Patent Publication Number: US-7907870-B2

Title: Fixing apparatus and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-068201 filed Mar. 17, 2008. 
     BACKGROUND OF THE PRESENT INVENTION 
     1. Technical Field 
     The present invention relates to a fixing apparatus and an image forming apparatus. 
     2. Related Art 
     Among fixing apparatuses each provided in an image forming apparatus such as a printer and a copier, there is one by an electromagnetic induction heat-generating method using, as heat sources, a coil that generates a magnetic field by energization and a heat generating body that generates heat by generating eddy current by electromagnetic induction of the magnetic field. 
     SUMMARY 
     A fixing apparatus of a first aspect of the present invention includes a magnetic-field generating part that generates a magnetic field, a fixing rotating body having a heat generating layer that generates heat by an electromagnetic induction action of the magnetic field, a pressurizing member that applies pressure to an outer circumferential surface of the fixing rotating body, a heating member that is arranged in contact with an inner side of the fixing rotating body so as to be opposed to the magnetic-field generating part, and heats the fixing rotating body, and a temperature sensing part that is located within a region where the fixing rotating body is opposed to the magnetic-field generating part and where the fixing rotating body is in contact with the heating member and senses a temperature of the fixing rotating body. 
     “Within the region where the fixing rotating body is in contact with the heating member” means a region smaller than a maximum range in the circumferential direction where the fixing rotating body and the heating member are in contact, and a notched portion may be included in the region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is an overall diagram of an image forming apparatus according to a first exemplary embodiment of the invention; 
         FIG. 2  is a cross-sectional diagram of a fixing apparatus according to the first exemplary embodiment of the invention; 
         FIG. 3A  is a cross-sectional diagram of a fixing belt according to the first exemplary embodiment of the invention, and  FIG. 3B  is a perspective diagram of a heat generating body according to the first exemplary embodiment of the invention. 
         FIG. 4A  is a partial cross-sectional diagram of the fixing apparatus according to the first exemplary embodiment of the present invention, and  FIG. 4B  is a connection diagram of a control circuit and an energization circuit according to the first exemplary embodiment of the invention; 
         FIG. 5A  is a schematic diagram showing temperature sensing positions of the fixing belt and the heat generating body according to the first exemplary embodiment of the invention, and  FIG. 5B  is a graph showing a relation between the temperature sensing positions and the temperature of the fixing belt and the heat generating body according to the first exemplary embodiment of the invention; 
         FIGS. 6A and 6B  are plane diagrams each showing another example of the heat generating body according to the first exemplary embodiment of the invention; 
         FIG. 7  is a cross-sectional diagram of a fixing apparatus according to a second exemplary embodiment of the present invention; 
         FIG. 8A  is a perspective diagram of a heat generating body according to the second exemplary embodiment of the invention, and  FIG. 8B  is a partial cross-sectional diagram of the fixing apparatus according to the second exemplary embodiment of the invention; 
         FIG. 9  is a graph showing a relation between positions in a width direction of the fixing belt and a fixing belt temperature according to the second exemplary embodiment of the invention; and 
         FIG. 10  is a plane diagram showing another example of the heat generating body according to the second exemplary embodiment of the invention. 
     
    
    
     DESCRIPTION 
     An example of a first exemplary embodiment of a fixing apparatus and an image forming apparatus of the present invention is described based on the drawings. 
       FIG. 1  shows a printer  10  as the image forming apparatus. In the printer  10 , optical scanning devices  14 Y,  14 M,  14 C and  14 K that emit optical beams corresponding to respective toners of yellow (Y), magenta (M), cyan (C), and black (K) are fixed inside of a housing  12  making up a body of the printer  10 . In a position adjacent to the optical scanning device  14 K, a controller  70  that controls operations of the respective parts of the printer  10  is provided. 
     The optical scanning devices  14 Y,  14 M,  14 C and  14 K scan optical beams emitted from light sources by rotating polygon mirrors (not shown), and the optical beams are reflected by a plurality of optical parts such as reflecting mirrors, so that optical beams  16 Y,  16 M,  16 C and  16 K corresponding to the respective toners are emitted. 
     The optical beams  16 Y,  16 M,  16 C and  16 K are guided to respective corresponding photoreceptors  18 Y,  18 M,  18 C and  18 K. The photoreceptors  18 Y,  18 M,  18 C and  18 K are each rotated in an arrow A direction by drive means made of a motor and a gear (not shown). 
     Chargers  20 Y,  20 M,  20 C and  20 K that charge surfaces of the photoreceptors  18 Y,  18 M,  18 C and  18 K are provided on the upstream side in a rotation direction of the photoreceptors  18 Y,  18 M,  18 C and  18 K. Also, developing units  22 Y,  22 M,  22 C and  22 K that develop the respective toners of Y, M, C and K on the photoreceptors  18 Y,  18 M,  18 C and  18 K are provided on the downstream side in the rotation direction of the photoreceptors  18 Y,  18 M,  18 C and  18 K. 
