JP2012073440A - Fixing device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fixing device using a planar resistance heating element, which is capable of detecting abnormality regardless of the malfunction of a temperature sensor and to provide an image forming apparatus.SOLUTION: The fixing device is one melting toner by the resistance heating element having such a positive characteristic that an electric resistance value is increased at predetermined temperature or more, to fix a toner image on a recording sheet and measures the temperature of the resistance heating element, to adjust the temperature of the resistance heating element to predetermined fixing temperature lower than the Curie point temperature of the resistance heating element. In the fixing device, the amount of current flowing in the resistance heating element is measured and when the amount of current is not within an appropriate range, it is determined that the abnormality occurs.

Description

  The present invention relates to a fixing device and an image forming apparatus, and more particularly to a technique for detecting defects in a sheet resistance heating element in a fixing device that heats and melts toner using a sheet resistance heating element.

In an electrophotographic image forming apparatus, a toner image is fixed on a recording sheet by melting and pressure-bonding toner carried on the recording sheet. Therefore, various devices such as a halogen lamp, an electromagnetic induction heater, and a sheet resistance heating element are used. A suitable heating means is used.
The planar resistance heating element is formed, for example, by forming a resistance heating pattern having a predetermined electric resistance on a long flat ceramic substrate, and melts toner by receiving power to cause Joule heating of the resistance heating pattern. Since the sheet resistance heating element has a short distance from the heat source to the recording sheet, the thermal efficiency is very high, the power consumption is low, and the warm-up time can be shortened (see, for example, Patent Document 1). .

  However, such a feature also has an adverse effect. For example, since the sheet resistance heating element has a high temperature rising rate, there is a possibility that an end region where the sheet does not pass is overheated when a small size sheet is passed. On the other hand, a technique has been proposed in which the sheet resistance heating element is divided into an end region and a central region, and power feeding is controlled while monitoring the temperature for each region using a temperature sensor. If it does in this way, the excessive temperature rise of an edge part can be suppressed.

JP 2008-40097 A JP 2009-244595 A JP 2000-39796 A

  However, if the temperature of the sheet resistance heating element cannot be measured accurately due to the vibration sensor or other causes that cause the temperature sensor to deviate from the original installation position or installation angle, the control described above is required. May not work properly. For example, when a non-contact type temperature sensor such as an infrared sensor deviates from an angle toward the planar resistance heating element, the temperature detected by the temperature sensor even if the temperature of the planar resistance heating element is increased Does not rise, there is a risk of causing an abnormal operation or failure of the peripheral device due to excessive temperature rise. Such an abnormality cannot be detected simply by monitoring the measurement result of the temperature sensor.

  The present invention has been made in view of the above problems, and in a fixing device using a planar resistance heating element, a fixing device capable of detecting an abnormality regardless of malfunction of a temperature sensor, and An object is to provide an image forming apparatus.

  In order to achieve the above object, a fixing device according to the present invention fixes a toner image on a recording sheet by melting toner with a resistance heating element having a positive characteristic in which an electrical resistance value rises above a predetermined temperature. With reference to the temperature measuring means for measuring the temperature of the resistance heating element, and the temperature measured by the temperature measurement means, the resistance heating element has a predetermined fixing temperature lower than the Curie point temperature of the resistance heating element. Temperature adjustment means for adjusting the temperature, current amount measurement means for measuring the amount of current flowing through the resistance heating element, and proper storage of the appropriate amount of current that should flow through the resistance heating element when the temperature is normally adjusted A range storage unit and an abnormality detection unit that determines that an abnormality has occurred if the current amount measured by the current amount measurement unit is not within the appropriate range are provided.

In this way, even when an abnormality occurs in the temperature sensor, an abnormality such as overheating can be detected by monitoring the amount of current flowing through the resistance heating element.
Specifically, at the time of temperature control, the appropriate range storage means stores a lower limit value of the amount of current that should flow to the resistance heating element when there is no abnormality as the appropriate range, and the abnormality detection means is fixed. During operation, it is desirable to determine that an abnormality has occurred if the measured effective value of the current amount falls below the lower limit.

