Image heating apparatus and image forming apparatus

Abstract

To appropriately manage the temperature of a rotating body.SOLUTION: The image heating apparatus includes a rotating member, a magnetic core material, an excitation coil wound around the magnetic core material along an axial direction of the rotating member, an inverter, a control unit configured to control the inverter to cause an alternating current to flow through the excitation coil and generate an alternating magnetic flux in the magnetic core material to heat the rotating member by electromagnetic induction, and a detection coil configured to be electromagnetically induced by the alternating magnetic flux to generate an induced electromotive force. When the induced electromotive force becomes larger than a predetermined value, the control means stops or reduces power supplied from the inverter to the exciting coil.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] In electrophotographic image forming devices such as printers and copiers, a toner image corresponding to image data is transferred onto a recording material such as recording paper or an overhead projector sheet, and then the toner image transferred onto the recording material is fixed to the recording material by applying heat and pressure in a fixing device. Generally, such a heating fixing device is often configured to include a heating element as a heat source, a power supply that supplies power to the heating element, a temperature detection means that detects the temperature near the heating element, and a control means that controls the current flowing through the heating element.

[0003] In such a configuration, in order to prevent excessive power from being supplied to the heating element when an abnormality occurs in the temperature detection means, a method has been devised to limit the power supplied to the heating element so that it does not exceed a predetermined value (see Patent Document 1).

[0004] Another heating method for fixing devices is an electromagnetic induction heating method in which an alternating magnetic field generated by a magnetic field generating means is supplied to the inside of an electromagnetic induction heating rotor, and heating is performed by Joule heat caused by eddy current loss inside the heating rotor. In a fixing device using this electromagnetic induction heating method, a method has been devised in which the longitudinal temperature distribution of the heating rotor is changed by changing the frequency of the alternating current supplied to the excitation coil that generates the alternating magnetic field (see Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-003396 [Patent Document 2] JP 2016-24348 A Summary of the Invention [Problem to be solved by the invention]

[0006] In a fixing device using an electromagnetic induction heating method as described in Patent Document 2, the temperature distribution in the longitudinal direction of the rotating body changes depending on the frequency of the alternating current supplied to the excitation coil. Therefore, even if the power is limited uniformly as described in Patent Document 1, the temperature of each part of the rotating body in the longitudinal direction varies depending on the frequency of the alternating current, making it difficult to properly control the temperature of the rotating body.

[0007] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image heating device and an image forming device that are capable of appropriately managing the temperature of a rotating body. [Means for solving the problem]

[0008] One aspect of the present invention is an image heating device that heats an image formed on a recording material, comprising: a cylindrical rotating body that is conductive; a magnetic core material that is installed inside the rotating body and forms an open magnetic path in the axial direction of the rotating body; an excitation coil that is wound around the magnetic core material along the axial direction of the rotating body; an inverter that passes an alternating current through the excitation coil; control means that controls the inverter to pass an alternating current through the excitation coil, thereby generating an alternating magnetic flux in the magnetic core material and electromagnetically inductively heating the rotating body; and a detection coil that is electromagnetically induced by the alternating magnetic flux and generates an induced electromotive force, wherein the control means is capable of changing the drive frequency of the inverter, and the control means stops or reduces the power supplied from the inverter to the excitation coil when the induced electromotive force becomes greater than a predetermined value.

[0009] One aspect of the present invention is an image heating device that heats an image formed on a recording material, comprising: a cylindrical rotating body that is conductive; a magnetic core material that is installed inside the rotating body and forms an open magnetic path in the axial direction of the rotating body; an excitation coil that is wound around the magnetic core material along the axial direction of the rotating body; an inverter that passes an alternating current through the excitation coil; control means that controls the inverter to pass an alternating current through the excitation coil, thereby generating an alternating magnetic flux in the magnetic core material and electromagnetically heating the rotating body; a detection coil that is electromagnetically induced by the alternating magnetic flux and generates an induced electromotive force; and temperature detection means that detects the temperature of the rotating body, wherein the control means compares the temperature of the rotating body detected by the temperature detection means over a predetermined period with the induced electromotive force detected by the detection coil over the predetermined period, and if the correspondence between the temperature of the rotating body and the induced electromotive force deviates from a predetermined relationship, the image heating device stops or reduces the supply of power from the inverter to the excitation coil.

[0010] One aspect of the present invention is an image heating device that heats an image formed on a recording material, comprising: a cylindrical rotating body that is conductive; a magnetic core material that is installed inside the rotating body and forms an open magnetic path in the axial direction of the rotating body; an excitation coil that is wound around the magnetic core material along the axial direction of the rotating body; an inverter that passes an alternating current through the excitation coil; control means that controls the inverter to pass an alternating current through the excitation coil, thereby generating an alternating magnetic flux in the magnetic core material and electromagnetically inductively heating the rotating body; a first heat generation amount detection unit that detects the amount of heat generated in a central portion of the rotating body; and a second heat generation amount detection unit that detects the amount of heat generated in an end portion of the rotating body, wherein the control means corrects the setting of the inverter's drive frequency to be higher when the difference between the heat generation amount in the central portion of the rotating body calculated based on the detection results of the first heat generation amount detection unit when the inverter is driven at a predetermined frequency and drive duty ratio and the heat generation amount in the end portion of the rotating body calculated based on the detection results of the second heat generation amount detection unit is greater than a predetermined difference. [Effects of the Invention]

[0011] According to the present invention, the temperature of the rotating body can be appropriately controlled. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment. [Figure 2] FIG. [Figure 3] FIG. [Figure 4] 2A and 2B are a perspective projection view and a connection circuit block diagram of the fixing device; [Figure 5] FIG. 2 is a schematic diagram showing the relationship between an exciting coil current and a magnetic field. [Figure 6] FIG. 4 is a diagram showing heat distribution in the longitudinal direction of the fixing film. [Figure 7] 1A and 1B are diagrams showing the relationship between the power input to the excitation coil and the voltage detected by the detection coil at the center and end of the fixing film, respectively. [Figure 8] FIG. 10 is a flowchart of control for limiting the amount of heat generated by the fixing film. [Figure 9] FIG. 10 is a flowchart of heat generation amount limiting control in a modified example. [Figure 10] FIG. 10 is a flowchart of a frequency correction determination process. [Figure 11] 10A and 10B are a perspective projection view and a connection circuit block diagram of a fixing device according to a second embodiment; [Figure 12] 1 is a graph showing the relationship between film temperature and induced electromotive force. [Figure 13] FIG. 10 is a flowchart of heat generation amount limit control of the fixing film according to the second embodiment. [Figure 14] FIG. 10 is a flowchart of a process for determining whether the temperature has risen or exceeded the predetermined value. DETAILED DESCRIPTION OF THE INVENTION

[0013] First Embodiment (Schematic configuration of image forming apparatus) A fixing device as an image heating device according to an embodiment of the present invention and an image forming apparatus 100 equipped with the same will be described below with reference to the drawings. As shown in FIG. 1, the image forming apparatus 100 is an electrophotographic process laser beam printer and includes a feed cassette 105, a feed roller 106, a registration roller 107, an image forming unit 120, a fixing device 130, and a control unit 31. The feed cassette 105 is a recording material supporting unit that supports the recording material P and is configured to hold and store the recording material P. The feed roller 106 is a feeding unit that feeds the recording material P stored in the feed cassette 105 and is configured to separate and feed the recording materials P stored in the feed cassette 105 one by one. The registration roller 107 is a recording material conveying unit that conveys the recording material fed from the feed cassette 105 toward the image forming unit 120 and is configured to convey the recording material P in accordance with the timing of image formation by the image forming unit 120.

[0014] The image forming means 120 is configured to form an image on a 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. The charging roller 102 uniformly charges the photosensitive drum 101, which rotates at a predetermined speed in the direction indicated by the arrow in the figure, to a desired polarity and potential. The exposure device 103 is a laser beam scanner that outputs an ON-OFF modulated laser beam corresponding to a time-series electric digital pixel signal of the target image information input from an external device such as a host computer, thereby scanning and exposing (irradiating) the charged surface of the photosensitive drum 101. The development device 104 includes a development 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 at the transfer portion, and is configured so that a transfer voltage is applied to the transfer roller 108, thereby transferring the toner image formed on the photosensitive drum 101 onto the recording material P. The cleaning device 110 is provided downstream of the transfer nip 108T in the rotation direction of the photosensitive drum 101, and is configured to remove residual toner, paper dust, etc. from the photosensitive drum 101.

