JP2017097032A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that can set the surface potentials of a plurality of photoreceptor drums to a target potential with a simple configuration.SOLUTION: An image condition adjustment part 53 executes a charging bias adjustment operation of adjusting the potentials of background parts of a plurality of photoreceptor drums 20 to a target potential V0. The image condition adjustment part 53 forms a plurality of toner images on a peripheral surface of a specific photoreceptor drum 20 from among the photoreceptor drums 20 in a plurality of units in an image forming part 13, and determines the value of the charging bias corresponding to the target potential V0 of the specific photoreceptor drum 20 from a result of measurement of the densities of the plurality of toner images performed by a density sensor 65. The image condition adjustment part 53 derives the value of the charging bias corresponding to the target potential V0 in the other photoreceptor drums 20 other than the specific photoreceptor drum 20 on the basis of the value of the determined charging bias.SELECTED DRAWING: Figure 7

Description

  The present invention relates to an image forming apparatus that forms an image on a sheet.

  2. Description of the Related Art Conventionally, as an image forming apparatus such as a printer or a copying machine adopting an electrophotographic system, an apparatus including a photosensitive drum, a charging device, an exposure device, a developing device, and a transfer device is known. The charging device uniformly charges the peripheral surface of the photosensitive drum. The exposure apparatus irradiates the photosensitive drum with exposure light according to image information to form an electrostatic latent image. The developing device supplies toner to the photosensitive drum and develops the electrostatic latent image into a toner image. The transfer device transfers the toner image from the photosensitive drum to a sheet or an intermediate transfer belt.

  In such an image forming apparatus, in order to obtain a good image, the surface potential of the photosensitive drum needs to be set to a desired potential. In particular, when the charging device includes a charging roller that rotates while being in contact with the surface of the photosensitive drum, the surface potential of the photosensitive drum is likely to fluctuate due to environmental fluctuations even when the voltage applied to the charging roller is the same. . In a charging roller in which an ionic conductive agent is blended, the resistance value of the roller is likely to fluctuate depending on the environment or the like.

  Patent Document 1 discloses an image forming apparatus provided with a surface potentiometer facing a peripheral surface of a photosensitive drum. The surface potential of the photosensitive drum is set to a desired potential by feeding back the potential measurement result of the surface potential meter to the applied voltage of the charging device.

JP 09-106142 A

  In the technique described in Patent Document 1, since the image forming apparatus includes a surface electrometer for the photosensitive drum, a space for arranging the electrometer in the image forming apparatus is required, and the cost of the image forming apparatus increases. There was a problem to do.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of setting the surface potentials of a plurality of photosensitive drums to target potentials with a simple configuration. .

  An image forming apparatus according to one aspect of the present invention includes an apparatus main body, a plurality of color units that are disposed in the apparatus main body and that form a color toner image, and a black unit that forms a black toner image. A transfer unit that transfers the color toner image and the black toner image from the image forming unit to a sheet, and the plurality of color units and the black unit of the image forming unit include: Each has a peripheral surface on which an electrostatic latent image including a background portion and an image portion is formed, and is in contact with or close to the photosensitive drum rotated in a predetermined rotation direction and the peripheral surface of the photosensitive drum. And a charging device that charges the peripheral surface to a predetermined potential, and a developing roller that faces the photosensitive drum, and supplies toner to the photosensitive drum. A developing device that develops the electrostatic latent image into a toner image, and charging the background portion of the electrostatic latent images of the plurality of photosensitive drums to a predetermined target potential. A bias adjustment unit that executes a bias adjustment operation; and a density measurement unit that measures the density of the toner image. The bias adjustment unit includes the plurality of image forming units in the charge bias adjustment operation. A plurality of toner images are formed on a peripheral surface of a specific photoconductor drum among the photoconductor drums of the unit, and the specific photoconductor drum is obtained from the density measurement results of the plurality of toner images measured by the density measurement unit. The charging bias value corresponding to the target potential is determined, and a photosensitive drum other than the specific photosensitive drum is applied to the photosensitive drum based on the determined charging bias value. Kicking characterized by deriving the value of the charging bias corresponding to the target potential.

  According to this configuration, the surface potential of the plurality of photosensitive drums can be set to the target potential with a simple configuration and in a short time.

  In the above configuration, a storage unit that stores information on replacement of the photosensitive drums of the plurality of units of the image forming unit, and a number of printed sheets of the sheet onto which the toner image is transferred are set to the plurality of photosensitive units. A count unit that counts for each drum, and the bias adjustment unit has a maximum number of printed sheets when any of the plurality of photosensitive drums is replaced during the charging bias adjustment operation. The charging drum value corresponding to the target potential is determined using the photosensitive drum having the smallest number of printed drums and the photosensitive drum as the specific photosensitive drum, and the charging bias value is determined based on the determined charging bias value. It is desirable to derive a value of the charging bias corresponding to the target potential in a photosensitive drum other than the specific photosensitive drum.

  According to this configuration, when some of the photosensitive drums have been replaced, a plurality of photosensitive drums can be obtained by performing the charging bias adjustment operation on the photosensitive drums with the maximum and minimum number of printed sheets. It becomes possible to set the surface potential of the target potential.

  In the above configuration, the bias adjustment unit may be configured to select one of the plurality of units of the photosensitive drum when the charging bias adjustment operation has not been replaced. As the photosensitive drum, a value of the charging bias corresponding to the target potential is determined, and the determined charging bias value is determined to correspond to the target potential in the other photosensitive drums of the plurality of units. It is desirable to use a charging bias.

  According to this configuration, when none of the photoconductor drums has been replaced, the surface potential of the plurality of photoconductor drums is set to the target potential only by performing the charging bias adjustment operation on one photoconductor drum. It becomes possible to set.

  In the above-described configuration, the transfer device includes the intermediate transfer belt on which the color toner image and the black toner image are superimposed and transferred from the plurality of photosensitive drums, and the plurality of photosensitive members sandwiching the intermediate transfer belt. A plurality of primary transfer rollers that respectively contact the body drum; and a contact / separation mechanism that collectively contacts and separates the primary transfer roller corresponding to the color unit among the plurality of primary transfer rollers with respect to the photosensitive drum. And the bias adjusting unit, when none of the plurality of photosensitive drums has been replaced during the charging bias adjustment operation, the photosensitive drum of the black unit and the plurality of colors The photosensitive drum of one unit of the units is set as the specific photosensitive drum, and each unit corresponds to the target potential. It is desirable to determine a value of the charging bias and use the charging bias determined in the one unit as the charging bias corresponding to the target potential in the other photosensitive drums of the plurality of color units. .

  According to this configuration, when any of the photosensitive drums is not replaced, the charging bias adjustment operation is performed only on the photosensitive drum of the black unit and the photosensitive drum of the color unit. It becomes possible to set the surface potential of the plurality of photosensitive drums to the target potential.

  In the above configuration, the image forming apparatus further includes: a charging bias applying unit that applies a predetermined charging bias to the charging device; and a developing bias applying unit that applies a predetermined developing bias to the developing roller. In the charging bias adjustment operation, the charging bias applying unit is controlled to form a plurality of potential regions having different potential magnitudes along the rotation direction on the peripheral surface of the photosensitive drum, and the developing bias By applying the predetermined developing bias corresponding to the target potential to the developing roller by controlling the applying unit, a plurality of toner images are formed by a potential difference between the developing bias and the plurality of potential regions, and the plurality of toner images are formed. From the relationship between the density measurement result of the toner image and the plurality of charging biases corresponding to the plurality of potential regions, the density of the toner image is zero. Deriving a value of the charging bias to be, it is desirable to determine the derived charging bias as the charging bias corresponding to the target potential.

  According to this configuration, it is utilized that the density of the toner image becomes zero when the development bias and the surface potential of the photosensitive drum coincide with each other with reference to the development bias for which the actual output voltage can be easily grasped. ing. For this reason, it is possible to realize surface potential setting with a simple configuration and a small error range.

  According to the present invention, there is provided an image forming apparatus capable of setting the surface potential of a plurality of photosensitive drums to a target potential with a simple configuration.

1 is a cross-sectional view illustrating an internal structure of an image forming apparatus according to an embodiment of the present invention. 2 is an electrical block diagram of a control unit of the image forming apparatus according to the embodiment of the present disclosure. FIG. It is the schematic diagram which showed the electric potential relationship in the charging bias adjustment operation | movement which concerns on the 1st Embodiment of this invention. 3 is a flowchart of a charging bias adjustment operation according to the first embodiment of the present invention. 6 is a graph showing the relationship between the toner image density and the charging bias in the charging bias adjustment operation according to the first embodiment of the present invention. It is a schematic diagram (A), (B) and (C) showing a pattern of a plurality of potential regions in the charging bias adjustment operation according to a modified embodiment of the present invention. 6 is a flowchart of inter-unit correction in the charging bias adjustment operation according to the first embodiment of the present invention. It is a flowchart of the correction | amendment between units of the charging bias adjustment operation | movement which concerns on the 2nd Embodiment of this invention. It is the schematic diagram which showed the electric potential relationship in the charging bias adjustment operation | movement which concerns on the 3rd Embodiment of this invention. It is a flowchart of the charging bias adjustment operation | movement which concerns on the 4th Embodiment of this invention. It is the schematic diagram which showed the electric potential relationship in the charging bias adjustment operation | movement which concerns on the 4th Embodiment of this invention. It is the schematic diagram which showed the electric potential relationship in the charging bias adjustment operation | movement which concerns on the deformation | transformation embodiment of this invention. It is a flowchart of the charging bias adjustment operation | movement which concerns on the 5th Embodiment of this invention. It is the schematic diagram which showed the electric potential relationship in the charging bias adjustment operation | movement which concerns on the 5th Embodiment of this invention. It is the schematic diagram which showed the electric potential relationship in the charging bias adjustment operation | movement which concerns on the deformation | transformation embodiment of this invention. It is a flowchart of the calibration operation | movement which concerns on embodiment of this invention.

  Hereinafter, an image forming apparatus 10 according to an embodiment of the present disclosure will be described in detail with reference to the drawings. In this embodiment, a tandem color printer is illustrated as an example of an image forming apparatus. The image forming apparatus may be, for example, a copying machine, a facsimile machine, and a complex machine of these.

  FIG. 1 is a cross-sectional view showing the internal structure of the image forming apparatus 10. The image forming apparatus 10 includes an apparatus main body 11 having a box-shaped housing structure. In the apparatus main body 11, a sheet feeding unit 12 that feeds the sheet P, an image forming unit 13 that forms a toner image to be transferred to the sheet P fed from the sheet feeding unit 12, and the toner image are primarily transferred. Intermediate transfer unit 14, secondary transfer roller 145, toner replenishing unit 15 for replenishing toner to the image forming unit 13, and fixing unit for performing a process of fixing an unfixed toner image formed on the sheet P to the sheet P 16 are decorated. Further, a discharge unit 17 that discharges the sheet P that has been subjected to the fixing process by the fixing unit 16 is provided on the upper portion of the apparatus main body 11.

