Abstract:
An image heating apparatus includes a heating member for heating an image on a recording material, a first temperature detecting element for detecting a first temperature of the heating member, a second temperature detecting element for detecting the second temperature of the heating member, wherein a thermal property of the second temperature detecting element differs from a thermal property of the first temperature detecting element, and a power supply controller for controlling the power supply to the heating member by utilizing the first temperature detecting element and the second temperature detecting element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image heating apparatus used in an image forming apparatus such as a copying machine, a printer and the like. 
     2. Related Background Art 
     In the past, a thermal fixing means including a heat roller was mainly used to fix a toner image developed in an image forming portion and transferred to a recording sheet to the recording sheet as a permanent image. In general, a fixing roller including a heater therein is urged against a pressure roller to form a fixing nip therebetween. A toner image is thermally fixed to the recording sheet by heat from the heater while the recording sheet is being passed through a nip between the fixing roller and the pressure roller. 
     Control of temperature of the heat roller is effected by a temperature sensor such as a thermistor contacted with the rotating heat roller. In the present day, the temperature control of the heat roller has been effected by using a CPU (central processing unit) in control of almost of all copying machines. 
     In the temperature control effected by the CPU, a heat amount given to the heat roller by the CPU is controlled by converting voltage (corresponding to a temperature of the heat roller) from the temperature sensor into a digital amount by means of an A/D (analogue/digital) converter. 
     [Explanation of Construction of Conventional Fixing Device] 
     FIG. 4 shows a conventional fixing device. 
     Here, explanation is made regarding an example of a fixing device in which an upper heat fixing roller and a lower pressure roller are urged against each other to form a fixing nip where a toner image (developed at an image forming portion and transferred to a recording sheet) is fixed to the recording sheet as a permanent image. 
     In FIG. 4, a fixing roller  40   a  contacted with the toner image has an outer diameter of 60 mm and is constituted by an aluminium core cylinder  418 , an HVT (high temperature vulcanizing type) silicone rubber layer  417  having a thickness of 1 mm and coated on the core cylinder, and a special adding type silicone rubber layer  416  coated on the rubber layer  417 . 
     On the other hand, the pressure roller  40   b  has an outer diameter of 60 mm and is constituted by an aluminium core cylinder  412 , an HVT silicone rubber layer having a thickness of 1 mm and coated on the core cylinder, and a special adding type silicone rubber layer  415  having a thickness of 1 mm and coated on the silicone rubber layer. 
     In the fixing roller  40   a,  a convey roller heater (heat generating means)  409  is disposed within the core cylinder  418 , and, in the pressure roller  40   b,  a heater  413  is disposed within the core cylinder  412 , so that the recording sheet is heated from both sides. A temperature of the fixing roller  40   a  is detected by a thermistor  410  contacted with the fixing roller  40   a  and a temperature of the pressure roller  40   b  is detected by a thermistor  411  contacted with the pressure roller  40   b.  The halogen heaters  409 ,  413  are controlled by a control device  414  on the basis of detected temperatures so that the temperature of the fixing roller  40   a  is maintained to 170° C. (constant) and the temperature of the pressure roller  40   b  is maintained to 165° C. (constant). The fixing roller  40   a  and the pressure roller  40   b  are urged against each other with total pressure of 80 kg by means of a pressurizing mechanism (not shown). In FIG. 4, symbol  0  denotes an oil applying device (mold releasing agent applying device); C denotes a cleaning device; and C 1  denotes a cleaning blade for removing oil and contamination from the pressure roller  40   b.  In the oil applying device  0 , dimethyl silicone oil  408  in an oil pan  407  is picked up by oil pick-up rollers  406  and  405  and is applied to the fixing roller  40   a  by an oil applying roller  404  while regulating an oil applying amount by means of an oil applying amount regulating blade  403 . In the cleaning device C, the surface of the fixing roller  40   a  is cleaned by a web  402  contacted with the fixing roller  40   a  by an abut roller  401 . In the above-mentioned fixing device, the recording sheet on which a non-fixed toner image was borne is conveyed to the fixing nip (between the fixing roller  40   a  and the pressure roller  40   b ), where the recording sheet is heated and pressurized from both sides, thereby fixing the toner image onto the recording sheet. In this case, toners adhered to the fixing roller  40   a  and the pressure roller  40   b  are removed by the cleaning device C and the cleaning blade C 1 , respectively. 
