JP2546261B2 - Copier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of use) The present invention relates to a visible image transfer type electrostatographic copying machine. More specifically, the exposure amount is controlled so as to obtain appropriate image reproducibility. The present invention relates to a copying machine having a function to perform.

(Prior Art) An electrostatic image copying machine of a visible image transfer type projects an original image on a photoconductor to form an electrostatic latent image, which is visualized by development and transferred onto a transfer sheet. Form an image. As a control for obtaining an appropriate image by this method, conventionally, the toner supply amount, the developing bias, the exposure amount and the like are controlled individually or in combination. Examples thereof include JP-A-58-23043 (conventional example 1) and JP-A-60-119580.
Japanese Unexamined Patent Publication No. 60-133475 (Prior Art 2), Japanese Unexamined Patent Publication No. 60-133475 (Prior Art 3), Japanese Unexamined Patent Publication No. 60-146256 (Prior Art 4), Japanese Unexamined Patent Publication No. 60
No. 260072 (Prior art example 5) is known.

These detect the density of the visible image formed on the photoconductor, and simply control the development conditions and exposure conditions to make it appropriate, and also change the temperature and humidity, the developer usage time, and the sensitivity of the photoconductor. One that controls the reproduced image density while correcting the reference level in consideration (conventional example 1, 3 to 5), one that detects the document density and determines the contrast, and controls the contrast by the exposure amount and the like (conventional example 2). ) And roughly divided.

(Problems to be Solved by the Invention) However, in the method of controlling the reproduced image density based on the detection result of the visible image, the reproduced image density is determined irrespective of the difference in the document density or the contrast. For this reason, for example, in a document with fine lines scattered on a pure white background, the fine lines are liable to blur or fly on the reproduced image. It is easy to be crushed by a fog, but these things cannot be dealt with.

Further, in the method in which the contrast of the reproduced image is controlled according to the detection result of the document density, the reproducibility of the fine line is improved. This is done by lowering the lamp voltage of the exposure lamp.However, if the optical system becomes dusty or the characteristics around the photoconductor change, fogging of the white original by lowering the lamp voltage, that is, Soil is likely to occur.

Therefore, each of the conventional ones has advantages and disadvantages, and it is desired to be able to reproduce an image without any of the above defects.

Therefore, the inventors of the present invention have considered that the reference image density when a reference pattern having a specific density is exposed and developed is detected, and this reference density is used to form an image. However, as a result of various experiments and studies, the relationship between the output of the sensor that detects the density of the visible image on the photoconductor and the copy image density when this visible image is copied is proportional to the sensor output and the reflected light amount of the visible image. When it is expressed by a specific exponential function, it is affected by the background surface of the photoconductor in areas where the image density is low,
Departing from the value represented by the exponential function, the accuracy of image density detection by the sensor deteriorates. Further, in a portion where the copy image density is high, the change in the sensor output with respect to the image density is small, so that the measurement accuracy deteriorates.

It is an object of the present invention to provide a copying machine with better image reproducibility, which can solve any of the problems described above by forming a visible image by using a halftone reference density. .

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present invention holds an exposure lamp for illuminating an original image and an original image illuminated by the exposure lamp as an electrostatic latent image. A photoconductor, a developing unit for developing an electrostatic latent image on the photoconductor, a document density detecting unit A for detecting the image density of the document as shown in FIG. 1, and an image density of the document image. A lamp that determines a proper lamp voltage for the original image based on the image density of the original image detected by the original density detecting means A using a predetermined lamp voltage characteristic with respect to the lamp voltage of the exposure lamp C. Forming a reference latent image for a halftone density on a photoconductor in a copying machine provided with a voltage control means D and visualizing the reference latent image by using a developing means set to a predetermined developing condition. Due to the photosensitivity The reference image forming means for forming the reference image on the body, the image density detecting means B for detecting the image density of the reference image, and the reference image detected by the image density detecting means B. A correction value determining means for determining a correction value of the lamp voltage and a correction value determined by the correction value determining means so that the output copy image substantially matches the image density of the reference visible image based on the image density. And a lamp voltage characteristic correction means for correcting the lamp voltage characteristic of the lamp voltage control means based on the above. A second feature of the present invention is to have a correction unit that corrects the image density of the reference visible image based on the background density of the photoconductor.

