Abstract:
A developing device includes a developer container that contains a two-component developer including a carrier and a toner attached to the carrier through a frictional electrification. The developing device also includes a developing roller that supplies the two-component developer from the developer container to an image carrier. Further, the developing device includes a first developer supplying unit, and a second developer supplying unit that is aligned with the first developer supplying unit and located more distant from the developing roller than the first developer supplying unit is from the developing roller. The first developer supplying unit and the second developer supplying unit circularly convey the two-component developer while stirring the two-component developer to supply the two-component developer to the developing roller. An absolute rotation speed of the second developer supplying unit is changeable without a change in a rotation speed of the developing roller.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of and claims the benefit of priority under 35 U.S.C. §120 from U.S. Ser. No. 11/737,417, filed Apr. 19, 2007, and claims the benefit of priority under 35 U.S.C. §119 from Japanese Patent Application Nos. 2006-122117, filed Apr. 26, 2006, and 2006-205370, filed Jul. 27, 2006. The entire contents of all of these documents are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a developing device, and an image forming apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    In an image forming apparatus, such as a copier, a printer, a facsimile machine, or a printer, an electrostatic latent image formed on a photosensitive member as an image carrier is visualized by a developing device to obtain a visible image, and the visible image is transferred onto a sheet and output. 
         [0006]    Some image forming apparatuses have such a configuration that one photosensitive member is provided for only a single color, and others have such a configuration that a plurality of photosensitive members are provided to form an image of a plurality of colors. The latter configuration is used to form a multicolor image including a full-color image. 
         [0007]    Meanwhile, developers for use in development include a one-component developer containing only a magnetic or non-magnetic toner and a two-component developer with a toner and a carrier being mixed therein. 
         [0008]    The two-component developer includes a toner and a carrier that carries the toner. The toner is charged through frictional electrification caused by stirring and mixing to be in a state in which the toner can be electrostatically attached to the electrostatic latent image on the photosensitive member. 
         [0009]    Japanese Patent Application Laid-Open No. 2003-270933, for example, discloses a developing device that supplies a two-component developer. The conventional developing device includes rotatable screw augers as first and second developer supplying units. The first and second developer supplying units are arranged in parallel from a position close to a developing roller disposed near a photosensitive member as an electrostatic-latent-image carrier in a developer container that contains the developer. 
         [0010]    In the conventional developing device, one of the first and second screw augers that is near the developing roller supplies the developer to the developing roller as a main function, whilst the other one that is located away from the developing roller stirs new toner and carrier supplied to the storage unit for mixing as a main function. 
         [0011]    Therefore, the first developer supplying unit is required to ensure a sufficient amount of drawn-up developer for the developing roller. The second developer supplying unit is required to keep an appropriate mixing ratio between the toner and the carrier to be stirred so as not to deteriorate a charging characteristic of the toner. 
         [0012]    However, if the fluidity of the developer is decreased with time or the density of the toner is increased (abnormality in the supply amount occurs), an exchange of the developer between the first and second screw augers may become uneven (the balance of the supply amount of the developer between the screw augers is disturbed), which causes an abnormal image. That is, if the developer is decreased at the screw auger located near the developing roller, the amount of the developer to be drawn up is decreased, resulting in deterioration in image density and unevenness in development due to nonuniformity of the amount of the developer carried on the developing roller (stripes according to a screw pitch of the screw auger and unevenness in density). 
         [0013]    Conversely, if the developer is increased at the screw auger located away from the developing roller, the fluidity may be decreased, which causes unevenness in density and an increase in density deviation. 
         [0014]    It has been generally known that toner supply control is performed by forming a test pattern or the like through development and, based on the density of the test pattern, determining the density of the developer. However, management of the amount of developer and its deviation state in the developing device is not performed, and it is often the case that only the management control over the developer with the test pattern is performed, and management of the balance of the actual supply amount has not yet been performed. Moreover, even if the balance is disturbed, there is no solution to this problem under the present circumstances. 
         [0015]    Many models of electrophotography devices in recent years have adopted a two-component development scheme using a powder toner and a carrier. Reasons for this include high durability and high responsivity to speedup, and that is why this scheme has been widely spread. 
         [0016]    However, unlike the one-component development scheme, this scheme requires a mechanism for evenly mixing the toner and the carrier. Therefore, various contrivances are provided to the developing device of the two-component development scheme. 
         [0017]    For example, a stirring member is provided to efficiently mix the toner and carrier, or a toner supply position is changed or a conveyor route is increased so as to increase the time of mixing the developer. More specifically, for example, a two-axis conveying scheme using two conveyor screws has been widely known and spread. 
