Patent Publication Number: US-11029623-B2

Title: Powder amount detector including a pair of measuring electrodes

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2019-141155, filed on Jul. 31, 2019, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure generally relate to a powder amount detector, a powder supply device, and an image forming apparatus. 
     Description of the Related Art 
     There is known a powder amount detector, which includes a pair of electrodes, configured to detect an amount of powder in a powder container based on capacitance between the pair of electrodes. 
     SUMMARY 
     Embodiments of the present disclosure describe an improved powder amount detector that detects an amount of powder in a powder container of a cylindrical shape arranged horizontally. The powder amount detector includes a pair of measuring electrodes configured to detect capacitance between the pair of measuring electrodes to detect the amount of powder. The pair of measuring electrodes is disposed around the powder container. One of the pair of measuring electrodes has a flat shape, and the other of the pair of measuring electrodes has an arc shape following a shape of the powder container. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic view of a printer as an example of an image forming apparatus according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic view of one of four image forming units included in the printer in  FIG. 1 ; 
         FIGS. 3A and 3B  are schematic views of one of four toner supply devices included in the printer in  FIG. 1 ; 
         FIG. 4  is a cross-sectional view along line A-A in  FIG. 3A ; 
         FIG. 5  is a perspective view of toner containers installed in a toner container mount of the printer in  FIG. 1 ; 
         FIGS. 6A and 6B  are schematic cross-sectional views of the toner container and a pair of arc-shaped electrodes to illustrate shortcomings of the arc shape; 
         FIG. 7  is a graph illustrating an example of a ratio of effects of the toner container and toner on capacitance; 
         FIG. 8  is a graph illustrating an example of a calibration curve; 
         FIG. 9  is a schematic cross-sectional view of an example of the toner supply device provided with ground electrodes disposed outboard of measuring electrodes according to an embodiment of the present disclosure; and 
         FIG. 10  is a schematic cross-sectional view of an example of the toner supply devices provided with ground electrodes between adjacent toner containers according to an embodiment of the present disclosure. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. In addition, identical or similar reference numerals designate identical or similar components throughout the several views, and redundant descriptions are omitted or simplified below as required. 
     DETAILED DESCRIPTION 
     Descriptions are given of embodiments of the present disclosure with reference to the drawings. 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that have the same function, operate in a similar manner, and achieve a similar result. 
     As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     It is to be noted that the suffixes Y, M, C, and K attached to each reference numeral indicate only that components indicated thereby are used for forming yellow, magenta, cyan, and black images, respectively, and hereinafter may be omitted when color discrimination is not necessary. 
       FIG. 1  is a schematic view of a printer  100  as an example of an image forming apparatus according to the present embodiment. The printer  100  includes a toner container mount  70 . Four replaceable toner containers  32 Y,  32 M,  32 C, and  32 K as powder containers (also collectively referred to as the “toner containers  32 ”) to contain yellow, magenta, cyan, and black toners, respectively, are removably installed in the toner container mount  70 . Below the toner container mount  70 , an intermediate transfer unit  15  is disposed. Four image forming units  6 Y,  6 M,  6 C, and  6 K (also collectively referred to as the “image forming units  6 ”) are arranged in parallel, facing an intermediate transfer belt  8  of the intermediate transfer unit  15  to form yellow, magenta, cyan, and black (Y, M, C, and K) toner images, respectively. Toner supply devices  60 Y,  60 M,  60 C, and  60 K as powder (developer) supply devices (also collectively referred to as the “toner supply devices  60 ”) are disposed below the toner containers  32 Y,  32 M,  32 C, and  32 K, respectively. The toner supply devices  60 Y,  60 M,  60 C, and  60 K supply toners contained in the corresponding toner containers  32 Y,  32 M,  32 C, and  32 K to developing devices  5  (see  FIG. 2 ), in which the toner as powder is used, of the corresponding image forming units  6 Y,  6 M,  6 C, and  6 K. 
     The four toner containers  32 Y,  32 M,  32 C, and  32 K, the four image forming units  6 Y,  6 M,  6 C, and  6 K, and the four toner supply devices  60 Y,  60 M,  60 C, and  60 K have similar configurations except for the color of toner used therein. Accordingly, in the description and drawings below, the suffixes Y, M, C, and K, each representing the color of toner, are omitted unless color discrimination is necessary. 
