JP2008224995A - Image forming apparatus and image forming method - Google Patents

Abstract

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, in which a proper bias voltage is stably applied to each charger with a simple device configuration. The technique which can be applied is provided.
An AC voltage output from a single AC voltage generator 2321 and a DC voltage generator 2325Y, 2325M, 2325C for each of charging rollers 231Y, 231M, and 231K corresponding to Y, M, and C colors. A charging bias superimposed with the DC voltage output from each is applied. A current detection resistor 2326Y is connected in series to the charging roller 231Y and the like, and a direct current flowing through the charging roller 231Y and the like is detected. Based on the current detection result, the CPU 101 gives a control signal to the DC voltage generator 2325Y and the like so as to output an output voltage obtained by adding the voltage drop due to the series resistor 2324Y and the like to the voltage applied to the charging roller 231Y and the like.
[Selection] Figure 5

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 generators corresponding to the respective units to the AC voltage generators via relatively high resistance series resistors. However, in this case, a 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 Patent Document 1, such a problem is not considered at all, and there is room for improvement in the above-described prior art in this respect.

  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 drop of the series resistance corresponding to the charger obtained based on the detection result of the direct current flowing through the charger. It is characterized in that a value.

  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. Further, the voltage drop due to the series resistance is compensated by outputting a voltage obtained by adding the voltage drop estimated from the detection result of the DC current flowing through the charger to the target voltage from the DC bias generating means. As a result, according to the present invention, it is possible to stably apply an appropriate bias voltage to each charger while having a simple device configuration.

  In the image forming apparatus configured as described above, the image forming apparatus further includes a current detection unit for individually detecting a direct current flowing through each of the chargers, and the control unit is configured based on a detection result of the current detection unit. You may make it obtain | require the voltage drop in each of each series resistance. Thus, the current flowing through each charger can be detected, and the voltage drop due to the series resistance can be obtained based on the detection result.

  For example, the current detection means includes a plurality of current detection resistors provided corresponding to the plurality of chargers, and each of the plurality of current detection resistors corresponds to the charger and the charger. It may be inserted in a series circuit including DC bias generating means. Thereby, the current flowing through the charger can be easily detected as the terminal voltage of the current detection resistor.

  Further, the control means outputs a predetermined DC voltage for testing from each of the DC bias generating means, and based on the current detection result of the DC current flowing through the charger corresponding to the DC bias generating means at that time. The setting process for setting the voltage value of the DC voltage output from the DC bias generating means may be executed. Thus, by detecting the current value when a known voltage is applied and setting the output voltage of the DC bias generating means based on the current detection result, the charging bias can be set appropriately.

  The image forming apparatus according to the present invention further includes an image carrier that circulates in a predetermined direction, and the plurality of image forming stations are arranged along a moving direction of the image carrier, and the plurality of image carriers are arranged. Each of the image forming stations may be configured to transfer toner images formed by executing the image forming operation onto the image carrier at transfer positions different from each other. In this way, it is possible to superimpose the toner images formed at each image forming station on the image carrier. For example, a single color toner image formed with a different toner color from each image forming station can be image-carrying. A full-color image superimposed on the body can be formed.

  In this case, the control means outputs a predetermined test DC voltage from each DC bias generating means for each of the DC bias generating means, and at that time, the charger corresponding to the DC bias generating means. The setting processing for setting the voltage value of the DC voltage output from the DC bias generating means based on the current detection result of the DC current flowing through the DC current may be executed at different timings. In an image forming apparatus in which a plurality of image forming stations are arranged along the moving direction of the image carrier, the operation timing of each image forming station is slightly different corresponding to the difference in position. Therefore, by allowing the setting process to be executed at different timings, the setting process can be executed at an appropriate timing according to the progress of the operation of each image forming station.

  For example, the control means performs the setting process for each of the DC bias generating means when the image forming station having the charger corresponding to the DC bias generating means is not performing the image forming operation. It may be configured to execute. By performing the setting process while avoiding the image forming operation, it is possible to avoid the change in the DC voltage generated by the DC bias generating means from affecting the image.

