JPS6311665B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image density control method in a transfer type electrophotographic copying machine.

As is well known, in a transfer type electrophotographic copying machine, an original placed on a transparent original table is projected onto the surface of a charged photoreceptor to create a latent image corresponding to the optical image of the original, and this latent image is transferred to a powder. The image is visualized using a body developer or a liquid developer, and the visible image is transferred to a transfer sheet to make a copy of the original.

What is important in such a copying machine is to always obtain a stable, clear copy image with a constant density. To this end, in the past, the toner concentration in the developer, that is, the mixing ratio of carrier and toner, was detected using changes in magnetic permeability, changes in electrical resistance, changes in light reflectance, changes in fluidity, etc. However, based on this, toner supply was controlled to maintain a constant toner concentration. However, no matter how much the toner density is maintained constant, the density of the copied image is not always constant. This is because the amount of toner attached to the latent image generally changes depending on developer fatigue, ambient temperature, humidity, etc., and the density of the copied image also changes. Therefore, in order to further improve accuracy, a method is used to measure the developing ability of the developer by detecting the amount of toner deposited on a reference surface that resembles an actual latent image, and to control toner replenishment based on this. is proposed.

The reference surface is generally formed in a non-image area of a photoreceptor or on the surface of a conductive member provided in a developer flow path. Although this method of measuring the developing ability of developer is certainly superior to the method of measuring toner concentration, in transfer type copying machines, the transfer process follows the development process, so this may be the cause. The copy density may change over time. That is, if the transfer voltage decreases due to changes over time, the transfer efficiency also decreases, and the copy density also decreases. Therefore, the method of controlling toner replenishment by detecting the developing ability of the developer has the disadvantage that the density of the final copy image cannot be maintained sufficiently constant.

Incidentally, the density of a copy image changes not only due to fluctuations in the development process and fluctuations in the transfer process, but also due to the amount of charge in the charging process, the amount of exposure in the exposure process, and the like. It also changes due to deterioration of the photoreceptor itself. Therefore, the optimum density maintenance and management of copy images must be carried out for the actual equipment used in each process of the copying machine.
For example, a method in which toner is deposited not on the photoconductor but on a detection electrode to which a reference voltage is applied and the amount of the toner deposited is measured, is subject to fluctuations in the charging process and exposure process of the copying machine, deterioration of the photoconductor, etc. It is not possible to maintain accurate copy density.

This invention provides a reference density mark in a non-image area of a transparent document table, reproduces this mark on the non-image area on the surface of a photoreceptor using various devices in an actual copying machine, and measures the density. The method is characterized in that it is transferred to a non-image area of a transfer sheet carrier, its density is measured, and the measured values are compared with each other to control the density of the copied image.

According to the present invention, it is possible to simply and easily maintain and manage the highly accurate and stable density of a copied image by compensating for various fluctuations that affect the density of a copied image in an actual copying machine.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved method for controlling image density.

It is a further object of the present invention to provide a stable and accurate method which provides direct comparison to the final copy image of the copier and compensates for variations in the copier.

Hereinafter, the present invention will be described with reference to the attached drawings. FIG. 1 is a graph showing the relationship between original density, latent image potential, toner adhesion amount, and copy density. This indicates that the higher the original density, the higher the latent image potential, and the higher the latent image potential, the greater the toner adhesion amount, and the greater the toner adhesion amount, the higher the copy density. If the copier is working properly,
Each relationship is proportional as shown in the diagram.
Copiers are also so designed and calibrated. However, if the latent image potential does not rise even though the density of the original is high, this may be due to insufficient charge on the photoreceptor, fatigue of the photoreceptor and inability to retain or erase the charge, or the amount of exposure. This is thought to be caused by a shortage, but in this case, the copy density also does not increase. Furthermore, if the amount of toner adhesion does not increase despite the latent image potential being high, this may be due to developer fatigue or bias voltage fluctuations, but in this case as well, the copy density does not increase. Furthermore, if the copy density does not increase even though the amount of toner adhesion is large, this is considered to be due to a decrease in transfer voltage.