     An intermediate transfer belt  28  to which developed toner images are primarily transferred is arranged on the downstream side of the developing units  22 Y,  22 M,  22 C and  22 K in the rotation direction of the photoreceptors  18 Y,  18 M,  18 C and  18 K. The intermediate transfer belt  28  is made of a film-like endless belt in which an antistatic agent such as carbon black is contained in a proper amount in a resin such as polyimide and polyamide. 
     Inside of the intermediate transfer belt  28  and in positions where the photoreceptors  18 Y,  18 M,  18 C and  18 K and the intermediate transfer belt  28  are opposed, the primary transfer rolls  24 Y,  24 M,  24 C and  24 K that transfer the respective color toner images formed on the photoreceptors  18 Y,  18 M,  18 C and  18 K to the intermediate transfer belt  28  are arranged. These primary transfer rolls  24 Y,  24 M  24 C and  24 K make up a primary transfer part  25  that performs the primary transfer from the photoreceptors  18 Y,  18 M,  118 C and  18 K to the intermediate transfer belt  18 . 
     The primary transfer rolls  24 Y,  24 M,  24 C and  24 K each have a shaft, and a sponge layer as an elastic layer fixed around the shaft, which are not shown. The shaft is a columnar rod made of a metal such as iron and SUS. The sponge layer is a cylindrical roll formed of a blend rubber of NBR, SBR and EPDM into which a conducting agent such as carbon black is compounded. 
     Moreover, the primary transfer rolls  24 Y,  24 M,  24 C and  24 K are brought into pressure contact with the respective photoreceptors  18 Y,  18 M,  18 C and  18 K with the intermediate transfer belt  28  sandwiched therebetween. A voltage (primary transfer bias) having a polarity reverse to charging polarity of the respective toners (negative polarity in exemplary embodiment, the same applies to the following) is applied to the primary transfer rolls  24 Y,  24 M,  24 C and  24 K by voltage applying means (not shown). 
     The toner images on the respective photoreceptors  18 Y,  18 M,  18 C and  18 K are sequentially attracted electrostatically, so that the toner images superimposed on the intermediate transfer belt  28  are formed. Cleaners  26 Y,  26 M,  26 C and  26 K that remove remaining toners on the photoreceptors  18 Y,  18 M,  18 C and  18 K are provided on the downstream side in the rotation direction of the photoreceptors  18 M,  18 M,  18 C and  18 K. 
     Inside of the intermediate transfer belt  28 , a drive roll  30  that is driven by a motor (not shown) excellent in speed constancy to move the intermediate transfer belt  28 , and a supporting roll  32  that extends substantially linearly along an arrangement direction of the respective photoreceptors  18 Y,  18 M,  18 C and  18 K to support the intermediate transfer belt  28  are provided. This allows the intermediate transfer belt  28  to be driven circularly at a predetermined speed in an arrow B direction. 
     Moreover, inside of the intermediate transfer belt  28 , a tension roll  34  that provides a constant tension to the intermediate transfer belt  28  and prevents meandering of the intermediate transfer belt  28 . A secondary transfer part  42  that transfers the toner images on the intermediate transfer belt  28  onto recording paper P is provided on the downstream side in the moving direction of the intermediate transfer belt  28 . 
     The secondary transfer part  42  is made of a secondary transfer roll  38  arranged on the side of a toner image carrying surface of the intermediate transfer belt  28  and a back-up roll  36 . 
     The secondary transfer roll  38  is made of a shaft, and a sponge layer as an elastic layer fixed around the shaft, which are not shown. The shaft is a columnar rod made of a metal such as iron and SUS. The sponge layer is a cylindrical roll formed of a blend rubber of NBR, SBR and EPDM into which a conducting agent such as carbon black is compounded. 
     Moreover, the secondary transfer roll  38  is brought into pressure contact with the back-up roll  36  with the intermediate transfer belt  28  sandwiched therebetween. The secondary transfer roll  38  is earthed, and a secondary transfer bias is applied between the secondary transfer roll  38  and the back-up roll  36 , so that the toner images are secondarily transferred on the recording paper P conveyed to the secondary transfer part  42 . 
     In the back-up roll  36 , a surface thereof is made of a tube of a blend rubber of EPDM and NBR with carbon dispersed, and an interior portion thereof is made of EPDM rubber. Hardness is set to 70° (ASKER C), for example. Moreover, the back-up roll  36  is arranged on the side of a back surface of the intermediate transfer belt  28 , to for an opposite electrode of the secondary transfer roll  38 , and the secondary transfer bias is stably applied through a metal electric supply roll  40  arranged in contact with the back-up roll  36 . 
     On the downstream side of the secondary transfer part  42  in the moving direction of the intermediate transfer belt  28 , an intermediate transfer belt cleaner  46  that removes remaining toners or paper powders on the intermediate transfer belt  28  after the secondary transfer is provided in such a manner that it can be brought into contact with, and be separated from the intermediate transfer belt  28 . A cleaning back-up roll  44  is provided inside of the intermediate transfer belt  28  in the intermediate transfer belt cleaner  46 . 
