Abstract:
An image forming device is provided with a photoreceptor, a belt, a back member, a front member, and a voltage change device. The belt faces the photoreceptor. The back member is disposed on a back side of the belt. The front member is disposed on a front side of the belt. The front member is disposed adjacent to the belt and faces the back member. The voltage change device is configured to change voltage between the back member and the front member within a range excluding zero. Based on detected conditions such as the number of sheets printed, the replacement of a developer cartridge, or the current between the back member and the front member, the voltage change device change the voltage between the back member and the front member to enhance the ability to clean the belt.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application NO. 2005-226300, filed on Aug. 4, 2005, the contents of which are hereby incorporated by reference into the present application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming device comprising a photoreceptor such as a laser printer etc. 
     2. Description of the Related Art 
     A laser printer forms an image on a printing sheet by transferring a developer developed on a photoreceptor onto the printing sheet. Some laser printers comprise a belt disposed so as to face the photoreceptor. This belt may be used to convey the printing sheet while causing the printing sheet to contact the photoreceptor. As the printing sheet is conveyed while contacting the photoreceptor, the developer is transferred onto the printing sheet from the photoreceptor. In the present specification, a belt for conveying a print medium (printing sheet or the like) will be referred to as a conveyor belt. 
     Furthermore, a belt is known which contacts the photoreceptor such that the developer is transferred onto the belt from the photoreceptor. The printing sheet contacts a part of the belt on which the developer has been transferred. The developer is thus transferred onto the printing sheet from the belt. In this technique, a primary transfer from the photoreceptor to the belt and a secondary transfer from the belt to the printing sheet are performed. In the present specification, a belt used in an image forming device which adopts this technique of performing the primary transfer and the secondary transfer will be referred to as an intermediate transfer belt. 
     Paper particles of the printing sheet adhere to the conveyor belt. If the paper particles remain on the conveyor belt, the printing quality may deteriorate. Further, an image forming device is known which evaluates the concentration of the developer by transferring the developer from the photoreceptor to the conveyor belt on a trial basis. Further, developer may adhere to the conveyor belt during a paper jam. If the developer remains on the conveyor belt, the printing sheet is stained when the conveyor belt conveys the printing sheet. Therefore, the conveyor belt must be cleaned to remove the paper particles and developer. 
     There is a possibility that the developer transferred onto the intermediate transfer belt during the primary transfer is not transferred entirely onto the printing sheet during the secondary transfer. If developer remains on the intermediate transfer belt, this developer may be transferred onto the printing sheet. In this case, the developer is transferred onto unintended parts of the printing sheet, and this causes deterioration of the printing quality. Therefore, the intermediate transfer belt must be cleaned to remove the developer not having been transferred to the printing sheet in the secondary transfer. 
     As described above, when the conveyor belt or the intermediate transfer belt is used, the belt must be cleaned. US Patent Application Publication NO. 2005/0074250 discloses a technique for cleaning the belt. This technique adopts a back roller disposed on the back side of the belt and a front roller disposed on the front side of the belt. The front roller faces the back roller. In this technique, a constant voltage is applied between the back roller and front roller. The paper particles and developer adhered to the belt move to the front roller by an electric field generated between the back roller and front roller. The paper particles and developer are thus trapped on the front roller, and the belt is cleaned. 
     BRIEF SUMMARY OF THE INVENTION 
     A front member (the front roller in the prior art described above) disposed on a front side of a belt traps paper particles and/or developer from the belt. If the paper particles and/or developer remain on the front member, the ability of the front member to clean the belt deteriorates. In the prior art described above, the front member is cleaned by another member. However, the paper particles and/or developer trapped on the front member cannot be removed completely by cleaning the front member, and paper particles and/or developer accumulate on the front member. Even when the front member is cleaned, its ability to clean the belt deteriorates steadily as the image forming device is used. 
     The present invention has been created in consideration of the circumstances described above, and it is a purpose thereof to provide a technique which enables an improvement in belt cleaning ability. 
     As a result of research, the present inventors learned that the belt cleaning ability of the front member is greatly affected by the magnitude of current flowing between a back member (the back roller in the prior art described above) and the front member. More specifically, it was discovered that even when a constant voltage is applied between the back member and front member such that an electric field having a fixed magnitude is generated, the belt cleaning ability of the front member changes when the current that flows between the back member and front member changes. Cleaning can be performed efficiently if maintaining the current between the back member and front member within a certain range. However, the cleaning ability deteriorates if the current deviates from this range. 
     When the front member becomes soiled, the electric resistance of the front member changes (usually increases). Therefore, in a case where the voltage between the front member and back member is regulated to a constant magnitude, the magnitude of the current between the front member and back member changes when the front member becomes soiled. When the magnitude of the current changes, the belt cleaning ability of the front member deteriorates such that the front member becomes unable to trap the paper particles and/or developer adhered to the belt satisfactorily. 
     An image forming device of the present invention has been created on the basis of the knowledge described above. 
     The image forming device of the present invention comprises a photoreceptor and a belt facing the photoreceptor. The photoreceptor may be a photoreceptor drum. The photoreceptor also may be a photoreceptor belt. The belt may be a conveyor belt or an intermediate transfer belt. The image forming device comprises a back member disposed on a back side of the belt and a first front member disposed on a front side of the belt. The first front member is disposed adjacent to the belt and facing the back member. The image forming device also comprises a first voltage change device which is capable of changing voltage between the back member and the first front member from a certain value other than zero to another value other than zero. In other words, the first voltage change device is capable of changing the voltage in a range excluding zero. 
     The above term “in a range excluding zero” is used to the exclusion of a structure in which the voltage is merely switched ON and OFF between zero and a predetermined value other than zero. 
     The first voltage change device may change the voltage among certain value other than zero, another value other than zero, and zero. The first voltage change device may change the voltage among more than three values. For example, the first voltage change device may change the voltage among a first value other than zero, a second value other than zero, and a third value other than zero. 
     In this image forming device, when the first front member becomes soiled, the magnitude of the voltage between the back member and first front member may be changed. By changing the magnitude of the voltage, the current flowing between the back member and first front member can be adjusted to a current which allows paper particles and/or developer adhered to the belt to be trapped by the first front member efficiently. This image forming device is able to maintain a favorable belt cleaning ability even when the first front member becomes soiled. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic side view of a laser printer of a first embodiment. 
         FIG. 2  shows a sectional view of a development device and an exposure device. 
         FIG. 3  shows a diagram illustrating a structure of a belt cleaning device. 
         FIG. 4  shows a flowchart illustrating voltage adjustment process executed by a controller. 
         FIG. 5  shows a view for explaining an experiment performed to evaluate cleaning ability. 
         FIG. 6  shows a relationship between voltage between a back roller and a first front roller, and the cleaning ability of the first front roller. 
         FIG. 7  shows a relationship between voltage between the first front roller and a second front roller, and the cleaning ability of the second front roller. 
         FIG. 8  shows a manner in which an electric potential of the first front roller and an electric potential of the second front roller vary over time. 
         FIG. 9  shows storage content of a memory according to a second embodiment. 
         FIG. 10  shows a diagram illustrating a structure of a belt cleaning device according to a third embodiment. 
         FIG. 11  shows a manner in which an electric potential of the first front roller and an electric potential of the second front roller vary over time (fourth embodiment). 
         FIG. 12  shows a manner in which an electric potential of the first front roller and an electric potential of the second front roller vary over time (fifth embodiment). 
         FIG. 13  shows a schematic side view of a laser printer according to a sixth embodiment. 
         FIG. 14  shows a schematic side view of a laser printer according to a seventh embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     An embodiment of the present invention will be described with reference to the drawings.  FIG. 1  is a view showing a simplification of the structure of a laser printer  10  according to this embodiment. 
     The laser printer  10  comprises an overall casing  12 . A paper feeding device  20 , a printing sheet conveying device  40 , development devices  50   a  to  50   d , exposure devices  80   a  to  80   d , a toner fixing device  100 , a belt cleaning device  120 , and so on are provided in the interior of the overall casing  12 . These devices  20 ,  40 , etc. will be described in sequence. 
