JP3217584B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to an image forming apparatus, and more particularly, it relates to image forming conditions for forming a toner image on an image carrier, such as primary charging, image exposure, and development of the image carrier. The present invention relates to an image forming apparatus such as an electrophotographic copying apparatus having a control system for controlling.

[0002]

2. Description of the Related Art In an image forming apparatus, in particular, an electrophotographic copying apparatus, the following measures are generally taken so as to stably obtain a good quality copy from the apparatus.

That is, before executing an image forming process for forming a toner image on a photosensitive drum as an image carrier provided in an image forming apparatus, for example, a charging means such as a primary charger or a laser beam scanner such as a laser beam scanner is used. The exposing means is driven to form a bright area and a dark area on the photosensitive drum, and a surface potential (i.e., a bright potential) in these bright areas.
And the surface potential in the dark area (i.e., the dark area potential) is detected by a known surface potential detecting means, and each detection value output from the detecting means is made to converge to a target value of a light area potential or a target value of a dark area potential set in advance. As described above, a method of controlling the high voltage output and / or the grid bias voltage of the primary charger according to each detection value is the method.

[0004]

By the way, the above-mentioned surface potential detecting means detects a light portion potential or a dark portion potential on the photosensitive drum and controls the surface potential on the photosensitive drum to converge to a target value. In the method, since the process of idling the photosensitive drum a predetermined number of times is an essential condition, when performing a continuous image forming process of continuously forming an image on the photosensitive drum,
The above control cannot be performed.

Therefore, if the characteristics of the photosensitive drum fluctuate for some reason while the image forming apparatus is performing the continuous image forming process, the fluctuation also changes the light portion potential and the dark portion potential. There is a drawback that the contrast potential of the latent image changes and the density of the toner image formed on the photosensitive drum changes.

Therefore, for the purpose of resolving such disadvantages, a proposal has been made as described in, for example, Japanese Patent Application Laid-Open No. 58-145972. This proposal,
The present invention relates to an electrophotographic copying apparatus using a so-called reversal developing method. In brief, during a continuous image forming process, a known surface potential detecting means detects a bright portion potential on a photosensitive drum to detect a potential. It is characterized in that a developing bias applied to a developing sleeve of a developing device for developing an electrostatic latent image formed on a photosensitive drum is corrected according to a detection value output from the means. For details of this content, refer to the above publication.

However, in the apparatus according to the above proposal, when the absolute value of the difference between the dark portion potential and the bright portion potential on the photosensitive drum, that is, | dark portion potential−bright portion potential |
The absolute value of the difference between the dark portion potential and the developing bias applied to the developing sleeve, ie, | dark portion potential-developing bias |, or the absolute value of the difference between the bright portion potential and the developing bias, ie, | bright portion potential-developing bias | Since either one of these changes, if the developing bias is corrected to maintain the electrostatic latent image contrast, fogging is likely to occur, and if the developing bias is corrected to suppress fogging, There is a problem that the electrostatic latent image contrast changes.

That is, before executing an image forming process for forming a toner image on a photosensitive drum which is an image carrier provided in an image forming apparatus, for example, a charging means such as a primary charger or a laser beam scanner. The exposure unit is driven to form a light area and a dark area on the photosensitive drum, and the surface potential in the light area (ie, the light area potential) and the surface potential in the dark area (ie, the dark area potential) are known. The primary charger according to each of the detected values so that each detected value detected by the surface potential detecting means and the detected value output from the detecting means converge to a preset target value of a light portion potential or a target value of a dark portion potential. This is a method of controlling the high-voltage output and / or the grid bias voltage of the power supply.

[0009]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image carrier, a charging unit having a grid, and charging the image carrier, an exposure unit for exposing the image carrier charged by the charging unit, and a charging unit formed on the image carrier. Developing means for developing the formed electrostatic image with toner, detecting means for detecting surface potentials of dark and light portions of the image carrier, and controlling a grid bias and a developing DC bias in accordance with an output of the detecting means. And a control unit, wherein during the continuous image formation, the detecting unit detects at least one surface potential of the dark part and the light part, and based on the detected surface potential, the grid bias and the grid bias. A correction target value of the development DC bias is determined, and the control unit controls the grid bias and the development DC bias each time an image is formed. When controlling so as to gradually approach the positive target value, the number of images formed until the grid bias and the developing DC bias reach the correction target value is fixed. Device. According to one embodiment of the present invention, before the continuous image formation is performed,
The grid bias and the developing DC bias are determined according to the surface potentials of the dark part and the light part.
According to another embodiment, before the continuous image formation is performed, charging by the charging unit and exposure by the exposure unit are performed with a plurality of different grid biases, and the dark portion and the bright portion obtained by the charging are performed. The grid bias and the developing DC bias are determined according to the plurality of surface potentials. According to another embodiment of the present invention, before the continuous image formation is performed, the grid bias and the developing DC bias are a potential difference between the surface potential of the dark part and the developing DC bias, and the developing DC bias. The potential difference from the surface potential of the bright portion is determined to be a predetermined value. According to another embodiment of the present invention, the correction target values of the grid bias and the developing DC bias are a potential difference between a surface potential of the dark part and the developing DC bias, and a potential difference between the developing DC bias and the surface of the bright part. The potential difference from the potential is determined to be a predetermined value. In the present invention, according to one embodiment, in the correction of the grid bias and the developing DC bias during the continuous image formation, the potential of the light portion detected during the continuous image formation exceeds a predetermined value. When done. According to another embodiment, the correction of the grid bias and the developing DC bias during the continuous image formation is performed when the potential of the dark portion detected during the continuous image formation exceeds a predetermined value. . Furthermore, in the present invention, according to one embodiment, the toner is a color toner, and the apparatus is capable of forming a color image.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an image forming apparatus according to the present invention will be described in detail with reference to the drawings.

