Patent Publication Number: US-2009226202-A1

Title: Fixing device, temperature controlling method and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from: U.S. provisional application 61/034,892, filed on Mar. 7, 2008, the entire contents of each of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     Described below is a temperature control technology in fixation in image formation for forming an image on a sheet. 
     BACKGROUND 
     In fixation for forming an image by fusing a developer image on a sheet, it is very important to control a heating temperature at a constant level in view of formation of a satisfactory image. 
     The fixation as described above is generally carried out by nipping and carrying the sheet formed with the developer image with a pair of rollers at least one of which is heated. As a heat source member for heating the rollers, halogen lamp, induction heat coil (so-called an IH coil), or the like is used. A heating system using the induction heat coil is able to respond to a demand for energy saving since it provides a high heat-exchanging efficiency. 
     A fixing device which carries out the fixation includes a pair of rollers having a length corresponding to a sheet of a maximum size that the device supports (hereinafter, referred to as “full size”), and the fixation for sheets of smaller sizes is also carried out with these rollers. Therefore, when carrying out the fixation for a sheet of a small size, it is necessary to control the temperature distribution of the roller in the direction of an axis of rotation such as preventing the temperature in areas where the sheets do not pass from being heated to an excessively high temperature. 
     JP-A-2000-206813 and JP-A-2001-312178 disclose technologies to uniformize the temperature distribution of fixing rollers in the direction of an axis of rotation thereof by arranging a plurality of induction heat coils at different positions in terms of the direction of the axis of rotation of the fixing rollers and changing the allocation of electric energy to be supplied to the respective coils. A device disclosed in JP-A-2000-206813 detects the temperature difference between the center and end portions of the fixing rollers and changes the allocation of the electric energy to be supplied to the coils according to the temperature difference. In contrast, A device disclosed in JP-A-2001-312178 determines the size of a sheet to be applied with the fixation and changes the allocation of the electric energy to be supplied to the coils according to the sheet size. 
     JP-A-2007-226125 discloses a technology to arrange degaussing coils at both end portions of fixing rollers and activate the degaussing coils when fixing sheets of smaller sizes thereby preventing areas where the sheets do not pass from being heated to an excessively high temperature. 
     However, the temperature control technologies disclosed in JP-A-2000-206813, JP-A-2001-312178, and JP-A-2007-226125 might be able to prevent a partial temperature rise (specifically, an excessive temperature rise on both sides) of the fixing rollers generated when processing the sheets of small sizes, but it might not be able to prevent a temperature fall at joints between the induction heat coils caused by the sheet size. 
     SUMMARY 
     Accordingly, it is an object of the present invention to provide a technology that is able to prevent a temperature fall at joints between heat source members caused by the sheet size and uniformize the temperature distribution when carrying out fixation for forming an image by fixing a developer on the sheet. 
     In order to solve the problem as described above, there is provided a fixing device including: a heat member that heats a sheet formed with a developer image thereon; a press member that is brought into press-contact with the heat member and nips and carries the sheet in cooperation with the heat member; a plurality of heat source members that are arranged at positions different from each other in a direction orthogonal to a direction to carry the sheet and heat the heat member; temperature detectors that are arranged corresponding to the respective heat source members and detect the temperatures of heat target areas that the respective heat source members heat; temperature control units that control outputs from the respective heat source members so that temperatures that the respective temperature detectors detect become preset controlled temperature; a processing target determining unit that determines whether at least one sheet to be subjected to fixation includes a medium size sheet which passes while lying astride both adjacent heat target areas but passes only one of temperature detection target areas; and a controlled temperature determining unit that sets the controlled temperature of the heat source member corresponding to the temperature detection target area where the sheet does not pass to a temperature higher than the controlled temperature of the heat source member corresponding to the temperature detection target area where the sheet passes if it is determined that the medium size sheet is included. 
     There is also provided a temperature controlling method to be performed when fixation is carried out by allowing the sheet to pass through a plurality of heat areas formed at different positions in a direction orthogonal to a direction to carry a sheet formed with a developer image thereon, including: determining whether at least one sheet to be subjected to the fixation includes a medium size sheet which passes while lying astride the both adjacent heat areas but passes only one of temperature detection target areas; and setting the controlled temperature of the heat area corresponding to the temperature detection target area where the sheet does not pass to a temperature higher than the controlled temperature of the heat area corresponding to the temperature detection target area where the sheet passes if it is determined that the medium size sheet is included. 
     The invention also provides an image forming apparatus including: an image data acquiring unit that acquires data of image to be formed; a developer image forming unit that forms a developer image on a sheet on the basis of the image data from the image data acquiring unit; the fixing device that carries out fixation on the sheet formed with the developer image according to claim  1 ; a sheet carrying unit that supplies the sheet to the developer image forming unit and discharges the sheet after the fixation from the apparatus; and an action control unit that controls an image forming action of the apparatus. 
