Image forming apparatus, bias voltage adjustment method, and program

Abstract

The present invention provides an image forming apparatus, a bias voltage adjustment method, and a control program that can more effectively prevent image defects such as white spots. [Solution] The image forming apparatus 10 comprises a DC setting processing unit 50 and an AC setting processing unit 51. The DC setting processing unit 50 performs DC calibration to determine the voltage value of the DC component Vdc of the development bias VB. The AC setting processing unit 51 performs AC calibration to determine the voltage value Vpp of the AC component Vac of the development bias VB. The AC setting processing unit 51 determines the voltage value Vpp based on the first voltage value if the first voltage value determined by the DC calibration is less than a predetermined upper limit voltage value VdcH, and determines the voltage value Vpp of the AC component Vac based on the upper limit voltage value VdcH if the first voltage value is equal to or greater than the upper limit voltage value VdcH.

Description

Technical Field

[0001] The present invention relates to an image forming apparatus, a bias voltage adjustment method, and a program.

Background Art

[0002] Conventionally, an image forming apparatus that moves toner to an electrostatic latent image formed on the surface of a photosensitive drum based on an electrophotographic method to form a toner image on the photosensitive drum is known. In the image forming apparatus, the charged photosensitive drum is exposed based on image data, and the electrostatic latent image is formed on the photosensitive drum. Then, the image forming apparatus applies a bias voltage (development bias) to a developing roller, and supplies the charged toner to an exposed portion (a portion where the electrostatic latent image is formed) of the photosensitive drum according to the electric field between the photosensitive drum and the developing roller. As a result, toner adheres to the exposed portion on the photosensitive drum, and the electrostatic latent image on the photosensitive drum is developed into a toner image.

[0003] As this type of image forming apparatus, in order to suppress deterioration of image quality, a technique for setting a direct current component (direct current voltage) and an alternating current component (peak-to-peak voltage) included in the bias voltage is disclosed (see Patent Document 1). In the image forming apparatus, a provisional reference direct current voltage that is a provisional reference for the direct current component of the development bias is determined, then a peak-to-peak voltage that is the alternating current component of the development bias is determined, and then a reference direct current voltage that is the direct current component of the development bias is determined.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, while the conventional technology described above can stabilize image density, it cannot sufficiently suppress image defects such as white spots on the image caused by the movement of carriers and toner clumps onto the photoreceptor drum.

[0006] The object of the present invention is to provide an image forming apparatus, a bias voltage adjustment method, and a program that can more effectively prevent image defects such as white spots. [Means for solving the problem]

[0007] An image forming apparatus according to one aspect of the present invention includes: an image forming unit that applies a bias voltage in which an AC component is superimposed on a DC component between a first carrier whose surface is charged and a second carrier that holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data; a DC calibration processing unit that performs DC calibration to determine the voltage value of the DC component of the bias voltage; and an AC calibration processing unit that performs AC calibration to determine the voltage value of the AC component of the bias voltage. The AC calibration processing unit determines the voltage value of the AC component based on the first voltage value if the first voltage value determined by the DC calibration processing unit is less than a predetermined upper limit voltage value with respect to the DC component, and determines the voltage value of the AC component based on the upper limit voltage value if the first voltage value is equal to or greater than the upper limit voltage value.

[0008] A bias voltage adjustment method according to another aspect of the present invention is applied to an image forming apparatus that applies a bias voltage in which an AC component is superimposed on a DC component between a first carrier whose surface is charged and a second carrier that holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data, wherein one or more processors perform the following steps: The method includes a DC calibration step of performing a DC calibration to determine the voltage value of the DC component of the bias voltage, and an AC calibration step of performing an AC calibration to determine the voltage value of the AC component of the bias voltage. The AC calibration step determines the voltage value of the AC component based on the first voltage value if the first voltage value determined by the DC calibration step is less than a predetermined upper limit voltage value with respect to the DC component, and determines the voltage value of the AC component based on the upper limit voltage value if the first voltage value is equal to or greater than the upper limit voltage value.

[0009] A program relating to another aspect of the present invention is applied to an image forming apparatus that applies a bias voltage having an AC component superimposed on a DC component between a first carrier whose surface is charged and a second carrier holding toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data, and is a program that causes one or more processors to perform the following steps: The program includes a DC calibration step that performs a DC calibration to determine the voltage value of the DC component of the bias voltage, and an AC calibration step that performs an AC calibration to determine the voltage value of the AC component of the bias voltage. The AC calibration step determines the voltage value of the AC component based on the first voltage value if the first voltage value determined by the DC calibration step is less than a predetermined upper limit voltage value with respect to the DC component, and determines the voltage value of the AC component based on the upper limit voltage value if the first voltage value is equal to or greater than the upper limit voltage value. [Effects of the Invention]

[0010] According to the present invention, it is possible to more effectively prevent image defects such as white spots. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a block diagram showing the configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] Figure 2 shows the configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 3] Figure 3 shows the configuration of the image forming section of an image forming apparatus according to an embodiment of the present invention. [Figure 4] Figure 4 is an explanatory diagram showing an example of the waveform of the development bias of an image forming apparatus according to an embodiment of the present invention. [Figure 5] Figure 5 is a flowchart showing an example of the procedure for bias voltage adjustment processing performed by the control unit of the image forming apparatus according to an embodiment of the present invention. [Figure 6] Figure 6 is a flowchart showing an example of a procedure for another example of the bias voltage adjustment process performed by the control unit of the image forming apparatus according to an embodiment of the present invention. [Figure 7] Figure 7 is an explanatory diagram showing an example of a method for measuring surface potential in an image forming apparatus according to an embodiment of the present invention. [Figure 8] Figure 8 is an explanatory diagram showing an example of AC calibration in an image forming apparatus according to an embodiment of the present invention. [Figure 9] Figure 9 is an explanatory diagram showing an example of DC calibration in an image forming apparatus according to an embodiment of the present invention. [Modes for carrying out the invention]

[0012] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The following embodiments are an example of embodying the present invention and are not intended to limit the technical scope of the present invention.

[0013] [1] Overall Configuration of Image Forming Apparatus First, the overall configuration of the image forming apparatus 10 according to the present embodiment will be described while referring to FIGS. 1 and 2.

[0014] For convenience of explanation, in the installed state (the state shown in FIG. 2) in which the image forming apparatus 10 can be used, the vertical direction is defined as the up-down direction D1. Also, the front-rear direction D2 is defined with the left side surface of the paper surface of the image forming apparatus 10 shown in FIG. 2 as the front (front surface). Also, the left-right direction D3 is defined based on the front surface of the image forming apparatus 10 in the installed state.

[0015] The image forming apparatus 10 according to the present embodiment is, as an example, a multifunction device having a plurality of functions such as a scan function for reading image data from a document, a print function for forming an image based on the image data, a facsimile function, and a copy function. The image forming apparatus 10 only needs to have a function of forming an image, and may be a printer, a facsimile apparatus, a copy machine, or the like.

[0016] As shown in FIG. 1, the image forming apparatus 10 includes an automatic document feeder 1, an image reading unit 2, an image forming unit 3, a paper feeding unit 4, a control unit 5, a storage unit 6, and an operation display unit 7. Since the automatic document feeder 1 is an ADF (Auto Document Feeder), it is denoted as "ADF" in FIG. 1 and is also referred to as "ADF1" in the following description.

[0017] ADF1 conveys the document whose image is read by the image reading unit 2. ADF1 has a document set unit, a plurality of conveyance rollers, a document holder, a paper discharge unit, and the like.

[0018] The image reading unit 2 reads an image from the original document and outputs image data corresponding to the read image. The image reading unit 2 includes a document table, a light source, a plurality of mirrors, an optical lens, a CCD (Charge Coupled Device), etc.

[0019] The image forming unit 3 realizes a printing function by forming a color or monochrome image on a sheet by an electrophotographic method. The image forming unit 3 forms an image on the sheet based on the image data output from the image reading unit 2. Also, the image forming unit 3 forms an image on the sheet based on the image data input from an information processing device outside the image forming apparatus 10, such as a personal computer.

[0020] The paper feeding unit 4 supplies a sheet to the image forming unit 3. The paper feeding unit 4 includes a paper feeding cassette, a manual feed tray, a sheet conveyance path, a plurality of conveyance rollers, etc. The image forming unit 3 forms an image on the sheet supplied from the paper feeding unit 4.

[0021] The control unit 5 comprehensively controls the image forming apparatus 10. The control unit 5 mainly consists of a computer system having one or more processors and one or more memories. In the image forming apparatus 10, the functions of the control unit 5 are realized by one or more processors executing a program. The program may be recorded in advance in a memory (storage unit 6), may be provided through an electric communication line such as the Internet, or may be recorded and provided in a non-temporary recording medium readable by a computer system, such as a memory card or an optical disk. One or more processors are composed of one or more electronic circuits including a semiconductor integrated circuit. Further, the computer system referred to here includes a microcontroller having one or more processors and one or more memories. The control unit 5 may be a control unit provided separately from the main control unit that comprehensively controls the image forming apparatus 10.

[0022] The storage unit 6 includes one or more non-volatile memories, and information such as control programs for causing the control unit 5 to execute various processes is pre-stored in it. Furthermore, the storage unit 6 is used as a temporary storage memory (work area) for the various processes executed by the control unit 5.

