JP7418122B2 - image forming device - Google Patents

Description

The present invention relates to a magnification correction method for correcting an optical system and the like in an image forming apparatus such as a digital copying machine.

(Principles of electrophotography and laser scanning)
In an electrophotographic image forming apparatus such as a digital copying machine, a laser is turned on and controlled in accordance with an image signal to form an electrostatic latent image on a photoreceptor drum, and an image is formed through development, transfer, and fixing steps.

Laser irradiation onto the photoreceptor drum is performed by deflection scanning in the longitudinal direction of the photoreceptor drum (hereinafter referred to as the main scanning direction) by rotation of a polygon mirror. Furthermore, by rotating the photosensitive drum, a two-dimensional latent image is formed by scanning in a direction perpendicular to the main scanning direction (hereinafter referred to as the sub-scanning direction). In addition, in the deflection caused by the rotation of the polygon mirror, the optical path length and the incident angle are made uniform in the longitudinal direction by irradiating the photosensitive drum with laser through the fθ lens.

(Residual correction of fθ characteristics)
The residual error of the optical correction by the fθ lens is finely adjusted by further correcting the magnification of the image data in the main scanning direction. For example, as described in Patent Document 1, pixel data is handled in units (hereinafter referred to as pixel pieces) divided in the main scanning direction. The pixel pieces are subjected to binary control, and the gradation of each pixel is expressed using PWM (Pulse Width Modulation). This method suppresses image quality deterioration by varying the magnification using zero-order interpolation at a high frequency in units of pixel pieces.

Points where pixel pieces are inserted and extracted by zero-order interpolation (hereinafter referred to as insertion/extraction points) occur at approximately constant intervals if the magnification correction amount is constant. Patent Document 1 proposes a method of controlling the insertion/extraction position to reduce the occurrence of local shading in order to prevent moiré due to interference between the cycle of the insertion/extraction location and the PWM cycle.

(Simplification of optical system)
On the other hand, as described in Patent Document 2, a system in which magnification correction is performed entirely electrically has been proposed as an optical configuration that does not use an fθ lens in pursuit of low cost. Patent Document 2 proposes a method of dividing the main scanning direction into predetermined areas and correcting the magnification by modulating the clock frequency according to the magnification correction amount for each area.

JP2013-22913A JP2004-338280A

Patent Document 2 proposes a method of dividing the main scanning direction into predetermined areas and correcting the magnification by modulating the clock frequency according to the magnification correction amount for each area. However, as will be explained below, there is a problem in that when the amount of magnification correction and the amount of optical correction by PWM increase, the gradation of the image density deteriorates.
(Unevenness in scanning speed in systems that do not use an fθ lens)
The amount of magnification correction in a configuration that does not use an fθ lens will be explained using FIG. 3 as an example. Angular speed of rotation of the polygon mirror ω, angle of incidence on the photosensitive drum θ, assuming that the angle of incidence perpendicular to the photosensitive drum is 0 degrees, the distance from the polygon mirror to the photosensitive drum at this time is R, distance L on the surface of the photosensitive drum, time. Equations 1 to 4 show the approximate derivation of the scanning speed v(θ) when the angle θ is expressed as t.

The magnification is proportional to v(θ). FIG. 4 shows a graph in which θ is plotted against the image height, which is the distance from the longitudinal center of the photosensitive drum, for a predetermined R. The horizontal axis shows the image height, and the vertical axis shows the magnification. The magnification at the ends of the drum is nearly 1.3 times that of the center. Therefore, in order to correct the magnification, it is necessary to change the pixel size as described below.

(pixel size control)
In FIG. 4, if the time to emit light for one pixel is the same at the center of the image and at the edge of the photoreceptor drum, and if the magnification is 1 at the center image height, the magnification is less than 1 at the image height of ±150 mm at the edge of the photoreceptor drum. big. This results in an image that is elongated at the image height at the edge of the drum compared to the center of the drum. Therefore, it is necessary to correct the size of one pixel according to the scanning speed v(θ). For example, as shown in FIG. 5, when one pixel is divided into 32 at the central image height, it becomes 24 at the image height of ±150 mm.

