Abstract:
An embodiment of a fixing apparatus according to the present invention is a fixing apparatus provided with a roller for fixing and a plurality of heat sources that heat a surface of the roller, in which a surface temperature of the roller is detected and controlled at a prescribed temperature while a sheet of recording paper is subjected to pressure and heat by the roller to fix a developer onto the sheet of recording paper, comprising: a plurality of temperature detection means for detecting the surface temperature of the roller, wherein at least one of the temperature detection means is arranged in a position of the roller that does not overlap any of the heat sources when viewed from a peripheral surface side of the roller, or is arranged in a position away from a region of the roller to be controlled at the prescribed temperature.

Description:
BACKGROUND OF THE INVENTION 
   This application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2005-230863 filed in Japan on Aug. 9, 2005, the entire contents of which are hereby incorporated by reference. 
   The present invention relates to fixing apparatus for fixing a development image onto a sheet of recording paper in electrophotographic image forming apparatuses such as copying machines, facsimile machines, and printers. 
   In this type of fixing apparatus, a sheet of recording paper is sandwiched between a heating roller and a pressure roller and subjected to heat and pressure, thereby thermally fusing and fixing the development image onto the sheet of recording paper. The surface temperature of the heating roller must be even to uniformly fix the development image onto the sheet of recording paper and it is necessary for substantially the entire heating roller to be heated uniformly. 
   However, the power consumption of a heater for heating in the fixing apparatus takes up a large proportion of the overall power consumption of the image forming apparatus, and therefore it is necessary to achieve reductions in the power consumption of the heater for heating in the fixing apparatus in order to reduce the power consumption of the image forming apparatus. 
   For example, in a heating roller of a fixing apparatus of Patent Document 1 (JP H08-262920A), a plurality of heaters are arranged in a line along a lengthwise direction of the heating roller, and the temperature is raised in only the heaters for heating required to heat surface portions of the heating roller that contact the sheet of recording paper for each sheet of recording paper of various widths, which avoids increasing the amount of heat produced by unneeded heaters, thereby reducing power consumption. Also, a plurality of temperature sensors for detecting the surface temperature of the heating roller in a plurality of locations are provided in order to control the heat produced by the plurality of heaters. 
   The temperature sensors are thermistors for example and, as disclosed in Patent Document 1 and Patent Document 2 (JP H05-188824A), it is common for the temperature sensors to be made to contact the surface of the heating roller to detect the surface temperature of the heating roller. 
   On the one hand, since only black developer is used in the case of monochrome images, a developer layer of a monochrome image on a sheet of recording paper is as thin as 20 to 30 μm. In contrast to this, since a color image is formed by superimposing a plurality of colors of developer onto a sheet of recording paper in the case of color images, a developer layer of a color image on a sheet of recording paper becomes as thick as 50 to 80 μm. 
   For this reason, for monochrome images, a nip region is formed between a metal heating roller coated in Teflon (registered trademark) or titanium and a pressure roller covered by a SiO 2  rubber for example, and even though a developer layer can be fixed onto a sheet of recording paper with this nip region, a developer layer of a color image cannot be fixed sufficiently with this nip region. 
   This is because although it is necessary for the heat of the heating roller to be transmitted sufficiently to the developer layer of a color image in order to thermally fuse the develop layer more than when thermally fusing the developer layer of a monochrome image since the developer layer of a color image is thick, the heat of the heating roller cannot be transmitted sufficiently to the developer layer with the above-described nip region. 
   Accordingly, in a fixing apparatus for a color image, the surfaces of both the heating roller and the pressure roller are covered with a SiO 2  rubber, which thereby achieves an increase in the width of the nip region. With a nip region whose width has been increased, the surface area of the sheet of recording paper that contacts the heating roller is increased, and therefore the heat of the heating roller is sufficiently transmitted to the developer layer on the sheet of recording paper and fixing can be achieved sufficiently by reliably carrying out thermal fusing of the developer layer. 
   However, by having the temperature sensors come in contact with the surface of the heating roller to control the surface temperature of the heating roller in this color image forming apparatus, damage is caused to the surface of the heating roller by contact with the temperature sensors since the surface of the heating roller is a SiO 2  rubber, thus causing unevenness in the fixing. 
   In particular, since there has been a call for increased printing speed in color image forming apparatuses in recent years and the rotational velocity of the heating roller and the pressure roller in fixing apparatus has been increased, there has been a tendency for more damage to occur on the surface of the heating roller due to contact with the temperature sensors and this has become a cause of reduced image quality. 
   For this reason, non-contact type temperature sensors have been employed that are capable of detecting the temperature of the heating roller surface without making contact with the surface. However, these non-contact type temperature sensors are large compared with contact-type temperature sensors. 
   Increasing the size of the heating roller temperature sensors is not desirable due to the advancement of miniaturization of color image forming apparatuses. Providing such large non-contact type temperature sensors in a plurality of locations to detect the surface temperature of the heating roller in the respective locations as described above would require a large space inside the fixing apparatus, which would increase the size of the fixing apparatus, and therefore in turn lead to an increase in size of the color image forming apparatus. 