         [0018]      FIG. 13  is a schematic diagram of a general developing device  2  of a two-component developing system. The developing device is explained as having a typical configuration including two screws and one developing roller. However, the developing device is not meant to be restricted to such a configuration. 
         [0019]    The developing device  2  includes a developing roller  3  and two developer conveyor screws  14  and  15 . Of these developer conveyor screws  14  and  15 , the one closer to the developing roller  3  is called a first developer conveyor screw  14  and the other closer to a toner supply opening (not shown) is called a second developer conveyor screw  15 . 
         [0020]    On the developer conveyor screws  14  and  15 , a two-component developer containing a toner and a carrier is input in an evenly mixed state, and is circulated as being stirred between the two developer conveyor screws  14  and  15 . This is a contrivance to efficiently stir the toner input from the toner supply opening provided at an end of the second developer conveyor screw  15 . 
         [0021]    In a one-axis conveyance scheme not depicted, the developer is drawn up by the developing roller to form an ear of the developer by a magnet incorporated in the developing roller. The ear strokes the surface of the photosensitive member opposing thereto with a predetermined distance being kept, thereby developing the toner onto the photosensitive member through an applied developing bias. 
         [0022]    In the two-axis conveyance scheme explained above, ideally, the first developer conveyor screw  14  has a sufficient amount of developer to be sufficiently drawn up by the developing roller, and the second developer conveyor screw  15  has a sufficient amount required for stirring the toner supplied for achieving the original purpose and the developer. 
         [0023]    For example, Japanese Patent Application Laid-Open Nos. 2003-270933 and 2005-227316 have proposed technologies related to the two-axis conveyor scheme for developer. Those technologies aim at ideally supplying to the first developer conveyor screw on a developer sleeve side a sufficient amount of developer to be drawn up by the developing roller and supplying to the second developer conveyor screw a sufficient amount of developer required to stir the toner and the developer. 
         [0024]    Japanese Patent Application Laid-Open No. 2003-270933 describes that, in order to reduce stress onto the toner, a developer conveyor screw and a developing roller can be drive-controlled at arbitrary timing. In the conventional technology, rotation is stopped in order to reduce a developer stirring time as much as possible other than the developing operation. Japanese Patent Application Laid-Open No. 2005-227316 describes that, in order to start up the developer in a short time, a developer conveyor screw is driven earlier than usual at the time of power-on or recovery from sleep. 
         [0025]    However, in the conventional technology explained in connection with  FIG. 13 , because of a decrease in fluidity of the developer with time, an increase in the amount of developer due to an increase in density of the toner, and the like, an appropriate balance of the amount of developer between the first and second developer conveyor screws  14  and  15  may be often decreased. With such a state, there is a problem of occurrence of an inconvenience on the image. 
         [0026]    When the developer on the first developer conveyor screw  14  side is decreased, the amount of drawing up by the developing roller  3  is decreased, thereby causing a decrease in image density and a screw-pitched abnormal image. 
         [0027]    Conversely, when the developer on the first developer conveyor screw  14  side is increased, fluidity is decreased, thereby causing unevenness in density on the right and left sides of the image and an increase in image density deviation in a page. Moreover, since the amount of developer and the state of deviation cannot be known from outside, only the abnormal image has to be used for determination. Therefore, there is no solution even when any problems explained above occur. 
       SUMMARY OF THE INVENTION 
       [0028]    It is an object of the present invention to at least partially solve the problems in the conventional technology. 
         [0029]    According to an aspect of the present invention, a developing device includes a developer container that contains two-component developer including carrier and toner attached to the carrier through frictional electrification, a developing roller that supplies the two-component developer from the developer container to an image carrier, a first developer supplying unit, and a second developer supplying unit that is aligned with the first developer supplying unit and located more distant from the developing roller than the first developer supplying unit is from the developing roller. The first developer supplying unit and the second developer supplying unit circularly convey the two-component developer while stirring the two-component developer to supply the two-component developer to the developing roller. The first developer supplying unit supplies a first amount of two-component developer, and the second developer supplying unit supplies a second amount of two-component developer different from the first amount. 
         [0030]    According to another aspect of the present invention, a developing device includes a developing roller that carries developer consisting of two component, a developer container that contains the developer, a first rotating member that is located near the developing roller in the developer container, a second rotating member that is located more distant from the developing roller than the first rotating member is from the developing roller, a first adjusting unit that deforms to adjust supply of the developer from the first rotating member to the second rotating member, and a second adjusting unit that deforms to adjust supply of the developer from the second rotating member to the first rotating member. The first rotating member and the second rotating member circularly convey the developer while stirring the developer in the developer container to supply the developer to the developing roller. 
