JP2020052240A - Image forming apparatus - Google Patents

Abstract

To accurately and efficiently measure the amount of charge of toner in an image forming apparatus that includes a developing device applied with a two-component developing system.SOLUTION: An image forming apparatus 10 comprises a mode control unit 984. The mode control unit 984 forms a plurality of toner images for measurement on a photoreceptor drum 20 in an image forming unit 13 in a non-developing operation period, and acquires the amount of charge of toner on the basis of the density of the plurality of toner images for measurement detected by a density sensor 100, or a DC component of a developing current flowing in a developing roller 231 when the plurality of toner images for measurement are formed in addition to the density of the plurality of toner images for measurement. The mode control unit 984 selects an image pattern for the toner images for measurement on the basis of predetermined selection information.SELECTED DRAWING: Figure 6

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, 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 the two-component development, a phenomenon is observed in which the developer is deteriorated and the toner charge amount is changed under the influence of the number of prints, environmental change, print mode (the number of continuous prints per job), print ratio, and the like. As a result, problems such as a decrease in image density, occurrence of toner fog, and an increase in toner scattering occur. To cope with such problems, conventionally, changes in the amount of developer charge are predicted from the number of prints, environmental fluctuations, print mode and print ratio, and the toner density, developing bias, photoconductor surface potential, developing roller rotation A technique has been adopted in which the speed, the output of a suction fan for collecting scattered toner, and the like are adjusted to suppress a decrease in image density, deterioration of toner fog, and deterioration of toner scattering.

  However, these techniques are only combinations of individual predictions under the respective conditions of the number of prints, environmental fluctuation, print mode, and print rate. It was difficult to fully predict the amount.

  For this reason, a technique for more accurately predicting the charge amount of the toner has 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, 4, and 5, a current value flowing into a developing roller that carries 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. Is done. 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 Patent No. 4480066

  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, 4, and 5, 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. Thus, it has been difficult to correctly measure the charge amount of the toner from the current flowing into the developing roller.

  Further, according to the techniques described in the above patent documents, an image pattern including a toner image for measurement is formed on a photosensitive drum in order to measure the charge amount of toner. In order to accurately measure the charge amount of the toner, it is desirable to form many measurement toner images. In this case, the time during which a normal image forming operation cannot be performed increases, and the amount of toner consumed during measurement increases. However, there is a problem that the number increases.

  SUMMARY An advantage of some aspects of the invention is to solve a problem as described above, and to accurately and efficiently measure a toner charge amount in an image forming apparatus including a developing device to which a two-component developing method is applied. Aim.

  The image forming apparatus according to one aspect of the present invention is rotated about a predetermined axis, has a preset image forming area on the surface, and an electrostatic latent image is formed in the image forming area, A photosensitive drum that carries the toner image in which the electrostatic latent image has been revealed, a charging device that charges the photosensitive drum to a predetermined charging potential, and the image of the photosensitive drum that has been charged to the charging potential An exposure device that forms the electrostatic latent image by exposing a formation area along a main scanning direction parallel to the axis and a sub-scanning direction orthogonal to the main scanning direction in accordance with predetermined image information; A developing device disposed opposite to the photosensitive drum at a developing nip portion, the developer device being rotated to carry a developer including a toner and a carrier on a peripheral surface and supplying toner to the photosensitive drum. toner And a developing device including a developing roller that forms a plurality of image forming units, each forming a different color toner image, and the plurality of image forming units carried on the photoconductor drum. An intermediate transfer unit including an intermediate transfer belt that allows the toner image to be primarily transferred in a superimposed manner and carries the superimposed toner image; and a secondary transfer that transfers the toner image from the intermediate transfer belt to a sheet. And a developing bias applying unit capable of applying a developing bias in which an AC voltage is superimposed on a DC voltage to the developing rollers of the plurality of image forming units, and the developing bias applying unit is disposed to face the photosensitive drum or the intermediate transfer belt. And at least one density detecting unit for detecting the density of the toner image, and a non-conducting device different from a developing operation in which the toner image is formed on the photosensitive drum. During operation, at least one of the plurality of image forming units controls the charging device, the exposing device, and the developing bias applying unit to form a plurality of measurement toner images on the photosensitive drum. Formed, based on the density of the plurality of measurement toner images detected by the density detection 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 for acquiring a charge amount of toner contained in the measurement toner image formed on the photosensitive drum based on a DC component of a development current flowing between the development roller and the development bias application unit. A charge amount acquiring unit for performing an operation; and a plurality of image patterns including information on at least the arrangement and the image density of the measurement toner image in the image forming area. A storage unit for storing, and the image pattern of the measurement toner image formed in the image forming area during the charge amount obtaining operation from the plurality of image patterns stored in the storage unit based on predetermined selection information. And an image pattern selection unit that selects

  According to this configuration, in the charge amount acquisition operation, the plurality of measurement toner images are detected based on the density of the plurality of measurement toner images detected by the density detection unit, or in addition to the densities of the plurality of measurement toner images. The amount of charge of the toner contained in the measurement toner image formed on the photosensitive drum is obtained based on the DC component of the developing current flowing between the developing roller and the developing bias applying unit when is formed. . As a result, the charge amount of the toner can be accurately measured. The image pattern selection unit selects an image pattern of the measurement toner image formed in the image forming area at the time of the charge amount acquisition operation from a plurality of image patterns stored in the storage unit based on predetermined selection information. Therefore, compared with the case where the same image pattern is always used for the charge amount acquisition operation, the balance between the accuracy of the charge amount of the toner and the time required for the charge amount acquisition operation is adjusted, and the charge amount of the toner is more efficiently used. Can be measured.

  In the above configuration, the image pattern selection unit may include, as the selection information, information on the number of prints on the sheet to which the toner image has been transferred, information on a print ratio of the toner image formed on the sheet, The measurement is performed based on at least one of driving time information, print mode information including at least information on an interval between the sheets on which the toner images are formed, and environmental information including at least one of temperature and humidity. It is desirable to select the image pattern of the toner image for use.

  According to this configuration, a suitable image pattern can be selected based on the selection information that can affect the charge amount acquisition operation. Therefore, it is possible to accurately measure the charge amount of the toner while minimizing the influence of disturbance.

  In the above configuration, the developing device is disposed in the housing that rotatably supports the developing roller, and the developer is transported along the developing roller in a first direction parallel to the main scanning direction. A first transport path that allows the developer to be transported, and a housing that is disposed adjacent to the first transport path and that allows the developer to be transported in a second direction opposite to the first direction. A second transport path, and a partitioning section that defines the first transport path and the second transport path so that both ends of the first transport path and the second transport path in the first direction communicate with each other. A first transport member rotatably disposed on the first transport path, transporting the developer in the first direction and supplying the developer to the developing roller; and a first transport member rotatably disposed on the second transport path; Developer A second conveying member for conveying in the second direction, wherein the at least one density detecting unit is disposed at a position corresponding to the first direction upstream portion of the image forming area in the main scanning direction. A first density detection unit, and a second density detection unit disposed at a position corresponding to the downstream portion of the image forming area in the main scanning direction in the first direction, and the storage unit includes: A first image pattern including a toner image for measurement continuously extending along the main scanning direction to include an area facing the first density detection section and the second density detection section in the image forming area is stored. Is desirable.

  In the above configuration, the storage unit is the measurement toner image formed in one of the plurality of image forming units, and the first density detection unit in the image forming region. A first toner image for measurement arranged in an area facing the image forming apparatus, and the toner image for measurement formed in another image forming unit different from the one image forming unit among the plurality of image forming units. A second measurement toner image disposed in an area of the image formation area facing the second density detection unit and on an extension of the first measurement toner image in the main scanning direction. It is desirable to store the pattern.

  In the above configuration, the storage unit is the measurement toner image formed in one of the plurality of image forming units, and the first density detection unit in the image forming region. A third toner image for measurement arranged in a region facing the image forming apparatus and the toner image for measurement formed in another image forming unit different from the one image forming unit among the plurality of image forming units. A third image including a fourth measurement toner image disposed in a region of the image formation region facing the second density detection unit and overlapping the third measurement toner image in the main scanning direction. It is desirable to store the pattern.

  Further, in the above configuration, the third measurement toner image and the fourth measurement toner image of the third image pattern are respectively arranged so as to straddle a center of the image forming area in the main scanning direction. Is desirable.

  In the above configuration, the storage unit is the measurement toner image formed in one of the plurality of image forming units, and the first density detection unit in the image forming region. A fifth measurement toner image disposed in a region facing the image forming unit and the measurement toner image formed in another image forming unit different from the one image forming unit among the plurality of image forming units. A sixth measurement toner image disposed in an area of the image formation area facing the second density detection unit and on an extension of the fifth measurement toner image in the main scanning direction, and the other image formation. A position in the sub-scanning direction with respect to the fifth measurement toner image in the measurement toner image formed in the image forming area and in the image forming area facing the first density detection section; Placed in the sixth A seventh measurement toner image having the same image density as the regular toner image, and the measurement toner image formed in the one image forming unit, wherein the measurement toner image faces the second density detection unit in the image forming area And an eighth measurement toner image having the same image density as that of the fifth measurement toner image, the fourth image pattern being arranged in a region to be scanned and on an extension of the seventh measurement toner image in the main scanning direction. It is desirable to have.

  Further, in the above-described configuration, the image pattern selection unit may include, as the print ratio information, the fourth image when the average value of the print ratio of the toner image at a predetermined number of prints exceeds a preset threshold. Preferably, a pattern is selected as the image pattern of the measurement toner image.

  In the above configuration, the storage unit is the measurement toner image formed in one of the plurality of image forming units, and the first density detection unit in the image forming region. A ninth measurement toner image arranged in a region facing the image forming apparatus and the measurement toner image formed in another image forming unit different from the one image forming unit among the plurality of image forming units. A tenth measurement toner image disposed in an area of the image formation area facing the second density detection unit and on an extension of the ninth measurement toner image in the main scanning direction; A position in the sub-scanning direction with respect to the ninth measurement toner image in an area of the image forming area facing the first density detection section, the measurement toner image being formed in the image forming area. Placed in the second An eleventh measurement toner image having an image density different from that of the zero measurement toner image, and the measurement toner image formed in the one image forming unit, wherein the second density detection unit is included in the image forming area. And a twelfth measurement toner image having an image density different from that of the ninth measurement toner image, the second twelfth measurement toner image being arranged on a region facing the first measurement toner image in the main scanning direction. It is desirable to store the pattern.

  In the above configuration, the ninth measurement toner image and the tenth measurement toner image may be formed of solid images, and the eleventh measurement toner image and the twelfth measurement toner image may be formed of a halftone image. Is desirable.

  Further, in the above configuration, the image pattern selection unit may include, as the printing ratio information, the fifth image when the average value of the printing ratio of the toner image at a predetermined number of printed sheets falls below a predetermined threshold. Preferably, a pattern is selected as the image pattern of the measurement toner image.

