Abstract:
An image heating apparatus includes an endless belt for heating an image on a sheet in a nip; a supporting roller for rotatably supporting the endless belt; a displacing mechanism for displacing the supporting roller so as to maintain the endless belt in a predetermined zone in a widthwise direction; an electrical discharging member for electrically discharging the endless belt; and a holding member for holding the electrical discharging member so as to displace together with the supporting roller.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus for heating an image on a sheet of recording medium. An image heating apparatus is employed by such an image forming apparatus as a copying machine, a printer, a facsimile machine, etc. It is also employed by a multifunction image forming apparatus, that is, an image forming apparatus capable of functioning as two or more of the preceding image forming apparatuses. 
     Generally, a fixing device (image heating apparatus) is structured so that a toner image formed on a sheet of recording medium through an electrophotographic process is fixed to the sheet by the application of heat and pressure to the sheet and the toner image thereon. 
     One of the fixing devices such as the one described above is disclosed in Japanese Laid-open Patent Application H11-45016. This fixing device employs a fixation belt (endless belt). In the case of a fixing belt such as the one disclosed in the abovementioned patent application, it is possible for the fixation belt to unwantedly shift in its widthwise direction. Thus, it is desired that a fixing device is controlled in terms of the unwanted shift of its fixation belt in the widthwise direction of the belt. 
     One of the known methods for controlling the widthwise shifting of the fixation belt is to displace (tilt) the roller which supports the fixation belt so that the belt is allowed to circularly move. 
     Another issue which concerns a fixation belt is that while a toner image on a sheet of recording medium is fixed, the fixation belt tends to become frictionally charged. As the fixation belt becomes frictionally charged, it is possible that the toner of which a toner image is formed will be attracted to the fixation belt, and therefore, it will become impossible for the fixing device to properly fix the toner image. 
     Thus, in the case of the fixing device disclosed in Japanese Laid-open Patent Application H11-45016, a means for removing electrical charge (which hereafter may be referred to simply as discharging member) from the fixation belt is positioned in the adjacencies of the fixation belt. 
     However, in a case where a fixing device is structured as disclosed in the aforementioned patent application, as the fixation belt is controlled in its widthwise shift, the gap between the fixation belt and discharging member changes, which possibly makes it difficult for the discharging member from properly removing the electrical charge from the fixation belt. 
     SUMMARY OF THE INVENTION 
     Thus, the primary object of the present invention is to provide an image heating apparatus capable of keeping its endless belt free of electrical charge. 
     According to an aspect of the present invention, there is provided an image heating apparatus comprising an endless belt for heating an image on a sheet in a nip; a supporting roller for rotatably supporting said endless belt; a displacing mechanism for displacing said supporting roller so as to maintain said endless belt in a predetermined zone in a widthwise direction; an electrical discharging member for electrically discharging said endless belt; and a holding member for holding said electrical discharging member so as to displace together with said supporting roller. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of the essential portions of the fixing device in the first embodiment of the present invention, at a plane perpendicular to the lengthwise direction of the fixing device, as seen from the right-hand side of the fixing device. 
         FIG. 2  is a perspective view of the fixing device, shown in  FIG. 1 , and shows the fixing device driving mechanism, which is on the right-hand side of the device. 
         FIGS. 3(   a ) and  3 ( b ) are schematic plan views of the left and right ends of the fixation belt unit as seen from the left and right sides, respectively, of the unit. 
         FIGS. 4(   a ) and  4 ( b ) are schematic plan views of the left and right ends of the pressure belt unit as seen from the left and right sides, respectively, of the unit. 
         FIG. 5  is a perspective view of the belt shift sensor and its adjacencies. 
         FIG. 6  is a drawing for showing the oscillatory movement of the steering roller. 
         FIG. 7  is a drawing for showing the change in the belt position of the pressure belt unit caused by the belt shift control. 
         FIG. 8  is a drawing which shows the change in the belt position of the pressure belt unit as seen from the steering roller side. 
         FIG. 9  is a drawing for describing the mechanism of the fixation belt unit, which holds the discharging member of the fixation belt unit. 
         FIG. 10  is a drawing for describing the mechanism of the pressure belt unit, which holds the discharging member of the pressure belt unit. 
         FIG. 11  is a drawing of the discharging member holding mechanism (of pressure belt unit) in the second embodiment of the present invention. 
         FIG. 12  is a drawing which shows the movement of the steering roller (of pressure belt unit) caused by the belt shift control, in the second embodiment. 
         FIG. 13  is a sectional view of the pressure belt unit in the second embodiment, which shows the twisting of the belt caused by the belt shift control. 
         FIG. 14  is a schematic drawing which shows the change in the amount of gap between the belt and discharging member (of the pressure belt unit) in the second embodiment. 
         FIG. 15  is a schematic sectional view of a typical image forming apparatus to which the present invention is applicable. 
         FIG. 16  is a drawing for describing in more detail the structure of the mechanism for holding the discharging member, shown in  FIG. 11 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [Embodiment 1] 
     (1) Image Forming Apparatus 
       FIG. 15  is a schematic sectional view of a typical image forming apparatus which has a fixing device A in accordance with the present invention, which functions as an image forming device. It shows the general structure of the apparatus. 
     This image forming apparatus  1  is an electrophotographic printer (which hereafter will be referred to simply as printer). The control portion  20  (CPU) of this printer  1  is in connection to an external host apparatus  22  through an interface  21 . The printer  1  can form an image, which reflects the image formation data (electrical information of image to be formed) inputted from the external host apparatus  22 , on a sheet S of recording medium, and output the combination of the sheet S and the toner image thereon, as a print. 
     The control portion  20  is a controller which integrally controls the various operations of the printer  1 . It exchanges various electrical information with the external host apparatus  22  and the control panel (unshown) of the printer  1 . Further, it controls the processing of electrical information inputted from the various processing devices and sensors (belt shift sensor, for example, which will be described later) of the printer  1 , processing of the command signals to be outputted to the various processing devices, preset initial sequence, and preset image formation sequence. The external host apparatus  22  is a personal computer, a network, an image reader, a facsimile machine, or the like. 
