JP2020034799A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To appropriately determine the necessity of executing a calibration operation in an image forming apparatus including a developing device to which a two-component developing method is applied, based on a change amount of a charge amount of toner. A determination unit 985 causes a mode control unit 984 to execute a charge amount measurement mode when execution of calibration is requested, and a previous charge amount of the toner acquired in the previous charge amount measurement mode is the previous charge amount. A determination mode for determining whether or not it is necessary to execute the requested calibration is performed based on the charge amount and the current charge amount, which is the charge amount of the toner acquired in the current charge amount measurement mode. [Selection diagram] Figure 2

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 for forming an image on a sheet, an image forming apparatus including a photosensitive drum (image carrier), a developing device including a developing roller, and a transfer member has been known. When the electrostatic latent image formed on the photosensitive drum is revealed by the developing device in the developing nip portion, a toner image is formed on the photosensitive drum. The toner image is transferred to the sheet by the transfer member. As a developing device applied to such an image forming apparatus, a two-component developing technique using a developer containing a toner and a carrier is known.

  In an image forming apparatus using such a developing technique, a so-called calibration operation is performed to improve the quality of image density. In the calibration operation, electrostatic latent images of a plurality of preset image patterns are formed on the image carrier, and the developing bias applied to the developing roller is changed according to each image pattern. Then, by measuring the density of the toner image of each image pattern, a developing bias corresponding to a suitable image density is selected.

  On the other hand, a technique for accurately estimating the charge amount of toner has conventionally been proposed. In Patent Literatures 1 and 2, the surface potential of the photosensitive drum before development and the surface potential of the toner layer on the photosensitive drum after development are measured, respectively. A development amount of the toner is calculated. Then, the toner charge amount is calculated from the measured surface potentials and the toner development amount.

  Further, in Patent Documents 3 and 4, a current value flowing into a developing roller carrying a developer is measured, and the measured current value is assumed to be a charge amount of the toner moved from the developing roller to the photosensitive drum. . Further, the amount of toner development is calculated from the image density measurement result of the developed toner layer. Then, the charge amount of the toner is calculated from the charge amount of the toner and the development amount of the toner.

JP 2003-345075 A JP 2004-37952 A Japanese Patent No. 5024192 Patent No. 5273542

  The calibration operation as described above is automatically executed at predetermined intervals in accordance with the usage status of the image forming apparatus such as the number of prints. Also, when the image forming apparatus is left unused all day and night, or when the surrounding environment of the image forming apparatus changes rapidly, such as the temperature and humidity around the image forming apparatus, the user may manually execute the calibration. Executed when an instruction is input.

  However, in the calibration operation, after forming electrostatic latent images of a plurality of image patterns on the image carrier, the developing bias applied to the developing roller is changed in accordance with each image pattern, and the toner image of each image pattern is formed. Generate. For this reason, if the calibration operation is performed in vain even though the image density is stable, the ratio of the time required to execute the calibration operation to the total use time of the image forming apparatus is large. As a result, the average printing speed of the image forming apparatus is reduced. Further, if the calibration operation is performed wastefully, the toner is wastefully consumed in order to generate toner images of a plurality of image patterns. Therefore, it is desired that the execution frequency of the calibration be minimized.

  In the techniques described in Patent Documents 1 and 2, a surface potential sensor is required to measure the surface potential on the photosensitive drum. Here, in order to measure the surface potential of the toner layer formed on the photosensitive drum, it is necessary to install a surface potential sensor downstream of the developing nip in the rotation direction of the photosensitive drum. However, if the surface potential sensor is installed at this position, the surface of the surface potential sensor is easily contaminated by the toner scattered from the developing roller, and it becomes difficult to accurately measure the surface potential over a long period of time.

  In the techniques described in Patent Documents 3 and 4, the current flowing into the developing roller includes the current flowing in the carrier in addition to the current flowing in the toner. Therefore, it is difficult to accurately calculate the charge amount of the toner from the current value. Further, when the resistance value of the carrier changes due to the peeling of the coat of the carrier or the contamination of the coat due to the repeated printing in the image forming apparatus, the current flowing through the carrier also changes. As described above, in the conventional method, it is difficult to correctly measure the charge amount of the toner from the current flowing into the developing roller.

  As described above, it is difficult to accurately measure the charge amount of the toner by the conventional technique for measuring the charge amount of the toner. For this reason, it has been difficult to avoid performing a useless calibration operation based on the amount of change in the amount of charge of the toner by combining the conventional calibration operation with a technique for measuring the amount of charge of the toner.

  SUMMARY An advantage of some aspects of the invention is to execute a calibration operation based on a change in a charge amount of a toner in an image forming apparatus including a developing device to which a two-component developing method is applied. The purpose is to appropriately determine the need for

  An image forming apparatus according to an aspect of the present invention includes an image carrier that is rotated to form an electrostatic latent image on a surface thereof, and that carries a toner image in which the electrostatic latent image is made visible, A charging device that charges the body to a predetermined charging potential; and a charging device that is disposed downstream of the charging device in the rotation direction of the image carrier, and converts the surface of the image carrier charged to the charging potential into predetermined image information. An exposure device that forms the electrostatic latent image by performing exposure in response to the exposure device, and a development device that is disposed to face the image carrier at a predetermined development nip portion downstream of the exposure device in the rotation direction. A developing device that includes a developing roller that forms a toner image by rotating and supporting a developer including a toner and a carrier on a peripheral surface and supplying toner to the image carrier; The obtained toner image A transfer unit that transfers the toner image to a developing roller, a developing bias applying unit that can apply a developing bias in which an AC voltage is superimposed on a DC voltage to the developing roller, a density detecting unit that detects the density of the toner image, and the charging device. And controlling the exposure device to form a plurality of electrostatic latent images for calibration on the image carrier and controlling the developing bias applying unit to control at least the developing bias for each electrostatic latent image for calibration. A plurality of calibration toner images are formed by varying the DC voltage, and image formation is performed based on a relationship between the densities of the plurality of calibration toner images detected by the density detection unit and the developing bias. A calibration for performing a calibration operation for determining the developing bias applied to the developing roller in operation. Controlling the charging device, the exposing device, and the developing bias applying unit to form a plurality of measurement toner images having different toner development amounts on the image carrier, and performing the density detection. The developing roller and the developing unit based on the density of the plurality of measurement toner images detected by the unit or when the plurality of measurement toner images are formed in addition to the density of the plurality of measurement toner images. A charge amount acquisition unit that executes a charge amount acquisition operation for acquiring a charge amount of toner based on a DC component of a development current flowing between the bias application unit, and a request unit that requests execution of the calibration operation. When the execution of the calibration operation is requested by the request unit, the charge amount acquisition unit is caused to execute the charge amount acquisition operation, and the charge amount acquisition operation is performed before the charge amount acquisition operation is performed. The necessity of executing the requested calibration operation is determined from the previous charge amount, which is the charge amount of the toner, and the current charge amount, which is the charge amount of the toner, acquired in the current charge amount acquisition operation. A calibration operation determination unit that performs a determination operation to be performed.

  According to this configuration, in the charge amount acquisition operation, the charge amount acquisition unit uses the densities of the plurality of measurement toner images, or the densities of the plurality of measurement toner images and the formation of the plurality of measurement toner images. Based on the DC component of the obtained development current, the charge amount of the toner included in the plurality of measurement toner images can be appropriately acquired.

  For this reason, in the determination operation, the calibration operation determination unit can grasp the change amount of the charge amounts of the two toners appropriately acquired in the current charge amount acquisition operation and the previous charge amount acquisition operation. Then, the necessity of the calibration operation requested this time can be appropriately determined according to the amount of change in the charge amount of the toner since the previous calibration operation was performed.

  In the above configuration, the calibration operation determination unit, in the determination operation, when the difference or ratio of the current charge amount to the previous charge amount is greater than a first threshold set in advance, the requested the said When it is determined that the calibration operation needs to be performed and the difference or ratio of the current charge amount to the previous charge amount is equal to or less than the first threshold, it is determined that the requested calibration operation need not be performed. It is desirable to do.

  According to this configuration, in the determination operation, the calibration operation determination unit is requested by the difference or ratio between the charge amounts of the toner obtained in the current charge amount acquisition operation and the previous charge amount acquisition operation in the determination operation. The necessity of the calibration operation can be appropriately determined.

  In the above configuration, the image forming apparatus further includes a toner concentration detection unit that detects a toner concentration indicating a ratio of a toner amount to a carrier amount contained in the developer, wherein the calibration operation determination unit performs the determination in the determination operation. When the difference or ratio of the current charge amount with respect to is less than or equal to the first threshold, the current charge amount acquisition operation for the first toner concentration detected by the toner concentration detection unit in response to the previous charge amount acquisition operation Accordingly, it is further determined whether the difference or ratio of the second toner concentration detected by the toner concentration sensor is smaller than a second threshold value set in advance, and the difference of the second toner concentration with respect to the first toner concentration or When the ratio is smaller than the second threshold value, it is desirable to determine that it is not necessary to execute the requested calibration operation.

  According to this configuration, the calibration operation determination unit determines in the determination operation not only the difference or ratio between the charge amounts of the toner obtained in the current charge amount acquisition operation and the previous charge amount acquisition operation, but also the current charge amount acquisition operation. The necessity of the calibration operation requested this time can be more appropriately determined based on the difference or ratio between the toner densities detected according to the charge amount acquisition operation and the previous charge amount acquisition operation.

  In the above configuration, when the execution of the calibration operation is requested by the requesting unit, the calibration operation determination unit may determine whether or not the execution of the determination operation is necessary prior to the determination operation. When the auxiliary determination operation determines that the execution of the determination operation is necessary, the determination operation is performed, and when it is determined that the execution of the determination operation is not necessary in the auxiliary determination operation, the previous charge amount In addition, it is desirable that the calibration operation execution unit be forced to execute the requested calibration operation regardless of the current charge amount.

  According to this configuration, the calibration operation determination unit performs the auxiliary determination operation prior to the determination operation, and in accordance with a change in the toner charge amount since the previous calibration operation was performed in the determination operation, The necessity of the calibration operation requested this time is determined, or the calibration operation requested this time is forcibly executed irrespective of a change in the charge amount of the toner since the previous calibration operation was executed. Can be determined.

  The above configuration further includes a temperature / humidity detecting unit that detects a characteristic value including at least one of ambient temperature and humidity, wherein the calibration operation determination unit performs the previous calibration operation in the auxiliary determination operation. The change amount of the second characteristic value detected by the temperature / humidity detecting unit in response to the auxiliary determination operation with respect to the first characteristic value detected by the temperature / humidity detecting unit is smaller than a third threshold value set in advance. It is determined whether it is small, and when the amount of change of the second characteristic value with respect to the first characteristic value is smaller than the third threshold value, it is determined that execution of the determination operation is necessary, and the first characteristic value is determined. When the amount of change of the second characteristic value is equal to or greater than the third threshold value, it is desirable to determine that the execution of the determination operation is not necessary.

  According to this configuration, the calibration operation determination unit determines that a change amount of the characteristic value detected in response to the request for performing the calibration operation with respect to the characteristic value detected in accordance with the previous calibration operation is When the value is larger than the third threshold value, the calibration operation requested this time can be forcibly executed without performing the determination operation.

  In the above configuration, when the calibration operation determination unit determines that the execution of the requested calibration operation is not necessary in the determination operation, the calibration operation determination unit determines a difference or a ratio between the previous charge amount and the current charge amount. Accordingly, it is preferable that the developing bias is corrected so as to change the difference in DC voltage between the image carrier and the developing roller.

