JPH06198978A - Color image forming apparatus - Google Patents

Abstract

PURPOSE:To prevent a color slippage from occurring at any region within a page. CONSTITUTION:In a color image forming apparatus, a first image carrier 20 is scanned by an electrostatic latent image forming device 24 to form an electrostatic latent image, and the latent image is transferred on to a second image carrier 21 after developed. Then, a toner image transferred on the second image carrier 21 is transferred on to a sheet of recording paper. The color image forming apparatus is provided with a speed detector 22 which converts the driving speed of the second image carrier 21 into a signal to detect it, and a driving device 23 which drives the second image carrier 21 so that the cycle of the signal detected by the speed detector 22 is synchronized with the scanning cycle of the electrostatic latent image forming device 24.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus which superimposes a plurality of color toner images on an intermediate transfer member and then collectively transfers them onto recording paper.

In recent years, an electrophotographic recording system has been used in color printers, color facsimiles, color copying machines, etc., and in an electrophotographic recording device using this system, a latent image formed on a photoconductor is sequentially colored. A process in which the toner images are developed with different color toners, the developed toner images are once transferred to the intermediate transfer body in the order of development, and all toner images are formed on the intermediate transfer body, and then transferred to the recording paper. There is.

As is well known, the electrophotographic recording apparatus has an image forming process and a recording paper conveying process. The image forming process includes an electrostatic latent image forming process, an electrostatic latent image developing process, and a transfer process. Processes and fixing processes are included.

In the electrostatic latent image forming process, an electrostatic latent image is formed by optically projecting an image on a photosensitive drum or photosensitive belt or by applying an electric charge on a dielectric drum.

In the electrostatic latent image developing process, the electrostatic latent image is developed by electrostatically attaching toner as a recording medium to the electrostatic latent image formed as described above. In the transfer process, the toner image developed as described above is transferred to the recording paper by applying a voltage having a polarity opposite to that of the toner to the recording paper, and in the fixing process, the toner image transferred to the recording paper is melted by heat or the like. Fix it on the recording paper.

By the way, when a plurality of color toner images are superposed on the intermediate transfer member and then transferred onto the recording paper at once, it is necessary that no color shift occurs in any area in one page. .

[0007]

2. Description of the Related Art FIG. 7 is a diagram for explaining a configuration example of a color image forming apparatus. FIG. 7 shows a case where the intermediate transfer belt 6 is used as the intermediate transfer member, and the photosensitive drum 1, the driving roller 8, the stretching rollers 9 and 10, and the transfer rollers 11a and 11b are used.
First, the charger 2 uniformly charges the photosensitive drum 1 to, for example, about -700 V under the control of a controller (not shown).

Next, the photosensitive drum 1 rotating as shown by an arrow B is scanned by a laser beam sent from the exposing section 4 as shown by an arrow A to form an electrostatic latent image. The image is developed by applying color toner by the developing units 3a to 3d.

The developing unit 3a is provided with yellow, the developing unit 3b is provided with magenta, the developing unit 3c is provided with cyan, and the developing unit 3d is provided with black toner. First, the photosensitive drum 1 makes one rotation to form a toner image developed in yellow. The toner image transferred to the intermediate transfer belt 6 and developed with magenta by the next one rotation is transferred to the intermediate transfer belt 6,
Next, the toner image developed with cyan is transferred to the intermediate transfer belt 6, and then the toner image developed with black is transferred to the intermediate transfer belt 6.
Transfer to.

That is, the intermediate transfer belt 6 is driven by the drive roller 8 as shown by the arrow C to run, and the transfer roller 1
A voltage having a polarity opposite to that of the toner, for example, about 2 KV is applied by 1a and 11b, and the toner image is transferred by adsorbing the toner at the contact point with the photosensitive drum 1.

The residual toner on the photosensitive drum 1 that has not been transferred is removed by the cleaner 5, and the photosensitive drum 1 is charged again by the charger 2 and the above process is repeated.

Therefore, the photosensitive drum 1 and the intermediate transfer belt 6
By respectively rotating four times in synchronization with each other, toner images of four colors are superimposed on the intermediate transfer belt 6, and all toner images are combined.

When all the toner images are combined on the intermediate transfer belt 6, the recording paper 15 is fed out under the control of the controller.
By the transfer roller 12 applying a voltage having a polarity opposite to that of the toner, for example, about 2 KV, to the back surface of the recording paper 15, the entire toner image is transferred.

