JP2017097033A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image formation device capable of setting surface potential of a photoreceptor drum to target potential with a simple configuration.SOLUTION: An image condition adjustment section 53 executes charging bias adjustment operation to adjust potential of a photoreceptor drum 20 to a target potential V0, forms a predetermined potential region on a periphery of the photoreceptor drum 20 in the charging bias adjustment operation, by applying certain charging bias to a charging bias application section 62, forms a toner image on the periphery of the photoreceptor drum 20 by controlling a developing bias application section 63 to change the developing bias in a manner to intersect with the potential region, and decides a value of the charging bias corresponding to the target potential V0 on the basis of a density measurement result of the toner image by a density sensor 65.SELECTED DRAWING: Figure 4

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 a surface potential of a photosensitive drum to a target potential with a simple configuration.

  An image forming apparatus according to one aspect of the present invention includes a photosensitive drum that includes an apparatus main body, a peripheral surface on which an electrostatic latent image including a background portion and an image portion is formed, and is rotated in a predetermined rotation direction. A charging device that is disposed in contact with or close to the peripheral surface of the photosensitive drum and charges the peripheral surface to a predetermined potential; and exposure light is applied to the peripheral surface of the photosensitive drum charged to the predetermined potential. An exposure device that forms the electrostatic latent image by irradiating and a developing roller disposed to face the photoconductive drum, and supplying the toner to the photoconductive drum allows the electrostatic latent image to be formed. A developing device that develops a toner image; a charging bias applying unit that applies a predetermined charging bias to the charging device; a developing bias applying unit that applies a predetermined developing bias to the developing roller; and the static of the photosensitive drum. The background of the electric latent image An image condition adjusting unit that performs a charging bias adjusting operation for adjusting the potential of the toner image to a predetermined target potential, and a density measuring unit that measures the density of the toner image, and the image condition adjusting unit includes the charging bias In the adjustment operation, a predetermined charging area is formed on the peripheral surface of the photosensitive drum by applying a constant charging bias to the charging bias applying section, and the developing bias applying section is controlled to control the potential area. By changing the developing bias so as to intersect with the photosensitive drum, a toner image is formed on the peripheral surface of the photosensitive drum, and the target potential is set based on the density measurement result of the toner image by the density measuring unit. The value of the corresponding charging bias is determined.

  According to this configuration, the surface potential of the photosensitive drum can be set to a target potential with a simple configuration. Further, a toner image is formed by the potential difference between the potential region on the photosensitive drum and the developing roller. At this time, in forming the potential region, the charging bias application unit applies a constant charging bias. For this reason, even when the potential characteristic of the photosensitive drum is difficult to be stabilized due to the physical properties and usage environment of the photosensitive drum, a toner image is formed by a change in the developing bias, and thus a toner image having a stable density is formed. be able to. As a result, the value of the charging bias corresponding to the target potential of the photosensitive drum can be accurately determined based on the density of the toner image.

  In the above configuration, the image condition adjustment unit controls the charging bias application unit to charge the peripheral surface of the photosensitive drum to a first background portion potential in the charging bias adjustment operation, and then The exposure apparatus is controlled to irradiate the exposure light on the peripheral surface to form a first potential region consisting of a first potential, and the developing bias application unit is controlled to the developing roller. By applying the developing bias obtained by adding a preset first differential potential to the first potential, a first toner image is formed by a potential difference between the first potential region and the developing roller, and A second potential region is formed by applying a first provisional charging bias that is set in advance corresponding to the target potential on the peripheral surface of the photosensitive drum by controlling a charging bias applying unit. The development bias applied to the development roller by controlling the development bias application unit is set to the first differential potential to the target potential from a value obtained by subtracting a preset second differential potential from the target potential. The second toner image is formed by the potential difference between the second potential region and the developing roller, and is measured by the density measuring unit. It is preferable to determine the value of the charging bias corresponding to the target potential from the density measurement results of the first toner image and the second toner image.

  According to this configuration, the first toner image is created by the potential difference between the first potential region on the photosensitive drum and the developing roller. The potential difference at this time is the first potential difference within a predetermined error range. On the other hand, a second toner image is formed by the potential difference between the second potential region on the photosensitive drum and the developing roller. At this time, a value obtained by adding the first differential potential to the target potential is applied to the developing roller. As a result, it is possible to determine the deviation of the first temporary charging bias from the target potential based on the relationship between the charging bias in the first toner image and the density of the toner image. For this reason, the surface potential of the photosensitive drum can be set to a target potential with a simple configuration.

