Patent Publication Number: US-2023142170-A1

Title: Image forming device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-184156, filed on Nov. 11, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an image forming device, an image forming method, and a contrast control device. 
     BACKGROUND 
     In the related art, an image forming device such as a digital multi-functional peripheral is maintained by a service person who visits an installation location. It is desired that such an image forming device be efficiently maintained by being visited by a service person at an appropriate timing. 
     An electrophotographic image forming device includes a developer containing toners of a plurality of colors (for example, yellow, magenta, cyan, and black) to form a color image. The electrophotographic image forming device adjusts, for a color, a contrast potential for developing an electrostatic latent image with a toner of the corresponding color so as to make a density (toner density) in an image of the corresponding color uniform. In the image forming device, if developers of colors have substantially the same charging properties, differences in contrast potential of colors are rarely increased in a state where a toner density of a color is made uniform. In other words, when the difference in contrast potential is large, in many cases, some kind of malfunction occurs in the electrophotographic image forming device. 
     However, an image forming device in the related art cannot detect a possibility of a failure or an abnormality based on the differences in contrast potential of colors. Therefore, the image forming device in the related art cannot be facilitated efficient maintenance by notifying the failure or the abnormality suggested by the differences in contrast potential of colors. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram showing a configuration example of a digital multi-functional peripheral as an image forming device according to an embodiment. 
         FIG.  2    is a diagram showing a configuration example of a printer. 
         FIG.  3    is a block diagram showing a configuration example of a control system. 
         FIG.  4    is a flowchart illustrating an operation example of image density adjustment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an image forming device that can be efficiently maintained is provided. 
     According to one embodiment, the image forming device includes a plurality of photoconductors, an exposure device, a developer, and a processor. The exposure device irradiates a corresponding surface of the plurality of photoconductors with light corresponding to an image formed on a corresponding photoconductor of the plurality of photoconductors. The developer supplies a toner to the corresponding surface of the plurality of photoconductors on which an electrostatic latent image is formed by the light emitted by the exposure device. The processor adjusts, for every photoconductor, a contrast potential for supplying the toner from the developer to the electrostatic latent image formed on the corresponding surface of the plurality of photoconductors, and notifies a warning when there is a contrast potential whose difference from another contrast potential exceeds a reference value. 
     Hereinafter, the present embodiment will be described with reference to the drawings. 
     First, a configuration of a digital multi-functional peripheral (MFP)  1  as the image forming device according to the embodiment will be described. 
       FIG.  1    is a block diagram showing a configuration example of the digital multi-functional peripheral  1  as the image forming device according to the embodiment. 
     As shown in  FIG.  1   , the digital multi-functional peripheral  1  includes a printer  2 , an operation panel  3 , a scanner  4 , and a system controller  5 . 
     The printer  2  is an image forming device that forms an image on a recording medium. The printer  2  included in the digital multi-functional peripheral  1  is an image forming device that forms an image on a recording medium by an electrophotographic method. The printer  2  forms an image (toner image) on a recording medium such as paper by using a toner. The recording medium on which the image is formed by the printer  2  may be any material as long as the image can be formed thereon, and is not limited to paper, and may be cloth, a plastic film or a sheet. 
     The scanner  4  is provided on an upper portion of a main body of the digital multi-functional peripheral  1 . The scanner  4  is a device that optically reads an image of a document. For example, the scanner  4  reads an image of a document set on a platen glass. Further, the scanner  4  may be configured to include a scanner that reads an image of a document to be conveyed by an auto document feeder (ADF). 
     The operation panel  3  is a user interface. The operation panel  3  includes a display unit (display), a touch panel, and an operation button. The operation panel  3  displays an operation guide on the display unit. The operation panel  3  receives an operation instruction from a user by using the touch panel and the operation button. For example, the operation panel  3  is provided with the touch panel on a display screen of the display unit, and detects a portion touched by the user on the display screen of the display unit. 
     The system controller  5  controls the entire digital multi-functional peripheral  1 . The system controller  5  receives the operation instruction input to the operation panel  3  and controls operations of the units. Further, the system controller  5  receives an operation instruction from an external device connected via an interface and controls the operations of the units. For example, when image formation on the recording medium is instructed, the system controller  5  controls the printer  2  to cause the printer  2  to perform the image formation on the recording medium. 
     Hereinafter, a configuration of the printer  2  will be described. 