(Light amount control)
In Fig. 4, it was mentioned above that when an fθ lens is not used, the magnification changes as the scanning speed changes depending on the distance from the longitudinal center position of the photoreceptor drum (referred to as image height). The amount of laser light irradiated per unit area of the drum also changes. As shown in FIG. 6, where the horizontal axis shows the image height and the vertical axis shows the relationship between the light amount on the drum surface, when the light amount at the center position in the longitudinal direction of the photoreceptor drum is 1, the light amount at both ends is less than 0.8 and is 20% or more. This results in a decrease. Therefore, in the conventional technology, the amount of light output from the laser (laser chip surface light amount) is changed for each image height so that the amount of light on the photoreceptor drum surface is constant between the center image height and the edge image height of the photoreceptor drum. change the laser chip surface light intensity. To this end, as shown in FIG. 7, the laser chip surface light quantity is made lowest at the center image height, and is increased to about 1.3 times at the ends of the photoreceptor drum compared to the center image height.

(Linearity of integrated light amount for PWM light emission)
FIG. 8 is a graph showing the relationship between the PWM duty and the integrated light amount when the laser is PWM modulated, for each chip surface light amount when CW light emission (continuous light emission) is performed. The vertical axis shows the value obtained by normalizing the integrated light amount, with the light amount when continuously emitting light being 100%. On the other hand, the horizontal axis represents the duty of the PWM modulation signal. It can be seen from this graph that even if the PWM modulation signal is 50%, the integrated light amount is less than 50%. This is largely due to the light emission delay of the laser. Furthermore, the integrated light amount varies depending on the chip surface light amount during continuous light emission even with the same PWM duty, and the integrated light amount tends to decrease as the chip surface light amount becomes lower.

As described in the above (pixel size control), when correcting the magnification by changing the number of pixel pieces of one pixel according to the image height, a PWM corresponding to each number of pixel pieces is generated. At this time, as explained with reference to FIG. 7, since the amount of light on the chip surface differs depending on the image height, there is a problem that the relationship between the pulse width and the integrated amount of light differs and the gradation of the image density deviates.

An object of the present invention is to provide a high-quality, low-cost system that can express good gradations for each pixel.

Taking into consideration changes in the amount of light on the chip surface for each image height, the PWM pulse width is determined according to the image data for each amount of light on the chip surface for each image height and the PWM pulse width LUT.

That is, in order to achieve the above object, an image forming apparatus according to claim 1 includes a rotating polygon mirror that deflects a light beam emitted from a light source , and a photoreceptor with the light beam deflected by the rotating polygon mirror. An image forming apparatus that scans a pixel in a main scanning direction, the image forming apparatus having a magnification table storing a correction magnification according to a scanning speed in the main scanning direction , and calculating the number of pixel pieces included in one pixel based on the magnification table. a pixel size calculation unit that determines the size of one pixel by determining the size of one pixel; A first gradation characteristic corresponding to one number of pixel pieces is selected, and a second number of pixel pieces is scanned at a second speed slower than the first speed and a second number of pixel pieces is larger than the first number. gradation characteristic selection means for selecting a second gradation characteristic corresponding to the second number of pixel pieces for the second pixel configured;
The present invention is characterized by comprising a gradation converting means for gradation converting the pixel value of each pixel using a gradation characteristic selected for each pixel, and outputting a pixel size determined for each pixel.

According to the present invention, it is possible to provide a high-quality, low-cost system that can express good gradations for each pixel even with a simple f-theta lens or a configuration without an f-theta lens.