     However, when the temperature of the heat roller detected by a single thermistor with high accuracy is unreliable, due to temperature errors in a series of conversion circuits from the thermistor to the CPU and temperature properties of the thermistor, if a low temperature reading accuracy of a thermistor selected for high temperature detecting is worsened, the low temperature cannot be measured accurately by the thermistor selected for the high temperature. Thus, when the surface temperature of the roller is heated to a desired temperature, due to poor abutment of a thermistor for low temperature and/or breakage of a signal line of the thermistor, it is difficult to quickly detect an abnormality of increase in temperature of the heat roller. 
     [Explanation of Causes for Worsening Low Temperature Reading Accuracy of Thermistor for High Temperature] 
     FIG. 1 shows a typical temperature detection circuit using a thermistor. In FIG. 1, the reference numeral  107  denotes a thermistor;  103 ,  104  and  106  denote fixed resistances;  105  denotes a circuit resistance component such as current limitter resistance;  102  denotes an OP amplifier; and  101  denotes detected voltage corresponding to detection temperature of the thermistor  107 . 
     When reference resistance value is R 0  and temperature is T 0 , a resistance value R th  of the thermistor is represented by the following equation (1): 
     
       
           R   th   =R   0 ×exp  B {(1/ T )−(1/ T   0 )}  (1) 
       
     
     where, B is a thermistor B constant (K) and T is an absolute temperature. 
     Further, when the above equation (1) is converted with respect to T, the following equation can be obtained: 
     
       
           T= ( T   0   ×B )/( T   0 ×{Ln  R− Ln  R   0   }+B )  (2) 
       
     
     The above equation (2) can be rewritten as follows: 
     
       
         1/ T= (1/ T   0 )+{(Ln  R   th /( B ×Ln  R   0 )}  (3) 
       
     
     When T 0 , R 0  and B are regarded as constants, and α=1/T 0  and β=B×Ln R 0 , the following equations are obtained: 
     
       
         1/ T=α+ (Ln  R   th /β)  (4) 
       
     
       T= 1/{(α×Ln  R   th )+β}  (5) 
     Thus, a reciprocal of natural logarithm of the resistance error of the thermistor in the circuit appears as an error of the reading temperature. Since the resistance value of the thermistor for high temperature is increased in an exponential function manner as the temperature is decreased, the error factor of the resistance value regarding the temperature is also increased. This is one error factor of the reading temperature. 
     Regarding the detected voltage  101 , voltage obtained by dividing power source voltage (Vcc) by a resistance value (R 1 ) of the division resistance  106  and the resistance value (R th ) of the thermistor  107  is outputted, and is represented by the following relation: 
     
       
         Detected voltage  101  (out)={ R   th /( R   1   +R   th )}× Vcc   (6) 
       
     
     FIG. 2A is a graph showing a relation between the measured temperature of the thermistor and the detected voltage, which represents a relation between the detected voltage  101  in the circuit shown in FIG.  1  and the temperature detected by the thermistor. In FIG. 2A, the abscissa indicates the detection temperature value of the thermistor and the ordinate indicates the detected voltage value. 
     The fixing device in electrophotography is normally used under a temperature of about 150° C. In order to improve the accuracy of the detected voltage  101  regarding the reading temperature in an area c in FIG. 2A (temperature of about 150° C.), the reference resistance value (R 0 ) and temperature (T 0 ) of the thermistor and the division resistance value R 1  for generating the detected voltage  101  are determined. 
     Consequently, as seen by referring to curve  211 , a change of the detected voltage as a function of a change of the temperature of the heat roller is greater outside the range d. However, in the case where the temperature of the heat roller is within the range d, the change of the detected voltage to the change of the temperature is small, thereby the temperature cannot be detected precisely. 
     Thus, conventionally, it was difficult to quickly find the abnormality of the fixing device, particularly abnormality regarding poor attachment of the thermistor and/or breakage of the signal line of the thermistor. 
     For example, it is assumed that the thermistor is not properly contacted with the fixing roller and is spaced apart from the fixing roller. If the temperature detecting accuracy of the thermistor is high under not only a high temperature but also under a low temperature, by measuring the degree of the temperature increase during the energization time period, it can be judged in a short time from the start of energization whether any abnormality exists (i.e., whether or not the thermistor is contacted with the roller). However, if the temperature detecting accuracy of the thermistor is inadequate for a low temperature, even if the temperature increases during the energization time period, reliability of the measured value is poor, and, thus, it is difficult to judge whether an abnormality exists. 
