Abstract:
A heating element includes an excitation coil disposed adjacent the heating element; a voltage source for applying to the excitation coil a high frequency electric power provided by modulating an input AC electric power with a high frequency, wherein the heating element is heated by induction by the excitation coil supplied with the high frequency electric power, wherein the heating element has a characteristic frequency which is unequal to integer multiple s of a frequency of the AC electric power.

Description:
FIELD OF THE INVENTION AND RELATED ART  
         [0001]    The present invention relates to a heating apparatus using induction heating as a heat generation source, and an image forming apparatus using the heating apparatus to heat and fix on a recording paper a toner image formed on a recording material with heat-fusing toner as a developer.  
           [0002]    An image forming apparatus such as an electrophotographic apparatus comprises image forming means (unshown) for forming a toner image on a recording paper with a developer (toner). The recording paper on which the toner image is formed is fed by paper feeding means (unshown) to a fixing device  801  shown in FIG. 10 in an image forming apparatus indicated by an arrow in the Figure, so that toner image  811  is heated, pressed and fixed on the recording paper  810 .  
           [0003]    In the fixing device  801 , a halogen heater  804  is disposed in a heating roller  802  (as an addition heat source) which is press-contacted to the pressing roller  803 , and the pressing roller  803  and the heating roller  802  are rotated in the direction indicated by an arrow by unshown driving source. In a widely used temperature adjustment method, a temperature sensor  805  is provided to detect a temperature, in response to which a halogen heater  804  is ON/OFF controlled such that surface of the heating roller is maintained at a predetermined temperature.  
           [0004]    [0004]FIG. 11 shows an ON/OFF control circuit for the halogen heater  804 , in which  802  designates a heating roller;  804  is a halogen heater;  905  is a thermister (temperature sensor);  901  is a sequence controller;  902  is a SSR (solid state relay)  903  is an AC voltage source;  904  is a comparator: and  906  is a reference resistance. The resistance value of the thermister  805  decreases with increase of a temperature. Therefore, a voltage (thermister detected voltage) between thermister and CND (ground) divided out by a reference resistance, decreases with increase of the temperature. Comparison is made between t reference voltage Vr set at a temperature control target-temperature and a thermister detected voltage, and when the thermister detected voltage is higher than the reference voltage Vr, an ON signal Is supplied to the SSR 902  from the comparator  904 .  
           [0005]    The output of the SSR 902  is ON when the output of the comparator  904  is ON, that is, when the inputted control signal is at H level, it is ON, and when the inputted control signal is at L level, it is OFF. When the output of SSR 902  is ON, an AC current supplied by the AC voltage applied by the AC voltage source  903  is applied through the halogen heater  804  by which the temperature of the heating roller  802  rises. When the roller surface temperature reaches the temperature control target temperature, and therefore, the thermister detected voltage becomes lower than t reference voltage Vr, the output of the comparator  904  renders OFF the SSR  902 . By such ON-OFF control, the heating roller surface temperature is maintained at the target temperature. In an alternative, the sequence controller is provided with an A/D (analog/digital) converter which functions to digitize the thermister detected voltage. The digitalized data are compared with the reference value by software, and an ON-OFF control is effected.  
           [0006]    A heating apparatus has been proposed in which as means for heating the heating roller  802 , the use is made with an excitation coil (unshown) disposed adjacent the heating heat roller  802 . A high frequency current is applied through the excitation coil to generate a high frequency magnetic field in the heating roller surface layer, so that eddy currents are produced in the electroconductive layer at the surface of the heating roller to generate Joule heat, which is used to heat the heating roller  802  (induction heating type).  
           [0007]    With such a heating apparatus of an induction heating type, the heating roller per se can be heated, and the electric power effective for the heating is controllable, and therefore, the target temperature can be quickly reached.  
           [0008]    In a conventional system in which a halogen heater is rendered ON and OF to control the heating roller temperature, the electric power usable for heating the heating roller is at most a consumption power of the halogen heater. The maximum consumption electric power is set to be within a predetermined range. Therefore, during the warming-up period immediately after the voltage source actuation in which the temperature of the heating roller is sufficiently lower than the operable temperature, the usable electric power is at most the electric energy consumption of the halogen heater, with the result that time period required for the fixable temperature to be reached is relatively long.  
           [0009]    In an induction heating type in which the electric power supply for the heating is variable, the electric power inputted from a commercial voltage source is applied to an excitation coil with switching at a predetermined high frequency, and the current induced by the high frequency electric power flows through the heating roller per se.  
