Patent Publication Number: US-9423729-B2

Title: Image forming apparatus and heat fixing device provided in the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2013-070014 filed Mar. 28, 2013. The entire content of the priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a heat fixing device having a temperature sensor, and an image forming apparatus having the heat fixing device. 
     BACKGROUND 
     A heat fixing device provided in an image forming apparatus includes an endless belt having a center portion and end portions in an axial direction thereof, a heater disposed within the endless belt, and temperature sensors adapted to detect temperature of the endless belt. 
     SUMMARY 
     One of the temperature sensors is positioned at a center portion within the endless belt, and another of the temperature sensor is positioned at end portions within the endless belt. Thus, the temperature sensor can efficiently detect the temperature of center portion and end portions within the endless belt. However, a decrease in temperature at the end portions on an outer peripheral surface of the endless belt adversely affects an image quality of the image forming apparatus. Therefore, it is desired that the temperature at the end portions on the outer peripheral surface of the endless belt can precisely be detected. 
     In view of the foregoing, it is an object of the present invention to provide an image forming apparatus that can precisely detect a temperature at the end portions on an outer peripheral surface of an endless belt. 
     In order to attain the above and other objects, the present invention provides an image forming apparatus. The image forming apparatus may include an endless belt, a heater, a first temperature sensor, and a second temperature sensor. The endless belt may be configured to circularly move about a rotational axis extending in an axial direction. The endless belt may have a center portion and end portions in the axial direction, and defines an internal space therein and an outer peripheral surface. The heater may be configured to heat the endless belt. The first temperature sensor may be positioned at the center portion and in the internal space. The second temperature sensor may be positioned at one of the end portions and facing the outer peripheral surface. 
     According to another aspect, the present invention provides a heat fixing device. The heat fixing device may include an endless belt, a nip member, a first temperature sensor, and a second temperature sensor. The endless belt may be configured to circularly move about a rotational axis extending in an axial direction. The endless belt may have a center portion and end portions in the axial direction. The endless belt may define an internal space therein, an inner peripheral surface, and an outer peripheral surface. The nip member may be configured to contact the inner peripheral surface of the endless belt. The first temperature sensor may be positioned at the center portion and in the internal space so as to face the nip member. The second temperature sensor may be positioned at one of the end portions and facing the outer peripheral surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional view of a color laser printer according to an embodiment of the invention; 
         FIG. 2  is a schematic cross-sectional of a heat fixing device of the color laser printer; 
         FIG. 3  is an exploded perspective view of a halogen lamp, a nip plate, a reflective plate, a stay, a center thermistor, and a thermostat; 
         FIG. 4A  is a schematic perspective view of the stay, a cover member, and side thermistors; 
         FIG. 4B  is a front view of the stay, the cover member, and the side thermistors; 
         FIG. 5  is a flowchart illustrating an operation of a control device; 
         FIG. 6  is a flowchart illustrating the operation of the control device; 
         FIG. 7  is a time chart of each parameter when a temperature of an end portion of an endless belt is larger than a second temperature and lower than a third temperature after a predetermined time has elapsed from a reception of a printing command for printing a plurality of sheets having a width lager than a predetermined width; 
         FIG. 8  is a time chart of each parameter when a second mode is performed during a printing control based on a printing command for printing a plurality of sheets having a width smaller than the predetermined width; 
         FIG. 9  is a time chart of each parameter when the temperature of the end portion is smaller than or equal to the second temperature after the predetermined time has elapsed from the reception of the printing command for printing a plurality of sheets having a width lager than the predetermined width; 
         FIG. 10A  is a cross-sectional view of a heat fixing device according to a first modification of the embodiment of the invention; 
         FIG. 10B  is a cross-sectional view of a heat fixing device according to a second modification of the embodiment of the invention; and 
         FIG. 11  is a cross-sectional view of a heat fixing device according to a third modification of the embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will be described in detail with reference to the accompanying drawings. In the following description, the top-bottom direction shown in  FIG. 1  is referred to as a top-bottom direction; the left side of  FIG. 1  is referred to as a rear side, the right side as a front side, the far side of the sheet as a right side, and the near side of the sheet as a left side. In this case, the directions are defined based on directions as viewed from a front side of a color laser printer  1 . 
     &lt;Schematic Configuration of Color Laser Printer&gt; 
     As shown in  FIG. 1 , the color laser printer  1  includes a device body  2 , a paper feed unit  5  adapted to feed sheets  51 , an image formation unit  6  adapted to form an image on the fed sheet  51 , a paper discharge unit  7  adapted to discharge the sheet  51  on which the image has been formed, a control device  300 ; and a motor  400 . These components are provided in the device body  2 . The control device  300  and the motor  400  will be described later. 
     The paper feed unit  5  includes a paper feed tray  50  and a conveyance mechanism M 1 . The paper feed tray  50  is slidingly attached to and detached from the device body  2  from a front side at a lower portion of the device body  2 . The conveyance mechanism M 1  lifts up a front side of the sheet  51  from the paper feed tray  50  and conveys the sheet  51  so as to turn the sheet  51  rearward. 
     The conveyance mechanism M 1  includes a pickup roller  52 , a separation roller  53 , and a separation pad  54 , which are provided at a front end portion of the paper feed tray  50 . Those components are adapted to separate one sheet  51  after another, and send the sheet  51  upward. As the sheet  51  that has been sent upward passes between a paper dust removing roller  55  and a pinch roller  56 , paper dust is removed from the sheet  51 . Subsequently, the sheet  51  travels along a conveyance route  57  while turning to the rear side. Then, the sheet  51  is fed onto a conveyance belt  73 , and is conveyed to a fusing belt  110 . 