(Operation) The present invention has the above-described structure, and the image density of the original to be reproduced at the time of copying is detected by the original density detecting means A, and is formed on the photoconductor by the developing means by the reference latent image forming means. The image density relating to the reference image of the halftone density is detected by the image density detecting means B. These are the original densities that provide information that uniquely obtains an appropriate lamp voltage based on the mutual relationship between the original to be reproduced and the lamp voltage of the exposure lamp C. The image density for the reference visible image on the photoconductor is the one when it is formed. Of the original image density is known, the information for knowing the condition change that causes the appropriate lamp voltage to fluctuate is provided, and the lamp voltage control means D sets the difference between the image density of the original image and the lamp voltage of the exposure lamp C. In order to determine the proper lamp voltage for the original image based on the image density of the original image detected by the original density detecting means A, using a predetermined lamp voltage characteristic of
The correction value determining means, based on the image density relating to the halftone reference visible image detected by the visible image density detecting means B, causes the output voltage of the copied image to substantially match the image density of the reference visible image. On the other hand, the lamp voltage characteristic correcting means corrects the lamp voltage characteristic of the lamp voltage controlling means D based on the correction value determined by the correction value determining means.

As a result, the exposure amount when reproducing the original image is
The proper lamp voltage that is uniquely determined according to the document density can be corrected and determined according to the change in the conditions that give the fluctuation. In particular, since the image density of the reference visible image adopts a halftone density, the necessary lamp voltage is determined by utilizing the parallel moving portion of the relational characteristic between the original density and the proper lamp voltage uniquely determined accordingly. It can be determined more accurately, and more appropriate image reproducibility can be guaranteed no matter how the conditions on the original side and the copying machine change.

Further, in the configuration of the second feature, the image density detection means corrects the image density of the reference image based on the background density of the photoconductor before forming the reference image by the correction means included therein. Even if there is toner stain on the background of the photoconductor, it can be prevented from affecting the detection result of the image density of the reference image by the image density detecting means, and more appropriate image reproducibility can be further stabilized. .

(Embodiment) An embodiment of the present invention will be described with reference to FIG.

This embodiment shows a case where the present invention is applied to a copying machine as shown in FIG. In this copying machine, a photosensitive drum 2 is provided substantially in the center of the machine body 1, an original document on a platen glass 3 on the upper surface of the machine body 1 is illuminated by an exposure lamp 4, and a scanner arranged under the document table glass 3 is used. First moving mirror 5, second and third moving mirrors 6 and 7, and a projection lens 8,
The exposure optical system 10 including the fixed mirror 9 exposes the original image on the photosensitive drum 2 by slit exposure. Reference numeral 11 denotes a slit plate for slit exposure, which is held together with the exposure lamp 4 and the first moving mirror 5 on the scanning moving base 12. Second and third moving mirrors 6, 7
Is held by the second moving table 13 and moves at half the speed of the first moving table 12 to keep the projection optical path length constant.

Around the photosensitive drum 2, a charger 14 for charging the photosensitive drum 2 to form an electrostatic latent image by the slit exposure, a developing device 15 for developing the electrostatic latent image with toner, and a photosensitive drum 2 on the photosensitive drum 2. A transfer charger 17 for transferring the developed visible image to the supplied transfer sheet 16, a cleaning device 18 for removing the toner remaining on the surface of the photosensitive drum 2 after the transfer, and the like are arranged.

A photo sensor 20 for detecting the original density is provided near the projection lens 8 in the projection optical system 10 so as to receive the reflected light from the original, and a reflection type for detecting the visible image density is provided around the photosensitive drum 2. A photo sensor 21 is provided to receive the reflected light from the visible image on the photosensitive drum 2.

These sensors 20 and 21 are connected via amplifiers 22 and 23 to an input port of a CPU 25 of a microcomputer (hereinafter referred to as a microcomputer) 24 as a control means, and a lamp voltage of the exposure lamp 4 is regulated to an output port of the CPU 25. An exposure amount control circuit 26 is connected. Further, a read-only storage circuit ROM 27 is connected to the CPU 25.

The microcomputer 24 can be shared with the one for controlling the operation of the copying machine, and the exposure amount is determined by the detection signals from the sensors 20 and 21 and the stored data from the ROM 27 on both the original side and the copying side of the image forming side. The automatic setting is made appropriately in response to changing conditions.