Embodiment 1 FIG. 1 is a schematic explanatory view showing an embodiment of the image forming apparatus of the present invention. In this embodiment, a three-color image forming apparatus will be described.

The image forming apparatus basically includes an optical system constituting exposure means for exposing given document information, a copying system for receiving and copying a document image exposed by the optical system, and an optical system. And a drive system for supplying a drive power supply for driving the copy system, and a control system for controlling the drive of the drive system.

As shown in FIG. 1, the optical system comprises a scanning mirror 2 and an fθ lens 3 disposed below a platen glass (not shown). The copying system includes a primary charging device (charging device) having an image carrier, that is, a photosensitive drum 4, a cleaning device 13, a static eliminator 5, and a grid 16 serving as an image forming device disposed near the outer peripheral surface of the photosensitive drum 4. It comprises six and three developing devices (developing means) 9A, 9B and 9C, a transfer drum 11, a transfer charger 10, a cassette 18, a separating means 19, a fixing device 23 and a tray 24. The driving system includes the developing bias voltage control circuits 8a, 8b and 8c, the laser control unit 1, the high voltage control units 15 and 17, and the like. The control system includes a surface potential detecting means, that is, a surface potential meter 14, an A / D converter 20, a potential control microcomputer (control means) 21 having functions as operation control means and correction means, and a D / A. It comprises a converter 22.

The scanning mirror 2 receives the modulated laser light output from the laser control unit 1 and moves along with the fθ lens 3 in the longitudinal direction of the photosensitive drum 4 (that is, from the near side in FIG. 1 to the back side in FIG. 1). Direction). The laser beam exposure method in this type of image forming apparatus includes a so-called image scanning method in which toner is adhered to a portion to which a light beam is applied by reversal development, and a normal method in which a light beam is applied to a background portion and an unexposed portion is applied. There is a so-called background scanning method in which toner is adhered by development.

The photosensitive drum 4 has a photosensitive member formed on an outer peripheral portion. The photosensitive drum 4 is supported by a shaft means (not shown) so as to be rotatable only in an arrow direction (clockwise direction) in FIG. . As described above, the cleaning unit 13, the static eliminator 5, and the grid 16 are provided around the photosensitive drum 4 as image forming means necessary for forming a visible toner image together with the photosensitive drum 4, as described above. A primary charger 6, developing units 9A, 9B, 9C and the like are provided.

The cleaning device 13 includes a photosensitive drum 4
And scrapes and collects the developer remaining on the photoreceptor, that is, the toner. The static eliminator 5 includes the cleaning device 1
The photosensitive drum 4 is disposed downstream of the photosensitive drum 4 in the rotation direction, and is configured to remove charges remaining on the photosensitive drum of the photosensitive drum 4.

The primary charger 6 is a photosensitive drum 4 of the static eliminator 5.
The grid 16 is disposed in an opposing gap formed between the primary charger 6 and the photosensitive drum 4. The primary charger 6 with the grid 16 is
The cleaning device 13 removes the toner, and the charge removal device 5 removes the remaining charge to uniformly charge the photosensitive member.

In a section from the position where the primary charger 6 is provided to the position where the developing device 9A is provided on the downstream side in the rotation direction of the photosensitive drum 4, the peripheral green portion of the photosensitive drum 4 is a space, The scanning on the photosensitive drum 4 by the scanning mirror 2 and the fθ lens 3 is enabled. A probe 14a of a surface voltmeter 14 is disposed close to the surface of the photosensitive drum 4 on the downstream side in the rotation direction of the photosensitive drum 4 from a scanning position on the photosensitive drum 4 by the optical system such as the scanning mirror 2 or the like. The surface potential of an electrostatic latent image formed on the photosensitive drum 4 by exposure with a laser beam can be detected.

