Anomaly detection device, anomaly detection method, and anomaly detection program

Abstract

To detect a local abnormality in a heater.SOLUTION: A heater ECU 10 as an abnormality detection device includes: a calculation unit that calculates a resistance value of a heater 51; a comparison unit that compares the resistance value calculated by the calculation unit this time with a resistance value calculated in the past; and a detection unit that detects an abnormality in the heater 51 when a comparison value from the comparison unit exceeds a threshold.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an abnormality determination device, an abnormality determination method, and an abnormality determination program.

Background Art

[0002] Conventionally, a heater used for the human body requires a technique for preventing overheating during use. For example, like the technique disclosed in Patent Document 1, there is a technique for cutting off the connection between a pair of electrodes and a power source when the temperature of the heater becomes a predetermined temperature or higher.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, even if a heater has a function of controlling at an upper limit of temperature like the technique disclosed in Patent Document 1, when the heater is locally damaged, current continues to flow, so there is a possibility that an abnormality may not be detected. Further, the technique disclosed in Patent Document 1 cannot detect local heat generation.

[0005] The present invention has been made in view of the above points, and an object thereof is to provide an abnormality determination device, an abnormality determination method, and an abnormality determination program capable of detecting a local abnormality of a heater.

Means for Solving the Problems

[0006] An abnormality determination device according to a first aspect includes a calculation unit that calculates a resistance value of a heater, a comparison unit that compares the past resistance value calculated by the calculation unit with the resistance value calculated this time, and a detection unit that detects an abnormality of the heater when a comparison value in the comparison unit exceeds a threshold value.

[0007] Here, the comparison value includes the difference between the past resistance value and the current resistance value, and the rate of change from the past resistance value to the current resistance value. According to the abnormality detection device of the first embodiment, abnormalities can be detected even when the heater is locally damaged.

[0008] The abnormality determination device of the second embodiment is the abnormality determination device of the first embodiment, wherein the calculation unit periodically calculates the resistance value, and the comparison unit compares the previously calculated resistance value calculated by the calculation unit with the resistance value calculated this time.

[0009] According to the abnormality detection device of the second embodiment, periodic measurements and calculations are performed and compared with the change in resistance value in the previous period, thereby enabling rapid detection of abnormalities during heater operation.

[0010] The abnormality detection device of the third embodiment is the abnormality detection device of the first embodiment, comprising an update unit that updates the minimum and maximum values ​​of the resistance calculated by the calculation unit, the comparison unit compares the minimum value with the resistance value calculated this time, and the maximum value with the resistance value calculated this time, and the detection unit detects an abnormality in the heater if either of the comparison values ​​exceeds a threshold.

[0011] According to the third embodiment of the abnormality detection device, the minimum and maximum resistance values ​​during normal operation are updated and saved, and these values ​​are used as thresholds to compare with the current resistance value, thereby enabling the detection of abnormalities during the initial operation of the heater.

[0012] The abnormality detection device of the fourth embodiment is the abnormality detection device of the first embodiment, wherein the heater is made of a material containing a conductive fine carbon structure.

[0013] The fifth embodiment of the abnormality detection device is the fourth embodiment of the abnormality detection device, wherein the heater is a planar heater made of a material containing carbon nanotubes.

[0014] According to the abnormality detection device of the fifth embodiment, it is possible to provide a heater that is easy to use because it can not only heat the entire surface uniformly but also heat up quickly.

[0015] The abnormality detection device of the sixth embodiment is the abnormality detection device of the first embodiment, comprising an acquisition unit that acquires the ambient temperature around the heater, and the detection unit detects an abnormality in the heater using the threshold value corrected based on the ambient temperature acquired by the acquisition unit or the resistance value corrected based on the ambient temperature.

[0016] According to the abnormality detection device of the sixth embodiment, the threshold or resistance value is corrected to an appropriate value according to the ambient temperature around the heater, thereby enabling detection of abnormalities with higher accuracy.

[0017] The abnormality determination device of the seventh embodiment is the abnormality determination device of the first embodiment, comprising a receiving unit that receives the voltage value and current value of the heater, and the calculation unit calculates the resistance value based on the maximum value of the fluctuating voltage value received by the receiving unit and the maximum value of the fluctuating current value received by the receiving unit.

