JPH03230177A - Image forming device - Google Patents

Abstract

PURPOSE:To control the quantity of developer sticking on an image forming body at a prescribed value even in the case that developing characteristic changes in the lapse of time and to obtain an image having good image quality by changing the gamma characteristic of an exposure device based on data on the number of copied sheets from a number of copied sheets detecting device. CONSTITUTION:A clock from a dot clock generation circuit 218 is delayed by a buffer circuit 217 so as to be inputted in a pulse pattern generation circuit 216, and the pulse pattern which is selectively generated from a pulse pattern selection circuit 219 by a means 400 for detecting the number of copied sheets is amplified 215 so as to be inputted in the negative terminal of a differential amplification circuit 211. A density signal whose pulse width is modulated is inputted in the input terminal of an APC circuit 220, and an output voltage is given to a PWM density signal at the prescribed amplification ratio based on an output signal from a differential amplification circuit 225 in an amplification circuit 221. Thus, a prescribed voltage is impressed on a laser semiconductor 222 so that light may be emitted. An optical semiconductor 223 senses the emitted light and conducts a current corresponding to the emitted light quantity, and then, the terminal voltage of a resistance element 224 is impressed on the positive terminal of the circuit 225. Thus, the differential amplification of a reference voltage and the voltage which is impressed on the positive terminal is performed. The laser semiconductor 224 emits the light with a prescribed laser power.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an image forming apparatus using electrophotography, and particularly to an image forming apparatus that controls the amount of exposure per dot in an image forming process.

[Conventional technology]

In image forming apparatuses using electrophotography, as is well known, a laser exposure device irradiates laser light onto the surface of a photoreceptor drum, which is coated with a photoconductor as an image forming member, to form an electrostatic latent image. The latent image is then developed by a developing device to obtain an image. In addition, in a color image forming apparatus, latent image formation and development are repeated on the image forming body according to the number of colors, and color toner images are superimposed on the image forming body and then transferred to complete the color image. . FIG. 4 is a graph showing the relationship between laser power from a general laser light source and image density. This graph shows the laser power (mW) of the laser light irradiated from the laser exposure device to form a latent image on the image forming body.
) and the amount of developer deposited on the image forming body (mg/cm
This shows the correlation with 2). The amount of adhesion tends to increase as the laser power increases. In addition, the exposure apparatus is equipped with an auto power control circuit that controls the amount of light emitted by the laser semiconductor, which is the light source, to a predetermined value.
The laser beam W) is controlled to be irradiated. In addition, the density of the image formed on the image forming body is set to a predetermined value.

[Problems to be solved by the invention]

However, in the above-mentioned image forming apparatus, the amount of developer attached to the surface of the image forming body changes due to changes over time even if development is performed under the same development conditions. When a latent image is formed on an image forming body using an exposure device, the image density on the image forming body and recording paper changes due to changes in development characteristics over time. There was a problem with that. The phenomenon of change in image density due to changes in development characteristics over time will be described in detail below. FIG. 5 is a graph showing an example of changes in developer over time. In the figure, the vertical axis shows the developer adhesion per unit area of the photoreceptor surface, El (mg/c+n2), and the horizontal axis shows the development potential gap (V). The stamp indicates the developer after making 30,000 copies, and the O mark indicates the new developer. For example, the amount of developer that adheres to the surface of the photoreceptor when developing with a development potential gap of 400 sq. is approximately 0.9 mg/cm2 after making 30,000 copies. The amount of developer that adheres to the surface of the photoreceptor (hereinafter referred to as the amount of adhesion) changes due to the change in the developer over time.Here, the amount of adhesion In other words, when developing under a predetermined development potential gap, the amount of adhesion may decrease or increase due to changes in development characteristics over time. Figure 6 is a graph showing changes in image density over time in a conventional image forming apparatus. In the figure, cases in which the image density decreases and cases in which it increases due to changes in development characteristics over time are shown. The vertical axis is It shows the image density (mg/cm2) on the recording paper, and the horizontal axis shows the number of copies as a measure of developer over time.Curve a shows the increase in adhesion Q (m
g/cm 2) increases, and the curved line indicates a developer whose adhesion amount decreases over time. For curves a and b, the image density gradually changes until about 4,300 copies, and becomes almost flat at about 5,000 copies. If the change in the developer over time is considered by reproducing a fine line on recording paper, the change will be traced as shown in FIG. FIG. 7 is a graph showing the correlation between blurring or skipping of images on recording paper and the number of copies. In the figure, the fine line reproduction of the toner image on the recording paper after fixing is such that the image produced by the developer shown by curve a becomes distorted as the number of copies increases (over time), and the image produced by the developer shown by the curve a There is a problem in that the number of copies increases as the number of copies increases. In view of the above-mentioned conventional problems, the present invention aims to control the amount of developer deposited on an image forming body to a predetermined value even if the development characteristics change over time to obtain high-quality images without skipping or collapse. The object of the present invention is to provide an image forming apparatus.