  Further, in the above configuration, the image pattern selecting unit, as the printing rate information, an average value of the printing rate of the toner image at a predetermined number of printed sheets is less than a predetermined threshold value, and as the printing mode information. It is preferable that the fifth image pattern is selected as the image pattern of the measurement toner image when an interval between the sheets on which the toner images are formed exceeds a preset threshold.

  Further, in the above configuration, it is preferable that the eleventh measurement toner image and the twelfth measurement toner image are arranged at positions overlapping in the main scanning direction.

  Further, in the above configuration, it is preferable that the eleventh measurement toner image and the twelfth measurement toner image are respectively arranged so as to straddle a center of the image forming area in the main scanning direction.

  In the above configuration, the plurality of image forming units may include a cleaning blade that contacts the photoconductor drum and collects toner attached to a surface of the photoconductor drum, and the toner collected by the cleaning blade. And a cleaning transporting member that transports the toner image in the first direction, wherein the ninth measurement toner image and the eleventh measurement toner image are solid images, and the tenth measurement toner image is Preferably, the twelfth measurement toner image is a halftone image.

  In the above configuration, the plurality of image forming units may include a cleaning blade that contacts the photoconductor drum and collects toner attached to a surface of the photoconductor drum, and the toner collected by the cleaning blade. And a cleaning conveying member that conveys the first and second density detecting units in the first image pattern in a region facing the first density detecting unit and the second density detecting unit. It is preferable that the width in the sub-scanning direction of an area between the first density detecting unit and the second density detecting unit is set smaller than the width in the scanning direction.

  In the above configuration, the plurality of image forming units may include a cleaning blade that contacts the photoconductor drum and collects toner attached to a surface of the photoconductor drum, and the toner collected by the cleaning blade. And a cleaning transporting member that transports the cleaning unit along the first direction or the second direction, wherein the storage unit is formed in one of the plurality of image forming units. A thirteenth measurement toner image disposed in a region of the image forming region facing the first density detection portion, the measurement toner image being measured, and the measurement formed in the one image forming portion. In the main scanning direction of the thirteenth measurement toner image in a region opposed to the second density detection portion in the image forming region. A fourteenth measurement toner image arranged on a long line and the measurement toner image formed in the one image forming unit, wherein the fourteenth measurement toner image is between the thirteenth measurement toner image and the fourteenth measurement toner image; It is preferable to store a sixth image pattern including a fifteenth measurement toner image arranged at an interval.

  Further, in the above-described configuration, the charge amount acquisition unit is configured to perform the non-developing operation when the photosensitive drum corresponds to a sheet interval between one sheet and another sheet following the one sheet. It is desirable that the toner image for measurement is formed thereon.

  According to this configuration, the charge amount of the toner can be measured without significantly affecting the operation time of the image forming operation.

  In the above configuration, the pattern image selection unit may be configured to perform the measurement formed in the image forming area at the time of the next charge amount acquisition operation according to the charge amount of the toner acquired by the charge amount acquisition operation. It is desirable to select the image pattern of the toner image for use.

  According to this configuration, it is possible to select an effective image pattern at the time of the next charge amount measurement acquisition operation according to the charge amount of the toner.

  In the above configuration, the storage unit stores the frequency 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 and the photosensitive drum is kept constant. Reference information relating to the inclination of a reference straight line indicating the relationship between the amount of change in the density of the toner image and the amount of change in the amount of change in the toner image. Forming the plurality of measurement toner images on the photosensitive drum while changing the frequency of the AC voltage of the developing bias while keeping the potential difference of the DC voltage between the drum and the drum constant; The gradient of the measurement straight line indicating the relationship between the density change amount of the measurement toner image and the density change amount of the measurement toner image is detected by the density detection unit. And obtaining the charge amount of the toner contained in the measurement toner image formed on the photosensitive drum from the obtained inclination of the measurement straight line and the reference information of the storage unit. desirable.

  According to this configuration, the charge amount of the toner can be obtained without using a surface potential sensor that measures the potential on the photosensitive drum or an ammeter that measures the developing current flowing into the developing roller.

  In the above configuration, the reference information stored in the storage unit is such that when the charge amount of the toner is the first charge amount, the slope of the reference straight line is negative, and the charge amount of the toner is When the second charge amount is smaller than the first charge amount, the inclination of the reference straight line is positive, and the inclination of the reference straight line is set to increase as the charge amount of the toner decreases. It is desirable to have been.

  According to this configuration, the charge amount of the toner can be accurately acquired from the relationship between the frequency of the AC voltage of the developing bias and the density of the toner image formed on the photosensitive drum (the amount of the developed toner).

  In the above-described configuration, the charge amount acquiring unit is configured to change the frequency of the AC voltage of the developing bias while maintaining a constant potential difference of the DC voltage between the developing roller and the photosensitive drum. Forming the plurality of measurement toner images on a drum, and forming the plurality of measurement toner images with respect to a difference in density between the plurality of measurement toner images detected by the density detection unit; And acquiring a charge amount of toner included in a measurement toner image formed on the photosensitive drum based on a ratio of a difference between DC components of the developing current flowing between the developing bias applying unit and the developing bias applying unit. Good.

  Further, in the above configuration, the charge amount acquisition unit controls the exposure device in a state where a potential difference of a DC voltage between the developing roller and the photosensitive drum is kept constant, and controls a printing rate per unit area. Forming the plurality of measurement toner images on the photosensitive drum while changing the density, and forming the plurality of measurement toner images with respect to a difference in density between the plurality of measurement toner images detected by the density detection unit. The toner contained in the measurement toner image formed on the photoconductor drum based on the ratio of the difference between the DC components of the developing current flowing between the developing roller and the developing bias applying unit. What acquires the charge amount may be used.

  According to the present invention, in an image forming apparatus including a developing device to which a two-component developing method is applied, it is possible to accurately and efficiently measure a charge amount of 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. 2 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. 2 is a plan view illustrating an internal structure of the developing device and the cleaning device according to the embodiment of the present disclosure. 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 a photosensitive drum 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. 5 is a graph showing the relationship between the inclination of the graph of FIG. 4 and the toner charge amount 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. 3 is a schematic diagram of an image pattern A formed in a charge amount measurement mode executed in the image forming apparatus according to one embodiment of the present disclosure. FIG. 4 is a schematic diagram of an image pattern B formed in a charge amount measurement mode executed in the image forming apparatus according to the embodiment of the present disclosure. FIG. 5 is a schematic diagram of an image pattern C formed in a charge amount measurement mode executed in the image forming apparatus according to the embodiment of the present disclosure. FIG. 3 is a schematic diagram of an image pattern D formed in a charge amount measurement mode executed in the image forming apparatus according to one embodiment of the present disclosure. FIG. 2 is a schematic diagram of an image pattern E formed in 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 an image pattern F formed in a charge amount measurement mode executed in the image forming apparatus according to one embodiment of the present disclosure. FIG. 4 is a schematic diagram of an image pattern G formed in 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 an image pattern H formed in a charge amount measurement mode executed in the image forming apparatus according to the embodiment of the present disclosure. FIG. 3 is a schematic diagram of an image pattern I formed in a charge amount measurement mode executed in the image forming apparatus according to one embodiment of the present disclosure. FIG. 4 is a schematic diagram illustrating a state in which the density of a measurement toner image formed in a charge amount measurement mode executed in the image forming apparatus according to an embodiment of the present invention changes according to the frequency of a developing bias. 6 is a flowchart of a charge amount adjustment mode executed in the image forming apparatus according to the embodiment of the present disclosure. FIG. 9 is a cross-sectional view of a developing device according to a 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 an image forming apparatus according to a modified embodiment of the present invention. FIG. 9 is a plan view illustrating an internal structure of a developing device and a cleaning device according to a modified embodiment of the present invention.

  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.

  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, a plurality of image forming sections 13 for forming toner images of respective colors to be transferred onto the sheet P fed from the sheet feeding section 12, respectively, An intermediate transfer unit 14 on which an image is primarily transferred, a secondary transfer roller 145 facing the intermediate transfer unit 14 in a secondary transfer unit, a toner replenishing unit 15 for replenishing toner to each image forming unit 13, and a sheet P A fixing unit 16 for performing a process of fixing the formed unfixed toner image to the sheet P is 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 (not shown) for inputting output conditions and the like for the sheet P is provided at an appropriate position on the upper surface of the apparatus main body 11. This operation panel is provided with a power key, a touch panel for inputting output conditions, and various operation keys.

  In the apparatus main body 11, a sheet conveying path 111 extending vertically is formed at a position on the right side of the plurality of image forming units 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 secondary transfer unit 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 plurality of image forming units 13 form toner images of different colors transferred to the sheet P. In the present embodiment, as the plurality of image forming units 13, magenta (M) arranged sequentially 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) a magenta unit 13M using a color developer, a cyan unit 13C using a cyan (C) developer, a yellow unit 13Y using a yellow (Y) developer, and black (Bk) color development. A black unit 13Bk using an agent is provided. Each of the units 13M, 13C, 13Y, and 13Bk includes a photosensitive drum 20, and a charging device 21, a developing device 23, and a cleaning device 25 disposed around the photosensitive drum 20. Further, an exposure unit 22 internally provided with an exposure device for each of the units 13M, 13C, 13Y, and 13Bk is disposed below the image forming unit. The exposure device of the exposure unit 22 emits exposure light of each color to the photosensitive drum 20.

  The photoreceptor drum 20 is driven to rotate around an axis extending in the front-rear direction, forms 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. An image forming area IA described later (see FIG. 7A) is set in advance on the surface of the photosensitive drum 20.

  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 of each color in the exposure unit 22 has various optical system devices such as a light source, a polygon mirror, a reflection mirror, and a deflection mirror. The exposure device irradiates the image forming area IA of the photosensitive drum 20 uniformly charged to the charging potential with light modulated based on image data (predetermined image information) to expose the image forming area IA, thereby forming an electrostatic latent image. Form an image. At this time, the exposure device performs the image forming area IA along a main scanning direction MS (FIG. 7A) parallel to the axis of the photosensitive drum 20 and a sub-scanning direction SS (FIG. 7A) orthogonal to the main scanning direction MS. Is exposed.

  The developing device 23 is opposed to the photosensitive drum 20 at a predetermined developing nip portion NP (FIG. 3A) located downstream of the optical path of the exposure light emitted by the exposure device in the rotation direction of the photosensitive drum 20. You. The developing device 23 includes a developing roller 231 that rotates 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 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 plurality of image forming units 13 and the toner supply unit 15 in the vertical direction, and is rotatably supported by the intermediate transfer belt 141 and a unit frame (not shown). And a primary roller 24, a driving roller 142, a driven roller 143, a backup roller 146, and a density sensor 100.

  The primary transfer roller 24 forms a primary transfer nip with the photosensitive drum 20 with the intermediate transfer belt 141 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 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. The intermediate transfer belt 141 allows the toner images carried on the photosensitive drums 20 of the plurality of image forming units 13 to be primarily transferred in a superimposed manner, and carries the superimposed toner images.