         [0031]    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 
         [0032]      FIG. 1  is a schematic diagram of an image forming apparatus according to a first embodiment of the present invention; 
           [0033]      FIG. 2  is a schematic diagram of a developing device shown in  FIG. 1 ; 
           [0034]      FIG. 3  is a perspective view of relevant part of the developing device; 
           [0035]      FIG. 4  is a side view for explaining a driving system of the developing device; 
           [0036]      FIG. 5  is a block diagram of a controlling unit of the developing device; 
           [0037]      FIG. 6  is a schematic diagram for explaining sensors shown in  FIG. 5 ; 
           [0038]      FIGS. 7 and 8  are schematic diagrams for explaining changes in the amount of developer; 
           [0039]      FIG. 9  is a schematic diagram of a developing device according to a second embodiment of the present invention; 
           [0040]      FIG. 10  is a perspective view of a first screw or a second screw moved upward or downward to change height positions of the screws; 
           [0041]      FIG. 11  is a schematic diagram of a partition plate that swings to open and close an opening; 
           [0042]      FIG. 12  is a schematic diagram of a partition plate that vertically moves to open and close the opening; and 
           [0043]      FIG. 13  is a schematic diagram of a general developing device of a two-component developing system. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. In the following, the present invention is explained taking as an example a tandem-type color printer capable of forming a full-color image. The image forming apparatus is not meant to be restricted to such a printer, but can be a copier, a facsimile machine, a printer, and the like. 
         [0045]      FIG. 1  is a schematic diagram of an image forming apparatus  120  according to a first embodiment of the present invention. The image forming apparatus  120  includes image forming devices  121 Y,  121 C,  121 M, and  121 K, a transfer device  122 , a feeding tray (not shown), a feeding cassette  124 A, a resist roller  130 , and a fixing device  110 . 
         [0046]    The image forming devices  121 Y,  121 C,  121 M, and  121 K each form an image for a relevant color according to an original image. The transfer device  122  is located opposite to each of the image forming devices  121 Y,  121 C,  121 M, and  121 K. The feeding tray and the feeding cassette  124 A are mounted on a feeding device  124  as supplying units that supply a recording medium to a transfer area opposed to the transfer device  122 . The resist roller  130  supplies a recording medium conveyed from the feeding tray or the feeding cassette  124 A at an image formation timing by the image forming devices  121 Y,  121 C,  121 M, and  121 K. The fixing device  110  fixes an image on a sheet-like recording medium in the transfer area after transfer. 
         [0047]    In the fixing device  110 , although not explained in detail, a belt fixing scheme is used including a fixing roller and a pressure roller that are opposed to each other and a fixing belt spread over each of the fixing roller and the heating roller. With the fixing belt heated by a heating roller being in contact with a sheet passing through a fixing nip portion formed by the fixing roller and the pressure roller, thereby melting an unfixed toner image through heat and pressure for fixing through penetrating action. The configuration of the fixing device  110  can be of a heat roller fixing scheme using rollers for heating and pressure instead of using a belt. 
         [0048]    In the transfer device  122 , a belt spread over a plurality of rollers as a transfer member (hereinafter, a transfer belt)  122 A is used. At positions opposing to photosensitive drums of the respective image-forming devices, transfer bias units  122 Y,  122 C,  122 M, and  122 K which apply the transfer bias are disposed. With a transfer bias having a polarity reverse to that of the toner being activated, toner images formed by the respective image forming devices are sequentially superposed for transfer. 
         [0049]    In the transfer device  122 , a secondary transfer bias unit  122 F for collectively transferring the toner images superposed for transfer on the transfer belt  122 A is disposed on a conveyor route of a recording medium. 
         [0050]    The image forming devices  121 Y,  121 C,  121 M, and  121 K perform development for colors of yellow, cyan, magenta, and black, respectively. Although using different toner colors, these devices are identical in configuration, and therefore the configuration of the image forming device  121 K is explained as a typical configuration among the image forming devices  121 Y,  121 C,  121 M, and  121 K. 
         [0051]    As shown in  FIG. 2 , the image forming device  121 K includes a photosensitive drum  125   k  as an image carrier, and also includes a charging device  127 K, a developing device  126 , and a cleaning device  128 K arranged in this order along a rotating direction of the photosensitive drum  125 . Between the charging device  127 K and the developing device  126 K, an electrostatic latent image according to image information corresponding to the color obtained through color separation by writing light  129 K from a writing device  129  (refer to  FIG. 1 ) is formed. The cleaning device  128 K includes, in addition to a known cleaning blade, a lubricant applying mechanism  128 K 1  for increasing foreign-matter removal efficiency. 
