JP6882006B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus including a fixing device for detecting a failure of a temperature detecting means.

Conventionally, a toner image formed on a recording material by using an image forming apparatus such as a copier or a printer, that is, a toner containing a heat-softening resin or the like as a component by an image process means such as electrophotographic, is obtained as a fixed image by heat treatment. There is a device to form. In such an image forming apparatus, a fixing apparatus that performs heat treatment is used. The fixing device includes a temperature detecting means, and the electric power supplied to the heating body is controlled by the control means based on the detection result of the temperature detecting means, and the calorific value of the heating body is controlled. Further, it has a configuration in which the power supply to the heating body is mechanically cut off by using the overheating protection element that is sensitive to the temperature of the heating body against the overheating (abnormal heating) in the fixing device (). For example, see Patent Document 1).

As the temperature detecting means, a thermistor having a negative resistance temperature coefficient (referred to as NTC characteristic) is used. FIG. 8A shows an example of a circuit that outputs the information of the temperature T of the thermistor 701 as the thermistor voltage Vth. Further, FIG. 8B is a graph showing the relationship between the thermistor temperature T of the thermistor 701 and the thermistor voltage Vth. The higher the thermistor temperature T, the lower the thermistor voltage Vth.

On the other hand, as a failure of the thermistor that occurs in the state of being incorporated in the fixing device, there is a case where the zero load resistance value R _{0 at the specified temperature T 0} changes n times due to the application of static electricity or the like to the thermistor. .. FIG. 9 shows the relationship between the thermistor voltage Vth and the thermistor temperature T when the zero load resistance value R _{0 is 1.5 times, 2.0 times, or 2.5 times that of a normal product.} For a certain thermistor voltage V _th0, compared with zero load resistance R ₀ of the normal product of the thermistor, the actual thermistor temperature is high degree of zero load resistance R ₀ becomes large, a temperature change in the same temperature range On the other hand, the change in the detection voltage Vth becomes small. When the control unit controls the power supply of the heating element using the thermistor that has an abnormality in the zero load resistance value R ₀ , the control unit detects the temperature as lower than the actual temperature by the thermistor, so the heating element overheats. It may become warm. When the overheated state exceeds a predetermined temperature, the overheated protection element operates to suppress excessive thermal damage to peripheral parts, and the power supply is cut off to ensure a safe state. ..

Further, in recent years, with the increase in speed of the image forming apparatus, it is required that the temperature of the fixing apparatus also rises at a high speed. For example, a thermal fuse is used as the overheating protection element in the overheating state when the heating body cannot be controlled due to the thermistor abnormality. When a thermal fuse is used, it takes time for the pellets to melt before the thermal fuse is activated. As a result, excessive thermal stress is applied to the pressurizing body that sandwiches the recording material together with the heating body, which may lead to damage. As a method of preventing an excessive temperature rise of the heating body due to an abnormality of the thermistor, there is a method of providing a plurality of temperature detecting means for the heating body. By providing a plurality of temperature detecting means, there is a means for determining a failure of the plurality of temperature detecting means and stopping the power supply to the heating body when the mutual detection results do not change by a predetermined value (for example,). See Patent Document 2). In this configuration, since the failure is determined based on the relationship between the detection results of each other, it is possible to detect an abnormality in the temperature detecting means before the heated body is in an overheated state.

Japanese Unexamined Patent Publication No. 06-314040 Japanese Unexamined Patent Publication No. 2001-305904

However, when power is supplied from a commercial AC power supply to the heating body and the control unit is configured at a potential insulated from the AC power supply, the temperature detecting means needs to be electrically insulated from the heating body. is there. Since it is necessary to secure the insulation distance determined by the AC voltage, the miniaturization of the temperature detecting means is limited. For this reason, the method using a plurality of temperature detecting means limits the miniaturization of the fixing device.

The present invention has been made under such a situation, and an object of the present invention is to accurately determine an abnormality of the temperature detecting means without hindering the miniaturization of the fixing device.

In order to solve the above-mentioned problems, the present invention includes the following configurations.

