Patent Publication Number: US-7596332-B2

Title: Fixing apparatus and image forming apparatus including power suppression

Description:
TECHNICAL FIELD 
     The present invention relates to a fixing apparatus that heat-fixes an unfixed image onto a recording material. 
     BACKGROUND ART 
     An image forming apparatus such as an electrophotographic copier, printer, or facsimile apparatus is equipped with a fixing apparatus that heat-fixes an unfixed toner image formed on the surface of a recording material. There is a fixing apparatus wherein a pressure nip is formed between a fixing roller and a heating roller pressing against that fixing roller, recording material bearing toner is gripped and transported to this pressure nip, and an unfixed image is heated from the fixing roller side and is heat-fixed onto the recording material surface. 
     A variety of methods have been developed as fixing roller heating methods. For example, a method is known whereby a fixing roller is composed of a film guide comprising an insulative cylindrical member that does not prevent the passage of magnetic flux and electromagnetic-induction heat-producing film (fixing film) wrapped around the outer periphery of this film guide, a magnetic field generated by a field generation section comprising an exciting coil and core provided outside the pressure nip area is applied and induction heating performed, and as the fixing roller rotates, the heated area moves to the pressure nip and heat-fixes the toner. Alternatively, a method is known whereby a fixing belt of electromagnetic-induction heat-producing film is suspended between a fixing roller and heating roller, the fixing belt is induction-heated by the application of a magnetic field to the fixing belt sliding over the heating roller by a field generation section provided opposite the heating roller, and the heated fixing belt moves to the pressure nip and heat-fixes the toner. 
     In both methods, a control circuit (microcomputer) generally performs temperature control in order to maintain the temperature of the rotating heating member (fixing film or fixing belt) at a temperature suitable for fixing. The control circuit not only controls the rotating heating member at the optimal temperature, but can also be given a control function of preventing the problem of erroneous heating when rotation of the rotating heating member stops. Specifically, a rotation detection section (optical sensor) is provided that detects rotation of the fixing film, and when rotation of the fixing film stops or falls to a predetermined speed or below, the control circuit (microcomputer) stops or suppresses the power supply to the exciting coil, and suppresses heat output (see Patent Document 1, for example).
     Patent Document 1: Unexamined Japanese Patent Publication No. 2001-203072   

     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     However, if the control circuit (microcomputer) fails or malfunctions, heat output will not be suppressed, and therefore if an excessive rise in temperature of the rotating heating member is predicted, heat output must be suppressed without the intermediation of the control circuit. 
     It is an object of the present invention to provide a fixing apparatus that makes it possible to diagnose whether or not a mechanism that prevents an excessive rise in temperature of the rotating heating member when the control circuit is in a normal state operates normally, and to suppress heat output dependably without the intermediation of the control circuit if the control circuit fails or malfunctions, thereby producing an excellent effect on safety. 
     Means for Solving the Problems 
     According to a fixing apparatus of the present invention, in an entity whereby a processor on the main body side controls heating of a rotating heating member that heat-fixes an unfixed image on a recording medium, a self-diagnosis function is provided whereby, when a condition for not heating the rotating heating member has been met, a directive to heat is given, and it is confirmed that the rotating heating member is not heated. 
     Advantageous Effect of the Invention 
     According to the present invention, a fixing apparatus can be provided that makes it possible to diagnose whether or not a mechanism that prevents an excessive rise in temperature of the rotating heating member when the control circuit is in a normal state operates normally, and to dependably suppress heat output without the intermediation of the control circuit if the control circuit fails or malfunctions, thereby producing an excellent effect on safety 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an overall configuration diagram of an image forming apparatus to which Embodiment 1 and Embodiment 2 of the present invention are applied; 
         FIG. 2  is a cross-sectional side view of a fixing apparatus provided in the image forming apparatus shown in  FIG. 1  according to Embodiment 1 of the present invention; 
         FIG. 3  is a functional block diagram of a fixing apparatus according to Embodiment 1 of the present invention; 
         FIG. 4  is a circuit configuration diagram of an IH power supply provided in the fixing apparatus shown in  FIG. 3  according to Embodiment 1 of the present invention; 
         FIG. 5  is a circuit configuration diagram of a rotation detection circuit provided in the IH power supply shown in  FIG. 4  according to Embodiment 1 of the present invention; 
         FIG. 6  is a flowchart for self-diagnosis of a fixing apparatus according to Embodiment 1 of the present invention; 
         FIG. 7  is a circuit configuration diagram of an IH power supply in a fixing apparatus according to Embodiment 2 of the present invention; 
         FIG. 8  is a flowchart for self-diagnosis of a fixing apparatus according to Embodiment 2 of the present invention; 
         FIG. 9  is a circuit configuration diagram of an IH power supply in a fixing apparatus according to Embodiment 3 of the present invention; and 
         FIG. 10  is a flowchart for self-diagnosis of a fixing apparatus according to Embodiment 3 of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the drawings, configuration elements and equivalent parts that have identical configurations or functions are assigned the same codes, and descriptions thereof are not repeated. 
     EMBODIMENT 1 
       FIG. 1  is schematic cross-sectional diagram showing the configuration of an image forming apparatus suitable for the installation of a fixing apparatus according to Embodiment 1 of the present invention. As shown in  FIG. 1 , this image forming apparatus  100  is a single-path image forming apparatus in which toner images of four colors contributing to coloring of a color image are formed separately on four image bearing elements, these toner images of four colors are successively superimposed onto an intermediate transfer element as a primary transfer process, and then blanket transfer (secondary transfer) of this primary image to the recording medium is performed. 
     A fixing apparatus according to Embodiment 1 is not limited solely to the above-described single-path type of image forming apparatus, but can be installed in any type of image forming apparatus. 
     In  FIG. 1 , symbols Y, M, C, and K appended to the reference codes assigned to various configuration elements of image forming apparatus  100  indicate configuration elements involved in formation of a yellow image (Y), magenta image (M), cyan image (C), and black image (K), respectively, with configuration elements assigned the same reference code having a common configuration. 
     Image forming apparatus  100  has photosensitive drums  110 Y,  110 M,  110 C, and  110 K as the above-described four image bearing elements, and an intermediate transfer belt (intermediate transfer element)  170 . Around photosensitive drums  110 Y,  110 M,  110 C, and  110 K are located image forming stations SY, SM, SC, and SK. Image forming stations SY, SM, SC, and SK comprise electrifiers  120 Y,  120 M,  120 C, and  120 K, an aligner (exposure apparatus)  130 , developing units  140 Y,  140 M,  140 C, and  140 K, transfer units  150 Y,  150 M,  150 C, and  150 K, and cleaning apparatuses  160 Y,  160 M,  160 C, and  160 K. 
