JP2010175847A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To shorten a period of time for detecting inter-peak voltage beginning discharging as much as possible, and to suppress an adverse influence due to occurrence of a simultaneous discharging in a plurality of image forming sections into a low level. <P>SOLUTION: The image forming apparatus is provided with: a photoreceptor drum; an image forming section including developing rollers facing each other with a gap; an AC voltage application section for applying voltage to the developing roller; a detection section for detecting the occurrence of discharge; and a control section. The control section executes repeated applications of AC voltage to the developing roller by varying the voltage between peaks at the amount of variation that is half the amount of the variation immediately before in detection of the voltage between peaks when discharging begins. Then, the variation is made in the direction making the voltage between peaks greater when the discharge occurs and in the direction making the voltage between the peaks smaller when the discharge does not occur in the AC voltage application immediately before. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

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

  2. Description of the Related Art Conventionally, some image forming apparatuses using toner, such as a copying machine, a printer, and a facsimile machine, are provided with a photosensitive drum and a developing roller facing the photosensitive drum with a gap. Then, for example, a so-called developing bias in which direct current and alternating current are superimposed is applied to the developing roller, charged toner flies from the developing roller to the photosensitive drum, and the electrostatic latent image on the photosensitive drum is developed and developed. Printing is performed by transferring and fixing the toner image to the paper.

  In order to supply sufficiently charged toner to the photosensitive drum, to secure 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 is increased. However, if it is too large, discharge occurs in the gap between the photosensitive drum and the developing roller. When a discharge occurs during printing, the electrostatic latent image is disturbed due to a potential change on the surface of the photosensitive drum, and the quality of the formed image is deteriorated. Therefore, even if the peak-to-peak voltage is increased, it must be kept within a range where no discharge occurs.

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 generating means and a leak detecting means for detecting a leak, and gradually increasing the maximum potential difference ΔVmax between the leak detection voltage and the surface potential of the image carrier 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 Patent Document 1: Claim 1).
Japanese Patent No. 3815356

  In order to apply an AC voltage having a peak-to-peak voltage as large as possible in a range where no discharge occurs during printing, as described in Patent Document 1, a discharge is intentionally generated, and the peak-to-peak voltage at which discharge starts is detected. There is an image forming apparatus. In general, such a discharge detection operation can be frequently performed at the time of inspection in a factory line, installation at a customer site, or when there is a large change in the environment (temperature, humidity, etc.). However, if it takes a long time to detect the peak-to-peak voltage at which discharge starts, the setup time during inspection and installation and the time during which the image forming apparatus cannot be used increase. There is a problem that should be.

  In addition, the image forming apparatus may include a plurality of image forming units (for example, four for black, yellow, cyan, and magenta) for color image formation. For example, in an image forming apparatus having four image forming units, if detection of the peak-to-peak voltage at which discharge starts is performed for each image forming unit (one by one), it takes a very long time.

  Therefore, in an image forming apparatus having a plurality of image forming units at the same time, detection of peak-to-peak voltage at which discharge starts can be performed in parallel by the plurality of image forming units to shorten the required time. However, if discharge occurs simultaneously in a plurality of image forming units, noise may become very large. For example, in an image forming apparatus, if each control circuit, a high-voltage board that generates a high voltage, etc. has a frame as a ground and a large current flows into the frame due to the simultaneous occurrence of discharge, malfunctions occur in each electric circuit. There is a possibility of adverse effects such as runaway. Therefore, there is a problem that simultaneous generation of discharge should be prevented in a plurality of image forming units.

  In Patent Document 1, although the maximum potential difference ΔVmax between the leak detection voltage and the surface potential of the image carrier is gradually increased (see Patent Document 1: Claim 1 and the like), ΔVmax is rapidly increased. However, since the peak-to-peak voltage at which discharge starts accurately cannot be detected, there is a problem that it is difficult to finish the detection in a short time. Further, when the invention described in Patent Document 1 is applied to an image forming apparatus having a plurality of image forming units and discharge detection operations are performed in parallel, it is unlikely that a plurality of image forming units simultaneously generate a discharge. Since it cannot be said, it cannot avoid the bad influence on the electric circuit due to the simultaneous occurrence of discharge. Therefore, the invention described in Patent Document 1 cannot solve the above problem.

  In view of the above problems, it is an object of the present invention to detect a peak-to-peak voltage at which discharge starts in as short a time as possible, and to suppress adverse effects due to simultaneous discharge in a plurality of image forming units to a low level.

  In order to achieve the above object, an image forming apparatus according to a first aspect of the present invention opposes a photoconductive drum with a gap provided in the photoconductive drum for supplying toner to the photoconductive drum. An image forming unit including a developing roller carrying toner, an AC voltage applying unit for applying a voltage to the developing roller, and a detecting unit for detecting the occurrence of discharge between the developing roller and the photosensitive drum. A control unit that controls the AC voltage application unit and recognizes the occurrence of discharge based on the output of the detection unit, and changes the peak-to-peak voltage of the AC voltage applied to the developing roller in a stepwise manner. When detecting the peak-to-peak voltage at which discharge starts between the photosensitive drum and the developing roller, the control unit firstly has a half of the adjustment range that is the range of the peak-to-peak voltage of the AC voltage applied to the developing roller. The AC voltage of the peak-to-peak voltage that is changed in the direction of increasing the minimum value of the adjustment width is applied to the developing roller by the amount of change, and then becomes less than the minimum amount of change that can be set by the AC voltage application unit. Until the change in peak-to-peak voltage is changed by half the amount of change just before, the application of AC voltage to the developing roller is repeated, and when changing the peak-to-peak voltage, discharge occurs when the previous AC voltage is applied In this case, the peak-to-peak voltage is increased, and when no discharge occurs, the peak-to-peak voltage is decreased.

  According to this configuration, since the amount of change in the peak-to-peak voltage is reduced by half, the peak-to-peak voltage at which discharge starts can be detected more efficiently than in the conventional case where the peak-to-peak voltage is gradually increased. Is called. Therefore, the time required for detecting the peak-to-peak voltage at which discharge starts is shortened.

  According to a second aspect of the present invention, in the first aspect of the invention, the image forming unit includes a plurality of the image forming units, and each of the image forming units is provided with the AC voltage application unit and the detection unit. The control unit applies an AC voltage to a plurality of the developing rollers simultaneously to detect discharge, and the number of the image forming units to be changed in the direction of increasing the peak-to-peak voltage is larger than a predetermined number. In this case, a part of the image forming units is set in a waiting state, the number of the image forming units that change the direction of increasing the peak-to-peak voltage is set to a predetermined number or less, and an AC voltage is applied to each developing roller.

  According to this configuration, even when a plurality of image forming units are provided, since the discharge detection is performed simultaneously, the time required for detecting the peak-to-peak voltage at which discharge starts can be shortened. In addition, if the peak-to-peak voltage is changed in the increasing direction, discharge is likely to occur when an AC voltage is applied. According to this configuration, the number of image forming units that change the peak-to-peak voltage in the increasing direction is less than a predetermined number. Therefore, simultaneous generation of a plurality of discharges exceeding the predetermined number can be prevented. Therefore, it is possible to prevent the occurrence of an abnormality due to the adverse effect of the discharge such as malfunction of the electric circuit and runaway.