Then, the recording paper 15 is conveyed to the fixing unit 13 and the transferred toner image is fixed. The toner remaining on the intermediate transfer belt 6 without being transferred to the recording paper 15 is removed by the cleaner 14 that operates when the transfer onto the recording paper 15 is completed.

FIG. 8 is a diagram for explaining another configuration example of the color image forming apparatus. FIG. 8 shows a case where the intermediate transfer drum 18 is used as the intermediate transfer member. When the intermediate transfer drum 18 is rotated four times in synchronization with the photosensitive drum 1, all toner images are combined on the intermediate transfer drum 18. Other operations are the same as those in FIG. 7, and detailed description thereof will be omitted.

[0016]

FIG. 9 is a diagram for explaining the exposure start timing detection. As described above, while the toner image formed on the photosensitive drum 1 is sequentially transferred to the intermediate transfer body for each color, the toner image already transferred onto the intermediate transfer body and the toner image to be transferred from now on are transferred. In some cases, the positions do not match completely, and the color shift of the image occurs.

Therefore, as shown in FIG. 9, for example, when the intermediate transfer belt 6 is used as the intermediate transfer member, a marker 16 is provided on the intermediate transfer belt 6 and the sensor 7 reads the marker 16 to output a marker detection signal. There is a method in which the control unit 17 sends an exposure start command each time as a trigger to the exposure unit 4 shown in FIG.

In an exposure system using a rotating mirror such as a laser optical system, since the laser beam is line-scanned on the photosensitive drum 1 at a constant time interval, the writing control of print data is performed by a beam detector in the laser optical system. Then, the arrival of the laser beam at the end of line scanning is detected, and timing is taken.

However, the position of the laser beam at the time when the marker 16 on the intermediate transfer belt 6 is detected is not always constant, and the time from the detection of the marker 16 by the sensor 7 to the actual start of exposure varies for each color. .

Therefore, the print start positions of the respective colors are not aligned,
A maximum of one line of color misregistration occurred in the sub-scanning direction.
As a countermeasure against this, conventionally, the time from the detection of the marker to the detection of the laser beam by the beam detector is detected, and this time is compared with a predetermined reference value to reduce the angle of the mirror in the optical path of the laser beam. JP-A-62-267774 discloses a method of adjusting the print start position by changing the above.

As another method, there has been proposed a method in which the print start position is adjusted by controlling the feeding speed of the intermediate transfer member between the marker detection and the exposure start. However, although it is possible to align the print start positions by using the above method, it is not possible to eliminate all color misregistration within the page.

This is because even if the alignment is performed at the leading edge of the page, the subsequent alignment cannot be controlled, so errors are accumulated due to various causes and color misregistration occurs at the trailing edge of the page. Because it will be.

The cause of this color misregistration is speed fluctuation of the intermediate transfer member. This is because the drive roller 8 and the intermediate transfer belt 6 are used when the intermediate transfer belt 6 is used as the intermediate transfer member.
This is caused by slippage between the roller and the drive roller 8, eccentricity of the drive roller 8, fluctuations in load torque, and the like.

On the other hand, when the intermediate transfer drum 18 is used as the intermediate transfer member, the causes are eccentricity of the drum, circularity, fluctuation of load torque, and the like. As described above, the conventional print start alignment method has a problem in that it is not possible to prevent the occurrence of color misregistration within a page.

Further, generally, the speed fluctuation is smaller when the intermediate transfer drum 18 is used as the intermediate transfer member, but the peripheral length of the intermediate transfer drum 18 needs to be equal to or larger than the length of the largest recording paper. However, the diameter of the intermediate transfer drum 18 becomes large, and it is difficult to increase the mechanical accuracy until sufficient stability is obtained. Therefore, there is a problem that an attempt to increase the mechanical accuracy to deal with the cost will result in an extremely high cost.

In view of the above problems, the present invention detects equidistant markers for speed detection provided at the end of the intermediate transfer member by a sensor and converts them into a signal, and the frequency and phase of this signal are exposed. The purpose is to economically prevent the occurrence of color misregistration within one page by controlling the speed of the motor that drives the intermediate transfer member so that the frequency and the phase of the line scanning clock of the system match. There is.

[0027]

FIG. 1 is a block diagram for explaining the principle of the present invention. The color image forming apparatus is the first
This image carrier 20 is scanned by the electrostatic latent image forming means 24 to form an electrostatic latent image, and after development, the image is transferred to the second image carrier 21. The toner image transferred to is transferred to recording paper.

