Abstract:
A powder conveying device includes a powder container, a powder conveying tube, a powder conveying member, and a detecting unit. The powder container is configured to contain powder. The powder conveying tube guides the powder from the powder container to a conveyance destination located downward from the powder container. The powder conveying member is located inside the powder conveying tube, and moves to convey the powder toward downstream in a conveying direction. The detecting unit is located in the powder conveying tube, and detects a remaining amount of powder. Time t 2  taken to convey the maximum amount of powder satisfies t 2 &lt;t 1  where t 1  is the sum of time required to feed recording sheets and time interval between feeding of the recording sheets.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to and incorporates by reference the entire contents of Japanese priority document, 2006-207636 filed in Japan on Jul. 31, 2006. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a powder conveying device, a developing apparatus, a process cartridge, and an image forming apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    Image forming apparatuses, such as copier, facsimile machine, and printer, that employ a toner conveying device have been known. The toner conveying device includes a toner discharging unit that discharges toner out of a toner container, and a conveying tube that connects the interior of a developing device that develops a latent image carried on a latent image carrier, such as a photosensitive drum, and the toner container. The toner discharging unit is activated as necessary, discharges the toner contained in the toner container to the conveying tube, and directly conveys the toner into the developing device through the conveying tube. If the toner container is arranged at a position lower than the position of the developing device in an image forming apparatus including the toner conveying device, it is required to convey the toner, which is the powder that has been passed through the conveying tube, by carrying the toner up against gravity toward the developing device. As a result, the conveying efficiency is degraded, or the toner becomes likely to get jammed inside the conveying tube. Therefore, generally, the toner container is arranged at a position higher than the position of the developing device to convey the toner in the direction of gravity. Japanese Patent Application Laid-Open No. H08-30097, for example, discloses a conventional toner conveying device that performs such toner conveyance in the direction of gravity. The conventional toner conveying device sends toner that has been discharged out of a toner box as a toner container into a conveying tube by a toner discharging device, to the developing device by letting the toner fall with weight thereof. 
         [0006]    With the conventional toner conveying device, however, the sectional area of a coil is small with respect to the sectional area of a space inside the conveying tube. Therefore, a part of the section that is not occupied by the section of the coil becomes a clearance through which the toner can pass. Accordingly, when a large amount of toner is discharged at once from the toner container, the toner can flow through the clearance, and can flow into the developing device regardless of rotation of the coil. As a result, a refilling control of toner to the developing device becomes unstable. 
         [0007]    Japanese Patent Application Laid-Open No. 2005-24665 discloses a conventional powder conveying device that includes a powder container that contains powder, and a powder conveying tube to guide the powder from the powder container to a destination that is positioned downward relative to the powder container, and that conveys the powder to the destination by passing the powder through the conveying tube. In the conventional powder conveying device, a powder conveying member is provided that is installed inside the conveying tube, and that provides, to the powder, a traveling force to travel downstream in the conveying direction by movement thereof to convey the powder, and a portion that has a higher performance in controlling the powder passage inside the tube than other portions of the powder conveying member in the powder conveying tube is provided at least at one part inside the powder conveying tube. 
         [0008]    However, because refilling of the powder is performed when the developing device, which is the destination of the powder, requires the powder according to the required amount, a driving time of a refilling motor becomes long when an image having large image area is processed. 
         [0009]    The powder has a small particle diameter to improve image quality. If such powder having a small particle diameter is stirred with air, the volume of the powder increases, and fluidity of the powder increases like liquid. On the other hand, if the powder is left for a long time, the powder enters in even a small space, the volume decreases, and the fluidity is deteriorated. 
         [0010]    If images having a large image area is successively printed, toner in a sub-hopper is constantly mixed with a small amount of air present in space, and fluidity increases. If the fluidity increases, friction with the wall of the powder container or with the powder conveying member decreases. Even if driving is stopped, the powder can still be conveyed (flowing) a little. This phenomenon problematically causes accumulation and increase of space at the uppermost stream portion (near a sensor) in the sub-hopper. 
