JP6882005B2 - 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 copying machine 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 fixed by heat treatment. There is a device to form as. 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. 7A 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. 7B 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. In the thermistor 701, for example, when assembling a unit in a factory, foreign matter may be caught in an electrical connection path, causing poor contact or the like, and a fixed resistance value R _s may be added in series. As shown in FIG. 8, the _{larger the added fixed resistance value R s} , the lower the thermistor temperature T converges. Therefore, the temperature of the heating body detected by the thermistor 701 may be detected to be lower than the actual temperature, resulting in an overheated state. An overheat protection element is provided in order to prevent thermal damage to peripheral parts when the heating body is in an overheated 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 for preventing excessive temperature rise of the heating body, for example, there are the following configurations. Comparing the detection result of the temperature detecting means when the first period elapses from the power-on and the detection result of the temperature detecting means when the second period elapses, the temperature gradient is smaller than the predetermined value. In some cases, it is determined that the fixing device is abnormal (see, for example, Patent Document 2). Since the presence or absence of an abnormality is determined based on the temperature gradient of the temperature detection result when the fixed time has elapsed from the power-on, the abnormality of the temperature detecting means can be detected before the heated body becomes overheated.

Japanese Unexamined Patent Publication No. 06-314040 Japanese Unexamined Patent Publication No. 02-309382

_{However, if the temperature converged by the combined resistance value RT + S} of the thermistor when the fixed resistance value R _s is added is close to the target temperature at the time of control, the control is normal assuming that the thermistor is normal. It will be done under the resistance value _RT. Therefore, it cannot be determined that an abnormality has occurred in the thermistor. As described above, in the method of detecting the thermistor abnormality by the temperature gradient based on the temperature detection result at the time when a predetermined time elapses, it may not be possible to determine the excessive temperature rise of the heated body.

The present invention has been made under such circumstances, and an object of the present invention is to accurately determine an abnormality in a thermistor.

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. A control means for controlling the electric power supplied to the heating element, a first temperature detected by the first detecting means when the first time elapses after the start of supplying electric power to the heating element, and a first temperature. Based on the difference value from the second temperature detected by the first detection means when the second time longer than the first time elapses and the electric power controlled by the control means. The first detecting means includes a determining means for determining whether or not the first abnormality is present, and the determining means is said to be in a state where the maximum electric power that can be supplied to the heating element is supplied. difference value is smaller than the first threshold value, said first detecting means determines that said first abnormality, the control means, based on a detection result by the first detecting means, Using P control, PI control, or PID control, the amount of power supplied to the heating element per unit time is calculated so that the heating element reaches the target temperature, and the calculated amount of supplying the electric power is plural. The image forming apparatus is characterized in that the determination means determines the first abnormality based on the difference value and the output level.
(2) 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. A control means for controlling the electric power supplied to the heating element, a first temperature detected by the first detecting means when the first time elapses after the start of supplying electric power to the heating element, and a first temperature. A difference value from the second temperature detected by the first detection means when a second time longer than the first time elapses, a cumulative value of electric power controlled by the control means, and the like. The first detecting means is provided with a determining means for determining whether or not the first abnormality is based on the above, and the determining means has a difference value smaller than the first threshold value and said. An image forming apparatus, characterized in that, when the cumulative value is equal to or greater than a second threshold value, the first detecting means determines that the first abnormality is present.

According to the present invention, it is possible to accurately determine an abnormality in the thermistor.

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 Graph showing the relationship between the power supply time of Example 1 and the thermistor-equivalent temperature Flow chart showing the thermistor failure determination process of the first embodiment Flow chart showing the thermistor failure determination process of the second embodiment Flow chart showing the thermistor failure determination process 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]
7 and 8 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. 7A 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. 7 (b). In FIG. 7B, 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, in a factory or the like, when the thermistor 701 is incorporated into a fixing device or the like, foreign matter is caught in an electrical connection path, resulting in poor contact, etc. The resistance value _RS may be added. _{Let RT + S} be the resistance value of the thermistor 701 to which the fixed resistance value Rs is added. _{The relationship between the combined resistance value RT + S} of the thermistor 701 and the thermistor temperature T at the time of this failure can be expressed by the following equation (3).