[0023] The operation display unit 7 is the user interface in the image forming apparatus 10. The operation display unit 7 has a display unit such as a liquid crystal display that displays various information in response to control instructions from the control unit 5, and an operation unit such as a switch or touch panel that inputs various information to the control unit 5 in response to user operations.

[0024] [2] Configuration of the image forming unit Next, the configuration of the image forming unit 3 will be explained in more detail with reference to Figures 1 to 4.

[0025] As shown in Figure 2, the image forming unit 3 includes four image forming units 31-34, an optical scanning device 35, an intermediate transfer device 36, a secondary transfer roller 37, a fixing device 38, and a paper output tray 39. In Figure 3, an enlarged schematic diagram of one of the four image forming units 31-34, image forming unit 34, is shown within the speech bubble.

[0026] The image forming unit 31 forms a yellow (Y) toner image. As shown in Figure 3, the image forming unit 31 includes a photoreceptor drum 311 (an example of the first carrier of the present invention), a developing device 313 including a charging roller 312 and a developing roller 313A (an example of the second carrier of the present invention), a primary transfer roller 314, and a drum cleaning unit 315. The image forming unit 31 also further includes a toner container 316 (see Figure 2).

[0027] The image forming unit 32 forms a cyan (C) toner image. As shown in Figure 3, the image forming unit 32 includes a photoreceptor drum 321 (an example of the first carrier of the present invention), a developing device 323 including a charging roller 322 and a developing roller 323A (an example of the second carrier of the present invention), a primary transfer roller 324, and a drum cleaning unit 325. The image forming unit 32 also further includes a toner container 326 (see Figure 2).

[0028] The image forming unit 33 forms a magenta (M) toner image. As shown in Figure 3, the image forming unit 33 includes a photoreceptor drum 331 (an example of the first carrier of the present invention), a developing device 333 including a charging roller 332 and a developing roller 333A (an example of the second carrier of the present invention), a primary transfer roller 334, and a drum cleaning unit 335. The image forming unit 33 also further includes a toner container 336 (see Figure 2).

[0029] The image forming unit 34 forms a K (black) toner image. As shown in Figure 3, the image forming unit 34 includes a photoreceptor drum 341 (an example of the first carrier of the present invention), a developing device 343 including a charging roller 342 and a developing roller 343A (an example of the second carrier of the present invention), a primary transfer roller 344, and a drum cleaning unit 345. The image forming unit 34 also further includes a toner container 346 (see Figure 2).

[0030] Furthermore, each of the multiple image forming units 31 to 34, in addition to the above configuration, further includes a power supply circuit 301 and a current detection circuit 302, as shown in Figure 1. In other words, the power supply circuit 301 and the current detection circuit 302 are provided in each of the multiple image forming units 31 to 34. Each power supply circuit 301 includes a developing power supply circuit 301A and a charging power supply circuit 301B.

[0031] Thus, the multiple (in this case, four) image forming units 31-34 each correspond to the four colors Y (yellow), C (cyan), M (magenta), and K (black), and basically employ a common configuration. Therefore, unless otherwise specified, the configuration described for image forming unit 34 is the same for the other image forming units 31-33. In the callouts in Figure 3, only the parts of image forming unit 34, such as the photoreceptor drum 341, developing roller 343A, power supply circuit 301, and current detection circuit 302, are shown.

[0032] An electrostatic latent image is formed on the photoreceptor drum 341. The photoreceptor drum 341 is rotatably supported by a unit housing that houses the photoreceptor drum 341, the charging roller 342, and the drum cleaning unit 345, around a rotation axis extending in the left-right direction D3. The photoreceptor drum 341 rotates in the rotation direction D5 shown in Figure 3, for example, by receiving a driving force supplied from a motor.

[0033] The charging roller 342 charges the surface (outer surface) of the photoreceptor drum 341 with positive polarity. Specifically, the charging roller 342 is electrically connected to the charging power supply circuit 301B of the power supply circuit 301, and charges the surface of the photoreceptor drum 341 by receiving high voltage from the charging power supply circuit 301B. However, the charging roller 342 is not limited to charging the surface of the photoreceptor drum 341 with positive polarity; it may also charge it with negative polarity.

[0034] The surface of the photoreceptor drum 341, which has been charged by the charging roller 342, is illuminated by light from the optical scanning device 35 based on image data. As a result, an electrostatic latent image is formed on the surface of the photoreceptor drum 341. In other words, the exposed portion of the surface of the photoreceptor drum 341 that is illuminated by light from the optical scanning device 35 becomes the "image portion".

[0035] The developing device 343 performs a developing process to develop the electrostatic latent image formed on the surface of the photoreceptor drum 341. In this embodiment, the developing device 343 specifically uses a two-component developer containing toner and carrier for development. For example, the developing device 343 includes a case, a pair of stirring members, a magnetic roller, and a developing roller 343A. The case supports the pair of stirring members, the magnetic roller, and the developing roller 343A so that they can rotate around a rotation axis extending in the left-right direction D3. The case also houses the K (black) toner and carrier. The pair of stirring members stir the toner and carrier housed in the case, thereby charging the toner. In this embodiment, the toner is positively charged. However, the charging polarity of the toner is not limited to positive polarity; it may also be negative polarity. The magnetic roller draws up the toner and carrier stirred by the pair of stirring members and supplies the toner to the surface (outer circumferential surface) of the developing roller 343A.

[0036] The developing roller 343A develops the electrostatic latent image formed on the photoreceptor drum 341 using charged toner. Specifically, the developing roller 343A is electrically connected to the developing power supply circuit 301A of the power supply circuit 301, and receives a developing bias VB (see Figure 4) from the developing power supply circuit 301A to supply toner to the surface of the photoreceptor drum 341. In other words, when a high-voltage developing bias VB is applied between the developing roller 343A and the photoreceptor drum 341 by the developing power supply circuit 301A, a developing electric field is formed, and the charged toner moves from the developing roller 343A to the photoreceptor drum 341. As a result, a toner image corresponding to the electrostatic latent image is formed on the surface of the photoreceptor drum 341.

[0037] In this embodiment, the photoreceptor drum 341 is an example of a "first carrier," and the developing roller 343A is an example of a "second carrier." That is, the image forming unit 3 moves charged toner from the developing roller 343A to the photoreceptor drum 341 using a developing electric field, developing the electrostatic latent image on the photoreceptor drum 341 with the toner, and forming a toner image (image) on the photoreceptor drum 341 that corresponds to the electrostatic latent image. In other words, in the development process, the image forming unit 3 moves charged toner from the second carrier to the first carrier and forms a toner image on the first carrier. Here, since the photoreceptor drum 341 rotates in the rotation direction D5 (see Figure 3), the portion of the surface of the photoreceptor drum 341 facing the developing roller 343A changes over time. In other words, the portion of the surface of the photoreceptor drum 341 facing the developing roller 343A moves in the rotation direction D5 of the photoreceptor drum 341.

[0038] Here, the developing bias VB applied between the developing roller 343A and the photoreceptor drum 341 is a voltage in which an AC component Vac is superimposed on a DC component Vdc, as shown in Figure 4. In other words, the developing power supply circuit 301A generates a developing bias VB in which an AC component Vac is superimposed on a DC voltage by superimposing an AC voltage on a DC voltage. Therefore, the developing bias VB has a fluctuating component due to the pulsation (ripple) of the AC component Vac superimposed on the value of its DC component Vdc, and periodically repeats between a value higher and a value lower than the value of the DC component Vdc. In the example in Figure 4, the AC component Vac of the developing bias VB is a square wave with a duty cycle of 50%, but it is not limited to this, and the AC component Vac may be a sine wave or a triangular wave, for example.

[0039] Furthermore, in the development bias VB, at least the voltage value of the DC component Vdc and the voltage value Vpp of the AC component Vac, which is the peak-to-peak value of the AC component Vac, are adjustable. For example, if the surface potential of the charged photoreceptor drum 341 is "Vs", and the potential of the exposed area on the surface of the photoreceptor drum 341 irradiated with light from the light scanning device 35 (exposure potential) is "VL", then the DC component Vdc is set to a value between the surface potential Vs and the exposure potential VL. In other words, the voltage value of the DC component Vdc is lower than the surface potential Vs and higher than the exposure potential VL of the exposed area.

[0040] In this embodiment, the developing bias VB is an example of a "bias voltage" applied between the developing roller 343A and the photoreceptor drum 341. That is, a developing bias VB, in which an AC component Vac is superimposed on a DC component Vdc, is applied as a "bias voltage" between the developing roller 343A and the photoreceptor drum 341. Furthermore, when the developing bias VB is applied, a developing current flows between the developing roller 343A and the photoreceptor drum 341. The developing current includes the toner current that flows as the toner moves. In this embodiment, the developing current that flows from the second carrier, the developing roller 343A, to the first carrier, the photoreceptor drum 341 is defined as "positive," and conversely, the developing current that flows from the photoreceptor drum 341 to the developing roller 343A is defined as "negative."

[0041] Here, a magnet is placed inside the developing roller 343A, and the developing roller 343A rotates around the stationary magnet. One of the magnetic poles of the magnet faces the photoreceptor drum 341 via the developing roller 343A and the developing gap. The developer carried on the developing roller 343A forms a magnetic brush in the developing gap. The magnetic brush is a magnetic carrier to which toner is attached. Therefore, the developing current includes a toner current that flows as the toner moves, a magnetic brush current that flows through the magnetic brush in the exposed area, and a reverse magnetic brush current that flows through the magnetic brush in the unexposed area in the opposite direction to that in the exposed area.

