Patent Publication Number: US-8543046-B2

Title: Fixing device, image forming apparatus incorporating same, and fixing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims priority to Japanese Patent Application No. 2010-140508, filed on Jun. 21, 2010, in the Japan Patent Office, which is hereby incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary aspects of the present invention relate to a fixing device, an image forming apparatus, and a fixing method, 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 fixing method for fixing a toner image on a recording medium. 
     2. Description of the Related Art 
     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 make 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 forming apparatuses may employ a fixing belt formed into a loop and a pressing roller pressed against the fixing belt to form a nip therebetween through which the recording medium bearing the toner image passes. 
     For example, Japanese patent publication no. JP-2002-251084-A proposes a configuration in which the fixing belt is stretched over and rotated around a rotatable fixing roller and a stationary heat generator (e.g., a resistance heat generator) and the pressing roller disposed outside the loop formed by the fixing belt is pressed against the fixing roller via the fixing belt to form the nip between the fixing belt and the pressing roller through which the recording medium bearing the toner image passes. With this configuration, the heat generator contacting the inner circumferential surface of the fixing belt heats the fixing belt; the fixing roller contacting the inner circumferential surface of the fixing belt rotates the fixing belt which in turn rotates the pressing roller by friction therebetween. As the fixing belt and the pressing roller rotate and convey the recording medium through the nip, they apply heat and pressure to the recording medium to fix the toner image on the recording medium. The fixing belt includes a ferromagnet that is attracted by a magnet of the heat generator, thus the fixing belt is adhered to the heat generator precisely with no gap therebetween, to improve heating efficiency of the fixing belt. 
     As another example, Japanese patent publication no. JP-2009-258453-A proposes a configuration in which the looped fixing belt is sandwiched between a heat generator (e.g., a temperature sensitive element) disposed inside the loop formed by the fixing belt and an exciting coil unit disposed outside the loop formed by the fixing belt. The heat generator contacts or is disposed opposite the inner circumferential surface of the fixing belt with a slight gap therebetween. As the heat generator generates heat by a magnetic flux from the exciting coil unit by electromagnetic induction, it heats the fixing belt. 
     However, the above-described configurations have a drawback in that the heat generator constantly contacting or disposed opposite the fixing belt may heat the fixing belt even in a standby mode in which the fixing belt is not rotated, resulting in localized overheating of the fixing belt. Accordingly, when a fixing process is started, the locally heated fixing belt, with a temperature not uniform and stable but instead varying in the direction of rotation of the fixing belt, may generate faulty fixing of the toner image on the recording medium. 
     BRIEF SUMMARY OF THE INVENTION 
     This specification describes below an improved fixing device. In one exemplary embodiment of the present invention, the fixing device includes a fixing rotary body to rotate in a predetermined direction of rotation and a pressing rotary body pressed against the fixing rotary body to rotate in a direction counter to the direction of rotation of the fixing rotary body and form a nip therebetween through which a recording medium bearing a toner image passes. A heat generator is disposed opposite the fixing rotary body at a section other than the nip to heat the fixing rotary body. A moving assembly is disposed opposite the heat generator to generate a magnetic force to move the heat generator with respect to the fixing rotary body so as to change one of a pressure and a distance between the heat generator and the fixing rotary body. 
     This specification further describes an improved image forming apparatus. In one exemplary embodiment, the image forming apparatus includes the fixing device described above. 
     This specification further describes an improved fixing method for fixing a toner image on a recording medium and including the steps of rotating a fixing rotary body in a predetermined direction of rotation; pressing a pressing rotary body against the fixing rotary body to rotate the pressing rotary body in a direction counter to the direction of rotation of the fixing rotary body and form a nip therebetween through which the recording medium bearing the toner image passes; heating the fixing rotary body with a heat generator disposed opposite the fixing rotary body at a section other than the nip; and moving the heat generator with respect to the fixing rotary body to change one of a pressure and a distance between the heat generator and the fixing rotary body with a moving assembly disposed opposite the heat generator and generating a magnetic force. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention 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 view of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  is a vertical sectional view of a fixing device included in the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3A  is a partially enlarged vertical sectional view of a fixing belt included in the fixing device shown in  FIG. 2  in a state in which the fixing belt is rotated; 
         FIG. 3B  is a partially enlarged vertical sectional view of the fixing belt included in the fixing device shown in  FIG. 2  in a state in which the fixing belt is not rotated; 
         FIG. 4  is a vertical sectional view of a fixing device according to another exemplary embodiment of the present invention; 
         FIG. 5A  is a partially enlarged vertical sectional view of the fixing belt included in the fixing device shown in  FIG. 4  in a state in which the fixing belt is rotated; 
         FIG. 5B  is a partially enlarged vertical sectional view of the fixing belt included in the fixing device shown in  FIG. 4  in a state in which the fixing belt is not rotated; 
         FIG. 6  is a vertical sectional view of a fixing device according to yet another exemplary embodiment of the present invention; 
         FIG. 7  is a vertical sectional view of a fixing device as a variation of the fixing device shown in  FIG. 6 ; 
         FIG. 8  is a vertical sectional view of a fixing device according to yet another exemplary embodiment of the present invention; 
         FIG. 9A  is a partially enlarged vertical sectional view of a fixing belt included in the fixing device shown in  FIG. 8  in a state in which the fixing belt is rotated; and 
         FIG. 9B  is a partially enlarged vertical sectional view of the fixing belt included in the fixing device shown in  FIG. 8  in a state in which the fixing belt is not rotated. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In describing exemplary 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 and achieve a similar result. 
     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 exemplary embodiment of the present invention is explained. 
       FIG. 1  is a schematic 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 exemplary embodiment of the present invention, the image forming apparatus  1  is a copier for forming an image on a recording medium. 
     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 an auto document feeder  10 , disposed atop the image forming apparatus  1 , which feeds an original document D bearing an original image placed thereon to an original document reader  2  disposed below the auto document feeder  10 . The original document reader  2  optically reads the original image on the original document D to generate image data and sends it to an exposure device  3  disposed below the original document reader  2 . The exposure device  3  emits light L onto a photoconductive drum  5  of an image forming device  4  disposed below the exposure device  3  according to the image data sent from the original document reader  2  to form an electrostatic latent image on the photoconductive drum  5 . Thereafter, the image forming device  4  renders the electrostatic latent image formed on the photoconductive drum  5  visible as a toner image with developer (e.g., toner). 
     Below the image forming device  4  is a transfer device  7  that transfers the toner image formed on the photoconductive drum  5  onto a recording medium P sent from one of paper trays  12 ,  13 , and  14 , each of which loads a plurality of recording media P (e.g., transfer sheets), disposed in a lower portion of the image foiling apparatus  1  below the transfer device  7 . The recording medium P bearing the transferred toner image is sent to a fixing device  20  disposed downstream from the transfer device  7  in a recording medium conveyance direction, where a fixing belt  21  and a pressing roller  31  disposed opposite each other apply heat and pressure to the recording medium P, thus fixing the toner image on the recording medium P. 
     Referring to  FIG. 1 , the following describes the operation of the image forming apparatus  1  having the above-described structure. 
     An original document D bearing an original image, placed on an original document tray of the auto document feeder  10  by a user, is conveyed by a plurality of conveyance rollers of the auto document feeder  10  in a direction D 1  above the original document reader  2 . As the original document D passes over an exposure glass of the original document reader  2 , the original document reader  2  optically reads the original image on the original document D to generate image data. 
     The image data is converted into an electric signal and then sent to the exposure device  3 . The exposure device  3 , serving as an image writer, emits light L (e.g., a laser beam) onto the photoconductive drum  5  of the image forming device  4  according to the electric signal, thus writing an electrostatic latent image on the photoconductive drum  5 . 
     The image forming device  4  performs a plurality of image forming processes as the photoconductive drum  5  rotates clockwise in  FIG. 1 : a charging process, an exposure process, and a development process. In the charging process, a charger of the image forming device  4  charges an outer circumferential surface of the photoconductive drum  5 , accordingly the exposure device  3  emits light L onto the charged outer circumferential surface of the photoconductive drum  5  to form an electrostatic latent image thereon as described above in the exposure process. Thereafter, in the development process, a development device of the image forming device  4  develops the electrostatic latent image formed on the photoconductive drum  5  into a toner image with toner. 
