JP2021179479A - Image forming apparatus - Google Patents

Abstract

[Problem] To provide an image forming apparatus capable of stably setting a peak-to-peak voltage based on a detection result of a development current in a developing device using a two-component development method. [Solution] A bias condition determination unit executes a first approximation formula determination operation for acquiring the actual development current Idr at at least three peak-to-peak voltages included in a first measurement range and determining a first approximation formula showing the relationship between the peak-to-peak voltage and the actual development current Idr, a second approximation formula determination operation for acquiring the actual development current Idr at at least three peak-to-peak voltages included in a second measurement range larger than the first measurement range and determining a second approximation formula showing the relationship between the peak-to-peak voltage and the actual development current Idr, and a reference voltage determination operation for determining a peak-to-peak voltage VT at a cross point where the first approximation formula and the second approximation formula cross each other as a reference peak-to-peak voltage. [Selected Figure] Figure 9

Description

The present invention relates to an image forming apparatus provided with a developing apparatus to which a two-component developing method is applied.

Conventionally, as an image forming apparatus for forming an image on a sheet, an image forming apparatus including a photoconductor drum (image carrier), a developing apparatus, and a transfer member is known. When the electrostatic latent image formed on the photoconductor drum is manifested by the toner by the developing device, the toner image is formed on the photoconductor drum. The transfer member transfers the toner image to the sheet. As a developing apparatus 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 developing technique, the developing apparatus has a developing roller, and a developing bias in which an AC bias is superimposed on a DC bias is applied to the developing roller to form a suitable toner image. In particular, when Vpp (inter-peak voltage) of the AC bias is set high, the image density tends to increase, the texture of the halftone image tends to be improved, and the half pitch unevenness that tends to occur in the rotation cycle of the developing roller tends to be improved. On the other hand, if Vpp is set too high, a leak may occur in the developing nip portion where the photoconductor drum and the developing roller face each other. Therefore, a technique for appropriately setting the Vpp of the AC bias among the development biases has been proposed.

In Patent Document 1, a development current when a test electrostatic latent image is developed is detected, and an image formation including a surface potential of a photoconductor drum and a development bias Vpp according to the detected development current is formed. The technology whose conditions are changed is disclosed. Further, Patent Document 2 describes a technique in which a toner charge amount is estimated from a developing current and a toner adhesion amount on a photoconductor drum, and at least one of Vpp and duty of AC bias is adjusted based on the toner charge amount. Is disclosed. Further, Patent Document 3 discloses a technique in which a development current at the time of development for a reference document is detected and the image density with respect to the reference document is determined by the detected development current. Further, Patent Document 4 discloses a technique in which development conditions such as exposure conditions and development gaps are changed according to the detected development current.

Japanese Unexamined Patent Publication No. 5-107835 Japanese Unexamined Patent Publication No. 2015-203731 Japanese Unexamined Patent Publication No. 9-138581 Japanese Unexamined Patent Publication No. 2006-64955

In the conventional technique as described above, the image forming conditions are changed based on the development current and the charge amount of the toner. As described above, when the development bias Vpp is increased, the amount of toner developed increases, so that the DC component of the development current increases. Therefore, in the conventional technique, the development current is confirmed while increasing the Vpp, and a suitable Vpp is set so as to obtain the target image density (development current).

However, in the method of determining Vpp in which the target image density can be obtained by detecting the development current while changing Vpp in this way, it is difficult to set an appropriate Vpp due to the detection error of the development current, and as a result, it is difficult to set an appropriate Vpp. There was a problem that the image density was liable to fluctuate.

The present invention is for solving the above-mentioned problems, and in a developing apparatus to which a two-component developing method provided in an image forming apparatus is applied, the peak voltage is stabilized based on the detection result of the developing current. The purpose is to set.

The developing apparatus according to one aspect of the present invention is an image forming apparatus capable of performing an image forming operation for forming an image on a sheet, and is allowed to be rotated to form an electrostatic latent image. An image carrier having a surface on which the electrostatic latent image carries a toner image manifested by toner, a charging device that charges the image carrier to a predetermined charging potential, and an image carrier rather than the charging device. An exposure device that forms the electrostatic latent image by exposing the surface of the image carrier charged with the charging potential on the downstream side in the rotation direction according to predetermined image information, and the exposure device. Is also a developing device arranged facing the image carrier at a predetermined developing nip portion on the downstream side in the rotation direction, and has a peripheral surface that is rotated and carries a developer composed of a toner and a carrier, and carries the image. An AC voltage is superimposed on a DC voltage, a developing device including a developing roller that forms the toner image by supplying toner to the body, a transfer unit that transfers the toner image supported on the image carrier to a sheet, and a DC voltage. A development bias application unit capable of applying the developed development bias to the development roller, a current detection unit capable of detecting a DC component of a development current flowing between the development roller and the development bias application unit, and the above-mentioned current detection unit. The development bias is applied to the developing roller corresponding to a predetermined latent image for measurement formed on the image carrier so as to include an image portion and a white background portion arranged at different positions in the axial direction of the developing roller. By doing so, the development current, which is the current detected by the current detection unit when the latent image for measurement is developed with toner, is placed on the image carrier so as to face at least the entire peripheral surface of the developing roller. In the image forming operation, the white background portion continuously formed along the axial direction is the current detected by the current detection unit when the white background portion passes through the developing nip portion. A bias condition determination unit for executing a bias condition determination mode for determining a reference peak-to-peak voltage, which is a reference of the peak-to-peak voltage of the AC voltage of the development bias applied to the developing roller, is provided.

According to this configuration, the reference peak voltage is accurately and stabilized based on the actual development current that cancels the influence of the white background on the development current corresponding to the latent image for measurement including the image and the white background. Can be set. Further, since the region of the image portion can be limited when the measurement toner image accompanied by the toner consumption is formed, the toner consumption associated with the formation of the measurement toner image can be reduced. Furthermore, assuming the same toner consumption, it is possible to form a measuring toner image that extends long along the rotation direction of the developing roller, so that fluctuations in the developing current due to the effects of runout and uneven rotation of the developing roller are leveled. It becomes possible to change.

In the above configuration, the bias condition determination unit executes the first approximate expression determination operation, the second approximate expression determination operation, and the reference voltage determination operation, respectively, in the bias condition determination mode. In the first approximation formula determination operation, the bias condition determination unit sets the peak voltage of the AC component of the development bias to at least three peak voltage for first measurement included in the predetermined first measurement range. Under the above conditions, the actual development current, which is the current obtained by correcting the development current with the white background current, is acquired, and the relationship between the peak voltage for the first measurement and the acquired actual development current in the first measurement range is obtained. The first approximate expression, which is the first-order approximate expression shown, is determined. In the second approximation formula determination operation, the bias condition determination unit is set so that the peak voltage of the AC component of the development bias has a minimum value larger than the maximum value in the first measurement range. The actual development current is acquired under the conditions set for at least three peak-to-peak voltages for second measurement included in the measurement range, and the peak-to-peak voltage for second measurement and the acquired development are acquired in the second measurement range. A second approximation formula, which is a first-order approximation formula showing the relationship with the actual current, is determined. In the reference voltage determination operation, the bias condition determination unit intersects the first approximation expression determined by the first approximation expression determination operation and the second approximation expression determined by the second approximation expression determination operation. The peak-to-peak voltage at the intersection is determined as the reference peak-to-peak voltage. Further, the actual peak-to-peak voltage during the image formation operation is the value of the reference peak-to-peak voltage as it is, the value obtained by multiplying the reference-peak-to-peak voltage by a certain ratio, or a constant value added to the reference-peak-to-peak voltage. Or a value obtained by multiplying by a certain ratio and adding a certain value may be used.

According to this configuration, the reference peak is obtained from the intersection of the first approximate expression and the second approximate expression showing the relationship between the peak voltage of the AC bias and the actual developing current in each of the first measurement range and the second measurement range. The voltage between them is set. In the vicinity of the above intersection, there is a change point in the relationship between the peak voltage of the AC bias and the actual developing current, so it is not easily affected by the inclination of the first approximation formula in the first measurement range, and the toner charge amount and the charge amount of the toner are not affected. It is possible to prevent the image density from changing due to fluctuations in the development gap. Further, the reference peak voltage is in the region where the slope of the second approximation formula becomes smaller than the predetermined threshold value according to the fluctuation of the carrier resistance and the development actual current tends to decrease as the peak voltage increases. Is suppressed. As a result, it becomes possible to set an AC bias of the development bias capable of outputting a stable image density in the image forming operation.

In the above configuration, the bias condition determining unit is the first approximate expression by the least squares method from the developed actual current acquired at at least three peak inter-peak voltages for the first measurement included in the first measurement range. It is desirable to determine.

According to this configuration, the first approximate expression can be determined from the first measurement peak-to-peak voltage included in the first measurement range by a simple arithmetic process.

