Abstract:
A belt driving apparatus includes a first rotatable belt member; a first supporting member rotatably supporting the first belt member; a first steering roller, rotatably supporting the first belt member, for adjusting a position, with respect to a widthwise direction perpendicular to a rotational direction, of the first belt member, wherein one end of the first steering roller is fixed, and the other end thereof is movable; control means for controlling movement of the other end of the first steering roller; a second rotatable belt member contacted to the first belt member; a second supporting member rotatably supporting the second belt member; a second steering roller, rotatably supporting the second belt member, for adjusting a position, with respect to the widthwise direction, of the second belt member, wherein an end of the second steering roller remote from the one end of the first steering roller is fixed, and an end thereof adjacent the other end the second steering roller is movable; and control means for controlling movement of the end of the second steering roller adjacent the other end the second steering roller.

Description:
FIELD OF THE INVENTION AND RELATED ART 
       [0001]    The present invention relates to a belt driving apparatus which is mountable in an image forming apparatus such as an electrophotographic copy machine, an electrophotographic printer, etc., to drive multiple (two) belts which are in contact with each other. It also relates to an image forming apparatus in which the above-described belt driving apparatus is mountable. 
         [0002]    There are various belt driving apparatuses which drive multiple (two) belts. One of such belt driving apparatuses is a fixing apparatus which is mounted in an image forming apparatus, such as an electrophotographic copy machine and an electrophotographic printer. More specifically, it is a fixing apparatus of the so-called belt-nip type. A fixing apparatus of this type has an endless heating belt and an endless pressing belt. The endless pressing belt (which hereafter may be referred to simply as pressure belt) is placed in contact with the heating belt (which hereafter may be referred to simply as fixation belt) to form a nip (Japanese Laid-open Patent Application 2007-079034). In operation, a sheet of recording medium which is bearing an unfixed toner image is conveyed through the nip while remaining pinched between the heating belt and pressing belt, whereby the unfixed toner image on the sheet of recording medium is thermally fixed to the sheet of recording medium. For size reduction and cost reduction, some fixing apparatuses of the belt-nip type are provided with only two (minimum number) rollers per belt. In other words, they are reduced in overall thermal capacity by using only two (minimum number) rollers per belt to minimize the length of time necessary for them to reach a temperature range in which they can properly fix a toner image. 
         [0003]    There are serious technical issues regarding fixing apparatuses of the belt-nip type, that is, fixing apparatuses which use an endless fixing belt and/or an endless pressing belt. One of these technical issues is how to prevent the belts of a fixing apparatus of the belt-nip type from shifting (snaking) in a specific direction. More specifically, if the belts shift in the direction perpendicular to their moving direction while they are driven, problems result sometimes such that the belts move out of their preset range, and/or that the belts become damaged across their edge or edges. There are various methods for preventing an endless belt from excessively shifting in the direction perpendicular to its moving direction. One of these methods is to change in attitude one of the two rollers around which the endless belt is wrapped, in such a manner that one of the lengthwise ends of this roller is changed in position to cause the belt to remain in its preset positional range (Japanese Laid-open Patent Application H04-104180). 
         [0004]    As described above, one of the methods for preventing the fixating belt and pressing belt of a fixing apparatus of the belt-nip type from excessively shifting in a specific direction is to structure the fixing apparatus so that the belt supporting rollers can be changed in attitude. However, if a fixing apparatus of the belt-nip type is structured so that its fixing belt and pressing belt can be changed in attitude by changing the upstream roller for the fixing belt, in terms of recording medium conveyance direction, and the upstream roller for the pressing belt, to change in position one of the lengthwise ends of the upstream roller for the fixing belt, and the corresponding lengthwise end of the upstream roller for the pressing belt, it is possible that when a sheet of recording medium is conveyed through the fixing apparatus by the belts, it will become unstable in attitude and behavior. The cause of this problem is that in a case of a fixing apparatus of the belt-nip type structured as described above, when the upstream fixation belt roller and upstream pressure belt roller are changed in attitude so that one of the lengthwise end of the upstream fixation belt roller and the corresponding lengthwise end of the upstream pressure belt roller come closer to each other, and when the upstream fixation belt roller and upstream pressure belt roller are changed in attitude so that one of the lengthwise end of the upstream fixation belt roller and the corresponding lengthwise end of the upstream pressure belt roller move away from each other, the amount by which the lengthwise ends are moved is rather large, and therefore, the amount by which the upstream fixation belt and upstream pressure belt are moved in the direction parallel to the lengthwise direction of the rollers to be corrected in their position is rather large, making it difficult for recording medium to remain stable in attitude and behavior while it is conveyed through the belt-nip. Therefore, in order to ensure that recording medium remains stable in attitude and behavior while it is conveyed through a fixing apparatus of the belt-nip type, it is desired to reduce the amount by which the distance between one end of the lengthwise ends of the upstream roller for the fixation belt, and the corresponding lengthwise end of the upstream roller for the pressure belt, that is, the distance between the fixation belt and pressure belt, on the upstream end of the fixing apparatus. 
         [0005]    Not only does this problem occur to the fixation belt and pressure belt, but also, to any belt driving apparatus which has two (multiple) belts (which are in contact with each other), and is structured so that the two belts are prevented from excessively shifting in the direction perpendicular to their moving direction. 
       SUMMARY OF THE INVENTION 
       [0006]    The primary object of the present invention is to provide a belt driving apparatus which is significantly smaller in the amount by which the distance between its first and second belts, on the recording medium entrance side of the apparatus, changes, and therefore, is significantly more stable in terms of recording medium conveyance than any of conventional belt driving apparatuses, and also, an image forming apparatus employing such a belt driving apparatus. 
         [0007]    According to an aspect of the present invention, there is provided a belt driving apparatus comprising a first rotatable belt member; a first supporting member rotatably supporting said first belt member; a first steering roller, rotatably supporting said first belt member, for adjusting a position, with respect to a widthwise direction perpendicular to a rotational direction, of said first belt member, wherein one end of said first steering roller is fixed, and the other end thereof is movable; control means for controlling movement of the other end of said first steering roller; a second rotatable belt member contacted to said first belt member; a second supporting member rotatably supporting said second belt member; a second steering roller, rotatably supporting said second belt member, for adjusting a position, with respect to the widthwise direction, of said second belt member, wherein an end of said second steering roller remote from said one end of said first steering roller is fixed, and an end thereof adjacent the other end said second steering roller is movable; and control means for controlling movement of said end of said second steering roller adjacent the other end said second steering roller. 
         [0008]    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 
         [0009]      FIG. 1  is a schematic sectional view of a typical image forming apparatus to which the present invention is related. It shows the general structure of the apparatus. 
           [0010]      FIG. 2  is a schematic sectional view (at plane perpendicular to moving direction of belts) of the fixing apparatus in the first preferred embodiment of the present invention, and shows the general structure of the apparatus. 
           [0011]      FIG. 3A  is an external perspective view of the combination of the mechanism for controlling the fixation belt in position, and the mechanism for controlling the pressure belt in position, as seen from the recording sheet entrance side, and shows the structure of the mechanisms.  FIG. 3B  is a left side view of the combination of the mechanism for controlling the fixation belt in position, and the mechanism for controlling the pressure belt in position, as seen from the recording sheet entrance side, and shows the structure of the mechanisms.  FIG. 3C  is a schematic drawing which depicts the movement of the pressure belt steering roller of the mechanism for controlling the pressure belt in position. 
