Fixing device and image forming apparatus

Abstract

To provide a fixing device that can keep the device in operation even when an abnormality in temperature is detected and reduce downtime.SOLUTION: A fixing device 50 has: a fixing roller 51; a heating member 53 that heats the fixing roller 51; temperature detection means 52a that detects a temperature of a surface of the fixing roller; and a control unit 54 that controls power to be supplied to the heating member 53 for each of a plurality of half wave periods in a control cycle including a predetermined number of cycles of AC voltage, and changes a lighting rate of the heating member 53. The fixing device determines the presence or absence of an abnormality in the temperature detection means 52a from a temperature difference ΔT that is a difference between a current temperature of the surface of the fixing roller detected by the temperature detection means 52a and a temperature in the immediately previous control cycle, and when determining that the temperature detection means 52a has an abnormality, changes a target period to calculate an average temperature of the surface of the fixing roller without stopping the operation of the device.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a fixing device and an image forming apparatus. [Background technology]

[0002] An electrophotographic image forming apparatus is equipped with a fixing device that heats and fixes a toner image transferred onto a recording medium such as paper. The fixing device includes a fixing member incorporating a heating element such as a halogen heater, a temperature sensor that detects the surface temperature of the fixing member, and a control means that controls the lighting of the heating element.

[0003] Temperature control of the fixing device is usually performed by feeding back the temperature detected by a temperature sensor, so for proper control it is necessary to detect the temperature rise state of the heating member with high precision and accuracy.

[0004] For example, Patent Document 1 discloses a configuration in which, for the purpose of detecting abnormalities in the temperature rise of a fixing belt of a fixing device, the temperature detected by a temperature sensor that monitors the temperature of the fixing belt is observed to detect a temperature drop that occurs when the fixing belt starts to rotate, and the temperature at the time when the fixing belt starts to rotate is used as a reference, and an abnormal temperature rise is detected based on whether or not the time measured by the temperature sensor reaches a threshold time. Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, a method is known in which the measurement value of the temperature sensor used to detect the temperature of the fixing member is obtained by averaging values ​​obtained multiple times at regular intervals in order to reduce the influence of disturbances such as noise.

[0006] However, if there is a large amount of noise due to a malfunction of the temperature sensor, the temperature may be detected as abnormal even if the device is within the normal range. If an abnormality is detected, the fixing device is usually stopped and control is performed to stop the image formation operation.

[0007] Patent Document 1 describes that if an abnormal temperature rise is detected, the temperature rise operation is stopped. Even with technology that accurately detects abnormal temperature rise in the fixing member, if an abnormal temperature rise is determined to be due to noise caused by a malfunction of the temperature sensor, the operation of the device will stop even if there is no problem with the device itself, resulting in a period of time when the device cannot be used, known as downtime.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a fixing device that allows the device to continue operating even when a temperature abnormality is detected, thereby reducing downtime. [Means for solving the problem]

[0009] In order to solve the above problem, a fixing device of the present invention includes a fixing roller, a heating element for heating the fixing roller, temperature detection means for detecting the temperature of the surface of the fixing roller, and a control unit for controlling the power supplied to the heating element for each of a plurality of half-wave periods within a control period including a predetermined number of periods of an AC voltage, and changing the lighting rate of the heating element, and the fixing device determines whether or not there is an abnormality in the temperature detection means from a temperature difference ΔT between the current temperature of the surface of the fixing roller detected by the temperature detection means and the temperature in the immediately previous control period, and when it is determined that there is an abnormality in the temperature detection means, changes the target period for calculating the average temperature of the surface of the fixing roller without stopping the operation of the device. The temperature detecting means changes the target period for calculating the average temperature of the surface of the fixing roller in accordance with the value of the temperature difference ΔT, and controls to reduce the number of copies per unit time and the lighting rate of the heating member in accordance with the value of the temperature difference ΔT. It is characterized by: [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a fixing device that can continue to operate even when a temperature abnormality is detected, thereby reducing downtime. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention; [Figure 2]FIG. 2 is a block diagram showing a main part of a control mechanism of a fixing device according to an embodiment of the present invention. [Figure 3] 10 is a graph illustrating an abnormality in the output of a temperature detection means. [Figure 4] 10 is a graph showing the output of the temperature detection means and the target temperature at start-up. [Figure 5] 10 is a flowchart showing an example of a flow for detecting an abnormality in a temperature detection means at start-up. [Figure 6] 10 is a graph showing the output of the temperature detection means and the target temperature when transitioning to a standby state. [Figure 7] 10 is a flowchart showing an example of a flow for detecting an abnormality in a temperature detection means when transitioning to a standby state. [Figure 8] 10 is a table showing an example of an average temperature calculation period, CPM, and maximum heater lighting rate controlled by the value of ΔT. [Figure 9] 10A and 10B are diagrams illustrating periods for calculating an average temperature when normal and when an abnormality is detected. [Figure 10] 10A and 10B are diagrams illustrating an example of a heater lighting pattern in a control period. [Figure 11] 10 is a table showing an example of fixed lighting control of the heater when an abnormality occurs in the temperature detection means. DETAILED DESCRIPTION OF THE INVENTION

[0012] The fixing device and image forming apparatus of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the following embodiments, and other modifications, additions, corrections, deletions, and other changes can be made within the scope of what a person skilled in the art can conceive. Any embodiment that achieves the functions and effects of the present invention is included in the scope of the present invention.

[0013] FIG. 1 is a block diagram showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 100 shown in FIG. 1 is, for example, an MFP, a printer, a copier, or a FAX machine, but is not limited to these. As shown in FIG. 1, the image forming apparatus 100 is composed of a scanner unit 10 that optically reads an image formed on a sheet-like recording medium (e.g., transfer paper, etc.), an engine unit 20 that transfers a toner image corresponding to the image after performing predetermined image processing on the image read by the scanner unit 10 onto the recording medium, a paper feed tray 30 in which the recording medium is stored, and a fixing device 50 according to the present invention that fixes the toner image transferred to the recording medium by the engine unit 20.