  Further, in the setting process, the AC bias generating means may generate the same AC voltage as that during execution of the image forming operation. By doing so, it is possible to detect the current flowing through the charger in a state closer to that during the image forming operation, so that the charging bias can be set more appropriately.

  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.

  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. In an image forming method for forming an image by developing each electrostatic latent image with toner, in order to achieve the above object, a plurality of DC bias generating means for generating a DC voltage are provided corresponding to each of the plurality of chargers. In addition, the DC voltage from each of the DC bias generating means is superimposed on the AC voltage from the AC bias generating means for generating an AC voltage via a series resistor, and is applied as a charging bias to each of the chargers. For each of the bias generation means, the voltage value of the DC voltage output from the DC bias generation means is set to the charger corresponding to the DC bias generation means. It is set to a voltage value obtained by adding a target voltage set in advance as a voltage to be applied and a voltage drop of the series resistor corresponding to the charger obtained based on a detection result of a direct current flowing through the charger. It is said.

  In the invention configured as described above, similarly to the above-described invention of the image forming apparatus, it is possible to stably apply an appropriate bias voltage to each charger with a simple apparatus configuration.

  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 the photosensitive member 21 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 and separate from 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.

  Furthermore, current detection resistors 2326Y, 2326M, 2326C, and 2326K are respectively connected to the series circuits including the DC voltage generators 2325Y, 2325M, 2325C, and 2325K and the charging rollers 231Y, 231M, 231C, and 231K, respectively. It is inserted. A voltage proportional to the direct current flowing through the charging rollers 231Y, 231M, 231C, and 231K is generated between the terminals of the current detection resistors 2326Y, 2326M, 2326C, and 2326K. Each of these voltages is input to a multi-channel AD converter (ADC) 103. The multi-channel ADC 103 converts the input analog voltage value into a digital value and outputs it to the CPU 101. By doing so, in this embodiment, the CPU 101 can individually detect the direct currents flowing through the charging rollers 231Y, 231M, 231C, and 231K. The CPU 101 sets the value of the control signal given to the multi-channel DAC 102 based on the value of the current flowing through each charging roller obtained in this way, so that the DC voltage output from each DC voltage generator 2325Y, 2325M, 2325C and 2325K. Control the value.

  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. 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.

  A current detection resistor 2326Y is interposed between the other end of the secondary winding of the transformer T1 and the ground potential. Then, the terminal voltage of the current detection resistor 2326Y is input to the AD converter 103. Thus, when the DC voltage generator 2326Y or the like is configured by a DC-DC converter, the secondary winding of the transformer T1 for taking out the output voltage is floated with respect to the primary side, so that the load (charging roller) is A current detection resistor can be interposed between the ground terminal opposite to the connected load side and the ground potential. In this way, current detection can be performed with little effect on the load side. Further, it is possible to detect only the direct current flowing through the charging roller 231Y to be controlled without being affected by the current flowing through the other charging rollers or the alternating current.

  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.

  FIG. 7 is a flowchart showing a setting process for controlling the output voltage of the DC voltage generator. The CPU 101 controls the DC voltage values output from the DC voltage generators 2325Y, 2325M, 2325C, and 2325K by executing the setting process shown in FIG. 7 for each toner color. Here, the setting process for the charging bias applied to the charging roller 231Y corresponding to the Y toner color will be described, but the contents of the setting process are the same for the other toner colors.

  In this setting process, in order to prevent the toner from being consumed unnecessarily during the process, the exposure by the line head 29 and the development by the developing unit 25 are stopped (step S101), and the charging roller 231Y is instructed. A test charging bias voltage is applied (step S102). The test charging bias preferably has a waveform similar to that of the charging bias used at the time of image formation in order to perform setting processing under conditions similar to those at the time of image formation. That is, the test charging bias applied here is obtained by superimposing an AC voltage on a DC voltage corresponding to the charging roller 231Y.