     In  FIG. 1 , photosensitive drums  110 Y,  110 M,  110 C, and  110 K are rotated in the direction indicated by arrows C. The surfaces of photosensitive drums  110 Y,  110 M,  110 C, and  110 K are uniformly charged to a predetermined potential by electrifiers  120 Y,  120 M,  120 C, and  120 K respectively. 
     The surfaces of charged photosensitive drums  110 Y,  110 M,  110 C, and  110 K are irradiated with laser beam scanning lines  130 Y,  130 M,  130 C, and  130 K corresponding to image data of specific colors by means of aligner  130 . By this means, electrostatic latent images of the aforementioned specific colors are formed on the surfaces of photosensitive drums  110 Y,  110 M,  110 C, and  110 K. 
     The electrostatic latent images of each of the specific colors formed on photosensitive drums  110 Y,  110 M,  110 C, and  110 K are developed by developing units  140 Y,  140 M,  140 C, and  140 K. By this means, unfixed images of the four colors contributing to the coloring of the color image are formed on photosensitive drums  110 Y,  110 M,  110 C, and  110 K. 
     The developed toner images of four colors on photosensitive drums  110 Y,  110 M,  110 C, and  110 K undergo primary transfer to above-described endless intermediate transfer belt  170  functioning as an intermediate transfer element by means of transfer units  150 Y,  150 M,  150 C, and  150 K. By this means, the toner images of four colors formed on photosensitive drums  110 Y,  110 M,  110 C, and  110 K are successively superimposed, and a full-color image is formed on intermediate transfer belt  170 . 
     After the toner images have been transferred to intermediate transfer belt  170 , photosensitive drums  110 Y,  110 M,  110 C, and  110 K have residual toner remaining on their surfaces removed by cleaning apparatuses  160 Y,  160 M,  160 C, and  160 K, respectively. 
     Here, aligner  130  is installed at a predetermined angle with respect to photosensitive drums  110 Y,  110 M,  110 C, and  110 K. Also, intermediate transfer belt  170  is suspended between a drive roller  171  and driven roller  172 , and is circulated in the direction indicated by arrow A in  FIG. 1  by rotation of drive roller  171 . 
     Meanwhile, at the bottom of image forming apparatus  100 , a paper feed cassette  180  is provided in which recording paper P such as printing paper functioning as a recording medium is held. Recording paper P is fed out from paper feed cassette  180  by a paper feed roller  181  one sheet at a time into a predetermined sheet path. 
     When recording paper P fed into this sheet path passes through a transfer nip formed between the outer surface of intermediate transfer belt  170  suspended on driven roller  172  and a secondary transfer roller  190  in contact with the outer surface of intermediate transfer belt  170 , the full-color image (unfixed image) formed on intermediate transfer belt  170  is blanket-transferred by secondary transfer roller  190 . 
     Recording paper P passes through a fixing nip N formed between the outer surface of a fixing belt  230  suspended between a fixing roller  210  and heating roller  220 , and a pressure roller  240  in contact with the outer surface of fixing belt  230 , in a fixing apparatus  200  shown in detail in  FIG. 2 . By this means, the unfixed full-color image blanket-transferred to recording paper P is heat-fixed onto recording paper P. 
     Image forming apparatus  100  is equipped with a freely opening and closing door  101  forming part of the housing of image forming apparatus  100 , and replacement or maintenance of fixing apparatus  200 , handling of recording paper P jammed in the above-described paper transportation path, and so forth, can be carried out by opening and closing this door  101 . 
     Next, fixing apparatus  200  according to Embodiment 1 installed in image forming apparatus  100  will be described with reference to  FIG. 2 . 
     Fixing apparatus  200  according to Embodiment 1 is an induction heating (IH) type of fixing apparatus, and, as shown in  FIG. 2 , is equipped with fixing roller  210 , heating roller  220  as a heat-producing element, fixing belt  230  as an image heating element, pressure roller  240 , an induction heating apparatus  250  as a heating section, a separator  260  as a sheet separation guide plate, sheet guide plates  281 ,  282 ,  283 , and  284  as sheet transportation path forming members, and so forth. 
     In this fixing apparatus  200 , heating roller  220  and fixing belt  230  are heated through the agency of a magnetic field generated by induction heating apparatus  250 , and an unfixed image on recording paper P transported along sheet guide plates  281 ,  282 ,  283 , and  284  is heat-fixed by fixing nip N between heated fixing belt  230  and pressure roller  240 . 
     A fixing apparatus according to this embodiment may also be configured so that fixing belt  230  is not used, fixing roller  210  also serves as heating roller  220 , and an unfixed image on recording paper P is heat-fixed directly by this fixing roller  210 . It also goes without saying that a heat source such as a halogen lamp may be used as the heating section. 
     In  FIG. 2 , heating roller  220  functioning as a heat-producing element is configured as a rotating element comprising a hollow cylindrical magnetic metallic member of iron, cobalt, nickel, or an alloy of these metals, for example, with both ends supported in rotatable fashion by bearings fixed to supporting side plates (not shown), and rotated by a drive section (not shown). Heating roller  220  has a configuration enabling a rapid rise in temperature with low thermal capacity, with an external diameter of 20 mm and thickness of 0.3 mm, and is regulated so that its Curie point is 300° C. or above. 
     Fixing roller  210  is configured with, for example, a core of stainless steel or another metal covered by a heat-resistant elastic member of solid or foam silicone rubber, and has an outer diameter of about 30 mm, larger than the outer diameter of heating roller  220 . The elastic member has a thickness of about 3 to 8 mm and hardness of about 15 to 50° (Asker hardness: 6 to 25° JIS A hardness). 
     Pressure roller  240  presses against fixing roller  210 . Due to the pressure between fixing roller  210  and pressure roller  240 , a fixing nip N of predetermined width is formed at the pressure location. 
     Fixing belt  230  is configured as a heat-resistant belt suspended between heating roller  220  and fixing roller  210 . Due to induction heating of heating roller  220  by induction heating apparatus  250  described later herein, the heat of heating roller  220  is transferred at the area of contact between fixing belt  230  and heating roller  220 , and fixing belt  230  is heated all around due to its circulation. 
     In fixing apparatus  200  configured in this way, the thermal capacity of heating roller  220  is smaller than the thermal capacity of fixing roller  210 , and therefore heating roller  220  is heated rapidly, and the warm-up time at the start of heat-fixing is shortened. 
     Fixing belt  230  is configured, for example, as a heat-resistant belt of multilayered construction, comprising a heat-producing layer, an elastic layer, and a release layer. The heat-producing layer has a magnetic metal such as iron, cobalt, nickel, or the like, or an alloy of these metals, as the base material. The elastic layer is of silicone rubber, fluororubber, or the like, fitted around the surface of the heat-producing layer. The release layer is formed of resin or rubber with good release characteristics, such as PTFE, PFY, FEP, silicone rubber, fluororubber, or the like, alone or mixed. 
