JP2000221431A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively and accurately correct a main scanning magnification by providing a transverse chromatic aberration compensation means for compensating the deviation of an image-formation position in a main scanning direction on a photoreceptor caused by the difference or the fluctuation of the wavelength of a light beam in a scanner optical system. SOLUTION: A toric lens 36 is arranged on a polygon mirror side and a composite optical device 37 is arranged on the side of a surface to be scanned. As for the device 37, its surface on an incident side is constituted of an aspherical surface (plane in a subscanning direction) having power only in a main scanning direction and the surface on an emitting side is constituted of a surface where a diffraction grating is added to the plane. A beam emitted from the device 37 scans to form an image on the surface to be scanned (photoreceptor) 38. The power arrangement of the lens 36 and the device 37 is decided so as to compensate the change of the image-formation position in the main scanning direction caused by the difference of the wavelength or the change of the wavelength of the beam. Namely, the transverse chromatic aberration of a scanning optical system is compensated by a refraction element 36 having positive dispersion in the main scanning direction and a diffraction element 37 having negative dispersion therein.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a color printer,
More particularly, the present invention relates to an electrophotographic image forming apparatus such as a color copying machine and a color facsimile, which has a plurality of laser scanner optical systems and forms an image on a plurality of lines simultaneously by a plurality of image forming units.

[0002]

2. Description of the Related Art An electrophotographic color image forming apparatus has a plurality of image forming units for speeding up, and holds toner images of different colors formed by the respective image forming units on a transport belt. Various methods for sequentially transferring images on a recording material have been proposed.

[0003] Problems with an apparatus having a plurality of image forming units include unevenness in the movement of a plurality of photosensitive drums and a conveyor belt and the outer circumference of the photosensitive drum at the transfer position of each image forming unit due to mechanical accuracy and the like. For example, the relationship between the surface and the movement amount of the conveyor belt may be different for each color, may not match when the images are superimposed, and may cause a color shift (position shift). In particular, there is an error in the optical distance between the laser scanner and the photoconductor in each image forming unit, and if this error is different between the image forming units, a difference occurs in the main scanning magnification of the beam on the photosensitive drum, Color shift (position shift) occurs. Further, in each image forming unit, the position of the laser scanner or the photoconductor in the beam scanning direction (hereinafter, referred to as the main scanning direction) is shifted. (Position shift) occurs.

In order to reduce the color shift (position shift) due to the difference in the main scanning magnification, a video clock generator for an image signal is provided for each color as shown in FIG. A method for correcting the main scanning magnification by adjusting the frequency has been proposed (for example, Japanese Patent Publication No.
-57040). FIG. 6 is a diagram for explaining a method of changing the video clock frequency. The figure shows P
FIG. 2 shows a block diagram of an LL (Phase Locked Loop) circuit. In the figure, reference numeral 14 denotes an output signal from the crystal oscillator, and reference numeral 15 denotes a video clock which is changed by an output of a video clock generator. A signal obtained by dividing the output of the crystal oscillator by M and a signal obtained by dividing the video clock by N are input to a phase comparator, and the output of the phase comparator is input to a voltage controlled oscillator through a low-pass filter. For example,
If the phase of the signal obtained by dividing the output of the crystal oscillator by M is earlier than the phase of the signal obtained by dividing the video clock by N, the input voltage of the voltage controlled oscillator rises and the phase of the video clock is advanced. Assuming that the frequency of the crystal oscillator is fin and the frequency of the video clock is fout, fout = fin × N / M. The video clock frequency is variable by adjusting the value of N / M according to the detected main scanning magnification.

In order to speed up image formation, a plurality of lines are simultaneously scanned using a plurality of beams. FIG. 7 is a schematic perspective view of a scanner optical system using a plurality of beams, particularly two beams. Laser light source 81
The plurality of beams 87a and 87b emitted from the beam are collimated by a collimator lens 82 and then scanned by a polygon mirror 83. Scanned beams 87a, 87
For b, the scanning speed is corrected by the f-θ lens 84, and a latent image corresponding to the image signal is finally formed on the photoconductor 85.