[0042] The primary transfer roller 344 transfers the toner image formed on the surface of the photoreceptor drum 341 by the developing device 343 to the outer surface of the intermediate transfer belt 361 (see Figure 3). Specifically, the primary transfer roller 344 is electrically connected to the power supply circuit 301, and by receiving high voltage from the power supply circuit 301, it transfers the toner image formed on the surface of the photoreceptor drum 341 to the outer surface of the intermediate transfer belt 361. In other words, a high-voltage transfer bias is applied between the photoreceptor drum 341 and the primary transfer roller 344 by the power supply circuit 301, creating a transfer electric field, and the charged toner moves from the photoreceptor drum 341 to the intermediate transfer belt 361. As a result, a toner image is formed (transferred) to the outer surface of the intermediate transfer belt 361.

[0043] The drum cleaning unit 345 cleans the surface of the photoreceptor drum 341 after the toner image has been transferred by the primary transfer roller 344. For example, the drum cleaning unit 345 has a blade-shaped cleaning member and a transport member. The cleaning member contacts the surface of the photoreceptor drum 341 and removes the toner adhering to the surface. The transport member transports the toner removed by the cleaning member to the toner container.

[0044] The toner container 346 supplies toner to the case of the developing device 343. In the image forming unit 34, which forms a K (black) toner image, the toner container 346 supplies K (black) toner.

[0045] The current detection circuit 302 detects the developing current flowing between the photoreceptor drum 341 and the developing roller 343A. The current detection circuit 302 is, for example, a circuit that includes a current sensor such as a shunt resistor or a current transformer, and is provided on the power supply path from the power supply circuit 301 to the developing roller 343A. The current detection circuit 302 outputs a detection signal to the control unit 5 that corresponds to the magnitude of the developing current flowing from the developing roller 343A (second carrier) to the photoreceptor drum 341 (first carrier).

[0046] Furthermore, in this embodiment, the current detection circuit 302 includes a filter circuit 302A. The filter circuit 302A is, for example, a low-pass filter, or integral filter, that attenuates frequency components above the cutoff frequency of the developing current flowing from the developing roller 343A (second carrier) to the photoreceptor drum 341 (first carrier). By including the filter circuit 302A, the target current detected by the current detection circuit 302 corresponds to the DC component of the developing current flowing from the developing roller 343A to the photoreceptor drum 341.

[0047] The optical scanning device 35 forms an electrostatic latent image on each of the photoreceptor drums 311, 321, 331, and 341 of the four image forming units 31 to 34. In this embodiment, the optical scanning device 35 includes two optical scanning units 351 and 352. Optical scanning unit 351 forms an electrostatic latent image on the photoreceptor drum 311 by irradiating it with light based on image data in response to the input of Y (yellow) image data. Optical scanning unit 351 forms an electrostatic latent image on the photoreceptor drum 321 by irradiating it with light based on image data in response to the input of C (cyan) image data. Optical scanning unit 352 forms an electrostatic latent image on the photoreceptor drum 331 by irradiating it with light based on image data in response to the input of M (magenta) image data. Furthermore, optical scanning unit 352 forms an electrostatic latent image on the photoreceptor drum 341 by irradiating it with light based on image data in response to the input of K (black) image data.

[0048] The toner images of each color formed by each of the multiple (in this case, four) image forming units 31 to 34 are transferred onto the outer surface of the intermediate transfer belt 361. As a result, a color image (toner image) is formed on the outer surface of the intermediate transfer belt 361.

[0049] As shown in Figure 3, the intermediate transfer device 36 includes an intermediate transfer belt 361, a drive roller 362, a tension roller 363, a belt cleaning unit 364, and a density detection unit 365. The intermediate transfer device 36 uses the intermediate transfer belt 361 to transport the toner image formed by the image forming units 31-34 to the transfer position P1 (see Figure 3) by the secondary transfer roller 37.

[0050] The intermediate transfer belt 361 is an endless belt on which toner images of each color are transferred from each of the photoreceptor drums 311, 321, 331, and 341. As shown in Figure 3, the intermediate transfer belt 361 is wrapped around drive rollers 362 and tension rollers 363 which are spaced apart from each other in the front-to-back direction D2 of the image forming apparatus 10. The drive rollers 362 rotate by receiving driving force supplied from a motor. As a result, the intermediate transfer belt 361 rotates in the rotation direction D4 shown in Figure 3. The toner image transferred to the outer surface of the intermediate transfer belt 361 is transported to the transfer position P1 by the secondary transfer rollers 37 as the intermediate transfer belt 361 rotates. The belt cleaning unit 364 cleans the outer surface of the intermediate transfer belt 361 after the toner image has been transferred at the transfer position P1.

[0051] The density detection unit 365 detects the density of the image (toner image) transferred to the outer surface of the intermediate transfer belt 361. For example, the density detection unit 365 includes a reflective optical sensor having a light-emitting unit that emits light directed toward the outer surface of the intermediate transfer belt 361, and a light-receiving unit that receives the light emitted from the light-emitting unit and reflected from the outer surface of the intermediate transfer belt 361. As shown in Figure 3, the density detection unit 365 is positioned downstream of the image forming unit 34 in the rotation direction D4 of the intermediate transfer belt 361 and upstream of the secondary transfer roller 37. The density detection unit 365 is also positioned opposite one end of the intermediate transfer belt 361 in the width direction (left-right direction D3) on the outer surface of the intermediate transfer belt 361. The density detection unit 365 may also be positioned opposite both ends of the intermediate transfer belt 361 in the width direction on the outer surface of the intermediate transfer belt 361. The density detection unit 365 may also detect the density of the toner image developed on the outer surface of the photoreceptor drum 341.

[0052] The secondary transfer roller 37 transfers the toner image formed on the outer surface of the intermediate transfer belt 361 to the sheet supplied by the paper feeding unit 4. As shown in Figure 3, the secondary transfer roller 37 is positioned opposite the tension roller 363, with the intermediate transfer belt 361 in between, so as to be in contact with the outer surface of the intermediate transfer belt 361. The secondary transfer roller 37 is pressed towards the tension roller 363 by a biasing member. The secondary transfer roller 37 is electrically connected to the power supply circuit, and by receiving a high voltage from the power supply circuit, it transfers the toner image formed on the outer surface of the intermediate transfer belt 361 to the sheet passing through the transfer position P1 (see Figure 3) where the secondary transfer roller 37 and the intermediate transfer belt 361 are in contact.

[0053] The secondary transfer roller 37 has a length in its axial direction (left-right direction D3) that is shorter than the width of the intermediate transfer belt 361. As a result, the outer surface of the intermediate transfer belt 361 has a contact area that contacts the secondary transfer roller 37 and a non-contact area (blank area) that does not contact the secondary transfer roller 37. The non-contact area is the area on both sides of the contact area on the outer surface of the intermediate transfer belt 361. The density detection unit 365 is positioned opposite one of the non-contact areas. The secondary transfer roller 37 transfers the image formed in the contact area to the sheet, but does not transfer the image formed in the non-contact area to the sheet. The secondary transfer roller 37 may have a length in its axial direction that is the same as the width of the intermediate transfer belt 361.

[0054] The fixing device 38 melts and fixes the toner image transferred to the sheet by the secondary transfer roller 37 to the sheet. For example, the fixing device 38 includes a fixing roller and a pressure roller. The fixing roller is positioned in contact with the pressure roller and heats the toner image transferred to the sheet to fix it to the sheet. The pressure roller presses the sheet as it passes through the contact area formed between it and the fixing roller.

[0055] The image-formed sheet is ejected into the output tray 39.

[0056] Incidentally, as a related technology, there is a known technique for setting the DC component (DC voltage) and AC component (peak voltage) included in the development bias voltage in an image forming apparatus in order to suppress the deterioration of image quality. In this image forming apparatus, a provisional reference DC voltage, which serves as a provisional reference for the DC component in the development bias, is determined, then the peak voltage, which is the AC component of the development bias used in the image forming operation, is determined, and then the reference DC voltage, which is the DC component of the development bias used in the image forming operation, is determined.

[0057] However, while the above-mentioned related technologies can stabilize image density, they cannot adequately suppress image defects such as white spots on the image caused by the movement of carriers or toner clumps onto the photoreceptor drum. For example, if the DC component of the development bias VB used in the image formation operation exceeds a predetermined upper limit due to a significant change in environmental conditions, and the AC component of the development bias remains at the specified value, image defects such as white spots are likely to occur in the developed image, and it is not possible to prevent or suppress these image defects.

[0058] In contrast, the image forming apparatus 10 according to this embodiment can stabilize the image density and more effectively prevent image defects such as white spots, through the configuration described below.

[0059] In other words, as shown in Figure 1, the image forming apparatus 10 according to this embodiment comprises at least an image forming unit 3, a DC setting processing unit 50 (an example of the DC calibration processing unit of the present invention), and an AC setting processing unit 51 (an example of the AC calibration processing unit of the present invention). In this embodiment, as an example, the DC setting processing unit 50 and the AC setting processing unit 51 are provided in the control unit 5 as a function of the control unit 5.