           [0012]      FIG. 4A  is an external perspective view of the combination of the mechanism for controlling the fixation belt in position, and the mechanism for controlling the pressure belt in position, as seen from the recording sheet exit side, and shows the structure of the mechanisms.  FIG. 4B  is a left side view of the combination of the mechanism for controlling the fixation belt in position, and the mechanism for controlling the pressure belt in position, as seen from the recording sheet exit side, and shows the structure of the mechanisms.  FIG. 4C  is a schematic drawing which depicts the movement of the pressure belt steering roller of the mechanism for controlling the pressure belt in position. 
           [0013]      FIG. 5  is a flowchart of the sequence for controlling the mechanism for controlling in position the fixation belt of the fixing apparatus in the first embodiment. 
           [0014]      FIG. 6  is a flowchart of the sequence for controlling the mechanism for controlling in position the pressure belt of the fixing apparatus in the first embodiment. 
           [0015]      FIGS. 7(   a )- 7 ( e ) are schematic drawings which depict the belt steering movement of the fixation belt steering roller and the pressure belt steering roller of the fixing apparatus in the first embodiment. 
           [0016]      FIG. 8(   a ) is an external perspective view of the fixation belt position controlling mechanism and pressure belt position control mechanism of a comparative fixing apparatus, as seen from the recording sheet entrance side of the apparatus.  FIG. 8(   b ) is a left side (as seen from recording sheet entrance side) view of the fixation belt position control mechanism and pressure belt position control mechanism of the fixing apparatus shown in  FIG. 8(   a ). 
           [0017]      FIG. 9(   a ) is an external perspective view of the fixation belt position control mechanism and pressure belt position control mechanism of the comparative fixing apparatus, as seen from the recording sheet exit side of the apparatus.  FIG. 9(   b ) is a left side (as seen from recording sheet entrance side) view of the fixation belt position control mechanism and pressure belt position control mechanism of the fixing apparatus shown in  FIG. 9(   a ). 
           [0018]      FIGS. 10(   a )- 10 ( e ) are schematic drawings which depict the belt steering movement of the fixation belt steering roller and pressure belt steering roller of the comparative fixing apparatus. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     (1) General Description of Image Forming Apparatus 
       [0019]    Hereafter, the first preferred embodiment of the present invention will be described with reference to the appended drawings.  FIG. 1  is a schematic sectional view of the image forming apparatus in the first embodiment of the present invention, the fixing apparatus of which is an image heating apparatus (device) in accordance with the present invention. It depicts the overall structure of the apparatus. The image forming apparatus depicted by  FIG. 1  is an electrophotographic laser beam printer. 
         [0020]    The image forming apparatus in the first embodiment can be roughly divided into an image forming portion  101  which forms a toner image on a sheet S (recording medium), and a fixing apparatus  111  (image heating apparatus) which fixes an unfixed toner image to the sheet S by heating and pressing the toner image. The image forming portion  101  has the following devices, which will be described next. They are an electrophotographic photosensitive member  102  (image bearing member), a charging device  103 , an exposing apparatus  104  (exposing means), and a developing device  105  (developing means). The photosensitive member  102  is in the form of a drum, and therefore, will be referred to as a photosensitive drum hereafter. The charging device  103 , exposing apparatus  104 , and developing device  105  are in the adjacencies of the peripheral surface of the photosensitive drum  101 . In an image forming operation, the peripheral surface of the photosensitive drum  101  is uniformly charged by the charging device  103 . Then, the uniformly charged portion of the peripheral surface of the photosensitive drum  102  is exposed by the exposing apparatus  104 . More specifically, the uniformly charged portion is scanned by a beam of laser light L projected by the exposing apparatus  104  while being modulated with the digital data of the image to be formed. Thus, an electrostatic latent image is formed on the charged portion of the peripheral surface of the photosensitive drum  101 . This electrostatic latent image is developed by the developing device  105  which uses toner. Thus, a visible image is formed of toner on the peripheral surface of the photosensitive drum  102  (this visible image hereafter will be referred to simply as toner image). 
         [0021]    The image forming apparatus  100  has also a recording sheet feeding-and-conveying cassette  107 , which is in the bottom portion of the apparatus  100 . The cassette  107  stores multiple sheets S in layers. As an image forming operation begins, the sheets S in the cassette  107  are fed into the main assembly of the apparatus  100  one by one, and conveyed to a pair of registration rollers  109 , by a pair of sheet feeder rollers  108 . Then, each sheet S is conveyed to a transfer nip which is between the photosensitive drum  102  and a transfer roller  110  (transferring means), by the pair of registration rollers  109  in synchronism with the arrival of the toner image on the photosensitive drum  102  at the transfer nip. Then, the sheet S is conveyed through the transfer nip while remaining pinched by the peripheral surface of the photosensitive drum  102  and the peripheral surface of the transfer roller  110 . While the sheet S is conveyed through the transfer nip, the toner image on the peripheral surface of the photosensitive drum  102  is electrostatically transferred onto the sheet S by the transfer roller  110 . In other words, the unfixed toner image is borne on one of the surfaces of the sheet S. Then, the sheet S bearing the unfixed toner image is conveyed to the fixing apparatus  111 , and is conveyed through the fixing apparatus  111 . In the fixing apparatus  111 , heat and pressure are applied to the unfixed toner image, whereby the unfixed toner image becomes thermally fixed to the sheet S. Then, the sheet S, bearing the fixed toner image, is conveyed by the fixing apparatus  111  to a pair of discharge rollers  112 . Then, the sheet S is discharged by the pair of discharge rollers  112  into a delivery tray  113  which makes up a part of the top portion of the image forming apparatus. The transfer residual toner, that is, the toner remaining on the peripheral surface of the photosensitive drum  102  after the transfer of the unfixed toner image onto the sheet S, is removed by a cleaning apparatus  106  (cleaning means). 
       (2) Description of Fixing Apparatus 
       [0022]    In the following description of the fixing apparatus and its structural members, the “lengthwise direction” means the direction perpendicular to the sheet conveyance direction (recording medium conveyance direction), whereas the “widthwise direction” means the direction parallel to the sheet conveyance direction. The “length” of a given member means the measurement of the member in the “lengthwise direction,” whereas the “width” of a given member means the measurement of the member in the “widthwise direction”. 
         [0023]      FIG. 2  is a schematic sectional view of the fixing apparatus, at a plane perpendicular to the lengthwise direction, and shows the general structure of the apparatus. This fixing apparatus is of the belt-nip type. That is, it has a belt driving apparatus, and a pair of belts which are placed in contact with each other to form a nip. 
