JP2010054740A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus for reducing false detection of electric discharge due to influence of noise. <P>SOLUTION: In a period when an AC voltage is applied from an AC voltage applying part 86 to a developing roller 81 until a photoreceptor drum 9 and the developer roller 81 are rotated at least twice, a control part 10 counts the number of times that a discharge detection signal output from a detection part 14 exceeds a first threshold, and when the result is one, an AC voltage equal to the AC voltage previously applied is applied from the AC voltage applying part 86 to the developing roller 81 until the photoreceptor drum 9 and the developing roller 81 are rotated at least twice, and during this period, the control part 10 counts the number of times that the discharge detection signal output from the detection part 14 exceeds a second threshold having an absolute value smaller than the first threshold, and when the result is ≤1, the control part 10 recognizes that no electric discharge has occurred. On the other hand, when the result is ≥2, the control part 10 recognizes that the electric discharge has occurred. <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 discloses a voltage adjustment device that changes a voltage applied between a toner carrier and an image carrier from a DC power source and an AC power source, and an image carrier and a toner carrier. Disclosed is a developing device provided with a current detector that detects a flowing current and a control device that determines the presence or absence of leakage (discharge) based on a result detected by the current detector and controls the voltage regulator. Has been. In this developing device, the control device controls the voltage adjusting device until the control device detects a leak, thereby changing the leak detection voltage to be applied between the toner carrier and the image carrier. A leak is generated between the body and the toner carrier. Then, based on the leak detection voltage at the time of the occurrence of the leak, the control device controls the voltage regulator, and it is appropriate under the condition that no leak occurs between the DC power source and the AC power source between the toner carrier and the image carrier. A developing bias capable of performing proper development is applied (see Patent Document 1: Paragraph 0023 to Paragraph 0026).
Japanese Patent No. 3815356

  However, the above-mentioned Patent Document 1 does not specifically describe how much current is detected to determine that a discharge has occurred. In the developing device of Patent Document 1, the current flows into the current detector. There is a risk that noise may be erroneously determined to be a discharge.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of suppressing erroneous detection of occurrence of discharge due to the influence of noise.

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 developing roller that faces the photoconductor drum with a gap, carries a toner during image formation, and is connected to an AC voltage application unit for supplying the toner to the photoconductor drum;
A control unit for controlling each part of the device;
A detection unit that converts a current that flows when a discharge occurs between the developing roller and the photosensitive drum into a voltage, and outputs the voltage to the control unit as a discharge detection signal;
The control unit instructs the AC voltage applying unit to change the AC voltage applied to the developing roller in one step, the AC voltage applying unit applies the AC voltage to the developing roller, and the photosensitive drum and the developing roller include During the application for at least two rotations, the control unit counts the number of times that the discharge detection signal output by the detection unit exceeds the first threshold value,
If the counting result is once, the AC voltage application unit applies the same AC voltage as the AC voltage applied immediately before to the developing roller for a time during which the photosensitive drum and the developing roller rotate at least twice, During the application, the control unit counts the number of times that the discharge detection signal output from the detection unit exceeds the second threshold value changed so that the absolute value becomes smaller than the first threshold value,
If the count result of the number of times the discharge detection signal exceeds the second threshold is 1 or less, the control unit recognizes that no discharge has occurred,
On the other hand, when the count result of the number of times that the discharge detection signal exceeds the second threshold is 2 or more, the control unit recognizes that a discharge has occurred.

  According to such a configuration, when an AC voltage is applied to the developing roller, no discharge occurs, but noise occurs in the discharge detection signal, and the number of times that the discharge detection signal exceeds the first threshold is one. In this case, since the same AC voltage as the AC voltage applied immediately before is applied and no discharge is generated, the number of times that the discharge detection signal exceeds the second threshold is 1 or less (in the case of 1 time, noise is considered to be generated). It is recognized that no discharge has occurred. That is, it is possible to suppress erroneous detection of the occurrence of discharge due to the influence of noise.

  In general, the photosensitive drum has “runout (shift)” from an ideal cylindrical shape or columnar shape due to an error in manufacturing a substrate, conditions in forming a photosensitive layer, or the like. Further, the developing roller also has “runout” due to an error during manufacture. Therefore, strictly speaking, the length of the gap that affects the occurrence of discharge changes during rotation of the photosensitive drum and the developing roller. Discharge is likely to occur when a portion with a large shake at the photosensitive drum or the developing roller reaches the opposing portion and the gap becomes short. During application of an AC voltage, at least the photosensitive drum and the developing roller Since it rotates twice, the part with large deflection can reach the opposing part at least twice. That is, when a discharge occurs, there is a high possibility that the discharge is detected twice or more.

