Patent Publication Number: US-9411297-B1

Title: Image formation apparatus having greater differential voltage for last station in a print conveyance direction

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority based on 35 USC 119 from prior Japanese Patent Application No. 2015-013116 filed on Jan. 27, 2015, entitled “IMAGE FORMATION APPARATUS”, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This disclosure relates to an image formation apparatus for forming an image. 
     2. Description of Related Art 
     Some image formation apparatuses have plural development devices and are configured to be capable of printing a color image (for example, Japanese Patent Application Publication No. 2014-32280). In such an image formation apparatus, each development device includes, for example, a supply roller, a development roller, a blade, and a photoreceptor drum. In each development device, for example, the supply roller supplies toner to the development roller, and the blade spreads the toner into a uniform toner layer on the surface of the development roller. Consequently, each development device develops a toner image on a surface of the photoreceptor drum on which an electrostatic latent image is formed. 
     SUMMARY OF THE INVENTION 
     An object of an embodiment of the invention is to provide an image formation apparatus capable of improving image quality. 
     An aspect of the invention is an image formation apparatus that includes: development devices placed on a one-to-one basis in stations arranged side by side, and configured to sequentially start the development in an order according to an arrangement order of the stations; and a power supply unit. Each development device includes a development unit, a supply unit configured to supply a developer to the development unit, and a regulation member configured to regulate an amount of the developer adhering on the development unit. The power supply unit applies voltages to each regulation member and each supply unit such that a first development device among the development devices has a greater differential voltage when the first development device is placed in a first station than when the first development device is placed in a second station in which the development is started earlier than in the first station, where the differential voltage denotes a voltage obtained by subtracting the absolute value of a voltage of the supply unit from the absolute value of a voltage of the regulation member. 
     According to an aspect of the invention, the first development device is set to have a greater differential voltage when being placed in the first station than when the first development device is placed in the second station, and thus, image quality can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram illustrating one example of a configuration of an image formation apparatus according to one embodiment of the invention. 
         FIG. 2  is an explanatory diagram illustrating one example of a configuration of the ID unit illustrated in  FIG. 1 . 
         FIG. 3A  is an explanatory diagram illustrating an example of the arrangement of the ID units. 
         FIG. 3B  is an explanatory diagram illustrating another example of the arrangement of the ID units. 
         FIG. 4  is a block diagram illustrating one example of a configuration of the image formation apparatus illustrated in  FIG. 1 . 
         FIG. 5A  is a table illustrating one example of an ID unit arrangement table illustrated in  FIG. 4 . 
         FIG. 5B  is a table illustrating another example of the ID unit arrangement table illustrated in  FIG. 4 . 
         FIG. 6A  is a table illustrating one example of a setting table illustrated in  FIG. 4 . 
         FIG. 6B  is a table illustrating another example of the setting table illustrated in  FIG. 4 . 
         FIG. 7  is an explanatory diagram illustrating an example of a supply of voltage to the ID unit illustrated in  FIG. 2 . 
         FIG. 8  is an explanatory diagram illustrating another example of a supply of voltage to the ID unit illustrated in  FIG. 2 . 
         FIG. 9  is a flowchart illustrating one example of the operation of the image formation apparatus illustrated in  FIG. 1 . 
         FIG. 10  is a schematic representation illustrating the behavior of toner in the image formation apparatus illustrated in  FIG. 1 . 
         FIG. 11  is a table illustrating one example of characteristics of the image formation apparatus illustrated in  FIG. 1 . 
         FIG. 12  is another schematic representation illustrating the behavior of the toner in the image formation apparatus illustrated in  FIG. 1 . 
         FIG. 13  is a table illustrating another example of characteristics of the image formation apparatus illustrated in  FIG. 1 . 
         FIG. 14  is still another schematic representation illustrating the behavior of the toner in the image formation apparatus illustrated in  FIG. 1 . 
         FIG. 15  is a table illustrating still another example of characteristics of the image formation apparatus illustrated in  FIG. 1 . 
         FIG. 16  is a table illustrating a further example of characteristics of the image formation apparatus illustrated in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Descriptions are provided hereinbelow for embodiments based on the drawings. In the respective drawings referenced herein, the same constituents are designated by the same reference numerals and duplicate explanation concerning the same constituents is omitted. All of the drawings are provided to illustrate the respective examples only. 
     An embodiment of the invention is described in detail below with reference to the drawings. 
     (Example of Configuration) 
       FIG. 1  illustrates one example of a configuration of an image formation apparatus (image formation apparatus  1 ) according to one embodiment of the invention. Image formation apparatus  1  functions as a printer to form an image on a recording medium, such as for example paper, using electrophotography. 
     Image formation apparatus  1  includes five image drum (ID) units  4  ( 4 K,  4 Y,  4 M,  4 C,  4 W), light sources  61  to  65 , primary transfer rollers  71  to  75 , intermediate transfer belt  11 , drive roller  12 , belt follower roller  13 , secondary transfer back-up roller  14 , cleaning blade  15 , and density sensor  17 . 
     ID units  4  are each operative to form a toner image. ID units  4  are placed in five stations S 1  to S 5 , respectively, on a one-to-one basis. In this example, five stations S 1  to S 5  are disposed in the order of stations S 5 , S 4 , S 3 , S 2 , S 1  in conveyance direction F. In this example, then, ID unit  4 K to form a toner image of a black color (K) is placed in station S 5 , ID unit  4 Y to form a toner image of a yellow color (Y) is placed in station S 4 , ID unit  4 M to form a toner image of a magenta color (M) is placed in station S 3 , ID unit  4 C to form a toner image of a cyan color (C) is placed in station S 2 , and ID unit  4 W to form a toner image of a white color (W) is placed in station S 1 . Each ID unit  4  is configured to be attachable to, and detachable from, any of the five stations S 1  to S 5 . Thus, image formation apparatus  1  is adapted to be capable of changing the order of ID units  4  (or the order of the colors) in five stations S 1  to S 5 , for example, as described later. 
       FIG. 2  illustrates one example of a configuration of ID unit  4 . ID unit  4  includes photoreceptor drum  41 , charge roller  42 , development roller  43 , supply roller  44 , toner container  45 , toner regulation blade  46 , cleaning blade  47 , and integrated circuit (IC) tag  48 . 