   SUMMARY OF THE INVENTION 
   The present invention has been devised in consideration of the conventional problems described above, and it is an object thereof to provide a fixing apparatus in which contact-type temperature sensors can be applied even when the surfaces of the heating roller and the pressure roller are easily damageable. 
   The present invention has been devised in consideration of the conventional problems described above, and it is an object thereof to provide a fixing apparatus in which contact-type and/or non-contact type temperature sensors can be applied and that is capable of miniaturization. 
   In order to solve these issues, in the present invention, a fixing apparatus provided with a roller for fixing and a plurality of heat sources that heat a surface of the roller, in which a surface temperature of the roller is detected and controlled at a prescribed temperature while a sheet of recording paper is subjected to pressure and heat by the roller to fix a developer onto the sheet of recording paper, comprises: a plurality of temperature detection means for detecting the surface temperature of the roller, wherein at least one of the temperature detection means is arranged in a position of the roller that does not overlap any of the heat sources when viewed from a peripheral surface side of the roller. 
   Another aspect of the present invention is a fixing apparatus provided with a roller for fixing and a plurality of heat sources that heat a surface of the roller, in which a surface temperature of the roller is detected and controlled at a prescribed temperature while a sheet of recording paper is subjected to pressure and heat by the roller to fix a developer onto the sheet of recording paper, comprises: a plurality of temperature detection means for detecting the surface temperature of the roller, wherein at least one of the temperature detection means is arranged in a position away from a region of the roller to be controlled at the prescribed temperature. 
   Furthermore, in the present invention, the temperature detection means arranged at the position is arranged in contact with the roller surface. 
   Further still, in the present invention, the position is a position away from a fixing region of a sheet of recording paper using the roller. 
   Furthermore, in the present invention, the heat sources include a first heat source that heats a central area in a lengthwise direction of the roller and a second heat source that heats both sides of the central area of the roller, and control means is provided for controlling the second heat source by performing correction on a temperature detected by the temperature detection means arranged in the position and determining a temperature of a region of the roller to be controlled to the prescribed temperature, wherein the second heat source is controlled based on the temperature determined by the correction. 
   Further still, in the present invention, the temperature detection means arranged at the region of the roller to be controlled to the prescribed temperature is arranged not contacting the roller surface. 
   With the present invention, a plurality of temperature detection means are provided for detecting the surface temperature of the roller, wherein at least one of the temperature detection means is arranged in a position of the roller that does not overlap any of the heat sources when viewed from a peripheral surface side of the roller, or arranged in a position away from a region of the roller to be controlled at the prescribed temperature. In such positions, there is no fixing of developer onto the sheet of recording paper and these positions are outside the fixing region of the sheet of recording paper using the roller, and therefore it is acceptable for the temperature detection means to contact the surface of the roller and the surface of the roller to be damaged by the temperature detection means. For this reason, contact type components that contact the surface of the roller may be used as the temperature detection means arranged in the aforementioned position. 
   Since such contact type temperature detection means are small sized, it is possible to avoid increasing the size of the fixing apparatus and the color image forming apparatus when compared to using large non-contact type components for all the temperature detection means. 
   For example, the heat sources may include a first heat source that heats a central area in a lengthwise direction of the roller and a second heat source that heats both sides of the central area of the roller. When the width of the sheet of recording paper is narrow, even when only the first heat source is caused to increase in temperature to heat only a central region of the roller corresponding to the width of the sheet of recording paper, the entire sheet of recording paper can be uniformly heated. Also, when the width of the sheet of recording paper is wide, by setting the first and second heat source to increase in temperature so that substantially the entire roller is heated to achieve a uniform surface temperature, the entire sheet of recording paper can be uniformly heated. By using the second heat source to increase the temperature only when the width of the sheet of recording paper is wide, the power consumption of the fixing apparatus can be reduced. 
   Then, the temperature detection means arranged in the aforementioned position does not directly measure the temperature of the region of the roller to be controlled at the prescribed temperature, but detects the temperature around that region. Accordingly, the temperature detected by this temperature detection means is corrected and the temperature of the region is determined. Further still, since the second heat source is controlled based on the temperature determined by correction in this manner, the surface temperatures on both sides of the central area of the roller can be controlled accurately, and the entire roller surface can be set to a uniform temperature. 
   Furthermore, the temperature detection means arranged at the region of the roller to be controlled to the prescribed temperature is arranged not contacting the roller surface. Consequently, in the fixing region of the sheet of recording paper using the roller, the temperature detection means does not contact the surface of the roller and there is no damage to the roller surface caused by contact with the temperature detection means. Thus, there is no unevenness in fixing caused by damage to the roller surface in regions where the roller can be set at the prescribed temperature. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a lateral view showing an image forming apparatus to which an embodiment of a fixing apparatus according to the present invention has been applied. 
       FIG. 2  is a lateral cross-section showing a vertical direction cutaway view of a heating roller and a pressure roller of the fixing apparatus according to the present embodiment. 
       FIG. 3  is a vertical cross-section showing a horizontal direction cutaway view of the heating roller of the fixing apparatus of  FIG. 2 . 