     The image formation unit  6  includes a scanner unit  61 , a process unit  62 , a transfer unit  63 , and a heat fixing device  100 . 
     The device body  2  has an upper portion provided with the scanner unit  61  including a laser emitting unit, a polygon mirror, a plurality of lenses and reflective mirrors (not shown in the drawings). Laser beams for each of the colors, cyan, magenta, yellow, and black, are emitted from the laser emitting unit in the scanner unit  61 , scans at high speed by the polygon mirror in a right-left direction, and then irradiates each photosensitive drum  31  after passing through or reflected by a plurality of lenses and reflective mirrors. 
     The process unit  62  is placed below the scanner unit  61  and above the paper feed unit  5 . The process unit  62  includes a photosensitive unit  3  that is movable in a front-rear direction with respect to the device body  2 . The photosensitive unit  3  includes four drum subunits  30  and developing cartridges  40 . The drum subunits  30  are provided at a lower portion of the photosensitive unit  3 , and each of the developing cartridges  40  is detachably mounted on each drum subunit  30 . 
     Each drum subunit  30  includes a photosensitive drum  31  and a scorotron-type charger  32 . Each developing cartridge  40  accommodates therein toner and includes a supply roller  41 , a developing roller  42 , and a layer thickness regulating blade  43 . 
     The process unit  62  functions as described below. The supply roller  41  supplies the toner in the developing cartridge  40  to the developing roller  42 . At this time, the toner is positively and frictionally charged between the supply roller  41  and the developing roller  42 . The toner supplied to the developing roller  42  is regulated by the layer thickness regulating blade  43  as the developing roller  42  is rotated. As a result, the toner is curried on a peripheral surface of the developing roller  42  as a uniform thin layer. 
     The photosensitive drum  31  is uniformly and positively charged by corona discharge of the scorotron-type charger  32  in the drum subunit  30 . The charged photosensitive drum  31  is irradiated with the laser beam emitted from the scanner unit  61  to form an electrostatic latent image corresponding to an image to be formed on the sheet  51  on the photosensitive drum  31 . 
     Furthermore, as the photosensitive drum  31  rotates, the toner carried on the developing roller  42  is supplied to the electrostatic latent image of the photosensitive drum  31 , e.g., to a portion of the surface of the positively charged photosensitive drum  31  whose potential is lowered due to the exposure of the laser beams. As a result, the electrostatic latent image of the photosensitive drum  31  is developed into a visible image, and a toner image is held on the peripheral surface of the photosensitive drum  31  for each color of the toner by reversal phenomena. 
     The transfer unit  63  includes a drive roller  71 , a driven roller  72 , an endless conveyance belt  73 , transfer rollers  74 , and a cleaning unit  75 . The drive roller  71  and the driven roller  72  are separated in the front-rear direction, and are disposed parallel to each other. The conveyance belt  73  is looped around the drive roller  71  and the driven roller  72 . The conveyance belt  73  has an outer surface in contact with each photosensitive drum  31 . The conveyance belt  73  defines an internal space therein provided with the transfer rollers  74  so that the conveyance belt  73  is sandwiched between the photosensitive drum  31  and the transfer roller  74 . The transfer rollers  74  are applied with a transfer bias from a high-voltage board not shown in the drawings. During the formation of the image, the sheet  51  conveyed by the conveyance belt  73  is held between the photosensitive drums  31  and the transfer rollers  74 , and the toner images on each of the photosensitive drums  31  are transferred and superimposed onto the sheet  51 . 
     The cleaning unit  75  is placed below the conveyance belt  73 . The cleaning unit  75  removes the toner adhering to the conveyance belt  73 , and collects the removed toner into a toner storage unit  76  disposed below the cleaning unit  75 . 
     The heat fixing device  100  is provided rearward of the transfer unit  63 . The heat fixing device  100  thermally fixes on the sheet  51  the toner images that have been transferred onto the sheet  51 . The heat fixing device  100  will be described later. 
     The paper discharge unit  7  defines a discharge path  91  of the sheet  51  extending from an outlet of the heat fixing device  100  toward upward and then turning frontward. A plurality of conveyance rollers  92  is disposed in the middle of the discharge path  91  to carry the sheets  51 . A paper discharge tray  93  is formed on an upper surface of the device body  2 . The sheet  51  discharged by the conveyance rollers  92  from the discharge path  91  is stacked on the paper discharge tray  93 . 
     &lt;Detailed Configuration of Heat Fixing Device&gt; 
     As shown in  FIG. 2 , the heat fixing device  100  includes a heating member  101 , a pressure roller  150  as an example of a rotation member, a fixing frame  200 , and a pair of side thermistors  210  as an example of a second temperature sensor and a third temperature sensor. 
     The heating member  101  includes a fusing belt  110  as an example of an endless belt, a halogen lamp  120  as an example of a heater, a nip plate  130  as an example of a nip member, a reflective plate  140 , a stay  160 , a cover member  170 , a center thermistor  180  as an example of a first temperature sensor, and a thermostat  190  as an example of an overheat prevention member (See  FIG. 3 ). 
     The fusing belt  110  is an endless belt having heat resistance and flexibility and defines an internal space therein in which above components are disposed. The fusing belt  110  contacts the pressure roller  150  so as to follow the same, thereby circularly moving in the clockwise direction in  FIG. 2 , i.e. moving rearward at a nip N described later. The fusing belt  110  rotates about an axis extending in the right-left direction, and has an inner peripheral surface  110 A in sliding contact with the nip plate  130  and an outer peripheral surface  110 B in sliding contact with the pressure roller  150 . The fusing belt  110  comprises a metal element tube made of stainless steel or the like. The fusing belt  110  may include a rubber layer covering a surface of the metal element tube, and may further include a non-metallic mold release layer such as fluorine coating for covering a surface of the rubber layer. 
