Patent Publication Number: US-11392065-B2

Title: Fixing device and image forming apparatus including the fixing device

Description:
INCORPORATION BY REFERENCE 
     This application is based on and claims the benefit of priority from Japanese patent application No. 2020-102824 filed on Jun. 15, 2020, which is incorporated by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates to a fixing device used for an image forming apparatus such a copying machine, a printer, a facsimile and a multifunctional peripheral and an image forming apparatus including the fixing device. 
     In an image forming apparatus such as a copying machine, a fixing device is widely used. The fixing device melts and fixes an unfixed toner image on a sheet, as a recording medium, by heating and pressing. As such a fixing device, for example, a configuration is known, in which an endless fixing belt to be heated (a heated rotational body) and a pressing roller (a pressing rotational body) come pressure contact with each other to form a fixing nip area, and the unfixed toner image is fixed on the sheet at the fixing nip area. 
     By the way, the fixing belt expands due to the heating. When the fixing belt thermally expands, a circumference of the fixing belt becomes longer than a reference circumference. In this case, if a rotational speed of the fixing belt is obtained by a period required for one rotation of the fixing belt and a predetermined circumference of the fixing belt, the obtained rotational speed contains a tolerance of variation in the circumference due to the thermal expansion of the fixing belt. Then, in a case where a rotational speed of the pressing roller coming into pressure contact with the fixing belt is adjusted based on the rotational speed of the fixing belt, it becomes difficult to perform the adjustment with high accuracy. Accordingly, in order to adjust the rotational speed of the pressing roller correctly, it is required to obtain the correct rotational speed of the fixing belt in view of the thermal expansion of the fixing belt. However, a technique for obtaining the correct rotational speed of the fixing belt is not disclosed. 
     SUMMARY 
     In accordance with an aspect of the present disclosure, a fixing device includes a heated rotational body, a heating unit, a pressing rotational body, a drive unit, a light emitting unit, a light receiving unit, and a drive control part. The heated rotational body has a reflection member on a portion in a circumferential direction. The heating unit heats the heated rotational body. The pressing rotational body comes into pressure contact with the heated rotational body, and a fixing nip area where an unfixed toner image on a recording medium is melted and fixed is formed between the pressing rotational body and the heated rotational body. The drive unit rotates the pressing rotational body. The light emitting unit emits infrared light toward the heated rotational body. The light receiving unit receives the infrared light emitted from the light emitting unit and reflected on the reflection member of the heated rotational body periodically when the heated rotational body is rotated, and receives radiation light generated by a heating of the heated rotational body. The drive control part is configured to control the drive unit based on a detection result of the light receiving unit. The light receiving unit detects a temperature of the heated rotational body based on a receiving of the radiation light. The drive control part obtains a rotational speed of the heated rotational body based on a light receiving period of the infrared light in the light receiving unit and a circumference determined depending on the temperature of the heated rotational body, and controls the drive unit based on the obtained rotational speed. 
     In accordance with an aspect of the present disclosure, an image forming apparatus includes the fixing device, and an image forming unit for forming the unfixed toner image on the recording medium to be conveyed to the fixing device. 
     The above and other objects, features, and advantages of the present disclosure will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present disclosure is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically showing an inner structure of an image forming apparatus including a fixing device according to one embodiment of the present disclosure. 
         FIG. 2  is a sectional view schematically showing a structure of the fixing device. 
         FIG. 3  is a sectional view schematically showing a fixing belt of the fixing device. 
         FIG. 4  is a block diagram schematically showing a control system of the fixing device. 
         FIG. 5  is a graph showing a relationship between a temperature and a circumference of the fixing belt. 
         FIG. 6  is a flow chart showing an operation for controlling a rotation of the fixing belt. 
         FIG. 7  is a timing chart schematically showing an example of timings of reflection of infrared light on a reflection member of the fixing belt and reception of the infrared light by a reception part. 
     
    
    
     DETAILED DESCRIPTION 
     [Structure of Image Forming Apparatus] Hereinafter, with reference to the attached drawings, one embodiment in the present disclosure will be described.  FIG. 1  is a sectional view schematically showing an inner structure of an image forming apparatus  100  including a fixing device  13  according to the embodiment of the present disclosure. In a main body of the image forming apparatus  100  (for example, a color printer in the embodiment), four image forming sections Pa, Pb, Pc and Pd are disposed in order along one direction (in a direction from the left side to the right side in  FIG. 1 ). These image forming sections Pa to Pd are provided corresponding to images of different four colors (cyan, magenta, yellow and black), and form cyan, magenta, yellow and black images in order by charging processing, exposure processing, development processing and transferring processing. 