(1) An image forming apparatus for fixing the toner on a recording material by heating, a tubular fixing film, a heating element arranged in the internal space of the fixing film and heating the fixing film, and the above. The pressure roller forming the fixing film and the nip portion, the first detecting means for detecting the temperature of the heating element, the temperature detected by the first detecting means, and the target temperature of the heating element. The control means for controlling the electric power supplied to the heating element, the second detecting means for detecting the environmental temperature, the temperature detected by the first detecting means before starting the supply of electric power to the heating element, and the said. The determination means includes a determination means for determining whether or not the first detection means is the first abnormality based on a difference value from the temperature detected by the second detection means, and the determination means is the difference. When the value is equal to or greater than the first threshold value, the first detecting means determines that the first abnormality is present, prohibits the supply of electric power to the heating element, and the difference value is the first abnormality. If it is smaller than the threshold value, the first detecting means determines that the abnormality is not the first abnormality, and starts supplying electric power to the heating element and rotating the pressurizing roller to the heating element. When the temperature detected by the first detecting means is equal to or less than the second threshold value when the first time elapses from the start of power supply, the first detecting means causes the first abnormality. The first time has elapsed since the supply of electric power to the heating element and the rotation of the pressurizing roller were stopped and the supply of electric power to the heating element was started. When the temperature detected by the detection means 1 is larger than the second threshold value, it is determined that the first detection means is not the first abnormality, and power is supplied to the heating element and the pressurizing roller. The temperature detected by the first detecting means is the third temperature when a second time longer than the first time elapses after the rotation of the heating element is continued and the power supply to the heating element is started. When it is smaller than the threshold value, the first temperature detected by the first detecting means and the fourth time longer than the third time when the third time longer than the second time elapses. The first detection means has a second abnormality based on the difference value from the second temperature detected by the first detection means and the electric power supplied to the heating element when An image forming apparatus, characterized in that it determines whether or not it is.

According to the present invention, it is possible to accurately determine an abnormality in the temperature detecting means without hindering the miniaturization of the fixing device.

Schematic of the image forming apparatus of Examples 1 and 2. Graph showing the relationship between the power supply time of Example 1 and the thermistor-equivalent temperature A flowchart showing a failure determination process of the thermistor of the first embodiment. Graph showing the relationship between the detection temperature of Example 1 and the thermistor voltage Graph showing the relationship between the detection temperature of Example 1 and the thermistor voltage A flowchart showing a failure determination process of the thermistor of the second embodiment. A flowchart showing a failure determination process of the connection state of the thermistor of the second embodiment. A circuit diagram for converting the thermistor temperature T of the conventional example into the thermistor voltage Vth, and a graph showing the relationship between the thermistor temperature T and the thermistor voltage Vth. Graph showing the relationship between the thermistor temperature T and the thermistor voltage Vth of the conventional example

ここで、Ｒ_Ｔは温度Ｔにおけるサーミスタの抵抗値、Ｒ_０は規定温度Ｔ_０におけるサーミスタのゼロ負荷抵抗値、Ｂはサーミスタの温度Ｔに対する感度を表すＢ定数を示す。規定温度Ｔ_０時のゼロ負荷抵抗値Ｒ_０に対し、温度Ｔが規定温度Ｔ_０よりも高い状態になると、サーミスタ抵抗値Ｒ_Ｔは小さくなる。 [Thermistor abnormality]
8 and 9 will be described in detail. The fixing device includes a heating element that generates heat when power is supplied, a temperature detecting means that detects the temperature of the heating element, and a control means that controls the power supply based on the detection result of the temperature of the heating element. There is. As the temperature detecting means, for example, a thermistor whose resistance value changes with respect to temperature may be used. For example, in the case of a thermistor having a negative temperature coefficient of resistance (hereinafter referred to as NTC characteristic), the relationship between the resistance value of the thermistor and the temperature is expressed by the following equation (1).

Here, _RT is the resistance value of the thermistor at the temperature T, R ₀ is the zero load resistance value of the thermistor at the specified temperature T ₀ , and B is the B constant representing the sensitivity of the thermistor to the temperature T. To zero load resistance R ₀ o'clock specified temperature T _0, the temperature T becomes higher than the specified temperature T _0, the thermistor resistance value R _T is small.

ここで、Ｒ_ｒｅｆは、基準抵抗７０２の抵抗値である。規定温度Ｔ_０と、規定温度Ｔ_０におけるサーミスタ７０１のゼロ負荷抵抗値Ｒ_０と、基準抵抗７０２の抵抗値Ｒ_ｒｅｆと、Ｂ定数と、電圧Ｖｃｃは定数であるため、サーミスタ電圧Ｖｔｈは温度Ｔによって一義的に決定できる。 FIG. 8A shows a general circuit diagram for converting information on the temperature T of the thermistor into a voltage signal (hereinafter referred to as the thermistor voltage Vth). One end of the thermistor 701 is connected to the reference resistor 702, and the other end is grounded. One end of the reference resistor 702 is connected to the thermistor 701, and a voltage Vcc is applied to the other end. The thermistor voltage Vth is represented by the following equation (2) because the voltage Vcc is a voltage dividing value between the thermistor 701 and the reference resistor 702.

Here, R _ref is the resistance value of the reference resistance 702. A specified temperature _{T 0,} the zero load resistance _{R 0} of the thermistor 701 in a specified temperature _{T 0,} the resistance value _{R ref} of the reference resistor 702, and the B constant, the voltage Vcc is a constant, the thermistor voltage Vth temperature T Can be uniquely determined by.