[Means to solve the problem]

The present invention, which achieves the above object, is an image forming apparatus that exposes images from an exposure device to form an electrostatic latent image on the surface of an image forming body, and develops the electrostatic latent image with a developing device to make it visible. The exposure apparatus is characterized in that the gamma characteristic of the exposure device is changed based on copy number data from a copy number detection means for detecting the number of copies.

[For use]

The third image is a graph explaining the operation of the image forming apparatus of the present invention. In the figure, the first quadrant is a graph showing the characteristics of the developing system, the vertical axis shows the copy density CD, and the horizontal axis shows the pulse width modulation output. The second quadrant shows the relationship between the copy density CD and the original density (original density), and the curve shown in the figure shows the ideal image output, that is, the multi-gradation image density on the recording paper. The third quadrant shows the relationship between the document density OD and the output level of the image scanner, that is, the characteristics of the image scanner. Needless to say, the present invention does not correct the characteristics of the image scanner. The fourth quadrant shows the relationship between the pulse width modulation output and the scanner level of the image scanner, and the gamma characteristics in the present invention. The characteristic curve A2 of the developing system is an ideal one, for example, when a new developer is loaded, and if the gamma characteristic of such a developing system is curve B2, it is the second
An ideal output characteristic shown in the quadrant is given. In other words, the image forming apparatus of the present invention indirectly detects the characteristics of the developing system, for example, curve AI or curve A3, from the copy number data, and changes the gamma characteristic curve to B3 or old according to the change in the characteristics of the developing system. Therefore, the amount of developer adhering to the image forming body can be controlled to a predetermined amount regardless of changes in the characteristics of the developing system.

