Abstract:
A fixing device configured to fix a toner image on a moving printing medium includes: a fixing heat generator that generates heat by electromagnetic induction; a fixing medium that rotates and is heated while being carried by the heating medium; and a pressing member that presses a back surface of the printing medium coming into contact with the fixing medium, the fixing heat generator including a center coil and a pair of side coils provided at a center portion and both side portions thereof along a direction vertical to the direction of movement of the printing medium for heating the fixing medium by induction heating, the pressing member including a center heater and a side heater that heat a center portion and both side portions thereof in a direction vertical to the direction of movement of the printing medium integrated therein, and includes a side temperature sensor that senses at least one of the temperatures of the both side portions in a widthwise direction vertical to the direction of movement of the printing medium, and a heater control unit that turns the side heater OFF when the temperature that the side temperature sensor senses exceeds a predetermined temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority from prior Application No. 61/043,803, filed on Apr. 10, 2008, the entire contents of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a fixing device used in an image forming apparatus of an electrophotographic system and, more specifically to a fixing device having a structure including a plurality of heaters in a press roller. 
       BACKGROUND 
       [0003]    In an image forming apparatus such as an electrophotographic apparatus or the like, an electrostatic latent image is formed, and then the electrostatic latent image is developed by toner or the like, and a developed toner image is fixed to a printing sheet. In order to fix the image to the printing sheet, it is necessary to apply heat to the toner image and, normally, the fixation is achieved by heating the printing sheet having the toner image held thereon from the front surface with a fixing roller and applying a pressure from the back surface with a press roller. 
         [0004]    For example, heating of the fixing roller is achieved by winding a fixing belt between the fixing roller and a heat roller, heating the heat roller by a heater integrated in the heat roller, transferring the heat to the fixing belt, and transferring the transferred heat to the fixing roller (for example, JP-A-2001-318557). 
         [0005]    The printing sheet to be subjected to the fixation of the toner image includes those in various sizes, and these printing sheets have different widths. For example, when the fixation is carried out on a printing sheet having a small width, the temperature of a portion of the fixing roller which actually carries out the fixation falls. In contrast, end portions which do not carry out the fixation are kept at a raised temperature, and hence might be overheated. 
         [0006]    In this manner, when printing an image using a small size sheet, the temperature of the portion of the fixing roller where the printing sheet passes falls because the printing sheet absorbs the heat therefrom, so that the temperature control is achieved by heating. In contrast, it is known that the portion of the fixing roller where the printing sheet does not pass is overheated because the heat is not absorbed by the sheet, so that the temperature rise is resulted. 
         [0007]    In order to prevent the temperature rise as such, JP-A-2001-318557 discloses an image forming apparatus in which a heat source (heat lamp) for a large size sheet is turns ON for the fixation of a large size sheet, and a heat source for a small size sheet and the heat source for the large size sheet controlled to a low power are used for the fixation of the small size sheet. In this publication, there are descriptions such that the heat source for the large size sheet may be provided within the press roller instead of within the heat roller (see  FIG. 9  and the paragraph [0023] in the specification) and the fixing belt is heated from the outside by a cleaning roller (having the heat source for the large size integrated therein) which comes into contact with the fixing belt (see  FIG. 6  and the paragraph [0022] in the specification). 
         [0008]    When heating the press roller as well, since the portion which is free from contact of the printing sheet might be overheated when the fixation of the printing sheet having a small width is carried out, it is also contemplated to prevent the temperature rise by providing a heat source for the large size sheet and a heat source for the small size sheet in the press roller. 
         [0009]    When the heat roller or the fixing belt (heat belt) is heated by the heat source (heater) provided in the heat roller or the cleaning roller as described above, a large heat capacity is required, and hence it takes a significantly long time from the power-on of the device until the temperature of the portion of the fixing belt which comes into contact with the press roller rises to a predetermined temperature. Therefore, there is a problem such that copying cannot be started immediately after the power-on of the image forming apparatus. 
         [0010]    In order to shorten the rising time after the power-on, a system to heat the fixing belt directly from the outside by an induction heating (IH) or the like is contemplated. According to the fixing belt direct heating system, since the heat capacity is small, the time required for heating the surface of the fixing belt to a predetermined temperature is shortened, so that the rising time after the power-on is shortened correspondingly. In contrast, since the temperature fall tends to occur in the portion of the fixing belt where the printing sheet comes into contact with, it is necessary to heat this portion, while the temperature rise tends to occur in the portion where the printing sheet does not come into contact with. Therefore, it is contemplated to use a separate induction heat device obtained by dividing the induction heat device which heats the fixing belt directly from the outside into a center portion and both side portions. 
         [0011]    The invention relates to a fixing device having a separate induction heat unit which divides the fixing belt into sections in the widthwise direction and induction-heats the same and a multiple-heater integrated press roller having a plurality of integrated heaters to be used according to the width of the printing sheet. 
         [0012]    Referring now to the drawings, problems of the fixing device having the structure as described above will be described. 