For example a specified temperature _{T 0} 25 ℃, _300kΩ zero load resistance _{R 0} of the thermistor 701 in a specified temperature _{T 0, 33kΩ} resistance value _{R ref} of the reference resistor 702, the constant B 4000K, the voltage Vcc to 3.3V .. In this case, the thermistor voltage Vth and the temperature T of the thermistor 701 show the relationship shown in FIG. 8 (b). In FIG. 8B, the horizontal axis represents the temperature of the thermistor 701 (thermistor temperature) T [° C.], and the vertical axis represents the thermistor voltage Vth [V]. It can be seen that the thermistor temperature T and the thermistor voltage Vth have a monotonically decreasing relationship, and that the change in the thermistor voltage Vth with respect to the temperature change differs depending on the temperature region.

On the other hand, as a failure of the thermistor that occurs in the state of being incorporated in the fixing device, the zero load resistance value R _{0 at the specified temperature T 0} of the thermistor changes n times due to the application of static electricity or the like to the thermistor. There is. Here, n is not limited to an integer. When static electricity or the like is applied to a normal thermistor, the zero load resistance value R ₀ is 1.5 times (n = 1.5), 2.0 times (n = 2), 2.5 times (n =). FIG. 9 shows the relationship between the thermistor temperature T and the thermistor voltage Vth when the value is 2.5). The horizontal axis and vertical axis of FIG. 9 are the same as those of FIG. 8 (b). In FIG. 9, the solid line indicates a normal product thermista, the dotted line indicates that the zero load resistance value R ₀ is 1.5 times, and the broken line indicates that the zero load resistance value R ₀ is 2.0 times. The one-point chain line shows that the zero load resistance value R ₀ is 2.5 times.

The thermistor temperature T with respect to a certain thermistor voltage V _{th0 is set as follows.} The thermistor temperature normal product of the thermistor _{T n0,} the thermistor temperature thermistor zero load resistance _{R 0} becomes 1.5 times the _{T e1.} Further, the thermistor temperature of the thermistor in which the zero load resistance value R _{0 is 2.0 times is Te 2} , and the thermistor temperature of the thermistor in which the zero load resistance value R ₀ is 2.5 times is _{Te 3} . For a certain thermistor voltage V _th0, as compared with a normal product of the thermistor, the larger the abnormal value of zero load resistance R _0, the actual thermistor temperature T increases. Further, the larger the abnormal value of the zero load resistance value R ₀ , the smaller the change in the thermistor voltage Vth with respect to the temperature change in the same temperature region. When the control unit controls the power supply to the heating element using a thermistor with an abnormality in the zero load resistance value R _{0, the control unit detects it at a temperature lower than the actual temperature.} Therefore, the heated body is in an overheated state. When the overheated state of the heating body exceeds a predetermined temperature, the overheated protection element operates to suppress excessive thermal damage to peripheral parts, and the power supply is cut off to ensure a safe state. doing.

Further, in recent years, as the speed of the image forming apparatus has increased, the temperature of the fixing apparatus is also required to rise at a high speed. For example, a thermal fuse may be used as the overheating protection element in order to cope with the overheating state when the uncontrollable state of the heating body occurs due to the thermistor abnormality as described above. It takes time for the pellets to melt before the thermal fuse is activated. As a result, excessive thermal stress is applied to the pressurizing body that sandwiches and conveys the recording material together with the heating body, which may lead to damage. As a method of preventing excessive temperature rise of the heating body due to an abnormality of the thermistor, a plurality of temperature detecting means are provided for the heating body, and when the detection results of each other do not change by a predetermined value, the plurality of temperature detecting means There is a means to stop the power supply to the heating body by judging that it is a failure. In this configuration, since the failure is determined based on the relationship between the detection results of each other, it is possible to detect an abnormality in the temperature detecting means before the heated body is in an overheated state. However, when power is supplied from a commercial AC power supply to the heating body and the control unit is configured at a potential insulated from the AC power supply, the temperature detecting means needs to be electrically insulated from the heating body. is there. Since it is necessary to secure the insulation distance determined by the AC voltage, the miniaturization of the temperature detecting means is limited. For this reason, the method using a plurality of temperature detecting means limits the miniaturization of the fixing device.

[Image forming device]
FIG. 1 is a cross-sectional view illustrating the image forming apparatus of the first embodiment. The photosensitive drum 201, which is an image carrier having a photosensitive layer formed on its surface, is charged with a surface layer by a charging roller 202, and then a latent image is formed by a laser beam emitted from a laser scanner 203. A toner 205 is applied to the latent image formed on the photosensitive drum 201 by the developing roller 204, and the latent image is formed on the photosensitive drum 201 as a toner image. The transfer roller 206 supplies an electric charge to the recording material 207. In the transfer nip portion formed by the photosensitive drum 201 and the transfer roller 206, the recording material 207 is transferred to the fixing device 300 while transferring the unfixed toner image onto the recording material 207 (on the recording material).