     The invention also relates to a fixing device including: a heating means for heating a sheet formed with a developer image thereon; a pressurizing means being brought into press-contact with the heating means for nipping and carrying the sheet in cooperation with the heating means; a plurality of heat source means being arranged at positions different from each other in a direction orthogonal to a sheet carrying direction for heating the heating means; temperature detecting means being arranged corresponding to the respective heat source means for detecting the temperatures of heat target areas that the respective heat source means heat; temperature control means for controlling outputs from the respective heat source means so that temperatures that the respective temperature detecting means detects become preset controlled temperature; and controlled temperature determining means for determining the controlled temperature so as to prevent the temperature of the heating member at a portion corresponding to a joint between the adjacent heating means from decreasing at the time of the fixation. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical cross-sectional view showing a schematic configuration of an image forming apparatus provided with a fixing device; 
         FIG. 2  is a vertical cross-sectional view of the fixing device according to a first embodiment; 
         FIG. 3  is a side view of the fixing device (including a functional block) according to the first embodiment; 
         FIG. 4  is a top view of the fixing device according to the first embodiment; 
         FIG. 5  is a drawing for explaining a relation among a sheet that the fixing device processes, heat target areas, and temperature detection positions according to the first embodiment; 
         FIG. 6  is a drawing for explaining the relation among the sheet that the fixing device processes, the heat target areas, and the temperature detection positions according to the first embodiment; 
         FIG. 7  is a drawing for explaining the relation among the sheet that the fixing device processes, the heat target areas, and the temperature detection positions according to the first embodiment; 
         FIG. 8  is a circuit drawing showing a control system of the fixing device according to the first embodiment; 
         FIG. 9  is a drawing for explaining a temperature distribution when the fixing device processes a sheet of a full size according to the first embodiment; 
         FIG. 10  is a drawing for explaining a temperature distribution when the fixing device processes a sheet of a medium size according to the first embodiment; 
         FIG. 11  is a drawing for explaining the temperature distribution when the fixing device processes the sheet of the medium size according to the first embodiment; 
         FIG. 12  is a drawing for explaining a temperature distribution when the fixing device processes a sheet of a small size according to the first embodiment; 
         FIG. 13  is a side view of the fixing device (including a functional block) according to a second embodiment; 
         FIG. 14  is a side view of the fixing device (including a functional block) according to a third embodiment; 
         FIG. 15  is a flowchart showing a flow of a temperature control by the fixing device according to the third embodiment; and 
         FIG. 16  is a side view of the fixing device (including a functional block) according to a fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, a fixing device will be described.  FIG. 1  is a vertical cross-sectional view of an image forming apparatus (MFP: Multi Function Peripheral) having the fixing device. 
     As shown in  FIG. 1 , the image forming apparatus includes an image scanning unit R and an image forming unit P. The image scanning unit R has a function to scan an image from a sheet-type original or a book-type original and acquire image data to be formed. The image forming unit P has a function to form a developer image on a sheet such as a printing paper or a film on the basis of the image data that the image scanning unit R acquires from the original or image data transmitted from an external device. In other words, the image forming apparatus includes an image data acquiring unit. 
     A flow of general image formation that the image forming apparatus carries out will be described with an example of a process of color copying an image on a sheet-type original on a sheet below. 
     The image scanning unit R scans the image of the original placed at an image scanning position by an Auto Document Feeder (ADF)  9  or manually by a scanning optical system  10  and acquires image data. 
     Subsequently, the image forming unit P forms electrostatic latent images on photoconductive surfaces of photoconductive drums ( 2 Y to  2 K) for yellow (Y), magenta (M), cyan (C), and black (K) on the basis of the image data that the image scanning unit R acquires. Subsequently, developer stirred by mixers ( 4 Y to  4 K) in a developing unit is supplied to the photoconductive surfaces of the photoconductive drums ( 2 Y to  2 K) by developing rollers ( 3 Y to  3 K) and the electrostatic latent images are visualized on the photoconductive surfaces by the developer. The developer images visualized on the photoconductor surfaces are transferred to a surface of a rotating intermediate transfer belt  6  (primary transfer), and are carried to a position T where the developer images are transferred to the sheet (secondary transfer). 
     On the other hand, the sheet picked up from a cassette by one of pickup rollers  51  to  54  is transferred to the secondary transfer position T by a plurality of roller pairs, and the developer images are transferred from the intermediate transfer belt  6  at the secondary transfer position T. 
     Subsequently, the sheet to which the developer images are transferred is supplied to a fixing device  7 . The fixing device  7  carries out fixation, described later in detail, to fix the developer images to the sheet. 
     The sheet to which the developer images are thermally fixed is carried in a carrying path by a plurality of carrying roller pairs and is discharged onto a discharge tray  8 . 
     These processes in the image formation are realized by a CPU  801  which carries out a program stored in a memory  802 . The memory  802  stores the program and various data to be used in the image processing. 
