Abstract:
A fixing device includes an endless belt with a resistance heating layer, a pressure roller fitting loosely in a belt circulation path, and a pressing roller pressing the pressure roller through the belt to form a fixing nip with a belt surface, and thermally fixes an unfixed image by passing a recording sheet through the fixing nip, comprises: a pair of annular electrodes provided circumferentially and sandwiching a sheet-passing region of the belt surface; a first power supply member pressurizing a given one of the electrodes; and a second power supply member positioned closer to the fixing nip than the first power supply member, also pressurizing the given one of the electrodes, and supplying power to the resistance heating layer in cooperation with the first power supply member, wherein the pressing force applied by the first power supply member is weaker than that applied by the second power supply member.

Description:
This application is based on an application No. 2011-121593 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention pertains to a fixing device and an image formation device using the fixing device, specifically to a fixing device including a resistance heating layer. 
     (2) Description of the Related Art 
     In recent years, greater energy economy than that offered by halogen heaters has been sought for the fixing devices used in image formation devices, such as printers. Propositions include the use of a fixing device having a fixing belt that includes a resistance heating layer (e.g., Japanese Patent Application Publication No. 2009-109997). 
       FIG. 7  is an overall perspective view diagram of such a fixing device  500 . 
     As shown, the fixing device  500  includes a fixing belt  554 , a pressure roller  550 , a pressing roller  560 , and a pair of power supply rollers  570  connected to an A/C power supply. 
     The fixing belt  554  is a cylindrical, resilient, and deformable belt that includes a resistance heating layer  554   b  and has electrodes  554   e  formed over the resistance heating layer  554   b  at each end thereof, in the width (Y-axial) direction. 
     The pressure roller  550  has a metal core  551  covered by a resilient layer  552 , and the fixing belt  554  is loosely fit therearound so as to circulate. 
     A pressing roller  560  is arranged outside the circulation path of the fixing belt  554  and pressurizes the pressure roller  550  through the fixing belt  554 , forming a fixing nip  530 . 
     Also, the pressing roller  560  receives a driving force from a (non-diagrammed) driving motor and thus rotates in the direction indicated by arrow P. The driving force is transmitted through the fixing belt  554  to the pressure roller  550 , such that the fixing belt  554  and the pressure roller  550  are driven to rotate in the direction indicated by arrow Q. 
     The pair of power supply rollers  570  is in contact with the electrodes  554   e  of the fixing belt  554 , outside the circulation path, pressing the fixing belt  554  downward, as shown (i.e., in the Z′ direction). Accordingly, power is supplied to the resistance heating layer  554   b  of the fixing belt  554 . 
     According to this configuration, the electrodes  554   e  supply power via the power supply roller  570  while the fixing belt  554  is driven to circulate. As this occurs, the electrical resistance in the electrodes  554   e  is much less than that of the resistance heating layer  554   b , to the extent that voltage drops in the electrodes  554   e  are safely ignored. Thus, electrical current flows over the entire circumference of each electrode  554   e  and Y-axially across the entire resistance heating layer  554   b , producing heat therein. 
     Given that the direction of the current I periodically reverses, the direction indicated in  FIG. 7  for the current I is simply an example at a given point in time. 
     Here, portions of the fixing belt  554  other than those pressed by the fixing nip  530  and the power supply roller  570  are not in contact with any other components and cannot avoid surrounding heat. As such, temperatures effectively increase in the region of the fixing nip  530  through Joule heating and, when a (non-diagrammed) recording sheet with a toner image formed thereon passes through the fixing nip  530 , the heat and pressure fixes the toner image to the recording sheet. 
     However, when the pressing roller  560  is driven to rotate while being pressed by the pressure roller  550  through the fixing belt  554 , the friction between the power supply roller  570  and the fixing belt  554 , which is deformed into an oval, leads to unstable running conditions for the fixing belt  554  and may cause running path fluctuations (hereinafter, ripples) in slack portions occurring in the fixing belt  554 . 
       FIGS. 8A and 8B  illustrate a somewhat exaggerated aspect of the ripple phenomenon occurring on the fixing belt  554 . 
     As shown in  FIG. 8A , the power supply roller  570  is normally pressed toward the pressure roller  550  by a compressed spring  571  or similar. When a rise portion  554   a  forms in the slack portion, having risen away from the pressure roller  550 , regains its original position upon passing through, and the motion of the fixing belt  554  follows with some delay, then as shown in  FIG. 8B , a space S may be produced between the power supply roller  570  and the fixing belt  554 . 
