Abstract:
An image heating apparatus has an endless belt for heating an image on a recording material in a nip formed with a roller, a steering control portion for swinging the belt in a widthwise direction of the belt; and an anomaly controller for reducing, when the belt is beyond a predetermined swinging range, a pressure between the belt and the roller to return the belt into a predetermined range.

Description:
FIELD OF THE INVENTION AND RELATED ART 
   The present invention relates to an image heating apparatus which heats an image on recording medium. As examples of an image heating apparatus, a fixing apparatus for fixing an image formed on recording medium with the use of an electrophotographic or electrostatic recording method, and a glossiness adding apparatus for increasing an image in the degree of glossiness by reheating the image having been fixed to recording medium, can be listed. As examples of an apparatus which employs an image heating apparatus, there are a copying machine, a printer, and a facsimileing machine, etc. 
   As one of the examples of an image forming apparatus in accordance with the prior art, there is a laser beam printer, shown in  FIG. 6 , which has multiple optical scanning means and four drums. 
     FIG. 16  is a schematic sectional view of an example of an image forming apparatus (laser beam printer) in accordance with the prior art. This image forming apparatus is provided with four image formation stations Pa′, Pb′, Pc′, and Pd′, as image forming means, which are disposed in parallel and tandem in the main assembly of the image forming apparatus, as shown in  FIG. 16 . 
   The abovementioned image formation stations Pa′, Pb′, Pc′, and Pd′ are the stations for forming images (which hereinafter will be referred to as toner images) of toners of magenta, cyan, yellow, and black colors, respectively, and have photosensitive drums  1   a ′,  1   b ′,  1   c ′,  1   d ′, respectively, as image bearing members. 
   In the adjacencies of each of the photosensitive drums  1   a ′,  1   b ′,  1   c ′,  1   d ′, a charging device, a developing apparatus, and a cleaner, listing from the upstream side in terms of the rotational direction of the photosensitive drum  1 , are disposed in a manner of surrounding the photosensitive drum  1 . Below the space for the photosensitive drums  1   a ′,  1   b ′,  1   c ′,  1   d ′, a transfer station is located. 
   In this image forming apparatus structured as described above, a sheet P of recording medium is fed into the main assembly of the image forming apparatus from a sheet feeder cassette. Then, the sheet P is conveyed through the image formation stations Pa′-Pd′. While the sheet P is conveyed through the Pa′-Pd′, the toner images formed on the abovementioned photosensitive drums  1   a ′- 1   d ′ are sequentially transferred onto the sheet P. After the completion of this toner image transferring step, the sheet P is conveyed to a fixing apparatus  500 . 
   In the fixing apparatus  500 , the toner images having been transferred onto the sheet P are fixed to the sheet P by heat and pressure. Thereafter, the sheet P is conveyed to an apparatus for processing the sheets delivered thereto (which hereinafter may be referred to simply as sheet processing apparatus). 
   Referring to  FIG. 17 , which shows in detail the fixing apparatus  500 , the fixing apparatus  500  comprises a fixation roller  510 , a halogen heater  520 , and a thermistor  525 . The fixation roller  510  is rotated in the direction indicated by an arrow mark A, by an unshown driving force source. It is heated by the halogen heater  520 . The fixing apparatus is controlled so that the temperature of the fixation roller  510  is kept at a preset level, based on the temperature level detected by the thermistor  525  disposed in contact with the peripheral surface of the fixation roller  510 . 
   Disposed on the underside of the fixation roller  510  is a belt unit  53 . A fixation belt  531 , which is an endless belt, is stretched around an entrance roller  532 , a separation roller  533 , and a steering roller  534 , being thereby suspended by them. 
   The separation roller  533  is formed of a metallic substance such as SUS (stainless steel), and is kept pressed in the direction indicated by an arrow mark SF against the fixation roller  510  with the presence of the fixation belt  531  between the two rollers. The steering roller  534  is structured so that it can be moved in a manner to move one of its lengthwise ends in the direction indicated by an arrow mark B, to correct the positional deviation of the fixation belt  531  in its width direction. 