       FIG. 4  is a graph illustrating temperature distribution characteristics A of the heating roller heated by the main heater and temperature distribution characteristics B of the heating roller heated by the sub-heater. 
       FIG. 5  conceptually illustrates a data table in which surface temperatures of an end portion of the heating roller and a ratio are associated. 
       FIG. 6  is a timing chart showing control of powering of the main heater and the sub-heater and change in the surface temperature of the heating roller when a sheet of recording paper passes a full fixing region. 
       FIG. 7  is a timing chart showing control of powering of the sub-heater and change in the surface temperature of the heating roller when a sheet of recording paper passes a narrow width fixing region. 
       FIG. 8  is a graph showing a surface temperature distribution of the heating roller when a sheet of recording paper passes through a nip region between the heating roller and the pressure roller. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1  is a lateral view showing an image forming apparatus to which an embodiment of a fixing apparatus according to the present invention has been applied. An image forming apparatus  100  is a color laser printer that records a color image on a sheet of recording paper and is provided with an exposure device  1 , image forming stations Pa, Pb, Pc, and Pd, an intermediate transfer belt device  2 , a fixing apparatus  3 , a paper transport system  4 , and a paper supply device  5 , for example. 
   The image forming stations Pa, Pb, Pc, and Pd respectively form toner images of black (K), cyan (C), magenta (M), and yellow (Y), and the toner image of each color is transferred to an intermediate transfer belt  11  of the intermediate transfer belt device  2 . The image forming stations Pa, Pb, Pc, and Pd are provided with items including developing devices  21   a  to  21   d,  photosensitive drums  23   a  to  23   d,  chargers  24   a  to  24   d,  and cleaning devices  25   a  to  25   d.    
   The photosensitive drums  23   a  to  23   d  press on respective primary transfer rollers  26   a  to  26   d  via the intermediate transfer belt  11  and rotate with the intermediate transfer belt  11  at a peripheral speed equivalent to the intermediate transfer belt  11 , which rotationally moves in the direction of arrow B. Furthermore, the primary transfer rollers  26   a  to  26   d  also rotate following the intermediate transfer belt  11  at a peripheral speed equivalent to the intermediate transfer belt  11 , which rotationally moves in the direction of arrow B. 
   The chargers  24   a  to  24   d  are roller-type or brush-type devices that contact the photosensitive drums  23   a  to  23   d,  or charger-type devices, and uniformly charge the surfaces of the photosensitive drums  23   a  to  23   d.    
   The exposure device  1  is provided with a laser light source  1   a  that irradiates laser light toward the respective photosensitive drums  23   a  to  23   d  and a plurality of mirrors  1   b  that guide the laser light onto the respective photosensitive drums  23   a  to  23   d,  for example. The laser lights are irradiated onto the surfaces of the respective photosensitive drums  23   a  to  23   d  while being modulated in accordance with the image data, such that respective electrostatic latent images are formed on the surfaces of the photosensitive drums  23   a  to  23   d.    
   It should be noted that a writing head in which light-emitting elements such as ELs and LEDs are arranged in an array may be used as the exposure device  1 . 
   The developing devices  21   a  to  21   d  hold the respective color toners and form toner images of these respective colors on the surfaces of the photosensitive drums  23   a  to  23   d  by causing toner of these respective colors to adhere to the electrostatic latent images on the photosensitive drums  23   a  to  23   d.  These toner images are transferred from the photosensitive drums  23   a  to  23   d  to the intermediate transfer belt  11  and superimposed there. 
   The intermediate transfer belt device  2  is provided with items such as the intermediate transfer belt  11 , the primary transfer rollers  26   a  to  26   d,  a drive support roller  31 , an idler support roller  32 , and a secondary transfer roller  33 . The intermediate transfer belt  11  is rotatably supported by being wound around the drive support roller  31  and the idler support roller  32 , and the primary transfer rollers  26   a  to  26   d  and the secondary transfer roller  33  are pressed against the intermediate transfer belt  11 . 
   The intermediate transfer belt  11  is made of a synthetic resin film of a thickness in the range of 100 μm to 150 μm for example. The secondary transfer roller  33  is supported so as to be movable laterally, and when it is moved rightward it sandwiches the intermediate transfer belt  11  between the drive support roller  31  and forms a nip region. The drive support roller  31  fulfills a role of being a backup roller of the secondary transfer roller  33  and rotates in the downstream of the respective nip regions between the primary transfer rollers  26   a  to  26   d  and the photosensitive drums  23   a  to  23   d  so that the intermediate transfer belt  11  is pulled and made to rotationally move in the B arrow direction. Thus, the nip regions are maintained stably. 
   It should be noted that, of the primary transfer rollers  26   a  to  26   d  and the photosensitive drums  23   a  to  23   d,  it is preferable for one of these to be formed of a hard material and the other to be formed of a flexible material in order to more stably form the respective nip regions between the primary transfer rollers  26   a  to  26   d  and the photosensitive drums  23   a  to  23   d.    
