JP5255952B2 - Image forming apparatus - Google Patents

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. Also, depending on the characteristics of the photosensitive drum, a large current may flow depending on the direction in which the discharge current flows, causing damage to the photosensitive drum. Therefore, a voltage that causes discharge should not be applied to the developing roller during image formation.

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 a developing device that applies an applied developing bias voltage and supplies toner to an image carrier to develop an electrostatic latent image, a leak that changes a leak detection voltage applied between the image carrier and the toner carrier A generation unit and a leak detection unit for detecting a leak are provided, and the maximum potential difference ΔVmax between the leak detection voltage and the surface potential of the image carrier is gradually increased to flow between the image carrier and the toner carrier. A developing device is described in which when a current continuously increases, a leak detection unit determines that a leak has occurred (see, for example, Patent Document 1: Claim 1).
Japanese Patent No. 3815356

  As described above, in order to increase development efficiency without causing adverse effects due to electric discharge, no electric discharge occurs between the photosensitive drum and the developing roller during image formation, and an AC voltage as large as possible is applied to the developing roller. Will do. Then, in order to set the application of an AC voltage as large as possible without causing a discharge, the presence or absence of the discharge is detected while changing the magnitude of the AC voltage applied to the developing roller, and the discharge is generated. It is necessary to determine the AC voltage applied to the developing roller so that the potential difference between the developing roller and the photosensitive drum at the AC voltage is grasped and slightly lower than the potential difference during image formation.

  Here, the length of the gap between the photosensitive drum and the developing roller changes for each solid of each image forming apparatus due to the shake of the photosensitive drum and the developing roller (deviation from an ideal cylindrical shape or a cylindrical shape). . Therefore, the magnitude of the AC voltage (the peak-to-peak voltage) at which discharge occurs varies from individual to individual. On the other hand, if a large AC voltage is applied unnecessarily and a large discharge current is applied, the photosensitive drum is damaged (for example, a hole penetrating the photosensitive layer and the substrate is generated). Therefore, when searching for an AC voltage that generates a discharge, normally, a small AC voltage is first applied to the developing roller, the amplitude of the AC voltage is gradually increased, and an AC voltage that generates a discharge (in other words, a photosensitive drum). And the developing roller potential difference). However, there is a problem that it takes time to reach the AC voltage at which discharge occurs.

  Therefore, looking at the invention described in Patent Document 1, "... gradually increase the maximum potential difference ΔVmax between the leak detection voltage and the surface potential of the image carrier ..." (Patent Document 1: Claim 1) , Paragraph [0015]), “... Control voltage regulator 21 with this control device 32 until leak is detected in control device 32” (see Patent Document 1: Paragraph [0025]). However, there is no description on how to shorten the time required to find an AC voltage that generates a discharge.

  In view of the above-described problems, an object of the present invention is to reduce the time required to detect the AC voltage at which discharge occurs by narrowing the range of the AC voltage applied to the developing roller.

In order to achieve the above object, an image forming apparatus according to a first aspect of the present invention opposes a photosensitive drum carrying a toner image on a peripheral surface thereof while providing a gap in the photosensitive drum, and carries the toner during image formation. For supplying toner to the photosensitive drum, a developing roller to which an AC voltage applying unit is connected, a detecting unit for detecting occurrence of discharge between the developing roller and the photosensitive drum, and for storing data And a control unit that controls each unit of the apparatus and recognizes the occurrence of discharge when the output of the detection unit is input, and the storage unit includes an AC voltage stage applied to the developing roller. The setting data for setting the upper limit value and the lower limit value of the peak-to-peak voltage of the AC voltage applied to the developing roller when the occurrence of discharge is detected by the detection unit is detected. , When the serial discharge generation detector, the developing roller does not carry toner, wherein the control unit, based on the setting data, narrowing the range of the peak voltage of the AC voltage applied to the developing roller, the alternating voltage applying unit An AC voltage was applied to the developing roller within a range narrowed down to

  When detecting the occurrence of discharge, the control unit narrows down the range of the peak voltage of the AC voltage applied to the developing roller based on the setting data, and applies the AC voltage to the developing roller within the range narrowed down to the AC voltage applying unit. Compared to the case where the occurrence of discharge is detected by applying an AC voltage from the lower limit value of the peak-to-peak voltage that can be set in the voltage application section to the upper limit value in order, it is possible to quickly find the AC voltage that generates the discharge. it can. Therefore, it is possible to shorten the time until the AC voltage at which discharge starts to be detected.