         [0011]    In the conventional technologies, if the flowing of the powder in the sub-hopper and the blocking of the powder container occur at the same time, the amount of air at the portion near the sensor exceeds the threshold. As a result, the sensor detects the air to cause the apparatus to erroneously recognize that there is “no toner”. 
       SUMMARY OF THE INVENTION 
       [0012]    It is an object of the present invention to at least partially solve the problems in the conventional technology. 
         [0013]    According to an aspect of the present invention, a powder conveying device includes a powder container that is configured to contain powder, a powder conveying tube that guides the powder from the powder container to a conveyance destination located downward relative to the powder container, a powder conveying member that is located inside the powder conveying tube and moves to apply a force to the powder to convey the powder toward downstream in a conveying direction, and a detecting unit that is located in the powder conveying tube and detects a remaining amount of powder. Time t 2  taken to convey the maximum amount of powder satisfies t 2 &lt;t 1  where t 1  is the sum of time required to feed recording sheets and time interval between feeding of the recording sheets. 
         [0014]    According to another aspect of the present invention, a powder conveying device includes a powder container that is configured to contain powder, a powder conveying tube that guides the powder from the powder container to a conveyance destination located downward relative to the powder container, a powder conveying member that is located inside the powder conveying tube and moves to apply a force to the powder to convey the powder toward downstream in a conveying direction, and a detecting unit that is located in the powder conveying tube and detects a remaining amount of powder. Time interval between feeding of recording sheets for successive printing of images with a large image area is set longer than time interval between feeding of recording sheets for normal printing. 
         [0015]    According to still another aspect of the present invention, a powder conveying device includes a powder container that is configured to contain powder, a powder conveying tube that guides the powder from the powder container to a conveyance destination located downward relative to the powder container, a powder conveying member that is located inside the powder conveying tube and moves to apply a force to the powder to convey the powder toward downstream in a conveying direction, and a detecting unit that is located in the powder conveying tube, and detects a remaining amount of powder. The maximum amount of powder to be conveyed is controlled. 
         [0016]    The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic diagram of an image forming apparatus according to an embodiment of the present invention; 
           [0018]      FIG. 2  is an enlarged view of a process cartridge shown in  FIG. 1 ; 
           [0019]      FIG. 3  is a perspective view of a toner bottle shown in  FIG. 1 ; 
           [0020]      FIG. 4  is a perspective view of a bottle holder and four toner bottles shown in  FIG. 1 ; 
           [0021]      FIG. 5  is a perspective view of toner conveying devices of the image forming apparatus; 
           [0022]      FIG. 6  is a perspective view of process cartridges and the toner conveying devices; 
           [0023]      FIG. 7  is an enlarged view of part of one of the toner conveying device; 
           [0024]      FIG. 8  is a schematic diagram of a powder conveying tube; 
           [0025]      FIG. 9  is a schematic diagram of a supply motor of the image forming apparatus; 
           [0026]      FIGS. 10 to 12  are timing charts for comparing ON/OFF control of a supply motor with that of the conventional technology. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings. In the following description, the present invention is applied to an electrophotographic printer as an example of an image forming apparatus. 
         [0028]      FIG. 1  is a schematic diagram of a printer  100  according to an embodiment of the present invention. The printer  100  includes four process cartridges  6 Y,  6 M,  6 C, and  6 K for forming toner images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. The process cartridges  6 Y,  6 M,  6 C, and  6 K are replaced when reaching their service life. The process cartridges  6 Y,  6 M,  6 C, and  6 K are of basically similar construction except that they use Y, M, C, and K toners of different colors, and thus but one of them, for example, the process cartridge  6 Y for forming a Y toner image is described in detail. 