[0015] The fixing device 130 is an image heating device of an electromagnetic induction heating type, and includes a fixing film 1 as a heating rotor, and a pressure roller 8 that forms a fixing nip N together with the fixing film 1. The fixing device 130 forms the fixing nip N with the fixing film 1 and the pressure roller 8, and is configured to heat and pressurize an unfixed toner image transferred onto a recording material P at the fixing nip N, thereby fixing the image to the recording material P.

[0016] The control means 31 is a controller that controls each unit of the image forming apparatus 100 described above, and includes a ROM 32a and a RAM 32b as memory units, a timer 32c, a CPU (Central Processing Unit) 32 as a calculation means, and various input / output control circuits (not shown), etc.

[0017] Since the image forming apparatus 100 is configured as described above, when a feed start signal is output from the control means 31 to the feed roller 106, the feed roller 106 is driven to separate and feed the recording materials P in the feed cassette 105 one by one. When the recording materials P are fed from the feed cassette 105, the registration roller 107 transports the recording materials P to the transfer nip 108T in time with the toner image on the photosensitive drum 101 being transported to the transfer nip 108T. Then, a transfer voltage (transfer bias) of opposite polarity to the toner is applied to the transfer roller 108, so that the toner image is transferred onto the recording material P at the transfer nip 108T.

[0018] Once the toner image has been transferred onto the recording material, the recording material P bearing the unfixed toner image is conveyed by a pre-fixing conveyance guide 109 to a fixing device 130, where the toner image is pressurized and heated to be fixed onto the recording material P. Then, the recording material with the fixed toner image is discharged from a discharge port 111 onto a discharge tray 112 serving as a discharge section.

[0019] (Configuration of fixing device) Next, the configuration of the fixing device 130 will be described with reference to Figures 2 to 4. As shown in Figure 2, the pressure roller 8 serving as a pressure member includes a core 8a, a heat-resistant elastic material layer 8b (hereinafter referred to as elastic material layer 8b) that is concentrically formed and coated around the core in the shape of a roller, and a release layer 8c provided on the surface. Both ends of the core 8a are rotatably held between chassis side plates (not shown) of the fixing device 130 via conductive bearings, allowing the pressure roller 8 to rotate freely.

[0020] On the other hand, the fixing film 1 is a cylindrical, electrically conductive rotating body, and a sleeve guide member 6 and a pressure stay 19 are installed inside the fixing film 1 in the longitudinal direction (direction of the rotation axis). The pressure stay 19 is disposed above the sleeve guide member 6, and both ends thereof protrude beyond the fixing film 1. The sleeve guide member 6 holds the fixing film 1 from the inner periphery below the fixing film 1, and as shown in FIG. 3, flange members 12a and 12b are attached to both ends of the sleeve guide member 6. The flange members 12a and 12b each have a cylindrical portion and a flange portion protruding radially from the cylindrical portion. The end of the fixing film 1 is rotatably fitted onto the cylindrical portion. The flange members 12a and 12b are restricted in their longitudinal positions (left and right positions) by restricting members 13a and 13b. The flange portions abut against the end of the fixing film 1, restricting longitudinal movement of the fixing film 1, thereby positioning the fixing film 1 in the longitudinal direction.

[0021] In addition, pressure springs 17a, 17b are respectively compressed between both ends of pressure stay 19 and spring bearing members 18a, 18b on the chassis side of fixing device 130, thereby biasing pressure stay 19 downward (applying a downward force). As a result, the lower surface of sleeve guide member 6 and the upper surface of pressure roller 8 are in pressure contact with each other, sandwiching fixation film 1 therebetween, forming fixation nip N of a predetermined width. Furthermore, pressure roller 8 is driven to rotate in the direction of the arrow in FIG. 1 (counterclockwise) by driving means (not shown), and as pressure roller 8 rotates, a rotational force is also applied to fixation film 1 due to frictional force.

[0022] As shown in FIG. 2, the fixing film 1 is a cylindrical rotating body with a composite structure, including a heat-generating layer 1a made of a conductive material and having a diameter of 10 to 50 mm. The heat-generating layer 1a is a metal film with a thickness of 10 to 50 μm, and the elastic layer 1b is made of silicone rubber with a hardness of 20 degrees (JIS-A, 1 kg load) and a thickness of 0.3 to 0.1 mm. 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 heat-generating layer 1a, an induced current is generated and the heat is generated. This heat is transferred to the elastic layer 1b and the release layer 1c, heating the entire fixing film 1. When a recording material P passes through the fixing nip N, the toner image T on the recording material P is heated and fixed.

[0023] Next, a mechanism for applying an alternating magnetic flux to the heat-generating layer 1a to generate an induced current will be described in detail. As shown in FIG. 2, a magnetic core 2 (an example of a magnetic core material) and an excitation coil 3 are provided in the center of the fixing film 1. As shown in FIG. 4, the magnetic core 2 is disposed so as to penetrate the hollow portion of the fixing film 1, forming a linear open magnetic path with magnetic poles. The magnetic core 2 is made of a material with low hysteresis loss and high relative permeability, such as sintered ferrite, ferrite resin, amorphous alloy, or high-permeability oxide or alloy material such as permalloy. The magnetic core 2 is preferably made of a ferromagnetic material. In this embodiment, sintered ferrite with a relative permeability of 1800 is used, and the magnetic core 2 is cylindrical with a diameter of 5 to 30 mm and a longitudinal length of 240 mm. Alternatively, the magnetic core 2 may be formed by arranging multiple cores, each 30 mm long, in the longitudinal direction.

[0024] The exciting coil 3 is a normal single conducting wire that is wound spirally around the magnetic core 2 along the rotational axis of the fixing film 1. The winding is performed so that the spacing is closer at the ends than at the center of the core. For example, for a magnetic core 2 with a longitudinal dimension of 240 mm, the exciting coil 3 is wound 18 times, with the winding spacing being 10 mm at the ends, 20 mm at the center, and 15 mm in between. In this way, the exciting coil 3 is wound in a direction that intersects with the axis X of the magnetic core 2.

[0025] Furthermore, the exciting coil 3 is connected to a high-frequency inverter 16 via power supply contacts 3a and 3b. The high-frequency inverter 16 is configured to receive AC power from a commercial power source 50 external to the image forming apparatus 100 via an inlet 51, convert the AC power from the commercial power source 50 into a high-frequency current, and supply the high-frequency current to the exciting coil 3. In other words, the high-frequency inverter 16 is an inverter that passes an alternating current through the exciting coil 3, and by passing a high-frequency alternating current through the exciting coil 3, an alternating magnetic flux can be generated in the magnetic path formed by the magnetic core 2.

[0026] The basic heat generation principle of the induction heating type fixing device 130 is similar to that of a transformer, with the excitation coil 3 corresponding to the primary coil (18 turns) on the input side of a transformer, and the fixing film 1 corresponding to the secondary coil (1 turn) on the output side. In this configuration, when an AC voltage of approximately 60 kHz to 90 kHz is applied to the primary coil (excitation coil 3), a current flows through the primary coil. When a current flows through this primary coil (excitation coil 3), a magnetic flux generated in a direction penetrating the fixing film 1 flows through the magnetic core 2, and this magnetic flux generates an induced electromotive force in the secondary coil (fixing film 1). When an induced electromotive force is generated in the secondary coil (fixing film 1), a current flows along the circumferential direction of the fixing film 1, causing heat to be generated due to resistance loss in the fixing film 1, which in turn causes the fixing film 1 to be induction heated.

[0027] More specifically, Figure 5 shows the magnetic field at the moment when the current in the excitation coil 3 is increasing in the direction of arrow I1. In this figure, the magnetic core 2 functions as a component that guides the magnetic field lines generated by the excitation coil 3 into the interior and forms a magnetic path. As a result, the magnetic field lines are concentrated along the magnetic path, then diffuse at the end of the magnetic core 2, and connect far away on the periphery (although some lines appear to be discontinued at the end in the figure). If a cylindrical circuit 61 with a small longitudinal width is placed so as to vertically surround this magnetic path, an alternating magnetic field (a magnetic field whose magnitude and direction repeatedly change over time) is generated inside the magnetic core.