     At the same time, a recording medium P is sent to a transfer nip formed between the photoconductive drum  5  and the transfer device  7  from one of the plurality of paper trays  12 ,  13 , and  14 , which is selected manually by the user using a control panel disposed atop the image forming apparatus  1  or automatically by an electric signal of a print request sent from a client computer. If the paper tray  12  is selected, for example, an uppermost recording medium P of a plurality of recording media P loaded in the paper tray  12  is conveyed to a registration roller pair disposed in a conveyance path K extending from each of the paper trays  12 ,  13 , and  14  to the transfer device  7 . 
     When the uppermost recording medium P reaches the registration roller pair, it is stopped by the registration roller pair temporarily and then conveyed to the transfer nip formed between the photoconductive drum  5  and the transfer device  7  at a time when the toner image formed on the photoconductive drum  5  is transferred onto the uppermost recording medium P by the transfer device  7 . 
     After the transfer of the toner image onto the recording medium P, the recording medium P bearing the toner image is sent to the fixing device  20  through a conveyance path extending from the transfer device  7  to the fixing device  20 . As the recording medium P passes through a fixing nip formed between the fixing belt  21  and the pressing roller  31  of the fixing device  20 , it receives heat from the fixing belt  21  and pressure from the fixing belt  21  and the pressing roller  31 , which fix the toner image on the recording medium P. Thereafter, the recording medium P bearing the fixed toner image is discharged from the fixing nip to an outside of the image forming apparatus  1 , thus completing a series of image forming processes. 
     Referring to  FIGS. 2 ,  3 A, and  3 B, the following describes the structure and operation of the fixing device  20  installed in the image forming apparatus  1  described above. 
       FIG. 2  is a vertical sectional view of the fixing device  20 .  FIG. 3A  is a partially enlarged vertical sectional view of the fixing belt  21  of the fixing device  20  in a state in which the fixing belt  21  is rotated.  FIG. 3B  is a partially enlarged vertical sectional view of the fixing belt  21  in a state in which the fixing belt  21  is not rotated.  FIGS. 3A and 3B  also illustrate multiple layers of the fixing belt  21  and a heat generator  23  of the fixing device  20 . 
     As illustrated in  FIG. 2 , the fixing device  20  includes the fixing belt  21  formed into a loop; a nip formation pad  22 , the heat generator  23 , a magnetic member  24 , and a tension spring  27 , which are disposed inside the loop formed by the fixing belt  21 ; and a permanent magnet  26 , the pressing roller  31 , guides  35  and  37 , a temperature sensor  40 , and a driver  45 , which are disposed outside the loop formed by the fixing belt  21 . 
     The fixing belt  21  is a flexible, thin, endless belt serving as a fixing member or a fixing rotary body that rotates or moves clockwise in  FIG. 2  in a rotation direction R 1 . As illustrated in  FIG. 3A , the fixing belt  21 , having a thickness not greater than about 1 mm and a loop diameter of about 40 mm when assuming its operative looped shape, is constructed of a base layer  21   a ; an elastic layer  21   b  disposed on the base layer  21   a ; and a release layer  21   c  disposed on the elastic layer  21   b.    
     The base layer  21   a  constitutes an inner circumferential surface of the fixing belt  21 , that is, a contact face sliding over the nip formation pad  22  and the heat generator  23  disposed inside the loop formed by the fixing belt  21 . The base layer  21   a  has a thickness of about 200 μm and is made of polyimide (PI). 
     The elastic layer  21   b , made of a rubber material such as silicon rubber, silicon rubber form, and/or fluorocarbon rubber, has a thickness in a range of from about 100 μm to about 300 μm. The elastic layer  21   b  eliminates or reduces slight surface asperities of the fixing belt  21  at a nip NP formed between the fixing belt  21  and the pressing roller  31 . Accordingly, heat is uniformly transmitted from the fixing belt  21  to a toner image T on a recording medium P passing through the nip NP, minimizing formation of a rough image such as an orange peel image. According to this exemplary embodiment, silicon rubber with a thickness of about 150 μm is used as the elastic layer  21   b.    
     The release layer  21   c  has a thickness in a range of from about 10 μm to about 50 μm, and is made of tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), polytetrafluoroethylene (PTFE), polyimide, polyetherimide, and/or polyether sulfide (PES). The release layer  21   c  releases or separates the toner image T from the fixing belt  21 . According to this exemplary embodiment, the release layer  21   c  has a thickness of about 30 μm and is made of PFA. 
     Inside the loop formed by the fixing belt  21  are disposed the nip formation pad  22 , the heat generator  23 , the magnetic member  24 , the tension spring  27 , and an insulator  29  depicted in  FIGS. 2 and 3A . Outside the loop formed by the fixing belt  21  is the permanent magnet  26  disposed opposite the fixing belt  21  with a predetermined gap between the permanent magnet  26  and a part of an outer circumferential surface of the fixing belt  21 . A lubricant is applied to the inner circumferential surface of the fixing belt  21  to reduce friction between an outer circumferential surface of the nip formation pad  22  and the heat generator  23  and the inner circumferential surface of the fixing belt  21  sliding over the nip formation pad  22  and the heat generator  23 . 
     The nip formation pad  22  contacting the inner circumferential surface of the fixing belt  21  is a stationary member fixedly disposed inside the loop formed by the fixing belt  21 ; thus, the rotating fixing belt  21  slides over the stationary, nip formation pad  22 . Further, the nip formation pad  22  presses against the pressing roller  31  via the fixing belt  21  to form the nip NP between the fixing belt  21  and the pressing roller  31  through which the recording medium P bearing the toner image T passes. Lateral ends of the nip formation pad  22  in a longitudinal direction thereof parallel to an axial direction of the fixing belt  21  are mounted on and supported by side plates of the fixing device  20 , respectively. The nip formation pad  22  is made of a rigid material that prevents substantial bending of the nip formation pad  22  by pressure applied from the pressing roller  31 . 
     The nip formation pad  22  is constituted by an opposed face (e.g., a contact face that contacts the inner circumferential surface of the fixing belt  21  sliding over the nip formation pad  22 ) facing the pressing roller  31  and having a concave shape corresponding to the curvature of the pressing roller  31 . The recording medium P moves along the concave opposed face of the nip formation pad  22  in conformity with the curvature of the pressing roller  31  and is discharged from the nip NP in a direction Y 11 . Thus, the concave shape of the nip formation pad  22  prevents the recording medium P bearing the fixed toner image T from adhering to the fixing belt  21 , thereby facilitating separation of the recording medium P from the fixing belt  21 . 
     As described above, according to this exemplary embodiment, the nip formation pad  22  has a concave shape to form the concave nip NP. Alternatively, the nip formation pad  22  may have a flat, planar shape to form a planar nip NP. Specifically, the opposed face of the nip formation pad  22  disposed opposite the pressing roller  31  may have a flat, planar shape. Accordingly, the planar nip NP formed by the planar opposed face of the nip formation pad  22  is substantially parallel to an imaged side of the recording medium P. Consequently, the fixing belt  21  pressed by the planar opposed face of the nip formation pad  22  is precisely adhered to the recording medium P to improve fixing performance. Further, the increased curvature of the fixing belt  21  at an exit of the nip NP facilitates separation of the recording medium P discharged from the nip NP from the fixing belt  21 . 
     As illustrated in  FIG. 2 , the substantially semi-cylindrical heat generator  23  is disposed opposite the permanent magnet  26  via the fixing belt  21  at a section of the fixing belt  21  other than the nip NP. In the present embodiment, the heat generator  23  and the permanent magnet  26  are disposed directly opposite the nip NP, although their location is not limited thereto. In this case, the heat generator  23  separably contacts the inner circumferential surface of the fixing belt  21 . Shafts protruding from lateral ends of the heat generator  23  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21 , respectively, engage slots provided in the side plates of the fixing device  20  via bearings, respectively, to slidably support the heat generator  23  in a diametrical direction of the fixing belt  21 . 
