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 stabilize image density, adjust the bias voltage in a timely manner in response to changes in process speed, and prevent a decrease in the productivity of the image forming process. [Solution] The image forming apparatus 10 comprises an image forming unit 3, a DC setting processing unit 50, and an AC setting processing unit 51. The DC setting processing unit 50 performs DC calibration for each of a plurality of process speeds that can be applied to the image forming unit, and the AC setting processing unit 51 determines an AC voltage value based on the DC voltage value corresponding to each of the plurality of process speeds determined by the DC calibration performed by the DC setting processing unit 50.

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 a latent electrostatic image formed on the surface of a photoreceptor drum based on an electrophotographic method to form a toner image on the photoreceptor drum is known. In the image forming apparatus, a charged photoreceptor drum is exposed based on image data, and the latent electrostatic image is formed on the photoreceptor drum. Then, the image forming apparatus applies a bias voltage (development bias) to a developing roller, and supplies charged toner to an exposed portion (a portion where the latent electrostatic image is formed) of the photoreceptor drum according to the electric field between the photoreceptor drum and the developing roller. As a result, toner adheres to the exposed portion on the photoreceptor drum, and the latent electrostatic image on the photoreceptor 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, with the conventional technology configuration described above, it is not possible to adjust the bias voltage in a timely manner in response to changes in process speed, and the productivity of the image formation process is also reduced.

[0006] The object of the present invention is to provide an image forming apparatus, a bias voltage adjustment method, and a control program that can stabilize image density, adjust the bias voltage in a timely manner in response to changes in process speed, and prevent a decrease in the productivity of the image forming process. [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 DC voltage value of the DC component of the bias voltage; and an AC calibration processing unit that performs AC calibration to determine the AC voltage value of the AC component of the bias voltage. The DC calibration processing unit is configured to perform the DC calibration for each of a plurality of process speeds that can be applied to the image forming unit, and the AC calibration processing unit determines the AC voltage value based on the DC voltage value corresponding to a predetermined process speed determined by the DC calibration processing unit.

[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 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, wherein one or more processors perform the following steps: a DC calibration step that performs a DC calibration to determine the DC voltage value of the DC component of the bias voltage; and an AC calibration step that performs an AC calibration to determine the AC voltage value of the AC component of the bias voltage. The DC calibration step is: The DC calibration is performed for a predetermined process speed among a plurality of process speeds that can be applied to the image forming unit, and the AC calibration step determines the AC voltage value based on the DC voltage value corresponding to the predetermined process speed determined in the DC calibration step.

[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: a DC calibration step that performs a DC calibration to determine the DC voltage value of the DC component of the bias voltage; and an AC calibration step that performs an AC calibration to determine the AC voltage value of the AC component of the bias voltage. The DC calibration step performs the DC calibration for a predetermined process speed among a plurality of process speeds that can be applied to the image forming unit, and the AC calibration step determines the AC voltage value based on the DC voltage value corresponding to the predetermined process speed determined in the DC calibration step. [Effects of the Invention]

[0010] According to the present invention, it is possible to stabilize the image density while adjusting the bias voltage in a timely manner in response to changes in the process speed, and to prevent a decrease in the productivity of the image formation process. [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 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 7] Figure 7 is an explanatory diagram showing an example of AC calibration 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 DC calibration in an image forming apparatus according to an embodiment of the present invention. [Modes for carrying out the invention]

[0012] The embodiments of the present invention will be described below with reference to the attached drawings. The following embodiments are examples that embody the present invention and are not intended to limit the technical scope of the present invention.

[0013] [1] Overall configuration of the image forming apparatus First, the overall configuration of the image forming apparatus 10 according to this embodiment will be described with reference to Figures 1 and 2.

[0014] For the sake of explanation, the vertical direction is defined as the up-down direction D1 when the image forming apparatus 10 is in a usable installation state (as shown in Figure 2). The front-to-back direction D2 is defined with the left side of the image forming apparatus 10 shown in Figure 2 as the front. The left-to-right direction D3 is defined with the front of the image forming apparatus 10 in its installed state as the reference point.

