JPS59136754A - Image forming device - Google Patents

Abstract

PURPOSE:To improve the sharpness of an image and the reproducibility of medium contrast and to obtain specified image quality by detecting the on and off state of the light beam on a photosensitive body and controlling the conditions for forming the image according to the detected values. CONSTITUTION:When potential control is started, residual potential on a photosensitive drum 4 is cleaned, and an electrostatic latent image is formed on the drum 4 via a scanning mirror 2 and an f-theta lens 3 by the laser light emitted from a laser control unit 1. The potentials VA, VB in an intermediate contrast part A and a bright part B are measured alternately by a probe 14. The measured values are inputted via an AD converter to a microcomputer and whether the differences between VA, VB and target values VAO, VBO are within permissible values C1, C2 or not is decided. The differential outputs thereof are supplied via a DA converter to a high voltage control unit 15 and the laser control circuit 1, by which the charging current of an electrostatic charger 6 and the intensity of the laser light are controlled. The sharpness of the image and the reproducibility of the medium contrast are thus improved and the specified image quality is obtd.

Description

[Detailed description of the invention] Technical field be.

In order to obtain the conventional technology, a bright area potential and a dark area potential are formed on the photoreceptor by turning on and off a laser beam, etc., each potential is detected by a sensor, and the laser beam, etc. is activated according to the detected value. A method is used to control the output of Jt or the high-voltage output of the charger to converge to a target bright area potential or dark area potential.

When such a surface potential control method is adopted, there are problems as described below.

That is, when an image is actually output after controlling the surface potential as described above, the laser light intensity or pulse width (hereinafter referred to as laser light intensity) and charging conditions may differ from each other.

This phenomenon will be explained using FIGS. 1 and 2.

Figure 1 (3) shows a background scan method in which the undeveloped area (background area) of a photoreceptor is scanned and exposed with a laser beam, and the photoreceptor surface is scanned by changing the interval between laser on and laser off, which is indicated by diagonal lines. - Shows the laser light intensity distribution during exposure.

Further, FIG. 1(B) shows the electrostatic latent image contrast corresponding to the photoreceptor surface (2) of the laser light intensity distribution shown in the first factor (5).

As shown in FIG. 1(B), when the dot density or line density (hereinafter referred to as dot density) becomes thick (the contrast of the latent image potential decreases).

On the other hand, FIG. 2 shows the relationship between image contrast and dot density (corresponding to MTF), and FIG.

It can be seen that the image contrast decreases as the dot density increases due to the phenomenon shown in (B). The solid curve indicated by the symbol A in FIG. 2 shows the image contrast when the charging condition is constant and the laser beam output is set to 4.5 mW, and the curve B shows the image contrast when the output is set to 3 mW.

When the intensity of the laser light is not appropriate as shown in curve A, the image contrast decreases even more with respect to the dot density. However, with the conventional potential control method as described above, it is difficult to accurately control the intensity of Raded light. This J"L is similar to the surface potential characteristic of the photoreceptor with respect to the laser exposure amount shown in Figure 3. Therefore, the laser diode accuracy determined in this non-linear region is often excessive, resulting in a large drop in image contrast as shown by curve A in FIG.

Purpose: The present invention eliminates the above-mentioned conventional drawbacks and improves image sharpness and halftone reproducibility.

Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 4 (8) explains a latent pattern used for potential control according to the present invention. During potential control, a pattern generator built in the device is used to control the direction of rotation of the photosensitive drum (
A repeating pattern of halftone portions A and white background portions B indicated by diagonal lines is formed. The width of parts A and B is, for example, 6
It is about 0.01 Am.

On the other hand, Fig. 4 (Φ) to ) shows a part of the dot pattern constituting the halftone part A, and B shows the laser output during scanning of the laser beam. The dot density is, for example, 16 dots/square. The D part corresponds to the dark part potential, and the L part corresponds to the bright part potential.

In addition, in Fig. 4(C), the laser output is weakened instead of turning off the laser output shown in No. 41J(B), and that part is indicated by the symbol H, and the part where the laser output is on is indicated by the symbol H. This is an indication. The H portion is at an intermediate potential as described above and the L portion.

In addition, in FIG. 4 (■), the pulse width of the laser output off shown in FIG. 4(B) is set to -, and the off portion is indicated by a symbol, and the on portion is indicated by a symbol.

All of the patterns in FIGS. 4(B) to 4(D) exhibit halftone density after development.

