linear portion is RA
M36, and the difference between these values is calculated to determine the first straight line portion in each of the registration marks 90K to 90C,
Times Tk to Tc required to detect a linear portion, and times T required to detect the first linear portion of the registration mark 90K to the first linear portions of the other registration marks 90Y to 90C.
ky, Tkm, and Tkc are obtained.

Here, assuming that the system speed is V,
The distance between the registration mark 90K and the first linear portion of the registration mark 90Y is V · Tky, and similarly, the distance between the first linear portion of the registration mark 90K and the registration marks 90M, 90
The distance between the first linear portions of C is V · Tkm and V
-It becomes Tkc. As described above, in a state where there is no color misregistration, the interval between each of the registration marks 90K to 90C should be D. Therefore, each of the registration marks 90Y, 90M, and 90 on the basis of the registration mark 90K.
The displacement amounts of the first linear portions of K (that is, the displacement amounts in the sub-scanning direction) are D1ky, D1km, and D1k, respectively.
Assuming that c, it is obtained by the following equations.

D1ky = DV · Tky D1km = 2D−V · Tkm D1k = 3D−V · Tkc On the other hand, the first linear portion and the second linear portion in each of the registration marks 90K to 90C on the detection line (reference line). The distance between the linear portions (hereinafter simply referred to as “line distance”) is D
Assuming that k, Dy, Dm, and Dc, these values are calculated using the times Tk to Tc required from the detection of the first linear portion to the detection of the second linear portion in each of the registration marks 90K to 90C described above. It is obtained by the formula.

Dk = V · Tk Dy = V · Ty Dm = V · Tm Dc = V · Tc Therefore, the line spacing Dk of black and the line spacing D of other colors
The differences from y, Dm, and Dc are D2ky and D2k, respectively.
Assuming m and D2kc, the following equations are obtained.

D2ky = Dk-Dy D2km = Dk-Dm D2kc = Dk-Dc As described above, the first linear portion of each of the registration marks 90K to 90C is orthogonal to the transport direction (sub-scanning direction),
Since the linear portion forms an angle of 45 ° with the first linear portion, the difference between the line interval of the black registration mark on the detection line (reference line) and the line interval of another color is determined in the main scanning direction. Is equal to the amount of displacement between the black image writing position and the image writing positions of the other colors.

As described above, the amount of displacement (D1ky, D1k) of the image writing position of another color in the sub-scanning direction with reference to the black image writing position.
m, D1kc) and the amount of displacement (D2ky, D2km, D2kc) in the main scanning direction are calculated by the CPU 31. Further, the CPU 31 determines the distance between the mark center of the black registration mark and the detection line (reference line), that is, the transfer belt traveling direction with respect to the detection line (reference line) of the mark center of the black registration mark based on the following equation. The shift amount D3k in the direction orthogonal to the above (hereinafter, referred to as “mark shift amount”) is calculated.

D3k = Dks-Dk Here, Dks is a line interval when the mark center and the detection line (reference line) coincide with each other, as in the case of the registration mark Rk1 shown in FIG. To R
It is stored in the OM 37. Therefore, D3k has a negative value when it is shifted to the front side in the width direction of the transfer belt 15 like Rk2, and D3k has a positive value when it is shifted to the back side like Rk3. .

The CPU 31 stores the calculated positional deviation amounts and mark deviation amounts in the RAM 36. The misregistration correction unit 34 internally includes an address change unit for each reproduced color and a corrected image memory unit. The address change unit
Based on the amount of displacement stored in the RAM 36, the address of the image data read from the image memory 33 is changed and stored in the corrected image memory to generate a corrected image, thereby correcting the image writing position. I have.

More specifically, considering the case where a corrected image is formed for a yellow image, the sub-scanning direction and the main scanning direction of the black registration mark image and the yellow registration mark image obtained by the above-described positional shift amount calculation. Are D1ky and D2ky, respectively, so that the addresses may be changed so that these displacements become substantially "0" when an image is reproduced on a recording sheet.

That is, if the distance between each pixel in the reproduced image is h (for example, if the image is reproduced at a density of 400 dpi, h is about 64 μm), then [D1ky / h ] In the main scanning direction [D2ky /
h] (where [N] indicates, for example, the maximum integer value not exceeding N. In addition, it may be an integer value obtained by rounding off the decimal part of N.) That is, the determined address may be determined and stored in the corrected image memory. Whether the address is changed in the positive or negative direction is determined according to the positive / negative position shift amount.

By executing the above-described corrected image based on the amount of misregistration between black and magenta, and between black and cyan, it is possible to reproduce a color image without color misregistration based on the black image. Become. Next, a processing procedure for detecting the amount of displacement by the control unit 30 will be described with reference to the flowcharts shown in FIGS.