     First Embodiment 
     Referring now to  FIG. 2  to  FIG. 4 , the fixing device according to the first embodiment will be described.  FIG. 2  is a vertical cross-sectional view of the fixing device;  FIG. 3  is a side view of the fixing device (including a functional block); and  FIG. 4  is a top view of the fixing device. 
     The fixing device includes a heat roller (so-called a fixing roller)  21  as a heat member that heats a sheet S on which developer images D are transferred and a press roller  22  as a press member which comes into press-contact with the heat roller  21 . The press roller  22  is brought into press-contact with the heat roller  21  by a pressurizing mechanism  23  so as to maintain a constant nip width so that a nip  24  is formed. The heat roller  21  is driven by a drive motor  25  and rotates in the direction of an arrow in the drawing. The press roller  22  is in press-contact with the heat roller  21 , and hence rotates in association with the rotating heat roller  21 . In other words, the heat roller  21  and the press roller  22  cooperatively nip and carry the sheet S formed with the developer images D, apply heat and a pressure to the sheet S passing through the nip  24 , and fuse and press-fit the developer images D. 
     The effective lengths of the heat roller  21  and the press roller  22  in the direction of an axis of rotation which substantially come into contact with the sheet S and heat the same are at least the same as or longer than the dimension of the full size sheet. The diameters of the rollers  21  and  22  may be 40 mm. The heat member is not limited to the heat roller  21  shown in  FIG. 1 , and a configuration including an endless belt (so-called a fixing belt) having a metallic conductive layer is also applicable. 
     The heat roller  21  has a configuration including a core metal  21   a , a foamed rubber (so-called sponge) layer  21   b , a metallic conductive layer  21   c , a solid rubber layer  21   d , and a mold release layer  21   e  laminated in sequence from the side of the axis of rotation. For example, a configuration in which the thickness of the foamed rubber layer  21   b  is 5 mm, the thickness of the metallic conductive layer  21   c  is 40 μm, the thickness of the solid rubber layer  21   d  is 200 μm, and the thickness of the mold release layer  21   e  is 30 μm is applicable. The material of the metallic conductive layer may be nickel, stainless, aluminum, or a composite material including stainless and aluminum. 
     The press roller  22  has a configuration including a core metal  22   a , a rubber layer  22   b  such as silicon rubber or fluorine-contained rubber, and a mold release layer  22   c  laminated in sequence from the side of the axis of rotation For example, a configuration in which the thickness of the rubber layer  22   b  is 1 mm and the thickness of the mold release layer  22   c  is 30 μm is applicable. 
     An induction heat coil (so called, an IH coil)  31  as a heat source member is arranged on the outer periphery of the heat roller  21  so as to oppose thereto via a space. The induction heat coil  31  includes a litz wire  32  including a plurality of wires such as copper wires insulated from each other by heat resistant polyamide imide or the like tied up together into a bundle. The induction heat coil  31  generates a magnetic flux and an eddy-current in the heat roller  21  if a high-frequency current is applied from an exciting circuit (so-called an inverter circuit) described later. The heat roller  21  is heated by Joule heat generated by the eddy-current and a resistance of the heat roller  21 . The induction heat coil  31  includes a magnetic body core  33  bent in cross-section so as to surround the outer periphery of the heat roller  21 , so that the generated magnetic flux concentrates on the heat roller  21 . 
     The induction heat coil  31  may have a plurality of induction heat coils arranged at different positions from each other in the direction of the axis of rotation of the heat roller  21 . As shown in  FIG. 3  and  FIG. 4 , a first induction heat coil  31   a  and second induction heat coils ( 31   b ,  31   c ) are arranged in the direction of the axis of rotation of the heat roller  21 . The first induction heat coil  31   a  is adapted to heat a center area in the direction of the axis of rotation of the heat roller  21  as a heat target area. The second induction heat coils ( 31   b ,  31   c ) heat both side areas in the direction of the axis of rotation of the heat roller  21  as heat target areas. The first induction heat coil  31   a  and the second induction heat coils ( 31   b ,  31   c ) are electrically isolated and are controlled in output independently. Two coils that constitute the second induction heat coils ( 31   b ,  31   c ) are electrically connected in series and are controlled equally in output. The entire length of the coil formed of the plurality of induction heat coils  31   a  to  31   c  is, for example, 320 mm, and the length of the first induction heat coil is, for example, 200 mm. 
     As shown in  FIG. 3 , the positions of a first temperature sensor  34   a  and a second temperature sensor  34   b  in the direction of the axis of rotation of the heat roller  21  are different from each other. The first temperature sensor  34   a  detects the temperature at the center of the heat roller  21  in the direction of the axis of rotation, that is, the temperature of the heat target area of the first induction heat coil  31   a . The second temperature sensor  34   b  detects the temperature of the sides of the heat roller  21  in the direction of the axis of rotation, that is, the temperature of the heat target areas of the second induction heat coil  31   b . The first and second temperature sensors  34   a  and  34   b  may be thermopile-type non-contact temperature sensors which detect infrared rays. The temperatures that the first and second temperature sensors  34   a  and  34   b  detect are used for output control of the first and second induction heat coils ( 31   a  to  31   c ). 
