Image heating apparatus and image forming apparatus

Abstract

It detects abnormal temperature readings in rotating parts. [Solution] The image heating device comprises a rotating body 1, a magnetic core material 2, an excitation coil 3, an inverter 16, a control means 300 for controlling the inverter 16, at least one temperature detection means 9 for detecting the temperature of the rotating body, and a storage means 23 for storing a reference value for the amount of change per unit time of the detected temperature detected by the temperature detection means 9. The control means acquires the amount of change per unit time of the detected temperature of the temperature detection means 9 and corrects the reference value based on the driving frequency of the inverter 16 when this amount of change is acquired. If the amount of change is smaller than the corrected reference value, the heating of the rotating body 1 is stopped.

Description

[Technical Field]

[0001] The present invention relates to an image heating apparatus and an image forming apparatus for heating an image formed on a recording material. [Background technology]

[0002] Conventionally, a fixing device using an electromagnetic induction heating method has been proposed, which directly heats a heating layer (conductive layer) provided on a heating rotating body by electromagnetic induction. In this method, an excitation coil and a magnetic core are placed inside a cylindrical heating rotating body, an alternating magnetic field is generated in the direction of the rotation axis of the heating rotating body, and the heating layer is heated by the current (circumferential current) generated in the direction surrounding the heating rotating body (see Patent Document 1).

[0003] Furthermore, it is known that the above electromagnetic induction heating method can control the heat generation distribution in the longitudinal direction of the heating rotating body by changing the driving frequency of the high-frequency current flowing through the excitation coil (see Patent Document 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-52308 [Patent Document 2] Japanese Patent Publication No. 2020-52233 [Overview of the project] [Problems that the invention aims to solve]

[0005] In electromagnetic induction heating systems, the heat distribution of the heating element changes with the drive frequency, and therefore the temperature detected by the temperature sensing element that monitors the temperature of the heating element also changes with the drive frequency. On the other hand, the temperature detected by the temperature sensing element may also change due to unexpected factors such as a malfunction of the temperature sensing element itself or, in the case of a contact-type temperature sensing element, poor contact with the target area. Therefore, it is difficult to determine whether a change in the temperature detected by the temperature sensing element is a normal result due to a change in drive frequency or an abnormal result due to a malfunction of the element.

[0006] Furthermore, if the temperature detected by the temperature sensing element is abnormal, there is a high possibility of malfunctions or failures in temperature control. Therefore, it was necessary to determine whether the detected temperature was normal or abnormal and then perform appropriate control.

[0007] Therefore, the present invention aims to provide an image heating device and an image forming device capable of detecting abnormal temperature detection of a rotating body. [Means for solving the problem]

[0008] One aspect of the present invention is an image heating device for heating an image formed on a recording material, comprising: a cylindrical rotating body having a conductive layer; a magnetic core material installed inside the rotating body and forming an open magnetic path in the axial direction of the rotating body; an excitation coil wound around the magnetic core material along the axial direction of the rotating body; an inverter for supplying an alternating current to the excitation coil; a control means for controlling the inverter to supply an alternating current to the excitation coil, thereby generating an alternating magnetic flux in the magnetic core material and electromagnetically inducing heating of the rotating body; at least one temperature detection means for detecting the temperature of the rotating body; and a storage means for storing a reference value of the amount of change per unit time of the detected temperature detected by the temperature detection means, wherein the control means can change the drive frequency of the inverter, and the control means acquires the amount of change per unit time of the detected temperature of the temperature detection means, corrects the reference value based on the drive frequency of the inverter when the amount of change was acquired, and stops heating of the rotating body if the amount of change is smaller than the corrected reference value. [Effects of the Invention]

[0009] According to the present invention, it is possible to detect abnormal temperature detection in a rotating body. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic cross-sectional view of an image forming apparatus according to the first embodiment. [Figure 2] This is a cross-sectional side view of a fixing device according to the first embodiment. [Figure 3] This is a front model view of a fixing device according to the first embodiment. [Figure 4] This is a perspective projection view and a connection circuit block diagram of a fixing device according to the first embodiment. [Figure 5] This figure shows the relationship between the excitation coil current and the magnetic field according to the first embodiment. [Figure 6] This is a physical model diagram relating to the induced current flowing through the fixing film according to the first embodiment. [Figure 7] This diagram shows the equivalent circuit of the excitation coil and the heat-generating layer of the fixing film. [Figure 8] This diagram shows the equivalent circuit of the excitation coil and the heat-generating layer of the fixing film. [Figure 9] This is an illustrative diagram illustrating the phenomenon where apparent magnetic permeability is lower at the edges than in the center. [Figure 10] This figure shows the magnetic field lines when a ferrite is placed in a uniform magnetic field H. [Figure 11] This figure shows the heat generation distribution in the longitudinal direction of the fixing film according to the first embodiment. [Figure 12] This figure shows the heat generation distribution in the longitudinal direction of the fixing film according to the first embodiment. [Figure 13] This is a schematic diagram of the temperature transition of the fixing film according to the first embodiment. [Figure 14] This figure shows the heat generation distribution in the longitudinal direction of the fixing film within section C. [Figure 15]It is a schematic diagram of the detected temperature transition of the temperature detection element within section C according to the first embodiment. [Figure 16] It is a flowchart diagram for explaining the drive sequence of the fixing device according to the first embodiment. [Figure 17] It is a schematic diagram of the detected temperature transition of the temperature detection element according to the second embodiment. [Figure 18] It is a schematic diagram of the detected temperature transition of the temperature detection element within section C according to the third embodiment.

Modes for Carrying Out the Invention

[0011] <First Embodiment> (Configuration of the Image Forming Apparatus) Hereinafter, a fixing device as an image heating device according to an embodiment of the present invention and an image forming apparatus 100 including the same will be described based on the drawings. As shown in FIG. 1, the image forming apparatus 100 is a laser beam printer using an electrophotographic method, and includes a paper feed cassette 105, a paper feed roller 106, a registration roller 107, an image forming unit 120, a fixing device 200, and a control unit 41. The paper feed cassette 105 is a recording material support means for supporting the recording material P, and is configured to stack and store the recording material P. The paper feed roller 106 is a paper feed means for feeding the recording material P stored in the paper feed cassette 105, and is configured to separate and feed the recording materials P stacked and stored in the paper feed cassette 105 one by one. The registration roller 107 is a recording material conveyance means for conveying the recording material fed from the paper feed cassette 105 toward the image forming unit 25, and is configured to convey the recording material P in accordance with the timing of image formation by the image forming unit 25.