     The printer  1  can be roughly divided into an electrophotographic image forming portion (image forming means) and a fixing device. The image forming portion forms an unfixed toner image and places the toner image on a sheet S of recording medium. The fixing device fixes the unfixed toner image formed on the sheet S, to the sheet S by applying heat and pressure to the sheet S and the unfixed toner image thereon. That is, it turns the unfixed toner image into a permanent image. 
     The electrophotographic image forming portion is provided with a photosensitive drum  2  (which hereafter may be referred to simply as drum) as an image bearing member which bears a latent image. The drum  2  is rotationally driven in the clockwise direction indicated by an arrow mark, at a preset speed. As the drum  2  is rotationally driven, its peripheral surface is uniformly charged to a preset polarity and potential level by a charging device  3 . 
     Then, the uniformly charged portion of the peripheral surface of the drum  2  is scanned by (exposed to) a beam  5  of laser light emitted by a laser scanner  4  (exposing device) while being modulated with the information of the image to be formed. As a result, an electrostatic latent image, which reflects the information of the image to be formed, with which the beam  5  of laser light is modulated, is effected on the peripheral surface of the drum  2 . Then, the electrostatic latent image is developed into a visible image, more specifically, an image formed of toner (toner image, hereafter), by a developing device  6 . 
     The toner image is electrostatically transferred from the drum  2  onto a sheet S of recording medium, in the transfer station of the image forming portion, which is the area of contact between the drum  2  and a transfer roller  7 . More specifically, as the sheet S is introduced into, and conveyed through the transfer station, the toner image on the drum  2  is transferred onto the sheet S as if it is peeled away from the drum  2 . To the transfer roller  7 , a preset transfer bias is applied with a preset control timing. 
     A sheet feeder cassette  9 , which is in the bottom portion of the main assembly of the printer  1  can store in layers a substantial number of sheets S of recording medium. As the feed roller  10  of the sheet feeder cassette  9  is driven with a preset sheet feeding timing, the sheets S in the cassette  9  are fed one by one into the main assembly of the printer  1  while being separated from the rest, and are conveyed to a pair of registration rollers  11  through a recording medium conveyance passage  10 . If a given sheet S of recording medium is askew when its leading edge reaches the pair of registration rollers  11 , it is corrected in attitude by the registration rollers  11 . Further, the sheet S is conveyed to the transfer station in synchronism with the progression of the formation of the toner image on the peripheral surface of the drum  2 . That is, the registration rollers  11  release the sheets S with such a timing that the leading edge of the toner image on the drum  2  arrives at the transfer station at the same time as the leading edge of the sheet S. 
     After being conveyed through the transfer station, the sheet S is separated from the drum  2 , and is conveyed to the fixing device A, by which the unfixed toner image on the sheet S is fixed, as a permanent image, to the surface of the sheet S by the heat and pressure applied by the fixing device A. Then, the sheet S is conveyed through a sheet conveyance passage  10   b , and is discharged by a pair of discharge rollers  12 , into a delivery tray  13 , which is a part of the top wall of the main assembly of the printer  1 . After the separation of the sheet S from the drum  2 , the residues, such as toner, remaining adhered to the peripheral surface of the drum  2 , are removed by a cleaning device  8 , so that the peripheral surface of the drum  2  can be repeatedly used for image formation. 
     (2) Fixing Device A 
       FIG. 1  is a schematic sectional view of the essential portions of the fixing device A, at a plane perpendicular to the lengthwise direction of the device A, as seen from the right-hand side of the device A. In the following description of the embodiments of the present invention, the lengthwise direction of the fixing device A and the structural components thereof (or their measurement) is such a direction that is perpendicular to the sheet conveyance direction D, at the sheet conveyance surface (recording medium conveyance surface) in which the sheet S is conveyed on the sheet conveyance surface. The widthwise direction of the fixing device A and structural components thereof (or their measurement) is such a direction that is parallel to the sheet conveyance direction D. 
     The front side of the fixing device A is the sheet entrance side of the fixing device A, and the rear side of the fixing device A is the sheet exit side of the fixing device A. The left and right sides of the fixing device A are the left and right sides of the fixing device A as seen from the front side of the fixing device A. In the description of this embodiment, the left side of the fixing device A in the drawings may be referred to as the front side, whereas the right side of the fixing device A in the drawings may be referred to as the rear side of the fixing device A. The top and bottom sides of the fixing device A are the top and bottom sides of the fixing device A in terms of the direction of gravity. Further, the upstream or downstream of the fixing device A is the upstream or downstream of the device A with reference to the direction in which recording medium is conveyed. 
     The fixing device A, as an image processing device, in this embodiment is of the so-called twin-belt-nip type, electromagnetic induction heating type (IH), and oil-less fixing type. 
     This fixing device A is provided with a fixation belt unit  100 A and a pressure belt unit  100 B, which are top and bottom units, respectively, of the device A. The fixation belt  130  of the fixation belt unit  100 A, and the pressure belt  120  of the pressure belt unit  100 B, are kept pressed upon each other to form a fixation nip N between the two belts  120  and  130 . 
     A sheet S of recording medium which is bearing an unfixed toner image t is conveyed through the fixation nip N, while being kept pinched between the fixation belt  130  and pressure belt  120 . As the sheet S is conveyed, the unfixed toner image on the sheet S is fixed to the sheet S, becoming a permanent toner image, by the heat from the fixation belt  130  which is being heated by electromagnetic induction, and the pressure generated in the fixation nip N by the pressure belt unit  100 B. 
     (2-1) Fixation Belt Unit  100 A 
     The fixation belt unit  100 A has the fixation belt  130  (endless belt), as a circularly movable heating means, which is flexible. It has also multiple rollers by which the fixation belt  130  is supported, and kept tensioned, in such a manner that the fixation belt  130  is circularly movable. More concretely, the fixation belt unit  100 A has a driver roller  131 , and a steering roller which functions also as a tension roller. Further, it has a stay  137  (pressure pad) which backs up the fixation belt  130  against the pressure belt unit  100 B, an induction heating coil  135  for heating the fixation belt  130  by electromagnetic induction, a discharging needle  170  for ridding the fixation belt  130  of electrical charge, etc. 
