JP2693422B2 - Image forming method and apparatus - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method and an apparatus thereof, and more particularly to an electrophotographic image forming method and an apparatus thereof for faithfully reproducing halftones. [Prior Art] Among image forming apparatuses, a laser printer is well known in which an image is exposed on a photoconductor with a laser and is developed to obtain an image. The laser printer has advantages such as high printing quality and high printing speed, and is widely known as an output device such as a personal computer or a word processor. FIG. 2 is a schematic configuration diagram of the image forming unit of the laser printer. The photoconductor 1 is rotating in the direction of arrow 2, and is first uniformly charged by the charger 3. After that, the signal generator 11
The laser light from the semiconductor laser driven according to the image data 12 output from the image is exposed by the image scanning exposure, which exposes the image area on the surface of the photoconductor and does not expose the non-image area, and is exposed on the photoconductor. An electrostatic latent image is formed. This latent image is developed by the developing device 5, and a toner image is formed on the photoconductor 1. This image is transferred onto the transfer paper 7 by the transfer charger 6.
After being transferred onto a sheet of paper and electrostatically separated by a separation charger 8, the toner is fixed by a fixing device 9 and discharged. Transfer residual toner remaining on the photoreceptor 1 is cleaned by a cleaner 10. [Problems to be Solved by the Invention] However, in an image forming apparatus equipped with such an image forming unit, if a charge amount on the photoconductor changes or sensitivity of the photoconductor changes, a high-quality image is stabilized. It has the drawback that it is difficult to obtain. The present invention has been made in view of the above-mentioned conventional example, in which the density of the formed image is appropriately changed even when the density level of the image signal is slightly changed, and the charging characteristic, the sensitivity characteristic, the developing characteristic, and the like are changed. On the other hand, it is an object of the present invention to provide an image forming method and an apparatus for stably forming a halftone image having excellent gradation. [Means for Solving Problems] and [Operation] In order to achieve the above object, the image forming apparatus of the present invention has the following configuration. That is, a pulse width modulation means for generating a pulse width modulation signal of a predetermined cycle according to an image signal whose density level fluctuates between a maximum value and a minimum value, and a pulse width of the pulse width modulation signal is a density level of the image signal. Is a minimum pulse width that is not 0 when it is the minimum value, and is gradually increased as the density level increases, and when the density level is the maximum value, a maximum pulse width shorter than the predetermined period. Pulse width control means for controlling so that the photoconductor is charged, the photoconductor is exposed based on the pulse width modulation signal to form an electrostatic latent image, and the electrostatic latent image is developed to form an image. Image forming means for forming and a potential information or density information is output by monitoring the surface potential of the exposed photoreceptor or the image density of the image formed based on the pulse width modulation signals of the minimum and maximum pulse widths. It is characterized by having monitor means for applying force and adjusting means for adjusting the image forming condition of the image forming means based on the output of the monitor means. In order to achieve the above object, the image forming method of the present invention includes the following steps. That is, a pulse width modulation step of generating a pulse width modulation signal of a predetermined cycle in accordance with an image signal whose density level fluctuates between a maximum value and a minimum value; An image forming method comprising: forming an electrostatic latent image by exposing a body, and developing the electrostatic latent image to form an image, wherein the pulse width of the pulse width modulation signal is the image. When the density level of the signal is the lowest value, the maximum pulse width is not 0 and becomes longer as the density level increases, and when the density level is the highest value, the maximum pulse shorter than the predetermined period. A pulse width control step for controlling the width of the photosensitive member, and a potential by monitoring the surface potential or the image density of the formed photoreceptor on the basis of the pulse width modulation signals of the minimum and maximum pulse widths. It is characterized by comprising a monitor step of outputting information or density information, and an adjusting step of adjusting the image forming condition of the image forming step based on the output of the monitor step. Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. [Description of Image Forming Unit of Laser Beam Printer (First
[FIG.]] FIG. 1 is a schematic configuration diagram of a laser beam printer according to an embodiment of the present invention. The same portions as those in FIG. 2 are indicated by the same symbols, and the description thereof will be omitted. In the figure, an image processing unit 20 receives digital image data 22 and a reference clock signal 23, and outputs binarized image data 21 which is almost continuously pulse width modulated according to the image density.
