JP2005091563A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming device of high reliability by surely detecting disconnection of a thermistor being a temperature detecting element which detects the surface temperature of a fixing roll. <P>SOLUTION: Power supply to a heater of the fixing roll is started, and an initial value R1 of a thermistor resistance value is read in (P11), and a first disconnection detection time TA is set (P12), and a resistance value R2 is read in after the elapse of the time TA (P16). A relation between the difference between values R1 and R2 and a limit value D ((R2-R1)>D) is discriminated (P17), and the thermistor is judged to be normal when the difference is larger than the limit value D, and disconnection is determined when the difference is equal to or smaller than the limit value D. A second disconnection detection time TB is set (P20), and a resistance value R3 is read in (P22). A relation between the difference between values R3 and R1 and a limit value E ((R3-R1)>E) is discriminated (P23). The thermistor is judged to be normal when the difference is larger than the limit value E, and power supply to the heater is started to terminate processing. When U(R3-R1) is smaller than the limit value E even after the elapse of the disconnection detection time TB, disconnection is determined (P25), and power to the heater is broken, and a warning is displayed (P27). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly to a fixing device thereof.

  A fixing device of an electrophotographic image forming apparatus has a configuration in which a toner image formed on a recording paper is heated and pressurized by a heating rotator and fixed, and the heating rotator is provided with a heating element. Heats by energizing the heating element. Further, since the fixing device needs to maintain the surface temperature of the heating rotator within a predetermined temperature range suitable for the fixing process, a temperature detecting element such as a thermistor is placed close to or in contact with the surface of the heating rotator. A temperature control device is provided for detecting the surface temperature of the heating rotator and controlling the current supplied to the heating element so as to maintain the surface temperature of the heating rotator within a predetermined temperature range based on the detected temperature. .

  A temperature detection element such as a thermistor shows a high resistance value when the temperature of the detection target is low, and shows a low resistance value when the temperature of the detection target is high. In addition, since the resistance value is high even when the temperature detection element is disconnected, a signal indicating a high resistance value is output even when the temperature of the detection target detected by the temperature detection element is higher than the predetermined temperature range. When this is done, it is because the temperature detecting element is disconnected, and the temperature control device energizes the heating element to increase the temperature. In order to prevent such a malfunction, a disconnection detection device that detects disconnection of the temperature detection element is required.

A conventional disconnection detection device for detecting disconnection of a temperature detection element is a contact-type temperature detection element arranged in contact with the surface of a heating rotator, with the resistance of the original temperature detection element in a state where the heating rotator is heated. A configuration is provided in which disconnection is detected from a change in the value and the resistance value of the temperature detection element after a predetermined time has elapsed. This is based on the premise that the resistance value of the temperature detection element changes immediately as the temperature of the heating rotator increases because the contact type temperature detection element does not have a large thermal impedance between the heating rotator and the temperature detection element. Thus, the disconnection detection time of the temperature detection element can be shortened, and this is a safe, inexpensive and efficient disconnection detection device (see Patent Document 1).
JP-A-8-76619.

  Unlike the contact type temperature detection element described above, in the non-contact type temperature detection element arranged in a non-contact state on the surface of the heating rotator, air exists between the surface of the heating rotator and the temperature detection element. Therefore, the thermal impedance between the two is large, and the temperature rise of the heating rotator does not immediately appear as a change in the resistance value of the temperature detection element, It is necessary to take a long time (called disconnection detection time). In particular, since the thermal impedance increases as the internal temperature of the image forming apparatus decreases, the disconnection detection time increases.

  In this case, if the temperature of the heating rotator is as low as room temperature, such as immediately after turning on the power of the image forming apparatus, the heating rotator is heated until the time for detecting disconnection of the temperature detecting element. However, there is no problem, but when the heated rotating body is heated to a certain temperature, for example, when a paper jam occurs during printing and the paper jam is processed quickly, the temperature detection element is disconnected. In such a case, even during the disconnection detection time of the temperature detection element, heating of the heating rotator is continued in the period until the disconnection is detected, and the temperature of the heating rotator rises abnormally, There is an inconvenience that even the devices around the heating rotator are damaged by heat.