         [0052]    As an image carrier, a belt-shaped member may be used in place of a drum-shaped member. Devices for image formation disposed around these photosensitive drums are collectively accommodated in a process cartridge (represented as a reference character PC for convenience) having a unit structure provided with a box as shown in  FIG. 2 . 
         [0053]    In the image forming apparatus  120 , one of the rollers over which the transfer belt  122 A for use in the transfer device  122  is spread (the roller represented by a reference character  122 A 1  in  FIG. 1 ) has an axial center taken as a base point, and the transfer device  122  is tilted so that a downstream side in a direction in which an extension surface of the transfer belt  122 A facing the image forming devices  121 Y,  121 C,  121 M, and  121 K moves is positioned lower than an upstream side, that is, the position of the roller  122 A 1 . With this, the occupied space of the transfer device  122  in the lateral direction is reduced to downsize the image forming apparatus. 
         [0054]    In the image forming apparatus  120  having the configuration explained above, image formation is performed through the following processes and conditions. In the following explanation, the image forming device  121 K for image formation using black toner is explained as a representative of the image forming devices, and it is assumed that this explanation can similarly apply to the other image forming devices. 
         [0055]    At the time of image formation, the photosensitive drum  125 K is driven for rotation by a main motor not shown, and is subjected to static elimination by an alternating-current (AC) bias applied to the charging device  127 K (with 0 direct-current (DC) component), with its surface potential being set at a reference potential of approximately −50 volts. 
         [0056]    Next, the photosensitive drum  125 K is applied with a DC bias having superposed thereon an AC bias on the charging device  127 K, thereby being uniformly charged at a potential approximately equal to that of the DC component, and its surface potential is charged at approximately −500 volts to −700 volts (a target charge potential is determined by a process controlling unit). 
         [0057]    After the photosensitive drum  125  is uniformly charged, a writing process is carried out. An image to be written is written by using the writing device  129  according to digital image information from a controller unit not shown for forming an electrostatic latent image. That is, in the writing device  129 , laser light from a light source that emits light based on a light emitting signal for laser diode binarized for each color corresponding to the digital image information is applied through a cylinder lens (not shown), a polygon mirror  129 A, an fθlens  129 B, first to third mirrors, and a WTL lens onto the photosensitive drum that carries an image for a relevant color, in this case, the photosensitive drum  125 K, for convenience. With this, the surface potential of a light-applied portion on the surface of the photosensitive drum becomes approximately-50 volts, thereby forming an electrostatic latent image corresponding to the image information. 
         [0058]    The electrostatic latent image formed on the photosensitive drum  125 K is subjected to a visible image process by the developing device  126 K using a toner having a complementary color relation with the color obtained through color separation. In the developing process, DC with an AC bias superposed on a developing sleeve of −300 volts to −500 volts is applied, thereby developing the toner only in an image portion with its potential being decreased due to radiation with writing light (Q/M: −20 to −30 μC/g) to form a toner image. 
         [0059]    The toner images of the respective colors subjected to a visible image process through the developing process are transferred to the recording medium let out with its resist timing being set by the resist roller  130 . The recording medium is electrostatically attached to the transfer belt  122 A with application of a bias for attachment by a bias unit for sheet attachment formed of a roller before the recording medium reaches the transfer belt  122 A. 
         [0060]    The transfer belt  122 A has electrostatically transferred thereon the toner images from the photosensitive drums through application of a bias with a polarity reverse to that of the toner by the relevant one of the transfer bias units  122 Y,  122 C,  122 M, and  122 K included in the transfer device  122  at a position facing to the photosensitive drum in each image forming device. The superposed and transferred toner images are then collectively transferred to the recording medium by the secondary transfer bias unit  122 F. 
         [0061]    The recording medium having the images of the respective colors collectively transferred thereto is self-stripped from the transfer belt  122 A by using an edge-surface curvature of a roller on a side facing to the secondary transfer bias unit  122 F from among rollers of the transfer belt unit, and is then conveyed toward the fixing device  110 . With the recording medium passing through the fixing nip formed of the fixing belt and the pressure roller, the toner image is fixed to the recording medium, and then the recording medium is delivered to a paper delivery tray  132 . 
         [0062]    In the image forming apparatus  120 , not only image formation on one side of the recording medium to be delivered after fixing but also image formation on both sides thereof can be performed. At the time of image formation on both sides, the recording medium after fixing is conveyed to a reverse circulation route RP, and is let out by a supply roller RP 1  toward the resist roller  130 , the supply roller RP 1  being positioned at an end of this circulation route to serve also as a sheet supplying unit from a feeding tray. Switching the conveying route for the recording medium between image formation on one side and image formation on both sides is performed by a conveyor route switching nail (not shown) disposed in a rearward position of the fixing device  110 . 