  At an appropriate position on the upper surface of the apparatus main body 11, an unillustrated operation panel for inputting an output condition for the sheet P and the like is provided. This operation panel is provided with a power key, a touch panel for inputting output conditions, and various operation keys. In the apparatus main body 11, a sheet conveying path 111 extending in the vertical direction is further formed on the right side of the image forming unit 13. The sheet conveyance path 111 is provided with a conveyance roller pair 112 that conveys a sheet to an appropriate position. A registration roller pair 113 that corrects the skew of the sheet and feeds the sheet to a nip portion of secondary transfer described later at a predetermined timing is also provided on the upstream side of the nip portion in the sheet conveyance path 111. The sheet conveyance path 111 is a conveyance path that conveys the sheet P from the paper feeding unit 12 to the paper discharge unit 17 via the image forming unit 13 (secondary transfer nip unit) and the fixing unit 16.

  The paper feed unit 12 includes a paper feed tray 121, a pickup roller 122, and a paper feed roller pair 123. The sheet feeding tray 121 is detachably mounted at a lower position of the apparatus main body 11 and stores a sheet bundle P1 in which a plurality of sheets P are stacked. The pickup roller 122 feeds the uppermost sheet P of the sheet bundle P1 stored in the sheet feeding tray 121 one by one. The pair of paper feed rollers 123 sends out the sheet P fed out by the pickup roller 122 to the sheet conveyance path 111. The paper feeding unit 12 includes a manual paper feeding unit attached to the left side surface of the apparatus main body 11 shown in FIG. The manual paper feed unit includes a manual feed tray 124, a pickup roller 125, and a paper feed roller pair 126. The manual feed tray 124 is a tray on which the manually fed sheet P is placed, and is opened from the side surface of the apparatus main body 11 as shown in FIG. 1 when the sheet P is manually fed. The pickup roller 125 feeds out the sheet P placed on the manual feed tray 124. The pair of paper feed rollers 126 sends out the sheet P fed out by the pickup roller 125 to the sheet conveyance path 111.

  The image forming unit 13 forms a toner image to be transferred to the sheet P, and includes a plurality of image forming units that form toner images of different colors. As this image forming unit, in the present embodiment, a developer of magenta (M) color sequentially disposed from the upstream side to the downstream side in the rotation direction of the intermediate transfer belt 141 (from the left side to the right side in FIG. 1). Magenta unit 13M (color unit) using cyan, cyan unit 13C (color unit) using cyan (C) developer, yellow unit 13Y (color unit) using yellow (Y) developer ) And a black unit 13Bk using a black (Bk) developer. The magenta unit 13M, the cyan unit 13C, and the yellow unit 13Y form color toner images of different colors. The black unit 13Bk forms a black toner image. Each of the units 13M, 13C, 13Y, and 13Bk includes a photosensitive drum 20 (image carrier), a charging device 21, a developing device 23, and a cleaning device 25 disposed around the photosensitive drum 20. An exposure device 22 common to the units 13M, 13C, 13Y, and 13Bk is disposed below the image forming unit.

  The photosensitive drum 20 is rotationally driven around its axis in the direction of the arrow in FIG. 1 (predetermined rotational direction), and an electrostatic latent image and a toner image are formed on the peripheral surface thereof. The electrostatic latent image formed on the photosensitive drum 20 includes a background portion and an image portion according to image information. The rotating shaft of the photoconductor drum 20 extends in the front-rear direction (a direction orthogonal to the paper surface of FIG. 1). As the photosensitive drum 20, a photosensitive drum using an organic photoconductor (OPC) -based material can be used. Also, as shown in FIG. 1, the plurality of photosensitive drums 20 corresponding to the respective colors are arranged at predetermined intervals in the left-right direction (horizontal direction).

  The charging device 21 uniformly charges the peripheral surface of the photosensitive drum 20 to a predetermined potential. As the charging device 21, a charging device using a contact charging method can be adopted. The charging device 21 includes a charging roller 21A that is disposed and rotated in contact with the peripheral surface of the photosensitive drum 20, and a charging cleaning brush 21B for removing toner attached to the charging roller 21A. In other embodiments, the charging roller 21 </ b> A may be disposed close to the peripheral surface of the photosensitive drum 20. The exposure device 22 has various optical system devices such as a light source, a polygon mirror, a reflection mirror, and a deflection mirror, and irradiates light that has been modulated based on image data onto the circumferential surface of the uniformly charged photoreceptor drum 20. Thus, the aforementioned electrostatic latent image is formed. The cleaning device 25 cleans the peripheral surface of the photosensitive drum 20 after the toner image is transferred.

  The developing device 23 supplies toner to the peripheral surface of the photosensitive drum 20 in order to develop the electrostatic latent image formed on the photosensitive drum 20. The developing device 23 is for a two-component developer composed of toner and carrier. The toner of the developing device 23 is supplied to the peripheral surface of the photosensitive drum 20, and the electrostatic latent image is developed. The developing device 23 includes a developing roller 23C that is disposed to face the photosensitive drum 20, a magnetic roller 23B, and a pair of screws 23A. As the developing device 23, another configuration including the developing roller 23C may be applied. In this embodiment, the toner has a characteristic of being charged to a positive polarity.

  The intermediate transfer unit 14 is disposed in a space provided between the image forming unit 13 and the toner supply unit 15. The intermediate transfer unit 14 includes an intermediate transfer belt 141, a drive roller 142, a driven roller 143, a plurality of primary transfer rollers 24 (transfer rollers), and a belt cleaning device 144.

  The intermediate transfer belt 141 is an endless belt-like rotating body, and is stretched between the driving roller 142 and the driven roller 143 so that the circumferential surface side thereof is in contact with the circumferential surface of each photosensitive drum 20. The intermediate transfer belt 141 is driven to rotate in one direction along the second direction, and carries a color toner image and a black toner image transferred from the plurality of photosensitive drums 20 on the surface. The intermediate transfer belt 141 is a conductive soft belt having a laminated structure including a base layer, an elastic layer, and a coat layer.

  The drive roller 142 stretches the intermediate transfer belt 141 on the right end side of the intermediate transfer unit 14 and drives the intermediate transfer belt 141 to rotate. The driving roller 142 is a metal roller. The driven roller 143 is driven to rotate on the left end side of the intermediate transfer unit 14. The driven roller 143 stretches the intermediate transfer belt 141. The driven roller 143 applies tension to the intermediate transfer belt 141. In the vicinity of the driven roller 143, a belt cleaning device 144 (FIG. 1) that removes toner remaining on the peripheral surface of the intermediate transfer belt 141 is disposed.

  The primary transfer roller 24 is disposed to face the photosensitive drum 20 with the intermediate transfer belt 141 interposed therebetween. As a result, the primary transfer roller 24 abuts on the photosensitive drum 20 to form a primary transfer nip portion with the photosensitive drum 20, and the toner image on the photosensitive drum 20 is transferred onto the intermediate transfer belt 141. Primary transfer. As shown in FIG. 1, a primary transfer roller 24 is disposed to face the photosensitive drum 20 of each color. The primary transfer roller 24 is a roller extending in the front-rear direction and is rotated together with the intermediate transfer belt 141.

  The secondary transfer roller 145 is disposed to face the driving roller 142 with the intermediate transfer belt 141 interposed therebetween. The secondary transfer roller 145 is pressed against the peripheral surface of the intermediate transfer belt 141 to form a secondary transfer nip portion. The toner image primarily transferred onto the intermediate transfer belt 141 is secondarily transferred to the sheet P supplied from the paper feeding unit 12 at the secondary transfer nip portion. In the present embodiment, the intermediate transfer unit 14 and the secondary transfer roller 145 constitute a transfer device. The transfer device transfers the above-described color toner image and black toner image from the photosensitive drum 20 to the sheet P.

  The toner replenishing unit 15 stores toner used for image formation, and includes a magenta toner container 15M, a cyan toner container 15C, a yellow toner container 15Y, and a black toner container 15Bk in this embodiment. These toner containers 15M, 15C, 15Y, and 15Bk store replenishment toners for each color of MCYBk, respectively, and image forming units 13M, 13C, and 13C corresponding to each color of MCYBk from a toner discharge port 15H formed on the bottom surface of the container. The 13Y and 13Bk developing devices 23 are supplied with toner of each color through a toner transport unit (not shown).

  The fixing unit 16 includes a heating roller 161 having a heating source therein, a fixing roller 162 oriented with the heating roller 161, a fixing belt 163 stretched between the fixing roller 162 and the heating roller 161, and a fixing belt A pressure roller 164 which is disposed to face the fixing roller 162 via the H.163 and forms a fixing nip portion. The sheet P supplied to the fixing unit 16 is heated and pressurized by passing through the fixing nip portion. As a result, the toner image transferred to the sheet P at the secondary transfer nip is fixed to the sheet P.

  The paper discharge unit 17 is formed by recessing the top of the apparatus main body 11, and a paper discharge tray 171 for receiving the discharged sheet P is formed at the bottom of the concave portion. The sheet P on which the fixing process has been performed is discharged toward the discharge tray 151 via the sheet conveyance path 111 extending from the upper part of the fixing unit 16.

  FIG. 2 is an electrical block diagram of the control unit 50 of the image forming apparatus 10 according to the present embodiment. The image forming apparatus 10 includes a control unit 50 that comprehensively controls the operation of each unit of the image forming apparatus 10. The controller 50 includes a CPU (Central Processing Unit), a ROM (Read Only Memory) that stores a control program, a RAM (Random Access Memory) that is used as a work area of the CPU, and the like. In addition to the photosensitive drum 20, the charging device 21, the exposure device 22, the developing device 23, the primary transfer roller 24, and the like of the image forming unit 13, the control unit 50 includes a driving unit 61, a charging bias applying unit 62, A development bias applying unit 63, an environment sensor 64 (environment detection unit), a density sensor 65 (density measurement unit), and the like are electrically connected.

  The drive unit 61 includes a motor and a gear mechanism that transmits torque thereof, and rotates each member such as the image forming unit 13 and the intermediate transfer unit 145 according to a control signal from the drive control unit 51 described later.

  The charging bias application unit 62 includes a DC power source, and applies a predetermined charging bias to the charging roller 21 </ b> A of the charging device 21 based on a control signal from the bias control unit 52 described later.

  The developing bias applying unit 63 includes a DC power source and an AC power source, and applies a predetermined developing bias to the developing roller 23C and the magnetic roller 23B of the developing device 23 based on a control signal from the bias control unit 52.

  The environmental sensor 64 (FIG. 1) is provided in the apparatus main body 11. The environment sensor 64 detects the temperature and humidity inside the apparatus main body 11. In other embodiments, the environment sensor 64 may detect the temperature and humidity around the apparatus main body 11.

  The density sensor 65 (FIG. 1) detects the image density of the toner image transferred onto the intermediate transfer belt 141 and converts it into an electrical signal. The density sensor 65 includes a light emitting unit that emits light to the belt surface of the rotationally driven intermediate transfer belt 141 and a light receiving unit that receives reflected light from the belt surface (not shown). Information regarding the image density output from the density sensor 65 is referred to by an image condition adjustment unit 53 described later, and is reflected in a charging bias adjustment operation and a calibration operation described later.

  The control unit 50 functions to include a drive control unit 51, a bias control unit 52, an image condition adjustment unit 53, a storage unit 54, and a count unit 55 by the CPU executing a control program stored in the ROM. .

  The drive control unit 51 controls the drive unit 61 in accordance with an image forming operation of the image forming apparatus 10 and a charging bias adjustment operation and a calibration operation described later. The drive control unit 51 controls a drive mechanism (not shown) in addition to the drive unit 61 to drive other drive members in the image forming apparatus 10.