  According to a second aspect of the present invention, in the first aspect of the invention, the setting data includes data relating to each shake of the photosensitive drum and the developing roller measured in advance, and the storage unit includes the storage unit When detecting occurrence of discharge, a table for determining a range of a peak-to-peak voltage to be applied to the developing roller by the AC voltage application unit corresponding to the gap is stored, and the control unit is based on the setting data The gap length is calculated, and the range of the peak-to-peak voltage of the AC voltage applied to the developing roller is narrowed down from the calculated gap length with the table.

  Based on the setting data, the gap length is calculated, and the range of the peak-to-peak voltage of the AC voltage applied to the developing roller is narrowed down from the calculated gap length in the table, so the gap length that affects the occurrence of discharge is reduced. Based on this, it is possible to set the range of the amplitude of the AC voltage. In addition, since the setting data includes data relating to pre-measured photoconductor drum and developing roller deflection, even if one or both of the photoconductive drum and the developing roller are replaced, the data can be rewritten so that the developing roller It is also possible to narrow the range of the peak-to-peak voltage of the AC voltage to be applied.

  According to a third aspect of the present invention, in the first aspect of the invention, the setting data defines a range of a peak-to-peak voltage that the AC voltage application unit should apply to the developing roller when the occurrence of discharge is detected. It was decided to be a thing.

  According to this configuration, the setting data defines the range of the peak-to-peak voltage that should be applied to the developing roller by the AC voltage application unit when the occurrence of discharge is detected. In addition, the range of the AC voltage to be applied to the developing roller at the time of detecting the discharge can be narrowed down.

  According to a fourth aspect of the invention, in the first to third aspects of the invention, when it is detected that a discharge has occurred when the occurrence of discharge is detected, the control unit applies the alternating current applied to the developing roller when the discharge has occurred. A potential difference between the photosensitive drum and the developing roller with respect to a voltage peak value is obtained, and development is performed 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 to be applied to the roller was determined.

  According to this configuration, based on the potential difference between the developing roller and the photosensitive drum where the accurately recognized discharge is generated, it is possible to set an appropriate alternating voltage that does not generate a discharge during image formation while improving the development efficiency. .

  According to the image forming apparatus of the present invention, when the occurrence of discharge is detected by applying an AC voltage to the developing roller, the range of the AC voltage to be applied is narrowed down in advance, so that it is possible to quickly find the AC voltage at which discharge occurs. it can.

  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. 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 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 accommodates various sheets such as copy sheets, OHP sheets, and label sheets, for example, and sends them to the conveyance path 2b by a sheet feeding roller 21 that is rotated by a drive mechanism (not shown) such as a motor. 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 (corresponding to a charging unit) has a charging roller 71 and charges the photosensitive drum 9 at 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) for removing foreign matters on the surface of each charging roller 71 is provided. The charging device 7 may be a device 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. 3). . 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 unit 84: see FIG. 4). 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.

  An exposure device 4 (corresponding to an exposure unit) 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 the converted light. A laser beam (illustrated by a broken line), which is a signal, is output, and the photosensitive drum 9 after being charged 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 beam on the photosensitive drum 9 and outside the irradiation range of the photosensitive drum 9. When this light receiving element is irradiated with laser light, it outputs a current (voltage), and this output is input to, for example, a CPU 11 to be described later and used as a synchronization signal when confirming whether or not a discharge has occurred (FIG. 5). reference).

  Returning to FIG. 1, the intermediate transfer unit 5 receives the primary transfer of the toner image from the photosensitive drum 9 and performs the secondary transfer onto the sheet, and each of the primary transfer rollers 51 a to 51 d, the intermediate transfer belt 52, and the driving roller 53. , Driven rollers 54, 55, 56, a secondary transfer roller 57, a belt cleaning device 58, and the like. Each of the primary transfer rollers 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 applying unit 59 (see FIG. 4) for applying a transfer voltage. The 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 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, and are 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, based on FIG. 3, a configuration relating to development bias application to each developing roller 81 and discharge detection between the respective photosensitive drums 9, 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 discharge between the photosensitive drums 9 according to the embodiment of the present invention.

  However, FIG. 3 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 This is input to the CPU 11 of the control unit 10 to be described later. Here, the direct-current voltage application unit 85, the alternating-current voltage application unit 86, the detection unit 14, and the amplifier 15 may be provided with symbols a, b, c, and d indicating the distinction between the image forming units. Since what is provided in each image forming unit 3 is the same, in order to avoid the complexity of the description, the symbols a, b, c, and d will be omitted below.