  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 density sensor 100 (density detecting unit) is disposed on the downstream side of the units 13M, 13C, 13Y, and 13Bk so as to face the intermediate transfer belt 141, and the toner of each color formed on the intermediate transfer belt 141 Detect the density of the image. In the present embodiment, the density sensor 100 includes a first density sensor S1 and a second density sensor S2 described later (see FIG. 7A). In another embodiment, the density sensor 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.

  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 secondary transfer nip with the drive 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 secondary transfer roller 145 transfers the toner image carried on the intermediate transfer belt 141 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 secondary transfer nip is fixed to the sheet P.

  The paper discharge unit 17 is formed by recessing the top of the apparatus main body 11, and a paper discharge tray 171 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 developing device and cleaning device>
FIG. 2A 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. 2B is a plan view showing the internal structure of the developing device 23 and the cleaning device 25 according to the present embodiment. The developing device 23 includes a developing roller 231, a first screw feeder 232 (first conveying member), a second screw feeder 233 (second conveying member), a developing roller 231, a first screw feeder 232, and a second screw feeder. A developing housing 230 (housing) that rotatably supports the developing member 233 and a regulating blade 234 are provided. 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 section 230H accommodates a two-component developer composed of a toner and a carrier. Further, the developer accommodating portion 230H is provided in a first transport direction (first direction, a direction orthogonal to the paper surface of FIG. 2A, that is, from the rear to the front) in which the developer moves from one axial end to the other axial end of the developing roller 231. Direction), and a first transfer path 230A at both ends in the axial direction, which communicates with the first transfer path 230A, and a second transfer direction (second direction) opposite to the first transfer direction. A second transport path 230B that allows the developer to be transported. The first transport direction is parallel to the main scanning direction MS in which the exposure device operates the image forming area IA of the photosensitive drum 20. The partition 230C defines the first and second transport paths 230A and 230B so that both ends of the first and second transport paths 230A and 230B in the first direction communicate with each other.

  The first screw feeder 232 is rotatably disposed on the first transport path 230A. The first screw feeder 232 is rotated in the direction of arrow D22 in FIG. 2A, and transports the developer in the first transport direction and supplies the developer to the developing roller 231. The second screw feeder 233 is rotatably disposed on the second conveyance path 230B. The second screw feeder 233 is rotated in the direction of arrow D23 in FIG. 2A, and transports the developer in the second transport direction.

  The developing roller 231 is arranged to face the photosensitive drum 20 at the developing nip NP (FIG. 3A). 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. 2A. 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 the embodiment, the developing roller 231 rotates in the same direction (with direction) at a position facing the photosensitive drum 20.

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

  The cleaning device 25 (FIG. 2B) includes a cleaning housing 250, a cleaning blade 251 that contacts the photosensitive drum 20 and collects toner adhered to the surface of the photosensitive drum 20, and the toner collected by the cleaning blade 251. And a cleaning screw 252 (cleaning transport member) that transports the toner in the first transport direction. The toner transported by the cleaning screw 252 is stored in a collection container (not shown). In FIG. 2B, an arrangement of a first density sensor S1 and a second density sensor S2 of the density sensor 100, which will be described later, in the main scanning direction is virtually shown.

  As described above, the density sensor 100 includes the first density sensor S1 and the second density sensor S2. The first density sensor S1 is arranged so as to face the intermediate transfer belt 141 at a position corresponding to the upstream portion in the first transport direction (main scanning direction) in the image forming area IA. Further, the second density sensor S2 is disposed so as to face the intermediate transfer belt 141 at a position corresponding to a downstream portion of the image forming area IA in the first transport direction (main scanning direction). The first density sensor S1 and the second density sensor S2 output a signal corresponding to the image density of the toner image (measurement toner image) on the intermediate transfer belt 141, which passes through a position facing the sensor.

  The image forming apparatus 10 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, an AC voltage is superimposed on the DC voltage on the developing roller 231 of the developing device 23 of each color. Apply a developing bias. The developing bias applying unit 971 can apply an individual developing bias to the developing roller 231 of the developing device 23 for each color.

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

  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 (charge amount acquisition unit, image pattern selection unit) by the CPU executing the control program stored in the ROM. And a determination unit 985.

  The drive control unit 981 controls the drive unit 972 to rotationally drive the developing roller 231, the first screw feeder 232, and the second screw feeder 233. 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. As an example, the storage unit 983 stores the value of the developing bias adjusted according to the rotation speed of the developing roller 981 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 on the inclination of the reference straight line indicating the relationship between the toner image density change amounts 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. Note that the data stored in the storage unit 983 may be in the form of a graph, a table, or the like.

  Further, the storage unit 983 stores in advance a plurality of image patterns including at least information on the arrangement and the image density of the measurement toner image in the image forming area IA. The image pattern will be described later in detail.

  The mode control unit 984 executes a charge amount measurement mode (charge amount acquisition operation) described later.

  The mode control unit 984 performs at least one of the plurality of image forming units 13 during a non-developing operation different from a developing operation in which the toner image to be transferred onto the sheet P on the photosensitive drum 20 is formed. In the image forming unit 13 (image forming unit), the charging device 21, the exposing device 22, the developing bias applying unit 971, and the like are controlled to form a plurality of measurement toner images having different toner development amounts on the photosensitive drum 20. Forming and developing when the plurality of measurement toner images are formed 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. Based on the DC component of the developing current flowing between the roller 231 and the developing bias applying unit 971, the developing current is included in the measurement toner image formed on the photosensitive drum 20. Executing a charge amount measurement mode for acquiring a charge amount of that toner.

  More specifically, 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 on the photosensitive drum 20. Then, the inclination of the measurement straight line indicating the relationship between the density change amount of the measurement toner image and the frequency change amount is obtained from the frequency change amount and the density detection result of the measurement toner image by the density sensor 100. At the same time, the charge amount of the toner included in the measurement toner image formed on the photosensitive drum 20 is acquired from the acquired inclination of the measurement straight line and the reference information in the storage unit 983.

  The mode control unit 984 is configured to perform the measurement toner image formation in the image forming area IA in the charge amount measurement mode from the plurality of image patterns stored in the storage unit 983 based on predetermined selection information. Is selected.

  The determination unit 985 performs various determination operations in the charge amount measurement mode.

<Developing operation>
FIG. 3A is a schematic diagram of the developing operation of the image forming apparatus 10 according to the present embodiment, and FIG. 3B is a schematic diagram illustrating the magnitude relationship between the potentials of the photosensitive drum 20 and the developing roller 231. Referring to FIG. 3A, a developing nip NP is formed between developing roller 231 and 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. Referring to FIG. 3B, the surface potential of photoconductor drum 20 is charged by charging device 21 to 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. 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. 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 is 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 in accordance with 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. 4 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. 5 is a graph showing the relationship between the inclination of the graph of FIG. 4 and the toner charge amount 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. 4). That is, as shown in FIG. 4, 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. 4 increases. Referring to FIG. 5, the relationship between the inclinations of the three graphs in FIG. 4 and the toner charge amounts is distributed on a straight line (approximate straight line). Therefore, if the information shown in FIG. 5 is stored in the storage unit 983 in advance, and the inclination of the straight line shown in FIG. 4 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 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 is reduced, 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 the 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”, “toner charge amount”. Rise ". In this case, if the toner density is increased in order to reduce the toner charge amount with respect to the image density decrease due to a factor other than the increase in the toner charge amount, problems such as toner scattering will newly occur. there is a possibility. It is desirable to reduce the toner charge amount by increasing the toner density to reduce the image density due to the increase in the toner charge amount, and to reduce the development electric field ( It is preferable to increase the developing bias. Further, by grasping the toner charge amount, 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 toner charge amount 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 small. 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 is reduced as described above, the number of reciprocating movements of the toner in the developing nip portion NP is reduced. Since it is difficult to separate from the carrier, 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. 6 is a flowchart of the charge amount measurement mode executed in the image forming apparatus 10 according to the present embodiment. 7A to 7I are schematic diagrams of image patterns A to I formed on the intermediate transfer belt 141 in the charge amount measurement mode executed in the image forming apparatus 10 according to the present embodiment. FIG. 8 is a schematic diagram illustrating a state in which the density of the measurement toner image formed in the charge amount measurement mode executed in the image forming apparatus 10 changes according to the frequency of the developing bias.

  Referring to FIG. 6, when the charge amount measurement mode is started, mode control unit 984 stores an image pattern including a measurement toner image formed in image forming area IA on photoconductor drum 20 in storage unit 983. The selected image pattern is selected from the image patterns A to I (step S01). The image patterns A to I will be described in more detail later.

  Next, 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 phenomenon 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 history of the immediately preceding image formation. Normally, a bias used for printing (image formation) is applied to the reference developing bias condition. When 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 frequency variable n 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. 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, when n = N (YES in step S08), mode control section 984 calculates the slope of the approximate straight line shown in FIG. 4 based on the information stored in storage section 983 (step S10). . Then, based on the graph (reference information) shown in FIG. 5 stored in the storage unit 983, the mode control unit 984 estimates the charge amount of the toner from the above inclination (step S11), and sets the charge amount measurement mode. To end.

  FIG. 8 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 toner charge amount, and is “positive (+)” when the toner charge amount is low, and “negative (−)” when the toner charge amount 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. In addition, the reference information shown in FIG.
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 toner charge amount per unit mass. In step S10, the toner charge amount Q / M is calculated by substituting the slope a of the approximate straight line calculated from equation 1 into equation 2. The charge amount measurement mode shown in FIG. 6 may be executed for each developing device 23 of each color shown in 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 the result of the previous toner charge amount measurement mode. In this case, the accuracy of the measured toner charge amount 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 automatic charge amount measurement mode is performed before and after execution of calibration (also referred to as setup or image quality adjustment operation) of the image forming apparatus 10 or between sheets during image formation, as described later.

  Further, it is desirable that the above-described charge amount measurement modes are respectively executed when the image forming apparatus 10 is shipped from a factory after manufacturing and when the image forming apparatus 10 is set up at a 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, the amount of charge of the developer tends to decrease when the idle period is long, and the level of the tendency often differs depending on the period in which the developer is left and the environment. Therefore, by measuring the toner charge amount at the time of shipment from the factory and at the time of set-up of the main body, a deterioration state due to leaving the developer is predicted.If the leaving time is very long or if the developer is left in a poor environment, A large difference between the two toner charge amounts (toner charge amount 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 encouraged.

