Patent Publication Number: US-11048197-B1

Title: Image forming apparatus with toner sensor and notification method for same

Description:
FIELD 
     Embodiments described herein relate generally to an image forming apparatus and a notification method. 
     BACKGROUND 
     In the related art, for example, a two-component developer including toner and carrier is used for an image forming apparatus. When the image forming apparatus using the two-component developer is delivered to a customer, a toner density sensor provided in a developing device is calibrated. In order to perform calibration, the toner density of the two-component developer needs to be appropriately maintained. For example, when the developer is inserted into the developing device, the developer is stirred to be uniform, and the toner density sensor is calibrated. In this calibration, a control input voltage of the toner density sensor is adjusted so that an output value of the toner density sensor becomes a target value. 
     Here, during the stirring operation of the developer, a predetermined bias is applied to a photosensitive drum so that the toner in the developing device is not transferred onto the photosensitive drum. 
     However, when transporting the image forming apparatus, if a wiring defect due to looseness of the connector, condensation occurring during the process leading up to delivery, or the like occurs, there may be a problem where the predetermined bias cannot be successfully applied to the photosensitive drum during the stirring operation. When such a problem occurs, the toner in the developing device is transferred onto the photosensitive drum during the stirring operation, and calibration is performed in a state where the toner density is not appropriate. Therefore, an appropriate toner density cannot be maintained, and the image quality may be deteriorated. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating an overall configuration example of an image forming apparatus according to an embodiment; 
         FIG. 2  is a diagram illustrating an example of an internal configuration of the image forming apparatus of  FIG. 1 ; 
         FIG. 3  is a diagram illustrating an example of a configuration around a developing device of the image forming apparatus of  FIG. 1 ; 
         FIG. 4  is a block diagram illustrating a functional configuration related to developer stirring processing of the image forming apparatus of  FIG. 1 ; 
         FIG. 5  is a graph illustrating an example of a timing chart of each unit when a toner density sensor is calibrated; 
         FIG. 6  is a flowchart illustrating an example of calibration processing performed by the image forming apparatus; 
         FIG. 7  is a flowchart illustrating another portion of the calibration processing of  FIG. 6 ; and 
         FIGS. 8A to 8D  are explanatory diagrams illustrating examples of a screen displayed on a display. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an image forming apparatus includes a storage unit, a stirring unit, a replenishment unit, a detection unit, a calibration unit, a comparison unit, and a notification unit. The storage unit contains a two-component developer including toner and carrier. The stirring unit stirs the two-component developer. The replenishment unit replenishes the storage unit with toner. The detection unit detects a toner density of the two-component developer in the storage unit. The calibration unit calibrates the detection unit using the two-component developer where the stirring is performed. The comparison unit compares a first detection value indicating a toner density at a first timing with a second detection value indicating a toner density at a second timing after a predetermined time period elapsed since the first timing. The notification unit issues a notification regarding the suitability of calibration based on a comparison result of the comparison unit. 
       FIG. 1  is a perspective view illustrating an overall configuration example of an image forming apparatus  100  according to an embodiment. The image forming apparatus  100  is, for example, a multifunction peripheral. The image forming apparatus  100  includes a display  110 , a control panel  120 , a printer  130 , a sheet storage unit  140 , and an image reading unit (image reader)  150 . 
     The display  110  is, for example, a liquid crystal display with a touch panel. The display  110  displays various information. The display  110  receives an operation from a user. The display  110  displays various operation screens, the image status, operation circumstances of various functions, and the like according to a display control signal output from a control unit  400  (see  FIG. 4 ). 
     The control panel  120  includes various operation keys such as a numeric keypad and a start key. The control panel  120  receives various input operations from the user. The control panel  120  outputs operation signals corresponding to various input operations received from the user to the control unit. 
     The printer  130  performs a series of printing operations using various information output from the display  110 , the control panel  120 , the image reading unit  150 , and the like. The series of printing operations includes an operation of inputting image information, an operation of forming an image, an operation of transferring the formed image to a sheet, and an operation of conveying the sheet. 
     The sheet storage unit  140  includes a plurality of sheet cassettes. Each sheet cassette accommodates sheets. 
     The image reading unit  150  includes an automatic document feeder and a scanner device. The automatic document feeder sends out a document placed on a document tray to the scanner device. The scanner device optically scans a document on a document glass table, and forms an image of light reflected from the document on a light receiving surface of a charge coupled device (CCD) sensor. As a result, the scanner device reads the document image on the document glass table. The image reading unit  150  generates image information (e.g., image data) using the reading result read by the scanner device. 
       FIG. 2  is a diagram illustrating an example of an internal configuration of the image forming apparatus  100 . As illustrated in  FIG. 2 , the image forming apparatus  100  (specifically, the printer  130 ) includes four image forming units  20   a  to  20   d  in parallel. The image forming apparatus  100  is a so-called quadruple tandem type image forming apparatus. The image forming apparatus  100  includes an image processing unit  10 , an image forming unit  20  ( 20   a  to  20   d ), an intermediate transfer unit  30 , a fixing unit  40 , a sheet conveying unit  50 , an alignment sensor, and an image density sensor. 
     The image processing unit  10  inputs image information. The image information to be input may include image information generated by the image reading unit  150  or image information transmitted from another device. The image processing unit  10  performs digital image processing that processes the input image information according to initial settings or user settings. For example, digital image processing includes gradation correction based on gradation correction data. In addition to gradation correction, digital image processing includes various correction processing such as color correction and shading correction, and compression processing on image data. 
