JP3204049B2 - Multicolor image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic multicolor image forming apparatus for forming a latent image on a photosensitive medium by scanning a light beam and developing the latent image with toner to form a multicolor image.

[0002]

2. Description of the Related Art Various color image forming apparatuses utilizing an electrophotographic system are known. For example, in a printer, a copying machine, or the like, a digital electrophotographic method is widely used as a method capable of providing high-speed and high image quality.
In the digital electrophotographic method, in order to perform optical scanning of a photosensitive medium using a light beam and reproduce the gradation of an image, pulse width modulation exposure using an analog screen generator or the like is often performed. (For example, see Japanese Patent Application Laid-Open No. 1-280965). In these, image formation is performed from a low density portion to a high density portion while keeping the light beam spot diameter and the number of lines constant. For this reason,
Exposure profiles in low-density areas have reduced contrast and become analog-like, and since the exposure amount itself is small, the reproducibility of dots and lines is deteriorated, and the stability of gradation and color reproduction is deteriorated. There was a problem.

[0003] In order to solve the above problem, stabilization of the light beam quantity,
A method to stabilize each element such as stabilization of the toner concentration in the developing unit, and to control the developing bias and transfer current value by measuring the temperature and humidity and the toner concentration in the developing unit, and to realize the gradation and color reproduction for the environment. A method called process control for increasing stability has been proposed (for example, Japanese Patent Application Laid-Open No. 4-3788).
No. 2, JP-A-4-36776). However, these systems require high-precision sensors and control mechanisms, and are disadvantageous in that they are complicated and expensive.

On the other hand, if an image is formed with a low number of lines in order to improve the contrast of an exposure profile in a low density portion, the reproducibility of dots and lines in the low density portion can be improved. A character image mixed in an image has a drawback that the image structure is easily recognized when the number of lines is reduced, and the image quality is reduced. In addition, to improve the reproducibility of character image reproduction, it is possible to improve the contrast of an exposure profile in a low-density part while maintaining the number of lines by reducing the light beam spot diameter sufficiently. An imaging optical system that forms a light beam spot on a photosensitive medium by condensing a beam becomes extremely precise and expensive, and is not suitable for practical use. In addition, a method has been proposed in which the light beam spot diameter and the light emission intensity of the light beam are varied to suppress a decrease in the contrast of the exposure profile and increase the reproducibility of dots and lines, but even with these methods,
A control mechanism for changing the light beam spot diameter and the light emission intensity is required, and there is a disadvantage that it is complicated and expensive.

[Problems to be solved by the invention]

In order to overcome such disadvantages of the prior art, the applicant has provided two or more alternating operating means having different characteristics for converting an image density signal, wherein at least one converting means is provided. A method was proposed in which the output of the image density signal was set to 0 for the low-density portion to improve the reproducibility of dots and lines in the low-density portion, and to improve the stability of gradation and color reproduction. (Japanese Patent Application 5-24
No. 8474). However, when an image is formed with a low number of lines, the reproducibility of dots and lines in a low density portion can be improved, but the image is formed on a line screen having a screen line number of 200 lines or less, particularly less than 130 lines. When the is configured, the image structure is visually recognized even in the low density portion,
It has been found that there is a problem that the image quality is sensuously reduced. In order to solve this problem, the present inventors have proposed a pulse width modulation unit that performs pulse width modulation on an image density signal, and an image forming apparatus that forms an image in accordance with a pulse width modulation signal output from the pulse width modulation unit. Using a plurality of image density signal conversion means having different tone reproduction characteristics, operating these conversion means sequentially in a time-division manner in the main scanning direction, and further, each time one or more light exposure scans, So that the permutation in which the signal conversion means operates is changed,
In addition to improving the stability of the gradation and color reproduction in the low-density part against the environment, the invention has an image structure that is hard to visually recognize even if the low-density part has a low pixel density, and can improve the image quality. A patent application was filed by the present applicant (Japanese Patent Application No. 6-278544).