     Even if foreign matter should be introduced between this fixing belt  230  and heating roller  220  for some reason, creating a gap, the fixing belt itself can still be heated by induction heating of its heat-producing layer by induction heating apparatus  250 . Thus, this fixing belt  230  can itself be heated directly by induction heating apparatus  250 , heating efficiency is good, and response is rapid, so that there is little unevenness of temperature, and reliability as a heat-fixing section is high. 
     Pressure roller  240  is configured with an elastic member of high heat resistance and high toner releasability fitted to the surface of a core comprising a cylindrical member of a highly heat conductive metal such as copper or aluminum, for example. Apart from the above-mentioned metals, SUS may also be used for the core. 
     This pressure roller  240  forms fixing nip N that grips and transports recording paper P by exerting pressure on fixing roller  210  via fixing belt  230 . In this fixing apparatus  200  according to Embodiment 1, the hardness of pressure roller  240  is greater than the hardness of fixing roller  210 , and fixing nip N is formed by the peripheral surface of pressure roller  240  biting into the peripheral surface of fixing roller  210  via fixing belt  230 . 
     For this reason, pressure roller  240  has an external diameter of about 30 mm, the same as fixing roller  210 , a thickness of about 2 to 5 mm, thinner than fixing roller  210 , and hardness of about 20 to 60° (Asker hardness: 6 to 25° JIS A hardness), harder than fixing roller  210 . 
     In fixing apparatus  200  with this kind of configuration, recording paper P is gripped and transported by fixing nip N so as to follow the surface shape of the peripheral surface of pressure roller  240 , with the resultant effect that the heat-fixing surface of recording paper P separates easily from the surface of fixing belt  230 . 
     A temperature detector  270  comprising a thermistor or similar heat-sensitive element with high thermal responsiveness is located in direct contact with the inner peripheral surface of fixing belt  230  in the vicinity of the entry side of fixing nip N. In this fixing apparatus  200 , the heating temperature of heating roller  220  and fixing belt  230  due to induction heating apparatus  250  is controlled so that the surface temperature of fixing belt  230 —that is, the unfixed image heat-fixing temperature—is maintained at a predetermined temperature based on the temperature of the inner peripheral surface of fixing belt  230  detected by temperature detector  270 . 
     Next, the configuration of induction heating apparatus  250  will be described. As shown in  FIG. 2 , induction heating apparatus  250  is located so as to face the outer peripheral surface of heating roller  220  via fixing belt  230 . Induction heating apparatus  250  is provided with a supporting frame  251  as a coil guide member of fire-retardant resin, curved so as to cover heating roller  220 . 
     In the center part of supporting frame  251 , a thermostat  252  is installed so that its temperature detecting part is partially expressed from supporting frame  251  toward heating roller  220  and fixing belt  230 . Thermostat  252  detects the temperature of heating roller  220  and fixing belt  230 , and if thermostat  252  detects that the temperature of heating roller  220  and fixing belt  230  is abnormally high, it forcibly breaks the connection between an exciting coil  253  functioning as a magnetic field generation section wound around the outer peripheral surface of supporting frame  251  and an inverter circuit (not shown). 
     Exciting coil  253  is configured with a long single exciting coil wire with an insulated surface wound alternately in the axial direction of heating roller  220  along supporting frame  251 . The length of the wound part of this exciting coil  253  is set so as to be approximately the same as the length of the area of contact between fixing belt  230  and heating roller  220 . 
     Exciting coil  253  is connected to an inverter circuit (not shown), and generates an alternating field by being supplied with a high-frequency alternating current of 10 kHz to 1 MHz (preferably, 20 kHz to 800 kHz). This alternating field acts upon the heat-producing layers of heating roller  220  and fixing belt  230  in the area of contact between heating roller  220  and fixing belt  230  and its vicinity. Through the agency of this alternating field, an eddy current with a direction preventing variation of the alternating field flows within these heat-producing layers. 
     This eddy current generates Joule heat corresponding to the resistance of the heating roller  220  and fixing belt  230  heat-producing layers, and causes induction heating of heating roller  220  and fixing belt  230  mainly in the area of contact between heating roller  220  and fixing belt  230  and its vicinity. 
     On the other hand, an arch core  254  and side core  255  are fitted on supporting frame  251  so as to surround exciting coil  253 . Arch core  254  and side core  255  increase the inductance of exciting coil  253  and provide good electromagnetic coupling of exciting coil  253  and heating roller  220 . Therefore, in this fixing apparatus  200 , it is possible to apply a larger amount of power to heating roller  220  with the same coil current through the agency of arch core  254  and side core  255 , enabling the warm-up time to be shortened. 
     Supporting frame  251  is also provided with a resin housing  256  formed in the shape of a roof so as to cover arch core  254  and thermostat  252  inside induction heating apparatus  250 . A plurality of heat release vents are formed in this housing  256 , allowing heat generated by supporting frame  251 , exciting coil  253 , arch core  254 , and so forth, to be released externally. Housing  256  may be formed of a material other than resin, such as aluminum, for example. 
     Supporting frame  251  is also fitted with a short ring  257  that covers the outer surface of housing  256  to prevent blockage of the heat release vents formed in housing  256 . Short ring  257  is located on the rear of arch core  254 . Through the generation of an eddy current in the direction in which slight leakage flux leaked externally from the rear of arch core  254  is canceled out, short ring  257  has the effect of generating a magnetic field that cancels out the magnetic field of that leakage flux, and preventing unwanted emission due to that leakage flux. 
     A rotary encoder  290  is installed coaxially with respect to the rotation axis of fixing roller  210 . A photointerrupter  291  is installed with its light-emitting section and light-receiving section positioned on opposite sides of the rotating blades of rotary encoder  290 . As rotary encoder  290  is installed coaxially with respect to the rotation axis of fixing roller  210 , it rotates integrally with fixing roller  210 . During rotation of rotary encoder  290 , the output signal of photointerrupter  291  is a square-wave phase signal in which the signal level rises each time a rotating blade of rotary encoder  290  cuts off a light beam that is emitted from the light-emitting section and strikes the light-receiving section. That is to say, photointerrupter  291  outputs a phase signal that has a period corresponding to the rotation speed of rotary encoder  290 , and takes on a flat signal waveform when rotation of rotary encoder  290  stops. 
     Next, the electrical configuration and function of parts that control the operation of induction heating apparatus  250  will be described.  FIG. 3  is a functional block diagram showing parts related to fixing apparatus  200 , comprising an IH power supply  300  that controls the operation of induction heating apparatus  250  and a main apparatus  400  of an image forming apparatus. 