     As such, the potential difference between the power supply roller  570  and the fixing belt  554  produces a spark across the space S, which is problematic in that a small hole may be opened thereby in the surface of the fixing belt  554 , reducing the useful life thereof. 
     SUMMARY OF THE INVENTION 
     In consideration of this problem, the present invention pertains to a fixing belt that includes a resistance heating layer and electrodes supplying electricity thereto in a fixing device, and an image formation device including the fixing belt circulating loosely around a pressure roller and heated by resistance heating, and aims to extend the useful life of the fixing belt. 
     In one aspect of the present invention, a fixing device includes an endless belt with a resistance heating layer, a pressure roller fitting loosely in a circulation path of the belt, and a pressing roller pressing the pressure roller through the belt to form a fixing nip in conjunction with a surface of the belt, and thermally fixes an unfixed image on a recording sheet by passing the recording sheet through the fixing nip, and comprises: a pair of annular electrodes provided circumferentially so as to sandwich a sheet-passing region on the surface of the belt therebetween; a first power supply member pressurizing a given one of the electrodes with pressing force; and a second power supply member positioned closer to the fixing nip than the first power supply member, pressurizing the given one of the electrodes with pressing force, and supplying power to the resistance heating layer in cooperation with the first power supply member, wherein the pressing force applied by the first power supply member is weaker than the pressing force applied by the second power supply member. 
     In another aspect of the present invention, an image formation device comprises a fixing device including an endless belt with a resistance heating layer, a pressure roller fitting loosely in a circulation path of the belt, and a pressing roller pressing the pressure roller through the belt to form a fixing nip in conjunction with a surface of the belt, and thermally fixes an unfixed image on a recording sheet by passing the recording sheet through the fixing nip, the fixing device comprising: a pair of annular electrodes provided circumferentially so as to sandwich a sheet-passing region on the surface of the belt therebetween; a first power supply member pressurizing a given one of the electrodes with pressing force; and a second power supply member positioned closer to the fixing nip than the first power supply member, pressurizing the given one of the electrodes with pressing force, and supplying power to the resistance heating layer in cooperation with the first power supply member, wherein the pressing force applied by the first power supply member is weaker than the pressing force applied by the second power supply member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and the other objects, advantages and features of the invention will become apparent from the following description thereof, taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. 
       In the drawings: 
         FIG. 1  is an overall diagram illustrating a tandem color printer serving as an example of an image formation device including a fixing device pertaining to an Embodiment of the present invention; 
         FIG. 2  is a partial cutaway perspective view diagram of the fixing device; 
         FIG. 3  is a partial cross-section of a fixing belt in the fixing device; 
         FIG. 4  is a side view diagram of the fixing device; 
         FIG. 5  illustrates the shape of the fixing belt in the fixing device when two power supply members that press electrodes are completely removed; 
         FIG. 6  illustrates a variant fixing device pertaining to the Embodiment; 
         FIG. 7  is a perspective-view diagram of a fixing device in a conventional image formation device; and 
         FIGS. 8A and 8B  illustrate a ripple phenomenon occurring in the fixing belt of the conventional fixing device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A fixing device and an image formation device pertaining to the present invention are described below, with reference to the accompanying drawings. 
     (Image Formation Device Configuration) 
       FIG. 1  is an overall configuration diagram illustrating a tandem colour printer (hereinafter, printer) serving as an example of an image formation device that includes a fixing device pertaining to the Embodiment of the present invention. 
     As shown, the printer  1  includes an image processing unit  3 , a feed unit  4 , a fixing unit  5 , and a control unit  60 . The printer  1  is connected to a network (e.g., a LAN), receives a print job execution instruction from a (non-diagrammed) external terminal, forms a toner image corresponding to the received instruction in each of yellow, magenta, cyan, and black, then creates a full-colour image through overlay transfer of these images. 
     The colours yellow, magenta, cyan, and black are hereinafter respectively abbreviated Y, M, C, and K. Components pertaining to reproduction of a given colour are marked with Y, M, C, or K as appropriate. 
     (Image Processing Unit) 
     The processing unit  3  includes imaging units  30 Y,  30 M,  30 C, and  30 K each corresponding to a colour Y, M, C, or K, an optics unit  10 , and an intermediate transfer belt  11 . 
     Imaging unit  30 Y includes a photosensitive drum  31 Y, a charger  32 Y, a developer  33 Y, a primary transfer roller  34 Y, and a cleaner  35 Y for cleaning the photosensitive drum  31 Y, all disposed at the periphery thereof. A yellow toner image is created on the photosensitive drum  31 Y. 
