JP2010054739A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus that measures a potential difference between the photoreceptor drum and a developing roller, which cause discharge, while suppressing cracking of a photoreceptive layer of a photoreceptor drum and, at the same time. <P>SOLUTION: The image forming apparatus 1 includes: the photoreceptor drum 9; a developing roller 81 to which a DC voltage application section 85 and an AC voltage application section 86 are connected; a detecting section 14 for detecting discharge between the developing roller 81 and the photoreceptor drum 9; and a control section 10. The control section 10 instructs the AC voltage application section 86 to gradually change an AC voltage applied to the developing roller 81. When the detecting section 14 detects discharge, the AC voltage application section 86 applies an AC voltage to the developing roller 81, the AC voltage being set such that a duty ratio is smaller than that for image formation and a frequency is lower than that for image formation so that the plus side time is equal to that for image formation. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile.

  2. Description of the Related Art Conventionally, in an image forming apparatus using toner such as a copying machine, a printer, and a facsimile, a developing roller facing a photosensitive drum is sometimes provided with a gap. A so-called developing bias in which direct current and alternating current are superimposed is applied to the developing roller, and the charged toner flies from the developing roller to the photosensitive drum, and the electrostatic latent image is developed.

  Here, in order to sufficiently supply the toner to the photosensitive drum, to ensure the density of the formed image and to improve the development efficiency, the peak-to-peak voltage of the AC voltage applied to the developing roller should be increased. However, if it is too large, discharge occurs in the gap between the photosensitive drum and the developing roller. When the discharge occurs, the electrostatic latent image is disturbed due to the potential change on the surface of the photosensitive drum, and the quality of the formed image is deteriorated.

Therefore, for example, Patent Document 1 provides a toner carrier that is opposed to the image carrier with a required distance in the development region, and a DC voltage and an AC voltage are superimposed between the toner carrier and the image carrier. In the developing device that applies the developed developing bias voltage and supplies the toner held on the toner carrier to the image carrier to develop the electrostatic latent image, it is applied between the image carrier and the toner carrier. Leak generation means for generating a leak between the image carrier and the toner carrier by changing the leak detection voltage and a leak detection means for detecting the leak are provided, and the leak detection voltage and the surface potential of the image carrier are A developing device is described in which when the maximum potential difference ΔVmax is gradually increased and the current flowing between the image carrier and the toner carrier continuously increases, the leak detection means determines that the leak has occurred (example) In Patent Document 1: see Japanese claim 1). According to such a developing device, it is possible to measure the potential difference between the image carrier and the toner carrier where discharge (leakage) occurs.
Japanese Patent No. 3815356

  Here, FIG. 10 shows the relationship between the potential difference between the photosensitive drum and the developing roller and the discharge current flowing between the photosensitive drum and the developing roller. As shown in FIG. 10, under the condition that the potential of the developing roller is lower than that of the photosensitive drum (in the negative direction), the discharge current increases rapidly when the potential difference between the developing roller and the photosensitive drum exceeds a certain threshold value. It has been confirmed that the discharge current gradually increases even when the potential difference between the developing roller and the photosensitive drum exceeds a certain threshold value under a condition where the potential is higher than that of the photosensitive drum (in the positive direction). Such a characteristic is likely to occur when the photosensitive layer of the photosensitive drum is made of amorphous silicon (it may occur even with other materials).

  In the developing device described in Patent Document 1, there is a possibility that discharge will occur in the negative direction by changing the leak detection voltage. At that time, a large discharge current flows due to the above characteristics, and a minute pin called a drum pinhole flows in the photosensitive layer. There is a high possibility that the photosensitive layer will be destroyed by forming a hole. If a drum pinhole is formed, the charge cannot be retained in that portion, leading to an image quality abnormality during image formation.

  SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of measuring a potential difference between a photosensitive drum and a developing roller where discharge occurs while suppressing destruction of a photosensitive layer of the photosensitive drum.

In order to achieve the above object, an image forming apparatus according to claim 1 includes a photosensitive drum carrying a toner image on a peripheral surface,
A direct current voltage application unit and a developing roller to which an alternating voltage application unit is connected to face the photoconductive drum with a gap, carry a toner during image formation, and supply the toner to the photoconductive drum. ,
A detector for detecting the occurrence of discharge between the developing roller and the photosensitive drum;
A control unit that controls each part of the device and recognizes the occurrence of discharge when the output of the detection unit is input;
Instructing the AC voltage application unit to change the AC voltage applied to the developing roller by the control unit to the AC voltage application unit, and detecting the occurrence of discharge by the detection unit,
The AC voltage application unit is configured to apply an AC voltage having a smaller duty ratio than that at the time of image formation and a frequency set to be lower than that at the time of image formation to the developing roller so that the plus time is the same as that at the time of image formation. .

  According to such a configuration, in order to grasp the potential difference between the developing roller where the discharge occurs and the photosensitive drum, when the occurrence of the discharge is detected and confirmed while changing the AC voltage applied to the developing roller, when the image is formed An AC voltage having a smaller duty ratio and a frequency set to be smaller than that at the time of image formation is applied to the developing roller so that the plus time is the same as that at the time of image formation.

  By making the duty ratio of the alternating voltage at the occurrence of discharge detection smaller than that at the time of image formation, it is possible to increase the difference between the positive side peak value and the area center in the alternating voltage, that is, the direct current bias by the direct current voltage application unit. Therefore, the potential difference between the positive peak value of the AC voltage and the surface potential of the photosensitive drum can be increased, and a discharge occurs between the developing roller and the photosensitive drum in the positive direction (a state where the potential of the developing roller is higher than that of the photosensitive drum). Can do. As described above, when the photosensitive drum has such a characteristic that current does not flow suddenly between the developing roller and the photosensitive drum when discharged in the plus direction, a drum pinhole is formed in the photosensitive drum due to the positive discharge. There is almost no risk that the formed photosensitive layer will be destroyed. That is, it is possible to measure the potential difference between the photosensitive drum and the developing roller, where discharge occurs, while suppressing the destruction of the photosensitive layer of the photosensitive drum.

  In addition, since an AC voltage whose frequency is set smaller than that at the time of image formation is applied to the developing roller so that the plus side time is the same as that at the time of image formation, even if it takes time to rise and fall of the AC voltage, the AC voltage The positive peak time of the voltage can be ensured in the same manner as in the image formation, and the application state of the AC voltage when the occurrence of discharge is detected can be matched during the image formation.