The developing units 9A, 9B and 9C are arranged in this order downstream of the probe 14a in the rotation direction of the photosensitive drum 4. The developing device 9A accommodates a yellow toner for developing the electrostatic latent image of the yellow component color formed on the photosensitive drum 4, and carries the yellow toner at a developing position facing the photosensitive drum 4. A developing sleeve 91 to be conveyed is provided. The yellow toner conveyed to the developing position adheres to the latent image on the photosensitive drum 4 by application of a developing bias to the developing sleeve 91 and is used to develop the latent image. Similarly, a magenta toner for developing a magenta component color latent image is accommodated in the developing device 9B, and a developing sleeve 92 is provided therein. In the developing device 9C, a cyan component color latent image is developed. Cyan toner is stored, and a developing sleeve 93 is provided.

The transfer drum 11 is disposed downstream of the developing device 9C in the rotation direction of the photosensitive drum 4 so that its outer peripheral surface can contact the outer peripheral surface of the photosensitive drum 4. This transfer drum 1
1 is supported by a shaft means (not shown) so as to be rotatable only in the arrow direction (counterclockwise direction) in FIG. The transfer drum 11 adsorbs and carries a transfer material such as transfer paper on the outer peripheral surface, and conveys the transfer material toward the photosensitive drum 4 by rotating in the direction of the arrow in FIG. . The transfer charger 10 transfers the transfer drum 1 near the position where the outer peripheral surface of the transfer drum 11 contacts the outer peripheral surface of the photosensitive drum 4.
1 is provided for transferring the toner image formed on the photosensitive drum 4 onto a transfer material carried on the transfer drum 11.

The cassette 18 is disposed upstream of the transfer drum 11 of the transfer charger 10 in the rotation direction.
To supply the transfer material. The separation unit 19 is provided near the outer peripheral surface of the transfer drum 11 at a position downstream of the transfer charger 10 in the rotation direction of the transfer drum 11, and transfers the transfer material adsorbed to the outer peripheral surface of the transfer drum 11 to the transfer drum 11. 11 is separated. The fixing device 23 receives the transfer material separated from the transfer drum 11 by the separation unit 19, performs color mixing and fixing of the toner images of each color on the transfer material to form a final color image, and then transfers the transfer material. Is discharged to the tray 24.

An image forming process for forming an electrostatic latent image on the photosensitive drum 4, developing the same, and transferring the obtained toner image onto a transfer material to obtain an image is already known.
First, a yellow component color electrostatic latent image is formed on the photosensitive drum 4 by image exposure corresponding to the yellow component color, and a developing bias is applied to the developing sleeve 91 of the developing device 9A corresponding to the electrostatic latent image to remove yellow toner. The toner image is adhered to the latent image and developed to form a yellow toner image, and the yellow toner image is transferred onto a transfer material on the transfer drum 11. The above operation is sequentially performed for the magenta component color and the cyan component color, and a color image is obtained in which three color toner images of yellow, magenta, and cyan are multiple-transferred and superimposed on the transfer material.

The developing bias voltage control circuits 8a, 8b and 8c are under the control of a control system described later, and based on output signals from the control system, the developing devices 9A,
The developing bias voltages applied to the developing sleeves 91, 92, and 93 of 9B and 9C are controlled. The laser control unit 1 controls the intensity of a laser beam supplied to an optical system based on an output signal from a control system and an image input signal given from the outside. The high voltage control units 15 and 17 are also under the control of the control system, and control the charging current applied to the primary charger 6 by the high voltage control unit 15 based on the output signal from the control system. Is to control the grid bias voltage applied to the grid 16.

The surface voltmeter 14 constitutes a part of a control system, detects the surface potential on the photosensitive drum 4 via a probe 14a, and controls the detected value via an A / D converter 20 for potential control. To the microcomputer 21 for use. A / D
The converter 20 converts a detection signal of the surface potential of the analog signal from the surface electrometer 14 into a digital signal. The output signal from the microcomputer 21 is D /
It is output to each drive system such as the developing bias voltage control circuit 8a via the A converter 22. The D / A converter 22 converts an output signal of a digital signal of the microcomputer 21 into an analog signal.

The potential control microcomputer 21 comprises:
The detection value data input from the surface voltmeter 14 from the A / D converter 20 is stored, and an arithmetic operation and a logical operation are performed according to an algorithm described later, and the data obtained by the operation is stored. Then, a predetermined signal is output to various drive systems via the D / A converter 22 as described above.

The control of each drive system including the above-described developing bias voltage control circuit 8a by the potential control microcomputer 21 includes the control of setting the initial image forming conditions before the image forming process is executed and the control of the continuous image forming. The control is roughly divided into the control at the time of correction of the initial image forming condition during the execution of the forming process, and is executed according to the flowcharts of FIGS. 2 and 3, respectively. Hereinafter, the control process at the time of setting the initial image forming condition will be described with reference to the flowchart of FIG. 2, and then the control process at the time of correcting the image forming condition will be described with reference to the flowchart of FIG.

