Patent Publication Number: US-2011064441-A1

Title: Temperature Control Method for Fixing Device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Provisional U.S. Application 61/242,731 filed on Sep. 15, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to temperature control for a fixing device used in an image forming apparatus such as a color copying machine or a MFP (multi-functional peripheral). 
     BACKGROUND 
     An image forming apparatus has a startup mode, a printing mode, a standby mode, a sleep mode, and the like. Set temperature for controlling the temperature of a fixing device is changed according to a change of a mode of the image forming apparatus. In conventional, while the image forming apparatus maintains the printing mode, fixing set temperature for controlling the temperature of the fixing device is fixed without being changed. The image forming apparatus controls the fixing device at the fixing set temperature until the last sheet passes through the fixing device and the image forming apparatus finishes the printing mode. When the image forming apparatus finishes the printing mode and changes the printing mode to the standby mode, the image forming apparatus changes the set temperature of the fixing device from the fixing set temperature to standby temperature during standby. 
     However, if the image forming apparatus controls the fixing device at the fixing set temperature until the image forming apparatus finishes the printing mode, the set temperature of the fixing device changes to the fixing set temperature when the last sheet passes through the fixing device. Even if the image forming apparatus changes the set temperature of the fixing device to standby set temperature after finishing the printing mode, it is likely that the fixing device is overshoot (abnormally overheated) after the passage of the last sheet because of time lag and electric power is wastefully consumed. It is also likely that influence on image quality and influence on peripheral apparatuses are caused by the overshoot. 
     Therefore, there is a demand for development of an image forming apparatus that prevents the overshoot of the fixing device, saves power consumption, and realizes improvement of image quality and safety of the apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a color copying machine according to a first embodiment; 
         FIG. 2  is a schematic diagram of a fixing device according to the first embodiment viewed from a side; 
         FIG. 3  is a block diagram of a control system for an IH coil according to the first embodiment; 
         FIG. 4  is a flowchart for explaining setting of a startup mode according to the first embodiment; 
         FIG. 5  is a flowchart for explaining setting of a standby mode  1  according to the first embodiment; 
         FIG. 6  is a flowchart for explaining setting of a printing mode according to the first embodiment; 
         FIG. 7  is a flowchart for explaining setting of a last paper printing mode according to the first embodiment; 
         FIG. 8  is a flowchart for explaining setting of a standby mode  2  according to the first embodiment; 
         FIG. 9  is a flowchart for explaining control of the fixing device according to the first embodiment; 
         FIG. 10  is a timing chart for explaining a control example 1 of the IH coil according to the first embodiment; 
         FIG. 11  is a flowchart for explaining setting of a startup mode of a comparative example 1 of the first embodiment; 
         FIG. 12  is a flowchart for explaining setting of a standby mode of the comparative example 1 of the first embodiment; 
         FIG. 13  is a flowchart for explaining setting of a printing mode of the comparative example 1 of the first embodiment; 
         FIG. 14  is a timing chart for explaining the comparative example 1 of the first embodiment; 
         FIG. 15  is a timing chart for explaining a control example 2 of the IH coil according to the first embodiment; 
         FIG. 16  is a timing chart for explaining a comparative example 2 of the first embodiment; 
         FIG. 17  is a schematic diagram of a fixing device according to a second embodiment viewed from a side; 
         FIG. 18  is a block diagram of a control system for a fixing heater according to the second embodiment; 
         FIG. 19  is a flowchart for explaining setting of a startup mode according to the second embodiment; 
         FIG. 20  is a flowchart for explaining setting of a standby mode  1  according to the second embodiment; 
         FIG. 21  is a flowchart for explaining setting of a printing mode according to the second embodiment; 
         FIG. 22  is a flowchart for explaining setting of a last paper printing mode according to the second embodiment; 
         FIG. 23  is a flowchart for explaining setting of a standby mode  2  according to the second embodiment; 
         FIG. 24  is a flowchart for explaining control of the fixing device according to the second embodiment; 
         FIG. 25  is a timing chart for explaining a control example 3 of the fixing heater according to the second embodiment; 
         FIG. 26  is a timing chart for explaining a comparative example 3 of the second embodiment; 
         FIG. 27  is a timing chart for explaining a control example 4 of the fixing heater according to the second embodiment; and 
         FIG. 28  is a timing chart for explaining a comparative example 4 of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to an embodiment, a fixing device includes: a fixing member configured to cause a recording medium to pass therethrough and subject the recording medium to fixing processing; a heating member configured to heat the fixing member; and a control member configured to control the heating member at two fixing set temperatures during a printing mode for the recording medium fed last. 
     Embodiments are explained below. 
     First Embodiment 
     A color copying machine  1  as an image forming apparatus shown in  FIG. 1  includes a printer section  2  configured to form an image, a paper discharge section  3  configured to accumulate sheets P, which are recording media, discharged from the printer section  2 , a scanner section  4  configured to read a document image, a paper feeding device  7  configured to feed the sheets P, and a bypass paper feeding device  8 . 
     The printer section  2  includes four image forming stations  11 Y,  11 M,  11 C, and  11 K arranged in parallel along a lower side of an intermediate transfer belt  10 . The image forming stations  11 Y,  11 M,  11 C, and  11 K respectively include photoconductive drums  12 Y,  12 M,  12 C, and  12 K. The image forming stations  11 Y,  11 M,  11 C, and  11 K respectively form toner images of Y (yellow), M (magenta), C (cyan), and K (black) on the photoconductive drums  12 Y,  12 M,  12 C, and  12 K. 
     The photoconductive drums  12 Y,  12 M,  12 C, and  12 K rotate in an arrow m direction. The image forming stations  11 Y,  11 M,  11 C, and  11 K respectively include, around the photoconductive drums  12 Y,  12 M,  12 C, and  12 K, electrifying chargers  13 Y,  13 M,  13 C, and  13 K, developing devices  14 Y,  14 M,  14 C, and  14 K, and photoconductive member cleaners  16 Y,  16 M,  16 C, and  16 K. 