     As shown in  FIG.  1   , the printer  2  includes a medium supply mechanism  13 , a conveyance mechanism  15 , a plurality of image forming stations SY, SM, SC, and SK, an intermediate transfer belt  21 , a secondary transfer roller  22 , a support roller  23 , a toner adhesion amount sensor  24 , a transfer belt cleaner  25 , and a fixing device  26 . 
     The medium supply mechanism  13  includes a plurality of paper feed cassettes  321 ,  322 , and  323 . Any number of paper feed cassettes may be used. The paper feed cassettes  321 ,  322 , and  323  separately store paper as a recording medium M. The paper as the recording medium M stored in the paper feed cassette may be designed such that different sizes or different types of paper can be stored. Pickup rollers  341 ,  342 , and  343  are respectively disposed on the paper feed cassettes  321 ,  322 , and  323 . The pickup rollers  341 ,  342 , and  343  pick up papers as the recording medium M respectively from the paper feed cassettes  321 ,  322 , and  323  one by one. The pickup rollers  341 ,  342 , and  343  separately supply the corresponding picked-up recording medium M to the conveyance mechanism  15 . 
     The conveyance mechanism  15  conveys the recording medium M. The conveyance mechanism  15  includes first conveyance rollers  521 ,  522 , and  523 , a second conveyance roller  54 , and a registration roller  56  in a conveyance path before image formation on the recording medium M. The conveyance mechanism  15  conveys the corresponding recording medium M supplied by the pickup rollers  341 ,  342 , and  343  from the first conveyance rollers  521 ,  522 , and  523  to the second conveyance roller  54 . In the conveyance mechanism  15 , the second conveyance roller  54  further conveys the recording medium M to the registration roller  56 . 
     The registration roller  56  of the conveyance mechanism  15  conveys the recording medium M to a secondary transfer position according to a timing of transferring an image from the intermediate transfer belt  21  to the recording medium M at a secondary transfer position described later. The conveyance mechanism  15  forms a conveyance path so as to convey, to the fixing device  26 , the recording medium M on which the image is transferred from the intermediate transfer belt  21 . Further, the conveyance mechanism  15  includes a third conveyance roller  58  that discharges the paper to a paper discharge unit, and a conveyance mechanism that conveys the recording medium M to a reversing unit for reversing the recording medium M. 
     The image forming stations SY, SM, SC, and SK separately form an image with a toner. In the present embodiment, the image forming station SY forms a yellow image. The image forming station SM forms a magenta image. The image forming station SC forms a cyan image. The image forming station SK forms a black image. The image forming stations SY, SM, SC, and SK transfer, to the intermediate transfer belt  21 , the images formed with the toners. 
     The intermediate transfer belt  21  is a medium that holds the images transferred by the image forming stations SY, SM, SC, and SK. The intermediate transfer belt  21  is an endless belt as shown in  FIG.  1   . The intermediate transfer belt  21  moves in a direction indicated by an arrow a in  FIG.  1   . The intermediate transfer belt  21  moves the corresponding image transferred by the image forming stations SY, SM, SC, and SK to a position where the secondary transfer roller  22  and the support roller  23  face each other. 
     The secondary transfer roller  22  and the support roller  23  form a transfer unit (secondary transfer unit) that transfers the image from the intermediate transfer belt  21  to a recording medium. The position where the secondary transfer roller  22  and the support roller  23  face each other is the secondary transfer position where the image is transferred from the intermediate transfer belt  21  to the recording medium. The secondary transfer roller  22  and the support roller  23  sandwich the intermediate transfer belt  21  and the recording medium at the secondary transfer position. 
     The support roller  23  supports the intermediate transfer belt  21 . The support roller  23  is a drive roller that drives the intermediate transfer belt  21 . The secondary transfer roller  22  faces the support roller  23  with the intermediate transfer belt  21  interposed therebetween. The secondary transfer roller  22  transfers (secondarily transfers), to a surface of the recording medium, the image formed with a toner on a transfer surface of the intermediate transfer belt  21 . 
     The toner adhesion amount sensor  24  is a sensor that detects a toner amount (density). The toner adhesion amount sensor  24  detects an amount of a toner adhered to the intermediate transfer belt  21 . The toner adhesion amount sensor  24  is disposed so as to face the transfer surface of the intermediate transfer belt  21 . The toner adhesion amount sensor  24  is provided at a position from an image transfer station (primary transfer position) to the secondary transfer position by the image forming station in the moving direction a of the intermediate transfer belt  21 . The toner adhesion amount sensor  24  outputs the detected toner adhesion amount to the system controller  5 . 