  The object of the present invention is to reliably detect disconnection by appropriately setting the time required for disconnection detection (disconnection detection time) even when a non-contact type temperature detecting element is used. If the internal environment temperature of the device is low and more time is required for the disconnection detection time, once the heat supply to the heating rotator is stopped, the disconnection detection estimated from the ambient temperature can be performed. By operating the disconnection detection device again during the time estimated to be necessary, the disconnection detection time is lengthened more than necessary, and even the devices around the heating rotator are heated by heat due to an abnormal rise in the temperature of the heating rotator. Temperature detection that will not be damaged, and will not be erroneously detected because the disconnection detection time is too short even when the ambient temperature is low and a longer time is required for the disconnection detection time. Device disconnection detector It is to provide an image forming apparatus having a.

  The present invention solves the above-mentioned problems, and the invention according to claim 1 detects the temperature of a heating body for fixing a toner image on a recording paper and a heating body arranged in a non-contact manner facing the heating body. First temperature detection means, first determination means for determining whether the temperature is normal based on a detection value by the first temperature detection means after a first predetermined time has elapsed from the start of heating of the heating body, and the first A shut-off means for reducing or turning off the energization of the heating element when the judgment means is judged to be not normal, and the first temperature detection means after a second predetermined time has elapsed since the shut-off means is activated. A second determination unit that determines whether or not it is normal based on a detection value obtained by the control unit, and a release unit that cancels the operation of the blocking unit when it is determined that the second determination unit is normal. An image forming apparatus characterized by the above.

  According to a second aspect of the present invention, there is provided a heating body for fixing a toner image on a recording paper, a first temperature detecting means for detecting the temperature of the heating body disposed in a non-contact manner facing the heating body, Based on a detection value obtained by the first temperature detection means after the first predetermined time has elapsed from the start of heating, the first determination means for determining whether or not normal and the first determination means have determined that the temperature is not normal. And determining whether or not it is normal based on a detected value by the first temperature detecting means after the elapse of a second predetermined time since the shut-off means is activated, and a shut-off means for reducing or turning off the energization to the heating body. An image forming apparatus comprising: a second determination unit that determines; and a notification unit that notifies that the second determination unit is abnormal when it is determined that the second determination unit is not normal.

  According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the second predetermined time is determined based on a value detected by the first temperature detection means at the start of heating of the heating body or before the heating is started. A time setting means for changing the value is provided.

  According to a fourth aspect of the present invention, in the image forming apparatus according to the first or second aspect, a second temperature detecting means for detecting an environmental temperature inside the image forming apparatus, and at the time of starting or before starting the heating body. And a time setting means for changing the value of the second predetermined time based on the detection value by the second temperature detection means.

  According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the detection value by the first temperature detection means at the start of heating of the heating body or before the start, the heating body and the temperature A time setting means for changing the value of the second predetermined time based on the distance to the detection means is provided.

  According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the first determination unit includes a value detected by the first temperature detection unit at the start of heating or before the start, and heating. It is characterized by determining whether it is normal or not by comparing the detected value by the first temperature detecting means after a predetermined time has elapsed from the start.

  As described above, according to the present invention, the power of the image forming apparatus is turned on to start the operation, the heating body of the fixing device is energized to start the heating, and the first temperature detection is performed within the first predetermined time. Based on the detection value of the means (thermistor), the first disconnection detection by the first determination means is performed. When the disconnection is detected, the energization to the heating body is cut off to prevent the heating body from becoming an abnormally high temperature. In the first disconnection detection by the first determination means within the first predetermined time, the disconnection may be detected in spite of the fact that the disconnection is not due to the characteristics of the temperature detection means. The second disconnection detection is performed based on the detection value of the first temperature detection means by the second determination means within the second predetermined time after the passage of time. Accordingly, when it is determined that the first temperature detection means is normal, the power supply to the heating body is released, and when it is determined that the first temperature detection means is disconnected, the heating body is disconnected. The power is cut off and a warning is displayed.