     A laser exposure device  17  irradiates exposure lights on the photoconductive drums  12 Y,  12 M,  12 C, and  12 K to form electrostatic latent images on the photoconductive drums  12 Y,  12 M,  12 C, and  12 K respectively. The electrifying chargers  13 Y,  13 M,  13 C, and  13 K and the laser exposure device  17  configure a latent-image forming section. The developing devices  14 Y,  14 M,  14 C, and  14 K respectively supply toners to the electrostatic latent images on the photoconductive drums  12 Y,  12 M,  12 C, and  12 K to visualize the electrostatic latent images. 
     The printer section  2  includes, above the respective developing devices  14 Y,  14 M,  14 C, and  14 K, toner cartridges  26 Y,  26 M,  26 C, and  26 K configured to respectively supply toners of Y, M, C, and K to the developing devices  14 Y,  14 M,  14 C, and  14 K through sub-hopper units  36 Y,  36 M,  36 C, and  36 K. The toner cartridges  26 Y,  26 M,  26 C, and  26 K are attached to and detached from a cartridge holding section  37 . 
     A backup roller  20  and a driven roller  21  of the intermediate transfer belt  10  stretch and suspend the intermediate transfer belt  10 . The intermediate transfer belt  10  rotates in an arrow n direction. Primary transfer rollers  18 Y,  18 M,  18 C, and  18 K respectively primarily transfer the toner images on the photoconductive drums  12 Y,  12 M,  12 C, and  12 K onto the intermediate transfer belt  10 . The photoconductive member cleaners  16 Y,  16 M,  16 C, and  16 K respectively remove and collect residual toners on the photoconductive drums  12 Y,  12 M,  12 C, and  12 K after the primary transfer. 
     The printer section  2  includes a secondary transfer roller  27  opposed to the intermediate transfer belt  10 . The printer section  2  secondarily transfers the toner images on the intermediate transfer belt  10  onto the sheet P passing between the intermediate transfer belt  10  and the secondary transfer roller  27 . Paper feeding cassettes  7   a  and  7   b  or the bypass paper feeding device  8  feeds the sheets P one by one in synchronization with the toner images on the intermediate transfer belt  10 . After the secondary transfer finishes, a belt cleaner  10   a  cleans the intermediate transfer belt  10 . 
     The color copying machine  1  includes, between the paper feeding device  7  and the secondary transfer roller  27 , pickup rollers  7   e , separation rollers  7   c , conveying rollers  7   d , and a registration roller pair  28 . The color copying machine  1  includes, between a manual feed tray  8   a  of the bypass paper feeding device  8  and the registration roller pair  28 , a manual feed pickup roller  8   b , a manual feed separation roller  8   c , and a manual feed conveying roller  8   d . The color copying machine  1  includes, along a conveying direction of the sheet P, a fixing device  30  further downstream than the secondary transfer roller  27 . 
     The color copying machine  1  includes, downstream of the fixing device  30 , a gate  33  configured to direct the sheet P to the direction of a paper discharge roller  31  or the direction of a re-conveying unit  32 . The color copying machine  1  discharges the sheet P, which reaches the paper discharge roller  31  from the gate  33 , to the paper discharge section  3 . The re-conveying unit  32  leads the sheet P to the direction of the secondary transfer roller  27  again. 
     The fixing device  30  includes, as shown in  FIG. 2 , a heat roller  37  and a press roller  38  included in a fixing member and an induction heating coil (hereinafter abbreviated as IH coil)  50 , which is a heating member, configured to heat the heat roller  37  with induction current. The fixing member is not limited to a roller type and may be a belt type. The heating member may heat the press roller  38 . 
     In the heat roller  37 , a foamed rubber (sponge) layer  37   b  having thickness of 5 mm, a metal layer  37   c  of nickel (Ni) having thickness of 40 μm, a solid rubber layer  37   d  made of silicon rubber having thickness of 20 μm, and a release layer  37   e  made of a PFA tube are sequentially laminated around, for example, a cored bar  37   a . A material of the metal layer  37   c  may be stainless steel, aluminum (Al), a composite material of stainless steel and aluminum, or the like. In the press roller  38 , a silicon sponge rubber layer  38   b  and a fluorine rubber layer  38   c  are laminated around, for example, a cored bar  38   a.    
     The press roller  38  comes into press-contact with the heat roller  37  to form a nip  40  having fixed width between the heat roller  37  and the press roller  38 . The press roller  38  is drivingly rotated in an arrow f direction. The heat roller  37  is driven to rotate in an arrow g direction following the press roller  38 . The press roller  38  and the heat roller  37  cause the sheet P to pass through the nip  40  and heat and press a toner image and fix the toner image on the sheet P. 
     High-frequency current is applied to the IH coil  50  to generate a magnetic flux, whereby eddy-current is generated in the metal layer  37   c  of the heat roller  37 . The surface of the heat roller  37  is heated by Joule heat generated by the eddy-current and the resistance of the metal layer  37   c . The IH coil  50  includes a center coil  50   a  configured to heat the center area of the heat roller  37  and side coils  50   b  configured to heat areas on both sides of the heat roller  37 . For example, when an image is fixed on the sheet P having width of the “A4” landscape size (210 mm) of the JIS standard, electric power is supplied to the center coil  50   a  to heat the center area of the heat roller  37 . When the entire length of the heat roller  37  is heated, electric power is alternately supplied to the center coil  50   a  and the side coils  50   b . Electric power may be simultaneously supplied to the center coil  50   a  and the side coils  50   b.    
     A peeling pawl  51 , non-contact first and second thermistors  52   a  and  52   b , which are temperature sensors, and first and second thermostats  53   a  and  53   b  are arranged on the outer circumference of the heat roller  37 . 