The flowchart shown in FIG. 9 is a routine for controlling the timing of detecting the amount of misalignment, and when the main power of the copying machine is turned on (Yes in step S1).
s), the flag provided in the RAM 36 is set to 0 (step S2), and the exposure waiting time TB for the type B registration mark is stored in the RAM 36 (step S2).
2), the amount of displacement is detected (step S4), and the CP
After the internal timer of U31 is reset and started (step S5), the internal timer is set to a predetermined time (for example,
When “30 minutes” is counted (Yes in step S6), the positional deviation amount is detected again (step S4). That is,
When the main power supply is turned on, first, the first positional deviation amount is detected, and thereafter, each time a predetermined time elapses, the positional deviation amount is detected.

The flowchart shown in FIG. 10 is a subroutine of step S4 of the flowchart described in FIG. First, whether the flag in the RAM 36 is 0,
It is determined whether or not the main power has just been turned on. If F = 0, that is, immediately after the main power has been turned on (Yes in step S41), the flag is set to 1 (step S42), and the first positional displacement is performed. Type A registration mark 9 to detect the amount
0C to 90K are formed on the transfer belt 15 (Step S43). At this time, the exposure waiting time TA stored in the RAM 36 is used.

Subsequently, the registration marks 90C to 90K are detected by the mark detector 60, and a waveform of a detection signal is obtained (step S44). From this, the amount of mark deviation and the amount of positional deviation in the main scanning direction and the sub-scanning direction of the image of another color based on the black image are obtained.
Then, the data of the mark shift amount and the data of each position shift amount in the previous position shift amount detection stored therein are updated (step S45). Until the next positional deviation amount is detected, the positional deviation is corrected based on the positional deviation amount detected this time.

On the other hand, when it is determined in step S41 that the flag is not 0 (F = 1), that is, in the case of detecting the position shift amount after the second power on, the mark shift amount D3k stored in the RAM 36 is detected. (Step S4)
6), whether or not D3k = 0, that is, whether or not the mark center of the black registration mark and the detection line (reference line) at the time of the previous position shift amount detection coincide with each other (whether there is no shift). And if they match (step S47)
Yes), type B registration marks 91C to 91K
Is formed on the transfer belt 15 (step S48), and the process proceeds to steps S44 and S45.

If it is determined in step S47 that there is a deviation, the TB in the RAM 36 according to D3k is adjusted so that the mark center of the registration mark formed on the transfer belt 15 coincides with the detection line (reference line). Is updated (step S49), and the type B registration marks 91C to 91C are updated.
K is formed on the transfer belt 15 (step S48),
Proceed to steps S44 and S45.

In any case, the exposure wait time TB stored in the RAM 36 is used in the process of step S48. As can be understood from the above description, first, the larger type A registration mark is used for the first positional deviation amount detection after the power is turned on, and the second and subsequent times use a smaller registration mark than the type A. The displacement amount is detected by switching to the B registration mark. Then, after the second time, the exposure waiting time (TB) is corrected according to the previous mark shift amount (D3k), and the registration mark is targeted to match the mark center of the registration mark with the detection line (reference line). Is forming.

That is, one of the reasons why the center of the mark does not coincide with the target detection line (reference line) is that the optical system in the optical unit or the member to which the optical system is attached contracts or expands (hereinafter referred to as expansion or contraction) due to a change in ambient temperature. However, it is difficult to predict how much the center of the mark deviates from the detection line (reference line) immediately after the power is turned on. Type A), that is, one having the same size as a conventional registration mark is used. Therefore, FIG.
The length of the type A registration mark in the main scanning direction shown in (a) is set to a length that can be detected by the mark detector 60 even if the maximum shift that can be expected occurs.

After the second time, the registration mark is switched to a smaller one (type B), and the position to be written on the photosensitive drum by the amount of the previous mark shift is corrected. In other words, since the correction is performed according to the previous mark shift amount, it is possible to use a small registration mark. Therefore, the length of the type B registration mark in the main scanning direction shown in FIG. 5B is set to a length that can correspond to the maximum expected mark shift amount occurring between the position shift amount detection processes. I have. The mark shift amount generated between the position shift amount detection processes depends on the time setting of the interval between the position shift amount detection processes.
It is considered that this is smaller than the mark shift amount expected at power-on. Considering the usage state of a normal copying machine,
Turn on the power in the morning, use it until the evening, turn off the power and finish using the day. After the power is turned on, the temperature inside the copying machine gradually rises, and when it rises to some extent, it changes to a substantially constant temperature. Therefore, after the power is turned on, the mark gradually deviates, but it is considered that when the temperature difference inside the copier from the end of use of the previous day is the largest, the start of use at which the temperature difference is the largest, the shift of the mark is also maximum. . Therefore, the second and subsequent registration marks can be made smaller than the first registration mark.