         [0063]    Being used for the image forming apparatus  120  having the configuration as explained above, the developing device  126 K that brings the developer in contact with the photosensitive drum  125 K includes, as shown in  FIG. 2 , a developer housing  101 A forming a developer container in a box  101  forming the process cartridge PC. In the developer housing  101 A, rotatable first and second screws  126 K 2  and  126 K 3  are used as first and second developer conveyor members in the order near a developing roller  126 K 1 . 
         [0064]    The first and second screws have their rotating directions being set in reverse so as to be able to stir the developer in reverse directions in the developer housing  101 A for transport (conveyance). 
         [0065]    Of these screws, the first screw  126 K 2  is used to supply the developer to the developing roller  126 K 1 , whilst the second screw  126 K 3  is used for stirring a new toner of black supplied from a relevant one of supply toner tanks represented by reference characters T 1  to T 4  in  FIG. 1  and a carrier for friction charge on the toner.  FIG. 3  is a perspective view of relevant part of the developing device  126 K. The first and second screws  126 K 2  and  126 K 3  can be drive-controlled independently. 
         [0066]      FIG. 4  depicts a mechanism for independent drive control. In  FIG. 4 , the developing roller  126 K 1  and the first screw  126 K 2  are driven by the same driving source and the second screw  126 K 3  is driven by using a driving source different from that on the side of the developing roller  126 K 1  and the first screw  126 K 2 . That is, in  FIG. 4 , a drive-side gear  126 K 1   a  provided to a rotational shaft of the developing roller  126 K 1  operates in conjunction with a driven-side gear  126 K 2   b  provided to a rotational shaft  126 K 2   a  of the first screw  126 K 2  via an idle gear  126 Kb. On the other hand, a gear  126 K 3   b  provided to a rotational shaft  126 K 3   a  of the second screw  126 K 3  is driven by an independent drive motor (not shown). 
         [0067]    The first and second screws  126 K 2  and  126 K 3  are drive-controlled independently. Therefore, as for the rotation speed, the absolute speed of the second screw  126 K 3  can be changed. That is, the driving sources of the first and second screws  126 K 2  and  126 K 3  have their rotation speeds set independently by a controlling unit  500  shown in  FIG. 5 . 
         [0068]    The controlling unit  500  is a unit that executes sequence programs, such as those for setting image forming conditions including toner supply control through image density detection. The controlling unit  500  has its input side to which a density sensor  501  for detecting a density of a density-detection test pattern formed on the photosensitive drum  125 K and developer-amount sensors  502  to  504  provided at a developer container are connected and its output side to which driving units  505  and  506  as driving sources forming supply amount adjusting units for the first and second screws  126 K 2  and  126 K 3  are connected. 
         [0069]    For the density sensor  501 , a light-receiving sensor capable of detecting reflected light from the density-detection test pattern formed on a non-image portion of the photosensitive drum  125 K is used for supplying toner to the developer, controlling a developing bias that defines an image formation condition, and others according to the detection result. 
         [0070]    The developer-amount sensors  502  to  504  are provided, as shown in  FIG. 6 , to a plurality of positions corresponding to the center in an axial direction parallel to an extending direction of the developing roller  126 K 1 , that is, in an axial direction in the first and second screws parallel to an axial direction of the developing roller  126 K 1 , and positions near image-area boundaries other than the center. These set-up positions correspond to the center of the image area in a horizontal direction and both boundary ends thereof. 
         [0071]    In the controlling unit  500 , the developer-amount sensors  502  to  504  each detect a height of the developer surface (corresponding to a draught surface) on the first screw  126 K 2  side in the developer container at arbitrary timing to find a developer amount. Based on the result of comparison between the detection result and a predetermined value, a threshold, the rotation speed of the second screw  126 K 3  is changed. 
         [0072]    That is, when the height of the developer surface on the first screw  126 K 2  side is below the threshold, it is determined that the developer amount is small on the first screw  126 K 2  side, and the absolute speed of the second screw  126 K 3  is increased to correct this situation. With this, the developer amount transported to the first screw  126 K 2  is increased, thereby increasing the height of the developer on the first screw  126 K 2 . 
         [0073]    Conversely, when the height of the developer surface is above the threshold, it is determined that the developer amount is large, and the absolute speed of the second screw  126 K 3  is decreased. With this, the developer amount on the first screw  126 K 2  side is made appropriate. 