     On the upstream side of the primary transfer roll  24 Y corresponding to the yellow toner and inside of the intermediate transfer belt  28 , a home position sensor  48  that generates a signal as a reference for timing of image formation corresponding to the respective toners is provided. The home position sensor  48  senses a predetermined mark provided on the back side of the intermediate transfer belt  28  to generate the reference signal. Based on this reference signal, the above-described controller  70  operates the respective parts of the printer  10  to start the image formation. Moreover, on the downstream side of the primary transfer roll  24 K corresponding to the black toner, an image density sensor  43  for performing image adjustment is provided. 
     On the other hand, on the lower side of the printer  10 , a paper tray  50  that contains the recording paper P is provided. On one end of the paper tray  50 , a pick-up roll  52  that takes out and conveys the recording paper P at predetermined timing is provided. Above the pick-up roll  52 , a plurality of conveyance rolls  54 ,  56  that are driven rotationally by driving means made of a motor and a gear not shown to convey the recording paper P sent out by the pick-up roll  52  to the above-described secondary transfer part  42  are provided. On the downstream side of the conveyance rolls  56  in a conveyance direction of the recording paper P, a conveyance chute  58  that sends the recording paper P to the secondary transfer part  42  is provided. 
     A conveyance belt  60  that conveys the recording paper P to a fixing apparatus  100  after completing the secondary transfer of the toner images is provided in a sending-out direction of the recording paper P in the secondary transfer part  42 . The conveyance belt  60  is provided so as to be tensioned by tensioning rolls  57  and  59  and be movable by drive means made of a motor or a gear not shown. 
     A guide  62  that guides the recording paper P to the fixing apparatus  100  is provided on the inlet side of the fixing apparatus  100 . On the outlet side of the fixing apparatus  100 , a paper output tray  64  fixed to the housing  12  of the printer  10  is provided. 
     Now, the image formation of the printer  10  is described. 
     Image data outputted from an image reading apparatus, a personal computer or the like not shown is subjected to predetermined image processing by an image processing apparatus not shown. In the image processing apparatus, the predetermined image processing including shading correction, displacement correction, brightness/color space conversion, gamma correction, frame deletion, and various types of image edition such as color edition and movement edition is performed for inputted reflectivity data. The image data subjected to the image processing is converted to colorant gradation data of four colors of Y, M, C and K, and is outputted to the optical scanning devices  14 Y,  14 M,  14 C and  14 K. 
     The optical scanning devices  14 Y,  14 M,  14 C and  14 K irradiate the optical beams  16 Y,  16 M,  16 C and  16 K to the respective photoreceptors  18 Y,  18 M,  18 C and  18 K in accordance with the inputted colorant gradation data. The surfaces of the photoreceptors  18 Y,  18 M,  18 C and  18 K are charged in advance by the chargers  20 Y,  20 M,  20 C and  20 K, and are exposed by the optical beams  16 Y,  16 M,  16 C and  16 K so as to form electrostatic latent images. The formed electrostatic latent images are developed as toner images of the respective colors of Y, M, C and K by the developing units  22 Y,  22 M,  22 C and  22 K. 
     Subsequently, the toner images formed on the photoreceptors  18 Y,  18 M,  18 C and  18 K are transferred onto the intermediate transfer belt  28  in the primary transfer part  25 . This transfer is performed by adding the voltage (primary transfer bias) having the polarity reverse to the charging polarity of the toners (negative polarity) to the intermediate transfer belt  28  by the primary transfer rolls  24 Y,  24 M,  24 C and  24 K and sequentially superimposing the toner images on the surface of the intermediate transfer belt  28 . The intermediate transfer belt  28  to which the toner images are transferred is conveyed to the secondary transfer part  42 . 
     On the other hand, at the timing at which the toner images are conveyed to the secondary transfer part  42 , the pick-up roll  52  is rotated to send out the recording paper P of a predetermined size from the paper tray  50 . The recording paper P sent out by the pick-up roll  52  is conveyed by the conveyance rolls  54 ,  56 , and reaches the secondary transfer part  42  via the conveyance chute  58 . Before reaching this secondary transfer part  42 , the recording paper P is once stopped to perform the alignment between the recording paper P and the toner images by rotating a resist roll (not shown) according to the movement timing of the intermediate transfer belt  28  carrying the toner images. 
     In the secondary transfer part  42 , the secondary transfer roll  38  is pressed by the back-up roll  36  with the intermediate transfer belt  28  sandwiched therebetween. At this time, the recording paper P, which has been timed to be conveyed, is sandwiched between the intermediate transfer belt  28  and the secondary transfer roll  38 . At this time, a voltage (secondary transfer bias) having the same polarity as the charging polarity (negative polarity) of the toners is applied from the electric supply roll  40 , so that a transfer electric field is formed between the secondary transfer roll  38  and the back-up roll  36 . The unfixed toner images carried on the intermediate transfer belt  28  is pressed by the secondary transfer roll  38  and the back-up roll  36  to be electrostatically transferred on the recording paper P collectively. 