In the above configuration, the bias condition determination unit is a first-order approximation determined by the least squares method from the developed actual current acquired at at least three second measurement peak-to-peak voltages included in the second measurement range. When the slope of the first determination approximation formula, which is an equation, is larger than the preset first threshold value, the average value of the actual development currents acquired at at least three of the peak inter-peak voltages for the second measurement is A linear equation that is constant with respect to a change in peak voltage is set as the second approximation equation, and when the inclination of the first determination approximation equation is smaller than the first threshold value, the first determination approximation equation is set. It is desirable to set the equation as the second approximate equation.

According to this configuration, in the process of determining the second approximate expression whose inclination is likely to change due to the influence of the resistance value of the carrier or the like, a more appropriate approximate expression is second-approximate according to the inclination of the first determination approximate expression. Can be selected as an expression.

In the above configuration, the interval between the plurality of first measurement peak voltages in the first measurement range and the interval between the plurality of second measurement peak voltages in the second measurement range are, respectively, the first. It is desirable that the interval is set smaller than the interval between the maximum value in one measurement range and the minimum value in the second measurement range.

According to this configuration, the first measurement range and the second measurement range are clearly distinguished, and the interval between peak voltages is finely set in each measurement range to obtain the first approximation formula and the second approximation formula. The determination accuracy can be improved.

In the above configuration, the number of the at least three first measurement peak voltage in the first measurement range is set to be larger than the number of the at least three second measurement peak voltage in the second measurement range. It is desirable to be.

According to this configuration, it is possible to determine a more accurate reference peak-to-peak voltage by acquiring a relatively large amount of data in the first measurement range in which the actual developing current is likely to change significantly.

In the above configuration, the bias condition determination unit is two currents constituting the actual development current, and the toner moves from the development roller to the image carrier in the image unit of the development nip unit. The toner transfer current, which is the generated current, flows in the same direction as the toner transfer current along the magnetic brush formed by the toner and the carrier so as to straddle the developing roller and the image carrier in the image unit. The voltage between reference peaks is determined by the balance with the magnetic brush current of the image unit, which is the current. At this time, the change point that changes according to the change of the peak voltage is acquired by the intersection of the first approximate expression and the second approximate expression, and the peak voltage corresponding to the change point is obtained between the reference peaks. It is desirable to determine it as a voltage.

According to this configuration, the change point at which the balance between the two currents of the toner transfer current and the magnetic brush current of the image unit changes can be predicted by the intersection of the two approximate expressions, and the reference peak voltage can be determined.

According to the present invention, in a developing apparatus to which a two-component developing method provided in an image forming apparatus is applied, it is possible to stably set an inter-peak voltage based on a detection result of a developing current.

It is sectional drawing which shows the internal structure of the image forming apparatus which concerns on one Embodiment of this invention. It is sectional drawing of the developing apparatus which concerns on one Embodiment of this invention, and is the block diagram which showed the electric structure of the control part. It is a schematic diagram which shows the development operation of the image forming apparatus which concerns on one Embodiment of this invention. It is a schematic diagram which shows the magnitude relation of the potential of the image carrier and the developing roller which concerns on one Embodiment of this invention. It is a flowchart of AC calibration performed in the image forming apparatus which concerns on one Embodiment of this invention. It is a flowchart of the first approximate expression determination step of AC calibration performed in the image forming apparatus which concerns on one Embodiment of this invention. It is a flowchart of the 2nd approximate expression determination step of AC calibration performed in the image forming apparatus which concerns on one Embodiment of this invention. It is a schematic diagram which shows the latent image for measurement including the image part and the white background part in AC calibration performed in the image forming apparatus which concerns on one Embodiment of this invention. It is a schematic diagram which shows the latent image for measurement only the white background part in AC calibration performed in the image forming apparatus which concerns on one Embodiment of this invention. It is a graph which shows the relationship between the Vpp of AC calibration performed in the image forming apparatus which concerns on one Embodiment of this invention, and the development actual current. It is a graph which shows the relationship between the Vpp of AC calibration performed in the image forming apparatus which concerns on one Embodiment of this invention, and the development actual current. It is a graph which shows the relationship between the Vpp of AC calibration performed in the image forming apparatus which concerns on one Embodiment of this invention, and the development actual current. It is a graph which shows the relationship between the Vpp of AC calibration performed in the image forming apparatus which concerns on one Embodiment of this invention, and the development actual current. It is a flowchart of the 2nd approximate expression determination step of AC calibration performed in the image forming apparatus which concerns on the modification embodiment of this invention. It is a flowchart of a part of the 2nd approximate expression determination step of AC calibration performed in the image forming apparatus which concerns on the modification embodiment of this invention.

Hereinafter, the image forming apparatus 10 according to the embodiment of the present invention will be described in detail based on the drawings. In this embodiment, a tandem color printer is exemplified as an example of the image forming apparatus. The image forming apparatus may be, for example, a copying machine, a facsimile apparatus, a multifunction device thereof, or the like. Further, the image forming apparatus may form a monochromatic (monochrome) image. The image forming apparatus 10 is capable of performing an image forming operation of forming an image on the sheet P.

FIG. 1 is a cross-sectional view showing the internal structure of the image forming apparatus 10. The image forming apparatus 10 includes an apparatus main body 11 having a box-shaped housing structure. In the apparatus main body 11, the paper feed unit 12 for feeding the sheet P, the image forming unit 13 for forming the toner image to be transferred to the sheet P fed from the paper feed unit 12, and the toner image are primarily transferred. The intermediate transfer unit 14 (transfer unit), the toner supply unit 15 that supplies toner to the image forming unit 13, and the fixing unit 16 that performs a process of fixing the unfixed toner image formed on the sheet P to the sheet P. It is decorated. Further, the upper part of the apparatus main body 11 is provided with a paper ejection portion 17 from which the sheet P subjected to the fixing treatment by the fixing portion 16 is discharged.

An operation panel (not shown) for inputting and operating output conditions and the like for the seat 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.

A sheet transport path 111 extending in the vertical direction is further formed in the apparatus main body 11 at a position on the right side of the image forming unit 13. The sheet transport path 111 is provided with a transport roller pair 112 that transports the sheet in a suitable place. Further, a resist roller pair 113 that performs skew correction of the sheet and feeds the sheet to the nip portion of the secondary transfer described later at a predetermined timing is provided on the upstream side of the nip portion in the sheet transport path 111. The sheet transport path 111 is a transport path for transporting the sheet P from the paper feed unit 12 to the paper discharge unit 17 via the image forming unit 13 and the fixing unit 16.

The paper feed unit 12 includes a paper feed tray 121, a pickup roller 122, and a paper feed roller pair 123. The paper feed tray 121 is removably mounted at a lower position of the apparatus main body 11 and stores a sheet bundle P1 in which a plurality of sheets P are laminated. The pickup roller 122 feeds out the uppermost sheet P of the sheet bundle P1 stored in the paper feed tray 121 one by one. The paper feed roller pair 123 feeds the sheet P unwound by the pickup roller 122 to the sheet transport path 111.

The paper feed unit 12 includes a manual paper feed unit attached to the left side surface of the apparatus main body 11 as shown in FIG. The manual paper feed unit includes a manual feed tray 124, a pickup roller 125, and a paper feed roller pair 126. The manual feed tray 124 is a tray on which the manually fed sheet P is placed, and is opened from the side surface of the apparatus main body 11 as shown in FIG. 1 when the sheet P is manually fed. The pickup roller 125 pays out the sheet P placed on the manual feed tray 124. The paper feed roller pair 126 feeds the sheet P unwound by the pickup roller 125 to the sheet transport path 111.

The image forming unit 13 forms a toner image to be transferred to the sheet P, and includes a plurality of image forming units that form toner images of different colors. As this image forming unit, in the present embodiment, magenta (M) colors are sequentially arranged (from the left side to the right side shown in FIG. 1) from the upstream side to the downstream side in the rotation direction of the intermediate transfer belt 141 described later. A magenta unit 13M using a developer, a cyan unit 13C using a cyan (C) color developer, a yellow unit 13Y using a yellow (Y) color developer, and a black (Bk) color developer are used. A black unit 13Bk is provided. Each unit 13M, 13C, 13Y, 13Bk includes a photoconductor drum 20 (image carrier), a charging device 21, a developing device 23, a primary transfer roller 24, and a cleaning device 25 arranged around the photoconductor drum 20, respectively. To prepare for. Further, an exposure apparatus 22 common to each unit 13M, 13C, 13Y, and 13Bk is arranged below the image forming unit.