  On the other hand, even if the toner charge amount is low at the time of factory shipment and when the main body is set up, if the difference between the two toner charge amounts is small, the possibility that the developer has deteriorated is low. Therefore, there is no need to replace the developer at the place of use, and the image quality can be improved by adjusting the toner concentration and the developing conditions (such as the developing bias). As described above, the toner charge amount measurement mode according to the present embodiment is executed after the image forming apparatus 10 is left unused for a predetermined period of time, so that a change in the state of the developer can be grasped. Becomes

<Selection of image pattern>
The charge amount measurement mode described above is executed during a non-developing operation (calibration or sheet interval) different from the developing operation in which the toner image transferred to the sheet P is formed on the photosensitive drum 20. In order to accurately measure the charge amount of the toner, it is desirable that the mode control unit 984 forms a large number of measurement toner images on the photosensitive drum 20. However, in this case, the time during which a normal image forming operation (developing operation) cannot be performed increases, and the amount of toner consumed during measurement increases. Therefore, in the present embodiment, in step S01 in FIG. 6, the mode control unit 984 selects an image pattern including a measurement toner image for accurately and efficiently measuring the charge amount of the toner.

  In particular, in the present embodiment, the mode control unit 984 includes, as the selection information, information on the number of prints on the sheet P on which the toner image has been transferred in the past, information on the print ratio of the toner image formed on the sheet P in the past, At least one of driving time information of the developing roller 231, print mode information including at least information on the interval between sheets P on which toner images are formed during continuous printing, and temperature and humidity around or inside the image forming apparatus 10. The image pattern of the measurement toner image is selected based on at least one piece of environmental information including the following. The printing ratio is a ratio of an image formed on the sheet surface (image-formable area) of the sheet S. Next, each image pattern stored in the storage unit 983 according to the present embodiment will be described with reference to FIGS. 7A to 7I.

  FIG. 7A shows an image pattern A (first image pattern) according to the present embodiment. The color and image density (%) of the image forming unit 13 are shown in the measurement toner image of the image pattern in each drawing. Note that the image density is a characteristic value indicating a low-density area and a high-density area, and is indicated from 0 (%) to 100 (%). An image density of 100 (%) is also called a solid image. That is, in each pattern diagram, C100 means a 100% (solid) image of cyan. In the image pattern A, a measurement toner image extending along the main scanning direction MS is formed on the photosensitive drum 20 of each color, and is primarily transferred onto the intermediate transfer belt 141 in order. On each of the photoconductor drums 20 of cyan, magenta, yellow, and black, the measurement toner image is developed during times t1, t2, t3, and t4, respectively. Further, it takes time T to detect the image densities of all the measurement toner images formed on the intermediate transfer belt 141. For example, by forming the image pattern A in order according to a plurality of frequencies of the developing bias, the charge amount of the toner can be measured in the charge amount measurement mode described above. Although FIG. 7A shows the image density of the measurement toner image of each color as 100%, that is, a solid image, the measurement toner image may be formed with a lower image density. As an example, the size of each measurement toner image of the image pattern A is 260 mm in the main scanning direction MS and 20 mm in the sub-scanning direction SS.

  Image pattern A as shown in FIG. 7A corresponds to a basic image pattern selected in the charge amount measurement mode. In particular, since the densities of the measurement toner images of each color are measured at the first density sensor S1 and the second density sensor S2, the charge amount of the toner can be measured from the average value of the detected densities. For this reason, it is possible to cancel the difference in the charge amount of the toner in the front-rear direction of the developing roller 231 (developing device 23). As described later, before the charge amount measurement mode is executed, if the printing rate of an image printed in a normal image forming operation is higher than a predetermined first threshold (for example, 15%), or When the printing rate of the image printed in the image forming operation of the above is further lower than a second threshold (for example, 7%) set lower than the first threshold, the toner in the developing housing 230 of the developing device 23 is Since a density gradient is likely to occur and a difference may occur between the detection densities of the first density sensor S1 and the second density sensor S2, it is desirable to select image patterns D and E described later.

  In other words, in other words, the image pattern A is in the main scanning direction MS so as to include a region facing the first density sensor S1 and the second density sensor S2 in the image forming area IA in the main scanning direction MS. And a measurement toner image extending continuously along the image.

  FIG. 7B shows an image pattern B (second image pattern) according to the present embodiment. In the image pattern B, a measurement toner image extending along the main scanning direction MS is formed on the photosensitive drum 20 of each color, and is primarily transferred onto the intermediate transfer belt 141 in order. In the image pattern B, a measurement toner image composed of a solid yellow image is detected by the first density sensor S1, and a measurement toner image composed of a solid magenta image is detected by the second density sensor S2. Thereafter, the measurement toner image composed of a black solid image is detected by the first density sensor S1, and the measurement toner image composed of the cyan solid image is detected by the second density sensor S2. As an example, the size of each measurement toner image of the image pattern B is 120 mm in the main scanning direction MS and 20 mm in the sub-scanning direction SS.

  The measurement toner images are developed on the yellow and magenta photosensitive drums 20 during the time t1. On the other hand, in each of the black and cyan photoconductor drums 20, the measurement toner image is developed for the time t2. Further, it takes time T to detect the image densities of all the measurement toner images formed on the intermediate transfer belt 141. The time T in the image pattern B is shorter than the time T in the previous image pattern A. For example, by forming the image pattern B in order according to a plurality of frequencies of the developing bias, the charge amount of the toner can be measured in the charge amount measurement mode described above. Although FIG. 7B also shows 100%, that is, a solid image, as the image density of the measurement toner image of each color, the measurement toner image may be formed with a lower image density.

  It is desirable that the image pattern B as shown in FIG. 7B be selected when the charge amount measurement mode is executed at a limited timing (time) such as between sheets. In particular, since it is possible to detect the image densities of the measurement toner images of two colors at the time t1 or t2, the time required for detecting the density of the measurement toner image is shorter than that of the image pattern A described above. Halve. Therefore, when an image is continuously formed on a plurality of sheets P, it is desirable that the image pattern B is selected and formed between sheets (during non-development operation).

  In other words, in other words, the image pattern B is a measurement toner image formed in one of the plurality of image forming units 13 and is the first toner image in the image forming area IA. A first measurement toner image (for example, Y100 in the upper left of FIG. 7B) arranged in a region facing the density sensor S1 and another image different from the one image forming unit 13 of the plurality of image forming units 13 A measurement toner image formed in the forming unit 13 and located in an area of the image forming area IA facing the second density sensor S2 and on an extension of the first measurement toner image in the main scanning direction. 2 measurement toner image (for example, M100 at the upper right of FIG. 7B).

  FIG. 7C shows an image pattern C (third image pattern) according to the present embodiment. In the image pattern C, a measurement toner image extending along the main scanning direction MS is formed on the photosensitive drum 20 of each color, and is primarily transferred onto the intermediate transfer belt 141 in order. In the image pattern C, a measurement toner image composed of a 50% half image of yellow (halftone image with an image density of 50%) is detected by the first density sensor S1, and a measurement toner composed of a 50% half image of cyan is detected. An image is detected by the second density sensor S2. Thereafter, the first density sensor S1 detects a measurement toner image including a black 50% half image, and the second density sensor S2 detects a measurement toner image including a magenta 50% half image.

  The measurement toner images are developed on the yellow and cyan photosensitive drums 20 during the time t1. On the other hand, the measurement toner image is developed on each of the black and magenta photosensitive drums 20 during the time t2. Further, it takes time T to detect the image densities of all the measurement toner images formed on the intermediate transfer belt 141. The time T in the image pattern C is shorter than the time T in the previous image pattern A, and is equal to the time in the image pattern B. For example, by forming the image pattern C in order according to a plurality of frequencies of the developing bias, the charge amount of the toner can be measured in the charge amount measurement mode described above. In FIG. 7C, the image density of the measurement toner image of each color is shown as a 50% half image, but the measurement toner image may be formed of a higher or lower half image.

  The image pattern C as shown in FIG. 7C is selected when the charge amount measurement mode is executed at a limited timing (time) such as a sheet interval, similarly to the image pattern B. In particular, since it is possible to detect the image densities of the measurement toner images of two colors at the time t1 or t2, the time required for detecting the density of the measurement toner image is shorter than that of the image pattern A described above. Halve. Further, the image pattern C is effective when the charge amount of the toner in the developing device 23 increases and the density of the measurement toner image tends to decrease. As an example, when the surrounding environment is at low temperature and low humidity, such a phenomenon is likely to occur.

  In other words, in other words, the image pattern C is the measurement toner image formed in one image forming unit 13 of the plurality of image forming units 13, and is included in the image forming area IA. A third measurement toner image (for example, Y50 in FIG. 7C) arranged in an area facing the first density sensor S1 and another image different from the one image forming unit 13 of the plurality of image forming units 13 A measurement toner image formed in the forming unit 13 and located in a region facing the second density sensor S2 in the image forming region IA and overlapping with the third measurement toner image in the main scanning direction. 4 measurement toner images (for example, C50 in FIG. 7C). For this reason, even when the detection of the image density of the half image or the detection of the developing current in the modified embodiment described later is likely to cause an error, the area of the toner image for measurement is largely secured along the main scanning direction MS. Therefore, the charge amount of the toner can be stably detected.

  As shown in FIG. 7C, the third measurement toner image (Y50) and the fourth measurement toner image (C50) of the image pattern C are positioned at the center CL in the main scanning direction MS of the image forming area IA. They are arranged so as to straddle each other. As a result, since the area of the measurement toner image is particularly large along the main scanning direction MS, the charge amount of the toner can be stably detected. At this time, since the single-color images of the respective colors are arranged in the area facing the first density sensor S1 and the second density sensor S2, no error occurs in the detected density by reading the secondary color image. Note that the image pattern C consumes a larger amount of toner than the image pattern B. Therefore, when the charge amount measurement mode using the image pattern B is executed a predetermined number of times, the image pattern C is selected once. The image patterns B and C may be selected in combination.

  FIG. 7D shows an image pattern D (fourth image pattern) according to the present embodiment. In the image pattern D, a measurement toner image extending along the main scanning direction MS is formed on the photosensitive drum 20 of each color, and is primarily transferred onto the intermediate transfer belt 141 in order. In the image pattern D, the first density sensor S1 detects a measurement toner image formed of a black solid image, and the second density sensor S2 detects a measurement toner image formed of a magenta solid image. Thereafter, the measurement toner image composed of the magenta solid image is detected by the first density sensor S1, and the measurement toner image composed of the black solid image is detected by the second density sensor S2.

  Note that the measurement toner image is developed on the first black and magenta photosensitive drums 20 during the time t1. On the other hand, the measurement toner image is developed on each of the second magenta and black photoconductor drums 20 during the time t2. Further, it takes time T to detect the image densities of all the measurement toner images formed on the intermediate transfer belt 141. The time T in the image pattern D is shorter than the time T in the previous image pattern A, and is equal to the time T of the image patterns B and C. For example, by forming the image pattern D in order according to a plurality of frequencies of the developing bias, the charge amount of the toner can be measured in the charge amount measurement mode described above. Although FIG. 7D also shows the image density of the measurement toner image of each color as 100%, that is, a solid image, the measurement toner image may be formed with a lower image density.

