Patent Publication Number: US-2023137743-A1

Title: Image forming apparatus capable of accurately acquiring electrical resistance value of transfer member, electrical resistance value acquisition method

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2021-177422 filed on Oct. 29, 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to an electrophotographic image forming apparatus and to an electrical resistance value acquisition method. 
     An electrophotographic image forming apparatus includes a transfer member such as a primary transfer roller that transfers toner images formed on an image-carrying member such as a photoconductor drum. The transfer member deteriorates with the number of pages printed by the image forming apparatus, resulting in an increase in the electrical resistance value. The increase in the electrical resistance value of the transfer member reduces the capability of the transfer member in transferring toner images, thereby reducing the image quality of the printed images. 
     To deal with this, a known image forming apparatus according to a related art acquires the electrical resistance value of the transfer member to set the voltage applied to the transfer member based on the acquired electrical resistance value of the transfer member. In the image forming apparatus according to the related art, the electrical resistance value of the transfer member is calculated based on the voltage applied to the transfer member and the current flowing in response to the application of the voltage to the transfer member. 
     SUMMARY 
     An image forming apparatus according to an aspect of the present disclosure includes an image-carrying member, a charging member, a transfer member, a first acquisition processing portion, a second acquisition processing portion, and a third acquisition processing portion. The image-carrying member includes a surface layer. The charging member is configured to charge the image-carrying member. The transfer member is configured to transfer a toner image formed on the image-carrying member. The first acquisition processing portion is configured to acquire a potential value of a charged area, charged by the charging member, on the image-carrying member. The second acquisition processing portion is configured to acquire a state value regarding a state of the surface layer based on the potential value of the charged area acquired by the first acquisition processing portion and a current value of a charging current flowing through the charging member during formation of the charged area. The third acquisition processing portion is configured to acquire an electrical resistance value of the transfer member based on the state value acquired by the second acquisition processing portion, a voltage value of a transfer voltage applied to the transfer member, and a current value of a transfer current flowing through the charged area in response to application of the transfer voltage. 
     An electrical resistance value acquisition method according to another aspect of the present disclosure, executed by an image forming apparatus including an image-carrying member including a surface layer, a charging member configured to charge the image-carrying member, and a transfer member configured to transfer a toner image formed on the image-carrying member, includes a first acquisition step, a second acquisition step, and a third acquisition step. In the first acquisition step, a potential value of a charged area, charged by the charging member, on the image-carrying member is acquired. In the second acquisition step, a state value regarding a state of the surface layer is acquired based on the potential value of the charged area acquired in the first acquisition step and a current value of a charging current flowing through the charging member during formation of the charged area. In the third acquisition step, an electrical resistance value of the transfer member is acquired based on the state value acquired in the second acquisition step, a voltage value of a transfer voltage applied to the transfer member, and a current value of a transfer current flowing through the charged area in response to application of the transfer voltage. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description with reference where appropriate to the accompanying drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view showing a configuration of an image forming apparatus according to an embodiment of the present disclosure. 
         FIG.  2    is a block diagram showing a system configuration of the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  3    is a cross-sectional view showing a configuration of an image forming unit in the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  4    is a cross-sectional view taken along line IV-IV in  FIG.  3   . 
         FIG.  5    shows a first development current detected in the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  6    shows a relationship between a DC component of a development bias voltage and the first development current detected in the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  7    shows an equivalent circuit diagram of a current carrying path passing through a charging roller and a photoconductor drum in the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  8    shows an equivalent circuit diagram of a current carrying path passing through a primary transfer roller and the photoconductor drum in the image forming apparatus according to the embodiment of the present disclosure. 
         FIG.  9    is a flowchart showing an example of a replacement timing determination process executed in the image forming apparatus according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes an embodiment of the present disclosure with reference to the accompanying drawings. It should be noted that the following embodiment is an example of a specific embodiment of the present disclosure and should not limit the technical scope of the present disclosure. 
     [Configuration of Image Forming Apparatus  100 ] 
     First, the configuration of an image forming apparatus  100  according to an embodiment of the present disclosure will be described with reference to  FIGS.  1  and  2   . 
     For purposes of illustration, the vertical direction in a state where the image forming apparatus  100  is installed and ready for use (state shown in  FIG.  1   ) is defined as an up-down direction D 1 . In addition, a front-rear direction D 2  is defined on the premise that the face of the image forming apparatus  100  on the left side of the page in  FIG.  1    serves as the front (front face). In addition, a left-right direction D 3  is defined relative to the front of the image forming apparatus  100  in the installed state. 
     The image forming apparatus  100  is a multifunction peripheral with multiple functions such as a scan function of reading images from document sheets, a print function of forming images based on image data, a facsimile function, and a copy function. The present disclosure may be applied to image forming apparatuses, such as printers, facsimile apparatuses, and copiers, capable of forming images by an electrophotographic method. 
     As shown in  FIGS.  1  and  2   , the image forming apparatus  100  includes an ADF (Automatic Document Feeder)  1 , an image reading portion  2 , an image forming portion  3 , a sheet feed portion  4 , an operation display portion  5 , a memory portion  6 , and a control portion  7 . 
     The ADF  1  feeds document sheets with images to be read by the image reading portion  2 . The ADF  1  includes a document sheet set portion, a plurality of conveying rollers, a document sheet holder, and a sheet discharge portion. 
     The image reading portion  2  implements the scan function. The image reading portion  2  includes a document sheet table, a light source, a plurality of mirrors, an optical lens, and a CCD (Charge Coupled Device). 
     The image forming portion  3  implements the print function. Specifically, the image forming portion  3  forms color or monochrome images on sheets supplied from the sheet feed portion  4  by an electrophotographic method. 
     The sheet feed portion  4  supplies sheets for the image forming portion  3 . The sheet feed portion  4  includes a sheet feed cassette, a manual feed tray, and a plurality of conveying rollers. 
     The operation display portion  5  is a user interface of the image forming apparatus  100 . The operation display portion  5  includes a display portion and an operation portion. The display portion displays various types of information according to control instructions from the control portion  7 . For example, the display portion is a liquid crystal display. The operation portion is used by a user for inputting various types of information to the control portion  7 . For example, the operation portion includes operation keys and a touch panel. 
