JP5081769B2 - 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. Further, 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, and an AC voltage as large as possible is applied to the developing roller. Become. Here, the gap between the developing roller and the photosensitive drum, which is a major factor that determines the potential difference in which electric discharge occurs, differs depending on each image forming apparatus due to an error in mounting and installation of the photosensitive drum and the developing roller. In addition, the potential difference at which discharge occurs varies under the influence of atmospheric pressure and the like. Therefore, the potential difference at which discharge occurs varies depending on each image forming apparatus, installation environment, and the like. 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. Based on the magnitude of the AC voltage, it is necessary to determine the potential difference between the developing roller and the photosensitive drum, and to set the AC voltage applied to the developing roller so that the potential difference is slightly smaller during printing.

  Here, it is necessary to stabilize the surface potential of the photosensitive drum in order to detect the occurrence of discharge and accurately grasp the potential difference between the developing roller where the discharge occurs and the photosensitive drum. This is because if the surface potential of the photosensitive drum varies, there may or may not be a discharge even if an AC voltage of the same magnitude is applied to the developing roller.

Incidentally, referring to Patent Document 1, for example, "the surface potential of the ... photoconductor _drum, - to 550 V ..." (Patent Document 1: see paragraph [0028], etc.) only been described such as, There is no description of a method for stabilizing the surface potential of the photosensitive drum. For example, in the invention described in Patent Document 1, there is a problem that whether the surface potential of the photosensitive drum is stable, such as when the surface of the photosensitive drum is soiled or scratched, or the bias circuit malfunctions or is abnormal.

  In general, an intermediate transfer member or a transfer roller for transferring the formed toner image is in contact with the photosensitive drum. However, in the invention described in Patent Document 1, negatively charged toner is used, and when the discharge is detected, the photosensitive drum is exposed to light. Since a negative voltage is applied to the body drum, at this time, toner remaining on the photosensitive drum may move to a transfer member such as an intermediate transfer body or a transfer roller. As a result, the friction coefficient between the photosensitive drum and the transfer member may fluctuate, which may cause uneven rotation of the intermediate transfer member and the photosensitive drum. This uneven rotation causes a problem such as toner image formation or transfer positional deviation during image formation, and there is a problem in that the quality of the formed image may be deteriorated.

  In view of the above problems, the present invention stabilizes the surface potential of the photosensitive drum when the occurrence of discharge is confirmed, accurately determines the potential difference between the photosensitive drum and the developing roller where the discharge occurs, and prevents occurrence of discharge. The object is to eliminate the deterioration of the quality of the formed image after confirmation.

  In order to achieve the above object, an image forming apparatus according to a first aspect includes a photosensitive drum carrying a toner image on a peripheral surface, a charging unit for charging the photosensitive drum at a constant potential, and the photosensitive after charging. The photosensitive drum is opposed to an exposure unit that exposes the photosensitive drum to form an electrostatic latent image with a gap provided therebetween, and carries toner at the time of image formation to supply toner to the photosensitive drum. A DC voltage application unit; a developing roller to which an AC voltage application unit is connected; a transfer unit connected to a transfer voltage application unit that applies a transfer voltage; and a transfer unit that transfers a toner image to an intermediate transfer member or sheet; A detection unit that detects the occurrence of discharge between the developing roller and the photosensitive drum; and a control unit that controls each unit of the apparatus and that receives an output of the detection unit and recognizes the occurrence of discharge. Part is the developing roller Instructing the AC voltage application unit to change the AC voltage to be applied stepwise, and detecting the occurrence of discharge by the detection unit, the control unit instructs the transfer voltage application unit to perform transfer. A voltage having a polarity opposite to that of the photosensitive drum is applied to the transfer unit, the charging unit charges the photosensitive drum, and the exposure unit exposes the entire peripheral surface of the photosensitive drum.

