JPS62127870A - Control method for density of copy image - Google Patents

Abstract

PURPOSE:To suppress the variation of an image density by compensating the influence of a photoelectric characteristic which is varied by the external environmental variation of a photosensitive body having the photoconductivity, by varying the frequency and voltage of an AC bias which is applied at the time of development. CONSTITUTION:When the temperature of the photosensitive body rises due to an environmental condition and a continuous use, etc., the sensitivity variation of the photosensitive body appears as a sensitivity rise on an exposure quantity and surface potential characteristic (E-V characteristic). In this state, the temperature of the photosensitive body is detected, and by a conversion value determined in advance, an AC bias value is varied. At the temperature of 20 deg.C, an AC bias frequency and a Vp-p value are set to 500Hz and 800V, respectively, and in case of the temperature of the photosensitive body is 33 deg.C, the AC bias frequency and the Vp-p are changed to 1.5kHz and 600V, respectively. Also, in case of between 20 deg.C and 30 deg.C, programming is executed in advance so that the frequency and the Vp-p value are changed at a roughly equal interval and also continuously.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for controlling the density of a copied image, and in particular, when a photoconductive photoreceptor used in electrophotography, which is mainly based on the curling process, changes in the external environment, such as temperature.

The present invention relates to a method for controlling the density of a copied image, which allows copies with stable image quality to be obtained despite changes in photoelectric characteristics due to humidity.

(Prior Art) Generally, the temperature inside a copying device changes depending on the state in which the device is first used and the state in which it is used continuously for a long time, and especially in cold regions, the temperature inside the copying device varies between 15 and 30°C. ℃ reached,
Along with this, for example, the sensitivity of the photoreceptor changes similarly. This photoreceptor sensitivity has a particularly large influence on the halftone original portion of the image, and tends to be more noticeable. Then, due to fluctuations in the source intensity of one image, phenomena such as changes in gradation occur. On the other hand, residual potential fluctuations occur due to humidity, for example, in Ia f@separation type OPG photoreceptors and the like.

Therefore, it is necessary to control the copying process in order to obtain copies with stable image quality despite the above-mentioned fluctuations in the photoreceptor's photoelectric properties. Conventionally, the following three methods have been commonly used as countermeasures for the above phenomenon. It was adopted as such.

(2) In a developing device that uses a two-component developer, when a change in the density of a developed image is detected, the toner density in the developer is temporarily varied to reduce the apparent change in the density of the developed image.

2) A method of reducing changes in image density by changing the developing DC bias voltage according to changes in the photoelectric characteristics of the photoreceptor.

3) A method in which the photoreceptor exposure amount is similarly controlled so that a constant surface potential is maintained in response to changes in photoreceptor sensitivity due to changes in photoreceptor temperature.

(Problems to be Solved by the Invention) However, the three conventional control methods described above have the following problems. That is.

In method 1), it is difficult to quickly control the toner density; It is difficult to reduce the toner concentration. In addition, an increase in toner concentration due to rapid toner replenishment may cause poor agitation of toner and carrier within the developing device.
1) Severe defects may cause image quality deterioration such as so-called fog.

Although method 2) is relatively easy to control.

Since the relationship between photoconductor surface potential and development density is nonlinear,
It has a characteristic of changing the gradation of the developed image by shifting the developing bias voltage. This is a point that must be kept in mind especially in the case of a color copying apparatus, and is not preferable from the viewpoint of color tone reproduction.

3) The method of varying the light source exposure amount is effective for analog exposure using a lens optical system, but it is not an effective method for digital exposure systems using a laser beam. There is a drawback that gradation changes in the image appear. This specific example is shown in the ninth
As shown in rA.

This example shows the results of a reversal development method in which an image area is exposed and developed with toner of the same polarity as the charge polarity charged on the photoreceptor, and the laser power is 2 mW (■) and 1 mW (
■) It shows the gradation variation when changing. As is clear from the figure, even if the laser power is changed, it is not possible to absorb the change in photoreceptor sensitivity, but rather causes a change in gradation.

Therefore, the present invention has been made in order to solve the above-mentioned problems of the conventional example, and its purpose is to reduce the image quality after development by reducing the influence of the photoelectric properties that vary due to external environmental changes of the photoconductive photoreceptor. The purpose is to reduce fluctuations.

(Means for Solving the Problems) In order to achieve the above object, the present invention detects fluctuations in the photoelectric characteristics of the photoreceptor, and based on the detection results,
The density of the copied image is controlled by changing the frequency and voltage of the AC bias voltage, or the value of either the frequency or the voltage, of the DC bias voltage and AC bias voltage applied during development of the r&minute developer. It is made up of.