     The paper feeding device  20  comprises a paper feeding tray  22 , three rollers  26 ,  30 ,  32 , a guide  28 , and so on. The paper feeding tray  22  can be pulled out from the overall casing  12 . When pulled out from the overall casing  12 , printing sheets  2  can be replenished in the paper feeding tray  22 . The paper feeding tray  22  comprises a base plate  24  on which a stack of the printing sheets  2  is placed. The uppermost sheet of the printing sheets  2  placed on the base plate  24  contacts the roller  26 . When the paper feeding tray  22  is stored inside the overall casing  12 , a right end portion of the base plate  24  is biased upward by a mechanism not shown in the drawing. Hence, when the number of the printing sheets  2  becomes low, the right end portion of the base plate  24  is raised upward. By means of this structure, the uppermost sheet of the printing sheets  2  can be kept in constant contact with the roller  26 . 
     The roller  26  will be referred to as a paper feeding roller. The rollers  30 ,  32  will be referred to as conveyance rollers. The paper feeding roller  26  is connected to a drive source not shown in the drawing. When feeding the printing sheet  2 , the paper feeding roller  26  rotates counterclockwise. As a result, the uppermost sheet of the printing sheets  2  is conveyed toward the guide  28  and conveyance rollers  30 ,  32  (arrow D 1 ). The guide  28  guides the printing sheet  2  conveyed by the paper feeding roller  26  toward between the conveyance rollers  30 ,  32 . The conveyance roller  32  is not connected to a drive source. The conveyance roller  30  is connected to a drive source, not shown in the drawing, and is rotated counterclockwise thereby. When the conveyance roller  30  rotates counterclockwise, the conveyance roller  32  rotates clockwise in response thereto. Thus the printing sheet  2  is conveyed between the conveyance rollers  30 ,  32  in the direction of the arrow D 1 . 
     The printing sheet conveying device  40  is disposed above the paper feeding tray  22 . The printing sheet conveying device  40  comprises two belt rollers  42 ,  44 , a belt  48 , a frame not shown in the drawing, and so on. The belt roller  42  and the belt roller  44  have a columnar shape extending in a perpendicular direction to the paper surface of  FIG. 1 . The belt roller  42  and the belt roller  44  are disposed in parallel and at an identical height. The belt  48  straddles the belt roller  42  and the belt roller  44 . The belt roller  42  is connected to a drive source not shown in the drawing, and is rotated counterclockwise thereby. The belt roller  44  is a driven roller. When the belt roller  42  rotates counterclockwise, the belt  48  rotates counterclockwise, and in response to the rotation of the belt  48 , the belt roller  44  rotates counterclockwise. 
     The printing sheet  2  conveyed by the conveyance rollers  30 ,  32  is placed on the upper surface of the belt  48  at the upper side thereof. The printing sheet  2  placed on the belt  48  is conveyed in a leftward direction as the belt  48  rotates (in the direction of arrows D 2  and D 3 ). Toner is transferred onto the printing sheet  2  in sequence from the four development devices  50   a  to  50   d.    
     The four development devices  50   a  to  50   d  are aligned in the horizontal direction. The development device  50   a  disposed furthest to the right transfers yellow toner onto the printing sheet  2 . The development device  50   b  disposed directly to the left of the development device  50   a  transfers magenta toner onto the printing sheet  2 . The development device  50   c  disposed directly to the left of the development device  50   b  transfers cyan toner onto the printing sheet  2 . The development device  50   d  disposed furthest to the left transfers black toner onto the printing sheet  2 . 
     The four development devices  50   a  to  50   d  are structured identically. Referring to  FIG. 2 , the structure of the development devices  50   a  to  50   d  will be described.  FIG. 2  is a vertical sectional view of the development device  50  and the exposure device  80 . Note that in  FIG. 2 , the reference numeral  50  is used to represent the development devices  50   a  to  50   d . The reference numeral  50  will be used similarly hereafter when there is no particular need to differentiate between the individual development devices  50   a  to  50   d . Also in  FIG. 2 , the reference numeral  80  is used to represent the exposure devices  80   a  to  80   d . The reference numeral  80  will be used similarly hereafter when there is no particular need to differentiate between the individual exposure devices  80   a  to  80   d.    
     The development device  50  comprises two cartridges  52 ,  56 , a transfer roller  66 , and so on. The upper side cartridge  52  will be referred to as a development cartridge. The lower side cartridge  56  will be referred to as a photoreceptor cartridge  56 . The development cartridge  52  and the photoreceptor cartridge  56  will be referred to together as a process cartridge. The process cartridge is mounted in the overall casing  12  detachably. An old process cartridge may be removed from the overall casing  12  and exchanged for a new one. The development cartridge  52  and photoreceptor cartridge  56  are connected to each other in a manner allowing the cartridges  52  and  56  to be separated. With this process cartridge, it is possible to exchange the development cartridge  52  alone and to exchange the photoreceptor cartridge  56  alone. The process cartridge may also be replaced as a whole. 
     The structure of the development cartridge  52  will now be described. The development cartridge  52  comprises a casing  53 . A toner chamber  53   a  is formed in the interior of the casing  53 . Toner is stored in the toner chamber  53   a . The respective development devices  50   a  to  50   d  each store a different colored toner. Yellow toner is stored in the toner chamber  53   a  of the development device  50   a . Magenta toner is stored in the toner chamber  53   a  of the development device  50   b . Cyan toner is stored in the toner chamber  53   a  of the development device  50   c . Black toner is stored in the toner chamber  53   a  of the development device  50   d.    
     In this embodiment, a positively-charged, non-magnetic single-component toner is used. A polymer toner is used which is obtained, for example, by subjecting a styrene monomer or an acrylic monomer to copolymerization using a polymerization method such as suspension polymerization. Acrylic acid, alkyl (C1 to C4) acrylate, alkyl (C1 to C4) methacrylate, and so on may be adopted as the acrylic monomer. This polymer toner has a substantially spherical shape and exhibits excellent fluidity. A colorant is blended with the polymer toner. As a result, toners of the four colors (yellow, magenta, cyan, black) are realized. A charge controlling agent is blended with the polymer toner. A resin obtained from a copolymer of an ionic monomer and another monomer (a styrene monomer or acrylic monomer) may be adopted as the charge controlling agent. A monomer having an ionic functional moiety such as ammonium salt may be adopted as an ionic monomer. Further, an external additive is added to the polymer toner. A metallic oxide powder, carbide powder, metallic salt powder, or another powder may be adopted as the external additive. Silica, aluminum oxide, strontium titanate, cerium oxide, magnesium oxide, or similar may be adopted as the metallic oxide. 
     An agitator  54  is housed in the toner chamber  53   a . The agitator  54  is attached to the casing  53  in a manner allowing its rotation. When the agitator  54  rotates, the toner in the toner chamber  53   a  is agitated. 
     A supply roller  60  and a developing roller  62  are housed in the casing  53 . The supply roller  60  is supported by the casing  53  in a manner allowing its rotation. The supply roller  60  comprises a supply roller main body  60   a  and a supply roller shaft  60   b . The supply roller main body  60   a  is formed from a conductive foamed material. The supply roller shaft  60   b  is made of metal. The supply roller shaft  60   b  is connected to a drive source not shown in the drawing, and thus the supply roller  60  rotates counterclockwise. 
     The developing roller  62  contacts the lower side of the supply roller  60 . The developing roller  62  is supported by the casing  53  in a manner allowing its rotation. The developing roller  62  comprises a developing roller main body  62   a  and a developing roller shaft  62   b . The developing roller main body  62   a  is made of a conductive rubber material. Conductive urethane rubber or silicone rubber containing carbon microparticles or the like may be adopted as the rubber material. The surface of the urethane rubber or silicone rubber is covered by urethane rubber or silicone rubber containing fluorine. The developing roller shaft  62   b  is made of metal. A voltage supply circuit, not shown in the drawing, is connected to the developing roller shaft  62   b . During development (when the toner is adhered to a photoreceptor drum  64  (to be described below)), a bias is applied to the developing roller  62  from the voltage supply circuit. The developing roller  62  is connected to a drive source not shown in the drawing, and is rotated counterclockwise thereby. 
     Next, the structure of the photoreceptor cartridge  56  will be described. The photoreceptor cartridge  56  comprises a casing  57 . A hole  57   a  which transmits a laser beam emitted by the exposure device  80  (to be described below) is formed between the casing  53  of the development cartridge  52  and the casing  57  of the photoreceptor cartridge  56 . Further, a hole  57   b  for exposing the photoreceptor drum  64  (to be described below) downward is formed in a lower surface of the casing  57 . 