  When the disconnection of the first temperature detection means is detected in the first disconnection detection, the energization to the heating body is cut off, and the second disconnection detection is performed after the elapse of a predetermined time. Even if the ambient temperature is low and it takes a long time to detect the disconnection of the temperature detection means, the heating element can be prevented from becoming abnormally high temperature and the peripheral device can be prevented from being destroyed, and the first temperature detection means can be disconnected. It is possible to provide a fixing device with high reliability and an image forming device with high reliability that can be detected accurately.

  Then, the value of the second predetermined time for performing the second disconnection detection is changed according to the detection value of the first temperature detection means or the detection value of the second temperature detection means for detecting the environmental temperature inside the image forming apparatus. When the environmental temperature inside the image forming apparatus is high, the time until the second disconnection detection can be shortened and the normal state can be quickly restored when there is no disconnection of the first temperature detection means. When the environmental temperature inside the image forming apparatus is low, the disconnection of the first temperature detection means can be accurately detected by lengthening the time until the second disconnection detection.

  Embodiments of the present invention will be described below.

  FIG. 1 is a diagram illustrating an outline of the configuration of a fixing device 10 of an image forming apparatus according to an embodiment of the present invention. As shown in FIG. 1, the fixing device 10 is a recording in which a fixing roller 11 as a heating body and a pressure roller 12 as a heating body are pressed against each other, and a toner image is formed in the nip portion N thereof. The medium P is passed, and the toner is melted and pressed to be fixed on the recording medium P.

  Inside the fixing roller 11 and the pressure roller 12, heaters 11a and 12a that are electrically heated are arranged, respectively, so that the surface temperature of the fixing roller 11 is heated to a temperature suitable for the fixing process. ing. The capacity of the heater is, for example, 430 W for the heater 11 a of the fixing roller 11 and 330 W for the heater 12 a of the pressure roller 12.

  The fixing device 10 approaches the fixing roller 11 and detects the surface temperature of the fixing roller 11 in a non-contact state, the thermistor 13 as a first temperature detecting means, and the environmental temperature inside the image forming apparatus. A temperature sensor 14 as second temperature detecting means is provided.

  The control circuit 15 controls the operation of the image forming apparatus. Here, the part related to the temperature control of the fixing device is shown. The control circuit 15 is arranged inside the fixing roller 11 and the pressure roller 12 based on the surface temperature of the fixing roller 11 detected by the thermistor 13 and the environmental temperature inside the image forming apparatus detected by the temperature sensor 14. The current supplied to the heaters 11a and 12a is turned on / off to maintain the surface temperature of the fixing roller 11 within a predetermined temperature range suitable for the fixing process.

  FIG. 2 is a block diagram showing a configuration of a part related to the temperature control of the fixing device in the control circuit 15 of the image forming apparatus. The control circuit 15 includes a CPU 21, and a thermistor 13, a temperature sensor 14, a heater drive circuit 22, and a heater drive circuit 23 are connected to an input / output port of the CPU 21, and the heater drive circuit 22 includes the fixing roller 11. The heater 11a is connected to the heater drive circuit 23, and the heater 12a of the pressure roller 12 is connected thereto. In addition, a motor control circuit 24 for controlling the motor 25 for driving the fixing roller is connected to the input / output port of the CPU 21.

  If the surface temperature of the fixing roller 11 detected by the thermistor 13 is equal to or lower than the lower limit value of the temperature range suitable for the fixing process, the CPU 21 sets the heater driving circuit 22 and the heater driving circuit 23 to ON. The heater 11a and the heater 12a are energized. Further, when the surface temperature of the fixing roller 11 detected by the thermistor 13 exceeds the upper limit value of the temperature range suitable for the preset fixing process, the heater driving circuit 22 and the heater driving circuit 23 are set to OFF. The energization to the heater 11a and the heater 12a is cut off. Thereby, the surface temperature of the fixing roller 11 is controlled so as to maintain a predetermined temperature range suitable for the fixing process.

  The heater 11a and the heater 12a can employ any known heating means such as a halogen lamp, a resistance heating element, and an electromagnetic induction heating element. The configuration of the fixing roller 11 and the pressure roller 12 is also a known configuration, and a heater may be disposed only on the fixing roller 11. Since these configurations are known configurations, detailed description thereof is omitted, and the disconnection detection process of the thermistor 13 executed by the CPU 21 will be described below. Since there are a plurality of embodiments of the disconnection detection process, they will be sequentially described below.