  Similarly, the bias control unit 52 controls the charging bias application unit 62 and the development bias application unit 63 according to the image forming operation, the charging bias adjustment operation, and the calibration operation of the image forming apparatus 10. The bias control unit 52 controls a bias application unit (not shown) in addition to the charging bias application unit 62 and the development bias application unit 63 and applies a predetermined bias to other members in the image forming apparatus 10. As an example, the bias controller 52 applies a primary transfer bias and a secondary transfer bias to the primary transfer roller 24 and the secondary transfer roller 145, respectively.

  The image condition adjustment unit 53 executes various image condition adjustment operations in the image forming apparatus 10. The image condition adjustment operation includes a charging bias adjustment operation and a calibration operation. In the charging bias adjustment operation, the image condition adjustment unit 53 adjusts the potential of the background portion of the electrostatic latent image on the photosensitive drum 20 to a predetermined target potential.

  The storage unit 54 stores various types of reference information referred to by the drive control unit 51, the bias control unit 52, and the image condition adjustment unit 53. As an example, the storage unit 54 stores potential information referred to in the charging bias adjustment operation. Further, the storage unit 54 stores information obtained by exchanging the photosensitive drums 20 of the plurality of units of the image forming unit 13 by maintenance workers or users, respectively. Specifically, an IC chip (not shown) is attached to the unit including the photosensitive drum 20. The IC chip can be transmitted to and received from an antenna (not shown) provided in the apparatus main body 11. By referring to the new product information stored in the IC chip, it is recognized that the photosensitive drum 20 is in an unreplaced state. When the photosensitive drum 20 is removed from the apparatus main body 11, the new product information is updated to the used information. Note that a maintenance worker of the image forming apparatus 10 may input replacement information of the photosensitive drum 20 to the storage unit 54 from an operation unit (not shown).

  The count unit 55 counts various pieces of accumulated information in the image forming operation and the image condition adjusting operation of the image forming apparatus 10. As an example, the count unit 55 may print the number of sheets to which a toner image is transferred, the sheet printing interval time (the time for which the image forming apparatus 10 is left), the cumulative number of rotations of the photosensitive drum 20, and the charging by the charging device 21. Count the cumulative application time of the bias. Further, the counting unit 55 counts the number of printed sheets P for each of the plurality of photosensitive drums 20 of the image forming unit 13. When only one of the plurality of photosensitive drums 20 is replaced, only the print sheet count value corresponding to the photosensitive drum 20 of the replaced unit is reset. The count result of the count unit 55 is stored in the storage unit 54 as needed.

<Charging bias adjustment operation>
Next, the charging bias adjustment operation according to the first embodiment of the present invention will be described. FIG. 3 is a schematic diagram showing a potential relationship between the photosensitive drum 20 and the developing roller 23C in the charging bias adjustment operation according to the present embodiment. FIG. 4 is a flowchart of the charging bias adjustment operation according to the present embodiment. FIG. 5 is a graph showing the relationship between the toner image density and the charging bias in the charging bias adjustment operation according to the present embodiment. As described above, in the present embodiment, the charging roller 21 </ b> A that rotates in contact with the circumferential surface of the photosensitive drum 20 is provided. In particular, in the present embodiment, an ionic conductive agent is blended in the charging roller 21A. Such an ion conductive type charging roller 21A has a characteristic that its resistance value is likely to change depending on environmental conditions such as temperature and humidity. Therefore, it is difficult to keep the surface potential of the photosensitive drum 20 constant. Become. In such a case, a known surface potentiometer is disposed opposite to the peripheral surface of the photosensitive drum 20, so that the charging bias applied to the charging roller 21A is feedback-controlled based on the measurement result of the surface potentiometer. It becomes possible to do. However, in this case, there arises a problem that the cost of the image forming apparatus 10 increases. In order to solve such a problem, in this embodiment, the photosensitive drum 20 is not provided with an electrometer for measuring the surface potential of the photosensitive drum 20, but is charged by a charging bias adjustment operation executed by the image condition adjusting unit 53. Is set to the target potential with high accuracy. In the present embodiment, as described later, after a charging bias adjustment operation is performed on a specific photoconductor drum 20, the result of the adjustment operation is applied to another photoconductor drum 20.

  With reference to FIGS. 3 and 4, the charging bias adjustment operation executed for a predetermined photosensitive drum 20 among the magenta unit 13M, the cyan unit 13C, the yellow unit 13Y, and the black unit 13Bk will be described. To do. Referring to FIG. 4, the charging bias adjustment operation includes formation of a band latent image (step S1), development of the latent image (step S2), density measurement of the band toner image (step S3), and determination of the charging bias (step S4). 4). The timing at which the charging bias adjustment operation is executed will be described in detail later.

  When the charging bias adjustment operation is executed, the image condition adjustment unit 53 executes the formation of a band latent image (step S1) in FIG. In order to form a good image in the image forming apparatus 10, a preset target potential of the background portion of the photosensitive drum 20 is defined as V0 (V). As described above, in this embodiment, the surface potential of the photosensitive drum 20 is not directly measured by an electrometer or the like. On the other hand, by controlling the input signal input from the bias control unit 52 to the charging bias application unit 62, the value of the charging bias applied to the charging roller 21A by the charging bias application unit 62 is controlled within a predetermined error range. Is possible. Therefore, in the charging bias adjustment operation, a charging bias value is derived such that the surface potential of the photosensitive drum 20 is V0 (V).

  In step S1, the image condition adjustment unit 53 refers to the charging bias Vref stored in the storage unit 54 (FIG. 2) in advance. The charging bias Vref is a value experimentally derived in advance so that the surface potential of the photosensitive drum 20 becomes V0 (V). In FIG. 2, the provisional potential of the background portion on the photosensitive drum 20 corresponding to the charging bias Vref is denoted as V0 (I). Then, the image condition adjusting unit 53 changes the charging bias by a plurality of levels using the charging bias Vref as a reference, and forms a predetermined latent image (a plurality of potential regions). In the present embodiment, four latent images are formed as shown in FIG. In particular, the image condition adjusting unit 53 uses a potential obtained by reducing the charging bias Vref by b (V) as the first charging bias. Further, a total of four latent images are formed by lowering the first charging bias by b (V) at equal intervals. Each latent image is applied for a time t1. In the present embodiment, a time t2 during which the charging bias Vref is applied is provided between the latent images.

  In this embodiment, b is set to 50 (V), t1 is set to 30 msec, and t2 is set to 60 msec. These values are stored in advance in the storage unit 54 (FIG. 2) and are referred to by the image condition adjustment unit 53. In this embodiment, the peripheral speed (system speed) of the photosensitive drum 20 is set to 166 mm / sec. Moreover, the range of preferable b is 10-100V, and 20-50V is still more preferable. Control is possible as long as the number of latent image levels is two or more, but it is preferable to provide three or more levels in order to improve the accuracy of the finally derived charging bias.

  In step S2, the image condition adjusting unit 53 sets the developing bias Vdc applied to the developing roller 23C to the above-described target potential V0, and then develops the band latent image formed in step S1. In other words, in FIG. 3, when the developing bias applied to the developing roller 23C during normal image forming operation (development) is Vdc (0), Vdc = V0 = Vdc (0) + a (V). A value is applied as the developing bias Vdc. In this embodiment, a = 100V is set, and the value of a is also stored in the storage unit 54 in advance. In addition, the range of the value of preferable a is 50-300V, and 100-200V is still more preferable. In the present embodiment, control is performed so that the potential difference (V0−Vdc (0)) between the potential V0 of the background portion of the photosensitive drum 20 and the developing bias Vdc (0) during the image forming operation is constant. In another embodiment, when the potential V0 of the background portion of the photosensitive drum 20 during the image forming operation is controlled to be constant, the value of a is not constant, and thus the value of V0 is set in step S2. You may set Vdc = V0 as it is. As shown in FIG. 3, a plurality of toner images (band toner images) are formed along the rotation direction on the peripheral surface of the photosensitive drum 20 due to the potential difference between the plurality of latent image potentials and the developing bias Vdc.

  In step S3, density measurement of the belt toner image formed in step S2 is executed. The toner image on the photosensitive drum 20 is transferred to the intermediate transfer belt 141 by a predetermined primary transfer bias applied to the primary transfer roller 24. The toner image carried on the intermediate transfer belt 141 passes immediately above the density sensor 65 in FIG. At this time, the density of the toner image is measured by the density sensor 65. The density result of each toner image measured by the density sensor 65 is stored in the storage unit 54 (FIG. 2).

  In step S4, a charging bias according to the target potential V0 is determined. The image condition adjusting unit 53 includes a plurality of charging biases applied to the charging bias applying unit 62 when a plurality of latent images are formed in step S1, and density measurement results (IDs) of the plurality of toner images measured in step S3. ) To determine the charging bias according to the target potential V0. Referring to FIG. 5, when the charging bias applied to charging roller 21 </ b> A by charging bias applying unit 62 decreases, the potential difference between developing bias Vdc (FIG. 3) and the surface potential of photosensitive drum 20 increases, so that the toner image. The concentration of increases. In the present embodiment, the following calculation is executed by a program executed by the image condition adjustment unit 53. That is, the image condition adjustment unit 53 excludes two data located in the region P from the plurality of data in FIG. 5 and performs linear regression on the remaining data. In the density data located in this region P, since the density of the toner image is low, the detection accuracy of the density sensor 65 is relatively low, so it is excluded during the calculation. The area where the density sensor 65 can accurately detect the density of the toner image varies depending on the performance of the sensor, but the range of ID = 0.1 to 1.0 is preferable, and the range of ID = 0.2 to 0.8. Is more preferable. The image condition adjusting unit 53 derives the horizontal axis intercept of the regression line (FIG. 5), that is, the value of the charging bias (R in FIG. 5) at which the density of the toner image is virtually zero. Then, the image condition adjustment unit 53 determines the derived charging bias as a charging bias corresponding to the target potential V0.

  Note that the derivation of the charging bias in step S4 is not limited to the above. Of the plurality of data in the graph of FIG. 5, a highly linear data portion may be extracted, and a regression line may be derived based on the data. Moreover, after selecting two pieces of data that are equal to or higher than a preset density (density threshold value) and have a higher charging bias among the plurality of data in the graph of FIG. 5, a regression line is selected based on the data. May be derived. Thus, by limiting the density range for performing linear regression, it is possible to employ data in a region where the detection accuracy of the density sensor 65 is high.

  As described above, in the present embodiment, in the charging bias adjusting operation, the image condition adjusting unit 53 controls the charging bias applying unit 62 so that the magnitude of the potential on the circumferential surface of the photosensitive drum 20 is increased along the rotation direction. A plurality of different potential regions are formed. Further, the image condition adjusting unit 53 controls the developing bias applying unit 63 to apply a predetermined developing bias corresponding to the target potential V0 of the photosensitive drum 20 to the developing roller 23C. As a result, a plurality of toner images are formed by the potential difference between the developing bias and the plurality of potential regions on the photosensitive drum 20. The charging bias value corresponding to the target potential V0 is determined from the density measurement results of the plurality of toner images measured by the density sensor 65. For this reason, it is possible to set the surface potential of the photosensitive drum 20 to the target potential V0 with a simple configuration without providing a surface potential meter facing the photosensitive drum 20.