         [0029]      FIG. 2  is an enlarged view of the process cartridge  6 Y. The process cartridge  6 Y includes a photosensitive drum  1 Y, a cleaning device  2 Y, an electrostatic discharger (not shown), a charger  4 Y, a developing device  5 Y. The process cartridge  6 Y is attachable to and detachable from the main body of the printer  100  to enable consumable parts to be replaced at one time. 
         [0030]    As described above, the process cartridges  6 Y,  6 M,  6 C, and  6 K each include a photosensitive drum, a cleaning device, an electrostatic discharger, a charger, and a developing device, and are configured to be integrally attached to and detached from the main body of the printer. These components have been separately attachable and detachable consumable parts, and to be replaced as necessary. However, such a configuration makes maintenance difficult because it is difficult to make operators understand attaching and detaching operations for each part. 
         [0031]    A process cartridge system has been proposed to integrally replace these components for easy maintenance, in which the time when toner in the developing device is exhausted is regarded as the end of its service life. However, in such a configuration, even a part that still has sufficient life at the point when toner is exhausted is replaced, and there has been a disadvantage that wasted parts increase. 
         [0032]    For example, Japanese Patent Application Laid-Open No. H10-239974 discloses an image forming apparatus having a toner container that is attachable to and detachable from the process cartridge. In such an image forming apparatus, however, even when only a toner container needs to be replaced, the process cartridge is required to be removed from the image forming apparatus. Thus, a problem arises in replacability of toner containers. 
         [0033]    In the printer  100 , these problems are solved by configuring the process cartridges  6 Y,  6 M,  6 C, and  6 K and toner bottles  32 Y,  32 M,  32 C, and  32 K to be separately attachable to and detachable from the main body of the printer. 
         [0034]    The charger  4 Y uniformly charges the surface of the photosensitive drum  1 Y that is rotated clockwise in  FIG. 1  by a driving unit (not shown). The surface of the photosensitive drum  1 Y that has been uniformly charged is exposed to be scanned by a laser beam L and carries a Y latent image. This Y latent image is developed by the developing device  5 Y to a Y toner image, and then intermediate transferred onto an intermediate transfer belt  8 . The cleaning device  2 Y cleans residual toner remained on the surface of the photosensitive drum  1 Y that is subjected to the intermediate transfer process. The electrostatic discharger discharges a residual electrical charge on the photosensitive drum  1 Y after the cleaning. This discharge initializes the surface of the photosensitive drum  1 Y to be ready for next image forming. Similarly in the other process cartridges  6 M,  6 C, and  6 K, M, C, and K toner images are formed on photosensitive drums  1 M,  1 C, and  1 K, and are transferred onto the intermediate transfer belt  8 . 
         [0035]    As shown in  FIG. 1 , an exposing device  7  is arranged below the process cartridges  6 Y,  6 M,  6 C, and  6 K. The exposing device  7  as a latent-image forming unit exposes the photosensitive drums  1 Y,  1 M,  1 C, and  1 K to the laser beam L emitted from a light source based on image information. By this exposure, Y, M, C, and K latent images are formed on the photosensitive drums  1 Y,  1 M,  1 C, and  1 K. The exposing device  7  irradiates the photosensitive drums  1 Y,  1 M,  1 C, and  1 K with the laser beam L through a plurality of optical lenses and a mirror while scanning the laser beam L with a polygon mirror that is rotated by a motor. 
         [0036]    Below the exposing device  7  in  FIG. 1  is arranged a sheet feeding unit that includes a sheet cassette  26 , a feeding roller  27 , and a resist roller pair  28 . The sheet cassette  26  contains a stack of transfer sheets P as recording medium, and the feeding roller  27  abuts a transfer sheet P on the top. When the feeding roller  27  is rotated counterclockwise in  FIG. 1  by a driving unit (not shown), the top one of the transfer sheets P is fed toward between rollers of the resist roller pair  28 . The resist roller pair  28  rotates both rollers to sandwich the transfer sheet P, and stops the rotation once, soon after the transfer sheet P is sandwiched therebetween. The resist roller pair  28  sends the transfer sheet P to a secondary transfer nip described later at appropriate timing. In the sheet feeding unit, a conveying unit is formed of a combination of the feeding roller  27  and the resist roller pair  28  serving as timing rollers. This conveying unit conveys the transfer sheet P from the sheet cassette  26  serving as a storing unit to the secondary transfer nip described later. 