  In the above-described configuration, the image condition adjustment unit may use the first provisional charging bias when the density of the first toner image is higher than the density of the second toner image in the charging bias adjustment operation. If the density of the first toner image is lower than the density of the second toner image, a smaller value is determined as the charging bias corresponding to the target potential. It is desirable to determine a value as the charging bias corresponding to the target potential.

  According to this configuration, the charging bias corresponding to the target potential can be easily determined from the density comparison result between the first toner image and the second toner image.

  In the above-described configuration, it is desirable that the exposure apparatus forms the first potential region by irradiating the first background portion potential with the exposure light corresponding to a 100% solid image.

  According to this configuration, the first potential in the first potential region is suppressed from being affected by the first background portion potential.

  In the above structure, the first potential is preferably 0V.

  According to this configuration, the first toner image is stably formed according to the first differential potential.

  In the above configuration, it is preferable that the first background portion potential is set smaller than the target potential.

  According to this configuration, even when a two-component developer is used in the developing device, a large amount of carrier is prevented from moving toward the photosensitive drum during the charging bias adjustment operation.

  In the above configuration, the image condition adjusting unit is preset in correspondence with the target potential on the peripheral surface of the photosensitive drum by controlling the charging bias applying unit in the charging bias adjusting operation. A third potential region is formed by applying a second provisional charging bias, and the developing bias applied to the developing roller by controlling the developing bias applying unit is set in advance from the target potential. The value obtained by subtracting the third potential region from the value obtained by subtracting the third potential region from the value obtained by subtracting the third potential region from the value obtained by subtracting the third potential region, A plurality of third toner images are formed by a potential difference with the developing roller, and the target voltage is determined based on the density measurement results of the plurality of third toner images measured by the density measuring unit. It is desirable to determine the value of the charging bias corresponding to.

  According to this configuration, it is possible to form toner images having different densities by providing a plurality of levels of the developing bias applied to the developing roller.

  In the above-described configuration, the image condition adjustment unit is configured so that, in the charging bias adjustment operation, the density of the toner image becomes zero based on the relationship between the density measurement result of the plurality of third toner images and the development bias. It is desirable to derive a value of the developing bias and determine that the derived value of the developing bias is a surface potential of the third potential region charged by the second temporary charging bias.

  According to this configuration, when the developing bias and the surface potential of the photosensitive drum are the same potential, the third potential charged by the second temporary charging bias is utilized by utilizing the fact that the density of the toner image becomes substantially zero. The surface potential of the region can be estimated.

  In the above configuration, the image condition adjustment unit sets a value obtained by correcting the second provisional charging bias based on a difference between the determined surface potential of the third potential region and the target potential as the target potential. It is desirable to determine the value of the corresponding charging bias.

  According to this configuration, the value of the charging bias corresponding to the target potential can be easily determined based on the difference between the surface potential of the third potential region and the target potential.

  In the above configuration, the image condition adjusting unit is preset in correspondence with the target potential on the peripheral surface of the photosensitive drum by controlling the charging bias applying unit in the charging bias adjusting operation. A fourth potential region is formed by applying a third provisional charging bias, and the developing bias applied to the developing roller by controlling the developing bias applying unit is set in advance from the target potential. The fourth toner image is formed by the potential difference between the fourth potential region and the developing roller by continuously increasing the value obtained by subtracting the difference potential between the fourth potential region and the developing roller. It is desirable to determine the value of the charging bias corresponding to the target potential from the density measurement result of the fourth toner image measured by the density measuring unit.

  According to this configuration, it is possible to form a toner image whose density changes continuously by the development bias that is continuously changed. Since the value of the charging bias corresponding to the target potential is determined based on the density of the toner image, the charging bias adjustment operation is completed in a short time.

  In the above-described configuration, the image condition adjusting unit may be configured such that, in the charging bias adjustment operation, the developing bias at which the density of the toner image becomes zero based on the relationship between the density measurement result of the fourth toner image and the developing bias. It is desirable that the value of the development bias is derived to be determined as the surface potential of the fourth potential region charged by the third provisional charging bias.

  According to this configuration, when the developing bias and the surface potential of the photosensitive drum are the same potential, the fourth potential charged by the third provisional charging bias is utilized by utilizing the fact that the density of the toner image becomes substantially zero. The surface potential of the region can be estimated.

  In the above configuration, the image condition adjustment unit sets a value obtained by correcting the third provisional charging bias based on a difference between the determined surface potential of the fourth potential region and the target potential as the target potential. It is desirable to determine the corresponding charging bias value.