Then, instead of making the mark smaller, the position to be written on the photosensitive drum is corrected by the previously detected mark shift amount. Here, the method of the correction will be described with reference to FIG. 11, taking the case of forming a latent image of the first linear portion of the registration mark as an example. FIG. 11A shows the SOS.
FIG. 2B shows the signal, the previous light emission timing, and FIG. 2C shows the current light emission timing. For example, if the registration mark is shifted by D3k toward the front side of the transfer belt in the previous detection of the position shift amount, the registration mark is displaced to the back side of the transfer belt at the time of forming the registration mark this time. Is lengthened by Ts corresponding to D3k (light emission timing is delayed). As a result, the mark center is corrected in a direction that matches the detection line (reference line).

As described above, according to the image forming apparatus of this embodiment, a small registration mark is used except for the first time of detecting the amount of displacement immediately after the power is turned on. Saving can be achieved, and the original image formation amount increases accordingly. In addition, since the mark writing timing is corrected according to the mark shift amount, it is possible to prevent a situation in which any part of the registration mark does not pass through the detection position of the mark detector, thereby reliably detecting the position shift amount. Can be executed.

The present invention is, of course, not limited to the above embodiment, and may be, for example, as follows. (1) In the above embodiment, one resist mark is formed for each color, but a plurality of resist marks may be formed. The reason why a plurality of marks are formed for each color is to improve the detection accuracy by averaging the detection data. In this case, the mark is formed as the first mark or several marks from the first mark. A may be used, and type B may be used for the remaining remaining marks. Also in this case, the exposure waiting time of the type B mark is adjusted according to the type A mark deviation amount. In addition, the above-described operation may be performed only when the first positional deviation is detected after the power is turned on, and only the type B may be used in the second and subsequent operations. Good. (2) In the above embodiment, the registration mark is
It was formed near one edge (front side) of the transfer belt,
A mark detector is also formed near the other edge and at the center of both edges, and a mark detector is provided to detect those registration marks, and detects the amount of displacement of the registration marks at each position in the main and sub-scanning directions. You may make it. By comparing the positional deviation amounts at these three locations, the amount of curvature (bow) or inclination (skew) of the image can be detected, and the relative positional deviation amount is calculated to perform bow correction or skew correction. May be executed. (3) In the above-described embodiment, a shape such as type B is adopted as a small-sized resist mark. However, the present invention is not limited to this, and may have a shape as shown in FIG. 12, for example. In the type C shown in FIG. 5A, both ends in the main scanning direction of the type A registration mark shown in FIG. 5A are deleted by a predetermined length. The type D shown in FIG. 12B has a longer deletion length than the type C. Further, the smaller registration mark may not have a shape similar to or larger than a part of the larger registration mark. In short, the length of the main scanning direction (the direction orthogonal to the detection line (reference line) of the mark detector) is shorter and shorter than the large registration mark, so that the amount of toner required for the formation is large. It is only necessary that the shape be smaller than the mark. However, if the shapes are not similar, it is needless to say that it may be necessary to change an arithmetic expression or the like for calculating the amount of displacement between large and small marks. (4) In the above embodiment, the positional deviation amount is detected each time a predetermined time elapses after the power is turned on. However, the detection may be performed not every time but every time a predetermined number of copies are performed. . Alternatively, the detection may be performed at predetermined time intervals in principle, and if the number of copies exceeds the predetermined amount even within the predetermined time period, the position shift amount may be detected. (5) In the above-described embodiment, a large registration mark is used at the time of detecting the first positional deviation amount after the power is turned on. However, not only at this time, but also immediately after maintenance or trouble such as paper jam. It is also possible to use a large-sized registration mark at the time of the first positional deviation detection immediately after the processing. Maintenance work involves removing and attaching units inside the copier, etc.At this time, the positional relationship between the components such as the optical system also changes slightly, and if the registration mark is small in size,
This is because the mark detector may not detect the mark. (6) In the above embodiment, the exposure waiting time is not updated only when there is no mark shift amount, that is, only when D3k = 0 (Yes in step S47 in FIG. 10). The present invention is not limited to this. Depending on whether or not the absolute value of the mark shift amount is within the predetermined allowable value, the update may not be performed if the absolute value is within the allowable value, and may be updated if the absolute value exceeds the allowable value. Note that this allowable value is set to a value that does not cause a situation in which the registration mark is not detected by the mark detector in the subsequent positional shift amount detection if the shift amount is within the allowable value. (7) In the above-described embodiment, the position shift correction amount is calculated based on the detection result of the mark detector 60, and the position shift correction unit 34 forms a corrected image based on the correction amount, whereby the position shift of each color is performed. Although the correction has been executed, similar misalignment correction can be achieved by controlling the timing of starting image writing in the main scanning direction and the sub-scanning direction based on the misalignment correction amount. (8) In the above embodiment, the registration mark is formed on the transfer belt. However, the present invention is not limited to this. The transfer mark is transferred onto a recording sheet conveyed by the transfer belt and detected. Is also good. (9) In the above embodiment, a tandem-type color copying machine has been described. However, a single-color copying machine having only one image forming unit may be used. In this case, of course, the problem of color misregistration does not occur. However, even in the case of a single color, by correcting the writing position as described above, it is possible to form a faithfully reproduced image on an original having excellent linearity. Becomes In this case, registration marks are formed at positions on the transfer belt 15 on the near side, the center, and the back side, respectively, on straight lines parallel to the main scanning direction, and these are detected by the corresponding three mark detectors. I do. In the main scanning direction, the amount of deviation from the detection line (reference line) is counted. In the sub-scanning direction, the count from the start of driving of the timing roller 84 to the detection of the registration mark is counted. It is only necessary to calculate the difference amount by calculating the difference between the predetermined count number and the position count when no position shift occurs.