The fixing film 209 is, for example, a film having a diameter of about 18 mm when the depth direction of FIG. 1 is the longitudinal direction. The pressure roller 210 forms a fixing nip portion by applying pressure to the fixing film 209. The heating member 211, which is a heating element, is arranged on the inner surface of the fixing film, and is composed of, for example, a ceramic base material, a heat generating layer, and a protective layer. The stay 212 holds the heat generating member 211. The member 213 is a reinforcing member. The thermistor 214, which is the first detection means, is a temperature detection element that detects the temperature of the heat generating member 211, and is, for example, an NTC characteristic thermistor. In the thermistor 214, for example, when the specified temperature T ₀ is 25 ° C., the zero load resistance value R ₀ is 300 kΩ, the reference resistance is 33 kΩ, the B constant is 4000 K, and the voltage Vcc is 3.3 V. The CPU 215, which is a control means, controls the amount of power supplied to the heat generating member 211 based on the detection result of the thermistor 214, for example, by a duty (hereinafter, power input duty). The heat generating member 211 is connected in series with a thermal fuse (not shown), which is an overheating protection element, and a power supply drive unit. By heating the heat generating member 211, the unfixed toner image is fixed to the recording material 207.

The recording material 207 that has been subjected to the fixing treatment is discharged from the fixing nip portion to the discharge portion 216 of the image forming apparatus via the discharge port. The environmental temperature sensor 217, which is the second detection means, is an environmental temperature detecting element that detects the environmental temperature of the place where the image forming apparatus is installed. For example, in the environmental temperature sensor 217, the zero load resistance value R _{0 at a specified temperature T 0} of 25 ° C. is 100 kΩ, the reference resistance R _ref (not shown) is 10 kΩ, the B constant is 10000 K, and the voltage Vcc is 3.3 V. It is a thermistor element. The CPU 215 adjusts image formation conditions such as the amount of charge of the charging roller 202 and the amount of transfer charge supplied to the transfer roller 206 based on the detection result of the environmental temperature sensor 217. The paper feed roller 218 is a roller for feeding the recording material 207, and the transport rollers 219 and 220 are rollers for transporting the recording material 207. The storage unit 230 is composed of a ROM, a RAM, or the like, and stores a program executed by the CPU 215 and various constants. The storage unit 230 is also used as a workspace when the CPU 215 executes processing. The configuration of the image forming apparatus is not limited to the configuration described with reference to FIG.

[Detected temperature and power supply time of thermistor 214]
FIG. 2 shows the detection temperature of the thermistor 214 when the CPU 215 controls the power supply to the heat generating member 211 in the normal product of the thermistor 214. In FIG. 2, the horizontal axis shows the power supply time [seconds], and the vertical axis shows the detection temperature (thermistor-equivalent temperature) [° C.] of the thermistor 214 converted from the thermistor voltage Vth. From the graph of FIG. 2, it can be seen that the detection temperature of the thermistor 214 rises as the power is supplied to the heat generating member 211.

However, as described with reference to FIG. 9, in _{the case of an abnormal product in which an abnormality has occurred in the zero load resistance value R 0} of the thermistor 214, the CPU 215 actually performs the heat generating member 211 based on the detection result of the thermistor 214 of the abnormal product. Detects a temperature lower than the temperature of. Therefore, the CPU 215 controls to increase the amount of power supplied to the heat generating member 211 per unit time so as to reach the target temperature of the fixing process based on the detected temperature (wrong temperature). In the first embodiment, for example, when the _{temperature control is performed in a state where the zero load resistance value R 0} is about twice or more that of the normal product, the thermal fuse blows.

As a means for detecting an abnormality in the thermistor 214 in which the zero load resistance value R ₀ is n times the resistance value of the normal product, in the first embodiment, the figure before starting the power supply to the heat generating member 211 is shown in FIG. The control shown in 3 is performed. The CPU 215 controls to determine an abnormality (first abnormality) in which the zero load resistance value R _{0 of the thermistor 214 is n times the resistance value of the normal product as shown in FIG.} When the user gives an instruction to print the image forming apparatus via, for example, a personal computer, image forming is started. Along with the image formation, the fixing device 300 is prepared for heating. When the printing instruction is given, the CPU 215, which is a determination means, executes the process of step 101 or less (hereinafter referred to as S). In S101, the CPU 215 receives a heating request to the heat generating member 211.

In S102, the CPU 215 compares the detection result of the thermistor 214 with the detection result of the environmental temperature sensor 217 before starting the heating of the heat generating member 211. The CPU 215 determines whether or not the difference value between the detection result of the environmental temperature sensor 217 and the detection result of the thermistor 214 is lower than the first threshold value Ta. Since it is a stage before starting the power supply to the heat generating member 211, when the thermistor 214 is a normal product, the detection result of the thermistor 214 should be the same as the detection result of the environmental temperature sensor 217.