Then, a speed detecting means 22 for converting the driving speed of the second image carrier 21 into a signal for detection, a cycle of the signal detected by the speed detecting means 22, and the electrostatic latent image forming means. The driving means 23 for driving the second image carrier 21 is provided so as to be synchronized with the scanning cycle of 24.

The speed detecting means 22 is composed of markers provided at equal intervals outside the area of the second image carrier 21 where the toner image is formed, and a sensor for reading this marker.

The distance between the markers is an integral multiple of the distance between scanning lines for scanning the surface of the first image carrier 20. Then, the sensor is the second image carrier 21.
Is attached to a position for reading the marker at a position immediately before contacting the first image carrier 20.

When the second image carrier 21 is composed of a belt, the driving roller 8 for driving the belt is at a position immediately after the belt comes into contact with the first image carrier 20. It is mounted to be driven.

[0032]

With the above-described structure, when the toner images of the respective colors formed on the first image carrier 20 are sequentially transferred to the second image carrier 21, the toner image is positioned at the leading edge of the page. Since the alignment is performed and the subsequent alignment control can be performed, it is possible to cancel the errors due to the various causes. Therefore, it is possible to prevent the occurrence of color misregistration within one page.

[0033]

2 is a block diagram of a circuit showing an embodiment of the present invention, FIGS. 3 and 4 are diagrams for explaining the operation of FIG. 2, and FIGS. 5 and 6 are diagrams of an intermediate transfer member of the present invention. It is a figure explaining an example.

FIG. 2 shows a case where the intermediate transfer belt 6 is used as an intermediate transfer member. The speed detecting means 22 in FIG. 1 is composed of a sensor 7, and the driving means 23 in FIG. 1 is each circuit shown by reference numerals 71 to 79. And a motor. The exposure unit 4 includes a laser diode 40, a lens 41, a polygon mirror 42,
Spindle motor 43, lens 44, mirror 45
And an optical system including the beam detector 46 and the reference oscillator 4
And an exposure control circuit 47 including eight.

The polygon mirror 42 is, for example, an eight-sided mirror, is rotated by a spindle motor 43, and a laser diode 40 reflects a laser beam projected through a lens 41 and projects it onto a mirror 45 through a lens 44.

Therefore, the laser beam reflected by the mirror 45 moves on the photosensitive drum 1 in the main scanning direction, for example, with a resolution of 600.
Line scanning is performed at dpi, and the beam detector 46 detects that the laser beam has reached the end of line scanning.

The spindle motor 43 generates, for example, 8 pulses for each rotation and sends them to the exposure control circuit 47. The exposure control circuit 47 divides this pulse and the frequency of, for example, 29.027 MHz generated by the reference oscillating unit 48. The spindle motor 43 is rotated at a predetermined rotation speed based on the rotated clock.

The photosensitive drum 1 has, for example, a diameter of 80 mm and rotates at a peripheral speed of 120 mm / s. The intermediate transfer belt 6 has, for example, an outer circumference of 251 mm and a thickness of 100 μm.
Made of conductive polycarbonate of DC motor 79
It is driven by the drive roller 8 driven by.

As shown in FIG. 5, the intermediate transfer belt 6 is provided with a marker 16a or 16b at its end.
In the marker 16a shown in FIG. 5A, two regions having different light reflectances at equal intervals, for example, a black region and a white region, are alternately coated. The sensor 7a reads the ON / OFF signals at equal intervals.

Therefore, in FIG. 3A, the horizontal axis represents time t.
When the voltage is plotted on the vertical axis, it is converted into an AC signal as shown by fg in FIG. 3 (A) and sent to the amplifying and waveform shaping circuit 71. f
It is converted into a pulse train as shown in _T.

The marker 16b shown in FIG. 5B is divided into two regions having different light transmittances at equal intervals, for example, a transparent region and an opaque region. A transparent area and an opaque area are read by a sensor 7b composed of a light receiving element and a light receiving element, converted into a signal as indicated by fg in FIG. . Then, it is converted into a pulse train as shown by f _T in FIG. 3A by the amplification and waveform shaping circuit 71.

FIG. 8 shows a case where the intermediate transfer drum 6 is used instead of the intermediate transfer belt 6. At the end of the intermediate transfer drum 18, 16c of FIG.
2, a marker 16c is provided, and two regions having different light reflectances at equal intervals, for example, black and white regions are alternately painted.