       FIG. 2  is a schematic view illustrating the configuration of one of the four image forming units  6 . Each image forming unit  6  includes a photoconductor  1  as an image bearer, and further includes a charging device  4 , the developing device  5 , a cleaning device  2 , a discharge device, and the like disposed around the photoconductor  1 . Image forming processes, namely charging, exposure, development, transfer, and cleaning processes, are performed on the photoconductor  1 , and thus a toner image of each color is formed on the photoconductor  1 . 
     The photoconductor  1  rotates clockwise in  FIG. 2 , driven by a drive motor. At the charging device  4 , the surface of the photoconductor  1  is uniformly charged (charging process). When the surface of the photoconductor  1  reaches a position where the surface of the photoconductor  1  is irradiated with a laser beam L emitted from an exposure device  7  (see  FIG. 1 ), the photoconductor  1  is scanned with the laser beam L, and thus an electrostatic latent image for each color is formed thereon (exposure process). Then, the surface of the photoconductor  1  reaches a position opposite the developing device  5 , where the electrostatic latent image is developed with toner into the toner image for each color (development process). At a primary transfer position at which the photoconductor  1  is opposed to a primary transfer roller  9  via the intermediate transfer belt  8 , the toner image on the photoconductor  1  is transferred onto the intermediate transfer belt  8  (primary transfer process). The respective toner images formed on the photoconductors  1 Y,  1 M,  1 C, and  1 K (see  FIG. 1 ) are sequentially transferred to and superimposed on the intermediate transfer belt  8 , thereby forming a multicolor toner image on the intermediate transfer belt  8 . 
     After the primary transfer process, a certain amount of untransferred toner remains on the surface of the photoconductor  1 . When the surface of the photoconductor  1  reaches a position opposite the cleaning device  2 , a cleaning blade  2   a  of the cleaning device  2  mechanically collects the untransferred toner remaining on the photoconductor  1  (cleaning process). Subsequently, the surface of the photoconductor  1  reaches a position opposite the discharge device, and the discharge device removes any residual potential on the photoconductor  1 . 
     The intermediate transfer unit  15  includes the intermediate transfer belt  8 , four primary transfer rollers  9 Y,  9 M,  9 C, and  9 K, a secondary transfer backup roller  12 , multiple tension rollers, and a belt cleaning device. The intermediate transfer belt  8  is stretched around and supported by the above-described multiple rollers and is rotated counterclockwise in  FIG. 1  as the secondary transfer backup roller  12 , which is one of the multiple rollers, rotates. The four primary transfer rollers  9 Y,  9 M,  9 C, and  9 K press against the corresponding photoconductors  1 Y,  1 M,  1 C, and  1 K (also collectively referred to as the “photoconductors  1 ”) via the intermediate transfer belt  8 , thereby forming primary transfer nips between the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K and the corresponding photoconductors  1 Y,  1 M,  1 C, and  1 K. 
     A transfer bias opposite in polarity to toner is applied to each of the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K. The intermediate transfer belt  8  rotates in the direction indicated by arrow A 1  in  FIG. 1  and sequentially passes through the primary transfer nips of the primary transfer rollers  9 Y,  9 M,  9 C, and  9 K. Thus, the single-color toner images on the respective photoconductors  1 Y,  1 M,  1 C, and  1 K are primarily transferred to and superimposed on the intermediate transfer belt  8 , thereby forming a multicolor toner image. 
     The intermediate transfer belt  8  carrying the multicolor toner image reaches a position opposite a secondary transfer roller  19 . The secondary transfer backup roller  12  and the secondary transfer roller  19  press against each other via the intermediate transfer belt  8 , and the contact portion therebetween is hereinafter referred to as a secondary transfer nip. The multicolor toner image on the intermediate transfer belt  8  is transferred onto a recording medium P such as a transfer sheet conveyed to the secondary transfer nip (secondary transfer process). After the secondary transfer process, a certain amount of untransferred toner, which is not transferred to the recording medium P, remains on the intermediate transfer belt  8 . When the intermediate transfer belt  8  reaches a position opposite the belt cleaning device, the untransferred toner is collected from the intermediate transfer belt  8  by the belt cleaning device to complete a series of transfer processes performed on the intermediate transfer belt  8 . 
