Patent Publication Number: US-8121498-B2

Title: Image forming apparatus and developer supply method therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent specification claims priority from Japanese Patent Application No. 2007-275555, filed on Oct. 23, 2007 in the Japan Patent Office, the entire contents of which are hereby incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an image forming apparatus, such as a facsimile machine, a copier, a printer, a multifunction machine including at least two of those functions, etc., and a developer supply method therefor. 
     2. Discussion of the Background 
     In general, an electrophotographic image forming apparatus, such as a copier, a printer, a facsimile machine, a multifunction machine including at least two of those functions, etc., includes an image forming mechanism for forming an electrostatic latent image on an image carrier, developing the latent image with developer, and transferring the developed image (toner image) onto a recording medium. As the developer, two-component developer in which toner and magnetic carrier are mixed is widely used. 
     The electronographic image forming apparatus has a developing unit, which typically includes a developing roller serving as a developer carrier that supplies the image carrier with the developer. The developing roller is partly exposed from an opening in the developing unit, and therefore, in such an image forming apparatus, the developer might spill out from the opening in the developing unit, particularly if a developer container part of the developing unit is filled with the developer before shipment and the developer container is shaken or the image forming apparatus tilts during transport. Further, the developer might deteriorate by being exposed to air. Therefore, the developer is typically put in the developer container part at a user&#39;s site. 
     In addition, because the magnetic carrier deteriorates over time and thus development capability is impaired with repeated use of the two-component developer, the developer including the degraded magnetic carrier should be periodically replaced. 
     As a typical developer replacement method, maintenance personnel visit the user each given cycle in order to collect the degraded developer and replenish the developing unit with unused developer. human error is inherent in such a method. 
     Herein, human error means the maintenance personnel might supply unused developer to the developer container part without removing the degraded developer therefrom, which is hereinafter referred to as redundant replenishment. Further, the maintenance personnel might forget having already filled it with the developer, and supply redundant developer to a developing unit of a newly installed image forming apparatus or a developing unit from which the degraded developer is removed. 
     As another example of human error, in a case of a color image forming apparatus including multiple developing units respectively corresponding to multiple color toners, the maintenance personnel might set a developer bottle of the wrong color to a developer supply port of the developing unit to which unused developer is to be supplied. 
     If redundant replenishment occurs, developer will spill over from the developing unit, and the developer remaining in the developer bottle will be spilled over the image forming apparatus when the developer bottle is removed from the developing unit, which might contaminate and damage the image forming apparatus. 
     Accordingly, there is a need to prevent such human error, as well as shorten a time period required to fill the developing unit with the developer and distribute the developer uniformly therein. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, in one illustrative embodiment of the present invention, an image forming apparatus includes an latent image carrier configured to carry a latent image thereon, a developing unit disposed facing the latent image carrier to develop the latent image with developer, a developer container configured to contain the developer and be attached to the image forming apparatus, a developer detector configured to detect the presence of the developer in the developing unit, and a developer supply controller. The developer supply controller prohibits supply of the developer from the developer container to the developing unit when the developer detector detects that the developer is present in the developing unit. 
     Another illustrative embodiment of the present invention describes a developer supply method used in the image forming apparatus described above. The developer supply method includes determining whether or not the developer is present in the developing unit, and prohibiting supply of the developer to the developing unit from the developer container when the developer is determined to be present therein. 
    
    
     
       BRIEF DESCRIPTION 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  schematically illustrates a configuration of an image forming apparatus according to an illustrative embodiment of the present invention; 
         FIG. 2  schematically illustrates configurations of a developing unit and a photoreceptor; 
         FIG. 3  schematically illustrates a flow of developer inside the developing unit shown in  FIG. 2 ; 
         FIG. 4  is a perspective view illustrating the developing unit shown in  FIG. 2 ; 
         FIG. 5  is a perspective view illustrating a toner supply unit; 
         FIG. 6  is a cross section view illustrating the toner supply unit shown in  FIG. 5 ; 
         FIG. 7  is a perspective view illustrating a toner bottle; 
         FIG. 8  illustrates setting of a black toner bottle on the toner supply unit shown in  FIG. 5 ; 
         FIG. 9  is a perspective view of the image forming apparatus shown in  FIG. 1 ; 
         FIG. 10  illustrates a full engagement mode in which an intermediate transfer belt contacts all photoreceptors; 
         FIG. 11  illustrates a partial disengagement mode in which the intermediate transfer belt contacts only the photoreceptor for black; 
         FIG. 12  illustrates a full disengagement mode in which the intermediate transfer belt is disengaged from all photoreceptors; 
         FIG. 13  illustrates the image forming apparatus from which the toner supply unit shown in  FIG. 5  is removed; 
         FIG. 14  illustrates a lever attached to a shaft of a second disengagement cam; 
         FIG. 15  illustrates a developer bottle set on a toner supply port of the developing unit shown in  FIG. 2 ; 
         FIG. 16  is a block diagram illustrating main elements of electrical circuitry of the image forming apparatus shown in  FIG. 1 ; 
         FIG. 17  illustrates a basic procedure to execute a developer supply mode; 
         FIG. 18  illustrates a procedure to execute a developer supply mode using a toner concentration sensor; 
         FIG. 19  illustrates a procedure to execute a developer supply mode using a development motor torque detector; 
         FIG. 20  illustrates a procedure to execute a developer supply mode using a toner adhesion detector; 
         FIG. 21  illustrates a basic procedure to execute a developer supply mode including detection of a position of the intermediate transfer belt; 
         FIGS. 22A and 22B  illustrate a disengagement detector according to an illustrative embodiment; 
         FIG. 23  illustrates a procedure to execute a developer supply mode using the disengagement detector shown in  FIGS. 22A and 22B ; 
         FIGS. 24A and 24B  illustrate a disengagement detector according to another illustrative embodiment; 
         FIG. 25  illustrates a procedure to execute a developer supply mode using the disengagement detector shown in  FIGS. 24A and 24B ; 
         FIG. 26  illustrates a procedure to execute a developer supply mode according to another illustrative embodiment; 
         FIG. 27  illustrates a second disengagement motor configured to rotate the second disengagement cam; and 
         FIG. 28  illustrates a procedure to execute a developer supply mode using the second disengagement motor shown in  FIG. 27 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In describing preferred 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 operate in a similar manner and achieve a similar result. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views thereof, and particularly to  FIG. 1 , an image forming apparatus according to an illustrative embodiment of the present invention is described. The image forming apparatus in the present embodiment is a tandem color laser copier in which multiple photoreceptors are arranged in parallel to each other. 
       FIG. 1  schematically illustrates a configuration of a tandem color laser copier  100  (hereinafter simply “copier  100 ”) according to the present embodiment. The copier  100  includes a print unit  150 , a sheet feeder  200  on which the print unit  150  is located, a scanner  300  fixed on the printer  150 , and an automatic document feeder (ADF)  400  fixed on the scanner  300 . 
     The print unit  150  includes an image forming unit  20 , an optical writing unit  21 , an intermediate transfer unit  17 , a secondary transfer unit  22 , a pair of registration rollers  49 , a belt type fixer  25 , and a sheet reverse unit  28  for reversing a transfer sheet that is a recording medium in a duplex print mode. The image forming unit  20  includes four process cartridges  18 Y,  18 M,  18 C, and  18 K for forming yellow, magenta, cyan, and black images, respectively. 
     It is to be noted that the reference characters Y, M, C, and K respectively represent yellow, magenta, cyan, and black, and may be omitted in the description below when color discrimination is not required. 
     Each process cartridge  18  includes a drum-shaped photoreceptor and a developing unit  4 . The optical writing unit  21  includes a light source, a polygon mirror, an f-θ lens, and a reflection mirror, and directs a laser light (exposure light) onto each photoreceptor  1  according to image data. 
     The process cartridge  18 K is described below in further detail. 
     The process cartridge  18 K further includes a charger, a drum cleaner that in the present embodiment is a cleaning blade, and a discharger, although not illustrated in  FIG. 1 . After the charger charges a surface of the photoreceptor  1 Y uniformly, the optical writing unit  21  directs a modulated and deflected laser light thereonto, and thus electrical potential of the exposed portions of the surface of the photoreceptor  1 Y is attenuated. Thus, an electrostatic latent image for yellow is formed thereon. Subsequently, the developing unit  4  develops the latent image into a yellow toner image. 