     Next, the image forming unit  20  (e.g., including image forming units  20   a  to  20   d ) will be described. The image forming unit  20  includes an image forming unit  20   a  corresponding to yellow (Y), an image forming unit  20   b  corresponding to magenta (M), an image forming unit  20   c  corresponding to cyan (C), and an image forming unit  20   d  corresponding to black K. The respective image forming units  20   a  to  20   d  include respective photosensitive drums  21   a  to  21   d , respective chargers  22   a  to  22   d , an exposure device  23 , respective developing devices  24   a  to  24   d , a drum cleaning device  25  (see  FIG. 3 ), and the like. In the following, the description will be made with the symbols a to d omitted. 
     The photosensitive drum  21  is, for example, a charged organic photoconductor (OPC) in which an undercoat layer, a charge generation layer, and a charge transport layer are sequentially laminated on the peripheral surface of an aluminum conductive cylinder. The photosensitive drum  21  has photoconductivity. 
     A charger  22  generates corona discharge. The charger  22  uniformly charges the surface of the photosensitive drum  21 . 
     The exposure device  23  is, for example, a semiconductor laser. The exposure device  23  irradiates the photosensitive drum  21  with laser light corresponding to the image of each color component. If the laser light is irradiated by the exposure device  23 , the potential of the region irradiated with the laser light in the region of the surface of the photosensitive drum  21  changes. Due to this potential change (e.g., a potential difference), an electrostatic latent image is formed on the surface of the photosensitive drum  21 . 
     The developing device  24  contains the developer. The developing device  24  attaches toner of each color component to the surface of the photosensitive drum  21 . Thus, a toner image is formed on the photosensitive drum  21 . That is, the electrostatic latent image formed on the surface of the photosensitive drum  21  is visualized. 
     Here, the developer will be described. For example, a two-component developer is used as the developer. The two-component developer contains non-magnetic toner and carrier. For the carrier, for example, iron powder having a particle size of several tens of μm or polymer ferrite particles are used. The carrier is mixed with the toner in the developing device  24  and is tribo-electrically charged to give the toner a charge (for example, a negative charge). Further, the carrier conveys the toner to the electrostatic latent image portion by a magnetic force. However, the developer is not limited to a two-component developer, and a one-component developer that does not use a carrier can also be used. 
     The drum cleaning device  25  (see  FIG. 3 ) includes a cleaning blade in contact with the surface of the photosensitive drum  21 . The cleaning blade removes residual toner remaining on the surface of the photosensitive drum  21  after the primary transfer. The removed residual toner is collected in a storage unit included in the drum cleaning device. 
     Next, the intermediate transfer unit  30  will be described. The intermediate transfer unit  30  includes an intermediate transfer body  31 , a primary transfer roller  32 , a plurality of support rollers  33 , a secondary transfer roller  34 , a belt cleaning device  35 , and the like. 
     The intermediate transfer body  31  is, for example, an endless belt (transfer belt). The intermediate transfer body  31  has conductivity and elasticity. 
     Support rollers  33   a  to  33   c  support the intermediate transfer body  31  so that tension is applied to the intermediate transfer body  31 . Thus, the intermediate transfer body  31  is formed in a continuous loop shape. One of the plurality of support rollers  33   a  to  33   c  (for example, the support roller  33   c ) is a driving roller. Rollers other than the driving roller are driven rollers. As the driving roller rotates, the intermediate transfer body  31  travels in the A direction at a predetermined speed and with a predetermined cycle. 
     Here, the direction in which the intermediate transfer body  31  moves can be defined as an upstream direction and a downstream direction. Specifically, the direction in which the intermediate transfer body  31  moves can be defined with the image forming unit  20   a  as the most upstream and the belt cleaning device  35  as the most downstream. 
     The primary transfer roller  32  is disposed to face the photosensitive drum  21  via the intermediate transfer body  31 . Specifically, the primary transfer roller  32  is disposed so that pressure is applied to the photosensitive drum  21  with the intermediate transfer body  31  interposed therebetween. As a result, a primary transfer portion that nips the intermediate transfer body  31  is formed by the primary transfer roller  32  and the photosensitive drum  21 . 
     When the intermediate transfer body  31  passes through the primary transfer portion, the toner image formed on the photosensitive drum  21  is transferred onto the intermediate transfer body  31 . When the intermediate transfer body  31  passes through the primary transfer portion, a primary transfer bias is applied to the primary transfer roller  32 . Specifically, for example, a charge having an opposite polarity (e.g., a positive polarity) to that of the toner is applied to the primary transfer roller  32 . As a result, the toner image formed on the photosensitive drum  21  is electrostatically transferred to the intermediate transfer body  31 . 
     The secondary transfer roller  34  is disposed to face the support roller  33   a  through the intermediate transfer body  31 . Specifically, the secondary transfer roller  34  is disposed so that pressure is applied to the support roller  33   a  with the intermediate transfer body  31  interposed therebetween. 
     As a result, the secondary transfer unit  38  that nips the intermediate transfer body  31  and the sheet is formed by the secondary transfer roller  34  and the support roller  33   a.    
     When the sheet passes through the secondary transfer unit  38 , the toner image on the intermediate transfer body  31  is transferred onto the sheet. When the sheet passes through the secondary transfer unit  38 , a secondary transfer bias is applied to the support roller  33   a . Specifically, a charge having the same polarity (e.g., a negative polarity) as that of the toner is applied to the support roller  33   a.    
     As a result, the toner image on the intermediate transfer body  31  is electrostatically transferred to the sheet. The secondary transfer roller  34  and the support roller  33   a  can be separated from each other. Thus, when a sheet is jammed in the secondary transfer unit  38 , the user can remove the sheet. 