         [0013]    According to the induction heat device having a divided structure, the fixing belt can be heated by selectively driving the plurality of heaters having a heating portion at positions different axially from each other according to the sheet width. However, the sheet width does not necessarily match the positions where a coil is divided, and the printing sheet might pass over part of the width of a divided induction coil. In such a case, the temperature fall occurs in the portion of the fixing belt where the printing sheet passes, while the temperature fall does not occur where the printing sheet does not pass, and hence partial overheating or temperature fall cannot be avoided if the belt is uniformly heated. 
         [0014]      FIG. 1  shows a state where the printing sheet passes over part of divided induction heat coils as described above. It is assumed that an induction heat coil  13  includes a center coil  13   a , and side coils  13   b  and  13   c  provided on both sides thereof with respect to a fixing belt  12 , and temperature sensors  15   a ,  15   b , and  15   c  sense passage of the printing sheet with respect to the center coil  13   a  and the side coils  13   b  and  13   c . In this case, as shown in the drawing, when a printing sheet P 1  having a width W 1  passes, that is, the printing sheet passes so that the side edges thereof match the divided positions of the divided coils, the temperature sensor  15   a  senses the passage of the printing sheet, and the temperature sensors  15   b  and  15   c  do not sense the passage of the printing sheet. Therefore, only the center coil  13   a  is energized, and hence only the center portion of the fixing belt is heated, so that an adequate temperature control is achieved. However, when a printing sheet P 2  having a width W 2  passes the induction heat device, the passage of the printing sheet is sensed or not sensed depending on the positions where the temperature sensors  15   b  and  15   c  are installed. For example, it is assumed that the temperature sensor  15   b  is provided at the center of the side coil  13   b  and the temperature sensor  15   c  is provided on the outer end of the side coil  13   c  as shown in  FIG. 1 . In this case, since the temperature sensor  15   c  is provided at the end, the temperature fall is not sensed even though the printing sheet P 2  passes and the side coil  13   c  is not heated. In contrast, since the temperature sensor  15   b  is provided at the substantially center of the side coil  13   b , the temperature sensor  15   b  senses the temperature fall due to the printing sheet P 2  and the passage of the printing sheet is sensed, so that the side coil  13   b  is heated. However, since the printing sheet does not pass a portion outside the center of the side coil  13   b , the temperature does not fall, and hence the overheated state is resulted. 
         [0015]    In particular, when heating directly from the outside of the fixing belt by an electromagnetic induction heating, the fixing belt (heat belt) as a heating object have a low heat capacity in many cases. Therefore, if the heating by the respective divided side coils continues, the temperature of the non-sheet-passing portion of the fixing belt might rise. Also, if an attempt is made to restrain the temperature rise of the non-sheet-passing portion by lowering the temperature of the side end portion of the fixing belt to be induction heated, the temperature rise in the non-sheet-passing both end portions is restrained, but the temperature of the fixing belt in the inner end of the side coils (the end portions of the side coils  13   b  and  13   c  near the center coil  13   a  in  FIG. 1 ) is also lowered. Therefore, the temperature of the portions of the fixing belt in the widthwise direction corresponding to the side coils  13   b  and  13   c  where the printing sheet passes falls, there is a problem such that the defective fixation in these portions or the defective image quality such that the gloss of the printed image is fluctuated might occur in a portion corresponding to the center coil and portions of the side coils near the center coil. 
         [0016]    The invention provides a fixing device which solves the problems of the fixing device of a separate induction heat type using a fixing belt or a fixing roller in the related art. 
       SUMMARY 
       [0017]    According to a first aspect of the invention, there is provided a fixing device configured to fix a toner image on a moving printing medium and including: a fixing heat generator that generates heat by electromagnetic induction; a fixing medium that rotates and is heated while being carried by the heating medium; and a pressing member that comes into press contact with a back surface of the printing medium which is brought into contact with the fixing medium, the fixing heat generator including a center coil and a pair of side coils provided at a center portion and both side portions thereof in terms of a vertical direction with respect to the direction of movement of the printing medium for heating the fixing medium by induction heating, the pressing member including a center heater and a side heater that heat the center portion and the both side portions thereof in the vertical direction in terms of the direction of movement of the printing medium respectively integrated therein, a side temperature sensor that senses at least one of the temperatures of the both side portions in terms of a widthwise direction vertical to the direction of movement of the printing medium, and a heater control unit that turns OFF the side heater if the temperature that the side temperature sensor senses exceeds a predetermined temperature. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a drawing for explaining a problem of a fixing device in the related art. 
           [0019]      FIG. 2  is a drawing showing a relation between an induction heat device and a press roller in an embodiment of the invention. 
           [0020]      FIG. 3  is a cross-sectional view showing a structure of the fixing device according to the embodiment of the invention. 
           [0021]      FIG. 4  is a drawing for explaining an operation of the fixing device according to the embodiment of the invention. 
