JP2000108406A - Imaging apparatus and control method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an imaging apparatus in which the needs of high speed and high image quality are satisfied while suppressing cost by lowering the resolution at the time of high speed image formation as compared with low speed image formation. SOLUTION: At an imaging section 15 provided with four LED arrays 6BK, 6C, 6M and 6Y corresponding to respective colors as an exposing light source, imaging speed is set at 1/2 for high image quality mode if the input image has 600 dpi, 256 gray levels per dot and the input image is magnified by a factor of 2 in the subscanning direction. Subsequently, binarization is performed by error diffusion method or dither method and a binarized image data is transferred to an LED driver at same operation clock thus performing image exposure. Assuming the quantity of light per dot is 1 in standard mode, it is reduced to 1/2 when the imaging speed is set at 1/2 thus enhancing resolution while sustaining image stability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital image forming apparatus, and more particularly to an electrophotographic image forming apparatus.

[0002]

2. Description of the Related Art In recent years, full-color image information has been developed.
There is a growing need for faster, cheaper, and cleaner hardcopy output.

At present, in an electrophotographic image forming apparatus, high resolution, high gradation, and high stability are progressing in order to realize the above needs.

While the resolution is standardized to 400 dpi and the image quality is further improved, the high stabilization technology is also advanced, and the two conflicting technologies of high resolution and high stability are compatible.

[0005] In addition, to meet the need for higher speed, 4
By performing image formation such as charging, exposing, and developing for each color by using one image carrier itself, four times as fast as the conventional system of one image carrier is achieved.

[0006] In addition, a mechanism is provided that can cope with various types of transfer materials by changing the fixing speed, and high image quality from the viewpoint of the transfer material has been achieved. For example, when forming an image on thick paper, an OHP sheet, or the like, it is general to lower the fixing speed.

In this case, as shown in FIG. 7, the fixing speed is changed while the rotation speed of the photosensitive member is kept constant by using the fixing and conveying belt 30. However, since the size of the entire apparatus tends to be large, a method of changing the rotation speed of the photoconductor has been proposed.

Further, in order to realize further space saving, an image forming apparatus using an LED array as an exposing means has become common.

For the need for cost reduction, 40
By realizing binary image formation at 0 dpi, the electric circuit configuration, the amount of memory, and the like are reduced to meet the requirements.

In this manner, an image forming apparatus for forming a binary image with high speed, low cost and high image quality can be realized.

[0011]

However, in the conventional image forming apparatus as described above, it is possible to reduce the cost and to meet the needs for high speed and high image quality. There was a problem that we could not respond.

In the conventional resolution, when a binary image is formed, the image quality cannot be said to be satisfactory, and further improvement in image quality is desired. For that purpose, several approaches have been proposed. One of them is to increase the image quality by increasing the resolution. However, if you simply increase the resolution,
There is a problem that some adverse effects described below occur.

As one of the adverse effects, when simply trying to increase the resolution, it is first necessary to increase the speed of the electric circuit.
There is a cost increase for that. Also, due to photoconductor characteristics,
If the resolution is simply increased, the stability of the electrostatic latent image is lost, and accordingly, the development stability is also lost, and it becomes difficult to stably maintain the output image at high image quality.

Achieving an exposure spot diameter according to the resolution also causes an increase in cost.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an image forming apparatus capable of responding to needs for high speed and high image quality while suppressing cost increase. And a control method thereof.

[0016]

An image forming apparatus and a control method thereof according to the present invention are configured as follows.

(1) In an electrophotographic image forming apparatus capable of switching the image forming speed, when forming an image at a high speed, the image is formed at a lower resolution than when forming an image at a low speed. .

(2) In the image forming apparatus of the above (1), the image carrier is uniformly charged, and the charged image carrier is exposed to an image by exposure means to form an electrostatic latent image. The electrostatic latent image is visualized by a developing unit, an image on the image carrier is transferred onto a transfer material by a transfer unit, and an image on the transfer material is fixed by a fixing unit to form an image. .

(3) In the image forming apparatus of (2), the resolution is changed in the sub-scanning direction, which is the rotation direction of the image carrier during image formation.

(4) In the image forming apparatus of (2) or (3), the exposure means is an LED array or a laser scanner.

(5) In any one of the image forming apparatuses (2) to (4), the operation clock of the image processing and exposure means is not changed even if the image forming speed is changed.

