JP4232651B2 - Heating device and fixing device - Google Patents

Description

  According to the present invention, a fluctuating magnetic field is generated by an exciting coil and heated by an eddy current generated in a conductive layer of an object to be heated, and powder toner is attached to a latent image due to a difference in electrostatic potential. The present invention relates to a fixing device in which a formed visible image is heated and melted by the heating device and fixed on a recording sheet.

  In general, the step of fixing a toner image in an image forming apparatus using powdered toner is to electrostatically transfer the toner image onto a recording medium, and then sandwich the recording medium between a heating member and a pressure member. A method in which a toner image is heated and melted and pressed onto a recording medium is widely used.

  As means for heating the heating member, Japanese Patent Application Laid-Open No. 10-254263 (Patent Document 1), Japanese Patent Application Laid-Open No. 11-352804 (Patent Document 2), etc. are provided with a conductive layer on the heating member, and electromagnetic induction heating is performed. In which the conductive layer generates heat is described.

  In the electromagnetic induction heating, an exciting coil that generates a variable magnetic field is disposed close to a conductive layer, and heat is generated by an eddy current generated in the conductive layer. According to this electromagnetic induction heating, the heating member can be directly heated, and the range of high temperature becomes a very limited range, and the heating member can be heated to a predetermined temperature in a short time. For this reason, compared with the case where a heating element such as a halogen lamp is used as a heating source, the warm-up time of the apparatus can be shortened and the power consumption can be reduced.

  On the other hand, as the heating member (fixing member), an endless fixing belt is generally used in addition to the heating roll, and the endless belt has a type stretched by a plurality of support rolls and a pressing member inside. However, there is a type that is driven around in a non-tensioned state. The fixing belt has a thin heat resistant resin or the like as a base layer and has a smaller heat capacity than the heating roll, so that it can warm up in a shorter time than the heating roll. Furthermore, the non-stretching type fixing belt can reduce the contact area with other members, and heat transfer to the other members is reduced. For this reason, more efficient warming up can be performed.

  As described above, in an apparatus that uses an endless belt as a heating member and electromagnetically heats the endless belt, when the endless belt is stretched by a plurality of rolls, the endless belt is stretched as described in Patent Document 2. An exciting coil is disposed so as to face the inner surface or the outer surface of the belt. On the other hand, in the case where the endless belt is driven to circulate in an unstretched state, it is described in Japanese Patent Application Laid-Open No. 2003-84591 (Patent Document 3) and Japanese Patent Application Laid-Open No. 2003-91185 (Patent Document 4). In addition, the exciting coil is disposed so as to face the outer peripheral surface of the endless belt in close proximity. Then, a fluctuating magnetic field is generated in a direction penetrating the belt, and an eddy current is induced around the magnetic field.

The high-frequency current supplied to the exciting coil is generally generated by switching direct current at high frequency, and constant current control or constant power control is performed. In addition, the power supply to the exciting coil is achieved by detecting the temperature of the fixing member, which is a heated object, with a temperature sensor, and controlling the amount of power supplied so that a predetermined temperature is maintained, or power supply ON / OFF control Is made.
Japanese Patent Laid-Open No. 10-254263 JP 11-352804 A JP 2003-84591 A JP 2003-91185 A

  Although the heating device using electromagnetic induction and the fixing device using the same have the advantages as described above, there are also problems. One of them is the advantage that the heating rate is fast. There is difficulty in safety measures when the heating body becomes abnormally high temperature (overheating). As a safety measure against this abnormally high temperature, in the case of a conventional fixing device using a halogen lamp, a thermostat or a thermal fuse is brought into contact with the fixing roller or arranged in the vicinity thereof, and the fixing roller reaches a certain temperature. In such a case, the current path to the halogen lamp is interrupted to prevent overheating of the fixing roller.

  However, thermostats and thermal fuses have a certain time delay in their operation. That is, the actual temperature of the object to be heated is detected with a delay, and when the thermistor or the like detects a predetermined reference temperature and operates, the object to be heated is higher than the reference temperature. In the case of a halogen lamp with a slow temperature rise rate, it is still possible to prevent overheating of the fixing roller sufficiently, but in the case of electromagnetic induction heating, the temperature rise rate is so fast that it cannot be controlled properly. Occurs. Further, when the object to be heated is a belt having a small heat capacity, the above problem becomes particularly remarkable.

FIG. 9 is a schematic diagram showing an example of the difference between the temperature of the object to be heated and the temperatures of the bimetal and the fuse that detect this temperature.
Even in the case of a thermostat that uses a bimetal in direct contact with the object to be heated, the object to be heated becomes an abnormally high temperature T0, and it takes about 50 to 60 seconds (t0 to t1) to detect the temperature T0. In the case of a fuse, since it cannot be brought into direct contact with the fixing roller, it takes approximately 100 seconds (t0 to t2). In the meantime, the fixing roller further increases from the abnormal temperature T0 to temperatures T1 and T2. In the case of a rotating body with a small heat capacity, since the gradient of temperature rise further increases, the temperature (T1, T2) that rises during such a heat transfer delay also increases, and high temperature heat resistance that can withstand these temperatures. It is necessary to use a member. On the other hand, if the reference temperature for control is set to a temperature (T3 or T4) that is lower than the temperature T0 at which the object to be heated is abnormal, the temperature detection error becomes large, and if the temperature rise is large, this tendency Becomes prominent.