  Here, the predetermined number is an integer that can be arbitrarily set. However, if the predetermined number and the total number of image forming units are the same, there is no difference from the case where the AC voltage is simply applied to all the image forming units simultaneously. It is assumed that the number is greater than 0 and is obtained by subtracting 1 from the total number of image forming units. Further, if the predetermined number is increased, the time required for detecting the peak-to-peak voltage at which discharge starts is shortened, but discharges are likely to occur simultaneously. On the other hand, if the predetermined number is reduced, the adverse effect due to the simultaneous occurrence of discharge can be reduced, but the time required for detecting the peak-to-peak voltage at which discharge starts increases. Therefore, the predetermined number can be set as appropriate in consideration of the adverse effect of simultaneous occurrence of discharge and the detection time.

  According to a third aspect of the present invention, in the second aspect of the present invention, priority is determined for each image forming unit, and the control unit waits from the image forming unit having a lower priority. did. According to this configuration, since the image forming unit having a low priority is set in a standby state, the order of the image forming units that detect the peak-to-peak voltage at which discharge starts can be controlled.

  According to a fourth aspect of the present invention, in the third aspect of the present invention, the image forming units are arranged in parallel, and the priority order is any one of the image forming units arranged at both ends. Is the first, and the priority of the image forming unit far from the first image forming unit is set to the second. According to this configuration, since the AC voltage application for detecting the peak-to-peak voltage at which the discharge starts is preferentially performed from the image forming units that are positioned apart from each other, even if the discharge occurs simultaneously, The adverse effect of the discharge can be reduced to some extent by shifting the timing at which current flows into the frame ground. Therefore, even if discharges occur at the same time, it is difficult for malfunctions and runaways in the electric circuit to occur.

  According to a fifth aspect of the present invention, in the first to fourth aspects of the invention, the adjustment width is set to a value obtained by multiplying the minimum change amount and a power of two. According to this configuration, when the peak-to-peak voltage is changed, the amount of change can be accurately halved.

  According to the present invention, the peak-to-peak voltage at which discharge starts can be detected in as short a time as possible. In addition, simultaneous occurrence of a discharge exceeding a predetermined number in a plurality of image forming units can be avoided, and adverse effects due to discharge can be suppressed to a low level.

1 is a schematic front sectional view showing a schematic configuration of a printer according to an embodiment. It is an expanded sectional view of each image forming part concerning an embodiment. It is explanatory drawing which shows the structure regarding the development bias application and discharge detection which concern on embodiment. FIG. 3 is a block diagram illustrating an example of a hardware configuration of a printer according to an embodiment. It is a block diagram for demonstrating the outline | summary of the discharge detection which concerns on embodiment. 6 is a timing chart for explaining an outline of an operation for detecting a peak-to-peak voltage at which discharge in each image forming unit according to the embodiment starts. 6 is a timing chart illustrating an example of an AC voltage applied to each developing roller according to the embodiment. It is a flowchart which shows an example of the basic flow of control of the detection operation of the peak-to-peak voltage where the discharge in the printer which concerns on embodiment starts. It is a flowchart which shows a detailed example of the detection operation | movement of the peak-to-peak voltage which the discharge in the printer which concerns on embodiment starts.

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

(Schematic configuration of image forming apparatus)
First, the outline of the printer 1 according to the embodiment of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic front 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. The printer 1 according to the present embodiment includes an operation panel 1a in the upper front portion, a sheet supply unit 2a, a conveyance path 2b, an image forming unit 3, an exposure device 4, an intermediate transfer unit 5, a fixing device 6 and the like in the main body. Provided.

  The operation panel 1a includes a liquid crystal display unit and various keys, and accepts display of the status (for example, error and mode) of the printer 1 and various inputs from the user. For example, the sheet supply unit 2a stores various sheets such as copy sheets and OHP sheets, and feeds the sheet 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 2b conveys the sheet, and guides the sheet from the sheet supply unit 2a to the discharge tray 22 through the intermediate transfer unit 5 and the like. 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 is opposed to the photosensitive drum 9 with a gap provided in order to supply toner to the photosensitive drum 9, and supplies toner at the time of image formation. A plurality of image forming units 3 (for four colors) are provided as a portion for forming a toner image based on image data of an image to be formed, including a developing roller 81 to be carried. Specifically, the printer 1 includes an image forming unit 3k that forms a black image (comprising a charging device 7k, a developing device 8k, a charge eliminating device 31k, and a cleaning device 32k), and an image forming unit 3y that forms a yellow image. (Including a charging device 7y, a developing device 8y, a charge eliminating device 31y, a cleaning device 32y, etc.) and an image forming unit 3c (charging device 7c, developing device 8c, charge eliminating device 31c, cleaning device 32c, etc.) for forming a cyan image. And an image forming unit 3m (including a charging device 7m, a developing device 8m, a charge removing device 31m, a cleaning device 32m, and the like) for forming a magenta image.

  Here, the image forming units 3k to 3m will be described in detail with reference to FIG. Each of the image forming units 3k to 3m 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 k, y, c, and m for identifying which image forming unit 3 belongs are omitted unless specifically described (in FIG. 2, the image forming unit 3k is not shown). 3), 3y, 3c, and 3m are identified by the symbols k, y, c, and m.)

  Each photosensitive drum 9 is rotatably supported, rotates by receiving a driving force from the motor M, carries a toner image on the peripheral surface, and has, for example, a photosensitive layer on the outer peripheral surface of an aluminum substrate. Then, it is driven to rotate counterclockwise at a predetermined speed by a driving device (not shown). Each photosensitive drum 9 of the present embodiment is a positively charged type.

  Each charging device 7 includes a charging roller 71 that charges the photosensitive drum 9 at a constant potential, and charges the photosensitive drum 9 at a constant potential. Each charging roller 71 rotates in contact with each 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). The charging device 7 may be one that charges the photosensitive drum 9 using a corona discharge device or the like.

  Each developing device 8 accommodates a developer (so-called two-component developer) containing toner and a magnetic carrier (the developing device 8k is black, the developing device 8y is yellow, the developing device 8c is cyan, and the developing device 8m is magenta. Of developer). Each developing device 8 includes a developing roller 81, a magnetic roller 82, and a conveying member 83. The developing roller 81 is opposed to the photosensitive drum 9 while being provided with a predetermined gap (for example, 1 mm or less), carries toner charged during image formation, and is supplied with AC voltage for supplying toner to the photosensitive drum 9. An application unit 86 (see FIG. 3) is connected. Each magnetic roller 82 faces the developing roller 81 and is connected to a magnetic roller bias unit 84 (see FIG. 3). The developing roller is applied with a voltage applied by superimposing a DC voltage and an AC voltage by the magnetic roller bias unit 84. Toner 81 is supplied. Each transport member 83 is provided below each magnetic roller 82.

  The roller shafts 811 and 821 of each developing roller 81 and each magnetic roller 82 are fixed and supported by a support shaft member (not shown) or the like. 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 at the time of printing or discharge detection, this sleeve 812, 822 is rotated. The magnets 813 and 823 at positions opposite to the developing roller 81 and the magnetic roller 82 have opposite polarities.

  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 applying a voltage to the magnetic roller 82, and a thin layer of toner is formed on the developing roller 81. 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 agitates the developer in each developing device 8 and charges the toner by friction between the toner and the carrier (in this embodiment). The toner is positively charged).

