JPH0158507B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a color image forming apparatus. In conventional color image forming devices, such as color copying devices, the surface potential on the photoreceptor on which a latent image is formed changes over time, making it difficult to obtain a copied image with stable color balance over a long period of time. was difficult. This problem will be explained below with reference to the gradation reproduction characteristics of color copying shown in FIG. In the figure, Do represents the original image density, Dp represents the print image density, E represents the exposure amount, and Vs represents the surface potential on the photoreceptor. Here, the first quadrant is the tone reproduction characteristic when a grayscale original is used. C, M, and Y are single colors of cyan, magenta, and yellow, respectively, and C+M+
Y indicates the tone reproduction characteristics of a copied image obtained by superimposing these three colors. The second quadrant when viewed clockwise is
The exposure characteristics of Do-E (common logarithm) are shown. The third quadrant shows the color separation latent image characteristics of red (R), green (G), and blue (B) on the photoreceptor. The fourth quadrant shows the development characteristics of cyan (C), magenta (M), and yellow (Y) toners. In the characteristics of the first quadrant, cyan, magenta,
The reason why the yellow print image density Dp is different from the same original image density Do is to correct unnecessary light absorption of each toner and maintain color balance. Currently, color balance is gray balance, so the unnecessary light absorption of blue and green in the cyan toner and the unnecessary light absorption of blue in the magenta toner are corrected, and the three colors of cyan, magenta and yellow are superimposed. The color balance in the image is maintained. Therefore, as shown in the third quadrant, the color separation latent image characteristics of red, green, and blue are almost the same, and as shown in the fourth quadrant, cyan,
There are differences in the development characteristics of magenta and yellow. Incidentally, in order to obtain a copied image with good color balance, it is necessary to keep the red, green, and blue color separation latent image characteristics of the photoreceptor constant for a long time. However, as mentioned above, this has been extremely difficult in the past. The reason for this was due to environmental changes such as temperature and humidity, deterioration of the photoreceptor, etc., and the latent image characteristics varied. Further, it is necessary to correct variations in development characteristics due to production lots of cyan, magenta, and yellow developers (toners), or variations in development characteristics due to changes in these developers over time.
To this end, in the past, the characteristics of the red, green, and blue color separation latent images were intentionally made different from the nearly uniform state shown in the third quadrant, and the characteristics were selected so as to have an appropriate relationship. However, it is extremely difficult to adjust the latent image characteristics of each color separation appropriately, and strictly speaking, it is necessary to carefully adjust the charger output, exposure amount, etc. of the color copying machine for each color using a surface electrometer and a recorder. It was necessary for the person to make adjustments. Therefore, it has been practically impossible for general users to make adjustments. SUMMARY OF THE INVENTION An object of the present invention is to provide a color image forming apparatus that eliminates the above-mentioned drawbacks and is capable of producing color images with good color balance regardless of environmental changes, deterioration of the photoreceptor, etc. be. That is, the present invention provides an electrophotographic means for forming a color image on a recording material by performing an image forming process for each color a plurality of times on a photoconductive photoreceptor, and a dark area surface potential on the photoreceptor after being charged. a detection means for detecting a bright area surface potential on the photoreceptor after static elimination; an A/D conversion means for converting an analog value outputted from the potential detection means into a digital value; and an output from the A/D conversion means. a predetermined control formula for the digital value in order to set operating conditions for each color of the electrophotographic means so that the digital value corresponding to the dark area surface potential or the bright area surface potential becomes a value corresponding to the target potential for each color; control means that calculates a control value according to the result of the calculation, D/A conversion means that converts the control value obtained by the control means into an analog value, and output from the D/A conversion means. A color image forming apparatus comprising: a driving means for driving the electrophotographic means based on an analog value; and a switching means capable of collectively switching the target potential for each color to another target potential for each color. This is what we provide. The present invention will be explained in detail below based on the drawings. FIG. 2 is a block diagram of a color copying apparatus according to an embodiment of the present invention. In the figure, a photosensitive member composed of a conductive layer, a CdS photoconductive layer, and an insulating layer is formed on the surface of a photosensitive drum 1 rotating in the direction a. A document to be copied is placed on document table glass 3 and illuminated by illumination lamp 5 . Scanning mirrors 7 and 9 that scan the original scan the original in synchronization with the rotation of the drum 1, and the mirrors 7 and 9 move to positions 7' and 9', respectively, and the illumination lamp 5 also moves to the position 5'. Move up to. The optical image of the scanned original is transmitted to a lens 11, a mirror 13, and a color separator 15.
