Patent Publication Number: US-6909862-B2

Title: Printer apparatus having fixing unit with heat controller for controlling temperature of heat-up roller

Description:
FIELD OF THE INVENTION 
   The present invention relates to a printer apparatus having a fixing unit, such as a copying machine, facsimile, or printer. 
   BACKGROUND OF THE INVENTION 
   Many image-forming apparatuses such as printers have fixing units. 
   Such a fixing unit fixes an unfixed toner image that has been formed by an imaging process, e.g. electrophotographic, electrostatic, and magnetic recording, on recording materials, e.g. a recording sheet, sensitized paper, and electrographic paper. Fixing units known as such employ a heating roller method and a film heating method. In recent years, an image-forming apparatus having a fixing unit using an electromagnetic induction heating method is known. 
   A fixing unit using the film heating method is disclosed in Japanese Patent Application Non-Examined Publication No. S63-313182 or No. H01-263679, for example. 
   As for a fixing unit using the electromagnetic induction heating method, a technique of causing a fixing roller to produce heat resulting from magnetic induction is disclosed in Japanese Patent Application Non-Examined Publication No. H11-297462. Herein, an alternating field causes the conductive layer of the fixing roller to produce eddy current and thus Joule heat, and this Joule heat causes the fixing roller to heat. 
   A fixing unit using the electromagnetic induction heating method is described below. 
     FIG. 22  is a schematic view of a fixing unit using a conventional electromagnetic induction heating method. 
   In the fixing unit shown in  FIG. 22 , exciting coil  42  is disposed along outer circumferential surface of fixing roller  41 . Magnetic substance  43  is disposed outside of and over this exiting coil  42 . Pressurizing roller  44  is disposed so as to press fixing roller  41  in contact therewith. Temperature sensor  45  detects temperatures on the surface of fixing roller  41 . 
   Alternating current at frequencies of 10 to 100 MHz is applied to exciting coil  42 . The magnetic field induced by this alternating current feeds eddy current through the conductive layer of fixing roller  41 , thereby causing Joule heat. 
   Temperature sensor  45  is disposed in contact with the front face of fixing roller  41 . Responsive to signals detected by temperature sensor  45 , power supply to exciting coil  42  is increased or decreased. Thus the temperatures on the front face of fixing roller  41  are automatically controlled so that a predetermined fixed value is maintained. 
   Recording material  46  carrying unfixed toner image  47  thereon is conveyed and placed by a carrier guide (not shown) in a position in which the recording material is guided to a nip portion “NI” between fixing roller  41  and pressurizing roller  44 . 
   In this manner, fixing roller  41  is rotated by a driving unit (not shown). At the same time, alternating current is applied to exciting coil  42  to heat up fixing roller  41 . Therefore, the fixing nip portion “NI” is heated to a predetermined temperature. In this state, recording material  46  carrying unfixed toner image  47  thereon is guided by the carrier guide (not shown) and introduced into the fixing nip portion “NI”. The recording material is further conveyed as fixing roller  41  rotates, and toner image  47  is melted and fixed onto recording material  46  by the heat of fixing roller  41  and the pressure of the nip portion. 
   As mentioned above, a fixing unit using the electromagnetic induction heating method heats fixing roller  41  with high heat transfer by utilizing eddy current produced by electromagnetic induction. Therefore, this method has such advantages as reducing warm-up time, allowing the unit to start earlier than a fixing unit using the film heating method, and contributing to energy saving. 
   In Japanese Patent Application Non-Examined Publication No. H08-286539, the following structure is disclosed. 
   A rotating heat-producing section has a conductive layer comprising a film containing a ferromagnetic metal, e.g. nickel, iron, ferromagnetic stainless steel, and nickel-cobalt alloy. Provided inside of the rotating heat-producing section is an electromagnetic induction heating section that has exciting coils wound along a core material in the direction of the rotating shaft of the rotating heat-producing section. 
   A fixing unit using the electromagnetic induction heating method disclosed in Japanese Patent Application Non-Examined Publication No. H11-297462 is structured so that the unit has an electromagnetic induction heating section outside of a fixing roller and substantially half of a circumferential area of the fixing roller is locally heated. In order to prevent abnormal temperature rise in a heat-producing section resulting from uncontrollable temperature, a heat controller comprising a heat-sensitive operation section, such as a thermostat, is provided in a position opposite to the electromagnetic induction heating section, i.e. inside of a heat-up roller. 
   In this structure, the surface of the heat-sensitive operation section may be worn by sliding thereof resulting from the rotation of the fixing roller. Therefore, it is difficult for the heat-sensitive operation section to be pressed onto the inside of the heat-up roller in contact therewith in a stable manner. 