  According to this configuration, in order to grasp the potential difference between the developing roller where the discharge occurs and the photosensitive drum, when detecting and confirming the occurrence of the discharge while changing the AC voltage applied to the developing roller, the charging unit includes a photosensitive member. Since the drum is charged and the exposure part is exposed over the entire surface of the photosensitive drum, the charging part charges the photosensitive drum at a constant potential, and the photosensitive drum is exposed. The surface potential of the body drum is stable (for example, approximately 0 V). Accordingly, since the surface potential of the reference photosensitive drum is stabilized, it is possible to accurately grasp the potential difference between the developing roller and the photosensitive drum where discharge occurs.

  Further, in this configuration, since a voltage having a polarity opposite to that at the time of transferring the toner image is applied to the transfer unit, it is possible to prevent the toner on the photosensitive drum from moving to the transfer member such as the intermediate transfer member or the transfer roller. Accordingly, it is possible to suppress the occurrence of rotation unevenness of the transfer member and the photosensitive drum in contact with the transfer member and the like, and toner image formation and transfer can be accurately performed without deviation.

  According to a second aspect of the present invention, in the first aspect of the invention, when the occurrence of discharge is detected, the control unit gives an instruction to reduce the charging output of the charging unit than during printing. According to this configuration, the control unit issues an instruction to reduce the charging output of the charging unit as compared with normal printing, so that the surface potential of the photosensitive drum can be reliably stabilized by exposure of the exposure unit. . Furthermore, in the charging unit, a relatively high voltage is applied to charge the photosensitive drum. Ozone may be generated by the application of the voltage for charging, and this ozone may generate dust (toner, development) in the machine. It reacts with an external additive in the agent to produce a charged product. When this charged product adheres to the photosensitive drum, the friction coefficient of the photosensitive drum or the transfer member may fluctuate, which may cause rotation unevenness. In addition, ozone reacts with the surface of the photosensitive drum to facilitate the adsorption of moisture. Then, if the charged product falls on the photosensitive drum, or if the charged product dissolves in moisture, the resistance of the photosensitive drum decreases, the charge on the surface of the photosensitive drum moves, and the quality of the image formed May decrease (so-called image flow). In the present invention, the charging unit performs charging for the detection and confirmation of the occurrence of discharge. If the detection operation takes a long time, a large amount of charged products may be generated. However, in this configuration, the control unit reduces the charging output of the charging unit as compared with normal printing, so the amount of ozone and charged products generated is reduced, and rotation unevenness and image formation after discharge detection are performed. Image quality degradation can be eliminated.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, a cleaning unit is provided for cleaning the photosensitive drum. According to this configuration, since the cleaning unit that cleans the photosensitive drum is provided, the toner adhering to the transfer member can be reduced. Even if the charged product adheres to the photosensitive drum, the amount of adhesion can be reduced. Therefore, it is possible to reduce the rotational speed unevenness of the photosensitive drum or the like and the degree of image quality deterioration.

  According to a fourth aspect of the present invention, in the first to third aspects of the invention, when it is detected that a discharge has occurred when the 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 alternating voltage that does not generate a discharge at the time of appropriate image formation with improved development efficiency. .

  According to the image forming apparatus of the present invention, when the occurrence of discharge between the developing roller and the photosensitive drum is detected and the potential difference between the developing roller and the photosensitive drum where the discharge is generated is grasped, the surface potential of the photosensitive drum is stabilized. Therefore, an accurate potential difference between the developing roller and the photosensitive drum can be grasped. Further, it is possible to provide an image forming apparatus in which there is no occurrence of uneven rotation of the transfer member or the photosensitive drum during image formation caused by the measurement of discharge, and deterioration in image quality due to the charged product.

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

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

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

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

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

  Each photosensitive drum 9 carries a toner image on its peripheral surface, and has, for example, a positively charged amorphous silicon photosensitive layer on the outer peripheral surface of an aluminum drum base. It is driven to rotate counterclockwise at the process speed. 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 applying unit 72 (see FIG. 5), and the surface of the photosensitive drum 9 has a predetermined positive potential (for example, 200V to 300V, dark potential). In addition, a cleaning brush 73 (for example, a resin brush or the like wound around a shaft) for removing foreign matters on the surface of each charging roller 71 is provided. The charging device 7 may be a device that charges the photosensitive drum 9 using a corona discharge type or a brush.