Dark portion potential V _D and light portion potential V _L is a linear, respectively relative to the experimental data (grid bias voltage V _G as illustrated prior, in FIG. 4, for example, in performing a series of processes shown in FIG. 2 it from the show), the variation of the grid bias voltage V _G applied to the grid 16 and, relational expression that indicates the relationship between the amount of change in dark-area potential V _D and light portion potential V _L on the photosensitive drum 4 at that time Is derived, and the obtained relational expression is stored in the memory of the potential control microcomputer 21.

That is, immediately after shifting to the routine for setting initial image forming conditions, the potential control microcomputer 2
When it is confirmed that the photosensitive drum 4 is in the pre-rotation state (step 101), the process proceeds to step 103.

In step 103, the microcomputer 21 sets the grid bias voltage to V _G1 (initial value) via the high-voltage control unit 17, and thereafter, the dark portion potential V _D1 from the surface electrometer 14 at that time, and and a light portion potential V _L1 from the surface electrometer 14 after the laser exposure by a laser control unit 1 against the non-image area of the photosensitive drum 4,
Read (ie, measure) each. In general the dark potential V _D when the grid bias voltage V _G1 V _D1, denoted the bright potential V _L and V _L1, hereinafter equivalent thereto.

Next, the grid bias voltage is set to V _G2 (V _G2
≠ V _G1 ), and similarly, the surface electrometer 1
The dark potential V _D2 from 4, after the new laser exposure under the same conditions of a light portion potential V _L2 read respectively (Step 10
5).

In step 107, the values of the read dark portion potentials V _D1 and V _D2 and the bright portion potentials V _L1 and V _L2 are read.
Index change from values for V _G of V _D alpha, a change rate β for V _G of V _L, is calculated using the pre-computing equation stored in the memory and V _G1, V _G2.

Α = (V _D2 −V _D1 ) / (V _G2 −V _G1 ) (1) β = (V _L2 −V _L1 ) / (V _G2 −V _G1 ) (2) (1) reacting a solving for V _D2, relationship for V _G and V _{D is, V D2 = α (V G2} -V G1) + V D1 ··· (3) , and the addition of the (2), relationship for V _G and V _{L is, V L2 = β (V G2} -V G1) + V L1 ··· (4) and it is learned.

In the image forming apparatus of this embodiment, as described above, the toner images of three colors, yellow, magenta, and cyan, are multiplex-transferred onto the transfer material, as shown in FIG.
Since the characteristics of the toner image are different depending on the colors of yellow, magenta and cyan, for example, three kinds of electrostatic latent image contrasts are prepared in order to obtain an image with good color balance.

The image density value of the toner image is determined by the DC voltage of the developing bias applied to each developing sleeve shown in FIG. 1 (hereinafter simply referred to as "developing bias voltage" unless confused).
Is V _DC , when the above-described image scanning method exposure is adopted, it corresponds to the latent image contrast potential V _Cont = V _DC −V _L shown in FIG. 6A, and the photosensitive image shown in FIG. In order to prevent fogging from occurring in the non-exposed area on the drum 4, the value of the background potential V _Back = V _D -V _DC needs to be equal to or higher than a predetermined fixed voltage value.

[0035] Therefore, in the present invention, the target value data of V _{Co nt} in advance in the memory of the potential control microcomputer 21,
V _Back target value data for each of three colors, that is, three types, are input, and V _Gi and V _DC corresponding to the three types of target values V _{Cont, i} , V _{Back , i} are obtained by the following process. _{, i}
(I is an integer of 1 to 3, for example, when yellow is the first color, magenta is the second color, and cyan is the third color, i = 1 is yellow, i = 2 is magenta. , I = 3 indicates the case of cyan).

First, as shown in FIG. 6, V _D , V _L ,
From the relationship between V _Cont and V _Back , V _Di -V _Li = V _{Cont, i} + V _{Back, i} (5) V _{DC, i} = V _Li + V _{Cont, i.} .

Next, equation (1), equation (2) (or equation (3),
(Equation (4)) is solved for V _G2 , and if V _G2 → V _Gi , V _Gi = {(V _Di −V _Li ) −V _D1 −V _L1 } / (α−β) + V _G1. since - (7) can be obtained, by substituting equation (5) into equation (7), the target value _{V Cont, i, V Back,} V Gi corresponding to _{i is,} V _Gi = {V _{Cont, i} + V _{Back, i−} (V _D1 −V _L1 )｝ / (α−β) + V _G1 (8) is obtained (step 109).

Since all the values in the expression (8) are known numbers, V _Gi is uniquely determined. If V _Gi is determined, the above expression (3), expression (4) (or (1) Equation, Equation (2)),
From equation (6), V _Di , V _Li , and V _{DC, i} are obtained.