           [0010]    [0010]FIG. 12 is a schematic diagram of the induction heating type system. The high frequency current Ip applied to the excitation coil corresponds to the frequency of the high frequency switching, but the average current Iav flowing through the excitation coil corresponds to twice the frequency of the frequency fp of the commercial voltage source (electric energy). The frequency of the commercial voltage source fp is a reciprocal of the cyclic period thereof. By doing so, between the heating roller and the excitation coil, a force corresponding to twice frequency of the frequency fp of the commercial voltage source. Here, the frequency fp of the commercial voltage source is generally 50 Hz or 60 Hz, and the twice frequency is 100 Hz or 120 Hz. The force is such that heating roller rotatably mounted on the beating apparatus is attracted or repelled relative to the excitation coil fixed to the heating apparatus. Particularly when the frequency of the applied force (or an integer multiple thereof) is the same as a characteristic frequency fn of the heating roller, there is a liability that resonance vibration of the heating roller occurs with the result of very large vibration or noise.  
         SUMMERY OF THE INVENTION  
         [0011]    Accordingly, it is a principal object of the present invention to provide a device wherein the resonance of the heating element due to the voltage source frequency of the AC power source is prevented, so that vibration or noise is prevented.  
           [0012]    According to an aspect of the present invention, there is provided a heating element includes an excitation coil disposed adjacent the heating element; a voltage source for applying to the excitation coil a high frequency electric power provided by modulating an input AC electric power with a high frequency, wherein the heating element is heated by induction by the excitation coil supplied with the high frequency electric power, wherein the heating element has a characteristic frequency which is unequal to integer multiples of a frequency of the AC electric power.  
           [0013]    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 THE DRAWINGS  
       [0014]    [0014]FIG. 1 illustrates a structure of a heating roller.  
         [0015]    [0015]FIG. 2 illustrates a general arrangement of an image forming apparatus.  
         [0016]    [0016]FIG. 3 is a block diagram of an induction heating voltage source.  
         [0017]    [0017]FIG. 4 is a block diagram showing flows of image processor in the image forming apparatus.  
         [0018]    [0018]FIG. 5 is an illustration of a magnetic circuit used in the present invention.  
         [0019]    [0019]FIG. 6 illustrates an illustration of a heat-fixing device according to an embodiment or the present invention.  
         [0020]    [0020]FIG. 7 illustrates a heating and fixing controller according to am embodiment of the present invention.  
         [0021]    [0021]FIG. 8 is a timing chart of a pulse generating portion according to an embodiment of the present invention.  
         [0022]    [0022]FIG. 9 shows a heating roller of a double wall structure type.  
         [0023]    [0023]FIG. 10 is an illustration of a conventional heat-fixing device.  
         [0024]    [0024]FIG. 11 is a circuit block diagram of a conventional heat-fixing device.  
         [0025]    [0025]FIG. 12 is a schematic diagram explaining the commercial power source voltage, the coil current and the force applied to the roller.  
         [0026]    [0026]FIG. 13 is a block diagram of a control system. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    The description will be made as to the preferred embodiments of the present invention.  
         [0028]    Embodiment 1  
         [0029]    [0029]FIG. 2 is a schematic view of a color image forming apparatus using a heat-fixing device having the heating apparatus according to this embodiment as the heat source, wherein designated by  201  is an image scanner portion, which reads an original and converts the image thereof to digital signals. Designated by  200  in addition a printer station which functions to effect full-color print corresponding to image data from an external device such as the image scanner  201 , a computer or the like.  
         [0030]    The image scanner portion  201  comprises an original pressure plate  202  which is effective to press the original  204  on t original supporting platen glass  203 . The original  204  on the original supporting platen glass  203  is illuminated by halogen lamp  205 . Light reflected by the reflected light is directed to mirrors  206 ,  207 , and is imaged on a 3 line sensor (CCD)  210 - 1 - 210 - 3  by a lens  208 . The lens  208  is provided with a far-infrared cutting filter  231 .  
         [0031]    The CCD s  200 - 1 - 210 - 3  color-separate the light information from t original and reads full-color information (red (R), green (G) and blue (B) components) and supplies output to a signal processing portion  209 . The halogen lamp  205 , the mirror  206  mechanical moves at a speed V, and the mirror  207  moves at a speed ½V, in a perpendicular direction ((sub-scan direction) relative to an electrical scanning direction (main scan direction) of the CCD sensors  210 - 1 - 210 - 3  to scan the whole surface of the original.  