The invention according to claim 2 further comprises a magnetic roller provided in the invention according to claim 1 so as to be opposed to the developing roller and holding toner charged positively.
When the occurrence of discharge is detected, the control unit causes the DC voltage application unit to apply a higher DC voltage to the developing roller than during image formation. According to such a configuration, since the toner is positively charged, it is possible to prevent the toner from being supplied from the magnetic roller to the developing roller when the occurrence of discharge is detected.

Further, in the invention according to claim 3, in the invention according to claim 1 or claim 2, when it is detected that discharge has occurred at the time of detection of the occurrence of discharge,
The control unit obtains a potential difference between the photosensitive drum and the developing roller with respect to a peak-to-peak voltage of the AC voltage applied to the developing roller at the time of occurrence of discharge, and determines the difference between the developing roller and the photosensitive drum during image formation. The AC voltage to be applied to the developing roller during image formation is determined so that the surface potential difference is smaller than the potential difference. According to such a configuration, based on the potential difference between the developing roller and the photosensitive drum where the recognized discharge is generated, the development efficiency is increased, and an AC voltage that does not cause a discharge during appropriate image formation can be set. it can.

  The invention according to claim 4 is the invention according to any one of claims 1 to 3, wherein the photosensitive drum includes a positively charged amorphous silicon photosensitive layer. According to such a configuration, since the photosensitive layer of the photosensitive drum is made of positively charged amorphous silicon, the characteristic that a large current hardly flows due to positive discharge is remarkably exhibited. The effect of suppressing the destruction of is increased.

  According to the image forming apparatus of the present invention, it is possible to measure the potential difference between the photosensitive drum and the developing roller, where discharge occurs, while suppressing the destruction of the photosensitive layer of the photosensitive drum.

  Hereinafter, embodiments of the present invention will be described with reference to FIGS. In this embodiment, an electrophotographic tandem color printer 1 (corresponding to an image forming apparatus) will be described as an example. However, each element such as the configuration and arrangement described in the present embodiment does not limit the scope of the invention and is merely an illustrative example.

(Schematic configuration of image forming apparatus)
First, the outline of the printer 1 according to the embodiment of the present invention will be described with reference to FIGS. 1 to 3. FIG. 1 is a cross-sectional view showing a schematic configuration of a printer 1 according to an embodiment of the present invention. FIG. 2 is an enlarged cross-sectional view of each image forming unit 3 according to the embodiment of the present invention. FIG. 3 is a schematic diagram showing an example of the exposure apparatus 4 according to the embodiment of the present invention. As shown in FIG. 1, the printer 1 according to the present embodiment includes a sheet supply unit 2 a, a conveyance path 2 b, an image forming unit 3, an exposure device 4, an intermediate transfer unit 5, a fixing unit 6 and the like in the main body. Provided.

  The sheet supply unit 2a stores various sheets such as copy sheets, OHP sheets, and label sheets toward the intermediate transfer unit 5 and the like, and is fed by a sheet feeding roller 21 that is rotated by a driving mechanism (not shown) such as a motor. It sends out to the conveyance path 2b. The conveyance path 2 b conveys the sheet in the printer 1 and guides the sheet supplied from the sheet supply unit 2 a to the discharge tray 22 through the intermediate transfer unit 5 and the fixing unit 6. The conveyance path 2b is provided with a pair of conveyance rollers 23, a guide 24, and a registration roller pair 25 that waits for the conveyed sheet in front of the intermediate transfer unit 5 and sends it in time.

  As shown in FIGS. 1 and 2, the printer 1 includes an image forming unit 3 for four colors as a part for forming a toner image based on image data of an image to be formed. Specifically, the printer 1 includes an image forming unit 3a (including a charging device 7a, a developing device 8a, a charge removing device 31a, a cleaning device 32a, and the like) that forms a black image, and an image forming unit 3b that forms a yellow image. (Equipped with a charging device 7b, a developing device 8b, a static eliminating device 31b, a cleaning device 32b, etc.) and an image forming unit 3c (charging device 7c, developing device 8c, static eliminating device 31c, cleaning device 32c, etc.) for forming a cyan image. And an image forming unit 3d (including a charging device 7d, a developing device 8d, a charge removing device 31d, a cleaning device 32d, and the like) that forms a magenta image.

  Here, the image forming units 3a to 3d will be described in detail with reference to FIG. Each of the image forming units 3a to 3d has basically the same configuration except that the color of the toner image to be formed is different. Therefore, in the following description, the symbols a, b, c, and d in each image forming unit 3 are omitted except for the case where they are specifically described (in FIG. 2, the image forming units 3a, 3b, 3c, and 3d). The reference numerals a, b, c, and d are given to the respective members in the same manner.)

  Each photosensitive drum 9 carries a toner image on its peripheral surface, and has, for example, a positively charged amorphous silicon photosensitive layer on the outer peripheral surface of an aluminum drum, and has a predetermined process speed by a driving device (not shown). Is rotated in the clockwise direction on the paper. Each photosensitive drum 9 of the present embodiment is a positively charged type.

  Each charging device 7 has a charging roller 71 and charges the photosensitive drum 9 with a constant potential. Each charging roller 71 is in contact with each photosensitive drum 9 and rotates in accordance with the photosensitive drum 9. In addition, a voltage in which direct current and alternating current are superimposed is applied to each charging roller 71 by a charging voltage applying unit 72 (see FIG. 5), and the surface of the photosensitive drum 9 has a predetermined positive potential (for example, 200V to 300V, dark potential). In addition, a cleaning brush 73 (for example, a resin brush or the like wound around a shaft) for removing foreign matters on the surface of each charging roller 71 is provided. The charging device 7 may be a device that charges the photosensitive drum 9 using a corona discharge type or a brush.

  Each developing device 8 accommodates a developer (so-called two-component developer) composed of toner and a magnetic carrier (the developing device 8a is black, the developing device 8b is yellow, the developing device 8c is cyan, and the developing device 8d is magenta. Of developer). Each developing device 8 includes a developing roller 81, a magnetic roller 82, and a conveying member 83. Each developing roller 81 faces the photosensitive drum 9 and is provided with a predetermined gap (for example, 1 mm or less). Each magnetic roller 82 faces the upper right of each developing roller 81 and is disposed with a predetermined gap. Each transport member 83 is provided above each magnetic roller 82.