     These image forming sections Pa to Pd include photosensitive drums (an image carrier)  1   a ,  1   b ,  1   c  and  1   d  on which a visible image (a toner image) of each color is carried. Further, an intermediate transferring belt  8  traveling in the counterclockwise direction in  FIG. 1  is provided adjacent to the image forming sections Pa to Pd. The toner images formed on the photosensitive drums  1   a  to  1   d  are primarily transferred in order and overlapped on the intermediate transferring belt  8  traveling while coming into contact with the photosensitive drums  1   a  to  1   d . After that, the toner images primarily transferred on the intermediate transferring belt  8  are secondarily transferred on a sheet S, as an example of a recording medium, by a second transferring roller  9 . The sheet S is discharged from the main body of the image forming apparatus  100  after the toner image is fixed in the fixing device  13 . The image forming processing for the photosensitive drums  1   a  to  1   d  is carried out as the photosensitive drums  1   a  to  1   d  are rotated in the clockwise direction in  FIG. 1  by a main motor (not shown). 
     The sheet S on which the toner image is secondarily transferred is stored in a sheet feeding cassette  16  disposed in the lower portion of the main body of the image forming apparatus  100 . The sheet S in the sheet feeding cassette  16  is conveyed to a nip area between the second transferring roller  9  and a drive roller  11  for driving the intermediate transferring belt  8  by a sheet feeding roller  12   a  and a resist rollers pair  12   b . As the intermediate transferring belt  8 , an endless (seamless) belt made of dielectric resin sheet is used conventionally. On a downstream side of the second transferring roller  9 , a blade shaped belt cleaner  19  is disposed so as to remove the toner remaining on the surface of the intermediate transferring belt  8 . 
     Next, the image forming sections Pa to Pd will be described. Around and below the rotatable photosensitive drums  1   a  to  1   d , charging devices  2   a ,  2   b ,  2   c  and  2   d , an exposure device  5 , development devices  3   a ,  3   b ,  3   c  and  3   d , and cleaning devices  7   a ,  7   b ,  7   c  and  7   d  are provided. The charging devices  2   a  to  2   d  charge the photosensitive drums  1   a  to  1   d . The exposure device  5  exposes the photosensitive drums  1   a  to  1   d  based on an image data. The development devices  3   a  to  3   d  form the toner images on the photosensitive drums  1   a  to  1   d . The cleaning devices  7   a  to  7   d  remove the developer (the toner) and the other remaining on the photosensitive drums  1   a  to  1   d.    
     When the image data is input from a host device such as a personal computer, first, the surfaces of the photosensitive drums  1   a  and  1   d  are uniformly charged by the charging devices  2   a  to  2   d . Secondary, the surfaces of the photosensitive drums  1   a  to  1   d  are exposed with light emitted from the exposure device  5  based on the image data. Then, electrostatic latent images based on the image data are formed on the photosensitive drums  1   a  to  1   d . The development devices  3   a  to  3   d  are filled with a predetermined amount of the developer (for example, a two-component developer) containing the cyan, magenta, yellow and black toner. The toner in the developer is supplied to the photosensitive drums  1   a  to  1   d  by the development devices  3   a  to  3   d  and electrostatically attracted to the photosensitive drums  1   a  to  1   d . Thus, the toner images corresponding to the electrostatic latent images formed by the exposing of the exposure device  5  are formed. When a rate of the toner in the two-component developer filled in each of the development devices  3   a  to  3   d  becomes less than a specified rate owing to the above toner image formation, the toner is replenished to the corresponding development device of the development devices  3   a  to  3   d  from the corresponding toner container of the toner containers  4   a  to  4   d.    
     When the primary transferring rollers  6   a  to  6   d  apply an electric field at a predetermined transferring voltage between the primary transferring rollers  6   a  to  6   d  and the photosensitive drums  1   a  to  1   d , the cyan, magenta, yellow and black toner images on the photosensitive drums  1   a  to  1   d  are primarily transferred on the intermediate transfer belt  8 . These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, in preparation to form a new electrostatic latent image subsequently, the toner and the others remaining on the surfaces of the photosensitive drums  1   a  to  1   d  after the primary transferring are removed by the cleaning devices  7   a  to  7   d.    
     The intermediate transferring belt  8  is wound between an upstream driven roller  10  and the downstream drive roller  11 . When the intermediate transferring belt  8  starts to travel in the counterclockwise direction as the drive roller  11  is rotated by a belt drive motor (not shown), the sheet S is conveyed from the resist rollers pair  12   b  to the nip area (a secondary transferring nip area) between the drive roller  11  and the secondary transferring roller  9  at a predetermined timing. In the nip area, the full-color image on the intermediate transferring belt  8  is secondarily transferred on the sheet S. The sheet S on which the toner image is secondarily transferred is conveyed to the fixing device  13 . 