   Each of the primary transfer rollers  26   a  to  26   d  is made of, for example, a metal shaft of a diameter in the range of 8 mm to 10 mm, the circumference of which is covered by a conductive elastic material (such as EPDM and urethane foam). With the intermediate transfer belt  11  sandwiched in the nip regions between the primary transfer rollers  26   a  to  26   d  and the photosensitive drums  23   a  to  23   d,  a bias voltage having a polarity inverse to the charged polarity of the toner is applied to the primary transfer rollers  26   a  to  26   d  such that the respective electrical fields are effected to the toner on the surfaces of the photosensitive drums  23   a  to  23   d  through intermediate transfer belt  11 , after which the toner on the surfaces of the photosensitive drums  23   a  to  23   d  is attracted and transferred to the intermediate transfer belt  11 . Thus, the toner images of these colors are transferred to the intermediate transfer belt  11  and superimposed there. 
   It should be noted that brushes or the like may be used instead of rollers as the primary transfer rollers  26   a  to  26   d.    
   A cleaning apparatus  34  includes, for example, a cleaning blade that slides in contact with the surface of the intermediate transfer belt  11 , and removes toner remaining on the surface of the intermediate transfer belt  11  to prevent such defects as fogging of the next image to be printed. 
   In this way, the color toner images that are transferred and superimposed onto the intermediate transfer belt  11  are transported to the nip region between the drive support roller  31  and the secondary transfer roller  33  along with the rotational movement of the intermediate transfer belt  11 . Then, the leading edge of the sheet of recording paper carried by register rollers  8  is aligned with the leading edge of the color toner images on the intermediate transfer belt  11 , and the color toner images and the sheet of recording paper are overlaid so that the color toner images are transferred to the sheet of recording paper. 
   After this, the sheet of recording paper is carried to the fixing apparatus  3  and is here sandwiched between a heating roller  3   a  and a pressure roller  3   b.  Thus, the color toners on the sheet of recording paper are thermally fused and mixed so that the color toner images are fixed to the sheet of recording paper as a color image. 
   The sheet of recording paper is carried to the paper discharge tray  35  by the paper transport system  4  and discharged here facedown. 
   On the other hand, sheets of recording paper are stacked and stored in the paper supply cassette  6  in the image forming apparatus  100 . In the paper transport system  4 , sheets of recording paper are drawn out sheet by sheet from the paper supply cassette  6  by a pickup roller  7 - 1  and the sheets of recording paper are carried to the register rollers  8  by carry rollers  4 - 1 . 
   Alternatively, sheets of recording paper are loaded into a manual paper supply tray  9 . In the paper supply device  5 , a sheet of recording paper is drawn out from the manual paper supply tray  9  by a pickup roller  7 - 2  and the sheet of recording paper is carried to the register rollers  8  of the paper transport system  4  by carry rollers  4 - 7  and  4 - 8 . 
   In the paper transport system  4 , the sheet of recording paper is temporarily stopped by the register rollers  8 , the leading edge of the sheet of recording paper is aligned, then the sheet of recording paper is carried to the secondary transfer roller  33  by the register rollers  8  with a timing in which the leading edge of the sheet of recording paper is superimposed on the leading edge of the toner image formed on the intermediate transfer belt  11  of the intermediate transfer belt device  2 . 
   It should be noted that it is also possible to use only the image forming station Pa to form a monochrome image and transfer the monochrome image to the intermediate transfer belt  11  of the intermediate transfer belt device  2 . As with the color image, the monochrome image is transferred from the intermediate transfer belt  11  to the sheet of recording paper and fixed to the sheet of recording paper. 
   Furthermore, when carrying out printing not only on the front surface of the sheet of recording paper but on both surfaces, after the image on the front surface of the sheet of recording paper is fixed by the fixing apparatus  3  and while carry rollers  4 - 3  of the paper transport system  4  are carrying the sheet of recording paper, the carry rollers  4 - 3  can be made to stop and then rotate in reverse. The front and back of the sheet of recording paper are inverted via an inversion route  4   r  of the paper transport system  4 , and the sheet of recording paper is guided to the register rollers  8  and an image is recorded and fixed on the back side of the sheet of recording paper in the same way as the front side of the sheet of recording paper, after which the sheet of recording paper is discharged to the paper discharge tray  35 . 
   The following is a detailed description of the fixing apparatus  3  of the present embodiment.  FIG. 2  is a lateral cross-section showing a vertical direction cutaway view of the heating roller  3   a  and the pressure roller  3   b  of the fixing apparatus  3  and  FIG. 3  is a vertical cross-section showing a horizontal direction cutaway view of the heating roller  3   a  of the fixing apparatus  3 . 
   In the fixing apparatus  3  of the present embodiment, the heating roller  3   a  and the pressure roller  3   b  are rotatably supported on shafts and are pressing against each other such that a nip region N is formed in which a sheet of recording paper is sandwiched between the heating roller  3   a  and the pressure roller  3   b.  A sheet of recording paper is guided into the nip region N between the heating roller  3   a  and the pressure roller  3   b  by one of the rollers  3   a  and  3   b  being rotationally driven and the other being driven as a follower, such that pressure and heat are applied to the sheet of recording paper by the rollers  3   a  and  3   b.  Thus, as described earlier, the color toners on the sheet of recording paper are thermally fused and mixed so that the color toner images are fixed to the sheet of recording paper as a color image. 