The photoconductor drum 20 is rotationally driven around its axis to allow the formation of an electrostatic latent image and has a cylindrical surface on which the electrostatic latent image carries a toner image manifested by toner. .. As the photoconductor drum 20, as an example, a known amorphous silicon (α-Si) photoconductor drum or an organic (OPC) photoconductor drum is used. The charging device 21 uniformly charges the surface of the photoconductor drum 20 to a predetermined charging potential. The charging device 21 includes a charging roller and a charging cleaning brush for removing toner adhering to the charging roller. The exposure device 22 is arranged on the downstream side in the rotation direction of the photoconductor drum 20 with respect to the charging device 21, and has various optical system devices such as a light source, a polygon mirror, a reflection mirror, and a deflection mirror. The exposure device 22 exposes the surface of the photoconductor drum 20 uniformly charged to the charging potential by irradiating the surface with light modulated based on image data (predetermined image information) to expose an electrostatic latent image. Form.

The developing device 23 is arranged facing the photoconductor drum 20 at a predetermined developing nip portion NP (FIG. 3A) on the downstream side in the rotation direction of the photoconductor drum 20 from the exposure device 22. The developing apparatus 23 includes a developing roller 231 that has a peripheral surface that is rotated and carries a developer composed of toner and a carrier, and forms the toner image by supplying toner to the photoconductor drum 20.

The primary transfer roller 24 forms a nip portion with the photoconductor drum 20 by sandwiching the intermediate transfer belt 141 provided in the intermediate transfer unit 14. Further, the primary transfer roller 24 primary transfers the toner image on the photoconductor drum 20 onto the intermediate transfer belt 141. The cleaning device 25 cleans the peripheral surface of the photoconductor drum 20 after the toner image is transferred.

The intermediate transfer unit 14 is arranged in a space provided between the image forming unit 13 and the toner replenishing unit 15, and includes an intermediate transfer belt 141 and a drive roller 142 rotatably supported by a unit frame (not shown). , A driven roller 143, a backup roller 146, and a density sensor 100. The intermediate transfer belt 141 is an endless belt-shaped rotating body, and is bridged over a drive roller 142, a driven roller 143, and a backup roller 146 so that the peripheral surface side thereof abuts on the peripheral surface of each photoconductor drum 20. Has been done. The intermediate transfer belt 141 is orbitally driven by the rotation of the drive roller 142. In the vicinity of the driven roller 143, a belt cleaning device 144 for removing the toner remaining on the peripheral surface of the intermediate transfer belt 141 is arranged. The density sensor 100 (concentration detection unit) is arranged to face the intermediate transfer belt 141 on the downstream side of the units 13M, 13C, 13Y, and 13Bk, and determines the density of the toner image formed on the intermediate transfer belt 141. Detected by reflected light (reflection type). In another embodiment, the density sensor 100 may detect the density of the toner image on the photoconductor drum 20 or may detect the density of the toner image fixed on the sheet P.

A secondary transfer roller 145 is arranged on the outside of the intermediate transfer belt 141 so as to face the drive roller 142. The secondary transfer roller 145 is pressed against the peripheral surface of the intermediate transfer belt 141 to form a transfer nip portion with the drive roller 142. The toner image primaryly transferred onto the intermediate transfer belt 141 is secondarily transferred to the sheet P supplied from the paper feed section 12 at the transfer nip section. That is, the intermediate transfer unit 14 and the secondary transfer roller 145 function as a transfer unit that transfers the toner image supported on the photoconductor drum 20 to the sheet P. Further, a roll cleaner 200 for cleaning the peripheral surface of the drive roller 142 is arranged on the drive roller 142.

The toner supply unit 15 stores toner used for image formation, and in the present embodiment, it 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 each store replenishment toner of each color of M / C / Y / Bk. From the toner discharge port 15H formed on the bottom surface of the container, toner of each color is supplied to the developing apparatus 23 of the image forming units 13M, 13C, 13Y, 13Bk corresponding to each color of M / C / Y / Bk.

The fixing portion 16 includes a heating roller 161 having a heating source inside, a fixing roller 162 arranged to face the heating roller 161 and a fixing belt 163 stretched on the fixing roller 162 and the heating roller 161. It is provided with a pressurizing roller 164 which is arranged to face the fixing roller 162 via 163 and forms a fixing nip portion. The sheet P supplied to the fixing portion 16 is heated and pressurized by passing through the fixing nip portion. As a result, the toner image transferred to the sheet P at the transfer nip portion is fixed to the sheet P.

The paper ejection portion 17 is formed by recessing the top of the apparatus main body 11, and a paper ejection tray 171 for receiving the ejected sheet P is formed at the bottom of the recess. The sheet P that has been subjected to the fixing process is discharged toward the output tray 151 via the sheet transport path 111 extending from the upper part of the fixing portion 16.

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

The developing housing 230 is provided with a developing agent accommodating portion 230H. The developer accommodating portion 230H accommodates a two-component developer composed of a toner and a carrier. Further, the developer accommodating portion 230H conveys the developer in the first transport direction (direction orthogonal to the paper surface in FIG. 2, direction from rear to front) in which the developer is directed from one end side to the other end side in the axial direction of the developing roller 231. The first transport section 230A to be processed and the second transport section 230B that communicates with the first transport section 230A at both ends in the axial direction and transports the developer in the second transport direction opposite to the first transport direction. include. The first screw feeder 232 and the second screw feeder 233 are rotated in the directions of arrows D22 and D23 in FIG. 2, and transport the developer in the first transport direction and the second transport direction, respectively. In particular, the first screw feeder 232 supplies the developing agent to the developing roller 231 while conveying the developing agent in the first conveying direction.

The developing roller 231 is arranged facing the photoconductor drum 20 in the developing nip portion NP (FIG. 3A). The developing roller 231 includes a rotated sleeve 231S and a magnet 231M fixedly arranged inside the sleeve 231S. The magnet 231M comprises S1, N1, S2, N2 and S3 poles. The N1 pole functions as a main pole, the S1 pole and the N2 pole function as a transport pole, and the S2 pole functions as a peeling pole. Further, the S3 pole functions as a pumping pole and a regulating pole. As an example, the magnetic flux densities of S1 pole, N1 pole, S2 pole, N2 pole and S3 pole 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 the arrow D21 in FIG. The developing roller 231 is rotated to receive the developing agent in the developing housing 230, support the developing agent layer, and supply toner to the photoconductor drum 20. In this embodiment, the developing roller 231 rotates in the same direction (with direction) at a position facing the photoconductor drum 20. Further, in the axial direction (width direction) of the developing roller 231 the range in which the magnetic brush of the two-component developer is formed is, for example, 304 mm.

The regulating blade 234 (layer thickness regulating member) is arranged on the developing roller 231 at a predetermined interval, 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 image forming apparatus 10 including the developing apparatus 23 further includes a developing bias applying unit 971, a driving unit 972, an ammeter 973 (current detecting unit), and a control unit 980. The control unit 980 is composed of 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 development bias application unit 971 is composed of a DC power supply and an AC power supply, and the development bias in which the AC voltage is superimposed on the DC voltage on the development roller 231 of the development device 23 based on the control signal from the bias control unit 982 described later. Is applied.

The drive unit 972 includes a motor and a gear mechanism for transmitting the torque thereof, and in response to a control signal from the drive control unit 981 described later, the developing roller 231 in the developing device 23 is added to the photoconductor drum 20 during the developing operation. The first screw feeder 232 and the second screw feeder 233 are rotationally driven.

The ammeter 973 detects a direct current (direct current component of the developing current) flowing between the developing roller 231 and the developing bias application unit 971.

The control unit 980 includes a drive control unit 981, a bias control unit 982, a storage unit 983, and a calibration execution unit 984 (bias condition determination unit) by executing a control program stored in the ROM by the CPU. Function.

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

The bias control unit 982 controls the development bias application unit 971 during the development operation (during the image formation operation) in which the toner is supplied from the development roller 231 to the photoconductor drum 20 to form the photoconductor drum 20 and the developing roller 231. A potential difference between the DC voltage and the AC voltage is provided between the two. Due to the potential difference, the toner is moved from the developing roller 231 to the photoconductor drum 20.

The storage unit 983 stores various information referred to by the drive control unit 981, the bias control unit 982, and the calibration execution unit 984. As an example, the rotation speed of the developing roller 231 and the value of the developing bias adjusted according to the environment are stored. Further, the storage unit 983 stores the print rate and the number of lines set according to each toner image when forming a plurality of measurement toner images. The data stored in the storage unit 983 may be in a format such as a graph or a table.

The calibration execution unit 984 executes AC calibration (bias condition determination mode) described later. In AC calibration, the calibration execution unit 984 forms a plurality of measurement toner images on the photoconductor drum 20 while controlling the photoconductor drum 20, the charging device 21, the exposure device 22, and the developing device 23. Then, the calibration execution unit 984 develops the measurement latent image with toner by applying the development bias to the developing roller 231 corresponding to the predetermined measurement latent image formed on the photoconductor drum 20. Based on the DC current detected by the current meter 973, the reference peak voltage which is the reference of the peak voltage of the AC voltage of the development bias applied to the developing roller 231 in the image forming operation is determined.