     The photoreceptor drum  64  and a charger  70  are disposed in the casing  57  of the photoreceptor cartridge  56 . The photoreceptor drum  64  contacts the lower side of the developing roller  62 . The photoreceptor drum  64  comprises a photoreceptor drum main body  64   a  and a photoreceptor drum shaft  64   b . The photoreceptor drum main body  64   a  has a cylindrical shape. The photoreceptor drum main body  64   a  is a positively-charged type. The surface of the photoreceptor drum main body  64   a  is constituted by polycarbonate or the like. The photoreceptor drum shaft  64   b  is made of metal. The photoreceptor drum shaft  64   b  is fixed to the casing  57  of the photoreceptor cartridge  56 . The photoreceptor drum main body  64   a  is attached to the photoreceptor drum shaft  64   b  in a manner allowing its rotation. The photoreceptor drum main body  64   a  is connected to a drive source not shown in the drawing, and is rotated clockwise thereby. A part of the photoreceptor drum  64  is exposed (downward) to the exterior of the casing  57  through the hole  57   b . When the printing sheet  2  is not carried on the belt  48 , the lowermost end of the photoreceptor drum  64  contacts the belt  48 . When the printing sheet  2  is carried on the belt  48 , the lowermost end of the photoreceptor drum  64  contacts the printing sheet  2 . The charger  70  is disposed on the left side of the photoreceptor drum  64 . The charger  70  is disposed at a position which is downstream of the belt  48  and upstream of the developing roller  62  in the rotation direction of the photoreceptor drum  64 . A gap is provided between the charger  70  and photoreceptor drum  64 . The charger  70  is a scorotron type charger. The charger  70  comprises a wire  74 . The wire  74  extends in a perpendicular direction to the paper surface of  FIG. 2 . A high voltage is applied to the wire  74 . By applying a high voltage to the wire  74  to perform corona discharge, the surface of the photoreceptor drum  64  (the photoreceptor drum main body  64   a ) is positively charged. 
     The transfer roller  66  contacts the belt  48  on the back side of the belt  48 . The transfer roller  66  is positioned directly below the photoreceptor drum  64 . The transfer roller  66  comprises a transfer roller main body  66   a  and a transfer roller shaft  66   b . The transfer roller main body  66   a  is formed from a conductive rubber material. The transfer roller shaft  66   b  is made of metal. The transfer roller shaft  66   b  is supported on the frame (not shown) of the printing sheet conveying device  40  in a manner allowing its rotation. The transfer roller shaft  66   b  is connected to a drive source not shown in the drawing. The transfer roller  66  rotates counterclockwise while the belt  48  rotates. The transfer roller shaft  66   b  is connected to a voltage supply circuit not shown in the drawing. During transfer (when the toner supported by the photoreceptor drum  64  is transferred onto the printing sheet  2 ), a bias is applied to the transfer roller  66  from the voltage supply circuit. 
     As shown in  FIG. 1 , the exposure device  80   a  is disposed on the left side of the development device  50   a . Similarly, the exposure devices  80   b  to  80   d  are disposed respectively on the left side of the other development devices  50   b  to  50   d . The exposure devices  80   a  to  80   d  have an identical structure. Here, the structure of the exposure device  80   a  will be described with reference to  FIG. 2 . In  FIG. 2 , the reference numeral  80  is used to represent the exposure devices  80   a  to  80   d.    
     The exposure device  80  is fixed to the overall casing  12  (see  FIG. 1 ). The exposure device  80  comprises a casing  82 . A through hole  82   a  is formed in the right surface of the casing  82 . A polygon mirror  84 , a lens  86 , a reflecting mirror  88 , a reflecting mirror  90 , a lens  92 , a reflecting mirror  94 , and so on are provided in the casing  82 . The exposure device  80  comprises a light source not shown in the drawing. A laser beam is emitted from the light source based on the content of print data. The laser beam emitted from the light source is deflected by the polygon mirror  84  toward the lens  86 . Having passed through the lens  86 , the laser beam is reflected by the reflecting mirror  88 . After being reflected by the reflecting mirror  88 , the laser beam is reflected by the reflecting mirror  90  toward the lens  92 . Having passed through the lens  92 , the laser beam is reflected by the reflecting mirror  94 . After being reflected by the reflecting mirror  94 , the laser beam passes through the through hole  82   a  and proceeds rightward out of the casing  82 . After emerging from the casing  82 , the laser beam passes through the hole  57   a  between the development cartridge  52  and the photoreceptor cartridge  56  and reaches the photoreceptor drum  64 . Thus the photoreceptor drum  64  is exposed to a predetermined pattern. The dot-dash line in  FIG. 2  depicts the trajectory of the laser beam. 
     Next, the actions of the development device  50  and exposure device  80  will be described. 
     The toner stored in the toner chamber  53   a  is adhered to the supply roller  60 . The toner adhered to the supply roller  60  is charged positively by the friction between the supply roller  60  and developing roller  62 . The positively charged toner covers the surface of the developing roller  62 . 
     Meanwhile, the surface of the photoreceptor drum main body  64   a  is charged positively by the charger  70 . The surface of the positively charged photoreceptor drum main body  64   a  receives the laser beam emitted from the exposure device  80 . Thus a predetermined part of the surface of the photoreceptor drum main body  64   a  is exposed. The electric potential of the exposed part of the photoreceptor drum main body  64   a  decreases. The part subjected to exposure varies according to the print content. An electrostatic latent image based on the print content is formed on the photoreceptor drum main body  64   a.    
     The toner covering the developing roller  62  becomes adhered to the exposed part of the photoreceptor drum main body  64   a . At this time, the toner does not become adhered to the non-exposed parts of the photoreceptor drum main body  64   a . As a result, the electrostatic latent image formed on the photoreceptor drum main body  64   a  is transformed into a visible image. 
     The toner carried on the photoreceptor drum main body  64   a  is transferred onto the printing sheet  2  between the photoreceptor drum  64  and belt  48 . At this time, a bias is applied to the transfer roller  66 . The toner is transferred onto the printing sheet  2  by the voltage between the photoreceptor drum  64  and transfer roller  66 . 
     In this embodiment, the four development devices  50   a  to  50   d  are used. Toner of each color is transferred onto the printing sheet  2  from the respective development devices  50   a  to  50   d . Thus full color printing can be realized. 
     Next, returning to  FIG. 1 , the structure of the toner fixing device  100  will be described. The toner fixing device  100  is disposed to the left of the leftmost development device  50   d . The toner fixing device  100  comprises two frames  102 ,  104  and two rollers  102   a ,  104   a . The frame  102  supports the pressure roller  102   a  in a manner allowing its rotation. The frame  104  supports the heating roller  104   a  in a manner allowing its rotation. 
     The surface of the pressure roller  102   a  is formed from rubber. The pressure roller  102   a  is biased to the heating roller  104   a  side by a mechanism not shown in the drawing. The pressure roller  102   a  is not connected to a drive source. The pressure roller  102   a  rotates counterclockwise in response to clockwise rotation of the heating roller  104   a.    
     A halogen lamp (not shown) is disposed in the interior of the heating roller  104   a . The halogen lamp heats the heating roller  104   a . The heating roller  104   a  is connected to a drive source not shown in the drawing, and is rotated clockwise thereby. 
     After being conveyed leftward by the printing sheet conveying device  40 , the printing sheet  2  is guided along a rail not shown in the drawing, and inserted between the pressure roller  102   a  and heating roller  104   a  (arrow D 4 ). When the heating roller  104   a  rotates clockwise, the printing sheet  2  between the pressure roller  102   a  and heating roller  104   a  is conveyed in the upward direction. The printing sheet  2  is heated by the high-temperature heating roller  104   a . As a result, the toner transferred onto the printing sheet  2  is fixed by the heat. Having passed through the toner fixing device  100 , the printing sheet  2  is conveyed in the upward direction. 
     A pair of eject rollers  110 ,  112  is disposed above the toner fixing device  100 . The lower side eject roller  112  is connected to a drive source not shown in the drawing, and is rotated clockwise thereby. The upper side eject roller  110  is not connected to a drive source. When the lower side eject roller  112  rotates clockwise, the upper side eject roller  110  rotates counterclockwise in response thereto. 
     Having passed through the toner fixing device  100 , the printing sheet  2  is guided along a rail not shown in the drawing, and inserted between the two eject rollers  110 ,  112 . When the lower side eject roller  112  rotates clockwise, the printing sheet  2  between the two eject rollers  110 ,  112  is conveyed in the rightward direction (arrow D 5 ). The printing sheet  2  is then conveyed to the exterior of the overall casing  12 . An eject tray  116  is formed on the upper surface of the overall casing  12 . Having been conveyed to the exterior of the overall casing  12 , the printing sheet  2  is delivered onto the eject tray  116 . 