(Environmental temperature sensor and normal / abnormal thermistor)
_{FIG. 4 shows the thermistor voltage V th true} when the thermistor 214 of the first embodiment is a normal product, and the thermistor voltage V when the zero load resistance value R ₀ is twice the resistance value of the normal product. _{The relationship between the false} and the temperature is shown. Further, FIG. 4 shows the relationship between _{the thermistor voltage V env} of the environmental temperature sensor 217 and the temperature. FIG. 4 shows the temperature [° C.] detected by the thermistor 214 or the temperature [° C.] detected by the environmental temperature sensor 217 on the horizontal axis. The vertical axis shows the thermistor voltage V _th [V] of the thermistor 214 and the thermistor voltage V _{th env} [V] of the environmental temperature sensor 217. The solid line with a black circle shows the graph of the environmental temperature sensor 217, the solid line shows the graph when the thermistor 214 is a normal product, and the broken line shows the resistance value of the thermistor 214 being an abnormal product (zero load resistance value R ₀ is the normal product). The graph in the case of 2 times) is shown.

In a state where the environmental temperature sensor 217 detects, for example, 10 ° C., the normal product of the thermistor 214 _{shows a thermistor voltage V th} truth = 3.1V whose detection result corresponds to about 10 ° C. On the other hand, in the case of the thermistor 214 of the abnormal product whose _{zero load resistance value R 0 is twice that of the normal product, the detection result shows the thermistor voltage V th false} = 3.2V corresponding to about 0 ° C., and the thermistor 214 of the normal product. It shows a value different from the thermistor voltage V _{th true = 3.1V of.} In this way, if _{the thermistor 214 whose zero load resistance value R 0} is twice the resistance value of the normal product is used, the temperature where it should be detected as 10 ° C. is detected as 0 ° C.

Returning to the description of the flowchart of FIG. The first threshold value Ta used for the determination of S102 is a value smaller than the difference value between the detection result by the thermistor 214 of the normal product and the detection result by the abnormal product in _{which the zero load resistance value R 0 is twice that of the normal product.} For example, 5 ° C. When the environmental temperature sensor 217 detects 10 ° C, the detection result of the environmental temperature sensor 217 (10 ° C) and the detection result of the abnormal product thermistor 214 in _{which the zero load resistance value R 0 is twice the resistance value of the normal product.} The difference from (0 ° C.) is 10 ° C. Therefore, an abnormal product in which the zero load resistance value R ₀ is twice the resistance value of the normal product can be distinguished from the normal product thermistor 214 by the first threshold value Ta.

When the CPU 215 determines in S102 that the difference value between the detection result of the environmental temperature sensor 217 and the detection result of the thermistor 214 is equal to or higher than the first threshold value Ta (or higher than the first threshold value), the process proceeds to S104. When the CPU 215 determines in S102 that the difference value between the detection result of the environmental temperature sensor 217 and the detection result of the thermistor 214 is smaller than the first threshold value Ta, the process proceeds to S103.

Detection results and zero load resistance R ₀ difference value is but not large zero load resistance R ₀ to the extent that the first threshold value Ta or higher compared to the normal product of the detection result of the thermistor 214 in ambient temperature sensor 217 Is large, the process of S103 is executed. However, with respect to such a thermistor 214, the process of S103 may be executed because it does not cause excessive thermal stress on the pressurizing roller 210. In S103, the CPU 215 starts supplying electric power to the heat generating member 211 and rotating the pressurizing roller 210, controls the fixing of the fixing device 300, and ends the failure determination process of the thermistor 214.

Further, in S102, an abnormality of the environmental temperature sensor 217 or the thermistor 214 is assumed as a factor that causes the difference value between the detection result of the environmental temperature sensor 217 and the detection result of the thermistor 214 of the CPU 215 to be equal to or higher than the first threshold value Ta. However, when power is supplied to the heat generating member 211 by using the thermistor 214 whose zero load resistance value R ₀ is larger than that of the normal product for temperature detection, excessive thermal stress may be applied to the pressurizing roller 210. There is. Therefore, in S104, the CPU 215 determines that the zero load resistance value R ₀ of the thermistor 214 is large and the thermistor 214 is out of order, prohibits power supply to the heat generating member 211 and rotation of the pressurizing roller 210, and prohibits the thermistor. The failure determination process of 214 is terminated.

With the above control, excessive thermal stress on the pressure roller 210 can be suppressed. Then, when it is determined that the zero load resistance value R _{0 of} the thermistor 214 is larger than that of the normal product, the user is notified to prompt the repair of the image forming apparatus, and the repair can be performed only by replacing the thermistor 214. It will be possible.

Although the first threshold value Ta is set as a fixed value in the first embodiment, the first threshold value Ta may be changed according to the detection result of the environmental temperature sensor 217. Further, in the first embodiment, the pressure roller 210 is mentioned as a target to be subjected to the excess thermal damage, but the excessive thermal damage to the fixing film 209, the stay 212 and the like is also suppressed. As described above, according to the first embodiment, it is possible to accurately determine the abnormality of the temperature detecting means without hindering the miniaturization of the fixing device.

