Abstract:
An image heating apparatus has an endless movable member together with a coil for generating a magnetic flux and a core for guiding the magnetic flux. A backup member for forming a nip with the movable member has associated therewith a temperature detecting device so that power supply to the coil may be controlled on the basis of an output of that temperature detecting device, the core being sandwiched by the coil at a position upstream of the nip with respect to a movement direction of an outer periphery of the movable member and the temperature detecting device being disposed downstream of the nip.

Description:
This application is a divisional of application Ser. No. 08/980,408, filed Nov. 28, 1997, now U.S. Pat. No. 6,072,964. 
    
    
     FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to an image heating apparatus suitable for an image forming apparatus such as a copying machine or a printer. In particular, it relates to an image heating apparatus which generates heat through electromagnetic induction. 
     For the sake of convenience, the present invention will be described with reference to an image heating apparatus (fixing apparatus) which is employed in such an image forming apparatus as a copying machine or a printer, to thermally fix a toner image to recording medium. 
     In an image forming apparatus, an image (toner image) is formed in an image forming station which employs a given image forming process such as an electrophotographic process, an electrostatic recording process, or a magnetic recording, is transferred onto, or directly deposited on, the recording medium (transfer sheet, electro-fax sheet, electrostatic recording sheet, OHP sheet, printing paper, formatted paper, and the like), and then is thermally fixed as a permanent image onto the surface of the recording medium by a fixing apparatus. As for such a fixing apparatus, a thermal roller type apparatus has been widely in use. However, recently, a heating apparatus which employs a film type heating system has been put to practical use, and also, a heating apparatus based on electromagnetic induction has been proposed. 
     FIG. 21 illustrates the essential structure of a typical electromagnetic induction based fixing apparatus in accordance with the prior technology on which the present invention is based. 
     A referential FIG. 10 designates a cylindrical fixing film as a rotatory member which generates heat through electromagnetic induction. The fixing film  10  comprises a heat generating layer (electrically conductive layer, magnetic layer, resistive layer) which electromagnetically generates heat. 
     A referential FIG. 16 designates a film guide in the form of a trough having a substantially semicircular cross section. The cylindrical fixing film  10  is loosely fitted around this film guide  16 . 
     A referential FIG. 15 designates a means for generating a magnetic field, which is disposed on the inward side of the film guide  16 , and is constituted of an excitation coil  18  and a magnetic core  17 . 
     A referential FIG. 30 designates an elastic pressure roller, which is disposed so that it presses, with a predetermined pressure, upon the bottom surface of the film guide  16 , with the fixing film interposed, and forms a fixing nip N having a predetermined width. The magnetic core  17  of the magnetic field generating means  15  is squarely aligned with the fixing nip N. 
     The pressure roller  30  is rotatively driven in the counterclockwise direction, indicated by an arrow mark, by a driving means M. As the pressure roller  30  is rotatively driven, the fixing film  10  is driven in the clockwise direction indicated by another arrow mark, by the friction between the pressure roller  30  and the outward surface of the fixing film  10 , with the inward surface of the fixing film  10  sliding flatly on the bottom surface of the film guide  16 ; the fixing film  10  is rotated along the outward surface of the film guide  16  at a peripheral velocity substantially equal to the peripheral velocity of the pressure roller  30  (pressure roller driving system). 
     The film guide  16  plays a role in generating pressure in the fixing nip N, supporting the excitation coil  18  and magnetic core  17  of the magnetic field generating means  15 , supporting the fixing film  10 , and stabilizing the conveyance of the fixing film  10  while the fixing film  10  is rotatively driven. The film guide  16  is formed of dielectric material which does not interfere with the permeation of magnetic flux, and also is capable of withstanding the load it must bear. 
     The excitation coil  18  generates an alternating magnetic flux as it is supplied with an alternating electric current by an unillustrated excitation circuit. Since the alternating magnetic flux is generated so as to be concentrated to the fixing nip N, the heat generated through electromagnetic induction is also concentrated to the fixing nip N. In other words, the fixing nip N is very efficiently heated. 
     The temperature of the fixing nip N is controlled by a temperature controlling system inclusive of a temperature detecting means; it is maintained at a predetermined level by controlling the current supplied to the excitation coil  18 . 
     Reviewing the above description, as the pressure roller  30  is rotatively driven, the cylindrical fixing film  10  is rotated around the film guide  16 , and electrical current is supplied to the excitation coil  18  from the excitation circuit to generate heat in the fixing film  10  through electromagnetic induction. As a result, the temperature of the fixing nip N is increased. As the temperature of the fixing nip N reaches the predetermined level, it is maintained at this level. With the heating apparatus in this state, a recording medium P, on which a toner image t has been just deposited without being fixed thereto, is introduced into the fixing nip N, between the fixing film  10  and the pressure roller  30 , with the image bearing surface of the recording medium P facing upward so that it will come in contact with the outward surface of the film  10 . Then, the recording medium P is passed through the fixing nip N, along with the fixing film  10 , while being compressed by the pressure roller  30  and the film guide  16 , with the image bearing surface being flatly in contact with the outward surface of the fixing film  10 . While the recording medium P with the toner image t is passed through the fixing nip N as described above, the toner image t which is borne on the recording medium P, but is yet to be fixed, is heated by the heat electromagnetically induced in the fixing film  10 , being thereby fixed to the recording medium P. After passing through the fixing nip N, the recording medium P separates from the outward surface of the rotating fixing film  10 , and is conveyed further to be discharged from the image forming apparatus. 
     In terms of preciseness in heating a toner image using a fixing apparatus which employs an electromagnetic induction system such as the system described above, it is desirable that the temperature detecting means of the fixing apparatus detects the temperature of the fixing film  10  itself, which actually comes in contact with the toner image t. However, if a temperature detection element for measuring the temperature of the fixing film  10  is placed in contact with the outward surface of the fixing film  10 , the film surface is liable to be damaged, and if the film surface is damaged, the damaged surface causes the offset of the fixed toner image. This is one of the problems of the image heating apparatus based on the prior art. In addition, if the fixing film  10  is rotated at an extremely high speed, it is rather difficult to maintain stable contact between the temperature detection element and the fixing film  10 , hence the accuracy of the detected temperature deteriorates. As a result, the temperature of the fixing film  10  cannot be reliably controlled, which is another problem. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an image heating apparatus capable of detecting the temperature of a moving member without damaging the surface of the moving member which generates heat through electromagnetic induction. 