<Development 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 showing the magnitude relationship between the potentials of the photoconductor drum 20 and the developing roller 231. With reference to FIG. 3A, a developing nip portion NP is formed between the developing roller 231 and the photoconductor drum 20. The toner TN and carrier CA supported 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 photoconductor drum 20 side, and the toner image TI is formed. With reference to FIG. 3B, the surface potential of the photoconductor drum 20 is charged to the background potential V0 (V) by the charging device 21. After that, when the exposure light is irradiated by the exposure apparatus 22, the surface potential of the photoconductor drum 20 is changed from the background potential V0 to the maximum image potential VL (V) according to the image to be printed. On the other hand, the DC voltage Vdc of the development 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 reverse development method, the potential difference between the surface potential V0 and the DC component Vdc (DC bias) of the development bias is the potential difference that suppresses toner fog on the background portion of the photoconductor drum 20. On the other hand, the potential difference between the surface potential VL after exposure and the DC component Vdc of the development bias becomes the development potential difference that moves the positive polarity toner to the image portion of the photoconductor drum 20. Further, the AC component (AC bias) of the development bias applied to the developing roller 231 promotes the transfer of toner from the developing roller 231 to the photoconductor drum 20.

<Relationship between development bias and image density>
Here, when the charge amount of the toner in the developing apparatus 23 changes, or when the developing gap changes due to the runout of the developing roller 231 or the like, the toner is applied to the toner in any of the above DC bias and AC bias. The moving force F (= toner charge amount Q × electric field magnitude E) changes, and the image density fluctuates. However, strictly speaking, the DC bias and the AC bias have different characteristics from each other. In the case of AC bias, when the Vpp (inter-peak voltage) is increased, the image density increases, but eventually the increase in the image density almost disappears, and when the Vpp (inter-peak voltage) is further increased, the image density decreases. On the other hand, when the development potential difference (Vdc-VL) in the DC bias is increased, the image density continues to increase, and the amount of increase in the image density becomes smaller, but the decrease in image density such as AC bias is not confirmed. .. This is because the AC electric field forms a bidirectional electric field (reciprocating electric field) between the photoconductor drum 20 and the developing roller 231 in the developing nip portion, while the DC electric field forms a unidirectional electric field. It is presumed that there is.

More specifically, the reciprocating electric field of the AC bias is two in opposite directions of the developing electric field for supplying the toner from the developing roller 231 to the photoconductor drum 20 and the recovery electric field for collecting the toner from the photoconductor drum 20 to the developing roller 231. It consists of two electric fields. When Vpp is increased, both electric fields increase, but eventually the amount of toner supplied by the developing electric field becomes maximum. After that, when Vpp is further increased, the amount of toner recovered increases due to the increase in the recovery electric field, but the amount of toner supplied by the developing electric field has already reached the maximum. As a result, the final developed amount of the toner decreases as the Vpp increases, depending on the magnitude relationship between the supply and recovery of the toner between the photoconductor drum 20 and the developing roller 231.

<Relationship between Vpp and development current>
In this way, while it is possible to grasp the relationship between the DC bias and AC bias and the amount of toner developed, when the Vpp of the AC bias is increased, the current flows between the developing roller 231 and the developing bias applying unit 971. It was not well known how the development current behaves.

The cause of this is that the development current generated in the development nip section NP is "tonner transfer current flowing due to the movement of toner" and "magnetic brush current flowing through the magnetic brush of the developer in the image section (image section magnetic brush current)". It is presumed that this is because it is composed of "magnetic brush current flowing through the magnetic brush of the developer in the non-image part (non-image part magnetic brush current or white background current)". This is because the toner transfer current changes according to the amount of toner transfer, so when Vpp is increased, the toner transfer current increases and then decreases, but the magnetic brush current in the image section is NP in the developing nip section. Since it is a current flowing through the magnetic brush in, it tends to increase with an increase in Vpp. Further, the non-image portion magnetic brush current tends to increase the current in the direction opposite to the image portion magnetic brush current as the Vpp increases in the non-image forming region (white background portion) adjacent to the image forming region. Therefore, it is sufficient to see how the developing current, which is complicatedly affected by the behavior of the total current of the toner transfer current, the magnetic brush current in the image part, and the magnetic brush current in the non-image part, behaves according to the increase in Vpp. It was not known.

Therefore, the present inventor has diligently conducted an experiment to confirm the behavior of the developing current when the Vpp of the AC bias of the developing bias is increased, and newly discovered that there are a plurality of patterns in the tendency. That is, when the Vpp of the AC bias is increased, the developing current (direct current) increases, but eventually the pattern reaches the change point where the gradient changes and the developing current gradually increases thereafter, or vice versa. It became clear that there was a pattern in which the developing current decreased from the change point.

The present inventor has newly focused on setting the AC bias Vpp in a region where the change in image density is small, based on such a pattern of development current. As a result, it is possible to reduce the change in image density even if the toner charge amount or the development gap changes. The details of AC calibration for setting such Vpp will be described below.

<About AC calibration>
FIG. 4 is a flowchart of AC calibration executed in the image forming apparatus 1 according to the present embodiment. FIG. 5 is a flowchart of the first approximate expression determination step of AC calibration executed in the image forming apparatus 1 according to the present embodiment. FIG. 6 is a flowchart of the second approximate expression determination step of AC calibration executed in the image forming apparatus 1 according to the present embodiment. FIG. 7 is a schematic view showing a latent image for measurement including an image portion and a white background portion in AC calibration performed in the image forming apparatus 1 according to the present embodiment. FIG. 8 is a schematic diagram showing a latent image for measurement of only a white background portion in the AC calibration according to the present embodiment.

In the present embodiment, the calibration execution unit 984 executes AC calibration (bias condition determination mode) at a timing when the image formation operation is not performed. The AC calibration is a mode for determining a reference peak voltage (target voltage) which is a reference of the peak voltage (Vpp) of the AC voltage of the development bias applied to the developing roller 231 in the image forming operation.

When the AC calibration is started, the calibration execution unit 984 performs a first approximate expression determination step (step S01 in FIG. 4), a second approximate expression determination step (step S02 in FIG. 4), and a target voltage determination step (FIG. 4). Step S03) of 4 is executed in order.

The first approximate expression determination step will be described in detail with reference to FIG. When the first approximation formula determination step is started, the calibration execution unit 984 acquires information regarding the first measurement range stored in the storage unit 983. The first measurement range is information regarding the range and interval of Vpp of the AC bias applied to the developing roller 231 in the first approximation formula determination step. In the present embodiment, as an example, information regarding the four first measurement peak-to-peak voltages is acquired by the calibration execution unit 984. As a result, the first measurement range in the first approximation formula determination step is determined (step S11).

Next, the calibration execution unit 984 measures the development current Idc (step S12). Specifically, the calibration execution unit 984 forms a latent image for measurement on the photoconductor drum 20 and develops the latent image for measurement by applying a development bias to the developing roller 231. At this time, the photoconductor drum 20 is rotated and the peripheral surface of the photoconductor drum 20 is uniformly charged to 250 V by the charging device 21 as in the case of image formation. As an example, the charging range of the photoconductor drum 20 in the axial direction (width direction) is set to 322 mm. Then, the potential of a part of the photoconductor drum 20 is lowered to 10 V by the exposure light emitted from the exposure device 22, and the measurement patch (measurement latent image, image portion) is formed on the photoconductor drum 20 as a solid black latent image. Is formed in. In this embodiment, as shown in FIG. 7, the width of the measurement patch is defined as M (mm) with respect to the magnetic brush width P (mm) on the developing roller 231. Then, on both sides of the measurement patch, white background portions on which the measurement patch is not formed are formed. In this case, the width (non-patch width) of the white background portion is calculated by PM (mm) by combining both sides of the measurement patch. This white background is the region where the above-mentioned non-image magnetic brush current (white background current) flows.

On the other hand, on the developing roller 231 as an example, an AC bias having a frequency of 10 kHz and a duty of 50% is superimposed on a DC voltage of 150 V. The AC bias Vpp is sequentially set to the four first measurement peak-to-peak voltages. As a result, with respect to each first measurement peak voltage, when the above measurement patch is developed by the developing roller 231, the ammeter 973 of the developing current flowing between the developing roller 231 and the developing bias application unit 971. Each DC component (development current Idc) is measured (step S12). As a result, four development currents Idc corresponding to the four first measurement peak-to-peak voltages are acquired. It is desirable that the development current Idc is measured with an average current of one round or more for the rotation of the developing roller 231 and more preferably averaged for an integral multiple of one round. The same applies to each current described later.