   The fixing apparatus  500  is also provided with a pressure pad unit  540 , which is between the entrance roller  532  and separation roller. The pressure pad unit  540  is made up of: a base  541  formed of a metallic substance such as SUS; a pressure pad  542  formed of silicon rubber or the like; and a slide sheet  543  formed of PI film or the like. The pressure pad unit  540  is kept pressed in the direction indicated by an arrow mark PF against the fixation roller  510 , with the fixing belt  531  sandwiched between the slide sheet  543  of the pressured pad unit  540  and fixation roller  510 . 
   Further, the fixing apparatus  500  is provided with an oil felt  536 , which is located between the entrance roller  532  and pressure pad unit  540 . The oil felt  536  is impregnated with silicon oil. It coats the inward surface of the fixation belt  531  with the silicon oil to reduce the friction between the fixation belt  531  and the slide sheet  543  of the pressure pad unit  540 . 
   The fixation belt  531  forms a fixation nip W in conjunction with the fixation roller  510 , pressure pad unit  540 , and separation roller  533 . With the employment of the fixation belt  531 , the nip width is greater than without it, making it possible to better melt the toner on the sheet P. In other words, the fixing apparatus  500  in this embodiment is very suitably structured as the one for such an image forming apparatus as a color image forming apparatus which uses a relatively large amount of toner. 
   Also regarding the structure of the fixing apparatus  500 , the fixation belt  531  is corrected in positional deviation by the steering roller  534 . In this method of correcting the positional deviation of the fixation belt  531 , the positional deviation of the fixation belt  531  is corrected in the manner proposed in Japanese Laid-open Patent Application 06-9096. That is, as the positional deviation of the fixation belt  531  is detected by a belt position detecting means, the correction roller (steering roller  534 ) is pivoted so that one of the lengthwise ends of the correction roller (steering roller  534 ), toward which the fixation belt  531  has shifted, is moved in the direction to shift the fixation belt  531  back toward the opposite lengthwise end of the correction roller. 
   However, the employment of this method makes it very difficult for the fixation belt  531  to last as long as the other mechanical components of the fixing apparatus  500 . 
   This is for the following reason. That is, the positional deviation of the fixation belt  531  is attributable to a large number of factors, for example, the state of alignment among the structural components, the physical property values of the materials for the structural components, the operational ambience, the cumulative length of usage of the fixation belt  531  (fixing apparatus  500 ). It is also attributable to whether the fixing apparatus has just been started up, or is being driven at a constant speed, etc. The positional deviation of the fixation belt  531  is a phenomenon that results from the complicated interaction among these factors. Therefore, there are a large number of issues to be overcome, in order to deal with all of the abovementioned factors. Presently, therefore, it is common practice to determine the lengths of the service lives of the components of the fixing apparatus  500 , and replace them with predetermined intervals, in order to ensure that the fixation belt  531  will be corrected in its positional deviation. 
   Further, the fixing apparatus  500  is designed so that if it becomes impossible to correct the positional deviation of the fixation belt  531  before the service life of the fixation belt  531  expires, this situation is detected, the mechanical operation is instantly ceased, and a warning suggesting the need for component exchange is issued (Japanese Laid-open Patent Application 10-109776). 
   There are essentially two types of phenomenon related to the anomaly in the fixation belt control in terms of positional deviation: the recurrent phenomenon, that is, the phenomenon that is expected to occur with predictable intervals (phenomenon with higher probability of occurrence), and the non-recurrent phenomenon, that is, the phenomenon that is expected to rarely occur (phenomenon with extremely low probability of occurrence). In the past, not only as the former occurred, but also, as the latter occurred (even though there was virtually no chance that the latter would occur), an image forming apparatus was left in the state in which it was stopped due to the occurrence of the above described phenomenon, until the maintenance operation inclusive of component replacement was carried out by a service person. In other words, even in the case of the latter, in which the belt deviation could be corrected without carrying out the above described maintenance operation, the copying operation had to be suspended by a user until the maintenance operation was carried out. 
   SUMMARY OF THE INVENTION 
   The primary object of the present invention is to provide an image heating apparatus which carries out a proper corrective operation if the oscillatory range of its fixation belt widens beyond a preset value. 
   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 perspective view of the fixation belt unit in one of the preferred embodiments of the present invention. 
       FIG. 2  is also a schematic perspective view of the fixation belt unit in the preferred embodiment. 
