JP2008224994A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology to stably apply an appropriate bias voltage to each charging device with a simple apparatus configuration, regarding an image forming apparatus having a plurality of image forming stations including a photoreceptor and the charging device to charge the photoreceptor with a prescribed surface potential and an image forming method thereof. <P>SOLUTION: A charging bias obtained by superimposing an AC voltage output from a single AC voltage generator 2321 and a DC voltage output from each of DC voltage generators 2325Y, 2325M and 2325C is applied to each of charging rollers 231Y, 231M and 231K corresponding to colors (Y, M and C). A value obtained by adding the estimated value of a voltage drop by a series resistance 2324Y or the like to the voltage applied to the charging roller 231Y or the like is stored in a lookup table 105 to set an output voltage of the DC voltage generator 2325Y or the like. Since the charging current is increased with the increase in the cumulative operation amount of a photoreceptor, the estimated amount of the voltage drop is increased. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus having a plurality of image forming stations having a photosensitive member and a charger for charging the photosensitive member to a predetermined surface potential, and an image forming method thereof. It relates to a technique for providing a bias.

  In an image forming apparatus that forms an electrostatic latent image by charging the surface of the photosensitive member to a predetermined surface potential and visualizes the electrostatic latent image to form an image, in order to efficiently charge the surface of the photosensitive member, Some chargers for charging a photosensitive member are configured to apply a charging bias in which a DC voltage and an AC voltage are superimposed. For example, in the image forming apparatus described in Patent Document 1, four toner image forming units corresponding to four toner colors of yellow (Y), magenta (M), cyan (C), and black (K) are used. Four types of DC voltage and two types of AC voltage are generated. A bias formed by superimposing one type of AC voltage on one type of DC voltage is supplied to the K toner image forming unit, while another type of AC voltage is superimposed on each of the three types of DC voltages. Supplied to Y, M and C toner image forming units.

JP 2006-220955 A (FIG. 3)

  The configuration of the apparatus can be simplified by adopting a configuration in which a bias generation circuit is shared by a plurality of units as in the above-described conventional technology. However, in this case, measures are required to prevent interference between the units. As such a countermeasure, for example, it is conceivable to connect the DC voltage generator corresponding to each unit to the AC voltage generator via a series resistor having a relatively high resistance. However, if this is done, voltage loss due to series resistance occurs due to the direct current flowing through the charger, and there is a possibility that a desired bias voltage cannot be applied to the photoreceptor. In particular, it is known that the magnitude of the charging current flowing from the charger to the photoconductor greatly varies with time depending on the degree of deterioration of the photoconductor, but Patent Document 1 completely considers such a problem. In this respect, there is still room for improvement in the above prior art.

  The present invention has been made in view of the above problems, and includes a charging bias in which a plurality of image forming stations having a photoreceptor and a charger for charging the photoreceptor to a predetermined surface potential are provided, and a DC voltage and an AC voltage are superimposed on the charger. In the image forming apparatus and the image forming method for providing the image forming apparatus, it is an object to provide a technique capable of stably applying an appropriate bias voltage to each charger while having a simple apparatus configuration.

  In order to achieve the above object, the image forming apparatus according to the present invention has a photoreceptor and a charger for charging the surface of the photoreceptor to a predetermined surface potential, and the photoreceptor is charged by the charger. A plurality of image forming stations for forming an electrostatic latent image on the surface and visualizing the electrostatic latent image with toner to form an image, and corresponding to each of the plurality of chargers A plurality of DC bias generating means for generating a DC voltage as a DC component of a charging bias to be provided to the charger, and provided corresponding to each of the plurality of chargers. A plurality of series resistors electrically connected to the DC bias generating means corresponding to the charger in a one-to-one relationship, and an AC component of the charging bias applied to each charger as an AC component. AC bias generating means for generating a voltage, and control means for controlling the voltage value of the DC voltage output from each of the DC bias generating means, each of the plurality of chargers corresponding to the charger A direct current voltage output from the direct current bias generation means via the series resistor corresponding to the charger and an alternating current voltage output from the alternating current bias generation means are superimposed and applied as the charging bias, and the control means For each of the DC bias generating means, a voltage value of a DC voltage output from the DC bias generating means is set as a target voltage set in advance as a voltage to be applied to the charger corresponding to the DC bias generating means. And a voltage corresponding to a voltage drop generated in the series resistor corresponding to the charger due to the direct current flowing through the charger. Set to a voltage value, moreover, a voltage value corresponding to the voltage drop, is characterized in that corrected in accordance with the operation history of the photoreceptor charging by the charger corresponding to the DC bias generator.

  The image forming method according to the present invention forms an electrostatic latent image by charging each of the photoconductors to a predetermined surface potential using a charger provided corresponding to each of the plurality of photoconductors. An image forming method for forming an image by developing each electrostatic latent image with toner, and in order to achieve the above object, a DC bias generating means for generating a DC voltage is provided for each of the plurality of chargers. A plurality of DC bias generators are provided, and a DC voltage from each of the DC bias generators is superimposed on an AC voltage from an AC bias generator that generates an AC voltage via a series resistor, and is applied to each charger as a charging bias. For each of the bias generation means, the voltage value of the DC voltage output from the DC bias generation means is applied to the charger corresponding to the DC bias generation means. Set to a voltage value obtained by adding a target voltage set in advance as a voltage and a voltage corresponding to a voltage drop generated in the series resistance corresponding to the charger due to a direct current flowing in the charger. The voltage value corresponding to the voltage drop is corrected in accordance with the operation history of the photoconductor charged by the charger corresponding to the DC bias generating means.

