JP4574741B2 - Heat fixing device - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, or a complex machine using an electrophotographic method, and more particularly to a heat fixing device in the image forming apparatus.

  The heating and fixing device in the image forming apparatus is based on an energization heating element that is a heat source, a power source that supplies current to the heating element, a temperature detection unit that detects a temperature near the heat generation, and a signal from the temperature detection unit. There is known a heating type fixing device having a control means for controlling the supply current of the power source, and fixing the image by heating a recording material on which an unfixed image is formed and supported by an image forming unit of the image forming device. ing. With the above configuration, the image fixing temperature is controlled to a predetermined temperature for image fixing.

  Such a heat fixing device does not function as a fixing device if any one of the heating element, power source, temperature detecting means, and control means does not function normally. Furthermore, when energization runaway occurs, there is a risk that the device may be destroyed due to overheating. Therefore, in such a fixing device, as disclosed in Patent Document 1 and the like, the following abnormal overheat safety device is provided to avoid overheating, smoke generation, and ignition during energization runaway.

  (1) A safety device (thermo protector) such as a thermal fuse or a thermo switch intervenes in the energization circuit of the heating element, and when overheating occurs due to energization runaway, the energization of the heating element is cut off.

  (2) If temperature detection means such as a thermistor is placed in the vicinity of the heating element, and the heating element is in an abnormally overheated state, current supply to the heating element is interrupted by a current interruption means such as a relay intervening in the energizing circuit. Let The temperature at which the abnormal overheat temperature safety device operates is set to a temperature higher than the temperature reached during normal operation to prevent malfunction during normal operation and to operate only during abnormal overheat.

JP 08-248813 A

  After the abnormal overheat safety device as described above is activated, the fixing component or unit (heating element, power supply, temperature detection means, control means, etc.) that originally caused the trouble, A serviceman may replace a used safety device such as a thermo switch.

  However, in actuality, not only the pressure roller and ceramic heater inside the fixing device due to the excessive temperature rise of the surrounding temperature after the safety device such as the thermo switch is activated after the fixing device is overheated. Also, the peripheral equipment may be damaged by overheating, such as deformation and deterioration, and in the worst case, the entire fixing device and the peripheral equipment may have to be repaired or replaced. is there. In order to suppress the damage that occurs during such energization runaway, the operating temperature of the abnormal overheat safety device may be set as low as possible. However, if the operating temperature is set low, the abnormal overheat safety device is activated during normal operation and the image forming apparatus malfunctions.

  Therefore, the present invention prevents the occurrence of a situation in which the entire fixing device and its peripheral devices must be repaired or replaced due to the above-described apparatus overheating without malfunction during normal operation. It is an object of the present invention to provide a heat fixing apparatus that can minimize damage and minimize the cost of replacement parts and service costs.

In order to solve such a problem, the heat fixing device of the present invention includes a cylindrical film, a heater having a heat generation pattern formed on a substrate, and being in contact with the inner surface of the cylindrical film, and the cylindrical film. A pressure roller that forms a nip portion that conveys the recording paper together with the heater through the film, and a heater temperature in a region through which a recording paper of a minimum size usable in the apparatus passes in the longitudinal direction of the heater. A temperature detecting element, a second temperature detecting element for detecting a heater temperature outside the passing area of the minimum size recording paper, and a power source so that the detected temperature of the first temperature detecting element maintains a target temperature. A control unit that controls energization of the heat generation pattern; a relay provided in a power supply circuit for the power source and the heat generation pattern; and a safety circuit that drives the relay independently of the control unit. And said safety circuit sensing the temperature of said second temperature sensing element to drive the relay to open the supply circuit to reach operating temperature,
The heat fixing device includes a rotation detection circuit that detects a rotation state of the pressure roller, and the operating temperature of the safety circuit is set according to a detection result of the rotation detection circuit. .

  Here, the operating temperature when the pressure roller is stopped is set lower than the operating temperature when the pressure roller is rotating.

  According to the present invention, the abnormal overheat detection temperature of the heating element is switched according to the operation state of the image forming apparatus, so that the safety device can malfunction even when the end temperature rise occurs during normal operation. In addition, it is possible to cut off the power supply to the heating element at a low temperature during energization runaway, minimizing damage to the device and minimizing the cost of replacement parts and service costs.

  In other words, by switching the detection temperature for detecting the abnormal overheating of the heating element according to the rotation state of the pressure roller, it is possible to cut off the power supply to the heating element at a low temperature during a runaway energization without causing the safety device to malfunction. This makes it possible to minimize damage to the apparatus and minimize the cost of replacement parts and service costs.