[0018] According to the abnormality detection device of the seventh embodiment, the resistance value can be calculated stably even when the current value and voltage value fluctuate, and the accuracy of heater abnormality detection can be ensured.

[0019] The eighth aspect is an abnormality detection method, wherein a computer performs a process to detect an abnormality in the heater if the comparison value exceeds a threshold, by calculating the resistance value of the heater, comparing the previously calculated resistance value with the currently calculated resistance value, and the comparison value exceeds a threshold.

[0020] The ninth aspect is an abnormality detection program, which causes a computer to perform a process of calculating the resistance value of a heater, comparing the previously calculated resistance value with the resistance value calculated this time, and detecting an abnormality in the heater if the comparison value exceeds a threshold. [Effects of the Invention]

[0021] According to the present invention, local abnormalities of the heater can be detected.

Brief Description of the Drawings

[0022] [Figure 1] It is a block diagram showing the hardware configuration of a vehicle. [Figure 2] It is a block diagram showing the functional configuration of the heater ECU. [Figure 3] It is a diagram for explaining the use of the maximum values of the periodically varying voltage value and current value. [Figure 4] It is a diagram for explaining an example of the temperature distribution at the time of local damage of the planar CNT heater. [Figure 5] It is a flowchart showing an example of the abnormality determination process during the operation of the CNT heater. [Figure 6] It is a flowchart showing an example of the abnormality determination process during the initial operation of the CNT heater. [Figure 7] It is a diagram for explaining an example of the temperature distribution at the time of half break of the nichrome wire.

Embodiments for Carrying Out the Invention

[0023] Hereinafter, an example of an embodiment of the present invention will be described while referring to the drawings. FIG. 1 shows the hardware configuration of a vehicle 100 including a heater ECU 10. As shown in FIG. 1, the vehicle 100 according to the present embodiment includes a heater ECU 10 as an abnormality determination device, a vehicle seat 11, a heater 51, and a temperature sensor 53. Furthermore, the vehicle seat 11 is configured to include a plurality of heaters 51 inside. Specifically, the heaters 51 include heater 51A, heater 51B, and heater 51C. Heaters 51A, 51B, and 51C are installed in various parts of the vehicle seat 11 and have the function of warming the occupant's body. The heaters 51 are planar and are composed of conductive fine carbon structures. Examples of conductive fine carbon structures include CNT (Carbon Nano Tube) and CPT (Carbon Pico Tube). Each heater 51 is connected to a heater ECU 10.

[0024] The temperature sensor 53 is a sensor for detecting the temperature around the heater 51. The temperature sensor 53 is connected to the heater ECU 10. The temperature sensor 53 is mounted on the heater 51 or the vehicle seat 11. Note that the number of heater ECUs 10, heaters 51, and temperature sensors 53 included in the vehicle 100 is not limited to the example in Figure 1. For example, there may be two or more heater ECUs 10 and temperature sensors 53, and any number of CNT heaters 51 may be used to configure the vehicle seat 11. Also, the vehicle 100 of this embodiment may have multiple vehicle seats 11. In this case, each heater 51 may be controlled by a single heater ECU 10, or each heater 51 may be controlled by a heater ECU 10 provided for each vehicle seat 11.

[0025] The heater ECU 10 has the function of controlling each heater 51 provided in the vehicle seat 11. As shown in Figure 1, the heater ECU 10 has a CPU (Central Processing Unit) 31, ROM (Read Only Memory) 33, RAM (Random Access Memory) 35, control circuit 37, and input / output I / F (Interface) 39. The CPU 31, ROM 33, RAM 35, control circuit 37, and input / output I / F 39 are interconnected via an internal bus 41, enabling them to communicate with each other. Each heater 51 is connected to the control circuit 37. The control circuit 37 controls the output of each heater 51 and also has the function of detecting the voltage and current values ​​of each heater 51. A temperature sensor 53, installed near the vehicle seat 11, is connected to the input / output interface 39.

[0026] The CPU 31 is a central processing unit that executes various programs and controls each component. Specifically, the CPU 31 reads program 43 from ROM 33 and executes program 43 using RAM 35 as a working area. The CPU 31 controls each of the above components and performs various calculations according to program 43 stored in ROM 33.

[0027] ROM33 stores various programs, including the operating system, and various data. ROM33 also stores an error detection program for executing the error detection process described later. In addition, recording media such as an HDD (Hard Disk Drive) and an SSD (Solid State Drive) may be provided instead of or in addition to the ROM33. RAM35 temporarily stores programs and data as a working area.