     The sheet S conveyed to the fixing device  13  is heated and pressed by a fixing belt  21  and a pressing roller  22  (see  FIG. 2 ). Thus, the toner image is fixed to the surface of the sheet S, and the predetermined full-color image is formed. The conveyance path of the sheet S on which the full-color image is formed is branched at a branch portion  14  branched in a plurality of directions, and is discharged to a discharge tray  17  by a discharge roller pair  15  as it is (alternatively, after the sheet is fed to a double-sided conveying path  18  and the images are formed on both sides). 
     [2. Structure of Fixing Device]  FIG. 2  is a sectional view schematically showing a structure of the fixing device  13 . The upper side of  FIG. 2  shows a downstream side in a sheet passing direction (a conveyance direction) for the fixing device  13 , and the lower side of  FIG. 2  shows an upstream side in the sheet passing direction for the fixing device  13 . The fixing device  13  includes the fixing belt  21  (a heated rotational body), the pressing roller  22  (a pressing rotational body), a heating unit  23 , a nip formation member  24 , a belt guide  25  and a frame member  26 . 
     The fixing belt  21  is supported by a housing (not shown) of the fixing device  13  in a rotatable manner around a horizontal axis. The fixing belt  21  is formed into an endless cylindrical shape having an outer diameter of 20 mm to 50 mm, for example. The fixing belt  21  has an axial length (a length in a width direction of the sheet S) almost equal to an axial length of the pressing roller  22 . The fixing belt  21  rotates in the counterclockwise direction in  FIG. 2  along the conveyance direction of the sheet S, as a recording medium. The rotational direction of the fixing belt  21  is also called a circumferential direction. 
       FIG. 3  is a sectional view schematically showing a structure of the fixing belt  21 . The fixing belt  21  has a layered structure having a heating layer  21   a  as a base layer, an elastic layer  21   b  and a release layer  21   c  which are provided around the heating layer  21   a  in order from the inside. The heating layer  21   a  is made of a metal film, such as a nickel film, having a thickness of 30 μm to 50 μm, or a polyimide film mixed with metal powder, such as copper, silver and aluminum, and having a thickness of 50 μm to 100 μm, for example. The elastic layer  21   b  is made of silicon rubber, and has a thickness of 100 μm to 500 μm, for example. The release layer  21   c  is made of fluorine-based resin, such as PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), and has a thickness of 30 μm to 50 μm, for example. 
     The fixing belt  21  includes a reflection member  21 R shown in  FIG. 2 . The reflection member  21 R is made of aluminum foil, for example, and is provided on the outer circumferential face (for example, on the release layer  21   c ) of the fixing belt  21  at an end portion in the axial direction of the fixing belt  21  (in the belt width direction, a direction perpendicular to the circumferential direction). In the circumferential direction of the fixing belt  21 , the reflection member  21 R is provided on a portion in the circumferential direction. The reflection member  21 R reflects infrared light emitted from a light emitting unit  51  on a light receiving unit  52 , described later. 
     The pressing roller  22  is supported by the housing of the fixing device  13  in a rotatable manner around a horizontal rotational axis. The pressing roller  22  is formed into a column shape, and has an axial length (a length in the sheet width direction) almost equal to the fixing belt  21 . 
     The pressing roller  22  has a layered structure having a core metal  22   a , an elastic layer and a release layer provided around the core metal  22   a  in order. The core metal  22   a  is made of metal, such as aluminum, and has a diameter of 20 mm, for example. The core metal  22   a  has an axial length longer than that of the elastic layer and the release layer. The elastic layer is made of silicon rubber, and has a thickness of 8 mm, for example. The release layer is made of fluorine-based resin, and has a thickness of 10 μm to 50 μm, for example. 
     A predetermined pressure is applied to the pressing roller  22  toward the fixing belt  21  by a pressing mechanism  30 . The outer circumferential face of the pressing roller  22  is pressed on the nip formation member  24  across the fixing belt  21 , and comes into pressure contact with the outer circumferential face of the fixing belt  21 . Then, between the outer circumferential faces, the fixing nip area N is formed. That is, the pressing roller  22  comes into pressure contact with the fixing belt  21 , and the fixing nip area N where an unfixed toner image IM on the sheet S is melt and fixed is formed between the pressing roller  22  and the fixing belt  21 . 