     Photoreceptor drum  41  is a member to carry an electrostatic latent image on its surface (or surface layer portion), and is constructed using a photoreceptor. Photoreceptor drum  41  rotates in a left-handed direction in this example, by power transmitted from a photoreceptor drum motor (not illustrated). Photoreceptor drum  41  is charged by charge roller  42 . Then, photoreceptor drum  41  of ID unit  4  placed in station S 1  is exposed to light by light source  61 , photoreceptor drum  41  of ID unit  4  placed in station S 2  is exposed to light by light source  62 , photoreceptor drum  41  of ID unit  4  placed in station S 3  is exposed to light by light source  63 , photoreceptor drum  41  of ID unit  4  placed in station S 4  is exposed to light by light source  64 , and photoreceptor drum  41  of ID unit  4  placed in station S 5  is exposed to light by light source  65 . Thus, an electrostatic latent image is formed on the surface of each photoreceptor drum  41 . 
     Charge roller  42  is a member to charge the surface (or the surface layer portion) of photoreceptor drum  41 . Charge roller  42  is arranged so as to contact the surface (or a peripheral surface) of photoreceptor drum  41 , and rotates in a right-handed direction in this example, in response to the rotation of photoreceptor drum  41 . Charge voltage CH is applied to charge roller  42  by charge voltage controller  314 , as described later. 
     Development roller  43  is a member to carry toner on its surface. Development roller  43  is arranged so as to contact the surface (or the peripheral surface) of photoreceptor drum  41 , and is adapted to rotate in the right-handed direction in this example, by the photoreceptor drum motor and a gear (not illustrated). In ID unit  4 , in this rotation, a gear ratio is set so as to produce friction between the surface of development roller  43  and the surface of photoreceptor drum  41 . On each photoreceptor drum  41 , a toner image is formed (or developed) according to the electrostatic latent image, by the toner supplied from development roller  43 . Development voltage DB is applied to development roller  43  by development voltage controller  313 , as described later. 
     Supply roller  44  is a member to supply the toner stored in toner container  45  to development roller  43 . Supply roller  44  is arranged so as to contact the surface (or a peripheral surface) of development roller  43 , and is adapted to rotate in the right-handed direction in this example, by the photoreceptor drum motor and the gear (not illustrated). Thus, in ID unit  4 , friction develops between the surface of supply roller  44  and the surface of development roller  43 , so that the toner, in turn, is charged by what is called frictional electrification. Supply voltage SB is applied to supply roller  44  by supply voltage controller  312 , as described later. 
     Toner container  45  is operative to store the toner. Specifically, toner container  45  in ID unit  4 K stores the toner of the black color (K), toner container  45  in ID unit  4 Y stores the toner of the yellow color (Y), toner container  45  in ID unit  4 M stores the toner of the magenta color (M), toner container  45  in ID unit  4 C stores the toner of the cyan color (C), and toner container  45  in ID unit  4 W stores the toner of the white color (W). The black toner contains carbon black for example, as a colorant. The yellow toner contains pigment yellow for example, as a colorant. The magenta toner contains pigment magenta for example, as a colorant. The cyan toner contains pigment cyan for example, as a colorant. The white toner contains titanium oxide for example, as a colorant. In this example, electrical conductivity of the white toner is higher than the electrical conductivity of the toner of the other colors. 
     Toner regulation blade  46  is a member configured to abut the surface of development roller  43  and thereby to form a layer made of the toner (or a toner layer) on the surface of development roller  43  and also regulate (or control or adjust) a thickness of the toner layer. Toner regulation blade  46  also has a function of adjusting the amount of electrostatic charge on the toner on the surface of development roller  43 . Toner regulation blade  46  is, for example, a plate-shaped elastic member (e.g. a leaf spring) made of stainless steel or the like, and is arranged in such a manner that a tip end portion of toner regulation blade  46  abuts on the surface of development roller  43 . As described later, supply voltage SB is applied by supply voltage controller  312  to toner regulation blades  46  of ID units  4  placed in stations S 2  to S 5 , and blade voltage BB is applied by blade voltage controller  311  to toner regulation blade  46  of ID unit  4  placed in station S 1 . 
     Cleaning blade  47  is a member to perform cleaning by scraping the toner remaining on the surface (or the surface layer portion) of photoreceptor drum  41 . Cleaning blade  47  is arranged so as to abut the surface of photoreceptor drum  41 , counter thereto (or protruding in the opposite direction to a direction of rotation of photoreceptor drum  41 ). 
     IC tag  48  is a tag to store data on an identification number of ID unit  4 , the color of the toner in toner container  45 , or the like. What is called RFID (Radio Frequency IDentifier), for example, can be used as IC tag  48 . The data stored in IC tag  48  is read for example by wire communication or radio communication, for example via interface  260  to be described later. 
     Light source  61  (see  FIG. 1 ) is a member to apply light to photoreceptor drum  41  of ID unit  4  placed in station S 1 , light source  62  is a member to apply light to photoreceptor drum  41  of ID unit  4  placed in station S 2 , light source  63  is a member to apply light to photoreceptor drum  41  of ID unit  4  placed in station S 3 , light source  64  is a member to apply light to photoreceptor drum  41  of ID unit  4  placed in station S 4 , and light source  65  is a member to apply light to photoreceptor drum  41  of ID unit  4  placed in station S 5 . Thus, photoreceptor drums  41  are exposed to light by light sources  61  to  65 , respectively. Consequently, the electrostatic latent image is formed on the surface of each photoreceptor drum  41 . 
     Primary transfer rollers  71  to  75  are members to electrostatically transfer the toner images formed by five ID units  4  placed in stations S 1  to S 5 , respectively, on a transfer surface of intermediate transfer belt  11 . Primary transfer roller  71  is arranged facing photoreceptor drum  41  of ID unit  4  placed in station S 1 , with intermediate transfer belt  11  in between. Likewise, primary transfer roller  72  is arranged facing photoreceptor drum  41  of ID unit  4  placed in station S 2 , with intermediate transfer belt  11  in between, primary transfer roller  73  is arranged facing photoreceptor drum  41  of ID unit  4  placed in station S 3 , with intermediate transfer belt  11  in between, primary transfer roller  74  is arranged facing photoreceptor drum  41  of ID unit  4  placed in station S 4 , with intermediate transfer belt  11  in between, and primary transfer roller  75  is arranged facing photoreceptor drum  41  of ID unit  4  placed in station S 5 , with intermediate transfer belt  11  in between. Transfer voltage TR 1  is applied to each of primary transfer rollers  71  to  75  by transfer controller  315 , as described later. Thus, in image formation apparatus  1 , the toner image formed by each ID unit  4  is transferred (or primarily transferred) on the transfer surface of intermediate transfer belt  11 . 