     The halogen lamp  120  is a separate member from the nip plate  130 . The halogen lamp  120  functions as a heating body for heating the toner on the sheet  51  by heating the nip plate  130  and the fusing belt  110 . The halogen lamp  120  is disposed in the internal space of the fusing belt  110  with a predetermined gap from inner peripheral surface  110 A of the fusing belt  110  and the nip plate  130 , i.e., separated from the inner peripheral surface  110 A of the fusing belt  110  and the nip plate  130 . 
     The nip plate  130  is a plate-like member for receiving radiation heat from the halogen lamp  120 , and is in sliding contact with the inner peripheral surface  110 A of the fusing belt  110 . The nip plate  130  transmits the radiation heat received from the halogen lamp  120  to the toner on the sheet  51  via the fusing belt  110 . The nip plate  130  is made of, for example, an aluminum plate having larger thermal conductivity than the stay  160  made of steel. The nip plate  130  mainly includes a base section  131  and protruding sections  132  shown in  FIG. 3 . 
     The base section  131  has a central section  131 A and end portions  131 B in a conveyance direction of the sheet  51 . The central section  131 A has a convex shape protruding from both end portions  131 B toward the pressure roller  150 . 
     The protruding sections  132  protrude rearward from a rear end  131 R of the base section  131  in the conveyance direction. As shown in  FIG. 3 , two protruding sections  132  are formed in the nip plate  130 . Specifically, one protruding section  132  is formed at a center portion of the rear end  131 R in the right-left direction, and another of the protruding section  132  is formed at a position slightly closer to the left side than the center portion in the right-left direction. 
     As shown in  FIG. 2 , the reflective plate  140  is a member adapted to reflect the radiation heat mainly emitted from the halogen lamp  120  in the front-rear direction and top direction to the nip plate  130 , e.g., an inner surface of the base section  131 . The reflective plate  140  is disposed in the internal space of the fusing belt  110  so as to surround the halogen lamp  120  with a predetermined gap therebetween. 
     The reflective plate  140  concentrates the radiation heat from the halogen lamp  120  on the nip plate  130 . Therefore, the radiation heat from the halogen lamp  120  can be efficiently utilized, allowing the nip plate  130  and the fusing belt  110  to be quickly heated. 
     The reflective plate  140  is formed by bending, for example, an aluminum plate having a high reflectance to infrared rays and far infrared rays or any other plate into an almost U-shape in cross-section. More specifically, the reflective plate  140  mainly includes a reflective section  141  having a curved shape (substantially U-shape in cross-section) and flange sections  142  outwardly extending from both end portions of the reflective section  141  in the front-rear direction. The reflective plate  140  may be made from a mirror-finished aluminum plate to increase heat reflectance. 
     The stay  160  is a member that enhances the rigidity of the nip plate  130  by supporting both end portions  131 B of the base section  131  of the nip plate  130  through the flange sections  142  of the reflective plate  140 . The stay  160  is so placed as to cover the reflective plate  140  from above. More specifically, the stay  160  has a U-shaped cross-section including an upper wall  160 A, a front wall  160 B, and a rear wall  160 C. The upper wall  160 A has a front end form which the front wall  160 B extends downward and a rear end from which the rear wall  160 C extends downward. 
     As shown in  FIG. 3 , the stay  160  is formed with two notches  161  on the rear wall  160 C to allow the center thermistor  180  and the thermostat  190  to be placed therein with a gap therebetween. More specifically, the notches  161  are formed at positions corresponding to the two protruding sections  132  of the nip plate  130 . 
     As shown in  FIGS. 4A and 4B , the cover member  170  is disposed for covering the upper wall  160 A and the front wall  160 B of the stay  160 . The cover member  170  includes an upper-side wall  171  and a front-side wall  172  extending downward from a front end of the upper-side wall  171 . The front-side wall  172 A has a front surface provided with a plurality of ribs  173 . 
     Seven ribs  173  in total are provided on the front surface of the front-side wall  172  in the right-left direction at equal intervals so as to protrude from the front surface of the front-side wall  172  toward the front side. Each rib  173  is formed into generally square shape and has a front surface as a guide surface  173 A for guiding the inner peripheral surface  110 A of the fusing belt  110 . The both end ribs  173  of the plurality of ribs  173  are opposite to the side thermistors  210  with respect to the fusing belt  110 . 
     As shown in  FIGS. 2 and 3 , the center thermistor  180  is contact-type thermistors, and is adapted to detect a temperature of the nip plate  130 . More specifically, the center thermistor  180  is positioned at internal space of the center portion of the fusing belt  110 , and is positioned inside of a minimum paper width W 2  (See  FIG. 4B ) in the right-left direction. The center thermistor  180  is adapted to output a center temperature TC as signals to the control device  300 . The center thermistor  180  has an upper portion provided with a fixing rib  183  protruding upward. The fixing rib  183  is fixed to the rear wall  160 C of the stay  160  with a screw  189 . The center thermistor  180  is disposed so as to face an upper surface of the protruding section  132  of the nip plate  130 , and has a bottom surface as a temperature detection surface  181  for detecting a temperature in contact with the upper surface of the protruding section  132 . The center thermistor  180  may be noncontact-type thermistors and be disposed away from the nip plate  130 , or may be infrared sensors. The minimum paper width W 2  is a minimum width of paper sheet that can be printed in the color laser printer  1 . 