In this way, even if there is a variation in one of the charging, exposure, development, and transfer steps, it will affect the density of the final copy image. Therefore, even if the charging, exposure, and development processes are normal, the copy density will change if there are fluctuations in the subsequent transfer process. It is understood that it is not possible to precisely control the

FIG. 2 is a schematic configuration diagram showing an example of a transfer type electrophotographic copying machine to which the present invention is applied. Around the photosensitive drum 1 having a photoconductive layer on its surface,
A main charging corona discharger 2, a developing device 3, a transfer corona discharger 4, a cleaning device 5, and a charge eliminating corona discharger 6 are arranged along the rotation direction. Below the photosensitive drum 1,
A transfer belt 7 whose outer surface is dielectric is stretched horizontally between grounded rollers 8 and 9, and the upper belt of the transfer belt 7 runs between the transfer corona discharger 4 and the photoreceptor drum 1. , comes into contact with the photoreceptor drum surface and moves at the same speed in the same direction as the moving direction of the photoreceptor drum surface at the contact point.
A scraping blade 10 is provided at the folded portion of the transfer belt 7 where the upper belt faces the lower belt, with its tip in contact with the belt. A grounded electrode plate 11 in contact with the lower belt of the transfer belt 7 is disposed inside the lower belt, and a corona discharger 12 for static elimination is disposed opposite to this on the outer side of the lower belt of the transfer belt 7. There is. Further, a pre-charging corona discharger 13 is disposed below the roller 8 located at the turn-back portion where the lower belt heads toward the upper belt.

The surface of the photosensitive drum 1, which rotates counterclockwise, is first uniformly charged to a predetermined polarity by the main charging corona discharger 2. When an optical image 14 of the document is irradiated thereon by an exposure optical system (not shown), a corresponding electrostatic latent image is formed thereon. This latent image is visualized by toner in the developer supplied from the developing device 3. On the other hand, the transfer belt 7, which rotates clockwise, electrostatically attracts the transfer sheet 15 fed toward the upper belt after being charged by the pre-charging corona discharger 12, and charges the photoreceptor. Head to the surface. Photosensitive drum 1
The above image formation timing and the feeding timing of the transfer sheet 15 are synchronized, and the leading edge of the developed image on the photosensitive drum and the leading edge of the transfer sheet match at the position of the transfer corona discharger 4. , the developed image on the photosensitive drum is transferred to the transfer sheet by the discharge from the transfer corona discharger 4. The transfer sheet is then peeled off from the transfer belt when the transfer belt suddenly changes direction at the roller 9 position, and
The transferred image is conveyed in a predetermined direction and permanently fixed onto the transfer sheet by a fixing device (not shown). The toner other than the image area transferred onto the transfer belt is removed by the blade 10, and then the surface of the transfer belt is neutralized by the static eliminating corona discharger 12, and then charged again by the preliminary charging corona discharger 13. receive. On the other hand, the toner remaining on the surface of the photoconductor after the transfer process is removed by a cleaning device 5.
At the same time, the charge remaining on the surface is removed by the static eliminating corona discharger 6, and one copying cycle is completed.

In such a copying machine, the present invention provides a third
As shown in the figure, a standard density mark 23 is placed in the non-image area of the transparent document table 22 on which the document 21 is placed.
A first density sensor 24 is provided opposite the mark image formed on the non-image area of the photosensitive drum 1 between the developing device 3 and the transfer corona discharger 4, and a first density sensor 24 is also provided opposite the mark image formed on the non-image area of the photosensitive drum 1 between the developing device 3 and the transfer corona discharger 4. Another second mark image 23a is placed opposite to the mark image 23a transferred onto the non-image area of
A feature is that a concentration sensor 25 is provided. Here, the non-image area means the location of each member that can be similarly copied, which is outside the image area, assuming that the maximum document size allowed for copying is the image area. The number of reference marks 23 provided on the document table 22 may be one or multiple with different densities. If there are multiple sensors, each concentration sensor 2
Naturally, there are a plurality of detection units 4 and 25. The second density sensor 25 faces the mark image remaining on the non-image area of the photosensitive drum 1 between the transfer corona discharger 4 and the cleaning device 5 instead of the non-image area of the transfer belt 7. It may be provided. The concentration sensors 24 and 25 are composed of a light emitting element 26 and a light receiving element 27, as shown in FIG. As the light emitting element 26, an LED, an incandescent lamp, etc. are used, and as the light receiving element 27, CdS, a phototransistor, etc. are used. The light from the light emitting element 26 is reflected on the transferred reference mark 23a, but the reflectance varies depending on the amount of toner attached.
This results in different photocurrents flowing through the light receiving element 27, and this current change is converted into voltage to detect the amount of toner adhesion. When the transfer belt 7 carrying the transferred mark image is transparent, these density sensors are configured so that the light receiving element 27 detects the change in the transmittance of light from the light emitting element 26, as shown in FIG. It's okay.

