Patent Publication Number: US-2009226201-A1

Title: Fixing device and temperature controlling method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority U.S. provisional application 61/034,903, filed on Mar. 7, 2008, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a fixing device, and, more particularly to a fixing device in which a heating member such as a belt or a heating roller is heated in each of different plural areas in a rotation axis direction. 
     BACKGROUND 
     In the past, there is known a technique for heating, in a fixing device, in order to make it possible to maintain temperature distribution in a rotation axis direction of a heating member such as a belt or a heating roller substantially the same, the heating member using plural coils and changing distribution of electric energy among the plural coils to thereby set the temperature distribution substantially the same (JP-A-2000-206813 and JP-A-2001-312178). 
     However, since electric currents having different frequencies are simultaneously supplied to the respective coils, interference sound occurs. 
     There is also known a technique for, rather than simultaneously feeding electric currents to plural coils, feeding an electric current to any one of the plural coils to heat a heating member (JP-A-2004-6353). There is also a technique for setting, in feeding an electric current to any one of plural coils to heat a heating member, timing for feeding electric currents to the respective coils to be equal to or larger than a minimum time interval to thereby enable more precise temperature control (JP-A-2004-273454). 
     However, in order to quicken rise to fixing temperature for toner image fixing and realize a reduction in size of a fixing device, a heating member with a heat capacity reduced from that in the past is used. Therefore, in order to reduce a difference in temperature distribution in a rotation axis direction of such a heating member with the reduced heat capacity, extremely long time is required when the temperature control techniques in the past are used. 
     SUMMARY 
     It is an object of an embodiment of the present invention to provide a temperature control technique that can keep temperature distribution in a rotation axis direction of a heating member such as a belt or a heating roller substantially constant more easily than in the past. 
     In order solve the problems, according to an aspect of the present invention, there is provided a fixing device including: a heating roller; a stretching and suspending roller that rotates around a rotation axis parallel to a rotation axis of the heating roller; a belt wound and suspended around the heating roller and the stretching and suspending roller; a pressing roller that nips and conveys, in cooperation with the belt, a sheet having a developer image formed thereon and heats and fixes the developer image on the sheet; plural heaters that heat plural areas different from one another in the rotation axis direction on the belt; a temperature-information acquiring unit that acquires temperature information concerning temperatures of areas on the belt heated by at least two among the plural heaters; and a heating control unit that alternately drives the respective plural heaters and sets, on the basis of the information acquired by the temperature-information acquiring unit, driving time for the heater that heats a second area having temperature lower than that of a first area among the plural areas longer than driving time for the heater that heats the first area. 
     According to another aspect of the present invention, there is provided a fixing device including: a heating roller; a pressing roller that nips and conveys, in cooperation with the heating roller, a sheet having a developer image formed thereon and heats and fixes the developer image on the sheet; plural heaters that heat plural areas different from one another in a rotation axis direction of the heating roller; a temperature-information acquiring unit that acquires temperature information concerning temperatures of areas on the heating roller heated by at least two among the plural heaters; and a heating control unit that alternately drives the respective plural heaters and sets, on the basis of the information acquired by the temperature-information acquiring unit, driving time for the heater that heats a second area having temperature lower than that of a first area among the plural areas longer than driving time for the heater that heats the first area. 
     According to still another aspect of the present invention, there is provided a temperature control method in a fixing device including a heating roller, a stretching and suspending roller that rotates around a rotation axis parallel to a rotation axis of the heating roller, a belt wound and suspended around the heating roller and the stretching and suspending roller, a pressing roller that nips and conveys, in cooperation with the belt, a sheet having a developer image formed thereon and heats and fixes the developer image on the sheet, and plural heaters that heat plural areas different from one another in the rotation axis direction on the belt, the temperature control method including: acquiring temperature information concerning temperatures of areas on the heating roller heated by at least two among the plural heaters; and alternately driving the respective plural heaters and setting, on the basis of the acquired temperature information, driving time for the heater that heats a second area having temperature lower than that of a first area among the plural areas longer than driving time for the heater that heats the first area. 
     According to still another aspect of the present invention, there is provided a temperature control program in a fixing device including a heating roller, a stretching and suspending roller that rotates around a rotation axis parallel to a rotation axis of the heating roller, a belt wound and suspended around the heating roller and the stretching and suspending roller, a pressing roller that nips and conveys, in cooperation with the belt, a sheet having a developer image formed thereon and heats and fixes the developer image on the sheet, and plural heaters that heat plural areas different from one another in the rotation axis direction on the belt, the temperature control program causing a computer to execute processing for: acquiring temperature information concerning temperatures of areas on the belt heated by at least two among the plural heaters; and alternately driving the respective plural heaters and setting, on the basis of the acquired temperature information, driving time for the heater that heats a second area having temperature lower than that of a first area among the plural areas longer than driving time for the heater that heats the first area. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional view of a schematic configuration of an image forming apparatus mounted with a fixing device according to a first embodiment of the present invention; 
         FIG. 2  is a longitudinal sectional view of a schematic configuration of the fixing device according to the first embodiment; 
         FIG. 3  is a front view of a schematic configuration in a belt rotation axis direction of the fixing device according to the first embodiment; 
         FIG. 4  is a block diagram of an electric configuration for control of temperature detection, excitation coils, and an oscillation circuit (an inverter circuit) of the fixing device according to the first embodiment; 
         FIG. 5  is a functional block diagram for explaining functions of the fixing device according to the first embodiment; 
         FIG. 6  is a driving control table corresponding to warm-up and ready states in the fixing device according to the first embodiment; 
         FIG. 7  is a driving control table corresponding to a paper passing state in the fixing device according to the first embodiment; 
         FIG. 8  is a flowchart of processing of excitation coil driving control corresponding to an operation state of the image forming apparatus mounted with the fixing device according to the first embodiment; 
         FIG. 9  is a flowchart of processing of excitation coil driving control in the warm-up state of the image forming apparatus mounted with the fixing device according to the first embodiment; 
         FIG. 10  is a flowchart of processing of electric energy control for the excitation coils in the ready state of the image forming apparatus mounted with the fixing device according to the first embodiment; 
         FIG. 11  is a flowchart of processing of electric energy control for the excitation coils in the paper passing state of the image forming apparatus mounted with the fixing device according to the first embodiment; 
         FIG. 12  is a flowchart of processing of excitation coil driving control in the paper passing state of the image forming apparatus mounted with the fixing device according to the first embodiment; 
         FIG. 13  is a flowchart of processing of excitation coil driving control corresponding to an operation state of an image forming apparatus mounted with a fixing device according to a second embodiment of the present invention; 
         FIG. 14  is a flowchart of processing of excitation coil driving control at conveying speed of 135 m/s in a paper passing state of the image forming apparatus mounted with the fixing device according to the second embodiment; 
         FIG. 15  is a driving control table corresponding to conveying speed of 135 m/s in the fixing device according to the second embodiment; 
         FIG. 16  is a flowchart of processing of excitation coil driving control corresponding to an operation state of an image forming apparatus mounted with a fixing device according to a third embodiment of the present invention; 
         FIG. 17  is a flowchart of processing of excitation coil driving control in a paper passing state of the image forming apparatus mounted with the fixing device according to the third embodiment in which twenty or more small-size sheets are continuously passed; 
         FIG. 18  is a driving control table corresponding to continuous paper passing of twenty or more small-size sheets in the fixing device according to the third embodiment; 
         FIG. 19  is a functional block diagram for explaining functions of a fixing device according to a fourth embodiment of the present invention; 
         FIG. 20  is a flowchart of processing of excitation coil driving control corresponding to internal temperature of an image forming apparatus mounted with the fixing device according to the fourth embodiment; 
         FIG. 21  is a flowchart of processing of excitation coil driving control at internal temperature equal to or lower than 10° C. of the image forming apparatus mounted with the fixing device according to the fourth embodiment; 
         FIG. 22  is a flowchart of processing of excitation coil driving control at internal temperature higher than 10° C. of the image forming apparatus mounted with the fixing device according to the fourth embodiment; 
         FIG. 23  is a driving control table corresponding to internal temperature equal to or lower than 10° C. of the image forming apparatus in the fixing device according to the fourth embodiment; 
         FIG. 24  is a driving control table corresponding to internal temperature higher than 10° C. of the image forming apparatus in the fixing device according to the fourth embodiment; 
         FIG. 25  is a front view of a schematic configuration in a belt rotation axis direction of a fixing device according to a fifth embodiment of the present invention; and 
         FIG. 26  is a front view of a schematic configuration on the belt rotation axis direction of the fixing device according to the fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are explained below with reference to the accompanying drawings. 
     First Embodiment 
     First, a first embodiment of the present invention is explained. 
       FIG. 1  is a longitudinal sectional view of a schematic configuration of an image forming apparatus (Multi Function Peripheral (MFP)) mounted with a fixing device according to the first embodiment of the present invention. 
     As shown in  FIG. 1 , the image forming apparatus according to this embodiment includes an image scanning unit R and an image forming unit P. 
     The image scanning unit R has a function of scanning an image of a sheet original document and a book original document. 
     The image forming unit P has a function of forming a developer image on a sheet on the basis of an image scanned from an original document by the image scanning unit R, image data transmitted from an external apparatus to the image forming apparatus, and the like. 
     The image scanning unit R includes an auto document feeder (ADF)  9  that can automatically feed an original document to a predetermined image scanning position. The image scanning unit R scans, using a scanning optical system  10 , images of an original document automatically fed by the auto document feeder  9  and an original document placed on a document table. 
