Image heating apparatus and image forming apparatus

Abstract

This prevents the rotating parts from overheating. [Solution] The image heating device comprises a rotating body 121, a magnetic core material 122, an excitation coil 123, an inverter 191, a control means 140 that controls the inverter 191 to supply an alternating current to the excitation coil 123, generating an alternating magnetic flux in the magnetic core material 122 to electromagnetically induce heating of the rotating body 121, and a power cutoff means 150 that, when the power supplied from the inverter 191 to the excitation coil 123 exceeds a threshold, cuts off the power supply from the inverter 191 to the excitation coil 123 regardless of the control state of the inverter 191 by the control means 140. The control means 140 can change the drive frequency of the inverter 191, and the power cutoff means 150 changes the value of the threshold according to the value of the drive frequency of the inverter 191.

Description

Technical Field

[0001] The present invention relates to an image heating device and an image forming device that heat an image formed on a recording material.

Background Art

[0002] In electrophotographic image forming devices such as printers and copiers, after transferring a toner image corresponding to image data onto a recording material such as recording paper or an OHP sheet, the toner image transferred onto the recording material is heated and pressed by a fixing device as an image heating device to be fixed onto the recording material. Among various types of fixing devices, a configuration using a fixing film provided with a resistive heating layer that generates heat when an electric current flows due to electromagnetic induction over the entire circumference in the circumferential movement direction has been proposed (see Patent Document 1).

[0003] Also, in an induction heating type image heating device, there is disclosed one that changes the heat generation distribution in the longitudinal direction by changing the driving frequency of the current flowing through a coil (see Patent Document 2).

[0004] Here, regardless of the configuration using a fixing film, in a general heat fixing device, if any one of the elements of the heating element, power supply, temperature detection means, and control means does not function properly, the fixing device does not function properly. Furthermore, when an abnormality occurs in the CPU, there is also a risk that the device may malfunction due to overheating. Therefore, in such a fixing device, by providing an overheating safety device (hereinafter referred to as an abnormal heating safety device) that operates in the following abnormal situations, overheating during a power run-away is avoided (see Patent Document 3).

[0005] Specifically, in Patent Document 3 mentioned above, it is proposed to intervene a safety device (thermo protector) such as a temperature fuse or a thermoswitch in the energization circuit of the heating element to cut off the energization to the heating element when overheating occurs due to a power run-away. That is, in the fixing device of Patent Document 3, when the temperature of a thermistor arranged near the heating element becomes a predetermined temperature or higher, the safety device cuts off the energization to the heating element.

Prior Art Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2014-026267 [Patent Document 2] Japanese Patent Publication No. 2016-24348 [Patent Document 3] Japanese Patent Application Publication No. 08-248813 [Overview of the project] [Problems that the invention aims to solve]

[0007] In this way, activating the safety device based on the temperature of the heating element detected by the thermistor can prevent the heating element from overheating. However, there was a risk that the safety device would be delayed if there was a lag between the temperature rise of the thermistor and the actual temperature of the heating element.

[0008] Therefore, the present invention aims to provide an image heating apparatus and an image forming apparatus capable of preventing excessive heat generation of the rotating body. [Means for solving the problem]

[0009] 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; and a power interruption means for interrupting the supply of power from the inverter to the excitation coil when the power supplied from the inverter to the excitation coil exceeds a threshold, regardless of the control state of the inverter by the control means, wherein the control means can change the drive frequency of the inverter, and the power interruption means changes the value of the threshold according to the value of the drive frequency of the inverter. [Effects of the Invention]

[0010] According to the present invention, it is possible to prevent the rotating body from overheating in an image heating device. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic cross-sectional view of an image forming apparatus according to the first embodiment. [Figure 2] This is a schematic cross-sectional view of the fixing device. [Figure 3] This is a schematic front view of the fixing device. [Figure 4] This is a perspective projection view and connection circuit block diagram of the fixing device. [Figure 5] This figure shows the drive frequency and the heat generation distribution in the longitudinal direction of the fixing film surface. [Figure 6] This is the equivalent circuit of the excitation coil and the heat-generating layer of the fixing film. [Figure 7] This is an illustrative diagram illustrating the phenomenon where apparent magnetic permeability is lower at the edges than in the center. [Figure 8] This figure shows the magnetic field lines when a ferrite is placed in a uniform magnetic field H. [Figure 9] It is a diagram for explaining the equivalent inductance of the central part and the end part of the coil. [Figure 10] It is a diagram showing the distribution in the longitudinal direction and its equivalent circuit in terms of apparent magnetic permeability and number of turns. [Figure 11] It is a diagram showing the frequency characteristics of the combined impedance of the central part and the end part. [Figure 12] It is a diagram showing the frequency characteristics of the heat generation amount of the central part and the end part. [Figure 13] It is a diagram showing the relationship between the film surface temperature and the thermistor temperature inscribed in the film. [Figure 14] It is a control flowchart showing the operation of the second safety circuit according to the first embodiment. [Figure 15] It is a diagram showing the limit input power setting table of the second safety circuit according to the first embodiment. [Figure 16] It is a diagram showing a modification example of the limit input power setting table of the second safety circuit. [Figure 17] It is a block diagram for explaining the perspective view of the excitation coil and magnetism and the control of the heating unit according to the second embodiment. [Figure 18] It is a schematic diagram for explaining a configuration for detecting the rotation / stop state of the fixing film. [Figure 19] It is a control flowchart showing the operation of the second safety circuit according to the second embodiment. [Figure 20] It is a diagram showing the limit input power setting table of the second safety circuit according to the second embodiment. [Figure 21] It is a block diagram for explaining the perspective view of the excitation coil and magnetism and the control of the heating unit according to the third embodiment. [Figure 22] It is a control flowchart showing the operation of the second safety circuit according to the third embodiment. [Figure 23] It is a diagram showing the limit input power setting table of the second safety circuit according to the third embodiment.

Embodiments for Carrying Out the Invention

[0012] <First Embodiment> (Outline configuration of an 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 equipped therewith will be described with reference to the drawings. As shown in Figure 1, the image forming apparatus 100 is an electrophotographic laser beam printer and comprises a feed cassette 105, a feed roller 106, a registration roller 107, an image forming means 20, a fixing device 200, and a control unit 31. The feed cassette 105 is a recording material support means for supporting recording material P and is configured to load and store recording material P. The feed roller 106 is a feed means for feeding the recording material P stored in the feed cassette 105 and is configured to separate and feed the recording material P loaded and stored in the feed cassette 105 one by one. The registration roller 107 is a recording material transport means for transporting the recording material fed from the feed cassette 105 toward the image forming means 20 and is configured to transport the recording material P in accordance with the image forming timing of the image forming means 20.

[0013] The image forming means 20 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.

[0014] 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.

[0015] The control unit 31 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.

[0016] Because the image forming apparatus 100 is configured in this way, when a feed start signal is output from the control unit 31 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.

[0017] 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.

[0018] (Configuration of the fixing device) Next, the configuration of the fixing device 200 will be explained with reference to Figures 2 and 3. As shown in Figure 2, the fixing device 200 as an image heating device comprises a heating unit 201 and a pressure roller 128 as a pressing member that is pressed against the heating unit 201. When the pressure roller 128 is pressed against the heating unit 201, a fixing nip section N is formed, and the recording material P on which the toner image is formed is held and conveyed in the fixing nip section N, and the toner image is heated and fixed. The heating unit 201 has a heat-generating layer 121a as a conductive layer and a fixing film 121 as a rotatable cylindrical rotating body. The heating unit 201 also has an excitation coil 123 and at least one temperature detection element for detecting the temperature of the fixing film 121, in this embodiment there are three temperature detection elements 129, 130, and 131.