     The pressing mechanism  30  includes a rod-shaped pressing lever  31  and a pressing spring  32 . The pressing levers  31  are provided in the axial end portions of a supporting shaft  31   s . The supporting shaft  31   s  extends in parallel to the rotational axis of the pressing roller  22  (the central axis of the core metal  22   a ), and is disposed separated away from the pressing roller  22 . One end portion  31   a  of the pressing lever  31  (the lower end portion in  FIG. 2 ) is connected to the supporting shaft  31   s . Then, the pressing lever  31  is turnable around the supporting shaft  31   s . The pressing lever  31  comes into contact with the core metal  22   a  between the one end portion  31   a  and the other end portion  31   b  (the upper end portion in  FIG. 2 ). 
     The pressing spring  32  is a biasing member which presses the other end portion  31   b  of the pressing lever  31  to bias the pressing lever  31  toward the core metal  22   a . By the biasing force of the pressing spring  32 , the pressing lever  31  turns in the counterclockwise direction in  FIG. 2  around the supporting shaft  31   s . This makes it possible to press the pressing roller  22  on the fixing belt  21 . 
     The pressing roller  22  rotates in the clockwise direction in  FIG. 2  by a drive unit  41  (see  FIG. 4 ) described later. The pressing roller  22  comes into contact with the outer circumferential face of the fixing belt  21 , and applies a rotational drive force in the counterclockwise direction to the fixing belt  21 . This makes it possible to drive the fixing belt  21  to be rotated. 
     The heating unit  23  is disposed on an area opposite to an area where the pressing roller  22  is disposed, with respect to the fixing belt  21 , and faces the outer circumferential face of the fixing belt  21  via a predetermined gap. The heating unit  23  extends along the axial direction of the fixing belt  21  slightly longer than the fixing belt  21 . The heating unit  23  applies heat to the heating layer  21   a  of the fixing belt  21  in an introduction heating manner, and heats the fixing belt  21 . 
     The heating unit  23  includes an excitation coil  23   a , a holder, a core (which are not shown) and the others. The excitation coil  23   a  and the core are held by the holder at a predetermined position. The excitation coil  23   a  is made of a litz wire made of conductive wires bundle, and is wound so as to extend along the axial direction of the fixing belt  21 . The excitation coil  23   a  is formed into an arc shape around the outer circumferential face of the fixing belt  21  along the circumferential direction of the fixing belt  21 . 
     The nip formation member  24  is disposed inside the fixing belt  21  so as to face the pressing roller  22  across the fixing belt  21 . The nip formation member  24  comes into contact with the inner circumferential face of the fixing belt  21 , and forms the fixing nip area N between the fixing belt  21  and the pressing roller  22 . 
     The nip formation member  24  has an approximately parallelepiped shape extending in the axial direction of the fixing belt  21  and having a length almost equal to the length of the fixing belt  21 . The nip formation member  24  has a base material made of metal such as aluminum, or heat resistant resin such as liquid crystal polymer, for example. The nip formation member  24  may have an elastic layer made of elastomer or silicon rubber, for example, on the surface facing the fixing belt  21 . The nip formation member  24  has a sheet member (a release layer) made of fluorine-based resin, such as PFA, on the face facing the fixing belt  21 . The nip formation member  24  has a sheet member (a release layer) made of fluorine-based resin, such as PFA, for example, on the surface facing the fixing belt  21 . The sheet member comes into contact with the inner circumferential face of the fixing belt  21  at the fixing nip area N, and extends in the upstream area and in the downstream area in the rotational direction of the fixing belt  21  from the fixing nip area N, with which the fixing belt  21  does not come into contact. 
     The belt guide  25  is disposed in the inside of the fixing belt  21  so as to face the heating unit  23  across the fixing belt  21 . The belt guide  25  comes into contact with the inner circumferential face of the fixing belt  21  other than the fixing nip area N, and supports the fixing belt  21  from the inside. The belt guide  25  is formed by a metal plate having a length almost equal to the fixing belt  21  in the axial direction of the fixing belt  21 . The belt guide  25  is made of magnetic elastic metal, such as SUS430, and has a thickness of 0.1 mm to 0.5 mm, for example. The belt guide  25  has a contact part  25   a  and a connection part  25   b.    
     The contact part  25   a  is disposed on an opposite side to the fixing nip area N with respect to a radial center of the fixing belt  21 . The contact part  25   a  is curved in an arc shape along the inner circumferential face of the fixing belt  21 . The contact part  25   a  comes into contact with the inner circumferential face of the fixing belt  21  with almost its outer circumferential face. The contact part  25   a  faces the excitation coil  23   a  across the fixing belt  21 . 
     The connection part  25   b  is disposed on the downstream side of the contact part  25   a  in the rotational direction of the fixing belt  21 . The connection part  25   b  is coupled to a circumferential end portion of the contact part  25   a . The connection part  25   b  bents from the circumferential end portion of the contact part  25   a  inward radially, and then bents toward the fixing nip area N adjacently the frame member  26 . The connection part  25   b  does not come into contact with the fixing belt  21 . 