     The fixing device includes a third temperature sensor  35  that detects the temperature of the heat roller  21 . The third temperature sensor  35  is arranged at a position outside the area where the sheet of a full size passes in the direction of the axis of rotation of the heat roller  21 . The third temperature sensor  35  may be a contact-type thermistor. The temperatures that the third temperature sensor  35  detects are used for stopping a heating action by interlocking when the temperature of the heat roller  21  rises to an abnormal level. 
     As shown in  FIG. 2 , the fixing device includes a separating claw  36  that prevents the sheet S from adhering to the heat roller  21 . 
     Referring now to  FIG. 3 , functions of the fixing device will be described. A first temperature control unit  41   a  has a function to feedback control the output from the first induction heat coil  31   a  so that the temperature that the first temperature sensor  34   a  detects is kept at a predetermined controlled temperature. A second temperature control unit  41   b  has a function to feedback control the outputs from the second induction heat coils ( 31   b ,  31   c ) so that the temperature that the second temperature sensor  34   b  detects is kept at a predetermined controlled temperature. 
     A controlled temperature determining unit  42  has a function to determine the controlled temperatures of the first induction heat coil  31   a  and the second induction heat coils ( 31   b ,  31   c ) on the basis of the size of the sheet S to be subjected to the fixation, and provide determined temperature data respectively to the first and second temperature control units  41   a  and  41   b . In particular, the controlled temperature determining unit  42  determines whether the controlled temperatures of the first induction heat coil  31   a  and the second induction heat coils ( 31   b ,  31   c ) are to be equalized or to be different temperatures if the size of the sheet S to be subjected to the fixation has a relation as shown in  FIG. 5  to  FIG. 7  described later between the heat target areas of the respective coils and the temperature detection positions of the respective temperature sensors. 
     A sheet size determining unit  43  as a processing target determining unit has a function to determine the size of the sheet S to be subjected to the fixation according to size data of the sheet S that the a user specifies via an operation panel (not shown) of the device or size data that a sensor (not shown) disposed in a sheet carrying path of the image forming apparatus shown in  FIG. 1  or a selected cassette identifies and provide determined size data to the controlled temperature determining unit  42 . A storage unit  44  includes, for example, a hard disk or a flush memory, and stores various programs or various data used for the fixation. The functions that the fixing device can use are realized by the CPU  801  activating the program stored in the storage unit  44 , the memory  802 , and the like. 
     Referring now to  FIG. 5  to  FIG. 7 , a method of determining the controlled temperature will be described. 
     As described above, the fixing device heats the center and the both sides of the heat roller  21  separately using the first induction heat coil  31   a  and the second induction heat coils ( 31   b ,  31   c ) controlled in output independently. Therefore, the sheet S to be subjected to the fixation is classified into three cases as schematically shown in  FIG. 5  to  FIG. 7  in terms of the relation between the heat target area and the temperature detection position in the direction of the axis of rotation of the heat roller  21 . 
     In a first case, as shown in  FIG. 5 , the sheet S to be subjected to the fixation passes both through a center heat target area  51   a  and a side heat target area  51   b  and also both through a center temperature-detection position  52   a  and a side-temperature detection position  52   b . The sheet size corresponding to the first case is referred to as “full size”. For example, in the case of sheets according to JIS standard, A4 size and A3 size correspond to the first case. The temperature detection positions  52   a  and  52   b  are target areas that the temperature sensors detect the temperature thereof. 
     In a second case, as shown in  FIG. 6 , the sheet S to be subjected to the fixation passes both through the center heat target area  51   a  and the side heat target area  51   b , but passes only the center temperature-detection position  52   a . In other words, an end of the sheet S passes a position shifted to the center from the side temperature-detection position  52   b . The sheet size corresponding to the second case is referred to as “medium size”. For example, in the case of the sheets according to JIS standard, A4-R size and B4 size correspond to the second case. 
     In a third case, as shown in  FIG. 7 , the sheet S to be subjected to the fixation passes only through the center heat target area  51   a  and the center temperature-detection position  52   a . In other words, it is a case in which the area where the sheet S passes is included within the center heat target area  51   a . The sheet size corresponding to the third case is referred to as “small size”. For example, in the case of the sheets according to JIS standard, B5-R size and a post card size correspond to the third case. 