[0028] An induced electromotive force is generated in the circumferential direction of this circuit 61 according to Faraday's law. Faraday's law states that "the magnitude of the induced electromotive force generated in the circuit 61 is proportional to the rate of change in the magnetic field that penetrates the circuit 61 perpendicularly." The heat generating layer 1a can be thought of as a number of these extremely short cylindrical circuits 61 connected in the longitudinal direction. Therefore, the induced electromotive force Vf generated in the heat generating layer 1a is expressed by the following equation 1.

number

[0029] Therefore, as shown in Figure 5, when an alternating current I1 (a current whose magnitude and direction repeatedly change over time) is passed through the excitation coil 3, an alternating magnetic field is generated inside the magnetic core 2, an induced electromotive force Vf is applied to the heating layer 1a in the circumferential direction along the entire length, and a circular current I2 flows along the entire length. Because the heating layer 1a has electrical resistance, the flow of this circular current I2 generates Joule heat. As long as the alternating magnetic field continues to be generated inside the magnetic core, the circular current I2 continues to be generated while changing direction. This is the heat generation principle of the heating layer 1a in the configuration according to this embodiment. For example, if the current I1 is a high-frequency AC current of 60 kHz, the circular current I2 will also be a high-frequency AC current of 60 kHz.

[0030] 4, the fixing device 130 is provided with a temperature detection element 9 as temperature detection means for detecting the temperature of the fixing film 1. In this embodiment, this temperature detection element 9 is disposed in the center of the fixing film 1 in the longitudinal direction so as to contact the inner surface of the fixing film 1 toward the upstream side in the conveyance direction of the recording material P. Furthermore, the CPU 32 is provided with a fixing temperature control means 44, a frequency control means 45, a power control means 46, an engine control means 43, and a detection result comparison unit 49, and the fixing temperature control means 44 is configured to receive the detection result of the temperature detection element 9.

[0031] The engine control means 43 is configured to calculate, based on the temperature of the fixing film 1 detected by the temperature detection element 9, the power that should be input (hereinafter also referred to as supply) so that the fixing film 1 reaches a target temperature. Furthermore, the frequency control means 45 outputs a control signal for the drive frequency of the high-frequency inverter 16 based on the power and other information calculated by the engine control means 43. This control signal enables the frequency control means 45 to change the drive frequency of the high-frequency inverter 16. The power control means 46 is configured to output a control signal for the power of the high-frequency inverter 16 to the high-frequency inverter 16 based on the power and other information calculated by the engine control means 43. More specifically, in this embodiment, the high-frequency inverter 16 controls the amount of power input to the fixing film 1 by PWM control, and the power control means 46 outputs, to the high-frequency inverter 16, control signals (e.g., drive duty ratios) related to this PWM control, inverter circuit switching signals, and the like. Based on the detected temperature detected by the temperature detection element 9, the high frequency inverter 16 is driven by control signals from the frequency control means 45 and the power control means 46, thereby maintaining and adjusting the surface temperature of the fixing film 1 to a predetermined target temperature.

[0032] (detection coil) Next, the detection coils 5A, 5B, and 5C will be described. As shown in FIG. 4, detection coils 5A, 5B, and 5C, which are different from the excitation coil 3, are wound around the magnetic core 2 in a direction intersecting the axis X. The detection coils 5A, 5B, and 5C are magnetic flux detection coils for detecting magnetic flux, and are provided at the center and both ends of the magnetic core 2, respectively. In the following description, the detection coil 5A will also be referred to as a center detection coil or a first detection coil. The detection coils 5B and 5C will also be referred to as end detection coils or second detection coils. While it is desirable to provide these detection coils 5A, 5B, and 5C at multiple locations in the direction of the axis X of the magnetic core 2, it is sufficient that they are provided at at least one location; for example, they may be provided only at the center. Furthermore, the detection coils 5A, 5B, and 5C need not necessarily be wound around the magnetic core 2, as long as they are configured to generate an induced current when an alternating magnetic flux is generated from the magnetic core 2; for example, they may be wound in a spiral shape near the outside of the magnetic core 2.

[0033] In this embodiment, the first detector coil 5A is located at the center of the fixing film 1, the second detector coil 5B is located 15 mm from the left end of the fixing film 1, and the third detector coil 5C is located 15 mm from the right end of the fixing film 1. Each detector coil 5A, 5B, and 5C is wound twice spirally around the magnetic core 2 in the gaps between the turns of the excitation coil 3. By arranging the detector coils in the gaps between the turns of the excitation coil 3 in this manner, the detector coils 5A, 5B, and 5C can be arranged in a more space-saving manner than in a configuration in which the detector coils are wound so as to cover the excitation coil 3. Note that while it is preferable to arrange the detector coils 5A, 5B, and 5C in the gaps between the turns of the excitation coil 3, they may also be wound so as to cover the excitation coil 3. Furthermore, it is desirable that the portion of the magnetic core 2 around which the detector coils 5A, 5B, and 5C are wound be no more than one-third of the longitudinal length of the magnetic core 2.

[0034] One end of each of the detection coils 5A, 5B, and 5C is connected to ground, and the other end is connected to an IV conversion circuit 47a, 47b, or 47c, respectively. The output of each of the IV conversion circuits 47a, 47b, or 47c is input to a detection result comparison unit 49 of the CPU 32.

[0035] When an alternating current flows through the excitation coil 3 and an alternating magnetic field is generated in the magnetic core 2, electromagnetic induction occurs, causing an induced current to flow through the detection coils 5A, 5B, and 5C. The IV conversion circuits 47a, 47b, and 47c are configured to convert the induced current flowing through the detection coils 5A, 5B, and 5C into a voltage, thereby making it possible to detect the induced electromotive force generated in each of the detection coils 5A, 5B, and 5C. The induced electromotive force Vd generated in the detection coils 5A, 5B, and 5C can be expressed by the following equation 2, similar to that of the heating layer 1a.

number

[0036] Furthermore, since approximately equal magnetic flux passes through each of the detection coils 5A, 5B, and 5C and the heat generating layer 1a, the induced electromotive force Vd can be expressed by the following equation 3 based on equations 1 and 2.

number

[0037] As shown above, the induced electromotive force Vd generated in the detection coils 5A, 5B, and 5C is proportional to the induced electromotive force Vf generated in the heat-generating layer 1a. In other words, the amount of heat generated by the fixing film 1 can be calculated from the induced electromotive force Vd. The conversion of the amount of heat generated by the fixing film 1 may be different between the center and the edges. That is, the voltage waveform of the induced electromotive force detected by the detection coils 5A, 5B, and 5C differs between the center and the edges. Specifically, the L component is large in the center, resulting in an integral waveform that resembles a rounded square wave. The L component is small in the edges, resulting in a differential waveform that resembles an overshoot of the square wave. Therefore, when averaging or converting the voltage waveform to an effective value, different correction coefficients may be used between the center and the edges.

[0038] (Frequency dependence of temperature distribution of fixing film) Next, we will explain the frequency dependence of the temperature distribution of the fixing film 1. If the fixing film 1 is configured so that most of the magnetic flux coming out of the end of the magnetic core 2 passes outside the heat-generating layer 1a and returns to the other end, so that an induced current flows in the heat-generating layer (conductive layer) 1a in the circumferential direction of the fixing film 1, the longitudinal temperature distribution of the fixing film 1 changes depending on the frequency of the induced current.

[0039] FIG. 6 shows a graph of the longitudinal temperature distribution of the fixing film 1 when the drive frequency of the high-frequency inverter 16 is changed. The lower the drive frequency of the high-frequency inverter 16, i.e., the frequency of the induced current flowing circumferentially through the fixing film 1, the lower the temperature at both ends of the fixing film 1. Taking advantage of this characteristic, the fixing device 130 according to this embodiment changes the drive frequency depending on the size of the recording material P and the temperature of the non-paper-passing area of ​​the fixing film 1. The non-paper-passing area refers to an area through which the maximum size recording material usable by the image forming apparatus 100 can pass but not smaller sizes. When fixing maximum-size recording materials, the entire longitudinal area of ​​the fixing film 1 is heated uniformly. When fixing small-size recording materials, the drive frequency is lowered to suppress the temperature at the ends of the fixing film 1. This suppresses temperature rise in the non-paper-passing area and saves energy when fixing small-size recording materials.