     Then, the yellow toner image is transferred from the photoreceptor  1 Y onto an intermediate transfer belt  110  serving as an intermediate transfer member by the intermediate transfer unit  17  serving as a transferor. Subsequent to this primary transfer process, the drum cleaner cleans the surface of the photoreceptor  1 Y, and the discharger removes electricity remaining thereon. Then, the surface of the photoreceptor  1 Y is again charged uniformly by the charger and thus initialized. 
     The sequence of processes described above are similarly performed in the process cartridges  18 M,  18 C, and  18 K, and thus magenta, cyan, and black toner images are respectively formed therein. 
     It is to be noted that the four process cartridges  18 Y,  18 M,  18 C, and  18 K have a similar configuration and operates in a similar manner except the color of toners used therein, and thus descriptions of the process cartridges  18 M,  18 C, and  18 K are omitted. 
     The intermediate transfer unit  17  is described below in further detail. 
     The intermediate transfer unit  17  includes the intermediate transfer belt  110 , a belt cleaner  90 , a roller  14 , a driving roller  15 , a back-up roller  16 , and primary transfer rollers  62 Y,  62 M,  62 C, and  62 K. 
     The intermediate transfer belt  110  is looped around the roller  14 , the driving roller  15 , and the back-up roller  16 , and endlessly travels clockwise in  FIG. 1  with the driving roller  15  that is driven by a motor. 
     The primary transfer rollers  62 Y,  62 M,  62 C, and  62 K are located to contact an inner surface of the intermediate transfer belt  110  and receives bias voltage from a power source. Further, the primary transfer rollers  62 Y,  62 M,  62 C, and  62 K press the intermediate transfer belt  110  against the photoreceptors  1 Y,  1 M,  1 C, and  1 K, respectively forming primary transfer nips where primary transfer electrical fields are formed between the photoreceptors  1 Y,  1 M,  1 C, and  1 K and the primary transfer rollers  62 Y,  62 M,  62 C, and  62 K. 
     The yellow toner image formed on the photoreceptor  1 Y is transferred onto the intermediate transfer belt  110  due to effects of the primary transfer electrical field and a nip pressure. On the yellow toner image, the magenta, cyan, and black toner images respectively formed on the photoreceptors  1 M,  1 C, and  1 K are superimposed one on another in the primary transfer process. Thus, a multicolor image, which in the present embodiment is a four-color image, is formed on the intermediate transfer belt  110 . 
     The four-color image is then transferred by the secondary transfer unit  22  onto a transfer sheet in a secondary transfer nip. The belt cleaner  90  faces the driving roller  15  via the intermediate transfer belt  110 , and removes toner remaining on the intermediate transfer belt  110  that has passed through the secondary transfer nip. 
     The secondary transfer unit  22  is described below in further detail. 
     The secondary transfer unit  22  is located beneath the intermediate transfer unit  17  in  FIG. 1 , and includes a transport belt  24  looped around a pair of rollers  23 . The transport belt  24  endlessly travels counterclockwise in  FIG. 1  with rotation of at least one of the rollers  23 . The intermediate transfer belt  110  and the transport belt  24  are sandwiched between the back-up roller  16  and the roller  23  on the right, forming the secondary transfer nip therebetween. Further, the right side roller  23  receives a secondary transfer bias having a polarity opposite a polarity of the toner from a power source. 
     With the secondary transfer bias, a secondary transfer electrical field for causing the four-color image on the intermediate transfer belt  110  to move to the side of the roller  23  is formed in the secondary transfer nip. Thus, due to effects of the secondary transfer electrical field and a nip pressure, the four-color image is transferred onto the transfer sheet that is forwarded by the registration rollers  49  in synchronization with the four-color image. 
     It is to be noted that, instead of the secondary transfer method in which the secondary transfer bias is applied to the roller  23  as described above, alternatively, a method using a charger that charges the transfer sheet in a non-contact manner can be adopted. 
     The sheet feeder  200  includes a paper bank  43  in which multiple sheet cassettes  44  are arranged one above another, and a sheet feed path  46  provided with multiple pairs of transport rollers  47 . Each sheet cassette  44  contains a stack of transfer sheets against which a feed roller  42  presses from above. The transfer sheets are fed from the top with rotation of the feed roller  42 , and a separation roller  45  separates the transfer sheets one by one. 
     Then, the sheet is transported along the sheet feed path  46  to the registration rollers  49 . While the registration rollers  49  sandwich the transfer sheet therebetween, the intermediate transfer belt  110  transports the four-color image to the secondary transfer nip. When the registration rollers  49  forwards the transfer sheet timely so that the transfer sheet laps over the four-color image in the secondary transfer nip, the four-color image is transferred from the intermediate transfer belt  110  onto a first side of the transfer sheet in the secondary transfer nip. This image becomes a full-color image (hereinafter also “toner image”) on the white transfer sheet. Subsequently, the transport belt  24  transports the transfer sheet to the fixer  25 . 
     The fixer  25  includes a belt unit including a fixing belt  26  looped around two rollers, and a pressure roller  27  that presses against one of those rollers. The fixing belt  26  contacts the pressure roller  27 , forming a fixing nip in which the transfer sheet forwarded by the transport belt  24  is sandwiched. A heat source is provided inside the roller against which the fixing roller  27  presses so as to heat the fixing belt  26 , which heats the transfer sheet. Thus, the full-color image is fixed on the transfer sheet with heat from the transfer belt  26  and a nip pressure. 
     Then, the transfer sheet whose first side carries the fixed toner image is either stacked on a stack part  57  provided outside a side plate of the print unit  150  on the left in  FIG. 1  or returned to the secondary transfer nip so that a toner image is formed on a second side thereof in the duplex print mode. 
     A copying operation using the copier  100  is described below with reference to  FIG. 1 . 
     For example, a stack of original documents is set on a document table  30  of the ADF  400 . Alternatively, when the original documents are bound like a book, the ADF  400  is lifted to expose a contact glass  32  of the scanner  300 , the original documents are set on the contact glass  32 , and then the ADF  400  is lowered to hold the original documents. 
     Then, when a user presses a start switch, the ADF  400  forwards the original documents set on the document table  30  one by one onto the contact glass  32 , and then the scanner  300  starts reading image information of the original documents. When the original documents are set on the contact glass  32 , pressing the start switch causes the scanner  300  to immediately read the original documents. 
     The scanner  300  includes a first carriage  33  including a light source, a second carriage  34  including a mirror, an imaging lens  35 , and a reading sensor  36 . In a document reading operation, both the first carriage  33  and the second carriage  34  start traveling, and the light source emits light toward the original document. The light is then reflected by the original document, and the mirror in the second carriage  34  further reflects the light to the imaging lens  35 . After passing through the imaging lens  35 , the light enters the reading sensor  36 , and thus the reading sensor  36  obtains image information based on the light. 
     In parallel to the document reading operation described above, components of the process cartridges  18 , the intermediate transfer unit  17 , the secondary transfer unit  22 , and the fixer  25  are activated. The optical writing unit  21  is controlled so as to form electrostatic latent images for yellow, magenta, cyan, and black on the photoreceptors  1 Y,  1 M,  1 C, and  1 K, respectively, according to the image information obtained by the reading sensor  36 . Then, the latent images are developed into toner images and further transferred onto the intermediate transfer belt  110 , forming a four-color image (toner image). 
     Further, simultaneously with the start of the document reading operation described above, the sheet feeder  200  starts to feed the transfer sheets. One of the feed rollers  42  is selected and rotates to feed the transfer sheets from the sheet cassette  44  corresponding thereto, and the transfer sheets are transported along the sheet feed path  46  one by one, separated by the separation roller  45 . Alternatively, the transfer sheets can be fed from a manual feed tray  51 . In this case, a manual feed roller  50  is selected to rotate, and the transfer sheets are transported along a manual feed path  53  one by one, separated by a separation roller  52 . 
     When forming a multicolor image using at least two different color toners, the copier  100  holds an upper side of the intermediate transfer belt  110  substantially horizontally so as to contact all photoreceptors  1 . 