     The controlled temperature determining unit  42  determines to equalize the controlled temperatures of the first induction heat coil  31   a  and the second induction heat coils ( 31   b ,  31   c ) if the size of the sheet S is the full size or the small size. In contrast, the controlled temperature determining unit  42  determines to set the controlled temperature of the second induction heat coil  31   b  to a higher temperature than the controlled temperature of the first induction heat coil  31   a  if the sheet size is the medium size. Since the lengths of the respective induction heat coils and the positions of the respective temperature sensors are defined on the basis of a device design, the sheets can be classified into the full size, the medium size, and the small size in advance. Therefore, data on the size-to-size basis relating to which control is to be carried out set in advance may be stored in the storage unit  44 . The sheets classified as the medium size include several sizes such as A4-R size or B4 size. Therefore, the temperature control which is in tune with the circumstances is achieved by changing the controlled temperature according to the fragmented sizes in the medium size when storing the data preset on the size-to-size basis. 
     Returning back to  FIG. 8 , a control system which realizes the above-described functions will be described. As shown in  FIG. 8 , resonant capacitors  61   a  and  61   b  are connected to the first induction heat coil  31   a  and the second induction heat coils ( 31   b ,  31   c ) respectively in parallel, and switching elements  62   a  and  62   b  are connected to a resonance circuit to form an exciting circuit (inverter circuit). A DC power obtained by smoothing an AC power from a commercial AC power source  63  by a rectifying circuit  64  is supplied to the inverter circuit. A transformer  65  arranged on the upstream side of the rectifying circuit  64  detects a total power consumption and feeds back a power consumed by heating. Drive circuits are connected respectively to control terminals of the switching elements  62   a  and  62   b . The drive circuits serve to apply a drive voltage to the control terminals of the switching elements  62   a  and  62   b  and drive (turn ON) the switching elements  62   a  and  62   b . Control circuits  66   a  and  66   b  output timings to apply the drive voltage to the control terminals. The control circuits  66   a  and  66   b  determine the ON time, and change the power by changing the ON time while feeding back an input power. If the ON time is changed, one cycle of the current flowing in the induction heat coils ( 31   a  to  31   c ) is changed correspondingly, so that the drive frequency is changed as well. In other words, the frequency is changed within a range from 20 to 100 kHz by controlling the ON time to control the power value (output value) to be supplied to the induction heat coils ( 31   a  to  31   c ). Such action is controlled by a CPU  67 . The CPU  67  reads a program or data from the memory  68  included in the storage unit  44  to carry out the control as described above. 
     Subsequently, a temperature controlling method when carrying out the fixation will be described. It is assumed that the fixing device is already completed with an initial activation and is in a ready state in which the temperature of the heat roller  21  is raised to a predetermined temperature. For example, the first and second induction heat coils  31   a  to  31   c  heat the heat roller  21  to a temperature of 160° C. to assume the ready state. It is also assumed that the sheet S to be subjected to the fixation is formed with the developer images D in advance, for example, by the image forming unit P shown in  FIG. 1 . 
     In such a state, the sheet size determining unit  43  of the fixing device determines the size of the sheet S to be subjected to the fixation. The controlled temperature determining unit  42  determines whether to equalize the controlled temperatures of the center and the both sides or to set the controlled temperature of the both sides to a higher temperature than the center on the basis of the determined size data. In the case of the full size sheet (for example, A4 size or A3 size), the controlled temperature determining unit  42  determines to equalize the controlled temperatures of the center and the both sides. The controlled temperature of the first induction heat coil  31   a  is set to 160° C. and the controlled temperature of the second induction heat coils ( 31   b ,  31   c ) is set to 160° C. 
     In the case of the full size sheet S, since the calorific power that the sheet S passing through the nip  24  absorbs is substantially the same between the center and the both sides, as shown schematically in  FIG. 9 , the temperature distribution in the area where the sheet S passes in the direction of the axis of rotation of the heat roller  21  is substantially uniform. 
     In contrast, in the case of the medium size sheet S (for example, A4R size or B4 size), the controlled temperature determining unit  42  determines to set the controlled temperature of the both sides to a temperature higher than that of the center. The controlled temperature of the first induction heat coil  31   a  is set to 160° C. and the controlled temperature of the second induction heat coils ( 31   b ,  31   c ) is set to 170° C. 
     As described above, although the medium size sheet S lies astride both the heat target areas  51   a  and  51   b  at the center and the both sides, the widthwise end of the sheet S passes a position shifted from the temperature detection position  52   b  on the side to the center. Therefore, if the controlled temperatures of the center and the both sides are equalized as in the case of the full size sheet, as shown in schematically in  FIG. 10 , a temperature fall occurs at the joints between the adjacent coils. In other words, since the sheet S passes the position shifted from the temperature detection position  52   b  on the side, the temperature that the temperature sensor  34   b  detects is higher than the temperature of a portion where the sheet S passes. Therefore, the calorific power to be supplied to the side by feedback is insufficient, and the temperature fall occurs at the joints between the coils. 