  The roller shafts 811 and 821 of each developing roller 81 and each magnetic roller 82 are fixed. Magnets 813 and 823 extending in the axial direction are attached to the roller shafts 811 and 821 inside the developing rollers 81 and the magnetic rollers 82, respectively. Each developing roller 81 and each magnetic roller 82 have cylindrical sleeves 812 and 822 that cover the magnets 813 and 823, respectively, and these sleeves 812 and 822 rotate during image formation (see FIG. 4). . In the magnet 813 of the developing roller 81 and the magnet 823 of the magnetic roller 82, different polarities face each other at a position where the developing roller 81 and the magnetic roller 82 face each other.

  Thereby, a magnetic brush is formed by the magnetic carrier between each developing roller 81 and each magnetic roller 82. The toner is supplied to the developing roller 81 by rotating the magnetic brush and the sleeve 822 of the magnetic roller 82 or applying a voltage to the magnetic roller 82 (magnetic roller bias applying unit 84: see FIG. 5). A thin layer of is formed. The toner remaining after the development is peeled off from the developing roller 81 by a magnetic brush. For example, each conveying member 83 is provided with a screw spirally with respect to the shaft, and conveys and stirs the developer in each developing device 8 to charge the toner to a predetermined level (in this embodiment, the toner is positive). Electrification).

  Each cleaning device 32 cleans the photosensitive drum 9 and has, for example, a cleaning member 33 made of a cylindrical material having elasticity on the outer peripheral portion. The cleaning member 33 abuts on each photosensitive drum 9, and the drum The transfer residual toner on the surface is removed and collected. Further, a neutralization device 31 (for example, an array of LEDs) that performs neutralization by irradiating the photosensitive drum 9 with light is provided below each cleaning device 32.

  The exposure device 4 above each image forming unit 3 converts an input color-separated image signal into an optical signal by a laser output unit (not shown), and laser light (converted optical signal). (Shown by a broken line) is output, and the photosensitive drum 9 after scanning is scanned and exposed to form an electrostatic latent image.

  Here, a schematic configuration of the exposure apparatus 4 will be described with reference to FIG. As shown in FIG. 3, the exposure device 4 includes a semiconductor laser device 41 (laser diode), a polygon mirror 42 (rotated by a polygon motor 43) that has a plurality of plane reflecting surfaces that reflect laser light, and rotates at high speed, fθ. A lens 44 and a mirror 45 for reflecting the laser light toward each photosensitive drum 9 are provided as appropriate (in FIG. 3, only one color configuration is shown. For example, in the case of four colors, the polygon mirror 42 is The other semiconductor laser device 41, the fθ lens 44, the mirror 45, etc. are provided for each color). With this configuration, laser light is irradiated from the exposure device 4 to each photosensitive drum 9, and an electrostatic latent image combined with image data is formed on the photosensitive drum 9. Specifically, each photosensitive drum 9 of the present embodiment is positively charged, the potential of the light irradiation portion is lowered, and the positively charged toner adheres to the portion where the potential is lowered (for example, in the case of a solid image, all lines Irradiate all pixels with laser light). The exposure device 4 may be composed of a large number of LEDs.

  Note that the exposure device 4 is provided with a light receiving element 46 within the irradiation range of the laser light and outside the irradiation range of the photosensitive drum 9. The light receiving element 46 outputs a current (voltage) when irradiated with laser light, and this output is input to, for example, a CPU 11 (Central Processing Unit) to be described later, and a synchronization signal at the time of confirming whether discharge has occurred or not. (See FIG. 5).

  Returning to FIG. 1, the intermediate transfer unit 5 receives the primary transfer of the toner image from the photosensitive drum 9 and performs secondary transfer onto the sheet. Each of the primary transfer rollers 51 a to 51 d, the intermediate transfer belt 52, The driving roller 53, driven rollers 54, 55, 56, a secondary transfer roller 57, a belt cleaning device 58, and the like are included. Each primary transfer roller 51a to 51d is in contact with each photosensitive drum 9 via an endless intermediate transfer belt 52, and is connected to a transfer voltage application unit (not shown) for applying a transfer voltage, thereby toning a toner image. Is transferred to the intermediate transfer belt 52.

  The intermediate transfer belt 52 is stretched around a driving roller 53 and driven rollers 54, 55, and 56, and rotates in the counterclockwise direction on the paper surface by rotational driving of the driving roller 53 connected to a driving mechanism (not shown) such as a motor. The intermediate transfer belt 52 is made of, for example, a dielectric resin. The driving roller 53 is in contact with the secondary transfer roller 57 via the intermediate transfer belt 52 to form a secondary transfer portion. The toner image transfer to the sheet will be described. Toner images (black, yellow, cyan, and magenta colors) formed by the image forming units 3 are sequentially applied by applying predetermined voltages to the primary transfer rollers 51. The primary transfer is performed on the intermediate transfer belt 52. At this time, the toner images of the respective colors are primarily transferred while being timed so as to be superimposed without deviation. The superimposed toner images are transferred onto the sheet by a secondary transfer roller 57 to which a predetermined voltage is applied. The residual toner remaining on the intermediate transfer belt 52 after the secondary transfer is removed and collected by the belt cleaning device 58 (see FIG. 1).

  The fixing unit 6 is disposed downstream of the secondary transfer unit in the transfer material conveyance direction, and fixes the toner image secondarily transferred to the sheet by heating and pressing. The fixing unit 6 is mainly composed of a fixing roller 61 having a built-in heat source and a pressure roller 62 pressed against the fixing roller 61 to form a nip. The sheet on which the toner image has been transferred is heated and pressurized as it passes through the nip, and as a result, the toner image is fixed on the sheet. The fixed sheet is discharged to the discharge tray 22 and the image forming process is completed.

(Configuration for discharge detection)
Next, a configuration relating to development bias application to each developing roller 81 and discharge detection between each photosensitive drum 9 and each developing roller 81, which is a feature of the present invention, will be described.