An image is formed on the surface of the photosensitive drum 1 via the mirror 17 and the secondary charger 19 for simultaneous charge removal during exposure. A latent image is formed by such an exposure means. Here, the color separator 15 includes a blue filter 15B and a green filter 15B according to each color separation.
G, red filter 15R and ND filter 15N
These filters are rotated and each color separates the light. The surface of the photosensitive drum 1 is cleaned in advance by a blade cleaner 31, and the influence of the previous latent image is removed by a pre-exposure lamp 33 and a pre-static eliminator 35. Next, the photoreceptor surface is uniformly charged by a primary charger 37 to have a uniform potential. This charged photoreceptor surface is transferred to a secondary charger 1 along with the optical image of the original.
The static electricity is removed by 9, and then the entire surface exposure lamp 3
9, the entire surface is uniformly exposed, and a high contrast electrostatic latent image is formed on the surface of the photoreceptor. The drum 1 is placed between the entire surface exposure lamp 39 and the developing device 41.
The intensity of the electrostatic latent image, that is, the electrostatic potential, is detected by an electrometer probe 43 placed near the surface of the electrostatic latent image. The developing device 41 consists of a yellow developing device 41Y, a magenta developing device 41M, a cyan developing device 41C, and a black developing device 41B, and each color developer (toner) is
is supplied for development. Transfer paper 51 placed in a cassette is sent to a transfer section 55 by a delivery roller 53.
In the transfer section 55, first, the gripper 57 grips the leading edge of the transfer paper 51. Transfer paper 51 held
Then, the developed image on the photoreceptor surface of the photoreceptor drum 1 is transferred by the transfer corona discharger 59 from the back side of the paper by corona discharge. In the case of monochrome copying, the separation claw 63 is activated immediately after the static electricity is removed by the separation static eliminator 61.
The transfer paper 51 is separated from the transfer section 55. In the case of multi-color copying, the grippers 57 of the transfer section 55 are not released and the separation claws 63 are not operated until the transfer paper 5 is completely transferred.
Keep 1. After the transfer is completed, release the separation claw 63
The transfer sheet 51 is separated from the transfer section 55 by the operation of the transfer sheet 51, and is sent to the heating roller fixing device 67 by the conveyor belt 65, where the image is fixed. Transfer paper 51 after fixing
is discharged onto the tray 69. After the transfer is completed, the toner remaining on the surface of the photosensitive drum 1 is cleaned by a blade cleaner 31 in preparation for the next copying cycle. FIG. 4 is a sectional view showing a schematic configuration of the secondary charger 19 shown in FIG. 2. As shown in the figure, groups of wires are arranged close to and along the surface of the photosensitive drum 1 at the opening of the secondary charger 19 on the photosensitive drum 1 side. The respective wire groups are designated as negative grid 191, zero grid 193, and positive grid 195. These grid biases are negative grid 191, zero grid 193,
Positive grid 195 is -50 to -300V, 0V respectively
(ground), +50 to +200V. Here, the color balance of a color copy image will be explained. As shown in the first quadrant of Figure 1,
It is assumed that the image densities (color filter densities) of cyan, magenta, and yellow are made different from each other. The image density of magenta should be located between cyan and yellow, and in terms of color balance, the image density of cyan and yellow must be considered with the image density of magenta as the center. For color balance, the image density ratio of cyan to magenta is in the range of 1.5:1 to 1:1, preferably around 1.2:1. The magenta to yellow image density ratio is in the range of 1:0.9 to 1:0.6, preferably around 1:0.8. For example, if the maximum image density of magenta is 1.2, the maximum image density of cyan suitable for color balance is 1.44, and the allowable range is between 1.8 and 1.32. Further, the maximum image density of yellow is 0.96, and the allowable range is between 1.08 and 0.72. In this way, the tolerance range for cyan image density is higher than the center value, and the tolerance range for yellow image density is lower than the center value. Therefore, even if the initial development characteristics are ideal as shown in the fourth quadrant of FIG. It is convenient to shift them slightly so that red>green>blue. For example, in the third quadrant of FIG. 1, the potential of the non-exposed portion of the latent image (dark potential) V _D is approximately 380 (V) in red, green, and blue. For example,
Making the voltages different such as 400 (V), 380 (V), and 360 (V) is effective in overcoming problems such as fluctuations in development characteristics and accuracy in automatic control of surface potential. In the apparatus according to the embodiment of the present invention, there are three potential setting levels: dark areas during full-color copying (illumination lamp 5 is turned off), intermediate density areas (lamp 5 is turned on at an intermediate voltage), and bright areas (lamp 5 is set to the maximum rating). V DO and V _DO , respectively.