     Therefore, in the case of the medium size sheet S, the temperature distribution in the area where the sheet S passes in the direction of the axis of rotation of the heat roller  21  is substantially uniformized as schematically shown in  FIG. 11  by setting the controlled temperature of the both sides to a temperature 10° C. higher. 
     Although the temperature of areas on the both sides where the sheet S does not pass is increased by 10° C. as shown in  FIG. 11  by setting the controlled temperature on the both sides to a temperature 10° C. higher, it is not a temperature to affect the heat-resistant temperature of the heat roller  21 , and not a temperature to cause a hot offset even though the full size sheet S passes subsequently. 
     In contrast, in the case of the small size sheet S (for example, B5-R size or post card size), the controlled temperature determining unit  42  determines to equalize the controlled temperatures between the center and the both sides. The controlled temperature of the first induction heat coil  31   a  is set to 160° C. and the controlled temperature of the second induction heat coils ( 31   b ,  31   c ) is set to 160° C. 
     In the case of the small size sheet S, the width of the sheet S in the direction of the axis of rotation of the heat roller  21  is included within the center heat target area  51   a . Therefore, the portion from which the heat is absorbed substantially by the sheet S is only the center heat target area  51   a . Therefore, even though the control to equalize the controlled temperatures of the center and the both sides, the temperature fall does not occur at the joints between the coils as schematically shown in  FIG. 12 , and the temperature distribution in the area where the sheet S passes in the direction of the axis of rotation of the heat roller  21  is substantially uniformized. 
     As described above, according to the first embodiment, the temperature fall at the joints between the coils caused by the size of the sheet S is prevented, so that the temperature distribution is uniformized. Consequently, the fixation which forms satisfactory images on the sheets S of any sizes such as the full size, the medium size, and the small size is achieved. 
     If the heat roller  21  is divided into sections for heating using the plurality of induction heat coils, the temperature fall at the joints between the coils is prevented by dividing the heated area into the number corresponding to the number of sizes of the sheets that the device processes. However, in the actual devices, the number of coils is smaller than the number of sizes of the sheets that the device processes in many cases due to the problems of rising of the cost or upsizing of the device. The device in the first embodiment has an advantage such that the fixation is carried out with uniform temperature distribution in the direction of the axis of the rotation with respect to the sheets S of various sizes even with a small number of induction heat coils. 
     Second Embodiment 
     Subsequently, a second embodiment will be described. 
     The second embodiment is a modification of the first embodiment described above. As shown in  FIG. 13 , the second embodiment is different from the first embodiment in that a quantity determining unit  45  that determines the numerical quantity of sheets to be subjected to the fixation is provided, and the temperature control is carried out on the basis of the size and the numerical quantity of the sheets to be subjected to the fixation. 
     The quantity determining unit  45  has a function to determine the numerical quantity of the sheets S to be subjected to the fixation according to the data on the number of copies that the user specifies via the operation panel (not shown) of the apparatus or data on the numerical quantity of the sheets S passed through the sheet carrying path or the like of the image forming apparatus shown in  FIG. 1  identified by a sensor (not shown) arranged therein or the selected cassette and provide the determined data on the number to the controlled temperature determining unit  42 . 
     If the numerical quantity of the sheet S to be subjected to the fixation continuously is more than a predetermined numerical quantity, the controlled temperature determining unit  42  determines the controlled temperature on the basis of the corresponding size of the sheet S. The numerical quantity of sheets to be subjected to the fixation continuously is, for example, the numerical quantity of sheets to be subjected to the fixation in one job. For example, if the numerical quantity of sheets is thirty or more, the controlled temperature is determined on the basis of the corresponding size of the sheets S. In other words, if thirty or more the full size or the small size sheets S are to be subjected to the fixation continuously, the controlled temperature determining unit  42  determines to equalize the controlled temperature of the first induction heat coil  31   a  and the second induction heat coils ( 31   b ,  31   c ). If thirty or more the medium size sheets S are to be subjected to the fixation continuously, the controlled temperature determining unit  42  determines to set the controlled temperature of the first induction heat coil  31   a  to be higher than the controlled temperature of the second induction heat coils ( 31   b ,  31   c ). 
     For example, if the fixation is to be carried out alternately for the full size sheets S and the medium size sheets S, occurrence of a state in which the controlled temperature is changed frequently is prevented by controlling only on the basis of the size of the sheet S, whereby the temperature control which is in tune with the circumstances is achieved. 
     Third Embodiment 
     Referring now to  FIG. 14  and  FIG. 15 , a third embodiment will be described. The third embodiment is a modification of the first embodiment described above. The third embodiment is different from the first embodiment in that the temperature of the press roller  22  is detected and the controlled temperature is determined on the basis of the detected temperature instead of determining the controlled temperature on the basis of the size of the sheet S as in the first embodiment. 
