Patent Publication Number: US-11397397-B1

Title: Image forming apparatus

Description:
FIELD 
     Embodiments described herein relate generally to an image forming apparatus and methods of operating an image forming apparatus. 
     BACKGROUND 
     For example, an image forming apparatus using an electrophotographic process includes a photoreceptor, a charging unit, an exposure unit, a developing unit, a transfer mechanism, and a fixing unit. In the image forming apparatus, a developer image is formed on a surface of the photoreceptor as an image carrier by the charging unit, the exposure unit, and the developing unit. In the image forming apparatus, the photoreceptor having an outer circumferential surface as the image carrier rotates. The charging unit uniformly charges the outer circumferential surface of the photoreceptor. The exposure unit exposes the uniformly charged surface of the photoreceptor to form an electrostatic latent image. The developing unit develops the electrostatic latent image formed on the surface of the photoreceptor with a developer to form a developer image. 
     The transfer mechanism transfers the developer image formed on the surface of the photoconductive drum to a medium such as an intermediate transfer belt. The transfer mechanism applies a transfer bias to the photoreceptor through the intermediate transfer belt to transfer the developer image on the surface of the photoconductive drum to the intermediate transfer belt. In this image forming apparatus, the photoreceptor may exhibit the memory effect for the transfer bias due to deterioration over time. If there is a surface potential difference due to deterioration over time during the application of the transfer bias, density unevenness may occur in the photoreceptor. In an image forming apparatus in the related art, if the density unevenness caused by deterioration over time increases, there is a problem in that a user needs to request a service person for maintenance such as replacement of the photoreceptor. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment; 
         FIG. 2  is a block diagram illustrating a configuration example of a control system; 
         FIG. 3  is a diagram illustrating a relationship between a surface potential of a photoconductive drum and an output of a transfer bias; 
         FIG. 4  is a diagram illustrating a relationship between the photoconductive drum, a charging unit, and a primary transfer roller; 
         FIG. 5  is a diagram illustrating a relationship between the photoconductive drum, the charging unit, and the primary transfer roller; 
         FIG. 6  is a diagram illustrating a relationship between the photoconductive drum, the charging unit, and the primary transfer roller; 
         FIG. 7  is a diagram illustrating a relationship between the photoconductive drum, the charging unit, and the primary transfer roller; 
         FIG. 8  is a diagram illustrating an example of a surface potential of the photoconductive drum where there is no deterioration over time; 
         FIG. 9  is a diagram illustrating an example of the surface potential of the photoconductive drum where there is deterioration over time; 
         FIG. 10  is a diagram illustrating an example of the surface potential of the photoconductive drum where there is deterioration over time; 
         FIG. 11  is a diagram illustrating density unevenness of an image caused by a difference in surface potential of the photoconductive drum; 
         FIG. 12  is a diagram illustrating a relationship between the surface potential of the photoconductive drum where there is no deterioration over time and a charging timing and an output timing of a transfer bias; 
         FIG. 13  is a diagram illustrating a relationship between the surface potential of the photoconductive drum where there is deterioration over time and the charging timing and the output timing of a transfer bias; 
         FIG. 14  is a diagram illustrating a relationship between the surface potential of the photoconductive drum where there is deterioration over time and the charging timing and the output timing of the transfer bias; 
         FIG. 15  is a diagram illustrating an adjusted value for a transfer bias applied to the photoconductive drum; 
         FIG. 16  is a flowchart illustrating a setting process such as deterioration detection and adjustment of the photoconductive drum; 
         FIG. 17  is a flowchart illustrating the setting process such as deterioration detection and adjustment of the photoconductive drum; and 
         FIG. 18  is a flowchart illustrating an operation example of deterioration detection of the photoconductive drum. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, there is provided an image forming apparatus including an image carrier, a charging unit, an exposure unit, a developing unit, a transfer device, a sensor, and a processor. The charging unit is configured to charge the image carrier. The exposure unit is configured to form an electrostatic latent image on the image carrier that is charged by the charging unit. The developing unit is configured to supply a developer to the image carrier. The transfer device is configured to apply a transfer bias for transferring a developer image to a medium, the developer image being formed by developing the electrostatic latent image on the image carrier with the developing unit. The sensor is configured to measure a surface potential on the image carrier. The processor is configured to compare a surface potential difference between a first surface potential and a second surface potential of the image carrier to a threshold to detect that a surface potential difference exceeding the threshold is generated in the image carrier in a state where the supply of the developer from the developing unit to the image carrier is inhibited, the first surface potential being a surface potential of the image carrier to which the transfer bias measured by the sensor is not applied, and the second surface potential being a surface potential of the image carrier to which the transfer bias is applied. According to one embodiment, method for operating an image forming apparatus involving charging an image carrier; forming an electrostatic latent image on the image carrier; supplying a developer to the image carrier; applying a transfer bias to transfer a developer image from the image carrier to a medium, the developer image being formed by developing the electrostatic latent image on the image carrier; measuring a surface potential on the image carrier; and comparing a surface potential difference between a first surface potential and a second surface potential of the image carrier to a threshold to detect that a surface potential difference exceeding the threshold is generated in the image carrier in a state where the supply of the developer from a developing component to the image carrier is inhibited, the first surface potential being a surface potential of the image carrier to which the transfer bias measured by the sensor is not applied, and the second surface potential being a surface potential of the image carrier to which the transfer bias is applied. 
       FIG. 1  is a schematic diagram illustrating a schematic configuration of an image forming apparatus  1  according to an embodiment. 
     The image forming apparatus  1  forms an image on a printing medium P through an electrophotographic process. The image forming apparatus  1  forms an image on the printing medium P by developing an image with toner. As the toner, a monochrome toner may be used or a plurality of color toners such as yellow, magenta, cyan, or black may be used.  FIG. 1  illustrates a multi-function peripheral using four color toners as an example of the image forming apparatus  1 . 