As described in the equation (1), as the thermistor temperature T increases, the resistance value _RT of the thermistor 701 decreases, so that the influence of the fixed resistance value _{RS added to the combined resistance RT + S increases.} .. That is, as the thermistor temperature T becomes higher, the temperature detection result converges to the temperature determined by the _{fixed resistance value RS.} FIG. 8 shows the relationship between the thermistor voltage Vth and the thermistor temperature T when several fixed resistance values _{RS having different values are added.} The horizontal axis and the vertical axis of FIG. 8 are the same as those of FIG. 7 (b). In FIG. 8, the solid line shows the relationship between the thermistor temperature T and the thermistor voltage Vth in the normal case where no contact failure or the like occurs (normal product), and is the same graph as in FIG. 7 (b). Dotted line, when the added fixed resistance R _S is 2.5 k, dashed line, when the added fixed resistance R _S is 5.0Keiomega, dashed line, the added fixed resistance R _S is 7 In the case of .5 kΩ, the long dashed line indicates the case where the added fixed resistance value _RS is 10 kΩ.

The temperature detected by the thermistor 701 includes a convergence value (hereinafter referred to as a temperature convergence value) (saturation value of the detection temperature) determined by the value _{of the added fixed resistance value RS. T 1 to} T ₄ shown on the horizontal axis of FIG. 8 indicate temperature convergence values corresponding to the above-mentioned four different fixed resistance values _RS of 2.5 kΩ to 10 kΩ, respectively. For example, when the fixed resistance value _RS is 10 kΩ, the thermistor voltage Vth output from the thermistor 701 converges to 0.8 V when the thermistor temperature T becomes high. In the thermistor characteristics of a normal product, the thermistor voltage Vth of 0.8 V _{corresponds to the temperature T 4} (about 124 ° C.), and the higher the actual temperature, the more the thermistor temperature T converges to the _{temperature T 4.} For the other fixed resistance value _{R S,} the temperatures _{T 1} when fixed resistance value _{R S} is 2.5 k, fixed resistance value _{R S} is the temperature _{T 2} in the case of 5.0Keiomega, the fixed resistance value _{R S} for 7.5kΩ temperature _{T 3,} converge respectively.

As described above, _{it can be seen that the larger the fixed resistance value RS} , the lower the thermistor temperature T converges. As a result, the _{larger the fixed resistance value RS} , the lower the temperature of the heating element is detected by the temperature detection by the thermistor 701, and the heating element of the fixing device may be in an overheated state. Therefore, in the overheated state of the heating body, when the temperature of the heating body exceeds a predetermined temperature, the overheating protection element operates to suppress excessive thermal damage to peripheral parts and supply power. A safe state is guaranteed by blocking it.

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. There are the following methods as a method for preventing an excessive temperature rise of the heating body. Compared after the power supply is turned with the detection result T _a0 of the temperature detection means at the time when the first time period t _a0 has elapsed, and a detection result T _b0 of the temperature detection means at the time when the second period t _b0 has elapsed This is a method of determining that the fixing device is abnormal when the temperature gradient is smaller than a predetermined value. Since the presence or absence of thermistor abnormality is determined based on the gradient of the temperature detection result when the fixed time elapses from the power-on, the abnormality of the temperature detecting means can be detected even if the heating body is not in an excessively heated state. _{However, if the detection temperature converged by the combined resistance value RT + S of} the thermistor to which the fixed resistance value _RS is added is close to the target temperature at the time of control, the control is normal assuming that the thermistor is normal. It is done under the resistance value _RT. In such a situation, it cannot be determined that the thermistor has an abnormality. Therefore, the method of detecting the thermistor abnormality based on the temperature gradient based on the temperature detection result when a predetermined time has elapsed may not be able to determine the excessive temperature rise of the heated body.

[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 to the recording material 207.

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. The fixing device 300 fixes the unfixed toner image on the recording material 207 (on the recording material) by heating the heat generating member 211.

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. The CPU 215 adjusts the image formation control 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 configuration of the image forming apparatus is not limited to the configuration described with reference to FIG.

The CPU 215, which is a control means, calculates the amount of power supplied to the heat generating member 211 based on the detection result of the thermistor 214, and supplies power from a commercial AC power source via a triac (not shown) according to the calculated power supply amount. To control. Power supply amount, and the target temperature T _obj is the target of the heat generating member 211, a P term proportional to the difference value between the detected temperature T _actual with the current thermistor 214, the integral with respect to the elapsed time from control start difference value It is calculated by PI control consisting of the I term proportional to the value and the PI control. According to the calculated amount of power supplied per unit time to the heat generating member 211, the output level is divided into a plurality of stages, for example, 10 stages of 0 to 9, and half of the commercial AC power supply according to the output level. Perform wave control. As a result, for example, power supply control with four sine waves as one control cycle is performed. 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.