     The memory portion  6  is a nonvolatile storage device. For example, the memory portion  6  is nonvolatile memory such as flash memory. 
     The control portion  7  provides integrated control over the image forming apparatus  100 . As shown in  FIG.  2   , the control portion  7  includes a CPU  11 , a ROM  12 , and a RAM  13 . The CPU  11  is a processor that executes various types of calculation processes. The ROM  12  is a nonvolatile storage device that stores in advance information including control programs for causing the CPU  11  to execute various types of processes. The RAM  13  is a volatile or nonvolatile storage device used as a temporary memory (work area) for the various types of processes executed by the CPU  11 . The CPU  11  executes the various types of control programs stored in the ROM  12  in advance to provide integrated control over the image forming apparatus  100 . 
     The control portion  7  may be provided separately from a main control portion that provides integrated control over the image forming apparatus  100 . In addition, the control portion  7  may be composed of an electronic circuit such as an integrated circuit (ASIC). 
     [Configuration of Image Forming Portion  3 ] 
     Next, the configuration of the image forming portion  3  will be described with reference to  FIGS.  1  to  3   .  FIG.  3    is a cross-sectional view showing the configuration of an image forming unit  24 . In  FIG.  3   , a current carrying path passing through a charging roller  32  and a first power source  61 , a current carrying path passing through a developing roller  44  and a second power source  63 , and a current carrying path passing through a primary transfer roller  34  and a third power source  65  are indicated by dash-dot lines. 
     As shown in  FIG.  1   , the image forming portion  3  includes a plurality of image forming units  21  to  24 , a laser scanning unit  25 , an intermediate transfer belt  26 , a secondary transfer roller  27 , a fixing device  28 , and a sheet discharge tray  29 , 
     The image forming unit  21  forms toner images of yellow (Y). The image forming unit  22  forms toner images of cyan (C). The image forming unit  23  forms toner images of magenta (M). The image forming unit  24  forms toner images of black (K). As shown in  FIG.  1   , the image forming units  21  to  24  are arranged side by side in the front-rear direction D 2  of the image forming apparatus  100  in the order of yellow, cyan, magenta, and black from the front side of the image forming apparatus  100 . 
     As shown in  FIG.  3   , the image forming unit  24  includes a photoconductor drum  31 , the charging roller  32 , a developing device  33 , the primary transfer roller  34 , and a drum cleaning member  35 . In addition, the image forming units  21  to  23  have configurations similar to that of the image forming unit  24 . 
     An electrostatic latent image is formed on the surface of the photoconductor drum  31 . The photoconductor drum  31  includes a surface layer  31 A. The photoconductor drum  31  is an example of an image-carrying member of the present disclosure. 
     For example, the surface layer  31 A is formed from an organic photosensitive material. The surface layer  31 A may be formed from a photosensitive material different from the organic photosensitive material. 
     The photoconductor drum  31  rotates in a rotation direction D 4  shown in  FIG.  3    under the rotational driving force supplied from a motor (not shown). Thus, the photoconductor drum  31  conveys the electrostatic latent image formed on the surface thereof. 
     The charging roller  32  electrically charges the surface layer  31 A of the photoconductor drum  31 . The charging roller  32  is an example of a charging member of the present disclosure. 
     The charging roller  32  is in contact with the surface layer  31 A of the photoconductor drum  31 . The charging roller  32  is driven to rotate as the photoconductor drum  31  rotates. The charging roller  32  electrically charges the surface layer  31 A of the photoconductor drum  31  in response to application of a preset charging bias voltage. For example, the charging roller  32  positively charges the surface layer  31 A of the photoconductor drum  31 . 
     The surface layer  31 A of the photoconductor drum  31  charged by the charging roller  32  is exposed to light beams, based on image data, emitted by the laser scanning unit  25 . This forms the electrostatic latent image on the surface layer  31 A of the photoconductor drum  31 . 
     The developing device  33  develops the electrostatic latent image formed on the surface layer  31 A of the photoconductor drum  31  using developer that contains toner and carrier. This forms a toner image on the surface layer  31 A of the photoconductor drum  31 . 
     The primary transfer roller  34  transfers the toner image formed on the surface layer  31 A of the photoconductor drum  31  by the developing device  33  to the intermediate transfer belt  26 . The primary transfer roller  34  is an example of a transfer member of the present disclosure. 
     The primary transfer roller  34  is in contact with the inner peripheral surface of the intermediate transfer belt  26 . In addition, the primary transfer roller  34  faces the surface layer  31 A of the photoconductor drum  31  with the intermediate transfer belt  26  therebetween. The primary transfer roller  34  is driven to rotate as the intermediate transfer belt  26  rotates. The primary transfer roller  34  transfers the toner image formed on the surface layer  31 A of the photoconductor drum  31  to the outer peripheral surface of the intermediate transfer belt  26  in response to application of a preset primary transfer bias voltage. 
     The drum cleaning member  35  removes remaining toner from the surface of the photoconductor drum  31  after the toner image is transferred by the primary transfer roller  34 . 
     The image forming portion  3  includes toner containers  36  (see  FIG.  1   ) respectively corresponding to the image forming units  21  to  24 . In addition, the image forming portion  3  includes the first power sources  61  (see  FIG.  2   ), first detection portions  62  (see  FIG.  2   ), the second power sources  63  (see  FIG.  2   ), second detection portions  64  (see  FIG.  2   ), the third power sources  65  (see  FIG.  2   ), and third detection portions  66  (see  FIG.  2   ) respectively corresponding to the image forming units  21  to  24 . 
     Here, the toner container  36 , the first power source  61 , the first detection portion  62 , the second power source  63 , the second detection portion  64 , the third power source  65 , and the third detection portion  66  corresponding to the image forming unit  24  will be described. 
     The toner container  36  stores toner of black (K). The toner container  36  supplies the toner of black (K) to the developing device  33  of the image forming unit  24 . 
     The first power source  61  (see  FIG.  3   ) applies the charging bias voltage to the charging roller  32 . Specifically, the charging bias voltage includes a direct current (DC) component. For example, the charging bias voltage includes a positive DC component. 