  The present invention has been made in view of the above problems, and its purpose is to detect abnormally high temperatures quickly even when the temperature of a heated object is rapidly increased by electromagnetic induction heating. The present invention provides a heating device that can effectively prevent overheating, or a fixing device using such a heating device.

In order to achieve the above object, the present invention according to claim 1 includes an exciting coil for generating an eddy current in a conductive layer of an object to be heated, and the exciting current is turned on / off by a switching element. A high frequency generation circuit for generating a high frequency current in the coil, a power value indicating means for specifying a power value to be supplied to the exciting coil, and an ON duty width corresponding to the power value specified by the power value indicating means. A drive circuit for outputting the drive signal to the switching element, voltage detection means for detecting a flyback voltage generated in a resonance circuit including the excitation coil in the high-frequency generation circuit, and a reference voltage value A reference voltage value output means, and compares the flyback voltage value detected by the voltage detection means with the voltage value output from the reference voltage value output means; When the voltage value exceeding or when the reference voltage value is equal to or higher than the reference voltage value, and a fault detection means for outputting a signal indicating said power value instruction means, the temperature of the object to be heated is a predetermined Different power values are specified when heating to increase the temperature and when the heated object is maintained at a predetermined temperature, and the reference voltage value is different as specified by the power value indicating means Provided is a heating device characterized in that, for each power value, the temperature of an object to be heated is set as a reference whether or not the temperature is abnormally high .
In the above configuration, the flyback voltage is a voltage generated in the resonance circuit after the switching element is turned from the ON state to the OFF state.
The reference voltage values are set in correspondence with the designated power values. For example , each of the reference voltage values is set as a function of the designated power value.

  In this heating device, a high-frequency current is supplied to the exciting coil by turning on / off the switching element, and the exciting coil generates a variable magnetic field. Due to this fluctuating magnetic field, an eddy current is generated in the conductive layer disposed in the vicinity of the exciting coil, and Joule heat is generated. The O / OFF timing of the switching element is set based on the power value designated by the power value indicating means, and the drive circuit adjusts the ON duty width so as to correspond to the designated power value. To output a drive signal.

  When the high frequency current is supplied to the exciting coil, a flyback voltage is generated when the switching element is turned off from the state where the switching element is turned on and the current is supplied, and this is detected by the voltage detection means. Is done. The detected voltage value is compared with a reference voltage value based on the power value designated by the power value indicating means. In other words, even if the ON duty width is set based on the specified power value, the specific resistance value of the excitation coil and the high frequency generation circuit increases and the magnetic coupling decreases when the heated object and the excitation coil are heated to a high temperature. As a result, the flyback voltage value increases. Therefore, by comparing this value with a reference voltage value set in advance corresponding to the designated power value, it is possible to immediately detect when the temperature of the object to be heated becomes an abnormally high temperature.

Further, in this fixing device, different reference voltage values are used when the object to be heated is heated from a low temperature to a predetermined temperature as in the initial heating of the object to be heated and when the object to be heated is maintained at a predetermined temperature. These are set, and these are switched and compared with the detected value of the flyback voltage. Thereby, efficient heating can be performed according to the situation, and the abnormal temperature can be detected accurately. For example, when this heating device is used in a fixing device for fixing a toner image, the power value to be input is often different between preheating (warming up) at the time of starting the device and during steady operation, and the coil The temperature and the heating rate are also different. During preheating, a large amount of electric power is applied and heating is performed rapidly. During steady operation, the input electric power is kept low and stable temperature control is performed. By setting the reference voltage value according to such a difference in situation, it is possible to take an appropriate countermeasure against abnormally high temperatures.

The invention according to claim 2 is the heating apparatus according to claim 1, wherein the abnormality detection unit is configured to check that the flyback voltage value is equal to or higher than the reference voltage value or exceeds the reference voltage value. When detecting multiple times, a signal indicating abnormality is output.

  Even if the abnormality detection means detects a value exceeding the reference voltage value, if this is a false signal due to breakthrough noise or the like, the device will malfunction. However, this heating device determines that the object to be heated has an abnormal temperature when a signal indicating that the flyback voltage value is equal to or higher than the reference voltage value is detected a plurality of times. Operation can be avoided. For example, the flyback voltage value is compared with the reference voltage value at a certain sampling period, and the number of times that an abnormal temperature is detected is counted. When the number of times reaches a predetermined value, it is determined that the object to be heated is at an abnormal temperature, and the operation of the high frequency generation circuit is stopped to prevent thermal runaway in advance.