  Therefore, if a discharge occurs when the AC voltage is changed by one level and applied to the developing roller, there is a high possibility that the peak of the discharge detection signal will be two or more. However, if the discharge is very small and only one of the peaks exceeds the first threshold value, the number of times the first threshold value is exceeded is one, and the same AC voltage as that applied immediately before is applied. To do. Here too, a small discharge occurs, but here the second threshold value is used for comparison so that the absolute value is smaller than the first threshold value, that is, in the direction approaching 0 V, so the first threshold value was not exceeded. The peak of the discharge detection signal also exceeds the second threshold, and the number of times the discharge detection signal exceeds the second threshold is two or more, so that it is recognized that a discharge has occurred. That is, although a slight discharge has actually occurred, even if the number of times that the discharge detection signal exceeds the first threshold is one, and it is unknown whether noise has occurred or discharge has occurred, it can be reduced by changing the threshold. The occurrence of discharge can be accurately detected.

  Further, when the invention according to claim 2 recognizes that discharge has occurred in the invention according to claim 1, the control unit changes the amount of change in one step from the state one step before the AC voltage applied immediately before. The AC voltage application unit is instructed to change the AC voltage applied to the developing roller step by step with the change amount obtained by dividing the above, and the occurrence of discharge is detected using the detection unit.

  According to such a configuration, when the control unit recognizes the occurrence of discharge, the AC voltage is applied in finer units from the state one stage before the AC voltage applied immediately before, and the vicinity of the AC voltage at which the discharge starts is generated. AC voltage can be found with high accuracy.

  Further, in the invention according to claim 3, in the invention according to claim 2, when the occurrence of discharge is detected, the control unit detects the peak-to-peak voltage of the AC voltage applied to the developing roller when the discharge occurs. A potential difference between the photosensitive drum and the developing roller is obtained and applied to the developing roller during image formation so that a potential difference between the surface potential of the developing roller and the photosensitive drum during image formation is smaller than the potential difference. The AC voltage is determined.

  According to such a configuration, the potential difference in the vicinity of the potential difference between the photosensitive drum where the occurrence of discharge starts and the developing roller can be accurately obtained based on the AC voltage near the alternating voltage where the occurrence of discharge starts with high accuracy. Further, it is possible to reliably set an AC voltage that has high development efficiency and does not generate discharge during image formation.

The invention according to claim 4 is the invention according to any one of claims 1 to 3, wherein the control unit is configured to output a discharge detection signal output from the detection unit toward the control unit. Monitor the change, calculate the rate of change of the voltage value of the discharge detection signal,
The first threshold value and the second threshold value are for the rate of change of the voltage value of the discharge detection signal.

  In the detection unit, the voltage on the signal line from the detection unit to the control unit may fluctuate (is not stable) due to noise, the circuit configuration of the detection unit, the influence of the AC voltage application unit, or the like. Therefore, there is a possibility that the occurrence of discharge cannot be accurately detected with an absolute threshold value (voltage value). However, according to this configuration, since the threshold is for the rate of change of the voltage value of the discharge detection signal, when a current is generated by the discharge and a large change in the state of the signal line from the detection unit to the control unit occurs. The control unit recognizes that a discharge has occurred. Therefore, the control unit can accurately detect the occurrence of discharge.

  According to the image forming apparatus of the present invention, it is possible to suppress erroneous detection of the occurrence of discharge due to the influence of noise.

  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 and 2. 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. As shown in FIG. 1, the printer 1 according to this 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 device 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 device 6. The conveyance path 2 b 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 application unit 72 (see FIG. 4), 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) is provided to remove foreign matters on the surface of each charging roller 71. 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 the sleeves 812 and 822 rotate during image formation. 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. 4). A thin layer of is formed. Further, 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. The exposure device 4 is provided with a light receiving element (not shown) within the irradiation range of the laser light and outside the irradiation range of the photosensitive drum 9. When the laser beam is irradiated, the light receiving element outputs a current (voltage), and this output is input to, for example, a CPU 11 (Central Processing Unit) described later as a synchronization signal when confirming whether or not a discharge has occurred. Used (see FIG. 4).