     Subsequently, the recording paper P with the toner images transferred electrostatically is conveyed by the secondary transfer roll  38  in a state where the toner images are stripped off from the intermediate transfer belt  28 , and is conveyed to the conveyance belt  60 . In the conveyance belt  60 , the recording paper P is conveyed to the fixing apparatus  100  so that the conveyance speed conforms to an optimal conveyance speed in the fixing apparatus  100 . The unfixed toner images on the recording paper P conveyed to the fixing apparatus  100  are fixed on the recording paper P by the fixing apparatus  100 . The recording paper P after the fixing is discharged in an arrow C direction to be accumulated in the paper output tray  64 . 
     After the transfer to the recording paper P is completed, the remaining toners on the intermediate transfer belt  28  are conveyed to the intermediate transfer belt cleaner  46  with the rotation movement of the intermediate transfer belt  28  to be removed from the intermediated transfer belt  28 . In this manner, the image formation of the printer  10  is performed. 
     Next, a description of the fixing apparatus  100  is given. 
     As shown in  FIG. 2 , the fixing apparatus  100  includes a housing  106  in which openings for taking in and discharging the recording paper P are formed. Inside of the housing  106 , an endless fixing belt  102 , cap-like supporting members (illustration is omitted) are fitted in both side end portions thereof, is supported rotatably in an arrow D direction. 
     A bobbin  108  made of an insulating material is arranged in a position opposed to an outer circumferential surface of the fixing belt  102 . The bobbin  108  is formed into a substantially circular arc following the outer circumferential surface of the fixing belt  102 , and is provided with a projected portion  108 A toward the opposite side of the fixing belt  102 . A distance between the bobbin  108  and the fixing belt  102  is set to 1 to 3 mm. 
     In the bobbin  108 , an exciting coil  110  that generates a magnetic field H by energization is wound in a plurality of times in an axial direction of the bobbin  108  centering on the projected portion  108 A. In positions faced to the exciting coil  110  and at the opposite side of the fixing belt  102 , magnetic-path forming members  112  each made of a magnetic body such as ferrite and formed into a substantially circular arc following the circular arc of the bobbin  108  are arranged and supported by the bobbin  108 . 
     As shown in  FIG. 4A , the plurality of magnetic-path forming members  112  are arranged along a width direction of the fixing belt  102 , and held by a holding member  113  made of a nonmagnetic body bridged in the width direction of the fixing belt  102 . The magnetic-path forming members  112  are arranged at even intervals in a central portion in a longitudinal direction of the holding member  113  while in both end portions in the longitudinal direction of holding member  113 , are arranged at narrower intervals or in contact with each other. This arrangement of the magnetic-path forming members  112  allows a distribution of the magnetic field H in the width direction of the fixing belt  102  to be adjusted. 
     As shown in  FIG. 3A , the fixing belt  102  is made of a base layer  130 , a heat generating layer  132 , a protecting layer  134 , an elastic layer  136 , and a releasing layer  138 , in order from the inside to the outside, and these are layered to be integrated. 
     The base layer  130  is a base having a strength of the fixing belt  102 , for which polyimide is used with a thickness set to 50 to 200 μm. For the base layer  130 , besides a resin such as polyimide, a metal such as iron, nickel, silicon, boron, niobium, copper, zirconium, and cobalt, and a soft magnetic metal material made of an alloy composed of these metals may be used. 
     The heat generating layer  132  is made of a metal material that generates heat by electromagnetic induction in which eddy current flows so as to generate a magnetic field which cancels out the above-described magnetic field H. Moreover, the heat generating layer  132  needs to be formed so as to be thinner than a so-called skin depth in order to pass through a magnetic flux of the magnetic field H. For the heat generating layer  132 , for example, a metal of gold, silver, copper, aluminum, zinc, tin, lead, bismuth, beryllium, antimony or an alloy of these may be used. In the present exemplary embodiment, as the heat generating layer  132  copper having a thickness of 10 μm is used. 
     The skin depth δ (m) is represented by the following expression using a frequency f(Hz), a relative magnetic permeability μr and a specific resistance ρ (Ωm) of an exciting circuit.
 
δ=503(ρ/( f×μr )) 1/2  
 
     The skin depth δ (m) represented by the above-described expression indicates a depth of absorption of an electromagnetic wave used in the electromagnetic induction, and in a point deeper than this, the intensity of the electromagnetic wave is 1/e or less. In other words, it means that most energy is absorbed until a layer reaches this depth. 
     For the protecting layer  134 , a material that has mechanical strength higher than the heat generating layer  132 , has high repeated distortion resistance, and has high rust and corrosion resistance is preferable, and in exemplary embodiment, nonmagnetic stainless steel having a thickness of 30 μm is used. 
     For the elastic layer  136 , a silicone-based rubber or a fluorine-based rubber is used in view of excellent elasticity, heat resistance and the like. In the present exemplary embodiment, a silicone rubber having a thickness of 200 μm is used. The thickness of the elastic layer  136  is preferably 200 to 600 μm. 
     The releasing layer  138  is provided to easily stripe off the recording paper P from the fixing belt  102  by weakening adhesion force to a toner T (see  FIG. 2 ) fused on the recording paper P. In order to attain excellent surface releasability, a fluoride resin, a silicone resin, or a polyimide resin is used for the releasing layer  138 , and in the present exemplary embodiment, PFA (tetrafluorethylene/perfluoroalkoxyethylene copolymer resin) is used. The thickness of the releasing layer  138  is 30 μm. 
