JPH09281419A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a sharp image owing to improvement of light write-in precision without increasing a cost by providing a light beam deflection means, a detection means detecting a width of a dot pattern and a focal position detection means. SOLUTION: A deflection mirror moving actuator 113 is moved within the movable range, and the dot patterns 202 in respective moving positions are formed successively. Then, the width of the developed dot patterns 202 are detected by a positional deviation detector 117, and one in a minimum width among the detection data is recognized by the detection means 203, and a focal position is obtained. The image is written in the state without positional deviation by writing in the image data in the position of the actuator 113 corresponding to the focal position, and since the focal position of a light beam is made adjustable, and positional deviation detection of main/sub-scan are also usable, the sharp image is written owing to a minute dot.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a full-color digital copying machine or a laser printer,
More specifically, by forming a specific dot pattern on the photoconductor and detecting the pattern width, the writing focus position of the light beam on the photoconductor is obtained, and the writing focus position is determined according to the image data. The present invention relates to an image forming apparatus that writes a light beam to form an image.

[0002]

2. Description of the Related Art Conventionally, in an electrophotographic digital copying machine or laser printer having an optical writing system, the accuracy of the image forming position of a light beam on a photosensitive member has been important. Particularly, in order to improve the image quality, the dot diameter is minimized to increase the pixel density, and in this case, more accurate optical writing at the focus position is required.

In general, an electrophotographic apparatus using an LD (laser diode) as a light emitting source has an fθ lens and a B lens arranged in the optical path in order to reduce the writing dot diameter.
The TL lens has a function of narrowing the beam diameter, and is designed so that an image is formed on the photoconductor at a position where the beam diameter is minimized.

As shown in FIG. 8, in each waist diameter of the LD, that is, the beam narrowed down by the lens action of the Gaussian beam, the spot diameter of the beam with respect to the distance from the waist position is given by the following mathematical expression 1.

[0005]

[Equation 1]

In the above, 2ω is the beam spot size, 2ω _{0 is the} beam waist, λ is the beam wavelength, and Z is the distance from the waist position.

FIG. 9 shows the spread of the beam with respect to the distance from the waist position of the beam based on this equation. As the waist diameter of the beam becomes smaller, the beam spread becomes sharper. That is, it can be seen that the tolerance of the image forming position becomes stricter as the dot diameter becomes smaller.

The intensity distribution at the distance from the center of the Gaussian beam is expressed by the following equation (2).

[0009]

[Equation 2]

In the above, I is the intensity of laser light, r is the distance from the center of the laser, and p is the power of the laser.

FIG. 7 shows the beam intensity distribution at each spot diameter when the LD emission power is constant based on this equation. Here, it can be seen that even at the same power level, the intensity of the center at the position deviated from the focus decreases, resulting in a blurred image.

On the other hand, with the cost reduction of products, the material of the optical lenses such as the fθ lens and the BTL lens described above is being replaced with plastic from glass.

As reference technical documents relating to the present invention, for example, Japanese Patent Laid-Open Nos. 2-140710 and 4-4.
No. 251213 is disclosed.

In Japanese Patent Laid-Open No. 140710/1990, a photodiode (PD) is irradiated with a light beam, the movement of an optical element is controlled while detecting the dot diameter, and the focus position is corrected.

Further, in Japanese Unexamined Patent Publication No. 4-251213, a detection plate capable of detecting the entire main scanning width of the light beam determines the defocus amount of the light beam over the entire main scanning.

[0016]

However, when the plastic lens is used in association with the cost reduction as in the prior art as described above, the plastic generally has a high coefficient of thermal expansion, and the When the temperature inside the machine rises, the focus position shifts due to the change in the lens shape, which often causes a problem of deviating from the allowable value.

For this reason, it becomes necessary to make the focal position of optical writing accurate and to adjust it sequentially over time. However, there is a problem in that cost reduction is hindered even if a special device is provided only for adjusting the focus position.

Further, in JP-A-2-140710 and JP-A-4-251213, since the PD is provided only for the purpose of adjusting the focus position, cost reduction is hindered. Further, if the positional relationship between the PD and the photoconductor in the beam irradiation direction is not accurate, correct focus position correction cannot be performed, and therefore the PD mounting position accuracy must be increased.

Further, the use of a special detection device only for correcting defocus as in Japanese Patent Laid-Open No. 4-251213 has a problem that cost reduction and space saving design are hindered.