  According to this configuration, if the difference or ratio between the previous charge amount and the current charge amount indicates a value that does not require execution of the requested calibration operation, the requested calibration operation is not performed. The developing bias is corrected so as to change the DC voltage difference between the image carrier and the developing roller according to the difference or the ratio. For this reason, the developing bias can be appropriately adjusted in consideration of a slight change in the charge amount of the toner without wasting the time required for executing the calibration operation and the toner used for forming the plurality of calibration toner images. Can be corrected.

  According to the present invention, in the image forming apparatus including the developing device to which the two-component developing method is applied, it is possible to appropriately determine the necessity of performing the calibration operation based on the amount of change in the charge amount of the toner.

FIG. 1 is a cross-sectional view illustrating an internal structure of an image forming apparatus according to an embodiment of the present disclosure. FIG. 1 is a cross-sectional view of a developing device according to an embodiment of the present invention and a block diagram illustrating an electrical configuration of a control unit. FIG. 1 is a plan view of a developing device according to an embodiment of the present invention. FIG. 3 is a schematic diagram illustrating a developing operation of the image forming apparatus according to the embodiment of the present disclosure. FIG. 3 is a schematic diagram illustrating a magnitude relationship between potentials of an image carrier and a developing roller according to an embodiment of the present invention. 4 is a graph showing a relationship between a developing bias frequency and an image density in the image forming apparatus according to the embodiment of the present disclosure. 7 is a graph showing the relationship between the inclination of the graph of FIG. 6 and the charge amount of toner in the image forming apparatus according to one embodiment of the present invention. 6 is a flowchart of a charge amount measurement mode executed in the image forming apparatus according to the embodiment of the present disclosure. FIG. 4 is a schematic diagram of a measurement toner image formed on an image carrier when a charge amount measurement mode is executed. 6 is a flowchart of a determination mode executed in the image forming apparatus according to the embodiment of the present disclosure. FIG. 4 is a cross-sectional view of a developing device according to a first modified embodiment of the present invention and a block diagram illustrating an electrical configuration of a control unit. 9 is a flowchart of a charge amount measurement mode executed in the image forming apparatus according to the first modified embodiment of the present invention. 9 is a flowchart of a determination mode executed in an image forming apparatus according to a second modified embodiment of the present invention. 13 is a flowchart of an auxiliary determination mode in an image forming apparatus according to a third modified embodiment of the present invention. 15 is a flowchart of a determination mode executed in an image forming apparatus according to a fourth modified embodiment of the present invention. FIG. 9 is a diagram illustrating an example of an experiment result of calibration.

  Hereinafter, an image forming apparatus 10 according to an embodiment of the present invention will be described in detail with reference to the drawings. In the present embodiment, a tandem type color printer will be described as an example of the image forming apparatus. The image forming apparatus may be, for example, a copying machine, a facsimile machine, or a multifunction peripheral thereof. Further, the image forming apparatus may form a single color (monochrome) image.

<About the image forming apparatus>
FIG. 1 is a sectional view showing an 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 section 12 for feeding a sheet P, an image forming section 13 for forming a toner image to be transferred onto the sheet P fed from the sheet feeding section 12, and the toner image is primarily transferred. An intermediate transfer unit 14, a toner replenishing unit 15 for replenishing toner to the image forming unit 13, and a fixing unit 16 for fixing an unfixed toner image formed on the sheet P to the sheet P are provided therein. . Further, at the upper part of the apparatus main body 11, there is provided a paper discharge unit 17 for discharging the sheet P subjected to the fixing processing by the fixing unit 16.

  An operation panel 18 for allowing a user to operate the image forming apparatus 10 is provided at an appropriate position on the upper surface of the apparatus main body 11. On the operation panel 18, a liquid crystal display that displays information about the image forming apparatus 10, such as an operation state of the image forming apparatus 10 (for example, during printing), an output condition of an image on a sheet P, and the like, relate to operations of the image forming apparatus 10. A touch panel for inputting information and various operation keys are provided.

  In the apparatus main body 11, a sheet conveying path 111 extending vertically is formed at a position on the right side of the image forming section 13. The sheet conveying path 111 is provided with a pair of conveying rollers 112 for conveying a sheet to an appropriate position. Further, a pair of registration rollers 113 for correcting the skew of the sheet and feeding the sheet at a predetermined timing to a nip portion for secondary transfer described later is provided upstream of the nip portion in the sheet conveying path 111. The sheet transport path 111 is a transport path that transports the sheet P from the paper supply unit 12 to the paper discharge unit 17 via the image forming unit 13 and the fixing unit 16.

  The paper supply unit 12 includes a paper supply tray 121, a pickup roller 122, and a paper supply roller pair 123. The paper feed tray 121 is removably mounted below 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 out the uppermost sheet P of the sheet bundle P1 stored in the sheet feeding tray 121 one by one. The paper feed roller pair 123 sends out the sheet P fed by the pickup roller 122 to the sheet transport path 111.

  The paper feeding unit 12 includes a manual paper feeding unit attached to the left side of the apparatus main body 11 shown in FIG. The manual sheet feeding unit includes a manual tray 124, a pickup roller 125, and a sheet feeding roller pair 126. The manual feed tray 124 is a tray on which sheets P to be manually fed are placed, and is opened from the side of the apparatus body 11 as shown in FIG. The pickup roller 125 feeds out the sheet P placed on the manual feed tray 124. The sheet feeding roller pair 126 sends out the sheet P fed by the pickup roller 125 to the sheet conveying 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. In the present embodiment, as the image forming units, a magenta (M) color is sequentially arranged from the upstream side to the downstream side (from left to right in FIG. 1) in the rotation direction of the intermediate transfer belt 141 described later. A magenta unit 13M using a developer, a cyan unit 13C using a cyan (C) developer, a yellow unit 13Y using a yellow (Y) developer, and a black (Bk) developer are used. A black 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, a primary transfer roller 24, and a cleaning device 25 disposed around the photosensitive drum 20. Is provided. An exposure device 22 common to the units 13M, 13C, 13Y, and 13Bk is disposed below the image forming unit.

  The photoreceptor drum 20 is driven to rotate around its axis to form an electrostatic latent image on the surface thereof, and carries a toner image in which the electrostatic latent image has been revealed. As the photosensitive drum 20, for example, a known amorphous silicon (α-Si) photosensitive drum or an organic (OPC) photosensitive drum is used. The charging device 21 uniformly charges the surface of the photosensitive drum 20 to a predetermined charging potential. The charging device 21 includes a charging roller and a charging cleaning brush for removing toner attached to the charging roller. The exposure device 22 is disposed downstream of the charging device 21 in the rotation direction of the photosensitive drum 20, and has various optical system devices such as a light source, a polygon mirror, a reflection mirror, and a deflection mirror. The exposure device 22 irradiates the surface of the photosensitive drum 20 uniformly charged to the charged potential with light modulated based on image data (predetermined image information) to expose the surface, thereby forming an electrostatic latent image. Form.

  The developing device 23 is arranged to face the photosensitive drum 20 at a predetermined developing nip portion NP (FIG. 4) downstream of the exposure device 22 in the rotation direction of the photosensitive drum 20. The developing device 23 includes a developing roller 231 that is rotated and carries a developer including a toner and a carrier on a peripheral surface and supplies the toner to the photosensitive drum 20 to form the toner image.

  The primary transfer roller 24 forms a nip with the photosensitive drum 20 with the intermediate transfer belt 141 provided in the intermediate transfer unit 14 interposed therebetween. Further, the primary transfer roller 24 primarily transfers the toner image on the photosensitive drum 20 onto the intermediate transfer belt 141. The cleaning device 25 cleans the peripheral surface of the photosensitive drum 20 after the transfer of the toner image.

  The intermediate transfer unit 14 is disposed in a space provided between the image forming unit 13 and the toner replenishing unit 15, and includes an intermediate transfer belt 141, a drive roller 142 rotatably supported by a unit frame (not shown), and , A driven roller 143, a backup roller 146, and a plurality of density sensors 100. The intermediate transfer belt 141 is an endless belt-shaped rotating body, and is stretched over the driving roller 142 and the driven rollers 143 and 146 such that the peripheral surface thereof comes into contact with the peripheral surface of each of the photosensitive drums 20. I have. The intermediate transfer belt 141 is driven to rotate by rotation of the driving roller 142. A belt cleaning device 144 that removes toner remaining on the peripheral surface of the intermediate transfer belt 141 is disposed near the driven roller 143.

  The plurality of density sensors 100 (density detection units) are positioned at predetermined intervals in the main scanning direction (the axial direction of the photosensitive drum 20) at a position downstream of the units 13M, 13C, 13Y, and 13Bk and facing the intermediate transfer belt 141. Are arranged side by side. Each density sensor 100 detects the density of a toner image formed on the intermediate transfer belt 141 at a position facing itself. In the control unit 980 described later, the average value of the density of the toner image detected by the plurality of density sensors 100 is used as the density of the toner image.

  In another embodiment, the plurality of density sensors 100 may detect the density of the toner image on the photosensitive drum 20 or may detect the density of the toner image fixed on the sheet P. Good. Hereinafter, for convenience of description, the average value of the density of the toner image detected by the plurality of density sensors 100 will be referred to as the density of the toner image detected by the density sensor 100.

  A secondary transfer roller 145 is disposed outside the intermediate transfer belt 141 so as to face the drive roller 142. The secondary transfer roller 145 is pressed against the peripheral surface of the intermediate transfer belt 141 to form a transfer nip with the driving roller 142. The toner image primarily transferred on the intermediate transfer belt 141 is secondarily transferred to a sheet P supplied from the paper supply unit 12 at a transfer nip. That is, the intermediate transfer unit 14 and the secondary transfer roller 145 function as a transfer unit that transfers the toner image carried on the photosensitive drum 20 to the sheet P. The drive roller 142 is provided with a roll cleaner 200 for cleaning the peripheral surface thereof.

  The toner replenishing unit 15 stores toner used for image formation. In the present embodiment, the toner replenishing unit 15 includes a magenta toner container 15M, a cyan toner container 15C, a yellow toner container 15Y, and a black toner container 15Bk. These toner containers 15M, 15C, 15Y, and 15Bk store replenishment toner of each color of M / C / Y / Bk. The toner of each color is supplied to the developing devices 23 of the image forming units 13M, 13C, 13Y, and 13Bk corresponding to each color of M / C / Y / Bk from the toner discharge port 15H formed on the bottom surface of the container.

  The fixing unit 16 includes a heating roller 161 having a heating source therein, a fixing roller 162 disposed to face the heating roller 161, a fixing belt 163 stretched between the fixing roller 162 and the heating roller 161, a fixing belt 163. And a pressure roller 164 that is arranged to face the fixing roller 162 with the fixing roller 162 interposed therebetween to form a fixing nip portion. The sheet P supplied to the fixing unit 16 is heated and pressed by passing through the fixing nip. As a result, the toner image transferred to the sheet P at the 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 that receives the discharged sheet P is formed at the bottom of the concave. The sheet P on which the fixing process has been performed is discharged to a discharge tray 151 via a sheet conveyance path 111 extending from an upper portion of the fixing unit 16.