However, a plurality of image density signal conversion means having different tone reproduction characteristics are used, and these image density signal conversion means are sequentially operated in a time-division manner in the main scanning direction. In an image forming apparatus (Japanese Patent Application No. 6-278544) in which the permutation in which the image density signal converting means operates is changed for each light exposure scan,
When a multi-color image is formed by a multi-color image forming apparatus provided with a rotary transfer body that performs a plurality of rotations and superimposes a plurality of color toner images to form a multi-color image, the dot writing timing and the rotation of the transfer body are considered. When a slight shift occurs, dots are discrete in a low-density portion where the number of lines is reduced, so that a slight shift between dots of a plurality of colors adversely affects color reproduction. For example, when the second color yellow dot is superimposed on the first color cyan dot, a slight shift occurs in the dots as shown in FIG. 10A, and if the dots overlap exactly, as shown in FIG. 10B. It looks green (G).
If the dot shifts as shown in FIG. 10A, it looks yellowish green due to the influence of the yellow component as shown in FIG. 10E. As described above, there is a disadvantage in that it is difficult to obtain a desired color due to color difference fluctuation and color misregistration.

Hereinafter, the occurrence of these problems will be described in detail. In the related art, a transfer member rotation reference signal is output for each rotation of a subject, and a first write timing signal (SOS signal = Start of Scanning) after the transfer member rotation reference signal is output.
Signal) in synchronization with the data write.
The transfer member rotation reference signal is generated when the reference mark on the rotating transfer member passes through a photosensor fixed externally. By detecting the rise of the transfer body rotation reference signal, it is determined that the sheet has reached a desired position.
Start writing. The writing is started in accordance with the rise of the SOS signal indicating that the light beam has reached the left end by the rotation of the polygon mirror of the optical scanning device. Since image data is transmitted serially from left to right,
Writing cannot be performed in the middle of the SOS signal.
It is always performed from the rise of the SOS signal. In the prior art, the rotation of the transfer member and the rotation of the polygon mirror are not synchronized and are driven separately. That is, as shown in FIG. 13, there is a variation in the time from the rise of the transfer body rotation reference signal to the rise of the first write timing signal (SOS signal). Since the transfer body is rotating at a constant speed, the variation in the time until the start of writing causes a shift in the paper feeding direction of the image. In the prior art, a line structure in which dots are continuous in the paper feed direction (vertical direction in FIG. 13) was employed, so that there was no problem even if a deviation occurred, but FIG. In such a staggered structure, there arises a problem that a desired color cannot be obtained due to a color shift as if it were made.

When the permutation of the operation of the image density signal converting means is changed, in order to make the dots of a plurality of colors for each pixel exactly coincide with each other, the position on the same rotary transfer member in the sub-scanning direction is required. Must be the same for any color. That is, when a multi-color image is formed by superimposing toner images of a plurality of colors by performing a plurality of rotations, not only the same pixel position coincides with the transfer of different colors to be superimposed, but also the permutation cycle ( Timing) must also match. If the permutation timings do not match, for example, one of the scan lines including a certain pixel
In cyan color image formation, the permutations for periodically operating the first image density conversion means (A) and the second image density conversion means (B) are (A), (B), (A), and (B). When the image shown in FIG. 9E is formed in the low-density portion, the second color yellow has the same image density conversion sequence (A), (B), (A), (B). ) ... then, there is no problem with both colors overlapping. However, if the permutation is (B), (A),
9B, the positions of the dots and the positions of the blanks are switched in FIG. 9E, and the colors of the first color and the second color do not overlap. Problems arise.

The present invention solves the above-mentioned problems of the prior art, and discloses a Japanese Patent Application No. Hei 6-278 filed earlier by the present applicant.
It is an object of the present invention to improve the invention of No. 544. That is, the present invention improves the stability of the gradation and color reproduction in the low-density portion to the environment without requiring complicated and expensive process control, a variable light emission intensity device, and a precise and expensive beam imaging optical system. That, even if the low-density part has a low pixel density, it has an image structure that is difficult to recognize visually, and furthermore, when obtaining a multicolor image, prevents a slight deviation between dot writing timing and rotation of the transfer body, In particular, an object of the present invention is to prevent a slight shift of dots of a plurality of colors in a low-density portion having a reduced number of lines to thereby improve image quality.