         [0074]    According to the first embodiment, the first and second screws  126 K 2  and  126 K 3  are drive-controlled independently at arbitrary timing. Therefore, such control may be performed at the time of developing operation. Thus, unlike the case where independent control is performed only during periods other than the period of developing operation, the balance of the developer supply amount can always be made appropriate. This is also true for the case in comparison with the configuration in which the rotation speed is changed between the screws only at the time of starting up the image forming apparatus. Therefore, unlike such conventional independent control, the developer supply balance can also always be monitored, and control can be performed based on the monitoring result. Thus, the occurrence of an abnormal image because the developer supply balance is disturbed and the occurrence of density unevenness between right and left of the image area can be prevented. Furthermore, the configuration for stabilizing the developer supply amount, that is, the configuration for keeping the balance of the supply amount between the first and second screws, can be achieved without additional providing a special member or device but only using the existing components, that is, the screws and their driving sources and the driving unit as a unit of adjusting the driving sources. 
       EXAMPLES 
       [0075]    Tests are conducted under the following conditions: 
         [0000]    (A) Linear velocity of the photosensitive drum: 180 mm/sec
 
(B) Linear velocity ratio between the photosensitive drum and the developing roller: variable range of 0.5 to 3.0
 
(C) Development gap: variable range of 0.25 millimeters to 0.50 millimeters
 
(D) Carrier for use: Iron powder carrier having a mass average particle diameter of 35 micrometers
 
(E) Toner density of the developer: approximately 7 weight percent
 
(F) Developing bias: DC bias
 
       Example 1 
       [0076]      FIG. 7  is a schematic diagram for explaining change in the amount of developer in Example 1. Specifically,  FIG. 7  depicts a state before the rotation speed of the second screw  126 K 3  is changed (A), and a state after the rotation speed of the second screw  126 K 3  is changed (B). 
         [0077]    In  FIG. 7 , a line of “threshold” indicates a developer supply distribution. When the developer contained in the developer container on the first screw  126 K 2  side is shifted to the developer container at which the second screw  126 K 3  is positioned ( FIG. 7  (A)), an entire solid image with the entire image area being taken as an image portion (a solid image formed in this case is hereinafter referred to as an image A) is formed. It is assumed in this case that the developer is not shifted in a horizontal direction of the image area (the state where the developer amount is not shifted in a horizontal direction with reference to the threshold in  FIG. 7A ). 
         [0078]    Next, the rotation speed of the second screw  126 K 3  is increased to continue transport of the developer until all of the developer-amount sensors  502  to  504  provided for the first screw  126 K 2  detect the threshold. At this time, as with the case explained above, an entire solid image is formed (this image is hereinafter referred to as an image B). 
         [0079]    The inventors compared the image densities of the images A and B to obtain the result of improvement such that the ID (image density) of the image A was 0.8, whilst the ID of the image B was 1.5. The inventors also confirmed that screw pitch unevenness or the like did not occur ( FIG. 7  (B)). It can be seen that, by increasing the supply amount from the second screw  126 K 3 , the uniform amount of developer becomes present over the entire image area of the first screw  126 K 2 . Note that a two-dot chain in (B) of  FIG. 7  represents the developer amount shown in (A) of  FIG. 7 . 
       Example 2 
       [0080]      FIG. 8  is a schematic diagram for explaining change in the amount of developer in Example 2. Specifically,  FIG. 8  depicts a state before the rotation speed of the second screw  126 K 3  is changed (A) and a state after the rotation speed of the second screw  126 K 3  is changed (B). In  FIG. 8 , as in  FIG. 7 , a line of “threshold” indicates an amount of developer. 
         [0081]      FIG. 8  depicts a case where the developer amount in a horizontal direction of the image area is shifted. When, with the right side of  FIG. 8  representing a back side of the developing roller  126 K and the left side representing a front side thereof, the image density on the back side is lower than that on the front side ( FIG. 8  (A)), an entire solid image similar to that in the case of  FIG. 7  is formed (this image is hereinafter referred to as an image C). 
         [0082]    Next, with the rotation speed of the second screw  126 K 3  being decreased, the rotation is kept until the developer amount on the first screw  126 K 2  side reaches the threshold to form an entire solid image (this image is hereinafter referred to as an image D). 
         [0083]    In comparison of image density between the images C and D obtained under the both conditions, as shown in (B) of  FIG. 8 , the transport amount is shifted from the front side to the back side of the image area at the second screw  126 K 3  through a decrease in rotation speed of the second screw  126 K 3 , thereby eliminating unevenness of the developer on the entire image area. The improved result was obtained such that, while the IDs (image densities) of the image C were 1.5 on the front side and 1.0 on the back side, the IDs of the image D were 1.5 on both of the front and back sides. With this, the developer amounts in the horizontal direction of the image area, that is, on the back and front sides, were able to be equalized to improve the image density. Note that a two-dot chain in (B) of  FIG. 8  represents the developer amount shown in (A) of  FIG. 8 . 