     As illustrated in  FIG. 1 , the image forming apparatus  1  includes a housing  11 , a communication interface  12 , a controller  13 , a plurality of paper trays  14 , a paper discharge tray  15 , a conveyance mechanism  16 , an image forming mechanism  17 , a fixing unit  18 , a scanner  19 , and a control panel  20 . 
     The housing  11  is a main body of the image forming apparatus  1 . The housing  11  accommodates, for example, the communication interface  12 , the controller  13 , the paper trays  14 , the conveyance mechanism  16 , the image forming mechanism  17 , and the fixing unit  18 . A part of an upper surface of the housing  11  functions as the paper discharge tray  15 . 
     The communication interface  12  is an interface for communication with another apparatus to which the image forming apparatus is connected via the network. The communication interface  12  is used for communication with an external apparatus. The external apparatus is, for example, a center as a management apparatus. The communication interface  12  is configured with, for example, a LAN connector. The communication interface  12  may execute wireless communication with another apparatus in accordance with a standard such as Bluetooth (registered trademark) or Wi-fi (registered trademark). 
     The controller  13  executes controls, data processing, and the like on the respective units of the image forming apparatus  1 . For example, the controller  13  is a computer including a processor, a memory, and various interfaces. The controller  13  executes controls, data processing, and the like on the respective units of the image forming apparatus  1  by the processor executing programs stored in the memory. The controller  13  is connected to the respective units in the housing  11  via various internal interfaces. For example, the controller  13  is connected to the communication interface  12 , the paper discharge tray  15 , the conveyance mechanism  16 , the image forming mechanism  17 , the fixing unit  18 , the scanner  19 , and the like. 
     The controller  13  generates a print job based on data acquired from an external apparatus via the communication interface  12 . The print job includes image data representing an image that is formed on the printing medium P. The image data may be data for forming an image on a single printing medium P or may be data for forming an image on a plurality of printing media P. The print job may include information representing whether the printing is color printing or monochrome printing. 
     The controller  13  includes an engine controller for controlling operations of the conveyance mechanism  16 , the image forming mechanism  17 , and the fixing unit  18 . For example, the controller  13  controls the conveyance mechanism  16  to convey the printing medium P. The controller  13  controls the image forming mechanism  17  to form a developer image and controls transfer of the developer image to the medium P. The controller  13  controls the fixing unit  18  to fix the developer image to the printing medium P. The controller  13  controls the operation of the conveyance mechanism  16 , the image forming mechanism  17 , and the fixing unit  18  such that an image of image data in a print job is formed on the printing medium P. 
     The image forming apparatus  1  may be configured to include an engine controller separately from the controller  13 . For example, in the image forming apparatus  1 , an engine controller that controls at least one of the conveyance mechanism  16 , the image forming mechanism  17 , the fixing unit  18 , and the like may be provided separately from the controller  13 . The engine controller provided separately from the controller  13  may acquire information required for the control from the controller  13 . 
     The paper trays  14  are cassettes accommodating the printing media P, respectively. The paper tray  14  is configured to supply the printing medium P from the outside of the housing  11 . For example, the paper tray  14  is configured to be drawn out from the housing  11 . 
     The conveyance mechanism  16  is a mechanism that conveys the printing medium P in the image forming apparatus  1 . As illustrated in  FIG. 1 , the conveyance mechanism  16  includes a plurality of conveyance paths. The conveyance mechanism  16  includes a paper feed conveyance path  31  and a paper discharge conveyance path  32 . 
     The paper feed conveyance path  31  and the paper discharge conveyance path  32  are configured with a plurality of rollers and a plurality of guides. The rollers rotate with power transmitted from a driving mechanism to convey the printing medium P. The guides control a conveying direction of the printing medium P conveyed by the rollers. 
     The paper feed conveyance path  31  picks up the printing medium P from the paper tray  14  and supplies the picked printing medium P to the image forming mechanism  17 . The paper feed conveyance path  31  includes a plurality of pickup rollers  33  corresponding to the paper trays  14 , respectively. Each of the pickup rollers  33  picks up the printing medium P of the paper tray  14  to the paper feed conveyance path  31 . 
     The paper discharge conveyance path  32  is a conveyance path through which the printing medium P on which an image is formed by the image forming mechanism  17  is discharged from the housing  11 . The paper discharge conveyance path  32  discharges the printing medium P to the paper discharge tray  15 . The paper discharge tray  15  is a tray that receives the printing medium P discharged from the image forming apparatus  1 . 
     The image forming mechanism  17  has a configuration for forming an image on the printing medium P. The details of the image forming mechanism  17  will be described below. 
     The fixing unit  18  includes a heating roller  34  and a pressurization roller  35 . The fixing unit  18  heats the printing medium P conveyed through the paper discharge conveyance path  32  to a predetermined temperature with the heating roller  34 . The fixing unit  18  further pressurizes the printing medium P heated by the heating roller  34  with the pressurization roller  35 . The fixing unit  18  heats and pressurizes the printing medium P to fix an image (developer image) on the printing medium P to the printing medium P. 
     The scanner  19  reads a document and converts the read image into image data. The scanner  19  is provided in an upper portion of the housing  11 . The scanner  19  includes an automatic document feeder  21 . The scanner  19  reads an image of a document conveyed by the automatic document feeder  21 . 
     The control panel  20  includes a touch panel  22 , a keyboard  23 , and the like. In the touch panel  22 , for example, a display such as a liquid crystal display or an organic EL display and a touch sensor that detects a touch input are laminated. The display including the touch panel  22  is a display device of the image forming apparatus  1 . 
     The keyboard  23  includes various keys for allowing a user to operate the image forming apparatus  1 . For example, the keyboard  23  includes a numeric keypad, a power key, a paper feed key, a function key, and the like. Each of the keys will also be referred to as “button”. The touch panel  22  and the keyboard  23  are input devices of the image forming apparatus  1 . 
     Next, the image forming mechanism  17  will be described. 