  Each cleaning device 32 extends in the axial direction of the photosensitive drum 9 in order to clean the photosensitive drum 9, and removes residual toner and the like by rubbing the surface of the photosensitive drum 9 and the blade 33 formed of resin, for example. A brush 34 is provided. The blade 33 and the brush 34 are in contact with the photosensitive drum 9 to remove and clean the residual toner after transfer by scraping off dirt such as the toner. In addition, above each cleaning device 32, a neutralization device 31 (for example, an array of LEDs) that performs neutralization by irradiating the photosensitive drum 9 with light is provided.

  An exposure device 4 below each image forming unit 3 is a laser unit that outputs laser light. Based on an input color-separated image signal, laser light (illustrated by a broken line) that is an optical signal is applied to each exposure unit 4. Output to the photosensitive drum 9 and scanning exposure of the charged photosensitive drum 9 is performed to form an electrostatic latent image. For example, the exposure apparatus 4 is provided with a semiconductor laser device, a polygon mirror, a polygon motor, an fθ lens, a mirror (not shown), and the like. 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.

  The exposure device 4 is provided with a light receiving element (not shown) within the irradiation range of the laser light and outside the irradiation range of the photosensitive drum 9. When the laser beam is irradiated, this light receiving element outputs a current (voltage), and this output is input to, for example, a CPU 11 (Central Processing Unit) to be described later as a synchronization signal when detecting the occurrence of discharge. Used (see FIG. 6).

  Returning to FIG. The intermediate transfer unit 5 receives the primary transfer of the toner image from the photosensitive drum 9 and performs secondary transfer onto the sheet. Each of the primary transfer rollers 51k to 51m, the intermediate transfer belt 52, the driving roller 53, and the driven roller 54 is performed. To 56, a secondary transfer roller 57, a belt cleaning device 58, and the like. Each primary transfer roller 51 sandwiches an endless intermediate transfer belt 52 between each photosensitive drum 9 and is connected to a transfer voltage applying unit (not shown) for applying a transfer voltage, and the toner image is transferred to the intermediate transfer belt 52. Transcript to.

  The intermediate transfer belt 52 is made of a dielectric resin or the like, is stretched around a driving roller 53 and driven rollers 54 to 56, and is rotated by a driving roller 53 connected to a driving mechanism (not shown) such as a motor. Orbit in the clockwise direction. The driving roller 53 and the secondary transfer roller 57 sandwich the intermediate transfer belt 52 to form a nip (secondary transfer portion). In the toner image transfer, a predetermined voltage is applied to each primary transfer roller 51, and the toner images (black, yellow, cyan, and magenta colors) formed in each image forming unit 3 are sequentially superimposed without deviation. The primary transfer is performed on the intermediate transfer belt 52. The superimposed toner images are transferred onto the sheet by a secondary transfer roller 57 to which a predetermined voltage is applied. Note that the residual toner and the like on the intermediate transfer belt 52 after the secondary transfer are removed by the belt cleaning device 58 and collected.

  The fixing device 6 is arranged downstream of the secondary transfer portion in the sheet conveying direction, and heats and presses the secondary transferred toner image to fix it on the sheet. The fixing device 6 is mainly composed of a fixing roller 61 having a built-in heat source and a pressure roller 62 pressed against the fixing roller 61, and forms a nip. The sheet on which the toner image has been transferred is heated and pressurized as it passes through the nip, and as a result, the toner image is fixed on the sheet. The fixed sheet is discharged to the discharge tray 22 and the image forming process is completed.

(Configuration for developing bias application and discharge detection)
Next, a configuration relating to development bias application and discharge detection will be described with reference to FIG. FIG. 3 is an explanatory diagram showing a configuration relating to development bias application and discharge detection according to the embodiment of the present invention. In the discharge detection, the voltage between the peaks of the AC voltage applied to the developing roller 81 is changed stepwise to check whether or not a discharge has occurred, and between the peaks when the discharge starts between the photosensitive drum 9 and the developing roller 81. This is done to detect the voltage.

  Here, FIG. 3 shows only one image forming unit 3, and each image forming unit 3 is provided with a DC voltage application unit 85, an AC voltage application unit 86, a detection unit 14, an amplifier 15, and the like. Here, the DC voltage application unit 85, the AC voltage application unit 86, the detection unit 14, and the amplifier 15 may be denoted by k, y, c, and m, which indicate the distinction between the image forming units 3. Since the same one is provided for each image forming unit 3, in order to avoid complicated description, the description of k, y, c, and m is omitted in the description of FIG.

  As shown in FIG. 3, the developing roller 81 facing the photoconductor drum 9 with a gap is provided with a roller shaft 811, a cap 814, and a sleeve 812 that carries toner. The roller shaft 811 is inserted through the sleeve 812, and circular caps 814 are fitted to both ends of the sleeve 812. 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.

  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 DC power from the power supply device 16 or the like (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. There is (for example, a DC-DC converter). Thereby, the alternating voltage applied to the developing roller 81 can be biased.

  The AC voltage application unit 86 has, for example, a rectangular wave shape (pulse shape), outputs an AC voltage having an average value of the DC voltage output from the DC voltage application unit 85, and applies a voltage to the developing roller 81. It is. 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 is a power supply circuit including a plurality of switching elements and the like, and inverts the polarity of the output by switching to output 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 AC voltage is increased / decreased by the DC voltage input from the power supply device 16 (see FIG. 4). The peak value on the positive side and the peak value on the negative side are varied according to instructions from the CPU 11. The configuration of the AC voltage application unit 86 only needs to change the peak-to-peak voltage, the duty ratio, and the frequency.

  The AC voltage application unit 86 can include, for example, a boosting circuit such as a boosting transformer in the output stage, and the developing bias in which the DC and the AC after boosting are superimposed 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 for detecting the occurrence of discharge between the developing roller 81 and the photosensitive drum 9, and converts the current flowing through the developing roller 81 into a voltage signal when the discharge occurs, and detects the occurrence of discharge ( Discharge detection signal). Then, the detection unit 14 outputs a discharge detection signal to the amplifier 15. The amplifier 15 amplifies the discharge detection signal from the detection unit 14 and outputs it to the CPU 11. If the CPU 11 has an amplifier circuit, the amplifier 15 is not necessary.

  The A / D converter 17 is a circuit that converts the analog output of the amplifier 15 into a digital signal and inputs it to the CPU 11. From the output of the A / D converted detection unit 14 (amplifier 15), the CPU 11 generates the discharge and the magnitude of the generated discharge (the magnitude of the current flowing between the developing roller 81 and the photosensitive drum 9). Can be recognized. If the CPU 11 has an A / D conversion function, an A / D converter is unnecessary.

  Next, a configuration for applying a voltage to the magnetic roller 82 and the magnetic roller 82 will be described. As described above, while providing a predetermined gap (a magnetic brush is formed in this gap), the magnetic roller 82 is disposed so as to face the developing roller 81 and the axial directions thereof are parallel to each other. The magnetic roller 82 includes a roller shaft 821, a sleeve 822 that carries toner and a carrier, and a cap 824. The roller shaft 821 is inserted through the sleeve 822, and circular caps 824 are fitted to both ends of the sleeve 822. In addition, a magnetic roller bias unit 84 that applies a voltage (magnetic roller bias) in which a DC voltage and an AC voltage are superimposed to the magnetic roller 82 is connected to the roller shaft 821 of the magnetic roller 82. The magnetic roller bias unit 84 applies the magnetic roller bias, and the charged toner is supplied to the developing roller 81 by electrostatic force.