[0012] The image forming means 25 is configured to form an image on the recording material P and includes a photosensitive drum 101, a charging roller 102, an exposure device 103, a developing device 104, a transfer roller 108, and a cleaning device 110. These components are arranged around the photosensitive drum 101, and the charging roller 102 uniformly charges the photosensitive drum 101, which is rotated at a predetermined speed in the direction of the arrow in the figure, to a specific polarity and potential. The exposure device 103 is a laser beam scanner that outputs ON-OFF modulated laser light corresponding to the time-series electrical digital pixel signal of the target image information input from an external device such as a host computer, and scans and exposes (irradiates) the charged surface of the photosensitive drum 101. The developing device 104 includes a developing roller 104a that supplies a developer (toner) to the surface of the exposure device 103, and is configured to develop the electrostatic latent image formed on the surface of the photosensitive drum 101 by the exposure device 103 with the developer. The transfer roller 108 forms an image transfer nip 108T with the photosensitive drum 101 in the transfer section, and is configured so that when a transfer voltage is applied to the transfer roller 108, the toner image formed on the photosensitive drum 101 is transferred onto the recording material P. The cleaning device 110 is provided downstream of the transfer nip 108T in the rotational direction of the photosensitive drum 101 and is configured to remove residual toner, paper dust, etc. from the photosensitive drum 101.

[0013] The fixing device 200 is an electromagnetic induction heating type image heating device and comprises a fixing film 121 as a heating rotating body and a pressure roller 8 that forms a fixing nip N together with the fixing film 121. The fixing device 200 forms a fixing nip N with the fixing film 121 and the pressure roller 8, and is configured to heat and pressurize the unfixed toner image transferred onto the recording material P with this fixing nip N to fix it to the recording material P.

[0014] The control unit 41 is a controller that controls each unit of the image forming apparatus 100 described above, and includes ROM and RAM as storage units, a CPU (Central Processing Unit) as a calculation means, and various input / output control circuits (not shown), etc.

[0015] Because the image forming apparatus 100 is configured in this way, when a feed start signal is output from the control unit 41 to the feed roller 106, the feed roller 106 is driven and the recording material P in the feed cassette 105 is separated and fed one sheet at a time. When the recording material P is fed from the feed cassette 105, the recording material P is transported to the transfer nip 108T by the registration roller 107 at the same time that the toner image on the photosensitive drum 101 is transported to the transfer nip 108T. Then, a transfer voltage (transfer bias) with the opposite polarity to the toner is applied to the transfer roller 108, and the toner image is transferred onto the recording material P at the transfer nip 108T.

[0016] Once the toner image is transferred onto the recording material, the recording material P carrying the unfixed toner image is transported to the fixing device 200 by the pre-fixing transport guide 109. In the fixing device 200, the toner image is pressurized and heated, fixing it to the recording material P. The recording material with the fixed toner image is then discharged from the discharge port 111 onto the discharge tray 112, which serves as the discharge section.

[0017] (Configuration of the fixing device) Next, the configuration of the fixing device 200 will be explained based on Figures 2 and 3. Figure 2 is a cross-sectional side view of the main part of the fixing device 200 of this embodiment, and Figure 3 is a front view of the fixing device 200. In Figure 3, the fixing film 1 is shown with its internal structure unfolded in a part of its longitudinal direction to explain its layer configuration. In addition, in this embodiment, the temperature detection element 9, which is a temperature detection means for detecting the temperature of the fixing film 1, is installed inside the rotating body 1, but in Figure 3, the temperature detection element 9 is shown outside the rotating body 1 for better visibility. Furthermore, regarding the components that make up the fixing device 200, the longitudinal direction is the direction perpendicular to the transport direction of the recording material, and this longitudinal direction corresponds to the width direction of the recording material.

[0018] In this embodiment, the fixing device 200 is an electromagnetic induction heating type image heating device. As shown in Figure 2, the fixing device 200 comprises a fixing film 1 which is a heat-generating rotating body and a pressure roller 8 which is a pressure rotating body, and a fixing nip N is formed between the fixing film 1 and the pressure roller 8 to pressurize and heat the recording material P. In addition, a film guide member 6, a pressure rigid stay 5, and a core unit 4 are provided on the inside of the fixing film 1. The film guide member 6 is made of heat-resistant PPS (polyphenylene sulfide) resin or the like and is pushed downward via the pressure rigid stay 5. Furthermore, a sliding member is provided on the lower surface of the film guide member 6, and this sliding member determines the shape of the fixing nip N.

[0019] The core unit 4 has an excitation coil 3 wound around a magnetic core 2, which serves as a magnetic core material (see Figure 4), and is inserted into the fixing film 1 between the film guide member 6 and the pressurizing rigid stay 5. The magnetic core 2 and the excitation coil 3 wound along the outer circumference of the magnetic core 2 are magnetic field generating means that generate an alternating magnetic flux (alternating magnetic field) in response to the AC power supplied from the high-frequency inverter 16, thereby inducing a current in the heating layer 1a of the rotating heating element. In the figure, the excitation coil 3 is shown as a single wire, but it is not limited to this, and may be made by bundling multiple wires together.

[0020] The pressure roller 8 is positioned opposite the film guide member 6 with the fixing film 1 in between, and by pressing against the film guide member 6, it forms a fixing nip N of a predetermined width between itself and the fixing film 1. The pressure roller 8 is driven to rotate counterclockwise by a driving means (not shown), and the fixing film 1 rotates in a driven manner due to the frictional force between itself and the pressure roller 8.

[0021] Figure 3 is a schematic front view of the fixing device 200. As shown in Figure 3, pressure springs 17a and 17b are positioned between both ends of the pressurizing rigid stay 5 and spring receiving members 18a and 18b configured on the chassis of the fixing device 200, and the film guide member 6 is biased downward by the biasing force of the pressure springs 17a and 17b. In addition, fixing film flanges (not shown) are provided at both ends of the fixing film 1, and these fixing film flanges, together with the film guide member 6, restrict the rotational trajectory of the fixing film 1. In this embodiment, the film guide member 6 is pressed down with a total pressure of approximately 100N to 250N (approximately 10kgf to approximately 25kgf).

[0022] Furthermore, a temperature detection element 9 is provided in the longitudinal center of the fixing film 1, and this temperature detection element 9 serves as a temperature detection means for detecting the surface temperature of the fixing film 1. The signal from the temperature detection element 9 is input to the control means 300 shown in Figure 4, and the control means 300 is configured to control the high-frequency inverter 16 based on the temperature detected by the temperature detection element 9. In this embodiment, the temperature of the fixing film 1 may be detected by any method, whether contact-type or non-contact-type.