The binarized image data 21 is input to the laser drive unit 24, drives the laser 4, and is output as modulated laser light.
As described with reference to FIG. 2, the laser beam from the laser 4 operates on the photoconductor 1 to form a two-dimensional electrostatic latent image as the photoconductor 1 uniformly charged rotates in the direction of arrow 2. obtain. Next, the electrostatic latent image is visualized by the developing device 5 and transferred onto the transfer paper 7. The toner remaining without being transferred is collected by the cleaner 10. After the transfer, the transfer paper 7 is separated from the photosensitive drum, passes through the fixing device 9, and is discharged. Reference numeral 25 denotes a potential sensor, which is provided near the surface of the photoconductor 1 at a position after the laser beam exposure of the photoconductor 1 and detects the potential of the electrostatic latent image on the surface of the photoconductor 1. Potential sensor 25
The output of is input to the potential measuring unit 26 and the potential is measured.
Since the measured potential is an analog value, it is converted into a digital signal by the A / D converter 27 and input to the control unit 28. The control unit 28 includes a CPU such as a microcomputer, a ROM storing a CPU control program and data, a RAM as a work area of the CPU, various I / O ports, a D / A conversion unit, and the like. It controls the entire system and outputs instruction signals to various control units to be described later as analog signals. Reference numeral 29 is a high voltage controller for controlling the charging current of the charger 3, and 30 is a high voltage controller for a grid bias voltage for controlling the grid bias voltage of the charger 3. Reference numeral 24 denotes a laser drive unit that drives the laser 4 and controls the intensity of the laser light.
Reference numeral 31 is a developing bias voltage control unit that controls the developing bias voltage of the developing device 5. [Description of Image Processing Unit (FIGS. 3 and 4)] FIG. 3 is a block diagram of the image processing unit 20, and the same parts as those in FIG. 1 are denoted by the same reference numerals. The digital image data 22 is converted into an analog image signal 203 by the D / A converter 202 and input to one terminal of the comparator 211. Reference numeral 207 denotes a timing signal generating circuit, which inputs the reference clock signal 23 and creates and outputs a screen clock 208 to the pixel clock 204 and the pattern signal generator 209. The pattern signal generator 209 is a screen clock 2
The pattern signal 210 is output based on 08 and is input to the other terminal of the comparator 211. The digital image data 21 is input in synchronization with the pixel clock 204, and the D / A converter 202 outputs an analog image signal 203 in synchronization with the pixel clock 204. Screen clock 2
Reference numeral 08 is a clock signal obtained by multiplying the period of the pixel clock 204 by an integer, and defines the period of the pattern signal 210, which is, for example, a triangular wave. The analog image signal 203 and the pattern signal 210 are compared by the comparator 211.
Are compared and the analog image signal 203 is set to 0 when the analog image signal 203 is larger, and is set to 1 when the analog image signal 203 is smaller. FIG. 4 is a timing chart showing the timing of each part of FIG. Here, the screen clock 208 is a clock having a period twice that of the pixel clock 204. When the digital image signal 21 changes stepwise from 00 (white) to hexadecimal FF (black), it is pulse width modulated by the pattern signal 210.
The pulse waveform of the binarized image data 21 is shown. By changing the amplitude of the pattern signal 210 in this way, the relationship between the input level of the digital image data 22 and the pulse width of the binarized image data 21 can be changed. [Explanation of sensitometry (Fig. 5)] Fig. 5 shows 8-bit digital image data 22 (00 to FF).