  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 through 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 device 6 is arranged on the downstream side of the secondary transfer portion in the transfer material conveyance direction, and fixes the toner image secondarily transferred to the sheet by heating and pressing. The fixing device 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, and forms 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. 3 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. 3 shows only one image forming unit 3, and each image forming unit 3 is provided with a DC voltage application unit 85, an AC voltage application unit 86, and a detection unit 14, and the output of each detection unit 14 will be described later. Input to the CPU 11 of the control unit 10. Here, each of the DC voltage application unit 85, the AC voltage application unit 86, and the detection unit 14 may be denoted by symbols a, b, c, and d indicating the distinction between the image forming units. Since the same part is provided in the section, in order to avoid the complexity of the description, in the following description, the symbols a, b, c, and d are omitted.

  As shown in FIG. 3, the developing roller 81 is opposed to the photosensitive drum 9 with a gap, and has 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. 4) 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-side 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 that varies the peak-to-peak voltage, the duty ratio, and the frequency of the AC voltage only need to change the peak-to-peak voltage, the duty ratio, and the frequency.

  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 to the photosensitive drum 9.

  The detection unit 14 detects a current that flows when a discharge occurs between the developing roller 81 and the photosensitive drum 9 into a voltage, and an amplifier 15 that amplifies the converted voltage signal and outputs the voltage signal to the CPU 11 as a discharge detection signal. It consists of. The CPU 11 A / D converts the discharge detection 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. 4 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. 4, 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 setting an AC voltage applied to the discharge detection and the developing roller 81 and thresholds TH1 and TH2 (details will be described later) for discharge detection. In addition, the timer unit 11a measures the time required for controlling the printer 1.

  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 6, the operation panel 13, and the like. Based on the above, the operation of each unit is controlled so that image formation is appropriately performed. The control unit 10 is also connected to the motor M, controls the ON / OFF of the power supply to the motor M, controls the supply of rotational driving force, and controls the rotation of the photosensitive drum 9, the developing roller 81, and the like. To do.

  Note that the operation panel 13 shown in FIG. 4 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. Also, the magnetic roller bias application unit 84 shown in FIG. 4 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.

  In addition, regarding the present invention, the control unit 10 (CPU 11) is connected to the detection unit 14 (amplifier 15). When the present invention is carried out, 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 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 development bias is stored in the storage unit 12.

(Discharge occurrence detection operation and setting of AC voltage applied to developing roller 81)
Next, an example of the 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. 5 is a timing chart for explaining the outline of the discharge occurrence detection operation according to the embodiment of the present invention. FIG. 6 is a timing chart for explaining 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, an outline of the discharge occurrence detection operation will be described with reference to FIG. Note that “developing roller (AC)” in FIG. 5 indicates the timing at which the AC voltage applying unit 86 applies an AC 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 outputs a DC voltage. “Magnetic roller (AC)” indicates the timing at which the magnetic roller bias applying unit 84 (see FIG. 4) 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 a light receiving element (not shown) 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 increased stepwise. 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, 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, and shifts to a 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. 6, 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 = dark potential, 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 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 the 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.

  At the time of 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. Moreover, the Vpp control part 88 sets Vpp2 of the alternating voltage output from the alternating voltage application part 86 by the instruction | indication of CPU11. In addition, the duty ratio / frequency control unit 89 uses the instruction of the CPU 11 to detect the duty ratio D2 of the AC voltage output from the AC voltage application unit 86 (the ratio of the plus-side time T2 with respect to the cycle T02, T2 / T02) for discharge generation detection. And 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 part of FIG. 6).

  Here, the duty ratio D2 is set to be smaller than the duty ratio D1 at the time of image formation (ratio of the positive side time T1 to the period T01, T1 / T01) (for example, D1 = 40%, D2 = 30%). The frequency f2 is set so that the positive time of the AC voltage is the same when the image is formed and when the discharge is detected (T1 = T2) (for example, when D1 = 40% and D2 = 30%) If the frequency f1 = 4 kHz during image formation, f2 = 3 kHz).

  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 the 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 FIGS. 7 and 8 are flowcharts 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. 5 is performed (step S2). Next, the process proceeds to the preparation state described with reference to FIG. 5 (step S3). For example, the charging voltage application unit 72 starts voltage application to the charging device 7 in accordance with an instruction from the CPU 11.

