JPS593457A - Control system of image density - Google Patents

Abstract

PURPOSE:To stabilize the density and gradation of a toner image, by setting the potential in the dark part of a V-D curve at a prescribed value in accordance with the measurement of the potential of a latent image, and controlling the inclination of the V-D curve in accordance with the measured density of the developed image. CONSTITUTION:A reference latent image is formed on a photoreceptor 1 by flickering of a blank exposing lamp after prescribed preparative operations upon turning on of a main switch. Both surface potentials in the bright and dark parts are measured alternately with a surface electrometer 7, and a primary electrostatic charger 3 is controlled to obtain the initially set surface potential. The standard white plate on the original plate is irradiated, and the quantity of the light of a lamp for illuminating the original is so controlled that the potential of the electrometer 7 attains 0V. The surface potential on the drum is measured and the potential for the DC component of a developing bias is determined. The potential for said DC component is applied on the drum to develop the latent image in the dark part and the density of the toner is measured with a detector 9 for reflection density, whereby the adequate surface potential in the dark part is determined. The similar control is successively accomplished in accordance with the surface potential determined in the above-mentioned way to control the height and inclination of the V-D curve, whereby the image having good density and gradation is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image density control system for an image forming apparatus such as an electrophotographic apparatus that forms and develops a latent image on an image carrier.

Conventionally, in this type of image forming apparatus, a reference latent image is formed on a photoreceptor, its surface potential is measured, and the charger output for forming the latent image and the original image exposure amount are adjusted according to the measured value. Alternatively, an image forming apparatus has been proposed in which stable images are obtained by controlling factors involved in image formation, such as developing bias voltage.

In such an image forming apparatus, the charger output is the difference in potential between the exposed area of the photoreceptor/body surface that is irradiated with light (hereinafter referred to as the bright area) and the unexposed area that is not irradiated with light (hereinafter referred to as the dark area), i.e. , which affects the contrast of the latent image, and development/earth effects which affect the degree of background fog in the resulting image.

However, even in an image forming apparatus in which the surface potential of the dark area of the photoconductor is kept constant through such photoconductor surface potential control, differences in toner, toner deterioration, humidity, It is not possible to provide a constant image density due to changes in the environment such as temperature. Among these, changes in image density due to changes in the amount of toner deposited on the photoreceptor during development pose a larger practical problem than changes in image density due to transfer.

Specifically, the relationship between the image density and the surface potential of the photoreceptor is V
The reason why the image density changes under the same surface potential condition is that the V-D cart changes due to the environment, toner deterioration, and other changes in the above-mentioned conditions. It is.

Even if the dark area surface potential is appropriate, if the actual dark area surface potential is too low, a phenomenon will occur in which the image becomes thin, and if it is too high, a sharp image with thick lines will not be obtained. Further, if the slope is different from the proper V-D curve, the gradation of the formed image cannot be changed properly. FIG. 1 is a V-D curve diagram showing the relationship between surface potential and image density due to the above-mentioned environmental changes.

In view of the above points, the present invention eliminates the above conventional drawbacks,
The purpose of this is to provide an image density control method for an image forming apparatus that constantly stabilizes the image density, and the dark area potential of the V-D curve is set to a predetermined value based on latent image potential measurement. By controlling the slope of the V-D curve based on the developed density measurement, the density and gradation of the toner image are stabilized and a good image is always provided.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a sectional view of the vicinity of a photoreceptor in an example of an electrophotographic copying apparatus to which the present invention can be applied. The surface of the drum 1 is composed of a three-layer seamless photoreceptor consisting of a conductive layer, a CdS photoconductive layer, and a surface insulating layer.

The drum 1 is rotatably supported on a shaft and rotated in the direction of the arrow by a main motor activated by closing the copy key. When the drum 1 rotates by a predetermined angle, the original placed on the original platen glass is illuminated by the illumination lamp, and the reflected image is formed into an image 4' on the drum 1 by the lens at the image exposure section.
do. Prior to this, the surface of the drum 10 is simultaneously neutralized by the pre-exposure lamp 2 and the pre-AC charger 3', and then corona-charged to a constant polarity (for example, positive) by the charger 3. Thereafter, the drum 1 is exposed to a reflected light image 4' of the document irradiated by the illumination lamp as described above in the image exposure section. At the same time, AC or corona charge removal with a polarity opposite to that of the secondary charge (in this case, for example, negative) is performed using a static eliminator (secondary charger) 4, and after that, the entire surface of the photoreceptor is uniformly exposed using an exposure lamp 6. , forming a high contrast electrostatic latent image on the surface of the drum 1. In the figure, reference numeral 5 indicates a blank exposure lamp for erasing black between document portions. The electrostatic latent image on the photosensitive drum 1 is then developed by a developing device 8 and visualized as a toner image. On the other hand, the transfer paper in the paper feed cassette is transferred to paper feed row 2.
It is sent into the machine by the registration metal roller 210 and transferred to the photosensitive drum 1 through the guide 11 so that the leading edge of the latent image on the photosensitive drum 1 and the leading edge of the transfer paper are aligned at the transfer section with accurate timing. sent to.