Up to 256 data), the image processing unit 20
The pulse width modulation is performed almost continuously by inputting the binarized image data 21 to the image forming unit of the laser beam printer to convert the exposure from the laser light to the surface potential and the surface potential to the density. It is a sensitometry showing the correspondence in case of going. Since the image processing unit 20 was intended to obtain an image output with good gradation, the digital image data
It is desirable that the relationship between the input level of 21 and the density of the reproduced image is linear. However, as can be seen in FIG. 5, the linear part is the part from the triangle (50) to the circle (51) in the figure. Therefore, by setting the input level of the portion of 50 to 00 and the input level of the portion of 51 to FF, it is possible to obtain a good gradation for an 8-bit digital image. The pulse width of the binarized image data 21 is made to correspond to the input level, and the pulse width for the white level (00) is PW0 and the pulse width for the black level (FF) is PW1 as shown in FIG. On the other hand, the surface potential measured by the potential sensor 24 of the photoconductor 1 is V0, and the potential corresponding to the black level is V0. These potentials V0 and V1 are the dark part potential (Vd) in the light-shielded state,
It is often different from the bright area potential (Vl) at full lighting. However, the area used for image formation is the area from the Δ mark (50) to the O mark (51), and V0 and V1 are brought close to the target value to form a stable and high-quality image. [Explanation for Potential Control Operation (FIG. 6)] FIG. 6 is a flow chart of the potential control operation program stored in the ROM of the control unit 28. First, when the potential control is started, the residual potential is cleaned by the pre-rotation of the photoconductor 1 in the direction of arrow 2 in step S1. Then, in step S2, initial values of the charging current of the charger 3 and the laser driving current of the laser 4 are set. In Steps S3 and S4, the laser light intensity initially set
Of the continuous pulse width modulation laser exposure by the binarized image data 21 obtained from the image processing unit 20, the average potentials V0 and V1 of the minimum pulse width exposure unit and the maximum pulse width exposure unit are measured. Then, in step S5, V measured in steps S3 and S4
The difference between 0 and V1 and their respective target potentials V _T and V _T ′ is examined. That is, | V0−V _T | ≦ C ₁ and | V1−V _T ′
｜ ≦ C ₂ is checked and the difference between them is the allowable value C ₁ , C
See if it is within ₂ . If it is not within the allowable value, the process proceeds to step S6, where a control signal is output to the high-voltage control units 29, 30 and the charging current I ₁ of the charger 3 is calculated by the following equation ΔI ₁ = α ₁ · ΔV 0 + α ₂ · ΔV 1 (a) Increase or decrease according to. Semiconductor laser 4 in step S7
A control signal is output to the laser drive unit 24 in order to increase or decrease the drive current of the following formula ΔI ₂ = β ₁ · ΔV 0 + β ₂ · ΔV 1 (b). Here, ΔV0 = V0−V _T , ΔV1 = V1−V _T ′
It is. The steps S3 to S7 will be described in more detail in accordance with the actual control procedure. First, the potential V0 of the exposed portion with the minimum pulse width (PW0) is measured and compared with the target potential V _T. When the difference from V _T is the allowable value C ₁ or more, a control signal is output to the high voltage control units 29 and 30 of the charger 3 to increase / decrease the charging current I _{1 according} to the equation (a). Then the maximum pulse width (PW1)
The electric potential V1 of the exposed portion is measured and compared with the target value V _T ′. When the difference from V _T ′ is at the boundary C ₂ or more, drive current
I ₂ is increased or decreased according to the equation (b). In this way the potential
If both V0 and V1 are within the allowable value, the process proceeds to step S8. In the case of the image scanning exposure method, in step S8, the developing bias voltage is set to V _B = V0−V _BK (V _BK is a necessary voltage value for preventing fog) and the potential control operation is completed. The coefficients α ₁ , α ₂ , β ₁ , β ₂ of the above-described control equations indicate the slopes of the functions in the respective relational expressions. Although the present embodiment has been described based on the laser beam printer using the image scanning method, the present invention is not limited to this, and may be a laser beam printer using the back ground scanning method. Not only that, but it may be a printer using another exposure method, for example, LED or LCD. The image data is halftone image data that is pulse width modulated, but even with halftone image data that uses the dither method or the density pattern method, the minimum and maximum input levels are set to faithfully reproduce the halftone. In some cases, it is not compatible with light blocking of exposure and full lighting, and in that case, the potential control shown in this embodiment is effective for stably obtaining a high quality image. In this embodiment, the process potential to be measured is the surface potential of the photoconductor, but the present invention is not limited to this, and the density of the reproduced image may be measured to control the image forming element of the image forming unit. You may As described above, according to this embodiment, in the image forming apparatus for digital electrophotography, by controlling the surface potential of the photoconductor in accordance with the input level of the image data,