Then, in step 111, from the above equations (8), (1), (2) and (6), i is i = i
V _Gi , V _Di , V _Li , and V _{DC, i} at 1, 2, and 3 are calculated (that is, V _Gi , V
_G , V _D , V _L , and V _DC ) are stored as initial image forming condition setting values. After that, step 113
In reading the V _Gi, V _{DC, i,} performs the image forming process on the basis thereof V _Gi, V _DC, the _i value (step 11
5).

Next, a control process for correcting image forming conditions during continuous image formation shown in FIG. 3 will be described. During the continuous image forming process, it is unavoidable that a change in the characteristics of the photoconductor of the photosensitive drum 4 is unavoidable. Such a change in characteristics is particularly remarkable in the case where an organic photoconductor is used as the photoconductor. is there. Therefore, during execution of the continuous image forming process, there is such a change in the characteristics of the photosensitive member, and the photosensitive drum 4
Even when the upper light portion potential _VL rises, the process shown in FIG. 3 is executed to keep the electrostatic latent image contrast constant without hindering image formation.

As described above, when _VL rises, the value of the light portion potential shifts from _VL to _VL 'as shown in FIG. 7, so that the developing bias voltages _VDC and _VL
And the latent image contrast potential V _Cont (= V _DC
−V _L ), and the density of the toner image also decreases.

Therefore, in the process shown in FIG. 3, it is confirmed that the above-described three color images of yellow, magenta and cyan have been formed (step 131).
If it is determined that the continuous color image forming process is being executed (step 133), it is recognized that the three-color image forming operation has been executed five times in a row (step 135). Then, primary charging and image exposure are performed, and a bright portion potential in the non-image area is detected by the surface voltmeter 14, and the detected value V _Li 'of the bright portion potential is read (step 137).

In step 139, the voltage increase V _I = V _Li ′ −V _{Li of the} bright portion potential V _Li is applied to a predetermined voltage (here, 10.
When it is confirmed that in excess of the _{V), V Di '-V Li} ' = V Cont, i + V Back, i + V I ··· (9) V DC, i '= V Di' -V Back, i ... so that (10), high-voltage control unit 17 via a with increasing the grid bias voltage V _G applied to the grid 16, the developing bias voltage control circuit 8a, 8b, each of the developing sleeve 91 through 8c , 92, 93 by increasing the developing bias voltage V _{DC, i} so that V _{Cont, i} and V _{Back, i} always coincide with the respective target values. What is necessary is just to correct the image forming conditions (step 141).

However, when the initial image forming conditions are corrected, the electrostatic latent image contrast changes before and after the correction, so that the image density greatly changes before and after the correction. That is, when a continuous image is formed, the electrostatic latent image contrast gradually decreases as compared with the initial stage, and the image density decreases. At this time, when the initial image condition is corrected, the electrostatic latent image contrast becomes the same as the initial stage. As shown in FIG. 8, a sharp density difference occurs before and after the correction. Also, when calculating the correction value, a large correction value may be calculated based on an abnormal value of the surface potential caused by a pinhole or the like of the photosensitive drum 4 or when the correction value is calculated based on erroneous detection of the surface potential due to noise or the like. There is a problem that an image density difference occurs.

Therefore, in the present embodiment, the target V _{Cont, i} , V
After calculating the target values of the grid bias voltage and the developing bias voltage necessary to achieve _Back, that is, the correction values of the grid bias voltage and the developing bias voltage by using the equations (9) and (10), the correction values are increased. Instead of all at once.

More specifically, as described above, if the initial image conditions are corrected during execution of the continuous image forming process, an image density difference occurs before and after the correction is performed. If this image density difference is simply caused by the electrostatic latent image contrast, the difference between the electrostatic latent image contrast before and after the correction is about 10 V, and when the image density difference is large, about 0.2 V.
About 0.3 occur. Also, when an abnormal value of the surface potential due to a pinhole or the like of the photosensitive drum 4 cannot be detected as abnormal, or when an erroneous detection due to noise or the like occurs, there is a difference in electrostatic latent image contrast before and after correction of about 10 V to 30 V. The image density has a difference of about 0.2 to 0.5. When there are two consecutive images having such a difference in image density, the difference is particularly noticeable. In order to make the difference in image density between two consecutive sheets inconspicuous, control as shown in FIG. 9B was performed.

Until now, as shown in FIG. 9A, for example, if the target value after the correction of the grid bias voltage has a difference of 20 V from that before the correction, the correction target value is immediately set when the correction operation is started. In the present embodiment, while FIG.
As shown in (b), the grid bias is approached to the target value by 5 V for each image forming sheet, increased by 20 V for four image forming sheets, and reached the target value for the fourth sheet. . The same applies to the developing bias voltage.