   SUMMARY OF THE INVENTION 
   The present invention provides a printer apparatus having a fixing unit using the electromagnetic induction heating method that allows stable detection of heat temperatures of a heat-up roller thereof. 
   In a printer apparatus in accordance with the present invention, an image-forming unit thereof forms an image that is to be transferred onto a recording material. The fixing unit fixes the image onto the recording material. The fixing unit has a heat-up roller, a heating section for heating the heat-up roller, a power supply section for supplying power to the heating section, and a heat controller for controlling the power supply section. 
   The heat controller is disposed in contact with the inner circumferential surface of the heat-up roller. When the heat controller is heated to temperatures above a predetermined value, it is thermally deformed and leaves the inner circumferential surface and thereby interrupts power supply from the power supply section to the heating section. 
   In the printer apparatus in accordance with the present invention, temperature control of the heat-up roller in the fixing unit is ensured in this manner. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an overall structure of a printer apparatus in accordance with an exemplary embodiment of the present invention. 
       FIG. 2  is a general view of a fixing unit in accordance with first and second exemplary embodiments of the present invention. 
       FIG. 3  is a cross-sectional view of the fixing unit in accordance with the first exemplary embodiment of the present invention. 
       FIG. 4A  is a plan view of an exciting coil of an electromagnetic induction heating section in accordance with the first exemplary embodiment of the present invention. 
       FIG. 4B  is a cross-sectional view of the exciting coil of the electromagnetic induction heating section in accordance with the first exemplary embodiment of the present invention. 
       FIG. 5A  is a plan view of an exciting coil core of the electromagnetic induction heating section in accordance with the first exemplary embodiment of the present invention. 
       FIG. 5B  is a cross-sectional view of the exciting coil core of the electromagnetic induction heating section in accordance with the first exemplary embodiment of the present invention. 
       FIG. 6  is a longitudinal sectional view of a heat controller in accordance with the first exemplary embodiment of the present invention. 
       FIG. 7  is a block diagram showing circuitry for causing the electromagnetic induction heating section in accordance with the first and second exemplary embodiments of the present invention to generate a magnetic field. 
       FIG. 8  illustrates how the heat controller in accordance with the first exemplary embodiment of the present invention performs an interruption operation. 
       FIG. 9  illustrates a heat controller in accordance with the first exemplary embodiment of the present invention. 
       FIG. 10  is a cross-sectional view of a fixing unit in accordance with the first exemplary embodiment of the present invention. 
       FIG. 11  is a longitudinal sectional view showing a structure of a heat controller in accordance with the second exemplary embodiment of the present invention. 
       FIG. 12  is a longitudinal sectional view of a heat controller in accordance with the second exemplary embodiment of the present invention. 
       FIG. 13A  illustrates a position of the heat controller in the fixing unit shown in  FIG. 12  when rotation is stopped. 
       FIG. 13B  illustrates a position of the heat controller in the fixing unit shown in  FIG. 12  when rotation is stopped. 
       FIG. 14  is a cross-sectional view of a fixing unit in accordance with the second exemplary embodiment of the present invention. 
       FIG. 15A  is a perspective view of a heat-sensitive operation section in accordance with the first exemplary embodiment of the present invention. 
       FIG. 15B  is a perspective view of a heat-sensitive operation section in accordance with the first exemplary embodiment of the present invention. 
       FIG. 16A  is a longitudinal sectional view of a periphery of a rotating shaft connected to a heat-up roller in accordance with the second exemplary embodiment of the present invention. 
       FIG. 16B  is a top view of the periphery of the rotating shaft connected to the heat-up roller in accordance with the second exemplary embodiment of the present invention. 
       FIG. 17  illustrates a structure of the heat-sensitive operation section in accordance with the exemplary embodiments of the present invention. 
       FIG. 18A  is a top view of a protrusion in the heat-sensitive operation section in accordance with the exemplary embodiments of the present invention. 
       FIG. 18B  is a side view of the protrusion in the heat-sensitive operation section in accordance with the exemplary embodiments of the present invention. 
       FIG. 19  illustrates another structure of the fixing unit in accordance with the first exemplary embodiment of the present invention. 
       FIG. 20  illustrates a printer apparatus in accordance with a third exemplary embodiment of the present invention. 
       FIG. 21  is a detailed drawing of a fixing unit for use in the printer apparatus in accordance with the third exemplary embodiment of the present invention. 