[0019] The fixing film 121 is a cylindrical rotating body comprising a heat-generating layer 121a made of a conductive material that serves as a conductive layer, an elastic layer 121b laminated on its outer surface, and a surface release layer 121c laminated on its outer surface. In this embodiment, the fixing film 121 is a cylindrical body with a diameter of 10 to 50 mm, the heat-generating layer 121a is a metal film with a thickness of 10 to 50 μm, and the elastic layer 121b is made of silicone rubber with a hardness of 20 degrees (JIS-A, 1 kg load) molded to a thickness of 0.3 mm to 0.1 mm. A fluororesin tube with a thickness of 50 μm to 10 μm is then coated on the elastic layer 121b as a release layer 121c (surface layer). When a high-frequency voltage is applied and an alternating magnetic flux with periodically reversing polarity acts on the fixing film 121, an induced current is generated and the heat-generating layer 121a generates heat. This heat is transferred to the elastic layer 121b and the release layer 121c, heating the entire fixing film 121 and heating the recording material P that is passed through the fixing nip N, thereby fixing the toner image T.

[0020] Furthermore, the heating unit 201 is provided with a magnetic core 122, which serves as a magnetic core material, inserted through the hollow portion of the fixing film 121 and arranged along the longitudinal direction of the fixing film 121. The excitation coil 123 is constructed by forming a winding around the magnetic core 122 in a direction intersecting the rotation axis of the fixing film 121. In addition, a sleeve guide 126 is inserted into the fixing film 121 along the fixing nip portion N, and a pressure stay 125 with an inverted U-shaped cross-section is attached to surround the excitation coil 123, with the lower end edge of the pressure stay 125 in contact with the sleeve guide 126.

[0021] (Composition of the pressurized system) Next, the pressurization configuration of the fixing device 200 will be explained. As shown in Figure 3, by compressing the pressurization springs 137a and 137b between both ends of the pressurization stay 135 and the spring receiving members 138a and 138b on the device chassis side, a downward force is applied to the pressurization stay 135. In this embodiment, the image heating unit 201 applies a total pressing force of approximately 100N to 250N (approximately 10kgf to approximately 25kgf). As a result, the lower surface of the sleeve guide 126, which is made of heat-resistant resin PPS or the like, and the upper surface of the pressurization roller 128 press against each other with the fixing film 121 in between, forming a fixing nip portion N of a predetermined width.

[0022] Furthermore, flange members 132a and 132b are fitted to both the left and right ends of the sleeve guide 126 from the outer end side, and are rotatably mounted while their left and right positions are fixed by regulating members 133a and 133b. The flange members 132a and 132b receive the ends of the fixing film 121 when the fixing film 121 rotates and restrict the movement of the fixing film 121 along the longitudinal direction of the sleeve guide. As the material of the flange members 132a and 132b, a heat-resistant material such as LCP (Liquid Crystal Polymer) resin is preferred.

[0023] The pressure roller 128 comprises a core metal 128a, a heat-resistant elastic material layer 128b formed and coated concentrically around the core metal in a roller shape, and a release layer 128c that forms the surface. The elastic material layer 128b is preferably made of a material with good heat resistance, such as silicone rubber, fluororubber, or fluorosilicone rubber. Both ends of the core metal 128a are rotatably supported between the chassis side sheet metal (not shown) of the device via conductive bearings. The pressure roller 128 is driven to rotate in the counterclockwise direction indicated by the arrow by a driving means (not shown), and as the pressure roller 128 rotates, a rotational force is applied to the fixing film 121 by friction with the outer surface of the fixing film 121.

[0024] (Explanation of temperature sensing element) Next, as shown in Figure 2, the temperature detection elements 129, 130, and 131 of the heating unit 201 are positioned upstream of the fixing film 121 (heating unit 201) in the transport direction in which the recording material P is transported to the heating unit 201. Furthermore, as shown in Figure 3, the temperature detection elements 129, 130, and 131 are positioned in the center and at both ends of the heating unit 201 in the longitudinal direction, facing the fixing film 121. In this embodiment, the temperature detection elements 129, 130, and 131 are composed of non-contact type thermistors, but they may also be composed of, for example, contact type thermistors embedded in the fixing film 121. With this configuration, for example, the surface temperature of the fixing film 121 can be maintained and adjusted to a predetermined target temperature based on the detection result of the central temperature detection element 129. In addition, the temperature detection elements 130 and 131, which are positioned near the ends of the fixing film 121, can detect the degree of temperature rise in the non-paper-feeding area when small-sized recording material P is continuously printed.

[0025] (Configuration of the induced current generation mechanism) Next, the induction current generation mechanism of the heating unit 201 will be explained. As shown in Figure 4, the high-frequency inverter 191 is supplied with AC power from an external AC power supply 160 via relays 161 and 162. Here, the AC power supply 160 is configured to have a first relay 161 and a second relay 162 on both sides, but a configuration with only the first relay 161 is also acceptable. The high-frequency inverter 191 is also equipped with a power detection circuit 190 that detects the power supplied to the excitation coil 123. This power detection circuit 190 calculates the power supplied to the excitation coil 123 from the current flowing through the excitation coil 123 and the AC voltage supplied from the AC power supply 160.

[0026] The magnetic core 122, as a magnetic core material, is positioned by fixing means (not shown) to penetrate the hollow portion of the fixing film 121 and functions as a member that guides magnetic field lines generated by the alternating magnetic field of the excitation coil 123 into the interior of the fixing film 121, forming a path for magnetic field lines (magnetic path). In particular, the magnetic core 122 in this embodiment is formed in a rod shape, and since the magnetic path does not pass through the magnetic core 122 outside the excitation coil 123, an open magnetic path is formed. That is, this magnetic core 122 is a magnetic core material that is installed inside the fixing film 121 and forms an open magnetic path in the axial direction (longitudinal direction) of the fixing film 121. The material of the magnetic core 122 is preferably a ferromagnetic material composed of a material with low hysteresis loss and high relative permeability, such as calcined ferrite, ferrite resin, amorphous alloy, or permalloy, which are oxides or alloys with high magnetic permeability. Because it is wound in a direction intersecting the rotation axis X, a high-frequency current is passed through this excitation coil 123 via the power supply contacts 123a and 123b using a high-frequency inverter 191 or the like, generating a magnetic flux.

[0027] The excitation coil 123 is formed by spirally winding a single ordinary wire around the magnetic core 122 in the hollow portion of the fixing film 121. It is wound inside the fixing film 121 in a direction intersecting the axis of rotation. Therefore, when a high-frequency voltage is applied to the excitation coil 123 via the high-frequency inverter 191 and power supply contacts 123a and 123b, an alternating current flows through the excitation coil 123, generating an alternating magnetic flux parallel to the axis of rotation of the fixing film 121. While the excitation coil 123 has been described using a single wire, it is not limited to this; it may also be constructed by bundling multiple wires together.

[0028] Furthermore, the control mechanism 300 for controlling the temperature of the heating unit 201 includes the control unit 31 (see Figure 1) of the image forming apparatus 100, which comprises a CPU 140 as a calculation means or control means, and first and second safety circuits 147 and 150. The CPU 141 functions as an engine control unit 143, a fixing temperature detection unit 144, a power control unit 145, and a frequency control unit 146 with respect to the temperature control of the heating unit 201.