[0040] (Relationship between the induced electromotive force of the detection coil and the power supplied to the excitation coil) Next, the influence of the drive frequency of the high-frequency inverter 16 on the relationship between the power supplied to the excitation coil 3 and the voltage detected by the detection coil will be explained. As described above, when power is supplied from the inverter 16 to the excitation coil 3, an alternating current flows through the excitation coil 3, and a voltage (induced electromotive force Vd) corresponding to the power supplied (input) to the excitation coil 3 is generated in the detection coil 5A. The power supplied to the excitation coil 3 is determined by a control signal sent from the CPU 32 to the high-frequency inverter 16, and is set to, for example, 1000 W at the start of printing and 500 W during continuous printing. The amount of power supplied (input power) is determined by the drive duty ratio, drive frequency, and AC voltage.

[0041] 7(a) is a graph showing the relationship between the power supplied to the excitation coil 3 and the voltage generated in the detection coil 5A, and shows the results when the drive frequency of the high-frequency inverter 16 is changed to 60 kHz, 75 kHz, and 90 kHz. In FIG. 7(a), the detected voltage of the detection coil 5A when generating the amount of heat required to raise the surface temperature of the center of the fixing film 1 to a predetermined target temperature is V1.

[0042] As can be seen from the graph in FIG. 7(a), the power supplied to the entire excitation coil 3 when the detection voltage of the detection coil 5A becomes V1 increases as the drive frequency of the high-frequency inverter 16 increases. That is, the power P2 at which the detection voltage of the detection coil 5A becomes V1 when the drive frequency is 75 kHz is greater than the power P1 at which the detection voltage of the detection coil 5A becomes V1 when the drive frequency is 60 kHz. Also, the power P3 at which the detection voltage of the detection coil 5A becomes V1 when the drive frequency is 90 kHz is greater than the power P2 at which the detection voltage of the detection coil 5A becomes V1 when the drive frequency is 75 kHz (P1 <P2<P3)。

[0043] This is because, as explained in FIG. 6, the temperature distribution in the longitudinal direction of the fixing film 1 changes depending on the drive frequency of the high-frequency inverter 16, and the higher the drive frequency, the higher the temperature at both ends of the fixing film 1. That is, the higher the drive frequency, the higher the surface temperature at the ends when the surface temperature at the center of the fixing film 1 reaches the target temperature. Furthermore, since a large amount of energy is required to raise the temperature at these ends of the fixing film 1, even if the surface temperature at the center is always the target temperature, the higher the drive frequency, the greater the power supplied to the exciting coil 3. Conversely, for the same supplied power, the lower the drive frequency of the inverter 16, the higher the surface temperature at the center of the fixing film 1.

[0044] (Power control to excitation coil) However, if an abnormality occurs in the temperature detection element 9 and the control means 31 erroneously detects a temperature lower than the actual temperature, and power control is performed to continue raising the temperature, there is a risk that the temperature of the fixing film 1 will exceed the target temperature and reach an abnormal temperature. For this reason, for example, a method can be considered in which the power supplied to the exciting coil 3 is limited to a predetermined value or less so as to prevent excessive power from being supplied to the fixing film 1.

[0045] Specifically, in FIG. 7(a), when the excitation coil 3 operates normally, the range of power usage is P1 to P3, and when an abnormality occurs in the temperature detection element 9, the limit power can be set to P4, which is larger than P1 to P3.

[0046] However, as described above, even if the same limited power P4 is supplied to the fixing film 1, the lower the driving frequency of the inverter 16, the greater the amount of heat generated in the center of the fixing film 1. This is because the lower the driving frequency of the high-frequency inverter 16, the less power is consumed at the ends of the fixing film 1, and the power of the exciting coil 3 is concentrated and supplied to the center of the fixing film 1, resulting in a greater amount of heat generated in the center of the fixing film 1.

[0047] For example, in the case of FIG. 7(a), when the drive frequency is 90 kHz, the detected voltage of the detection coil 5A is V2. However, when the drive frequency is 75 kHz, the detected voltage of the detection coil 5A is V3, which is greater than V2. Also, when the drive frequency is 60 kHz, the detected voltage of the detection coil 5A is V4, which is greater than V3. The induced electromotive force (detected voltage) Vd generated in the detection coil 5A is proportional to the induced electromotive force Vf generated in the heat generating layer 1a at the central portion of the fixing film 1, that is, the heat generation amount at the central portion of the fixing film 1. Therefore, the greater the detected voltage, the greater the heat generation amount at the central portion of the fixing film 1.

[0048] Thus, even if the power supplied to the excitation coil 3 is limited to a predetermined value P4, the heat generation amount (temperature) at the central portion of the fixing film 1 will vary depending on the drive frequency of the inverter 16. And, for example, consider the case where the detected voltage detected by the detection coil 5A at the maximum allowable heat generation amount when the temperature of the fixing film 1 is within a temperature range that does not damage the device is V3. In this case, when the drive frequencies are 90 kHz and 75 kHz, the detected voltage will be below V3. However, when the drive frequency is 60 kHz, the detected voltage will be V4, which is a voltage value greater than V3, that is, the heat generation amount at the central portion of the fixing film 1 will exceed the maximum allowable heat generation amount.

[0049] On the other hand, for example, when the drive frequency of the high-frequency inverter 16 is 60 kHz and the heat generation amount at the central portion of the fixing film 1 is to be not more than the maximum allowable heat generation amount, that is, when trying to make the detected voltage of the detection coil 5A not more than V3, it is necessary to make the power supplied to the excitation coil 3 not more than P5. However, as can be seen from FIG. 7(a), this supply power P5 is smaller than the normal supply power P3 when the drive frequency of the high-frequency inverter 16 is 90 kHz (P5 < P3). Therefore, in this case, it is necessary to narrow the power consumption range of the excitation coil 3 during normal operation, and due to insufficient required power, a decrease in the print speed or the like will occur.

[0050] It is also possible that an abnormality occurs not in the temperature detection element 9 but in the frequency control means 45, causing the high-frequency inverter 16 to be driven at a frequency higher than the set driving frequency. In this case, when the driving frequency of the high-frequency inverter 16 becomes high, the temperature of the fixing film 1 at its end becomes high.

[0051] 7(b) is a graph showing the relationship between the power supplied to the excitation coil 3 and the detected voltages of the detection coils 5B and 5C when the drive frequency of the high-frequency inverter 16 is changed. For example, in the example of FIG. 7(b), when the high-frequency inverter 16 is driven within the normal drive frequency range, the highest drive frequency is 90 kHz.

[0052] When the drive frequency is 90 kHz, if the power supplied to the exciting coil 3 is limited to a predetermined value P4, the detected voltage value of the detection coils 5B and 5C at that time is V2. As can be seen from the temperature distribution in Figure 6, when the drive frequency is 90 kHz, the relationship between the amount of heat generated at the end of the fixing film 1 and the detected voltage can be considered to be the same as that of the detection coil 5A. Furthermore, since the detected voltage value V2 is lower than the detected voltage value V3, it can be seen that when operating at a drive frequency of 90 kHz or less, if the power supplied to the exciting coil 3 is limited to P4 or less, the amount of heat generated at the end of the fixing film 1 will not exceed the maximum allowable heat value.

[0053] However, if the high-frequency inverter 16 enters a runaway state by being driven at a frequency equal to or higher than the maximum driving frequency, for example, at 120 kHz as shown by the dotted line in FIG. 7B, the detected voltage of the detection coils 5B and 5C will be greater than V3. More specifically, if the high-frequency inverter 16 is driven at a driving frequency of 120 kHz and the power supplied to the excitation coil 3 is P4, the voltage detected by the detection coils 5B and 5C will be V5, which is greater than V3. Therefore, if the high-frequency inverter 16 is driven at a frequency equal to or higher than the maximum driving frequency, even if the power supplied to the excitation coil 3 is limited to a predetermined value, the edge of the fixing film 1 may be excessively heated depending on the actual driving frequency.