     Intermediate transfer belt  11  is, for example, an endless elastic belt constructed of a semiconducting plastic film of high resistance. Intermediate transfer belt  11  is stretched in tension by drive roller  12 , belt follower roller  13 , and secondary transfer back-up roller  14 . Then, intermediate transfer belt  11  is adapted to cyclically rotate in conveyance direction F in response to the rotation of drive roller  12 . In this rotation, intermediate transfer belt  11  is adapted to move between ID unit  4  placed in station S 5  and primary transfer roller  75 , between ID unit  4  placed in station S 4  and primary transfer roller  74 , between ID unit  4  placed in station S 3  and primary transfer roller  73 , between ID unit  4  placed in station S 2  and primary transfer roller  72 , and between ID unit  4  placed in station S 1  and primary transfer roller  71 . 
     Drive roller  12  is operative to cyclically rotate intermediate transfer belt  11 . In this example, drive roller  12  is arranged on the downstream side of five ID units  4  in conveyance direction F, and rotates in the right-handed direction in this example, by power transmitted from a belt drive motor (not illustrated). Thus, drive roller  12  is adapted to cyclically rotate intermediate transfer belt  11  in conveyance direction F. 
     Belt follower roller  13  makes the follower roller rotation in the right-handed direction in this example, in response to the cyclic rotation of intermediate transfer belt  11 . In this example, belt follower roller  13  is arranged on the upstream side of the five ID units  4  in conveyance direction F. 
     Secondary transfer back-up roller  14  makes the follower roller rotation in the right-handed direction in this example, in response to the cyclic rotation of intermediate transfer belt  11 . Secondary transfer back-up roller  14  is arranged facing secondary transfer roller  25  (to be described later), with conveyance path  20  for the conveyance of recording medium  9  and with the intermediate transfer belt  11  in between, as described later. 
     Cleaning blade  15  is a member to perform cleaning by scraping a deposit, such as the toner, adhering on the transfer surface of intermediate transfer belt  11 . In this example, cleaning blade  15  is arranged so as to abut the transfer surface of intermediate transfer belt  11  at a position facing belt follower roller  13 . The deposit scraped off by cleaning blade  15  is accommodated in container  16 . 
     Density sensor  17  is operative to detect the density of the toner of each color on the transfer surface of intermediate transfer belt  11 . Density sensor  17  is used for density correction, for example at power-on or the like, as described later. 
     Image formation apparatus  1  further includes hopping roller  21 , registration sensor  22 , registration roller  23 , conveyance roller  24 , secondary transfer roller  25 , fixation device  50 , ejection sensor  26 , separator  27 , and ejection roller  28 . These members are arranged along conveyance path  20  for the conveyance of recording medium  9 . 
     Hopping roller  21  is a member to take out recording media  9  contained in paper feed tray  2 , one by one, starting at the topmost one, and to send out taken-out recording medium  9  to conveyance path  20 . Registration sensor  22  is a mechanical sensor to detect the passage of recording medium  9 . Registration roller  23  is constructed of a pair of rollers with conveyance path  20  in between, and is operative to correct an oblique position of recording medium  9  fed from hopping roller  21 . Conveyance roller  24  is constructed of a pair of rollers with conveyance path  20  in between, and is operative to convey recording medium  9  so that recording medium  9  reaches a nip portion between secondary transfer back-up roller  14  and secondary transfer roller  25  at an appropriate time. 
     Secondary transfer roller  25  is a member to transfer a toner image on the transfer surface of intermediate transfer belt  11 , onto a transfer surface of recording medium  9 . Secondary transfer roller  25  is arranged facing secondary transfer back-up roller  14 , with intermediate transfer belt  11  and conveyance path  20  in between. Transfer voltage TR 2  is applied to secondary transfer roller  25  by transfer controller  315 , as described later. Thus, in image formation apparatus  1 , the toner image on the transfer surface of intermediate transfer belt  11  is transferred (or secondarily transferred) on the transfer surface of recording medium  9 . 
     Fixation device  50  is a member to fix the toner image transferred on recording medium  9 , to recording medium  9 , by applying heat and pressure to recording medium  9 . Fixation device  50  includes heat roller  51 , press roller  52 , and temperature sensor  53 . Heat roller  51  is, for example, a member internally including a heater such as a halogen lamp, and configured to apply heat to the toner on recording medium  9 . Press roller  52  is a member arranged in such a way as to form a pressure contact portion between press roller  52  and heat roller  51 , and configured to apply pressure to the toner on recording medium  9 . Temperature sensor  53  is operative to detect a surface temperature of heat roller  51  or press roller  52 . Thus, in fixation device  50 , the toner on recording medium  9  is heated and thus fused and is pressed. Consequently, the toner image is fixed on recording medium  9 . 
     Ejection sensor  26  is a mechanical sensor to detect passage of recording medium  9 . Separator  27  is operative to perform a control to determine whether to guide recording medium  9  to a conveyance path for ejection of recording medium  9  out of image formation apparatus  1  or to guide recording medium  9  to re-conveyance path  30  (to be described later). Ejection roller  28  is a member to eject recording medium  9  out of image formation apparatus  1 , when separator  27  guides recording medium  9  to the conveyance path for an ejection of recording medium  9  out of image formation apparatus  1 . 
     Image formation apparatus  1  further includes re-conveyance roller  31 , separator  32 , and re-conveyance rollers  33 ,  35 ,  36 . These members are arranged along re-conveyance path  30 . Re-conveyance path  30  is used, for example, to retransfer a toner image to the surface of recording medium  9  on which a toner image has been fixed once, or to transfer a toner image to a surface opposite from the surface having a toner image fixed thereon (i.e. to perform what is called two-sided printing). 