[First embodiment]
A first embodiment of the disconnection detection process of the thermistor 13 will be described. FIG. 3 is a flowchart for explaining disconnection detection processing executed by the CPU 21. The resistance value R of the thermistor is measured based on the voltage and current applied to the thermistor by the CPU 21. Here, it is assumed that the thermistor uses a negative temperature characteristic (a characteristic in which the resistance value decreases as the temperature rises).

  The disconnection detection time Z is a waiting time required for the thermistor 13 as the first temperature detection means to reach a temperature suitable for performing disconnection detection in conformity with the environmental temperature. The resistance value of the thermistor 13 varies with temperature. After the power is turned on, the in-machine temperature of the image forming apparatus gradually increases from the cooled state, and the temperature of the thermistor 13 also gradually increases. However, the time until the thermistor 13 is warmed to the in-machine temperature, that is, the environmental temperature, is set by the image forming apparatus. It varies depending on the environment and season. Therefore, it is assumed that an appropriate disconnection detection time Z until reaching a temperature suitable for disconnection detection is set in advance through experiments.

  The image forming apparatus is activated to start energization of the heaters 11a and 12a, the initial value of the resistance value R of the thermistor 13 at that time is measured by the CPU 21, and R = R1 is read (step P11), and the first predetermined time A time TA determined beforehand by an experiment is set as the first disconnection detection time Z (Z ← TA) (step P12). Further, an initial value is set to the count value Z of the timer that measures the disconnection detection time Z (Z ← 1) (step P13). The timer starts counting to determine the count value Z (step P14). If the count value Z has not reached (Z = TA), the count value is incremented (step P15). When the count value Z becomes (Z = TA) in the determination of step P14, it is determined that the first predetermined time has elapsed, that is, the time TA set for the first disconnection detection time Z has elapsed. Then, the thermistor resistance value R = R2 at that time is measured and read (step P16).

  Whether or not the determination by the first determination means, that is, the difference between the thermistor resistance values R2 and R1 (R2-R1) is larger than a limit value D set in advance for detecting the disconnection of the thermistor ((R2-R1 )> D) is determined (step P17), and if it is greater than the limit value D, it is determined that the thermistor is normal, that is, not disconnected, and the process is terminated. In step P17, if it is equal to or less than the limit value D, it is determined that the thermistor is disconnected (first detection), and the heaters 11a and 12a are energized for safety, ie, for safety. And a warning display to that effect is displayed (step P18).

  A time TB determined in advance by experiment is set to the second disconnection detection time Z which is the second predetermined time (Z ← TB) (step P20). Further, an initial value is set (Z ← 1) to the count value (Z) of the timer that measures the time until disconnection is detected (step P21). The timer starts counting, and the thermistor resistance value R = R3 at that time is read (step P22). Whether the determination by the second determination means, that is, the difference between the thermistor resistance values R3 and R1 (R3-R1) is larger than a limit value E set in advance for detecting the disconnection of the thermistor ((R3-R1 )> E) is determined (step P23), and if it is greater than the limit value E, it is determined that the thermistor is normal, that is, it is not disconnected, the interruption by the interruption means is released, and energization to the heaters 11a and 12a is started. (Step P24), and the process ends.

  In step P23, if the value is equal to or less than the limit value E, the timer count value (Z) is determined (step P25). If the count value (Z) has not reached (Z = TB), the count value (Z) is determined. Is incremented (step P26), and when the count value (Z) reaches (Z = TB), the elapse of the second predetermined time, the predetermined time (Z = TB) for the second disconnection detection Even if it has elapsed, it is determined that the difference between the thermistor resistance values R3 and R1 (R3-R1) is smaller than the limit value E of the thermistor disconnection detection, that is, the thermistor disconnection is detected (the second detection), For safety, the energization of the heaters 11a and 12a by the shut-off means is shut off, and a thermistor disconnection warning is displayed (step P27).

  It is determined whether or not the thermistor disconnection detection process has been reset (step P28), and when reset, the process returns to step P11 to prepare for the next disconnection detection. If not reset, the process returns to step P27, and the interruption of the energization to the heaters 11a and 12a by the interruption means and the disconnection warning display of the thermistor are continued.

