Image forming apparatus

Abstract

The present application provides an image forming apparatus. A bias condition determining section performs: a direct current voltage determining mode (DC calibration) that determines a reference direct current voltage that is a reference of a direct current voltage to be applied to a developing roller as a developing bias in an image forming operation; and a peak-to-peak voltage determining mode (AC calibration) that determines a reference peak-to-peak voltage that is a reference of a peak-to-peak voltage to be applied to the developing roller as the developing bias in the image forming operation. A calibration performing section determines whether or not the peak-to-peak voltage determining mode needs to be performed based on a value of the reference direct current voltage.

Description

Technical Field

[0001] This invention relates to an image forming apparatus having a developing apparatus that employs a two-component developing method. Background Technology

[0002] Conventionally, image forming apparatuses for forming images on a sheet are known to include a photosensitive drum (image carrier), a developing unit, and a transfer unit. When an electrostatic latent image formed on the photosensitive drum is developed by a developer in the developing unit, a toner image is formed on the photosensitive drum. The toner image is then transferred onto the sheet via the transfer unit. As a developing apparatus used in such an image forming apparatus, a two-component developing technique using a developer comprising a toner and a carrier is known.

[0003] In two-component development technology, the developing apparatus includes a developing roller. A suitable toner image is formed by applying a developing bias, which is a DC bias superimposed with an AC bias, to this developing roller. Conventionally, techniques have been known to measure the image density of a halftone image while varying the DC bias, and to select a DC bias that yields the target image density based on its characteristics. In particular, the image density is affected by changes in the toner charge due to environmental changes and printing conditions; therefore, it is necessary to set a DC bias corresponding to the toner charge. On the other hand, if the Vpp (peak voltage) in the AC bias is set higher, the image density increases, the texture of the halftone image improves, and there is a tendency to improve the uneven half-image spacing that easily occurs during the rotation cycle of the developing roller. However, if Vpp is set too high, leakage sometimes occurs at the developing gap between the photosensitive drum and the developing roller, and a so-called development ghosting degradation, where the printing history of the developing roller appears one week prior, appears on the image. Furthermore, if Vpp is set too low, image density variations (uneven half-image spacing) will occur on the halftone image corresponding to the circumferential vibration of the developing roller and photosensitive drum. Therefore, it is necessary to appropriately set the Vpp of the AC bias in the developing bias. At this time, developing ghosting and uneven half-image spacing are also easily affected by the charge of the toner, so it is necessary to adjust the Vpp of the AC bias at the appropriate time, just like with the DC bias.

[0004] In existing technologies, Vpp of the AC bias is adjusted to suppress image defects. However, if the difference between the DC bias and the background potential of the photosensitive drum becomes too large, the aforementioned ghosting will worsen, while uneven half-image spacing will be improved. Since both the DC bias of the development bias and Vpp of the AC bias affect image quality, adjusting only Vpp is insufficient to obtain a stable image. In other words, even with appropriate adjustment of Vpp of the AC bias, image defects that should be eliminated may worsen depending on the value of the DC bias. Summary of the Invention

[0005] The present invention was made to solve the above-mentioned problems. Its purpose is to set the peak voltage between the DC bias and AC bias of the development bias, which both affect image defects, at appropriate times in an image forming apparatus having a developing apparatus that applies a two-component developing method.

[0006] An image forming apparatus according to one aspect of the present invention is capable of performing an image forming operation on a thin sheet, characterized in that it comprises: an image carrier, which is rotated and has a surface that allows the formation of an electrostatic latent image and carries a toner image after the electrostatic latent image is developed by a toner; a charging device that charges the image carrier to a predetermined charging potential; an exposure device disposed downstream of the charging device in the rotation direction of the image carrier, which forms the electrostatic latent image by exposing the surface of the image carrier charged to the charging potential according to predetermined image information; and a developing device disposed opposite the image carrier in a predetermined developing slit portion downstream of the exposure device in the rotation direction. The system includes a developing roller that rotates and has a circumferential surface that carries a developer composed of a toner and a carrier, forming a toner image by supplying the toner to the image carrier; a transfer unit that transfers the toner image carried on the image carrier onto a sheet; a developing bias application unit that applies a developing bias, consisting of an AC voltage superimposed on a DC voltage, to the developing roller; a current detection unit that detects the DC component of the developing current flowing between the developing roller and the developing bias application unit; a concentration detection unit that detects the concentration of the toner image; and a bias condition determination unit that executes a bias condition determination mode, wherein the bias condition determination mode is such that, when the toner formed on the image carrier... When the latent image for measurement is applied to the developing roller in accordance with the specified measurement bias and the toner develops the latent image for measurement into a toner image, a reference voltage is determined based on the DC component of the developing current detected by the current detection unit or the concentration of the toner image for measurement detected by the concentration detection unit. This reference voltage serves as a reference for both the peak voltage of the AC voltage applied to the developing roller and the DC voltage during the image forming operation. The bias condition determination unit can execute the following modes as the bias condition determination mode: DC voltage determination mode, which determines the concentration of the toner image for measurement detected by the concentration detection unit. The reference DC voltage of the DC voltage of the development bias applied to the developing roller during the image forming operation; and the peak voltage determination mode, when the latent image for measurement is developed into the toner image by the toner through the development bias applied to the developing roller, the reference peak voltage of the AC voltage of the development bias applied to the developing roller during the image forming operation is determined based on the DC component of the developing current detected by the current detection unit, and the bias condition determination unit determines whether the peak voltage determination mode needs to be executed based on the reference DC voltage determined in the DC voltage determination mode. Attached Figure Description

[0007] Figure 1This is a cross-sectional view showing the internal structure of an image forming apparatus according to one embodiment of the present invention.

[0008] Figure 2 This is a cross-sectional view of a developing apparatus and a block diagram showing the electrical structure of the control unit according to one embodiment of the present invention.

[0009] Figure 3A This is a schematic diagram illustrating the developing operation of an image forming apparatus according to one embodiment of the present invention.

[0010] Figure 3B This is a schematic diagram illustrating the magnitude relationship between the potentials of the image carrier and the developing roller according to one embodiment of the present invention.

[0011] Figure 3C This is a schematic diagram illustrating the relationship between the DC bias and AC bias of the developing bias of an image forming apparatus according to an embodiment of the present invention.

[0012] Figure 4 This is a flowchart of development bias calibration performed in an image forming apparatus according to one embodiment of the present invention.

[0013] Figure 5 This is a graph illustrating the relationship between DC bias and image density during DC calibration performed in an image forming apparatus according to one embodiment of the present invention.

[0014] Figure 6 This is a flowchart of AC calibration performed in an image forming apparatus according to one embodiment of the present invention.

[0015] Figure 7 This is a flowchart of the first approximate determination step of AC calibration performed in an image forming apparatus according to an embodiment of the present invention.

[0016] Figure 8 This is a flowchart of the second approximation determination step of AC calibration performed in an image forming apparatus according to one embodiment of the present invention.

[0017] Figure 9 This is a graph showing the relationship between Vpp and developing current during AC calibration performed in an image forming apparatus according to one embodiment of the present invention.

[0018] Figure 10 This is a graph showing the relationship between Vpp and developing current during AC calibration performed in an image forming apparatus according to one embodiment of the present invention.

[0019] Figure 11This is a graph showing the relationship between Vpp and developing current during AC calibration performed in an image forming apparatus according to one embodiment of the present invention.

[0020] Figure 12 This is a flowchart of the second approximation determination step of AC calibration performed in an image forming apparatus according to a modified embodiment of the present invention.

[0021] Figure 13 This is a flowchart of a second approximation determination step of AC calibration performed in an image forming apparatus according to a modified embodiment of the present invention. Detailed Implementation

[0022] Hereinafter, the image forming apparatus 10 according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In this embodiment, a serial color printer is illustrated as an example of an image forming apparatus. The image forming apparatus may also be, for example, a copier, a fax machine, and a digital multifunction printer thereof. Furthermore, the image forming apparatus may also form monochrome (black and white) images. The image forming apparatus 10 is capable of performing an image forming operation that forms an image on a sheet body P.

[0023] Figure 1 This is a cross-sectional view showing the internal structure of the image forming apparatus 10. The image forming apparatus 10 includes a main body 11 with a box-shaped housing structure. Inside the main body 11 are: a paper supply unit 12 for supplying a sheet P; an image forming unit 13 for forming a toner image transferred onto the sheet P supplied from the paper supply unit 12; an intermediate transfer unit 14 (transfer unit) for the first transfer of the toner image; a toner replenishment unit 15 for replenishing toner to the image forming unit 13; and a fixing unit 16 for fixing the unfixed toner image formed on the sheet P onto the sheet P. Furthermore, a paper discharge unit 17 is provided at the top of the main body 11, to which the sheet P, which has undergone fixing in the fixing unit 16, is discharged.

[0024] An operation panel (not shown) is provided at an appropriate position on the upper surface of the device body 11 for inputting and operating conditions such as output conditions for the sheet body P. This operation panel includes a power button, a touch panel for inputting and output conditions, and various operation keys.

[0025] Within the main body 11 of the apparatus, a sheet transport channel 111 extending vertically is formed at a position to the right of the image forming unit 13. A pair of transport rollers 112 is provided in the sheet transport channel 111 to transport the sheet to the appropriate position. Furthermore, an alignment roller pair 113 is provided upstream of the slit portion in the sheet transport channel 111. The alignment roller pair 113 corrects the skew of the sheet and feeds the sheet into the slit portion for secondary transfer, described later, at a predetermined time. The sheet transport channel 111 is the transport channel that transports the sheet P from the paper feeding unit 12 via the image forming unit 13 and the fixing unit 16 to the paper output unit 17.

[0026] The paper feeding unit 12 includes a paper feeding tray 121, a paper feed roller 122, and a paper feeding roller pair 123. The paper feeding tray 121 is detachably mounted below the main body 11 of the device and stores a stack of sheets P1 containing multiple sheets P. The paper feed roller 122 extracts the top sheet P1 from the stack of sheets P1 stored in the paper feeding tray 121 one by one. The paper feeding roller pair 123 delivers the sheets P extracted by the paper feed roller 122 to the sheet transport channel 111.