     By contrast, when forming a monochrome image using only black toner, the upper side of the intermediate transfer belt  110  is disengaged from the photoreceptors  1 Y,  1 M, and  1 C by inclining the intermediate transfer belt  110  so that its left side is lowered. Then, only the photoreceptor  1 K is rotated counterclockwise in  FIG. 1  and a black image is formed thereon. In each of the process cartridges  18 Y,  18 M, and  18 C, the developing unit  4  is deactivated as well so as to save the developer and prevent wear of the photoreceptor  1 . 
     Although not shown in  FIG. 1 , the copier  100  further includes a controller  198  for controlling operations of respective parts thereof and an operating unit (operation display)  194  including a display and various keys, and these are described below with reference to  FIG. 16 . 
     Regarding single print modes in which an image is formed only one side of the transfer sheet, the copier  100  can offer three different mode: a direct discharge mode, a reverse discharge mode, and a reverse decal discharge mode. The user can select one of the modes by sending a command to the controller  198  from the operating unit  194 . 
       FIG. 2  is an end-on view illustrating the developing unit  4  and the photoreceptor  1  included in each process cartridge  18  shown in  FIG. 1 . 
     Referring to  FIGS. 1 and 2 , while the photoreceptor  1  rotates in a direction indicated by arrow G, the charger charges the surface of the photoreceptor  1  and an electrostatic latent image is formed thereon with the laser light emitted from the optical writing unit  21 . Further, the developing unit  4  supplies the latent image with the toner, forming a toner image. 
     As shown in  FIG. 2 , the developing unit  4  includes a developer container part and a developing roller  5  serving as a developer carrier that supplies the electrostatic latent image on the photoreceptor  1  with the toner while rotating in a direction indicated by arrow I. 
     The developer container part (hereinafter also “developer transport path”) forms a collection path  7 , a supply path  9 , and an agitation path  10  provided with a collection screw  6 , a supply screw  8 , and an agitation screw  11 , respectively. Each of the collection screw  6 , the supply screw  8 , and the agitation screw  11  serves as a developer transporter and a blade part is provided on its rotary shaft so as to transport the developer in an axial direction by rotating. 
     While supplying the toner to the developing roller  5 , the supply screw  8  in the supply path  9  transports the developer toward a back side of the sheet on which  FIG. 2  is drawn, that is, in a direction perpendicular to and rearward of the sheet on which  FIG. 2  is drawn. The developer supplied to the developing roller  5  is adjusted to have a desired or given thickness by a developer doctor  12 , serving as a developer regulator, located downstream from a portion where the developing roller  5  faces the supply screw  8  in the direction indicated by arrow I in which the developing roller  5  rotates (hereinafter also “developing roller rotational direction”). 
     The collection path  7  is located downstream from a development area where the developing roller  5  faces the photoreceptor  1  in the developing roller rotational direction. The developing unit  4  further includes a development bias applicator configured to apply a developing bias for forming an electrical field that causes the toner to adhere to the electrostatic latent image on the photoreceptor  1 . 
     The collection screw  6  collects the developer that has passed through the development area and transports the collected developer in a direction identical or similar to the direction in which the developer is transported (hereinafter simply “developer transport direction”) by the supply screw  8 . 
     The developing roller  5  and the supply path  9  are arranged laterally, and the collection path  7  is located beneath the developing roller  5 . 
     The agitation path  10  including the agitation screw  11  is located beneath the supply path  9 , parallel to the collection path  7 . While agitating the developer, the agitation screw  11  transports the developer toward a front side of the sheet on which  FIG. 2  is drawn, which is a direction opposite to the developer transport direction of the supply screw  8 . 
     The developing unit  4  further includes a first separation wall  133  and a second separation wall  134 , serving as separators, and a toner concentration sensor  191 . The copier  100  further includes a toner adhesion sensor  190  serving as a toner adhesion detector. The first separation wall  133  includes a portion separating the supply path  9  from the agitation path  10  and a portion separating the supply path  9  from the collection path  7 . The second separation wall  134  separates the collection path  7  from the agitation path  10 . 
     For example, the toner adhesion detector  190  is located downstream from the development area in the direction indicated by arrow G in which the photoreceptor  1  rotates, and faces the photoreceptor  1  at a location in the axial direction that is within the width of the developing roller  5 . The toner concentration sensor  191  is located on a bottom portion of the agitation path  10 , in a downstream portion in the developer transport direction of the agitation screw  11 , which is a back side portion in  FIG. 2 . 
     Circulation of the developer in the developing unit  4  is described below with reference to  FIGS. 2 and 3 . 
       FIG. 3  illustrates a flow of the developer inside the developing unit  4 , and arrows therein indicate directions in which the developer flows. Further, the back surface and the front side in  FIG. 2  are located on the left and right in  FIG. 3 , respectively. 
     The first separation wall  133  includes openings  91  and  92 , shown in  FIG. 3 , respectively provided in both end portions in the axial direction thereof, enabling the supply path  9  and the agitation path  10  to communicate with each other. The second separating wall  134  includes an opening  93  provided in an end portion on the left in  FIG. 3  so as to connect the collection path  7  and the agitation path  10 . 
     The developer that has passed through the development area is collected in the collection path  7  and transported to the left in  FIG. 3 . Subsequently, the developer is further transported to the agitation path  10  through the opening  93  of the first separation wall  133 , which is located in a non-image area. 
     The first separation wall  133  has no opening in the portion separating the supply path  9  and the collection path  7 , and thus the supply path  9  and the collection path  7  are kept separate and do not communicate with each other. 
     As shown in  FIG. 3 , a toner supply port  95  is provided in an upper portion of the agitation path  10  near the opening  93 , and premixed toner including magnetic carrier is supplied to the agitation path  10  through the toner supply port  95  as indicated by arrow L, as needed. 
     The developer is transported from the agitation path  10  to the supply path  9  as indicated by arrow D in  FIG. 3 , where the supply screw  8  transports the developer downstream, that is, to the right in  FIG. 3 , while supplying the developing roller  5  with the toner. The developer that is not used for development (hereinafter also “excess developer”) is transported to a downstream end portion of the supply path  9 . 
     Then, the excess developer is transported to the agitation path  10  as indicated by arrow E through the opening  92  (hereinafter also “excess developer opening  92 ”) provided on the first separation wall  133 , located on the left in  FIG. 3 . 
     By contrast, the developer supplied to the developing roller  5  is collected in the collection path  7  as the developing roller  5  rotates, and then the collected developer (hereinafter also “used developer”) is transported by the collection screw  6  to a downstream end portion of the collection path  7  located on the left in  FIG. 3 . The collected developer is further transported to the agitation path  10  as indicated by arrow F though the opening  93  (hereinafter also “collection opening  93 ”) provided on the second separation wall  134 . 
     Subsequently, in the agitation path  10 , the agitation screw  11  agitates the excess developer and the collected developer as well as transports the agitated developer downstream, which is upstream in the developer transport direction of the supply screw  8 . In a downstream end portion of the agitation path  10 , the developer is supplied to the supply path  9  as indicated by arrow D through the opening  91  provided on the first separation wall  133  (hereinafter also “supply opening  91 ”). 
     Thus, in the agitation path  10 , the agitation screw  11  agitates and transports the collected developer, the excess developer, and the premixed toner (hereinafter also “unused toner”) supplied through the toner supply port  95  in the opposite direction to the developer transport direction of the collection screw  6  and the supply screw  8 . Then, the developer is transported from the downstream end portion of the agitation path  10  to an upstream portion of the supply path  9  that communicate with each other. 
     It is to be noted that the toner is supplied according to an output from the toner concentration sensor  191  that can be provided, for example, in a downstream portion of the agitation path  10 . 
     As described above, the developing unit  4  shown in  FIG. 3  includes the supply path  9  and the collection path  7  so that supply and collection of the developer can be performed in separate paths, preventing the used developer from entering the supply path  9  directly. Thus, the toner concentration in the developer to be supplied to the developing roller  5  does not decrease as the developer is transported downstream in the supply path  9 . 
     Further, the developer  4  includes the collection path  7  and the agitation path  10  so that collection and agitation of the developer can be performed in separate paths, and thus the used developer can be fully mixed with the excess developer and the unused developer to supply well-agitated developer to the supply path  9 . 
     Consequently, image density during development can be kept constant by maintaining the toner concentration in the developer as well as fully agitating the developer. 