  The image pattern D as shown in FIG. 7D can also be selected when the charge amount measurement mode is executed at a limited timing (time) such as between sheets. In particular, since it is possible to detect the image densities of the measurement toner images of two colors at the time t1 or t2, the time required for detecting the density of the measurement toner image is shorter than that of the image pattern A described above. Halve. In this case, the measurement toner images of four colors are formed using two continuous paper intervals.

  Further, in the image pattern D, the first density sensor S1 detects the density of the measurement toner image that is made visible by the toner on the upstream side in the first transport direction of the developing roller 231 (FIG. 2B) for each color, and the developing roller 231 ( The density of the measurement toner image that is revealed by the toner on the downstream side in the first transport direction in FIG. 2B) is detected by the second density sensor S2. For this reason, it is possible to individually measure the amount of charge of the toner on the upstream side and the downstream side of the developing roller 231 of the same color from the detection result of each density sensor. Here, when the charge amounts of the toner on the upstream side and the downstream side of the developing roller 231 are largely different, the charging performance of the toner in the developing device 23 may be deteriorated. Therefore, it is desirable that the mode control unit 984 prompts the exchange of the developing device 23 and the exchange of the developer through a display unit (not shown) of the image forming apparatus 10. Further, a later-described developer recovery mode may be prompted. Such information may be transmitted to a service center via a telephone line or an Internet line.

  The above-described deterioration in the charging performance of the toner is likely to occur when images with a high printing ratio are continuously printed. For this reason, the mode control unit 984 sets the image pattern D as the print rate information for selecting an image pattern when the average value of the print rates of the toner images at a predetermined number of prints exceeds a preset threshold. It is desirable to select On the other hand, if an image having an extremely high printing rate is not continuously formed and a printing operation in which the toner density in the developing device 23 is likely to change stably is performed, the above-described image pattern B causes a It is desirable that the color measurement toner image is formed between one sheet of paper.

  Note that, in FIG. 7D, even when the measurement toner images of the same color are formed at positions adjacent to each other in the main scanning direction MS, the effects described above are exerted. However, when the charge amount of the toner is measured by using the developing current as in a modified embodiment described later, a plurality of toner particles that are made visible by the upstream and downstream toners in the first transport direction of the developing roller 231 of the same color are used. It is preferable that the measurement toner images are arranged at positions shifted from each other in the sub-scanning direction SS as shown in FIG. 7D. This is for individually measuring a developing current when each measurement toner image on the upstream side and the downstream side in the first transport direction is developed.

  In other words, in other words, the image pattern D is a measurement toner image formed in one of the plurality of image forming units 13 and is the first toner image in the image forming area IA. A fifth measurement toner image (BK100 at the upper left of FIG. 7D) arranged in an area facing the density sensor S1 and another image formation of the plurality of image formation units 13 that is different from the one image formation unit 13 A sixth portion of the image forming area IA facing the second density sensor S <b> 2 and an extension of the fifth measuring toner image in the main scanning direction MS, which is the measurement toner image formed in the section 13. A measurement toner image (M100 at the upper right of FIG. 7D) and a measurement toner image formed in the other image forming unit 13 and opposed to the first density sensor S1 in the image forming area IA. A seventh measurement toner image (M100 at the lower left of FIG. 7D) arranged at a position spaced in the sub-scanning direction SS with respect to the fifth measurement toner image in the area, An eighth measurement toner image which is a measurement toner image to be formed and which is located in an area of the image forming area IA opposed to the second density sensor S2 and on an extension of the seventh measurement toner image in the main scanning direction MS. (BK100 at the lower right of FIG. 7D).

  FIG. 7E illustrates an image pattern E (a fifth image pattern) according to the present embodiment. In the image pattern E, a measurement toner image extending along the main scanning direction MS is formed on the photosensitive drum 20 of each color, and is primarily transferred onto the intermediate transfer belt 141 in order. For example, in the image pattern E, a measurement toner image composed of a cyan solid image is detected by the first density sensor S1, and a measurement toner image composed of a black solid image is detected by the second density sensor S2. Thereafter, the measurement toner image formed of the 50% half image of black color is detected by the first density sensor S1, and the measurement toner image formed of the 50% half image of cyan color is detected by the second density sensor S2. Note that similar image patterns are formed for the yellow color and the magenta color. The combination of colors used for the image pattern is not limited to these.

  The cyan and black photosensitive drums 20 corresponding to the first-stage measurement toner image are each developed with the measurement toner image during the time t1. On the other hand, on the black and cyan photosensitive drums 20 corresponding to the second-stage measurement toner image, the measurement toner image is developed during the time t2. Further, it takes time T to detect the image densities of all the measurement toner images formed on the intermediate transfer belt 141. The time T in the image pattern E is shorter than the time T in the previous image pattern A, and is equal to the time T of the image patterns B, C, and D. For example, by forming the image pattern E in order according to a plurality of frequencies of the developing bias, the charge amount of the toner can be measured in the charge amount measurement mode described above. Although the solid image and the 50% half image are shown as the image density of the measurement toner image of each color in FIG. 7E, the measurement toner image may be formed with an image density different from this.

  The image pattern E as shown in FIG. 7E can also be selected when the charge amount measurement mode is executed at a limited timing (time) such as between sheets. In particular, since the image densities of the two color measurement toner images can be detected at the time t1 or t2, the time required for the density detection of the measurement toner image is shorter than that of the image pattern A described above. Halve. Further, in the image pattern E, between t1 and t2, the density of the solid image and the 50% half image of each color can be detected by the first density sensor S1 or the second density sensor S2. For this reason, when the developing performance of the toner in the developing device 23 decreases and the balance between the developing performance of the solid image and the half image is easily lost, the charge amount of the toner can be stably measured by the image pattern E. .

  In addition, the above-mentioned reduction in the balance between the development performance of the solid image and the half image is caused by the fact that such an image is printed in the intermittent printing mode when an image having a low printing rate is continuously printed. Is likely to occur when Here, the intermittent printing mode means that, when an image is formed on a sheet P (thick paper or the like) thicker than a sheet P having a normal thickness, the interval between sheets is large in order to secure a fixing time in the fixing unit 16. This is the mode to be set. In the developing device 23, the developing roller 231 and the like also rotate at the timing corresponding to the sheet interval. Therefore, when the sheet interval increases, the idle time of each member of the developing device 23 increases, and the deterioration of the toner is promoted. In this case, the balance between the development performance of the solid image and the half image tends to be lost.

  In such a case, the mode control unit 984 sets in advance the average value of the toner image print rates at a predetermined number of prints as the print rate information as the print rate information for selecting an image pattern. It is desirable to select the image pattern E when the value falls below the threshold value. In this case, when a solid image hardly causes a density difference according to the frequency of the developing bias, and a half image has a too low density to be easily detected, the toner charge amount can be stably measured.

  More specifically, the mode control unit 984 includes, as the printing rate information for selecting an image pattern, an average value of the printing rates of the toner images at a predetermined number of prints that is lower than a predetermined threshold value, and It is preferable to select the image pattern E when the interval between the sheets on which the toner images are formed exceeds a preset threshold value as the print mode information for selecting. Further, in order to form the image pattern E between the sheets during normal image formation, it is necessary to form the measurement toner image over a plurality of sheets. Therefore, when the print job is completed, the image pattern E is selected. It is desirable that the charge amount measurement mode is performed.

  In other words, in other words, the image pattern E is a measurement toner image formed in one of the plurality of image forming units 13 and is the first toner image in the image forming area IA. The ninth measurement toner image (C100 at the upper left of FIG. 7E) arranged in the area facing the density sensor S1 and another image formation of the plurality of image formation units 13 that is different from the one image formation unit 13 A tenth image, which is a measurement toner image formed in the section 13 and is located in an area of the image forming area IA facing the second density sensor S2 and on an extension of the ninth measurement toner image in the main scanning direction MS. The toner image for measurement (BK100 at the upper right of FIG. 7E) and the toner image for measurement formed in the other image forming unit 13 and opposed to the first density sensor S1 in the image forming area IA. An eleventh measurement toner image having a different image density from the tenth measurement toner image at a position spaced apart from the ninth measurement toner image in the sub-scanning direction SS in the area (lower left of FIG. 7E). BK50), an area of the image forming area IA facing the second density sensor S2, which is the measurement toner image formed in the one image forming section 13, and the main scanning direction of the eleventh measurement toner image. A twelfth measurement toner image (C50 at the lower right of FIG. 7E) having an image density different from that of the ninth measurement toner image, which is arranged on an extension of the MS.

  FIG. 7F shows an image pattern F (fifth image pattern) according to the present embodiment. In the image pattern F, a measurement toner image extending along the main scanning direction MS is formed on the photosensitive drum 20 of each color, and is primarily transferred onto the intermediate transfer belt 141 in order. For example, in the image pattern F, a measurement toner image composed of a black solid image is detected by the first density sensor S1, and a measurement toner image composed of a cyan 50% half image is detected by the second density sensor S2. You. Thereafter, the measurement toner image composed of the cyan solid image is detected by the first density sensor S1, and the measurement toner image composed of the black 50% half image is detected by the second density sensor S2.

  In the black and cyan photosensitive drums 20 corresponding to the first-stage measurement toner image, the measurement toner image is developed during the time t1. On the other hand, in the cyan and black photosensitive drums 20 corresponding to the second-stage measurement toner image, the measurement toner image is developed during the time t2. Further, it takes time T to detect the image densities of all the measurement toner images formed on the intermediate transfer belt 141. The time T in the image pattern E is shorter than the time T in the previous image pattern A, and is equal to the time T of the image patterns B, C, D, and E. For example, by forming the image pattern F in order according to a plurality of frequencies of the developing bias, the charge amount of the toner can be measured in the charge amount measurement mode described above. Although the solid image and the 50% half image are shown in FIG. 7F as the image density of the measurement toner image of each color, the measurement toner image may be formed with an image density different from this.

  The image pattern F as shown in FIG. 7F can also be selected when the charge amount measurement mode is executed at a limited timing (time) such as between sheets. In particular, since it is possible to detect the image densities of the measurement toner images of two colors at the time t1 or t2, the time required for detecting the density of the measurement toner image is shorter than that of the image pattern A described above. Halve. Further, in the image pattern F, between t1 and t2, the density of the solid image and the 50% half image of each color can be detected by the first density sensor S1 or the second density sensor S2. For this reason, when the developing performance of the toner in the developing device 23 is reduced and the balance between the developing performance of the solid image and the half image is easily lost, the charge amount of the toner can be stably measured by the image pattern F. In FIG. 7F, since the positions of the 100% image and the 50% half image of the same color in the sub-scanning direction are shifted, the developing current may be used for measuring the toner charge amount as in a modified embodiment described later. it can.