[0027] The paper feeding unit 12 is equipped with a paper feeding unit installed on the main body 11 of the device. Figure 1 The manual paper feeding unit is shown on the left side. The manual paper feeding unit includes a manual tray 124, a paper feed roller 125, and a pair of feed rollers 126. The manual tray 124 is a tray for holding the manually fed sheet P, as shown... Figure 1 As shown, when the sheet body P is manually supplied, it is opened from the side of the device body 11. The paper feed roller 125 pulls out the sheet body P placed on the manual tray 124. The paper feed roller pair 126 delivers the sheet body P pulled out by the paper feed roller 125 to the sheet body transport channel 111.

[0028] The image forming unit 13 forms a toner image transferred to the sheet body P, and includes multiple image forming units for forming toner images of different colors. In this embodiment, this image forming unit is configured to extend from the upstream side of the rotation direction of the intermediate transfer belt 141 (described later) towards the downstream side. Figure 1 The following units are arranged sequentially from left to right (as shown): a magenta unit 13M using magenta (M) developer, a cyan unit 13C using cyan (C) developer, a yellow unit 13Y using yellow (Y) developer, and a black unit 13Bk using black (Bk) developer. Each unit 13M, 13C, 13Y, and 13Bk includes a photosensitive drum 20 (image carrier), a charging device 21 disposed around the photosensitive drum 20, a developing device 23, a primary transfer roller 24, and a cleaning device 25. Furthermore, the exposure device 22 shared by all units 13M, 13C, 13Y, and 13Bk is disposed below the image forming unit.

[0029] A photosensitive drum 20 is driven to rotate about its axis and has a cylindrical surface that allows the formation of an electrostatic latent image and carries the toner image after the electrostatic latent image is developed by the toner. As an example, this photosensitive drum 20 uses a known amorphous silicon (α-Si) photosensitive drum or an organic (OPC) photosensitive drum. A charging device 21 uniformly charges the surface of the photosensitive drum 20 to a predetermined charging potential. The charging device 21 includes a charging roller and a charged cleaning brush for removing toner adhering to the charging roller. An exposure device 22 is positioned downstream of the charging device 21 in the direction of rotation of the photosensitive drum 20 and includes various optical devices such as a light source, a polyhedral prism, a mirror, and a deflector. The exposure device 22 forms an electrostatic latent image by irradiating the surface of the photosensitive drum 20, which is uniformly charged to the predetermined charging potential, with light modulated based on image data (predetermined image information).

[0030] The developing apparatus 23 has a defined developing slit section NP located downstream of the photosensitive drum 20 in the rotational direction of the exposure apparatus 22. Figure 3A The developing apparatus 23 is positioned opposite the photosensitive drum 20. The developing apparatus 23 includes a developing roller 231 that rotates and has a circumferential surface that carries a developer consisting of a toner and a carrier, which forms the toner image by supplying the toner to the photosensitive drum 20.

[0031] The primary transfer roller 24 forms a gap with the photosensitive drum 20 through the intermediate transfer belt 141 of the intermediate transfer unit 14. Then, the primary transfer roller 24 transfers the toner image on the photosensitive drum 20 onto the intermediate transfer belt 141 in one pass. The cleaning device 25 cleans the peripheral surface of the photosensitive drum 20 after the toner image transfer.

[0032] The intermediate transfer unit 14 is disposed in the space between the image forming unit 13 and the toner supply unit 15, and includes an intermediate transfer belt 141, a drive roller 142 supported by a unit frame (not shown) for rotation, a driven roller 143, a support roller 146, and a density sensor 100. The intermediate transfer belt 141 is an annular strip-shaped rotating body, mounted on the drive roller 142, driven roller 143, and support roller 146 such that its circumferential side abuts against the circumferential surface of each photosensitive drum 20. The intermediate transfer belt 141 is driven to rotate by the rotation of the drive roller 142. A belt cleaning device 144 for removing toner residue on the circumferential surface of the intermediate transfer belt 141 is disposed near the driven roller 143. The concentration sensor 100 (concentration detection unit) is disposed opposite to the intermediate transfer belt 141 on the downstream side of the comparison units 13M, 13C, 13Y, and 13Bk, and detects the concentration of the toner image formed on the intermediate transfer belt 141 using reflected light (reflective type). In other embodiments, the concentration sensor 100 can detect the concentration of the toner image on the photosensitive drum 20, and also the concentration of the toner image fixed on the film body P.

[0033] A secondary transfer roller 145 is disposed opposite to the drive roller 142 on the outer side of the intermediate transfer belt 141. The secondary transfer roller 145 makes pressure contact with the circumferential surface of the intermediate transfer belt 141, forming a transfer gap between itself and the drive roller 142. The toner image transferred to the intermediate transfer belt 141 in the first step is transferred a second time in the transfer gap to the sheet body P supplied to the paper feed unit 12. That is, the intermediate transfer unit 14 and the secondary transfer roller 145 function as a transfer section for transferring the toner image carried on the photosensitive drum 20 to the sheet body P. In addition, a roller cleaner 200 for cleaning its circumferential surface is disposed on the drive roller 142.

[0034] The toner replenishment unit 15 stores toner for image formation. In this 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 respectively store supplementary toner for each of the colors M / C / Y / Bk. Toner for each color is replenished to the developing apparatus 23 of the image forming units 13M, 13C, 13Y, and 13Bk corresponding to each of the colors M / C / Y / Bk through the toner discharge port 15H formed on the bottom of the container.

[0035] The fixing unit 16 includes: a heating roller 161 with a heating source inside; a fixing roller 162 disposed opposite to the heating roller 161; a fixing belt 163 tensioned and mounted on the fixing roller 162 and the heating roller 161; and a pressure roller 164 disposed opposite to the fixing roller 162 via the fixing belt 163 and forming a fixing slit. A sheet P supplied to the fixing unit 16 is heated and pressurized through the fixing slit. As a result, the toner image transferred to the sheet P in the transfer slit is fixed onto the sheet P.

[0036] The paper discharge section 17 is formed by a recess in the top of the device body 11, and a paper discharge tray 171 for receiving the discharged sheet body P is formed at the bottom of the recess. The sheet body P, which has undergone fixing treatment, is discharged toward the paper discharge tray 151 via a sheet body transport channel 111 extending from the upper part of the fixing section 16.

[0037] (Regarding the developing apparatus)

[0038] Figure 2 This is a cross-sectional view of the developing apparatus 23 according to this embodiment and a block diagram showing the electrical structure 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 limiting scraper 234. The developing apparatus 23 employs a two-component developing method.

[0039] The developing housing 230 includes a developer storage section 230H. The developer storage section 230H stores a two-component developer consisting of a toner and a carrier. Furthermore, the developer storage section 230H includes a first conveying section 230A, which conveys the developer in a first conveying direction (in line with the axial direction of the developing roller 231) from one end towards the other end. Figure 2 The first conveyor 230A delivers developer in a direction perpendicular to the paper surface (from back to front); and the second conveyor 230B, connected to the first conveyor 230A at both ends in the axial direction, delivers developer in a second conveying direction opposite to the first conveying direction. The first screw feeder 232 and the second screw feeder 233... Figure 2 The screw feeder rotates in the directions of arrows D22 and D23, respectively conveying the developer in the first conveying direction and the second conveying direction. In particular, the first screw feeder 232 conveys the developer in the first conveying direction while simultaneously supplying the developer to the developing roller 231.

[0040] The developing roller 231 is in the developing gap section NP ( Figure 3A The developing roller 231 is positioned opposite the photosensitive drum 20. It includes a rotating sleeve 231S and a magnet 231M fixedly disposed inside the sleeve 231S. The magnet 231M has poles S1, N1, S2, N2, and S3. Pole N1 functions as the main pole, poles S1 and N2 function as transport poles, and pole S2 functions as a stripping pole. Pole S3 functions as both a pick-up pole and a limit pole. For example, the magnetic flux densities of poles S1, N1, S2, N2, and S3 are set to 54 mT, 96 mT, 35 mT, 44 mT, and 45 mT, respectively. The sleeve 231S of the developing roller 231... Figure 2 The developing roller 231 rotates in the direction of arrow D21. It receives and carries the developer layer from the developing housing 230, supplying toner to the photosensitive drum 20. In this embodiment, the developing roller 231 rotates in the same direction (same direction) at a position opposite to the photosensitive drum 20. Furthermore, the range of the magnetic brush forming the two-component developer along the axial (width) direction of the developing roller 231 is, for example, 304 mm.

[0041] The scraper 234 is configured to be spaced apart from the developing roller 231 by a predetermined interval, thereby limiting the thickness of the developer layer supplied from the first screw feeder 232 to the circumferential surface of the developing roller 231.

[0042] The image forming apparatus 10, which includes the developing apparatus 23, also includes a developing bias application unit 971, a driving unit 972, a current meter 973 (current detection unit), and a control unit 980. The control unit 980 consists of a CPU (Central Processing Unit), a ROM (Read Only Memory) storing the control program, and RAM (Random Access Memory) used as the working area of ​​the CPU.

[0043] The developing bias application unit 971 is composed of a DC power supply and an AC power supply. Based on the control signal from the bias control unit 982 described later, it applies a developing bias to the developing roller 231 of the developing apparatus 23, which is a DC voltage (DC bias) superimposed with an AC voltage (AC bias).

[0044] The drive unit 972 consists of a motor and a gear mechanism that transmits its torque. According to the control signal from the drive control unit 981 described later, during the developing operation, in addition to the photosensitive drum 20, it also rotates and drives the developing roller 231, the first spiral feeder 232, and the second spiral feeder 233 in the developing device 23.

[0045] The ammeter 973 detects the DC current (DC component of the developing current) flowing between the developing roller 231 and the developing bias application section 971.

[0046] The control unit 980 executes the control program stored in the ROM through the CPU, and functions in a manner that 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).

[0047] The drive control unit 981 controls the drive unit 972 to rotate the developing roller 231, the first spiral feeder 232, and the second spiral feeder 233. Furthermore, the drive control unit 981 controls a drive mechanism (not shown) to rotate the photosensitive drum 20.