<About the developing device>
FIG. 2 is a cross-sectional view of the developing device 23 according to the present embodiment and a block diagram illustrating an electrical configuration of the control unit 980. FIG. 3 is a plan view of the developing device 23. The developing device 23 includes a developing housing 230, a first screw feeder 233, a second screw feeder 232, a developing roller 231, and a regulating blade 234. The developing device 23 employs a two-component developing method.

  The developing housing 230 is provided with a developer accommodating portion 230H. The developer accommodating portion 230H is covered by a top plate 235 (FIG. 2) and is divided into a first transport portion 230A and a second transport portion 230B by a partition plate 230E extending in the front-rear direction. The partition plate 230E is shorter than the width of the developing housing 230 in the front-rear direction. The first transport unit 230A and the second transport unit 230B are provided in front of the front end (hereinafter, the front end) of the partition plate 230E and behind the rear end (hereinafter, the rear end) of the partition plate 230E. A first communication portion 230C and a second communication portion 230D are provided to communicate with each other. As a result, a circulation path is formed in the developer accommodating portion 230H for repeatedly moving the developer in the order of the first transport portion 230A, the first communication portion 230C, the second transport portion 230B, and the second communication portion 230D. That is, the toner is conveyed clockwise in FIG. 3 in the circulation path. Further, a toner supply port 236 is provided in the developer accommodating section 230H.

  The toner supply port 236 is an opening formed in the top plate 235 (FIG. 2), and is disposed above the vicinity of the upstream end (front end) in the developer conveyance direction (D1 direction) in the first conveyance section 230A. (FIG. 3). The toner supply port 236 is arranged to face the above-described circulation path, and receives the supply toner supplied from the toner supply section 15 (FIG. 1) into the developer accommodating section 230H. For example, the toner supply port 236 has an opening of 14 mm × 8 mm in plan view.

  The first screw feeder 233 is provided in the first transport section 230A. The first screw feeder 233 includes a first rotating shaft 233A, and first spiral blades 233B (screw blades) protruding in a spiral shape around the first rotating shaft 233A. The first screw feeder 233 is rotated in the direction of the arrow D23, and is transported in the first transport direction D1 while stirring and mixing the new toner flowing from the toner supply port 236 and the developer in the developer accommodating portion 230H. I do.

  A first paddle 233c is disposed downstream of the first screw feeder 233 in the toner transport direction (direction D1). The first paddle 233c is a rib member extending in the axial direction for one pitch of the first spiral blade 233B. The first paddle 233c is rotated together with the first rotation shaft 233A, and transfers the toner from the first transport unit 230A to the second transport unit 230B in the direction of arrow D3 in FIG.

  The second screw feeder 232 is disposed to the left and below the developing roller 231 so as to face the developing roller 231. As the second screw feeder 232 rotates in the direction of the arrow D22, toner is supplied from the second screw feeder 232 to the developing roller 231. In the present embodiment, the toner supply path to the developing roller 231 is formed only by the path supplied from the second screw feeder 232. Therefore, the second screw feeder 232 supplies the toner to the developing roller 231 by pumping the toner upward from below.

  A second paddle 232C is disposed downstream of the second screw feeder 232 in the toner transport direction (D2 direction). The second paddle 232C is a plate-like member provided on the rotation axis of the second screw feeder 232. The second paddle 232C is rotated together with the rotation shaft, and transfers the toner from the second transport unit 230B to the first transport unit 230A in the direction of arrow D4.

  Further, a toner density sensor 101 (toner density detection unit) is disposed near the downstream end (rear end) of the outer surface of the developing housing 230 in the developer transport direction (D1 direction). The toner density sensor 101 detects the toner density of the developer delivered from the first transport unit 230A to the second transport unit 230B by the first paddle 233c. The toner density is the ratio of the amount of toner to the amount of carrier contained in the developer (= toner amount / carrier amount).

  The developing roller 231 is arranged to face the photosensitive drum 20 at the developing nip NP (FIG. 4). The developing roller 231 includes a sleeve 231S that is rotated, and a magnet 231M that is fixed and disposed inside the sleeve 231S. The magnet 231M has S1, N1, S2, N2 and S3 poles. The N1 pole functions as a main pole, the S1 pole and the N2 pole function as carrier poles, and the S2 pole functions as a peeling pole. The S3 pole functions as a pumping pole and a regulating pole. As an example, the magnetic flux densities of the S1, N1, S2, N2, and S3 poles are set to 54 mT, 96 mT, 35 mT, 44 mT, and 45 mT. The sleeve 231S of the developing roller 231 is rotated in the direction of arrow D21 in FIG. The developing roller 231 is rotated, receives the developer in the developing housing 230, carries the developer layer, and supplies toner to the photosensitive drum 20. In this embodiment, the developing roller 231 rotates in the same direction (with direction) at a position facing the photosensitive drum 20.

  The regulating blade 234 is arranged at a predetermined interval on the developing roller 231, and regulates the layer thickness of the developer supplied from the second screw feeder 232 onto the peripheral surface of the developing roller 231.

  The image forming apparatus 10 including the developing device 23 further includes a developing bias applying unit 971, a driving unit 972, and a control unit 980. The control unit 980 includes a CPU (Central Processing Unit), a ROM (Read Only Memory) for storing a control program, a RAM (Random Access Memory) used as a work area of the CPU, and the like.

  The developing bias applying unit 971 is composed of a DC power supply and an AC power supply. Based on a control signal from a bias control unit 982 described later, the developing bias in which the AC voltage is superimposed on the DC voltage is applied to the developing roller 231 of the developing device 23. Is applied.

  The drive unit 972 is composed of a motor and a gear mechanism for transmitting the torque thereof. In response to a control signal from a drive control unit 981 described below, the developing unit (image forming operation), the charge amount measurement mode, and the execution of calibration are performed. In addition to the body drum 20 and the like, the developing roller 231 and the first screw feeder 233 and the second screw feeder 232 in the developing device 23 are rotationally driven. The driving unit 972 further generates a driving force for driving (rotating) other members of the image forming apparatus 10.

  When the CPU executes the control program stored in the ROM, the control unit 980 controls the drive control unit 981, the bias control unit 982, the storage unit 983, and the mode control unit 984 (the charge amount acquisition unit, the calibration operation execution unit). ), A requesting unit 986, and a determining unit 985 (calibration operation determining unit).

  The drive control unit 981 controls the drive unit 972 to rotationally drive the developing roller 231, the first screw feeder 233, and the second screw feeder 232. Further, the drive control unit 981 controls a drive mechanism (not shown) to rotate the photosensitive drum 20.

  The bias control unit 982 controls the developing bias applying unit 971 during a developing operation in which toner is supplied from the developing roller 231 to the photosensitive drum 20, so that a DC voltage and an AC voltage are applied between the photosensitive drum 20 and the developing roller 231. A voltage potential difference is provided. The toner is moved from the developing roller 231 to the photosensitive drum 20 by the potential difference.

  The storage unit 983 stores various types of information referred to by the drive control unit 981, the bias control unit 982, the mode control unit 984, and the determination unit 985. For example, the storage unit 983 stores the value of the developing bias adjusted according to the rotation speed of the developing roller 231 and the environment.

  Further, the storage unit 983 stores the amount of change in the frequency of the AC voltage of the developing bias when the frequency of the AC voltage of the developing bias is changed while the potential difference of the DC voltage between the developing roller 231 and the photosensitive drum 20 is kept constant. Reference information relating to the inclination of the reference straight line indicating the relationship between the toner image density change amount and the toner image density change amount is stored in advance for each toner charge amount. The reference information stored in the storage unit 983 has a negative slope of the reference line when the charge amount of the toner is the first charge amount, and the charge amount of the toner is smaller than the first charge amount. When the charge amount is the second charge amount, the inclination of the reference straight line is positive, and further, the inclination of the reference straight line is set to increase as the charge amount of the toner decreases. The data stored in the storage unit 983 may be in the form of a graph, a table, or the like.

  The mode control unit 984 executes a charge amount measurement mode (charge amount acquisition operation) and calibration (calibration operation).

  In the charge amount measurement mode, the mode control unit 984 controls the charging device 21, the exposure device 22, the developing bias applying unit 971, and the like, to form a plurality of measurement toner images having different toner development amounts on the photosensitive drum 20 from each other. Form. The mode control unit 984 forms the plurality of measurement toner images based on the density of the plurality of measurement toner images detected by the density sensor 100 or in addition to the densities of the plurality of measurement toner images. At this time, the charge amount of the toner is obtained based on the DC component of the developing current flowing between the developing roller 231 and the developing bias applying unit 971.

  More specifically, in the charge amount measurement mode, the mode control unit 984 changes the photosensitive bias while changing the frequency of the AC voltage of the developing bias while maintaining the potential difference of the DC voltage between the developing roller 231 and the photosensitive drum 20 constant. A plurality of measurement toner images are formed on the body drum 20. Thereafter, the mode control unit 984 calculates the inclination of the measurement straight line indicating the relationship between the amount of change in the frequency and the amount of change in the density of the toner image for measurement. The charge amount of the toner contained in the measurement toner image formed on the photoconductor drum 20 is acquired from the density detection result and from the acquired inclination of the measurement straight line and the reference information in the storage unit 983. .

  In calibration, the mode control unit 984 controls the charging device 21 and the exposure device 22 to form a plurality of electrostatic latent images for calibration on the photosensitive drum 20 and controls the developing bias application unit 971. Thus, a plurality of calibration toner images are formed by making at least the DC voltage of the developing bias different for each calibration electrostatic latent image. Thereafter, the mode control unit 984 controls the developing bias applied to the developing roller 231 in the image forming operation based on the relationship between the densities of the plurality of calibration toner images detected by the density sensor 100 and the developing bias. To determine.

  The request unit 986 requests execution of calibration at a predetermined timing. The timing at which the request unit 986 requests the execution of calibration will be described later.

  The determination unit 985 executes the determination mode (determination operation) when the request unit 986 requests the execution of the calibration. The determination unit 985 causes the mode control unit 984 to execute the charge amount measurement mode in the determination mode, and determines the charge amount of the toner obtained in the previous charge amount measurement mode (hereinafter, the previous charge amount) and the current charge amount measurement. The necessity of executing the requested calibration is determined based on the toner charge amount acquired in the mode (hereinafter, the current charge amount). In particular, in the present embodiment, in the determination mode, when the difference or ratio of the current charge amount to the previous charge amount is larger than a preset first threshold in the determination mode, the execution of the requested calibration is performed. If the difference is determined to be necessary and the difference or ratio of the current charge amount to the previous charge amount is equal to or less than the first threshold value, it is determined that the execution of the requested calibration is not necessary.

<Developing operation>
FIG. 4 is a schematic diagram of a developing operation of the image forming apparatus 10 according to the present embodiment. FIG. 5 is a schematic diagram illustrating the magnitude relationship between the potentials of the photosensitive drum 20 and the developing roller 231. As shown in FIG. 4, a developing nip NP is formed between the developing roller 231 and the photosensitive drum 20. The toner TN and the carrier CA carried on the developing roller 231 form a magnetic brush. In the developing nip portion NP, the toner TN is supplied from the magnetic brush to the photosensitive drum 20 side, and a toner image TI is formed. As shown in FIG. 5, the surface potential of the photosensitive drum 20 is charged by the charging device 21 to the background portion potential V0 (V). Thereafter, when exposure light is irradiated by the exposure device 22, the surface potential of the photosensitive drum 20 is changed from the background portion potential V0 to the image portion potential VL (V) at the maximum according to the image to be printed. On the other hand, a DC voltage Vdc of a developing bias is applied to the developing roller 231, and an AC voltage (not shown) is superimposed on the DC voltage Vdc.