[0028] The control circuit 37 is a circuit consisting of a PWM (Pulse Width Modulation) controller, a voltage detection unit, and a current detection unit.

[0029] Next, with reference to Figure 2, the functional configuration of the heater ECU 10 according to this embodiment will be described. As shown in Figure 2, the heater ECU 10 of this embodiment functions as a reception unit 12, acquisition unit 14, calculation unit 16, update unit 18, comparison unit 22, and detection unit 24 when the CPU 31 executes an abnormality determination program stored in the ROM 33.

[0030] The reception unit 12 has the function of receiving voltage and current values ​​detected by the control circuit 37. The reception unit 12 receives voltage and current values ​​that fluctuate due to PWM control. Figure 3 is a diagram illustrating the change in voltage and current values ​​over time in the heater 51. As a result of PWM control, the graph of voltage and current values ​​received by the reception unit 12 takes the shape of a pulse wave (see Figure 3).

[0031] As shown in Figure 2, the acquisition unit 14 has the function of acquiring the ambient temperature around the heater 51. The acquisition unit 14 acquires temperature information measured by the temperature sensor 53 via the input / output interface 39.

[0032] The calculation unit 16 has the function of calculating the resistance value of the heater 51. The calculation unit 16 periodically calculates the resistance value of the heater 51. Specifically, the calculation unit 16 calculates the resistance value by dividing the voltage value received by the reception unit 12 by the current value received by the reception unit 12. Since the voltage and current values ​​received by the reception unit 12 fluctuate, the calculation unit 16 calculates the resistance value based on their maximum values, as shown in Figure 3.

[0033] As shown in Figure 2, the update unit 18 has the function of updating the minimum and maximum resistance values ​​calculated by the calculation unit 16 and saving the values ​​in the ROM 33. Specifically, the update unit 18 updates and saves the minimum and maximum resistance values ​​during normal operation, thereby setting these values ​​as thresholds described later when the heater 51 first starts operating.

[0034] The comparison unit 22 has the function of comparing past resistance values ​​calculated by the calculation unit 16 with the resistance value calculated in the current cycle. Specifically, during the operation of the heater 51, the comparison unit 22 compares the resistance value calculated in the previous cycle by the calculation unit 16 with the resistance value calculated in the current cycle. In addition, during the initial operation of the heater 51 (when the power is turned on), the comparison unit 22 compares the minimum and maximum values ​​of the resistance values ​​updated by the update unit 18 with the resistance value calculated in the current cycle. Note that each comparison value obtained as a result of the comparison by the comparison unit 22 is not limited to a difference, but may also be a rate of change. When the comparison value is expressed as a rate of change, it is also set as a threshold for the rate of change in the setting of the first threshold and second threshold described later.

[0035] As a preprocessing step, the detection unit 24 corrects the threshold value or resistance value based on the ambient temperature of the heater 51 acquired by the acquisition unit 14. The detection unit 24 has the function of detecting a heater abnormality when any of the comparison values ​​in the comparison unit 22 exceeds a threshold. Here, the thresholds are set as a first threshold for detection during the operation of the heater 51 and a second threshold for detection during the initial operation of the heater 51. The detection unit 24 controls the shutdown of the heater 51's power supply by detecting an abnormality.

[0036] With the above functions, the heater ECU 10 can detect and control abnormalities in the heater 51. For example, Figure 4 is a diagram illustrating the ambient temperature distribution of a localized failure in a planar heater 51. As shown in Figure 4, when the heater 51 is locally damaged, the temperature rises locally around the damaged area 61. In Figure 4, as an example, the state where the temperature is higher closer to the damaged area 61 is represented by varying shades of gray. On the other hand, since current flows through the heater 51 outside the damaged area 61, localized damage cannot be detected by simple wire break detection. Therefore, in this embodiment, abnormalities in the heater 51 are detected using a resistance value that fluctuates in response to the localized damage of the heater 51. Thus, because it is difficult to detect temperature changes in the heater 51 even when it is locally damaged because current continues to flow, in this embodiment, the accuracy of heater 51 abnormality detection is improved by detecting abnormalities using the change in resistance value as the control method by the heater ECU 10. Furthermore, the heater ECU10 can not only detect abnormalities in the event of localized damage, but also, of course, detect abnormalities in the event of complete failure because the resistance value exceeds the threshold.