[0060] As described above, the image forming unit 3 applies the development bias VB between the photoreceptor drum 341, whose surface is charged, and the developing roller 343A, which holds the toner to be attached to the photoreceptor drum 341, thereby moving the toner from the developing roller 343A to the exposed portion of the photoreceptor drum 341 and forming a toner image on the photoreceptor drum 341 based on the image data. Here, the development bias VB is a voltage in which an AC component Vac is superimposed on a DC component Vdc.

[0061] [3] Configuration of the control unit Next, with reference to Figure 1, each functional unit included in the control unit 5 will be described in more detail. The control unit 5 includes a DC setting processing unit 50, an AC setting processing unit 51, a potential measurement processing unit 52, an upper limit voltage determination processing unit 54 (an example of the upper limit voltage determination unit of the present invention), and a potential adjustment processing unit 55, among other processing units. In other words, the image forming apparatus 10 is equipped with each of these processing units as a function of the control unit 5.

[0062] The DC setting processing unit 50 performs DC calibration to determine and set the voltage value of the DC component Vdc of the development bias VB. The DC setting processing unit 50 performs the DC calibration when predetermined adjustment execution conditions are met. These adjustment execution conditions include, for example, that the initial operation performed after the main power of the image forming apparatus 10 is turned on has been executed, or that a predetermined number of print cycles (for example, 1000) has been counted.

[0063] The DC calibration involves causing the image forming unit 3 to perform a process to create a predetermined test toner image (an example of a measurement toner image of the present invention), then measuring the density of the test toner image, and determining and setting the voltage value of the DC component Vdc of the development bias VB in the image forming unit 3 according to the measured density.

[0064] For example, the DC setting processing unit 50 causes the test toner image to be formed on each of the image forming units 31 to 34. As a result, the four color test toner images are developed on the surface of each photoreceptor drum and then transferred to the intermediate transfer belt 361. For example, the test toner image includes a toner patch. The test toner image is formed, for example, on the non-contact area on the outer circumferential surface of the intermediate transfer belt 361. Alternatively, the test toner image may be formed on the contact area of ​​the intermediate transfer belt 361.

[0065] Furthermore, the DC setting processing unit 50 acquires the density detection results of the four test toner images by the density detection unit 365. Furthermore, the DC setting processing unit 50 determines the voltage value of the DC component Vdc according to the density difference between the density detection results of the four test toner images and a predetermined target image density. Specifically, if the density of the test toner image is lighter than the target image density, the DC setting processing unit 50 increases the voltage value of the DC component Vdc by the density difference from its current value in order to match the image density to the target image density. Also, if the density of the test toner image is darker than the target image density, the DC setting processing unit 50 decreases the voltage value of the DC component Vdc by the density difference from its current value.

[0066] Furthermore, the DC setting processing unit 50 sets the first voltage value determined by the DC calibration to the voltage value of the DC component Vdc of the development bias VB (set voltage value) if the first voltage value determined by the DC calibration is less than a predetermined upper limit voltage value VdcH with respect to the DC component Vdc of the development bias VB. Also, the DC setting processing unit 50 sets the upper limit voltage value VdcH to the voltage value of the DC component Vdc of the development bias VB (set voltage value) if the first voltage value determined by the DC calibration is equal to or greater than the upper limit voltage value VdcH. The set voltage value is stored in the storage unit 6.

[0067] If the image forming apparatus 10 is a monochrome printer, the test toner image is formed on the photoreceptor drum 341, and the density detection unit 365 detects the density of the test toner image on the photoreceptor drum 341.

[0068] Here, the DC setting processing unit 50 may perform the DC calibration after measuring the surface potential Vs. This sets the voltage value of the DC component Vdc in the image forming apparatus 10 to an appropriate value according to the measurement result of the surface potential Vs.

[0069] The AC setting processing unit 51 performs AC calibration to determine and set the voltage value Vpp (peak voltage) of the AC component Vac of the development bias VB.

[0070] In this embodiment, the AC setting processing unit 51 determines the voltage value Vpp of the AC component Vac based on the first voltage value determined by the DC calibration if the first voltage value determined by the DC calibration is less than a predetermined upper limit voltage value VdcH with respect to the DC component Vdc of the development bias VB. Also, the AC setting processing unit 51 determines the voltage value Vpp of the AC component Vac based on the upper limit voltage value VdcH if the first voltage value determined by the DC calibration is equal to or greater than the upper limit voltage value VdcH.

[0071] Furthermore, the AC setting processing unit 51 sets the voltage value Vpp of the AC component Vac based on the development current. Specifically, the AC setting processing unit 51 sequentially sets the voltage value Vpp of the AC component Vac to several predetermined values ​​within a range, and causes the current detection circuit 302 to detect the development current that flows when developing a solid black image. Then, the AC setting processing unit 51 determines an appropriate voltage value Vpp of the AC component Vac from this change in development current.

[0072] Here, the DC component Vdc of the developing bias VB used in the AC calibration is the voltage value of the DC component Vdc determined and set in the most recent DC calibration performed in advance. That is, when the first voltage value is less than the upper limit voltage value VdcH, the voltage value of the DC component Vdc applied to the AC calibration is the first voltage value. Also, when the first voltage value is equal to or greater than the upper limit voltage value VdcH, the voltage value of the DC component Vdc applied to the AC calibration is the upper limit voltage value VdcH.

[0073] In this embodiment, the image forming apparatus 10 performs AC calibration based on the development current, and DC calibration based on the density detected by the density detection unit 365. The reason for this is that the AC calibration adopts conditions in which the image saturation density is stable, and the DC calibration selects a method for setting the saturation density level. This method can stabilize image quality the most. In other words, in order to set conditions in which the image saturation density is stable, it is necessary to measure the image saturation density, but there is a problem that measurement errors tend to be large because the measurement sensitivity of the optical sensor by the density detection unit 365 is low at the saturation density. Therefore, in this embodiment, this problem is resolved by using the development current instead of image density for the AC calibration.

[0074] Furthermore, when developing toner, the DC component of the development current includes the toner current associated with the movement of the toner and the current flowing through the magnetic brush. Simultaneously, a reverse magnetic brush current flows in the unexposed areas of the photoreceptor drum 341. Therefore, the AC setting processing unit 51 measures the development current while changing the voltage value Vpp of the AC component Vac, and from the state of change, determines the voltage value Vpp of the AC component Vac that results in the peak image density.

[0075] In this embodiment, the AC setting processing unit 51 determines the voltage value Vpp of the AC component Vac based on the development current when the development bias VB is applied while the voltage value Vpp of the AC component Vac is varied within a first measurement range, and the development current when the development bias VB is applied while the voltage value Vpp of the AC component Vac is varied within a second measurement range. Here, the second measurement range is a higher frequency range than the first measurement range. In this way, the variable range of the voltage value Vpp of the AC component Vac is divided into a first measurement range and a second measurement range, and the relationship between the voltage value Vpp of the AC component Vac and the development current is obtained in each range. By considering the relationship obtained in each range, it is possible to set the optimal voltage value Vpp of the AC component Vac.

[0076] The AC calibration performed by the AC setting processing unit 51 may be performed based on the density of the test toner image detected by the density detection unit 365, without using the development current.

[0077] In another embodiment, the AC setting processing unit 51 performs the AC calibration when the absolute value of the voltage difference between the voltage value (set voltage value) of the DC component Vdc of the development bias VB set by the DC setting processing unit 50 and a predetermined voltage value which is the smaller of the second voltage value obtained by the DC calibration performed earlier and the upper limit voltage value VdcH is greater than or equal to a predetermined threshold a. Furthermore, if the absolute value of the voltage difference is less than the threshold a, the AC setting processing unit 51 corrects the currently set voltage value Vpp of the AC component Vac using the DC component Vdc set by the DC setting processing unit 50. In other words, the voltage difference is the difference between the voltage value (set voltage value) of the DC component Vdc of the development bias VB set by the DC setting processing unit 50 and the voltage value (set voltage value) of the DC component Vdc of the development bias VB set by the DC setting processing unit 50 in the previous instance.

[0078] The predetermined voltage value may be the second voltage value obtained by the DC calibration performed immediately prior to the first voltage calibration. The threshold value a is an arbitrary value determined by experimentation or other means.

[0079] Specifically, the AC setting processing unit 51 calculates a correction amount ΔVpp to correct the voltage value Vpp when the absolute value of the voltage difference is less than the threshold a. Then, if the set voltage value is greater than the predetermined voltage value, the corrected voltage value Vpp is set by adding the correction amount ΔVpp to the set current voltage value Vpp of the AC component Vac. On the other hand, if the first voltage value is less than the predetermined voltage value, the corrected voltage value Vpp is set by subtracting the correction amount ΔVpp from the set current voltage value Vpp of the AC component Vac.

[0080] Here, the correction amount ΔVpp can be calculated by the following equation (1), where ΔVdc is the absolute value of the voltage difference between the set voltage value and the predetermined voltage value, and b is an arbitrary coefficient determined by experiment or the like.

[0081] ΔVpp = ΔVdc × b ... (1)

[0082] In addition, as another embodiment, the AC setting processing unit 51 may perform the AC calibration if the absolute value of the voltage difference between the first voltage value obtained by the DC calibration and the second voltage value obtained by the DC calibration performed earlier is greater than or equal to a predetermined threshold a, and if the absolute value of the voltage difference is less than the threshold a, it may correct the set current voltage value Vpp of the AC component Vac with the first voltage value. In this case, ΔVdc in the above formula (1) is the voltage difference between the first voltage value and the second voltage value.