           [0022]      FIG. 5  is a drawing showing the timing of the passage of electric current through a heater integrated in the press roller according to the embodiment of the invention. 
           [0023]      FIG. 6  is a drawing showing the timing of the passage of electric current through a coil of an induction heat device according to the embodiment of the invention. 
           [0024]      FIG. 7  is a drawing for explaining an operation of another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Referring now to the drawings, an embodiment of an image forming apparatus according to the invention will be described. The invention is applied to a case of heating a fixing belt and a fixing roller by an electromagnetic induction heating (IH) using divided coils, and is specifically effective for a fixing device in which the fixing belt and the fixing roller is directly heated from the outside. 
         [0026]    Referring now to  FIG. 2  and  FIG. 3 , a configuration of the fixing device according to the embodiment of the invention will be described.  FIG. 3  is a cross-sectional view showing the structure of the fixing device. 
         [0027]    The fixing device includes a fixing roller  30 , a band-shaped fixing belt  31  wound around the fixing roller  30  and heated, a tension roller  32  around which the fixing belt  31  is wound for providing a tension to the belt, an induction heat coil  33  that heats the fixing belt  31  directly from the outside by the electromagnetic induction heating, an induction heat power source  34  that supplies a power to the induction heat coil  33 , a fixing belt temperature sensor  35  that senses the surface temperature of the fixing belt  31 , a fixing belt temperature control unit  36  that controls the induction heat power source  34  for controlling the temperature of an outer surface of the fixing belt according to the temperature sensed by the fixing belt temperature sensor  35 , a press roller  37  provided so as to oppose the fixing roller  30  on which the fixing belt  31  is wound and brought into press contact with a printing sheet P from the back side thereof, a center heater  38   a  and a side heater  38   b  integrated in the press roller  37 , a press roller temperature sensor  39  that senses the temperatures of the outer surfaces of the press roller  37 , and a heater control unit  40  that controls the passage of electric current through the center heater  38   a  and the side heater  38   b  on the basis of the temperature that the press roller temperature sensor  39  senses. 
         [0028]      FIG. 2  shows a relation between the structure of the induction heat coil  33  and the fixing belt temperature sensor  35 , and a relation between the press roller  37  and the press roller temperature sensor  39 . As shown in  FIG. 2 , the induction heat coil  33  is divided into three parts in the axial direction and includes one center coil  33   a  and two side coils  33   b  and  33   c  disposed on both sides thereof. Part or the entire portion of the coils are driven according to the size of the printing sheet, and the fixing belt  31  is heated in the widthwise direction correspondingly. 
         [0029]    The induction heat coil  33  is divided into the center coil  33   a  that heats the center and side coils  33   b  and  33   c  that heat the sides, and the fixing belt  31  is electromagnetic induction heated by the divided center coil  33   a  and the side coils  33   b  and  33   c . The center coil  33   a  and the side coils  33   b  and  33   c  are alternately and repeatedly driven to perform the electromagnetic induction heating, and repeat heating continuously. 
         [0030]    The fixing belt temperature sensor  35  includes a fixing belt center temperature sensor  35   a  provided at a position corresponding to the center of the center coil  33   a  of the fixing belt  31 , a fixing belt side temperature sensor  35   b  provided at a position corresponding to the center of the side coil  33   b , and a fixing belt abnormal temperature sensor  35   c  provided near the outer end of the side coil  33   c  for sensing abnormality. 
         [0031]    The press roller  37  which is opposed to the fixing belt  31  and is to be brought into press contact therewith includes the center heater  38   a  having a heating portion that mainly heats the axially center portion of the surface thereof, and the side heater  38   b  having a heating portion that mainly heats both end portions integrated therein. The heating portion of the center heater  38   a  corresponds to the center coil  33   a  of the induction heat coil  33 , and the heating portions of the side heater  38   b  correspond to the side coils  33   b  and  33   c  of the induction heat coil  33 . 
         [0032]    The press roller temperature sensor  39  that senses the surface temperature of the press roller  37  includes a press roller center temperature sensor  39   a  provided near the center of the press roller for sensing the temperature of the center portion thereof, a press roller side temperature sensor  39   b  provided near the center of one of the heating portions of the side heater  38   b , and a press roller abnormal temperature sensor  39   c  provided near an end of the other heating portion of the side heater  38   b.    
         [0033]    The surface temperature sensed axially of the press roller  37  by the press roller center temperature sensor  39   a  and the press roller side temperature sensor  39   b  is inputted to the heater control unit  40 , and the heater control unit  40  selectively passes electric current through the center heater  38   a  and the side heater  38   b  correspondingly. In other words, if only the press roller center temperature sensor  39   a  senses the temperature fall of the surface of the press roller  37 , the heater control unit  40  passes electric current through the center heater  38   a , and if the press roller center temperature sensor  39   a  and the press roller side temperature sensor  39   b  sense the temperature fall of the surface of the press roller  37 , the heater control unit  40  passes electric current through the center heater  38   a  and the side heater  38   b.    