         [0084]    Therefore, according to the embodiment, with rotatable screws being used as first and second developer supplying units, the occurrence of an abnormal image can be prevented through a simple control of only changing the rotation speed with the use of the existing developer-supply components without adding a special structure. 
         [0085]    Also, the controlling unit having provided on its input side a density detecting unit that detects a density of a visible image subjected to a visible image process by the developing roller controls the first and second developer supplying units independently. With this, the occurrence of an abnormal image can be prevented according to changes in the state of image density. 
         [0086]    Furthermore, a sensor that detects a developer amount in the developer container is provided for each of the positions near the center and both boundary ends of the image area. Therefore, by detecting the developer amount in the entire image area to detect an uneven state of the developer in the image area, the uneven state can be eliminated. 
         [0087]    Still further, with such a developing device being incorporated in an image forming apparatus, the occurrence of an abnormal image can be prevented by making the developer supply balance appropriate. 
         [0088]      FIG. 9  is a schematic diagram of a developing device according to a second embodiment of the present invention. According to the second embodiment, the problems in the developing device explained above can be avoided. 
         [0089]    Specifically, the first screw (first shaft)  126 K 2  and the second screw (second shaft)  126 K 3 , which have been driven simultaneously, are driven independently, and the rotation speed of the second screw  126 K 3  is changed according to the developer amount of the first screw  126 K 2 , thereby making it possible to always keep the developer amount at one shaft appropriate. 
         [0090]    In a developing device of  FIG. 9 , the developing roller  126 K 1  receives mobile power via the drive transmission gear  126 Kb receiving drive power from the image forming apparatus body ( FIG. 1 ) via the drive gear  126 K 1   a  mounted at one end of the developing roller  126 K 1  and, via the drive transmission gear  126 K 2   b , further transfers the mobile power to the gear  126 K 2   b  of the first screw  126 K 2  as a first rotator. The developing roller  126 K 1  and the first screw  126 K 2  are driven simultaneously. 
         [0091]    The second screw  126 K 3  as a second rotator having the drive gear  126 K 3   b  is driven by a driving unit not shown independently of the developing roller  126 K 1  and the first screw  126 K 2 , and can be rotated at a speed different from the speed of the developing roller  126 K 1  and the first screw  126 K 2 . 
         [0092]    As shown in  FIG. 9 , the developing device is provided with a (variable) first adjusting unit  2   b  that deforms so as to adjust the capability of supplying the developer from the first screw  126 K 2  disposed near the developing roller  126 K 1  to the second screw  126 K 3  disposed at a position away from the developing roller  126 K 1  by a distance longer than a distance between the first screw  126 K 2  and the developing roller  126 K 1 , and a second adjusting unit  2   c  that deforms so as to adjust the capability of supplying the developer from the second screw  126 K 3  to the first screw  126 K 2 . 
         [0093]    Near the first screw  126 K 2  in a developer container  2   a , the developer-amount sensors  502  to  504  are mounted for measuring and detecting the developer amount, and sample a draught surface (height of the developer surface) of the developer of the first screw  126 K 2  at an arbitrary cycle. 
         [0094]    For example, if the draught surface of the developer is below a certain threshold, it is determined that the developer amount of the first screw  126 K 2  is small, and the rotation speed of the second screw  126 K 3  is increased in order to increase the developer on the first screw  126 K 2 . With that, the developer on the second screw  126 K 3  side flows into the first screw  126 K 2  side, thereby increasing the height of the draught surface of the developer on the first screw  126 K 2  side. 
         [0095]    Conversely, if the draught surface of the developer is above a certain threshold, it is determined that the developer amount of the first screw  126 K 2  is large, and the rotation speed of the second screw  126 K 3  is decreased. In this manner, through adjustment as appropriate so that the developer amount of the first screw  126 K 2  (first shaft) is appropriate, an excellent image can be obtained over a period of time without a decrease in ID, screw-pitch unevenness, image density deviation in a horizontal direction, or others. 
         [0096]    In the second embodiment, a threshold of the developer amount on the first screw  126 K 2  side is provided, and the rotation speed of the second screw  126 K 3  is changed when the developer amount is below or above the threshold. When the developer amount on the first screw  126 K 2  side is desired to be changed successively, the rotation speed of the second screw  126 K 3  can be changed successively based on the values read by the developer-amount sensors  502  to  504 . 
         [0097]    Other schemes for changing the capability of supplying the developer between the first screw  126 K 2  and the second screw  126 K 3  include changing the opening size of an opening between the first screw  126 K 2  and the second screw  126 K 3  and changing a height position of the screw. 