The invention according to claim 3 is the heating apparatus according to claim 1, wherein the abnormality detection unit is configured to check that the flyback voltage value is equal to or more than the reference voltage value or exceeds the reference voltage value. When it is detected that the reference voltage value is again exceeded or exceeded after a predetermined time has passed, a signal indicating an abnormality is output.

  As in the invention according to claim 3, the fixing device prevents malfunction due to noise or the like, and the abnormality detection means detects a value equal to or higher than the reference voltage value, and further, after the predetermined time has elapsed, the reference voltage is also detected. When a value equal to or greater than the value is detected, it is determined that the abnormal state continues, and the operation of the high frequency generation circuit can be appropriately stopped without causing a malfunction.

According to a fourth aspect of the present invention, there is provided a fixing member having an endless peripheral surface and contacting a toner image formed of powder toner, a heating device for heating the fixing member, and being pressed against the fixing member. A fixing device that presses a toner image heated in contact with the fixing member against a recording medium that passes between the pressing member and the fixing member; 4 has a conductive layer along an endless peripheral surface, and the heating device provides a fixing device which is the heating device according to any one of claims 1 to 3 .

  In this fixing device, the abnormal temperature of the fixing member heated by the electromagnetic induction heating device can be quickly detected and the heating can be stopped appropriately. Even if a thin belt or the like is used for the fixing member, Damage can be reliably prevented.

  As described above, in the heating device according to the present invention, the flyback voltage generated in the resonance circuit having the exciting coil in the high frequency generation circuit is detected, and a reference set in advance corresponding to the power value to be supplied is detected. By comparing with the voltage value, the abnormality can be detected instantaneously. Furthermore, by setting a plurality of reference voltage values, abnormal high temperatures can be accurately detected even when the indicated value of supplied power is changed according to the situation, and the accuracy of abnormal high temperature detection is improved.

Hereinafter, embodiments of the invention according to the present application will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of an image forming apparatus using a heating device and a fixing device according to an embodiment of the present invention.
The image forming apparatus includes a cylindrical photosensitive drum 1 on which a latent image is formed on the surface by irradiation with image light after uniform charging, and around the photosensitive drum 1. A charging device 2 for uniformly charging the surface of the photosensitive drum 1, an exposure device 3 for irradiating the photosensitive drum 1 with image light to form a latent image on the surface, and a toner on the latent image on the photosensitive drum. Is transferred to the developing unit 4 to form a toner image, the endless belt-shaped intermediate transfer member 5 that is opposed to the photosensitive drum 1 and is supported so as to be able to circulate on the peripheral surface, and after the toner image is transferred. A cleaning device 6 that removes toner remaining on the photosensitive drum 1 and a static elimination exposure device 7 that neutralizes the surface of the photosensitive drum 1 are provided.

  Further, inside the intermediate transfer member 5, a transfer charger 8 for primarily transferring the toner image formed on the photosensitive drum to the intermediate transfer member 5, two support rolls 9a and 9b, and secondary transfer are performed. A transfer counter roll 10 for carrying out is disposed, and the intermediate transfer body 5 is stretched around these rolls so as to be able to go around. A transfer roll 11 for transferring a toner image on the intermediate transfer body onto a recording sheet is disposed at a position facing the transfer opposing roll 10 via the intermediate transfer body 5. The transfer opposing roll 10 and the transfer roll The recording paper P is fed from a paper tray (not shown) into the pressure contact portion with the paper 11. Further, on the downstream side of the press contact portion, a fixing device 12 that heats and melts the toner image on the recording paper and press-bonds it to the recording paper, and removes the toner remaining on the intermediate transfer member at a position along the intermediate transfer member 5. A cleaning device 13 is provided.

  The photosensitive drum 1 has a metal drum surface with various inorganic photosensitive materials such as Se, a-Si, a-SiC, and Cds, organic photosensitive materials, amorphous selenium photosensitive materials, amorphous silicon photosensitive materials, and the like. What formed the layer can be used.

  The charging device 2 is obtained by coating a metal roll having conductivity such as stainless steel or aluminum with a coating of a high resistance material. The charging device 2 is in contact with the photosensitive drum 1 and is driven to rotate. When a predetermined voltage is applied, a continuous discharge is generated in a minute gap in the vicinity of the contact portion between the roll and the photosensitive drum 1, and the surface of the photosensitive drum 1 is charged almost uniformly. It is.

  The exposure device 3 generates blinking laser light based on the image signal, and scans this in the main scanning direction of the photosensitive drum 1 by a polygon mirror. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 1.

  In the developing unit 4, four developing devices 4a to 4d containing yellow, magenta, cyan, and black toners are rotatably supported, and the developing devices 4a to 4d are in close proximity to the photosensitive drum 1 in order. Oppositely, toner is transferred to a latent image corresponding to each color to form a visible image.