  The appropriate range storage means stores a lower limit value of the amount of current that should flow through the resistance heating element after a predetermined time has elapsed since the start of warm-up as an appropriate range, and the abnormality detection means During operation, if the maximum value of the measured current amount falls below the lower limit value after the predetermined time has elapsed, it is possible to detect an abnormality during warm-up if it is determined that an abnormality has occurred.

Further, when the electrical resistance of the resistance heating element increases when the fixing temperature is exceeded, the appropriate range storage means stores the lower limit value of the rate of decrease in the amount of current that should flow through the resistance heating element as the appropriate range, Even if it is determined that an abnormality has occurred when the abnormality detection means determines that an abnormality has occurred when the rate of decrease in the measured current amount exceeds the lower limit during the fixing operation, an abnormality during temperature adjustment can be detected.
Further, when the electrical resistance of the resistance heating element decreases when the fixing temperature is exceeded, the appropriate range storage means stores the lower limit value of the rate of increase in the amount of current that should flow through the resistance heating element as the appropriate range, When the abnormality detection means determines that an abnormality has occurred when the increase rate of the measured current amount exceeds the lower limit during the fixing operation, the resistance heating element has a negative temperature characteristic near the fixing temperature. But it can detect anomalies.

  An image forming apparatus according to the present invention includes the fixing device according to the present invention. Therefore, the image forming apparatus according to the present invention can achieve all the effects of the fixing device according to the present invention.

1 is a diagram illustrating a main configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 6 is a cross-sectional view illustrating a main configuration of the fixing device 115 in a direction orthogonal to a recording sheet conveyance direction. It is a figure which shows the main structures of the resistance heating element 200, Comprising: (a) is a top view, (b) is a front view. It is a circuit diagram which shows the main structures for making the resistance heating element 200 generate heat. It is a graph showing the temperature-resistance characteristic of resistance heating pattern 301a-301c. It is a graph which illustrates the measured value of the ammeter 401 when constant electric power is continuously supplied from the AC power source 402 when the resistance heating element 200 is at room temperature in the initial state. 3 is a block diagram illustrating a main configuration of a control unit 112. FIG. It is a flowchart which shows the abnormality detection operation | movement which exists at the time of temperature control. It is a graph which illustrates the typical change of the electric current amount (effective value) until abnormality is detected after warm-up is started. It is a flowchart which shows the abnormality detection operation | movement in warm-up. It is a graph which illustrates the typical time change of the maximum value Irpm of the electric current amount at the time of warm-up. It is a flowchart which shows the abnormality detection operation | movement which concerns on the modification of this invention. It is a graph which illustrates the typical time change of the maximum value Irpm of the electric current amount at the time of temperature control. It is a flowchart which shows the abnormality detection operation | movement which concerns on the modification of this invention. It is a graph which illustrates the typical time change of the maximum value Irpm of the electric current amount at the time of temperature control.

Hereinafter, embodiments of a fixing device and an image forming apparatus according to the present invention will be described with reference to the drawings.
[1] Configuration of Image Forming Apparatus First, the configuration of the image forming apparatus according to the present embodiment will be described.
FIG. 1 is a diagram illustrating a main configuration of the image forming apparatus according to the present embodiment. As shown in FIG. 1, the image forming apparatus 1 includes a document reading unit 100, an image forming unit 110, and a paper feeding unit 120. The document reading unit 100 optically reads a document and generates image data.

The image forming unit 110 includes image forming units 111Y to 111K, a control unit 112, an intermediate transfer belt 113, a secondary transfer roller pair 114, a fixing device 115, a paper discharge roller 116, a paper discharge tray 117, a cleaner 118, and a timing roller 119. ing.
The image forming units 111Y to 111K form toner images of Y (yellow), M (magenta), C (cyan), and K (black), respectively, under the control of the control unit 112. Specifically, for example, each of the image forming units 111Y to 111K includes a photosensitive drum, a charging device, an exposure device, a developing device, and a primary transfer roller. First, when the photosensitive surface of the photosensitive drum is uniformly charged to a predetermined potential by the charging device, an electrostatic latent image is formed by exposing the charging area in accordance with the image data.