[0083] In this way, the AC calibration is performed only when the absolute value of the potential difference ΔVdc is greater than or equal to the threshold a, and when the absolute value of the potential difference ΔVdc is less than the threshold a, the AC calibration is not performed, and the set current voltage value Vpp of the AC component Vac is corrected by the correction amount ΔVpp, thereby shortening the time required to set the voltage value Vpp of the AC component Vac.

[0084] The potential measurement processing unit 52 measures the surface potential Vs of the photoreceptor drum 341 based on the developing current flowing between the developing roller 343A and the photoreceptor drum 341. In other words, the potential measurement processing unit 52 measures the surface potential Vs based on the developing current detected by the current detection circuit 302. Specifically, the potential measurement processing unit 52 utilizes the fact that when the surface potential Vs and the DC component Vdc of the developing bias VB match, the DC component of the developing current becomes 0 (zero). In other words, the potential measurement processing unit 52 identifies the DC component Vdc of the developing bias VB when the DC component of the developing current is 0 as the surface potential Vs.

[0085] Here, in order to improve the accuracy of measuring the surface potential Vs, it is preferable that the AC component Vac of the development bias VB applied during potential measurement is the same as the AC component Vac of the development bias VB used during image formation (development). This is because the AC component Vac of the development bias VB has the effect of changing the surface potential Vs by injecting charge into the photoreceptor drum 341. Therefore, it is preferable that the AC component Vac of the development bias VB be set to a value that stabilizes the movement of the toner. This suppresses excessive charge injection into the photoreceptor drum 341, making it possible to use it in a stable state.

[0086] The upper limit voltage determination processing unit 54 performs a process to correct the predetermined upper limit voltage value VdcH with respect to the DC component Vdc of the development bias VB, based on the environmental conditions of the location where the image forming apparatus 10 is installed. The upper limit voltage value VdcH is the upper limit of the allowable range (allowable upper limit) that the voltage value of the DC component Vdc of the development bias VB can take. If the voltage value of the DC component Vdc of the development bias VB is too high, carriers and nerite clumps contained in the developer are more likely to move to the photoreceptor drum 341, making it easier for image defects such as white spots on the image to occur. On the other hand, if the DC component Vdc of the development bias VB is too low, the image density will be insufficient, making it impossible to obtain the appropriate density and degrading the image quality. For this reason, the upper limit voltage value VdcH is set for the DC component Vdc of the development bias VB so as to ensure the appropriate density and so as not to cause image defects.

[0087] The aforementioned upper limit voltage value VdcH is initially set to, for example, a reference upper limit voltage value (200V as an example) corresponding to average environmental conditions. In this embodiment, the upper limit voltage determination processing unit 54 corrects the reference upper limit voltage value or the upper limit voltage value VdcH that was set immediately before, based on the environmental conditions. The reference upper limit voltage value is determined by changing the voltage value of the DC component Vdc to form a test toner image under average environmental conditions and setting it to the highest voltage value within the range in which no image loss occurs.

[0088] Here, the environmental conditions are, for example, the absolute humidity of the location where the image forming apparatus 10 is installed. In this case, the upper limit voltage determination processing unit 54 corrects the upper limit voltage value VdcH according to the measured absolute humidity. The higher the absolute humidity, the more difficult it is for the toner to move to the photoreceptor drum 341, and the lower the absolute humidity, the easier it is for the toner to move to the photoreceptor drum 341. Therefore, in this case, if the measured humidity is higher than the absolute humidity measured previously, the upper limit voltage determination processing unit 54 increases the upper limit voltage value VdcH according to the amount of change (humidity difference). Also, if the measured humidity is lower than the absolute humidity measured previously, the upper limit voltage determination processing unit 54 decreases the upper limit voltage value VdcH according to the amount of change (humidity difference).

[0089] Furthermore, the aforementioned environmental conditions are, for example, the process speed during the image formation process in the image forming unit 3. In this embodiment, when the process speed is changed, the upper limit voltage determination processing unit 54 corrects the upper limit voltage value VdcH according to the amount of change in the process speed. The faster the process speed, the more difficult it is for the toner to move to the photoreceptor drum 341, and the slower the process speed, the easier it is for the toner to move to the photoreceptor drum 341. Therefore, in this case, when the process speed becomes faster than the previous speed, the upper limit voltage determination processing unit 54 increases the upper limit voltage value VdcH according to the amount of change in speed (speed difference). Also, when the process speed becomes slower than the previous speed, the upper limit voltage determination processing unit 54 decreases the upper limit voltage value VdcH according to the amount of change in speed (speed difference).

[0090] Here, the process speed is changed, for example, according to the content of the image data. For example, if the image data is a high-density photograph or a high-density solid image, more toner is used compared to a low-density text image, so the process speed is changed to a slower speed than the speed used when printing the text image (e.g., the reference speed). By changing the process speed to a different process speed, the degradation of image quality when printing high-density images can be reduced. The process speed is, for example, the linear velocity of the photoreceptor drum 341.

[0091] The potential adjustment processing unit 55 adjusts the surface potential Vs based on the surface potential Vs measured by the potential measurement processing unit 52. Specifically, the potential adjustment processing unit 55 sets the magnitude of the voltage applied by the charging power supply circuit 301B to the charging roller 342 based on the surface potential Vs measured by the potential measurement processing unit 52. As a result, the image forming apparatus 10 can maintain the surface potential Vs in an appropriate state and suppress deterioration of image quality due to fluctuations or unevenness in the surface potential Vs.

[0092] [4] Method for adjusting bias voltage in an image forming apparatus The following describes a bias voltage adjustment method performed in the image forming apparatus 10, along with an example of the procedure for bias voltage adjustment processing performed by the control unit 5 in the image forming apparatus 10, with reference to the flowchart in Figure 5. Here, steps S1, S2, etc., represent the processing procedure (step) performed by the control unit 5. In the following explanation, the processing procedure for K (black) toner is used as an example, but the control unit 5 performs similar processing for Y (yellow), C (cyan), and M (magenta).

[0093] <Step S1> First, in step S1, the control unit 5 determines whether the conditions for performing the adjustment of the development bias VB have been met.

[0094] <Step S2> If the adjustment execution conditions are determined to be met in step S1, the control unit 5 performs a process (surface potential measurement process) in the next step S2 to measure the surface potential Vs of the photoreceptor drum 341 based on the developing current flowing between the developing roller 343A and the photoreceptor drum 341. At this time, the potential measurement processing unit 52 of the control unit 5 uses the voltage values ​​set immediately before as the DC component Vdc and AC component Vac of the developing bias VB. Details of the surface potential Vs measurement method are explained in the section "[5] Details of each process".

[0095] <Step S3> In the next step S3, the control unit 5 performs a process to adjust the surface potential Vs of the photoreceptor drum 341 (potential adjustment process). At this time, the potential adjustment processing unit 55 of the control unit 5 adjusts the magnitude of the voltage applied by the charging power supply circuit 301B to the charging roller 342 based on the surface potential Vs measured in step S3, and adjusts the surface potential Vs to an appropriate value.

[0096] <Step S4> In the next step S4, the control unit 5 performs the DC calibration (DC calibration process). The process in step S4 is performed by the DC setting processing unit 50. This determines the voltage value of the DC component Vdc of the development bias VB (hereinafter sometimes referred to as the first voltage value). Step S4 is an example of the DC calibration step of the present invention. Details of the DC calibration performed in step S4 are described in the section "[5] Details of Each Process".

[0097] <Step S5> Once the first voltage value of the DC component Vdc is determined in step S4, in the next step S5, the control unit 5 performs a process (voltage determination process) to determine whether the first voltage value is greater than or equal to the upper limit voltage value VdcH. If it is determined that the first voltage value is greater than or equal to the upper limit voltage value VdcH, the control unit 5 proceeds to step S6. If it is determined that the first voltage value is less than the upper limit voltage value VdcH, the control unit 5 proceeds to step S7.

[0098] <Step S6> In step S6, the control unit 5 sets the DC component Vdc of the development bias VB to the same voltage value as the upper limit voltage value VdcH. After that, the control unit 5 proceeds to step S8.

[0099] <Step S7> In step S7, the control unit 5 sets the DC component Vdc of the development bias VB to the first voltage value. After that, the control unit 5 proceeds to step S8.

[0100] <Step S8> In step S8, the control unit 5 performs the AC calibration (AC calibration process) based on the set voltage value set in step S6 or S7, and determines the voltage value Vpp of the AC component Vac of the development bias VB. That is, in step S8, if the set voltage value is set to the first voltage value, the control unit 5 performs the AC calibration based on the first voltage value. On the other hand, if the set voltage value is set to the same voltage value as the upper limit voltage value VdcH, the control unit 5 performs the AC calibration based on the same voltage value as the upper limit voltage value VdcH. The process in step S8 is performed by the AC setting processing unit 51. After that, the process moves on to step S9. Steps S6 to S8 are an example of the AC calibration steps of the present invention.

[0101] Further details of the AC calibration performed in step S8 will be explained in section "[5] Details of each process".

[0102] <Step S9> In step S9, the control unit 5 controls the optical scanning device 35 and performs light intensity calibration (light intensity calibration process) to adjust the exposure amount. With this, the control unit 5 completes the series of processes.