         [0032]    Designated by  211  is a standard white color plate, which is used to generate correction data for correcting the data provided by the reading. The standard white color plate  211  has a substantially exhibits a substantially uniform reflection particularly property in a range from the visual light to the infrared light, and is white in the visual range. Using the standard white color plate  211 , the output data of the visual sensor of the CCD sensors  210 - 1 - 210 - 3  is corrected (shading). Designated by  230  is a photo-sensor, which cooperates with a flag plate  229  to generate an image top signal VTOP.  
         [0033]    The image signal (electric signal) is processed in the image processor  209  in accordance with the flow shown in FIG. 4. The signals from the CCD sensors  210 - 1 - 210 - 3  are converted to digital data in A/D &amp; S/H portion  410 , the image data is corrected by the shading correction portion  411  and the input masking portion  412 . During variable magnifying operation, the variable magnification process is carried out it the variable magnification processing portion  413 .  
         [0034]    Subsequently, a LOG conversion portion  414  converts the RGB data to magenta (M), cyan (C) and yellow data, which are inputted to a compressing and elongating portion  415  for compressing, storing and elongating the image data. The stored image data is read out in synchronism with the respective color printing portions of a printer which will be described hereinafter. After the image data are subjected to a masking process by a masking-UCR portion  416 , they are further corrected by a &amp;c&amp;correction portion  417  and an edge stressing portion  418  to generate M, C, Y and black (K) output image data. Then, they are fed to a printer station  200 . Here, for original scanning by the image scanner portion  201 , one of M, C, Y, K components is fed to the printer station  200 . By four original scanning operations, one print is produced.  
         [0035]    The description will be made as to an operation of the printer station  200 . The image signal from an external device such as a scanner portion  201  or an unshown computer or the like, is fed to an image writing timing control circuit  101 . The image writing timing control circuit  101  modulates and actuates the semiconductor laser  102  in response to a magenta (M), cyan (C), yellow (Y) and black, (K) image signal. The laser beam is reflected by a polygonal mirror  103  rotated by a polygonal motor  106 , and is subjected to a fθcorrection by a f-θ lens  104 . It is reflected by the folding mirror  216  to scan the photosensitive drum  105 .  
         [0036]    The photosensitive drum  105  has been uniformly charged by a primary charger  242 , and therefore, an electrostatic latent image is formed on the photosensitive drum  105  by the laser exposure. Around the photosensitive member  105 , there are provided magenta (M)  219 , cyan (C)  220 , yellow (Y)  221  and black (K)  222  developing devices. With four full-turns of the  219 , cyan, the four developing devices are contacted to the circumference sequentially to develop the M, C, Y, K electrostatic latent images formed on the photosensitive drum  105  with the corresponding toner particles.  
         [0037]    On the other hand, the recording paper fed from recording paper sheet feeders  224 ,  225  is electrostatically attracted on a transfer drum  108  having been electrically charged in a sheet attracting polarity by an attraction charging blade  245  connected with an unshown attraction high voltage generating portion, at timing in synchronism with the image formation on the photosensitive drum.  
         [0038]    It is pushed up toward the photosensitive drum  105  by a transfer charging blade  240  connected to an unshown transfer high voltage generating portion at a transfer position  246 , so that toner is transferred onto the transfer material. The image formation and transferring operations are repeated four times, and thereafter, the recording paper is separated from the transfer drum  108 , and is fed to a fixing device  226  (fixing means) which heat-fixes the toner image on the recording paper. Then, the recording material in addition discharged as a print. The cleaner  241  functions to remove from the photosensitive drum the residual toner which has not been transferred and toner of specified patch (for various controls) which has been formed on the photosensitive drum  105  but is not to be transferred onto the transfer material.  
         [0039]    [0039]FIG. 7 shows a heating apparatus of this embodiment. Designated by  701  is a heating roller;  702  is an excitation coil;  706  is a thermister;  707  is an A/D converter;  706  is a CPU;  710  is an induction heating voltage source for supplying high frequency electric power to the excitation coil  702 ;  713  is a fixing device;  715  is a reference resistor;  716  is a pulse generator;  721  is an AC voltage source.  
         [0040]    The CPU 708  is connected by bus lines with the A/D converter  707  and the pulse generator  716 , and effects sequence control in accordance with a program stored in an unshown ROM connected in the same bus. The excitation coil  702  is an induction heating coil which generates a high frequency magnetic field by application of a high frequency current. It is magnetically connected with a core (I core)  703  having an I-shaped section disposed as shown in FIG. 6. The generated high frequency magnetic field is connected with the heating roller  701  to constitute a magnetic circuit.  