[0010]

The present invention (claim 1) provides:
Pulse width modulation means for pulse width modulating the image density signal;
A multi-color image forming apparatus for forming a multi-color image on a rotary transfer member in accordance with a pulse width modulation signal output from the pulse width modulation means; In the apparatus , two or more image density converting means (45 and 46 in FIG. 4A) are provided, and at least one image density converting means outputs 0 to the low density portion.
Alternatively, the image forming apparatus has a density conversion characteristic having a value in a range where the image is not visualized (see FIGS. 5B and 8B), and further includes forming the two or more image density conversion units on the rotary transfer member. Sa
Toner image on the recording medium surface on the rotary transfer body
Selection means (113 in FIG. 1) for periodically operating the image density signal so as to be periodically arranged in the first direction .
And a permutation for periodically operating the two or more image density conversion units by a toner formed on the rotary transfer member.
The image is formed on the first surface of the recording medium on the rotary transfer member.
To be periodically arranged in a second direction different from the direction ,
A permutation control means (112 in FIG. 1) for generating a control signal to be changed periodically is provided, and means for synchronizing a control signal generated by the permutation control means with a control signal for controlling rotation of the rotary transfer member is provided. It is characterized by having.

Also, in the present invention (claim 2), the synchronizing means includes a control signal generated by the permutation control means,
A control signal for controlling the rotation of the rotary transfer member is generated by the same clock.

Further, according to the present invention (claim 3), the synchronizing means sets the rotation cycle of the rotary transfer member to the permutation system.
It is characterized in that it is an integral multiple of the period of the control signal generated by the control means .

Further, the present invention (Claim 4), which means for the synchronization, the generated control signal of the permutation control means, wherein the controls the rotation of the rotary transfer member It is.

[0014]

In the present invention, the number of lines is reduced substantially in a low density portion by converting the image density signal. That is, two or more image density signal conversion units have different characteristics, for example, as shown in FIGS. As shown in FIG. 5B, one of the conversion means has a characteristic that the output of the image density signal for the low density portion is set to 0, so that there is no output when the low density signal comes. These image density signal conversion means are selected by the selection means and operate alternately in a time-division manner as shown in FIGS. 6B and 6C. Therefore, as shown in FIG. In other words, the input image signal is thinned out, and the pulse width modulation output has a low number of lines for a low density portion and a high number of lines for a middle and high density portion. FIG. 9G shows an example of a pixel arrangement in which the low density portion is thinned out.

Then, the permutation control means changes the permutation in which two or more image signal conversion means operate for each one or a plurality of light exposure scans. That is, FIG.
(A) The image density signal conversion means having the characteristics shown in (b) is time-divisionally multiplexed, for example, in the main scanning direction.
(B), (A), (B), (A), (B)..., And then in the next light exposure scanning, in the main scanning direction,
(B), (A), (B), (A), (B), (A)... By such an operation of changing the permutation of the image density signal converting means operating in a time-division manner every other line or every plural lines in the sub-scanning direction, uneven distribution of pixels in the low-density portion is suppressed, and Prevents the number of lines from changing. For example, in the pixel arrangement as shown in FIG. 9 (g), the uneven distribution of pixels can be suppressed as shown in FIG. 9 (e) or (f) by adding an operation of changing the permutation.

Further, a major feature of the present invention is that the image signal conversion means includes a rotating transfer member which performs a plurality of rotations and superimposes a plurality of color toner images to form a multicolor image. In the means, a control signal for periodically changing the permutation for periodically operating the image density converting means in the sub-scanning direction and a control signal for controlling the rotation of the rotary transfer member are synchronized. That is, the paper transfer position by the rotary transfer member and the dot writing timing by the laser are matched to prevent the dots of a plurality of colors from being slightly shifted to cause a color difference variation. This is achieved by generating a control signal for periodically changing the permutation for periodically operating the image density conversion means in the sub-scanning direction and a control signal for controlling the rotation of the rotary transfer member with the same clock. Achieved. Further, the present invention is achieved by setting the cycle of the control signal for periodically changing the order in which the image density conversion means is periodically operated in the sub-scanning direction and the rotation cycle of the rotary transfer member to be an integral multiple. . Further, the present invention is achieved by controlling the rotational position of the rotary transfer member with reference to a control signal for periodically changing the permutation for periodically operating the image density converting means in the sub-scanning direction.

According to the present invention, as described above, the control signal for controlling the rotation of the rotary transfer member is synchronized with the control signal for periodically changing the image density signal conversion means in the sub-scanning direction. The purpose of the present invention is to match the upper position, that is, the paper transport position, with the dot writing timing by the laser, and to prevent the dots of a plurality of colors from being slightly shifted to cause color difference fluctuation. As described above,
Improves the stability of color reproduction in the environment and improves the reproducibility of dots and lines in low-density areas, and improves image quality with an image structure that is difficult to recognize visually even when the low-density areas have a low number of lines. Further, by synchronizing the control signal for periodically changing the sub-scanning direction with the control signal for controlling the rotation of the rotary transfer member, it is possible to prevent the occurrence of color difference fluctuation due to the displacement of dots of a plurality of colors. can do.