As a problem when using a plurality of beams, it has been pointed out that the main scanning magnification differs depending on the emission wavelength of each semiconductor laser as a light source (Japanese Patent Laid-Open No. 6-2).
27037). As a countermeasure, a method has been proposed in which the main scanning magnification is corrected for each beam by changing the video clock frequency for each beam (Japanese Patent Laid-Open No. Hei 6-227).
037).

[0007]

A configuration having a plurality of image forming units, a configuration for simultaneously scanning a plurality of lines with a plurality of beams, and a main scanning magnification correction by changing a video clock frequency, as described in the section of the prior art. , It is possible to realize higher-speed image formation with less color misregistration. For example, a configuration of a video clock frequency adjusting unit as shown in FIG. 8 can be considered. The clock output from the oscillator 90 is input to the video clock frequency adjusting means 91. The video clock frequency adjusting means corrects the clock frequency so that the main scanning magnification of each beam becomes the same. The clock output from the video clock frequency varying unit is input to a horizontal synchronizing unit (BD synchronizing unit) 95, and a video clock synchronized with the horizontal synchronizing signal (BD signal) is input to a laser driving unit 93. Laser driver 93
Drives the laser generator 94 according to the image signal. In this case, the same laser scanner optical system has two laser generators, and the main scanning magnification can be corrected independently for each laser. The above is a description of the correction of the main scanning magnification for one color, and the same configuration for the other three colors is as shown.

However, the above configuration has the following problems.

Since each image forming unit has a plurality of light sources, it is necessary to have a large number of video clock frequency variable means, resulting in high cost. For example, when scanning is performed with four beams by four image forming units, 16 video clock frequency variable units are required.

On the other hand, when the video clock is shared by several lasers, the main scanning magnification differs for each beam due to the difference in wavelength for each laser, as described in JP-A-6-227037. The resulting image has low accuracy.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to correct the main scanning magnification accurately at low cost.

[0012]

According to a first aspect of the present invention, there is provided a modulating means for modulating each of light beams generated from a plurality of light sources by an image signal, and generating a clock of the image signal. In an image forming apparatus having a clock generating unit and irradiating the plurality of light beams onto a photosensitive member through a common scanner optical system to form an image on the photosensitive member, a difference in wavelength of the light beam is provided. A magnification color compensating means for compensating for a shift of an image forming position in the main scanning direction on the photoconductor due to fluctuation or fluctuation is provided in the scanner optical system,
At least two clocks among a plurality of clocks of the image signal for modulating each of the plurality of light beams are commonly used.

According to a second aspect of the present invention, there is provided a modulation means for modulating each of light beams generated from a plurality of light sources with an image signal, and a clock generation means for generating a clock for the image signal. A plurality of image forming units for forming an image on the photosensitive member by irradiating the plurality of light beams onto the photosensitive member via a common scanner optical system, and forming each of the image forming units. An image forming apparatus that superimposes a plurality of images formed on each other, wherein the scanner optical system includes a magnification color compensation unit that compensates for a shift in an imaging position in the main scanning direction on the photoconductor due to a difference or variation in the wavelength of the light beam. And at least two clocks among a plurality of clocks of the image signal in the same image forming unit are commonly used.

According to a third aspect of the present invention, in the second aspect, there is provided an adjusting means for adjusting the clock of the image signal to be different between the plurality of image forming units. .

According to a fourth aspect of the present invention, in the third aspect, the adjustment means fixes a clock of a reference image forming unit among the plurality of image forming units, and adjusts a clock of another image forming unit. The clock of the image signal is adjusted to be different between the plurality of image forming units by making the clock of the unit variable.

According to a fifth aspect of the present invention, in the third aspect, the adjusting means is a PLL.

According to a sixth aspect of the present invention, in the fifth aspect, a synchronization signal output means for detecting a scanned light beam at a predetermined position and outputting a horizontal synchronization signal of the light beam, There is provided a means for generating a clock synchronized with the horizontal synchronization signal of each of the light beams by dividing the frequency of the clock output from the PLL.