[0037] Next, the operation of the heater ECU 10 according to this embodiment will be described. In the vehicle 100, the abnormality detection process during operation of the heater 51 shown in Figure 5 and the abnormality detection process during initial operation of the heater 51 shown in Figure 6 are executed. Each process in the heater ECU 10 is executed by the CPU 31 functioning as a reception unit 12, acquisition unit 14, calculation unit 16, update unit 18, comparison unit 22, and detection unit 24. Note that each abnormality detection process is an example of the abnormality detection method of the present invention.

[0038] Referring to Figure 5, the abnormality detection process during the operation of the heater 51 will be explained.

[0039] In step S101, the CPU 31 determines whether the power to the heater 51 is ON or OFF. If the CPU 31 determines that the power to the heater 51 is OFF (step S101: NO), the process terminates. On the other hand, if the CPU 31 determines that the power to the heater 51 is ON (step S101: YES), the process proceeds to step S103.

[0040] Next, in step S103, the CPU 31 assigns an initial setting value to the first threshold value of the heater 51's resistance. Here, the initial setting value is the default value, and any value can be set.

[0041] Next, in step S105, the CPU 31 receives the voltage value and the current value.

[0042] Next, in step S107, the CPU 31 obtains the ambient temperature around the heater 51 from the temperature sensor 53 and corrects the first threshold based on that ambient temperature.

[0043] Next, in step S109, the CPU 31 calculates the current resistance value of the heater 51.

[0044] Next, in step S111, the CPU 31 determines whether it is the first time the resistance value of the heater 51 has been calculated after power-on. If the CPU 31 determines that it is the first time the resistance value of the heater 51 has been calculated after power-on (step S111: YES), the process proceeds to step S113. On the other hand, if the CPU 31 determines in step S111 that it is not the first time the resistance value of the heater 51 has been calculated after power-on (step S111: NO), the process proceeds to step S115. In step S113, the CPU 31 substitutes the current resistance value for the previous resistance value.

[0045] Next, in step S115, the CPU 31 stores the change amount, which is the difference between the previous resistance value and the current resistance value of the heater 51, as a comparison value.

[0046] Next, in step S117, the CPU 31 determines whether the comparison value, which is the change in resistance value compared to the previous cycle, exceeds the first threshold. If the CPU 31 determines that the comparison value does not exceed the first threshold (step S117: NO), the process proceeds to step S119. On the other hand, if the CPU 31 determines that the comparison value exceeds the first threshold (step S117: YES), the process proceeds to step S121. In step S119, the CPU 31 updates the minimum and maximum values ​​of the resistance, and returns to step S105. These actions from step S105 to step S119 are repeated at predetermined intervals.

[0047] Next, in step S121, the CPU 31 performs an anomaly detection. In this case, the detection of an anomaly can trigger notification to the crew. However, it is not always necessary to detect an anomaly; that is, step S121 is unnecessary, and the process may proceed from step S117 (if the judgment result is YES) to step S123. In step S123, the CPU 31 turns off the power to the heater 51. Then the abnormality detection process ends.

[0048] Referring to Figure 6, the abnormality detection process during the initial operation of the heater 51 will be explained.

[0049] In step S201, the CPU 31 determines whether the heater 51 is powered on or off. If the CPU 31 determines that the heater 51 is not powered on, i.e., off (step S201: NO), the process terminates. On the other hand, if the CPU 31 determines that the heater 51 is powered on (step S201: YES), the process proceeds to step S203.

[0050] Next, in step S203, the CPU 31 assigns an initial setting value to the second threshold value of the heater 51's resistance. Here, the initial setting value is the default value, and any value can be set.

[0051] Next, in step S205, the CPU 31 receives the voltage and current values.

[0052] Next, in step S207, the CPU 31 obtains the ambient temperature around the heater 51 from the temperature sensor 53 and corrects the second threshold based on that ambient temperature.

[0053] Next, in step S209, the CPU 31 calculates the current resistance value of the heater 51.