Next, while the transfer paper passes between the transfer charger 12 and the drum 1, the toner image on the drum l is transferred onto the transfer paper. After the transfer is completed, the transfer paper is separated from the sheets 2 and 1 by a separation roller 13, sent to a conveyance roller, fixed by pressure and heat in a fixing device, and then discharged to a tray. After the transfer, the drum 1 continues to rotate, its surface is cleaned by the cleaning device 14, and the process proceeds to the next cycle.

FIG. 3 shows the developing device used in the above illustrated example arrangement.

The developing device 8 has a permanent magnet roller 82 disposed inside a non-magnetic sleeve 81. 83 is a developer hopper, 84 is a one-component magnetic developer, and 85 is a magnetic blade. Reference numeral 86 denotes a power source that applies a frequency i voltage, which will be described later, between the sleeve 81 and the photosensitive drum 1. developer hotuno f-83
The developer inside is conveyed by a sleeve 81, and the coating thickness is regulated to, for example, about 100 μm by a magnetic blade 85 disposed close to the sleeve, and then conveyed to the photoreceptor development area 70. The sleeve 81 and the photosensitive drum 1 rotate at a constant peripheral speed in the directions of the arrows. Also, the gap between the two is approximately 300
Janging development is performed in which the developer is maintained at approximately μ and the developer is selectively applied only to the image area without contacting the non-image area. In the illustrated device, the frequency of the alternating voltage can be changed from 100 Hz to 1000 Hz using a power source 86.

When the frequency of this alternating voltage is changed, γ changes under high humidity and low humidity environments as shown in FIGS. 4 and 5. FIG. 4 shows changes in high humidity, and FIG. 5 shows changes in low humidity. By utilizing this phenomenon, the deviation of γ from the ideal value due to environmental changes as shown in FIG. 1 can be corrected by changing the frequency of this alternating voltage. At this time, in order to prevent a foggy image and obtain a stable toner image, development may be performed by applying an appropriate DC voltage to the developing sleeve. Next, a control mechanism for switching the frequency will be described.

In FIG. 2, a surface potential side 7 for measuring the surface potential of the photoreceptor is provided close to the surface of the drum 1 between the full-surface exposure rung 6 and the developing device 80, and also on the surface of the photoreceptor drum 1 after development. A reflection density detector 9 for measuring the density of the toner image is provided close to the surface of the drum l between the developing device 8 and the transfer charger 120.

The reflection density detector 9 irradiates the photosensitive drum surface with a lamp and receives the diffusely reflected light generated by the irradiation with a light receiving element such as a photodiode. The output characteristic of the light receiving element with respect to the density of this toner image is approximately linear.

FIG. 6 is a block diagram illustrating a control mechanism based on detection signals of these detection means. 15 is a potential measuring circuit connected to the surface electrometer 7, and 16 is a concentration measuring circuit connected to the concentration detector 9. The output ends of these circuits are connected to the control circuit 17.

The control circuit 17 is configured to provide control signals to the high voltage transformer 18, the developing bias circuit 3 connected to the developing bias circuit 19, and also to each process means (not shown), if necessary.

In the image density control method of the present invention, as a cycle executed prior to the copy cycle (hereinafter referred to as pre-rotation), the drum 1 is rotated and the pre-exposure lamp 2, the pre-AC static eliminator 3', etc. There is a step of erasing residual charges and memory of the drum 1, and cleaning the surface of the drum 1 with a cleaning device 14. This is drum 1
This is to ensure appropriate sensitivity and to form an image on a clean drum surface. The pre-rotation time (rotation speed) is automatically changed according to various conditions. Also, as a cycle after the copy cycle for making copies of the set number of sheets is completed, the drum 1 is rotated several times to remove residual charges and memory from the citram 1 using an AC charger 4 or the like, and the drum surface is cleaned by a cleaning device 14. There is a cleaning step. Hereinafter, this will be referred to as post-rotation. This is to leave the drum 1 electrostatically and physically clean.

Next, the basic sequence of operations based on the control method of the present invention will be explained in the order in which the operations are performed.