         [0041]    [0041]FIG. 6 is a view of a heat-fixing device using the heating apparatus  713  according to an embodiment of the present invention. The heating roller  701  which is a heating roller  701  is a hollow pipe roller of steel, and is rotatably mounted on a fixing device frame and is rotated by an unshown driving means. On an inside wall of the roller, rib members  101   a - 101   d  (reinforcing member) for changing the characteristic frequency are provided in the shown example, the rib members  101   a - 101   d  are mounted is on the inner wall at respective four positions. The rib member  101   a - 101   d  are made of non-magnetic material so as not to be influential to the magnetic circuit.  
         [0042]    The magnetic circuit generating structure constituted by the excitation coil  702  and the I core  703  is disposed in the heating roller and is supported by the supporting member  704 , and the magnetic field generated by the excitation coil  702  is imparted in the surface of the heating roller. The supporting member  704  is made of a non-magnetic material such as a heat resistive resin material, and is fixed on a frame of the heating apparatus at the opposite ends thereof.  
         [0043]    The excitation coil  702  and the I core  703  extend in the longitudinal direction of the heating roller  701 , and encloses the I core  703 . In FIG. 6, through the wire portions indicated by circles with dots, the current flows in the same direction, aid through the wire portions indicated by circles with x, the current flow in the same direction, which however is opposite from the direction of the wire portions indicated by circles with dots. I core  703  comprises a ferrite having a high magnetic permeability. Designated by  502  is a pressing roller, and is urged to the heating roller  701 . A recording paper  810  carrying a toner image  811  is passed through between is the pressing roller  502  and the heating roller  701  driven by an unshown driving source, by which the toner image is heat-fixed on the recording paper.  
         [0044]    [0044]FIG. 1, (a) is a sectional view of the heating roller  701  having an inner wall on which rib members  101   a - 101   d  are provided. As shown in FIG. 1, (b), the rib members  101   a - 101   d  are elongated in t longitudinal direction of the roller. The rib members  101   a - 101   d  are effective to change the characteristic frequency of the heating roller  701  determined by the elasticity thereof. By properly selecting the number and width of the rib members  101   a - 101   d,  the characteristic frequency of the heating roller  701  is selectable. When the characteristic frequency of the heating roller is selected, it is preferable that frequency is not equal to an integer multiple of the frequency of the AC electric power source. Particularly, it is desirably not equal to even number multiple. Further particularly, the frequency which is twice the frequency of the power source is greatly influential to the force imparted to the heating roller, and it is particularly desirable that the frequency is not equal to twice the frequency of the power source. The characteristic frequency of the heating roller is measured by mounting an acceleration sensor to the heating roller and detecting a frequency of vibration caused by lightly hammering the heating roller.  
         [0045]    The operation of the device according to this embodiment will be described. In FIG. 7, AC electric power is supplied from the AC voltage source  721  to the induction heating voltage source  710 . When an ON signal and a PWM signal are fed to t induction heating voltage source  710  through the pulse generator  716  from the CPU 70 S ruling the sequence control, high frequency AC electric power is generated in response to the PWM signal at an output terminal of the induction heating voltage source  710  connected to the excitation coil  702 .  
         [0046]    [0046]FIG. 3 is a detailed block diagram of the induction heating voltage source  710 . Designated by  301 - 304  are diodes;  305  is a reactor for a noise filter;  306  is a capacitor for the noise filter;  307  is an electric power switching MOS-FET;  308  is a diode;  309  is a capacitor  311  is a logical product (AND) gate;  721  is an AC voltage source (commercial power source) for energizing the induction heating voltage source;  702  is an excitation coil which is supplied with an output from the induction heating voltage source  710 ;  716  is a pulse generator connected so as to control the induction heating voltage source  710 .  
         [0047]    The AC current applied from the AC voltage source  721  is converted to a pulsating flow rectified by the diodes  301 - 304 , and the waveform thereof is rectified by passing through the coil  305  and the capacitor  306  which constitute a noise filter. The parameters of the coil  305  and the capacitor  306  constituting the noise filter are set such that sufficient attenuation amount is assured for the switching frequency of the MOS-FET  307  and that no attenuation of passage is assured for the voltage source frequency fp of the AC voltage source  721 .  