     As noted above and illustrated in  FIG. 3A , the heat generator  23  is constructed of multiple layers: a base layer  23   a  constituting an inner circumferential surface disposed opposite the insulator  29 ; a heat generation layer  23   b , including a resistance heat generator, disposed on the base layer  23   a ; and a protective layer  23   c , that is, an insulating layer disposed on the heat generation layer  23   b . According to this exemplary embodiment, the heat generator  23  has a length of about 320 mm in the longitudinal direction thereof and a length, that is, an arcuate length, of about 10 mm in a circumferential direction thereof. In the present embodiment, the base layer  23   a  is made of aluminum oxide (alumina) and/or aluminum nitride. The heat generation layer  23   b  is made of a resistance heat generator, that is, a laminated heat generator made of ceramic. Lateral ends of the heat generation layer  23   b  in a longitudinal direction thereof parallel to the axial direction of the fixing belt  21  are connected to a power source. When the heat generation layer  23   b  is supplied with an electric current, it is heated by its electric resistance, thus heating the fixing belt  21  that either contacts or is disposed opposite the heat generator  23 . It is to be noted that the heat generation layer  23   b  may be any device capable of generating heat, such as a metal dispersion resin with an adjusted resistance. 
     The protective layer  23   c  is made of an insulating material, such as glass, that prevents the electric current applied to the heat generator  23  from flowing to the fixing belt  21 . The base layer  23   a  of the heat generator  23  is mounted with the magnetic member  24  via the insulator  29 . 
     With the above-described configuration, the heat generator  23  generates heat by itself, conducting the heat therefrom to the fixing belt  21 . Then, the heat is applied from the outer circumferential surface of the heated fixing belt  21  to a toner image T on a recording medium P depicted in  FIG. 2  as the recording medium P passes through the nip NP formed between the fixing belt  21  and the pressing roller  31 . 
     The temperature sensor  40 , disposed opposite the outer circumferential surface of the fixing belt  21 , serves as a temperature detector that detects a temperature of the outer circumferential surface of the fixing belt  21 . The temperature sensor  40  may be for example, a thermistor, a thermopile, or the like. Based on the temperature detected by the temperature sensor  40 , a controller  6  depicted in  FIG. 1 , that is, a central processing unit (CPU) provided with a random-access memory (RAM) and a read-only memory (ROM), for example, controls output of the power source that applies the electric current to the heat generator  23 , thus adjusting the temperature of the fixing belt  21  to a desired fixing temperature. 
     As described above, according to this exemplary embodiment, the heat generator  23  has multiple layers including the heat generation layer  23   b . Alternatively, the heat generator  23  may have a single layer, that is, the heat generation layer  23   b  only. 
     As illustrated in  FIGS. 2 ,  3 A, and  3 B, the permanent magnet  26  is disposed opposite the magnetic member  24  via the fixing belt  21  and the heat generator  23 . The permanent magnet  26  may be a ferromagnetic magnet, for example, a rare-earth magnet or a magnet made of a hard magnetic material such as neodymium-iron-boron alloy. 
     The permanent magnet  26  is slidably moved over a frame of the fixing device  20 , for example, by the driver  45  bidirectionally as indicated by the double-headed arrow A 1  in  FIG. 2  to change a distance between the permanent magnet  26  and the magnetic member  24 . As the driver  45  moves the permanent magnet  26  in the diametrical direction of the fixing belt  21 , the permanent magnet  26  alternately applies or ceases to apply a magnetic force to the magnetic member  24  or changes a magnitude of the magnetic force exerted on the magnetic member  24 , thus moving the heat generator  23  together with the magnetic member  24  in the diametrical direction of the fixing belt  21 , a detailed description of which is deferred. 
     The driver  45  that moves the permanent magnet  26  may be a mechanism that includes a cam contacting the permanent magnet  26  biased upward in  FIG. 2  by a spring. 
     Optionally, a fan that cools the permanent magnet  26  may be added to minimize the decrease in magnetic permeability due to the heated permanent magnet  26 . 
     As illustrated in  FIG. 2 , the substantially semi-cylindrical magnetic member  24  is attached to the heat generator  23  and is disposed opposite the fixing belt  21  via the heat generator  23 . The magnetic member  24  may be made of soft ferrite, but preferably is made of hard ferrite. The magnetic member  24  made of hard ferrite need to be disposed with respect to the permanent magnet  26  in such a manner that an attractive force is generated between the magnetic member  24  and the permanent magnet  26 . For example, the south pole of the magnetic member  24  is disposed opposite the north pole of the permanent magnet  26 , thus moving the heat generator  23  attached to the magnetic member  24  bidirectionally in the diametrical direction of the fixing belt  21  precisely by slidable movement of the permanent magnet  26 , a detailed description of which is deferred. 
     As illustrated in  FIG. 3A , the insulator  29  is provided between the heat generator  23  and the magnetic member  24 . The insulator  29 , made of an insulating material such as sponge rubber or urethane rubber, minimizes the decrease in magnetic permeability due to the heated magnetic member  24  by heat conduction from the heat generator  23  to the magnetic member  24 . 
     With the above-described configuration of the insulator  29  combined with the heat generator  23  and the magnetic member  24 , in accordance with the bidirectional movement of the permanent magnet  26  as indicated by the double-headed arrow A 1  in  FIG. 2 , the insulator  29  also moves bidirectionally in the diametrical direction of the fixing belt  21  as indicated by the double-headed arrow A 1  together with the heat generator  23  and the magnetic member  24 . 
     As illustrated in  FIG. 2 , the tension spring  27  has one end in a longitudinal direction thereof which is attached to the heat generator  23 , the magnetic member  24 , and the insulator  29  and another end in the longitudinal direction thereof which is attached to a frame of the fixing device  20 . Thus, the tension spring  27  serves as a biasing member that biases the magnetic member  24 , the insulator  29 , and the heat generator  23 , against a magnetic force of the permanent magnet  26  to separate the heat generator  23  combined with the magnetic member  24  and the insulator  29  from the fixing belt  21  downward in  FIG. 2  in a direction D 2 . 
     As illustrated in  FIG. 2 , the pressing roller  31  serves as a pressing rotary body that presses against the nip formation pad  22  via the fixing belt  21  by contacting the outer circumferential surface of the fixing belt  21  at the nip NP. The pressing roller  31  is constructed of a hollow metal core  32  and an elastic layer  33  disposed on the metal core  32 . The elastic layer  33 , having a thickness of about 3 mm, is made of silicon rubber form, silicon rubber, and/or fluorocarbon rubber. Optionally, a thin surface release layer made of PFA and/or PTFE may be disposed on the elastic layer  33 . With the above-described configuration, the pressing roller  31  is pressed against the nip formation pad  22  via the fixing belt  21  to form the desired nip NP between the pressing roller  31  and the fixing belt  21 . 
     On the pressing roller  31  is mounted a gear engaging a driving gear of a driving mechanism that drives and rotates the pressing roller  31  counterclockwise in  FIG. 2  in a rotation direction R 2  counter to the rotation direction R 1  of the fixing belt  21 . Lateral ends of the pressing roller  31  in a longitudinal direction, that is, an axial direction thereof, are rotatably supported by the side plates of the fixing device  20  via bearings, respectively. Optionally, a heat source, such as a halogen heater, may be disposed inside the pressing roller  31 . 
     With the elastic layer  33  of the pressing roller  31  made of a sponge material such as silicon rubber form, the pressing roller  31  applies decreased pressure to the nip formation pad  22  via the fixing belt  21  at the nip NP to decrease bending of the nip formation pad  22 . Further, the pressing roller  31  provides increased heat insulation that minimizes heat conduction thereto from the fixing belt  21 , improving heating efficiency of the fixing belt  21 . 
     As a mechanism to convey the recording medium P bearing the toner image T to and from the nip NP formed between the fixing belt  21  and the pressing roller  31 , the fixing device  20  includes two guide plates, the guide  35 , that is, an entry guide plate, disposed at an entry to the nip NP and the guide  37 , that is, an exit guide plate, disposed at an exit of the nip NP. The guide  35  is directed to the entry to the nip NP to guide the recording medium P conveyed in a direction Y 10  from the transfer device  7  depicted in  FIG. 1  to the nip NP. The guide  37  is directed to a conveyance path downstream from the fixing device  20  in the recording medium conveyance direction to guide the recording medium P discharged from the nip NP in the direction Y 11  to the conveyance path. Both the guides  35  and  37  are mounted on the frame (e.g., a body) of the fixing device  20 . 