[0048] During the developing operation (image forming operation) when toner is supplied from the developing roller 231 to the photosensitive drum 20, the bias control unit 982 controls the developing bias application unit 971 to establish a potential difference between a DC voltage and an AC voltage between the photosensitive drum 20 and the developing roller 231. This potential difference causes the toner to move from the developing roller 231 to the photosensitive drum 20.

[0049] The storage unit 983 stores various information referenced by the drive control unit 981, the bias control unit 982, and the calibration execution unit 984. For example, it stores the rotational speed of the developing roller 231 and the value of the developing bias adjusted according to the environment. Furthermore, the storage unit 983 stores the print rate and screen ruling set for each toner image when multiple measurement toner images are formed on the photosensitive drum 20. Additionally, the data stored in the storage unit 983 can be in the form of graphs, tables, etc.

[0050] The calibration execution unit 984 performs development bias calibration, including DC calibration and AC calibration described later.

[0051] Furthermore, during AC calibration, the calibration execution unit 984 controls the photosensitive drum 20, the charging device 21, the exposure device 22, and the developing device 23 while forming multiple measurement toner images on the photosensitive drum 20. Then, when the measurement latent image is developed into a measurement toner image by the toner using the toner by applying the development bias to the developing roller 231 corresponding to the predetermined measurement latent image formed on the photosensitive drum 20, the calibration execution unit 984 determines a reference peak-to-peak voltage, which serves as a reference to the peak-to-peak voltage of the AC voltage applied to the developing roller 231 during the image forming operation, based on the DC current detected by the ammeter 973. Alternatively, during DC calibration after AC calibration or during the image forming operation, the aforementioned reference peak-to-peak voltage can be used directly, or a value obtained by multiplying the reference peak-to-peak voltage by a predetermined safety factor can be used.

[0052] (Regarding the developing process)

[0053] Figure 3A This is a schematic diagram of the developing operation of the image forming apparatus 10 according to this embodiment. Figure 3B This is a schematic diagram showing the magnitude relationship between the potentials of the photosensitive drum 20 and the developing roller 231. Figure 3C This is a schematic diagram illustrating the relationship between DC bias and AC bias in the development bias. (Refer to...) Figure 3A A developing gap NP is formed between the developing roller 231 and the photosensitive drum 20. Toner TN and carrier CA, carried on the developing roller 231, form a magnetic brush. In the developing gap NP, toner TN is supplied from the magnetic brush to the photosensitive drum 20 side, forming a toner image TI. (See reference...) Figure 3B The surface potential of the photosensitive drum 20 is charged to the background potential V0 (V) by the charging device 21. Then, when exposure light is emitted through the exposure device 22, the surface potential of the photosensitive drum 20 changes from the background potential V0 (non-image forming portion) to the image potential VL (V) (image forming portion) according to the image to be printed. On the other hand, referring to… Figure 3CA DC bias voltage Vdc is applied to the developing roller 231, and an AC bias voltage is superimposed on the DC bias voltage Vdc. As an example, ... Figure 3C As shown, the AC bias consists of a periodic rectangular wave, and its peak-to-peak voltage (Vpp) has an amplitude exceeding that of the background potential V0 and the image potential VL of the photosensitive drum 20.

[0054] In this reverse development method, the potential difference between the surface potential V0 and the DC component of the development bias Vdc (DC bias) is the potential difference that suppresses toner fogging towards the background area of ​​the photosensitive drum 20. On the other hand, the potential difference between the surface potential VL after exposure and the DC component of the development bias Vdc becomes the development potential difference that causes the positively polarized toner to move towards the image area of ​​the photosensitive drum 20. Furthermore, by applying the AC component of the development bias (AC bias) to the developing roller 231, the movement of toner from the developing roller 231 to the photosensitive drum 20 is promoted.

[0055] (Regarding developer bias calibration)

[0056] The DC and AC biases in the aforementioned development biases both exhibit the property of increasing image density when their respective voltage values ​​are increased. In particular, increasing the AC bias voltage value intensifies the reciprocating motion of the toner in the developing region, resulting in half-image homogenization, etc. Therefore, it is preferable to set the AC bias to a condition that ensures active reciprocating motion of the toner while maintaining stable image density. Furthermore, by adjusting the image density according to the DC bias value, a more stable image can be obtained.

[0057] Here, the region where image density is stabilized by AC bias becomes a region of high image density. However, optical sensors like density sensor 100 are generally suitable for use in half-images. This is because it is difficult to ensure the measurement sensitivity of optical sensors in regions of high image density. Therefore, even if an appropriate value for AC bias is set based on the output of the optical sensor, high-precision setting conditions cannot be obtained.

[0058] On the other hand, as described above, in order to adjust the image density of the image to be printed onto the sheet body via DC bias, it is necessary to set an appropriate DC bias based on the output of the density sensor 100 (optical sensor) while changing the DC bias. Here, it is assumed that the appropriate DC bias is set based on the value of the developing current detected by the ammeter 973. When the charge of the toner changes, even with the same developing current, the amount of toner adhering to the photosensitive drum 20 will change. Furthermore, if the carrier in the developer is, for example, a ferrite-coated carrier, if the film thickness of the carrier changes due to the cutting of the coating on the surface of the carrier, the resistance of the carrier changes, and the amount of current flowing in the carrier changes. As a result, the relationship between the developing current and the amount of toner adhering is disrupted, and the amount of toner adhering cannot be accurately predicted based solely on the developing current. Therefore, it is not suitable to detect the developing current as a characteristic value for adjusting image density via DC bias.

[0059] Through this research, the inventors first determined the peak-to-peak voltage (Vpp) of the AC bias based on the value of the developing current. Specifically, they obtained the magnitude of the developing current that caused the Vpp to change, and set the Vpp that resulted in a smaller change in the developing current relative to the change in Vpp as an appropriate Vpp for image formation. Next, the DC bias was performed by changing the value of Vdc while measuring the half-image formed on the photosensitive drum 20 using a density sensor 100 on the intermediate transfer belt 141, and setting the Vdc that yielded an appropriate image density as an appropriate Vdc for image formation. As a result, a stable image was obtained.

[0060] Furthermore, when the charge of the toner changes due to changes in the operating conditions of the image forming apparatus 10 and the surrounding environment, it is necessary to change Vpp. The inventors have newly discovered an alternative characteristic for the change of the charge of the toner, and the timing of the change of Vpp is determined based on the appropriate change of Vdc mentioned above.

[0061] While changes in the charge of the toner can be predicted to some extent based on factors such as the surrounding environment, print rate, and printing status, the actual impact of these changes on the image forming system varies constantly. Therefore, even when adjusting image forming conditions, especially the development bias, based on such predictions, it is difficult to obtain a stable image.

[0062] Here, the inventors of the present invention have newly discovered a "development bias calibration" in an image forming apparatus 10 having a developing apparatus 23 employing a two-component developing method, which allows for setting the peak-to-peak voltages of the DC bias and AC bias of the development bias at appropriate times. In this development bias calibration, the timing of the change in the peak-to-peak voltage of the AC bias is determined by using the DC bias (appropriate Vdc) that changes directly in response to changes in the charge of the toner.

[0063] Figure 4 This is a flowchart of the development bias calibration performed by the calibration execution unit 984 in the image forming apparatus 10 according to this embodiment. The development bias calibration is performed when no image is formed on the sheet body P during non-image formation.

[0064] Specifically, during the development bias calibration, the calibration execution unit 984 determines whether the prescribed calibration start conditions are met (step S01). For example, when the number of prints in the image forming apparatus 10 exceeds a predetermined threshold number, the calibration execution unit 984 performs development bias calibration. Alternatively, the calibration start conditions may be conditions where development bias calibration is performed under conditions of significant changes in the surrounding environment (temperature and humidity) of the image forming apparatus 10. Furthermore, if the above-mentioned calibration start conditions are not met, the calibration execution unit 984 does not perform development bias calibration and terminates the process, waiting for the next execution time.

[0065] Reference Figure 4 When development bias calibration begins, calibration execution unit 984 performs DC calibration (step S01). In this DC calibration, the optimal DC bias (Vdc) (reference DC voltage Vdc1) that will yield the desired image density and image quality in subsequent image forming operations is determined (step S02). Here, DC calibration is performed using a fixed Vpp preset and stored in storage unit 983 or the Vpp used in the preceding image forming operation. The same applies to the settings for other AC biases.

[0066] When performing DC calibration, the calibration execution unit 984 determines the relationship between the determined Vdc1 and the preset lower threshold (VdcL) and upper threshold (VdcH) of Vdc (step S03). Here, if Vdc1 is above the lower threshold VdcL (e.g., 40V) and below the upper threshold VdcH (e.g., 200V) ("Yes" in step S03), the calibration execution unit 984 determines the relationship between the absolute value of the difference between Vdc1 determined in step S02 and Vdc1 (=Vdc0) determined in the previous DC bias calibration, and the preset threshold s (e.g., 50V) (step S04). Here, if the absolute value is below the threshold s ("Yes" in step S04), the calibration execution unit 984 determines that the reference DC voltage Vdc1 determined in step S02 is appropriate and ends the development bias calibration. In the image formation process after DC calibration, the aforementioned reference DC voltage Vdc1 can be used directly, or a voltage equal to Vdc1 multiplied by a specified safety factor (safety rate) can be used. Furthermore, this safety factor can be varied according to the value of the reference DC voltage Vdc1. Specifically, increasing the safety factor when Vdc1 is small and decreasing it when Vdc1 is large helps to minimize variations in the set range, thereby suppressing significant changes in image quality.

[0067] Furthermore, in Figure 4 In step S03, if Vdc1 is less than the lower threshold VdcL (e.g., 40V) or exceeds the upper threshold VdcH (e.g., 200V) (No in step S03), the calibration execution unit 984 determines the correction DC voltage Vdc1' instead of the reference DC voltage Vdc1 determined in step S02 (step S05). For example, the correction DC voltage Vdc1' can be the midpoint between the lower threshold VdcL and the upper threshold VdcH, i.e., (VdcH - VdcL) / 2. Furthermore, if Vdc1 is less than the lower threshold VdcL, the correction DC voltage Vdc1' can be determined from the range of the midpoint to the upper threshold VdcH; if Vdc1 exceeds the upper threshold VdcH, the correction DC voltage Vdc1' can be determined from the range of the lower threshold VdcL to the midpoint.