     As shown in  FIG.  1   , the transfer belt cleaner  25  is disposed at a position from the secondary transfer position to the primary transfer position in the moving direction a of the intermediate transfer belt  21 . The transfer belt cleaner  25  removes the toner on the intermediate transfer belt  21 . For example, the transfer belt cleaner  25  removes the toner remaining on the transfer surface of the intermediate transfer belt  21  after the image is transferred from the intermediate transfer belt  21  to the recording medium. 
     The fixing device  26  fixes, onto the recording medium, the image formed with the toner transferred to the recording medium. The fixing device  26  is disposed in the conveyance path of the recording medium after passing through the secondary transfer position. The fixing device  26  includes a pressure roller and a heating roller facing each other. The fixing device  26  provides heat and pressure to the recording medium by conveying the recording medium between the pressure roller and the heating roller facing each other. The fixing device  26  fixes the toner image transferred to the recording medium by heating in a pressurized state. 
     Next, the corresponding configuration of the image forming stations SY, SM, SC, and SK in the digital multi-functional peripheral  1  as the image forming device according to the embodiment will be described in detail. 
       FIG.  2    is a diagram showing the corresponding configuration example of the image forming stations SY, SM, SC, and SK in the printer  2 . 
     As shown in  FIG.  2   , the image forming stations SY, SM, SC, and SK separately include an exposure device  100 , a developer  110 , a photoconductor drum  122 , a charger  126 , a primary transfer roller  128 , a photoconductor cleaner  130 , and a charge remover  132 . In the present embodiment, the image forming stations SY, SM, SC, and SK separately include a configuration as shown in  FIG.  2   . 
     The photoconductor drum  122  is an image carrier including a photoconductor layer  124  on the surface thereof. The photoconductor drum  122  rotates in a direction (direction indicated by an arrow b in  FIG.  2   ) according to a movement of the intermediate transfer belt  21  in the moving direction a. The charger  126 , the exposure device  100 , the developer  110 , the primary transfer roller  128 , the intermediate transfer belt  21 , the photoconductor cleaner  130 , and the charge remover  132  are disposed around the photoconductor drum  122 . 
     The charger  126  uniformly charges the photoconductor layer  124  on the surface of the photoconductor drum  122 . For example, the charger  126  uniformly negatively charges the photoconductor layer  124  on the surface of the photoconductor drum  122 . 
     The exposure device  100  forms an electrostatic pattern (electrostatic latent image) corresponding to an image on the surface of the photoconductor drum  122 . The exposure device  100  irradiates the surface of the photoconductor drum  122  with light L whose emission is controlled based on image data. For example, the exposure device  100  irradiates, by an optical system such as a polygon mirror, the surface of the photoconductor drum  122  with the light L emitted based on the image data. The exposure device  100  may be configured to include a device that emits a plurality of laser beams guided to the corresponding photoconductor drum  122  of the plurality of image forming stations. Further, the exposure device  100  may be a light emitting device provided for every one of the plurality of image forming stations. 
     The developer  110  develops, with a developer, the electrostatic latent image formed on the surface of the photoconductor drum  122 . The developer  110  supplies a developer D to the surface of the photoconductor drum  122  exposed by the exposure device  100 . The corresponding developer  110  of the image forming stations develops an image in a corresponding color. For example, the developer  110  of the image forming station SY develops an electrostatic latent image on the photoconductor drum  122  with a yellow toner. The developer  110  of the image forming station SM develops an electrostatic latent image on the photoconductor drum  122  with a magenta toner. The developer  110  of the image forming station SC develops an electrostatic latent image on the photoconductor drum  122  with a cyan toner. The developer  110  of the image forming station SK develops an electrostatic latent image on the photoconductor drum  122  with a black toner. 
     In the configuration example shown in  FIG.  2   , the developer  110  includes a developer container  112 , a developing roller  114 , a first mixer  116 , a second mixer  118 , and a toner density sensor  120 . 
     The developer container  112  is a container that contains the developer D. The developer D is a mixture of a toner and a carrier made of magnetic fine particles. When the developer D is stirred, the toner is frictionally charged. Accordingly, the toner adheres to a surface of the carrier by electrostatic force. 