     The recording medium P is conveyed from a sheet feeding tray  26  disposed in a lower portion of the printer  100  to the secondary transfer nip via a sheet feeding roller  27  and a registration roller pair  28 . More specifically, the sheet feeding tray  26  contains multiple recording media P piled one on another. As the sheet feeding roller  27  rotates counterclockwise in  FIG. 1 , the sheet feeding roller  27  feeds a top recording medium P in the sheet feeding tray  26  to a roller nip between the registration roller pair  28 . The registration roller pair  28  stops rotating temporarily, stopping the recording medium P with a leading edge of the recording medium P nipped in the registration roller pair  28 . Then, the registration roller pair  28  rotates to convey the recording medium P to the secondary transfer nip, timed to coincide with the arrival of the multicolor toner image on the intermediate transfer belt  8 . Thus, the multicolor toner image is transferred onto the recording medium P. 
     The recording medium P onto which the multicolor toner image is transferred at the secondary transfer nip is conveyed to a fixing device  20 . In the fixing device  20 , a fixing belt and a pressure roller apply heat and pressure to the recording medium P to fix the multicolor toner image on the recording medium P. Subsequently, the recording medium P is ejected by an output roller pair  29  to the exterior of the printer  100 . The ejected recording media P are sequentially stacked as output images on a stack tray  30  to complete a sequence of image forming processes performed in the printer  100 . 
     Next, the configuration and operation of the developing device  5  of the image forming unit  6  are described in further detail below. As illustrated in  FIG. 2 , the developing device  5  includes a developing roller  51  disposed opposite the drum-shaped photoconductor  1 , a doctor blade  52  disposed opposite the developing roller  51 , and two conveying screws  55  respectively disposed in a first developer containing compartment  53  and a second developer containing compartment  54 . The developing device  5  further includes a toner concentration sensor  56  to detect a concentration of toner in a developer G in the second developer containing compartment  54 . The developing roller  51  includes stationary magnets therein, a sleeve that rotates around the magnets, and the like. The first and second developer containing compartments  53  and  54  contain the two-component developer G including carrier and toner. The second developer containing compartment  54  communicates, via an opening on an upper side thereof, with a downward toner passage  64 . 
     The sleeve of the developing roller  51  rotates counterclockwise as indicated by arrow A 2  in  FIG. 2 . The developer G is carried on the developing roller  51  by a magnetic field generated by the magnets. As the sleeve rotates, the developer G moves along a circumference of the developing roller  51 . The percentage (concentration) of toner in the developer G (ratio of toner to carrier) in the developing device  5  is adjusted within a predetermined range. More specifically, the toner supply device  60  (see  FIG. 3A ) supplies toner from the toner container  32  to the second developer containing compartment  54  according to the consumption of the toner in the developing device  5 . The configuration and operation of the toner supply device  60  are described in detail later. 
     The two conveying screws  55  stir and mix the developer G with the toner supplied to the second developer containing compartment  54  while circulating the developer G in the first and second developer containing compartments  53  and  54 . The toner in the developer G is triboelectrically charged by friction with the carrier and electrostatically attracted to the carrier. Then, the toner is carried on the developing roller  51  together with the carrier by magnetic force generated on the developing roller  51 . The developer G on the developing roller  51  is carried in the direction indicated by arrow A 2  in  FIG. 2  to the doctor blade  52 . 
     An amount of developer G on the developing roller  51  is adjusted by the doctor blade  52 . Then, the developer G is carried to a development range opposite the photoconductor  1 , and toner in the developer G is attracted to the latent image on the photoconductor  1  by an electric field generated in the development range. Subsequently, as the sleeve rotates, the developer G remaining on the developing roller  51  reaches an upper portion of the first developer containing compartment  53  and separates from the developing roller  51 . 
     Next, the toner supply device  60  and the toner container  32  are described in further detail.  FIGS. 3A and 3B  are schematic views of one of the four toner supply devices  60 .  FIG. 4  is a cross-sectional view along line A-A in  FIG. 3 .  FIG. 5  is a perspective view of toner containers  32 Y,  32 M,  32 C, and  32 K installed in the toner container mount  70 . The respective color toners in the toner containers  32  installed in the toner container mount  70  of the printer  100  are supplied to the corresponding developing devices  5  by the toner supply devices  60  provided for the respective color toners according to an amount of toner consumption in the developing devices  5 . 