  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 whose average value is the DC voltage applied by the DC voltage application unit 85. 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 AC and DC 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 includes a detection circuit 14a that detects the occurrence of discharge by converting a current flowing when a discharge occurs between the developing roller 81 and the photosensitive drum 9 into a voltage signal, and an amplifier 15 that amplifies the converted voltage signal. Is done. For example, the detection circuit 14a compares a voltage obtained by converting the current flowing through the developing roller 81 when no discharge is generated into a voltage using a resistor or the like, and a voltage obtained by converting the current flowing through the developing roller 81 when a discharge is generated into a voltage. The difference between the two voltages is output to the amplifier 15. That is, the change in the current flowing through the developing roller 81 when the discharge occurs is converted into a voltage and output. In addition, the structure which converts the electric current which flowed by discharge into a voltage is not restricted to this.

  In the printer 1 of the present embodiment, the photosensitive drum 9 to be used has a positively charged amorphous silicon photosensitive layer, and discharging with the developing roller 81 at a higher potential causes the photosensitive drum 9 to discharge. It has the characteristic that a large current is difficult to flow suddenly. Therefore, in order to avoid damage to the photosensitive drum 9 due to a large current, the duty ratio and frequency are adjusted to detect the presence or absence of discharge when the potential of the developing roller 81 is high (details will be described later). Accordingly, in the present embodiment, the discharge current flows only from the developing roller 81 toward the photosensitive drum 9, and the discharge current appears as a variation in the DC voltage applied to the developing roller 81.

  The amplifier 15 amplifies the voltage signal from the detection unit 14 and outputs it to the CPU 11 as a discharge detection signal. The CPU 11 A / D converts the discharge detection signal from the 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) from the output of the amplifier 15 that has been subjected to the A / D conversion unit.

(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, data, and the like for the printer 1. In addition, the present invention stores a program for detecting discharge and setting an AC voltage applied to the developing roller 81. Further, a table TB for determining the range of the AC voltage applied to the developing roller 81 when the occurrence of discharge is detected from the setting data (details will be described later) relating to the pre-measured photoconductor drum 9 and developing roller 81 and the setting data. (Details will be described later).

  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 unit 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. A magnetic roller bias 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.

  Further, with respect to the present invention, the control unit 10 (CPU 11) is connected to the detection unit 14 (amplifier 15). Further, when detecting the occurrence of discharge, the CPU 11 gives an instruction to the AC voltage application unit 86 to stepwise change the peak-to-peak voltage of the AC voltage applied to the developing roller 81, and the analog output of the detection unit 14 (amplifier 15) is output to / D conversion is performed to detect the presence or absence of discharge and to determine the magnitude of the discharge. When the occurrence of discharge is detected, the control unit 10 grasps the potential difference between the developing roller 81 and the photosensitive drum 9 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 of the developing bias is stored in the storage unit 12.

(Basic discharge occurrence detection operation and setting of AC voltage when discharge occurrence is detected)
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. 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 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, an outline of the discharge occurrence detection operation will be described with reference to FIG. Note that “developing roller 81 (AC)” in FIG. 5 indicates the timing at which the AC voltage application 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 to the developing roller 81. “Developing roller 81 (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 the timing at which the magnetic roller bias 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 unit 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 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 the detection of the occurrence of discharge, basically, toner is not carried on the developing roller 81, but if no toner is carried, 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 control unit 10 changes the peak-to-peak voltage of the AC voltage applied to the developing roller 81 in a stepwise manner (for example, increase). 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.

<Discharge detection status>
In the discharge detection state, a developing bias is applied to the developing roller 81, and the exposure device 4 continuously performs exposure (exposure of the entire surface of the photosensitive drum 9) in order to stabilize the surface potential 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, 0.5 seconds 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 peak-to-peak voltage of the AC voltage applied to the developing roller 81 is basically higher than the previous time. The condition change state and the discharge detection state are repeated until the AC voltage to be discharged is recognized, and the peak-to-peak voltage of the AC voltage to be applied to the developing roller 81 is increased stepwise during the repetition. 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 to light, for example, about 200 to 300 V). “V ₊₁ ” indicates a potential difference between V 0 and the positive peak value of the developing bias at the time of 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 the time of the high state (positive state) 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 value 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 time of the high state (positive state) 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 is instructed by 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 high time T2 to the period 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 high 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 (ie, T1 = T2, for example, when D1 = 40% and D2 = 30%, If the frequency f1 = 4 kHz during image formation, f2 = 3 kHz). As a result, the same positive voltage as that during image formation is applied to the developing roller 81.