     The belt cleaning device  35  includes a cleaning blade that contacts the surface of the intermediate transfer body  31 . The cleaning blade removes residual toner remaining on the surface of the intermediate transfer body  31  after the secondary transfer. The removed residual toner is collected in a storage unit included in the belt cleaning device  35 . 
     The fixing unit  40  heats and pressurizes the sheet on which the toner image is transferred. As a result, the fixing unit  40  fixes the toner image on the sheet. The fixing unit  40  may be a system in which a toner image is fixed on a sheet by heating the sheet through a film-like member. 
     Next, the sheet conveying unit  50  will be described. The sheet conveying unit  50  includes a sheet feeding unit  51 , a registration unit  52 , a first guide unit  53 , a second guide unit  54 , and a sheet discharge unit  55 . 
     The sheet feeding unit  51  conveys the sheets accommodated in the sheet storage unit  140  one by one to the registration unit  52 . The registration unit  52  stops the sheet conveyed from the sheet feeding unit  51  and sends out the sheet to the secondary transfer unit  38  at a predetermined timing. The predetermined timing is a timing at which the toner image formed on the intermediate transfer body  31  is secondarily transferred. 
     The first guide unit  53  regulates the conveyance direction of the sheet sent out from the registration unit  52 . Further, the first guide unit  53  sends out the sheet whose conveyance direction is regulated to the secondary transfer unit  38 . 
     The secondary transfer unit  38  transfers the toner image to the sheet whose conveyance direction is regulated by the first guide unit  53 . Further, the secondary transfer unit  38  sends out the sheet on which the toner image is transferred to the fixing unit  40 . 
     The second guide unit  54  regulates the conveyance direction of the sheet sent out from the secondary transfer unit  38 . The fixing unit  40  heats and pressurizes the sheet whose conveyance direction is regulated by the second guide unit  54 , and sends out the sheet to the sheet discharge unit  55 . The sheet discharge unit  55  sends out the sheet to the discharge tray. 
       FIG. 3  is a diagram illustrating an example of a configuration around the developing device  24 . As illustrated in  FIG. 3 , the developing device  24  includes a developer storage unit  240 , stirring rollers  241 , a developing roller  242 , and a toner density sensor  243 . 
     The developer storage unit  240  (i.e., a developer container) contains a two-component developer. The developer storage unit  240  is an example of a storage unit or container (e.g., a bucket, a box, a tray, an enclosure, etc.). The stirring roller  241  is provided at the bottom of the developer storage unit  240 . The stirring roller  241  includes a developing device motor  241   a . The stirring roller  241  stirs the two-component developer by the driving force of the developing device motor  241   a . Thus, the toner and the carrier of the two-component developer become uniform. The toner is charged. The stirring roller  241  is an example of a stirring unit or agitator. 
     The developing roller  242  is a cylindrical rotating roller. A part of the developing roller  242  is exposed from the opening of the developer storage unit  240 . The exposed portion is disposed to face the photosensitive drum  21 . The developing roller  242  carries the two-component developer contained in the developer storage unit  240  and conveys the two-component developer to a portion facing the photosensitive drum  21 . The photosensitive drum  21  is connected to a drum motor  210  and rotates in the opposite direction to the developing roller  242 . Further, the developing roller  242  is driven to rotate while being in contact with the photosensitive drum  21  in a state of holding the toner. 
     A predetermined bias (for example, negative voltage) is applied to the developing roller  242  by a high-voltage power supply  300 . A predetermined bias (for example, positive voltage) is applied to the charger  22 . The charger  22  charges the surface of the photosensitive drum  21 . Due to the bias applied by the high-voltage power supply  300 , a potential difference is generated between the surface of the developing roller  242  and the surface of the photosensitive drum  21 , and the toner negatively charged on the developing roller  242  is transferred onto the photosensitive drum  21 . In this way, the developing roller  242  causes the toner to adhere to the electrostatic latent image formed on the photosensitive drum  21  to form a toner image. That is, the developing roller  242  develops the electrostatic latent image. 
     Further, the moving speed of the surface of the photosensitive drum  21  is equal to the moving speed of the sheet, that is, the processing speed in the image forming apparatus  100 . Further, the support roller  33   c  of the intermediate transfer body  31  is a driving roller of the intermediate transfer body  31 . The support roller  33   c  is rotated by the driving force of an intermediate transfer motor  330  so that the moving speed of the surface of the photosensitive drum  21  and the moving speed of the intermediate transfer body  31  are the same. 
     The developing device  24  can be replenished with toner from a toner replenishment unit  250 . The toner replenishment unit  250  includes a toner replenishment roller  251 . The toner replenishment roller  251  is provided between a toner replenishment port of a toner cartridge (not illustrated) and a toner replenishment port of the developer storage unit  240 . The toner replenishment roller  251  is rotated by the driving force of a toner replenishment motor  252  and causes the developer storage unit  240  to be replenished with toner. The toner replenishment unit  250  is an example of a replenishment unit or toner supply. 
     The toner density sensor  243  is a sensor that detects the toner density in the developer storage unit  240 . The toner density sensor  243  is provided at the bottom of the developer storage unit  240 . The toner density sensor  243  outputs a sensor output (e.g., an output voltage) corresponding to the control input voltage and the toner density in the developer storage unit  240 . The toner density sensor  243  outputs a sensor output corresponding to the magnetic permeability (e.g., a toner density) by detecting the magnetic permeability having a correlation with the toner density. The toner density sensor  243  is an example of a detection unit. 