  According to this configuration, the value of the charging bias corresponding to the target potential can be determined based on the difference between the surface potential of the fourth potential region and the target potential.

  According to the present invention, there is provided an image forming apparatus capable of setting the surface potential of the photosensitive drum 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. 3 is a flowchart of a charging bias adjustment operation according to the first embodiment of the present invention. 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. It is a flowchart 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 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. FIG. 6 is a schematic diagram showing a relationship between a developing bias and a toner image density in a charging bias adjustment operation according to a modified embodiment of the present 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). 13M for magenta, 13C for cyan using cyan (C) developer, 13Y for yellow using yellow (Y) developer, and black using black (Bk) developer A unit 13Bk is provided. 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 driving roller 142, a tension roller 143, a plurality of primary transfer rollers 24, and a belt cleaning device 144.

  The intermediate transfer belt 141 is an endless belt-like rotator, and is stretched around the drive roller 142 and the tension 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 left-right direction, and carries the toner images 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 tension roller 143 is driven to rotate on the left end side of the intermediate transfer unit 14. The tension roller 143 stretches the intermediate transfer belt 141. The tension roller 143 applies tension to the intermediate transfer belt 141. In the vicinity of the tension 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 forms a primary transfer nip portion with the photosensitive drum 20, and primarily transfers the toner image on the photosensitive drum 20 onto the intermediate transfer belt 141. 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 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 respectively store replenishment toners for each color of MCYBk, 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 formed on 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 on the image density output from the density sensor 65 is referred to by an image condition adjusting unit 53 described later, and is reflected in a charging bias adjusting 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 according to an image forming operation of the image forming apparatus 10 and a charging bias adjusting 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. 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 V0.

  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.

  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.

<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 flowchart of the charging bias adjustment operation according to the present embodiment. FIG. 4 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. In FIG. 4, the surface potential of the photosensitive drum 20 is Vdr, and the DC bias potential of the developing roller 23C is Vdc. 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, the charging bias adjustment operation is sequentially performed on the photosensitive drums 20 of the respective colors. In other embodiments, the charging bias adjustment operations on the photosensitive drums 20 of a plurality of colors may be executed in parallel.

  Referring to FIG. 3, the charging bias adjustment operation includes formation of band latent image 1 (step S1), development of band latent image 1 (step S2), density measurement of band toner image 1 (step S3), and band toner image. 2 (step S4), density measurement of the band toner image 2 (step S5), and determination of the charging bias (step S6). Broadly speaking, the charging bias adjustment operation includes a first stage until the density measurement of the belt toner image 1 (step S3), a second stage until the density measurement of the band toner image 2 (step S5), and determination of the charging bias. It is classified into the third stage of (Step S6). 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 the band latent image 1 in FIG. 3 (step S1). 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). The storage unit 54 (FIG. 2) stores a value of the charging bias Vref 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). Even if the charging bias Vref is applied to the charging roller 21A of the charging device 21, the surface potential of the photosensitive drum 20 is not necessarily set to V0 (V). For this reason, the above-described charging bias adjustment operation is required.

  In step S1, 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 the intermediate potential (Vm) (first background portion potential) (FIG. 4). 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. If 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 S4 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. Therefore, it is desirable that the surface potential Vdr (Vm) of the background portion of the photosensitive drum 20 in step S1 is higher than the developing bias Vdc. 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 band latent image 1 (first potential region) composed of the image portion potential V1 (first potential) is formed on the peripheral surface of the photosensitive drum 20.

  In step S2, the belt latent image 1 is developed. The image condition adjusting unit 53 controls the developing bias applying unit 63 to add a preset potential a (V) (first differential potential) to the developing roller 23C to the image unit potential V1 (V). By applying the developing bias Vdc (V1 + a), the band latent image 1 (first potential region) formed in step S1 is developed. As a result, the band toner image 1 (I1 in FIG. 4) (first) 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 V1 + a (V) is applied and the band latent image 1. 1 toner image) is formed. 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 50-200V, and 100-150V is still more preferable. Further, a plurality of levels of patch toner images may be formed while the value of a is changed.