  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.

(Shake data of the photosensitive drum 9 and the developing roller 81)
Next, setting data relating to the shake of the photosensitive drum 9 and the developing roller 81 according to the embodiment of the present invention will be described with reference to FIGS. FIG. 7 is an explanatory diagram for explaining shake of the photosensitive drum 9 and the developing roller 81 according to the embodiment of the present invention. FIG. 8 is an example of a table TB for setting the range of the AC voltage applied to the developing roller 81 when the occurrence of discharge is detected according to the embodiment of the present invention.

  Here, according to the present invention, the range of the AC voltage applied to the developing roller 81 upon detecting the occurrence of discharge is narrowed according to the length of the gap between the photosensitive drum 9 and the developing roller 81. Measurement of the gap with the developing roller 81 attached is difficult due to the space in the machine. For this reason, in the printer 1 of the present embodiment, the measurement relating to the shake of the photosensitive drum 9 and the developing roller 81 is performed in advance before mounting, and the gap length in the mounting state in the printer 1 is predicted from the measurement result.

  First, with reference to FIGS. 7A and 7B, the deflection of the developing roller 81 will be described. FIG. 7A is an explanatory diagram for explaining the measurement related to the deflection of the developing roller 81. FIG. 7B is an enlarged view of a portion surrounded by a broken line in FIG.

  First, when measuring the deflection of the developing roller 81, the roller shaft 811 of the developing roller 81 is attached to the support shaft member 91 so that the developing roller 81 faces the dummy drum 90. The dummy drum 90 is a reference drum, has no vibration, and is positioned with high accuracy.

  Here, the developing roller 81 of the present embodiment is configured by the sleeve 812 or the like. For example, when viewed in a cross section perpendicular to the axial direction due to manufacturing errors of the sleeve 812 or the like, the roller shaft 811 is formed. The radius from the center point to the circumferential surface varies in the entire circumferential direction. In other words, the sleeve 812 is not an ideal cylindrical shape but has a deviation, and the deviation is a shake. Therefore, for example, when the developing roller 81 is rotated, strictly speaking, the gap length between the developing roller 81 and the photosensitive drum 9 constantly changes.

  Therefore, in the present embodiment, the developing roller 81 is attached while facing the dummy drum 90, and in this state, in the axial direction of the developing roller 81, for example, as shown in FIG. The developing roller 81 is rotated at a fixed angle (for example, 1 ° to 15 °), and the gap length is measured each time as shown as an example in FIG. 7B. Note that the number of measurement points is not limited to three points A, B, and C, and a plurality of measurement points may be provided. In addition, it is possible to arbitrarily determine how many gap lengths are measured on one circumference of the developing roller 81. As an instrument for measuring the length of the gap, for example, a plurality of laser beams are irradiated from one direction toward the gap in a row at a minute interval in a direction perpendicular to the axial direction of the developing roller 81. However, it is possible to use a device in which a plurality of light receiving elements for receiving light that has passed through the gap are similarly arranged in a row and the gap length is measured from the number of light receiving elements received, but this is only an example. It is sufficient if the length of the gap can be measured. Then, the deviation (difference) between the measured gap length between the dummy drum 90 and the developing roller 81 and the ideal gap in design becomes a shake of the developing roller 81 itself.

  Next, the measurement of the shake of the photosensitive drum 9 will be described with reference to FIGS. 7 (c) and 7 (d). FIG. 7C is a conceptual diagram when measuring the shake of the photosensitive drum 9, and FIG. 7D is an enlarged view of a broken line portion in FIG. 7C.

  First, FIG. 7C will be described. In FIG. 7C, an ideal photosensitive drum 9 ′ with no shake is illustrated by a two-dot chain line. An example of the photosensitive drum 9 with actual vibration is shown by a solid line. As illustrated by the solid line, the photosensitive drum 9 has a slight shake. The A, B, and C lines on the photosensitive drum 9 correspond to the A, B, and C lines on the developing roller 81, respectively.

  The cause of this shake is, for example, that it is difficult to form the base portion of the photosensitive drum 9 so that the cross section thereof is a perfect circle, and it becomes slightly elliptical. In the photosensitive drum 9 of the present embodiment, a positively charged amorphous silicon photosensitive layer is formed on a substrate such as aluminum by vapor deposition or the like (for example, an OPC photosensitive member is formed by coating). It is difficult to make the layer thickness completely uniform. Including these points, the photosensitive drum 9 usually has a shake (shift) as compared with an ideal cylindrical shape and a cylindrical shape.