In the above description, for the sake of simplicity, the description has been made on the assumption that the characteristic of the image density signal converting means is a characteristic in which the output to the low density part of one converting means is set to 0. Generally, in an electrophotographic apparatus that involves light beam scanning, the laser diode does not respond to a minute input signal, and the developing bias potential is applied to suppress the toner adhesion to the base. There is a value in a range where the output is not visualized even if the output is not 0. In other words, it is not an absolute condition that the characteristics of the image density signal conversion means have a characteristic in which the output of one of the conversion means to the low density portion is 0, but, for example, as shown in FIGS. 8A and 8B. Alternatively, the output of one of the conversion means to the low-density portion may be converted to a value in a range that is not visualized.
In the present invention, the intention is to reduce the number of lines by using the characteristic that the output of one of the conversion means to the low density portion is set to a value within a range in which the image is not visualized.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is applied to a color image forming apparatus will be described below with reference to the drawings. FIG. 2 is a diagram showing a schematic configuration of an embodiment of the present invention. In FIG.
A charger 2 for forming an electrostatic latent image, a rotary developing device 3, a transfer drum 4, a cleaner 5, and the like are arranged around the photoreceptor 1 rotating in the direction of the arrow. The photoconductor 1 is uniformly charged by the charger 2 in a dark portion. Light beam scanning device 20
Scans the photosensitive member 1 with a light beam. The light beam is turned on and off by the light beam pulse width modulator 30 according to the density signal supplied from the document reading unit 10 or the like. As a result, the photosensitive member 1 is exposed, and an electrostatic latent image is formed. The spot diameter (1 / e ² ) of the light beam in the main scanning direction on the photoconductor 1 was set to 75 μm.

The rotary developing device 3 includes four developing devices having yellow, cyan, magenta, and black toners, respectively. Each developing device employs a reversal development system using two-component magnetic plus development. Average toner particle size is 7μ
m. The rotary developing device 3 rotates as appropriate to develop the electrostatic latent image with a desired color toner. At this time, a bias voltage is applied to the developing roll to suppress toner adhesion to a white background. The transfer drum 4 rotates with the sheet mounted on the outer periphery. The developed toner image on the photoconductor is transferred to a transfer device 4.
b is transferred to the sheet. For each color of yellow, cyan, magenta, and black, formation, development, and transfer of an electrostatic latent image are performed. The toner on the paper obtained by this operation is fixed by the fixing device 9, and a multicolor image is formed.

FIG. 3 is a detailed view of the light beam scanning device 20, which comprises a semiconductor laser 21, a collimator lens 22, a polygon mirror 23, an fθ lens 24, and the like.
A scanning start signal generation sensor 26 for generating an S signal is provided. As shown in FIG. 4A, the pulse width modulation device 30 for turning on and off the light beam includes a triangular wave oscillator 4
1, waveform selection circuit 47, comparison circuit 42, first look-up tape (LUTA) 45, second look-up table (LUTB) 46, D / A converter 48, selection circuit 4
4. The selection circuit 44 includes a counter, a flip-flop circuit, etc., counts the reference clock signal and the SOS signal, and
And the like, and periodically distributes and outputs the digital image density signal to the first and second LUTs 45 and 46.

FIG. 4B shows an example of the circuit configuration of the selection circuit 44. Flip-flop frequency divider 44a
Is a control signal that periodically changes the permutation for periodically operating the first and second LUTs in the sub-scanning direction. The knot circuit 44b is for obtaining a signal obtained by inverting the image clock CLK. The AND circuit 44c allows the image clock CLK to pass during the scanning period in which the frequency divider 44a is in the first state.
4d, during an operation cycle in which the frequency divider 44a is in the second state,
The inverted image clock is passed, and the OR circuit 44e supplies those outputs to the LUT selection circuit 113. Each time the SOS signal is generated, the phase of the image clock CLK is inverted. In a case where the permutation for alternately distributing the image signal to the first and second LUTs is changed every two sub-scans, the frequency divider 44 including a one-stage flip-flop is used.
Instead of a, a frequency divider composed of two-stage flip-flops may be used. The output of the frequency divider 44a is a control signal for periodically changing the permutation for periodically operating the LUT in the sub-scanning direction.