         [0024]    The fixing apparatus  111  in the first embodiment has a fixation belt unit  10  and a pressure belt unit  20 . The fixation belt unit  10  has a fixation belt  11 , which is an endless belt and is one (first) of the two belts of the fixing apparatus  111 . The fixation belt  11  is supported by a pair of rollers  12  and  13  in such a manner that it can be circularly moved. It is kept stretched also by the pair of rollers  12  and  13  which apply a preset amount of tensile force (120 N for example) to the fixation belt  11 . The roller  12  is a fixation belt driving roller, whereas the roller  13  is a fixation belt steering roller. In other words, the roller  13  has a function of steering the fixation belt  11  and a function of keeping the fixation belt  11  stretched. The fixation belt  11  is a laminar belt. It is made up of a metallic substrate layer, and a silicon rubber layer coated on the substrate layer. The substrate layer is 75 μm in thickness, 380 mm in width, and 200 mm in length. It is made of a magnetic metallic substance such as nickel or stainless steel. The silicon rubber layer is 300 μm in thickness. The fixation belt  11  does not need to be limited in structure and material to the above described ones. That is, any belt may be employed, as long as it is heat resistant and can generate heat by being subjected to the magnetic flux generated by an inductive heating coil  30  as a heat generation source, as will be described later. The fixation belt driver roller  12  is made up of a metallic core  12   a  and an elastic layer  12   b . The metallic core  12   a  is a solid cylindrical member made of stainless steel, and is 18 mm in external diameter. The elastic layer  12   b  is made of heat resistant silicon rubber, and was molded on the peripheral surface of the metallic core  12   a  in a manner to entirely cover the peripheral surface of the metallic core  12   a . The fixation belt steering roller  13  is made up of a hollow roller made of stainless steel, for example, and is 20 mm in external diameter and roughly 18 mm in internal diameter. The fixation belt steering roller  13  has the belt steering function and belt tensioning function as described previously. That is, not only does the fixation belt steering roller  13  function as a steering roller for correcting the fixation belt  11  in the position in the “lengthwise direction” of the fixing apparatus  111  (widthwise direction of belt  11 ), but also, it functions as a belt tensioning roller for adjusting the fixating belt  111   n  tension. 
         [0025]    The pressure belt unit  20  has an endless pressure belt as the second belt ( FIG. 2 ) of the fixing apparatus  111 . The pressure belt  21  is supported by a pair of rollers  22  and  23 , as supporting members, in such a manner that it can be circularly moved. It is kept stretched also by the pair of rollers  22  and  23  which apply a preset amount of tensile force (100 N for example) to the pressure belt  21 . The roller  22  is a pressure belt driving roller, whereas the roller  23  is a pressure belt steering roller. In other words, the roller  23  has a function of steering the pressure belt  21  and a function of keeping the pressure belt  21  stretched. The pressure belt  21  also is a laminar belt. It is made up of a substrate layer, and a silicon rubber layer coated on the substrate layer. The substrate layer is 75 μm in thickness, 380 mm in width, and 200 mm in length. It is made of polyimide film. The silicon rubber layer is 300 μm in thickness. The pressure belt  21  does not need to be limited in structure and material to the above described ones. That is, any belt may be employed as the pressure belt  21 , as long as it is heat resistant. The pressure belt driving roller  22  is a solid roller made of stainless steel, for example, and is 20 mm in external diameter. The pressure belt steering roller  23  is made up of a hollow roller made of stainless steel, for example, and is 20 mm in external diameter and roughly 18 mm in internal diameter. The pressure belt steering roller  23  has the belt steering function and belt tensioning function as described previously. That is, not only does the pressure belt steering roller  23  function as a steering roller for correcting the pressure belt  21  in its position in the “lengthwise direction” of the fixing apparatus  111  (widthwise direction of belt  21 ), but also, it functions as a belt tensioning roller for adjusting the pressure belt in tension. 
         [0026]    The fixation belt  11  is suspended by the fixation belt driving roller  12  and fixating belt steering roller  13  in such a manner that the portion of the fixation belt  11 , which is moving through the top portion of the loop it forms, remains roughly horizontal. The pressure belt  21  is under the fixation belt  11 , and is in contact with the fixation belt  11 . It is suspended by the pressure belt driving roller  22  and pressure belt steering roller  23  in such a manner that its portion which is moving through the top portion of the loop it forms, is tilted in such a manner that its upstream end, in terms of the moving direction of the fixation belt  21 , is positioned lower than its downstream end. The pressure belt driving roller  22  opposes the fixation belt driving roller  12  with the presence of the fixation belt  11  and pressure belt  21  between the two rollers  22  and  12 . It is kept pressed against the fixation belt driving roller  12  by a pair of springs  71 F and  71 R (which will be described later) so that the outward surface of the pressure belt  21  is kept in contact with the outward surface of the fixation belt  11 . That is, the pressure from the springs  71 F and  71 R is applied to the elastic layer  12   b  of the fixation belt driving roller  12  through the pressure belt  21  and fixation belt  11 , whereby the elastic layer  21   b  is elastically deformed, forming thereby a part of the fixation nip N. The fixation belt unit  10  is provided with a stay  14  (pressure applying member) formed of stainless steel (SUS), for example. The stay  14  is on the inward side of the fixation belt loop, and is positioned so that its lengthwise direction coincides with the widthwise direction of the fixation belt  11 . The pressure belt unit  11  is provided with a pressure pad  24  (pressing member) formed of silicon rubber, for example. The stay  24  is in on the inward side of the pressure belt loop, and is positioned so that its lengthwise direction coincides with the widthwise direction of the pressure belt  21 . The stay  14  opposes the pressure pad  24  with the presence of the fixation belt  11  and pressure belt  21  between the stay  14  and pressure pad  24 . More specifically, the stay  14  is kept pressed upon the inward surface of the fixation belt  11  by unshown compression springs so that a preset amount (400 N, for example) of contact pressure is maintained between the stay  14  and fixation belt  11 . The pressure pad  24  is kept pressed upon the inward surface of the pressure belt  21  and the peripheral surface of the pressure belt driving roller  22  so that a preset mount (400 N, for example) of contact pressure is maintained between the pressure pad  24  and pressure belt  21 , and between the pressure pad  24  and the pressure belt driving roller  22 . By not only pressing stay  14  upon the inward surface of the fixation belt  11 , but also pressing the pressure pad  24  upon the inward surface of the pressure belt  21  and peripheral surface of the pressure belt driving roller  22 , it is possible to provide a long area of contact between the outward surface of the fixation belt  11  and the outward surface of the pressure belt  21  in terms of the sheet conveyance direction. In other words, it is possible to form a large fixation nip N, the size of which is proportional to the size of the abovementioned area of contact, by the outward surface of the fixation belt  11  and the outward surface of the pressure belt  21 . With the formation of the long and wide fixation nip N, it is possible to make longer the length of time it takes for the sheet S, which is bearing an unfixed toner image T, to be conveyed through the fixation nip N while remaining pinched between the two belts  11  and  21 . Therefore, toner images which are significantly superior in glossiness than those obtainable with the use of any of conventional fixing apparatuses, can be outputted at a significantly higher speed than those reachable by any of conventional fixing apparatuses. 
         [0027]    The typical operation of the fixing apparatus  111  in this embodiment is as follows. As the fixation belt driving roller  12  is rotated by a fixation motor, it circularly moves the fixation belt  11  in the direction indicated by an arrow mark ( FIG. 2 ). The force given to the fixation belt  11  by the fixation motor through the fixation belt driving roller  12  is transmitted from the fixation belt  11  to the pressure belt  21  through the fixation nip N, whereby the pressure belt  21  is rotated in the direction indicated by the arrow mark. In other words, the pressure belt  21  is rotated by the circular movement of the fixation belt  11 . Through the inductive heating coil  30 , high frequency electric current is flowed from an exciter circuit, causing the inductive heating coil  30  to generate magnetic flux, which heats the fixation belt  11 . The surface temperature of the fixation belt  11  is detected by a temperature detecting member, such as a thermistor, which is in the adjacencies of the surface of the fixation belt  11 . The output signal from the temperature detecting member is picked up by a control portion  200  made up of a CPU and memories, such as a RAM, a ROM, and the like. Then, the control portion  200  controls the exciter circuit, based on the output signals, so that the surface temperature of the fixation belt  11  remains in a preset fixation temperature range (target temperature range). While the surface temperature of the fixation belt  11  is kept in the preset fixation range, the sheet S on which the unfixed toner image T is present is introduced into the fixation nip N of the fixing apparatus  111  with the toner image bearing surface of the sheet S facing upward, and is conveyed through the fixation nip N while remaining pinched by the outward surface of the fixation belt  11  and the outward surface of the pressure belt  21 . While the sheet S is conveyed through the fixation nip N, the sheet S and the unfixed toner image T thereon are subjected to the heat from the fixation belt  11  and the pressure from the combination of the pressure belt  21  and fixation belt  11 . Thus, the toner image T becomes thermally fixed to the surface of the sheet S. 