FIG. 1 is a diagram showing the entire image forming apparatus in this embodiment. FIG. 2 is a diagram showing the configuration around a photoreceptor drum 102 and an optical scanning device 104 in this embodiment. FIG. 2 is a diagram illustrating an optical scanning device without a θ lens. FIG. 3 is a diagram showing magnification characteristics of an optical scanning device without an fθ lens. FIG. 6 is a diagram showing changes in the number of divisions depending on the scanning position of the optical scanning device. FIG. 3 is a diagram showing light quantity characteristics of an optical scanning device without an fθ lens. FIG. 7 is a diagram showing a chip surface light amount correction amount of an optical scanning device without an fθ lens. FIG. 3 is a diagram showing the relationship between PWM duty and integrated light amount. FIG. 4 is a diagram for setting gradation characteristics based on run length. FIG. 3 is a diagram showing an image control flow. FIG. 3 is a diagram showing gradation characteristics depending on bit patterns. FIG. 6 is a diagram showing changes in split nests depending on the scanning position of the optical scanning device. FIG. 3 is a diagram showing the relationship between input gradation and the number of output divisions. This is a graph for setting gradation characteristics of 32 divisions and 24 divisions. It is a figure which shows the change of Duty of PWM. FIG. 7 is a diagram for setting gradation characteristics based on run lengths of only black.

Hereinafter, preferred embodiments of the present invention will be described in detail by way of example with reference to the drawings.

(Configuration of entire image forming apparatus)
FIG. 1 is a schematic cross-sectional view of a digital full-color printer (color image forming apparatus) that forms images using toners of multiple colors. An image forming apparatus 100 of this embodiment will be explained using FIG. 1. The image forming apparatus (100) is equipped with four image forming units (image forming means) (101Y), (101M), (101C), and (101Bk) that form images for each color.

Here, Y, M, C, and Bk represent yellow, magenta, cyan, and black, respectively. Image forming units (101Y), (101M), (101C), and (101Bk) form images using yellow, magenta, cyan, and black toners, respectively. The image forming units (101Y), (101M), (101C), and (101Bk) are equipped with photosensitive drums (102Y), (102M), (102C), and (102Bk), which are photosensitive members. Around the photosensitive drums (102Y), (102M), (102C), and Bk (102), there are charging devices (103Y), (103M), (103C), (103Bk), optical scanning devices (104Y), and (104M). ), (104C), (104Bk), developing devices (105Y), (105M), (105C), and (105Bk), respectively.

Furthermore, drum cleaning devices (106Y), (106M), (106C), and (106Bk) are arranged around the photosensitive drums (102Y), (102M), (102C), and (102Bk). An endless intermediate transfer belt (107) is arranged below the photosensitive drums (102Y), (102M), (102C), and (102Bk). The intermediate transfer belt (107) is stretched around a driving roller (108) and driven rollers (109) and (110), and rotates in the direction of arrow B in the figure during image formation.

In addition, primary transfer devices (111Y) and (111M) are located at positions facing the photosensitive drums (102Y), (102M), (102C), and (102Bk) via the intermediate transfer belt (107) (intermediate transfer body). , (111C), and (111Bk) are provided. The image forming apparatus (100) of the present embodiment also includes a secondary transfer device (112) for transferring the toner image on the intermediate transfer belt (107) to the recording medium S, and a secondary transfer device (112) for transferring the toner image on the recording medium S. A fixing device (113) is provided for.

Here, the image forming process from the charging process to the developing process of the image forming apparatus (100) having such a configuration will be described. Since the image forming process in each image forming unit is the same, the image forming process will be explained using the image forming unit (101Y) as an example, and the image forming process in the image forming units (101M), (101C), and (101Bk) will be explained below. The explanation will be omitted. First, the photosensitive drum (102Y), which is rotationally driven by the charging device of the image forming section (101Y), is charged. The charged photosensitive drum (102Y) (on the image carrier) is exposed to laser light emitted from the optical scanning device (104Y). As a result, an electrostatic latent image is formed on the rotating photoreceptor.

Thereafter, the electrostatic latent image is developed as a yellow toner image by a developing device (105Y). The image forming process after the transfer step will be described below using the image forming section as an example. The primary transfer devices (111Y), (111M), (111C), and (111Bk) apply a transfer bias to the transfer belt, so that the photosensitive drums (102Y), (102M), (102C), and (102Bk) of each image forming section are ) are respectively transferred to the intermediate transfer belt (107). As a result, the toner images of each color are superimposed on the intermediate transfer belt (107).