  The belt-like intermediate transfer member 5 has a two-layer structure of a base layer and a surface layer, and the base layer includes polyimide, polyamide, polyamideimide, etc. in consideration of heat resistance, strength, and surface smoothness. The surface layer is made of silicone rubber, fluorine rubber, or the like which has high heat resistance and high releasability from the toner.

  The transfer charger 8 applies a voltage to the back surface of the intermediate transfer member 5 by applying a voltage so that the toner image on the photosensitive drum 1 is efficiently transferred to the intermediate transfer member 5. The transfer roll 11 is made of a conductive or semiconductive roll-shaped member, and a toner image on the intermediate transfer member 5 is collectively applied to the recording paper P by applying a voltage between the transfer opposing roll 10. Transcript.

In such an image forming apparatus, the toner image is formed by the following process.
First, the surface of the photosensitive drum 1 is charged almost uniformly by the charging device 2, and subsequently, image light is irradiated from the exposure device 3 to form a latent image on the surface of the photosensitive member 1 due to the difference in electrostatic potential. . Then, the rotation of the photosensitive drum 1 moves to a position facing one developing device 4a of the developing unit 4, and the toner of the first color is transferred from the developing device 4a to form a toner image. This toner image is conveyed to a position facing the intermediate transfer member 5 by the circumferential movement of the photosensitive drum 1 and is electrostatically primary-transferred onto the intermediate transfer member. On the other hand, the toner remaining on the photosensitive drum 1 after the primary transfer is removed by the cleaning device 6, and the surface of the photosensitive drum 1 is potentialally initialized by the static elimination exposure device 7, and again at a position facing the charging device 2. Moving.

  Thereafter, the three developing devices 4b, 4c, and 4d of the developing unit 4 sequentially move to positions facing the photosensitive drum 1, and similarly, second, third, and fourth color toner images are sequentially formed, and intermediate transfer is performed. It is transferred onto the body 5 in a superimposed manner. At this time, the transfer roll 11 for secondary transfer is retracted to a position where it does not come into contact with the intermediate transfer member 5.

  The toner image superimposed on the intermediate transfer member 5 is conveyed to a position where the transfer roller 11 and the transfer opposite roller 10 face each other by the circumferential movement of the intermediate transfer member 5, and comes into contact with the recording paper P fed from the paper tray. Is done. A transfer bias voltage is applied between the transfer roll 11 and the intermediate transfer member 5, and the toner image is secondarily transferred onto the recording paper. Then, the recording paper P on which the toner image is transferred is conveyed to the fixing device 12.

Next, the configuration of the fixing device 12 will be described with reference to FIG.
The fixing device 12 includes an endless fixing belt 21, an electromagnetic induction heating device 22 supported at a position facing the peripheral surface of the fixing belt 21, a pressure roll 23 pressed against the fixing belt 21, and a fixing belt. 21, a pressure facing member 24 that sandwiches the fixing belt 21 with the pressure roll 23, and a peeling member 26 that is supported near the downstream side of the pressure contact portion between the pressure roll 23 and the fixing belt 21. And have. Further, a temperature sensor 27 that measures the temperature of the peripheral surface of the fixing belt 21 is provided on the upstream side of the pressure contact portion between the pressure roll 23 and the fixing belt 21. An inverter circuit 28 that generates a high-frequency current and supplies it to the electromagnetic induction heating device 22 and a control device 29 that outputs a drive signal to the inverter circuit 28 and controls the operation of the electromagnetic induction heating device are provided. The detection signal of the temperature sensor 27 is input to the control device 28. The control device 29 may have a function of controlling the driving of the entire image forming apparatus.

  As shown in FIG. 3 (a), the fixing belt 21 has a base layer 21a made of a sheet member having high heat resistance, represented by polyimide, polyimide amide, etc., and a conductive layer 21b thereon. The elastic layer 21c and the surface release layer 21d are laminated. The conductive layer 21b is made of, for example, a magnetic metal such as iron, magnetic stainless steel, nickel or the like, or a main alloy thereof, a nonmagnetic metal such as nonmagnetic stainless steel, aluminum, or copper, or a main alloy thereof having a thickness of 5 μm to 50 μm. The material is selected so that the specific resistance value is such that sufficient heat can be obtained by electromagnetic induction. The elastic layer 21c adheres the surface of the fixing belt 21 to the toner image on the recording paper, and has heat resistance, high thermal conductivity such as silicone rubber, fluorine rubber, fluorosilicone rubber, etc., and has a thickness. Is preferably 10 μm to 500 μm and the hardness is 600 (JIS-KA type tester) or less. As the surface release layer 21d, a layer having a thickness of 1 μm to 100 μm, such as a fluororesin, a silicone resin, or the like and having high release properties and heat resistance can be used.