         [0037]    Above the process cartridges  6 Y,  6 M,  6 C, and  6 K in  FIG. 1  is arranged an intermediate transfer unit  15  that endlessly moves the intermediate transfer belt  8  as the intermediate transfer medium kept in a tensioned state. The intermediate transfer unit  15  includes four primary-transfer bias rollers  9 Y,  9 M,  9 C, and  9 K, and a cleaning device  10  in addition to the intermediate transfer belt  8 . The intermediate transfer unit  15  further includes a secondary-transfer backup roller  12 , a cleaning backup roller  13 , a tension roller  14 , and the like. The intermediate transfer belt  8  is held in a tensioned manner by these three rollers and is endlessly moved counterclockwise in  FIG. 1  by a rotation driving force of at least one of the rollers. The primary-transfer bias rollers  9 Y,  9 M,  9 C, and  9 K catch the intermediate transfer belt  8 , which is thus endlessly moved, between the primary-transfer bias rollers  9 Y,  9 M,  9 C, and  9 K and the photosensitive drums  1 Y,  1 M,  1 C, and  1 K, thereby forming a primary transfer nip. Such a configuration is to apply a transfer bias having an opposite polarity (for example, positive) to that of the toner on a rear surface (inner periphery surface) of the intermediate transfer belt  8 . The rollers except the primary-transfer bias rollers  9 Y,  9 M,  9 C, and  9 K are all electrically grounded. On the intermediate transfer belt  8 , the Y, M, C, and K toner images on the photosensitive drums  1 Y,  1 M,  1 C, and  1 K are superimposed in the process of sequentially passing through the primary transfer nips for Y, M, C, and K as the intermediate transfer belt  8  is endlessly moved, thereby performing the primary transfer. As a result, a superimposed toner image of four colors (hereinafter, “four-color toner image”) is formed on the intermediate transfer belt  8 . 
         [0038]    The secondary-transfer backup roller  12  forms the secondary transfer nip with a secondary transfer roller  19  by sandwiching the intermediate transfer belt  8  therebetween. The four-color toner image formed on the intermediate transfer belt  8  is transferred to the transfer sheet P at this secondary transfer nip. On the intermediate transfer belt  8  that has passed the secondary transfer nip, transfer residual toner that has not been transferred to the transfer sheet P adheres. This residual toner is cleaned by the cleaning device  10 . 
         [0039]    At the secondary transfer nip, the transfer sheet P is conveyed in the opposite direction relative to the resist roller pair  28 , being sandwiched between the intermediate transfer belt  8  and the secondary transfer roller  19  that make surface movement in a forward direction. The transfer sheet P sent out from the secondary transfer nip passes through between rollers of a fixing device  20 . At this time, the four-color toner image that has been transferred onto the surface of the transfer sheet P is fixed by heat and pressure. Thereafter, the transfer sheet P is discharged out of the apparatus through between rollers of a discharge roller pair  29 . A stack  30  is formed on a top of the main unit of the printer, and the transfer sheet P that has discharged out of the apparatus by the discharge roller pair  29  is sequentially stacked on the stack  30 . 
         [0040]    The developing device  5 Y in the process cartridge  6 Y is explained below. The developing device  5 Y has a magnetic-field generating unit inside, and includes a developing sleeve  51 Y as a developer carrier that conveys two component developer containing magnetic particles and toner carrying on the surface, and a doctor  52 Y as a developer control member that controls the layer thickness of the developer that is carried on the developing sleeve  51 Y. A developer container  53 Y that holds developer that is not carried to the developing area to become a subject of the control is formed at a portion upstream in the direction of conveying the developer. Furthermore, the developing device  5 Y includes a toner container  54 Y that is arranged adjacent to the developer container  53 Y and that contains toner, and a toner carrying screw  55 Y to agitate and carry the toner. 