  Therefore, in this embodiment, the amount of shake is measured before the photosensitive drum 9 is attached to the printer 1. Specifically, as shown in FIG. 7 (d), the photosensitive drum 9 is installed between the light emission side measuring device 92 and the light receiving side measuring device 93 (FIG. 7 (d) shows the photosensitive drum 9 as an axis. View from the direction). Then, from the light emission side measuring device 92, for example, in a direction perpendicular to the axial direction of the photosensitive drum 9 (direction perpendicular to the paper surface of FIG. 7C), it corresponds to points A, B, and C of the developing roller 81. At the position, a plurality of laser beams are radiated in rows at a minute interval, and the light receiving elements are similarly arranged in rows on the other light receiving side measuring device 93.

  A specific example will be described with reference to FIG. FIG. 7 (e) is an enlarged view of a portion surrounded by a broken line at point A in FIG. 7 (c). Specifically, the light receiving side measuring device 93 is caused to detect the actual deviation (shake) of the photosensitive drum 9 from the light receiving position of the light receiving element with respect to the ideal peripheral surface position (illustrated by a two-dot chain line) of the photosensitive drum 9 ′. taking measurement. As an example of the shake amount, an example of the shake amount at point A is DA in FIG. Then, the amount of shake of the photosensitive drum 9 is measured at a plurality of points per rotation while rotating by a fixed amount at each point A, B, and C of the photosensitive drum 9.

  Next, an example of the range setting (range narrowing) of the AC voltage applied to the developing roller 81 when the occurrence of discharge is detected will be described with reference to FIG.

  First, the upper table G1 in FIG. 8 will be described. First, as described with reference to FIGS. 7A and 7B, the values of A1, B1, and C1 are set so that the developing roller 81 is rotated once at each of the points A, B, and C. However, it is the shortest gap length among the measurement results of the gap length between the dummy drum 90 and the developing roller 81.

  Also, the values of A2, B2, and C2 in the upper table G1 in FIG. 8 are the values of the points A, B, and C of the photosensitive drum 9, as described with reference to FIGS. 7 (c) to (e). At each point, the maximum deflection in the direction in which the photosensitive drum 9 becomes thicker (in the direction in which the radius of the photosensitive drum 9 becomes longer) is measured when the shake amount is measured while rotating the photosensitive drum 9 once. Amount.

As shown in the upper table of FIG. 8, the photosensitive drum 9 and the developing roller 81 are attached to the printer 1 by subtracting the shake amount of the photosensitive drum 9 from the gap length between the dummy drum 90 and the developing roller 81. In this case, the gap lengths at points A, B, and C (shown in the table of FIG. 8 as DsA, DsB, and DsC) can be predicted. Specifically, the following equation is obtained.
A1-A2 = DsA, B1-B2 = DsB, C1-C2 = DsC
Then, the shortest gap length is selected from DsA, DsB, and DsC.

  Based on the selected shortest gap length, the range of the AC voltage applied to the developing roller 81 when the occurrence of discharge is detected is narrowed based on the table TB. For example, X1, X2, X3, X4... Shown in FIG. 8 indicate the shortest selected gap length (for example, a value of about 60 to 200 μm). Y1, Y2, Y3, Y4... Indicate the lower limit value of the peak-to-peak voltage of the AC voltage applied to the developing roller 81 when the occurrence of discharge is detected (for example, about 700 to 1500 V). Z1, Z2, Z3, Z4... Indicate the upper limit value of the peak-to-peak voltage of the AC voltage applied to the developing roller 81 when the occurrence of discharge is detected (for example, about 1200 to 3000 V). Note that “...” Indicates that it can be subsequently set, and the number is not particularly limited.

  For example, assuming that X1 = 80 μm, X2 = 90 μm, X3 = 100 μm, and X4 = 110 μm (relationship of X1 <X2 <X3 <X4), the longer the gap length, the less likely discharge occurs, so Y1 <Y2 <Y3 < Y4 may be set (for example, Y1 = 800V, Y2 = 850V, Y3 = 900V, Y4 = 950V, etc.). Also, the value of Z may be Z1 <Z2 <Z3 <Z4 (for example, Z1 = 1400V, Z2 = 1450V, Z3 = 1500V, Z4 = 1550V, etc.).