     In this way, if the abnormal condition such as the poor attachment (in which the thermistor is not contacted with the fixing roller) and/or breakage of the signal line of the thermistor has occurred, it is difficult to find the abnormal condition until the temperature of the fixing roller is increased up to the temperature area where the temperature detecting accuracy of the thermistor becomes high. 
     SUMMARY OF THE INVENTION 
     The present invention aims to eliminate the above-mentioned conventional drawbacks, and an object of the present invention is to provide an image heating apparatus which can quickly detect an abnormality such as poor attachment of a thermistor and breakage of a signal line of the thermistor. 
     To achieve the above object, according to the present invention, there is provided an image heating apparatus comprising a heating member for heating an image on a recording material, a first temperature detecting element for detecting a first temperature of the heating member, a second temperature detecting element for detecting a second temperature of the heating member, thermal property of the second temperature detecting element differing from thermal property of the first temperature detecting element, and an energization control means for controlling energization of the heating member by utilizing the first temperature detecting element and the second temperature detecting element. 
     The other objects of the present invention will be apparent from the following detailed explanation of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing a conventional temperature detecting circuit using a thermistor; 
     FIG. 2A is a graph showing a relation between a temperature measured by the conventional thermistor and detected voltage, and FIG. 2B is a graph showing a relation between a measured temperature and detected voltage by using a combination of a thermistor for low temperature and a thermistor for high temperature of a heat fixing apparatus according to a first embodiment of the present invention; 
     FIG. 3A is a Table showing a relation between temperatures of the thermistors of the heat fixing apparatus according to the first embodiment and division voltage values, and FIG. 3B is a graph showing a relation between temperatures measured by the thermistors for high and low temperatures and detected voltage in the same apparatus as FIG. 3A; 
     FIG. 4 is a schematic view of a conventional fixing device; 
     FIG. 5 is a circuit block diagram for measurement control in the heat fixing apparatus according to the first embodiment; 
     FIG. 6 is a schematic view of a fixing device of the heat fixing apparatus according to the first embodiment; 
     FIG. 7 is a schematic view of a fixing device of a heat fixing apparatus according to a third embodiment of the present invention; 
     FIG. 8 is a circuit block diagram for measurement control in the heat fixing apparatus according to the third embodiment; 
     FIG. 9 which is comprised of FIGS. 9A and 9B is a flow chart for explaining an operation of a CPU of the heat fixing apparatus according to the third embodiment; 
     FIG. 10 which is comprised of FIGS. 10A and 10B is a flow chart for explaining an operation of a CPU of the heat fixing apparatus according to the first embodiment; 
     FIG. 11 is a circuit diagram for switching division resistance in a heat fixing apparatus according to a second embodiment of the present invention; 
     FIG. 12A is a Table showing a relation between measured temperatures and division voltage values obtained by switching the division resistance in the heat fixing apparatus according to the second embodiment, and FIG. 12B is a graph showing property of the detected voltage in FIG. 12A; and 
     FIG. 13 is a schematic view of a color copying machine to which the heat fixing apparatus according to the present invention can be applied. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be explained in connection with embodiments thereof with reference to the accompanying drawings. However, dimensions, materials, configurations and relative positional relations of constructural elements shown in the embodiments do not limit the present invention so long as special limitation is not given. 
     FIG. 13 is a schematic sectional view of a color image forming apparatus to which a heat fixing apparatus according to the present invention can be applied. 
     The image forming apparatus includes an upper digital color image reader portion and a lower digital color image printer portion. 
     In the reader portion, after an original  30  is rested on an original glass support  31 , by exposure-scanning the original by an exposure lamp  32 , an image reflected from the original is focused on a full-color sensor  34  through a lens  33 , thereby obtaining color decomposed image signals. The color decomposed image signals are sent, through an amplifier circuit (not shown), to a video process unit (not shown), where the signals are treated. Then, the signals are sent to the printer portion. 
     In the printer portion, a photosensitive drum (image bearing member)  1  is supported for rotation in a direction shown by the arrow. Around the photosensitive drum  1 , there are disposed a pre-exposure lamp  11 , a corona charger  2 , a laser exposure optical system  3 , a potential sensor  12 , four developing devices  4   y,    4   c,    4   m,    4   Bk  including different color toners, a drum light amount detecting means  13 , a transfer device  5  and a cleaning device  6 . 