[0029] The temperature signal from the first temperature detection element 129, which is provided to detect the temperature of the area (paper-feeding area) through which the recording material P passes in the rotation axis direction of the fixing film 121, is input to the fixing temperature detection unit 144. Based on this temperature signal from the first temperature detection element 129, the high-frequency inverter 191 is controlled by the power control unit 145 and the frequency control unit 146, and an appropriate high-frequency voltage is applied to the power supply contacts 123a and 123b. As a result, the fixing film 121 is inductively heated, and the surface temperature is maintained and adjusted (temperature controlled) to a predetermined target temperature.

[0030] More specifically, the power control unit 145 controls the pulse period and pulse-on time for applying the high-frequency voltage based on the detection results of the fixing temperature detection unit 144 and the frequency setting in the frequency control unit 146, thereby controlling the power of the high-frequency inverter 191. In Figure 4, the engine control unit 143, fixing temperature detection unit 144, power control unit 145, and frequency control unit 146 are all shown as being included in the CPU 140, but the present invention is not limited to this configuration and may be configured by other circuits or calculation means.

[0031] Next, the reception of image data will be explained. The control unit 31 is equipped with a printer controller 141 that communicates with the host computer 142. The printer controller 141 receives image data from the host computer 142 and converts the received image data into information that can be printed by the image forming apparatus 100. The printer controller 141 also simultaneously exchanges signals and performs serial communication with the engine control unit 143.

[0032] The engine control unit 143 exchanges signals with the printer controller 141 and also performs various controls on the image forming apparatus 100 via serial communication. The host computer 142 transfers image data to the printer controller 141 and sets various print conditions, such as the size of the recording material P, to the printer controller 141 in response to user requests.

[0033] (Drive frequency and heat distribution) Figure 5 shows the characteristics of how output power changes with driving frequency, and illustrates the heat generation distribution of the fixing film 121 when a high-frequency voltage is applied from the high-frequency inverter 191 to the power supply contacts 123a and 123b. Here, the heat-generating layer 121a of the fixing film 121 is stainless steel with a thickness of 30 μm, a diameter of 30 mm, and a length of 220 mm. The magnetic core 122 is a ferrite core with a diameter of 12 mm, a length of 240 mm, and a relative permeability of 1800. The excitation coil 123 is a conductor wound with a tightly packed winding pitch at the ends and a loosely packed winding pitch in the center. For example, the excitation coil 123 is wound 18 times around the magnetic core 122, with winding intervals of 10 mm at the ends, 20 mm in the center, and 15 mm in between. When current I1 is passed through the excitation coil 123, an alternating magnetic field is formed inside the magnetic core 122, and a circulating current I2, shown by the dotted line, flows throughout the entire longitudinal direction of the fixing film 121, causing the fixing film 121 to heat up.

[0034] The relationship between driving frequency and heat generation is briefly explained below. Figures 6(a) to 6(d) show the equivalent circuits of the excitation coil 123 and the heat-generating layer 121a of the fixing film 121. In Figure 6(a), L1 is the inductance of the primary winding, L2 is the inductance of the secondary winding, M is the mutual inductance of the primary and secondary windings, and R is the resistance. The circuit diagram in Figure 6(a) can be equivalently transformed into the circuit diagram in Figure 6(b). To consider a simpler model, assume that the mutual inductance M is sufficiently large, and L1 ≈ L2 ≈ M. In that case, (L1-M) and (L2-M) become sufficiently small, so the circuit can be approximated from Figure 6(b) to Figure 6(c).

[0035] Now, let's explain resistance. In the circuit diagram in Figure 6(a), the impedance on the secondary side is the electrical resistance R in the circumferential direction of the heating layer 121a. In a transformer, the impedance on the secondary side, when viewed from the primary side, is N 2The equivalent resistance R' is (where N is the turns ratio of the transformer). Here, the turns ratio N of the transformer can be considered as the number of turns of the heating layer 121a, where the number of turns of the primary winding is equal to the number of turns of the excitation coil 123 in the heating layer 121a, and the heating layer 121a is considered to have 1 turn. Therefore, R' = N 2 This can be considered as R, and the more turns there are, the larger the equivalent resistance R shown in Figure 6(c).

[0036] Figure 6(d) shows a simplified definition of the combined impedance X. The combined impedance X is obtained from the following equation (1).

[0037]

number

[0038] According to this, the combined impedance X is (1 / ωM) 2 The term exhibits frequency dependence. This means that both the resistance R' and the inductance M contribute to the combined impedance, and since the dimension of impedance is [Ω], it means that the load resistance is frequency dependent.

[0039] By varying the number of coil turns per unit length depending on the position along the longitudinal direction, it becomes possible to create a nearly uniform heat distribution along the longitudinal direction in combination with frequency control of the high-frequency voltage (drive frequency control of the high-frequency inverter 191). As shown in Figure 5, it can be seen that the amount of heat generated at both ends of the fixing film decreases as the drive frequency is lowered below 75 kHz, and increases as the drive frequency is raised above 75 kHz.

[0040] Next, we will explain the phenomenon where "the apparent permeability μ decreases at the edges of the magnetic core." The graph in Figure 7 is an illustrative diagram of the phenomenon in which the "apparent permeability μ" at both ends of the magnetic core 122 is lower than at the center. The reason why this phenomenon occurs is explained in detail below. 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 (2).

[0041]

number

[0042] 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 invention, 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. This embodiment is characterized by the use of open magnetic paths.

[0043] Figure 8 shows the shape of the magnetic flux when ferrite 201 and air 202 are placed in a uniform magnetic field H. The ferrite has open magnetic paths with respect to air, having 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 122, the magnetic field lines are less dense in the air and denser in the central part 201C of the magnetic core, as shown in Figure 8. Furthermore, the magnetic flux density is lower at the ends 201E compared to the central part 201C of the magnetic core.

[0044] The reason why the magnetic flux density decreases at the edges lies in the boundary conditions between air and ferrite. At the interface surfaces NP⊥ and SP⊥ perpendicular to the magnetic field lines, the magnetic flux density is continuous. Therefore, near the interface, the portion of air in contact with the ferrite has a high magnetic flux density, while the ferrite edge 201E in contact with the air has a low magnetic flux density. As a result, the magnetic flux density at the ferrite edge 201E decreases. This phenomenon appears as if the permeability at the edge is decreasing due to the decrease in magnetic flux density. Therefore, in the description of this embodiment, it is expressed as "the apparent permeability decreases at the edge of the magnetic core." Furthermore, the equivalent inductance L from both ends of the coil is expressed by the following equation (3).

[0045]

number

[0046] Here, μ is the permeability of the magnetic core, N is the number of turns in the coil, l is the length of the coil, and S is the cross-sectional area of ​​the coil. Consequently, the equivalent inductance L also exhibits a bell-shaped distribution as shown in Figure 9, due to the fact that "the apparent permeability is smaller at the ends of the magnetic core."

[0047] Next, we will explain how "making the number of coil turns dense at the ends of the magnetic core and sparse in the center" results in "changing the balance between inductance and resistance at the ends and in the center." This configuration has a distribution in the longitudinal direction in terms of apparent permeability and number of turns. To explain these using a simple model, we will use the configuration shown in Figure 10(a). Figure 10(a) shows the configuration for induction heating of the embodiment of the present invention divided into approximately three parts in the longitudinal direction. The longitudinal dimensions are equally divided into three parts, and the shape and physical properties of both ends are the same. The magnetic core is divided into an end part 192e (permeability μe) and a central part 192c (permeability μc), and the longitudinal dimension of each is 80 mm.