  As described above, the range of the AC voltage applied to the developing roller 81 when the occurrence of discharge can be narrowed down based on the setting data relating to the shake of the developing roller 81 and the photosensitive drum 9. Therefore, in the above example, for example, when the occurrence of discharge is detected, the peak-to-peak voltage as the lower limit value is not always started from (for example, Vpp 700 to 800 V), and the range is narrowed. The values of Xn, Yn, Zn in the table (n = 1, 2, 3, 4,..., The maximum value of n can be arbitrarily set) are experimental results and AC voltage application units. It can be set as appropriate in accordance with the configuration of 86, and is not limited to the above example.

  The printer 1 of the present embodiment determines the range of AC voltage applied to the developing roller 81 when the occurrence of discharge is detected for the combination of the developing roller 81 and the photosensitive drum 9 for each color. The shortest gap length between the dummy drum 90 and the developing roller 81 at each point and the largest shake amount of the photosensitive drum 9 are stored in the storage unit 12 as setting data (upper table G1 in FIG. 8). Then, the table TB in the lower part of FIG.

  In summary, the storage unit 12 changes the AC voltage applied to the developing roller 81 in a stepwise manner, and the detection unit 14 detects the occurrence of discharge, and the AC voltage applied to the developing roller 81 is detected. Setting data for setting the upper limit value and the lower limit value of the peak-to-peak voltage is stored, and the control unit 10 determines the range of the peak-to-peak voltage range of the AC voltage applied to the developing roller 81 based on the setting data when the occurrence of discharge is detected. The AC voltage is applied to the developing roller 81 within a range narrowed down and narrowed down to the AC voltage application unit 86.

  More specifically, the setting data includes data measured in advance for each of the photosensitive drum 9 and the developing roller 81, and the storage unit 12 applies an AC voltage corresponding to the gap when the occurrence of discharge is detected. The table 86 for determining the range of the peak-to-peak voltage to be applied to the developing roller 81 by the unit 86 is stored, and the control unit 10 calculates the gap length based on the setting data, and calculates the gap calculated from the table TB. From this length, the range of the peak-to-peak voltage of the AC voltage applied to the developing roller 81 is narrowed down.

(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 discharge occurrence detection operation is started by a predetermined operation or the like in the operation unit (start), setting data is read from the storage unit 12 and, at the instruction of the CPU 11, a photosensitive drum is driven by a drive mechanism (not shown). 9. The rotation of various rotating bodies in the image forming unit 3 and the intermediate transfer unit 5 such as the developing roller 81, the magnetic roller 82, and the intermediate transfer belt 52 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 control unit 10 sets the range of the AC voltage applied to the developing roller 81 based on the setting data and the table TB, and the initial operation described with reference to 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, it is confirmed that the occurrence of discharge is not detected (step S5). This default measurement is performed in a state where discharge does not occur at all (for example, the AC voltage having the lowest peak-to-peak voltage among settable AC voltages is applied to the developing roller 81), and the occurrence of discharge is detected by default measurement. If so (No in step S5), an abnormality in the gap or an abnormality in the hardware of the detection unit 14 or the like is 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 a signal indicating that the discharge has occurred is not input to the CPU 11 (Yes in step S5), the process proceeds 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). In the first step S7, the control unit 10 instructs the Vpp control unit 88 to match the output of the AC voltage application unit 86 with the lower limit value in the AC voltage range set in step 2.

  Then, the process proceeds to the discharge detection state, and specifically, an AC voltage in which the peak-to-peak voltage is increased by ΔV1 (in the first discharge detection state, an AC voltage having a set lower limit peak-to-peak voltage) is developed. This is applied to the roller 81, and exposure is performed during this time according to an instruction from the CPU 11. During this time, the CPU 11 counts the number of times that the output voltage (discharge detection signal) of the amplifier 15 exceeds a predetermined threshold (step S8).

  Then, it is confirmed whether the count number is 0 times between the photosensitive drum 9 and the developing roller 81 (step S9). If the count number is 0 (No in step S9), the current peak-to-peak voltage is determined as no discharge. CPU 11 confirms whether or not the upper limit value in the previously set range has been reached (step S10), and if it has reached (Yes in step S10), the process proceeds to step S11 (details will be described later). If not reached (No in step S10), the process returns to step S7.