(6) In the image forming apparatus according to any one of the above (2) to (5), when changing the resolution, the amount of exposure by the exposure means per unit area of the image carrier is determined by the resolution When the value was high, it was more or equal than when the value was lower.

(7) In the image forming apparatus of (2), the resolution is changed in the main scanning direction, which is the longitudinal direction of the image carrier during image formation.

(8) In the image forming apparatus of (7), the exposure means is a laser scanner.

(9) In the image forming apparatus of (8), the resolution in the main scanning direction is switched by switching the rotation speed of the polygon motor of the laser scanner.

(10) In any one of the above-mentioned image forming apparatuses (1) to (9), the number of reproduced gradations per pixel is binary.

(11) A method for controlling an electrophotographic image forming apparatus capable of switching the image forming speed, wherein image forming at high speed is performed at a lower resolution than image forming at low speed. To do.

(12) In the method for controlling an image forming apparatus according to the above (11), the image carrier is uniformly charged, and the charged image carrier is exposed to an image by an exposure means to form an electrostatic latent image. Formed, and the electrostatic latent image is visualized by developing means.
The image on the image carrier is transferred onto a transfer material by a transfer unit, and the image on the transfer material is fixed by a fixing unit to form an image.

(13) In the control method of the image forming apparatus of (12), the resolution is changed in the sub-scanning direction, which is the rotation direction of the image carrier during image formation.

(14) In the method of controlling an image forming apparatus according to the above (12) or (13), the exposing means uses an LED array or a laser scanner, and performs image processing even if the image forming speed is changed. The operation clock was not changed.

(15) In the method of controlling an image forming apparatus according to any one of the above (12) to (14), when changing the resolution, the amount of exposure by the exposure means per unit area of the image carrier is determined by the resolution When the value was high, it was more or equal than when the value was lower.

(16) In the control method of the image forming apparatus of (12), the resolution is changed in the main scanning direction, which is the longitudinal direction of the image carrier during image formation.

(17) In the method for controlling an image forming apparatus according to the above (11) or (12), a laser scanner is used as an exposure means, and the resolution in the main scanning direction is increased by switching the rotation speed of a polygon motor of the laser scanner. I changed it.

(18) In the method of controlling an image forming apparatus according to any one of the above (11) to (17), the number of reproduced gradations per pixel is binary.

[0035]

(Embodiment 1) FIG. 1 is an explanatory view showing a main part of an image forming apparatus according to Embodiment 1 of the present invention. This image forming apparatus is a full-color image using a four-drum system. It is a forming device.

In FIG. 1, reference numeral 15 denotes an image reading unit for reading a document image, and has a CCD sensor unit 13. Then, the document image read by the image reading unit 15 is recorded on a recording sheet by the image forming unit 12. Here, an image forming process in the image forming unit 12 will be described.

First, the photosensitive drum 10, which is an image carrier, is uniformly charged by the primary charger 5, and is exposed to an LED array (exposure means) 6 to form an electrostatic latent image. The electrostatic latent image is visualized by a developing device (developing means) 7 and is transferred onto transfer paper (recording paper) by transfer means such as a transfer charger 8, a paper feed path 11, and a transfer belt 12. Then, the toner image transferred onto the transfer paper by the fixing device (fixing means) 9 is fixed. Note that the number of reproduced gradations per pixel is 2
Value.

In this embodiment, the above-described image forming process is simultaneously performed for each of the four colors to achieve high speed. The first station 1 is Yellow and the second station 2 is Magenta (Yellow). Magenta), the third station 3 is cyan (cyan), and the fourth station 4 is black (black). In FIG. 1, members common to the stations are denoted by Y, M, C, and BK, respectively.

Therefore, four LED arrays 6 are used as exposure sources. The light 6 emitted from the LED array is imaged on the surface of the photosensitive drum 10 through a lens eye 16 of selfoc (registered trademark of Nippon Sheet Glass) as shown in FIG.

The resolution of the image forming apparatus in the main scanning direction (the longitudinal direction of the photosensitive drum 10, the direction from the front side to the back side in FIG. 1) is determined by the resolution of the LED array 6, and is 600 dpi in the present image forming apparatus. ing. selfo
The lens array 15 of c. forms an image of the LED 6 on the photosensitive drum surface in a one-to-one manner.
The resolution of i is obtained.