As the intermediate transfer drum 18 rotates, the reflection type sensor 7a reads the light and shade at equal intervals. As a result, similarly to the above, it is converted into a signal as shown by fg in FIG. 3 (A) and sent to the amplification and waveform shaping circuit 71. Then, by the amplification and waveform shaping circuit 71, as shown in FIG.
It is converted into a pulse train as shown by f _T in (A).

The frequency of the converted pulse train changes when the running speed of the intermediate transfer belt 6 or the peripheral speed of the intermediate transfer drum 18 changes, as indicated by f _T in FIG. 3B.

The F-V (frequency-voltage) conversion circuit 72 is
When the traveling speed of the intermediate transfer belt 6 or the peripheral speed of the intermediate transfer drum 18 is increased and the frequency of the pulse train is increased, as shown by V _AFC in FIG. , Convert so that the voltage drops.

That is, as shown in FIG. 3C, when the frequency f _T is plotted on the horizontal axis and the voltage V _AFC is plotted on the vertical axis, the FV conversion circuit 72 is centered on the set frequency f _T0. This is because, when the frequency f _T is on the underspeed side, the voltage is high, and on the overspeed side, the voltage is low.

Therefore, the voltage V _AFC that changes in accordance with the traveling speed of the intermediate transfer belt 6 or the peripheral speed of the intermediate transfer drum 18 is sent from the FV conversion circuit 72 to the mixing circuit 77.

Further, the frequency dividing circuit 74 sends the signal f _T1 obtained by dividing the frequency f _T to a frequency of (1 / integer) to the phase comparison circuit 73, and the frequency dividing circuit 75 sends the reference signal sent by the exposure control circuit 47. The signal f _{S1 obtained} by dividing the clock f _S is used for the phase comparison circuit 73.
Send to.

The frequency division ratios of the frequency dividing circuits 74 and 75 are set so that the frequencies of the signal f _T1 and the signal f _S1 match, and the interval between the markers 16a or 16b of the intermediate transfer belt 6 and the intermediate transfer drum. The interval between the 18 markers 16c is an integral multiple of the interval of the line scanning of the exposure unit 4.

When the resolution is 600 dpi, the marker 16a
Alternatively, if the interval of 16b or 16c is, for example, 1.27 mm, this corresponds to 30 times the interval of line scanning. Therefore, the traveling speed of the intermediate transfer belt 6 or the intermediate transfer drum 1
When the peripheral speed of 8 is 120 mm / s, the frequency of the signal fg sent by the sensor 7 is 94.488 Hz.

Therefore, if the frequency dividing ratio of the frequency dividing circuit 74 is 1/1, the frequency of the signal f _T1 becomes 94.4488 Hz. Therefore, the frequency of the signal f _S1 is the same as that of the exposure control circuit 47 and the frequency dividing circuit 75. 29.027M generated by the reference oscillator 48
The frequency of Hz is divided into 1/4 × 30 × 128 × 20 and supplied.

As shown in FIG. 4 (A), the phase comparison circuit 73 has a frequency dividing circuit 75, where the horizontal axis represents time t and the vertical axis represents voltage.
Signal f _S1 sent by the frequency divider 74 and the signal f sent by the frequency divider circuit 74
_The low-pass filter 76 shown in FIG.
The voltage as indicated by Vp in (A) is transmitted.

That is, when the phase of the signal f _{T1 leads} the phase of the signal f _S1 , a pulse is sent so as to send a voltage lower than the reference value, and the phase of the signal f _T1 lags behind the phase of the signal f _S1. If so, the pulse is sent so as to send a voltage higher than the reference value, and if the phase is the same, only the reference value is sent.

Therefore, the low-pass filter 76 has the configuration shown in FIG.
As shown in (B), if the horizontal axis is the phase φ and the vertical axis is the voltage,
When the phase comparison circuit 73 sends out only the reference value as described above, the mixing circuit 7 outputs the control voltage V _APC corresponding to 0 of the phase φ.
If there is a phase difference, the control voltage V _APC that changes according to the advance or delay of the phase difference is sent to the mixing circuit 77.