[When using a normal thermistor]
FIG. 2 shows the transition between the output level and the detection result of the thermistor 214 with respect to the power supply time when the normal thermistor 214 is used _{when the target temperature Tobj} when controlling the fixing device 300 is 180 ° C. .. In FIG. 2, the horizontal axis shows the power supply time [seconds], and the right vertical axis shows the output levels in 10 steps according to the calculated power supply amount. Further, the left vertical axis of FIG. 2 shows the temperature converted based on the detection result of the thermistor 214 (thermistor equivalent temperature) [° C.]. In the graph of FIG. 2, the solid line shows the converted temperature when the normal thermistor 214 is used, the broken line shows the output level when the normal thermistor 214 is used, and the alternate long and short dash line shows the target temperature _Tobj (180 ° C.). Shown.

From the start of power supply to about 2 seconds, the output level is the maximum value (for example, 9) of the output level corresponding to the maximum power that can be supplied to the heat generating member 211. Here, to reduce the output level at reaching the target temperature _{T obj} versus about 15 ℃ low temperature (target temperature _T obj -15 ℃). The output level is controlled to be the maximum value in the area on the left side of the figure, and the output level is between 0 and 6 in the area on the right side of the figure, with the time point of about 2 seconds after the start of power supply as the boundary. Is controlled. Approximately 2.3 seconds after the start of power supply, the detection result of the _{thermistor 214 reaches the target temperature obj} , and thereafter the detection result of the thermistor 214 is controlled to move around the _{target temperature obj.} The output level has been increased or decreased.

On the other hand, FIG. 3 shows how the detection temperature of the normal thermistor 214 rises in a state where the temperature rise is slower than in the case of FIG. 2 due to the low AC voltage of the AC power supply, although the output level is the maximum value. Shown. The horizontal axis and the vertical axis of FIG. 3 are the same as those of FIG. The condition of FIG. 3 is the state in which the temperature rises at the slowest in Example 1, and the target temperature _Tobj is reached at about 2.3 seconds in FIG. 2, whereas 8 seconds have elapsed in FIG. Even at that time, the target temperature _Tobj has not been reached. As shown in FIG. 3, the temperature rises by about 17 ° C. (Δ17 ° C.) in about 2 seconds from 8 seconds to 10 seconds, and this graph shows about 8.5 ° C./sec (= Δ17 ° C./Δ2 seconds). Has a temperature gradient of.

At maximum power level, the temperature gradient of 8.5 ° C./sec is the smallest gradient in the configuration of Example 1. Therefore, for example, when the temperature gradient based on the temperature detected by the thermistor 214 at the time points of 8 seconds and 10 seconds from the start of power supply is less than 8.5 ° C./sec, it is estimated as follows. That is, it is presumed that the fixed resistance value Rs is associated with the thermistor 214, and that the thermistor 214 has an abnormality. However, for example, when the timing at which the recording material 207 reaches the fixing nip portion is delayed, the output level is lowered to reduce the temperature gradient in order to suppress excessive overshoot of the temperature of the heat generating member 211. Control may take place. The control for lowering the output level to reduce the temperature gradient is not limited to the purpose of suppressing the temperature overshoot of the heat generating member 211. Therefore, if a control that reduces the temperature gradient occurs between the time point of 8 seconds and the time point of 10 seconds after the power supply is started under the temperature transition condition of FIG. 3, other than the fixed resistance value Rs. In some cases, the temperature gradient may be lower than 8.5 ° C./sec due to the above factors. Therefore, it may not be possible to determine the anomaly of the thermistor 214 based solely on the temperature gradient.

[Thermistor failure judgment processing]
To distinguish whether the temperature gradient below 8.5 ° C./sec is due to the control of the CPU 215 (control to suppress overshoot of the temperature of the heat generating member 211, etc.) or the addition of the fixed resistance value Rs. , The control shown in FIG. 4 is performed. The CPU 215 controls to determine an abnormality (first abnormality) in which a _{fixed resistance value R s} is added in series with the thermistor 214, as shown in FIG. FIG. 4 is a flowchart showing a failure determination process of the thermistor 214, more specifically, a failure determination process of the thermistor 214 due to the fixed resistance value Rs, and the CPU 215 functions as a determination means. When the CPU 215 receives an instruction to start heating the heat generating member 211, the CPU 215 starts the processes after step 101 (hereinafter referred to as S). In S101, the CPU 215 starts heating the heat generating member 211 by supplying electric power to the heat generating member 211 as the image is formed, and starts the rotation of the pressurizing roller 210. Further, the CPU 215 resets and starts a timer (not shown), and starts measuring the time t after starting the power supply.