   Furthermore, cleaning rollers  71  and  71  press against the heating roller  3   a  and the pressure roller  3   b  respectively to remove such substances as toner and paper debris adhering to the surfaces of the rollers  3   a  and  3   b.    
   Further still, paper separating claws  72  and  72  press against the heating roller  3   a  and the pressure roller  3   b  respectively to take off sheets of recording paper from the surfaces of the rollers  3   a  and  3   b  and prevent sheets of recording paper from winding around the rollers  3   a  and  3   b.    
   The heating roller  3   a  is a component in which a peripheral surface of a metal cylinder  51  is covered in an elastic layer  52  made of a SiO 2  rubber. Collars  51   a  are provided at both ends of the cylinder  51  and the elastic layer  52  is arranged between the collars  51   a.  Furthermore, pipe shafts  51  are provided protruding from the center of the collars  51   a  of the cylinder  51  and these pipe shafts  51   b  are rotatably supported. Further still, a main heater  53  and a sub-heater  54  are arranged inside the cylinder  51  along a lengthwise direction of the heating roller  3   a  and the heating roller  3   a  is heated by the heat produced by the main heater  53  or the sub-heater  54 . Terminals  53   a  at the ends of the main heater  53  and terminals  54   a  at the ends of the sub-heater  54  are connected to a control section  55  through the pipe shafts  51   b.    
     FIG. 4  is a graph illustrating temperature distribution characteristics A of the heating roller  3   a  heated by the main heater  53  and temperature distribution characteristics B of the heating roller  3   a  heated by the sub-heater  54 . The main heater  53  is arranged at a central vicinity of the heating roller  3   a  and heats the heating roller  3   a  at a fixing region L 1  at this central vicinity. When the heating roller  3   a  is heated by only the main heater  53 , the surface temperature of the heating roller  3   a  rises uniformly at the fixing region L 1  of the central vicinity, and the surface temperature of the heating roller  3   a  is lower at both ends for further distances from the fixing region L 1 . 
   Furthermore, the sub-heater  54  has two heater sections  54   b  and  54   b  and these heater sections  54   b  are distributed and arranged at the ends of the heating roller  3   a  such that the heating roller  3   a  is heated at two end side fixing regions L 2 . When the heating roller  3   a  is heated by only the heater sections  54   b,  the surface temperature of the heating roller  3   a  rises uniformly at the two end side fixing regions L 2 , and the surface temperature of the heating roller  3   a  is lower for further distances from these fixing regions L 2 . 
   Accordingly, when only the main heater  53  produces heat, the surface temperature of the heating roller  3   a  is uniform at only the fixing region L 1  with the temperature distribution characteristics A. 
   Furthermore, when the sub-heater  54  produces heat, the main heater  53  is also always made to produce heat. Thus, the surface temperature of the heating roller  3   a  becomes uniform at a full fixing region L, which includes the fixing region L 1  and the fixing regions L 2 . As shown in  FIG. 4 , when viewed from the peripheral surface of the heating roller  3   a,  the full fixing region L is slightly wider than a heat-producing region M where the main heater  53  and the sub-heater are present. 
   When fixing the color toners on the sheet of recording paper, the surface temperature of the heating roller  3   a  at the fixing region L 1 , or the surface temperature of the heating roller  3   a  at the full fixing region L, is maintained at a prescribed temperature T 1 . By maintaining the surface temperature of the heating roller  3   a  at the prescribed temperature T 1 , the color toners are suitably heated and fused to be fixed onto the sheet of recording paper. 
   Similar to the heating roller  3   a,  the pressure roller  3   b  is also a component in which a peripheral surface of a metal cylinder  56  is covered in an elastic layer  57  made of a SiO 2  rubber. Similar to the cylinder  51  of the heating roller  3   a,  the cylinder  56  has collars (not shown in drawings) at both ends and pipe shafts (not shown in drawings), and the pipe shafts are rotatably supported. Furthermore, a single heater  58  is arranged inside the cylinder  56  along a lengthwise direction of the pressure roller  3   b  and the pressure roller  3   b  is heated by the heat produced by the heater  58 . Terminals at the ends of the heater  58  are also connected to the control section  55  through the pipe shafts at the ends of the cylinder  56 . 
   The heater  58  heats substantially the entire pressure roller  3   b  and the surface temperature of the pressure roller  3   b  is made to rise uniformly in the full fixing region L shown in  FIG. 4 . 
   When fixing the color toners on the sheet of recording paper, the surface temperature of the pressure roller  3   b  of the full fixing region L is maintained at the same prescribed temperature T 1  as the heating roller  3   a.  By maintaining the surface temperature of the pressure roller  3   b  at the prescribed temperature T 1 , the sheet of recording paper is heated and the fixing properties of the toner with respect to the sheet of recording paper are improved. 