  In the case of such a reversal developing method, a potential difference between the surface potential V0 and the DC component Vdc of the developing bias is a potential difference that suppresses toner fogging on the background portion of the photosensitive drum 20. The background portion of the photoconductor drum 20 is an area on the surface of the photoconductor drum 20 where no electrostatic latent image is formed. On the other hand, the potential difference between the surface potential VL after the exposure and the DC component Vdc of the developing bias is a developing potential difference for moving the positive polarity toner to the image portion of the photosensitive drum 20. The image portion of the photosensitive drum 20 is an area on the surface of the photosensitive drum 20 where an electrostatic latent image is formed. Further, the movement of the toner from the developing roller 231 to the photosensitive drum 20 is promoted by the AC voltage applied to the developing roller 231.

  On the other hand, each toner is frictionally charged with the carrier while being circulated and transported in the developing housing 230. The amount of charge of each toner affects the amount of toner (development amount) that moves toward the photosensitive drum 20 due to the above-described development bias. Therefore, if it becomes possible to accurately predict the charge amount of the toner in the image forming apparatus 10, by adjusting the developing bias and the toner density according to the number of prints, environmental fluctuation, print mode, print ratio, etc. Image quality can be maintained. For this reason, it has conventionally been desired to accurately predict the charge amount of the toner.

<Estimation of toner charge amount>
The inventor of the present invention has conducted intensive studies in view of the above situation, and has newly found that when the frequency of the AC voltage of the developing bias is changed, the change in the amount of developed toner varies depending on the amount of charge of the toner. did. Specifically, when the charge amount of the toner is low, increasing the frequency of the AC voltage increases the development amount of the toner. On the other hand, it was newly found that when the charge amount of the toner is high, increasing the frequency of the AC voltage decreases the development amount of the toner. By utilizing this characteristic, it is possible to accurately predict the charge amount of the toner by measuring the change in the image density when the frequency of the AC voltage is changed.

  FIG. 6 is a graph showing the relationship between the frequency of the developing bias and the image density in the image forming apparatus 10 according to the present embodiment. FIG. 7 is a graph showing the relationship between the inclination of the graph of FIG. 6 and the charge amount of the toner in the image forming apparatus 10 according to the present embodiment.

  The potential difference between the DC voltage of the developing bias applied to the developing roller 231 and the DC voltage between the electrostatic latent image on the photosensitive drum 20 is kept constant, and the peak-to-peak voltage Vpp of the AC voltage of the developing bias and the duty ratio are reduced. In the fixed state, the frequency of the AC voltage is changed. As a result, the image density of the toner image detected by the density sensor 100 tends to be different depending on the charge amount of the toner on the developing roller 231 (FIG. 6). That is, as shown in FIG. 6, when the charge amount of the toner is “27.5 μc / g”, the image density decreases as the frequency f decreases. On the other hand, when the charge amount of the toner is “34.0 μc / g” or “37.7 μc / g”, the image density increases as the frequency f decreases. Then, as the charge amount of the toner decreases, the slope of the graph illustrated in FIG. 6 increases.

  As shown in FIG. 7, the relationship between the slopes of the three graphs in FIG. 6 and the charge amount of each toner is distributed on a straight line (approximate straight line). Therefore, if the information shown in FIG. 7 is stored in the storage unit 983 in advance, and the inclination of the straight line shown in FIG. 6 is derived in the charge amount measurement mode described later, the charge amount of the toner at that time is measured (predicted). It is possible to do.

<About the effect of predicting the charge amount of the toner>
In the present embodiment, it is not necessary to provide a surface potential sensor for measuring the surface potential on the photosensitive drum 20 in order to predict the charge amount of the toner. Further, in order to estimate the charge amount of the toner, it is not necessary to measure the current flowing into the developing roller 231 according to the developing bias. Therefore, it is possible to stably predict the charge amount of the toner without being affected by the change in the current flowing into the developing roller 231 due to the contamination of the surface potential sensor and the change in the resistance of the carrier. For this reason, when the image density printed in the image forming apparatus 10 decreases, it is desirable to increase the image density by increasing the toner density of the developing device 23 to reduce the charge amount of the toner. It is easy to select whether it is desirable to increase the image density by increasing the development potential difference Vdc-VL in the portion NP.

  In general, the causes of the decrease in image density in the image forming apparatus 10 include “decrease in development potential difference”, “decrease in transport amount of developer passing through the regulating blade 234”, “increase in carrier resistance”, and “charge of toner”. Increase in the amount ”. In this case, if the image density is decreased due to factors other than the increase in the charge amount of the toner, and if the toner concentration is increased in order to reduce the charge amount of the toner, a new problem such as toner scattering occurs. there's a possibility that. It is desirable to reduce the toner charge amount by increasing the toner density to reduce the image density caused by the increase in the toner charge amount, and to develop the image density by reducing the image density due to other factors. It is preferable to increase the electric field (development bias). Further, by grasping the charge amount of the toner, the transfer current applied to the secondary transfer roller 145 can be optimized, so that the entire system of the image forming apparatus 10 can be further stabilized.

<Relationship between frequency and toner charge amount>
The inventor of the present invention estimates that the charge amount of the toner contributes to the change in image density when the frequency of the AC voltage of the developing bias is changed as follows.

(1) When the charge amount of the toner is low When the charge amount of the toner is low, the toner is easily separated from the carrier because the electrostatic adhesion acting between the toner and the carrier is small. However, when the frequency of the AC voltage of the developing bias decreases, the number of reciprocating movements of the toner in the developing nip NP decreases. For this reason, the image density decreases. Note that when the frequency decreases, the reciprocating movement distance of the toner per one cycle of the AC voltage increases. However, when the charge amount of the toner is low, the original movement distance of the toner is small, and the influence on the reduction of the image density is not affected. Few. As described above, when the charge amount of the toner is low, the image density decreases when the frequency of the AC voltage of the developing bias decreases.

(2) When the charge amount of the toner is high When the frequency of the AC voltage of the developing bias decreases as described above, the number of reciprocating movements of the toner in the developing nip portion NP decreases. Is hardly detached from the carrier, so that the influence of the decrease in the number of reciprocating movements is small. On the other hand, when the frequency decreases, the reciprocating distance of the toner per one cycle of the AC voltage increases, so that the image density increases according to the high toner charge amount. As described above, when the charge amount of the toner is high, the image density increases as the frequency of the AC voltage of the developing bias decreases.

<About the toner charge amount measurement mode>
FIG. 8 is a flowchart of the charge amount measurement mode executed in the image forming apparatus 10 according to the present embodiment. FIG. 9 is a schematic diagram of a measurement toner image formed on the photosensitive drum 20 when the charge amount measurement mode is executed.

  As shown in FIG. 8, when the charge amount measurement mode is started (step S01), the mode control unit 984 sets a variable n for changing the frequency of the AC voltage of the developing bias to n = 1 (step S02). ). Then, the mode control unit 984 controls the drive control unit 981 and the bias control unit 982 to rotate the developing roller 231 by one or more rotations in a state where a preset reference phenomenon bias is applied. The frequency of the AC voltage is set to the first frequency (n = 1) (step S03).

  The reference phenomenon bias is set so that the charge amount measurement mode is not affected by the previous image forming history. Normally, a bias used for printing (image formation) is applied to the reference developing bias condition. If only a DC voltage is applied as the reference developing bias, the effect of eliminating the above history is weak. Therefore, it is preferable that the DC voltage and the AC voltage are applied in a superimposed manner.

  Next, a predetermined measurement toner image is developed with the developing bias having the frequency of the AC voltage set to the first frequency (step S04), and the toner image is transferred from the photosensitive drum 20 to the intermediate transfer belt 141. (Step S05). Then, the image density of the toner image for measurement is measured by the density sensor 100 (step S06), and the acquired image density is stored in the storage unit 983 together with the value of the first frequency (step S07).

  Next, the mode control unit 984 determines whether or not the variable n relating to the frequency has reached a preset specified number N (step S08). Here, if n ≠ N (NO in step S08), the value of n is counted up by one (n = n + 1, step S09), and steps S03 to S07 are repeated. Note that, in order to increase the accuracy of the charge amount measurement, it is preferable that the prescribed number of times N = 2 or more, and it is more preferable that 3 ≦ N be set.

  On the other hand, if n = N (YES in step S08), mode control section 984 calculates the slope of the approximate straight line shown in FIG. 6 based on the information stored in storage section 983 (step S10). . Then, the mode control unit 984 estimates the charge amount of the toner from the inclination based on the graph (reference information) shown in FIG. 7 stored in the storage unit 983 (step S11), and sets the charge amount measurement mode. Is completed (step S12).

  FIG. 9 shows an example in which, when the prescribed number of times N = 3, the image density of the measurement toner image is increased by increasing the frequency f. In this case, the charge amount of the toner is relatively low, such as “27.5 μc / g” in FIG.

When N = 2, the image densities measured in step S06 are defined as ID1 and ID2, respectively. Further, the first frequency is defined as f1 (kHz), and the second frequency is defined as f2 (kHz) (f2 <f1). In this case, the slope a of the straight line shown in FIG.
Slope a = (ID1-ID2) / (f1-f2)) (Equation 1)

The slope a varies depending on the charge amount of the toner, and is “positive (+)” when the charge amount of the toner is low, and “negative (−)” when the charge amount of the toner is low. When the measurement is performed under the condition of 3 ≦ N, the slope of the approximate straight line of the linear expression obtained by the least square method may be used. The reference information shown in FIG. 7 is expressed by Expression 2.
Q / M = A × Slope of straight line + B (Equation 2)

  Here, A and B are values specific to the developer, and are determined in advance by experiments. Q / M means the amount of charge of the toner per unit mass. By substituting the slope a of the approximate straight line calculated from Equation 1 in Step S10 into Equation 2, the toner charge amount Q / M is calculated.

  The charge amount measurement mode shown in FIG. 8 may be executed for each of the developing devices 23 of FIG. 1, and the frequency set during execution of the mode may be a value unique to each developing device 23. May be set. In particular, when a desired frequency is known according to the temperature and humidity around the image forming apparatus 10 and the number of endurance sheets, the frequency set during the execution of the mode may be set near the known frequency. Further, a frequency used in a new measurement mode may be selected with reference to a result of the previous toner charge amount measurement mode. In this case, the accuracy of the measured charge amount of the toner can be improved.

<Execution timing of charge amount measurement mode>
The execution timing of the charge amount measurement mode according to the present embodiment includes one that is automatically started and one that is manually started. The charge amount measurement mode which is automatically started is performed in a determination mode described later, before execution of a calibration (also referred to as a setup or an image quality adjustment operation) described later.

  In addition, it is desirable that the charge amount measurement mode be executed when the image forming apparatus 10 is shipped from the factory after manufacturing and when the image forming apparatus 10 is set up at the place where the image forming apparatus 10 is used. As a result, it is possible to predict the effect of the image forming apparatus 10 during the suspension period. That is, when the idle period is long, the amount of charge of the toner tends to decrease, and the level of the tendency often differs depending on the period during which the developer is left and the environment. Therefore, by measuring the charge amount of the toner at the time of shipment from the factory and at the time of set-up of the main body, the deterioration state due to leaving the developer is predicted, and when the leaving time is extremely long or when the developer is left in a poor environment, The difference between the charge amounts of the two toners (the charge amounts of the toner at the time of shipment from the factory and at the time of setup of the main body) is detected to be large. In such a case, replacement of the developer at the place of use can be promoted in the same manner as described above.

