JPH02157771A - Picture quality controller for picture forming device - Google Patents

Abstract

PURPOSE:To improve the tone reproduction of a picture forming device by providing a bias power source which selectively applies a developing bias voltage produced by superimposing a DC and AC voltages upon another or another developing bias voltage of the DC voltage only access a developing device and, at the same time, a means which switches the quantity of exposure light in an interlocking way with the selection of the developing bias voltage. CONSTITUTION:A bias power source 21 which selectively applies a developing bias voltage produced by superimposing a DC and AC voltages upon another or another developing bias voltage of the DC voltage only across a developing device 5 and a means 22a which switches the quantity of the exposure light to a photosensitive body 1 in an interlocking way with the selection of the developing bias voltage are provided. When an original D is, for instance, a character original, the voltage produced by superimposing the DC and AC voltages upon another is used as the developing bias voltage. Therefore, the character of the original D is reproduced with a sufficient density and, since toner is circulated between the photosensitive body 1 and developing device 5, an appropriate edge effect can be obtained and the reproducibility of very narrow lines can also be improved.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an electrophotographic copying machine and a Rede printer.

The present invention relates to an image forming apparatus using electrophotographic technology such as an LED (light emitting, diode) printer, and particularly to a device for controlling image quality after development.

[Conventional technology]

For example, in an electrophotographic copying machine, an electrostatic latent image is formed by exposing a charged photoreceptor to light according to the density of an image on a document. A developed image is obtained by developing this electrostatic latent image with toner or the like.

There are various methods of development, but in the case of dry development, there are two main types: two-component development and one-component development. The two-component development method uses a developer containing a mixture of toner and a carrier for transporting the toner, while the -component development method uses only the toner.

According to the one-component development method, devices for toner concentration adjustment, stirring, etc. are not required, and the cost and size can be reduced. For this reason, in low-cost copying machines, the -component development method is increasingly being adopted.

One of the -component development methods uses magnetic toner and charges the toner by friction. A fixed sleeve made of high-resistance resin is installed around the outer periphery of a rotating magnet roll, and a friction charging blade is brought into contact with the toner moving as it is pulled by the magnet roll to charge the toner. is applied. Then, at the nip between the photoreceptor and the sleeve, due to the potential difference between the sleeve and the photoreceptor,
Development is performed by transferring toner from the sleeve to the electrostatic latent image area on the photoreceptor.

[Problem to be solved by the invention]

Such an electrophotographic copying machine is provided with a mechanism for arbitrarily varying the density of an image by controlling, for example, a developing bias, an amount of exposure light, etc., and the image density can be selected. However, in addition to text-only documents with binary density, there are various types of originals, such as photographs with subtle shading. , it was difficult to obtain images of the desired quality.

On the other hand, it is known that by changing one or both of the image exposure light amount and the charging potential to change the light attenuation curve of the photoreceptor, the gamma characteristics are lowered and good gradation is obtained (for example, (See JP-A-55-7709, JP-A-55-9544, JP-A-56-133746, JP-A-56-133759, etc.). Note that this light attenuation curve shows the exposure light amount vs. photoreceptor surface potential characteristic, and is P I D C (Photo.

(Induced Discharge Curve)

However, in the above-mentioned method, since only changes in the light attenuation curve of the photoreceptor are utilized, there is a limit to the amount of adjustment, and sufficient gradation reproducibility cannot be obtained.

The present invention was devised to solve the above problems, and an object of the present invention is to improve gradation reproducibility by controlling a plurality of development conditions.

[Means to solve the problem]

In order to achieve the above object, an image quality control device in an image forming apparatus according to the present invention is an image forming apparatus that develops a latent image formed on a photoreceptor using a one-component developer. A bias power source is provided for selectively applying a developing bias voltage and a developing bias voltage consisting only of a DC voltage to the developing device, and means is provided for switching the amount of exposure light to the photoreceptor in conjunction with the selection of the developing bias voltage. It is characterized by

controlling means for switching the bias power supply and exposure light amount depending on whether the original is a text original or a photographic original;
For character originals, a developing bias voltage obtained by superimposing DC voltage and AC voltage is used, and for photographic originals, a developing bias voltage of only DC voltage is used, and the amount of exposure light is lowered compared to that for character originals.

[Effect]

In the present invention, when the original is a character original, a superimposed DC voltage and an AC voltage is used as the developing bias voltage. Therefore, development is mainly performed on solid areas based on the potential difference of DC voltage.
Characters are reproduced with sufficient density. In addition, since the toner moves back and forth between the photoreceptor and the developer using AC voltage,
A moderate edge effect is also produced and fine line reproducibility is improved.

Further, when the original is a photographic original, only a DC voltage is used as the developing bias voltage. Therefore, the low density portion does not become excessively dark. At this time, since the exposure light amount to the photoreceptor is lowered, the apparent exposure light amount vs. photoreceptor surface potential characteristic changes, and the T in the gamma characteristic changes.
The value of will be low. Therefore, gradation reproducibility becomes good.