[0023] The high-frequency inverter 16 supplies a frequency and amplitude switching current to the excitation coil 3 via power supply contacts 3a and 3b (not shown) based on the control signal from the control means 300. As a result, the heating layer 1a (see Figure 2) of the fixing film 1 is heated by electromagnetic induction, and the surface temperature is controlled to a predetermined target temperature.

[0024] In this embodiment, the fixing film 1 is a cylindrical rotating body with a composite structure comprising a heating layer 1a made of a conductive base material with a diameter of 10 to 50 mm, an elastic layer 1b laminated on its outer surface, and a release layer 1c laminated on its outer surface. The heating layer 1a, which acts as a conductive layer through which alternating current flows, is formed as a ring-shaped heating pattern divided in the longitudinal direction of the rotating body 1, as shown in Figure 3. This heating pattern is made of metal and has a thickness of 10 to 50 μm. The elastic layer 1b is a silicone rubber with a hardness of 20 degrees (JIS-A, 1 kg load) and a thickness of 0.3 mm to 0.1 mm. Furthermore, the surface layer 1c (release layer) is a fluororesin tube with a thickness of 50 μm to 10 μm. When an alternating magnetic flux acts on the heating layer 1a, an induced current is generated and heat is produced. This heat is transferred to the elastic layer 1b and the release layer 1c, heating the entire fixing film 1. When the recording material P passes through the fixing nip N, the toner image T on the recording material P is heated and fixed.

[0025] Furthermore, the pressure roller 8 comprises a core metal 8a, a heat-resistant elastic material layer 8 molded and coated concentrically around the core metal 8a in a roller shape, and a release layer 8c on the roller surface. It is preferable that it be formed from a heat-resistant material such as silicone rubber, fluororubber, fluorosilicone rubber, or fluororesin.

[0026] (Heating principle) Next, the heating principle of the fixing film 1 will be explained in detail. Figure 5 is a conceptual diagram of the magnetic field around the magnetic core 2 and the induced current induced in the heating layer 1a. The magnetic core 2 forms a path of magnetic field lines (magnetic path) in the axial direction (longitudinal direction) O of the fixing film 1. At the moment when the current in the excitation coil 3 increases in the direction of arrow I1, the magnetic core 2 induces magnetic field lines shown by the dotted line B in the figure. This change in the magnetic field causes an induced current I2 to flow through the heating layer 1a of the rotating body 1, resulting in Joule heating.

[0027] The heating layer 1a-1 is shown for illustrative purposes as one of many arranged heating patterns. The heating principle of heating layer 1a-1 follows Faraday's law. The induced electromotive force V that attempts to drive current through the circuit of heating layer 1a-1 is proportional to the time change of the magnetic flux perpendicular to the circuit. The induced electromotive force V can be expressed by the following equation (1). That is, the induced electromotive force V is proportional to the product of the change in magnetic flux perpendicular to heating layer 1a-1 over a small time interval Δt ΔΦ / Δt and the number of turns N.

[0028]

number

[0029] Due to this induced electromotive force V, if the heating layer 1a-1 is connected in the circumferential direction, a current flows and Joule heating occurs. On the other hand, if the heating layer 1a-1 is not connected in the circumferential direction, no current flows and no Joule heating occurs.

[0030] (Frequency control of heat distribution in the longitudinal direction of the fixing film) The electromagnetic induction heating type fixing device 200 can control the heat generation distribution in the longitudinal direction of the fixing film 1 by changing the drive frequency of the high-frequency inverter 16 using the control means 300. The principle by which the heat generation distribution of the fixing film 1 changes by changing the drive frequency will be explained below. Specifically, this principle involves the frequency dependence of the load resistance of the fixing film 1 and the magnetic flux density in the open magnetic circuit, so these matters will be explained in order.

[0031] (Frequency dependence of the load resistance of rotating body 1) Figure 6 shows a physical model relating to the induced current I2. This can be said to be equivalent to the magnetic coupling of a concentric transformer with a primary winding coil (excitation coil) 3 shown by a solid line and a secondary winding coil 31 shown by a dashed line. The secondary winding coil 31 also forms a circuit and has a resistor 32. The alternating voltage generated from the high-frequency inverter 16 generates a high-frequency current in the primary winding coil 3, which in turn generates an induced electromotive force in the secondary winding coil 31, and this is dissipated as heat by the resistor 32. Here, the secondary winding coil 31 and the resistor 32 model the Joule heat generated in the heat-generating layer 1a.

[0032] Next, the equivalent circuit of the model diagram shown in Figure 6 is shown in Figure 7(a). Here, L1 is the inductance of the primary winding coil 3 (see Figure 6), L2 is the inductance of the secondary winding coil 31 (see Figure 6), M is the mutual inductance of the primary winding coil 3 and the secondary winding coil 31, and R is the resistor 32 (see Figure 6). The circuit diagram shown in Figure 7(a) can be equivalently transformed into Figure 7(b). Furthermore, in order to consider a simpler model, if the mutual inductance is sufficiently large and L1 ≈ L2 ≈ M, then (L1-M) and (L2-M) become sufficiently small, and the circuit diagram can be approximated from Figure 7(b) to Figure 7(c). For this reason, the configuration of the fixing film 1, magnetic core 2, and excitation coil 3 in this embodiment can be considered by replacing it with the equivalent circuit model shown in Figure 7(c).

[0033] Next, let's explain resistance. In Figure 7(a), the impedance of the secondary side is the electrical resistance R in the circumferential direction of the heating layer 1a. Also, the impedance of the secondary side of the transformer, when viewed from the primary side, is N 2 (N is the turns ratio of the transformer) Equivalent resistance R' = N 2 R is given by R. Here, the turns ratio of the transformer can be considered as N=n, where n is the number of turns of the excitation coil 3 in this embodiment, and the heating layer 1a is considered to have 1 turn. Therefore, R'=N 2 R=n 2 This can be considered as R, and the more turns the excitation coil 3 has, the larger the equivalent resistance R' shown in Figure 7(c).

[0034] Next, Figure 8(a) shows a further simplified model of Figure 7(c). Figure 8(b) defines the combined impedance X and is a further simplified model of Figure 8(a). The combined impedance X can be calculated as shown in equation (2) below.

[0035]

number

[0036] According to equation (2), the equivalent impedance X is (1 / ωM) 2 It has a frequency dependence. This means that the inductance M, along with the resistance R', contributes to the combined impedance, and since the dimension of impedance is [Ω], this is equivalent to saying that the load resistance of the rotating body 1 has frequency dependence.

[0037] (Magnetic flux density in an open magnetic circuit) As shown in Figure 9, when the magnetic core 2 and excitation coil 3 are configured to form an open magnetic path, as in this embodiment, the effect of "reducing the apparent permeability μ at the ends of the magnetic core" is obtained. This effect will be explained below.