The present invention has been made in view of the above, and by forming a plurality of dot patterns using a writing light beam and recognizing one of the dot patterns having the minimum width, In addition to adjusting the focus position of the light beam, it is also possible to detect misalignment between main and sub-scans.
The purpose is to obtain a sharp image by improving the optical writing accuracy without increasing the cost.

[0021]

In order to achieve the above object, in the image forming apparatus according to the first aspect of the present invention, a specific dot pattern is formed on the photoconductor and the pattern width is detected. By determining the writing focus position of the light beam on the photoconductor, writing the light beam according to the image data at the writing focus position to form an electrostatic latent image, and attaching toner to the latent image. In an image forming apparatus for forming an image, a light beam deflecting means for deflecting the light beam to move an image forming position on a surface of a photosensitive member to write a specific dot pattern, and a width of the developed dot pattern are detected. And a focus position detecting means for detecting and recognizing the minimum width pattern of the dot pattern from the output of the detecting means.

That is, the optical writing position is moved by the light beam deflecting means to form a plurality of dot patterns, and the width of the developed dot pattern is detected by the detecting means. The focus position is obtained by recognizing one of them, and the image data is written at the position of the light beam deflecting means corresponding to the focus position, so that the image writing is performed without any positional deviation, so that it is possible to sharpen with fine dots. You can write a nice image.

Further, in the image forming apparatus according to the second aspect, the light emission intensity of the light beam at the time of forming the dot pattern is set to a value higher than that at the time of image formation.

That is, by writing with an increased light emission intensity of the light beam at the time of dot pattern formation, the shape of the dot pattern at the time of development is stabilized, and the focus / defocus state of the beam spot can be detected more accurately. Since this is possible, the waist position of the laser can be accurately detected.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

[Embodiment] In this embodiment, an arrangement for detecting a positional deviation in the main scanning direction is aimed at an image forming apparatus using a plurality of photoconductors having a particularly high degree of difficulty in preventing the positional deviation from occurring.・ Describe the operation.

(Structure of Embodiment) FIG. 1 is an explanatory view showing the structure of an image forming apparatus suitable for applying the present invention. This image forming apparatus shows a full-color digital copying machine using a plurality of photoconductors. The image forming apparatus is mainly a scanner 100 that optically scans a document and reads the image for each scanning line, and a scanner 100 that reads the image. The image processing circuit 101 performs predetermined image processing on image data, a paper feeding unit 102 that feeds and conveys recording paper, an image forming system described later, and a fixing unit 103.

The image forming system is a writing optical system 1 for performing optical writing according to image data output from the image processing circuit 101.
05, an electrostatic latent image is formed, and photosensitive drums 106BK, M, Y, and C are provided for four colors of BK (black), M (magenta), Y (yellow), and C (cyan), respectively. ，
Toners are attached to the chargers 107BK, M, Y, and C that charge the photosensitive drums 106BK, M, Y, and C and the latent images formed on the photosensitive drums 106BK, M, Y, and C to form a visible image. Developing device 108BK, M, Y, C
And the photosensitive drums 106BK, M, Y, C after the transfer processing
Cleaning device 109B for removing the residual toner on the surface
K, M, Y, C, and the photoconductor drum 106BK, M, Y,
It is composed of a transfer belt 110 and the like for transferring the images formed on C to the recording paper in color order.

The writing optical system 105 has a polygon scanner 1 for scanning light from an LD (laser diode).
11, a deflection mirror 112 that guides the light beam scanned by the polygon scanner 111 to the photosensitive drums 106BK, M, Y, and C and irradiates it, and a light beam deflection that deflects the light beam by moving the deflection mirror 112. A deflection mirror moving actuator 113 as a means is provided.

Further, in the transfer belt 110 portion, a photodiode (PD) 115, which will be described later, provided facing the surface of the transfer belt 110, and a light emitting diode (LE).
D) 116 is provided with a positional deviation detector 117 as a detecting means.

FIG. 2 is an explanatory diagram in which the focus position detecting portion in FIG. 1 is taken out, and shows a portion for one color. In the figure, 201 is an LD (laser diode) unit, 202 is a developed / formed dot pattern on the transfer belt 110, and 203 is a dot pattern to be described later, and 203 is a focus position detection means for detecting / recognizing the dot pattern from the detection output of the positional deviation detector 117. The reference numeral 204 denotes a drive circuit for driving and controlling a drive source for driving the deflection mirror moving actuator 113. In addition, this misalignment detection unit 1
FIG. 3 is an enlarged view of 17 parts.