This will be described in detail. In the automatic exposure, the voltage of the exposure lamp 4 is uniquely controlled according to the document density. If the relationship between the document density OD and the lamp voltage V _{L in} this case is shown,
As shown in the figure.

However, if only this is done, if the dirt on the projection optical system 10 becomes dirty, the characteristics of the photoconductor drum 2, the charging charger 14, the developing device 15 and the like change, that is, if there are changes in the image forming conditions on the copying machine side, the original density will be reduced. Even if they are the same, the density of the reproduced image, which is a reproduced image, changes. Specifically, even if the proper exposure amount is set by the conventional method, thin lines or thin characters on the original may be faint or blown on the copied image, and fogging occurs even if it is a white background. It may stain the ground.

Therefore, in order to correct the proper lamp voltage _VL which is uniquely obtained according to the document density OD in consideration of the change in the image forming condition on the copying machine side as described above, the photoconductor drum 2 by the sensor 21 is corrected. The halftone density of the visible image formed above is detected.

For ease of detection and reliability, the visible image for detecting the visible image density is provided with a white pattern 31 at a position where the first moving mirror 5 on the side of the platen glass 3 stands by, and the white pattern 31 is provided each time. It is obtained by exposing and developing on the photosensitive drum 2 prior to copying.

In this case, the white pattern 31 is exposed with a certain lamp voltage V _L ,
When actually copied, the relationship between the lamp voltage V _L and the copy image density ID is as shown in FIG.

On the other hand, the relationship between the output V _{S of the} sensor 21 that detects the visible image density on the photosensitive drum 2 and the copy image density ID when the visible image is copied is known in advance. This output V _S and concentration ID
The relationship with and is stored in ROM 27, and the concentration ID can be known from the output V _S.

However if V _S is proportional to the amount of light reflected visible, V _S is ID
It is expressed by the exponential function of, but at the low density of ID less than 0.2, it is affected by the surface of the photoconductor drum 2,
As shown in FIG. 5, the value deviates from the value represented by the exponential function (broken line portion), and the accuracy of ID measurement by V _S deteriorates.
In addition, at high concentrations where the ID exceeds 0.7, the change in V _S with respect to the change in ID is small, so the measurement accuracy of ID deteriorates.

Therefore, when measuring the density of the visible image on the photosensitive drum 2,
Exposure with a lamp voltage that gives an intermediate density of ID of approximately 0.2 to 0.7,
develop. The relationship of the ID and V _S shown in FIG. 5 the solid line are stored in the ROM27 as described above, the halftone visible sensor 2
From the detection output V _ST by 1 to the copy image density ID _{T at} that time
Can be measured accurately.

Projection optical system 10, photoconductor drum 2, charging charger 14,
When the characteristics of the developing device 15 change and the _VL- ID characteristic line shown in FIG. 4 changes from the solid line in FIG. 5 to the one-dot chain line,
The lamp voltage (threshold value) at which the output V _{S of the} sensor 21 becomes a value corresponding to ID = 0.1 (the limit density at which the background of the copied image does not fog) changes from V _L0 to V _L1 . At this time ID = 0.2 ~
In the half-tone density range of 0.7, it is considered that the relational characteristic of ID with respect to V _L moves substantially in parallel, so the initial characteristic of the relation between V _L and ID shown by the solid line in Fig. 6 can be expressed as V _L = g (ID). For example, the lamp voltage V _L1 for ID = 0.1 when the image density for V _LT becomes IDT by changing like the one-dot chain line is calculated as V _L1 = V _LT + (V _L0 −g (ID _T )) be able to. Here, the constant V _L0 and the function g (I
D) is known in advance and is stored in the ROM 27.

In this way, when the white pattern 31 is exposed and developed, ID = 0.
Rather than directly _{obtaining the} lamp voltage V _L1 that becomes 1, half-tone density
It is more accurate to obtain it from ID _T and the lamp voltage V _LT at that time.
V _L1 can be _calculated .

Also, a sensor for measuring the density of the visible image on the photosensitive drum 2.
21 is easily soiled with toner, the output V _{S of the} sensor 21 gradually decreases, and the measurement error of the visible image density increases.