[0015] The image forming apparatus 10 according to this embodiment is, for example, a multifunction device having multiple functions such as a scanning function for reading image data from a document, a printing function for forming an image based on the image data, a facsimile function, and a copying function. The image forming apparatus 10 only needs to have the function of forming an image, and may be a printer, a facsimile machine, a copier, etc.

[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 conveying rollers, a document presser, a paper discharge unit, and the like.

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

[0019] The image forming unit 3 realizes the 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. Further, 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 has a paper feeding cassette, a manual feed tray, a sheet conveyance path, a plurality of conveying rollers, and the like. 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 primarily 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 pre-recorded in memory (storage unit 6), provided via a telecommunication line such as the Internet, or provided on a non-temporary recording medium readable by a computer system, such as a memory card or optical disc. One or more processors consist of one or more electronic circuits including a semiconductor integrated circuit. Furthermore, 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 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 portion of the surface of the photoreceptor drum 341 irradiated with light from the light scanning device 35 is "VL", then the DC component Vdc will be a value between the surface potential Vs and the potential VL. In other words, the voltage value of the DC component Vdc will be lower than the surface potential Vs and higher than the potential VL of the exposed portion.

[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 non-exposed 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, with the above-mentioned related technologies, it is not possible to adjust the development bias in a timely manner in response to changes in the process speed in the image forming unit, and the productivity of the image forming process is also reduced.

[0058] In contrast, the image forming apparatus 10 according to this embodiment, with the configuration described below, can stabilize the image density while adjusting the bias voltage in a timely manner in response to changes in the process speed, and can prevent a decrease in the productivity of the image forming process.

[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 a DC calibration processing unit according to the present invention), and an AC setting processing unit 51 (an example of an AC calibration processing unit according to 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, an AC voltage correction processing unit 52 (an example of the AC component correction processing unit of the present invention), a speed change processing unit 53, a potential measurement processing unit 54, 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 (DC 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 processing times (for example, 1000 prints) 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 according to 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. The development bias VB at the time of creating the test toner image is the bias voltage currently set.

[0064] In this embodiment, if the image forming unit 3 performs image forming processing at one of a plurality of process speeds, the DC setting processing unit 50 performs the DC calibration for the process speed that can be applied to the image forming unit 3.

[0065] In the image forming apparatus 10, the process speed during image forming in the image forming unit 3 is changed, for example, according to the content of the image data. For example, if the image data is a dark photograph or a dark solid image, more toner is used compared to a light text image, so the process speed is changed to a slower speed than when printing the text image (for example, the reference process speed). In this embodiment, the process speed applied in the image forming unit 3 is set by the speed change processing unit 53, which will be described later. The process speed is, for example, the linear velocity of the photoreceptor drum 341.

[0066] For example, the DC setting processing unit 50 causes each of the image forming units 31 to 34, which operate at a preset process speed, to create the test toner image. 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, in the non-contact area on the outer circumferential surface of the intermediate transfer belt 361. Alternatively, the test toner image may be formed in the contact area of ​​the intermediate transfer belt 361.

[0067] 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.

[0068] The DC setting processing unit 50 performs the DC calibration process described above for a predetermined process speed applied to the image forming unit 3. That is, for example, if the process speed of the image forming unit 3 is set to a predetermined process speed, the DC setting processing unit 50 operates the image forming unit 3 at that predetermined process speed, performs the DC calibration, and determines the voltage value of the DC component Vdc corresponding to the predetermined process speed. Then, the DC setting processing unit 50 instructs the speed change processing unit 53 to change the process speed of the image forming unit 3 from the predetermined process speed to another process speed. After that, the DC setting processing unit 50 operates the image forming unit 3 at the other process speed, performs the DC calibration, and determines the voltage value of the DC component Vdc corresponding to the other process speed.

[0069] The voltage value of the DC component Vdc determined by the DC setting processing unit 50 is stored in the storage unit 6.

[0070] 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.

[0071] 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.