     Re-conveyance roller  31  is a member to convey recording medium  9  along re-conveyance path  30 , when separator  27  guides recording medium  9  to re-conveyance path  30 . Separator  32  is operative to perform a control to determine whether to guide recording medium  9  as it is to re-conveyance path  30  or to turn recording medium  9  over and then guide recording medium  9  to re-conveyance path  30 . Re-conveyance roller  33  is a member provided in conveyance path  34  to turn recording medium  9  over. Re-conveyance roller  33  and conveyance path  34  are used for what is called two-sided printing. Re-conveyance roller  35  is a member to convey recording medium  9  guided by separator  32 , along re-conveyance path  30 . Re-conveyance roller  36  is a member to guide recording medium  9  conveyed by re-conveyance roller  35 , again to conveyance path  20 . 
     By this configuration, in image formation apparatus  1 , the toner image formed by each ID unit  4  is transferred (or primarily transferred) on the transfer surface of intermediate transfer belt  11 , and the toner image on the transfer surface of intermediate transfer belt  11  is transferred (or secondarily transferred) on the transfer surface of recording medium  9 . When so doing, image formation apparatus  1  can change the order of ID units  4  (or the order of the colors) in five stations S 1  to S 5 , as described below. 
       FIGS. 3A and 3B  illustrate examples of an arrangement of ID units  4  in five stations S 1  to S 5 ;  FIG. 3A  illustrates the example in which ID units  4 K,  4 Y,  4 M,  4 C,  4 W are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as is the case with  FIG. 1 .  FIG. 3B  illustrates the example in which ID units  4 W,  4 Y,  4 M,  4 C,  4 K are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively. 
     Image formation apparatus  1  allows a user to change the order of ID units  4  (or the order of the colors) in five stations S 1  to S 5 , for example according to the type of recording medium  9 . For example, when recording medium  9  is paper, ID units  4 K,  4 Y,  4 M,  4 C,  4 W can be placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3A . In other words, when ID units  4  are placed in this manner, a toner image of the black color (K), a toner image of the yellow color (Y), a toner image of the magenta color (M), a toner image of the cyan color (C) and a toner image of the white color (W) are sequentially transferred in this order on the transfer surface of intermediate transfer belt  11 . In other words, the toner image of the white color (W) is transferred as the uppermost layer on the transfer surface of intermediate transfer belt  11 . In a subsequent secondary transfer, therefore, the toner image of the white color (W) is transferred as the lowest layer on a paper sheet (i.e. recording medium  9 ). This, for example when the color of paper (i.e. recording medium  9 ) is not white, enables reducing the likelihood of the color of the paper affecting image quality, thus improving the image quality. 
     Moreover, for example, when recording medium  9  is a transparent film, ID units  4 W,  4 Y,  4 M,  4 C,  4 K can be placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3B . In other words, when ID units  4  are placed in this manner, a toner image of the white color (W), a toner image of the yellow color (Y), a toner image of the magenta color (M), a toner image of the cyan color (C) and a toner image of the black color (K) are sequentially transferred in this order on the transfer surface of intermediate transfer belt  11 . In other words, the toner image of the white color (W) is transferred as the lowermost layer on the transfer surface of intermediate transfer belt  11 . In a subsequent secondary transfer, therefore, the toner image of the white color (W) is transferred as the uppermost layer on the transparent film (i.e. recording medium  9 ). This enables improving image quality, for example when the user observes a printed image from the surface of the transparent film (i.e. recording medium  9 ) opposite from the transfer surface thereof. 
     Note that, in this instance, the arrangement of ID units  4  is described by two examples (i.e.  FIGS. 3A and 3B ) given, but the invention is not so limited, and various arrangements are possible. 
       FIG. 4  illustrates one example of a control unit in image formation apparatus  1 . Image formation apparatus  1  includes system controller  200  and process controller  300 . 
     System controller  200  is operative to control the overall operation of image formation apparatus  1 . System controller  200  includes central processing unit (CPU)  210 , read only memory (ROM)  220 , random access memory (RAM)  230 , timer  240 , host interface  250 , and interface  260 . These components are interconnected via internal bus  270 . 
     CPU  210  is operative to control the overall operation of image formation apparatus  1  according to a printing processing program stored in ROM  220 , based on print data fed, for example, from a personal computer (not illustrated) via host interface  250 . Specifically, CPU  210  is adapted to control RAM  230  and timer  240  according to the printing processing program and also control operation of the members in image formation apparatus  1  via interface  260 . 
     ROM  220  is a nonvolatile memory and is operative to store the printing processing program. ROM  220  also stores ID unit arrangement table  221  and setting table  222 . 
     ID unit arrangement table  221  indicates the arrangement of ID units  4  in five stations S 1  to S 5 . 
       FIGS. 5A and 5B  illustrate examples of ID unit arrangement table  221 .  FIG. 5A  illustrates the example in a case where ID units  4 K,  4 Y,  4 M,  4 C,  4 W are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3A .  FIG. 5B  illustrates the example in a case where ID units  4 W,  4 Y,  4 M,  4 C,  4 K are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3B . ID unit arrangement table  221  indicates the correspondence between five stations S 1  to S 5  and five ID units  4  ( 4 K,  4 Y,  4 M,  4 C,  4 W). In the example of  FIG. 5A , stations S 5 , S 4 , S 3 , S 2 , S 1  are associated with ID units  4 K,  4 Y,  4 M,  4 C,  4 W, respectively, and in the example of  FIG. 5B , stations S 5 , S 4 , S 3 , S 2 , S 1  are associated with ID units  4 W,  4 Y,  4 M,  4 C,  4 K, respectively. 
     Setting table  222  contains various parameters to cause image formation apparatus  1  to operate. 
       FIGS. 6A and 6B  illustrate information on four parameters (i.e. blade voltage BB, supply voltage SB, development voltage DB, and charge voltage CH) in setting table  222 .  FIG. 6A  illustrates an example in a case where ID units  4 K,  4 Y,  4 M,  4 C,  4 W are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3A .  FIG. 6B  illustrates an example in a case where ID units  4 W,  4 Y,  4 M,  4 C,  4 K are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3B . Note that  FIGS. 6A and 6B  also illustrate differential voltage ΔV (=|BB|−|SB|) between an absolute value of blade voltage BB and an absolute value of supply voltage SB, for convenience of explanation. 