       FIG. 3  is a schematic drawing of the fixation belt unit in the preferred embodiment, showing the general structure thereof. 
       FIG. 4  is a schematic drawing of the fixation belt unit, showing its structural arrangement for pressure application. 
       FIG. 5  is a schematic drawing showing the oscillatory ranges of the fixation belt detected by the detecting means, in the preferred embodiment. 
       FIG. 6  is a schematic drawing of the fixation belt unit, showing its structural arrangement for pressure application. 
       FIG. 7  is a schematic drawing of the fixation belt unit, showing its structural arrangement for pressure application. 
       FIG. 8  is a schematic drawing of the fixation belt unit, showing its structural arrangement for pressure application. 
       FIG. 9  is a schematic drawing of the fixation belt unit, showing its structural arrangement for pressure application. 
       FIG. 10  is a schematic drawing of the fixation belt unit, showing its structural arrangement for pressure application. 
       FIG. 11  is a schematic drawing of the fixation belt unit, showing its structural arrangement for pressure application. 
       FIG. 12  is a diagrammatic drawing showing the control system of the fixation belt unit in the preferred embodiment. 
       FIG. 13  is a graph showing the positions of the fixation belt of the fixation belt unit in the preferred embodiment. 
       FIG. 14  is a schematic sectional view of the image forming apparatus in the preferred embodiment. 
       FIG. 15  is a schematic sectional view of the image forming portion of the image forming apparatus in the preferred embodiment. 
       FIG. 16  is a schematic sectional view of one of the typical image forming apparatuses in accordance with the prior art. 
       FIG. 17  is a schematic sectional view of one of the fixing apparatuses in accordance with the prior art. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, one of the preferred embodiments of the present invention will be described in detail with reference to the appended drawings. Incidentally, the dimensions, materials, and shapes of the structural components, and the positional relationship among the structural components, of the apparatuses in this embodiment, are not intended to limit the scope of the present invention. That is, they are to be modified as necessary, according to the structure of an apparatus to which the present invention is applied, and various conditions under which the apparatus is operated. 
   First, a laser beam printer, as one of the examples of the image forming apparatuses in accordance with the present invention, which employs four drums and multiple optical scanning means will be described. 
     FIG. 14  is a schematic sectional view of the image forming apparatus A (laser beam printer) in this preferred embodiment.  FIG. 15  is a schematic sectional view of the image forming portion of the image forming apparatus A in this embodiment. Referring to  FIG. 14 , the image forming apparatus is provided with four image formation stations Pa, Pb, Pc, and Pd, as image forming means, which are disposed in parallel in the main assembly of the image forming apparatus. 
   The abovementioned image formation stations Pa, Pb, Pc, and Pd are the stations for forming images of toners of magenta, cyan, yellow, and black colors, respectively. They have photosensitive drums  1   a ,  1   b ,  1   c ,  1   d , respectively, as image bearing members, which are rotated in the direction indicated by an arrow mark in  FIG. 15 . 
   In the adjacencies of the photosensitive drums  1   a ,  1   b ,  1   c ,  1   d , charging devices  12   a ,  12   b ,  12   c , and  12   d , developing apparatuses  2   a ,  2   b ,  2   c , and  2   d , and cleaners  4   a ,  4   b ,  4   c , and  4   d , listing from the upstream side in terms of the rotational direction of the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , are disposed in a manner of surrounding the photosensitive drums  1   a ,  1   b ,  1   c , and  1   d , respectively. Below the space for the photosensitive drums  1   a ,  1   b ,  1   c ,  1   d , a transfer station  3  is located. The transfer station  3  is has a transfer belt  31  as a recording means conveying means, which is shared by the image formation stations Pa, Pb, Pc, and Pd, and charging devices  3   a ,  3   b ,  3   c , and  3   d  for transfer. 
   In this image forming apparatus structured as described above, a sheet P of recording medium is fed into the main assembly of the image forming apparatus from a sheet feeder cassette as a sheet feeding means shown in  FIG. 14 . Then, the sheet P is placed on the transfer belt  31 , and is conveyed through the image formation stations Pa-Pd. While the sheet P is conveyed through the Pa-Pd, the toner images formed on the abovementioned photosensitive drums  1   a - 1   d  are sequentially transferred onto the sheet P. After the completion of this toner image transferring step, the sheet P is separated from the transfer belt  31 , and is conveyed to a fixing apparatus  5 . 