【Example】

Next, embodiments of the present invention will be described based on the accompanying drawings. FIG. 1 is a schematic configuration diagram showing the schematic configuration of an embodiment of an image forming apparatus of the present invention. In the figure, an image forming apparatus 100 has a function of indirectly detecting, for example, changes over time in characteristics of a developing system based on copy number data from a copy number detection means for detecting the number of copies, and changing the gamma characteristics of the exposure device. It consists of an image reading unit IO1, an exposure device 20, a process unit 30, and a paper feeding unit 40. The image reading unit IO includes a carriage 11 to which a halogen lamp 12.13 and a mirror 14 are attached, a movable mirror unit 15 to which a mirror 16.17 is attached,
Lens barrel 18, color separation filter 19a, CCD 19
The reading section 19 is comprised of a reading section 19 made up of a.b. In the image reading unit 10, reference numeral 90 denotes a document table, and the document placed on the document table 90 is illuminated by halogen lamps 12 and 13 attached to a carriage 11 that slides in the horizontal direction. Movable mirror unit 15 is mirror 1
6.17 is attached, which also slides in the horizontal direction, and in combination with the mirror 14 attached to the carriage 11 guides the optical image of the original to the lens barrel 18. The optical image of the original transmitted by the mirrors 14, 16 and 17 is focused by the lens barrel 18, and transmitted through each of the blue and green filters constituting the color separation filter 19a,
An image is formed on the light receiving surface of the CCD 19b. Therefore, when exposing the surface of one document, scanning by the carriage 12 and the movable mirror unit 15 described above is performed once. In this way, the light image passes through each of the blue, green, and red monochromatic light filters, and each image signal output from the CCD 19b is subjected to aging correction, gradation correction, and dither processing in a signal processing section (not shown). The image data obtained by applying this process is output. The image signal is input to an exposure device 20 that irradiates a laser beam having an emission wavelength of 800 r+m. The exposure apparatus 20 of this embodiment uses a semiconductor laser (not shown)
The laser beam generated is rotated and scanned by a polygon mirror rotated by a drive motor, passes through an Fθ lens, has its optical path bent by the mirror, and is projected onto the circumferential surface of the image forming body 31, which is charged in a negative direction. to form a latent image. A drum-shaped photoreceptor (hereinafter simply referred to as a photoreceptor) that rotates in a direction, a scorotron charger 32 that applies a negative charge to the photoreceptor 31, and developing devices 33A to 33C;
It consists of a static eliminator 34 and a cleaning device 35. The paper feed unit 40 includes a paper feed cassette 41. Paper feed roller 4
2, a registration roller 43, a transfer device 44, a separator 45, a fixing device 46, etc. For example, image exposure is performed using laser light from the exposure device 20, and electrostatic latent images corresponding to each color are formed on the photoreceptor 31. Among the electrostatic latent images corresponding to the respective colors, the electrostatic latent image corresponding to blue is formed by irradiation with laser light modulated by blue data (density data). The electrostatic latent image corresponding to blue is developed by the first developing device 33A, and a first toner image (blue toner image) is formed on the photoreceptor 31. This first toner image is not transferred onto the recording paper P, but is charged again onto the photoreceptor 31 by the scorotron charger 32. Red data (density data) is then irradiated onto the laser to form an electrostatic latent image. This electrostatic latent image
The developing device 33B develops the second toner image (
A red toner image) is formed. Third developing device in the same way as above! 33C to form a third toner image (black toner image), and three-color toner images sequentially stacked on the photoreceptor 31 are formed. These three-color toner images are charged again by a scorotron charger 32 after electricity is removed from the photoreceptor by a charge removal lamp in the same manner, and then transferred onto recording paper P fed from a paper feeder by the action of a transfer device 44. Ru. The recording paper P carrying the transferred toner image is separated from the photoreceptor 31 by the separation electrode 45, conveyed by a guide and a conveyor belt, carried into a fixing device 46, heated and fixed, and discharged onto a paper plate. On the other hand, the photoreceptor 31, which has completed the transfer, is the one that was not used during the toner image formation. After the static electricity is removed by the device 34, the toner remaining on the surface is removed by the blade, fur brush, or magnetic brush of the cleaning device 35, which was released during toner image formation, so that it will not interfere with the next multicolor image formation. be made into The developing devices 33A, 33B, and 33C of this embodiment have similar configurations, and the configuration of the developing device 33A will be described below as a representative. The developing device 33A includes a stirring member, a supply roller, a scraper, a thin layer forming plate, and a developer transport carrier in the developer tank. In order to prevent fogging, a developing bias circuit is provided that applies a DC bias voltage to the sleeve via a protective resistor. The developing bias circuit is an AC bias circuit that supplies AC bias to vibrate between the sleeve and the photoreceptor 31 in the developing area. [
and a high-voltage DC power supply that supplies DC bias. In this way, the developing bias circuit connects the sleeve and the photoreceptor 31.
Since an oscillating electric field is generated between the sleeve and the photoreceptor, the developer particles vibrate between the sleeve and the photoreceptor. A toner image is formed. Note that the developing device 33A~
The development characteristics of 33C are similar to those shown in the first quadrant of FIG. Hereinafter, the configuration and function of the laser light source drive circuit in the exposure apparatus 20 of this embodiment will be explained. FIG. 2 is a diagram applied to the image forming apparatus of this embodiment. In the figure, the number of copies detecting means 400 supplies data on the number of copies after the developer is replaced with new developer to the pulse pattern selection circuit 219 in order to indirectly detect changes in the developer over time. A counter counts the output signal from the passage detection sensor installed in the paper feed system,
Output data indicating this count value. This may be an analog signal. The laser light source drive circuit 200 of this embodiment includes a light emission amount control circuit (hereinafter referred to as A20 circuit) 210 that controls the light emission amount of a laser semiconductor 222 constituting an exposure apparatus to a predetermined value.
and a pulse width modulation circuit (hereinafter referred to as PWM circuit) 22
By operating in combination with 0 and 0, the light emission time per dot from the laser semiconductor 222 is controlled, thereby increasing the image density of the intermediate tone by 3[＾≠, so the PWM circuit 210 has gradation information. This is a circuit that outputs digital data (referred to as a density signal) of a predetermined number of pixels, and is particularly equipped with a pulse pattern selection circuit 219, and also includes a differential amplifier circuit 211, a D/A conversion circuit 212,
The dot clock generation circuit 218 is a reference circuit. This is a circuit that generates a clock.Since the image forming apparatus 100 of this embodiment superimposes toner images of a plurality of colors on the photoreceptor 31 in a predetermined order, the laser light source drive circuit 200 of this embodiment A pulse pattern generation circuit 21 generates pulse patterns of different frequencies.
6, it is necessary to supply a signal for selecting a color to the pulse pattern selection circuit 219. The APC circuit 220 is a circuit whose purpose is to obtain a stable amount of light emission by controlling the output voltage from the amplifier circuit 222 to a predetermined value and controlling the current flowing through the semiconductor laser 222 as a load to a predetermined value. It consists of an amplifier circuit 2211, an optical semiconductor 223, a resistance element 224, and a differential amplifier circuit 227, which constitute a feedback differential amplification loop. The feedback amplification loop controls the voltage applied to the laser semiconductor 222 as a load to a predetermined control value, and constitutes a photocoupler from the laser semiconductor 222, which is the laser emission source of the exposure apparatus 20, and the optical semiconductor 223. , a resistive element 224 is connected to the anode terminal of the optical semiconductor 223, and the optical semiconductor 223 conducts a current according to the amount of light emitted from the laser semiconductor 222, and the terminal voltage of the resistive element 224 is connected to the positive terminal of the differential amplifier circuit 225. input to the terminal. The output terminal of the differential amplifier circuit 225 is connected to the input terminal of the amplifier circuit 221, and the output terminal of the amplifier circuit 221 is connected to the laser semiconductor 2.
It is connected to the cathode terminal of 22. The operation of the laser light source drive circuit 200 will be described below. In the PWM circuit 210, the digital data generation circuit 2
14 is a circuit that A/D converts image data from an image sensor, a video camera, etc., and outputs digital data of predetermined bits having density information. This digital data may or may not be stored in memory once, but
In this embodiment, the pixel data of one line is continuously output to the line buffer 7213b, and the image data of a predetermined pixel is continuously output to the latch circuit 213a based on the dot clock. The image data from the latch circuit 213a is input with a half cycle delay from the dot clock, and
The /A conversion circuit 212 converts the signal into an analog signal and inputs it to the positive terminal of the differential amplifier circuit 211. On the other hand, the dot clock from the dot clock generation circuit 21g is delayed by the buffer circuit 217 and inputted to the pulse pattern generation circuit 216, and the pulse pattern selectively generated by the selection signal from the pulse pattern selection circuit 219 is inputted to the amplifier circuit 215. do. The pulse pattern is amplified by the amplifier circuit 215 and inputted to the negative terminal of the differential amplifier circuit 2+1. The concentration signal pulse width modulated in this manner is input to the input terminal of the APC circuit 220. In the APC circuit 220, the above-mentioned pulse width modulated concentration signal is outputted by an amplifier circuit 221, which amplifies the concentration signal by a predetermined amplification factor based on the output signal from the differential amplifier circuit 225. As a result, a voltage of a predetermined value is applied to the laser semiconductor 222, a current is conducted to the laser semiconductor 222, and the laser semiconductor 222 emits light. The optical semiconductor 223 is sensitive to this light emission, conducts a current corresponding to the amount of light emission, and applies the terminal voltage of the resistive element 224 to the positive terminal of the differential amplifier circuit 225. Thereby, the differential amplifier circuit 225 differentially amplifies the reference voltage applied to the negative terminal and the voltage applied to the positive terminal. In this way, the amplifier circuit 2 performs feedback differential amplification.
By stabilizing the amplification factor of 22 and controlling the voltage applied to the laser semiconductor 224 to a predetermined value, the laser semiconductor 224 emits light with a predetermined laser power. In this way, the laser light source drive circuit 200 indirectly detects the change in the developing system over time based on the number of copies and corrects the gamma characteristics to calculate the exposure amount per dot obtained from the density signal of the same gradation. It is also possible to control the amount of exposure per dot obtained from the density signal of the same gradation to increase as the number of copies increases. As described above, the image forming apparatus lOO of this embodiment changes the gamma characteristic of the exposure device based on the copy number data from the copy number detection means that detects the number of copies, thereby controlling the change over time of the developer. Accordingly, the amount of developer deposited on the image forming body 31 can be controlled to a predetermined value, so that it is possible to obtain a high-quality image without collapse or skipping.