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

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

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

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

  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.

  Here, a schematic configuration of the exposure apparatus 4 will be described with reference to FIG. As shown in FIG. 3, the exposure device 4 includes a semiconductor laser device 41 (laser diode), a polygon mirror 42 (rotated by a polygon motor 43) that has a plurality of planar reflection surfaces that reflect laser light, and rotates at high speed, and fθ. A lens 44 and a mirror 45 for reflecting the laser light toward each photosensitive drum 9 are provided as appropriate (in FIG. 3, only one color configuration is shown. For example, in the case of four colors, the polygon mirror 42 is The other semiconductor laser device 41, the fθ lens 44, the mirror 45, etc. are provided for each color). With this configuration, laser light is irradiated from the exposure device 4 to each photosensitive drum 9, and an electrostatic latent image combined with image data is formed on the photosensitive drum 9. Specifically, each photosensitive drum 9 of the present embodiment is positively charged, the potential of the light irradiation portion is lowered (for example, approximately 0 V), and the positively charged toner is attached to the portion of the potential decrease (for example, solid image) In this case, all lines and all pixels are irradiated with laser light). The exposure device 4 may be composed of a large number of LEDs.

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

  Returning to FIG. 1, the intermediate transfer unit 5 receives the primary transfer of the toner image from the photosensitive drum 9 and performs secondary transfer onto the sheet, and each primary transfer roller 51 a to 51 d (corresponding to the transfer unit). And an intermediate transfer belt 52 (corresponding to an intermediate transfer member), a 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. 9) that applies a transfer voltage. The toner image is transferred to the intermediate transfer belt 52.

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

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

(Configuration for discharge detection)
Next, the configuration relating to the application of the developing bias to each developing roller 81 and the detection of the discharge between the respective photosensitive drums 9, which are the features of the present invention, will be described.

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

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

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

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

  The AC voltage application unit 86 is, for example, a rectangular wave (pulse shape), and is a circuit that outputs an AC voltage 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. 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.

  The detection unit 14 is a circuit that converts a current that flows when a discharge occurs between the developing roller 81 and the photosensitive drum 9 into a voltage signal and detects the occurrence of the discharge, and outputs the converted voltage signal to the amplifier 15. The amplifier 15 amplifies the voltage signal from the detection unit 14 and outputs it to the CPU 11. The CPU 11 A / D converts the voltage signal from the amplifier 15. 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. 5 is a block diagram illustrating an example of a hardware configuration of the printer 1 according to the embodiment of the present invention.

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

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

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

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

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

  First, the outline of the discharge occurrence detection operation will be described with reference to FIG. In FIG. 6, “developing roller (alternating current)” indicates the timing at which the alternating voltage application unit 86 applies an alternating voltage to the developing roller 81. “Vpp” indicates a change in the peak-to-peak voltage of the AC voltage to the developing roller 81. “Developing roller (DC)” indicates a timing at which the DC voltage application unit 85 applies a DC voltage to the developing roller 81. “Magnetic roller (AC)” indicates the timing at which the magnetic roller 82 bias unit (see FIG. 5) applies an AC voltage to the magnetic roller 82. “Magnetic roller (DC)” indicates a timing at which the magnetic roller 82 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 46 of the exposure apparatus 4. “Exposure” indicates the exposure (laser beam irradiation) timing of the photosensitive drum 9 in the exposure apparatus 4. “Discharge detection (detector output)” indicates a discharge occurrence detection timing by the detector 14.

