Patent Publication Number: US-8971782-B2

Title: Fixing device, image forming apparatus incorporating same, and method for heating fixing rotary body

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Application No. 2011-002892, filed on Jan. 11, 2011, in the Japanese Patent Office, the entire disclosure of which is hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     Example embodiments generally relate to a fixing device, an image forming apparatus, and a method for heating a fixing rotary body, and more particularly, to a fixing device for fixing a toner image on a recording medium, an image forming apparatus including the fixing device, and a method used by the fixing device. 
     BACKGROUND OF THE INVENTION 
     Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium. 
     The fixing device used in such image foaming apparatuses may employ an induction heater to warm up the fixing device quickly to a predetermined fixing temperature with reduced energy consumption. For example, the induction heater is disposed opposite a fixing roller that presses against a pressing roller to form a fixing nip between the fixing roller and the pressing roller. As a recording medium bearing a toner image passes through the fixing nip, the fixing roller heated by the induction heater and the pressing roller apply heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium. 
     Specifically, the induction heater includes an exciting coil that generates a magnetic flux toward a conductive layer of the fixing roller. As the magnetic flux reaches the conductive layer of the fixing roller, the conductive layer generates an eddy current that heats the conductive layer throughout the entire width of the fixing roller in the axial direction thereof. However, if a small recording medium having a width smaller than the entire width of the fixing roller in the axial direction thereof is conveyed through the fixing nip, the lateral ends of the fixing roller in the axial direction thereof over which the small recording medium is not conveyed may be overheated because the small recording medium does not draw heat from the lateral ends of the fixing roller in the axial direction thereof 
     To address this circumstance, degaussing coils may be disposed between the exciting coil and the fixing roller in such a manner that the degaussing coils are disposed opposite the lateral ends of the fixing roller in the axial direction thereof, respectively, to offset the magnetic flux generated by the exciting coil toward the fixing roller, thus minimizing the magnetic flux that reaches the conductive layer of the fixing roller and therefore preventing overheating of the lateral ends of the fixing roller in the axial direction thereof For example, when the image forming apparatus receives a print job for forming a toner image on a small recording medium, the degaussing coils are turned on. Conversely, when the image forming apparatus receives a print job for forming a toner image on a large recording medium, the degaussing coils are turned off. 
     However, such configuration has a drawback in that the degaussing coils cannot be turned on and off while the exciting coil is turned on because serially-connected relays used to turn on and off the degaussing coils may be short-circuited and melted. To address this circumstance, it is necessary to turn off the exciting coil temporarily while the degaussing coils are turned on and off, generating variation in the temperature of the fixing roller in the direction of rotation of the fixing roller. Specifically, since the fixing roller rotates even while the exciting coil is turned off temporarily, a section of the fixing roller that passes through the induction heater while the exciting coil is turned off is not heated by the induction heater. Accordingly, the fixing roller has a heated section heated by the induction heater and a non-heated section not heated by the induction heater, resulting in temperature variation of the fixing roller in the direction of rotation of the fixing roller. Consequently, the fixing roller heats the toner image on the recording medium unevenly, thus forming a faulty toner image on the recording medium. 
     SUMMARY OF THE INVENTION 
     At least one embodiment may provide a fixing device that includes a fixing rotary body rotatable in a predetermined direction of rotation; an induction heater disposed opposite the fixing rotary body to heat the fixing rotary body; and a controller operatively connected to the induction heater. The induction heater includes an exciting coil to generate a magnetic flux toward the fixing rotary body; a first pair of degaussing coils disposed opposite lateral ends of the exciting coil in an axial direction of the fixing rotary body to offset the magnetic flux generated by the exciting coil, each degaussing coil of the first pair having a first width in the axial direction of the fixing rotary body; a second pair of degaussing coils disposed opposite the lateral ends of the exciting coil in the axial direction of the fixing rotary body to offset the magnetic flux generated by the exciting coil, each degaussing coil of the second pair having a second width in the axial direction of the fixing rotary body greater than the first width of each degaussing coil of the first pair; an exciting coil switch connected to the exciting coil and a power supply to connect and disconnect the exciting coil to and from the power supply to turn on and off the exciting coil; a first degaussing coil switch connected to the first pair of degaussing coils to turn on and off the first pair of degaussing coils; and a second degaussing coil switch connected to the second pair of degaussing coils to turn on and off the second pair of degaussing coils. The controller causes the exciting coil switch to turn off the exciting coil while the controller turns on one of the first degaussing coil switch and the second degaussing coil switch and at the same time turns off the other one of the first degaussing coil switch and the second degaussing coil switch, and then causes the exciting coil switch to turn on the exciting coil for an extra time period corresponding to reserved power not supplied to the exciting coil while the exciting coil is turned off. 