(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. In FIG. 4, only one image forming unit 3 is shown for convenience.

  As shown in FIG. 4, the printer 1 according to the present embodiment includes a control unit 10 inside. For example, the control unit 10 controls the output of the AC voltage application unit 86 and its timing, recognizes the occurrence of discharge based on the output of the detection unit 14, and controls each unit of the apparatus. 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 in the control unit 10, and controls and calculates each unit of the printer 1 based on a control program stored in the storage unit 12 and developed. In addition, since it is necessary to perform a large number of controls in the printer 1, a control is provided such as having a plurality of CPUs 11 in a function-divided form such as a board that controls the entire printer 1 and a board that controls the driving of each motor. The unit 10 may be configured in a distributed manner.

  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 various data in addition to the control program of the printer 1. In the present invention, the storage unit 12 stores a program for detecting a peak-to-peak voltage at which discharge starts, a setting of a voltage applied to the developing roller 81 during printing, and the like.

  The control unit 10 is connected to the sheet supply unit 2 a, the conveyance path 2 b, the image forming unit 3, the exposure device 4, the intermediate transfer unit 5, the fixing device 6, and the like, and is appropriately based on the control program and data in the storage unit 12. The operation of each unit is controlled so that image formation is performed. The motor M is connected to the control unit 10 and supplies a driving force for rotating the photosensitive drums 9 and the like of the image forming units 3. Then, the control unit 10 drives the motor M at the time of printing or discharge detection to rotate each of the photosensitive drums 9 described above. Further, the sleeves of the developing roller 81 and the magnetic roller 82 can be rotated using the driving of the motor M.

  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 via the I / F unit 18 (interface unit). The image data is processed, and the exposure device 4 receives the image data and forms an electrostatic latent image on the photosensitive drum 9.

  The output of the detection unit 14 (amplifier 15) is input to the control unit 10 (CPU 11). At the time of detecting the discharge, the CPU 11 gives an instruction to change the peak-to-peak voltage of the AC voltage applied to the developing roller 81 to the AC voltage application unit 86, and from the output after the A / D conversion of the detection unit 14 (amplifier 15). Then, the presence / absence of discharge in the image forming unit 3 is detected and the magnitude of the discharge is determined. Each charging device 7 (charging roller 71) of each image forming unit 3 is individually connected with a charging voltage applying unit 72. The charging roller 71 contacts the photosensitive drum 9 to charge the photosensitive drum 9. .

(Outline of development bias application and discharge detection)
Next, an outline of discharge detection according to the embodiment of the present invention will be described with reference to FIG. FIG. 5 is a block diagram for explaining an outline of discharge detection according to the embodiment of the present invention.

  As shown in FIGS. 2 and 5, the printer 1 according to the present embodiment is provided with a total of four image forming units 3 k to 3 m including a developing roller 81 facing the photosensitive drum 9. Then, for each image forming unit 3, an AC voltage applying unit 86 (86k to 86m) for applying a developing bias to the developing roller 81, a DC voltage applying unit 85 (85k to 85m), and a detection for detecting a discharge. A unit 14 (14k to 14m), an amplifier 15 (15k to 15m), and an A / D converter 17 (17k to 17m) are provided. That is, the printer 1 according to the present embodiment includes a plurality of image forming units 3, and each image forming unit 3 is provided with an AC voltage application unit 86, a detection unit 14, and the like.

  Here, as shown in FIGS. 2 and 3, a minute gap (for example, about 100 to several hundred μm) is provided between each developing roller 81 and each photosensitive drum 9. However, in each image forming unit 3, there are errors in attaching the photosensitive drum 9 and the developing roller 81, errors in parallelism, and the like. Each photosensitive drum 9 has a photosensitive layer formed by vapor deposition or the like, and each developing roller 81 uses a sleeve 812 formed by processing aluminum or the like, but the photosensitive drum 9 or developing roller 81 (sleeve) is used. It is difficult to make a complete cylindrical shape or a columnar shape in manufacturing. In other words, each photosensitive drum 9 and each developing roller 81 has a shake (deviation from an ideal shape).

  Due to the presence of such errors and shakes, there is a slight difference in the gap length between each developing roller 81 and each photosensitive drum 9 in each image forming unit 3. As a result, the peak-to-peak voltage of the AC voltage at which the discharge starts is different in each image forming unit 3. Therefore, it is necessary to detect the peak-to-peak voltage at which discharge starts for each image forming unit 3.

  Specifically, while changing the peak-to-peak voltage, application of an alternating voltage to each developing roller 81 is repeated, the occurrence of discharge is confirmed, and the peak-to-peak voltage at which discharge starts is detected. In addition, a discharge detection signal from each A / D converter 17 is input to the CPU 11 for recognition of the image forming unit 3 where the discharge has occurred. A discharge detection circuit that generates a signal indicating the image forming unit 3 in which discharge is detected may be provided between the CPU 11 and each A / D converter 17 and the signal may be input to the CPU 11.

  Each image forming unit 3 determines the magnitude of the AC voltage applied to each developing roller 81 during printing based on the detected peak-to-peak voltage at which discharge starts. Since each image forming unit 3 is provided with an AC voltage application unit 86, an AC voltage having a different peak-to-peak voltage can be applied to each developing roller 81 during printing.

(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 the outline of the peak-to-peak voltage detection operation at which discharge is started in each image forming unit 3 according to the embodiment of the present invention. FIG. 7 is a timing chart showing an example of an AC voltage applied to each developing roller 81 according to the embodiment of the present invention. 6 and 7 are common to each image forming unit 3, and therefore the symbols k, y, c, and m indicating the distinction of the image forming unit 3 are omitted.

  First, an outline of an operation for detecting a peak-to-peak voltage at which discharge starts will be described with reference to FIG. In FIG. 6, “developing roller (AC)” indicates the timing at which each AC voltage application unit 86 applies an AC voltage to each developing roller 81. “Developing roller (DC)” indicates the timing at which each DC voltage application unit 85 applies a DC voltage to the developing roller 81. “Magnetic roller (AC)” indicates the timing at which each magnetic roller bias unit 84 (see FIG. 4) applies an AC voltage to each magnetic roller 82. “Magnetic roller (DC)” indicates the timing at which each magnetic roller bias unit 84 applies a DC voltage to the magnetic roller 82. “Charging roller” indicates the timing at which each charging device 7 charges each 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 each photosensitive drum 9 in the exposure apparatus 4. “Discharge detection (detection unit output)” indicates a discharge occurrence detection timing by each detection unit 14.

  Here, although details will be described later, in the printer 1 of the present embodiment, an alternating voltage is applied to the developing roller 81 at the same time for each image forming unit 3 to confirm whether or not a discharge has occurred. Accordingly, each operation described below is basically performed simultaneously in each image forming unit 3.

<Initial operation>
When the operation for detecting the peak-to-peak voltage at which discharge starts 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 AC and DC voltages are applied to 82 as a test. 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. After this initial operation, a transition is made to the preparation state.

<Ready state>
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 detection of the peak-to-peak voltage at which discharge starts is completed. Thereafter, the condition change state (first time) is entered.