         [0041]    The operation of the developing device  5 Y is explained next. In the developing device  5 Y, a developer layer is formed on the developing sleeve  51 Y. The toner is taken in the developer from the developer container  53 Y by movement of the developer layer that is carried by rotation of the developing sleeve  51 Y. The toner is taken in to make the toner concentrations of the developer within a predetermined toner concentration range. The toner taken into the developer is charged by triboelectric charge with carrier. The developer containing the charged toner is supplied to the surface of the developing sleeve  51 Y having a magnetic pole inside, and is carried by a magnetic force. The developer layer that is carried on the developing sleeve  51 Y is conveyed in a direction of an arrow along the rotation of the developing sleeve  51 Y. After the layer thickness is controlled by the doctor  52 Y on the way, the developer layer is conveyed to the developing area. In the developing area, development based on a latent image formed on the photosensitive drum  1 Y is performed. The developer layer remained on the developing sleeve  51 Y is conveyed to the developer container  53 Y at an upstream portion in the direction of conveying the developer. 
         [0042]    As shown in  FIG. 1 , a bottle holder  31  is arranged between the intermediate transfer unit  15  and the stack  30  that is arranged upstream to this unit. The bottle holder  31  holds the toner bottles  32 Y,  32 M,  32 C, and  32 K that contain Y, M, C, and K toners, respectively. The toner bottles  32 Y,  32 M,  32 C, and  32 K are set by putting on the bottle holder  31  from the top for each color. The toner in each of the toner bottles  32 Y,  32 M,  32 C, and  32 K are supplied to the developing device of the process cartridges  6 Y,  6 M,  6 C, and  6 K as necessary by the toner conveying device. The toner bottles  32 Y,  32 M,  32 C, and  32 K are attachable to and detachable from the main unit of the printer  100  independently from the process cartridges  6 Y,  6 M,  6 C, and  6 K. 
         [0043]      FIG. 3  is a perspective view of the toner bottle  32 Y.  FIG. 4  is a perspective view of the bottle holder  31  where the toner bottle  32 K is to be set. The toner bottle  32 Y includes a bottle main body  33 Y, a resin case  34 Y, a handle  35 Y, a shutter  36 Y, and a gear  37 Y. The resin case  34 Y is located at the end of the bottle main body  33 Y. The handle  35 Y is integrally formed on the resin case  34 Y. The gear  37 Y integrally rotates with the bottle main body  33  on a side of the resin case  34 Y. When the toner bottle  32 Y is to be set in the main unit of the printer  100 , it is ensured to open the stack  30  upward to expose the bottle holder  31 . After the toner bottle  32 Y is set on the bottle holder  31  as shown in  FIG. 4 , the handle  35 Y is rotated. As a result, the resin case  34 Y that is integrally formed with the handle  35 Y is rotated, the shutter  36 Y moves in a circumferential direction of the resin case  34 Y to be open a toner discharge port (not shown). At the same time the resin case  34 Y and the bottle holder  31  are connected and fixed. On the other hand, when the toner bottle  32 Y is to be removed from the main unit of the printer  100 , by rotating the handle  35 Y in the opposite direction, the connection between the resin case  34 Y and the bottle holder  31  is released, and at the same time, the shutter  36 Y closes the toner discharge port. The toner bottle  32 Y can be removed from the main unit of the printer  100  while holding the handle  35 Y. As described above, the toner bottle  32 Y can be attached and detached by handling from the top side of the main unit of the printer  100 , whereby the toner bottle  32 Y can be easily replaced. Besides, because the handle  35 Y is formed on the resin case  34 Y, it is possible to fix the toner bottle  32 Y to the bottle holder  31  easily by rotating the resin case  34 Y. In a state in which the toner bottle  32 Y is removed from the main unit of the printer  100 , the shutter  36 Y does not open even if the handle  35 Y of the resin case  34 Y is rotated. This enables to prevent the shutter  36 Y from opening by mistake during the replacement work and the toner inside from spilling out of the toner bottle  32 Y. 