  In the invention thus configured, the AC voltage output from the AC bias generating means is superimposed on each of the DC voltages from the plurality of DC bias generating means and applied to each charger corresponding to the DC bias generating means. Is done. For this reason, the AC bias generating means can be shared among the plurality of chargers, so that the apparatus configuration can be simplified and the apparatus can be reduced in size and cost. In this case, since the output voltage from each DC bias generating means is superimposed on the output voltage from the AC bias generating means via a series resistor, interference between the DC bias generating means and from the DC bias generating means. Interference with the AC bias generating means is suppressed. In addition, since the voltage corresponding to the voltage drop due to the series resistance is added to the DC voltage generated from the DC bias generating means, each charger is appropriately charged without being affected by the voltage drop due to the series resistance. A bias will be applied. In addition, the voltage drop due to the series resistance is corrected according to the operation history of the photoconductor, so that even if the current flowing through the charger fluctuates as a result of its characteristics changing over time due to operation of the photoconductor, the effect Can prevent the charging bias from fluctuating.

  In the image forming apparatus configured as described above, for example, the control unit has in advance a correction table that associates the operation history of the photoconductor with the corresponding output voltage of the DC bias generation unit. The output voltage value of the DC bias generating means may be set based on the correction table. This makes it possible to easily set the output voltage of the DC bias generating means at that time by referring to the correction table based on the operation history of the photosensitive member, and a special process for obtaining the voltage drop. And control becomes unnecessary.

  The operation history of the photoconductor can be expressed by parameters that change monotonously with the operation of the photoconductor, such as the operation time and the number of images formed. Based on such parameters, the operation history of the photoreceptor can be appropriately determined. Also, a combination of these parameters may be used.

  In addition, for example, when each of the image forming stations can selectively execute a plurality of operation modes having different process speeds as the image forming operation, the control unit performs the operation according to the operation mode. You may make it vary the correction amount by the said correction | amendment. Between operation modes with different process speeds, the amount of charge transferred to the photoconductor per time is different, so the amount of current flowing through the charger is also different. Therefore, the magnitude of the voltage drop due to the series resistance also changes. Therefore, by making the correction amount of the voltage for guaranteeing the voltage drop due to the series resistance also different depending on the operation mode, it is possible to perform more appropriate correction according to the mode of the image forming operation.

  In general, as the operation of the photoconductor progresses and the film thickness decreases due to wear, the amount of current flowing from the charger to the photoconductor tends to increase even when the same voltage is applied. Therefore, the control means may increase the absolute value of the output voltage of the DC bias generating means as the operation amount of the photoconductor determined based on the operation history increases. By so doing, it is possible to compensate for an increase in the current flowing through the charger over time and an increase in voltage drop due to the series resistance, so that a predetermined charging bias can be stably applied to the charger.

  In addition, the image forming apparatus according to the present invention includes a single operation photoreceptor and a single operation charger for charging the surface of the single operation photoreceptor to a predetermined surface potential separately from the plurality of image forming stations. And forming an electrostatic latent image on the surface of the single operation photosensitive member charged by the single operation charger in a state where the plurality of image forming stations are not operated, and visualizing the electrostatic latent image. A single operation image forming station for performing a single image forming operation for forming a monochrome image, and a single operation bias generating means for applying a charging bias in which a DC voltage and an AC voltage are superimposed on the single operation charger. May be configured.

  In the image forming apparatus configured as described above, it is possible to form a monochrome image by executing a single image forming operation in which only the single image forming station is operated. In addition, by independently providing a single operation bias generating means for applying a charging bias to the single operation charger provided in the single color operation image forming station, when executing the single image forming operation, other images are provided. Application of the charging bias to the forming station can be stopped.

  FIG. 1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a diagram showing the configuration of the main part of the image forming station in the image forming apparatus of FIG. This apparatus uses a color mode in which four color toners of yellow (Y), magenta (M), cyan (C) and black (K) are superimposed to form a color image, and only black (K) toner. Thus, the image forming apparatus can selectively execute a monochrome mode for forming a monochrome image. In this image forming apparatus, when an image forming command is given from an external device such as a host computer to a main controller having a CPU, a memory, etc., the main controller gives a control signal to the engine controller, and based on this, the engine controller Each part of the apparatus such as the part EG is controlled to execute a predetermined image forming operation, and an image corresponding to the image forming command is formed on a sheet as a recording material such as copy paper, transfer paper, paper, and an OHP transparent sheet.

  In the housing main body 3 of the image forming apparatus according to this embodiment, an electrical component box 5 containing a power circuit board, a controller board, and the like is provided. An image forming unit 2, a transfer belt unit 8, and a paper feed unit 7 are also disposed in the housing body 3. In FIG. 1, a secondary transfer unit 12, a fixing unit 13 and a sheet guide member 15 are disposed on the right side in the housing body 3. The paper feed unit 7 is configured to be detachable from the housing body 3. The paper feeding unit 7 and the transfer belt unit 8 can be removed and repaired or exchanged.

  The image forming unit 2 includes four image forming stations 2Y (for yellow), 2M (for magenta), 2C (for cyan) and 2K (for black) that form a plurality of images of different colors. In FIG. 1, since the image forming stations of the image forming unit 2 have the same configuration, only some of the image forming stations are denoted by reference numerals for convenience of illustration, and the reference numerals are omitted for other image forming stations. To do.