After the image clock CLK or its inverted signal is frequency-divided by the flip-flop 44f, the image density signal is converted into the first and second LUTs 4 by a circuit including AND circuits 44g and 44i and a knot circuit 44h.
The control signal is alternately distributed to 5, 46 and output.
The distributed digital image density signal is converted into digital data by first and second LUTs 45 and 46 having different characteristics, converted into an analog image density signal via a D / A converter 48, and synthesized, and is compared with a comparison circuit 42. Is input to

FIG. 5 shows the conversion characteristics of the first and second LUTs 45 and 46 for converting the input digital data for 8 pits into the output digital data for 8 pits. The triangular wave oscillator 41 generates two kinds of triangular wave pattern signals. The period of each pattern signal was made to correspond to 400 lines and 200 lines, respectively. Since the spot diameter (1 / e ² ) of the light beam in the main scanning direction on the photoconductor 1 is set to 75 μm, the distance dP (m) between adjacent pixels is set.
The values of D defined by the ratio between m) and the light beam spot diameter dB are 1.17 and 0.59, respectively. It is known that, when the value of D is set to D ≦ １ ／, the reproduction of dots and lines in low-density portions is improved, and the stability of gradation and color reproduction with respect to the environment is increased. The above values of D in the example satisfy the above conditions. Waveform selection circuit 47
Selects a waveform based on a signal from the halftone image / character image discriminating means. When it is determined from the halftone image-character image determination signal that the image is a character image, a pattern signal corresponding to 400 lines is selected. On the other hand, if it is determined that the image is a medium tone image, a pattern signal corresponding to 200 lines is selected. The comparison circuit 42 compares each pattern signal with the analog image density signal to generate a pulse width modulation signal.

FIG. 6 shows a waveform generation process of the pulse width modulation device in this embodiment. First, the image signal Sig (01) is input to the LUT selection circuit 44 based on the counts of the reference clock signal and the SOS signal, and is alternately sorted in the order of the first and second LUTs 45 and 46, respectively. Sig (21) is generated. Sig (11) and Sig (21) are D / A converters 4
8 to form an analog image signal Sig (31). The analog image signal Sig (31) is compared with a triangular wave corresponding to 200 lines generated by the triangular wave oscillator 41 and the magnitude of the image density signal by the comparing circuit 42 (Si
g (41)), the pulse width modulation signal Sig (51) is generated, and the semiconductor laser is driven by the pulse width modulation signal Sig (51).
, The optical scanning for one line is completed, and a latent image for one line is formed.

Subsequently, an SOS signal for detecting the optical scanning start timing is generated, and the optical scanning for the next one line is started. The SOS signal is input to the LUT selection circuit 44, and based on this, the first and second LUTs are determined as described above.
The permutation for distributing the image signal Sig (02) to the UTs 45 and 46 is changed. That is, the image signal Sig (02) is input to the LUT selection circuit 44, and the second and first LUs are input.
T46 and 45 are alternately sorted in the order of
g (12) and Sig (22) are generated. Thereafter, a pulse width modulation signal Sig (52) is generated in the same manner as described above,
The semiconductor laser is turned on and off based on the pulse width modulation signal Sig (52), the optical scanning is completed, and a latent image for another two lines is formed. The third line is similar to the modulation processing of the image signal Sig (01), the fourth line is similar to the modulation processing of the image signal Sig (02), and so on. L
The permutation alternately assigned to the UTs 45 and 46 is changed.

The pulse width modulation device for turning on and off the light beam can be realized by another configuration example. That is, FIG. 7 shows another configuration of the pulse width modulation device. In this pulse width modulation device, image signals are input in parallel to the first and second LUTs 75 and 76, respectively, and signal conversion is performed. Then, a signal is alternately selected by an LUT selection circuit 74.
The signals are combined and output to the D / A converter 78. Although the triangular wave oscillator generates only a pattern signal corresponding to 200 lines, a configuration similar to the example of FIG. 4A may be used.

When a halftone image is generated by a pulse width modulation device using an LUT having the conversion characteristic shown in FIG. 5, if a digital image density signal is a halftone region of 50% or more, pulse width modulation which is usually performed is performed. A halftone image is generated on a 200-line screen without any difference from the method.
FIG. 9A shows a pixel arrangement when the image density signal is 60%.