     As illustrated in  FIG. 1 , the image forming mechanism  17  includes a plurality of image forming stations  41  and a transfer mechanism  42 . Each of the image forming stations  41  forms a toner image. Each of the image forming stations  41  is provided for each type of toner. For example, the image forming stations  41  correspond to the respective color toners such as yellow, magenta, cyan, and black in this order from the left side in  FIG. 1 , respectively. The image forming stations  41  include toner cartridges  2  containing corresponding color toners, respectively.  FIG. 1  illustrates the image forming apparatus  1  including four image forming stations  41  corresponding to four color toners of yellow, magenta, cyan, and black. 
     Next, the image forming stations  41  will be described. 
     Each of the image forming stations  41  includes a photoconductive drum  71 , a cleaner  72 , a charging unit  73 , an exposure unit  74 , and a developing unit  75 . 
     The photoconductive drum  71  includes: a cylindrical drum; and a photosensitive layer that is formed on an outer circumferential surface of the drum. The photoconductive drum  71  is a photoreceptor. The outer circumferential surface of the photoconductive drum  71  is an image carrier. The photoconductive drum  71  rotates at a fixed speed with power transmitted from the driving mechanism. 
     The cleaner  72  includes a blade in contact with the surface of the photoconductive drum  71 . The cleaner  72  removes toner remaining on the surface of the photoconductive drum  71  using the blade. 
     The charging unit  73  uniformly charges a surface of the photoconductive drum  71 . The charging unit  73  will also be referred to as “electrostatic charger”. For example, the charging unit  73  applies a grid bias voltage output from a grid electrode to the photoconductive drum  71  such that the photoconductive drum  71  is charged with uniform negative potential. 
     The exposure unit  74  includes a plurality of light emitting elements. The light emitting element is, for example, a laser diode (LD), a light emitting diode (LED), or an organic EL (OLED). The light emitting elements are arranged in a main scanning direction that is a direction parallel to a rotation axis of the photoconductive drum  71 . Each of the light emitting elements is configured to irradiate one point on the photoconductive drum  71  with light. 
     The exposure unit  74  forms an electrostatic latent image corresponding to one line on the photoconductive drum  71  by irradiating the charged surface of the photoconductive drum  71  with light from the light emitting elements arranged in the main scanning direction. Further, the exposure unit  74  forms an electrostatic latent image corresponding to a plurality of lines by continuously irradiating the rotating photoconductive drum  71  with light. 
     The developing unit  75  attaches the toner to the photoconductive drum  71 . The developing unit  75  contains a developer containing a toner and a carrier. The developing unit  75  stirs the toner and the carrier supplied from the toner cartridge  2  with an agitating mechanism. The developing unit  75  supplies the toner to the photoconductive drum  71  from a developing roller to which the developer containing the toner and the carrier agitated by the agitating mechanism is attached. The developing unit  75  develops the electrostatic latent image on the photoconductive drum  71  with the toner by supplying the toner to the photoconductive drum  71 . The photoconductive drum  71  holds the toner image (developer image) developed with the toner by the developing unit  75 . The photoconductive drum  71  rotates to transfer the toner image to a transfer position of a transfer belt  91 . 
     The transfer mechanism (transfer device)  42  transfers the toner image formed on the surface of the photoconductive drum  71  to the printing medium P. As illustrated in  FIG. 1 , the transfer mechanism  42  includes the transfer belt  91 , a driving roller  92 , a plurality of primary transfer rollers  93 , and a secondary transfer roller  94 . 
     The transfer belt  91  is a medium to which the toner image formed on the surface of the photoconductive drum  71  of each of the image forming stations  41  is transferred. The transfer belt  91  is an intermediate transfer medium that holds the image to be transferred to the printing medium P. In the configuration example illustrated in  FIG. 1 , the transfer belt  91  is an endless belt that is wound around the driving roller  92  and a plurality of winding rollers. A back surface of the transfer belt  91  as an inner surface comes into contact with the driving roller  92  and the winding rollers. A front surface of the transfer belt  91  as an outer surface faces the photoconductive drum  71  of each of the image forming stations  41 . 
     The driving roller  92  rotates with power transmitted from the driving mechanism. The driving roller  92  rotates to convey the transfer belt  91 . In the configuration example illustrated in  FIG. 1 , the driving roller  92  rotates counterclockwise. Due to the rotation of the driving roller  92 , the transfer belt  91  as the endless belt is conveyed to rotate counterclockwise. The winding rollers are configured to be freely rotatable. The winding rollers rotate according to the movement of the transfer belt  91  by the driving roller  92 . 
     The primary transfer rollers  93  are provided for each of the image forming stations  41 . Each of the primary transfer rollers  93  is provided to face the photoconductive drum  71  of the corresponding image forming station  41 . Each of the primary transfer rollers  93  is provided at a position facing the photoconductive drum  71  of the corresponding image forming station  41  with the transfer belt  91  interposed therebetween. 
     The primary transfer roller  93  comes into contact with an inner circumferential surface of the transfer belt  91 . The primary transfer roller  93  presses the transfer belt  91  against the photoconductive drum  71  from the inner circumferential surface side. The surface (outer circumferential surface) of the transfer belt  91  pressed by the primary transfer roller  93  abuts against the photoconductive drum  71 . If the image (toner image) is transferred from the photoconductive drum  71 , the primary transfer roller  93  applies the transfer bias to the photoconductive drum  71  through the transfer belt  91 . The toner image is transferred from the photoconductive drum  71  to the transfer belt  91  by the transfer bias applied from the primary transfer roller  93 . 
     The secondary transfer roller  94  is provided at a position facing the driving roller  92 . The inner circumferential surface of the secondary transfer roller  94  comes into contact with the surface of the conveyed transfer belt  91  due to the driving roller  92 . The secondary transfer roller  94  presses the transfer belt  91  against the driving roller  92  side. The surface of the transfer belt  91  interposed between the driving roller  92  and the secondary transfer roller  94  adheres closely to the secondary transfer roller  94 . A transfer nip is formed at a place where the surface of the transfer belt  91  and the secondary transfer roller  94  adhere closely to each other. 