     The thermostat  190  is a temperature detection element using bimetal and is disposed so as to detect the temperature of the nip plate  130 . More specifically, the thermostat  190  is placed in an area slightly closer to the left side than the center portion of the fusing belt  110  in the right-left direction, and is positioned an inner side of a minimum paper width W 2  in the right-left direction (See  FIG. 4B ). The thermostat  190  has an upper portion provided with a fixing rib  193  protruding upward. The fixing rib  193  is fixed to the rear wall  160 C of the stay  160  with a screw  199 . 
     The thermostat  190  is disposed so as to face an upper surface of the protruding section  132  of the nip plate  130  and has a bottom surface as a temperature detection surface  191  in contact with the upper surface of the protruding section  132 . The thermostat  190  is provided on a circuit that supplies power to the halogen lamp  120 . If the thermostat  190  detects a temperature larger than or equal to a predetermined value, then the thermostat  190  interrupts the supply of power to the halogen lamp  120 , thereby preventing an excessive rise in the temperature of the heat fixing device  100 . 
     The pressure roller  150  is in sliding contact with the outer peripheral surface  110 B of the fusing belt  110  so as to form the nip N therebetween. The pressure roller  150  is disposed immediately below the nip plate  130  and sandwiches the fusing belt  110  in cooperation with the nip plate  130 . 
     The fixing frame  200  is disposed so as to cover the heating member  101  from diagonally upward and frontward of the same, as shown in  FIG. 1 . The fixing frame  200  has a front wall  201  in front of the heating member  101 , and the front wall  201  is provided with the pair of side thermistors  210 . 
     The pair of side thermistors  210  is a noncontact-type thermistor and has an upper portion provided with a fixing rib  213  extending upward. The fixing rib  213  is fixed to the front wall  201  of the fixing frame  200  with a screw  219 . The pair of side thermistors  210  has a rear surface as a temperature detection surface  211  in confrontation with the outer peripheral surface  110 B with a gap therebetween. 
     More specifically, as shown in  FIGS. 2, 4A, and 4B , the temperature detection surface  211  of each of the pair of side thermistors  210  is disposed on a front side of the nip N, i.e. on an upstream side of the moving-direction (rotational direction) of fusing belt  110  relative to the nip N. The pair of side thermistors  210  faces the right and left end portions of the outer peripheral surface  110 B of the fusing belt  110  in the right-left direction. The fact that the pair of side thermistors  210  faces the right and left end portions of the outer peripheral surface  110 B of the fusing belt  110  means that the temperature detection surface  211  is close to the outer peripheral surface  110 B capable of detecting the temperature of the outer peripheral surface  110 B of the fusing belt  110 . 
     The pair of side thermistors  210  is disposed on the outside of a maximum paper width W 1  in the right-left direction. The pair of side thermistors  210  may be a contact-type thermistor in direct contact with the fusing belt  110 , or an infrared sensor. The pair of side thermistors  210  is adapted to respectively output end-portion temperatures TS as signals to the control device  300 . The pair of side thermistors  210  and the center thermistors  180  may generate analog values corresponding to the temperatures, or generate digital values based on the analog values. The analog or digital values are transmitted to the control device  300  as signals. The maximum paper width W 1  is a maximum width of paper sheet that can be printed in the color laser printer  1 . 
     &lt;Control Device&gt; 
     The control device  300  will be described in detail. The control device  300  includes, for example, a storage unit having a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory). The control device  300  is adapted to control the halogen lamp  120 , the pickup roller  52 , and the motor  400  by performing arithmetic process, based on previously prepared programs and the signals from each side thermistor  210  and the center thermistor  180 . The signals may represent the temperatures acquired by the side thermistors  210  and the center thermistor  180 . The ROM stores instructions for executing various control processes (described later) as programs. The CPU reads the instructions from the ROM, and performs various arithmetic processes. 
     The control device  300  controls the halogen lamp  120  based on signals from the center thermistor  180 . For example, the control device  300  controls the halogen lamp  120  to maintain the output of the halogen lamp  120  constant until the center temperature TC obtained from the center thermistor  180  reaches a target temperature TH 0 . After the center temperature TC reaches the target temperature TH 0 , the control device  300  controls the halogen lamp  120  to maintain the center temperature TC at the target temperature TH 0 . The target temperature TH 0  is a temperature within a range where a favorable heat fixing can be performed. The target temperature TH 0  can be arbitrarily determined based on results of experiments or simulation. According to the present embodiment, TH 0 =180 degrees Celsius. The target temperature TH 0  may be preferably any value within the range of 160 to 240 degrees Celsius, depending on characteristics of the heat fixing device  100 , and be more preferably any value within the range of 175 to 200 degrees Celsius. 
     The control device  300  determines based on signals from each side thermistor  210  that a failure has occurred, i.e., an edge overheat has occurred, if the end-portion temperature TS obtained from at least one of side thermistors  210  is larger than or equal to a first temperature TH 1  higher than the target temperature TH 0 . The first temperature TH 1  is higher than temperatures at which the favorable fixing operation can be performed. The first temperature TH 1  can be arbitrarily determined based on results of experiments or simulation. According to the present embodiment, TH 1 =220 degrees Celsius. The first temperature TH 1  may be preferably any value within the range of 190 to 270 degrees Celsius, depending on characteristics of the heat fixing device  100 , and be more preferably any value within the range of 200 to 230 degrees Celsius. 