     Location to supply the premixed toner to the developer transport path including the supply path  9 , the agitation path  10 , and the collection path  7  is described below in further detail. 
       FIG. 4  is a perspective view of the developer  4  shown in  FIGS. 2 and 3 . 
     With reference to  FIGS. 3 and 4 , the toner supply port  95  is located in the upper portion of an upstream end portion of the agitation path  10 , which is outside an end portion of the developing roller  5  in the axial direction. 
     It is to be noted that the location of the toner supply port  95  is not limited to that described above, and alternatively, the toner supply port  95  may be provided on an upper portion of the downstream end portion of the collection path  7 , for example. Alternatively, the toner supply port  95  may be provided above the collection opening  93  where the developer is transported from the collection path  7  to the agitation path  10 . Because the newly supplied developer and the existing developer can be easily mixed together around the collection opening  93 , the developer can be agitated more effectively by providing the toner supply port  95  there. 
     A toner supply unit to supply the premixed toner to the developing unit  4  through the toner supply port  95  is described below. 
     The copier  100  shown in  FIG. 1  further includes a toner supply unit  500 .  FIG. 5  is a perspective view illustrating the toner supply unit  500  including multiple toner bottles  120  serving as toner containers or powder containers, and  FIG. 6  is a schematic illustration of a configuration thereof. Further,  FIG. 7  is a perspective view illustrating the toner bottle  120 ,  FIG. 8  illustrates installation of the toner bottle  120 , and  FIG. 9  is a perspective view of the copier  100 . 
     Referring to  FIG. 5 , each toner bottle  120  contains the premixed toner including the toner and the carrier. The toner concentration of the premixed toner is higher than that of the developer in the developing unit  4 . In  FIG. 5 , reference character T f  indicates a flow of the premixed toner. 
     As shown in  FIG. 5 , the multiple toner bottles  120  for respective colors are arranged in the copier  100  that is a tandem image forming apparatus. The toner supply unit  500  further includes supply units each including a toner pump  60 , a toner supply tube  65 , and a sub hopper  68  for each color, and each toner bottle  120  is connected to the supply unit. The developing unit  4  is located beneath the supply unit. The toner supply unit  500  further includes a nozzle  80  for each color whose tip portion is inserted into the toner bottle  120 , and each toner pump  60  is connected to a driving motor  66 . 
     As shown in  FIG. 6 , the toner supply unit  500  further includes a suction port  63 , a universal joint  64 , and a toner outlet  67 . The toner pump  60  in the present embodiment is either a mohno-pump, which is a type of screw pump, or a suction-type uniaxial eccentric screw pump, and includes a rotor  61  and a cylindrical stator  69  whose inner surface includes a spiral groove as main components. The rotor  61  is the shape of a shaft having a circular cross-section twisted into a spiral, and is connected to the driving motor  66  via a driving transmission and the universal joint  64 . The rotor  61  transports the premixed toner axially by rotating inside the stator  69 . The stator  69  is elastic and includes a hole whose cross-section is an ellipse twisted into a spiral. The pitch of the spiral of the stator  69  is twice that of the spiral of the rotor  61 . 
     By engaging the rotor  61  with the stator  69  and then rotating the rotor  61 , the premixed toner is transported through a space formed between the rotor  61  and the stator  69 . In other words, in the toner pump  60 , one of the main components is caused to slidably move as the other main component is rotated, generating a negative pressure at the suction port  63 , which causes airflow inside the toner supply tube  65 . 
     More specifically, when the rotor  61  is rotated, the premixed toner in the toner bottle  120  enters the toner pump  60  through the suction port  63 , is aspirated and transported from the left to the right in  FIG. 6 , and then supplied to the developing unit  4  through the toner outlet  67 , the sub hopper  68 , and the toner supply port  95 . 
     It is to be noted that the configuration of the toner pump  60  is not limited to the description above, and various known pumps such as those disclosed in Japanese Patent Publication No. 2000-098721 can be used, the contents of which are hereby incorporated by reference herein. 
     Referring to  FIGS. 6 and 7 , each toner bottle  120  includes a toner container  121 , a toner outlet coupling  122  serving as a single powder outlet, and a base  130  attached to the toner outlet coupling  122 . 
     Referring to in  FIGS. 8 and 9 , the toner supply unit  500  further includes four bottle holders  75 Y,  75 M,  75 C, and  75 K, each of which can pivot on a rotary shaft so as to partly disengage from the toner supply unit  500 . As shown in  FIG. 9 , outer side surface  76 Y,  76 M,  76 C, and  76 K of the bottle holders  75 Y,  75 M,  75 C, and  75 K are exposed on a front side of the copier  100 . The bottle holders  75 Y,  75 M,  75 C, and  75 K hold the toner bottles  120 Y,  120 M,  120 C, and  120 K, respectively. The toner supply unit  500  further includes multiple screw holes  77  and  78  into which screws are respectively inserted, attaching the toner supply unit  500  to the side walls of the copier  100 . 
     Setting of the toner bottle  120  on the bottle holder  75  is described below. 
     Referring to  FIGS. 6 and 8 , for example, to set the toner bottle  120 K on the bottle holder  75 K, the user unlocks and pivots the bottle holder  75 K so that the bottle holder  75 K rotates down and out on the front side of the copier  100 . Then, the user holds the toner bottle  120 K with the side of the base  130  facing down and inserts the toner bottle  120 K down the bottle holder  75 K. 
     When the toner bottle  120  is on the bottle holder  75 , the tip portion of the nozzle  80 , which serves as a connecter of the copier  100  to be connected to the base  130 , is inserted into the toner bottle  120 . Thus, the toner outlet coupling  122  and a toner inlet of the nozzle  80  communicate with each other. The nozzle  80  includes a joint to be connected to the toner supply tube  65  that communicates with the toner pump  60 , and further, the toner pump  60  communicates with the developing unit  4  via the sub hopper  68 . Thus, when the toner bottle  120  is set on the bottle holder  75 , the toner bottle  120  communicates with the developing unit  4 . 
     Next, a disengagement mechanism to disengage the intermediate transfer belt  110  from the photoreceptors  1  is described below. 
       FIG. 10  illustrates a full engagement mode, in which the intermediate transfer belt  110  supported by the multiple rollers contacts all the photoreceptors  1 Y,  1 M,  1 C, and  1 K. 
     Referring to  FIG. 10 , a disengagement unit  140  includes a first arm  141 , a first disengagement cam  142 , a second arm  143 , a second disengagement cam  144 , a driving controller  145 , and a first disengagement motor  146 . The first arm  141  and the first disengagement cam  142  are for engaging or disengaging the intermediate transfer belt  110  with or from the photoreceptors  1 Y,  1 M, and  1 C simultaneously, and the second arm  143  and the second disengagement cam  144  are for engaging or disengaging the intermediate transfer belt  110  with or from the photoreceptor  1 K. The first disengagement cam  142  is rotated by the first disengagement motor  146  according to a control signal from the driving controller  145 . The second disengagement cam  144  can be manually rotated using a lever  147  (shown in  FIG. 14 ) that is attached to a tip portion of a shaft  144   a  of the second disengagement cam  144 . 
     It is to be noted that hereinafter the right and the left sides of the first arm  141  in  FIGS. 10 through 12  are simply referred to as the right and the left sides of the first arm  141 . 
     An end portion of the first arm  141  is pivotally supported by a pivot point  148  provided on the second arm  143 , and the location of the pivot point  148  is closer to the primary transfer roller  62 K than a center portion of the second arm  143  in a longitudinal direction is. The second arm  143  is pivotally supported by a pivot point  149 , and the pivot point  148  provided on the second arm  143  swings as the second arm  143  pivots. 
     The driving roller  15  is provided on a left end portion of the first arm  141 , which is the side opposite the pivot point  148 . Further, the primary transfer rollers  62 Y,  62 M, and  62 C are located between the driving roller  15  and the pivot point  148  in a longitudinal direction of the first arm  141 , and rotatably supported and biased toward the photoreceptors  1 Y,  1 M, and  1 C simultaneously by pressure springs  163 Y,  163 M, and  163 C, respectively. A tension spring  13   a  biases a tension roller  13  provided to contact the intermediate transfer belt  110  from outside so as to tension the intermediate transfer belt  110 . 