After the developing device supplies toner to the electrostatic latent image to form a toner image (visualization), the primary transfer roller electrostatically transfers to the intermediate transfer belt 113 so that the toner images of the respective colors overlap ( Primary transfer). The intermediate transfer belt 113 is an endless rotating body, and rotates in the direction of arrow A to convey the toner image to the secondary transfer position.
The paper feeding unit 120 includes a paper feeding cassette 121 that stores the recording paper P for each paper size, and supplies the recording paper P to the image forming unit 110. The supplied recording paper P is conveyed to the secondary transfer position by the timing roller 119 at an appropriate timing in parallel with the intermediate transfer belt 113 conveying the toner image.

The secondary transfer roller pair 114 is composed of a pair of rollers having a potential difference, and these roller pairs are pressed against each other to form a transfer NIP portion. In the transfer NIP portion, the toner image on the intermediate transfer belt 113 is electrostatically transferred (secondary transfer) to the recording paper P. The recording paper P to which the toner image has been transferred is conveyed to the fixing device 115.
The fixing device 115 is an electromagnetic induction heating type fixing device, and heats and melts the toner image and presses the toner image on the recording paper P. The recording paper P to which the toner image is fused is discharged onto a paper discharge tray 117 by a paper discharge roller 116.

[2] Configuration of Fixing Device 115 Next, the configuration of the fixing device 115 will be described.
FIG. 2 is a cross-sectional view illustrating a main configuration of the fixing device 115 in a direction orthogonal to the recording sheet conveyance direction. As shown in FIG. 2, the fixing device 115 includes a resistance heating element 200, a heat resistant film 201, a pressure roller 202, and a temperature sensor 203, and an endless belt-like resistance heating element 200 having a long flat rectangular shape. The toner image is fused to the recording sheet S by passing the recording sheet S through the NIP unit 204 formed by pressing the pressure roller 202 through the heat-resistant film 201.

  The pressure roller 202 forms an elastic layer made of an elastic material such as silicone rubber on the outer peripheral surface of a cylindrical metal core made of a metal material such as aluminum, and further, PFA is formed on the outer peripheral surface of the elastic layer. A release layer made of a tube or the like is formed. For example, the outer diameter may be 20 mm, the thickness of the elastic layer may be 3 mm, and the thickness of the release layer may be 30 μm. The pressure roller 202 is pressed against the resistance heating element 200 via the heat resistant film 201 by being urged by a bearing means (not shown). The pressure roller 202 is rotationally driven in the direction of arrow B by a driving unit (not shown).

  The heat-resistant film 201 is made of a heat-resistant resin film having a film thickness of 100 μm or less in order to reduce the heat capacity and improve the rate of temperature increase, and the outer diameter is 18 mm, for example. When the heat-resistant film 201 is a single layer film, PTFE (Polytetrafluoroethylene), PFA (Perfluoroalkoxy), FEP (Tetrafluoroethylene hexafluoropropylene copolymer), or the like may be used. In the case of a composite layer film, PTFE or the like is formed on the surface corresponding to the outer peripheral surface of the heat-resistant film 201 of a film made of polyimide, polyamideimide, PEEK (polyetheretherketone), PES (Polyethersulphone), PPS (Polyphenylene sulfide), or the like. PFA, FEP or the like may be coated. The heat-resistant film 201 rotates in the direction of arrow D following the pressure roller 202 due to friction with the pressure roller 202 that is in pressure contact. At the time of fixing, the heat-resistant film 201 also rotates in the direction of arrow D due to friction with the recording sheet S conveyed in the direction of arrow C due to friction with the pressure roller 202.

  The resistance heating element 200 is held by a holding member (not shown). This holding member is made of a material having both heat resistance and rigidity, such as polyimide, polyamideimide, PEEK, PES, and PPS, and also has a structure as a film guide. A plurality of temperature sensors 203 are provided and measure the temperature of the resistance heating element 200. The control unit 112 refers to the measurement result of the temperature sensor 203 and adjusts the fixing temperature.

[3] Configuration of Resistance Heating Element 200 Next, the configuration of the resistance heating element 200 will be described.
3A and 3B are diagrams showing a main configuration of the resistance heating element 200, where FIG. 3A is a plan view and FIG. 3B is a front view. In FIG. 2A, a heat-resistant glass layer to be described later is omitted.