The image forming apparatus has a standard mode and a high image quality mode as image modes.

Sub-scanning direction (rotation direction of photosensitive drum 10)
Has a resolution of 600 dpi in the standard mode and 1200 dpi in the high quality mode.
i. The image forming speed (process speed) at this time is 20 cpm in the standard mode and 1 in the high image quality mode.
0 cpm.

That is, by changing the image forming speed, the resolution is changed, and the image quality according to the needs is provided.

FIG. 3 is a schematic diagram summarizing the relationship between the image forming mode and each control parameter changed in accordance with the image forming mode in the present image forming apparatus.

The present image forming apparatus has an image memory for adjusting the phase of the drum interval. In the high image quality mode, this memory amount is required in excess of the resolution in the standard mode. In the present image forming apparatus,
Since it is 1200 dpi and 600 dpi, the memory amount is required twice.

In order to set the high image quality mode, it is necessary to increase the memory amount. In other cases, for example, the clock speed at the time of image processing and the light emission clock of the LED are changed depending on the image mode (image forming mode). I have not switched. That is, the cost can be increased only by the memory amount. By doing so, high image quality or high speed can be achieved while minimizing cost increase.

The image processing in the image forming apparatus will be described.

In the present image forming apparatus, an image signal input with multiple values per pixel is binarized by using an error diffusion method or a dither method, and image exposure is performed by the LED array 6.

FIG. 4 shows a flowchart of image processing in the standard mode and the high image quality mode. This operation is executed based on a program stored in a ROM (not shown) in accordance with an instruction from a CPU (not shown).

As an example, a case where the input image has 600 dpi and 256 gradations per dot will be described.

First, in the high image quality mode (step S1)
08), the image forming speed is set to （(step S1)
09) The input image is enlarged twice in the sub-scanning direction. Here, the image is enlarged by linear interpolation, and the image
The data is converted into 256 gradation data at 0 dpi and 1200 dpi in the sub-scanning direction (step S110). After this, binarization is performed by the error diffusion method or the dither method (steps S111 and S112).

The binarized image data is transferred to the LED driver at the same operation clock regardless of the image forming speed, and the image is exposed.

At this time, the amount of light per dot is changed according to the image forming speed. For example, when the light amount per dot in the standard mode is 1, when the image forming speed is に, the light amount per dot is 1 /
2 (step S113). By doing so, the potential attenuation amount per unit area with respect to the photosensitive drum can be made constant. Then, an image forming operation is performed (step S114). As a result, it is possible to increase the resolution without losing image stability or the like.

Here, a brief description will be given of how high image quality is achieved by increasing the resolution.

In general, when an image is formed by using the error diffusion method, the graininess of dots becomes a problem. The result is a so-called rough image. One way to reduce this is to reduce the size of the smallest isolated dot. In the case of this image forming apparatus, the standard 600 dpi
By making the resolution 1200 dpi in the sub-scanning direction,
The area can be reduced to half, and the graininess of the image has been successfully reduced.

Here, the dither method at 1200 dpi will be described.

FIG. 5 shows a dither matrix in the present image forming apparatus.

The matrix in the standard mode has 150 lines,
256 gradations are achieved.

In this image forming apparatus, the same dither matrix is used for four colors of yellow, magenta, cyan and black, and an image is formed at a screen angle of 0 degrees.

By using a matrix as shown in FIG. 5, it is possible to increase the number of screen lines while maintaining the number of reproduced gradations, thereby achieving high image quality. When the dither matrix shown in FIG. 5 is adopted, a change in the dither pattern in the halftone is slightly noticeable, which is one of the causes of the deterioration of the image quality. Therefore, by changing the resolution to 1200 dpi, the same pattern change occurs, but the pattern can be made smaller, so that the pattern change becomes less noticeable and the image quality can be improved.

In the standard mode (step S10)
1) Set the standard image forming speed (step S10)
2). Then, an image signal of 600 dpi is input (step S103), and the error signal 2 or the dither method is used.
Value conversion is performed (steps S104 and S105). afterwards,
The exposure amount per dot is set to the standard (step S1).
06), an image forming operation is performed (step S107).

(Embodiment 2) FIG. 6 is a sectional view showing the internal structure of an image forming apparatus according to Embodiment 2 of the present invention. This image forming apparatus is a full-color image forming apparatus using a four-drum system as in the first embodiment, but uses a laser as an exposure source unlike the first embodiment.