     The secondary transfer roller  94  is conveyed in a state where the printing medium P supplied from the paper feed conveyance path  31  is interposed between the secondary transfer roller  94  and the transfer belt  91 . The printing medium P passes through the transfer nip. The secondary transfer roller  94  presses the printing medium P passed through the transfer nip against the surface of the transfer belt  91 . If the toner image of the transfer belt  91  is transferred to the printing medium P in the transfer nip, the secondary transfer roller  94  applies the transfer bias to the transfer belt  91  through the printing medium P. The toner image on the transfer belt  91  is transferred to the printing medium P by the transfer bias. 
     At a position where the surface of the transfer belt  91  is in contact with the surface of the photoconductive drum  71 , the transfer mechanism  42  transfers the toner image on the surface of the photoconductive drum  71  to the surface of the transfer belt  91 . The transfer mechanism  42  transfers the toner image on the photoconductive drum  71  to the transfer belt  91  abutting against the photoconductive drum  71  due to the transfer bias applied from the primary transfer roller. If the image forming stations  41  are provided, the transfer mechanism  42  transfers the toner images to the transfer belt  91  from the photoconductive drums  71  of the image forming stations  41 . 
     The transfer mechanism  42  conveys the toner image transferred to the surface of the transfer belt  91  up to the transfer nip. The transfer mechanism  42  transfers the toner image transferred to the surface of the transfer belt  91  to the printing medium P present in the transfer nip due to the transfer bias applied from the secondary transfer roller. The transfer belt  91  is an example of the image carrier that holds the toner image transferred to the printing medium P. 
     In the image forming mechanism  17 , each of the image forming stations  41  includes a sensor  100  that measures a potential of the surface (surface potential) of the photoconductive drum. The sensor  100  measures the surface potential of the photoconductive drum  71 . In order to calculate a first surface potential described below, the sensor  100  measures the surface potential of the photoconductive drum  71  to which the transfer bias is not applied from the primary transfer roller  93 . In order to calculate a second surface potential described below, the sensor  100  measures the surface potential of the photoconductive drum  71  to which the transfer bias is applied from the primary transfer roller. 
     Next, a configuration of a control system in the image forming apparatus  1  will be described. 
       FIG. 3  is a block diagram illustrating the configuration example of the control system in the image forming apparatus  1 . 
     As illustrated in  FIG. 3 , in the image forming apparatus  1 , the communication interface  12 , the image forming mechanism  17 , the fixing unit  18 , the scanner  19 , the control panel  20 , a motor  30 , and the like are connected to the controller  13 . 
     The controller  13  includes a processor  131 , a read only memory (ROM)  132 , a random-access memory (RAM)  133 , and an auxiliary storage device  134 . The controller  13  configures a computer with the processor  131 , the ROM  132 , the RAM  133 , and the auxiliary storage device  134 . 
     The processor  131  corresponds to a central part of the computer as the controller  13 . The processor  131  controls the respective units of the image forming apparatus  1  in accordance with an operating system or an application program. The processor  131  is, for example, a central processing unit (CPU). 
     The ROM  132  and the RAM  133  correspond to a main memory of the computer as the controller  13 . The ROM  132  is a nonvolatile memory area, and the RAM  133  is a volatile memory area. The ROM  132  stores an operating system or an application program. The ROM  132  stores control data required to execute a process for allowing the processor  131  to control the respective units. The RAM  133  is used as a work area where data is appropriately rewritten by the processor  131 . The RAM  133  has a work area for storing, for example, image data. 
     The auxiliary storage device  134  corresponds to an auxiliary storage part of the computer as the controller  13 . The auxiliary storage device  134  is configured with, for example, an electric erasable programmable read-only memory (EEPROM), a hard disk drive (HDD), or a solid state drive (SSD). The auxiliary storage device  134  stores data such as setting data used for allowing the processor  131  to execute various processes. The auxiliary storage device  134  stores data generated through the process executed by the processor  131 . The auxiliary storage device  134  may store the application program. 
     The controller  13  is connected to the toner cartridge  2 , the photoconductive drum  71 , the cleaner  72 , the charging unit  73 , the exposure unit  74 , and the developing unit  75  in each of the image forming stations  41 . The controller  13  controls the toner cartridge  2 , the photoconductive drum  71 , the cleaner  72 , the charging unit  73 , the exposure unit  74 , and the developing unit  75 . For example, the controller  13  controls the charging unit  73  of each of the image forming stations  41  to start or stop charging. The controller  13  controls the exposure unit  74  of each of the image forming stations  41  to start or stop irradiation of laser light with which the photoconductive drum is irradiated. In addition, the controller  13  controls the developing unit  75  of the each of the image forming stations  41  to start or stop application of a developing bias. 
     The controller  13  is connected to the transfer belt  91 , the driving roller  92 , the primary transfer rollers  93 , and the secondary transfer roller  94  in the transfer mechanism  42 . The controller  13  is also connected to the fixing unit  18 . For example, the controller  13  controls the start or stop of application of the transfer bias to the photoconductive drum  71  facing each of the primary transfer rollers  93 . The controller  13  controls a value (applied value) of the transfer bias that is applied by the primary transfer roller  93 . The controller  13  controls the start or stop of application of the transfer bias by the secondary transfer roller  94 . 
     The motor  30  operates the respective units. The motor  30  is connected to the controller  13 . The motor  30  operates in accordance with the control from the controller  13 . The motor  30  includes, for example, a first motor, a second motor, and a third motor. The first motor as the motor  30  drives the conveyance mechanism  16 . The second motor as the motor  30  rotates the photoconductive drum  71 . The third motor as the motor  30  rotates the driving roller  92 . A plurality of second motors are provided to correspond to the photoconductive drums  71  of the image forming stations  41 , respectively. The motor  30  may include a motor other than the first, second, and third motors. 
     Next, the surface potential of the photoconductive drum  71  in the image forming apparatus  1  according to the embodiment will be described. 