Note that FIGS. 4(B) to 4) show some examples of dot arrangement, and of course many other variations are possible.

Particularly when the dither method is used, the dot pattern of the halftone portion A requires a number of settings such as dot arrangement, dot area ratio, and potential of each dot.

On the other hand, FIG. 5 shows an embodiment of a copying apparatus to which the present invention is applied.

In the figure, the symbol 1 is a laser control unit.
The electrical signal inputted to this is outputted as a modulated laser beam, which is carried and scanned in the longitudinal direction of the photosensitive drum 4 by the scanning mirror 2 and f-θ'' lens 3. Photosensitive drum 4
rotates clockwise in the figure, making it possible to scan the laser beam two-dimensionally.

The photosensitive drum 4 uses an organic photoconductor, and after uniformizing the potential on the surface of the photoconductor with an AC static eliminator 5, it is charged with a charger 6 (→), and then exposed to a laser beam to make the photoconductor static. Forms an electrolatent image.

Next, the electrostatic latent image is visualized by a developing device 9, transferred onto a transfer paper 11 by a transfer charger 10, and then transferred to a fixing device 1.
Fixed by 2. Photosensitive drum 4 without being transferred
The toner remaining on the top is collected in a pile by the cleaner 13.

On the other hand, the probe 14 of the surface electrometer is provided close to the surface of the photosensitive drum 4 at a position after laser beam exposure, and the probe 14 shown in FIG. 4 is formed on the photosensitive drum 4 during surface potential control. The potential of the latent image pattern is detected.

Since the detected potential is an analog value, the control output of )() is converted into a digital signal and input to the potential control microcomputer, and the control output is converted to an analog value again. Input to the unit 15, l't, laser light intensity and charging current of the charger 6 are controlled.

On the other hand, FIGS. 6(8) to (C) are schematic diagrams for explaining potential detection according to the present invention. Showing the distribution, the 6th
Figure [F]) shows a latent image pattern when the photosensitive drum is exposed to a laser beam having the distribution shown in Figure 6 (A). Further, FIG. 6(C) shows the detection output when the latent image pattern shown in FIG. 6(B) is relatively (two-scanned) by the probe 14.

As shown in FIG. 6(C), the probe 14 is as shown in FIG. 6(B).
The potential of the non-exposed portions is not detected individually, but the potential of the non-exposed portions is detected as an average value or a constant value smaller than the average value.

For example, when the potentials of a large area where the laser output is off and on are -750V and -170V, respectively, the dot density is reduced to 1 using the dot pattern shown in Figure 4).
6 dots) / am, the detected average potential is -38
It is 0V.

Of course, the smoothness of this detected value may vary depending on the dot density of the latent image pattern and the response speed of the probe 14, but it is easy to detect the approximate average value of the latent image potential at a dot density of 1 dot/home or less. The response speed of the surface electrometer globe can be adjusted to allow for

In this way, the slope 14 alternately measures the approximate average potential of the A area and the exposed part potential of the B area shown in FIG. 4 (8),
The measured value is input to the potential control microcomputer as described above.

This potential control microcomputer performs calculations according to a predetermined control formula so that the measured potential value converges to a target value. The signal of the calculation result passes through the D/A converter and is supplied to the high voltage control unit and the laser output control circuit in the laser control unit 1, so that the charging current of the charger 6 and the laser light intensity are controlled. Of course, instead of controlling the laser light intensity, the pulse width for turning on and off the laser output may be controlled.

This control is performed prior to exposure of the microscopic image, and the seventh control operation will be explained with reference to the flowchart of FIG.

First, when potential control is started, residual potential is cleaned in step S1 by forward rotation of the photosensitive drum 4.

Next, the process proceeds to step S2, where an electrostatic latent image as shown in FIG. 4 (8) is formed on the photosensitive drum by the initial charging current and laser light intensity, and the probe 14 forms an electrostatic latent image as shown in FIG. 4 (8). The approximate average potential W of the A area shown in is measured.Subsequently, in step S3, the potential Vll of the bright area that is the B area
Measure.

Next, the process proceeds to step S4, where VA and VI are measured.
3 and the target values vAo and VBo are the allowable values C,
, it is determined whether or not it is within C2. Ding, IV
It is determined whether A-VAOl≦C1 and whether Vu-Vuol≦c2 or not.