     A control system  70  for the IH coil  50  includes, as shown in  FIG. 3 , an inverter driving circuit  72  configured to supply electric power to the center coil  50   a  and the side coils  50   b , a noise filter  74  configured to rectify electric current from a commercial AC power supply  73  and supply the electric current to the inverter driving circuit  72 , a coil control circuit  76 , which is a control member, configured to control the inverter driving circuit  72 , a power supply and detection circuit  77  configured to detect an output of the noise filter  74  and feed back the output to the noise filter  74  to fix electric power from the commercial AC power supply  73 , and a fuse  78 . The first or second thermostat  53   a  or  53   b  interrupts power supply from the commercial AC power supply  73  to the IH coil  50  when the heat roller  37  is abnormally overheated. 
     The coil control circuit  76  is connected, via an interface  80 , to a CPU  71  configured to control the entire color copying machine  1 . The CPU  71  controls a high-voltage power supply necessary for image formation of the color copying machine  1 , a motor used for, for example, conveyance of the sheet P, and other operations of the color copying machine  1 . Temperature detection results of the first and second thermistors  52   a  and  52   b  are input to the CPU  71 . 
     The CPU  71  notifies the coil control circuit  76  of an operation state according to a mode of the color copying machine  1  such as a startup mode, a printing mode, a standby mode, or a sleep mode. 
     Setting for control of the IH coil  50  is explained with reference to  FIGS. 4 to 8 . According to power-on or return from the sleep mode, the CPU  71  starts the startup mode. The startup mode is in a period from a cooled state of the fixing device  30  to the standby mode. At the start of the startup mode, the coil control circuit  76  sets the set temperature T of the IH coil  50  to set temperature Tw 1  (e.g., 170° C.) and sets initial set power Pu of the IH coil  50  to 1000 W (ACT  100 ). In ACT  101 , the first and second thermistors  52   a  and  52   b  measure heat roller temperature t. In ACT  102 , the CPU  71  waits for the heat roller temperature t to be equal to or higher than the set temperature Tw 1 . If the heat roller temperature t is equal to or higher than the set temperature Tw 1  in ACT  102 , the CPU  71  shifts to a standby mode  1 . 
     In the standby mode  1 , the CPU  71  keeps the set temperature T of the IH coil  50  at the same set temperature Tw 1  as set temperature for startup and the coil control circuit  76  sets the initial set power Pu to 800 W (ACT  110 ). The CPU  71  stands by for occurrence of a print interrupt (ACT  111 ). If a print interrupt occurs (Yes in ACT  111 ), the CPU  71  shifts to the printing mode. 
     At the start of the printing mode, the CPU  71  sets the set temperature T of the IH coil  50  to set temperature Tn (e.g., 180° C.), which is first fixing set temperature, and sets the initial set power Pu by the coil control circuit  76  to 800 W (ACT  120 ). The color copying machine  1  performs print processing (ACT  121 ). The CPU  71  subtracts 1 from the number of prints n every time the print processing is performed (ACT  122 ) and, if the number of prints n is equal to or smaller than 1 (Yes in ACT  123 ), shifts to a last paper printing mode. 
     During the last paper printing mode, the CPU  71  sets the set temperature T of the IH coil  50  to set temperature T 1  (e.g., 160° C.), which is second fixing set temperature (ACT  130 ). The color copying machine  1  performs print processing for last paper (ACT  131 ). The CPU  71  shifts to a standby mode  2 . 
     In the standby mode  2 , the CPU  71  sets the set temperature T of the IH coil  50  to set temperature Tw 2  (e.g., 160° C.), which is standby set temperature (ACT  140 ). The CPU  71  stands by for occurrence of a print interrupt (ACT  141 ). If a print interrupt occurs (Yes in ACT  141 ), the CPU  71  shifts to the printing mode. 
     While the setting for control of the IH coil  50  shown in  FIGS. 4 to 8  is carried out, the fixing device  30  performs fixing control according to a flowchart shown in  FIG. 9 . In the fixing device  30 , the first and second thermistors  52   a  and  52   b  measure heat roller temperature t in fixing by the heat roller  37  (ACT  150 ). If the measured heat roller temperature t is equal to or higher than the set temperature T in ACT  151 , the coil control circuit  76  reduces the set power P of the IH coil  50  by, for example, 100 W (ACT  152 ) and proceeds to ACT  153 . The set power P of the IH coil  50  fluctuates when the heat roller temperature t changes. 
     The set temperature T in ACT  151  is, for example, T=Tw 1  in the startup mode and the standby mode  1 , T=Tn in the printing mode, T=T 1  in the last paper printing mode, and T=Tw 2  in the standby mode  2 . If the set power P of the IH coil  50  is smaller than 400 W in ACT  153 , the coil control circuit  76  sets electric power of the IH coil  50  to OFF (the set power P=0 W) (ACT  154 ). If the set power P of the IH coil  50  is equal to or larger than 400 W in ACT  153 , the coil control circuit  76  returns to ACT  150 . 
     If the measured heat roller temperature t is lower than the set temperature T in ACT  151 , the coil control circuit  76  proceeds to ACT  156 . If the set power P of the IH coil  50  is smaller than 400 W in ACT  156 , the coil control circuit  76  sets the set power P of the IH coil  50  to 400 W, resets the set power P to a lower limit of the electric power (ACT  157 ), and returns to ACT  150 . 
     If the set power P of the IH coil  50  is equal to or larger than 400 W in ACT  156 , the coil control circuit  76  increases the set power P of the IH coil  50  by, for example, 100 W (ACT  158 ) and proceeds to ACT  160 . If the set power P of the IH coil  50  is equal to or larger than the initial set power Pu of the IH coil  50  in ACT  160 , in ACT  161 , the coil control circuit  76  sets the electric power of the IH coil  50  to an upper limit of the electric power (the set power P=the initial set power Pu) and returns to ACT  150 . If the set power P of the IH coil  50  is smaller than the initial set power Pu of the IH coil  50  in ACT  160 , the coil control circuit  76  returns to ACT  150 . 