         [0034]    The fixing belt center temperature sensor  35   a , the fixing belt side temperature sensor  35   b , the fixing belt abnormal temperature sensor  35   c , the press roller center temperature sensor  39   a , the press roller side temperature sensor  39   b , and the press roller abnormal temperature sensor  39   c  are configured of a thermistor or a thermopile. The fixing belt abnormal temperature sensor  35   c  and the press roller abnormal temperature sensor  39   c  are temperature sensors for sensing abnormal heating of the side portions of the side coil  33   c  and the side heater  38   b.    
         [0035]    The fixing belt center temperature sensor  35   a  and the press roller center temperature sensor  39   a  sense the temperature change (rise and fall) caused by the passage of sheets in the center portions of the center coil  33   a  and the press roller  37 . The fixing belt side temperature sensor  35   b  and the press roller side temperature sensor  39   b  sense the temperature change due to the passage of sheets in the side end portions of the side coil  33   b  and the press roller. 
         [0036]    The temperature fall which occurs when the printing sheet passes may be sensed on one of the IH coil side or the press roller side. Therefore, if the press roller center temperature sensor  39   a  and the press roller side temperature sensor  39   b  are provided, the fixing belt center temperature sensor  35   a  and the fixing belt side temperature sensor  35   b  are not necessarily required. Since the center coil  33   a  and the side coils  33   b  and  33   c  achieve significant thermal fluctuations since they are adapted to heat by AC current passed therethrough, and the temperature sensors  39   a  and  39   b  on the side of the press roller exhibit less abrupt fluctuations in sensed temperatures than the temperature sensors  35   a  and  35   b  on the side of the IH coil, so that the stable temperature detection is advantageously ensured. Description of this embodiment will be given below under the assumption that the temperature is sensed by the temperature sensors  39   a  and  39   b  on the side of the press roller. 
         [0037]    It is assumed that the induction heat coil  33  is induction-heated by an alternate drive system which drives the center coil  33   a  and the side coils  33   b  and  33   c  alternately. 
         [0038]    Referring now to a flowchart shown in  FIG. 4 , an operation of this embodiment of the invention will be described on the basis of the width of the printing sheet separately for (A) the case of a sheet width W 4  and (B) the case of a sheet width W 5  shown in  FIG. 2 . (A) The case of sheet width W 4  shown in  FIG. 2   
         [0039]      FIG. 5  shows a state in which the center heater  38   a  in the press roller  37  is heated (part (A)) and a state in which the side heater  38   b  is heated (part (B)). In other words, as is clear from the  FIG. 2 , the part (A) in  FIG. 5  shows the case where the center portion of the press roller  37  is heated and the part (B) in  FIG. 5  shows the case where the both side portions of the press roller  37  are heated. 
         [0040]    A time TI 51  in the parts (A) and (B) in  FIG. 5  shows a state in which the press roller  37  is heated to a constant temperature. The reason why the timing of the passage of electric current is opposite between the center heater  38   a  and the side heater  38   b  (ON state and OFF state appear at opposite timings) is because the alternate drive system is employed. By employing the alternate drive system, high-power lamps may be used in the center portion and the side portion. In other words, there is a limit in available power. For example, assuming that the available power is 300 W, if the center portion and the side portion are turned ON simultaneously, only the lamps of 150 W maximum each can be used in a simultaneous drive system. In contrast, by employing the alternate drive system, the high-power lamps of 300 W may be used respectively in the center portion and the side portion. 
         [0041]      FIG. 6  shows the timing of the passage of electric current through the center coil  33   a  (part (A)), and the timing of the passage of electric current through the side coils  33   b  and  33   c  (part (B)). In other words, the center coil and the side coil are driven alternately. The side coil  33   b  and the side coil  33   c  are handled completely in the same manner, and are passed with the electric current therethrough at the same timing. A time TI 61  shows a case of maintaining the constant temperature. In the case of the alternate drive system, since the center coil  33   a  and the side coils  33   b  and  33   c  are driven alternately, the center portion and the side portion of the fixing belt  31  are heated alternately. 
         [0042]    As the percentage of heating between the center portion and the side portion of the fixing belt  31 , the periods of the passage of electric current through the center coil  33   a  and the side coils  33   b  and  33   c  are changed according to a duty ratio, and an optimal duty ratio is selected. Therefore, in the time TI 61  in which the predetermined temperature is maintained, a current-passing time T 61   c  through the center coil  33   a  (turned ON) and a time T 61   s  in which electric current is passed through the side coils  33   b  and  33   c  are the same, and the duty ratio of the passage of electric current is T 61   c /(T 61   c +T 61   s ), which is 50 percent. 
         [0043]    In  FIG. 4 , the press roller center temperature sensor  39   a  and the press roller side temperature sensor  39   b  sense (measure) the temperature of the surface of the press roller  37  at a predetermined temporal intervals (timing). In Act A 401 , whether there comes such the temperature sensing timing or not is sensed. If there comes the temperature sensing timing, the press roller center temperature sensor  39   a  senses a temperature Tpc of the surface of the press roller  37  in Act A 402 . 