  On the other hand, even if the charge amount of the toner at the time of factory shipment and the setup of the main body is low, if the difference between the charge amounts of the two toners is small, the possibility that the developer has deteriorated is low. Therefore, it is not necessary to change the developer at the place of use, and the image quality can be improved by adjusting the toner concentration and the developing conditions (developing bias and the like). As described above, the change in the state of the developer can be grasped by executing the charge amount measurement mode according to the present embodiment after the image forming apparatus 10 is left unused for a predetermined period of time without being used. Become.

  As described above, in the charge amount measurement mode according to the present embodiment, the developing device does not use the surface potential sensor that measures the potential on the photosensitive drum 20 or the ammeter that measures the developing current flowing into the developing roller 231. The charge amount of the toner stored in the storage unit 23 can be obtained. As a result, it is possible to accurately determine whether the developer needs to be replaced in the developing device 23 or the need for adjusting the developing bias.

  In particular, the reference information stored in the storage unit 983 has a negative slope of the reference straight line when the charge amount of the toner is the first charge amount, and the charge amount of the toner is smaller than the first charge amount. The inclination of the reference straight line is positive when the second charge amount is small, and the inclination of the reference straight line is set to increase as the charge amount of the toner decreases. According to such a configuration, the charge amount of the toner is accurately determined based on the relationship between the frequency of the AC voltage of the developing bias and the density (developed toner amount) of the toner image formed on the photosensitive drum 20 (the intermediate transfer belt 141). Can get better.

<Improvement of efficiency in charge amount measurement mode>
As described above with reference to FIG. 3, the second screw feeder 232 supplies the toner to the developing roller 231 while transporting the toner in the direction of arrow D2 in the second transport unit 230B. Therefore, the toner concentration on the downstream side in the developer transport direction of the second transport unit 230B decreases in accordance with the amount of toner consumed on the upstream side (rear end side) of the developer roller 231 in the developer transport direction. Therefore, the toner concentration on the downstream side in the developer transport direction in the second transport unit 230B has a large variation. Further, since the charge amount of the toner increases when the toner concentration decreases, the measurement result of the charge amount of the toner on the downstream side in the developer transport direction in the second transport unit 230B also has large dispersion.

  Therefore, in the charge amount measurement mode, the toner charge amount is measured by detecting the density of the measurement toner image only by the density sensor 100 that is arranged at the most upstream side in the developer transport direction among the plurality of density sensors 100. It is desirable to do. In this case, the toner consumed by the execution of the charge amount measurement mode and the time required to execute the charge amount measurement mode can be reduced, and the charge amount measurement mode can be efficiently executed.

<Example of charge amount measurement mode>
Hereinafter, an example of the charge amount measurement mode will be described. The conditions of the experiment performed are as follows.

<Common experimental conditions>
Print speed: 55 sheets / min. Photoconductor drum 20: amorphous silicon photoconductor (α-Si)
Developing roller 231: Outer diameter 20 mm, surface knurl processing, 80 rows of recesses (grooves) formed along the circumferential direction.
・ Regulatory blade 234: SUS430, magnetic, 1.5mm thick
Amount of developer transported after the regulating blade 234: 250 g / m ²
A peripheral speed of the developing roller 231 with respect to the photosensitive drum 20 is 1.8 (in the trailing direction at the opposing position).
-Distance between the photosensitive drum 20 and the developing roller 231: 0.30 mm
-White background (background) potential V0 of the photosensitive drum 20: +270 V
-Image portion potential VL of photosensitive drum 20: + 20V
・ Toner: positively charged polar toner, volume average particle diameter 6.8 μm, toner concentration 8%
Carrier: volume average particle diameter 35 μm, ferrite resin-coated carrier developing bias of developing roller 231: frequency = 4.2 kHz, duty = 50%, Vpp = 1000 V AC square wave, Vdc (DC voltage) = 200 V

<About developer>
Similar effects have been confirmed regardless of whether the toner is a pulverized toner or a toner having a core-shell structure. It was also confirmed that the same effect was obtained in the range of “3%” to “12%” for the toner density. Since the movement of the toner due to the AC electric field is more likely to occur as the magnetic brush becomes finer, the volume average particle diameter of the carrier is preferably "45 μm" or less, more preferably "30 μm" or more and "40 μm" or less. The measurement (actual measurement) of the charge amount of the toner during the experiment was performed using a suction type small charge amount measuring device Model 212HS manufactured by Trek.

<About Career>
The carrier is a ferrite core having a volume average particle diameter of “35 μm” coated with silicon, fluorine, or the like. Specifically, the carrier was prepared by the following procedure. A coating solution is prepared by dissolving 20 parts by mass of a silicone resin KR-271 (manufactured by Shin-Etsu Chemical Co., Ltd.) in 200 parts by mass of toluene in 1000 parts by mass of a carrier core EF-35 (manufactured by Powder Tech). Then, the coating liquid was spray-coated by a fluidized bed coating apparatus, and then heat-treated at “200 ° C.” for “60 minutes” to obtain a carrier. In this coating liquid, the resistance adjustment and the charge adjustment are performed by mixing and dispersing the conductive agent and the charge control agent in the range of “0 to 20 parts” with respect to “100 parts” of the coating resin, respectively. .

  Under the above conditions, an experiment was performed in which the amount of the external additive of the toner was changed to adjust the charge amount of the toner, and the printing operation was performed. The results of the experiment are shown in FIGS. 6 and 7 described above. In FIG. 6, the image density of the toner image on the intermediate transfer belt 141 is measured by the density sensor 100, and the image density (sensor output) of the toner image acquired in advance and the toner formed on the printing paper (paper) are obtained. Using the correlation curve with the image density of the fixed image, the image density of the toner image is calculated based on the image density of the toner-fixed image. It is represented as D.

FIG. 7 shows the relationship between the charge amount of each toner and the slope of the straight line (approximate straight line) in FIG. Equation 3 (described below) of the approximate straight line illustrated in FIG. 7 is stored in the storage unit 983 in advance. Using Equation 3, the charge amount of the toner can be predicted.
Toner charge amount Q / M (μc / g) = − 442.32 × inclination + 29.87 (Equation 3)
Note that the slope of Equation 3 = the change amount of the image density / the change amount of the frequency (slope of the graph in FIG. 6).

<Calibration>
Generally, calibration in an image forming apparatus includes development bias calibration for adjusting image density, laser light amount calibration for maintaining dot reproducibility, data correction calibration for gradation adjustment, and multiple calibration. This includes color registration (registration) calibration of color toner images. The calibration according to the present invention may include at least a developing bias calibration for adjusting the image density. In other words, the calibration means an image density optimizing process for optimizing the image density on the sheet by the developing bias.

  In the present embodiment, the mode control unit 984 performs the development bias calibration in the calibration. More specifically, the mode control unit 984 fixes each parameter (potential, laser light amount, rotation speed, etc.) other than the background portion potential V0 and the DC component Vdc of the developing bias in FIG. Then, a plurality of electrostatic latent images for calibration are formed on the photosensitive drum 20 by controlling the charging device 21 and the exposure device 22.

  Further, the mode control unit 984 controls the developing bias applying unit 971 while maintaining V0-Vdc (background portion side potential difference) in FIG. 5 at a constant value to control at least the DC of the developing bias for each calibration electrostatic latent image. A plurality of calibration toner images are formed by changing the voltage Vdc. At this time, it is desirable that three or more DC components Vdc of the developing bias are set and three or more calibration toner images are formed.

  The mode control unit 984 is applied to the developing roller 231 in a subsequent image forming operation based on the relationship between the densities of the plurality of calibration toner images detected by the density sensor 100 and the DC component Vdc of the developing bias. Is determined. As an example, three Vdc and image density data are linearly regressed on a graph, and a DC component Vdc of a developing bias suitable for a preset target image density is determined from the straight line.

<Calibration execution timing>
Hereinafter, the timing at which the request unit 986 requests execution of calibration will be described. The request unit 986 automatically requests the execution of the calibration at predetermined intervals according to the time counted by a timer (not shown) provided in the image forming apparatus 10 and the number of prints printed in the image forming apparatus 10. I do. Further, the operation panel 18 is used when the image forming apparatus 10 is left unused all day and night in an unused state, or when the surrounding environment of the image forming apparatus 10 suddenly changes, such as the temperature and humidity around the image forming apparatus 10. When the user manually inputs a calibration execution instruction, the request unit 986 requests execution of calibration.

<Determining the necessity of performing calibration>
As described above, in the calibration, after generating a plurality of electrostatic latent images for calibration, the developing bias applied to the developing roller 231 is changed in accordance with each electrostatic latent image for calibration, and thus a plurality of calibration electrostatic latent images are generated. A toner image is generated. For this reason, if the calibration is performed unnecessarily in spite of the state where the image density is stable, the proportion of the time required to execute the calibration in the total use time of the image forming apparatus 10 increases, The average printing speed of the image forming apparatus 10 will be reduced. Further, if the calibration is performed wastefully, the toner is wastefully consumed to generate a plurality of calibration toner images. Therefore, it is desired that the execution frequency of the calibration be minimized. Therefore, the inventor of the present invention has intensively studied a method of appropriately determining whether or not the execution of the calibration is necessary, and recalled that the determination unit 985 executes a determination mode described later.

<About the flow of the judgment mode>
Hereinafter, the flow of the determination mode executed by the determination unit 985 will be described. FIG. 10 is a flowchart of the determination mode executed in the image forming apparatus 10 according to the present embodiment. When the execution of the calibration is requested by the request unit 986, the determination unit 985 starts the determination mode. Upon starting the determination mode, the determination unit 985 causes the mode control unit 984 to execute the charge amount measurement mode as shown in FIG. 10 (step S21).

  Next, the determination unit 985 determines the current charge amount, which is the toner charge amount acquired in the charge amount measurement mode in the immediately preceding (current) step S21, and the previous charge amount in the charge amount measurement mode executed in the immediately preceding step S21. It is determined from the previous charge amount, which is the charge amount of the toner acquired in the charge amount measurement mode executed in step (a), whether or not the requested calibration needs to be performed (step S22).

  The charge amount measurement mode executed immediately before the charge amount measurement mode executed in the immediately preceding step S21 is the charge amount measurement mode manually executed last time in the charge amount measurement mode executed in the immediately preceding step S21. Or the charge amount measurement mode automatically executed in step S21 immediately after the start of the previous determination mode.

  Specifically, in step S22, the determination unit 985 calculates a difference (= current charging amount−previous charging amount) or a ratio (= current charging amount / previous charging amount) of the current charging amount with respect to the previous charging amount. Hereinafter, the difference or ratio of the current charge amount to the previous charge amount may be referred to as a charge amount change amount ΔQ. Then, the determination unit 985 determines whether or not the charge amount change amount ΔQ is greater than a predetermined first threshold value TH1. Then, when the charge amount change amount ΔQ is larger than the first threshold value TH1, the determination unit 985 determines that the execution of the requested calibration is necessary (YES in step S22). On the other hand, when the charge amount change amount ΔQ is equal to or less than the first threshold value TH1, the determination unit 985 determines that the execution of the requested calibration is not necessary (NO in step S22).