(Example) The present invention will be described below based on the illustrated embodiment. FIG. 1 shows a first embodiment of the present invention, and is a schematic sectional view of a digital copying apparatus using the same embodiment. The digital copying apparatus of this embodiment incorporates four sets of electrophotographic laser beam printer mechanisms as multiple sets of image forming apparatuses, and in FIG. denotes four sets of image forming devices arranged sequentially from the right side to the left side in the figure in the main body case 10;
Reference numeral 11 denotes a drive roller for driving the conveyor belt 8, and this drive roller 11 is rotationally driven by a drive source (not shown). The conveyor belt 8 is made of Tetron tea mesh, and is stretched between a driving roller 11 and driven rollers 12 and 76 and moves in the direction of the arrow shown in the figure. Reference numeral 13 designates a paper feeding mechanism disposed on the right side of the main body case 10, 7 an image fixing device disposed on the left side of the main body case 10, and 14 a transfer member discharge port.

Each image forming device Pa, Pb, Pc and Pd has a mechanism,
The configuration itself is substantially the same, and each of the image forming apparatuses Pa, Pb, Pc, and Pd has a photosensitive drum la as an image carrier that is rotationally driven in the direction of the arrow around an axis.
, lb, lc and 1d, and the photosensitive drums la, lb
, lc, and 1d, chargers 15a are sequentially arranged in the drum rotation direction.

15b, 15c and 15d, developing units 3a and 3b.

3C and 3d, transfer dischargers 4a, 4b, 4c and 4d
, cleaning devices 5a, 5b, 5C, and 5d, and laser beam scanners 16a, 16b, 16c, and 16d arranged above the photoreceptor drums la, lb, lc, and 1d. Laser beam scanner 16a.

16b, 16c, and 16d are composed of a semiconductor laser, a polygon mirror, an fθresen, etc., and upon receiving the input of an electric digital pixel signal, a laser beam modulated according to the input signal is applied to the chargers 15a, 15b, 15c, and the like. 15d and the developing devices 3a, 3b, 3c, and 3d, the drum surface is exposed by scanning in the drum generatrix direction.

The developing device 3a of the image forming device Pa is loaded with yellow toner, that of the image forming device pb is loaded with magenta toner, that of the image forming device Pc is loaded with cyan toner, and that of the image forming device Pd is loaded with black toner. It can hold toner. The laser beam scanner 16a of the image forming apparatus Pa has a pixel signal corresponding to the yellow component image of the color image, the image forming apparatus pb has a pixel signal corresponding to the magenta component image, and the image forming apparatus Pc has a pixel signal corresponding to the magenta component image. A signal corresponding to a cyan component image is inputted to one image forming device 'l1Pd, and a signal corresponding to a black component image is input to that of the image forming device 'l1Pd.

Here, when a cut sheet-shaped transfer paper 6 is inserted as a transfer member onto the paper feed guide 51 of the paper feed mechanism 13, the leading end of the paper is detected by the sensor 52, and this is used as a start signal for each image forming apparatus Pa.

Pb, Pc and Pd photosensitive drums la, lb.

IC and ld start rotating. At the same time, the drive roller 11 is also driven and the conveyor belt 8 begins to move in the direction of the arrow. Then, the transfer paper 6 passes through the paper feed guide 57 and is supplied onto the conveyor belt 8.

Therefore, the transfer paper 6 receives corona discharge from the attraction charger 59 and is reliably attracted to the conveyor belt 8.

Furthermore, each sensor 60a is located at the leading end of the transfer paper 6.

When 60b, 60c, and 60d are sequentially shut off, the signals cause each photoreceptor drum la of each image forming apparatus Pa, Pb, Pc, and Pd to rotate in advance.

Image formation for lb, lc, and 1d is sequentially started. That is, there is a yellow image on the photosensitive drum la of the image forming apparatus Pa, a magenta image on that of the image forming apparatus pb, a cyan image on that of the image forming apparatus ii Pc, and a black image on that of the image forming apparatus Pd. are formed by sharing their respective roles. Note that each image forming apparatus Pa, Pb, Pc, and P
Since the image forming principle in d is already well known as the Carlson process, its explanation will be omitted.

Then, the transfer paper 6 is moved toward the one-image room applicator 7 by the rotation of the conveyor belt 8 to each image forming apparatus Pa.

Pb, Pc and Pd are conveyed sequentially through the lower part,
By the transfer dischargers 4a, 4b, 4c and 4d of each image forming apparatus Pa, Pb, Pc and Pd.