[0072] The AC setting processing unit 51 performs AC calibration by determining and setting the voltage value Vpp (AC voltage value) of the AC component Vac of the development bias VB. The voltage value Vpp of the AC component Vac of the development bias VB is the voltage between the peaks of adjacent AC waveforms.

[0073] In this embodiment, the AC setting processing unit 51 determines the voltage value Vpp of the AC component Vac based on the voltage value (DC voltage value) determined by the DC calibration. As described above, the DC setting processing unit 50 performs the DC calibration process and determines the voltage value of the DC component corresponding to the predetermined process speed. Therefore, when the voltage value of the DC component corresponding to the predetermined process speed is determined, the AC setting processing unit 51 determines the voltage value Vpp of the AC component based on the voltage value (DC voltage value) of the DC component corresponding to the predetermined process speed.

[0074] 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 changes the DC component of the development bias VB to the voltage value of the DC component Vdc determined by the DC setting processing unit 50, and then sequentially sets the voltage value Vpp of the AC component Vac to one of several values ​​within a predetermined range, thereby creating a predetermined solid black image on the photoreceptor drum 341. The current detection circuit 302 detects the development current flowing between the development roller 343A and the photoreceptor drum 341 during development. The AC setting processing unit 51 then determines an appropriate voltage value Vpp of the AC component Vac from this change in development current.

[0075] 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.

[0076] Furthermore, when developing toner, the DC component Vdc 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.

[0077] 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.

[0078] 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.

[0079] The AC voltage correction processing unit 52 performs a process to correct the voltage value Vpp of the AC component Vac determined by the AC calibration of the AC setting processing unit 51. Specifically, the AC voltage correction processing unit 52 corrects the voltage value Vpp of the AC component Vac determined by the AC calibration based on the voltage difference (voltage change) between the voltage value of the DC component Vdc used before the DC calibration of the DC setting processing unit 50 was performed (hereinafter referred to as the first DC voltage value) and the voltage value of the DC component Vdc determined by the DC calibration of the DC setting processing unit 50 (hereinafter referred to as the second DC voltage value). Specifically, the first DC voltage value is the voltage value determined by the DC calibration performed immediately before the DC calibration that determines the second DC voltage value.

[0080] In this embodiment, the AC voltage correction processing unit 52 determines whether the absolute value of the voltage difference between the first DC voltage value and the second DC voltage value is greater than a predetermined determination threshold, and if the absolute value is greater than the determination threshold, it corrects the voltage value Vpp of the AC component Vac determined by the AC calibration of the AC setting processing unit 51.

[0081] Specifically, the AC voltage correction processing unit 52 calculates a correction amount ΔVpp to correct the voltage value Vpp when the absolute value of the voltage difference is equal to or greater than the determination threshold. Then, if the second DC voltage value is greater than the first DC voltage value, the corrected voltage value Vpp is set to the value obtained by adding the correction amount ΔVpp to the set current voltage value Vpp of the AC component Vac. On the other hand, if the second DC voltage value is less than the first DC voltage value, the corrected voltage value Vpp is set to the value obtained by subtracting the correction amount ΔVpp from the set current voltage value Vpp of the AC component Vac.

[0082] The speed change processing unit 53 performs a process to change the process speed of the image forming unit 3 (speed change processing). The speed change processing unit 53 performs a process to change the process speed when the image forming unit 3 is imaged at multiple applicable process speeds, while the calibration of the development bias VB is performed at each of the multiple applicable process speeds.

[0083] Furthermore, when a print job is input to the image forming unit 3, the speed change processing unit 53 changes the process speed of the image forming unit 3 according to the content of the image data included in the print job.

[0084] For example, when image data such as a photograph or solid image is input, the speed change processing unit 53 performs a deceleration process to change the process speed to another process speed (first process speed) that is slower than the reference process speed in order to mitigate the degradation of image quality.

[0085] For example, when the speed change processing unit 53 acquires image data to be printed, it determines whether the image data is a solid image or a text image. If it is a text image, it maintains the process speed at the initial reference process speed. If it is a solid image, it changes the process speed from the reference process speed to the first process speed. The first process speed is, for example, 3 / 4 of the reference process speed.