     The first detection portion  62  (see  FIG.  3   ) detects current flowing through the charging roller  32 . As shown in  FIG.  3   , the first detection portion  62  is disposed on the current carrying path passing through the charging roller  32  and the first power source  61 . The first detection portion  62  inputs an electrical signal indicating the current value of the detected current to the control portion  7 . 
     The second power source  63  (see  FIG.  3   ) applies a preset development bias voltage to the developing roller  44  (see  FIG.  3   ) of the developing device  33 . Specifically, the development bias voltage includes a DC component and an alternating current (AC) component. For example, the development bias voltage includes a positive DC component and an AC component with a rectangular waveform. 
     The second power source  63  can separately output the DC component and the AC component included in the development bias voltage. In addition, the second power source  63  can adjust the voltage value of the DC component included in the development bias voltage within a preset range. 
     The second detection portion  64  (see  FIG.  3   ) detects current flowing through the developing roller  44 . As shown in  FIG.  3   , the second detection portion  64  is disposed on the current carrying path passing through the developing roller  44  and the second power source  63 . The second detection portion  64  inputs an electrical signal indicating the current value of the detected current to the control portion  7 . 
     The third power source  65  (see  FIG.  3   ) applies the primary transfer bias voltage to the primary transfer roller  34 . Specifically, the primary transfer bias voltage includes a DC component. For example, the primary transfer bias voltage includes a negative DC component. 
     The third detection portion  66  (see  FIG.  3   ) detects current flowing through the primary transfer roller  34 . As shown in  FIG.  3   , the third detection portion  66  is disposed on the current carrying path passing through the primary transfer roller  34  and the third power source  65 . The third detection portion  66  inputs an electrical signal indicating the current value of the detected current to the control portion  7 . 
     The laser scanning unit  25  emits light that illuminates the charged area, charged by the charging roller  32 , on the surface layer  31 A of the photoconductor drum  31 . The laser scanning unit  25  is an example of a light emitting portion of the present disclosure. 
     Specifically, the laser scanning unit  25  emits light based on the image data to the surface layers  31 A of the photoconductor drums  31  in the respective image forming units  21  to  24 . 
     The intermediate transfer belt  26  is an endless belt member to which the toner images formed on the surfaces of the photoconductor drums  31  in the respective image forming units  21  to  24  are transferred. The intermediate transfer belt  26  is stretched by a drive roller and a tension roller with a predetermined tension. The intermediate transfer belt  26  rotates in a rotation direction D 5  shown in  FIG.  3    as the drive roller rotates under the rotational driving force supplied from a motor (not shown). 
     The secondary transfer roller  27  transfers the toner images from the surface of the intermediate transfer belt  26  to a sheet supplied from the sheet feed portion  4 . 
     The fixing device  28  fixes the toner images transferred to the sheet by the secondary transfer roller  27  onto the sheet. 
     The sheet with the toner images fixed thereon by the fixing device  28  is discharged to the sheet discharge tray  29 . 
     Configuration of Developing Device  33 ] 
     Next, the configuration of the developing device  33  in the image forming unit  24  will be described with reference to  FIGS.  3  and  4   . The developing devices  33  in the image forming units  21  to  23  also have configurations similar to that of the developing device  33  described below. 
     As shown in  FIGS.  3  and  4   , the developing device  33  includes a housing  41 , a first conveyance member  42 , a second conveyance member  43 , the developing roller  44 , a restricting member  45 , and a toner sensor  46 . 
     The housing  41  houses the first conveyance member  42 , the second conveyance member  43 , the developing roller  44 , and the restricting member  45 . The housing  41  also stores the developer. The housing  41  extends in the left-right direction D 3 . 
     As shown in  FIGS.  3  and  4   , the housing  41  includes a first conveyance path  52  and a second conveyance path  53  extending in the left-right direction D 3 . Specifically, a partition  54  (see  FIG.  4   ) that partitions a lower part of the housing  41  into the first conveyance path  52  and the second conveyance path  53  is disposed on the bottom surface  51  of the housing  41 . 
     The first conveyance member  42  conveys the developer stored in the first conveyance path  52  in a conveying direction D 6  (see  FIG.  4   ) along the first conveyance path  52 . In addition, the first conveyance member  42  stirs the developer to triboelectrically charge the toner and the carrier contained in the developer. For example, the first conveyance member  42  is a screw-shaped member disposed in the first conveyance path  52  to be rotatable around a rotation axis along the first conveyance path  52 . The first conveyance member  42  rotates under the rotational driving force supplied from a motor (not shown), thereby conveying and stirring the developer. For example, the toner contained in the developer stirred by the first conveyance member  42  is positively charged by the friction with the carrier contained in the developer. 
     The second conveyance member  43  conveys the developer stored in the second conveyance path  53  in a conveying direction D 7  (see  FIG.  4   ) along the second conveyance path  53 . In addition, the second conveyance member  43  stirs the developer to triboelectrically charge the toner and the carrier contained in the developer. For example, the second conveyance member  43  is a screw-shaped member disposed in the second conveyance path  53  to be rotatable around a rotation axis along the second conveyance path  53 . The second conveyance member  43  rotates under the rotational driving force supplied from a motor (not shown), thereby conveying and stirring the developer. 
     A first path  55  (see  FIG.  4   ) leading to the second conveyance path  53  is disposed at the downstream end, in the conveying direction D 6 , of the first conveyance path  52 . In addition, a second path  56  (see  FIG.  4   ) leading to the first conveyance path  52  is disposed at the downstream end, in the conveying direction D 7 , of the second conveyance path  53 . The first conveyance path  52 , the first path  55 , the second conveyance path  53 , and the second path  56  form a circulating conveyance path in which the developer is circulated in one direction. 
     The developing roller  44  faces the photoconductor drum  31 . The developing roller  44  conveys the developer to a facing portion R 1  (see  FIG.  3   ) between itself and the photoconductor drum  31 . The developing roller  44  is an example of a developing member of the present disclosure. 