(Operation of Embodiment) Next, the operation and the focus position detecting operation of the image forming apparatus configured as described above will be described.

The surfaces of the photosensitive drums 106BK, M, Y and C are uniformly charged by the charging chargers 107BK, M, Y and C, and then a pattern corresponding to an image to be output is exposed by the writing optical system 105. To be done. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 106BK, M, Y, C. This electrostatic latent image is developed by the developing devices 108BK, M,
A visible image of a toner image is obtained by developing with Y and C.

Further, recording paper is fed from the paper feeding unit 102.
The toner image is transferred onto the recording paper that is transported and transported by the transfer belt 110. After transfer, photoconductor drum 106B
The toner remaining on the surface of K, M, Y, C is removed by the cleaning device 109BK, M, Y, C.

In the above, the writing optical system 105
The polygon scanner 111 rotates the light beam output from the LD unit 201 to scan the photosensitive drums 106BK, M, Y, and C in the axial direction (main scanning), and the photosensitive drums 106BK, M, and By the rotations of Y and C, sub-scanning is performed in the direction orthogonal to the axes of the photoconductor drums 106BK, M, Y and C.

The alignment of the colors BK (black), M (magenta), Y (yellow), and C (cyan) is performed at the timing when the recording paper is conveyed from the registration roller section to the transfer position of each color by the transfer belt 110. , Photoconductor drum 106B
This is performed by setting the exposure start time so that the timings at which the K, M, Y, and C images are moved to the transfer position are the same for all colors.

In addition, the misregistration can be detected and corrected by, for example, the photoconductor drum 106B in the misregistration correction mode.
A latent image of a specific dot pattern is formed on K, M, Y, and C, and the latent image is developed by the developing device 108BK, M, Y, and C to form a visible image, and this pattern is formed by the LED 116 and PD1.
The positional deviation detector 117 composed of 15 and 15 detects and stores this detection data. Then, based on this detection data, the deflection mirror moving actuator 113
Use to move the optical path of the light beam and perform dot misregistration correction.

Further, the above-mentioned positional deviation detection will be described in detail.

In FIG. 2, the light beam emitted from the LD unit 201 is the deflection mirror moving actuator 1
An image is formed on the surface of the photoconductor drum 106 via a deflection mirror 112 equipped with a dot pattern 13
An electrostatic latent image of 2 (1 dot line in this case) is formed.

This electrostatic latent image is developed by the developing device 108 to form a visible image. Further, this image is transferred to the transfer belt 110.
And is made to pass through the measurement surface of the positional deviation detector 117 constituted by the LED 116 and the PD 115 arranged so as to face the surface of the transfer belt 110.

The positional deviation in the main scanning direction is detected by using the dot pattern shown in FIG.
Specifically, the amount of deviation is detected based on the time at the center of the time width of the output value that seems to pass the dots. This detection state is shown in FIG.

On the other hand, in detecting the shift of the focus position, the deflection mirror moving actuator 113 is moved within the movable range, and the dot pattern 202 at each moving position is detected.
Are sequentially formed. By performing such a process, the dot pattern gradually becomes thinner or becomes a thick dot line pattern.

That is, the position of the deflection mirror moving actuator 113 corresponding to the formation of the thinnest dot line in this pattern is the position where the optimum focus position is obtained, and the optimum image formation is performed at this position. It can be performed.

Therefore, the detection method is as shown in FIG.
As shown in (b), the deviation is detected based on the dot width, specifically, the time width of the output value through which the dot passes, and the one having the shortest output time width is selected. As a result, the images in each optical path length can be relatively evaluated, and the focus position can be accurately recognized.

In this embodiment, the deflection mirror 1
Although the optical path length is deflected by 12 to change the focal position, another mechanism for changing the focal position by a lens or the like may be used.

Further, by performing the focusing by using such a position shift detector 117, it becomes possible to detect the main scanning direction and the sub scanning direction, the detection in both directions is realized, and its technical value is high. Become.

By the way, the dot pattern 202 to be detected is a toner image developed by adhering toner having electrostatic force to the latent image electrically written on the surface of the photosensitive drum 106. Therefore, normally, the reproducibility of the developed dot pattern 202 deteriorates with respect to the latent image dot pattern 202.