[0054] (Limitation of heat generation from the fixing film due to the induced electromotive force of the detection coil) Therefore, in this embodiment, to solve these problems, the induced electromotive force Vd generated in the detection coils 5A, 5B, and 5C arranged at each longitudinal portion of the fixing film 1 is limited to a predetermined value or less, rather than the power supplied to the excitation coil 3. By limiting the induced electromotive force Vd detected by the detection coils 5A, 5B, and 5C to, for example, a predetermined value V3 or less, the heat generation amount at each longitudinal portion of the fixing film 1 can be limited to the maximum allowable heat generation amount or less, regardless of the driving frequency of the high-frequency inverter 16.

[0055] 8, the control for limiting the amount of heat generated by the fixing film 1 in this embodiment will be described together with the drive sequence of the fixing device 130. The control process shown in this flowchart is executed in accordance with a program stored in advance in a storage unit (for example, ROM 32a) of the CPU 32.

[0056] When heating the fixing film 1, the CPU 32 first detects the current temperature based on the detection result of the temperature detection element 9 (step S101 in FIG. 8). Next, the CPU 32 determines the input power based on the difference between the target temperature of the fixing film 1 and the current temperature (S102), and drives the high-frequency inverter 16 to supply power to the exciting coil 3 (S103). At the same time, the induced currents flowing through the detection coils 5A, 5B, and 5C are converted into voltages by the IV conversion circuits 47a, 47b, and 47c, and detection of the induced electromotive forces Vd generated in the detection coils 5A, 5B, and 5C is started (S104).

[0057] Then, the CPU 32 functions as the detection result comparison unit 49 and determines whether the induced electromotive force Vd of each detection coil converted by the IV conversion circuits 47a, 47b, 47c exceeds a predetermined voltage V3 (S105). If the induced electromotive force Vd of each detection coil 5A, 5B, 5C exceeds the predetermined voltage V3 (Yes in S105), it determines that abnormal heat generation is occurring in the fixing film 1 (S106) and prohibits the supply of power to the fixing device (S107). In addition, it urgently stops the image forming operation and notifies the user of the abnormality via an operation panel (not shown) or the like (S108).

[0058] If the induced electromotive force Vd of each of the detection coils 5A, 5B, and 5C is equal to or less than the predetermined voltage V3, the state is determined to be normal (No in S105).Then, the image formation operation is continued and the induced electromotive forces Vd of each of the detection coils 5A, 5B, and 5C converted by the IV conversion circuits 47a, 47b, and 47c are continuously monitored by the CPU 32.

[0059] On the other hand, after step S104, the CPU 32 simultaneously resets the timer T to 0 and starts counting up the timer (S109). Then, it waits for a predetermined time Ttemp to elapse until it is time to detect the current temperature to determine the next power (No in S110). When the time Ttemp has elapsed (Yes in S110), the next power to be supplied is determined from the detected temperature information, as in the previous case (S111 to S112). Then, if the supply of fixing power continues (No in S113), the process returns to step S105 and step S109, and the above-described control is repeated.

[0060] Furthermore, if it is determined in step S113 that power supply to the fixing device 113 is to be terminated (S113 Yes), the control of power supply to the fixing device is terminated. Note that the termination of power supply to the fixing device 113 means that supply of fixing power is prohibited when printing is completed in the image forming apparatus 100 or due to an emergency stop factor such as a jam or an error.

[0061] In the above-described film heat generation amount limiting control, when it is determined that the fixing film 1 is generating abnormal heat, power supply to the fixing device is prohibited. However, the drive duty ratio of the high-frequency inverter 16 may be limited to limit the supplied power so that the induced electromotive force is equal to or less than a predetermined voltage V3. For example, the CPU 32 may control the high-frequency inverter 16 by reducing the drive duty ratio. Furthermore, taking into consideration noise, instantaneous power supply at startup, and the like, it may be determined that an abnormal amount of heat is being supplied to the fixing film 1 only when the induced electromotive force Vd exceeds the predetermined voltage V3 for a certain period of time.

[0062] Furthermore, while the detection coils 5B and 5C at the ends of the fixing film 1 are positioned 15 mm from the left and right ends of the fixing film 1 in the above embodiment, they may be positioned at different distances from the ends, for example, 15 mm from the left end of the fixing film 1 and 30 mm from the right end of the fixing film 1. Additionally, the number of turns of the detection coils 5A, 5B, and 5C may be different. Similarly, the predetermined value V3 for limiting the amount of heat generated by the fixing film 1 may be set to a different predetermined value for each induced electromotive force detected by each detection coil.

[0063] In the above-described embodiment, an abnormality is determined when the induced electromotive force Vd of any one of the detection coils 5A, 5B, and 5C exceeds a predetermined voltage V3. However, for example, a detection coil may be provided at least at one position along the longitudinal direction of the magnetic core 2, and when the detection voltage of at least one of the detection coils exceeds a predetermined voltage, it may be determined that abnormal heat is occurring in the fixing film 1.

[0064] As described above, in this embodiment, the detection coils 5A, 5B, and 5C are provided to detect the heat generation amount at each longitudinal portion of the fixing film 1, and the induced electromotive force Vd detected by each of the detection coils 5A, 5B, and 5C is limited to a predetermined value or less. Specifically, when the induced electromotive force Vd detected by the detection coils 5A, 5B, and 5C exceeds a predetermined value, the CPU 32 stops or reduces the power supplied from the inverter 16 to the exciting coil 3. As a result, when a current runaway state occurs, the heat generation amount at each longitudinal portion of the fixing film 1 can be limited to a maximum allowable heat generation amount or less without changing the power range used by the exciting coil 3 during normal operation, regardless of the driving frequency of the high-frequency inverter 16.

[0065] (Longitudinal temperature distribution adjustment) In this embodiment, the induced electromotive force Vd generated in the detection coils 5A, 5B, and 5C is used not only to limit and control the amount of heat generated by the fixing film, but also to adjust the longitudinal temperature distribution of the fixing film 1. The longitudinal temperature distribution of the fixing film 1 can be adjusted by changing the drive frequency of the high-frequency inverter 16, as described in FIG.

[0066] That is, the fixing device 130 is set in advance so that the temperature of each longitudinal portion of the fixing film 1 will be a desired temperature for each drive frequency of the high-frequency inverter 16, based on the number of turns and winding interval of the exciting coil 3, the relative magnetic permeability of the magnetic core 2, and other characteristics. However, there is a risk that the induced electromotive force generated in each longitudinal portion of the fixing film 1 will differ from the predetermined value due to the winding position tolerance during assembly of the exciting coil 3 and variations in characteristics such as the relative magnetic permeability of the magnetic core 2. In this case, the longitudinal temperature distribution of the fixing film 1 will differ from the desired temperature distribution.

[0067] Therefore, in this embodiment, it is determined from the induced electromotive force Vd generated in the detection coils 5A, 5B, and 5C whether the induced electromotive forces generated in the center and left and right ends of the fixing film 1 are desired values ​​corresponding to the drive frequencies of the high-frequency inverter 16. If the induced electromotive force Vd generated in the detection coils 5A, 5B, and 5C is not the desired value, the drive frequency of the high-frequency inverter 16 is corrected so that it becomes the desired value, thereby adjusting the longitudinal temperature distribution of the fixing film 1 to the desired distribution.

[0068] Specifically, the control for correcting the drive frequency of high-frequency inverter 16 in this modification will be described using the flowchart of Fig. 9. In Fig. 9, the processes of steps S101 to S113 are the same as the processes of S101 to S113 in Fig. 8, and therefore their description will be omitted. After determining the next energization input power in step S112, CPU 32 detects the induced electromotive forces Vd in each of detection coils 5A, 5B, and 5C converted by IV conversion circuits 47a, 47b, and 47c (S114). Based on the induced electromotive forces in each of detection coils 5A, 5B, and 5C detected by CPU 32, a frequency correction determination is started to determine whether or not to change the drive frequency of high-frequency inverter 16 (S115).