     As shown in  FIG. 2 , inside of the fixing belt  102 , a prismatic support  114  made of aluminum, which is a nonmagnetic material, is arranged in a noncontact state with respect to the fixing belt  102  with a longitudinal direction of the support  114  corresponding to the width direction of the fixing belt  102 . Both ends of the support  114  are fixed to the housing  106  of the fixing apparatus  100 . The support  114  is formed with a depressed portion  114 A along the longitudinal direction on the bottom side. A pressing pad  116  made of resin for pressing the fixing belt  102  outwards at a predetermined pressure is fixed to the depressed portion  114 A. The pressing pad  116  is made of a member having elasticity, and one end surface thereof is in contact with an inner circumferential surface of the fixing belt  102  to press the fixing belt outwards. 
     Moreover, inside of the fixing belt  102  and above the support  114 , a heating member is provided so as to be opposed to the exciting coil  110 , and in the present exemplary embodiment, a heat generating body  118  formed into a circular arc as shown in  FIGS. 2 and 3B  is used. 
     As shown in  FIGS. 2 and 3B , the heating generating body  118  is a substantially semicylindrical member whose longitudinal direction corresponds to the width direction of the fixing belt  102 , and is arranged so that a surface thereof is in contact with the inner surface of the fixing belt  102 . Moreover, the heat generating body  118  is made of an iron-based alloy, and forms a closed magnetic path by the above-described magnetic field H between the magnetic-path forming members  112  and the heat generating body  118 , and generates heat by the electromagnetic induction of the magnetic field H. The contact of the heat generating body  118  with the fixing belt  102  keeps temperature decrease of the fixing belt  102  smaller even if the heat of the fixing belt  102  is consumed by the recording paper P passing. 
     For the heat generating body  118 , a magnetic metal material having a thickness more than the skin depth noted above is preferably used. The thickness more than the skin depth brings about sufficient heat generation by the action of the magnetic field, and the heat is accumulated inside of the heat generating body  118 , which suppresses temperature decrease of the fixing belt  102  more. The magnetic metal material is desirably a ferromagnetic body having a relative magnetic permeability of 100 or more, more desirably a ferromagnetic body of a relative magnetic permeability of 500 or more, for example. 
     In the heat generating body  118 , a notched portion  120  as an opening portion is formed at a central portion in the longitudinal direction and at an end portion in a circumferential direction, and at the upstream side thereof with respect to the rotation direction of the fixing belt  102 . The notched portion  120  is formed in a position opposed to one of the magnetic-path forming members  112  which is provided at the central portion in the longitudinal direction of the holding member  113  (see  FIG. 4A ). Since the notched portion  120  is formed only by cutting the end portion of the heat generating body  118 , the attachment of the temperature sensor  124  (described later) is easier as compared with a case where a through-hole is formed in the heat generating body  118 . 
     Moreover, although it is expected that the formation of the opening portion in the heat generating body  118  will decrease a quantity of heat value of the heat generating body  118 , the heating member such as the heat generating body  118  has larger heat release in a position closer to the end portion in a circumferential direction, and thus, the provision of the opening portion (notched portion) in the end portion keeps decrease of the quantity of heat value smaller in comparison with a case where the opening portion is provided at an intermediate part in a circumferential direction of the heat generating body  118 . Furthermore, when the heat generating body  118  generates heat by the electromagnetic induction of the magnetic field H as in the present exemplary embodiment, the quantity of heat value becomes larger in a position closer to a central portion in a winding width of the exciting coil  110 , and thus, the provision of the notched portion  120  in the end portion keeps decrease in the quantity of heat value smaller as compared with a case where a through-hole is provided at a position in the heating generating body  118  closer to the central portion in the winding width of the exciting coil  110 . 
     A supporting member  122  is provided at a predetermined position in the longitudinal direction of the heat generating body  118  and at each end portion in the circumferential direction at the inner circumferential side of the heat generating body  118 . At one-end of the supporting member  122 , a substantially L-shaped supporting portion is formed and is attached at the respective inner circumferential side end of the heat generating body  118 . The other end of the supporting member  122  is jointed to each side (right and left sides in  FIG. 2 ) of the support  114  by screws  123  and  128 , by which the support  114  supports the heat generating body  118 . 
     In the notched portion  120  of the heat generating body  118 , the temperature sensor  124  that makes contacts with the inner circumferential surface of the fixing belt  102  to sense a temperature of the fixing belt  102  surface is disposed. The temperature sensor  124  measures the temperature of the fixing belt  102  surface by changing resistance value in accordance with an amount of heat given from the fixing belt  102  surface. 
     Moreover, the temperature sensor  124  is fixed in a terminal portion of a plate spring  126  made of a resin such as polyimide, and a base end portion of the plate spring  126  is jointed to the side of the support  114  (right side in  FIG. 2 ) by the screw  128 . This allows the plate spring  126  to extend from the downstream side to the upstream side in the rotation direction of the fixing belt  102  along the inner circumferential surface of the fixing belt  102 , and the temperature sensor  124  is located in the notched portion  120  along the inner circumferential surface of the fixing belt  102 . 