       FIG. 4  is a block diagram illustrating a functional configuration related to the developer stirring processing of the image forming apparatus  100  according to the embodiment. In  FIG. 4 , the image forming apparatus  100  includes the display  110 , the developing device motor  241   a , the toner density sensor  243 , the toner replenishment motor  252 , the high-voltage power supply  300 , the controller or control unit  400 , and the storage unit  410 . The control unit  400  includes a calibration unit  401 , a comparison unit  402 , a notification unit  403 , a decision unit  404 , a replenishment control unit  405 , and a determination unit  406 . 
     The control unit  400  is realized by a processor such as a CPU. The control unit  400  functions as the calibration unit  401 , the comparison unit  402 , the notification unit  403 , the decision unit  404 , the replenishment control unit  405 , and the determination unit  406  by executing a program using the processor. The storage unit  410  is realized by a storage device such as a magnetic hard disk device or a semiconductor storage device. 
     When the image forming apparatus  100  is shipped and delivered to a customer (i.e., a delivery is completed), a stirring operation and a calibration operation of the toner density sensor  243  are performed according to the operation of an operator such as a service person. In the stirring operation, the control unit  400  controls the toner replenishment motor  252  to replenish toner into the developer storage unit  240 . The control unit  400  controls the developing device motor  241   a  to stir the developer in the developer storage unit  240 . As a result, the toner in the developer storage unit  240  is negatively charged. 
     The control unit  400  controls the high-voltage power supply  300  to apply a bias (for example, −600 V) to the photosensitive drum  21  (charger  22 ) during the calibration operation. The control unit  400  controls the high-voltage power supply  300  to apply a bias (for example, −500 V) to the developing roller  242  during the calibration operation. Thereby, it is possible to prevent the toner (e.g., having a negative polarity) from being transferred to the photosensitive drum  21  during the calibration operation. 
     The calibration unit  401  calibrates the toner density sensor  243  using the two-component developer in which the toner is replenished into the storage unit and stirring is performed. The calibration is an operation of adjusting the control input voltage of the toner density sensor  243  so that the output value of the toner density sensor  243  becomes a target value (e.g., is within a target value range). The calibration unit  401  uses the output value of the toner density sensor  243  to adjust the control input voltage so that the toner density becomes 6 to 10 weight percent (wt %), for example. Here, the toner density can be expressed by a ratio of the toner weight to the toner and carrier weight. That is, the toner density can be expressed as “toner density”=“toner weight/(toner weight+carrier weight)”. 
     The toner density can be increased as the volume of the developer storage unit  240  increases. For this reason, the toner density is a value that varies depending on the model. In this embodiment, as an example, the control input voltage is adjusted so that the toner density becomes about 8 wt %. Further, when the toner density is 8 wt %, the control input voltage is adjusted so that the output value of the toner density sensor  243  becomes, for example, 2.5 V. 
     Here, when transporting the image forming apparatus  100 , if a wiring defect due to looseness of the connector, condensation during the process leading up to the delivery, or the like occurs, there may be a problem that a −600 V bias cannot be applied to the photosensitive drum  21  during the stirring operation, and the voltage applied to the photosensitive drum  21  becomes, for example, 0 V. When such a problem occurs, the toner in the developer storage unit  240  is transferred onto the photosensitive drum  21  during the stirring operation, and calibration is performed in a state where the toner density is not appropriate. 
     Therefore, the image forming apparatus  100  according to the present embodiment issues a notification regarding the suitability of calibration based on the comparison result between a first detection value indicating the toner density at a first timing (e.g., a first point in time relative to a reference, such as the beginning of time measurement, a first time, etc.) and a second detection value indicating the toner density at a second timing (e.g., a second point in time relative to a reference, such as the beginning of time measurement, a second time, etc.) after a predetermined time elapsed since the first timing. Specifically, the image forming apparatus  100  issues a notification regarding the suitability of calibration based on the comparison result between the first detection value of the toner density when the first time elapsed and the second detection value of the toner density when the second time elapsed since the start of the developer stirring. A detailed description will be made below. 
     The control unit  400  starts the measurement of time when toner is replenished into the developer storage unit  240  and stirring is started. The comparison unit  402  compares the first detection value indicating the toner density at the first timing with the second detection value indicating the toner density at the second timing. 
     The first timing is, for example, a timing at which a first time (e.g., a first time period) has elapsed since the start of developer stirring. More specifically, the first timing is a timing at which the first time period has elapsed since the control unit  400  starts the measurement of time. The first time period is, for example, 60 seconds. However, the first timing is not limited to the timing triggered by the developer stirring, and may be the timing triggered by another operation. 
     The second timing is a timing at which a first predetermined time period (for example, 60 seconds) has elapsed since the first timing. Specifically, the second timing is, for example, a timing at which a second time period has elapsed since the control unit  400  starts the measurement of time, where the second timing elapses after the first time has elapsed. The second time period is, for example, 120 seconds. Both the first detection value and the second detection value are represented by the output value (V) of the toner density sensor  243 . In the following description, the detection value of the toner density and the output value of the toner density sensor  243  will be described as having the same meaning unless specifically distinguished. 
     For example, the comparison unit  402  compares the output value (e.g., a first detection value) of the toner density sensor  243  when 60 seconds have elapsed from the start of the measurement of time and the output value (e.g., a second detection value) of the toner density sensor  243  when 120 seconds have elapsed from the start of the measurement of time. 
     More specifically, the comparison by the comparison unit  402  is a comparison as to whether or not the comparison value obtained by comparing the first detection value and the second detection value is equal to or greater than a threshold value. This comparison value is a value based on the difference between the first detection value and the second detection value, for example. The value based on the difference is, for example, an absolute value of a difference between the first detection value and the second detection value, in other words, a value indicating an increase or a decrease between the first detection value and the second detection value. The threshold value is a value that can determine that the toner density is abnormal. The storage unit  410  stores the first detection value, the second detection value, and the threshold value. The comparison unit  402  refers to each value stored in the storage unit  410  when performing the comparison. 