In FIG. 3, the reference mark 23 provided in the non-image area of the document table 22 is reproduced in the non-image area on the photoreceptor drum 1 through the above-described charging, exposure, and development steps together with the regular document 21. The concentration is measured by the first concentration sensor 24. This density is compared with a reference value representing the optimal image density after development by an appropriate comparison circuit, and if it is determined that the image density is low, the necessary amount of toner is replenished into the developer. Aim to optimize image density. Next, the developed mark image is transferred to a transfer corona discharger 4.
is transferred onto the transfer belt 7, and its density is measured by the second density sensor 25. This density is also compared with a reference value representing the optimum image density after transfer by a suitable comparison circuit in relation to the above-mentioned density after development. That is, when the apparatus is operating normally, both the image density after development and the image density after transfer represent the optimum density, although the image density after transfer is of course slightly lower. However,
If the image density after development is optimal but the image density after transfer is lower than the standard, this is considered to be caused by poor transfer, and as a countermeasure, try increasing the transfer voltage, for example. Furthermore, even though the image density after development is higher than the standard, the density after transfer may be optimal. In this case, although the copied image is good, the operating state of the device is not normal. This is because if the density after development is high,
This is because the density after transfer should also be high. Therefore, in this case, it can be said that the operating state of the apparatus is poor in transfer, and the developer density is also too high, so some adjustment is required. If such a state is left as it is, there will be a large amount of residual toner remaining after transfer, which will place a burden on the photoreceptor cleaning device and cause the copying machine to malfunction. Possible causes of the development density being too high include high toner density, excessive charging of the photoreceptor, insufficient exposure, poor development bias, fatigue of the photoreceptor, etc., and these should be checked and adjusted in descending order of probability.

An example of such a comparison circuit will be explained with reference to FIG. The lamp 31, which is the light emitting element of the first sensor 24, is connected between the positive power source and the ground, and the CdS cell 32, which is the light receiving element, is connected between the positive power source and the ground via the resistor 33. There is. A resistor 34, a variable resistor 35, and a resistor 36 are also connected in series between the positive power source and the ground. The connection point between the CdS cell 32 and the resistor 33 is connected to the positive input terminal of the first operational amplifier 37, and the movable terminal of the variable resistor 35 is connected to the negative input terminal of the same operational amplifier 37. Two diodes 3 are connected between the positive and negative input terminals of the first operational amplifier 37.
8 and 39 are connected in parallel with their respective polarities reversed. The output side of the first operational amplifier 37 is connected to the input side of a suitable hold circuit 40, and the output side of the hold circuit 40 is grounded via resistors 41 and 42 connected in series. CdS cells 45 connected in series to the lamp 43 and resistor 44 of the second concentration sensor 25 are each connected between the positive power source and ground.
The connection point between the resistors 41 and 42 is connected to the positive input terminal of the second operational amplifier 47 via the resistor 46, and the connection point between the resistor 44 and the CdS cell 45 is connected to the positive input terminal of the second operational amplifier 47 via the resistor 48. Connected to the negative input terminal. Second operational amplifier 47
A resistor 49 is connected between the positive input terminal and ground, and a resistor 50 is connected between the negative input terminal and the output terminal. A resistor 52 is connected between the output terminal of the second operational amplifier 47 and an external terminal 51, and a diode 53 is connected between the external terminal 51 and ground.

If the density of the mark image after development is low, CdS cell 3
The current flowing through the first operational amplifier 37 is also large, so there is little difference from the voltage of the positive power supply input to the first operational amplifier 37, and its output becomes a voltage close to ground. In this case, the output voltage is inputted from the external terminal 54 to a suitable drive circuit to control the toner replenishment tank to replenish the required amount of toner into the developer, thereby restoring the image density. When the density of the mark image after development is appropriate, the current flowing through the CdS cell 32 is small, so the output voltage of the first operational amplifier 37 is close to the voltage of the positive power supply, and this voltage is temporarily applied to the hold circuit 4.
After being held at 0, it is input to the second operational amplifier 47. This becomes a reference voltage, and the second operational amplifier 47 compares this with the input voltage from the CdS cell 45 representing the density of the mark image after transfer. When the input voltage from the CdS cell 45 is higher, that is, when the density of the mark image is lower, the output voltage is inputted from the external terminal 51 to a suitable drive circuit to increase the transfer voltage, for example. When the input voltage from the CdS cell 45 approaches the reference voltage, the second operational amplifier 4
7's output voltage also approaches zero.

This circuit is also applied to the case where the second sensor 25 is opposed to the surface of the photoreceptor drum rather than on the transfer belt to check the residual density of the mark image after transfer.