  In step S3, density measurement of the belt toner image 1 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, the band toner image 2 is formed. Here, the image condition adjusting unit 53 controls the charging bias applying unit 62 to apply the charging bias Vref (first provisional charging bias) to the charging roller 21A. In the present embodiment, on the right side of FIG. 4, the region where the surface potential Vdr of the photoconductor is constant corresponds to the second potential region. At this stage, the surface potential Vdr 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 developing bias applying unit 63 to subtract a preset c (V) (second differential potential) from the target potential V0 as the developing bias applied to the developing roller 23C. From the measured value (V0−c) to the value (V0 + a) obtained by adding a (V) (first differential potential) to the target potential V0, the surface potential Vdr (second potential region) of the photosensitive drum 20 is crossed. To change. As a result, the belt toner image 2 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 + a (V) is applied and the surface potential Vdr (Vref) of the photosensitive drum 20. (I2 in FIG. 4) (second toner image) is formed. In this embodiment, c = 50 V is set, and the value of c is also stored in the storage unit 54 in advance. In addition, the range of the value of preferable c is 20-100V, and 50-70V is still more preferable.

  In step S5, the image condition adjusting unit 53 controls the density sensor 65 to execute the density measurement of the belt toner image 2 formed in step S4.

  In step S6, the density D1 of the band toner image 1 measured in step S3 is compared with the density D2 of the band toner image 2 measured in step S5, and the charging bias Vref is corrected as necessary. As described above, in step S1, the potential difference between the image portion potential V1 and the developing bias Vdc is a (V). Therefore, the density D1 of the belt toner image 1 is formed by the movement of the toner with respect to the potential difference a (V) between the photosensitive drum 20 and the developing roller 23C. In step S4, 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. The potential difference from (V0 + a) is a (V). For this reason, since the density D2 of the belt toner image 2 is formed by the movement of the toner with respect to the potential difference a (v), the density D1 = the density D2.

  On the other hand, when the density D2 measured in step S5 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 S4 to S6 are performed 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 S5 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 S4 to S6, a plurality of levels of belt toner images may be formed while the value of a (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 a.

  As described above, in the present embodiment, the image condition adjusting unit 53 applies a constant charging bias to the charging bias applying unit 62 so that a predetermined potential region (second potential region) is formed on the peripheral surface of the photosensitive drum 20. ) And by changing the developing bias Vdc so as to cross the potential region by controlling the developing bias applying unit 63, a toner image (band toner image 2) is formed on the peripheral surface of the photosensitive drum 20. Form. Then, the image condition adjustment unit 53 determines a charging bias value corresponding to the target potential V 0 based on the density measurement result of the toner image by the density sensor 65. Therefore, the image forming apparatus 10 that can set the surface potential Vdr of the photosensitive drum 20 to the target potential V0 with a simple configuration without providing the surface potential meter facing the photosensitive drum 20 is provided. Is done. In forming a predetermined potential region, the charging bias application unit 62 applies a constant charging bias. For this reason, even if the potential characteristics of the photosensitive drum 20 are difficult to stabilize due to the physical properties of the photosensitive drum 20 and the usage environment, a toner image is formed by a change in the developing bias. Can be formed. As a result, the charging bias value corresponding to the target potential V0 of the photosensitive drum 20 can be accurately determined based on the density of the toner image.

  In the present embodiment, the belt toner image 1 is formed by the potential difference between the belt latent image 1 on the photosensitive drum 20 and the developing roller 23C. The potential difference at this time is the first potential difference (a) within a predetermined error range. As a result, based on the relationship between the charging bias of the belt toner image 1 and the density of the toner image, it is possible to determine the deviation of the first temporary charging bias (Vref) from the target potential V0. For this reason, the surface potential of the photosensitive drum 20 can be set to the target potential V0 with a simple configuration.

  Further, in the charging bias adjustment operation, when the density of the band toner image 1 is higher than the density of the band toner image 2, the image condition adjustment 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 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.

  Further, in the present embodiment, the exposure device 22 forms the image portion potential V1 by irradiating exposure light corresponding to a 100% solid image with respect to the intermediate potential Vm. For this reason, the image portion potential V1 is suppressed from being affected by the intermediate potential Vm. In another modified embodiment, the exposure device 22 may form the image portion potential V1 with exposure light corresponding to a 50% to 100% halftone image. Further, the image portion potential V1 may be set to 0V. In this case, the developing bias Vdc is set to a (V). Therefore, the belt toner image 1 is stably formed according to a (V) (first differential potential).

  Next, the charging bias adjustment operation according to the second embodiment of the present invention will be described. FIG. 5 is a flowchart of the charging bias adjustment operation according to the present embodiment. FIG. 6 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. In the present embodiment, the description will focus on differences from the first embodiment.

  Referring to FIG. 5, the charging bias adjustment operation according to the present embodiment includes three operations: band toner image formation (step S11), band toner image density measurement (step S12), and charging bias determination (step S13). It consists of steps. Also in this 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).

  When the charging bias adjustment operation is executed, the image condition adjustment unit 53 executes the formation of the band toner image (step S11) in FIG.