     Here, for example, an example of a threshold value for determining that the toner density is abnormal when the toner density changes by ±1 wt % or more will be described. For example, in the image forming apparatus  100 , it is assumed that the output value of the toner density sensor  243  is set to change by 0.25 V when the toner density changes by 1 wt %. In this case, when the toner density is set to ±1 wt % as a threshold value, the threshold value becomes ±0.25 V when converted to the output value of the toner density sensor  243 . 
     More specifically, for example, in the image forming apparatus  100 , when the toner density is 8 wt %, it is assumed that the output value of the toner density sensor  243  is set to, for example, 2.5 V. For example, if the output value of the toner density sensor  243  is 2.75 V, the fact indicates that the toner density is 7 wt %. For example, if the output value of the toner density sensor  243  is 2.25 V, the fact indicates that the toner density is 9 wt %. Therefore, if the output value of the toner density sensor  243  is within the range of 2.25 V to 2.75 V (e.g., if the absolute value of the difference between the first detection value and the second detection value is less than the threshold value), the toner density can be determined to be normal. On the other hand, if the output value of the toner density sensor  243  is not within the range of 2.25 V to 2.75 V (e.g., if the absolute value of the difference between the first detection value and the second detection value is equal to or greater than the threshold value), the toner density can be determined to be abnormal. The toner density, the output value, and the threshold value described here are merely examples, and can be set to different values according to the specifications or the like of the image forming apparatus  100 . 
     Further, the comparison value is not limited to a value based on the difference between the first detection value and the second detection value. For example, the comparison value may be a value based on the ratio between the first detection value and the second detection value. The value based on the ratio is, for example, the ratio of the second detection value to the first detection value or the ratio of the first detection value to the second detection value. 
     The notification unit  403  issues a notification regarding the suitability of calibration based on the comparison result by the comparison unit  402 . The notification regarding the suitability of calibration is, for example, a notification regarding that calibration is appropriate or notification regarding that calibration is not appropriate. Specifically, the notification regarding that calibration is appropriate is, for example, a notification that the calibration is performed appropriately, a notification that the toner density is appropriate, and a notification that printing is possible. In addition, the notification regarding that calibration is not appropriate is, for example, a notification that the calibration is inappropriate, a notification that the toner density is not appropriate, a notification that printing is not possible, or a notification prompting the developer to be replaced. 
     The notification unit  403  issues a notification regarding that the calibration is appropriate when a comparison result in which the comparison value is less than the threshold value is obtained. In addition, the notification unit  403  issues a notification regarding that the calibration is not appropriate when a comparison result in which the comparison value is equal to or greater than the threshold value is obtained. 
     The notification unit  403  controls the display  110  to issue a notification regarding the suitability of calibration. The notification regarding the suitability of calibration is not limited to the notification by displaying of the display  110 . The notification regarding the suitability of calibration may be a voice notification in addition to or instead of the display notification. 
     The decision unit  404  determines whether the toner density decreased or increased during stirring when a comparison result in which the comparison value is equal to or greater than the threshold value is obtained. During the stirring operation, for example, when the toner is transferred onto the photosensitive drum  21  and the amount of toner in the developer storage unit  240  decreases, the magnetic permeability increases. Thus, when the amount of toner in the developer storage unit  240  decreases, that is, when the toner density decreases, the output value of the toner density sensor  243  increases. Accordingly, the decision unit  404  can determine that the toner density decreases when the output value of the toner density sensor  243  increases. 
     In addition, during the stirring operation, for example, when the carrier is transferred onto the photosensitive drum  21  and the amount of the carrier in the developer storage unit  240  decreases, the magnetic permeability decreases. For this reason, when the amount of the carrier in the developer storage unit  240  decreases, that is, when the toner density increases, the output value of the toner density sensor  243  decreases. Accordingly, the decision unit  404  can determine that the toner density increases when the output value of the toner density sensor  243  decreases. 
     The notification unit  403  issues different notifications depending on when the decision unit  404  determines that the toner density decreases and that the toner density increases during stirring. For example, when it is determined that the toner density decreased, the notification unit  403  issues a notification regarding that the toner decreased during the stirring operation, that the toner was transferred onto the photosensitive drum  21 , and the like. In addition, if it is determined that the toner density increased, the notification unit  403  issues a notification regarding that the carrier decreased during the stirring operation, that the carrier was transferred onto the photosensitive drum  21 , and the like. 
     The replenishment control unit  405  causes the toner replenishment unit  250  to replenish toner. Specifically, the replenishment control unit  405  drives the toner replenishment motor  252  to replenish toner. For example, the replenishment control unit  405  drives the toner replenishment motor  252  to replenish toner when the decision unit  404  determines that the toner density decreased during stirring. 
     The amount of toner to be replenished is, for example, an amount corresponding to a magnitude of the decrease in the toner density. The amount corresponding to the magnitude of the decrease in the toner density can be obtained from the output value of the toner density sensor  243 . For example, the storage unit  410  stores a table in which the value increased by the toner density sensor  243  is associated with the amount of toner to be replenished. The replenishment control unit  405  refers to the table stored in the storage unit  410  and can obtain the amount of toner to be replenished from the value increased by the toner density sensor  243 . 