  According to the first embodiment described above, even if a thermistor disconnection is detected in the first disconnection detection, the thermistor is also low because the ambient temperature inside the apparatus is low, and the thermistor resistance value R2 is If it is detected that the difference between the initial value R1 and the initial value R1 is smaller than the limit value D, and the thermistor is detected as disconnected even though it is not disconnected, it is predetermined in the second disconnection detection. Since the thermistor is heated while the time (Z = TB) elapses, it is determined that the thermistor is normal, that is, not disconnected in the second disconnection detection. Further, in the second disconnection detection, when it is determined that the disconnection is not detected, energization to the heaters 11a and 12a is started immediately and the thermistor is also warmed, so that the normal state is restored more quickly.

[Second Embodiment]
A second embodiment of the detection process for the presence or absence of disconnection of the thermistor 13 will be described. The difference from the first embodiment is that, in the first embodiment, in step P20, the second disconnection detection time Z, which is the second predetermined time, is unilaterally set to the predetermined time TB that is determined in advance. Although set (Z ← TB), in the second embodiment, the environmental temperature inside the image forming apparatus at the start of heating by the heaters 11a and 12a or before the start of heating is detected by the temperature sensor 14 as the second temperature detecting means. Then, the temperature of the thermistor is estimated based on the detected ambient temperature, and a predetermined time TV is set as the second disconnection detection time Z that is the second predetermined time based on the estimated temperature of the thermistor (Z ← TV) There is in point to do.

  Specifically, a table (or an arithmetic expression) showing a relationship between the environmental temperature (which is also the thermistor temperature) and a predetermined time suitable for the second disconnection detection is stored in the memory of the control device in advance by experiments. When the temperature of the thermistor is estimated from the ambient temperature detected by the temperature sensor 14 and the temperature of the thermistor is estimated to be low (low temperature), a long time TV is set as the second disconnection detection time Z. When the temperature of the thermistor is estimated to be high (at a high temperature), a short time TV is set to the predetermined time Z.

  Accordingly, when it is estimated that the ambient temperature is high and the thermistor is high and sufficiently warm, the second disconnection detection time Z can be set short, and the thermistor disconnection can be detected quickly, When the ambient temperature is low and the thermistor is low, the second disconnection detection time Z can be lengthened to accurately detect the thermistor disconnection.

  FIG. 4 is a flowchart for explaining a second embodiment of the detection process for the presence or absence of disconnection of the thermistor 13 executed by the CPU 21. Steps P11 to P18 and steps P27 and P28 are the same processes as those in the flowchart of the first embodiment, and thus description thereof will be omitted, and steps for performing different processes will be described.

  As described above, the temperature of the thermistor is estimated from the environmental temperature (also the thermistor temperature) detected by the temperature sensor 14, and the second disconnection detection time Z that is the second predetermined time corresponding to the temperature is timed. TV is set (Z ← TV) (step P30). An initial value (Z ← 1) is set to the count value (Z) of the timer that measures the disconnection detection time Z (step P31). The timer starts counting, and the thermistor resistance value R = R3 at that time is read (step P32). Whether or not the determination by the second determination means, that is, the difference between the thermistor resistance values R3 and R1 (R3-R1) is larger than the limit value E set in advance for detecting the disconnection of the thermistor ((R3-R1 )> E) is determined (step P33), and if it is greater than the limit value E, it is determined that the thermistor is normal, that is, it is not disconnected, the interruption by the interruption means is released, and energization to the heaters 11a and 12a is started. (Step P34), and the process ends.

  If it is determined in step P33 that the count value (Z) of the timer is equal to or less than the limit value E, it is determined whether the timer count value (Z) is (Z = TV) (step P35). When the count value (Z) does not reach (Z = TV), the count value (Z) is incremented (step P36). When the count value (Z) becomes (Z = TV), the second time Even if the disconnection detection time (Z = TV) elapses, the difference (R3−R1) between the thermistor resistance values R3 and R1 is smaller than the limit value E of the thermistor disconnection detection, that is, the thermistor disconnection is detected (the second time It is determined that it has been detected), and the process proceeds to step P27 and subsequent steps.