  Instead of the fixing belt 21, a fixing belt 121 as shown in FIG. 3B may be used. In this fixing belt, heat-resistant resin layers 121a and 121c are formed in two layers, and a conductive layer 121b is formed between them. And the elastic layer 121d and the surface release layer 121e are laminated | stacked on the surface. In this fixing belt, even if the metal layer which is a conductive layer is formed thin, deterioration due to repeated bending deformation can be suppressed.

  The pressure roll 23 includes an elastic layer 23b such as sponge or rubber on the surface of a metal core 23a having a circular cross section, and a surface layer 23c covering the surface. The pressure roll 23 is rotationally driven by a drive motor (not shown), and the fixing belt 21 is driven to rotate accordingly.

  The electromagnetic induction heating device 22 is arranged so that the gap with the outer peripheral surface is 1 to 3 mm along the outer peripheral surface of the fixing belt 21, and heats the conductive layer 21b of the fixing belt 21. is there. The electromagnetic induction heating device 22 includes a magnetic core 22b made of ferrite or the like supported on a pedestal 22a, and an excitation coil 22c wound around the magnetic core 22b.

  The pressure facing member 24 supports an elastic member 24a having a contact surface curved in a shape along the peripheral surface of the pressure roll 23, and supports the elastic member 24a and is fixed at both ends in the width direction of the fixing belt. A support member 24b that is supported, and a belt traveling guide (not shown) that is supported by the support member 24b, abuts against the inner peripheral surface at both ends in the width direction of the fixing belt 21, and regulates the position of the fixing belt 21. The main part is composed.

  The peeling member 26 is disposed so that the tip thereof is close to or in contact with the fixing belt 21, and the tip portion enters between the recording paper P and the fixing belt 21 that have passed through the pressure contact portion, and the recording paper P is placed on the fixing belt 21. Peel from.

  The temperature sensor 27 is disposed to face the fixing belt 21, detects the temperature of the fixing belt 21, and outputs the detected value to the control device 29. The temperature sensor 27 detects the temperature in a non-contact manner using infrared rays, but a sensor that measures the temperature in contact with the measurement object can also be used. For example, a thermocouple using the Seebeck effect, a thermistor using an element having a large temperature coefficient of resistance, or the like can be used. The temperature sensor 27 may detect the temperature at one place, but it is desirable to provide a plurality of temperature sensors so as to detect the temperature at a plurality of positions.

FIG. 4 is a schematic configuration diagram showing the inverter circuit 28 and the control device 29.
A direct current (DC) obtained by rectifying an alternating current from a commercial power supply 31 (AC 100 V) by a rectifier circuit 32 is input to the inverter circuit 28 which is a high frequency generation circuit. And supplied to the exciting coil 22c. The exciting coil 22c is connected in parallel with the resonance capacitor 34, and forms an LC resonance circuit. The switching element 33 is driven by a drive signal output from the control device 29.

FIG. 5 shows the current value and voltage value in each part when the switching element 33 of the inverter circuit 28 is turned ON / OFF.
When the switching element 33 is turned on, a current is generated in the exciting coil 22c, and the amount of current gradually increases due to the inductance. When the switching element 33 is turned off, the current amount of the exciting coil (L) 22c gradually decreases, and the capacitor 34 is charged. As a result, the flyback voltage values at Q and D in FIG. 4 increase and then decrease. Further, the current of the exciting coil 22c turns in the opposite direction, and a regenerative current in a very short time is generated in the diode 35 inserted in parallel with the switching element 34. Then, the switching element 34 is turned on again. In such switching, the LC of the inverter circuit determines the timing from the OFF state to the ON state, and the time during which the switching element 34 is turned ON by setting one switching period, that is, the ON duty width is set. Determined. When the ON duty width increases, the amount of supplied power increases. The one period of the switching or the ON duty width is set based on the power value (indicated power value) to be supplied to the exciting coil. It has become.

  The control device 29 attempts to supply power to the excitation coil 22c based on the control circuit 43 that performs ON / OFF control of power supply to the excitation coil based on the detected temperature of the fixing belt 21 that is a heated body. Based on a signal corresponding to the command power value output from the power value command means and the power value command means 42 for setting the power value to be used as the command power value, a drive signal is sent to the switching element 33 by adjusting the ON duty width. And a drive circuit 41 that outputs (pulse signal). Furthermore, a voltage detection means 36 for detecting a flyback voltage generated in a resonance circuit including the excitation coil 22c, and a reference voltage value for outputting a reference voltage value corresponding to the indicated power value output from the power value indicating means 42 An output unit 38, and an abnormality detection unit 37 that compares the voltage values output from the voltage detection unit 36 and the reference voltage value output unit 38 and detects overheating of the fixing belt 21 as a heated body are provided. Is.

  The power value indicating means 42 reduces the indicated value of the power to be supplied when heating is rapidly performed when power is supplied to the exciting coil 22c based on the detected temperature of the fixing belt 21 that is a heated object. However, when the amount of heat taken by the recording medium or the like is large and the temperature tends to decrease, the instruction power value is set large. Further, different command power values are set when the fixing belt 21 is preheated and when the fixing operation is performed.