<Initial operation>
When the discharge generation detecting operation according to the present invention is started, after the photosensitive drum 9, the developing roller 81, the intermediate transfer belt 52, and the like start rotating, in the initial operation, the developing roller 81 and the magnetic roller 82 are respectively connected to the alternating current. A DC voltage is applied. By applying a voltage to the magnetic roller 82 in this initial operation, a small amount of toner is supplied from the magnetic roller 82 to the developing roller 81. In 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 in the default measurement. Next, the process moves to the default measurement and confirms whether or not a discharge is detected. Note that the default measurement is performed to detect an abnormality in the detection position of the detection unit 14 or the like, the member installation position, the circuit, or the like. After the default measurement, the condition shifts to the condition change state (first time).

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

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

  The discharge detection state continues for a certain time (for example, 1 second to several seconds), 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 AC peak-to-peak voltage change instruction. Thereby, in the discharge detection state after the next time, basically, 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. In other words, the condition change state and the discharge detection state are repeated until the AC voltage to be discharged is recognized. During the repetition, the peak-to-peak voltage of the AC voltage applied to the developing roller 81 is basically stepwise with a constant step size. Is increased. FIG. 5 shows that the discharge is detected in the nth discharge detection state.

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

First, the rectangular wave in the timing chart at the time of image formation is an example of the waveform of the developing bias (AC + DC) applied to the developing roller 81. “Vdc1” indicates the bias potential of the DC voltage application unit 85. “V0” indicates the potential of the photosensitive drum 9 after exposure by the exposure device 4 (approximately 0 V = bright potential). “V1” indicates a potential after charging of the photosensitive drum 9 (potential of a portion not exposed to light, for example, about 200 to 300 V). “V ₊₁ ” indicates a potential difference between 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 8.

  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 FIG. FIG. 8 is a flowchart showing an example of the control flow of the discharge occurrence detection operation of the printer 1 according to the embodiment of the present invention. Note that this flowchart is a control for one image forming unit 3 and is repeated four times in the present embodiment when all colors are used.

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

  First, when a predetermined operation is performed in the operation unit and the discharge generation detection operation is started (start), the photosensitive drum 9, the developing roller 81, the magnetic roller 82, and the intermediate are driven by a drive mechanism (not shown) according to an instruction from the CPU 11. The rotation of various rotating bodies in the image forming unit 3 such as the 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 according to an instruction from the CPU 11.

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

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

  Then, the state shifts to a discharge detection state. Specifically, an AC voltage whose peak-to-peak voltage is increased by ΔV1 is applied to the developing roller 81, and exposure is performed for a predetermined time according to an instruction from the CPU 11, during which the CPU 11 Counts the number of times the output voltage of the amplifier 15 exceeds a predetermined threshold (step S8). Then, it is confirmed whether the count number is not 0 (step S9). If it is 0 (No in step S9), the current peak-to-peak voltage can be set to a maximum value (for example, 1500 to 1500) as no discharge occurs. CPU11 confirms whether it has reached 3000V (step S10), and if it has reached (Yes in step S10), the process proceeds to step S11 (details will be described later). If not reached (No in step S10), the process returns to step S7.