     In IH power supply  300 , a commercial AC power supply  302  is connected to a rectifier circuit  304  via a filter  303 , and alternating current (AC) is converted to direct current (DC). The DC side of rectifier circuit  304  is connected to an inverter circuit  305 , and a high-frequency alternating current is supplied to induction heating apparatus  250  from inverter circuit 
     Meanwhile, a phase signal output from a rotation signal generation section  301  comprising rotary encoder  290  and photointerrupter  291  is captured by a rotation detection circuit  306 . A rotation detection signal output by rotation detection circuit  306  is input to an oscillation stop circuit  307 , and when rotation of fixing roller  210  (fixing belt  230 ) is detected to have stopped or to have fallen to a predetermined rotation speed or below, oscillation of inverter circuit  305  is forcibly stopped. The rotation detection signal output by rotation detection circuit  306  is also input to a CPU  401  of main apparatus  400 . 
     For purposes of self-diagnosis described later herein, a detection section  308  is also provided that detects the voltage value and current value supplied to rectifier circuit  304  from commercial AC power supply  302 . A detection signal output by detection section  308  is converted to a digital signal by an A/D converter  309 , and is then input to CPU  401  of main apparatus  400 . 
     When power is turned on, and at regular intervals during standby, CPU  401  of main apparatus  400  performs self-diagnosis to confirm that fixing roller  210  is not heated when stopped or when rotating at a predetermined rotation speed or below. A drive section  402  of main apparatus  400  has the function of rotating pressure roller  240  on receiving a drive request from CPU  401 . 
       FIG. 4  is a drawing showing the circuit configuration of IH power supply  300 . 
     A power supply switch  326  is provided between an inlet  325  that is physically connected to commercial AC power supply  302 , and filter  303 . When power supply switch  326  is turned on, alternating current flows from commercial AC power supply  302  to rectifier circuit  304 . In inverter circuit  305 , a capacitor  327  is connected in parallel to exciting coil  253 , one electrode of capacitor  327  is grounded via another capacitor  328 , and the other electrode of capacitor  327  is grounded via a switching element  329  comprising an IGBT in the forward direction. DC-side terminals of rectifier circuit  304  are connected to both ends of exciting coil  253 , and a high-frequency alternating current can be supplied to exciting coil  253  by switching element  329  on and off. Also, a thermostat  330  is inserted in series between a positive-electrode-side terminal of rectifier circuit  304  and exciting coil  253 . 
     Switching element  329  has its gate electrode driven on and off by an IGBT drive circuit  331 . IGBT drive circuit  331  controls the on/off drive period (the width of the on period and the width of the off period) by sending switching element  329  a square-wave PWM signal supplied from a PWM circuit  332 . In a high-level period of IGBT drive circuit  331  output, switching element  329  is turned on and alternating current flows in exciting coil  253 . In a low-level period, switching element  329  is turned off and the coil current flowing in exciting coil  253  falls abruptly. PWM circuit  332  outputs a PWM signal composed of pulses when an ON/OFF signal supplied from CPU  401  is ON, and stops pulse output when the ON/OFF signal is OFF. When the level of a power signal supplied from CPU  401  is high, the high-level period of the PWM signal is lengthened, and conversely, when the level of a power signal is low, the high-level period of the PWM signal is shortened. Varying the length of the high-level period of the PWM signal enables the size of the coil current flowing in exciting coil  253  to be varied, and the strength of the generated field to be varied, making it possible to vary the calorific value of heating roller  220  and fixing belt  230 . 
     Oscillation stop circuit  307  is composed of a first transistor  334  and a second transistor  335 . First transistor  334 , IGBT drive circuit  331  and second transistor  335  are connected in series between +Vcc and ground, the collector side of first transistor  334  is maintained at +Vcc potential, and the emitter side of second transistor  335  is connected to ground potential. IGBT drive circuit  331  is configured so as to generate switching element  329  drive pulses using voltage Vcc applied via first transistor  334 . Meanwhile, the output signal from rotation detection circuit  306  is applied to the base of first transistor  334 , and the output signal from rotation detection circuit  306  is inverted by an inverter circuit  336  and then applied to the base of second transistor  335 . Thus, while the output signal from rotation detection circuit  306  is active (while fixing roller  210  is rotating steadily), the base of first transistor  334  is in a conducting state, and the base of second transistor  335  is in a non-conducting state, and therefore operating voltage Vcc is applied to IGBT drive circuit  331 . Conversely, while the output signal from rotation detection circuit  306  is non-active (while fixing roller  210  is stopped or is at a predetermined speed or below), the base of first transistor  334  is in a non-conducting state, and the base of second transistor  335  is in a conducting state, and therefore operating voltage Vcc ceases to be applied to IGBT drive circuit  331 , and the PWM signal output from PWM circuit  332  ceases to be input. 
       FIG. 5  is a drawing showing the actual configuration of rotation detection circuit  306 . For the sake of explanation, the configuration of the parts connected before and after rotation detection circuit  306  is also shown in the drawing. In rotation detection circuit  306 , a phase signal from photointerrupter  291  is input to an edge extraction circuit  340 . The phase signal from photointerrupter  291  has a square-wave signal waveform while rotary encoder  290  is rotating, and has a flat signal waveform maintained at a low level or high level when rotation of rotary encoder  290  is stopped as described above. The period of the phase signal has a larger value as the rotation speed of rotary encoder  290  falls. Edge extraction circuit  340  extracts (rising or falling) edges of a phase signal output from photointerrupter  291 , and an edge interval measuring circuit  341  measures the edge interval detected by edge extraction circuit  340 —that is, the period of the phase signal. To be specific, edge interval measuring circuit  341  counts the number of clocks from detection of one edge until detection of the next edge, and inputs a count value indicating the edge interval (period) to a comparator  342 . Meanwhile, a numeric value corresponding to an arbitrary rotation speed of fixing roller  210  is stored in a stipulated time data storage section  343 . In this example, a numeric value is stored that corresponds to the period of a phase signal output from photointerrupter  291  when the rotation speed of fixing roller  210  is a value at which heating should be suppressed. Comparator  342  compares the count value output by edge interval measuring circuit  341  with the numeric value stored in stipulated time data storage section  343 , and outputs a non-active drive signal while the count value exceeds the stored value, and an active drive signal while the count value is less than the stored value. The drive signal is applied to the base of a driver  344  comprising a transistor. Driver  344  generates a rotation detection signal that is low-level while the drive signal is non-active (while the count value exceeds the stored value), and high-level while the drive signal is active (while the count value is less than the stored value). Rotation detection circuit  306  shown here is configured as a digital circuit, but the same kind of function may also be implemented using an analog circuit. 
     Next, the operation of fixing apparatus  200  configured as described above will be described. 