  In particular, the image condition adjusting unit 53 controls the developing bias applying unit 63 to apply the developing bias Vdc (= V0 = Vdc (0) + a) having the same value as the target potential V0 of the photosensitive drum 20 to the developing roller 23C. . Then, from the relationship between the density measurement results of the plurality of toner images and the plurality of charging biases corresponding to the plurality of potential regions, a charging bias value at which the density of the toner image becomes zero is derived, and the derived charging bias is targeted. It is determined as a charging bias corresponding to the potential V0. In other words, in this embodiment, when the developing bias Vdc and the surface potential V0 (I) of the photosensitive drum 20 coincide with each other with the developing bias Vdc for which the actual output voltage can be easily grasped as a reference, the density of the toner image Is used to become zero. For this reason, it is possible to realize surface potential setting with a simple configuration and a small error range.

  Further, the image condition adjustment unit 53 applies a charging bias Vref set in advance corresponding to the target potential of the photosensitive drum 20 to the charging bias application unit 62, and a plurality of values are set so that the absolute value decreases sequentially from the charging bias Vref. Are applied to form a plurality of latent images (potential regions) (FIG. 3). As a result, the charging bias is gradually lowered, so that the surface potential of the photosensitive drum 20 is stabilized, and the correlation between the charging bias and the surface potential of the photosensitive drum 20 is increased. Therefore, the surface potential can be set with higher accuracy.

  4, the latent image pattern formed on the peripheral surface of the photoconductor drum 20 is not limited to the form shown in FIG. 3, and the potential of the latent image pattern along the rotation direction of the photoconductor drum 20 is not limited. A plurality of potential regions having different sizes may be formed. 6A to 6C are schematic diagrams showing patterns of a plurality of potential regions (latent images) in the charging bias adjustment operation according to the modified embodiment of the present invention. In any case, since a plurality of latent image potentials are formed from the high potential side toward the low potential side, the surface potential of the photosensitive drum 20 is easily stabilized, and the charging bias and the surface of the photosensitive drum 20 are stabilized. Correlation with potential increases. As a result, the surface potential can be set with higher accuracy.

  In FIG. 6A, the potential of the background portion on the photosensitive drum 20 is denoted as V0 (I). Then, the image condition adjusting unit 53 changes the charging bias by a plurality of levels with the above-described charging bias Vref as a reference, and forms a predetermined latent image. In FIG. 6A, three latent images are formed. In particular, the image condition adjustment unit 53 uses the potential that is reduced by b1 (V) from the charging bias Vref corresponding to the target potential V0 as the first charging bias A1. Further, the image condition adjusting unit 53 applies the second charging bias A2 and the third charging bias A3 by decreasing the first charging bias A1 by b2 (V) at equal intervals, for a total of three latent voltages. An image is formed. In FIG. 6A, the relationship b1> b2 is satisfied. In other words, the image condition adjusting unit 53 applies the first charging bias A1 having an absolute value smaller than Vref (first provisional charging bias) in the charging bias adjusting operation, and then applies the first charging bias A1. A plurality of potential regions are formed by applying the second charging bias A2 having a small absolute value. The potential difference between Vref and the first charging bias A1 is larger than the potential difference between the first charging bias A1 and the second charging bias A2. As described above, when the first latent image is formed, the charging bias is greatly reduced from Vref, thereby suppressing the formation of a low-density toner band image in which the density sensor 65 cannot accurately detect the density. Can do. As a result, the charging bias adjustment control can be executed efficiently.

  On the other hand, in FIG. 6B, the interval between the charging biases is set to gradually decrease from the first charging bias B1, the second charging bias B2, and the third charging bias B3. When the intervals between the charging biases are set at equal intervals as shown in FIG. 6A, there is an advantage that the calculation of the regression line in step S4 in FIG. 4 is simplified. However, in the case of an equal interval, the density of the toner band in the third charging bias A3 becomes high, and the density detection accuracy by the density sensor 65 may slightly decrease. On the other hand, as shown in FIG. 6B, the interval between the charging biases is gradually reduced, so that the density of the toner band corresponding to the third charging bias B3 is prevented from becoming excessively high.

  In FIG. 6C, the first charging bias C1, the second charging bias C2, the third charging bias C3, and the fourth charging bias C4 are continuously applied. In this way, latent images can be continuously formed without applying Vref between the charging biases. In this case, since the time t2 in FIG. 3 can be reduced, the control time of the charging bias adjustment operation can be greatly reduced. On the other hand, as shown in FIGS. 6A and 6B, when Vref is applied between the charging biases, the edge of the band latent image becomes clear, so that the density detection accuracy by the density sensor 65 is improved. To do.

<Inter-unit correction for charging bias adjustment operation>
By performing the charging bias adjustment operation as described above, the surface potential of the photosensitive drum 20 can be stably set to the target potential V0. On the other hand, when such a charging bias adjustment operation is executed for all the photosensitive drums 20 of the magenta unit 13M, the cyan unit 13C, the yellow unit 13Y, and the black unit 13Bk, the image forming apparatus 10 The stop time (time during which image formation is impossible) increases. For this reason, in this embodiment, a charging bias adjustment operation is performed on a predetermined photoconductor drum 20, and the execution result of the charging bias adjustment operation is used for deriving a charging bias of another photoconductor drum 20. .

  FIG. 7 is a flowchart of inter-unit correction in the charging bias adjustment operation according to this embodiment. The image condition adjustment unit 53 first refers to the storage unit 54 when performing the above-described charging bias adjustment operation, and determines whether any of the plurality of units (photosensitive drums 20) of the image forming unit 13 has been replaced. Determination is made (step S11 in FIG. 7). If any of the units has been replaced (YES in step S11), the image condition adjustment unit 53 determines that the unit with the largest number of prints (hereinafter referred to as the maximum unit) among the plurality of units and the number of prints. 4 is executed in both the unit with the smallest unit (hereinafter referred to as the smallest unit) (A) (step S12). As a result, in the maximum unit and the minimum unit, the value of the charging bias corresponding to the target potential V0 of the photosensitive drum 20 is determined.

  Next, the image condition adjustment unit 53 calculates the value of the charging bias corresponding to the target potential V0 of the photosensitive drum 20 in the other units from the result of step S12. The number of prints corresponding to each photoconductor drum 20 is proportional to the cumulative rotation time of the photoconductor drum 20. The accumulated rotation time of the photosensitive drum 20 has a correlation with the amount of wear of the surface layer of the photosensitive drum 20. That is, the photosensitive drum 20 having a long accumulated rotation time is more worn. Depending on the amount of wear on the surface layer of the photosensitive drum 20, the charged potential changes even when the same amount of exposure light is irradiated. Therefore, the optimum charging bias of the other photosensitive drums 20 can be calculated by performing the charging bias adjustment operation on the photosensitive drums 20 having the maximum number of printed sheets and the minimum number of printed sheets.

  Formula 1 is a calculation formula for deriving the charging bias of the other photosensitive drum 20 from the charging bias of the maximum unit and the minimum unit. The formula 1 is stored in the storage unit 54 in advance.

  In Equation 1, Vx is the derived charging bias value of the other photoconductive drum 20, Vmax is the charging bias value determined through the charging bias adjustment operation in the maximum number of printed sheets unit, and Vmin is The charging bias value determined through the charging bias adjusting operation in the minimum number of printed sheets unit. Pmax is the number of printed sheets of the maximum number of printed sheets unit, Pmin is the number of printed sheets of the minimum number of printed sheets unit, and Px is the number of printed sheets of the other photosensitive drums 20 to be derived. As described above, when the charging bias adjustment operation is executed on the photosensitive drum 20 with the maximum number of printed sheets and the minimum number of printed sheets, the optimal charging of the other photosensitive drums 20 (the remaining two photosensitive drums 20) is obtained from Equation 1. The bias can be calculated. As a result, the charge bias adjustment operation is performed as compared with the case where the charge bias adjustment operation is executed for all the photosensitive drums 20 of the magenta unit 13M, the cyan unit 13C, the yellow unit 13Y, and the black unit 13Bk. The operation can be completed in about half the time.

  On the other hand, if none of the units has been replaced in step S11 of FIG. 7 (NO in step S11), the image condition adjustment unit 53 performs a charging bias adjustment operation on the photosensitive drum 20 of the black unit 13Bk. (Step S14). Thereafter, the image condition adjusting unit 53 applies the charging bias value determined by the black unit 13Bk as it is as the charging bias corresponding to the target potential V0 of the magenta unit 13M, the cyan unit 13C, and the yellow unit 13Y. (Step S15). In this case, since the number of printed sheets (cumulative rotation time) of all the photosensitive drums 20 is substantially the same, the same charging bias value can be applied without using Equation 1. Note that the unit for which the charging bias adjustment operation is executed in step S14 may be other than the black color unit Bk.

  As described above, in the present embodiment, the image condition adjusting unit 53 has a plurality of photosensitive drums 20 on the peripheral surface of the plurality of units of the image forming unit 13 in the charging bias adjustment operation. A toner image is formed, and the charging bias value corresponding to the target potential V0 of the specific photosensitive drum 20 is determined from the density measurement results of the plurality of toner images measured by the density sensor 65. Thereafter, the image condition adjusting unit 53 derives a value of the charging bias corresponding to the target potential V0 in the other photosensitive drum 20 other than the specific photosensitive drum 20 based on the determined charging bias value. As a result, it is possible to set the surface potentials of the plurality of photosensitive drums 20 to the target potential V0 with a simple configuration and in a short time.

  Further, when any of the plurality of photosensitive drums 20 is replaced, the image condition adjusting unit 53 selects the photosensitive drum 20 having the maximum number of prints and the photosensitive drum 20 having the minimum number of prints as a specific photosensitive drum. As the body drum, a charging bias adjustment operation is executed to determine a charging bias value corresponding to the target potential V0. Then, the image condition adjusting unit 53 derives a charging bias value corresponding to the target potential V0 in the other photosensitive drums 20 other than the specific photosensitive drum 20 based on the determined charging bias value. For this reason, when some of the photosensitive drums 20 have been replaced, the charging bias adjustment operation is executed on the photosensitive drums 20 with the maximum number of printed sheets and the minimum number of photosensitive drums 20, whereby a plurality of the photosensitive drums 20. Can be set to the target potential V0.

  In addition, when none of the plurality of photosensitive drums 20 has been replaced, the image condition adjusting unit 53 performs the charging bias adjustment operation using one photosensitive drum 20 of the plurality of units as a specific photosensitive drum. The charging bias value corresponding to the target potential V0 is determined. Then, the image condition adjusting unit 53 sets the determined charging bias value as the charging bias value corresponding to the target potential V0 in the other photoconductive drum 20. For this reason, when none of the photosensitive drums 20 is exchanged, the surface potential of the plurality of photosensitive drums 20 is set to the target potential V0 only by performing the charging bias adjustment operation on one photosensitive drum 20. It becomes possible to set to.

  Next, the charging bias adjustment operation according to the second embodiment of the present invention will be described. Since the present embodiment differs from the first embodiment in the correction between units in the charging bias adjustment operation, only the difference will be described, and the description of other common features will be omitted. FIG. 8 is a flowchart of inter-unit correction in the charging bias adjustment operation according to this embodiment.