[0054] Next, in step S211, the CPU 31 determines whether the difference between the current resistance value and the minimum value or the difference between the current resistance value and the minimum value or the maximum value exceeds the second threshold. Here, the minimum value and maximum value refer to the minimum and maximum values ​​of the resistance value that were updated in step S119 in Figure 5 when the heater 51 was last used. Note that when an unused heater 51 is used for the first time, arbitrary initial values ​​are set for the minimum and maximum values. If the CPU 31 determines that neither difference exceeds the second threshold (step S211: NO), the process ends. On the other hand, if the CPU 31 determines in step S211 that either difference exceeds the second threshold (step S211: YES), the process proceeds to step S213.

[0055] Next, in step S213, the CPU 31 performs an anomaly detection. In this case, the detection of an anomaly can trigger notification to the crew. However, it is not always necessary to detect an anomaly; that is, step S213 is unnecessary, and the process may proceed from step S211 (if the judgment result is YES) to step S215. In step S215, the CPU 31 turns off the power to the heater 51. Then the abnormality detection process ends. (summary) As described above, this embodiment calculates the resistance value of the heater 51, compares the previously calculated resistance value with the resistance value calculated this time, and detects an abnormality in the heater 51 if the comparison value exceeds a threshold. Therefore, according to this embodiment, an abnormality can be detected even if the CNT heater is locally damaged.

[0056] Furthermore, this embodiment periodically calculates the resistance value and compares the previously calculated resistance value with the resistance value calculated this time. If the amount of change in the resistance value due to the comparison exceeds a first threshold, an abnormality in the heater 51 is detected. Under normal conditions, the amount of change in the resistance value each period is small, whereas under abnormal conditions, the amount of change in the resistance value each period is large, so the abnormality can be detected by comparing it with the first threshold. In other words, according to this embodiment, periodic measurement and calculation are performed and the change in resistance value with that of the previous period is compared, so an abnormality in the operation of the heater 51 can be detected quickly.

[0057] Furthermore, this embodiment updates the minimum and maximum values ​​of the calculated resistance, compares the minimum value with the currently calculated resistance, and compares the maximum value with the currently calculated resistance, respectively, and detects a heater abnormality if either of the comparison values ​​exceeds the second threshold. Therefore, according to this embodiment, the minimum and maximum values ​​of the resistance during normal operation are updated and saved, and compared with the current resistance as the second threshold, making it possible to detect abnormalities during the initial operation of the heater 51. In this embodiment, the second threshold for detection based on the minimum value and the second threshold for detection based on the maximum value are set to the same value, but this is not limited to this, and they may be set to different values.

[0058] Furthermore, the heater 51 of this embodiment is made of a material containing a conductive fine carbon structure. In particular, the heater 51 of this embodiment is a planar heater made of a material containing carbon nanotubes. Therefore, according to this embodiment, it is possible to provide a heater that is not only able to heat the entire surface uniformly but also provides instant heating, thus offering a pleasant user experience.

[0059] Furthermore, this embodiment acquires the ambient temperature around the heater 51 and detects heater abnormalities using a threshold value corrected based on the acquired ambient temperature or a resistance value corrected based on the ambient temperature. Therefore, according to this embodiment, since the threshold value or resistance value is corrected to an appropriate value according to the ambient temperature around the heater 51, abnormalities can be detected with higher accuracy.

[0060] Furthermore, this embodiment receives the voltage and current values ​​of the heater 51 and calculates the resistance value based on the maximum value of the received fluctuating voltage value and the maximum value of the received fluctuating current value. Therefore, according to this embodiment, even when the current and voltage values ​​fluctuate, such as in PWM control, the resistance value can be calculated stably, and the accuracy of abnormality detection of the heater 51 can be ensured.

[0061] In this embodiment, a CNT heater is used as an example, but this method can also be used to detect abnormalities in the case of partial breakage of a nichrome wire, which is a heating element. Figure 7 illustrates the ambient temperature distribution when a nichrome wire 62 is partially broken, showing an example where heat continues to be transferred even when damaged. In Figure 7, as an example, the temperature is higher closer to the partially broken portion 63, and this is represented by varying shades of gray. If the nichrome wire 62 is completely broken, the current will stop flowing, so it is possible to detect an abnormality based on the breakage. However, in cases where the current continues to flow even when damaged, such as when the nichrome wire 62 is partially broken, the heater ECU 10 can be used to detect abnormalities due to localized damage, similar to the case of a CNT heater. Furthermore, although this embodiment uses a vehicle seat as an example, the heater ECU can also be used for other products that can utilize a CNT heater, such as clothing and chairs other than those in vehicles.