  In step S11, 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 of this embodiment corresponds to the second provisional charging bias of the present invention. The image condition adjusting unit 53 controls the charging bias applying unit 62 to apply the charging bias Vref, so that a potential region (third potential region) where the surface potential is constant as shown in FIG. It is formed on the peripheral surface. The image condition adjustment unit 53 forms a plurality of toner images by changing the value of the developing bias applied to the developing roller 23C a plurality of times. In the present embodiment, as shown in FIG. 6, three belt toner images I1, I2, and I3 (a plurality of third toner images) are formed.

  Specifically, the image condition adjusting unit 53 controls the developing bias applying unit 63 to change the developing bias applied to the developing roller 23C from the target potential V0 of the photosensitive drum 20 to a preset potential c (V) (first). 3 (differential potential of 3) to a value (V0 + b) obtained by adding a preset potential b (V) (fourth differential potential) to the target potential V0 from the value (V0−c). It is changed so as to intersect with the surface potential Vdr (third potential region). As a result, a first belt toner image I1 (third toner image) is formed by the potential difference between the surface potential Vdr of the photosensitive drum 20 and the developing roller 23C. Thereafter, the image condition adjusting unit 53 once returns the developing bias Vdc to the above-described V0-c (V), and then adds a preset potential 2b (V) (fourth differential potential) to the target potential V0. Set to the value (V0 + 2b). As a result, a second belt toner image I2 (third toner image) is formed by the potential difference between the surface potential Vdr of the photosensitive drum 20 and the developing roller 23C.

  Thereafter, similarly, the image condition adjusting unit 53 sets the developing bias Vdc to a value (V0 + 3b) obtained by adding a preset potential 3b (V) (fourth differential potential) to the target potential V0. As a result, a third belt toner image I3 (third toner image) is formed by the potential difference between the surface potential Vdr of the photosensitive drum 20 and the developing roller 23C. In the present embodiment, as described above, since each band toner image is formed, the developing bias Vdc is set to V0 + n × b (V) (n: natural number). Each developing bias Vdc for forming a belt toner image is applied for a time t1. In this embodiment, a time t2 during which the developing bias Vdc is returned to V0-c (V) is provided between the toner image forming times of the respective bands.

  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 if the number of levels of the belt toner image 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 S <b> 12, the density sensor 65 performs density measurement of the plurality of band toner images formed in step S <b> 11.

  In step S13, a charging bias according to the target potential V0 is determined. The image condition adjustment unit 53 measures the densities of the plurality of development bias Vdc applied to the development bias application unit 63 when the plurality of band toner images are formed in step S11 and the plurality of band toner images measured in step S12. From the result (ID), the charging bias corresponding to the target potential V0 is determined.

  When the value of the developing bias Vdc increases, the potential difference between the developing roller 23C and the surface potential Vdr of the photosensitive drum 20 increases, so that the density of the belt toner image 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 adjusting unit 53 virtually forms a graph with the horizontal axis representing the value of the developing bias Vdc and the vertical axis representing the density of the belt toner image. These data are linearly regressed. At this time, data in a region where the detection accuracy of the density sensor 65 is relatively low may be excluded at the time of 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 adjustment unit 53 derives the horizontal axis intercept of the regression line, that is, the value of the developing bias Vdc at which the density of the belt toner image is virtually zero.

  Then, the image condition adjusting unit 53 determines that the value of the developing bias Vdc derived above is the surface potential Vdr of the photosensitive drum 20 charged by the charging bias Vref in Step 11. Then, the image condition adjusting unit 53 determines the value of the charging bias corresponding to the target potential V0 from the value obtained by correcting the charging bias Vref based on the difference between the surface potential Vdr and the target potential V0. That is, when the surface potential Vdr is larger than the target potential V0, a value smaller than the charging bias Vref is set as a charging bias value corresponding to the target potential V0. Conversely, when the surface potential Vdr is smaller than the target potential V0, a value larger than the charging bias Vref is set as a charging bias value corresponding to the target potential V0.