  In the present embodiment, the drive unit 61 (FIG. 2) includes a contact / separation mechanism (not shown) provided in the intermediate transfer unit 14. The contact / separation mechanism collectively contacts and separates the primary transfer roller 24 corresponding to the color unit among the plurality of primary transfer rollers 24 with respect to the photosensitive drum 20. In the image forming apparatus 10, when a full color image is formed, the contact / separation mechanism causes the primary transfer roller 24 for the black unit to abut on the photosensitive drum 20 and the primary transfer roller 24 for the color unit is bundled. To contact with the photosensitive drum 20. On the other hand, when a monochrome image is formed in the image forming apparatus 10, the contact / separation mechanism collectively separates the primary transfer roller 24 for the color unit from the photosensitive drum 20. As a result, only the primary transfer roller 24 for the black unit is brought into contact with the photosensitive drum 20. In the color unit in which the primary transfer roller 24 is separated from the photosensitive drum 20, the rotation of the photosensitive drum 20 can be stopped during printing of a monochrome image.

  Also in the present embodiment, the charging bias adjustment operation is performed on a predetermined photoconductor drum 20, and the execution result of the charging bias adjustment operation is used for deriving the charging bias of another photoconductor drum 20. When executing the above-described charging bias adjustment operation, the image condition adjustment unit 53 first refers to the storage unit 54 and determines whether any of the plurality of units of the image forming unit 13 has been replaced (see FIG. 8). Step S21). If any of the units has been replaced (YES in step S21), the image condition adjusting unit 53 includes both the unit with the largest number of printed sheets and the unit with the smallest number of printed sheets among the plurality of units. 4A and 4B, the charging bias adjustment operation shown in FIG. 4 is executed (B) (step S22). As a result, in the maximum unit and the minimum unit, the value of the charging bias corresponding to the target potential V0 of the photosensitive drum 20 is determined. Next, the image condition adjusting unit 53 calculates the value of the charging bias corresponding to the target potential V0 of the photosensitive drum 20 in the other units from the result of step S22 (step S23). The calculation method of the charging bias of the other units is executed using Expression 1 as in the first embodiment.

  On the other hand, if no unit has been replaced in step S21 of FIG. 8 (NO in step S21), the image condition adjustment unit 53 charges the photosensitive drums 20 of the yellow unit 13Y and the black unit 13Bk. A bias adjustment operation is executed (step S24). Thereafter, the image condition adjusting unit 53 applies the value of the charging bias determined by the yellow unit 13Y as it is as the charging bias corresponding to the target potential V0 of the magenta unit 13M and the cyan unit 13C (step S25). . In this case, since the number of printed sheets (cumulative rotation time) of the photosensitive drum 20 of the color unit is substantially the same, the same charging bias value can be applied without using Equation 1. Note that the unit on which the charging bias adjustment operation is executed in step S24 may be a unit other than the yellow unit Y among the color units.

  As described above, in the present embodiment, the image condition adjusting unit 53 determines whether the photosensitive drum 20 of the black unit 13Bk and the plurality of color units are in a state where none of the plurality of photosensitive drums 20 has been replaced. The photosensitive drum 20 of one of the units is set as a specific photosensitive drum 20, and the value of the charging bias corresponding to the target potential V0 is determined. Thereafter, the image condition adjusting unit 53 sets the charging bias determined in one unit of the color unit as a charging bias corresponding to the target potential V0 in the other photosensitive drum 20 of the plurality of color units. For this reason, the surface potential of the plurality of photosensitive drums 20 is set to the target potential only by performing the charging bias adjustment operation on the photosensitive drum 20 of the black unit 13Bk and the photosensitive drum 20 of the color unit. It becomes possible to do.

  Next, the charging bias adjustment operation according to the third embodiment of the present invention will be described. Since the present embodiment is different from the first and second embodiments in the flow of the charging bias adjustment operation, only the difference will be described and description of other common features will be omitted. Note that the flow of the charging bias adjustment operation according to the present embodiment can be applied to any inter-unit correction in FIG. 7 of the first embodiment and FIG. 8 of the second embodiment. FIG. 9 is a schematic diagram showing a potential relationship between the photosensitive drum 20 and the developing roller 23C in the charging bias adjusting operation according to the present embodiment.

  Referring to FIG. 9, the present embodiment is characterized by the values of the surface potential V0 (I) and developing bias Vdc of the photosensitive drum 20 during the charging bias adjustment operation. In FIG. 9, for comparison, the surface potential of the photosensitive drum 20 during the charging bias adjustment operation in the first embodiment is indicated as V0 (I) ′. On the other hand, the surface potential of the photosensitive drum 20 during the charging bias adjustment operation according to the present embodiment is shown as V0 (I). Here, the relationship V0 (I) ′ − V0 (I) = a (V) is satisfied. In other words, the image condition adjusting unit 53 uses the value obtained by subtracting a (V) from the preset charging bias Vref as described in the first embodiment as the charging bias Vref in the present embodiment. 62 is applied (step S1 in FIG. 4). That is, the purpose of the charging bias Vref in the present embodiment is to form a surface potential on the photosensitive drum 20 that is smaller than the original target potential V0 by a (V). After that, as shown in FIG. 9, the charging bias is sequentially reduced to form a plurality of latent images.

  Further, the image condition adjusting unit 53 applies the developing bias Vdc to the developing roller 23C in step S2 (FIG. 4). At this time, in this embodiment, Vdc = V0−a (V) is set. In the charging bias adjustment operation, if the developing bias Vdc applied to the developing roller 23C is set to the value of the target potential V0 of the photosensitive drum 20 as in the first embodiment, it is simple and highly accurate. Control is possible. On the other hand, in a normal image forming operation, the developing bias Vdc at the time of development is rarely set higher as the target surface potential V0 of the photosensitive drum 20. Therefore, the target potential V0 may exceed the control range of the developing bias Vdc during the normal image forming operation. Further, depending on the use environment, the target potential V0 of the photosensitive drum 20 needs to be set to a value higher than usual, and therefore, it may be difficult to set the developing bias Vdc to the same value as V0. In such a case, if the control range of the developing bias Vdc is to be expanded, the cost of the high-voltage substrate of the developing bias applying unit 63 is increased. On the other hand, in the present embodiment, the development of the latent image is performed after the development bias Vdc is set a (V) lower than the target potential V0 of the photosensitive drum. Therefore, an increase in the cost of the developing bias applying unit 63 is suppressed.

  If there is a large potential difference between the developing bias Vdc and the surface potential V0 (I) of the photosensitive drum 20 due to the charging bias Vref, the charging bias adjusting operation is performed when a two-component developer is used in the developing device 23. The carrier easily moves to the photosensitive drum 20 side. In this case, an image defect may occur during image formation after the adjustment operation. For this reason, it is preferable that the reference charging bias Vref is also set low according to the value of the developing bias Vdc. In the present embodiment, as described above, a bias that is lower by a (V) than the charging bias Vref of the first embodiment is set as a new Vref.

  In the present embodiment, the difference potential a (V) (V0 (I) ′ − V0 (I)) when the charging bias Vref is reduced and the developing bias Vdc are photosensitive as compared with the first embodiment. In the embodiment (p = a), the difference potential p (V) at the time of reduction from the target surface potential V0 of the body drum 20 is set to the same value. However, in other embodiments, both constants are constant. The value may be a different value. As in the present embodiment, when both constant values are set to the same value, the relationship between the developing bias Vdc during the charging bias adjustment operation and the surface potential V0 of the photosensitive drum 20 is the same as during normal development ( Therefore, it is preferable from the viewpoint of carrier development, toner fogging, and gradation reproducibility.

  The image condition adjustment unit 53 determines the charging bias in step S4 after performing step S3 (FIG. 4) similar to that in the first embodiment. At this time, as in the first embodiment, the image condition adjustment unit 53 excludes two data located in the region P from the plurality of data in FIG. 5 and performs linear regression on the remaining data. Then, the image condition adjustment unit 53 derives an intercept on the horizontal axis of the regression line (FIG. 5), that is, a charging bias value (R in FIG. 5) at which the density of the toner image is virtually zero. . Here, as described above, in the present embodiment, the developing bias Vdc = V0−a (V) is set. Therefore, the charging bias having the same value as the developing bias Vdc during the charging bias adjustment operation is a value lower by a (V) than the target potential V0. For this reason, in this embodiment, the image condition adjustment unit 53 determines a value obtained by adding a (V) to the charging bias value at which the density of the toner image becomes zero as the charging bias value corresponding to the target potential V0. To do. As a result, the charging bias corresponding to the target potential V0 of the photosensitive drum 20 is accurately derived even if the developing bias Vdc at the time of the charging bias control operation is set low in order to suppress the cost increase of the developing bias applying unit 63. be able to.

  As described above, in the present embodiment, the image condition adjusting unit 53 controls the developing bias applying unit 63 to develop the developing bias Vdc that is smaller by a (V) that is set in advance from the target potential V0 of the photosensitive drum 20. (V0-a) is applied to the developing roller 23C. Further, the image condition adjustment unit 53 derives a charging bias value at which the density of the toner image becomes zero from the relationship between the density measurement results of the plurality of toner images and the plurality of charging biases corresponding to the plurality of potential regions. A value obtained by adding a (V) to the derived charging bias is determined as a charging bias corresponding to the target potential V 0 of the photosensitive drum 20.

  Next, the charging bias adjustment operation according to the fourth embodiment of the present invention will be described. Since the present embodiment is different from the first and second embodiments in the flow of the charging bias adjustment operation, only the difference will be described and description of other common features will be omitted. Note that the flow of the charging bias adjustment operation according to the present embodiment can be applied to any inter-unit correction in FIG. 7 of the first embodiment and FIG. 8 of the second embodiment. FIG. 10 is a flowchart of the charging bias adjustment operation according to the present embodiment. FIG. 11 is a schematic diagram showing a potential relationship between the photosensitive drum 20 and the developing roller 23C in the charging bias adjusting operation according to the present embodiment. In FIG. 11, the surface potential of the photosensitive drum 20 is Vdr, and the DC bias potential of the developing roller 23C is Vdc.

  Referring to FIG. 10, the charging bias adjustment operation includes formation of band latent image 1 (step S31), development of band latent image 1 (step S32), density measurement of band toner image 1 (step S33), and band latent image. 2 (step S34), development of the latent band image 2 (step S35), density measurement of the band toner image 2 (step S36), and determination of the charging bias (step S37). Broadly speaking, the charging bias adjustment operation includes a first stage until the density measurement of the belt toner image 1 (step S33), a second stage until the density measurement of the band toner image 2 (step S36), and determination of the charging bias. It is classified into the third stage of (Step S37).

  When the charging bias adjustment operation is executed, the image condition adjustment unit 53 executes the formation of the band latent image 1 in FIG. 10 (step S31). Also in the present embodiment, in the charging bias adjustment operation, a charging bias value is derived such that the surface potential of the photosensitive drum 20 is V0 (V).

  In step S31, the image condition adjusting unit 53 refers to the intermediate charging bias Vm stored in advance in the storage unit 54 (FIG. 2), controls the charging bias applying unit 62, and applies the intermediate charging bias Vm. The intermediate charging bias Vm has a smaller absolute value than the charging bias Vref. As a result, the surface of the photosensitive drum 20 is charged to an intermediate potential (Vm). This intermediate potential can be set with a certain degree of freedom. When the two-component development method is used as the development method, if the intermediate potential is too high, carrier development occurs due to the potential difference between the surface potential Vdr of the photosensitive drum 20 and the potential Vdc of the developing roller 23C. Cheap. For this reason, it is desirable that the intermediate potential of the photosensitive drum 20 is a value around 50% of the target potential V0. Further, when the two-component development method is not used as the development method, the photosensitive drum 20 may be charged with the charging bias Vref as in step S34 described later.