     The developing roller  114 , the first mixer  116 , the second mixer  118 , and the toner density sensor  120  are disposed inside the developer container  112 . 
     The toner density sensor  120  is disposed inside the developer container  112 . The toner density sensor  120  detects a toner density in the developer D contained in the developer container  112 . The toner density is represented by, for example, a ratio (toner/carrier) of the toner to the carrier in the developer D in the developer container  112 . The system controller  5  controls the toner density detected by the toner density sensor  120  to be a predetermined value. 
     The developing roller  114  includes, for example, a magnetic body (for example, a magnet) in which a positive electrode and a negative electrode are alternately arranged circumferentially. The developing roller  114  rotates counterclockwise. The first mixer  116  and the second mixer  118  stir the developer D in the developer container  112 . Further, the first mixer  116  and the second mixer  118  convey the developer D. The second mixer  118  disposed below the developing roller  114  supplies the developer D to a surface of the developing roller  114 . 
     The developer D adheres to the surface of the developing roller  114  in a napped state according to a magnetic field distribution generated by the magnetic body of the developing roller  114 . The developing roller  114  rotates while carrying the developer D. A layer of the developer D adhering to the developing roller  114  is limited to a predetermined thickness by a blade provided such that a distance from the surface of the developing roller  114  is a predetermined width. The developer D carried by the developing roller  114  limited to the predetermined thickness by the blade moves to a position (development position) facing the surface of the photoconductor drum  122 . 
     A development bias is applied to the developing roller  114  carrying the developer D. A potential of the surface of the developing roller  114  is controlled by the development bias. The toner in the developer D carried by the developing roller  114  adheres to the electrostatic latent image due to a potential difference between the potential of the surface of the developing roller  114  and a potential of the electrostatic latent image formed on the surface of the photoconductor drum  122 . As the developing roller  114  rotates in a predetermined direction, the developer D carried by the developing roller  114  approaches the surface of the photoconductor drum  122  on which the electrostatic latent image is formed. The toner contained in the developer D carried by the developing roller  114  develops the electrostatic latent image on the photoconductor drum  122  when the toner approaches the surface of the photoconductor drum  122 . Accordingly, a toner image obtained by developing the electrostatic latent image with the toner is formed on the photoconductor drum  122 . 
     Here, the potential difference between the potential of the surface of the developing roller  114  and the potential of the electrostatic latent image formed on the surface of the photoconductor drum  122  is referred to as a contrast voltage. The contrast voltage is in association with the density of the toner moving from the developing roller  114  to the electrostatic latent image on the photoconductor drum  122 . That is, the density of the toner image formed on the photoconductor drum  122  is adjusted by controlling the contrast voltage. The contrast voltage is adjusted by controlling the development bias. Further, the contrast voltage may be adjusted by controlling the potential of the electrostatic latent image. 
     The image (toner image) developed with the toner on the surface of the photoconductor drum  122  is moved to a position corresponding to the primary transfer roller  128  by the rotation of the photoconductor drum  122 . The primary transfer roller  128  faces the photoconductor drum  122  with the intermediate transfer belt  21  interposed therebetween. The primary transfer roller  128  abuts on the surface of the photoconductor drum  122  with the intermediate transfer belt  21  interposed therebetween. The primary transfer roller  128  transfers, to the intermediate transfer belt  21 , the toner image on the surface of the photoconductor drum  122  (primary transfer). 
     The photoconductor cleaner  130  is disposed downstream of a position where the toner image on the surface of the photoconductor drum  122  is transferred to the intermediate transfer belt  21  in a circumferential direction of the photoconductor drum  122 . The photoconductor cleaner  130  removes the toner on the surface of the photoconductor drum  122 . That is, the photoconductor cleaner  130  removes the toner remaining on the surface of the photoconductor drum  122  after the primary transfer of the toner image from the photoconductor drum  122  to the intermediate transfer belt  21  is performed. 
     The charge remover  132  is disposed downstream of a position of the photoconductor cleaner  130  in the circumferential direction of the photoconductor drum  122 . The charge remover  132  irradiates the surface of the photoconductor drum  122  with light. Accordingly, the charge remover  132  removes a charge remaining in the photoconductor layer  124  on the surface of the photoconductor drum  122 . 
     Next, a configuration of a control system in the digital multi-functional peripheral  1  as the image forming device according to the embodiment will be described. 