[0068] Next, the calibration execution unit 984 performs AC calibration (step S06) based on the determined correction DC voltage Vdc1'. In this AC calibration, the optimal AC bias Vpp (reference peak voltage) that can obtain the desired image density and image quality in subsequent image forming operations is determined.

[0069] Next, the calibration execution unit 984 performs DC calibration again (step S07). In this DC calibration, the optimal DC bias Vdc (reference DC voltage Vdc1) that can obtain the desired image density and image quality in subsequent image forming operations is determined using Vpp determined in the preceding AC calibration.

[0070] Thus, in this embodiment, if the Vdc1 determined by DC calibration meets the preset conditions (steps S03 and S04), the subsequent image forming operation is performed with this Vdc1. On the other hand, if the Vdc1 determined by DC calibration does not meet the preset conditions (steps S03 and S04), this Vdc1 is corrected because it does not correspond to the charge of the current toner (step S05). Furthermore, the calibration execution unit 984 presumes that the charge of the current toner also does not correspond to the Vpp determined in the previous AC calibration, and thus performs AC calibration (step S06). Then, the calibration execution unit 984 performs DC calibration again based on the determined Vpp (step S07). Therefore, the DC bias (Vdc) and AC bias Vpp can be set at appropriate times according to the change in the charge of the toner.

[0071] That is, as described above, when Vdc1 is attached to the upper limit threshold VdcH of the preset range, it is difficult to display image density. Therefore, image density is easily displayed when AC calibration is performed. At this time, when AC calibration (Vpp adjustment) is performed with Vdc set to the range between the lower limit threshold VdcL and the middle value of the preset range, after setting this Vpp, a margin range for adjusting image density is obtained in the DC bias (Vdc).

[0072] Similarly, when Vdc is set to the lower limit threshold VdcL of the preset range, image density is easily affected, making it difficult to achieve image density during AC calibration. In this case, when AC calibration (Vpp adjustment) is performed with Vdc set to a range between the upper limit threshold VdcH and the intermediate value of the preset range, a margin for adjusting image density is obtained in the DC bias (Vdc) after setting this Vpp.

[0073] Furthermore, as described above, in this embodiment, the calibration execution unit 984 determines the appropriate Vpp based on the developing current detected by the ammeter 973, and on the other hand, determines the appropriate Vdc based on the image density detected by the density sensor 100 (optical sensor). This is because the stability of the image's saturation density is set as the condition for determining Vpp, and the level (magnitude) of the set saturation density is selected as the condition for determining Vdc. This selection method enables more stable image quality.

[0074] Furthermore, when the image saturation concentration is stabilized as a condition determined by Vpp, it is difficult to accurately measure the image concentration in the saturation region using a concentration sensor 100 composed of an optical sensor. Therefore, it is necessary to determine the image saturation state using methods other than image concentration. Thus, the inventors of this invention have discovered a new method for determining Vpp based on the developing current. The DC calibration and AC calibration described above will be explained in detail below.

[0075] (Regarding DC calibration)

[0076] Figure 5 This is a graph illustrating the relationship between the DC bias Vdc and the image density D during DC calibration performed in the image forming apparatus 10 according to this embodiment. The calibration execution unit 984 starts DC calibration when ( Figure 4 In steps S02 and S04, based on setting the surface potential of the photosensitive drum 20 to VL, the DC bias (Vdc) of the developing bias is sequentially changed to V1, V2, V3, and V4. Toner images corresponding to each DC bias are formed on the photosensitive drum 20 and transferred to the intermediate transfer belt 141. Then, the concentration of each toner image is detected by the concentration sensor 100. The image concentrations at this time (or the reflection concentration detected by the concentration sensor 100, or the output voltage of the concentration sensor 100) are defined as D1, D2, D3, and D4. Then, as... Figure 5 As shown, the DC bias Vdc is set as the horizontal axis and the image density as the vertical axis, and the relationship between Vdc and image density D is made into a first-order approximation. Based on this approximation, the Vdc (Vdc1, reference DC voltage) that can achieve the desired target image density D0 during image formation is determined. Furthermore, if the Vdc1 obtained at this time is less than the preset lower threshold of Vdc (VdcL: for example, 40V), it is replaced by Vdc1 = VdcH. Similarly, if Vdc1 exceeds the preset upper threshold of Vdc (VdcH: for example, 200V), it is replaced by Vdc1 = VdcL. Furthermore, as described above, in DC calibration, DC calibration is performed using a fixed Vpp pre-stored in the storage unit 983 or Vpp (Vppi) used in the preceding image formation operation. Furthermore, the same values ​​as during image formation are used for other AC bias parameters. The Vdc1 determined as described above is used as the reference DC voltage. Additionally, Figure 5 The graph can be plotted with the horizontal axis as ΔV (Vdc - VL).

[0077] (Regarding changes in toner adhesion and developing current)

[0078] When the charge of the toner changes within the developing apparatus 23, or when the developing gap changes due to the vibration of the developing roller 231, both DC and AC bias exhibit characteristics such as changes in the toner's moving force F (= toner charge Q × electric field magnitude E) and variations in image density. Strictly speaking, DC and AC biases have distinct characteristics. In the case of AC bias, increasing Vpp (peak voltage) increases image density, but the increase almost disappears shortly, and further increases result in a decrease in image density. On the other hand, increasing the developing potential difference (Vdc - VL) in DC bias causes a continuous increase in image density, which soon diminishes, but the decrease in image density observed in AC bias is not confirmed. This is presumably because the AC electric field forms a bidirectional electric field (reciprocating electric field) between the photosensitive drum 20 and the developing roller 231 in the developing gap, while the DC electric field forms a unidirectional electric field.

[0079] More specifically, the AC biased reciprocating electric field consists of two opposing electric fields: a developing electric field that supplies toner from the developing roller 231 to the photosensitive drum 20, and a recovering electric field that recovers toner from the photosensitive drum 20 to the developing roller 231. Furthermore, when Vpp increases, both electric fields increase, but the toner supply based on the developing electric field soon reaches its maximum. Subsequently, when Vpp increases further, the amount of toner recovered increases due to the increase in the recovering electric field, but the toner supply based on the developing electric field has already reached its maximum. As a result, depending on the relationship between the toner supply and recovery between the photosensitive drum 20 and the developing roller 231, the final amount of toner developed decreases as Vpp increases.

[0080] (Regarding the relationship between Vpp and developing current)

[0081] In this way, the relationship between DC bias and AC bias and the amount of toner developed can be grasped. On the other hand, it is not very clear what kind of behavior the developing current flowing between the developing roller 231 and the developing bias application section 971 will exhibit when the Vpp of AC bias is increased.

[0082] The reason for this is speculated to be that the developing current generated in the developing slit section NP consists of "toner moving current flowing due to the movement of toner," "brush current of the brushes flowing through the developing section (image section brush current)," and "brush current of the brushes flowing through the non-image section (non-image section brush current)." This is because the toner moving current varies depending on the amount of toner movement; therefore, when Vpp increases, the toner moving current decreases after the increase. However, the image section brush current, being the current flowing through the brushes in the developing slit section NP, tends to increase with increasing Vpp. Furthermore, the non-image section brush current, existing at both ends of the long side of the image forming area in the non-image forming region, tends to increase in the opposite direction as Vpp increases. Therefore, it is unclear how the developing current, which is complexly affected by the combined current of the toner moving current, the image section brush current, and the non-image section brush current, behaves in response to an increase in Vpp.

[0083] Therefore, through dedicated experiments to confirm the behavior of the developing current when the AC bias Vpp of the developing bias is increased, the inventors have newly discovered several patterns in this tendency. That is, it is clearly known that there are patterns in which the developing current (DC current) rises when the AC bias Vpp is increased, but soon reaches a point of change in its gradient, and then the developing current also rises slowly, and in the opposite pattern in which the developing current decreases from the point of change.

[0084] The inventors have refocused on patterns based on such developing currents, setting the AC bias Vpp in a region where image density variations are small. As a result, even with changes in toner charge and developing gap, variations in image density can be reduced. The details of the AC calibration used to set such Vpp are explained below.

[0085] (Regarding AC calibration)

[0086] Figure 6 This is a flowchart of AC calibration performed in the image forming apparatus 10 according to this embodiment. Figure 7 This is a flowchart of the first approximation determination step (first approximation determination action) of AC calibration performed in the image forming apparatus 10 according to this embodiment. Figure 8 This is a flowchart of the second approximation determination step (second approximation determination action) of AC calibration performed in the image forming apparatus 10 according to this embodiment.

[0087] In this embodiment, Figure 4In step S02, the calibration execution unit 984 performs AC calibration. AC calibration is a mode that determines the reference peak-to-peak voltage (target voltage) as a reference for the peak-to-peak voltage (Vpp) of the AC voltage applied to the developing roller 231 during the image forming operation.

[0088] When AC calibration begins, the calibration execution unit 984 sequentially executes the first approximation determination step ( Figure 6 Step S11), the second approximation determination step ( Figure 6 Step S12), Target Voltage Determination Step ( Figure 6 Step S13).

[0089] Reference Figure 7 The first approximation determination step will be described in detail below. At the start of the first approximation determination step, the calibration execution unit 984 acquires information related to the first measurement range stored in the storage unit 983. The first measurement range is information related to the range and interval of Vpp, the AC bias applied to the developing roller 231 in the first approximation determination step. In this embodiment, as an example, the calibration execution unit 984 acquires four pieces of information related to the peak voltage for the first measurement. As a result, the first measurement range in the first approximation determination step is determined (step S21).

[0090] Next, the calibration execution unit 984 forms a measurement latent image consisting of a solid image on the photosensitive drum 20. By applying a development bias to the developing roller 231, the measurement latent image is developed into a measurement toner image. Specifically, similar to image formation, the photosensitive drum 20 is rotated, and the peripheral surface of the photosensitive drum 20 is uniformly charged to 250V by the charging device 21. Furthermore, as an example, the axial (width direction) charging range of the photosensitive drum 20 is set to 322mm. Then, the potential of a portion of the photosensitive drum 20 is reduced to 10V by exposure light irradiated from the exposure device 22, forming the measurement latent image on the photosensitive drum 20. In this embodiment, the width of the measurement latent image is set to 287mm relative to the sheet width of 297mm (A4 transverse), and the width of the magnetic brush of the developing roller is set to 304mm. The difference between the width of the magnetic brush and the width of the measurement latent image is the area through which the magnetic brush current flows in the non-image section.