According to a seventh aspect of the present invention, in the first or the second aspect, the magnification color compensating means comprises a refracting element and a diffractive element.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments.

(First Embodiment) In the first embodiment, a configuration will be described in which the video clock generating means for all colors is a video clock generating means capable of changing the video clock frequency.

FIG. 1 is a view for explaining the whole image forming apparatus according to an embodiment of the present invention.

The embodiment of the present invention shows a color image forming apparatus provided with image forming sections for four colors, that is, yellow Y, magenta M, cyan C, and black K. In FIG. Photosensitive drums (k, c, m,
y indicates K, C, M, and Y), 2 indicates a laser scanner that forms an electrostatic latent image on the photosensitive drum 1 by exposing according to an image signal, and 3 indicates that a sheet is to be formed in an image forming unit for each color. An endless transport belt that also serves as a transfer belt, which is sequentially transported, 4
A driving roller connected to a driving means including a motor and gears (not shown) for driving the transport belt 3, a driven roller 5 for rotating according to the movement of the transport belt 3 and applying a constant tension to the transport belt 3, And a pair of optical sensors provided on both sides of the conveyor belt for detecting a misregistration detection pattern formed on the conveyor belt 3.

The operation of the embodiment of the present invention will be described below.

When data to be printed is sent from a computer or the like to the printer, and the image signal generation according to the printer engine system is completed and the printer becomes ready for printing, the paper is supplied from the paper cassette and reaches the transport belt 3.
The sheet is sequentially conveyed to the image forming units of each color by the conveying belt 3. Image signals of the respective colors are sent to the respective laser scanners 2 in synchronism with the sheet conveyance by the conveyor belt 3 and
An electrostatic latent image is formed on the photosensitive drum 3, toner is developed by a developing unit (not shown), and is transferred onto a sheet by a transfer unit (not shown). In FIG. 1, the toner images formed by the respective image forming units are sequentially superimposed on a sheet in the order of Y, M, C, and K. Thereafter, the sheet is separated from the transport belt, the toner image is fixed on the sheet by heat in a fixing device (not shown), and is discharged to the outside.

The scanner optical system of each image forming unit detects a beam at a predetermined scanning position and outputs a horizontal synchronizing signal as described in the section of the prior art.

Since there are a plurality of image forming units, if the main scanning magnification of the scanner optical system of each image forming unit is different, a color shift appears in the image as a result.

In order to reduce the color misregistration caused by the difference in the main scanning magnification, a pattern for detecting a misregistration as shown in FIG. 2 is formed on the conveyor belt 3 and one sensor 6 provided on both sides of the conveyor belt. Is read, and the displacement amount of each color is detected. 2, 7, 8, 9, and 10 are linear patterns extending in the laser scanning direction and the transport direction of K, C, M, and Y, respectively, and a and b are patterns on the front side and the back side of the transport belt 3, respectively. is there. The pattern of a is the optical sensor 6a and the pattern of b
The pattern is read by the optical sensor 6b, and the positional deviation in the transport direction is detected from the linear pattern extending in the laser scanning direction, and the positional deviation in the laser scanning direction is detected from the linear pattern extending in the transport direction. The main scanning magnification of each color is calculated from the distance between the linear patterns a and b extending in the transport direction.