1 is a diagram illustrating a configuration example of an image forming apparatus in Embodiment 1. FIG. FIG. 3 is a diagram illustrating a configuration example of a fixing device according to the first exemplary embodiment. It is a figure which shows the structural example of the ceramic heater in Embodiment 1. FIG. It is a figure explaining the heat_generation | fever distribution in Embodiment 1. FIG. 3 is a diagram illustrating a configuration example of a power control circuit in Embodiment 1. FIG. 3 is a diagram illustrating a configuration example of a temperature detection circuit in Embodiment 1. FIG. It is a figure explaining the electric power ratio in Embodiment 1. FIG. FIG. 3 is a diagram for explaining edge temperature rise in the first embodiment. It is a figure explaining the safety circuit in Embodiment 1. FIG. It is a figure which shows the operating temperature setting table | surface of the safety circuit in Embodiment 1. FIG. It is a figure which shows the structural example of the power control circuit in Embodiment 2. FIG. FIG. 6 is a diagram illustrating a configuration example of a fixing drive motor rotation detection circuit according to a second embodiment. It is a figure explaining the safety circuit in Embodiment 2. FIG. It is a figure which shows the operating temperature setting table | surface of the safety circuit in Embodiment 2. FIG. It is a figure explaining the operating point of a thermo switch. It is a figure explaining operation | movement of the thermoswitch in embodiment. It is a flowchart which shows the example of a procedure of the relay control of embodiment.

[Embodiment 1]
Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(1) Configuration Example of Image Forming Apparatus FIG. 1 is a configuration diagram of a laser beam printer 100 according to the present embodiment.

  The laser printer 100 includes a deck 101 that stores recording paper P, a deck paper presence sensor that detects the presence or absence of the recording paper P in the deck 101, and a paper size detection sensor that detects the size of the recording paper P in the deck 101. 103, a pickup roller 104 that feeds the recording paper P from the deck 101, a deck paper feeding roller 105 that transports the recording paper P fed by the pickup roller 104, and a pair of the deck paper feeding roller 105. A retard roller 106 is provided to prevent feeding.

  A deck 101, a paper feed sensor 107 that detects a paper feed conveyance state from a double-side reversing unit (to be described later), and a paper feed conveyance roller for conveying the recording paper P further downstream of the deck paper supply roller 105. 108, a registration roller pair 109 that conveys the recording paper P in synchronization with the printing timing, and a pre-registration sensor 110 that detects the conveyance state of the recording paper P to the registration roller pair 109 are provided.

  Further, downstream of the pair of registration rollers, a process cartridge 112 that forms a toner image on the photosensitive drum 1 based on the laser beam from the laser scanner unit 111, and a toner image formed on the photosensitive drum 1 are recorded on the recording paper P. A roller member 113 (hereinafter referred to as a transfer roller) for transferring up and a discharge member 114 (hereinafter referred to as a charge eliminating needle) for removing charges on the recording paper P and promoting separation from the photosensitive drum 1 are provided. Has been.

  Further, downstream of the static elimination needle 114, a conveyance guide 115, a fixing device 116 that thermally fixes the toner image transferred onto the recording paper P, a fixing paper discharge sensor 119 that detects the conveyance state from the fixing device 116, and the fixing device 116. A double-sided flapper 120 for switching the destination of the recording paper P conveyed from the paper discharge unit to the double-side reversing unit is provided, and the paper conveyance state of the paper discharge unit is detected downstream of the paper discharge unit side. A paper discharge sensor 121 and a paper discharge roller pair 122 for discharging the recording paper are provided.

  On the other hand, the recording paper P after one-sided printing is reversed in order to print on both sides of the recording paper P, and the recording paper P is fed to the double-side reversing part side for feeding again to the image forming part by forward / reverse rotation. A pair of reversing rollers 123 to be switched back, a reversing sensor 124 for detecting the state of paper conveyance to the reversing rollers 123, and a lateral registration portion (not shown) for aligning the lateral position of the recording paper P to transport the recording paper P. A D-cut roller 125, a double-sided sensor 126 for detecting the recording paper P conveyance state of the double-side reversing unit, and a double-sided conveyance roller pair 127 for conveying the recording paper P from the double-side reversing unit to the paper feeding unit. .

(2) Configuration Example of Fixing Device 116 FIG. 2 is a schematic diagram of a schematic configuration of the fixing device 116. The fixing device of this example is a film heating type device disclosed in, for example, Japanese Patent Application Laid-Open Nos. 4-44075 to 44083 and Japanese Patent Application Laid-Open No. 4-204800 to 204984.