[0062] (remarks) In the above embodiment, the first and second threshold values ​​were corrected based on the ambient temperature obtained from the heater 51, but this is not limited to this. For example, the resistance value calculated by the calculation unit 16 may be corrected based on the ambient temperature.

[0063] In the above embodiment, the abnormality detection process, which is executed by the CPU after it reads the software (program), may be executed by various processors other than the CPU. Examples of such processors include PLDs (Programmable Logic Devices) such as FPGAs (Field-Programmable Gate Arrays) whose circuit configuration can be changed after manufacturing, and dedicated electrical circuits that are processors with circuit configurations specifically designed to execute specific processes, such as ASICs (Application Specific Integrated Circuits). Furthermore, the transmission process may be executed by one of these various processors, or by a combination of two or more processors of the same or different types (for example, multiple FPGAs, and a combination of a CPU and an FPGA). More specifically, the hardware structure of these various processors is an electrical circuit that combines circuit elements such as semiconductor elements.

[0064] Furthermore, although the above embodiment describes a configuration in which the abnormality detection program is pre-stored (installed) in a storage device, the system is not limited to this configuration. The program may be provided in a form recorded on a recording medium such as a CD-ROM, DVD-ROM (Digital Versatile Disc Read Only Memory), or USB (Universal Serial Bus) memory. Alternatively, the program may be provided in a form that can be downloaded from an external device via a network. [Explanation of Symbols]

[0065] 10. Heater ECU (Anomaly Detection Device) 12 Reception Department 14 Acquisition Department 16 Calculation Section 18 Update section 22 Comparison Section 24 Detection unit 51 CNT heater

Claims

1. A calculation unit for calculating the heater resistance value, An update unit updates and stores the minimum and maximum values ​​of the resistance during normal operation, calculated by the calculation unit, at predetermined intervals. A comparison unit compares the current resistance value with the minimum value and the maximum value of the past resistance values ​​stored by the calculation unit, respectively. A detection unit detects an abnormality in the heater when any of the comparison values ​​in the comparison unit exceeds a threshold, An anomaly detection device that includes this.

2. The calculation unit periodically calculates the resistance value, The abnormality determination device according to claim 1, wherein the comparison unit compares the previously calculated resistance value with the currently calculated resistance value during the operation of the heater.

3. The comparison unit, upon powering on the heater, compares the minimum and maximum values ​​stored from the previous use with the resistance value calculated this time. The abnormality determination device according to claim 1, wherein the detection unit detects an abnormality in the heater when the difference between the resistance value calculated this time and the minimum value, or the difference between the resistance value calculated this time and the maximum value, exceeds the threshold value.

4. The abnormality detection device according to claim 1, wherein the heater is made of a material containing a conductive fine carbon structure.

5. The abnormality detection device according to claim 4, wherein the heater is a planar heater made of a material containing carbon nanotubes.

6. The system includes an acquisition unit that acquires the ambient temperature around the heater, The abnormality determination device according to claim 1, wherein the detection unit detects an abnormality in the heater using the threshold value corrected based on the ambient temperature acquired by the acquisition unit or the resistance value corrected based on the ambient temperature.

7. The heater is equipped with a receiving unit that receives the voltage and current values ​​of the heater, The abnormality determination device according to claim 1, wherein the calculation unit calculates the resistance value based on the maximum value of the fluctuating voltage value received by the reception unit and the maximum value of the fluctuating current value received by the reception unit.

8. Calculate the heater's resistance, The minimum and maximum values ​​of the resistance during normal operation, calculated by the calculation, are updated and saved at predetermined intervals. The calculated minimum value and the stored maximum value of the past resistance values ​​are compared with the resistance value calculated this time. An abnormality detection method wherein a computer executes a process to detect an abnormality in the heater if any of the comparison values ​​exceeds a threshold.

9. Calculate the heater's resistance, The minimum and maximum values ​​of the resistance during normal operation, calculated by the calculation, are updated and saved at predetermined intervals. The calculated minimum value and the stored maximum value of the past resistance values ​​are compared with the resistance value calculated this time. An abnormality detection program that causes a computer to execute a process to detect an abnormality in the heater if any of the comparison values ​​exceeds a threshold.

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