     As shown in  FIG.  3   , the developing roller  44  faces the second conveyance path  53  and the photoconductor drum  31 . The developing roller  44  draws up the developer from the second conveyance path  53 . The developer drawn up by the developing roller  44  forms a magnetic brush on the outer peripheral surface of the developing roller  44  by the magnetic force of magnetic poles disposed inside the developing roller  44 . 
     The developing roller  44  is rotatably supported by the housing  41 . The developing roller  44  rotates in a rotation direction D 8  shown in  FIG.  3    under the rotational driving force supplied from a motor (not shown). Thus, the developing roller  44  conveys the magnetic brush formed on the outer peripheral surface thereof to the facing portion R 1 . 
     As the photoconductor drum  31  rotates, the electrostatic latent image formed on the surface layer  31 A of the photoconductor drum  31  is conveyed to the facing portion R 1 . Here, the electrostatic latent image includes an exposed area and an unexposed area. The exposed area is an area illuminated with the light emitted by the laser scanning unit  25  in the charged area, charged by the charging roller  32 , on the surface layer  31 A of the photoconductor drum  31 . In addition, the unexposed area is an area that is not illuminated with the light emitted by the laser scanning unit  25  in the charged area. 
     When the development bias voltage is applied to the developing roller  44 , a first electric field that causes toner included in the magnetic brush to move to the exposed area is generated between the developing roller  44  and the exposed area that face each other at the facing portion R 1 . In addition, when the development bias voltage is applied to the developing roller  44 , a second electric field that causes the toner included in the magnetic brush to move to the developing roller  44  is generated between the developing roller  44  and the unexposed area that face each other at the facing portion R 1 . The toner included in the magnetic brush is selectively moved to the exposed area formed on the surface layer  31 A of the photoconductor drum  31  by the effect of the first electric field and the second electric field generated at the facing portion R 1 . Thus, the electrostatic latent image formed on the surface layer  31 A of the photoconductor drum  31  is developed. 
     The restricting member  45  restricts the thickness of the magnetic brush formed on the outer peripheral surface of the developing roller  44 . As shown in  FIG.  3   , the restricting member  45  is disposed downstream, in the rotation direction D 8 , of a position where the second conveyance member  43  and the developing roller  44  face each other and upstream, in the rotation direction D 8 , of the facing portion R 1 . The restricting member  45  faces the outer peripheral surface of the developing roller  44  such that a predetermined gap is left between the restricting member  45  and the outer peripheral surface of the developing roller  44 . 
     An opening  57  is provided at an upper part of the first conveyance path  52 . As shown in  FIG.  3   , the opening  57  is provided in an outer wall of the housing  41  that covers the upper part of the first conveyance path  52 . The opening  57  faces the upstream end, in the conveying direction D 6 , of the first conveyance path  52 . The toner supplied from the toner container  36  is carried through the opening  57  to a carry-in position P 1  (see  FIG.  4   ) facing the opening  57  in the first conveyance path  52 . 
     The toner sensor  46  detects toner at a detection position P 2  (see  FIG.  4   ) downstream, in the conveying direction D 6 , of the carry-in position P 1  in the first conveyance path  52 . For example, as shown in  FIG.  3   , the toner sensor  46  is disposed on a bottom part of the housing  41 . For example, the toner sensor  46  is a permeability sensor including an LC oscillator circuit that outputs an electrical signal according to the permeability of the developer stored inside the housing  41 . The control portion  7  uses the toner sensor  46  to control toner supply from the toner container  36  to the developing device  33 . 
     [Configuration of Control Portion  7 ] 
     Next, the configuration of the control portion  7  will be described with reference to  FIG.  2   . 
     As shown in  FIG.  2   , the control portion  7  includes a second detection processing portion  71 , a first detection processing portion  72 , a potential value acquisition portion  73 , a state value acquisition portion  74 , a first resistance value acquisition portion  75 , a second resistance value acquisition portion  76 , a first timing determination portion  77 , a second timing determination portion  78 , and a third timing determination portion  79 . 
     Specifically, the ROM  12  of the control portion  7  stores in advance a replacement timing determination program for causing the CPU  11  to function as the above-described portions. The CPU  11  executes the replacement timing determination program stored in the ROM  12  to function as the above-described portions. 
     The replacement timing determination program is recorded in a computer-readable recording medium, such as a CD, a DVD, and a flash memory, and may be read from the recording medium to be stored in a storage device such as the memory portion  6 . In addition, part or all of the second detection processing portion  71 , the first detection processing portion  72 , the potential value acquisition portion  73 , the state value acquisition portion  74 , the first resistance value acquisition portion  75 , the second resistance value acquisition portion  76 , the first timing determination portion  77 , the second timing determination portion  78 , and the third timing determination portion  79  may be composed of an electronic circuit such as an integrated circuit (ASIC). 
     The following describes an example of the portions included in the image forming unit  24  and the portions corresponding to the image forming unit  24  among the image forming units  21  to  24 . The description below also applies to the image forming units  21  to  23 . 
     When a DC voltage is not applied to the developing roller  44 , the second detection processing portion  71  detects a second development current flowing through the facing portion R 1  (see  FIG.  3   ) including the developer and an uncharged area, which is not charged by the charging roller  32 , on the photoconductor drum  31 . 
     For example, the second detection processing portion  71  detects the second development current when a preset determination timing arrives. For example, the determination timing is a timing when the number of pages printed by the image forming apparatus  100  exceeds multiples of a preset specific number of pages. The determination timing may be a timing when the image forming apparatus  100  is powered on, for example. 
     For example, the second detection processing portion  71  detects the second development current using the following procedure. 
     First, the second detection processing portion  71  conveys the uncharged area on the photoconductor drum  31  to the facing portion R 1  and, at the same time, conveys the developer to the facing portion R 1 . Specifically, the second detection processing portion  71  rotates the photoconductor drum  31  while the output from the first power source  61  and the laser scanning unit  25  is halted. In addition, the second detection processing portion  71  drives the developing device  33 . The second detection processing portion  71  may eliminate static charge from the uncharged area conveyed to the facing portion R 1  using the laser scanning unit  25  or a static eliminator (not shown) that eliminates static charge from the surface layer  31 A of the photoconductor drum  31 . 