  If the count value is 1 or more in step S9 (Yes in step S9), the discharge voltage is generated and the Vpp control unit 88 (FIG. 4) 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, as in step S8, the discharge detection state is set, and the CPU 11 counts the number of times that the output voltage of the amplifier 15 exceeds a predetermined threshold (step 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 the 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, it is confirmed whether the number of counts is not 0 (step S15). If it is 0 (No in step S15), the current peak-to-peak voltage is the peak-to-peak voltage at which discharge is detected first as no discharge has occurred. CPU11 confirms whether it has reached (step S16). If it has reached (Yes in step S16), the process proceeds to step S11. If not reached (No in step S17), the process returns to step S12. On the other hand, if the count value is 1 or more (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 the discharge is detected (Yes in step S15, Yes in step S16), or when the maximum peak-to-peak voltage that can be set cannot be detected (Yes in step S9), the CPU 11 determines the maximum peak-to-peak voltage or discharge 5 is obtained from the peak-to-peak voltage Vpp2, the frequency f2, the duty ratio D2, and the bias set value Vdc2 recognized as the potential difference V ₊₂ (photosensitive drum when discharge 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 at the time of discharge occurrence detection operation is changed in stages. Therefore, 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 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, based on the obtained V ₊₂ , the CPU 11 develops the developing roller 81 during image formation so that both V ₊₁ and V ₋ shown in FIG. 5 are smaller than the obtained V _+2. A peak-to-peak voltage Vpp1 of the AC voltage applied to is set (step S17). Specifically, a method determining the Vpp1 are diverse, for example, V ₊₁ and V _- than the V _+2, or discharge if much smaller does not occur (or to take much margin), the toner used For example, based on an experiment at the time of development, for example, for V ₊₂ obtained, a value of Vpp1 recognized that no discharge occurs during image formation is tabulated, and the CPU 11 refers to the table. Vpp1 may be determined. 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, 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.

  Next, the advantages of the present invention will be described with reference to FIG. FIG. 9A is a partially enlarged view of the image forming unit 3 when the occurrence of discharge is detected, and FIG. 9B is a relationship between the amount of toner adhering to the intermediate transfer belt 52 and the fluctuation of the friction coefficient of the intermediate transfer belt 52. FIG. 6C is a partially enlarged view of the image forming unit 3 when discharge occurrence is detected, and FIG. 4D is an image forming unit when discharge occurrence is detected according to the embodiment of the present invention. 3 is a partially enlarged view of FIG.

  First, FIG. 9A will be described. FIG. 9A shows an example of voltage application in the image forming unit 3 when the occurrence of discharge is detected. FIG. 9A shows that no charging voltage is applied to the charging device 7 and the transfer roller when the occurrence of discharge is detected.

  If the potential of the photosensitive drum 9 is not stable when the occurrence of discharge is detected, even if the same developing bias is applied, a case where a discharge occurs and a case where no discharge occurs are likely to occur. Accordingly, the accuracy of setting the peak-to-peak voltage Vpp1 of the AC voltage at the time of image formation described above decreases, and even if the peak-to-peak voltage Vpp1 of the AC voltage at the time of image formation is set, discharge may occur. is there.

  Therefore, in order to stabilize the surface potential of the photosensitive drum 9, there is a method in which the voltage of the charging device 7 and the primary transfer roller 51 is not applied when the occurrence of discharge is detected, as shown in FIG. As a result, the potential of the photosensitive drum 9 is stabilized at substantially zero (ground level). Here, in the printer 1 of the present embodiment, when the occurrence of discharge is detected, the developing roller 81 basically does not carry toner. Specifically, when a positive voltage is applied to the magnetic roller 82, the toner is positively charged, so that the toner moves from the magnetic roller 82 to the developing roller 81 by repulsion. Therefore, no voltage is applied to the magnetic roller 82, and the developing roller 81 is directed not to carry toner.

  However, residual toner is present on the surface of the photosensitive drum 9. In addition, the toner on the sleeve 812 is not completely lost because the toner is dragged by the rotation of the sleeve 812, and the toner on the developing roller 81 may fly to the photosensitive drum 9. Then, since the potential of the intermediate transfer belt 52 is lower than that of the toner, toner that moves in the direction of the intermediate transfer belt 52 appears as shown in FIG. Then, the toner continues to move to the intermediate transfer belt 52 little by little while the occurrence of discharge is detected on the intermediate transfer belt 52.

  Then, as shown in an example in FIG. 9B, there is a problem that the friction coefficient (mu) of the intermediate transfer belt 52 varies depending on the toner adhesion amount of the intermediate transfer belt 52. In the printer 1 of the present embodiment, for example, a rubber belt can be adopted as the intermediate transfer belt 52, and the friction coefficient of the rubber belt itself is large. However, when an appropriate amount of toner is applied, minute particles of toner adhere to the surface of the rubber belt, and the rubber belt tends to have a reduced friction coefficient. However, when the toner amount is further increased, the friction coefficient is increased (when the toner amount is excessively large, for example, the belt cleaning device 58 is operated and scraped off).