  If the count value is not 0 in step S10 (Yes in step S9), the discharge voltage is generated and the Vpp control unit 88 (FIG. 3) applies the AC voltage application unit 86 to the developing roller 81 in accordance with an instruction from the CPU 11. The peak-to-peak voltage of the AC voltage is decreased from the current state by a predetermined step size ΔV1 (step S12), and further set to a value increased by a predetermined step size ΔV2 (step S13). 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, similarly to step S8, the discharge detection state is set, and the CPU 11 counts the number of times that the output voltage (discharge detection signal) of the amplifier 15 exceeds a predetermined threshold (step S14). In other words, if a discharge is detected during the stepwise change of the peak-to-peak voltage at the step size Δ1, the discharge is detected at the step size Δ2 in order to obtain a peak-to-peak voltage at which discharge occurs in more detail. Until this occurs, the discharge detection state and the condition change state are repeated.

  Next, as in step S9, it is confirmed whether the count number is 0 times between the photosensitive drum 9 and the developing roller 81 (step S15). If it is 0 times (No in step S15), no discharge is generated. Then, the CPU 11 confirms whether or not the current peak-to-peak voltage has reached the peak-to-peak voltage at which discharge was detected previously (step S16). If it has reached (Yes in step S16), the process proceeds to step S11. If not reached (No in step S16), the process returns to step S12. On the other hand, if the count value is not 0 (Yes in step S15), the CPU 11 determines that a discharge occurs at the current peak-to-peak voltage, and proceeds to step S11.

Next, step S11 will be described in detail. When discharge is detected (Yes in step S15, Yes in step S16), or even when the maximum peak-to-peak voltage that can be set cannot be detected (Yes in step S10), the CPU 11 determines the maximum peak-to-peak voltage or discharge From the peak-to-peak voltage Vpp2, the frequency f2, the duty ratio D2, and the bias setting value Vdc2 that are recognized to occur, the potential difference V ₊₂ shown in FIG. 6 (photosensitive drum when discharging is detected or when Vpp2 is applied at the maximum settable value) 9 and the developing roller 81) (step S11).

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 a set value and Vdc2 as a reference, the positive side area and the negative side area are made equal to obtain the positive side peak value of Vpp2 and the potential difference between Vdc2. If this potential difference is added with the potential difference between Vdc2 and V0 (V0 is almost 0 V, it can be treated as Vdc2), V ₊₂ is obtained.

Specifically, Vpp2 during the discharge occurrence detection operation is changed in stages, and if the duty ratio D2 and the bias setting value Vdc2 are constant, V _{+ 2} is calculated in advance according to the magnitude of each Vpp2. The data may be converted into a lookup table, and the CPU 11 may obtain V ₊₂ by referring to the lookup table. 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 causes the developing roller 81 to make an adjustment so that both V ₊₁ and V ₋ shown in FIG. 6 are smaller than the obtained V _+2. A peak-to-peak voltage Vpp1 of the AC voltage to be applied is set (step S17). Specifically, there are various methods for determining Vpp1. 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, for the obtained V ₊₂ , a value of Vpp1 that is recognized as causing no discharge during image formation may be tabulated, and the CPU 11 may determine Vpp1 by referring to the table. This table may also be stored in the storage unit 12. As a result, during image formation, no discharge is generated, and as much AC voltage as possible can be applied.

  In short, when the printer 1 according to the present embodiment detects that a discharge has occurred at the time of occurrence of discharge, the control unit 10 causes the photosensitive drum to correspond to the peak value of the AC voltage applied to the developing roller 81 at the time of occurrence of discharge. The AC voltage to be applied to the developing roller 81 during image formation so that the potential difference between the surface potential of the developing roller 81 and the photosensitive drum 9 during image formation is smaller than the potential difference. It is determined.

  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.

  In this way, the control unit 10 narrows down the range of the peak voltage of the AC voltage applied to the developing roller 81 and detects the AC voltage within the range narrowed down to the AC voltage application unit 86 based on the setting data when the occurrence of discharge is detected. Since it is applied to the developing roller 81, compared to the case where the AC voltage is applied to the developing roller 81 in order from the lower limit value of the peak-to-peak voltage that can be set by the AC voltage application unit 86 to detect the occurrence of discharge. It is possible to quickly find the AC voltage at which discharge occurs. Therefore, it is possible to shorten the time until the AC voltage at which discharge starts to be detected.

  Further, the gap length is calculated based on the setting data, and the range of the peak-to-peak voltage of the AC voltage applied to the developing roller 81 is narrowed down from the calculated gap length with the table TB. The range of AC voltage can be set based on the length of the gap. Further, since the setting data includes data relating to the pre-measured shake of the photosensitive drum 9 and the developing roller 81, the shake data is rewritten even if one or both of the photosensitive drum 9 and the developing roller 81 are replaced. Thus, the range of the peak-to-peak voltage of the AC voltage applied to the developing roller 81 can be narrowed down. Further, based on the potential difference between the developing roller 81 and the photosensitive drum 9 where the accurately recognized discharge is generated, it is possible to set an appropriate alternating voltage that does not generate a discharge during image formation while improving the development efficiency.