     Next, the second detection processing portion  71  causes the second power source  63  to apply an AC voltage to the developing roller  44  while the uncharged area and the developer lie at the facing portion R 1 . Specifically, the second detection processing portion  71  causes the second power source  63  to output the AC component included in the development bias voltage. 
     The second detection processing portion  71  then detects the second development current flowing through the current carrying path that passes through the second power source  63  and the developing roller  44  in response to the application of the AC voltage using the second detection portion  64 . The second detection processing portion  71  may detect the second development current flowing through the current carrying path that passes through the second power source  63  and the developing roller  44  while the AC voltage is not applied to the developing roller  44 . 
     The first detection processing portion  72  detects a first development current for each of a plurality of specific voltages with different DC voltage values applied to the developing roller  44 . The first development current flows through the facing portion R 1  (see  FIG.  3   ) including the developer and the unexposed area on the photoconductor drum  31  in response to the application of the specific voltages. 
     For example, the specific voltages include a DC component and an AC component. The specific voltages may include only the DC component in a case where the second development current flows through the current carrying path passing through the second power source  63  and the developing roller  44  while the AC voltage is not applied to the developing roller  44 . 
     For example, the first detection processing portion  72  detects the first development current when the second development current is detected by the second detection processing portion  71 . 
     For example, the first detection processing portion  72  detects the first development current using the following procedure. 
     First, the first detection processing portion  72  conveys the unexposed area on the photoconductor drum  31  to the facing portion R 1  and, at the same time, conveys the developer to the facing portion R 1 . Specifically, the first detection processing portion  72  causes the first power source  61  to apply the charging bias voltage to the charging roller  32  and rotates the photoconductor drum  31  while the output from the laser scanning unit  25  is halted. In addition, the first detection processing portion  72  drives the developing device  33 . 
     Next, the first detection processing portion  72  causes the second power source  63  to output any of the specific voltages while the unexposed area and the developer lie at the facing portion R 1 . Specifically, the first detection processing portion  72  causes the second power source  63  to output the development bias voltage including the DC component of which the voltage value is adjusted. 
     The first detection processing portion  72  then detects the first development current flowing through the current carrying path that passes through the second power source  63  and the developing roller  44  in response to the application of the specific voltages using the second detection portion  64 . 
       FIG.  5    shows an example of the first development current detected by the first detection processing portion  72  for each of the plurality of specific voltages with different DC voltage values. 
     The potential value acquisition portion  73  acquires the potential value of the charged area, charged by the charging roller  32 , on the photoconductor drum  31 . The potential value acquisition portion  73  is an example of a first acquisition processing portion of the present disclosure. 
     Specifically, the potential value acquisition portion  73  acquires the potential value of the unexposed area based on the DC voltage values of the specific voltages and the current values of the first development current, detected by the first detection processing portion  72 , corresponding to the respective specific voltages. 
     Here, the relationship between the first development current and the potential difference between the developing roller  44  and the unexposed area will be described with reference to  FIG.  6   .  FIG.  6    shows an approximate straight line that indicates the relationship between the DC voltage values of the specific voltages and the current values of the first development current based on the data shown in  FIG.  5   . In  FIG.  6   , the approximate straight line is indicated by a dash-dot line. 
     When the potential difference between the developing roller  44  and the unexposed area is small, the first development current including a first toner current and a first carrier current described below flows. The first toner current flows as the toner lying at the facing portion R 1  mechanically adheres to the unexposed area. The first carrier current flows through the carrier lying at the facing portion R 1 . When the potential of the developing roller  44  is higher than the potential of the unexposed area, the first carrier current flows from the developing roller  44  to the unexposed area, whereas when the potential of the developing roller  44  is lower than the potential of the unexposed area, the first carrier current flows from the unexposed area to the developing roller  44 . 
     In addition, when the potential difference between the developing roller  44  and the unexposed area is zero, the first development current including only the first toner current flows. 
     Here, the second development current flowing between the developing roller  44  to which the DC voltage is not applied and the uncharged area on the photoconductor drum  31  can be regarded as the same as the first development current flowing when the potential difference between the developing roller  44  and the unexposed area is zero. 
     Accordingly, the DC voltage value of a specific voltage on the approximate straight line shown in  FIG.  6    corresponding to the current value of the first development current substantially equal to the current value of the second development current detected by the second detection processing portion  71  can be assumed to be the potential value of the unexposed area on the photoconductor drum  31 . 
     For example, the potential value acquisition portion  73  acquires the DC voltage value of a specific voltage, that is assumed based on the DC voltage values of the specific voltages and the current values of the first development current, detected by the first detection processing portion  72 , corresponding to the respective specific voltages and that corresponds to the current value of the first development current of which the difference from the current value of the second development current detected by the second detection processing portion  71  is less than or equal to a preset permissible value, as the potential value of the unexposed area. The permissible value may be any value including zero. 
     It is noted that when the potential difference between the developing roller  44  and the unexposed area is large, the first development current including a second toner current or a second carrier current described below flows. The second toner current flows as the toner lying at the facing portion R 1  electrostatically adheres to the unexposed area. The second carrier current flows as the carrier lying at the facing portion R 1  electrostatically adheres to the unexposed area. In a case where the first development current detected by the first detection processing portion  72  includes the second toner current or the second carrier current, the accuracy of the potential value acquisition portion  73  in acquiring the potential value of the unexposed area decreases. 
     Accordingly, it is desirable that the DC voltage values of the specific voltages be determined within a preset specific range so that the first development current detected by the first detection processing portion  72  does not include the second toner current or the second carrier current. For example, the specific range is based on the potential value of the unexposed area last acquired by the potential value acquisition portion  73 . For example, the specific range is a range of ±25 V (volts) centered on the potential value of the charged area last acquired by the potential value acquisition portion  73 . 
     It is noted that the first toner current is very small and may be ignored. That is, the potential value acquisition portion  73  may acquire the DC voltage value of the specific voltage, assumed based on the DC voltage values of the specific voltages and the current values of the first development current, detected by the first detection processing portion  72 , corresponding to the respective specific voltages, when the current value of the first development current is zero as the potential value of the unexposed area. In this case, the control portion  7  does not need to include the second detection processing portion  71 . 