Therefore, the number of sheets reaching the set target value is not necessarily the same between the grid bias voltage and the developing bias voltage. By performing such control, the density difference between two consecutive images can be reduced to within 0.05. It can be suppressed.

In the present embodiment, when the correction target value is controlled stepwise, a constant value (5 V in this example) is determined and stepped up, but the number of sheets until reaching the correction target value is fixed. Alternatively, the correction amount per sheet may be determined from the number of sheets, and the same effect can be obtained. In particular, according to this configuration, the number of image formations performed before the grid bias and the development DC bias reach the correction target value is fixed, so that a sharp difference in image density before and after correction is suppressed, and Even when the grid bias and the developing DC bias are largely different from the correction target value, it is possible to prevent the current grid bias and the developing DC bias from converging on the correction target value.

The contents described above relate only to the image forming apparatus according to one embodiment of the present invention, and do not mean that the present invention is limited to only the above contents. For example, when it is recognized that the three-color image forming operation has been performed five times, the operation of measuring the bright portion potential only once in the non-image forming area on the photosensitive drum 4 or the increase of _VL becomes 10 V or more. In this case, the operation of correcting the initial image forming conditions so that V _{Cont, i} and V _Back , _i match the target values is merely one example for explaining the contents of the present invention. It is not intended to limit the present invention. In the above embodiment, V _L during continuous image formation
Has been described only measures for increasing the sequence according to the present invention (i.e., V _G and V _D, V _G and V _L leave derive the relationship, V _D running continuous image formation process,
Process shake the proper values of V _G according to the measured value of V _L)
Is of course also applicable to measures changes in the change and / or V _L the V _D running continuous imaging process.

As described above, in the image forming apparatus according to one embodiment of the present invention, the grid bias voltage V _G is changed as V _G1 and V _G2 at the beginning of the control sequence,
Dark potential V _D at that time, the bright potential V _L measured, V _G
And V _D, although derived the relationship between V _L, as compensation in the case of an unevenness of local surface potential due to causes such as pinholes potential measurement position of the photosensitive drum, the same charging conditions, the same the different sites of the photosensitive drum above in exposure condition as measured a plurality of times, based on the data processed by excluding the effect of the abnormal value of the surface potential, V _G and V _D, is possible to derive the relationship between V _L More desirable. As an example of such data processing, there is a method of truncating the lowest value among a plurality of measurement data and averaging the remaining data.

[0052] In the one embodiment of the present invention described above, it is assumed that control the grid bias voltage V _G and the developing bias voltage V _DC, the laser control unit 1, controls the drive of the high-voltage control unit 15 To control the laser light intensity and / or the charging current of the primary charger 6, or to control one or more of the laser light intensity, the charging current of the primary charger 6, the grid bias voltage, and the developing bias voltage In this case, it is possible to perform the same process as described above.

When controlling the charging current to the primary charger 6 or controlling the laser beam intensity, a pattern of a plurality of curves as experimental data showing the relationship between the charging current and the surface potential shown in FIG. And a plurality of curve patterns, which are experimental data indicating the relationship between the laser beam intensity (exposure amount) and the surface potential shown in FIG. 11, are stored in advance in the memory of the potential control microcomputer 21 and detected. One curve pattern is selected from the plurality of curve patterns in accordance with the surface potential value to control the charging current value and the laser beam intensity value.

The present invention can of course be applied to an image forming apparatus using a background scanning method and an analog image forming apparatus in addition to the image forming apparatus using the above-described image scanning method.

Embodiment 2 FIG. 12 is a schematic structural view showing another embodiment of the image forming apparatus of the present invention. The image forming apparatus according to the present embodiment is applied to a multicolor electrophotographic copying machine having four image units I to IV. FIG. 13 is an enlarged view of the first image forming unit I.

In the present multicolor electrophotographic copying machine, each of the image forming units I to IV has photosensitive drums 4a to 4d, around which primary chargers 6a to 6d with grids, exposing means 7a to 7d, developing devices 9a to 9d, transfer chargers 10a to 1
0d, cleaners 13a to 13d are arranged, and the photosensitive drums 4a to 4d are further penetrated through the image forming units I to IV.
An endless transfer material carrying belt 4 is provided as a conveying means below d.
1 to transfer the transfer material P fed by the feed roller so that the transfer material P comes into contact with the photosensitive drums 4a to 4d of the image forming units I to IV at the positions where the transfer chargers 10a to 10d are provided. It is configured.

Further, in order to charge the photosensitive drums 4a to 4d and simultaneously expose the entire surface, auxiliary chargers 15a to 15d and a discharge lamp (only the discharge lamp 25a of the first color is shown in FIG. 11) are provided on the surfaces of the photosensitive drums 4a to 4d. Are provided one above the other at the same location.