   Next, a non-contact type thermistor  61  is arranged at a location slightly apart from the surface of the elastic layer  52  of the heating roller  3   a  at substantially the central vicinity of the heating roller  3   a.  Also, a contact type thermistor  62  is arranged in contact with an end portion of the elastic layer  52  of the heating roller  3   a.  As shown in  FIG. 3 , when viewed from the peripheral surface of the heating roller  3   a,  this end portion of the heating roller  3   a  corresponds to a location where the end portion does not overlap the main heater  53  nor the sub-heater, that is, a location further outside from the heat-producing region M. 
   Two thermistor elements are mounted inside the non-contact type thermistor  61 . The surface temperature of the elastic layer  52  of the heating roller  3   a  is detected by one of the thermistor elements and the temperature of the non-contact type thermistor  61  is detected by the other thermistor element, and detection output indicating the surface temperature of the elastic layer  52  and the temperature of the non-contact type thermistor  61  respectively is outputted to the control section  55 . The control section  55  determines a surface temperature t 1  of the heating roller  3   a  at the fixing region L 1  based on the two items of detection output from the non-contact type thermistor  61 . 
   Furthermore, the contact type thermistor  62  directly detects the surface temperature of the end portion of the elastic layer  52  of the heating roller  3   a.  However, as is evident from the temperature distribution characteristics B shown in  FIG. 4 , at the end portion of the elastic layer  52  of the heating roller  3   a,  a surface temperature t 3  of the end portion is lower than a surface temperature t 2  of the fixing regions L 2 . Consequently, the control section  55  corrects the surface temperature t 3  of the end portion indicated by the detection output of the contact type thermistor  62  and determines the surface temperature t 2  of the fixing regions L 2 . 
   For example, while the main heater  53  and the sub-heater  54  of the heating roller  3   a  are caused to produce heat and the surface temperature of the heating roller  3   a  is gradually raised from a normal temperature, the surface temperature t 3  of the end portion of the heating roller  3   a  and the surface temperature t 2  of the fixing regions L 2  are measured, and each time the surface temperature t 3  of the end portion of the heating roller  3   a  incrementally changes, a ratio α between the surface temperature t 3  of the end portion and the surface temperature t 2  of the fixing regions L 2  is obtained. A data table Tb is created in advance in which the surface temperature t 3  of the end portion and the ratio α are associated as shown in  FIG. 5 , and this data table Tb is stored in the control section  55 . 
   When the main heater  53  and the sub-heater  54  produce heat, the control section  55  searches the data table Tb for the ratio α that corresponds to the surface temperature t 3  of the end portion indicated by the detection output of the contact type thermistor  62  and multiplies the surface temperature t 3  by this ratio α to determine the temperature t 2  of the fixing regions L 2 . 
   Here, reductions in the surface temperature of the end portion of the heating roller  3   a  are caused by such factors as direct heat release from the heating roller  3   a  and by heat release due to thermal conduction from the heating roller  3   a  to surrounding members. The amount of heat released is substantially specified and therefore the temperature t 2  of the fixing regions L 2  can be determined based on the ratio α with the surface temperature t 3  of the end portion of the heating roller  3   a  using the data table Tb as described above. 
   The non-contact type thermistor  61  is arranged slightly apart from the surface of the elastic layer  52  of the heating roller  3   a  in the fixing region L 1 , and therefore there is no damage to the surface of the elastic layer  52 . Furthermore, by arranging the contact type thermistor  62  in contact with the end portion of the elastic layer  52  of the heating roller  3   a  away from the full fixing region L, this end portion has no effect on the fixing of the toner onto the sheet of recording paper and even if the end portion is damaged by contact with the contact type thermistor  62 , damage to the end portion does not become a cause of unevenness of fixing on the sheet of recording paper. 
   Furthermore, since only one large non-contact type thermistor  61  is used in combination with the small contact type thermistor  62 , it is possible to avoid increasing the size of the fixing apparatus  3  as well as the image forming apparatus  100 . 
   On the other hand, a contact type thermistor  63  is arranged in contact with a central vicinity of the elastic layer  57  of the pressure roller  3   b . The contact type thermistor  63  directly detects the surface temperature of the central vicinity of the elastic layer  57 . Furthermore, as mentioned earlier, the heater  58  of the pressure roller  3   b  uniformly raises the surface temperature of the pressure roller  3   b  in the full fixing region L. Consequently, the control section  55  can determine the surface temperature of the pressure roller  3   b  indicated by the detection output of the contact type thermistor  63  as the surface temperature of the full fixing region L. 
   It should be noted that even if the elastic layer  57  of the pressure roller  3   b  is damaged by contact with the contact type thermistor  63 , the pressure roller  3   b  heats the sheet of recording paper only and does not directly heat the toner on the sheet of recording paper, and therefore damage on the elastic layer  57  of the pressure roller  3   b  does not become a cause of unevenness in the fixing of the sheet of recording paper. 
   In a fixing apparatus  3  configured in this manner, the surface temperature of the heating roller  3   a  and the surface temperature of the pressure roller  3   b  are kept below the prescribed temperature T 1  during standby mode or power saving mode of the image forming apparatus  100 . 