Then, the DC current (charging current) flowing through the charging roller 231Y at this time is detected as the terminal voltage of the current detection resistor 2326Y (step S103), and the DC voltage generator 2325Y is detected from the value of the charging current thus detected. A voltage value corresponding to a voltage drop due to the connected series resistance 2324Y is calculated (step S104). The voltage value Vdrop corresponding to the voltage drop is expressed as follows: Rs is the resistance value of the series resistor connected to the DC voltage generator 2325Y, Rd is the resistance value of the current detection resistor 2326Y, and Vd is the detected terminal voltage. The following formula:
Vdrop = Vd × Rs / Rd
It can ask for. The resistance value Rd of the current detection resistor 2326Y is set to such a value that the charging current in the assumed current range can be converted into a DC voltage of 5 V or less and output. Specifically, it is desirable that the resistance value Rd of the current detection resistor 2326Y be a value sufficiently smaller than the resistance value of the series resistor 2324Y.

  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 order to apply a desired DC voltage to the charging roller 231Y, the voltage output from the DC voltage generator 2325Y needs to be a value obtained by adding a voltage drop to the voltage value to be applied to the charging roller 231Y. is there. That is, a DC voltage to be applied to the charging roller 231Y is set as a target voltage (step S106), and a voltage value obtained by adding a voltage drop due to the target voltage and the series resistance 2324Y is output from the DC voltage generator 2325Y. A reference voltage value to be output from the multichannel DAC 102 is determined (step S106). Then, by outputting a control signal corresponding to the set value of the reference voltage to the multi-channel DAC 102 (step S107), the DC voltage generator 2325Y adds the voltage drop due to the series resistor 2324Y to the target voltage. Is output, and a desired DC voltage, that is, a 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.

  The above-described charging bias setting process 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 setting process is performed when the image forming operation is not executed in all the image forming stations, for example, immediately after the apparatus is turned on or when the apparatus is in a standby state where no image forming instruction is given. It is desirable. Further, in order to maintain good image quality, it is desirable that the charging bias setting process is executed every certain number of image formations or every certain 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 setting process is executed, the throughput of image formation is reduced. In order to prevent this, for example, the following can be performed.

  FIG. 8 is a timing chart showing the execution timing of the setting process. This figure shows the operation timing of each part of the apparatus when two full-color images are continuously formed by the operation of taking two pages (forming images of two pages on the transfer belt 81). 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 the charging operation is not performed at any image forming station and there is no timing at which the setting process can be performed. . In such a case, as shown in the figure, when the charging operation corresponding to the image forming station is not performed at each image forming station, the charging bias setting process for the charging roller is performed. Can be done.

  As a result, the charging bias setting process for each toner color is started at different timings. In this case, the setting process for one toner color may be performed during the execution of the charging operation for another toner color. Therefore, there is a concern that changing the DC voltage of the charging bias by the setting process may affect 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.

  In addition, a DC current flowing through each charging roller 231Y is detected by a current detection resistor 2326Y, and a voltage drop due to the series resistance 2324Y is obtained from the detection result, and a DC voltage obtained by adding the voltage drop to the target voltage is generated for each DC voltage. Therefore, a desired DC voltage can be applied to each charging roller 231Y and the like without being affected by a voltage drop due to the series resistance.

  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, while the resistor 2326Y and the like for current detection correspond to the “current detecting means of the present invention. And “current detection resistor”. In this embodiment, the transfer belt 81 functions as the “image carrier” of the present invention, and the primary transfer position TR1y corresponding to each image forming station corresponds to the “transfer position” of the present invention. Yes.

  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.

  Further, when the charging bias setting process is performed, when switching from the charging bias for image forming operation to the charging bias for testing or vice versa, the bias output may be interrupted once, or the bias output may be You may make it switch a voltage, continuing. For example, the charging bias may be functioned as a test charging bias by extending the application of the charging bias for forming the electrostatic latent image until after the latent image is formed.

  Further, for example, the charging of the photoconductor for forming the electrostatic latent image and the setting process may be performed simultaneously. That is, while charging the photosensitive member by applying a charging bias for forming an electrostatic latent image, the charging current flowing through the charging roller at this time is detected, and the subsequent charging bias is determined based on the result. May be.