The image forming process by the copying machine of the present embodiment
Although the detailed description is omitted because it is basically the same as that of the first embodiment, the photosensitive drums 4a to 4d are also used in this embodiment.
Means that the residual toner on the surface is cleaner 13a-1
After being removed by 3d, the auxiliary chargers 15a to 15d
As a result, the photosensitive drums 4a to 4d are charged to the same polarity as the electrostatic latent image formed thereon, that is, to the negative polarity, and at the same time, are uniformly exposed by the neutralizing lamps 25a to 25d, so that the memory-effect area and the normal Both of the regions are neutralized so that the surface potential becomes approximately 0V. Thereafter, the primary chargers 6a to 6d,
By the operation of the laser beam exposure devices 7a to 7d of the image exposure means, a color-separated electrostatic latent image corresponding to the image exposure pattern is formed on the photosensitive drums 4a to 4d.
9d, the electrostatic latent images are visualized by developing with yellow, magenta, cyan, and black toners, respectively, and then these visible images are carried by the transfer chargers 10a to 10d. The image is sequentially transferred onto the transfer material P carried on the belt 41, and a full color is formed on the transfer material P.

The image forming apparatus of this embodiment can form a continuous image of up to 999 sheets. The correction of the initial image setting conditions at the time of continuous image formation is performed every 100 continuous images in order to increase the number of output images per unit time.

In this embodiment, the correction at the time of continuous image formation is
It is characterized in that it is performed only at the dark portion potential in the non-image area. This will be specifically described below.

FIG. 14 is a diagram showing a case where the dark portion potential is corrected in the non-image area during the continuous image forming process. In the present embodiment, the relationship between the grid bias voltage and the dark portion potential stored at the time of setting the initial image forming conditions is applied at the time of correction, and the initial dark portion potential at the initial grid bias voltage is compared with the dark portion potential at the time of correction. Let it. That is, at the time of correction, primary charging is performed with the initial grid bias voltage V _Gi , exposure is performed, and the dark portion potential V _Di ′ of the non-image area is measured. Then, the dark portion potential V _Di ′ and the initial grid bias voltage V _Gi
Calculates a difference V _I = V _Di -V _Di 'between the initial dark potential V _Di when the.

[0062] The image forming apparatus of this embodiment, for the dark portion potential of the correction time, the correction since the linearity of the dark potential with respect to the grid bias voltage is compensated, based on the difference value V _I of the dark area potential obtained The grid bias voltage V _Gi 'necessary to obtain the target dark portion potential V _Di when the initial image forming conditions are set can be calculated from the relationship between the dark portion potential at the time and the grid bias.

As described above, according to the present embodiment, correction of the developing bias voltage is not required by setting the dark portion potential to be constant. Further, by performing the correction of the grid bias voltage stepwise in the same manner as in the first embodiment, it is possible to minimize a sudden change in density due to the correction during continuous image formation, and to detect an abnormal value when it occurs. Even when this is not possible, a rapid change in image density and a change in color can be minimized.

Embodiment 3 In this embodiment, as in Embodiment 2, correction is performed at the dark portion potential, but it is characterized in that laser emission is not performed when performing the dark portion potential correction. The basic configuration of the image forming apparatus according to this embodiment is the same as that of the second embodiment, and a description thereof will be omitted.

Conventionally, the digital image forming apparatus emits a very small amount of light even in a non-image area in order to ensure linearity of the laser emission with respect to the image level. That is, when charging is performed by controlling the grid bias voltage and laser light emission in the image area is performed, the non-image light-emission area receives minute laser light emission. As a result, the dark part potential, which is the surface potential of the non-image area, is attenuated by several volts.

Therefore, in this embodiment, the correction of the dark portion potential is performed without performing laser emission at the time of correcting the dark portion potential. First, a dark portion potential (complete dark portion potential) where primary charging is performed using the grid bias voltage calculated under the initial image setting conditions and laser light emission is not performed is measured and stored. Then, after performing the prescribed image formation,
In the same manner as in the second embodiment, the dark portion potential is corrected.

In this embodiment, as described above, it is possible to minimize the decrease in image density during continuous image formation. Of course, by performing the correction stepwise, it is possible to minimize the sudden change in image density before and after the correction, and even when an abnormal value or the like cannot be detected, the sudden calculation by the abnormal value It is possible to suppress a large density fluctuation.

[0068]

As described above, according to the present invention,
When the control means controls the grid bias and the development DC bias so as to gradually approach the correction target value each time image formation is performed, the control is performed until the grid bias and the development DC bias reach the correction target value. Since the number of formed images is fixed, the current grid bias and the development DC bias are greatly different from the correction target value while suppressing the sharp image density difference before and after the correction. Further, it is possible to prevent the development DC bias from converging on the correction target value.