     The image forming unit P includes pickup rollers  61  to  64 , photoconductive members  2 Y to  2 K, developing rollers  3 Y to  3 K, mixers  4 Y to  4 K, an intermediate transfer belt  6 , a fixing device  7 , and a discharge tray  8 . 
     A CPU  45  has a role of performing various kinds of processing in the image processing apparatus and also has a role of realizing various functions by executing programs stored in a memory  54 . 
     The memory  54  can be, for example, a RAM (Random Access Memory), a ROM (Read Only Memory), a DRAM (Dynamic Random Access Memory), an SRAM (Static Random Access Memory), or a VRAM (Video RAM). The memory  54  has a role of storing various kinds of information and programs that are used in the image processing apparatus. 
     As an example of the fixing device according to this embodiment, an overview of copy processing in the image forming apparatus mounted with the fixing device is explained. 
     First, sheets picked up from cassettes by the pickup rollers  61  to  64  are fed into a sheet conveying path. The sheets fed into the sheet conveying path are conveyed in a predetermined conveying direction by plural roller pairs. 
     Images of plural sheet original documents automatically conveyed continuously by the auto document feeder  9  are scanned by the scanning optical system  10  in the predetermined image scanning position. 
     On the basis of image data of the images scanned from the original documents by the image scanning unit R, electrostatic latent images are formed on photoconductive surfaces of the photoconductive members  2 Y,  2 M,  2 C, and  2 K for transferring developer images of yellow (Y), magenta (M), cyan (C), and black (K) onto the sheets. 
     Subsequently, developers agitated by the mixers  4 Y to  4 K (equivalent to agitating units) in a developing device are supplied to the photoconductive members  2 Y to  2 K, on which the electrostatic latent images are formed as explained above, by the developing rollers (so-called mug rollers)  3 Y to  3 K. Consequently, the electrostatic latent images formed on the photoconductive surfaces of the photoconductive members  2 Y,  2 M,  2 C, and  2 K are visualized. 
     Developer images formed on the photoconductive members  2 Y,  2 M,  2 C, and  2 K in this way are transferred onto a belt surface of the intermediate transfer belt  6  (so-called primary transfer). The developer images carried according to the rotation of the intermediate transfer belt  6  are transferred on to the conveyed sheets in a predetermined secondary transfer position T. 
     The developer images transferred onto the sheets are heated and fixed on the sheets by the fixing device  7 . 
     The sheets on which the developer images are heated and fixed are conveyed through a conveying path by plural conveying roller pairs and sequentially discharged onto the discharge tray  8 . 
     Details of the fixing device  7  according to the first embodiment are explained below. 
       FIG. 2  is a sectional view of a schematic configuration of the fixing device  7  according to the first embodiment. 
     The fixing device  7  includes a heating roller  11  (φ 50 mm), a pressing roller  12  (φ 50 mm), a stretching and suspending roller  13  (φ 18 mm), a belt  14 , and a heating unit  21 . 
     The pressing roller  12  is driven in an arrow direction by a driving motor (not shown). The heating roller  11 , the stretching and suspending roller  13 , and the belt  14  are driven to rotate in the arrow direction. The pressing roller  12  is set in press contact with the heating roller  11  across the belt  14  by a pressing mechanism  15  and maintained to have fixed nip width. Therefore, in the first embodiment, the heating roller  11  does not come into direct contact with a sheet. 
     The stretching and suspending roller  13  is arranged further on a downstream side in a sheet conveying direction than the heating roller  11  and rotates around a rotation axis parallel to a rotation axis of the heating roller  11 . The belt  14  is wound and suspended between the heating roller  11  and the stretching and suspending roller  13  with predetermined tension by a tension mechanism. Heating parts  48  are arranged over the periphery of the heating roller and heat the belt  14 . 
     The heating roller  11  includes, in order from an inner side, a core bar  11   a  and foamed rubber (sponge)  11   b.  In the first embodiment, core bar thickness is set to 2 mm and foamed rubber thickness is set to 5 mm. 
     The belt  14  includes, in order from an inner side, a metal conductive layer  14   a,  a solid rubber layer  14   b,  and a release layer  14   c.  In the first embodiment, nickel (40 μm) is used as a material of the metal conductive layer  14   a.  Besides, stainless steel, aluminum, a composite material of stainless steel and aluminum, and the like may be used. In the first embodiment, the solid rubber layer  14   b  is formed of 200 μm of silicon rubber and the release layer  14   c  is formed of 30 μm of a PFA tube. 
     The pressing roller  12  is configured by coating a core bar with silicon rubber, fluorine rubber, or the like. 
     The stretching and suspending roller  13  is configured by coating the surface of a metal pipe  13   a  with a coating layer  13   b.  As a material of the metal pipe  13   a,  in this embodiment, aluminum is used. However, the material may be iron, copper, stainless steel, and the like. A heat pipe and the like having higher thermal conductivity may be used instead of the metal pipe  13   a.    
     When a sheet P passes a fixing point as a press contact portion (a nip portion) between the heating roller  11  and belt  14  and the pressing roller  12 , a developer on the sheet is fused and pressed to be fixed. 
     A peeling blade  16   a  that peels the sheet P from a belt surface of the belt  14  is provided further on a downstream side in a rotating direction than a contact position (the nip portion) between the belt  14  and the pressing roller  12 . A peeling blade  16   b  that peels the sheet P from the pressing roller  12  is provided further on the downstream side in the rotating direction than the nip portion. 
     Plural non-contact temperature sensors  17  ( 17   a  and  17   b ) are arranged in a position near the belt  14  on the stretching and suspending roller  13  and different from one another in the rotation axis direction of the stretching and suspending roller  13 . Although the two sensors are arranged in the first embodiment as an example, three or more sensors maybe arranged. In the first embodiment, as the non-contact temperature sensors  17 , a thermopile type for detecting an infrared ray is used. The non-contact temperature sensors  17 , more specifically, a thermopile  17   a  and a thermopile  17   b  detect temperatures of plural heating target areas on the surface of the belt  14 . 
     A configuration of the heating unit  21  according to the first embodiment is specifically explained ( FIG. 3 ). 
     The heating unit  21  is an induction heating member including plural excitation coils ( 21   a,    21 - 1 , and  21 - 2 ). 
     As shown in  FIG. 3 , as the heating unit  21 , an induction heating type for performing heating making use of electromagnetic induction is used. In the heating unit  21 , an excitation coil (an electromagnetic induction coil) is divided into three areas in one rotation axis direction. End coils  21 - 1  and  21 - 2  except a center coil  21   a  are connected in series. The end coils  21 - 1  and  21 - 2  are described as end coils  21   b  below. 
     The coils  21   a  and  21   b  intensify a magnetic field using a magnetic core  22  such that the coils  21   a  and  21   b  can display performance even if the number of windings of an electric wire is reduced. 
     Magnetic fluxes can be concentrated by this coil shape. The belt  14  is locally concentratedly heated. 
     In this embodiment, the belt  14  is heated by alternately driving the plural coils  21   a  and  21   b  (so-called alternate lighting). Driving of the coils is explained later. 
     The configuration of the excitation coils is more specifically explained. As the excitation coils  21   a  and  21   b,  a copper wire material having the diameter of 0.5 mm is used. The excitation coils  21   a  and  21   b  are formed as litz wires obtained by binding plural wire materials insulated from one another. Since the excitation coils  21   a  and  21   b  are formed as the litz wires, it is possible to further reduce the wire diameter to be smaller than penetration depth and effectively feed an alternating current. In the first embodiment, sixteen wire materials having the diameter φ 0.5 mm are bound. As a coating wire for the coils, heat-resistant polyamide-imide is used. 
     In the first embodiment, magnetic fluxes and eddy-currents are generated on the belt  14  to prevent a change in a magnetic field by magnetic fluxes generated by a high-frequency current applied the excitation coils  21   a  and  21   b  from a not-shown excitation circuit (an inverter circuit). Joule heat is generated by the eddy-currents and heating roller resistance and the belt  14  is heated. In the first embodiment, an electric current having a high frequency in a range of 20 to 100 kHz is fed to the excitation coils  21   a  and  21   b.  An output can be changed from 200 W to 1500 W by changing a driving frequency of the inverter circuit. 
     The excitation coils  21   a  and  21   b  respectively heat the plural heating target areas on the belt  14 . In this specification, an area heated by the excitation coil  21   a  is referred to as “center” and areas heated by the excitation coils  21   b  are referred to as “ends”. When the center coil  21   a  is driven, an eddy-current is generated in the center of the belt  14 , the center of the belt  14  is heated by Joule heat, and the temperature thereof rises. On the other hand, when the end coils  21   b  are driven, eddy-currents are generated at the ends of the belt  14 , the ends of the belt  14  are heated by Joule heat, and the temperature thereof rises. 
     In the first embodiment, according to detected temperatures in the thermopiles  17   a  and  17   b  as temperature sensors, the coils  21   a  and  21   b  are selectively switched and driven to raise the temperature of the belt  14 , whereby control temperature for fixing is maintained. 
     When the belt  14  is heated, usually, the belt  14  is rotated together with the pressing roller  12 , the heating roller  11 , and the stretching and suspending roller  13 . 
       FIG. 4  is a diagram of an overview of an electric configuration concerning a control method for temperature detection, the excitation coils, and an oscillation circuit (an inverter circuit). 