【Effect of the invention】

As explained above, the present invention provides an image forming apparatus in which an exposure device performs image exposure to form an electrostatic latent image on the surface of an image forming body, and a developing device develops the electrostatic latent image to make it visible. By changing the gamma characteristics of the exposure device based on copy number data from a copy number detection means that detects the number of copies, the amount of developer deposited on the image forming body can be maintained at a predetermined value even if the development characteristics change over time. Accordingly, it has been possible to provide an image forming apparatus that aims to obtain high-quality images without skipping or blurring by controlling the image quality.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing a schematic configuration of an embodiment of an image forming apparatus of the present invention, and FIG. 2 is a block diagram showing a mechanical configuration of a driving circuit for a laser light source applied to the image forming apparatus of this embodiment. Figure 3 is a graph explaining the operation of the image forming apparatus of the present invention, Figure 4 is a graph showing the relationship between laser power from a general laser light source and image density, and Figure 5 is a graph showing the relationship between the developer and the developer over time. A graph showing an example of changes; FIG. 6 is a graph showing changes in image density over time in a conventional image forming apparatus; FIG. 7 is a graph showing the correlation between image collapse or skipping on recording paper and the number of copies. It is.

Claims (1)

[Claims] Copy number detection means for detecting the number of copies in an image forming apparatus that exposes images from an exposure device to form an electrostatic latent image on the surface of an image forming body, and develops the electrostatic latent image with a developing device to make it visible. An image forming apparatus characterized in that the gamma characteristic of the exposure device is changed based on copy number data from.