     There is no restriction with respect to the material and structure of the fixation belt  130 , as long as the fixation belt  130  can be heated by magnetic induction heating coil  135  and is heat resistant. For example, an endless belt which is made up of a layer of magnetic metal, such as nickel or stainless, is 75 μm in thickness, 380 mm in width, and 300 mm in length (circumferential length), a silicone rubber layer which is coated on the outward surface of the metallic layer to a thickness of 300 μm, for example, and a surface layer (piece of PFA tube) which covers the outward surface of the silicone rubber layer, may be used as the fixation belt  130 . 
     The driver roller  131  is rotatably supported in the fixing device A, on the sheet outlet side of the fixing device A. The driver roller  131  in this embodiment is a solid roller which is made up of stainless steel and is 18 mm in external diameter, and an elastic layer which is molded on the peripheral surface of the metallic core, of heat resistant silicone rubber, and is 1 mm in thickness. 
     The tension roller  132  is rotatably supported in the fixing device A, on the sheet entrance side of the fixing device A. It is kept pressured by a tension generating mechanism (which will be described later) in the direction to provide the fixation belt  130  with a preset amount of tension. Further, it can be changed in its attitude relative to the driver roller  131  (belt supporting other member), as a member which is movable in an oscillatory manner, by the steering mechanism (attitude changing mechanism), which will be described later. With the fixing device A being structured as described above, it is possible to control the fixation belt  130  in its movement in the direction parallel to the axial line of the driver roller  131 , which occurs while the fixation belt  130  is circularly moved. That is, the fixation belt  130  is controlled by the steering mechanism in such a manner that when it is circularly moved, it remains in a preset range in terms of its widthwise direction. 
     The stay  137  is made of stainless steel, for example. It is placed in the inward side of the loop which the fixation belt  130  forms. It is placed between the driver roller  131  and steering roller  132 . More specifically, it is positioned next to the driver roller  131 , with its belt backing surface facing downward. 
     The fixation belt  130  is suspended by the driver roller  131 , tension roller  132 , and belt backing stay  137 , being tensioned by a preset amount of force applied to the tension roller  132  by the belt tensioning mechanism in the direction to provide the fixation belt  130  with a preset amount of tension. The downwardly facing surface of the belt backing stay  137  remains in contact with the inward surface of the fixation belt  130 , in terms of the belt loop, across the portion of the belt  130 , which is moving through the bottom portion of the belt loop. 
     (2-2) Pressure Belt Unit  100 B 
     The pressure belt unit  100 B has the pressure belt  120  (endless belt), as a circularly movable pressure applying means, which is flexible. It has also multiple rollers by which the pressure belt  120  is supported so that the pressure belt  120  is circularly movable. More concretely, the pressure belt unit  100 B has a pressure roller  121 , and a steering roller  122  which functions as a tension roller. Further, it has a pressure pad  125  which keeps the pressure belt  120  pressured toward the fixation belt unit  100 A, a discharging needle  164  for ridding the pressure belt  120  of electrical charge, etc. 
     There is no restriction with respect to the material and structure of the pressure belt  120 , as long as the pressure belt  120  can be heat resistant. For example, an endless metallic belt which is made of nickel, for example, is 50 μm in thickness, 380 mm in width, and 200 mm in length (circumferential length), a silicone rubber layer which is coated on the outward surface of the metallic belt to a thickness of 300 μm, for example, and a surface layer (piece of PFA tube) which covers the outward surface of the silicone rubber layer, may be used as the pressure belt  120 . The electrical resistance of the surface layer of the pressure belt  120  in this embodiment is in a range of 10 9 - 10   11 Ω. 
     The pressure roller  121  is rotatably supported in the fixing device A, on the sheet outlet side of the fixing device A. The pressure roller  121  in this embodiment is a solid roller which is made up of stainless steel and is 20 mm in external diameter. 
     The tension roller  122  is rotatably supported in the fixing device A, on the sheet entrance side of the fixing device A. It is kept pressured by a tension generating mechanism (which will be described later) in the direction to provide the pressure belt  120  with a preset amount of tension. Further, the tension roller  122  be changed in its attitude relative to the pressure roller  121  (belt supporting other member), as a member which is movable in an oscillatory manner, by the steering mechanism (attitude changing mechanism), which will be described later. With the pressure belt unit  100 B being structured as described above, it is possible to control the pressure belt  120  in its movement in the direction parallel to the axial line of the pressure roller  121 , which occurs while the pressure belt  120  is circularly moved. That is, the pressure belt  120  can be controlled by the steering mechanism in such a manner that when it is circularly moved, it remains in a preset range in terms of its widthwise direction. 
     The pressure pad  125  is an elastic pad formed of silicone rubber, for example. It is held to a metallic base  125   a  of the pressure pad  125 , and is placed in the inward side of the loop which the pressure belt  120  forms. It is placed between the pressure roller  121  and steering roller  122 . More specifically, it is positioned next to the pressure roller  121 , with its belt backing surface facing upward. 
     The pressure belt  120  is suspended by the pressure roller  121 , tension roller  122 , and pressure pad  125 . It is kept tensioned by a preset amount of force applied to the tension roller  122  by the belt tensioning mechanism in the direction to provide the pressure belt  120  with a preset amount of tension. The upwardly facing surface of the pressure pad  125  remains in contact with the inward surface of the pressure belt  120 , in terms of the belt loop, across the portion of the belt  120 , which is moving through the top portion of the belt loop. 
     In this embodiment, the pressure belt unit  100 B is kept pressed upon the fixation belt unit  100 A by 400 N of pressure generated by a pressure application mechanism (unshown). Thus, the pressure roller  121  is kept pressed against driver roller  131  with the presence of the pressure belt  120  and fixation belt  130  between the two rollers  121  and  131 . The elastic layer of the driver roller  131  is kept elastically deformed, in the nip N between the driver roller  131  and pressure roller  121 , by a preset amount, by the pressure applied by the pressure roller  121 . 