         [0000]    (3) Description of Fixation Belt Position Control Mechanism and Pressure Belt position Control Mechanism 
         [0028]      FIG. 3A  is an external perspective view of the combination of the mechanism for controlling the fixation belt in position, and the mechanism for controlling the pressure belt in position, as seen from the recording sheet entrance side, and shows the structure of the mechanisms.  FIG. 3B  is a left side view of the combination of the mechanism for controlling the fixation belt in position, and the mechanism for controlling the pressure belt in position (shown in  FIG. 3A ), as seen from the recording sheet entrance side, and shows the structure of the mechanisms.  FIG. 3C  is a schematic drawing which depicts the movement of the pressure belt steering roller of the mechanism for controlling the pressure belt in position.  FIG. 4A  is an external perspective view of the combination of the mechanism for controlling the fixation belt in position, and the mechanism for controlling the pressure belt in position, as seen from the recording sheet exit side, and shows the structure of the mechanisms.  FIG. 4B  is a left side view of the combination of the mechanism for controlling the fixation belt in position, and the mechanism for controlling the pressure belt in position, as seen from the recording sheet exit side, and shows the structure of the mechanisms.  FIG. 4C  is a schematic drawing which depicts the movement of the pressure belt steering roller of the mechanism for controlling the pressure belt in position. 
         [0029]    First, the mechanism  50  (first correctional means) for controlling the fixation belt in position will be described. This mechanism  50  will be referred to hereafter as a fixation belt position controlling means  50 . The fixation belt position controlling means  50  has a pair of lateral plates  40 F and  40 R (front and rear plates  40 F and  40 R), a pair of steering roller supporting arms  51 F and  51 R, the fixation belt driving roller  12 , and the fixation belt steering roller  13 . The steering roller supporting arms  51 F and  51 R are attached to the front and rear plates  40 F and  40 R, respectively. The front end of metallic core  12   a F of the fixation belt driving roller  12  and the front end  13   a F of the metallic core  13   a  of the fixation belt steering roller  13  are supported by the steering roller supporting arm  51 F. The fixation belt steering roller  13  is supported in such a manner that the fixation belt steering roller  13  can be tilted to vertically move the front end  13   a F of its metallic core  13   a . More specifically, the fixation belt driving roller  12  is rotatably supported by the front plate F of the fixing apparatus  111 , and the steering roller supporting arm  51 F of the fixation belt position controlling mechanism  50  of the fixing apparatus  111 , by the lengthwise front end portion  12   a F of its metallic core  12   a  ( FIGS. 3A and 3B ). Further, the fixation belt driving roller  12  is rotatably supported by the rear plate  40 R of the fixing apparatus  111  and the steering roller supporting arm  51 R of the fixation belt position control mechanism  50 , by the other lengthwise end (right lengthwise end)  12   a R of the metallic core  12   a  ( FIGS. 4A and 4B ). The lengthwise end  13   a F of the metallic core  13   a  of the fixation belt steering roller  13  is rotatably supported by the front plate  41 F of the fixing apparatus  111 , and the steering roller supporting front arm  51 F of the fixing apparatus  111 , with the placement of a bearing  52 F between the metallic core end  13   a F and steering roller supporting arm  51 F to make the fixation belt steering roller  13  rotatable ( FIGS. 3A and 3B ). The rear end  13   a R of the metallic core  13   a  of the fixation belt steering roller  13  is rotatably supported by the rear plate  41 R of the fixing apparatus  111 , and the steering roller supporting rear arm  51 R, with the placement of a bearing  52 R between the rear front end  13   a R of the metallic core  13   a , and the fixation belt steering roller supporting rear arm  51 R ( FIGS. 4A and 4B ). The bearing  52 F is supported by the steering roller supporting arm  51 F on the front plate  40 F in such a manner that the bearing  52 F can be slid in the direction in which the fixation belt  11  is kept stretched ( FIGS. 3A and 3B ). The frontal plate  40 F is provided with a hole through which the front end portion  13   a F of the metallic core  13   a  of the fixation belt steering roller  13  is put. The hole is shaped so that as the fixation belt steering roller  13  is tilted to steer the fixation belt  11 , the front end portion  13   a F is allowed to be vertically displaced. Further, a tension spring  53 F for keeping the bearing  52 F pressed in the belt tensioning direction to provide the fixation belt  11  with a preset amount of tension is attached to the steering roller support arm  51 F. Therefore, of the steering roller support arm  51 F and  51 R, the steering roller support arm  51 F can tilt the fixation belt driving roller  12  in such a manner that the front end  12   a F of the metallic core  12   a  of the fixation belt driving roller  12  vertically displaces in an oscillatory manner. Thus, the fixation belt steering roller  13  can be tilted in such a manner that the lengthwise end  13   a R, by which the fixation belt steering roller  13  is supported by the steering roller support arm  51 R, is moved upward or downward as indicated by a pair of arrow marks A 1  or A 2 , respectively, to steer the fixation belt  11  by a preset amount ( FIG. 3B ). That is, the fixation belt position control mechanism  50  is structured so that as the steering roller support arm  51 F is moved in an oscillatory manner, the fixation belt steering roller  13  tilts by a preset angle in such a manner, that the lengthwise end  13   a F of the metallic core  13   a  of the fixation belt steering roller  13  moves upward or downward indicated by the pair of arrow marks A 1  and A 2 , respectively ( FIG. 3C ). The steering roller support arm  51 F is provided with a fan-shaped gear  54 , the gear portion of which faces away from the metallic core  13   a . The fan-shaped gear  54  is in engagement with a worm gear  56  attached to the output shaft of a stepping motor  55 . Further, the steering roller support arm  51 R supported by the rear plate  40 R is fitted with a bearing  52 R, which is supported by the steering roller support arm  51 R in such a manner that it is slid in the belt tension direction ( FIGS. 4A and 4B ). It is by this bearing  52 R that the lengthwise rear end portion  13   a R of the metallic core  13   a  of the fixation belt steering roller  13 , which is put through the rear plate  40 R in such a manner that it cannot be moved upward or downward, is rotatably supported. Further, the steering roller support arm  51 R is fitted with a tension spring  53 R for keeping the bearing  52 R pressed in the belt tension direction to provide the fixation belt  11  with a preset amount of tension. 