         [0044]      FIG. 5  is a perspective view of the toner bottles  32 Y,  32 M,  32 C, and  32 K and toner conveying devices  40 Y,  40 M,  40 C, and  40 K.  FIG. 6  is a perspective view of the toner bottles  32 Y,  32 M,  32 C, and  32 K, the intermediate transfer unit  15 , and the toner conveying devices  40 Y,  40 M,  40 C, and  40 K viewed from another angle. The toner conveying devices  40 Y,  40 M,  40 C, and  40 K are provided in the main unit of the printer  100  on a side of the intermediate transfer unit  15 . Therefore, the process cartridges  6 Y,  6 M,  6 C, and  6 K or the toner bottles  32 Y,  32 M,  32 C, and  32 K are not required to be equipped with the toner conveying unit. Thus, the process cartridges  6 Y,  6 M,  6 C, and  6 K and the toner bottles  32 Y,  32 M,  32 C, and  32 K can be minimized compared with those of a conventional technology. With the conventional technology, because the process cartridges and the toner bottles are arranged adjacent to each other, there is a limit on design flexibility. In contrast, according to the embodiment, the process cartridges and the toner bottles can be arranged at portions away from each other, and therefore, flexibility in design is improved and miniaturization of the printer can be achieved. 
         [0045]    Furthermore, the discharge port of the toner bottles  32 Y,  32 M,  32 C, and  32 K, the toner conveying devices  40 Y,  40 M,  40 C, and  40 K, and a toner supply port of toner containers  54 Y,  54 M,  54 C, and  54 K in developing devices  5 Y,  5 M,  5 C, and  5 K are disposed on a side of one end of the intermediate transfer unit  15 . Therefore, a toner conveyance route of the toner conveying devices  40 Y,  40 M,  40 C, and  40 K can be minimized, miniaturization of the printer can be achieved, and clogging during the toner conveyance can be prevented. 
         [0046]    The toner conveying devices  40 Y,  40 M,  40 C, and  40 K are of basically similar construction, and thus but one of them, for example, the toner conveying device  40 Y for the Y toner is explained. As shown in  FIG. 5 , the toner conveying device  40 Y includes a driving motor  41 Y, a driving gear  42 Y, and a toner conveying pipe  43 Y. Inside the toner conveying pipe  43 Y is provided a resin coil (not shown). The driving gear  42 Y engages with the gear  37 Y of the toner bottle  32 Y, and when the driving motor  41 Y is rotated, the bottle main body  33 Y that integrally rotates with the gear  37 Y is rotated. When a concentration sensor  56 Y detects insufficient toner concentration in the toner container  54 Y, the driving motor  41 Y rotates according to a supply signal from a controller  57 Y. Because a spiral developer guiding groove  38 Y is formed on the inner surface of the bottle main body  33 Y, toner inside is carried from the back side to the end side of the resin case  34 Y by rotation. The toner inside the bottle main body  33 Y falls into a toner catcher (not shown) of the toner conveying device  40 Y through a discharge port (not shown) of the resin case  34 Y. The toner catcher is connected to the toner conveying pipe  43 Y, and when the driving motor  41 Y is rotated, a coil inside the toner conveying pipe  43 Y rotates synchronously with the rotation of the bottle main body  33 Y. The toner fell into the toner catcher by the rotation of the coil is conveyed through the toner conveying pipe  43 Y, and supplied to the toner supply port (not shown) of the toner container  54 Y in the developing device  5 Y. Thus, the toner concentration inside the developing device  5 Y is adjusted. 