  Each of the image forming stations 2Y, 2M, 2C, and 2K is provided with a drum-shaped photoconductor 21 on which a toner image of each color is formed. Each photoconductor 21 is connected to a dedicated drive motor (not shown) and is driven to rotate at a predetermined speed in the direction of arrow D21 in the figure. A charging unit 23, a line head 29, a developing unit 25, a static elimination light source 27, and a photoconductor cleaner 28 are disposed around the photoconductor 21 along the rotation direction. Then, a charging operation, a latent image forming operation, and a toner developing operation are executed by these functional units. When the color mode is executed, the toner images formed in all the image forming stations 2Y, 2M, 2C, and 2K are superimposed on the transfer belt 81 provided in the transfer belt unit 8 to form a color image. When the monochrome mode is executed, only the image forming station 2K is operated to form a black monochrome image.

  FIG. 3 is a diagram illustrating a configuration of the charging unit. The charging unit 23 includes a charging roller 231 whose surface is made of a metal material such as iron, aluminum, or stainless steel. Rollers 234 made of an insulating material are respectively attached to both ends of the charging roller 231, and the rollers 234 come into contact with the surface of the photosensitive member 21, so that the charging roller 231 and the surface of the photosensitive member 21 are in contact with each other. Is provided with a certain gap GP. Further, one end portion of the sliding terminal 233 made of an elastic conductive plate material such as stainless steel or phosphor bronze is slidably contacted with the end face of the charging roller 231 and the other end portion of the sliding terminal 233 is charged. Connected to the bias generator 232, the charging bias voltage from the charging bias generator 232 is applied to the charging roller 231 via the sliding terminal 233. As a result, the surface of the photoreceptor 21 is charged to a predetermined surface potential.

  Returning to FIG. 1, the description of the apparatus configuration will be continued. The line head 29 includes a plurality of light emitting elements arranged in the axial direction of the photoconductor 21 (direction X perpendicular to the paper surface of FIG. 1), and is disposed to face the photoconductor 21. Then, light L is emitted from these light emitting elements toward the surface of the photoreceptor 21 charged by the charging unit 23 to form an electrostatic latent image on the surface.

  The developing unit 25 has a developing roller 251 on which toner is carried. Then, the charged toner is applied to the developing position where the developing roller 251 and the photosensitive member 21 come into contact with each other by the developing bias applied to the developing roller 251 from a developing bias generating unit (not shown) electrically connected to the developing roller 251. The electrostatic latent image formed on the surface of the photosensitive member 21 by moving from the developing roller 251 is visualized.

  The toner image visualized at the development position is conveyed in the rotation direction D21 of the photosensitive member 21, and then is primarily applied to the transfer belt 81 at a primary transfer position TR1 where the transfer belt 81 described later and each photosensitive member 21 abut. Transcribed.

  Further, on the downstream side of the primary transfer position TR1 in the rotation direction D21 of the photosensitive member 21 and on the upstream side of the charging unit 23, the photosensitive member 21 is in contact with the surface of the static electricity removing light source 27 and the photosensitive member 21 toward the photosensitive member 21. 28 are provided in this order. The neutralization light source 27 resets the surface potential of the photosensitive member 21 by irradiating the surface of the photosensitive member 21 after the primary transfer with the neutralizing light Le, and the photosensitive member cleaner 27 comes into contact with the surface of the photosensitive member so that it is exposed after the primary transfer. The toner remaining on the surface of the body 21 is removed by cleaning. The surface of the photoconductor 21 from which the charge has been removed by removing the toner is conveyed again to the position facing the charging roller 231, charged by the charging unit 23, and used for forming an electrostatic latent image.

  The transfer belt unit 8 includes a driving roller 82, a driven roller 83 (blade facing roller) disposed on the left side of the driving roller 82 in FIG. 1, and an arrow D81 illustrated in FIG. And a transfer belt 81 that is circulated in the direction (conveyance direction). The transfer belt unit 8 is disposed inside the transfer belt 81 in a one-to-one relationship with each of the photoreceptors 21 included in the image forming stations 2Y, 2M, 2C, and 2K when the cartridge is mounted. Primary transfer rollers 85Y, 85M, 85C and 85K are provided. Each of these primary transfer rollers is electrically connected to a primary transfer bias generator (not shown).

  FIG. 4 is a diagram showing the primary transfer position. When the color mode is executed, as shown in FIG. 4A, all the primary transfer rollers 85Y, 85M, 85C, and 85K are positioned on the image forming stations 2Y, 2M, 2C, and 2K, so that the transfer belt 81 is imaged. The primary transfer positions TR1y, TR1m, TR1c, and TR1k are formed between the photoconductors 21 and the transfer belt 81 by pushing them into contact with the photoconductors 21 of the forming stations 2Y, 2M, 2C, and 2K. Then, by applying a primary transfer bias from the primary transfer bias generating unit to the primary transfer roller 85Y or the like at an appropriate timing, the toner images formed on the surface of each photoconductor 21 are transferred to the corresponding primary transfer positions. Transfer onto the surface of the transfer belt 81. That is, in the color mode, the single color toner images of the respective colors are superimposed on the transfer belt 81 to form a color image.

  On the other hand, when executing the monochrome mode, as shown in FIG. 4B, among the four primary transfer rollers, the primary transfer rollers 85Y, 85M, and 85C are separated from the image forming stations 2Y, 2M, and 2C facing each other. In addition, only the primary transfer roller 85K corresponding to the black color is brought into contact with the image forming station 2K, so that only the monochrome image forming station 2K is brought into contact with the transfer belt 81. As a result, the primary transfer position TR1k is formed only between the primary transfer roller 85K and the image forming station 2K. Then, by applying a primary transfer bias from the primary transfer bias generator to the primary transfer roller 85K at an appropriate timing, a black toner image formed on the surface of the photoreceptor 21 provided in the image forming station 2K is converted into a primary toner image. A transfer image is transferred to the surface of the transfer belt 81 at the transfer position TR1k to form a monochrome image.

