Abstract:
A fixing device has a magnetic field generator; an electric power supplier for electric power supply to the magnetic field generator; electric power controller for controlling an electric power value to be supplied by the electric power supplier; a fixing member, disposed in the magnetic field, having an electroconductive layer which generates heat by eddy current generated by the magnetic field; a temperature detecting member for detecting a temperature of the fixing member; a discriminator for discriminating whether the apparatus is in order by comparing a detected temperature to a reference temperature in a period from a start of heating the fixing member to arrival at a predetermined temperature; and a reference temperature changer for changing the reference temperature on the basis of the supplied electric power value upon the electric power controller changing the electric power value.

Description:
This application is a divisional of U.S. patent application Ser. No. 10/412,394, filed on Apr. 14, 2003 now U.S. Pat. No. 6,959,158. 

   FIELD OF THE INVENTION AND RELATED ART 
   The present invention relates to a fixing device for heating and fixing of a recording paper (recording material) with heat-fusing powder such as toner using induction heating as a heat generation source. An image forming apparatus comprises image forming means for forming a visualized image (toner image) on a recording material (recording paper) with visualizing material (toner), recording paper feeding means for feeding recording paper on which the toner image is formed and fixing means for heating and fixing the toner image on the recording paper. 
   Recently, an induction heating type comprising a fixing roller (heating member)  100  and a pressing roller  101  which are press-contacted to each other as shown in  FIG. 8  is noted from the standpoint of saving energy consumption. In the induction heating type, the high frequency current is applied to the induction heating coil L 1 , the generated high frequency magnetic field acts on the electroconductive layer which is an inner surface layer of the fixing roller, by which eddy currents are generated in the electroconductive layer, and the eddy current causes self-heat-generation in the fixing roller  100  by joule heat. 
   With the induction heating type, the electroconductive layer  100   a  (inner surface layer) of the fixing roller is itself a heat generating element (direct heating), and therefore, the heat generating efficiency is high. This easily accomplished quick heating of the fixing roller  100  to a required fixing temperature, and therefore, quick start-up is possible. In addition, the high efficiency of electric power using can significantly reduce the electric energy consumption. 
   Such a fixing device of induction heating type is provided with both of a software safety means using CPU or the like as temperature abnormality detecting means, and a hardware safety means such as temperature detection means (mechanical contact) using bimetal or the like or a temperature detecting means using a constant melting point metal which fuses at a predetermined temperature. However, when the fixing roller is rapidly heated as with the said induction heating type, the conventional software safety means is insufficient. The temperature rise of the heating member can be so quick that temperature of the heating member may rise to such a temperature as to cause a mechanical malfunction before the actuation of the hardware temperature detecting means using a mechanical contact for the excessive temperature rise detection, is actuated. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is a principal object of the present invention to provide a fixing device wherein the temperature abnormality detection accuracy of induction heating apparatus is enhanced, and an image forming apparatus capable of high quality image formation. 
   According to an aspect of the present invention, there is provided a fixing device comprising magnetic field generating means for generating a high frequency magnetic field; electric power supplying means for electric power supply to said magnetic field generating means; electric power control means for controlling an electric power value to be supplied by said electric power supplying means; a fixing member, disposed in the magnetic field generated by said magnetic field generating means, having an electroconductive layer which generates heat by eddy currents generated by the magnetic field; a temperature detecting member for detecting a temperature of said fixing member; discriminating means for making discrimination as to whether or not said apparatus is in order by comparing a detected temperature provided by said temperature detecting member a reference temperature in a period from start of heating said fixing member to arrival at a predetermined temperature of said fixing member; and reference temperature changing means for changing the reference temperature on the basis of the supplied electric power value upon said electric power control means changing the electric power value. 
   According to another aspect of the present invention, there is provided an image forming apparatus comprising image forming means for forming a toner image on a recording material; magnetic field generating means for generating a high frequency magnetic field; electric power supplying means for electric power supply to said magnetic field generating means; electric power control means for controlling an electric power value to be supplied by said electric power supplying means; a fixing member, disposed in the magnetic field generated by said magnetic field generating means, having an electroconductive layer which generates heat by eddy currents generated by the magnetic field; a temperature detecting member for detecting a temperature of said fixing member; and discriminating means for making discrimination as to whether or not said apparatus is in order by comparing a detected temperature provided by said temperature detecting member a reference temperature in a period from start of heating said fixing member to arrival at a predetermined temperature of said fixing member; and reference temperature changing means for changing the reference temperature on the basis of the supplied electric power value upon said electric power control means changing the electric power value. 