Set V _WLO and V _SLO as shown in Table 1 below.

[Table] FIG. 3 is a block diagram of a circuit that controls the potential. In the figure, a chopper disk 101 for detecting the rotation angle of the photosensitive drum 1 outputs a drum clock pulse 105 corresponding to the rotation angle from a photo interrupter 103. This clock pulse 105 is counted by the main sequence controller 107 of this copying machine, and information necessary for copying,
For example, it controls information signals such as the set number of sheets. The controller 107 supplies the potential control microcomputer 109 with timing signals necessary for switching high voltage and halogen light intensity, and measurement timing signals of the dark potential V _D , intermediate density potential V _WL , and light potential V _SL . . The latent image potential detected by the surface electrometer probe 43 is measured by the surface potential measuring circuit 111 as a potential of 1/300 of the surface potential, and
After being digitally converted by the converter 113, the signal is supplied to the computer 109. computer 109
performs calculations according to a control formula so that the measured potential value converges to the target value selected by the switch board 115. The signal resulting from the calculation is supplied to the DA converter 119 via the bus line 117 and converted into analog. Each analog-converted signal is supplied to high voltage control circuits 121, 123, 125 and mix circuit 127. Also computer 10
9 is the control signal 1 to the analog multiplexer 129.
31 and switches the image fine adjustment signal 135 from the fine adjustment board 133 to the high voltage control circuits 121 to 125.
and mix circuit 127. Analog signals 137I, 137G, 137V obtained from high voltage control circuits 121, 123, 125
Addition voltage signal 139 of and image fine adjustment signal 135
I, 139G, 139V is high voltage transformer 14
1, 143, and 145, respectively, and then supplied to the primary charger 37, the negative grid 191 of the secondary charger, and the secondary charger 19, respectively. This results in a primary charging current I ₁ , a negative grid V _G- and a secondary charging voltage
V ₂ is controlled. Also, an analog signal 137H and an image fine adjustment signal 135 from the mixer circuit 127 are provided.
The mixed signal 139H with the halogen control circuit 155
is supplied to control the halogen voltage V _Hl applied to the illumination (halogen) lamp 5. The computer 109 also has I/O data via a bus line 117.
A digital signal is supplied to the driver 161. By this I/O driver 161, 7 segments 8
The digit display 163 is scanned, the display signal 165 from the computer 109 is input to the BCD-7 segment driver 167, and its output signal 1
69, the photosensitive drum surface potential is displayed on the display 163. Further, the diode switch board 171 is scan-controlled via the I/O driver 161 to sequentially and individually select the target values set on the switch board 115. Send the voltage signal of this selected target value to the computer 109,
Perform calculations according to individual control formulas (described later),
It operates to converge to those target values. The signals based on the target values converged by these computers 109 are converted into analog signals by the D/A converter 119, and each of the analog-converted voltage signals is sent to each of the high voltage control circuits 121 to 125 and the mixer circuit 127. Supplied. Let's look at the control sequence of the control circuit shown in FIG. Before activating this control circuit, the operator of the copying machine performs the following operations. Place a blank sheet of paper (transfer paper) on the document table glass 3. Set the aperture on the copying machine to “5”
(Standard). Next, a target value is set using a target value setting changeover switch (connected to the switch board 115) provided outside the copying machine. After performing these operations, a control button (not shown) is pressed to activate the control circuit. The control operation is performed according to the flowchart shown in FIG. First, the power to the potential control circuit is turned on (block 401). Next, potential cleaning is performed by pre-rotating the photosensitive drum 1 (block 403).
In this state, the document illumination lamp 5 is turned on with the voltage V _Hl set to the maximum rated voltage, and the amount of light emitted from this lamp 5 is maximized. The color separator 15 is set as a filter so that the document image light with the maximum light amount passes through the ND filter 15N of the color separator 15.