     The frame member  26  is disposed in almost the radial center portion of the fixing belt  21  between the contact part  25   a  of the belt guide  25  and the nip formation member  24 . The frame member  26  extends slightly longer than the fixing belt  21  along the axial direction of the fixing belt  21 . 
     The frame member  26  holds the nip formation member  24  and the belt guide  25 . The nip formation member  24  is fixed to a nip side wall portion  26   a  of the frame member  26  facing the fixing nip area N. The connection part  25   b  of the belt guide  25  is fixed to a side wall portion  26   b  of the frame member  26  on the upstream side of the rotational direction of the fixing belt  21 . 
     On the downstream side (the upper side in  FIG. 2 ) of the fixing nip area N in the sheet conveyance direction, a separator  29  is disposed. The separator  29  separates the sheet S passed through the fixing nip area N from the outer circumferential face of the fixing belt  21 . 
     [3. Control System of Fixing Device]  FIG. 4  is a block diagram schematically showing a configuration of a control system of the fixing device  13 . The fixing device  13  includes the drive unit  41 , a light emitting unit  51 , a light receiving unit  52  and a control unit  60 , in addition to the above-described configuration. The drive unit  41  includes a motor, a gear train and the others, and drives the pressing roller  22  to rotate it. The pressing roller  22  is rotated with a drive force from the motor. 
     The light emitting unit  51  is a light source for emitting infrared light (infrared ray) toward the fixing belt  21 , and is constituted of, for example, an LED (a light emitting diode) or a laser light source for emitting the infrared light. In the present embodiment, the light emitting unit  51  is controlled by a main control part  60   a , described later, of the control unit  60  so as to emit the infrared light at a constant period. 
     The light receiving unit  52  receives the infrared light emitted from the light emitting unit  51  and reflected on the reflection member  21 R of the fixing belt  21  periodically owing to the rotation of the fixing belt  21 , and also receives radiation light generated by heat generation of the fixing belt  21  (heat generated by the heating unit  23 ). In particular, the light receiving unit  52  detects a temperature of the fixing belt  21  by receiving the radiation light from the fixing belt  21 . The light receiving unit  52  is constituted of an infrared sensor having sensitivity in both wavelength ranges of the infrared light and the radiation light. In the present embodiment, the light emitting unit  51  and the light receiving unit  52  are disposed on the downstream side of the fixing nip area N around the fixing belt  21  (see  FIG. 2 ), but may be disposed on the upstream side. 
     The control unit  60  includes, for example, a central processing unit (CPU) and a memory. Specifically, the control unit  60  includes the main control part  60   a , a drive control part  60   b , and a storage part  60   c.    
     The main control part  60   a  controls the operations of the fixing device  13  and other parts of the image forming apparatus  100 . The main control part  60   a  controls the heating unit  23  based on the temperature of the fixing belt  21  detected by the light receiving unit  52 . This makes it possible to control the temperature of the fixing belt  21  within a predetermined temperature range suitable for the fixing. 
     The drive control part  60   b  controls the drive unit  41  based on the detection result by the light receiving unit  52  to control the rotation of the pressing roller  22 . Thus, the rotation of the fixing belt  21  rotated by being driven by the rotation of the pressing roller  22  can be indirectly controlled. The rotation control of the fixing belt  21  by the drive control of the drive unit  41  will be described later in detail. 
     The storage part  60   c  is a memory for storing an operation program of the control unit  60  and various kinds of information, and includes a ROM (a Read Only Memory), a RAM (a Random Access Memory), a nonvolatile memory, and the like. In particular, the storage part  60   c  stores a table showing a relationship between the temperature of the fixing belt  21  and the circumference (the circumferential length) of the fixing belt  21 . 
       FIG. 5  is a graph showing the relationship between the temperature BT (° C.) of the fixing belt  21  and the circumference L (mm) of the fixing belt  21 . As shown in  FIG. 5 , the circumference L of the fixing belt  21  changes in accordance with a change in the temperature BT of the fixing belt  21 . For example, when the temperature BT of the fixing belt  21  increases from a normal temperature (for example, 23° C.) to a temperature necessary for the fixing (for example, 160° C.), the circumference L of the fixing belt  21  extends from L0 (mm) to L1 (mm) due to the thermal expansion of the fixing belt  21 . The relationship between the temperature BT of the fixing belt  21  and the circumference L of the fixing belt  21  varies depending on the layer structure, the material forming each layer, and the others of the fixing belt  21  to be used. The relationship between the temperature BT and the circumference L, specific to the fixing belt  21  to be used is stored in the storage part  60   b  in a table state. 