Each color is sequentially superimposed and transferred onto the surface of the transfer paper 6 to synthesize a color image. When the transfer paper 6 passes through the image forming apparatus Pd, the charge is removed by a charge remover 61 to which an AC voltage is applied, and the transfer paper 6 is separated from the conveyor belt 8.

Next, the transfer paper 6 enters the feeder 7, fixes one image, and is discharged from the discharge port 14 to the outside of the main body case lO.
One print cycle is completed.

Next, there are developing devices 3a and 3b in the present invention.

3c and 3d will be explained based on FIG. In the figure, 20 is a photosensitive drum, 21 is a developing sleeve, 22 is a cutting plate, 23 is a stirring blade, and the developing sleeve 21
is rotating counterclockwise, and the stirring blade 23 is rotating clockwise. Inside the developing sleeve 21, a large number of magnets are arranged in the circumferential direction with alternating polarities. As a developer, a coating carrier made of spherical iron powder coated with resin is used as a carrier, and as a toner, polyester resin is used as the main ingredient, with the addition of cyan, magenta, yellow, and black pigments. In this embodiment, the photoreceptor is charged to negative polarity, the image area is exposed to a laser beam, and the image area is developed using the negative polarity.

A DC bias voltage and an AC bias voltage are simultaneously applied to the developing sleeve 21 from a power source (not shown). (Hereinafter, all DC bias voltages are -150
It is fixed at V. ) Also, the distance between the developing sleeve 21 and the ear cutting blade 22 is 200 JLm, and the distance between the developing sleeve 21 and the photosensitive drum 2 is 200 JLm.
0 is set to 300 pm, and in this embodiment, the developing device is used.

It is possible to reproduce a digital pattern with 16 gradations. Since this digital pattern is created by a method using a well-known dither matrix or the like, its explanation will be omitted. Furthermore, since the four image forming apparatuses Pa, Pb, Pc, and Pd have the same structure, the following explanation is applicable to all of these four image forming apparatuses Pa, Pb, Pc, and Pd. The relationship between the developed image density and each gradation dot number is shown in FIG. 3 using data when cyan toner is used. That is, FIG. 3 shows how the gradation changes by changing the frequency of the AC bias voltage and the value of the peak-to-peak value (hereinafter referred to as VPp value). If the changes in typical gradation dot numbers from the data in FIG. 3 are summarized in FIG. 4, it can be seen that the developing characteristics change depending on the AC bias value. That is.

l) When the frequency of the AC bias value is high, and the -2 value is also high (in Fig. 4, the frequency is 1.5 KHz).

(3) (-3 value 800V), the developing performance deteriorates in both the high-density part and the low-density part of the two images, and a decrease in the density of one image is observed.

2) When the frequency of the AC bias value is relatively low (around 500H2 in this example) and the Vp-p value is about 800V, an increase in image density is observed, especially in low density areas.

In this example, the characteristics described below are utilized, and the photoreceptor (
When the temperature of the OPC photoreceptor (in this example) increases due to the environmental conditions in which it is placed, continuous use, etc., the sensitivity change of the photoreceptor will change as the sensitivity increases, as shown in Figure 5. .

This is called the E-V characteristic. In the same figure, 2
The curves in this book show cases where the photoreceptor temperature was 20°C (0) and 33°C (0). The temperature of the photoreceptor is detected, and the alternating current bias value is changed according to a predetermined conversion value. At a temperature of 20°C in Figure 5, the AC bias frequency is 500
Hz, Vp-p value is set to 800v, and the photoreceptor temperature is 3.
In the case of 3°C (Fig. 5 0), the AC bias frequency is 1.5.
Kl(z.

vp-p value 600Vl: Change. Also, this 20℃～
Between 30°C, the frequency is approximately equal and continuous.

It is preprogrammed to change the vp-p value.

A method of controlling image density by changing the AC bias value as described above is shown in Fig. gg6. In the same figure, [F] is 20'
0, AC bias frequency * 500 Hz.

Figure 6 shows the gradation characteristics when 800V is applied to the -3 value, and the temperature of the photoreceptor rises from this state, and the Uyama image density at which the sensitivity change as shown in Figure 5 0 occurs, for example, is shown in Figure 6 [ F] appears as an increase in concentration.

Therefore, as mentioned above, change the AC bias value to a frequency of 500.
Hz +1.5 KHz, Vp-, value 800V-6
When the voltage is changed to 00V, the image density changes as shown in FIG. 4, and the density control as shown in FIG. 6 is performed. The gradation properties after density control are shown in FIG. 6G, and as is clear from this, fairly good density control has become possible.