<Condition change state>
In the condition change state, the setting of the peak-to-peak voltage of the AC voltage output from the AC voltage application unit 86 is performed based on an instruction from the CPU 11. And in each condition change state, the Vpp control part 88 adjusts and sets an output so that the alternating voltage application part 86 may output with the set peak-to-peak voltage. In the second and subsequent condition change states, the Vpp control unit 88 adjusts and sets the output by changing the voltage between the peaks applied immediately before (for example, rising and falling). 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 at the peak-to-peak voltage set in the condition change state, 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. is there. 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). Note that the application of the AC voltage is repeated to the developing roller 81 of each image forming unit 3 until the application of the AC voltage with the changed peak-to-peak voltage reaches a certain number of times.

  The discharge detection state continues for a certain period of time (for example, 1 second to several seconds), during which the photosensitive drum 9 and the developing roller 81 rotate a plurality of times. As described above, “shake” exists in the photosensitive drum 9 and the developing roller 81, but a state in which the gap length between the photosensitive drum 9 and the developing roller 81 is shortened by the rotation appears in the discharge detection state. In FIG. 6, it is shown that a discharge is detected by any one of the image forming units 3 in the first or n-th discharge detection state.

  Next, application of a voltage to the developing roller 81 will be described with reference to FIG. In FIG. 7, a timing chart at the time of printing 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 printing 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 (for example, about 50 to 100 V). “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 during printing. “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 printing. “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, the rectangular wave in the timing chart at the time of occurrence of discharge indicates the waveform of the developing bias applied to each developing roller 81 in the discharge detection state. “Vdc2” indicates the bias potential of the DC voltage application unit 85 at the time of detection (for example, about 50 to 200 V). “V0” indicates a potential (approximately 0 V) of the photosensitive drum 9 after exposure by the exposure device 4. “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 be the set value Vdc2 for detecting the occurrence of discharge in accordance with an instruction from the CPU 11. Further, in response to an instruction from the CPU 11, the Vpp control unit 88 sets the Vpp2 of the AC voltage output from the AC voltage application unit 86 (Note that the value of Vpp2 changes for each condition change state). 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 a value for detecting the occurrence of discharge (lower stage in FIG. 7).

  Here, the duty ratio D2 is set smaller than the duty ratio D1 at the time of printing (ratio of High time T1 to period T01, T1 / T01) (for example, D1 = 40%, D2 = 30%). In this way, the duty ratio D2 is set because the photosensitive drum 9 of the present embodiment is charged with a positive polarity and a large current is generated when a discharge occurs when the potential of the developing roller 81 is low (at the negative peak). This is because the absolute value of the negative peak voltage is made as small as possible. The frequency f2 is set so that the positive time of the AC voltage is the same at the time of printing and when the occurrence of discharge is detected (ie, T1 = T2. For example, when D1 = 40% and D2 = 30%, printing is performed. If the frequency at the time f1 = 4 kHz, then f2 = 3 kHz). As a result, a positive voltage is applied to the developing roller 81 for the same time as during printing.

(Basic flow of control of detection of peak-to-peak voltage at which discharge starts)
Next, an example of a basic flow of control of the detection operation of the peak-to-peak voltage at which discharge of the printer 1 according to the embodiment of the present invention starts will be described with reference to FIG. FIG. 9 is a flowchart illustrating an example of a basic flow of control of an operation for detecting a peak-to-peak voltage at which discharge starts in the printer 1 according to the embodiment of the present invention, based on FIG. 8.

  The detection operation of the peak-to-peak voltage at which the discharge starts is 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. It may also be performed when the main power is turned on. 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 highlands of Mexico), 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.

  In order to cope with environmental changes, the image forming apparatus may include a temperature sensor and a humidity sensor. Although not shown, the image forming apparatus also includes the printer 1 of this embodiment. Then, the temperature and humidity at the time of the previous discharge detection operation are stored in the storage unit 12 or the like, and the peak-to-peak voltage at which discharge starts when the temperature and humidity at the time of the previous discharge detection operation differ by a certain value or more. May be done.

  FIG. 8 will be described. When a predetermined operation is performed on the operation panel 1a and the like and a peak-to-peak voltage detection operation at which discharge starts is started (start), the CPU 11 instructs all the image forming units 3 to drive the motor M or a driving mechanism (not shown). Then, rotation of various rotating bodies such as the photosensitive drum 9, each sleeve, and each roller in the intermediate transfer portion 5 is started (step S1). The driving of each rotating body is continued until the detection operation of the peak-to-peak voltage at which discharge starts is completed. Next, the initial operation described with reference to FIG. 6 is executed in all the image forming units 3 (step S2). Next, the transition to the preparation state described in FIG. 6 is executed in all the image forming units 3 (step S3). For example, each charging voltage applying unit 72 applies a voltage to each charging device 7 in accordance with an instruction from the CPU 11. Start.

  Next, the condition change state and the discharge detection state described in FIG. 6 are repeated in all the image forming units 3, and the peak-to-peak voltage at which discharge starts is detected in each image forming unit 3 (step S <b> 4). Details of step S4 will be described later.

Then, when the peak-to-peak voltage at which discharge of each image forming unit 3 starts is determined in step S4, the detected peak-to-peak voltage Vpp2, frequency f2, duty ratio D2, and bias setting value of each image forming unit 3 are determined. From Vdc2, the potential difference V _{+2 in} FIG. 7 (potential difference between the photosensitive drum 9 and the developing roller 81 when Vpp2 is applied) is obtained for each image forming unit 3 (step S5).

Here, V ₊₂ can be easily obtained. The CPU 11 designates the magnitude of the peak-to-peak voltage and issues an instruction to each 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 the discharge 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 at the time of printing so that V ₊₁ and V ₋ shown in FIG. 7 are smaller than the obtained V _+2. The peak-to-peak voltage of the AC voltage applied to 81 is set for each image forming unit 3, and the set value is stored in the storage unit 12 or the like (step S6). Specifically, there are various methods for determining the peak-to-peak voltage. For example, how much less V ₊₁ and V ₋ than V ₊₂ can cause no discharge (how much margin should be taken) Taking into account the toner used, etc., based on the experiment at the time of development, for example, for the obtained V ₊₂ , the table shows the value of the peak-to-peak voltage that is recognized that no discharge occurs during printing, and the CPU 11 refers to the table In addition, the peak-to-peak voltage in each image forming unit 3 may be determined. This table may also be stored in the storage unit 12.

  Thereby, it is possible to apply an alternating voltage that is as large as possible without causing discharge during printing and that is different for each image forming unit 3. When the setting of the peak-to-peak voltage during printing is completed, the detection of discharge occurrence and the setting of the peak-to-peak voltage during printing are completed (end). In this way, the determined peak-to-peak voltage is Vpp1 in FIG.

(Details of detection control of peak-to-peak voltage at which discharge starts)
Next, a detailed example of the control of the peak-to-peak voltage detection operation at which the printer 1 according to the embodiment of the present invention starts discharging will be described with reference to FIG. That is, the details of step S4 in FIG. 8 will be described. FIG. 9 is a flowchart showing a detailed example of the peak-to-peak voltage detection operation at which discharge starts in the printer 1 according to the embodiment of the present invention.