[0091] On the other hand, an AC bias with a frequency of 10 kHz and a duty of 50% is superimposed on the developing roller 231 at a DC voltage of 150V. Furthermore, the Vpp of the AC bias is sequentially set to the four first measurement peak voltages mentioned above. As a result, regarding each first measurement peak voltage, when the latent image for measurement is developed into a toner image for measurement by the developing roller 231, the ammeter 973 measures the DC component (DC current Idc) of the developing current flowing between the developing roller 231 and the developing bias application section 971 (step S22). As a result, four developing currents corresponding to the four first measurement peak voltages are obtained, and four sets of data related to the first measurement peak voltages and the developing current are obtained. Furthermore, for the calculation of the developing current, it is preferable to perform the average current over one revolution or more of the rotation of the developing roller 231, and more preferably to perform the average over revolutions that are integer multiples of one revolution.

[0092] Next, the calibration execution unit 984 uses a linear regression to regress the relationship between the four peak voltages of the first measurement and the four development currents, and calculates the correlation coefficient R (step S23). As an example, the calibration execution unit 984 calculates the linear regression using the least squares method to obtain the correlation coefficient R.

[0093] Next, the calibration execution unit 984 compares the obtained correlation coefficient R with the threshold R1 pre-stored in the storage unit 983 (step S24). For example, the threshold R1 is set to 0.90. Here, if the threshold R1 ≤ the correlation coefficient R ("Yes" in step S24), the calibration execution unit 984 determines the linear regression equation as a first approximation (step S25). On the other hand, if the threshold R1 > the correlation coefficient R in step S24 ("No" in step S24), the calibration execution unit 984 recalculates the correlation coefficient R based on the remaining three data points from the four sets of data after removing the data with the largest Vpp. Then, the calibration execution unit 984 performs steps S24 and S25 in the same manner as above. Furthermore, if the relationship of threshold R1 ≤ correlation coefficient R is still not satisfied after removing the data with the largest Vpp in step S26, the calibration execution unit 984 can further remove a portion of the data and repeat the steps, or it can interrupt the AC calibration execution and use the result of the previous AC calibration.

[0094] As described above, the second approximation determination step begins at the end of the first approximation determination step. (Refer to...) Figure 8The second approximation determination step will be described in detail. When the second approximation determination step begins, the calibration execution unit 984 acquires information related to the second measurement range stored in the storage unit 983. The second measurement range is information related to the range and interval of Vpp, the AC bias applied to the developing roller 231 in the second approximation determination step. In this embodiment, as an example, the calibration execution unit 984 acquires information related to the voltage between the three second measurement peaks. As a result, the second measurement range in the second approximation determination step is determined (step S31). Furthermore, the minimum value of the second measurement range (the three second measurement peaks) is set to be larger than the maximum value of the first measurement range (the four first measurement peaks).

[0095] Next, calibration execution unit 984 and Figure 7 Similar to step S12, a latent image for measurement is formed on the photosensitive drum 20. A developing bias is applied to the developing roller 231, and the latent image for measurement is developed into a toner image for measurement. At this time, an AC bias with a frequency of 10 kHz and a duty cycle of 50% is superimposed on the developing roller 231 at a DC voltage of 150V. The Vpp of the AC bias is sequentially set to the three second measurement peak voltages. As a result, regarding each second measurement peak voltage, when the latent image for measurement is developed by the developing roller 231, the ammeter 973 measures the DC component (DC current Idc) of the developing current flowing between the developing roller 231 and the developing bias application section 971 (step S32). As a result, three developing currents corresponding to the three second measurement peak voltages are obtained, and three sets of data related to the second measurement peak voltages and the developing current are obtained.

[0096] Next, the calibration execution unit 984 uses a linear formula (the first determination approximation formula) to regress the relationship between the three second measurement peak voltages and the three development currents, and calculates its slope L (step S33). As an example, the calibration execution unit 984 calculates the linear formula using the least squares method to obtain the slope L.

[0097] Next, the calibration execution unit 984 compares the obtained slope L with the threshold L1 pre-stored in the storage unit 983 (step S34). As an example, the threshold L1 is set to 0 (zero). Here, if the slope L < the threshold L1 ("Yes" in step S34), the calibration execution unit 984 determines the linear regression equation as the second approximation equation (step S35). On the other hand, if the slope L ≥ the threshold L1 in step S34 ("No" in step S34), the calibration execution unit 984 calculates the average value of Vpp of the three sets of data and sets the average value as the second approximation equation, which is a linear expression relative to the change in voltage between peaks (step S36).

[0098] exist Figure 7 , Figure 8 When the first approximation determination step and the second approximation determination step shown are completed, the calibration execution unit 984 executes the target voltage determination step. Figure 6 Step S13). In the target voltage determination step, the calibration execution unit 984 determines the peak-to-peak voltage at the intersection of the first approximation and the second approximation as the reference peak-to-peak voltage (target voltage VT). As a result, the peak-to-peak voltage during image formation can be set near the boundary (peak-to-peak vicinity) of the relationship between the peak-to-peak voltage and the developing current in the first and second measurement ranges, respectively. In addition, in this embodiment, the reference peak-to-peak voltage determined as described above, which is set to 1.2 times including a predetermined safety factor, is applied as the actual peak-to-peak voltage during image formation.

[0099] Figure 9 , Figure 10 and Figure 11 These are graphs showing the relationship between Vpp, the AC calibration performed in the image forming apparatus 10 according to this embodiment, and the developing current. In each graph, the developing current is represented by the vertical axis (Y-axis), and Vpp is represented by the horizontal axis (X-axis).

[0100] Tables 1 and 2 show... Figure 9 The relationship between Vpp and developing current is shown in the first and second measurement ranges.

[0101] (Table 1)

[0102]

[0103] (Table 2)

[0104]

[0105] exist Figure 9 In Figure 7 In the first approximation determination step shown, the linear form of y = 0.01x + 7 is calculated as the first approximation. On the other hand, in... Figure 8In the second approximation determination step shown, the slope L is negative (L < L1 = 0). Therefore, in step S35, the linear expression y = -0.0075x + 20.767 is calculated as the second approximation. As a result, in the target voltage determination step S13, as the intersection of the first and second approximations, Vpp = target voltage VT = 787V is calculated. By setting 1.2 as a safety factor, Vpp = 787 × 1.2 = 944 (V) is selected for the image forming operation. Furthermore, the safety factor can be changed according to the target voltage VT. Increasing the safety factor when the target voltage VT is low and decreasing it when the target voltage VT is high reduces the variation in the set Vpp, which also helps to suppress large changes in image quality.

[0106] Tables 3 and 4 show... Figure 10 The relationship between Vpp and developing current is shown in the first and second measurement ranges.

[0107] (Table 3)

[0108]

[0109] (Table 4)

[0110]

[0111] exist Figure 10 In Figure 7 In the first approximation determination step shown, the linear expression y = 0.01x + 7 is calculated as the first approximation. On the other hand, in Figure 8 In the second approximation determination step shown, the slope L is positive (L > L1 = 0). Therefore, in step S36, the average value of the developing current is calculated as a linear expression for the second approximation to calculate y = 14.1. As a result, in the target voltage determination step S13, as the intersection of the first and second approximations, Vpp = target voltage VT = 710V is calculated. A safety factor of 1.2 is set, and thus Vpp = 710 × 1.2 = 852 (V) is selected for the image forming operation.

[0112] Tables 5 and 6 show... Figure 11 The relationship between Vpp and developing current is shown in the first and second measurement ranges.

[0113] (Table 5)

[0114]

[0115] (Table 6)

[0116]

[0117] exist Figure 11 In Figure 7 In the first approximation determination step shown, the linear expression y = 0.0042x + 6.71 is calculated as the first approximation. On the other hand, in Figure 8 In the second approximation determination step shown, the slope L is positive (L > L1 = 0). Therefore, in step S36, the average value of the developing current is calculated as a linear expression of y = 12.4 in the second approximation. As a result, in the target voltage determination step S13, as the intersection of the first and second approximations, Vpp = target voltage VT = 1310V is calculated. A safety factor of 1.2 is set, and thus Vpp = 1310 × 1.2 = 1572 (V) is selected for the image forming operation.

[0118] (The reason why the developing current (DC component) has peak intervals (points of change))

[0119] Next, as with the data above, we will infer the reason why the developing current (DC component) has peak intervals (points of change) relative to Vpp. As mentioned above, the developing current consists of "toner moving current + image section brush current + non-image section brush current." However, when the developing current is obtained, both "toner moving current + image section brush current" flow in the portion of the electrostatic latent image corresponding to the image section (solid image portion), while in the white background portion at the width end, only the "non-image section brush current" flows in the opposite direction to the image section. Therefore, when Vpp increases, the non-image section brush current in this white background portion increases, and the total developing current decreases.

[0120] Furthermore, the image section brush current also increases accordingly with the increase of Vpp. However, the toner layer formed by the toner adhering to the surface of the photosensitive drum 20 becomes a resistive layer, suppressing the extreme increase of the image section brush current. On the other hand, in the white background area, a small amount of toner moves towards the sleeve surface of the developing roller 231, but its amount is very small compared to the image section. Therefore, the toner layer adhering to the sleeve surface does not become a relatively high resistance compared to the image section. As a result, the non-image section brush current in the white background area increases significantly with the increase of Vpp. This brush current flows in the opposite direction to the toner movement current. Therefore, it is speculated that the developing current has a point of variation (between peaks).

[0121] Through dedicated and repeated experiments, the inventors have newly discovered the aforementioned relationship between the developing current and Vpp. Furthermore, they found that this phenomenon is more likely to occur when the resistivity of the carrier is lower, particularly on parallel plates with a 1mm gap (area 240mm²). 2 When 0.2g of carrier is filled between the two sides and a voltage of 1000V is applied, the current flowing through the carrier is used to determine the resistance value. This phenomenon is significant below 10 to the power of 9 ohms.