  Next, another embodiment will be described. In the above embodiment, the positively charged photosensitive drum 9 and toner have been described as examples. However, the present invention can also be applied to the case where a negatively charged photosensitive drum 9 and toner are used. Specifically, when the occurrence of discharge is detected, the charging device 7 applies a negative voltage to the photosensitive drum 9, and the exposure device 4 stabilizes the surface potential at approximately zero volts. In addition, a negative reverse bias voltage is applied to the primary transfer roller 71. In the above embodiment, the color image forming apparatus has been described. However, the present invention can be applied to, for example, a monocolor image forming apparatus having only the image forming unit 3a (black).

  In the above embodiment, the range of the AC voltage applied to the developing roller 81 when the occurrence of discharge is detected is obtained based on the setting data with reference to the table TB. However, the pre-measured fluctuations of the developing roller 81 and the photosensitive drum 9 are determined. On the basis of the above, the range of the AC voltage applied to the developing roller 81 when the occurrence of discharge is detected (for example, the peak-to-peak voltage from 800 V to 1400 V) may be stored in the storage unit 12 as setting data. That is, the setting data defines the range of the peak-to-peak voltage that the AC voltage application unit 86 should apply to the developing roller 81 when the occurrence of discharge is detected. Thereby, it is possible to directly narrow the range of the AC voltage to be applied to the developing roller 81 when discharge is detected without performing processing such as calculation.

  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 is applicable to an image forming apparatus that includes a photosensitive drum and a developing roller and applies a developing bias (DC + AC) to the developing roller.

1 is a cross-sectional view illustrating a schematic configuration of a printer according to an embodiment. It is an expanded sectional view of each image forming part concerning an embodiment. 2 shows a configuration around a developing roller related to development bias application to the developing roller and detection of electric discharge between the photosensitive drums according to the embodiment. FIG. 3 is a block diagram illustrating an example of a hardware configuration of a printer according to an embodiment. It is a timing chart for demonstrating the outline of the discharge generation detection operation which concerns on embodiment. 6 is a timing chart illustrating details of an AC voltage applied to the developing roller according to the embodiment. FIG. 6 is an explanatory diagram for explaining shake of the photosensitive drum and the developing roller according to the embodiment. It is an example of the table for setting the range of the alternating voltage applied to the developing roller at the time of discharge generation detection which concerns on embodiment. 6 is a flowchart illustrating an example of a control flow of a discharge occurrence detection operation of the printer according to the embodiment.

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)
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 detector for detecting the occurrence of discharge between the developing roller and the photosensitive drum;
A storage unit for storing data;
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;
In the storage unit, the AC voltage applied to the developing roller is changed stepwise, and a peak-to-peak voltage of the AC voltage applied to the developing roller at the time of occurrence of discharge detected by the detection unit is detected. The setting data for setting the upper and lower limit values is stored,
Upon detection of the occurrence of discharge,
The developing roller does not carry toner,
The control unit narrows a range of a peak-to-peak voltage of an AC voltage applied to the developing roller based on the setting data, and causes the developing roller to apply an AC voltage within a range narrowed to the AC voltage applying unit. An image forming apparatus.
The setting data includes data relating to each shake of the photosensitive drum and the developing roller measured in advance,
The storage unit stores a table for determining a range of a peak-to-peak voltage that the AC voltage application unit should apply to the developing roller in correspondence with the gap when the occurrence of discharge is detected,
The control unit calculates the length of the gap based on the setting data, and narrows down the range of the peak-to-peak voltage of the AC voltage applied to the developing roller from the calculated length of the gap with the table. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 1, wherein the setting data defines a range of a peak-to-peak voltage that the AC voltage application unit should apply to the developing roller when the occurrence of discharge is detected. When it is detected that a discharge has occurred when the discharge is detected,
The control unit obtains a potential difference between the photosensitive drum and the developing roller with respect to a peak value of the AC voltage applied to the developing roller at the time of occurrence of discharge, and surfaces of the developing roller and the photosensitive drum during image formation The image forming apparatus according to claim 1, wherein an AC voltage to be applied to the developing roller at the time of image formation is determined so that a potential difference between the potentials is smaller than the potential difference.

Images