     The initial set power Pu of the ACT  160  is, for example, Pu=1000 W in the startup mode, Pu=800 W in the standby mode  1  and the printing mode, Pu=0 W in the last paper printing mode, and Pu=800 W in the standby mode  2 . 
     Control Example 1 
     As a control example 1 of the IH coil  50 , control for printing five sheets is explained with reference to  FIG. 10 . The coil control circuit  76  sets the initial set power Pu of the IH coil  50  to 1000 W, sets the set temperature T to Tw 1 =170° C., and starts the startup mode of the IH coil  50 . When the heat roller temperature t input to the CPU  71  from the first or second thermistor  52   a  or  52   b  reaches Tw 1 =170° C. from the cooled state as indicated by α 1 , the CPU  71  changes the initial set power Pu of the IH coil  50  to 800 W. A control mode of the heat roller  37  shifts from the startup mode to the standby mode  1 . Thereafter, while the standby mode  1  is maintained, the coil control circuit  76  controls power setting for the IH coil  50  to be swung between, for example, 0 W to 800 W and keeps the heat roller temperature t at 170° C. 
     If a print interrupt occurs, the CPU  71  sets the set temperature T of the IH coil  50  to Tn=180° C. and shifts the fixing device  30  from the standby mode  1  to the printing mode. When printing is started immediately after completion of the startup mode, the heat roller temperature t of the heat roller  37  falls, for example, as indicated by α 2 . In general, until first several sheets P pass the nip  40  of the fixing device  30  after the start of printing, the heat roller temperature t is particularly low. An amount of fall of fixing temperature at the start of printing indicated by α 2  changes according to conditions such as environment temperature and the thickness of the sheet P. Therefore, the set temperature Tw 1  in the standby mode  1  is set in advance such that the heat roller temperature t (α 2 ) at the start of printing keeps temperature equal to or higher than fixing lowest temperature TL indicated by a solid line β 1  even during worst conditions such as allowed lowest environment temperature and allowed maximum sheet thickness. The fixing lowest temperature TL is lowest temperature necessary for the fixing device  30  to obtain satisfactory fixing performance. 
     The heat roller temperature t fallen at the start of printing rises with time and stabilizes at the set temperature Tn as indicated by α 3  while a first sheet P 1  to a fourth sheet P 4  are printed. Thereafter, before a fifth last sheet P 5  enters the nip  40 , the CPU  71  shifts the fixing device  30  from the printing mode to the last paper printing mode and sets the set temperature T to T 1 =160° C. Timing of the shift from the printing mode to the last paper printing mode only has to be timing before the trailing end of the last sheet P 5  passes through the nip  40 . The timing of the shift from the printing mode to the last paper printing mode may be timing after the last sheet P 5  enters the nip  40 . 
     The set temperature T 1  in the last paper printing mode is set lower than the set temperature Tn in the printing mode and set higher than the fixing lowest temperature TL (Tn&gt;T 1 &gt;TL). If the set temperature T of the fixing device  30  is set to lower T 1  from Tn during the last paper printing mode, the heat roller temperature t is higher than the set temperature T 1 =160° C. as indicated by α 4 . Therefore, the set power P to the IH coil  50  is sequentially reduced in order to lower the heat roller temperature t. The set power P sequentially falls from 800 W to 400 W and eventually falls to 0 W. Setting the set power P to 0 W is substantially the same as a state in which the IH coil  50  is turned off. The IH coil  50  stops as indicated by γ 1 . 
     The heat roller temperature t falls according to the fall of the set power P. When the heat roller temperature t is lower than the set temperature T 1  in the last paper printing mode as indicated by α 5 , the set power P is larger than 0 W as indicated by γ 2 . The set power P is controlled to swing between, for example, 0 W to 800 W according to the change of the heat roller temperature t of the fixing device  30 . Since the set power P of the IH coil  50  is controlled to swing, at the finishing of the last paper printing mode, the heat roller temperature t stabilizes at the set temperature T 1  as indicated by α 6 . 
     After finishing the last paper printing mode, the CPU  71  shifts the IH coil  50  from the last paper printing mode to the standby mode  2 . Since the heat roller temperature t stabilizes at T 1  lower than the set temperature Tn at the finishing of the last paper printing mode, the heat roller  37  is suppressed from being overshoot during the shift from the last paper printing mode to the standby mode  2 . 
     During the standby mode  2 , the coil control circuit  76  controls supply power to the IH coil  50  to swing and keeps the heat roller temperature t at Tw 2 =T 1 =160° C. Subsequently, when the IH coil  50  shifts from the standby mode  2  to the printing mode, after the start of printing, the heat roller temperature t of the fixing device  30  is low, for example, as indicated by α 7 . The set temperature Tw 2  in the standby mode  2  is set in advance such that the heat roller temperature t (α 17 ) keeps temperature equal to or higher than the fixing lowest temperature TL indicated by a solid line β 2 . 
     However, if the set temperature Tw 2  in the standby mode  2  is set higher than the set temperature T 1  in the last paper printing mode, when the next printing mode is started immediately after the finishing of the last paper printing mode (before the heat roller temperature t stabilizes at Tw 2 ), it is likely that the heat roller temperature t falls below the fixing lowest temperature TL and causes a fixing failure. Therefore, a relation between the set temperature Tw 2  in the standby mode  2  and the set temperature T 1  in the last paper printing mode needs to be T 1 ≧Tw 2 . When the relation between the set temperature Tw 2  in the standby mode  2  and the set temperature T 1  in the last paper printing mode is T 1 =Tw 2 , efficiency is improved in terms of electric power. 