  Various factors are involved in the friction coefficient, such as the type and particle size of the toner, the material of the intermediate transfer belt 52, the material of the surface of the photosensitive drum 9, the operating conditions of the belt cleaning device 58, and the characteristics shown in FIG. As an example, it is clear that the friction coefficient of the intermediate transfer belt 52 varies as the amount of toner on the intermediate transfer belt 52 changes. Since the toner is generally not distributed uniformly on the intermediate transfer belt 52, the friction coefficient of the belt surface varies. As a result, the rotation unevenness (rotation) of the intermediate transfer belt 52 and the photosensitive drum 9 in contact with the intermediate transfer belt 52 occurs. Speed unevenness) occurs.

  If rotation irregularities occur, fluctuations in the peak-to-peak voltage at which discharge occurs (that is, a decrease in the accuracy of setting the AC voltage during image formation), and transfer of the toner image in image formation after detection of occurrence of discharge This may cause a shift in position and toner image formation position. Therefore, as shown in FIG. 9C, the control unit 10 instructs the transfer voltage application unit 59 to prevent the toner from being applied to the intermediate transfer belt 52, so that the voltage having a polarity opposite to the charging polarity of the toner is obtained. (Hereinafter referred to as “reverse bias”. In this embodiment, positive polarity) may be applied to the primary transfer roller 5151. This reverse bias can prevent the toner from moving to the intermediate transfer belt 52.

  However, in order to reliably prevent the toner from moving, it is necessary to apply a relatively large reverse bias (for example, reliably exceeding the charging potential of the toner) to the primary transfer roller 51. Then, the photosensitive drum 9 is charged due to the reverse bias application. Note that the neutralization device 31 exists between the facing portion of the photosensitive drum 9 and the primary transfer roller 51 and the cleaning device 32. However, the toner, dust, etc. on the photosensitive drum 9 obstruct light and reliably eliminate static electricity. There are cases where it is not possible. Therefore, when the occurrence of discharge is detected, the surface potential of the photosensitive drum 9 is not easily stabilized by the method of FIG. 9C.

  Therefore, in the invention according to the present embodiment, when the occurrence of discharge is detected, as shown in FIG. 9D, the charging voltage application unit 72 charges the photosensitive drum 9 with the charging device 7 in accordance with an instruction from the CPU 11, thereby exposing the exposure device. 4, the exposure of the photosensitive drum 9 is continued (over the entire circumferential surface). As a result, in the section L where the photosensitive drum 9 and the developing roller 81 face each other, the surface potential of the photosensitive drum 9 is stabilized at substantially zero V. As a result, there is no variation in whether or not the discharge occurs, and the potential difference between the photosensitive drum 9 and the developing roller 81 when the discharge occurs can be accurately grasped. In the present invention shown in FIG. 9, the reverse bias of the primary transfer roller 51 is naturally applied.

  Further, due to charging by the charging device 7, ozone or a charged product is usually generated. Ozone reacts with the surface of the photosensitive drum 9, adsorbs moisture, and when a charged product dissolves therein, the resistance of the photosensitive drum 9 decreases, disturbs the electrostatic latent image, and causes image quality deterioration. (Image flow). In addition, if the generated charged product falls on and adheres to the photosensitive drum 9, the friction coefficient of the photosensitive drum 9 is changed, which causes rotation unevenness. Note that, during image formation, the toner-like action of the toner and the polishing and cleaning action of the cleaning device 32 can prevent the image flow and the charged product from sticking to some extent.