     Referring to  FIGS. 1 and 2 , the following describes the operation of the fixing device  20  having the above-described structure. 
     When the image forming apparatus  1  is powered on, the power source supplies an electric current to the heat generator  23 ; at the same time, the pressing roller  31  starts rotating in the rotation direction R 2 . Accordingly, the fixing belt  21  rotates in accordance with rotation of the pressing roller  31  in the rotation direction R 1  counter to the rotation direction R 2  of the pressing roller  31  due to friction therebetween at the nip NP. 
     Thereafter, at the transfer nip formed between the photoconductive drum  5  and the transfer device  7 , the toner image T formed on the photoconductive drum  5  as described above is transferred onto a recording medium P sent from one of the paper trays  12 ,  13 , and  14 . Being guided by the guide  35 , the recording medium P bearing the toner image T is conveyed from the transfer nip in the direction Y 10  toward the nip NP, entering the nip NP formed between the fixing belt  21  and the pressing roller  31  pressed against each other. 
     As the recording medium P bearing the toner image T passes through the nip NP, it receives heat from the heated fixing belt  21  and pressure from the fixing belt  21 , the nip formation pad  22 , and the pressing roller  31  that form the nip NP. Thus, the toner image T is fixed on the recording medium P by the heat and the pressure applied at the nip NP. Thereafter, the recording medium P bearing the fixed toner image T is discharged from the nip NP and conveyed in the direction Y 11  as guided by the guide  37 . 
     Referring to  FIGS. 2 ,  3 A, and  3 B, the following describes the configuration of the fixing device  20  according to a first illustrative embodiment of the present invention. 
     As illustrated in  FIG. 2 , the fixing device  20  includes a moving assembly  60 , constructed of the magnetic member  24 , the permanent magnet  26 , the tension spring  27 , and the driver  45 , which moves the heat generator  23  combined with the magnetic member  24  and the insulator  29  to change the pressure with which the heat generator  23  presses against the fixing belt  21  or, if separated from the fixing belt  21 , a distance between the heat generator  23  and the fixing belt  21  disposed opposite the heat generator  23 . For example, the moving assembly  60  moves the heat generator  23  bidirectionally in a direction D 3  shown in  FIG. 3A  and a direction D 5  shown in  FIG. 3B . 
     As illustrated in  FIG. 3A , the permanent magnet  26  is disposed opposite the magnetic member  24  via the fixing belt  21 , the heat generator  23 , and the insulator  29 , and is slidably moved by the driver  45  depicted in  FIG. 2  bidirectionally toward and away from the fixing belt  21 , changing a distance between the permanent magnet  26  and the magnetic member  24 . The magnetic member  24 , together with the insulator  29 , is attached to the heat generator  23  in such a manner that it is disposed opposite the fixing belt  21  via the insulator  29  and the heat generator  23 . As illustrated in  FIG. 2 , the magnetic member  24  and the heat generator  23  are biased by the tension spring  27  in the direction D 2  away from the fixing belt  21 . 
     As illustrated in  FIG. 3A , as the driver  45  depicted in  FIG. 2  moves the permanent magnet  26  downward in a direction D 4  toward the fixing belt  21  and the magnetic member  24 , the permanent magnet  26  exerts an increased magnetic attractive force on the magnetic member  24  against a biasing force of the tension spring  27  depicted in  FIG. 2 , thus moving the heat generator  23  together with the magnetic member  24  upward in the direction D 3 . Simultaneously, the heat generator  23  presses against the fixing belt  21  with an increased pressure or, if separated from the fixing belt  21 , is disposed opposite the fixing belt  21  with a decreased distance between the heat generator  23  and the fixing belt  21 , thus improving heat conductivity from the heat generator  23  to the fixing belt  21 , that is, activating heat conduction from the heat generator  23  to the fixing belt  21 . 
     By contrast, as illustrated in  FIG. 3B , as the driver  45  depicted in  FIG. 2  moves the permanent magnet  26  upward in a direction D 6  away from the fixing belt  21  and the magnetic member  24 , the permanent magnet  26  exerts a decreased magnetic attractive force on the magnetic member  24  against a biasing force of the tension spring  27 , thus moving the heat generator  23  together with the magnetic member  24  downward in the direction D 5 . Simultaneously, the heat generator  23  presses against the fixing belt  21  with a decreased pressure or is disposed opposite the fixing belt  21  with an increased distance between the heat generator  23  and the fixing belt  21 . That is, the heat generator  23  is isolated from the fixing belt  21  with no pressure therebetween, thus degrading heat conductivity from the heat generator  23  to the fixing belt  21 , that is, deactivating heat conduction from the heat generator  23  to the fixing belt  21 . 
     Accordingly, instead of a moving mechanism including a cam that contacts and moves the heat generator  23 , the fixing device  20  employs the permanent magnet  26  that moves the heat generator  23  by magnetic force without contacting the heat generator  23 , preventing elements of the fixing device  20  other than the fixing belt  21  from drawing heat generated by the heat generator  23  and thereby maintaining heating efficiency of the fixing belt  21 . 
     For example, even when the entire heat generator  23  does not contact the fixing belt  21 , with a gap therebetween of about 0.2 mm or smaller, preferably about 0.1 mm or smaller, an air layer of the gap degrades heat conductivity to an extent that can be ignored, maintaining high heat conductivity from the heat generator  23  to the fixing belt  21 . Accordingly, the driver  45  moves the permanent magnet  26  in such a manner that the position of the permanent magnet  26  is switchable between the two positions: a first position shown in  FIG. 3A , where the permanent magnet  26  is disposed closer to the fixing belt  21  and the magnetic member  24  with a gap of about 0.2 mm or smaller, preferably about 0.1 mm or smaller, between the fixing belt  21  and the heat generator  23 ; and a second position shown in  FIG. 3B , where the permanent magnet  26  is disposed away from the fixing belt  21  and the magnetic member  24  with a greater gap of at least 0.2 mm between the fixing belt  21  and the heat generator  23 . 
     Optionally, the fixing device  20  may further include a stopper that restricts an amount of movement of the heat generator  23  moving upward in the direction D 3  and downward in the direction D 5  in accordance with movement of the permanent magnet  26  as described above, thus facilitating adjustment of the pressure with which the heat generator  23  presses against the fixing belt  21  or the distance between the heat generator  23  and the fixing belt  21  within a target range. 
     The moving assembly  60  that moves the heat generator  23  is controlled by the controller  6  depicted in  FIG. 1  according to rotation of the fixing belt  21 . For example, when the fixing belt  21  does not rotate, the moving assembly  60  moves the heat generator  23  to a position where the heat generator  23  presses against the fixing belt  21  with a pressure smaller than that when the fixing belt  21  rotates or to a position where the heat generator  23  is disposed opposite the fixing belt  21  with a distance greater than that when the fixing belt  21  rotates. 
     Specifically, when the fixing device  20  is warmed up or a recording medium P passes through the fixing device  20  and therefore the fixing belt  21  rotates clockwise in  FIG. 2  in the rotation direction R 1 , the driver  45  moves the permanent magnet  26  to the first position shown in  FIG. 3A  where the permanent magnet  26  is disposed closer to the fixing belt  21 , causing the heat generator  23  to contact the fixing belt  21  or causing the heat generator  23  to be disposed opposite the fixing belt  21  with a slight gap therebetween allowing heat conduction from the heat generator  23  to the fixing belt  21 . Simultaneously, as the fixing belt  21  rotates clockwise in  FIG. 2  in the rotation direction R 1 , a contact section on the inner circumferential surface of the fixing belt  21  where the fixing belt  21  contacts the heat generator  23  and is heated by the heat generator  23  moves in the circumferential direction of the fixing belt  21 , resulting in efficient and uniform heating of the fixing belt  21  over the circumferential direction thereof. 