     At least one embodiment may provide a fixing device that includes a fixing rotary body rotatable in a predetermined direction of rotation; an induction heater disposed opposite the fixing rotary body to heat the fixing rotary body; and a controller operatively connected to the induction heater. The induction heater includes an exciting coil to generate a magnetic flux toward the fixing rotary body; a first degaussing coil disposed opposite one lateral end of the exciting coil in an axial direction of the fixing rotary body to offset the magnetic flux generated by the exciting coil and having a width in the axial direction of the fixing rotary body; a second degaussing coil disposed opposite the one lateral end of the exciting coil in the axial direction of the fixing rotary body to offset the magnetic flux generated by the exciting coil and having a width in the axial direction of the fixing rotary body greater than the width of the first degaussing coil; an exciting coil switch connected to the exciting coil and a power supply to connect and disconnect the exciting coil to and from the power supply to turn on and off the exciting coil; a first degaussing coil switch connected to the first degaussing coil to turn on and off the first degaussing coil; and a second degaussing coil switch connected to the second degaussing coil to turn on and off the second degaussing coil. The controller causes the exciting coil switch to turn off the exciting coil while the controller turns on one of the first degaussing coil switch and the second degaussing coil switch and at the same time turns off the other one of the first degaussing coil switch and the second degaussing coil switch, and then causes the exciting coil switch to turn on the exciting coil for an extra time period corresponding to reserved power not supplied to the exciting coil while the exciting coil is turned off. 
     At least one embodiment may provide an image fanning apparatus that includes the fixing device described above. 
     At least one embodiment may provide a method for heating a fixing rotary body with an induction heater including an exciting coil and a plurality of degaussing coils. The method includes steps of rotating the fixing rotary body; turning on the exciting coil; identifying a size of a recording medium to be conveyed to the fixing rotary body; turning off the exciting coil; turning on one of the plurality of degaussing coils and turning off the other one of the plurality of degaussing coils according to the identified size of the recording medium; turning on the exciting coil; and supplying reserved power not supplied to the exciting coil while the exciting coil is turned off to the exciting coil to turn on the exciting coil for an extra time period corresponding to the supplied power. 
     Additional features and advantages of example embodiments will be more fully apparent from the following detailed description, the accompanying drawings, and the associated claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of example embodiments and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic sectional view of an image forming apparatus according to an example embodiment; 
         FIG. 2  is a vertical sectional view of a fixing device installed in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  is a horizontal sectional view of one example of a coil assembly incorporated in the fixing device shown in  FIG. 2 ; 
         FIG. 4  is a horizontal sectional view of another example of the coil assembly incorporated in the fixing device shown in  FIG. 2 ; 
         FIG. 5  is a graph showing a relation between time and power supplied to the coil assembly shown in  FIG. 3 ; 
         FIG. 6  is a flowchart showing one example of a control method employed by the fixing device shown in  FIG. 2 ; and 
         FIG. 7  is a flowchart showing another example of a control method employed by the fixing device shown in  FIG. 2 . 
     
    
    
     The accompanying drawings are intended to depict example embodiments and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION OF THE INVENTION 
     It will be understood that if an element or layer is referred to as being “on”, “against”, “connected to”, or “coupled to” another element or layer, then it can be directly on, against, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, if an element is referred to as being “directly on”, “directly connected to”, or “directly coupled to” another element or layer, then there are no intervening elements or layers present. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Spatially relative terms, such as “beneath”, “below”, “lower”, “above”, “upper”, and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, term such as “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein are interpreted accordingly. 
     Although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, it should be understood that these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are used only to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present invention. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     In describing example embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, in particular to  FIG. 1 , an image forming apparatus  1  according to an example embodiment of the present invention is explained. 
       FIG. 1  is a schematic sectional view of the image forming apparatus  1 . As illustrated in  FIG. 1 , the image forming apparatus  1  may be a copier, a facsimile machine, a printer, a multifunction printer having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like. According to this example embodiment, the image forming apparatus  1  is a tandem color copier for forming a color image on a recording medium by electrophotography. 
     Referring to  FIG. 1 , the following describes the structure of the image forming apparatus  1 . 
     As illustrated in  FIG. 1 , the image forming apparatus  1  includes four image forming devices  10 Y,  10 M,  10 C, and  10 K disposed in a center portion of the image forming apparatus  1  and aligned in a horizontal direction. The image forming devices  10 Y,  10 M,  10 C, and  10 K that form yellow, magenta, cyan, and black toner images include drum-shaped photoconductors  11 Y,  11 M,  11 C, and  11 K surrounded by chargers  12 Y,  12 M,  12 C, and  12 K, development devices  13 Y,  13 M,  13 C, and  13 K, and cleaners  15 Y,  15 M,  15 C, and  15 K, respectively. Yellow, magenta, cyan, and black toner bottles disposed in an upper portion of the image forming apparatus  1  supply yellow, magenta, cyan, and black toners in a predetermined amount to the development devices  13 Y,  13 M,  13 C, and  13 K through toner supply tubes, respectively. 