       FIG. 22  illustrates a structure of a fixing unit in accordance with a conventional technique. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Exemplary embodiments of the present invention are hereinafter demonstrated with reference to the accompanying drawings. 
   In the following drawings, elements that perform the same operations have the same reference marks, and repetitive descriptions of those elements are omitted. 
   (First Embodiment) 
     FIG. 1  shows a structure of a printer apparatus in accordance with a first exemplary embodiment of the present invention. 
   In this color printer apparatus, four image-forming units  101   a ,  101   b ,  101   c , and  101   d  are disposed. Image-forming units  101   a ,  101   b ,  101   c , and  101   d  have photosensitive drums  201   a ,  201   b ,  201   c , and  201   d , respectively. A latent image is formed on each of the drums by irradiation of exposure light. 
   Provided in the periphery of photosensitive drums 201 a  to  201   d  are electric chargers, an exposure unit, development sections, transfer sections, and cleaning sections. 
   Electric chargers  301   a ,  301   b ,  301   c , and  301   d  electrically charge the surface of each of photosensitive drums  201   a  to  201   d  to a predetermined electric potential uniformly. Exposure unit  131  irradiates charged photosensitive drums  201   a  to  201   d  with exposure light  91 Y,  91 M,  91 C, and  91 K corresponding to image data of particular colors to form electrostatic latent images. Development sections  41   a ,  41   b ,  41   c , and  41   d  visualize the electrostatic latent images formed on photosensitive drums  201   a  to  201   d.    
   Transfer sections  81   a ,  81   b ,  81   c , and  81   d  transfer the toner images visualized on photosensitive drums  201   a  to  201   d  onto endless band-like inter-stage transfer belt  121 . Cleaning sections  51   a ,  51   b ,  51   c , and  51   d  remove the residual toner on photosensitive drums  201   a  to  201   d  after the toner images have been transferred from photosensitive drums  201   a  to  201   d  to inter-stage transfer belt  121 . 
   In this drawing, inter-stage transfer belt  121  is rotated in the direction of arrow “A” by driving rollers  105  and  111 . In image-forming units  101   a  to  101   d , a yellow image, a magenta image, a cyan image, and a black image are formed, respectively. 
   Next, single-color images of each color that have been formed on photosensitive drums  201   a  to  201   d  are transferred onto inter-stage transfer belt  121  sequentially one on another to form a full-color image. 
   Provided below inter-stage belt  121  is a paper feed cassette  161  for housing sheet material  11  such as printing paper. Sheet material  11  is fed from paper feed cassette  161  into paper-carrying passage by paper feed roller  181  sheet by sheet. 
   In the paper-carrying passage, sheet material transfer roller  191  is in contact with the outer peripheral surface of inter-stage transfer belt  121  so as to transfer the color image formed on this inter-stage transfer belt  121  onto the above-mentioned sheet material  11 . Then, fixing unit  401  fixes onto sheet material  11  the toner image that has been transferred onto sheet material  11 , using the pressure and heat produced by rotation of sheet material transfer roller  191 . 
   Next, fixing unit  401  of the above-mentioned printer apparatus is detailed using  FIGS. 2 and 3 . 
     FIG. 2  is a general view showing fixing unit  401  in accordance with the first exemplary embodiment of the present invention. 
     FIG. 3  is a cross-sectional view showing a structure of fixing unit  401 . 
   In the cross-sectional view of  FIG. 3 , heat-up roller  1 , fixing roller  2 , pressurizing roller  4  are shown as they are vertically aligned for convenience in explanation. 
   In FIG.  2  and  FIG. 3 , heat-up roller  1 , fixing roller  2 , endless band-like heat-resistant belt  3 , pressurizing roller  4 , heating section  6  are shown. Heating section  6  is an electromagnetic induction heating section and provided with exciting coil  7 , coil guide  8 , exciting coil core  9 , and exciting coil core support  10 . 
   In fixing unit  401  of  FIG. 3 , cylindrical heat-up roller  1  is heated along the outer circumferential surface thereof by the electromagnetic induction of heating section  6 . Fixing roller  2  is disposed in parallel with heat-up roller  1 . Endless band-like heat-resistant belt (belt section)  3  is held by heat-up roller  1  and fixing roller  2  under a tension, heated by heat-up roller  1 , and rotated in the direction of arrow “A2” by the rotation of fixing roller  2 . Pressurizing roller (pressurizing section)  4  is in contact with heat-resistant belt  3  to form a nip portion (shown by the portion “N” in the drawing). The pressurizing roller is also pressurized onto fixing roller  2  in contact therewith, and rotated in the direction identical with that of heat-resistant belt  3 . 
   In  FIG. 3 , recording material  11 , toner image  210 , and temperature-detecting section  5  are also shown. Driving section  37  rotates pressurizing roller  4 . 
   The material and other details of each component of fixing unit  401  are described below. 