[0048] The magnetic permeability of each core 192e, 192c has a relationship where the permeability at the end μe < the permeability at the central part μc. For simplicity, assume that the change in the individual apparent magnetic permeability inside each magnetic core 192e, 192c is not considered. For the winding, the excitation coil 193e is wound Ne times around the magnetic core 192e, and the excitation coil 192c is wound Nc times around the magnetic core 192c. Here, consider a simple physical model. In Fig. 10(a), the heating layer 121a has fixing films 194e, 194c with the same shape and the same physical properties arranged respectively, and the circular resistance is Re = Rc (= R). Since the magnetic permeability of the magnetic cores 192e, 192c has a relationship of μe < μc, the relationship of the mutual inductance is also Me < Mc. A further simplified equivalent circuit is shown in Fig. 10(b). Looking at the equivalent resistance seen from the primary side of each circuit, at the end, it is Ne 2 R, and at the central part, it is Nc 2 R. Therefore, when calculating the composite impedances Xe and Xc, they are respectively as shown in the following formulas (4) and (5).

[0049]

Equation

[0050]

Equation

[0051] These composite impedances Xe and the composite impedance Xc have different frequency characteristics. When plotting the frequency characteristics of the composite impedance Xe and the composite impedance Xc on a graph, it becomes as shown in Fig. 11. The behavior of the composite impedance Xc changes like a frequency filter. When it is lower than the cut-off frequency f, it monotonically increases, and when it is higher, it no longer changes. Qualitatively explain this phenomenon. When the frequency is low, the circuit responds similarly to a series circuit. That is, the inductor approaches a short circuit, current flows through the inductor side, and as a result, the composite impedance becomes low. Conversely, when the frequency is high, the inductor approaches an open circuit, current flows through the resistance R side, and as a result, the composite impedance becomes high and no longer changes further.

[0052] When a constant voltage is applied to each circuit from the high-frequency inverter 191, the relative magnitudes of heat generation are determined by the combined impedance. The behavior of the combined impedance Xe also changes at the cutoff frequency f1, similar to the combined impedance Xc. However, the equivalent resistances of the combined impedance Xe and the combined impedance Xc are different, and their mutual inductance Me, Different Mc values ​​result in different cutoff frequencies.

[0053] Figure 12 shows the amount of heat generated when the same high-frequency voltage is supplied to the center and the edges. Qc is the amount of heat generated in the center, and Qe is the amount of heat generated at the edges, exhibiting the same behavior as the change in combined impedance. We will now explain how to control the heat distribution of the fixing film 121 using this phenomenon. When frequency f3 is selected, Figure 12 shows that the magnitude of the heat generated in the center Qc and the heat generated at the edges Qe are the same. As a result, the amount of heat generated in the center and the edges is equal in the longitudinal direction, creating a flat distribution shape. When frequency f2 is selected, Figure 12 shows that the amount of heat generated at the edges Qe is smaller than Qc. As a result, the amount of heat generated at the edges is smaller in the longitudinal direction, resulting in a bell-shaped distribution shape. On the other hand, for example, when frequency f4 is selected, Figure 12 shows that the amount of heat generated at the edges Qe is larger than Qc. As a result, the heat distribution shape is such that both ends in the longitudinal direction are raised.

[0054] This mechanism allows for control of the heat distribution of the fixing film 121 by changing the drive frequency of the high-frequency inverter 191. When using this for controlling the heat distribution of the fixing film 121, it is preferable to set the variable range of the drive frequency to, for example, the range of f2 to f3. This allows for the use of a range from a flat distribution shape to a mountain-shaped distribution shape. Furthermore, using a frequency band greater than f3 allows for a higher heat distribution shape at the edges. This characteristic is utilized to change the drive frequency according to the size of the recording material P and the temperature of the non-paper-passing region of the fixing film 121. The non-paper-passing region is the area 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 direction of the fixing film 121 is heated uniformly, and when fixing small-sized recording material, the drive frequency is lowered to suppress the temperature at the edges of the fixing film 121. This makes it possible to suppress the temperature rise in the non-paper-passing region when fixing small-sized recording material.

[0055] (safety circuit) Next, we will explain the safety circuits 147 and 150 for preventing abnormal overheating of the heating unit 201. As mentioned above, the temperature control of the heating unit 201 is mainly performed by the CPU 140, which detects the signal from the temperature detection element 129 and determines the input power from the difference with the fixing target temperature. A limit input power (hereinafter also called the limit input power of FW control) is set for this input power, and the CPU 140 controls the input power based on the power detected by the power detection circuit 190, thereby suppressing the input power. In addition, the drive frequency of the high-frequency inverter 191 is determined by the frequency control unit 146 based on the paper size to be printed and the temperature difference between the temperature detection element 129 in the center of the film and the temperature detection element 130 or temperature detection element 131 at the edge of the film.

[0056] In this way, the high-frequency inverter 191 is controlled by the CPU 140 to ensure that the fixing film 121 generates heat appropriately. However, if a malfunction occurs in the CPU 140 or the sensors that communicate with the CPU 140, the fixing film 121 may overheat and reach an abnormal temperature. Specifically, this could occur if the control program of the CPU 140 malfunctions, or if an output port constituting the power control unit of the CPU 140 fails and a PWM (Pulse width Modulation) signal is continuously output to the high-frequency inverter 191. It could also occur if the power detection circuit 190 fails and sends a value lower than the actual input power to the CPU 140, or if the receiving circuit that receives signals from the temperature detection elements 129-131 and the power detection circuit 190 fails.

[0057] Therefore, as shown in Figure 4, the control mechanism 300 that controls the temperature of the heating unit 201 is equipped with the first and second safety circuits 147 and 150 described above. The first safety circuit 147 is a means for preventing overheating of the fixing film 121 based on the surface temperature of the fixing film 121. Specifically, the first safety circuit 147 is configured to detect abnormal heat generation of the fixing film 121 using temperature detection elements 129, 130, and 131 in addition to a thermoswitch or thermal fuse (not shown), and to cut off the power supply when abnormal heat generation occurs. In other words, in this embodiment, the first safety circuit 147 is a power cut-off means (second power cut-off means) that cuts off the power supply from the inverter 191 to the excitation coil 123 regardless of the control state of the inverter 191 by the CPU 140 when the temperature detected by the temperature detection elements 129, 130, and 131 exceeds a threshold.

[0058] Specifically, the first safety circuit 147 includes a temperature comparison unit 148 and a first forced power cut-off circuit 149. The temperature comparison unit 148 receives detection signals from temperature detection elements 129, 130, and 131. When the temperature comparison unit 148 determines that any of the temperature detection elements 129, 130, or 131 has exceeded an abnormal temperature, the first forced power cut-off circuit 149 is activated. When the first forced power cut-off circuit 149 is activated, it forcibly cuts off (turns off) the signal (for example, the PWM signal) output from the power control unit 145 to drive the high-frequency inverter 191. Note that the configuration of the first forced power cut-off circuit 149 is not limited to this; any configuration is acceptable as long as it can stop abnormal heat generation.

[0059] In this way, by providing the first safety circuit 147, it is possible to prevent abnormal overheating of the fixing film 121 based on the detected temperature detected by the temperature detection elements 129, 130, and 131. However, in the case of a heating unit 201 that heats the fixing film 121 while changing the drive frequency of the high-frequency inverter 191, as in this embodiment, there are the following concerns.

[0060] First, the fixing film 121 that comes into contact with the toner image on the recording material is thin and has a small heat capacity. Because the fixing film 121 has a small heat capacity, if an abnormality occurs in which the fixing film 121 does not rotate, less heat is lost to the pressure roller 128, etc., and the rate at which the fixing film 121 heats up becomes significantly faster. For example, an example of this is shown in Figures 13(a) and (b).