     In the first embodiment, capacitors  31  and  32  for resonance are connected to the excitation coils  21   a  and  21   b  shown in  FIG. 1  in parallel to each other. Switching elements  33  and  34  are connected to this resonant circuit to configure an inverter circuit. As the switching elements  33  and  34 , IGBT (Insulated Gate Bipolar Transistor), MOS-FET, or the like used under high withstanding pressure and large current is used. In the first embodiment, IGBT is used. 
     DC power obtained by smoothing a commercial AC power supply  35  with a rectifying circuit  36  is supplied to the inverter circuit. A transformer  37  is arranged at a pre-stage of the rectifying circuit  36 . Total power consumption can be detected via the input detecting unit  37   a.  Electric power is fed back by this power detection. 
     Driving circuits  38  and  39  are connected to control terminals of the switching elements  33  and  34 , respectively. The driving circuits  38  and  39  apply driving voltage to the control terminals of the switching elements  33  and  34  to turn on the switching elements. Control circuits  41  and  42  output timing for the application of the driving voltage. The control circuits  41  and  42  control ON time, change a frequency in a range of 20 to 100 kHz, and change an output value. 
     The thermopiles  17   a  and  17   b  that detect temperature as explained above are arranged in a heated object (in the first embodiment, the belt  14 ) heated by the coils  21   a  and  21   b.  Temperature detection signals (voltage values) of the thermopiles are input to the CPU  45 . According to values of the thermopiles  17   a  and  17   b,  the CPU  45  sends, to the control circuits  41  and  42 , a command for instructing which coil ( 21   a  or  21   b ) should be driven, whether all the coils ( 21   a  and  21   b ) should be turned off, and to which value an output value should be set. 
     Functions related to driving control for the excitation coils of the fixing device according to the first embodiment are explained with reference to  FIG. 5 . Functions of respective blocks in a functional block diagram shown in  FIG. 5  are realized by causing the CPU  45  to execute, for example, various computer programs stored in the memory  54 . 
     As shown in  FIG. 5 , the driving control for the excitation coils of the fixing device according to the first embodiment is performed by a temperature-information acquiring unit  51 , a heating control unit  52 , and an operation-information acquiring unit  53 . 
     The temperature-information acquiring unit  51  acquires, as temperature information, the temperature of the belt  14  detected by the thermopiles  17 , more specifically, the thermopile  17   a  that detects the temperature in the center of the belt  14  and the thermopile  17   b  that detects the temperature at the ends of the belt  14 , and sends the temperature information to the heating control unit  52 . 
     The operation-information acquiring unit  53  acquires information concerning an operation state such as a warm-up state, a ready state, or a paper passing state of an image forming apparatus  1  mounted with the fixing device  7  according to the first embodiment from the inside or the outside of the image forming apparatus  1  and sends the information to the heating control unit  52 . 
     Driving control tables concerning driving times for the excitation coils associated with temperature differences between the center of the belt  14  heated by the excitation coil  21   a  and the ends of the belt  14  heated by the excitation coils  21   b  ( 21 - 1  and  21 - 2 ) and the operation states of the image forming apparatus  1  are stored in the memory  54 . A driving control table in the warm-up and ready states of the image forming apparatus  1  as an example of the driving control table is shown in  FIG. 6 . A driving control table in the paper passing state of the image forming apparatus  1  is shown in  FIG. 7 . 
     The heating control unit  52  controls driving of the excitation coils  21   a  and  21   b  of the heating unit  21  on the basis of the temperature information acquired from the temperature-information acquiring unit  51  and the information concerning the operation state of the image forming apparatus  1  acquired from the operation-information acquiring unit  53  to thereby control temperature in the rotation axis direction of the belt  14 . Specifically, the heating control unit  52  alternately drives the excitation coils  21   a  and  21   b  of the heating unit  21  and sets, on the basis of the temperature information acquired by the temperature-information acquiring unit  51 , driving time for the excitation coil(s) that heats (heat) a second area (one of the center and the ends) having temperature lower than that of a first area (the other of the center and the ends) on the surface of the belt  14  longer than driving time for the excitation coil(s) that heats (heat) the first area. 
     An example of the temperature control by the heating control unit  52  is more specifically explained. First, the heating control unit  52  determines an operation state of the image forming apparatus  1  on the basis of the information concerning the operation state of the image forming apparatus  1  acquired from the operation-information acquiring unit  53 . Subsequently, the heating control unit  52  calculates a temperature difference between the first area and the second area of the belt  14  on the basis of the temperature information acquired from the temperature-information acquiring unit  51 . On the basis of the calculated temperature difference and the operation state of the image forming apparatus  1  and referring to the driving control tables stored in the memory  54 , the heating control unit  52  selects, as driving time for the excitation coils that heat the first and second areas, driving time associated with the temperature difference and the operation state among plural kinds of driving times set as driving times of the excitation coils that heat the first and second areas. 
     In the first embodiment, concerning plural kinds of driving times for the excitation coil(s) that heats (heat) the second area having temperature lower than that of the first area on the surface of the belt  14 , driving time for the excitation coil(s) that heats (heat) the second area is set longer as operation states associated with the plural kinds of driving times have a larger degree of causing a temperature difference between the areas in the rotation axis direction on the belt  14 . For example, in the paper passing state, a degree of causing a temperature difference is large compared with those in the warm-up and ready states. Therefore, as shown in  FIGS. 6 and 7 , even when a temperature difference is in the same range, it is preferable to set driving time for the excitation coil(s) that heats (heat) the second area corresponding to the paper passing state longer than driving time for the excitation coil(s) that heats (heat) the second area corresponding to the warm-up state and the read state. 
     As explained above, driving time for the excitation coil(s) that heats (heat) an area having lower temperature in the paper passing state is set longer than driving time for the excitation coils(s) in the warm-up and the ready state. This is explained more in detail below. 
     During paper passing, a ratio of a heat quantity deprived in the rotation axis direction of the belt  14  in the fixing device  7  substantially changes according to a size of a sheet subjected to fixing and conveyed. When a sheet having width far smaller than the width of the belt  14  in the rotation axis direction such as an A4R, A5, or B5 sheet is passed, heat is deprived on the belt  14  differently in the center and at the ends. 
     More specifically, a sheet of the size A4R, B5, A5, or the like relatively small in the rotation axis direction comes into contact with only the center in the rotation axis direction of the belt  14  as a paper passing area. Therefore, the temperature in the center tends to be low compared with those at the ends. The temperature at the ends on the belt  14  tends to be higher than control temperature explained later. 
     Therefore, when the image forming apparatus  1  is in the paper passing state, the temperature difference rather increases even if the driving control table corresponding to the warm-up and ready states is used. 
     Therefore, in the first embodiment, driving time corresponding to the paper passing state is set longer than driving times corresponding to the warm-up and ready states. Further, in the first embodiment, driving time at a temperature difference equal to or larger than 20° C. is set in the driving control table corresponding to the paper passing state. 
     Consequently, it is possible to keep temperature distribution in the rotation axis direction of the belt  14  substantially constant even when a small-size sheet is passed. 
     An example of a flowchart of processing by the fixing device  7  according to the first embodiment is explained. 
     The heating control unit  52  determines, according to an operation state of the image forming apparatus  1 , based on which of the driving control tables stored in the memory  54  driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  should be performed. 
     Specifically, as shown in  FIG. 8 , in Act  101 , the heating control unit  52  determines, on the basis of information concerning an operation state of the image forming apparatus  1  acquired from the operation-information acquiring unit  53 , whether the operation state of the image forming apparatus  1  is the warm-up state. If the operation state of the image forming apparatus  1  is the warm-up state, the heating control unit  52  proceeds to (1) and performs, on the basis of processing of a flowchart shown in  FIG. 9 , driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  using the driving control table corresponding to the warm-up and ready states. 
     If it is determined in Act  101  that the operation state of the image forming apparatus  1  is not the warm-up state, in Act  102 , the heating control unit  52  determines, on the basis of the information concerning the operation state of the image forming apparatus  1  acquired from the operation-information acquiring unit  53 , whether the operation state of the image forming apparatus  1  is the ready state. If the operation state of the image forming apparatus  1  is the ready state, the heating control unit  52  proceeds to (2) and performs, on the basis of the processing of the flowchart shown in  FIG. 9 , driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  using the driving control table corresponding to the warm-up and ready states. 
     On the other hand, if it is determined in Act  102  that the operation state of the image forming apparatus  1  is not the ready state, the heating control unit  52  proceeds to (3), determines that the image forming apparatus  1  is in the paper passing state, and performs, on the basis of processing of a flowchart shown in  FIG. 12 , driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  using the driving control table corresponding to the paper passing state. 
     Driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  in the warm-up state of the image forming apparatus  1  is explained in detail with reference to  FIG. 9 . 
     First, in Act  201 , the temperature-information acquiring unit  51  acquires, as temperature information, the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  and sends the temperature information to the heating control unit  52 . In Act  202 , the heating control unit  52  determines, on the basis of the temperature information acquired by the temperature-information acquiring unit  51 , whether the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  reaches control temperature (in the first embodiment, 160° C.). In the first embodiment, the detection of temperature is executed by the CPU  45  at every 200 (ms). 
     If the temperature exceeds 160° C. at this point, the image forming apparatus  1  finishes the warm-up state and enters (returns to) the ready. 