[0038] In a uniform magnetic field H, in a magnetic field region where the magnetization of an object is approximately proportional to the external magnetic field, the magnetic flux density B in space follows the following equation (3).

[0039]

number

[0040] In other words, by placing a material with high magnetic permeability μ in a magnetic field H, it is possible to create a magnetic flux density B that is ideally proportional to the height of the magnetic permeability. In this embodiment, this space with high magnetic flux density is utilized as a "magnetic path". In particular, when creating a magnetic path, there are closed magnetic paths, which are created by connecting the magnetic paths themselves in a loop, and open magnetic paths, which are created by discontinuing the magnetic path by leaving an open end, etc., but this embodiment is characterized by the use of open magnetic paths.

[0041] Figure 10 shows the shape of the magnetic flux when ferrite 41 and air 42 are placed in a uniform magnetic field H. The ferrite has open magnetic paths with respect to air, with interface surfaces NP⊥ and SP⊥ perpendicular to the magnetic field lines. When the magnetic field H is generated parallel to the longitudinal direction of the magnetic core, the density of magnetic field lines is low in the air, as shown in Figure 10, meaning the magnetic flux density B is low. Also, the density of magnetic field lines is high in the central part 41C of the magnetic core, meaning the magnetic flux density B is high. Furthermore, the magnetic flux density B is lower at the edges 41E compared to the central part 41C of the magnetic core.

[0042] The reason why the magnetic flux density B decreases at the edges lies in the boundary conditions between air and ferrite. Since the magnetic flux density is continuous at the interface surfaces NP⊥ and SP⊥ perpendicular to the magnetic field lines, the magnetic flux density is high in the air portion in contact with the ferrite near the interface. Consequently, the magnetic flux density decreases at the ferrite edge 41E in contact with the air. This phenomenon appears as if the permeability at the edges is low due to the decrease in magnetic flux density, and in this embodiment, it is expressed as "the apparent permeability decreases at the edges of the magnetic core."

[0043] Note that this phenomenon does not occur in the case of a closed magnetic circuit. This is because, unlike an open magnetic circuit, in a closed magnetic circuit, the magnetic field lines only pass through the open magnetic circuit and therefore do not have any "boundaries perpendicular to the magnetic field lines (boundaries perpendicular to the magnetic field lines NP⊥, SP⊥ shown in Figure 10)".

[0044] Generally, the relationship between the mutual inductance M and the permeability is M ∝ μ. As described above, the apparent permeability μ in the magnetic core 2 has a distribution as shown in FIG. 9 in the longitudinal direction. That is, the composite impedance |X|, that is, the load resistance of the fixing film 1 also has a distribution in the longitudinal direction. Here, FIG. 11 shows an example of the heat generation distribution in the longitudinal direction of the magnetic core 2. In this figure, for the sake of explanation, the magnetic core 2 is divided into three parts in the longitudinal direction, and it is assumed that the magnetic core has different permeabilities in the end region and the central region, respectively. Also, the permeability within each region is assumed to be uniform. If the apparent permeability and the mutual inductance in the end magnetic core 2e are μe and Me, and those in the central magnetic core 2c are μc and Mc, then from Equation (2), the composite impedances Xe and Xc in each region are as shown in the following Equations (4) and (5).

[0045] [Number]

[0046] [Number]

[0047] According to these Equations (4) and (5), the composite impedances Xe and Xc in each region have a frequency dependence of (1 / ωM e )<00000​​​​​​​​​

[0049] Furthermore, in Figure 11, for the sake of simplicity, it is assumed that the apparent permeability is equal within each divided region, resulting in a discrete heat generation distribution. However, in reality, the apparent permeability changes continuously from the center to the edges, and the heat generation distribution follows a similar pattern.

[0050] From equations (4) and (5) above, the load resistance of the fixing film 1 has different frequency characteristics from the center to the edges of the magnetic core 2. Therefore, by changing the driving frequency, the load resistance in each region can be changed, and the heat generation distribution can be controlled. Furthermore, in this embodiment, as shown in Figure 12, the excitation coil 3 on the magnetic core 2 is wound densely at the edges and loosely in the center. In this way, by changing the winding density of the excitation coil 3 in the longitudinal direction, the apparent permeability at each location can be changed, and the balance of the amount of heat generated by the rotating body 1 in the longitudinal direction of the magnetic core 2 can be adjusted. As a result, in this embodiment, the heat generation distribution can be changed by frequency in the manner shown in Figure 12.

[0051] Furthermore, in this embodiment, the control means 300 takes advantage of the feature that "heat distribution can be controlled by the drive frequency" and changes the drive frequency according to the size of the recording material P and the temperature of the non-paper-passing region of the rotating body 1. The non-paper-passing region is the region through which the largest size of recording material usable by the device passes, but recording material smaller than the maximum size does not. When fixing large-sized recording material, the entire longitudinal region of the rotating body 1 is heated uniformly, and when fixing small-sized recording material, the drive frequency is lowered to suppress the temperature of the edges of the fixing film 1. This makes it possible to reduce power consumption by suppressing the temperature rise in the non-paper-passing region when fixing small-sized recording material.

[0052] (Temperature increase process for fixing film) Next, the heating process of the fixing film 1 will be explained based on Figures 13 and 14. Figure 13 shows a schematic diagram of the temperature progression of the fixing film 1 until it is maintained at a constant temperature by temperature control. Here, section C is the section in which the high-frequency inverter 16 applies the maximum power it can normally output to heat the fixing film 1, and the application of constant power continues until the temperature detected by the temperature detection element 9, which is responsible for temperature control, reaches a predetermined value. Section D is the section in which the power from the high-frequency inverter 16 is appropriately adjusted and applied so that the temperature detected by the temperature detection element 9 always reaches a predetermined value.

[0053] Furthermore, Figure 14 shows the heat generation distribution in the longitudinal direction of the fixing film 1 at a certain time within section C in Figure 13. In section C, power adjustment has not yet been performed because the temperature detected by the temperature detection element 9 has not yet reached the target temperature, and the power input from the high-frequency inverter 16 is constant. Therefore, at a certain time within section C, the edges or the center of the fixing film 1 will have different temperatures depending on the drive frequency. Although the temperature of the fixing film 1 is mentioned here, the temperature detected by the temperature detection element 9 will also follow the temperature change of the fixing film 1 and will have a similar general shape.

[0054] (Control of high-frequency inverters) Next, the control configuration of the high-frequency inverter 16 will be explained based on Figure 4. Figure 4 is a block diagram of the control circuit of the high-frequency inverter 16 in this embodiment. In Figure 4, the temperature detection element 9 is shown outside the rotating body 1, as in Figure 3, for improved visibility.