[0061] Figure 13(a) shows the actual surface temperature of the fixing film and the temperature changes of the inscribed thermistor embedded in the fixing film. The horizontal axis represents the time elapsed since power was applied, and the vertical axis represents the temperature, showing an example of the temperature changes of the film surface temperature and the inscribed thermistor. As shown in Figure 13(a), when power is applied when the film is not rotating, the film surface temperature and the temperature detected by the inscribed thermistor spread apart as time passes. Therefore, if, for example, a gear or CPU malfunctions and an abnormal condition occurs where the film does not rotate, there is a risk that the film surface temperature will reach the abnormal temperature before the thermistor, which acts as a temperature sensing element, reaches the print temperature.

[0062] Secondly, in the electromagnetic induction heating type heating unit 201 as in this embodiment, the heat distribution in the longitudinal direction of the fixing film 121 changes depending on the drive frequency of the high-frequency inverter 191, as shown in Figure 5. That is, even if the power input from the high-frequency inverter 191 to the excitation coil 123 is the same, if the drive frequency of the high-frequency inverter 191 is low, the amount of heat generated in the center of the fixing film 121 increases, while the heat generated at the edges of the fixing film 121 is less. In other words, excessive power is more likely to be locally applied to the center of the fixing film 121.

[0063] Figure 13(b) shows the surface temperature of the center of the fixing film and the temperature of the thermistor inscribed in the center, at different high-frequency inverter drive frequencies. The solid line shows the surface temperature change of the center of the film when a drive frequency of 75 kHz and 1300 W of power are applied, and the dotted line shows the surface temperature change of the center of the film when a drive frequency of 60 kHz and 1300 W of power are applied. By lowering the drive frequency, the temperature gradient of the center of the film becomes steeper, and the temperature reached is higher even with the same drive time.

[0064] On the other hand, the temperature trend of the inscribed thermistor in the center of the film is shown as follows: the dashed line represents the temperature trend of the inscribed thermistor when a power of 1300W is applied at a drive frequency of 75kHz. The double dashed line represents the temperature trend of the inscribed thermistor when a power of 1300W is applied at a drive frequency of 60kHz. Although the temperature slope of the inscribed thermistor is slightly steeper when the drive frequency is lower, the temperature change is slower compared to the change in surface temperature, indicating that the inscribed thermistor is not following the surface temperature as closely. In other words, even with the same power input, a lower drive frequency causes the temperature in the center of the fixed film to rise in a shorter time, but this means that the activation of the abnormal temperature detection by the inscribed thermistor is delayed. Therefore, the lower the drive frequency, the easier it is for the temperature in the center of the film to reach an abnormal temperature.

[0065] Therefore, in this embodiment, a second safety circuit 150 is provided in addition to the first safety circuit 147 to prevent delays in the operation of the abnormal high temperature detection. The second safety circuit 150 includes a power comparison unit 151, a second forced power cutoff circuit 152, a frequency comparison unit 153, and a power threshold setting unit 154. The power comparison unit 151 and the frequency comparison unit 153 are connected to the high-frequency inverter 191, and the detection signal from the power detection circuit 190 is input to the power comparison unit 151. The frequency comparison unit 153 is also capable of detecting the drive frequency of the high-frequency inverter 191.

[0066] Furthermore, the second forced power cut-off circuit 152 is configured to forcibly turn off the first relay 161 and the second relay 162 regardless of instructions from the engine control unit 143 when the circuit is activated. Similar to the first forced power cut-off circuit 149, the means for turning off the second forced power cut-off circuit 152 can be any configuration that can stop abnormal heat generation; for example, it may be configured to cut off the signal from the power control unit 145.

[0067] Next, the operation of the second safety circuit 150 will be explained based on the flowchart in Figure 14. When a print job is input from the host computer 142 to the printer controller 141, temperature control of the heating unit 201 according to the input print job is started. Then, the CPU 140 functions as a fixing temperature detection unit 144 and detects the surface temperature of the fixing film 121 based on the temperature signal of the temperature detection element 129 (step S100 in Figure 14).

[0068] Next, the CPU 140 functions as a frequency control unit 146 capable of changing the drive frequency, and sets the drive frequency of the high-frequency inverter 191 based on the detection results of the temperature sensing elements 129, 130, 131 and the print job (step S101).

[0069] Once the above drive frequency is set, the CPU 140 functions as a power control unit 145 and sets the input power according to the difference between the current temperature and the fixing target temperature and the drive frequency (step S102). The input power setting value set by the CPU 140 is set with the limit input power of FW control as the upper limit, as described above, and the CPU 140 sets the value of the input power (target value) by the control program within this limit input power of FW control.

[0070] Furthermore, in this embodiment, the limit input power for FW control is set as shown in the table in Figure 15, with the limit input power determined for each drive frequency. Also, as can be seen from Figure 15, the limit input power for FW control is set such that the amount of power that can be input decreases as the drive frequency decreases below 65.1 kHz, and also decreases when the drive frequency is above 75 kHz.

[0071] Then, once the power to be supplied to the excitation coil 123 is determined, the CPU 140, acting as the power control unit 145, outputs a control signal to the high-frequency inverter 191 and controls the high-frequency inverter 191 at the set drive frequency (step S103). When the above control signal is input to the high-frequency inverter 191, a voltage corresponding to the control signal is applied to the excitation coil 123, causing current to flow through the excitation coil 123 and inductively heating the fixing film 121.

[0072] Furthermore, when the high-frequency inverter 191 is driven, the frequency comparison unit 153 of the second safety circuit 150 functions as a frequency detection means and detects the drive frequency of the high-frequency inverter 191 at that time (step S104). When the drive frequency of the high-frequency inverter 191 is detected, the second safety circuit 150 switches the setting of the power threshold setting unit 154 to a setting corresponding to the value of the drive frequency of the high-frequency inverter 191 according to the detection result (step S105). That is, the power threshold setting unit 154 is set to a power threshold at which the second forced power cutoff circuit 152 described above will operate. As shown in Figure 15 as the limit power threshold of the safety circuit, the value of this power threshold is set for each drive frequency of the high-frequency inverter 191, and the power threshold of the power threshold setting unit 154 is set to the power threshold value corresponding to the drive frequency detected by the frequency comparison unit 153.

[0073] More specifically, the limit power threshold of the safety circuit is greater than the limit input power setting of the FW control, which is the upper limit of the input power setting that the CPU 140 can set, for the same drive frequency. Furthermore, the lower the drive frequency value is below 65.1kHz, the lower the power threshold value becomes, and the power threshold is also set to be lower when the drive frequency is above 75kHz.

[0074] Specifically, in this embodiment, when the drive frequency is 65.1kHz to 75kHz, which is a uniform heat distribution region where the difference in heat distribution between the longitudinal edges and the center of the fixing film 121 is small, the power threshold is set to 1300W. When the drive frequency is 50.1kHz to 65kHz, the power threshold is set to 1100W. Furthermore, when the drive frequency is 50kHz or less, where the heat generation in the center of the fixing film 121 is greater than that at the edges, the power threshold is set to 500W. In other words, the second safety circuit 150, as a power cutoff means, sets the power threshold to a first value when the drive frequency of the high-frequency inverter 191 is a first drive frequency (for example, the above drive frequency of 65.1kHz to 75kHz). Furthermore, when the drive frequency of the high-frequency inverter 191 is a second drive frequency that is smaller than the first drive frequency (for example, the drive frequency is 50kHz or less), the power threshold is set to a second value that is smaller than the first value.