  Reference numeral 204 denotes a heat-resistant, heat-insulating, and rigid body stay for fixing the ceramic heater and guiding the inner surface of the film, and is a horizontally long member having the longitudinal direction in the direction crossing the conveyance path of the recording paper 210 (perpendicular to the drawing). 205 is a ceramic heater, which will be described later, and is a horizontally long member that is inserted in a groove formed along the length of the lower surface of the stay and is fixedly supported by a heat-resistant adhesive and has a length in the direction crossing the transfer material conveyance path. is there. 201 is a cylindrical heat-resistant film material (hereinafter referred to as a fixing film), which is loosely fitted on a stay 204 to which a ceramic heater 205 is attached. For example, a cylindrical single layer film such as PTFE, PFA, FEP, etc. having a thickness of about 40 to 100 μm and having heat resistance, releasability, strength, durability, etc., or a cylinder such as polyimide, polyamide, PEEK, PES, PPS It is a composite layer film in which PTFE, PFA, FEP, etc. are coated on the outer peripheral surface of the film-like film.

  A pressure roller 202 is an elastic roller in which a heat-resistant elastic layer 204 such as silicone rubber is provided concentrically and integrally on the outer periphery of the core metal 203. The pressure roller 202 and the ceramic heater 205 on the stay 204 side are pressed against the elasticity of the pressure roller with the fixing film 201 interposed therebetween. A range indicated by an arrow N is a fixing nip portion formed by the pressure contact. The pressure roller 204 is rotationally driven in the direction of arrow B at a predetermined peripheral speed by a fixing drive motor M2 (118). A rotational force acts directly on the film 201 by the frictional force between the roller 204 and the outer surface of the film 201 at the fixing nip portion N due to the rotational driving of the pressure roller 204 (the recording paper 210 moves in the fixing nip in the direction of arrow A). When the film 201 is introduced into the portion N, the rotational force is indirectly applied to the film 201 via the recording paper 210), and the film 201 is rotationally driven in the clockwise direction C as indicated by the arrow while sliding against the lower surface of the ceramic heater 205. The

  The stay 204 also functions as a film inner surface guide member to facilitate the rotation of the film 201. In order to reduce the sliding resistance between the inner surface of the film 201 and the lower surface of the ceramic heater 205, a small amount of lubricant such as heat-resistant grease can be interposed between them. The fixing nip portion N formed by the ceramic heater 205 and the pressure roller 202 with the film 201 interposed therebetween in a state where the rotation of the film 201 by the rotation of the pressure roller 202 becomes steady and the temperature of the ceramic heater 205 rises to a predetermined level. The recording paper 210 to be image-fixed is introduced between the film 201 and the pressure roller 202, and the fixing nip portion N is nipped and conveyed together with the film 201, so that the heat of the ceramic heater 205 passes through the film 201. The recording sheet 210 is applied to the unfixed image and the unfixed image on the recording sheet 210 is heated and fixed on the surface of the recording sheet 210. The recording paper 210 that has passed through the fixing nip N is separated from the surface of the film 201 and conveyed. 2 indicates the conveyance direction of the recording paper 210.

(3) Configuration Example of Ceramic Heater 205 FIG. 3 is a diagram illustrating a configuration example of the ceramic heater 205. The ceramic heater 205 is long disposed in a direction orthogonal to the recording paper conveyance direction.

Alumina (Al ₂ O ₃ ) is used as the substrate 301, and two heat generation patterns 302a and 302b are formed on one side by printing. The heat generation patterns 302a and 302b are covered with a glass protective film as an electrical insulating layer. In the present embodiment, the heater portion formed by the heat generation pattern 302a is referred to as a main heater, and the heater portion formed from the heat generation pattern 302b is referred to as a sub-heater. 303a, 303b, and 303c are power supply electrodes, and are formed so that a voltage can be applied to both ends of the heat generation pattern. The two main heaters 302a and main heater 302b have greatly different heat generation distributions.

  FIG. 4 shows the heat distribution of the main heater 302a and the sub heater 302b. The main heater 302a is formed with a large calorific value at the center of the ceramic heater 205. On the other hand, the sub-heater 301b is formed to generate a large amount of heat at the end of the ceramic heater 205.

(4) Example of thermistor The fixing device of this embodiment has three thermistors for measuring the temperature of the ceramic heater. Each thermistor is pressed onto the ceramic heater 205 with a predetermined pressure.

  FIG. 4 shows the arrangement relationship of the thermistor, and the arrangement of the thermistor in the longitudinal direction of the ceramic heater is indicated by arrows E, F, and G. The thermistor 1 is disposed at the center of the ceramic heater 205. On the other hand, the thermistors 2 and 3 are arranged at the end of the ceramic heater 205. Each thermistor is connected to a temperature detection circuit (not shown).