  As shown in FIG. 2A, the resistance heating element 200 has resistance heating patterns 301a to 301c and power feeding electrodes on one main surface (hereinafter referred to as “heat-resistant film sliding surface”) of the ceramic substrate 300. 302a to 302d are formed. The resistance heating element 200 has a long flat rectangular shape as described above, and is arranged such that its longitudinal direction is perpendicular to the recording sheet conveyance direction and the heat-resistant film sliding surface faces the recording sheet. The The plurality of temperature sensors 203 are provided one for each of the resistance heating patterns 301a to 301c, and the temperatures of the resistance heating patterns 301a to 301c are individually measured.

The ceramic substrate 300 is made of a ceramic material such as alumina (Al ₂ O ₃ ), aluminum nitride (AlN), or silicon nitride (Si ₃ N ₄ ) that has heat resistance, insulation, and good thermal conductivity. It is a rectangular plate. The dimensions are, for example, a width of 7 mm, a length of 260 mm, and a thickness of 1.5 mm. In addition, the power supply electrodes 302a to 302d are made of a low-resistance good conductor such as a screen-printed silver pattern, for example. Supply. In the present embodiment, the case where the resistance heating patterns 301a to 301c share the power supply electrode 302d will be described. However, the effect of the present invention is the same even if the power supply electrode is not shared. If the power feeding electrode is shared, the pattern area can be reduced and the man-hour for connecting the terminal to the power feeding electrode can be reduced in addition to the effect of the present invention.

  As shown in FIG. 3B, the heat-resistant film sliding surface of the resistance heating element 200 is covered with an overcoat layer 310 except for the terminal portions of the power supply electrodes 302a to 302d. The overcoat layer 310 is made of heat resistant glass, and electrical insulation and slidability between the resistance heating element 200 and the heat resistant film 2 are ensured. The thickness of the overcoat layer 310 may be 60 μm, for example.

  FIG. 4 is a circuit diagram showing a main configuration for causing the resistance heating element 200 to generate heat. As shown in FIG. 4, switches 403 a to 403 c are connected in series to the resistance heating patterns 301 a to 301 c, respectively, and series circuits composed of combinations of these resistance heating patterns and switches are connected in parallel to each other. . This parallel circuit is further connected in series to an ammeter 401 and an AC power source 402. The ammeter 401 measures the amount of current supplied to the resistance heating element 200.

  The switches 403a to 403c are under the control of the control unit 112, and are on / off controlled according to the temperature of each resistance heating pattern 301a to 301c measured by the temperature sensor 203 and the amount of current measured by the ammeter 401. The AC power source 402 is also under the control of the control unit 112 and is controlled by the duty ratio. For example, a triac may be used for the switches 403a to 403c.

[4] Resistance Heating Patterns 301a to 301c Next, the resistance heating patterns 301a to 301c will be described.
The resistance heating patterns 301a to 301c are made of a material having a PTC (Positive Temperature Coefficient) characteristic, for example, a ceramic material such as barium titanate (BaTiO ₃ ), a conductive polymer in which carbon is dispersed, or the like. FIG. 5 is a graph showing the temperature-resistance characteristics of the resistance heating patterns 301a to 301c, where the vertical axis indicates the resistance value [Ω] and the horizontal axis indicates the temperature [° C.]. In the figure, CP indicates a Curie Point. As shown in FIG. 5, the resistance heating patterns 301 a to 301 c gradually decrease the resistance value as the temperature rises, and then suddenly increase the resistance value when the temperature exceeds the Curie point (CP). Has increased PTC characteristics.

  In this embodiment, the case where the fixing temperature is 180 ° C. and the material is used for the resistance heating patterns 301 a to 301 c having a Curie point temperature of 200 ° C. will be described as an example. A material having a fixing temperature or other Curie point temperature may be used. Further, when the electrical resistance of the circuit in which the resistance heating patterns 301a to 301c are connected in parallel as shown in FIG. 4 is 10Ω, the area resistivity of the resistance heating patterns 301a to 301c is assumed to be constant regardless of the location. Then, it can be estimated that by applying AC 100 V, a heat amount of about 1000 W is generated in the entire resistance heating patterns 301a to 301c.