   In the fixing apparatus  5 , the toner images having been transferred onto the sheet P are fixed to the sheet P by heat and pressure, in the fixation nip. Thereafter, the sheet P is conveyed to a sheet processing apparatus  6 . In the sheet processing apparatus  6 , the sheet P is discharged by a pair of discharge rollers  61  into a delivery tray  62 , which can be moved downward to allow a large number of sheets P to be cumulatively discharged into the delivery tray  62 . The sheet processing apparatus  6  is enabled to variously process the sheets P of recording medium, for example, to staple the sheets P. 
   Next, the fixing apparatus  5  in this embodiment will be described. 
   The fixing apparatus  5  comprises a fixation roller  80 , which is rotatably disposed, and a fixation belt unit  100  disposed below the fixation roller  80 . 
   The fixation roller  80  is rotatably disposed so that it can be rotationally driven by a driving force source. It is heated by a heater disposed in its hollow. The fixing apparatus  5  is controlled so that the temperature of the fixation roller  80  is kept at a preset level, based on the temperature level detected by the thermistor disposed in contact with the peripheral surface of the fixation roller  80 . 
   Next, the fixation belt unit  100  will be described. 
     FIGS. 1 and 2  are schematic drawings of the fixation belt unit  100  (fixation belt  101  is not shown) in this embodiment. 
   In the fixation belt unit  100 , the fixation belt  101  is stretched around multiple rotational members: an inlet roller  130 , a separation roller  141 , and a steering roller  151 , being thereby suspended by them. The fixation belt  101  is kept pressed on the fixation roller  80 , forming thereby a fixation nip. 
   The fixation belt unit  100  is provided with front and rear plates  110  and  120 , and a fixation pad portion  160  to which the front and rear plates  110  and  120  are fastened. The inlet roller  13  is rotatably supported by the front and rear plates  110  and  120 . 
   The separation roller portion  140  comprises a separation roller  141 , and a pair of bearings  142   a  and  142   b , which are fitted around the lengthwise end portions  141   a  and  141   b , respectively, of the separation roller  141 . The steering roller portion  150  comprises the steering roller  151 , a bearing  152   a  fitted around one of the lengthwise end portions of the steering roller  151 , and a pressure application front arm  153  having an elongated hole  153   a  in which the bearing  152   a  is fitted. 
   The steering roller  151  is movable in the elongated hole  153   a  of the pressure application front arm  153  in the direction indicated by an arrow mark B. Further, the pressure application front arm  153  applies pressure on the steering roller  151  in the direction indicated by an arrow mark C, with the use of a spring  154   a , tensioning thereby the fixation belt  101 . 
   Similarly on the rear plate side, the other lengthwise end portion  151   b  of the steering roller  151  is fitted with a bearing  152   b , which is fitted in the elongated hole  155   a  of the pressure application rear arm  155 . The steering roller  151  is movable in the elongated hole  155   a  of the pressure application rear arm  155  in the direction indicated by an arrow mark B. Further, the pressure application rear arm  155  applies pressure on the steering roller  151  in the direction indicated by an arrow mark C, with the use of a spring  154   b , tensioning thereby the fixation belt  101 . 
   A steering control portion  200  as a fixation belt oscillating means is provided with a steering roller supporting front member  210 , a steering roller supporting rear member  220 , and a control shaft  230  which connects the steering roller supporting front and rear members  210  and  220 . More specifically, the steering roller supporting front member  210  is rotatably supported by one of the lengthwise end portions of the control shaft  230 , whereas the steering roller supporting rear member  220  is rigidly attached to the other lengthwise end portion of the control shaft  230 . 
     FIG. 3  is a drawing of the fixation belt unit  100  shown in  FIG. 1 , as seen from the direction indicated by a referential symbol X in  FIG. 1 . 