         [0044]    Subsequently, in Act A 403 , the press roller side temperature sensor  39   b  senses a temperature Tps of the surface of the press roller  37 . The sensed temperature Tpc and Tps are inputted to the heater control unit  40 . In Act A 404 , the heater control unit  40  obtains a temperature difference ΔTp between the sensed temperature Tps and the sensed temperature Tpc. 
         [0000]    
       
      
       Tps−Tpc=ΔTp  
      
     
         [0045]    Then, in Act A 404 , the heater control unit  40  determines whether the temperature difference ΔTp is 10° C. or higher. 
         [0046]    There, the reason to inspect the temperature difference between the press roller center temperature sensor  39   a  and the press roller side temperature sensor  39   b  will be described. In the case (A), since the side end of the printing sheet is positioned inside the press roller side temperature sensor  39   b , the press roller side temperature sensor  39   b  which is located outside thereof cannot sense the temperature fall at the time of the passage of the printing sheet accurately, so that the press roller  37  is increased in temperature to a relatively high temperature. In contrast, since the printing sheet comes into contact with the portion of the press roller that the press roller center temperature sensor  39   a  senses, the temperature does not rise, and hence continuous heating is necessary. 
         [0047]    In Act A 404 , if the temperature difference ΔTp=(Tps−Tpc) is 10° C. or higher, the procedure goes to Act A 405 , where the minimum value of the duty ratio of the passage of electrical current between the side coils  33   b  and  33   c  is, for example, increased. 
         [0048]    When the printing sheet having a width wider than that of the center coil  33   a  and narrower than that of the center position of the side coil  33   b  (W 4 ) is passed continuously, the printing sheets absorb heat from the area of the fixing belt  31  where the sheets pass. Therefore, in the longitudinal direction of the fixing belt  31 , the temperature falls and is brought into a state of being controlled in temperature. In contrast, in the area where the printing sheets do not pass, heat supply to the printing sheets is not carried out, and hence the temperature rises due to the excessive heat. 
         [0049]    The center coil  33   a  and the side coils  33   b  and  33   c  are driven alternately, and the induction heating is carried out constantly by passing electric current through either one of the center coil  33   a  and the side coils  33   b  and  33   c , so that the area of the fixing belt  31  where the sheets do not pass is also heated constantly. 
         [0050]    If the passage of electrical current through the side coils  33   b  and  33   c  is cut, the temperature of the areas of the side coils  33   b  and  33   c  where the sheets pass (SW 4 ) also falls, so that heating of the side coils is necessary. 
         [0051]    When the drive system of the induction heat coil is the simultaneous drive system, since the temperature maintenance in the side heating areas of the fixing belt  31  where the sheets pass is difficult if the passage of electrical current through the side coils is cut, heating is necessary. 
         [0052]    When the width of the printing sheet is W 4  in  FIG. 2 , since the press roller side temperature sensor  39   b  is positioned out of the area where the sheet pass, it is determined that the heat is not absorbed by the passage of the printing sheet, whereby the temperature is controlled to be maintained or to be lowered. Actually, however, parts SW 4  of the side coils  33   b  and  33   c  are included in an area where the sheets pass, the temperature in this area falls and hence a state of requiring heat supply to enable the fixation of the toner image is resulted. 
         [0053]    In this case, when the sheets are passed continuously, a duty ratio is selected so as to maintain the temperature for the center portion of the fixing belt  31 , and keep the temperature in the area where the sheets pass within a range in which the offset does not occur and hence the defective image is not made and the temperature rise is minimized in the non-sheet-passing portion for the side portions. 
         [0054]    However, in order to maintain the temperature of the areas in the side portions of the fixing belt  31  where the sheets pass within the range in which the offset does not occur and hence the defective image is not made, it is necessary to heat the side portions of the fixing belt  31  by distributing the duty ratio to the side coils  33   b  and  33   c  at a ratio larger than a certain level. In this manner, if electric current is passed through the side coils  33   b  and  33   c  so as to heat the area where the sheets pass, the areas of the side portions of the fixing belt  31  where the sheets do not pass is brought into an excessively heated state, whereby the temperature rise might occur. 
         [0055]    In this manner, while the temperature is controlled in the center portion of the fixing belt  31  so as to prevent the temperature fall due to the passage of the sheets, the press roller side temperature sensor  39   b  does not sense the temperature fall due to the passage of the sheets in the side portions and hence the temperature rises continuously. Therefore, the difference between the temperature Tps that the press roller side temperature sensor  39   b  senses and the temperature Tpc that the press roller center temperature sensor  39   a  senses is increased. 
         [0056]    The reason why the threshold temperature as a criterion of the temperature difference is set to 10° C. is because the erroneous control may be resulted due to the temperature fluctuations such as temperature ripples or the control temperature unit if it is lower than 10° C. 