  Next, the default measurement described in FIG. 5 is performed (step S4). At this time, the CPU 11 confirms whether or not a discharge has occurred based on the output of the detection unit 14 (step S5). This default measurement is performed in a state in which no discharge occurs (for example, the magnitude of the AC voltage applied to the developing roller 81 is extremely low), and if the occurrence of discharge is detected in the default measurement (No in step S5). An abnormality in the gap or an abnormality in the hardware of the detection unit 14 or the like can be considered. 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 occurrence of discharge is not detected in the default measurement (Yes in step S5), the process shifts to the condition change state described in FIG. 5, and the Vpp control unit 88 outputs the AC voltage application unit 86 in response to the instruction from the CPU 11. Setting is made to increase the peak-to-peak voltage of the AC voltage by a predetermined step size ΔV1 (for example, 30 to 100 V) (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 that the output voltage (discharge detection signal) of the amplifier 15 exceeds the predetermined threshold TH1 (step S8). The time for the discharge detection state here is a time for which the photosensitive drum 9 rotates at least twice (for example, a time for two rotations). Further, since the developing roller 81 has a smaller radius and a shorter circumference than the photosensitive drum 9 (see FIG. 3 and the like), the developing roller 81 rotates more than that (for example, photosensitive drum 9) while the photosensitive drum 9 rotates at least twice. The developing roller 81 rotates 5 times while the body drum 9 rotates 2 times). The discharge detection state time may be stored in the storage unit 12 in advance, and the time measuring unit 11a may measure the time.

  Then, the CPU 11 confirms how many times the count is (step S9), and if it is 0, it is determined that no discharge has occurred and the current peak-to-peak voltage can be set to a maximum value (for example, 1500). The CPU 11 confirms whether or not it has reached (˜3000 V) (step S10), and if it has reached (Yes in step S10), the process proceeds to step S18 (FIG. 8) (details will be described later). If not reached (No in step S10), the process returns to step S7.

  In step S9, if the count is 2 or more, it is determined that a discharge has occurred, and the process proceeds to step S13 (FIG. 8) described later. In general, the photosensitive drum 9 has an “out-of-shake” from an ideal cylindrical shape or columnar shape due to an error in manufacturing a substrate, conditions in forming a photosensitive layer (for example, amorphous silicon or organic photosensitive member), or the like. ) ”. Further, the developing roller 81 also has “runout” due to an error during manufacture. Therefore, strictly speaking, the length of the gap that influences the occurrence of discharge changes even while the photosensitive drum 9 and the developing roller 81 are rotating. Then, the discharge is likely to occur when a portion with a large shake at the photosensitive drum 9 or the developing roller 81 reaches the opposite portion and the gap is shortened. Since the developing roller 81 rotates more than that, the portion with a large shake can reach the opposite portion at least twice. That is, when a discharge occurs, there is a high possibility that the discharge is detected twice or more.

  In step S9, if the count number is 1, it is unknown whether noise has occurred or discharge has occurred, and the process proceeds to step S11. Here, the state shifts to a discharge detection state. Specifically, the same AC voltage as in step S8 is applied to the developing roller 81, and exposure is performed for a predetermined time in accordance with an instruction from the CPU 11, while the CPU 11 outputs the output voltage ( The number of times that the discharge detection signal) exceeds a predetermined threshold value TH2 is counted (step S11). The time for the discharge detection state here is the time for at least two rotations of the photosensitive drum 9 as in step S8. Further, the threshold value TH2 is made smaller than the threshold value TH1 in step S8. Specifically, the threshold value TH2 may be about 90% to 50% of the threshold value TH1 (for example, TH1 = 1.0V, TH2 = 0.5V, etc.).

  Then, the CPU 11 confirms how many times the count is (Step S12). If it is less than 1, the count is 1 in Step S9 because of noise, and discharge occurs. If not, the process proceeds to step S10. In step S12, if the count is 2 or more, it is determined that a discharge has occurred, and the process proceeds to step S13 (FIG. 8).

  For example, in the discharge detection state in step S8, as shown in FIG. 9A, it is assumed that noise does not occur but noise exceeding the threshold TH1 occurs in the output voltage (discharge detection signal) of the detection unit 14. Then, the count number becomes 1 in step S9, and the process shifts to the discharge detection state in step S11. If the same noise is generated here, the noise exceeds the threshold TH2 smaller than the threshold TH1, so that the count is one in step S12, and it is assumed that no discharge has occurred, and the process proceeds to step S10. If no noise is generated in the discharge detection state in step S11, the discharge detection signal does not exceed the threshold value TH2. Therefore, the count number is 0 in step S12, and it is assumed that no discharge has occurred, and the process proceeds to step S10. To do. Thus, even if noise is generated in the discharge detection state, it is possible to prevent erroneous detection that discharge has occurred.