FIG. 3 is a sectional view in the main scanning direction of the laser scanner optical system of the embodiment. For simplicity, 1 is shown in the figure.
Only one beam is shown. In the case of a plurality of beams, the beams are emitted from the laser light source 31 and scan the surface to be scanned 38 through a very close portion of the optical system. The figure shows an example using a refraction element (lens) and a diffraction element.
The beam emitted from the laser light source 31 is converted by the collimator lens 32 into substantially parallel light. The light quantity of this beam is limited by an aperture 33 and a cylinder lens 34
Incident on. The beam changed by the polygon mirror 35 enters an optical system including a refraction element and a diffraction element. In this embodiment, as shown in the figure, a toric lens 36 is disposed on the polygon mirror side, and a composite optical element 37 is disposed on the surface to be scanned. The compound optical element 37 has an aspheric surface having a power only in the main scanning direction (a plane in the sub-scanning direction) on the incident side, and a plane on which a diffraction grating is added on the plane on the outgoing side. Here, as the grating shape, for example, a Fresnel-like grating shape made of a sawtooth-shaped diffraction grating by surface ablation, or a step-like diffraction grating shape made by photo-etching is suitable.
The beam emitted from the composite optical element 37 forms an image and scans on a surface to be scanned (photoconductor) 38. In the present embodiment, the toric lens 3 is used to compensate for a change in the imaging position in the main scanning direction caused by a difference in the wavelength of the beam or a change in the wavelength.
6 and the power arrangement of the composite optical element 37 are determined. That is, the chromatic aberration of magnification of the scanning optical system is compensated for by the refractive element having positive dispersion in the main scanning direction and the diffractive element having negative dispersion in the main scanning direction (magnification chromatic compensation). With the configuration of the laser scanner optical system described above, fluctuations in the main scanning magnification due to differences or fluctuations in the wavelength of the beam can be kept small, and the main scanning magnification for each beam of the same optical system can be made substantially the same. As a result, the video clocks corresponding to the beams of the same optical system can be made common. In addition, a short optical path length can be realized by a high angle of view aberration correction using an aspheric surface.

FIG. 4 shows a configuration for independently generating a video clock for each image forming unit according to the embodiment. Laser 5
4a and 54b are lasers included in the same optical system.
The two lasers 54a and 54b have a common video clock.

The clock generated from the crystal oscillator 50 is P
It is input to LL51. The PLL has been described above. The PLL changes the internal dividing ratio by an output clock frequency control means (not shown), and outputs a clock having a frequency such that the main scanning magnification of all colors becomes the same. The clock output from the PLL is supplied to the horizontal synchronization section (BD
(Synchronizing section) 52. The BD synchronization unit is an IC,
The clock is set to a horizontal synchronization signal (B
D signal). Inside the IC, the clock frequency is multiplied by L by the PLL to divide the clock frequency by L while synchronizing with the BD signal. The clock output from the BD synchronizing unit is input to laser driving units 53a and 53b, and the lasers 54a. 54b is modulated and driven by image data (not shown).

FIG. 4 shows an example of the case of four colors, so there are four sets of the above-mentioned configurations. By adjusting the respective video clocks, the main scanning magnification can be made uniform for each color, and the color shift can be reduced. Can be reduced.

In the case of the above configuration, the frequency of the clock input to the PLL 51 can be increased by the PLL 51. Therefore, the oscillation frequency of the crystal oscillator 50 is made smaller than the oscillation frequency of the video clock to reduce radiation noise. It is also possible to use

According to the first embodiment, since there is no main scanning magnification deviation between beams of the same color and there is no main scanning magnification deviation for each color, a high-precision image with little color deviation can be obtained. it can.

(Second Embodiment) In the second embodiment, the reference color video clock generating means generates a video clock having a fixed frequency, and the video clock generating means other than the reference color has a variable frequency. A configuration for generating a video clock will be described.

The video clock generating means of the second embodiment will be described with reference to FIG.

The video clock generating means for the color used as the standard for the main scanning magnification does not include a PLL, and a clock having a constant frequency is always input to the BD synchronization unit. The video clock generating means for colors other than the reference color is the same as the video clock generating means shown in the first embodiment. The oscillation frequency of the crystal oscillator may be set to the video clock frequency in an ideal mechanical arrangement.

For example, black (K) is used as a reference color for the main scanning magnification. In that case, the PLL is set so that the main scanning magnification of cyan (C), magenta (M), and yellow (Y) becomes the same as K.
To adjust the video clock frequency.

Since the number of PLLs used for generating the video clock is reduced, an inexpensive and highly accurate image can be obtained.