     The first disengagement cam  142  contacts a portion of the first arm  141  located between the driving roller  15  and a center portion thereof in the longitudinal direction, on the side opposite the side of the primary transfer rollers  62 Y,  62 M, and  62 C. As the first disengagement cam  142  rotates, the first arm  141  pivots on the pivot point  148 , which causes the primary transfer roller  62 Y,  62 M, and  62 C to engage or disengage the intermediate transfer belt  110  with or from the photoreceptors  1 Y,  1 M, and  1 C simultaneously. 
     The second arm  143  is pivotally attached to a frame of the intermediate transfer unit  17  shown in  FIG. 1  by the pivot point  149  provided on one portion thereof. The primary transfer roller  62 K is located on another end portion of the second arm  143  on the side opposite the pivot point  149 , and a pressure spring  163  rotatably supports the primary transfer roller  62 K, as well as biasing it toward the photoreceptors  1 K. The second disengagement cam  144  contacts the second arm  143  at a position between the primary transfer roller  62 K and the center portion in the longitudinal direction, on the side opposite the side of the photoreceptor  1 K. As the second disengagement cam  144  rotates, the second arm  143  pivots on the pivot point  149 , which causes the primary transfer roller  62 K to engage or disengage the intermediate transfer belt  110  with or from the photoreceptor  1 K. 
       FIG. 11  illustrates a partial disengagement mode, in which the intermediate transfer belt  110  engages only the photoreceptor  1 K and is disengaged from the photoreceptors  1 Y,  1 M, and  1 C. 
     When the first disengagement cam  142  makes a half revolution from the state illustrated in  FIG. 10 , the first arm  141  pivots around the pivot point  148  downward in  FIG. 10 , and thus the primary transfer rollers  62 Y,  62 M, and  62 C move away from the photoreceptors  1 Y,  1 M, and  1 C, respectively. In this state, the second disengagement cam  144  is at an engagement position so as to press the primary transfer roller  62 K against the photoreceptor  1 K via the intermediate transfer belt  110 . Accordingly, the intermediate transfer belt  110  disengages from the photoreceptors  1 Y,  1 M, and  1 C and engages only the photoreceptor  1 K as shown in  FIG. 11 , and thus the intermediate transfer belt  110  enters the partial disengagement mode for forming monochrome black images. 
     In the partial disengagement mode, deterioration of the photoreceptors  1 Y,  1 M, and  1 C can be prevented or reduced because the intermediate transfer belt  110  does not contact them. Further, the photoreceptors  1 Y,  1 M, and  1 C can be deactivated, extending the life of the chargers, the developing units  4 , and the drum cleaners therefor as well as the photoreceptors  1 Y,  1 M, and  1 C. 
       FIG. 12  illustrates a full disengagement mode, in which the intermediate transfer belt  110  is disengaged from all the photoreceptors  1 Y,  1 M,  1 C, and  1 K. 
     When unused developer is supplied to an empty developing unit  4 , the intermediate transfer belt  110  is disengaged from all the photoreceptors  1 Y,  1 M,  1 C, and  1 K in the present embodiment. The intermediate transfer belt  110  is set to the full disengagement mode at the factory, and, at the user&#39;s site, the maintenance person rotates the second disengagement cam  144  using the lever  147  (shown in  FIG. 14 ) attached to the shaft  144   a  so as to set the intermediate transfer belt  110  to the partial disengagement mode shown in  FIG. 11 . 
     When the second disengagement cam  144  makes a half revolution from the engagement position illustrated in  FIG. 11  to a disengagement position, the second arm  143  pivots on the pivot point  149  clockwise in  FIG. 11 , and accordingly the primary transfer roller  62 K descends away from the photoreceptor  1 K, disengaging the intermediate transfer belt  110  from the photoreceptor  1 K. That is, the intermediate transfer belt  110  is disengaged from all the photoreceptors  1 Y,  1 M,  1 C, and  1 K. 
     Further, in this state, the first arm  141  is inclined so that its right side on which the pivot point  148  is provided is lowered because the right side end portion is supported by the second arm  143 , and thus the first arm  141  moves downward in  FIGS. 11 and 12  to a position parallel or substantially parallel to the position shown in  FIG. 10 . 
     It is to be noted that, if the pivot point  148  of the first arm  141  is not connected to the second arm  143  as in the configuration described above, the first arm  141  would be inclined to the lower left in  FIG. 12 , and thus the primary transfer roller  62 C would be closest to the corresponding photoreceptor  1  among the primary transfer rollers  62 Y,  62 C,  62 M, and  62 K. Accordingly, the distance between the intermediate transfer belt  110  and the photoreceptor  1 C would be shorter than the distance between the intermediate transfer belt  110  and either of the photoreceptors  1 Y and  1 M. 
     Replacement of the developer in the developing unit  4  is described below. 
     The developer in the developing unit  4  is replaced periodically because the developer, the carrier in particular, deteriorates over time while being used. Used developer is removed from the developing unit  4  and then unused developer is supplied to the empty developing unit  4 . 
     The used developer is collected from the developing unit  4  as follows: Referring to  FIGS. 1 and 4 , first, the developing unit  4  is detached from the copier  100 , and then the used developer is collected through the toner supply port  95 . 
     Alternatively, a developer outlet and a shutter to open/close this developer outlet can be provided on a bottom portion of the developing unit  4 , and the copier  100  can be configured to offer a developer discharge mode that is selectable via the operating unit  194  (shown in  FIG. 16 ). When this developer discharge mode is executed, for example, the shutter opens and each screw in the developing unit  4  starts rotating, discharging the used developer from the developing unit  4  through the developer outlet. 
     It is to be noted that, if the developing unit  4  is filled with the developer before shipment, and the copier  100  is shaken or tilts during transport, the developer might spill out from the opening of the developing unit  4 . Further, the developer might deteriorate by being exposed to air. Therefore, the developer is supplied to the developing unit  4  at the user&#39;s site. 
     A developer supply operation is described below with reference to  FIGS. 9 and 12  through  15 . 
     Referring to  FIG. 9 , a front door is provided on the front side of the copier  100  so as to detach the toner supply unit  500  from the copier  100 . First, the front door is opened and power to the copier  100  is turned off, after which the toner supply unit  500  is detached therefrom. 
     After the toner supply unit  500  is thus removed from the copier  100 , the toner supply port  95  of each developing unit  4  appears as shown in  FIG. 13 . Subsequently, as shown in  FIG. 14 , the maintenance person attaches the lever  147  to the tip portion of the shaft  144   a  of the second disengagement cam  144 , and then rotates the lever  147  counterclockwise in  FIG. 14  so as to disengage the intermediate transfer belt  110  from all the photoreceptors  1 Y,  1 M,  1 C, and  1 K as shown in  FIG. 12 . 
     Then, referring to  FIG. 15 , the maintenance person attaches a toner bottle  120  serving as a developer container containing unused developer to the developing unit  4  so that a developer supply port of the toner bottle  120  engages the toner supply port  95 . 
     In an initialization operation that is performed when the copier  100  arrives at the user&#39;s site, the toner bottles  120  for yellow, magenta, cyan, and black are respectively set on the toner supply ports  95  of the corresponding colors. In developer replacement work, a corresponding toner bottle  120  is set on the toner supply ports  95  of the developing unit  4  from which the used developer is removed. 
     Then, the maintenance person removes a heat seal covering the developer supply port of the toner bottle  120 , closes the front door of the copier  100 , and then turns on the power. Further, the maintenance person calls up a hidden menu via a display of the operating unit (operation panel)  194  (shown in  FIG. 16 ), selects the color of the toner bottle  120  set on the developing unit  4 , and then executes a developer supply mode. It is to be noted that the developer supply mode is performed for each color in the initialization operation. 
     When the developer supply mode is executed, each screw in the developing unit  4  starts rotating so as to transport and uniformly distribute the unused developer supplied from the toner bottle  120  in the developing unit  4 . 
     It is to be noted that, while the developer supply mode is executed, the photoreceptor  1  can be rotated as well so as not to be damaged by newly supplied carrier adhered to the developing roller  5  (shown in  FIG. 2 ). Further, because the cleaning blade (drum cleaner) might turn outward or inward if the photoreceptor  1  is kept rotating while the toner is not supplied thereto, a belt-shaped image can be formed on the photoreceptor  1  after a sufficient or given amount of the developer is supplied thereto so that the cleaning blade is supplied with the toner. 