In the first embodiment, when the difference value between the detection result of the environmental temperature sensor 217 and the detection result of the thermistor 214 is equal to or greater than the first threshold value Ta, it is determined that _{the zero load resistance value R 0 of the thermistor 214 is abnormal.} In the second embodiment, a configuration will be described in which an abnormality of the thermistor 214 is detected more accurately and thermal damage to parts other than the thermistor 214 is suppressed in consideration of the usage environment and the performance of the parts to be used.

[Voltage values of environmental temperature sensor 217 and thermistor 214]
FIG. 5 shows a graph of the voltage values with respect to the temperature of the normal product of the environmental temperature sensor 217 and the thermistor 214 and the abnormal product in which the zero load resistance value R _{0 is twice the resistance value of the normal product.} FIG. 5 is an enlarged view of a region where the temperature region is low in the graph of FIG. 4, and the description of the horizontal axis and the vertical axis is omitted. The lower the temperature of the thermistor 214, the closer the thermistor voltage Vth approaches to the voltage Vcc, and the change in the thermistor voltage Vth with respect to temperature changes becomes smaller for both _{normal products and abnormal products in which the zero load resistance value R 0 is larger than the resistance value of the normal product.} .. For example, the thermistor voltages Vth corresponding to 0 ° C. and 5 ° C. in a normal thermistor show about 3.2 V and 3.17 V, respectively, which is a small potential difference of about 30 mV with respect to a temperature difference of 5 ° C. As a result, accurate temperature detection by the thermistor in a low temperature region becomes difficult.

[Thermistor failure judgment processing]
FIG. 6 is a flowchart showing a failure determination process of the thermistor 214 of the second embodiment. The CPU 215 controls to determine an abnormality (first abnormality) in which the zero load resistance value R _{0 of the thermistor 214 is n times the resistance value of the normal product as shown in FIG.} In the second embodiment, the failure of the thermistor 214 is accurately determined even in a temperature region where the difference in the thermistor voltage Vth between the normal product and the abnormal product of the thermistor 214 as described with reference to FIG. 5 becomes small.

The processing of S601 to S604 of FIG. 6 is the same as the processing of S101 to S104 of FIG. 3 described in the first embodiment, and the description thereof will be omitted. The CPU 215 has a timer (not shown), and at the timing when the power supply to the heat generating member 211 and the rotation of the pressurizing roller 210 are started in S603, the timer is reset and started to measure the time. Keep it. The time measured by the timer, that the power supply time _{t heat. In S605, the CPU 215 refers to the timer, and the first time t 0} has elapsed from the start of power supply to the heat generating member 211 and the start of rotation of the pressurizing roller 210, that is, the power supply time t _heat is the first. It is determined whether or not the time t _{0 or more of 1 is satisfied.}

If the CPU 215 determines in S605 that the first time t ₀ has not elapsed, the process returns to S605, and if it determines that the first time t ₀ has elapsed, the process proceeds to S606. In S606, the CPU 215 compares the detection result of the thermistor 214 with the second threshold Tb. The CPU 215 determines whether or not the temperature detected by the thermistor 214 is larger than the second threshold value Tb. As the first time t ₀ , a time during which the range heated by the heat generating member 211 reaches the fixing nip portion in the circumferential direction of the pressurizing roller 210, for example, 400 milliseconds is set. As a result, the temperature of the thermistor 214 rises at the timing _{when the first time t 0 elapses.} Referring to FIG. 2, at a timing when the power supply time has elapsed 400 milliseconds, the temperature detected by the thermistor 214 (the case of the normal product) (thermistor reduced temperature in FIG. 2) is about 10 ° C. at a first time t ₀ point It can be confirmed that the degree has risen.

(Voltage change with temperature change)
For example, as shown in FIG. 5, when the environmental temperature in the installation environment of the image forming apparatus is 0 ° C., the temperature change of the thermistor 214 and the change of the thermistor voltage Vth are as follows. That is, when the temperature of the thermistor 214 rises by 10 ° C., the thermistor voltage Vth of the normal product changes by about 80 mV from about 3.2 V corresponding to 0 ° C. to about 3.12 V corresponding to 10 ° C. On the other hand, in _{the case of an abnormal thermistor 214 in which the zero load resistance value R 0} is twice the resistance value of the normal product, about 30 mV from about 3.24 V corresponding to 0 ° C to about 3.21 V corresponding to 10 ° C. It becomes a change of. The thermistor voltage of 3.21 V indicates about 1.5 ° C. in the normal product thermistor 214. Second threshold Tb used in S606 is a thermistor 214 for normal products, the temperature at which the first time t ₀ is detected when the elapsed from start of rotation of the power supply start and a pressure roller 210 of the heat generating member 211 A low value is defined as the threshold value Tb.