     Conversely, in a standby mode in which the fixing belt  21  does not rotate, the driver  45  moves the permanent magnet  26  to the second position shown in  FIG. 3B  where the permanent magnet  26  is disposed away from the fixing belt  21 , thus isolating the heat generator  23  from the fixing belt  21  or separating the heat generator  23  from the fixing belt  21  with a substantial gap therebetween that prohibits heat conduction from the heat generator  23  to the fixing belt  21 . Simultaneously, the fixing belt  21 , although it does not rotate, is not heated by the heat generator  23  locally, preventing temperature variation of the fixing belt  21  in the circumferential direction thereof, that is, the rotation direction R 1 . Moreover, heat radiated from the heat generator  23  isolated from the fixing belt  21  sufficiently reaches the fixing belt  21  substantially uniformly over the circumferential direction of the fixing belt  21 , thus heating the fixing belt  21  uniformly over the circumferential direction thereof although heating efficiency is degraded compared to when the heat generator  23  contacting the fixing belt  21  conducts heat to the fixing belt  21 . Accordingly, even when a recording medium P is conveyed to the nip NP for the fixing process immediately after the standby mode is finished, faulty fixing does not occur due to variation in the temperature of the fixing belt  21  in the circumferential direction thereof. 
     In addition to the above-described control, even when the fixing belt  21  rotates after conveyance of the recording medium P through the nip NP is finished, the controller  6  controls the moving assembly  60  to move the heat generator  23  to the position where the heat generator  23  presses against the fixing belt  21  with a decreased pressure or is disposed opposite the fixing belt  21  with a greater distance therebetween compared to when conveyance of the recording medium P through the nip NP is ongoing. 
     For example, when the fixing process is performed at the nip NP while a recording medium P is conveyed through the nip NP or until the fixing process is finished on the last recording medium P when a plurality of recording media P is conveyed through the nip NP continuously, the driver  45  moves the permanent magnet  26  to the first position shown in  FIG. 3A  where the permanent magnet  26  is disposed closer to the fixing belt  21 , causing the heat generator  23  to contact the fixing belt  21  or causing the heat generator  23  to be disposed opposite the fixing belt  21  with a slight gap therebetween allowing heat conduction from the heat generator  23  to the fixing belt  21 . Simultaneously, as the fixing belt  21  rotates clockwise in  FIG. 2  in the rotation direction R 1 , the contact section on the inner circumferential surface of the fixing belt  21  where the fixing belt  21  contacts the heat generator  23  and is heated by the heat generator  23  moves in the circumferential direction of the fixing belt  21 , resulting in efficient and uniform heating of the fixing belt  21  over the circumferential direction thereof. 
     Conversely, immediately after the fixing process is finished at the nip NP while a recording medium P is conveyed through the nip NP or immediately after the fixing process is finished on the last recording medium P when a plurality of recording media P is conveyed through the nip NP continuously, the driver  45  moves the permanent magnet  26  to the second position shown in  FIG. 3B  where the permanent magnet  26  is disposed away from the fixing belt  21 , thus isolating the heat generator  23  from the fixing belt  21  or moving the heat generator  23  downward in the direction D 5  to the position where the heat generator  23  presses against the fixing belt  21  with a slight pressure of about 0.1 kgf/cm 2  or smaller. Simultaneously, the fixing belt  21 , although it rotates, does not contact the heat generator  23  or presses against it with the slight pressure therebetween, preventing deterioration or wear of the fixing belt  21  and the heat generator  23  and an increased torque of drivers installed in the fixing device  20  due to friction between the fixing belt  21  and the heat generator  23  that arises as the fixing belt  21  slides over the heat generator  23 . 
     As described above, the configuration according to the first illustrative embodiment changes the pressure with which the heat generator  23  presses against the fixing belt  21  or the distance between the heat generator  23  and the fixing belt  21  disposed opposite the heat generator  23 . Thus, even when the heat generator  23  presses against the fixing belt  21  or is disposed opposite the fixing belt  21  to heat the fixing belt  21 , the heat generator  23  can heat the fixing belt  21  efficiently. Further, even when the fixing belt  21  does not rotate, temperature variation of the fixing belt  21  does not arise in the rotation direction R 1  thereof. 
     Additionally, according to the first illustrative embodiment, the permanent magnet  26  generates an attractive force between the permanent magnet  26  and the magnetic member  24  and at the same time the tension spring  27  exerts a biasing force on the magnetic member  24  and the heat generator  23  downward in  FIG. 2  in the direction D 2  to separate the heat generator  23  from the fixing belt  21 . Alternatively, the permanent magnet  26  may generate a repulsive force between the permanent magnet  26  and the magnetic member  24  and at the same time a biasing member (e.g., a compression spring) may exert a biasing force (e.g., a compressive force) on the magnetic member  24  and the heat generator  23  upward in  FIG. 2  in a direction opposite the direction D 2  to move the heat generator  23  closer to the fixing belt  21 , thus attaining effects equivalent to the effects of the first illustrative embodiment. 
     Further, the configuration according to the first illustrative embodiment uses the permanent magnet  26  as a magnet that slidably moves over the frame of the fixing device  20  in the diametrical direction of the fixing belt  21  and exerts a magnetic force on the magnetic member  24  to cause the heat generator  23  to contact and separate from the fixing belt  21  or change pressure with which the heat generator  23  presses against the fixing belt  21 . Alternatively, an electromagnet or a superconducting magnet may be used as a magnet that exerts a magnetic force on the magnetic member  24 . Such magnets can also slidably move to cause the heat generator  23  to contact and separate from the fixing belt  21  or change pressure with which the heat generator  23  presses against the fixing belt  21 , thus attaining effects equivalent to the effects of the first illustrative embodiment. 
     Referring to  FIGS. 4 ,  5 A, and  5 B, the following describes a fixing device  20 S according to a second illustrative embodiment. 
       FIG. 4  is a vertical sectional view of the fixing device  20 S.  FIG. 5A  is a partially enlarged vertical sectional view of the fixing belt  21  of the fixing device  20  in a state in which the fixing belt  21  is rotated.  FIG. 5B  is a partially enlarged vertical sectional view of the fixing belt  21  in a state in which the fixing belt  21  is not rotated. Instead of the permanent magnet  26  depicted in  FIG. 2  of the fixing device  20  according to the first illustrative embodiment, which is slidably movable, the fixing device  20 S according to the second illustrative embodiment includes a permanent magnet  26 S that is rotatably movable. 
     As illustrated in  FIGS. 4 ,  5 A, and  5 B, like the fixing device  20  according to the first illustrative embodiment shown in  FIG. 2 , the fixing device  20 S according to the second illustrative embodiment includes the fixing belt  21  formed into a loop; the nip formation pad  22 , the heat generator  23 , and the magnetic member  24 , which are disposed inside the loop formed by the fixing belt  21 ; and the permanent magnet  26 S, the pressing roller  31 , the temperature sensor  40 , and a driver  46 , which are disposed outside the loop formed by the fixing belt  21 . 
     The fixing device  20 S further includes a moving assembly  60 S that moves the heat generator  23  combined with the magnetic member  24  and the insulator  29  to change pressure with which the heat generator  23  presses against the fixing belt  21  or a distance between the heat generator  23  and the fixing belt  21  disposed opposite the heat generator  23 . 
     For example, the moving assembly  60 S includes the permanent magnet  26 S, the magnetic member  24 , and the driver  46  that drives and rotates the permanent magnet  26 S. 
     The permanent magnet  26 S, disposed opposite the magnetic member  24  via the fixing belt  21  and the heat generator  23 , is rotated about a rotary shaft  26   a  by the driver  46  to change the magnetic pole, that is, the north pole or the south pole, of the permanent magnet  26 S disposed opposite the magnetic member  24 . The magnetic member  24 , together with the insulator  29  depicted in  FIG. 5A , is adhered to the heat generator  23  in such a manner that the magnetic member  24  is disposed opposite the fixing belt  21  via the insulator  29  and the heat generator  23 . 