     The toner containers  32  are inserted into the toner container mount  70  of the printer  100  in the direction indicated by arrow Q in  FIG. 5 , thereby installing the toner containers  32  in the toner container mount  70 . The toner container  32  is supported by two guides  72  illustrated in  FIG. 4 . The toner container  32  is substantially cylindrical and mainly includes a cap  34  held stationary by the toner container mount  70  so as not to rotate and a container body  33  formed together with a gear  33   c . The container body  33  is rotatably supported so as to rotate relative to the cap  34 , and the gear  33   c  meshes with an output gear  81  of the toner supply device  60 . As a drive motor  91  rotates the output gear  81 , driving force is transmitted to the gear  33   c  of the container body  33 , and the container body  33  is rotated while the guides  72  guide an outer circumference of the container body  33 . The drive motor  91 , the output gear  81 , the gear  33   c , and the like construct a rotary drive device. 
     The container body  33  includes a helical rib  331  protruding inward from an inner circumference face of the container body  33 . As the container body  33  rotates, the helical rib  331  conveys toner in the container body  33  from the container rear end to the container front end (from the left to the right in  FIG. 3A ) in a longitudinal direction of the container body  33 . The conveyed toner is discharged from the toner container  32  and supplied to a hopper  61  of the toner supply device  60 . That is, the drive motor  91  rotates the container body  33  of the toner container  32  as required, thereby supplying the toner to the hopper  61 . The toner containers  32 Y,  32 M,  32 C, and  32 K are replaced with new ones when the respective service lives thereof have expired, that is, when almost all toner contained in the toner container  32  has been depleted. 
     As illustrated in  FIG. 3A , the toner supply device  60  includes the toner container  32 , the toner container mount  70  (see  FIG. 5 ), the hopper  61 , a toner conveying screw  62 , and the rotary drive device including the drive motor  91 . The hopper  61  stores the toner supplied from the toner container  32 , and the toner conveying screw  62  is disposed in the hopper  61 . 
     A controller  150  (see  FIG. 2 ) controls various operations in the printer  100 , for example, toner supply, toner amount detection, toner concentration adjustment, and the like. As the controller  150  detects that a toner concentration in the developing device  5  has decreased based on a detection result obtained by the toner concentration sensor  56  (see  FIG. 2 ), the controller  150  causes the toner conveying screw  62  to rotate in a predetermined period, thereby supplying the toner to the developing device  5 . Since the toner conveying screw  62  is rotated to supply toner, the amount of toner supplied to the developing device  5  can be calculated accurately by detecting the number of rotations of the toner conveying screw  62 . 
     The toner end sensor is disposed on a side wall of the hopper  61  and detects that the amount of toner stored in the hopper  61  has fallen below a predetermined amount. For example, a piezoelectric sensor can be used as the toner end sensor. As the toner end sensor detects that the amount of toner stored in the hopper  61  has fallen below the predetermined amount, the drive motor  91  is driven. As a result, the container body  33  of the toner container  32  is rotated in the predetermined period, thereby supplying toner to the hopper  61 . In the present embodiment, the hopper  61  stores toner discharged from the toner container  32 , but alternatively, toner discharged from the toner container  32  may be directly supplied to the developing device  5 . 
     In certain image forming apparatuses, an amount of toner remaining in a toner container is estimated and reported to a user. A method to estimate the amount of toner remaining in the toner container is based on cumulative drive duration of a toner conveying screw. Since an amount of toner conveyed by the toner conveying screw is approximately proportional to a rotation angle (a rotation duration), an amount of toner usage can be calculated based on a record of the total rotation duration of the toner conveying screw. Therefore, the amount of toner remaining in the toner container can be calculated by subtracting the amount of toner usage from an initial amount of toner filling the toner container. However, since the amount of toner conveyed by the toner conveying screw varies depending on the environment, drive duration, supply frequency (supply interval), and the like, the estimated value of the amount of toner remaining in the toner container also varies. 
     Another method to estimate the amount of toner remaining in the toner container is based on an output image pattern. An amount of toner usage to output a printed image can be calculated because an amount of toner adhering to a photoconductor per image area is approximately constant. Therefore, the amount of toner usage can be calculated based on a cumulative image area. However, with this method, it is difficult to accurately estimate the amount of toner remaining in the toner container because the amount of toner adhering to the photoconductor varies due to various errors. 
     In a comparative example of a toner amount detector, electrodes are disposed on an upper and a lower inner walls of a box-shaped toner container, and the amount of toner remaining in the toner container is estimated by measuring capacitance corresponding to an amount of toner. However, toner may adhere to the electrodes because the electrodes are disposed on the inner walls of the toner container, and the toner is not removed by light force such as vibration and remains on the electrodes. If a lot of toner adheres to the electrodes under certain environmental conditions, for example, a false detection may occur that toner still remains in the toner container even though, in fact, the toner in the toner container is depleted. 