     A fixing roller  210  rotation directive is issued from CPU  401  of main apparatus  400  to drive section  402 . Drive section  402  performs normal rotation of pressure roller  240  by controlling a drive system (not shown). Fixing roller  210  pressed against pressure roller  240  is rotated together. Fixing roller  210  and heating roller  220  rotate in synchronization via fixing belt  230 . 
     At this time, rotary encoder  290  installed coaxially with respect to the rotation axis of fixing roller  210  also rotates in synchronization. Through the rotation of rotary encoder  290 , a phase signal with a period corresponding to the rotation speed of fixing roller  210  is output from photointerrupter  291 . Rotation detection circuit  306  outputs a low-level signal until the rotation speed of fixing roller  210  reaches a predetermined value, and changes the signal to a high-level signal when the rotation speed exceeds the predetermined value. When the rotation detection signal from rotation detection circuit  306  becomes high-level, first and second transistors  334  and  335  of oscillation stop circuit  307  go to the on state. As a result, inverter circuit  305  goes to a state in which oscillation is possible in accordance with the output signal from PWM circuit  332 . 
     When the need for heating by induction heating apparatus  250  arises, CPU  401  starts supplying an ON/OFF signal and power signal to inverter circuit  305  of IH power supply  300 . PWM circuit  332  generates a pulsed PWM signal based on the ON/OFF signal and power signal, and supplies this PWM signal to IGBT drive circuit  331 . IGBT drive circuit  331  sends the PWM signal to switching element  329  and performs on/off control. As a result, a high-frequency alternating current is supplied to exciting coil  253  of induction heating apparatus  250 . 
     In induction heating apparatus  250 , an alternating field generated by exciting coil  253  causes an eddy current to flow in the heat-producing layers of heating roller  220  and fixing belt  230 , and induction heating of heating roller  220  and fixing belt  230  is performed mainly in the area of contact between heating roller  220  and fixing belt  230  and its vicinity. 
     Control during a rise in temperature of fixing belt  230  (in the period from the start of heating until the target temperature is reached) will now be described. When fixing belt  230  rises in temperature, in order to shorten the time taken to reach the target temperature as much as possible, CPU  401  controls the level of the ON/OFF signal and power signal so that the power supplied to IH power supply  300  is maintained at the highest level that can be supplied. That is to say, the current value and voltage value supplied to IH power supply  300  are detected by detection section  308 , and a detection signal is input to CPU  401  by A/D converter  309 . CPU  401  controls the level of the power signal based on the detected current value and voltage value so that the supply of predetermined power to IH power supply  300  is maintained. Power is controlled by means of this kind of feedback control. 
     Next, control during fixing belt  230  temperature regulation (in the period in which the target temperature is maintained) will be described. 
     The temperature of fixing belt  230  is detected by temperature detector  270 . A temperature detection signal output by temperature detector  270  is input to CPU  401 . CPU  401  determines the ON/OFF signal and power signal that should be output to PWM circuit  332  based on the relevant temperature detection signal. That is to say, the ON period and OFF period of the ON/OFF signal and the level of the power signal are controlled so that the target temperature is achieved. Basically, the fixing temperature is controlled by means of this kind of feedback control. 
     However, there is a possibility of the above-described feedback control not working if CPU  401  fails or malfunctions. If control becomes impossible after CPU  401  has issued a heating oriented directive to PWM circuit  332 , induction heating apparatus  250  will continue heating. In particular, if the drive system of pressure roller  240 , fixing roller  210 , and heating roller  220  stops when induction heating apparatus  250  is performing heating, an area that continues to be heated directly by induction heating apparatus  250  will be damaged due to overheating, and it is therefore necessary to perform emergency stopping of heating by induction heating apparatus  250 . 
     In a case such as this, in this embodiment, oscillation of inverter circuit  305  is halted and emergency stopping of heating by induction heating apparatus  250  is performed forcibly, without the intermediation of CPU  401 , through the operation of oscillation stop circuit  307 . That is to say, stopping of rotation of fixing roller  210  is detected directly by rotation signal generation section  301 . At the point at which the rotation speed detected by rotation signal generation section  301  falls to a predetermined value, rotation detection circuit  306  changes the signal level of the rotation detection signal to the low level. As a result, first and second transistors  334  and  335  of oscillation stop circuit  307  go to the off state, and the supply of operating voltage Vcc and the PWM signal to IGBT drive circuit  331  is stopped. As a result, switching operations by switching element  329  stop, and therefore oscillation of inverter circuit  305  stops, and a high-frequency alternating current ceases to be supplied to exciting coil  253 . Exciting coil  253  ceases to generate an alternating field, and therefore induction heating also stops. 
     As inverter circuit  305  oscillation is forcibly stopped without the intermediation of CPU  401  in this way when the rotation speed detected by rotation signal generation section  301  is at or below a predetermined value, even if CPU  401  fails or malfunctions, heat output can be dependably suppressed without the intermediation of CPU  401  if an excessive rise in temperature of the fixing belt is predicted. 
     The kind of function described above is only effectuated when the detection system that detects the rotation speed (rotation detection circuit  306  and so forth) and oscillation stop circuit  307  are operating normally. It is therefore desirable for self-diagnosis of these functions to be carried out before operation of induction heating apparatus  250  and so forth is performed. In Embodiment 1, the configuration provides for CPU  401  to perform self-diagnosis each time power is turned on and/or the system is restored from the sleep state, and/or at regular intervals during standby. 
       FIG. 6  is a flowchart for self-diagnosis performed by CPU  401 . This self-diagnosis is performed when power is turned on and/or at regular intervals during standby. When self-diagnosis is started, CPU  401  issues a stop command to drive section  402 , and stops driving of pressure roller  240  (S 100 ). After driving of pressure roller  240  is stopped and rotation of fixing roller  210  is stopped, a rotation detection signal output by rotation detection circuit  306  is captured, and it is determined whether or not fixing belt  230  (fixing roller  210 ) has stopped (S 101 ). If the rotation detection signal is low-level, this indicates that the rotation speed of fixing roller  210  is at or below a predetermined value, but is here treated as indicating that rotation of fixing belt  230  has stopped. If rotation of fixing belt  230  is determined to have stopped (S 101 : YES), CPU  401  gives a directive for heat output to inverter circuit  305  by sending an ON/OFF signal and power signal to PWM circuit  332  (S 102 ) That is to say, a heating directive is given when a condition for not heating fixing belt  230  has been met. 
     Here, if rotation detection circuit  306  and oscillation stop circuit  307  are operating normally, a state should be in effect in which operating voltage Vcc and a PWM signal are not input to IGBT drive circuit  331 . Therefore, since oscillation of inverter circuit  305  has stopped, the current flowing from rectifier circuit  304  to inverter circuit  305  becomes a stipulated value or less. 