     In the laser exposure optical system  3 , the image signal from the reader portion is converted into a light signal in a laser output portion (not shown), and the converted laser beam is reflected by a polygon mirror  3   a  and is projected onto a surface of the photosensitive drum  1  through a lens  3   b  and a mirror  3   c.    
     During image formation in the printer portion, the photosensitive drum  1  is rotated in the direction shown by the arrow, and, electricity is removed from the photosensitive drum by the pre-exposure lamp  11 , the photosensitive drum  1  is uniformly charged by the charger  2 . Then, for each decomposed color, a light image E is illuminated on the photosensitive drum, thereby forming a latent image. 
     Then, a selected developing device is operated to develop the latent image on the photosensitive drum  1 , thereby forming a toner image based on resin on the photosensitive drum  1 . The developing devices can selectively be approached to the photosensitive drum  1  for each decomposed color by driving eccentric cam  24   y,    24   c,    24   m  or  24   Bk.    
     The toner image on the photosensitive drum  1  is transferred, by the transfer device  5 , onto a recording material supplied to a transfer position (where the transfer device is opposed to the photosensitive drum  1 ) from a recording material cassette  7  through a convey system. In the illustrated embodiment, the transfer device  5  includes a transfer drum  5   a,  a transfer charger  5   b,  an absorb charger  5   c  and an opposed absorb roller  5   g  for electrostatically absorbing the recording material, an inner charger  5   d  and an outer charger  5   e,  and a dielectric recording material bearing sheet  5   f  is mounted in a cylindrical form to a peripheral opening zone of the transfer drum  5   a  supported for rotation. A dielectric sheet such as polycarbonate film is used as the recording material bearing sheet  5   f.    
     As the drum-shaped transfer material (i.e., transfer drum  5   a ) is rotated, the toner image on the photosensitive drum  1  is transferred, by the transfer charger  5   b,  onto the recording material born on the recording material bearing sheet  5   f.    
     A desired number of color toner images are transferred to the recording material born on the recording material bearing sheet  5   f  in this way to form a full-color image. 
     In the full-color image formation, after four color toner images were transferred, the recording material is separated from the transfer drum  5   a  by a separation pawl  8   a,  a separation push-up roller  8   b  and a separation charger  5   h.  Then, the recording sheet is passed through a heat roller fixing device  9  and then is discharged onto a tray  10 . 
     After the transferring, residual toner remaining on the photosensitive drum  1  is removed by the cleaning device  6  for the preparing for next image formation. 
     When images are formed on both surfaces of the recording material, after the recording material is discharged from the fixing device  9 , a convey path switching guide  19  is driven immediately, so that the recording material is temporarily introduced into a reverse path  21   a  through a convey vertical path  20 . Then, by rotating a reverse roller  21   b  reversely, the recording material is returned to a return direction opposite to an introduction direction with a trail end of the recording material facing the return direction, thereby resting on an intermediate tray  22 . Thereafter, the image is formed on the other surface of the recording material in the same image forming process as mentioned above. 
     In order to remove powder adhered to the recording material bearing sheet  5   f  of the transfer drum  5   a  and to prevent oil from adhering to the recording material, the recording material bearing sheet  5   f  is cleaned by a fur brush  14  and a back-up brush  15  opposed to the fur brush  14  with the interposition of the recording material bearing sheet  5   f,  and, an oil removing roller  16  and back-up brush  17  opposed to the roller  16  with the interposition of the recording material bearing sheet  5   f.  Such cleaning is effected before or after the image formation and is always effected whenever a sheet jam occurs. 
     In the illustrated embodiment, by operating a cam follower  5   i  integral with the transfer drum  5   a  by driving an eccentric cam  25  at a desired timing, a gap between the recording material bearing sheet  5   f  and the photosensitive drum, can be selected appropriately. For example, in a stand-by condition or in a power OFF condition, the transfer drum is separated from the photosensitive drum. 
     (First Embodiment) 
     In a heat fixing apparatus according to a first embodiment of the present invention, as a temperature detecting means for achieving high accurate temperature reading until a surface temperature of a fixing device is changed from low temperature to high temperature, a combination of a thermistor for low temperature and a thermistor for high temperature is used. 
     FIG. 2B shows a relation between a measured temperature and detected voltage obtained by using the combination of the thermistor for low temperature and the thermistor for high temperature. In FIG. 2B, the abscissa indicates a temperature value detected by the thermistors and the ordinate indicates a detected voltage value. A curve  221  represents-detected voltage property of the thermistor for high temperature and a curve  222  represents detected voltage property of the thermistor for low temperature. 