         [0030]    There is a fixation belt position sensor  90 F (belt position detecting first member) for detecting the position of front edge of the fixation belt  11 , on the inward surface of the front plate  40 F. The fixation belt position sensor  90 F is structured so that it can detect the presence of front edge of the fixation belt  11  when the edge is within its preset range, and at the preset limit position in terms of the lengthwise direction of the fixation belt driving roller  13  and fixation belt steering roller  13 . There is also a fixation belt position sensor  90 R (belt position detecting first member) for detecting the position of rear edge of the fixation belt  11 , on the inward surface of the front plate  40 F. The fixation belt position sensor  90 R is structured so that it can detect the presence of the rear edge of the fixation belt  11  when the edge is within its preset range, and at the preset limit position, in terms of the lengthwise direction of the fixation belt driving roller  12  and fixation belt steering roller  13 . The abovementioned preset limit position is on the outward side of the preset range, in terms of the lengthwise direction of the fixation roller driving roller  12  and fixation roller steering roller  13 . As for the angle of the tilt of the fixation belt steering roller  13 , the output of the fixation belt position sensor  90 F and that of the fixation belt position sensor  90 R are inputted into the control portion  200  (controlling means) so that the control portion  200  can control the operation of the stepping motor  55  to keep the fixation belt steering roller  13  in a preset range in terms of tilt. 
         [0031]    Next, the mechanism  60  (second controlling means) for controlling the pressure belt in position will be described. This mechanism hereafter will be referred to as a pressure belt position control mechanism  60 . The pressure belt position control mechanism  60  is made up of a front plate  41 F, a rear plate  41 R, a pair of steering roller supporting arms  61 F and  61 R, the pressure belt driving roller  22 , and the pressure belt steering roller  23 . The steering roller supporting arms  61 F and  61 R are attached to the front and rear plates  41 F and  41 R, respectively. The pressure belt driving roller  22  and pressure belt steering roller  23  are supported by the front and rear plates  41 F and  41 R. The rear end portion  22   a R of the metallic core  22   a  of the pressure belt driving roller  22  and the rear end portion  13   a R of the metallic core  13   a  of the pressure belt steering roller  23  are supported by the steering roller support arm  61 R. The pressure belt steering roller  23  is supported in such a manner that the pressure belt steering roller  23  can be tilted to vertically move the rear end  23   a R of its metallic core  23   a . The front end portion  23   a F of the metallic core  23   a  of the pressure roller driving roller  23  is rotatably supported by the front plate  41 F of the fixing apparatus  111 , and the steering roller support arm  61 F of the pressure belt position control mechanism  60  of the fixing apparatus  111  ( FIGS. 3A and 3B ). The other lengthwise end portion  23   a R of the metallic core  23   a  of the pressure belt driving roller  23  is rotatably supported by the rear plate  41 R of the fixing apparatus  111  and the steering roller support arm  61 R of the pressure belt position control mechanism  60  of the fixing apparatus  111  ( FIGS. 4A and 4B ). The lengthwise front end portion  23   a F of the metallic core  23   a  of the pressure belt steering roller  23  is rotatably supported by the front plate  41 F of the fixing apparatus  111  and the steering roller support arm  61 F of the fixing apparatus  111 , with the presence of a bearing  62 F between the metallic core end portion  23   a F and steering roller support arm  62 F ( FIGS. 3A and 3B ). The lengthwise other end portion  23   a R of the metallic core  23   a  of the pressure belt steering roller  23  is rotatably supported by the rear plate  41 R of the fixing apparatus  111  and the steering roller support arm  61 R, with the presence of a bearing  62 R between the metallic core end portion  23   a R and steering roller support arm  61 R ( FIGS. 4A and 4B ). The bearing  62 R is supported by the steering roller support arm  61 R attached to the rear plate  41 R, in such a manner that it can be slid in the belt tension direction ( FIGS. 4A and 4B ). Further, the metallic core end portion  23   a R of the pressure belt steering roller  23  is put through the rear plate  40 R and is rotatably supported by the bearing  62 R. The steering roller support arm  61 R is fitted with a tension spring  63 R for keeping the bearing  61 R pressed in the belt tension direction to provide the pressure belt  21  with a preset amount of tension. Therefore, of the pair of steering roller support arm  61 F and  61 R, the steering roller support arm  61 R is rotationally movable about the axis of the metallic core end portion  22   a R of the pressure belt driving roller  22 . Therefore, the steering roller support arm  61 R is rotationally (virtually vertically) movable about the axis of the metallic core end portion  22   a R of the pressure belt driving roller  22 . Therefore, the pressure belt steering roller  23  can be tilted (rotationally moved) about the center of the lengthwise metallic core end portion  23   a F of the pressure belt steering roller  23 , which is supported by the steering roller supported by the steering roller support arm  61 F, so that the rear end portion  23   a R of the metallic core  23   a  of the pressure belt steering roller  23  moves in the upward or downward direction as indicated by a pair of arrow marks B 1  and B 2 , respectively, to steer the pressure belt  21  by a preset amount ( FIG. 4B ). That is, the pressure belt steering mechanism  60  is structured so that as the steering roller support arm  61 R is rotationally moved, the pressure belt steering roller  23  is rotationally moved (tilted) by a preset angle (amount) about the center of the front end portion  23   a F of the metallic core  23   a  of the pressure belt steering roller  23 , in the upward or downward direction indicated by the pair of arrow marks. B 1  and B 2 , respectively ( FIG. 4C ). The steering roller support arm  61 R is provided with a fan-shaped gear  64 , which is on its surface facing away from its rotational axis. The fan-shaped gear  64  is in mesh with a worm gear  66  attached to the output shaft of a stepping motor  65  (for pressure belt steering roller) supported by the rear plate  41 R. Further, steering roller support arm  61 F attached to the front plate F is fitted with a bearing  62 F in such a manner that the bearing  62 F can be slid in the belt tension direction ( FIGS. 4A and 4B ). It is by the bearing  62 F that the front end portion  23   a F of the metallic core  23   a  of the pressure belt steering roller  23 , which is put through the front plate F in such a manner that it cannot be vertically moved, is rotatably supported. Further, the steering roller support arm  61 F is fitted with a tension spring  63 F for keeping the bearing  62 F pressed in the belt tension direction to provide the pressure belt  21  with a preset mount of tension. 
         [0032]    There is a pressure belt position sensor  91 R (belt position detecting second member) for detecting the position of the rear edge of the pressure belt  13 , on the inward surface of the rear plate  41 R. The pressure belt position sensor  91 R is structured so that it can detect in position the range in which the lengthwise rear edge of the pressure belt  13  is allowed to move, and the preset positional limit. There is also a pressure belt position sensor  91 F (belt position detecting second member) for detecting the position of the front edge of the pressure belt  13 , on the inward surface of the front plate  41 F. The pressure belt position sensor  91 R is structured so that it can detect in position the range in which the front edge of the fixation belt steering roller  13  is allowed to move, and the positional limit for the shifting of the pressure belt  21 . The above-mentioned position limit for the shifting of the pressure belt  21  is on the outward side of the preset range for the shifting of the pressure belt  13 , in terms of the lengthwise direction of the pressure belt driving roller  22  and pressure roller steering roller  23 . As for the angle of the tilt of the pressure roller steering roller  23 , the outputs of the pressure belt position sensors are inputted into the control portion  200  (controlling means) so that the control portion  200  can control the operation of the stepping motor  65  to keep the pressure roller steering roller  23  in a preset range in terms of tilt. 
       (4) Description of Belt Position Control of Fixation Belt Position Control Mechanism and Pressure Belt Position Control Mechanism 
       [0033]      FIG. 5  is a flowchart of an example of belt position control of the fixation belt position control mechanism.  FIG. 6  is a flowchart of an example of the belt position control of the pressure belt position control mechanism. 