     If the temperature does not exceed 160° C. in Act  202 , first, in Act  203 , the heating control unit  52  sets an output of the heating unit  21  to 1300 W. Subsequently, in Act  204 , the heating control unit  52  executes, on the basis of the temperature information of the belt  14 , comparison of temperatures in the center and at the ends of the belt  14  detected by the thermopiles  17   a  and  17   b.  The heating control unit  52  selects, on the basis of a result of the temperature comparison, driving time for each of the temperature differences set in the driving control tables and performs driving control for the excitation coils  21   a  and  21   b  of the heating unit  21 . 
     Specifically, processing can be performed as explained below on the basis of the driving control table shown in  FIG. 6 . 
     First, in Act  204 , the heating control unit  52  determines whether a temperature difference between the center and the ends of the belt  14  is equal to or smaller than 5° C. If it is determined that the temperature difference is equal to or smaller than 5° C., the heating control unit  52  alternately drives the excitation coils  21   a  and  21   b  of the heating unit  21  for 20 ms (Act  205 ). The heating control unit  52  returns to Act  201  and repeats the processing until it is determined in Act  202  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  204  that the temperature difference between the center and the ends of the belt  14  is larger than 5° C., the heating control unit  52  proceeds to Act  206  and determines whether the temperature difference between the center and the ends is 5 to 10° C. If it is determined that the temperature difference is 5 to 10° C., the heating control unit  52  drives, for 40 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature (in the first embodiment, one of the center and the ends and equivalent to the second area) among the excitation coils  21   a  and  21   b  of the heating unit  21 . The heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (in the first embodiment, the other of the center and the ends and equivalent to the first area) (Act  207 ). The heating control unit  52  returns to Act  201  and repeats the processing until it is determined in Act  202  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  206  that the temperature difference between the center and the ends of the belt  14  is larger than 10° C., the heating control unit  52  proceeds to Act  208  and determines whether the temperature difference between the center and the ends of the belt  14  is 10 to 15° C. If it is determined that the temperature difference is 10 to 15° C., the heating control unit  52  drives, for 60 ms, the excitation coils that heat the side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  209 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil that heats the side determined as having higher temperature (Act  209 ). The driving of the excitation coils  21   a  and  21   b  of the heating unit  21  that heats the belt  14  is alternately performed. The temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  210 ). Subsequently, the heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends of the belt  14  is equal to or smaller than 5° C. (Act  211 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  209  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  201  and repeats the processing until it is determined in Act  202  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  208  that the temperature difference between the center and the ends of the belt  14  is larger than 15° C., the heating control unit  52  drives, for 80 ms, the excitation coils that heat the side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  212 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil that heats the side determined as having higher temperature (Act  212 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  213 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends of the belt  14  is equal to or smaller than 5° C. (Act  214 ). If it is determined that the temperature difference between the center and the ends of the belt  14  is larger than 5° C., the heating control unit  52  returns to Act  212  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  201  and repeats the processing until it is determined in Act  202  that the temperature of the belt  14  reaches 160° C. 
     As a result of performing the processing explained above, when the temperature of the belt  14  exceeds 160° C., the image forming apparatus  1  finishes the warm-up state and shifts the operation state to the ready (returns to the ready). 
     A flowchart indicating a flow of processing of power control for the excitation coils  21   a  and  21   b  of the heating unit  21  in the ready state of the image forming apparatus  1  is explained with reference to  FIG. 10 . 
     When the image forming apparatus  1  is in the ready state, the fixing apparatus  7  according to the first embodiment uses a driving control table same as that used in the warm-up state. 
     The ready state is different from the warm-up state in that, in the ready state, the CPU  45  sends a command for reducing electric power (output) to both the control circuits  41  and  42  to maintain the control temperature (160° C.). In other words, the CPU  45  reduces ON time of the switching elements and reduces electric power to control the temperature of the belt  14  to be equal to or lower than 160° C. 
     Concerning an output of the heating unit  21 , in the case of the warm-up state, the center coil and the side coils are driven at a frequency for heating at 1300 W. In the case of the ready state, the center coil and the side coils are driven at a frequency for heating at MAX 700 W. Since the temperature of the belt  14  already reaches the control temperature (160° C.), a large heat quantity is not required. Therefore, a heat quantity is limited to be equal to or smaller than 700 W. In order to maintain the temperature of the belt  14  at the control temperature 160° C., electric power is gradually reduced. When the temperatures in the center and at the ends exceed 160° C. even if the heat quantity decreases to a minimum output 200 W, the heating unit  21  is turned off. 
     An example of a flowchart for changing electric power according to the first embodiment is shown in  FIG. 10 . 
     In Act  301 , first, the CPU  45  sets output power of the excitation coils  21   a  and  21   b  of the heating unit  21  to 700 W. Subsequently, in Act  302 , the CPU  45  acquires temperatures in the center and at the ends of the belt  14  detected by the thermopiles  17   a  and  17   b  and determines whether the temperature of the belt  14  is higher than 160° C. 
     If it is determined in Act  303  that the temperature of the belt  14  is equal to or lower than 160° C., the CPU  45  proceeds to Act  304  and determines whether electric energy is the maximum output 700 W. 
     If it is determined in Act  304  that the electric energy is not 700 W, the CPU  45  proceeds to Act  305  and increases the electric energy by a predetermined value (e.g., 100 W). The CPU  45  returns to Act  302  and repeats the processing. 
     On the other hand, if it is determined in Act  304  that the electric energy is 700 W, the CPU  45  proceeds to Act  306  and maintains the electric energy at 700 W. The CPU  45  returns to Act  302  and repeats the processing. 
     If it is determined in Act  303  that the temperature of the belt  14  is higher than 160° C., the CPU  45  proceeds to Act  307  and determines whether the electric energy is the minimum output 200 W. 
     If it is determined in Act  307  that the electric energy is not 200 W, the CPU  45  proceeds to Act  308  and reduces the electric energy by a predetermined value (e.g., 100 W). The CPU  45  returns to Act  302  and repeats the processing. 
     On the other hand, if it is determined in Act  307  that the electric energy is 200 W, the CPU  45  proceeds to Act  309  and turns off a power supply for the excitation coils  21   a  and  21   b.  The CPU  45  returns to Act  302  and repeats the processing. 
     In the first embodiment, the CPU  45  sets timing for switching driving of the excitation coil  21   a  and the excitation coil  21   b  to timing when the voltage of the commercial AC power supply falls to 0 volt. By switching the driving at 0 volt, since sudden voltage and current are not applied to the energization coils, it is possible to eliminate a phenomenon that the heating roller  11  oscillates. It is also possible to reduce switching losses of the inverter circuit. 
     In the warm-up and ready states, since paper passing is not performed, heat is not deprived by a sheet. Therefore, temperature distribution in the rotation axis direction of the belt  14  does not change because of a size of the sheet. Heat is relatively often deprived from the entire rotation axis direction of the roller. Therefore, since a large temperature difference rarely occurs in the roller center and the roller ends of the belt  14 , driving times set in the driving control tables corresponding to the warm-up and ready states are the four types explained above. Conversely, if the driving times are set with a larger ratio difference (e.g., 20 ms and 110 ms), it is likely that fluctuation in a temperature difference between the center and the sides increases. 
     This is because, since temperature detection time is long with respect to switching time for excitation coil driving, time lag occurs between temperature detection and driving, a temperature difference cannot be detected on a real time basis, and the detection delays a little. Therefore, the four kinds of driving times are at least set in the driving control tables to prevent a large change from occurring. 
     Even when there is a temperature difference, a high temperature side of the belt  14  is always heated as well. This is also because, if temperature response performance of the temperature detecting means (the thermopiles) and switching timing for the excitation coils shift, a mode for heating one of the center and the ends may be more often used to cause a temperature difference. As a roller heat capacity is smaller, it is more highly likely that temperature ripple increases. Therefore, the temperature distribution fluctuation in the roller rotation axis direction could be reduced by setting switching time for the excitation coils  21   a  and  21   b  as fine as possible and limiting time for driving only one of the excitation coils  21   a  and  21   b.    
     An example of power control for the excitation coils  21   a  and  21   b  of the heating unit  21  in the paper passing state is explained. In the case of the paper passing state, as in the warm-up and ready states, the temperature of the belt  14  is detected by the thermopile  17   a  and the thermopile  17   b.  In the first embodiment, the detection of temperature is executed by the CPU  45  at every 200 (ms). 
     Electric power in the paper passing state is set to 1100 W at the maximum. Since various motors, other fans, and the like are more often used than at the warm-up time, electric power used in fixing is slightly reduced. 
     In the first embodiment, the control temperature is 160° C. When the temperatures both in the center and on the ends of the belt  14  exceed 160° C., electric power is gradually reduced to perform temperature control. When the temperature is equal to or lower than 160° C., electric power of the excitation coils  21   a  and  21   b  is changed according to driving control for the excitation coils  21   a  and  21   b  explained later (a flowchart of power control is shown in  FIG. 11 ). 
     As shown in  FIG. 11 , in Act  401 , first, the CPU  45  sets output power of the excitation coils  21   a  and  21   b  of the heating unit  2  to 1100 W. Subsequently, in Act  402 , the CPU  45  acquires the temperatures in the center and at the ends of the belt  14  detected by the thermopiles  17   a  and  17   b  and determines whether the temperature of the belt  14  is higher than 160° C. (Act  403 ). 
     If it is determined that the temperature of the belt  14  is equal to or lower than 160° C., the CPU  45  proceeds to Act  404  and determines whether electric energy is the maximum output 1100 W. 
     If it is determined in Act  404  that the electric energy is not 1100 W, the CPU  45  proceeds to Act  405  and increases the electric energy by a predetermined value (e.g., 100 W). The CPU  45  returns to Act  402  and repeats the processing. 