     Next, the structure of the device  120  for cleaning the belt  48  will be described. The belt  48  contacts the printing sheet  2 , and therefore paper particles of the printing sheet  2  may adhere to the belt  48 . Furthermore, after a long period during which no printing is executed, the laser printer  10  of this embodiment executes an operation of transferring the toner from each of the photoreceptor drums  64  to the belt  48  prior to the next printing operation. The electrostatic charge of the toner following a long period during which no printing is executed differs from the electrostatic charge of the toner when printing is executed frequently. Hence, when printing has not been executed for a long time, the concentration of the toner transferred onto the printing sheet  2  differs. By transferring the toner onto the belt  48 , the printer  10  of this embodiment checks the concentration of the toner of each color. When the toner concentration is not within a desired range, the voltage applied to the toner is altered. In other words, the applied voltage of the charger  70  is altered. Note that checking the toner concentration is a well-known technique, and hence a detailed description thereof has been omitted. 
     The belt cleaning device  120  removes the paper particles and toner adhered to the belt  48 . The belt cleaning device  120  comprises a casing  122 , three rollers  130 ,  132 ,  134 , a blade  136 , and so on. The casing  122  is disposed below the belt  48 . A part of the upper surface of the casing  122  is open. The lower surface of the casing  122  may be opened by a mechanism not shown in the drawing. This structure allows the toner and paper particles that have accumulated in the casing  122  to be removed. The casing  122  houses the rollers  132 ,  134  and the blade  136 . 
     Referring to  FIG. 3 , the structure of the three rollers  130 ,  132 ,  134  and the blade  136  will be described in detail. 
     The roller  130  will be referred to as a back roller. The back roller  130  contacts the back surface of the belt  48  on the lower side thereof. The back roller  130  is supported by a frame (not shown) of the printing sheet conveying device  40  (see  FIG. 1 ) via a bearing. The bearing is biased downward. Thus the back roller  130  is biased in a downward direction. The back roller  130  is supported by the flame in a manner allowing its rotation. The back roller  130  rotates counterclockwise when the belt  48  rotates. The back roller  130  is made of metal, and the surface thereof is nickel plated. The back roller  130  is connected to a first high-voltage power circuit  140 . 
     The roller  132  will be referred to as a first front roller. The first front roller  132  is exposed upward from an upper surface opening of the casing  122  (see  FIG. 1 ). The first front roller  132  contacts the belt  48  on the front surface side of the belt  48 . The first front roller  132  is disposed in a position facing the back roller  130 . The first front roller  132  comprises a first front roller main body  132   a  and a first front roller shaft  132   b . The first front roller main body  132   a  is formed from a foamed material. A silicone or urethane type material may be adopted as the foamed material. The first front roller shaft  132   b  is made of metal. The first front roller shaft  132   b  is supported by the casing  122  (see  FIG. 1 ) in a manner allowing its rotation. A power source, not shown in the drawing, is connected to the first front roller  132 . The first front roller  132  rotates counterclockwise when the belt  48  rotates. The first front roller shaft  132   b  is connected to the first high-voltage power circuit  140  and a second high-voltage power circuit  142 . 
     The roller  134  will be referred to as a second front roller. The second front roller  134  contacts the lower side of the first front roller  132 . The second front roller  134  is supported by the casing  122  (see  FIG. 1 ) in a manner allowing its rotation. The second front roller  134  rotates clockwise when the belt  48  rotates (when the first front roller  132  rotates). The second front roller  134  is made of metal, and its surface is nickel-plated. The second front roller  134  is connected to the second high-voltage power circuit  142 . 
     The blade  136  contacts the lower side of the second front roller  134 . The blade  136  extends in a diagonally rightward and upward direction. The blade  136  is made of rubber. The blade  136  extends in a perpendicular direction to the paper surface of  FIG. 3 , and contacts the second front roller  134  over substantially the entire axis direction of the second front roller  134 . The blade  136  knocks adhered paper particles and toner off from the second front roller  134 . The paper particles and toner knocked off by the blade  136  drop onto the bottom surface of the interior of the casing  122 . The paper particles and toner that have accumulated in the interior of the casing  122  can be removed by opening the bottom surface of the casing  122 . 
     It is possible to omit the second front roller  134  by making the blade  136  contact the first front roller  132 . However, since the surface of the first front roller  132  (the first front roller main body  132   a ) is constituted by a foamed material, the surface of the first front roller  132  would be damaged if the blade  136  makes contact with the first front roller  132 . Cleaning must be performed without damaging the surface of the first front roller  132 , and therefore the second front roller  134  is used. The second front roller  134  cleans the first front roller  132  using electric force. Thus the first front roller  132  can be cleaned without damage to its surface. 
     If the first front roller  132  were formed from metal, the surface of the first front roller  132  would not be damaged even when contacted by the blade  136 . In so doing, the blade  136  could be brought into contact with the first front roller  132  and the second front roller  134  could be omitted. However, if the first front roller  132  were made of metal, the cleaning ability in relation to the belt  48  would be poorer than that of a foamed material, and therefore in this embodiment, the first front roller  132  is not made of metal. 
     In this embodiment, the two front rollers  132 ,  134  and the blade  136  are adopted in consideration of the circumstances described above. 
     The belt cleaning device  120  comprises a controller  150 , the first high-voltage power circuit  140 , the second high-voltage power circuit  142 , a first D/A converter  160 , a second D/A converter  162 , and so on. 
     The controller  150  controls the voltage between the back roller  130  and first front roller  132 , and the voltage between the first front roller  132  and second front roller  134 . The first D/A converter  160  and second D/A converter  162  are connected to the controller  150 . The controller  150  outputs a digital signal to the first D/A converter  160  to control the voltage between the back roller  130  and first front roller  132 . The controller  150  also outputs a digital signal to the second D/A converter  162  to control the voltage between the first front roller  132  and second front roller  134 . The content of the processing executed by the controller  150  will be described later. 
     The first D/A converter  160  inputs the digital signal output by the controller  150 , converts the input digital signal into an analog signal (voltage), and outputs the converted analog signal to the first high-voltage power circuit  140 . The second D/A converter  162  inputs the digital signal output by the controller  150 , converts the input digital signal into an analog signal (voltage), and outputs the converted analog signal to the second high-voltage power circuit  142 . 
     The first high-voltage power circuit  140  is connected to the back roller  130  and first front roller  132 , and also earthed. The first high-voltage power circuit  140  inputs the analog signal (voltage) output by the first D/A converter  160 , and amplifies the analog signal into a high voltage. As a result, a high voltage is applied between the back roller  130  and first front roller  132 . In this embodiment, the potential of the back roller  130  is set to zero, and the potential of the first front roller  132  is set to a negative value. 
     The second high-voltage power circuit  142  is connected to the first front roller  132  and second front roller  134 , and also earthed. The second high-voltage power circuit  142  inputs the analog signal (voltage) output by the second D/A converter  162 , and amplifies the analog signal into a high voltage. As a result, a high voltage is applied between the first front roller  132  and second front roller  134 . In this embodiment, the potential of the second front roller  134  is set to be lower than the potential of the first front roller  132 . 
     Note that a first current sensor  170  is disposed between the back roller  130  and first high-voltage power circuit  140 . A current value measured by the first current sensor  170  is equal to the current flowing between the back roller  130  and first front roller  132 . Further, a second current sensor  172  is disposed between the second front roller  134  and second high-voltage power circuit  142 . A current value measured by the second current sensor  172  is equal to the current flowing between the first front roller  132  and second front roller  134 . The current sensors  170 ,  172  are connected to the controller  150 . The controller  150  is capable of inputting the measurement values of the respective current sensors  170 ,  172 . 
     According to the belt cleaning device  120  structured as described above, the potential of the first front roller  132  is lower than the potential of the back roller  130 . Thus an electric field is generated from the back roller  130  in the direction of the first front roller  132 . The toner  6  and paper particles adhered to the belt  48  receive an electrostatic force between the back roller  130  and first front roller  132  in the direction of the first front roller  132 . As a result, the toner  6  and paper particles on the belt  48  are trapped by the first front roller  132 , and the belt  48  is cleaned. 
     Further, the potential of the second front roller  134  is lower than the potential of the first front roller  132 . Thus an electric field is generated from the first front roller  132  in the direction of the second front roller  134 . The toner  6  adhered to the first front roller  132  receives an electrostatic force between the first front roller  132  and second front roller  134  in the direction of the second front roller  134 . As a result, the toner  6  adhered to the first front roller  132  is trapped by the second front roller  134 , and the first front roller  132  is cleaned. 