     Comparative Example 1 
     As a comparative example 1 for the control example 1, control performed when the last paper printing mode is not provided in the control mode for the IH coil  50  is explained with reference to  FIGS. 11 to 14 . In the comparative example 1, at the startup mode, the CPU  71  sets the set temperature T to the startup set temperature Tw (e.g., 170° C.) and sets the initial set power Pu to 1000 W (ACT  200 ). In ACT  201 , the CPU  71  measures the heat roller temperature t. In ACT  202 , the CPU  71  waits for the heat roller temperature t to be equal to or higher than the startup set temperature Tw. If the heat roller temperature t is equal to or higher than the startup set temperature Tw in ACT  202 , the CPU  71  shifts to the standby mode. 
     In the comparative example 1, the CPU  71  keeps the set temperature T at the startup set temperature Tw and sets the initial set power Pu to 800 W (ACT  210 ) and stands by for occurrence of a print interrupt (ACT  211 ). If a print interrupt occurs (Yes in ACT  211 ), the CPU  71  shifts to printing mode. 
     During the printing mode, the CPU  71  sets the set temperature T to the fixing set temperature Tn (e.g., 180° C.) and sets the initial set power Pu to 800 W (ACT  220 ) and performs print processing (ACT  221 ). The CPU  71  subtracts 1 from the number of prints n every time the print processing is performed (ACT  222 ). If the CPU  71  finishes all printings (Yes in ACT  223 ), the CPU  71  shifts to the standby mode. 
     In the comparative example 1, since the set temperature T is Tn at the finishing of the printing mode, the heat roller temperature t in the finishing of printing of the last sheet P 5  stabilizes at Tn higher than T 1  as indicated by δ 1  shown in  FIG. 14 . In the comparative example 1, the fixing device  30  shifts to the standby mode in a state in which the heat roller temperature t is Tn. Therefore, as indicated by δ 2 , the heat roller  37  causes overshoot and waste of power consumption occurs. To prevent the influence of the overshoot, cooling operation for the fixing device  30  by, for example, a cooling fan is necessary after the finishing of the printing mode. This prevents power consumption saving. 
     Control Example 2 
     As a control example 2 for the IH coil  50 , control performed when the number of prints is one is explained with reference to  FIG. 15 . While the standby mode  1  at Tw 1 =170° C. is maintained after the startup mode, if a print interrupt for one sheet occurs, the CPU  71  shifts to the printing mode and sets the set temperature T of the fixing heater  37  to Tn=180° C. The heat roller temperature t falls as indicated by α 8  according to the start of printing immediately after the startup mode but keeps temperature equal to or higher than the fixing lowest temperature T 1  indicated by a solid line β 3 . 
     Since the number of prints is one and the last paper is printed, after the start of the printing mode, before a first sheet P 1 - 1  (last paper) enters the nip  40  of the fixing device  30 , the fixing device  30  shifts from the printing mode to the last paper printing mode. The CPU  71  sets the set temperature T to T 1 =160° C. lower than Tn. However, during the shift to the last paper printing mode, the heat roller temperature t does not reach T 1 =160° C. Therefore, during the shift to the last paper printing mode, although the set temperature T is lowered to T 1 =160° C., the coil control circuit  76  keeps set power 800 W supplied to the IH coil  50  as indicated by γ 3 . 
     After finishing the last paper printing mode, the fixing device  30  shifts from the last paper printing mode to the standby mode  2 . However, at the end of the last paper printing mode, the heat roller temperature t falls to T 1 =160° C. lower than the set temperature T. Therefore, the heat roller  37  is suppressed from being overshoot during the shift from the last paper printing mode to the standby mode  2 . 
     Thereafter, during the standby mode  2 , the coil control circuit  76  controls set power of the IH coil  50  to swing between, for example, 0 W to 800 W as indicated by γ 4  and keeps the heat roller temperature t at Tw 2 =160° C. If a print interrupt for one sheet occurs during the standby mode  2 , the CPU  71  shifts the fixing device  30  to the printing mode. The CPU  71  sets the set temperature T of the fixing heater  37  to Tn=180° C. The coil control circuit  76  changes the initial set power of the IH coil  50  to 800 W. 
     After the start of the printing mode, before a first sheet P 1 - 2  (last paper) enters the nip  40  of the fixing device  30 , the fixing device  30  shifts from the printing mode to the last paper printing mode. However, during the shift to the last paper printing mode, the heat roller temperature t does not reach T 1 =160° C. Therefore, during the shift to the last paper printing mode, although the set temperature T is lowered to T 1 =160° C., the coil control circuit  76  keeps the set power P to the IH coil  50  at 800 W as indicated by γ 5 . 
     When the heat roller temperature t reaches T 1  as indicated by α 9  while the sheet P 2 - 2  is passing through the nip  40 , the coil control circuit  76  controls set power of the IH coil  50  to swing as indicated by γ 6  and keeps the heat roller temperature t at T 1 . 
     After finishing the last paper printing mode, the fixing device  30  shifts from the last paper printing mode to the standby mode  2 . However, at the finishing of the last paper printing mode, the heat roller temperature t keeps T 1 =160° C. lower than the set temperature Tn. Therefore, at the finishing of the last paper printing mode, the fixing device  30  is suppressed from being overshoot. 
     Comparative Example 2 
     As a comparative example 2 for the control example 2, control for keeping the set temperature T of the fixing heater  37  at Tn=180° C. and printing one sheet without providing the last paper printing mode is explained with reference to  FIG. 16 . In the comparative example 2, if a print interrupt for one sheet occurs while the standby mode at Tw is maintained after the startup mode, the fixing heater  37  shifts to the printing mode. During the passage of a first sheet P 2 - 1  (last paper), the heat roller temperature t does not reach T 1  as indicated by δ 3 . Therefore, the heat roller  37  does not cause overshoot during the shift from the printing mode to the standby mode. 