   That is, the control section  55  determines the surface temperature t 1  of the heating roller  3   a  in the fixing region L 1  from the detection output of the non-contact type thermistor  61  and controls the ON/OFF of powering to the main heater  53  of the heating roller  3   a  so that the surface temperature t 1  is a constant temperature below the prescribed temperature T 1 , and also determines the surface temperature t 2  of the heating roller  3   a  in the fixing regions L 2  from the detection output of the contact type thermistor  62  and controls the ON/OFF of powering to the sub-heater  54  of the heating roller  3   a  so that the surface temperature t 2  is a constant temperature below the prescribed temperature T 1 . Similarly, the control section  55  controls the ON/OFF of powering to the heater  58  of the pressure roller  3   b  so that the surface temperature of the pressure roller  3   b  detected by the contact type thermistor  63  is a constant temperature below the prescribed temperature T 1 . Thus, during standby mode or power saving mode, the surface temperature of the heating roller  3   a  and the surface temperature of the pressure roller  3   b  in the full fixing region L are maintained at a constant temperature and power consumption of the fixing apparatus  3  is reduced. 
   Next, when a printing request occurs, the control section  55  performs ON/OFF control of the powering to the heater  58  of the pressure roller  3   b  in response to the surface temperature of the pressure roller  3   b  detected by the contact type thermistor  63  and raises the surface temperature of the pressure roller  3   b  to the prescribed temperature T 1 . 
   Furthermore, the control section  55  inputs the width of the sheet of recording paper from an external section and in response to that width it judges whether to carry out control of the powering of the main heater  53  and the sub-heater  54  of the heating roller  3   a  based on only the detection output of the non-contact type thermistor  61  or whether to carry out control of the powering of the main heater  53  and the sub-heater  54  of the heating roller  3   a  based on the detection output of the non-contact type thermistor  61  and the detection output of the contact type thermistor  62 . 
   For example, when the sheet of recording paper is a standard B4 size and the sheet of recording paper passes a full fixing region L that is slightly narrower than the full width of the heating roller  3   a,  a judgment is made to carry out control of the powering based on only the detection output of the non-contact type thermistor  61  and when the sheet of recording paper is a standard A4 size and the sheet of recording paper passes the fixing region L 1  that has a narrow width, a judgment is made to carry out control of the powering based on the detection output of the non-contact type thermistor  61  and the detection output of the contact type thermistor  62 . 
   Then, if the sheet of recording paper is a standard B4 size and a judgment is made to carry out control of powering based on only the detection output of the non-contact type thermistor  61 , the control section  55  determines the surface temperature t 1  of the heating roller  3   a  in the fixing region L 1  based on the detection output of the non-contact type thermistor  61  and controls the ON/OFF of the powering to both the main heater  53  and the sub-heater  54  of the heating roller  3   a  so that the surface temperature t 1  becomes the prescribed temperature T 1 . 
   That is, when the sheet of recording paper is a standard B4 size and the sheet of recording paper passes the full fixing region L, control of the powering to both the main heater  53  and the sub-heater  54  is carried out based on the surface temperature t 1  in the fixing region L 1  indicated by the detection output of the non-contact type thermistor  61  such that the surface temperature t 1  in the fixing region L 1  is controlled at the prescribed temperature T 1 . Thus, the surface temperature t 1  of the heating roller  3   a  becomes the prescribed temperature T 1  in the full fixing region L. 
     FIG. 6  is a timing chart showing control of powering of the main heater  53  and the sub-heater  54  and change in the surface temperature t 1  of the heating roller  3   a  when the sheet of recording paper passes the full fixing region L. As shown in the timing chart, immediately prior to the start of printing, ON/OFF control of the powering to the main heater  53  and the sub-heater  54  is performed and the surface temperature t 1  in the full fixing region L is substantially maintained at the prescribed temperature T 1 . 
   When printing starts, the sheet of recording paper passes through the nip region N between the heating roller  3   a  and the pressure roller  3   b,  and therefore an amount of heat of the heating roller  3   a  is lost due to the sheet of recording paper and the toner on the sheet of recording paper, such that the surface temperature t 1  of the heating roller  3   a  corresponding to the detection output of the non-contact type thermistor  61  begins to fall. For this reason, continuous powering to the main heater  53  and the sub-heater  54  is performed and the surface temperature t 1  rises to the prescribed temperature T 1 . Then, when the surface temperature t 1  returns to the prescribed temperature T 1 , ON/OFF control of the powering to the main heater  53  and the sub-heater  54  is performed and the surface temperature t 1  is again maintained at the prescribed temperature T 1 . 
     FIG. 8  is a graph showing a surface temperature distribution of the heating roller  3   a  when a sheet of recording paper passes through the nip region N between the heating roller  3   a  and the pressure roller  3   b.  When the sheet of recording paper passes the full fixing region L, the surface temperature t 1  of any location of the full fixing region L is maintained at the prescribed temperature T 1  as shown by a solid line C. However, although an amount of heat of the heating roller  3   a  is lost due to the sheet of recording paper and the toner on the sheet of recording paper in the full fixing region L, an amount of heat of the heating roller  3   a  is not lost at the outer sides of the full fixing region L, and therefore the heating roller  3   a  becomes overheated due to continuous powering to the main heater  53  and the sub-heater  54  and the surface temperature t 1  becomes higher than the prescribed temperature T 1 . 