     Another object of the present invention is to provide an image heating apparatus in which stable contact is maintained between a moving member which generates heat through electromagnetic induction, and a temperature detecting means. 
     Another object of the present invention is to provide an image heating apparatus in which a temperature detecting means is in contact with the inward facing surface of an endless moving member which generates heat through electromagnetic induction. 
     These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic illustration of an image forming apparatus which employs the fixing apparatus in an embodiment of the present invention, and it depicts the general structure the fixing apparatus. 
     FIG. 2 is a schematic cross section of the essential portion of a fixing apparatus as a heating apparatus. 
     FIG. 3 is a schematic front view of the essential portion of the heating apparatus illustrated in FIG.  2 . 
     FIG. 4 is a schematic longitudinal section of the essential portion of the heating apparatus illustrated in FIG.  2 . 
     FIG. 5 is a perspective view of a film guide, an excitation coil, and a magnetic core. 
     FIG. 6 is an explanatory drawing which depicts the relationship between magnetic flux and the amount of heat generated by a fixing film. 
     FIG. 7 is an enlarged view of the section surrounded by a dotted line in FIG.  2 . 
     FIG. 8 is an explanatory drawing which depicts a temperature detecting means. 
     FIG. 9 is a schematic drawing of a temperature sensor. 
     FIG. 10 is a picture of a mounted temperature sensor as seen from the direction in which the fixing film is moved in a fixing nip. 
     FIG. 11 is an explanatory drawing which depicts another embodiment of the present invention. 
     FIG. 12 is an explanatory drawing which depicts another embodiment of the present invention. 
     FIG. 13 is an explanatory drawing which depicts a temperature detecting means. 
     FIG. 14 is a schematic vertical section of a fixing film. 
     FIG. 15 is a graph which shows the relationship between the depth in a heating layer and the strength of the electromagnetic wave. 
     FIG. 16 is a schematic vertical section of another fixing film. 
     FIG. 17 is a schematic cross section of the essential portion of the heating apparatus in another embodiment of the present invention. 
     FIG. 18 is an explanatory drawing which depicts another temperature detecting means. 
     FIG. 19 is a schematic cross section of the fixing apparatus in another embodiment of the present invention. 
     FIG. 20 is a schematic cross section of the fixing apparatus in another embodiment of the present invention. 
     FIG. 21 is a schematic cross section of an electromagnetic induction type heating apparatus based on the prior technology, or the background technology of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the embodiments of the present invention will be described with reference to the drawings. 
     (1) Image Forming Apparatus in Accordance with the Present Invention 
     FIG. 1 is a schematic vertical section of a typical image forming apparatus compatible with any of the image heating apparatuses in the following embodiments of the present invention. 
     A referential figure 101 designates a photosensitive drum (image bearing member) composed of organic photosensitive material, or amorphous silicon, and rotatively driven in the counterclockwise direction indicated by an arrow mark, at a predetermined process speed (peripheral velocity). 
     The photosensitive drum  101  is uniformly charged to predetermined polarity and potential by a charging apparatus  102  such as a charge roller. 
     The uniformly charged surface of the photosensitive drum  101  is exposed to a scanning laser beam  103  which carries the image data of a target image, and is projected from a laser optical box (laser scanner)  110 ; the laser optical box  110  projects the laser beam  103  while modulating it (on/off) in accordance with sequential electrical digital signals which reflect the image data of the target image. As a result, an electrostatic latent image correspondent to the image data of the target image is formed on the peripheral surface of the rotatory photosensitive drum  101 . The sequential electrical digital signals are supplied from an image signal generation apparatus such as an image reading apparatus, which is not illustrated in the drawing. A referential figure 109 designates a mirror which deflects to the laser beam projected from the laser optical box  110 , onto a point to be exposed on the photosensitive drum  101 . 
     In full-color image formation, a target image is subjected to a color separation process in which the color of the target image is separated into, for example, four primary color components. Then, the above described scanning exposure and image formation processes are carried out for each of the primary color components, starting from, for example, yellow component. The latent image correspondent to the yellow color component is developed into a yellow toner image by the function of a yellow color component developing device  104 Y of a color developing device  104 . Then, the yellow toner image is transferred onto the peripheral surface of an intermediary transfer drum  105 , at a primary transfer point T 1 , which is the contact point of the photosensitive drum  101  and the intermediary transfer drum  105  (or the point at which the distance between the photosensitive drum  101  and the intermediary transfer drum  105  becomes smallest). After the toner image is transferred onto the surface of the intermediary transfer drum  105 , the peripheral surface of the photosensitive drum  101  is cleaned by a cleaner  107 ; foreign matters such as the residual toner particles from the transfer are removed from the peripheral surface of the photosensitive drum  101  by the cleaner  107 . 
     Next, a process cycle comprising the above described charging process, scanning/exposing process, developing process, primary transfer process, and cleaning process is also carried out for the rest (second, third, and fourth) of the primary color components of the target image. More specifically, for the latent image correspondent to the second primary color component, that is, magenta color component, a magenta color component developing device  104 M is activated; for the latent image correspondent to the third primary color components, a cyan color component developing device  104 C; and for the latent image for the fourth color component, a black color component developing device  104 BK is activated. As a result, a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image are superposed in the aforementioned order on the peripheral surface of the intermediary transfer drum  105 , effecting a compound full-color toner image of the target image. 
     The intermediary transfer drum  105  comprises a metallic drum, an elastic middle layer with medium resistance, and a surface layer with high resistance. It is disposed so that its peripheral surface is placed in contact with, or extremely close to, the peripheral surface of the photosensitive drum  101 . It is rotatively driven in the counterclockwise direction indicated by the arrow mark, at substantially the same peripheral velocity as that of the photosensitive drum  101 . The toner image on the photosensitive drum  101  is transferred onto the peripheral surface of the intermediary transfer drum  105  using the potential difference created by applying a bias voltage to the metallic drum of the intermediary transfer drum  105 . 