Next, the calibration execution unit 984 measures the white background current Idw (step S13). Also at this time, the photoconductor drum 20 is rotated, and the peripheral surface of the photoconductor drum 20 is uniformly charged to 250 V by the charging device 21. On the other hand, as shown in FIG. 8, the non-image portion (white background portion) is in the axial direction of the magnetic brush on the developing roller 231 without forming an electrostatic latent image corresponding to the image portion on the photoconductor drum 20. It is formed so as to face the whole. That is, the exposure light is not emitted from the exposure apparatus 22. In this case, the width of the white background (non-patch width) coincides with the magnetic brush width P. On the other hand, the development roller 231 is superposed with an AC bias having a frequency of 10 kHz and a duty of 50% on a DC voltage of 150 V, as in the case of measuring the development current Idc. Further, the AC bias Vpp is sequentially set to the four first measurement peak-to-peak voltages. When the white background portion as described above passes through the development nip portion N, the ammeter 973 measures the DC component (white background current Idw) of the development current flowing between the development roller 231 and the development bias application portion 971. As a result, four white background currents Idw corresponding to the four first measurement peak-to-peak voltages are acquired.

Next, the calibration execution unit 984 calculates four development actual currents Idr corresponding to the four first measurement peak-to-peak voltages (step S14). Specifically, the calibration execution unit 984 calculates the actual developing current Idr at each first measurement peak-to-peak voltage using the following equation 1.
Idr = Idc-Idw × M / P ... (Equation 1)
As a result, four sets of data regarding the first measurement peak voltage and the actual developing current Idr are acquired.

Next, the calibration execution unit 984 regresses the relationship between the above four peak-to-peak voltages for first measurement and the four actual developing currents Idr with a linear equation, and calculates the correlation coefficient R (step S15). .. As an example, the calibration execution unit 984 calculates the linear equation by the least squares method and acquires the correlation coefficient R.

Next, the calibration execution unit 984 compares the magnitude relationship between the correlation coefficient R acquired above and the threshold value R1 previously stored in the storage unit 983 (step S16). As an example, the threshold value R1 is set to 0.90. Here, when the threshold value R1 ≤ the correlation coefficient R (YES in step S16), the calibration execution unit 984 determines the linear expression regressed above as the first approximate expression (step S17). On the other hand, when the threshold value R1> the correlation coefficient R in step S16 (NO in step S16), the calibration execution unit 984 removes the largest Vpp data from the above four sets of data and the remaining three. The correlation coefficient R is calculated again based on the two data. After that, the calibration execution unit 984 executes steps S16 and S17 in the same manner as described above. If the relationship of the threshold value R1 ≤ correlation coefficient R is not satisfied even after the maximum Vpp data is removed in step S18, the calibration execution unit 984 may further remove some data and repeat the step. , The execution of AC calibration may be interrupted and the result of the previous AC calibration may be used.

As described above, when the first approximate expression determination step is completed, the second approximate expression determination step is started. The second approximate expression determination step will be described in detail with reference to FIG. When the second approximate expression determination step is started, the calibration execution unit 984 acquires information regarding the second measurement range stored in the storage unit 983. The second measurement range is information regarding the range and interval of Vpp of the AC bias applied to the developing roller 231 in the second approximate expression determination step. In the present embodiment, as an example, information regarding the three peak-to-peak voltages for the second measurement is acquired by the calibration execution unit 984. As a result, the second measurement range in the second approximate expression determination step is determined (step S21). The minimum value of the second measurement range (three second measurement peak voltage) is set larger than the maximum value of the first measurement range (four first measurement peak voltage).

Next, the calibration execution unit 984 forms a measurement patch (FIG. 7) on the photoconductor drum 20 and applies a development bias to the developing roller 231 in the same manner as in step S12 of FIG. Develop a latent image. At this time, an AC bias having a frequency of 10 kHz and a duty of 50% is superimposed on the DC voltage 150V on the developing roller 231, and the Vpp of the AC bias is sequentially set to the three second measurement peak-to-peak voltages. As a result, with respect to each second measurement peak-to-peak voltage, when the above-mentioned measurement latent image is developed by the developing roller 231, the ammeter 973 flows between the developing roller 231 and the developing bias application unit 971. DC components (development current Idc) of the above are measured (step S22). As a result, three development currents Idc corresponding to the three peak-to-peak voltages for the second measurement are acquired.

Next, the calibration execution unit 984 measures the white background current Idw in the same manner as in step S13 of FIG. 5 (step S23). Also at this time, the photoconductor drum 20 is rotated, and the peripheral surface of the photoconductor drum 20 is uniformly charged to 250 V by the charging device 21. On the other hand, as shown in FIG. 8, the non-image portion (white background portion) is in the axial direction of the magnetic brush on the developing roller 231 without forming an electrostatic latent image corresponding to the image portion on the photoconductor drum 20. It is formed so as to face the whole. On the other hand, the development roller 231 is superposed with an AC bias having a frequency of 10 kHz and a duty of 50% on a DC voltage of 150 V, as in the case of measuring the development current Idc. Further, the Vpp of the AC bias is sequentially set to the three peak-to-peak voltages for the second measurement. When the white background portion as described above passes through the development nip portion N, the ammeter 973 measures the DC component (white background current Idw) of the development current flowing between the development roller 231 and the development bias application portion 971. As a result, three white background currents Idw corresponding to the three peak-to-peak voltages for the second measurement are acquired.

Next, the calibration execution unit 984 calculates the three actual developing currents Idr corresponding to the three peak-to-peak voltages for the second measurement based on the above equation 1 (step S24). As a result, three sets of data regarding the second measurement peak voltage and the actual developing current Idr are acquired.

Next, the calibration execution unit 984 regresses the relationship between the above three peak-to-peak voltages for second measurement and the three actual developing currents Idr by a linear equation (approximate equation for first determination), and determines the slope L. Calculate (step S25). As an example, the calibration execution unit 984 calculates the linear expression by the method of least squares and acquires the slope L.

Next, the calibration execution unit 984 compares the magnitude relationship between the inclination L acquired above and the threshold value L1 previously stored in the storage unit 983 (step S26). As an example, the threshold value L1 is set to 0 (zero). Here, when the slope L <threshold value L1 (YES in step S26), the calibration execution unit 984 determines the linear expression regressed above as the second approximate expression (step S27). On the other hand, when the slope L ≥ the threshold value L1 in step S26 (NO in step S26), the calibration execution unit 984 calculates the average value of Vpp of the above three sets of data, and the average value is the change in the voltage between peaks. A linear equation that is constant with respect to the above is set as the second approximate equation (step S28).

When the first approximate expression determination step and the second approximate expression determination step shown in FIGS. 5 and 6 are completed, the calibration execution unit 984 executes the target voltage determination step (step S03 in FIG. 4). In the target voltage determination step, the calibration execution unit 984 determines the peak-to-peak voltage at the intersection where the first approximate expression and the second approximate expression intersect each other as the reference peak voltage (target voltage VT). As a result, it is possible to set the peak voltage during the image formation operation near the boundary (near the peak) of the relationship between the peak voltage and the developing current in each of the first measurement range and the second measurement range. In this embodiment, the reference peak voltage determined as described above is multiplied by 1.2, including a predetermined safety factor, and the peak voltage is applied as the actual peak voltage during the image formation operation.

Hereinafter, the AC calibration in this embodiment will be described in more detail based on the data. The data below are based on the following conditions.

<Common conditions>
-Printing speed: 55 sheets / minute-Photoreceptor drum 20: Amorphous silicon photoconductor (α-Si)
・ Development roller 231: Outer diameter 20 mm, surface shape knurled groove processing + blast processing (80 rows of recesses (grooves) are formed along the circumferential direction),
-Regulatory blade 234: Made of SUS430, magnetic, thickness 1.5 mm
-Developer transport amount after regulation blade 234: 250 g / m ²
Peripheral speed of the developing roller 231 with respect to the photoconductor drum 20: 1.8 (in the trail direction at the opposite position)
Distance between the photoconductor drum 20 and the developing roller 231: 0.25 mm
-White background (background) potential V0: + 250V of the photoconductor drum 20
-Image potential VL of the photoconductor drum 20: + 10V
Development bias of developing roller 231: frequency = 7 kHz, duty = 50% AC voltage square wave (Vpp is adjusted according to each experimental condition), Vdc (direct current voltage) = 150 V
-Toner: Positively charged polar toner, volume average particle size 6.8 μm, toner concentration 6%
-Carrier: Volume average particle diameter 35 μm, ferrite resin coated carrier

<About the developer>
The same effect has 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 exhibited in the toner concentration in the range of 3% to 12%. Since the movement of toner due to an AC electric field is more likely to occur as the magnetic brush is finer, the volume average particle size of the carrier is preferably 45 μm or less, and more preferably 30 μm or more and 40 μm or less. Further, a resin carrier having a smaller true specific density than a ferrite carrier is more preferable.