  As for the dimensions of the resistance heating patterns 301a to 301c, for example, if the maximum recording sheet width to be fixed is 216 mm in consideration of the letter size, the entire resistance heating patterns 301a to 301c The width (size in a direction orthogonal to the recording sheet conveyance direction) may be 220 mm. Further, when a general envelope is assumed as a narrow recording sheet, the width of the resistance heating pattern 301a may be set to 118 mm. In this way, if the dimensions of the resistance heating patterns 301a to 301c are determined according to the width of the recording sheet to be fixed, the excessive temperature rise at the end when the toner image is continuously fixed on the small size recording sheet. It can be effectively prevented.

  FIG. 6 is a graph illustrating the measured value of the ammeter 401 when a constant power is continuously supplied from the AC power source 402 when the resistance heating element 200 is at room temperature in the initial state, and the horizontal axis indicates the power supply. Represents the elapsed time since the start of, and the vertical axis represents the measured value of the ammeter 401. As shown in FIG. 6, when a constant power is supplied to the resistance heating element 200 at room temperature, the electrical resistance of the resistance heating patterns 301a to 301c is shown in FIG. 5 as the temperature of the resistance heating element 200 increases. Since the current decreases, the current value increases.

  Thereafter, when the temperature of the resistance heating element 200 further increases, the resistance values of the resistance heating patterns 301a to 301c start to increase as shown in FIG. Therefore, after taking the peak value at the temperature where the resistance value becomes the smallest, the current value starts to decrease. Even if the temperature of the resistance heating element 200 exceeds the Curie point temperature, if power is continuously supplied, the temperature further increases, and the resistance values of the resistance heating patterns 301a to 301c rapidly increase accordingly. For this reason, current does not flow so much and the current value is low.

The normal temperature is a temperature within an environmental temperature range in which normal operation of the image forming apparatus 1 is guaranteed, for example, within a range from 0 ° C. to 40 ° C.
[5] Configuration of Control Unit 112 Next, the configuration of the control unit 112 will be described.
FIG. 7 is a block diagram showing the main configuration of the control unit 112. As shown in FIG. 7, the control unit 112 includes a CPU (Central Processing Unit) 701, a ROM (Read Only Memory) 702, a RAM (Random Access Memory) 703, and an I / O (Input / Output) interface 704. Yes. When the power is turned on, the CPU 701 reads a program stored in the ROM 702 and executes the program using the RAM 703 as a working storage area. Further, the CPU 701 accesses the temperature sensor 203, the ammeter 401, the AC power supply 402, the switches 403a to 403c, etc. via the I / O interface 704 to determine the temperature of each part of the resistance heating element 200 and the amount of current to be energized. To obtain, to control the duty ratio of the AC power source 402, and to turn on / off the switches 403a to 403c.

[6] Abnormality detection Next, an operation for detecting an abnormal state will be described.
(1) Abnormality detection during temperature adjustment First, an operation for detecting an abnormality during temperature adjustment will be described.
FIG. 8 is a flowchart showing an abnormality detection operation during temperature control. As shown in FIG. 8, the control unit 112 refers to the temperature sensor 203 and completes the warm-up when the temperature of the resistance heating element 200 reaches the fixing temperature. This operation assumes that the fixing temperature is higher than the temperature at which the resistance value in the graph of FIG. 5 is minimum (hereinafter referred to as “minimum resistance temperature”) and lower than the Curie point (CP) temperature. ing.

  As shown in FIG. 5, the resistance heating patterns 301 a to 301 c show positive temperature characteristics in a temperature range higher than the minimum resistance temperature (hereinafter referred to as “high temperature range”), and a temperature range lower than the minimum resistance temperature ( Hereinafter, it is referred to as “low temperature region”), and shows a negative temperature characteristic. In the high temperature range, the resistance value increases rapidly as the temperature increases. In the low frequency range, the resistance value gradually decreases as the temperature increases.