     With this configuration, when the fixing belt  21  rotates, the driver  46  depicted in  FIG. 4  rotates the permanent magnet  26 S to a first position shown in  FIG. 5A  where the north pole of the permanent magnet  26 S is disposed opposite the fixing belt  21  and the magnetic member  24 ; thus, the permanent magnet  26 S exerts a magnetic attractive force on the magnetic member  24 , which moves the heat generator  23 , together with the magnetic member  24 , upward in a direction D 7  as shown in  FIG. 5A . Simultaneously, the heat generator  23  presses against the fixing belt  21  with an increased pressure or is disposed opposite the fixing belt  21  with a decreased distance therebetween, improving heat conducting efficiency from the heat generator  23  to the fixing belt  21 . 
     By contrast, when the fixing belt  21  does not rotate, the driver  46  rotates the permanent magnet  26 S to a second position shown in  FIG. 5B  where the south pole of the permanent magnet  26 S is disposed opposite the fixing belt  21  and the magnetic member  24 ; thus, the permanent magnet  26 S exerts a magnetic repulsive force on the magnetic member  24 , which moves the heat generator  23 , together with the magnetic member  24 , downward in a direction D 8  as shown in  FIG. 5B . Simultaneously, the heat generator  23  presses against the fixing belt  21  with a decreased pressure or is disposed opposite the fixing belt  21  with an increased distance therebetween, that is, the heat generator  23  separates from the fixing belt  21 , rendering pressure between the heat generator  23  and the fixing belt  21  to zero. Accordingly, the fixing belt  21 , which is heated by heat conduction from the heat generator  23 , is now heated by heat radiation from the heat generator  23 , thus minimizing localized overheating of the fixing belt  21  while the fixing belt  21  does not rotate. 
     It is to be noted that, according to the second illustrative embodiment, the south pole of the magnetic member  24  is disposed opposite the permanent magnet  26 S. 
     According to the second illustrative embodiment, since the permanent magnet  26 S biases the magnetic member  24  and the heat generator  23  attached to the magnetic member  24  by its magnetic repulsive force to separate the heat generator  23  from the fixing belt  21 , the tension spring  27  of the fixing device  20  according to the first illustrative embodiment shown in  FIG. 2  is not attached to the magnetic member  24 . Alternatively, the tension spring  27  may be attached to the magnetic member  24  to add a supplementary biasing force that separates the heat generator  23  and the magnetic member  24  from the fixing belt  21 . 
     As described above, like the configuration according to the first illustrative embodiment, the configuration according to the second illustrative embodiment changes the pressure with which the heat generator  23  presses against the fixing belt  21  or the distance between the heat generator  23  and the fixing belt  21  disposed opposite the heat generator  23 . Thus, even when the heat generator  23  presses against the fixing belt  21  or is disposed opposite the fixing belt  21  to heat the fixing belt  21 , the heat generator  23  can heat the fixing belt  21  efficiently. Further, even when the fixing belt  21  does not rotate, temperature variation of the fixing belt  21  does not arise in the rotation direction R 1  thereof. 
     Referring to  FIGS. 6 and 7 , the following describes a fixing device  20 T according to a third illustrative embodiment and a fixing device  20 TV as a variation of the fixing device  20 T. 
       FIG. 6  is a vertical sectional view of the fixing device  20 T.  FIG. 7  is a vertical sectional view of the fixing device  20 TV as a variation of the fixing device  20 T shown in  FIG. 6 . Instead of the permanent magnet  26  depicted in  FIG. 2  of the fixing device  20  according to the first illustrative embodiment, the fixing devices  20 T and  20 TV according to the third illustrative embodiment include an electromagnet  28 . 
     As illustrated in  FIG. 6 , like the fixing device  20  according to the first illustrative embodiment shown in  FIG. 2 , the fixing device  20 T according to the third illustrative embodiment includes the fixing belt  21  formed into a loop; the nip formation pad  22 , the heat generator  23 , the magnetic member  24 , and the tension spring  27 , which are disposed inside the loop formed by the fixing belt  21 ; and the electromagnet  28 , the pressing roller  31 , the temperature sensor  40 , a power source  50 , and a variable resistor  51 , which are disposed outside the loop formed by the fixing belt  21 . 
     The fixing device  20 T further includes a moving assembly  60 T that moves the heat generator  23  combined with the magnetic member  24  and the insulator  29  depicted in  FIG. 3A  to change pressure with which the heat generator  23  presses against the fixing belt  21  or a distance between the heat generator  23  and the fixing belt  21  disposed opposite the heat generator  23 . 
     For example, the moving assembly  60 T includes the electromagnet  28 , the magnetic member  24 , the tension spring  27 , the power source  50 , and the variable resistor  51 . 
     The electromagnet  28  is disposed opposite the magnetic member  24  via the fixing belt  21  and the heat generator  23 . The variable resistor  51  changes an amount of electric current applied to the electromagnet  28  (e.g., an electromagnetic coil) from the power source  50  to change a magnetic force exerted on the magnetic member  24 . The magnetic member  24 , together with the insulator  29  depicted in  FIG. 3A , is adhered to the heat generator  23  in such a manner that the magnetic member  24  is disposed opposite the fixing belt  21  via the insulator  29  and the heat generator  23 . 
     With this configuration, when the fixing belt  21  rotates, the controller  6  depicted in  FIG. 1  controls the variable resistor  51  to supply an increased amount of electric current from the power source  50  to the electromagnet  28 ; thus, the electromagnet  28  exerts an increased magnetic attractive force on the magnetic member  24  against a biasing force of the tension spring  27 , moving the heat generator  23 , together with the magnetic member  24 , upward in  FIG. 6 . Simultaneously, the heat generator  23  presses against the fixing belt  21  with an increased pressure or is disposed opposite the fixing belt  21  with a decreased distance therebetween, improving heat conducting efficiency from the heat generator  23  to the fixing belt  21 . 
     By contrast, when the fixing belt  21  does not rotate, the controller  6  controls the variable resistor  51  to supply a decreased amount of electric current from the power source  50  to the electromagnet  28 ; thus, the electromagnet  28  exerts a decreased magnetic attractive force on the magnetic member  24 , moving the heat generator  23 , together with the magnetic member  24 , downward in  FIG. 6  with a biasing force of the tension spring  27 . Simultaneously, the heat generator  23  presses against the fixing belt  21  with a decreased pressure or is disposed opposite the fixing belt  21  with an increased distance therebetween, that is, the heat generator  23  separates from the fixing belt  21 , rendering pressure between the heat generator  23  and the fixing belt  21  to zero. Accordingly, the fixing belt  21 , which is heated by heat conduction from the heat generator  23 , is now heated by heat radiation from the heat generator  23 , thus minimizing localized overheating of the fixing belt  21  while the fixing belt  21  does not rotate. 
     According to the above-described fixing device  20 T according to the third illustrative embodiment, the controller  6  controls the variable resistor  51  to change the amount of electric current supplied from the power source  50  to the electromagnet  28 , thus causing the heat generator  23  to contact and separate from the fixing belt  21 . Alternatively, the controller  6  may change a direction in which the electric current is applied to the electromagnet  28  to change the magnetic pole thereof, that is, the north pole or the south pole, which exerts a magnetic force on the magnetic member  24 , thus causing the heat generator  23  to contact and separate from the fixing belt  21 . 
     For example, as illustrated in  FIG. 7 , the electromagnet  28  is disposed opposite the magnetic member  24  via the fixing belt  21  and the heat generator  23 . Instead of the variable resistor  51  shown in  FIG. 6 , the fixing device  20 TV includes a switching circuit  52  that changes the direction in which the power source  50  applies the electric current to the electromagnet  28 , thus changing the magnetic polarity of the electromagnet  28  that exerts a magnetic force on the magnetic member  24 . 
     As illustrated in  FIG. 7 , the fixing device  20 TV as a variation of the fixing device  20 T according to the third illustrative embodiment includes the fixing belt  21  formed into a loop; the nip formation pad  22 , the heat generator  23 , and the magnetic member  24 , which are disposed inside the loop formed by the fixing belt  21 ; and the electromagnet  28 , the pressing roller  31 , the temperature sensor  40 , the power source  50 , and the switching circuit  52 , which are disposed outside the loop formed by the fixing belt  21 . 
     The fixing device  20 TV further includes a moving assembly  60 TV that moves the heat generator  23  combined with the magnetic member  24  and the insulator  29  depicted in  FIG. 3A  to change pressure with which the heat generator  23  presses against the fixing belt  21  or a distance between the heat generator  23  and the fixing belt  21  disposed opposite the heat generator  23 . 