   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 
       FIG. 1  is a schematic block diagram of an induction heating apparatus in Embodiment 1 of the present invention. 
       FIG. 2  is a detailed illustration of an inside structure of the fixing roller. 
       FIG. 3  is an illustration of a heat generation distribution of the fixing roller. 
       FIG. 4  is an illustration of a normal sequence profile upon temperature raising. 
       FIG. 5  is an illustration of an abnormality sequence profile upon temperature raising. 
       FIG. 6  is a schematic block diagram of an induction heating apparatus in Embodiment 1 of the present invention. 
       FIG. 7  is an illustration of abnormality discrimination on the basis of temperature information wherein a temperature reference profile is produced by an electric power control circuit. 
       FIG. 8  shows an induction heating apparatus in the form of a fixing device. 
       FIG. 9  illustrates an induction heating apparatus in the form of an image forming apparatus. 
       FIG. 10  shows a rising curve of the fixing roller connected with an abnormality detection circuit. 
       FIG. 11  shows a temperature rising curve which is a reference when a high temperature abnormality detection level THL and the low temperature abnormality detection level TLL are set. 
       FIG. 12  is a temperature rising curve of a fixing roller when the presence or absence of the heat release is taken into consideration. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The description will be made as to the preferred embodiment of the present invention. 
   Embodiment 1: 
     FIG. 1  is a schematic block diagram of an induction heating apparatus in Embodiment 1 of the present invention.  FIG. 2  is a detailed illustration of an inside structure of the fixing roller.  FIG. 3  is an illustration of a heat generation distribution of the fixing roller. FIG.  4  is an illustration of a normal sequence profile upon temperature raising.  FIG. 5  is an illustration of an abnormality sequence profile upon temperature raising.  FIG. 6  is a schematic block diagram of an induction heating apparatus in Embodiment 1 of the present invention.  FIG. 7  is an illustration of abnormality discrimination on the basis of temperature information wherein a temperature reference profile is produced by an electric power control circuit.  FIG. 8  shows an induction heating apparatus in the form of a fixing device.  FIG. 9  illustrates an induction heating apparatus in the form of an image forming apparatus. In these Figures, the same reference numerals are assigned to the elements having corresponding functions, and the detailed description is not repeated for simplisity. 
     FIG. 9  is a cross-section of an image forming apparatus provided with a fixing device (heating apparatus) according to an embodiment of the present invention, wherein the image forming apparatus is a laser beam printer as an exemplary apparatus. The description will be made as to the image forming apparatus. The electrophotographic photosensitive member in the form of a drum (image bearing member)  51  is rotatable Lines supported on a main assembly M of the apparatus, and is rotated at a predetermined process speed in the direction indicated by an arrow R 1  by driving means (unshown). Around the photosensitive drum  51 , there are provided a charging roller (charging device)  52 , exposure means  53 , a developing device  54 , a transfer roller (transferring device)  55  and a cleaning device  56  in the order named. Below the main assembly M of the apparatus, there is provided a cassette  57  accommodating material to be heated  3  in the form of sheets. The feeding path for the material to be heated P comprises, from the upstream side, a sheet feeding pick-up  65 , feeding rollers  58 , a top sensor  59 , a feeding guide  60 , fixing device  61  using the heating apparatus of the present invention and including a pair of a feeding roller  62  and a discharging roller  63 , and discharge tray  64 . The description will be made as to the operation of the image forming apparatus. 