  Further, in the image pattern F, a measurement toner image having a high image density is formed on the upstream side (the left side in FIG. 7F) of the image forming area IA of the photosensitive drum 20 in the first transport direction. For this reason, when the toner of the toner image for measurement is collected by the cleaning blade 251 of the cleaning device 25 and transported downstream by the cleaning screw 252 in the first transport direction, the cleaning is performed over the entire first transport direction. The toner can be interposed at the tip of the blade 251 as a lubricant. For this reason, it is possible to prevent the cleaning blade 251 from being turned up and the toner from slipping through. As described above, by forming the measurement toner image having a high image density on the upstream side in the transport direction of the cleaning screw 252, it is possible to measure the charge amount of the toner and prevent problems in the cleaning device 25. The image patterns E and F may be alternately combined in a plurality of charge amount measurement modes.

  In other words, in other words, the image pattern F is a measurement toner image formed in one of the plurality of image forming units 13 and is the first toner image in the image forming area IA. The ninth measurement toner image (BK100 in the upper left of FIG. 7F) arranged in the area facing the density sensor S1 and another image forming of the plurality of image forming units 13 different from the one image forming unit 13 A tenth image, which is a measurement toner image formed in the section 13 and is located in an area of the image forming area IA facing the second density sensor S2 and on an extension of the ninth measurement toner image in the main scanning direction MS. The measurement toner image (C50 at the upper right of FIG. 7F) and the measurement toner image formed in the other image forming unit 13 and opposed to the first density sensor S1 in the image forming area IA. An eleventh measurement toner image having a different image density from the tenth measurement toner image at a position spaced from the ninth measurement toner image in the sub-scanning direction SS with respect to the ninth measurement toner image (lower left in FIG. 7F) C100), a measurement toner image formed in the one image forming section 13, an area of the image forming area IA facing the second density sensor S <b> 2, and a main scanning direction of the eleventh measurement toner image. And a twelfth measurement toner image (BK50 at the lower right of FIG. 7F) having an image density different from that of the ninth measurement toner image. The image density of the ninth measurement toner image and the eleventh measurement toner image in FIG. 7F is set to be higher than the image densities of the tenth measurement toner image and the twelfth measurement toner image.

  FIG. 7G shows an image pattern G (fifth image pattern) according to the present embodiment. The image pattern G differs from the previous image pattern E (FIG. 7E) in the size of the measurement toner image. Therefore, the description will be focused on the difference. As shown in FIG. 7G, in the image pattern G, the 50% half image of the black color (the eleventh measurement toner image) and the 50% half image of the cyan color (the twelfth measurement toner image) in the second stage are: They are arranged at positions overlapping in the main scanning direction MS. More specifically, each of these measurement toner images is arranged so as to straddle the center CL in the main scanning direction MS of the image forming area IA.

  According to such an image pattern G, when the developing performance of the half image is low, the toner charge amount can be stably measured by setting the area of the measurement toner image of the half image large. . In particular, when the surrounding environment is a severe environment such as high temperature and high humidity or low temperature and low humidity, the developing performance of the developing device 23 is likely to change greatly. Therefore, it is desirable to select the image pattern G. Note that the length of the second-stage measurement toner image of the image pattern G in the sub-scanning direction SS may be extended as necessary. In this case, it is desirable that the charge amount measurement mode in which the image pattern G is selected is executed at the end of the print job. This is because the time required for the printing operation is not affected.

  When the charge amount of the toner is measured using the developing current as in a modified embodiment described later, if the width of the measurement toner image in the main scanning direction MS is set to be large as shown in FIG. Since the developing current flowing during the developing operation at t2 increases, the current detection value is stabilized. For this reason, it is more desirable that the image pattern G of FIG. 7G be selected. Also in this case, a single-color measurement toner image is arranged at a position where the first density sensor S1 and the second density sensor S2 face each other.

  FIG. 7H illustrates an image pattern H (first image pattern) according to the present embodiment. It should be noted that the image pattern H differs from the previous image pattern A (FIG. 7A) in the shape of the measurement toner image, and therefore the description will be focused on the difference. As shown in FIG. 7H, in the image pattern H, the width of the measurement toner image in the sub-scanning direction SS in a region of the measurement toner image of each color that is not opposed to the first density sensor S1 and the second density sensor S2. It is set small. By forming such an image pattern H on the photosensitive drum 20, the density of each toner image can be stably measured by the first density sensor S1 and the second density sensor S2, and the central portion CL The toner can be supplied to the cleaning blade 251 of the cleaning device 25 in a wide area sandwiching. For this reason, as described above, it is possible to prevent the cleaning blade 251 from being turned up and the toner from slipping through. Further, by setting the width of the toner image in a region sandwiching the central portion CL to be small, unnecessary consumption of toner can be suppressed.

  In other words, in other words, in the image pattern H (FIG. 7H), the width of the area of the image pattern H (FIG. 7H) facing the first density sensor S1 and the second density sensor S2 is smaller than the width in the sub-scanning direction SS. The width of the area between the second density sensor S2 and the second density sensor S2 in the sub-scanning direction SS is set small. The image pattern H is desirably selected when the image forming operation for printing an image with a low printing rate is continuous.

  FIG. 7I illustrates an image pattern I (sixth image pattern) according to the present embodiment. The image pattern I is different from the previous image pattern A (FIG. 7A) in the shape of the measurement toner image. Therefore, the description will be made focusing on the difference. As shown in FIG. 7I, in the image pattern I, in the area of the measurement toner image of each color that is not opposed to the first density sensor S1 and the second density sensor S2, the measurement toner images are spaced in the main scanning direction MS. And are formed separately. Even when such an image pattern I is formed on the photosensitive drum 20, the density of each toner image can be stably measured by the first density sensor S1 and the second density sensor S2, and the central portion CL The toner can be supplied to the cleaning blade 251 of the cleaning device 25 in a wide area sandwiching. Note that, even when the measurement toner image is divided and arranged, the toner is transported in the first transport direction by the cleaning screw 252, so that the toner can be continuously interposed at the tip of the cleaning blade 251. . For this reason, as described above, it is possible to prevent the cleaning blade 251 from being turned up and the toner from slipping through. Further, since the measurement toner image is divided and arranged, unnecessary consumption of toner can be suppressed.

  In other words, in other words, the image pattern I is a measurement toner image formed in one of the plurality of image forming units 13 and is the first toner image in the image forming area IA. The thirteenth measurement toner image (the leftmost toner image in FIG. 7I) arranged in the area facing the density sensor S1 and the measurement toner image formed in the one image forming unit 13 and the image forming area IA. And a fourteenth measurement toner image (toner image at the right end in FIG. 7I) which is arranged in a region facing the second density sensor S2 and on an extension of the thirteenth measurement toner image in the main scanning direction MS. And a fifteenth measurement toner image, which is formed in the image forming unit 13 of the first embodiment, and is disposed at an interval between the thirteenth measurement toner image and the fourteenth measurement toner image. No. The fifteenth measurement toner image may be one toner image or a plurality of toner images arranged at intervals. The image pattern I is desirably selected when the image forming operation for printing an image with a low printing rate is continuous.

  As described above, in the present embodiment, in the charge amount measurement mode, based on the densities of the plurality of measurement toner images detected by the density sensor 100, the charge amount is included in the measurement toner images formed on the photosensitive drum 20. The charge amount of the toner to be obtained is obtained. As a result, the charge amount of the toner can be accurately measured. The mode control unit 984 selects an image pattern of the measurement toner image formed in the image forming area IA in the charge amount measurement mode from a plurality of image patterns stored in the storage unit 983 based on predetermined selection information. I do. Therefore, compared with the case where the same image pattern is always used in the charge amount measurement mode, the balance between the accuracy of the charge amount of the toner and the time required for the charge amount measurement mode is adjusted, and the charge amount of the toner is efficiently used. Can be measured.

  In the present embodiment, the mode control unit 984 includes, as the selection information, information on the number of prints on the sheet P to which the toner image has been transferred, information on the printing ratio of the toner image formed on the sheet P, and driving of the developing roller 231. Based on time information, at least one of print mode information including at least information on the interval between sheets P on which toner images are formed, and environmental information including at least one of temperature and humidity, the measurement toner image Select the image pattern of According to such a configuration, a suitable image pattern can be selected based on the selection information that can affect the charge amount measurement mode. It is possible to accurately measure the charge amount of the toner while removing the influence of disturbance as much as possible.

  The mode control unit 984 sets each of the above-described image patterns as the non-developing operation when corresponding to a sheet interval between one sheet P and another sheet P next to the one sheet P. It is desirable to form it on the photosensitive drum 20. As a result, it is possible to stably and efficiently measure the toner charge amount while preventing the image forming operation in the image forming apparatus 10 from being hindered.

  Further, in the toner charge amount measurement mode according to the present embodiment, the developing device 23 does not use a surface potential sensor that measures the potential on the photosensitive drum 20 or an ammeter that measures the developing current flowing into the developing roller 231. The amount of charge of the toner stored in the printer 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.

  Further, the mode control unit 984 performs the measurement formed in the image forming area IA in the next charge amount measurement mode according to the charge amount of the toner acquired in the charge amount measurement mode executed at a predetermined time. The image pattern of the toner image for use may be selected from the above image patterns A to I. In this case, it is possible to select an effective image pattern in the next charge amount measurement mode according to the charge amount of the toner. For example, when the charge amount of the toner is low, an error is likely to occur in the measurement of the toner consumption amount from the image density because the development amount in the developing device 23 is too large. Therefore, it is desirable to select a pattern C having an image density of 50%. On the other hand, when the charge amount of the toner is high, the measurement error of the toner consumption amount from the image density is small, so it is desirable to select the pattern B having the image density of 100%.

<Adjustment of toner charge amount>
FIG. 9 is a flowchart of the charge amount adjustment mode executed in the image forming apparatus 10 according to the present embodiment. The mode control unit 984 (charge amount adjustment unit) can adjust the charge amount of the toner in the developing device 23 based on the charge amount of the toner obtained from the charge amount measurement mode. That is, when the acquired charge amount of the toner is higher than a predetermined threshold value (predetermined range), the mode control unit 984 executes a charge amount reduction operation of reducing the charge amount of the toner. If the acquired charge amount of the toner is lower than the predetermined threshold, a charge amount increasing operation for increasing the charge amount of the toner is executed. The instruction to execute the charge amount adjustment mode may be input by a maintenance worker or a user from an operation unit (not shown) of the image forming apparatus 10 in addition to the comparison result with the threshold value.

  Referring to FIG. 9, when the charge amount adjustment mode is executed (step S21), mode control unit 984 sets in advance the charge amount Q / M (also referred to as q) of the toner acquired from the charge amount measurement mode. It is determined whether or not it is equal to or less than the threshold value a1 (step S22). If Q / M ≦ a1 (YES in step S22), the mode control unit 984 further determines whether the charge amount Q / M is equal to or less than a preset threshold a2 (step S23). Note that a1 and a2 are preset so as to satisfy a2 <a1, and are stored in the storage unit 983.