     For example, the moving assembly  60 TV includes the electromagnet  28 , the magnetic member  24 , the power source  50 , and the switching circuit  52 . 
     With this configuration, when the fixing belt  21  rotates, the controller  6  depicted in  FIG. 1  controls the switching circuit  52  to change the direction in which the power source  50  applies the electric current to the electromagnet  28 , causing the north pole of the electromagnet  28  to be disposed opposite the fixing belt  21  and the magnetic member  24 ; thus, the electromagnet  28  exerts a magnetic attractive force on the magnetic member  24 , moving the heat generator  23 , together with the magnetic member  24 , upward in  FIG. 7 . Simultaneously, the heat generator  23  presses against the fixing belt  21  with an increased pressure or is disposed opposite the fixing belt  21  with a decreased distance therebetween, improving heat conducting efficiency from the heat generator  23  to the fixing belt  21 . 
     By contrast, when the fixing belt  21  does not rotate, the controller  6  controls the switching circuit  52  to change the direction in which the power source  50  applies the electric current to the electromagnet  28 , causing the south pole of the electromagnet  28  to be disposed opposite the fixing belt  21  and the magnetic member  24 ; thus, the electromagnet  28  exerts a magnetic repulsive force on the magnetic member  24 , moving the heat generator  23 , together with the magnetic member  24 , downward in  FIG. 7 . Simultaneously, the heat generator  23  presses against the fixing belt  21  with a decreased pressure or is disposed opposite the fixing belt  21  with an increased distance therebetween, that is, the heat generator  23  separates from the fixing belt  21 , rendering pressure between the heat generator  23  and the fixing belt  21  to zero. Accordingly, the fixing belt  21 , which is heated by heat conduction from the heat generator  23 , is now heated by heat radiation from the heat generator  23 , thus minimizing localized overheating of the fixing belt  21  while the fixing belt  21  does not rotate. 
     It is to be noted that, in the fixing devices  20 T and  20 TV, the south pole of the magnetic member  24  is disposed opposite the electromagnet  28 . 
     As described above, like the configuration according to the above-described illustrative embodiments, the configurations according to the third illustrative embodiment and the variation thereof change the pressure with which the heat generator  23  presses against the fixing belt  21  or the distance between the heat generator  23  and the fixing belt  21  disposed opposite the heat generator  23 . Thus, even when the heat generator  23  presses against the fixing belt  21  or is disposed opposite the fixing belt  21  to heat the fixing belt  21 , the heat generator  23  can heat the fixing belt  21  efficiently. Further, even when the fixing belt  21  does not rotate, temperature variation of the fixing belt  21  does not arise in the rotation direction R 1  thereof. 
     Referring to  FIGS. 8 ,  9 A, and  9 B, the following describes a fixing device  20 U according to a fourth illustrative embodiment. 
       FIG. 8  is a vertical sectional view of the fixing device  20 U.  FIG. 9A  is a partially enlarged vertical sectional view of a fixing belt  21 U installed in the fixing device  20 U in a state in which it is rotated.  FIG. 9B  is a partially enlarged vertical sectional view of the fixing belt  21 U in a state in which it is not rotated. Unlike the fixing device  20  depicted in  FIG. 2  according to the first illustrative embodiment in which the heat generator  23  generates heat by its resistance, the fixing device  20 U according to the fourth illustrative embodiment has the configuration in which a heat generator  23 U is heated by an exciting coil unit  25  by electromagnetic induction. 
     As illustrated in  FIG. 8 , the fixing device  20 U includes the fixing belt  21 U faulted into a loop; the nip formation pad  22 , the heat generator  23 U, the magnetic member  24 , and the tension spring  27 , which are disposed inside the loop formed by the fixing belt  21 U; and the permanent magnet  26 , the driver  45 , the pressing roller  31 , the temperature sensor  40 , and the exciting coil unit  25 , which are disposed outside the loop formed by the fixing belt  21 U. 
     Like the fixing device  20  according to the first illustrative embodiment depicted in  FIG. 2 , the fixing device  20 U further includes the moving assembly  60  that moves the heat generator  23 U combined with the magnetic member  24  and the insulator  29  depicted in  FIG. 3A  to change pressure with which the heat generator  23 U presses against the fixing belt  21 U or a distance between the heat generator  23 U and the fixing belt  21 U disposed opposite the heat generator  23 U. For example, the moving assembly  60  includes the permanent magnet  26 , the magnetic member  24 , the tension spring  27 , and the driver  45 . 
     The exciting coil unit  25 , serving as an induction heater, includes an exciting coil  25   a  and an exciting coil core  25   b . The exciting coil  25   a , extending in a longitudinal direction of the exciting coil unit  25  parallel to the axial direction of the fixing belt  21 U, is constructed of litz wire formed by bundling thin wire and wound around the exciting coil core  25   b  that covers a part of an outer circumferential surface of the fixing belt  21 U. The exciting coil core  25   b , made of ferromagnet (e.g., ferrite) having a relative permeability of about 2,500, generates a magnetic flux toward a heat generation layer of the fixing belt  21 U and a heat generation layer of the heat generator  23 U efficiently. 
     Referring to  FIG. 9A , a detailed description is now given of the fixing belt  21 U. 
     The fixing belt  21 U is constructed of three layers: a base layer  21   d  constituting an inner circumferential surface of the fixing belt  21 U, that is, a contact face that slides over the nip formation pad  22  and the heat generator  23 U; the elastic layer  21   b  disposed on the base layer  21   d ; and the release layer  21   c  disposed on the elastic layer  21   b.    
     For example, the base layer  21   d , having a thickness of from about several microns to about several hundred microns, is made of a magnetic material, such as SUS420 stainless steel or Fe—Ni alloy, thus serving as a heat generation layer heated by the exciting coil unit  25  by electromagnetic induction. The configuration of the elastic layer  21   b  and the release layer  21   c  of the fixing belt  21 U is identical to that of the fixing belt  21  depicted in  FIG. 2  installed in the fixing device  20  according to the first illustrative embodiment. 
     Referring to  FIG. 9A , a detailed description is now given of the heat generator  23 U. 
     The heat generator  23 U is constructed of three layers like the heat generator  23  of the fixing device  20  shown in  FIG. 3A , however, the configuration of the three layers is different from that of the heat generator  23 . For example, the heat generator  23 U includes an antioxidant layer  23   e  constituting an inner circumferential surface of the heat generator  23 U, that is, an opposed face disposed opposite the magnetic member  24 ; a heat generation layer  23   f  disposed on the antioxidant layer  23   e ; and an antioxidant layer  23   g  disposed on the heat generation layer  23   f.    
     The heat generation layer  23   f , having a thickness of about 10 μm, is made of copper. As an exciting magnetic flux generated by the exciting coil unit  25  passes through the heat generation layer  23   f , it induces an eddy current that heats the heat generation layer  23   f  by electromagnetic induction. 
     Each of the antioxidant layers  23   e  and  23   g , having a thickness of about 30 μm, is made of nickel plate; the antioxidant layers  23   e  and  23   g  sandwich the heat generation layer  23   f , inhibiting oxidation of the heat generation layer  23   f.    
     With this configuration, the heat generator  23 U is heated by electromagnetic induction by an alternating magnetic field generated by the exciting coil unit  25 , thus heating the fixing belt  21 U contacting the heat generator  23 U. That is, the exciting coil unit  25  heats the heat generator  23 U directly by electromagnetic induction and heats the fixing belt  21 U indirectly via the heat generator  23 U by heat conduction from the heat generator  23 U to the fixing belt  21 U. 
     Further, since the fixing belt  21 U has the base layer  21   d  that functions as a heat generation layer, the fixing belt  21 U itself, that is, the base layer  21   d , is also heated directly by electromagnetic induction by the alternating magnetic field generated by the exciting coil unit  25 . That is, the fixing belt  21 U is heated directly by electromagnetic induction by the exciting coil unit  25  and at the same time is heated indirectly by the exciting coil unit  25  by heat conduction from the heat generator  23 U heated by electromagnetic induction by the exciting coil unit  25 , improving heating efficiency of the fixing belt  21 U. 