     CPU  401  captures a detection signal from detection section  308  (S 103 ), and determines whether or not the current value indicated by the detection signal is less than or equal to the stipulated value (S 104 ). If the current value is less than or equal to the stipulated value (S 104 : YES), this means that rotation detection circuit  306  and oscillation stop circuit  307  are operating normally. Therefore, in this case, CPU  401  determines that the results of the self-diagnosis are normal, and stops transmission of the ON/OFF signal and power signal being output to PWM circuit  332  (S 105 ). 
     On the other hand, if the current value is greater than the stipulated value (S 104 : NO), this means that oscillation stop circuit  307  is not operating normally and oscillation of inverter circuit  305  has not stopped. In this case, CPU  401  immediately stops heating by stopping transmission of the ON/OFF signal and power signal being output to PWM circuit  332  (S 106 ), and executes error notification processing (S 107 ). For example, a message indicating that a failure has occurred may be displayed on an operation panel (not shown). Then CPU  401  performs control so that no subsequent printing (heating) is performed (S 108 ). Alternatively, a warning voice message may be issued. 
     If CPU  401  determines in the processing in step S 101  that the rotation detection signal does not indicate that fixing belt  230  has stopped (S 101 : NO), this means that rotation detection circuit  306  has detected rotation even though rotation of pressure roller  240  and so forth has stopped, indicating that a failure has occurred in rotation detection circuit  306  or rotation signal generation section  301 . In this case, also, CPU  401  gives an error notification (S 107 ) and performs control so that no subsequent printing (heating) is performed (S 108 ). 
     By thus performing diagnosis of oscillation stop circuit  307  and rotation detection circuit  306  that stop oscillation of inverter circuit  305  dependably even if CPU  401  fails, the reliability of fixing apparatus  200  can be further increased. 
     Embodiment 2 
     Next, a fixing apparatus according to Embodiment 2 will be described. In Embodiment 2, a power-supply-side CPU is incorporated in the IH power supply, and rotation detection circuit and oscillation stop circuit functions are implemented by the power-supply-side CPU. An image forming apparatus to which this fixing apparatus is applied may be the above-described apparatus shown in  FIG. 1  and  FIG. 2 , or may be of another type. 
       FIG. 7  is a functional block diagram showing parts related to fixing apparatus  200 , comprising an IH power supply  500  that controls the operation of induction heating apparatus  250  and a main apparatus  400  of an image forming apparatus. Parts having the same function as parts in above-described Embodiment 1 are assigned the same codes as in Embodiment 1. 
     IH power supply  500  basically has the same configuration as above-described IH power supply  300 , except that rotation detection circuit  306  and oscillation stop circuit  307  are replaced by a power-supply-side CPU  501 , but the control method is somewhat different, with CPU  401  on the main apparatus side indicating the desired power during a rise in temperature and during temperature regulation to power-supply-side CPU  501  by means of a power signal  1 , and power-supply-side CPU  501  sending a power signal  2  to inverter circuit  305  so that the power indicated by CPU  401  is effected. That is to say, power-supply-side CPU  501  inputs an ON/OFF signal  2  and power signal  2  to inverter circuit  305  based on an ON/OFF signal  1  and power signal  1  sent from CPU  401  of main apparatus  400 . Also, power-supply-side CPU  501  is configured so as to enable data exchange by serial communication with CPU  401  of main apparatus  400 , and a detection signal output by detection section  308  is converted to a digital signal by an A/D converter  502  and sent to CPU  401  by serial communication. Furthermore, power-supply-side CPU  501  captures an output signal from rotation signal generation section  301  and performs determination of the rotation speed of fixing roller  210  (fixing belt  230 ), and in the case of a value at which oscillation of inverter circuit  305  should be stopped (a value less than or equal to a predetermined value), stops output of ON/OFF signal  2  and power signal  2  without regard to ON/OFF signal  1  and power signal  1  from CPU  401 . Thus, when the rotation speed of fixing roller  210  (fixing belt  230 ) is at or below a predetermined value, power-supply-side CPU  501  acts to stop oscillation of inverter circuit  305  independently of a directive from CPU  401 . 
     Since oscillation is controlled by having ON/OFF signal  1  and power signal  1  from CPU  401  relayed and supplied to inverter circuit  305  by power-supply-side CPU  501  installed on the IH power supply  500  side in this way, when the rotation speed of fixing roller  210  (fixing belt  230 ) detected from an output signal from rotation signal generation section  301  is at or below a predetermined value, oscillation of inverter circuit  305  is stopped by discontinuing output of ON/OFF signal  2  and power signal  2  even though CPU  401  is outputting ON/OFF signal  1  and power signal  1 , enabling oscillation of inverter circuit  305  to be stopped dependably, and heat output to be suppressed dependably, even if CPU  401  fails. 
     The kind of function described above is only effectuated when the detection system that detects the rotation speed and power-supply-side CPU  501  are operating normally. It is therefore desirable for self-diagnosis of these functions to be carried out before operation of induction heating apparatus  250  and so forth is performed. In this embodiment, the configuration provides for CPU  401  to perform self-diagnosis each time power is turned on and/or the system is restored from the sleep state, and/or at regular intervals during standby. 
       FIG. 8  is a flowchart for self-diagnosis performed by CPU  401 . This self-diagnosis is performed when power is turned on and/or at regular intervals during standby. When self-diagnosis is started, CPU  401  issues a stop command to drive section  402 , and stops driving of pressure roller  240  (S 200 ). After driving of pressure roller  240  is stopped and rotation of fixing roller  210  is stopped, a request is made to power-supply-side CPU  501  for fixing belt  230  rotation status information, and the rotation speed of fixing roller  210  (fixing belt  230 ) is acquired from power-supply-side CPU  501  (S 201 ). The rotation status information request and rotation speed response between CPU  401  and power-supply-side CPU  501  are implemented by serial communication. 
     If CPU  401  detects from the rotation speed data that fixing belt  230  has stopped (S 202 : YES), CPU  401  outputs ON/OFF signal I and power signal  1  for heat output to power-supply-side CPU  501  (S 203 ). Even though power-supply-side CPU  501  receives ON/OFF signal  1  and power signal  1 , since the rotation speed of fixing roller  210  is at or below a predetermined value, power-supply-side CPU  501  does not send ON/OFF signal  2  or power signal  2  to inverter circuit  305 . That is to say, inverter circuit  305  is controlled so as not to oscillate. 