  Further, the transfer belt unit 8 includes a downstream guide roller 86 disposed on the downstream side of the black primary transfer roller 85K and on the upstream side of the driving roller 82. The downstream guide roller 86 is on a common tangent line between the primary transfer roller 85K and the black photoconductor 21 (K) at the primary transfer position TR1 formed by the primary transfer roller 85K contacting the photoconductor 21 of the image forming station 2K. 2 is configured to abut against the transfer belt 81.

  A patch sensor 89 is provided opposite to the surface of the transfer belt 81 wound around the downstream guide roller 86. The patch sensor 89 is composed of, for example, a reflection type photosensor, and optically detects a change in the reflectance of the surface of the transfer belt 81, so that the position and density of the patch image formed on the transfer belt 81 as necessary. Is detected.

  The sheet feeding unit 7 includes a sheet feeding unit having a sheet feeding cassette 77 capable of stacking and holding sheets and a pickup roller 79 that feeds sheets one by one from the sheet feeding cassette 77. The sheet fed from the sheet feeding unit by the pickup roller 79 is adjusted in sheet feeding timing by the registration roller pair 80, and then the drive roller 82 and the secondary transfer roller 121 abut along the sheet guide member 15. Paper is fed to the secondary transfer position TR2.

  The secondary transfer roller 121 is provided so as to be able to come into contact with and separate from the transfer belt 81 and is driven to come into contact with a secondary transfer roller drive mechanism (not shown). The fixing unit 13 includes a heating roller 131 that includes a heating element such as a halogen heater and is rotatable, and a pressure unit 132 that presses and biases the heating roller 131. The sheet on which the image is secondarily transferred is guided to the nip formed by the heating roller 131 and the pressure belt 1323 of the pressure unit 132 by the sheet guide member 15, and in the nip, a predetermined value is formed. The image is heat-fixed at temperature. The pressure unit 132 includes two rollers 1321 and 1322 and a pressure belt 1323 stretched between them. A nip portion formed by the heating roller 131 and the pressure belt 1323 is formed by pressing the belt tension surface stretched by the two rollers 1321 and 1322 out of the surface of the pressure belt 1323 against the peripheral surface of the heating roller 131. Is configured to be widely taken. Further, the sheet thus subjected to the fixing process is conveyed to a paper discharge tray 4 provided on the upper surface portion of the housing body 3.

  The drive roller 82 circulates and drives the transfer belt 81 in the direction of the arrow D81 in the figure, and also serves as a backup roller for the secondary transfer roller 121. A rubber layer having a thickness of about 3 mm and a volume resistivity of 1000 kΩ · cm or less is formed on the peripheral surface of the drive roller 82, and secondary transfer is omitted by grounding through a metal shaft. A conductive path of a secondary transfer bias supplied from the bias generation unit via the secondary transfer roller 121 is used. Thus, by providing the driving roller 82 with a rubber layer having high friction and shock absorption, image quality deterioration caused by transmission of the impact to the transfer belt 81 when the sheet enters the secondary transfer position TR2. Can be prevented.

  Further, in this apparatus, a cleaner portion 71 is disposed to face the blade facing roller 83. The cleaner unit 71 includes a cleaner blade 711 and a waste toner box 713. The cleaner blade 711 removes foreign matters such as toner and paper dust remaining on the transfer belt 81 after the secondary transfer by bringing the tip of the cleaner blade 711 into contact with the blade facing roller 83 via the transfer belt 81. The foreign matter removed in this way is collected in a waste toner box 713. Further, the cleaner blade 711 and the waste toner box 713 are integrally formed with the blade facing roller 83.

  In this embodiment, the image forming stations 2Y, 2M, 2C, and 2K of the photoconductor 21, the charging roller 231, the developing unit 25, the static elimination light source 27, and the photoconductor cleaner 28 are integrated into a unit as a cartridge. Yes. The cartridge is configured to be detachable from the apparatus main body. Each cartridge is provided with a nonvolatile memory for storing information related to the cartridge. Based on these pieces of information, the usage history of each cartridge and the lifetime of consumables are managed.

  FIG. 5 is a diagram showing an electrical configuration of the charging unit. In this figure, reference numerals 231Y, 231M, 231C, and 231K are assigned to distinguish the four charging rollers 231 corresponding to the Y, M, C, and K toner colors, respectively.

  The charging bias generator 232 includes AC voltage generators 2321K and 2321YMC and DC voltage generators 2325Y, 2325M, 2325C, and 2325K controlled by the CPU 101 that controls the operation of the entire apparatus. More specifically, a control signal for determining a voltage value generated from each voltage generator is output from the CPU 101 to the multi-channel DA converter (DAC) 102, and the multi-channel DAC 102 applies an analog voltage corresponding to the control signal to each voltage. Output to the generator. Each of the voltage generators 2321K, 2321YMC, 2325Y, 2325M, 2325C, and 2325K outputs an AC voltage or a DC voltage having a voltage value corresponding to the reference voltage using the input analog voltage as a reference voltage. The AC voltage generator 2321Y and the like can be configured by, for example, an inverter or a high-frequency amplifier that amplifies a reference clock given from the outside. Further, the DC voltage generator 2325Y and the like can be configured by, for example, a DC-DC converter.