     The toner  6  and paper particles trapped by the second front roller  134  are knocked off by the blade  136 . Thus the second front roller  134  is cleaned. 
     Note that a toner exchange sensor  152 , a counter  154 , and a memory  156  are connected to the controller  150 . 
     The toner exchange sensor  152  outputs a toner exchange signal when the development cartridge  52  is exchanged for a new one. When the toner exchange signal is input into the controller  150 , the controller  150  learns that the toner has been exchanged. 
     The counter  154  counts the number of printing sheets printed by the laser printer  10 . The value of the counter  154  is latched by the controller  150 . 
     The storage content of the memory  156  will be described later in a second embodiment and so on. The actions of the counter  154  and memory  156  will be described in detail in the second embodiment. 
     Next, referring to  FIG. 4 , the manner in which the controller  150  performs voltage control will be described.  FIG. 4  is a flowchart of voltage control process executed by the controller  150 . 
     The controller  150  monitors the measurement value of the first current sensor  170  (step S 2 ). More specifically, the controller  150  monitors a current i A  flowing between the back roller  130  and first front roller  132 . When the current i A  is not within a range of I A2  to I A1  (NO in the step S 2 ), the routine advances to S 4 . In S 4 , a voltage V A  between the back roller  130  and first front roller  132  is adjusted. Here, the voltage V A  is adjusted to make the current i A  an intermediate value I Am  between I A2  and I A1  (i.e. I Am  is a value obtained by dividing the sum of I A2  and I A1  by 2). This adjustment is performed specifically in the following manner. The present voltage V A  between the back roller  130  and first front roller  132  is known. The present current i A  flowing between the back roller  130  and first front roller  132  is also known. Hence, a present resistance R A  between the back roller  130  and first front roller  132  can be calculated (R A =V A /i A ). Next, a target voltage between the back roller  130  and first front roller  132  is obtained by multiplying R A  by I Am  (an intermediate value between I A2  and I A1 ). The controller  150  outputs a digital signal corresponding to the target voltage to the first D/A converter  160 . Thus the voltage between the back roller  130  and first front roller  132  is adjusted to the target voltage. The current flowing between the back roller  130  and first front roller  132  becomes the intermediate value I Am  between I A2  and I A1 . Note that the manner in which I A2  and I A1  are set will be described later. 
     When the step S 4  is complete, the routine advances to a step S 6 . The routine also advances to S 6  when it is determined in the step S 2  that the current i A  is within the range of I A2  to I A1 . In the step S 6 , the value of the second current sensor  172  is monitored. More specifically, a current i B  flowing between the first front roller  132  and second front roller  134  is monitored. When the current i B  is not within a range of I B2  to I B1  (NO in the step S 6 ), the routine advances to S 8 . In S 8 , a voltage V B  between the first front roller  132  and second front roller  134  is adjusted. Here, the voltage V B  is adjusted to make the current i B  an intermediate value I Bm  between I B2  and I B1 . This adjustment is performed specifically in the following manner. The present voltage V B  between the first front roller  132  and second front roller  134  is known. The present current i B  flowing between the first front roller  132  and second front roller  134  is also known. Hence, a present resistance R B  between the first front roller  132  and second front roller  134  can be calculated (R B =V B /i B ). Next, a target voltage between the first front roller  132  and second front roller  134  is obtained by multiplying R B  by I Bm . The controller  150  outputs a digital signal corresponding to the target voltage to the second D/A converter  162 . Thus the voltage between the first front roller  132  and second front roller  134  is adjusted to the target voltage. The current flowing between the first front roller  132  and second front roller  134  becomes the intermediate value I Bm  between I B2  and I B1 . The manner in which I B2  and I B1  are set will be described later. 
     Once the step S 8  is complete, the routine advances to a step S 10 . The routine also advances to S 10  when it is determined in the step S 6  that the current i B  is within the range of I B2  to I B1 . In the step S 10 , a determination is made as to whether or not the development cartridge  52  (see  FIG. 2 ) has been exchanged. When the toner exchange signal output by the toner exchange sensor  152  (see  FIG. 3 ) has been input into the controller  150 , the controller  150  determines YES in the step S 10 . Upon a determination of YES in the step S 10 , the routine advances to a step S 12 . 
     In the step S 12 , the voltage V A  between the back roller  130  and first front roller  132  is adjusted, and the voltage V B  between the first front roller  132  and second front roller  134  is adjusted. This adjustment is performed specifically in the following manner. First, V A  is adjusted such that the current i A  matches I A1 . A target voltage can be obtained by obtaining the present resistance R A  between the back roller  130  and first front roller  132  (R A =V A /i A ), and multiplying R A  by I A1 . The controller  150  outputs a digital signal corresponding to the target voltage to the first D/A converter  160 . As a result, the voltage between the back roller  130  and first front roller  132  is adjusted to the target voltage, and the current flowing between the back roller  130  and first front roller  132  becomes I A1 . 
     Further, the voltage V B  between the first front roller  132  and second front roller  134  is adjusted such that the current i B  matches I B1 . A target voltage can be obtained by obtaining the present resistance R B  between the first front roller  132  and second front roller  134  (R B =V B /i B ), and multiplying R B  by I B1 . The controller  150  outputs a digital signal corresponding to the target voltage to the second D/A converter  162 . As a result, the voltage between the first front roller  132  and second front roller  134  is adjusted to the target voltage, and the current flowing between the first front roller  132  and second front roller  134  becomes I B1 . 
     After the controller  150  has executed the step S 12  or determined NO in the step S 10 , the routine returns to the step S 2 . 
     Next, the manner in which above described I A1 , I A2 , I B1 , and I B2  are set will be described. The relationship between the magnitude of the voltage between the back roller  130  and first front roller  132 , and the cleaning ability of the first front roller  132  in relation to the belt  48  was obtained through experiment in a case where the resistance between the back roller  130  and first front roller  132  was constant. Further, the relationship between the magnitude of the voltage between the first front roller  132  and second front roller  134 , and the cleaning ability of the second front roller  134  in relation to the first front roller  132  was obtained through experiment in a case where the resistance between the first front roller  132  and second front roller  134  was constant. 
     Referring to  FIG. 5 , the methods of these experiments will be described. First, transparent adhesive tape was affixed to a new belt  48  free from adhered toner. The adhesive tape was then removed from the belt  48 . The removed adhesive tape was set in a digital reflection densitometer and the density thereof was measured. Note that hereafter, this density will be referred to as a reference density. 
     Next, the toner  6  was adhered to the new belt  48  ( FIG. 5A ). Three new rollers  130 ,  132 ,  134  were prepared. The voltage between the back roller  130  and first front roller  132  was set to −0.2 kV. The voltage between the first front roller  132  and second front roller  134  was set to −0.2 kV. The belt  48  adhered with the toner  6  was rotated, and the three rollers  130 ,  132 ,  134  were rotated. The toner  6  on the belt  48  was trapped by the first front roller  132  ( FIG. 5B ). Note that in  FIG. 5A , a point P on the first front roller  132  indicates the point of contact with the tip of the toner adhered part of the belt  48 . When the point P enters the state shown in  FIG. 5B , this point P is located at a position where the point P makes contact with the second front roller  134 . In the state shown in  FIG. 5B , the toner  6  trapped on the first front roller  132  has not yet been removed by the second front roller  134 . 
     In the state in  FIG. 5B , the part of the belt  48  that has passed the first front roller  132  is shown by a reference symbol S 1 . Adhesive tape was affixed to the S 1  part. The adhesive tape was removed from the belt  48 , and the removed adhesive tape was set in the digital reflection densitometer to measure its density. Note that hereafter, this density will be referred to as a first measured density. When a large amount of toner remains in the part S 1 , the first measured density increases. Conversely, when little toner remains in the S 1  part, the first measured density decreases. In other words, the cleaning ability of the first front roller  132  in relation to the belt  48  is indicated to be steadily more favorable as the first measured density is low. 