     On the other hand, if it is determined in Act  404  that the electric energy is 1100 W, the CPU  45  proceeds to Act  406  and maintains the electric energy at 1100 W. The CPU  45  returns to Act  402  and repeats the processing. 
     If it is determined in Act  403  that the temperature of the belt  14  is higher than 160° C., the CPU  45  proceeds to Act  407  and determines whether electric energy is the minimum output 200 W. 
     If it is determined in Act  407  that the electric energy is not 200 W, the CPU  45  proceeds to Act  408  and reduces the electric energy by a predetermined value (e.g., 100 W). The CPU  45  returns to Act  402  and repeats the processing. 
     On the other hand, if it is determined in Act  407  that the electric energy is 200 W, the CPU  45  proceeds to Act  409  and turns off the power supply for the coils  21   a  and  21   b.  The CPU  45  returns to Act  402  and repeats the processing. 
     Driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  in the paper passing state is explained in detail with reference to  FIG. 12 . In the first embodiment, Act  401  to Act  406  in the flowchart shown in  FIG. 11  correspond to Act  501  to Act  503  in a flowchart shown in  FIG. 12 . 
     First, in Act  501 , the temperature-information acquiring unit  51  acquires, as temperature information, the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  and sends the temperature information to the heating control unit  52 . In Act  502 , the heating control unit  52  checks, on the basis of the temperature information acquired by the temperature-information acquiring unit  51 , whether the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  reaches the control temperature (160° C.). 
     If it is determined in Act  502  that the temperature of the belt  14  is equal to or lower than 160° C., first, in Act  503 , the heating control unit  52  sets an output of the heating unit  21  to 1100 W. Subsequently, in Act  504 , the heating control unit  52  executes, on the basis of the temperature information of the belt  14 , comparison of the temperatures in the center and at the ends of the belt  14  detected by the thermopiles  17   a  and  17   b.  The heating control unit  52  selects, on the basis of a result of the temperature comparison, driving time for each of the temperature differences set in the driving control table ( FIG. 7 ) corresponding to the paper passing state and performs control of the excitation coils of the heating unit  21 . In the driving control table corresponding to the paper passing state, when the temperature difference between the center and the ends is larger than 10° C., driving time for the excitation coils that heats an area having lower temperature is set longer than the driving time in the driving control table corresponding to the warm-up and ready states. 
     Specifically, processing can be performed as explained below on the basis of the driving control table shown in  FIG. 7 . 
     First, in Act  504 , the heating control unit  52  determines whether a temperature difference between the center and the ends of the belt  14  is equal to or smaller than 5° C. If it is determined that the temperature difference is equal to or smaller than 5° C., the heating control unit  52  drives each of the excitation coils  21   a  and  21   b  of the heating unit  21  for 20 ms (Act  505 ). Subsequently, the heating control unit  52  returns to Act  501  and repeats the processing until it is determined in Act  502  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  504  that the temperature difference between the center and the ends of the belt  14  is larger than 5° C., the heating control unit  52  proceeds to Act  506  and determines whether the temperature difference between the center and the ends of the belt  14  is 5 to 10° C. If it is determined that the temperature difference is 5 to 10° C., the heating control unit  52  drives, for 40 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21 . The heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  507 ). The heating control unit  52  returns to Act  501  and repeats the processing until it is determined in Act  502  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  506  that the temperature difference between the center and the ends of the belt  14  is larger than 10° C., the heating control unit  52  proceeds to Act  508  and determines whether the temperature difference between the center and the ends of the belt  14  is 10 to 15° C. If it is determined that the temperature difference is 10 to 15° C., the heating control unit  52  drives, for 80 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  509 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  509 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  510 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  511 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  509  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  501  and repeats the processing until it is determined in Act  502  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  512  that the temperature difference between the center and the ends of the belt  14  is larger than 15° C., the heating control unit  52  proceeds to Act  512  and determines whether the temperature difference between the center and the ends of the belt  14  is 15 to 20° C. If it is determined that the temperature difference is 15 to 20° C., the heating control unit  52  drives, for 120 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  513 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  513 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  514 ). The heating control unit  52  determines, on the basis of the acquired information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  515 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  513  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  501  and repeats the processing until it is determined in Act  502  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  512  that the temperature difference between the center and the ends of the belt  14  is larger than 20° C., the heating control unit  52  drives, for 160 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  516 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  516 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  517 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  518 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  516  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  501  and repeats the processing until it is determined in Act  502  that the temperature of the belt  14  reaches 160° C. 
     As a result of performing the processing explained above, even when temperature distribution in the rotation axis direction of the belt  14  is more likely to be non-uniform than in the warm-up and ready states, it is possible to keep the temperature distribution substantially constant. 
     As explained above, in the first embodiment, driving time for heater(s) (the excitation coil(s)) that heats (heat) the second area having temperature lower than that of the first area among the plural areas of the belt  14  is set longer than driving time for heater(s) that heats (heat) the first area. Consequently, it is possible to keep temperature distribution in the rotation axis direction on the belt substantially constant more easily than in the past. 
     Driving time for the heaters is selected according to an operation state of the image forming apparatus as well as a temperature difference of temperature distribution in the rotation axis direction on the belt. Consequently, it is possible to control temperature distribution in the rotation axis direction on the belt more substantially uniform. 
     Even if time lag occurs between thermal response time of the temperature detecting means (the thermopiles) and temperature rise of the belt, since driving times of the heaters that heat the first area and the second area are switched at timing finer than timing (200 ms) for temperature detection to perform temperature control, it is possible to perform control such that a large temperature difference less easily occurs. 
     Second Embodiment 
     In a second embodiment of the present invention, the heating control unit  52  is configured to perform driving control for the excitation coils  21   a  and  21   b  corresponding to reduced sheet conveying speed in addition to the driving control for the excitation coils  21   a  and  21   b  illustrated in the explanation of the first embodiment. 
     In the second embodiment, the image forming apparatus  1  is configured to change conveying speed according to a type of a sheet to be passed. For example, when thick paper or glossy paper is passed, conveying speed is set smaller (e.g., 135 mm/s) than that in a paper passing state of a normal sheet (e.g., 270 mm/s). When the conveying speed is changed in this way, time for the belt  14  coming closer to the excitation coils also changes. In other words, when the conveying speed decreases, a heat quantity that the belt  14  receives from the excitation coils increases. 
     Therefore, for example, when the thick paper or the glossy paper is passed, if driving control for the excitation coils is performed on the basis of a driving control table same as that in the paper passing state of the normal sheet, a temperature difference among plural areas in the rotation axis direction of the belt  14  may increase. Therefore, when the conveying speed is set low, time for driving the excitation coils that heat an area having lower temperature is set shorter than that in the paper passing state of the normal sheet. 
     A flowchart indicating a flow of processing according to the second embodiment is explained with reference to FIGS.  13  and  14 . As a driving control table at conveying speed set to 270 mm/s, the driving control table shown in  FIG. 7  is used. On the other hand, as a driving control table at conveying speed set to 135 mm/s, the driving control table shown in  FIG. 15  is used. 
     First, the heating control unit  52  determines, according to an operation state of the image forming apparatus  1 , based on which of the driving control tables stored in the memory  54  driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  should be performed. In the flowchart, acts for determining which of the warm-up state, the ready state, and the paper passing state the image forming apparatus  1  is in are the same as those in the flowchart shown in  FIG. 8  in the first embodiment. Therefore, explanation of the acts is omitted. 
     In the second embodiment, if it is determined that the operation state of the image forming apparatus  1  is the paper passing state, the heating control unit  52  proceeds to (3) in  FIG. 8 . Further, as shown in  FIG. 13 , in Act  601 , the heating control unit  52  determines whether sheet conveying speed is 270 mm/s. 
     If it is determined in Act  601  that the sheet conveying speed is 270 mm/s, the heating control unit  52  proceeds to Act  501 . Driving control corresponding to a driving control table at conveying speed set to 270 mm/s is performed. The driving control table is the driving control table shown in  FIG. 7  and is the same as that in the paper passing state in the first embodiment. 
     Driving control for the excitation coils at conveying seed 270 mm/s is the same as the driving control in the paper passing state in the first embodiment. Therefore, explanation of the driving control is omitted. 
     On the other hand, if it is determined in Act  601  that the conveying speed is not 270 mm/s, the heating control unit  52  proceeds to (A) in  FIG. 13  and determines that the image forming apparatus  1  performs paper passing processing at, for example, 135 mm/s, which is speed for paper passing of the thick paper and the glossy paper. The heating control unit  52  proceeds to Act  701  as shown in  FIG. 14  and performs driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  using the driving control table corresponding to the paper passing processing at 135 mm/s. 
     Driving control for the excitation coils  21   a  and  21   b  is specifically explained below. As shown in  FIG. 14 , in Act  701 , the temperature-information acquiring unit  51  acquires, as temperature information, the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  and sends the temperature information to the heating control unit  52 . In Act  702 , the heating control unit  52  checks, on the basis of the temperature information acquired by the temperature-information acquiring unit  51 , whether the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  reaches control temperature (in the second embodiment, 160° C.) 
     If it is determined in Act  702  that the temperature of the belt  14  is equal to or lower than 160° C., first, in Act  703 , the heating control unit  52  sets an output of the excitation coils of the heating unit  21  to 1100 W. Subsequently, in Act  704 , the heating control unit  52  executes, on the basis of the temperature information of the belt  14 , comparison of temperatures in the center and at the ends of the belt  14  detected by the thermopiles  17   a  and  17   b.  The heating control unit  52  selects, on the basis of a result of the temperature comparison, driving time for each of temperature differences set in the driving control table corresponding to a paper passing state at conveying speed set to 135 mm/s and performs driving control for the excitation coils of the heating unit  21 . 