First, before entering the description of the flowchart shown in FIG. In this description, it is assumed that the peak-to-peak voltage at which discharge starts is detected by changing the peak-to-peak voltage of the AC voltage in the developing bias in each image forming unit 3 in the range of 800V to 1056V. The Vpp control unit 88 of the AC voltage application unit 86 can change the peak-to-peak voltage in units of 1V. Therefore, in the printer 1 of the present embodiment, the range of 256 V (256 steps) is set as the adjustment range when detecting the peak-to-peak voltage at which discharge starts. That is, the adjustment width is set to a value (1V × 2 ⁸ ) obtained by multiplying the minimum change amount and the power of 2.

  Next, each symbol (symbol) in the flowchart shown in FIG. 9 will be described. First, “Vof” indicates the minimum value of the peak-to-peak voltage in the discharge detection, and is 800 V in this description. “Vm”, “Vk”, “Vy”, and “Vc” indicate peak-to-peak voltages in the next discharge detection state (actual AC voltage application). “Vd_m”, “Vd_k”, “Vd_y”, and “Vd_c” indicate the amount of change in the peak-to-peak voltage that is changed in the condition change state in each image forming unit 3. The symbols “m”, “k”, “y”, and “c” are “magenta image forming unit 3 m”, “black image forming unit 3 k”, “yellow image forming unit 3 y”, “ This is the value relating to “cyan image forming portion 3c”, and so on.

  Here, in the printer 1 of the present embodiment, in order to detect the peak-to-peak voltage at which discharge starts, adjustment is performed when an AC voltage is first applied to each developing roller 81 in each image forming unit 3 (first discharge detection state). An AC voltage is applied at a peak-to-peak voltage obtained by adding half the width (128 V in this description) to Vof (in this description, 928 V). Next, when the AC voltage is applied to the developing roller 81, the peak-to-peak voltage of the AC voltage is changed by a change amount that is a half of the half of the adjustment width compared to the previous setting (the change amount is 64 V in this description). (Specifically, 928 + 64 = 992V or 928−64 = 864V). Thereafter, every time the setting of the peak voltage of the AC voltage applied to the developing roller 81 is performed, the peak voltage of the AC voltage changes by half of the amount of change compared to the previous setting (in this description, the AC voltage is applied). Each time, the amount of change changes from 64V → 32V → 16V → 8V → 4V → 2V → 1V).

  That is, in order to detect the peak-to-peak voltage at which discharge starts, the control unit 10 first adjusts the adjustment width with an amount of change that is half the adjustment width that is the range of the peak-to-peak voltage of the AC voltage applied to the developing roller 81. The AC voltage of the peak-to-peak voltage changed in the direction of increasing the minimum value of the AC voltage is applied to the developing roller 81, and then half of the immediately previous change amount until the AC voltage application unit becomes equal to or less than the minimum change amount that can be set. The peak-to-peak voltage is changed by the amount of change, and the application of the AC voltage to the developing roller 81 is repeated.

“Nm”, “Nk”, “Ny”, and “Nc” are the number of times that each image forming unit 3 applied the AC voltage to detect the presence or absence of discharge (= the discharge detection state was reached). Number of times). Each Vpp control unit 88 can change the peak-to-peak voltage in units of 1 V, the adjustment range is 256 (2 ⁸ ) V, and the AC to the developing roller 81 is halved while the amount of change in the peak-to-peak voltage is halved. Since voltage application is repeated, the maximum value of Nm to Nc is 8 (times) in this example. “Pm”, “Pk”, “Py”, and “Pc” respectively indicate the direction of change in the peak-to-peak voltage when the next AC voltage is applied (next discharge detection state) in each image forming unit 3. It is shown that “+1” is set when the voltage is larger than the peak-to-peak voltage of the AC voltage applied immediately before, and “−1” is set when the voltage is made smaller. Whether it is “+1” or “−1” is determined by whether or not a discharge has occurred when an AC voltage is applied to the developing roller 81 (see step S419).

  In addition, “Lm”, “Lk”, “Ly”, and “Lc” respectively indicate the probability of occurrence of a discharge in the next AC voltage application (next discharge detection state) in each image forming unit 3. Specifically, when “+1” is set in Pm, Pk, Py, and Pc, the peak-to-peak voltage changes in the increasing direction, so that the corresponding colors Lm, Lk, Ly, and Lc are “+1”. Is set. On the other hand, when “−1” is set in Pm, Pk, Py, and Pc, “0” is set in Lm, Lk, Ly, and Lc of the corresponding color. First, when an AC voltage is applied to each developing roller 81, since it is unclear whether or not a discharge occurs, Pm, Pk, Py, Pc, Lm, Lk, Ly, and Lc are all set to “+1”.

  “Fm”, “Fk”, “Fy”, and “Fc” indicate flags that determine whether or not an AC voltage is actually applied when an AC voltage is applied to the developing roller 81. In this description, “+1” is set when an AC voltage is applied to the developing roller 81 in the discharge detection state, and “0” is set when no AC voltage is applied.

  Based on the above assumptions, the flowchart shown in FIG. 9 will be described. First, the start in FIG. 9 is the time when the transition to the preparation state (step S3) in all the image forming units 3 is completed in the discharge detection operation described with reference to FIG. When the condition change state is reached (steps S401 to S416 in FIG. 9 correspond to the condition change state), the control unit 10 (CPU 11 or the like) performs initial setting for detecting the peak-to-peak voltage at which discharge starts. Do. Specifically, Vof = 800V, Vd_m = Vd_k = Vd_y = Vd_c = 128V are set. Further, Pm = Pk = Py = Pc = + 1 and Lm = Lk = Ly = Lc = + 1 are set (step S401). In this setting, with respect to 800 V, which is the minimum peak-to-peak voltage that can be applied to the developing roller 81, the amount of change with respect to 800 V is set to 128 V in all the image forming units 3. Accordingly, in the first discharge detection state, an alternating voltage having a peak-to-peak voltage of 800 + 128 = 928V is actually applied to the developing roller 81 in each image forming unit 3. Nm to Nc indicate the number of times the AC voltage is actually applied to the developing roller 81 in order to detect the peak-to-peak voltage at which discharge starts (the number of discharge detection states).

  Next, the control unit 10 checks whether Nm <8 with respect to the number of discharge detection executions in the magenta image forming unit 3m (step S402). That is, since the amount of change in the peak-to-peak voltage when the previous AC voltage is applied is the same as the unit (1V) that can be changed by each Vpp control unit 88, it is confirmed whether the amount of change can no longer be halved. To do. In other words, it is confirmed whether or not the discharge detection state has been reached a maximum number of times. If Nm ≧ 8 (No in step S402), it is not necessary to check whether or not a discharge has occurred, so the control unit 10 sets the magenta image forming unit 3m flag to Fm = 0 (step S403). Even if it becomes a discharge detection state, it sets so that an alternating voltage may not be applied.

  On the other hand, if Nm <8 (Yes in step S402), the control unit 10 sets Vm = Vof + (Pm × Vd_m), Vd_m = Vd_m / 2, Nm = Nm + 1, and Fm = + 1 (step S404). In step S404, the peak-to-peak voltage of the AC voltage applied to the developing roller 81 in the discharge detection state is determined. In accordance with this determination, control unit 10 instructs Vpp control unit 88, and Vpp control unit 88 sets and changes the peak-to-peak voltage of the AC voltage so as to output the determined peak-to-peak voltage. Further, the change amount of the peak-to-peak voltage in the next discharge detection state is halved by the calculation formula of Vd_m = Vd_m / 2. Further, Fm indicating the flag is set to +1, and in the next discharge detection state, the AC voltage of the peak-to-peak voltage set to the developing roller 81 is applied.