(Third Embodiment) In the third embodiment, a video clock frequency variable means outputs N times the frequency of a video clock, and a BD synchronizing section multiplies the frequency by 1 / N to generate a video clock. The configuration will be described.

The video clock generating means of the third embodiment will be described with reference to FIG. In this example, an example in which the BD synchronization accuracy is 1/4 will be described.

The clock output from the crystal oscillator 50 is input to the PLL 51. The PLL 51 has a frequency of 4 such that the main scanning magnification of the scanning line of each beam is the same.
Outputs a double frequency clock. The clock output from the PLL 51 is input to the BD synchronization unit 52. BD
The synchronizer 52 divides the frequency of the clock input from the PLL 51 by 4 so as to synchronize with the BD signals 55a and 55b, and outputs video clocks corresponding to the lasers 54a and 54b, respectively.

The one-color video clock generating means has been described above. The other three colors of video clock generating means have the same structure.

Here, the accuracy of the BD synchronization is 1/4.
Although the case of dots has been described, it is determined by this accuracy and the required image quality, and may be 1/2 dot or 1/8 dot accuracy.

With the above configuration, the BD synchronization unit uses the PLL
There is no need to multiply the clock. Generally, if the frequency of the clock is changed by the PLL, the jitter of the clock increases. Therefore, it is better to reduce the number of PLLs in the video clock generating means. In the configuration of the third embodiment, since the number of PLLs in the video clock generating means is one, deterioration of jitter can be suppressed to a minimum, and a highly accurate image can be obtained.

(Other Embodiments) In the above-described embodiment, an example has been described in which the magnification color compensating means includes a refracting element and a diffractive element. However, any other optical system that can perform magnification color compensation can be realized. As an example, the present invention is effective even if the magnification color compensation means is configured to include only a refraction element, for example, the magnification color compensation is performed using a combination of a concave lens and a convex lens.

The correction of the main scanning magnification is performed at an appropriate time. For example, when the environment such as temperature and humidity changes, after a certain period of time elapses between printing one sheet and printing the next paper,
When starting up the image forming apparatus, after replacing the unit of the image forming unit, after opening and closing the main body door of the image forming apparatus,
The effect can be obtained by performing the adjustment before shipment at the factory. Pre-shipment adjustment at a factory can be expected to be performed at lower cost than mechanical adjustment of optical components and scanners.

In the above-described embodiment, the configuration including the image forming units of four colors has been described. However, the present invention is also effective in a configuration including image forming units of two or three colors. The present invention is also effective in a configuration including image forming units of five or more colors.

In the above-described embodiment, the cause of the change in the main scanning magnification between the colors is that the distance between the scanner and the photosensitive member is shifted. However, the present invention is not limited to this cause, and the present invention is also effective for correction when the main scanning magnification is changed due to, for example, a displacement of an optical component or a change or deformation of a refractive index of a lens or the like.

In the above-described embodiment, this configuration is used for correcting the main scanning magnification. However, this configuration is also effective when the main scanning magnification is to be actively adjusted, such as when it is desired to finely adjust the image magnification.

In the above-described embodiment, the configuration in which the laser scanner optical system and the photoconductor correspond one-to-one is shown. However, the present invention is effective even if the laser scanner optical system and the photoconductor do not correspond one-to-one. For example, by using an endless photoconductor or a drum-shaped photoconductor, an image forming unit can be configured by a plurality of laser scanner optical systems and one photoconductor. The present invention is effective even in this case.

In the above-described embodiment, the case where the source oscillation of the PLL is one crystal oscillator has been described. However, the number of source oscillations of the PLL need not be one, for example, one for each PLL.
There may be one source oscillation, or there may be source oscillation for every several PLLs.

In the above-described embodiment, the PLL is used as the video clock frequency varying means. However, the video clock frequency varying means may be constituted by a VCO or the like.
Since the oscillation frequency of the VCO changes depending on the input voltage, the input voltage may be changed according to the correction amount. If an oscillator with small jitter such as a voltage controlled crystal oscillator is used as the VCO, a video clock with small jitter can be obtained.