The reference voltage value output means 38 outputs a reference voltage value, that is, a reference voltage value for determining whether the temperature of the exciting coil is normal or abnormal, and calculates a value corresponding to the indicated power value. Output. When the command power value is large, the ON duty width of the drive signal output from the drive circuit is increased, and the flyback voltage is increased accordingly. Therefore, in order to detect an increase in flyback voltage due to an abnormal temperature increase, it is necessary to set the reference voltage value corresponding to a difference in the indicated power value.
In addition, when the fixing belt as a heated body is heated rapidly and when heating is performed to maintain the fixing belt at a predetermined temperature, the amount of power to be supplied, the temperature of the exciting coil, and the temperature The speed of change is different, and taking these into account, different settings are used for the calculation of the reference voltage value during preheating and during driving of the fixing device.

  The abnormality detection unit 37 compares the flyback voltage value detected by the voltage detection unit 36 with a reference voltage value. When the reference voltage value is exceeded, the temperatures of the fixing belt 21 and the excitation coil 22c are increased. Judge that the impedance is excessively high due to abnormally high temperature. In order to prevent malfunction due to noise or a sudden abnormal signal, a signal indicating an abnormality is output when a value exceeding the reference voltage value is detected a predetermined number of times.

Next, the operation of the fixing device 12 will be described.
A high-frequency current is supplied from the inverter circuit 28 to the exciting coil 22c substantially simultaneously with or after the pressurizing roll 23 starts driving and the fixing belt 21 is driven to start circulation. When a high frequency current is supplied to the exciting coil 22c, the magnetic flux indicated by the arrow H around the exciting coil 22c repeats generation and disappearance. Then, when this magnetic field H crosses the conductive layer of the fixing belt 21, an eddy current is generated in the conductive layer so as to prevent a change in the magnetic field, and the skin resistance and the conductive layer of the conductive layer are generated. It generates heat in proportion to the current flowing through it.

  The recording paper P sent to the fixing device is heated and pressed by the fixing belt 21 and the pressure roll 23 heated to a predetermined temperature by the electromagnetic induction heating device 22, and the toner image T is melted and pressed on the recording paper P. The When the recording paper P is fed out from the pressure contact portion between the pressure roll 23 and the elastic member 24a, the recording paper P tends to move straight in the fed-out direction due to its rigidity, and is easily peeled off from the bent fixing belt 21. Further, it is peeled off from the fixing belt 21 by the peeling member 26.

The fixing belt 21 needs to be maintained at an appropriate temperature for fixing the toner image, and the control of the temperature and the control for preventing overheating in the event of an abnormality are based on the output of the temperature sensor 27 and the like. Based on the following.
In the fixing device 12, for example, when the power switch is turned on, initial heating (warming up) is performed, an ON signal is output from the control circuit 43, and the power value indicating means 42 drives the drive circuit. The command power value is input to 41. In the drive circuit 41, a drive signal in which the ON duty width is set based on the command power value is output, and the switching element 33 of the inverter circuit 28 is turned on / off. As a result, a high frequency current is generated in the inverter circuit 28. As a result, the exciting coil generates a variable magnetic field, and an eddy current is generated in the conductive layer of the fixing belt 21 to generate heat. At this time, the temperature of the fixing belt 21 is detected by the temperature sensor 27, and when the temperature reaches a predetermined temperature, preparation for the fixing operation is completed.

  Thereafter, the setting is switched to the normal driving setting for fixing the unfixed toner image, and when the recording paper P carrying the unfixed toner image is fed, the fixing belt 21 is pressed against the recording paper P to add the toner image. Fix by heating with pressure. In these steps, the temperature of the fixing belt 21 is detected by the temperature sensor 27, and based on these outputs, the power value indicating means outputs the indicated power value. That is, if the detected temperature is high, it is considered that the supplied power is excessive, and the power value indicating means 42 changes the command power value to a smaller value. Further, when the temperature of the fixing belt 21 decreases, the supplied power is insufficient, and the indicated power value is changed to a larger value.

  In addition, even if the specified power value is set to a small value, if the supplied power becomes excessive, the control circuit 43 outputs a signal for ON / OFF control of the supply of the high-frequency current to the power value indicating means 42. The As a result, the drive signal from the drive circuit 41 is temporarily stopped, and the temperature of the fixing belt 21 decreases due to heat transfer and heat dissipation from the fixing belt 21 to the recording paper P and the toner image. Thereafter, when the value falls below a predetermined control lower limit value, the control circuit 43 instructs the power value to be supplied from the power value instruction means 42 and outputs the drive signal from the drive circuit 41, that is, the signal for turning on the supply of the high-frequency current. The instructing signal is output to the drive circuit 41. As a result, a drive signal is output, a high-frequency current is supplied to the exciting coil, and the fixing belt 21 is heated. When the temperature of the fixing belt 21 rises and reaches the control upper limit value, the signal from the control circuit changes to an OFF signal, and the drive signal from the drive circuit 41 is stopped. Accordingly, the temperature of the fixing belt 21 is controlled between the predetermined control upper limit value and the control lower limit value.