  Now, when the warm-up is completed (S801: YES), the control unit 112 refers to the output of the ammeter 401, measures the effective current value Iw when the warm-up is completed (S802), and then starts paper feeding. (S803). When paper feeding is started, the control unit 112 refers to the ammeter 401 to measure the effective current value Irpm (S804) and compares it with the current amount Iw. If Irpm does not satisfy Iw (S805: YES), the abnormality detection is notified (S807), and the fixing operation is stopped (S808). If Irpm is equal to or greater than Iw (S805: NO) and fixing is not completed (S806: NO), the process proceeds to step S804 and the above process is repeated. For the current amount (effective value) Irpm, for example, the current is measured for 300 milliseconds, and the minimum effective value during that time may be adopted.

  FIG. 9 is a graph illustrating a typical change in the amount of current (effective value) from when warm-up is started until abnormality is detected. In the figure, a broken line is a graph illustrating a change in the amount of current when no abnormality has occurred. As shown in FIG. 9, when the warm-up is started, the resistance heat generation patterns 301a to 301c show negative temperature characteristics in the low temperature range, so that the resistance value decreases as the temperature increases, and the amount of current increases. . Then, when the temperature of the resistance heating element 200 exceeds the minimum resistance temperature and reaches a high temperature range, the resistance heating patterns 301a to 301c exhibit positive temperature characteristics, so that the current amount starts to decrease.

  When the temperature of the resistance heating element 200 reaches the fixing temperature, the warm-up is completed, and when the sheet feeding is started, the temperature of the resistance heating element 200 is somewhat lowered by taking heat away from the recording sheet. In the high temperature range, the resistance heat generation patterns 301a to 301c exhibit positive temperature characteristics, so that the electrical resistance of the resistance heat generation patterns 301a to 301c also decreases somewhat and the amount of current increases as the temperature decreases.

After that, if the temperature sensor 203 is unable to continue normal temperature control due to a malfunction or the like and the temperature of the resistance heating element 200 rises, the current amount (effective value) Irpm is increased due to the positive temperature characteristics. Decrease. When Irpm falls below Iw, the control unit 112 detects an abnormality and the fixing operation is stopped.
In this way, even if the temperature control operation cannot be normally performed due to a failure of the temperature sensor 203, the resistance heating element 200 is prevented from being overheated by monitoring the current amount (effective value) Irpm. be able to.

(2) Abnormality detection during warm-up Next, an operation for detecting an abnormality during warm-up will be described.
FIG. 10 is a flowchart showing an abnormality detection operation in warm-up. As shown in FIG. 10, when the warm-up is started (S1001), the control unit 112 measures the temperature of the resistance heating element 200 with the temperature sensor 203 (S1002). If the temperature does not reach the fixing temperature (S1003: NO), referring to the output of the ammeter 401, the maximum value Irpm of the amount of current (effective value) is measured (S1004).

  If the measured maximum value Irpm does not reach the maximum value Ij of the normal current amount (S1005: YES), it is determined that an abnormality has occurred, so that the abnormality is detected (S1006), and the warm The up is finished (S1007). Also, when the fixing temperature is reached without detecting an abnormality (S1003: YES), the warm-up is ended (S1007). Note that the maximum value Ij of the normal current amount is prepared in advance, for example, as a value every 0.1 second from the start to the completion of warm-up, and in step S1005, the maximum value at the corresponding time is prepared. Compare with the value Irpm.

  FIG. 11 is a graph illustrating a typical time change of the maximum value Irpm of the current amount at the time of warm-up, where a solid line represents an abnormal time and a broken line represents a normal time. When the temperature reaches the fixing temperature, the warm-up is completed. This operation also assumes a case where the fixing temperature is higher than the minimum resistance temperature and lower than the Curie point (CP) temperature. As shown in FIG. 9, at the normal time, the maximum value Irpm of the current amount increases with the temperature rise due to the start of warm-up, then decreases when the minimum resistance temperature is exceeded, and slightly increases according to the temperature control after reaching the fixing temperature. Repeat slight reduction.

  On the other hand, when a portion of the resistance heating element 200 outside the range where the temperature sensor 203 detects the temperature is damaged, even if the output result of the temperature sensor 203 reaches the fixing temperature, a fixing failure due to uneven fixing temperature occurs. As a result, image quality may be deteriorated. In such a case, since the electrical resistance of the entire resistance heating pattern 301a to 301c increases, even if the maximum value Irpm of the current amount increases as the temperature rises, it does not reach the normal current amount Ij. Therefore, an abnormality can be detected by monitoring the maximum current value Irpm and comparing it with the normal value Ij.