     Above the image forming devices  10 Y,  10 M,  10 C, and  10 K in the upper portion of the image forming apparatus  1  is a reader  4  that reads an image on an original document D placed on an exposure glass  5  disposed atop the image forming apparatus  1 . Specifically, the reader  4  includes a light source, a polygon mirror, an f theta lens, and reflection mirrors to read the image on the original document D into yellow, magenta, cyan, and black image data. Below the image forming devices  10 Y,  10 M,  10 C, and  10 K is an optical writer  2  electrically connected to the reader  4 . The optical writer  2  emits laser beams onto an outer circumferential surface of the respective photoconductors  11 Y,  11 M,  11 C, and  11 K charged by the chargers  12 Y,  12 M,  12 C, and  12 K according to the yellow, magenta, cyan, and black image data sent from the reader  4  in such a manner that the laser beams scan the charged outer circumferential surface of the photoconductors  11 Y,  11 M,  11 C, and  11 K, respectively, as the photoconductors  11 Y,  11 M,  11 C, and  11 K rotate clockwise in  FIG. 1  in a rotation direction R 1 . Thus, an electrostatic latent image is formed on the outer circumferential surface of the respective photoconductors  11 Y,  11 M,  11 C, and  11 K. The development devices  13 Y,  13 M,  13 C, and  13 K supply the yellow, magenta, cyan, and black toners to the photoconductors  11 Y,  11 M,  11 C, and  11 K to render the electrostatic latent images formed thereon visible as yellow, magenta, cyan, and black toner images, respectively. 
     Above the image forming devices  10 Y,  10 M,  10 C, and  10 K is an endless intermediate transfer belt  17  looped over a plurality of support rollers including a driving roller that drives and rotates the intermediate transfer belt  17 . For example, a driver (e.g., a motor) is connected to a rotation shaft of the driving roller. As the driver drives the driving roller, the driving roller rotates the intermediate transfer belt  17  counterclockwise in  FIG. 1  in a rotation direction R 2  in a state in which the rotating intermediate transfer belt  17  rotates the plurality of support rollers over which the intermediate transfer belt  17  is looped. 
     Primary transfer rollers  3 Y,  3 M,  3 C, and  3 K disposed inside a loop formed by the intermediate transfer belt  17  transfer the yellow, magenta, cyan, and black toner images formed on the photoconductors  11 Y,  11 M,  11 C, and  11 K onto an outer circumferential surface of the intermediate transfer belt  17  in such a manner that the yellow, magenta, cyan, and black toner images are superimposed on the same position on the intermediate transfer belt  17 , thus forming a color toner image on the intermediate transfer belt  17 . After the transfer of the yellow, magenta, cyan, and black toner images, the cleaners  15 Y,  15 M,  15 C, and  15 K remove residual toner not transferred onto the intermediate transfer belt  17  and therefore remaining on the photoconductors  11 Y,  11 M,  11 C, and  11 K therefrom. 
     Downstream from the primary transfer rollers  3 Y,  3 M,  3 C, and  3 K in the rotation direction R 2  of the intermediate transfer belt  17  is a secondary transfer roller  6 . A secondary transfer opposed roller  18  is disposed opposite the secondary transfer roller  6  via the intermediate transfer belt  17  in such a manner that the secondary transfer opposed roller  18  presses against the secondary transfer roller  6  via the intermediate transfer belt  17 . 
     A paper tray  7  disposed in a bottom portion of the image forming apparatus  1  loads a plurality of recording media P (e.g., sheets). Above the paper tray  7  is a feed roller  8  that picks up and feeds an uppermost recording medium P from the paper tray  7  to a registration roller pair  14 . The registration roller pair  14  feeds the recording medium P to a secondary transfer nip formed between the secondary transfer opposed roller  18  and the intermediate transfer belt  17  at a time when the secondary transfer roller  6  transfers the color toner image formed on the intermediate transfer belt  17  onto the recording medium P. After the transfer of the color toner image onto the recording medium P, a belt cleaner  16  disposed opposite the intermediate transfer belt  17  removes residual toner not transferred onto the recording medium P and therefore remaining on the intermediate transfer belt  17  therefrom. 
     Downstream from the secondary transfer nip in a conveyance direction of the recording medium P is a fixing device  19  that fixes the color toner image on the recording medium P and conveys the recording medium P bearing the fixed toner image to an output roller pair  9  disposed downstream from the fixing device  19  in the conveyance direction of the recording medium P. The output roller pair  9  discharges the recording medium P onto an outside of the image forming apparatus  1 . 
     Referring to  FIG. 2 , the following describes the fixing device  19  installed in the image forming apparatus  1  described above. 