  Further, if the surface potential Vdr of the background portion of the photosensitive drum 20 becomes lower than the developing bias Vdc, background portion fogging occurs, and an error is likely to occur in the density measurement in step S3 described later. For this reason, it is desirable that the surface potential Vdr of the background portion of the photosensitive drum 20 in step S31 is higher than the developing bias Vdc. Next, the image condition adjusting unit 53 controls the charging bias applying unit 62 to form a region of charging bias 0 V (charging bias OFF) for a predetermined time. As a result, as shown in FIG. 11, an uncharged region having a surface potential of approximately 0 V is formed on the peripheral surface of the photosensitive drum 20.

  In step S32, the belt latent image 1 is developed. The image condition adjusting unit 53 sets the developing bias Vdc applied to the developing roller 23C to a preset potential b (V), and then the latent image formed in step S1 (band latent image 1, uncharged region). develop. As a result, a belt toner image 1 (I1 in FIG. 11) is formed on the peripheral surface of the photosensitive drum 20 by the potential difference between the developing roller 23C to which the developing bias Vdc of b (V) is applied and the non-charged area. The In this embodiment, b = 100 V is set, and the value of b is also stored in the storage unit 54 in advance. In addition, the range of the value of preferable b is 50-200V, and 100-150V is still more preferable.

  In step S33, density measurement of the belt toner image 1 formed in step S32 is executed. Also at this time, the density of the toner image is measured by the density sensor 65. The density result of the toner image measured by the density sensor 65 is stored in the storage unit 54 (FIG. 2).

  In step S34, the latent band image 2 is formed. Here, the image condition adjusting unit 53 controls the charging bias applying unit 62 to apply the charging bias Vref to the charging roller 21A. At this stage, the surface potential of the photosensitive drum 20 may be set to a value that deviates from the target potential V0. Further, the image condition adjusting unit 53 controls the charging bias applying unit 62 to apply a value obtained by subtracting the above-described b (V) from the charging bias Vref to the charging bias applying unit 62 for a predetermined time. As a result, as shown in FIG. 11, a band latent image 2 (potential region) is formed on the peripheral surface of the photosensitive drum 20.

  In step S35, the belt latent image 2 is developed. The image condition adjusting unit 53 sets the developing bias Vdc applied to the developing roller 23C to the target potential V0 (V) of the photosensitive drum 20, and then develops the latent image (band latent image 2) formed in step S34. . As a result, the belt toner image 2 (I2 in FIG. 11) is formed on the peripheral surface of the photosensitive drum 20 due to the potential difference between the developing roller 23C to which the developing bias Vdc of V0 (V) is applied and the band latent image 2. It is formed.

  In step S36, the image condition adjusting unit 53 controls the density sensor 65 to measure the density of the belt toner image 2 formed in step S35.

  In step S37, the density D1 of the band toner image 1 measured in step S33 is compared with the density D2 of the band toner image 2 measured in step S6, and the charging bias Vref is corrected as necessary. As described above, when the potential of the non-charged region formed in step S1 is 0 (V), the density D1 of the belt toner image 1 is the potential difference b (V) between the photosensitive drum 20 and the developing roller 23C. It is formed by the movement of the toner with respect to. In step S34, assuming that the surface potential Vdr of the photosensitive drum 20 is set to the target potential V0 (V) when the charging bias Vref is applied, the surface potential Vdr of the background portion of the photosensitive drum 20 and the developing roller 23C. Are the same potential. For this reason, the density D2 of the belt toner image 2 is formed by the movement of the toner with respect to the potential difference b (V), so that density D1 = density D2.

  On the other hand, when the density D2 measured in step S36 is larger than the density D1, the surface potential Vdr of the background portion of the photosensitive drum 20 in step S34 is smaller than the target potential V0. Therefore, in this case, the image condition adjusting unit 53 determines a value larger than the charging bias Vref as the charging bias for the target potential V0 (V). Specifically, steps S34 to S36 are executed again by applying a charging bias obtained by adding a preset step value m (V) to the charging bias Vref to the photosensitive drum 20. As described above, the image condition adjusting unit 53 extracts the charging bias such that the density D1 = the density D2 while correcting the value of the charging bias applied to the charging roller 21A. If the density D2 measured in step S36 is smaller than the density D1, the image condition adjusting unit 53 determines a value smaller than the charging bias Vref as the charging bias for the target potential V0 (V). As a result, the charging bias value corresponding to the target potential V0 of the photosensitive drum 20 is determined.

  As described above, in the present embodiment, the image condition adjustment unit 53 controls the charging bias application unit 62 in the charging bias adjustment operation, so that the charging bias is not applied to the peripheral surface of the photosensitive drum 20. Form. Further, the image condition adjusting unit 53 controls the developing bias applying unit 63 to apply the developing bias Vdc consisting of b (V) to the developing roller 23C, so that the belt toner is generated by the potential difference between the non-charged area and the developing roller 23C. Image 1 is formed. Further, the image condition adjusting unit 53 controls the charging bias applying unit 62 to set the charging bias Vref to b (V) on the peripheral surface of the photosensitive drum 20 in advance corresponding to the target potential V0 (V). The band latent image 2 is formed by applying a charging bias minus. The image condition adjusting unit 53 controls the developing bias applying unit 63 to apply the target potential V0 (V) to the developing roller 23C, so that the band toner image 2 is generated by the potential difference between the band latent image 2 and the developing roller 23C. Form. Then, the image condition adjusting unit 53 determines the value of the charging bias corresponding to the target potential V0 from the density measurement results (D1, D2) of the band toner image 1 and the band toner image 2 measured by the density sensor 65. Therefore, based on the relationship between the charging bias in the belt toner image 1 and the density of the toner image, it is possible to determine the difference between the charging bias Vref and the target potential V0. As a result, it is possible to set the surface potential of the photosensitive drum 20 to a target potential with a simple configuration without providing a surface potential meter facing the photosensitive drum 20.

  In particular, when the density of the band toner image 1 is higher than the density of the band toner image 2 in the charging bias adjustment operation, the image condition adjusting unit 53 sets a value smaller than the charging bias Vref to the charging corresponding to the target potential V0. Determine as bias. Further, when the density of the band toner image 1 is lower than the density of the band toner image 2, the image condition adjusting unit 53 determines a value larger than the charging bias Vref as the charging bias corresponding to the target potential V0. Therefore, the charging bias corresponding to the target potential V0 can be easily determined from the density comparison result between the band toner image 1 and the band toner image 2.

  In the present embodiment, the image condition adjusting unit 53 controls the charging bias applying unit 62 to apply a predetermined intermediate charging bias Vm in the charging bias adjusting operation, thereby rotating the non-charging region (0 V) in the rotation direction. Set the background potential before and after. The intermediate charging bias Vm is set smaller than the target potential V0. For this reason, even when a two-component developer is used in the developing device, a large amount of carrier from the developing roller 23C is prevented from moving toward the photosensitive drum 20 during the charging bias adjustment operation.

  Note that the fourth embodiment may be modified as follows. FIG. 12 is a schematic diagram showing a potential relationship between the photosensitive drum 20 and the developing roller 23C in the charging bias adjustment operation according to the present modified embodiment. In this modified embodiment, compared to the previous fourth embodiment, the formation of the band latent image 2 in step S34 in FIG. 10 and the development of the band latent image 2 in step S35 are partially different. Only the said difference is demonstrated and description is abbreviate | omitted about the other common control aspect.

  Referring to FIG. 12, in this variation, in the charging bias adjustment operation, the surface potential Vdr of the photosensitive drum 20 when forming the band latent image 2 and the value of the developing bias Vdc in developing the band latent image 2 are described. It has the characteristics. Note that the values of the surface potential Vdr and the developing bias Vdc of the photosensitive drum 20 when the band latent image 1 is formed and developed are the same as those in the fourth embodiment.

  In step S34 of FIG. 10, the image condition adjustment unit 53 controls the charging bias application unit 62 to apply a value (Vref−a) obtained by subtracting a (V) from a preset charging bias Vref to the charging roller 21A. By applying this, the background potential is set. Further, the image condition adjusting unit 53 charges a value (Vref−a−b) obtained by subtracting a (V) and b (V) described above from a preset charging bias Vref when forming the band latent image 2. Apply to roller 21A. As a result, the band latent image 2 is formed on the photosensitive drum 20. Note that a (V) is stored in the storage unit 54 in advance. Further, the image condition adjusting unit 53 controls the developing bias applying unit 63 in step S35 of FIG. 10 to apply a value obtained by subtracting a (V) from the target potential V0 of the photosensitive drum 20 to the developing roller 23C. . As a result, the band toner image 2 is formed by the potential difference (V0−Vref−b) between the developing roller 23C and the band latent image 2 (I2 in FIG. 12). Therefore, the image condition adjusting unit 53 can determine the charging bias corresponding to the target potential V0 after comparing the density D1 and the density D2, as in the fourth embodiment.

  Furthermore, in this modified embodiment, the developing bias applied by the developing bias applying unit 63 is reduced by a (V) when adjusting the charging bias, as compared with the fourth embodiment. For this reason, it is possible to suppress an increase in the cost of the developing bias applying unit 63 including a high voltage power source.

  Next, the charging bias adjustment operation according to the fifth embodiment of the present invention will be described. Since the present embodiment is different from the first and second embodiments in the flow of the charging bias adjustment operation, only the difference will be described and description of other common features will be omitted. Note that the flow of the charging bias adjustment operation according to the present embodiment can be applied to any inter-unit correction in FIG. 7 of the first embodiment and FIG. 8 of the second embodiment. FIG. 13 is a flowchart of the charging bias adjustment operation according to the present embodiment. FIG. 14 is a schematic diagram showing a potential relationship between the photosensitive drum 20 and the developing roller 23C in the charging bias adjusting operation according to the present embodiment. In FIG. 14, the surface potential of the photosensitive drum 20 is Vdr, and the DC bias potential of the developing roller 23C is Vdc.

  Referring to FIG. 13, the charging bias adjustment operation includes patch latent image formation (step S41), patch latent image development (step S42), patch toner image density measurement (step S43), and band latent image formation (step S43). Step S44), development of the band latent image (Step S45), density measurement of the band toner image (Step S46), and determination of the charging bias (Step S47). Generally, the charging bias adjustment operation includes a first stage up to a patch toner image density measurement (step S43), a second stage up to a belt toner image density measurement (step S46), and a charging bias determination (steps). It is classified into the third stage of S47).

  When the charging bias adjusting operation is executed, the image condition adjusting unit 53 executes the patch latent image formation (step S41) in FIG. Also in the present embodiment, in the charging bias adjustment operation, a charging bias value is derived such that the surface potential of the photosensitive drum 20 is V0 (V).

  In step S41, the image condition adjusting unit 53 refers to the intermediate charging bias Vm stored in advance in the storage unit 54 (FIG. 2), controls the charging bias applying unit 62, and applies the intermediate charging bias Vm. The intermediate charging bias Vm has a smaller absolute value than the charging bias Vref. As a result, the surface of the photosensitive drum 20 is charged to an intermediate potential (Vm) (FIG. 14). When the two-component development method is not used as the development method, the photosensitive drum 20 may be charged with the charging bias Vref as in step S44 described later.

