JP4311155B2 - Image forming apparatus - Google Patents

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).

In addition, a conventional disconnection detection device that detects disconnection of a contact-type temperature detection element exhibits a high resistance value even when the temperature detection element is not in a disconnected state because the temperature detection element is also at a low temperature when the entire apparatus is at a low temperature. Indistinguishable from the disconnection state. In order to solve this inconvenience, there is known a configuration in which detection of disconnection is prohibited for a certain time after power-on, and detection of disconnection is performed after a lapse of a certain time (see Patent Document 2).
JP-A-8-76619. Japanese Patent Laid-Open No. 55-164867.

  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, the temperature rise of the heating rotator does not immediately appear as a change in the resistance value of the temperature detecting element, and the time from the start of heating of the heating rotator to the detection of the disconnection of the temperature detecting element (hereinafter, 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.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems. The invention according to claim 1 is a heating body for fixing a toner image on a recording sheet, and a first temperature detecting means for detecting the temperature of the heating body arranged in non-contact with the heating body. A temperature detecting means; a second temperature detecting means for detecting an environmental temperature inside the image forming apparatus; and whether a change in a detected value of the heating body temperature by the first temperature detecting means exceeds a predetermined temperature range set in advance. Temperature determination means for determining whether or not the determination by the temperature determination means is repeatedly executed, and when the determination result indicates that the detected value of the heating body temperature exceeds a predetermined temperature range set in advance, the first temperature It is determined that the detection unit is normal, the repeated determination operation by the temperature determination unit is stopped, and the detection value of the heating body temperature is set to a predetermined value even if the determination result has passed a predetermined disconnection detection period. Within temperature range Period for changing the abnormality determination means determines that the first temperature sensing means is abnormal, said predetermined disconnection detection period based on the detection value of the second temperature detecting means for detecting the environmental temperature when indicating that the stay An image forming apparatus comprising: a changing unit.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the temperature determination unit includes an initial temperature detection value by the first temperature detection unit at the start of heating of the heating body or before the start, and a temperature determination. An image forming apparatus comprising: temperature determination means for determining whether or not a difference between a temperature detection value at a determination time by the first temperature detection means at a determination time by the means exceeds a predetermined range.

According to a third aspect of the present invention, in the image forming apparatus according to the first aspect, in addition to the detection value of the second temperature detection unit, the period changing unit includes a distance between the heating body and the first temperature detection unit. An image forming apparatus comprising: a period changing unit that changes the predetermined disconnection detection period based on the period.

According to a fourth aspect of the present invention, in the image forming apparatus according to the third aspect, the period changing unit includes a distance estimating unit that estimates a distance between the heating body and the first temperature detecting unit. The image forming apparatus according to Item 1 .

According to a fifth aspect of the present invention, in the image forming apparatus according to the first , third, or fourth aspect, the period changing unit increases the predetermined disconnection detection period as the detection value of the second temperature detecting unit indicates a lower temperature. An image forming apparatus characterized by being a period changing means for lengthening.

  According to a sixth aspect of the present invention, in the image forming apparatus according to the first aspect, the period changing unit changes the predetermined temperature range according to a detection value of the first temperature detecting unit or the second temperature detecting unit. An image forming apparatus characterized by the above.

As described above, according to the present invention, the change in the detection value of the first temperature detection means for detecting the temperature of the heating body after the start of energization of the heating body for fixing the toner image on the recording paper in a non-contact manner is set in advance. When the temperature determination means repeatedly determines whether or not the temperature range exceeds the predetermined temperature range, and the determination result indicates that the detected value of the heating body temperature exceeds a preset predetermined temperature range, the abnormality determination means It is determined that the temperature detection means is normal, and the determination operation is stopped. When the determination result indicates that the detected value of the heating body temperature remains within the predetermined temperature range set in advance even after the predetermined disconnection detection period set in advance, the first temperature detection means is abnormal. It is determined.

With this configuration, when the first temperature detecting means is determined to be normal by stopping the repetition determining operation that by the temperature determining means, it is possible to start the temperature control of the immediate heating body from that point, the In order to determine normality / abnormality of the one temperature detection means, the image forming apparatus can be operated quickly without waiting for the elapse of a predetermined disconnection detection period.

In the present invention, the predetermined disconnection detection period in which the above determination is repeated is configured to be changed by the detection value of the second temperature detection means for detecting the ambient temperature in the image forming apparatus, so the first temperature detection means It takes a long time to detect abnormalities in the fixing device and there is no danger of overheating and damage to the surrounding equipment, and the abnormality detection time is too short, so there is no false detection and a highly reliable fixing device Can be provided.