     By reading the surface temperature of the fixing device by combining a range dl where the detected voltage property of the thermistor for low temperature is linear and a range d 2  where the detected voltage property of the thermistor for high temperature is linear, the thermistors can be used within the range having small temperature error, thereby increasing the resolving power of the detected voltage at the low temperature area. 
     By using the combination of the thermistor for low temperature and the thermistor for high temperature in this way, it is possible to provide a detecting apparatus which can achieve high accurate temperature reading within a wide temperature range (a to b). 
     Tests were carried out by using a thermistor having a measurement range of −25° C. to 70° C. as the thermistor for low temperature and a thermistor having a measurement range of 70° C. to 250° C. as the thermistor for high temperature. It was found that, in FIG. 2B, “a” becomes −50° C. and “b” becomes 150° C., and, thus, high accurate temperature reading can be effected within a wide range. 
     FIG. 3A is a Table showing an example of division voltages values vs thermistor temperatures. The Table indicates a relation between the temperatures measured by the thermistor for low temperature and the thermistor for high temperature, and resistance values of the thermistors and division voltage values when resistance of 10 kΩ is used as the division resistance. 
     FIG. 3B is a graph showing a relation between the temperatures measured by the thermistor for low temperature and the thermistor for high temperature, and the detected voltage. The graph indicates detected voltage property of the division voltage value of the thermistor for high temperature and detected voltage property of the division voltage value of the thermistor for low temperature within a temperature range of −50° C. to 200° C. when the resistance of 10 kΩ is used as the division resistance. 
     It can be seen from FIGS. 3A and 3B that the temperature can be measured with high accuracy within a wide range by using the combination of the thermistor for low temperature and the thermistor for high temperature, with the result that a low surface temperature of the roller (which could not measured correctly up to date) can be measured correctly, and, thus, even when the surface temperature of the roller is low, the abnormality such as poor attachment of the thermistor and/or breakage of a signal line of the thermistor. 
     &lt;Explanation of Fixing Device&gt; 
     FIG. 6 schematically shows a fixing device according to the illustrated embodiment. 
     In the illustrated embodiment, a thermistor  505  for high temperature and a thermistor  506  for low temperature are contacted with a fixing roller  511 , and a thermistor  531  for high temperature and a thermistor  532  for low temperature are contacted with a pressure roller  540 . The thermistor  506  is disposed in the vicinity of the thermistor  505  and the thermistor  532  is disposed in the vicinity of the thermistor  531 . 
     &lt;Explanation of Control Circuit&gt; 
     FIG. 5 shows a control circuit according to the illustrated embodiment. 
     In FIG. 5, the reference numerals  501 ,  503 ,  507 ,  508 ,  509  and  510  denote fixed resistances;  505  and  506  denote thermistors;  525  and  526  denote capacitors;  513  and  514  denote OP amplifiers;  520  denotes a multiplexer;  511  and  540  denote heat rollers;  515  denotes an A/D converter;  516  denotes a decoder;  517  denotes a CPU;  518  denotes an I/O port; and  519  denotes a heater drive circuit. 
     A lower heater temperature detecting circuit  530  has the same circuit arrangement as an upper heater temperature detecting circuit  521 . 
     [Explanation of Upper Heater Control Circuit] 
     The surface temperature of the heat roller is read by the thermistor  505  contacted with the roller. The thermistor  505  has a variable resistance value varied with the temperature of the roller. The resistance value of the thermistor is great at a low temperature and is small at a high temperature. The power voltage (5V in the illustrated embodiment) is divided by the resistance value (corresponding to the temperature) of the thermistor  505  and the resistance value of the fixed resistance  501 , thereby outputting division voltage corresponding to the surface temperature of the roller. The fixed resistance  503  and the capacitor  525  constitute a filter for removing a noise component included in the division voltage. The OP amplifier  513  acts as a buffer for effecting impedance conversion. An output of the OP amplifier  513  serves to supply division voltage corresponding to detected temperature of the thermistor for high temperature for the upper roller to an A channel of the multiplexer  520 . The circuit for the thermistor for low temperature has the similar arrangement and serves supply division voltage corresponding to detected temperature of the thermistor for low temperature for the upper roller to a B channel of the multiplexer  520 . The voltage values inputted to the A and B channels of the multiplexer  520  are outputted from the multiplexer in a time-sharing manner and are inputted to a channel  1  of the A/D converter  515 . In the A/D converter  515 , the inputted analog voltage is converted into a digital value. The detected temperature of the thermistor for high temperature for the upper roller and the detected temperature of the thermistor for low temperature for the upper roller which were converted into the digital values are sent to the CPU  517 . The CPU  517  adjusts an amount of electric power to be applied to the upper heater  511  on the basis of the detected temperature of the thermistor for high temperature for the upper roller and the detected temperature of the thermistor for low temperature for the upper roller. 