       FIG.  3    is a block diagram showing a configuration example of the control system in the digital multi-functional peripheral  1  as the image forming device according to the embodiment. 
     As shown in  FIG.  3   , the system controller  5  includes a processor  101 , a ROM  102 , a RAM  103 , a storage device  104 , and a communication interface (I/F)  105 . Further, the processor  101  of the system controller  5  is connected to the units in the digital multi-functional peripheral  1  via various interfaces. 
     The processor  101  executes various processes by executing a program. The processor  101  is, for example, a CPU. The processor  101  is connected to the ROM  102 , the RAM  103 , the storage device  104 , and the communication interface (I/F)  105 . Further, the processor  101  is connected to the units in the printer  2 , the operation panel  3 , and the scanner  4  via interfaces. 
     The ROM  102  is a non-volatile memory that is not rewritable. The ROM  102  operates as a program memory for storing a program. The RAM  103  operates as a working memory or a buffer memory. The processor  101  executes various processes by executing a program stored in the ROM  102  or the storage device  104  by using the RAM  103 . 
     The storage device  104  is a non-volatile memory that is rewritable. For example, the storage device  104  includes a storage device such as a hard disk drive (HDD) or a solid state drive (SSD). The storage device  104  stores data such as control data, a control program, and setting information. The storage device  104  also stores image data. 
     The communication I/F  105  is an interface for performing data communication with the external device. For example, the communication I/F  105  communicates with a user terminal such as a PC and a mobile terminal via a network. The communication I/F  105  may input an image print request (print job) from the user terminal such as a PC. 
     As shown in  FIG.  3   , the printer  2  includes a power supply  140  in addition to the configurations shown in  FIGS.  1  and  2   . 
     The power supply  140  separately supplies a voltage to the developer  110 , the charger  126 , the primary transfer roller  128 , and the secondary transfer roller  22 . As shown in  FIG.  3   , the power supply  140  includes a high-voltage power supply  141 , a development bias transformer  142 , a charging bias transformer  143 , a primary transfer bias transformer  144 , and a secondary transfer bias transformer  145 . The development bias transformer  142 , the charging bias transformer  143 , and the primary transfer bias transformer  144  are provided for every one of the image forming stations SY, SM, SC, and SK. 
     The high-voltage power supply  141  supplies a high voltage to the various transformers  142 ,  143 ,  144 , and  145 . The high voltage is, for example, a voltage of several hundreds of V to several kV. The high-voltage power supply  141  generates the high voltage from an input voltage of several tens of V, for example. 
     The development bias transformer  142  supplies a development bias voltage to the developer  110 . The development bias transformer  142  converts the high voltage generated by the high-voltage power supply  141  into a development bias voltage having a voltage value set by the system controller  5 . The development bias transformer  142  supplies, to the developer  110 , the development bias voltage specified by the system controller  5 . 
     The charging bias transformer  143  supplies a charging bias voltage to the charger  126 . The charging bias transformer  143  converts the high voltage generated by the high-voltage power supply  141  into a charging bias voltage having a voltage value set by the system controller  5 . The charging bias transformer  143  supplies, to the charger  126 , the charging bias voltage specified by the system controller  5 . 
     The primary transfer bias transformer  144  supplies a primary transfer bias voltage to the primary transfer roller  128 . The primary transfer bias transformer  144  converts the high voltage generated by the high-voltage power supply  141  into a primary transfer bias voltage having a voltage value set by the system controller  5 . The primary transfer bias transformer  144  supplies, to the primary transfer roller  128 , the primary transfer bias voltage specified by the system controller  5 . 
     The secondary transfer bias transformer  145  supplies a secondary transfer bias voltage to the secondary transfer roller  22 . The secondary transfer bias transformer  145  converts the high voltage generated by the high-voltage power supply  141  into a secondary transfer bias voltage having a voltage value set by the system controller  5 . The secondary transfer bias transformer  145  supplies, to the secondary transfer roller  22 , the secondary transfer bias voltage having a value specified by the system controller  5 . 
     Next, an operation of forming an image in the digital multi-functional peripheral  1  as the image forming device according to the embodiment will be described. 
     The digital multi-functional peripheral  1  forms an image by acquiring an image to be formed on the recording medium M, and printing the acquired image on the recording medium M by the printer  2 . For example, when a copying instruction is given from the operation panel  3 , the processor  101  of the system controller  5  prints, on the recording medium M by the printer  2 , the image of the document read by the scanner  4 . 