The image forming process will be briefly described with reference to FIG.

First, the photosensitive drum, which is an image carrier, is uniformly charged by the primary charger 31 and is exposed to a laser scanner (exposure means) 35 to form an electrostatic latent image. The electrostatic latent image is visualized by a developing device (developing means) 32,
The image is transferred onto transfer paper (recording paper) by transfer means such as a transfer charger and a transfer conveyance belt. Then, the fixing conveyance belt 30
Thus, the toner image transferred to the fixing device (fixing means) 37 and transferred onto the transfer paper is fixed. Note that the number of reproduced gradations per pixel is binary.

Also in this embodiment, the above-described image forming process is performed simultaneously for each of the four colors to realize a high speed. The first station is set to Y in the same order as in FIG.
yellow (yellow), the second station is Mage
nta (magenta), the third station is cyan (cyan), and the fourth station is black (black). In FIG. 6, members common to the stations are denoted by Y, M, C, and BK, respectively.

Therefore, four lasers are used as an exposure source.

As in the first embodiment, the present image forming apparatus has a standard mode and a high image quality mode as image forming modes. FIG. 7 shows parameter settings in each mode.

The image forming speed is 20 cp in the standard mode.
m, 10 cpm in the high image quality mode.

The resolution of the image forming apparatus in the standard mode is 600 dpi in both the main scanning direction and the sub-scanning direction.
It has become. When the high image quality mode is selected, the resolution is 1200 dpi in the main scanning direction and 600 dpi in the sub scanning direction.

To achieve this, the laser scanner 3
In the high image quality mode, the rotation speed of the polygon motor 5 is half the speed in the standard mode, so that the resolution in the main scanning direction can be increased without changing the image processing clock and the like. Has become.

By doing so, 1200 lines in the main scanning direction,
It is possible to output a two-tone image without changing the image processing configuration or the image clock.

At this time, the amount of light per dot is changed according to the image forming speed. For example, when the light amount per dot in the standard mode is 1, when the image forming speed is に, the light amount per dot is 1 /
I am 2. By doing so, the potential attenuation amount per unit area with respect to the photosensitive drum can be made constant. As a result, it is possible to increase the resolution without losing image stability or the like.

However, the memory amount is used for the phase adjustment for the drum interval as in the first embodiment.
In the high image quality mode, twice the amount of memory is required.

In the high image quality mode, the resolution can be increased in the sub-scanning direction by forming an image at a normal rotation speed without setting the rotation speed of the polygon motor to half speed.

In this embodiment, in the standard mode, it is possible to meet the need for high speed, and in the high image quality mode, it is possible to meet the need for high image quality. It is possible to provide an image forming apparatus capable of responding to the above.

As described above, in the first and second embodiments, the image carrier is uniformly charged, and an image is exposed on the image carrier uniformly charged by the exposure means to form an electrostatic latent image. Means for visualizing the electrostatic latent image, transferring the image on the image carrier onto the transfer material by the transfer means, and finally fixing the image on the transfer material to the transfer material by the fixing means to form an image, Further, at a variable image forming speed, the image forming speed of the image forming apparatus is switched between the standard speed and half the speed, and the resolution in the sub-scanning direction or the main scanning direction is increased accordingly. , High image quality by reducing the minimum isolated dot in the error diffusion method,
High image quality can be achieved by making the dither pattern change less noticeable by reducing the dither matrix by the dither processing method.

When the resolution is increased, the image formation can be performed without losing the stability of the image by adjusting the exposure amount per unit area to be substantially equal.

The only cost increase at that time is an increase in the amount of memory for phase adjustment at the drum interval.

Therefore, it is possible to provide an image forming apparatus capable of responding to needs for high speed and needs for image quality with a single image forming apparatus while minimizing cost increase.

[0080]

As described above, according to the present invention,
In an electrophotographic image forming apparatus in which the image forming speed can be switched, when forming an image at a high speed, image formation is performed at a lower resolution than when forming an image at a low speed, thereby suppressing cost increase. There is an effect that one image forming apparatus can meet the above-mentioned needs of high image quality and high speed.