     If a print interrupt for a first sheet P 2 - 2  occurs during the standby mode, the fixing heater  37  shifts to the printing mode. At the finishing of the printing mode for the first sheet P 2 - 2  (last paper), the heat roller temperature t reaches the set temperature Tn as indicated by δ 4 . Therefore, when the fixing heater  37  finishes the printing mode and shifts to the standby mode, the heat roller  37  causes overshoot as indicated by δ 5  and waste of power consumption occurs. To prevent excessively high fixing temperature after the finishing of printing due to the overshoot, cooling operation for the fixing device  30  by, for example, a cooling fan is necessary after the finishing of printing. This prevents power consumption saving. 
     According to the first embodiment, during the last paper printing mode, the set temperature T is set from Tn in the printing mode to lower T 1 . The set temperature T 1  is lowered to T 1 , whereby set power supplied to the IH coil  50  during the last paper printing mode is reduced and power consumption is reduced. Since the set temperature is set to T 1  during the last paper printing mode, the fixing device  30  is suppressed from being overshoot during the shift from the last paper printing mode to the standby mode  2  and waste of power consumption is eliminated. Power consumption for cooling the fixing device  30  is reduced. A reduction in power consumption in the standby mode  2  is realized by setting the set temperature T 1  in the last paper printing mode and the set temperature Tw 2  in the standby mode  2  to be the same. 
     Second Embodiment 
     A second embodiment is explained below. The second embodiment is different from the first embodiment in a heating member. In the second embodiment, the same components as the components explained in the first embodiment are denoted by the same reference numerals and signs and detailed explanation of the components is omitted. 
     In the second embodiment, as shown in  FIG. 17 , a fixing device  81  includes a fixing heater  83  that is a lamp heater configured to heat a heat roller  82  of the fixing device  81  with radiation heat. The heat roller  82  has, for example, a surface layer of fluorine resin or the like around a roller of aluminum having thickness of 0.8 mm. A press roller  84  has, for example, an elastic surface layer of silicon rubber or the like on a cored bar of iron. The heat roller  82  and the press roller  84  come into press-contact with each other to form a nip  85 . The press roller  84  drivingly rotates in an arrow h direction. The heat roller  82  is driven to rotate in an arrow j direction following the press roller  84 . 
     The heat roller  82  includes the fixing heater  83  on the inside of a hollow. The fixing heater  83  includes, for example, a center lamp  83   a  having power consumption of 600 W configured to heat a center area of the heat roller  82 , side lamps  83   b  having power consumption of 600 W configured to heat areas on both sides of the heat roller  82 , an auxiliary lamp  83   c  having power consumption of 280 W configured to cover the center area and the areas on both the sides of the heat roller  82 . For example, when an image is fixed on the sheet P having width of the “A4” landscape size (210 mm) of the JIS standard, the center lamp  83   a  is turned on to heat the center area of the heat roller  82 . When the entire length of the heat roller  82  is heated, the center lamp  83   a  and the side lamps  83   b  are turned on. When a larger heat quantity is necessary, for example, at the startup mode or during fixing on thick paper, the auxiliary lamp  83   c  is turned on. 
     A control system  86  for the fixing heater  83  shown in  FIG. 18  includes a switching circuit  88  configured to on-off control power supply from a power supply  87  to the center lamp  83   a , the side lamps  83   b , and the auxiliary lamp  83   c . The control system  86  includes a heater control board  90  configured to feed back control based on temperature detection results by the first and second thermistors  52   a  and  52   b  to the switching circuit  88 . 
     The switching circuit  88  includes a center lamp control circuit  91   a , a side lamp control circuit  91   b , and an auxiliary lamp control circuit  91   c  configured to respectively on-off control the center lamp  83   a , the side lamps  83   b , and the auxiliary lamp  83   c . The switching circuit  88  connects the center lamp control circuit  91   a , the side lamp control circuit  91   b , and the auxiliary lamp control circuit  91   c  to the power supply  87  via a relay  88   a , a noise filter  88   b , and a power switch  88   c.    
     The heater control board  90  includes an A/D converter  92 , a CPU  93 , a relay off circuit  94 , and an ASIC  96  for the center lamp control circuit  91   a , the side lamp control circuit  91   b , and the auxiliary lamp control circuit  91   c . The CPU  93  controls the entire color copying machine  1 . 
     Setting of control for the fixing heater  83  is explained below with reference to  FIGS. 19 to 24 . At the start of the startup mode, the ASIC  96  sets the set temperature T of the fixing heater  83  to the set temperature Tw 1  (e.g., 170° C.) (ACT  300 ). 
     In ACT  301 , the first and second thermistors  52   a  and  52   b  measure the heat roller temperature t. The CPU  93  waits for the heat roller temperature t to be equal to or higher than the set temperature Tw 1 . If the heat roller temperature t is equal to or higher than the set temperature Tw 1  in ACT  302 , the CPU  93  shifts to the standby mode  1 . 
     In the standby mode  1 , the ASIC  96  keeps the set temperature T of the standby mode  1  of the fixing heater  83  at the same set temperature Tw 1  as set temperature for start up (ACT  310 ). The CPU  93  stands by for occurrence of a print interrupt (ACT  311 ). If a print interrupt occurs (Yes in ACT  311 ), the CPU  93  shifts to the printing mode. 
     At the start of the printing mode, the CPU  93  sets the set temperature T of the fixing heater  83  to the set temperature Tn (e.g., 180° C.), which is first fixing set temperature (ACT  320 ). The color copying machine  1  performs print processing (ACT  321 ). The CPU  93  subtracts from the number of prints n every time the print processing is performed (ACT  322 ) and, if the number of prints n is equal to or smaller than 1 (Yes in ACT  323 ), shifts to the last paper printing mode. 