     When the image is to be formed, the processor  101  of the system controller  5  takes in, by the medium supply mechanism  13 , the recording medium M stored in a storage unit. The processor  101  conveys, by the conveyance mechanism  15 , the recording medium M supplied from the medium supply mechanism  13 , to a position in front of the registration roller  56  in the printer  2 . 
     Further, the processor  101  of the system controller  5  generates the corresponding image formed by the image forming stations SY, SM, SC, and SK based on an image (printed image) to be printed on the recording medium M. For example, the processor  101  generates images of colors (yellow, magenta, cyan, and black) formed by the image forming stations SY, SM, SC, and SK based on the printed image. When the processor  101  generates the images of the colors based on the printed image, the processor  101  causes the image forming stations to form the generated images of the colors. 
     In the image forming stations SY, SM, SC, and SK, the charger  126  receives the charging bias voltage from the charging bias transformer  143  to charge the photoconductor layer  124  of the photoconductor drum  122 . The exposure device  100  irradiates the corresponding photoconductor drum  122  of the image forming stations SY, SM, SC, and SK with light that forms an electrostatic latent image corresponding to an image of a color. In the image forming stations SY, SM, SC, and SK, the electrostatic latent image is formed on the photoconductor layer  124  of the photoconductor drum  122  by the light emitted from the exposure device  100 . 
     The image forming stations SY, SM, SC, and SK separately develop the electrostatic latent image on the photoconductor drum  122  with a toner of a color contained in the developer  110 . In the image forming stations SY, SM, SC, and SK, the developing roller  114  rotates while carrying a developer containing a toner of a color supplied from the developer container  112 . A development bias voltage from the development bias transformer  142  is applied to the developing roller  114  that carries the developer. The developer  110  supplies, to the electrostatic latent image, the toner in the developer carried by the developing roller  114 , by the potential difference (contrast potential) between the potential on the developing roller  114  and the electrostatic latent image on the photoconductor drum  122 . 
     In the image forming stations SY, SM, SC, and SK, the photoconductor drum  122  moves the image (toner image) developed by the developer  110  to a position (primary transfer position) facing the primary transfer roller  128 . At the primary transfer position, the photoconductor drum  122  faces the primary transfer roller  128  with the intermediate transfer belt  21  interposed therebetween. The primary transfer bias voltage from the primary transfer bias transformer  144  is applied to the primary transfer roller  128 . The toner image on the photoconductor drum  122  is transferred to the intermediate transfer belt  21  by the primary transfer roller  128  to which the primary transfer bias voltage is applied at the primary transfer position. When a color image is to be formed, the image forming stations SY, SM, SC, and SK superimpose and transfer the toner images of the colors on the intermediate transfer belt  21 . Accordingly, the color image in which the toner images of the colors are superimposed is transferred to the intermediate transfer belt  21 . 
     The intermediate transfer belt  21  moves the transferred toner image to a position (secondary transfer position) facing the secondary transfer roller  22 . The registration roller  56  feeds the recording medium M to the secondary transfer position according to the timing and the position of the image transferred to the intermediate transfer belt  21 . Accordingly, the recording medium M is conveyed in a state where the overlapping intermediate transfer belt  21  and recording medium M are sandwiched between the secondary transfer roller  22  and the support roller  23  at the secondary transfer position. The secondary transfer bias voltage from the secondary transfer bias transformer  145  is applied to the secondary transfer roller  22 . The toner image on the intermediate transfer belt  21  is transferred to the recording medium M at the secondary transfer position by the secondary transfer roller  22  to which the secondary transfer bias voltage is applied. 
     The recording medium M passing through the secondary transfer position is conveyed to the fixing device  26 . The fixing device  26  fixes, onto the recording medium M, the toner image transferred from the intermediate transfer belt  21  to the recording medium M at the secondary transfer position. The fixing device  26  applies heat and pressure to the recording medium M, on which the toner image is transferred, to fix the toner image onto the recording medium M. The recording medium M passing through the fixing device  26  is discharged from the paper discharge unit with the toner image fixed thereon. 
     Next, image density adjustment in the digital multi-functional peripheral  1  as the image forming device according to the embodiment will be described. 