   With reference to  FIG. 3 , heat-up roller  1  is made of a cylindrical hollow ferromagnetic metal, e.g. Fe, Ni, and stainless steel. For example, the outer diameter thereof is 20 mm and the thickness thereof is 0.3 mm. Therefore, heat-up roller  1  has low heat capacity and is heated up quickly. 
   Fixing roller  2  includes metallic core  2   a  made of such a metal as stainless steel, and elastic material portion  2   b  made of heat-resistant silicon rubber covering metallic core  2   a . This silicon rubber is in the form of a solid or foam. Moreover, the pressurizing force of pressurizing roller  4  forms a predetermined width of contact portion between fixing roller  2  and this pressurizing roller  4 . For this reason, the outer diameter of the fixing roller is approx. 30 mm, which is larger than that of heat-up roller  1 . In fixing roller  2 , elastic material portion  2   b  has a thickness of approx. 3 mm to 8 mm and a hardness of approx. 15° to 50 ° (Asker C). 
   With such a structure, because the heat capacity of heat-up roller  1  is smaller than that of fixing roller  2 , heat-up roller is heated quickly and thus the warm-up time is reduced. 
   Heat-resistant belt  3  that is held between heat-up roller  1  and fixing roller  2  under a tension is heated in a portion “W” in which the belt is in contact with heat-up roller  1  heated by heating section  6  disposed along the outer circumferential surface of heat-up roller  1 . Moreover, the inner surface of heat-resistant belt  3  is continuously heated by the rotation of heat-resistant belt  3  resulting from the rotation of fixing roller  2 . As a result, belt  3  is heated entirely. 
   Heat-resistant belt  3  is a multilayer belt containing a base layer and a releasing layer (hereinafter referred to as a surface layer). The base layer is made of a heat-resistant material, e.g. fluorine plastic, polyimide resin, polyamide resin, polyamideimide resin, polyether ketone (PEEK) resin, polyether sulfone (PES) resin, and polyphenylene sulfide (PPS) resin. The surface layer is made of an elastic material, e.g. silicone rubber and fluoro rubber, covering the surface. 
   Such a structure allows heat-resistant belt  3  to conform to the curvature of heat-up roller  1  easily. In addition, because the base layer is made of a highly heat-resistant resin material, heat kept by heat-up roller  1  can be transferred to belt  3  efficiently. 
   In this case, the preferable thickness of the base layer is approx. 10 μm to 250 μm . Especially, approx. 75 μm is most preferable. On the other hand, the preferable thickness of the surface layer is approx. 30 μm to 400 μm . Especially, approx. 200 μm is most preferable. Such a structure allows toner image  210  formed on recording material  11  to be heated and melted uniformly. 
   As the base layer of heat-resistant belt  3 , a ferromagnetic metallic material, e.g. Ni, Co, Cr and stainless steel, can also be used instead of a heat-resistant resin material, e.g. fluorine plastic, polyimide resin, polyamide resin, polyamideimide resin, PEEK resin, PES resin, and PPS resin. 
   With the use of a metallic material, even when a gap is formed by entry of foreign matter between heat-resistant belt  3  and heat-up roller  1 , the base layer of heat-resistant belt  3  containing the metallic material produces heat resulting from electromagnetic induction. This ensures heat-up of belt  3  without variation in temperatures. 
   The preferable thickness of the metallic material is approx. 10 μm to 60 μm. Especially, approx. 30 μm is most preferable. 
   Pressurizing roller  4  includes cylindrical metallic core  4   a  that is made of a metal having high thermal conductivity, such as stainless steel and Al, and elastic material  4   b  having high heat resistance and toner releasability that is provided over the surface of this metallic core  4   a.    
   Such a pressurizing roller  4  pressurizes fixing roller  2  in contact with heat-resistant belt  3  to form the fixing nip portion “N”. 
   In this embodiment, the outer diameter of pressurizing roller  4  is approx. 30 mm, which is similar to that of fixing roller  2 , so that the toner releasing action at the exit of the fixing nip portion “N” is enhanced. On the other hand, the thickness of pressurizing roller  4  is approx. 2 mm to 5 mm, which is smaller than that of fixing roller  2 . The hardness of the pressurizing roller is approx. 20° to 60° (Asker C), which is larger than that of fixing roller  2 . 
   Heating section  6  for heating heat-up roller  1  using electromagnetic induction has exciting coil  7  for generating a magnetic field, and coil guide  8  having this exciting coil  7  wound thereon, as shown in  FIGS. 4A and 4B . 
   Coil guide  8  is shaped like a semi-circular arc disposed adjacent to the outer circumferential surface of heat-up roller  1 . Exciting coil  7  is made by a length of an exciting coil material wound along this coil guide  8  in a direction of the rotating shaft of heat-up roller  1 . The area around which exciting coil  7  is wound is identical with the contact area between heat-resistant belt  3  and heat-up roller  1 . 
   This structure maximizes the area of heat-up roller  1  to be heated by heating section  6 . In addition, this structure maximizes the time when the heating surface of heat-up roller  1  is in contact with heat-resistant belt  3 . Therefore, transfer efficiency of heat from heating section  6  to heat-resistant belt  3  is increased. 