     Therefore, the fixation nip N, which has a preset width in terms of the recording medium conveyance direction D, is formed, and maintained, between the fixation belt  130  and pressure belt  120 . 
     (2-3) Fixing Operation 
     The driver roller  131  is rotationally driven at a preset peripheral velocity in the clockwise direction indicated by an arrow mark, by the driving force transmitted to the driver roller  131  from a fixation motor M, which is under the control of the control portion  20 , through a driving force transmission mechanism (unshown). By this rotation of the driver roller  131 , the fixation belt  130  is circularly moved in the clockwise direction indicated by the arrow mark at the speed which corresponds to that of the driver roller  131 . The steering roller  132  is rotated by the circularly movement of the fixation belt  130 , with the portion of the fixation belt  130 , which is moving through the bottom portion of the belt loop, sliding on the downwardly facing surface of the belt backing stay  137 . 
     In order to ensure that a sheet S of recording medium is reliably conveyed through the fixation nip N, it is ensured that the rotational of the driver roller  131  is reliably transmitted to the fixation belt  130 . Further, the driving force from the fixation nip N is also transmitted to the pressure roller  121  through the driving force transmission mechanism (unshown), whereby the pressure roller  121  is rotationally driven in the counterclockwise direction indicated by an arrow mark. By this rotation of the pressure roller  121  along with the friction between the rotating fixation belt  130  and pressure roller  121 , the pressure belt  120  is circularly moved in the counterclockwise direction indicated by the arrow mark. The fixation belt  130  and pressure belt  120  are the same in their moving speed in the fixation nip N, and are roughly the same in the moving direction in the fixation nip N. 
     As the induction heating coil  135  is supplied with electric power by an electric power source  135 A which is under the control of the control portion  20 , it generates an alternating magnetic field, which heats the circularly moving fixation belt  130  by magnetic induction. The temperature of the fixation belt  130  is detected by a thermistor (temperature detecting means: unshown), and the information regarding the temperature of the fixation belt  130  is inputted into the control portion  20 . Based on the inputted information regarding the temperature of the fixation belt  130 , the control portion  20  controls the electric power supply to the induction heating coil  135  to increase the temperature of the fixation belt  130  to a preset target level, and keep it at the target level. 
     As soon as the peripheral velocity of the fixation belt  130  and pressure belt  120  reach their target level, and the temperature of the fixation belt  130  reaches it target level, after they began to be circularly moved, a sheet S of recording medium, on which an unfixed toner image t has just been formed by the image formation station, is introduced into the fixing device A. As the sheet S is introduced into the fixing device A, it is advanced into the fixation nip N while being guided by an entrance guide  101  which is located at the sheet entrance portion of the fixing device A. 
     Then, the sheet S is conveyed through the fixation nip N, with its image bearing surface facing the fixation belt  130 , and its opposite surface (back surface) from the image bearing surface facing the pressure belt  120 , while remaining pinched between the fixation belt  130  and pressure belt  120 . While the sheet S is conveyed through the fixation nip N, the unfixed toner image t on the sheet S is fixed to the surface of the sheet S by the heat from the fixation belt  130  and the nip pressure; the unfixed toner image is turned into a permanent image. After being conveyed through the fixation nip N, the sheet S is separated from the surface of the fixation belt  130 , and then, is moved out of the fixing device A through the sheet exit of the fixing device A, to be conveyed further. 
     (2-4) Belt Tensioning Mechanism and Belt Steering Mechanism 
     Next, referring to  FIGS. 2 and 3 , the belt tensioning mechanism of the fixation belt unit  100 A and the belt steering mechanism of the fixation belt unit  100 A are described. Then, referring to  FIGS. 2 and 4 , the belt tensioning mechanism of the pressure belt unit  100 B and the belt steering mechanism of the pressure belt unit  100 B are described.  FIG. 2  is a perspective view of the combination of the belt driving mechanism of the fixation belt unit  100 A and that of the pressure belt unit  100 B, which are on the right-hand end of the fixing device A.  FIGS. 3(   a ) and  3 ( b ) are plan views of the fixation belt unit  100 A as seen from the left and right sides, respectively, of the fixation belt unit  100 A.  FIGS. 4(   a ) and  4 ( b ) are plan views of the pressure belt unit  100 B as seen from the left and right sides, respectively, of the unit  100 B. 
     1) Belt Tensioning Mechanism and Belt Steering Mechanism of Fixation belt unit  100 A 
     The driver roller  131  of the fixation belt unit  100 A is positioned between the top left and top right plates  140 L and  140 R of the fixing device frame, and is rotatably supported by the left and right plates  140 L and  140 R, respectively, with the placement of bearings  103 L and  103 R between the left and right end portions  131   a L and  131 R, respectively, of the shaft  131   a  of the driver roller  131 . It should be noted here that  FIG. 2  does not show the top left plate  140 L of the fixation unit frame. 
     The fixation belt unit  100 A is provided with the steering roller supporting left and right arms  154 L and  154 R, which are attached to the outward sides of the top left and top right plates  140 L and  140 R of the fixation unit frame in such a manner that they are symmetrically positioned. These left and right supporting arms  154 L and  154 R are provided with bearings  153 L and  153 R, respectively, which are slidable relative to the arms  154 L and  154 R in the direction of the belt tension. The left and right end portions  132   a L and  132 R of the shaft  132  of the steering roller  132  are rotatably borne by these left and right bearings  153 L and  153 R, respectively. 
     The above-described left and right bearings  153 L and  153 R are kept under a preset amount of pressure generated by the tension springs  156 L and  156 R in the direction to provide the fixation belt  130  with tension. Thus, the steering roller  132  remains pressed in the direction to keep tensioned the fixation belt  130 , whereby the fixation belt  130  always remains under 200 N of tensional force, for example. 