     The printer  2  of the digital multi-functional peripheral  1  adjusts a density of the image formed on the recording medium M by the image formation as described above. The density of the image formed on the recording medium M varies depending on an amount (density) of the toner supplied from the developing roller  114  to the electrostatic latent image when the electrostatic latent image on the photoconductor drum  122  is developed. 
     The density of the toner supplied from the developing roller  114  to the electrostatic latent image is adjusted by the contrast potential, which is the potential difference between the electrostatic latent image on the photoconductor drum  122  and the developing roller  114 . The processor  101  of the system controller  5  executes image density adjustment of adjusting the density of the image of a color formed on the recording medium M by controlling the contrast potential for a color. The image density adjustment may be performed periodically or at any timing. 
     The processor  101  of the system controller  5  in the digital multi-functional peripheral  1  according to the present embodiment detects differences in contrast potential of the colors after executing the image density adjustment. The processor  101  notifies a warning when there is a contrast potential whose difference from another contrast potential exceeds a reference value. 
       FIG.  4    illustrates an operation example of the image density adjustment in the digital multi-functional peripheral  1  as the image forming device according to the embodiment. 
     The processor  101  of the system controller  5  executes the image density adjustment to uniformize the toner density of the image of a corresponding color formed by the image forming stations SY, SM, SC, and SK. The processor  101  transfers, to the intermediate transfer belt  21 , the corresponding toner image formed by the image forming stations SY, SM, SC, and SK as the image density adjustment (ACT  11 ). 
     The corresponding toner image formed by the image forming stations SY, SM, SC, and SK in the image density adjustment may be an image having a predetermined test pattern or any image. The toner image of a corresponding color formed by the image forming stations SY, SM, SC, and SK is transferred to the intermediate transfer belt  21  at the respective primary transfer positions. 
     The processor  101  of the system controller  5  detects the toner density of a color by the toner adhesion amount sensor  24  after transferring the toner image of a corresponding color to the intermediate transfer belt  21  (ACT  12 ). The toner adhesion amount sensor  24  detects the density (toner density) of the toner image of a color transferred to the intermediate transfer belt  21 . The toner adhesion amount sensor  24  supplies, to the processor  101 , a detection result indicating the toner density of a color. 
     The processor  101  determines whether to adjust the density for every image forming station based on the toner density of a color detected by the toner adhesion amount sensor  24  (ACT  13 ). For example, the processor  101  determines whether the toner density of a color detected by the toner adhesion amount sensor  24  is a predetermined density (within a predetermined density range). The processor  101  determines that the density is to be adjusted for the image forming station of the color determined to have a toner density that is not the predetermined density. 
     When there is an image forming station that is determined to be adjusted in density (YES in ACT  13 ), the processor  101  adjusts the contrast potential of the image forming station to be adjusted in density (ACT  14 ). For example, when the density of the yellow (magenta, cyan, black) toner is not the predetermined density, the processor  101  adjusts the contrast potential in the image forming station SY (SM, SC, SK). 
     In the density adjustment, the processor  101  changes (adjusts) the contrast potential such that the toner density of the toner image formed by the image forming station is the predetermined density. For example, the processor  101  changes the contrast potential by controlling the development bias voltage applied to the developing roller  114  by the development bias transformer  142 . Further, the system controller  5  may change the contrast potential by controlling the charging bias voltage applied to the charger  126  by the charging bias transformer  143 . Furthermore, the system controller  5  may change the contrast potential by controlling the light emitted to the photoconductor drum  122  by the exposure device  100 . 
     When the image density adjustment is executed, the processor  101  stores, in the storage device  104 , an adjustment result of the corresponding contrast potential in the image forming stations SY, SM, SC, and SK (ACT  15 ). For example, when the contrast potential in the image forming station SY is adjusted, the processor  101  stores, in the storage device  104 , an adjustment result of a contrast potential of the yellow color (contrast potential corresponding to the image forming station SY). Similarly, when the contrast potential in the image forming station SM (SC, SK) is adjusted, the processor  101  stores, in the storage device  104 , an adjustment result of a contrast potential of the magenta (cyan, black) color. 
     Further, when the contrast potential is adjusted, the processor  101  calculates a difference in contrast potential among the image forming stations SY, SM, SC, and SK (ACT  16 ). The processor  101  calculates a difference between the contrast potential of a color (contrast potential corresponding to an image forming station) and a contrast potential of another color (contrast potential corresponding to another image forming station). 