     When the toner is replenished by the replenishment control unit  405 , the control unit  400  drives the developing device motor  241   a  again, stirs the developer, and starts the measurement of time. Further, the comparison unit  402  compares the first detection value with the second detection value again when the toner is replenished by the replenishment control unit  405 . 
     The determination unit  406  determines whether or not printing is possible based on the comparison result by the comparison unit  402 . For example, the determination unit  406  determines that printing is not possible when a comparison result in which the comparison value is equal to or greater than the threshold value is obtained in the second comparison. In response to a determination that printing is not possible, the control unit  400  may prevent printing. 
     However, the determination unit  406  may determine that printing is not possible when a comparison result in which the comparison value is equal to or greater than the threshold value is obtained in the first comparison. Specifically, when the decision unit  404  determines that the toner density decreased during stirring, the toner replenishment by the replenishment control unit  405  or the second comparison by the comparison unit  402  may not be performed. In this case, the determination unit  406  may determine that printing is not possible when a comparison result in which the comparison value is equal to or greater than the threshold value is obtained in the first comparison. 
     Further, the determination unit  406  determines that printing is possible when a comparison result in which the comparison value is less than the threshold value is obtained. Specifically, the determination unit  406  determines that printing is possible when a comparison result in which the comparison value is less than the threshold value is obtained in the second comparison. When a comparison result in which the comparison value is less than the threshold value is obtained in the second comparison, the determination unit  406  may determine that printing is possible by setting an upper limit number of prints and a printable period because the comparison result in which the comparison value is equal to or greater than the threshold value in the first comparison is obtained (an abnormality is detected). 
     Further, when a comparison result in which the comparison value is less than the threshold value is obtained in the second comparison, the control unit  400  may cause the comparison unit  402  to perform the comparison again without replenishing the toner, as a precaution. Then, the determination unit  406  may determine that printing is possible when a comparison result in which the comparison value is less than the threshold value is obtained in this comparison. Thereby, the accuracy of the determination indicating whether or not the calibration is performed appropriately can be improved. 
     Further, when a comparison result in which the comparison value is less than the threshold value is obtained in the second comparison, the notification unit  403  issues a notification regarding that printing is possible because the printing is temporarily possible. However, since the comparison result in which the comparison value is equal to or greater than the threshold value is obtained in the first comparison, that is, an abnormality occurred in the first stirring operation, the notification unit  403  issues a notification regarding that the comparison result in which the comparison value is equal to or greater than the threshold value was obtained in the first comparison. This notification is, for example, a notification that prompts the developer to be replaced as a precaution, although printing is possible. Further, the notification unit  403  may prompt the developer to be replaced by displaying a predetermined icon or mark on the display  110  until the developer is replaced. 
     In addition, when the notification unit  403  issues a notification regarding that the calibration is not appropriate, the control unit  400  may control a communication unit (e.g., a communication interface or bus) to transmit a developer order, the fact that a problem occurred during the developer stirring operation, and information indicating that the calibration was not normally performed to a predetermined service center or the like. 
     Further, the determination unit  406  determines that printing is not possible when the decision unit  404  determines that the toner density increased during stirring. That is, during the stirring operation, for example, when the carrier is transferred onto the photosensitive drum  21  and an abnormality occurs in which the amount of the carrier in the developer storage unit  240  is reduced, the carrier cannot be replenished and thus, the determination unit  406  immediately determines that printing is not possible. 
       FIG. 5  is an explanatory diagram illustrating an example of a timing chart of each unit when the toner density sensor  243  is calibrated. As illustrated in  FIG. 5 , a timing t 0  is a timing at which calibration is started. At the timing t 0 , the drum motor  210  is driven and the photosensitive drum  21  rotates. Further, when a predetermined bias (for example, a voltage of −600 V) is applied to the charger  22 , the surface of the photosensitive drum  21  is charged. At the timing t 0 , the intermediate transfer motor  330  is also driven, and the intermediate transfer body  31  also travels. Furthermore, at the timing t 0 , the timer starts the measurement of time. 
     At a timing t 10 , the developing device motor  241   a  is driven, and the stirring roller  241  rotates. Further, at the timing t 10 , a developing bias (for example, −500 V) is applied to the developing roller  242 . At the timing t 10 , the power of the toner density sensor  243  is turned on. Further, at the timing t 10 , the control input voltage (e.g., the control voltage) of the toner density sensor  243  is also turned on, and the control input voltage is adjusted. 
     At a timing t 1  (for example, 60 seconds), the comparison unit  402  acquires the output value (i.e., a first detection value) of the toner density sensor  243  and stores the acquired output value in the storage unit  410 . Further, at a timing t 2  (for example, 120 seconds), the comparison unit  402  acquires the output value (i.e., a second detection value) of the toner density sensor  243 , and compares the comparison value obtained by comparing the output value at the timing t 1  with the output value at the timing t 2 , with the threshold value stored in the storage unit  410 . 
     At the timing t 2 , the charging bias and the developing bias are turned off. Further, at a timing t 20 , the drum motor  210 , the developing device motor  241   a , the toner density sensor  243 , the control voltage of the toner density sensor  243 , the intermediate transfer motor  330 , and the timer are turned off. Thus, the stirring operation of the developer and the calibration operation of the toner density sensor  243  are completed. 
     If the toner density decreases during the stirring operation, the toner is replenished and the stirring operation and the calibration operation similar to the above are performed again. 
       FIGS. 6 and 7  are flowcharts illustrating examples of calibration processing performed by the image forming apparatus  100 . As illustrated in  FIG. 6 , the control unit  400  determines whether or not the start timing of the initialization (e.g., the stirring operation and the calibration operation) of the toner density sensor  243  is reached (ACT  601 ). This start timing is a timing at which an initial setting start operation such as a calibration operation is received from an operator such as a service person. 