<About career>
The carrier is a ferrite core having a volume average particle diameter of 35 μm coated with silicon, fluorine, or the like, and was specifically prepared by the following procedure. A coating liquid is prepared by dissolving 20 parts by mass of silicon resin KR-271 (manufactured by Shin-Etsu Chemical Co., Ltd.) in 200 parts by mass of toluene in 1000 parts by weight of carrier core EF-35 (manufactured by Powdertech). Then, after spray-coating the coating liquid with a fluidized bed coating device, heat treatment was performed at 200 ° C. for 60 minutes to obtain carriers. Resistance adjustment and charge adjustment are performed by mixing and dispersing a conductive agent and a charge control agent in the coating liquid in the range of 0 to 20 parts with 100 parts of each coating resin.

9 to 12 are graphs showing the relationship between Vpp of AC calibration executed in the image forming apparatus 1 according to the present embodiment and the actual developing current Idr, respectively.

Tables 1 and 2 show the relationship between Vpp and the developing current in the first measurement range and the second measurement range shown in FIGS. 9 and 10, respectively.

In Tables 1 and 2, the development current Idc and the white background current Idw detected in the first measurement range and the second measurement range, respectively, and the actual development current Idr are calculated based on the above-mentioned formula 1. The white background current Idw in each table is a value after conversion with respect to the width based on the second term on the right side of the equation 1. Further, the white background current has a negative sign because the current flows from the magnetic brush in the direction opposite to the developing current. That is, the actual development current Idr after the development current Idc is corrected with respect to the white background current based on Equation 1 has a larger absolute value than the development current Idc.

Here, in FIG. 9, the actual developing current Idr is shown on the vertical axis (Y-axis), and Vpp is shown on the horizontal axis (X-axis). On the other hand, in FIG. 10, for comparison with FIG. 9, the developing current Idc is shown on the vertical axis (Y axis), and Vpp is shown on the horizontal axis (X axis).

In FIG. 9, in the first approximate expression determination step shown in FIG. 5, a linear expression of y = 0.0123x + 4.5129 is calculated as the first approximate expression. On the other hand, in the second approximate expression determination step shown in FIG. 6, since the slope L is plus (L> L1 = 0), the average value of the developed actual current Idr is calculated in step S28, and y is used as the second approximate expression. The linear equation of = 18.204 has been calculated. As a result, in the target voltage determination step S03, Vpp = target voltage VT = 1113V is calculated as the intersection of the first approximate expression and the second approximate expression, and 1.2 is set as the safety factor to form an image. Vpp = 1113 × 1.2 = 1335 (V) at the time of operation is selected.

On the other hand, FIG. 10 shows how the target voltage VT is calculated based on the development current Idc without performing the correction based on the white background current Idw according to the present embodiment. That is, in the first approximate expression determination step shown in FIG. 5, a linear expression of y = 0.0099x + 4.32 is calculated as the first approximate expression. On the other hand, in the second approximate expression determination step shown in FIG. 6, since the slope L is plus (L> L1 = 0), the average value of the developed actual current Idr is calculated in step S28, and y is used as the second approximate expression. = The linear equation of 14.233 has been calculated. As a result, in the target voltage determination step S03, Vpp = target voltage VT = 1001V is calculated as the intersection of the first approximate expression and the second approximate expression, and 1.2 is set as the safety factor to form an image. Vpp = 1001 × 1.2 = 1201 (V) at the time of operation is selected.

That is, if the correction based on the white background current Idw is not performed, the Vpp set in the subsequent image forming operation will be 1335-1201 = 134 (V) lower. As a result, in the embodiment shown in FIG. 10, image quality defects are likely to occur based on the Vpp setting error. On the other hand, in the embodiment shown in FIG. 9, by excluding (cancelling) the originally unnecessary white background current, it is possible to set Vpp more accurately at the time of image formation.

Tables 3 and 4 show the relationship between Vpp and the developing current in the first measurement range and the second measurement range shown in FIGS. 11 and 12, respectively, as in Tables 1 and 2, respectively. ..

Similarly, in FIG. 11, the actual developing current Idr is shown on the vertical axis (Y-axis), and Vpp is shown on the horizontal axis (X-axis). On the other hand, in FIG. 12, for comparison with FIG. 11, the developing current Idc is shown on the vertical axis (Y axis), and Vpp is shown on the horizontal axis (X axis).

In FIG. 11, in the first approximate expression determination step shown in FIG. 5, a linear expression of y = 0.0159x + 8.3653 is calculated as the first approximate expression. On the other hand, in the second approximate expression determination step shown in FIG. 6, since the slope L is minus (L <L1 = 0), the linear expression of y = −0.0006x + 24.273 is used as the second approximate expression in step S27. It has been calculated. As a result, in the target voltage determination step S03, Vpp = target voltage VT = 964V is calculated as the intersection of the first approximate expression and the second approximate expression, and 1.2 is set as the safety factor to form an image. Vpp = 964 × 1.2 = 1156 (V) at the time of operation is selected.

On the other hand, FIG. 12 shows how the target voltage VT is calculated based on the development current Idc without performing the correction based on the white background current Idw according to the present embodiment. That is, in the first approximate expression determination step shown in FIG. 5, a linear expression of y = 0.0135x + 8.1 is calculated as the first approximate expression. On the other hand, in the second approximate expression determination step shown in FIG. 6, since the slope L is minus (L <L1 = 0), the linear expression y = −0.0045x + 24.9 is used as the second approximate expression in step S27. It has been calculated. As a result, in the target voltage determination step S03, Vpp = target voltage VT = 933V is calculated as the intersection of the first approximate expression and the second approximate expression, and 1.2 is set as the safety factor to form an image. Vpp = 933 × 1.2 = 1119 (V) at the time of operation is selected.

That is, if the correction based on the white background current Idw is not performed, the Vpp set in the subsequent image forming operation will be 1156-1119 = 37 (V) lower. As a result, in the embodiment shown in FIG. 12, image quality defects are likely to occur based on the Vpp setting error. On the other hand, in the embodiment shown in FIG. 11, by excluding (cancelling) the originally unnecessary white background current, it is possible to set Vpp more accurately at the time of image formation. The above safety factor may be changed according to the target voltage VT, and is set by increasing the safety factor when the target voltage VT is low and decreasing the safety factor when the target voltage VT is high. It is also useful to suppress a large change in image quality by keeping the change width of Vpp small.

<Reason why the development current (DC component) has a peak (change point)>
Next, as in each of the above-mentioned data, the reason why the developing current (DC component) has a peak (change point) with respect to Vpp is inferred. As described above, the development current is composed of "toner transfer current + image part magnetic brush current + non-image part magnetic brush current", but when the development current is acquired, it corresponds to the image part of the electrostatic latent image. In the part (solid image part), both of this "toner transfer current + image part magnetic brush current" flows, but in the white background part at the end in the width direction, the "non-image part magnetic brush current" flows in the direction opposite to the image part. Only flows. Therefore, as Vpp is increased, the magnetic brush current in the non-image portion of the white background portion increases, and the total development current decreases.

The magnetic brush current in the image section of the image section also increases as the Vpp increases, but the toner layer formed by the toner adhering to the surface of the photoconductor drum 20 becomes a resistance layer, and the magnetic brush current in the image section Extreme increase is suppressed. On the other hand, in the white background portion, some toner moves to the sleeve surface of the developing roller 231 but the amount is overwhelmingly smaller than that of the image portion, so that the toner layer adhering to the sleeve surface is higher than that of the image portion. It doesn't become a resistance. As a result, the magnetic brush current in the non-image area on the white background increases significantly with the increase in Vpp, and this magnetic brush current flows in the direction opposite to the toner transfer current, so that the development current has a change point (peak). It is presumed that it will be.

The present inventor has newly discovered the above-mentioned relationship between the developing current and Vpp through repeated diligent experiments. Further, this phenomenon is more likely to occur as the resistance of the carrier is lower, and the carrier is based on the current flowing when 0.2 g of the carrier is filled between ^{parallel plates (area 240 mm 2) having a gap of 1 mm and a voltage of 1000 V is applied.} It was further found that this phenomenon remarkably appears at 10 9 ohms or less when the resistance value of is obtained.

That is, a two-component developer is interposed between the photoconductor drum 20 and the developing roller 231 and a latent image for measurement is formed at the center of the electrostatic latent image in the axial direction (width direction), and both ends thereof. When a white background portion is arranged on the surface, the above-mentioned change points occur at the boundary between the two ranges of the first measurement range and the second measurement range in the present embodiment. In particular, the phenomenon that the slope of the second approximation formula is distributed over a wide range of positive and negative is due to the fact that the current in the direction opposite to the central part flows through both ends of the developing roller 231 in the axial direction as described above. There is. In particular, in the present embodiment, the range of the magnetic brush on the developing roller 231 is narrower than the charging range on the photoconductor drum 20 in the axial direction, and a latent image for measurement formed on the photoconductor drum 20 is further formed. Of these, the range of the image section (solid image section) is set to be even narrower than the range of the magnetic brush. As a result, as described above, a region is formed at both ends of the developing roller 231 in the axial direction in which a current in the direction opposite to the image portion flows through the magnetic brush.