   As shown in  FIG. 19 , heating section  6  may also be disposed along the inner circumferential surface of heat-up roller  1 . 
   Exciting coil  7  is connected to a driving power source (not shown) that has an oscillation circuit having variable frequencies. 
   Moreover, as shown in  FIG. 3 , exciting coil core  9  shaped like a semi-circular arc is disposed adjacent to exciting coil  7  outside thereof, and is secured to exciting coil core support  10 . In this embodiment, used as exciting coil core  9  is a core integrally formed of the mixture of a ferromagnetic powder, e.g. iron, nickel, and ferromagnetic stainless steel, and a heat-resistant resin, e.g. PEEK resin, PES resin and PPS resin. Exciting coil core  9  can also be formed of such ferromagnetic materials as ferrite and permalloy. 
   Such a structure allows downsizing of exciting coil core  9  and reduction of material cost, and moreover, substantial reduction of the number of man-hours in assembling the core. 
   In addition, because exciting coil core  9  can be machined precisely to have a variety of shapes, heat-up roller  1  can be made to have uniform temperature distribution. 
   Moreover, as shown in  FIGS. 5A and 5B , a plurality of openings K can be provided on exciting coil core  14  to partially expose exciting coil  7 . Such a structure allows heat produced by copper loss of exciting coil  7  and other causes to be released outside of heating section  6 . 
   Exciting coil  7  receives high-frequency alternating current at frequencies of 10 kHz to 1 MHz, more preferably 20 kHz to 800 kHz, and generates an alternating magnetic field. This alternating magnetic field acts on heat-up roller  1  in the contact area “W” between heat-up roller  1  and heat-resistant belt  3  and a periphery thereof, as shown in FIG.  3 . Inside of these areas, eddy current flows in the direction that hinders this change in magnetic field. 
   This eddy current produces Joule heat according to the resistance of heat-up roller  1 . Heat-up roller  1  is heated mainly in the contact area between heat-up roller  1  and heat-resistant belt  3  and the periphery thereof, by the heat resulting from electromagnetic induction. 
   The temperature on the inner surface of heated heat-resistant belt  3  is detected by temperature-detecting section  5  that is made of a temperature-sensitive element having high thermal responsibility, such as a thermistor, on the entry side of the fixing nip portion “N”. 
   As mentioned above, temperature-detecting section  5  is disposed on the back face of belt  3 . Thus, temperature-detection section  5  does not damage the front face of belt  3 . Therefore, fixing performance is continuously ensured and the temperature just before belt  3  enters the fixing nip portion “N” is detected. 
   Furthermore, the temperature of heat-resistant belt  3  can be maintained at 180° C., for example, in a stable manner, by the control of the power supply to heating section  6  based on the signals showing this temperature information. 
   In this embodiment, as shown in  FIGS. 1 and 3 , toner image  210  that has been formed on recording material  11  by image-forming units  101   a  to  101   d  is introduced into the fixing nip portion “N”. At this time, recording material  11  is fed into the fixing nip portion “N” with little temperature difference between the front face and back face of heat-resistant belt  3  that has been heated by heating section  6 . This can inhibit so-called overshoot, in which the temperature on the front face of the belt is excessively higher than a preset temperature, and thus stable temperature control can be performed. 
   Next, a heat controller of this embodiment is described. 
     FIG. 6  is a longitudinal sectional view of heating section  6  and heat-up roller  1 . 
   As shown in the drawing, heat controller  13 A for overheat prevention is provided at an end of heat-up roller  1 . As hereinafter detailed, this heat controller  13 A may be provided on both ends of heat-up roller  1 . 
   Now, a case where the heat controller is provided at an end of the heat-up roller is described. 
   Heat controller  13 A has heat-sensitive operation section  30  shaped like a flat spring, and electrode  15 . This heat-sensitive operation section  30  includes a bimetal that is thermally deformed in a direction farther from the inner circumferential surface of heat-up roller  1  when heated to temperatures above a predetermined value. This heat-sensitive operation section  30  is made of two different kinds of metals bonded together as shown in FIG.  17 . In other words, a metal having high thermal expansion coefficient is used as metal  220  in contact with the inner surface of the heat-up roller. A metal having low thermal expansion coefficient is used as metal  222  that does not contact the inner surface of the heat-up roller. Therefore, with a temperature rise, the bimetal is deformed so that the length of the metal having higher thermal expansion coefficient increases. 
   Metals having high thermal expansion coefficients are made of such composite materials as Ni—Mo—Fe, Ni—Cr, and FeNi—Mn—Fe. Metals having low thermal expansion coefficients are made of such composite materials as Ni—Fe and Cr—Fe. Details on the combinations of materials are shown in Table 1. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Metal with high thermal 
               Metal with low thermal 
             