     In another comparative example, a cylindrical ink container to store ink that is liquid rather than powder is arranged such that a discharge port disposed on one end of the ink container in the longitudinal direction faces vertically downward. An amount of the ink is detected based on change of capacitance between two electrodes. The two electrodes have a curved shape along the side face of the ink container. However, if this structure is directly applied to a powder amount detector, toner as powder may clog the discharge port under gravity, thereby preventing the toner from being discharged. 
     In the present embodiment, as illustrated in  FIGS. 3A and 4 , a pair of measuring electrodes  65  and  66  is arranged below and above the toner container  32  in the vertical direction to detect capacitance between the measuring electrodes  65  and  66 . The toner container  32  has a cylindrical shape and is arranged horizontally. The measuring electrodes  65  and  66  are not attached to the toner container  32 , but are attached to walls  67  and  68  of the printer  100 . Since the measuring electrodes  65  and  66  are disposed around the toner container  32 , toner is prevented from adhering to the measuring electrodes  65  and  66 . In  FIG. 4 , toner in the toner container  32  is discharged from the lower side of the toner container  32  as the toner container  32  rotates counterclockwise. 
     In the present embodiment, one of the pair of measuring electrodes  65  and  66 , that is, the upper measuring electrode  65  has an arc shape following the shape of the toner container  32 . The other measuring electrode  66 , which is the lower measuring electrode  66 , has a flat shape. In another embodiment, the shapes of the measuring electrodes  65  and  66  may be inverted. That is, the upper measuring electrode  65  may have the flat shape, and the lower measuring electrode  66  may have the arc shape following the shape of the toner container  32 . The measuring electrodes  65  and  66  are secured to the walls  67  and  68  of the printer  100  with double-sided tape or the like, respectively. The measuring electrodes  65  and  66  are made of any conductive material, for example, iron plate. The projected areas of the upper and lower measuring electrodes  65  and  66  projected onto the horizontal plane by projection light L 1  directed in the vertical direction have the same size, but are not limited thereto. 
     Since only one of the measuring electrodes  65  and  66  is arranged along the toner container  32 , the distance between both ends of the upper and lower measuring electrodes  65  and  66  can be increased as compared with the case in which both of the measuring electrodes  65  and  66  are arranged along the toner container  32 . In the case in which both of the measuring electrodes  65  and  66  are arranged along the toner container  32 , as illustrated in  FIG. 6B , lines of electric force are denser in an area A between the ends of the measuring electrodes  65  and  66  than lines of electric force in an area B between center portions of the measuring electrodes  65  and  66 . This is because the distance between the ends of the measuring electrodes  65  and  66  is shorter than the distance between the center portions of the measuring electrodes  65  and  66 . Therefore, when the toner container  32  rotates and toner T in the toner container  32  is unevenly distributed, for example, on the right side as illustrated in  FIG. 6A , the capacitance is greater than that when the toner T is evenly distributed, causing the capacitance to vary widely. 
     Therefore, in the present embodiment, only one of the measuring electrode  65  and  66  is arranged along the toner container  32 . As a result, the distance between both ends of the upper and lower measuring electrodes  65  and  66  can be increased, and the difference of the lines of electric force between the end and the center portion can be reduced. With this configuration, the difference of the capacitance is decreased between when the toner in the toner container  32  is unevenly distributed to the left or right and when the toner in the toner container  32  is evenly distributed, thereby improving the measurement accuracy. 
       FIG. 7  is a graph illustrating an example of a ratio of effects of objects to be measured on the capacitance. As illustrated in  FIG. 7 , the objects to be measured between the measuring electrodes  65  and  66  are toner, the toner container  32 , and air. A certain voltage is applied to the measuring electrode  65  and  66  to measure the capacitance. If the voltage varies, the capacitance also varies, and the amount of toner calculated from the capacitance also varies greatly. The variation of the amount of toner can be reduced by lowering the capacitance of the object other than the measurement target (i.e., toner) or by increasing the sensitivity of the measurement target (i.e., toner). One of the measuring electrodes  65  and  66  arranged along the toner container  32  can make the measurement region of air smaller and increase the sensitivity of the measurement target (i.e., toner) as compared with the case of the pair of flat electrodes, thereby improving the measurement accuracy. 
     For example, in the case of flat upper and lower electrodes, the calculated amount of toner varies as follows. 