       FIG. 2  is a vertical sectional view of the fixing device  19 . As illustrated in  FIG. 2 , the fixing device  19  (e.g., a fuser unit) includes a fixing roller  20 ; a pressing roller  30  pressed against the fixing roller  20  to form a fixing nip N therebetween; an induction heater  25  disposed opposite an outer circumferential surface of the fixing roller  20  to heat the fixing roller  20 ; a separator  41  disposed opposite the outer circumferential surface of the fixing roller  20  to separate a recording medium P discharged from the fixing nip N from the fixing roller  20 ; a temperature detector  62  disposed opposite the outer circumferential surface of the fixing roller  20  to detect a temperature of the fixing roller  20 ; and a controller  40  operatively connected to the induction heater  25  and the temperature detector  62 . The controller  40 , that is, a central processing unit (CPU) provided with a random-access memory (RAM) and a read-only memory (ROM), for example, controls the induction heater  25  based on the temperature of the fixing roller  20  detected by the temperature detector  62  so as to adjust the temperature of the outer circumferential surface of the fixing roller  20  to a predetermined fixing temperature. 
     The fixing roller  20  is constructed of a metal core  23 , an elastic layer  22 , made of sponge, disposed on the metal core  23 , and a fixing sleeve  21  disposed on the elastic layer  22 . The pressing roller  30  is constructed of a metal core  32  and an elastic layer  31 , made of rubber, disposed on the metal core  32 . The fixing roller  20  and the pressing roller  30  may be made of a foam material such as sponge and an elastic material such as rubber to attain desired pressure and nip length at the fixing nip N in the conveyance direction of the recording medium P. According to this example embodiment, the elastic layer  22  of the fixing roller  20  has a hardness of about 35 Hs; the elastic layer  31  of the pressing roller  30  has a hardness of about 60 Hs and a thickness of about 3 mm. Both the fixing roller  20  and the pressing roller  30  have an outer diameter of about 40 mm. Generally, the fixing sleeve  21  of the fixing roller  20  is constructed of a metal layer, having a thickness of about  15  micrometers, that generates heat, a silicone rubber layer having a thickness of about 200 micrometers, and a surface layer, having a thickness of about 30 micrometers, made of tetrafluoroethylene perfluoroalkylvinylether copolymer (PFA). 
     The fixing sleeve  21  is sandwiched by side guides shown in the broken line in  FIG. 2  disposed in proximity to lateral edges of the fixing sleeve  21  in an axial direction of the fixing roller  20 . If the fixing sleeve  21  is skewed, the skewed fixing sleeve  21  contacts the side guide that prohibits the fixing sleeve  21  from skewing farther. Alternatively, the fixing sleeve  21  may adhere to the elastic layer  22  to prevent potential skewing of the fixing sleeve  21 . 
     The induction heater  25  that heats the fixing roller  20  by electromagnetic induction is disposed opposite the outer circumferential surface of the fixing roller  20  at a side opposite a fixing nip side of the fixing roller  20  disposed opposite the pressing roller  30  at the fixing nip N. For example, the induction heater  25  includes a coil assembly  26  that heats the fixing sleeve  21  partially. According to this example embodiment, a thermopile is used as the temperature detector  62  operatively connected to the controller  40 . The controller  40  controls a power supply shown below to adjust an amount of power supplied to the induction heater  25  according to the temperature of the fixing roller  20  detected by the thermopile. 
     For example, the thermopile is composed of several thermocouples connected usually in series. The thermocouples are provided with a hot junction where infrared rays radiated from an object are collected. A cold junction is disposed at an inner position of the thermopile where temperature fluctuation barely arises. The thermocouples measure the temperature of the hot junction and the cold junction and generate an electromotive force according to the temperature differential between the temperature of the hot junction and the temperature of the cold junction. Thus, the thermopile serves as a time-responsive sensor. The thermopile includes an ambient sensor to address temperature fluctuation of the cold junction of the thermopile. According to this example embodiment, a single thermopile is used as the temperature detector  62 . Alternatively, a plurality of temperature sensors may be used to detect the temperature of the fixing roller  20  at a plurality of positions thereon to correspond to various sizes of the recording medium P. 
     Referring to  FIG. 3 , the following describes the coil assembly  26  of the induction heater  25  incorporated in the fixing device  19  described above. 
       FIG. 3  is a horizontal sectional view of the coil assembly  26 . As illustrated in  FIG. 3 , the coil assembly  26  includes an exciting coil  26 A serving as a main coil and three pairs of degaussing coils serving as sub coils, that is, a first pair of degaussing coils  26 B 1 , a second pair of degaussing coils  26 B 2 , and a third pair of degaussing coils  26 B 3 . The exciting coil  26 A extends throughout the entire width of the coil assembly  26  corresponding to the axial length of the fixing roller  20  depicted in  FIG. 2  and generates a magnetic flux throughout the entire width of the exciting coil  26 A. Accordingly, when a small recording medium P is conveyed through the fixing nip N, lateral ends of the fixing roller  20  in the axial direction thereof heated by the magnetic flux generated by the exciting coil  26 A are overheated because the small recording medium P does not pass over the lateral ends of the fixing roller  20  and therefore does not draw heat from the lateral ends of the fixing roller  20 . 