     Specifically, processing can be performed as explained below on the basis of a driving control table shown in  FIG. 15 . 
     First, in Act  704 , the heating control unit  52  determines whether a temperature difference between the center and the ends of the belt  14  is equal to or smaller than 5° C. If it is determined that the temperature difference is equal to or smaller than 5° C., the heating control unit  52  drives each of the excitation coils  21   a  and  21   b  of the heating unit  21  for 20 ms (Act  705 ). The heating control unit  52  returns to Act  701  and repeats the processing until it is determined in Act  702  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  704  that the temperature difference between the center and the ends of the belt  14  is larger than 5° C., the heating control unit  52  proceeds to Act  706  and determines whether the temperature difference between the center and the ends of the belt  14  is 5 to 10° C. If it is determined that the temperature difference is 5 to 10° C., the heating control unit  52  drives, for 40 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21 . The heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  707 ). The heating control unit  52  returns to Act  701  and repeats the processing until it is determined in Act  702  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  706  that the temperature difference between the center and the ends of the belt  14  is larger than 10° C., the heating control unit  52  proceeds to Act  708  and determines whether the temperature difference between the center and the ends of the belt  14  is 10 to 15° C. If it is determined that the temperature difference is 10 to 15° C., the heating control unit  52  drives, for 60 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  709 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having high temperature (Act  709 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  710 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  711 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  709  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  701  and repeats the processing until it is determined in Act  702  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  708  that the temperature difference between the center and the ends of the belt  14  is larger than 15° C., the heating control unit  52  proceeds to S 712  and determines whether the temperature difference between the center and the ends of the belt  14  is 15 to 20° C. If it is determined that the temperature difference is 15 to 20° C., the heating control unit  52  drives, for 80 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  713 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determine as having high temperature (Act  713 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  714 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  715 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  713  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  701  and repeats the processing until it is determined in Act  702  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  712  that the temperature difference between the center and the ends of the belt  14  is larger than 20° C., the heating control unit  52  drives, for 100 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  716 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  716 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  717 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  718 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  716  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the end is equal to or smaller than 5° C., the heating control unit  52  returns to Act  701  and repeats the processing until it is determined in Act  702  that the temperature of the belt  14  reaches 160° C. 
     As a result of performing the processing explained above, even when the image forming apparatus has an operation mode with reduced conveying speed, it is possible to keep temperature distribution in the rotation axis direction on the belt  14  substantially constant. 
     Third Embodiment 
     In a third embodiment of the present invention, the heating control unit  52  is configured to perform driving control for the excitation coils  21   a  and  21   b  corresponding to continuous paper passing of small-size sheets in addition to the driving control for the excitation coils  21   a  and  21   b  explained in the first embodiment. 
     When small-size sheets such as A4R sheets are continuously passed, a heat quantity to be deprived is different in an area in contact with the small-size sheets in the rotation axis direction of the belt  14 , for example, the center, and an area not in contact with the small-size sheets, for example, the ends. Specifically, whereas a consumed heat quantity is large in the center, almost no heat quantity is consumed at the ends. Therefore, when the small-size sheets are continuously passed, a difference in temperature distribution in the rotation axis direction of the belt  14  tends to be larger than that in paper passing of normal sheets. Therefore, in an operation state in which the small-size sheets are continuously passed, driving control for the excitation coils is performed on the basis of the driving control table corresponding to the operation state. 
     In the third embodiment, a sheet having an area of contact with the belt  14  in the rotation axis direction of the belt  14  larger than that of the A4R sheet is referred to as normal sheet. A sheet having an area of contact with the belt  14  equal to or smaller than that of the A4R sheet is referred to as small-size sheet. 
     A flowchart indicating a flow of processing according to the third embodiment is explained with reference to  FIGS. 16 and 17 . When paper larger than the small-size sheet is passed or when the number of small-size sheets to be passed is smaller than a predetermined number, for example, smaller than twenty, the driving control table shown in  FIG. 7  is used. When the predetermined number of small-size sheets are continuously passed, for example, when twenty or more sheets having a size equal to or smaller than A4-R are continuously passed, a driving control table shown in  FIG. 18  is used. It goes without saying that twenty is only an example and is a value that could fluctuate according to fixing temperature, a type of a sheet, or the like. 
     First, the heating control unit  52  determines, according to an operation state of the image forming apparatus  1 , based on which of the driving control tables stored in the memory  54  driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  should be performed. In the flowchart, acts for determining which of the warm-up state, the ready state, and the paper passing state the image forming apparatus  1  is in are the same as those in the flowchart shown in  FIG. 8  in the first embodiment. Therefore, explanation of the acts is omitted. 
     In  FIG. 8 , if it is determined that the operation state of the image forming apparatus  1  is the paper passing state, the heating control unit  52  proceeds to (3). Further, as shown in  FIG. 16 , in Act  801 , the heating control unit  52  determines whether a sheet to be passed is the small-size sheet. 
     If it is determined in Act  801  that the sheet to be passed is not the small-size sheet, the heating control unit  52  proceeds to Act  501  and performs driving control corresponding to a driving control table in a paper passing state of the normal sheet. 
     If it is determined in Act  801  that the sheet to be passed is the small-size sheet, the heating control unit  52  proceeds to Act  802  and determines whether the number of sheets to be passed in one job is equal to or larger than twenty. If it is determined that the number of sheets to be passed is smaller than twenty, the heating control unit  52  proceeds to Act  501  and performs driving control corresponding to the driving control table in the paper passing state of the normal sheet. 
     Driving control for the excitation coils of the heating unit  21  corresponding to the driving control table in the paper passing state of the normal sheet is the same as that in the first embodiment. Therefore, explanation of the driving control is omitted. 
     On the other hand, if it is determined in Act  802  that the number of small-size sheets to be passed in one job is equal to or larger than twenty, the heating control unit  52  proceeds to (B). Further, as shown in  FIG. 17 , the heating control unit  52  proceeds to Act  901  and performs, on the basis of a flowchart shown in  FIG. 17 , driving control for the excitation coils of the heating unit  21  using the driving control table shown in  FIG. 18  corresponding to the paper passing of twenty or more small-size sheets. 
     First, in Act  901 , the temperature-information acquiring unit  51  acquires, as temperature information, the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  and sends the temperature information to the heating control unit  52 . In Act  902 , the heating control unit  52  determines, on the basis of the temperature information acquired from the temperature-information acquiring unit  51 , the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  reaches control temperature (in the third embodiment, 160° C.). 
     If it is determined in Act  902  that the temperature of the belt  14  is equal to or lower than 160° C., first, in Act  903 , the heating control unit  52  sets an output of the excitation coils of the heating unit  21  to 1100 W. Subsequently, in Act  904 , the heating control unit  52  executes, on the basis of the temperature information of the belt  14 , comparison of temperatures in the center and at the ends of the belt  14  detected by the thermopiles  17   a  and  17   b.  The heating control unit  52  selects, on the basis of a result of the temperature comparison, driving time for each of the temperature differences set in the driving control table for continuous paper passing of small-size sheets and performs driving control for the excitation coils  21   a  and  21   b  of the heating unit  21 . 
     Specifically, processing can be performed as explained below on the basis of the driving control table shown in  FIG. 18 . 
     First, in Act  904 , the heating control unit  52  determines whether a temperature difference between the center and the ends of the belt  14  is equal to or smaller than 5° C. If it is determined that the temperature difference is equal to or smaller than 5° C., the heating control unit  52  drives each of the excitation coils  21   a  and  21   b  of the heating unit  21  for 20 ms (Act  905 ). Subsequently, the heating control unit  52  returns to Act  901  and repeats the processing until it is determined in Act  902  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  904  that the temperature difference between the center and the ends of the belt  14  is larger than 5° C., the heating control unit  52  proceeds to Act  906  and determines whether the temperature difference between the center and the ends of the belt  14  is 5 to 10° C. If it is determined that the temperature difference is 5 to 10° C., the heating control unit  52  drives, for 40 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21 . The heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  907 ). The heating control unit  52  returns to Act  901  and repeats the processing until it is determined in Act  902  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  906  that the temperature difference between the center and the ends of the belt  14  is larger than 10° C., the heating control unit  52  proceeds to Act  908  and determines whether the temperature difference between the center and the ends of the belt  14  is 10 to 15° C. If it is determined that the temperature difference is 10 to 15° C., the heating control unit  52  drives, for 60 ms, the excitation coils that heat the side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  909 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil that heats the side determined as having higher temperature (Act  909 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  910 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  911 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  909  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  901  and repeats the processing until it is determined in Act  902  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  908  that the temperature difference between the center and the ends of the belt  14  is larger than 15° C., the heating control unit  52  proceeds to Act  912  and determines whether the temperature difference between the center and the ends of the belt  14  is 15 to 20° C. If it is determined that the temperature difference between the center and the ends is 15 to 20° C., the heating control unit  52  drives, for 160 ms, the excitation coils that heat the side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  913 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil that heats the side determined as having higher temperature (Act  913 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  914 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  915 ). If it is determined that the temperature difference between the center and the ends of the belt  14  is larger than 5° C., the heating control unit  52  returns to Act  913  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  901  and repeats the processing until it is determined in Act  902  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  912  that the temperature difference between the center and the ends of the belt  14  is larger than 20° C., the heating control unit  52  drives, for 200 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  916 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  916 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  917 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  918 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  916  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the end is equal to or smaller than 5° C., the heating control unit  52  returns to Act  901  and repeats the processing until it is determined in Act  902  that the temperature of the belt  14  reaches 160° C. 