     In addition, the potential value acquisition portion  73  may acquire the potential value of the charged area using a surface potential sensor that can detect the surface potential of the photoconductor drum  31 . 
     The state value acquisition portion  74  acquires a state value regarding the state of the surface layer  31 A based on the potential value of the charged area acquired by the potential value acquisition portion  73  and the current value of a charging current flowing through the charging roller  32  during the formation of the charged area. The state value acquisition portion  74  is an example of a second acquisition processing portion of the present disclosure. 
     For example, the state value is the capacitance value of the surface layer  31 A. The state value may be the thickness value of the surface layer  31 A. For example, the thickness value of the surface layer  31 A can be acquired based on the capacitance value of the surface layer  31 A and a preset dielectric constant of the surface layer  31 A. 
     For example, the state value acquisition portion  74  acquires the state value using Equation (1) below. Here, Cp is the capacitance of the surface layer  31 A, Idc is the charging current detected by the first detection portion  62 , Vo is the potential of the charged area acquired by the potential value acquisition portion  73 , ΔV 1  is a decrement in the potential due to dark decay while the charged area is conveyed from a position facing the charging roller  32  to the facing portion R 1 , v is a linear velocity of the photoconductor drum  31 , and L is the width of the charged area. 
         Cp=Idc /[( Vo+ΔV 1)·v·L]  (1)
 
     It is noted that ΔV 1  may be preset based on the linear velocity of the photoconductor drum  31 . ΔV 1  may also be calculated based on the number of pages printed by the image forming apparatus  100  or the temperature inside the apparatus. 
     The charging roller  32  deteriorates with the number of pages printed by the image forming apparatus  100 , resulting in an increase in the electrical resistance value. The increase in the electrical resistance value of the charging roller  32  reduces the capability of the charging roller  32  to charge the photoconductor drum  31 , thereby reducing the image quality of the printed images. 
     To deal with this, a known image forming apparatus according to a related art acquires the electrical resistance value of the charging roller  32  to determine whether the timing of replacing the charging roller  32  has arrived based on the acquired electrical resistance value of the charging roller  32 . 
     Here, in the image forming apparatus according to the above-described related art, two pulsed voltages with different frequencies are applied to the charging roller  32  for acquisition of the electrical resistance value of the charging roller  32 . That is, for acquisition of the electrical resistance value of the charging roller  32  using the above-described related art, a power source that can apply the two pulsed voltages with different frequencies to the charging roller  32  is required. 
     In contrast, in the image forming apparatus  100 , the electrical resistance value of the charging roller  32  can be acquired as described below without a power source that has a special function. 
     The first resistance value acquisition portion  75  acquires the electrical resistance value of the charging roller  32  based on the state value acquired by the state value acquisition portion  74 , the current value of the charging current, and the voltage value of the charging bias voltage applied to the charging roller  32  during the formation of the charged area. 
     For example, a current carrying path passing through the charging roller  32  and the photoconductor drum  31  can be expressed by an equivalent circuit shown in  FIG.  7   . Equation (2) below can be derived from the equivalent circuit shown in  FIG.  7   . Here, Vdc is the DC component of the charging bias voltage, R 1  is the electrical resistance of the charging roller  32 , Vth 1  is the potential difference between the charging roller  32  and the photoconductor drum  31 , and Vp 1  is the potential of the charged area at the position facing the charging roller  32 . It is noted that Vth 1  can be calculated based on Cp and the dielectric constant of vacuum. 
         Vdc=Idc·R 1+ Vth 1+ Vp 1   (2)
 
     Rearranging Equation (2) yields Expression (3) below. It is noted that Vp 1  is replaced with (Vo+ΔV 1 ) in Equation (3). 
         R 1={ Vdc −[ Vth 1+( Vo+ΔV 1)]}/ Idc    (3)
 
     The first resistance value acquisition portion  75  acquires the electrical resistance value of the charging roller  32  using Equation (3). 
     The primary transfer roller  34  deteriorates with the number of pages printed by the image forming apparatus  100 , resulting in an increase in the electrical resistance value. The increase in the electrical resistance value of the primary transfer roller  34  reduces the capability of the primary transfer roller  34  to transfer toner images, thereby reducing the image quality of the printed images. 
     To deal with this, a known image forming apparatus according to a related art acquires the electrical resistance value of the primary transfer roller  34  to set the primary transfer bias voltage applied to the primary transfer roller  34  based on the acquired electrical resistance value of the primary transfer roller  34 . In the image forming apparatus according to the related art, the electrical resistance value of the primary transfer roller  34  is calculated based on the primary transfer bias voltage applied to the primary transfer roller  34  and the current flowing in response to the application of the primary transfer bias voltage to the primary transfer roller  34 . 
     Here, the current flowing in response to the application of the primary transfer bias voltage to the primary transfer roller  34  changes according not only to the electrical resistance value of the primary transfer roller  34  but to the capacitance of the photoconductor drum  31 . However, in the image forming apparatus according to the above-described related art, the capacitance of the photoconductor drum  31  is not considered during the calculation of the electrical resistance value of the primary transfer roller  34 , and thus the electrical resistance value cannot be accurately acquired. 
     In contrast, in the image forming apparatus  100 , the electrical resistance value of the primary transfer roller  34  can be accurately acquired as described below. 
     The second resistance value acquisition portion  76  acquires the electrical resistance value of the primary transfer roller  34  based on the state value acquired by the state value acquisition portion  74 , the voltage value of the primary transfer bias voltage applied to the primary transfer roller  34 , and the current value of a transfer current flowing through the charged area in response to the application of the primary transfer bias voltage. The second resistance value acquisition portion  76  is an example of a third acquisition processing portion of the present disclosure. In addition, the primary transfer bias voltage is an example of a transfer voltage of the present disclosure. 