[0103] The bias voltage adjustment method performed in the image forming apparatus 10 will be described below, along with an example of another example of the bias voltage adjustment process performed by the control unit 5 in the image forming apparatus 10, with reference to the flowchart in Figure 6. In the following description, the processing procedure for K (black) toner will be used as an example, but the control unit 5 will perform the same processing for Y (yellow), C (cyan), and M (magenta). Steps that are the same as those in the flowchart shown in Figure 5 will be indicated by reference numerals and their explanations will be omitted, with only the different steps being described.

[0104] <Step S21> In steps S6 and S7, once the voltage value (set voltage value) of the DC component Vdc of the development bias VB is set, in the next step S21, the control unit 5 determines whether the potential difference ΔVdc between the set voltage value and the predetermined voltage value is greater than or equal to the threshold a. If the potential difference ΔVdc is determined to be greater than or equal to the threshold a, the control unit 5 moves the process to step S8 and executes the processes from step S8 onward. The predetermined voltage value is, as described above, the smaller of the second voltage value determined by the DC calibration performed before the DC calibration in step S4 and the upper limit voltage value VdcH.

[0105] <Step S22> On the other hand, if in step S21 it is determined that the potential difference ΔVdc is less than the threshold a, in the next step S22 the control unit 5 corrects the voltage value Vpp of the AC component Vac using the set voltage value set in step S6 or S7 without performing the AC calibration. In this embodiment, as described above, the control unit 5 calculates the correction amount ΔVpp, corrects the set current voltage value Vpp of the AC component Vac using the correction amount ΔVpp, and sets the corrected voltage value as the voltage value Vpp of the AC component Vac. After that, the process moves to step S9.

[0106] The bias voltage adjustment procedure described above is merely an example, and the order of the processes shown in the flowcharts in Figures 5 and 6 may be changed as appropriate, or processes may be added or omitted as needed. For example, steps S2 and S3 are not mandatory. Therefore, steps S2 and S3 may be omitted.

[0107] Furthermore, in step S21 of Figure 6, for example, it may be determined whether the absolute value of the voltage difference between the first voltage value obtained by the DC calibration and the second voltage value obtained by the DC calibration performed earlier is greater than or equal to the threshold a.

[0108] Furthermore, the bias voltage adjustment process described above may be performed not only when the adjustment execution conditions are met, but also, for example, when the following individual trigger conditions are met. Examples of the individual trigger conditions include the image forming apparatus 10 reaching a predetermined time (for example, 4 hours) during power-up, the number of continuous prints reaching a certain number (for example, 200 sheets), or the internal temperature changing to a certain value (for example, 10 degrees) or more.

[0109] [5] Details of each process The details of each process performed by the control unit 5 in the image forming apparatus 10 will be described below with reference to Figures 7 to 9.

[0110] [5.1] Measurement of surface potential There are several methods for measuring (calculating) the surface potential Vs based on the development current. The first method uses the DC component Vdc of the development current when the DC component is 0 (zero) as the value of the surface potential Vs. The second method applies a development bias VB with the surface potential Vs on the photoreceptor drum 341, and uses the DC component Vdc of the development current that becomes equal to the target current when the DC component Vdc is changed as the value of the surface potential Vs. In the second method, when determining the target current, the DC component of the development current is used as the target current when the DC component Vdc of the development bias VB is set to 0 and only the AC component Vac is applied, with the surface potential Vs not on the photoreceptor drum 341.

[0111] The first method is simpler than the second method because it does not require pre-measuring the target current, but it is affected by the difference in length between the photoreceptor drum 341 and the developing roller 343A. For example, there are parts where the developing roller 343A and the photoreceptor drum 341 face each other via magnetic brushes, and parts where the developing roller 343A faces the photoreceptor drum 341 without the magnetic brushes, such as the end of the developing roller 343A. Due to this effect, the DC component Vdc when the DC component of the developing current is 0 (zero) is not exactly the same as the surface potential Vs value. If this point is considered strictly, the second method is superior. Also, setting the target current not only at the beginning but also during use can ensure higher measurement accuracy.

[0112] In this embodiment, the potential measurement processing unit 52 measures the surface potential Vs using the first method. For example, as shown in Figure 7, the potential measurement processing unit 52 sweeps the DC component Vdc of the development bias VB and determines the magnitude of the DC component Vdc when the DC component of the development current becomes 0 (zero). In the example in Figure 7, the development current becomes 0 when the voltage value of the DC component Vdc is 272V, so the potential measurement processing unit 52 identifies this voltage value of the DC component Vdc (272V) as the surface potential Vs.

[0113] In a two-component development system, the development current includes both a carrier current and a toner current, and the carrier current may flow even when there is little to no toner movement. The direction in which the carrier current flows is determined by the difference between the potential of the developing roller 343A and the surface potential Vs of the photoreceptor drum 341. That is, if the potential of the developing roller 343A is higher than the surface potential Vs of the photoreceptor drum 341, the development current will be a "positive" current flowing from the developing roller 343A to the photoreceptor drum 341. Conversely, if the potential of the developing roller 343A is lower than the surface potential Vs of the photoreceptor drum 341, the development current will be a "negative" current flowing from the photoreceptor drum 341 to the developing roller 343A.

[0114] Therefore, with the surface potential Vs on the photoreceptor drum 341, the potential of the developing roller 343A is changed in steps to obtain the developing current, the correlation between the potential of the developing roller 343A and the developing current is determined, and the potential of the developing roller 343A when the developing current becomes 0 is found to be the surface potential Vs. However, if the difference between the surface potential Vs and the potential of the developing roller 343A is large, the prediction accuracy of the surface potential Vs decreases due to the influence of the toner current associated with toner movement. Therefore, it is preferable that the difference between the surface potential Vs and the potential of the developing roller 343A is small. However, if the potential difference is too small, the resolution of the developing current decreases, and problems with measurement accuracy still arise.

[0115] Therefore, more preferably, the variation range of the DC component Vdc is set so as to sandwich the expected surface potential Vs above and below it. When the DC component Vdc is higher than the surface potential Vs, and when the DC component Vdc is lower than the surface potential Vs, the direction of the development current is reversed. Therefore, the potential measurement processing unit 52 acquires the change in development current when the DC component Vdc is varied within the variation range, and by identifying the DC component Vdc at which the development current becomes 0, the surface potential Vs can be determined. In this case, since the DC component Vdc is set so as to sandwich the surface potential Vs (for example, a potential difference of 100V or less is preferable, and 50V or less is more preferable), it becomes easier to set conditions that eliminate phenomena that cause errors in the development current, and the measurement accuracy is improved.

[0116] Furthermore, when measuring the surface potential Vs, it is preferable to use a sine wave rather than a square wave as the AC component Vac of the development bias VB. This is because, in the case of a square wave, changes in the developer resistance or development gap can change the impedance of the development region, easily causing distortion in the bias waveform. This distortion of the bias waveform affects the development current. In contrast, with a sine wave, such effects are less likely, allowing for stable measurement of the development current. However, when developing, a square wave is more advantageous than a sine wave because it provides higher toner movement efficiency. Therefore, it is more preferable for the image forming apparatus 10 to have a power supply circuit 301 that can switch between two types of waveforms, a square wave and a sine wave, as the AC component Vac.

[0117] Furthermore, in this embodiment, the photoreceptor drum 341 is a so-called amorphous silicon photoreceptor drum (a-Si drum) in which an amorphous silicon film is formed on the outer surface. In this case, as shown in Figure 7, the developing current is approximated linearly on the positive and negative sides, and the surface potential Vs is the point at which the developing current becomes 0. On the other hand, if the photoreceptor drum 341 is an organic photoconductor (OPC), there is a difference in the ease with which the developing current flows on the positive and negative sides. Therefore, it is preferable to set the surface potential Vs to the midpoint between the point at which the developing current becomes 0 in the linear approximation on the positive side and the point at which the developing current becomes 0 in the linear approximation on the negative side.

[0118] Furthermore, it is preferable to measure the development current on the development power supply circuit 301A side, which applies the development bias VB. Although the development current associated with toner movement can also be detected on the photoreceptor drum 341 side, current flows into the photoreceptor drum 341 from the charging member or transfer member, making it difficult to isolate the toner current. For this reason, it is preferable to measure the development current on the development power supply circuit 301A side.

[0119] [5.2] AC calibration <Changes in toner deposition and development current> When the charge amount of the toner or the development gap changes, the force of the toner's movement due to the electric field changes, causing the image density to fluctuate. However, the AC component Vac and the DC component Vdc of the development bias VB exhibit different characteristics. With respect to the AC component Vac, increasing its magnitude Vpp increases the image density, but eventually the increase in image density almost disappears, and further increasing the magnitude Vpp actually decreases the image density. On the other hand, with respect to the DC component Vdc, increasing the difference (Vdc-VL) between the potential VL of the exposed area on the photoreceptor drum 341 and the DC component Vdc causes the image density to continue to increase. Even if the rate of increase in image density decreases as the DC component Vdc continues to increase, it has not been confirmed that the image density decreases. This is because the electric field due to the AC component Vac is a reciprocating electric field, while the electric field due to the DC component Vdc is a unidirectional electric field.

[0120] More specifically, with respect to the AC component Vac, there are electric fields in opposite directions to the developing electric field and the recovery electric field. When the magnitude Vpp of the AC component Vac increases, both of these electric fields increase, but eventually the amount of toner moved by the developing electric field reaches its upper limit. Subsequently, if the magnitude Vpp of the AC component Vac increases further, the amount of toner recovered increases due to the increase in the recovery electric field, but since the amount of toner moved by the developing electric field has reached its upper limit, the amount of toner moved from the developing roller 343A to the photoreceptor drum 341 remains unchanged. As a result, the total amount of toner moved decreases.