         [0048]    Prom t pulse generator  716 , a PWM signal and an ON signal of a predetermined pulse width is fed to t induction heating voltage source  710 . When t ON signal is at a H level, the PWM signal is applied across the source and the gate of the MOS-FET  307  through the AND gate  311 , and the MOS-FET  307  becomes conductive during the H level section of the PWM signal, so that rectified inputting current is drain current to energize the excitation coil  702 .  
         [0049]    When the MOS-FET  307  becomes open in the L level section of the PWM signal, a back electromotive force is generated by the excitation coil  702  accumulating the current flowing when the MOS-FET  307  is ON, and the back electromotive force is charged in the resonance capacitor  309  connected in parallel with the excitation coil  702 . By the coil accumulating current, the voltage across the resonance capacitor  309  increases, and a maximum AC voltage is generated when the accumulation energy of the excitation coil  702  becomes zero.  
         [0050]    The current flown out of the excitation coil  702  attenuates in inverse proportion to the increase of the voltage; at a certain instance, no coil current flows, and after that, the charge accumulated in the resonance capacitor  309  flows out to the excitation coil  702  and produces a current thereby.  
         [0051]    Simultaneously with the charge accumulated in the resonance capacitor  309  returns to t excitation coil  702 , the voltage of the resonance capacitor  300  decreases. When the drain voltage of the MOS-FET  307  lowers beyond the source voltage, a flywheel diode  308  is rendered ON so that forward current flows. Then, the MOS-FET  307  is reactuated so that current flows through the excitation coil  702 , so that AC current of the frequency corresponding to the PWM signal continues to flow through the excitation coil  702 .  
         [0052]    By the AC electric power of the predetermined frequency from t induction heating voltage source  710  being applied across t excitation coil  702 , the excitation coil  702  generates an AC magnetic field  5 . FIG. 5 shows this. The AC electric power supplied to the excitation coil  702  increases with decrease of the frequency of the AC electric power applied to the excitation coil  702 , and it is normally 200W to several kW approx.  
         [0053]    The eddy currents  52  are generated in the surface of the heating roller  701  to which the AC magnetic field  51  produced by the AC electric power is opposed. By t eddy currents  52  flowing in the surface of the heating roller, Joule heart is produced in the surface of the heating roller leaving due to the resistivity of the heating roller  701 , that is, the surface of the heating roller generates heat by itself. At this time, the magnetic field is concentrated at the I core  703  having a high magnetic permeability, by which a large amount of the heat is generated by the eddy currents at a portion of the heating roller  701  opposed to the I core  703 . The larger the electric power supplied to the excitation coil  702 , the larger the amounts of the generated AC magnetic field and joule heat.  
         [0054]    By the heat generation of the surface of the heating roller thus provided, the resistance value of the thermister  706  disposed on t surface of the heating roller decreases with the increase of the temperature. As shown in FIG. 7, a voltage (detected thermister voltage) between the thermister and GND divided out with the aid of the reference resistance disposed substantially at a longitudinal center of the heating roller  701  decreases with increase of the temperature. The detected thermister voltage is digitalized by an A/D converter  707  and is supplied to t CPU 708 , where the digitalized data is software compared with the reference temperature, and a set point for determining ON/OFF pulse width of the PWM signal to t induction heating voltage source  710  is outputted to t pulse generator  716 .  
         [0055]    The pulse generating portion  716  compares the CLK signal with the set point provided by the CPU 708  and the predetermined set point, and counts with a proper set value, to produce a PWM signal of proper ON and OFF widths. FIG. 13 is a block diagram showing details or the pulse generator  716 , wherein designated by b 101 ,  106  and  114  are D latches;  103  and  108  are down counters;  104 ,  109 ,  112  and  113  are logical product (AND) gates:  105  and  110  are logical sum (OR) gate;  111  is a SR latch.  