When the CPU 215 determines in S606 that the detection temperature of the thermistor 214 is larger than the second threshold value Tb, the process proceeds to S607. _{The thermistor 214 has a zero load resistance value R 0} that is larger than the resistance value of a normal product, although the detection temperature of the thermistor 214 is not so high as to be equal to or less than the second threshold value Tb (below the second threshold value). However, it does not cause excessive thermal stress on the pressurizing roller 210. Therefore, the process proceeds to S607 including such a thermistor 214. In S607, the CPU 215 continues the power supply control to the heat generating member 211 and performs the fixing control. Further, in S606, when the CPU 215 determines that the detection temperature of the thermistor 214 is equal to or less than the second threshold value Tb, the CPU 215 determines that the zero load resistance value R ₀ is twice or more the resistance value of the normal product. it can. The CPU 215 advances the process to S608, stops the power supply to the heat generating member 211 in S608, stops the rotation of the pressurizing roller 210, and ends the failure determination process of the thermistor 214.

With the above control, excessive thermal stress on the pressure roller 210 can be suppressed. As a result, when it is determined that the thermistor 214 has a zero load resistance value R ₀ , the user is notified to prompt the repair of the image forming apparatus, and the repair can be performed only by replacing the thermistor 214.

[Foreign matter detection processing at the thermistor connection]
Further, after continuing the power supply control to the heat generating member 211 in S607 of FIG. 6, the control may be performed based on the change in the detection result of the thermistor 214 with respect _{to the power supply time teat as shown in FIG.} The process of FIG. 7 may be executed after the power supply control is started in S103 of FIG. The process of FIG. 7 is a process of detecting that there is no foreign matter or the like that causes the fixed resistance value to be added in the connection portion of the thermistor 214. The CPU 215 controls to determine an abnormality (second abnormality) in which a _{fixed resistance value R s} is added in series with the thermistor 214, as shown in FIG. In S701, the CPU 215 continues to supply electric power to the heat generating member 211 following the processing of S607.

In S702, the CPU 215 determines whether or not _{the power supply time t heat} is equal to or greater than the second time t _{1 by referring to the timer.} Here, the second time t ₁ is set to a time longer than the first time t _{0 (t 1} > t ₀ ). When the CPU 215 determines in S702 that the second time t ₁ has not elapsed, the CPU 215 returns the process to S702. When the CPU 215 determines in S702 that the second time t ₁ has elapsed, the CPU 215 proceeds to the process in S703.

In S703, the CPU 215 compares the detection result of the thermistor 214 with the third threshold value Tc. When the CPU 215 determines in S703 that the detection result of the thermistor 214 is equal to or higher than the third threshold value Tc, the process proceeds to S704. In S704, the CPU 215 continues to supply electric power to the heat generating member 211 and ends the process. When the CPU 215 determines in S703 that the detection result of the thermistor 214 is lower than the third threshold value Tc, the processing proceeds to S705.

In S705, the CPU 215 refers to the timer to determine whether or not a third time t ₂ (t ₂ > t _{1 ) longer than the second time t 1} has elapsed, that is, the power supply time t _heat is the second. It is determined whether or not the time t _{2 or more of 3 is satisfied.} When the CPU 215 determines in S705 that the third time t ₂ has not elapsed, the process returns to S705 and the power supply to the heat generating member 211 is continued. When the CPU 215 determines in S705 that the third time t ₂ has elapsed, the CPU 215 proceeds to the process in S706. In S706, the CPU 215 _{stores the detection result T t2} , which is the first temperature of the thermistor 214 at the timing _{when the third time t 2} has elapsed, in the storage unit 230.

In S707, the CPU 215 refers to the timer to determine whether or not a fourth time t ₃ (t ₃ > t _{2 ) longer than the third time t 2} has elapsed, that is, the power supply time t _heat is the second. It is determined whether or not the time t _{3 or more of 4 is satisfied.} In S707 CPU 215, when it is determined that the fourth time _{t 3} has not elapsed, the process proceeds to S 708. In S708, the CPU 215 determines whether or not the power input duty to the heat generating member 211 is less than the maximum value. When the CPU 215 determines in S708 that the power input duty is less than the maximum value, the process proceeds to S709. In S709, the CPU 215 continues to supply power to the heat generating member 211, and ends the thermistor failure determination process. When the CPU 215 determines in S708 that the power input duty is equal to or greater than the maximum value, the process returns to S707.

S707 in CPU215, when it is determined that the fourth time _{t 3} has elapsed, the process proceeds to S710. In S710, the CPU 215 _{stores the detection result T t3} , which is the second temperature of the thermistor 214 at the timing _{when the fourth time t 3} has elapsed, in the storage unit 230. In S711 CPU 215 reads out a detection result _{T t3} from the storage unit 230 thermistor 214 and the detection result _{T t2,} the detection result difference value between _{T t3} and the detection result _{T t2} and _(T t3 _{-T t2)} fourth threshold Compare with Td. The CPU 215 determines whether or not the difference value is equal to or greater than the fourth threshold value Td.