[0121] As described above, the developing current includes the toner current that flows as the toner moves, the magnetic brush current that flows through the magnetic brush in the image area, and the reverse magnetic brush current that flows through the magnetic brush in the non-image area in the opposite direction to that of the image area. When the magnitude Vpp of the AC component Vac is increased, the developing current increases in proportion to the increase in the magnitude Vpp of the AC component Vac, but eventually the change becomes smaller. It is known that the current can then exhibit a first behavior in which it hardly changes, a second behavior in which it continues to increase slowly, and a third behavior in which it decreases (see Figure 8).

[0122] Regarding the magnitude Vpp of the AC component Vac, it is preferable to set it in a region where the change in image density is small. This is because under such conditions, even if the toner charge amount or the developing gap changes, the change in image density is small. And as shown in FIG. 8, the intersection of the first approximation formula F1 and the second approximation formula F2 respectively obtained in the first measurement range R1 and the second measurement range R2 indicates the magnitude Vpp of the AC component Vac. Also, this stable state changes with the setting of the DC component Vdc. That is, when the DC component Vdc changes, the optimum value of the magnitude Vpp of the AC component Vac also changes. Therefore, it is preferable to perform the setting of the DC component Vdc (DC calibration) and the setting of the magnitude Vpp of the AC component Vac (AC calibration) as a set.

[0123] <Method for setting Vpp> As a specific method, as shown in FIG. 8, the AC setting processing unit 51 first changes (Vp1, Vp2, Vp3, Vp4) the magnitude Vpp of the AC component Vac of the developing bias VB within the first measurement range R1 and acquires the developing current at that time. Then, the AC setting processing unit 51 changes (Vp5, Vp6, Vp7) the magnitude Vpp of the AC component Vac of the developing bias VB within the second measurement range R2 higher than the first measurement range R1 and also acquires the developing current at that time. Then, the AC setting processing unit 51 creates the first approximation formula F1 from the plots obtained in the first measurement range R1 and creates the second approximation formula F2 from the plots obtained in the second measurement range R2 in a graph with the horizontal axis being the magnitude Vpp of the AC component Vac and the vertical axis being the developing current. And the AC setting processing unit 51 uses the value (Vpp) of the intersection of the first approximation formula F1 and the second approximation formula F2 as the "reference value" and determines the magnitude Vpp of the AC component Vac based on this reference value.

[0124] In this process, the first approximation formula F1 is a linear approximation formula for the plot obtained within the first measurement range R1, and its slope is "positive". For the second approximation formula F2, a linear approximation formula for the plot obtained within the second measurement range R2 is determined, and if the slope of this linear approximation formula is "greater than or equal to a certain value", this linear approximation formula is used as the second approximation formula F2. If the slope of this linear approximation formula is "less than a certain value", the average of the developing currents of the plot obtained in the second measurement range R2 is used as the y-intercept value of the linear approximation formula, and a linear approximation formula with a slope of "0" is used as the second approximation formula F2. Here, the certain value is "0" as an example. The AC setting processing unit 51 then determines the Vpp (reference value) at the point where the first approximation formula F1 and the second approximation formula F2 intersect.

[0125] Furthermore, the AC setting processing unit 51 does not acquire the developing current between the upper limit (of Vpp) of the first measurement range R1 and the lower limit (of Vpp) of the second measurement range R2. Therefore, in order to improve the calculation accuracy of the first approximation formula F1 and the second approximation formula F2 and to correctly determine the intersection point, the interval between the upper limit of the first measurement range R1 and the lower limit of the second measurement range R2 must be wide with respect to the average interval of Vpp used for measurement in the first measurement range R1 and the average interval of Vpp used for measurement in the second measurement range R2.

[0126] Furthermore, while it is simple to read and set the image density to set the magnitude Vpp of the AC component Vac in a region where the image density is stable, the optical sensor of the density detection unit 365, which detects the image density of the test toner image on the photoreceptor drum 341 or the intermediate transfer belt 361, has reduced measurement accuracy as the image density increases. Therefore, the accuracy of setting the magnitude Vpp of the AC component Vac tends to decrease. In other words, since these optical sensors are usually used for halftone images, even if the first measurement range R1 is measurable, measuring the second measurement range R2 is difficult. For this reason, it is preferable to use the development current rather than the image density to set the magnitude Vpp of the AC component Vac.

[0127] Furthermore, the approximation formula is set to calculate in regions where the developing current is increasing, stabilizing, or decreasing. However, if the calculation is performed in a region where the change in developing current is different due to some malfunction, the setting of the magnitude Vpp of the AC component Vac will be incorrect. Therefore, if the deviation of the measurement data from the approximation formula becomes large, it is necessary to exclude some of the measurement data as error values ​​from the calculation. Thus, when calculating the approximation formula, the correlation coefficient is also calculated at the same time. If the correlation coefficient falls below a specified value (for example, 0.9), in the case of the first measurement range R1, the measurement data with the highest Vpp within that range is excluded, and the approximation formula calculation is performed again. In the case of the second measurement range R2, there is a possibility of abnormal current due to leakage, so the approximation formula calculation is performed separately for the case where the measurement data with the lowest Vpp is excluded and the case where the measurement data with the highest Vpp is excluded. Then, the one with the higher correlation coefficient and a correlation coefficient equal to or greater than the specified value is used. In the case of the second measurement range R2, this is only done if the slope is less than a specified value. If the correlation coefficient does not equal or greater than the specified value even after excluding the error values, the error values ​​are excluded again using the same method, and the calculation is performed again.

[0128] Furthermore, the first measurement range R1 is where the developing current changes significantly, so it is preferable to make it as wide as possible. On the other hand, the second measurement range R2 shows little change, and further increasing Vpp may cause developing leakage. In addition, to improve measurement accuracy, it is desirable to increase the number of measurement points over a wide range, but increasing the number of measurement points leads to an increase in toner consumption and measurement time. Taking these factors into consideration, it is preferable to make the second measurement range R2 narrower than the first measurement range R1 and to reduce the number of measurement points.

[0129] [5.3] DC Calibration As shown in Figure 9, the DC setting processing unit 50 changes the voltage value of the DC component Vdc of the development bias VB, for example, as detected values ​​V1, V2, V3, V4, to create the test toner image on the photoreceptor drum 341. Then, with the test toner image transferred to the intermediate transfer belt 361, the DC setting processing unit 50 obtains the density of the test toner image for each detected value V1, V2, V3, V4 from the density detection unit 365 as sensor outputs Op1, Op2, Op3, Op4, respectively.

[0130] The DC setting processing unit 50 creates a first-order approximation equation on a graph with the DC component Vdc on the horizontal axis and the sensor output (output of the density detection unit 365) on the vertical axis. Using this approximation equation, it determines the voltage Vdc0 when the sensor output Op0 is equal to the target image density, and sets the voltage value of the DC component Vdc. However, if the voltage Vdc0 obtained at this time is less than or equal to the lower limit voltage value VdcL of the DC component Vdc (for example, 40V), the DC setting processing unit 50 determines the voltage value of the DC component Vdc to be the lower limit voltage value VdcL. Also, if the voltage Vdc0 is greater than or equal to the upper limit voltage value VdcH (for example, 200V), the DC setting processing unit 50 determines the voltage value of the DC component Vdc to be the upper limit voltage value VdcH.

[0131] [Notes on the invention] The following is an overview of the invention extracted from the above-described embodiments. Note that each configuration and processing function described below can be selected and combined as desired.

[0132] <Note 1> An image forming unit applies a bias voltage in which an AC component is superimposed on a DC component between a first carrier whose surface is charged and a second carrier which holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data. A DC calibration processing unit that performs DC calibration to determine the voltage value of the DC component of the bias voltage, The system includes an AC calibration processing unit that performs AC calibration to determine the voltage value of the AC component of the bias voltage, The AC calibration processing unit is An image forming apparatus that determines the voltage value of the AC component based on the first voltage value when the first voltage value determined by the DC calibration processing unit is less than a predetermined upper voltage limit value for the DC component, and determines the voltage value of the AC component based on the upper voltage limit value when the first voltage value is equal to or greater than the upper voltage limit value.

[0133] <Note 2> The DC calibration processing unit is as follows: The voltage value of the DC component of the bias voltage is determined based on the density value of a predetermined measurement toner image developed by applying the bias voltage before performing the DC calibration. The AC calibration processing unit is The image forming apparatus according to Appendix 1, wherein, after performing the DC calibration, the voltage value of the AC component is determined based on the density value of the measurement toner image developed by applying the bias voltage including the first voltage value or the upper limit voltage value, or the current value of the development current flowing between the first and second carriers during the development of the measurement toner image.