  Next, the image condition adjustment unit 53 controls the exposure device 22 to irradiate the peripheral surface of the photosensitive drum 20 with exposure light. At this time, the exposure device 22 irradiates exposure light corresponding to a 100% solid image. As a result, a patch latent image having the image portion potential VL is formed on the peripheral surface of the photosensitive drum 20.

  In step S42, the patch latent image is developed. The image condition adjusting unit 53 controls the developing bias applying unit 63 to apply a developing bias Vdc (VL + b) obtained by adding a preset potential b (V) to the image unit potential VL (V) to the developing roller 23C. By applying, the patch latent image formed in step S41 is developed. As a result, a patch toner image (I1 in FIG. 14) is formed on the peripheral surface of the photosensitive drum 20 by the potential difference between the developing roller 23C to which the developing bias Vdc of VL + b (V) is applied and the patch latent image. . In this embodiment, b = 100 V is set, and the value of b is also stored in the storage unit 54 in advance. In addition, the range of the value of preferable b is 50-200V, and 100-150V is still more preferable. Further, a plurality of levels of patch toner images may be formed while the value of b is changed.

  In step S43, the density measurement of the patch toner image formed in step S42 is executed. The toner image transferred to the intermediate transfer belt 141 passes immediately above the density sensor 65 in FIG. At this time, the density of the toner image is measured by the density sensor 65. The density result of each toner image measured by the density sensor 65 is stored in the storage unit 54 (FIG. 2).

  In step S44, a latent band image is formed. Here, the image condition adjusting unit 53 controls the charging bias applying unit 62 to apply the charging bias Vref to the charging roller 21A. Further, the image condition adjusting unit 53 controls the charging bias applying unit 62 to apply a value (Vref−b) obtained by subtracting the above-described b (V) from the charging bias Vref to the charging bias applying unit 62 for a predetermined time. As a result, as shown in FIG. 14, a band latent image is formed on the peripheral surface of the photosensitive drum 20.

  In step S45, the belt latent image is developed. The image condition adjusting unit 53 develops the band latent image formed in step S44 after setting the developing bias Vdc applied to the developing roller 23C to the target potential V0 (V) of the photosensitive drum 20. As a result, a belt toner image (I2 in FIG. 14) is formed on the peripheral surface of the photosensitive drum 20 by the potential difference between the developing roller 23C to which the developing bias Vdc of V0 (V) is applied and the belt latent image. The

  In step S46, the image condition adjusting unit 53 controls the density sensor 65 to measure the density of the belt toner image formed in step S45.

  In step S47, the density D1 of the patch toner image measured in step S43 is compared with the density D2 of the belt toner image measured in step S46, and the charging bias Vref is corrected as necessary. As described above, in step S41, the potential difference between the image portion potential VL and the developing bias Vdc is b (V). Therefore, the density D1 of the patch toner image is formed by the movement of the toner with respect to the potential difference b (V) between the photosensitive drum 20 and the developing roller 23C. In step S44, assuming that the surface potential Vdr of the photosensitive drum 20 is set to the target potential V0 (V) when the charging bias Vref is applied, the surface potential Vdr of the background portion of the photosensitive drum 20 and the developing roller 23C. Are the same potential. For this reason, the density D2 of the belt toner image is formed by the movement of the toner with respect to the potential difference b (v), so that density D1 = density D2.

  On the other hand, when the density D2 measured in step S46 is larger than the density D1, the surface potential Vdr of the background portion of the photosensitive drum 20 in step S4 is smaller than the target potential V0. Therefore, in this case, the image condition adjusting unit 53 determines a value larger than the charging bias Vref as the charging bias for the target potential V0 (V). Specifically, steps S44 to S46 are executed again by applying a charging bias obtained by adding a preset step value m (V) to the charging bias Vref to the photosensitive drum 20. As described above, the image condition adjusting unit 53 extracts the charging bias such that the density D1 = the density D2 while correcting the value of the charging bias applied to the charging roller 21A. When the density D2 measured in step S46 is smaller than the density D1, the image condition adjusting unit 53 determines a value smaller than the charging bias Vref as the charging bias for the target potential V0 (V). As a result, the charging bias value corresponding to the target potential V0 of the photosensitive drum 20 is determined. In steps S44 to S46, a plurality of levels of belt toner images may be formed while the value of b (V) is changed. In this case, a charging bias value satisfying D1 = D2 may be derived by linear regression of the relationship between the density D2 of the plurality of belt toner images and the value of each b.

  As described above, in this embodiment, the image condition adjusting unit 53 controls the charging bias applying unit 62 to charge the peripheral surface of the photosensitive drum 20 to the first background portion potential (Vm), and then performs exposure. The apparatus 22 is controlled to irradiate the peripheral surface of the photosensitive drum 20 with exposure light, thereby forming a first potential region composed of a first potential (VL). Then, the image condition adjusting unit 53 controls the developing bias applying unit 63 to apply a developing bias obtained by adding a preset first differential potential (b) to the first potential with respect to the developing roller 23C. By applying this, a first toner image (I1) is formed by the potential difference between the first potential region and the developing roller 23C. Further, the image condition adjusting unit 53 controls the charging bias applying unit 62 to start the above-described operation from the first provisional charging bias (Vref) set in advance corresponding to the target potential V0 on the circumferential surface of the photosensitive drum 20. A second potential region is formed by applying a charging bias (Vref-b) obtained by subtracting the first differential potential. Further, the image condition adjusting unit 53 controls the developing bias applying unit 63 to apply the target potential V0 (V) to the developing roller 23C, so that the second toner is generated by the potential difference between the second potential region and the developing roller 23C. An image (I2) is formed. Thereafter, the image condition adjustment unit 53 determines the value of the charging bias corresponding to the target potential V0 from the density measurement results (D1, D2) of the patch toner image and the belt toner image measured by the density sensor 65. For this reason, the deviation of the charging bias Vref from the target potential V0 can be determined based on the relationship between the charging bias in the patch toner image and the density of the toner image. As a result, it is possible to set the surface potential of the photosensitive drum 20 to a target potential with a simple configuration without providing a surface potential meter facing the photosensitive drum 20.

  In particular, when the density of the patch toner image is higher than the density of the band toner image in the charging bias adjustment operation, the image condition adjusting unit 53 sets a value smaller than the charging bias Vref as the charging bias corresponding to the target potential V0. decide. In addition, when the density of the patch toner image is lower than the density of the belt toner image, the image condition adjusting unit 53 determines a value larger than the charging bias Vref as the charging bias corresponding to the target potential V0. Therefore, the charging bias corresponding to the target potential V0 can be easily determined from the density comparison result between the patch toner image and the belt toner image.

  In the present embodiment, the image condition adjustment unit 53 controls the charging bias application unit 62 to apply a predetermined intermediate charging bias Vm in the charging bias adjustment operation, so that the image portion potential VL is rotated before and after in the rotation direction. Set the background potential. The intermediate charging bias Vm is set smaller than the target potential V0. For this reason, even when a two-component developer is used in the developing device, a large amount of carrier from the developing roller 23C is prevented from moving toward the photosensitive drum 20 during the charging bias adjustment operation.

  Furthermore, in this embodiment, the exposure device 22 forms the image portion potential VL by irradiating the intermediate potential Vm with exposure light corresponding to a 100% solid image. For this reason, the image portion potential VL is prevented from being affected by the intermediate potential Vm.

  Note that the fifth embodiment may be modified as follows. FIG. 15 is a schematic diagram showing a potential relationship between the photosensitive drum 20 and the developing roller 23C in the charging bias adjustment operation according to the present modified embodiment. In this modified embodiment, as compared with the previous fifth embodiment, there is a partial difference from the band latent image in step S44 in FIG. 13 to the determination of the charging bias in step S47. And other common control modes will not be described.

  Referring to FIG. 15, this embodiment is particularly characterized by the surface potential Vdr of the photosensitive drum 20 during the formation of the band latent image and the value of the developing bias Vdc in the development of the band latent image during the charging bias adjustment operation. Have Note that the surface potential Vdr (Vm, VL) and the value (VL + b) of the developing bias Vdc of the photosensitive drum 20 when the patch latent image is formed and developed are the same as those in the fifth embodiment.

  In step S44 of FIG. 13, the image condition adjusting unit 53 controls the charging bias applying unit 62 to apply a value (Vref−a) obtained by subtracting a (V) from a preset charging bias Vref to the charging roller 21A. By applying this, the background potential is set. Further, the image condition adjusting unit 53 applies a value (Vref−ab) obtained by subtracting the above a (V) and b (V) from the charging bias Vref to the charging roller 21 </ b> A when forming the band latent image. . As a result, a band latent image is formed on the photosensitive drum 20. Note that the value of a (V) is set in advance and stored in the storage unit 54. Further, in step S45 of FIG. 13, the image condition adjusting unit 53 controls the developing bias applying unit 63 to develop a value (V0-a) obtained by subtracting a (V) from the target potential V0 of the photosensitive drum 20. Apply to roller 23C. As a result, a band toner image is formed by the potential difference (V0−Vref + b) between the developing roller 23C and the band latent image (I2 in FIG. 15). Therefore, the image condition adjusting unit 53 can determine the charging bias corresponding to the target potential V0 after comparing the density D1 and the density D2, as in the fifth embodiment.

  In the present modified embodiment, the developing bias applied by the developing bias applying unit 63 is reduced by a (V) when adjusting the charging bias, as compared with the fifth embodiment. Therefore, even when the development bias Vdc cannot be set to the target potential V0 of the photoconductor drum 20 due to environmental conditions or restrictions on the control range of the high-voltage power supply, the target potential V0 of the photoconductor drum 20 can be set with high accuracy. . For this reason, it is possible to suppress an increase in the cost of the developing bias applying unit 63 including a high voltage power source.

<Performance timing of charging bias adjustment operation>
Next, the execution timing of the charging bias adjustment operation according to the first to fifth embodiments (hereinafter referred to as the present embodiment) will be described. In the image forming apparatus 10, when the surface potential of the photosensitive drum 20 varies, image defects such as density variations occur. For this reason, it is desirable that the charging bias adjustment operation be executed under conditions where the surface potential of the photosensitive drum 20 is likely to vary from the target potential V0. Hereinafter, preferable conditions will be described.

  First, it is desirable that the charging bias adjustment operation is executed when the image forming apparatus 10 is left for a long time since the end of the previous image forming operation. In this case, the temperature and humidity environment inside and outside the image forming apparatus 10 may fluctuate, or the characteristics of the charging roller 21A of the charging device 21 may change. In the present embodiment, the image forming apparatus 10 includes a counting unit 55 (FIG. 2). The count unit 55 calculates the difference between the previous image forming operation end time and the next image forming operation request time. In other words, the counting unit 55 counts the printing interval time between sheets. Then, when the printing interval time of the count unit 55 exceeds a preset threshold value stored in the storage unit 54, the image condition adjustment unit 53 performs the charging bias adjustment operation prior to the next image forming operation. Good. As a result, even when the image forming apparatus 10 is left unused for a long period of time, the occurrence of image defects due to fluctuations in the surface potential of the photosensitive drum 20 is prevented.