     During the last paper printing mode, the CPU  93  sets the set temperature T of the fixing heater  83  to set temperature T 1  (e.g., 160° C.), which is second fixing set temperature (ACT  330 ). The color copying machine  1  performs print processing for last paper (ACT  331 ). The CPU  93  shifts to the standby mode  2 . 
     In the standby mode  2 , the CPU  93  sets the set temperature T of the fixing heater  83  to the set temperature Tw 2  (e.g., 170° C.) in the standby mode  2  (ACT  340 ). The CPU  93  stands by for occurrence of a print interrupt (ACT  341 ). If a print interrupt occurs (Yes in ACT  341 ), the CPU  93  shifts to the printing mode. 
     While the setting for control of the fixing heater  83  shown in  FIGS. 19 to 23  is carried out, the fixing device  81  performs fixing control according to a flowchart shown in  FIG. 24 . In the fixing device  81 , the first and second thermistors  52   a  and  52   b  measure the heat roller temperature t of the heat roller  82  (ACT  350 ). If the measured heat roller temperature t is equal to or higher than the set temperature T in ACT  351 , the ASIC  96  turns off the fixing heater  83  (ACT  352 ) and returns to ACT  350 . 
     If the measured heat roller temperature t is lower than the set temperature T in ACT  351 , the ASIC  96  proceeds to ACT  353 . In ACT  353 , the ASIC  96  turns on the fixing heater  83  and returns to ACT  350 . 
     Control Example 3 
     As a control example 3 of the fixing heater  83 , control for printing five sheets is explained with reference to  FIG. 25 . The ASIC  96  turns on the fixing heater  83  and starts the startup mode of the fixing heater  83 . The heat roller temperature t rises as indicated by all. When the heat roller temperature t reaches Tw 1 =170° C., the fixing heater  83  shifts from the startup mode to the standby mode  1  and the ASIC  96  turns off the fixing heater  83 . Thereafter, while the standby mode  1  is maintained, the ASIC  96  on-off controls the fixing heater  83  such that the heat roller temperature t keeps Tw 1 =170° C. 
     If a print interrupt occurs, the CPU  93  sets the set temperature T to Tn=180° C. and turns on and off the fixing heater  83  in the printing mode. When the fixing device  81  starts printing immediately after the startup mode, the heat roller temperature t falls, for example, as indicated by α 12 . However, the set temperature Tw 1  of the standby mode  1  is set in advance such that the heat roller temperature t (α 12 ) keeps temperature equal to or higher than the fixing lowest temperature TL indicated by a solid line β 11  even during worst conditions. 
     The heat roller temperature t fallen at the start of printing rises with time and stabilizes at the set temperature Tn as indicated by α 13 . Thereafter, before the fifth last sheet P 5  enters the nip  85 , the CPU  93  shifts the fixing device  81  from the printing mode to the last paper printing mode. The CPU  93  sets the set temperature T of the fixing motor  83  to T 1 =160° C. lower than Tn. 
     Since the heat roller temperature t is higher than the set temperature T 1 , the ASIC  96  turns off the fixing heater  83  as indicated by γ 3 . The heat roller temperature t falls as indicated by α 14 . At the finishing of the last paper printing mode, the heat roller temperature t stabilizes at the set temperature T 1  in the last paper printing mode as indicated by α 16 . The heat roller  82  is suppressed from being overshoot during shift from the last paper printing mode to the standby mode  2 . 
     After finishing the last paper printing mode, the CPU  93  shifts the control of the fixing heater  83  from the last paper printing mode to the standby mode  2 . During the standby mode  2 , the ASIC  96  on-off controls the fixing heater  83  and keeps the heat roller temperature t at Tw 2 =T 1 =160° C. Further, when the CPU  93  shifts the control of the fixing heater  83  from the standby mode  2  to the printing mode, after the start of printing, the heat roller temperature t of the fixing device  81  falls, for example, as indicated by α 17 . The set temperature Tw 2  in the standby mode  2  is set in advance such that the heat roller temperature t (α 17 ) keeps temperature equal to or higher than the fixing lowest temperature TL indicated by a solid line β 12  under any fixing condition. 
     However, a relation between the set temperature Tw 2  in the standby mode  2  and the set temperature T 1  in the last paper printing mode needs to be T 1 ≧Tw 2 . When the relation between the set temperature Tw 2  in the standby mode  2  and the set temperature T 1  in the last paper printing mode is T 1 =Tw 2 , efficiency is improved in terms of electric power. 
     Comparative Example 3 
     As a comparative example 3 for the control example 3, control performed when the last paper printing mode is not provided in the control mode for the fixing heater  83  is explained with reference to  FIG. 26 . In the comparative example 3, at the finishing of the last paper printing mode, the heat roller temperature t stabilizes at Tn (e.g., 180° C.) higher than T 1  as indicated by δ 11 . In the comparative example 3, the fixing device  81  shifts to the standby mode in a state in which the heat roller temperature t is Tn. Therefore, as indicated by δ 12 , the heat roller  82  causes overshoot and waste of power consumption occurs. To prevent the influence of the overshoot, cooling operation for the fixing device  81  by, for example, a cooling fan is necessary after the finishing of the printing. This prevents power consumption saving. 
     Control Example 4 
     As a control example 4 for the fixing heater  83 , control performed when the number of prints is one is explained with reference to  FIG. 27 . While the standby mode  1  at Tw 1 =170° C. is maintained after the startup mode, if a print interrupt for one sheet occurs, the CPU  93  shifts to the printing mode and sets the set temperature T of the fixing heater  83  to Tn=180° C. When the fixing device  81  starts printing immediately after completion of the startup mode, the heat roller temperature t falls, for example, as indicated by α 18 . Therefore, the ASIC  96  turns on the fixing heater  83  such that the heat roller  82  keeps Tn=180° C. Although the heat roller temperature t falls according to the start of printing immediately after the startup mode, the ASIC  96  keeps temperature equal to or higher than the fixing lowest temperature TL indicated by a solid line β 21 . 