[0134] <Note 3> An image forming unit applies a bias voltage in which an AC component is superimposed on a DC component between a first carrier whose surface is charged and a second carrier which holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data. A DC calibration processing unit that performs DC calibration to determine the voltage value of the DC component of the bias voltage, The system includes an AC calibration processing unit that performs AC calibration to determine the voltage value of the AC component of the bias voltage, The DC calibration processing unit sets the first voltage value determined by the DC calibration to the voltage value of the DC component of the bias voltage if the first voltage value is less than a predetermined upper limit voltage value with respect to the DC component, and sets the upper limit voltage value to the voltage value of the DC component of the bias voltage if the first voltage value is equal to or greater than the upper limit voltage value. The AC calibration processing unit is An image forming apparatus that corrects the voltage value of the AC component based on a set voltage value set by the DC calibration processing unit, and the smaller of the second voltage value obtained by the DC calibration performed earlier and the upper limit voltage value.

[0135] <Note 4> The image forming apparatus according to any one of the appendices 1 to 3, further comprising an upper limit voltage determination unit that corrects the upper limit voltage value based on the environmental conditions of the location where the image forming apparatus is installed.

[0136] <Note 5> The image forming apparatus according to Appendix 4, wherein, when the environmental condition is absolute humidity, the upper limit voltage determination unit corrects the upper limit voltage value according to the absolute humidity.

[0137] <Note 6> The image forming apparatus according to Appendix 4, wherein, when the aforementioned environmental conditions are the amount of change in the process speed of the image forming unit, the upper limit voltage determination unit corrects the upper limit voltage value according to the amount of change.

[0138] <Note 7> An image forming unit applies a bias voltage in which an AC component is superimposed on a DC component between a first carrier whose surface is charged and a second carrier which holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data. A DC calibration processing unit that performs DC calibration to determine the voltage value of the DC component of the bias voltage, The system includes an AC calibration processing unit that performs AC calibration to determine the voltage value of the AC component of the bias voltage, The DC calibration processing unit sets the first voltage value determined by the DC calibration to the voltage value of the DC component of the bias voltage if the first voltage value is less than a predetermined upper limit voltage value with respect to the DC component, and sets the upper limit voltage value to the voltage value of the DC component of the bias voltage if the first voltage value is equal to or greater than the upper limit voltage value. The AC calibration processing unit is If the voltage difference between the set voltage value set by the DC calibration processing unit and the smaller of the second voltage value obtained by the DC calibration performed earlier and the upper limit voltage value is greater than or equal to a predetermined threshold, the voltage value of the AC component is determined based on the set voltage value. An image forming apparatus that corrects the voltage value of the AC component using the set voltage value when the voltage difference is less than the threshold.

[0139] <Note 8> A bias voltage adjustment method applied to an image forming apparatus in which a bias voltage having an AC component superimposed on a DC component is applied between a first carrier whose surface is charged and a second carrier that holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data, wherein one or more processors perform the following steps: A DC calibration step is performed to determine the voltage value of the DC component of the bias voltage, The AC calibration step includes performing an AC calibration to determine the voltage value of the AC component of the bias voltage, The aforementioned AC calibration step is, A bias voltage adjustment method comprising: determining the voltage value of the AC component based on the first voltage value when the first voltage value determined by the DC calibration step is less than a predetermined upper voltage limit value with respect to the DC component; and determining the voltage value of the AC component based on the upper voltage limit value when the first voltage value is equal to or greater than the upper voltage limit value.

[0140] <Note 9> A program applied to an image forming apparatus that applies a bias voltage having an AC component superimposed on a DC component between a first carrier whose surface is charged and a second carrier that holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data, causing one or more processors to perform the following steps: A DC calibration step is performed to determine the voltage value of the DC component of the bias voltage, The AC calibration step includes performing an AC calibration to determine the voltage value of the AC component of the bias voltage, The aforementioned AC calibration step is, A program, or a non-temporary computer-readable storage medium on which such program is stored, which determines the voltage value of the AC component based on the first voltage value if the first voltage value determined by the DC calibration step is less than a predetermined upper voltage limit for the DC component, and determines the voltage value of the AC component based on the upper voltage limit if the first voltage value is equal to or greater than the upper voltage limit. [Explanation of symbols]

[0141] 3 Image forming unit 10 Image forming apparatus 50 DC setting processing unit 51 AC setting processing unit 52 Potential Measurement Processing Unit 54 Upper limit voltage determination processing unit 54 55 Potential adjustment section 311, 321, 331, 341 Photoconductor drum 313A, 323A, 333A, 343A Developing Roller VB Development Bias Vac AC component Vdc DC component

Claims

1. An image forming unit applies a bias voltage in which an AC component is superimposed on a DC component between a first carrier whose surface is charged and a second carrier which holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data. A DC calibration processing unit that performs DC calibration to determine the voltage value of the DC component of the bias voltage, The system includes an AC calibration processing unit that performs AC calibration to determine the voltage value of the AC component of the bias voltage, The AC calibration processing unit is An image forming apparatus that determines the voltage value of the AC component based on the first voltage value when the first voltage value determined by the DC calibration processing unit is less than a predetermined upper voltage value for the DC component, and determines the voltage value of the AC component based on the upper voltage value when the first voltage value is equal to or greater than the upper voltage value.

2. The DC calibration processing unit is as follows: The voltage value of the DC component of the bias voltage is determined based on the density value of a predetermined measurement toner image developed by applying the bias voltage before performing the DC calibration. The AC calibration processing unit is The image forming apparatus according to claim 1, wherein, after performing the DC calibration, the voltage value of the AC component is determined based on the density value of the measurement toner image developed by applying the bias voltage including the first voltage value or the upper limit voltage value, or the current value of the development current flowing between the first and second carriers during the development of the measurement toner image.

3. An image forming unit applies a bias voltage in which an AC component is superimposed on a DC component between a first carrier whose surface is charged and a second carrier which holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data. A DC calibration processing unit that performs DC calibration to determine the voltage value of the DC component of the bias voltage, The system includes an AC calibration processing unit that performs AC calibration to determine the voltage value of the AC component of the bias voltage, The DC calibration processing unit sets the first voltage value determined by the DC calibration to the voltage value of the DC component of the bias voltage if the first voltage value is less than a predetermined upper limit voltage value with respect to the DC component, and sets the upper limit voltage value to the voltage value of the DC component of the bias voltage if the first voltage value is equal to or greater than the upper limit voltage value. The AC calibration processing unit is An image forming apparatus that corrects the voltage value of the AC component based on a set voltage value set by the DC calibration processing unit, and the smaller of the second voltage value obtained by the DC calibration performed earlier and the upper limit voltage value.

4. The image forming apparatus according to any one of claims 1 to 3, further comprising an upper limit voltage determination unit that corrects the upper limit voltage value based on the environmental conditions of the location where the image forming apparatus is installed.

5. The image forming apparatus according to claim 4, wherein, when the environmental condition is absolute humidity, the upper limit voltage determination unit corrects the upper limit voltage value according to the absolute humidity.

6. The image forming apparatus according to claim 4, wherein, if the environmental conditions are the amount of change in the process speed of the image forming unit, the upper limit voltage determination unit corrects the upper limit voltage value according to the amount of change.

7. An image forming unit applies a bias voltage in which an AC component is superimposed on a DC component between a first carrier whose surface is charged and a second carrier which holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data. A DC calibration processing unit that performs DC calibration to determine the voltage value of the DC component of the bias voltage, The system includes an AC calibration processing unit that performs AC calibration to determine the voltage value of the AC component of the bias voltage, The DC calibration processing unit sets the first voltage value determined by the DC calibration to the voltage value of the DC component of the bias voltage if the first voltage value is less than a predetermined upper limit voltage value with respect to the DC component, and sets the upper limit voltage value to the voltage value of the DC component of the bias voltage if the first voltage value is equal to or greater than the upper limit voltage value. The AC calibration processing unit is If the voltage difference between the set voltage value set by the DC calibration processing unit and the smaller of the second voltage value obtained by the DC calibration performed earlier and the upper limit voltage value is greater than or equal to a predetermined threshold, the voltage value of the AC component is determined based on the set voltage value. An image forming apparatus that corrects the voltage value of the AC component using the set voltage value when the voltage difference is less than the threshold.

8. A bias voltage adjustment method applied to an image forming apparatus in which a bias voltage having an AC component superimposed on a DC component is applied between a first carrier whose surface is charged and a second carrier that holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data, wherein one or more processors perform the following steps: A DC calibration step is performed to determine the voltage value of the DC component of the bias voltage, The AC calibration step includes performing an AC calibration to determine the voltage value of the AC component of the bias voltage, The aforementioned AC calibration step is, A bias voltage adjustment method comprising: determining the voltage value of the AC component based on the first voltage value when the first voltage value determined by the DC calibration step is less than a predetermined upper voltage limit value with respect to the DC component; and determining the voltage value of the AC component based on the upper voltage limit value when the first voltage value is equal to or greater than the upper voltage limit value.

9. A program applied to an image forming apparatus that applies a bias voltage having a DC component superimposed on an AC component between a first carrier whose surface is charged and a second carrier that holds toner to be attached to the first carrier, thereby moving toner from the second carrier to the first carrier and forming a toner image on the first carrier based on image data, and causing one or more processors to perform the following steps: A DC calibration step is performed to determine the voltage value of the DC component of the bias voltage, The AC calibration step includes performing an AC calibration to determine the voltage value of the AC component of the bias voltage, The aforementioned AC calibration step is, A program that determines the voltage value of the AC component based on the first voltage value if the first voltage value determined by the DC calibration step is less than a predetermined upper voltage limit for the DC component, and determines the voltage value of the AC component based on the upper voltage limit if the first voltage value is equal to or greater than the upper voltage limit.

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