     FIGS. 10A and 10B are flowchart showing an operation of the CPU  517 . 
     When the fixing temperature control is started, initial setting is performed, the variable T which is a flag showing whether an abnormality of the thermistor is checked is changed to “1”, an initial temperature of the roller when the power supply is set ON is measured and the temperature data is reserved. Thereafter, a main routine is carried out. In the main routine, a sub routine for adjusting the temperature of the fixing device is repeated at a predetermined interval. 
     In the subroutine for adjusting the temperature of the fixing device, temperature control of upper and lower heaters is performed. Here, although the flowchart regarding the temperature control of the upper heater will be explained, regarding the lower heater, the same treatment is carried out. 
     When the subroutine for adjusting the temperature of the fixing device is carried out, first of all, P 1 , P 2  of the I/O port are set to “L” to select input A of the multiplex of the temperature adjusting circuits for the upper and lower heaters (step  1201 ). By selecting the input A of the multiplex of the temperature adjusting circuits for the upper and lower heaters, temperature voltages of the thermistors for high temperature for the upper and lower heaters are sent to the A/D converter. In the A/D converter, the temperature voltages (analog values) are converted into digital values. The temperatures (converted into the digital values) of the thermistors for high temperature for the upper and lower heaters is sent to the CPU (step  1202 ). 
     Then, P 1 , P 2  of the I/O port are set to “H” to select input B of the multiplex of the temperature adjusting circuits for the upper and lower heaters (step  1203 ). By selecting the input B of the multiplex of the temperature adjusting circuits for the upper and lower heaters, temperature voltages of the thermistors for low temperature for the upper and lower heaters are sent to the A/D converter. In the A/D converter, the temperature voltages (analog values) are converted into digital values. The temperatures (converted into the digital values) of the thermistors for low temperature for the upper and lower heaters is sent to the CPU (step  1204 ). 
     On the basis of the temperatures of the thermistors for high and low temperatures for the upper and lower heaters sent to the CPU, it is judged whether the present surface temperature of the upper roller is greater or smaller than a predetermined reference temperature (for example, 70° C.) (step  1205 ). When a temperature range in which the linear property of the thermistor for low temperature is obtained is (−25° C. to 70° C.) and a temperature in which the linear property of the thermistor for high temperature is obtained is (70° C. to 250° C.) and the present temperature of the roller is equal to or smaller than 70° C., the temperature read by the thermistor for low temperature is used (step  1206 ), and, when the present temperature of the roller is equal to or larger than 70° C., the temperature read by the thermistor for high temperature is used (step  1207 ), with the result that the roller temperature reading can be effected with high accuracy within a wide temperature range of −25° C. to 250° C. 
     Then, it is judged whether or not the check for finding the abnormality such as poor attachment of the thermistors and/or breakage of a signal line of the thermistor is performed till now after the power supply is set ON (step  1208 ). 
     If the check has been performed (T=0), “heater electric power control” is effected and then the program is returned to the main routine. 
     If the check has not yet been performed (T=1), it is judged whether or not a predetermined time period is elapsed from an initial condition when the power supply is set ON (step  1209 ). 
     If the predetermined time period is not elapsed, the “heater electric power control” is effected and then the program is returned to the main routine. 
     On the other hand, if the predetermined time period is elapsed, the reserved initial surface temperature of the roller is compared with the present surface temperature of the roller and it is judged whether a difference in surface temperature is equal to or larger than a predetermined value or not (step  1210 ). 
     If the present temperature is increased beyond the initial temperature by the predetermined value, the “heater electric power control” is effected and then the program is returned to the main routine. 
     If the present temperature has not increased by the predetermined value, it is judged that there is an abnormality such as poor attachment of the thermistor and/or breakage of the signal line of the thermistor and an upper roller ERR signal is generated (step  1213 ), and the electric power applied to the upper and lower heaters is stopped (step  1214 ). 