   Furthermore, if the sheet of recording paper is a standard A4 size and a judgment is made to carry out control of the powering based on the detection output of the non-contact type thermistor  61  and the detection output of the contact type thermistor  62 , the control section  55  determines the surface temperature t 1  of the heating roller  3   a  in the fixing region L 1  from the detection output of the non-contact type thermistor  61  and controls the ON/OFF of powering to the main heater  53  so that the surface temperature t 1  becomes the prescribed temperature T 1 , and also determines the surface temperature t 2  of the heating roller  3   a  in the fixing regions L 2  from the detection output of the contact type thermistor  62  and controls the ON/OFF of powering to the sub-heater  54  of the heating roller  3   a  so that the surface temperature t 2  becomes the prescribed temperature T 1 . 
   That is, when the sheet of recording paper is a standard A4 size and the sheet of recording paper passes the fixing region L 1  having a narrow width, in addition to the control of the powering to the main heater  53  of the heating roller  3   a  carried out based on the detection output of the non-contact type thermistor  61 , control of the powering to the sub-heater  54  of the heating roller  3   a  is carried out based on the detection output of the contact type thermistor  62 . Thus, the surface temperature t 1  of the heating roller  3   a  in at least the fixing region L 1  is controlled at the prescribed temperature T 1 . 
     FIG. 7  is a timing chart showing control of powering of the sub-heater  54  and change in the surface temperature t 1  when the sheet of recording paper passes the narrow width fixing region L 1 . As shown in the timing chart, immediately prior to the start of printing, ON/OFF control of the powering to the sub-heater  54  is performed and the surface temperature t 2  in the fixing regions L 2  is substantially maintained at the prescribed temperature T 1 . Furthermore, since the contact type thermistor  62  is arranged in contact with the end portion of the heating roller  3   a,  the surface temperature t 3  of the end portion indicated by the detection output of the contact type thermistor  62  is delayed and follows the surface temperature t 2 . 
   The surface temperature t 3  of the end portion falls delayed compared to the surface temperature t 2 , and therefore control of the powering to the sub-heater  54  is also delayed and as a result the surface temperature t 2  in the fixing regions L 2  is substantially maintained at the prescribed temperature T 1 . 
   Assuming that control of powering to the main heater  53  and the sub-heater  54  of the heating roller  3   a  was carried out based on only the detection output of the non-contact type thermistor  61  when the sheet of recording paper passes the narrow width fixing region L 1 , then as shown by a dashed line D in  FIG. 8 , even though the surface temperature t 1  in the fixing region L 1  can be held at the prescribed temperature T 1 , at the outer sides of the fixing region L 1  an amount of heat of the heating roller  3   a  is not lost due to the sheet of recording paper and the toner, and therefore overheating of the heating roller  3   a  occurs due to continuous powering to the main heater  53  and the sub-heater  54  such that the surface temperature t 2  becomes extremely higher than the prescribed temperature T 1 . 
   However, since control of the powering to the sub-heater  54  is carried out based on the detection output of the contact type thermistor  62 , it is possible to avoid overheating of the heating roller  3   a  at the outer sides of the fixing region L 1  and the surface temperature t 1  can be maintained around the prescribed temperature T 1 . Moreover, the power consumption of the sub-heater  54  can be reduced. 
   In the fixing apparatus  3  according to the present embodiment, the non-contact type thermistor  61  in the fixing region L 1  is arranged slightly apart from the surface of the elastic layer  52  of the heating roller  3   a,  and therefore there is no damage to the surface of the elastic layer  52 . Furthermore, since the contact type thermistor  62  is arranged in contact with the end portion of the elastic layer  53  of the heating roller  3   a  away from the full fixing region L, and even if the end portion of the elastic layer  53  is damaged by contact with the contact type thermistor  62 , damage to the end portion does not become a cause of unevenness of fixing on the sheet of recording paper. 
   Furthermore, since only one large non-contact type thermistor  61  is used in combination with the small contact type thermistor  62 , it is possible to avoid increasing the size of the fixing apparatus  3  as well as the image forming apparatus  100 . 
   It should be noted that the present invention is not limited to the above-described embodiment and includes other various variations. For example, one each of a non-contact type thermistor and a contact type thermistor were provided on the heating roller  3   a  but it is also possible to provide a larger number of non-contact type thermistors and contact type thermistors. 
   Furthermore, non-contact type thermistors and contact type thermistors may be provided not only on the heating roller  3   a  but on the pressure roller  3   b  as well. 
   The present invention can be embodied and practiced in other different forms without departing from the spirit and essential characteristics thereof. Therefore, the above-described examples are considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All variations and modifications falling within the equivalency range of the appended claims are intended to be embraced therein.