The digital image density signal is less than 50% 2
In the halftone region of 0% or more, after the data is converted by the first LUT, the D / A converted portion and the second LUT
The digital image density signal is periodically formed by a D / A-converted portion after data conversion by
In the vicinity of 0%, the portion subjected to D / A conversion after data conversion by the second LUT hardly contributes to image formation. Also, by changing the permutation for alternately distributing the image signal to the first and second LUTs 45 and 46 for each sub-scan (line), large and small pixels can be moved in the main scanning direction.
It is formed alternately in the sub-scanning direction. In FIG. 9B,
The pixel arrangement when the image density signal is 30% is shown. Also,
FIG. 9C shows a pixel arrangement formed by changing the permutation for alternately distributing the image signal to the first and second LUTs 45 and 46 every two sub-scans (lines).

Further, the digital image density signal is 20%
In the halftone region of less than, only the portion that has been data-converted by the first LUT and then D / A-converted by the D / A converter contributes to image formation, and the image signal is converted for each sub-scan (line). By changing the permutation that is alternately allocated to the first and second LUTs 45 and 46, apparently the number of lines is prevented from being reduced, and one-half of 200 lines.
A halftone image is generated at the same pixel density as the 00-line screen, improving the reproducibility of dots and lines in low-density areas, and improving the stability of gradation and color reproduction in environments. In addition, the image quality can be prevented from being lowered by reducing the number of lines. FIG. 9E shows that the image density signal is 1
The pixel arrangement in the case of 5% is shown. Also, in FIG. 9 (f),
The image signal is converted into the first and second L signals every two sub-scans (lines).
This shows a pixel arrangement formed by alternately assigning the UTs 45 and 46 and changing the permutation. FIG. 9D,
(G) shows an image density signal of 30% and 15% when pixels are formed without changing the permutation of alternately distributing image signals to the first and second LUTs 45 and 46 for each sub-scan (line). 1 shows a pixel arrangement. Image density signal is 2
In a halftone area of less than 00, the screen is completely converted into a 100-line screen, which is half of the 200-line screen, and the screen structure can be recognized, which is not preferable.

FIG. 1 shows a control signal for synchronizing a control signal for the transfer drum and a control signal for periodically changing the permutation for periodically operating the image density conversion means in the sub-scanning direction in the present embodiment. 1 shows an example of the configuration. Hereinafter, the sub-scanning LUT permutation control signal (the output of the frequency divider 44a in FIG. 5) is set so that the permutation for periodically operating the image density conversion means LUTs 45 and 46 is changed for each sub-scanning (line). The following describes the case. The clock signal output from the reference clock generator 101 is frequency-divided by the frequency divider 102, and the frequency-divided output controls the rotation of the drive motor 105 for driving the transfer drum 4. Drive motor 10
5 is controlled by the motor current control circuit 104. The drive motor 105 is provided with an encoder 106 for detecting its rotational position. The output of the encoder 106 and the output of the frequency divider 103 are compared by a comparator 103, and based on the comparison output, the motor current control circuit 104 Controls the rotation of the drive motor. Thereby, the drive motor 105
Rotates at an accurate speed according to the output of the frequency divider 102, and therefore, the transfer drum 4 performs rotation in synchronization with the frequency-divided output obtained by dividing the reference clock.

On the other hand, the output of the reference clock generator 101 is frequency-divided by the frequency divider 107, and the frequency-divided output controls the rotation of the drive motor 110 for driving the polygon mirror 23. The drive motor 110 is controlled by a motor current control circuit 109. The drive motor 110 is provided with an encoder 111 for detecting its rotation. The output of the encoder 111 and the output of the frequency divider 107 are
08, and the motor current control circuit 109 controls the rotation of the drive motor 110 to be constant according to the comparison output. As a result, the driving motor 110
The polygon mirror 23 rotates at an accurate speed corresponding to the output of the reference clock 72, and accordingly, rotates in synchronization with the divided output obtained by dividing the reference clock of the reference clock generator 101 by the divider 107. The scanning of the laser beam is performed by mirrors formed by the respective sides of the polygon of the polygon mirror 23, and the scanning start signal SOS is output to the sensor 2 every time the mirror passes.
6 caused by In this embodiment, the scanning start signal SOS
The frequency divider output determined by the frequency of the reference clock and the frequency division ratio of the frequency divider 107 is set so that the occurrence cycle Tsos of the signal becomes 400 μsec.