     The compound full-color toner image formed on the peripheral surface of the intermediary transfer drum  105  is transferred onto the surface of a recording medium P, at a secondary transfer point T 2 , that is, a contact nip between the intermediary transfer drum  105  and a transfer roller  106 . The recording medium P is delivered to the secondary transfer point T 2  from an unillustrated sheet feeding portion with a predetermined timing. The transfer roller  106  transfers all at once the compound color. toner image from the peripheral surface of the intermediary transfer drum  105  onto the recording medium P by supplying the recording medium P with charge having such polarity that is opposite to the polarity of the toner, from the back side of the recording medium P. 
     After passing through the secondary transfer point T 2 , the recording medium P is separated from the peripheral surface of the intermediary transfer drum  105 , and then is introduced into an image heating apparatus (fixing apparatus)  100 , in which the compound full-color toner image composed of layers of toner particles of different color is thermally fixed to the recording medium P. Thereafter, the recording medium P is discharged from the image forming apparatus into an unillustrated delivery tray. The fixing apparatus  100  will be described in detail in section (2). 
     After the compound full-color toner image has been transferred onto the recording medium P, the intermediary transfer drum  105  is cleaned by a cleaner  108 ; the residue, such as the residual toner from the secondary transfer or paper dust, on the intermediary transfer drum  105  is removed by the cleaner  108 . Normally, the cleaner  108  is kept away from the intermediary transfer drum  105 , and when the full-color toner image is transferred from the intermediary transfer drum  105  onto the recording medium P (secondary transfer), the cleaner  108  is placed in contact with the intermediary transfer drum  105 . 
     Also, the transfer roller  106  is normally kept away from the intermediary transfer drum  105 , and when the full-color toner image is transferred from the intermediary transfer drum  105  onto the recording medium P (secondary transfer), the transfer roller  106  is pressed on the intermediary transfer drum  105 , with the interposition of the recording medium P. 
     The image forming apparatus illustrated in FIG. 1 can be operated in a monochromatic mode, for example, a black-and-while mode. It also can be operated in a double-sided mode, as well as a multi-layer printing mode. 
     In a double-sided mode, after an image is fixed to one (first) of the surfaces of the recording medium P, the recording medium P is delivered to an unillustrated recirculating mechanism, in which the recording medium P is turned over, and then, is fed into the secondary transfer point T 2  for the second time so that another toner image is transferred onto the other (second) surface. Then, the recording medium P is sent into the image heating apparatus for the second time, in which the second toner image is fixed. Therefore, the recording medium P is discharged as a double-side print from the main assembly of the image forming apparatus. 
     In a multi-layer mode, after coming out of the image heating apparatus  100 , with the first image on the first surface, the recording medium P is sent into the secondary transfer point T 2  for the second time, without being turned over through the recirculating mechanism. Then, the second image is transferred onto the first surface, to which the first image has been fixed. Then, the recording medium P is introduced into the image heating apparatus  100  for the second time, in which the second toner image is fixed. Thereafter, the recording medium P is discharged as a multi-layer image print from the main assembly of the image forming apparatus. 
     The toner used in this embodiment is such toner that contains ingredients which control the excessive softening of the toner. 
     (2) Fixing Apparatus  100   
     FIG. 2 is a schematic cross section of the essential portion of the fixing apparatus  100  in this embodiment, and FIG. 3 is a schematic front view of the portion illustrated in FIG.  2 . FIG. 4 is a longitudinal, vertical section of the portion illustrated in FIG.  2 . 
     The fixing apparatus  100  is the same type of apparatus as the fixing apparatus illustrated in FIG. 21, hence it employs a cylindrical film, that is, the rotatory member, which generates heat through electromagnetic induction, and is driven by a pressure roller. Therefore, its components or portions which are the same as those of the apparatus illustrated in FIG. 21 are designated with the same referential codes to eliminate repetition of the same descriptions. 
     Magnetic cores  17   a ,  17   b  and  17   c  are members with high magnetic permeability. As for the material for these cores, material such as ferrite or permalloy which is used as the material for a transformer core is desirable; preferably, ferrite in which loss is small even when operational frequency is above 100 kHz. 
     A referential code  16   a  designates a film guide in which the magnetic cores  17   a ,  17   b  and  17   c , and an excitation coil  18 , are disposed. A referential code  16   b  designates a top film guide, which is in the form of a trough with a substantially semicircular cross section, and is placed on top of the film guide  16   a  in a manner to cover the opening of the film guide  16   a , forming a substantially cylindrical column, together with the film guide  16   a.    
     Around the assembly constituted of the film guides  16   a  and  16   b , the electromagnetic induction based heat generating endless (cylindrical) film  10  (fixing film), that is, the movable member, is loosely fitted. 
     A referential figure 22 designates a rigid pressing stay, which is oblong and is placed in contact with the flat top portions of the film guide  16   a  in which the magnetic cores  17   a ,  17   b , and  17   c , and the excitation coil  18 , are disposed. 
     Designated with a referential figure 19 is an electrically insulative member which electrically insulates between the magnetic core  18  and the rigid pressing stay  22 . 
     Referential codes  23   a  and  23   b  designate flanges, which are fitted, one for one, around the longitudinal ends of the assembly constituted of the film guides  16   a  and  16   b , to regulate the edges of the fixing film  10  and retain the fixing film  10 . They are capable of following the rotation of the fixing film  10 . 
     The pressure roller  30  as a backup member comprises a metallic core  30   a  and an elastic layer  30   b . The elastic layer  30   b  is concentrically formed around the metallic core  30   a , covering the peripheral surface of the core  30   a , and is composed of heat resistant material such as silicone rubber, fluorinated rubber, fluorinated resin, or the like. The pressure roller  30  is fitted between unillustrated side plates of the main assembly of the image forming apparatus, being rotatively supported by bearings, at the respective longitudinal ends of the metallic core  30   a.    