[0069] The frequency correction determination process will be described with reference to FIG. 10. As described with reference to FIG. 6, when the driving frequency of the high-frequency inverter 16 is 90 kHz, the target temperatures of the fixing film 1 at which the center detection coil 5A and the edge detection coils 5B and 5C are located are the same. Therefore, the heat generation amounts, i.e., the induced electromotive forces, of the center detection coil 5A and the edge detection coils 5B and 5C are set to be approximately equal. As the driving frequency of the high-frequency inverter 16 decreases, the target temperature of the fixing film 1 at the edges decreases, and the induced electromotive forces of the edge detection coils 5B and 5C also decrease. Therefore, the lower the driving frequency of the high-frequency inverter 16, the greater the difference in induced electromotive forces between the center detection coil 5A and the edge detection coils 5B and 5C. In this embodiment, the CPU 32 presets a predetermined value for the expected difference in induced electromotive forces between the center detection coil 5A and the edge detection coils 5B and 5C for each driving frequency of the high-frequency inverter 16.

[0070] Then, in the frequency correction determination process, the CPU 32 compares the estimated value of the induced electromotive force difference corresponding to the current setting of the drive frequency of the high-frequency inverter 16 with the difference between the actual induced electromotive forces (S116). That is, the CPU 32 compares the difference between the actual induced electromotive forces of the center detection coil 5A and the edge detection coils 5B and 5C detected in S114 with the predetermined estimated value. If the difference between the induced electromotive forces is approximately equal to the predetermined estimated value (if the difference is within a predetermined range), it determines that the amount of heat generated in the fixing film 1 is as expected based on the induced electromotive forces, and does not change the drive frequency of the high-frequency inverter 16 (S117), and ends the frequency correction determination process.

[0071] If the difference in induced electromotive force is different from a predetermined assumed value, it is determined whether it is greater than the predetermined assumed value (S118). If it is greater than the predetermined assumed value, it is determined that the temperature difference between the center and end of the fixing film 1 is greater than assumed, and the driving frequency of the high-frequency inverter 16 is increased to increase the amount of heat generated at the end, thereby reducing the temperature difference (S119). Thereafter, the frequency correction determination process is terminated.

[0072] If the temperature difference is smaller than the predetermined estimated value, it is determined that the temperature difference between the center and end portions of the fixing film 1 is smaller than expected, and the drive frequency of the high-frequency inverter 16 is lowered to reduce the amount of heat generated at the end portions and increase the temperature difference (S120). Thereafter, the frequency correction determination process is terminated. The frequency may be changed by a predetermined fixed amount, or the change amount may be determined based on the difference between the estimated induced electromotive force of the end detection coils 5B and 5C and the actually detected induced electromotive force.

[0073] As described above, by correcting the drive frequency based on the induced electromotive force Vd generated in the detection coils 5A, 5B, and 5C, the desired longitudinal temperature distribution of the fixing film 1 can be achieved even if there are winding position tolerances during assembly of the excitation coil 3 or variations in the characteristics of the magnetic core 2. Note that the above estimated value is typically calculated by adding a tolerance to the difference between the induced electromotive force at the center and the induced electromotive force at the end portions, which is experimentally or theoretically determined when the high-frequency inverter 16 is driven at a predetermined frequency and a predetermined drive duty ratio. The drive frequency can also be corrected based on the induced electromotive force of each of the detection coils 5A, 5B, and 5C individually. However, by correcting the drive frequency based on the difference between the induced electromotive forces of the detection coils 5A, 5B, and 5C at the center and the end portions, as in this embodiment, the drive frequency can be corrected more effectively. For example, when printing continuously, the induced electromotive force decreases over time as the components around the fixing film gradually warm up. However, if the induced electromotive force of the detection coils 5A, 5B, and 5C is measured alone, it is difficult to determine whether the change in induced electromotive force is simply due to a temperature rise or whether the drive frequency needs to be adjusted. On the other hand, as in this embodiment, if the difference in induced electromotive force between the center and end detection coils 5A, 5B, and 5C is measured, the difference between the center and end at that time (i.e., the difference in temperature distribution along the length of the fixing film) can be effectively detected. Then, by adjusting the drive frequency based on the difference in induced electromotive force, it becomes possible to adjust the drive frequency to more evenly distribute heat.

[0074] In the above-described embodiment, the driving frequency of the high-frequency inverter 16 is corrected based on the induced electromotive force detected by the detection coils 5A, 5B, and 5C. However, for example, temperature detection elements may be provided at the center and edge of the fixing film 1, and the driving frequency may be corrected if the temperature difference between the center and edge of the fixing film 1 detected by these temperature detection elements is greater than a predetermined difference. In other words, the detection coils 5A, 5B, and 5C and the temperature detection elements function as heat generation detectors that detect the heat generation amounts at the center and edge of the fixing film 1. The difference between the heat generation amounts at the center and edge of the fixing film 1 is calculated based on the detection results of a first heat generation detector that detects the heat generation amount at the center of the fixing film 1 and a second heat generation detector that detects the heat generation amount at the edge. The driving frequency of the high-frequency inverter 16 is corrected based on the fact that this difference in heat generation amount is greater than an expected value.

[0075] <Second embodiment> Next, a second embodiment will be described. Note that in the second embodiment, the film heat generation amount limiting control is different from that in the first embodiment. Therefore, in the following explanation, only the parts that are different from the first embodiment will be explained, and explanations of similar configurations will be omitted and the same reference numerals as in the first embodiment will be used.

[0076] 11 is a perspective view of the fixing film 1 according to the second embodiment and a diagram showing the circuit configuration to which the fixing film is connected. In this embodiment, the fixing device 130 is provided with temperature detection elements 201 and 202 at the left and right ends of the fixing film 1 in addition to the temperature detection element 9 disposed in the center of the fixing film 1. These temperature detection elements 201 and 202 at the left and right ends are located 15 mm from the left and right ends of the fixing film 1, respectively, and are disposed so as to contact the inner surface of the fixing film 1 toward the upstream side of the conveyance path of the recording material P, similar to the temperature detection element 9.

[0077] Furthermore, the temperature detection elements 9, 201, and 203 are each connected to fixing temperature control means 44 within the CPU 32. Engine control means 43 of the CPU 32 calculates the power and drive frequency to be applied to the fixing film 1 based on signals from the temperature detection elements 9, 201, and 203 and signals from IV conversion circuits 47a, 47b, and 47c that convert the induced current generated in each detection coil into a voltage. Then, the engine control means 43 drives the high-frequency inverter 16 at the calculated power and frequency via control signals output from frequency control means 45 and power control means 46, thereby maintaining and adjusting the surface temperature of each portion of the fixing film 1 in the longitudinal direction to a predetermined target temperature.

[0078] (Temperature rise abnormality judgment) As described above, in this embodiment, the temperature detection elements 9, 201, and 202 are arranged in positions corresponding to the detection coils 5A, 5B, and 5C in the longitudinal direction. This makes it possible to detect the temperature and induced electromotive force of the fixing film 1 at each position (center and both ends) of the fixing film 1 where the detection coils 5A, 5B, and 5C and the temperature detection elements 9, 201, and 202 are arranged. In this embodiment, by comparing the detected temperature and induced electromotive force at each portion of the fixing film 1, it is possible to determine whether the amount of temperature rise is abnormal.

[0079] For example, Fig. 12 is a graph showing the relationship between the temperature of the fixing film 1 and the detected voltages of the detection coils 5A, 5B, and 5C when the high-frequency inverter 16 starts to drive at a drive frequency of 60 kHz. Note that a drive frequency of 60 kHz is used, for example, when fixing an image on a small-sized recording material, and in the example of Fig. 12, the target temperature at the center of the film 1 is 200°C, and the target temperature at the end of the fixing film 1 is 100°C.

[0080] When the high-frequency inverter 16 starts driving (time t0 in FIG. 12) and power is supplied to the exciting coil 3, the temperature of the fixing film 1 starts to rise toward the target temperature. When the driving frequency is 60 kHz, the amount of heat generated at the ends of the fixing film 1 is smaller than the amount of heat generated at the center of the fixing film 1. Therefore, for example, between times t0 and t3, the temperature of the fixing film 1 rises more slowly at the ends than at the center. Also, the induced electromotive force Vd generated at the center of the fixing film 1 is smaller than the induced electromotive force generated at the ends of the fixing film 1.