[0075] Once the power threshold setting is complete, the power comparison unit 151 detects the power supplied by the high-frequency inverter 191 to the excitation coil 123 based on the detection signal from the power detection circuit 190, and determines whether the detected supplied power exceeds the power threshold (step S106). The power comparison unit 151 may detect the power supplied to the excitation coil 123 by any method. For example, the power detection circuit 190 may be configured as a current detection circuit that detects the current flowing through the excitation coil 123, and the supplied power may be estimated based on the magnitude of the current detected by this current detection circuit.

[0076] If the power comparison unit 151 detects an input power greater than the above power threshold (Yes in S106), an abnormal condition is detected (step S107), and the power comparison unit 151 activates the second forced power cutoff circuit 152 (step S108). When the second forced power cutoff circuit 152 is activated, the relays 161 and 162 are turned off as described above, power is no longer supplied from the AC power supply 160 to the high-frequency inverter 191, and the heating of the fixing film 121 is forcibly stopped.

[0077] Furthermore, if the above abnormal condition is detected, the CPU 140 will emergency stop the printing operation of the image forming apparatus 100 and terminate by displaying a fault on an unshown display panel (step S109).

[0078] On the other hand, if the power input in step S106 does not exceed the power threshold (No. in step S106), the CPU 140 determines whether or not to continue supplying power in order to continue the print operation (step S110).

[0079] Furthermore, if it is determined to continue power supply (No in step S110), the CPU 140 detects the temperature information of the fixing film 121 after a predetermined time has elapsed (step S111). Then, based on the detected temperature information, it resets the drive frequency and power supply (S112). Once the resetting is complete, it returns to S106 and repeats steps S106 and S110-S112 until power supply is terminated (Yes in step S110). In addition, if the recording material being transported during the printing operation jams, an emergency stop occurs, and the CPU 140 determines that power supply should be stopped and terminates (Yes in step S110).

[0080] As described above, in this embodiment, if an abnormal amount of power is supplied to the excitation coil 123 based on the power supplied to it, the second safety circuit 150 forcibly stops the heating of the fixing film 121. That is, in this embodiment, the second safety circuit is a power cutting means (first power cutting means) that cuts off the power supply from the inverter 191 to the excitation coil 123 when the power supplied from the high-frequency inverter 191 to the excitation coil 123 exceeds a threshold, regardless of the control state of the inverter 191 by the CPU 140. Therefore, even if the fixing film 121 has a small heat capacity and a fast heating rate, heating can be stopped at an appropriate timing. Furthermore, especially when the drive frequency of the high-frequency inverter 191 is low, power concentrates in the center, and even in the induction heating type heating unit 201 which is easily heated, the lower the drive frequency, the smaller the power threshold. Therefore, it is possible to prevent the central part of the fixing film 121 from rising to an abnormal temperature locally.

[0081] Furthermore, because abnormal temperature increases in the fixing film 121 can be appropriately prevented in this way, it is possible to prevent the detection of overheating from being delayed, which could cause the temperature of the fixing film 121 to become extremely high and damage surrounding components.

[0082] Furthermore, in this embodiment, not only the power threshold of the second safety circuit, but also the value of the limit input power set by the control program of the CPU 140 is switched in accordance with the drive frequency. Therefore, when the drive frequency is high, which requires a lot of power to raise the fixing film 121 to the target temperature, the value of the power supplied to the excitation coil 123 can be set higher. This allows the fixing film 121 to be raised to the target temperature quickly, without reducing the printing speed of the image forming apparatus 100. In other words, the image forming apparatus 100 according to this embodiment can achieve both safety, which minimizes damage to the apparatus due to heat generation, and high-speed printing.

[0083] Furthermore, as the drive frequency decreases, the upper limit of the input power set by the control program of the CPU 140 also decreases. Therefore, even at low drive frequencies where the amount of heat generated in the center of the film tends to increase, the temperature rise in the center of the film can be slowed down, and the first safety circuit 147 can be activated before it damages the surrounding components.

[0084] In the embodiment described above, only the case where the drive frequency is up to 75 kHz is considered. However, for example, cases where the drive frequency is higher than 75 kHz may also be considered. As described above, when the drive frequency is higher than frequency f3 (for example, 75 kHz), the heat distribution becomes such that the temperature of the edges of the fixing film 121 is higher than that of the center. Therefore, in such cases, taking into account the temperature rise at the edges of the fixing film 121, the power threshold of the second safety circuit 150 may be set lower compared to the case where the heat distribution at the center and edges is approximately constant.

[0085] For example, as shown in Figure 16, when the drive frequency of the high-frequency inverter 191 is 75.1kHz to 90kHz, the power threshold of the second safety circuit 150 may be set to 1100W, which is lower than when the drive frequency is 65.1kHz to 75.1kHz. In other words, the second safety circuit 150 sets the power threshold to a first value (e.g., 1300W) when the drive frequency of the high-frequency inverter 191 is a first drive frequency (e.g., 75.1kHz), and sets the power threshold to a third value (e.g., 1100W), which is lower than the first value, when the drive frequency of the high-frequency inverter 191 is a third drive frequency (e.g., 90kHz) that is higher than the first drive frequency.

[0086] Furthermore, in the embodiment described above, the limit input power is reduced to 500W when the drive frequency is 50kHz or less. However, for example, if the control range for the drive frequency is set to a range of 50kHz to 75kHz, the CPU 140 may be judged to be abnormal when the drive frequency falls outside the control range, and the power threshold may be reduced to 0W. This forces the second forced power cutoff circuit 152 to activate, preventing abnormal heat generation.

[0087] <Second Embodiment> Next, the second embodiment will be described with reference to Figures 17 to 20. This embodiment differs from the first embodiment described above in that it switches the power threshold of the second safety circuit 150 while also considering the rotation state of the fixing film. 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.

[0088] As shown in Figure 17, the image forming apparatus 100 includes an optical sensor 211 and a film rotation detection unit 212 to detect the rotation of the fixing film 121. Furthermore, as shown in Figure 18, the outer circumferential surface of one end of the fixing film 121 is provided with rotation detection marks 213 arranged at equal intervals in a different color from other parts in the circumferential direction. For example, in this embodiment, black detection marks 213 measuring 3 mm square are arranged on the outer circumferential surface of the fixing film 121. Note that if the recording material becomes wrapped around the fixing film 121 due to an accidental transport malfunction, the detection marks 213 will not be detectable, so it is desirable that the detection marks 213 be located outside (towards the end) the area where the paper is transported.

[0089] The optical sensor 211 described above includes a light-emitting element 214 and a light-receiving element 215, and is positioned to detect the detection mark 213. The light-emitting element 214 and light-receiving element 215 are configured such that light emitted from the light-emitting element 214 is reflected by the fixing film 121 and detected by the light-receiving element 215. As the fixing film 121 rotates, the intensity of the reflected light changes periodically, and the optical sensor 211 converts this into an electrical signal and outputs it to the film rotation detection unit 212.

[0090] The film rotation detection unit 212 determines that the fixing film 121 is rotating if there is a large change in the intensity of the reflected light, and determines that the fixing film 121 is stopped if there is almost no change in intensity. In this embodiment, the configuration for detecting film rotation is described using an optical sensor 211, but the rotation detection configuration for the fixing film 121 is not limited to this configuration and can be any configuration.