[0122] That is, when a two-component developer is sandwiched between the photosensitive drum 20 and the developing roller 231, and a measurement latent image is formed at the center of the electrostatic latent image in the axial (width direction), with white background portions arranged at both ends, the aforementioned variation point occurs at the boundary between the first measurement range and the second measurement range in this embodiment. In particular, the phenomenon that the slope distribution of the second approximation is distributed over a wide range of positive and negative values ​​is due to the current flowing in the opposite direction to the central portion at both ends of the axial direction of the developing roller 231. In particular, in this embodiment, the range of the magnetic brush on the developing roller 231 is set narrower than the charged range on the photosensitive drum 20 in the axial direction, and consequently, the range of the image portion (solid image portion) formed in the measurement latent image on the photosensitive drum 20 is set narrower than the range of the magnetic brush. As a result, as described above, areas where current flows through the magnetic brush in the opposite direction to the image portion are formed at both ends of the axial direction of the developing roller 231. Furthermore, this phenomenon, inherent in the developing gap where the discharge current does not occur between the photosensitive drum 20 and the charged roller that abuts its peripheral surface, was discovered through the aforementioned repeated experiments. In particular, since there is no developer between the charged roller and the photosensitive drum 20 where the resistance of the carrier is the main factor causing the change, it is difficult to produce the characteristic of a decrease in current shortly after the peak voltage is increased.

[0123] As described above, in this embodiment, the calibration execution unit 984 performs developer bias calibration (bias condition determination mode) when the prescribed execution conditions are met. Developer bias calibration includes DC calibration (direct current voltage determination mode) and AC calibration (peak voltage determination mode).

[0124] In DC calibration, the calibration execution unit 984 determines a reference DC voltage, which serves as a reference for the DC voltage used for developing bias applied to the developing roller 231 during subsequent image forming operations, based on the concentration of the toner image detected by the concentration sensor 100. Furthermore, in AC calibration, the calibration execution unit 984 determines a reference peak-to-peak voltage, which serves as a reference for the peak-to-peak voltage used as a reference for the AC voltage used for developing bias applied to the developing roller 231 during subsequent image forming operations, based on the DC component of the developing current detected by the ammeter 973 when the toner develops the latent image for measuring into a toner image for measuring by applying a developing bias to the developing roller 231.

[0125] More specifically, in each DC calibration, the calibration execution unit 984 controls the photosensitive drum 20, the charging device 21, the exposure device 22, and the developing device 23, while forming multiple measurement toner images on the photosensitive drum 20. Then, the calibration execution unit 984 applies the development bias to the developing roller 231 corresponding to the predetermined measurement latent image formed on the photosensitive drum 20, and develops the measurement latent image into a measurement toner image using toner, which is then transferred to the photosensitive drum 20 and the intermediate transfer belt 141. Subsequently, based on the concentration of each measurement toner image on the intermediate transfer belt 141 detected by the concentration sensor 100, a reference DC voltage Vdc1, which serves as the reference for the DC voltage of the development bias applied to the developing roller 231 during the image forming operation, is determined.

[0126] That is, during DC calibration, the calibration execution unit 984 determines a reference DC voltage Vdc1, which serves as the reference for the DC voltage used for the developing bias in subsequent AC calibration and image forming operations. Furthermore, in the AC calibration and image forming operations following DC calibration, the aforementioned reference DC voltage can be used directly, or the reference DC voltage multiplied by a predetermined safety factor can be used.

[0127] Then, the calibration execution unit 984 determines whether AC calibration needs to be performed based on the reference DC voltage Vdc1 determined in the DC calibration. Figure 4 Steps S03 and S04).

[0128] Based on this configuration, even when image formation conditions such as the distance between the developing roller 231 and the photosensitive drum 20 (DS gap), the charge of the toner, and the resistance of the carrier change, the calibration execution unit 984 can set the DC bias and AC bias (Vpp) corresponding to each image formation condition by performing developing bias calibration as needed. As a result, the peak-to-peak voltages of the developing bias DC bias and AC bias, which both affect image defects, can be set stably, thereby stabilizing and improving image quality. Furthermore, the reference DC voltage Vdc1 can be used as a substitute characteristic value for the charge of the toner to effectively determine whether AC calibration is required. Therefore, the peak-to-peak voltages of the developing bias DC bias Vdc and AC bias Vpp can be set at appropriate times.

[0129] Furthermore, in this embodiment, when a new reference peak-to-peak voltage Vpp is determined during AC calibration, the calibration execution unit 984 performs DC calibration again based on this reference peak-to-peak voltage Vpp. Figure 4 Step S07) prevents deviations in image density.

[0130] Furthermore, if the difference between the reference DC voltage determined by the nth (n is a natural number) DC calibration and the reference DC voltage determined by the (n+1)th DC calibration exceeds a preset threshold, the calibration execution unit 984 determines that AC calibration needs to be performed after the (n+1)th DC calibration. If the reference DC voltage Vdc1, which serves as a substitute characteristic value for the toner's charge, changes significantly from the previous Vdc0, it is presumed that the toner's charge has changed significantly, and AC calibration can be reliably performed. Additionally, the aforementioned nth calibration can be either a case that simply represents the number of DC calibration operations, or a case that represents the number of DC calibration operations performed continuously with AC calibration. In the case where the nth calibration simply represents the number of DC calibration operations, the probability of the reference DC voltage changing slightly each time exceeding the preset threshold is low, but it adapts to the actual change in the toner's charge. Therefore, for example, if the reference DC voltage increases sharply while decreasing slightly, the AC calibration operates accordingly. On the other hand, when the nth time mentioned above represents the number of DC calibration operations performed consecutively with AC calibration, since the results of DC calibration alone are ignored, even if the reference DC voltage gradually changes due to performing DC calibration alone, it is possible to accurately determine whether the reference peak voltage needs to be changed regardless of this change. In this case, even if the aforementioned drastic change occurs, since the change is based on the reference DC voltage at the time of the last DC calibration, AC calibration will not be performed. However, the probability of such an event occurring is extremely small, so there is no problem.

[0131] Furthermore, in this embodiment, when the reference DC voltage Vdc1, which is a substitute characteristic value of the charge of the toner, deviates from the preset allowable range (VdcL to VdcH), the calibration execution unit 984 assumes that the charge of the toner has changed significantly and can reliably perform AC calibration.

[0132] In particular, when the reference DC voltage Vdc1, which serves as a substitute characteristic value for the charge of the toner, is lower than a preset lower threshold VdcL, the calibration execution unit 984 sets the DC voltage for development bias to a value greater than the determined reference DC voltage Vdc1 during the subsequent AC calibration. Therefore, AC calibration can be stably performed based on a larger DC voltage corresponding to the charge of the toner. At this time, by setting the DC voltage to a value within the range from the midpoint between the lower threshold VdcL and the upper threshold VdcH to the upper threshold VdcH, AC calibration can be performed more stably. Furthermore, it is more preferable to set the DC voltage to the upper threshold VdcH.

[0133] Furthermore, if the reference DC voltage Vdc1, which serves as a substitute characteristic value for the charge of the toner, is greater than a preset upper threshold VdcH, then in the subsequent AC calibration, the calibration execution unit 984 sets the DC voltage for development bias to a value less than the determined reference DC voltage Vdc1. Therefore, AC calibration can be stably performed based on a smaller DC voltage corresponding to the charge of the toner. At this time, by setting the DC voltage to a value within a range from the lower threshold VdcL to the midpoint between the lower threshold VdcL and the upper threshold VdcH, AC calibration can be performed more stably. Furthermore, it is more preferable to set the DC voltage to the lower threshold VdcL.

[0134] Furthermore, in this embodiment, during AC calibration (peak voltage determination mode), a reference peak voltage is set at the intersection of a first approximation and a second approximation representing the relationship between the peak voltage of the AC bias and the developing current within each range of the first and second measurement ranges. Since there are points of variation in the relationship between the peak voltage of the AC bias and the developing current near these intersection points, the developing current is less susceptible to the influence of the slope of the first approximation within the first measurement range, thus suppressing changes in image density due to variations in toner charge and developing gap. Furthermore, the reference peak voltage is set in regions where the slope of the second approximation is less than a predetermined threshold due to variations in carrier resistance, etc., and in regions where the developing current easily decreases with an increase in peak voltage. As a result, an AC bias that provides a stable image density during image formation can be set. Additionally, the actual peak voltage during image formation, relative to the reference peak voltage, can be obtained by using the original value of the reference peak voltage, or by multiplying the reference peak voltage by a certain ratio, or by adding a certain value, or by multiplying by a certain ratio and adding a certain value.

[0135] Furthermore, in this embodiment, the calibration execution unit 984 determines the first approximation formula using the least squares method based on the DC component of the developing current obtained at each of the at least three first measurement peak voltages included in the first measurement range. According to this structure, the first approximation formula can be determined through simple calculation based on the first measurement peak voltages included in the first measurement range.

[0136] Furthermore, in this embodiment, the calibration execution unit 984 sets the average value of the DC component of the developing current obtained at the at least three second measurement peak voltages within the second measurement range as a linear expression with a fixed change relative to the peak voltage as the second approximation if the slope of the first approximation (a first approximation determined by least squares) is greater than a preset first threshold L1. If the slope of the first approximation is less than the first threshold L1, the first approximation is set as the second approximation. According to this structure, in the process of determining the second approximation, whose slope is easily changed due to factors such as the resistance of the carrier, a more appropriate approximation can be selected as the second approximation, corresponding to the slope of the first approximation.

[0137] Furthermore, in this embodiment, the interval between the plurality of first measurement peak voltages within the first measurement range and the interval between the plurality of second measurement peak voltages within the second measurement range are respectively set to be less than the interval between the maximum value of the first measurement range and the minimum value of the second measurement range. According to this structure, by clearly distinguishing between the first and second measurement ranges, and further subdividing the interval between peak voltages within each measurement range, the determination accuracy of the first approximation and the second approximation can be improved.

[0138] In the first approximation determination operation, if the correlation coefficient of the first approximation is less than a preset second threshold, the calibration execution unit 984 determines the first approximation based on the DC component of the developing current of the peak-to-peak voltage remaining after removing at least one peak-to-peak voltage from the at least three first measurement peak-to-peak voltages. According to this structure, when the correlation coefficient is small during the determination of the first approximation, a more accurate first approximation can be determined by removing data from at least one peak-to-peak voltage.