  In the above-described embodiment, the temperature sensor 14 as the second temperature detecting unit is used to detect the environmental temperature inside the image forming apparatus at the start of heating or before the start of heating. You may make it obtain | require environmental temperature from the resistance value of the thermistor 13 which is a 1st temperature detection means. According to this configuration, it is not necessary to provide the temperature sensor 14.

[Third embodiment]
A third embodiment of detection processing for the presence or absence of disconnection of the thermistor 13 will be described. The difference from the first embodiment is that in step P20 in the first embodiment, the second disconnection detection time (Z ← TB), which is a second predetermined time, is set to a predetermined time. Although set unilaterally, in the third embodiment, the thermistor temperature T estimated from the environmental temperature inside the image forming apparatus detected by the temperature sensor 14 as the second temperature detecting means, and the surface temperature of the fixing roller 11. The second disconnection detection time TW is set based on the interval M between the thermistor 13 and the fixing roller 11 that detects the above.

  The interval M between the thermistor 13 and the fixing roller 11 is detected by energizing the heater to start temperature control, and measuring the relationship between the time interval H at which the heater is turned ON / OFF and the interval M between the thermistor and the fixing roller. A table (or empirical formula) showing the relationship between the time interval H at which the heater is turned ON / OFF and the interval M between the thermistor and the fixing roller is created, and the interval M is estimated from the time interval H. Since the time interval H becomes longer as the interval M is larger and the time interval H is shorter as the interval M is smaller, the interval M between the thermistor and the fixing roller can be estimated from the time interval H. When the distance M between the thermistor and the fixing roller is small, the temperature of the thermistor rises quickly, so that the second disconnection detection time TW can be set short.

  In addition to the thermistor temperature T estimated from the environmental temperature inside the image forming apparatus detected by the temperature sensor 14, the thermistor temperature T is high or thermistor When the distance M between the fixing roller and the fixing roller is approaching, it is presumed that the thermistor temperature is high and sufficiently warm, and the second disconnection detection time TW is set short to quickly detect the thermistor disconnection. be able to.

  FIG. 5 is a flowchart for explaining a third embodiment of the detecting process for the presence or absence of disconnection of the thermistor 13 executed by the CPU 21. Steps P11 to P18 and steps P27 and P28 are the same processes as those in the flowchart of the first embodiment, and thus description thereof will be omitted, and steps for performing different processes will be described.

  The second predetermined time is the second predetermined time based on the temperature T of the thermistor 13 detected by the temperature sensor 14 and the estimated value M of the thermistor and the fixing roller when the image forming apparatus set in advance by experiment is started. The predetermined time TW is set by a processing routine for calculating the disconnection detection time TW (see the flowchart of FIG. 6) (step P40).

  An initial value (Z ← 1) is set as the count value (Z) of the timer that measures the disconnection detection time Z (step P41). The timer starts counting, and the thermistor resistance value R = R3 at that time is read (step P42). Whether the determination by the second determination means, that is, the difference between the thermistor resistance values R3 and R1 (R3-R1) is larger than a limit value E set in advance for detecting the disconnection of the thermistor ((R3-R1 )> E) is determined (step P43), and if it is greater than the limit value E, it is determined that the thermistor is normal, that is, it is not disconnected, the interruption by the interruption means is released, and energization to the heaters 11a and 12a is started. (Step P44), and the process ends.

  If it is determined in step P43 that the count value (Z) of the timer is equal to or less than the limit value E, it is determined whether the timer count value (Z) is (Z = TW) (step P45). When the count value (Z) does not reach (Z = TW), the count value (Z) is incremented (step P46). When the count value (Z) becomes (Z = TW), the second time Even if the disconnection detection time (Z = TW) elapses, the difference (R3−R1) between the thermistor resistance values R3 and R1 is smaller than the limit value E of the thermistor disconnection detection, that is, the thermistor disconnection is detected (the second time It is determined that it has been detected), and the process proceeds to step P27 and subsequent steps.