When the CPU 215 determines in S711 that the difference value (T _t3- T _t2 ) is equal to or greater than the fourth threshold value Td, the process proceeds to S712. In S712, the CPU 215 continues to supply electric power to the heat generating member 211, and ends the thermistor failure determination process. When the CPU 215 determines in S711 that the difference value (T _t3- T _t2 ) is less than the fourth threshold value Td, the process proceeds to S713. In S713, the CPU 215 determines that there is a foreign substance or the like that causes a fixed resistance value in the connection portion of the thermistor 214, stops the power supply to the heat generating member 211 and the rotation of the pressurizing roller 210, and the thermistor 214 fails. The judgment process ends.

By executing the above control following the process of FIG. 6, it is possible to _{determine the failure of the thermistor 214 based on the zero load resistance value R 0} and also to determine the failure of the connection state that causes the fixed resistance value. .. As a result, the factor of applying excessive thermal stress to the pressure roller 210 can be suppressed. In Example 2, the first threshold value Ta or the second threshold value Tb is set as a fixed value, but it may be changed according to the detection result of the environmental temperature sensor 217. Further, in the second embodiment, the pressure roller 210 is mentioned as a target to be subjected to the excess thermal damage, but the excessive thermal damage to the fixing film 209, the stay 212 and the like is also suppressed. As described above, according to the second embodiment, it is possible to accurately determine the abnormality of the temperature detecting means without hindering the miniaturization of the fixing device.

209 Fixing film 210 Pressurizing roller 211 Heat generating member 214 Thermistor 215 CPU
217 Environmental temperature sensor

Claims (6)

An image forming apparatus that fixes toner on a recording material by heating it.
Cylindrical fixing film and
A heating element that is placed in the internal space of the fixing film and heats the fixing film,
The pressure roller forming the fixing film and the nip portion,
A first detecting means for detecting the temperature of the heating element and
A control means for controlling the electric power supplied to the heating element based on the temperature detected by the first detecting means and the target temperature of the heating element.
A second detection means for detecting the environmental temperature,
Before starting the supply of electric power to the heating element, the first detecting means is the first based on the difference value between the temperature detected by the first detecting means and the temperature detected by the second detecting means. Judgment means for determining whether or not the abnormality is 1 and
With
The judgment means is
When the difference value is equal to or greater than the first threshold value, the first detecting means determines that the first abnormality is present, and the supply of electric power to the heating element is prohibited .
When the difference value is smaller than the first threshold value, the first detecting means determines that the abnormality is not the first abnormality, and supplies electric power to the heating element and rotates the pressurizing roller. Start and
When the temperature detected by the first detecting means is equal to or less than the second threshold value when the first time elapses after starting the supply of electric power to the heating element, the first detecting means Determining that this is the first abnormality, the supply of electric power to the heating element and the rotation of the pressurizing roller are stopped.
When the temperature detected by the first detection means is larger than the second threshold value when the first time elapses after starting the supply of electric power to the heating element, the first detection means. Determined that it was not the first abnormality, and continued the supply of electric power to the heating element and the rotation of the pressurizing roller.
When the temperature detected by the first detection means is smaller than the third threshold value when a second time longer than the first time elapses after starting the supply of electric power to the heating element. The first temperature detected by the first detection means when a third time longer than the second time elapses and the fourth time longer than the third time elapses. Whether or not the first detecting means is the second abnormality is determined based on the difference value from the second temperature detected by the first detecting means and the electric power supplied to the heating element. An image forming apparatus characterized by making a judgment. When the difference value is smaller than the fourth threshold value in a state where the maximum electric power is supplied among the electric power supplied to the heating element, the first detecting means is the first detecting means. The image forming apparatus according to claim 1 , wherein the image forming apparatus is determined to be abnormal in 2, and the supply of electric power to the heating element is stopped. When the electric power supplied to the heating element is lower than the maximum electric power after the lapse of the third time and before the lapse of the fourth time, the determination means is said to be the first. The image forming apparatus according to claim 2 , wherein the detection means 1 is determined not to be the second abnormality and continues to supply electric power to the heating element. The first detection means is a thermistor.
The image forming apparatus according to any one of claims 1 to 3 , wherein the first abnormality is an abnormality of the zero load resistance value of the thermistor at a specified temperature. The first detection means is a thermistor.
The second abnormality, the image forming apparatus according to any one of claims 1 to 3, characterized in that the thermistor is abnormal fixed resistance in series is added. The image forming apparatus according to any one of claims 1 to 5, wherein the nip portion is formed by sandwiching the fixing film between the heating element and the pressurizing roller.

Images