In addition, as shown in Fig. 3, two reference marks 23 are provided, and the density of one of them is set to the highest image density of the original.
By setting one to 1.8 or higher and setting the other to 0.05 or lower, which corresponds to the lowest density of the document, that is, the background density, the following can be done. In other words, in the mark images 23a after development or transfer, even though one mark image has the highest image density, the other mark image has the lowest image density higher than the reference value. This means that the developed image has background stains due to some reason. The main causes are considered to be insufficient exposure, fatigue of the photoreceptor, or defective developing bias, so adjustments are made accordingly. On the other hand, if the minimum image density is within the standard value but the maximum image density is lower than the standard value, this may be due to insufficient toner or poor transfer, so make adjustments accordingly. do.

FIG. 7 shows another example of an electrophotographic copying machine to which the present invention is applied. The difference from the device shown in FIG. 2 is that a bias power source 5 is used instead of the transfer belt.
1, using a conductive transfer roller 52 connected to 1. This is a so-called bias transfer method and is a well-known method. When applying the present invention to this device, similarly, a reference mark is provided in the non-image area of the document table, and this is reproduced on the non-image area on the photoreceptor using each device of the copying machine, and the density is is measured by the first density sensor 53, then transferred to the non-image area of the transfer roller 52, and the density is measured by the second density sensor 54.
It can be measured by

The present invention can be applied to color copying machines in addition to ordinary copying machines for monochrome images. Most color copying machines obtain color copies by overlapping and transferring developed images. Usually, a magenta image is superimposed on a yellow image, and a cyan image is further superimposed on that to obtain a so-called natural color image. In this case, if the image density of a certain color becomes higher than the appropriate value, that color becomes dominant and faithful color reproduction becomes impossible. Therefore, controlling image density is an extremely important technique in color copying machines.

Fig. 8 shows an example of a color copying machine, the details of which are shown in the patent application filed in 1972 by the applicant.
No. 84525. Briefly, the surface of the photoreceptor drum 62 is charged by the charging device 61, first exposed to image light through a blue filter, the formed latent image is developed by the yellow developing device 63, and this image is transferred. The image is transferred onto a transfer paper held on a drum 64 by a transfer device 65. Next, the surface of the photoreceptor drum is cleaned by a cleaning device 66, then neutralized by a static eliminator 67, and then charged again by a charging device 61.
Next, in the same manner, after exposing the image through a green filter, it is developed by the magenta developing device 68, and the developed image is superimposed on the yellow image on the transfer paper.
After image exposure through a red filter, the image is developed by a cyan developing device 69, and the developed image is superimposed on the image on the transfer paper. The transfer drum 64 is the ninth
As shown in the figure, the middle part of a cylinder 71 is cut away leaving a continuous band-shaped part at both ends, and the cut part is covered with a dielectric sheet 72 such as Mylar, and the transfer paper 73 is made of It is wound and clamped onto this dielectric sheet. A fiducial mark is also transferred to the non-image area on this dielectric sheet as shown at 74 for each image process. The density of the mark image is detected by density sensors 75 and 76 in FIG.

The present invention can also be applied to a wet development type copying machine.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between document density, latent image potential, toner adhesion amount, and copy density in an electrophotographic copying machine, and FIG. 2 is a schematic diagram showing an example of an electrophotographic copying machine to which the present invention is applied. 3 is a schematic perspective view showing an embodiment of the present invention together with the copying machine shown in FIG. 2, and FIGS. 4 and 5 are
FIG. 6 is a schematic diagram showing the configuration of a concentration sensor, FIG. 6 is a circuit diagram showing an example of a detection circuit used in carrying out the present invention, and FIGS. 7 and 8 are diagrams showing another electrophotographic copy to which the present invention is applied. FIG. 9, a schematic configuration diagram showing an example of the machine, is a schematic perspective view of the transfer drum in FIG. 8. 21...Original, 22...Original table, 23...Reference density marker, 24, 25...Density sensor.

Claims (1)

[Claims] 1. A means to create a latent image by projecting an original placed on a transparent document table onto the surface of a charged photoreceptor, a means to make this latent image visible using a developer, and a means to transfer this visible image to a transfer sheet. In an electrophotographic copying machine equipped with means for transferring onto a transfer sheet on a carrier, a reference density mark is provided in a non-image area of the document table, and this is applied to the surface of the photoreceptor using each of the above means. reproduce it in the non-image area and measure its density,
This is transferred to the non-image area of the transfer sheet carrier, its density or the density of the mark remaining on the photoreceptor is measured, and the density signal of the developed mark is compared with a reference signal representing the appropriate developed density. At the same time, the output signal is further compared with the density signal of the transferred mark or the density signal of the residual mark to control the density of the copied image.