         [0057]    It is also possible to set the threshold temperature as the criterion of the temperature difference ΔTp is a value lower than 10° C. depending on the degree of uniformization of the temperature ripple or the temperature fluctuation in the longitudinal direction, and might be set to 10° C. or higher depending on the environment. 
         [0058]    If Tps−Tpc is lower than 10° C. in the next Act A 404 , the procedure goes back to Act A 401 , where the next timing of the temperature sensing is waited. 
         [0059]    If the fixation to the printing sheets is continuously carried out, the sensed temperature Tpc in the center portion of the press roller  37  is lowered, and the sensed temperature Tps outside the side portions of the press roller  37  rises, so that the temperature difference ΔTp between these temperatures is gradually increased. In contrast, the temperature inside the side portions of the press roller  37  falls. 
         [0060]    Therefore, if the temperature difference ΔTp reaches or exceeds 10° C. in Act A 404 , the minimum value of the duty ratio of the passage of electric current through the side coils  33   b  and  33   c  is heightened to increase the temperature in Act A 405 . This will be described on the basis of the timing chart of a time TI 62  in  FIG. 6 . The current-passing time of the center coil  33   a  at this time is T 62   c  and the current-passing time of the side coils  33   b  and  33   c  is T 62   s . Therefore, the duty ratio of the passage of electrical current through the side coils  33   b  and  33   c  is T 62   s /(T 62   c +T 62   s ). For example, T 62   c =180 msec, and T 62   s =20 msec. The duty ratio of the passage of electrical current through the side coils  33   b  and  33   c  at this time is 10%. 
         [0061]    In this state, if the ΔTp reaches or exceeds 10° C., for example, the current-passing time through the center coil and the side coils are controlled to achieve, for example, T 62   c =160 msec and T 62   s =40 msec. The duty ratio of the passage of electrical current through the side coils  33   b  and  33   c  at this time is T 62   s /(T 62   c +T 62   s )=20%. In other words, the temperature fall in the portion SW 4  in  FIG. 2  is prevented by increasing supply of electric power to the side coils  33   b  and  33   c  and reducing the supply of electric power to the center coil  33   a.    
         [0062]    In the time TI 62  in  FIG. 6 , as indicated by arrows  64  and  65 , the current-passing time to the center coil  33   a  is reduced and the current-passing time to the side coils  33   b  and  33   c  is increased. It means that the temperature fall is resulted since the printing sheet passes through the SW 4  in  FIG. 2 , so that the side coils are further heated in order to rise the temperature of this portion. However, since a feedback control is carried out on the basis of the sensed temperature, the overheated state is not resulted. 
         [0063]    Although not shown in the drawing, the heater control unit  40  includes a duty ratio control unit that prevents the temperature fall due to the printing sheet passing through the inside the side temperature sensor by increasing the duty ratio of the passage of electrical current through the side coils  33   b  and  33   c  if the temperature difference (Tps−Tpc) is a predetermined temperature difference or more as a result of subtracting the temperature Tpc sensed by the center temperature sensor from the temperature Tps sensed by the side temperature sensor. There is also a method of keeping at a constant temperature in Act A 405 . 
         [0064]    Since the temperature Tps that the press roller side temperature sensor  39   b  senses rises in this manner, in the next Act A 406 , the heater control unit  40  shown in  FIG. 3  determines whether the sensed temperature Tps is equal to or higher than a predetermined temperature Tth or not. When the sensed temperature Tps is lower than the predetermined temperature Tth, the procedure goes back to Act A 401 , where the next timing of temperature sensing is waited. 
         [0065]    “When the sensed temperature Tps is lower than the predetermined temperature Tth” means that the side portions of the fixing belt  31  is not heated to an excessively high temperature. In contrast, if the temperature Tps that the press roller side temperature sensor  39   b  senses is the predetermined temperature Tth or higher, it means that the temperature of the side portions of the fixing belt  31  is excessively high. 
         [0066]    If it is sensed that the sensed temperature Tps reaches or exceeds the predetermined temperature Tth, the side heater  38   b  is turned OFF in the next Act A 407 . Then, in the Act A 408 , the next timing of temperature sensing is waited. 
         [0067]    When the next timing of temperature sensing arrives, in the next Act A 410 , the press roller side temperature sensor  39   b  senses the temperature Tps of the surface of the side portion of the press roller  37  as in Act A 403  before. 
         [0068]    Then, in the next Act A 411 , whether or not the sensed temperature Tps is lowered to a temperature lower than the predetermined temperature Tth is determined. If the sensed temperature Tps is still equal to or higher than the predetermined temperature Tth, the side heater  38   b  is kept in OFF and the procedure goes back to Act A 408 , where the next timing of sensing is waited. If the Tps is lowered to a temperature lower than the predetermined temperature Tth, the operation is restored to the normal control in Act A 412 . The predetermined temperature Tth is normally set to a temperature higher than the temperature Tpc that the press roller center temperature sensor  39   a  senses. (B) The case of printing sheet width W 5  in  FIG. 2   
         [0069]    In this case, since the printing sheet extends to the outside the press roller side temperature sensor  39   b , the press roller side temperature sensor  39   b  is able to sense the temperature fall when the printing sheet passes. 