  Among these, the AC voltage output from the AC voltage generator 2321K is boosted by the transformer 2322K, and the boosted AC voltage is superimposed on the DC voltage output from the DC voltage generator 2325K via the series resistor 2325K. In this way, the charging bias voltage formed by superimposing the AC voltage and the DC voltage is applied to the charging roller 231K. A DC blocking capacitor 2323K is inserted in the conductive path from the secondary side of the transformer 2322K to the mixing point with the DC voltage.

  The DC voltage applied to the charging roller 231K is a negative voltage of about (−500) V, for example, and determines the charging potential of the photosensitive member 21. On the other hand, the AC voltage applied to the charging roller 231K is, for example, a sinusoidal AC voltage having a peak-to-peak voltage of about 1500 V and a frequency of about 1 to 2 kHz, and is not directly related to the charging potential of the photosensitive member 21, but the charging roller 231K By causing a discharge in the gap GP with the photoconductor 21, the charge transfer to the photoconductor 21 is promoted, and the photoconductor 21 is charged efficiently.

  On the other hand, the AC voltage output from AC voltage generator 2321YMC is boosted by transformer 2322YMC, and then input to charging rollers 231Y, 231M, and 231C corresponding to Y, M, and C toner colors, respectively. That is, when viewed from the AC voltage generator 2321YMC, the charging rollers 231Y, 231M, and 231C are connected in parallel as loads.

  More specifically, the charging roller 231Y corresponding to the yellow image forming station 2Y is connected to the AC voltage output from the transformer 2322YMC via the DC blocking capacitor 2323Y and the DC voltage generator 2325Y via the series resistor 2324Y. Is applied as a charging bias in a superimposed manner. Similarly, the charging bias applied to the charging roller 231M corresponding to magenta color includes the AC voltage output from the transformer 2322YMC via the DC blocking capacitor 2323M and the DC voltage generator 2325M via the series resistor 2324M. The output DC voltage is superimposed. Further, the charging bias applied to the charging roller 231C corresponding to the cyan color is output from the transformer 2322YMC via the DC blocking capacitor 2323C and from the DC voltage generator 2325C via the series resistor 2324C. DC voltage to be superimposed.

  By connecting each charging roller 231Y, 231M and 231C to the transformer 2322YMC via capacitors 2323Y, 2323M and 2323C, respectively, the DC voltage applied to each charging roller affects the bias circuit to the other charging rollers. Can be suppressed. Further, by connecting a series resistor to each of the DC voltage generators 2325Y, 2325M, and 2325C, it is possible to prevent the AC voltage from entering the DC voltage generator.

  As described above, in this embodiment, the AC voltage generator 2321YMC and the transformer 2322YMC are common to the three colors Y, M, and C, thereby reducing the size and cost of the apparatus. On the other hand, the DC voltage generators 2325Y, 2325M, and 2325C are provided independently for the respective charging rollers, so that the charging bias is individually set and the charging potential of the photosensitive member 21 is independently set for each toner color. It is possible to set.

  For the charging roller 231K corresponding to the black color used alone in the monochrome mode, a bias generation circuit independent of the other toner colors is provided. By doing so, it is possible to stop the application of bias to the charging roller 231Y and the charging rollers 231M and 231C which are not related to the operation when the monochrome mode is executed. As a result, it is possible to reduce power consumption when the monochrome mode is executed, and to suppress the deterioration of the photoreceptor 21 provided in each of the image forming stations 2Y, 2M, and 2C, thereby extending the life of these. Can do.

  FIG. 6 is a diagram illustrating a circuit example of a DC voltage generator. The DC voltage generator 2325Y and the like described above are configured by a DC-DC converter as shown in FIG. 6, for example. Here, the structure of the DC voltage generator 2325Y for yellow will be described as an example, but the structures of the DC voltage generators 2325M, 2325C, and 2325K corresponding to other toner colors are the same. Further, since the circuit configuration itself of the DC-DC converter shown in FIG. 6 is known, only the outline of the configuration and operation will be briefly described here.

  In the DC voltage generator 2325Y, the transistor Q1 and the transformer T1 constitute a self-excited oscillator. Then, the AC voltage appearing at one end of the secondary winding of the transformer T1 is rectified by the diode D1 and the capacitor C1 and output as a DC voltage. The other end of the secondary winding of the transformer T1 is grounded. Thereby, for example, a DC voltage of 24V is boosted to a predetermined high voltage (for example, −500V). This DC high voltage is applied to the charging roller 231Y as the DC component of the charging bias through the series resistor 2324Y as described above. As shown in FIG. 6, the direction of the rectifying diode D1 is a direction in which a negative voltage is applied to the charging roller 231Y. As a result, the photosensitive member 21 is charged to a negative potential.

  Further, a voltage proportional to the output voltage taken out from another winding of the transformer T1 is rectified by the diode D2 and the capacitor C2, and appropriately divided by the resistor R2, and is input as a feedback signal to the error amplifier Q2. A reference voltage output from the multi-channel DAC 102 is given to the error amplifier Q2 based on a control signal from the CPU 101, and the error amplifier Q2 outputs an error signal obtained by comparing this reference voltage with a feedback signal. This error signal adjusts the injection power to the base of the transistor Q1 via the current amplifier Q3 and the driver transistor Q4, and thereby the output voltage is controlled to a voltage proportional to the reference voltage.

  Further, the DC voltage generator 2325Y is provided with a switch element Q5 that operates based on a control signal supplied to turn on / off the high-voltage output from the CPU 101. The switch element Q5 is connected to the output of the error amplifier Q2. When the control signal for turning on the high voltage output is given from the CPU 101, the switch element Q5 is opened to form the feedback control loop described above. When a control signal for turning off the high-voltage output is given from, the output of the error amplifier Q2 is grounded and forced to zero potential so that no high voltage is output.