When the four-color toner image is transferred to the intermediate transfer belt (107), the four-color toner image transferred onto the intermediate transfer belt (107) is transferred to the manual feed cassette (114) by the secondary transfer device (112). ) or onto the recording medium S conveyed from the paper feed cassette (115) to the secondary transfer section (secondary transfer). Then, the toner image on the recording medium S is heated and fixed by a fixing device (113), and is discharged to a paper discharge section (116), so that a full-color image is obtained on the recording medium S.

(Photosensitive drum 102 and optical scanning device 104)
FIG. 2 shows the configuration of the photosensitive drum (102), the optical scanning device (104), and the control section of the optical scanning device (104). Note that since the configurations of the photosensitive drums and optical scanning devices for each color are the same, the subscripts Y, M, C, and Bk indicating the colors will be omitted in the following description.

The optical scanning device (104) includes a multi-beam laser light source (201) that generates a plurality of laser beams (light beams), a collimator lens (202) that shapes the laser beams into parallel beams, and a collimator lens (202) that passes through the collimator lens (202). It includes a cylindrical lens (203) that focuses the laser beam in the sub-scanning direction (direction corresponding to the rotation direction of the photoreceptor) and a polygon mirror (rotating polygon mirror) (204).

In this embodiment, a multi-beam light source in which a plurality of beams are arranged is used as an example of the laser light source (201), but the same operation is performed when a single light source is used. The polygon mirror (204) consists of a rotating motor and a reflecting mirror attached to the motor shaft. In this embodiment, there are five mirrors, but other numbers may be used.

Furthermore, a beam detector (207) is a signal generating means that detects the laser beam deflected by the polygon mirror (204) and outputs a horizontal synchronization signal (hereinafter referred to as BD signal) in response to the detection of the laser beam. (hereinafter referred to as BD207). The laser light emitted from the optical scanning device 104 scans and exposes the photoreceptor drum 102. The optical scanning device (104) and the photoreceptor drum (102) are positioned so that the scanning direction of the laser beam is parallel to the rotation axis of the photoreceptor drum (102). Each time the mirror surface of the polygon mirror (204) scans once on the photosensitive drum, scanning lines corresponding to the number of laser elements of the multi-beam laser are simultaneously formed.

Next, the control unit (CPU 303) of the optical scanning device (104) will be explained. Image data is input to the CPU (303) from an image controller (not shown) that generates image data, and the CPU (303) inputs image data to the BD sensor (207), memory (302), laser drive unit (304), polygon mirror drive unit (305), and the like. Connected.

(polygon control)
The CPU (303) detects the writing start position of the scanning line based on the BD signal output from the BD sensor (207), and detects the rotation speed of the polygon mirror (204) by counting the time interval of the BD signal. Then, the controller instructs the polygon mirror drive unit (305) to accelerate and decelerate so that the polygon mirror reaches a predetermined speed. The polygon mirror drive section (305) supplies a drive current to the motor section of the polygon mirror in response to the input acceleration/deceleration signal to drive the motor.

(image control)
Further, the CPU (303) converts image data from the image controller into a PWM signal according to the flow shown in the block diagram of FIG.

The main scanning counter (703) shown in FIG. 10, which is reset for each BD signal, counts each pixel and outputs a count value. The magnification table (707) has a table in which magnifications to be corrected for each image height are prepared in advance, and a pixel size calculation unit (708) calculates the pixel size based on the magnification output from the magnification table (707). Then, upon receiving the count value of the main scanning counter (703), a corresponding pixel size is read out from a gradation characteristic table (705) prepared in advance by a gradation characteristic selector (706) and output.

In this embodiment, the pixel size ranges from 24 to 32, and gradation characteristics 1 to N (N=9) are associated with each pixel size. For example, if the pixel size is 24, the gradation characteristic is 1, if the pixel size is 25, the gradation characteristic is 2, and thereafter, each time the pixel size is increased by 1, the gradation characteristic number is incremented by 1.