         [0047]    Instead of arranging the concentration sensor  56 Y, a reference image can be formed on the photosensitive drum  1 Y and an optical sensor or a charge coupled device (CCD) camera to measure pixels of the reference image can be provided to supply toner based on a result of measurement. 
         [0048]    Salient features of the embodiment are explained next.  FIG. 7  is an enlarged view of part of the toner conveying device  40 Y serving as a powder conveying device for Y. As shown in  FIG. 7 , a conveyance coil  70 Y as a powder conveying member is arranged to contact the inner wall of the toner conveying pipe  43 Y as a powder conveying tube. A space between the toner conveying pipe  43 Y and the conveyance coil  70 Y is approximately 0.1 millimeter to 0.2 millimeter. 
         [0049]    The conveyance coil  70 Y inside the toner conveying pipe  43 Y applies a force to the toner such that the toner travels in the conveying direction. Thus, the toner can be prevented from accumulating inside the toner conveying pipe  43 Y. Accordingly, it is possible to prevent a malfunction caused by Y toner that has been accumulated inside the toner conveying pipe  43 Y flowing into the developing device  5 Y in the process cartridge  6 Y all at once. 
         [0050]    Furthermore, because influence of bend stress is small for a coil shape, even if the toner conveying pipe  43 Y is bent, the conveyance coil  70 Y can rotate. Accordingly, the toner conveying pipe  43 Y is not required to be arranged in a straight shape, which improves flexibility of layout and minimizes the developing device. 
         [0051]    Even if a conveying unit that has a screw-like axis instead of the conveyance coil  70 Y is used, the toner may be conveyed in a conveyance route that is not straight. However, compared with a conveying unit having an axis, a conveying unit having a coil is easier to be bent. Therefore, when the conveyance coil is used, a repulsive force to deformation at the time of rotating in a bent portion of the toner conveying pipe  43 Y becomes smaller. Thus, when the conveyance coil  70 Y is used, a sliding load with respect to the toner conveying pipe  43 Y can be reduced compared to a case where a conveying unit having an axis is used. 
         [0052]      FIG. 8  is a schematic diagram of a powder conveying tube. The powder conveying tube includes a sub-hopper  45  that is a large space, a sensor  46  that detects presence or absence of powder inside the sub-hopper  45 , and an agitator  47  that agitates powder in the sub-hopper  45  so that the powder does not harden. The agitator  47  is formed with an elastic material and is arranged such that the agitator  47  bites a detecting surface of the sensor. By rotation of the agitator  47 , the detecting surface of the sensor is scraped (cleaned). 
         [0053]      FIG. 9  is a schematic diagram of a supply motor  48 . As shown in  FIG. 9 , the single supply motor  48  drives a spare-container driving gear  49 , an agitator driving gear  50 , and a powder-conveying-member driving gear  58 . 
         [0054]    The supply motor  48  drives when the developing device  5 , which is a powder conveyance destination, requires powder, corresponding to a required amount. When an image having a large image area is processed, the driving time becomes long. The powder to be used has small diameter to improve image quality. If such powder having small particle diameter is stirred with air, volume of the powder increases and fluidity of the powder increases like liquid. On the other hand, if the powder is left for a long time, the powder enters in even a small space, the volume decreases, and the fluidity is deteriorated. If an image having a large image area is successively, toner in a sub-hopper is constantly mixed with air present in a small amount in space, and fluidity increases. If the fluidity increases, friction with the wall of the powder container or with the powder conveying member decreases, and even if driving is stopped, the powder is still conveyed (flowing) a little. This phenomenon causes accumulation and increase of space at the uppermost stream portion (near a sensor) in the sub-hopper. 
         [0055]    Furthermore, if a powder container that has been stored for a long time is set to the main unit, powder can harden near an outlet because of poor fluidity thereof, and may cause a state (blocking) in which the powder is not conveyed even if driving is applied. In such a case also, space at the uppermost stream portion (near a sensor) in the sub-hopper is accumulated to be increased. However, if the powder is kept agitated, the fluidity increases in a short time, and the powder is soon filled also in the accumulated space. 