     Thereafter, the heated fixing belt  21 U heats a recording medium P bearing a toner image T. 
     The controller  6  depicted in  FIG. 1  controls output of the exciting coil unit  25  based on a detection result provided from the temperature sensor  40  disposed opposite the outer circumferential surface of the fixing belt  21 U to detect a temperature thereof, thus adjusting the temperature of the fixing belt  21 U to a desired fixing temperature. 
     Referring to  FIGS. 1 and 8 , the following describes the operation of the fixing device  20 U having the above-described configuration. 
     When the image forming apparatus  1  is powered on, a high-frequency power source supplies an alternating electric current to the exciting coil  25   a  of the exciting coil unit  25 , and at the same time the pressing roller  31  starts rotating in the rotation direction R 2 . Accordingly, the fixing belt  21 U rotates in accordance with rotation of the pressing roller  31  in the rotation direction R 1  counter to the rotation direction R 2  of the pressing roller  31  due to friction therebetween at the nip NP. 
     Thereafter, at the transfer nip formed between the photoconductive drum  5  and the transfer device  7 , the toner image T formed on the photoconductive drum  5  as described above is transferred onto a recording medium P sent from one of the paper trays  12 ,  13 , and  14 . The recording medium P bearing the toner image T is conveyed from the transfer nip in the direction Y 10  toward the nip NP, entering the nip NP formed between the fixing belt  21 U and the pressing roller  31  pressed against each other. 
     As the recording medium P bearing the toner image T passes through the nip NP, it receives heat from the heated fixing belt  21 U and pressure from the fixing belt  21 U, the nip formation pad  22 , and the pressing roller  31  that form the nip NP. Thus, the toner image T is fixed on the recording medium P by the heat and the pressure applied at the nip NP. Thereafter, the recording medium P bearing the fixed toner image T is discharged from the nip NP and conveyed in the direction Y 11 . 
     With the above-described configuration of the fixing device  20 U shown in  FIGS. 8 ,  9 A, and  9 B, when the fixing belt  21 U rotates, the driver  45  moves the permanent magnet  26  to a first position shown in  FIG. 9A  where the permanent magnet  26  is disposed closer to the fixing belt  21 U, thus increasing a magnetic attractive force of the permanent magnet  26  exerted on the magnetic member  24  against a biasing force of the tension spring  27 , which moves the heat generator  23 U, together with the magnetic member  24 , upward in a direction D 9 . Simultaneously, the heat generator  23 U presses against the fixing belt  21 U with an increased pressure or is disposed opposite the fixing belt  21 U with a decreased distance therebetween, thus improving heat conductivity from the heat generator  23 U to the fixing belt  21 U. 
     By contrast, when the fixing belt  21 U does not rotate, the driver  45  moves the permanent magnet  26  to a second position shown in  FIG. 9B  where the permanent magnet  26  is disposed away from the fixing belt  21 U, thus decreasing a magnetic attractive force of the permanent magnet  26  exerted on the magnetic member  24  and moving the heat generator  23 U, together with the magnetic member  24 , downward in a direction D 10  with a biasing force of the tension spring  27 . Simultaneously, the heat generator  23 U presses against the fixing belt  21 U with a decreased pressure or is disposed opposite the fixing belt  21 U with an increased distance therebetween, that is, the heat generator  23 U separates from the fixing belt  21 U, rendering pressure between the heat generator  23 U and the fixing belt  21 U to zero. Accordingly, the fixing belt  21 U, which is heated by heat conduction from the heat generator  23 U, is now heated by heat radiation from the heat generator  23 U, thus minimizing localized overheating of the fixing belt  21 U while the fixing belt  21 U does not rotate. 
     Even when the heat generator  23 U is isolated from the fixing belt  21 U, it is constantly disposed within a magnetic field indicated by the broken line in  FIGS. 9A and 9B , which is generated by the exciting coil unit  25 . Accordingly, the fixing belt  21 U is heated precisely both during rotation and non-rotation. For example, while the fixing belt  21 U rotates, it is heated by heat conduction from the heat generator  23 U; while the fixing belt  21 U does not rotate, it is heated by heat radiation from the heat generator  23 U. 
     Preferably, the heat generation layer  23   f  of the heat generator  23 U may be made of a magnetic shunt alloy. 
     For example, the base layer  21   d , that is, the heat generation layer, of the fixing belt  21 U is made of a ferromagnetic, magnetic shunt alloy such as iron, nickel, cobalt, or an alloy of these. 
     With such materials of the heat generation layer  23   f  of the heat generator  23 U and the base layer  21   d  of the fixing belt  21 U, the base layer  21   d  of the fixing belt  21 U has a Curie temperature near an upper temperature limit of the fixing temperature with which the toner image T is fixed on the recording medium P, preventing overheating of the fixing belt  21 U with self temperature control of the magnetic shunt alloy and thereby minimizing thermal degradation of the fixing belt  21 U. Further, the base layer  21   d  of the fixing belt  21 U has a Curie temperature equivalent to a temperature that maintains magnetic permeability against the heated magnetic member  24 , rendering the insulator  29  disposed between the heat generator  23 U and the magnetic member  24  unnecessary. 
     According to the fourth illustrative embodiment, the fixing belt  21 U includes the heat generation layer, that is, the base layer  21   d , heated by the exciting coil unit  25  by electromagnetic induction. Alternatively, the fixing belt  21 U may not include the heat generation layer. For example, the fixing belt  21 U is heated solely by the heat generator  23 U by heat conduction or heat radiation, which is heated by the exciting coil unit  25  by electromagnetic induction, thus further enhancing prevention of localized overheating of the fixing belt  21 U when the fixing belt  21 U does not rotate. 
     As described above, like the configuration according to the above-described illustrative embodiments, the configuration according to the fourth illustrative embodiment changes the pressure with which the heat generator  23 U presses against the fixing belt  21 U or the distance between the heat generator  23 U and the fixing belt  21 U disposed opposite the heat generator  23 U. Thus, even when the heat generator  23 U presses against the fixing belt  21 U or is disposed opposite the fixing belt  21 U to heat the fixing belt  21 U, the heat generator  23 U can heat the fixing belt  21 U efficiently. Further, even when the fixing belt  21 U does not rotate, temperature variation of the fixing belt  21 U does not arise in the rotation direction R 1  thereof. 
     According to the above-described exemplary embodiments, the fixing belts  21  and  21 U are used as a fixing rotary body that rotates in the predetermined direction of rotation; the pressing roller  31  is used as a pressing rotary body disposed opposite the fixing rotary body to form the nip NP therebetween and rotating in the direction counter to the direction of rotation of the fixing rotary body. Alternatively, a fixing film, a fixing roller, or the like may be used as a fixing rotary body; a pressing belt or the like may be used as a pressing rotary body, attaining effects equivalent to the effects of the fixing devices  20 ,  20 S,  20 T,  20 TV, and  20 U according to the above-described exemplary embodiments. 
     Further, the fixing devices  20 ,  20 S,  20 T,  20 TV, and  20 U according to the above-described exemplary embodiments are installed in the image forming apparatus  1  serving as a monochrome copier. Alternatively, they may be installed in color image forming apparatuses such as copiers, printers, facsimile machines, and multifunction printers having at least one of copying, printing, scanning, plotter, and facsimile functions, or the like. 
     Further, according to the above-described exemplary embodiments, the fixing devices  20 ,  20 S,  20 T, and  20 TV include the heat generator  23  that generates heat; the fixing device  20 U includes the heat generator  23 U heated by the exciting coil unit  25  by electromagnetic induction. Alternatively, the fixing devices  20 ,  20 S,  20 T,  20 TV, and  20 U may include a heat generator heated by a heater (e.g., a halogen heater) by radiant heat, attaining effects equivalent to the effects of the fixing devices  20 ,  20 S,  20 T,  20 TV, and  20 U according to the above-described exemplary embodiments. 
     The present invention has been described above with reference to specific exemplary embodiments. Note that the present invention is not limited to the details of the embodiments described above, but various modifications and enhancements are possible without departing from the spirit and scope of the invention. It is therefore to be understood that the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.