Next, a second embodiment of the present invention will be described based on FIGS. 7 and 8. The first embodiment detects the temperature of the photoreceptor and prevents the effects of changes in photoreceptor sensitivity.
This embodiment detects the humidity of the environment in which the photoreceptor is installed to prevent changes in image density due to fluctuations in residual potential. That is, in the function-separated type OPC photoreceptor that has come into use in recent years, the residual potential characteristics change depending on the humidity, causing a change in the EV characteristics as shown in FIG. In the figure, ■ indicates the case where the environmental humidity is 55%, and ■ indicates the case where the environmental humidity is 90%.

Such EV characteristic fluctuations naturally also cause image density fluctuations, as shown in FIG.

The gradation changes as shown in (2). ■ is environmental humidity 55
Indicates the density gradation of the image developed at %RH, and ■
indicates the density gradation of an image developed at an environmental humidity of 90% RH. ■ and ■ are both AC bias value frequency 500 Hz
, when a Vp-p value of 400V is applied. Other conditions are the same as in the first embodiment. Here, if the environmental humidity is 90% RH as shown in FIG. Then, from the characteristics shown in FIG. 4, a decrease in density is observed, especially in high-density areas, and selective image density control as shown in FIG. Such a program can be stored in a ROM installed inside the copier using a well-known method.
The program can be changed by replacing it as necessary, and each condition may be selected using a DIP switch provided on a control board or the like.

It should be noted that the present invention is not limited to the above-mentioned embodiments.1 For example, the present invention can be effectively used not only for the purpose of compensating for changes in photoreceptor characteristics, but also as a method for controlling delicate reproduction of image quality gradation. It's a good finger. Further, the variation in the photoelectric properties of the photoconductor can be applied to the variation in sensitivity of the photoconductor, the variation in the residual potential of the photoconductor, and the variation in dark resistance of the photoconductor.

(Effects of the Invention) The copied image density control method according to the present invention has the following configuration and operation, and the influence of the photoelectric characteristics that vary due to changes in the external environment of the photoconductive photoreceptor can be reduced by changing the AC bias applied during development. Varying the frequency and voltage can compensate and reduce developed image density changes. This makes it possible to suppress changes in image density without causing problems associated with various conventional control methods, and is particularly effective in digital copying compared to analog copying. This is an effective control method for digital copying systems since the method of controlling the exposure amount cannot be used.

In addition, since it is possible to selectively control the image density of high-density areas and low-density areas, this control method is particularly advantageous for low-density areas, which have traditionally been difficult to control and change sensitively due to various fluctuation factors. This has the effect of being able to provide the following.

[Brief explanation of drawings]

FIG. 1 is a schematic sectional view of the digital copying apparatus used in the first embodiment of the present invention, FIG. 2 is an explanatory diagram showing the structure of the developing device used in the same embodiment, and FIG. 3 is an AC A graph showing the gradation when the bias value is varied; FIG. 4 is a graph showing how the developed image density changes depending on the AC bias frequency and voltage value in the same example; FIG. FIG. 6 is a graph showing the effect of the control method of the same embodiment. FIG. 7 shows the second embodiment of the present invention. Figure 8 is a graph showing the residual potential fluctuation of the body; Figure 9 is a graph showing the effect of the control method of the same embodiment;
The figure is a graph showing the gradation of an image in a conventional example in which the laser intensity is varied in a digital copying method using a laser beam. Explanation of symbols la, lb, lc, ld--photosensitive drums 3a, 3
b, 3c, 3d--Developing units 4a, 4b, 4c, 4d--Transfer exposure device "I'I"
lt Ken 5a, 5b, 5c, 5d・-Cleaning Sae 6・
...Transfer paper 7...Image setting device 8...Transport belt lO...Main body case 11...Drive rollers 15a, 15b, 15c, 15d...Charger 1
6a, 16b, 16c, 16d・-Laser beam scanner

Claims (4)

[Claims] (1) Detect photoelectric property fluctuations of the photoreceptor, and based on the detection results, determine the frequency and voltage of the AC bias voltage among the DC bias voltage and AC bias voltage applied during development of the two-component developer. A method for controlling the density of a copied image, comprising controlling the density of the copied image by changing the value of one of the voltages. (2) The copy image density control method according to claim (1), wherein the variation in the photoelectric characteristics of the photoconductor is a variation in the sensitivity of the photoconductor. (3) The copy image density control method according to claim (1), wherein the variation in photoelectric characteristics of the photoconductor is a variation in the residual potential of the photoconductor. (4) The copy image density control method according to claim (1), wherein the variation in photoelectric properties of the photoconductor is a variation in dark resistance of the photoconductor.