     As a result of performing the processing explained above, even when the image forming apparatus continuously performs paper passing of a predetermined number or more of small-size sheets, it is possible to keep temperature distribution in the rotation axis direction on the belt  14  substantially constant. 
     Fourth Embodiment 
     In a fourth embodiment of the present invention, the heating control unit  52  is configured to perform driving control for the excitation coils  21   a  and  21   b  corresponding to a temperature state in the image forming apparatus  1  in the paper passing state in addition to the driving control for the excitation coils  21   a  and  21   b  explained in the first embodiment. 
     More specifically, a degree of temperature rise of the belt  14  tends to be different according to a difference in temperature in the inside of the image forming apparatus  1 . When the temperature in the inside of the image forming apparatus  1  is lower than usual, the temperature less easily rises even if the belt  14  is heated in the same driving time. Therefore, the heating control unit  52  performs driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  using driving control tables corresponding to temperatures in the inside of the image forming apparatus, i.e., temperature equal to or lower than predetermined temperature, for example, equal to or lower than 10° C. and temperature higher than the predetermined temperature, for example, higher than 10° C. 
     First, functional blocks according to the fourth embodiment are explained with reference to  FIG. 19 . Functions of the respective blocks shown in  FIG. 19  are realized by causing the CPU  45  to execute, for example, various computer programs stored in the memory  54 . 
     As shown in  FIG. 19 , the driving control for the excitation coils of the fixing device according to the fourth embodiment is performed by the temperature-information acquiring unit  51 , the heating control unit  52 , and a second temperature-information acquiring unit  55 . 
     The functional blocks other than the second temperature-information acquiring unit  55  and the heating control unit  52  are the same as those in the first embodiment. Therefore, explanation of the functional blocks is omitted. 
     The second temperature-information acquiring unit  55  acquires, as temperature information in the inside of the image forming apparatus  1 , temperature detected by a temperature sensor (not shown) arranged in the inside of the image forming apparatus  1  and sends the temperature information to the heating control unit  52 . The temperature sensor can be arranged near, for example, photoconductive members, transfer rollers, or the like in the image forming apparatus  1 . In this case, the temperature sensor detects temperature around a process unit. 
     The heating control unit  52  controls, on the basis of temperature information of the belt  14  acquired from the temperature-information acquiring unit  51  and temperature information in the inside of the image forming apparatus  1  acquired by the second temperature-information acquiring unit  55 , driving time for the excitation coils  21   a  and  21   b  of the heating unit  21  to thereby control temperature in the rotation axis direction of the belt  14 . In the fourth embodiment, as in the other embodiments, the excitation coils  21   a  and  21   b  are alternately driven. 
     An example of the temperature control by the heating control unit  52  is more specifically explained. First, the heating control unit  52  determines an operation state of the image forming apparatus  1  on the basis of information concerning an operation state of the image forming apparatus  1  acquired from the operation-information acquiring unit  53 . If it is determined that the operation state of the image forming apparatus is the warm-up and ready states, the heating control unit  52  performs driving control for the excitation coils according to a method same as that in the first embodiment. On the other hand, if it is determined that the operation state of the image forming apparatus  1  is the paper passing state, the heating control unit  52  determines, on the basis of information concerning temperature in the inside of the image forming apparatus  1  acquired by the second temperature-information acquiring unit  55 , whether temperature in the inside of the heating control unit  52  is equal to or lower than a predetermined value. Subsequently, the heating control unit  52  calculates, on the basis of temperature information acquired from the temperature-information acquiring unit  51 , a temperature difference between the first area (one of the center and the ends) and the second area (the other of the center and the ends) of the belt  14 . Referring to the driving control tables stored in the memory  54 , the heating control unit  52  selects, as driving time for the respective excitation coils that heat the first and second areas, driving time associated with the temperature difference on the belt  14  and the temperature in the inside of the image forming apparatus  1  among plural kinds of driving times set as driving times of the respective excitation coils that heat the first and second area. 
     A flowchart indicating a flow of processing according to the fourth embodiment is explained with reference to  FIGS. 20 ,  21 , and  22 . A driving control table at internal temperature equal to or lower than 10° C. of the image forming apparatus  1  is shown in  FIG. 23 . A driving control table at internal temperature higher than 10° C. is shown in  FIG. 24 . 
     First, the heating control unit  52  determines, according to an operation state of the image forming apparatus  1 , based on which of the driving control tables stored in the memory  54  driving control for the excitation coils  21   a  and  21   b  of the heating unit  21  should be performed. In the flowchart, acts for determining which of the warm-up state, the ready state, and the paper passing state the image forming apparatus  1  is in are the same as those in the flowchart shown in  FIG. 8  in the first embodiment. Therefore, explanation of the acts is omitted. 
     In  FIG. 8 , in the fourth embodiment, if it is determined that the operation state of the image forming apparatus  1  is the paper passing state, the heating control unit  52  proceeds to (3) in  FIG. 8 . Further, as shown in  FIG. 20 , in Act  1001 , the heating control unit  52  determines whether internal temperature of the image forming apparatus  1  is equal to or lower than 10° C. 
     If it is determined in Act  1001  that the internal temperature of the image forming apparatus  1  is equal to or lower than 10° C., the heating control unit  52  proceeds to (C), further proceeds to Act  1101  as shown in  FIG. 21 , and performs driving control corresponding to a driving control table at internal temperature equal to or lower than 10° C. of the image forming apparatus  1 . The driving control table is the driving control table shown in  FIG. 23 . 
     On the other hand, if it is determined in Act  1001  that the internal temperature of the image forming apparatus  1  is higher than 10° C., the heating control unit proceeds to (D), further proceeds to Act  1201  as shown in  FIG. 22 , and performs driving control corresponding to internal temperature higher than 10° C. of the image forming apparatus  1 . The driving control table is the driving control table shown in  FIG. 24 . 
     First, the heating control unit  52  determines, according to temperature information in the inside of the image forming apparatus  1 , based on which of the driving control tables stored in the memory  54  driving control for the heating unit  21  should be performed. 
     Driving control for the excitation coils of the heating unit  21  corresponding to the driving control table at internal temperature equal to or lower than 10° C. of the image forming apparatus  1  is explained in detail with reference to  FIG. 21 . 
     First, in Act  1101 , the temperature-information acquiring unit  51  acquires, as temperature information, the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  and sends the temperature information to the heating control unit  52 . In Act  1102 , the heating control unit  52  determines, on the basis of the temperature information acquired from the temperature-information acquiring unit  51 , whether the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  reaches control temperature (in the fourth embodiment, 160° C.). 
     If it is determined in Act  1102  that the temperature of the belt  14  is equal to or lower than 160° C., first, in Act  1103 , the heating control unit  52  sets an output of the excitation coils  21   a  and  21   b  of the heating unit  21  to 1100 W. Subsequently, in Act  1104 , the heating control unit  52  executes, on the basis of the temperature information of the belt  14 , comparison of temperatures in the center and at the ends of the belt  14  detected by the thermopiles  17   a  and  17   b.  The heating control unit  52  selects, on the basis of a result of the temperature comparison, driving time for each of temperature differences set in the driving control table and performs driving control for the excitation coils of the heating unit  21 . 
     Specifically, processing can be performed as explained below on the basis of a driving control table shown in FIG.  23 . 
     First, in Act  1104 , the heating control unit  52  determines whether a temperature difference between the center and the ends of the belt  14  is equal to or smaller than 5° C. If it is determined that the temperature difference is equal to or smaller than 5° C., the heating control unit  52  drives each of the excitation coils  21   a  and  21   b  of the heating unit  21  for 20 ms (Act  1105 ). The heating control unit  52  returns to Act  1101  and repeats the processing until it is determined in Act  1102  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  1104  that the temperature difference between the center and the ends of the belt  14  is larger than 5° C., the heating control unit  52  proceeds to Act  1106  and determines whether the temperature difference between the center and the ends of the belt  14  is 5 to 10° C. If it is determined that the temperature difference is 5 to 10° C., the heating control unit  52  drives, for 60 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21 . The heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  1107 ). The heating control unit  52  returns to Act  1101  and repeats the processing until it is determined in Act  1102  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  1106  that the temperature difference between the center and the ends of the belt  14  is larger than 10° C., the heating control unit  52  proceeds to Act  1108  and determines whether the temperature difference between the center and the ends of the belt  14  is 10 to 15° C. If it is determined that the temperature difference is 10 to 15° C., the heating control unit  52  drives, for 100 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  1109 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having high temperature (Act  1109 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  1110 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  1111 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  1109  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the sides is equal to or smaller than 5° C., the heating control unit  52  returns to Act  1101  and repeats the processing until it is determined in Act  1102  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  1112  that the temperature difference between the center and the ends of the belt  14  is larger than 15° C., the heating control unit  52  proceeds to S 1112  and determines whether the temperature difference between the center and the ends of the belt  14  is 15 to 20° C. If it is determined that the temperature difference is 15 to 20° C., the heating control unit  52  drives, for 140 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  1113 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determine as having high temperature (Act  1113 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  1114 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  1115 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  1113  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  1101  and repeats the processing until it is determined in Act  1102  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  1112  that the temperature difference between the center and the ends of the belt  14  is larger than 20° C., the heating control unit  52  drives, for 180 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  1116 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  1116 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  1117 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  1118 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  1116  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the end is equal to or smaller than 5° C., the heating control unit  52  returns to Act  1101  and repeats the processing until it is determined in Act  1102  that the temperature of the belt  14  reaches 160° C. 