     By performing the above experiment under various voltages between the back roller  130  and first front roller  132 , the relationship between the magnitude of the voltage between these rollers  130 ,  132 , and the cleaning ability of the first front roller  132  in relation to the belt  48  can be obtained. The results thereof are shown in  FIG. 6 . The abscissa of  FIG. 6  is the potential of the first front roller  132  relative to the potential of the back roller  130 . The ordinate of  FIG. 6  is the difference (Y 1 ) between the reference density and first measured density. As Y 1  is low, the first measured density is low, indicating that the cleaning ability of the first front roller  132  is favorable. As is evident from  FIG. 6 , when the voltage between the back roller  130  and first front roller  132  is too small, the cleaning ability of the first front roller  132  deteriorates. The cleaning ability of the first front roller  132  also deteriorates when the voltage between the back roller  130  and first front roller  132  is too large. In this embodiment, if the resistance between the back roller  130  and first front roller  132  has a predetermined value (R s1 ) and the voltage between the rollers  130 ,  132  is within a range of −0.4 kV to −1.6 kV, the first front roller  132  is evaluated as exhibiting an excellent cleaning performance. The resistance R s1 , between the new back roller  130  and the new first front roller  132  was measured in advance. I A1  (see  FIG. 4 ) was obtained by dividing −0.4 kV by R s1 , and I A2  (see  FIG. 4 ) was obtained by dividing −1.6 kV by R s1 . 
     The first front roller  132  cannot be cleaned completely by the second front roller  134 . The first front roller  132  becomes soiled over time. When the first front roller  132  becomes soiled, the electric resistance thereof increases. When the voltage between the back roller  130  and first front roller  132  is fixed and the electric resistance of the first front roller  132  increases, it becomes difficult for current to flow between the rollers  130 ,  132 . In this case, the cleaning ability of the first front roller  132  deteriorates. It has been discovered as a result of research performed by the present inventors that, even when the electric resistance of the first front roller  132  increases, the first front roller  132  can be made to exhibit an excellent cleaning performance continuously by keeping the current flowing between the back roller  130  and first front roller  132  within the range of I A2  to I A1 . 
     In this embodiment, when the electric resistance of the first front roller  132  increases, the voltage V A  between the back roller  130  and first front roller  132  is increased to keep the current within the range of I A2  to I A1 . As a result, the first front roller  132  exhibits an excellent cleaning performance at all times. 
     When the belt  48  is rotated further from the state shown in  FIG. 5B , the state shown in  FIG. 5C  is reached. In this state, the point P on the first front roller  132  has performed one revolution and moved back into contact with the belt  48 . The part of the first front roller  132  following the point P is cleaned by the second front roller  134 . Following the state shown in  FIG. 5C , the first front roller  132  cleaned by the second front roller  134  cleans the belt  48 .  FIG. 5D  shows a state following the state shown in  FIG. 5C . In this state, the part of the belt  48  cleaned by the first front roller  132  that has been cleaned by the second front roller  134  is shown by a reference symbol S 2 . Adhesive tape was affixed to the S 2  part. The adhesive tape was removed from the belt  48 , and the removed adhesive tape was set in the digital reflection densitometer to measure its density. Note that hereafter, this density will be referred to as a second measured density. When a large amount of toner remains in the part S 2 , the second measured density increases, and when no toner remains in the S 2  part, the second measured density decreases. When the second measured density is low, this indicates that the first front roller  132  has been cleaned thoroughly, and hence that the cleaning ability of the second front roller  134  is favorable. 
     By performing the above experiment under various voltages between the first front roller  132  and second front roller  134 , the relationship between the magnitude of the voltage between the first front roller  132  and second front roller  134 , and the cleaning ability of the second front roller  134  in relation to the first front roller  132  can be obtained. The results thereof are shown in  FIG. 7 . The abscissa of  FIG. 7  is the potential of the second front roller  134  relative to the potential of the first front roller  132 . The ordinate of  FIG. 7  is the difference (Y 2 ) between the reference density and second measured density. As Y 2  is low, the second measured density is low, indicating that the cleaning ability of the second front roller  134  is favorable. When the voltage between the first front roller  132  and second front roller  134  is too small, the cleaning ability of the second front roller  134  deteriorates. The cleaning ability of the second front roller  134  also deteriorates when the voltage between the first front roller  132  and second front roller  134  is too large. In this embodiment, if the resistance between the first front roller  132  and second front roller  134  has a predetermined value (R s2 ) and the voltage between the rollers  132 ,  134  is within a range of −0.4 kV to −0.8 kV, the second front roller  134  is evaluated as exhibiting an excellent cleaning performance. The resistance R s2  between the new first front roller  132  and the new second front roller  134  was measured in advance. I B1  (see  FIG. 4 ) was obtained by dividing −0.4 kV by R s2 , and I B2  (see  FIG. 4 ) was obtained by dividing −0.8 kV by R s2 . 
     The second front roller  134  is cleaned by the blade  136 , but becomes soiled over time. When the second front roller  134  becomes soiled, the electric resistance thereof increases. When the voltage between the first front roller  132  and second front roller  134  is fixed and the electric resistance of the first front roller  132  or the second front roller  134  increases, it becomes difficult for current to flow between the rollers  132 ,  134 . In this case, the cleaning ability of the second front roller  134  deteriorates. It has been discovered as a result of research performed by the present inventors that, even when the electric resistance of the second front roller  134  increases, the second front roller  134  can be made to exhibit an excellent cleaning performance continuously by keeping the current flowing between the first front roller  132  and second front roller  134  within the range of I B2  to I B1 . 
     In this embodiment, when the electric resistance of the second front roller  134  increases, the voltage between the first front roller  132  and second front roller  134  is increased to keep the current within the range of I B2  to I B1 . As a result, the second front roller  134  exhibits an excellent cleaning performance at all times. 
     The cleaning ability of the first front roller  132  is dependent on the magnitude of the current flowing between the back roller  130  and the first front roller  132 . In this embodiment, the current flowing between the back roller  130  and the first front roller  132  is maintained within a range (I A2  to I A1 ) at which the first front roller  132  exhibits an excellent cleaning performance. Even when the first front roller  132  becomes soiled with paper particles and toner such that the electric resistance of the first front roller  132  increases, the current flowing between the back roller  130  and first front roller  132  is maintained within I A2  to I A1 . According to the laser printer  10  of this embodiment, the cleaning ability of the first front roller  132  can be maintained at a high level. 
     Further, the cleaning ability of the second front roller  134  is dependent on the magnitude of the current flowing between the first front roller  132  and second front roller  134 . The current flowing between the first front roller  132  and second front roller  134  is maintained within a range (I B2  to I B1 ) at which the second front roller  134  exhibits an excellent cleaning performance. Even when the second front roller  134  becomes soiled such that the electric resistance of the second front roller  134  increases, the current flowing between the first front roller  132  and second front roller  134  is maintained within I B2  to I B1 . Hence, the cleaning ability of the second front roller  134  can be maintained at a high level. 
       FIG. 8  shows the manner in which the potentials of the first front roller  132  and second front roller  134  change over time when the laser printer  10  of this embodiment is used. The abscissa of  FIG. 8  shows the number of printing sheets having been printed, and the ordinate shows the potential. A graph L 1  shows the potential of the first front roller  132 , while a graph L 2  shows the potential of the second front roller  134 . Note that the potential of the back roller  130  is maintained at zero. 
     As is evident from the graph L 1 , the potential of the first front roller  132  decreases steadily as the number of printed sheets increases. This means that the potential difference (voltage) between the back roller  130  and first front roller  132  increases over time. When the number of printed sheets reaches A and B, the potential of L 1  changes greatly. This indicates that the development cartridge  52  (see  FIG. 2 ) has been exchanged for a new one. When the development cartridge  52  is exchanged for a new one such that new toner is used, it becomes difficult for the first front roller  132  to trap the toner. It was learned from an experiment performed by the present inventors that, within the current range (I A2  to I A1 ) at which the first front roller  132  exhibits an excellent cleaning performance, it is easier for the first front roller  132  to trap new toner with a large current. Hence in this embodiment, when new toner is replenished, the voltage between the back roller  130  and first front roller  132  is increased such that the current flowing between these rollers  130 ,  132  reaches I A1  (see  FIG. 4 ). As a result, the first front roller  132  is able to trap new toner efficiently. 
     The potential of the second front roller  134  decreases over time (L 2  in  FIG. 8 ). The voltage (the difference between L 1  and L 2 ) between the first front roller  132  and second front roller  134  increases over time. When the development cartridge  52  (see  FIG. 2 ) is exchanged for a new one, the potential difference between the first front roller  132  and second front roller  134  increases greatly (when the number of printed sheets reaches A and B). At the timings A and B, a change amount of L 2  is greater than a change amount of L 1 . That is, a potential difference (voltage) between the first front roller  132  and second front roller  134  increases at the timing A and B. It was learned from an experiment performed by the present inventors that, within the current range (I B2  to I B1 ) at which the second front roller  134  exhibits an excellent cleaning performance, it is easier for the second front roller  134  to trap new toner with a large current. Hence in this embodiment, when new toner is replenished, the voltage between the first front roller  132  and second front roller  134  is increased such that the current flowing between these rollers  132 ,  134  reaches I B1  (see  FIG. 4 ). As a result, the second front roller  134  is able to trap new toner efficiently. 