[0055] As shown in Figure 4, in this embodiment, the control means 300 is composed of calculation means such as a CPU, and the storage unit (storage means) 23 stores the CPU's control program and the like. The control means 300 then operates based on the program stored in the storage unit 23 to function as a power control unit 17, a frequency control unit 18, a fixing temperature control unit 19, a detection result comparison unit 20, an engine control unit 21, and a reference value correction unit 22.

[0056] The power control unit 17 is configured to control the power supplied by the high-frequency inverter 16 to the excitation coil 3, and outputs a control signal, such as a PWM signal, to the high-frequency inverter 16. The frequency control unit 18 is configured to change the drive frequency of the high-frequency inverter 16. The fixing temperature control unit 19 is connected to the temperature detection element 9 and is configured to supply temperature information to the engine control unit 21 based on the detection result of the temperature detection element 9. The engine control unit 21 is configured to receive print job information as well as temperature information from the fixing temperature control unit 19. The engine control unit 21 also calculates the power to be supplied from the high-frequency inverter 16 to the excitation coil 3 based on the print job information and temperature information, and inputs it to the power control unit 17. Furthermore, the engine control unit 21 inputs the print job information and temperature information to the frequency control unit 18 so that the frequency control unit 18 can change (set) to an appropriate drive frequency.

[0057] In this embodiment, the surface temperature of the fixing film 1 is maintained and adjusted to a predetermined target temperature (approximately 150°C to 200°C) by supplying a high-frequency current from the high-frequency inverter 16 to the excitation coil 3 via the power control unit 17. In addition, information on an appropriate drive frequency is supplied to the high-frequency inverter 16 from the frequency control unit 18 according to the size of the recording material P and the temperature of the non-paper-feeding area of ​​the rotating body 1. As a result, the frequency of the high-frequency current supplied from the high-frequency inverter 16 to the excitation coil 3 changes, and the heat generation distribution in the longitudinal direction of the fixing film 1 changes.

[0058] Incidentally, as shown in Figure 14, in this embodiment, the temperature detection element 9, which is located in the longitudinal center of the fixing film 1, will experience a decrease in detection temperature if the drive frequency is high. On the other hand, it is also possible that the detection temperature of the temperature detection element 9 may decrease regardless of the drive frequency due to a malfunction of the temperature detection element itself or poor contact with the fixing film 1. In this case, it is difficult to determine whether the decrease in the detection temperature of the temperature detection element 9 is a normal result as specified in this embodiment, which is a change in drive frequency, or an abnormal result such as a malfunction or poor contact of the temperature detection element 9. Furthermore, if it is not possible to determine whether the detection temperature of the temperature detection element 9 is normal or abnormal, there is a risk of false detection or inability to perform appropriate temperature control. Note that in this embodiment, the temperature detection element 9 is located in the longitudinal center of the fixing film 1, but if it were located at the edge, the detection temperature would decrease as shown in Figure 14 if the drive frequency decreases, and similarly, it would become difficult to determine whether the detection result is normal or abnormal.

[0059] The detection result comparison unit 20 is configured to determine whether the fluctuation in the temperature detected by the temperature detection element 9, as described above, is due to a malfunction of the temperature detection element 9. Specifically, the temperature information detected by the temperature detection element 9 is also input to the detection result comparison unit 20, and the unit is configured to determine that there is an abnormality if the temperature detected by the temperature detection element 9 falls below a reference value stored in the storage unit 23. Furthermore, the detection result comparison unit 20 is configured to transmit an abnormality detection signal to the engine control unit 21 when it detects an abnormality in the temperature detection element 9. When the engine control unit 21 receives the abnormality detection signal, it prohibits power supply to the excitation coil 3 and stops the heating operation of the fixing film 1.

[0060] As described above, the memory unit 23 stores a reference value S of the temperature gradient used to determine whether the temperature detected by the temperature detection element 9 is normal or abnormal. The reference value correction unit 22 is configured to correct this reference value S according to the frequency information from the frequency control unit 18. Specifically, the reference value correction unit 22 receives information on the drive frequency from the frequency control unit 18 and performs the correction by multiplying the reference value by a correction coefficient corresponding to a predetermined drive frequency.

[0061] For example, Figure 15 shows the temperature change of the temperature sensing element 9 in interval C (see Figure 13) according to the driving frequency. Here, the temperature slope S1 at 60kHz is taken as the reference value S, and the reference values ​​at 80kHz and 100kHz, corrected according to equation (6) below and the predetermined correction coefficients listed in Table 1, are taken as Sf2 and Sf3, respectively.

[0062]

number

[0063] [Table 1]

[0064] Furthermore, s1, s2, and s3 in Figure 15 are the detected temperature slopes at 60kHz, 80kHz, and 100kHz drive frequencies, respectively, at a certain time within section C. The reference value S1 and the reference values ​​Sf2 and Sf3 corrected based on frequency information are compared with the detected temperature slopes s1, s2, and s3 at each drive frequency. If the slope falls below the reference value (S>s) as shown by the dashed line in Figure 15, the detection result comparison unit 20 determines that there is an abnormality and stops the heating operation of the fixing film 1. In this way, even if the drive frequency changes, malfunctions and temperature control failures due to an abnormal drop in the detected temperature of the temperature detection element 9 can be prevented.

[0065] Note that the temperature gradient used as the reference value S may be different from that at 60kHz. Also, when multiple temperature sensing elements are used, the change in heat distribution according to the drive frequency will differ depending on the placement of each temperature sensing element. Therefore, the correction coefficient α for the reference value S according to the drive frequency, as described above, does not have to be the value listed in Table 1, and must be set individually for each temperature sensing element.

[0066] For example, if each temperature sensing element detects the temperature of the edge, as described above, the temperature of the edge of the fixing film 1 will rise more easily when the driving frequency f is higher. Therefore, in this case, the correction coefficient α will also be smaller when the driving frequency is lower than the first value than when the driving frequency is the first value.

[0067] (Anomaly detection and control of temperature sensing element) Next, the operation of the abnormality detection control of the temperature detection element 9 described above will be explained along with the drive sequence of the fixing device 200, based on the flowchart shown in Figure 16. Note that the control process shown in this flowchart is executed according to a program stored in the memory unit 23 in advance.

[0068] When a print job is input to the image forming apparatus 100, the control means 300 controls the high-frequency inverter 16 to supply the excitation coil 3 with the maximum power that can be supplied under normal conditions in order to raise the temperature of the fixing film 1 to the target temperature (step S101 in Figure 16). Next, the control means 300 detects the current temperature of the fixing film 1 based on the detection result of the temperature detection element 9 (step S102), and determines the power to be supplied to the excitation coil 3 after the maximum power has been supplied, based on the difference between the target temperature and the current temperature (step S103).