       FIG. 3  is a diagram illustrating timings of charging, exposure, and transfer bias application for the photoconductive drum  71 . 
     As illustrated in  FIG. 3 , the photoconductive drum  71  is charged by the charging unit  73 . After the charging unit  73  chares the surface of the photoconductive drum, the exposure unit  74  projects an optical image to the uniformly charged surface of the photoconductive drum. An electrostatic latent image corresponding to the optical image is formed on the surface of the photoconductive drum  71  to which the optical image is projected from the exposure unit  74 . After a predetermined time is elapsed from the start of the exposure from the exposure unit  74 , the primary transfer roller  93  applies the transfer bias to the photoconductive drum  71 . The transfer belt  91  interposed between the primary transfer roller  93  and the photoconductive drum  71  transfers the toner image to the surface abutting against the photoconductive drum  71  to which the transfer bias is applied. 
       FIGS. 4 to 7  are schematic diagrams illustrating the primary transfer roller  93 , the photoconductive drum  71 , and the exposure unit  74  in the image forming apparatus  1 . 
     In the examples illustrated in  FIGS. 4 to 7 , the photoconductive drum  71  rotates clockwise.  FIG. 4  is a diagram illustrating a state where the exposure to the surface of the photoconductive drum  71  starts. In the image forming apparatus  1 , the exposure unit  74  starts the exposure after the charging unit  73  charges the surface of the photoconductive drum  71 . As illustrated in  FIG. 4 , a start position where the exposure to the surface of the photoconductive drum  71  starts will be referred to as “position a”. 
       FIG. 5  is a diagram illustrating a state where the transfer mechanism  42  starts the application of the transfer bias to the photoconductive drum  71  after a predetermined time is elapsed from the start of the exposure. 
     When the predetermined time is elapsed from the start of the exposure to the photoconductive drum  71 , the transfer mechanism  42  applies the transfer bias to a place where the primary transfer roller  93  and the photoconductive drum  71  face each other. As illustrated in  FIG. 5 , a position where the application of the transfer bias starts in the photoconductive drum  71  will be referred to as “position b”. 
     A fan-shaped region interposed between the position b and the position a in the photoconductive drum  71  is a region that is not yet exposed by the exposure unit  74 . The fan-shaped region interposed between the position b and the position a passes through a position corresponding to the primary transfer roller  93  before being exposed by the exposure unit  74 . In addition, a region c other than the fan-shaped region interposed between the position b and the position a does not apply the transfer bias until the position b subsequently passes the position facing the primary transfer roller  93 . While the position b reaches the position facing the primary transfer roller  93 , the surface of the photoconductive drum  71  is exposed by the exposure unit  74  before the primary transfer bias is applied. 
       FIG. 6  is a diagram illustrating a state where the position a where the exposure by the exposure unit  74  starts reaches initially the position facing the primary transfer roller  93 .  FIG. 7  is a diagram illustrating a state where the position b on the photoconductive drum  71  reaches again the position corresponding to the primary transfer roller  93  after rotating once. 
     After the position a where the exposure starts passes through the position facing the primary transfer roller  93 , the toner image formed after the position a is transferred to the transfer belt  91 . In addition, while the position b reaches again the position corresponding to the primary transfer roller  93 , a region before the position b in the rotation direction of the photoconductive drum  71  is a region where the transfer bias is not applied. Accordingly, the region c from the position a to the position b is a region that is exposed by the exposure unit  74  in a state where the transfer bias is not applied. 
     As illustrated in  FIG. 7 , if the position b where the application of the transfer bias starts rotates once, the photoconductive drum  71  applies the transfer bias to the entire surface. In other words, a region after the position b where the application of the transfer bias starts in the rotation direction of the photoconductive drum  71  is a region that is exposed by the exposure unit  74  in a state where the transfer bias is applied. 
       FIGS. 8 to 10  are diagrams illustrating examples of the surface potential of the photoconductive drum  71  that operates as illustrated in  FIGS. 4 to 7 . 
     In  FIGS. 8 to 10 , a region A is a portion that is exposed in a state where the transfer bias is not applied in the photoconductive drum  71 . A region B is a portion that is exposed after the transfer bias is applied in the photoconductive drum  71  once. 
     In the photoconductive drum  71  where there is no deterioration over time, the surface potential does not change depending on whether or not the transfer bias is applied. Therefore, in the photoconductive drum  71  where there is no deterioration over time, there is no difference between the surface potential of the portion corresponding to the region A and the surface potential of the portion corresponding to the region B as illustrated in  FIG. 8 . 
     On the other hand, in the photoconductive drum where there is deterioration over time, the memory effect (referred to as “transfer memory”) caused by the application of the transfer bias occurs on the surface. In the photoconductive drum where the transfer memory occurs, the surface potential changes between the region where the transfer bias is not applied and the region where the transfer bias is applied. Even when the photoconductive drum  71  where there is deterioration over time is charged and exposed at the same potential, there may be a potential difference between the surface potential of the portion corresponding to the region A and the surface potential of the portion corresponding to the region B. 
       FIGS. 9 and 10  illustrate examples where there is a potential difference in the surface potential of the photoconductive drum  71  depending on whether or not the transfer bias is applied. In addition,  FIG. 11  schematically illustrates density unevenness that occurs in the image that is transferred from the photoconductive drum  71  to the transfer belt  91  due to the surface potential difference of the photoconductive drum  71  as illustrated in  FIG. 9 . As illustrated in  FIG. 11 , the surface potential difference of the photoconductive drum  71  that occurs due to deterioration over time appears as the density unevenness in the image to be printed. 
     Some image forming apparatus may include a charge eraser that erases charge before charging. The image forming apparatus including the charge eraser executes charging and exposure again after erasing charge with the charge eraser. Even in the image forming apparatus including the charge eraser, the portion corresponding to the region B is charged after the transfer bias is applied once. Therefore, even in the image forming apparatus including the charge eraser, the transfer memory may occur due to the deterioration over time of the photoconductive drum. 