   Referring to  FIGS. 1   3 , the steering control portion  200 , which may serve as swing means, is to be mounted by displacing the fixation belt unit  100  in the direction indicated by an arrow mark Y. As the steering control portion  200  is mounted, the steering control portion accommodating front portion  151   c  of the steering roller  151  fits into the U shaped groove  211  of the steering roller supporting front member  210 , and the steering control portion accommodating rear portion  151   d  of the steering roller  151  fits into the U shaped groove  221  of the steering roller supporting rear member  220 . 
   With the provision of the above described structural arrangement, as an input gear  241  is rotated, the steering roller supporting front member  210  is rotated in the opposite direction from the rotational direction of the input gear  241 . Further, the control arm  243  is rotated by the rotation of the input gear  241  in the same direction as the rotation direction of the input gear  241 , through an idler gear  242 . Since the control arm  243  is rigidly attached to the control shaft  230 , it rotates the steering roller supporting rear member  220  in the same direction as the rotational direction of the control arm  243 . As a result, the steering control portion accommodating front portion  151   c  of the steering roller  151 , which is in the U-shaped groove of the steering roller supporting front member  210 , and the steering control portion accommodating rear portion  151   d  of the steering roller  151 , which is in the U-shaped groove  221  of the steering roller supporting rear member  220 , are moved in the opposite directions. 
     FIG. 4  is a drawing of pad pressuring front and rear plates  331  and  332 , respectively, which support the fixation belt unit  100 . 
   The fixation pad portion  160  is supported by the fixation pad portion accommodating portion  331   a  of the pad pressuring front plate  331 , and the fixation pad accommodating portion  332   a  (which is the same in shape as pad pressuring front plate, and is not shown) of the pad pressuring rear plate  332 . Further, the pad pressuring rear plate  332  is provided with a belt deviation detecting portion  270  as a means for detecting the positional deviation of the fixation belt  101  (in terms of the direction parallel to the axial lines of the rollers). The control shaft  230  is rotatably supported by the pad pressuring front and rear plates  331  and  332 , with a pair of bearings placed between the lengthwise end portions of the control shaft  230  and the pad pressuring front and rear plates  331  and  332 , one for one. 
     FIG. 5  is a drawing showing the extent of the positional deviation of the fixation belt  101 , which is detected by the belt deviation detecting portion  270  as a means for detecting the positional deviation of the fixation belt  101 . A referential symbol W 1  stands for the width of the fixation roller  101 , and a referential symbol W 2  stands for the normal range with a preset value, in which the fixation is oscillated in its width direction by the changing of the position of the steering roller  151 . A referential symbol W 3  stands for the abnormal oscillatory range of the fixation belt  101 , which is substantially wider than the range W 2 . A referential symbol W 4  stands for the maximum range in which the fixation belt  101  can be oscillated (beyond this range, belt is not rotatable, and forceful rotation of belt results in damage to belt). The abovementioned belt deviation detecting portion  270  is made up of a position sensor capable of detecting the ranges W 2  and W 3 . 
     FIG. 6  is a side view of the fixation belt unit  100 , which has been attached to the pad pressuring front and rear plates  331  and  332 , as seen from the front plate side of the fixing apparatus. The steering control portion accommodating front portion  151   c  of the steering roller  151  is in the U-shaped groove  211  of the steering roller supporting front member  210 . The control arm  243  is provided with a positioning hole  243   c , which is similar to the hole with which the steering roller supporting front member  210  is provided, and the hole with which the pad pressuring front plate  331  is provided. By aligning this positioning hole  243   c  of the control arm  243  with the hole of the steering roller supporting front member  210  and the hole of the pad pressuring front plate  331 , it is possible to synchronize the steering roller supporting front member  210  and control arm  243  in the phase relative to the pad pressuring front arm  331 . Further, the input gear  241  and idler gear  242  are also attached to the pad pressuring front plate  331 . 
     FIG. 7  is a drawing of a pressure application mechanism  300  as a fixation belt unit moving means, which is for pressing the fixation belt unit  100  against the fixation roller  80 . The pressure application mechanism  300  is provided with a rotational axle  301  attached to an unshown fixating apparatus frame. As a cam  304  rotates about a cam shaft  303 , the fixation belt unit  100  is placed in contact with, or moved away from, the fixation roller  80 . More specifically, the pressure application mechanism  300  is enabled to take a pressure application position in which it presses the fixation belt unit  100  upon the fixation roller  80 , and a position in which it eliminates or reduces the pressure it applied to the fixation belt unit  100 . More specifically, as the cam  304  is rotated about the cam shaft  303 , the roller  305  of the pressure application front plate  311  is lifted, causing the pressure application mechanism  300  to pivot in the direction indicated by an arrow mark U. As a result, the fixation belt unit  100  is pressed on the fixation roller  80 . 