     For example, a current carrying path passing through the primary transfer roller  34  and the photoconductor drum  31  can be expressed by an equivalent circuit shown in  FIG.  8   . Equation (4) below can be derived from the equivalent circuit shown in  FIG.  8   . Here, Vt is the DC component of the primary transfer bias voltage, It is the transfer current detected by the third detection portion  66 , R 2  is the electrical resistance of the primary transfer roller  34 , Vth 2  is the potential difference between the primary transfer roller  34  and the photoconductor drum  31 , and Vp 2  is the potential of the charged area at a transfer position where toner images are transferred by the primary transfer roller  34 . 
         Vt=It·R 2+ Vth 2+ Vp 2   (4)
 
     Rearranging Equation (4) yields Expression (5) below. It is noted that Vp 2  is replaced with (Vo−ΔV 2 ) in Equation (5). ΔV 2  is a decrement in the potential due to dark decay while the charged area is conveyed from the facing portion R 1  to the transfer position where toner images are transferred by the primary transfer roller  34 . 
         R 2={ Vt −[ Vth 2+( Vo−ΔV 2)]}/ It    (5)
 
     The second resistance value acquisition portion  76  acquires the electrical resistance value of the primary transfer roller  34  using Equation (5). 
     The first timing determination portion  77  determines whether the timing of replacing the photoconductor drum  31  has arrived based on the state value acquired by the state value acquisition portion  74 . The first timing determination portion  77  is an example of a first determination processing portion of the present disclosure. 
     For example, the first timing determination portion  77  determines that the timing of replacing the photoconductor drum  31  has arrived when the state value acquired by the state value acquisition portion  74  exceeds a preset first threshold. 
     The second timing determination portion  78  determines whether the timing of replacing the charging roller  32  has arrived based on the electrical resistance value of the charging roller  32  acquired by the first resistance value acquisition portion  75 . 
     For example, the second timing determination portion  78  determines that the timing of replacing the charging roller  32  has arrived when the electrical resistance value of the charging roller  32  acquired by the first resistance value acquisition portion  75  exceeds a preset second threshold. 
     The third timing determination portion  79  determines whether the timing of replacing the primary transfer roller  34  has arrived based on the electrical resistance value of the primary transfer roller  34  acquired by the second resistance value acquisition portion  76 . The third timing determination portion  79  is an example of a second determination processing portion of the present disclosure. 
     For example, the third timing determination portion  79  determines that the timing of replacing the primary transfer roller  34  has arrived when the electrical resistance value of the primary transfer roller  34  acquired by the second resistance value acquisition portion  76  exceeds a preset third threshold. 
     [Replacement Timing Determination Process] 
     An electrical resistance value acquisition method of the present disclosure will now be described with reference to  FIG.  9    using an example of a procedure of a replacement timing determination process executed by the control portion  7  in the image forming apparatus  100 . Here, steps S 11 , S 12 , . . . represent the numbers of processing procedures (steps) executed by the control portion  7 . 
     It is noted that the replacement timing determination process is executed when the determination timing arrives. 
     &lt;Step S 11 &gt; 
     First, in step S 11 , the control portion  7  detects the second development current. Here, the process in step S 11  is executed by the second detection processing portion  71  of the control portion  7 . 
     Specifically, the control portion  7  detects the second development current using the following procedure. 
     First, the control portion  7  conveys the uncharged area on the photoconductor drum  31  to the facing portion R 1  and, at the same time, conveys the developer to the facing portion R 1 . Specifically, the control portion  7  rotates the photoconductor drum  31  while the output from the first power source  61  and the laser scanning unit  25  is halted. In addition, the control portion  7  drives the developing device  33 . 
     Next, the control portion  7  causes the second power source  63  to apply an AC voltage to the developing roller  44  while the uncharged area and the developer lie at the facing portion R 1 . Specifically, the control portion  7  causes the second power source  63  to output the AC component included in the development bias voltage. 
     The control portion  7  then detects the second development current flowing through the current carrying path that passes through the second power source  63  and the developing roller  44  in response to the application of the AC voltage using the second detection portion  64 . 
     &lt;Step S 12 &gt; 
     In step S 12 , the control portion  7  detects the charging current. 
     Specifically, the control portion  7  causes the first power source  61  to apply the charging bias voltage to the charging roller  32 . The control portion  7  then detects the charging current flowing through the current carrying path that passes through the first power source  61  and the charging roller  32  in response to the application of the charging bias voltage using the first detection portion  62 . 
     &lt;Step S 13 &gt; 
     In step S 13 , the control portion  7  detects the first development current for each of the plurality of specific voltages. Here, the process in step S 13  is executed by the first detection processing portion  72  of the control portion  7 . 
     Specifically, the control portion  7  detects the first development current using the following procedure. 
     First, the control portion  7  conveys the unexposed area on the photoconductor drum  31  to the facing portion R 1  and, at the same time, conveys the developer to the facing portion R 1 . Specifically, the control portion  7  causes the first power source  61  to apply the charging bias voltage to the charging roller  32  and rotates the photoconductor drum  31  while the output from the laser scanning unit  25  is halted. In addition, the control portion  7  drives the developing device  33 . 
     Next, the control portion  7  causes the second power source  63  to output any of the specific voltages while the unexposed area and the developer lie at the facing portion R 1 . Specifically, the control portion  7  causes the second power source  63  to output the development bias voltage including the DC component of which the voltage value is adjusted. 
     The control portion  7  then detects the first development current flowing through the current carrying path that passes through the second power source  63  and the developing roller  44  in response to the application of the specific voltages using the second detection portion  64 . 
     &lt;Step S 14 &gt; 
     In step S 14 , the control portion  7  detects the transfer current. 
     Specifically, the control portion  7  causes the third power source  65  to apply the primary transfer bias voltage to the primary transfer roller  34  when the unexposed area is conveyed to the transfer position where toner images are transferred by the primary transfer roller  34 . The control portion  7  then detects the transfer current flowing through the current carrying path that passes through the third power source  65  and the primary transfer roller  34  in response to the application of the primary transfer bias voltage using the third detection portion  66 . 
     &lt;Step S 15 &gt; 
     In step S 15 , the control portion  7  acquires the potential value of the charged area on the photoconductor drum  31 . Here, the process in step S 15  is an example of a first acquisition step of the present disclosure and is executed by the potential value acquisition portion  73  of the control portion  7 . 