     As shown in  FIG. 4B , the temperature sensor  124  is connected to a control circuit  142  provided inside of the above-described controller  70  (see  FIG. 1 ) through wiring  140 . Moreover, the control circuit  142  is connected to an energization circuit  146  through wiring  144 , and the energization circuit  146  is connected to the above-described exciting coil  110  through wiring  148  and  150 . 
     The control circuit  142  measures a temperature on the inner circumferential side of the fixing belt  102  based on an amount of electricity sent from the temperature sensor  124 , and converts it to a temperature on the outer circumferential side of the fixing belt  102 , and then, compares this converted temperature with a fixing setting temperature stored in advance (170° C. in the present exemplary embodiment). When the converted temperature is lower than the fixing setting temperature, the energization circuit  146  is driven to energize the exciting coil  110 , and generate the magnetic field H as a magnetic circuit (see  FIG. 2 ). When the converted temperature is higher than the fixing setting temperature, the energization circuit  146  is stopped. 
     The energization circuit  146  is driven or the driving thereof is stopped based on the electric signal sent from the control circuit  142 , and an alternating current of a predetermined frequency is supplied to the exciting coil  110  through the wiring  148 ,  150 , or the supply thereof is stopped. 
     As shown in  FIG. 2 , a pressurizing roll  104  that pressurizes the fixing belt  102  toward the pressing pad  116 , and is rotated in an arrow E direction by a driving mechanism made of a motor and a gear not shown is arranged in a position opposed to the outer circumferential surface of the fixing belt  102 . 
     The pressurizing roll  104  is constituted so that a silicone rubber and PFA are covered around a cored bar  105  made of a metal such as aluminum. The pressurizing roll  104  pressurizes the fixing belt  102  to the pressing pad  116  side, so that the fixing belt  102  is in a state depressed inwards at a contact portion (nip portion) formed between the fixing belt  102  and the pressurizing roll  104 . 
     A shape of this nip portion is curved in a direction where the recording paper P with the toner T placed thereon is stripped off from the fixing belt  102  when it passes this nip portion. Thereby, the recording paper P conveyed from an arrow IN direction is discharged in an arrow OUT direction while following the shape of the nip portion because of stiffness of the paper itself. 
     Next, operations of the first exemplary embodiment of the present invention are described. First, a fixing operation of the fixing apparatus  100  is described. 
     As shown in  FIGS. 1 to 4B , the recording paper P (or envelope) to which the toner T is transferred via the image forming process of the above-described printer  10  is sent to the fixing apparatus  100 . In the fixing apparatus  100 , a drive motor not shown is driven by the controller  70 , so that the pressurizing roll  104  is rotated in an arrow E direction, and following this, the fixing belt  102  is rotated in the arrow D direction. At this time, the energization circuit  146  is driven based on an electric signal from the control circuit  142 , and an alternating current is supplied to the exciting coil  110 . 
     When the alternating current is supplied to the exciting coil  110 , the magnetic field H as a magnetic circuit is repeatedly generated and extinguished in the vicinity of the exciting coil  110 . When the magnetic field H crosses the heat generating layer  132  of the fixing belt  102 , an eddy current is generated in the heat generating layer  132  so as to generate a magnetic field preventing change of the magnetic field H. The heat generating layer  132  generates heat in proportion to the magnitudes of the skin resistance of the heat generating layer  132  and the eddy current flowing the heat generating layer  132 , resulting in the heating of the fixing belt  102 . 
     Similarly, the heat generating body  118  generates heat by the electromagnetic induction action of the magnetic field H to heat the fixing belt  102 . Thus, since the heat generating layer  132  and the heat generating body  118  are heated by the same exciting coil  110 , power consumption is lower as compared with a case where the heat generating layer  132  and the heat generating body  118  are heated by different heat sources. 
     The temperature of the fixing belt  102  surface is sensed by the temperature sensor  124  and when it does not reach the fixing setting temperature, the control circuit  142  controls the driving of the energization circuit  146  to apply an alternating current of a predetermined frequency to the exciting coil  110 . Moreover, when the temperature of the fixing belt  102  surface reaches the fixing setting temperature, the control circuit  142  stops the control of the energization circuit  146 . 
     The temperature sensor  124  is fixed to the terminal portion of the plate spring  126 , and the base portion of the plate spring  126  is jointed to the support  114 . Thereby, the plate spring  126  is extended out from the downstream side to the upstream side in the rotation direction of the fixing belt  102  along the inner circumferential surface of the fixing belt  102 . The temperature sensor  124  is arranged in the end portion of the heat generating body  118  at the upstream side in the rotation direction of the fixing belt  102  along the inner circumferential surface of the fixing belt  102 . Therefore, even if the temperature sensor  124  is pulled by the rotation of the fixing belt  102 , external force acts to the temperature sensor  124  in a direction where the temperature sensor  124  is contained in the notched portion  120 . This allows the temperature sensing position by the temperature sensor  124  to fall into the heating region opposed to the exciting coil  110 . 