  Further, for example, in the discharge detection state in step S8, as shown in FIG. 9B, the discharge occurred four times, but the discharge was very small, and the peak of the output voltage (discharge detection signal) of the detection unit 14 was 1 Assume that only one exceeds the threshold TH1. Then, the count number becomes 1 in step S9, and the process shifts to the discharge detection state in step S11. In this case as well, a slight discharge is generated, but it is assumed that the four peaks of the output voltage of the detection unit 14 exceed a threshold value TH2 smaller than the threshold value TH1. Then, the count number in step S12 is four times or more, and it is assumed that discharge has occurred, and the process proceeds to step S13. In this way, a slight discharge has occurred in the discharge detection state, but even if it is unclear whether discharge has occurred or noise has occurred in comparison with the threshold TH1, the comparison with the threshold TH2 with a smaller threshold TH1 It can be detected that a slight discharge has occurred.

  Next, in step S13, the Vpp control unit 88 reduces the peak-to-peak voltage of the AC voltage applied to the developing roller 81 by the AC voltage application unit 86 by a predetermined step size ΔV1 from the current state in accordance with an instruction from the CPU 11 (step S13). ) And a value increased by a predetermined step size ΔV2 (step S14). 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, the state shifts to a discharge detection state. Specifically, an AC voltage whose peak-to-peak voltage is increased by ΔV2 is applied to the developing roller 81, and exposure is performed for a predetermined time in accordance with an instruction from the CPU 11. The number of times that the output voltage (discharge detection signal) exceeds a predetermined threshold value TH1 is counted (step S15). Here, the time for the discharge detection state is a time for at least two rotations of the photosensitive drum 9 (for example, a time for two rotations), as in step S8. 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 1 or more (step S16). If it is 0 (No in step S16), it is assumed that no discharge has occurred, and the current peak-to-peak voltage is discharged first. The CPU 11 confirms whether or not the detected peak-to-peak voltage has been reached (step S17). If it has reached (Yes in step S17), the process proceeds to step S18. If not reached (No in step S17), the process returns to step S14. On the other hand, if the count value is 1 or more (Yes in step S16), the CPU 11 determines that a discharge occurs at the current peak-to-peak voltage, and proceeds to step S18.

  Next, step S18 will be described in detail. When the discharge occurrence is detected (Yes in step S16, Yes in step S17), or even if the maximum peak-to-peak voltage that can be set cannot be detected (Yes in step S10), the CPU 11 recognizes that the discharge occurs, the peak-to-peak voltage Vpp2. Or from the maximum peak-to-peak voltage, frequency f2, duty ratio D2, and bias setting value Vdc2, the potential difference V + 2 shown in the lower part of FIG. 6 (photosensitive drum 9 and developing roller when discharge is detected or when Vpp2 is applied at the maximum settable value) 81) is obtained (step S18).

  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, Vpp2 at the time of discharge occurrence detection operation is changed in stages, and the magnitude of Vpp2 is determined to some extent. If the duty ratio D2 and the bias setting value Vdc2 are constant, the Vpp2 depends on the magnitude of each Vpp2. Alternatively, V + 2 may be calculated in advance, converted into data as a lookup table, and the CPU 11 may refer to the table 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, the CPU 11 sets the peak-to-peak voltage Vpp1 of the AC voltage applied to the developing roller 81 during image formation so that both V + 1 and V− shown in the upper part of FIG. 6 are smaller than the obtained V + 2. (Step S19). 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). 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.

  Next, another embodiment will be described. In the above embodiment, the threshold value having a constant value (absolute threshold value) has been described as an example. However, the occurrence of discharge may be detected using a relative threshold value (voltage value change rate). That is, the control unit 10 monitors the change in the signal output from the detection unit 14 toward the control unit 10, calculates the change rate of the voltage value of the discharge detection signal (for example, calculated by the CPU 11), and the threshold value is Suppose that the rate of change in the voltage value of the discharge detection signal.