     FIG. 8  is a drawing of the fixation belt unit  100  and fixation roller  80  shown in  FIG. 7 , as seen from the direction indicated by an arrow mark K in  FIG. 7 . The pressure application cam  304  lifts the pressure application front plate  311 , which is provided with a pad pressing spring  312  and a pad separating spring  313 . The pad pressing spring  312  acts in the direction to lift the pad pressing plate front plate  331  in the direction indicated by an arrow mark F, and the pad separating spring  313  acts in the direction to lift a pad separation front plate  321  in the direction indicated by the arrow mark F. In this case, the structural arrangement on the pad pressing rear plate side is the same as that on the pad pressing front plate side. 
     FIG. 9(   a ) is a schematic top view of the fixing apparatus  5 , and  FIG. 9(   b ) is a schematic front view of the fixing apparatus  5 .  FIG. 9(   c ) is a sectional view of the fixing apparatus, at a plane A-A in  FIG. 9(   a ). 
   The separation roller  141  is provided with a pressure application bearing  143 , which is attached to one of the lengthwise ends of the separation roller  141 . The pressure application bearing  143  is in contact with the slanted surface  321   a  of the pad separation front plate  321 . Here, the structure of the fixation belt unit  100  on the front side has been described. However, the structure of the fixation belt unit  100  on the rear side is the same as that on the front side. With provision of this structural arrangement, the separation roller  141  presses the fixation roller  80  in the direction indicated by an arrow mark J, which is perpendicular to the slanted surface  321   a  of the pad separation front plate  321 . The pad pressing front plate  331  is provided with a groove  331   a , in which a pad holder  161  is fitted. This structural arrangement is the same on the rear side of the drawing. With the provision of this structural arrangement, the pad portion  160  presses the fixation roller  80  in the direction indicated by an arrow mark K. 
     FIG. 10  shows the fixation roller  80 , and the fixation belt unit  100  which has been separated from the fixation roller. 
   The pressure application mechanism  300  is structured so that as the cam  304  rotates about the cam shaft  303 , the pressure application front plate  311  is moved in the direction indicated by an arrow mark V, causing thereby the fixation belt unit  100  to be separated from the fixation roller  80 . With the fixation belt unit  100  separated from the fixation roller  80 , the paper jam or the like which sometimes occurs to the fixing apparatus can be easily dealt with. 
     FIG. 11  shows the state (which is the same as that shown in  FIG. 6 ) of the pad pressing front plate  331 , which has occurred as the fixation belt unit  100  was separated from the fixation roller  80 . To the pad pressing front plate  331 , the input gear  241 , idler gear  242 , and control arm  243  are attached, and further, the control shaft  230  is held by the pad pressing front and rear plates  331  and  332 . Therefore, as the pad pressing front and rear plates  331  and  332  are rotated in the same manner by the rotation of the pressure application front and rear plates  311  and  312 , respectively, the steering control portion  200  is also rotated. Therefore, even after the separation of the fixation belt unit  100  from the fixation roller  80 , the steering roller portion  150  can be controlled in the same manner as the manner in which the steering roller portion  150  can be controlled while the fixation belt unit  100  is kept pressed on the fixation roller. The rotational axis of the input gear  241  coincides with the rotational axis of the rotational shaft  301 . Therefore, the position of the input gear  241  is not affected by the pivoting of the pad pressing front plate  331 . 
     FIG. 12  is a diagrammatic drawing showing the control system in this embodiment. The signals outputted by the belt deviation detecting portion  270  are inputted into a CPU as an anomaly controlling means  400 , through a belt deviation detecting means  401 . The anomaly controlling means  400  corrects the positional deviation of the fixation belt  101  in response to the signals from the belt deviation detecting portion  270 , by transmitting driving force to the steering roller  151  through an alignment adjusting means  402  and a motor M 3 . The belt deviation detecting means  401  is a detecting means, whereas the alignment adjusting means  402  and motor M 3  are adjusting means. Further, the anomaly controlling means  400  constitutes a first decision making means as well as a second decision making means. 