     When all yellow, magenta, cyan, and black are selected in the developer supply mode, the yellow, magenta, cyan, and black developers are supplied, sequentially or simultaneously, to the respective developing units  4  in the present embodiment. 
     After all unused developer in the toner bottle  120  is supplied to the developing unit  4  and the developer supply mode is completed, the maintenance person turns the power off, opens the front door, and then attaches the toner supply unit  500  to the copier  100 . Then the maintenance person closes the front door, turns on the power again, and then performs an initial setting operation to achieve a proper or desired image density. The initial setting operation includes calibrating the sensitivity of the toner concentration sensor  191  (shown in  FIG. 2 ), setting image forming condition, etc. 
     The sensitivity of the toner concentration sensor  191  can be calibrated as follows: The unused developer contained in the toner bottle  120  has a predetermined or given toner concentration, and thus toner concentration in the developing unit  4  equals that value after the unused developer is supplied thereto. For example, the unused developer has a toner concentration of 7% in the present embodiment. Therefore, the sensitivity of the toner concentration sensor  191  is adjusted so that an output value thereof indicates a toner concentration of 7%. 
     Further, the image forming conditions are set as follows: Referring to  FIG. 2 , a predetermined or given test pattern is formed on the photoreceptor  1 , and then the toner adhesion detector  190  detects the amount of the toner adhered to the test pattern. Based on results of the detection, development bias, charge bias, intensity of the exposure light, etc., are adjusted. 
     After the initial setting operation is completed, the maintenance person opens the front door of the copier  100 , rotates the lever  147  shown in  FIG. 14  so as to set the intermediate transfer belt  110  to the partial disengagement mode shown in  FIG. 11  from the full disengagement mode shown in  FIG. 12 , and then closes the front door. 
       FIG. 16  is a block diagram illustrating main elements of control circuitry of the copier  100 . 
     Referring to  FIG. 16 , the controller  198  includes a CPU (Central Processing Unit) serving as a computing unit, a nonvolatile RAM (Random Access Memory) serving as a data storage unit, a ROM (Read Only Memory) serving as another data storage unit, etc. The controller  198  is connected to the toner adhesion detector  190 , the toner concentration sensor  191 , a photoreceptor motor  192  that drives the photoreceptor  1 , a development motor  193  that drives the developing unit  4 , the operating unit  194 , a position detector  195  that detects position of the intermediate transfer belt  110 , a photoreceptor motor torque detector  196 , a development motor torque detector  197 , and a belt driving motor torque detector  199 . 
     It is to be noted that, although the controller  198  performs overall control of the copier  100  and various devices and sensors are connected thereto, only the devices and the sensors that concern features of the copier  100  are shown in  FIG. 16 . 
     The controller  198  implements functions of the respective parts according to control programs stored in the RAM and the ROM. More specifically, when execution of the developer supply mode is instructed via the operating unit  194 , the controller  198  drives the photoreceptor motor  192  and the development motor  193 , thus serving as a developer supply controller. 
     As noted previously, in the developer supply operation described above, if unused developer is supplied to the developing unit  4  in which developer is present (redundant replenishment) due to human error, the copier  100  might be seriously contaminated or damaged. 
     In view of the foregoing, in a process whose steps are illustrated in the flow chart shown in  FIG. 17 , at S 1  the controller  198  confirms that the developer container part of the developing unit  4  is empty, and then at S 2  executes the developer supply mode only when the developer is not present therein, that is, the developing unit is empty (YES at S 1 ) in order to prevent such human error. 
     The present embodiment is described below in further detail with reference to  FIGS. 16 and 18 . 
       FIG. 18  illustrates a procedure to execute the developer supply mode, in which the controller  198  checks for the presence of the developer in the developer container part of the developing unit  4  based on an output value V t  from the toner concentration sensor  191  serving as a developer detector. 
     First, the maintenance person sets the toner bottle  180  (shown in  FIG. 15 ) on the toner supply port  95  (shown in  FIG. 14 ) of the developing unit  4 , and then instructs the copier  100  to execute the developer supply mode via the operating unit  194 . Subsequently, referring to  FIG. 18 , at S 11  the controller  198  activates the toner concentration sensor  191  that in the present embodiment is a magnetic permeability sensor, and at S 12  checks whether or not an output value V t  therefrom is lower than a predetermined or given threshold V ref . 
     When the toner concentration in the developing unit  4  is lower, that is, a relatively large amount of the carrier is present in an area detected by the toner concentration sensor  191 , magnetic permeability thereof is relatively high, and accordingly the output value V t  is higher. By contrast, when the toner concentration in the developing unit  4  is higher, that is, a relatively small amount of the carrier is present in the area detected by the toner concentration sensor  191 , magnetic permeability thereof is lower, and accordingly the output value V t  is lower. 
     In other words, when the developing unit  4  is empty, the output value V t  as well as the magnetic permeability therein are significantly low. Therefore, when the output value V t  of the toner concentration sensor  191  is lower than the threshold V ref  (YES at S 12 ), the developing unit  4  can be regarded as being empty. Then, at S 13  the controller  198  executes the developer supply mode, driving the collection screw  6 , the supply screw  8 , the agitation screw  11  (developer transporters), etc., in the developing container part shown in  FIG. 2 . 
     By contrast, when the output value V t  of the toner concentration sensor  191  is higher than the threshold value V ref  (NO at S 12 ), the controller  198  displays an error message on the operating unit  194 , etc., and terminates the procedure. Thus, redundant replenishment of the developer can be prevented. Further, because the toner concentration detector  191  serves as the developer detector to detect presence of the developer in the developer container part of the developing unit  4 , a separate developer detector is not required, saving both the number of components used in the copier  100  as well as the cost thereof. 
     Another illustrative embodiment is described below with reference to  FIGS. 16 and 19 . 
       FIG. 19  illustrates another procedure to execute the developer supply mode, in which the presence of the developer in the developer container part is detected based on torque of the developer transporter or the development motor  193 . 
     First, the maintenance person sets the toner bottle  120  (shown in  FIG. 15 ) on the toner supply port  95  (shown in  FIG. 14 ) of the developing unit  4 , and then instructs the copier  100  to execute the developer supply mode via the operating unit  194 . Subsequently, referring to  FIG. 19 , at S 21  the controller  198  activates the development motor  193  so as to drive the developing roller  5 , the collection screw  6 , the supply screw  8 , and the agitation screw  11  shown in  FIG. 2 . 
     At S 22 , the controller  198  activates the development motor torque detector  197  so as to detect torque of the development motor  193 . The development motor torque detector  197  monitors a driving current of the development motor  193  and then converts it into torque, which is used to detect an abnormal state of the development motor  193  and the developing unit  4 . 
     At S 23 , the controller  198  checks whether or not the detected torque T of the development motor  193  is lower than a predetermined or given threshold T ref . Because the torques of the collection screw  6 , the supply screw  8 , and the agitation screw  11  are higher, and accordingly the detected torque T is higher when the developer is present in the developing unit  4 , the presence of the developer can be detected based on the torque of the development motor  193 . 
     When the detected torque T is higher than the threshold T ref  (NO at S 23 ), that is, the developer is present in the developer container part of the developing unit  4 , the controller  198  displays an error message on the operating unit  194 , etc., stops the development motor  193  at S 25 , and terminates the procedure. 
     By contrast, when the detected torque T is lower than the threshold T ref  (YES at S 23 ), that is, the developer is not present in the developer container part, at S 24  the controller  198  executes the developer supply mode and then stops the development motor  193  at S 25 . 
     It is to be noted that, although the development motor  193  is driven so as to detect the torque, the developer does not spill over from the developing unit  4  even if the developer is already present therein because driving time of the development motor is very short, supplying a very small amount of the developer, if any. 
     As described above, redundant replenishment of the developer can be prevented as well in the present embodiment because, when the developer is present in the developing unit  4 , the developer is not supplied from the toner bottle  120  thereto. Further, because the development motor  193  and the development motor torque detector  197  serve as the developer detector, a separate developer detector is not required, saving both the number of components used in the copier  100  as well as the cost thereof. 