  FIG. 6 is an example of an internal circuit of the temperature detection circuit.

  The thermistors 1, 2 and 3 are connected in series with resistors 604, 606 and 607, respectively. S6, S7, and S8 are detection signals, which vary according to the resistance value of the thermistor that varies with temperature. The detection signals S6, S7, and S8 are connected to the CPU 501 and a safety circuit 509 described later.

(5) Example of Thermo Switch The fixing device of the present embodiment has one thermo switch (not shown) as a current interrupting means during abnormal heating. The thermo switch is pressed against the ceramic heater 205 with a predetermined pressure.

  In FIG. 4, the position of the thermo switch in the longitudinal direction of the ceramic heater is indicated by an arrow D. The operating temperature of the thermoswitch is 250 ° C.

  Here, the operating temperature of the thermoswitch will be described. The operating temperature of the thermoswitch is largely related to the rate of temperature rise until the operating temperature is reached.

  FIG. 15 is a diagram showing the relationship between the temperature of the ceramic heater 205 and the temperature at which the thermoswitch actually operates. LINE-B is a case where the heater is heated at a low temperature rise rate, and the thermo switch operates at a point B where the ceramic heater 205 is heated from 250 ° C. by ΔTb. On the other hand, LINE-A indicates a case where the ceramic heater 205 is heated at a high temperature rise rate. In this case, the thermo switch does not operate at 250 ° C., and operates at the point A where ΔTa is heated more than 250 ° C. The size of ΔTa is larger than ΔTb. That is, the thermoswitch has a characteristic of operating at a temperature closer to the operating temperature (250 ° C.) as the temperature increase rate until reaching the operating temperature is lower. Such characteristics are caused by the heat capacity of the thermoswitch itself.

(6) Example of Power Control Circuit Next, a power control circuit that supplies power to the ceramic heater 205 will be described. The power control circuit is configured to be controlled independently by the main heater 302a and the sub heater 302b.

  FIG. 5 is a diagram illustrating a connection example of the power control circuit.

  Reference numeral 501 denotes an arithmetic control CPU, 502 and 503 are first and second triacs, 504 is an AC power source, 505 is a relay (RL), and 507 is a current detection circuit. The first triac 502 and the main heater 302a are connected in series, the second triac 503 and the sub-heater 302b are connected in series, and they are connected to the AC power source 504 in parallel. The first and second triacs 502 and 503 are ON / OFF controlled by the high and low levels of the first and second heater drive signals S1 and S2 from the CPU 501, respectively. The relay 505 is inserted between the first and second triacs 502 and 503 and the AC power source 504, and is configured to cut off the energization of the main heater 302a and the sub heater 302b by driving the relay 505. The control signal S4 of the relay 505 is connected to a safety circuit 509 described later. The current detection circuit 507 is inserted between the relay 505 and the AC power source 504, and sends a current detection signal S5 to the safety circuit. The operation of the current detection circuit 507 will be described later. The safety circuit 509 is controlled by a control signal S3 from the CPU 501.

(7) Example of Power Control Sequence A power control method in the image forming apparatus will be described. In the present embodiment, the main heater 302a and the sub heater 302b are turned on / off at a phase angle within one half wave, thereby performing phase control for controlling the power applied to each heater.

(7-1) Control at Startup When the CPU 501 receives a print start signal from a controller (not shown), it executes an image forming sequence. At the same time, the first and second heater drive signals S1, S2 are controlled to turn on the first and second triacs 502, 503, and the ceramic heater 205 is heated. As the temperature of the ceramic heater 205 rises, the resistance value of the thermistor 1 decreases, and the CPU 501 detects and recognizes the temperature state at the center of the ceramic heater 205 by monitoring the output signal S6 from the thermistor 1. At this time, the applied power (Pup) to the ceramic heater 205 is set from the difference between the temperature detected by the thermistor 1 and a predetermined fixing target temperature. By setting the Pup level large, the temperature of the ceramic heater 205 can be raised in a short time. Pup is set at a power of 80% or more, assuming that the power is 100% when all phases are ON.

(7-2) Steady Temperature Control When it is detected that the temperature detected by the thermistor 1 has risen to a predetermined fixing temperature, the power applied to the ceramic heater 205 is reduced to suppress the temperature of the ceramic heater 205. Thereafter, the applied power is increased or decreased according to the difference between the detected temperature of the thermistor 1 and a predetermined fixing target temperature, so that the temperature at the center of the ceramic heater 205 is controlled to be the fixing target temperature. The applied power (Psat) at this time is smaller than the applied power (Pup) at the start-up, and is controlled at 0% to 60%.