     Capacitance:
         Air (without the toner container  32  and toner): 3000 counts (79%)   Air and the toner container  32 : 3100 counts   Air, the toner container  32 , and toner: 3800 counts (100%)       

     Toner sensitivity of capacitance: 2.0 counts/g 
     If the voltage variation is ±0.5%, the amount of toner varies from ±7.8 g to 9.5 g. 
     On the other hand, in the case of a flat lower electrode and an arc-shaped upper electrode, the calculated amount of toner varies as follows. 
     Capacitance:
         Air (without the toner container  32  and toner): 3500 counts (74%)   Air and the toner container  32 : 3650 counts   Air, the toner container  32 , and toner: 4700 counts (100%)       

     Toner sensitivity of capacitance: 3.0 counts/g 
     If the voltage variation is ±0.5%, the amount of toner varies from ±6.1 g to 7.8 g. 
     With the arc-shaped upper measuring electrode  65 , the space between the upper and lower measuring electrodes  65  and  66  can be narrowed. Accordingly, the sensitivity of measuring capacitance is increased, so that the toner sensitivity is increased. Further, although the capacitance of only air increases, the ratio of the capacitance of air to the capacitance including the toner container  32  and toner decreases. As a result, the variation of the amount of toner can be reduced by ±1.7 g. 
     When the amount of toner in the toner container  32  is large, the difference of the variation of the capacitance between the flat electrode and the arc-shaped electrode is not large, but when the amount of toner is small, the difference of the variation is large. Therefore, the arc-shaped measuring electrode  65  is useful for detecting amount of toner because high detection accuracy is required when the amount of toner is small. 
     As illustrated in  FIGS. 3A and 3B , each of the measuring electrodes  65  and  66  is connected to a capacitance detection circuit  111  included in a powder amount detection unit  110 . The capacitance detection circuit  111  applies electric power to the pair of measuring electrodes  65  and  66 , thereby detecting the capacitance between the pair of measuring electrodes  65  and  66 . A known method of detecting capacitance can be used. In the present embodiment, a charging method is used in which the capacitance is measured by a relation between the time of charge arrival point and the voltage or current while a constant voltage or a constant current is applied between the pair of measuring electrodes  65  and  66 . 
     The detection result obtained by the capacitance detection circuit  111  is transmitted to a toner amount calculation circuit  112 , and a toner amount calculation circuit  112  calculates the amount of toner remaining in the toner container  32  based on the detected capacitance. The detected capacitance varies depending on a dielectric constant between the measuring electrodes  65  and  66 . Toner has a higher dielectric constant than air. Therefore, the dielectric constant varies according to the amount of toner in an electric field between the measuring electrodes  65  and  66 . As a result, the capacitance varies according to the amount of toner in the toner container  32  sandwiched by the pair of measuring electrodes  65  and  66 . Thus, the amount of toner in the toner container  32  can be calculated by detecting the capacitance. 
     In the present embodiment, the toner amount calculation circuit  112  calculates the amount of toner remaining in the toner container  32  based on a calibration curve stored in a memory  113  and the capacitance obtained by the capacitance detection circuit  111 . The calibration curve preliminarily acquired indicates the relation between the capacitance and the amount of toner in the toner container  32 . A temperature and humidity sensor  114  is provided to detect temperature and humidity around the toner container  32 , and the amount of toner remaining in the toner container  32  is corrected based on a detection result obtained by the temperature and humidity sensor  114 . The amount of toner obtained by the toner amount calculation circuit  112  is displayed on a display  115  (e.g., a control panel). 
     As described above, in the present embodiment, a powder amount detector (a toner amount detector) includes the measuring electrodes  65  and  66  and the powder amount detection unit  110  including the capacitance detection circuit  111 , the toner amount calculation circuit  112 , the memory  113 , the temperature and humidity sensor  114 , and the display  115 . In the present embodiment, the measuring electrodes  65  and  66  are disposed outboard of the toner container  32 , thereby preventing toner from adhering to the measuring electrodes  65  and  66 . Therefore, the amount of toner can be detected accurately. The number of components and the cost of the toner container  32  can be reduced. Under high temperature environment, the amount of toner remaining in the toner container  32  can be accurately detected without being affected by thermal expansion of the toner container  32 . 