As described above, in the present embodiment, the calibration execution unit 984 is formed on the photoconductor drum 20 so as to include an image unit and a white background portion arranged at different positions in the axial direction of the developing roller 231. The development current Idc, which is the current detected by the current meter 973 when developing the measurement latent image with toner by applying a development bias to the developing roller 231 corresponding to the measurement latent image, and at least the developing roller. It is a current detected by the current meter 973 when a white background portion continuously formed along the axial direction on the photoconductor drum 20 so as to face the entire peripheral surface of 231 passes through the developing nip portion NP. AC calibration (bias condition determination mode) that determines the reference peak-to-peak voltage, which is the reference of the AC voltage of the development bias applied to the developing roller 231 in the image forming operation, based on each of the white background current Idw. To execute.

According to such a configuration, the reference peak voltage can be accurately set based on the actual developing current Idr that cancels the influence of the white background portion on the developing current corresponding to the latent image for measurement including the image portion and the white background portion. And it can be set stably. Further, since the region of the image portion can be limited to a part in the axial direction when the measurement toner image accompanied by the toner consumption is formed, the toner consumption associated with the formation of the measurement toner image can be reduced. Further, on the premise of the same toner consumption, it is possible to form a measurement toner image that extends long along the rotation direction of the developing roller 231. Therefore, fluctuations in the developing current due to the influence of runout and rotation unevenness of the developing roller 231 Can be leveled.

Further, in the present embodiment, the intersection of the first approximate expression and the second approximate expression representing the relationship between the peak voltage of the AC bias and the actual developing current Idr in each of the first measurement range and the second measurement range. The reference peak voltage is set from. In the vicinity of the above intersection, there is a change point in the relationship between the peak voltage of the AC bias and the actual developing current Idr, so that it is not easily affected by the inclination of the first approximation formula in the first measurement range, and the toner charge amount. It is possible to prevent the image density from changing due to fluctuations in the development gap and the development gap. Further, between the reference peaks in the region where the slope of the second approximation formula becomes smaller than the predetermined threshold value according to the fluctuation of the carrier resistance and the like, and the development actual current Idr tends to decrease as the inter-peak voltage increases. Setting the voltage is suppressed. As a result, it becomes possible to set an AC bias of the development bias capable of outputting a stable image density in the image forming operation. The actual peak-to-peak voltage during the image formation operation is the value of the reference peak-to-peak voltage as it is, the value obtained by multiplying the reference-peak-to-peak voltage by a certain ratio, or a constant value added to the reference-peak-to-peak voltage. Or a value obtained by multiplying a certain ratio and adding a certain value, or a value obtained by increasing the coefficient to be multiplied (for example, 1 or more) to improve pitch unevenness when the reference peak voltage is low, or a reference peak. When the voltage is high, a value obtained by reducing the multiplication factor (for example, less than 1) can be used in order to suppress the occurrence of leakage. Further, the upper and lower limits of the actual peak-to-peak voltage during the image formation operation may be determined based on the initially set peak-to-peak voltage (initial setting value). At the time of initial setting, the characteristics are the most stable because it does not include much influence of environmental factors and usage history. Therefore, it is desirable to set the upper and lower limits of the actual peak voltage in advance based on the initial setting value so that the voltage does not become a voltage at which problems such as pitch unevenness and leakage may occur in the future.

Further, in the present embodiment, the calibration execution unit 984 uses the least squares method from the developed actual current Idr acquired at at least three peak-to-peak voltages for the first measurement included in the first measurement range. 1 Determine the approximate expression. According to this configuration, the first approximate expression can be determined from the first measurement peak-to-peak voltage included in the first measurement range by a simple arithmetic process.

Further, in the present embodiment, the calibration execution unit 984 is determined by the least squares method from the developed actual current Idr acquired at at least three peak-to-peak voltages for the second measurement included in the second measurement range. When the inclination of the first determination approximation formula, which is a first-order approximation formula, is larger than the preset first threshold value L1, the actual development currents acquired at the at least three second measurement peak-to-peak voltages are obtained. When a linear equation in which the average value of Idr is constant with respect to the change in the peak voltage is set as the second approximation equation and the slope of the first determination approximation equation is smaller than the first threshold L1. The first determination approximate expression is set as the second approximate expression. According to this configuration, in the process of determining the second approximate expression whose inclination is likely to change due to the influence of the resistance value of the carrier or the like, a more appropriate approximate expression is second-approximate according to the inclination of the first determination approximate expression. Can be selected as an expression.

Further, in the present embodiment, the interval between the plurality of first measurement peak voltages in the first measurement range and the interval between the plurality of second measurement peak voltages in the second measurement range are, respectively. It is set smaller than the interval between the maximum value in the first measurement range and the minimum value in the second measurement range. According to this configuration, the first measurement range and the second measurement range are clearly distinguished, and the interval between peak voltages is finely set in each measurement range to obtain the first approximation formula and the second approximation formula. The determination accuracy can be improved.

Further, in the first approximate expression determination operation, when the correlation coefficient of the first approximate expression is smaller than the preset second threshold value, the calibration execution unit 984 is used for at least three first measurements. The first approximation formula is determined based on the developed actual current Idr with respect to the remaining inter-peak voltage obtained by excluding at least one inter-peak voltage from the inter-peak voltage. According to this configuration, when the correlation coefficient is small in the process of determining the first approximate expression, it is possible to determine the first approximate expression with higher accuracy by excluding the data of at least one peak-to-peak voltage. can.

In particular, when the correlation coefficient of the first approximate expression is smaller than the preset second threshold value R1 in the first approximate expression determination operation, the calibration execution unit 984 performs the at least three first measurements. The first approximate expression is determined based on the developed actual current Idr with respect to the remaining inter-peak voltage excluding the largest inter-peak voltage among the peak inter-peak voltages. According to this configuration, when the correlation coefficient is small in the process of determining the first approximate expression, the first approximate expression with higher accuracy is determined by excluding the data of the peak voltage near the second measurement range. can do.

Further, the calibration execution unit 984 excludes the largest inter-peak voltage or the smallest inter-peak voltage excluded in the second approximation formula determination operation from the second measurement range in advance, and determines the next bias condition. Run the mode. According to this configuration, the data excluded in the previous bias condition determination mode is excluded from the beginning in the next bias condition determination mode, so that the mode execution time is shortened and the reference peak voltage with high accuracy is determined. be able to.

Further, in the present embodiment, the number of the at least three peak inter-peak voltages for the first measurement in the first measurement range is larger than the number of the three peak inter-peak voltages for the second measurement in the second measurement range. Many are set. According to this configuration, the slope of the first approximation formula is positive, and by acquiring a relatively large amount of data in the first measurement range where the actual developing current Idr is likely to change significantly, more accurate reference peak interval is obtained. The voltage can be determined.

Further, in the present embodiment, after removing the influence of the non-image portion magnetic brush current from the three currents of the toner transfer current, the image portion magnetic brush current and the non-image portion magnetic brush current (white background current Idw), the toner transfer current The change point at which the balance (total of each current) of the image unit magnetic brush current changes can be predicted by the intersection of the two approximate equations, and the reference peak voltage can be determined.

In this embodiment, the setting of the reference peak voltage is determined based on the actual developing current Idr. In the past, it was considered to measure the image density and determine the reference peak voltage from its stability, but for example, a density sensor that measures the image density on the photoconductor drum 20 or the intermediate transfer belt 141 has an image density. When the value is high, the measurement accuracy tends to decrease, and the image density in the second measurement range of the present invention cannot be detected accurately. From this point as well, it is preferable that the data for determining the reference peak voltage in the first measurement range and the second measurement range is the development current, particularly the actual development current Idr.

Further, since the actual developing current Idr is likely to change significantly in the first measurement range, it is desirable to perform the measurement in the widest possible peak-to-peak voltage range. On the other hand, in the second measurement range, the change in the actual developing current Idr is relatively small, and if the peak voltage is set excessively large, a leak may occur in the developing nip portion. Therefore, it is desirable that the second measurement range is narrower than the first measurement range and the number of measurement points is set to be small. As a result, it is possible to shorten the mode execution time and reduce the amount of toner consumed.

Further, the actual development current Idr, the development current Idc for calculating the development current Idc, and the white background current Idw may be measured in the circuit in the development bias application unit 971. The moving current of the toner can also be measured on the photoconductor drum 20 side, but since the photoconductor drum 20 also includes the current flowing from the transfer roller, these currents cannot be separated. Therefore, it is desirable to measure each current on the development bias application unit 971 side.

Although the embodiment of the present invention has been described above, the present invention is not limited to this, and for example, the following modified embodiment can be adopted.

(1) In the above embodiment, the embodiment in which the surface of the developing roller 231 is knurled and blasted has been described, but the surface of the developing roller 231 has a concave shape (dimple) + blasting. It may be blasted only, knurled groove only, concave shape (dimple) only, or plated.