     To address this problem, the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  are disposed at lateral ends of the coil assembly  26  corresponding to the lateral ends of the fixing roller  20  in the axial direction thereof. For example, the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  generate a repulsive magnetic flux that offsets a magnetic flux generated by the exciting coil  26 A toward the fixing sleeve  21 , preventing the lateral ends of the fixing roller  20  in the axial direction thereof from overheating while the small recording medium P is conveyed through the fixing nip N. 
     Specifically, the exciting coil  26 A is connected to a power supply  50  via an exciting coil switch  54 . The first pair of degaussing coils  26 B 1  is connected to a first degaussing coil switch  51 . The second pair of degaussing coils  2682  is connected to a second degaussing coil switch  52 . The third pair of degaussing coils  26 B 3  is connected to a third degaussing coil switch  53 . The first degaussing coil switch  51 , the second degaussing coil switch  52 , the third degaussing coil switch  53 , and the exciting coil switch  54  are operatively connected to the controller  40  depicted in  FIG. 2 . When the exciting coil  26 A is connected to the power supply  50  via the exciting coil switch  54  in a state in which all of the first degaussing coil switch  51 , the second degaussing coil switch  52 , and the third degaussing coil switch  53  is open and therefore all of the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  is turned off, a magnetic flux generated by the exciting coil  26 A penetrates the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  and reaches the fixing roller  20 , thus heating the fixing roller  20  throughout the entire width thereof. Conversely, when one of the first degaussing coil switch  51 , the second degaussing coil switch  52 , and the third degaussing coil switch  53  is closed, one of the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  connected to the closed switch is turned on and generates a repulsive magnetic flux that offsets a magnetic flux generated by the exciting coil  26 A, thus minimizing heat generation from lateral ends of the fixing roller  20  in the axial direction thereof disposed opposite the one of the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 . 
     For example, in order to offset the magnetic flux according to various sizes of the recording medium P, the three pairs of degaussing coils, that is the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 , are available in the fixing device  19 . Each degaussing coil  26 B 1  of the first pair has a width W 1  in the axial direction of the fixing roller  20 ; each degaussing coil  26 B 2  of the second pair has a width W 2  greater than the width W 1  in the axial direction of the fixing roller  20 ; each degaussing coil  26 B 3  of the third pair has a width W 3  greater than the width W 2  in the axial direction of the fixing roller  20 .. When the controller  40  depicted in  FIG. 2  identifies that a letter size recording medium P (215.9 mm×279.4 mm) is conveyed through the fixing nip N based on image data sent from the reader  4  depicted in  FIG. 1  or information sent from a control panel  42  depicted in  FIG. 1  with which a user inputs a print job, the controller  40  drives the first pair of degaussing coils  26 B 1  having the smallest width W 1  via the first degaussing coil switch  51 ; when the controller  40  identifies that an A4 size recording medium P (210 mm×297 mm) is conveyed through the fixing nip N, the controller  40  drives the second pair of degaussing coils  26 B 2  having the medium width W 2  via the second degaussing coil switch  52 ; when the controller  40  identifies that a B5 size recording medium P (182 mm×257 mm) is conveyed through the fixing nip N, the controller  40  drives the third pair of degaussing coils  26 B 3  having the greatest width W 3  via the third degaussing coil switch  53 . Thus, the controller  40  drives the three pairs of degaussing coils, that is, the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 , independently. 
     In order to offset the magnetic flux according to the various sizes of the recording medium P more precisely, four or more pairs of degaussing coils having four or more widths in the axial direction of the fixing roller  20  may be incorporated in the fixing device  19 . According to the example embodiment described above, the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  are disposed at the lateral ends of the coil assembly  26  in the axial direction of the fixing roller  20 , respectively, because the recording medium P is conveyed over a center portion of the fixing sleeve  21  in the axial direction of the fixing roller  20 . Alternatively, a first degaussing coil  26 B 1 ′, a second degaussing coil  26 B 2 ′, and a third degaussing coil  26 B 3 ′ may be disposed at one lateral end of a coil assembly  26 ′ in the axial direction of the fixing roller  20  as shown in  FIG. 4 . 
       FIG. 4  is a horizontal sectional view of the coil assembly  26 ′ with such arrangement of the first degaussing coil  26 B 1 ′, the second degaussing coil  26 B 2 ′, and the third degaussing coil  26 B 3 ′. For example, if the recording medium P is configured to be conveyed over the fixing sleeve  21  along one lateral edge of the fixing sleeve  21 , the first degaussing coil  26 B 1 ′, the second degaussing coil  26 B 2 ′, and the third degaussing coil  26 B 3 ′ may be disposed at one lateral end of the coil assembly  26 ′ through which the recording medium P is not conveyed. Like the arrangement shown in  FIG. 3 , the first degaussing coil  26 B 1 ′, the second degaussing coil  26 B 2 ′, and the third degaussing coil  26 B 3 ′ have different widths in the axial direction of the fixing roller  20  that correspond to various non-conveyance regions of the fixing roller  20  through which recording media P of various sizes are not conveyed. 