〔Example〕

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, features of the present invention will be specifically described based on examples with reference to the drawings.

FIG. 1 is an explanatory diagram showing a schematic cross section and a control block of an electrophotographic copying machine to which an image quality control device of the present invention is applied.

In the figure, reference numeral 1 indicates a drum-shaped photoreceptor, and around the photoreceptor 1 there is a charger 2. 3. Exposure device consisting of a lens, etc.; 4. Static eliminator; Developing device 5
．． Pre-transfer lamp 6, transfer device 7. Cleaner 8
．． Static elimination lamps 9 and the like are sequentially arranged. Further, a paper feed roll 10 is provided on the input side of the transfer device 70, and a conveyance belt 11 is provided on the output side.

Then, the document placed on the platen glass 12 and covered with the platen cover 13 is irradiated by the exposure lamp 3a, and the light reflected from the document is imaged on the photoreceptor 1 by the exposure device 3. Note that the electrophotographic copying machine of this embodiment is an original moving type, and the original is moved together with the platen glass 12 and platen cover 13. However, in the case of a fixed original type, the exposure device 3 is equipped with movable lenses, mirrors, etc. It consists of a scanning optical system.

The developing device 5 includes a fixed sleeve 52 made of, for example, a high-resistance resin containing phenol resin, carbon, or glass fiber, on the outer periphery of a magnet roll 51 that rotates in the direction of the arrow as shown in FIG. , a frictional charging blade 53 is provided in contact with the sleeve 52.

Further, a developing bias voltage from the developing bias power supply 21 is applied to the sleeve 52 . The developing bias power supply 21 includes an AC power supply 21& and a DC power supply 21b.

In this embodiment, as shown in FIG. 1, the AC power source 21a can be turned on and off from the outside, and the exposure amount of the exposure lamp 3& can be controlled by the exposure lamp power source 23. In addition, these AC power sources 21
a, m! A control circuit 22 for controlling a light lamp power source 23 is provided. Then, this control device 22 turns on the AC power supply 21a when the original to be copied is a normal original, and turns off the AC power supply 21& when it is a photographic original, and controls the exposure lamp power supply 23 to control the exposure lamp 3.
The exposure light amount of a is lowered.

For example, a switch 2 connected to a control circuit 22 for switching between normal mode and photo mode on the console of a copying machine.
2& is provided, and the user presses switch 2 while checking the type of document.
By switching 2a, the developing bias voltage and the amount of exposure light can be switched.

Next, the operation of the electrophotographic copying machine having the above configuration will be explained.

First, the photoreceptor 1 is uniformly charged by the charger 2 . Next, the light reflected from the original is applied to the photoreceptor 1 by the exposure device 3.
An electrostatic latent image is formed on the photoreceptor 1. Next, after the charge on the non-image portion is eliminated by the static eliminator 4, the non-image portion is developed with toner in the developer 5.

After development, the toner image on the photoreceptor 1 is transferred to the paper conveyed by the paper feed roll 10 in the transfer device 7 section, and then conveyed to the fixing device (not shown) by the conveyance belt 11, where the toner image is transferred to the paper. The final copy is obtained.

The toner remaining on the photoreceptor 1 after transfer is collected by a cleaner 8, and the charge remaining on the photoreceptor 1 is removed by a static elimination lamp 9.
The charge is removed by the process, and it is ready for the next exposure process.

Next, the developing mechanism in the developing device 5 will be explained with reference to FIGS. 1 and 2. Note that r+J and r-J in FIG. 2 indicate positive and negative charging states.

In this embodiment, the developing device 5 performs development using a magnetic component developer as the toner T.

The toner T stored in the hopper 5a of the developing device 50 is
It is pulled by the rotating magnet roll 51 and moves along the surface of a fixed, ie, non-rotating, sleeve 520. Then, the position passes between the frictional charging blade 53 and the sleeve 52, forms a thin layer, and faces the photoconductor 1;
That is, it is carried to the nip section. At this time, the frictional charging blade 53 comes into sliding contact with the toner T attached to the sleeve 52, so that the toner T is triboelectrically charged. Then, at the nip between the photoreceptor 1 and the developing device 5, the toner T is transferred from the sleeve 52 to the electrostatic latent image portion of the photoreceptor 1, and development is performed.

Next, the developing bias voltage will be explained.

In the normal mode, a developing bias voltage in which an alternating current voltage is superimposed on a direct current voltage is applied to the sleeve 52 of the developing device 5, and a potential difference is generated between the sleeve 52 and the photoreceptor 1. Due to this potential difference, the toner T moves back and forth between the sleeve 52 and the photoconductor 1, and finally the toner T in the image area adheres to the photoconductor 1 to perform development. Potential difference for AC component shown in the figure V A C *
DC component potential difference VIIC and AC frequency f (=1/T)
(where T is the period of the AC component.

For example, in the case of a normal document, since a dark character image is required, the alternating current potential difference V A e is set to be relatively high. This is to increase the electric field between the photoreceptor 1 and the sleeve and improve toner flying efficiency.