  The first threshold value TH1 can be edited using the operation panel 18. With this, for example, when giving priority to printing of a monochrome image or a text document, etc., for which low image quality stability is required, the user can suppress the shortening of the printable time by executing the calibration. , A large value can be set. In addition, when giving priority to printing a photograph or a high-quality image, etc., for which high image quality stability is required, the user may set a small value in order to increase the execution frequency of calibration and improve the stability of image density. Can be set.

  That is, as described above, the developing device 23 develops the developing potential difference Vdc-VL between the exposed surface potential VL (FIG. 5) and the DC component Vdc of the developing bias (FIG. 5) so as to satisfy the toner charge. The operation is performed. For this reason, when the charge amount of the toner is high, the amount of toner required to fill the development potential difference with the charge of the toner is small, and the image density of the developed toner image is low. Conversely, when the charge amount of the toner is low, a large amount of toner is required to satisfy the development potential difference with the charge of the toner, and the image density of the developed toner image increases. Therefore, if the charge amount of the toner changes even though the developing conditions such as the developing bias are kept constant, the image density of the developed toner image changes.

  Therefore, when the charge amount change amount ΔQ is larger than the first threshold value TH1 (ΔQ> TH1) in step S22, the determination unit 985 determines that the image density of the toner image greatly changes. It is determined that the requested calibration needs to be performed in order to make it constant. On the other hand, when the charge amount change amount ΔQ is equal to or smaller than the first threshold value TH1 (ΔQ ≦ TH1), it is considered that the image density of the toner image remains substantially constant and does not change. It is determined that it is not necessary.

  If it is determined in step S22 that the requested calibration needs to be performed (YES in step S22), the mode control unit 984 executes the calibration as described above (step S23). On the other hand, if it is determined in step S22 that the execution of the requested calibration is not necessary (NO in step S22), the mode control unit 984 does not execute the calibration, and executes the previous calibration. The determined developing bias is continuously used as it is (step S24).

  In step S22, when the charge amount change amount ΔQ is much larger than the first threshold value TH1, it is estimated that the deterioration of the developer is progressing. For this reason, the flow of the determination mode may be interrupted, and a predetermined service center may be notified of the necessity of the developer replacement via the operation panel 18 or the Internet line or telephone line.

  As described above, in the present embodiment, in the charge amount measurement mode, the mode control unit 984 determines the densities of the plurality of measurement toner images, or the densities of the plurality of measurement toner images, and the plurality of measurement toner images. Based on the DC component of the developing current used for the formation, the charge amount of the toner included in the plurality of measurement toner images can be appropriately acquired.

  Therefore, in the determination mode, the determination unit 985 determines the calibration requested this time based on the difference or ratio between the charge amounts of the two toners appropriately acquired in the current charge amount measurement mode and the previous charge amount measurement mode. Can be appropriately determined.

  As described above, the embodiment of the present invention has been described, but the present invention is not limited to this. For example, the following modified embodiment can be adopted.

<First Modified Embodiment>
In the above-described embodiment, in the charge amount measurement mode, the mode control unit 984 charges the toner included in the measurement toner image formed on the photosensitive drum 20 from the inclination of the measurement straight line and the reference information in the storage unit 983. Although described in the aspect of obtaining the amount, the present invention is not limited to this. FIG. 11 is a cross-sectional view of the developing device 23 and a block diagram illustrating an electrical configuration of the control unit 980 according to the first modified embodiment of the present invention. FIG. 12 is a flowchart of a charge amount measurement mode executed in the image forming apparatus 10 according to the first modified embodiment of the present invention.

  As shown in FIG. 11, in the present modified embodiment, the image forming apparatus 10 further includes an ammeter 973. The ammeter 973 detects a DC current flowing between the developing roller 231 and the developing bias applying unit 971.

  In the charge amount measurement mode, the mode control unit 984 changes the frequency of the AC voltage of the developing bias while maintaining the potential difference of the DC voltage between the developing roller 231 and the photosensitive drum 20 constant. A plurality of measurement toner images are formed thereon. Then, the mode control unit 984 controls the distance between the developing roller 231 and the developing bias applying unit 971 when a plurality of measurement toner images corresponding to a difference in the density of the plurality of measurement toner images detected by the density sensor 100 are formed. Of the toner contained in the measurement toner image formed on the photosensitive drum 20 is acquired based on the ratio of the difference between the DC components of the developing current flowing through the photosensitive drum 20.

  As shown in FIG. 12, when the mode control unit 984 starts the charge amount measurement mode (step S31), the mode control unit 984 sets a variable n for forming a plurality of measurement toner images to n = 1 (n = 1). Step S32). Then, the mode control unit 984 selects the image 1 corresponding to n = 1 stored in the storage unit 983 in advance (step S33). The storage unit 983 stores image information of an electrostatic latent image for forming the image n and information on the frequency of the AC voltage of the developing bias. Note that the other parameters relating to the image forming operation are set to the same values as in the immediately preceding image forming operation. Next, the mode control unit 984 controls the exposure device 22 (FIG. 1), the drive control unit 981 and the bias control unit 982 to perform development while applying the phenomenon bias for forming the image 1 to the development roller 231. After rotating the roller 231 by one or more rotations, an electrostatic latent image of the measurement toner image corresponding to the image 1 is formed on the photosensitive drum 20. As the photoconductor drum 20 rotates, when the measurement toner image passes through the development nip NP where the photoconductor drum 20 and the development roller 231 face each other, toner is supplied to the electrostatic latent image, The toner image is developed (Step S34). During this developing operation, the developing current (DC current) is measured by the ammeter 973 (step S35).

  Thereafter, the toner image is transferred from the photosensitive drum 20 to the intermediate transfer belt 141 (Step S36). Then, the image density of the measurement toner image is measured by the density sensor 100 (step S37), and the acquired image density is stored in the storage unit 983 together with the value of the developing current measured in step S35 (step S38). ).

  Next, the mode control unit 984 determines whether or not the variable n for forming a plurality of measurement toner images has reached a preset specified number N (step S39). Here, if n ≠ N (NO in step S39), the value of n is counted up by one (n = n + 1, step S40), and steps S33 to S39 are repeated. In order to increase the accuracy of the charge amount measurement, it is preferable that the specified number of times N is equal to or greater than 2, and it is more preferable that 3 ≦ N. On the other hand, if n = N (YES in step S39), the mode control unit 984 estimates the charge amount of the toner (step S41), and ends the charge amount measurement mode (step S42).

As an example, when N = 2, the development currents (DC currents) of n = 1 and 2 measured in step S35 are defined as I1 and I2, respectively. The image densities of n = 1 and 2 measured in step S37 are defined as ID1 and ID2, respectively. At this time, the charge amount of the toner in step S41 corresponds to the slope a obtained from the following Expression 4.
Slope a = (I1-I2) / (ID1-ID2) (Equation 4)
The above gradient a corresponds to the gradient of a straight line passing through two points when data (ID, I) at n = 1 and 2 are plotted, with the horizontal axis representing the image density ID and the vertical axis representing the developing current I. When the charge amount of the toner is measured under the condition of N = 3 or more, the slope a of the approximate straight line of the linear expression obtained by the least square method is used as the charge amount of the toner.

  As another modified embodiment, the parameter that is changed when a plurality of measurement toner images are formed is not the frequency of the AC voltage of the developing bias, but the printing of the electrostatic latent image formed by the exposure device 22. It may be a rate.

  That is, in the modified embodiment, the mode control unit 984 controls the exposure device 22 in a state where the potential difference of the DC voltage between the developing roller 231 and the photosensitive drum 20 is kept constant to control the printing rate per unit area. Are formed, a plurality of measurement toner images are formed on the photosensitive drum 20. Then, the mode control unit 984 controls the distance between the developing roller 231 and the developing bias applying unit 971 when a plurality of measurement toner images corresponding to a difference in the density of the plurality of measurement toner images detected by the density sensor 100 are formed. The charge amount of the toner included in the measurement toner image formed on the photoconductor drum 20 may be obtained based on the ratio of the difference between the DC components of the developing current flowing through the photoconductor drum 20. In this case, similarly to the above-described modified embodiment, the charge amount of the toner can be obtained based on Expression 4.

<Second modified embodiment>
The execution of the calibration by the mode control unit 984 adjusts the image density to be stable at the target image density in consideration of various factors such as the toner charge amount and the toner density change amount. However, the amount of change in the charge amount of the toner from the previous execution of the calibration may be small, but the amount of change in the toner density may be large. In this case, in step S22 of the above-described determination mode, it is determined that the difference or ratio ΔQ of the current charge amount with respect to the previous charge amount is equal to or less than the first threshold value TH1, and it is not necessary to execute the calibration requested by the request unit 986. Since it is determined that there is no calibration, the requested calibration may not be performed. As a result, there is a possibility that the image density is largely changed due to a large change amount of the toner density. Therefore, the image forming apparatus 10 of the above-described embodiment and each of the modified embodiments may be configured as follows in consideration of the influence of the change in image density due to the amount of change in toner density. FIG. 13 is a flowchart of a determination mode executed in the image forming apparatus 10 according to the second modified embodiment of the present invention.

  In the present modification, the mode control unit 984 stores the toner density detected by the toner density sensor 101 in the storage unit 983 in accordance with the execution of the charge amount measurement mode. Specifically, the mode control unit 984 stores the representative value of the toner density detected by the toner density sensor 101 in the storage unit 983 during the execution of the charge amount measurement mode. Here, the representative value indicates an average value, a maximum value, a minimum value, or the like. The mode control unit 984 may store the toner density detected by the toner density sensor 101 in the storage unit 983 immediately after the end of the charge amount measurement mode.

  Then, when the execution of the calibration is requested by the request unit 986, the determination unit 985 starts the determination mode as shown in FIG. 13, and differs from the flow shown in FIG. If it is determined that the difference or the ratio ΔQ of the current charge amount with respect to is less than or equal to the first threshold TH1 (NO in step S22), step S51 is further executed. The determination unit 985 determines in step S51 that the toner density sensor 101 has detected the first toner density detected by the toner density sensor 101 during the previous execution of the charge amount measurement mode and performed by the toner density sensor 101 during the execution of the current charge amount measurement mode. Whether the difference between the two toner densities (= second toner density-first toner density) or the ratio (= second toner density / first toner density) ΔT / C is smaller than a preset second threshold value TH2 Is determined (step S51).

  Note that the current charge amount measurement mode is the charge amount measurement mode in step S21 immediately before. The previous charge amount measurement mode is a charge amount measurement mode in which the previous charge amount was acquired, and is a charge amount measurement mode executed manually or automatically last time in the charge amount measurement mode executed in step S21 immediately before. . The second threshold value TH2 can be edited using the operation panel 18 in the same manner as the first threshold value TH1.

  Then, when the difference or ratio ΔT / C of the second toner concentration with respect to the first toner concentration is smaller than the second threshold TH2, the determination unit 985 determines that the execution of the requested calibration is not necessary (step S51). YES). As described above, when the determination unit 985 determines that the execution of the requested calibration is not necessary, step S24 is performed similarly to the flow illustrated in FIG.

  On the other hand, when the difference or ratio ΔT / C of the second toner concentration with respect to the first toner concentration is equal to or greater than the second threshold TH2, the determination unit 985 determines that the execution of the requested calibration is necessary (step). (NO in S51). As described above, when the determination unit 985 determines that the execution of the requested calibration is necessary, step S23 is performed as in the flow illustrated in FIG.