[0139] In particular, during the first approximation determination operation, if the correlation coefficient of the first approximation is less than a preset second threshold R1, the calibration execution unit 984 determines the first approximation based on the DC component of the developing current of the peak-to-peak voltage remaining after removing the largest peak-to-peak voltage under the at least three first measurement peak-to-peak voltages. According to this structure, when the correlation coefficient is small during the determination of the first approximation, a more accurate first approximation can be determined by removing data of peak-to-peak voltages close to the second measurement range.

[0140] Furthermore, the calibration execution unit 984 pre-removes the maximum or minimum inter-peak voltage removed in the second approximation determination operation from the second measurement range, and executes the next bias condition determination mode. According to this structure, by removing the data removed in the previous bias condition determination mode first in the next bias condition determination mode, the mode execution time can be shortened, and a high-accuracy reference inter-peak voltage can be determined.

[0141] Furthermore, in this embodiment, the number of the at least three first inter-peak voltages for measurement within the first measurement range is set to be greater than the number of the at least three second inter-peak voltages for measurement within the second measurement range. According to this structure, by obtaining a relatively large amount of data within the first measurement range where the slope of the first approximation is positive and the developing current is prone to significant changes, a more accurate reference inter-peak voltage can be determined.

[0142] Furthermore, according to this embodiment, the point of change in the balance (total of each current) of the toner moving current, the image unit brush current, and the non-image unit brush current can be predicted by the intersection of two approximation formulas, and the reference peak voltage can be determined.

[0143] Furthermore, in this embodiment, the setting of the reference peak voltage is determined based on the developing current. Conventionally, it has been considered to measure image density and determine the reference peak voltage based on its stability; however, for example, density sensors that measure image density on the photosensitive drum 20 and the intermediate transfer belt 141 tend to have reduced accuracy when image density increases, making it impossible to accurately detect image density within the second measurement range of this invention. Therefore, the data used to determine the reference peak voltage within the first and second measurement ranges is preferably the developing current.

[0144] Furthermore, since the developing current tends to vary significantly within the first measurement range, it is preferable to perform the measurement over the widest possible range of inter-peak voltage. On the other hand, within the second measurement range, when the variation in the developing current is relatively small and the inter-peak voltage is set too high, leakage may occur in the developing gap. Therefore, it is preferable that the second measurement range is narrower than the first measurement range, and that fewer measurement points are set. As a result, the mode execution time can be shortened, and the consumption of toner dosage can be suppressed.

[0145] Alternatively, the developing current can be measured within the circuitry of the developing bias application section 971. While the toner movement current can also be measured on the photosensitive drum 20 side, it is impossible to separate this current because the photosensitive drum 20 also contains current flowing in from the transfer roller. Therefore, it is preferable to measure the developing current on the developing bias application section 971 side.

[0146] Furthermore, in this embodiment, during DC calibration, the calibration execution unit 984 determines the DC voltage corresponding to a specified target concentration as the reference DC voltage Vdc1 based on the relationship between multiple measuring DC voltages and the concentrations of multiple measuring toner images. Therefore, the reference DC voltage Vdc1 can be easily and stably determined.

[0147] The embodiments of the present invention have been described above, but the present invention is not limited thereto. For example, the following modified embodiments may be adopted.

[0148] (1) In the above embodiment, it is described that the surface of the developing roller 231 is subjected to rolling groove processing + shot peening processing. However, it can also be a developing roller with a concave shape (pit) on the surface of the developing roller 231 and shot peening processing, or a developing roller subjected to shot peening processing, rolling groove processing, concave shape (pit) processing, or electroplating processing.

[0149] (2) For example Figure 1 As shown, when the image forming apparatus 10 has multiple developing devices 23, the AC calibration involved in the above embodiment can also be performed by one or two developing devices 23, and the results can be used in other developing devices 23.

[0150] (3) Figure 12 This is a flowchart of the second approximation determination step of AC calibration performed in an image forming apparatus according to a modified embodiment of the present invention. Figure 13 This is a flowchart of the second approximation determination step. In this modified embodiment, compared with the previous embodiment, in Figure 12 The steps S32A, S32B, and S32C differ. Specifically, in step S32, the developing DC current Idc is measured. In this modified embodiment, the same as the first approximation determination step is used, obtaining four developing currents corresponding to the four second measurement peak voltages, and obtaining four sets of data related to the second measurement peak voltages and developing currents.

[0151] Here, the calibration execution unit 984 calculates the correlation coefficient R in the same way as the first approximation determination step (step S32A). Then, it compares the correlation coefficient R with the threshold R2 pre-stored in the storage unit 983 (step S32B). As an example, the threshold R2 is set to 0.90. Here, if the threshold R2 ≤ the correlation coefficient R ("Yes" in step S32B), the calibration execution unit 984 calculates the slope L in step S33, as in the previous embodiment, and calculates the second approximation in step S35 or S36 based on the determination result in step S34. On the other hand, if R2 > R in step S32B ("No" in step S32B), the calibration execution unit 984 determines the corrected correlation coefficient R in step S32C.

[0152] Reference Figure 13 When starting the step of determining the corrected correlation coefficient R, the calibration execution unit 984 calculates the correlation coefficient Rm based on the remaining three data points from the four sets of data after removing the data with the largest Vpp (step S41). Next, the calibration execution unit 984 calculates the correlation coefficient Rn based on the remaining three data points from the four sets of data after removing the data with the smallest Vpp (step S42). Then, the calibration execution unit 984 compares the calculated correlation coefficients Rm and Rn and selects the larger correlation coefficient as the corrected correlation coefficient R (step S43). Afterwards, it returns to... Figure 12 Based on the selected corrected correlation coefficient R, the processing after step S32B is repeated.

[0153] Thus, in this modified embodiment, during the second approximation determination step, when the correlation coefficient is small, data with a high correlation coefficient is selected, and the second approximation is set based on this data. Therefore, by removing data from at least one peak-to-peak voltage interval, a more accurate second approximation can be determined.

[0154] In particular, the calibration execution unit 984 compares the correlation coefficient Rm of the second determination approximation with the correlation coefficient Rn of the third determination approximation, and determines the determination approximation with the larger correlation coefficient between the second and third determination approximations as the second approximation. The second determination approximation is determined based on the DC component of the developing current of the peak-to-peak voltage remaining after removing the largest peak-to-peak voltage for the at least three second measurement peak-to-peak voltages. The third determination approximation is determined based on the DC component of the developing current of the peak-to-peak voltage remaining after removing the smallest peak-to-peak voltage for the at least three second measurement peak-to-peak voltages. According to this structure, when the correlation coefficient is small during the determination of the second approximation, a more accurate second approximation can be determined by removing data from either the smallest peak-to-peak voltage within the second measurement range that is closest to the first measurement range, or the largest peak-to-peak voltage that is prone to noise that could cause discharge leakage.

[0155] (4) In the above implementation method, based on Figure 4 The flowchart illustrates the development bias calibration, which includes DC calibration and AC calibration respectively, but the implementation of the development bias calibration is not limited to this. Figure 4 For example, it can be done Figure 4 After performing AC bias and DC bias calibration in steps S06 and S07 respectively, the following steps are performed again. Figure 4 The steps following S03. In this case, it can be confirmed in steps S03 and S04 whether the reference DC voltage Vdc1 determined in step S07 is an appropriate value. Furthermore, if Vdc1 is not within the range of the lower threshold VdcL and the upper threshold VdcH in step S03, the Vdc1 determined in the previous step S07 is no longer an appropriate value, therefore the calibration execution unit 984 displays calibration failure information on a display unit (not shown) of the image forming apparatus 10. As a result, it is preferable that the maintenance personnel of the image forming apparatus 10 perform the confirmation of related components and the maintenance of the developing apparatus 23. This maintenance includes replacing the developer, etc.

[0156] In addition, the data is subjected to the development bias calibration as described in the previous embodiments under the following conditions.

[0157] (General conditions)

[0158] • Printing speed: 55 pages / minute

[0159] • Photosensitive drum 20: Amorphous silicon photoreceptor (α-Si)

[0160] • Developing roller 231: 20mm outer diameter, surface shape with rolling groove processing + shot peening (forming 80 rows of recesses (grooves) along the circumference)

[0161] • Restricting scraper 234: Made of SUS430, magnetic, 1.5mm thick

[0162] • Developer delivery rate after scraper 234: 250g / m 2

[0163] • Circumferential speed of developing roller 231 relative to photosensitive drum 20: 1.8 (track direction at the opposite position)

[0164] • Distance between photosensitive drum 20 and developing roller 231: 0.25mm

[0165] • Background potential V0 of the white base of the photosensitive drum 20: +250V

[0166] • Image section potential VL of photosensitive drum 20: +10V

[0167] • Developing bias of developing roller 231: AC voltage rectangular wave with frequency = 10kHz, duty = 50% (Vpp adjusted according to experimental conditions), Vdc (DC voltage) = 150V

[0168] • Toner: Positively polarized toner, volume average particle size 6.8 μm, toner concentration 6%

[0169] • Carrier: Ferrite / resin coated carrier with a volume average particle size of 35 μm.

[0170] (Regarding developer)

[0171] The toner, whether it be a pulverized toner or a core-shell toner, has been confirmed to have the same effect. Furthermore, regarding toner concentration, it has been confirmed that a concentration in the range of 3% to 12% produces the same effect. Since finer magnetic brushes more easily generate significant movement of the toner caused by the alternating electric field, it is preferable that the volume average particle size of the carrier is 45 μm or less, more preferably 30 μm or more but less than 40 μm. Furthermore, a resin carrier with a lower true specific gravity than the ferrite carrier is preferred.