  In the above process, the energization method of the sub-heater 302b is changed according to the length in the direction perpendicular to the recording sheet conveyance direction of the recording sheet, that is, the width size.

  FIG. 7 is a diagram showing the relationship between the width size of the recording paper and the energization setting of the sub-heater 302b. The control method is classified into four according to the width size. Here, the sub-heater power ratio indicates the power ratio applied to the sub-heater 302b with respect to the power applied to the main heater 302a. The second heater drive signal S2 is controlled according to this setting. By setting the power ratio of the sub-heater 302b to be smaller as the paper width is smaller, a phenomenon in which the temperature at the end of the fixing device increases during printing (hereinafter referred to as end temperature rise) is suppressed. Edge temperature rise is largely related to the width size of the recording paper. When the width of the recording paper is smaller than the width of the heating region of the fixing device, the end of the fixing device becomes a non-sheet passing region. Since the amount of heat taken away greatly differs between the portion where the recording paper is passed and the portion where the recording paper is not passed, a phenomenon occurs in which the temperature at the end of the ceramic heater increases. This edge temperature rise may cause various problems such as wrinkles and offsets. If the recording paper to be passed is a narrow recording paper, the non-uniformity of heat of the ceramic heater 205 tends to become larger.

(8) Example of edge temperature rise countermeasure control In this image forming apparatus, the temperature of the ceramic heater 205 at the edge is detected by the thermistors 2 and 3, and the temperature at the edge of the ceramic heater is equal to or higher than a predetermined temperature due to the edge temperature rise. In such a case, control is performed to increase the interval between recording sheets. By increasing the paper passing interval, the temperature difference between the portion where the recording paper is passed and the portion where the recording paper is not passed is reduced, and the temperature rise at the edge can be suppressed.

  FIG. 8 is a diagram showing the transition of the temperature rise at the edge when the recording paper whose paper width is classified as “A” in FIG. 7 is passed. The horizontal axis represents the time from the start of printing, and the vertical axis represents the detected temperature of the thermistor 1 disposed at the center of the ceramic heater 205 and the thermistor 3 disposed at the end. After printing is started, electric power is supplied to the ceramic heater 205 so that the detected temperature of the thermistor 1 becomes a predetermined value, and the detected temperature of the thermistor 1 rises to 200 ° C. (point a).

  Thereafter, the detected temperature of the thermistor 1 is controlled to be constant. On the other hand, the temperature detected by the thermistor 3 rises gradually after printing is started. This is because the sub-heater 302b is not energized when the recording paper whose paper width is classified as “A” is passed. Even if the detection temperature of the sub-thermistor 3 exceeds the detection temperature of 200 ° C. of the thermistor 1 (point b), it reaches 240 ° C. (point c). When the temperature of the thermistor 3 reaches 240 ° C., control for increasing the sheet passing interval is started. This stops the temperature rise at the end.

(9) Example of Current Detection Circuit 507 The current detection circuit 507 is inserted between the relay 505 and the AC power source 504. The current detection circuit 507 detects the total current value of the currents flowing through the main heater 302a and the sub heater 302b, and outputs a detection signal S5. The detection signal S5 is connected to a safety circuit 508 described later. The detection signal S5 is in a high state when the heater current value is larger than the reference current (Ipr). The reference current (Ipr) corresponds to the heater current that flows when the applied power is 80%. As described above, the applied power becomes 80% or more only in the case of control at the time of start-up, so that the detection signal S5 is in a high state only at the time of start-up.

(10) Example of Safety Circuit 509 In the image forming apparatus of this embodiment, a safety device is provided to avoid overheating of the ceramic heater 205 during energization runaway. As a safety device, in addition to the above-described thermoswitch, a circuit that detects abnormal overheating of the ceramic heater 205 by a thermistor and interrupts energization is provided.

  FIG. 9 is a circuit diagram of a safety circuit 509 that controls the interruption of energization by thermistor detection.

  The detection signals S6, S7, and S8 of the thermistors 1, 2, and 3 are input to the negative inputs of the comparators 901, 902, and 903, respectively, and are compared with the comparison voltage (Vref) input to the positive electrodes to cause abnormal overheating. Determine the state. The comparison voltage (Vref1) for the detection signal of the thermistor 1 is compared with a voltage value obtained by dividing Vcc by resistors 904 and 905. When the comparator 901 is turned on as a result of the voltage comparison, a base current flows to the transistor 915 through the resistor 912 and the transistor 915 is turned on. As a result, the relay control signal S4 becomes a low state, the energization to the relay is stopped, and the relay 505 is cut off.