Frequency divider 44a in permutation control circuit 112
Is a control signal for periodically changing the permutation for periodically operating the first and second LUTs in the sub-scanning direction, that is, a sub-scanning LUT permutation control signal. An SO
A pair of a first scan that is started by the S signal and performs image density conversion in the first permutation and a second scan that is started by the next SOS signal and that is used in image density conversion of the second permutation is performed. This is the cycle of permutation change. In the present embodiment, this cycle is twice as long as the SOS signal, that is, 800 μsec, because the frequency divider 44a is composed of one flip-flop.

When a multi-color image is formed by superimposing toner images of a plurality of colors by performing a plurality of rotations, not only the position of the same pixel coincides with the transfer of different colors to be superimposed, but also the period of the permutation change. (Timing) must also match. If the timings of the permutation change do not match, for example, the first LUT (A) and the second LUT (B) are periodically arranged in the first color cyan image forming of the scanning line including a certain pixel. Are operated in the order of (A), (B), (A), (B)..., And when the image shown in FIG. Permutations (A), (B),
(A), (B)..., There is no problem in that the two colors overlap. However, the permutations are (B), (A), (B), (A) ...
The image formed is the same as that shown in FIG.
In, the position of the dot and the position of the blank are switched,
There is a problem that the first color and the second color do not overlap. In order to avoid this problem, in this embodiment, the sub-scanning LUT permutation control signal and the control signal for controlling the rotation of the transfer drum are synchronized. For this purpose, the rotation cycle Tctr of the transfer drum 4 is set to an integral multiple of 800 μsec of the control signal for periodically changing the sub-scanning direction. Specifically, for example, 800 μsec × 83
08. Incidentally, in the present embodiment, the transfer drum 4
If the SOS signal (400 μs
If c) is an odd multiple, the permutation will be reversed.

FIG. 11 shows another configuration for synchronizing the control signal for the transfer drum and the control signal for periodically changing the permutation for periodically operating the image density conversion means in the sub-scanning direction. This is an example. The configuration is almost the same as that of FIG. 1 except that a reference clock generator for the transfer drum and a generator for the polygon mirror reference clock are independently provided instead of the reference clock generator 101 of FIG. In the figure, the same reference numerals are used for elements having the same function.

FIG. 12 shows still another example, which differs from that of FIG. 1 in the specific configuration for synchronizing the sub-scanning LUT permutation control signal and the control signal for controlling the rotation of the transfer drum. The transfer drum 4 is directly controlled by the scanning LUT permutation control signal. That is, synchronization is achieved by connecting the output of the frequency divider 44a to the input of the frequency divider 102. As a result, the rotation cycle of the transfer drum becomes
This is an integral multiple of the period of the sub-scanning LUT permutation control signal for periodically changing the permutation for operating the LUT periodically in the sub-scanning direction. The permutations match, and the dots do not shift even when a plurality of colors are overlapped.

In the present embodiment, the case where the rotary transfer member is a transfer drum has been described, but the same applies to a case where an endless pelt-like transfer member is used.

[0038]

As described in detail above, according to the present invention,
A plurality of image density signal conversion means having different tone reproduction characteristics are used, these conversion means are sequentially operated in a time-division manner in the main scanning direction, and further, one or more light exposure scanning When the multi-color image is obtained by changing the permutation in which the converting means operates and further obtaining a multi-color image with the rotary transfer member forming the multi-color image, the permutation in which the image signal converting means operates is periodically changed in the sub-scanning direction. Since the control signal to be controlled and the signal for controlling the rotary transfer body are configured to be synchronized, there is no need for complicated and expensive process control, a light emission intensity variable device, a precise and expensive beam imaging optical system, and the like.
In addition to improving the stability of gradation and color reproduction in low-density areas with respect to the environment, even if the low-density parts have a low pixel density, they have an image structure that is difficult to visually recognize. A small deviation between the writing timing of the image and the rotation of the transfer member, and in particular, in a low-density portion with a reduced number of lines, it is possible to prevent a slight deviation of the dots of a plurality of colors and improve the image quality. Play.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a main part of an embodiment of the present invention.