     [4. Rotation Control of Fixing Belt] Next, the rotation control of the fixing belt  21  in the present embodiment will be described.  FIG. 6  is a flowchart showing an operation for controlling the rotation of the fixing belt  21 .  FIG. 7  is a timing chart schematically showing an example of the timings of the emitting of infrared light in the light emitting unit  51 , the reflecting of the infrared light on the reflecting member  21 R of the fixing belt  21 , and the receiving of the infrared light in the light receiving unit  52 . 
     First, the drive control part  60   b  (see  FIG. 4 ) of the control unit  60  controls the drive unit  41  to rotate the pressing roller  22  in the clockwise direction in  FIG. 2  (S 1 ). As a result, the fixing belt  21  on which the pressure roller  22  is pressed rotates in the counterclockwise direction in  FIG. 2  (S 2 ). A timing at which the drive control part  60   b  starts the rotation of the pressure roller  22  is appropriately controlled at a timing determined in accordance with the image forming operation in the image forming sections Pa to Pd. 
     Next, the main control part  60   a  controls the heating unit  23  to heat the heat generating layer  21   a  of the fixing belt  21 , and heats the fixing belt  21  to a predetermined temperature (for example, 160° C.) (S 3 ). The fixing belt  21  may be heated in parallel with S 2  or before the pressing roller  22  is rotated in S 1 . 
     Next, the main control part  60   a  of the control unit  60  controls the light emitting unit  51  to perform an emitting of the infrared light and a stopping of the emitting of the infrared light (S 4 ). By this control, the light emitting unit  51  performs the emitting of the infrared light and the stopping of the emitting of the infrared light within a prescribed period TL (sec) shown in  FIG. 7 , and repeats the emitting of the infrared light and the stopping of the emitting of the infrared light at the period TL. When a period in which the light emitting unit  51  emits the infrared light within the above period TL is set to a light emitting period T 1  (sec) and a period in which the light emitting unit  51  stops the emitting of the infrared light within the above period TL is set to a light emitting stop period T 2  (sec), TL=T 1 +T 2 . The light emitting timing of the infrared light in the light emitting unit  51  is controlled such that a period T 3  (sec), described later, in which the infrared light is reflected on the reflection member  21 R of the fixing belt  21  and then received by the light receiving unit  52  is contained within the light emitting period T 1 . 
     Next, the light receiving unit  52  detects the temperature BT of the fixing belt  21  (S 5 ). More specifically, it is as follows. 
     The infrared light emitted from the light emitting unit  51  advances toward the fixing belt  21  and is emitted on the fixing belt  21 . In the period T 3  in which the infrared light is emitted on the reflection member  21 R circulating with the rotation of the fixing belt  21 , of the light emitting period T 1  in which the infrared light is emitted from the light emitting unit  51 , the infrared light is reflected on the reflection member  21 R toward the light receiving unit  52 , and then received by the light receiving unit  52 . Therefore, the period T 3  constitutes an infrared light receiving period when the light receiving unit  52  periodically receives the infrared light emitted from the light emitting unit  51  via the reflection member  21 R. Hereinafter, the period T 3  is also called the infrared light receiving period T 3 . 
     On the other hand, in a period T 4  (=T 1 −T 3 ) in which the infrared light is emitted on a portion other than the reflection member R as the fixing belt  21  is rotated, of the light emitting period T 1 , the infrared light is not received by the light receiving unit  52  because it is not reflected on the reflection member  21 R. Further, in the light emitting stop period T 2 , because the infrared light is not emitted from the light emitting unit  51 , the infrared light is not received by the light receiving unit  52 . Therefore, the light emitting stop period T 2  and the period T 4  constitutes a period in which the infrared light emitted from the light emitting unit  51  is not received. Hereinafter, a total period of the light emitting stop period T 2  and the period T 4  is called a non-infrared light receiving period Toff (sec) (Toff=T 2 +T 4 ). 
     In the non-infrared light receiving period Toff, infrared light generated by the heating of the fixing belt  21  is radiated from the fixing belt  21 . The above infrared light generated by the heating of the fixing belt  21  is called a radiation light in order to separate it from the infrared light emitted from the light emitting unit  51 . In the non-infrared light receiving period Toff, the radiation light radiated from the fixing belt  21  is only received by the light receiving unit  52 . Then, the light receiving unit  52  makes it possible to detect the temperature BT of the fixing belt  21  based on the receiving of the radiation light in the non-infrared light receiving period Toff. 
     However, in the infrared light receiving period T 3 , the reception belt  21  radiates the radiation light due to the heating, and the light receiving unit  52  receives the radiation light. Then, an amount of the light detected by the light receiving unit  52  in the infrared light receiving period T 3  is an amount of the above radiation light added with an amount of the infrared light received via the reflection member  21 R from the light emitting unit  51 . Therefore, the light receiving unit  52  allows to separate the infrared light receiving period T 3  in which both the infrared light and the radiation light are received from the non-infrared light receiving period Toff where the above radiation light is only received, based on the amount of the detected light. Then, the light receiving unit  52  allows to detect the temperature BT of the fixing belt  21  based on the amount of the radiation light detected in the non-infrared light receiving period Toff. 
     Next, the drive control part  60   b  detects a light receiving period Tc (sec) of the infrared light when the fixing belt  21  is rotated, that is a period required for one rotation of the fixing belt  21 , based on the receiving of the infrared light by the light receiving unit  52  (S 6 ). As described above, the light receiving unit  52  allows to separate the infrared light receiving period T 3  from the non-infrared light receiving period Toff in which the infrared light is not received while the radiation light is only received. Then, the drive control part  60   b  allows to obtain the light receiving period Tc of the infrared light based on a light receiving starting timing of the infrared light in the infrared light receiving period Tc of the light receiving unit  52 . 
     Next, the drive control part  60   b  obtains a circumference L corresponding to the temperature BT of the fixing belt  21  detected in S 5 , based on the table stored in the storage part  60   c  (S 7 ). Then, the drive control part  60   b  obtains a rotational speed V (mm/sec) of the fixing belt  21  based on the light receiving period Tc of the infrared light and the circumference L of the fixing belt  21  obtained in S 7  (S 8 ). For example, the rotational speed V of the fixing belt is obtained by L/Tc. 
     Then, the drive control part  60   b  controls the drive unit  41  (for example, a motor) based on the rotational speed V of the fixing belt  21  obtained in S 8  to adjust a rotational speed of the pressing roller  22  (S 9 ). For example, when it is determined that the rotational speed V of the fixing belt  21  is faster than a predetermined speed range due to the thermal expansion of the fixing belt  21 , the drive control part  60   b  controls the drive unit  41  to decrease the rotational speed of the pressing roller  22  such that the sheet S is conveyed at a conveyance speed within the predetermined range. 
     As described above, in the fixing device  13  in the present embodiment, the light receiving unit  52  detects the temperature BT of the fixing belt  21  by the receiving of the radiation light generated by the heating of the heating belt  21  (S 5 ). Therefore, even if the fixing belt  21  is heat-expanded, the drive control part  60   b  allows to obtain the circumference L corresponding to the temperature BT at the heat-expansion (S 7 ). That is, it becomes possible to obtain the circumference L (a standard circumference+an extended length due to the heat-expansion) at the heat-expansion. Then, the drive control part  60   b  obtains the rotational speed V of the fixing belt  21  based on the light receiving period Tc in which the infrared light emitted from the light emitting unit  51  is received by the light receiving unit  52  and the above circumference L of the fixing belt  21  (S 8 ). As described above, because the circumference L of the fixing belt  21  is obtained in consideration of a variation in length due to the heat-expansion of the fixing belt  21 , it becomes possible to obtain the rotational speed V of the fixing belt  21  correctly based on the light receiving period Tc and the above circumference L. 
     Accordingly, the drive control part  60   b  controls the drive unit  41  based on the obtained rotational speed V, and it becomes possible to adjust the rotational speed of the pressing roller  22  coming into pressure contact with the fixing belt  21  with high accuracy. As a result, it becomes possible to keep the conveyance speed of the sheet S passed through the fixing nip area N within a predetermined range with high accuracy. 
     In particularly, when the period other than the infrared light receiving period T 3  when the light receiving unit  52  periodically receives the infrared light emitted from the light emitting unit  51  via the reflection member  21 R is defined as the non-infrared light receiving period Toff, the light receiving unit  52  detects the temperature BT of the fixing belt  21  based on the receiving of the radiation light in the non-infrared light receiving period Toff in S 5 . 
     In the non-infrared light receiving period Toff, the radiation light radiated from the fixing belt  21  is received by the light receiving unit  52  while the infrared light emitted from the light emitting unit  51  is not received by the light receiving unit  52  via the reflection member  21 R. Therefore, the light receiving unit  52  allows to detect the temperature of the fixing belt  21  correctly based on the amount of the received radiation light. 
     Especially, as shown in  FIG. 7 , when the light emitting unit  51  repeats the emitting of the infrared light and the stopping of the emitting at the predetermined period TL and the non-infrared light receiving period Toff contains the light emitting stop period T 2  of the infrared light, because the emitting of the infrared light from the light emitting unit  51  is not carried out in the light emitting stop period T 2 , the light receiving unit  52  is prevented from erroneously detecting the infrared light owing to diffused reflection in the casing, for example. Accordingly, the light receiving unit  52  may preferably detect the temperature BT of the fixing belt  21  based on the receiving of the radiation light in the light emitting stop period T 2  even in the non-infrared light receiving period Toff. In this case, the light receiving unit  52  allows to detect the temperature of the fixing belt  21  correctly based on the amount of the received radiation light. 
     Further, the storage part  60   c  previously stores the table showing the relationship between the temperature BT and the circumference L of the fixing belt  21 . The drive control part  60   b  obtains the circumference L corresponding to a detected temperature BT of the fixing belt  21  by the light receiving unit  52  based on the above table (S 7 ). By using the table in which the relationship is previously set, it becomes possible to obtain the circumference L corresponding to the temperature BT of the fixing belt  21  obtained in S 5  easily. 
     Further, the drive control part  60   b  controls the drive unit  41  based on the rotational speed V of the fixing belt  21  obtained in S 8  to adjust the rotational speed of the pressing roller  22 , so that the conveyance speed of the sheet S passed through the fixing nip area N is kept within the predetermined range (S 9 ). Even if the fixing belt  21  is heat-expanded and the circumference L is varied, the conveyance speed of the sheet S can be kept within the predetermined range by the adjustment of the rotational speed of the pressing roller  22 , so that it becomes possible to achieve an excellent conveyance of the sheet S. 
     Further, the light receiving unit  52  is constituted of the infrared light sensor having sensitivity in both wavelength ranges for the infrared light emitted from the light emitting unit  51  and the radiation light radiated from the fixing belt  21 . In this case, it becomes possible to detect both the infrared light and the radiation light using a single light receiving unit  52  (the infrared light sensor), so that it becomes possible to make the structure of the fixing device  13  simple compared with a case where the infrared light and the radiation light are detected by separate sensors. 
     Further, in the present embodiment, the fixing belt  21  is an example of a heated rotational body heated by the heating unit  23 . Because the fixing belt  21  is easily changed in circumference depending on the temperature BT, an effect of the present embodiment is remarkably exhibited, in which the rotational speed V of the fixing belt  21  is correctly obtained and an adjustment of the rotational speed of the pressing roller  22  is carried out with high accuracy. 
     The image forming apparatus  100  of the present embodiment includes the fixing device  13  having the above-described structure and the image forming sections Pa to Pd which forms an unfixed toner image IM on the sheet S conveyed to the fixing device  13 . Even if the fixing belt  21  is heat-expanded and the circumference L is changed, the pressing roller  22  is rotated based on the accurate rotational speed V of the fixing belt  21 , so that the sheet S conveyed from the image forming sections Pa to Pd can be conveyed at the conveyance speed within the predetermined range and discharged from the fixing device  13 . 
     The present disclosure is not limited to the configuration of the present embodiment, and various modifications can be made without departing from the spirit of the present disclosure. For example, in the present embodiment, the belt-heating type fixing device  13  provided with the endless fixing belt  21  as a rotational heated body is exemplified, but it is needless to say that the present invention can also be applied to a fixing device provided with a heated rotational body other than the fixing belt  21 , such as a fixing roller. The heating unit  23  is not limited to an induction heating type including an excitation coil and a core, and a halogen heater, for example, may be used. 
     In this embodiment, an example in which the table showing the relationship in  FIG. 5  is stored in the storage part  60   c  (see  FIG. 4 ) of the control unit  60  is described, but the present disclosure is not limited to this embodiment. For example, a memory may be provided outside the control unit  60  in the fixing device  13 , and the table may be stored in the memory. Further, a memory may be provided outside the fixing device  13  in the image forming apparatus  100 , and the table may be stored in the memory. Further, the configuration may be such that the table is stored in a server (for example, a cloud server) outside the image forming apparatus  100 , and the control unit  60  communicates with the server to refer to the table. 
     In the present embodiment, although the vertical conveyance type fixing device  13  in which the sheet S passes through the fixing nip area from the lower side to the upper side is described, the configuration described in the present embodiment can also be applied to a horizontal conveyance type fixing device in which the sheet S passes horizontally through the fixing nip area N. 
     The image forming apparatus  100  is not limited to a tandem type color printer as shown in  FIG. 1 , but can be applied to various image forming apparatuses equipped with a fixing device, such as a monochrome copying machine, a digital multifunctional peripheral, a facsimile, a laser printer, and the like. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure can be used, for example, in a fixing device of an image forming apparatus such as a copying machine, a printer, a facsimile, and a multifunctional peripheral.