     Another illustrative embodiment is described below with reference to  FIGS. 16 and 20 . 
       FIG. 20  illustrates another procedure to execute the developer supply mode, in which a predetermined or given image is formed on the photoreceptor  1  as a developer detection pattern, the amount of the toner adhered to the developer detection pattern is detected, and then the presence of the developer in the developer container part is detected based on results of the detection. 
     First, the maintenance person sets the toner bottle  120  (shown in  FIG. 15 ) on the toner supply port  95  (shown in  FIG. 14 ) of the developing unit  4 , and then instructs the copier  100  to execute the developer supply mode via the operating unit  194 . Subsequently, referring to  FIG. 20 , at S 31  the controller  198  activates the photoreceptor motor  192  and the development motor  193 , and simultaneously, causes the charger and the development bias applicator to generate the charge bias and the development bias, respectively, at S 32 . Then, the developer detection pattern is formed on the photoreceptor  1  at S 33 , and then the controller  198  activates the toner adhesion detector  190  to detect the developer detection pattern at S 34 . At S 35 , the controller  198  checks whether or not an output value Vsp of the toner adhesion detector  190  is higher than a predetermined or given threshold Vref at S 35 . 
     The toner adhesion detector  190  in the present embodiment is a reflection optical sensor, and the output value V sp  is lower when the amount of the toner adhered to the photoreceptor  1  is larger and higher when the amount of the toner adhered to the photoreceptor  1  is smaller. When the developer is not present in the developing unit  4 , the toner does not adhere to the developer detection pattern on the photoreceptor  1 , and accordingly the output value V sp  is higher than the threshold V ref . 
     Therefore, when the output value V sp  is higher than the threshold V ref  (YES at S 35 ), the controller  198  executes the developer supply mode at S 36 . By contrast, when the output value V sp  is lower than the threshold V ref  (NO at S 35 ), that is, there is some toner adhered to the developer detection pattern, the controller  198  determines that the developer is present in the developing unit  4 . Consequently, the controller  198  displays an error message on the operating unit  194 , etc. 
     Then, the controller  198  turns off the charger and the development bias applicator at S 37 , and further turns off the photoreceptor motor  192  and the development motor  193  at S 38 . 
     It is to be noted that, although the development motor  193  is driven so as to form the developer detection pattern, the developer does not spill over from the developing unit  4  even if the developer is already present therein because driving time of the development motor is very short, supplying a very small amount of the developer, if any. 
     As described above, redundant replenishment of the developer can be prevented as well in the present embodiment because, when the developer is present in the developing unit  4 , the developer is not supplied from the toner bottle  120  thereto. Further, because the toner adhesion detector  190  serves as the developer detector, a separate developer detector is not required, saving both the number of components used in the copier  100  as well as the cost. 
     A procedure to execute the developer supply mode according to another illustrative embodiment is described below with reference to  FIGS. 21  though  23 . 
     If the maintenance person forgets to disengage the intermediate transfer belt  110  from the photoreceptors  1  before executing the developer supply mode, the intermediate transfer belt  110  might rub against the photoreceptors  1 , damaging the surfaces thereof. Therefore, in the present embodiment, the controller  198  checks whether or not the intermediate transfer belt  110  is disengaged therefrom before executing the developer supply mode as shown in  FIG. 21 . 
     Referring to  FIG. 21 , at S 41  the controller  198  activates the toner concentration sensor  191 , and checks whether or not the output value V t  is lower than the threshold V ref  so as to confirm that the developer is not present in the developing unit  4  at S 42 . When the output value V t  is lower than the threshold V ref  (YES at S 42 ), the controller  198  checks whether or not the intermediate transfer belt  110  is disengaged from the photoreceptors  1  at S 43 . After confirming that the intermediate transfer belt  110  is disengaged from the photoreceptors  1  (YES at S 43 ), the controller  198  executes the developer supply mode at S 44 . 
     It is to be noted that, alternatively, presence of the developer may be determined based on the amount of the toner adhered to the photoreceptor  1  or the torque of one of the development motor  193  and the photoreceptor motor  192  as described above. 
     Further, the procedure described above is performed when black or all colors is selected in the developer supply mode because the partial disengagement mode shown in  FIG. 11  is set as the default mode in the present embodiment. 
     This procedure is described in further detail below. 
     Referring to  FIG. 22A , a disengagement unit  140 A according to the present embodiment includes a filler  182  protruding from a bottom surface of the second arm  143  and a position detector  195  as a disengagement detector to detect disengagement between an intermediate transfer belt  110  and photoreceptors  1 . Other than that, the disengagement unit  140 A has a configuration similar to that of the disengagement unit  140  shown in  FIG. 10 , and thus a description thereof is omitted. 
     The position detector  195  in the present embodiment is a transmissive optical sensor including a light emitting element and a light receiving element arrayed to face each other at a predetermined or given distance apart, and detects a position of the intermediate transfer belt  110 . 
     In the partial disengagement mode shown in  FIG. 22A , in which the second disengagement cam  144  is at the engagement position to engage the intermediate transfer belt  110  with the photoreceptor  1 K, the light receiving element receives light emitted from the light emitting element, and the position detector  195  outputs a predetermined or given value. 
     By contrast, when the intermediate transfer belt  110  is disengaged from the photoreceptor  1 K by rotating the second disengagement cam  144  to the disengagement position using the lever  147  shown in  FIG. 14 , the filler  182  moves to between the light emitting element and the light receiving element. In this state, the filler  182  interrupts the light emitted from the light emitting element, and the output value from the light receiving element decreases. 
     Thus, the position detector  195  can detect that the intermediate transfer belt  110  is disengaged from the photoreceptor  1 K based on the rotational position of the second disengagement cam  144 . 
     The procedure using the filler  182  and the position detector  195  is described below with reference  FIGS. 22A ,  22 B, and  23 . 
     When the developer supply mode is executed regarding the developing unit  4 K for black, the controller  198  performs steps S 51  and S 52  that are similar to the steps S 41  and S 42  shown in  FIG. 21 , and thus descriptions thereof are omitted. After thus confirming that the developer is not present in the developing unit  4 K, the controller  198  checks whether or not the position detector  195  detects the filler  182  at S 53 . When the position detector  195  detects the filler  182  (YES at S 53 ), that is, the intermediate transfer belt  110  is disengaged from the photoreceptor  1 K, the controller  198  executes the developer supply mode at S 54 . 
     By contrast, when the position detector  195  does not detect the filler  182  (NO at S 53 ), the intermediate transfer belt  110  is in the partial disengagement mode and contacts the photoreceptor  1 K, that is, the maintenance person has forgotten to rotate the lever  147  shown in  FIG. 14 . Therefore, the controller  198  causes the operating unit (operation display)  194  to display an error message, and terminates the procedure. 
     As described above, in the present embodiment, the developer supply mode is not executed while the intermediate transfer belt  110  engages the photoreceptors  1 , preventing damage to the photoreceptors  1  and the intermediate transfer belt  110  caused by rubbing against each other. 
     Another illustrative embodiment in which the disengagement between the intermediate transfer belt  110  and the photoreceptor  1  is confirmed based on the rotational position of the second disengagement cam  144  is described below with reference to  FIGS. 24A ,  24 B, and  25 . 
     As shown in  FIG. 24A , a disengagement unit  140 B according to the present embodiment includes a filler  181  fixed to the shaft  144   a  and a position detector  195 A. The position detector  195 A has a configuration similar to that of the position detector  195  shown in  FIGS. 22A and 22B . The filler  181  and the position detector  195 A serve as a disengagement detector. Other than that, the disengagement unit  140 B has a configuration similar to that of the disengagement unit  140  shown in  FIG. 10 , and thus a description thereof is omitted. 
     When the intermediate transfer belt  110  contacts the photoreceptor  1 K, a part of the filler  181  is located between a light emitting element and a light receiving element of the position detector  195 A, interrupting the light emitted from the light emitting element. Therefore, the position detector  195 A does not output a predetermined or given output value. 