The photosensitive drum 1 is rotated once to expose the surface of the photosensitive member. The electrometer probe 43 detects the bright area potential V _SL on the surface of the photoreceptor of the photosensitive drum 1, and supplies the detection signal to the potential measuring circuit 111 to measure the bright area potential V _SL (block 405). This measured light area potential V _SL and the target value of the light area potential V _SLO
It is determined whether the difference (|V _SL −V _SLO |) is within the allowable error (C ₁ ) (block 407). If the determination is negative, the secondary charging voltage V ₂ of the secondary charger 19
is controlled according to the control formula ΔV ₂ =δΔV _SL (block 409). Then, the process returns to block 405 and repeats the operation. Blocks 405, 407 and 409 are executed until the bright area potential V _SL obtained again by the secondary charging voltage V ₂ controlled by block 409 falls within the tolerance C ₁ and converges to its target value V _SLO . The operations in are repeated. allowable error
If the image is within _C1 and an affirmative determination is made in block 407, the color separator 15 is rotated and the blue filter 15B is set so that the original image light passes therethrough (block 411). In this step, the filter is switched in the order of green, red, ND, green, and red. The illumination lamp 5 is turned off and the photosensitive drum 1 is rotated once without exposing the original. Since the surface potential of the photosensitive member surface of the photosensitive drum 1 is the dark potential V _D , this dark potential V _D is detected and measured by the electrometer probe 43 (block 413). It is determined whether the difference between the measured dark potential V _D and its target value V _DO is within the tolerance C ₂ (block 415). If this determination is negative, the primary charging current I ₁ of the primary charger 37 is determined by the control formula ΔI ₁ =αΔV _D
(block 417). After that,
Returning to block 413, the operation is repeated. dark potential
When V _D converges to its target value V _DO within the tolerance C ₂ , the decision at block 415 becomes affirmative, exiting the loop and proceeding to the next operation. The illumination lamp 5 is emitted at the maximum rated voltage to maximize the exposure amount of the original. The photosensitive drum 1 is rotated and the surface of the photosensitive drum 1 is exposed to the maximum amount of light. In this state, the bright area potential _VSL, which is the surface potential, is detected and measured by the electrometer probe 43 (block 41).
9). The measured bright area potential V _SL and its target value V _SLO
It is determined whether the difference (|V _SL −V _SLO |) is within the tolerance C ₃ (block 421). If the determination is negative, the negative grid 191 in the secondary charger 19
The negative grid voltage V _G- is expressed by the control formula ΔV _G- = β ₁ ΔV _D
+β ₂ ΔV _SL (block 423).
The process then returns to block 419 to repeat the loop. The bright area potential V _SL is within the tolerance of its target value V _SLO
If it converges within _C3 , an affirmative determination is made in block 421 and the loop exits. The illumination lamp 5 is turned on with an intermediate halogen voltage V _Hl , and the original exposure amount is set to the standard light amount. Under such an amount of light, the photosensitive drum 1 is rotated to expose the surface of the photosensitive member. The potential on the surface of the photoreceptor becomes the intermediate density potential V _WL , and this potential V _WL is measured by the electrometer probe 43 and the potential measuring circuit 111 (block 425). This measured intermediate concentration potential
The difference between V _WL and its target value (|V _WL − V _WLO |) is
It is determined whether the error is within the tolerance _C4 (block 427). If the judgment is negative, the halogen voltage V _Hl for lighting the illumination lamp 5 is determined by the control formula ΔV _Hl =
control according to γΔV _WL (block 429). The process then returns to block 425 to repeat the loop. If the intermediate concentration portion potential V _WL converges to its target value V _WLO within the tolerance C ₄ , an affirmative determination is made in block 427 and the loop exits. Through such an operating procedure, the primary charging current in the blue filter 15B is reduced based on blocks 411 to 427.
I ₁ , the negative grid voltage V _G- and the halogen voltage V _Hl are controlled and set. Next, it is determined whether the set filter in the color separator 15 is the last one (block 431). In this case, since the determination is negative, the process returns to block 411. At block 411, the color separator 15 is rotated in the order of blue → green → red → ND → green → red to switch and set the filter. Each time a filter is set, the operations in blocks 413 to 429 are performed to determine the primary charging current I ₁ and negative grid voltage in that filter.
Control setting of V _G- and halogen voltage V _Hl . Such a loop operation is repeated, and when the control of each voltage is completed for the last filter, block 4
An affirmative determination is made in step 31, and the control operation of the control circuit ends. In this way, the tricolor blue,
The voltage relationship is set so that the surface potentials based on green, red, black and white, and the two colors magenta and black are set to predetermined values. Then, under the set control voltage relationship, the image of the original placed on the original platen glass 3 is copied in color onto the transfer paper 51. It is preferable to provide a changeover switch outside the copying apparatus so that the three colors, blue, green, red, and black and white, and the two colors, magenta and black, can be set individually in, for example, three levels. In the case of the device of this example, the switching setting can be made in three stages as shown in Table 1. To the target value set in this way,
The potentials respectively converge. Note that the bright area potential V _SL and intermediate density area potential V _WL in blocks 405, 419, and 425 are measured with the transfer paper placed on the document table glass 3.