     With such a configuration in which the pair of measuring electrodes  65  and  66  sandwiches the toner container  32 , the capacitance does not vary due to the shape error or rotational eccentricity of the toner container  32 . Therefore, the amount of toner remaining in the toner container  32  can be detected accurately. In the present embodiment, the pair of measuring electrodes  65  and  66  covers almost the entire toner container  32 . Specifically, the projection areas of the measuring electrodes  65  and  66  projected on the horizontal plane by the projection light L 1  directed in the vertical direction include the projection area of the toner container  32 . With this configuration, since almost all toner in the toner container  32  is included in the lines of electric force between the pair of measuring electrodes  65  and  66  (i.e., electric field), the amount of toner remaining in the toner container  32  can be detected accurately even if the toner is unevenly distributed in the toner container  32 , and the accurate amount of toner remaining in the toner container  32  can be reported to a user. 
       FIG. 8  is a graph illustrating an example of the relation between the amount of toner in the toner container  32  and the capacitance. As illustrated in  FIG. 8 , the relation between the amount of toner in the toner container  32  and the capacitance is approximately linear. Therefore, the amount of toner remaining in the toner container  32  can be accurately calculated based on the capacitance. A distance between the measuring electrodes  65  and  66  may be different for each device due to assembly tolerances. Therefore, in the present embodiment, the powder amount detector employs a calibration curve calculation mode to acquire the calibration curve as illustrated in  FIG. 8 . Before factory shipment, the calibration curve calculation is performed, and the calibration curve is acquired and stored in the memory  113 . The calibration curve calculation can be performed by a certain operation on the display  115  (e.g., the control panel) of the printer  100  as the image forming apparatus. 
     As the calibration curve calculation starts, the controller  150  causes the display  115  to display an instruction to install an empty toner container  32  in the toner container mount  70 . After setting the empty toner container  32  in the toner container mount  70 , an operator operates the display  115 , for example, pushes a start button, thereby measuring capacitance. After measuring the capacitance of the empty toner container  32 , the controller  150  causes the display  115  to display an instruction to install a full toner container  32  in the toner container mount  70 . After setting the full toner container  32  in the toner container mount  70 , the operator operates the display  115 , thereby measuring capacitance. After measuring the capacitance of the full toner container  32 , the controller  150  acquires a calibration curve based on the capacitances of the empty and full toner containers  32  and stores the calibration curve in the memory  113 . The calibration curve calculation is performed for each color of Y, M, C, and K. 
     Alternatively, the controller  150  may acquire a calibration curve based on capacitance of a toner container  32  containing a small amount of toner instead of the empty toner container  32  or capacitance without the toner container  32 , and the capacitance of the full toner container  32 . That is, the capacitance between the pair of measuring electrodes  65  and  66  is measured in at least two states in which the amount of toner between the pair of measuring electrodes  65  and  66  is different from each other to acquire the calibration curve. Further, the calibration curve may be acquired by an imitation of the toner container  32  in which an amount of material, such as an acrylonitrile-butadiene-styrene (ABS) resin, is adjusted so as to have the capacitance identical to that of the toner container  32 . As described above, the controller  150  performs the calibration curve calculation. 
     In the present embodiment, the temperature and humidity sensor  114  is provided to detect temperature and humidity around the toner container  32 , and the amount of toner is corrected based on a detection result obtained by the temperature and humidity sensor  114 . This is because the distance between the measuring electrodes  65  and  66  varies due to the thermal expansion of components to which the measuring electrodes  65  and  66  are secured (i.e., components constructing the upper and lower walls  67  and  68 ). Further, moisture between the measuring electrodes  65  and  66  varies. As a result, the capacitance between the measuring electrodes  65  and  66  varies. 
     The temperature and humidity at the time of measuring the above-described calibration curve are stored in the memory  113 , and the amount of toner is corrected according to the difference of temperature and humidity between at the time of measuring the capacitance of the toner container  32  actually used and at the time of measuring the calibration curve in consideration of a predetermined temperature and humidity correction factor. As a result, the calculation error of the amount of toner due to ambient temperature and humidity is minimized, thereby acquiring the amount of toner accurately. 
     For example, a correction factor α at high temperature and high humidity and a correction factor β at low temperature and low humidity are stored in the memory  113 . If temperature and humidity detected by the temperature and humidity sensor  114  are equal to or higher than a predetermined first threshold, the amount of toner is corrected by multiplying the calculated amount of toner by the correction factor α at high temperature and high humidity. If temperature and humidity detected by the temperature and humidity sensor  114  are equal to or less than a second threshold which is lower than the first threshold, the amount of toner is corrected by multiplying the calculated amount of toner by the correction factor β at low temperature and low humidity. As a result, the calculation error of the amount of toner due to ambient temperature and humidity is minimized, thereby acquiring the amount of toner accurately. As described above, the calculated amount of toner is corrected according to temperature and humidity, but alternatively, the detected capacitance can be corrected according to temperature and humidity. 