In order to deal with this, the background of the photosensitive drum 2 having no visible image is measured, and the output of the sensor 21 at that time is set as V _S0 , The calculation result of is used as the visible image density V _S. k is sensor 21
This is the output of the sensor 21 when the background of the photosensitive drum 2 is measured when there is no toner stain on the surface. That is, when there is toner stain, V _S and V _S0 both include the factor due to toner stain, so the factor of stain is canceled by the equation. By using the expression as V _S , not only the influence of toner stain but also the influence of temperature characteristics of the sensor 21 and the amplifier 23 can be eliminated.

As shown in FIG. 3 described above, the lamp voltage _{VL with} respect to the document density during normal automatic exposure is such that the lamp voltage is increased as the document density increases in order to prevent the background of the copy from being fogged. And, as mentioned, this property is uniquely defined. In the present invention, in order to increase the contrast of the copied image and improve the fine line reproducibility even for a low-contrast original, the lamp voltage is set to a limit so that the copied image does not fog at any original density. It is done as follows.

That is, when the white lamp pattern 31 is exposed and the halftone density of the developed image is used as a reference as described above, the limit lamp voltage at which the copy image does not fog changes from V _L0 to V _L1 , The control characteristic line of OD- _VL as shown in FIG. 3 is displaced from the solid line position to the alternate long and short dash line position as shown in FIG. That is, the lamp voltage corresponding to the density of the white pattern 31 is shifted so as to change from V _L0 to V _L1 .

By doing so, when the image forming characteristics on the copying machine side such as the projection optical system 10, the photoconductor drum 2, the charging charger 14, and the developing device 15 are changed, OD and _VL which is uniquely determined in accordance therewith. In order to obtain a proper copy image by shifting and correcting the relational characteristic with respect to, the required lamp voltage can be more accurately determined by using the parallel movement portion of the characteristic. Therefore, regardless of the density of the original, fine line reproducibility can always be obtained, and a clearer copy image without background fog can be obtained.

It should be noted that, even if the lamp voltage is maximized when the original density is higher than a certain density, the background of the copied image cannot be skipped, but in FIG. 7, the higher density is represented by a broken line.

Next, specific control will be described based on the flowchart shown in FIG.

When the print key is pressed in step # 1, step #
In step 2, the lamp voltage data V _L is set to the initial value V _LT0, and in step # 3, the background density of the photoconductor drum 2 is measured without _{forming a} visible image on the photoconductor drum 2, and the sensor 21 at that time is measured.
The output of is V _S0 . After that, the photosensitive drum 2 rotates,
The charging charger 14 operates (step # 4). Next, the white pattern 31 is exposed and developed with the voltage V _L = V _LT0 of the exposure lamp 4, and the density of the visible image formed on the photoconductor drum 2 is measured, and then the output of the sensor 21 at that time is V _ST (steps # 5, # 6).

Now move to step # 6, Is calculated. k is the output of the sensor 21 when the background density of the drum surface is measured when there is no toner stain. As a result, the influence of toner stains can be removed from the output V _ST of the sensor 21.

Since the relationship between V _S and ID as shown in FIG. 5 is stored in the ROM 27, ID _T is next obtained from V _ST (step # 8).

When ID _T is a halftone density of 0.2 <ID _T <0.7, the process proceeds to step # 11 via steps # 9 and # 10, and V _LT = V _L. After this, in step # 12, the calculation of V _L1 = V _LT + (V _L0 −g) (ID _T )) is performed.

The function g (ID) is a white pattern as shown by the solid line in FIG.
31 is an initial characteristic of a function of ID with respect to V _L when 31 is exposed and developed, and V _L0 is a constant as a lamp voltage at which ID = 0.1. Both g (ID) and V _L0 are known in advance and stored in the ROM 27. In this way, the lamp voltage (equation) VL1 at which ID = 0.1 can be accurately obtained from ID _T and V _LT that are halftone concentrations.

Next, in step # 13, the characteristic (set value) of the lamp voltage with respect to the document density is shifted so that the lamp voltage at the density of the white pattern 31 becomes V _L1 as shown by the alternate long and short dash line in FIG. That is, assuming that the initial state of the characteristic of the lamp voltage V _L with respect to the document density OD is V _L = f (OD) and the lamp voltage at the density of the white pattern 31 is V _L0 , V _L = f (OD) + (V _L1 −V _L0 ) ... Performs the calculation.