     Subsequently, the recording paper P sent by the fixing apparatus  100  is heated and pressed by the fixing belt  102  and the pressurizing roll  104  at the predetermined fixing setting temperature so that the toner images are fixed on the recording paper P surface, and the fixed recording paper P is discharged into the paper output tray  64 . 
     Next, a temperature in the circumferential direction of the fixing belt  102  is described. 
       FIG. 5A  is a schematic diagram showing positions A to E that are sensing positions of the temperature of the fixing belt  102  in the circumferential direction and a sensing position of the temperature of the heat generating body  118 . The temperatures of the respective portions excluding B are sensed using temperature sensors not shown. 
     The position A is a sensing position at the inner circumferential surface of the fixing belt  102 . The position A is located at the upstream side of a region opposed to the exciting coil  110  in the circumferential direction of the fixing belt  102 . The position B is a sensing position where the temperature sensor  124  (see  FIG. 2 ) senses and located in a region opposed to the exciting coil  110  and the heat generating body  118 . The position B is located at the inner circumferential surface of the fixing belt  102 . 
     The position C is a sensing position corresponding to the projected portion  108 A of the bobbin  108  (see  FIG. 2 ), and located in a region not opposed to the exciting coil  110  at the inner circumferential surface of the fixing belt  102 . The position D is a sensing position symmetric to the position A centering on the position C, and located in the region not opposed to the exciting coil  110  at the inner circumferential surface of the fixing belt  102 . 
     The position E is a sensing position adjacent to the position B at an inner circumferential surface (opposite side of the fixing belt  102 ) of the heat generating body  118 . The position E is set for comparing the temperatures at the inner circumferential surface of the heat generating body  118  and at the inner circumferential surface of the fixing belt  102 . 
     For example, a relation between the positions in the circumferential direction of the fixing belt  102  and the sensed temperature when the fixing belt  102  is heated in a state where the rotation is stopped is shown in  FIG. 5B . 
     In  FIG. 5B , in the position A, the sensed temperature of the fixing belt  102  is T 1 . In the position B, since the heat generating layer  132  (see  FIG. 3A ) of the fixing belt  102  and the heat generating body  118  generate heat by the electromagnetic induction action of the magnetic field H created by the exciting coil  110 , the sensed temperature of the fixing belt  102  is T 2 , which is higher than T 1 . 
     In the position C, while the heat generating body  118  releases heat by heat conduction from a region heated by the magnetic field H to heat the fixing belt  102 , the exciting coil  110  does not exit thus, a heating amount is smaller and the sensed temperature of the fixing belt  102  is T 3  lower than T 2 . At the point D, since it is out of the region heated by the magnetic field H, the sensed temperature of the fixing belt  102  is the temperature T 1  equivalent to that of the position A. 
     At the point E, since heat capacity of the fixing belt  102  is smaller than heat capacity of the heat generating body  118 , temperature rising of the fixing belt  102  is faster. Therefore, the sensed temperature of the heat generating body  118  is T 4  lower than the sensed temperature T 2  at the point B of the fixing belt  102 . 
     Normally, since during fixing, the fixing belt  102  is rotated, the temperature of the heat generating body  118  is higher. However, when heating is performed in the state where the rotation of the fixing belt  102  is stopped as described above, a temperature rising rate of the fixing belt  102 , which has smaller heat capacity, is faster than that of the heat generating body  118 , and as a result, the temperature on the fixing belt  102  side becomes higher. 
     Since the magnetic-path forming members  112  collect the magnetic fields of the magnetic field H to form the closed magnetic path, the temperature on the inner circumferential surface of the fixing belt  102  in the heated region opposed to the exciting coil  110  becomes the highest, and the temperature in this region is sensed by the temperature sensor  124  (see  FIG. 2 ) to control the temperature by the control circuit  142  (see  FIG. 4B ). This will suppress excessive temperature rising of the fixing belt  102 . 
     As another example of the heating generating body  118  of the fixing apparatus  100 , for example, heat generating bodies  152  and  156  shown in  FIGS. 6A and 6B  may be used. In the heat generating body  152 , a through-hole  154  is formed closer to the central portion in the circumferential direction of the heat generating body  152  and at the upstream side in the rotation direction of the fixing belt  102  than the notched portion  120  of the above-described heat generating body  118 . Moreover, in the heat generating body  156 , a notched portion  158  is formed in an end portion at the opposite side of the notched portion  120  (at the downstream side in the rotation direction of the fixing belt  102 ). When the heating generating body  156  is used, the temperature sensor  124  is caused to be adhered, or a frame or the like for fixing is provided. 
     Next, a second exemplary embodiment of the fixing apparatus and the image forming apparatus of the invention is described based on the drawings. Basically the same parts as those in the above-described first exemplary embodiment are given the same numerals and signs as those of the first exemplary embodiment, and their descriptions are omitted. 
     A fixing apparatus  160  is shown in  FIG. 7 . The fixing apparatus  160  has a constitution using a heat generating body  162  in place of the heat generating body  118  of the fixing apparatus  100  of the first exemplary embodiment. 