If the result is a negative determination, the process proceeds to step s5, and the charging current i1 of the charger 6 is changed to the control formula ΔI, =α1 +α2Δ
VB, and then in step S6 the drive current 2 of the semiconductor laser is controlled according to the control formula ΔI2=β1ΔvA
Control according to 1β2ΔVB. Then, the process returns to step S2 and the above operations are repeated.

Further, if an affirmative determination is made in step S4, the control loop is completed and the potential control operation ends.

In this way, the surface potentials of VA and VB are set to predetermined values (the charging current 11 of the second charger 6 and the drive current 12 of the semiconductor laser are set, and the laser beam is scanned based on the set values 11 and I2). A printed image of the original is obtained and processed.

It should be noted that the coefficient α1 of the control equation described above. α2. β1. I2 indicates the slope of the function in each relational expression.

Furthermore, once the currents 11 and I2 reach a predetermined value, they are configured so that they do not increase any further (a two-J Mitsuter circuit operates to protect the high-voltage power supply or semiconductor laser).

Based on the above configuration, the potential control target value for the A area in Figure 4 (8) (the dot density is 16 dots/tomo in the dot pattern in Figure 4 (B)) is -380V, and the potential for the B area is set to -380V. Potential control was performed with the control target value set at -170V. After potential control, the potentials of area A and B are converged to the target value; the potential of the dark area (laser off) is -72
The range of 0 V to -800 V (varied by 2. However, this variation in dark area potential has almost no effect on the image density after development.

This is because the development characteristics tend to be saturated at high potentials.

On the other hand, using the conventional method, the potential control target values for bright and dark areas are set to -Cn 3-170V, -750V, ! :L, potential control was performed. Area A after potential control (Fig. 4 (B)
With a dot pattern of 16 dots/dragon), the potential ranges from -320V to -400V (2 variations.
there was. The image after development had low sharpness and poor midtone reproducibility. Further, even if the potential was controlled, a constant halftone image could not be obtained. Incidentally, in the tree embodiment, the value is not limited to the carbon fi% detected by the surface potential on the photoreceptor. For example, it may be configured to detect the image density after development.

Effects As is clear from the above explanation, according to the present invention (=), the first surface state on the photoreceptor in which the light beam is on and the off part coexist, and the second surface state on the photoreceptor where the light beam is on and off are mixed. Since the system uses a configuration that detects each surface condition and controls image forming conditions according to the detected values, it is possible to obtain images with high sharpness and good midtone reproducibility. Good image quality can be maintained at a constant level.

[Brief explanation of the drawing]

Figure 1 (5) is a diagram showing the relationship between scanning distance and laser beam intensity, Figure 1) is a diagram showing the relationship between scanning distance and potential contrast), Figure 2 is a diagram showing the relationship between dot density and 4(8) is an explanatory diagram of a latent image pattern, and FIG. 4(B) to ) are explanatory diagrams showing different examples of dot arrangement patterns in halftone areas. 5 is a schematic configuration diagram of a copying apparatus to which the present invention is applied, and FIG.
) is a diagram showing the relationship between scanning distance and laser beam intensity, 6th factor [F]) is a diagram showing the relationship between scanning distance and surface potential, and Figure 6 (Q is scanning distance and latent image pattern FIG. 7 is a flowchart explaining the control operation. 1... Laser control unit 4... Photosensitive drum 5
...AC static eliminator 6...Charger 9...Developing device 10...Transfer charger 11, = transfer paper
12... Fixing device 14... Potential tear globe 1 (A) Fig. 2 (B) Fig. 3 Fig. 6 Fig. 7

Claims (1)

[Scope of Claims] (1) In a light beam scanning type image forming apparatus that scans a light beam to form an electrostatic latent image on a photoreceptor and visualizes this electrostatic latent image, the light beam is turned on. The first surface state on the photoreceptor in the part where the light beam is on and the off part are mixed, and the second surface state in the part where the light beam is on are detected, and the image forming conditions are controlled according to the detected values (2). An image forming apparatus characterized in that: 1?- (2) The first surface state is a substantially average potential of an electrostatic latent image on the photoreceptor in which light beam on parts and off parts coexist. The image forming apparatus according to claim 1, characterized in that (4) the image forming conditions are a light beam intensity, a light beam emission pulse width, and an amount of charged electric charge. The image forming apparatus according to item 1. (5) The electrostatic latent image on the photoreceptor, in which light beam on parts and off parts are mixed, is a halftone part of an original image formed by a dithering method. Claim 1 characterized by
The image forming apparatus described in .