     The left supporting arm  154 L is solidly attached to the top left plate  140 L. The right supporting arm  154 R is pivotally supported by the top right plate  140 R. More specifically, the lengthwise end of the right supporting arm  154 R is pivotally supported by a shaft  151  (pivot) attached to the top right plate  140 R. Thus, the right supporting arm  154 R is allowed to vertically pivot about the shaft  151  (pivot). Further, the opposite end of the shaft  151  from the right support arm  154 R is fitted with a sector gear  152 , which is in engagement with a worm gear rotatable by a stepping motor  155 . The motor  155  is solidly attached to the right top plate  140 R. 
     The motor  155  can be controlled by the control portion  20  so that it rotates forward or in reverse. Thus, as the motor  155  is rotated in the forward or in reverse by the motor  155 , the right supporting arm  154 R is pivotally moved about the shaft  151  upward or downward by the force conveyed thereto from the motor  155  by way of the worm gear  157  and sector gear  152 . That is, the right bearing  153 R, by which the right end portion  132   a R of the shaft  132   a  of the steering roller  132  is borne, is moved upward or downward. 
     Thus, the steering roller  132  is pivotally moved about the left bearing  153 L, by which the end portion  132   a L of the shaft  132   a  of the steering roller  132 , is moved upward or downward. Consequently, the steering roller  132 , which functions as an oscillatory roller, is changed in its attitude relative to the driver roller  131 . It is by this oscillatory movement of the steering roller  132  that the lateral shift of the fixation belt  130 , that is, the movement of the fixation belt  130  in the direction parallel to the axial line of the driver roller  131  (belt supporting member), which occurs while the fixation belt  130  is circularly moved, is controlled. 
     To describe in more detail, referring to  FIG. 5 , the fixation belt unit  100 A is provided with a belt shift sensor  150  for detecting the position of the fixation belt  130  in terms of the direction perpendicular to the direction of the circular movement of the fixation belt  130 . The belt shift sensor  150  is in the adjacencies of the left end portion (front end portion) of the fixation belt  130 . The control portion  20  detects the position of one of the lateral edges of the fixation belt  130  with the use of this sensor  150 , and controls the fixation belt  130  in position in terms of the direction perpendicular to the circular movement of the fixation belt  130 . More specifically, the control portion  20  controls the fixation belt  130  in position by tilting the steering roller  132  as shown in  FIG. 6  by rotating the motor  155 . 
     2) Belt Tensioning Mechanism and Belt Steering Mechanism of Pressure Belt Unit  100 B 
     The pressure roller  121  of the pressure belt unit  100 B is positioned between the left and right bottom plates  104 L and  104 R of the fixing device frame, and is rotatably supported by the left and right bottom plates  104 L and  104 R, respectively, with the placement of bearings  105 L and  105 R between the left and right end portions  121   a L and  121   a R, respectively, of the shaft  121   a  of the pressure roller  121 . It should be noted here that  FIG. 2  does not show the bottom left plate  104 L of the fixation device frame. 
     The pressure belt unit  100 B is provided with the steering roller supporting left and right arms  126 L and  126 R, which are attached to the outward sides of the top left and top right plates  140 L and  140 R of the fixation device frame in such a manner that they are symmetrically positioned. These left and right supporting arms  126 L and  126 R are provided with bearings  106 L and  106 R, respectively, which are slidable relative to the arms  126 L and  126 R in the direction of the belt tension. The left and right end portions  122   a L and  122   a R of the shaft  122  of the pressure roller  121  are rotatably borne by these left and right bearings  106 L and  106 R, respectively. 
     The above-described left and right bearings  106 L and  106 R are kept under a preset amount of pressure generated by the tension springs  127 L and  127 R in the direction to provide the pressure belt  120  with tension. Thus, the steering roller  122  remains pressed in the direction to keep the pressure roller  122  tensioned, whereby the pressure belt  120  always remains under 200 N of tensional force, for example. 
     The left supporting arm  126 L is solidly attached to the bottom left plate  140 L. The right supporting arm  126 R is pivotally supported by the bottom right plate  140 R. More specifically, the lengthwise end of the right supporting arm  126 R is pivotally supported by a shaft  107  (pivot) attached to the bottom right plate  140 R. Thus, the right supporting arm  126 R is allowed to vertically pivot about the shaft  107  (pivot). Further, the opposite end of the shaft  107  from the right support arm  126 R is fitted with a sector gear  108 , which is in engagement with a worm gear  109  rotatable by a stepping motor  110 , The motor  110  is solidly attached to the bottom right plate  140 R. 
     The motor  110  can be controlled by the control portion  20  so that it rotates forward or in reverse. Thus, as the motor  110  is rotated in the forward or in reverse by the control portion  20 , the right supporting arm  126 R is pivotally moved about the shaft  107  upward or downward by the force conveyed thereto from the motor  110  by way of the worm gear  109  and sector gear  108 . That is, the right bearing  106 R, by which the right end portion  122   a R of the shaft  122  of the steering roller  122  is borne, is moved upward or downward. 
     Thus, the steering roller  122  is pivotally moved upward or downward, about the left bearing  106 L, by which the end portion  121   a L of the shaft  121   a  of the steering roller  122  is borne. Consequently, the steering roller  122 , which functions as an oscillatory roller, is changed in its attitude relative to the pressure roller  121 . It is by this oscillatory movement of the steering roller  122  that the lateral shift of the pressure belt  120 , that is, the movement of the pressure belt  120  in the direction parallel to the axial line of the pressure roller  120  (belt supporting member), which occurs while the pressure belt  120  is circularly moved, is controlled. 
     To describe in more detail, referring to  FIG. 5 , the pressure belt unit  100 B also is provided with a belt shift sensor (which is similar to sensor shown in  FIG. 5 ) for detecting the position of the pressure belt  120  in terms of the direction perpendicular to the circular movement of the pressure belt  120 . The belt shift sensor is in the adjacencies of the right end portion (front end portion) of the pressure belt  120 . The control portion  20  detects the position of one of the lateral edges of the pressure belt  120  with the use of this sensor, and controls the fixation belt  130  in position in terms of the direction perpendicular to the circular movement of the pressure belt  120 . More specifically, the control portion  20  controls the pressure belt  120  in position by changing the steering roller  122  in angle by rotating the motor  110 . 