(2) When the image forming apparatus 10 has a plurality of developing devices 23 as shown in FIG. 1, the AC calibration according to the above embodiment is performed by one or two developing devices 23, and the result is obtained by another developing device 23. It may be used in. Further, the processing procedure of the first approximate expression determination step and the second approximate expression determination step is not limited to the above. As an example, after continuously measuring the development current Idc in the first approximate expression determination step and the second approximate expression determination step, the white background current Idw in the first approximate expression determination step and the second approximate expression determination step is continuously measured. Then, the actual development current Idr may be calculated after that.

(3) FIG. 13 is a flowchart of a second approximate expression determination step of AC calibration executed in the image forming apparatus according to the modified embodiment of the present invention. FIG. 14 is a flowchart of a part of the second approximate expression determination step. This modified embodiment is different in steps S24A, S24B and S24C of FIG. 13 as compared with the previous embodiment. That is, the actual developing current Idr is calculated in step S24. At this time, in the present modification embodiment, the four development currents Idc and the white background current Idw corresponding to the four second measurement peak-to-peak voltages are acquired, respectively, as in the first approximation formula determination step, and the second measurement peak is obtained. Four sets of data regarding the inter-voltage and the actual developing current Idr are acquired.

Here, the calibration execution unit 984 calculates the correlation coefficient R in the same manner as in the first approximate expression determination step (step S24A). Then, the magnitude relationship between the correlation coefficient R and the threshold value R2 previously stored in the storage unit 983 is compared (step S24B). As an example, the threshold value R2 is set to 0.90. Here, in the case of the threshold R2 ≤ correlation coefficient R (YES in step S24B), the calibration execution unit 984 calculates the slope L in step S25 and is based on the determination result in step S26, as in the previous embodiment. Then, in step S27 or step S28, the second approximate expression is calculated, respectively. On the other hand, in step S24B, when R2> R (NO in step S24B), the calibration execution unit 984 determines the correction correlation coefficient R in step S24C.

With reference to FIG. 14, when the step of determining the modified correlation coefficient R is started, in step S31, the calibration execution unit 984 removes the largest Vpp data from the above four sets of data. In, the correlation coefficient Rm is calculated based on the remaining three data (step S31). Next, the calibration execution unit 984 calculates the correlation coefficient Rn based on the remaining three data with the smallest Vpp data removed from the above four sets of data (step S32). Then, the calibration execution unit 984 compares the magnitude relations of the correlation coefficients Rm and Rn calculated above, and selects the larger correlation coefficient as the modified correlation coefficient R (step S33). After that, returning to FIG. 13, the processes after step S24B are repeated based on the selected modified correlation coefficient R.

As described above, in the present modification embodiment, when the correlation coefficient is small in the second approximation formula determination step, data having a high correlation coefficient is selected, and the second approximation formula is set based on the data. .. Therefore, by excluding the data of at least one peak-to-peak voltage, a more accurate second approximation formula can be determined.

In particular, the calibration execution unit 984 determines the second determination based on the actual development current for the remaining peak-to-peak voltage excluding the largest inter-peak voltage among the at least three second-measurement peak-to-peak voltages. A third determined based on the correlation coefficient Rm of the approximate expression and the actual developed current for the remaining peak voltage excluding the smallest peak voltage among the at least three second measurement peak voltages. The correlation coefficient Rn of the determination approximation formula is compared with each other, and the judgment approximation formula having the larger correlation coefficient among the second judgment approximation formula and the third judgment approximation formula is used as the second approximation formula. decide. According to this configuration, when the correlation coefficient is small in the determination process of the second approximation formula, the minimum peak-to-peak voltage closest to the first measurement range in the second measurement range, or the noise that is likely to cause a discharge leak. By excluding any of the data of the maximum peak voltage that is likely to include, a more accurate second approximation can be determined.

10 Image forming device 20 Photoreceptor drum 23 Developing device 231 Developing roller 971 Developing bias application unit 972 Driving unit 973 Ammeter (current detection unit)
980 Control unit 981 Drive control unit 982 Bias control unit 983 Storage unit 984 Calibration execution unit (bias condition determination unit)

Claims (7)

An image forming apparatus capable of performing an image forming operation for forming an image on a sheet.
An image carrier having a surface that allows the electrostatic latent image to be rotated to form an electrostatic latent image and that carries the toner image manifested by the toner.
A charging device that charges the image carrier to a predetermined charging potential, and
The electrostatic latent image is formed by exposing the surface of the image carrier, which is arranged on the downstream side in the rotation direction of the image carrier from the charging device and is charged with the charging potential, according to predetermined image information. Exposure device and
A developing device arranged facing the image carrier at a predetermined developing nip portion on the downstream side in the rotation direction of the exposure device, and has a peripheral surface that is rotated and carries a developer composed of toner and a carrier. A developing apparatus including a developing roller that forms the toner image by supplying toner to the image carrier, and
A transfer unit that transfers the toner image supported on the image carrier to a sheet, and a transfer unit.
A development bias application unit capable of applying a development bias in which an AC voltage is superimposed on a DC voltage to the development roller, and a development bias application unit.
A current detection unit capable of detecting a DC component of a current flowing between the development roller and the development bias application unit, and a current detection unit.
The developing bias is applied to the developing roller corresponding to a predetermined latent image for measurement formed on the image carrier so as to include an image portion and a white background portion arranged at different positions in the axial direction of the developing roller. By applying this, the development current, which is the current detected by the current detection unit when the latent image for measurement is developed with toner, is applied onto the image carrier so as to face at least the entire peripheral surface of the developing roller. In the image forming operation, based on each of the white background current, which is the current detected by the current detection unit when the white background portion continuously formed along the axial direction passes through the developing nip portion. A bias condition determination unit that executes a bias condition determination mode for determining a reference peak-to-peak voltage that is a reference of the peak-to-peak voltage of the AC voltage of the development bias applied to the developing roller.
An image forming apparatus. The bias condition determination unit is in the bias condition determination mode.
The development current is corrected by the white background current under the condition that the peak voltage of the AC component of the development bias is set to at least three peak voltage for first measurement included in the predetermined first measurement range. A first approximation formula is obtained by acquiring a certain development actual current and determining a first approximation formula which is a first-order approximation formula showing the relationship between the first measurement peak voltage in the first measurement range and the acquired development actual current. Approximate formula determination operation and
At least three peak-to-peak voltages for the second measurement included in the second measurement range in which the peak-to-peak voltage of the AC component of the development bias is set to have a minimum value larger than the maximum value of the first measurement range. The second approximation, which is a first-order approximation equation showing the relationship between the peak voltage for the second measurement and the acquired actual development current in the second measurement range by acquiring the actual development current under the conditions set in 1. The second approximate expression determination operation to determine the expression and
The peak-to-peak voltage at the intersection where the first approximate expression determined by the first approximate expression determination operation and the second approximate expression determined by the second approximate expression determination operation intersect with each other is defined as the reference peak voltage. To determine, the reference voltage determination operation,
The image forming apparatus according to claim 1, wherein each of the above is executed. The bias condition determining unit determines the first approximate expression from the developed actual current acquired at at least three first measurement peak voltage included in the first measurement range by the least squares method. Item 2. The image forming apparatus according to Item 2. The bias condition determining unit is a first-order approximate expression determined by the least squares method from the developed actual current acquired at at least three peak-to-peak voltages for the second measurement included in the second measurement range. When the slope of the approximation formula for determination is larger than the preset first threshold value, the average value of the actual developed currents acquired at the at least three peak-to-peak voltages for the second measurement is the change in the peak-to-peak voltage. A linear equation that is constant with respect to the above is set as the second approximation equation, and when the inclination of the first determination approximation equation is smaller than the first threshold value, the first determination approximation equation is used as the second approximation equation. The image forming apparatus according to claim 2, which is set as an approximate expression. The interval between the plurality of first measurement peak voltages in the first measurement range and the interval between the plurality of second measurement peak voltages in the second measurement range are the above-mentioned intervals in the first measurement range, respectively. The image forming apparatus according to any one of claims 2 to 4, which is set to be smaller than the interval between the maximum value and the minimum value in the second measurement range. Claimed that the number of the at least three first measurement peak voltage in the first measurement range is set to be larger than the number of the at least three second measurement peak voltage in the second measurement range. The image forming apparatus according to any one of 2 to 5. The bias condition determining unit is two currents constituting the actual development current, which are currents generated by the movement of the toner from the development roller to the image carrier in the image unit of the development nip unit. The moving current and the image section magnetic current that flows in the same direction as the toner moving current along the magnetic brush formed by the toner and the carrier so as to straddle the developing roller and the image carrier in the image section. The change point, which is the point where the balance with the brush current changes according to the change in the voltage between peaks, is acquired by the intersection of the first approximation formula and the second approximation formula, and the peak corresponding to the change point is obtained. The image forming apparatus according to any one of claims 2 to 6, wherein the intercurrent voltage is determined as the reference peak intercurrent voltage.

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