  FIG. 6 is a flowchart for explaining a subroutine for calculating and determining the second disconnection detection time TW, which is the second predetermined time, based on the temperature T of the thermistor 13 and the estimated value of the interval M between the thermistor and the fixing roller. .

  In this subroutine, the relationship between the time interval H at which the heater is turned ON / OFF by temperature control and the interval M between the thermistor and the fixing roller is examined in advance to estimate the interval M from the time interval H, and the estimated temperature of the thermistor 13. A predetermined time TW suitable for the second disconnection detection is experimentally determined from T and the interval M, and a table for determining the predetermined time TW from the estimated temperature T and the interval M is created and stored in the memory of the control device. It shall be.

  A flowchart will be described. A control target value TX for the surface temperature of the fixing roller is set (step P61), the heater is energized to start temperature control (step P62), and a time interval H at which the heater is turned on / off by temperature control is measured. (Step P63). Further, the temperature T of the thermistor 13 is estimated from the environmental temperature measured by the temperature sensor 14 (step P64). From the obtained time interval H and the estimated temperature T of the thermistor 13, a table for determining the predetermined time TW stored in the memory is referred to obtain the second disconnection detection time TW (step P65).

[Fourth embodiment]
In the above-described third embodiment, the second predetermined time is determined based on the estimated temperature T of the thermistor 13 estimated from the environmental temperature measured by the temperature sensor 14 and the estimated value of the interval M between the thermistor 13 and the fixing roller 11. A certain second disconnection detection time TW is set. In the fourth embodiment, the estimated temperature T of the thermistor 13 estimated from the environmental temperature measured by the temperature sensor 14 and the interval M between the thermistor 13 and the fixing roller 11. The second disconnection detection time TW is set based on the actual measurement value. Note that the measured value of the interval M between the thermistor and the fixing roller is measured at the time of assembling the image forming apparatus and stored in the memory of the temperature control circuit 15.

  In the fourth embodiment, the estimated temperature T of the thermistor 13 estimated from the environmental temperature measured by the temperature sensor 14 can be used as a variable, and can be handled by a linear expression using a measured value of the distance M between the thermistor and the fixing roller as a constant. The second disconnection detection time TW can be obtained immediately by simply inputting the estimated temperature T of the thermistor 13.

  In the third embodiment described above, a table for estimating the interval M from the time interval H by measuring the relationship between the time interval Z at which the heater is turned ON / OFF by temperature control and the interval M between the thermistor and the fixing roller in advance. Further, it is necessary to determine a predetermined time W suitable for disconnection detection from the estimated temperature T of the thermistor 13 and the interval M, collect it in a table, and store it in the memory of the control device. In the fourth embodiment, since such a complicated process is not required, the first output (first copy) is quickly output in the image forming operation.

  Since the process of the fourth embodiment is different only in the content of the setting method of the predetermined time W executed in step P40 in the process of the third embodiment, the flowchart of the third embodiment is used and the description is omitted. .

  As described above, the configuration of the fixing device of the image forming apparatus according to the embodiment of the present invention and the plurality of examples of the detection process of the thermistor disconnection for detecting the surface temperature of the fixing roller in a non-contact manner have been described. As described in the embodiment, even if energization of the heater of the fixing roller is started, the temperature rise of the thermistor starts to rise with a considerable delay. FIG. 7 is a diagram showing the rise of the surface temperature of the fixing roller and the temperature of the thermistor after energization of the fixing roller is started. Line (a) shows the rise of the surface temperature of the fixing roller. b) shows the temperature rise of the thermistor.

  As shown in FIG. 7, when the energization of the heater of the fixing roller is started, the surface temperature of the fixing roller starts to rise with a delay of about 5 seconds from the start of energization, but the surface temperature of the fixing roller is not contacted. It can be seen that the temperature of the thermistor detected in (1) begins to rise after about 45 seconds from the start of energization. FIG. 7 shows the case where the environmental temperature of the image forming apparatus is 0 ° C. The lower the environmental temperature, the later the time when the temperature starts to rise. In FIG. 7, it can be seen that after about 45 seconds have elapsed from the start of energization of the heater of the fixing roller, the surface temperature of the fixing roller reaches 60 ° C. (actually measured value), but the temperature of the thermistor begins to gradually increase from 0 ° C.