     The control unit  400  waits until the initialization start timing is reached (ACT  601 : NO). If the initialization start timing is reached (ACT  601 : YES), the control unit  400  starts the operation of each unit illustrated in  FIG. 5  (ACT  602 ). In ACT  602 , for example, the operation of each unit is started at the timing t 0  and the timing t 10  illustrated in  FIG. 5 . Thereby, for example, the stirring operation and the calibration operation are started. 
     Then, the comparison unit  402  determines whether or not a measured value T of the timer is reached the timing t 1  (e.g., 60 seconds) (ACT  603 ). The comparison unit  402  waits until the measured value T of the timer reaches the timing t 1  (ACT  603 : NO). If the measured value T of the timer reaches the timing t 1  (ACT  603 : YES), the comparison unit  402  acquires an output value Vt 1  (i.e., the first detection value) of the toner density sensor  243  (ACT  604 ). 
     Then, the comparison unit  402  determines whether or not the measured value T of the timer is reached the timing t 2  (e.g., 120 seconds) (ACT  605 ). The comparison unit  402  waits until the measured value T of the timer reaches the timing t 2  (ACT  605 : NO). If the measured value T of the timer reaches the timing t 2  (ACT  605 : YES), the comparison unit  402  acquires an output value Vt 2  (i.e., the second detection value) of the toner density sensor  243  (ACT  606 ). 
     Then, the comparison unit  402  determines whether or not the comparison value |Vt 2 −Vt 1 | that is the difference between the output value Vt 1  and the output value Vt 2  is equal to or greater than a threshold value Vs (ACT  607 ). If the comparison value |Vt 2  −Vt 1 | is less than the threshold value Vs (ACT  607 : NO), the determination unit  406  determines that printing is possible (ACT  608 ). Then, the replenishment control unit  405  determines whether or not the toner was replenished (ACT  609 ). If the toner was replenished (ACT  609 : YES), that is, if the comparison in ACT  607  is the second comparison, the notification unit  403  controls the display  110  to issue a notification regarding calibration error  3  (see  FIG. 8C ) (ACT  610 ). 
     In ACT  609 , if the toner was not replenished (ACT  609 : NO), that is, if the comparison in ACT  607  is the first comparison, the notification unit  403  controls the display  110  to issue a notification regarding that the calibration is normally completed (see  FIG. 8D ) (ACT  611 ). Then, the control unit  400  ends the initialization of the toner density sensor  243  (ACT  612 ) and ends a series of processes. 
     In ACT  607 , if the comparison value |Vt 2  −Vt 1 | is equal to or greater than the threshold value Vs (ACT  607 : YES), as illustrated in  FIG. 7 , the decision unit  404  determines whether or not the output value Vt 2  is greater than the output value Vt 1  (ACT  613 ). If the output value Vt 2  is greater than the output value Vt 1  (ACT  613 : YES), that is, for example, if the toner in the developer storage unit  240  is transferred onto the photosensitive drum  21  and the magnetic permeability increases (i.e., the toner density decreases), the replenishment control unit  405  determines whether the toner was replenished (ACT  614 ). 
     If the toner was not replenished (ACT  614 : NO), the replenishment control unit  405  drives the toner replenishment motor  252  to replenish toner into the developer storage unit  240  (ACT  615 ). 
     Then, the control unit  400  starts the operation of each unit again, such as driving of the developing device motor  241   a  and measuring of the timer (ACT  616 ), and the process proceeds to ACT  603  of  FIG. 6 . By proceeding to ACT  603 , the output value Vt 1  and the output value Vt 2  are compared again. Because this second comparison occurs after the first comparison, the output value Vt 1  and the output value Vt 2  may be considered an example of a third detection value taken at a third timing or third time and a fourth detection value occurring at a fourth timing or a fourth time, respectively, with a second predetermined time period elapsing between the third time and the fourth time. The second predetermined time period may accordingly be equal to the first predetermined time period of the first comparison. The comparison may be used to generate a second comparison value, which is compared to a second threshold value. 
     On the other hand, if the toner was replenished in ACT  614  (ACT  614 : YES), that is, if the comparison in ACT  607  is the second comparison, the notification unit  403  controls the display  110  to issue a notification regarding calibration error  1  (see  FIG. 8A ) (ACT  617 ). Further, in ACT  613 , if the output value Vt 2  is smaller than the output value Vt 1  (ACT  613 : NO), that is, for example, if the carrier in the developer storage unit  240  is transferred onto the photosensitive drum  21  and the magnetic permeability decreases (i.e., the toner density increases), the notification unit  403  controls the display  110  to issue a notification regarding calibration error  2  (see  FIG. 8B ) (ACT  618 ). Thereafter, the determination unit  406  determines that printing is not possible (ACT  619 ), and the process proceeds to ACT  612  of  FIG. 6 . 
     Through the processing described above, the image forming apparatus  100  can calibrate the toner density sensor  243  when the comparison value (e.g., the difference) between the output value Vt 1  and the output value Vt 2  is less than the threshold value, that is, in a state where the toner density is normal. Therefore, the image forming apparatus  100  can perform printing with an appropriate toner density. 
       FIGS. 8A to 8D  are explanatory diagrams illustrating examples of a screen displayed on the display  110 .  FIG. 8A  illustrates a display screen  801  for the calibration error  1  of the toner density sensor  243 . The display screen  801  shows a notification screen indicating the type of error, the fact that printing is not possible, the fact that toner decreases during the stirring operation, and the prompt for developer replacement. 