  Note that the derivation of the developing bias Vdc at which the density of the belt toner image becomes zero in step S13 is not limited to the above. A data portion with high linearity may be extracted from the plurality of data, and a regression line may be derived based on the data. In addition, a regression line is derived on the basis of two pieces of data selected from the plurality of data which are equal to or higher than a preset density (density threshold) and have a higher development bias Vdc. Also good. 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 adjustment operation, the image condition adjustment unit 53 changes the development bias Vdc a plurality of times after forming a constant potential region (third potential region) on the photosensitive drum 20. As a result, a plurality of band toner images are formed. Then, the image condition adjusting unit 53 determines a charging bias value corresponding to the target potential V0 from the density measurement results of the plurality of band toner images. For this reason, 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. Further, by providing a plurality of levels of the developing bias Vdc applied to the developing roller 23C, band toner images having different densities can be formed. Further, when the developing bias Vdc and the surface potential Vdr of the photosensitive drum 20 are the same potential, the surface of the photosensitive drum 20 charged by the charging bias Vref is utilized by utilizing the fact that the density of the toner image becomes substantially zero. The potential Vdr can be estimated. Further, the value of the charging bias corresponding to the target potential V0 can be determined based on the difference between the surface potential Vdr of the photosensitive drum 20 and the target potential V0.

  Next, the charging bias adjustment operation according to the third embodiment of the present invention will be described. FIG. 7 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. Note that this embodiment is mainly different from the previous second embodiment in the formation of the band toner image in step S11 in FIG. 5, and therefore, the difference will be mainly described and other common points will be described. Explanation of the points is omitted.

  Also in this embodiment, the image condition adjusting unit 53 refers to the charging bias Vref stored in the storage unit 54 (FIG. 2) in advance. The charging bias Vref of the present embodiment corresponds to the third provisional charging bias of the present invention. The image condition adjusting unit 53 controls the charging bias applying unit 62 to apply the charging bias Vref, so that a potential region (fourth potential region) where the surface potential Vdr is constant as shown in FIG. 20 on the peripheral surface. The image condition adjusting unit 53 forms a belt toner image by changing the value of the developing bias Vdc applied to the developing roller 23C.

  Specifically, the image condition adjusting unit 53 controls the developing bias applying unit 63 to change the developing bias Vdc applied to the developing roller 23C from a target potential V0 to a preset potential c (V) (fifth differential potential). ) Is continuously increased from time t0 so as to intersect the surface potential Vdr of the photosensitive drum 20 from the value (V0-c), so that the surface potential Vdr of the photosensitive drum 20 and the developing roller 23C are increased. A band toner image It (fourth toner image) is formed by the potential difference. In the present embodiment, as shown in FIG. 7, the developing bias Vdc is increased by ΔV (V) from V0−c (V) during the time t. As an example, the time t is set to 120 msec. In this embodiment, the peripheral speed (system speed) of the photosensitive drum 20 is set to 166 mm / sec. Here, the toner image is formed on the peripheral surface of the photosensitive drum 20 from time t1 when the developing bias Vdc exceeds the surface potential Vdr of the photosensitive drum 20. Thereafter, the toner image density is increased until time t2. In the present embodiment, at time t2, the developing bias Vdc is lowered to V0-c (V).

  Also in this embodiment, the density of the belt toner image It is measured by the density sensor 65 in step S12 of FIG. Thereafter, in step S13, a charging bias value corresponding to the target potential V0 of the photosensitive drum 20 is determined. As described above, the density of the belt toner image It is detected after time t1 in FIG. Therefore, the image condition adjusting unit 53 calculates the value of the developing bias Vdc corresponding to the time t1 based on the slope (ΔV / t) in FIG. As a result, the value of the developing bias Vdc from time t1 to time t2 and the density measurement result of the band toner image It are prepared. As in the second embodiment, the image condition adjustment unit 53 virtually forms a graph with the horizontal axis representing the value of the developing bias Vdc and the vertical axis representing the density of the band toner image It. Then, the image condition adjusting unit 53 derives the horizontal axis intercept of the regression line drawn by these data, that is, the value of the developing bias Vdc at which the toner image density is virtually zero. The image condition adjusting unit 53 determines that the value of the developing bias Vdc derived as described above is the surface potential Vdr of the photosensitive drum 20 charged by the charging bias Vref in step S11. Then, the image condition adjusting unit 53 determines the value of the charging bias corresponding to the target potential V0 from the value obtained by correcting the charging bias Vref based on the difference between the surface potential Vdr and the target potential V0.

  As described above, in this embodiment, it is possible to form a belt toner image It whose density changes continuously by the development bias Vdc that is continuously changed. Therefore, the image forming apparatus 10 that can set the surface potential Vdr of the photosensitive drum 20 to the target potential V0 with a simple configuration without providing the surface potential meter facing the photosensitive drum 20 is provided. Is done. Since the value of the charging bias corresponding to the target potential V0 is determined based on the density of the belt toner image It, the charging bias adjustment operation is completed in a short time. In other words, in this embodiment, the standby time corresponding to the time t2 in FIG. 6 is reduced, and the charging bias adjustment operation can be reduced.