  Next, the setting of the output voltage of each DC voltage generator 2325Y, 2325M, 2325C, and 2325K by the CPU 101 will be described. In this embodiment, when an image forming command is given from an external device, the CPU 101 controls each image forming station to execute an image forming operation. Prior to that, a direct current output from each direct current voltage generator 2325Y or the like is executed. Set the voltage value. In this embodiment, together with the CPU 101, storage means (RAM) 104 for storing information representing the operation history of the photoconductor 21 provided in each image forming station, the operation history of the photoconductor, the set voltage of the DC voltage generator, Are associated with each other, and the CPU 101 refers to the look-up table 105 based on the information related to the operation history of the photosensitive member 21 stored in the RAM 104 and outputs the output of the DC voltage generator. Set the voltage. Specifically, when an image formation command is given from an external device, the CPU 101 executes the following operation. Hereinafter, control of the image forming station 2Y for yellow will be described, but the same can be applied to other image forming stations.

  FIG. 7 is a flowchart showing the operation when an image formation command is given. When an image formation command is given, the CPU 101 reads out the cumulative operation amount of the photosensitive member 21 of the image forming station 2Y that is to execute the image forming operation to be used from the RAM 104 that stores the information (step S101). The cumulative operation amount of the photoconductor 21 can be represented by a physical quantity that monotonously increases or decreases according to the change in the operation history of the photoconductor. As such a physical quantity, for example, the total operation time of the photoconductor 21, The cumulative number of rotations, the cumulative number of image formations, etc. can be used. Moreover, you may combine these suitably. Then, based on the read operation amount of the photosensitive member, the lookup table 105 is referred to determine the output voltage from the DC voltage generator 2325Y (step S102).

  FIG. 8 is a diagram illustrating an example of a lookup table. More specifically, FIG. 8A is a lookup table applied when executing a normal image forming operation (normal mode). Further, FIG. 8B is applied when an image forming operation is performed in a low speed mode in which a process proceeds at a lower speed in order to form a higher definition image or to form an image on a thick recording material. Lookup table. In these look-up tables, the output voltage value of the DC voltage generator 2325Y corresponding to the combination of the target voltage to be applied to the photoconductor 21 and the cumulative operation amount of the photoconductor (photoconductor operation amount) is tabulated. Yes.

  Here, the photosensitive member operating amount is shown as a percentage when the value of the operating amount that is considered as the lifetime is 100%. For example, when the photoconductor is new, the operation amount is 0%, and when the cumulative operation amount of the photoconductor reaches the design life, the operation amount is evaluated as 100%. In addition, the target voltage is not constant, and is appropriately set within a predetermined variable range based on a temperature measurement result at room temperature or a result of a known concentration control operation that will not be described.

  In this embodiment, as shown in FIG. 5, a series resistance 2324Y is connected to the DC voltage generator 2325Y, and when a charging current flows through the DC circuit from the DC voltage generator 2325Y to the charging roller 231Y, the series resistance Because of the voltage drop due to 2324Y, the DC voltage actually applied to the charging roller 231Y is the voltage output from the DC voltage generator 2325Y minus this voltage drop. In general, the direct current flowing through the charging roller is about 100 μA, but if the resistance value of the series resistance is set to 1 MΩ, for example, the voltage drop becomes 100 V and cannot be ignored.

  Therefore, in the look-up table, a voltage obtained by adding a voltage corresponding to a voltage drop due to the series resistor 2324Y inserted between the DC voltage generator 2325Y and the charging roller 231Y to the target voltage to be applied to the photoconductor 21. Values are listed. The CPU 101 determines the voltage value described in the column where the voltage value to be applied to the photoconductor 21 and the photoconductor operating amount at that time intersect as the output voltage of the DC voltage generator 2325Y at that time. By doing so, a DC voltage with a voltage drop caused by the series resistor 2324Y added is output from the DC voltage generator 2325Y, and a target voltage is applied to the charging roller 231Y.

  Further, in view of the knowledge that the charging current increases when the film thickness of the photoconductor 21 decreases due to wear as the photoconductor 21 is used, the absolute value of the voltage increases as the photoconductor operating amount increases, that is, the absolute value of the output voltage. The table has been created to increase. Further, in the low-speed mode where the process speed is low, the charging operation to the photoconductor 21 is performed at a slower speed, so that the amount of charge flowing into the photoconductor 21 per unit time is small, so the charging current is also small. Become. In consideration of this, the table corresponding to the low speed mode shown in FIG. 8B estimates the voltage drop due to the series resistance 2324Y less than the table corresponding to the normal mode shown in FIG. .

  Returning to FIG. 7, the description will be continued. The CPU 101 outputs a control signal corresponding to the output voltage thus determined to the multi-channel DAC 102 (step S103). As a result, the DC voltage generator 2325Y outputs a DC voltage obtained by adding the voltage drop due to the series resistor 2324Y to the target voltage, and a desired DC voltage, that is, the target voltage is applied to the charging roller 231Y. .

  The multi-channel DAC 102 preferably has a latch function for input data, that is, once a digital value corresponding to the reference voltage is given, that value is retained unless rewritten by the CPU 101. For this purpose, a general-purpose digital tuning DA converter can be used. By doing so, the amount of transmission data from the CPU 101 to the multi-channel DAC 102 can be reduced, and the CPU 101 does not need to constantly monitor the reference voltage, and thus the processing is simplified.

  In a state where a desired charging bias can be applied to the charging roller 231Y in this manner, the image forming station 2Y is caused to execute an image forming operation to form an image (step S104). When the image formation is completed, the operation amount of the photoconductor 21 in the image forming operation is calculated, and the accumulated operation amount stored in the RAM 104 is updated and stored (step S105).