  Second, it is desirable that the charging bias adjustment operation is executed when the temperature and humidity inside and outside the image forming apparatus 10 change greatly. In this case, the characteristics of the charging roller 21A of the charging device 21 may change due to fluctuations in the temperature and humidity environment. In the present embodiment, the image forming apparatus 10 includes an environment sensor 64 (FIG. 2). Therefore, when the temperature or humidity detected by the environment sensor 64 exceeds a preset threshold value stored in the storage unit 54, the image condition adjustment unit 53 performs the charging bias adjustment operation prior to the next image forming operation. Just do it. As a result, even when the temperature and humidity inside and outside the image forming apparatus 10 change greatly, the occurrence of image defects due to the fluctuation of the surface potential of the photosensitive drum 20 is prevented. The temperature / humidity detection timing by the environment sensor 64 may be executed at regular time intervals. In addition, the temperature and humidity at the time when the previous charging bias adjustment operation was performed are stored in the storage unit 54, and the execution of the charging bias adjustment operation may be determined when the amount of variation from the stored temperature and humidity is large. .

  Third, the image condition adjustment unit 53 may execute the charging bias adjustment operation when the number of prints printed within a predetermined period exceeds a threshold value set in advance and stored in the storage unit 54. . When a long-time image forming operation is continuously performed, the surface potential of the photosensitive drum 20 is likely to fluctuate due to a temperature rise of the photosensitive drum 20 or a change in characteristics of the charging roller 21A. Therefore, when the number of printed sheets within a predetermined time is large, the surface potential V0 of the photosensitive drum 20 is adjusted with high accuracy, thereby preventing image defects.

  Note that the execution timing of the charging bias adjustment operation as described above is substantially the same as the timing of the calibration operation (adjustment of developability, exposure amount, and color misregistration) executed in the image forming apparatus 10. For this reason, the image condition adjustment unit 53 may perform the charging bias adjustment operation simultaneously with the execution of the calibration operation. FIG. 16 is a flowchart of the calibration operation according to the present embodiment. As an example, when the image forming apparatus 10 is left unused from the previous night, and the image forming apparatus 10 is turned on the next morning, the image condition adjustment unit 53 performs the calibration operation of FIG. Run. The image condition adjustment unit 53 executes development bias calibration (step S51). In the calibration, the DC bias value of the developing bias, the AC bias waveform, and the like are adjusted according to the temperature / humidity detection result of the environment sensor 64. Next, the image condition adjustment unit 53 executes a charging bias adjustment operation (charging bias correction) (step S52) according to the present embodiment. Thereafter, the image condition adjustment unit 53 performs light amount calibration of the exposure device 22 (step S53). Here, the laser light amount of the exposure device 22 is adjusted so that the density of the halftone image is appropriate. Thereafter, the image condition adjustment unit 53 executes gradation table correction (density gradation adjustment calibration) (step S54). Here, continuous gradation density is adjusted from a low density area to a high density area. Thereafter, the image condition adjustment unit 53 performs registration correction (step S55). Here, a color shift or the like of the full color image is adjusted.

  Thus, in this embodiment, after the charging bias adjustment operation (step S52) is executed by the image condition adjustment unit 53, the calibration operation (step S54) for adjusting the density gradation of the toner image is executed. Therefore, the density gradation of the toner image is adjusted while the surface potential V0 of the photosensitive drum 20 is stably maintained. As a result, stable image quality can be obtained in subsequent image forming operations.

<Regarding Correction of Charging Bias Vref>
Next, a sixth embodiment of the present invention will be described. Note that this embodiment is different from the previous embodiments in that the charging bias Vref is predicted and controlled in advance prior to the charging bias adjustment operation. Therefore, only the difference will be described and other common controls will be described. Description of the aspect is omitted. Vref used in the charging bias adjustment operation is desirably a value that can accurately reproduce the target surface potential V0 of the photosensitive drum 20. However, the charging bias Vref necessary to reproduce the same target potential V0 is likely to change greatly depending on the environment (temperature and humidity), the usage time of the photosensitive drum 20 (the degree of deterioration of the surface layer of the photosensitive drum 20), and the like. Therefore, in the present embodiment, the image condition adjustment unit 53 corrects the value of the charging bias Vref according to a predetermined correction condition prior to the charging bias adjustment operation.

  Table 1 shows the correction amount of the charging bias Vref corrected by the image condition adjustment unit 53 when the temperature and humidity detected by the environment sensor 64 change. The correction amount is stored in the storage unit 54 in advance. As an example, when the detected temperature and humidity is 18 degrees and 30% RH, a value obtained by adding 76 V to a predetermined reference value is set as the charging bias Vref, and the charging bias adjustment operation is started. . According to such correction, the adjustment operation is performed in a potential region close to the actual target potential V0 even when the characteristics of the photosensitive drum 20 and the charging device 21 change according to temperature and humidity. The charging bias adjustment operation is realized quickly and accurately.

  Table 2 shows the correction amount of the charging bias Vref corrected by the image condition adjusting unit 53 in accordance with the driving time of the photosensitive drum 20 detected by the counting unit 55. The correction amount is stored in the storage unit 54 in advance. As an example, when the detected driving time of the photosensitive drum 20 is 50 hours, a charging bias adjusting operation is started after a value obtained by adding 50 V to a predetermined reference value is set as the charging bias Vref. The In this case, even when the charging characteristics of the photosensitive drum 20 change according to the driving time of the photosensitive drum 20, the charging bias adjustment operation is realized quickly and accurately. In another modified embodiment, the counting unit 55 may count the cumulative application time of the charging bias by the charging device 21. Then, the correction values as shown in Table 2 may be stored in the storage unit 54 in advance according to the cumulative application time of the charging bias. Also in this case, even when the charging characteristic of the charging roller 21A changes according to the cumulative application time of the charging bias, the charging bias adjustment operation is realized quickly and accurately. Note that the above-described correction amounts may be combined with each other, and the charging bias Vref may be adjusted according to the temperature and humidity outside the image forming apparatus 10 and outside the image forming apparatus, the driving time of the photosensitive drum 20, and the like. Further, the charging bias Vref may be adjusted according to other correction conditions. Each correction value may be stored as a predetermined correction expression instead of a table. Then, after the charging bias Vref is corrected, the same charging bias adjustment operation as in the previous embodiments is executed.

  The image forming apparatus 10 according to the embodiment of the present invention has been described in detail above, but the present invention is not limited to this. The present invention can take, for example, the following modified embodiments.

In each of the above-described embodiments, the toner is charged with a positive polarity. However, the present invention is not limited to this. When the toner is charged to a negative polarity, the same charging bias adjustment control can be executed with the polarity of each bias reversed.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 13 Image forming part 14 Intermediate transfer unit 141 Intermediate transfer belt 145 Secondary transfer roller 20 Photosensitive drum 21 Charging device 21A Charging roller 22 Exposure device 23 Developing device 23C Developing roller 24 Primary transfer roller 50 Control unit 51 Drive control Unit 52 bias control unit 53 image condition adjustment unit (bias adjustment unit)
54 Storage Unit 55 Counting Unit 61 Drive Unit 62 Charging Bias Application Unit 63 Development Bias Application Unit 64 Environmental Sensor (Environment Detection Unit)
65 Concentration sensor (concentration measurement unit)

Claims (5)

The device body;
An image forming unit disposed in the apparatus main body and including a plurality of color units for forming a color toner image and a black unit for forming a black toner image;
A transfer device for transferring the color toner image and the black toner image from the image forming unit to a sheet;
Have
The plurality of color units and the black unit of the image forming unit are respectively
A photosensitive drum having a peripheral surface on which an electrostatic latent image including a background portion and an image portion is formed, and rotated in a predetermined rotation direction;
A charging device disposed in contact with or in proximity to the peripheral surface of the photosensitive drum, and charging the peripheral surface to a predetermined potential;
A developing roller disposed opposite to the photosensitive drum, and a developing device that develops the electrostatic latent image into a toner image by supplying toner to the photosensitive drum;
A bias adjusting unit for performing a charging bias adjusting operation for adjusting the potential of the background portion to a predetermined target potential among the electrostatic latent images of the plurality of photosensitive drums;
A density measuring unit for measuring the density of the toner image;
In the charging bias adjustment operation, the bias adjustment unit forms a plurality of toner images on a peripheral surface of a specific photosensitive drum among the photosensitive drums of the plurality of units of the image forming unit, and the density measurement unit The charging bias value corresponding to the target potential of the specific photosensitive drum is determined from the density measurement results of the plurality of toner images measured by the step, and based on the determined charging bias value, An image forming apparatus, wherein a value of the charging bias corresponding to the target potential in a photosensitive drum other than the specific photosensitive drum is derived. A storage unit for storing information on replacement of the photosensitive drums of the plurality of units of the image forming unit;
A counting unit that counts the number of printed sheets of the sheet onto which the toner image is transferred for each of the plurality of photosensitive drums;
With
In the charging bias adjustment operation, the bias adjusting unit is configured to output the photosensitive drum having the maximum number of printed sheets and the photosensitive drum having the minimum number of printed sheets when any of the plurality of photosensitive drums is replaced. , The charging bias value corresponding to the target potential is determined, and other photosensitive drums other than the specific photosensitive drum are determined based on the determined charging bias value. The image forming apparatus according to claim 1, wherein a value of the charging bias corresponding to the target potential is derived.   In the charging bias adjustment operation, when none of the plurality of photosensitive drums has been replaced, the bias adjustment unit uses one photosensitive drum of the plurality of units as the specific photosensitive drum, A value of the charging bias corresponding to the target potential is determined, and the determined charging bias is set as the charging bias corresponding to the target potential in the other photosensitive drums of the plurality of units. The image forming apparatus according to claim 2. The transfer device includes:
An intermediate transfer belt on which the color toner image and the black toner image are superimposed and transferred from the plurality of photosensitive drums;
A plurality of primary transfer rollers that respectively contact the plurality of photosensitive drums with the intermediate transfer belt interposed therebetween,
A contact / separation mechanism that collectively contacts and separates the primary transfer roller corresponding to the color unit among the plurality of primary transfer rollers with respect to the photosensitive drum,
In the charging bias adjustment operation, the bias adjustment unit, when none of the plurality of photosensitive drums has been replaced, of the photosensitive drum of the black unit and the plurality of color units. The photosensitive drum of one unit is used as the specific photosensitive drum, the value of the charging bias corresponding to the target potential is determined, and the charging bias determined in the one unit is used as the plurality of colors. The image forming apparatus according to claim 2, wherein the charging bias corresponding to the target potential in the photosensitive drum of the other unit is used. A charging bias applying unit that applies a predetermined charging bias to the charging device;
A developing bias applying unit for applying a predetermined developing bias to the developing roller;
Further comprising
In the charging bias adjusting operation, the bias adjusting unit controls the charging bias applying unit to form a plurality of potential regions having different potential magnitudes along the rotation direction on the peripheral surface of the photosensitive drum. In addition, by controlling the developing bias application unit and applying the predetermined developing bias corresponding to the target potential to the developing roller, a plurality of toner images are generated according to a potential difference between the developing bias and the plurality of potential regions. A charging bias value at which the density of the toner image becomes zero is derived from the relationship between the density measurement results of the plurality of toner images and the plurality of charging biases corresponding to the plurality of potential regions, 5. The derived charging bias is determined as the charging bias corresponding to the target potential. 6. Image forming apparatus.

Images