     Specifically, the control portion  7  acquires the potential value of the unexposed area based on the DC voltage values of the specific voltages and the current values of the first development current, detected in step S 13 , corresponding to the respective specific voltages. 
     For example, the control portion  7  acquires a linear expression corresponding to the approximate straight line (see  FIG.  6   ) that indicates the relationship between the DC voltage values of the specific voltages and the current values of the first development current based on the DC voltage values of the specific voltages and the current values of the first development current, detected in step S 13 , corresponding to the respective specific voltages. The control portion  7  then acquires the DC voltage value of a specific voltage, that is assumed based on the acquired linear expression and that corresponds to the current value of the first development current of which the difference from the current value of the second development current detected in step S 11  is less than or equal to the permissible value, as the potential value of the unexposed area. 
     &lt;Step S 16 &gt; 
     In step S 16 , the control portion  7  acquires the state value based on the potential value of the charged area acquired in step S 15  and the current value of the charging current detected in step S 12 . Here, the process in step S 16  is an example of a second acquisition step of the present disclosure and is executed by the state value acquisition portion  74  of the control portion  7 . 
     Specifically, the control portion  7  acquires the state value using Equation (1). 
     &lt;Step S 17 &gt; 
     In step S 17 , the control portion  7  acquires the electrical resistance value of the charging roller  32  based on the state value detected in step S 16 , the current value of the charging current detected in step S 12 , and the voltage value of the charging bias voltage. Here, the process in step S 17  is executed by the first resistance value acquisition portion  75  of the control portion  7 . 
     Specifically, the control portion  7  acquires the electrical resistance value of the charging roller  32  using Equation (3). 
     &lt;Step S 18 &gt; 
     In step S 18 , the control portion  7  acquires the electrical resistance value of the primary transfer roller  34  based on the state value acquired in step S 16 , the voltage value of the primary transfer bias voltage, and the current value of the transfer current detected in step S 14 . Here, the process in step S 18  is an example of a third acquisition step of the present disclosure and is executed by the second resistance value acquisition portion  76  of the control portion  7 . 
     Specifically, the control portion  7  acquires the electrical resistance value of the primary transfer roller  34  using Equation (5). 
     &lt;Step S 19 &gt; 
     In step S 19 , the control portion  7  executes a first determination process of determining whether the timing of replacing the photoconductor drum  31  has arrived based on the state value acquired in step S 16 . Here, the process in step S 19  is executed by the first timing determination portion  77  of the control portion  7 . 
     Specifically, the control portion  7  determines that the timing of replacing the photoconductor drum  31  has arrived when the state value acquired in step S 16  exceeds the first threshold. 
     &lt;Step S 20 &gt; 
     In step S 20 , the control portion  7  executes a second determination process of determining whether the timing of replacing the charging roller  32  has arrived based on the electrical resistance value of the charging roller  32  acquired in step S 17 . Here, the process in step S 20  is executed by the second timing determination portion  78  of the control portion  7 . 
     Specifically, the control portion  7  determines that the timing of replacing the charging roller  32  has arrived when the electrical resistance value of the charging roller  32  acquired in step S 17  exceeds the second threshold. 
     &lt;Step S 21 &gt; 
     In step S 21 , the control portion  7  executes a third determination process of determining whether the timing of replacing the primary transfer roller  34  has arrived based on the electrical resistance value of the primary transfer roller  34  acquired in step S 18 . Here, the process in step S 21  is executed by the third timing determination portion  79  of the control portion  7 . 
     Specifically, the control portion  7  determines that the timing of replacing the primary transfer roller  34  has arrived when the electrical resistance value of the primary transfer roller  34  acquired in step S 18  exceeds the third threshold. 
     &lt;Step S 22 &gt; 
     In step S 22 , the control portion  7  causes the process to branch out depending on the results of the first determination process executed in step S 19 , the second determination process executed in step S 20 , and the third determination process executed in step S 21 . 
     Specifically, upon determining that the replacement timing has arrived in one or more determination processes (Yes in step S 22 ), the control portion  7  moves the process to step S 23 . If it is not determined that the replacement timing has arrived in any of the determination processes (No in step S 22 ), the control portion  7  ends the replacement timing determination process. 
     &lt;Step S 23 &gt; 
     Upon determining that the timing of replacing any of the members has arrived in steps S 19  to S 21 , the control portion  7  executes an informing process of informing that the timing of replacing the member has arrived in step S 23 . 
     Specifically, the control portion  7  causes the operation display portion  5  to display the name of the member determined to be replaced and a message that the timing of replacing the member has arrived. 
     In the image forming apparatus  100 , the potential value of the charged area on the photoconductor drum  31  is acquired as described above. The state value is then acquired based on the acquired potential value of the charged area and the current value of the charging current. Subsequently, the electrical resistance value of the charging roller  32  is acquired based on the acquired state value, the current value of the charging current, and the charging bias voltage. Thus, the electrical resistance value of the charging roller  32  can be acquired without a power source that has a special function compared with a configuration in which two pulsed voltages with different frequencies are applied to the charging roller  32  for acquisition of the electrical resistance value of the charging roller  32 . 
     In addition, the potential value of the charged area on the photoconductor drum  31  is acquired in the image forming apparatus  100 . The state value is then acquired based on the acquired potential value of the charged area and the current value of the charging current. Subsequently, the electrical resistance value of the primary transfer roller  34  is acquired based on the acquired state value, the current value of the transfer current, and the primary transfer bias voltage. Thus, the electrical resistance value of the primary transfer roller  34  can be accurately acquired compared with a configuration in which the electrical resistance value is calculated based on the primary transfer bias voltage and the transfer current without regard to the state value. 
     In addition, in the image forming apparatus  100 , the first development current is detected for each of the plurality of specific voltages, and the potential value of the unexposed area is acquired based on the DC voltage values of the specific voltages and the current values of the first development current corresponding to the respective specific voltages. This enables the potential value of the charged area to be acquired without a surface potential sensor that can detect the surface potential of the photoconductor drum  31 . 
     The present disclosure may be applied to an image forming apparatus that forms images using single-component developer, which does not contain carrier. 
     It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of the disclosure is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.