     Referring to  FIGS. 1 and 2 , the following describes the operation of the image forming apparatus  1  installed with the fixing device  19  having the above-described configuration. 
     As the photoconductors  11 Y,  11 M,  11 C, and  11 K rotate in the rotation direction R 1 , the chargers  12 Y,  12 M,  12 C, and  12 K uniformly charge the outer circumferential surface of the respective photoconductors  11 Y,  11 M,  11 C, and  11 K. Then, the optical writer  2  emits laser beams onto the charged outer circumferential surface of the respective photoconductors  11 Y,  11 M,  11 C, and  11 K according to image data sent from the reader  4 , thus forming an electrostatic latent image on the outer circumferential surface of the respective photoconductors  11 Y,  11 M,  11 C, and  11 K. Thereafter, the development devices  13 Y,  13 M,  13 C, and  13 K supply yellow, magenta, cyan, and black toners to the electrostatic latent images on the photoconductors  11 Y,  11 M,  11 C, and  11 K, thus visualizing the electrostatic latent images as yellow, magenta, cyan, and black toner images, respectively. 
     As the driver rotates the driving roller over which the intermediate transfer belt  17  is looped, the driving roller rotates the intermediate transfer belt  17  in the rotation direction R 2  which in turn rotates the driven rollers, such as the primary transfer rollers  3 Y,  3 M,  3 C, and  3 K and the secondary transfer roller  6 . As the intermediate transfer belt  17  rotates in the rotation direction R 2 , the primary transfer rollers  3 Y,  3 M,  3 C, and  3 K transfer the yellow, magenta, cyan, and black toner images formed on the photoconductors  11 Y,  11 M,  11 C, and  11 K onto the intermediate transfer belt  17  successively in such a manner that the yellow, magenta, cyan, and black toner images are superimposed on the same position on the intermediate transfer belt  17 , thus forming a color toner image on the intermediate transfer belt  17 . After the transfer of the yellow, magenta, cyan, and black toner images from the photoconductors  11 Y,  11 M,  11 C, and  11 K, the cleaners  15 Y,  15 M,  15 C, and  15 K remove residual toner not transferred onto the intermediate transfer belt  17  and therefore remaining on the photoconductors  11 Y,  11 M,  11 C, and  11 K therefrom, respectively. Thus, the photoconductors  11 Y,  11 M,  11 C, and  11 K become ready for the next image forming processes performed thereon. 
     The feed roller  8  picks up and feeds an uppermost recording medium P from a plurality of recording media P loaded on the paper tray  7  to the registration roller pair  14 . When the uppermost recording medium P reaches the registration roller pair  14 , it stops the recording medium P temporarily. Then, the registration roller pair  14  resume rotating to feed the recording medium P to the secondary transfer nip formed between the secondary transfer opposed roller  18  and the intermediate transfer belt  17  at a time when the color toner image formed on the intermediate transfer belt  17  is transferred onto the recording medium P. As the recording medium P is conveyed through the secondary transfer nip, the secondary transfer roller  6  transfers the color toner image formed on the intermediate transfer belt  17  onto the recording medium P. 
     Thereafter, the recording medium P bearing the color toner image is conveyed to the fixing device  19 . As shown in  FIG. 2 , as the recording medium P is conveyed through the fixing nip N formed between the fixing roller  20  and the pressing roller  30 , the fixing roller  20  and the pressing roller  30  apply heat and pressure to the recording medium P, thus melting and fixing a toner image T on the recording medium P. The recording medium P bearing the fixed toner image T is discharged from the fixing nip N as the separator  41  separates the recording medium P from the fixing roller  20 . Then, the output roller pair  9  discharges the recording medium P onto the outside of the image forming apparatus  1 . After the transfer of the color toner image from the intermediate transfer belt  17 , the belt cleaner  16  removes residual toner not transferred from the intermediate transfer belt  17  and therefore remaining on the intermediate transfer belt  17  therefrom. Thus, the intermediate transfer belt  17  becomes ready for the next image forming processes performed thereon. 
     With the above-described configuration of the fixing device  19 , the controller  40  powers on and off the induction heater  25  according to the temperature of the fixing roller  20  detected by the temperature detector  62 , thus adjusting the temperature of the fixing roller  20  to a desired fixing temperature. 
     With the fixing device  19  in which the induction heater  25  heats the fixing roller  20  at a part of the fixing roller  20  with a smaller thermal capacity and a smaller thermal conduction, as soon as power is supplied to the induction heater  25 , the temperature of the fixing roller  20  increases quickly. Conversely, as soon as power supply to the induction heater  25  is stopped, the temperature of the fixing roller  20  decreases quickly. For example, as shown in  FIG. 3 , a switching element is used as the first degaussing coil switch  51 , the second degaussing coil switch  52 , and the third degaussing coil switch  53  to turn on and off the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 . With this configuration, it is necessary to turn off the exciting coil  26 A while switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 . Otherwise, serially-connected relays used to turn on and off the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  may be short-circuited and melted. However, if the exciting coil  26 A is turned off while switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 , a part of the fixing roller  20  may not be heated by the exciting coil  26 A, resulting in variation in the temperature of the fixing roller  20  in a circumferential direction, that is, a direction of rotation of the fixing roller  20  that rotates counterclockwise in  FIG. 2 , thus causing so-called temperature ripple of the fixing roller  20 . 