     The other imaging units  30 M,  30 C, and  30 K are configured similarly to imaging unit  30 Y. The reference signs therefor are thus omitted from  FIG. 1 . 
     The intermediate transfer belt  11  is an endless belt overspanning a driving roller  12  and a driven roller  13  and driven to rotate in the direction indicated by arrow A. 
     The optics unit  10  includes a light-emitting element, which is a laser diode and so on. The optics unit  10  produces laser light L for forming the image in the colours Y, M, C, K in accordance with a drive signal from the control unit  60  by scanning the photosensitive drums  31 Y,  31 M,  31 C, and  31 K. 
     Exposure to the laser light L causes each of the photosensitive drums  31 Y,  31 M,  31 C,  31 K, charged by the corresponding charger  32 Y,  31 M,  31 C, or  32 K, to form a latent static image. Each latent static image is formed by performing a primary transfer of the Y, M, C, K-colored toner images developed on the photosensitive drums  31 Y,  31 M,  31 C, and  31 K by the developers  33 Y,  33 M,  33 C, and  33 K onto the intermediate transfer belt  11 , the transfer timed so as to overlap at a common position. 
     A full-color toner image is formed by sequential transfer of the toner images on the intermediate transfer belt  11 , operated by the primary transfer rollers  34 Y,  34 M,  34 C, and  34 K through the action of static electricity. The full-color toner image is then shifted toward a secondary transfer position  46 . 
     The feed unit  4  includes a paper feed cassette  41  containing recording sheets, a pick-up roller  42  picking up the recording sheets in the paper feed cassette  41  one by one for passage into a transport path  43 , and a pair of timing rollers  44  for adjusting the timing at which each recording sheet is sent to the secondary transfer position  46 . A recording sheet is fed to the secondary transfer position from the feed unit  4  at timing matching that of the toner image transfer on the intermediate transfer belt  11 , and the toner images undergo a secondary transfer as a batch, through the action of a secondary transfer roller  45 . 
     Having passed through the secondary transfer position  46 , the recording sheet is transported to the fixing unit  5 . The (unfixed) toner image on the recording sheet is heated and pressurized by the fixing unit  5 , thus becoming fixed in place. Afterward, the recording sheet is taken to an exit tray  72  by the action of a pair of exit rollers  71 . 
     (Fixing Unit Configuration) 
       FIG. 2  is a partial cross-section of the aforementioned fixing unit  5 . 
     As shown, the fixing unit  5  includes a fixing belt  154 , a pressure roller  150 , a pressing roller  160 , and power supply members  170   a  and  170   b.    
     The pressure roller  150  is arranged loosely inside the fixing belt  154  so as to be parallel to the pressing roller  160 . A fixing nip N is formed between the fixing belt  154  and the pressing roller  160  through bias applied to the pressing roller  160  by a non-diagrammed biasing mechanism toward the pressure roller  150  through the fixing belt  154 . The toner image formed on the (non-diagramed) recording sheet is heated and pressurized by passing through the fixing nip N. 
     The components of the fixing unit  5  are described in detail, below. 
     (Pressing Roller Configuration) 
     The pressure roller  150  is driven by a non-diagrammed drive mechanism to rotate in the direction indicated by arrow C. The pressure roller  150  applies pressure to the fixing belt  154  from the outside. 
     Accordingly, the fixing belt  154  and the pressure roller  150  are driven to rotate in the direction indicated by arrow D. 
     As shown in  FIG. 2 , the pressing roller  160  includes a metal core  161  covered by resilient layer  162 , everywhere but the two ends thereof. 
     The metal core  161  is a solid shaft made of a metal such as aluminum, steel, or stainless steel, 30 mm in diameter, driven to rotate by the non-diagrammed drive mechanism. 
     The solid shaft may be replaced with a hollow shaft of a thickness ranging from 0.1 mm to 10 mm, inclusive, or with a hollow shaft having a support rib with a Y-shaped cross-section installed therein. 
     Resilient layer  162  is a tube made of silicone rubber, having a thickness that is ideally between 1 mm and 20 mm, inclusive. 
     In the present Embodiment, the thickness of resilient layer  162  is 3 mm, and the outer diameter thereof is 36 mm. 
     Resilient layer  162  is 374 mm long as measured along the Y-axis. 
