JPS60178469A - Electrophotographic control method - Google Patents

Abstract

PURPOSE:To correct and control automatically the density of a formed picture in accordance with the state of an original picture by controlling variably the quantity of illuminating and exposure light of the original or development conditions on a basis of a fundamental characteristic curve determined in accordance with the classification of the original. CONSTITUTION:Frequencies Ndc, potential differences Vac, and DC potential differences Vdc of AC biases for diazo, photographic, and document originals are arranged into a table 1 by characteristic curves selected by switches 291-293. For example, in case that the diazo original is copied, the signal which turns on the contact 291 to control the frequency Ndc and the voltage Vac to 1kHz and 1,600V respectively is sent from an operation control circuit 28 to a power source 20, and the signal which controls the lighting voltage of a lamp 11 to 63V is sent from the operation control circuit 28 to a power source 22. A photosensitive body 1 is rotated to start the electrostatic latent image forming operation, and an optical system is scanned to measure an electrostatic latent image potential Vs of the original on the photosensitive body 1 by a potentiometer 25. The operation control circuit 28 calculates the voltage Vdc on a basis of a change rate, which is selected by turning-on of the contact 291, and the electrostatic latent image potential Vs and sends the control signal to the power source 20.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to control of image density in an electrophotographic method for forming an electrostatic latent image on a photoreceptor and developing the latent image to obtain a hard copy.

Conventionally, this type of control was disclosed in Japanese Patent Application Laid-Open No. 50-4285.
As disclosed in Japanese Patent No. 8, No. 54-2134, a light receiving element is provided in the middle of the optical path from the position where the original is exposed to the photoreceptor. The light receiving element measures the density of the original, and the amount of light from the original illumination lamp is adjusted based on the density obtained. For example, when a document has a light color in the background, such as a document obtained by diasocopy or a newspaper document, so-called fog tends to occur in the background of the copy due to the light color. To prevent this, the exposure amount is increased.

However, if such control is performed, in originals with halftones such as photographs, the halftone portions that should originally be reproduced and expressed will be omitted. As described above, since the light receiving element is placed in the middle of the optical path, a portion of the optical path is cut. Furthermore, it is necessary to correct the difference between the spectral characteristics of the light receiving element and the spectral characteristics of the photoreceptor. Since the document characteristics change depending on the temperature and humidity, it is not possible to obtain a good copy image even if only the exposure amount is controlled in a place where the temperature and humidity change drastically.

The present invention was developed in view of these facts, and is an electrophotographic method that can obtain a proper copy image of any type of original.

The types of manuscripts can be roughly divided into the following three types, for example.

(1) The original paper is blurred, such as diazo copy paper or newspaper, and the contrast is low. It is called a diazo manuscript. A sharp copy image is required by increasing the contrast and eliminating capri.

(2) In a document such as a photograph, a large portion of the document image has halftones of black and white (light and dark). Also called a solid manuscript,
Hereinafter, this will be referred to as a photographic manuscript. Copy images require gradation,
Ru.

(3) For general documents, the manuscript paper is clean and white, and the text is dark. It is called a document manuscript. Copied images do not require halftones, but require sharp images with high contrast and no fog.

It is an object of the present invention to provide an electrophotographic method that can be controlled to meet the requirements for these types of originals.

In order to achieve this object, the electrophotographic control method of the present invention manually sorts document images according to their image conditions.
Basic characteristic curves corresponding to each category and different from each other are determined in advance according to the selected document type, and the document illumination exposure amount or development conditions are variably controlled based on the determined basic characteristic curve, and the document is processed. The gist of the present invention is to automatically correct and control the density of an image formed according to the state of the image.

Hereinafter, embodiments of the present invention will be described in detail to clarify the gist of the present invention.

FIG. 1 shows an electrophotographic apparatus for implementing the present invention.

The drum-shaped photoreceptor 1 forms an image while rotating in the direction of the arrow. First, while being exposed by the pre-exposure lamp 9, residual charges are removed by the charge remover 3. +7 there
The surface is uniformly primary charged to 1500 to 2000 V by the primary corona charger 4 applying KV. Next, the light reflected from the original O exposed by the halogen lamp 11, which is an exposure light source, is exposed to -8K light by the optical system IO.
After the charge is removed by a charger 5 applying V, the entire surface is exposed to light by a light source 6 to pick up an electrostatic latent image. The potential of the electrostatic latent image is approximately OV in the bright area of the original image.
Details of the formation of an electrostatic latent image of 0 or more, which is about +500 V in a dark area, are disclosed in Japanese Patent Publication No. 42-23910 and are well known, so a detailed explanation will be omitted.

The electrostatic latent image is visualized with the developing toner supplied from the developing device 13. The toner image is transferred to transfer paper P,
The image is fixed by a fixing device (not shown) to obtain a hard copy. On the other hand, toner remaining on the photoreceptor 1 without being transferred is scraped off by the cleaning blade 8 and cleaned.