             
                 
               expansion coefficient 
               expansion coefficient 
             
             
                 
                 
             
           
          
             
                 
               Ni-Cr-Fe 
               Ni-Fe 
             
             
                 
               Ni-Cr-Fe 
               Cr-Fe 
             
             
                 
               Ni-Mn-Fe 
               Ni-Fe 
             
             
                 
                 
             
          
         
       
     
   
     FIGS. 15A and 15B  show structures of heat-sensitive operation sections  30  disposed in heat-up roller  1 . 
     FIG. 15A  shows a case where a single heat-sensitive operation section  30  is disposed in heat-up roller  1 .  FIG. 15B  shows an example of cases where a plurality of heat-sensitive operation sections  30  are disposed in heat-up roller  1 . In these drawings, support member  20  is also shown. 
   As shown in the drawings, the heat-sensitive operation section  30  is shaped so that the dimension thereof along the longitudinal direction of heat-up roller  1  is larger than the dimension perpendicular to the longitudinal direction. Therefore, heat-sensitive operation section  30  has a shape that is deformed by heat without fail. However, when the heat-sensitive operation section  30  is shaped so that the dimension thereof along the longitudinal direction of heat-up roller  1  is shorter than the dimension perpendicular to the longitudinal direction, the operation section works. 
   In the case of the heat controller having a plurality of heat-sensitive operation sections, each heat-sensitive operation section may be structured to have different materials bonded together. 
   For example, when a plurality of bimetals are used as shown in  FIG. 15B , one bimetal is made of a metal a having high thermal expansion coefficient and a metal having a low thermal expansion coefficient bonded together while the other bimetal is made of two metals bonded together that are different from the aforementioned metal materials. 
   In this case, a metal having a higher thermal expansion coefficient is used as a metal in contact with the inner surface of the heat-up roller. 
   In addition, with reference to  FIG. 6 , protrusions  16  and  17  are formed by press work and the like, on the tips of heat-sensitive operation section  30  and electrode  15 , respectively. The tips of heat-sensitive operation section  30  and electrode  15  are brought into contact with the inner circumferential surface of heat-up roller  1  and pressurized thereto with a predetermined load via these protrusions  16  and  17 . 
   This keeps electrical connection between the heat-up roller, i.e. a cylindrical rotating body, and heat-sensitive operation section  30 , and thus heat controller  13 A operates in a stable manner. 
   Now, protrusions  16  and  17  are detailed using  FIGS. 18A and 18B . Forming protrusions  16  and  17  in contact with the inner circumferential surface of heat-up roller  1  as a semi-spherical shape ensures intimate contact of protrusions  16  and  17  with the inner circumferential surface of the heat-up roller. 
   Over protrusions  16  and  17  formed on the tips of heat-sensitive operation section  30  and electrode  15 , a metal having high thermal conductivity and low electrical resistance, e.g. Cu, Ag, and Pt, is crimped. This can reduce electrical resistance in the contact portions between protrusions  16  and  17  and the inner circumferential surface of heat-up roller  1 . Thus heat production of heat-sensitive operation section  30  is reduced. In this manner, the temperature detection accuracy of heat-sensitive operation section  30  can be improved. The crimped portions are crushed and thus the protrusions are secured to the bimetals by crimping. 
   Next, the operation of the heat controller of this embodiment is described below. 
   With reference to  FIG. 6 , heat-sensitive operation section  30  and electrode  15  are secured to the body of fixing unit  401  via support  20  and structured to slide on the inner surface of heat-up roller  1 . 
   As mentioned above, heat-sensitive operation section  30  as a heat controller is normally in contact with heat-up roller  1 , in a position opposite to electromagnetic induction heating section  6  as a heating section, sandwiching heat-up roller  1 . Therefore, the heat controller performs excellent temperature control against rapid and local heating of heat-up roller  1 . 
   Next, peripheral devices of the fixing unit using the electromagnetic induction heating method structured as above and a control method thereof are described using the circuit diagram of FIG.  7 . 
   The circuit diagram of  FIG. 7  shows heat-up roller  1 , exciting coil  7 , heat controller  13 , commercial power source  21 , rectifier element  22 , resonant capacitor  23 , switching element  24 , switching element driver  25 , DC power source  26 , and controller  27 . 
   Rectifier element  22  performs full-wave rectification of commercial power source  21 . Resonant capacitor  23  is connected in parallel to exciting coil  7 . Connected in series with rectifier element  22  in order to pass high-frequency current through capacitor  23  and exciting coil  7  in parallel is switching element  24  for insulated gate bipolar transistor (IGBT) driving. In the IGBT driving, the switching element is driven. Switching element driver  25  contains such an IC for the IGBT. Driver  25  drives the gate of switching element  24 . Connected to driver  25  via heat controller  13  is 20-V DC power source  26 , for example. Controller  27  feeds on/off signals to switching element driver  25 . Thus, high-frequency current flows through exciting coil  7 . 