The abnormality detection means is controlled as described above, and prevents overheating of the fixing belt, which is an object to be heated, as follows when the drive signal is output.
The fixing belt 21 including the conductive layer has a characteristic that the specific resistance value increases as the temperature increases due to electromagnetic induction heating. When the induction coil 22c side is the primary side and the fixing belt 21 side to be heated is the secondary side, the resistance and inductance components are R1, L1, R2, and L2, and the coupling factor is A. The load impedance Z is
Z = (R1 + A ² * R2) + jω (L1−A ² * L2)
However, A ≒ M / L2 (M is mutual inductance)
It is. Since the resistance component of the load impedance decreases and the inductance component increases due to the temperature rise, it can be seen that this phenomenon occurs when the coupling factor A decreases.

  As described above, the flyback voltage increases when the magnetic coupling decreases as the temperature of the fixing belt increases. This voltage is detected by the voltage detection means (ST1).

  As shown in FIG. 6, the abnormality detection unit 37 compares the detected flyback voltage value Vin with the reference voltage value Vset with respect to the command power value (ST2), and the flyback voltage value Vin at the command power value is When it becomes larger than the reference voltage value Vset with respect to the command power value, a pulse signal indicating this is output. The pulse signal is counted by a counter (ST3), and when the predetermined number is exceeded, it is determined that the fixing belt as a heated body is at an abnormal temperature (ST4) and output of the drive signal is stopped (ST5). ). As a result, all the operations of the inverter circuit 28 are stopped, the supply of the high-frequency current to the exciting coil 22c is cut off, and the overheating of the fixing belt 21 is prevented. On the other hand, when the pulse count does not increase at a predetermined time, power supply to the inverter circuit is continued as an erroneous signal due to noise or the like.

  In the above abnormality detection step, the pulse signal indicating that the reference voltage value has been exceeded is counted, but as shown in FIG. 7, the output time of the pulse signal indicating that the reference voltage value has been exceeded continues. (ST6), when this time exceeds the reference time, it may be determined that the heating is abnormal (ST7), and the power supply to the inverter circuit may be stopped.

  As described above, by detecting the abnormal temperature based on the variation factor due to the temperature of the fixing belt itself, there is no time lag in detecting the abnormal temperature, and a quick response can be made. Therefore, it is possible to provide a more appropriate protection means against overheating.

FIG. 8 is a schematic configuration diagram showing another example of an image forming apparatus using the heating device of the present invention.
This image forming apparatus is substantially the same as the image forming apparatus shown in FIG. 1 with respect to the photosensitive drum 51, the charging device 52, the exposure device 53, the developing unit 54, the cleaning device 56, the static elimination exposure device 57, and the transfer device 58. Have the same thing. However, in this image forming apparatus, an intermediate transfer member 55 having a conductive layer along an endless peripheral surface is used, and an electromagnetic induction heating device that heats the toner image on the intermediate transfer member 55. 62 and a temperature sensor 63 for measuring the temperature of the intermediate transfer member 55 are provided along the peripheral surface of the intermediate transfer member 55. And the control apparatus 64 controls the electric power supply to the inverter circuit which the electromagnetic induction heating apparatus 62 has based on the output of the temperature sensor 63. FIG.

  The intermediate transfer member 55 is composed of three layers: a base layer made of a sheet member having high heat resistance, a conductive layer laminated thereon, and a surface release layer as the uppermost layer. The material of the layer is selected so that sufficient heat can be obtained by electromagnetic induction. The surface release layer is preferably a highly releasable sheet or coat layer. In consideration of electrostatic transfer properties of the toner image during primary transfer, a conductive material such as carbon black is dispersed to provide resistance. You may use the sheet | seat which adjusted.

  Further, the control device 64 is the same as that used in the fixing device of the image forming apparatus shown in FIG. 1, and the electromagnetic induction heating device 62 is different from that shown in FIG. An inverter circuit having the same configuration is provided.

  In this image forming apparatus, the toner image transferred from the photosensitive drum 51 onto the intermediate transfer member 55 and overlaid with a plurality of colors is moved in a heating area facing the electromagnetic induction heating device 62 by the circular movement of the intermediate transfer member 55. pass. The toner image is heated and melted on the intermediate transfer member, and is conveyed to the pressure contact portion between the transfer facing roll 60 and the pressure roll 61. The recording paper P is sent from a paper tray (not shown) between the intermediate transfer body 55 and the pressure roll 61 in time with this conveyance, and the toner image is pressed against the recording paper P, and the transfer and fixing are performed. Done at the same time.