  FIG. 4 shows a configuration around the developing roller 81 relating to the application of the developing bias to the developing roller 81 and the detection of the occurrence of discharge between the photosensitive drum 9 and the developing roller 81 according to the embodiment of the present invention. However, FIG. 4 shows only one image forming unit 3. Each image forming unit 3 is provided with a DC voltage application unit 85, an AC voltage application unit 86, a detection unit 14, and an amplifier 15, and the output of each amplifier 15 is The data is input to a CPU 11 of the control unit 10 described later. Here, each of the DC voltage application unit 85, the AC voltage application unit 86, the detection unit 14, and the amplifier 15 may be affixed with symbols a, b, c, and d indicating the distinction between the image forming units. Since the same components are provided in each image forming unit, in order to avoid complicated description, the following description will be made with the symbols a, b, c, and d omitted.

  As shown in FIG. 4, the developing roller 81 is opposed to the photosensitive drum 9 with a gap, and includes a roller shaft 811, a sleeve 812 that carries toner during image formation, and a cap 814. The roller shaft 811 is inserted through the sleeve 812, and circular caps 814 are fitted to both ends of the sleeve 812. Further, a DC voltage application unit 85 and an AC voltage application unit 86 are connected to the roller shaft 811 of the developing roller 81 for supplying toner to the photosensitive drum 9.

  The DC voltage application unit 85 is a circuit that generates a DC component to be applied to the developing roller 81, and its output is input to the AC voltage application unit 86. The DC voltage application unit 85 includes an output control unit 87, and the output control unit 87 controls the bias value output from the DC voltage application unit 85 in accordance with an instruction from the CPU 11.

  The DC voltage application unit 85 is a circuit that receives supply of DC power from the power supply device 16 (see FIG. 5) in the printer 1 and whose output voltage is variable under the control of the output control unit 87 in accordance with an instruction from the CPU 11. (For example, there are a plurality of paths to output terminals with different output voltages, and the selection of the path is changed between image formation and discharge detection). Thereby, the alternating voltage applied to the developing roller 81 can be biased.

  The AC voltage application unit 86 is, for example, a rectangular wave (pulse shape), and is a circuit that outputs an AC voltage having the DC voltage output from the DC voltage application unit 85 as an average value (area center value). AC voltage application unit 86 includes a Vpp control unit 88 and a duty ratio / frequency control unit 89. The Vpp control unit 88 controls the peak-to-peak voltage of the AC voltage according to an instruction from the CPU 11. Further, the duty ratio / frequency control unit 89 controls the duty ratio and frequency of the AC voltage in accordance with an instruction from the CPU 11.

  For example, the AC voltage application unit 86 includes a switching element or the like, inverts the output polarity by switching, and outputs an AC voltage. The duty ratio / frequency control unit 89 can control the duty ratio and frequency of the AC voltage by controlling, for example, the positive / negative switching timing of the output of the AC voltage application unit 86. Further, the Vpp control unit 88 uses the voltage between the peaks of the AC voltage to be applied to the developing roller 81 and the duty ratio, and the positive peak in the AC voltage by the step-up / step-down of the DC voltage input from the power supply device 16. The value and the negative peak value are varied according to an instruction from the CPU 11. In addition, the configuration of the AC voltage application unit 86 and the configuration in which the peak-to-peak voltage, the duty ratio, and the frequency of the AC voltage can be varied as long as the peak-to-peak voltage, the duty ratio, and the frequency can be changed.

  In the AC voltage application unit 86, for example, a boosting circuit such as a boosting transformer can be provided in the output stage, and the developing bias in which the DC and AC are superimposed after the boosting is, for example, the roller of the developing roller 81. Applied to the shaft 811. As a result, a developing bias is also applied to the sleeve 812, and the charged toner carried on the sleeve 812 flies.

  The detection unit 14 is a circuit that converts a current that flows when a discharge occurs between the developing roller 81 and the photosensitive drum 9 into a voltage signal and detects the occurrence of the discharge, and outputs the converted voltage signal to the amplifier 15. The amplifier 15 amplifies the voltage signal from the detection unit 14 and outputs it to the CPU 11. The CPU 11 A / D converts the voltage signal from the amplifier 15. From the output of the A / D converted amplifier 15, the CPU 11 can recognize the magnitude of the generated discharge (the magnitude of the current flowing between the developing roller 81 and the photosensitive drum 9).

(Hardware configuration of printer 1)
Next, the hardware configuration of the printer 1 according to the embodiment of the present invention will be described with reference to FIG. FIG. 5 is a block diagram illustrating an example of a hardware configuration of the printer 1 according to the embodiment of the present invention.

  As shown in FIG. 5, the printer 1 according to the present embodiment includes a control unit 10 inside. The control unit 10 controls each unit of the printer 1 and receives the output of the detection unit 14 to recognize the occurrence of discharge. For example, the control unit 10 includes a CPU 11, a storage unit 12, and the like. The CPU 11 is a central processing unit, and controls and calculates each unit of the printer 1 based on a control program stored in the storage unit 12 and developed. The storage unit 12 is configured by a combination of nonvolatile and volatile storage devices such as ROM, RAM, and flash ROM. For example, the storage unit 12 stores a control program, control data, and the like for the printer 1. In the present invention, the storage unit 12 also stores a program for detecting discharge and setting an AC voltage applied to the developing roller 81.

  The control unit 10 is connected to the sheet supply unit 2a, the conveyance path 2b, the image forming unit 3, the exposure device 4, the intermediate transfer unit 5, the fixing device, the operation panel 13, and the like. Based on this, the operation of each unit is controlled so that image formation is performed appropriately.

  The operation panel 13 shown in FIG. 5 is provided, for example, at the upper front of the printer 1 and has a liquid crystal screen to display various setting information, warnings, and the like. The operation panel 13 has various operation buttons and accepts operations from the user. The control unit 10 is connected to a user terminal 100 (personal computer or the like) as a transmission source of image data to be printed, and the control unit 10 performs image processing on the received image data and transmits it to the exposure apparatus 4. Then, the exposure device 4 forms an electrostatic latent image on the photosensitive drum 9 based on the image data. A magnetic roller bias applying unit 84 shown in FIG. 5 is a circuit that applies a voltage in which alternating current and direct current are superimposed on the magnetic roller 82. The charging voltage application unit 72 is a circuit that applies a charging voltage to the charging roller 71.