     In this way, since two thermistors having different temperature properties are used, the accurate temperature detection can be achieved within a wide temperature range. Thus, even when the temperature of the fixing roller is low, the abnormality of the apparatus can be detected correctly. 
     &lt;Heater Electric Power Control&gt; 
     The electric power amount applied to the heater is determined so that the detected temperature of the thermistor maintains a target temperature in accordance with the temperature of the upper roller sent to the CPU (step  1211 ). ON/OFF control of the heater is performed on the basis of the determined electric power amount (step  1212 ). 
     (Second Embodiment) 
     FIG. 11 shows a circuit including a thermistor of a heat fixing apparatus according to a second embodiment of the present invention. 
     In FIG. 11, the reference numerals  1302 ,  1304 ,  1306  and  1309  denote fixed resistances;  1303  denotes an OP amplifier;  1310  denotes a thermistor;  1307  denotes a transistor;  1305  denotes a circuit resistance component;  1301  denotes detected voltage corresponding to a temperature measured by the thermistor; and  1308  denotes a resolving power variable signal. 
     Regarding the detected voltage  1301 , voltage obtained by dividing power source voltage (Vcc) by a resistance value R α =(R 1 ) of the division resistance  1306  or resultant resistance of the division resistance  1306  and  1309 , {(R 1 ×R 2 )/(R 1 +R 2 )} and the resistance value (R th ) of the thermistor  1310  is outputted, and is represented by the following equation: 
     
       
         Detected voltage  1301  (out)={ R   th /( R   α   +R   th )}× Vcc   
       
     
     The transistor  1307  is used to switch the resistance value R α  to R 1  or {(R 1 ×R 2 )/(R 1 +R 2 )}. 
     For example, resistance of 100 MΩ is used as R 1  and resistance of 10 KΩ is used as R 2 . In the low temperature area, by setting a control signal G-SW 1308  of the transistor  1307  to “H”, the resultant resistance value R α  of R 1  and R 2  becomes 100 MΩ, and division voltage value regarding the temperature becomes as shown in a Table  1402  in FIG.  12 A and has a property curve  1422  shown in FIG.  12 B. 
     In the high temperature area, by setting the control signal G-SW 1308  of the transistor  1307  to “L”, the resultant resistance value R α  of R 1  and R 2  becomes about 10 KΩ, and division voltage value regarding the temperature becomes as shown in a Table  1401  in FIG.  12 A and has a property curve  1421  shown in FIG.  12 B. 
     In the first embodiment, while the temperature detecting range was widened by using two thermistors for high and low temperatures, in the second embodiment, the temperature detecting range is widened by using two fixed resistances having different resistance values. 
     When a plurality of fixed resistances having different resistance values are provided, it is ideal that thermistors capable of measuring a sufficient wide range of temperatures as the temperature detecting means, and for example, when a thermistor capable of measuring a temperature range of −25° C. to 250° C. is used, high resolving power can be obtained within a temperature range sufficient to achieve the object of the present invention. 
     As mentioned above, by using the combination of higher resolving powers of the detected voltage (regarding the measured temperatures) of two temperature detecting properties, it is possible to provide a temperature detecting apparatus for the fixing apparatus capable of detecting the temperature with high accuracy within a wide temperature range. 
     (Third Embodiment) 
     In a heat fixing apparatus according to a third embodiment of the present invention, in addition to the thermistors contacted with the fixing roller and the pressure roller, there is an arrangement in which an abnormality of the thermistor contacted with the fixing roller can be detected by measuring an environmental temperature of the surface of the fixing roller in a non-contact (with the fixing roller) manner. 
     &lt;Explanation of Fixing Device&gt; 
     FIG. 7 is a constructural view of a fixing apparatus according to a third embodiment of the present invention. 
     Roller environmental temperature measuring plates  812 ,  832  formed from metallic plates having good heat conductivity such as iron are disposed in the vicinity of the fixing rollers in a fixed relation with respect to movable parts, i.e., rollers. Thermistors  806 ,  833  are contacted with the roller environmental temperature measuring plates so that the temperature measurement can be effected. 
     &lt;Explanation of Control Circuit&gt; 
     FIG. 8 shows a control circuit according to the illustrated embodiment. 