[0172] (Regarding the medium)

[0173] The carrier is formed by coating a ferrite core with a volume average particle size of 35 μm with materials such as silicon and fluorine, and is specifically manufactured according to the following steps: A coating solution is prepared by dissolving 20 parts by mass of silicone resin KR-271 (manufactured by Shin-Etsu Chemical Co., Ltd.) in 200 parts by mass of toluene in 1000 parts by mass of the carrier core EF-35 (manufactured by Powdertech Co., Ltd.). Then, after spraying the coating solution through a fluidized bed coating apparatus, it is heat-treated at 200°C for 60 minutes to obtain the carrier. In this coating solution, a conductive agent and a charge control agent are mixed in a range of 0 to 20 parts per 100 parts of coating resin and dispersed to adjust resistance and charge.

Claims

1. An image forming apparatus capable of performing an image forming operation of forming an image on a sheet, characterized by comprising: include: Like a carrier, it rotates and has a surface that allows the formation of electrostatic latent images and carries the toner image after the electrostatic latent images are revealed by the toner; A charging device that charges the image carrier to a predetermined charging potential; An exposure device is positioned downstream of the image carrier in the direction of rotation, relative to the charging device, and forms the electrostatic latent image by exposing the surface of the image carrier, which is charged to the charged potential, according to predetermined image information. A developing apparatus is disposed opposite to an image carrier in a predetermined developing slit portion downstream of the exposure apparatus in the direction of rotation. The developing apparatus includes a developing roller that rotates and has a circumferential surface that carries a developer composed of a toner and a carrier. The toner image is formed by supplying the toner to the image carrier. The transfer section transfers the toner image carried on the image carrier onto the sheet body; The developing bias application section is capable of applying a developing bias, in which an AC voltage is superimposed on a DC voltage, to the developing roller; The current detection unit is capable of detecting the DC component of the developing current flowing between the developing roller and the developing bias application unit; The concentration detection unit is capable of detecting the concentration of the toner image; as well as The bias condition determination unit executes a bias condition determination mode, which determines a reference voltage that serves as a reference for the inter-peak voltage of the AC voltage applied to the developing roller during the image forming operation and the DC component of the developing current detected by the current detection unit or the concentration of the measuring toner image detected by the concentration detection unit. The bias condition determination unit, as the bias condition determination mode, can execute the following modes respectively: The DC voltage determination mode determines a reference DC voltage as a reference for the DC voltage of the developing bias applied to the developing roller during the image forming operation, based on the concentration of the toner image detected by the concentration detection unit. as well as In the peak-to-peak voltage determination mode, when the latent image for measurement is developed into an image for measurement toner by the toner through the development bias applied to the developing roller, a reference peak-to-peak voltage is determined based on the DC component of the developing current detected by the current detection unit, which serves as a reference for the peak-to-peak voltage of the AC voltage of the development bias applied to the developing roller during the image forming operation. The bias condition determination unit determines whether the peak voltage determination mode needs to be executed based on the reference DC voltage determined in the DC voltage determination mode.

2. The image forming apparatus according to claim 1, characterized in that, After executing the peak voltage determination mode, the bias condition determination unit can execute the DC voltage determination mode again by applying a development bias to the developing roller, which includes a peak voltage set according to the determined reference peak voltage.

3. The image forming apparatus according to claim 1, characterized in that, If the difference between the reference DC voltage determined by the nth DC voltage determination mode and the reference DC voltage determined by the (n+1)th DC voltage determination mode is greater than a preset threshold, the bias condition determination unit determines that the peak voltage determination mode needs to be executed after the (n+1)th DC voltage determination mode is executed, where n is a natural number.

4. The image forming apparatus according to claim 1, characterized in that, If the reference DC voltage determined by the DC voltage determination mode is less than a preset lower threshold, or if the determined reference DC voltage is greater than a preset upper threshold, the bias condition determination unit determines that the peak voltage determination mode needs to be executed after the DC voltage determination mode is executed.

5. The image forming apparatus according to claim 4, characterized in that, If the reference DC voltage determined by the DC voltage determination mode is less than the lower threshold, the bias condition determination unit will set the DC voltage of the developing bias applied to the developing roller in the peak voltage determination mode executed after the DC voltage determination mode to a value greater than the determined reference DC voltage.

6. The image forming apparatus according to claim 5, characterized in that, The bias condition determination unit sets the DC voltage of the developing bias applied to the developing roller in the peak voltage determination mode executed after the execution of the DC voltage determination mode to a value that is included in the range from the midpoint between the lower threshold and the upper threshold to the upper threshold.

7. The image forming apparatus according to claim 4, characterized in that, If the reference DC voltage determined by the DC voltage determination mode is greater than the upper limit threshold, the bias condition determination unit will set the DC voltage of the developing bias applied to the developing roller in the peak voltage determination mode executed after the DC voltage determination mode to a value smaller than the determined reference DC voltage.

8. The image forming apparatus according to claim 7, characterized in that, The bias condition determination unit sets the DC voltage of the developing bias applied to the developing roller in the peak voltage determination mode, which is executed after the DC voltage determination mode, to a value that is included in the range from the midpoint between the lower threshold and the upper threshold to the lower threshold.

9. The image forming apparatus according to claim 1, characterized in that, The bias condition determination unit performs the following actions in the peak voltage determination mode: The first approximation determines the action by setting the peak-to-peak voltage of the AC voltage used for development bias to at least three first measurement peak-to-peak voltages that are included in a predetermined first measurement range, obtaining the DC component of the development current, and determining the first approximation. The first approximation is a first-order approximation that represents the relationship between the first measurement peak-to-peak voltage within the first measurement range and the obtained DC component of the development current. The second approximation determines the operation by obtaining the DC component of the developing current under the condition that the peak-to-peak voltage of the AC voltage used for development bias is set to include at least three second measurement peak-to-peak voltages within a second measurement range, and determining the second approximation. The second measurement range is set such that it has a minimum value that is larger than the maximum value of the first measurement range. The second approximation is a first-order approximation that represents the relationship between the second measurement peak-to-peak voltage within the second measurement range and the obtained DC component of the developing current. The reference voltage determination action determines the peak-to-peak voltage at the intersection point where the first approximation determined by the first approximation determination action and the second approximation determined by the second approximation determination action intersect.

10. The image forming apparatus according to claim 9, characterized in that, The bias condition determination unit determines the first approximation by least squares based on the DC component of the imaging current obtained at the at least three first measurement peak voltages included in the first measurement range.

11. The image forming apparatus according to claim 9, characterized in that, When the slope of the first determination approximation, which is a first approximation, is greater than a preset first threshold, the bias condition determination unit sets the average value of the DC component of the developing current obtained at the at least three second measurement peak voltages as a fixed linear change relative to the peak voltage as the second approximation. When the slope of the first determination approximation is less than the first threshold, the first determination approximation is set as the second approximation. The first approximation is determined by the least squares method based on the DC component of the developing current obtained at the at least three second measurement peak voltages included in the second measurement range.

12. The image forming apparatus according to claim 9, characterized in that, 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 set to be smaller than the interval between the maximum value in the first measurement range and the minimum value in the second measurement range.

13. The image forming apparatus according to claim 9, characterized in that, In the first approximation determination operation, if the correlation coefficient of the first approximation is smaller than a preset second threshold, the bias condition determination unit determines the first approximation based on the DC component of the imaging current corresponding to the peak-to-peak voltage remaining after removing at least one peak-to-peak voltage from the at least three first measurement peak-to-peak voltages.

14. The image forming apparatus according to claim 13, characterized in that, In the first approximation determination operation, when the correlation coefficient of the first approximation is smaller than the second threshold, the bias condition determination unit determines the first approximation based on the DC component of the imaging current corresponding to the peak-to-peak voltage remaining after removing the largest peak-to-peak voltage among the at least three first measurement peak-to-peak voltages.

15. The image forming apparatus according to claim 9, characterized in that, In the second approximation determination operation, if the correlation coefficient of the second approximation is smaller than a preset third threshold, the bias condition determination unit determines the second approximation based on the DC component of the imaging current corresponding to the peak-to-peak voltage remaining after removing at least one peak-to-peak voltage from the at least three second measurement peak-to-peak voltages.

16. The image forming apparatus according to claim 15, characterized in that, In the second approximation determination operation, if the correlation coefficient of the second approximation is smaller than the third threshold, the bias condition determination unit compares the correlation coefficient of the second determination approximation based on the DC component of the developing current corresponding to the peak inter-voltage remaining after removing the largest peak inter-voltage among the at least three second measurement peak inter-voltages, and the correlation coefficient of the third determination approximation based on the DC component of the developing current corresponding to the peak inter-voltage remaining after removing the smallest peak inter-voltage among the at least three second measurement peak inter-voltages. The determination approximation with the larger correlation coefficient between the second determination approximation and the third determination approximation is determined as the second approximation.

17. The image forming apparatus according to claim 16, characterized in that, The bias condition determination unit pre-removes the maximum or minimum inter-peak voltage removed in the second approximation determination action from the second measurement range, and executes the next bias condition determination mode.

18. The image forming apparatus according to claim 9, characterized in that, The number of the at least three first measurement peak voltages within the first measurement range is set to be greater than the number of the at least three second measurement peak voltages within the second measurement range.

19. The image forming apparatus according to claim 9, characterized in that, The bias condition determination unit obtains, based on the intersection of the first approximation and the second approximation, the point where the balance of the three currents constituting the DC component of the developing current changes corresponding to the change in the peak voltage, i.e., the change point, and determines the peak voltage corresponding to the change point as the reference peak voltage. The three currents are: toner moving current, which is the current generated in the image forming section of the developing slit section by the movement of toner from the developing roller to the image carrier; image section brush current, which is the current flowing in the image forming section along a brush formed by the toner and the carrier across the developing roller and the image carrier, in the same direction as the toner moving current; and non-image section brush current, which is the current flowing in the non-image forming section of the developing slit section along a brush formed by the toner and the carrier across the developing roller and the image carrier, in the opposite direction to the toner moving current.

20. The image forming apparatus according to claim 1, characterized in that, The bias condition determination unit is in the DC voltage determination mode. With the DC voltage for the development bias set to a plurality of measurement DC voltages, the measurement latent image is developed into a measurement toner image by the toner through the development bias applied to the development roller. The concentration of the measurement toner image detected by the concentration detection unit is obtained. Based on the relationship between the plurality of measurement DC voltages and the concentrations of the plurality of measurement toner images, the DC voltage corresponding to the specified target concentration is determined as the reference DC voltage.

Images