  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. The flowchart which shows the setting process which controls the output voltage of a DC voltage generator. The timing chart which shows the execution timing of a setting process.

Explanation of symbols

  2Y, 2M, 2C ... image forming station (image forming station), 2K ... image forming station (image forming station for single operation), 21 ... photoconductor, 81 ... transfer belt (image carrier), 101 ... CPU (control means) 231 ... Charging roller (charger), 231K ... Charging roller (single operation charger), 2321YMC ... AC voltage generator (AC bias generating means), 2322YMC ... Transformer (AC bias generating means), 2324Y, 2324M, 2324C... Resistance (series resistance), 2325Y, 2325M, 2325C... DC voltage generator (DC bias generation means), 2326Y, 2326M, 2326C... (For current detection) resistance (current detection means, current detection resistance), 2321K. AC voltage generator (Bias generation means for single operation) 2322K ... trans (independent operation for bias generating means), 2325K ... DC voltage generator (independent operation for bias generating means), TR1y, TR1m, TR1c ... primary transfer position (transfer position)

Claims (10)

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 for performing an image forming operation for forming an image by developing the image with toner;
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, respectively. ,
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,
In each of the plurality of chargers, a DC voltage output from the DC bias generation unit corresponding to the charger via the series resistor corresponding to the charger, and an 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. An image forming apparatus comprising: a target voltage and a voltage value obtained by adding a voltage drop of the series resistor corresponding to the charger obtained based on a detection result of a direct current flowing through the charger.   Further comprising current detection means for individually detecting a direct current flowing through each of the chargers, wherein the control means obtains a voltage drop in each of the series resistors based on a detection result of the current detection means. The image forming apparatus according to 1.   The current detection means includes a plurality of current detection resistors provided corresponding to the plurality of chargers, and each of the plurality of current detection resistors includes the charger and the DC bias corresponding to the charger. The image forming apparatus according to claim 2, wherein the image forming apparatus is interposed in a series circuit including a generating unit.   The control means outputs a predetermined test DC voltage from each of the DC bias generation means, and at the time, based on the current detection result of the DC current flowing through the charger corresponding to the DC bias generation means, 4. The image forming apparatus according to claim 1, wherein a setting process for setting a voltage value of a DC voltage output by the DC bias generating unit is executed. Further comprising an image carrier that moves around in a predetermined direction,
The plurality of image forming stations are arranged along the moving direction of the image carrier, and each of the plurality of image forming stations transfers toner images formed by executing the image forming operation to each other. The image forming apparatus according to claim 1, wherein the image forming apparatus is configured to transfer the image onto the image carrier at a position.   The control means outputs a predetermined test DC voltage from each DC bias generating means for each of the DC bias generating means, and at that time, a DC current flowing through the charger corresponding to the DC bias generating means The image forming apparatus according to claim 5, wherein setting processing for setting a voltage value of a DC voltage output from the DC bias generating unit is executed at different timings based on the current detection result.   The control means executes the setting process for each of the DC bias generating means when the image forming station having the charger corresponding to the DC bias generating means is not performing the image forming operation. The image forming apparatus according to claim 6.   8. The image forming apparatus according to claim 4, wherein in the setting process, the AC bias generating unit generates the same AC voltage as that during execution of the image forming operation.   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. The image according to any one of claims 1 to 8, further comprising an image forming station for single operation, and a bias generating unit 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 photoreceptors is charged to a predetermined surface potential by a charger provided corresponding to each of the plurality of photoreceptors to form an electrostatic latent image, and each of the electrostatic latent images 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. Applied to each charger as a charging bias superimposed on the AC voltage from
For each of the DC bias generators, a voltage value of a DC voltage output by the DC bias generator is set in advance as a target voltage to be applied to the charger corresponding to the DC bias generator, and An image forming method comprising: setting a voltage value obtained by adding a voltage drop of the series resistor corresponding to the charger obtained based on a detection result of a direct current flowing through the charger.

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