  The charging bias setting change described above is desirably executed at a timing different from the execution of the image forming operation. This is because if the charging bias is changed during the image forming operation, the surface potential of the photoconductor 21 may change, thereby affecting the image density and image quality. Therefore, the charging bias is changed when the image forming operation is not executed in all the image forming stations, for example, immediately after the power of the apparatus is turned on or in a standby state where no image forming instruction is given to the apparatus. It is desirable that In order to maintain good image quality, it is desirable to set the charging bias for every fixed number of image formations or every fixed time.

  However, the image forming apparatus of this embodiment is a so-called tandem color image forming apparatus in which the image forming stations 2Y, 2M, 2C, and 2K are arranged in a line along the moving direction of the transfer belt 81. In such an image forming apparatus, the image forming operation by each image forming station can be performed in parallel, but due to the difference in position, the progress timing of the operation is slightly shifted. Therefore, for example, when a large number of color images are continuously formed, no image is formed in any of the image forming stations. To be more precise, a photosensitive for forming an electrostatic latent image is formed. There is not always a timing when the charging operation of the body 21 is not performed. In such a case, if all the image forming stations are stopped and the charging bias is set, the image forming throughput is lowered. In order to prevent this, for example, the following can be performed.

  FIG. 8 is a timing chart showing the execution timing of the charging bias setting. As shown in FIG. 8, the timing at which the charging operation is performed at each image forming station is slightly shifted, and there is a case where there is no timing at which the charging operation is not performed at any image forming station. In such a case, as shown in the figure, when the charging operation corresponding to the image forming station is not performed in each image forming station, the charging bias change setting for the charging roller is performed. Can be done.

  As a result, the setting of the charging bias for each toner color is started at different timings. In this case, the setting for one toner color may be performed during the execution of the charging operation for the other toner color. Therefore, there is a concern that changing the DC voltage of the charging bias affects the charging operation for other toner colors being executed. However, it is rarely necessary to greatly change the charging bias during continuous use of the apparatus, and the amount of change in the DC voltage is not so large, so that the possibility of such a problem is extremely low.

  As described above, in this embodiment, the AC voltage generator 2321YMC that generates an AC component of the charging bias applied to the charging rollers 231Y, 231M, and 231C used only in the color mode is shared between these charging rollers. As a result, the size and cost of the apparatus are reduced. A DC voltage generator for generating a DC component of the charging bias is provided for each charging roller, so that the surface potential of the photoconductor 21 for each toner color corresponding to each charging roller can be independently set. It is possible to set.

  For the DC voltage output from each DC voltage generator 2325Y, etc., a voltage corresponding to a voltage drop caused by a series resistor 2324Y etc. connected in series to the voltage generator is applied to the target voltage to be applied to the charging roller 231Y, etc. The added value. Therefore, a desired DC voltage can be applied to each charging roller 231Y and the like without being affected by the voltage drop due to the series resistance.

  The voltage drop due to the series resistor 2324Y or the like is estimated to increase as the cumulative operating amount of the photoconductor 21 increases in view of the increase in charging current as the photoconductor 21 is used. More specifically, a look-up table in which the accumulated operating amount and target voltage of the photosensitive member are associated with a voltage value obtained by adding a corresponding voltage drop is prepared in advance, and a DC voltage generator is generated based on this table. Determine the output voltage from. By doing so, the voltage drop of the series resistance caused by the charging current can be canceled without having a configuration for detecting the charging current, and the change in the charging current with time can be dealt with. Can do.

  In addition, for the black color that is used alone in the monochrome mode, by providing a bias generation circuit independently, it is not necessary to charge the other photoconductors 21 when the monochrome mode is executed. be able to.

  As described above, in this embodiment, each of the image forming stations 2Y, 2M, and 2C functions as an “image forming station” of the present invention, and the photosensitive member 21 and the charging roller 231 provided in each image forming station. Respectively function as the “photoreceptor” and “charger” of the present invention. The AC voltage generator 2321YMC and the transformer 2322YMC function as an “AC bias generating unit” of the present invention, while the DC voltage generators 2325Y, 2325M, and 2325C function as the “DC bias generating unit” of the present invention. is doing. In this embodiment, the CPU 101 functions as the “control unit” of the present invention.

  In this embodiment, the resistor 2324Y and the like connected in series to each DC voltage generator 2325Y and the like correspond to the “series resistor” of the present invention, and the lookup table 105 corresponds to the “correction table” of the present invention. ing.

  In this embodiment, the black image forming station 2K used alone in the monochrome mode corresponding to the “single image forming operation” of the present invention corresponds to the “image forming station for single operation” of the present invention. The photoconductor 21 and the charging roller 231K provided in the image forming station 2K function as the “single operation photoconductor” and the “single operation charger” of the present invention, respectively. An AC voltage generator 2321K that applies a charging bias to the charging roller 231K, a transformer 2322K, a DC voltage generator 2325K, and the like integrally function as the “single operation bias generating means” of the present invention.

  The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, in the above embodiment, a bias generation circuit for applying a charging bias to the black image forming station 2K is provided independently of the other colors, but the bias generation circuit is shared for all colors. Also good.

  The image forming apparatus according to the above-described embodiment is a so-called tandem type image forming apparatus in which four image forming stations each having a photoconductor are arranged side by side along the moving direction of the intermediate transfer belt 81. The present invention also relates to a so-called rotary type image forming apparatus in which a plurality of developing devices are mounted on a rotatable developing rotary, and these are selectively positioned at positions facing the photosensitive member to form an image. Can be applied.