     (Pressure Roller) 
     As shown in  FIG. 2 , the pressure roller  150  is made of a metal core  151  shaped as an elongated cylinder and enveloped by resilient layer  152 . 
     The metal core  151  is a solid shaft made of a metal such as aluminum, steel, or stainless steel, 20 mm in diameter. The two axial ends of the shaft are supported by non-diagrammed bearings in the frame of the fixing unit  5  so as to be able to rotate freely. 
     The solid shaft may be replaced with a hollow shaft of a thickness ranging from 0.1 mm to 10 mm, inclusive, or with a hollow shaft having a support rib with a Y-shaped cross-section installed therein. 
     Resilient layer  152  is made of a heat-resistant, adiabatic material, such as a resilient foam of silicone rubber or fluorine rubber, having a thickness of 1 mm to 20 mm. Accordingly, the outer diameter of the pressure roller  150  is between 20 mm and 100 mm, inclusive. In the present Embodiment, the outer diameter is 30 mm. 
     Resilient layer  152  is 374 mm long as measured along the Y-axis. 
     The length of resilient layer  152 , as measured along the Y-axis, should of course be longer than the maximum passing width of a recording sheet. 
     Resilient layer  152  is less film than resilient layer  162  of the pressing roller  160 . The nip N is formed primarily by elastic deformation of resilient layer  152 . 
     (Fixing Belt) 
       FIG. 3  is a partial cross section indicating the layer structure of the fixing belt  51 . 
     The fixing belt  51  is illustrated in  FIG. 3  with particular attention to one end in the roller-axial direction. The other end of the fixing belt  51  is configured identically. 
     Also, in  FIG. 3 , the thickness is greatly exaggerated for ease of comprehension. The dimensions of each component are given as examples and do not necessarily correspond to those of actual components. 
     The fixing belt  154  is an endless, elastically-deformable belt having a layered structure. As shown, a resistance heating layer  154   b  is layered over the outer circumferential surface of an insulating layer  154   a . Further, an electrode layer  154   e  is layered on each of the Y-axial ends of the resistance heating layer  154   b.    
     Furthermore, resilient layer  154   c  and a release layer  154   d  are sequentially layered over areas the resistance heating layer  154   b  not covered by the electrode layer  154   e.    
     The components of the fixing belt  154  are described in detail, below. 
     The insulating layer  154   a  is made of a material that does not conduct electricity, such as PI (polyimide), PPS (polyphenylene sulfide), PEEK (polyether ether ketone), having a thickness of approximately 50 μm and a length of 374 mm along the Y axis. 
     The resistance heating layer  154   b  is a heat-producing tube that undergoes Joule heating as a result of electric current producing differences in electric potential between the Y-axial ends. 
     Specifically, the resistance heating layer  154   b  has a thickness of 5 mm to 200 mm inclusive and is made of a PI resin having one or more varieties of conductive filler dispersed throughout, each variety having a different electrical resistance. 
     The Y-axial length of the resistance heating layer  154   b  is 374 mm, similar to that of the insulating layer  154   a.    
     The material used for the base of resistance heating layer  154   b  (hereinafter, base material) may also be PPS or PEEK. 
     The different varieties of conductive filler may be a powdered metal, such as silver, copper, aluminum, magnesium, or nickel, a powdered carbon compound, such as graphite, carbon black, carbon nanofibers, or carbon nanotubes, or may be an inorganic compound that is also a fast ion conductor, such as silver iodide or copper iodide. The conductive filler is preferably shaped as fibers so as to increase the probability of contact at the unit content level. 
     In the present Embodiment, the fibrous conductive filler is, for example, nickel scattered uniformly throughout the above-described base material. 
     The volume resistivity of the resistance heating layer  154   b  is preferably on the order of 10×10 −6  Ω·m to 1.0×10 −2  Ω·m, inclusive, and for the fixing unit  5  pertaining to the present Embodiment, is ideally between 10×10 −5  Ω·m to 5.0×10 −3  Ω·m, inclusive. 
     Resilient layer  154   c  is, for example, made of a resilient and heat-resistant material such as silicone rubber and has a thickness of approximately 500 μm. 
     The material used for resilient layer  154   c  may alternatively be fluorine rubber or the like. 
     The release layer  154   d  is made, for example, of a fluorine-based resin such as PTFE or PFA, and has a thickness of 5 μm to 100 μm, inclusive. 
     The electrode layer  154   e  is formed as a loop so as to cover the Y-axial ends of the resistance heating layer  154   b , and is provided as a pair of annular electrodes for supplying electricity to the resistance heating layer  154   b.    