  On the other hand, the comparison voltage (Vref2) for the detection signal of the thermistor 2 is compared with a value determined by the resistors 906, 907, and 908. A resistor 907 has a transistor 925 connected in series. The transistor 925 is driven by the output signal of the current detection circuit 507, and the comparison voltage (Vref2) is switched according to the detection result of the current detection circuit 507. Since the current detection signal S5 is in a high state when the heater current is larger than a predetermined value, the comparison voltage (Vref2) is the combined resistance value of the parallel connection of the resistor 907 and the resistor 908 when the heater current value is large. The voltage is divided with the resistor 906. On the other hand, when the heater current value is small, the current detection signal S5 is in a low state, and the comparison voltage is determined by the divided voltage of the resistors 908 and 906.

  The comparison voltage (Vref3) for the detection signal of the thermistor 3 is compared with a value determined by the resistors 909, 910, and 911. As in the case of the thermistor 2, the comparison voltage (Vref3) is a combined resistance value of the resistor 910 and the resistor 911 connected in parallel and a divided voltage value of the resistor 909 when the heater current value is large. On the other hand, when the heater current value is small, the comparison voltage is determined by the divided voltage of the resistors 909 and 911.

  FIG. 10 is a setting table of the operating temperature of the safety circuit for each thermistor. The operating temperature of the safety circuit 509 is different for each thermistor.

  The operating temperature of the thermistor 1 is 220 ° C. regardless of the heater current state. As described above, in the image forming apparatus of the present embodiment, the ceramic heater 205 is controlled so that the detection value of the thermistor 1 is 200 ° C. Accordingly, the safety circuit 509 does not operate during normal operation, and the safety circuit 509 operates at a detected temperature of 220 ° C. during energization runaway, and the energization to the ceramic heater 205 can be cut off.

  The operating temperature of the thermistor 2 and the thermistor 3 is switched depending on the heater current state. The operating temperature when the heater current is at a low level is set to 260 ° C. During normal operation, even if end temperature rise occurs, the detection values of the thermistors 2 and 3 do not reach 260 ° C., so the safety circuit 509 does not operate. During energization runaway, the thermoswitch is activated at the same time as the ceramic heater 205 reaches 250 ° C., and the energization to the ceramic heater 205 is cut off.

  FIG. 16 is a diagram showing the relationship between the temperature of the ceramic heater 205 of the thermo switch unit and the operating point of the thermo switch during energization runaway. LINE-C is a line when only the main heater 302a is energized, LINE-D is a line when the main heater 302a and the sub heater 302b are energized, and LINE-E is a line when only the sub heater 302b is energized. . The difference between the three lines is caused by the heat distribution of each heater.

  As shown in FIG. 4, since the thermistor 1 is disposed in the heat distribution region of the main heater 302a, the energization runaway of the main heater 302a increases at a faster temperature rise rate than the energization runaway of the sub heater 302b. In any energization runaway, the thermo switch can be operated simultaneously with the temperature of the ceramic heater 205 reaching the operating temperature (250 ° C.) of the thermo switch. This is because when the heater current is at a low level, the applied power of the ceramic heater 205 is low and the temperature rise rate of the ceramic heater 205 is low.

  On the other hand, when the heater current is at a high level, the operating temperature is set to 220 ° C. The heater current becomes high only during the heater start-up operation. As shown in FIG. 8, the temperature rise at the edge is a phenomenon that occurs when the paper is continuously fed, so the temperature at the edge does not exceed 220 ° C. when the heater is started up. Therefore, the safety circuit 509 does not operate during normal operation. During energization runaway, the safety circuit 509 operates at a detected temperature of 220 ° C., and energization of the ceramic heater 205 is interrupted.

  As described above, in the image forming apparatus according to the present embodiment, the detection temperature of the abnormal overheat detection circuit using a plurality of thermistors is switched according to the level of the current flowing through the ceramic heater 205. As a result, even when the temperature rise at the end occurs during normal operation, the energization to the ceramic heater 205 can be cut off at a low temperature during energization runaway without causing the safety device to malfunction.

[Embodiment 2]
A second embodiment of the present invention will be described. In the first embodiment, the determination level of the abnormal overheat temperature of the plurality of thermistors is switched according to the current detection result of the ceramic heater. The basic configuration of this embodiment is the same as that of the first embodiment, and is characterized in that the determination level of the abnormal superheat temperature is switched according to the rotation state of the pressure roller. Only parts different from the first embodiment will be described.

(1) Example of Fixing Drive Motor Rotation Detection FIG. 11 is a circuit configuration diagram of this embodiment.