The present invention is not limited to a copying machine, but can be applied to all image forming apparatuses using a transfer belt such as a laser printer.

[0055]

As described above, according to the image forming apparatus of the present invention, one of the first registration mark and the second registration mark smaller than the first registration mark is selected and the registration is performed. Since the mark is formed, the first
By correcting the formation position of the second registration mark on the image carrier by an amount corresponding to the amount of deviation of the registration mark from the reference line, it is possible to minimize the consumption of toner due to the formation of the registration mark and to ensure the color misregistration. Corrections can be made.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a copying machine according to an embodiment.

FIG. 2 is a diagram showing a schematic configuration of an optical unit of the copying machine.

FIG. 3 is a diagram showing a schematic configuration of a mark detector of the copying machine.

FIG. 4 is a functional block diagram of a control unit of the copying machine.

FIG. 5 is a diagram illustrating an example of the shape of a registration mark.

FIG. 6 is a diagram illustrating an example of a registration mark formed on a transfer belt.

FIG. 7 is a diagram illustrating an example of a sampling waveform of a registration mark.

FIG. 8 is a diagram for explaining a method of detecting a shift amount of a registration mark from a reference line.

FIG. 9 is a flowchart illustrating a position shift amount detection timing routine;

FIG. 10 is a flowchart illustrating a position shift amount detection routine.

FIG. 11 is a diagram illustrating an example of light emission timing of a laser diode for forming a registration mark.

FIG. 12 is a diagram illustrating an example of the shape of a registration mark.

[Explanation of symbols]

 Reference Signs List 30 control unit 31 CPU 36 RAM 37 ROM 40C to 40K Imaging unit 50C to 50K Optical unit 60 Mark detector 90, 91, 92 Registration mark

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Katsuaki Goto 2-3-13 Azuchicho, Chuo-ku, Osaka-shi, Osaka Inside Osaka International Building Minolta Co., Ltd. (72) Inventor Takeshi Minami Azuchi-cho, Chuo-ku, Osaka-shi, Osaka 2-3-1-3 Osaka International Building Minolta Co., Ltd. (72) Inventor Shiro Tsujioka 2-3-1-13 Azuchicho, Chuo-ku, Osaka-shi, Osaka F-term in Osaka International Building Minolta Co., Ltd. 2H027 DA09 DE02 EB06 EC03 EC06 EC07 2H030 AA01 AB02 BB16 BB56

Claims (5)

[Claims] A control means for controlling the image writing means to write an image on the image carrier and writing a registration mark when detecting a displacement amount of the image; An image forming apparatus for transferring a mark to a transfer target, wherein the control means selects one of a first registration mark and a second registration mark smaller than the first registration mark according to a predetermined condition. And an image forming device for writing the selected registration mark to an image writing means. 2. The control device according to claim 1, wherein after the power of the image forming apparatus is turned on, the first registration mark is selected for the first detection of the positional deviation amount, and the first registration mark is selected for the other positional deviation amounts. 2. The image forming apparatus according to claim 1, wherein a second registration mark is selected. 3. The control means forms a plurality of registration marks in succession in one positional deviation amount detection, selects at least one first registration mark from the head, and thereafter selects a second registration mark. 2. The image forming apparatus according to claim 1, wherein a registration mark is selected. 4. The image forming apparatus further comprises: a detection unit for detecting the registration mark transferred to the transfer target; and an acquisition unit for obtaining a shift amount of the registration mark from a reference line based on a detection signal of the detection unit. 4. The image according to claim 1, wherein the controller corrects a position of the second registration mark on the image carrier by an amount of displacement of the first registration mark. Forming equipment. 5. The image forming apparatus according to claim 1, wherein the second registration mark has a shape in which a part of the first registration mark is deleted.