  Further, with respect to the present invention, the control unit 10 (CPU 11) is connected to the detection unit 14 (amplifier 15). When the present invention is implemented, the CPU 11 gives an instruction to the AC voltage application unit 86 to change the peak-to-peak voltage of the AC voltage applied to the developing roller 81 in a stepwise manner, and discharges from the output of the detection unit 14 (amplifier 15). Detection of occurrence and determination of discharge magnitude. When the occurrence of discharge is detected, the control unit 10 grasps the potential difference between the photosensitive drum 9 and the developing roller 81 at the time of occurrence of discharge based on values such as the DC voltage and the peak-to-peak voltage of the AC voltage at that time. The setting of the developing bias to be applied during the image forming operation is determined so that no discharge occurs during the image formation. The setting value of the developing bias can be stored in the storage unit 12.

(Discharge occurrence detection operation and setting of AC voltage applied to developing roller 81)
Next, an example of an operation for detecting the occurrence of discharge between the photosensitive drum 9 and the developing roller 81 will be described with reference to timing charts shown in FIGS. FIG. 6 is a timing chart for explaining an outline of the discharge occurrence detection operation according to the embodiment of the present invention. FIG. 7 is a timing chart illustrating details of the AC voltage applied to the developing roller 81 according to the embodiment of the present invention. This discharge occurrence detection operation is sequentially performed for each image forming unit 3 one by one.

  First, the outline of the discharge occurrence detection operation will be described with reference to FIG. In FIG. 6, “developing roller (alternating current)” indicates the timing at which the alternating voltage application unit 86 applies an alternating voltage to the developing roller 81. “Vpp” indicates a change in the peak-to-peak voltage of the AC voltage applied to the developing roller 81. “Developing roller (DC)” indicates a timing at which the DC voltage application unit 85 applies a DC voltage to the developing roller 81. “Magnetic roller (AC)” indicates a timing at which the magnetic roller bias applying unit 84 (see FIG. 5) applies an AC voltage to the magnetic roller 82. “Magnetic roller (DC)” indicates the timing at which the magnetic roller bias applying unit 84 applies a DC voltage to the magnetic roller 82.

  “Charging roller” indicates the timing at which the charging device 7 charges the photosensitive drum 9. The “synchronization signal” is a synchronization signal output from the light receiving element 46 of the exposure apparatus 4. “Exposure” indicates the exposure (laser beam irradiation) timing of the photosensitive drum 9 in the exposure apparatus 4. “Discharge detection (detector output)” indicates a discharge occurrence detection timing by the detector 14.

<Initial operation>
When the discharge generation detecting operation according to the present invention is started, after the photosensitive drum 9, the developing roller 81, the intermediate transfer belt 52, and the like start rotating, in the initial operation, the developing roller 81 and the magnetic roller 82 are respectively connected to the alternating current. A DC voltage is applied. By applying a voltage to the magnetic roller 82 in this initial operation, a small amount of toner is supplied from the magnetic roller 82 to the developing roller 81. In detection of occurrence of discharge, basically, toner is not carried on the developing roller 81, but if no toner is carried at all, the friction between the photosensitive drum 9 and the rotating member (intermediate transfer belt 52, etc.) in contact therewith becomes too large. Therefore, a small amount of toner is supplied to the photosensitive drum 9. After initial operation, transition to the ready state.

<Preparation state> and <Default measurement>
In the ready state, charging of the photosensitive drum 9 by the charging device 7 is started. Note that the voltage applied to the charging device 7 remains ON until the discharge occurrence detection operation is completed. Further, the peak-to-peak voltage of the AC voltage applied to the developing roller 81 is increased to the peak-to-peak voltage to be measured. Next, the process moves to the default measurement and confirms whether or not a discharge is detected. Note that the default measurement is performed to detect an abnormality in the detection position of the detection unit 14 or the like, the member installation position, the circuit, or the like. After the default measurement, the condition shifts to the condition change state (first time).

<Condition change state>
When the condition is changed, the peak-to-peak voltage of the AC voltage applied to the developing roller 81 is changed stepwise (for example, increased). Then, in the middle of the condition change state, the synchronization signal that becomes a guide for the start of exposure of the exposure apparatus 4 becomes High. After the synchronization signal is high, the state shifts to the discharge detection state (first time).

<Discharge detection status>
In the discharge detection state (first time), a developing bias is applied to the developing roller 81, and the exposure device 4 continues exposure (exposure of the entire surface of the photosensitive drum 9). In the printer 1 of this embodiment, the toner and the photosensitive drum 9 are charged with positive polarity, and the toner is deposited on the exposed portion. Therefore, the continuous exposure is the same as the formation of the electrostatic latent image of the solid image. It is. Therefore, in the discharge detection state, for example, solid image data is sent from the control unit 10 to the exposure apparatus 4 (solid image data is stored in, for example, the storage unit 12).

  At this time, the discharge detection state continues for a certain time (for example, 1 second to several seconds), and shifts to the condition change state, for example, when the occurrence of discharge is not recognized from the input of the amplifier 15 to the CPU 11. In the condition change state, the control unit 10 again instructs the AC voltage application unit 86 to issue an instruction to change the peak-to-peak voltage of the AC voltage. Thereby, in the discharge detection state after the next time, the presence or absence of discharge is confirmed in a state where the peak-to-peak voltage of the AC voltage applied to the developing roller 81 is higher than the previous time. Thereafter, until the AC voltage to be discharged is recognized, the condition change state and the discharge detection state are repeated, and during the repetition, the peak-to-peak voltage of the AC voltage applied to the developing roller 81 at a constant step size is basically determined. Enhanced. FIG. 5 shows that discharge is detected in the nth discharge detection state.

  Next, application of a voltage to the developing roller 81 in the discharge detection state will be described with reference to FIG. In FIG. 7, a timing chart at the time of image formation is shown in the upper part, and a timing chart in the discharge detection state is shown in the lower part.

  First, the rectangular wave in the timing chart at the time of image formation is an example of the waveform of the developing bias (AC + DC) applied to the developing roller 81. “Vdc1” indicates the bias potential of the DC voltage application unit 85. “V0” indicates the potential of the photosensitive drum 9 after exposure by the exposure device 4 (approximately 0 V = bright potential). “V1” indicates a potential after charging of the photosensitive drum 9 (potential of a portion not exposed to light, for example, about 200 to 300 V). “V + 1” indicates a potential difference between V0 and the positive peak of the developing bias during image formation. “V−” indicates a potential difference between V1 and the negative peak value of the developing bias. “Vpp1” indicates the peak-to-peak voltage of the AC voltage applied to the developing roller 81 during image formation. “T1” is a plus time in the rectangular wave. “T01” indicates the period of the rectangular wave.