  In addition, the image forming apparatus of the above-described embodiment is an image forming apparatus including a drum-shaped photoconductor, but the electrostatic latent image carrier of the present invention is not limited to such a drum-shaped member, for example, a belt-shaped member. The photoreceptor may be used.

  Furthermore, in the above embodiment, the present invention is applied to a color image forming apparatus using YMCK four-color toner, but the application target of the present invention is not limited to this, and the types of colors and the number of colors The present invention can also be applied to different image forming apparatuses.

1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a diagram illustrating a main part of an image forming station in the image forming apparatus of FIG. 1. The figure which shows the structure of a charging part. The figure which shows a primary transfer position. The figure which shows the electrical constitution of a charging part. The figure which shows the circuit example of a DC voltage generator. 6 is a flowchart showing an operation when an image formation command is given. The figure which shows the example of a lookup table. The timing chart which shows the execution timing of charging bias setting.

Explanation of symbols

  2Y, 2M, 2C ... image forming station (image forming station), 2K ... image forming station (single-operation image forming station), 21 ... photoconductor, 81 ... transfer belt, 101 ... CPU (control means), 105 ... look Up table (correction table), 231 ... charge roller (charger), 231K ... charge roller (charger for single operation), 2321YMC ... AC voltage generator (AC bias generation means), 2322YMC ... transformer (AC bias generation means) 2324Y, 2324M, 2324C, resistors (series resistors), 2325Y, 2325M, 2325C, DC voltage generator (DC bias generating means), 2321K, AC voltage generator (bias generator for single operation), 2322K, transformer (single) Operation bias generation means) 2325K ... DC voltage generator (independent operation for bias generation means)

Claims (8)

Each of the photosensitive member and a charger for charging the surface of the photosensitive member to a predetermined surface potential is formed, and an electrostatic latent image is formed on the surface of the photosensitive member charged by the charger, and the electrostatic latent image is A plurality of image forming stations that perform an image forming operation of forming an image by developing a toner image;
A plurality of DC bias generating means provided corresponding to each of the plurality of chargers and generating a DC voltage as a DC component of a charging bias applied to the charger;
A plurality of series resistors provided corresponding to each of the plurality of chargers and electrically connecting the plurality of chargers and the DC bias generating means corresponding to the chargers in a one-to-one relationship; ,
AC bias generating means for generating an AC voltage as an AC component of the charging bias applied to each charger;
Control means for controlling the voltage value of the DC voltage output by each of the DC bias generating means,
Each of the plurality of chargers outputs a DC voltage output from the DC bias generation unit corresponding to the charger via the series resistor corresponding to the charger, and is output from the AC bias generation unit. AC voltage is superimposed and applied as the charging bias,
The control means presets the voltage value of the DC voltage output from the DC bias generating means as a voltage to be applied to the charger corresponding to the DC bias generating means for each of the DC bias generating means. Set to a voltage value obtained by adding a target voltage and a voltage corresponding to a voltage drop generated in the series resistance corresponding to the charger due to a direct current flowing in the charger;
An image forming apparatus, wherein a voltage value corresponding to the voltage drop is corrected according to an operation history of the photosensitive member charged by the charger corresponding to the DC bias generating unit.   The control means has in advance a correction table associating the operation history of the photosensitive member with the corresponding output voltage of the DC bias generating means, and based on the correction table, the output voltage of the DC bias generating means. The image forming apparatus according to claim 1, wherein a value is set.   The image forming apparatus according to claim 1, wherein the control unit determines an operation history of the photoconductor based on an operation time.   The image forming apparatus according to claim 1, wherein the control unit determines an operation history of the photoconductor based on a number of image formations. Each of the image forming stations can selectively execute a plurality of operation modes having different process speeds as the image forming operation,
The image forming apparatus according to claim 1, wherein the control unit varies a correction amount by the correction according to the operation mode.   6. The image forming apparatus according to claim 1, wherein the control unit increases the absolute value of the output voltage of the DC bias generation unit as the operation amount of the photoconductor determined based on the operation history increases. apparatus.   Separately from the plurality of image forming stations, a single operation photosensitive member and a single operation charger for charging the surface of the single operation photosensitive member to a predetermined surface potential, and operating the plurality of image forming stations. A single image forming operation for forming a monochrome image by forming an electrostatic latent image on the surface of the single operation photosensitive member charged by the single operation charger in a state in which the electrostatic latent image is visualized. 7. The image according to claim 1, further comprising an image forming station for single operation, and bias generating means for single operation for applying a charging bias in which a DC voltage and an AC voltage are superimposed on the single operation charger. Forming equipment. Each of the photosensitive members is charged to a predetermined surface potential by a charger provided corresponding to each of the plurality of photosensitive members to form an electrostatic latent image, and the electrostatic latent image is visualized with toner. In an image forming method for forming an image by
A plurality of DC bias generating means for generating a DC voltage corresponding to each of the plurality of chargers, and an AC bias generating means for generating an AC voltage from each DC bias generating means via a series resistor. Is applied to each charger as a charging bias superimposed on the AC voltage from
For each of the DC bias generating means, a voltage value of a DC voltage output from the DC bias generating means is set as a target voltage set in advance as a voltage to be applied to the charger corresponding to the DC bias generating means, and Set to a voltage value obtained by adding a voltage corresponding to a voltage drop generated in the series resistance corresponding to the charger due to a direct current flowing in the charger, and
An image forming method, wherein a voltage value corresponding to the voltage drop is corrected according to an operation history of the photosensitive member charged by the charger corresponding to the DC bias generating means.

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