     As illustrated in  FIGS. 6A and 6B , blade voltage BB, supply voltage SB, development voltage DB, and charge voltage CH are set for each of the five ID units  4 . Moreover, for example, a voltage to be set for ID unit  4  associated with a toner of a certain color can be set so as to vary according to the station in which ID unit  4  is placed. Specifically, in this example, blade voltage BB and supply voltage SB for ID unit  4 W, when ID unit  4 W is placed in station S 1  ( FIG. 6A ), are set to their respective values which are different from those when ID unit  4 W is placed in station S 5  ( FIG. 6B ). More specifically, for example, differential voltage ΔV for ID unit  4 W has a positive value (120 V) when ID unit  4 W is placed in station S 1  ( FIG. 6A ), whereas differential voltage ΔV is 0 V when ID unit  4 W is placed in station S 5  ( FIG. 6B ). Thus, image formation apparatus  1  is adapted to set blade voltage BB, supply voltage SB, development voltage DB and charge voltage CH to be applied to each ID unit  4 , based on the arrangement of ID units  4  in the five stations S 1  to S 5 . 
     RAM  230  is a volatile memory to function as what is called a working memory. Specifically, RAM  230  is adapted to store print data, various types of control timing measured by timer  240 , or the like, for example. 
     Timer  240  is operative to measure time and feed the measured time to CPU  210 . 
     Host interface  250  is operative to receive print data from the personal computer (not illustrated) and also communicate various control signals to and from the personal computer. 
     Interface  260  is an interface to allow system controller  200  to control the operation of the members of image formation apparatus  1 . Specifically, system controller  200  feeds control signals to high-voltage controller  310  (to be described later), exposure controller  320  and motor controller  330  of process controller  300  via interface  260 , and also receives detected results from density sensor  17 , registration sensor  22 , ejection sensor  26  and temperature sensor  53  via interface  260 . System controller  200  is also operative to communicate information to and from IC tag  48  of each ID unit  4  via interface  260 . 
     Process controller  300  is operative to control a printing process such as the conveyance of recording medium  9 , charging, development, transfer, or fixing. Process controller  300  includes high-voltage controller  310 , exposure controller  320 , and motor controller  330 . 
     High-voltage controller  310  is operative to apply voltages to each ID unit  4 , primary transfer rollers  71  to  75 , and secondary transfer roller  25 , respectively. High-voltage controller  310  includes blade voltage controller  311 , supply voltage controller  312 , development voltage controller  313 , charge voltage controller  314 , and transfer controller  315 . 
     Blade voltage controller  311  is operative to apply blade voltage BB to toner regulation blade  46  of ID unit  4  placed in station S 1 . 
     Supply voltage controller  312  is operative to apply supply voltage SB to toner regulation blades  46  and supply rollers  44  of ID units  4  placed in stations S 2  to S 5 , and to apply supply voltage SB to supply roller  44  of ID unit  4  placed in station S 1 . 
       FIGS. 7 and 8  illustrate an application of voltage to toner regulation blades  46  and supply rollers  44  in ID units  4 .  FIG. 7  illustrates an example of ID units  4  placed in stations S 2  to S 5 , and  FIG. 8  illustrates an example of ID unit  4  placed in station S 1 . In ID units  4  placed in stations S 2  to S 5 , supply voltage controller  312  applies the same voltage (i.e. supply voltage SB) to toner regulation blades  46  and supply rollers  44 , as illustrated in  FIG. 7 . In other words, in each of ID units  4  placed in stations S 2  to S 5 , blade voltage BB of toner regulation blade  46  becomes equal to supply voltage SB of supply roller  44 . Meanwhile, in ID unit  4  placed in station S 1 , as illustrated in  FIG. 8 , blade voltage controller  311  applies blade voltage BB to toner regulation blade  46 , and supply voltage controller  312  applies supply voltage SB to supply roller  44 . Thus, in ID unit  4  placed in station S 1 , blade voltage BB of toner regulation blade  46  and supply voltage SB of supply roller  44  can be individually set. 
     Development voltage controller  313  is operative to apply development voltage DB to development rollers  43  of ID units  4  placed in stations S 1  to S 5 . Charge voltage controller  314  is operative to apply charge voltage CH to charge rollers  42  of ID units  4  placed in stations S 1  to S 5 . Transfer controller  315  is operative to apply transfer voltage TR 1  to primary transfer rollers  71  to  75 , and to apply transfer voltage TR 2  to secondary transfer roller  25 . 
     Exposure controller  320  is operative to control the exposure operation of light sources  61  to  65 . 
     Motor controller  330  is operative to control operation of the motors in image formation apparatus  1 . Thus, motor controller  330  is adapted to rotate each photoreceptor drum  41 , drive roller  12 , hopping roller  21 , registration roller  23 , conveyance roller  24 , ejection roller  28 , and press roller  52 . Motor controller  330  is also adapted to rotate separator  27  and separator  32  at certain rotation angels. 
     As employed herein, ID units  4 K,  4 Y,  4 M,  4 C,  4 W correspond to one specific example of “development devices” of the invention. ID unit  4 W corresponds to one specific example of a “first development device” of the invention. Development roller  43  corresponds to one specific example of a “development unit” of the invention. Supply roller  44  corresponds to one specific example of a “supply unit” of the invention. Toner regulation blade  46  corresponds to one specific example of a “regulation member” of the invention. Photoreceptor drum  41  corresponds to one specific example of an “image carrier” of the invention. Charge roller  42  corresponds to one specific example of a “charging unit” of the invention. High-voltage controller  310  corresponds to one specific example of a “power supply unit” of the invention. Blade voltage controller  311  corresponds to one specific example of a “first power supply” of the invention. Supply voltage controller  312  corresponds to one specific example of a “second power supply” and a “third power supply” of the invention. 
     (Operation and Function) 
     Next, a description is given with regard to the operation and function of image formation apparatus  1  according to the embodiment. 
     (General Outline of Operation) 
     Firstly, a description is given with reference to  FIGS. 1, 2 , etc. with regard to a general outline of the operation of image formation apparatus  1 . In system controller  200 , CPU  210  controls the operation of overall image formation apparatus  1  according to the printing processing program stored in ROM  220 , based on print data fed from personal computer PC via host interface  250 . When so doing, CPU  210  uses ID unit arrangement table  221  and setting table  222  stored in ROM  220 , to control the operation. 