  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 known, and a heater may be disposed only on the fixing roller 11. Since these configurations are known configurations, a detailed description thereof will be omitted, and the disconnection detection process of the thermistor 13 that is the first temperature detection means executed by the CPU 21 will be described below.

  A disconnection detection process of the thermistor 13 will be described. 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).

  Terms used in the following configurations will be described. First, the disconnection detection period Z is a period sufficient for the thermistor 13 serving as the first temperature detection means to reach a temperature suitable for disconnection detection in conformity with the environmental temperature. In the disconnection detection period Z, an appropriate period TA is set in advance by experiment (Z = TA). However, when the ambient temperature of the thermistor 13 is relatively low, the temperature reaches a temperature suitable for disconnection detection. If it takes a long time and the environmental temperature is relatively high, it takes a short time to reach a temperature suitable for detecting disconnection. Therefore, the period TA set in the disconnection detection period Z is configured to change according to the environmental temperature Y detected by the temperature sensor 14, and the period TA is set longer as the detected environmental temperature Y is lower. It shall be. Further, in addition to the environmental temperature Y, the temperature may be changed according to the temperature of the thermistor, that is, the resistance value.

  In addition, when the thermistor is at a low temperature just after the power is turned on, the resistance value R is high. Therefore, even if the thermistor is not disconnected, it may be detected as a disconnection. R gradually decreases and is detected as not being disconnected.

  Therefore, in this embodiment, the disconnection detection is repeatedly performed within the range of the disconnection detection period Z, for example, every several milliseconds, and within the range of the disconnection detection period Z, the heating of the fixing roller 11 is started or before the start of heating. When the difference between the temperature detection value (initial temperature detection value) of the thermistor 13 and the temperature detection value (initial temperature detection value) at the time of disconnection determination that is repeatedly executed thereafter exceeds a predetermined temperature range, The thermistor 13 determines that it is normal, ends the disconnection detection process, and immediately shifts to normal temperature control of the fixing roller. Even if the disconnection detection period Z elapses, the difference between the temperature detection values is set to a predetermined value. When it remains within the temperature range, the thermistor 13 determines that it is abnormal (disconnected) and stops the temperature control of the fixing roller.

  Specifically, since the temperature of the thermistor 13 can be treated as the resistance value of the thermistor 13, the resistance value R1 of the thermistor 13 at the start of heating of the fixing roller 11 or before the start of heating, and the resistance of the thermistor 13 at the time of disconnection determination. When the difference (R2−R1) from the value R2 exceeds a predetermined value B set in advance ((R2−R1) ≧ B), it is determined that the thermistor 13 is normal and the disconnection detection period Z has elapsed. The thermistor 13 is determined to be abnormal (disconnected) when the difference between the resistance values (R2−R1) remains within a predetermined range of a predetermined value B.

  Here, the predetermined value B indicates the amount of change in the resistance value as the temperature of the thermistor 13 rises, and is a reference value for detecting disconnection of the thermistor 13, so that an optimum value is previously determined by experiment. It shall be decided.

  The resistance value of the thermistor 13 varies with temperature. After the power is turned on, the internal temperature of the image forming apparatus gradually rises from the cold state, and the temperature of the thermistor 13 also gradually rises. However, the time until the thermistor 13 is warmed to the internal temperature, that is, the environmental temperature, is It varies according to the environment and the season where it is installed. Further, the interval M between the thermistor 13 that detects the surface temperature of the fixing roller 11 and the fixing roller 11 also affects the disconnection detection period Z. That is, when the distance M between the thermistor and the fixing roller is small, the temperature of the thermistor rises quickly, and when the distance M is large, the temperature rise of the thermistor becomes slow.

  Therefore, the relationship between the ambient temperature Y, the distance M between the thermistor and the fixing roller, and the disconnection detection period Z suitable for detecting disconnection (Z = f (Y, M)) is confirmed in advance by experiments, and the experimental results are obtained. It is assumed that an appropriate disconnection detection period Z is preset based on the table M and stored in the memory of the control circuit 15 in a table format for obtaining the disconnection detection period Z from the interval M and the environmental temperature Y.

  The detection of the interval M between the thermistor 13 and the fixing roller 11 starts the temperature control by energizing the heater of the fixing roller, the time interval H when the heater is turned ON / OFF, and the interval M between the thermistor and the fixing roller. The interval M is estimated from the relationship. 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. The interval M is detected by an interval detection subroutine described later. In addition, as the interval M, a value obtained by actually measuring the interval M between the thermistor 13 and the fixing roller 11 at the time of assembling the image forming apparatus can be used. In this case, the measured value of the interval M is stored in the memory of the control device 15.