     In the laser printer  10  of this embodiment, the voltage between the back roller  130  and first front roller  132  is subjected to constant current control, and hence the cleaning performance of the first front roller  132  in relation to the belt  48  is favorable. The voltage between the first front roller  132  and second front roller  134  is also subjected to constant current control, and hence the cleaning performance of the second front roller  134  in relation to the first front roller  132  is also favorable. By keeping the first front roller  132  clean, the first front roller  132  is able to clean the belt  48  efficiently and continuously. The ability of the laser printer  10  of this embodiment to clean the belt  48  is therefore extremely high. 
     Second Embodiment 
     Here, description will focus on parts that are different to the first embodiment. In this embodiment, the controller  150  does not monitor i A  and i B . The controller  150  varies the voltage between the back roller  130  and first front roller  132 , and the voltage between the first front roller  132  and second front roller  134  in accordance with information stored in the memory  156  (see  FIG. 3 ).  FIG. 9  shows an example of the information stored in the memory  156 . The word “Sheets” in the drawing shows the number of printed sheets. The term “Potential 1 ” shows the potential of the first front roller  132 . The term “Potential 2 ” shows the potential of the second front roller  134 . Note that the potential of the back roller  130  is maintained at zero. 
     The controller  150  of this embodiment monitors the number of printed sheets, which is counted by the counter  154  (see  FIG. 3 ). When the count value reaches the number of printed sheets stored in the memory  156 , the controller  150  adjusts the potentials to values corresponding to the number of printed sheets. For example, when the number of printed sheets reaches 10,000, the potential of the first front roller  132  is adjusted to −1050V and the potential of the second front roller  134  is adjusted to −1700V. In other words, the voltage between the back roller  130  and first front roller  132  is adjusted to 1050V, and the voltage between the first front roller  132  and second front roller  134  is adjusted to 650V. 
     According to this embodiment, the current sensors  170 ,  172  are unnecessary. In this embodiment also, the cleaning ability of the first front roller  132  and second front roller  134  can be maintained at a high level. 
     Third Embodiment 
     In this embodiment, description will focus on parts that are different to the first embodiment.  FIG. 10  is a diagram illustrating the structure of the belt cleaning device  120  of this embodiment. In  FIG. 10 , identical elements to those of the first embodiment have been allocated identical reference symbols. 
     A first high-voltage power circuit  240  is connected to the back roller  130  and also connected to the first front roller  132 . The first high-voltage power circuit  240  applies a voltage between the back roller  130  and first front roller  132  by applying a negative potential to the first front roller  132 . Note that the potential of the back roller  130  is maintained at zero. 
     A second high-voltage power circuit  242  is connected to the back roller  130  and also connected to the second front roller  134 . The second high-voltage power circuit  242  applies a high voltage between the back roller  130  and second front roller  134  by applying a negative potential to the second front roller  134 . As a result, the voltage between the first front roller  132  and second front roller  134  is adjusted. 
     In this embodiment also, the voltage between the back roller  130  and first front roller  132  can be adjusted, and the voltage between the first front roller  132  and second front roller  134  can also be adjusted. 
     Fourth Embodiment 
     In this embodiment, description will focus on parts that are different to the first embodiment. In the first embodiment, the voltage between the first front roller  132  and second front roller  134  increases steadily as the number of printed sheets increases. However, when the second front roller  134  does not easily become soiled, the voltage between the first front roller  132  and second front roller  134  may be maintained at a constant value. In this case, only the voltage between the back roller  130  and first front roller  132  is subjected to constant current control. 
       FIG. 11  shows the relationship between the number of printed sheets and the potentials in this embodiment. The abscissa in  FIG. 11  shows the number of printed sheets. The ordinate shows negative potentials, the absolute values of which increase as the values of the ordinate increase. L 1  shows the potential of the first front roller  132 . L 2  shows the potential of the second front roller  134 . The voltage between the first front roller  132  and second front roller  134  is constant. 
     Fifth Embodiment 
     In the fourth embodiment, the voltage between the first front roller  132  and second front roller  134  is maintained at a constant value. When the first front roller  132  does not easily become soiled, the voltage between the back roller  130  and first front roller  132  may be maintained at a constant value. In this case, only the voltage between the first front roller  132  and second front roller  134  is subjected to constant current control. 
       FIG. 12  shows the relationship between the number of printed sheets and the potential in this embodiment. The abscissa in  FIG. 12  shows the number of printed sheets. The ordinate shows negative potentials, the absolute values of which increase as the values of the ordinate increase. L 1  shows the potential of the first front roller  132 . L 2  shows the potential of the second front roller  134 . The voltage between the back roller  130  and the first front roller  132  is constant. 
     Sixth Embodiment 
     Referring to  FIG. 13 , a laser printer  310  of this embodiment will be described. The laser printer  310  is a secondary transfer type. In other words, in this laser printer  310 , toner is transferred from a photoreceptor drum  364  to an intermediate transfer belt  348  (primary transfer), whereupon the primary-transferred toner is transferred from the intermediate transfer belt  348  to a sheet of printing sheet  302  (secondary transfer). 
     The structure of the laser printer  310  will be described below. Identical names have been used for members that are similar to those of the first embodiment, and detailed description thereof has been omitted. Furthermore, the rotation direction of each roller is indicated in the drawing, and hence detailed description relating to the rotation direction has been omitted. 
     The laser printer  310  comprises a paper feeding device  320 . The printing sheet  302  stored in the paper feeding device  320  is conveyed in the direction of an arrow E 1  by a paper feeding roller  326 . The printing sheet  302  conveyed in the direction of the arrow E 1  is inserted between two conveyance rollers  330 ,  332 . The printing sheet  302  between the two conveyance rollers  330 ,  332  is conveyed rightward. 
     Printing sheet transfer rollers  334 ,  336  are provided to the right of the conveyance rollers  330 ,  332 . Having been conveyed rightward by the conveyance rollers  330 ,  332 , the printing sheet  302  is inserted between the printing sheet transfer rollers  334 ,  336  (arrow E 2 ). The lower side printing sheet transfer roller  334  contacts the front surface side of the intermediate transfer belt  348 . The upper side printing sheet transfer roller  336  contacts the back surface side of the intermediate transfer belt  348 . The printing sheet transfer roller  334  is connected to a voltage supply circuit not shown in the drawing. When the toner is to be transferred onto the printing sheet  302  from the intermediate transfer belt  348 , a transfer bias is applied to the printing sheet transfer roller  334 . The printing sheet transfer rollers  334 ,  336  are disposed facing each other. 
     A toner fixing device  400  is provided to the right of the printing sheet transfer rollers  334 ,  336 . The toner fixing device  400  comprises a pressure roller  402   a  and a heating roller  402   b . Having been conveyed in the direction of the arrow E 2 , the printing sheet  302  is inserted between the pressure roller  402   a  and heating roller  402   b . The toner transferred onto the printing sheet  302  is fixed on the printing sheet  302  by heat. Having passed through the toner fixing device  400 , the printing sheet  302  is conveyed in the direction of an arrow E 3  and ejected. The laser printer  310  comprises the intermediate transfer belt  348  and two belt rollers  342 ,  344 . The belt roller  344  is connected to the ground of a voltage supply circuit not shown in the drawing. 
     The laser printer  310  comprises four development devices  350   a  to  350   d  and four exposure devices  380   a  to  380   d . By means of this structure, full color printing is realized. The reference numeral  360  shows a supply roller. The reference numeral  362  shows a developing roller. The reference numeral  364  shows the photoreceptor drum. The reference numeral  366  shows a transfer roller. 
     The toner is transferred from the photoreceptor drum  364  to the intermediate transfer belt  348  (primary transfer). The toner transferred onto the intermediate transfer belt  348  is then transferred onto the printing sheet  302  between the printing sheet transfer rollers  334 ,  336  (secondary transfer). Thus the toner is transferred onto the printing sheet  302 . 