     On the other hand, if it is determined in Act  1101  that the temperature in the inside of the image forming apparatus  1  is higher than 10° C., the heating control unit  52  proceeds to Act  1201  and performs, on the basis of a flowchart shown in  FIG. 22 , driving control for the excitation coils of the heating unit  21  using the driving control table corresponding to temperature equal to or higher than 10° C. in the inside of the image forming apparatus  1 . 
     First, in Act  1202 , the temperature-information acquiring unit  51  acquires, as temperature information, the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  and sends the temperature information to the heating control unit  52 . In Act  1202 , the heating control unit  52  determines, on the basis of the temperature information acquired by the temperature-information acquiring unit  51 , whether or not the temperature of the belt  14  detected by the thermopiles  17   a  and  17   b  reaches 160° C. 
     If it is determined in Act  1202  that the temperature of the belt  14  is equal to or lower than 160° C., first, in Act  1203 , the heating control unit  52  sets an output of the heating unit  21  to 1100 W. Subsequently, in Act  1204 , the heating control unit  52  executes, on the basis of the temperature information of the belt  14 , comparison of temperatures in the center and at the ends of the belt  14  detected by the thermopiles  17   a  and  17   b.  The heating control unit  52  selects, on the basis of a result of the temperature comparison, driving time for each of the temperature differences set in the driving control table corresponding to temperature higher than 10° C. in the inside of the image forming apparatus  1  and performs driving control for the excitation coils  21   a  and  21   b  of the heating unit  21 . 
     Specifically, processing can be performed as explained below on the basis of the driving control table shown in  FIG. 24 . 
     First, in Act  1204 , the heating control unit  52  determines whether a temperature difference between the center and the ends of the belt  14  is equal to or smaller than 5° C. If it is determined that the temperature difference is equal to or smaller than 5° C., the heating control unit  52  drives each of the excitation coils  21   a  and  21   b  of the heating unit  21  for 20 ms (Act  1205 ). Subsequently, the heating control unit  52  returns to Act  1201  and repeats the processing until it is determined in Act  1202  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  1204  that the temperature difference between the center and the ends of the belt  14  is larger than 5° C., the heating control unit  52  proceeds to Act  1206  and determines whether the temperature difference between the center and the ends of the belt  14  is 5 to 10° C. If it is determined that the temperature difference is 5 to 10° C., the heating control unit  52  drives, for 40 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21 . The heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  1207 ). The heating control unit  52  returns to Act  1201  and repeats the processing until it is determined in Act  1202  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  1206  that the temperature difference between the center and the ends of the belt  14  is larger than 10° C., the heating control unit  52  proceeds to Act  1208  and determines whether the temperature difference between the center and the ends of the belt  14  is 10 to 15° C. If it is determined that the temperature difference is 10 to 15° C., the heating control unit  52  drives, for 80 ms, the excitation coils that heat the side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  1209 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil that heats the side determined as having higher temperature (Act  1209 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  1210 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  1211 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  1209  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  1201  and repeats the processing until it is determined in Act  1202  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  1208  that the temperature difference between the center and the ends of the belt  14  is larger than 15° C., the heating control unit  52  proceeds to Act  1212  and determines whether the temperature difference between the center and the ends of the belt  14  is 15 to 20° C. If it is determined that the temperature difference between the center and the ends is 15 to 20° C., the heating control unit  52  drives, for 100 ms, the excitation coils that heat the side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  1213 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil that heats the side determined as having higher temperature (Act  1213 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  1214 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  1215 ). If it is determined that the temperature difference between the center and the ends of the belt  14  is larger than 5° C., the heating control unit  52  returns to Act  1213  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the ends is equal to or smaller than 5° C., the heating control unit  52  returns to Act  1201  and repeats the processing until it is determined in Act  1202  that the temperature of the belt  14  reaches 160° C. 
     If it is determined in Act  1212  that the temperature difference between the center and the ends of the belt  14  is larger than 20° C., the heating control unit  52  drives, for 120 ms, the excitation coil(s) that heats (heat) a side determined as having lower temperature among the excitation coils  21   a  and  21   b  of the heating unit  21  (Act  1216 ). Further, the heating control unit  52  drives, for 20 ms, the excitation coil(s) that heats (heat) a side determined as having higher temperature (Act  1216 ). Subsequently, the temperature-information acquiring unit  51  acquires temperature information of the belt  14  (Act  1217 ). The heating control unit  52  determines, on the basis of the acquired temperature information, whether a temperature difference between the center and the ends is equal to or smaller than 5° C. (Act  1218 ). If it is determined that the temperature difference between the center and the ends is larger than 5° C., the heating control unit  52  returns to Act  1216  and repeats the processing. On the other hand, if it is determined that the temperature difference between the center and the end is equal to or smaller than 5° C., the heating control unit  52  returns to Act  1201  and repeats the processing until it is determined in Act  1202  that the temperature of the belt  14  reaches 160° C. 
     As a result of performing the processing explained above, even when internal temperature of the image forming apparatus  1  in the paper passing state is lower than usual, it is possible to keep temperature distribution in the rotation axis direction on the belt  14  substantially constant. 
     Fifth Embodiment 
     In the embodiments explained above, the position of the thermopiles is near the belt surface of the belt  14  ( FIG. 2 ). However, the position of the thermopiles is not limited to this. It is also possible to arrange the thermopiles in other positions. 
     In a fifth embodiment of the present invention, the thermopiles  17   a  and  17   b  arranged near the belt surface of the belt  14  in the first embodiment are arranged in a position near the heating roller  11 . 
     As shown in  FIG. 25 , in a fixing device according to the fifth embodiment, the thermopiles  17   a  and  17   b  are arranged in a position near a roller surface of the heating roller  11  and detect surface temperature of areas on the heating roller  11  corresponding to plural areas of the belt  14 , for example, areas corresponding to the center and the ends of the belt  14 . The temperature-information acquiring unit  51  acquires the surface temperature as temperature information of the belt  14 . 
     Sixth Embodiment 
     In a sixth embodiment of the present invention, the thermopiles  17   a  and  17   b  arranged near the belt surface of the belt  14  in the first embodiment are arranged in a position near the pressing roller  12 . 
     As shown in  FIG. 26 , in a fixing device according to the sixth embodiment, the thermopiles  17   a  and  17   b  are arranged in a position near a roller surface of the pressing roller  12  and detect surface temperature of areas on the pressing roller  12  corresponding to plural areas of the belt  14 , for example, areas corresponding to the center and the ends of the belt  14 . The temperature-information acquiring unit  51  acquires the surface temperature as temperature information of the belt  14 . 
     Seventh Embodiment 
     In the first to sixth embodiments, the heating device  7  includes the stretching and suspending roller  13  and the belt  14 . However, the present invention is not limited to this. As shown as a seventh embodiment of the present invention, the stretching and suspending roller  13  and the belt  14  may be removed from the fixing device  7  according to the first to sixth embodiments. 
     Specifically, the fixing device  7  can be configured to include the heating roller  11 , the pressing roller  12 , the heating unit  21  including the excitation coils  21   a  and  21   b,  the thermopiles  17   a  and  17   b,  the temperature-information acquiring unit  51 , the heating control unit  52 , and the operation-state acquiring unit  53 . The temperature-information acquiring unit  51  acquires temperature information in the rotation axis direction of the heating roller  11 . The heating control unit  52  performs, on the basis of the temperature information, driving control for the excitation coils  21   a  and  21   b  in the same manner as the first to fourth embodiments to thereby perform temperature control in the rotation axis direction of the heating roller  11 . 
     As in the fifth and sixth embodiments, the position of the thermopiles  17   a  and  17   b  can be set in a position near the heating roller  11  or a position near the pressing roller  12 . 
     The present invention has been explained with reference to the embodiments. However, the present invention is not limited to the embodiments and various modifications are possible. 
     The operations in the processing in the fixing device are realized by causing the CPU  45  to execute a temperature control program stored in the memory  54 . 
     A computer program for causing a computer configuring the fixing device to execute the operations explained above can be provided as the temperature control program. In the example explained in the first to fourth embodiment, the computer program for realizing the functions for carrying out the present invention is recorded in advance in a storage area provided in the device. However, present invention is not limited to this. The same computer program may be downloaded from a network to the device or the same program stored in a computer-readable recording medium may be installed in the device. A form of the recording medium may be any form as long as the recording medium can store the computer program and can be read by the computer. Specifically, examples of the recording medium include internal storage devices implemented in the computer such as a ROM and a RAM, portable storage media such as a CD-ROM, a flexible disk, a DVD disk, a magneto-optical disk, and an IC card, a database that stores a computer program, other computers and databases for the computers, and a transmission medium on a line. Functions obtained by the installation and the download in this way may realize the functions in cooperation with an OS (operating system) in the apparatus. 
     The program in this embodiment includes a program for dynamically generating an execution module. 
     The present invention has been explained in detail with reference to the specific forms. However, it would be obvious for those skilled in the art that various modifications and alterations are possible without departing from the spirit and the scope of the present invention. 
     As explained above in detail, according to the present invention, it is possible to provide a fixing device that can keep temperature distribution in a rotation axis direction of a heating member such as a belt or a heating roller substantially constant.