   The photosensitive drum  51  is rotated in the direction indicated by arrow R 1  by driving means (unshown), during which it is uniformly charged to a predetermined potential of a predetermined polarity by the charging roller  52 . The photosensitive drum  51  having been electrically charged by the charging roller, is exposed to image light by exposure means  53  including a laser optical system or the like, so that charge of the exposed portion is removed, and therefore, an electrostatic latent image is formed. The electrostatic latent image is developed by the developing device  54 . The developing device  54  comprises a developing roller  54   a , and the developing roller  54   a  thereof is supplied with a developing bias, so that toner is electrostatically deposited onto the electrostatic latent image on the photosensitive drum  51 , by which a toner image is formed. The toner image is transferred onto the material to be heated  3  by the transfer roller  55 . The material to be heated  3  is accommodated in the cassette  57 , and are fed out by the pick-up roller  65  and the feeding rollers  58 , and is then fed to a transfer nip formed between the photosensitive drum  51  and the transfer roller  55  at the proper timing using the top sensor  59 . At this time, the leading end of the material to be heated  3  is detected by the top sensor  59  and is synchronized with the unfixed toner image on the photosensitive drum  51 . The transfer roller  55  is supplied with a transfer bias, so that toner image is transferred to the material to be heated  3  at the predetermined position from the photosensitive drum  51 . The material to be heated  3  carrying the toner image (unfixed) on its surface, is fed to the fixing device  61  along the feeding guide  60 , where the unfixed toner image is heated and pressed, by which it is fixed on the surface of the material to be heated  3 . The material to be heated  3 , after the toner image is fixed, is fed and discharged onto the discharge tray  64  of the main assembly M of the apparatus. On the other hand, the photosensitive drum  51  after the toner image is transferred, the residual toner (untransferred toner) remaining on the surface of the photosensitive drum  51  is removed by a cleaning blade  56   a  of the cleaning device  56 , and the photosensitive drum  51  is prepared for the next image forming operation. By repeating the above-described operations, the image forming operations are sequentially carried out. Referring to  FIG. 1 , the description will be made as to the induction heating apparatus according to the embodiment of the present invention. In  FIG. 1 , designated by TR 1  is an electric power switching element such as a MOS-FET or the like; C 2  is a resonance capacitor for providing a resonance waveform from a high frequency AC to be applied to the dielectric heating coil L 1  which is a load; and D 5  is a flywheel diode for regenerating the electric power accumulates in the induction heating coil L 1  connected in parallel with the electric power switching element TR 1 . 
   Designated by TH 1  is a temperature detecting element (temperature detecting means) and is disposed proposed to the pollution of the fixing roller (heating member)  100  which generates the largest amount of heat. The temperature detecting element TH 1  is generally a temperature sensing resistance element such as a thermister or the like, and the output there is inputted to the temperature detecting circuit IC 2 . 
   The temperature detecting circuit IC 2  outputs a voltage value which corresponds to the change in the electrical resistance of the temperature detecting element TH 1  which changes with the temperature, the output is a temperature signal T-MON. The temperature signal T-MON is surprised to an electric power control circuit (electric power control means)  110 , to a resonance control circuit IC 1  of the electric power application circuit (electric power applying means)  90  and to an abnormality detection circuit (abnormality detecting means)  111 . The electric power control means  110  determines an electric power supplying operation through the fixing roller  100  in accordance with the state operation of the image forming apparatus (unshown), and determines the electric power amount (electric power instruction value Pcont) upon the electric power application and during the electric power supply. The electric power instruction value Pcont is determined in accordance with the state of operation of the image forming apparatus, and the target value is changed upon necessity. 
   The resonance control circuit IC 1  includes a one-shot pulse generating circuit  11 , a processing circuit  12  and a comparison circuit  13 , and the processing circuit  12  receives the temperature signal T-MON and the electric power instruction value Pcont outputted from the electric power control means  110 . The one-shot pulse generating circuit  11  is supplied with the operation permission signal IH-ON output from the electric power control means  110 . 
   Here, the electric power instruction value Pcont inputted from the electric power control means  110  to the resonance control circuit IC 1  is inputted to the pulse modulation (PFM) oscillation circuit as an electric power control signal. The resonance control circuit IC 1  generates PFM pulses corresponding to the electric power control signal value to the gate of the electric power switching element TR 1  to rendering on and off the electric power switching element. 
   The electric power supplied from the commercial AC voltage source AC is rectified by a rectifying circuit  1  constituted by diode D 1 –D 4  which are connected into a bridge circuit, and is converted to a DC by a smoothing circuit  2  comprising a noise filter NF 1  and a smoothing capacitor C 1 . The noise filter NF 1  and the smoothing capacitor C 1  are such that sufficient attenuation amount is assured for the frequency of the electric power switching element TR 1  and such that substantially no attenuation is assured fro the voltage source frequency. The electric power application circuit  90  is constituted by the rectifying circuit  1 , the smoothing circuit  2 , the resonance capacitor C 2 , the temperature detecting element TH 1 , the resonance control circuit IC 1  and so on. 