[0069] Here, the control means 300 compares the temperature gradient s detected by the temperature detection element 9, obtained during the temperature detection in step S102, with the temperature gradient reference value S (step S104). At this time, the control means 300 also detects the drive frequency of the high-frequency inverter 16, and if correction is necessary, uses the corrected reference value Sf as described above as the temperature gradient reference value.

[0070] Then, if the temperature gradient s detected by the temperature detection element 9 falls below the temperature gradient reference value S (S>s, Yes in step S104), the control means 300 determines that there is an abnormality in the output of the temperature detection element 9 (S105).

[0071] If the control means 300 determines that there is an abnormality in the output of the temperature detection element 9, it prohibits power supply to the fixing device 200 (step S106). The control means 300 also stops the heating operation of the fixing film 1 and terminates control by notifying the malfunction via the operation panel (not shown) or the like (step S107).

[0072] On the other hand, if the temperature gradient s detected by the temperature detection element 9 is greater than or equal to the temperature gradient reference value S (S ≤ s, No. in step S104), the output of the temperature detection element is judged to be normal, and the heating operation of the rotating body 1 continues.

[0073] Subsequently, as the temperature of the rotating body 1 approaches the target temperature, the control means 300 starts controlling the input power to reduce the input power (step S108). Specifically, it controls the high-frequency inverter 16 so that the input power is the same as that determined in step S103.

[0074] At the same time, the control means 300 resets timer T to zero and starts counting up the timer (step S109), and waits for the timing Ttemp time to elapse for detecting the current temperature to determine the next power (step S110).

[0075] After the Ttemp time has elapsed, the control means 300 again determines the next power input from the detected temperature information (steps S111 to S112) and resets the power input from the high-frequency inverter 16 to the excitation coil 3.

[0076] When the above power supply is reset, the control means 300 determines whether or not to terminate the power supply (step S113). If the power supply is to continue (No. in step S113), the operations from step S109 to step S113 are repeated. Then, in step S113, if the control means 300 determines that the power supply has ended, it terminates the above control. Note that in step S113, the control means 300 determines that the power supply has ended when the print job is completed or when the fuser power supply is prohibited due to an emergency stop factor such as a jam or error.

[0077] (Detection of damage to the fixing film) If the rotating body 1 is damaged, such as by a crack, and the conductivity of the heating pattern of the heating layer 1a (Figure 3) is interrupted, the circulating current will no longer flow through that heating pattern. As a result, the damaged area of ​​the rotating body 1 and the area along its circumference will stop generating heat. Now, if we consider a case where the temperature detection element 9 is located in this area that no longer generates heat, the temperature detection element 9 itself will detect low temperatures, and the engine control unit 21 (Figure 4) will increase the power input to try to maintain the target temperature. However, since the area outside the detection region of the temperature detection element 9 is generating heat normally, the temperature will rise in line with the increased power input, and there is a risk that it will reach an abnormal temperature. Therefore, if the heating pattern of the detection region of the temperature detection element 9 on the rotating body 1 is damaged, it is necessary to detect this and stop the operation.

[0078] In contrast, if we focus on the temperature detected by the temperature detection element 9 when the detection area is damaged and no longer generates heat, even if the temperature detection element 9 is functioning normally, it should detect a lower temperature than when functioning normally. In other words, the method described above, in which the temperature slope s detected by the temperature detection element 9 is compared with a temperature slope reference value S corrected based on frequency information, and if the slope falls below the reference value, it is judged to be abnormal and the operation is stopped, can be used in the same way.

[0079] As described above, in this embodiment, abnormal temperature detection of the fixing film 1 caused by a failure of the temperature detection element 9 or damage to the fixing film can be appropriately detected by focusing on the temperature gradient of the temperature detection element 9, which is the amount of change in the temperature detected by the temperature detection element 9 per unit time. That is, in this embodiment, the temperature gradient of the temperature detection element 9 is compared with a reference value stored in the memory unit 23, and if the temperature gradient is smaller than the reference value, it is determined that an abnormal temperature detection of the fixing film 1 has occurred.

[0080] Furthermore, considering that the heat distribution of the fixing film 1 differs depending on the setting of the drive frequency of the high-frequency inverter 16, the system is configured to correct the value of the reference value based on this drive frequency information. As a result, temperature detection abnormalities of the fixing film 1 can be detected with high accuracy. In addition, since temperature detection abnormalities of the fixing film 1 can be detected from the temperature change of the temperature detection element 9 in this way, appropriate action can be taken in response to the temperature detection abnormality of the fixing film 1, such as stopping the heating control of the fixing film 1.

[0081] <Second Embodiment> Next, the second embodiment will be described with reference to Figure 17. In the first embodiment, an example was described in which a value previously stored in the memory unit was used as the temperature gradient reference value. However, this embodiment differs from the first embodiment in that the temperature gradient of the rotating body during heating is used as the reference value. For this reason, in the following description, only the configurations that differ from the first embodiment will be described, and the same reference numerals will be used for other configurations, and their descriptions will be omitted.

[0082] Figure 17 is a schematic diagram of the temperature detection change of the temperature detection element 9 until the fixing film 1 is maintained at a constant temperature by temperature control. If an abnormality occurs in the temperature detection element 9 or on the same circumference as the detection area of ​​the temperature detection element 9 in the fixing film 1 while the fixing film 1 is heating up, the temperature detected by the temperature detection element 9 will be as shown by the dashed line in the figure.

[0083] In such abnormal situations, the time during which the high-frequency inverter 16 is supplied with a constant maximum power is extended compared to normal conditions, which may lead to overheating. Therefore, in this embodiment, the control means 300 stores the temperature gradient S4 detected by the temperature detection element 9 immediately after the start of heating as a temperature gradient reference value S in the storage unit 23 (Figure 4). Then, the reference value correction unit 22 corrects the reference value S based on the drive frequency information of the high-frequency inverter 16, similar to the first embodiment, and compares it with the temperature gradient s4 at the end of section C.

[0084] Then, if the temperature gradient s4 is smaller than the detected temperature gradient S4 which has been corrected using the reference value (S4 > s4), the control means 300 determines that an abnormality has occurred and stops the heating operation of the fixing film 1.

[0085] This allows for detection of any abnormalities in the temperature detected by the temperature detection element 9 during the heating of the fixing film 1, preventing malfunctions and overheating. Furthermore, by correcting the reference value S based on frequency information, abnormalities can be detected in the same way as in the first embodiment, even if the frequency and the heat generation distribution of the rotating body 1 change due to a malfunction or other reason during the heating of the fixing film 1.