       FIG. 9  is a schematic cross-sectional view of an example of a toner supply device  60  provided with ground electrodes disposed outboard of the measuring electrodes  65  and  66 . As illustrated in  FIG. 9 , the measuring electrodes  65  and  66  are attached to the upper and lower walls  67  and  68  via insulators  69 . Components constructing the upper and lower walls  67  and  68  are electrically grounded, thereby functioning as ground electrodes. As illustrated in  FIG. 1 , the photoconductors  1 , the charging devices  4  (see  FIG. 2 ), the intermediate transfer unit  15 , and the like are disposed below the toner containers  32 . This configuration may cause capacitance to vary. In the present embodiment, since the component constructing the lower wall  68  is electrically grounded as the ground electrode, electrical noises from the photoconductors  1 , the charging devices  4 , and the intermediate transfer unit  15  can be cut off. Above the toner containers  32 , the printed recording media P are stacked, the control panel is disposed, and an operator may put the hand on the stack tray  30 . This configuration may cause capacitance to vary. In the present embodiment, since the component constructing the upper wall  67  is electrically grounded as the ground electrode, electrical noises from above can be cut off. Therefore, the variation of capacitance due to the electrical noises can be minimized, and the amount of toner can be accurately detected. Note that, preferably, the ground electrodes (i.e., the upper and lower walls  67  and  68 ) are larger than the measuring electrodes  65  and  66 , and cover the measuring electrodes  65  and  66  as viewed from the ground electrodes (i.e., the upper and lower walls  67  and  68 ). 
       FIG. 10  is a schematic cross-sectional view of an example of a plurality of toner supply devices  60  provided with a plurality of ground electrodes  120  that partition a plurality of toner containers  32 Y,  32 M,  32 C, and  32 K disposed adjacent to each other. Without the ground electrodes  120 , some of the lines of electric force between the measuring electrodes  65  and  66  (i.e., the lines of electric force near the adjacent toner container  32 ) may be changed due to toner in the adjacent toner container  32 . That is, current may flow through the toner in the adjacent toner container  32 . As a result, the capacitance may vary according to the amount of toner in the adjacent toner container  32 , and the amount of toner may not be accurately detected. 
     However, as illustrated in  FIG. 10 , since the ground electrodes  120  partition the adjacent toner containers  32 , the lines of electric force between the measuring electrodes  65  ( 65 Y,  65 M,  65 C, and  65 K) and  66  ( 66 Y,  66 M,  66 C, and  66 K) are cut off by the ground electrodes  120 . That is, some of the lines of electric force between the measuring electrodes  65  and  66  is directed toward the ground electrode  120  but does not go to the adjacent toner container  32  beyond the ground electrode  120 . Therefore, this configuration can prevent the capacitance to be detected from being affected by the amount of toner in the adjacent toner container  32 , and the amount of toner can be accurately detected. 
     In addition to the ground electrodes  120  on the left and right side in  FIG. 10 , ground electrodes  120  may be disposed in the direction perpendicular to the surface of the paper on which  FIG. 10  is drawn so as to surround the four toner containers  32 Y,  32 M,  32 C, and  32 K. Therefore, the ground electrodes  120  can cut off electrical noises caused by human passing by or another device disposed on the side, front, or back of the printer  100 , and the amount of toner can be more accurately detected. 
     In the example in  FIG. 10 , the component constructing the lower wall  68  is not electrically grounded as a ground electrode, and the insulator  69  is not provided, but the lower wall  68  may functions as a ground electrode similarly to the upper wall  67  in another example. Conversely, in yet another example, only the lower wall  68  may functions as a ground electrode, and the upper wall  67  may not function as a ground electrode. 
     In the above-described embodiments, the measuring electrodes  65  and  66 , one of which has the flat shape and the other of which has the arc shape following the shape of the powder container  32 , are arranged vertically, but, alternatively, measuring electrodes may be arranged horizontally. 
     As described above, according to the present disclosure, a powder amount detector can accurately detect an amount of powder. 
     The above-described embodiments are illustrative and do not limit the present disclosure. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the present disclosure.