  In addition to the environmental temperature Y (also the temperature of the thermistor 13) detected by the temperature sensor 14, data on the distance M between the thermistor 13 and the fixing roller 11 is used to obtain the environmental temperature Y (thermistor). When the distance M between the thermistor and the fixing roller is close, it is estimated that the thermistor temperature is sufficiently high and the disconnection detection period Z can be set short. It becomes possible, and the disconnection of the thermistor can be detected quickly. The period changing means in the present invention means means for changing the disconnection detection period Z based on information such as the environmental temperature Y and the interval M.

  FIG. 3 is a flowchart for explaining disconnection detection processing of the thermistor 13 executed by the CPU of the control circuit 15.

  The image forming apparatus is activated to start energization of the heaters 11a and 12a to start temperature control (step P11), and an interval M between the thermistor and the fixing roller is input (step P12). The interval M uses a measurement subroutine which will be described later, or a measurement value measured during assembly and stored in the memory. Further, the environmental temperature detection value Y detected by the temperature sensor 14 is read (step P13), 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 P14).

  With reference to the table stored in the memory of the control circuit 15 from the input interval M and environmental temperature Y, the period TA is set in the disconnection detection period Z (Z ← TA) (step P15), and the disconnection detection period Z An initial value is set to the count value N of the timer for measuring (N ← 1) (step P16), the resistance value of the thermistor 13 at that time is measured by the CPU 21, and the resistance value R = R2 is read (step P17).

  Further, a predetermined value B (limit value) which is a reference value for detecting disconnection of the thermistor 13 determined in advance by an experiment is read (step P18).

  It is determined whether or not the difference (R2−R1) between the thermistor resistance values R2 and R1 is larger than a predetermined value B (limit value) for detecting disconnection of the thermistor ((R2−R1) ≧ B) (step) P19) (step of configuring temperature determination means). If the difference between the thermistor resistance values R2 and R1 (R2-R1) is greater than a predetermined value B (limit value), it is determined that the thermistor is normal, that is, not disconnected, and the thermistor abnormality determination operation is stopped. (Step P20) (Step of configuring an abnormality determination unit), the disconnection detection process is terminated. Since it is determined that the thermistor is normal and the disconnection detection process is completed, the temperature control of the fixing roller can be started immediately from this point.

  If the difference (R2−R1) between the thermistor resistance values R2 and R1 is equal to or smaller than the predetermined value B (limit value) in the determination of step P19, whether the period TA set in the disconnection detection period Z has elapsed (N = TA) It is determined whether or not (step P21). If the period TA has not elapsed, the timer count value (N) is incremented by 1 (step P22), and the process returns to step P17.

  If the period TA has elapsed in the determination in step P21, it is determined that the thermistor is disconnected, the power supply to the heaters 11a and 12a is cut off, and a warning to that effect is displayed (step P23) ( A step of constituting an abnormality determination means).

  It is determined whether or not the thermistor disconnection detection process has been reset (step P24), and if reset, the process returns to step P11 to prepare for the next disconnection detection. If not reset, the process returns to step P23, and the energization interruption to the heaters 11a and 12a and the disconnection warning display of the thermistor are continued.

  FIG. 4 is a diagram for explaining an interval detection subroutine for detecting the interval M between the thermistor 13 and the fixing roller 11. The interval M between the thermistor 13 and the fixing roller 11 is detected by energizing the heater of the fixing roller to start temperature control, and 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. Measure and estimate the interval M from 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. It should be noted that, in advance, 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 examined by experiment, and a table for obtaining the interval M from the time interval H is created. I shall keep it.

  The control target value TX of the fixing roller surface temperature is set (step P31), the temperature control is started by energizing the heater of the fixing roller (step P32), and the time interval H at which the heater is turned on / off by the temperature control is measured. (Step P33). An interval M between the thermistor and the fixing roller is obtained from the time interval H with reference to a previously created table (step P34).

  In the above-described embodiment, a predetermined value B (limit value), which is a reference value for detecting disconnection of the thermistor 13, that is, a value indicating a change in resistance value accompanying a rise in the temperature of the thermistor 13 is previously set to an optimum value through experiments. Has been decided.