   When the signal from the belt deviation detecting means  270  indicates the range W 3  (abnormal range) shown in  FIG. 5 , the anomaly controlling means  400  moves the fixation belt unit  100  in the direction to separate the fixation belt  101  from the fixation roller  80 , reducing thereby the amount of the contact pressure between them, or separating them, with the use of the pressure application mechanism  300  through a pressure removing means  405  and a motor M 2 . The anomaly controlling means  400  also controls the rotation of the fixation roller  80  through a driving means  406  and a motor M 1 . Here, the pressure removing means  405  constitutes a movement controlling means, whereas the motor M 2  constitutes a moving means. 
     FIG. 13  is a drawing for roughly describing the state of the positional deviation of the fixation belt  101 :  FIG. 13(   a ) is a drawing for describing the transition of the positional state of the fixation belt  101  from the well controlled state to the uncontrollable state;  FIG. 13(   b ) is a drawing for describing the transition (restoration) of the positional state of the fixation belt  101  from the uncontrollable state, shown in  FIG. 13(   a ), to the normal state; and  FIG. 13(   c ) is a drawing for describing the transition of the positional state of the fixation belt  101  from the normal state to the uncontrollable state, after the returning to the normal state. In each drawing, the axis of ordinates represents a belt position L, and axis of abscissa represents the length t of elapsed time. 
     FIG. 13  shows the positions of the fixation belt  101  detected by the belt deviation detecting portion  270 . As for the method for detecting the position of the fixation belt  101 , the surface of the fixation belt  101  is provided with a belt position marker, and the position of the fixation belt  101  is detected by detecting the position of this belt position marker by the belt deviation detecting means  270 . 
   Referring to  FIG. 13(   a ), during the period T 1 , the belt position marker was moving within the range W 2 . This means that the fixation belt  101  was being satisfactorily controlled in terms of positional deviation, by the steering roller portion  150  which was being controlled by the belt deviation detecting portion  270  in response to the signals from the belt deviation detecting portion  270 . During the period T 2 , that is, the period immediately after the period T 1 , the fixation belt unit  100  became uncontrollable, allowing the fixation belt  101  to move out of the normal range of oscillation into the abnormal range of oscillation. 
   In this situation, the belt deviation detecting portion  270  detected at a point ta in time that the aforementioned belt deviation detection marker moved out of the normal range W 2 . As a result, the anomaly controlling means  400  determined, based on the signals from the belt deviation detecting means  270 , that the fixation belt  101  moved into the control anomaly range. In the case of an image forming apparatus in accordance with the prior art, when an anomaly such as this one occurs, the anomaly was detected by the main assembly of the image forming apparatus, and such steps as immediately turning off the fixing apparatus, informing an operator of the occurrence of the anomaly, etc., were carried out. 
   Referring to  FIG. 13(   b ), as the anomaly controlling means  400  detected that the fixation belt unit was in an abnormal state of control, it outputted a pressure removing signal to the pressure removing means  405 , separating thereby the fixation belt unit  100  from the fixation roller  80 , at a point tb in time. As a result, the fixation belt  101  was freed from the pressure and restraint placed by the fixation roller  80  and pad portion  160 , which were the major causes of the belt deviation.  FIG. 13(   b ) shows that in the period T 3 , the fixation belt unit  100 , which was in the uncontrollable state, recovered to the normal state. 
   Incidentally, regarding the method for restoring the fixation belt unit  100  in terms of the function of controlling the belt deviation, the structural arrangement therefor does not need to be limited to such that the fixation belt is separated from the fixation roller. For example, the fixation belt unit  100  may be structured to reduce the contact pressure between the fixation belt and fixation roller while allowing the two to remain in contact with each other. Such a structural arrangement also makes it possible to restore, in a short length of time, the fixation belt unit into its normal state in terms of the belt deviation control. 