   The fixing roller  100 , as shown in  FIG. 2 , includes a roller core metal  109 , a rubber layer  108  on the outer surface thereof, a ferrite core  106  having a T-shaped cross-section disposed at the inner central portion, a supporting member  101  supporting the ferrite core  106 , and an arcuate induction heating coil L 1  extending along the inner surface of the roller core metal  109  between the opposite ends of the ferrite core. By this structure, the heat generation distribution is generated on the surface of the fixing roller  100 . 
   The description will be made as to the operation. 
   When the electric power control circuit  110  receives a heating signal upon the start of the copying operation, the operation permission signal IH-ON and the electric power instruction value Pcont are outputted to resonance control circuit IC 1  of the electric power application circuit  90  and to the abnormality detection circuit  111  in accordance with the state of the copying operation. The circuit  111  receives the operation permission signal IH-ON and the electric power instruction value Pcont to produce a relay operation signal RL-ON to close the relay RL 1 , thus supplying the AC input voltage to the electric power application circuit  90 . 
   When the AC input voltage is supplied to the input contact of the electric power application circuit  90  by this operation, the voltage rectified by the rectifying circuit  1  constituted by the diode D 1 –D 4  into a pulsating flow voltage, is applied across the capacitor C 1  through the noise filter NF 1  of the smoothing circuit  2 . By this, the end-to-end voltage of the capacitor C 1  forms a waveform provided by rectifying the AC input voltage. 
   From the electric power control circuit  110 , the electric power instruction value Pcont corresponding to the state of the operation of the apparatus is applied to the resonance control circuit IC 1  as a control signal, and the resonance control circuit IC 1  generates a PFM pulse corresponding to the electric power instruction value Pcont. The PFM pulse generated by the resonance control circuit IC 1  is applied across the gate-sources, by which the electric power switching element TR 1  is switched to permit flow of the drain current ID, thus supplying the electric power to the induction heating coil L 1 . 
   The induction heating coil L 1  stores the current provided by actuation of the electric power switching element TR 1 , and therefore, when the electric power switching element TR 1  is deactuated, a counterelectromotive force is generated to electrically charge the resonance capacitor C 2  with the cumulative current, thus raising the charged voltage of the resonance capacitor. 
   The current flown out of the induction heating coil L 1  attenuates inverse-proportionally to the rising of the voltage across the resonance capacitor C 2 . After passing through an instance when no coil current flows, the current provided by the charge accumulated in the resonance capacitor C 2  inversely flows out into the induction heating coil L 1 . 
   Simultaneously with the charge accumulated in the resonance capacitor C 2  returning to the induction heating coil L 1 , the voltage of the resonance capacitor C 2  lowers so that the drain voltage of the electric power switching element TR 1  lowers beyond the source voltage to actuate the flywheel diode D 5 , thus flowing the forward current. 
   When the electric power switching element TR 1  is actuated thereafter, the current flows through the induction heating coil L 1 , and the current is accumulated in the induction heating coil. These operations are repeated, with the result that induced current flows through the fixing roller  100  which is a load opposed to and therefore electromagnetically connected with the induction heating coil L 1 . By this, joule heat is generated therein which is the resistance value of itself multiplied with the induced current square, so that inner surface efficiently generates heat, and the entirety of the fixing roller which is rotation is heated. 
   Here, the current flowing through the electric power switching element TR 1  and the induction heating coil L 1  is smoothed by the capacitor C 1  charging and discharging the high frequency component. Accordingly, the high frequency current does not flows through the noise filter NF 1 , and only the AC having the rectified input current waveform flows. 
   The current waveform of the current flowing through the electric power switching element TR 1  and the induction heating coil L 1  is the one filtered by the smoothing circuit  2  including the capacitor C 1  and the noise filter NF 1 , and therefore, the AC input current waveform before the rectification is close to the AC input voltage waveform, so that higher harmonics wave component included in the input current can be significantly decreased, and the power factor of the input current of the smoothing circuit  2  can be significantly improved. 
   The smoothing circuit  2  comprising the noise filter NF 1  and the capacitor C 1  may be any if the filtering effect functions to the oscillation frequency of high frequency provided by the resonance control circuit IC 1  since the capacity of the capacitor C 1  and the inductance value of the noise filter NF 1  can be small, and therefore, downswing and weight saving is accomplished. 
   By inputting the electric power temperature control signal to the induction heating actuating power source circuit, a high frequency AC electric power of approx. 20 KHz-1 MHz is generated at the output terminal of the induction heating power source. 