         [0034]    First, referring to  FIG. 5 , the belt position control carried out by the control portion  200  to control the fixation belt position controlling means  50  will be described. Referring to  FIG. 3A , if the front edge of the fixation belt  11  moves out of the preset range for the front edge of the fixation belt  11  because of the shifting of the fixation belt  11  in the direction indicated by an arrow mark F, for example, the fixation belt position sensor  90 F detects the position of the front edge of the fixation belt  11 , and outputs a signal Sf 1 . Further, if the other edge of the fixation belt  11  moves out of the range preset for the fixation belt  11 , the fixation belt position sensor  90 R detects the position of the other edge of the fixation belt  11 , and outputs a signal Sf 2 . 
         [0035]    In step S 1 , as the signal Sf 1  outputted from the fixation belt position sensor  90 F is picked up by the control portion  200 , the control portion  200  moves to step S 2 . If the control portion  200  takes in the signals Sf 1  and Sf 2 , it takes step S 5 . In step S 2 , the control portion  200  rotates the stepping motor  55  in the direction to cause the output shaft of the stepping motor  55  to rotate in the direction indicated by an arrow mark CW. The rotation of the output shaft of the stepping motor  55  causes the worm gear  56 , whereby the steering roller supporting arm  51 F is rotationally moved, along with the fan-shaped gear  54 , in the downward direction indicated by an arrow mark A 2 . As the steering roller supporting arm  51 F is rotationally moved in the direction indicated by the arrow mark A 2 , the fixation belt steering roller  13  is tilted by the movement of the steering roller supporting arm  51 F, in the direction to cause its front end to move also in the direction indicated by the arrow mark A 2 . As the fixation belt steering roller  13  is tilted as described above, the fixation belt  11  begins to shift toward the other end, that is, in the direction indicated by the arrow mark R. If the other edge of the fixation belt  11  moves beyond the preset range for the other edge, the edge is detected by the fixation belt position sensor  90 R, and the fixation belt position sensor  90 R outputs a signal Sr 2 . In step S 3 , as the control portion  200  takes in the signal Sr 1  from the fixation belt position sensor  90 R, it proceeds to step S 4 , whereas if the control portion  200  takes in signals Sr 1  and Sr 2 , the control portion  200  proceeds to step S 5 . In step S 4 , in response to the signal Sr 1 , the control portion  200  rotates the stepping motor  55  in the direction to cause the output shaft of the stepping motor  55  rotates in the direction indicated by an arrow mark CC. The rotation of the output shaft of the stepping motor  55  causes the worm gear  56 , whereby the steering roller supporting arm  51 F is rotationally moved, along with the fan-shaped gear  54 , in the direction indicated by the arrow mark A 1 . As the steering roller supporting arm  51 F is rotationally moved in the direction indicated by the arrow mark A 1 , the fixation belt steering roller  13  is tilted by the movement of the steering roller supporting arm  51 F, in the direction to cause its front end to move also in the direction indicated by the arrow mark A 1 . As the fixation belt steering roller  13  is tilted as described above, the fixation belt  11  begins to shift toward the other end, that is, in the direction indicated by the arrow mark R. If the front edge of the fixation belt  11  moves beyond the preset range for the front edge, the edge is detected by the fixation belt position sensor  90 F, and the fixation belt position sensor  90 F outputs a signal Sf 1 . If the other edge of the fixation belt  11  moves beyond the preset range for the other edge, the edge is detected by the fixation belt position sensor  90 R, and the fixation belt position sensor  90 R outputs the signal Sf 2 . In step S 1 , as the control portion  200  takes in the output signal Sf 1  of the fixation belt position sensor  90 F, it moves to step S 2 , whereas if it takes in output signals Sf 1  and Sf 2 , it moves to step S 5 . The processes in steps S 2 -S 4  are repeatedly carried out. Thus, the fixation belt  11  continuously and alternately sways frontward and rearward (it continues to snake) while remaining in the preset range in which the fixation belt  11  is allowed to move. In step S 5 , the control portion  200  stops driving the stepping motor  55 , and also, stops the operation of the fixing apparatus  111  by stopping the electric power supply to the exciter coil  30 . 
         [0036]    Next, referring to  FIG. 6 , the control carried out by the control portion  200  to make the pressure belt position control mechanism  60  control the pressure belt  13  in position will be described. Referring to  FIG. 3A , if one of the edges of the pressure belt  21  moves out of the preset belt movement range because of the shifting of the pressure belt  21  in the direction indicated by the arrow mark F, for example, the belt edge is detected by the pressure belt position sensor  91 F, and the pressure belt position sensor  91 F outputs a signal Sf 3 . Further, if the other edge of the pressure belt  21  moves out of the preset belt range, the belt edge is detected by the pressure belt position sensor  91 R, and the pressure belt position sensor  91 R outputs a signal Sf 4 . In step S 11 , if the control portion  200  picks up the output signal Sf 3  from the pressure belt position sensor  91 F, it moves to step S 12 , whereas if it picks up the output signals Sf 3  and Sf 4 , it moves to step S 15 . In step S 12 , the control portion  200  rotates the stepping motor  65  to rotate the output shaft of the stepping motor  65  in the direction indicated by the arrow mark CW to move the fan-shaped gear  64  downward, that is, the direction indicated by the arrow mark B 2  ( FIG. 4B ) by a preset amount. Thus, the worm gear  66  rotates in response to the rotation of the output shaft of the stepping motor  65 , whereby the steering roller support arm  61 R is moved, along with the fan-shaped gear  64 , in the direction indicated by the arrow mark B 2 . As the steering roller support arm  61 R is rotationally moved in the direction indicated by the arrow mark B 2 , the pressure belt steering roller  23  is tilted as indicated by the arrow mark B 2 . As the pressure roller steering roller  23  is tilted as indicated by the arrow mark B 2 , the pressure belt  21  begins to shift rearward, that is, the direction indicated by an arrow mark R. Then, if the rear edge of the pressure belt  21  moves beyond the preset range for the pressure belt  21 , the rear edge is detected by the pressure belt position sensor  91 R, and the pressure belt position sensor  91 R outputs a signal Sr 3 . Further, if the front rear edge of the pressure belt  21  moves beyond the preset range for the front edge, the front edge detected by the pressure belt position sensor  91 F, and the pressure belt position sensor  91 F output a signal Sr 4 . In step S 13 , if the control portion  200  picks up the output signal Sr 3  from the pressure belt position sensor  91 R, it moves to step S 14 , whereas if it picks up the output signals Sr 3  and Sr 4 , it moves to step S 15 . In step S 14 , the 200 rotates the stepping motor  65  in response to the output signal Sr 3 , to rotate the output shaft of the stepping motor  65  in the direction indicated by an arrow mark CCW to move the fan-shaped gear  64  upward, that is, the direction indicated by the arrow mark B 1  ( FIG. 4B ) by a preset amount. With this rotation of the output shaft of the stepping motor  65 , the worm gear  66  rotates, causing the steering roller support arm  61 R to rotationally move as indicated by the arrow mark B 1 . By this rotational movement of the steering roller support arm  61 R in the direction indicated by the arrow mark B 1 , the pressure roller steering roller  23  is tilted as indicated by the arrow mark B 1 . As the pressure roller steering roller  23  is tilted as indicated by arrow mark B 1 , the pressure belt  21  begins to shift frontward, that is, in the direction indicated by the arrow mark F. If the front edge of the pressure belt  21  moves beyond the preset range while the pressure belt  21  is moving frontward, the front edge is detected by the pressure belt position sensor  91 F, and the pressure belt position sensor  91 F output the signal Sf 3 . Further, if the rear edge moves beyond the preset range, the pressure belt position sensor  91 R detects the rear edge, and outputs the signal Sf 4 . In step S 11 , if the control portion  200  takes in the output signal Sf 3  from the pressure belt position sensor  91 R, it moves to step S 12 , whereas if it takes in both the output signals Sf 3  and Sf 4 , it moves to step S 15 , and repeats the above described processes in steps S 12 -S 14 . Thus, the pressure belt  21  continues to alternately shift frontward and rearward (to snake) within preset pressure belt movement range. In step S 15 , the control portion  200  stops driving the stepping motor  65 , and also, stops the operation of the fixing apparatus  111  by stopping the electric power supply to the inductive heating coil  30 . 