■Forward rotation After a predetermined period of time has passed after the main switch is turned on, the dorat starts rotating forward. During the pre-rotation, the pre-exposure lamp 2, the blank exposure lamp 5, and the full-surface exposure lamp 6 are turned on, and each charger 3', 3, 4 for the pre-discharge, primary charge, secondary charge, and transfer charge reservoir described above is turned on. , 12 to electrostatically clean the drum surface, and the cleaning device 14
to prepare for later surface potential control.

■ Controlled rotation (1) In controlled rotation (1), the blank exposure lamp 5 is blinked to form a reference latent image on the photoconductor, and the surface potential of both the bright and dark areas of the photoconductor is alternately measured using the surface potentiometer 7. The corona current of the primary charger (and, if necessary, the secondary charger) is controlled so that the initially set surface potential is obtained based on the measured value.

■Controlled rotation (2) In controlled rotation (2), light from a document illumination lamp (not shown) is emitted from, for example, a standard white plate provided on the document table while the corona current controlled in controlled rotation (1) is flowing. The light is reflected onto the drum, and the amount of light from the document illumination rung is controlled so that the surface potential measured by the surface electrometer 7 at this time becomes Ov. Finally, the light from the document illumination 2 that was controlled in this way is applied once again to the standard white plate, and the surface potential when the reflected light is irradiated onto the drum is measured, and based on the measured value, the DC component of the developing bias is determined. Determine the voltage of

■Controlled rotation (3) In controlled rotation (3), while the corona current controlled in controlled rotation (1) is flowing, the developing bias DC voltage determined in controlled rotation (2) is applied to form a dark electrostatic latent image. After development, the density of the toner image is measured by a reflection density detector 9, and an appropriate dark area surface potential value is determined from the measured value.

(2) Controlled rotation (1') In controlled rotation (1'), the corona current of the charger is controlled in the same way as described above so that the dark area surface potential determined in controlled rotation (3) is obtained.

(2) Controlled Rotation (2') In the controlled rotation (2'), similarly to the controlled rotation (2), the light amount of the original illumination lamp is controlled, and the developing bias DC voltage is determined.

■Controlled rotation (3') In controlled rotation (3'), a controlled rotation (1) applies a controlled corona current, a developing bias DC voltage component determined in (2) is applied, and the above-mentioned surface A standard white plate is irradiated with the original illumination lamp at 1/2 the light intensity of the appropriate exposure amount with the potential being Ov, or with other light intensity as necessary, and the reflected light is exposed onto the photosensitive drum. The body is developed, and the toner density resulting from the development is measured by the reflection density detector 9. Thereby, the frequency of the alternating current component of the developing bias is changed based on the measured value to control the slope of the V-D curve. ◎Repeat the above operations from ■ to ■ as many times as necessary (second step)
In the example shown in the figure, K is applied once) to obtain appropriate image density and gradation. Once this is done, the wait lamp goes out and the copy cycle as previously described can then be performed. After the copy cycle ends, the above-mentioned post-rotation is performed. Each of the above-described controlled rotation operations may be performed when the image forming apparatus starts to be used.

As described above, according to the present invention, V- Since the height and slope of the D curve are controlled, images with stable density and gradation can always be obtained.

[Brief explanation of the drawing]

FIG. 1 is a V-D curve diagram showing the relationship between surface potential and image density due to environmental changes, FIG. 2 is a cross-sectional view of the vicinity of the photosensitive drum in an example of an electrophotographic copying apparatus embodying the present invention, and FIG.
The figure shows the developing device used in the apparatus shown in Figure 2, and
Figure 4 and Figure 5 are diagrams showing changes in image density due to changing the frequency, Figure 4 is a diagram showing changes in high humidity, Figure 5 is a diagram showing changes in low humidity, and Figure 6 is a block diagram showing the control mechanism. It is. 1... Photosensitive drum 2... Pre-exposure rung 3'
...Front AC charger 3...Primary charger 4...
・Secondary charger 4'...Original image exposure 5'...
Blank exposure lamp 6...Full exposure lamp 7...
Surface electrometer 8...Developer 81...Nonmagnetic sleeper 82...Permanent magnet roller 83...Hora/
4'-84...-Component magnetic developer 85... Magnetic blade 86... Power supply 9... Reflection density detector 1
0...Register roller 11...Guide 1
2...Transfer charger 13...Separation row 2 14.
...Cleaning device Figure 1 Surface potential (V) Figure 4 Surface potential (V) Surface potential (V)

Claims (1)

[Claims] In the image density control system of an image forming apparatus that forms and develops a latent image on a large carrier, the dark area potential of the V-D cart is set to a predetermined value based on the latent image potential measurement, and the V
- An image density control method characterized by controlling the slope of the D curve.