         [0070]    In the case of the alternate drive system, since the center coil  33   a  and the side coils  33   b  and  33   c  are driven alternately, the center portion and the side portions of the fixing belt  31  are heated alternately. As the percentage of heating between the center portions and the side portion of the fixing belt  31 , the periods of the passage of electric current through the center coil  33   a  and the side coils  33   b  and  33   c  are changed according to the duty ratio, and an optimal duty ratio is selected. 
         [0071]    In the case of the induction heating system from the outside, the heated portion of the fixing belt  31  to be heated by the electromagnetic induction cope with a rapid heating, and hence has a small heat capacity. Therefore, if the passage of electrical current of the induction heating is cut, the temperature falls abruptly, and if the passage of electrical current is started again, the temperature rises abruptly, whereby significant temperature ripples are resulted. In the induction heating, as many as several seconds of time lag occurs from the start of the passage of electrical current until a predetermined output is reached. Therefore, in view of maintaining the temperature at a constant level, it is preferable to maintain the passage of electrical current and avoid the passage of electrical current from stopping as much as possible. It is also the reason of not stopping the passage of electrical current through the side coils in this embodiment of the invention. 
         [0072]    If the fact that the timing to sense the temperature arrives is detected in Act A 401 , the press roller center temperature sensor  39   a  senses the temperature of the center portion of the press roller  37  in Act A 402  and the sensed temperature Tpc is obtained as described above in (A). In the subsequent Act A 403 , the press roller side temperature sensor  39   b  senses the temperature of the side portion of the press roller  37 , and the sensed temperature Tps is obtained. In Act A 404 , whether the difference between the both sensed temperatures (Tps−Tpc)=ΔTp is 10° C. or higher is determined. 
         [0073]    If the width of the printing sheet corresponds to W 5 , the difference between the sensed temperature Tpc that the press roller center temperature sensor  39   a  senses and the sensed temperature Tps sensed by the press roller side temperature sensor  39   b  is normally small, and does not reach or exceed 10° C. Therefore, the procedure goes to Act A 406 , where the heater control unit  40  shown in  FIG. 3  determines whether the sensed temperature Tps that the press roller side temperature sensor  39   b  senses reaches or exceeds the predetermined temperature Tth. When the sensed temperature Tps is lower than the predetermined temperature Tth, the procedure goes back to Act A 401 , where the next timing of temperature sensing is waited. 
         [0074]    If the sensed temperature Tps reaches or exceeds the predetermined temperature Tth, the procedure goes from Act A 406  to Act A 407 , where the side heater  38   b  is turned OFF. The side heater  38   b  has a function to heat the side portions of the press roller  37  as shown in  FIG. 2 . Therefore, the sensed temperature Tps that the press roller side temperature sensor  39   b  senses should be lowered. After the side heater  38   b  is turned OFF, the procedure goes to Act A 408 , where the next timing of temperature sensing is waited. In Act A 410 , the press roller side temperature sensor  39   b  senses the temperature of the side portion of the press roller  37  as described above, and in Act A 411 , whether or not the sensed temperature Tps is lowered to a temperature lower than the predetermined temperature Tth. If no, the next timing of sensing is waited in the state in which the side heater  38   b  is turned OFF. 
         [0075]    In Act A 411 , if the sensed temperature Tps is lowered to a temperature lower than the predetermined temperature Tth, the normal temperature control is restored. 
         [0076]    In Act A 407 , the side heater  38   b  is turned OFF. However, it is to be noted that the side coils  33   b  and  33   c  of the induction heat coil  33  are driven as in the normal state. As described above, the heat capacity of the induction heat coil  33  is small. Therefore, if the passage of electrical current through the side coils  33   b  and  33   c  of the induction heat coil  33  is stopped or reduced, the temperature of the side portions of the transfer belt falls abruptly, and if the side portions of the transfer belt is heated for compensating the temperature fall, the temperature thereof rises abruptly this time, so that abrupt temperature variations are resulted. In contrast, the heat capacity of the press roller  37  is relatively large. Therefore, even though the heating of the side portions of the press roller  37  is stopped temporarily, the temperature falls gradually. This is the reason why turning OFF the side heater  38   b  without stopping the driving of the side coils  33   b  and  33   c  is preferable. 
         [0077]    While the side heater  38   b  on the side of the press roller  37  is turned OFF, heating of the side coil on the side of the fixing belt  31  is continued, and the temperature of the entire fixing belt  31  is kept uniformly, so that the disadvantages such as partially defective fixation of images or uneven gloss of the image between the center portion and the side portions are prevented. Even when the temperature of the sheet passing portion SW 5  of the fixing belt  31  is lowered when the side heater  38   b  on the side of the press roller  37  is turned OFF, the duty ratio of the passage of electrical current through the center coil  33   a  and the side coils  33   b  and  33   c  is changed, so that the temperature fall in the side portions of the fixing belt  31  is prevented. Accordingly, maintenance of the temperature in the sheet passing portion is enabled while lowering the temperature of the non-sheet-passing portion of the fixing belt  31 , which is specifically effective for the alternate driving in the separated induction heating system. 