     On the top side of the pressure roller  30 , a heating means unit, which comprises the aforementioned film guide  16   a , magnetic cores  17   a ,  17   b  and  17   d , excitation coil  18 , tip film guide  16   b , rigid pressure stay  22 , insulative member  19 , fixing film  10 , flanges  23   a  and  23   b , etc., is disposed with the semicircular bottom side of the film guide  16   a  facing downward. Between the longitudinal ends of the rigid pressing stay  22 , and the spring seats  29   a  and  29   b , springs  25   a  and  25   b  are fitted, respectively, in a state of compression, to press the rigid pressing stay  22  downward. With this arrangement, a fixing nip N with a predetermined width is formed, in which the fixing film  10  is sandwiched between the bottom surface of the film guide  16   a  and the upward facing peripheral surface of the pressure roller  30 . The bottom surface of the magnetic core  17   a  is squarely aligned with the fixing nip N, sandwiching the bottom portion of the film guide  16   a.    
     The pressure roller  30  is rotatively driven by a driving means M in the counterclockwise direction indicated by an arrow mark. As the pressure roller  30  is rotationally driven, rotational force is applied to the fixing film  10  by the friction between the pressure roller  30  and the outward surface of the fixing film  10 , whereby the fixing film  10  is rotated along the peripheral surfaces of the film guides  16   a  and  16   b  in the clockwise direction indicated by another arrow mark, at a peripheral velocity substantially equal to the peripheral velocity of the pressure roller  30 . In the fixing nip N, the inward surface of the fixing film  10  slides on the bottom surface of the film guide  16   a , flatly in contact with the surface. 
     With the above setup, in order to reduce the friction between the bottom surface of the film guide  16   a  and the inward surface of the fixing film  10 , lubricant such as heat resistant grease may be placed between the bottom surface of the film guide  16   a  and the inward surface of the fixing film  10 , or the bottom surface of the film guide  16   a  may be coated with lubricous material such as mold releasing agent. 
     The film guide  16   a  applies pressure to the fixing nip N, and supports the magnetic cores  17   a ,  17   b  and  17   c , and the excitation coil  18 . Also, it supports the fixing film  10  in cooperation with the top film guide  16   b , playing a role in providing the fixing film  10  with stability when the fixing film  10  is rotated. 
     FIG. 5 is a perspective view of the film guide  16   a , in which the magnetic cores  17   b  and  17   c  are not illustrated. A referential code  16   e  designates each of a plurality of ribs which protrude outward from the peripheral surface of the film guide  16   a , and run in parallel in the circumferential direction, with equal intervals. These protuberant ribs  16   e  are effective to reduce the friction between the outward surface of the film guide  16   a  and the inward surface of the fixing film  10 , so that the rotational load borne by the fixing film  10  is reduced. The film guide  16   b  may also be provided with protuberant ribs similar to these ribs  16   b.    
     The excitation coil  18  disposed within the film guide  16   a  is connected to an excitation circuit  27  through the power supply lead wires  18   a  and  18   b  of the excitation coil  18 . This excitation circuit  27  is capable of generating high frequency waves ranging from 20 kHz to 500 kHz with the use of a switching power source. The excitation coil  18 , the magnetic cores  17   a ,  17   b , and  17   c , the excitation circuit  27 , etc., constitute a means for generating magnetic flux. 
     The excitation coil  18  within the film guide  16   a  is caused to generate alternating magnetic flux, by alternating current (high frequency current) supplied from the excitation circuit  27 . 
     FIG. 6 schematically depicts the direction and distribution of the alternating magnetic flux adjacent to the fixing nip N. A magnetic flux C represents a portion of the alternating magnetic flux. 
     As for the distribution of the alternating magnetic flux (C), the alternating magnetic flux (C) is guided by the magnetic cores  17   a ,  17   b , and  17   c  to be concentrated between the magnetic cores  17   a  and  17   b , and between the magnetic cores  17   a  and  17   c , generating eddy current in the electromagnetic induction based heat generating layer  1  of the fixing film  10 . This eddy current generates Joule heat (eddy current loss) in the electromagnetic induction based heat generating layer  1 , in accordance with the specific resistance of the heat generating layer  1 . The amount of the heat generated by the electromagnetic induction based heat generating layer  1  is determined by the density of the magnetic flux which permeates through the electromagnetic induction based heat generating layer  1 , and is distributed as shown by the graph in FIG.  6 . In FIG. 6 which is a graph, the locational points on the fixing film  10  are plotted on the abscissa, being expressed by the angle θ from the center (0°) of the fixing nip, and the amount of the heat generated in the electromagnetic induction based heat generating layer  1  of the fixing film  10  is plotted on the axis of ordinates. 
     FIG. 7 is an enlarged view of the section adjacent to a temperature detecting element  50 , surrounded by a dotted line in FIG.  2 . FIG. 8 is a detailed picture of the temperature detecting element  50  illustrated in FIG.  7 . 
     The temperature of the fixing nip N is maintained at a predetermined level by a CPU which controls the electric current supplied to the excitation coil  8  through the excitation circuit, while detecting the temperature data through the temperature detecting element  50 . The temperature detecting element  50 , which detects the temperature of the fixing film  10 , is a temperature sensor such as a thermistor. In this embodiment, a temperature detecting means which comprises the temperature sensor  50  is placed in contact with the inward surface of the fixing film  10 , on the area immediately before the fixing nip N, and the temperature of the fixing film  10  is controlled based on the temperature data from the temperature sensor  50  placed as described above. 
     FIG. 9 depicts the structure of the temperature sensor  50 . The structure of the temperature sensor  50  is such that a thermistor portion  50   b , that is, the temperature sensing portion, which has a negative temperature coefficient, and an electrode  50   a , are printed, in a pattern, on the ceramic substrate  50   c.    
     The electrode  50   a  of the temperature sensor  50 , and a thin metallic electrode  51   a , are glued together with unillustrated electrically conductive adhesive. The temperature sensor  50  is attached to an elastic, thermally conductive, thin metallic plate  51  as a supporting member. These components constitute a temperature detecting means  60 . 
     The thin metallic plate  51  comprises the thin metallic plate electrode  51   a , and a thin metallic guide plate  51   b  for protecting the thin metallic electrode  51   a , and this thin metallic plate  51  is sandwiched between electrically insulative coats  52  to electrically insulate the thin metallic plate  51  from the fixing film  10 . In this embodiment, the thin metallic plate  51  is a gold plated 0.07 mm thick plate of SUS  304 . The thickness of the thin metallic plate  51  is desired to be no more than 0.2 mm since the smaller the thermal capacity of the thin metallic plate  51 , the more advantageous the thin metallic plate  51 , in terms of thermal responsiveness. As for the material for the insulative coat  52 , 50 μm thick polyimide film is used. Since the insulative coat  52  has only to provide electrical insulation, the thinner the better. 