  However, when the occurrence of discharge is detected, the developing roller 81 basically does not carry toner, so that the adverse effects caused by the charged products are likely to appear significantly. Here, since it is sufficient that the surface potential of the photosensitive drum 9 can be stabilized when the occurrence of discharge is detected, the charging voltage application unit 72 applies a voltage smaller than that at the time of image formation upon detection of the occurrence of discharge. (For example, 20 to 80% during image formation). Therefore, since the voltage applied to the charging roller 71 is smaller than that during image formation, the generation amount of ozone and charged products is reduced. Therefore, in the present invention, adverse effects caused by the charged product and ozone are reduced. Further, the surface potential of the photosensitive drum 9 can be sufficiently lowered without increasing the laser power of the laser light from the exposure device 4 as compared with the image formation.

  In short, in the present invention, the control unit 10 instructs the AC voltage application unit 86 to stepwise change the AC voltage applied to the developing roller 81, and the control unit 10 detects the occurrence of discharge by the detection unit 14. 10 designates the transfer voltage application unit 59 to apply a voltage having a polarity opposite to that at the time of transfer to the transfer roller, causes the charging device 7 to charge the photoconductor drum 9, and causes the exposure unit to rotate the circumference of the photoconductor drum 9. Expose the entire surface. At this time, the control unit 10 gives an instruction to reduce the charging output of the charging device 7 as compared with printing.

  As described above, according to the embodiment of the present invention, in order to grasp the potential difference between the developing roller 81 and the photosensitive drum 9 where the discharge is generated, the discharge is generated while changing the AC voltage applied to the developing roller 81. When detecting and checking, the charging unit (charging device 7) charges the photosensitive drum 9, and the exposure unit (exposure device 4) exposes the entire circumferential surface of the photosensitive drum 9, so that the charging unit is the photosensitive member. When the drum 9 is charged at a constant potential and the photosensitive drum 9 is exposed, the surface potential (V0) of the photosensitive drum 9 after exposure is stabilized (for example, approximately 0 V). Accordingly, since the surface potential of the photosensitive drum 9 serving as a reference is stabilized, the potential difference between the developing roller 81 and the photosensitive drum 9 where discharge occurs can be accurately grasped.

  Further, since a voltage having a polarity opposite to that at the time of transfer of the toner image is applied to the transfer portion (primary transfer roller 51), the photosensitive member is exposed to a transfer member such as an intermediate transfer member or a transfer roller (for example, the intermediate transfer belt 52). The movement of the toner on the body drum 9 can be prevented, the occurrence of rotation unevenness of the transfer member and the photosensitive drum 9 in contact therewith can be suppressed, and the toner image formation and transfer of the image formation can be shifted. Can be done accurately. Further, since the control unit 10 issues an instruction to reduce the charging output of the charging unit as compared with normal printing, the surface potential of the photosensitive drum 9 can be reliably stabilized by exposure of the exposure unit. Further, in the charging unit, a relatively high voltage is applied to charge the photosensitive drum 9, and ozone may be generated by the application of the voltage for the charging. It reacts with external additives in the developer, etc., to produce a charged product. If this charged product adheres to the photosensitive drum 9, the friction coefficient of the photosensitive drum 9 or the transfer member may fluctuate, causing uneven rotation. Further, ozone reacts with the surface of the photosensitive drum 9 to facilitate the adsorption of moisture. Then, when the charged product is deposited on the photosensitive drum 9 or when the charged product is dissolved in moisture, the resistance of the photosensitive drum 9 is lowered, and the charge on the surface of the photosensitive drum 9 is moved and formed. Image quality may be degraded (so-called image flow). In the present invention, the charging unit performs charging for the detection and confirmation of the occurrence of discharge. If the detection operation takes a long time, a large amount of charged products may be generated. However, in this configuration, the control unit 10 reduces the charging output of the charging unit as compared with normal printing, so the generation amount of ozone and charged products is reduced, and the occurrence of rotation unevenness and image formation after discharge detection is performed. It is possible to eliminate the deterioration of image quality.