     Process controller  300  controls the printing process. Specifically, high-voltage controller  310  applies voltages to each ID unit  4 , primary transfer rollers  71  to  75 , and secondary transfer roller  25 , respectively. Exposure controller  320  controls the exposure operation of light sources  61  to  65 . Motor controller  330  controls operation of the motors in image formation apparatus  1 . 
     In each of ID units  4 , photoreceptor drum  41  carries an electrostatic latent image on the surface (or the surface layer portion). Charge roller  42  charges the surface (or the surface layer portion) of photoreceptor drum  41 . Development roller  43  carries toner on the surface. Supply roller  44  supplies the toner stored in toner container  45  to development roller  43 . Toner regulation blade  46  forms a toner layer on the surface of development roller  43 , and regulates a thickness of the toner layer. Further, toner regulation blade  46  adjusts the amount of electrostatic charge on the toner on the surface of development roller  43 . IC tag  48  stores data on the identification number of ID unit  4 , the color of the toner in toner container  45 , or the like. 
     (Detailed Operation) 
       FIG. 9  illustrates one example of an operation of image formation apparatus  1 . At power-on, system controller  200  first checks the order of ID units  4  (or the order of the colors) in stations S 1  to S 5 . Then, system controller  200  controls the overall operation of image formation apparatus  1  according to the order of ID units  4 . This operation is described in detail below. 
     When the user powers image formation apparatus  1  on (at step S 1 ), system controller  200  first obtains data stored in IC tag  48  of each ID unit  4  (at step S 2 ). Thus, system controller  200  grasps the color of the toner of each ID unit  4 . Then, system controller  200  generates an ID unit arrangement table as illustrated in  FIG. 5A or 5B , based on the color of the toner of each ID unit  4 . 
     Then, system controller  200  checks whether or not the arrangement of ID units  4  has been changed (at step S 3 ). Specifically, system controller  200  compares the ID unit arrangement table generated at step S 2  with ID unit arrangement table  221  stored in ROM  220 , thereby to check whether or not the arrangement of ID units  4  has been changed. If at step S 3  a determination is made that the arrangement of ID units  4  has not been changed (“N” at step S 3 ), the operation goes to step S 6 . 
     If at step S 3  a determination is made that the arrangement of ID units  4  has been changed (“Y” at step S 3 ), image formation apparatus  1  performs a density correction (at step S 4 ). Specifically, system controller  200  first controls process controller  300  thereby to cause each ID unit  4  to form a toner image for the density correction. Thus, the toner image for the density correction is transferred to intermediate transfer belt  11 . Then, system controller  200  obtains a detected result of the density of the toner of each color on intermediate transfer belt  11 , from density sensor  17 . Then, system controller  200  corrects the density of the toner of each color, based on the detected result. System controller  200  performs such operation one or more times thereby to perform the density correction. Then, CPU  210  of system controller  200  updates a density parameter contained in setting table  222  stored in ROM  220 , based on the result of the density correction. 
     Then, system controller  200  writes the ID unit arrangement table generated at step S 2 , as ID unit arrangement table  221 , to ROM  220  (at step S 5 ). 
     Then, system controller  200  checks whether or not print data has been received via host interface  250  (at step S 6 ). Then, system controller  200  repeats step S 6  until the print data is received. 
     If at step S 6  a determination is made that the print data has been received (“Y” at step S 6 ), CPU  210  of system controller  200  reads setting table  222  from ROM  220  (at step S 7 ). 
     Then, image formation apparatus  1  performs a printing operation (at step S 8 ). Specifically, system controller  200  first controls high-voltage controller  310  of process controller  300 , based on setting table  222  read at step S 7 , so that high-voltage controller  310  applies blade voltage BB to toner regulation blade  46  of each ID unit  4 , applies supply voltage SB to supply roller  44  of each ID unit  4 , applies development voltage DB to development roller  43  of each ID unit  4 , applies charge voltage CH to charge roller  42  of each ID unit  4 , applies transfer voltage TR 1  to primary transfer rollers  71  to  75 , and applies transfer voltage TR 2  to secondary transfer roller  25 . System controller  200  also controls motor controller  330  of process controller  300  so that motor controller  330  operates the motors. Thus, recording medium  9  is fed and conveyed along conveyance path  20 . Then, system controller  200  controls exposure controller  320  of process controller  300  so that photoreceptor drums  41  are exposed to light by light sources  61  to  65 , respectively. Thus, toner images are formed by ID units  4 , and the toner images are each primarily transferred on the transfer surface of intermediate transfer belt  11 , and further, the toner images on intermediate transfer belt  11  are secondarily transferred on the transfer surface of recording medium  9 . Then, the toner images transferred to recording medium  9  are fixed by fixation device  50 . After that, recording medium  9  is ejected. 
     Thereafter, while image formation apparatus  1  is in its power-on state (“N” at step S 9 ), image formation apparatus  1  repeats the operation of steps S 6  to S 8 . 
     (With Regard to Blade Voltage BB and Supply Voltage SB) 
     Supply roller  44  supplies the toner stored in toner container  45  to development roller  43 . Then, toner regulation blade  46  forms a toner layer on the surface of development roller  43 , and regulates a thickness of the toner layer. Here, it is desirable that the toner on the surface of development roller  43  be sufficiently negatively charged (or be normally charged) by frictional electrification. However, toner having a high electrical conductivity due to the toner containing a metallic material, such for example as white toner, may be insufficiently negatively charged or be positively charged (or be unnormally charged) due to the fact that electric charge escapes even if an attempt is made to charge the toner. Thus, when the toner on the surface of development roller  43  is insufficiently negatively charged or is positively charged, image quality may deteriorate. This operation is described in detail below by giving several examples of the operation. 
     Firstly, as Operation Example 1, a description is given with regard to an example in which ID units  4 W,  4 Y,  4 M,  4 C,  4 K are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3B . 