   Thereafter, pressure was applied again on the fixation belt unit  100  at a point tc in time. During the period T 4  in  FIG. 13(   b ), it was possible to control the belt deviation, even though the fixation belt unit  100  was under the pressure. In other words,  FIG. 13(   b ) shows that the anomaly which occurred during the period T 2  was not recurrent, and normalcy was restored by eliminating the anomaly with the use of the pressure removing means  405 . 
   The period T 3  for determining whether or not the normal state of belt oscillation was restored after the elimination of the anomaly by the pressure removing means  405  was set by a control period setting means  403  as a time setting means shown in  FIG. 12 , and during this period, the anomaly controlling means  400  determined, based on the results of the detection by the belt deviation detecting portion  270 , whether or not the normal belt oscillation had been restored. 
     FIG. 13(   c ) shows a case in which what occurred during the period T 4 , which followed the period T 3  (in which pressure was removed in response to detection of anomaly), and which began with the re-application of the pressure, was different from what occurred in the period T 4  in  FIG. 13(   b ). 
   In the case shown in  FIG. 13(   c ), as the anomaly was detected again by the belt deviation detection portion  270  at a point td in time, the anomaly controlling means  400  determined that this anomaly in the control of the positional deviation of the fixation belt  101  was recurrent, and immediately stopped the driving of the fixing apparatus with the use of the anomaly processing means  407 , and informed a user of the need for maintenance. 
   In this case, an arrangement may be made so that if the length of time (td-dc) it takes for the belt oscillation to become abnormal after the re-application of the pressure is no more than a preset length of time, the anomaly controlling means  400  determines that the anomaly in the belt deviation control is recurrent. In this case, the referential length of time should be set to be a value which is not large enough to allow the image forming means to restart the interrupted image forming operation. Also in this case, such an arrangement may be made that if the length of time (td-dc) it takes for the belt deviation detecting portion  270  to detect the anomaly after the re-application of the pressure is greater than the abovementioned preset length of time, the fixation belt unit  100  is separated again from the fixation roller  80  by the pressure removing means  405  after the second detection of the anomaly at the point td in time. 
   Also referring to  FIG. 13(   c ), an arrangement may be made so that if the belt deviation detecting portion  270  detects again the anomaly at the point td in time, the fixation belt unit  100  is separated from the fixation roller  80  by the pressure removing means  405  regardless of the length (td-tc) of the time. In this case, for example, an arrangement may be made so that the number of times the pressure removing means  405  separates the fixation belt unit  100  from the fixation roller  80  is counted, and if the count reaches a preset value, the anomaly controlling means  400  determines that the anomaly in the control of the positional deviation of the fixation belt  101  is recurrent. 
   The referential value with which the elapsed time (td-tc) is compared, or the like values, are set by the means  404  for setting referential values used for determining whether or not the positional deviation of the fixation belt  101  is abnormal; whether or not the anomaly in the control of the positional deviation of the fixation belt  101  is recurrent is determined by the anomaly controlling means  400  with reference to the preset referential value. 
   Further, such an arrangement is advisable that if the belt deviation detecting portion  270  detects the anomaly in the control of the positional deviation of the fixation belt  101  after the separation of the fixation belt unit  100  from the fixation roller  80  by the pressure removing means  405 , the anomaly controlling means  407  determines that the control of the fixation belt  101  is abnormal, immediately stops the driving of the fixing apparatus, with the use of the anomaly processing means  407 , and informs a user of the need for maintenance. 
   As described above, according to this embodiment of the present invention, as soon as the anomaly in the control of the positional deviation of the fixation belt  101  is detected, the anomaly in the belt deviation can be immediately dealt with, with the use of the pressure removing means. Further, by checking whether or not the anomaly in the belt deviation, which occurs while the pressure is being applied, is recurrent, not only can the service lives of the structural components of the fixation belt unit  100  be extended, but also, the image forming apparatus can be reduced in downtime. 
   Also according to the above described embodiment, whether or not the anomaly in the control of the belt oscillation is recurrent is efficiently determined. Therefore, the measures to be taken when there is an anomaly in the control of the belt oscillation can be optimized. Therefore, not only can the service life of the fixing apparatus (image forming apparatus) be extended, but also, the apparatus can be reduced in downtime. 
   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. 358410/2004 filed Dec. 10, 2004 which is hereby incorporated by reference.