  However, since the resistance value of the thermistor 13 varies depending on the temperature of the thermistor 13, the predetermined value B described above is closely related to the resistance value R1 at the start of heating of the thermistor 13 or before the start of heating, or the resistance value of the thermistor 13. The detection value Y of the temperature sensor 14 may be changed. When the detection value of the resistance value R1 at the start of heating of the thermistor 13 or before the start of heating is high, or the temperature detection value Y of the temperature sensor 14 is When the value is low, the predetermined value B may be set small.

  Specifically, in the flowchart of FIG. 3, when the predetermined value B shown as Step P18 is input, the resistance value R1 at the start of heating of the thermistor 13 or before the start of heating by the experiment or the temperature detection value of the temperature sensor 14 in advance. A predetermined value B set based on Y is used. The predetermined value B may be determined in consideration of both the detection value of the resistance value R1 and the temperature detection value Y of the temperature sensor 14.

  As the image forming apparatus is in a low temperature environment and the temperature of the fixing device is lower, the temperature rise is smaller even when heating of the fixing roller is started. However, the detected value of the resistance value R1 or the detected temperature value Y of the temperature sensor 14 If used to change the predetermined value B, the thermistor 13 can be determined to be normal, i.e., not disconnected, even if a slight temperature rise is detected, and the thermistor state can be quickly detected. It can be carried out.

  The configuration of the fixing device of the image forming apparatus according to the embodiment of the present invention and the detection process of thermistor disconnection for detecting the surface temperature of the fixing roller in a non-contact manner have been described above. As already described, even if energization of the heater of the fixing roller is started, the temperature rise of the thermistor begins to rise with a considerable delay. FIG. 5 is a diagram showing how the surface temperature of the fixing roller and the temperature of the thermistor rise after energization of the fixing roller is started. Line (a) shows the increase in the surface temperature of the fixing roller. b) shows the temperature rise of the thermistor.

  As shown in FIG. 5, when 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. 5 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. 5, it can be seen that when about 45 seconds have elapsed after the energization of the heater of the fixing roller, the surface temperature of the fixing roller reaches 60 ° C. (actual measurement value), but the temperature of the thermistor begins to gradually increase from 0 ° C.

  Thermistor disconnection detection device, which is a temperature detection element that detects the temperature of the fixing roller in the image forming apparatus, repeatedly performs thermistor disconnection detection processing in consideration of the characteristics of the thermistor whose resistance value varies with temperature. It is possible to reliably detect the presence or absence of disconnection of the thermistor. When the thermistor is determined to be normal, the temperature control of the fixing roller can be immediately started to operate the image forming apparatus with high reliability. An image forming apparatus can be provided.

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 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 a fixing roller after the 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 body arranged in non-contact with the heating body;
A second temperature detecting means for detecting an environmental temperature inside the image forming apparatus;
Temperature determining means for determining whether or not the change in the detected value of the heating body temperature by the first temperature detecting means exceeds a predetermined temperature range set in advance;
The determination by the temperature determination means is repeatedly executed, and when the determination result indicates that the detected value of the heating body temperature exceeds a predetermined temperature range set in advance, it is determined that the first temperature detection means is normal. When the repeated determination operation by the temperature determination means is stopped and the determination result indicates that the detection value of the heating body temperature remains within the predetermined temperature range set in advance even after a predetermined disconnection detection period has elapsed. Is an abnormality determination means for determining that the first temperature detection means is abnormal,
An image forming apparatus comprising: a period changing unit that changes the predetermined disconnection detection period based on a detection value of a second temperature detecting unit that detects the environmental temperature . The temperature determination means includes an initial temperature detection value by the first temperature detection means at the start or before heating of the heating body, and a temperature detection value at the determination time by the first temperature detection means at the determination time by the temperature determination means. The image forming apparatus according to claim 1, wherein the image forming apparatus is a temperature determining unit that determines whether or not a difference between the first and second differences exceeds a predetermined range. The period changing means is a period changing means for changing the predetermined disconnection detection period based on a distance between the heating body and the first temperature detecting means in addition to a detection value of the second temperature detecting means. The image forming apparatus according to claim 1, wherein: The image forming apparatus according to claim 3, wherein the period changing unit includes a distance estimating unit that estimates a distance between the heating body and the first temperature detecting unit. Said period changing means is any one of claims 1, 3 or 4, wherein the detection value of the second temperature detecting means is a period changing means for lengthening the predetermined disconnection detection period as indicating the low temperature The image forming apparatus described in 1. The image forming apparatus according to claim 1, wherein the period changing unit changes the predetermined temperature range according to a detection value of the first temperature detecting unit or the second temperature detecting unit.

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