JPH0425546B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrophotographic apparatus using a reversal development method, and more particularly to an electrophotographic apparatus having an image adjustment mechanism.

Typically, in electrophotographic apparatuses, fogged images or images with low density appear due to deterioration of the photoreceptor or dirt on the exposure optical system. This is a phenomenon that occurs because the latent image potential formed on the photoreceptor deviates from an appropriate value, and the development bias voltage that had been appropriate until then is no longer sufficient. When a photoreceptor becomes fatigued or deteriorated, the charges existing on the surface of the photoreceptor do not disappear completely and remain even if a sufficient amount of light is irradiated during exposure. Additionally, if the illumination lamps and mirrors of the exposure optical system become dirty with toner, etc., they will not be able to irradiate the photoreceptor with a sufficient amount of light, and in the same way, it will not be possible to completely dissipate the charge. Put it away. Furthermore, the ability to retain charges decreases and the dark potential shifts.

In particular, in electrophotographic equipment using the reversal development method,
Since the dark area potential corresponds to the background area of the document, a shift in the dark area potential causes a fogged image, and a shift in the bright area potential causes blurring, skipping, etc. of the image. In order to solve this problem, a method has been known in which the surface potential of the background portion in the image forming area is detected and a developing bias voltage is applied in accordance with the detected value. However, as the number of prints increases, the photoreceptor deteriorates as described above, and as a result, the potential contrast between bright and dark areas changes. Therefore, if the developing bias is corrected only based on the dark area, which is the background area, there is a drawback that the image density decreases and, conversely, the thin lines become thicker.

SUMMARY OF THE INVENTION In view of the above-mentioned drawbacks, an object of the present invention is to provide an electrophotographic apparatus that is free from background fog, has sufficient image density, and has sharp fine lines.

The present invention will be explained in detail below based on the drawings.

FIG. 1 shows an embodiment of the present invention. here,
A laser beam printer is shown as an electrophotographic device. In the figure, after the photoconductor surface 13 of the photoconductor drum 11 rotating in the direction of arrow a is positively charged by the primary charger 15, an electrostatic latent image is formed on the photoconductor surface 13 depending on whether or not the laser beam 17 is irradiated. do.
This latent image appears as a change in electrostatic potential, and this electrostatic potential can be detected by the potential sensor 19. Since the detection signal 21 output from the sensor 19 is an analog signal, it is converted into a digital signal 25 by an analog-to-digital (hereinafter referred to as AD) converter 23 and then supplied to the microcomputer 30 .

In this way, the surface potential of the portion of the photoreceptor surface 13 of the photosensitive drum 11 that is not exposed to the laser beam 17 is set to the dark area potential _VD , and the surface potential of the portion exposed to the laser beam 17 is set to the bright area potential VD. _L , and these potentials V _D and V _L are detected by the potential sensor 19. These two potentials V _D and V _L are calculated by the calculation unit 3 of the microcomputer 30.
3 to calculate an appropriate developing bias B _DC . Note that the calculation results are stored in a random access memory (hereinafter referred to as RAM) 35 within the microcomputer 30. At this time, both potentials V _D
The values of V _L and developing bias B _DC are given by the following equation (1).

B _DC =V _D −α(V _D −V _L ) (1) Here, α is a constant.

Next, let's examine the validity of equation (1). in general,
If B _DC ≦V _D −100 (V) (2) for the value of the dark potential V _D which is the background potential, a clear white background without fogging can be obtained.
On the other hand, regarding the thickening of thin lines, the condition for no thickening is B _DC -V _L ≦500 (V) (3) with respect to the value of the bright area potential V _L which is the image area potential. Furthermore, regarding solid black density and fine line thinning, if B _DC -V _L ≧300 (V) (4), the density can be reduced and image quality without thinning can be obtained. Now, in this example, the initial dark potential V _D =750
(V), bright area potential (V) _L = 150 (V), appropriate developing bias that satisfies equations (2) to (4) above.
B _DC1 is 350 (V)≦B _DC1 ≦450 (V) (5).

From equation (5), if the developing bias B _DC is fixed at 400 (V), a good initial image can be obtained.

By the way, when the photosensitive drum 11 is continuously rotated, the dark area potential V _D and the light area potential V _L shift as shown in FIG. 2. At this time, if the developing bias B _DC is fixed, fogging will occur due to a decrease in the dark area potential V _D. Therefore, according to the conventional method in which the developing bias B _DC is controlled in accordance with the dark area potential V _D as shown by the dotted line in FIG. 2, no fogging occurs. However, the condition of equation (4) is no longer satisfied, resulting in a decrease in density and thinning of the fine lines. On the other hand, if the developing bias B _DC is controlled by the dark area potential V _D and the light area potential V _L as in the present invention, a good image can be maintained.

In this embodiment, an 8-bit microcomputer 30 is used, and in order to improve calculation accuracy, the coefficient is moved to the denominator and the following equation is used: B _DC =V _D -V _D -V _L /β (6). Here, β is a constant. This is because calculations by the microcomputer 30 are performed using integers from 0 to 255, so the form of equation (6) above is more convenient. Furthermore, in this example, the value of β was determined by experiment and was 2.5, so it was calculated as 5/2. Generally, β takes a value in the range of 2-5.