This is performed after scanning the original at the same speed as during normal copying and forming a potential. Further, the coefficients γ, δ, α, β ₁ and β ₂ of each control equation shown in FIG. 5 indicate the slope of the function based on each relational equation. As shown in the flowchart of Figure 5, the secondary charging voltage V ₂
The reason why is controlled prior to the grid voltage V _G- , which is the grid bias voltage of the secondary charger 19, will be described. Refer to the configuration of the secondary charger 19 in FIG. 4. The distance between the wires of the grids 191 to 195 and the surface of the photosensitive drum 1 is usually 1.0±0.1 mm.
Within this distance tolerance, for example, the voltage applied to the discharge wire of the secondary charger 19 is -8.5kV, and the voltage applied to the discharge wire of the secondary charger 19 is set to -8.5kV, and
The applied voltages of 91, 193, and 195 are -
When set to 120V, 0V, and 100V, the bright area potential V _SL is −
Varies within the range of 120±30V. Therefore, the secondary charging voltage V ₂ is first controlled so as to offset variations between devices and obtain a constant bright area potential V _SL with a constant grid bias voltage. Thereafter, by applying a negative grid voltage V _G- , which is a bias voltage of the negative grid 191, the present applicant
The bright area potential _VSL is controlled based on the gradation control method presented in the specification of 54-14237 "Electrophotographic Method and Apparatus". In this manner, by setting the surface potential on the surface of the photoreceptor of the photosensitive drum 1 for each color separation exposure with the transfer paper placed on the document platen glass 3, an appropriate color balance can be automatically obtained. Thereafter, by placing the original to be copied on the original table glass 3 and copying it, a copied image can be obtained on the transfer paper 51 with appropriate color balance. In addition, since the surface potential is set for each color, the contrast of the color copy image can be changed arbitrarily, and even if you want to add strength or weakness to only a certain color, you can change the color by that color alone. This can be handled by changing the potential characteristics of the resolved latent image. Furthermore, even if there is a change in the development characteristics of the toner, it is possible to change the target potential value and change the set value of the surface potential for each color, so it is possible to deal with a wide range of changes. As described above, according to the present invention, the operating conditions of the image forming means are set for each color according to the surface potential of the dark or bright part of the photoreceptor after charging or neutralization and the target potential for each color. Since the target potential for each color can be switched to another target potential at once, it is possible to produce color images with an appropriate color balance depending on the density of the original, regardless of photoreceptor deterioration or environmental changes. It becomes possible to obtain.

[Brief explanation of drawings]

Figure 1 is a diagram showing the gradation reproduction characteristics of color copying.
FIG. 2 is a device configuration diagram of a color copying apparatus according to an embodiment of the present invention, FIG. 3 is a block diagram of a control circuit in the device shown in FIG. 2, and FIG. 4 is a schematic configuration diagram of a secondary charger. FIG. 5 is a flowchart showing the operation of FIG. 3. 1...Photosensitive drum, 3...Original table glass, 5...
...Illumination lamp, 15...Color separator, 19...Secondary charger, 41...Developer, 43...Electrometer probe, 51...Transfer paper, 55...Transfer section, 59...
Transfer corona discharger, 67...Heating roller fixing device,
101... Chippad disk, 107... Main sequence controller, 109... Microcomputer, 111... Surface potential measurement circuit, 11
5...Switch board, 133...Fine adjustment board,
161...I/O driver, 163...display device,
171...Diode switch board.

Claims (1)

[Scope of Claims] 1. An electrophotographic means for forming a color image on a recording material by performing an image forming process for each color multiple times on a photoconductive photoreceptor, a dark area surface on the photoreceptor after being charged. Detection means for detecting a potential or a bright area surface potential on the photoreceptor after neutralization; A/D conversion means for converting an analog value output from the potential detection means into a digital value; and an output from the A/D conversion means. A predetermined value is set for the digital value in order to set the operating conditions for each color of the electrophotographic means so that the digital value corresponding to the dark area surface potential or the bright area surface potential to be applied becomes a value corresponding to the target potential for each color. A control means that calculates a control value according to a control formula and obtains a control value according to the result of the calculation; a D/A conversion means that converts the control value obtained by the control means into an analog value; and an output from the D/A conversion means. a driving means for driving the electrophotographic means based on an analog value obtained by the color image formation, and a switching means capable of collectively switching the target potential for each color to another target potential for each color. Device.