[7] Modifications Although the present invention has been described based on the embodiments, it is needless to say that the present invention is not limited to the above-described embodiments, and the following modifications can be implemented. .
(1) In the above embodiment, the case where an abnormality is detected by monitoring the minimum value Irpm of the amount of current flowing through the resistance heating patterns 301a to 301c during temperature control has been described. However, the present invention is limited to this. Needless to say, the following may be used instead.

  That is, instead of the minimum value Irpm of the current amount, the time change of the minimum value Irpm of the current amount may be monitored. FIG. 12 is a flowchart showing an abnormality detection operation according to this modification. As shown in FIG. 12, the control unit 112 according to the present embodiment completes warm-up (S1201: YES), starts paper passing (S1202), and then changes with time ΔIrpm of the effective value of the current amount. Is measured (S1203). For example, the time change may be obtained by measuring the effective value Irpm of the current amount at intervals of 1 second and determining the time change for 1 second.

  The control unit 112 stores in advance a minimum value ΔImin of a time variation of the effective value of the current amount at normal time. If the time change ΔIrpm is smaller than the minimum value ΔImin, in other words, the effective value Irpm exceeds a predetermined threshold value. If it decreases rapidly (S1204: YES), it is determined that the resistance heating element 200 is overheated, so an abnormality detection is notified (S1206), and the fixing operation is stopped (S1207). If the time change ΔIrpm is equal to or greater than the minimum value ΔImin (S1204: NO), it is determined that no abnormality has occurred. If fixing has not been completed (S1205: NO), the process proceeds to step S1203 and the above operation is performed. repeat.

FIG. 13 is a graph illustrating a typical time change of the maximum value Irpm of the current amount during temperature control. As shown in FIG. 13, after the abnormality occurs, the temperature increase rate of the resistance heating element 200 increases, and when ΔIrpm falls below ΔImin, the abnormality is detected. Even if it does in this way, the effect of the present invention is the same.
(2) In the above embodiment, the case where the fixing temperature is between the minimum resistance temperature and the Curie point temperature has been described. Needless to say, the present invention is not limited to this, and the minimum resistance temperature is higher than the fixing temperature. May be high.

FIG. 14 is a flowchart showing an abnormality detection operation according to this modification. As shown in FIG. 14, the control unit 112 according to the present embodiment completes the warm-up (S1401: YES), starts the sheet passing (S1402), and then the maximum effective value Irpm of the current amount. Is measured (S1403).
The control unit 112 stores in advance the maximum value Im of the effective value of the current amount in the normal state. If the measured value Irpm is larger than the threshold value Im (S1404: YES), the resistance heating element 200 is overheated. It is judged. This is because when the fixing temperature is lower than the minimum resistance temperature, the electrical resistance of the resistance heating patterns 301a to 301c decreases as the temperature of the resistance heating element 200 increases, so that the current flowing through the resistance heating patterns 301a to 301c. This is because the amount increases.

Therefore, in such a case, the abnormality detection is notified (S1406), and the fixing operation is stopped (S1407). If the measured value Irpm is less than or equal to the threshold value Im (S1404: NO), it is determined that no abnormality has occurred. If fixing has not been completed (S1405: NO), the process proceeds to step S1203 and the above operation is performed. repeat.
FIG. 15 is a graph illustrating a typical time change of the maximum current value Irpm during temperature control. As shown in FIG. 15, in this modification, since the fixing temperature is lower than the minimum resistance temperature, unlike the above embodiment, the warm-up is completed before the maximum current value Irpm reaches its peak. To do. Since the resistance heating patterns 301a to 301c have negative temperature characteristics in a low temperature range lower than the minimum resistance temperature, when the resistance heating element 200 is overheated, the electrical resistance of the resistance heating patterns 301a to 301c is reduced, and the current When the maximum value Irpm of the quantity increases and exceeds the threshold value Im, it is determined that there is an abnormality.