     Next, an operation of detecting deterioration of the photoconductive drum  71  in the image forming apparatus  1  according to the embodiment will be described. 
     The image forming apparatus  1  detects deterioration over time of the photoconductive drum  71  based on the surface potential difference of the photoconductive drum  71 . In the image forming apparatus  1 , the surface potential of the photoconductive drum  71  is measured by the sensor  100 . The image forming apparatus  1  detects that the surface potential of the photoconductive drum  71  that is charged by the charging unit  73  changes depending on the application of the transfer bias. 
     Separately from an operation of actually printing an image on the printing medium P, the image forming apparatus  1  executes a deterioration detection operation of detecting the deterioration over time of the photoconductive drum  71  based on the surface potential of the photoconductive drum  71 . The image forming apparatus  1  compares the surface potential of the photoconductive drum  71  if the transfer bias is not applied and the surface potential of the photoconductive drum  71  if the transfer bias is applied to each other. 
       FIG. 12  illustrates a relationship between the surface potential of the photoconductive drum where there is no deterioration over time and an output timing of the transfer bias. As illustrated in  FIG. 12 , in the photoconductive drum where there is no deterioration over time, the surface potential does not change depending on the application of the transfer bias. 
       FIGS. 13 and 14  illustrate a relationship between the surface potential of the photoconductive drum where there is deterioration over time and an output timing of the transfer bias. As illustrated in  FIG. 13 or 14 , in the photoconductive drum  71  where the deterioration over time is accelerated, the surface potential changes if the transfer bias is applied. 
     The processor  131  detects deterioration of the photoconductive drum  71  based on the surface potential change caused by the application of the transfer bias illustrated in  FIG. 13 or 14 . The processor  131  measures the surface potential of the photoconductive drum in a period (non-application portion) where the transfer bias is not applied. The processor  131  measures the surface potential of the photoconductive drum in a period (application portion) where the transfer bias is applied. If a difference (surface potential difference) between the surface potential of the non-application portion and the surface potential of the application portion exceeds a threshold, the processor  131  detects the deterioration of the photoconductive drum. 
     In addition, if the deterioration of the photoconductive drum is detected, the processor  131  can change (adjust) a control value used for the application control of the transfer bias. As the control value used for the application control of the transfer bias, an output timing of the transfer bias, an applied value of the transfer bias, and the like can be changed. 
       FIG. 15  is a diagram illustrating an adjusted value relative to the control value for applying the transfer bias to the photoconductive drum  71  in the image forming apparatus  1 . 
     As illustrated in  FIG. 15 , the output timing and the applied value of the transfer bias can be changed. By changing the output timing and the applied value of the transfer bias, an oblique line portion C illustrated in  FIG. 15  can be changed. 
     As the change in the output timing of the transfer bias, a timing at which the application of the transfer bias starts can be advanced. In addition, the timing at which the application of the transfer bias stops can be delayed. A timing at which the application of the transfer bias starts or stops can be freely set. 
     For example, the output timing of the transfer bias is adjusted such that the transfer bias is applied to the start position of the exposure. By applying the transfer bias to the exposure start position, the occurrence of density unevenness caused by whether or not the application of the transfer bias can be prevented. 
     In addition, the applied value of the transfer bias can be set in the period where the start of the application of the transfer bias is advanced and in the period where the stop of the application of the transfer bias is delayed. For example, in the period where the start of the application of the transfer bias is advanced, an applied value of the transfer bias that is applied to the transfer roller in a normal condition is a reference value. Assuming that the same applied value as the reference value is 100%, the effect of preventing density unevenness increases as the applied value of the transfer bias in the period where the start of the application of the transfer bias is advanced approaches the upper limit of 100%. On the other hand, as the applied value increases, the bias that is applied to the transfer roller increases. Therefore, the deterioration of the transfer roller may be accelerated. Accordingly, the applied value of the transfer bias in the period where the start of the application of the transfer bias is advanced can be set in consideration of the effect of preventing density unevenness and the deterioration of the transfer roller. 
     Next, the setting of deterioration detection for the photoconductive drum  71  in the image forming apparatus  1  according to the embodiment will be described. 
       FIG. 16  is a flowchart illustrating a setting operation of the deterioration detection for the photoconductive drum  71  in the image forming apparatus  1 . 
     The processor  131  of the image forming apparatus  1  receives various settings relating to the deterioration detection (ACT  11 ). If various settings relating to the deterioration detection are designated (ACT  11 , YES), the processor  131  receives an input of setting information representing the deterioration detection (ACT  12 ). The setting information representing the deterioration detection is stored in the auxiliary storage device  134 . If the deterioration detection for the photoconductive drum starts, the processor  131  executes the deterioration detection in accordance with the setting information stored in the auxiliary storage device  134 . 
     The processor  131  receives designation of a condition where the deterioration detection starts. If the condition where the deterioration detection starts is input (ACT  13 , YES), the processor  131  stores information representing the condition where the deterioration detection starts in the auxiliary storage device  134  as the setting information (ACT  14 ). For example, an operator (user or manager) designates the number of images to be formed as the condition where the deterioration detection operation starts. If the number of images to be formed is designated as the condition where the deterioration detection starts, the processor  131  stores the number of images to be formed as the setting information for executing the deterioration detection. 
     The processor  131  receives designation of a comparison method of comparing the surface potential of the photoconductive drum to a threshold as the deterioration detection. If the comparison method to be executed is designated (ACT  15 , YES), the processor  131  stores setting information representing the comparison method that is designated to be executed in the auxiliary storage device  134  (ACT  16 ). The processor  131  of the image forming apparatus  1  includes a plurality of comparison methods as the comparison method of the surface potential using a plurality of pre-installed programs. The processor  131  receives designation of the operator for whether or not to execute each of the executable comparison methods. For example, if the processor  131  includes four comparison methods as described below, the processor  131  receives designation for whether or not to execute each of the four comparison methods. The processor  131  stores setting information representing whether or not to execute each of the comparison methods in the auxiliary storage device  134 . 