         [0037]    Referring to  FIG. 7 , the distance between the fixation belt  11  and pressure belt  21 , on the sheet entrance side of the fixing apparatus is as follows.  FIG. 7(   a ) is a schematic drawing which shows the positional and attitudinal relationship between the fixation belt  11  and pressure belt  21  of the fixing apparatus  111  in the first embodiment of the present invention before the starting of the steering of the fixation belt  11  and pressure belt  21 .  FIG. 7(   b ) is a schematic drawing which shows the positional and attitudinal relationship between the fixation belt  11  and pressure belt  21  of the fixing apparatus  111  after the fixation belt steering roller  13  was tilted so that its front end was moved downward, and the pressure belt steering roller  23  was tilted so that its rear end was moved upward.  FIG. 7(   c ) is a schematic drawing which shows the positional and attitudinal relationship between the fixation belt  11  and pressure belt  21  of the fixing apparatus  111  after the fixation belt steering roller  13  was tilted so that its front end was moved upward, and the pressure belt steering roller  23  was tilted so that its rear end was moved downward.  FIG. 7(   d ) is a schematic drawing which shows the positional and attitudinal relationship between the fixation belt  11  and pressure belt  21  of the fixing apparatus  111  after the fixation belt steering roller  13  was tilted so that its front end was moved upward, and the pressure belt steering roller  23  was tilted so that its rear end was moved upward.  FIG. 7(   e ) is a schematic drawing which shows the positional and attitudinal relationship between the fixation belt  11  and pressure belt  21  of the fixing apparatus  111  after the fixation belt steering roller  13  was tilted so that its front end was moved downward, and the pressure belt steering roller  23  was tilted so that its rear end was moved downward. 
         [0038]    Referring to  FIG. 7(   a ), Xf stands for the distance between the front end of the fixation belt steering roller  13  and front end of the pressure roller steering roller  23 , and Xr stands for the distance between the rear end of the fixation belt steering roller  13  and rear end of the pressure roller steering roller  23 . Yu stands for the distance the front end of the fixation belt steering roller  13  moves upward as the steering roller supporting arm  51 F is rotationally moved, and Yd stands for the distance the front end of the fixation belt steering roller  13  moves downward as the steering roller supporting arm  51 F is rotationally moved. Further, Zu stands for the distance the rear end of the pressure roller steering roller  23  moves upward as the steering roller support arm  61 R is rotationally moved, and Zd stands for the distance the rear end of the pressure roller steering roller  23  moves downward as the steering roller support arm  61 R is rotationally moved. 
         [0039]    When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 7(   b ), 
         [0000]        Xf=Xf−Yd , and  Xr=Xr−Zu.    
         [0040]    When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 7(   c ), 
         [0000]        Xf=Xf+Yu , and  Xr=Xr+Zd.    
         [0041]    When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 7(   d ), 
         [0000]        Xf=Xf+Yu , and  Xr=Xr−Zu    
         [0042]    When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 7(   e ), 
         [0000]        Xf=Xf−Yd , and  Xr=Xr+Zd.    
         [0043]    Substituting actual values for the terms in the formulas given above, for example, if Xf=Xr=20 mm, and Yu=Yd=Zd=Zu=5 mm. 
         [0044]    When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 7(   b ), 
         [0000]      Xf=15 mm, and Xr=15 mm. 
         [0045]    When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 7(   c ), 
         [0000]      Xf=25 mm, and Xr=25 mm. 
         [0046]    When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 7(   d ), 
         [0000]      Xf=25 mm, and Xr=15 mm. 
         [0047]    When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 7(   e ), 
         [0000]      Xf=15 mm, and Xr=25 mm. 
         [0048]    In other words, the fixing apparatus  111  in the first embodiment changes by no more than 10 mm in the distance between the front end of its fixation belt  11  and pressure belt  21  on the sheet entrance side. 
       (5) Description of Comparative Fixing Apparatus 
       [0049]    Next, a conventional fixing apparatus as a comparative fixing apparatus to the fixing apparatus  111  in this embodiment will be described about its fixation belt position control mechanism and pressure belt position control mechanism, and their belt position control.  FIG. 8(   a ) is an external perspective view of the fixation belt position control mechanism and pressure belt position control mechanism of a typical conventional fixing apparatus as a comparative fixing apparatus, as seen from the recording sheet entrance side of the apparatus. It depicts the structure of the conventional fixing apparatus.  FIG. 8(   b ) is a left side (as seen from recording sheet entrance side) view of the fixation belt position control mechanism and pressure belt position control mechanism of the comparative fixing apparatus shown in  FIG. 8(   a ).  FIG. 9(   a ) is an external perspective view of the fixation belt position control mechanism and pressure belt position control mechanism of the comparative fixing apparatus, as seen from the recording sheet exit side of the apparatus.  FIG. 9(   b ) is the left side (as seen from recording sheet entrance side) view of the fixation belt position control mechanism and pressure belt position control mechanism of the comparative fixing apparatus shown in  FIG. 9(   a ). 
         [0050]    The comparative fixing apparatus is the same in structure as the fixing apparatus  111  in the first embodiment, except for the fixation belt position controlling means  50 . The members, portions, etc., of the comparative fixing apparatus, which are the same as the counterparts of the fixing apparatus in the first embodiment are given the same referential codes as those given to the counterparts, one for one, and will not be described here. The lengthwise rear end portion  12   a R of the metallic core  12   a  of the fixation belt driving roller  12  is rotatably supported by the rear plate  40 R and the steering roller support arm  51 R of the fixing apparatus  111  ( FIGS. 8(   a ) and  8 ( b )). The front end portion  12   a F of the fixation belt driving roller  12  is rotatably supported by the front plate  40 F of the fixation belt position controlling means  50  of the fixing apparatus  111 , and the steering roller supporting arm  51 F of the fixation belt position controlling means  50  of the fixing apparatus  111  ( FIGS. 9(   a ) and  9 ( b )). Of the rear end portion  12   a R of the metallic core  12   a  of the driving roller  12  and the rear end portion  13   a R of the metallic core  13   a  of the fixation belt steering roller  13 , which are supported by the steering roller support arm  51 R of the rear plate  40 R, the rear end portion  13   a R of the metallic core of the fixation belt steering roller  13  is movable upward and downward. Further, the steering roller support arm  51 R is rotationally moved upward or downward, respectively, about the axis of the rear end portion of the metallic core  12   a R of the fixation belt driving roller  12 . Thus, as the rear end portion  13   a R of the metallic core  13   a  of the fixation belt steering roller  13  is moved upward or downward, the fixation belt steering roller  13  is rotationally moved about the center of the front end portion  13   a F of the metallic core  13   a  supported by the steering roller support arm  51 F, in the upward or downward indicated by arrow marks A 1  and A 2 , respectively by a preset amount to steer the fixation belt  11  ( FIG. 8(   b )). That is, the fixation belt position controlling means  50  is structured so that as the steering roller support arm  51 R is rotationally moved upward or downward, the fixation belt steering roller  13  is rotationally moved (tilted) by a preset angle about the center of the front portion  13   a F in such a manner that the rear end portion  13   a R moves upward or downward as indicated by the arrow marks A 1  and A 2 , respectively. The fixation belt position controlling means  50  is provided with a stepping motor  55 , which is on the rear plate  40 R, and the worm gear  56  attached to the output shaft of the stepping motor  55  is in mesh with the fan-shaped gear  54  solidly attached to the steering roller support arm  51 R. That is, in the case of the comparative fixing apparatus, the steering roller supporting arm  51 R attached to the rear plate  40 R of the fixation belt position controlling means  50  is enabled to swing upward or downward about the center of the rear portion  12   a R of the metallic core of the driving roller  12 . The principle of the belt position control of the fixation belt position controlling means  50  of the comparative fixing apparatus is the same as that of the belt position control of the fixation belt position controlling means  50  of the fixing apparatus  111 . Thus, the belt shift control of the fixation belt position controlling means  50  of the comparative fixing apparatus will not be described here. 