     To address this problem, a proportional-integral-derivative controller (PID controller) may be employed to calculate power used for the next temperature control cycle by measuring the present temperature of the fixing roller  20 . The PID controller has an advantage of maintaining the temperature of the fixing roller  20  at a predetermined temperature in the long view, but has a disadvantage of a slow thermal response in detecting temperature variation of the fixing roller  20  in the circumferential direction thereof and power decrease due to switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  so as to adjust the temperature of the fixing roller  20 . Accordingly, the PID controller cannot eliminate temperature ripple of the fixing roller  20  completely. 
     For example, there is a time lag after the temperature detector  62  detects the temperature of the fixing roller  20 . That is, the controller  40  does not adjust an amount of magnetic flux generated by the induction heater  25  at the same time when the temperature detector  62  detects the temperature of the fixing roller  20 . Accordingly, it is difficult to complement an amount of power decreased during switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 , resulting in temperature ripple of the fixing roller  20 . To address this circumstance, there is a need for an improved control method for adjusting the amount of power supplied to the coil assembly  26  earlier than a conventional control method for adjusting the amount of power supplied to the coil assembly  26  after the controller  40  identifies the temperature of the fixing roller  20  detected by the temperature detector  62 . 
     Referring to  FIG. 5 , a description is now given of the improved control method for adjusting the amount of power supplied to the coil assembly  26 . 
       FIG. 5  is a graph showing a relation between time and power supplied to the coil assembly  26 . In  FIG. 5 , the grid pattern area shows power supplied to the exciting coil  26 A and a time period for power supply; the diagonally shaded area shows power supplied to one of the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  and a time period for power supply. As shown in  FIG. 5 , power not supplied to the exciting coil  26 A to turn off the exciting coil  26 A during switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  is added to power supplied to the exciting coil  26 A to turn on the exciting coil  26 A the next time. Accordingly, the exciting coil  26 A is turned on for an extra time period corresponding to the supplied power. Thus, power not supplied to the exciting coil  26 A while it is turned off is consumed in the same PID control cycle in which power is not used during switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 , that is, turning on one of the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  and at the same time turning off the others. Accordingly, reserved power is used to turn on the exciting coil  26 A the next time. Consequently, temperature ripple of the fixing roller  20  caused by interruption of power supply to the exciting coil  26 A to turn off the exciting coil  26 A during switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  is minimized. 
     If the exciting coil  26 A is driven in a pulse width modulation (PWM), duty is increased by a time when the exciting coil  26 A is turned off during switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 . By contrast, if the exciting coil  26 A is driven by a pulse amplitude modulation (PAM), an amount of power requested by a PID operation that is multiplied by an amount of power not supplied to the exciting coil  26 A during switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  is added to an amount of power used to turn on the exciting coil  26 A the next time. 
     Referring to  FIGS. 1 ,  2 ,  3 , and  6 , a description is now given of a first example of the improved control method for heating the fixing roller  20  as described above. 
       FIG. 6  is a flowchart showing steps of the first example of the improved control method. In step S 11 , the fixing roller  20  rotates counterclockwise in  FIG. 2 . In step S 12 , the controller  40  turns on the exciting coil switch  54  to connect the power supply  50  to the exciting coil  26 A, thus turning on the exciting coil  26 A. In step S 13 , the controller  40  identifies the size of a recording medium P to be conveyed to the fixing roller  20  according to image data sent from the reader  4  or information sent from the control panel  42 . In step S 14 , the controller  40  turns off the exciting coil switch  54  to disconnect the power supply  50  from the exciting coil  26 A, thus turning off the exciting coil  26 A. Simultaneously, in step S 15 , the controller  40  switches between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  to turn on one of them corresponding to the size of the recording medium P identified in step S 13 . In step S 16 , after switching is finished, the controller  40  turns on the exciting coil switch  54  to connect the power supply  50  to the exciting coil  26 A, thus turning on the exciting coil  26 A. In step S 17 , the controller  40  retains the exciting coil switch  54  on to supply reserved power not supplied to the exciting coil  26 A while the exciting coil  26 A is turned off in step S 14  to the exciting coil  26 A, thus turning on the exciting coil  26 A for an extra time period corresponding to the supplied power. 