[0091] The film rotation detection unit 212 transmits the detection result to the second safety circuit 150, and the second safety circuit 150 considers the drive frequency and the rotation / stopping state of the fixing film 121, and sets the threshold for the limit input power in the power threshold setting unit 154. The operation of the second safety circuit 150 in this embodiment will be described below based on the flowchart in Figure 19. Note that the operation of steps S200 to S202 in the flowchart in Figure 19 differs from the flowchart in Figure 14. For this reason, the following explanation will mainly describe steps S200 to S202, and the explanation of the other steps will be omitted.

[0092] When a print job is input from the host computer 142, in parallel with step S100, the CPU 140 checks whether the pressure roller 128 is being driven (step S200). If the pressure roller 128 is being driven (Yes in step S200), the film rotation detection unit 212 detects the rotation of the fixing film 121 based on the detection signal from the optical sensor 211 (step S201).

[0093] When rotation of the fixing film 121 is detected, the CPU 140 determines whether the fixing film 121 is stopped or not based on the detection result of the film rotation detection unit 212 (step S202). If the CPU 140 determines that the fixing film 121 is rotating (No in step S202), it returns to step S200. If the CPU 140 determines that the fixing film 121 is stopped (Yes in step S202), it sets the power threshold of the second safety circuit 150, taking into account the stopped state of the fixing film 121 (step S105).

[0094] In this embodiment, as shown in Figure 20, the limit input power setting for FW control and the limit power threshold of the second safety circuit 150 are set separately depending on whether or not the fixing film 121 is rotating. When the fixing film 121 is stopped, these limit input power settings for FW control and the limit power threshold of the second safety circuit 150 are set in the same way as in the first embodiment. Specifically, when the drive frequency is 65.1kHz to 75kHz, the limit power threshold of the second safety circuit 150 is set to 1300W. Similarly, when the drive frequency is 50.1kHz to 65kHz, the limit power threshold of the second safety circuit 150 is set to 1100W. Furthermore, when the drive frequency is 50kHz or less, the limit power threshold of the second safety circuit 150 is set to 500W. This makes it possible for the second safety circuit 150 to activate faster than the first safety circuit 147 when the rotation of the fixing film 121 is stopped.

[0095] On the other hand, when the fixing film 121 is rotating, its apparent heat capacity increases as heat is easily absorbed by the pressure roller 128, etc., thus improving the responsiveness of the temperature detected by the temperature detection elements 129, 130, and 131. As a result, the difference between the temperature detected by the temperature detection elements 129, 130, and 131 and the actual surface temperature of the fixing film 121 becomes smaller. Therefore, when the fixing film 121 is rotating, the detection of abnormal heat generation in the heating unit 201 is delayed, and the operation of the first safety circuit 147 is unlikely to be delayed. As a result, the first safety circuit 147 alone can stop abnormal heat generation before the fixing film 121 reaches an abnormal temperature.

[0096] For the reasons stated above, in this embodiment, the threshold value of the limit input power is not switched according to the drive frequency when the fixing film 121 is rotating. Specifically, it is set to 1300W regardless of the drive frequency. This 1300W is the highest limit input power threshold when the fixing film 121 is stopped. In this embodiment, the threshold value of the limit input power is not switched, but it is also possible to configure it so that the threshold value of the limit input power decreases as the drive frequency decreases. Even in this case, the threshold value of the limit input power when the fixing film 121 is rotating is set to be higher than or equal to the threshold value of the limit input power when it is stopped.

[0097] As described above, in this embodiment, the safety circuit is configured to switch the threshold value of the limiting power input by considering two factors: the rotation state of the fixing film 121 and the drive frequency. This prevents the central part of the film from rising to an abnormally high temperature and damaging the surrounding components of the fixing film 121. Furthermore, since power can be supplied without suppressing the input power when the fixing film 121 is rotating, printing can be performed on various paper sizes without reducing the printing speed.

[0098] <Third Embodiment> Next, the third embodiment will be described with reference to Figures 21 to 23. This embodiment differs from the second embodiment described above in that it switches the threshold value of the limit input power of the second safety circuit 150 according to the temperature difference between the center and the edges of the fixing film 121. For this reason, in the following description, only the configurations that differ from the second embodiment will be described, and the same reference numerals will be used for other configurations, and their descriptions will be omitted.

[0099] As described above, the relative magnitudes of heat generation in the fixing film 121 depend on the combined impedance. The combined impedance is influenced by factors such as the circumferential electrical resistance R of the heating layer 121a, as shown by equation (4) or equation (5). On the other hand, the fixing film 121 has a resistance variation of several percent due to manufacturing variations, so even at the same frequency, the combined impedance will differ, and the temperature distribution in the longitudinal direction will naturally differ.

[0100] In this embodiment, to absorb this variation, the temperature distribution for each fixing film is estimated from the detection results of two temperature sensing elements, and a decision is made as to whether or not to change the threshold value of the limit input power.

[0101] As shown in Figure 21, in this embodiment, the control mechanism 300 includes a temperature difference calculation means 300 that calculates the temperature difference between the temperature detection element 129 and the temperature detection element 130 or the temperature detection element 131. The operation of the second safety circuit 150 according to this embodiment will be described below based on the flowchart in Figure 22. Note that the operation of steps S300 to S301 in the flowchart in Figure 22 differs from that in the flowchart in Figure 19. For this reason, the following description will mainly focus on steps S300 to S301, and the explanation of the other steps will be omitted.

[0102] If it is determined in step S110 to continue power supply (No. of step S110), the surface temperature of the fixing film 121 is detected again in S111 using the temperature detection elements 129, 130, and 131.

[0103] When the surface temperature of the fixing film 121 is detected again, the temperature difference calculation means 300 determines the temperature difference between the center and the edge of the film and determines whether the temperature difference between the center and the edge of the fixing film 121 is 20°C or more (step S300).

[0104] Here, if the temperature difference between the center and edges of the fixing film 121 is not 20°C or more (No. in step S300), the heat distribution of the fixing film 121 is not as centrally heated as the standard (design). In this case, it can be seen that the heating unit 201 is a unit in which the center of the fixing film 121 is less likely to overheat abnormally, even if the drive frequency of the high-frequency inverter 191 is low, due to the influence of component variations, etc.

[0105] Therefore, if the temperature difference between the center and the edges of the fixing film 121 is not 20°C or more, the CPU 140 resets the threshold value of the limit input power of the second safety circuit 150 (step S301), and proceeds to step S105. At the same time, the CPU 140 also resets the drive frequency and input power values ​​of the high-frequency inverter 119 based on the temperature information, similar to step S112.

[0106] In step S105 described above, for example, if the fixing film 121 is in a stopped state, in this embodiment, the threshold value for limit input power is reset according to the table in Figure 23(a). More specifically, as shown in Figure 23(a), the threshold value for limit input power of the second safety circuit 150 is configured such that, if the temperature difference between the center and the edges of the fixing film 121 is 20°C or more, the value decreases as the driving frequency of the high-frequency inverter 191 decreases. For example, in this embodiment, it is set to 1100W when the driving frequency is 50.1kHz to 65kHz, and to 500W when it is 50kHz or less. However, if the temperature difference between the center and the edges of the fixing film 121 is not 20°C or more, even when the driving frequency is 50kHz or less, the threshold value for limit input power is set to 1100W, similar to the case when the driving frequency is 50.1kHz to 65kHz.

[0107] Therefore, for example, if the drive frequency reset in step S302 is 50kHz or less, the CPU 140 resets the threshold value of the limit input power of the second safety circuit 150 from 500W to 1100W in step S105.