  Next, the control unit 10 confirms whether or not Nk <8 with respect to the number of discharge detection executions in the black image forming unit 3k (step S405). Note that steps S405 to S407 are different from steps S402 to 404 only in the image forming unit 3, and the basic concept can be applied in the same way, so that the description of steps S405 to 407 will be omitted.

  After step S407, the control unit 10 confirms whether (Lm + Lk + Ly) ≦ a predetermined number (step S408). In step S419, which will be described later, whether to change the peak-to-peak voltage of the AC voltage to be applied next to the developing roller 81 in a direction to increase or decrease is determined depending on whether or not a discharge occurs. When the voltage is changed in the direction of increasing the discharge voltage, the probability of occurrence of discharge increases. If it does so, the probability that discharge will generate | occur | produce simultaneously in the several image formation part 3 will become large. Therefore, the control unit 10 checks whether or not (Lm + Lk + Ly) ≦ predetermined number, and suppresses the number of image forming units 3 to be changed in the direction of increasing the peak-to-peak voltage to a predetermined number or less.

  Here, the predetermined number is an integer that can be arbitrarily set to an integer value larger than 0, and can be set to 2 to 3 in the present description. The predetermined number is determined by the predetermined number and the image forming unit 3. If the number is the same, there is no difference from the case where the AC voltage is simply applied to all the image forming units 3 at the same time. Therefore, the number is set to a number obtained by subtracting 1 from the total number of image forming units 3. If the predetermined number = 2, the number of image forming units 3 that can simultaneously generate discharge is recognized up to two. If the predetermined number = 3, the simultaneous occurrence of discharge is more likely to occur than in the case where the predetermined number is 1 or 2, but the time required to detect the peak-to-peak voltage at which the discharge starts is the shortest. It should be noted that the predetermined number is the degree of noise countermeasures for the frame or the like in the printer 1, the likelihood of malfunction or runaway due to noise in the electric circuit such as the control unit 10, and the detection time of the peak-to-peak voltage at which discharge starts. It can be set as appropriate in consideration of trade-offs.

  Since the number of simultaneous discharges is minimized, the predetermined number = 1 can be set. However, when the predetermined number = 1, (Lm + Lk) ≦ the predetermined number (= 1) is confirmed before step S405. If a step is separately provided and the condition is satisfied, a step of setting Fk = 0 indicating the flag of the black image forming unit 3k needs to be provided.

  If (Lm + Lk + Ly) ≦ predetermined number (Yes in step S408), the number of simultaneously occurring discharges can be kept below the predetermined number. Therefore, the process proceeds to step S409, and the control unit 10 uses the yellow image forming unit 3y. It is checked whether Ny <8 with respect to the number of discharge detection executions. Since step S409 is the same as steps S402 and S405, detailed description thereof is omitted. On the other hand, if (Lm + Lk + Ly)> predetermined number (No in step S408) and Ny ≧ 8 (No in step S409), the control unit 10 sets the flag of the yellow image forming unit 3y to Fy = 0. (Step S410), and in the next discharge detection state, the AC voltage is not applied and a standby state is entered. As a result, the probability of simultaneous occurrence of discharge can be reduced.

  If Ny <8 (Yes in step S409), the process proceeds to step S411, and the control unit 10 sets each value for the yellow image forming unit 3. Note that step S411 is the same as steps S404 and 407 described above, and detailed description thereof is omitted. After step S410 or step S411, the control unit 10 checks whether (Lm + Lk + Ly + Lc) ≦ predetermined number (step S412). In step S412, as in step S408, the control unit 10 checks whether (Lm + Lk + Ly + Lc) ≦ predetermined number, and reduces the number of image forming units 3 to be changed in the direction of increasing the peak-to-peak voltage to a predetermined number or less. This reduces the probability of simultaneous discharge in the plurality of image forming units 3.

  As described above, the control unit 10 applies the AC voltage to the plurality of developing rollers 81 at the same time to perform discharge detection, and the number of the image forming units 3 to be changed in the direction of increasing the peak-to-peak voltage is larger than a predetermined number. In this case, a part of the image forming units 3 is set in a waiting state, the number of image forming units 3 that change in a direction of increasing the peak-to-peak voltage is set to a predetermined number or less, and an AC voltage is applied to each developing roller 81.

  Further, as a result of the execution of this flowchart, AC voltage application and discharge detection are preferentially performed in the magenta and black image forming units 3. That is, in the printer 1 of the present embodiment, the priority order is determined for each image forming unit 3, and the control unit 10 waits from the image forming unit 3 with the lower priority order. The priority order in the printer 1 of the present embodiment is specifically magenta> black> yellow> cyan. Here, looking at the positional relationship between the image forming units 3 arranged in parallel (see FIG. 1), the magenta image forming unit 3m and the black image forming unit 3k, which are the image forming units 3 arranged at both ends, are seen. The priority is set high for program execution. That is, the priority order is that of any one of the image forming units 3 arranged at both ends is the first, and the image forming unit 3 farthest from the first image forming unit 3 has priority. The rank is set to the second rank. Thereby, even if discharge occurs simultaneously, the adverse effect of noise due to discharge is reduced.

  After step S412, when (Lm + Lk + Ly + Lc) ≦ predetermined number (Yes in step S412), the process proceeds to step S413, and when (Lm + Lk + Ly + Lc)> predetermined number, the process proceeds to step S414. Step S413 is the same as steps S402, 405, and 409. Step S414 is the same as steps S403, 406, and 411. Step S415 is the same as steps S404, 407, and 410. Since only the image forming unit 3 is different, the description is omitted.

  Then, after step S414 or S415, the control unit 10 checks whether Nm + Nk + Ny + Nc ≧ 32 (step S416). That is, the control unit 10 confirms whether the number of times the AC voltage is applied to the developing roller 81 (the number of discharge detection states) has reached 32 times, and discharge detection is performed 8 times in all the image forming units 3. Make sure. If Nm + Nk + Ny + Nc <32 (No in step S416), the control unit 10 applies an AC voltage to the developing roller 81 with the set peak-to-peak voltage only in the +1 image forming unit 3 among Fm to Fc. Then, discharge detection is executed (step S417).

  And the control part 10 confirms the presence or absence of discharge generation in each image formation part 3 from the output of the detection part 14 (step S418). Then, the control unit 10 sets Px = −1 and Lx = 0 for the image forming unit 3 that has been discharged (however, any of x: m, k, y, and c; the same applies hereinafter). For the image forming unit 3 that has not been discharged, Px = + 1 and Lx = + 1 are set. As a result, in the image forming unit 3 that has been discharged, the peak-to-peak voltage of the AC voltage applied to the developing roller 81 in the next discharge detection state changes in a decreasing direction. On the other hand, in the image forming unit 3 where there was no discharge, the peak-to-peak voltage of the AC voltage applied to the developing roller 81 in the next discharge detection state changes in an increasing direction. That is, when changing the peak-to-peak voltage, the control unit 10 increases the peak-to-peak voltage if a discharge occurs due to the application of the previous AC voltage, and decreases the peak-to-peak voltage if no discharge occurs. Change direction.