  On the other hand, a rectangular wave in the timing chart when the occurrence of discharge is detected indicates the waveform of the developing bias applied to the developing roller 81 when the presence or absence of discharge is detected. “Vdc2” indicates the bias potential of the DC voltage application unit 85 at the time of detection. “V0” indicates a potential (approximately 0 V) after exposure of the photosensitive drum 9 by the exposure device 4 as in the upper part of FIG. “V + 2” indicates a potential difference between the positive peak of the developing bias at the time of detection and V0. “Vpp2” indicates a peak-to-peak voltage of the AC voltage applied to the developing roller 81 at the time of detection. “T2” is the plus time in the rectangular wave. “T02” is a period of a rectangular wave.

  First, when detecting the occurrence of discharge, the output control unit 87 sets the output of the DC voltage application unit 85 to a set value Vdc2 (for example, 100 V to 200 V) for detecting the occurrence of discharge according to an instruction from the CPU 11. Further, in response to an instruction from the CPU 11, the Vpp control unit 88 sets the Vpp 2 of the AC voltage output from the AC voltage application unit 86. Further, the duty ratio / frequency control unit 89 instructs the AC voltage duty unit D2 (the ratio of the positive side time T2 with respect to the cycle T02, T2 / T02) to be generated in response to an instruction from the CPU 11. Then, the frequency f2 (= 1 / T02) of the AC voltage output from the AC voltage application unit 86 is set to the setting value for detecting the occurrence of discharge (lower stage in FIG. 7).

  Here, the duty ratio D2 is set to be smaller than the duty ratio D1 at the time of image formation (ratio of the high time T1 to the period T01, T1 / T01) (for example, D1 = 40%, D2 = 30%). Here, the area center (average value) of the AC voltage is the set value of the DC bias (Vdc1 at the time of image formation, Vdc2 at the time of detecting the occurrence of discharge). As described above, the difference between the positive-side peak value and the area center, that is, the DC bias setting value Vdc2 in the AC voltage is increased by making the duty ratio of the AC voltage at the time of detecting the occurrence of discharge smaller than that during image formation. Can do. Further, since the duty ratio is made smaller than that at the time of image formation, even if the peak-to-peak voltage is increased, the minus side potential is less likely to fall than the plus side potential. Therefore, the potential difference V + 2 between the positive peak value of the AC voltage and the bright potential V0 (approximately 0 V) on the surface of the photosensitive drum can be made larger than the potential difference between the negative peak value and the bright potential V0 (lower row in FIG. 7). , It is possible to cause a discharge between the developing roller and the photosensitive drum in the plus direction (a state where the potential of the developing roller is higher than that of the photosensitive drum).

  The photosensitive drum of this embodiment has a positively charged amorphous silicon photosensitive layer. As described above, it has been confirmed that such a photosensitive drum has a characteristic that even if a discharge occurs in the plus direction, the discharge current does not increase rapidly and a large current does not flow easily (see FIG. 10). Therefore, there is almost no possibility that a large current flows through the photosensitive drum due to the positive discharge and a drum pin hole (a minute hole) is formed in the photosensitive drum, and the photosensitive layer is destroyed. Thus, by making the duty ratio of the AC voltage at the time of detecting the occurrence of discharge smaller than that at the time of image formation, it is possible to discharge in the plus direction and suppress the destruction of the photosensitive layer. Further, since the photosensitive drum is not damaged even if the occurrence of discharge is repeated, the occurrence detection of discharge can be repeated frequently, and the developing efficiency of the printer 1 can always be maintained at a high level.

  Here, in practice, it can be considered that the AC voltage is also applied to the capacitive load such as the toner adhering to the developing roller 81 and the developer between the developing roller 81 and the magnetic roller 82. Actually, the rise and fall of the voltage are not steep and take a certain time. As shown in FIG. 8A, if the cycle remains the same as that during image formation (T01 = T02), the duty ratio D2 Is smaller than the duty ratio D1 at the time of image formation, the time of the positive side peak of the AC voltage is considerably shortened. Therefore, in the present embodiment, as shown in FIG. 8B, the frequency f2 is set so that the positive side time of the AC voltage is the same between the image formation and the discharge occurrence detection (T1 = T2) ( For example, when D1 = 40% and D2 = 30%, f2 = 3 kHz if the frequency f1 = 4 kHz during image formation. As a result, the time of the positive side peak of the AC voltage at the time of detecting the occurrence of discharge can be as close as possible to that at the time of image formation, and can be ensured in the same way, and the application state of the AC voltage at the time of detecting discharge occurrence can be matched during the image formation .

  It should be noted that the setting value Vdc2 for detecting the occurrence of bias discharge is desirably set higher than the setting value Vdc1 at the time of image formation. This is because the toner is positively charged, and the amount of toner supplied from the magnetic roller 82 to the developing roller 81 when the occurrence of discharge is detected can be suppressed.

(Flow of control of discharge occurrence detection operation)
Next, an example of a control flow of the discharge occurrence detection operation of the printer 1 according to the embodiment of the present invention will be described with reference to FIG. FIG. 9 is a flowchart showing an example of the control flow of the discharge occurrence detection operation of the printer 1 according to the embodiment of the present invention. Note that this flowchart is a control for one image forming unit 3 and is repeated four times in the present embodiment when all colors are used.

  This discharge occurrence detection operation can be performed, for example, at the time of manufacturing as initial defect detection or initial setting, when the printer 1 is installed, or when the developing device 8 or the photosensitive drum 9 is replaced. The reason why the printer 1 is installed is that the atmospheric pressure changes depending on the altitude of the installation environment (for example, the difference between Japan and the Mexican highlands), and there is a difference in the voltage at which discharge occurs. The reason for performing the replacement of the developing device 8 and the like is that the gap between the photosensitive drum 9 and the developing roller 81 is different from that before the replacement. Note that the present invention is not limited to the above example. For example, it may be performed every time the printer 1 prints a certain number of sheets, and the execution timing can be set as appropriate.

  First, when a predetermined operation is performed on the operation panel 13 and a discharge generation detection operation is started (start), the photosensitive drum 9, the developing roller 81, the magnetic roller 82, The rotation of various rotating bodies in the image forming unit 3 such as the intermediate transfer belt 52 and the intermediate transfer unit 5 is started (step S1). The driving of each rotating body is continued until the discharge occurrence detecting operation is completed. In the discharge occurrence detection operation, the developing roller 81 basically does not carry toner. Next, the initial operation described in FIG. 6 is performed (step S2). Next, the process proceeds to the preparation state described with reference to FIG. 6 (step S <b> 3). For example, the charging voltage application unit 72 starts voltage application to the charging device 7 according to an instruction from the CPU 11.