     (2-5) Discharging Member 
     As described above, if the surface of the fixation belt  130  or pressure belt  120 , with which the sheet S of recording medium comes into contact, is nonuniform in potential level, the toner on a sheet S of recording medium sometimes moves in the pattern of the nonuniformity of the potential level of the fixation belt  130 , which results in the formation of an image which is abnormal in density. This is why the fixation belt unit  100 A is provided with the aforementioned discharging needle  170  for discharging the surface of the fixation belt  130 , which comes into contact with the sheet S of recording medium, in order to make the surface uniform in potential level. Further, the pressure belt unit  100 B is provided with the aforementioned discharging needle  164  for discharging the surface of the pressure belt  120 , with which the sheet S comes into contact, in order to make the surface uniform in potential level. 
     The discharging members  170  and  164  in this embodiment are in the form of a needle (discharging needle array), which are positioned in the adjacencies of the fixation belt  130  and pressure belt  120 , respectively. The discharging members  170  and  164  are of the non-contact type, and extend in the widthwise direction of the corresponding belts. In order to ensure that the discharging members  170  and  164  are uniform in effectiveness in terms of its lengthwise direction, it is desired that the fixation belt unit  100 A and pressure belt unit  100 B are structured so that a preset amount of gap g ( FIG. 1 ) is maintained between the discharging members  170  and  164 , and the fixation belt  130  and pressure belt  120 , respectively, which are the objects to be discharged. 
     In the case of the fixing device A designed so that its fixation belt  130  and pressure belt  120  are controlled in their lateral shift, the track of the fixation belt  130  of the fixation belt unit  100 A and the track of the pressure belt  120  of the pressure belt unit  100 B, change in position in response to the changes in the angle of the steering rollers  132  and  122 , respectively, although the portion of the track of the fixation belt  130 , which corresponds in position to the driver roller  131 , and the portion of the track of the pressure belt  120 , which corresponds in position to the pressure roller  121 , do not change in position. 
       FIG. 7  is a drawing for describing the pressure belt unit  100 B. The area indicated by a broken line in  FIG. 7 , is the area in which the track of the pressure belt  120  does not change in position in response to the changes in the angle of the steering roller  122 . Thus, the thermistor (unshown) for detecting the belt temperature, and the separation guide (unshown), are positioned so that the positional relationship between the pressure roller  121  or driver roller  131  is unchanged. 
     Further, the track of the fixation belt  130 , as seen from the side of the members of the fixation belt unit  100 A, which move with the steering roller  132 , hardly changes in position relative to the roller of the pressure belt unit  100 B, which opposes the steering roller  132 . The belt track changes in position in the area in which the fixation belt  130  is in contact with the fixation belt unit members which contribute to the formation of the fixation nip N.  FIG. 8  is a drawing for describing the pressure belt unit  100 B. The belt track changes in position in a range between the belt tracks indicated by referential codes a and b. 
     It is desired that the discharging members  170  and  164  are placed in an area in which rollers and metallic components are not present on the opposite side of the fixation belt  130  and pressure belt  120  from the discharging members  170  and  164 , respectively. 
     The characteristic feature of this embodiment is that the discharging members  170  and  164  are held to the steering roller  132  and  122 , which are oscillatory members, and also, that they move with the steering rollers  132  and  122 , respectively. Therefore, the preset amount of gap between the belts  130  and  120 , and the discharging member  170  and  164 , respectively, can be maintained without using the limited space available in the adjacencies of the fixation nip N. Further, the belts  130  and  120  can be discharged without requiring that the metallic components such as rollers, laminar plates, etc., are positioned in a manner to oppose the discharging members  170  and  164 . In other words, this embodiment of the present invention makes it possible to satisfactorily discharge the belts  130  and  120  by expertly using the limit space available in the adjacencies of the fixation nip N. 
     1) Positioning and Structure of Discharging Member of Fixation Belt Unit  100 A 
       FIG. 9(   a ) is a perspective view of the discharging member of the fixation belt unit  100 A, and shows the positioning and structure of the discharging member.  FIGS. 9(   b ) and  9 ( c ) are plan views of the left and right ends, respectively, of the fixation belt unit  100 A. 
     The fixation belt unit  100 A is provided with left and right members  165 L and  165 R, which function as the members for holding the discharging member  170 . The discharging member supporting members  165 L and  165 R are attached to the left and right ends, respectively, of the fixation belt unit  100 A, and are symmetrically positioned. The left and right members  165 L and  165 R are long and narrow pieces of plate, one for one, the lengthwise direction of which is parallel to the recording medium conveyance direction. They are long enough to cover the area between the shaft of the driver roller  131  and the shaft of the steering roller  132 , and slightly beyond the area. 
     The end portion of the left supporting member  165 L, which corresponds in position to the steering roller  132 , is provided with a round hole  165   a L, in which the left end portion  132   a L of the shaft  132   a  of the steering roller  132  is fitted. The end portion of the left supporting member  165 L, which corresponds in position to the driver roller  131 , is shaped like a two-pronged fork (forked portion  165   b L, which may be elongated hole). The left end portion  131   a L of the shaft  131   a  of the driver roller  131  is fitted in the gap between the two-prongs of the forked portion  165   b L. 
     That is, the left supporting member  165 L is supported by the left end portion  131   a L of the shaft  131   a  of the driver roller  131 , and left end portion  132   a L of the shaft  132   a  of the steering roller  132 , in such a manner that it is allowed to pivot about the left end portion  132   a L of the shaft  132   a  of the steering roller  132  in an oscillatory manner. Further, the left supporting member  165 L is provided with an arm portion  165   c L, which extends upward from the portion of the main portion of the left supporting member  165 L, which is adjacent to the steering roller  132 . 
     The end portion of the right supporting member  165 R, which corresponds in position to the steering roller  132 , is provided with a round hole  165   a R, in which the right end portion  132   a R of the shaft  132   a  of the steering roller  132  is fitted. The end portion of the right supporting member  165 R, which corresponds in position to the driver roller  131 , is shaped like a two-pronged fork (forked portion  165   b R, which may be elongated hole). The right end portion  131   a R of the shaft  131   a  of the driver roller  131  is fitted in the gap between the two-prongs of the forked portion  165   b R. 