     The electrode layer  154   e  is, for example, formed of a metal having lower electrical resistivity than the resistance heating layer  154   b , such as copper, aluminum, nickel, brass, or phosphor bronze, and may be formed over the ends of the resistance heating layer  154   b  through chemical plating, electroplating, or the like. 
     Alternately, a strip from a sheet of any of the above materials may be applied to the Y-axial ends of the resistance heating layer  154   b  by means of a conductive adhesive, thus forming the electrode layer  154   e.    
     The width (Y-axial length) of the electrode layer  154   e  is 18 mm. 
     Further, the thickness of the electrode layer  154   e  is preferably between 5 μm and 100 μm, inclusive, so as to preserve the flexibility needed to deform the belt, particularly when forming the fixing nip N, while providing appropriate rigidity. In this example, the electrode layer  154   e  is 20 μm thick. 
     (Power Supply Member) 
     Again, as shown in  FIG. 2 , the power supply members  170   a  and  170   b  are provided as a pair on each of two pieces of the electrode layer  154   e  on the below-described fixing belt  154 . 
     Specifically, the power supply members  170   a  and  170   b  are electrically connected to an external A/C power supply  180  through respective lead wires  175   a  and  175   b  and press against the electrode layer pieces, thus supplying power thereto. 
     Power supply member  170   a  presses against a piece of the electrode layer  154   e  at a position closer to the fixing nip N than power supply member  170   b.    
     Accordingly, two power supply members press against the pieces of the electrode layer  154   e  and supply power thereto. This enables power to be supplied in a more stable and reliable manner. 
     The lead wires  175   a  and  175   b  branch off from a common lead wire (hereinafter, main lead unit)  175 . The main lead unit  175  is connected to the A/C power supply  180  via a non-diagrammed relay switch. 
     The control unit  60  switches the relay switch ON or OFF in accordance with the surface temperature of the fixing belt  154 , read by a non-diagrammed temperature sensor, thus maintaining a target temperature for the fixing belt  154 . 
     Power supply member  170   a  includes a brush unit  171   a , a flexible member  172   a , a support plate  173   a , and a shaft unit  174   a.    
     Similarly, power supply member  170   b  includes a brush unit  171   b , a flexible member  172   b , a support plate  173   b , and a shaft unit  174   b . All components other than flexible member  172   b  are identical to those of power supply member  170   a.    
     The brush unit  171   a  is, for example, a block-like conductor having a thickness of 30 mm, a Y-axial width of 10 mm, and a length of 5 mm along a sliding-motion direction thereof. The brush unit  171   a  is a carbon brush made of a slidable, conductive material such as copper graphite or carbon graphite. 
     The current density may grow to excess and produce undesirable holes in the electrode layer  154   e  not only when, for example, spark discharge occurs between the pieces of the electrode layer  154   e  and the power supply members  170   a  and  170   b , but also when, for example, the contact surface therebetween temporarily becomes excessively small and causes a locally-concentrated current density to occur. 
     As described above, the block-like brush unit  171   a  is pressed by the piece of the electrode layer  154   e , thus securing a wider area for surface contact such that the probability of temporarily reducing the surface contact area to an extremely small size is reduced, even under unstable connection conditions. 
     The shaft unit  174   a  is a conductive shaft made of metal or similar, fixedly incorporated into the brush unit  171   a  at one end and connected to the lead wire  175   a  at the other end. 
     The support plate  173   a  is joined to the main frame of the fixing unit shaft unit, and has a (non-diagrammed) through-hole through which the shaft unit  174   a  passes so as to be freely slideable. 
     The flexible member  172   a  is, for example, a compression spring interposed between the brush unit  171   a  and the support plate  173   a . As shown in  FIG. 2 , the brush unit  171   a  presses against the outer circumferential surface of the piece of the electrode layer  154   e.    
     Here, pressing force F 1  is given as follows, in Newtons.
 
 F 1= Fs+Fc   (Math. 1)
 
     where Fs is the pressing force securable for stable electricity application while the fixing belt is stopped, given in Newtons, and Fc is maximum force pulling the brush unit  171   a  away from the fixing belt toward the normal direction of the contact surface through ripple phenomena occurring on the fixing belt during circulation, also given in Newtons. 
     The value of Fs must be on the order of 0.2 N to 0.5 N, as determined experimentally. 
     When the value of Fs falls below the above-described range for pressing force, the contact resistance increases, producing heat at the contact portion between the brush unit  171   a  and the piece of the electrode layer  154   e  and decreasing the efficacy of the power supply. 