         [0051]    Referring to  FIG. 10 , on the sheet entrance side of the comparative fixing apparatus, the distance between the fixation belt steering roller  13  and pressure roller steering roller  23  changes as follows.  FIG. 10(   a ) is a schematic drawing which shows the positional and attitudinal relationship between the fixation belt steering roller  13  and pressure roller steering roller  23  of the comparative fixing apparatus before the starting of the belt steering operation by the fixation belt steering roller  13  and pressure roller steering roller  23 .  FIG. 10(   b ) is a schematic drawing which shows the positional and attitudinal relationship between the fixation belt steering roller  13  and pressure roller steering roller  23  after the fixation belt steering roller  13  was tilted so that its rear end was moved upward, and the pressure belt steering roller  23  was tilted so that its rear end was moved downward.  FIG. 10(   c ) is a schematic drawing which shows the positional and attitudinal relationship between the fixation belt steering roller  13  and pressure roller steering roller  23  after the fixation belt steering roller  13  was tilted so that its rear end was moved downward, and the pressure belt steering roller  23  was tilted so that its rear end was moved upward.  FIG. 10(   d ) is a schematic drawing which shows the positional and attitudinal relationship between the fixation belt steering roller  13  and pressure roller steering roller  23  after the fixation belt steering roller  13  was tilted so that its rear end was moved upward, and the pressure belt steering roller  23  was tilted so that its rear end was moved upward.  FIG. 10(   e ) is a schematic drawing which shows the positional and attitudinal relationship between the fixation belt steering roller  13  and pressure roller steering roller  23  after the fixation belt steering roller  13  was tilted so that its rear end was moved downward, and the pressure belt steering roller  23  was tilted so that its rear end was moved downward. 
         [0052]    Referring to  FIG. 10(   a ), Xf stands for the distance between the front end of the fixation belt steering roller  13  and front end of the pressure roller steering roller  23 , and Xr stands for the distance between the rear end of the fixation belt steering roller  13  and rear end of the pressure roller steering roller  23 . Yu stands for the distance the rear end of the fixation belt steering roller  13  moves (upward) as the steering roller supporting arm  51 F is rotationally moved, and Yd stands for the distance the rear end of the fixation roller  13  moves (downward) as the steering roller supporting arm  51 R is rotationally moved. Further, Zu stands for the distance the rear end of the pressure roller steering roller  23  moves (upward) as the steering roller support arm  61 R is rotationally moved, and Zd stands for the distance the rear end of the pressure roller steering roller  23  moves (downward) as the steering roller support arm  61 R is rotationally moved. When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIGS. 10(   b ) and  10 ( c ), Xf does not change in value. When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 10(   b ), Xr=Xf+Yu+Zd. When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 10(   c ), Xr=Xf−Yu−Zd. Substituting actual values for the terms in these equations, for example, Xf=Xr=20 mm, and Yu=Yd=Zu=Zd=5 mm. 
         [0053]    When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 10(   b ), Xr=30 mm. 
         [0054]    When the fixation belt steering roller  13  and pressure roller steering roller  23  are in the state shown in  FIG. 10(   c ), Xr=10 mm. 
         [0055]    In other words, in the case of the comparative fixing apparatus, the distance between the fixation belt  11  and pressure belt  21  changes no less than 20 mm on the sheet entrance side. The changes in the distance between the fixation belt  11  and pressure belt  21  on the sheet entrance side makes the sheet S change in attitude and behavior when the sheet S is introduced into, and conveyed through, the fixing apparatus. Even if the fixing apparatus is enabled to tolerate the behavioral instability of the sheet S, problems sometimes occur when recording medium (sheet S) which is small in basis weight is used for a two-sided printing operation. More specifically, a sheet of recording medium, which is small in basis weight, is likely to curl. Thus, it is likely to be curled while it is conveyed through a fixing apparatus. Thus, if it is used as recording medium for a two-sided printing (image forming) operation, it sometimes comes into contact with the surface of the fixation belt  11  when the distance between the fixation belt  11  and fixation belt steering roller  13  reduces. This contact sometimes causes an image forming apparatus to output a print with an unsatisfactory image. Further, even if a sheet of recording medium is small in the amount of the curl which occurred along the leading edge, it may be large enough in the amount of the curl which occurred along the trailing edge for its trailing end to rub against the surface of the fixation belt  11 . 
         [0056]    Compared with the comparative fixing apparatus, the fixing apparatus  111  in the first embodiment was half in the amount of changes in the distance between the fixation belt  11  and pressure belt  21 , on the sheet entrance side. Thus, the sheet S remained more stable in behavior when it was conveyed through the fixing apparatus  111  than when it was conveyed through the comparative fixing apparatus. Thus, the employment of the fixing apparatus  111  in this embodiment by an image forming apparatus can substantially reduce the image forming apparatus in the number of unsatisfactory images. 
       [Miscellanies] 
       [0057]    In the first embodiment of the present invention, an image heating apparatus in accordance with the present invention was used as a fixing apparatus for an image forming apparatus. More specifically, the first and second belts of the belt driving apparatus in accordance with the present invention were used as the fixation belt and pressure belt, respectively, of the fixing apparatus. However, the usage of the first and second belts of the belt driving apparatus in accordance with the present invention does not need to be limited to the fixation belt and pressure belt of an image heating apparatus (fixing apparatus). For example, they may be used as the endless intermediary transfer belt and endless image bearing belt (image bearing member, which comes into contact with endless intermediary transfer belt) of an image forming apparatus. 
         [0058]    As described above, the present invention can significantly reduce the amount of change in the distance between the first and second belts of a recording medium conveying apparatus (mechanism), and therefore, it can provide a recording medium conveying apparatus (mechanism) which can more reliably convey recording medium than any of conventional recording medium conveying apparatuses (mechanisms). 
         [0059]    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. 
         [0060]    This application claims priority from Japanese Patent Application No. 175202/2009 filed Jul. 28, 2009 which is hereby incorporated by reference.