  According to the configuration of the present embodiment, in the determination mode, the determination unit 985 determines not only the difference or ratio between the charge amounts of the toner obtained in the current charge amount measurement mode and the previous charge amount measurement mode, but also the current charge amount measurement mode. The necessity of the calibration requested this time can be more appropriately determined based on the difference or ratio between the toner densities detected according to the charge amount measurement mode and the previous charge amount measurement mode.

<Third Modified Embodiment>
When the temperature or humidity around the image forming apparatus 10 greatly changes, the fluidity of the developer greatly changes, and there is a possibility that the charge amount, the transport amount, and the toner density of the toner greatly change. As a result, the image density of the toner image may be greatly changed. In consideration of such a case, the image forming apparatus 10 of the above embodiment and each of the modified embodiments may be configured as follows.

  In the present modified embodiment, the image forming apparatus 10 includes an environment sensor (temperature and humidity detecting unit) that detects a characteristic value including at least one of ambient temperature and humidity in an arbitrary space in the apparatus main body 11 (FIG. 1). ) Is further provided.

  The mode control unit 984 stores the characteristic values detected by the environment sensor in accordance with the execution of the calibration in the storage unit 983. Specifically, the mode control unit 984 stores the representative value of the characteristic value detected by the environment sensor in the storage unit 983 during the execution of the calibration. Here, the representative value indicates an average value, a maximum value, a minimum value, or the like. The mode control unit 984 may store the characteristic value detected by the environment sensor in the storage unit 983 immediately after the end of the calibration.

  Then, when the execution of the calibration is requested by the request unit 986, the determination unit 985 performs an auxiliary determination mode (auxiliary determination operation) for determining whether or not the determination mode needs to be performed, prior to the determination mode. The determination unit 985 performs the determination mode when it is determined that the execution of the determination mode is necessary in the auxiliary determination mode, and when the determination unit 985 determines that the execution of the determination mode is not necessary in the auxiliary determination mode, the previous charge amount and the current charge amount Regardless of the above, the mode control unit 984 is forced to execute the requested calibration.

  Specifically, in the auxiliary determination mode, the determination unit 985 determines whether the second characteristic value detected by the environment sensor according to the auxiliary determination mode corresponds to the first characteristic value detected by the environment sensor according to the previous calibration. It is determined whether or not the amount of change is smaller than a third threshold set in advance, and if the amount of change of the second characteristic value with respect to the first characteristic value is smaller than the third threshold, it is necessary to execute the determination mode. If it is determined that the change amount of the second characteristic value with respect to the first characteristic value is equal to or larger than the third threshold value, it is determined that the execution of the determination mode is not necessary.

  In the present modified embodiment, the second characteristic value detected by the environment sensor in accordance with the auxiliary determination mode is a characteristic value detected by the environment sensor immediately after the start of the auxiliary determination mode. However, the second characteristic value detected by the environment sensor in accordance with the auxiliary determination mode is not limited to this. The representative value of the characteristic value detected by the environment sensor before a predetermined time elapses after the start of the auxiliary determination mode. It may be. Here, the representative value indicates an average value, a maximum value, a minimum value, or the like.

  FIG. 14 is a flowchart of the auxiliary determination mode in the image forming apparatus 10 according to the third modified embodiment of the present invention. Specifically, as shown in FIG. 14, when the execution of the calibration is requested by the request unit 986, the determination unit 985 performs the auxiliary determination including the steps S61 and S62 prior to the determination mode after the step S21. Perform mode.

  Upon starting the auxiliary determination mode, the determination unit 985 acquires the characteristic value detected by the environment sensor (Step S61). Then, the determination unit 985 calculates the change amount ΔCV of the second measurement value, which is the characteristic value acquired in step S61, with respect to the first characteristic value, which is the characteristic value detected by the environment sensor according to the previous calibration, It is determined whether it is smaller than a preset third threshold value TH3 (step S62). The change amount ΔCV of the second characteristic value with respect to the first characteristic value is the difference (= second characteristic value−first temperature) or ratio (= second characteristic value / second characteristic value) of the second characteristic value with respect to the first characteristic value. (One characteristic value). Further, the third threshold value TH3 can be edited using the operation panel 18 in the same manner as the first threshold value TH1.

  Then, when the change amount ΔCV of the second characteristic value with respect to the first characteristic value is smaller than the third threshold value TH3 in Step S62 (YES in Step S62), the determining unit 985 determines that the execution of the determination mode is necessary, The determination mode (the processing after step S21) is executed. On the other hand, when the change amount ΔCV of the second characteristic value with respect to the first characteristic value is equal to or more than the third threshold value TH3 in Step S62 (NO in Step S62), the determining unit 985 determines that the execution of the determination mode is not necessary. Step S23 (calibration) is forcibly executed regardless of the previous charge amount and the current charge amount.

  For example, it is assumed that the characteristic value detected by the environment sensor is the ambient temperature. In this case, in step S62, the determination unit 985 determines whether the change amount ΔCV of the temperature obtained in step S61 with respect to the temperature detected by the environment sensor according to the previous calibration is smaller than the third threshold TH3. Is determined. Similarly, it is assumed that the characteristic value detected by the environment sensor is ambient humidity. In this case, in step S62, the determination unit 985 determines whether the change amount ΔCV of the humidity obtained in step S61 with respect to the humidity detected by the environment sensor according to the previous calibration is smaller than the third threshold TH3. Is determined.

  Alternatively, it is assumed that the characteristic values detected by the environment sensor are ambient temperature and humidity. In this case, in step S62, the determination unit 985 determines the amount of change in the temperature (hereinafter, the first amount of change) obtained in step S61 with respect to the temperature detected by the environment sensor in accordance with the previous calibration, or In accordance with the calibration, it is determined whether or not the amount of change in the humidity (hereinafter, the second amount of change) obtained in step S61 with respect to the humidity detected by the environment sensor is smaller than a third threshold value TH3. In this case, the determination unit 985 determines that the execution of the determination mode is necessary when at least one of the first change amount and the second change amount is smaller than the third threshold TH3. On the other hand, when both the first change amount and the second change amount are equal to or greater than the third threshold value TH3, the determination unit 985 determines that the execution of the determination mode is not necessary.

  According to the configuration of the present modified embodiment, the determination unit 985 performs the auxiliary determination mode (steps S61 and S62) prior to the determination mode, thereby charging the toner after the previous calibration was performed in the determination mode. The necessity of the calibration requested this time is determined according to the change in the amount, or the calibration requested this time is forcibly performed regardless of the change in the charge amount of the toner since the previous calibration was executed. Can be determined.

  Specifically, when the change amount of the characteristic value detected in response to the request for performing the calibration with respect to the characteristic value detected in the previous calibration is larger than the third threshold value, Thus, the calibration operation requested this time can be forcibly executed without performing the determination mode.

<Fourth modified embodiment>
In step S22 of the determination mode, it is determined that the difference or ratio ΔQ of the current charge amount with respect to the previous charge amount is equal to or less than the first threshold value TH1, and it is determined that the execution of the requested calibration is not necessary. Assume that no calibration has been performed. However, even in this case, the image density may change due to a slight change in the charge amount of the toner. Therefore, even when calibration is not executed because the amount of change in the amount of charge of the toner is small, the image forming apparatus according to the embodiment described above can stabilize the image density in accordance with the amount of change in the amount of charge of the toner. The image forming apparatus 10 of each modified embodiment may be configured as follows. FIG. 15 is a flowchart of the determination mode executed in the image forming apparatus 10 according to the fourth modified embodiment of the present invention.

  Specifically, as shown in FIG. 15, the determination unit 985 executes the requested calibration because the difference or ratio ΔQ of the current charge amount with respect to the previous charge amount is not less than or equal to the first threshold value TH1 in step S22. Is determined to be unnecessary (NO in step S22), the DC voltage between the photosensitive drum 20 and the developing roller 231 is determined according to the difference or the ratio ΔQ of the current charge amount with respect to the previous charge amount used in step S22. The developing bias is corrected so as to change the difference (step S71).

  For example, the charge amount of the toner becomes larger than when the previous charge amount measurement mode was executed, and the difference ΔQ1 (= current charge amount−previous charge amount) of the current charge amount with respect to the previous charge amount indicates a positive value. Assume that the ratio of the current charge amount to the amount ΔQ2 (= current charge amount / previous charge amount) is a value of 1 or more.

  In this case, the determination unit 985 determines in step S71 that the product of the predetermined positive first coefficient c1 (> 0) and the difference ΔQ1 (> 0) between the previous charge amount and the current charge amount, and the DC component Vdc of the developing bias And the sum of (c1 × ΔQ1 + Vdc) is determined as the DC component Vdcm of the corrected developing bias. Alternatively, in step S71, the determination unit 985 determines the product of the second coefficient c2 (> 1) larger than the predetermined value 1, the ratio ΔQ2 (≧ 1) of the current charge amount to the previous charge amount, and the DC component Vdc of the developing bias ( c2 × ΔQ2 × Vdc) is determined as the DC component Vdcm of the development bias after the correction.

  As a result, when it is considered that the charge amount of the toner becomes larger and the image density is lower than when the charge amount measurement mode was last executed, the developing bias is set in accordance with the change amount of the charge amount of the image. A large correction can be made. As a result, it is possible to increase the image density of the toner image formed using the corrected development bias, and to approach the target image density.

  Conversely, for example, the charge amount of the toner becomes smaller than that in the previous execution of the charge amount measurement mode, and the difference ΔQ1 (= current charge amount−previous charge amount) of the current charge amount with respect to the previous charge amount becomes negative. Suppose that the ratio ΔQ2 of the current charge amount to the previous charge amount (= current charge amount / previous charge amount) is less than 1.

  In this case, the determination unit 985 determines in step S71 that the product of the predetermined positive third coefficient c3 (> 0) and the difference ΔQ1 (<0) between the previous charge amount and the current charge amount, and the DC component Vdc of the developing bias And the sum of (c3 × ΔQ1 + Vdc) is determined as the DC component Vdcm of the corrected developing bias. Alternatively, in step S71, the determination unit 985 determines the product (the product of the fourth coefficient c4 (<1) of less than 1), the ratio ΔQ2 (<1) of the current charge amount to the previous charge amount, and the DC component Vdc of the developing bias. c4 × ΔQ2 × Vdc) is determined as the DC component Vdcm of the development bias after the correction.

  As a result, when it is considered that the charge amount of the toner becomes smaller and the image density becomes higher than when the previous charge amount measurement mode was executed, the developing bias is set in accordance with the change amount of the charge amount of the toner of the image. Small corrections can be made. As a result, it is possible to lower the image density of the toner image formed using the corrected development bias and to approach the target image density.

  As described above, according to the configuration of the present embodiment, when the difference or the ratio ΔQ between the previous charge amount and the current charge amount indicates a value that does not require the execution of the requested calibration, the requested calibration is performed. No development is performed, but the developing bias is corrected so as to change the DC voltage difference between the photosensitive drum 20 and the developing roller 231 according to the difference or the ratio ΔQ. For this reason, the developing bias can be appropriately adjusted in consideration of a slight change in the charge amount of the toner without wasting the time required for executing the calibration and the toner used for forming the plurality of calibration toner images. Can be corrected.

<Other modified embodiments>
(1) In the above embodiment, the mode in which the surface of the developing roller 231 is knurled is described. However, the surface of the developing roller 231 having a concave shape (dimple) or the surface of the developing roller 231 is blasted. It may be something.

  (2) When the image forming apparatus 10 has a plurality of developing devices 23 as shown in FIG. 1, the charge amount measurement mode according to the above-described embodiment is performed by one or two developing devices 23, and the result is determined by another developing device. 23 may be used.