     When the differences in contrast potential of the colors are calculated, the processor  101  determines whether there is a contrast potential whose difference from the contrast potential of another color exceeds the reference value (ACT  17 ). The reference value to be compared with a difference in contrast potential is a threshold value for determining that there is a possibility that a malfunction such as a failure or an abnormality occurs in the digital multi-functional peripheral  1 . An image forming station including a device having a failure or an abnormality may have a large difference in contrast potential as compared with a contrast potential of another image forming station. 
     For example, in a developer having an abnormality in a toner density sensor, a toner density in the developer cannot be maintained at a predetermined value. In an image forming station where the toner density in the developer is not maintained at the predetermined value, the contrast potential is significantly changed in order to adjust the toner density to the predetermined value. A contrast potential of an image forming station including the developer having the abnormality in the toner density sensor may have a large difference from the contrast potential of another image forming station. Further, a contrast potential of an image forming station in which a charger, an exposure device or a developing roller is not operating in a normal state may have a large difference from another contrast potential. 
     When there is no contrast potential whose difference from another contrast potential exceeds the reference value (NO in ACT  17 ), the processor  101  ends the image density adjustment. That is, when the difference in contrast potential of the colors is within the reference value, the processor  101  ends the series of operations in the image density adjustment. 
     When the difference in contrast potential exceeds the reference value (YES in ACT  17 ), the processor  101  notifies a warning that the difference in contrast potential exceeds the reference value (ACT  18 ). The warning to be notified may prompt verification or maintenance of a malfunction suggested by the difference in contrast potential exceeding the reference value. For example, the warning may be an inspection or maintenance guide, or may be a message notifying that there is a possibility of a failure or an abnormality in the digital multi-functional peripheral. Further, the warning may include a message indicating an image forming station or a color whose difference in contrast potential from another contrast potential exceeds the reference value. 
     In addition, even when the difference in contrast potential exceeds the reference value, the digital multi-functional peripheral  1  can form an image having a normal density as long as the toner density of a color is adjusted to a normal value. Therefore, the processor  101  may continue the operation of the image formation even when the warning indicating that the difference in contrast potential exceeds the reference value is notified. Accordingly, the digital multi-functional peripheral  1  can maintain the image formation at a normal density and notify a warning that the difference in contrast potential is increased. 
     Further, the processor  101  may notify, without notifying the user, a service person or an administrator that the difference in contrast potential exceeds the reference value. Accordingly, the digital multi-functional peripheral  1  can provide the user with normal image formation and prompt the service person to perform maintenance for the abnormality suggested by the difference in contrast potential. 
     For example, the processor  101  notifies a terminal device (external device) possessed by the service person via the communication I/F  105  that the difference in contrast potential exceeds the reference value. Further, the processor  101  may notify a system managing an operating state of the digital multi-functional peripheral via the communication I/F  105  that the difference in contrast potential exceeds the reference value. Furthermore, the processor  101  may display on the operation panel  3  that the difference in contrast potential exceeds the reference value when the service person or the administrator logs in. 
     It should be noted that the processor  101  may execute the ACTS  11  to  15  as the image density adjustment, and may execute the ACTS  16  to  18  in response to a request from the service person. Accordingly, the digital multi-functional peripheral  1  can notify a possibility of a failure or an abnormality based on the differences in contrast potential of the colors in response to the request from the service person. 
     As described above, an image forming device according to the embodiment includes a plurality of photoconductor drums, a plurality of developing rollers, and a system controller. A photoconductor drum carries an electrostatic latent image formed by light from an exposure device. A developing roller is provided facing a photoconductor drum. A developing roller supplies a toner to the electrostatic latent image by a contrast potential which is a potential difference from the electrostatic latent image carried by the facing photoconductor drum. The system controller adjusts a contrast potential corresponding to a photoconductor drum such that a density of a toner image developed on a photoconductor drum is uniform. The system controller notifies a warning when a difference in contrast potential corresponding to a photoconductor drum exceeds a reference value. 
     With the above-described configuration, when image density is to be adjusted, the image forming device according to the embodiment can notify that a difference in contrast potential, which may be a failure or an abnormality, occurs. As a result, according to the image forming device of the embodiment, the service person can predict a portion where there is a possibility of a failure or an abnormality based on the difference in contrast potential. In addition, the image forming device can be facilitated, by notifying that the difference in contrast potential exceeds the reference value, rapid maintenance for a portion where a failure or an abnormality may occur. 
     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 inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments 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.