  Here, the detection circuit 14a is connected to the DC voltage application unit 85, the AC voltage application unit 86, and the like, and is detected by the influence of these application units, the influence of noise, the circuit configuration of the detection circuit 14a, and the like. The voltage on the signal line from the unit 14 to the control unit 10 may fluctuate (is not stable). Therefore, there is a case where the discharge cannot be accurately detected with the absolute threshold. Even in such a case, if the relative threshold value is used, when the current is generated by the discharge and a large change in the state of the signal line from the detection unit 14 to the control unit 10 occurs, the control unit 10 generates the discharge. Recognize. Therefore, the control unit 10 can accurately detect the occurrence of discharge.

  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.

  For example, in the above-described embodiment, the entire surface of the photosensitive drum 9 is exposed to 0 V in the discharge detection state, and the discharge is generated by the potential difference from the positive peak of the AC voltage applied to the developing roller 81. However, in the discharge detection state, the photosensitive drum 9 may be positively charged and not exposed, and a discharge may be generated by a potential difference from the negative peak of the AC voltage applied to the developing roller 81. In this case, the discharge current flows in the opposite direction to the above embodiment, and the positive / negative of the discharge detection signal is also opposite to that in the above embodiment. Therefore, the positive and negative values of the threshold values TH1 and TH2 may be reversed from those in the above embodiment. That is, the threshold value TH2 is a threshold value that has been changed in a direction closer to 0V (so that the absolute value becomes smaller) than the threshold value TH1.

  The present invention is applicable to an image forming apparatus that includes a photosensitive drum and a developing roller and applies a developing bias to the developing roller.

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. FIG. 4 shows a configuration around the developing roller relating to the application of a developing bias to the developing roller and detection of occurrence of discharge between the photosensitive drum and the developing roller according to the 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 the flowcharts which show an example of the flow of control of discharge generation detection operation in the printer which concerns on this embodiment. These are the flowcharts which show an example of the flow of control of discharge generation detection operation in the printer which concerns on this embodiment. FIG. 4A is a schematic waveform diagram showing an example where noise is generated in a discharge detection signal, and FIG. 4B is a schematic waveform diagram showing an example of a discharge detection signal when a discharge is generated.

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 developing roller that faces the photoconductor drum with a gap, carries a toner during image formation, and is connected to an AC voltage application unit for supplying the toner to the photoconductor drum;
A control unit for controlling each part of the device;
A detection unit that converts a current that flows when a discharge occurs between the developing roller and the photosensitive drum into a voltage, and outputs the voltage to the control unit as a discharge detection signal;
The control unit instructs the AC voltage applying unit to change the AC voltage applied to the developing roller in one step, the AC voltage applying unit applies the AC voltage to the developing roller, and the photosensitive drum and the developing roller include During the application for at least two rotations, the control unit counts the number of times that the discharge detection signal output by the detection unit exceeds the first threshold value,
If the counting result is once, the AC voltage application unit applies the same AC voltage as the AC voltage applied immediately before to the developing roller for a time during which the photosensitive drum and the developing roller rotate at least twice, During the application, the control unit counts the number of times that the discharge detection signal output from the detection unit exceeds the second threshold value changed so that the absolute value becomes smaller than the first threshold value,
If the count result of the number of times the discharge detection signal exceeds the second threshold is 1 or less, the control unit recognizes that no discharge has occurred,
On the other hand, when the count result of the number of times the discharge detection signal exceeds the second threshold is 2 or more, the control unit recognizes that a discharge has occurred.   When recognizing that the discharge has occurred, the control unit gradually changes the AC voltage applied to the developing roller in a change amount obtained by dividing the change amount of one step from the state one step before the AC voltage applied immediately before. The image forming apparatus according to claim 1, wherein a change is instructed to the AC voltage application unit, and occurrence of discharge is detected using the detection unit.   When the occurrence of discharge is detected, the control unit obtains a potential difference between the photosensitive drum and the developing roller with respect to the peak-to-peak voltage of the AC voltage applied to the developing roller at the time of occurrence of the discharge, and the developing at the time of image formation The image forming apparatus according to claim 2, wherein an AC voltage to be applied to the developing roller during image formation is determined so that a potential difference between a surface potential of the roller and the photosensitive drum is smaller than the potential difference. The control unit monitors the change in the discharge detection signal output from the detection unit toward the control unit, calculates the rate of change in the voltage value of the discharge detection signal,
4. The image forming apparatus according to claim 1, wherein the first threshold value and the second threshold value are for a change rate of a voltage value of a discharge detection signal. 5.

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