     In FIG. 8, in order to make it easier to identify the insulative coat  52 , it is drawn as if separated from the thin metallic plate  51 . However, in reality, the insulative coat  52  is placed perfectly in contact with the thin metallic plate  51 ; it may be glued to the thin metallic plate  51 . 
     A referential figure 51 designates the mount for the thin metallic plate  51 , and the lead wires to the temperature detection circuit are extended from this mount. 
     The thin metallic plate  51  is placed so that its longitudinal direction becomes parallel to the direction of the magnetic field (moving direction of the fixing film), and its widthwise direction becomes perpendicular to the magnetic field. This is due to the fact that eddy current is generated by electromagnetic induction, in the direction perpendicular to the direction of the magnetic flux, hence the amount of the eddy current to be generated can be reduced by reducing the dimension of the thin metallic plate  51  in the direction perpendicular to the direction of the magnetic flux (widthwise direction of the thin metallic plate  51 ). As long as the width of the thin metallic plate  51  is no more than 10 mm, the amount of the heat generated in the thin metallic plate  51  itself is so small that it does not have a negative effect on the temperature detection of the fixing film  10  by the temperature sensor  50 . The contact area between the thin metallic plate  51  and the fixing film  10  is larger than the surface area of the temperature sensor  50 . 
     The thin metallic plate  51  is bent at a point  54  and follows the curvature of the fixing film  10 , in contact with the inward surface of the fixing film  10 . The point  54  corresponds to the edge of the film guide  16   a  in FIG.  7 . The temperature sensing portion  50   b  in this embodiment is between two thin metallic electrodes  51   a , and the thin metallic plate  51  makes contact with the fixing film  10 , by the surface opposite to the surface to which the temperature sensor  50  is attached. 
     Referring to FIG. 10, an angle θ1, that is, the angle at which the thin metallic plate  51  is attached relative to the rotational direction of the fixing film  10 , in other words, the angle of the line connecting the point  54  of the thin metallic plate  51  and the temperature sensor  50 , relative to the rotational direction of the fixing film  10 , is desired to satisfy the following formula: −30°≦θ1 ≦30°. This is because if the angle θ1 is out of the above range, the thin metallic plate  51  is liable to be turned over by the friction, and if the thin metallic plate  51  is turned over, the thin metallic plate  51  and the fixing film  10  fail to make proper surface-to-surface contact with each other. 
     As for the relationship between the point  54  and the thin metallic plate  51 , the shortest distance L 1  between the point  54  and the fixing film  10 , and the length L 2  of the thin metallic plate  51 , are desired to satisfy a formula: L 2 ≧2×L 1 . This is because a thin metallic plate  51  which satisfies a formula: L 2 &lt;2×L 1 , is too short to be placed satisfactorily in contact with the fixing film  10 ; the thin metallic plate  51  is liable to remain partially separated from the fixing film  10  due to the friction between the thin metallic plate  51  and the fixing film  10 . Thus, it is desirable that the formula: L 2 ≧2×L 1 , is satisfied. 
     With the provision of the above described structure, the size of the area, by which the thin metallic plate  51  makes surface-to-surface contact with the fixing film  10 , becomes greater as the thin metallic plate  51  is pressured by the fixing film  10 , and therefore, not only the contact between the thin metallic plate  51  and the fixing film  10  becomes more stable, but also the thermal conductivity between the fixing film  10  and the temperature sensor  50  is improved. As a result, the accuracy and responsiveness of the temperature sensor  50  in detecting the temperature of the fixing film  10  are greatly improved. 
     According to this embodiment, the temperature sensor  50  constitutes a protrusion on the thin metallic plate  51 . However, the thin metallic plate  51  makes contact with the fixing film  10  by the surface opposite to the surface with the temperature sensor  50 , and therefore, the fixing film  10  is not in danger of being damaged by the protrusion. 
     Also, the temperature sensing portion  50   b  of the temperature sensor  50  is embedded between the two thin plate electrodes  50   a , and therefore, the temperature sensing portion  50   b  can be placed much closer to the fixing film  10  than otherwise, to improve the responsiveness of the temperature sensor  50 . 
     Further, according to this embodiment, the temperature detecting means is substantially immune to the effects of the generated magnetic field, and therefore, the thicknesses of the members which constitute the temperature detecting means can be reduced to produce a temperature detecting means, such as the one described in this embodiment, which is small in thermal capacity, and is very efficient in terms of space utilization, so that it can be placed in a minuscule space between the fixing film  10  and the film guide  16   a.    
     Further, according to this embodiment, the temperature sensor  50  is placed virtually in contact with the fixing film  10 , with the interposition of the thin metallic plate  51  and the insulative coat  52 . However, when a reasonable degree of responsiveness is all that is necessary as it is in the case of a slow image forming apparatus like a low speed laser beam printer, and also there is no danger of the fixing film  10  being damaged, the positional relationship between the temperature sensor  50  and thin metallic plate  51  may be reversed; the temperature sensor  50  may be placed directly in contact with the fixing film  10 , in other words, without the interposition of the thin metallic plate  51 . In this case, only the temperature sensor  50  may be placed in contact with the fixing film  10  as illustrated in FIG. 11, or both the thin metallic plate  51  and the temperature sensor  50  may be placed in contact with the fixing film  10  as illustrated in FIG. 12, in order to increase the thermal conductivity between the two components. FIG. 13 is a detailed illustration of the temperature sensing portion extracted from FIG. 11 or  12 . 