     If the edge overheat has occurred, the control device  300  reduces the output of the halogen lamp  120 . More specifically, the control device  300  reduces a duty ratio of pulse current supplied to the halogen lamp  120 . 
     The control device  300  is configured to selectively perform either a first mode or a second mode as a print mode. When the edge overheat does not occur, the control device  300  performs the first mode in which a plurality of sheets  51  are supplied at first intervals T 10 . On the other hand, when the edge overheat occurs, the control device  300  performs the second mode in which a plurality of sheets  51  are supplied at second intervals T 11  longer than the first intervals T 10 . That is, when the edge overheat occurs, the control device  300  controls the conveyance mechanism M 1  to delay the conveyance timing of the sheets  51 . 
     In the first mode, the time from the conveyance of the sheet  51  to the conveyance of the subsequent sheet  51  is set to the first interval T 10 . The first intervals T 10  can be arbitrarily determined based on results of experiments, simulation, or the like. 
     In the second mode, the time from the conveyance of the sheet  51  to the conveyance of the subsequent sheet  51  is set to the second interval T 11 . More specifically, the control device  300  controls the conveyance mechanism M 1  to convey the sheet  51  in the second mode after the second interval T 11  has elapsed since the start of the conveying of the previous sheet  51 . The second interval T 11  is a period of time larger than or equal to the time required for the heat to be transferred from the right and left end portions of the fusing belt  110  toward the central portion thereof upon the occurrence of the edge overheat. The second interval T 11  can be arbitrarily determined based on results of experiments or simulation. Incidentally, the control device  300  of the present embodiment is initially set in the first mode. When the edge overheat has occurred in the initial state or in the first mode, the control device  300  then changes the print mode from the first mode to the second mode. 
     The control device  300  control the motor  400  to be continuously turned ON, i.e., rotates the fusing belt  110  so as to follow the rotation of the pressure roller  150 , and control the conveyance mechanism M 1  not to supply the sheet  51  during the second interval T 11  of the second mode. Therefore, when the edge overheat has occurred, the rotation of the fusing belt  110  has been continued to agitate the air, thereby dispersing the heat. The situation where the control device  300  controls the conveyance mechanism M 1  not to supply the sheet  51  means the conveyance mechanism M 1  suspends or forbids the supply of sheets  51  after the control device receives the print command. 
     When a sheet  51  having a width larger than or equal to a predetermined width W in the right-left direction is to be printed, the control device  300  determines whether the end-portion temperature TS obtained from at least one of side thermistors  210  is lower than or equal to a second temperature TH 2  lower than the target temperature TH 0  after a predetermined time T 00  has elapsed since the halogen lamp  120  is turned ON. The second temperature TH 2  is a temperature at which the favorable heat fixing cannot be performed. The second temperature TH 2  can be arbitrarily determined based on results of experiments or simulation. The predetermined time T 00  is, for example, the time required for the end-portion temperature TS and the center temperature TC to rise to the target temperature TH 0  after the halogen lamp  120  is turned ON in a low-temperature environment. The predetermined time T 00  can be arbitrarily determined based on results of experiments or simulation. The predetermined width W is larger than the minimum paper width W 2  and smaller than the maximum paper width W 1 . According to the present embodiment, TH 2 =160 degrees Celsius. The second temperature TH 2  may be preferably any value within the range of 130 to 200 degrees Celsius, depending on characteristics of the heat fixing device  100 , and be more preferably any value within the range of 150 to 180 degrees Celsius. 
     If the end-portion temperature TS is lower than or equal to the second temperature TH 2 , the control device  300  increases the output of the halogen lamp  120 . More specifically, the control device  300  increases a duty ratio of pulse current supplied to the halogen lamp  120 . 
     When a sheet  51  having a width larger than or equal to the predetermined width W in the right-left direction is to be printed, the control device  300  determines whether the end-portion temperature TS obtained from at least one of side thermistors  210  is larger than or equal to a third temperature TH 3  lower than the target temperature TH 0  after the predetermined time T 00  has elapsed since the halogen lamp  120  is turned ON. In this case, the third temperature TH 3  of the present embodiment is a temperature within a range higher than the second temperature TH 2  and slightly lower than the target temperature TH 0 . The third temperature TH 3  can be arbitrarily determined based on results of experiments or simulation. The third temperature TH 3  may be equal to the second temperature TH 2 . According to the present embodiment, TH 3 =170 degrees Celsius. The third temperature TH 3  may be preferably any value within the range of 140 to 210 degrees Celsius, depending on characteristics of the heat fixing device  100 , and be more preferably any value within the range of 160 to 190 degrees Celsius. 
     When the end-portion temperature TS is larger than or equal to the third temperature TS 3 , the control device  300  controls the conveyance mechanism M 1  to start conveying a sheet  51 , that is, the pickup roller  52  conveys the sheet  51 . 
     The control device  300  having the above configuration performs the control processes in accordance with a flowchart shown in  FIGS. 5 and 6 . The halogen lamp  120  is basically controlled by a normal control process in which the detection temperature of the center thermistor  180  is maintained substantially constant based on signals from the center thermistor  180 . Upon starting a temperature control process shown in  FIGS. 5 and 6  for controlling the halogen lamp  120 , the temperature control process is applied instead of the normal control process. At the time of print operation, the process returns to the normal control process. 
     As shown in  FIG. 5 , the control device  300  determines whether to receive a print command (S 1 ). If not (S 1 : No), then the control device  300  ends the temperature control process (See  FIG. 6 ). If so (S 1 : Yes), the halogen lamp  120  is turned ON (S 2 ), and then the motor  400  is turned ON after a predetermined small amount of time has elapsed from step S 2  (S 3 ). The output of the halogen lamp  120  at step S 2  is smaller than a maximum output thereof. 