  Next, the default measurement described in FIG. 6 is performed (step S4). At this time, it is confirmed that the occurrence of discharge is not detected (step S5). This default measurement is performed in a state in which no discharge occurs (for example, the magnitude of the AC voltage to the developing roller 81 is extremely low). If the occurrence of discharge is detected in the default measurement (No in step S5), There may be a gap abnormality or a hardware abnormality of the detection unit 14 or the like. In this case, an error display (step S6) is performed on the operation panel 13 or the like, and the discharge occurrence detection operation ends (end).

  On the other hand, if the signal indicating that the discharge has occurred is not input to the CPU 11 (Yes in step S5), the process shifts to the condition change state described with reference to FIG. Setting is made to increase the peak-to-peak voltage of the AC voltage output by 86 by a predetermined step width ΔV1 (for example, 30 to 100 V, etc.) from the current state (step S7).

  Then, the state shifts to a discharge detection state. Specifically, an AC voltage whose peak-to-peak voltage is increased by ΔV1 is applied to the developing roller 81, and exposure is performed for a predetermined time according to an instruction from the CPU 11, during which the CPU 11 Counts the number of times the output voltage of the amplifier 15 exceeds a predetermined threshold (step S8). Then, it is confirmed whether the count number is not 0 (step S9). If it is 0 (No in step S9), the current peak-to-peak voltage can be set to a maximum value (for example, 1500 to 1500) as no discharge occurs. CPU11 confirms whether it has reached 3000V (step S10), and if it has reached (Yes in step S10), the process proceeds to step S16 (details will be described later). If not reached (No in step S10), the process returns to step S7.

  In step S9, if the count value is once or more (Yes in step S9), the discharge is generated, and the Vpp control unit 88 instructs the AC voltage application unit 86 to apply the peak of the AC voltage to the developing roller 81 in response to an instruction from the CPU 11. The inter-voltage is decreased from the current state by a predetermined step size ΔV1 (step S11), and further set to a value increased by a predetermined step size ΔV2 (step S12). Here, the predetermined step width ΔV2 can be obtained by dividing the predetermined step width ΔV1 (for example, ΔV2 = 10V when ΔV1 = 50V). In other words, in order to find the peak-to-peak voltage at which discharge occurs more finely, the step size is stepped back and the step size of the step-by-step change in the peak-to-peak voltage is reduced.

  Thereafter, as in step S8, the discharge detection state is set, and the CPU 11 counts the number of times that the output voltage of the amplifier 15 exceeds a predetermined threshold (step S13). In other words, if a discharge is detected in the stepwise change of the peak-to-peak voltage at the step size ΔV1, the discharge is detected at the step size ΔV2 in order to obtain a more detailed peak-to-peak voltage at which discharge occurs. Until this occurs, the discharge detection state and the condition change state are repeated.

  Next, it is confirmed whether the count number is not 0 (step S14). If it is 0 (No in step S14), the current peak-to-peak voltage is the peak-to-peak voltage at which discharge is detected first as no discharge has occurred. CPU11 confirms whether it has reached (step S15). If it has been reached (Yes in step S15), the process proceeds to step S16. If not reached (No in step S15), the process returns to step S12. On the other hand, if the count value is one or more times (Yes in step S14), the CPU 11 determines that a discharge occurs at the current peak-to-peak voltage, and proceeds to step S16.

  Next, step S16 will be described in detail. When the discharge is detected (Yes in step S14, Yes in step S15) or even when the maximum peak-to-peak voltage that can be set cannot be detected (Yes in step S10), the CPU 11 recognizes that the discharge is generated. From the voltage Vpp2, or the maximum peak-to-peak voltage, the frequency f2, the duty ratio D2, and the bias setting value Vdc2, the potential difference V + 2 shown in FIG. 7 (photosensitive drum 9 and development at the time of detecting discharge or applying Vpp2 at the maximum settable value) A potential difference between the rollers 81 is obtained (step S16).

  Here, V + 2 can be easily obtained. CPU 11 designates the magnitude of the peak-to-peak voltage and issues an instruction to Vpp control unit 88. Therefore, the control part 10 grasps | ascertains Vpp2 at that time, when discharge generation | occurrence | production is detected. Then, based on the duty ratio D2 as the set value and Vdc2 as a reference, the positive side area and the negative side area are made equal to obtain the potential difference between the positive side peak value and Vdc2. If this potential difference is added with the potential difference between Vdc2 and V0 (V0 is almost 0V, it can be treated as Vdc2), V + 2 is obtained.

  Specifically, if the frequency f2, the duty ratio D2, and the bias setting value Vdc2 are constant, V + 2 is calculated in advance according to the size of each Vpp2, converted into data as a lookup table, and the CPU 11 stores the table. Reference may be made to obtain V + 2. In addition, what is necessary is just to memorize | store this table in the memory | storage part 12, for example.

  Next, based on the obtained V + 2, the CPU 11 determines the AC voltage applied to the developing roller 81 during image formation so that V + 1 and V− shown in FIG. 7 are smaller than the obtained V + 2. The peak-to-peak voltage Vpp1 is set (step S17). Specifically, there are various methods for determining Vpp1, but for example, how much smaller V + 1 and V− than V + 2 will not cause discharge (how much margin should be taken) depends on the toner used. For example, based on an experiment at the time of development, for example, for V + 2 obtained, a value of Vpp1 that is recognized as causing no discharge during image formation is tabulated, and the CPU 11 refers to that table and Vpp1 is determined. good. This table may also be stored in the storage unit 12. This makes it possible to apply as much AC voltage as possible without causing discharge during image formation.

  In short, in the printer 1 of the present embodiment, when it is detected that a discharge has occurred when the occurrence of a discharge is detected, the control unit 10 causes the photosensitive drum 9 and the development roller with respect to the AC voltage applied to the developing roller 81 when the discharge occurs. A potential difference between 81 is determined, and an AC voltage to be applied to the developing roller 81 during image formation is determined so that the potential difference between the surface potentials of the developing roller 81 and the photosensitive drum 9 during image formation is smaller than the determined potential difference. It is.