[0086] Furthermore, the speed change processing unit 53 determines whether the image data to be printed is a solid image and whether it contains a standard number or more of high-density pixels exceeding a predetermined gradation. If the number of high-density pixels is less than the standard number, the process speed is changed from the standard process speed to the first process speed. If the number of pixels is equal to or greater than the standard number, the process speed is changed from the standard process speed to another process speed (second process speed) that is slower than the first process speed. The second process speed is, for example, half the speed of the standard process speed.

[0087] Furthermore, the speed change processing unit 53 performs a process to change the process speed of the image forming unit 3 to a process speed corresponding to the input setting value when, for example, a process speed setting instruction is input from the operation display unit 7 of the image forming apparatus 10.

[0088] The potential measurement processing unit 54 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 54 measures the surface potential Vs based on the developing current detected by the current detection circuit 302. Specifically, the potential measurement processing unit 54 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 54 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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).

[0093] 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).

[0094] 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.

[0095] The potential adjustment processing unit 55 adjusts the surface potential Vs based on the surface potential Vs measured by the potential measurement processing unit 54. 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 54. As a result, the image forming apparatus 10 can maintain the surface potential Vs in an appropriate state and suppress image quality degradation due to fluctuations or unevenness in the surface potential Vs.

[0096] [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).

[0097] <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.

[0098] <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 54 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".

[0099] <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.

[0100] <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 second DC 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".

[0101] <Step S5> In step S5, the control unit 5 performs a process to calculate the voltage difference (voltage change) between the voltage value of the DC component Vdc of the development bias VB that was used before the DC calibration was performed in step S4 (first DC voltage value) and the voltage value of the DC component Vdc of the development bias VB determined by the DC calibration in step S4 (second DC voltage value).

[0102] <Step S6> In step S6, the control unit 5 determines whether the voltage difference (voltage change) is greater than or equal to a predetermined determination threshold. The determination threshold is, for example, a threshold used to determine whether or not to perform a correction of the voltage value Vpp of the AC component Vac. If the voltage difference is greater than or equal to the determination threshold, the process proceeds to step S7; if the voltage difference is less than the determination threshold, the process proceeds to step S8.

[0103] <Step S7> If it is determined in step S6 that the voltage difference is equal to or greater than the determination threshold, in the next step S7, the control unit 5 performs the AC calibration (AC calibration process) based on the second DC voltage value and determines the voltage value Vpp of the AC component Vac of the development bias VB. That is, the DC component of the development bias VB used for development in the image forming unit 3 is changed to the second DC voltage value determined in step S4, a predetermined image (solid black image) is imaged on the photoreceptor drum 341 using the changed development bias VB, and the voltage value Vpp of the AC component Vac of the development bias VB is determined based on the development current at that time. The process in step S7 is performed by the AC setting processing unit 51. After that, a series of bias voltage adjustment processes are completed. Note that step S7 is an example of the AC calibration step of the present invention.

[0104] <Step S8> If it is determined in step S6 that the voltage difference is less than the threshold for determination, the AC calibration in step S7 is not performed, and in the next step S8, the control unit 5 performs a process to correct the voltage value Vpp of the AC component Vac. This correction process is performed by the AC voltage correction processing unit 52. After that, the series of bias voltage adjustment processes are completed.

[0105] The bias voltage adjustment procedure described above is merely an example, and the order of the processes shown in the flowchart of Figure 5 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.

[0106] 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.

[0107] [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 6 to 8.

[0108] [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.

[0109] 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.

[0110] In this embodiment, the potential measurement processing unit 54 measures the surface potential Vs using the first method. For example, as shown in Figure 6, the potential measurement processing unit 54 sweeps the DC component Vdc of the development bias VB and identifies the magnitude of the DC component Vdc when the DC component of the development current becomes 0 (zero). In the example in Figure 6, the development current becomes 0 when the voltage value of the DC component Vdc is 272V, so the potential measurement processing unit 54 identifies this voltage value of the DC component Vdc (272V) as the surface potential Vs.