  Also in the present embodiment, when the developing bias Vdc and the surface potential Vdr of the photosensitive drum 20 are the same potential, the toner image is charged by the charging bias Vref using the fact that the density of the toner image becomes substantially zero. The surface potential Vdr of the photosensitive drum 20 can be estimated. Based on the difference between the surface potential Vdr of the photosensitive drum 20 and the target potential V0, the value of the charging bias corresponding to the target potential V0 can be determined.

  In the present embodiment, the development bias Vdc data and the band toner image It density data are associated with each other based on the times t1 and t2 in step S13 of FIG. Is not limited to this. FIG. 7 is a schematic diagram showing the relationship between the developing bias Vdc and the density of the belt toner image in the charging bias adjustment operation according to the modified embodiment of the present embodiment.

  In the present modified embodiment, the development bias Vdc is continuously increased until time tm (Vdc1 in FIG. 8) and continuously decreased (Vdc2 in FIG. 8) with the transition of time t. As a result, the density data D detected by the density sensor 65 also has a value that continuously increases (D1 in FIG. 8) and then continuously decreases (D2 in FIG. 8). Therefore, the image condition adjusting unit 53 aligns the peak value Vt of the developing bias Vdc and the peak value Dt of the density data D as data at the same time tm. Thereafter, the development bias Vdc and the density data D are associated with each other at times t1, t2 and times t3, t4 after the time tm.

  According to such processing, even when there is a deviation in the application start time of the development bias Vdc by the development bias application unit 63, the development bias Vdc and the density data D can be associated with each other with high accuracy. In step S13 of FIG. 5, the value of the charging bias corresponding to the target potential V0 can be determined through the same calculation as that of the third embodiment. At this time, the image condition adjustment unit 53 may selectively use a plurality of collected data as in the third embodiment.

<Performance timing of charging bias adjustment operation>
Next, the execution timing of the charging bias adjustment operation according to each of the above 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 Vdr of the photosensitive drum 20 is accurately adjusted to the target potential V0, thereby preventing the occurrence of 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. 9 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 night before and the power of 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 first executes development bias calibration (step S21). 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 S22) according to the present embodiment. Thereafter, the image condition adjustment unit 53 performs light amount calibration of the exposure device 22 (step S23). 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 S24). 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 S25). Here, a color shift or the like of the full color image is adjusted.

  As described above, in this embodiment, after the charging bias adjustment operation (step S22) is executed by the image condition adjustment unit 53, the calibration operation (step S24) 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 fourth embodiment of the present invention will be described. Note that this embodiment is different from the first to third embodiments in that the charging bias Vref is predicted and controlled in advance, so only the difference will be described and other common control modes will be described. Will not be described. The charging bias 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, the charging bias adjustment operation is started after a value obtained by adding 50 V to a predetermined reference value is set as the charging bias Vref. . 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 characteristics of the charging roller 21A change 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 / humidity inside and outside the image forming apparatus 10 and the driving time of the photosensitive drum 20. 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.

  (1) In each of the above embodiments, the toner has been described as being charged with a positive polarity, but 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.

  (2) In each of the above embodiments, the image forming apparatus 10 is described as a full-color image forming apparatus, but the present invention is not limited to this. The image forming apparatus 10 may form a single color image such as a monochrome printer.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 13 Image forming part 14 Intermediate transfer unit (transfer apparatus)
141 Intermediate transfer belt 145 Secondary transfer roller (transfer device)
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 54 Storage unit 55 Count unit 61 Drive unit 62 Charging Bias application unit 63 Development bias application unit 64 Environmental sensor 65 Density sensor (density measurement unit)

Claims (12)