     Thus, as the pressure roller  30  is rotatively driven, the cylindrical fixing film  10  is rotated along the outward surfaces of the film guide  16   a  and the top film guide  16   b , and electrical current is supplied to the excitation coil  18  within the film guide from the excitation circuit to generate heat in the fixing film  10  through electromagnetic induction. As a result, the temperature of the fixing nip N is increased. As the temperature of the fixing nip N reaches the predetermined level, it is maintained at this level. With the heating apparatus in this state, a recording medium P, on which a toner image t has been just deposited without being fixed thereto, is introduced into the fixing nip N. between the fixing film  10  and the pressure roller  30 , with the image bearing surface of the recording medium P facing upward so that it will come in contact with the outward surface of the film  10 . Then, the recording medium P is passed through the fixing nip N, along with the fixing film  10 , while being compressed by the pressure roller  30  and the film guide  16 , with the image bearing surface being flatly in contact with the outward surface of the fixing film  10 . While the recording medium P, bearing the yet-to-be-fixed toner image t, is passed through the fixing nip N as described above, this toner image t borne on the recording medium P is heated by the heat electromagnetically induced in the fixing film  10 , being thereby fixed to the recording medium P. After passing through the fixing nip N, the recording medium P separates from the outward surface of the rotating fixing film  10 , and is conveyed further to be discharged from the image forming apparatus. After passing through the fixing nip N while being thermally fixed to the recording medium P, the toner image cools down and becomes a permanently fixed image. 
     In this embodiment, such toner that contains ingredients, which control the excessive softening of the toner, is used, and therefore, the fixing apparatus is not provided with an oil coating mechanism for offset prevention. When toner which does not contain the softening controlling ingredient is used, the fixing apparatus may be provided with an oil coating mechanism. Further, even when the toner which contains the softening controlling ingredient is used, the oil may be applied and the recording medium P may be separated by cooling. 
     Next, the excitation coil  18  and fixing film  10  will be described. 
     (A) Excitation Coil  18   
     The material for the excitation coil  18  is copper. More specifically, a plurality of fine copper wires, each of which is individually coated with electrically insulative material, are bundled, and this bundle of insulator coated fine wires is wound a given number of turns to form the excitation coil  18 . In this embodiment, the bundle of wires is wound 12 times. 
     As for the insulator for coating the copper wires, heat resistant insulator is recommendable in consideration of the conduction of the heat generated in the fixing film  10 . In this embodiment, polyimide is used to coat the fine wires. The thermal deformation point of the insulative coat is 220° C. 
     The density of the coil wires may be increased by applying external pressure to the excitation coil  18 . 
     In order to make the heat generating layer of the fixing film  10  efficiently absorb the magnetic field generated by the excitation coil  18  and the magnetic cores  17   a ,  17   b , and  17   c , the distances between the excitation coil  18  and the heat generating layer  1  of the fixing film  10 , and between the magnetic cores  17   a ,  17   b , and  17   c  and the heat generating layer  1  of the fixing film  10 , are desired to be as small as possible. 
     Therefore, in this embodiment, the excitation coil  18  is shaped to conform to the curvature of the heat generating layer  1 , as illustrated in FIG.  2 . The distance between the heat generating layer  1  of the fixing film  10  and the excitation coil  18  is set at approximately 1 mm. 
     As for the material for the film guides  16   a  and  16   b , electrically insulative and heat resistant material is recommendable in order to satisfactorily insulate the excitation coil  18  from the fixing film  10 . For example, phenol resin, fluorinated resin, polyimide resin, polyamide resin, polyamide-imide resin, PEEK resin, PES resin, PPS resin, PFA resin, PTFE resin, FEP resin, LCP, and the like are desirable candidates for the selection. 
     The wires  18   a  and  18   b , which lead from the excitation coil  18 , and are put through the film guide  16   a , are covered with insulative coating, on the portions outside the film guide  16   a.    
     (B) Fixing Film  10   
     FIG. 14 is a schematic vertical section of the fixing film  10  in this embodiment. This fixing film  10  has a compound (laminar) structure, that is, an electrically conductive layer, comprising: the heat generating layer  1 , which is formed of metallic film or the like, and constitutes the base layer of the fixing film  10 ; the elastic layer  2  laid on the outward surface of the heat generating layer  1 ; and the lubricous layer  3  laid on the outward surface of the elastic layer  2 . In order to assure the adhesion between the heat generating layer  1  and the elastic layer  2 , and the adhesion between the elastic layer  2  and the lubricous layer  3 , primer layers (unillustrated) may be placed between the correspondent layers. The heat generating layer  1  is on the inward side of the cylindrical fixing film  10 , and the lubricous layer  3  is on the outward side. As described above, as alternating magnetic flux acts on the heat generating layer  1 , eddy current is generated in the heat generating layer  1 , and this eddy current generates heat in the heat generating layer  1 . The thus generated heat heats the fixing film  10  through the elastic layer  2  and the lubricous layer  3 , and in turn, the fixing film  10  heats the recording medium, that is, an object to be heated, which is being passed through the fixing nip N, to thermally fix the toner image. 
     a. Heat Generating Layer  1   
     The heat generating layer  1  may be composed of nonmagnetic metal, but usage of highly magnetic material such as nickel, iron, magnetic SUS, nickel-cobalt alloy, or the like is preferable. 
     As for the thickness of the heat generating layer  1 , it is desired to be no less than the skin depth σ (m) expressed by the formula given below, and no more than the 200 μm: 
     
       
         σ=503×(ρ/fμ) ½   
       
     
     wherein, a referential code f stands for the frequency (Hz) of the excitation circuit; μ, the magnetic permeability; and ρ stands for specific resistance (Ωm). 
     The thickness of the heat generating layer  1  is desired to be in a range of 1-100 μm. If the thickness of the heat generating layer  1  is no more than 1 μm, all the electromagnetic energy cannot be a absorbed; heat generating efficiency deteriorates. If the thickness of the heat generating layer  1  exceeds 100 μm, the heat generating layer  1  becomes too rigid; in other words, its flexibility is lost too much to be practically used as a rotatory member. Hence, it is desirable that the thickness of the heat generating layer  1  is in a range of 1-100 μm. 
     b. Elastic Layer  2   
     The elastic layer  2  is composed of such material that is good in heat resistance and thermal conductivity; for example, silicone rubber, fluorinated rubber, fluoro-silicone rubber, and the like. 
     The thickness of the elastic layer  2  is desired to be in a range of 10-500 μm, which is necessary to assure the quality of the fixed image after fixation. 