[0086] As described above, in this embodiment, the amount of change per unit time of the detected temperature of the temperature detection element 9 during the first period, such as section C, is stored in the storage means 23 as a reference value for the detected temperature slope. Therefore, even if the reference value for the detected temperature slope is not stored in the storage unit 23 in advance, it is possible to detect temperature detection abnormalities of the fixing film 1 during the second period following the first period.

[0087] <Third Embodiment> Next, a second embodiment will be described with reference to Figure 18. This embodiment differs from the first embodiment in that it uses multiple temperature detection elements to detect abnormalities. For this reason, in the following description, only the configurations that differ from the first embodiment will be described, and other configurations will be described using the same reference numerals.

[0088] As shown in Figure 18(a), the fixing device 200 according to this embodiment includes a temperature detection element 9 for detecting the temperature of the longitudinal center of the fixing film 1, and temperature detection elements 10 and 11 for detecting the temperature of the longitudinal ends of the fixing film 1.

[0089] Figure 18(b) is a graph showing the change in detected temperature in section C between the temperature detection element 9 located in the longitudinal center of the fixing film 1 and the temperature detection element 10 located at the longitudinal end of the fixing film 1.

[0090] In this embodiment, the control means 300 stores the temperature gradient S5 detected by the temperature detection element 10 located at the end as a temperature gradient reference value S in the storage unit 23. Since a temperature difference occurs between the center and the edges of the fixing film 1 due to the drive frequency of the high-frequency inverter 61, the control means 300 corrects the temperature gradient reference value S based on information regarding the correlation between the detected temperatures of the temperature detection element 9 and the temperature detection element 10, and the frequency information of the drive frequency. In the following description, the temperature gradient reference value for the central temperature detection element 9 obtained by correcting the temperature gradient S5 detected by the temperature detection element 10 at the edge will be referred to as Sf6.

[0091] Next, the control means 300 compares the detected temperature slope s6 of the temperature detection element 9 located in the center with the temperature slope reference value Sf6, and determines that an abnormality has occurred if the detected temperature slope s6 is lower than the temperature slope reference value Sf6 (Sf6 > s6).

[0092] By doing this, the temperature gradients detected by temperature detection elements located in places with different heat generation levels can be compared, and an abnormality in the temperature detected by one of the temperature detection elements can be detected, thereby stopping the heating operation of the fixing film 1.

[0093] As described above, in this embodiment, the fixing device 200 includes a first temperature detection means 9 that detects the temperature of the fixing film 1 at least at a first position in the longitudinal direction, and a second temperature detection means 10 that detects the temperature of the fixing film 1 at a second position in the longitudinal direction that is different from the first position. The storage unit 23 stores a reference value as the amount of change per unit time of the detected temperature detected by the second temperature detection means 10. The control means 300 corrects the reference value based on information regarding the correlation between the detected temperatures of the first temperature detection means 9 and the second temperature detection means 10, and information regarding the drive frequency. As a result, it is possible to detect temperature detection abnormalities in the temperature of the fixing film 1 detected by the first temperature detection means 9 using the reference value of the amount of change per unit time of the detected temperature detected by the second temperature detection means 10.

[0094] In the above-described embodiment, the temperature gradient S5 detected by the end temperature detection element 10 was corrected to obtain a temperature gradient reference value for the central temperature detection element 9, but this is not limited to this. For example, the temperature gradient detected by the end temperature detection element 11 may be corrected to obtain a temperature gradient reference value for the central temperature detection element 9. Alternatively, the temperature gradient detected by the central temperature detection element 9 may be corrected to obtain a temperature gradient reference value for the end temperature detection elements 10 / 11. In other words, the temperature gradient reference value S can be the temperature gradient detected by any temperature detection element, and the comparison between the reference value and the detected temperature gradient may be performed between any temperature detection elements. Furthermore, the comparison of the temperature gradient reference value S may target the temperature gradients detected by two or more temperature detection elements.

[0095] <Fourth Embodiment> Next, a fourth embodiment will be described. In the following description, only the configurations that differ from the first embodiment will be described, and other configurations will be described using the same reference numerals.

[0096] In the electromagnetic induction heating method, the heat generation distribution of the fixing film 1 changes depending on the drive frequency. For this reason, in the first to third embodiments described above, an example was explained in which the temperature gradient reference value S is corrected based on the drive frequency information of the high-frequency inverter 16 when detecting an abnormal output of the temperature detection element. On the other hand, in addition to the drive frequency described above, there are several other parameters that can change the heat generation distribution in the electromagnetic induction heating method. These parameters will be described below.

[0097] (Power supplied from the high-frequency inverter 16 to the excitation coil 3) In the electromagnetic induction heating method, the amount of heat generated by the fixing film 1 changes depending on the amount of power supplied from the high-frequency inverter 16 to the excitation coil 3. Since the temperature detection element monitors the temperature of the fixing film 1, if the amount of heat generated by the fixing film 1 changes, the temperature detected by the temperature detection element 9 (10, 11) also changes accordingly.

[0098] (Rotation speed of the fixing film) When the fixing film 1 is pressed against the pressure roller 8 and the pressure roller 8 is driven to rotate, it rotates while forming a nip portion N. Here, because the fixing film 1 is thin, it has a small heat capacity, and since it rotates while in contact with the pressure roller 8, which has a large heat capacity, the heat from the fixing film 1 is continuously absorbed by the pressure roller 8 through the nip portion N.

[0099] Furthermore, in the electromagnetic induction heating method, the entire circumference of the fixing film 1 generates heat, so heat is not lost anywhere except the nip, and the temperature continues to rise. In other words, if we focus on any surface of the fixing film 1, the temperature continues to rise due to the heating around the entire circumference, and each time it enters the nip N due to rotation, heat is lost by the pressure roller 8 and the temperature drops.

[0100] Therefore, a change in rotational speed, that is, a change in the speed at which the film enters the nip portion N, means that the rate at which the fixing film 1 heats up changes, and consequently, the temperature detected by the temperature sensing elements 9 (10, 11) also changes.

[0101] (Variation in the resistance value of the fixing film) In the heating pattern of the heating layer 1a in the fixing film 1, there is a slight variation in resistance value from one strand to the next. This is due to variations in width and thickness when forming the heating layer 1a of the fixing film 1, and is unavoidable in the manufacturing process. Incidentally, if we focus on equations (4) and (5) relating to the combined impedance X derived in the first embodiment, these equations include a term that represents the resistance value R of the fixing film 1.