     As shown in  FIGS. 7 ,  8 A and  8 B, the heat generating body  162  is a substantially semicylindrical member whose longitudinal direction corresponds to the width direction of the fixing belt  102 , and is arranged so that a surface thereof is in contact with the inner surface of the fixing belt  102 . Moreover, the heat generating body  162  is made of an iron-based alloy and forms a closed magnetic path by the above-described magnetic field H between the magnetic-path forming members  112  and the heat generating body  162 , and at the same time, generates heat by the electromagnetic induction action of the magnetic field H 1 . 
     In the heat generating body  162 , a plurality of notched portions  164 ,  166  and  168  are formed in an end portion in a circumferential direction and at the upstream side in the rotation direction of the fixing belt  102 . The notched portion  166  is located substantially in the center in the longitudinal direction of the heat generating body  162 , and is within a region of a width W 2  when recording paper of a small size passes on the fixing belt  102 . Moreover, the notched portions  164  and  168  are located in both end portions of the heat generating body  162  in the longitudinal direction, which are outside of the region of the width W 2 , and inside of a region of a width W 1 +W 2 +W 3  when recording paper of a large size passes on the fixing belt  102 . 
     In the respective notched portions  164 ,  166  and  168 , the above-described temperature sensors  124  ( 124 A,  124 B and  124 C) are disposed, respectively, and the respective temperature sensors  124  are fixed to the support  114  through the plate springs  126  ( 126 A,  126 B and  126 C). Moreover, the respective sensors  124 A,  124 B and  124 C are arranged so as to be opposed to the magnetic-path forming members  112 . 
     Next, operations of the second exemplary embodiment of the invention are described. 
     As shown in  FIGS. 7 and 8 , in the fixing apparatus  160 , when fixing of the toners is continuously performed to the recording paper P of the small size, in the region of the width W 2  of the fixing belt  102 , an amount of heat is deprived by the recording paper P, so that the temperature of the fixing belt  102  becomes lower than the fixing setting temperature. 
     At this time, since the temperature sensed by the temperature sensor  124 B becomes the lowest within the temperature sensors  124 A to  124 C, the control circuit  142  (see  FIG. 4B ) controls the energization circuit  146  (see  FIG. 4B ) so as to bring the temperature of the fixing belt  102  closer to the fixing setting temperature, based on a difference between the sensed temperature by the temperature sensor  124 B and the fixing setting temperature, and the heat generating body  162  generates heat. Although this raises a temperature of the entire fixing belt  102 , in the regions of the widths W 1  and W 3  (non-paper-conveyance region) in the fixing belt  102 , the heat is not deprived by the recording paper P, and thus, an amount of heat is accumulated, which makes the temperature higher than that in the region of the width W 2 . As a result, as shown in  FIG. 9 , a graph exhibits a high temperature on the both end portion sides. 
     Since the temperature sensors  124 A and  124 C are located outside of the passage region (W 2 ) of the recording paper P of the small size, and inside of the passage region (W 1  W 3 ) of the recording paper P of the large size, a temperature of a highest temperature portion of the fixing belt  102  is sensed. 
     On the other hand, when the recording paper P of the large size is fixed, even if there is partially a position at a high temperature, any of the temperature sensors  124 A to  124 C senses the temperature of the high-temperature portion because the plurality of temperature sensors  124 A,  124 B and  124 C are arranged in the longitudinal direction of the heat generating body  162 . 
     As another example of the heat generating body  162 , for example, a heat generating body  170  shown in  FIG. 10  may be used. In the heat generating body  170 , through-holes  172 ,  173 ,  174 ,  175  and  176  are formed in positions closer to the central portion in the circumferential direction than the notched portions  164 ,  166  and  168  of the heat generating body  162 , and on the upstream side in the rotation direction of the fixing belt  102 . In the through-holes  172  to  176 , the temperature sensors  124  ( 124 A to  124 E) are provided, respectively. In this manner, by arranging the plurality of temperature sensors  124  in the longitudinal direction of the fixing belt  102 , a temperature in a highest temperature portion of the fixing belt  102  in the longitudinal direction is sensed. 
     The present invention is not limited to the above-described exemplary embodiments. 
     The printer  10  may be not only of a dry electrophotographic system using a solid developer, but of a type using a liquid developer. Moreover, the heat generating bodies  118  and  162  may be sheet heat generating bodies that generate heat by supplying electricity. Furthermore, as sensing means of the temperature of the fixing belt  102 , a thermocouple may be used in place of the temperature sensor  124 . 
     A shape of the notched portion  120  may be not only rectangular but circular arc or multangular. Moreover, in place of the notched portion  120 , a depressed portion as an opening portion may be formed in an outer circumferential surface of each of the heat generating bodies  118  and  162 , where the temperature sensor  124  may be arranged. When the magnetic field H is the most intensive in a central portion in a width direction of the exciting coil  110  in a bundle, the notched portion  120  may be advantageously formed in a position opposed to the central portion of the exciting coil  110  to arrange the temperature sensor  124 .