     When printing a color image, in particular, a photographic image, a large proportion of the recording medium P surface is likely to be solidly covered with toner. In such a case, if the actual heating surface (lubricous surface layer  3 ) cannot conforms to the irregularities of the recording medium P surface, or toner layer, heating becomes nonuniform, creating difference in glossiness between the areas to which a relatively large amount of heat is conducted, and the areas to which a relatively small amount of heat is conducted; the areas which receive a relatively large amount of heat displays a higher degree of glossiness than the areas which receive relatively small amount of heat. As for the thickness of the elastic layer  2 , if it is no more than 10 μm, it fails to conform to the irregularities of the toner layer, and causes glossiness to be uneven across the images. If it exceeds 1,000 μm, the thermal resistance of the elastic layer  2  becomes too large for a fixing apparatus to be quickly started up. Therefore, the thickness of the elastic layer  2  is preferably in a range of 50-500 μm. 
     As for the hardness of the elastic layer  2 , the excessive hardness of the elastic layer  2  does not allow the elastic layer  2  to conform to the irregularities of the recording medium surface or the toner layer, causing glossiness to be uneven across an image. Hence, it is desirable that the hardness of the elastic layer  2  is no more than 60° (JIS-A), preferably, no more than 45° (JIS-A). 
     The thermal conductivity λ of the elastic layer  2  is desired to be 6×10 −4 ˜2×10 −3  (cal/cm·sec·deg.): 
     
       
         λ=6×10 −4 ˜2×10 −3  (cal/cm·sec·deg.). 
       
     
     When the thermal conductivity λ is no more than 6×10 −4  (cal/cm·sec·deg.), the thermal resistance becomes large, which slows down the speed at which the temperature of the surface layer (lubricous layer  3 ) of the fixing film  10  rises. 
     When the thermal conductivity λ is no less than 2×10 −3  (cal/cm·sec·deg.), the hardness of the elastic layer  2  increases too much, and also the permanent deformation of the elastic layer  2  caused by compression worsens. 
     Therefore, it is desirable that the heat conductivity is in the range of 6×10 −4 ˜2×10 −3  (cal/cm·sec·deg.), preferably in a range of 8×10 −4 ˜1.5×10 −3  (cal/cm·sec·deg.). 
     c. Lubricous Layer  3   
     As for the material for the lubricous layer  3 , it can be selected from among such material as fluorinated resin, silicone resin, fluoro-silicone rubber, fluorinated rubber, silicone rubber, PFA, PTFE, FEP, or the like, which is desirable in terms of lubricity (mold releasing properties) and heat resistance. 
     The thickness of the lubricous layer  3  is desired to be in a range of 1-100 μm. If the thickness of the lubricous layer  3  is no more than 1 μm, the unevenness of the lubricous layer  3  manifests as lubricous unevenness, creating spots inferior in lubricity or durability. On the other hand, if the thickness of the lubricous layer  3  is no less than 100 μm, thermal conductivity deteriorates; in particular, if the lubricous layer  3  is composed of resin, the hardness of the lubricous layer  3  becomes too high to be effective as the elastic layer  2 . 
     Referring to FIG. 16, in the laminar structure of the fixing film  10 , a thermally insulative layer  4  may be disposed on the exposed surface (surface opposite to the elastic layer  2 ) of the heat generating layer  1 . 
     As for the material for the thermally insulative layer  4 , heat resistant resin, for example, fluorinated resin, polyimide resin, polyamide resin, polyamide-imide resin, PEEK resin, PES resin, PPS resin, PFA resin, PTFE resin, FEP resin, or the like is recommendable. 
     As for the thickness of the thermally insulative layer  4 , it is desired to be in a range of 10-1,000 μm. If the thickness of the thermally insulative layer  4  is no more than 10 μm, the layer  4  is not effective as a thermally insulative layer, and also lacks durability. On the other hand, if the thickness of the thermally insulative layer  4  exceeds 1,000 μm, the distance from the magnetic cores  17   a ,  17   b , and  17   c  to the heat generating layer  1  becomes too large to allow the magnetic flux to be sufficiently absorbed by the heat generating layer  1 . 
     The thermally insulative layer  4  prevents the heat generated in the heat generating layer  1  from conducting inward of the loop of the fixing film  10 , and therefore, the ratio of the heat conducted toward the recording medium P increases compared to when the thermally insulative layer  4  is not present. As a result, power consumption decreases. 
     As is evident from the above description, according to this embodiment, the temperature detecting means is placed in contact with the inward surface of the fixing film, and therefore, the film temperature can be detected without fear of damaging the outward surface of the film, eliminating negative effect of the contact between the temperature detecting means and the fixing film. Further, the temperature detection element is first attached to a resilient thin metallic plate, and then, the thin metallic film is placed in contact with the fixing film. Therefore, the thermal relationship between the temperature detection element and the fixing film is stabilized. In addition, since the thin metallic film which has a wider contact area than the temperature detection element itself is interposed between the temperature detection element and the fixing film, the heat from the fixing film is more reliably conducted to the temperature detection element. Therefore, the responsiveness of the temperature detection element in terms of temperature detection is improved, hence the fixing film temperature can be controlled with high accuracy. 
     Next, another embodiment of the present invention will be described. 
     Referring to FIGS. 7 and 8, in this embodiment, a temperature sensor  50  is disposed after the fixing nip N relative to the rotational direction of the fixing film. Otherwise, the structure of the fixing apparatus in this embodiment is identical to that in the preceding embodiment. Therefore, the components and the portions thereof which are identical to those in the preceding embodiment are designated with the identical referential codes to omit the repetition of the same description. 
     Also in this embodiment, the thin metallic plate  51  is fixed to the mount  53  by one of the longitudinal ends, leaving the other end as a free end. However, in this embodiment, the thin metallic plate  51  is installed in a manner to oppose the rotational direction of the fixing film  10 ; the free end of the thin metallic plate  51  is on the upstream side relative to the rotational direction of the fixing film  10 . With this arrangement, the thin metallic plate  51  is more firmly pressed against the fixing film  10  by the friction between the thin metallic plate  51  and the fixing film  10  than otherwise. Therefore, the size of the contact area between the fixing film  10  and the thin metallic plate  51  is further increased, hence more effectively conducting the heat, and in addition, the contact between the fixing film  10  and thin metallic plate  51  is more stabilized. 