     Since the number of prints is one and the last paper is printed, after the start of the printing mode, before a first sheet P 3 - 1  (last paper) enters the nip  85  of the fixing device  81 , the fixing device  81  shifts from the printing mode to the last paper printing mode. The CPU  93  sets the set temperature T of the fixing heater  83  to T 1 =160° C. lower than Tn. However, during the shift to the last paper printing mode, the heat roller temperature t does not reach T 1 =160° C. Therefore, during the shift to the last paper printing mode, although the set temperature T of the fixing heater  83  is lowered to T 1 =160° C., the ASIC  96  keeps the fixing heater  83  on as indicated by γ 14 . 
     Thereafter, when the heat roller temperature t reaches T 1 =160° C. during execution of the last paper printing mode, as indicated by γ 15 , the ASIC  96  turns off the fixing heater  83 . When the fixing device  81  finishes the last paper printing mode, the control of the fixing heater  83  of the fixing device  81  shifts from the last paper printing mode to the standby mode  2 . However, when the sheet P 3 - 1  passes through the nip  85  of the fixing device  81 , the heat roller temperature t does not reach T 1 =160° C. lower than the set temperature Tn. Therefore, the fixing device  81  is suppressed from being overshoot during the shift from the last paper printing mode to the standby mode  2 . 
     Thereafter, as indicated by α 19 , while the heat roller temperature t keeps Tw 2 =160° C. in the standby mode  2 , if a print interrupt for one sheet occurs, the CPU  93  shifts the fixing device  81  to the printing mode. The CPU  93  sets the set temperature T of the fixing heater  83  to Tn=180° C. The ASIC  96  turns on the fixing heater  83 . 
     For example, as indicated by α 20 , the heat roller temperature t falls according to the start of the printing mode. The ASIC  96  on-off controls the fixing heater  83  such that the heat roller  82  keeps Tn=180° C. After the start of the printing mode, before a first sheet P 3 - 2  (last paper) enters the nip  85  of the fixing device  81 , the CPU  93  shifts the fixing device  81  from the printing mode to the last paper printing mode. The CPU  93  sets the set temperature T of the fixing heater  83  to T 1 =160° C. lower than Tn. However, during the shift to the last paper printing mode, the heat roller temperature t does not reach T 1 =160° C. Therefore, during the shift to the last paper printing mode, although the set temperature T of the fixing heater  83  is lowered to T 1 =160° C., the ASIC  96  keeps the fixing heater  83  on as indicated by γ 16 . 
     While the sheet P 3 - 2  is passing through the nip  85 , when the heat roller temperature t reaches T 1 =160° C. as indicated by α 21 , the ASIC  96  turns off the fixing heater  83  as indicated by γ 17 . Thereafter, the ASIC  96  on-off controls the fixing heater  83  such that the heat roller temperature t keeps T 1 . After finishing the last paper printing mode, the fixing device  81  shifts from the last paper printing mode to the standby mode  2 . When the sheet P 3 - 2  passes through the nip  85  of the fixing device  81 , the heat roller temperature t is T 1 =160° C. lower than Tn. During the shift from the last paper printing mode to the standby mode  2 , the heat roller  82  is suppressed from being overshoot. 
     Comparative Example 4 
     As a comparative example 4 for the control example 4, control for keeping the set temperature T of the fixing heater  83  at Tn and printing one sheet without providing the last paper printing mode is explained. In the comparative example 4, if a print interrupt for one sheet occurs while the standby mode  1  at Tw 1  is maintained after the startup mode, the fixing heater  83  shifts to the printing mode. During the passage of a first sheet P 4 - 1  (last paper), the heat roller temperature t does not reach T 1  as indicated by δ 13 . Therefore, the heat roller  82  does not cause overshoot during the shift from the printing mode to the standby mode. 
     When the printing of one sheet is finished, the fixing heater  83  shifts to the standby mode. The fixing heater  83  is on-off controlled and keeps the heat roller temperature t at the set temperature Tw. If a print interrupt for the first sheet P 4 - 2  occurs during the standby mode, the fixing heater  83  shifts to the printing mode. When the printing mode of the first sheet P 4 - 2  (last paper) finishes, as indicated by δ 14 , the heat roller temperature t reaches the set temperature Tn. Therefore, when the fixing heater  83  finishes the printing mode and shifts to the standby mode, as indicated by δ 15 , the heat roller  82  causes overshoot and waste of power consumption occurs. To prevent excessively high fixing temperature after the finishing of printing due to the overshoot, cooling operation by, for example, a cooling fan is necessary after the finishing of printing. This prevents power consumption saving. 
     According to the second embodiment, during the last paper printing mode, the set temperature T of the fixing heater  83  is set from Tn in the printing mode to lower T 1 , whereby on-time of the fixing heater  83  during the last paper printing mode can be reduced and power consumption can be reduced. Since the set temperature T is lowered to T 1  during the last paper printing mode, the fixing device  81  is suppressed from being overshoot during the shift from the last paper printing mode to the standby mode  2  and waste of power consumption is eliminated. Power consumption for cooling the fixing device  81  is reduced. Power consumption in the standby mode  2  is reduced by setting the set temperature T 1  in the last paper printing mode and the set temperature Tw 2  in the standby mode  2  to be the same. 
     While certain embodiments have been described these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel apparatus and methods described herein may be embodied in a variety of other forms: furthermore various omissions, substitutions and changes in the form of the apparatus and methods described herein may be made without departing from the spirit of the inventions. The accompanying claims and there equivalents are intended to cover such forms of modifications as would fall within the scope and spirit of the invention.