         [0056]    In the conventional technologies, if the flowing of the powder in the sub-hopper and the blocking of the powder container occur at the same time, the amount of air at the portion near the sensor exceeds the threshold, and as a result, the sensor detects the air to cause the apparatus to erroneously recognize that there is “no toner”. 
         [0057]      FIG. 10  is a timing chart for comparing an example of ON/OFF control of the supply motor  48  with that of a conventional technology. In the example of  FIG. 10 , recording sheets in A4 size are successively fed, and it takes 1.6 seconds (sec) to feed or pass the respective recording sheets with an interval of 0.3 second between feeding of them. 
         [0058]      FIG. 10  depicts, as an example in which the flowing most significantly occurs, ON/OFF timing of the supply motor  48  for forming a single-color full-page solid image (image formed by adhering powder on the entire area of a page except page margins). It is assumed in this example that 0.40 gram of powder is required for a single-color full-page solid image. In the conventional technology, 0.2857 gram of powder is supplied per second, and driving of the supply motor is required for 1.4 seconds in total. The 1.4-second driving is divided into two times of driving: 1-second driving and 0.4-second driving, with a 0.3-second interval therebetween. The recording sheets are fed at 0.2-second intervals (OFF time). 
         [0059]    On the other hand, according to the embodiment, powder-supply speed is slowed down such that 0.2353 gram of powder is supplied per second, and the same amount, i.e., 0.40 gram of powder is slowly conveyed taking 1.7 seconds. Thus, the fluidity of powder in the sub-hopper is not excessively promoted, and the flowing does not occur. Although the more slowly the powder is conveyed, the higher effect is obtained, it is not desirable in terms of control if supply for a next page is performed continuously to the current supply without an interval. Therefore, considering an error of 0.1 second in measurement of control time and variation in the supply amount, OFF time of 0.2 second is provided so that the supply for the next page is not performed continuously to the current supply. The slowdown can be achieved either by reducing rotation of the motor or by increasing the number of teeth of the gear thereafter. 
         [0060]      FIG. 11  is a timing chart for comparing another example of ON/OFF control of the supply motor  48  with that of the conventional technology. While in the conventional technology, the recording sheets are fed at 0.2-second intervals, in the embodiment, the interval is extended to 0.5 second. Accordingly, OFF time (OFF ( 2 ) in  FIG. 11 ) of the supply motor  48  is extended by 0.2 second. 
         [0061]    Thus, when images having a large image area are successively processed, a time for powder in the sub-hopper  45  to be settled is given by extending the interval between feeding of recording sheets. This enables to ease the flowing of powder in the sub-hopper  45 . Because the extension of the interval between feeding of recording sheets decreases productivity of an image forming apparatus such as a copier and a printer, in this example, the interval is extended only when 80% or more of the area of an image is occupied by an image area. Thus, there is no influence of productivity decrease on regular images. 
         [0062]      FIG. 12  is a timing chart for comparing still another example of ON/OFF control of the supply motor  48  with that of the conventional technology. While in the conventional technology, 0.40 gram of powder is used for an A4 single-color full-page solid image as described above, in the embodiment, the upper limit of supply is defined so that up to 0.34 gram of powder is to be supplied. Therefore, the total ON time of the supply motor  48  is 1.2 seconds, and the second OFF time is extended by 0.2 second. This gives a time for powder in the sub-hopper  45  to be settled, and the flowing of powder in the sub-hopper  45  caused by continuous driving can be eased. 
         [0063]    As set forth hereinabove, according to an embodiment of the present invention, flowing can be suppressed, and the powder concentration in the developing device can be maintained stably. Thus, the amount of powder adhering on a photosensitive drum can be maintained uniformly. Therefore, density variation (excessively high density) of an output image can be suppressed. 
         [0064]    Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.