In S102, the CPU 215 determines whether or not _{the first time t 1} , for example, 8 seconds has elapsed (t ≧ t ₁ ) since the start of power supply by referring to the timer. In S102, when the CPU 215 _{determines that the first time t 1} has not elapsed since the start of power supply, the process is returned to S102, and when it is determined that the first time t ₁ has elapsed, the process is performed. Proceed to S103. In S103, the CPU 215 stores in the storage unit 230 the _{detection temperature T t1} , which is the first temperature of the thermistor 214 at the time when the first time t _{1 has elapsed.}

In S104, the CPU 215 determines whether or not a _{second time t 2} , for example, 10 seconds has elapsed (t ≧ t 2) since the start of power supply by referring to the timer. In S104, when the CPU 215 _{determines that the second time t 2} has not elapsed since the start of power supply, the process proceeds to S105, and when it is determined that the second time t ₂ has elapsed, the process is performed. Proceed to S107. In S105, the CPU 215 determines whether or not the output level is smaller than the maximum value in the period until the _{second time t 2 elapses.} When the CPU 215 determines in S105 that the output level is the maximum value, the process returns to S104, and when it is determined that the output level is smaller than the maximum value, the process proceeds to S106. When a decrease in the output level is confirmed in S105, the CPU 215 determines that the temperature increase is suppressed by the control for decreasing the output level. In S106, the CPU 215 determines that the thermistor 214 is operating normally, controls the power supply of the heat generating member 211, continues the rotation of the pressurizing roller 210, and ends the failure determination process of the thermistor 214.

On the other hand, when the elapsed time from the start of power supply with the output level at the maximum value _{reaches the second} time t 2, the CPU 215 in S107 causes the thermistor 214 at _{the time when the second time t 2 elapses. The detection temperature T t2} , which is the second temperature, is stored in the storage unit 230. S108 in CPU215, the difference value of the read and detected temperature _{T t1} from the storage unit 230 and the detected temperature _{T t2} at the second time _{t 2} in the first time _{t 1,} the detected temperature _{T t2} and the detected temperature _{T t1} ( T _t2- T _t1 ) is compared with the first threshold value T _{temperature. Here, the threshold value Tthres is set} to, for example, 13 ° C. based on the smallest temperature gradient of 8.5 ° C./sec in Example 1.

In S108 CPU 215, the difference value _T t2 _{-T t1} is equal to or a first threshold value _{T thres} more (or the first threshold value). S108 in CPU215, if the difference value _T t2 _{-T t1} is determined that the first threshold value _{T thres} less than the output level for it and the temperature gradient the maximum value is small, the process proceeds to S110. In S110, the CPU 215 determines that the thermistor 214 has failed because it is assumed that the fixed resistance value Rs is attached to the thermistor 214. The CPU 215 stops the power supply to the heat generating member 211 and the rotation of the pressurizing roller 210, and ends the failure determination process of the thermistor 214.

S108 in CPU215, if the difference value _T t2 _{-T t1} is determined that the first threshold value _{T thres} above, the process proceeds to S109. In S109, the CPU 215 determines that the thermistor 214 is a normal product. The CPU 215 continues to control the power supply of the heat generating member 211 and the rotation of the pressurizing roller 210, and ends the failure determination process of the thermistor 214.

With the above control, the output level during the period for calculating the temperature gradient is determined in a state where the normal thermistor 214 and the thermistor 214 in the fault state in which the fixed resistance value Rs is attached to the thermistor 214 cannot be distinguished only by the temperature gradient. It can be distinguished by making it a requirement of. Although the first threshold value _Tthres is set as a fixed value in the first embodiment, it may be changed according to the detection result of the environmental temperature sensor 217. Further, in the first embodiment, the control when calculating the power supply amount to the heat generating member 211 is the PI control, but the same effect can be obtained in the P control or the PID control. Further, although the power supply control is wave number control, it may be phase control. As described above, according to the first embodiment, it is possible to accurately determine the abnormality of the thermistor.

In the first embodiment, the failure of the thermistor 214 is determined according to the output level. If noise or the like is added to the detection result of the state where the fixed resistance value Rs is attached to the thermistor 214 and the calculated output level becomes an output other than the maximum value, the thermistor is judged to be a normal product. There is a risk. In the second embodiment, as a means for avoiding an erroneous determination due to the influence of noise or the like, the CPU 215 controls to determine an abnormality (first abnormality) in which a _{fixed resistance value R s is added in series with the thermistor 214, as shown in FIG.} I do.