<Experimental results>
Hereinafter, the results of a calibration experiment performed by the inventor will be described. FIG. 16 is a diagram illustrating an example of an experiment result of the calibration.

  In the image forming apparatus 10, as a result of causing the mode control unit 984 to execute the calibration, the toner charge amount Q / M, the DC component of the developing bias voltage (hereinafter referred to as developing DC voltage) Vdc, the toner density T / C, the image unit The potential VL and the development potential difference Vdc-VL are “23 μC / g”, “180 V”, “8%”, “20 V”, and “160 V” described in the record “last time” in the first row of the table shown in FIG. It became.

  Thereafter, when the charge amount Q / M of the toner and the toner concentration T / C change so as to be the charge amount Q / M and the toner concentration T / C of the toner of each record in the second and subsequent rows, the image portion potential VL is obtained. The calibration was performed by the mode control unit 984 while maintaining the voltage at “20 V”. The execution result of the calibration includes the charge amount change amount ΔQ, the development DC voltage Vdc, the toner density change amount ΔT / C, the development potential difference Vdc−VL, and the development DC voltage change of each record of the second and subsequent rows in the table of FIG. The quantity ΔVdc is shown in the evaluation EV.

  The charge amount change amount ΔQ of each record is the toner charge amount Q / M of each record (for example, “Experiment 1”) with respect to the toner charge amount Q / M “23 μC / g” of the record “previous” in the first row. For example, the difference is “24 μC / g” (eg, “1 μC / g”), and corresponds to the difference of the current charge amount with respect to the previous charge amount (= current charge amount−previous charge amount) used in step S22.

  The toner density change amount ΔT / C is the toner density T / C (for example, “7.9%”) of each record (for example, “Experiment 1”) with respect to the toner density T / C of “8%” for the record “last time” in the first row. )) (For example, “0.1%”), which is the difference between the second toner density and the first toner density (= second toner density−first toner density) used in step S51 (FIG. 13). Equivalent to.

  The development DC voltage change amount ΔVdc is the difference (for example, “185 V”) between the development DC voltage Vdc (for example, “185 V”) of each record (for example, “Experiment 1”) and the development DC voltage Vdc of “180 V” for the record “last time” in the first row "5 V"), which indicates the adjustment amount (voltage difference) of the DC component of the developing bias by the calibration.

  The evaluation EV is “◎” indicating that the image density is very stable without calibration, “○” indicating that the image density is stable, and that the image density is not stable , And "x" indicating that the image density is degraded. In other words, the evaluation EV is classified into “「 ”,“ ○ ”,“ △ ”, and“ × ”in ascending order of the adjustment amount of the DC component of the developing bias by the calibration.

  In the example shown in FIG. 16, the evaluation EV of each record is classified as “◎” when the adjustment amount of the DC component of the development bias by the calibration indicated by the development DC voltage change amount ΔVdc of each record is 5 V or less. . The evaluation EV of each record is classified as “分類” when the adjustment amount is greater than 5 V and equal to or less than 10 V, and is classified as “△” when the adjustment amount is greater than 10 V and equal to or less than 20 V. When the adjustment amount is 20 V or more, it is classified as “×”.

  From the results of such calibration, as shown in FIG. 16, five records “Experiment 1”, “Experiment 2”, “Experiment 9”, and “Experiment 10” with evaluation EVs of “」 ”and“ O ”are shown. The present inventor has found that the charge amount change amount ΔQ in “Experiment 11” is “1 μC / g” or less. From this, the present inventor sets the first threshold value TH1 to “1 μC / g” and executes the determination mode according to the flow shown in FIG. 10 immediately before the next calibration is executed, for example. It has been found that unnecessary execution of calibration can be avoided.

  That is, when the charge amount Q / M of the toner has changed by not more than “1 μC / g” after the execution of the previous calibration as shown in the five records, in step S 22, Since the difference ΔQ in the charge amount this time is equal to or smaller than the first threshold value TH1 “1 μC / g”, it can be determined that the execution of the calibration is not necessary. Thus, it is possible to avoid performing the calibration uselessly.

  On the other hand, if the charge amount Q / M of the toner has changed more than “1 μC / g” after the previous calibration, as shown in a record different from the five records, step S22 Since the difference ΔQ between the current charge amount and the previous charge amount is larger than the first threshold value TH1 “1 μC / g”, it can be determined that the calibration needs to be performed. Thereby, the calibration is executed in step S23, and the developing bias can be appropriately adjusted so that the image density becomes the target image density.

  Further, based on the result of the calibration, as shown in FIG. 16, the toner density change amount ΔT / C of three records “Experiment 1”, “Experiment 9”, and “Experiment 10” with the evaluation EV “Ｖ” Has been found to be less than “0.5%”. On the other hand, the inventor has found that the toner density change ΔT / C of the two records “Experiment 2” and “Experiment 11” with the evaluation EV “O” is “0.5%” or more. For this reason, the present inventor sets the second threshold value TH2 to “0.5%”, for example, and executes the determination mode according to the flow shown in FIG. 13 immediately before the next calibration is executed. It is possible to appropriately adjust the image density even when the image density has not changed due to the change in the charge amount Q / M, but has changed due to the change in the toner density T / C. Was found.

  That is, although the charge amount Q / M of the toner has changed only by 1 μC / g or less after the previous calibration as shown in the two records “Experiment 2” and “Experiment 11”, If the toner density T / C has changed by “0.5%” or more, in step S51, the difference ΔT / C between the first toner density and the second toner density becomes the second threshold TH2 “0.5%”. As described above, it can be determined that the execution of the calibration is necessary. As a result, although the charge amount Q / M of the toner changed only by 1 μC / g or less, the toner density T / C changed by 0.5% or more, and the image density was extremely stable. Even if it cannot be said that there is, calibration can be executed in step S23. That is, even when the image density is not changed due to the change in the toner charge amount Q / M, but the image density is changed due to the change in the toner density T / C, the image density is appropriately adjusted. can do.

10 Image forming apparatus 14 Intermediate transfer unit (transfer section)
20 Photoconductor drum (image carrier)
Reference Signs 22 Exposure device 23 Developing device 100 Density sensor (density detector)
101 Toner density sensor (toner density detector)
145 Secondary transfer roller (transfer section)
231 Developing roller 971 Developing bias applying unit 973 Ammeter 980 Control unit 984 Mode control unit (charge amount acquisition unit, calibration operation execution unit)
985 Judgment unit (calibration operation judgment unit)
986 Request section

Claims (6)

Rotated, an electrostatic latent image is formed on the surface, and an image carrier that carries a toner image in which the electrostatic latent image has been revealed;
A charging device for charging the image carrier to a predetermined charging potential,
The electrostatic latent image is formed by exposing the surface of the image carrier charged to the charging potential in accordance with predetermined image information, which is disposed downstream of the charging device in the rotation direction of the image carrier. An exposure apparatus,
A developing device arranged opposite to the image carrier at a predetermined developing nip portion on the downstream side in the rotation direction from the exposure device, the developer device being rotated and carrying a developer composed of toner and carrier on a peripheral surface thereof. A developing device that includes a developing roller that forms the toner image by supplying toner to the image carrier;
A transfer unit that transfers the toner image carried on the image carrier to a sheet,
A developing bias application unit that can apply a developing bias in which an AC voltage is superimposed on a DC voltage to the developing roller;
A density detector for detecting the density of the toner image,
Controlling the charging device and the exposing device to form a plurality of electrostatic latent images for calibration on the image carrier; and controlling the developing bias applying unit to control the developing for each electrostatic latent image for calibration. A plurality of calibration toner images are formed by making at least the DC voltage of a bias different, based on a relationship between the density of the plurality of calibration toner images detected by the density detection unit and the developing bias. A calibration operation execution unit that executes a calibration operation to determine the developing bias applied to the developing roller in an image forming operation;
The charging device, the exposure device, and the developing bias application unit are controlled to form a plurality of measurement toner images having different toner development amounts on the image carrier, and the plurality of measurement toner images are detected by the density detection unit. Based on the density of the measurement toner image, or when the plurality of measurement toner images are formed in addition to the densities of the plurality of measurement toner images, the gap between the developing roller and the developing bias applying unit is formed. A charge amount acquisition unit that performs a charge amount acquisition operation of acquiring a charge amount of the toner based on a DC component of a flowing developing current;
A requesting unit for requesting execution of the calibration operation,
When the execution of the calibration operation is requested by the request unit, the charge amount acquisition unit is caused to execute the charge amount acquisition operation, and the charge amount of the toner acquired in the previous charge amount acquisition operation is the charge amount. Calibration for performing a determination operation of determining whether or not the requested calibration operation needs to be performed based on a previous charge amount and a current charge amount which is a charge amount of the toner acquired in the current charge amount acquisition operation. An operation determining unit;
An image forming apparatus comprising: The calibration operation determination unit may execute the requested calibration operation when the difference or ratio of the current charge amount to the previous charge amount is larger than a first threshold value set in advance in the determination operation. Is determined to be necessary, and when the difference or ratio of the current charge amount to the previous charge amount is equal to or smaller than the first threshold, it is determined that the execution of the requested calibration operation is not necessary. The image forming apparatus as described in the above. A toner concentration detection unit that detects a toner concentration indicating a ratio of a toner amount to a carrier amount included in the developer,
If the difference or ratio of the current charge amount to the previous charge amount is equal to or less than the first threshold in the determination operation, the calibration operation determination unit may determine the toner density detection unit in accordance with a previous charge amount acquisition operation. It is further determined whether the difference or ratio of the second toner concentration detected by the toner concentration sensor according to the current charge amount acquisition operation with respect to the detected first toner concentration is smaller than a second threshold value set in advance. 3. The method according to claim 2, further comprising: determining that it is not necessary to execute the requested calibration operation when a difference or a ratio of the second toner concentration to the first toner concentration is smaller than the second threshold. Image forming apparatus. When the execution of the calibration operation is requested by the requesting unit, the calibration operation determination unit performs an auxiliary determination operation of determining whether the determination operation is necessary prior to the determination operation. When it is determined in the determination operation that the execution of the determination operation is necessary, the determination operation is performed. When it is determined that the execution of the determination operation is not necessary in the auxiliary determination operation, the previous charge amount and the current charge amount are determined. 4. The image forming apparatus according to claim 2, wherein the calibration operation execution unit is configured to execute the requested calibration operation irrespective of the condition. 5. Further comprising a temperature and humidity detector for detecting a characteristic value including at least one of ambient temperature and humidity,
The calibration operation determination unit, in the auxiliary determination operation, for the first characteristic value detected by the temperature and humidity detection unit according to the previous calibration operation, the temperature and humidity detection unit according to the auxiliary determination operation Determined whether the detected change amount of the second characteristic value is smaller than a preset third threshold value,
When the amount of change of the second characteristic value with respect to the first characteristic value is smaller than the third threshold, it is determined that the determination operation needs to be performed, and the amount of change of the second characteristic value with respect to the first characteristic value 5. The image forming apparatus according to claim 4, wherein when is equal to or more than the third threshold value, it is determined that the execution of the determination operation is not necessary. The calibration operation determination unit, when determining in the determination operation that the execution of the requested calibration operation is not necessary, according to a difference or ratio between the previous charge amount and the current charge amount, The image forming apparatus according to claim 2, wherein the developing bias is corrected so as to change a difference in a DC voltage between the carrier and the developing roller.