     After step S 3 , the control device  300  determines whether the width of the sheet  51  is larger than or equal to the predetermined width W (S 4 ). If the width of the sheet  51  is larger than or equal to the predetermined width W (S 4 : Yes), then the control device  300  determines whether the predetermined time T 00  of time has elapsed (S 5 ). At step S 5 , if the predetermined time T 00  of time has not elapsed (S 5 : No), the process of step S 5  is repeatedly performed. 
     If the predetermined time T 00  of time has elapsed (S 5 : Yes), the control device  300  determines whether the end-portion temperature TS is lower than or equal to the second temperature TH 2  (S 6 ). If the end-portion temperature TS is lower than or equal to the second temperature TH 2  (S 6 : Yes), the control device  300  increases the duty ratio of pulse current supplied to the halogen lamp  120 , i.e., the output of the halogen lamp  120  (S 7 ). 
     If the width of the sheet  51  is not larger than or equal to the predetermined width W (S 4 : No), or if the end-portion temperature TS at step S 6  is not lower than or equal to the second temperature TH 2  (S 6 : No), or after the process of step S 7  is performed, as shown in  FIG. 6 , the control device  300  determines whether the end-portion temperature TS is larger than or equal to the first temperature TH 1  (S 8 ). 
     At step S 8 , if the end-portion temperature TS is larger than or equal to the first temperature TH 1  (S 8 : Yes), the control device  300  determines that the edge overheat has occurred, sets an overheat flag ON (S 9 ), and then decreases the duty ratio of pulse current supplied to the halogen lamp  120  (S 10 ). The control device  300  set the print mode to the second mode (S 11 ). 
     In step S 8 , if the end-portion temperature TS is not larger than or equal to the first temperature TH 1  (S 8 : No), the control device  300  sets the overheat flag OFF (S 12 ) and sets the print mode to the first mode (S 13 ). 
     After step S 11  and step S 13 , the control device  300  determines whether the width of the sheet  51  is larger than or equal to the predetermined width W (S 14 ). If the width of the sheet  51  is larger than or equal to the predetermined width W (S 14 : Yes), the control device  300  determines whether the end-portion temperature TS is larger than or equal to the third temperature TH 3  (S 15 ). If the end-portion temperature TS is not larger than or equal to the third temperature TH 3  (S 15 : No), the control device  300  repeatedly performs step S 15 . If the end-portion temperature TS is larger than or equal to the third temperature TH 3  (S 15 : Yes), the control device  300  performs a print control process in the print mode set in step S 11  or S 13  (S 16 ). In step S 16 , one paper sheet is printed under the print control process among the paper sheets specified by the print command. In step S 14 , if the width of the sheet  51  is not larger than or equal to the predetermined width W (S 14 : No), the control device  300  performs the process of step S 16  without carrying out the process of step S 15 . 
     After step S 16 , the control device  300  determines whether or not all sheets specified by the print command have been completely printed (S 17 ). If the printing for the print command is not yet completed (S 17 : No), the control device  300  returns to the process of step S 8 . If the printing for the print command is completed (S 17 : Yes), the control device  300  ends the print control process. 
     With reference to  FIG. 7 , changes of each parameter over time will be described in a state where the end-portion temperature TS is larger than the second temperature TH 2  and lower than the third temperature TH 3  after the predetermined time T 00  of time has passed since the control device  300  receives the print command for printing a plurality of sheets  51  having a width larger than or equal to the predetermined width W. In  FIGS. 7 to 9 , the end-portion temperature TS is indicated by solid line, and the center temperature TC by dashed line. 
     When the control device  300  receives the print command (time t 11 ), the halogen lamp  120  is turned ON and the output of the halogen lamp  120  is set to a predetermined value (PD value), and subsequently the motor  400  is turned ON (time t 12 ). When the predetermined time T 00  has elapsed from time t 11  (time t 13 ), the output of the halogen lamp  120  is maintained at the predetermined value if the end-portion temperature TS is larger than the second temperature TH 2 . If the end-portion temperature TS is lower than the third temperature TH 3 , the conveyance of the sheet  51  by the pickup roller  52  is suspended until the end-portion temperature TS reaches the third temperature TH 3  (time t 13  to t 14 ). The end-portion temperature TS and the center temperature TC gradually rise due to the constant output of the halogen lamp  120 . 
     After the end-portion temperature TS reaches the third temperature TH 3  (time t 14 ), the pickup roller  52  conveys the sheets  51  at the first intervals T 10  in the first mode. The control device  300  controls the halogen lamp  120  to maintain the center temperature TC at the target temperature TH 0  by adjusting the output of the halogen lamp  120  from time t 14  to time t 19 . After the print control process comes to an end (time t 19 ), the halogen lamp  120  is turned OFF, and thereafter the motor  400  is turned OFF (time t 20 ). 
     With reference to  FIG. 8 , changes of each parameter over time will be described when the control device  300  sets the print mode to the second mode during the print control process based on print command for printing a plurality of sheets  51  having a width smaller than the predetermined width W. 