[Thermistor failure judgment processing]
The processes from S501 to S507 in FIG. 5 are the same as the processes in S101 to S104 and S107 to S109 in FIG. 4, and the description thereof will be omitted. In the second embodiment, when the CPU 215 determines in S504 that the second time t ₂ has not elapsed, the process returns to S504. In S506, the difference value (T _t2- T _{t1 ) between the detected temperature T t2} and the detected temperature T _t1 is compared with the first threshold value T _thres. Here, also with a 13 ° C. first threshold _{T thres} in Example 2. CPU215 difference value _(T t2 _{-T t1)} is determined to the first threshold value _{T thres} above, if it is determined that the normal thermistor 214, the power supply and the pressure roller 210 of the heat generating member 211 in S507 Continue to control rotation.

On the other hand, if the difference value in S506 _(T t2 _{-T t1)} is determined to below a first threshold value _{T thres,} CPU 215 advances the process to S508. In S508, the CPU 215 confirms the cumulative value of the output level up to the time when the _{second time t 2 has elapsed.} The CPU 215 adds the output level calculated for each control cycle to the previous output level stored in the storage unit 230 until the _{second time t 2 elapses.} Here, the power supply control to the heat generating member 211 is set to 4 sine wave units as in the first embodiment. Therefore, when the frequency of the commercial power supply is 50 Hz, the time of one control cycle is 80 milliseconds. Assuming that the second time t ₂ is, for example, 10 seconds, the _{number of times of control until the time when the second time t 2} elapses is 125 times (= 10000/80). When such control is performed, the cumulative value of the output level is 1125 (= 9 × 125) when the maximum output level (= 9) is always reached.

Here, in order to suppress the influence of noise, the second threshold value _{Nthres is set} to 1125-n. The larger the value of n, the more the influence of noise is suppressed, but it is necessary to distinguish it from the state where the output level is reduced in the normal state. As shown in FIG. 2, when the output level is lowered in the normal state, it can be seen that the output level is gradually lowered by about 2 to 4 levels over a plurality of control cycles. If noise is generated in the detection result of the thermistor 214, it is assumed that the value will return to a value close to the detection result before the noise generation when the noise generation is completed, and the output level is also before the noise generation. It is thought to return to the level. _{In the second embodiment, n is set to, for example, 4 and the first threshold value Tthres} is 1121 in order to determine that the thermistor 214 is a normal product when the output level is lowered in the normal state. As a result, even when the output level drops by 4 levels in one control cycle due to noise, it is possible to separate the fixed resistance value Rs from the state associated with the thermistor 214 and the case caused by noise and shift to S510. ..

S508 in CPU215, when the accumulated value of the output level is determined to be the second threshold value _{N thres} more (larger than or equal to the second threshold), the process proceeds to S510. In S510, the CPU 215 determines that the thermistor 214 has failed, stops the power supply to the heat generating member 211 and the rotation of the pressurizing roller 210, and ends the failure determination process of the thermistor 214. On the other hand, when the output level is lowered in normal control, the output level is gradually lowered over a plurality of control cycles as shown in FIG. Therefore, the cumulative value of the output level is smaller than the second threshold value _Nthres. S508 in CPU215, when the accumulated value of the output level is determined as the second threshold value _{N thres} smaller, the process proceeds to S509. In S509, the CPU 215 determines that the thermistor 214 is a normal product, continues to control the power supply of the heat generating member 211 and the rotation of the pressurizing roller 210, and ends the failure determination process of the thermistor 214. From the above control, by setting the temperature gradient of the thermistor 214 and the cumulative value of the output level as the requirements for determining the abnormality of the thermistor 214, it is possible to detect the abnormality of the thermistor 214 which is not easily affected by noise.

[Thermistor failure judgment processing before power supply]
Further, prior to the processing of S501 in FIG. 5, the control for determining the abnormality (second abnormality) of the zero load resistance value R _{0 of the thermistor 214 at the specified temperature T 0, which is shown in FIG. 6, is performed.} Thereby, by determining whether or not the zero load resistance value R ₀ of the thermistor 214 is defective, the presence or absence of an abnormality of the thermistor 214 can be confirmed more accurately. In the state before heating the heat generating member 211, the difference between the temperature detected by the thermistor 214 and the temperature detected by the environmental temperature sensor 217 is _{predetermined if the zero load resistance value R 0} of the thermistor 214 is normal. It is considered to be within the range.