   The output of the temperature detecting element TH 1  for detecting the temperature of the surface of the fixing roller is inputted to the temperature detecting circuit IC 2  at proper timing, and is inputted to the electric power control circuit  110  as a temperature signal T-MON the detected temperature is compared with the target temperature at proper timing. The difference from the target value is fed back to the resonance control circuit IC 1  as an electric power instruction value Pcont. 
   When the detected temperature detected by the temperature detection circuit IC 2  approaches to the predetermined temperature information (set target temperature), the electric power control circuit  110  produces a feed-back signal to decrease the applied high frequency electric power so as to keep the surface temperature of the fixing roller at a constant level through a control system proportional control or so-called PID control. The resonance control circuit IC 1  is supplied with the difference from the set target temperature detected by the electric power control circuit  110 , that is, the electric power instruction value Pcont. In accordance with the electric power instruction value Pcont, the gate ON time of the electric power switching element TR 1  is determined, so that supplied electric power of the electric power switching element TR 1  is adjusted. As a result, the electric power inputted to the induction heating coil L 1  is controlled, and the heating value of the fixing roller is controlled, by which the toner fixing temperature is stabilized. 
   In the fixing device of the induction heating type, the temperature control is carried out through the above-described sequence. The material of the fixing roller  100  used with the induction heating is usually steal, ferro-alloy or the like from the standpoint of cost and/or heat generation property. 
   However, the fixing roller  100  of ferro-material exhibits low thermo-conductivity. In order to uniformly heat the surface of fixing roller, the fixing roller  100  is rotated from the start of the application of electric power with the pressing roller  101  contacted thereto. This makes the surface temperature of the fixing roller  100  uniform. 
   The description will be made as to the operation at the start of the heating operation. 
   The fixing roller is supplied with the electric energy by the electric power supplying means until the fixable temperature is reached.  FIG. 4  shows a normal sequence profile upon the temperature raising operation. In this Figure, a temperature signal T-MON is indicative of the temperature of the fixing device actually sensed by the temperature detecting element TH 1 . The curve indicates that temperature rising in the case of the normal operation. Designated by THL is a high temperature abnormality detection level (first temperature discrimination reference) of the abnormality detection circuit  111 , and TLL is a low temperature abnormality detection level (second temperature discrimination reference). The description will be made as to the sequential start-up operation. The electric power control circuit  110  receives the electric power supply signal of the main assembly of the image forming apparatus and starts the start-up operation for the fixing device. At this time, the electric power control circuit  110  detects the state of the temperature of the fixing roller  100  on the basis of the output voltage value of the temperature detecting circuit IC 2 . For example, when the surface temperature of the fixing roller  100  upon the main switch actuation is lower than the fixable temperature, the start-up situation is discriminated and a rotational driving signal for the fixing roller  100  (unshown) is outputted, and also outputs an electric power instruction value Pcont and an operation permission signal IH-ON. 
   The operation permission signal IH-ON and the electric power instruction value Pcont are received by the abnormality detection circuit  111 , and a relay operation signal RL-ON is outputted to close the relay RL 1  so that AC input voltage is applied to the electric power application circuit  90 , by which the electric power supply to the induction heating coil L 1  is started. At this time, the electric power instruction value Pcont in  FIG. 4  showing the electric power supply sequence is the maximum electric power P 1  that is applicable upon the start-up of the image forming apparatus, since various electrical and mechanical load elements of the image forming apparatus are at rest. 
   After a certain time period collapses, the surface temperature of the fixing roller  100  reaches a predetermined temperature, the signal indicative of this event is received by the electric power control circuit  110 , and then various electrical and mechanical elements required for image forming operation of the copying machine start to receive the electric power supply. This timing is indicated by T 1   FIG. 4 . In this example of the sequential operation, an example is shown wherein in order to control for stabilization of the image forming process, for example, the stabilization of the photosensitive drum or the like in the image formation system, the photosensitive drum is rotated (preliminary multi-rotation) which leads to increase of the mechanical load. In the image fixing system, the electric power instruction value Pcont is P 2  which means a decreased electric energy consumption of the mechanical elements to supply sufficient electric power to the mechanical elements in the image formation system. 
   The preliminary multi-rotation step ends at T 2  in  FIG. 4 , and thereafter, the potential control or the like operation for adjusting the charge amount on the photosensitive drum begins. In this state, the electric energy consumption for the mechanical load element is smaller than before, the electric power instruction value Pcont which can be applied to the fixing system is larger, and therefore, P 3  which is larger than P 2  is used. 