     A belt cleaning device  420  is provided to the right of the belt roller  344 . The belt cleaning device  420  removes residual toner that has been transferred during the primary transfer onto the intermediate transfer belt  348  but not transferred during the secondary transfer. Further, the printing sheet  302  contacts the intermediate transfer belt  348  between the printing sheet transfer rollers  334 ,  336 , and hence paper particles also may become adhered to the intermediate transfer belt  348 . The belt cleaning device  420  also removes paper particles that have become adhered to the intermediate transfer belt  348 . 
     The belt cleaning device  420  comprises a first front roller  432 , a second front roller  434 , a blade  436 , and so on. In this embodiment, the belt roller  344  functions as the back roller of the belt cleaning device  420 . 
     A voltage is applied between the belt roller  344  and the first front roller  432 . The first front roller  432  has a lower potential than the belt roller  344 . A voltage is also applied between the first front roller  432  and second front roller  434 . The second front roller  434  has a lower potential than the first front roller  432 . 
     The voltage between the belt roller  344  and first front roller  432  is subjected to constant current control similarly to the control performed on the voltage between the back roller  130  and first front roller  132  in the first embodiment. Further, the voltage between the first front roller  432  and second front roller  434  is subjected to constant current control similarly to the control performed on the voltage between the first front roller  132  and second front roller  134  in the first embodiment. 
     In the laser printer  310  of this embodiment, the ability of the first front roller  432  to clean the intermediate transfer belt  348  is high. The ability of the second front roller  434  to clean the first front roller  432  is also high. Hence, the ability of the laser printer  310  of this embodiment to clean the intermediate transfer belt  348  is extremely high. 
     Seventh Embodiment 
     A laser printer  510  of this embodiment will be described with reference to  FIG. 14 . The laser printer  510  is a secondary transfer type. The laser printer  510  does not adopt a photoreceptor drum. Instead, a photosensitive belt  710  is used. Toner is transferred from the photosensitive belt  710  to an intermediate transfer belt  750  (primary transfer), whereupon the primary-transferred toner is transferred from the intermediate transfer belt  750  to a printing sheet  502  (secondary transfer). 
     The structure of the laser printer  510  will be described below. Identical names have been used for members that are similar to those of the first embodiment, and detailed description thereof has been omitted. Furthermore, the rotation direction of each roller is indicated in the drawing, and hence detailed description relating to the rotation direction has been omitted. 
     The laser printer  510  comprises a paper feeding device  520 . The printing sheet  502  stored in the paper feeding device  520  is conveyed in the direction of an arrow F 1  by a paper feeding roller  526  and conveyance rollers  530 ,  532 . 
     A pair of secondary transfer rollers  534 ,  536  is disposed above the conveyance rollers  530 ,  532 . The secondary transfer roller  534  contacts the front surface side of the intermediate transfer belt  750 . The secondary transfer roller  534  is connected to a voltage supply circuit not shown in the drawing. When the toner is to be transferred onto the printing sheet  502  from the intermediate transfer belt  750 , a transfer bias is applied to the secondary transfer roller  534 . The secondary transfer roller  536  contacts the back surface side of the intermediate transfer belt  750 . The secondary transfer roller  536  faces the secondary transfer roller  534 . Having been conveyed in the direction of the arrow F 1 , the printing sheet  502  is inserted between the secondary transfer rollers  534 ,  536 . When the secondary transfer rollers  534 ,  536  are rotated, the printing sheet  502  is conveyed in the direction of an arrow F 2 . 
     A toner fixing device  600  is provided above the secondary transfer rollers  534 ,  536 . The toner fixing device  600  comprises a pressure roller  602   a  and a heating roller  602   b . Having been conveyed in the direction of the arrow F 2 , the printing sheet  502  is inserted between the pressure roller  602   a  and heating roller  602   b . The toner transferred onto the printing sheet  502  is fixed on the printing sheet  502  by heat. Having passed through the toner fixing device  600 , the printing sheet  502  is conveyed in the direction of an arrow F 3  and ejected. 
     Four development devices  550   a  to  550   d  are disposed in vertical series. Each of the development devices  550   a  to  550   d  comprises a supply roller  560  and a developing roller  562 . Each of the development devices  550   a  to  550   d  is structured to be capable of movement in a left-right direction. 
     The photosensitive belt  710  is disposed on the left side of the development devices  550   a  to  550   d . Five rollers  700 ,  702 ,  704 ,  706 ,  722  are disposed on the back surface side of the photosensitive belt  710 . When the development devices  550   a  to  550   d  move in a leftward direction, the developing rollers  562  contact the photosensitive belt  710 . In  FIG. 14 , the second development device  550   c  from the top has moved leftward so as to contact the photosensitive belt  710 . 
     A charger  570  is provided below and to the left of the photosensitive belt  710 . The charger  570  electrifies the photosensitive belt  710 . An exposure device  580  is disposed below the charger  570 . A laser beam emitted from the exposure device  580  is reflected by a reflecting mirror  580   a . The laser beam reflected by the reflecting mirror  580   a  reaches the photosensitive belt  710 . As a result, the photosensitive belt  710  is exposed to a pattern corresponding to the print content. The toner carried on the developing roller  562  is developed in the exposed part of the photosensitive belt  710 . 
     A photosensitive belt cleaning device  720  is disposed above the charger  570 . The photosensitive belt cleaning device  720  comprises a back roller  722 , a first front roller  724 , a second front roller  726 , a blade  728 , and so on. The voltage between the back roller  722  and first front roller  724  is subjected to constant current control similarly to the control performed on the voltage between the back roller  130  and first front roller  132  in the first embodiment. The voltage between the first front roller  724  and second front roller  726  is subjected to constant current control similarly to the control performed on the voltage between the first front roller  132  and second front roller  134  in the first embodiment. 
     In the laser printer  510  of this embodiment, the ability of the first front roller  724  to clean the photosensitive belt  710  is high. The ability of the second front roller  726  to clean the first front roller  724  is also high. Hence, according to this embodiment, the ability to clean the photosensitive belt  710  is extremely high. 
     The intermediate transfer belt  750  is disposed on the left side of the photosensitive belt  710 . Five rollers  730 ,  732 ,  734 ,  536 ,  742  are provided on the back surface side of the intermediate transfer belt  750 . The roller  732  faces the roller  706 . The photosensitive belt  710  and the intermediate transfer belt  750  contact each other between the roller  732  and the roller  706 . Thus, the toner developed on the photosensitive belt  710  can be transferred onto the intermediate transfer belt  750  (primary transfer). The toner transferred onto the intermediate transfer belt  750  is then transferred onto the printing sheet  502  between the pair of secondary transfer rollers  534 ,  536  (secondary transfer). 
     An intermediate transfer belt cleaning device  740  is disposed on the left side of the intermediate transfer belt  750 . The intermediate transfer belt cleaning device  740  comprises a back roller  742 , a first front roller  744 , a second front roller  746 , a blade  748 , and so on. The voltage between the back roller  742  and first front roller  744  is subjected to constant current control similarly to the control performed on the voltage between the back roller  130  and first front roller  132  in the first embodiment. The voltage between the first front roller  744  and second front roller  746  is subjected to constant current control similarly to the control performed on the voltage between the first front roller  132  and second front roller  134  in the first embodiment. 
     In the laser printer  510  of this embodiment, the ability of the first front roller  744  to clean the intermediate transfer belt  750  is high. The ability of the second front roller  746  to clean the first front roller  744  is also high. Hence, the ability of the laser printer  510  according to this embodiment to clean the intermediate transfer belt  750  is extremely high. 
     Specific examples of the present invention were described in detail above. However, these are merely illustrations, and do not limit the scope of the claims. The technology described in the claims includes various alternatives and modifications of the specific examples described above. 
     For, example, in the first embodiment, the potential of the back roller  130  (see  FIG. 3 ) is maintained at zero. However, when the potential of the first front roller  132  has been reduced to its limit, the potential of the back roller  130  may be adjusted to a larger value than zero. In so doing, the voltage between the back roller  130  and first front roller  132  can be increased even after the first front roller  132  has reached its minimum potential. 
     Note that the following image forming device is also useful. This image forming device comprises a photosensitive belt, a back member disposed on the back side of the photosensitive belt, a front member disposed on the front side of the photosensitive belt so as to face the back member, and a device for adjusting a voltage between the back member and front member such that a current flowing between the back member and front member is maintained within a predetermined range. 
     Further, the technical elements described in the present specification and drawings exhibit technical usefulness either individually or in various combinations, and are not limited to the combinations in the claims at the time of filing. Moreover, the technology illustrated in the present specification and drawings achieves a plurality of objects simultaneously, and possesses technical usefulness simply by achieving one of these objects.