The CPU 215 executes the processing after S601 before executing the processing of S501. In S601, the CPU 215 receives the heating request of the heat generating member 211. In S602, 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 third threshold value Ta.

In S602, when the CPU 215 determines that 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 third threshold value Ta, the CPU 215 determines that the thermistor 214 is a normal product and performs processing. Proceed to S603. In S603, the CPU 215 starts supplying electric power to the heat generating member 211 and starts heating. Further, the CPU 215 starts the rotation of the pressure roller 210. When the CPU 215 determines in S602 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 third threshold value Ta (or higher than the third threshold value), the process proceeds to S604. In S604, the CPU 215 determines that the thermistor 214 has failed (R ₀ ), and prohibits the power supply to the heat generating member 211 and the rotation of the pressurizing roller 210. The process of FIG. 6 may be executed prior to the process of S101 of FIG.
As described above, according to the second embodiment, it is possible to accurately determine the abnormality of the thermistor.

209 Fixing film 210 Pressurizing roller 211 Heat generating member 214 Thermistor 215 CPU

Claims (11)

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.
The first temperature detected by the first detecting means when the first time elapses from the start of supplying electric power to the heating element, and the second time longer than the first time. Based on the difference value from the second temperature detected by the first detecting means and the electric power controlled by the controlling means when the lapse has passed, the first detecting means has a first abnormality. Judgment means to judge whether or not there is
With
When the difference value is smaller than the first threshold value in a state where the maximum electric power that can be supplied to the heating element is supplied, the first detecting means is the first abnormality. Judging that there is
Based on the detection result by the first detection means, the control means uses P control, PI control, or PID control so that the heating element reaches the target temperature per unit time of the heating element. The power supply amount of the above is calculated, and the calculated power supply amount is distributed to the output levels of a plurality of stages.
The image forming apparatus is characterized in that the determination means determines the first abnormality based on the difference value and the output level. In the determination means, the output level is higher than the maximum output level corresponding to the maximum electric power that can be supplied to the heating element between the lapse of the first time and the lapse of the second time. If turned down, according to claim 1, wherein said first detecting means is judged not the first abnormality, characterized in that to continue the rotation of the pressure roller and the supply of power to the heating element The image forming apparatus according to. 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.
The first temperature detected by the first detecting means when the first time elapses from the start of supplying electric power to the heating element, and the second time longer than the first time. The first detecting means is the first based on the difference value from the second temperature detected by the first detecting means when the lapse has passed and the cumulative value of the electric power controlled by the controlling means. As a means of determining whether or not the power is abnormal,
With
The determination means determines that the first detection means is the first abnormality when the difference value is smaller than the first threshold value and the cumulative value is equal to or larger than the second threshold value. An image forming apparatus characterized in that. Based on the detection result by the first detection means, the control means uses P control, PI control, or PID control so that the heating element reaches the target temperature per unit time of the heating element. The power supply amount of the above is calculated, and the calculated power supply amount is distributed to the output levels of a plurality of stages.
The image forming apparatus according to claim 3 , wherein the determination means determines the first abnormality based on the difference value and the cumulative value of the output level. The fourth aspect of the present invention is characterized in that the determination means obtains the output level for each control cycle for controlling the electric power supplied to the heating element by the control means and calculates the cumulative value of the output level. The image forming apparatus according to the description. The determination unit, when the difference value is equal to or larger than the first threshold value, said first detecting means determines that there is not the first abnormality, the pressure roller and the supply of power to the heating element The image forming apparatus according to any one of claims 1 to 5 , wherein the rotation of the image is continued. The image forming apparatus according to claim 6 , wherein the first threshold value is set according to the environmental temperature. Equipped with a second detection means to detect the environmental temperature
When the determination means determines that the difference value between the temperature detected by the second detection means and the temperature detected by the first detection means is equal to or greater than the third threshold value, the first detection means determines. The image forming apparatus according to any one of claims 1 to 7 , wherein it is determined to be the second abnormality. The first detection means is a thermistor.
The image forming apparatus according to any one of claims 1 to 8 , wherein the first abnormality is an abnormality in which a fixed resistance value is added in series with the thermistor. The first detection means is a thermistor.
The image forming apparatus according to claim 8 , wherein the second abnormality is an abnormality of the zero load resistance value of the thermistor at a specified temperature. The image forming apparatus according to any one of claims 1 to 10, wherein the nip portion is formed by sandwiching the fixing film between the heating element and the pressurizing roller.

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