     In FIG. 8, the reference numerals  801 ,  803 ,  807 ,  808 ,  809  and  810  denote fixed resistance;  805  and  806  denote thermistors detecting the surface temperature of the fixing roller;  825  and  826  denote capacitors;  813  and  814  denote OP amplifiers;  820  denotes a multiplexer;  811  and  840  denote heat rollers;  831  denotes a thermistor detecting the surface temperature of the pressing roller;  815  denotes an A/D converter;  816  denotes a decoder;  817  denotes a CPU;  818  denotes an I/O port; and  819  denotes a heater drive circuit. 
     A lower heater temperature detecting circuit  830  has the same circuit arrangement as an upper heater temperature detecting circuit  821 . 
     [Explanation of Heater Control Circuit] 
     The surface temperature of the heat roller is read by the thermistor  805  contacted with the roller. The thermistor  805  has a variable resistance value varied with the temperature of the roller. The resistance value of the thermistor is high at a low temperature and is low at a high temperature. The power voltage 5V is divided by the resistance value (corresponding to the temperature) of the thermistor  805  and the resistance value of the fixed resistance  801 , thereby outputting division voltage corresponding to the surface temperature of the roller. The fixed resistance  803  and the capacitor  825  constitute a filter for removing a noise component included in the division voltage. The OP amplifier  813  acts as a buffer for effecting impedance conversion. An output of the OP amplifier  813  serves to supply division voltage corresponding to the surface temperature of the upper roller to an A channel of the multiplexer  820 . The thermistor  806  of non contact type has the similar arrangement and serves supply division voltage corresponding to the temperature of the non contact metallic plate  812  to a B channel of the multiplexer  820 . The upper roller surface temperature voltage and the upper roller environmental temperature voltage inputted to the multiplexer  820  are outputted from the multiplexer in a time-sharing manner and are inputted to a channel  1  of the A/D converter  815 . In the A/D converter  815 , the inputted analog voltage is converted into a digital value. The upper roller surface temperature and the upper roller environmental temperature which were converted into the digital values are sent to the CPU  817 . The CPU  817  adjusts an amount of electric power to be applied to the upper heater  811  on the basis of the upper roller surface temperature and the upper roller environmental temperature. 
     FIGS. 9A and 9B are flowcharts showing an operation of the CPU  817 . 
     When the fixing temperature control is started, P 1 , P 2  of the I/O port are set to “L” to convert the voltage corresponding to the roller surface temperature into a digital value by the A/D converter (step  1101 ). Data regarding the surface temperatures of the upper and lower rollers are read out from the A/D converter (step  1102 ). Then, P 1 , P 2  of the I/O port are set to “H” to convert the voltage corresponding to the roller environmental temperature into a digital value by the A/D converter (step  1103 ). Data regarding the environmental temperatures of the upper and lower rollers are read out from the A/D converter (step  1104 ). 
     Then, it is judged whether the roller environmental temperature data is included within a predetermined range with respect to the upper roller surface temperature data (step  1105 ). This is done because, if the roller surface temperature data is considerably different from the upper and lower roller environmental temperature data, there is a danger of an abnormality recurring such as, poor contact between the rollers and the thermistors/and or breakage of the signal line of the thermistor. 
     If the upper roller environmental temperature data is not included within a predetermined range with respect to the upper roller surface temperature data, an upper roller ERR signal is generated (step  1112 ). When the upper roller ERR signal is generated, the electric power applied to the upper heater is stopped (step  1113 ). 
     If the upper roller environmental temperature data is included within a predetermined range with respect to the upper roller surface temperature data, the electric power amount to be applied to the heater is determined on the basis of the upper roller temperature and the predetermined target temperature (step  1106 ). The heater drive circuit is ON/OFF-controlled via the I/O port in accordance with the determined electric power amount (step  1107 ). 
     Thereafter, it is judged whether the lower roller environmental temperature data is included within a predetermined range with respect to the lower roller surface temperature data (step  1108 ). 
     If the lower roller environmental temperature data is not included within a predetermined range with respect to the lower roller surface temperature data, a lower roller ERR signal is generated (step  1109 ). When the lower roller ERR signal is generated, the electric power applied to the lower heater is stopped (step  1113 ). 
     If the lower roller environmental temperature data is included within a predetermined range with respect to the lower roller surface temperature data, the electric power amount to be applied to the heater is determined on the basis of the lower roller temperature and the predetermined target temperature (step  1110 ). The heater drive circuit is ON/OFF-controlled via the I/O port in accordance with the determined electric power amount (step  1111 ). 
     The above-mentioned operation is repeated at a predetermined interval, thereby keeping the surface temperatures of the upper and lower rollers of the fixing device constant.