  When the setting of Vpp1 is completed, the detection of discharge occurrence and the setting of Vpp1 at the time of image formation are completed (END). Then, after the completion of this control, the printer 1 returns to a state where image formation is possible. As described above, according to the present invention, it is possible to automatically set Vpp1 to be applied to the developing roller 81, which has high development efficiency and does not generate discharge during image formation.

  As described above, according to the embodiment of the present invention, in order to grasp the potential difference between the developing roller 81 and the photosensitive drum 9 where the discharge is generated, the discharge is generated while changing the AC voltage applied to the developing roller 81. When detecting and checking, an AC voltage having a duty ratio smaller than that at the time of image formation and a frequency set smaller than that at the time of image formation is applied to the developing roller 81 so that the plus side time is the same as that at the time of image formation.

  By making the duty ratio of the AC voltage at the time of detecting the occurrence of discharge smaller than that at the time of image formation, the difference between the positive peak value and the center of the AC voltage, that is, the DC bias by the DC voltage application unit 85 can be increased. . Accordingly, the potential difference between the positive side peak value of the AC voltage and the surface potential of the photosensitive drum 9 can be increased, and between the developing roller 81 and the photosensitive drum 9 in the positive direction (a state where the potential of the developing roller 81 is higher than that of the photosensitive drum 9). Can cause a discharge. Here, the photosensitive drum 9 has an amorphous silicon photosensitive layer, and the photosensitive drum 9 has such a characteristic that a current does not suddenly flow between the developing roller 81 and the photosensitive drum 9 when discharged in the plus direction. Therefore, there is almost no possibility that a drum pinhole is formed in the photosensitive drum 9 due to the positive discharge and the photosensitive layer is destroyed. That is, it is possible to measure the potential difference between the photosensitive drum 9 and the developing roller 81 where the discharge occurs while suppressing the destruction of the photosensitive layer of the photosensitive drum 9.

  In addition, since the AC voltage whose frequency is set smaller than that at the time of image formation is applied to the developing roller 81 so that the plus side time is the same as that at the time of image formation, even if it takes time to rise and fall of the AC voltage, The time of the positive side peak of the AC voltage can be secured in the same manner as in the image formation, and the application state of the AC voltage when the occurrence of discharge is detected can be matched during the image formation.

  In addition, when the occurrence of discharge is detected, the control unit 10 causes the DC voltage application unit 85 to apply a higher DC voltage to the developing roller 81 than during image formation, so that when the occurrence of discharge is detected, the magnetic roller is positively charged from the developing roller. It is possible to prevent the toner from being supplied. In addition, based on the potential difference between the developing roller 81 and the photosensitive drum 9 where the detected discharge is generated, it is possible to set an AC voltage that enhances the development efficiency and does not generate a discharge during appropriate image formation.

  Further, since the photosensitive layer of the photosensitive drum 9 is composed of positively charged amorphous silicon, the characteristic that a large current hardly flows due to the positive discharge is remarkably exhibited, and the destruction of the photosensitive layer of the photosensitive drum 9 is suppressed. The effect is increased.

  The embodiment of the present invention has been described above, but the scope of the present invention is not limited to this, and various modifications can be made without departing from the spirit of the invention.

The present invention includes a photosensitive drum and a developing roller, and a developing bias (DC +
It can be used for an image forming apparatus that applies (AC).

These are sectional views showing a schematic configuration of a printer according to an embodiment of the present invention. These are the expanded sectional views of the image formation part concerning the embodiment of the present invention. These are schematic diagrams showing an example of an exposure apparatus according to the present embodiment. FIG. 4 shows a configuration around the developing roller related to application of a developing bias to the developing roller and detection of discharge between the photosensitive drum and the developing roller according to the present embodiment. FIG. 2 is a block diagram illustrating an example of a hardware configuration of a printer according to the present embodiment. These are timing charts for explaining the outline of the discharge occurrence detection operation according to the present embodiment. These are timing charts explaining the details of the AC voltage applied to the developing roller according to the present embodiment. These are timing charts showing the actual waveform of the AC voltage applied to the developing roller. These are the flowcharts which show an example of the flow of control of discharge generation detection operation of the printer which concerns on this embodiment. These are graphs showing the relationship between the potential difference between the photosensitive drum and the developing roller and the discharge current flowing between the photosensitive drum and the developing roller.

Explanation of symbols

1 Printer (image forming device)
3 (3a, 3b, 3c, 3d) Image forming unit 8 (8a, 8b, 8c, 8d) Developing device 81 (81a, 81b, 81c, 81d) Developing roller 85 DC voltage applying unit 86 AC voltage applying unit 9 (9a) 9b, 9c, 9d) Photosensitive drum 10 Control unit 11 CPU (part of control unit 10)
14 detector

Claims (4)

A photosensitive drum carrying a toner image on its peripheral surface;
A direct current voltage application unit and a developing roller to which an alternating voltage application unit is connected to face the photoconductive drum with a gap, carry a toner during image formation, and supply the toner to the photoconductive drum. ,
A detector for detecting the occurrence of discharge between the developing roller and the photosensitive drum;
A control unit that controls each part of the device and recognizes the occurrence of discharge when the output of the detection unit is input;
Instructing the AC voltage application unit to change the AC voltage applied to the developing roller by the control unit to the AC voltage application unit, and detecting the occurrence of discharge by the detection unit,
The AC voltage application unit applies an AC voltage having a smaller duty ratio than that at the time of image formation and a frequency set to be lower than that at the time of image formation so that the plus time is the same as that at the time of image formation. An image forming apparatus. A magnetic roller provided opposite to the developing roller and holding a positively charged toner;
The image forming apparatus according to claim 1, wherein when the occurrence of discharge is detected, the control unit causes the DC voltage application unit to apply a higher DC voltage to the developing roller than during image formation. When it is detected that a discharge has occurred during the occurrence of discharge,
The control unit obtains a potential difference between the photosensitive drum and the developing roller with respect to a peak-to-peak voltage of the AC voltage applied to the developing roller at the time of occurrence of discharge, and determines the difference between the developing roller and the photosensitive drum during image formation. 3. The image forming apparatus according to claim 1, wherein an AC voltage to be applied to the developing roller during image formation is determined so that a potential difference of the surface potential is smaller than the potential difference.   The image forming apparatus according to claim 1, wherein the photosensitive drum has a positively charged amorphous silicon photosensitive layer.

Images