  Reference numeral 118 denotes a fixing drive motor M2 that rotationally drives the pressure roller 204. The fixing drive motor M2 (118) is controlled at a constant speed by an ACC signal, a BLK signal, and an FG (Frequency Generator) signal. The ACC signal is an acceleration signal and is output from the CPU 501. When the ACC signal is turned on, the rotation speed of the fixing drive motor M2 (118) is accelerated. On the other hand, the BLK signal is a deceleration signal and is output from the CPU 501. When the BLK signal is turned on, the rotational speed is decelerated. The FG signal is a rotation speed detection signal and outputs a pulse having a frequency proportional to the rotation speed. The CPU 501 controls the rotation speed to a predetermined level by controlling the ACC signal and the BLK signal so that the frequency of the received FG signal becomes a predetermined value.

  On the other hand, the FG signal is connected to a motor rotation detection circuit 1102. The motor rotation detection circuit 1102 detects whether or not the motor is stopped from the FG signal, and sends a detection signal S10 to the safety circuit 509. FIG. 12 shows an internal circuit of the motor rotation detection circuit 1102. The input FG signal is frequency-divided into a half-frequency rectangular wave by the D flip-flop circuit 1201 to drive the gate voltage of the FET 1201. A rectangular wave having an amplitude of 24 V is applied to the capacitor 1204 by driving the FET 1204, and a current flows through the diode 1205. The current is input to an integrating circuit composed of an operational amplifier 1211, a resistor 1209, and a capacitor 1210, where it is converted into a DC voltage. The output value (Vop) of the operational amplifier 1211 can be expressed by the following equation.

Vop = Vt− (24−Vt) × C ₁₂₀₄ × R ₁₂₀₉ × f ÷ 2
Here, Vt is the positive input voltage of the operational amplifier 1211, C ₁₂₀₄ is the capacitance value of the capacitor 1204, R ₁₂₀₉ is the resistance value of R1209, and f is the frequency of the FG signal. As is clear from the above equation, the output value (Vop) of the operational amplifier 1211 is a value corresponding to the frequency of the FG signal. The output of the operational amplifier 1211 is further input to the positive input of the comparator 1214 and is compared with the reference voltage set in the negative input, and the high / low state of the comparator output is switched according to the frequency of the FG signal. In this way, the rotation detection signal S10 for detecting the presence or absence of rotation of the fixing drive motor M2 (118) is output.

(2) Example of Safety Circuit 509 ′ FIG. 13 is a circuit diagram of a safety circuit 509 ′ that controls the interruption of energization by thermistor detection.

  The difference from the safety circuit 509 of the first embodiment is a method of switching the comparison voltage: Vref for the detection signal of the thermistor 2. The comparison voltage (Vref2) for the detection signal of the thermistor 2 is switched according to the detection result of the motor rotation detection circuit 1102. That is, the operating temperature of the safety circuit 509 'is switched according to the rotation state of the motor. The rotation detection signal S10 is in a low state when the fixing drive motor M2 (118) is rotating. Therefore, at the time of rotation, the comparison voltage (Vref2) becomes a combined resistance value of the parallel connection of the resistor 907 and the resistor 908 and a divided value of the resistor 906. On the other hand, at the time of non-rotation, the rotation detection signal S10 is in the HIgh state, and the comparison voltage (Vref2) is determined by the divided voltage of the resistor 908 and the resistor 906.

  FIG. 14 is a setting table of the operating temperature of the safety circuit 509 ′ for each thermistor. The judgment level of abnormal overheating is different for each thermistor. The operating temperature is different from that of the first embodiment only for the thermistor 2.

  When the fixing drive motor M2 (118) rotates, the operating temperature is set to 260 ° C. Even when the end temperature rise occurs during normal operation, the detected value of the thermistor 2 does not reach 260 ° C., so that the safety circuit 509 ′ does not operate during normal operation.

  During energization runaway, the ceramic heater 205 reaches 250 ° C., and at the same time, the thermo switch is activated to cut off the energization to the ceramic heater 205. This is because when the fixing drive motor M2 (118) rotates, that is, when the pressure roller 204 is in a rotating state, the temperature rise of the pressure roller 204 becomes small, the temperature rise rate of the ceramic heater 205 becomes low, and the ceramic heater As soon as the temperature of 205 reaches the operating temperature (250 ° C.) of the thermo switch, the thermo switch can be operated.