  Therefore, each time the discharge detection state is repeated, the peak-to-peak voltage of the AC voltage approaches the peak-to-peak voltage at which discharge starts. In addition, the values of Px and Lx are maintained for the image forming unit 3 (waiting state or the like) in which discharge detection is not performed without applying an AC voltage. Further, since the image forming unit 3 that has performed discharge detection eight times is unnecessary for the calculation of a predetermined number, Lx = 0 is set (step S419). Then, it returns to step S402.

  On the other hand, if Nm + Nk + Ny + Nc ≧ 32 (Yes in step S416), the control unit 10 determines the peak-to-peak voltage at which discharge starts for each image forming unit 3 (step S420 → end). The control unit 10 grasps the peak-to-peak voltage of the AC voltage applied at the eighth time in each image forming unit 3. If no discharge is detected in the eighth discharge detection state, for example, the control unit 10 applies the eighth voltage. The peak-to-peak voltage that is 1 V larger than the peak-to-peak voltage of the AC voltage thus determined can be determined as the peak-to-peak voltage at which discharge starts. On the other hand, if discharge is detected at the eighth time, for example, the peak-to-peak voltage of the AC voltage applied at the eighth time can be determined as the peak-to-peak voltage at which discharge starts. Note that, as in this embodiment, the Vpp controller 88 repeats halving the amount of change in peak-to-peak voltage to the smallest unit that can be changed, so that the peak-to-peak voltage of the AC voltage applied the eighth time is discharged. It can also be regarded as the peak-to-peak voltage starting. The method for determining the peak-to-peak voltage at which discharge starts is not limited to the above method, and a slight margin may be provided for the peak-to-peak voltage of the AC voltage applied the eighth time. Thereafter, the peak-to-peak voltage of the AC voltage at the time of printing is set in each image forming unit 3 (step S6 in FIG. 8).

  In this way, according to the present invention, the amount of change in the peak-to-peak voltage is reduced by half, so that the peak-to-peak discharge can be started more efficiently than when the peak-to-peak voltage is gradually increased as in the prior art. Voltage detection is performed. Therefore, the time required for detecting the peak-to-peak voltage at which discharge starts is shortened. Even when a plurality of image forming units 3 are provided, since the discharge detection is performed at the same time, the time required for detecting the peak-to-peak voltage at which the discharge starts can be shortened. Further, when the peak-to-peak voltage is changed in the increasing direction, discharge easily occurs when an AC voltage is applied. According to this configuration, the number of image forming units 3 that change the peak-to-peak voltage in the increasing direction is less than a predetermined number. Therefore, simultaneous generation of a plurality of discharges exceeding the predetermined number can be prevented. Therefore, it is possible to prevent the occurrence of abnormality due to the adverse effects of discharge such as malfunction of electric circuit and runaway.

  Further, since the image forming unit 3 having a low priority is set in a waiting state, the order of the image forming unit 3 that detects the occurrence of discharge can be controlled. In addition, since an AC voltage is applied for detecting the peak-to-peak voltage at which discharge starts preferentially from the image forming units 3 that are separated from each other in position, even if a discharge occurs simultaneously, the current due to the discharge is not reduced. The adverse effect of the discharge can be reduced to some extent by shifting the timing of flowing into the ground. Therefore, even if discharges occur simultaneously, malfunctions and runaways in the electric circuit are less likely to occur. Further, since the adjustment width is set to a value obtained by multiplying the minimum change amount by a power of 2, the change amount can be exactly halved when the peak-to-peak voltage is changed.

  Next, another embodiment will be described. In the above embodiment, the minimum change width of the Vpp control unit 88 has been described as 1 V. However, it varies depending on the function of the Vpp control unit 88 (for example, 2 V to 10 V, for example). In addition, although the adjustment range when detecting the peak-to-peak voltage at which discharge starts is 256 V, the adjustment range may be determined according to the function of the Vpp control unit 88. For example, if the minimum change width is 10 V and the change in the peak-to-peak voltage in 256 steps is possible, the adjustment width is 2.56 kV.

  In the above embodiment, the adjustment width is a value obtained by multiplying the minimum change amount by a power of 2. However, the change amount is exactly halved. The Vpp control unit can be set, and the peak-to-peak voltage may be changed with a change amount closest to a value obtained by halving the previous change amount.

  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 can be used in an image forming apparatus such as a printer or a multi-function machine that performs printing by applying an AC voltage to a developing roller.

DESCRIPTION OF SYMBOLS 1 Printer (image forming apparatus) 10 Control part 12 Memory | storage part 14 (14a, 14b, 14c, 14d) Detection part 3 (3k, 3y, 3c, 3m) Image formation part 81 (81a, 81b, 81c, 81d) Developing roller 86 (86a, 86b, 86c, 86d) AC voltage application unit 9 (9a, 9b, 9c, 9d) Photosensitive drum

Claims (5)

An image forming unit including: a photosensitive drum; and a developing roller that is opposed to the photosensitive drum with a gap for supplying toner to the photosensitive drum and carries the toner during image formation;
An alternating voltage application unit for applying a voltage to the developing roller;
A detecting unit for detecting occurrence of discharge between the developing roller and the photosensitive drum;
A control unit that controls the AC voltage application unit, and recognizes the occurrence of discharge based on the output of the detection unit;
When the peak-to-peak voltage of the AC voltage applied to the developing roller is changed stepwise, and the peak-to-peak voltage at which discharge starts between the photosensitive drum and the developing roller is detected,
The control unit first changes the peak of the adjustment width in the direction of increasing the amount of change of the half of the adjustment width that is a range of the peak-to-peak voltage of the AC voltage applied to the developing roller. An alternating voltage of an inter-voltage is applied to the developing roller, and then the peak-to-peak voltage is changed by a change amount that is half of the immediately preceding change amount until the AC voltage application unit is equal to or less than the minimum change amount that can be set. When the AC voltage is repeatedly applied to the developing roller and the peak-to-peak voltage is changed, if a discharge occurs due to the application of the previous AC voltage, no discharge occurred in the direction of increasing the peak-to-peak voltage. In the case, the image forming apparatus is characterized in that the voltage between the peaks is changed in the direction of decreasing. A plurality of the image forming units, and each of the image forming units is provided with the AC voltage applying unit and the detecting unit,
The control unit applies an AC voltage to a plurality of the developing rollers at the same time to detect discharge, and when the number of the image forming units to be changed in the direction of increasing the peak-to-peak voltage is larger than a predetermined number, The AC voltage is applied to each developing roller after setting a part of the image forming units in a waiting state and setting the number of the image forming units to be changed in a direction of increasing the peak-to-peak voltage to a predetermined number or less. The image forming apparatus according to 1. Priorities are set for each image forming unit,
The image forming apparatus according to claim 2, wherein the control unit waits from the image forming unit having a low priority. Each image forming unit is arranged in parallel,
Among the image forming units arranged at both ends, one of the image forming units is the first priority, and the priority of the image forming unit far from the first image forming unit is set to the second priority. The image forming apparatus according to claim 3, wherein:   5. The image forming apparatus according to claim 1, wherein the adjustment width is set to a value obtained by multiplying the minimum change amount by a power of 2. 6.

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