[0081] 12, when the temperature of the central portion of the fixing film 1 detected by the temperature detection element 9 reaches 180° C., the CPU 32 adjusts the power supplied to the exciting coil 3 to decrease in order to prevent an overshoot in the temperature of the fixing film 1. Furthermore, as the supplied power decreases, the induced electromotive force Vd detected by the detection coils 5A, 5B, and 5C also decreases accordingly.

[0082] Then, at time t4, when the temperature of the central portion of the fixing film 1 detected by the temperature detection element 9 reaches the target temperature of 200°C, the CPU 32 drives the high-frequency inverter 16 with the input power required to maintain the temperature of the central portion of the fixing film 1 at the target temperature. At this time, the induced electromotive force Vd detected by the detection coils 5A, 5B, and 5C also has a magnitude corresponding to the input power.

[0083] Now, consider a case where an abnormality occurs in the temperature detection element 201 at the left end of the fixing film 1, making it unable to correctly detect the inner surface temperature at the left end of the fixing film 1. In this case, as shown in Fig. 12, the control means 31 erroneously detects the inner surface temperature at the left end of the fixing film 1 as a value lower than the actual temperature. On the other hand, the induced electromotive force Vd detected by the detection coil 5B at the left end does not depend on the state of the temperature detection element 201, and has a magnitude proportional to the drive frequency and input power of the high-frequency inverter 16.

[0084] As described in the first embodiment, even if the amount of electric power input to the excitation coil 3 is the same, the amount of temperature rise at each longitudinal portion of the fixing film 1 varies depending on the drive frequency of the high-frequency inverter 16. For this reason, the amount of temperature rise at each longitudinal portion of the fixing film 1 cannot be calculated from the amount of electric power. However, if the induced electromotive force Vd detected by each of the detection coils 5A, 5B, and 5C is known, the amount of temperature rise at each longitudinal portion of the fixing film 1 can be calculated.

[0085] Therefore, the amount of temperature rise is calculated from the amount of heat generated in each longitudinal portion of the fixing film 1 due to the induced electromotive force Vd detected by each of the detection coils 5A, 5B, and 5C between time t1 and time t2, and is compared with the amount of temperature rise detected by the temperature detection elements 9, 201, and 202. This allows the CPU 32 to determine whether the amount of temperature rise is abnormal with respect to the induced electromotive force Vd.

[0086] The heat quantity restriction control and frequency correction control in this embodiment will be described using the flowcharts of Figures 13 and 14. In Figure 13, the processes of steps S101 to S115 are the same as the processes of steps S101 to S115 in Figure 9, and so a description thereof will be omitted. After step S104, the CPU 32 starts a temperature rise amount abnormality determination process (S201) in parallel with steps S105 and S109.

[0087] Next, the temperature rise amount abnormality determination process will be described with reference to Figure 14. When the temperature rise amount abnormality determination process is started, the CPU 32 clears the timer T2 to 0 (S202) and stores the temperatures detected by the temperature detection elements 9, 201, and 202 at that time and the induced electromotive forces detected by the detection coils 5A, 5B, and 5C in the CPU 32 (S203). Thereafter, the timer starts counting up (S204) and waits for the passage of a predetermined time Ttemp2 (No in S205). Then, when the predetermined time Ttemp2 has passed (Yes in S205), the temperatures detected by the temperature detection elements 9, 201, and 202 at that time and the induced electromotive forces detected by the detection coils 5A, 5B, and 5C are again stored in the CPU 32 (S206).

[0088] The CPU 32 calculates the temperature rise and average induced electromotive force at each longitudinal portion of the fixing film 1 over a predetermined time period from the temperatures detected by the temperature detection elements 9, 201, and 202 at T2=0 and Ttemp2 and the induced electromotive forces detected by the detection coils 5A, 5B, and 5C (S207).The CPU 32 then calculates the estimated temperature rise at each longitudinal portion of the fixing film 1 from the average induced electromotive force between T2=0 and Ttemp2, and compares this with the actual temperature rise of each temperature detection element between T2=0 and Ttemp2 (S208).

[0089] If the temperature rise amount is different from the expected amount, for example, if it is too large or too small compared to the expected amount, the CPU 32 determines that the temperature rise amount of the corresponding portion in the longitudinal direction of the fixing film 1 is abnormal (S209). Then, it prohibits power supply to the fixing device 130 (S210), urgently stops the image forming operation, and notifies the user of the malfunction via an operation panel (not shown) or the like (S211). If the temperature rise amount is the expected amount (No in S208), it determines that the state is normal, ends the temperature rise amount abnormality determination process, and continues the image forming operation.

[0090] As described above, in this embodiment, by comparing the induced electromotive force detected by each detection coil over a predetermined period with the temperature rise of each portion of the fixing film 1, it is possible to determine whether the temperature rise of the fixing film 1 is abnormal relative to the induced electromotive force. That is, if the correspondence between the temperature of the fixing film 1 detected by the temperature detection elements 9, 201, and 202 and the induced electromotive force of the detection coils 5A, 5B, and 5C deviates from the predetermined correspondence, the CPU 32 stops or reduces the supply of power to the excitation coil 3. This makes it possible to determine whether the temperature detected by each temperature detection element is abnormal, regardless of the control drive frequency of the high-frequency inverter 16. Furthermore, because it is possible to make a determination even when the temperature detected by each temperature detection element is lower than the target temperature, it is possible to determine whether the target temperature is reached after starting the printing operation and driving the high-frequency inverter 16, and then to issue a fault notification.

[0091] The inventions described in the above-mentioned embodiments may be combined in any manner. Furthermore, the heating mechanism of the fixing device 130 as an image heating device can be used not only for fixing an image onto a recording material, but also for heating the image fixed onto the recording material to gloss it, or for correcting curl of the recording material on which an image has been formed.

[0092] [Configuration 1] an inverter that supplies an 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, thereby electromagnetically inducing heating of the rotor; and a detection coil that is electromagnetically induced by the alternating magnetic flux and generates an induced electromotive force, wherein the control means is capable of changing a drive frequency of the inverter, and when the induced electromotive force exceeds a predetermined value, the control means stops or reduces the power supplied from the inverter to the excitation coil.

[0093] [Configuration 2] 2. The image heating device according to claim 1, wherein the control means stops driving the inverter or reduces a driving duty ratio of the inverter when the induced electromotive force exceeds a predetermined value.

[0094] [Configuration 3] 3. An image heating device according to configuration 1 or 2, further comprising a temperature detection means for detecting the temperature of the rotating body, wherein the control means compares the temperature of the rotating body detected by the temperature detection means during a predetermined period with the induced electromotive force detected by the detection coil during the predetermined period, and when the correspondence between the temperature of the rotating body and the induced electromotive force deviates from a predetermined relationship, stops or reduces the supply of power from the inverter to the excitation coil.

[0095] [Configuration 4] The image heating device according to configuration 3, characterized in that the control means calculates the amount of temperature rise of the rotating body based on the induced electromotive force detected by the detection coil during the predetermined period, and stops or reduces the supply of power from the inverter to the excitation coil when the amount of temperature rise calculated based on the induced electromotive force differs by a predetermined value or more from the amount of temperature rise of the rotating body calculated based on the detection result of the temperature detection means during the predetermined period.

[0096] [Configuration 5] 5. The image heating device according to any one of configurations 1 to 4, wherein the detection coil is provided at a central portion in the direction of the rotation axis of the rotating body.

[0097] [Configuration 6] The image heating device according to any one of configurations 1 to 5, characterized in that the control means corrects the setting of the drive frequency of the inverter to be higher based on the fact that the difference between the heat generation amount at the center of the rotating body and the heat generation amount at the end of the rotating body when the inverter is driven at a predetermined frequency and drive duty ratio is greater than a predetermined difference amount.

[0098] [Configuration 7] the control means corrects the setting of the drive frequency of the inverter to be lower based on the fact that the difference between the heat generation amount at the center of the rotating body and the heat generation amount at the end of the rotating body when the inverter is driven at a predetermined frequency and drive duty ratio is smaller than a predetermined difference amount. 7. The image heating apparatus according to any one of configurations 1 to 6.