   Now, DC power source  26  and switching element driver  25  are connected in series with heat controller  13  via heat-up roller  1 . Because the operating current thereof is approx. 20 mA, as heat-sensitive operation section  30 , a small one having excellent heat response and low heat capacity is used. 
   As mentioned above, normally, electrical conduction of both ends of heat-sensitive operation section  30  is continued. However, when the operation section exceeds a predetermined temperature while the current is carried, electrical conduction of both ends of heat-sensitive operation section  30  is discontinued. In this embodiment, used is heat-sensitive operation section  30  that leaves the inner surface of heat-up roller  1  at a temperature of 200° C. 
   In such a circuit, under normal conditions, heat-up roller  1  is controlled to keep temperatures of approx. 180° C. Heat-sensitive operation section  30  is in contact with the inner surface of heat-up roller  1 . 
   When some causes disable temperature control and bring heat-up roller  1  into a thermal runaway state during the rotation of the heat-up roller, the temperature thereof rapidly increases and then the temperature of heat-sensitive operation section  30  also increases following the temperature of heat-up roller  1 . 
   Furthermore, when the temperature rise continues and the temperature of heat-sensitive operation section  30  exceeds 200° C., heat-sensitive operation section  30  is thermally deformed in the direction of arrow “D” as shown in FIG.  8  and leaves the inner circumferential surface of heat-up roller  1 . As a result, electrical connection between heat-sensitive operation section  30  and heat-up roller  1  is discontinued, and thus no current flows. Therefore, no power is supplied from DC power source  26  to switching element driver  25 . At this time, the output of switching element driver  25  is reduced, and thus the gate of switching element  24  is turned off. As a result, no current flows through exciting coil  7  and heating section  6  stops operation. 
   As mentioned above, because heat-sensitive operation section  30  is disposed in the power line of switching element driver  25  having low current and voltage values in this embodiment, heat-sensitive operation section  30  is small and the heat capacity thereof can be made small. Therefore, temperature control against rapid temperature rise in heat-up roller  1  can be ensured. This allows prevention of rapid temperature rise in heat-resistant belt  3  heated by electromagnetic induction of heating section  6  and thus thermal deformation of the fixing unit. 
   However, when some causes hinder heat controller  13  having heat-sensitive operation section  30  from operating normally at a predetermined temperature, heat-up roller  1  may be heated rapidly. For this reason, it is desirable to provide a plurality of heat controllers  13  in fixing unit  401  using heating section  6 . 
   Therefore, in this embodiment, another heat controller  13 B including heat-sensitive operation section  29  and electrode  15  is provided at the other end of heat-up roller  1 , as shown in FIG.  9 . Heat controllers  13 A and  13 B are connected in series. 
   In addition, a metal having high thermal conductivity and low electrical resistance may be crimped over protrusions  27  and  17  formed on the tips of this heat-sensitive operation section  29  and electrode  15 , respectively. 
   As a result, even when heat-sensitive operation section  30  provided at one end of heat-up roller  1  does not operate normally at a predetermined temperature, heat-sensitive operation section  29  provided on the other end performs an interrupting operation. Therefore, overheating of heat-up roller  1  can be prevented and safe operation of fixing unit  401  can be further ensured. 
   The advantage of the above-mentioned structure of the heat controller can be obtained as well when fixing unit  401  is structured as shown in FIG.  10 . 
   In this structure, fixing unit  401  includes heat-up roller  1  heated by electromagnetic induction of heating section  6  along the outer circumferential surface of the heat-up roller, and pressurizing roller  4 . Pressurizing roller  4  is in contact with heat-up roller  1  to form a nip portion “N” and rotated in the direction identical with that of heat-up roller  1 . 
   (Second Embodiment) 
   Next, a second exemplary embodiment of the present invention is described below. 
     FIG. 11  is a longitudinal sectional view of a heat-up roller in accordance with the second exemplary embodiment of the present invention. 
   What differs from the first embodiment is only the structure of the end of the heat-up roller, and only this part is described with reference to FIG.  11 . 
   Heat-up roller  1  shown in  FIG. 11  is secured to the body of the fixing unit via flange  20 , which is an end of heat-up roller  1 , and heat-sensitive operation section  30  is rotated together with heat-up roller  1 . 
   With this structure, when heat-up roller  1  is rotated, heat-sensitive operation section  30  can be brought into intimate contact with heat-up roller  1 , without sliding. Therefore, the portion in which heat-sensitive operation section  30  is in contact with heat-up roller  1  will not be worn and thus an excellent contact condition is always maintained. This further ensures temperature adjustment against rapid temperature rise in heat-up roller  1 . 
   However, in the fixing unit of the second embodiment, when some causes hinder normal operation of temperature controller  13  having heat-sensitive operation section  30  shown in  FIG. 11 , like the first embodiment, heat-up roller  1  may be heated rapidly and damaged. For this reason, it is desirable to take countermeasures for the fixing unit using heating section  6 , assuming such accidents. 