Therefore, the mixing circuit 77 outputs f _T1 and the signal f _S1.
The control voltage V _APC , which is an automatic phase control signal for controlling the number of revolutions of the DC motor 79, and the frequency f _T from the FV conversion circuit 72 become the set frequency f _T0 , as described above, so that the phases of the DC motor 79 match. As described above, since the control voltage V _AFC , which is the automatic frequency control signal for controlling the rotation speed of the DC motor 79, is applied, the frequency and phase of the signal fg read by the sensor 7 is the frequency of the clock generated by the reference oscillator 48. And a drive voltage V _C capable of performing frequency phase synchronization control so as to match the phase with the phase are sent to the motor drive circuit 78.

Therefore, the motor drive circuit 78 is driven by the drive voltage V
A drive current for controlling the rotation speed of the DC motor 79 corresponding to _C is sent to the DC motor 79. It is to be noted that the fluctuation of the traveling speed of the intermediate transfer member most affects the occurrence of the color misregistration in the portion where the intermediate transfer member and the photosensitive drum 1 contact each other. Therefore, it is desirable to control the traveling speed of the surface of the intermediate transfer member by detecting the speed fluctuation in this portion. However, since it is difficult to provide the sensor 7 at this portion, it is advisable to mount the sensor 7 at a position immediately before the surface of the intermediate transfer member comes into contact with the photosensitive drum 1.

That is, the mounting position of the sensor 7 shown in FIG. 7 is preferably located closest to the transfer roller 11a, and the mounting position of the sensor 7 shown in FIG. 8 is such that the surface of the intermediate transfer drum 18 is the photosensitive drum. It is better to set the position just before touching 1.

When the intermediate transfer belt 6 is used as the intermediate transfer member, a delay does not occur until the control result of the DC motor 79 appears at the position where it contacts the photosensitive drum 1.
The mounting position of the drive roller 8 is preferably a position immediately after the intermediate transfer belt 6 contacts the photosensitive drum 1, that is, a position closest to the transfer roller 11b shown in FIG.

[0059]

As described above, according to the present invention, when the toner images of the respective colors formed on the photosensitive drum are sequentially transferred onto the intermediate transfer member, the position adjustment is performed at the leading edge of the page, and the subsequent position is adjusted. Since the alignment control is also performed, it is possible to cancel the running speed error between the photosensitive drum and the intermediate transfer member.

Therefore, it is possible to prevent image distortion and color shift in one page.

[Brief description of drawings]

FIG. 1 is a block diagram illustrating the principle of the present invention.

FIG. 2 is a block diagram of a circuit showing an embodiment of the present invention.

FIG. 3 is a diagram for explaining the operation of FIG. 2 (No. 1)

FIG. 4 is a diagram for explaining the operation of FIG. 2 (No. 2)

FIG. 5 illustrates an example of the intermediate transfer member of the present invention (No. 1)

FIG. 6 is a diagram for explaining an example of the intermediate transfer member of the present invention (No. 2)

FIG. 7 is a diagram illustrating a configuration example of a color image forming apparatus.

FIG. 8 is a diagram illustrating another configuration example of the color image forming apparatus.

FIG. 9 is a diagram for explaining exposure start timing detection.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Charging devices 3a to 3d Developing unit 4 Exposure unit 5, 14 Cleaner 6 Intermediate transfer belt 7, 7a, 7b Sensor 8 Driving roller 9, 10 Stretching roller 11a, 11b, 12 Transfer roller 13 Fixing unit 15 Recording paper 16, 16a to 16c Markers 17 Control unit 18 Intermediate transfer drum 20 First image carrier 21 Second image carrier 22 Speed detecting means 23 Driving means 24 Electrostatic latent image forming means 40 Laser diode 41, 44 Lens 42 Polygon Mirror 43 Spindle motor 45 Mirror 46 Beam detector 47 Exposure control circuit 48 Reference oscillator 71 Amplification and waveform forming circuit 72 FV conversion circuit 73 Phase comparison circuit 74, 75 Dividing circuit 76 Low pass filter 77 Mixing circuit 78 Motor drive circuit 79 DC motor

Claims (1)

[Claims] 1. A first image carrier (20) is an electrostatic latent image forming means.
After scanning with (24) to form an electrostatic latent image and development, transfer is performed to the second image carrier (21), and the second image carrier (21)
A color image forming apparatus for transferring a toner image transferred onto a recording sheet, the speed detecting means (22) for converting a driving speed of the second image carrier (21) into a signal for detection. Driving means (for driving the second image carrier (21) so that the cycle of the signal detected by the speed detecting means (22) and the scanning cycle of the electrostatic latent image forming means (24) are synchronized. 23), and a color image forming apparatus comprising: