Patent Publication Number: US-2023145933-A1

Title: Temperature control device and temperature control method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-181145, filed Nov. 5, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate to a temperature control device and a temperature control method. 
     BACKGROUND 
     An image forming apparatus includes a fuser that fixes a toner image onto a print medium by applying heat and pressure. A controller for the fuser controls a surface temperature of a fixing belt to be at a target value based on a detection signal (temperature sensor signal) from a temperature sensor. A fuser may also be referred to as a fixing device or the like. 
     If the temperature sensor fails, generally the temperature reported by the temperature sensor drops sharply. The controller increases power supplied to the fuser in order to bring the detected (reported) temperature back to the target value. In such a case of sensor failure, the temperature as reported by the temperature sensor remains below the target value, but the actual surface temperature of the fixing belt may exceed a normal operating temperature. If the surface temperature of the fixing belt approaches an ignition temperature, another sensor (different from the failed temperature sensor) responds, and the image forming apparatus stops operations immediately. 
     But due to such possible behavior of the image forming apparatus, the fixing belt and also the parts around the fixing belt may be subjected to potentially damaging thermal stresses, and the useful life of such parts of the image forming apparatus may be shortened. In addition, the image forming apparatus of such design normally cannot detect the cause of the emergency stop and repeated failures may be expected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of a maintenance system according to a first embodiment. 
         FIG.  2    is a diagram of an image forming apparatus. 
         FIG.  3    is a diagram of a temperature control circuit. 
         FIG.  4    is a flowchart of an operation of a temperature control circuit. 
         FIG.  5    is a graph for explaining aspects of an operation of a temperature control circuit. 
         FIG.  6    is a graph for explaining aspects of an operation of a temperature control circuit. 
         FIG.  7    is a flowchart of an operation of a temperature control circuit. 
         FIG.  8    is a graph for explaining aspects of a frequency generation process of a temperature control circuit. 
         FIG.  9    is a graph for explaining aspects of a conversion process of a temperature control circuit. 
         FIG.  10    is a graph for explaining aspects of a correction process of a temperature control circuit. 
         FIG.  11    is a diagram illustrating aspects of a drive pulse signal. 
         FIG.  12    is flowchart of a process based on an abnormality detection by a temperature sensor. 
         FIG.  13    is a graph illustrating aspects related to a temperature detection result and a temperature estimation result. 
         FIG.  14    is a diagram for explaining an example of incorporation of a correction value in a temperature difference. 
         FIG.  15    is a diagram illustrating an example display of a message to a user. 
         FIG.  16    depicts an image forming apparatus according to a second embodiment. 
         FIG.  17    depicts a temperature control circuit according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An object to be solved by an exemplary embodiment is to provide a temperature control device and a temperature control method, which are capable of preventing an abnormal temperature of an object being subjected to temperature control. 
     In general, according to one embodiment, a temperature control device includes a temperature estimation unit configured to provide an estimated temperature for an object based on energization of elements related to temperature control of the object; a comparison unit configured to compare a reference value to a difference between a detected temperature of the object the estimated temperature to a reference value; and an energization control unit configured to stop energizing the elements related to temperature control of the if the difference exceeds the reference value. 
     First Embodiment 
     Hereinafter, a temperature control device according to a first embodiment will be described with reference to the drawings. 
       FIG.  1    is a diagram for explaining an example of configuration of a maintenance system S according to the first embodiment. The maintenance system S includes a plurality of image forming apparatus  1  and a maintenance server  2 . 
     The image forming apparatuses  1  are communicably connected to the maintenance server  2  via a network NW with a firewall  3  interposed. For example, the network NW is the Internet. 
     The maintenance server  2  is an electronic device used by a management company to manage the image forming apparatuses  1 . The maintenance server  2  is communicably connected to each image forming apparatus  1  via the network NW. The maintenance server  2  is an example of an external device. 
       FIG.  2    depicts an image forming apparatus  1  in the first embodiment. The image forming apparatus  1  is one example of the temperature control device. 
     For example, the image forming apparatus  1  is a multifunction peripheral (MFP) that performs various processes such as image forming (printing) or the like on a print medium P. For example, the image forming apparatus  1  is a solid-state scanning printer (for example, an LED printer) that scans a light emitting diode (LED) array while conveying a print medium P. 
     For example, the image forming apparatus  1  includes a configuration that receives toner from a toner cartridge and forms an image on the print medium P with the received toner. The toner may be a monochromatic toner, or may be a color toner having a color such as cyan, magenta, yellow, black, or the like. Further, the toner may be a decolorable toner that decolorizes if heat is applied. 
     As illustrated in  FIG.  2   , the image forming apparatus  1  includes a housing  10 , a power conversion circuit  11 , a communication interface  12 , a system controller  13 , a temperature control circuit  14 , a display unit  15 , an operation interface  16 , a plurality of paper trays  17 , a paper discharge tray  18 , a conveyance unit  19 , an image forming unit  20 , and a fuser  21 . 
     The housing  10  is the main body of the image forming apparatus  1 . The housing  10  houses the power conversion circuit  11 , the communication interface  12 , the system controller  13 , the temperature control circuit  14 , the display unit  15 , the operation interface  16 , the plurality of paper trays  17 , the paper discharge tray  18 , the conveyance unit  19 , the image forming unit  20 , and the fuser  21 . 
     First, the configuration of a control system of the image forming apparatus  1  will be described. 
     The power conversion circuit  11  uses AC voltage from an AC power supply that supplies power to the image forming apparatus  1  to supply DC voltage to various components in the image forming apparatus  1 . 
     The communication interface  12  is for communicating with other devices. For example, the communication interface  12  is used for communication with a higher-level device (an external device). For example, the communication interface  12  is a Local Area Network (LAN) connector or the like. Further, the communication interface  12  may perform wireless communication with other devices in accordance with a standard such as Bluetooth®, Wi-fi, or the like. 
     The system controller  13  controls the image forming apparatus  1 . For example, the system controller  13  includes a processor  22  and a memory  23 . 
     The processor  22  is an arithmetic element that executes arithmetic processes. For example, the processor  22  is a central processing unit (CPU). The processor  22  performs various processes based on programs, data, and the like stored in the memory  23 . The processor  22  serves as a control unit capable of executing various operations by executing a program stored in the memory  23 . 
     The processor  22  executes the program stored in the memory  23  to perform various information processing functions or operations. For example, the processor  22  generates a print job based on an image acquired from an external device via the communication interface  12 . The processor  22  stores the generated print job in the memory  23 . 
     The print job includes image data indicating an image to be formed on the print medium P. The image data may be data for forming an image on one print medium P, or may be data for forming an image on a plurality of print media P. In addition, the print job includes information indicating whether it is a color print job or a monochrome print job. The print job may include information such as the number of copies to be printed (the number of page sets), the number of prints per copy (the number of pages), and the like. 
     Further, the processor  22  generates print control information for controlling the operations of the conveyance unit  19 , the image forming unit  20 , and the fuser  21  based on the generated print job. The print control information includes information indicating the timing of paper to be printed. The processor  22  supplies the print control information to the temperature control circuit  14 . 
     Further, the processor  22  serves as a print engine controller (engine controller) that executes a program stored in the memory  23  to control the operations of the conveyance unit  19  and the image forming unit  20 . That is, the processor  22  controls the conveyance of the print medium P by the conveyance unit  19  and controls the formation of an image on the print medium P by the image forming unit  20 , and the like. 
     The memory  23  is a storage medium for storing programs, data used in the programs, and the like. In addition, the memory  23  also serves as a working memory. That is, the memory  23  temporarily stores the data being processed by the processor  22 , the program executed by the processor  22 , and the like. 
     The image forming apparatus  1  in other examples may be configured to include an engine controller separately from the system controller  13 . In this case, the engine controller controls the conveyance of the print medium P by the conveyance unit  19  and controls the formation of an image on the print medium P by the image forming unit  20 , and the like. 
     Furthermore, in this case, the system controller  13  supplies the engine controller with information necessary for control of a print operation. 
     The temperature control circuit  14  controls the temperature of the fuser  21 . For example, the temperature control circuit  14  includes a processor  24  and a memory  25 . Like the processor  22 , the processor  24  is an arithmetic element that executes arithmetic processes. The processor  24  performs various processes based on programs, data, and the like stored in the memory  25 . The processor  24  executes programs stored in the memory  25  to execute various operations and functions. Like the memory  23 , the memory  25  is a storage medium for storing programs, data used in the programs, and the like. 
     The display unit  15  includes a display that displays a screen according to a video signal input from a display control unit such as the system controller  13 , a graphic controller, or the like. For example, the display of the display unit  15  displays screens for various settings of the image forming apparatus  1 . 
     The operation interface  16  is connected to an input operation member. The operation interface  16  supplies operation signals to the system controller  13  corresponding to the user operations made using the input operation member(s). For example, an input operation member can be a touch sensor, a numeric keypad, a power key, a paper feed key, various function keys, a keyboard, or the like. The touch sensor acquires information indicating a designated position in a certain area of a display screen or the like. The touch sensor can be configured integrally with the display unit  15  as a touch panel, and thus inputs a signal indicating a touched position on the screen displayed on the display unit  15  to the system controller  13 . 
     Each of the paper trays  17  is a cassette that houses print media P. The paper tray  17  is configured to be inserted into and removed from the housing  10  to permit loading and unloading of print media P. 
     The paper discharge tray  18  supports a print medium P discharged from the image forming apparatus  1 . 
     Next, a configuration for conveying the print medium P of the image forming apparatus  1  will be described. 
     The conveyance unit  19  is a mechanism for conveying the print medium P within the image forming apparatus  1 . As illustrated in  FIG.  2   , the conveyance unit  19  provides a plurality of conveyance paths. For example, the conveyance unit  19  includes a paper feed conveyance path  31  and a paper discharge conveyance path  32 . 
     The paper feed conveyance path  31  and the paper discharge conveyance path  32  are each formed of motors, rollers, and guides. The motors rotate shafts under the control of the system controller  13  to rotate the rollers linked to the shafts. As the rollers are rotated, the print medium P is moved along a conveyance path. The guides serve to limit the conveyance direction of the print medium P on a conveyance path. 
     The paper feed conveyance path  31  picks up a print medium P from the paper tray  17 , and then supplies the picked up print medium P to the image forming unit  20 . The paper feed conveyance path  31  includes pickup rollers  33  corresponding to the respective paper trays. Each pickup roller  33  cans send a print medium P on a paper tray  17  into the paper feed conveyance path  31 . 
     The paper discharge conveyance path  32  is a conveyance path for discharging the print medium P from the housing  10  after printing. The print medium P discharged by the paper discharge conveyance path  32  can be supported on the paper discharge tray  18 . 
     The image forming unit  20  is configured to form an image on the print medium P. Specifically, the image forming unit  20  forms an image on the print medium P based on a print job generated by the processor  22 . 
     The image forming unit  20  includes a plurality of process units  41 , a plurality of exposure devices  42 , and a transfer mechanism  43 . The image forming unit  20  includes the exposure device  42  for each process unit  41 . One process unit  41  and one exposure device  42  will be described as representative of the plurality of process units  41  and the plurality of exposure devices  42 . 
     The process unit  41  is configured to form a toner image. For example, a separate process unit  41  is provided for each type of toner. For example, one of the process units  41  corresponds to each of the colors of toner such as cyan, magenta, yellow, black, and the like, respectively. Specifically, a toner cartridge for one color of toner can be connected to each process unit  41 . 
     The toner cartridge includes a toner container and a toner delivery mechanism. The toner container is a container that stores toner therein. The toner delivery mechanism is a mechanism formed of a screw or the like that delivers toner from the toner container to the process unit  41 . 
     Each process unit  41  includes a photosensitive drum  51 , an electrostatic charger  52 , and a developing device  53 . The photosensitive drum  51  is a cylindrical drum with a photosensitive layer formed on an outer peripheral surface of the drum. The photosensitive drum  51  can be rotated at a constant speed by a drive mechanism. 
     The electrostatic charger  52  uniformly charges the surface of the photosensitive drum  51 . For example, the electrostatic charger  52  applies a voltage (development bias voltage) to the photosensitive drum  51  using an electrostatic roller to charge the photosensitive drum  51  to a uniform negative polarity potential (contrast potential). The electrostatic roller is rotated by the rotation of the photosensitive drum  51  with a predetermined pressure being applied to the photosensitive drum  51 . 
     The developing device  53  is a device for adhering the toner onto the photosensitive drum  51 . The developing device  53  includes a developer container, an agitating mechanism, a developing roller, a doctor blade, an auto toner control (ATC) sensor, and the like. 
     The developer container is a container that receives and stores the toner delivered from the toner cartridge. A carrier is stored in the developer container in advance. The toner delivered from the toner cartridge is agitated (mixed) with the carrier by the agitating mechanism to form a developer in which the toner and the carrier are mixed. In general, the carrier is placed in the developer container when the developing device  53  is manufactured and is not replenished (replaced) over time, but rather is used over and over (recycled). 
     The developing roller is rotated in the developer container to attach the developer onto the roller surface. The doctor blade is a member arranged at a predetermined interval from the surface of the developing roller. The doctor blade removes a portion of the developer adhered onto the surface of the rotating developing roller. As a result, a developer layer having a thickness corresponding to the distance between the doctor blade and the surface of the developing roller is formed on the surface of the developing roller. 
     For example, an ATC sensor is a magnetic flux sensor that has a coil and detects a voltage value generated in the coil. The detected voltage of the ATC sensor changes according to the density of the magnetic flux from the toner in the developer container. That is, the system controller  13  determines the concentration ratio (toner concentration ratio) of the toner to the carrier still remaining in the developer container based on the detected voltage of the ATC sensor. The system controller  13  operates a motor to drive a toner cartridge delivery mechanism based on the detected toner concentration ratio to deliver additional toner from the toner cartridge to the developer container of the developing device  53  if the toner concentration ratio is low. 
     The exposure device  42  includes a plurality of light emitting elements. The exposure device  42  selectively irradiates the charged photosensitive drum  51  with light from the light emitting elements to form a latent image on the photosensitive drum  51 . For example, the light emitting elements are light emitting diodes (LEDs) or the like. One light emitting element is configured to irradiate one point on the photosensitive drum  51  with light. The plurality of light emitting elements are arranged in a main scanning direction which is a direction parallel to the rotation axis of the photosensitive drum  51 . 
     The exposure device  42  irradiates the photosensitive drum  51  with light with the plurality of light emitting elements arranged in the main scanning direction to form a latent image on the photosensitive drum  51  for one line. The exposure device  42  continuously irradiates the rotating photosensitive drum  51  with light to form a plurality of lines of the latent images line-by-line. 
     When the electrostatically charged surface of the photosensitive drum  51  is irradiated with the light from the exposure device  42 , an electrostatic latent image can be formed since exposure changes the conductivity of the photosensitive layer of the photosensitive drum  51 . When the layer of the developer formed on the surface of the developing roller approaches the surface of the photosensitive drum  51 , the toner contained in the developer is selectively adhered onto the surface of the photosensitive drum  51  in a manner corresponding to the latent image. As a result, a toner image is formed on the surface of the photosensitive drum  51 . 
     The transfer mechanism  43  is configured to transfer the toner image formed on the surface of the photosensitive drum  51  to the print medium P. 
     For example, the transfer mechanism  43  includes a primary transfer belt  61 , a secondary transfer facing roller  62 , a plurality of primary transfer rollers  63 , and a secondary transfer roller  64 . 
     The primary transfer belt  61  is an endless belt wound around the secondary transfer facing roller  62  and a plurality of winding rollers. An inner surface (inner peripheral surface) of the primary transfer belt  61  is in contact with the secondary transfer facing roller  62  and the plurality of winding rollers, and an outer surface (outer peripheral surface) of the primary transfer belt faces the photosensitive drum  51  of the process unit  41 . 
     The secondary transfer facing roller  62  is rotated by a motor. The secondary transfer facing roller  62  rotates to convey the primary transfer belt  61  in a predetermined conveyance direction. The plurality of winding rollers are configured to be freely rotatable. The plurality of winding rollers are rotated according to the movement of the primary transfer belt  61  by the secondary transfer facing roller  62 . 
     The plurality of primary transfer rollers  63  are configured to bring the primary transfer belt  61  into contact with the photosensitive drum  51  of the process unit  41 . The plurality of primary transfer rollers  63  are provided so as to correspond to the photosensitive drums  51  of the plurality of process units  41 . Specifically, the plurality of primary transfer rollers  63  are provided at positions facing the photosensitive drums  51  of the corresponding process units  41 , respectively, with the primary transfer belt  61  interposed therebetween. The primary transfer roller  63  comes into contact with the inner peripheral surface side of the primary transfer belt  61  and displaces the primary transfer belt  61  toward the photosensitive drum  51 . As a result, the primary transfer roller  63  brings the outer peripheral surface of the primary transfer belt  61  into contact with the photosensitive drum  51 . 
     The secondary transfer roller  64  is provided at a position facing the primary transfer belt  61 . The secondary transfer roller  64  contacts the outer peripheral surface of the primary transfer belt  61  and applies pressure thereto. As a result, a transfer nip is formed at which the secondary transfer roller  64  and the outer peripheral surface of the primary transfer belt  61  are in close contact with each other. When the print medium P passes through the transfer nip, the secondary transfer roller  64  presses the print medium P passing through the transfer nip against the outer peripheral surface of the primary transfer belt  61 . 
     The secondary transfer roller  64  and the secondary transfer facing roller  62  are rotated to convey the print medium P supplied from the paper feed conveyance path  31  while holding the print medium P therebetween. As a result, the print medium P passes through the transfer nip. 
     In the above configuration, when the outer peripheral surface of the primary transfer belt  61  comes into contact with the photosensitive drum  51 , the toner image formed on the surface of the photosensitive drum is transferred onto the outer peripheral surface of the primary transfer belt  61 . If the image forming unit  20  includes the plurality of process units  41 , the primary transfer belt  61  receives the toner images from each of the photosensitive drums  51  of the plurality of process units  41 . The toner image transferred onto the outer peripheral surface of the primary transfer belt  61  is conveyed by the primary transfer belt  61  to the transfer nip where the secondary transfer roller  64  and the outer peripheral surface of the primary transfer belt  61  are in close contact with each other. If the print medium P is in the transfer nip, the toner image on the outer peripheral surface of the primary transfer belt  61  is transferred onto the print medium P in the transfer nip. 
     Next, a configuration related to fixing of the image forming apparatus  1  will be described. 
     The fuser  21  is an induction heating type fuser that fixes the toner image on the print medium P. The fuser  21  is operated under the control of the system controller  13  or the temperature control circuit  14 . 
     The fuser  21  includes a pressure roller  70 , a pressure pad  71 , a magnetic alloy shunt position adjustment mechanism  72  (“shunt adjuster  72 ”), an aluminum member  73 , a magnetic alloy shunt  74 , a ferrite core  75 , an induction heating coil  76 , a fixing belt  77 , a frame  78 , and a temperature sensor  79 . 
     The pressure roller  70  is positioned so as to face the fixing belt  77  from a radial direction. The width of the pressure roller  70  in the longitudinal direction is greater than the width of the print medium P to be conveyed. The longitudinal direction of the pressure roller  70  is a direction orthogonal to the rotation direction of the pressure roller  70 . The pressure roller  70  comes into contact with the fixing belt  77  by the pressure of springs at both ends. The pressure roller  70  includes a metal member, as a core material, and an elastic layer, such as a rubber layer or the like, on the outside thereof. The pressure roller  70  includes a release layer on the outside surface. The pressure roller  70  is rotationally driven. The rotation of the pressure roller  70  may drive the fixing belt  77 . The pressure roller  70  may include a one-way clutch such that a speed difference from the fixing belt  77  does not occur. 
     The pressure pad  71  is positioned inside the fixing belt  77 . The pressure pad  71  presses against the fixing belt  77  toward the pressure roller  70 . A fixing nip is formed between the fixing belt  77  and the pressure roller  70 . The shape of the portion of the pressure pad  71  facing the pressure roller  70  is substantially the same as the outer peripheral shape of the pressure roller  70 . The width of the pressure pad  71  in the longitudinal direction is greater than the width of the print medium P to be conveyed. The longitudinal direction of the pressure pad  71  is a direction parallel to the longitudinal direction of the fixing belt  77  corresponding to the direction orthogonal to the rotation direction of the fixing belt  77 . The pressure pad  71  has a low friction material between itself and the pressure roller  70  in order to improve the slidability (reduce friction). The pressure pad  71  is made of a heat resistant resin material. For example, the heat-resistant resin is polyetheretherketone (PEEK), phenol resin, or the like. 
     The shunt adjuster  72  is fixed to the frame  78 . The shunt adjuster  72  is a position adjustment mechanism for the magnetic alloy shunt  74 . The shunt adjuster  72  includes a spring. The shunt adjuster  72  adjusts the position of the magnetic alloy shunt  74  by the force of the spring. 
     The aluminum member  73  is connected to shunt adjuster  72 . The aluminum member  73  blocks the magnetic field generated by the induction heating coil  76 . 
     The magnetic alloy shunt  74  faces the induction heating coil  76  with a portion of the fixing belt  77  interposed therebetween. For example, the width of the magnetic alloy shunt  74  in the longitudinal direction is greater than the width of the fixing belt  77  in the longitudinal direction. The longitudinal direction of the magnetic alloy shunt  74  is a direction parallel to the longitudinal direction of the fixing belt  77 . The magnetic alloy shunt  74  is a sheet material made of a temperature-sensitive magnetic material. The inductance value of the magnetic alloy shunt  74  is substantially constant at less than a saturation temperature, but drops sharply at the saturation temperature or higher. 
     The ferrite core  75  is positioned outside the induction heating coil  76 . The ferrite core  75  blocks the magnetic field generated by the induction heating coil  76 . 
     The induction heating coil  76  is positioned on the outside of the fixing belt  77 . The induction heating coil  76  forms a magnetic field by the supply of power from an inverter  82 . The power supplied to the induction heating coil  76  is also referred to as IH power. The induction heating coil  76  is an example of an element related to temperature control. 
     The fixing belt  77  is an endless belt. The fixing belt  77  is rotated counterclockwise in  FIG.  2   . The width of the fixing belt  77  in the longitudinal direction is greater than the width of the print medium P to be conveyed. The fixing belt  77  includes a plurality of layers. The fixing belt  77  includes a conductive layer that generates heat in response to the magnetic field of the induction heating coil  76 . For example, the conductive layer is made of a conductive material such as iron, nickel, copper, or the like. The fixing belt  77  may be formed by laminating a copper layer on a nickel layer. The fixing belt  77  also includes an elastic layer on the conductive layer and a release layer. The release layer is a layer that comes into direct contact with the toner. As the release layer, a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (PFA) or the like having good releasability is preferable. 
     The frame  78  is positioned inside the region surrounded by the fixing belt  77  (interior region). The frame  78  holds the pressure pad  71 . 
     The temperature sensor  79  detects the surface temperature of the fixing belt  77 . The surface of the fixing belt  77  is an example of a temperature to be controlled. The surface temperature of the fixing belt  77  is a temperature of the fixing belt  77 . The temperature of the fixing belt  77  is an example of a temperature to be controlled. For example, the temperature sensor  79  is positioned outside the fixing belt  77 . The temperature sensor  79  may be positioned at the center of the fixing belt  77  in the longitudinal direction. The temperature sensor  79  may be positioned at the end of the fixing belt  77  in the longitudinal direction. The temperature sensor  79  may be positioned on a downstream side of a heating portion including the magnetic alloy shunt  74  and the induction heating coil  76 , and an upstream side of the fixing nip formed between the fixing belt  77  and the pressure roller  70 . The number of the temperature sensors  79  is not limited to one and there may be a plurality of temperature sensors  79 . The temperature sensor  79  may be a contact type thermistor. 
     With the configuration described above, the fixing belt  77  and the pressure roller  70  apply heat and pressure to the print medium P passing through the fixing nip. The toner on the print medium P is melted by the heat applied from the fixing belt  77  and is applied to the surface of the print medium P by the pressure applied by the fixing belt  77  and the pressure roller  70 . As a result, the toner image is fixed on the print medium P at the fixing nip. The print medium P after the fixing nip is sent into the paper discharge conveyance path  32  and discharged to the outside of the housing  10 . 
     In some examples, the fuser  21  may include a belt having the same function as the pressure roller  70 , instead of the roller such as the pressure roller  70 . Likewise, the fuser  21  may include a roller having the same function as the fixing belt  77  instead of a belt such as the fixing belt  77 . 
     An automatic temperature control function of the fuser  21  will be described. 
     If the induction heating coil  76  is driven at a high frequency by an inverter  82 , a composite inductance of the magnetic alloy shunt  74 , the induction heating coil  76 , and the fixing belt  77  is generated. A resonance phenomenon occurs due to the composite inductance and a resonance capacitor  83 . If the resonance frequency and the frequency for driving the induction heating coil  76  are appropriate, the induction heating coil  76  is supplied with a large amount of power. In this embodiment, it is assumed that a narrow print medium P passes through the fuser  21 . The portion of the fixing belt  77  through which the print medium P passes is deprived of heat by the passage of the print medium P. On the other hand, since the portion of the fixing belt  77  where the print medium P does not pass (contact) continues to accumulate heat, the temperature increases. At this time, the magnetic alloy shunt  74  reacts to the high temperature and changes inductance value. As a result, the relationship between the resonance frequency and the frequency for driving the induction heating coil  76  is changed, and the heat generation in the high temperature portion of the fixing belt  77  is suppressed. As a result, the end of the fixing belt  77  in the longitudinal direction does not reach an abnormal high temperature. 
     The temperature control circuit  14  controls the temperature of the fuser  21 .  FIG.  3    is a diagram for explaining an example of the configuration of the temperature control circuit  14  according to the first embodiment. The temperature control circuit  14  includes a converter  81 , the inverter  82 , and the resonance capacitor  83 . 
     The converter  81  is a circuit that converts the AC voltage of the AC power supply into a DC voltage. For example, the converter  81  is a diode bridge. The converter  81  is connected to the AC power supply. The converter  81  is connected to the inverter  82 . 
     The inverter  82  is a circuit that converts the DC voltage converted by the converter  81  into an AC voltage. The inverter  82  supplies power to the induction heating coil  76  and drives the induction heating coil  76 . For example, the inverter  82  is a half-bridge inverter that includes a switch  821  and a switch  822 . The inverter  82  is connected to the converter  81 . The inverter  82  is connected to a series resonant circuit that includes the resonance capacitor  83  and the induction heating coil  76 . The series resonant circuit is connected between a connection point M 1  of the inverter  82  and GND. The connection point M 1  is a between the switch  821  and the switch  822 . If a high frequency alternating signal is supplied to the gates of the switch  821  and the switch  822 , a high frequency alternating voltage is generated between the connection point M 1  of the inverter  82  and GND. The series resonant circuit resonates with the high frequency, and a high power is supplied to the induction heating coil  76 . This high power is used for induction heating based on the magnetic field formed by the induction heating coil  76 . 
     For example, the switch  821  and the switch  822  are power semiconductors such as an insulated gate bipolar transistor (IGBT) or a silicon carbide (SiC) transistor, or the like. The inverter  82  is not limited to a half-bridge inverter, and may be a full-bridge inverter, a half-wave voltage resonance inverter, a quasi-resonance inverter, or the like in other examples. 
     The temperature control circuit  14  includes a temperature estimation unit  801 , an estimation history storage unit  802 , a high frequency component extraction unit  803 , a coefficient addition unit  804 , a target temperature output unit  805 , a difference comparison unit  806 , a frequency generation unit  807 , a conversion unit  808 , a correction unit  809 , a pulse generation unit  810 , a buffer  811 , a buffer  812 , and a determination unit  813 . The temperature control circuit  14  acquires a temperature detection result Td from the temperature sensor  79 . The temperature detection result Td indicates the surface temperature of the fixing belt  77  as detected by the temperature sensor  79 . The temperature control circuit  14  acquires a voltage value ACV of the AC voltage of the AC power supply. For example, the voltage value ACV is an effective value. Since the AC power supply generally has an allowable variation range, the voltage value ACV varies within a predetermined range. However, if the voltage value ACV varies, the IH power changes. Therefore, it can be said that the heating operation of the induction heating coil  76  varies according to the voltage value ACV. If it is assumed that the duty control for the inverter  82  is the same, the amount of heat generation of the fixing belt  77  for voltage value ACV of 90 V is less than that for voltage value ACV of 100 V. Similarly, for voltage value ACV of 110 V, the amount of heat generation of the fixing belt  77  is greater than that for voltage value ACV of 100 V. 
     The temperature estimation unit  801  performs a temperature estimation process for estimating the surface temperature of the fixing belt  77 . An estimation history PREV from the estimation history storage unit  802  and a power estimation result ESTPB from the correction unit  809  are input to the temperature estimation unit  801 . The estimation history PREV is the history of temperature estimation result EST generated by the temperature estimation unit  801  for each short space of time dt (time periods dt). The temperature estimation result EST indicates the surface temperature of the fixing belt  77  as estimated by the temperature estimation unit  801 . The power estimation result ESTPB indicates an estimated value of the currently generated IH power according to the voltage value ACV corresponding to the frequency FRQ. The power estimation result ESTPB is an example of the power estimation result showing the estimated value of the IH power corresponding to the frequency FRQ. The frequency FRQ indicates the frequency of drive pulse signal of the inverter  82  to which the induction heating coil  76  is connected. For example, the frequency FRQ is an analog voltage or digital numerical value representing the frequency. The drive pulse signal is an example of a drive signal. The drive pulse signal includes a high frequency drive pulse signal PU and a high frequency drive pulse signal PD that alternately output High. 
     The temperature estimation unit  801  estimates the surface temperature of the fixing belt  77  based on the estimation history PREV and the power estimation result ESTPB. Estimating the surface temperature of the fixing belt  77  based on the estimation history PREV and the power estimation result ESTPB is an example of estimating the surface temperature of the fixing belt  77  by the correction unit  809  based on the power estimation result ESTPB. The power estimation result ESTPB is based on the frequency FRQ which will be described below. Therefore, estimating the surface temperature of the fixing belt  77  based on the estimation history PREV and the power estimation result ESTPB is an example of estimating the surface temperature of the fixing belt  77  based on the frequency FRQ. The power estimation result ESTPB and the frequency FRQ are related to the energization of the induction heating coil  76 . Therefore, estimating the surface temperature of the fixing belt  77  based on the estimation history PREV and the power estimation result ESTPB is an example of estimating the surface temperature of the fixing belt  77  based on the energization of the induction heating coil  76 . 
     For example, the temperature estimation unit  801  estimates the amount of temperature change in the surface temperature of the fixing belt  77  based on the power estimation result ESTPB at the current time for each time period dt. The temperature estimation unit  801  adds the amount of temperature change to a temperature estimation result EST for the time period dt before the current time, which is included in the estimation history PREV. The temperature estimation unit  801  estimates the surface temperature of the fixing belt  77  at the current time based on the addition of the amount of temperature change to the temperature estimation result EST for the time period dt before the current time. The temperature estimation unit  801  reuses the temperature estimation result EST of the time period dt before the current time to obtain the temperature estimation result EST at the current time. The temperature estimation unit  801  outputs the temperature estimation result EST to the estimation history storage unit  802  and the high frequency component extraction unit  803 . 
     The estimation history storage unit  802  holds the history of the temperature estimation result EST. The estimation history storage unit  802  outputs the estimation history PREV to the temperature estimation unit  801 . 
     The high frequency component extraction unit  803  performs a high-pass filter process for extracting the high frequency component of the temperature estimation result EST. For example, the high frequency component extraction unit  803  cancels the DC component of the temperature estimation result EST and extracts only the high frequency component. The high frequency component extraction unit  803  outputs the high frequency component HPF, which is a signal indicating the extracted high frequency component, to the coefficient addition unit  804 . 
     The coefficient addition unit  804  performs a coefficient addition process for correcting the temperature detection result Td. The temperature detection result Td from the temperature sensor  79  and the high frequency component HPF from the high frequency component extraction unit  803  are input to the coefficient addition unit  804 . The coefficient addition unit  804  corrects the temperature detection result Td based on the high frequency component HPF. Specifically, the coefficient addition unit  804  calculates the correction temperature value WAE based on the temperature detection result Td and the high frequency component HPF. The high frequency component HPF is based on the temperature estimation result EST. Therefore, it can be said that the correction temperature value WAE is based on the temperature estimation result EST and the temperature detection result Td. The coefficient addition unit  804  is an example of a calculation unit for calculating the correction temperature value WAE. The coefficient addition unit  804  outputs the correction temperature value WAE to the difference comparison unit  806 . 
     The target temperature output unit  805  performs an output process for outputting a preset target temperature TGT to the difference comparison unit  806 . The target temperature TGT is a target value of the surface temperature of the fixing belt  77 . The target temperature TGT can be changed by rewriting by a command from the processor  22 . The target temperature TGT may be stored in the memory  23  or stored in the memory  25 . 
     For example, the target temperature TGT can be set separately for each printing process. 
     In one example, the target temperature TGT to be used varies according to the characteristics of the print medium P used in each printing process. For example, one variable characteristic of a print medium P is sheet thickness. Generally, the target temperature TGT is set such that a predetermined temperature can be maintained when the print medium P is plain paper (e.g., basic or standard paper type). In general, the amount of heat withdrawn from the fixing belt  77  by the print medium P when the print medium P passes through the fuser  21  increases for thick paper as compared to plain paper. Thus, the surface temperature of the fixing belt  77  tends to become lower when printing on thick paper than when printing on plain paper. If the print medium P is known to be thick paper, the target temperature TGT is set higher than the target temperature TGT associated with plain paper, in consideration of the greater amount of heat withdrawn from the fixing belt  77  by the thick paper. As a result, the surface temperature of the fixing belt  77  can be more easily maintained at a predetermined temperature. If the print medium P is known to be thinner than plain paper, the target temperature TGT can be set lower than the target temperature TGT associated with plain paper. 
     In another example, the target temperature TGT may vary according to the statuses of the printing process. 
     In this context, the possible statuses of the printing process include, for example, an inrush current prevention state, a start-up heating state, a ready state, a print start state, a printing state, and an energy saving ready state, and the like, but is not limited thereto. 
     In the inrush current prevention state, the target temperature TGT is set to increase stepwise such that a large current does not flow suddenly. In the start-up heating state, the target temperature TGT is set to be higher such that the reference temperature suitable for printing can be reached quickly. In the ready state, the target temperature TGT is set to be slightly lower than the target temperature TGT in the start-up heating state to save energy after the printer is ready. In the printing start state, the target temperature TGT is set to be higher than the target temperature TGT for the printing state shortly before printing begins such that the temperature does not decrease below the appropriate temperature at the beginning of printing. In the printing state, the target temperature TGT is set to the reference temperature considered suitable for printing. In the energy-saving ready state, the target temperature TGT is set to be lower than the target temperature TGT in the ready state if the ready state continues for a long time. 
     The difference comparison unit  806  performs a difference calculation process. The difference comparison unit  806  compares the target temperature TGT from the target temperature output unit  805  with the correction temperature value WAE from the coefficient addition unit  804 . The difference comparison unit  806  calculates a difference DIF based on the comparison between the target temperature TGT and the correction temperature value WAE. The difference DIF is an example of the comparison result by the difference comparison unit  806 . The difference comparison unit  806  is an example of the temperature comparison unit. In this example, the difference DIF will be described as a value obtained by subtracting the correction temperature value WAE from the target temperature TGT, but the opposite may be true in other examples. If the correction temperature value WAE is lower than the target temperature TGT, the difference DIF is a positive value. If the correction temperature value WAE is higher than the target temperature TGT, the difference DIF is a negative value. The difference DIF shows the relationship between the target temperature TGT and the correction temperature value WAE. The difference comparison unit  806  outputs the difference DIF to the frequency generation unit  807 . 
     The frequency generation unit  807  performs a frequency generation process for generating a frequency FRQ. The frequency generation unit  807  generates the frequency FRQ based on the difference DIF. The generating the frequency FRQ includes determining the frequency FRQ. For example, if the correction temperature value WAE is higher than the target temperature TGT, the frequency generation unit  807  raises the frequency FRQ to be higher than if the correction temperature value WAE is equal to the target temperature TGT. This is to reduce the IH power. If the correction temperature value WAE is lower than the target temperature TGT, the frequency generation unit  807  decreases the frequency FRQ to be lower than if the correction temperature value WAE is equal to the target temperature TGT. This is to increase the IH power. The difference DIF is based on the target temperature TGT and the correction temperature value WAE. Therefore, the generating the frequency FRQ based on the difference DIF is an example of generating the frequency FRQ based on the temperature estimation result EST by the temperature estimation unit  801 , the temperature detection result Td by the temperature sensor  79 , and the target temperature TGT. The frequency generation unit  807  outputs the frequency FRQ to the conversion unit  808  and the pulse generation unit  810 . 
     The conversion unit  808  performs a conversion process of converting the frequency FRQ into a power estimation result ESTPA. The power estimation result ESTPA indicates an estimated value of the currently generated IH power corresponding to the frequency FRQ if it is assumed that the voltage value ACV is 100 V. The power estimation result ESTPA is an example of the power estimation result showing the estimated value of the IH power corresponding to the frequency FRQ. The converting the frequency FRQ to the power estimation result ESTPA is an example of estimating the IH power based on the frequency FRQ. The conversion unit  808  is an example of a power estimation unit that estimates IH power. The conversion unit  808  outputs the power estimation result ESTPA to the correction unit  809  based on the conversion from the frequency FRQ to the power estimation result ESTPA. 
     The correction unit  809  performs a correction process for correcting the power estimation result ESTPA based on the voltage value ACV. The correcting the power estimation result ESTPA based on the voltage value ACV includes converting the power estimation result ESTPA based on the voltage value ACV into the power estimation result ESTPB. The correcting the power estimation result ESTPA based on the voltage value ACV is an example of estimating the IH power based on the voltage value ACV. The correction unit  809  is an example of the power estimation unit that estimates IH power. The correction unit  809  outputs the power estimation result ESTPB to the temperature estimation unit  801 . 
     The pulse generation unit  810  performs a pulse generation process for generating a pulse signal based on the frequency FRQ. The pulse signal includes a high frequency first pulse signal and a high frequency second pulse signal that alternately output High. The second pulse signal is a pulse train obtained by inverting High and Low of the first pulse signal. The first pulse signal and the second pulse signal are pulse trains having a predetermined duty corresponding to the frequency FRQ. The first pulse signal and the second pulse signal are pulse trains that repeat a High period and a Low period according to a predetermined duty. For example, the predetermined duty is 50%. If the first pulse signal and the second pulse signal include a dead time, the predetermined duty may be a value less than 50%. The dead time includes the time if both the first pulse signal and the second pulse signal are Low between the timing at which the first pulse signal transitions from High to Low and the timing at which the second pulse signal transitions from Low to High. The dead time includes the time if both the first pulse signal and the second pulse signal are Low between the timing at which the second pulse signal transitions from High to Low and the timing at which the first pulse signal transitions from Low to High. The pulse generation unit  810  outputs the first pulse signal to the buffer  811 . The pulse generation unit  810  outputs the second pulse signal to the buffer  812 . The pulse signal is an example of the drive signal because it is the source of the drive pulse signal including the drive pulse signal PU and the drive pulse signal PD. 
     The buffer  811  supplies the drive pulse signal PU obtained by converting the first pulse signal into the gate voltage of the switch  821  of the inverter  82  to the gate of the switch  821 . The buffer  812  supplies the drive pulse signal PD obtained by converting the second pulse signal into the gate voltage of the switch  821  of the inverter  82  to the gate of the switch. The drive pulse signal PD is a pulse train obtained by inverting High and Low of the drive pulse signal PU. The drive pulse signal PU and the drive pulse signal PD are pulse trains having a predetermined duty corresponding to the frequency FRQ. The drive pulse signal PU and the drive pulse signal PD are pulse trains that repeat a High period and a Low period according to a predetermined duty. Note that, in this example, since the inverter  82  is described as a half-bridge inverter, two drive signals are supplied to the inverter  82 , but embodiments are not limited thereto. If the inverter  82  is a full bridge inverter, four drive signals are supplied to the inverter  82 . 
     The determination unit  813  performs a process for detecting an abnormality in the temperature sensor  79 . For example, the abnormality of the temperature sensor  79  includes a failure of the temperature sensor  79  such as a disconnection or the like of the temperature sensor  79 . The determination unit  813  compares a temperature difference based on the temperature estimation result EST by the temperature estimation unit  801  and the temperature detection result Td by the temperature sensor  79 , with a reference. The determination unit  813  is an example of the comparison unit that compares the temperature difference with the reference. 
     The temperature difference may be a value based on at least the temperature estimation result EST and the temperature detection result Td. Here, the temperature difference will be described as being a value based on a correction value, in addition to the temperature estimation result EST and the temperature detection result Td. The temperature difference is a value obtained by correcting the difference between the temperature estimation result EST and the temperature detection result Td with the correction value. The value obtained by correcting the difference between the temperature estimation result EST and the temperature detection result Td with the correction value corresponds to a difference between a value obtained by correcting one of the temperature estimation result EST and the temperature detection result Td with the correction value and the other of the temperature estimation result EST and the temperature detection result Td. 
     The correction value is a value corresponding to the temperature estimation result EST in the normal operation of the image forming apparatus  1  and the temperature detection result Td in the normal operation of the image forming apparatus  1 . The normal operation is the operation of the image forming apparatus  1  if the temperature sensor  79  is in a normal state. The correction value is a value for bias correction that reflects, in the temperature difference, the difference between the temperature estimation result EST in the normal operation and the temperature detection result Td in the normal operation. The correction value may include a correction value for each status of the printing process described above. The correction value may be calculated in advance by the image forming apparatus  1  based on the past temperature estimation result EST and the past temperature detection result Td at any timing such as once a day, once a week, or the like. The correction value may be stored in the memory  25  or stored in the memory  23 . 
     The determination unit  813  uses the correction value for detecting the temperature difference so as to correct the individual difference for each image forming apparatus. As illustrated in  FIG.  5   , there is a difference between the temperature estimation result EST in the normal operation and the temperature detection result Td in the normal operation. However, the temperature estimation result EST in the normal operation and the temperature detection result Td in the normal operation change while maintaining a certain correlation. The relationship between the temperature estimation result EST in normal operation and the temperature detection result Td in normal operation differs for each image forming apparatus. In some image forming apparatuses, the temperature estimation result EST in normal operation is higher than the temperature detection result Td in normal operation, while it is lower in the other image forming apparatuses. The determination unit  813  can standardize the process of comparing the temperature difference with the reference by using the temperature difference obtained by correcting the individual difference for each image forming apparatus based on the correction value. 
     In this example, the temperature difference will be described as a value obtained by correcting, with a correction value, a value obtained by subtracting the temperature detection result Td from the temperature estimation result EST. The correction value is a value obtained by subtracting the temperature estimation result EST in the normal operation from the temperature detection result Td in the normal operation. In this example, if the temperature estimation result EST in the normal operation is higher than the temperature detection result Td in the normal operation, the correction value is a negative value. If the temperature estimation result EST in the normal operation is lower than the temperature detection result Td in the normal operation, the correction value is a positive value. The temperature difference is a value obtained by adding the correction value to the value obtained by subtracting the temperature detection result Td from the temperature estimation result EST. Note that, conversely, the correction value may be a value obtained by subtracting the temperature detection result Td in the normal operation from the temperature estimation result EST in the normal operation. In this example, the temperature difference is a value obtained by subtracting the correction value from the value obtained by subtracting the temperature detection result Td from the temperature estimation result EST. 
     Note that, conversely, the temperature difference may be a value by correcting, with a correction value, a value obtained by subtracting the temperature estimation result EST from the temperature detection result Td. In this example, the correction value may be a value obtained by subtracting the temperature estimation result EST in the normal operation from the temperature detection result Td in the normal operation. Conversely, the correction value may be a value obtained by subtracting the temperature detection result Td in the normal operation from the temperature estimation result EST in the normal operation. 
     Note that the temperature difference is not limited to the temperature difference at one time point. The temperature difference may be an amount of change in the temperature difference over a predetermined time. The amount of change may be a difference between the temperature difference at the time of beginning of the predetermined time and the temperature difference at the time of end of the predetermined time. In this example, the determination unit  813  can detect a sudden change in the temperature difference. The predetermined time can be set as appropriate. 
     Note that the temperature difference may be a difference between the temperature estimation result EST and the temperature detection result Td, without considering the correction value. In this example, a predetermined temperature may be a temperature in consideration of the correction value. 
     The reference is a reference for detecting an abnormality of the temperature sensor  79  based on the temperature difference. In one example, the reference includes that it is a predetermined temperature or above, such as 30 degrees or above, or the like. In another example, the reference may include that the temperature change over a predetermined time is greater than or equal to a predetermined temperature. The predetermined temperature can be set as appropriate. 
     If the temperature difference satisfies the reference, the determination unit  813  outputs an abnormality detection signal to an energization control unit  131  and a transmission unit  132 . The fact that the temperature difference satisfies the reference includes that the temperature difference is equal to or higher than the predetermined temperature. The fact that the temperature difference does not satisfy the reference includes that the temperature difference is less than the predetermined temperature. The abnormality detection signal is a signal indicating the detection of the abnormality of the temperature sensor  79 . 
     As described above, the temperature control circuit  14  adjusts the IH power based on the temperature detection result Td, the estimation history PREV, and the frequency FRQ. As a result, the temperature control circuit  14  controls the surface temperature of the fixing belt  77  by induction heating based on the magnetic field formed by the induction heating coil  76 . This control will be referred to as weighted average control with estimate temperature (WAE control) in this example. 
     Note that the temperature estimation unit  801 , the estimation history storage unit  802 , the high frequency component extraction unit  803 , the coefficient addition unit  804 , the target temperature output unit  805 , the difference comparison unit  806 , the frequency generation unit  807 , the conversion unit  808 , the correction unit  809 , the pulse generation unit  810 , and the determination unit  813  of the temperature control circuit  14  are not limited to those implemented by software, and may be configured by hardware by an electric circuit. 
     Each component implemented in the system controller  13  by executing the program stored in the memory  23  by the processor  22  of the system controller  13  will be described. The system controller  13  includes the energization control unit  131 , the transmission unit  132 , a reception unit  133 , and a display control unit  134 . 
     The energization control unit  131  controls the power conversion circuit  11  to control energization of various configurations in the image forming apparatus  1 . In one example, the energization control unit  131  stops energizing the induction heating coil  76  based on the abnormality detection signal from the determination unit  813 . That is, the energization control unit  131  stops energizing the induction heating coil  76  if the temperature difference satisfies the reference. The energization control unit  131  controls the power conversion circuit  11  and stops energizing the induction heating coil  76 . The energization control unit  131  may stop energizing the temperature control circuit  14  and stop the operation of the temperature control circuit  14  to stop energizing the induction heating coil  76 . As long as the energization control unit  131  is able to stop energizing the induction heating coil  76 , the mode of stopping energizing the induction heating coil  76  is not limited to the above. 
     In another example, the energization control unit  131  continues to energize the elements related to the communication function based on the abnormality detection signal from the determination unit  813 . That is, if the temperature difference satisfies the reference, the energization control unit  131  continues to energize the elements related to the communication function. The elements related to the communication function are elements of the image forming apparatus  1  for maintaining the state in which the image forming apparatus  1  can communicate with the maintenance server  2  via the network NW. For example, the elements related to the communication function are the processor  22 , the communication interface  12 , or the like, but are not limited thereto. While stopping energizing the induction heating coil  76 , the energization control unit  131  controls the power conversion circuit  11  to continue to energize the elements related to the communication function. The elements related to the communication function are maintained in an operable state while the energization of the induction heating coil  76  is stopped. 
     The transmission unit  132  transmits information to the maintenance server  2  via the network NW. For example, the transmission unit  132  transmits an abnormality notification to the maintenance server  2  based on the abnormality detection signal from the determination unit  813 . That is, if the temperature difference satisfies the reference, the transmission unit  132  transmits the abnormality notification to the maintenance server  2 . 
     The abnormality notification is a notification indicating the occurrence of an abnormality in the temperature sensor  79 . The abnormality notification may include the information to be exemplified below. The abnormality notification may include a serial number of the image forming apparatus  1 . The abnormality notification may include a model number of the image forming apparatus  1 . The abnormality notification may include the name of a purchasing company of the image forming apparatus  1 . The abnormality notification may include the name of the service company in charge of maintenance. 
     The abnormality notification may include information indicating the content of the abnormality. The information indicating the content of the abnormality is the information indicating the abnormality of the temperature sensor  79 . If the image forming apparatus  1  includes the plurality of temperature sensors  79 , the information indicating the abnormality of the temperature sensor may include information for identifying the temperature sensor  79  in which the abnormality occurred. The abnormality notification may include information indicating how to deal with the abnormality of the temperature sensor  79 . The information indicating how to deal with the abnormality may include information indicating replacement of the temperature sensor  79 . The information indicating how to deal with the abnormality may include information indicating the model number of the temperature sensor  79  to be replaced. 
     The reception unit  133  receives information from the maintenance server  2  via the network NW. For example, the reception unit  133  receives the communication result from the maintenance server  2  as a response to the abnormality notification. The communication result includes information indicating the maintenance schedule of the image forming apparatus  1 . The information indicating the maintenance schedule includes information indicating the scheduled visit time of the serviceman. 
     The display control unit  134  controls a display of image on the display unit  15 . For example, the display control unit  134  controls the display of a message on the display unit  15  based on the communication result. 
     Hereinafter, WAE control will be described in detail.  FIG.  4    is a flowchart for explaining output of the frequency FRQ in WAE control.  FIGS.  5  and  6    are explanatory views for explaining each signal and the like in WAE control. The horizontal axis of  FIGS.  5  and  6    represents the time. The vertical axis of  FIGS.  5  and  6    represents the temperature. 
     The temperature control circuit  14  generates a trigger for starting the process for every time period dt (ACT  1 ). At ACT  1 , for example, the temperature control circuit  14  starts counting by the timer based on an instruction to start WAE control from the system controller  13 . The temperature control circuit  14  ends the counting by the timer based on an instruction to end WAE control from the system controller  13 . The temperature control circuit  14  generates triggers at time period dt intervals based on the counts by the timer during the operation of the image forming apparatus  1 . 
     The temperature control circuit  14  acquires the temperature detection result Td (ACT  2 ). At ACT  2 , for example, the temperature control circuit  14  acquires the temperature detection result Td from the temperature sensor  79 . 
     The temperature control circuit  14  acquires the voltage value ACV (ACT  3 ). At ACT  3 , for example, the temperature control circuit  14  acquires the voltage value ACV from the voltage detection unit that detects the voltage value ACV. 
     The temperature control circuit  14  acquires the target temperature TGT (ACT  4 ). At ACT  4 , for example, the temperature control circuit  14  acquires the target temperature TGT based on the signal from the system controller  13 . 
     The temperature estimation unit  801  performs a temperature estimation process (ACT  5 ). For example, the temperature estimation unit  801  acquires the power estimation result ESTPB at the current time from the correction unit  809 . The temperature estimation unit  801  acquires the temperature estimation result EST for the time period dt before the current time as the estimation history PREV from the estimation history storage unit  802 . The temperature estimation unit  801  estimates the surface temperature of the fixing belt  77  based on the estimation history PREV and the power estimation result ESTPB. The temperature estimation unit  801  outputs the temperature estimation result EST to the estimation history storage unit  802  and the high frequency component extraction unit  803  based on the estimation of the surface temperature of the fixing belt  77 . 
     The heat transfer can be expressed equivalently by the RC time constant or the like of the electric circuit. The heat capacity is replaced by the capacitor C. The resistance of heat transfer is replaced by resistance R. The heat source is replaced by a voltage source. The temperature estimation unit  801  simulates a RC circuit in which the values of individual elements are set in advance in real time. The temperature estimation unit  801  uses the power estimation result ESTPB based on the frequency FRQ. The power estimation result ESTPB corresponds to the voltage value applied to the RC circuit. That is, the IH power increases as the frequency FRQ decreases, and accordingly, as a means of simulating this, the temperature estimation unit  801  increases the voltage applied to the RC circuit. On the other hand, the IH power decreases as the frequency FRQ increases, and accordingly, as a means of simulating this, the temperature estimation unit  801  decreases the voltage applied to the RC circuit. The temperature estimation unit  801  estimates the amount of heat applied to the fixing belt  77  based on the RC circuit and the power estimation result ESTPB. The temperature estimation unit  801  estimates the surface temperature of the fixing belt  77  based on the amount of heat applied to the fixing belt  77  and the estimation history PREV. As described above, the temperature estimation unit  801  estimates the surface temperature of the fixing belt  77  based on the RC circuit and the power estimation result ESTPB. 
     As illustrated in  FIG.  5   , there is a difference between the temperature detection result Td and the actual surface temperature of the fixing belt  77 . The actual surface temperature of the fixing belt  77  changes with a short cycle because the driving frequency of the induction heating changes frequently. On the other hand, there are circumstances that the temperature sensor  79  may have poor responsiveness to temperature changes due to its own heat capacity and the characteristics of the temperature-sensitive material. In particular, cheaper temperature sensors tend to have poorer responsiveness. As a result, the temperature detection result Td cannot accurately follow the actual surface temperature of the fixing belt  77  which changes at a high frequency. That is, the temperature detection result Td detected by the temperature sensor  79  is a delayed result which may differ from the actual surface temperature of the fixing belt  77 . Due to such delay (the lack of sensor responsiveness), the temperature detection result Td as detected by the temperature sensor  79  corresponds to a smoothed state lacking details of the fine (high frequency) changes in the actual surface temperature of the fixing belt  77 . 
     However, as illustrated in  FIG.  5   , the temperature estimation result EST more appropriately follows the changes in the actual surface temperature of the fixing belt  77  corresponding to the frequency of the drive pulse signal supplied to the inverter  82  (or the IH power based on the frequency). However, since the temperature estimation result EST is only a simulation result, its absolute value may differ from the actual surface temperature of the fixing belt  77  due to differences in actual conditions from simulation parameters and the like. 
     The high frequency component extraction unit  803  performs a high-pass filter process (ACT  6 ). At ACT  6 , for example, the high frequency component extraction unit  803  extracts the high frequency component of the temperature estimation result EST. As illustrated in  FIG.  5   , the high frequency component HPF appropriately follows the change in the actual surface temperature of the fixing belt  77 . The high frequency component extraction unit  803  outputs just the high frequency component HPF to the coefficient addition unit  804 . 
     The coefficient addition unit  804  performs a coefficient addition process (ACT  7 ). At ACT  7 , for example, the coefficient addition unit  804  acquires the temperature detection result Td (as acquired by the temperature control circuit  14 ) at ACT  2 . The coefficient addition unit  804  acquires the high frequency component HPF from the high frequency component extraction unit  803 . The coefficient addition unit  804  calculates the correction temperature value WAE based on the temperature detection result Td and the high frequency component HPF. In a typical example, the coefficient addition unit  804  multiplies the high frequency component HPF by a preset coefficient KA. The coefficient addition unit  804  adjusts the value of the high frequency component HPF to be added to the temperature detection result Td with the coefficient KA. The coefficient addition unit  804  adds the high frequency component HPF multiplied by the coefficient KA to the temperature detection result Td. The coefficient addition unit  804  calculates the correction temperature value WAE based on this addition process. 
     For example, if the coefficient KA is 1, the coefficient addition unit  804  directly adds the high frequency component HPF to the temperature detection result Td. If the coefficient KA is 0.1, the coefficient addition unit  804  adds a value of 1/10 of the high frequency component HPF to the temperature detection result Td. In such a case, the correction temperature value WAE incorporates little to no effect of the high frequency component HPF and is thus close to the temperature detection result Td. When the coefficient KA is 1 or more, the correction temperature value WAE can more strongly reflect the effect of the high frequency component HPF. Experiments have shown that the coefficient KA set in the coefficient addition unit  804  is preferably not a very extreme value (high or low), but rather a value near 1. 
       FIG.  6    is an explanatory diagram for explaining an example of the actual surface temperature of the fixing belt  77 , the temperature detection result Td, and the correction temperature value WAE. In the WAE control, the temperature control circuit  14  estimates a fine temperature change of the surface temperature of the fixing belt  77  based on the temperature detection result Td and the high frequency component HPF of the temperature estimation result EST. Therefore, as illustrated in  FIG.  6   , the correction temperature value WAE is a value that more appropriately follows the actual surface temperature of the fixing belt  77 . 
     The difference comparison unit  806  performs a difference calculation process (ACT  8 ). For example, at ACT  8 , the difference comparison unit  806  acquires the target temperature TGT from the target temperature output unit  805 . The difference comparison unit  806  acquires the correction temperature value WAE from the coefficient addition unit  804 . The difference comparison unit  806  compares the target temperature TGT with the correction temperature value WAE. The difference comparison unit  806  calculates the difference DIF obtained by subtracting the correction temperature value WAE from the target temperature TGT. The difference comparison unit  806  outputs the difference DIF to the frequency generation unit  807 . 
     The frequency generation unit  807  performs a frequency generation process (ACT  9 ). At ACT  9 , for example, the frequency generation unit  807  acquires the difference DIF from the difference comparison unit  806 . The frequency generation unit  807  generates the frequency FRQ based on the difference DIF. The frequency generation unit  807  may generate the frequency FRQ based on the difference DIF and the voltage value ACV. The frequency generation unit  807  outputs the frequency FRQ to the conversion unit  808 . The frequency generation unit  807  stores the frequency FRQ until the timing of outputting the frequency FRQ to the pulse generation unit  810  is reached. 
     The conversion unit  808  performs a conversion process (ACT  10 ). At ACT  10 , for example, the conversion unit  808  acquires the frequency FRQ from the frequency generation unit  807 . The conversion unit  808  converts the frequency FRQ into the power estimation result ESTPA. The conversion unit  808  outputs the power estimation result ESTPA to the correction unit  809 . 
     The correction unit  809  performs a correction process (ACT  11 ). At ACT  11 , for example, the correction unit  809  acquires the power estimation result ESTPA from the conversion unit  808 . The correction unit  809  acquires the voltage value ACV acquired by the temperature control circuit  14  at ACT  3 . The correction unit  809  corrects the power estimation result ESTPA based on the voltage value ACV. The correction unit  809  acquires the power estimation result ESTPB based on the correction of the power estimation result ESTPA. The correction unit  809  outputs the power estimation result ESTPB to the temperature estimation unit  801 . 
     The temperature control circuit  14  determines whether or not a time period dt elapses (ACT  12 ). If the time period dt has not yet elapsed (ACT  12 , NO), the temperature control circuit  14  waits until the time period dt elapses. If a time period dt has elapsed (ACT  12 , YES), the frequency generation unit  807  outputs the frequency FRQ to the pulse generation unit  810  (ACT  13 ). At ACT  12 , for example, the frequency generation unit  807  outputs the frequency FRQ generated at the time period dt intervals to the pulse generation unit  810  at the time period dt intervals. Further, the value of the frequency FRQ output by the frequency generation unit  807  is stored by the frequency generation unit  807  until it is updated after the elapse of the next time period dt interval. 
     The temperature control circuit  14  determines whether or not to execute the WAE control stop process (ACT  14 ). At ACT  14 , for example, the temperature control circuit  14  stops the WAE control based on the instruction to stop the WAE control from the system controller  13 . If the temperature control circuit  14  does not execute the WAE control stop process (ACT  14 , NO), the process proceeds from ACT  14  to ACT  1 . The temperature control circuit  14  repeats the processes illustrated in  FIG.  4    for each time period dt during the operation of the image forming apparatus  1 . If the temperature control circuit  14  executes the WAE control stop process (ACT  14 , YES), the temperature control circuit  14  ends the process illustrated in  FIG.  4   . 
       FIG.  7    is a flowchart for explaining the output of the drive pulse signal in the WAE control. 
     The pulse generation unit  810  acquires the frequency FRQ from the frequency generation unit  807  (ACT  14 ). At ACT  14 , for example, the pulse generation unit  810  acquires the frequency FRQ from the frequency generation unit  807  at time period dt intervals. 
     The pulse generation unit  810  generates a first pulse signal based on the frequency FRQ (ACT  15 ). At ACT  15 , for example, the pulse generation unit  810  generates a first pulse signal having a duty of 50% corresponding to the frequency FRQ. If the frequency FRQ is 50 kHz, one cycle is 20 μs. The pulse generation unit  810  allocates 10 μs as High and 10 μs as Low in one cycle of 20 μs. 
     The pulse generation unit  810  generates a second pulse signal based on the frequency FRQ (ACT  16 ). At ACT  16 , for example, the pulse generation unit  810  generates a second pulse signal obtained by inverting High and Low of the first pulse signal. 
     The pulse generation unit  810  inserts a dead time into the pulse signal (ACT  17 ). At ACT  17 , for example, the pulse generation unit  810  inserts a dead time into the first pulse signal having a duty of 50% and generates a first pulse signal having a duty of 48%. The pulse generation unit  810  inserts a dead time into the second pulse signal having a duty of 50% and generates a second pulse signal having a duty of 48%. The dead time is provided in order to prevent a short circuit if the switch  821  and the switch  822  of the inverter  82  are turned on at the same time. The pulse generation unit  810  outputs the first pulse signal to the buffer  811 . The pulse generation unit  810  outputs the second pulse signal to the buffer  812 . 
     The buffer  811  outputs a drive pulse signal PU, and the buffer  812  outputs a drive pulse signal PD (ACT  18 ). For example, at ACT  18 , the buffer  811  acquires the first pulse signal from the pulse generation unit  810 . The buffer  811  supplies the drive pulse signal PU obtained by converting the first pulse signal into the gate voltage of the switch  821  of the inverter  82  to the gate of the switch  821 . The buffer  812  acquires the second pulse signal from the pulse generation unit  810 . The buffer  812  supplies the drive pulse signal PD obtained by converting the second pulse signal into the gate voltage of the switch  822  of the inverter  82  to the gate of the switch  822 . 
     The temperature control circuit  14  determines whether or not to execute the WAE control stop process (ACT  19 ). At ACT  19 , for example, the temperature control circuit  14  stops the WAE control based on the instruction to stop the WAE control from the system controller  13 . If the temperature control circuit  14  does not execute the WAE control stop process (ACT  19 , NO), the process proceeds from ACT  19  to ACT  14 . The temperature control circuit  14  repeats the processes illustrated in  FIG.  7    at time period dt intervals during the operation of the image forming apparatus  1 . If the temperature control circuit  14  executes the WAE control stop process (ACT  19 , YES), the temperature control circuit  14  ends the process illustrated in  FIG.  7   . 
     An example of the frequency generation process by the frequency generation unit  807  will be described. 
       FIG.  8    illustrates a graph of a function for each of three different voltage values ACV showing the relationship between the control amount and the frequency of the drive pulse signal of the inverter  82 . 
     The horizontal axis represents the control amount of IH power. The control amount is a power increase and decrease coefficient indicating the degree of increase and decrease in IH power. The control amount may be the value of the difference DIF itself or a value having a correlation with the difference DIF. As the difference DIF increases, the control amount also increases. The control amount of 0 indicates that the correction temperature value WAE is the same as the target temperature TGT, so that the IH power may be kept as it is. The control amount being positive indicates a situation in which the IH power needs to be increased because the correction temperature value WAE is lower than the target temperature TGT. The control amount being negative indicates a situation in which the IH power needs to be decreased because the correction temperature value WAE is higher than the target temperature TGT. The vertical axis is the frequency of the drive pulse signal of the inverter  82  corresponding to the frequency FRQ. 
     Since the inverter  82  utilizes the LC resonance phenomenon, the relationship between the frequency FRQ and the IH power is non-linear. Therefore, as illustrated in  FIG.  8   , a function showing the relationship between the control amount and the frequency of the drive pulse signal of the inverter  82  is prepared. The solid line shows a graph line of a function (also referred to as a FRQ100 function) for voltage value ACV of 100 V. The broken line shows a graph line of a function (also referred to as a FRQ110 function) for voltage value ACV of 110 V. The alternate long and short dash line shows a graph line of a function (also referred to as a FRQ90 function) for voltage value ACV of 90 V.  FIG.  8    shows three functions for three different voltage values ACV, but four or more functions corresponding to different voltage values ACV may be prepared. 
     According to the characteristics of the inverter  82 , in a situation where the control amount is positive and the IH power needs to be increased, the frequency FRQ needs to be lower than the frequency FRQ for control amount of 0. According to the characteristics of the inverter  82 , in a situation where the control amount is negative and the IH power needs to be decreased, the frequency FRQ needs to be higher than the frequency FRQ for control amount of 0. 
     The frequency generation unit  807  generates a frequency FRQ based on the difference DIF and the voltage value ACV, as illustrated below. The frequency generation unit  807  selects a function associated with the voltage value ACV from a predetermined plurality of functions corresponding to different voltage values ACV. The frequency generation unit  807  determines the control amount based on the difference DIF. The frequency generation unit  807  determines the frequency FRQ according to the control amount based on the selected function. For example, for voltage value ACV of 90 V, the frequency generation unit  807  selects the predetermined FRQ90 function. The frequency generation unit  807  determines (sets) the frequency FRQ according to the control amount using the FRQ90 function. The frequency FRQ determined according to the control amount based on the FRQ90 function is lower than the frequency FRQ that would be determined according to the same control amount using the FRQ100 function. The decrease in the IH power due to the voltage value ACV being 90 V (lower than 100 V) is offset by the increase in the IH power that accompanies the decrease of the frequency FRQ for voltage value ACV 90 V from the voltage value ACV of 100 V. 
     The frequency generation unit  807  can generate the frequency FRQ according to the variation of the voltage value ACV by generating the frequency FRQ based on different voltage values ACV. As a result, the frequency generation unit  807  can generate a frequency FRQ for appropriately controlling the IH power even if the voltage value ACV varies. 
     The frequency generation unit  807  preferably generates a frequency FRQ based on the difference DIF and the voltage value ACV, but embodiments are not limited thereto. The frequency generation unit  807  may generate a frequency FRQ based on the difference DIF without considering the voltage value ACV. In this example, the frequency generation unit  807  may use the FRQ100 function for voltage value ACV of 100 V. 
     The frequency generation unit  807  may generate a frequency FRQ by reference to table data instead of calculation from a selected predetermined function. The table data may be data in which the control amount and the frequency of the drive pulse signal of the inverter  82  are associated with each other. The table data may include data for each of several voltage values ACV with the control amount and the frequency of the drive pulse signal of the inverter  82  associated with each other. The table data may be stored in the memory  25 . 
     An example of the conversion process by the conversion unit  808  will be described. 
       FIG.  9    illustrates a graph line of a function for different voltage values ACV showing the relationship between the frequency of the drive pulse signal of the inverter  82  and the IH power. 
     The horizontal axis is the frequency of the drive pulse signal of the inverter  82  corresponding to the frequency FRQ. The vertical axis represents the IH power. 
     The solid line shows a graph line of a function (also referred to as an F2P100 function) for voltage value ACV of 100 V. The broken line shows a graph line of a function (also referred to as an F2P110 function) for voltage value ACV of 110 V. The alternate long and short dash line shows a graph line of a function (also referred to as a F2P90 function) for voltage value ACV of 90 V. 
     Since the inverter  82  utilizes the LC resonance phenomenon, the relationship between the frequency FRQ and the IH power is non-linear. The IH power increases as the frequency FRQ decreases, and the IH power decreases as the frequency FRQ increases. 
     The conversion unit  808  converts the frequency FRQ into the power estimation result ESTPA, as exemplified below. The conversion unit  808  acquires the IH power corresponding to the frequency FRQ as the power estimation result ESTPA based on the F2P100 function for voltage value ACV of 100 V. 
     The conversion unit  808  may convert the frequency FRQ into the power estimation result ESTPA with reference to the table data instead of the function. The table data is data in which the frequency of the drive pulse signal of the inverter  82  and the IH power are associated with each other. The table data may be stored in the memory  25 . 
     An example of the correction process by the correction unit  809  will be described. 
       FIG.  10    illustrates a graph line of a function for different voltage values ACV showing the relationship between the IH power before correction and the IH power after correction. 
     The horizontal axis represents the IH power before correction. The IH power before correction corresponds to the power estimation result ESTPA. The vertical axis represents the IH power after correction. The IH power after correction corresponds to the power estimation result ESTPB. 
     The solid line shows a graph line of a function (a function with slope of 1) for voltage value ACV of 100 V. The broken line shows a graph line of a function (a function with slope of 1.1) for voltage value ACV of 110 V. The alternate long and short dash line shows a graph line of a function (a function with slope of 0.9) for voltage value ACV of 90 V.  FIG.  10    shows three functions for three different voltage values ACV, but four or more functions corresponding to different voltage values ACV may be prepared. 
     The correction unit  809  corrects the power estimation result ESTPA based on the voltage value ACV, as illustrated below. The correction unit  809  selects a function associated with the voltage value ACV from the plurality of functions based on the voltage value ACV. The correction unit  809  converts the IH power before correction corresponding to the power estimation result ESTPA into the IH power after correction based on the selected function. The correction unit  809  acquires the IH power after correction obtained by converting the IH power before correction corresponding to the power estimation result ESTPA, as the power estimation result ESTPB. 
     For example, it is assumed that the IH power before correction corresponding to the power estimation result ESTPA is 1000 W. For the voltage value ACV of 90 V, the correction unit  809  converts 1000 W into 900 W based on the function associated with the voltage value ACV. The correction unit  809  acquires 900 W as the power estimation result ESTPB. The power estimation result ESTPB is decreased to be lower than the power estimation result ESTPA. For voltage value ACV of 110 V, the correction unit  809  converts 1000 W into 1100 W based on the function associated with the voltage value ACV. The correction unit  809  acquires 1100 W as the power estimation result ESTPB. The power estimation result ESTPB is increased to be higher than the power estimation result ESTPA. 
     The correction unit  809  can estimate the IH power according to the variation of the voltage value ACV by correcting the power estimation result ESTPA based on the voltage value ACV. As a result, the correction unit  809  can prevent the power estimation result ESTPB from deviating from the IH power used for the actual heat generation operation even if the voltage value ACV varies. Since the estimation accuracy of the IH power by the correction unit  809  is improved, it is possible to prevent the temperature estimation result EST from the temperature estimation unit  801  from deviating significantly from the actual surface temperature of the fixing belt  77 . 
     The coefficient KB to be multiplied by the IH power before correction is not limited to a fixed value corresponding to the voltage value ACV representing a linear relationship as illustrated in  FIG.  10   . The coefficient KB may be expressed by any function for each voltage value ACV. 
     The correction unit  809  may correct the power estimation result ESTPA by reference to table data instead of a function. The table data may be data in which the IH power before correction obtained by actual measurement and the IH power after correction for each voltage value ACV are associated with each other. The table data may be stored in the memory  25 . 
     An example of a drive pulse signal will be described. 
       FIG.  11    is a diagram illustrating a drive pulse signal.  FIG.  11    illustrates the drive pulse signal PU in the upper section of the figure and the drive pulse signal PD in the lower section of the figure. 
     The horizontal axis represents the time. The vertical axis represents the voltage. 
     If the frequency FRQ is 50 kHz, one cycle of the drive pulse signal PU and the drive pulse signal PD is 20 μs. The drive pulse signal PU and the drive pulse signal PD are pulse signals having a duty of 48% obtained by subtracting the dead time from the duty of 50% of the original signal. The drive pulse signal PU and the drive pulse signal PD alternately output 
     High. 
     In an example, the conversion unit  808  and the correction unit  809  are illustrated as separate, but embodiments are not limited thereto. The temperature control circuit  14  may include a power estimation unit that estimates the IH power based on the frequency FRQ and the voltage value ACV, instead of the conversion unit  808  and the correction unit  809 . Estimating the IH power based on the frequency FRQ and the voltage value ACV includes converting the frequency FRQ into a power estimation result ESTPB according to the voltage value ACV. 
     In an example, as illustrated in  FIG.  9   , a plurality of functions corresponding to the relationship between the frequency of the drive pulse signal of the inverter  82  and the IH power are prepared in advance for different voltage values ACV.  FIG.  9    shows three functions according to three different voltage values ACV, but additional functions corresponding to other possible voltage values ACV may be prepared. 
     The power estimation unit can estimate the IH power based on the frequency FRQ and the voltage value ACV. To do so, the power estimation unit selects a function associated with a particular voltage value ACV from the plurality of prepared functions. The power estimation unit then converts the frequency FRQ into IH power based on the selected function. The power estimation unit acquires (calculates) the IH power obtained based on the frequency FRQ by using the selected function. The calculated IH power is taken as the power estimation result ESTPB. 
     For example, for a voltage value ACV of 90 V, the power estimation unit selects the F2P90 function (see  FIG.  9   ). The power estimation unit then acquires the power estimation result ESTPB based on the frequency FRQ and the F2P90 function. The power estimation result ESTPB acquired based on the F2P90 function will be lower than the power estimation result ESTPB acquired a based on the F2P100 function (see  FIG.  9   ) for the same frequency FRQ. For a voltage value ACV of 110 V, the power estimation unit would select the F2P110 function (see  FIG.  9   ). The power estimation unit would thus calculate (acquire) the power estimation result ESTPB according to the frequency FRQ and the F2P110 function. The power estimation result ESTPB based on the F2P110 function will be higher than the power estimation result ESTPB based on the F2P100 function at the same frequency FRQ. 
     In some examples, the power estimation unit may estimate the IH power based on the frequency FRQ and the voltage value ACV by reference to table data instead of by calculation of a value from a function. The table data may include data entries for each voltage value ACV in which a frequency of the drive pulse signal of the inverter  82  and an IH power are associated with each other. The table data may be stored in the memory  25 . 
     In one example, the temperature estimation unit  801  estimates the surface temperature of the fixing belt  77  based on the estimation history PREV and the power estimation result ESTPB, but embodiments are not limited thereto. In other examples, the temperature estimation unit  801  may estimate the surface temperature of the fixing belt  77  based on the estimation history PREV and the power estimation result ESTPA. 
     In one example, the system controller  13  and the temperature control circuit  14  are illustrated as separate components, but embodiments are not limited thereto. The system controller  13  may include some or all of the functions of the temperature control circuit  14 . In such an example, the processor  22  may implement a part or all of the described functions of the temperature control circuit  14  as implemented by the processor  24 . The memory  23  may store programs stored in the memory  25 , data used in the programs, and the like. 
     A process based on detection of an abnormality of the temperature sensor  79  will be described. 
       FIG.  12    is a flowchart for explaining an example of a process related to detection of an abnormality of the temperature sensor  79 . 
     The system controller  13  executes an initial setting (e.g., startup process) of the image forming apparatus  1  after on a power-on (turning on) of the image forming apparatus  1  (ACT  21 ). The system controller  13  printing operations can be started (ACT  22 ) after completion of initial setting process. 
     At ACT  23 , the determination unit  813  acquires the temperature estimation result EST from the temperature estimation unit  801  and the temperature detection result Td from the temperature sensor  79 . The determination unit  813  also acquires a correction value from the memory  25  or the memory  23 . The determination unit  813  may acquire the correction value according to the present state of the printing process. The determination unit  813  detects the temperature difference based on the temperature estimation result EST, the temperature detection result Td, and the correction value. The determination unit  813  may continuously detect the temperature difference during the operations of the image forming apparatus  1  regardless of the present state of the image forming apparatus  1 . 
     The determination unit  813  compares the temperature difference to the reference value (ACT  24 ). At ACT  24 , if the temperature difference matches (satisfies) the reference value or exceeds the reference value, the determination unit  813  outputs an abnormality detection signal to the energization control unit  131  and the transmission unit  132 . If the temperature difference does not satisfy (meet or exceed) the reference value (ACT  24 , NO), the system controller  13  continues the printing operation (ACT  25 ). 
     If a power off signal for the image forming apparatus  1  is input (ACT  26 , YES), the process ends. If a power off signal of the image forming apparatus  1  is not input (ACT  26 , NO), the process returns from ACT  26  to ACT  23 . 
     If the temperature difference satisfies the reference (ACT  24 , YES), the energization control unit  131  stops energizing the induction heating coil  76  upon receiving the abnormality detection signal from the determination unit  813  (ACT  27 ). At ACT  27 , the energization control unit  131  stops energizing the induction heating coil  76 , but continues to energize the elements related to communication functions or the like even after the abnormality detection signal from the determination unit  813 . 
     The transmission unit  132  transmits an abnormality notification to the maintenance server  2  based on the abnormality detection signal from the determination unit  813  (ACT  28 ). The management company may then transmit the abnormality notification received at the maintenance server  2  to a company (maintenance company) responsible for performing maintenance on the image forming apparatus  1 . The maintenance company sets the maintenance schedule for the image forming apparatus  1  by reference to the information included in the abnormality notification. The abnormality notification may include information indicating the type of the abnormality (e.g., an error code, an error type notice, or the like), a preferred manner of dealing with such an abnormality, and the like. The maintenance company generates a return message (response message) based on the maintenance schedule of the image forming apparatus  1 . The maintenance company transmits the return message to the management company. The maintenance server  2  transmits the return message (or information corresponding thereto) to the image forming apparatus  1  via the network NW. The reception unit  133  receives the return message from the maintenance server  2  in response to the previously transmitted abnormality notification (ACT  29 ). 
     The display control unit  134  controls a display of a message on the display unit  15  based on the return message received (ACT  30 ). At ACT  30 , for example, the display control unit  134  causes the display of a message on the display unit  15  is maintained until the replacement of the temperature sensor  79  is completed. 
     The changes in the temperature detection result Td and the temperature estimation result EST due to the occurrence of the abnormality of the temperature sensor  79  will be described.  FIG.  13    is a diagram illustrating the temperature detection result Td and the temperature estimation result EST according to the first embodiment. 
     The horizontal axis of  FIG.  13    represents the time. The vertical axis of  FIG.  13    represents the temperature. 
     During normal operation of the image forming apparatus  1 , the temperature estimation result EST and the temperature detection result Td are kept to be at a near constant correlation with one another. However, if an abnormality occurs in the temperature sensor  79 , the temperature estimation result EST will generally begin to increase, but the temperature detection result Td begins to decrease sharply. Although the actual surface temperature of the fixing belt  77  is not actually being measured at this time (due to failure of the temperature sensor  79 ), the actual temperature can be expected to rise in a manner basically maintaining the same correlation to the temperature estimation result EST, as before the sensor failure. The presumed actual temperature of the fixing belt  77  (as opposed to the measured temperature) is illustrated as dashed line continuing upward in  FIG.  13    from the point of abnormality of the temperature sensor  79 . 
     An example reflecting the incorporation of a correction value in the temperature difference will be described.  FIG.  14    is a diagram for explaining an example reflecting use of a correction value in the temperature difference. 
     The horizontal axis of  FIG.  14    represents the time. The vertical axis of  FIG.  14    represents the temperature. 
     The drawing in the upper-left side of  FIG.  14    illustrates an example in which the temperature estimation result EST is lower than the temperature detection result Td in the normal operation of the image forming apparatus  1 . The drawing in the lower-left side of  FIG.  14    illustrates an example in which the temperature estimation result EST is higher than the temperature detection result Td in the normal operation of the image forming apparatus  1 . 
     The drawing on the right side of  FIG.  14    illustrates a state in which the temperature estimation result EST or the temperature detection result Td has been corrected by the correction value. 
     In the normal operation of the image forming apparatus  1 , the temperature difference is adjusted to be almost zero (0) by use of the correction by the correction value. However, if an abnormality occurs in the temperature sensor  79 , the temperature difference rapidly increases even when correction is attempted. 
     An example of displaying a message based on the return message (response message) will be described. 
       FIG.  15    is a diagram illustrating an example of a display of a message corresponding to a return message received via the maintenance server  2 . For example, the display unit  15  displays the scheduled time for the serviceman to visit as provided in the return message. 
     A temperature control device according to the first embodiment includes a temperature estimation unit that estimates a temperature of an object being controlled based on the energization levels of elements related to the temperature control. The temperature control device includes a comparison unit that compares a temperature difference to a reference value. The temperature difference in this context is the difference in the temperature estimation result from the temperature estimation unit and the temperature detection result from the temperature sensor. The temperature control device includes an energization control unit that stops energizing the elements related to temperature control (e.g., heater elements or the like), if the temperature difference meets or exceeds the reference value. 
     According to such a configuration, the temperature control device can stop energizing the elements related to temperature control before the object exceeds a normal operating temperature range. Therefore, the temperature control device can prevent the occurrence of an abnormal, damaging temperature in the object. By this control, the temperature control device prevents not only the object but also the surrounding parts from being subjected to possibly damaging thermal stresses. This allows the temperature control device to ensure the useful life of the various components. 
     The temperature control device includes a transmission unit that transmits an abnormality notification to an external device if the temperature difference meets exceeds a threshold (reference) value. 
     According to such a configuration, the temperature control device can transmit an abnormality notification to the external device without delay after the occurrence of the abnormality in the temperature sensor. 
     The abnormality notification that can be sent may include information indicating an abnormality of the temperature sensor has occurred. The abnormality notification may include information indicating a preferred manner of dealing with the detected abnormality in the temperature sensor. 
     According to such a configuration, since the temperature control device can notify a maintenance company of a specific error type, the time until the temperature control device is restored to service can be shortened. 
     If the temperature difference meets or exceeds the reference value, the energization control unit may still keep energizing the elements related to communication functions to permit transmission of the abnormality notification to the maintenance server  2  and receiving of the return message from the maintenance server  2 . 
     According to such a configuration, the temperature control device can wait for response from the external device after the abnormality notification is sent. 
     In this context, the relevant temperature difference is the difference between a temperature estimation result and a temperature detection result, as corrected as compared to difference between the temperature estimation result and the temperature detection result in the normal operation of the temperature control device. According to such a configuration, since the temperature control device can use the temperature difference obtained by correcting the individual difference for each image forming apparatus based on the correction value, it is possible to standardize the process of comparing the temperature difference with the reference. 
     Second Embodiment 
     In description of the second embodiment, aspects different from those of the first embodiment will be mainly described. The components or operations of the second embodiment that may be the same as those of the first embodiment are denoted by the same reference numerals, and the description thereof will generally be omitted. 
       FIG.  16    depicts an image forming apparatus  1  according to the second embodiment. 
     The fuser  21  according to the second embodiment is a different type of fuser from the first embodiment. 
     The fuser  21  in the second embodiment includes a pressure roller  70 , a temperature sensor  79 , a heat roller  91 , and a heater  92 . 
     The pressure roller  70  is different from the first embodiment in that it is positioned so as to face the heat roller  91 , but may otherwise be the same as the first embodiment in other respects. 
     The temperature sensor  79  is different from the first embodiment in that it detects the surface temperature of the heat roller  91 , but may otherwise be the same as the first embodiment in other respects. The surface of the heat roller  91  is an example of a temperature controlled object. The surface temperature of the heat roller  91  is an example of a temperature of the heat roller  91 . The temperature of the heat roller  91  is an example of a temperature controlled object. 
     The heat roller  91  is a fixing rotating body rotated by a motor. The heat roller  91  includes a core metal formed of hollow metal and an elastic layer formed on the outer periphery of the core metal. In the heat roller  91 , the inside of the core metal is heated by the heater  92  arranged inside the core metal formed in hollow shape. The heat generated inside the core metal is transferred to the surface of the heat roller  91  (that is, the surface of the elastic layer). 
     The heater  92  is a device that generates heat using energizing power PC supplied from the temperature control circuit  14 . For example, the heater  92  is a halogen lamp heater. When the energizing power PC is supplied to the halogen lamp heater, the light from the halogen lamp heater heats the inner side of the core metal of the heat roller  91 . The heater  92  is an example of an element related to temperature control on the surface of the heat roller  91 . 
     Next, the temperature control circuit  14  in the second embodiment will be described. 
     The temperature control circuit  14  controls the energization of the heater  92  of the fuser  21 . The temperature control circuit  14  generates and supplies an energizing power PC to the heater  92  of the fuser  21 . 
       FIG.  17    is a diagram for explaining an example of the configuration of the temperature control circuit  14  according to the second embodiment. 
     The temperature control circuit  14  includes the temperature estimation unit  801 , the estimation history storage unit  802 , the high frequency component extraction unit  803 , the coefficient addition unit  804 , the target temperature output unit  805 , the difference comparison unit  806 , the determination unit  813 , a control signal generation unit  814 , and a power supply circuit  815 . The temperature control circuit  14  acquires the temperature detection result Td from the temperature sensor  79 . 
     The temperature estimation unit  801  performs a temperature estimation process for estimating the surface temperature of the heat roller  91 . The estimation history PREV from the estimation history storage unit  802  and the energization pulse Ps from the control signal generation unit  814  are input to the temperature estimation unit  801 . The temperature estimation unit  801  estimates the surface temperature of the heat roller  91  based on the estimation history PREV and the energization pulse Ps, and generates the temperature estimation result EST. Further, the temperature estimation unit  801  may be configured to generate the temperature estimation result EST based on the estimation history PREV, the energization pulse Ps, and the voltage (rated voltage) supplied to the heater  92  when the energization pulse Ps is on. The energization pulse Ps is related to energization of the heater  92 . Therefore, estimating the surface temperature of the heat roller  91  based on the estimation history PREV and the energization pulse Ps is an example of estimating the surface temperature of the heat roller  91  based on the energization of the heater  92 . The temperature estimation unit  801  outputs the temperature estimation result EST to the estimation history storage unit  802  and the high frequency component extraction unit  803 . 
     The estimation history storage unit  802 , the high frequency component extraction unit  803 , the coefficient addition unit  804 , the target temperature output unit  805 , the difference comparison unit  806 , and the determination unit  813  may be the same as in the first embodiment. 
     The control signal generation unit  814  generates the energization pulse Ps as a pulse signal for controlling energization of the heater  92  based on the difference DIF. The control signal generation unit  814  outputs the energization pulse Ps to the power supply circuit  815  and the temperature estimation unit  801 . 
     The power supply circuit  815  supplies the energizing power PC to the heater  92  based on the energization pulse Ps. The power supply circuit  815  energizes the heater  92  of the fuser  21  by using the DC voltage supplied from the power conversion circuit  11 . The power supply circuit  815  supplies the energizing power PC to the heater  92  by switching between a state in which the DC voltage from the power conversion circuit  11  is supplied to the heater  92  and a state in which the DC voltage from the power conversion circuit  11  is not supplied based on the energization pulse Ps, for example. That is, the power supply circuit  815  changes the time of energizing the heater  92  of the fuser  21  according to the energization pulse 
     Ps. 
     Since the processes of the image forming apparatus  1  in the second embodiment may be the same as those in the first embodiment, additional description thereof will be omitted. Since the effects obtained by the image forming apparatus  1  in the second embodiment may be the same as those in the first embodiment, additional description thereof will be omitted. 
     Other Embodiments 
     A program incorporating instructions for implementing the various described functions above may be transferred already stored in a device according to an embodiment, or may be transferred to such a device subsequently. In the latter case, the program may be transferred via a network or may be transferred as stored on a recording medium. The recording medium is a non-transitory tangible medium. The recording medium is a computer-readable medium. The recording medium may be any medium such as a CD-ROM, a memory card, or the like, which can store a program and can be read by a computer, without limited to any form. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.