The device body;
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;
An exposure apparatus that forms the electrostatic latent image by irradiating exposure light onto a peripheral surface of the photosensitive drum charged to the predetermined potential;
A developing device provided with a developing roller disposed to face the photosensitive drum, and developing the electrostatic latent image into a toner image by supplying toner to the photosensitive drum;
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;
An image condition adjustment unit that performs a charging bias adjustment operation for adjusting the potential of the background portion of the electrostatic latent image on the photosensitive drum to a predetermined target potential;
A density measuring unit for measuring the density of the toner image;
Have
In the charging bias adjusting operation, the image condition adjusting unit forms a predetermined potential region on the peripheral surface of the photosensitive drum by applying a constant charging bias to the charging bias applying unit, and the developing A toner image is formed on the peripheral surface of the photosensitive drum by controlling the bias applying unit to change the developing bias so as to cross the potential region, and the density of the toner image by the density measuring unit An image forming apparatus that determines a value of the charging bias corresponding to the target potential based on a measurement result. The image condition adjustment unit, in the charging bias adjustment operation,
By controlling the charging bias application unit to charge the peripheral surface of the photosensitive drum to a first background portion potential, the exposure device is controlled to irradiate the peripheral surface with the exposure light. The first potential region including the first potential is formed, and the developing bias application unit is controlled to add a preset first difference potential to the first potential with respect to the developing roller. By applying a developing bias, a first toner image is formed by a potential difference between the first potential region and the developing roller,
A second potential region is formed by controlling the charging bias application unit and applying a first provisional charging bias set in advance corresponding to the target potential on the peripheral surface of the photosensitive drum, The development bias applied to the development roller by controlling the development bias application unit is set to the first differential potential to the target potential from a value obtained by subtracting a preset second differential potential from the target potential. Is changed so as to intersect the second potential region up to a value added with the above, thereby forming a second toner image by a potential difference between the second potential region and the developing roller,
2. The charging bias value corresponding to the target potential is determined from density measurement results of the first toner image and the second toner image measured by the density measurement unit. The image forming apparatus described.   In the charging bias adjustment operation, the image condition adjusting unit sets a value smaller than the first provisional charging bias when the density of the first toner image is higher than the density of the second toner image. When the charging bias corresponding to the target potential is determined and the density of the first toner image is lower than the density of the second toner image, a value larger than the first temporary charging bias is set to the target potential. The image forming apparatus according to claim 2, wherein the charging bias is determined as the charging bias corresponding to the charging bias.   4. The exposure apparatus according to claim 2, wherein the first potential region is formed by irradiating the exposure light corresponding to a 100% solid image with respect to the first background portion potential. The image forming apparatus described.   The image forming apparatus according to claim 2, wherein the first potential is 0V.   The image forming apparatus according to claim 2, wherein the first background portion potential is set to be smaller than the target potential. The image condition adjustment unit, in the charging bias adjustment operation,
A third potential region is formed by controlling the charging bias application unit and applying a second provisional charging bias set in advance corresponding to the target potential on the peripheral surface of the photosensitive drum,
The development bias applied to the development roller by controlling the development bias application unit is set to a fourth value preset to the target potential from a value obtained by subtracting a preset third differential potential from the target potential. A plurality of third toner images are formed by a potential difference between the third potential region and the developing roller, by changing a plurality of times so as to intersect the third potential region up to a value added with the difference potential of
2. The image forming apparatus according to claim 1, wherein the charging bias value corresponding to the target potential is determined from density measurement results of the plurality of third toner images measured by the density measurement unit. .   In the charging bias adjustment operation, the image condition adjustment unit calculates the value of the development bias at which the density of the toner image is zero based on the relationship between the density measurement result of the plurality of third toner images and the development bias. 8. The image forming apparatus according to claim 7, wherein the image forming apparatus derives and determines that the derived developing bias value is a surface potential of the third potential region charged by the second temporary charging bias. 9. .   The image condition adjustment unit calculates a value obtained by correcting the second temporary charging bias based on a difference between the determined surface potential of the third potential region and the target potential, and the charging bias corresponding to the target potential. The image forming apparatus according to claim 8, wherein the image forming apparatus determines the value of the image forming apparatus. The image condition adjustment unit, in the charging bias adjustment operation,
A fourth potential region is formed by controlling the charging bias application unit and applying a third provisional charging bias set in advance corresponding to the target potential on the peripheral surface of the photosensitive drum.
The development bias applied to the development roller by controlling the development bias application unit is set to intersect the fourth potential region from a value obtained by subtracting a preset fifth differential potential from the target potential. By continuously increasing, a fourth toner image is formed by the potential difference between the fourth potential region and the developing roller,
The image forming apparatus according to claim 1, wherein the charging bias value corresponding to the target potential is determined from a density measurement result of the fourth toner image measured by the density measuring unit.   In the charging bias adjustment operation, the image condition adjustment unit derives the value of the developing bias at which the density of the toner image becomes zero from the relationship between the density measurement result of the fourth toner image and the developing bias. The image forming apparatus according to claim 10, wherein the derived developing bias value is determined to be a surface potential of the fourth potential region charged by the third provisional charging bias.   The image condition adjustment unit calculates a value obtained by correcting the third provisional charging bias based on the difference between the determined surface potential of the fourth potential region and the target potential, and the charging bias corresponding to the target potential. The image forming apparatus according to claim 11, wherein the value is determined.

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