A so-called jumping development method is used for development. The one-component magnetic toner in the hopper 15 of the developing device 13 is drawn by the rotating magnetic roller 17 and is deposited near the blade 18 along the surface of the non-rotating non-magnetic sleeve 16 that is loosely fitted onto the roller 17. transported to. Toner is in sleeve 1
6, blade 18, or mutual contact friction. This charged toner forms a thin layer in the gap between the sleeve 16 and the blade 18, and is further conveyed on the surface of the sleeve 16 to a position facing the photoreceptor 1. The surface of the sleeve 16 and the surface of the photoreceptor 1 face each other with a gap therebetween. sleeve 1
6, a biased AC bias as shown in FIG. 2 is applied from a power source 20, and a potential difference is created between the biased AC bias and the electrostatic latent image on the surface of the photoreceptor l. Although a portion of the charged toner is drawn back to the surface potential of the sleeve 16 due to the bias voltage, it jumps to the electrostatic latent image portion on the surface of the photoreceptor 1 where the potential difference is relatively large, and is developed.

The developing conditions are the AC bias waveform shown in FIG. 2, which is determined by the potential difference Vac between the top of the peak and the bottom of the valley, the potential difference Vdc of the biased DC component, and the frequency N ac (=α).

The surface electrometer 25 faces the surface of the photoreceptor 1, and sequentially measures the electrostatic latent image potential Vs as the photoreceptor 1 rotates while scanning along the rotational axis direction of the photoreceptor 1. It is. Note that the electrometer 25 can also sequentially measure the electrostatic latent image potential Vs only according to the rotation of the photoreceptor 1 without scanning the photoreceptor 1 in the axial direction. Electrometer 25
An arithmetic control circuit 28 is connected to the arithmetic control circuit 28 via an amplifier circuit 26 . A switch 29 is attached to the arithmetic control circuit 28 for manually changing the exposure amount and development conditions depending on the type of document. Turn on contact 291 for diazo originals, and turn on contact 292 for photo originals.
When the contact point 293 is turned on, the document original case is selected. The arithmetic control circuit 28 determines development conditions based on the type of document and the value of the surface potential, and sends the signal to the bias power supply 2o.

The voltage #i22 of the lamp 11 is set to voltage V by the calculation control circuit 28.
r is controlled. Then, the document exposure amount is controlled.

An example of implementing the method to which the present invention is applied using the apparatus shown above is as follows.

“Example-1” (1) If you select a diazo manuscript.

Set the AC bias frequency Nac to a high frequency, IKHz. When the charged toner repeatedly adheres and separates between the surface of the developing sleeve 16 and the surface of the photoreceptor 1 due to the bias voltage, the electric field is small in the areas where the electrostatic latent image potential is relatively low (ground force blur areas). , it takes time for the charged toner to reach the surface of the photoreceptor. If the cycle time T of the AC bias is short (if the frequency Nac is high), the charged toner will not reach that part. Therefore, as shown in FIG. 3, the so-called γ (the slope of the curve of image density versus electrostatic latent image potential) becomes steep. If γ is steep, as shown in Figure 5, the electrostatic latent image potential of the dark image part of the original will be a high potential A, and the slightly thinner part (for example, characters written in pencil) will be correspondingly lower. The potential becomes B, but the densities of the copied images become A1 and Bl, respectively, and both become darker. Fog in relatively thin parts of the original (ground blur parts) disappears.

The potential difference Vac of AC bias is relatively high 1800cm
Make it. The electric field between the surface of the photoreceptor 1 and the surface of the developing sleeve 16 increases, and as shown in FIG. 4, the image density increases. The black of the copied image can be made darker and clearer.

The potential difference Vdc of the DC component of the AC bias is changed in proportion to the background potential Vs (the minimum value of the electrostatic latent image potential or an integral value close to it) measured by the electrometer 25 (Vdc
= aVs). The rate of change is shown by straight line a in FIG.

The voltage V of the lamp power supply 22 is set to 83V.

(2) If you select a photo manuscript.

The AC frequency Nac is set to a low 400 Hz. If the frequency Nac is low, the period time T becomes long, and the electric field is maintained for a sufficient period of time for the charged toner to arrive.

The density of the copied image changes in proportion to the electrostatic latent image potential, and γ becomes gradual (see FIG. 3).

As shown in FIG. 6, the electrostatic latent image potential AB is faithfully reproduced as the copy image density A2 and B2.

The voltage Vac is set to a rather low value of 1300V. This is because the black of the photographic original is sufficiently deep.

The potential difference Vdc is changed proportionally to the potential of the background portion measured by the electrometer 25. In this case, since there are many intermediate tones, the measured potential Vs tends to be high. In order to correct this, the seventh
As shown by the straight line in the figure, the rate of change of the potential difference Vdc with respect to the measured potential Vs is made smaller than that of the diazo manuscript reference document (change straight line a).

In order to increase the exposure amount of the original, the voltage Vr of the power supply 22 is increased to 70V.

(3) When a document manuscript is selected.

The AC frequency Nac is set to a high IKHz. This is because there is no need to copy halftones.