[0102] This means that if the resistance of the fixing film 1 changes, the combined impedance X will also change accordingly. Furthermore, because the combined impedance X has different frequency characteristics in each region along the longitudinal direction of the magnetic core 2 due to differences in apparent permeability, if the resistance of the fixing film 1 changes, the combined impedance X will change in each region. Consequently, if the resistance of the fixing film 1 varies, even if operated at the same power and driving frequency, the heat generation distribution along the longitudinal direction will differ for each fixing film 1, which will also lead to changes in the detection temperature of the temperature sensing element.

[0103] In this embodiment, the reference value correction unit 22 corrects the temperature gradient reference value S using at least one of the following parameters in addition to the drive frequency: input power information, rotation speed information of the fixing film 1, and resistance variation information of the fixing film 1. This makes it possible to further improve the detection accuracy of abnormal output from the temperature detection element 9.

[0104] (summary) [Configuration 1] An image heating device for heating an image formed on a recording material, A cylindrical rotating body having a conductive layer, A magnetic core material installed inside the rotating body and forming an open magnetic path in the axial direction of the rotating body, An excitation coil wound around the magnetic core material along the axial direction of the rotating body, An inverter that supplies alternating current to the excitation coil, A control means that controls the inverter to supply an alternating current to the excitation coil, thereby generating an alternating magnetic flux in the magnetic core material and electromagnetically inducing heating of the rotating body, The rotating body comprises at least one temperature detection means for detecting the temperature of the rotating body, The system includes a storage means that stores a reference value for the amount of change per unit time of the detected temperature detected by the temperature detection means, The control means is capable of changing the drive frequency of the inverter. The control means is The amount of change per unit time of the temperature detected by the temperature detection means is obtained, The reference value is corrected based on the drive frequency of the inverter when the amount of change is obtained. If the amount of change is smaller than the corrected reference value, the heating of the rotating body is stopped. An image heating device characterized by the following features.

[0105] [Configuration 2] The control means is The amount of change per unit time of the temperature detected by the temperature detection means during the first period is stored in the storage means as the reference value. If the change per unit time of the temperature detected by the temperature detection means during the second period, which is later than the first period, is smaller than the reference value corrected based on the drive frequency, the heating of the rotating body is stopped. The image heating device according to configuration 1, characterized in that it is a device that provides an image heating device.

[0106] [Configuration 3] The at least one temperature detection means comprises a first temperature detection means for detecting the temperature of the rotating body at a first position in the longitudinal direction of the rotating body, and a second temperature detection means for detecting the temperature of the rotating body at a second position in the longitudinal direction different from the first position. The storage means stores, as the reference value, a reference value of the amount of change per unit time of the detected temperature detected by the second temperature detection means. The control means is The reference value is corrected based on information relating to the correlation between the detected temperatures of the first temperature detection means and the second temperature detection means, and the drive frequency. If the rate of change per unit time of the temperature detected by the first temperature detection means is smaller than the corrected reference value, the heating of the rotating body is stopped. An image heating device according to configuration 1 or 2, characterized by the above.

[0107] [Structure 4] The control means corrects the reference value based on the power supplied from the inverter to the excitation coil, the rotational speed of the rotating body, and at least one of the resistance value of the conductive layer, and the drive frequency. An image heating device according to any one of configurations 1 to 3, characterized by the above.

[0108] [Composition 5] The control means notifies of the occurrence of an abnormality when the amount of change is smaller than the corrected reference value. An image heating device according to any one of configurations 1 to 4, characterized by the above.

[0109] [Composition 6] Image forming means for forming a toner image on a recording material, The image heating device comprises one of the configurations 1 to 5, The image heating device is a fixing device that heats the recording material on which the toner image is formed to fix the toner image to the recording material. An image forming apparatus characterized by the following: [Explanation of Symbols]

[0110] 1: Rotating body (fixing film) / 2: Magnetic core material (magnetic core) / 3: Excitation coil / 9: Temperature detection means (temperature detection element) / 16: Inverter (high-frequency inverter) / 200: Image heating device (fixing device) / 300: Control means

Claims

1. An image heating device for heating an image formed on a recording material, A cylindrical rotating body having a conductive layer, A magnetic core material installed inside the rotating body and forming an open magnetic path in the axial direction of the rotating body, An excitation coil wound around the magnetic core material along the axial direction of the rotating body, An inverter that supplies alternating current to the excitation coil, A control means that controls the inverter to supply an alternating current to the excitation coil, thereby generating an alternating magnetic flux in the magnetic core material and electromagnetically inducing heating of the rotating body, The rotating body comprises at least one temperature detection means for detecting the temperature of the rotating body, The system includes a storage means that stores a reference value for the amount of change per unit time of the detected temperature detected by the temperature detection means, The control means is capable of changing the drive frequency of the inverter. The control means is The amount of change per unit time of the temperature detected by the temperature detection means is obtained, The reference value is corrected based on the drive frequency of the inverter when the amount of change is obtained. If the amount of change is smaller than the corrected reference value, the heating of the rotating body is stopped. An image heating device characterized by the following features.

2. The control means is The amount of change per unit time of the temperature detected by the temperature detection means during the first period is stored in the storage means as the reference value. If the change per unit time of the temperature detected by the temperature detection means during the second period, which is after the first period, is smaller than the reference value corrected based on the drive frequency, the heating of the rotating body is stopped. The image heating apparatus according to claim 1, characterized by the features described above.

3. The at least one temperature detection means comprises a first temperature detection means for detecting the temperature of the rotating body at a first position in the longitudinal direction of the rotating body, and a second temperature detection means for detecting the temperature of the rotating body at a second position in the longitudinal direction different from the first position. The storage means stores, as the reference value, a reference value of the amount of change per unit time of the detected temperature detected by the second temperature detection means. The control means is The reference value is corrected based on the information relating to the correlation between the detected temperatures of the first temperature detection means and the second temperature detection means, and the drive frequency. If the rate of change per unit time of the temperature detected by the first temperature detection means is smaller than the corrected reference value, the heating of the rotating body is stopped. The image heating apparatus according to claim 1, characterized by the features described above.

4. The control means corrects the reference value based on the power supplied from the inverter to the excitation coil, the rotational speed of the rotating body, and at least one of the resistance value of the conductive layer, and the drive frequency. The image heating apparatus according to claim 1, characterized by the features described above.

5. The control means notifies of the occurrence of an abnormality when the amount of change is smaller than the corrected reference value. The image heating apparatus according to claim 1, characterized by the features described above.

6. Image forming means for forming a toner image on a recording material, The image heating device comprises the image heating device according to any one of claims 1 to 5, The image heating device is a fixing device that heats the recording material on which the toner image is formed to fix the toner image to the recording material. An image forming apparatus characterized by the following:

Images