In the case where the fixing temperature is lower than the minimum resistance temperature, when the time change of the current amount Irpm is used as in the modified example (1), an abnormality occurs depending on whether the increase rate of the current amount exceeds a predetermined threshold value. You may determine the presence or absence of.
(3) Although not specifically mentioned in the above embodiment, it is desirable to compare the measured current amount Irpm with the current amount (Iw, Ij, etc.) when the open / close states of the switches 403a to 403c match. This is because the amount of current flowing through the resistance heating patterns 301a to 301c changes according to the open / closed state of the switches 403a to 403c.

  (4) Although not particularly mentioned in the above embodiment, the image forming apparatus according to the present invention may be a copier or a printer. Further, the present invention may be applied to a facsimile apparatus or an MFP (Multi Function Peripheral) having a plurality of functions, and any apparatus that performs image formation using an electrophotographic method, and the effects thereof can be achieved. Can be obtained.

  The fixing device and the image forming apparatus according to the present invention are useful as a device that can detect an abnormality regardless of the malfunction of the temperature sensor when a sheet resistance heating element is used.

1 …………………… Image forming apparatus 100 …………………… Original reading unit 110 …………………… Image forming units 111 </ b> Y to 111 </ b> K… Image forming unit 112 ………………… Control unit 113 ……………… Intermediate transfer belt 114 …………………… Secondary transfer roller pair 115 ……………… Fusing device 116 ……………… Discharge roller 117 …… …………… Discharge tray 118 ………………… Cleaner 119 ………………… Timing roller 120 …………………… Paper feed section 121 ………………… Paper feed cassette 122 ………………… Pickup roller 200 ………………… Resistance heating element 201 ………………… Heat-resistant film 202 ………………… Pressure roller 203 ………………… Temperature Sensor 204 ... NIP unit 300 ... Ceramic substrates 301a to 301c ... Antipyretic pattern 302a-302d ... feeding electrode 310 ..................... overcoat layer 401 ..................... ammeter 402 ..................... AC power 403a to 403c ... switch

Claims (6)

A fixing device that melts toner with a resistance heating element having a positive characteristic in which an electric resistance value increases at a predetermined temperature or higher and fixes a toner image on a recording sheet,
Temperature measuring means for measuring the temperature of the resistance heating element;
Referring to the temperature measured by the temperature measuring means, a temperature adjusting means for adjusting the temperature of the resistance heating element to a predetermined fixing temperature lower than the Curie point temperature of the resistance heating element;
Current amount measuring means for measuring the amount of current flowing through the resistance heating element;
Appropriate range storage means for storing an appropriate range of the amount of current that should flow through the resistance heating element when the temperature is normally adjusted;
A fixing device comprising: an abnormality detection unit that determines that an abnormality has occurred if the amount of current measured by the current amount measurement unit is not within an appropriate range. The appropriate range storage means stores, as an appropriate range, a lower limit value of the amount of current that should flow through the resistance heating element when there is no abnormality,
2. The fixing device according to claim 1, wherein the abnormality detection unit determines that an abnormality has occurred when an effective value of a measured current amount falls below the lower limit value during a fixing operation. The appropriate range storage means stores, as an appropriate range, a lower limit value of the amount of current that should flow to the resistance heating element after a predetermined time has elapsed since the start of warm-up,
2. The abnormality detection unit according to claim 1, wherein the abnormality detection unit determines that an abnormality has occurred when a maximum value of the measured current amount falls below the lower limit value after the predetermined time has elapsed during the warm-up operation. The fixing device described. When the electrical resistance of the resistance heating element increases when the fixing temperature is exceeded,
The appropriate range storage means stores, as an appropriate range, a lower limit value of the rate of decrease in the amount of current that should flow through the resistance heating element,
The fixing device according to claim 1, wherein the abnormality detection unit determines that an abnormality has occurred when a decrease rate of the measured current amount exceeds the lower limit value during the fixing operation. When the electrical resistance of the resistance heating element decreases when the fixing temperature is exceeded,
The appropriate range storage means stores a lower limit value of an increase rate of the amount of current that should flow through the resistance heating element as an appropriate range,
2. The fixing device according to claim 1, wherein the abnormality detection unit determines that an abnormality has occurred when the increase rate of the measured current amount exceeds the lower limit during the fixing operation. An image forming apparatus comprising the fixing device according to claim 1.

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