     If the end-portion temperature TS is lower than the first temperature TH 1  after the print command is received at time t 11  and the center temperature TC reaches the target temperature TH 0 , the control device  300  performs the first mode (time t 15 ). In this case, the sheets  51  are conveyed at the first intervals T 10 . Then, if the end-portion temperature TS becomes larger than or equal to the first temperature TH 1  (time t 16 ), the output of the halogen lamp  120  is set to low, and the control device  300  set the print mode to the second mode. At this time, no sheets  51  are conveyed during the second interval T 11  longer than the first interval T 10 , while the motor  400  has continuously driven. As a result, the rotation of the fusing belt  110  stirs the air, thereby dispersing the heat in the end portion thereof. Furthermore, as the output of the halogen lamp  120  becomes smaller, the end-portion temperature TS and the center temperature TC gradually fall. 
     Then, at an appropriate timing during the second interval T 11 , the process returns to the control of the output of the halogen lamp  120  based on the center temperature TC (time t 17 ). After that, as in the case of  FIG. 7 , the print control process comes to an end. 
     With reference to  FIG. 9 , changes of each parameter over time will be described when the end-portion temperature TS is lower than or equal to the second temperature TH 2  after the predetermined time T 00  has elapsed since the control device  300  receives the print command for printing a plurality of sheets  51  having a width larger than or equal to the predetermined width W. 
     If the end-portion temperature TS is lower than or equal to the second temperature TH 2  after the predetermined time T 00  of time has elapsed since the print command is received, the control process is performed based on the end-portion temperature TS, and the output of the halogen lamp  120  is set to high (time t 13 ). In response to increasing the output of the halogen lamp  120 , an increase ratio of the end-portion temperature TS and the center temperature TC, i.e., slope of the temperatures, gradually rise rather than before time t 13 . After the end-portion temperature TS reaches the third temperature TH 3  (time t 18 ), the process returns to the control of the output of the halogen lamp  120  based on the center temperature TC, e.g., the output of the halogen lamp  120  returns to the predetermined value, and then the conveyance of the sheets  51  starts. After that, as in the case of  FIG. 7 , the print control process comes to an end. 
     According to those described above, the present embodiment can achieve the following advantageous effects. 
     The side thermistors  210  are positioned facing the end portions of the outer peripheral surface  110 B of the fusing belt  110 , the end-portion temperature TS can be precisely detected. Moreover, the center thermistor  180  provided at the internal space of the fusing belt  110 , thereby accurately detecting the temperature of the internal space. Moreover, the center thermistor  180  is positioned at the internal space of the fusing belt  110 , thereby accurately detecting the temperature of the internal space of the fusing belt  110 . 
     The center thermistor  180  can accurately detect the temperature of the nip plate  130 . The control device  300  determines that the edge overheat occurs at the end portions of the outer peripheral surface  110 B of the fusing belt  110  based on the side thermistor  210  positioned outside of the fusing belt  110 , which can suppress the effect of the heat generated on the outer peripheral surface  110 B on the pressure roller  150 . 
     If the control device  300  determines that the temperature obtained from at least one of the side thermistors  210  is larger than or equal to the first temperature TH 1 , then the control device  300  determines that the edge overheat occurs and decreases the output of the halogen lamp  120 . Therefore, this configuration can prevent an excessive rise in temperature in the end portions of the fusing belt  110 . 
     If the control device  300  determines that the edge overheat occurs, the conveyance timing of the sheets  51  is delayed. Therefore, the heat at the end portions of the fusing belt  110  can be transferred to the central portion thereof before the subsequent sheet  51  is conveyed. 
     If the control device  300  determines that the edge overheat occurs, the rotation of the fusing belt  110  has been continued. The rotation of the fusing belt  110  agitates the air, dispersing the heat. Therefore, the heat in the internal space of the end portions of the fusing belt  110  can be reduced during the process of not feeding the sheets  51 . 
     If the end-portion temperature TS on the outer peripheral surface  110 B of the fusing belt  110  is low after the predetermined time T 00  has elapsed since the halogen lamp  120  is turned ON, the control device  300  increases the output of the halogen lamp  120 . Therefore, when the sheet  51  having a width larger than or equal to the predetermined width W is printed, the end-portion temperature TS on the outer peripheral surface  110 B of the fusing belt  110  can easily become a suitable temperature, thereby improving an image quality. 
     If the end-portion temperature TS on the outer peripheral surface  110 B of the fusing belt  110  is low, no sheets  51  are conveyed. Therefore, when the sheet  51  having a width larger than or equal to the predetermined width W is printed, the end-portion temperature TS on the outer peripheral surface  110 B of the fusing belt  110  can easily become a suitable temperature, thereby improving an image quality. 
     While the invention has been described in detail with reference to the embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. The same components as those in the above embodiment are represented by the same reference symbols, and will not be described again in the description below. 
     According to the above embodiment, the pair of side thermistors  210  is placed on upstream side in the moving-direction of the fusing belt  110  relative to the pressure roller  150 . However, the present invention is not limited to this configuration. For example, as shown in  FIG. 10A , a fixing frame  202  extends rearward of the fusing belt  110 . A pair of side thermistors  210 A may be placed on downstream side in the moving-direction of the fusing belt  110  relative to the pressure roller  150 . The pair of side thermistors  210 A has an upper portion provided with a fixing rib  213 A fixed to the rear wall of the fixing frame  200  with a screw. Each side thermistor  210 A has a front surface as a temperature detection surface  211 A configured to detect a temperature of the outer peripheral surface  110 B of the fusing belt  110 . 
     Alternatively, as shown in  FIG. 10B , a pair of side thermistors  210 B may be placed on the opposite side of the nip N (pressure roller  150 ) with respect to the fusing belt  110 . The pair of side thermistors  210 B has both end portions each provided with a fixing rib  213 B fixed to the upper wall of the fixing frame  200  with screws. The pair of side thermistors  210 B has a bottom surface as a temperature detection surface  211 B configured to detect a temperature of the outer peripheral surface  110 B of the fusing belt  110 . 