     That is, the right supporting member  165 R is supported by the right end portion  131   a R of the shaft  131   a  of the driver roller  131 , and right end portion  132   a R of the shaft  132   a  of the steering roller  132 , in such a manner that it is allowed to pivot about the right end portion  132   a R of the shaft  132   a  of the steering roller  132  in an oscillatory manner. Further, the right supporting member  165 R is provided with an arm portion  165   c R, which extends upward from the portion of the main portion of the right supporting member  165 R, which is adjacent to the steering roller  132 . 
     The aforementioned discharging member  170  (discharge needle array) is held between the upwardly extending arm portion  165   c L of the left supporting member  165 L and the upwardly extending arm portion  165   c R of the right supporting member  165 R. That is, the discharging member  170  is held so that it extends in the widthwise direction of the fixation belt  130 , in parallel to the steering roller  132 , with the presence of a preset amount of gap g ( FIG. 1 ) between itself and fixation belt  130 . That is, the discharging member  170  is parallel to the steering roller  132 , and its distance from the steering roller  132  is within 20 mm. It is positioned so that there is no object within the belt loop, that opposes the discharging member across the fixation belt  130 . 
     The fixation belt unit  100 A may be provided with a member which connects the upward arm portion  165   c L of the left supporting member  165 L and the upward arm portion  15   c R of the right supporting member  165 R, so that the discharging member  170  can be attached to the connective member. 
     With the fixation belt unit  100 A being structured as described above, the discharging member  170  is held to the steering roller  132 , which is an oscillatory member. Thus, it moves with the steering roller  132 . Therefore, the preset amount of gap g can be maintained between the discharging member  170  and fixation belt  130  without using the limited space available in the adjacencies of the fixation nip N, and also, it is possible to discharge the fixation belt  130  without requiring that the mechanical component such as a roller, a laminar plate, or the like, is placed on the inward side of the belt loop in a manner to oppose the discharging member  170 . Therefore, the fixation belt  170  can be satisfactorily discharged while expertly utilizing the limited space available in the fixation belt unit  100 A. 
     2) Positioning and Structure of Discharging Member of Pressure Belt Unit  100 B 
       FIG. 10(   a ) is a perspective view of the discharging member of the pressure belt unit  100 B, and shows the positioning and structure of the discharging member.  FIGS. 10(   b ) and  10 ( c ) are plan views of the left and right ends, respectively, of the pressure belt unit  100 B. 
     The pressure belt unit  100 B is provided with left and right members  166 L and  166 R, which function as the members for holding the discharging member  164 . The discharging member holding members  166 L and  166 R are attached to the left and right ends, respectively, of the pressure belt unit  100 B, and are symmetrically positioned. The left and right members  166 L and  166 R are long and narrow pieces of plate, one for one, the lengthwise direction of which is parallel to the recording medium conveyance direction. They are long enough to cover the area between the shaft of the pressure roller  121  and the shaft of the steering roller  122 , and slightly beyond the area. 
     The end portion of the left supporting member  166 L, which corresponds in position to the steering roller  122 , is provided with a round hole  166   a L, in which the left end portion  122   a L of the shaft  122   a  of the steering roller  122  is fitted. The end portion of the left supporting member  166 L, which corresponds in position to the pressure roller  121 , is shaped like a two-pronged fork (forked portion  166   b L, which may be elongated hole). The left end portion  121   a L of the shaft  121   a  of the pressure roller  121  is fitted in the gap between the two-prongs of the forked portion  166   b L. 
     That is, the left supporting member  166 L is supported by the left end portion  121   a L of the shaft  121   a  of the pressure roller  121 , and the left end portion  122   a L of the shaft  122   a  of the steering roller  122 , in such a manner that it is allowed to pivot about the left end portion  122   a L of the shaft  122   a  of the steering roller  122  in an oscillatory manner. Further, the left supporting member  166 L is provided with an arm portion  166   c L, which extends upward from the portion of the main portion of the left supporting member  166 L, which is adjacent to the steering roller  122 . 
     The end portion of the right supporting member  166 R, which corresponds in position to the steering roller  122 , is provided with a round hole  166   a R, in which the right end portion  122   a R of the shaft  122   a  of the steering roller  122  is fitted. The end portion of the right supporting member  166 R, which corresponds in position to the pressure roller  121 , is shaped like a two-pronged fork (forked portion  166   b R, which may be elongated hole). The right end portion  121   a R of the shaft  121   a  of the pressure roller  121  is fitted in the gap between the two-prongs of the forked portion  166   b R. 
     That is, the right supporting member  166 R is supported by the right end portion  121   a R of the shaft  121   a  of the pressure roller  121 , and the right end portion  122   a R of the shaft  122   a  of the steering roller  122 , in such a manner that it is allowed to pivot about the right end portion  122   a R of the shaft  122   a  of the steering roller  122  in an oscillatory manner. Further, the right supporting member  166 R is provided with an arm portion  166   c R, which extends downward from the portion of the main portion of the right supporting member  166 R, which is adjacent to the steering roller  122 . 
     The aforementioned discharging member  164  (discharge needle array) is held between the downwardly extending arm portion  166   c L of the left supporting member  166 L and the downwardly extending arm portion  166   c R of the right supporting member  166 R. That is, the discharging member  164  is held so that it extends in the widthwise direction of the pressure belt  120 , in parallel to the steering roller  122 , with the presence of a preset amount of gap g ( FIG. 1 ) between itself and pressure belt  120 . That is, the discharging member  164  is parallel to the steering roller  122 , and its distance from the steering roller  122  is within 20 mm. It is positioned so that there is no object within the belt loop, that opposes the discharging member 164  across the pressure belt  120 . 
     The pressure belt unit  100 B may be structured so that it is provided with a member which connects the downward arm portion  166   c L of the left supporting member  166 L and the downward arm portion  166   c R of the right supporting member  166 R, and the discharging member  164  is attached to the connective member. 