         [0098]      FIG. 10  is a schematic diagram for explaining change in screw-height position by vertically moving the first or second screw. In the second embodiment, to change the height position of the developer conveyor screws  126 K 2  or  126 K 3  (first or second screws  126 K 2  and  126 K 3 ), cams  19  that rotates using an appropriate scheme are disposed under the first screw  126 K 2  or the second screw  126 K 3  so as to vertically move the screw. The capability of supplying the developer at this time is determined by a height position of the developer conveyor screw and a height of the developer. 
         [0099]      FIG. 11  is a schematic diagram of a partition plate that swings to open and close an opening. As shown in  FIG. 11 , a partition plate  20  capable of swinging is provided between the first screw  126 K 2  and the second screw  126 K 3  at a position where an opening (not shown) can be open or closed. The partition plate  20  is swung as required to open and close the opening. 
         [0100]      FIG. 12  is a schematic diagram of a partition plate that vertically moves to open and close the opening. As shown in  FIG. 12 , the partition plate  20  provided between the first screw  126 K 2  and the second screw  126 K 3  is moved vertically by an appropriate mechanism in a longitudinal direction not shown, thereby achieving opening and closing the opening (not shown). Although not shown, the opening and closing method and direction of the partition plate can be changed for each of the first and second adjusting units. 
         [0101]    A test was conducted by using an image forming apparatus having a photosensitive drum in which a two-component developer was conveyed and developed by a developing roller including a fixed magnet roller and the developing bias was DC under the following conditions:
       Liner velocity of the photosensitive drum: 180 mm/sec   Linear velocity ratio between the photosensitive drum and the developing roller: variable range of 0.5 to 3.0   The amount of drawing up the developer by the developing roller: 55 to 60 mg/cm 2      Development gap: variable range of 0.25 millimeters to 0.50 millimeters   Carrier: Iron powder carrier having a mass average particle diameter of 35 micrometers   Toner density of the developer: approximately 7 weight percent   Developing bias: DC bias       
 
         [0109]    In Example 1, the developer in the developer container on the first screw  126 K 2  ( FIG. 9 ) side was shifted to the developer container on the second screw  126 K 3  ( FIG. 9 ) side, and then the entire solid image A was output. 
         [0110]    Next, the rotation speed of the second screw  126 K 3  was increased, and an amount of the developer in the developer container on the first screw  126 K 2  side is increased until the values of the three developer-amount sensors  502  to  504  ( FIG. 9 ) (one disposed at each of the positions near both boundaries of the image area in a horizontal direction and one disposed at a position near the center of the developing roller) become a threshold specified in advance. However, no deviation in developer amount was provided in the horizontal direction. Then, the entire solid image B was output. 
         [0111]    The number of developer-amount sensors is cited by way of example and without limitation, and any number of sensors can be utilized. 
         [0112]    As a result of measurement of the output images A and B, the image A had an ID of 0.80 with the occurrence of screw pitch unevenness, whilst the image B had an ID of 1.50 without the occurrence of screw pitch unevenness. This indicates that the occurrence of screw pitch unevenness and reduction in ID do not occur if the developer amount on the first screw  126 K 2  side is above the certain threshold, but image quality is significantly degraded if the developer amount is below the threshold. 
         [0113]    In the second embodiment, a deviation in ID between right and left sides was produced due to a factor other than a factor associated with the developing device, and the entire solid image C was output with its ID on the back side lower than that on the front side. Next, the rotation speed of the second screw  126 K 3  ( FIG. 9 ) was decreased to shift the developer in the developer container on the first screw  126 K 2  ( FIG. 9 ) side from the front side to the back side until the values of the three developer-amount sensors  502  to  504  ( FIG. 9 ) became the threshold specified in advance to output the entire solid image D. 
         [0114]    Then, as a result of measurement of the output images C and D, the image C had an ID on the front side of 1.5 and an ID on the back side of 1.0, whilst the image D had an ID on the front side of 1.5 and an ID on the back side of 1.5. As such, the deviation of the right and left IDs was improved. 
         [0115]    As set forth hereinabove, according to an embodiment of the present invention, the supply amount of the first and second developer supplying units can be controlled independently. Therefore, a shortage of developer at the first developer supplying unit can be solved, and also a state in which the supply amount is uneven over the entire supply area can be solved. Therefore, by preventing deterioration in image density due to a decrease in the amount of drawing up the developer and preventing the occurrence of unevenness in density on the right and left sides of the image area, the occurrence of an abnormal image can be avoided in advance. 
         [0116]    Moreover, the developer can be shifted to the photosensitive member side, that is, the developing roller side. Therefore, a decrease in image density in association with a decrease in the drawn-up amount due to supply shortage of the developer to the developer can be prevented. Furthermore, reduction in screw pitch unevenness due to unevenness of the developer supplied to the developing roller. 
         [0117]    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.