A gradation characteristic table (705) is also prepared according to the amount of light. Details will be described later together with the light amount calculation unit (709).

As shown in FIG. 12, pixels are shown on the horizontal axis, with pixels divided into 24 at both ends in the main scanning direction of the horizontal axis, and 32 at the center. The pixels in between are set by changing the number of divisions according to the characteristics of the optical system. Note that the number of divisions is simplified for ease of viewing in the figure.

The gradation characteristic is a profile that associates the input pixel value with the output density, and can be realized using a table, a function, or the like. Since this embodiment uses PWM output, the gradation characteristics are expressed by a bit pattern table. Examples of gradation characteristic 9 and gradation characteristic 1 are shown in FIGS. 11(a) and 11(b), respectively.

In FIG. 11, one column on the left shows the input gradation, and each row corresponding to each input gradation shows the PWM ON/OFF pattern as 1/0. In FIG. 11(b), since the register has 32 patterns for each gradation, the 24th to 31st rows are not used and are therefore filled with 0. 13(a) and (b) show the gradation characteristics (a) and (b) in FIG. 11 in terms of input gradation and pulse width. In this example, the gradation characteristics can be approximated even with different numbers of divisions. It is set so that

The gradation characteristic selector (706) in FIG. 10 selects and outputs gradation characteristics 1 to N, respectively, according to the input pixel sizes 24 to 32. PWM conversion (701) outputs a PWM bit pattern according to a table selected for each pixel by a gradation characteristic selector (706) according to the gradation of each pixel.

The parallel/serial converter (702) extracts the PWM bit pattern output by the PWM converter (701) by a certain number of bits (24 bits in this example), converts it into a serial signal, and outputs the serial signal to the laser driver (304). .

For example, when consecutive pixels have pixel size = 32, 24, 24 and gradation = 10, 1, 5, the gradation characteristic selector (706) will set gradation characteristic 9 (FIG. 11(a)), gradation characteristic 1 (FIG. 11(b)), gradation characteristic 1 is selected in order.

The PWM conversion (701) outputs PWM data of the corresponding gradation, and the parallel/serial conversion (702) converts it into a serial string as shown in Figure 16 and outputs it as a PWM signal with 1 as High and 0 as Low. do.

Run length may be used to indicate the gradation characteristics, for example, the method shown in FIGS. 9(a) and 9(b) may be used. In FIG. 9, the left column shows the input gradation, the W column shows the length of the leading white of PWM white→black→white, and B shows the length of black.

W' indicates the length of the trailing white, but it does not exist as a register, and is determined by W' = pixel size - WB. Incidentally, if it is determined in advance that the white width of each of the PWM white → black → white is divided into two equal parts, then only the relationship between the black width and the input pixel value as shown in FIG. 16 is sufficient. PWM conversion (701) converts a combination of white and black run lengths into a bit pattern and outputs it to parallel/serial conversion (702).

In this embodiment, the pixel size table (704) and gradation characteristics (705) are stored in a hard disk (not shown), and the CPU 303 copies them to the memory (302) at startup, and stores them in the memory (302) during image processing. 302) for high-speed processing.

(gradation characteristics according to the amount of light)
As described in the above section (Linearity of integrated light amount with respect to PWM light emission), there is a problem that the integrated light amount with respect to PWM pulse duty differs depending on the laser chip surface light amount. For example, considering the gradation characteristics of PWM with 32 divisions per pixel used at the central image height, when the horizontal axis represents the number of input gradations and the vertical axis represents the pulse width, as shown in FIG. 13(a), Based on the relationship between the PWM duty and the integrated light amount, gradation characteristics are prepared when the light amount is 50% to 100% so as to be linear. Similarly, the PWM gradation characteristics for one pixel divided into 24 are prepared as shown in FIG. 13(b).