     In other words, the temperature distribution of the press roller  22  in the direction of the axis of rotation which is changed by the sheet S passing through the nip  24  is detected, and the controlled temperatures of the first induction heat coil  31   a  and the second induction heat coils ( 31   b ,  31   c ) are determined on the basis of a change in temperature distribution. 
     As shown in  FIG. 14 , the fixing device includes a fourth temperature sensor  71   a  and a fifth temperature sensor  71   b  that detect the temperatures of the press roller  22  at different positions in the direction of the axis of rotation. The fourth and fifth temperature sensors  71   a  and  71   b  are arranged at positions corresponding to the first and second temperature sensors  34   a  and  34   b  in the direction of the axis of rotation of the rollers. The fourth and fifth temperature sensors  71   a  and  71   b  may be, for example, non-contact thermistors, or thermopile-type non-contact temperature sensors which detect infrared rays. A sixth temperature sensor  72  is arranged at a position outside the area where the full size sheet passes in the direction of the axis of rotation of the press roller  22 . The sixth temperature sensor  72  may be a contact-type thermistor. 
     The fixing device includes a first halogen lamp  73   a  and a second halogen lamp  73   b  arranged at positions different from each other in the direction of the axis of rotation of the press roller  22 . The first halogen lamp  73   a  is adapted to heat the center area in the direction of the axis of rotation of the press roller  22 . The second halogen lamp  73   b  is adapted to heat the both side areas in the direction of the axis of rotation of the press roller  22 . The heat source member that heats the press roller  22  is not limited to the halogen lamp. 
     A temperature data acquiring unit  74  calculates the temperature difference between these positions from temperatures that the fourth temperature sensor  71   a  and the fifth temperature sensor  71   b  detect. The controlled temperature determining unit  42  determines the controlled temperatures of the first induction heat coil  31   a  and the second induction heat coils ( 31   b ,  31   c ) on the basis of the temperature difference described above. 
     Referring now to  FIG. 15 , a method of determining the controlled temperature when carrying out the fixation will be described. It is assumed that the fixing device is completed with the initial activation and is in a ready state in which the temperatures of the heat roller  21  and the press roller  22  are raised to predetermined temperatures (Act  101 ). The controlled temperatures of the heat roller  21  are 160° C. at the center and 160° C. at the both sides. The controlled temperatures of the press roller  22  are 130° C. at the center and 130° C. at the both sides. 
     When the fixation is started in such a state, the temperature data acquiring unit  74  determines whether the temperature difference (=temperature at the sides (° C.)−temperature at the center (° C.)) that the fourth and fifth temperature sensors  71   a  and  71   b  detect is 15° C. or more or not (Act  102 ). If the temperatured at a acquiring unit  74  determines that the temperature difference is smaller than 15° C. (No in Act  102 ), the controlled temperature determining unit  42  maintains the controlled temperature in Act  101 . 
     In contrast, if the temperature data acquiring unit  74  determines that the temperature difference is 15° C. or more (Yes in Act  102 ), the temperature data acquiring unit  74  determines whether the temperature difference is 20° C. or more or not. If the temperature data acquiring unit  74  determines that the temperature difference is smaller than 20° C. (No in Act  103 ), the controlled temperature determining unit  42  determines to change the controlled temperature of the both ends of the heat roller  21  to 170° C. for the control (Act  104 ). 
     If the temperature data acquiring unit  74  determines that the temperature difference is 20° C. or more (Yes in Act  103 ), the controlled temperature determining unit  42  determines to change the controlled temperature of the both sides of the heat roller  21  to 175° C. for the control (Act  105 ). 
     As described above, when the full size sheet S passes through the nip  24 , the temperatures at the center and the both sides fall by substantially the same temperature. Therefore, the temperature difference described above does not often exceed 15° C. In contrast, when the medium size and small size sheets S pass through the nip  24 , the temperature of the press roller  22  falls more at the center than at the both sides thereof in the direction of the axis of rotation. Such the temperature change is remarkable when a plurality of (or a plurality of pages of) the sheets S pass continuously through the nip  24 . Therefore, the temperature distribution of the press roller  22  in the direction of the axis of rotation is monitored, and if the temperature difference reaches or exceeds the predetermined value, it is determined that the sheets are the medium size sheets or the small size sheets, so that the controlled temperature of the both sides is determined to be risen. In other words, the temperature data acquiring unit  74  constitutes a processing target determining unit together with the fourth and fifth temperature sensors  71   a  and  71   b . As described above, the small size sheet S does not pass the side heat target area ( 51   b ) on the both sides, even though the controlled temperature of the both sides is changed to 175° C., it does not often affect the temperature distribution of the area where the sheet passes. On the other hand, the temperature fall between the coils occurring when the medium size sheet S passes is prevented by rising the controlled temperature of the both sides as described above. 