   At a certain point of time thereafter, or when the predetermined temperature with which the image fixing operation is capable, the original carriage scanner, the polygonal mirror motor driving of the laser writing system or the like begins. The operation timing is indicated by T 3  on the electric power supply sequence in  FIG. 4 . Then, the directly preliminary operations are carried out, the mechanical and electrical load elements consume corresponding electric power. Therefore, the mechanical and electrical load elements consume increased electric power. Then, in the fixing system, the electric energy consumption of the fixing system is decreased to P 4  (electric power instruction value Pcont) so as to permit the increase of the electric energy consumption. 
   Thereafter, when the surface temperature of the fixing roller  100  reaches the predetermined fixing temperature, the copying operation of the copying machine is enabled, so that print output becomes possible in response to instructions on an operating panel or remote instructions. 
   When such a raising sequence is used, the electric power instruction value Pcont, the high temperature abnormality detection level (first temperature discrimination reference) corresponding to the Pcont target value THL value and the low temperature abnormality detection level (second temperature discrimination reference) TLL value, are set in accordance with the various states of operations. Examples of setting the abnormality detection level THL, high temperature abnormality detection level (first temperature discrimination reference), TLL low temperature abnormality detection level (second temperature discrimination reference.  FIG. 10  shows an ideal temperature rising curve when the fixing roller is heating in good order. Curves P 1 , P 2 , P 3  are fixing roller temperature rising curves when the fixing roller is heated with the P 1 , P 2 , P 3  of f electric power instruction value Pcont. The abnormality detection circuit  111  has data indicative of the temperature risings with such determined electric power supplies. The abnormality detection circuit  111  sets THL, TLL on the basis of the electric power instruction value Pcont from the electric power control circuit  110  and the heating time, and compares the detected temperature fed from the temperature detecting circuit IC with them. Referring to  FIG. 11 , the description will be made as to the specific setting method for the THL, TLL.  FIG. 11  shows a temperature rising curve which constitutes a reference to be used for setting the high temperature abnormality detection level THL and the low temperature abnormality detection level TLL. Curves P 1 , P 2  represent ideal rising curves when the fixing roller is heated in good order with P 1 , P 2  of the electric power instruction value Pcont. When the electric power instruction value Pcont is P 1 , the temperature of the fixing roller traces the thick curve P 1 . When the electric power instruction value Pcont changes to P 2  at the point of time T 1 , the temperature of the fixing roller traces the thick line P 2  which is a line translated in the direction of time axis. A temperature rising curve by connecting such ideal temperature rising curves for the respective electric power instruction values is used as a reference line, and THL is determined as a curve which is higher than the reference line by a predetermined temperature, and TLL is determined as a curve which is lower than that by a predetermined temperature. The THL line and the TLL line may be determined as lines which are higher and lower by predetermined percentages relative to the reference line. The abnormality detecting means  111  keeps the thus determined THL and TLL curves obtained from the reference curve as time series temperature rising data, and the abnormality detection circuit  111  reads out the THL and TLL data in accordance with the electric power instruction value Pcont from the electric power control circuit  110  and the heating time, and compares the detected temperature from the temperature detecting circuit IC with them, thus discriminating whether the apparatus is normal or not. As another example of THL and TLL setting, the abnormality detecting means does not keep such time series data, but keeps a mathematical expression indicative of the ideal temperature rising curve on the assumption that fixing roller is heated in order. An example of a mathematical expression of the ideal temperature rising curve on the assumption that fixing roller is heated in order, will be described. Such an ideal temperature rising curve is supposed to be determined as a phantom temperature rising curve (line  11 ) of the fixing roller on the assumption that there is no heat release (curve  12  in  FIG. 12 ), deducted by a curve of released heat quantity. Then, the temperature TR of the fixing roller which has been heated for t hours from the start of heating is expressed as the following mathematical expression:
 
 TR =( P/C )× t−k ( TR−T 0)
 
   where TR is a temperature of the fixing roller on the assumption that there is no heat release in the heating period; T 0  is an ambient temperature of the apparatus; P is the electric power supplied to the fixing roller; C is a thermal capacity of the fixing roller; and k is a proportional constant. The temperature TR of the fixing roller obtained in consideration of the heat release is used as a reference, and the THL curve is determined as a curve which is higher by a predetermined temperature than the reference curve, and the TLL curve is determined as a curve which is lower by a predetermined temperature than the reference curve. The THL line and the TLL line may be determined as lines which are higher and lower by predetermined percentages relative to the reference line. In this manner, the circuit  111  calculates at proper timing the reference fixing roller temperature TR on the basis of the electric power instruction value Pcont from the electric power control circuit  110  and the heating time, and the THL and TLL are set on the basis of the calculation. The detected temperature from the temperature detecting circuit IC is compared with them, and the discrimination is made as to whether or not the apparatus is in order. 