In particular, the dot pattern 202 written at a position deviated from the focus has a small electrostatic energy difference from the non-written portion, as is clear from the beam intensity distribution shown in FIG. As shown in (a), an actual image is often a faint image, or a so-called dusty and unclear image in which toner is scattered around the dots. If these are detected by the PD 115, accurate detection may not be possible.

Therefore, dots are formed at a level in which the output power of the LD unit 201 is raised more than in the actual image formation, and toner is firmly attached even in a dot pattern 202 that is out of focus to form a dot pattern 202 with high contrast. Let it form. This state is shown in FIG.
By doing so, accurate detection is realized.

Therefore, according to the above-described embodiment, it is possible to solve one problem of the focal point shift of the light beam, that is, the positional shift in the irradiation direction.

By the way, particularly in the full-color digital copying machine as shown in FIG. 1, the photosensitive drum 106BK,
Since each color image is written in M, Y and C and each color is formed respectively, a relative displacement of each color occurs. This relative misregistration between the colors is visually noticeable as a color misregistration, which significantly deteriorates the image quality.

Therefore, by using the focus position shift detection mechanism of the above-described embodiment and detecting the main / sub scanning shift from the detection data based on the writing time as described in FIG. 6A, the efficiency can be improved. Detection is realized.

[0053]

As described above, according to the image forming apparatus of the present invention (Claim 1), the optical writing position is moved by the light beam deflecting means to form a plurality of dot patterns and the development is performed. The focus position is obtained by detecting the width of the formed dot pattern by the detection means and recognizing one of the detection data having the minimum width, and the image data is obtained at the position of the light beam deflection means corresponding to the focus position. By writing
Image writing is performed without misalignment, the focus position of the light beam can be adjusted, and misregistration detection for main and sub-scanning can be used at the same time. You can write a nice image.

Further, according to the image forming apparatus (Claim 2) of the present invention, writing is performed by increasing the light emission intensity of the light beam at the time of dot pattern formation, thereby stabilizing the shape of the dot pattern at the time of development, Since the focused / defocused state of the beam spot can be detected more accurately, the waist position of the laser can be accurately detected.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of an image forming apparatus suitable for applying the present invention.

FIG. 2 is an explanatory diagram in which a focus position detection portion in FIG. 1 is extracted.

FIG. 3 is an enlarged view showing a position deviation detecting portion in FIG.

FIG. 4 is an explanatory diagram showing an example of a dot pattern according to the embodiment.

FIG. 5 is an explanatory diagram showing an example of a dot pattern depending on the presence / absence of a writing intensity process according to the embodiment.

FIG. 6 is an explanatory diagram showing a method of detecting a focus position shift by a PD detection signal according to the embodiment.

FIG. 7 is a graph showing the intensity distribution of LD output light at each dot diameter with a constant emission power.

FIG. 8 is an explanatory diagram showing a beam focused by the lens action of a Gaussian beam.

FIG. 9 is a graph showing the relationship between the distance from the waist position of a Gaussian beam and the spread of the dot diameter.

[Explanation of symbols]

 105 Writing Optical Meter 112 Polarizing Mirror 113 Polarizing Mirror Moving Actuator 115 Photodiode (PD) 116 Light Emitting Diode (LED) 117 Displacement Detector 202 Dot Pattern 203 Detection Circuit

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Satoshi Shinohara 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Company (72) Inventor Yutaka Shio 10-3 Kitamura, Tottori City, Tottori Prefecture Ricoh Microelectronics Co., Ltd. (72) Inventor Tomonori Yabuta 10-3 Kitamura, Tottori City, Tottori Prefecture Ricoh Microelectronics Co., Ltd.

Claims (2)

[Claims] 1. A specific dot pattern is formed on a photoconductor, and a writing focus position of a light beam on the photoconductor is obtained by detecting the pattern width, and the writing focus position is determined according to image data. In the image forming apparatus which forms an electrostatic latent image by writing a light beam and adheres toner to the latent image, the light beam is deflected to move the image forming position on the surface of the photoconductor. Then, a light beam deflecting means for writing a specific dot pattern, a detecting means for detecting the width of the developed dot pattern, and a minimum width pattern of the dot pattern from the output of the detecting means.
An image forming apparatus comprising: a focus position detecting unit for recognizing. 2. The image forming apparatus according to claim 1, wherein the light emission intensity of the light beam at the time of forming the dot pattern is set to a value higher than that at the time of image formation.