In the above embodiment, an example is shown in which the number of laser light sources in the same optical system is two. However, it is possible to further increase the speed by further increasing the number of laser light sources.

[0054]

As described above, as described above, the first embodiment according to the present application is described.
According to the first or second aspect, a light beam generated from each of the plurality of light sources can be formed at low cost and with high accuracy, and a good image can be obtained.

According to the third aspect of the present invention,
The light beam of each image forming unit can be imaged with high precision.

According to the fourth invention of the present application,
The light beam of each image forming unit can be formed at low cost with high accuracy.

According to the sixth aspect of the present invention,
Clock jitter can be reduced.

According to the seventh aspect of the present invention,
The light beam can be imaged with high precision.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an entire image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a misregistration detection pattern.

FIG. 3 is a cross-sectional view in the main scanning direction of the laser scanner optical system according to the first embodiment.

FIG. 4 is a view for explaining video clock generation means of the first and third embodiments.

FIG. 5 is a view for explaining a video clock generating means of the second embodiment.

FIG. 6 is a diagram illustrating a video clock generator.

FIG. 7 is a schematic perspective view of a multiple beam scanner optical system.

FIG. 8 is a view for explaining a video clock frequency varying means which can be considered from the related art.

[Explanation of symbols]

Reference Signs List 1 photosensitive drum 2 laser scanner 3 transport belt 6 optical sensor 7, 8, 9, 10 misregistration detection pattern 14 output signal from crystal oscillator 15 video clock changed by output of video clock generator 31 laser light source 32 collimator lens 33 Aperture 34 Cylinder lens 35 Polygon mirror 36 Refraction element 37 Composite optical element 38 Scanned surface 81 Laser beam light source 82 Collimator lens 83 Polygon mirror 84 f-θ lens 85 Photoconductor 86 BD sensor 87 Beam

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04N 1/113 H04N 1/04 104A 1/23 103

Claims (7)

[Claims] And a clock generator for generating a clock for the image signal, wherein the light beam generated by each of the plurality of light sources is modulated by an image signal. An image forming apparatus that forms an image on the photoconductor by irradiating the photoconductor with the light through a scanner optical system of the present invention. A magnification color compensating means for compensating for a displacement of an image position is provided in the scanner optical system, and at least two or more clocks of a plurality of image signal clocks for modulating each of the plurality of light beams are commonly used. An image forming apparatus characterized by being used. 2. A light source comprising: a modulator for modulating each of light beams generated from each of a plurality of light sources with an image signal; and a clock generator for generating a clock for the image signal, wherein the plurality of light beams are shared. Image forming unit comprising a plurality of image forming units for forming an image on the photosensitive body by irradiating the photosensitive body through the scanner optical system, and superimposing a plurality of images formed in each of the image forming units. In the apparatus, magnification color compensating means for compensating for a shift of an imaging position in the main scanning direction on the photoconductor due to a difference or variation in the wavelength of the light beam is provided in the scanner optical system, An image forming apparatus, wherein at least two or more clocks of a plurality of image signal clocks are commonly used. 3. The image forming apparatus according to claim 2, further comprising an adjusting unit that adjusts a clock of the image signal so as to be different between the plurality of image forming units. 4. The image forming apparatus according to claim 1, wherein the adjusting unit fixes a clock of the image forming unit serving as a reference among the plurality of image forming units, and changes a clock of the other image forming units to thereby change the clock of the image signal. The image forming apparatus according to claim 3, wherein the image forming units are adjusted so as to be different from each other. 5. The image forming apparatus according to claim 3, wherein said adjusting means is a PLL. 6. A synchronizing signal output means for detecting a light beam to be scanned at a predetermined position and outputting a horizontal synchronizing signal of the light beam; and dividing the clock output from the PLL by dividing the light beam. 6. The image forming apparatus according to claim 5, further comprising: means for generating a clock synchronized with each of the horizontal synchronization signals. 7. The image forming apparatus according to claim 1, wherein said magnification color compensating means comprises a refraction element and a diffraction element.