     As described above, according to the third embodiment, the temperature fall at the joints between the coils caused by the size of the sheet S is prevented, so that the temperature distribution in the area where the sheet passes is uniformized in the same manner as the first embodiment. In particular, since the controlled temperature is determined according to the temperature difference on the side of the press roller  22 , the controlled temperature is changed only when the medium size and small size sheets S are brought into the fixation continuously until a constant temperature difference is achieved. Therefore, if the fixation is to be carried out alternately for the sheets S of different sizes, occurrence of a state in which the controlled temperature is changed frequently is prevented by controlling only on the basis of the size of the sheet S the same as the first embodiment, whereby the temperature control which is in tune with the circumstances is achieved. 
       FIG. 14  shows an example in which the fourth and fifth temperature sensors  71   a  and  71   b  are arranged at almost the same positions as the first and second temperature sensors  34   a  and  34   b  in the direction of the axis of rotation of the rollers, the positions of arrangement are not limited thereto. For example, the fourth and fifth temperature sensors  71   a  and  71   b  may be arranged at positions closer to the joints between the coils than the first and second temperature sensors  34   a  and  34   b . In this manner, by arranging the fourth and fifth temperature sensors  71   a  and  71   b  to the positions closer to the joints between the coils, the temperature fall at the joints between the coils is prevented. 
     The invention is not limited to detecting the temperatures of the plurality of positions of the press roller  22  by the fourth and fifth temperature sensors  71   a  and  71   b  and, for example, it is also possible to detect the temperatures near the positions corresponding to the joints between the coils on the press roller by one temperature sensor and determine the controlled temperature on the basis of a change in detected temperatures. 
     Fourth Embodiment 
     Subsequently, a fourth embodiment will be described. 
     The fourth embodiment is a modification of the third embodiment described above. As shown in  FIG. 16 , a sheet size determining unit  75  that determines the size of the sheet S to be subjected to the fixation is provided. The sheet size determining unit  75  has the same configuration as the sheet size determining unit  43  in the first embodiment. 
     The fixing device carries out the method of determining the controlled temperature as shown in  FIG. 15  as a general rule. However, if it is determined that the sheet S to be subjected to the fixation is of a small size, the controlled temperature determining unit  42  determines that the controlled temperature is set to a default value (that is, the controlled temperature in Act  101  in  FIG. 15 ). In the method of determining the controlled temperature shown in  FIG. 15 , the controlled temperature of the both sides is set to a high temperature also for the small size sheet. However, if it is the small size sheet, the temperature fall at the joints between the coils does not occur often. Therefore, if it is determined to be the small size, the controlled temperature is returned to the original value, so that the temperature distribution in the area where the sheet passes in the direction of the axis of rotation of the heat roller  21  is uniformized. 
     Fifth Embodiment 
     Subsequently, a fifth embodiment will be described. 
     The fifth embodiment is a modification of the first to fourth embodiments. In other words, a quantity determining unit that determining the numerical quantity of the sheets S to be subjected to the fixation is provide, and if it is determined that the numerical quantity of the sheets S to be subjected to the fixation continuously is determined to be a predetermined numerical quantity or more, a control to reduce the numerical quantity of the sheets S to be passed through the nip  24  per unit time (for example, lowers the process speed) is carried out. The quantity determining unit has the similar configuration to the quantity determining unit  45  in the second embodiment. 
     The temperature change occurring, for example, when the medium size sheets S pass through the nip  24  continuously is reduced by reducing the numerical quantity of the sheets S to be passed through the nip  24  per unit time, so that the temperature distribution in the area where the sheet passes in the direction of the axis of rotation of the rollers is uniformized. 
     In the respective embodiments described above, a configuration in which the center of the sheet S passes through the position near the center of the heat roller, so-called a “central paper feeding system” is employed is exemplified, the invention is not limited thereto. For example, a configuration in which the sheet S is carried at a position shifted on one of the sides in the sheet carrying direction, so-called a “one-side paper feeding system” is employed is also applicable. 
     In the embodiments described above, the device that senses the temperature of the heat roller or the press roller is not limited to the examples shown above and, needless to say, any of the contact type or non-contact type may be employed as appropriate. 
     The determination of the sheet size or the determination of the throughput of the sheets to be subjected to the fixation may be carried out during the fixation instead of performing prior to the actual fixation. In other words, the determination must simply be completed as a result before the controlled temperature determining unit determines the controlled temperature. 
     Specifically, as the sheet S to be subjected to the fixation in the respective embodiments described above, various sheets which are usable as printing media in the image forming apparatus such as copying sheets (so-called normal sheets), OHP films, card boards, post cards, and so on may be employed. 
     Although the specific aspects is described in detail, it is apparent for those skilled in the art that various modifications or improvement are possible as long as it does not deviate from the spirit and the scope in the invention. 
     According to the embodiments described above in detail, when carrying out the fixation for fixing developer on the sheet and forming an image, the temperature fall at the joints between the heat source members caused by the size of the sheet is prevented, so that the temperature distribution is uniformized.