         [0056]    The PWM generation timing chart will be described with respect to the operation of the pulse generator shown in FIG. 8. Here, designated by CS 1 - 3  is a selection signal of a register, and WR is a light signal. CS 1 - 3 , Data, WR, CLK is included in the bus between the CPU 708  and the pulse generator  716 . Designated by  101 -Q is a Q output of the D latch  101 ;  102 -Q is a Q output of the D latch  102 ;  103 -CNT is a count of the counter  103 ;  103 -RC is a ripple-output of the counter  103 ;  111 -Q is a Q output of the DSR latch  111 ;  108 -CNT is a count of the counter  108 ;  108 -RC is a ripple-output of the counter  108 ;  114 -Q is a Q output of the D latch  104 ;  115 -Q is an output of the and gate  115 ; and  112 -Q is an output of the logical product (AND) gate  112 -CLK is a signal having a frequency of several MHz, and is inputted to each D latch and counter as reference signals, and PWM pulses of approx. 20 kHz-100 MHz using counts of the signals. The data=N outputted to the Data path at the time when the selection signal CS 1  is selected with H level, and the light signal WR rises. Are latched on the D latch  10 . The register CS 8  is selected with H level indicative of the driving voltage source being ON, and data=1 is latched by the D latch  114  at the rising or the light signal WR, and the data=N is loaded in the counter  103 .  
         [0057]    Since the enablement EN of the counter  103  connected to the Q output of the SR latch  111  is at the H level, the counter  103  carries out the down count operations in accordance with the CLK. When the count becomes 0, it makes the ripple carrying signal RC=H. By this output, the SR latch  111  is reset, Q=L level and Q*=H level result, and in addition, count=M is loaded into one  108  of the counters. The operations of the D latch  106  are the same as the D latch  101 .  
         [0058]    The counter  108  is by the loading of the count=M carries out downcounting operation in accordance with the CLK, and when count=0, the ripple carrying signal RC becomes H. By this output, the SR latch  111  is set, and Q=H level and Q=L level result. By repeating this, the PWM pulses having ON width=N and OFF width=M count are generated as an output of the SR latch  111 .  
         [0059]    The PWM signal and the ON signal are fed to t induction heating voltage source, a high frequency AC electric power of approx. 20 kHz-100 kHz (converted so as to correspond to the PWM signal) at the output terminal of the induction heating voltage source  710 . By such operations, the temperature of the surface of the heating roller can be maintained at the predetermined temperature. Here, the characteristic frequency of the heating roller  701  is selected so as not to be equal to the frequency fp or an integer *multiple of the commercial electric power, and therefore, great vibration or noise due to resonance of the heating roller  701  can be prevented.  
         [0060]    Embodiment 2  
         [0061]    In order to deviate the characteristic frequency of the heating roller  701  from the integer multiple of the frequency fp of the commercial power source, the thickness of the heating roller  701  may be changed, thus changing the elasticity of the heating roller per se by which the characteristic frequency of t heating roller  701  is changed. When t induction heating type heating roller  701  is made of steel, the proper thickness is 0, 3 mm-1.0 mm degree. In this range, the characteristic frequency fn of the heating roller  701  can be deviated from integer multiples of the frequency of the commercial power source by changing the thickness of the heating roller  701  while maintaining the fixing property of the apparatus.  
         [0062]    Embodiment 3  
         [0063]    In order to deviate the characteristic frequency fn of the heating roller  701 , the material of the heating roller  701  may be changed so that elasticity of the heating roller per se is changed by which the characteristic frequency fn of the heating roller  701  is changed. For example, when the steel is used as a core metal of the heating roller  701 , the mechanical properties such as tensile strength or Young&#39;s modulus of a steel tube may be changed by changing the content or contents of the chromium, molybdenum, the niobium, the vanadium or the tungsten. Thus, by properly selecting the steel tube, the characteristic frequency of the heating roller  701  can be deviated from integer multiples of the frequency fp of the frequency of the commercial power source  
         [0064]    Embodiment 4  
         [0065]    In order to deviate the characteristic frequency Fn of the heating roller  701  from the commercial electric power source, the heating roller  701  may be made of a plurality of materials, so that elasticity of the heating roller per se is changed by which the characteristic frequency of the heating roller  701  is changed. For example, the surface of the heating roller may be coated with a resin material which is selected so as to change the characteristic frequency of the heating roller  701 . The coating may have a surface parting property of the entire surface of the heating roller. The coating material may be PTFE or PFA, and the thickness thereof is 10-50 μm, preferably.  
         [0066]    Alternatively, the core metal portion of the heating roller may be made of a plurality of metal materials, so that elasticity of the heating roller per se is changed, by which the characteristic frequency Fn of the heating roller is changed. As shown in FIG. 9, the heating roller  720  may comprise a steel material  721  (constituting a part of the magnetic circuit) and an aluminum material  722  on an outer surface thereof, which are integrated with each other by interference shrink fitting. By doing so, the characteristic frequency is made different from that made of a steel only.  
         [0067]    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 purpose of the improvements or the scope of the following claims.