  If Q / M ≦ a2 in step S23 (YES in step S23), the mode control unit 984 determines that the charge amount of the toner in the developing device 23 is low, and executes the forced stirring operation (charge amount increasing operation). (Step S24). At this time, the mode control unit 984 forcibly agitates the developer in the developing housing 230 by rotating at least the first screw feeder 232 and the second screw feeder 233 in the developing device 23. As a result, the charge amount of the toner in the developing device 23 can be stably increased, and a good image can be formed in the image forming apparatus 10. After step S24, the mode control unit 984 ends the charge amount adjustment mode (step S25). After step S24, the mode control unit 984 may execute steps S22 and S23 again to check whether the toner charge amount has returned to the appropriate range.

  On the other hand, if Q / M> a2 in step S23 (NO in step S23), the mode control unit 984 determines that the charge amount of the toner in the developing device 23 is within the appropriate range, and maintains the current state ( The charge amount adjustment mode is ended (step S25) with step S26).

  If Q / M> a1 in step S22 (NO in step S22), the mode control unit 984 determines that the charge amount of the toner is high, and performs the charge amount reduction operation from the toner supply unit 15 to the developing device 23. A toner supply operation is performed to supply toner (step S27). As a result, by increasing the amount of toner in the developing device 23, the charge amount of each toner can be stably reduced. As a result, a good image can be formed in the image forming apparatus 10. After step S27, the mode control unit 984 ends the charge amount adjustment mode (step S25). Note that even after step S27, the mode control unit 984 may execute steps S22 and S23 again to check whether the toner charge amount has returned to the appropriate range.

  By executing the charge amount adjustment mode as described above, the charge amount of the toner can be accurately adjusted according to the charge amount of the toner acquired in the charge amount measurement mode. If the charge amount of the toner does not fall within the predetermined range even if the charge amount adjustment mode of FIG. 9 is executed a plurality of times in succession, the toner may be significantly deteriorated or the carrier in the developer may be deteriorated. There is a possibility that the external additive is stuck to the carrier or the carrier is missing. In this case, the mode control unit 984 may display a developer recovery operation or a developer replacement operation on a display unit (not shown) of the image forming apparatus 10 to urge the operator. The developer recovery operation is to supply toner from the toner supply unit 15 to the developing device 23 while forcibly discharging the toner in the developing device 23 as a toner image. This is the operation of switching.

  As mentioned above, although one Embodiment of this invention was described, this invention is not limited to this, For example, the following modified embodiment can be taken.

  (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.

  (3) Further, in the above-described embodiment, in the charge amount measurement mode, the mode control unit 984 converts the inclination of the measurement straight line and the reference information in the storage unit 983 into the measurement toner image formed on the photosensitive drum 20. Although the embodiment has been described in which the charge amount of the contained toner is obtained, the present invention is not limited to this. FIG. 10 is a cross-sectional view of the developing device 23 according to a modified embodiment of the present invention and a block diagram showing an electrical configuration of the control unit 980. FIG. 11 is a flowchart of a charge amount measurement mode executed in the image forming apparatus 10 according to the present modified embodiment.

  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 based on the ratio of the difference between the DC components of the developing current flowing through the photoconductor drum 20.

  Referring to FIG. 11, when the mode control unit 984 starts the charge amount measurement mode, the mode control unit 984 selects an image pattern including the toner image for measurement as in the previous embodiment (step S31). The variable n for forming the measurement toner image is set to 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 unit 22 (exposure device) (FIG. 1), the drive control unit 981 and the bias control unit 982 to apply a phenomenon bias for forming the image 1 to the developing roller 231. After the phenomenon roller 231 is rotated by one or more rotations in this state, 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 equation 3.
Slope a = (I1-I2) / (ID1-ID2) (Equation 3)
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. In the charge amount measurement mode using the developing current in this way, for example, as shown in FIG. 7C, an image pattern in which the amount of toner developed during the times t1 and t2 increases due to the measurement toner image extending in the main scanning direction MS. It is desirable to be selected.

  Further, as another modified embodiment, the parameter changed when a plurality of measurement toner images are formed is not the frequency of the AC voltage of the developing bias but the electrostatic latent image formed by the exposure device of the exposure unit 22. The printing ratio of the image may be used.

  That is, in the modified embodiment, the mode control unit 984 controls the exposure device 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 reduce the printing rate per unit area. A plurality of measurement toner images are formed on the photosensitive drum 20 while being changed. 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 3.

  (4) FIG. 12 is a plan view showing the internal structure of a developing device 23M and a cleaning device 25 according to a modified embodiment of the present invention. Like the developing device 23M, two circulation paths of the developer may be formed in the developing housing 230. Also in this case, the first density sensor S1 and the second density sensor S2 are disposed on the upstream side in the transport direction, and the third density sensor S3 is disposed on the downstream side in the transport direction with respect to the two transport directions provided in the first screw feeder 232. Is done. Therefore, if the measurement toner images are arranged at the position where the first density sensor S1 and the second density sensor S2 face each other and the position where the third density sensor S3 faces each other, the charge amount of the toner at each position is obtained. Therefore, the charging performance of the toner in the developing device 23 (the distribution of the charge amount of the toner) can be detected.

  Hereinafter, embodiments of the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples. In addition, each experimental condition in the implemented comparative experiment is 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 a trailing direction at the opposed 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
A developing bias of the developing roller 231: frequency = 4.2 kHz, Duty = 50%, Vpp = 900 V AC voltage rectangular wave, Vdc (DC voltage) = 180 V
・ Toner: positively charged polar toner, volume average particle diameter 6.8 μm, toner concentration 8%
・ Carrier: Volume average particle diameter 35μm, ferrite / resin coated carrier

<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 toner concentration range of 3% to 12%. 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. Further, a resin carrier having a smaller true specific gravity than a ferrite carrier is more preferable. 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, a conductive agent and a charge control agent are mixed and dispersed in a range of 0 to 20 parts with respect to 100 parts of the coat resin, respectively, to thereby perform resistance adjustment and charge adjustment.

<Experiment 1>
Under the above conditions, the charge amount of the toner was adjusted by changing the amount of the external additive of the toner, and the printing operation was performed. The results of this experiment are shown in FIGS. 4 and 5 described above. In FIG. 4, 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 measured. Using the correlation curve with the image density of the fixed image, the toner image density is calculated based on the I.I. It is represented as D.

FIG. 5 shows the relationship between each toner charge amount and the slope of the straight line (approximate straight line) in FIG. Equation 4 (described below) of the approximate straight line shown in FIG. 5 is stored in the storage unit 983 in advance. Using this formula 4, the toner charge amount can be predicted.
Toner charge amount Q / M (μc / g) = − 442.32 × inclination + 29.87 (Equation 4)
Note that the slope of equation 4 = Δimage density / Δfrequency (see the slope of the graph in FIG. 4)

<Experiment 2>
In this experiment, continuous printing was performed on the sheet P using an image having a printing rate of 5%, and the charge amount of the toner was measured every time a predetermined number of durable sheets were printed. In the charge amount measurement mode, a plurality of measurement toner images having different printing rates (image densities) are formed according to the above-described modified embodiment, and the image density and the developing current of the measurement toner image are measured. In this experiment, the printing rates at which the potential differences Vo-Vdc in FIG. 3B are 50 V, 100 V, and 150 V, respectively, are selected. Table 1 shows the image patterns selected in the first and second embodiments.

  In the first embodiment, after the image pattern A is selected in the initial charge amount measurement mode, the image pattern B is selected after the number of durable sheets reaches 100. On the other hand, in the second embodiment, the image pattern A is continuously selected after the initial charge amount measurement mode. In each of the examples, the result was that the charge amount of the toner was measured with high accuracy after each elapse of the number of durable sheets. The total amount of toner consumed in the charge amount measurement mode was 262.8 mg in Example 1 and 514.8 mg in Example 2. As described above, the mode control unit 984 selects the image pattern B and the like as necessary, so that the toner consumption can be suppressed.

10 Image forming apparatus 100 Density sensor (density detection unit)
13 Image forming unit 14 Intermediate transfer unit 141 Intermediate transfer belt 145 Secondary transfer roller (secondary transfer unit)
20 Photoconductor drum (Photoconductor drum)
21 charging device 22 exposure unit (exposure device)
23 Developing device 230 Developing housing (housing)
230A First transport path 230B Second transport path 230C Partitioning part 231 Developing roller 232 First screw feeder (first transport member)
233 Second screw feeder (second transport member)
25 Cleaning Device 251 Cleaning Blade 252 Cleaning Screw (Cleaning Conveying Member)
971 developing bias applying section 972 drive section 980 control section 981 drive control section 982 bias control section 983 storage section 984 mode control section (charge amount acquisition section, image pattern selection section)
985 determination unit S1 first density sensor (first density detection unit)
S2 Second density sensor (second density detection unit)

Claims (23)