         [0078]    While the side heater  38   b  on the side of the press roller  37  is turned OFF, heating of the side portions on the side of the fixing belt  31  is continued, and the temperature of the entire fixing belt  31  is kept uniformly, so that the disadvantages such as partially defective fixation of images or uneven gloss of the image between the center portion and the side portions are prevented. 
         [0079]    Also, when the induction heating employs the simultaneous drive system, the passage of electrical current is maintained as in the case of the alternate drive since the side portions of the fixing belt  31  need to be heated while the sheet is passing, so that the same control as described above may be applied. 
       Other Embodiment 
       [0080]    In the embodiment described above, the duty ratio of the passage of electrical current through the side coils  33   b  and  33   c  is changed when the difference between the temperature Tpc that the press roller center temperature sensor  39   a  senses and the temperature Tps that the press roller side temperature sensor  39   b  senses reaches or exceeds the predetermined temperature difference (10° C.), and the side heater  38   b  is turned OFF if the temperature Tps that the press roller side temperature sensor  39   b  senses is equal to or higher than the predetermined temperature Tth only when the temperature difference is within the predetermined temperature difference. However, it is also applicable to turn the side heater  38   b  OFF when the sensed temperature Tps is equal to or higher than the predetermined temperature without obtaining the temperature difference between the sensed temperatures Tpc and Tps. 
         [0081]    Referring now to a flowchart in  FIG. 7 , an operation of another embodiment as described above will be described. The configuration of the fixing device in this embodiment is the same as that in the embodiment descried above, and is shown in  FIG. 2  and  FIG. 3 . 
         [0082]    In Act A 701 , whether there comes the temperature sensing timing on the basis of the predetermined time at regular intervals is determined. If there comes the sensing timing, the press roller side temperature sensor  39   b  senses the temperature of the side portion of the fixing belt  31  in Act A 702 . It is assumed that the temperature sensed at this time is Tps. The heater control unit  40  determines whether the sensed temperature Tps is equal to or higher than the predetermined temperature Tth or not (A 703 ). If the sensed temperature Tps does not exceed the predetermined temperature Tth, the procedure goes back to Act A 701 , where the next timing of temperature sensing is waited. 
         [0083]    If it is sensed that the sensed temperature Tps is equal to or higher than the predetermined temperature Tth in Act A 703 , that is, if the fixing belt  31  is about to be overheated, the procedure goes to Act A 704 , where the side heater  38   b  is turned OFF. Then, the procedure goes to Act A 705 , where the next timing of temperature sensing is waited. In Act A 706 , the press roller side temperature sensor  39   b  senses the temperature Tps of the side portion of the press roller  37  as in Act A 702 . 
         [0084]    Then, in Act A 707 , whether the sensed temperature Tps is lowered to a temperature lower than the predetermined temperature Tth is sensed. As long as the sensed temperature Tps is equal to or higher than the predetermined temperature Tth, the side heater  38   b  is continuously kept in a state of being turned OFF, and in Act A 705 , the next timing of temperature sensing is waited. On the other hand, when the fact that the sensed temperature Tps is lowered to a temperature lower than the predetermined temperature Tth is sensed, the normal fixing temperature control is restored. 
         [0085]    According to this embodiment, if the width of the printing sheet is small and the both side portions in terms of the direction vertical to the direction of rotation of the fixing belt  31  (widthwise direction) are heated, the side heater provided in the press roller is turned OFF. Therefore, overheating of the both side portions is prevented. In addition, since the temperature of the both side portions is lowered gradually, it has an advantage such that the temperature variations are small. 
         [0086]    In the embodiment described above, the separated structure having one coil at the center and two side coils on both sides thereof is employed as the induction heat device. However, two or four or more divided coils may be used as the induction heating device in the invention. 
         [0087]    In the embodiment descried above, the fixing device using the fixing belt and the press roller is described. However, it is also applicable to employ a fixing roller instead of the fixing belt and heat the surface of the fixing roller directly from the outside by the electromagnetic induction heating. Also, in the embodiment described above, the device configured to bring the press roller into contact with the back surface of the printing sheet is described. However, it is also applicable to use a press belt instead of the press roller. 
         [0088]    In the embodiment descried above, the case of using the alternate drive system which drives the center coil and the side coils of the induction heat device is described. However, the simultaneous drive system which passes electric current having the same phase through the both coils may be employed. In the embodiment described above, the device of a (simultaneous drive) system in which electric current of the same phase is passed through the side coils provided on the both sides of the center coil is described. However, these side coils may be driven independently in the invention. 
         [0089]    Obviously, many modifications and variations of this invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, this invention may be practiced otherwise than as specification.