     The processor  131  receives designation of an adjusted value for the application control of the transfer bias to be applied if the deterioration of the photoconductive drum is detected. If the adjusted value for the application control of the transfer bias is designated (ACT  17 , YES), the processor  131  stores setting information representing the designated adjusted value in the auxiliary storage device  134  (ACT  18 ). As the adjusted value for the application control of the transfer bias, an adjusted value for the output timing and the applied value of the transfer bias is designated. 
     In addition, if the deterioration of the photoconductive drum is detected, the processor  131  also receives designation of whether or not to change the application control of the transfer bias. As a result, the operator can designate the application control of the transfer bias to be automatically changed based on the adjusted value designated if the deterioration of the photoconductive drum is detected. 
     Further, the processor  131  receives designation of the content of notification to be notified to a center as a management apparatus if the deterioration of the photoconductive drum is detected. If the content of notification is designated (ACT  19 , YES), the processor  131  stores setting information representing the designated content of notification in the auxiliary storage device  134  (ACT  20 ). If the deterioration of the photoconductive drum is detected, the processor  131  notifies the content of notification representing the setting information stored in the auxiliary storage device  134  to the sensor. 
     If the setting end is not instructed (ACT  21 , NO), the processor  131  continues the above-described various settings. If the setting end is instructed (ACT  21 , YES), the processor  131  ends the setting operation. In the above-described setting operation, the various setting information for executing the deterioration detection can be stored in accordance with the instruction of the operator. 
     Next, the deterioration detection operation of the photoconductive drum  71  in the image forming apparatus  1  according to the embodiment will be described. 
       FIGS. 17 and 18  are flowcharts illustrating the deterioration detection operation of the photoconductive drum  71  in the image forming apparatus  1 . 
     The processor  131  of the image forming apparatus  1  determines whether or not to start the deterioration detection (ACT  31 ). The processor  131  determines the start of the deterioration detection based on the setting information stored in the auxiliary storage device  134 . For example, the auxiliary storage device  134  sets the number of images to be formed as the condition where the deterioration detection starts that is the execution setting of the deterioration detection. If the number of images to be formed reaches the set number, the processor  131  starts the deterioration detection. In addition, even when the execution of the deterioration detection is instructed at any timing, the processor  131  may start the deterioration detection operation. 
     If the deterioration detection operation starts (ACT  31 , YES), the processor  131  sets the comparison method of the surface potential for calculating the surface potential difference (ACT  32 ). Based on the setting information stored in the auxiliary storage device  134 , the processor  131  specifies the comparison method that is set to be executed from the comparison methods. One comparison method or a plurality of comparison methods may be executed. 
     If the deterioration detection operation starts, the processor  131  drives the motor  30  to rotate the photoconductive drum  71  that is a target of the deterioration detection (ACT  33 ). If the deterioration detection is executed on the photoconductive drums, the processor  131  executes the deterioration detection on each of the photoconductive drums. The processor  131  may execute the deterioration detection operation on the photoconductive drums at the same time. 
     If the photoconductive drum  71  is rotated, the processor  131  causes the charging unit  73  corresponding to the rotating photoconductive drum  71  to start charging the photoconductive drum  71  (ACT  34 ). The processor  131  causes the charging unit  73  to uniformly charge the surface of the photoconductive drum  71  at a predetermined potential. The processor  131  causes the charging unit  73  to continuously execute a charging operation at a given value during the measurement of the surface potential of the photoconductive drum  71 . 
     In addition, the processor  131  applies the developing bias to the developing unit  75  in order to inhibit the developer from being transferred to the surface of the photoconductive drum  71  (ACT  35 ). If the rotating photoconductive drum  71  is charged, the developer in the developing unit  75  is transferred to the surface of the photoconductive drum. Therefore, the processor  131  applies the developing bias in order to inhibit the supply of the developer to the photoconductive drum. The image forming apparatus  1  may execute a control other than the application of the developing bias such that the supply of the developer to the photoconductive drum is inhibited. 
     After the start of charging, the processor  131  causes the sensor  100  to measure the surface potential of the photoconductive drum  71  in a state where the photoconductive drum is continuously charged at a predetermined potential without applying the transfer bias (ACT  36 ). The processor  131  causes the sensor  100  to measure the surface potential of the photoconductive drum in the period where the transfer bias is not applied. For example, in order to specify the first surface potential, the processor  131  measures the surface potential of the photoconductive drum at a predetermined interval in a predetermined sampling period where the transfer bias is not applied. In addition, a method (for example, the number of times of measurement or the sampling period) of measuring the surface potential to specify the first surface potential may be set for each of the comparison methods. 
     After the start of charging, the processor  131  measures the output timing of the transfer bias. If a predetermined output timing is reached, the processor  131  causes the transfer mechanism  42  to apply a predetermined potential as the transfer bias (ACT  37 ). If the transfer bias is applied, the processor  131  causes the sensor  100  to measure the surface potential of the photoconductive drum  71  in the period where the transfer bias is applied (ACT  38 ). For example, in order to specify the second surface potential, the processor  131  measures the surface potential of the photoconductive drum  71  at a predetermined interval in a predetermined sampling period where the transfer bias is applied. In addition, a method (for example, the number of times of measurement or the sampling period) of measuring the surface potential to specify the second surface potential may be set for each of the comparison methods. 
     The processor  131  holds the charging potential, the developing bias, and the transfer bias at fixed values during the measurement of the surface potential. In this case, each of the charging potential, the developing bias, and the transfer bias may be any set value designated by a user or a manager. 
     After measuring the surface potential in the period where the transfer bias is not applied and the surface potential in the period where the transfer bias is applied, the processor  131  executes the comparison using the set comparison method. 