For example, assuming that the laser chip surface light intensity is 100% at an image height of ±150 mm and 80% at the center image height, the gradation characteristic at an image height of ±150 mm is the 24-divided input gradation shown in Figure 14. The characteristics of 100% light amount are used in the relationship between and pulse width. Further, at the central image height, a characteristic of 80% light amount is used in the relationship between the input gradation divided into 32 divisions and the pulse width.

The actual number of divisions in one pixel is 24 to 32 divisions that exist continuously at image heights of -150 mm to +150 mm as shown in Figure 5, and the amount of light for each pixel is approximately 24 to 32 divisions with respect to the central image height as shown in Figure 6. The amount of light changes continuously up to 1.3 times. Furthermore, the amount of light at the center image height is also adjusted according to process conditions. Therefore, the number of gradation characteristic tables required is equal to the number of divisions for each image height and the amount of light. Therefore, the gradation characteristics for each amount of light may be calculated using a mathematical formula depending on the amount of light.

(Image control according to the amount of light)
The aforementioned light amount calculation section (709) and gradation characteristic table (705) will be explained with reference to FIG. As described in the section (Light Amount Control), the light amount calculation section has a function of changing the chip surface light amount for each image height so that the light amount per unit area on the drum surface is constant. It also has a function to change the amount of light required for the photoreceptor drum depending on process conditions. For example, if 60% of the light amount is required at the photoreceptor drum center image height, and if the image height is ±150 mm, the light amount required is 1.3 times the center image height, then the required light amount is 78%. Become. The required light amount value is sent to the gradation characteristic selector (706). A gradation characteristic selector (706) selects a gradation characteristic table (705) based on image height in consideration of the amount of light. For example, gradation characteristic tables (705) are prepared in increments of 1% light intensity, and the gradation characteristic table (705) is selected according to the light intensity at each image height. Alternatively, in order to reduce the amount of tables, calculation may be performed using an approximation formula such as linear interpolation from the 100% gradation characteristic value and the 50% gradation characteristic value.

In the above embodiment, the maximum number of divisions was set to 32, but the same implementation is possible even when there is a larger number of divisions using techniques such as digital control using DLL (Delay Locked Loop).

102 Photoreceptor 201 Light source 204 Rotating polygon mirror 706 Gradation characteristic determining means 707 Magnification table 708 Arithmetic unit

Claims (5)

An image forming apparatus comprising a rotating polygon mirror that deflects a light beam emitted by a light source, and scanning a photoreceptor in a main scanning direction with the light beam deflected by the rotating polygon mirror ,
a magnification table storing correction magnifications according to scanning speeds in the main scanning direction ;
a pixel size calculation unit that determines the size of one pixel by determining the number of pixel pieces included in one pixel based on the magnification table;
A first gradation characteristic corresponding to the first number of pixel pieces is selected for a first pixel that is scanned in the main scanning direction at a first speed and is composed of a first number of pixel pieces. However, for a second pixel that is scanned at a second speed slower than the first speed and is composed of a second number of pixel pieces that is larger than the first number, the second number of pixel pieces is gradation characteristic selection means for selecting a second gradation characteristic corresponding to the pixel piece;
gradation conversion means for converting the pixel value of each pixel into a gradation characteristic selected for each pixel, and outputting the pixel size determined for each pixel;
An image forming apparatus comprising : 2. The image forming apparatus according to claim 1, wherein the relationship between the cumulative light amount of the laser and the PWM duty is used to correct the gradation characteristics of each magnification. The first gradation characteristic is a table derived from the relationship between the size of the first pixel and the amount of light ,
The second gradation characteristic is a table derived from the relationship between the size of the second pixel and the amount of light;
The image forming apparatus according to claim 1, characterized in that: The first gradation characteristic is a relational expression derived from the relationship between the size of the first pixel and the amount of light ,
The second gradation characteristic is a relational expression derived from the relationship between the size of the second pixel and the amount of light,
The image forming apparatus according to claim 1, characterized in that: The image forming apparatus according to claim 1, wherein the second pixel is closer to the center of the photoreceptor than the first pixel in the main scanning direction.

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