FIG. 2 is a diagram showing an overall schematic configuration of an embodiment of the image forming apparatus of the present invention.

FIG. 3 is a diagram illustrating a configuration example of a light beam scanning device according to an embodiment.

4A is a diagram illustrating a configuration of a pulse width modulation unit in the image forming apparatus according to the embodiment, and FIG. 4B is a diagram illustrating a configuration of an LUT selection circuit 44;

FIGS. 5 (a) and (b) show LUs according to an embodiment.
The figure which shows the data conversion characteristic of T

FIG. 6 is a diagram showing a waveform generation process of the pulse width modulation device.

FIG. 7 is a diagram illustrating another configuration example of the pulse width modulation unit in the image forming apparatus according to the embodiment;

FIGS. 8 (a) and (b) show LUs in the embodiment.
The figure which shows the other example of the data conversion characteristic of T

FIGS. 9A to 9G are diagrams for explaining pixel arrangement in the image forming apparatus of the present invention.

10A is a diagram for explaining pixel arrangement in a conventional image forming apparatus, and FIGS. 10B and 10C are diagrams for explaining color recognition of an image structure.

FIG. 11 is a diagram showing a modified configuration example of the embodiment of the image forming apparatus of the present invention.

FIG. 12 is a diagram illustrating a modified configuration example of the image forming apparatus according to the embodiment of the invention.

FIGS. 13A and 13B are diagrams for explaining a problem of the related art.

[Explanation of symbols]

101, 101a, 101b ... reference clock generator,
102, 107: frequency divider, 103, 108: comparator, 104, 109: motor current control circuit, 105,
110: drive motor, 106, 111: encoder,
112: Permutation control circuit, 113: LUT selection circuit,
23: polygon mirror, 30: pulse width modulation circuit,
4: Transfer drum.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI G03G 21/14 G03G 21/00 372 H04N 1/46 H04N 1/40 D 1/60 1/46 Z (72) Inventor Kazuhiro Iwaoka 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox Co., Ltd. (72) Inventor Masahiko Kubo 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox Co., Ltd. (56) References JP-A-62-39973 (JP, A) JP-A-62-39974 (JP, A) JP-A-1-286675 (JP, A) JP-A-4-269075 (JP, A) A) JP-A-5-176103 (JP, A) JP-A-5-176164 (JP, A) JP-A-7-147640 (JP, A) JP-A-1-2044741 (JP, A) JP-A-6 −71948 (J , A) (58) investigated the field (Int.Cl. ^7, DB name) B41J 2/44 B41J 2/52 G03G 15/00 G03G 15/01 G03G 21/14 H04N 1/46 H04N 1/60

Claims (4)

(57) [Claims] 1. A pulse width modulation means for pulse width modulating an image density signal, and a rotary transfer member for superimposing toner images of a plurality of colors by a plurality of rotations, according to a pulse width modulation signal output from the pulse width modulation means. In a multicolor image forming apparatus for forming a multicolor image on a recording medium held by a rotary transfer member, two or more image density conversion means are provided, and at least one image density conversion means is provided with a low image density signal in an input image density signal. The output for the density portion was set to 0 or a value in a range where no visualization was performed.
Having a density conversion characteristic, wherein the two or more image density conversion units are provided on the rotary transfer member.
The toner image formed on the recording medium surface on the rotary transfer body
The first to be periodically arranged in a direction, a selection means for cyclically selected operation for the image density signal, a permutation for periodically selecting operation of said two or more image density converting means, the The toner image formed on the rotary transfer
The first direction on the recording medium surface on the rotary transfer member;
Is provided with a permutation control means for generating a control signal to be periodically changed so as to be periodically arranged in a different second direction, and controls a control signal generated by the permutation control means and a rotation of the rotary transfer member. A means for synchronizing the control signals to be performed. 2. The apparatus according to claim 1, wherein the synchronizing unit generates a control signal generated by the permutation control unit and a control signal controlling the rotation of the rotary transfer member using the same clock. 2. The multicolor image forming apparatus according to 1. 3. The apparatus according to claim 1, wherein said synchronizing means sets a rotation cycle of said rotary transfer body to an integral multiple of a cycle of a control signal generated by said permutation control means. Multicolor image forming apparatus. 4. The multicolor image forming apparatus according to claim 1, wherein said synchronizing means controls rotation of said rotary transfer member by a control signal generated by said permutation control means.