     Given that influential elements such as the position at which force is applied by the power supply member  170   b  to the fixing belt  154  and the value of pressing force F 2  are prone to fluctuations, the value of Fc is determined experimentally upon investigation of these elements. 
     This is because the ripple phenomenon occurring on the fixing belt  154  is likely to occur at a position farther from the fixing nip N (hereinafter, nip-distal portion) at greater separation from the pressure roller  150 . The ripple phenomenon produced at the nip-distal portion is then likely to propagate to the vicinity of the fixing nip N (hereinafter, nip-proximal portion). 
     Therefore, the inventor has mainly set the value of pressing force F 1  on the electrode layer  154   e  so as to provide reliable electricity supply from the power supply member  170   a  provided at the nip-proximal portion, where the ripple phenomenon is less likely to occur. Consequently, the value of pressing force F 2  for the power supply member  170   b  provided at the nip-distal portion, where the ripple phenomenon is more likely to occur, is sufficient when set high enough to constrain ripple propagation from the nip-distal portion to the nip-proximal portion. 
     Accordingly, pressing force F 2  is set to be smaller than pressing force F 1 , satisfying the following.
 
 F 2&lt; F 1  (Math. 2)
 
     That is, in contrast to conventional technology where only one power supply member is provided for each electrode, the present Embodiment features two power supply members for the fixing unit  5 . Thus, there is no need to apply the double pressing force required in conventional technology when only one power supply member is present. 
     As a result, the load on the motor driving the pressing roller  160  is reduced, the abrasion of the pieces of the electrode layer  154   e  is mitigated, and the abrasive deterioration of the fixing belt  154  is correspondingly decreased. 
     As described above, the fixing unit  5  pertaining to the present Embodiment has at least one member of a pair of annular pieces of the electrode layer  154   e  pressurized by power supply member  170   b  and power supply member  170   a , which is located closer to the fixing nip, both supplying electrical power thereto. The pressing force of power supply member  170   a  is weaker than that of power supply member  170   b.    
     The ripple phenomenon occurring on the fixing belt  154  is more likely to occur at positions farther from the nip portion. Thus, the pressing force of power supply member  170   b  serves to reduce the amplitude of the ripple phenomenon, which places a constraint on the amplitude of the ripple phenomenon propagated as far as power supply member  170   a , arranged closer to the nip portion than power supply member  170   b . As such, power supply member  170   a  serves as the main power supply member and the reliability of contact between power supply member  170   a  and the piece of the electrode layer  154   e  can be improved. Therefore, the risk of spark discharge damaging the electrode layer  154   e  is diminished, promoting a longer useful life for the fixing belt  154 . 
     Here, power supply member  170   b  need only press the belt with force sufficient to constrain the propagation of the ripple phenomenon, and does not require as much pressing force as power supply member  170   a . Thus, the pressing force of power supply member  170   b  may be weaker than that of power supply member  170   a . As described above, this enables reduction of the abrasion imposed on the electrode layer  154   e  by power supply member  170   a.    
     Also, both power supply members  170   a  and  170   b  are in contact with a single piece of the electrode layer  154   e . Thus, the total contact surface area between the electrode layer  154   e  and the power supply members is increased in comparison to conventional technology. This leads to improved stability for the power supply and reduced likelihood of spark discharge, in turn extending the useful life of the belt. 
     Also, the inventor has arranged the position at which the power supply member  170   b  presses the piece of the electrode layer  154   e  to be far from the fixing nip N so as to constrain ripple propagation, and therefore set the position of the power supply member  170   b  as described below. 
     When the pressure roller  150  is viewed along the length of the rotational axis, four regions are defined by line L 1  passing through center O 1  of the pressure roller  150  and center O 2  of the pressing roller  160 , and by line L 2  passing through center O 1  of the pressure roller  150  perpendicular to line L 1 . Of these, region R 1  includes upstream edge P 1  of the sheet-passing portion of the fixing nip N, region R 2  includes downstream edge P 2  of the sheet-passing portion of the fixing nip N, and the remaining regions R 3  and R 4  are so numbered in counterclockwise sequential order. At least one portion of the contact surface between power supply member  170   b  and the fixing belt  154  is positioned within region R 1  or region R 2 , while power supply member  170   a  is positioned closer to the fixing nip N than power supply member  170   b.    