It has become possible to stably obtain high-quality images. [Effects of the Invention] As described above, according to the present invention, the density of a formed image is appropriately changed even with a slight change in the density level of an image signal, and charging characteristics, sensitivity characteristics, development characteristics, etc. There is an effect that a halftone image having excellent gradation can be stably formed against the change.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram of a laser beam printer according to an embodiment of the present invention, FIG. 2 is a schematic view of an image forming unit, and FIG. 3 is image processing according to an embodiment of the present invention. FIG. 4 is a block diagram of a part, FIG. 4 is a timing chart of signals of each part of the image processing part, FIG. 5 is a sensitometry showing correspondence between an input level and an output density, and FIG. 6 is a potential control operation of one embodiment of the present invention. It is a flow chart. In the figure, 1 ... Photoconductor, 3 ... Charger, 4 ... Laser, 5
…… Developer, 6 …… Transfer charger, 7 …… Transfer paper, 8 ……
Separation charger, 9 ... Transfer device, 10 ... Cleaner, 11 ... Signal generation unit, 20 ... Image processing unit, 21 ... Binary image data, 22 ... Digital image data, 23 ... Standard Clock signal, 24 …… Laser driver, 25 …… Potential sensor, 26 …… Potential measuring section, 27 …… A / D converter, 28 …… Control section, 29,30 ……
High voltage controller, 31 ... Development bias voltage controller, 202 ... D
A / A converter, 203 ... Analog image data, 204 ... Pixel clock, 207 ... Timing signal generating circuit, 209 ... Pattern signal generator, 210 ... Pattern signal.

   ────────────────────────────────────────────────── ─── Continuation of front page    (56) References JP-A-60-54567 (JP, A)                 JP-A-55-14728 (JP, A)                 JP-A-49-22806 (JP, A)                 JP-A-54-119241 (JP, A)                 JP 58-14669 (JP, A)                 JP-A-51-13518 (JP, A)

Claims (1)

(57) [Claims] A pulse width modulation means for generating a pulse width modulation signal of a predetermined cycle according to an image signal whose density level fluctuates between a maximum value and a minimum value; and a pulse width of the pulse width modulation signal, wherein the density level of the image signal is the minimum When the value is a value, the minimum pulse width is not 0, and when the density level is increased, the pulse width is gradually increased. When the density level is a maximum value, the maximum pulse width is shorter than the predetermined period. And a pulse width control means for controlling the photoconductor, charging the photoconductor, exposing the photoconductor on the basis of the pulse width modulation signal to form an electrostatic latent image, and developing the electrostatic latent image to form an image. An image forming unit and a potential information or density information is output by monitoring the surface potential of the exposed photoreceptor or the image density of the image formed based on the pulse width modulation signals of the minimum and maximum pulse widths. And Nita unit, the image forming apparatus characterized by having an adjustment means for adjusting the image forming condition of said image forming means based on an output of said monitor means. 2. A pulse width modulation step of generating a pulse width modulation signal of a predetermined cycle in accordance with an image signal whose density level fluctuates between a maximum value and a minimum value; charging the photoconductor; An image forming method comprising: forming an electrostatic latent image by exposure and developing the electrostatic latent image to form an image, wherein the pulse width of the pulse width modulation signal is equal to that of the image signal. When the density level is the minimum value, the minimum pulse width is not 0, and when the density level is the maximum value, the pulse width is gradually increased, and when the density level is the maximum value, the maximum pulse width is shorter than the predetermined period. And a pulse width control step for controlling so that the surface potential of the exposed photoreceptor or the image density formed is monitored based on the pulse width modulation signals of the minimum and maximum pulse widths to monitor potential information or The image forming method characterized in that it comprises a monitor step of outputting the density information, and a adjustment step of adjusting the image forming condition of said image forming step based on the output of said monitor process.