The developing bias B _DC value calculated by the calculation unit 33 in the microcomputer 30 is once stored in the RAM 35.
The output signal 37 from this RAM 35 is stored in a digital-to-analog (hereinafter referred to as DA) converter 3.
9, it is converted into an analog signal 41. This signal 41
By applying this to the developer 43, the developer 43 reversely develops the photoreceptor surface 13 of the photoreceptor drum 11 with positive toner in accordance with the analog signal value (corresponding to the development bias B _DC ).

FIG. 3 shows another embodiment of the present invention, in which the same reference numerals as in FIG. 1 indicate the same circuit parts. In this embodiment, the dark potential V _D and the light potential V _L shift in a relatively short time, and the contrast -V _D -V _L - does not change in a short time. This example is a so-called NP
This is an electrophotographic apparatus using a process, and a photosensitive drum 51 has a three-layered photosensitive drum surface 53 with an insulating layer provided on its surface. Here, the primary charger 15
The photoconductor surface 53 of the photoconductor drum 51 that is negatively charged is positively charged by the secondary charger 55 and forms a measurable bright pattern on the surface by the laser beam 17 . Next, the potential sensor 19 detects the dark area (portion not exposed by the laser 17) potential V _D and the bright area potential on the photoreceptor surface 53.
Detect V _L. These detected dark area potential V _D and bright area potential V _L are calculated by a calculation unit 63 in the microcomputer 60 to obtain an appropriate developing bias.
B _DC is calculated. At this time, the values of the detected V _D and V _L and the developing bias B _DC are given in the same format as equations (1) and (6), but the values of both the detected potentials V _D and V _L
Since the polarity of is opposite to that in the example shown in Fig. 1, B _DC = V _D + α (V _L − V _D ) (7) B _DC = V _D + (V _L − V _D )/β (8) and Become. Furthermore, the characteristics of this photoreceptor surface 53 are as follows:
As shown in the figure, the dark potential V _D and the bright potential V _L
changes. On the other hand, the contrast −V _D −V _L − is
Contrast - V _D - V _L
- Corrections can be made from time to time as necessary. Therefore, in this embodiment, the correction of the developing bias B _DC by the dark potential V _D and the light potential V _L is performed during the pre-rotation before printing starts, and at this time, the second term on the right side of equation (8) (V _L −V _D )/β is stored in the RAM 65, and based on this value, the dark area potential V _D
A method was used in which only the value in the RAM 65 was detected and the value added thereto was output as the developing bias B _DC value. In other words, the second term (V _L −V _D )/β of equation (8) stores the calculation result of the calculation unit 63 in the RAM 65 .
Detection of the dark potential V _D corresponding to the first term of equation (8) is as follows:
A digital signal 25 based on the potential detected by the potential sensor 19 is introduced. The calculation of equation (8) is performed by the adder 69 based on the digital signal 25 and the output signal 67 from the RAM 65. The added signal 71 obtained by this addition is converted into an analog signal 73 by the DA converter 39, and development is performed by the developing device 43 based on the developing bias obtained thereby.

According to this method, the dark potential between image areas is
Just measure V _D , reduce the paper spacing,
It is possible to simplify the control of the developing bias B _DC at the paper interval. In the case of non-reversal development, the background potential (the potential relative to the white background of the document) must be created at paper intervals, so the above-mentioned method is particularly effective in the reversal development method in which the dark area becomes the white background area.

In addition, in the embodiment shown in FIG.
In the embodiment shown in Fig. 3, S _{e is} used.
A laminated photoreceptor with a three-layer structure in which an insulating layer was applied to the surface of the S _e −T _e photoreceptor doped with T _e was used, but when the material of these photoreceptors is changed, the dark area potential V _D and the bright area potential It is necessary to create a correction formula for the developing bias B _DC , taking into account that the polarity of V _L changes. Of course, the effect of the present invention does not change depending on the polarity, and the present invention can be applied to any type of electrophotographic apparatus using a reversal development method that uses the dark potential V _D area as the background.

As described above in detail, according to the present invention, an electrophotographic apparatus that can obtain high-quality images can be realized by appropriately correcting and controlling the developing bias.

[Brief explanation of drawings]

FIGS. 1 and 3 are block diagrams showing embodiments of an electrophotographic apparatus according to the present invention, and FIGS. 2 and 4 are characteristic diagrams showing changes in potential on the surface of the photoreceptor with respect to the number of rotations of the photoreceptor drum, respectively. be. 11, 51... Photosensitive drum, 13, 53... Photoconductor surface, 15... Primary charger, 17... Laser beam, 19... Potential sensor, 30, 60... Microcomputer, 33, 63... Calculation Part, 3
5, 65... Random access memory, 43...
Developing device, 55... Secondary charger, 69... Adding section.

Claims (1)

[Scope of Claims] 1. A developing bias for developing an electrostatic latent image formed by exposing a photoreceptor to light is set to be shifted by a predetermined amount with respect to the dark area potential of the electrostatic latent image. An electrophotographic apparatus that performs reversal development and includes a charging means, an exposing means, and a developing means, comprising: a detecting means for detecting a bright area potential and a dark area potential of an electrostatic latent image formed on the photoreceptor; and a control means for setting the developing bias based on both the bright area potential and the dark area potential detected by the means, and the controlling means sets the amount of change in the developing bias with respect to the dark area potential according to the change in the bright area potential. An electrophotographic device characterized in that it can be changed.