It is also possible to increase the electric field by lowering the resistance value of the sleeve.

However, photographic originals do not require such high density because the density of the original is originally high.

What is required here is continuous tone reproducibility.

That is, it is important that the value of T in so-called gamma characteristics is low and stable.

Therefore, in this embodiment, in the photo mode for copying a photographic original, the AC component of the developing bias is removed and development is performed using only the DC component. That is, in the normal mode, development is performed by adding a DC component of AC superimposition to the sleeve, but in the photographic mode, development is performed using only the DC component.

In this embodiment, in the normal mode, the AC component is
kVp-p / 2 k) Iz, DC component -200
-400V, and in photo mode, the DC component is -350 to -450V (however, the photoreceptor dark potential knee is 800V).

A development bias of background potential: -150 V) was used.

Further, the exposure light amount is adjusted in order to change the apparent PIDC characteristic, that is, the exposure light amount vs. photoreceptor surface potential characteristic. In this embodiment, in the photo mode,
The amount of light is reduced by 30% compared to the normal mode, and as a result, the background potential is increased to 150V.

Jones plot in Figure 4
To explain this with reference to a characteristic curve called , the exposure light amount characteristic curve shown in the fourth quadrant changes from A to B4 as the exposure light amount is reduced. Since the exposure light amount versus photoconductor surface potential characteristic shown in the third quadrant does not change, the photoconductor surface potential versus copy density characteristic shown in the second quadrant changes from A2 to B! Changes to Here, the DC component of the developing bias is from V to V.
, and further remove the AC component, the photoreceptor surface potential vs. copy density characteristic further changes to B,l. Such a change occurs because development is performed only by the development electric field, resulting in a decrease in development efficiency. Therefore, as a result, the original density versus copy density characteristic shown in the first quadrant changes from A1 to B+'.

That is, by removing the AC component, the photoreceptor 1 and sleeve 5
It was confirmed that a low T can be obtained by reducing the movement of toner between the two and lowering the amount of exposure light, and changing the optical attenuation curve while giving a better appearance. Note that when the exposure light amount is lowered, the background potential increases, and the appearance is upward, and the photoreceptor 10P
It appears that the IDC characteristics are changing.

As a result, reproduction characteristics that continuously and gradually change from low density to high density were obtained, making it suitable for reproducing photographic originals, and copies with excellent gradation reproducibility were obtained.

The volume resistivity (specific resistivity) of the sleeve 520 of the developing device 5 is IO'1 to IQ+20'e1m, preferably 1
It can be set to 0' to IQIOΩ. If the volume resistivity value is smaller than 1030.ci+, the image quality of the copy will become binary shading, making it difficult to reproduce halftone images, and if it is larger than 10 Ω/mouth, the manufacturing of the sleeve 52 will be difficult. Become.

In the above-described embodiment, the image forming conditions are changed depending on the type of document, but the image forming conditions may be changed according to the user's preference.

[Effects of the Invention] As described above, according to the present invention, when forming an image from a photographic original, the alternating current component of the developing bias is turned off and the amount of exposure light is lowered. A rapid increase in concentration towards medium concentration can be suppressed. Therefore, a gradation reproduction characteristic that continuously and gradually changes from low density to high density can be obtained, and is suitable for reproducing a photographic original having delicate shading, for example.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing a schematic cross section and control block of an electrophotographic copying machine to which the image quality control device of the present invention is applied, and FIG. 2 is a schematic cross-sectional view of main parts of a developing device used in the electrophotographic copying machine. , FIG. 3 is a graph showing the developing bias voltage, and FIG. 4 is a graph showing changes in copying characteristics when the amount of exposure light is changed and the alternating current component of the developing bias is turned on and off. 1: Photoreceptor 2: Charger 3: Exposure device 3a: IT light lamp 4: Static eliminator 5: Developing device 5a: Hopper 6: Transfer lamp 7: Transfer device 8: Cleaner 9: Static eliminator lamp 1O: Paper feed roll 11 : Conveyor belt 13 Nibraten cover 21a; AC power supply 22: Control circuit 23: L light lamp power supply 52 Ni sleeve D: Document

Claims (1)

[Scope of Claims] 1. In an image forming apparatus that develops a latent image formed on a photoreceptor using a one-component developer, a developing bias voltage that is a superimposition of a DC voltage and an AC voltage, and a developing bias voltage that is only a DC voltage. An image quality control device for an image forming apparatus, comprising: a bias power source for selectively applying a voltage to a developing device; and means for switching an amount of exposure light to the photoreceptor in conjunction with selection of the developing bias voltage. . 2. Controlling the bias power supply and the means for switching the exposure light amount depending on whether the document is a text document or a photo document, and in the case of a text document, a developing bias voltage obtained by superimposing a DC voltage and an AC voltage is used; 2. The image quality control device for an image forming apparatus according to claim 1, wherein a developing bias voltage of only a DC voltage is used for photographic originals, and the amount of exposure light is lower than that for text originals.