     With a configuration of the fixing device  19  in which the induction heater  25  heats a part of the fixing roller  20  at a heating position where the induction heater  25  is disposed opposite the fixing roller  20  in a state in which heat is barely conducted in the circumferential direction of the fixing roller  20 , power not supplied to the exciting coil  26 A during switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  may be added to power used to turn on the exciting coil  26 A the next time after the heated part of the fixing roller  20  rotates and returns to the heating position where the induction heater  25  heats the fixing roller  20 . By doing so, a part of the fixing roller  20  that is not heated by the induction heater  25  while the exciting coil  26 A is turned off during switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  is heated when that part of the fixing roller  20  rotates counterclockwise in  FIG. 2  by about 360 degrees. Accordingly, variation in the temperature of the fixing roller  20  is minimized in the circumferential direction thereof, reducing temperature ripple of the fixing roller  20 . 
     Referring to  FIGS. 1 ,  2 ,  3 , and  7 , a description is now given of a second example of the improved control method for heating the fixing roller  20  as described above. 
       FIG. 7  is a flowchart showing steps of the second example of the improved control method. In step S 21 , the fixing roller  20  rotates counterclockwise in  FIG. 2 . In step S 22 , the controller  40  turns on the exciting coil switch  54  to connect the power supply  50  to the exciting coil  26 A, thus turning on the exciting coil  26 A. In step S 23 , the controller  40  identifies the size of a recording medium P to be conveyed to the fixing roller  20  according to image data sent from the reader  4  or information sent from the control panel  42 . In step S 24 , the controller  40  turns off the exciting coil switch  54  to disconnect the power supply  50  from the exciting coil  26 A, thus turning off the exciting coil  26 A. Simultaneously, in step S 25 , the controller  40  switches between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  to turn on one of them corresponding to the size of the recording medium P identified in step S 23 . In step S 26 , after switching is finished, the controller  40  turns on the exciting coil switch  54  to connect the power supply  50  to the exciting coil  26 A, thus turning on the exciting coil  26 A. In step S 27 , the fixing roller  20  rotates counterclockwise in  FIG. 2  by about 360 degrees. When a non-heated part of the fixing roller  20  not heated by the exciting coil  26 A while the exciting coil  26 A is turned off in step S 24  returns to the heating position where the exciting coil  26 A is disposed opposite the non-heated part of the fixing roller  20 , the controller  40  retains the exciting coil switch  54  on to supply reserved power not supplied to the exciting coil  26 A while the exciting coil  26 A is turned off in step S 24  to the exciting coil  26 A, thus turning on the exciting coil  26 A for an extra time period corresponding to the supplied power in step S 28 . 
     With the first and second examples of the improved control method described above, the fixing device  19  completes the fixing process of fixing the toner image T on the recording medium P precisely with minimized energy. It is to be noted that the first and second examples of the improved control method described above are also applicable to the configuration of the coil assembly  26 ′ shown in  FIG. 4 . 
     Referring to  FIGS. 2 to 4 , the following describes advantages of the fixing device  19  according to the example embodiments described above. 
     The fixing device  19  includes the induction heater  25  disposed opposite the fixing roller  20  serving as a fixing rotary body. The induction heater  25  includes the exciting coil  26 A that generates a magnetic flux toward the fixing roller  20  to heat the fixing roller  20  and the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  disposed between the exciting coil  26 A and the fixing roller  20  to generate a repulsive magnetic flux that offsets the magnetic flux generated by the exciting coil  26 A toward the fixing roller  20 . The controller  40  operatively connected to the induction heater  25  turns on and off the exciting coil  26 A, the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 . Within a control cycle that turns on one of the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 , the controller  40  turns off the exciting coil  26 A while the one of the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  is switched to other one thereof. The controller  40  adds power not supplied to the exciting coil  26 A and therefore reserved during switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3  to power used to turn on the exciting coil  26 A the next time, thus turning on the exciting coil  26 A for an extra time period corresponding to the reserved power. As a result, temperature ripple, that is, temperature variation, of the fixing roller  20  in the circumferential direction thereof is minimized. 
     The fixing rotary body may be the fixing roller  20  or an endless belt that rotates in a predetermined direction of rotation. The induction heater  25  heats the rotating fixing rotary body at a heating position where the induction heater  25  is disposed opposite the fixing rotary body. Accordingly, while the controller  40  turns off the exciting coil  26 A during switching between the first pair of degaussing coils  26 B 1 , the second pair of degaussing coils  26 B 2 , and the third pair of degaussing coils  26 B 3 , a part of the fixing rotary body in the circumferential direction thereof is not heated by the exciting coil  26 A. To address this circumstance, when that part of the fixing rotary body rotates and returns to the heating position where the induction heater  25  is disposed opposite and heats the fixing rotary body, the controller  40  adds power not supplied to the exciting coil  26 A while it is turned off to power used to turn on the exciting coil  26 A the next time, thus minimizing temperature ripple of the fixing rotary body more effectively. 
     The present invention has been described above with reference to specific example embodiments. Nonetheless, the present invention is not limited to the details of example embodiments described above, but various modifications and improvements are possible without departing from the spirit and scope of the present invention. It is therefore to be understood that within the scope of the associated claims, the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative example embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.