     With the pressure belt unit  100 B being structured as described above, the discharging member  164  is held to the steering roller  122 , which is an oscillatory member. Thus, it moves with the steering roller  122 . Therefore, the preset amount of gap g can be maintained between the discharging member  164  and pressure belt  120  without using the limited space available in the adjacencies of the fixation nip N, and also, it is possible to discharge the pressure belt  120  without requiring the mechanical component such as a roller, a laminar plate, or the like, to be placed on the inward side of the belt loop in a manner to oppose the discharging member  164 . Therefore, the fixation belt  164  can be satisfactorily discharged while expertly utilizing the limited space available in the pressure belt unit  100 B. 
     [Embodiment 2] 
       FIG. 11  is a drawing of the mechanism, in the second embodiment of the present invention, for holding a discharging member (of pressure belt unit). Hereafter, the discharging member holding mechanism in the second embodiment is described with reference to the pressure belt unit  100 B, for the sake of convenience. The description of the discharging member holding mechanism of the fixation belt unit  100 A is similar to that of the discharging member holding mechanism of the pressure roller unit  100 B.  FIG. 11  does not show the portions of the mechanism, which are not essential to the description of the second embodiment. That is, it shows only the portion of the mechanism, at which the discharging member holding member is placed in contact with the belt to prevent the discharging member holding member from being rotationally moved. Referring to  FIG. 16 , the discharging member holding member  166  is enabled to maintain a preset amount of gap between the discharging member  164  and belt  120  ( 130 ), by the contact between itself and belt  120 , or the contact between a member held to (by) the discharging member holding member  166  and the belt  120 . 
     In a case where the discharging member  164  is positioned in parallel to the steering roller  122 , the steering roller  122  moves as shown in  FIG. 13  when the belt  120  is controlled in its lateral movement. Therefore, the steering roller  122 , by which the pressure belt  120  is suspended at its opposite end from where it is suspended by the pressure roller  121 , becomes angled relative to the pressure roller  121  as shown in  FIG. 12 . Thus, the pressure belt  120  becomes twisted relative to the surface which is parallel to the pressure roller  121 , and the surface which is parallel to the steering roller  122 . Referring to  FIG. 12 , “A” stands for the angle of the steering roller  122  relative to the pressure roller  121 , which suspends, and keeps tensioned, the pressure belt  120  at the opposite end of the pressure belt  120  from where the pressure belt  120  is suspended by the pressure roller  121 . “L 1 ” stands for the width (dimension in terms of direction parallel to axial line of pressure roller  121 ) of the pressure roller  121 , and “L 2 ” stands for the amount of gap between the steering roller  122  and discharging member  164 . Further, “L 3 ” stands for the distance between the steering roller  122 , and the pressure roller  121  which suspends, and keeps tensioned, the pressure belt  120  at the opposite end of the belt loop from where the pressure belt  120  is suspended by the steering roller  122 . 
     The amount of difference in phase ( FIG. 13 ) between the steering roller  122 , and the pressure roller  121  which suspends, and keeps tensioned the pressure roller  120  at the opposite end of the belt loop from where the pressure belt  120  is suspended by the steering roller  122  is L 1 ·sin(θ/2). Next, referring to  FIG. 14  which is a schematic drawing for showing the changes in the amount of gap between the discharging member  164  and pressure belt  120 , the changes which occurs in the amount of the gap between the discharging member  164  and pressure belt  120  can be expressed as (L 2 /L 3 )×L 1 ·sin(θ/2). 
     For example, when the angle θ by which the steering roller  121  pivots is 0.8 [degrees], and the amount of gap to be maintained between the discharging member  164  and steering roller  122  is ±0.25, the value of (L 2 /L 3 ) which is obtainable by substituting actual values for L 1 , L 2 , L 3 , and θ in the mathematical formula given above is roughly 0.19, since the belt width in this embodiment is 380 mm. 
     If the distance L 3  between the steering roller  122 , and the pressure roller  121  by which the pressure belt  120  is suspended, and kept tensioned, at the opposite end of the belt loop from where the pressure belt  120  is suspended by the steering roller  122  is 50 mm, the discharging member  164  is desired to be positioned no more than 9.4 mm away from the steering roller  122 . 
     In this embodiment, the pressure belt  120 , which is to be discharged, is in a range of 500-1,000 V in potential level. Therefore, the amount of the gap g between the discharging member  164  and steering roller  122  is desired to be kept within roughly ±0.5. In consideration of this requirement, the proper position for the discharging member  164  is no more than roughly 20 mm from the steering roller  122  in terms of the moving direction of the pressure belt  120 . 
     Up to this point, the present invention has been described with reference to the fixing devices in two embodiments of the present invention. However, the present invention is also applicable to various known fixing devices which are different in structure from those in the preceding embodiments, within the gist of the present invention. 
     For example, in the preceding embodiments, the fixing member and pressing member which form the nip N are both endless belts. However, the present invention is also applicable to a fixing device and the like which uses a roller as either the fixing member or pressing member. 
     Further, the present invention is also applicable to a fixing device and the like which uses a belt as its fixing member, and a nonrotational member such as a pad or flat plate which is small in surface friction (relative to fixation belt and sheet of recording medium), as its pressing member. 
     Further, the preceding embodiments are not intended to limit the present invention in terms of the heating system for heating the endless belts. That is, not only is the present invention applicable to a fixing device and the like which employs an electromagnetic induction heating system to heat the belts, but also, a fixing device and the like which uses other heating system, such as a halogen heater, than an electromagnetic induction heating system. 
     Further, in the description of the preceding embodiments, the fixing device was described as an image heating device. However, the present invention is also applicable to a device (apparatus) which is for reheating a fixed image on a sheet of recording medium to improve the image in surface properties. 
     Further, not only is the present invention compatible with an image forming apparatus which uses the above described electrophotographic image formation system, as the method for forming a toner image on a sheet of recording medium, but also, an image forming apparatus which uses an electrostatic recording system or a magnetic recording system. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 098677/2012 filed Apr. 24, 2012, which is hereby incorporated by reference.