[0108] By performing this control, the fixing film 121 can achieve a safety circuit configuration that can absorb variations even if the electrical resistance value R in the circumferential direction varies slightly due to manufacturing variations. On the other hand, if the temperature difference is 20°C or more, the process proceeds to step S112 as before, and the CPU 140 only resets the drive frequency and input power. In this case, the threshold value of the limit input power of the second safety circuit 150 is not reset, and the current value is maintained.

[0109] As described above, in this embodiment, by also considering the detection results of the temperature difference between the center and edges of the fixing film 121, the threshold value of the limit input power can be changed to an appropriate drive frequency suited to each individual fixing film 121. As a result, even if there are manufacturing variations in the fixing film 121, it is possible to prevent the center of the film from rising to an abnormally high temperature locally and damaging the surrounding components of the film.

[0110] In the embodiment described above, an example was explained in which the threshold value of the limit input power of the second safety circuit 150 is not lowered even when the drive frequency is 50 kHz or less, by using the table shown in Figure 23(a) to reset the threshold value of the limit input power of the second safety circuit 150. However, for example, the table shown in Figure 23(b) may be used. That is, when the drive frequency decreases, the threshold value of the limit input power of the second safety circuit 150 is also lowered, similar to the case where the temperature difference is 20°C or more, but the amount of the reduction may be smaller. For example, in the example in Figure 23(b), when the temperature difference is 20°C or more, the threshold value of the limit input power is set to 1100W when the drive frequency is 50.1 kHz to 65 kHz, and the threshold value is set to 500W when the drive frequency is 50 kHz or less.

[0111] On the other hand, when the temperature difference is less than 20°C, the threshold for limit input power is set to 1100W when the drive frequency is 50.1kHz to 65kHz, and when the drive frequency is 50kHz or less, the threshold is set to 900W. Thus, as can be seen in the table shown in Figure 23(b), when the temperature difference between the center and the edges is less than 20°C, the suppression range of the limit input power of the second safety circuit 150 at low drive frequencies is smaller compared to when the temperature difference is 20°C or more.

[0112] The second safety circuit 150 may be composed of any electrical circuit, such as a CPU or ASIC. Furthermore, for fail-safe purposes, it is desirable that the second safety circuit 150 be configured separately from the CPU 140 that controls the high-frequency inverter 191; however, these may be implemented as functions of each program using the same control means. Moreover, the contents of the above-described embodiment may be combined in any way.

[0113] (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 system includes a power interruption means that, when the power supplied from the inverter to the excitation coil exceeds a threshold, interrupts the power supply from the inverter to the excitation coil, regardless of the control state of the inverter by the control means. The control means is capable of changing the drive frequency of the inverter. The power cut-off means changes the value of the threshold according to the value of the drive frequency of the inverter. An image heating device characterized by the following features.

[0114] [Configuration 2] The power cut-off means sets the threshold to a first value when the drive frequency of the inverter is a first drive frequency, and sets the threshold to a second value which is smaller than the first value when the drive frequency of the inverter is a second drive frequency which is smaller than the first drive frequency. The image heating device according to configuration 1, characterized in that it is a device that provides an image heating device.

[0115] [Configuration 3] The power cut-off means sets the threshold to a first value when the drive frequency of the inverter is a first drive frequency, and sets the threshold to a third value that is smaller than the first value when the drive frequency of the inverter is a third drive frequency that is greater than the first drive frequency. An image heating device according to configuration 1 or 2, characterized by the above.

[0116] [Structure 4] The power interruption means includes a frequency detection means for detecting the drive frequency of the inverter. An image heating device according to any one of configurations 1 to 3, characterized by the above.

[0117] [Composition 5] The system includes a power detection means that calculates the power supplied from the inverter to the excitation coil based on the current flowing through the excitation coil. An image heating device according to any one of configurations 1 to 4, characterized by the above.

[0118] [Composition 6] The power detection means calculates the power supplied from the inverter to the excitation coil based on the current flowing through the excitation coil and the AC voltage supplied to the inverter. The image heating device according to configuration 5, characterized in that it is a device that provides image heating.

[0119] [Composition 7] The system includes a rotation detection means for detecting the rotational state of the rotating body, The power cut-off means changes the value of the threshold based on the value of the drive frequency of the inverter and the rotation state of the rotating body. An image heating device according to any one of configurations 1 to 6, characterized by the above.

[0120] [Structure 8] A first temperature detection element for detecting the temperature of the central part of the rotating body in the axial direction, The rotating body comprises a second temperature detection element for detecting the temperature of the end of the rotating body in the axial direction, The power cut-off means changes the value of the threshold based on the value of the drive frequency of the inverter and the temperature difference between the detected temperatures of the first and second temperature sensing elements. An image heating device according to any one of configurations 1 to 7, characterized by the above.

[0121] [Composition 9] Image forming means for forming a toner image on a recording material, The image heating device comprises one of the configurations 1 to 8, The image heating device is a fixing device that heats the recording material on which a toner image has been formed by the image forming means to fix the toner image to the recording material. An image forming apparatus characterized by the following features. [Explanation of Symbols]

[0122] 121: Rotating body (fixing film) / 122: Magnetic core material (magnetic core) / 123: Excitation coil / 140: Control means (CPU) / 150: Power cutoff means (second safety circuit) / 191: Inverter (high-frequency inverter) / 200: Image heating device (fixing device)

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 system includes a power interruption means that, when the power supplied from the inverter to the excitation coil exceeds a threshold, interrupts the power supply from the inverter to the excitation coil, regardless of the control state of the inverter by the control means. The control means is capable of changing the drive frequency of the inverter. The power cut-off means changes the value of the threshold according to the value of the drive frequency of the inverter. An image heating device characterized by the following features.

2. The power interruption means sets the threshold to a first value when the drive frequency of the inverter is a first drive frequency, and sets the threshold to a second value which is smaller than the first value when the drive frequency of the inverter is a second drive frequency which is smaller than the first drive frequency. The image heating apparatus according to claim 1, characterized by the features described above.

3. The power cut-off means sets the threshold to a first value when the drive frequency of the inverter is a first drive frequency, and sets the threshold to a third value that is smaller than the first value when the drive frequency of the inverter is a third drive frequency that is greater than the first drive frequency. The image heating apparatus according to claim 1, characterized by the features described above.

4. The power interruption means includes a frequency detection means for detecting the drive frequency of the inverter. The image heating apparatus according to claim 1, characterized by the features described above.

5. The system includes a power detection means that calculates the power supplied from the inverter to the excitation coil based on the current flowing through the excitation coil. The image heating apparatus according to claim 1, characterized by the features described above.

6. The power detection means calculates the power supplied from the inverter to the excitation coil based on the current flowing through the excitation coil and the AC voltage supplied to the inverter. The image heating apparatus according to claim 5, characterized in that it is a feature of the present invention.

7. The system includes a rotation detection means for detecting the rotational state of the rotating body, The power cut-off means changes the value of the threshold based on the value of the drive frequency of the inverter and the rotation state of the rotating body. The image heating apparatus according to claim 1, characterized by the features described above.

8. A first temperature detection element for detecting the temperature of the central part of the rotating body in the axial direction, The system includes a second temperature detection element for detecting the temperature of the end of the rotating body in the axial direction, The power interruption means changes the value of the threshold based on the value of the drive frequency of the inverter and the temperature difference between the detected temperatures of the first and second temperature sensing elements. The image heating apparatus according to claim 1, characterized by the features described above.

9. 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 8, The image heating device is a fixing device that heats the recording material on which a toner image has been formed by the image forming means to fix the toner image to the recording material. An image forming apparatus characterized by the following features.

Images