     In the fixing unit  5  of the present Embodiment, at least portion A 1  of the contact surface between power supply member  170   b  and the fixing belt  154  is positioned within region R 1 . 
     The following describes the reasoning behind this positioning of power supply member  170   b.    
       FIG. 5  illustrates the fixing unit  5  with power supply members  170   a  and  170   b , which press the same piece of the electrode layer  154   e , removed from the fixing belt  154 . 
     Given that the pressing roller  160  presses the surface of the sheet leftward in the fixing nip N, center O 3  of the fixing belt  154  is offset leftward with respect to center O 1  of the pressure roller  150 . 
     As such, gap d 0  between the fixing belt  154  and the pressure roller  150  in region R 1  is narrower than otherwise similar gap d 1  in region R 2 . The positions thereof change very little, the fixing nip N notwithstanding. 
     Accordingly, when the power supply member  170   b  presses the piece of the electrode layer  154   e  in regions R 1  and R 2 , the fixing belt  154  comes into contact with the pressure roller  150  and thus, ripples can be constrained without excessive pressing force F 2 . 
     However, in regions R 3  and R 4 , gaps d 2  and d 3  between the fixing belt  154  and the pressure roller  150  widen with increasing distance from the fixing nip. Thus, in regions R 3  and R 4 , when the piece of the electrode layer  154   e  is pressed by the power supply member  170   b , a greater pressing force F 2  is required to bring the fixing belt  154  and the pressure roller  150  into contact. As such, these areas are poor choices for the suppression of ripple propagation. 
     Thus, inventor has concluded that in order to suppress ripple propagation to the nip-proximal portion, at least part of the contact surface between the power supply member  170   b  and the fixing belt  154  should preferably be in region R 1  or in region R 2 . 
     Of course, the power supply member  170   a  must be positioned in the same region as power supply member  170   b  and closer to the fixing nip N than the power supply member  170   b.    
     In fact, Math. 1 and Math. 2 may be satisfied with experimentally obtained minimal values of Fc and F 2 , found by having the fixing belt  154  be driven to circulate, having the piece of the electrode layer  154   e  be pressed by the entire contact surface of power supply member  170   a  and by at least one portion of power supply member  170   b  in regions R 1  and R 2 , and moving the pressing positions of the power supply members. 
     Power supply member  170   a  is positioned as close as possible to the fixing nip N, where the fixing belt  154  ripple phenomenon is less likely to occur, and preferably arranged as not to cause interference with the pressing roller  160 . 
     Also, positioning the power supply members  170   a  and  170   b  in region R 1  rather than in region R 2  is preferable for securing a stable power supply. 
     This is because, in region R 1 , tensile force on the fixing belt  154  is produced between power supply member  170   b  and power supply member  170   a  as well as between power supply member  170   a  and the fixing nip N, making the fixing belt  154  less flexible. In contrast, in region R 2 , the fixing belt  154  experiences pressing-out due to the fixing nip N. Thus, the fixing belt  154  is easily flexible between the fixing nip N and power supply member  170   a , which renders the contact surface between the fixing belt  154  and power supply member  170   a  unstable. Thus, pressing force F 1  must be made larger. 
     (Variations) 
     The present invention is not limited to the above-described Embodiment. The following variations are also possible. 
     (1) In the above-described Embodiment, the power supply members  170   a  and  170   b  independently pressurize the piece of the electrode layer  154   e . However, no limitation is intended. The power supply members  170   a  and  170   b  may also be molded integrally. 
       FIG. 6  illustrates an example of such a configuration. 
     As shown, a power supply member  270  includes a brush unit  271 , a support member  282 , a guide member  290 , and a compression member  300 . 
     The support member  282 , here shown as a partial cut-away, is U-shaped as seen in a cross-section taken along a plane that intersects the W axis. Side plates  282   b  and  282   c  are arranged to face each other on either side of the U-shape, and each have a semispherical notch  282   d.    
     The brush unit  271  is also U-shaped, as seen along the axis of rotation of the pressure roller  150 , and has arms  271   a  and  271   b  facing each other at either side of the U-shape. The aims  271   a  and  271   b  correspond to power supply members  170   a  and power supply member of the brush unit  171   a  shown in  FIG. 4 . 
     Further, the brush unit  271  has a support shaft  271   d  that is parallel to the axis of rotation of the pressure roller  150  and protrudes from the side plates at the front and back, as seen in  FIG. 6 . The support shaft  271   d  is supported in the notch  282   c  of the support member  282  so as to be freely rotatable. 