A toner image rotated around a predetermined axis, having a predetermined image forming area on the surface, forming an electrostatic latent image in the image forming area, and displaying the electrostatic latent image. A photosensitive drum carrying the
A charging device for charging the photosensitive drum to a predetermined charging potential,
Exposing the image forming area of the photosensitive drum charged to the charging potential along a main scanning direction parallel to the axis and a sub-scanning direction orthogonal to the main scanning direction according to predetermined image information; An exposure device that forms the electrostatic latent image at
A developing device arranged opposite to the photosensitive drum at a predetermined developing nip portion, the developing device being rotated to carry a developer composed of toner and carrier on a peripheral surface and supplying toner to the photosensitive drum. A developing device including a developing roller for forming the toner image;
Each having a plurality of image forming units for forming toner images of different colors,
An intermediate transfer unit including an intermediate transfer belt that allows the toner images carried on the photosensitive drums of the plurality of image forming units to be primarily transferred in a superimposed manner and carries the superimposed toner images; ,
A secondary transfer unit that transfers the toner image from the intermediate transfer belt to a sheet,
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 rollers of the plurality of image forming units,
At least one density detection unit that is arranged to face the photoconductor drum or the intermediate transfer belt and detects the density of the toner image;
In a non-developing operation different from a developing operation in which the toner image is formed on the photosensitive drum, in at least one image forming unit of the plurality of image forming units, the charging device, the exposing device, and the Controlling a developing bias application unit to form a plurality of measurement toner images on the photosensitive drum, and based on the densities of the plurality of measurement toner images detected by the density detection unit, or based on the plurality of measurement toner images. On the photosensitive drum, based on a DC component of a developing current flowing between the developing roller and the developing bias applying unit when the plurality of measuring toner images are formed in addition to the density of the measuring toner image. A charge amount acquisition unit that performs a charge amount acquisition operation of acquiring a charge amount of toner included in the formed measurement toner image,
A storage unit that stores in advance a plurality of image patterns including information on at least the arrangement and the image density of the measurement toner image in the image forming area,
Image pattern selection for selecting the image pattern of the measurement toner image formed in the image forming area at the time of the charge amount acquisition operation from the plurality of image patterns stored in the storage unit based on predetermined selection information. Department and
An image forming apparatus comprising:   The image pattern selection unit, as the selection information, information on the number of prints of the sheet on which the toner image has been transferred, print rate information of the toner image formed on the sheet, drive time information of the developing roller, at least The print mode information including information on the interval between the sheets on which the toner images are formed, based on at least one of environmental information including at least one of temperature and humidity, based on the toner image for measurement. The image forming apparatus according to claim 1, wherein the image forming apparatus selects an image pattern. The developing device includes:
A housing that rotatably supports the developing roller;
A first transport path that is arranged in the housing and that allows the developer to be transported along the developing roller in a first direction parallel to the main scanning direction;
A second transport path that is arranged in the housing so as to be adjacent to the first transport path, and that allows the developer to be transported in a second direction opposite to the first direction;
A partitioning section that defines the first transport path and the second transport path so that both ends of the first transport path and the second transport path in the first direction communicate with each other;
A first transport member rotatably disposed in the first transport path and transporting the developer in the first direction and supplying the developer to the developing roller;
A second transport member rotatably disposed in the second transport path and transporting the developer in the second direction;
Has,
The at least one concentration detector,
A first density detection unit disposed at a position corresponding to an upstream portion of the image forming area in the first scanning direction in the main scanning direction;
A second density detection unit disposed at a position corresponding to a downstream portion of the image forming area in the first direction in the main scanning direction;
Has,
The storage unit includes a measurement toner image continuously extending along the main scanning direction so as to include an area facing the first density detection unit and the second density detection unit in the image forming area. The image forming apparatus according to claim 1, wherein one image pattern is stored.   The storage unit is the measurement toner image formed in one of the plurality of image forming units and is arranged in an area of the image forming area facing the first density detection unit. A first measurement toner image, and the measurement toner image formed in another image forming unit different from the one image forming unit of the plurality of image forming units, and A second image pattern including an area facing the second density detection unit and a second measurement toner image disposed on an extension of the first measurement toner image in the main scanning direction. The image forming apparatus according to claim 3.   The storage unit is the measurement toner image formed in one of the plurality of image forming units and is arranged in an area of the image forming area facing the first density detection unit. A third toner image for measurement, and the toner image for measurement formed in another image forming unit different from the one image forming unit among the plurality of image forming units. A third image pattern including an area opposed to the second density detection unit and a fourth measurement toner image disposed in a position overlapping the third measurement toner image in the main scanning direction; The image forming apparatus according to claim 3.   6. The toner image for third measurement and the toner image for fourth measurement of the third image pattern are each arranged so as to straddle a center of the image forming area in the main scanning direction. 7. Image forming apparatus.   The storage unit is the measurement toner image formed in one of the plurality of image forming units and is arranged in an area of the image forming area facing the first density detection unit. A fifth measurement toner image, and the measurement toner image formed in another image forming unit different from the one image forming unit of the plurality of image forming units. A sixth measurement toner image disposed in an area facing the second density detection unit and on an extension of the fifth measurement toner image in the main scanning direction, and the measurement formed in the another image forming unit; The fifth measurement toner image in the sub-scanning direction in a region of the image forming region facing the first density detection portion, the sixth measurement The same image density as the toner image for A seventh measurement toner image having the following formula: and the measurement toner image formed in the one image forming unit, wherein the measurement image is an area of the image forming area facing the second density detection unit and the seventh measurement toner image. 4. A fourth image pattern including an eighth measurement toner image having the same image density as the fifth measurement toner image, the fourth image pattern being stored on an extension of the toner image in the main scanning direction. 4. 7. The image forming apparatus according to any one of 6.   The image pattern selecting unit, as the printing rate information, when the average value of the printing rate of the toner image at a predetermined number of printed sheets exceeds a predetermined threshold, the fourth image pattern is the toner image for measurement The image forming apparatus according to claim 7, wherein the image pattern is selected as the image pattern.   The storage unit is the measurement toner image formed in one of the plurality of image forming units and is arranged in an area of the image forming area facing the first density detection unit. A ninth measurement toner image, and the measurement toner image formed in another image forming unit different from the one image forming unit of the plurality of image forming units, and A tenth measurement toner image disposed in an area facing the second density detection unit and on an extension of the ninth measurement toner image in the main scanning direction, and the measurement formed in the other image forming unit The tenth measurement toner image, which is arranged in the sub-scanning direction with respect to the ninth measurement toner image in an area of the image forming area that faces the first density detection unit. Different from toner image for An eleventh measurement toner image having an image density, and the measurement toner image formed in the one image forming unit, the image formation region being opposed to the second density detection unit and the eleventh measurement toner image. A fifth image pattern including a twelfth measurement toner image having an image density different from that of the ninth measurement toner image and arranged on an extension of the measurement toner image in the main scanning direction. Item 9. The image forming apparatus according to any one of Items 3 to 8.   The image according to claim 9, wherein the ninth measurement toner image and the tenth measurement toner image are formed of solid images, and the eleventh measurement toner image and the twelfth measurement toner image are formed of halftone images. Forming equipment.   The image pattern selecting unit, as the printing rate information, when the average value of the printing rate of the toner image at a predetermined number of prints is less than a predetermined threshold value, the fifth image pattern to the measurement toner image The image forming apparatus according to claim 10, wherein the image pattern is selected as the image pattern.   The image pattern selection unit is configured such that, as the printing rate information, the average value of the printing rate of the toner image at a predetermined number of printed sheets is lower than a predetermined threshold value, and the toner image is formed as the printing mode information. 12. The image forming apparatus according to claim 11, wherein when the interval between the sheets exceeds a preset threshold, the fifth image pattern is selected as the image pattern of the toner image for measurement.   The image forming apparatus according to claim 10, wherein the eleventh measurement toner image and the twelfth measurement toner image are arranged at positions overlapping in the main scanning direction.   The image forming apparatus according to claim 13, wherein the eleventh measurement toner image and the twelfth measurement toner image are arranged so as to straddle a center of the image forming area in the main scanning direction. A cleaning blade that contacts the photoconductor drum and collects toner adhered to the surface of the photoconductor drum; and transports the toner collected by the cleaning blade in the first direction. Further comprising a cleaning device having a cleaning conveyance member, and
The image according to claim 9, wherein the ninth measurement toner image and the eleventh measurement toner image are formed of solid images, and the tenth measurement toner image and the twelfth measurement toner image are formed of halftone images. Forming equipment. A cleaning blade that contacts the photoconductor drum and collects toner adhered to the surface of the photoconductor drum; and a cleaning blade that collects the toner collected by the cleaning blade along the first direction. Further comprising a cleaning device having a cleaning transport member that transports
The distance between the first density detection unit and the second density detection unit is larger than the width in the sub-scanning direction of an area of the first image pattern facing the first density detection unit and the second density detection unit. The image forming apparatus according to claim 3, wherein a width of the region in the sub-scanning direction is set to be small. A cleaning blade that contacts the photoconductor drum and collects toner adhered to the surface of the photoconductor drum; and a cleaning blade that collects the toner collected by the cleaning blade in the first direction or the first direction. A cleaning transporting member that transports along the second direction;
The storage unit is the measurement toner image formed in one of the plurality of image forming units and is arranged in an area of the image forming area facing the first density detection unit. A thirteenth measurement toner image, the measurement toner image formed in the one image forming unit, an area facing the second density detection unit in the image forming area, and the thirteenth measurement toner image A fourteenth measurement toner image disposed on an extension of the image in the main scanning direction, the measurement toner image formed in the one image forming unit, the thirteenth measurement toner image and the fourteenth measurement toner image. The image forming apparatus according to claim 3, wherein a sixth image pattern including a fifteenth measurement toner image disposed at a distance from the measurement toner image is stored.   The charge amount acquisition unit is configured to perform the non-development operation, and when the non-developing operation corresponds to a sheet interval between one sheet and another sheet next to the one sheet, the measurement toner image is formed on the photosensitive drum. The image forming apparatus according to claim 1, wherein the image forming apparatus forms the image.   The pattern image selecting unit, according to the charge amount of the toner acquired by the charge amount acquisition operation, the measurement toner image formed in the image forming area at the time of the next charge amount acquisition operation performed The image forming apparatus according to claim 1, wherein an image pattern is selected. The storage unit stores the DC voltage between the developing roller and the photosensitive drum in a state where the potential difference of the DC voltage is held constant, and the frequency of the AC voltage of the developing bias is changed. Reference information on the inclination of the reference straight line indicating the relationship between the density change amounts of the toner images is stored in advance for each charge amount of the toner,
The charge amount acquisition unit is configured to change the frequency of the AC voltage of the developing bias while maintaining the potential difference of the DC voltage between the developing roller and the photosensitive drum constant, and the plurality of the plurality of charge amounts on the photosensitive drum. The measurement toner image is formed, and the inclination of the measurement straight line indicating the relationship between the amount of change in the density of the measurement toner image and the amount of change in the frequency is expressed by the amount of change in the frequency and the measurement toner by the density detection unit. The toner charge obtained from the image density detection result and the inclination of the obtained measurement straight line and the reference information in the storage unit are used to calculate the charge amount of the toner contained in the measurement toner image formed on the photosensitive drum. The image forming apparatus according to claim 1, wherein the image forming apparatus acquires the image.   The reference information stored in the storage unit is such that when the charge amount of the toner is the first charge amount, the slope of the reference straight line is negative, and the charge amount of the toner is the first charge amount. The inclination of the reference straight line is positive when the second charge amount is smaller than the second charge amount, and the inclination of the reference straight line is set to increase as the charge amount of the toner decreases. Item 21. An image forming apparatus according to Item 20.   The charge amount acquisition unit is configured to change the frequency of the AC voltage of the developing bias while maintaining the potential difference of the DC voltage between the developing roller and the photosensitive drum constant, and the plurality of the plurality of charge amounts on the photosensitive drum. Forming the measurement toner image, and applying the developing roller and the developing bias when the plurality of measurement toner images are formed with respect to the density difference of the plurality of measurement toner images detected by the density detection unit. 20. The charge amount of toner contained in a measurement toner image formed on the photosensitive drum based on a ratio of a difference between DC components of the development current flowing between the photoconductor drum and the developing unit. The image forming apparatus according to claim 1. The charge amount acquisition unit controls the exposure device in a state in which the potential difference of the DC voltage between the developing roller and the photosensitive drum is kept constant, and changes the printing rate per unit area while controlling the photosensitive member. Forming the plurality of measurement toner images on a drum, and forming the plurality of measurement toner images with respect to a difference in density between the plurality of measurement toner images detected by the density detection unit; And acquiring a charge amount of toner included in a measurement toner image formed on the photosensitive drum based on a ratio of a difference between DC components of the development current flowing between the photoconductor drum and the developing bias application unit. Item 20. The image forming apparatus according to any one of Items 1 to 19.