As a result, the change in the image forming characteristics on the copying machine side is corrected to the relationship of the proper lamp voltage V _{L with} respect to the document density OD during automatic exposure, and in the next step # 14, the document density OD is measured, In step # 15, exposure is performed with a lamp voltage based on the above formula according to the document density, and copying is performed. When the image density on the photoconductor drum 2 is not a halftone of 0.2 <ID _T <0.7, if ID _T ≦ 0.2, the process proceeds from step # 10 to step # 17, and the lamp voltage V _L is reduced by ΔV. , The white pattern 31 is exposed and developed, and repeated until the visible image density becomes 0.2 <ID _T <0.7. If ID _T ≧ 0.7, the process proceeds from step # 9 to step # 16, the lamp voltage V _L is increased by ΔV, exposure and development are performed, and the same process is repeated until the visible image becomes 0.2 <ID _T <0.7.

When the image density on the photosensitive drum 2 becomes a halftone of 0.2 <ID _T <0.7, copying is performed as described above (step # 9-
# 15).

(Effect of the Invention) According to the present invention, since it has the above-described structure and operation, even if there is a change in the image forming characteristics of the copying machine, fine line reproducibility is good according to the density of the original, and proper copying without fogging on a white background. Images can be obtained stably.

Furthermore, by correcting the image density of the reference image on the basis of the background density of the photoconductor before the formation of the reference image, even if toner particles are present on the background of the photoconductor, the image density of the reference image is detected by the image density detecting means. It is possible to prevent the detection result of the image density from being affected, and more appropriate image reproducibility can be further stabilized.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of the present invention, FIG. 2 is a side view of an exposure and image forming unit of a copying machine showing an embodiment of the present invention, and FIG. 3 is a lamp for general document density in automatic exposure. FIG. 4 is a graph showing the relationship between the voltages, FIG. 4 is a graph showing the relationship between the sensor output when detecting the visible image density and the copy image density of the visible image, and FIG. 5 is a copy of the visible image density detection output and the visible image. FIG. 6 is a graph showing the relationship between the image density and the lamp voltage, FIG. 6 is a graph showing an example of changes in the characteristic line of FIG. 4 when the image forming characteristics of the copying machine are changed, and FIG. FIG. 8 is a graph showing an example in which the characteristic line of the lamp voltage is corrected and displaced according to the change in the image forming property on the copying machine side, and FIG. 8 is a flowchart of automatic exposure control. A: original density detecting means B: visible image density detecting means C: exposure lamp D: lamp voltage control means 1 ... copier 2 ... photoconductor drum 10 ... projection optical system 16 ... transfer sheet 20 , 21 …… Sensor 24 …… Microcomputer 25 …… CPU 26 …… Exposure control circuit 27 …… Memory circuit

Continuation of the front page (56) Reference JP 61-103169 (JP, A) JP 60-184240 (JP, A) JP 60-260072 (JP, A) JP 61-213865 (JP , A) JP 60-95453 (JP, A) JP 54-10743 (JP, A) JP 62-269943 (JP, A) JP 60-10269 (JP, A) JP 62-169182 (JP, A) Japanese Patent Sho 61-50425 (JP, B2)

Claims (2)

(57) [Claims] 1. An exposure lamp for illuminating an original image, a photoconductor for holding the original image illuminated by the exposure lamp as an electrostatic latent image, and a developing for visualizing the electrostatic latent image on the photoconductor. Means for detecting the image density of the original, and the original density detecting means for detecting a relationship between the image density of the original image and the lamp voltage of the exposure lamp by the original density detecting means. In a copying machine provided with a lamp voltage control means for determining an appropriate lamp voltage for the original image based on the image density of the original image thus formed, a reference latent image for the halftone density is formed on the photoconductor and the reference latent image is formed. A reference image forming unit that forms a reference image on the photoconductor by developing the latent image using a developing unit set to a predetermined developing condition, and detects the image density of the reference image. Visible image Based on the density detecting means and the image density of the reference image detected by the image density detecting means, the correction value of the lamp voltage is set so that the output copy image substantially matches the image density of the reference image. Based on the correction value determining means to be determined and the correction value determined by the correction value determining means,
And a lamp voltage characteristic correction means for correcting the lamp voltage characteristic of the lamp voltage control means. 2. The image density detecting means comprises correction means for correcting the image density of the reference image based on the background density of the photoconductor before the formation of the reference image. Copy machine.