     By contrast, when the second disengagement cam  144  is rotated by 90 degrees or about 90 degrees to the disengagement position shown in  FIG. 24B , and accordingly the intermediate transfer belt  110  is disengaged from the photoreceptor  1 K, the light receiving element of the position detector  195 A receives the light emitted from the light emitting element. In this state, the position detector  195 A outputs a predetermined or given output value. Thus, based on the output value of the position detector  195 A, the rotational position of the second disengagement cam  144  can be detected, and accordingly disengagement between the intermediate transfer belt  110  and the photoreceptor  1 K can be detected. 
     The procedure using the filler  181  and the position detector  195 A is described below with reference  FIGS. 24A ,  24 B, and  25 . 
     When the developer supply mode is executed regarding the developing unit  4 K for black, the controller  198  performs steps S 61  and S 62  that are similar to the steps S 41  and S 42  shown in  FIG. 21 , and thus descriptions thereof are omitted. After thus confirming that the developer is not present in the developing unit  4 K (YES at S 62 ), the controller  198  checks whether or not the second disengagement cam  144  is at the disengagement position shown in  FIG. 24B  at S 63 . 
     More specifically, when the position detector  195 A outputs the predetermined output value, it is known that the second disengagement cam  144  is at the disengagement position as described above, and thus the controller  198  can confirm that the intermediate transfer belt  110  is disengaged from the photoreceptor  1 K. When the second disengagement cam  144  is at the disengagement position (YES at S 63 ), the controller  198  executes the developer supply mode at S 64 . 
     By contrast, when the second disengagement cam  144  is not at the disengagement position (NO at S 63 ), that is, the intermediate transfer belt  110  is in the partial disengagement mode and contacts the photoreceptor  1 K, the controller  198  causes the operating unit (operation display)  194  to display an error message, and terminates the procedure. 
     As described above, the developer supply mode is not executed while the intermediate transfer belt  110  engages the photoreceptors  1 , preventing damage to the photoreceptors  1  and the intermediate transfer belt  110  caused by rubbing against each other as well in the present embodiment. 
     Another illustrative embodiment is described below with reference to  FIG. 26 . 
       FIG. 26  is a flow chart illustrating a procedure in which disengagement of the intermediate transfer belt  110  from the photoreceptors  1  is detected based on an output value from the photoreceptor motor torque detector  196  that is used to detect an abnormal state of the image forming apparatus  100  shown in  FIG. 1  by detecting that the load of the photoreceptor motor  192  is abnormally high. 
     That is, the photoreceptor motor torque detector  196  serves as a disengagement detector to detect disengagement of the intermediate transfer belt  110  from the photoreceptors  1 . The photoreceptor motor torque detector  196  monitors and converts a driving current of the photoreceptor motor  192  into torque. 
     When the developer supply mode is executed for the developing unit  4 K for black, the controller  198  performs steps S 71  and S 72  that are similar to the steps S 41  and S 42  shown in  FIG. 21 , and thus descriptions thereof are omitted. After confirming that the developer is not present in the developing unit  4 K (YES at S 72 ), the controller  198  activates the photoreceptor motor  192  at S 73  and further activates the photoreceptor motor torque detector  196  at S 74  so as to detect torque of the photoreceptor motor  192 . 
     Then, at S 75  the controller  198  checks whether or not a detected torque T of the development motor  193  is lower than a predetermined or given threshold T ref . When the intermediate transfer belt  110  contacts the photoreceptor  1 K, the torques of the photoreceptor  1 K is higher, and accordingly the detected torque T is higher than the threshold T ref . Because it is known that the detected torque T is higher than the threshold T ref  (NO at S 75 ) when the intermediate transfer belt  110  contacts the photoreceptor  1 K, the controller  198  displays an error message on the operating unit  194 , etc., and does not execute the developer supply mode. 
     By contrast, when the detected torque T is lower than the threshold T ref  (YES at S 75 ), that is, the intermediate transfer belt  110  is disengaged from the photoreceptor  1 K, at S 76  the controller  198  executes the developer supply mode and then stops the photoreceptor motor  192  at S 77 . 
     Alternatively, disengagement of the intermediate transfer belt  110  from the photoreceptor  1  can be determined based on driving torque of the intermediate transfer belt  110  using the belt driving motor torque detector  199  shown in  FIG. 16 . Alternatively, disengagement between the intermediate transfer belt  110  and the photoreceptor  1  may be detected based on driving torque of both the photoreceptor  1  and the intermediate transfer belt  110 . 
     As described above, the developer supply mode is not executed while the intermediate transfer belt  110  engages the photoreceptors  1  as well in the procedure described above, preventing damage to the photoreceptors  1  and the intermediate transfer belt  110  caused by rubbing against each other. Further, because the photoreceptor motor torque detector  196  serves as the disengagement detector, a separate developer detector is not required, saving both the number of components used in the copier  100  as well as the cost. 
     A procedure to execute the toner supply mode according to another illustrative embodiment is described below with reference to  FIGS. 27 and 28 . 
     As shown in  FIG. 27 , a disengagement unit  140 C according to the present embodiment includes a second disengagement motor  183  to rotate the second disengagement cam  144  and a driving controller  184  to control the second disengagement motor  183 , and the intermediate transfer belt  110  can be automatically disengaged from the photoreceptor  1 K. Other than that, the disengagement unit  140 A has a configuration similar to that of the disengagement unit  140  shown in  FIG. 10 , and thus a description thereof is omitted. 
     When the developer supply mode is executed regarding the developing unit  4 K for black, the controller  198  performs steps S 81  and S 82  that are similar to the steps S 41  and S 42  shown in  FIG. 21 , and thus descriptions thereof are omitted. After confirming that the developer is not present in the developing unit  4 K (YES at S 82 ), the controller  198  activates the second disengagement motor  183  at S 83  so as to disengage the intermediate transfer belt  110  from the photoreceptor  1 K. Then, at S 84  the controller  198  executes the developer supply mode. 
     Thus, the intermediate transfer belt  110  can be automatically disengaged from the photoreceptors  1  before executing the developer supply mode in the procedure described above, preventing damage to the photoreceptors  1  and the intermediate transfer belt  110  caused by rubbing against each other. 
     As described above, the image forming apparatus  100  according to the illustrative embodiments of the present invention includes the photoreceptors  1  serving as the latent image carriers, the developing units  4  to respectively develop the latent images formed on the photoreceptors  1  with the developer, the developer detector to detect whether or not the developer is present in the developer container part of the developing unit  4 , and the controller  198  that supplies the developer to the developer container part from the toner bottle  120  set on the image forming apparatus  100  when the developer detector detects that the developer is not present therein. 
     In the configuration described above, because the developer is not supplied to the developer container part in which the developer is present, redundant replenishment of the developer due to human error can be prevented. 
     Further, the toner concentration sensor  191  shown in  FIG. 2  can be used as the developer detector, and presence of the developer in the developer container part can be detected based on the output value thereof as shown in  FIG. 18 . 
     Alternatively, as shown in  FIG. 19 , the developer detector can be configured to detect presence of the developer in the developer container part based on the torque of the supply screw  8 , agitation screw  11 , and collection screw  6  serving as the developer transporters shown in  FIG. 2 . 
     Alternatively, a predetermined or given image can be formed on the photoreceptor  1  as the detection pattern. Presence of the developer can be detected by detecting the amount of the toner adhered to the image with the toner adhesion detector  190  shown in  FIG. 2 . 
     The image forming apparatus  100  further includes the disengagement unit  140  to engage/disengage the intermediate transfer belt  110  with/from the photoreceptors  1 , the disengagement detector to detect whether or not the intermediate transfer belt  110  is disengaged therefrom. The controller  198  can be configured to supply the developer to the developer container part from the toner bottle  120  only when the developer detector detects that the developer is not present therein. With this configuration, the developer supply operation is not performed unless the intermediate transfer belt  110  is disengaged from the photoreceptors  1 , preventing damage to the photoreceptors  1  and the intermediate transfer belt  110  caused by rubbing against each other. 
     The disengagement detector can be configured to detect disengagement between the intermediate transfer belt  110  and the photoreceptor  1  based on driving torque of one of the photoreceptor  1  and the intermediate transfer belt  110 . 
     Alternatively, the disengagement detector can be configured to detect disengagement between the intermediate transfer belt  110  and the photoreceptor  1  based on a detection result generated by the position detector that detects position of the intermediate transfer belt  110 . 
     Alternatively, position of the intermediate transfer belt  110  can be detected based on a rotational position of the disengagement cam. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.