  At this time, the amount of electric power supplied to the electromagnetic induction heating device 62 is controlled by the detection value of the temperature sensor 63, so that the surface temperature of the intermediate transfer member 5 maintains a desired temperature. The control of power supply to the electromagnetic induction heating device and the detection of abnormal heating of the intermediate transfer member 55, which is the object to be heated, are performed in the same manner as that performed by the fixing device of the image forming apparatus shown in FIG.

1 is a schematic configuration diagram illustrating an image forming apparatus using a fixing device and a heating device according to an embodiment of the present invention. FIG. 2 is a schematic configuration diagram illustrating a fixing device used in the image forming apparatus illustrated in FIG. 1. FIG. 3 is an enlarged cross-sectional view of a fixing belt used in the fixing device shown in FIG. 2. It is the schematic which shows the structure of a control apparatus and an inverter circuit. 6 is a timing chart showing changes in current and voltage in each part of the inverter circuit shown in FIG. 4. It is a flowchart of the operation | movement which prevents the abnormal heating in the control apparatus shown in FIG. It is a flowchart which shows the other example of the operation | movement which prevents the abnormal heating in the control apparatus shown in FIG. It is a schematic block diagram which shows the other example of the image forming apparatus with which the heating apparatus which concerns on this invention is used. It is the schematic which shows the change of the temperature of the to-be-heated body, the bimetal provided near this, and the temperature fuse over time.

Explanation of symbols

1: Photosensitive drum, 2: Charging device, 3: Exposure device, 4: Development unit, 5: Intermediate transfer member, 6: Cleaning device, 7: Static elimination exposure device, 8: Transfer charger, 9: Support roll, 10 : Transfer facing roll, 11: Transfer roll, 12: Fixing device, 13: Cleaning device,
21: fixing belt, 21a: base layer, 21b: conductive layer, 21c: elastic layer, 21d: surface release layer, 22: electromagnetic induction heating device, 23: pressure roll (pressure member), 24: pressure Opposing member, 26: peeling member, 27: temperature sensor, 28: inverter circuit,
29: Control device,
31: Commercial power supply 32: Rectifier circuit 33: Switching element 34: Capacitor 35: Diode 36: Voltage detection means 37: Abnormality detection means 38: Reference voltage value output means 41: Drive circuit 42: Power value indicating means, 43: control circuit,
51: Photosensitive drum 52: Charging device 53: Exposure device 54: Development unit
55: Intermediate transfer member, 56: Cleaning device, 57: Destaticizing exposure device, 58: Transfer charger, 59: Support roll, 60: Transfer facing roller, 61: Pressure roll, 62: Electric induction heating device, 63: Temperature Sensor, 64: control device,
121: Intermediate transfer member 121a: First polyimide layer 121b: Conductive layer 121c: Second polyimide layer 121d: Elastic layer 121e: Surface release layer

Claims (4)

A high-frequency generating circuit that includes an exciting coil for generating an eddy current in the conductive layer of the object to be heated, and that generates a high-frequency current in the exciting coil by turning on and off direct current by a switching element;
Power value indicating means for specifying a power value to be supplied to the exciting coil;
A drive circuit that outputs to the switching element a drive signal in which an ON duty width is set corresponding to the power value specified by the power value indicating means;
Voltage detection means for detecting a flyback voltage generated in a resonance circuit including the excitation coil in the high-frequency generation circuit;
A reference voltage value output means for outputting a reference voltage value;
The flyback voltage value detected by the voltage detection means is compared with the voltage value output from the reference voltage value output means, and the flyback voltage value is greater than or equal to the reference voltage value or exceeds the reference voltage value. And an abnormality detection means for outputting a signal to that effect,
The power value indicating means designates a different power value when heating the heated body so that the temperature of the heated body rises to a predetermined temperature and when maintaining the heated body at the predetermined temperature,
The heating is characterized in that the reference voltage value is set as a reference for whether or not the temperature of the object to be heated is abnormally high for each of the different power values specified by the power value indicating means. apparatus.   The abnormality detection means outputs a signal indicating an abnormality when the flyback voltage value is detected a plurality of times when the flyback voltage value is equal to or higher than the reference voltage value or exceeds the reference voltage value. The heating device according to claim 1.   The abnormality detecting means detects that the flyback voltage value is equal to or higher than the reference voltage value or exceeds the reference voltage value, and further becomes equal to or higher than the reference voltage value after a predetermined time has elapsed. The heating apparatus according to claim 1, wherein a signal indicating abnormality is output when it is detected that the reference voltage value is exceeded. A fixing member having an endless peripheral surface and contacting a toner image formed by powder toner;
A heating device for heating the fixing member;
A pressure member pressed against the fixing member,
A fixing device for pressing a toner image heated in contact with the fixing member to a recording medium passing between the pressure member and the fixing member;
The fixing member has a conductive layer along an endless peripheral surface;
The fixing device according to claim 1 , wherein the heating device is the heating device according to claim 1 .

Images