[0069]

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an outline of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a flowchart showing a control process when setting initial image forming conditions before executing the image forming process in the embodiment of FIG. 1;

FIG. 3 is a flowchart showing a control process at the time of initial image forming condition correction during the continuous image forming process.

FIG. 4 is a diagram showing a relationship between a grid bias voltage of a primary charger and a dark portion potential and a bright portion potential on the surface of a photosensitive drum.

FIG. 5 is a diagram illustrating a relationship between a contrast potential of an electrostatic latent image and an optical density of an obtained toner image.

FIG. 6 is a diagram showing a potential level relationship between a bright portion potential and a dark portion potential of a latent image formed on a photosensitive drum by exposure and a developing DC bias, and that a toner image is formed in the bright portion potential portion. is there.

FIG. 7 is a diagram illustrating a relationship between a grid bias voltage and a surface potential of a photosensitive drum for explaining correction when a bright portion potential is increased due to a change in photosensitive drum characteristics during execution of a continuous image forming process.

FIG. 8 is a diagram illustrating a change in image density when correction is performed during execution of a continuous image forming process.

FIG. 9 is a diagram showing a case where correction based on a grid bias voltage during the execution of a continuous image forming process is performed once for each correction and a case where the correction is performed stepwise based on the number of formed images.

FIG. 10 is a diagram illustrating a relationship between a charging current flowing through a primary charger and a surface potential of a photosensitive drum.

FIG. 11 is a diagram illustrating a relationship between an exposure amount for a photosensitive drum and a surface potential.

FIG. 12 is an explanatory diagram showing an outline of an image forming apparatus according to another embodiment of the present invention.

FIG. 13 is an explanatory diagram illustrating a first image forming unit of the image forming apparatus of FIG. 12;

FIG. 14 is a diagram illustrating a relationship between a grid bias voltage and a dark portion potential and a light portion potential of a photosensitive drum for explaining that correction is performed based on a dark portion potential during execution of a continuous image forming process.

[Explanation of symbols]

 Reference Signs List 1 laser control unit 4 photosensitive drum 6 primary charger 8a, 8b, 8c developing bias voltage control circuit 9A, 9B, 9C developing device 14 surface voltmeter 14a surface voltmeter probe 16 primary charger grid 21 potential control microcomputer 91, 92, 93 developing sleeve

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields investigated (Int. Cl. ⁷ , DB name) G03G 15/00 G03G 15/02 G03G 15/06 G03G 21/00-21/04 G03G 21/14

Claims (8)

(57) [Claims] An image carrier, a charging unit having a grid, and charging the image carrier; an exposing unit for exposing the image carrier charged by the charging unit; and an image forming unit formed on the image carrier. Developing means for developing the electrostatic image with toner; detecting means for detecting the surface potential of dark and light portions of the image carrier; and control means for controlling a grid bias and a developing DC bias in accordance with an output of the detecting means. Wherein, during continuous image formation, at least one surface potential of the dark part and the light part is detected by the detection means, and the grid bias and the development are performed based on the detected surface potential. A correction target value of the DC bias is determined, and the grid bias and the developing DC bias are changed by the control unit each time image formation is performed. When controlled so as to approach gradually toward the value, the image forming apparatus, characterized in that the grid bias and the developing DC bias is a constant number of image formation to be performed until reaching the corrected target value. 2. The image according to claim 1, wherein before the continuous image formation is performed, the grid bias and the developing DC bias are determined in accordance with the surface potentials of the dark part and the light part. Forming equipment. 3. Before the continuous image formation is performed, charging by the charging unit and exposure by the exposing unit are performed with a plurality of different grid biases, and a plurality of each of the dark portion and the bright portion obtained by the charging are provided. 2. The image forming apparatus according to claim 1, wherein the grid bias and the developing DC bias are determined according to a surface potential. 4. Before the continuous image formation is performed, the grid bias and the developing DC bias are a potential difference between a surface potential of the dark area and the developing DC bias, and a potential difference between the developing DC bias and the surface of the bright area. 4. The image forming apparatus according to claim 1, wherein a potential difference from the potential is determined to be a predetermined value. 5. The correction target values of the grid bias and the developing DC bias include a potential difference between a surface potential of the dark part and the developing DC bias, and a potential difference between the developing DC bias and a surface potential of the bright part. The image forming apparatus according to claim 1, wherein the image forming apparatus is determined to be a predetermined value. 6. The correction of the grid bias and the developing DC bias during the continuous image formation is performed when a potential of the bright portion detected during the continuous image formation exceeds a predetermined value. The image forming apparatus according to claim 1. 7. The correction of the grid bias and the developing DC bias during the continuous image formation is performed when a potential of the dark portion detected during the continuous image formation exceeds a predetermined value. The image forming apparatus according to claim 1. 8. The image forming apparatus according to claim 1, wherein the toner is a color toner, and the apparatus is capable of forming a color image.