  Further, since the cleaning unit (cleaning device 32) for cleaning the photosensitive drum 9 is provided, the toner adhering to the transfer member can be reduced. Even if the charged product adheres to the photosensitive drum 9, the amount of adhesion can be reduced. Accordingly, it is possible to reduce the rotational speed unevenness of the photosensitive drum 9 and the like and the degree of image quality deterioration. In addition, based on the potential difference between the developing roller 81 and the photosensitive drum 9 where the accurately grasped discharge is generated, it is possible to set an AC voltage that enhances the development efficiency and does not generate a discharge during appropriate image formation.

  Next, another embodiment will be described. In the above-described embodiment, an example in which primary transfer is performed from each photoconductive drum 9 to the intermediate transfer belt 52 and then secondary transfer is performed to the sheet is described. However, a toner image is directly transferred from each photoconductive drum 9 to the sheet. The present invention can also be applied to the configuration (for example, a mode in which a transfer roller is in direct contact with each photoconductor drum 9 and a sheet passes through the nip, or a conveyance belt is in contact with each photoconductor drum 9 and On a conveying belt, and the sheet passes through the nip.

  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 transfer roller. 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).

  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 unit according to the present embodiment. It is a schematic diagram which shows an example of the exposure apparatus which concerns on this embodiment. 2 shows a configuration around a developing roller in relation to application of a developing bias to the developing roller and detection of electric discharge between photosensitive drums according to the present embodiment. FIG. 2 is a block diagram illustrating an example of a hardware configuration of a printer according to the present embodiment. 5 is a timing chart for explaining an outline of a discharge occurrence detection operation according to the embodiment. 6 is a timing chart illustrating details of an AC voltage applied to the developing roller according to the present 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. (A) is a partially enlarged view of the image forming unit when the occurrence of discharge is detected, and (b) is a graph showing an example of the relationship between the amount of toner adhering to the intermediate transfer belt and the fluctuation of the friction coefficient of the intermediate transfer belt. (C) is a partially enlarged view of the image forming unit at the time of detecting discharge occurrence, and (d) is a partially enlarged view of the image forming unit at the time of detecting discharge occurrence according to the embodiment of the present invention. is there.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Printer (image forming apparatus) 4 Exposure apparatus (exposure part)
3 (3a, 3b, 3c, 3d) Image forming unit 32 (32a, 32b, 32c, 32d) Cleaning device 51 (51a, 51b, 51c, 51d) Primary transfer roller 51 (transfer unit)
52 Intermediate transfer belt (intermediate transfer member) 59 Transfer voltage application unit 7 (7a, 7b, 7c, 7d) Charging device (charging unit)
71 (71a, 71b, 71c, 71d) Charging roller (part of charging unit)
8 (8a, 8b, 8c, 8d) Developing device 81 (81a, 81b, 81c, 81d) Developing roller 85 DC voltage application unit 86 AC voltage application 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 charging unit for charging the photosensitive drum at a constant potential;
An exposure unit that forms an electrostatic latent image by exposing the photosensitive drum after charging;
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 transfer unit that is connected to a transfer voltage application unit that applies a transfer voltage, and that transfers a toner image to an intermediate transfer member or sheet;
A detector for detecting the occurrence of discharge between the developing roller and the photosensitive drum;
A control unit that controls each part of the device and recognizes the occurrence of discharge when the output of the detection unit is input;
Instructing the AC voltage application unit to change the AC voltage applied to the developing roller by the control unit to the AC voltage application unit, and detecting the occurrence of discharge by the detection unit,
The control unit instructs the transfer voltage application unit to apply a voltage having a reverse polarity to the transfer unit during transfer, causes the charging unit to charge the photosensitive drum, and causes the exposure unit to perform the photosensitive operation. An image forming apparatus that exposes the entire peripheral surface of a body drum. When discharge occurrence is detected,
The image forming apparatus according to claim 1, wherein the control unit gives an instruction to reduce a charging output of the charging unit than during printing.   The image forming apparatus according to claim 1, further comprising a cleaning unit that cleans the photosensitive drum. 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