     Placing the thin metallic plate  51  in contact with the fixing film  10  in the counter direction to the rotational direction of the fixing film  10  increases the contact pressure between the thin metallic plate  51  and the fixing film  10 , and therefore, heat is more effectively conducted. As a result, the responsiveness of the temperature sensor  50  is improved; heat detection accuracy is improved. It should be noted here that if the revolution of the fixing film  10  reaches a high level, with the thin metallic plate  51  being fitted in conformity with the rotational direction of the fixing film as it is in the preceding embodiment, the friction between the thin metallic plate  51  and the fixing film works in the direction to cause the thin metallic plate  51  to become separated from the fixing film, whereas in the case of the structure in this embodiment, the friction works in the direction to cause the thin metallic plate  51  to adhere to the fixing film, and therefore, the thin metallic plate  51  does not separate from the fixing film. However, in consideration of the fact that that the thin metallic plate  51  is installed in a manner to oppose the rotational direction of the fixing film, it is desirable that the attachment angle of the thin metallic plate  51  relative to the rotational direction of the fixing film  10 , in other words, the angle θ (FIG. 10) of the line connecting the point  54  of the thin metallic plate  51  and the temperature sensor  50 , relative to the rotational direction of the fixing film  10 , satisfies the following formula: −20°≦θ≦20°. This is because if the angle θ is outside the above range, it is easier for the thin metallic plate  51  to be turned over, and if turned over, the thin metallic plate  51  and the fixing film  10  fail to make satisfactory surface-to-surface contact with each other. 
     The relationship between the point  54  and the thin metallic plate  51  is desirable to be such that the shortest distance L 1  between the point  54  and the fixing film  10  and the length L 2  of the thin metallic plate  51  satisfies the following formula: L 2 ≧2×L 1 . This is because, if L 2 &lt;2×L 1 , the thin metallic plate  51  is too short to prevent the thin metallic plate  51  from being turned over by the friction between the fixing film  10  and the thin metallic plate  51 , and if turned over, the temperature of the fixing film  10  cannot be detected. Thus, it is desirable that the relation between L 2  and L 1  satisfies the above formula: L 2 ≧2×L 1 . 
     In the case of a slow apparatus, satisfactory results can be obtained even when the thin metallic plate  51  is arranged in conformity with the rotational direction of the fixing film  10  as it is in the preceding embodiment, but in the case of a high speed apparatus, it is desirable that the thin metallic plate  51  is arranged in the direction opposite (counter) to the rotational direction of the fixing film  10  as it is in this embodiment, so that a contact area of a satisfactory size can be reliably maintained between the thin metallic plate  51  and the fixing film  10  to assure accurate detection of the temperature of the fixing film  10  by the temperature sensor  50 . 
     The advantage of the structure of this embodiment is more apparent when the structure is applied to a high speed apparatus, but the same effect can be also obtained even when applied to a medium speed apparatus. However, in the case of a slow speed apparatus, the positional relationship between the temperature sensor  50  and the thin metallic plate  51  may be reversed; the temperature sensor  50  may be placed directly in contact with the fixing film  10 . In such a case, it may be only the temperature sensor  50  that is placed in contact with the fixing film  10 , or the thin metallic plate  51  may also be placed in contact with the fixing film  10  for the sake of effective heat conduction. 
     The temperature sensor  50  may be disposed both before and after the fixing nip N. 
     With this arrangement, the difference ΔT between the fixing film temperature measured before the fixing nip N and the fixing film temperature measured after the fixing nip N can be obtained to determine the amount of the heat robbed by the recording medium P in the fixing nip N. 
     Thus, a predetermined amount of heat can be supplied to the recording medium P by controlling the temperature of the fixing film so that the temperature difference ΔT remains the same. With such temperature control, it does not occur that an excessive amount of heat is applied to the recording medium P. In other words, electric power consumption is reduced. 
     Also, the temperature difference ΔT can be varied according to the type of the recording medium to control the temperature of the fixing apparatus to suit the properties of the recording medium P. 
     Further, according to the present invention, the elastic layer  2  of the electromagnetic induction based fixing film  10  may be omitted when the heating apparatus is to be used for thermally fixing a monochromatic image or a single pass multicolor image. The heat generating layer  1  may be formed of compound material composed by mixing metallic filler into resin. Further, the fixing film  10  may be constituted of a heat generating layer only. 
     The positioning of the magnetic field generating means (magnetic flux generating means) does not need to limited to the positioning described in the preceding embodiment. For example, it may be as illustrated in FIG.  19 . 
     Also, the film driving system employed in the heating apparatus as the fixing apparatus  100  does not need to be limited to the pressure roller based driving system. 
     For example, the film driving system may be such as the one illustrated in FIG. 20, in which an electromagnetic induction based fixing film  10  in the form of an endless belt is suspended around a film guide  16 , a driving roller  31 , and a tension roller  32 , and a pressure roller  30  as a pressing member is pressed upon the downward facing surface of the film guide  16 , forming a fixing nip N, with the fixing film  10  sandwiched between the film guide  16  and the pressure roller  30 , wherein the fixing film  10  is rotatively driven by the driving roller  31 . In this setup, the pressure roller  30  is a follower roller. 
     Further, the pressing member  30  does not need to be in the form of a roller; it may take other forms such as a rotatory belt. 
     The thermal energy to be supplied to the recording medium may come from the pressing member side, as well as from the fixing film side. In such a case, the heat generating means such as the electromagnetic induction based heating means is provided not only on the fixing film side, but also, on the pressing member side, to heat the pressing means side to a predetermined temperature level and maintain the temperature of the pressing member side at the predetermined level. 
     Further, application of the heating apparatus in accordance with the present invention is not limited to the image forming apparatus described in the embodiments of the present invention. Instead, the heating apparatus in accordance with the present invention can be applicable to a wide range of means or apparatuses for thermally processing an object to be heated; for example, an image heating apparatus that heats a printed recording medium to improve its surface properties, such as glossiness, an image heating apparatus that temporarily fixes an image, and other types of heating apparatuses, for example, a drying apparatus that thermally dries an object to be heated, or a thermal laminating apparatus. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.