     CPU  401  then acquires a detection signal (current value) from detection section  308  of IH power supply  500  (S 204 ). Current value acquisition is performed by means of serial communication via power-supply-side CPU  501 . CPU  401  compares the current value supplied to rectifier circuit  304  with a stipulated value (S 205 ). As fixing belt  230  is currently stopped, if power-supply-side CPU  501  is operating normally, inverter circuit  305  should be being controlled so as not to oscillate, and therefore the current value supplied to rectifier circuit  304  should be less than or equal to the stipulated value. Therefore, if the current value is less than or equal to the stipulated value (S 205 : YES), CPU  401  determines that power-supply-side CPU  501  is operating normally, and executes heating stop processing (S 206 ). To be specific, CPU  401  stops ON/OFF signal  1  and power signal  1  being output to power-supply-side CPU  501 , and returns to the normal state. 
     However, if the current value exceeds the stipulated value (S 205 : NO), it can be determined that power-supply-side CPU  501  is not operating normally. In this case, CPU  401  executes heating stop processing (S 207 ), and then executes error notification processing (S 208 ). For example, a message indicating that a failure has occurred may be displayed on an operation panel (not shown) Then CPU  401  performs control so that no subsequent printing (heating) is performed (S 209 ). 
     If CPU  401  determines in the processing in step S 202  that stoppage of fixing belt  230  is not indicated (S 202 : NO), this means that power-supply-side CPU  501  has detected rotation even though rotation of pressure roller  240  and so forth has stopped, indicating that a failure has occurred in power-supply-side CPU  501  or rotation signal generation section  301 . In this case, also, CPU  401  gives an error notification (S 208 ) and performs control so that no subsequent printing (heating) is performed (S 209 ). 
     By having CPU  401  perform diagnosis of power-supply-side CPU  501  in this way, a failure of power-supply-side CPU  501  can be detected in advance, and IH power supply  500  can be operated with the certainty that power-supply-side CPU  501  is normal, enabling the reliability of fixing apparatus  200  to be further increased. 
     Embodiment 3 
     Next, a fixing apparatus according to Embodiment 3 will be described. In Embodiment 3, a power suppression circuit is incorporated in the IH power supply, and a function is implemented by the CPU of the main apparatus that performs self-diagnosis to confirm that power input to a fixing apparatus that has a temperature maintaining mode in which fixing belt stoppage or rotation at or below a threshold value is set is suppressed to stipulated power or below. An image forming apparatus to which this fixing apparatus is applied may be the above-described apparatus shown in  FIG. 1  and  FIG. 2 , or may be of another type. 
       FIG. 9  is a functional block diagram showing parts related to fixing apparatus  200 , comprising an IH power supply  600  that controls the operation of induction heating apparatus  250  and a main apparatus  400  of an image forming apparatus. Parts having the same function as parts in above-described Embodiment 1 are assigned the same codes as in Embodiment 1. 
     IH power supply  600  basically has the same configuration as above-described IH power supply  300 , except that oscillation stop circuit  307  is replaced by a power suppression circuit  601 , but the control method is somewhat different, with CPU  401 , when fixing apparatus  200  is in temperature maintaining mode, sending a power signal that inputs predetermined power to power suppression circuit  601  when fixing belt  230  stops or is rotating at or below a threshold value each time power is turned on or at regular intervals during standby, and performing self-diagnosis to confirm that the input power has been suppressed to the stipulated power or below. Also, during fixing belt  230  rotation when fixing apparatus  200  is not in the temperature maintaining mode, power suppression circuit  601  outputs an operating voltage of a level based on a power signal from CPU  401  to PWM circuit  332  in inverter circuit  305 . Furthermore, when rotation of fixing belt  230  stops or is at or below a threshold value while fixing apparatus  200  is in the temperature maintaining mode, if the power signal from CPU  401  is at or below a stipulated level, power suppression circuit  601  outputs an operating voltage of a level based on that power signal to PWM circuit  332  in inverter circuit  305 , and if the power signal is above the stipulated level, power suppression circuit  601  outputs an operating voltage of the stipulated level to PWM circuit  332  in inverter circuit  305 . Thus, when fixing roller  210  (fixing belt  230 ) stops rotating, or its rotation speed is at or below a threshold value, while fixing apparatus  200  is in temperature maintaining mode, power suppression circuit  601  acts to stop oscillation of inverter circuit  305  independently of a directive from CPU  401 . 
     Since, when fixing apparatus  200  is in temperature maintaining mode, oscillation is controlled by having a power signal from CPU  401  suppressed by power suppression circuit  601  installed on the IH power supply  600  side and supplied to inverter circuit  305  in this way, when the rotation speed of fixing roller  210  (fixing belt  230 ) detected from an output signal from rotation signal generation section  301  is zero or is at or below a predetermined value, oscillation of inverter circuit  305  is suppressed by suppressing the operating voltage output to a stipulated level even though CPU  401  is outputting a power signal of the stipulated level or above, enabling oscillation of inverter circuit  305  to be suppressed dependably, and heat output to be suppressed dependably, even if CPU  401  fails. 
     The kind of function described above is only effectuated when the detection system that detects the rotation speed and power suppression circuit  601  are operating normally. It is therefore desirable for self-diagnosis of these functions to be carried out before operation of induction heating apparatus  250  and so forth is performed. In this embodiment, the configuration provides for CPU  401  to perform self-diagnosis each time power is turned on, and/or at regular intervals during standby, when fixing apparatus  200  is in temperature maintaining mode. 
       FIG. 10  is a flowchart for self-diagnosis performed by CPU  401 . This self-diagnosis is performed when power is turned on, and/or at regular intervals during standby, when fixing apparatus  200  is in temperature maintaining mode. When self-diagnosis is started, CPU  401  issues a stop command to drive section  402 , and stops driving of pressure roller  240  (S 300 ). After driving of pressure roller  240  is stopped and rotation of fixing roller  210  is stopped, a rotation detection signal output by rotation detection circuit  306  is captured, and it is determined whether or not fixing belt  230  (fixing roller  210 ) has stopped (S 301 ). If the rotation detection signal is low-level, this indicates that the rotation speed of fixing roller  210  is at or below a predetermined value, but is here treated as indicating that rotation of fixing belt  230  has stopped. If rotation of fixing belt  230  is determined to have stopped (S 301 : YES), CPU  401  gives a directive for heat output to inverter circuit  305  by sending an ON/OFF signal to PWM circuit  332 , and sending a power signal that inputs predetermined power to power suppression circuit  601  (S 302 ). That is to say, a heating directive is given when a condition for not heating fixing belt  230  has been met. 
     CPU  401  captures a detection signal from detection section  308  (S 303 ), and determines whether or not a power value obtained by multiplying together the current value and voltage value indicated by the detection signal (current value×voltage value) is less than or equal to the stipulated value (S 304 ). If the power value is less than or equal to the stipulated value (S 304 : YES), this means that rotation detection circuit  306  and power suppression circuit  601  are operating normally. Therefore, in this case, CPU  401  determines that the results of the self-diagnosis are normal, and stops transmission of the ON/OFF signal and power signal being output to PWM circuit  332  and power suppression circuit  601  (S 305 ). 