   Then, in a fixing unit in accordance with the second embodiment, temperature controller  13  that has heat-sensitive operation section  29  as shown in  FIG. 12  instead of electrode  15  is disposed. Heat-sensitive operation sections  29  and  30  are electrically connected in series with each other. Thus, even when heat-sensitive operation section  29  does not normally work at a predetermined temperature because of failure, the other heat-sensitive operation section  30  performs an interrupting operation. Therefore, overheating of heat-up roller  1  can be prevented and safe operation of fixing unit  401  can be further ensured. 
   A metallic material having high thermal conductivity and low electrical resistance can also be crimped over protrusion  27  formed on the tip of this heat-sensitive operation section  29 . 
   Furthermore, also in this second embodiment, two heat-sensitive operation sections  29  and  30  including bimetals are provided in opposite positions along the circumferential direction of heat-up roller  1 , as shown in the cross-sectional views of the electromagnetic induction heating section of  FIGS. 13A and 13B . 
   This configuration allows either one of heat-sensitive operation sections  29  and  30  to be always disposed in a position opposite to heating section  6  via heat-up roller  1 . It is assumed that some causes may stop the rotation of heat-up roller  1 . At that time, heating section  6  will heat half of a circumferential area of heat-up roller  1  locally. In such a case, heat-sensitive operation sections  29  and  30  can immediately respond to the rapid and local temperature rise in heat-up roller  1 . 
   Now, how the periphery of rotating shaft  230  is when the heat controller of this embodiment is rotated together with heat-up roller  1  is detailed with reference to  FIGS. 16A and B . 
     FIG. 16A  is a longitudinal sectional view of a periphery of rotating shaft  230  along rotating shaft  230 . 
     FIG. 16B  is a top view of the periphery of rotating shaft  230 . 
   Inside electrode  53 B coupled to electrodes  52  is fixed. On the other hand, electrode  53 A coupled to electrodes  51  rotates with respect to fixed electrode  53 B. In other words, while electrodes  51  coupled to heat-up roller  1  and ring-like electrode  53 A rotate, ring-like electrode  53 B is fixed. 
   In addition, protrusions  54  are provided on the ends of electrodes  51  and  52 . Protrusions  54  are in contact with the inner surfaces of ring-like electrodes  53 , utilizing resilient force in the direction of arrow “A3” of the flat springs of electrodes  51  and  52 . Current flows through these electrodes  51  and  52  and the current is carried to heat-up roller  1 . 
   The advantage of the above-mentioned structure of the heat controller can be obtained as well when fixing unit  401  is structured as shown in FIG.  14 . The structure of this fixing unit has been described at the end of the explanation of the first embodiment. 
   (Third Embodiment) 
   Next, a third exemplary embodiment of the present invention is described. 
     FIG. 20  shows a structure of an example of a printer apparatus having a fixing unit in accordance with the first and second embodiments of the present invention. 
     FIG. 21  is a detailed drawing of fixing unit  401  shown in FIG.  20 . 
   The printer apparatus shown in  FIG. 20  has only one image-forming unit  101   a  for convenience in explanation. A plurality of image-forming units  101   a  can be disposed. 
   The name and function of each component is omitted because they have already been described in the first embodiment. 
   A printer apparatus having a fixing unit in accordance with the third embodiment of the present invention is described below. 
   As shown in the drawing, electric charger  301   a  disposed in the periphery of photosensitive drum  201   a  electrically charges the surface of photosensitive drum  201   a . Next, exposure unit  61   a  irradiates the surface of the above-mentioned charged photosensitive drum  201   a  with laser light  91 Y corresponding to image data to form a latent image. As a result, development section  41   a  visualizes the latent image formed on photosensitive drum  201   a  as a toner image. Then, transfer section  81   a  transfers the visualized toner image onto inter-stage transfer belt  121 . 
   Next, sheet material transfer roller  191  transfers onto recording material  11  the toner image  210  that has been transferred onto inter-stage transfer belt  121 . Thereafter, as shown in  FIG. 21 , fixing unit  401  fixes onto recording material  11  the toner image that has been transferred onto recording material  11 . 
   At this time, in the same manner as in the first and second embodiments, the heat of fixing roller  2  that is produced by heat-up roller  1  heated by heating section  6  and the pressurizing force of pressurizing roller  4  fix the toner image onto recording material  11 . 
   As mentioned above, in accordance with the first and second embodiments of the present invention, heating section  6  ensures the heating of heat-up roller  1  and temperature control. Therefore, fixing unit  401  fixes toner image onto recording material  11  in an excellent manner. In addition, when the heat-sensitive operation section leaves heat-up roller  1 , power supply to the heating section is interrupted. Therefore, the temperature of heat-up roller  1  can be detected immediately and excessive temperature rise in heat-up roller  1  can be prevented. 
   Moreover, by the use of the heat-sensitive operation section, the power supply interruption section as a heat controller is downsized and thus the component cost is reduced. 
   Shown in the above-mentioned first to third embodiments are cases where an electromagnetic induction heating method is used in heating section  6  for heating heat-up roller  1 . However, heating methods are not limited to this method, and the advantages of this invention can also be obtained with a lamp heating method using a halogen lamp.