[0099] [Configuration 8] 8. The image heating apparatus according to any one of configurations 1 to 7, wherein the detection coil is wound around the magnetic core material with a gap equal to the winding interval of the excitation coil.

[0100] [Configuration 9] 8. The image heating apparatus according to any one of configurations 1 to 7, wherein the detection coil is wound around the excitation coil so as to overlap with the excitation coil in the circumferential direction.

[0101] [Configuration 10] an inverter that supplies an 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 rotor; a detection coil that generates an induced electromotive force when electromagnetically induced by the alternating magnetic flux; and a temperature detection means that detects the temperature of the rotor; wherein the control means compares the temperature of the rotor detected by the temperature detection means over a predetermined period with the induced electromotive force detected by the detection coil over the predetermined period, and if the correspondence between the temperature of the rotor and the induced electromotive force deviates from a predetermined relationship, the control means stops or reduces the supply of power from the inverter to the excitation coil.

[0102] [Configuration 11] an inverter that supplies an 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 rotor; a first heat generation amount detection unit that detects the amount of heat generated in a central portion of the rotor; and a second heat generation amount detection unit that detects the amount of heat generated in an end portion of the rotor, wherein the control means corrects the drive frequency of the inverter to be higher when the inverter is driven at a predetermined frequency and drive duty ratio and the difference between the amount of heat generated in the central portion of the rotor calculated based on the detection result of the first heat generation amount detection unit and the amount of heat generated in the end portion of the rotor calculated based on the detection result of the second heat generation amount detection unit is greater than a predetermined difference.

[0103] [Configuration 12] the control means corrects the setting of the drive frequency of the inverter to be lower based on the fact that the difference between the heat generation amount at the center of the rotating body and the heat generation amount at the end of the rotating body when the inverter is driven at a predetermined frequency and drive duty ratio is smaller than a predetermined difference amount. 12. The image heating apparatus according to claim 11,

[0104] [Configuration 13] an image forming means for forming an image on a recording material; an image heating apparatus according to any one of configurations 1 to 12; The image forming apparatus is characterized in that the image heating device is a fixing device that heats and pressurizes the image formed on the recording material by the image forming means to fix it to the recording material. [Explanation of symbols]

[0105] 1: Rotating body (fixing film) / 2: Magnetic core material (magnetic core) / 3: Magnetic coil / 5A to 5C: Detection coil (heat generation amount detection unit) / 6: Inverter (high frequency inverter) / 9, 201, 203: Temperature detection means (temperature detection element, heat generation amount detection unit) / 31: Control means / 130: Image heating device (fixing device)

Claims

1. An image heating device that heats an image formed on a recording material, a cylindrical rotor having electrical conductivity; a magnetic core material disposed inside the rotor and forming an open magnetic path in the axial direction of the rotor; an excitation coil wound around the magnetic core material along the axial direction of the rotor; an inverter that supplies an alternating current to the excitation coil; a control means for controlling the inverter to pass an alternating current through the excitation coil, thereby generating an alternating magnetic flux in the magnetic core material and electromagnetically inductively heating the rotor; a detection coil that is electromagnetically induced by the alternating magnetic flux to generate an induced electromotive force; the control means is capable of changing the drive frequency of the inverter, the control means stops or reduces the power supplied from the inverter to the excitation coil when the induced electromotive force exceeds a predetermined value. An image heating device characterized by:

2. When the induced electromotive force becomes larger than a predetermined value, the control means stops driving the inverter or reduces the drive duty ratio of the inverter.

2. The image heating apparatus according to claim 1, wherein the heating element is a heat-generating element.

3. temperature detection means for detecting the temperature of the rotating body; the control means compares the temperature of the rotating body detected by the temperature detection means during a predetermined period with the induced electromotive force detected by the detection coil during the predetermined period, and when the correspondence relationship between the temperature of the rotating body and the induced electromotive force deviates from a predetermined relationship, stops or reduces the supply of power from the inverter to the excitation coil.

2. The image heating apparatus according to claim 1, wherein the heating element is a heat-generating element.

4. the control means calculates a temperature rise of the rotating body based on the induced electromotive force detected by the detection coil during the predetermined period, and when the temperature rise calculated based on the induced electromotive force differs by a predetermined value or more from the temperature rise of the rotating body calculated based on the detection result of the temperature detection means during the predetermined period, stops or reduces the supply of power from the inverter to the excitation coil.

4. The image heating apparatus according to claim 3, wherein the heating medium is a heat-sensitive material.

5. the detection coil is provided at a central portion in a direction of a rotation axis of the rotating body; 2. The image heating apparatus according to claim 1, wherein the heating element is a heat-generating element.

6. the control means corrects the setting of the drive frequency of the inverter to be higher based on the fact that the difference between the heat generation amount at the center of the rotating body and the heat generation amount at the end of the rotating body when the inverter is driven at a predetermined frequency and drive duty ratio is larger than a predetermined difference amount.

2. The image heating apparatus according to claim 1, wherein the heating element is a heat-generating element.

7. the control means corrects the setting of the drive frequency of the inverter to be lower based on the fact that the difference between the heat generation amount at the center of the rotating body and the heat generation amount at the end of the rotating body when the inverter is driven at a predetermined frequency and drive duty ratio is smaller than a predetermined difference amount.

2. The image heating apparatus according to claim 1, wherein the heating element is a heat-generating element.

8. The detection coil is wound around the magnetic core material with a gap equal to the winding interval of the excitation coil.

2. The image heating apparatus according to claim 1, wherein the heating element is a heat-generating element.

9. the detection coil is wound around the excitation coil so as to overlap with the excitation coil in the circumferential direction; 2. The image heating apparatus according to claim 1, wherein the heating element is a heat-generating element.

10. An image heating device that heats an image formed on a recording material, a cylindrical rotor having electrical conductivity; a magnetic core material disposed inside the rotor and forming an open magnetic path in the axial direction of the rotor; an excitation coil wound around the magnetic core material along the axial direction of the rotor; an inverter that supplies an alternating current to the excitation coil; a control means for controlling the inverter to pass an alternating current through the excitation coil, thereby generating an alternating magnetic flux in the magnetic core material and electromagnetically inductively heating the rotor; a detection coil that is electromagnetically induced by the alternating magnetic flux to generate an induced electromotive force; a temperature detection means for detecting the temperature of the rotating body, the control means compares the temperature of the rotating body detected by the temperature detection means during a predetermined period with the induced electromotive force detected by the detection coil during the predetermined period, and when the correspondence relationship between the temperature of the rotating body and the induced electromotive force deviates from a predetermined relationship, stops or reduces the supply of power from the inverter to the excitation coil. An image heating device characterized by:

11. An image heating device that heats an image formed on a recording material, a cylindrical rotor having electrical conductivity; a magnetic core material disposed inside the rotor and forming an open magnetic path in the axial direction of the rotor; an excitation coil wound around the magnetic core material along the axial direction of the rotor; an inverter that supplies an alternating current to the excitation coil; a control means for controlling the inverter to pass an alternating current through the excitation coil, thereby generating an alternating magnetic flux in the magnetic core material and electromagnetically inductively heating the rotor; a first heat generation amount detection unit that detects a heat generation amount at a central portion of the rotating body; a second heat generation amount detection unit that detects the heat generation amount at the end of the rotating body, the control means corrects the setting of the drive frequency of the inverter to be higher when a difference between the heat generation amount at the center of the rotating body calculated based on the detection result of the first heat generation amount detection unit and the heat generation amount at the end of the rotating body calculated based on the detection result of the second heat generation amount detection unit when the inverter is driven at a predetermined frequency and drive duty ratio is larger than a predetermined difference amount. An image heating device characterized by:

12. the control means corrects the setting of the drive frequency of the inverter to be lower based on the fact that the difference between the heat generation amount at the center of the rotating body and the heat generation amount at the end of the rotating body when the inverter is driven at a predetermined frequency and drive duty ratio is smaller than a predetermined difference amount.

12. The image heating apparatus according to claim 11.

13. an image forming means for forming an image on a recording material; and an image heating apparatus according to any one of claims 1 to 12, the image heating device is a fixing device that heats and pressurizes the image formed on the recording material by the image forming means to fix it to the recording material; An image forming apparatus characterized by:

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