     On the other hand, if the power value is greater than the stipulated value (S 304 : NO), this means that power suppression circuit  601  is not operating normally and oscillation of inverter circuit  305  has not been suppressed. In this case, CPU  401  immediately stops heating by stopping transmission of the ON/OFF signal being output to PWM circuit  332  and the power signal being output to power suppression circuit  601  (S 306 ), and executes error notification processing (S 307 ). For example, a message indicating that a failure has occurred may be displayed on an operation panel (not shown). Then CPU  401  performs control so that no subsequent printing (heating) is performed (S 308 ). Alternatively, a warning voice message may be issued. 
     If CPU  401  determines in the processing in step S 301  that the rotation detection signal does not indicate that fixing belt  230  has stopped (S 301 : NO), this means that rotation detection circuit  306  has detected rotation even though rotation of pressure roller  240  and so forth has stopped, indicating that a failure has occurred in rotation detection circuit  306  or rotation signal generation section  301 . In this case, also, CPU  401  gives an error notification (S 307 ) and performs control so that no subsequent printing (heating) is performed (S 308 ). 
     By having CPU  401  perform diagnosis of power suppression circuit  601  in this way when fixing apparatus  200  is in temperature maintaining mode, a failure of power suppression circuit  601  can be detected in advance, and IH power supply  600  can be operated with the certainty that power suppression circuit  601  is normal, enabling the reliability of fixing apparatus  200  to be further increased. 
     In this embodiment, a case has been described in which the power suppression circuit is provided on the IH power supply side, but the power suppression circuit may also be provided on the main apparatus side. Also, a power-supply-side processor may be provided on the IH power supply side, separately from the main apparatus, and made to perform the same operations as the power suppression circuit. 
     A first aspect of a fixing apparatus of the present invention has a configuration that includes: a rotating heating element that heat-fixes an unfixed image on a recording medium; a heating section that heats the rotating heating element; a power supply that supplies power to the heating section; and a self-diagnosis section that issues a directive for heating when a condition for not heating the rotating heating element has been met, and confirms that the rotating heating element is not heated. 
     According to this configuration, when a condition for not heating the rotating heating element has been met, a directive for heating is issued, and it is confirmed that the rotating heating element is not heated, enabling the safety of the apparatus to be confirmed before the apparatus is operated. 
     A second aspect of a fixing apparatus of the present invention has a configuration wherein, in the fixing apparatus described in the first aspect above, the power supply has: an inverter circuit that supplies a high-frequency alternating current to the heating section; and an oscillation stop circuit that stops oscillation of the inverter circuit when the rotating heating element stops or has a rotation speed less than or equal to a threshold value. 
     According to this configuration, the oscillation stop circuit can be installed in the power supply, not the main apparatus, making possible a design in which independence from the CPU of the main apparatus is increased compared with a case in which an oscillation stop function is provided on the main apparatus side. 
     A third aspect of a fixing apparatus of the present invention has a configuration wherein, in the fixing apparatus described in the second aspect above, there are provided: a signal generation section that outputs a phase signal corresponding to the rotation speed of the rotating heating element; and a rotation detection section that is provided independently of the processor, and detects from the phase signal that the rotating heating element has stopped rotating or has a rotation speed less than or equal to a threshold value. 
     According to this configuration, since the rotation detection section is provided independently of the processor, it is possible to determine whether or not a condition for stopping heating has been met without being affected by the reliability of the processor, enabling reliability to be improved. 
     A fourth aspect of a fixing apparatus of the present invention has a configuration wherein, in the fixing apparatus described in the first aspect above, the power supply has: an inverter circuit that supplies a high-frequency alternating current to the heating section; and a power-supply-side processor that controls oscillation of the inverter circuit in accordance with a control signal supplied from a processor, and when the rotating heating element stops or has a rotation speed less than or equal to a threshold value, stops oscillation of the inverter circuit without regard to the control signal. 
     According to this configuration, since a power-supply-side processor equivalent to an oscillation stop circuit is installed in the power supply, not the main apparatus, a design is possible in which independence from the CPU of the main apparatus is increased compared with a case in which an oscillation stop function is provided on the main apparatus side. 
     A fifth aspect of a fixing apparatus of the present invention has a configuration wherein, in the fixing apparatus described in the fourth aspect above, a signal generation section is provided that outputs a phase signal corresponding to the rotation speed of the rotating heating element; and the power-supply-side processor detects from the phase signal that the rotating heating element has stopped rotating or has a rotation speed less than or equal to a threshold value. 
     According to this configuration, since a rotation detection function is provided independently of the processor, it is possible to determine whether or not a condition for stopping heating has been met without being affected by the reliability of the processor, enabling reliability to be improved. 
     A sixth aspect of a fixing apparatus of the present invention has a configuration wherein, in the fixing apparatus described in the first aspect above, the self-diagnosis section executes self-diagnosis each time power is turned on and/or the system is restored from the sleep state, and/or at regular intervals during standby. 
     According to this configuration, self-diagnosis can be performed when the load on the CPU on the main apparatus side is light, enabling self-diagnosis to be performed without imposing a heavy load on the CPU. 
     A seventh aspect of a fixing apparatus of the present invention has a configuration wherein, in the fixing apparatus described in the first aspect above, the power supply has: an inverter circuit that supplies a high-frequency alternating current to the heating section; and a power suppression circuit that controls oscillation of the inverter circuit in accordance with a power control signal supplied from a processor, and when the rotating heating element stops or has a rotation speed less than or equal to a threshold value, suppresses oscillation of the inverter circuit without regard to the power control signal. 
     According to this configuration, since the power suppression circuit is provided independently of the processor, it is possible to determine whether or not a condition for suppressing heating has been met without being affected by the reliability of the processor, enabling reliability to be improved. 
     An eighth aspect of the present invention is an image forming apparatus that includes: an image forming section that forms an unfixed image on a recording medium; and a fixing apparatus that heat-fixes by means of a rotating heating element an unfixed image formed on the recording medium by the image forming section; wherein the fixing apparatus described in the first aspect above is used as the fixing apparatus. 
     The present application is based on Japanese Patent Application No. 2004-059754 filed on Mar. 3, 2004, entire content of which is expressly incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     The present invention performs self-diagnosis to confirm the normal operation of a mechanism that suppresses heating in the event of a condition for stopping heating in a fixing apparatus that can be applied to an image forming apparatus such as an electrophotographic copier, printer, or facsimile apparatus, and makes it possible to prevent an excessive rise in temperature of a rotating heating member dependably without the intermediation of a control circuit.