  On the other hand, when the fixing drive motor M2 (118) is not rotating, the temperature is set to 220 ° C. Since the temperature rise at the edge is a phenomenon that occurs when the paper is continuously passed, the temperature at the edge does not become 220 ° C. or higher when the motor is not rotating. Therefore, the safety circuit 509 'does not operate during normal operation. During energization runaway, the safety circuit 509 'operates at a detected temperature of 220 ° C., and the energization to the ceramic heater 205 is interrupted.

  As described above, in the image forming apparatus of the present embodiment, the detection temperature of one detection circuit is switched among the abnormal overheat detection circuits using a plurality of thermistors according to the rotation state of the pressure roller. In addition, the detection temperature of the detection circuit other than the above is switched among the abnormal overheat detection circuits using a plurality of thermistors according to the detection value of the current flowing through the ceramic heater 205.

  As a result, even when the temperature rise at the end occurs during normal operation, the energization to the ceramic heater 205 can be cut off at a low temperature during energization runaway without causing the safety device to malfunction.

  The circuit elements of the safety circuit 509 (FIG. 9) and 509 ′ (FIG. 13) and the motor rotation detection circuit 1102 (FIG. 12) do not show specific characteristic values. The characteristic value corresponding to the temperature value to be switched can be set according to the Vcc value, etc., and those skilled in the art can easily set the characteristic value.

  In this embodiment, the safety circuits 509 (FIG. 9), 509 ′ (FIG. 13) and the motor rotation detection circuit 1102 (FIG. 12) are shown as analog circuits, but information such as temperature, current, and rotation is digitally displayed. It may be converted into data and relay control may be performed by a program.

  FIG. 17 is a flowchart showing a procedure example of relay control by a program. FIG. 17 shows the case of the second embodiment, but the case of the first embodiment can be realized by a similar flowchart.

  In step S171, it is determined whether the output data from the thermistor 1 is 220 ° C. or higher. If it is 220 degreeC or more, it will progress to step S178 and will make the relay 505 cut off. If it is less than 220 ° C., the process proceeds to step S172, and it is determined from the output data from the motor rotation detection circuit whether the pressure roller is rotating / non-rotating.

  If it is rotation, it is determined in step S173 whether the output data from the thermistor 2 is 260 ° C. or higher. If it is 260 degreeC or more, it will progress to step S178 and the relay 505 will be cut off. If it is less than 260 ° C., the process proceeds to step S175. On the other hand, if it is not rotating, it is determined in step S174 whether the output data from the thermistor 2 is 220 ° C. or higher. If it is 220 degreeC or more, it will progress to step S178 and will make the relay 505 cut off. If it is less than 220 ° C., the process proceeds to step S175.

  In step S175, it is determined whether the heater current from the current detection circuit 507 is higher than a predetermined value. If the current is low, it is determined in step S176 whether the output data from the thermistor 3 is 260 ° C. or higher. If it is 260 degreeC or more, it will progress to step S178 and the relay 505 will be cut off. If it is less than 260 ° C., the process proceeds to step S179 and the relay 505 is brought into contact. On the other hand, if the current is high, it is determined in step S177 whether the output data from the thermistor 2 is 220 ° C. or higher. If it is 220 degreeC or more, it will progress to step S178 and will make the relay 505 cut off. If it is less than 220 ° C., the process proceeds to step S175, and the relay 505 is brought into contact.

  101 Image forming apparatus, 116 fixing apparatus, 205 ceramic heater, 206 thermistor, 204 pressure roller, 502 triac, 501 CPU, 505 relay, 901 operational amplifier, 915 transistor, 1205 diode

Claims (2)

A tubular film,
A heater in which a heat generation pattern is formed on the substrate and contacting the inner surface of the cylindrical film;
A pressure roller that forms a nip portion for conveying recording paper together with the heater through the cylindrical film;
A first temperature detecting element for detecting a heater temperature in a region through which a recording sheet of a minimum size usable in the apparatus passes in the longitudinal direction of the heater;
A second temperature detecting element for detecting a heater temperature outside the passing area of the minimum size recording paper;
A control unit for controlling energization from a power source to the heat generation pattern so that a detection temperature of the first temperature detection element maintains a target temperature;
A relay provided in a power supply circuit of the power source and the heat generation pattern;
A safety circuit that drives the relay independently of the control unit, and that drives the relay to open the power supply circuit when the detected temperature of the second temperature detecting element reaches an operating temperature. Circuit,
In the heat fixing apparatus having
A rotation detection circuit for detecting a rotation state of the pressure roller;
The heating and fixing device, wherein the operating temperature of the safety circuit is set according to a detection result of the rotation detection circuit.   The heat fixing device according to claim 1, wherein the operating temperature when the pressure roller is stopped is set lower than the operating temperature when the pressure roller is rotating.

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