[0111] 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.

[0112] 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.

[0113] 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 54 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.

[0114] 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.

[0115] 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 6, 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.

[0116] 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.

[0117] [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.

[0118] More specifically, regarding the alternating current component Vac, there exist electric fields in opposite directions between the development electric field and the recovery electric field. When the magnitude Vpp of the alternating current component Vac increases, both of these electric fields increase. However, eventually, the amount of toner movement due to the development electric field reaches its upper limit. After that, when the magnitude Vpp of the alternating current component Vac further increases, the amount of toner recovery increases due to the increase in the recovery electric field. However, on the development electric field side, since the amount of toner movement has reached its upper limit, the amount of toner movement from the development roller 343A to the photoreceptor drum 341 does not change. As a result, the total amount of toner movement decreases.

[0119] As described above, the development current includes the toner current flowing with the movement of the toner, the magnetic brush current flowing through the magnetic brush in the image portion, and the reverse magnetic brush current flowing through the magnetic brush in the non-image portion in the opposite direction to the image portion. The development current when increasing the magnitude Vpp of the alternating current component Vac rises as the magnitude Vpp of the alternating current component Vac increases. However, eventually, the change becomes small, and it is known to exhibit the first behavior of remaining almost unchanged, the second behavior of continuing to rise more slowly, and the third behavior of decreasing conversely (see Fig. 7).

[0120] Regarding the magnitude Vpp of the alternating current 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 charge amount of the toner or the development gap changes, the change in image density is small. And as shown in Fig. 7, 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 this alternating current component Vac. Also, this stable state also changes with the setting of the direct current component Vdc. That is, when the direct current component Vdc changes, the optimal value of the magnitude Vpp of the alternating current component Vac also changes. Therefore, it is preferable to perform the setting of the direct current component Vdc (DC calibration) and the setting of the magnitude Vpp of the alternating current component Vac (AC calibration) as a set.

[0121] <Method for Setting Vpp> Specifically, as shown in Figure 7, the AC setting processing unit 51 first changes the magnitude Vpp of the AC component Vac of the development bias VB within the first measurement range R1 (Vp1, Vp2, Vp3, Vp4) and obtains the development current at each of these values. Subsequently, the AC setting processing unit 51 changes the magnitude Vpp of the AC component Vac of the development bias VB within the second measurement range R2, which is higher than the first measurement range R1 (Vp5, Vp6, Vp7), and obtains the development current at each of these values. Then, in a graph with the magnitude Vpp of the AC component Vac on the horizontal axis and the development current on the vertical axis, the AC setting processing unit 51 creates a first approximation formula F1 from the plot obtained in the first measurement range R1 and a second approximation formula F2 from the plot obtained in the second measurement range R2. The AC setting processing unit 51 then uses the value (Vpp) at 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.

[0122] 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.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] 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.

[0127] [5.3] DC Calibration As shown in Figure 8, 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 acquires 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.

[0128] 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.

[0129] [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.

[0130] <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 DC 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 AC voltage value of the AC component of the bias voltage, The DC calibration processing unit is as follows: The image forming unit is configured to be able to perform the DC calibration for each of the multiple process speeds that can be applied to it. The AC calibration processing unit is An image forming apparatus that determines the AC voltage value based on the DC voltage value corresponding to a predetermined process speed determined by the DC calibration processing unit.

[0131] <Note 2> The DC calibration processing unit is as follows: The DC 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 the AC 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 DC voltage value determined in the execution of the DC calibration, or the current value of the developing current flowing between the first and second carriers during the development of the measurement toner image.

[0132] <Note 3> The image forming apparatus according to Appendix 1, further comprising an AC component correction processing unit that corrects the AC voltage value determined by the AC calibration processing unit based on the amount of voltage change between a first DC voltage value used before the DC calibration was performed and a second DC voltage value determined by the DC calibration.