   The image forming apparatus use the induction heating type is different from the heating method using a heat generation source having a heat generating electric power which is inherent to the heat source such as the halogen lamp in that temperature control function can be accomplished using any electric power instruction value Pcont. Therefore, the electric power value supplied to the heating fixing device can be changed during the period from the heating roller  100  cold state which occurs at the time of start of the electric power supply to the image forming apparatus to the image-fixable temperature state, and therefore, the start-up time period of the image forming apparatus can be reduced. 
   Referring to  FIG. 5  showing the electric power supply sequence, in the fixing device using the induction heating, if the motor (M. M) for rotating the fixing device stops for some reason or another, for example, only a part of the fixing roller generates heat, as shown in  FIG. 3  with the result of non-uniform temperature distribution, and the temperature rising at this part is very steep as shown in  FIG. 5 . If the use is made with a conventional temperature detecting means such as those using bimetal or temperature sensing fusing metal, a significantly long time is required until the cutting temperature is reached with the result that excessive temperature rise detection is not correctly carried out, and therefore, a high sensitivity element which is expensive has to be used. 
   According to the embodiment of the present invention, a temperature detecting element such as thermister is disposed substantially at a position of the fixing roller where the heating value is the largest. The abnormality detecting means  111  shuts off the electric power supply to the electric power applying means  90  when the detected temperature which is detected in predetermined time series upon the start-up of the heating member becomes out of a temperature range which is predetermined in accordance with the electric power control signal Pcont. Thus, the temperature abnormality can be detected, and the temperature can be raised at high speed. In this manner, the safety of the fixing device can be significantly improved. 
   The electric power control signal Pcont determines the electric power supplying operation to the fixing roller  100  in accordance with the operational state of the image forming apparatus (unshown), and the electric power amount (electric power instruction value) during the electric power application and electric power supply are determined. By doing so, when the electric power is required by other than the fixing device, the electric power control signal Pcont changes correspondingly, and the abnormality detection range can be set. Therefore, even when the electric power instruction value changes due to the change of the operational state of the apparatus, the corresponding abnormality detection range can be set. Thus, the safety is significantly improved in the fixing device wherein the temperature can be quickly raised. 
   Embodiment 2: 
     FIG. 6  is a schematic block diagram of the device and method in another embodiment of the present invention. In this embodiment, the electric power control circuit  110  is given the function of keeping and outputting in time series the temperature rising level in the case of normal operation (reference). Simultaneously with outputting the operation permission signal IH-ON and the electric power instruction value Pcont, the normal temperature rising output level is supplied to the abnormality detecting circuit  111  as a temperature reference signal T-Ref. 
   In the abnormality detecting circuit  111 , the abnormality detection level is set on the basis of the temperature reference signal TRef supplied from the electric power control circuit  110 . For example, as shown in  FIG. 7 , the high temperature abnormality detection level THL is determined as 1.2 times the temperature reference signal T-Ref, and the low temperature abnormality detection level TLL is determined as 0.8 times the temperature reference signal T-Ref, thus providing the upper and lower limits. 
   With this structure, it is not necessary for the abnormality detection circuit  111  to keep the time series temperature rising profile, so that abnormality detection circuit  111  can be made simple and can be made more widely usable. In addition, the detection levels are provided by the temperature reference signal T-Ref multiplied by coefficients, and therefore, when the fixing roller temperature is low (low temperature side), the detection width is small, and when it is close to the target temperature (high temperature side), the detection width is large, the detection accuracy is improved. 
   The temperature detecting element in the Figure has been described as being a thermister, but another temperature detecting element such as thermocouple, platinum temperature measuring wire, thermo pile or the like is usable. 
   In the foregoing embodiments, the heating apparatus of the present invention is used for the fixing device. However, the present invention is applicable to a heating apparatus for uncreasing a material to be heated or for heating it by increasing surface gloss to improve the quality. 
   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.