       FIG. 8B  illustrates a display screen  802  for the calibration error  2  of the toner density sensor  243 . The display screen  802  shows a notification screen indicating the type of error, the fact that printing is not possible, the fact that the carrier decreases during the stirring operation, and the prompt for developer replacement. 
       FIG. 8C  illustrates a display screen  803  of the calibration error  3  of the toner density sensor  243 . The display screen  803  shows a notification screen indicating the type of error, the fact that printing is possible, the fact that an abnormality occurs during the calibration operation, and the prompt for early developer replacement. 
       FIG. 8D  illustrates a display screen  804  when the calibration of the toner density sensor  243  is normally completed. The display screen  804  shows a notification screen indicating that calibration is completed normally and printing is possible. 
     By displaying such display screens  801  to  804 , the operator can easily grasp whether or not the calibration operation is completed normally, the type of abnormality, whether or not printing is possible, the measures thereafter, and the like. 
     As described above, the image forming apparatus  100  according to the embodiment issues a notification regarding the suitability of calibration based on the comparison result between the first detection value indicating the toner density at the first timing and the second detection value indicating the toner density at the second timing. As a result, calibration can be performed in a state where the toner density is appropriate. Therefore, since the image forming apparatus  100  can maintain an appropriate toner density, that is, printing can be performed with an appropriate toner density, the deterioration in image quality can be suppressed. 
     Further, when a comparison result in which the comparison value obtained by comparing the first detection value and the second detection value is less than the threshold value is obtained, the image forming apparatus  100  according to the present embodiment issues a notification regarding that the calibration is appropriate. Therefore, it is possible to issue a notification regarding that the calibration is appropriate when the difference between the first detection value and the second detection value is small. Thereby, an operator such as a service person can easily grasp that the calibration is performed appropriately. On the other hand, the image forming apparatus  100  issues a notification regarding that the calibration is not appropriate when a comparison result in which the comparison value obtained by comparing the first detection value and the second detection value is equal to or greater than the threshold value is obtained. Therefore, it is possible to issue a notification regarding that the calibration is not appropriate when the difference between the first detection value and the second detection value is large. Thereby, an operator such as a service person can easily grasp that the calibration is not performed appropriately. 
     In addition, the image forming apparatus  100  according to the present embodiment issues different notifications when a comparison result in which the comparison value is equal to or greater than the threshold value is obtained, depending on whether the toner density is decreased or increased during the developer stirring. Accordingly, it is possible to notify the operator of whether an abnormality indicating a decrease in toner density occurs or an abnormality indicating an increase in toner density occurs during the stirring operation. As a result, the operator can grasp the type of abnormality and can take measures according to the type of abnormality. 
     Further, when a comparison result in which the comparison value is equal to or greater than the threshold value is obtained and the toner density decreases during the developer stirring, the image forming apparatus  100  according to the present embodiment replenishes the developer storage unit  240  with toner, and compares the first detection value and the second detection value again. Therefore, even when a comparison result in which the comparison value is equal to or greater than the threshold value is obtained in the first comparison, the second comparison can be performed. Thereby, when a comparison result in which the comparison value is less than the threshold value is obtained in the second comparison, it is possible to make printing possible and issue a notification regarding that the calibration is performed appropriately. Therefore, when the image forming apparatus  100  is delivered to a customer, since the printing can be made possible for the time being, it is possible to prevent a situation in which the customer cannot use the image forming apparatus  100  after the delivery. 
     Further, in the image forming apparatus  100  according to the present embodiment, when the toner density decreases during the developer stirring, the developer storage unit  240  is replenished with an amount of toner corresponding to the decrease in toner density. As a result, the image forming apparatus  100  can replenish the amount of toner reduced during the first stirring and compare the first detection value and the second detection value again. 
     Further, when the comparison result in which the comparison value is less than the threshold value is obtained in the second comparison, the image forming apparatus  100  according to the present embodiment issues a notification regarding that the comparison result in which the comparison value is equal to or greater than the threshold value is obtained in the first comparison (see  FIG. 8C ). As a result, it is possible to issue a notification regarding that the first calibration is not normal although the second calibration is normal. Therefore, the operator can be notified of the type of abnormality in more detail. 
     In addition, the image forming apparatus  100  according to the present embodiment determines that printing is not possible when a comparison result in which the comparison value is equal to or greater than the threshold value is obtained in the second comparison. Here, it is assumed that the same comparison result can be obtained even if the second comparison is performed twice or more. For this reason, it is possible to make printing impossible by performing the second comparison only once. Therefore, it is possible to quickly determine whether or not printing is possible. 
     In addition, the image forming apparatus  100  according to the present embodiment determines that printing is possible when a comparison result in which the comparison value is less than the threshold value is obtained in the second comparison. As a result, when the image forming apparatus  100  is delivered to a customer, since the printing can be made possible for the time being, it is possible to prevent a situation in which the customer cannot use the image forming apparatus  100  after the delivery. 
     Further, when a comparison result in which the comparison value is equal to or greater than the threshold value is obtained and it is determined that the toner density increases during the developer stirring, the image forming apparatus  100  according to the present embodiment determines that printing is not possible. As a result, when the carrier in the developer storage unit  240  decreases, since the carrier cannot be replenished, printing cannot be performed immediately. Therefore, it is possible to quickly determine whether or not printing is possible, and it is possible to prevent printing when there is a possibility of image failure. 
     In the image forming apparatus  100  according to the present embodiment, the comparison value is a value based on the difference between the first detection value and the second detection value. Thereby, a comparison value can be obtained by a simple calculation process. Therefore, it is possible to reduce the load related to the process of issuing a notification regarding the suitability of calibration. 
     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.