[0133] <Note 4> 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 DC 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 AC voltage value of the AC component of the bias voltage, The DC calibration processing unit is as follows: The image forming unit is configured to be able to perform the DC calibration for each of the multiple process speeds that can be applied to it. The AC calibration processing unit is Based on the DC voltage value corresponding to a predetermined process speed determined by the DC calibration processing unit, the AC voltage value is determined. An image forming apparatus further comprising an AC component correction processing unit that corrects the AC voltage value determined by the AC calibration processing unit based on the amount of voltage change between a first DC voltage value used before the DC calibration was performed and a second DC voltage value determined by the DC calibration.

[0134] <Note 5> 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 DC voltage value of the DC component of the bias voltage, The AC calibration step includes performing an AC calibration to determine the AC voltage value of the AC component of the bias voltage, The DC calibration step is as follows: The DC calibration is performed for a predetermined process speed among a plurality of process speeds that can be applied to the image forming unit. The aforementioned AC calibration step is, A bias voltage adjustment method for determining the AC voltage value based on the DC voltage value corresponding to the predetermined process speed determined in the DC calibration step.

[0135] <Note 6> 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 DC voltage value of the DC component of the bias voltage, The AC calibration step includes performing an AC calibration to determine the AC voltage value of the AC component of the bias voltage, The DC calibration step is as follows: The DC calibration is performed for a predetermined process speed among a plurality of process speeds that can be applied to the image forming unit. The aforementioned AC calibration step is, A program, or a non-temporary computer-readable storage medium in which such a program is stored, which determines the AC voltage value based on the DC voltage value corresponding to the predetermined process speed determined in the DC calibration step. [Explanation of symbols]

[0136] 3 Image forming unit 10 Image forming apparatus 50 DC setting processing unit 51 AC setting processing unit 52 AC voltage correction processing unit 53 Speed ​​change processing unit 54 Potential Measurement Processing Unit 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 DC 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 AC voltage value of the AC component of the bias voltage, The DC calibration processing unit is as follows: The image forming unit is configured to be able to perform the DC calibration for each of the multiple process speeds that can be applied to it. The AC calibration processing unit is An image forming apparatus that determines the AC voltage value based on the DC voltage value corresponding to a predetermined process speed determined by the DC calibration processing unit.

2. The DC calibration processing unit is as follows: The DC 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 the AC 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 DC voltage value determined in the execution of the DC calibration, or the current value of the developing current flowing between the first and second carriers during the development of the measurement toner image.

3. The image forming apparatus according to claim 1, further comprising an AC component correction processing unit that corrects the AC voltage value determined by the AC calibration processing unit based on the amount of voltage change between a first DC voltage value used before the DC calibration was performed and a second DC voltage value determined by the DC calibration.

4. 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 DC 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 AC voltage value of the AC component of the bias voltage, The DC calibration processing unit is as follows: The image forming unit is configured to be able to perform the DC calibration for each of the multiple process speeds that can be applied to it. The AC calibration processing unit is Based on the DC voltage value corresponding to a predetermined process speed determined by the DC calibration processing unit, the AC voltage value is determined. An image forming apparatus further comprising an AC component correction processing unit that corrects the AC voltage value determined by the AC calibration processing unit based on the amount of voltage change between a first DC voltage value used before the DC calibration was performed and a second DC voltage value determined by the DC calibration.

5. 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 DC voltage value of the DC component of the bias voltage, The AC calibration step includes performing an AC calibration to determine the AC voltage value of the AC component of the bias voltage, The DC calibration step is as follows: The DC calibration is performed for a predetermined process speed among a plurality of process speeds that can be applied to the image forming unit. The aforementioned AC calibration step is, A bias voltage adjustment method for determining the AC voltage value based on the DC voltage value corresponding to the predetermined process speed determined in the DC calibration step.

6. 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 DC voltage value of the DC component of the bias voltage, The AC calibration step includes performing an AC calibration to determine the AC voltage value of the AC component of the bias voltage, The DC calibration step is as follows: The DC calibration is performed for a predetermined process speed among a plurality of process speeds that can be applied to the image forming unit. The aforementioned AC calibration step is, A program that determines the AC voltage value based on the DC voltage value corresponding to the predetermined process speed determined in the DC calibration step.

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