       FIG. 10  schematically represents the behavior of white toner TW in Operation Example 1. As illustrated in  FIG. 10 , white toner TW, in this example, is primarily transferred on intermediate transfer belt  11  by ID unit  4 W placed in station S 5 . Then, toner TW on intermediate transfer belt  11  passes sequentially through ID units  4 Y,  4 M,  4 C,  4 K arranged downstream of ID unit  4 W, as intermediate transfer belt  11  moves in conveyance direction F. Here, positively charged toner or insufficiently negatively charged toner, in white toner TW, is adsorbed on photoreceptor drums  41  in ID units  4 Y,  4 M,  4 C,  4 K. In short, photoreceptor drums  41  are negatively charged, and therefore, the positively charged toner or the insufficiently negatively charged toner is adsorbed on photoreceptor drums  41 . Thus, white toner TW on intermediate transfer belt  11  decreases each time white toner TW passes through ID units  4 Y,  4 M,  4 C,  4 K. 
       FIG. 11  illustrates blade voltage BB and supply voltage SB in ID unit  4 W placed in station S 5 , and printed image quality in a case of printing performed under such setting conditions. In this example, three parameters (i.e. a gray background level, a stain level, and a white density) are used to evaluate the printed image quality. The gray background level is a parameter indicating the amount of white toner TW transferred to an undesired region on which white toner TW should not be transferred, due to the fact that white toner TW is insufficiently negatively charged. The gray background level is such that its higher value indicates that the toner is more sufficiently negatively charged. In this example, it is desirable that the gray background level be equal to or more than “9.” The stain level is a parameter indicating the degree of stain in a so-called white background region having no image, and its higher value indicates less stain. In this example, it is desirable that the stain level be equal to or more than “9.” The white density is a parameter indicating the density of white toner TW, and its lower value indicates higher density. In this example, it is desirable that the white density be equal to or less than “0.3.” 
     In this example, blade voltage BB and supply voltage SB in ID unit  4 W are both set to −430 [V]. In this case, the gray background level is “9” and the stain level is “9,” and they are both good. In other words, in this example, after the passage of white toner TW through ID units  4 Y,  4 M,  4 C,  4 K, positively charged toner or insufficiently negatively charged toner decreases, and thus, the gray background level has a good value. However, the amount of white toner TW on intermediate transfer belt  11  decreases, and thus, the white density is “0.35,” which is slightly more than its desirable level “0.3.” 
     Next, as Operation Example 2, a description is given with regard to an example in which ID units  4 K,  4 Y,  4 M,  4 C,  4 W are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3A . 
       FIG. 12  schematically represents the behavior of white toner TW in Operation Example 2. As illustrated in  FIG. 12 , white toner TW is primarily transferred on intermediate transfer belt  11  by ID unit  4 W placed in station S 1 . Then, toner TW on intermediate transfer belt  11  moves rearward as intermediate transfer belt  11  moves in conveyance direction F. Here, positively charged toner or insufficiently negatively charged toner, in white toner TW, remains as it is, on intermediate transfer belt  11 . In other words, in Operation Example 2, as distinct from the case of Operation Example 1, ID unit  4  is absent downstream of ID unit  4 W, and thus, such toner remains as it is, on intermediate transfer belt  11 . 
       FIG. 13  illustrates blade voltage BB and supply voltage SB in ID unit  4 W placed in station S 1 , and printed image quality in a case of printing performed under such setting conditions. In this example, blade voltage BB and supply voltage SB in ID unit  4 W are both set to −430 [V]. In this case, the white density is “0.25” and the stain level is “9,” and they are both good. In other words, in this example, positively charged toner or insufficiently negatively charged toner remains, as it is, on intermediate transfer belt  11  and does not decrease unlike Operation Example 1, and thus, the white density has a good value. Thus, however, such positively charged toner or insufficiently negatively charged toner remains on intermediate transfer belt  11 , and thus, the gray background level is “5,” which is less than its desirable level “9.” 
     In image formation apparatus  1 , therefore, toner regulation blade  46  adjusts the amount of electrostatic charge on the toner on the surface of development roller  43 . Specifically, blade voltage BB of toner regulation blade  46  is set to a negative voltage having a larger value. 
       FIG. 14  schematically represents the behavior of white toner TW on the surface of development roller  43 . In  FIG. 14 , TW 1  indicates sufficiently negatively charged toner, TW 2  indicates positively charged toner, and TW 3  indicates insufficiently negatively charged toner. 
     In this example, blade voltage BB is set to −550 [V]. In other words, in this example, blade voltage BB is set to a negative voltage having a larger value (−550 [V]), although in Operation Example 2 blade voltage BB is set to −430 [V] as illustrated in  FIG. 13 . Thus, positively charged toner TW 2  and insufficiently negatively charged toner TW 3  on the surface of development roller  43 , for example, can be sufficiently negatively charged. Consequently, the amount of positively charged toner TW 2  and insufficiently negatively charged toner TW 3  on the surface of development roller  43  can be reduced. 
     Next, as Operation Example 3, a description is given with regard to an example in which blade voltage BB is set to −550 [V] in a case where ID units  4 K,  4 Y,  4 M,  4 C,  4 W are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3A . 
       FIG. 15  illustrates blade voltage BB and supply voltage SB in ID unit  4 W placed in station S 1 , and printed image quality in the case of printing performed under such setting conditions. In this example, blade voltage BB and supply voltage SB in ID unit  4 W are both set to −550 [V]. In this case, the gray background level is “9” and the white density is “0.25,” and they are both good. In other words, in this example, blade voltage BB is set low, and thus, the amount of positively charged toner TW 2  and insufficiently negatively charged toner TW 3  on the surface of development roller  43  decreases, so that the gray background level and the white density have their respective good values. In this example, however, supply voltage SB is also set to −550 [V], and thus, supply roller  44  supplies an excessive amount of toner to development roller  43 . Consequently, toner is developed even in a so-called white background region having no image, and thus, the stain level is “5,” which is less than its desirable level “9.” 
     Next, as Operation Example 4, a description is given with regard to an example in which blade voltage BB is set to −550 [V] and supply voltage SB is set to −430 [V] in a case where ID units  4 K,  4 Y,  4 M,  4 C,  4 W are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3A . Note that Operation Example 4 corresponds to the settings in  FIG. 6A . 
       FIG. 16  illustrates blade voltage BB and supply voltage SB in ID unit  4 W placed in station S 1 , and printed image quality in the case of printing performed under such setting conditions. In this example, blade voltage BB in ID unit  4 W is set to −550 [V], and supply voltage SB is set to −430 [V]. In this case, as is the case with Operation Example 3, the gray background level is “9” and the white density is “0.25,” and they are both good. Further, the stain level is “9,” which is a good value. In other words, as distinct from the case of Operation Example 3, supply voltage SB is set to −430 [V], and thus, supply roller  44  can supply a proper amount of toner to development roller  43 , so that the stain level can be improved. 