The potential difference Vac is set to a rather low value of 1300V. This is because the black is sufficiently dark, just like a photographic original.

The potential difference Vdc is changed proportionally to the potential of the background portion measured by the electrometer 25. Since the halftone of the background area does not increase the measured potential, the rate of change is the same as that of the diazo original (change straight line a in Figure 7).

The voltage Vr of the lamp power supply 22 is set to 63V.

Table 1 summarizes the above selections.

Table 1 Among (1) to (3) of this "Table 1", for example, (1) the case of a diazo original is selected and copied according to the following procedure.

■Turn on contact 291 of switch 29. Frequency Nd
The signal that controls c to lKH2 and the voltage Vac to 16OO
A signal to control the V is sent from the arithmetic control circuit 28 to the voltage
sent to. Further, a signal for controlling the lighting voltage of the lamp 11 to 63V is sent from the arithmetic control circuit 28 to the power supply @22. ■Set the diazo original on the original table (not shown). ■Turn on the copy start button (not shown). (2) The photoreceptor 1 rotates and the electrostatic latent image forming operation begins. Optical system (lamp 11
and lens lO) to form an electrostatic latent image. Note that this latent image is not developed (idle rotation).

(2) The electrostatic latent image potential Vs of the original formed on the photoreceptor l is measured with an electrometer 25. ■ In the arithmetic control circuit 28, the contact 29
The voltage Vdc is calculated from the rate of change a selected when the switch 1 is turned on and the electrostatic latent image potential Vs, and a control signal is sent to the power source 20. (2) While the electrostatic latent image formed during idle rotation is erased, the electrostatic latent image is formed, developed, transferred, fixed, and ejected in the usual manner to complete copying. At this time, copies are made under the exposure amount and development conditions according to the above control.

"Example 2" When changing the potential difference Vdc of the DC component of the AC bias in proportion to the measured potential Vs, the rate of change a is constant regardless of the type of document. That is, in Table 1, Vdc in the column (2) Photo original changes to aVs. The other developing conditions, exposure amount, and operation order are the same as in "Example-1".

"Example-3" In "Example-1" and "Example-2", the idling operation of the photoreceptor l of [Corporation] ■ is omitted. While forming an electrostatic latent image for copying, the electrometer 25 measures the electrostatic latent image potential Vs. The operating time of idle rotation can be saved, but more accurate control can be achieved by measuring after idle rotation.

"Example 4" This example is an example in which the exposure amount is controlled according to the value of the electrostatic latent image potential Vs formed by idle rotation. The developing conditions are selected as shown in Table 2 by operating the switch 29 depending on the type of document. Note that the voltage of the power supply 22 that determines the amount of exposure during idle rotation is controlled to be set to [i3V] regardless of the type of document.

Table 2 The operating procedure is the same as ■ to ■ and ■ in "Example-1", but in ■, the arithmetic control circuit 28 calculates the voltage Vr=f(Vs ) and sends a signal to control the correction to that value to the electric power i22.

"Example-5" The setting value of the voltage of the power supply 22, which determines the exposure amount during idle rotation, is
Change depending on the type of manuscript. Other than that, “Example-4
” is the same. For example, the voltage of the power supply 22 is initially set to 63 V in the case of a photo original or a document original, and is set to 70 V in the case of a photographic original. In this way, depending on the type of document, the control value is set to approximately 1. By setting the % value, the time during control becomes smaller and accuracy improves.

"Example 6" The rate of change when calculating the voltage Vr from the electrostatic latent image potential Vs is changed depending on the type of document. For example, diazo manuscript
To make the rate of change smaller in the case of a photo original than in the case of a document original. Others are "Example-4" or "Example-5"
is the same as

As explained above, according to the electrophotographic method of the present invention, regardless of the type of original, the exposure amount and development conditions are optimally controlled, making it possible to obtain extremely good copied images. .

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of an apparatus implementing the present invention, FIG. 2 is a waveform diagram of developing bias AC, FIGS. 3 to 6 are diagrams explaining the relationship between electrostatic latent image potential and image density, and FIG. The figure is a diagram illustrating the relationship between the measured potential of an electrostatic latent image and the DC component voltage of the deviation/<Iass AC. 1 is a photoreceptor, 11 is an exposure halogen lamp, 13 is a developer,
16 is the sleeve, 20 is a bias power supply, 22 is a lamp power supply, 25 is a surface electrometer, 28 is an arithmetic control circuit, 2
9 is a manual changeover switch. Figure 3

Claims (3)

[Claims] (1) Manually sort original images according to their image conditions, determine in advance basic characteristic curves that correspond to each category and differ from each other according to the selected document type, and use the determined basic characteristic curves. 1. A method of controlling electrophotography, comprising: variably controlling illumination exposure amount or developing conditions of an original based on the above, and automatically correcting and controlling the density of an image formed according to the state of the original image. (2) The electrophotographic control method according to claim 1, wherein the controlled developing condition is a voltage amount of a developing bias voltage. (3) The electrophotographic control method according to claim 1, wherein the controlled developing condition is the frequency of a developing bias AC voltage.