     According to the above embodiment, the center thermistor  180  is fixed to the stay  160  with screws  189 . However, the present invention is not limited to this configuration. For example, as shown in  FIG. 11 , a center thermistor  180  may be urged by a compression spring  220  toward the nip plate  130 . 
     A cover member  170  in this configuration includes a rear wall  175 , a support wall  176 , and an extending wall  177  in addition to the configuration of  FIG. 2 . The rear wall  175  extends downward from a rear end of the upper-side wall  171 , the support wall  176  extends rearward from a lower end of the rear wall  175 , and the extending wall  177  extends downward from a rear end of the support wall  176 . The compression spring  220  is provided between the support wall  176  and the center thermistor  180  placed on the nip plate  130 . The compression spring  220  is held by a projection  176 A extending downward from a lower surface of the support wall  176  and a projection  184  extending upward from an upper surface of the center thermistor  180 . More specifically, the compression spring  220  is held by the projections  176 A and  184  each inserted into the compression spring  220 . The compression spring  220  presses the center thermistor  180  toward the nip plate  130 . Therefore, the center thermistor  180  can detect the temperature of the nip plate  130 . 
     According to the above embodiment, the control device  300  determines whether the edge overheat occurs based on the end-portion temperature TS obtained by at least one of side thermistors  210 . However, the present invention is not limited to this configuration. For example, the control device  300  may be configured to determine whether the center temperature TC acquired from the center thermistor  180  is larger than or equal to a fourth temperature TH 4  after the predetermined time T 00  has elapsed since the control device  300  controls the halogen lamp  120  to be turned ON, and configured to determine whether the end-portion temperature TS obtained by at least one of side thermistors  210  is larger than or equal to the fourth temperature TH 4 . 
     If the control device  300  in this configuration determines that the center temperature TC is not larger than or equal to the fourth temperature TH 4  after the predetermined time T 00  has elapsed, and that the end-portion temperature TS is larger than or equal to the fourth temperature TH 4 , then the control device  300  performs a control process to determine that a failure (edge overheat) has occurred. In order to perform such a control process, the control device  300  determines whether the center temperature TC is not larger than or equal to the fourth temperature TH 4 , and whether the end-portion temperature TS is larger than or equal to the fourth temperature TH 4  in step S 8  of the flowchart shown in  FIGS. 5 and 6 . Only if the center temperature TC is not larger than or equal to the fourth temperature TH 4 , and the end-portion temperature TS is larger than or equal to the fourth temperature TH 4 , the control device  300  then proceeds to step S 9 . In other cases, the control device  300  may proceed to step S 12 . 
     According to the above embodiment, the control device  300  basically controls the halogen lamp  120  to maintain the output of the halogen lamp  120  constant until the center temperature TC reaches the target temperature TH 0 . However, the present invention is not limited to this configuration. For example, the control device may compare the center temperature acquired from the center thermistor with the target temperature. The control device may control the halogen lamp to increase the output thereof as a difference between the target temperature and the center temperature becomes larger. In this case, the halogen lamp may be controlled so as to increase or decrease the output thereof by changing the target temperature of the halogen lamp. 
     According to the above embodiment, the side thermistors  210  are positioned facing the right and left end portions of the outer peripheral surface  110 B of the fusing belt  110 . Instead, a side thermistor  210  may be positioned facing one of end portions of the outer peripheral surface  110 B of the fusing belt  110 . Moreover, the pair of side thermistors  210  is located outside of the maximum paper width W 1  in the right-left direction. Instead, the pair of the side thermistors may be placed inside of the maximum paper width W 1  and outside of the minimum paper width W 2  in the right-left direction. 
     According to the above embodiment, the halogen lamp  120  is illustrated as one example of a heater. However, the present invention is not limited to this configuration. For example, the heater may be an IH (Induction Heating) heater or a ceramic heater. In this case, the IH heater is a device that does not generate heat by itself but uses an electromagnetic induction heating method to heat the metallic fusing belt and the nip plate. 
     According to the above embodiment, the first intervals T 10  and the second intervals T 11  are defined as time. Instead, for example, a distance between sheets may be employed. 
     According to the above embodiment, the thermostat is illustrated as an overheat prevention member. However, the present invention is not limited to this configuration. For example, a fuse may be used. 
     According to the above embodiment, the pressure roller  150  is illustrated as one example of a rotation member. However, the present invention is not limited to this configuration. For example, a belt-like member may be used. 
     According to the above embodiment, the nip plate  130  is illustrated as a nip member. However, the present invention is not limited to this configuration. For example, a thick member that is not a plate may be used as the nip member. 
     According to the above embodiment, the present invention is applied to the color laser printer  1 . However, the present invention is not limited to this configuration. The present invention may be applied to other image formation devices, such as copying devices or multifunctional devices. 
     According to the above embodiment, sheets  51 , such as cardboard, postcards, or thin paper, are illustrated as recording sheets. However, the present invention is not limited to those. For example, OHP sheets may be used. 
     According to the above embodiment, the control device  300  includes single CPU configured to perform the processes of  FIGS. 5 and 6 . However, the present invention is not limited to this configuration. The control device may include a plurality of CPUs configured to perform the processes of  FIGS. 5 and 6 , or may include a hardware circuit, such as ASIC (Application Specific Integrated Circuit) configured to perform the processes of  FIGS. 5 and 6 . The control device may include a CPU and a hardware circuit each configured to perform the processes of  FIGS. 5 and 6 .