When the resolution is changed, the exposure amount per unit area of the image carrier by the exposure means is set to be larger or equal when the resolution is high compared to when the resolution is low. There is an effect that an image can be formed without losing the stability of the image.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a main configuration of an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a positional relationship among an LED array, a lens array, and a photosensitive drum.

FIG. 3 is a diagram illustrating control parameters for an image forming mode according to the first exemplary embodiment.

FIG. 4 is a flowchart illustrating an image forming operation according to the first exemplary embodiment.

FIG. 5 is an explanatory diagram illustrating a dither matrix according to the first embodiment.

FIG. 6 is a cross-sectional view illustrating an internal configuration of an image forming apparatus according to a second embodiment of the present invention.

FIG. 7 illustrates control parameters for an image forming mode according to the second exemplary embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st station 2 2nd station 3 3rd station 4 4th station 5 Primary charger 6 LED array (exposure means) 7 Developing unit (developing unit) 8 Transfer charger (transfer unit) 9 Fixing unit (fixing unit) 10 Photosensitive drum (image carrier) 12 Transfer / transport belt (transfer means) 30 Fixing / transport belt 35 Laser scanner (exposure means) 37 Fixing device (fixing means)

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G03G 21/14 9A001

Claims (18)

[Claims] 1. An image forming apparatus of an electrophotographic type capable of switching an image forming speed, wherein an image is formed at a lower resolution when forming an image at a higher speed than when forming an image at a lower speed. Characteristic image forming apparatus. 2. An electrostatic latent image is formed by uniformly charging an image carrier, exposing the charged image carrier on an image by exposure means, forming an electrostatic latent image by developing means, and developing the electrostatic latent image by developing means. 2. The image forming apparatus according to claim 1, wherein an image on the image carrier is transferred onto a transfer material by a transfer unit, and an image on the transfer material is fixed by a fixing unit to form an image. 3. The image forming apparatus according to claim 2, wherein the resolution is changed in a sub-scanning direction, which is a rotation direction of the image carrier during image formation. 4. An image forming apparatus according to claim 2, wherein said exposure means is an LED array or a laser scanner. 5. The image forming apparatus according to claim 2, wherein the operation clock of the image processing and exposure means is not changed even if the image forming speed is changed. 6. When changing the resolution, it is necessary that the exposure amount per unit area of the image carrier by the exposure means is higher or equal when the resolution is high compared to when the resolution is low. The image forming apparatus according to any one of claims 2 to 5, wherein: 7. The image forming apparatus according to claim 2, wherein the resolution is changed in a main scanning direction which is a longitudinal direction of the image carrier during image formation. 8. An image forming apparatus according to claim 7, wherein said exposure means is a laser scanner. 9. The image forming apparatus according to claim 8, wherein the resolution in the main scanning direction is switched by switching a rotation speed of a polygon motor of the laser scanner. 10. The image forming apparatus according to claim 1, wherein the number of reproduced gradations per pixel is binary. 11. A method for controlling an electrophotographic image forming apparatus capable of switching an image forming speed, wherein image forming is performed at a lower resolution when forming an image at a higher speed than when forming an image at a lower speed. And controlling the image forming apparatus. 12. An image carrier is uniformly charged, an image is exposed on the charged image carrier by an exposure unit to form an electrostatic latent image, and the electrostatic latent image is visualized by a developing unit. 12. The image forming apparatus according to claim 11, wherein the image on the image carrier is transferred onto a transfer material by a transfer unit, and the image on the transfer material is fixed by a fixing unit. Control method. 13. The method according to claim 12, wherein the resolution is changed in a sub-scanning direction, which is a rotation direction of the image carrier during image formation. 14. The image according to claim 12, wherein the exposure means uses an LED array or a laser scanner, and does not change the image processing and the operation clock of the exposure means even if the image forming speed is changed. A method for controlling a forming apparatus. 15. When changing the resolution, the exposure amount per unit area of the image carrier by the exposure means is higher or equal when the resolution is higher than when the resolution is lower. 15. The method for controlling an image forming apparatus according to claim 12, wherein: 16. The method according to claim 12, wherein the resolution is changed in a main scanning direction which is a longitudinal direction of the image carrier during image formation. 17. The control of the image forming apparatus according to claim 11, wherein a resolution in the main scanning direction is switched by switching a rotation speed of a polygon motor of the laser scanner as the exposure unit. Method. 18. The method according to claim 11, wherein the number of reproduced gradations per pixel is binary.