     In image formation apparatus  1 , for example, when ID units  4 K,  4 Y,  4 M,  4 C,  4 W are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3A , blade voltage BB in ID unit  4 W placed in station S 1  is set to −550 [V] and supply voltage SB is set to −430 [V], as illustrated in  FIG. 6A . In other words, differential voltage ΔV is set to a positive value (120 [V]). Thus, image formation apparatus  1  can achieve the gray background level, the stain level and the white density all having their respective good values, as illustrated in Operation Example 4 ( FIG. 16 ), thereby enabling an improvement in image quality. 
     Moreover, for example, when ID units  4 W,  4 Y,  4 M,  4 C,  4 K are placed in this order in five stations S 5 , S 4 , S 3 , S 2 , S 1 , respectively, as illustrated in  FIG. 3B , blade voltage BB and supply voltage SB in ID unit  4 W placed in station S 5  are both set to −430 [V], as illustrated in  FIG. 6B . In other words, differential voltage ΔV is set to 0 [V]. Thus, image formation apparatus  1  can achieve the gray background level and the stain level having their respective good values and can also suppress deterioration in the white density, as illustrated in Operation Example 1 ( FIG. 11 ), thereby enabling an improvement in image quality. 
     In other words, in image formation apparatus  1 , as illustrated in  FIGS. 6A and 6B , differential voltage ΔV in ID unit  4 W when ID unit  4 W is placed in station S 1  ( FIG. 6A ) is set greater than differential voltage ΔV in ID unit  4 W when ID unit  4 W is placed in station S 5  ( FIG. 6B ). Thus, image formation apparatus  1  can improve image quality. 
     (Advantageous Effect) 
     According to the embodiment, as described above, differential voltage ΔV when ID unit  4 W is placed in station S 1  is set greater than differential voltage ΔV when ID unit  4 W is placed in station S 5 , and thus, image quality can be improved. 
     (Modification 1) 
     In the above-described embodiment, the white toner is described byway of example; however, the invention is not so limited. In other words, the black toner, the yellow toner, the magenta toner, and the cyan toner, for example, may also be insufficiently negatively charged or be positively charged although there is a difference in extent. Therefore, the invention may be applied to anything other than the white toner. For example when the invention is applied to the black toner, differential voltage ΔV when ID unit  4 K is placed in station S 1  is set greater than differential voltage ΔV when ID unit  4 K is placed in station S 5 . The same goes for application to other-colored toners. 
     Also, when a transparent toner (or clear toner), gold toner, silver toner, mica toner, UV toner, or the like, for example, is used, the invention may be applied to such toner. The transparent toner is used for example to partially add luster. The gold toner or the silver toner is used for example to give a glossy luster. The gold toner contains, for example, copper as a colorant, and the silver toner contains, for example, aluminum as a colorant. The mica toner is toner containing a magnetic material. The UV (Ultra Violet) toner is toner which reacts with ultraviolet light. Such toner is somewhat high in electrical conductivity due to it containing a metallic material. Consequently, such toner may be insufficiently negatively charged or be positively charged due to the fact that electric charge escapes even if an attempt is made to charge the toner. When such toner is used, it is therefore desirable that the invention be applied to the toner. 
     (Modification 2) 
     In the above-described embodiment, differential voltage ΔV when ID unit  4 W associated with the white toner is placed in station S 1  is set greater than differential voltage ΔV when ID unit  4 W is placed in station S 5 ; however, the invention is not so limited. Instead, for example, differential voltage ΔV when ID unit  4 W associated with the white toner is placed in station S 1  may be set greater than differential voltage ΔV when ID unit  4 W is placed in a station (i.e. at least one of stations S 2  to S 5 ) upstream of station S 1 . Also, for example, differential voltage ΔV when ID unit  4 W associated with the white toner is placed in station S 2  may be set greater than differential voltage ΔV when ID unit  4 W is placed in a station (i.e. at least one of stations S 3  to S 5 ) upstream of station S 2 . Also, for example, differential voltage ΔV when ID unit  4 W associated with the white toner is placed in station S 3  may be set greater than differential voltage ΔV when ID unit  4 W is placed in a station (i.e. at least one of stations S 4  and S 5 ) upstream of station S 3 . Also, for example, differential voltage ΔV when ID unit  4 W associated with the white toner is placed in station S 4  may be set greater than differential voltage ΔV when ID unit  4 W is placed in station S 5  upstream of station S 4 . 
     (Modification 3) 
     In the above-described embodiment, a toner image formed by each ID unit  4  is transferred (or primarily transferred) on the transfer surface of intermediate transfer belt  11 , and thereafter, the toner image on the transfer surface of intermediate transfer belt  11  is transferred (or secondarily transferred) on the transfer surface of recording medium  9 ; however, the invention is not so limited. Instead, the toner image formed by each ID unit  4  may be transferred directly on the transfer surface of recording medium  9 . 
     (Modification 4) 
     In the above-described embodiment, as illustrated in  FIGS. 6A and 6B , negative voltages are applied to charge roller  42 , development roller  43 , supply roller  44 , and toner regulation blade  46 , respectively; however, the invention is not so limited. Instead, for example, positive voltages may be applied to charge roller  42 , development roller  43 , supply roller  44 , and toner regulation blade  46 , respectively. Also in this case, differential voltage ΔV (=|BB|−|SB|=BB−SB) when ID unit  4 W is placed in station S 1  is set greater than differential voltage ΔV when ID unit  4 W is placed in station S 5 , and thereby, image quality can be improved. 
     Although the invention is described above with reference to the embodiment and Modifications, it should be understood that the invention is not limited to the embodiment and the like, and various modifications are possible. 
     For example, in the above-described embodiment and the like, the invention is applied to the printer but is not so limited, and instead, for example, the invention may be applied to a multifunction peripheral having functions such as a printer, a facsimile, and a scanner. 
     The invention includes other embodiments in addition to the above-described embodiments without departing from the spirit of the invention. The embodiments are to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. Hence, all configurations including the meaning and range within equivalent arrangements of the claims are intended to be embraced in the invention.