     If the comparison is executed using the average value of measured values as a first comparison method (ACT  41 , YES), the processor  131  calculates the average value of surface potentials measured in the period where the transfer bias is not applied. The processor  131  calculates the average value of surface potentials measured in the period where the transfer bias is not applied as the first surface potential. The processor  131  calculates the average value of surface potentials measured by the sensor  100  in the period where the transfer bias is applied as the second surface potential. 
     After calculating the first surface potential and the second surface potential as the average values, the processor  131  calculates a difference (surface potential difference) between the calculated first surface potential and the calculated second surface potential. After calculating the surface potential difference, the processor  131  compares the calculated surface potential difference to a threshold (ACT  42 ). The threshold may be set for each of the comparison methods. If the threshold for the first comparison method is set, the processor  131  compares the threshold for the first comparison method and the surface potential difference to each other. 
     If the comparison is executed using the maximum value of measured values as a second comparison method (ACT  43 , YES), the processor  131  specifies the maximum value of surface potentials measured in the period where the transfer bias is not applied as the first surface potential. In addition, if the comparison is executed using the second comparison method, the processor  131  specifies the maximum value of surface potentials measured by the sensor  100  in the period where the transfer bias is applied as the second surface potential. 
     After specifying the first surface potential and the second surface potential as the maximum values, the processor  131  calculates a difference (surface potential difference) between the specified first surface potential and the specified second surface potential. After calculating the surface potential difference, the processor  131  compares the calculated surface potential difference to a threshold (ACT  44 ). If the threshold for the second comparison method is set, the processor  131  compares the calculated surface potential difference and the threshold for the second comparison method to each other. 
     If the comparison is executed using the minimum value of measured values as a third comparison method (ACT  45 , YES), the processor  131  specifies the minimum value of surface potentials measured in the period where the transfer bias is not applied as the first surface potential. In addition, if the comparison is executed using the third comparison method, the processor  131  specifies the minimum value of surface potentials measured by the sensor  100  in the period where the transfer bias is applied as the second surface potential. 
     After specifying the first surface potential and the second surface potential as the minimum values, the processor  131  calculates a difference (surface potential difference) between the specified first surface potential and the specified second surface potential. After calculating the surface potential difference, the processor  131  compares the calculated surface potential difference to a threshold (ACT  46 ). If the threshold for the third comparison method is set, the processor  131  compares the calculated surface potential difference and the threshold for the third comparison method to each other. 
     If the comparison is executed using the median value of measured values as a fourth comparison method (ACT  47 , YES), the processor  131  specifies the median value of surface potentials measured multiple times in the period where the transfer bias is not applied as the first surface potential. In addition, if the comparison is executed using the fourth comparison method, the processor  131  specifies the median value of surface potentials measured by the sensor  100  multiple times in the period where the transfer bias is applied as the second surface potential. 
     After specifying the first surface potential and the second surface potential as the median values, the processor  131  calculates a difference (surface potential difference) between the specified first surface potential and the specified second surface potential. After calculating the surface potential difference, the processor  131  compares the calculated surface potential difference to a threshold (ACT  48 ). If the threshold for the fourth comparison method is set, the processor  131  compares the calculated surface potential difference and the threshold for the fourth comparison method to each other. 
     If the comparison is executed using the set comparison method, the processor  131  determines whether or not the surface potential difference exceeds the threshold (ACT  49 ). If a plurality of comparison methods are set, the processor  131  determines whether or not the surface potential difference exceeds the threshold in any one of the comparison methods. 
     If the surface potential difference exceeds the threshold (ACT  49 , NO), the processor  131  ends the deterioration detection operation. 
     If the surface potential difference exceeds the threshold (ACT  49 , YES), the processor  131  determines whether or not to change (adjust) the control value used for the application control of the transfer bias (ACT  50 ). If the control value is not changed (ACT  50 , NO), the processor  131  causes the communication interface  12  to notify the management apparatus that the surface potential difference exceeds the threshold (ACT  52 ). If the surface potential difference exceeds the threshold, the processor  131  may store the fact that the surface potential difference exceeds the threshold in the auxiliary storage device  134 . 
     If the processor  131  determines to change the control value used for the application control of the transfer bias (ACT  50 , YES), the processor  131  changes the control value based on the preset setting information (ACT  51 ). As described above, the processor  131  changes the output timing of the transfer bias and the applied value of the transfer bias in accordance with the setting information stored in the auxiliary storage device  134 . In addition, the processor  131  may change the control value based on the adjusted value designated by the user or the manager. 
     If the control value used for the application control of the transfer bias is changed, the processor  131  notifies the management apparatus of the fact that the surface potential difference exceeds the threshold, the content of change of the control value, and the like (ACT  52 ). The processor  131  may notify not only the content of change of the control value but also the time of use of the photoconductive drum, the number of images to be formed by the photoconductive drum, and the like to the management apparatus. If this content is notified to the management apparatus, the processor  131  ends the deterioration detection for the photoconductive drum  71  as a target. 
     The processor  131  executes the above-described deterioration detection operation on each of the photoconductive drums  71  mounted on the image forming apparatus  1 . For example, if the deterioration operation on one photoconductive drum ends, the processor  131  executes the deterioration detection on another photoconductive drum where the deterioration detection is not executed. For example, if image forming stations corresponding to four colors are provided, the processor  131  executes the above-described deterioration detection operation on the photoconductive drums of the four image forming stations. 
     As described above, the image forming apparatus detects the deterioration over time of the photoconductive drum based on the difference between the surface potential in the period where the transfer bias is not applied and the surface potential in the period where the transfer bias is applied. In addition, by starting charging and starting the application of the developing bias, the image forming apparatus specifies the surface potentials and the surface potential difference in a state where the supply of the developer to the photoconductive drum is stopped. 
     As a result, the image forming apparatus according to the embodiment can detect the deterioration over time of the photoconductive drum while reducing the deterioration of the developer. In addition, if the deterioration over time of the photoconductive drum is detected, the image forming apparatus according to the embodiment can prevent density unevenness or the like by changing the application control of the transfer bias. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel apparatus and methods described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the apparatus and methods described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.