     As shown, the brush unit  271  is connected to a lead wire  272  for supplying power thereto. 
     The guide member  290  is a tubular body shaped to have a rectangular cross-section as taken in the plane intersecting the V axis of  FIG. 6 , and guides the support member  282  with respect to the V-axis. 
     The compression member  300  is a compression spring joined to the inner bottom surface of the guide member  290 . When the support member  282  is inserted within the guide member  290 , the support member  282  is biased along the V-axis by pressing force F 3 . 
     With respect to the W-axis orthogonal to the V-axis, the distances from the center of the support shaft  271   d  to the center of each arm  271   a  and  271   b , respectively labeled distance d 4  and distance d 5 , are such that d 5  is longer than d 4 . 
     Therefore, pressing force F 3  applied by the compression member  300  is divided along the arms  271   a  and  271   b  into pressing forces F 4  and F 5  such that pressing force F 5  is smaller than pressing force F 4 . 
     Thus, the power supply member  270  serves the same functions as the individually provided power supply members  170   a  and  170   b.    
     The configuration illustrated in  FIG. 6  describes the brush unit  271  as provided in the support shaft  271   d . However, no limitation is intended. A protrusion may be provided on one of the pair of side plates that face each other in the support member  282  and the brush unit  271  may have a recess provided to fit into this protrusion. Also, if needed, the support shaft  271   d  may be provided on any of these portions. 
     Further,  FIG. 6  illustrates an example in which power supply members  170   a  and  170   b  form a common whole. However, a single conductive support member may also be used to support each of the power supply members  170   a  and  170   b.    
     (2) In the above-described Embodiment, a single piece of the electrode layer  154   e  is pressurized by two power supply members. However, depending on circumstances, three or more power supply members may also be used for this purpose. 
     In such circumstances, the third and subsequent power supply members are provided at positions farther away from the fixing nip N than power supply member  170   b . Also, the pressing force pressurizing the piece of the electrode layer  154   e  of the fixing belt  154  is set lower than or equal to the pressing force of the power supply member  170   b . This is done in order to reduce the abrasive degradation of the fixing belt, following the same reasoning as that given for setting the pressing force of power supply member  170   b  to be weaker than that of power supply member  170   a.    
     (3) In the above-described embodiment, the power supply members  170   a  and  170   b  are provided in pairs on each of a pair of pieces of the electrode layer  154   e . However, depending on the circumstances, the power supply members  170   a  and  170   b  may be provided in a pair on only one piece of the electrode layer  154   e  while a single power supply member is provided on the other piece of the electrode layer  154   e.  
 
(4) In the above-described Embodiment, the fixing belt  154  includes the insulating layer  154   a , the resistance heating layer  154   b , the resilient layer  154   c , the release layer  154   d , and the electrode layer  154   e . However, no limitation is intended, provided that the fixing belt  154  includes at least the resistance heating layer  154   b  and the electrode layer  154   e.  
 
     For example, a monochrome copier does not require as wide a fixing nip, and fixing quality degradation is not as noticeable as in a color copier. Thus, the resilient layer  154   c  may be omitted from the fixing belt  154 . 
     (5) In the above-described Embodiment, the brush unit  171   a  used in each power supply members  170   a  and  170   b  has the same shape. However, no limitation is intended. The shape and size of the brush unit may vary. 
     (6) In the above-described Embodiment, the pressing roller  160  drives the rotation and the pressure roller  150  is driven to rotate accordingly. However, no limitation is intended. 
     For example, the pressure roller  150  may drive the rotation while the pressing roller  160  is driven to rotate accordingly, or the pressure roller  150  and the pressing roller  160  may both drive the rotation. 
     (7) In the above-described Embodiment, resilient layer  152  of the pressure roller  150  is made less firm than resilient layer  162  of the pressing roller  160  so that the fixing nip N deformation occurs in resilient layer  152  of the pressure roller  150 . However, no limitation is intended. In some circumstances, provided that the fixing quality does not deteriorate, the firmness of resilient layer  152  may be made greater than that of resilient layer  162 , or the two resilient layers  152  and  162  may be equally firm.
 
(8) In the above-described Embodiments, the image formation device pertaining to the present invention is described using an example of a tandem color digital printer. However, the invention is also applicable to a monochrome printer or to any general image formation device that includes a fixing device having a resistance heating layer with an electrode layer supplying electricity thereto and having a fixing belt circulating loosely around a pressure roller and heated by resistance heating.
 
     Further, the above-described Embodiment and variations may be freely combined. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.