  For this reason, in the above-described first embodiment, in the first disconnection detection, even when the thermistor disconnection is detected because the temperature of the thermistor is low, in the second disconnection detection in which more time has passed, It has been described that the thermistor may be determined to be normal, that is, not disconnected because the ambient temperature inside the image forming apparatus has risen and the thermistor has been warmed. In the above-described plurality of embodiments, the thermistor disconnection detection process is performed twice to detect thermistor disconnection for the reason described above.

  This device detects the disconnection of the thermistor, which is a temperature detection element for detecting the temperature of the fixing roller in the image forming apparatus. Thermistor disconnection detection processing is performed twice in consideration of the characteristics of the thermistor whose resistance value changes depending on the temperature. Therefore, it is possible to reliably detect the presence or absence of disconnection of the thermistor, and to provide a highly reliable fixing device for an image forming apparatus.

1 is a diagram illustrating an outline of a configuration of a fixing device of an image forming apparatus according to an embodiment of the present invention. The block diagram which shows the structure of a temperature control circuit. The flowchart explaining 1st Example of the detection process of the presence or absence of the disconnection of a thermistor. The flowchart explaining 2nd Example of the detection process of the presence or absence of the disconnection of a thermistor. The flowchart explaining the 3rd Example of the detection process of the presence or absence of the disconnection of a thermistor. 9 is a flowchart for explaining a process for estimating a distance M between the thermistor and the fixing roller. The figure which shows the mode of the raise of the surface temperature of the fixing roller after energization start of a fixing roller, and the temperature rise of a thermistor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Fixing device 11 Fixing roller 11a, 12a Heater 12 Pressure roller 13 Thermistor 14 Temperature sensor 15 Control circuit 21 CPU
22 Heater Drive Circuit 23 Heater Drive Circuit 24 Motor Control Circuit 25 Motor

Claims (6)

A heating element for fixing the toner image on the recording paper;
First temperature detecting means for detecting the temperature of the heating element disposed in a non-contact manner facing the heating element;
First determination means for determining whether or not normal based on a detection value by the first temperature detection means after a first predetermined time has elapsed from the start of heating of the heating body;
When it is determined that the first determination unit is not normal, a blocking unit that reduces or turns off the power to the heating body;
Second determination means for determining whether or not normal based on a detection value by the first temperature detection means after a second predetermined time has elapsed since the shut-off means is activated;
An image forming apparatus comprising: a release unit that cancels the operation of the blocking unit when the second determination unit determines that the second determination unit is normal. A heating element for fixing the toner image on the recording paper;
First temperature detecting means for detecting the temperature of the heating element disposed in a non-contact manner facing the heating element;
First determination means for determining whether or not normal based on a detection value by the first temperature detection means after a first predetermined time has elapsed from the start of heating of the heating body;
When it is determined that the first determination unit is not normal, a blocking unit that reduces or turns off the power to the heating body;
Second determination means for determining whether or not normal based on a detected value by the first temperature detection means after a second predetermined time has elapsed since the shut-off means is activated;
An image forming apparatus comprising: an informing unit for informing that the second determining unit is abnormal when it is determined that the second determining unit is not normal. The time setting means which changes the value of the said 2nd predetermined time based on the detected value by the said 1st temperature detection means at the time of the heating start of the said heating body, or before the start is characterized by the above-mentioned. The image forming apparatus described in 1. A second temperature detecting means for detecting an environmental temperature inside the image forming apparatus;
2. A time setting unit that changes a value of the second predetermined time based on a value detected by the second temperature detection unit at the start of heating of the heating body or before the start of heating. The image forming apparatus according to 2. Time setting means for changing a value of the second predetermined time based on a detection value by the first temperature detection means at the start of heating of the heating body or before the heating body and a distance between the heating body and the temperature detection means. The image forming apparatus according to claim 1, further comprising an image forming apparatus. The first determination means is normal by comparing the detection value by the first temperature detection means at the start of heating or before the start with the detection value by the first temperature detection means after a predetermined time has elapsed from the start of heating. 6. The image forming apparatus according to claim 1, wherein the image forming apparatus determines whether or not the image forming apparatus is used.

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