[0014] The scanner unit 10 scans and exposes the document, converts the image or text contained in the document into an image signal, and outputs the converted image signal to the engine unit 20. The engine unit 20 performs image processing such as color conversion and gradation correction on the image signal output from the scanner unit 10. The engine unit 20 creates an electrostatic latent image on an image carrier (not shown) in accordance with the image that has been subjected to image processing, and forms a toner image by attaching toner to the created electrostatic latent image. The engine unit 20 then transfers the formed toner image onto a recording medium transported from the paper feed tray 30 via a transport path 40, and sends the recording medium with the transferred toner image toward the fixing device 50 via the transport path 40.

[0015] The fixing device 50 uses fixing rollers 51a and 51b, which are a pair of cylindrical fixing members equipped with a heating element, to fix the toner image transferred onto the recording medium sent from the engine unit 20 via the conveying path 40 onto the recording medium, and then discharges the recording medium toward the discharge tray 60. In the fixing device 50, a pair of fixing rollers 51a and 51b rotate in a state of pressure contact, thereby fixing the toner image transferred onto the recording medium onto the recording medium. The fixing rollers 51a and 51b are an example of an object to be heated, and a heating member is provided inside at least one of the pair of fixing rollers 51a and 51b.

[0016] FIG. 2 is a block diagram showing an example of a control mechanism for the heating member (hereinafter referred to as "heater") 53 provided in at least one of the fixing rollers 51a and 51b of the fixing device 50 shown in FIG. 2, the control mechanism includes a heater 53 of the fixing device 50, temperature detection means (hereinafter referred to as "temperature sensors") 52a and 52b, a relay 55, a triac 56, a control unit 54, and a zero-cross detection unit 57. In this embodiment, the heater 53 is disposed inside the fixing roller 51b.

[0017] The heater 53 heats the fixing rollers 51a and 51b based on the power supplied from the AC power source 58, and melts and fixes the toner image transferred onto the recording medium. The heater 53 may be, for example, a halogen lamp that is heated by radiant heat generated when turned on.

[0018] The AC power supply 58 outputs an AC voltage to be supplied to the heater 53. In this embodiment, the AC power supply 58 is, for example, a commercial power supply, and supplies an AC voltage that changes sinusoidally over time to the heater 53, which is a load.

[0019] The triac 56 turns on / off the AC voltage supplied to the heater 53 from the AC power supply 58 at a timing instructed by the control unit 54. When the triac 56 is on, the heater 53 is energized and generates heat, and when the triac 56 is off, the heater 53 is deenergized and stops generating heat.

[0020] The temperature sensors 52a and 52b measure the surface temperature of the fixing roller 51b, which has a built-in heater 53. The temperature sensors 52a and 52b may be, for example, non-contact detection elements that detect temperature by receiving infrared rays, such as thermopiles. The result detected by the temperature sensor 52a is input to a temperature detection unit 54a of the control unit 54. The temperature sensor 52b is for protection and is connected to a relay 55. It monitors the surface temperature of the fixing roller 51b, and if an abnormality in temperature is detected, it shuts off the relay 55 and stops the power supply to the heater 53 in a hardware manner.

[0021] The zero-cross detection unit 57 detects zero-cross timings, which are the timings at which the AC voltage supplied from the AC power supply 58 crosses 0 volts from the positive side to the negative side and the timings at which it crosses from the negative side to the positive side. When the zero-cross detection unit 57 detects a zero-cross timing, it outputs a zero-cross detection signal to the control unit 54.

[0022] The control unit 54 includes a temperature detection unit 54a, a lighting rate determination unit 54b for the heater 53, and a heater supply voltage control unit 54c. The temperature detection unit 54a monitors the sensor for abnormalities and performs noise removal processing (averaging of the detected temperature) on the input information on the detected temperature of the temperature sensor 52a. The temperature detected by the temperature sensor 52a at a temperature detection unit 54a every fixed time (for example, 20 ms) is averaged over a predetermined period (for example, 100 ms), and the average value is used by the lighting rate determination unit 54b.

[0023] The lighting rate determination unit 54b and the heater supply voltage control unit 54c perform control to set the surface temperature of the fixing roller 51 to a predetermined target temperature when fixing a toner image on a recording medium sent from the engine unit 20. The lighting rate determination unit 54b and the heater supply voltage control unit 54c may be realized by a heater control program executed by a CPU (Central Processing Unit, not shown) provided in the fixing device 50 to realize a heater control method, or may be realized by hardware.

[0024] The lighting rate determination unit 54b determines the period for supplying AC voltage to the heater 53 (the lighting period of the halogen lamp) for each AC voltage control period, i.e., the lighting rate, based on the surface temperature of the fixing roller 51b obtained by the temperature detection unit 54a and the target temperature. The AC voltage control period is, for example, two periods (2T). Based on the lighting rate calculated by the lighting rate determining unit 54b, the triac 56 is controlled to be turned on / off, and the heater 53 is controlled to reach a target temperature.

[0025] Hereinafter, the power ratio PS / Pmax, which is the ratio of the amount of power PS supplied to the heater 53 in each control cycle to the maximum amount of power Pmax that can be supplied in the control cycle, is referred to as lighting duty (Do: 0≦Do≦100%). For example, "lighting duty 50%" indicates that AC voltage (power) is supplied to the heater 52 for a period corresponding to 50% of the area surrounded by the sine wave of the control period and 0V.

[0026] The lighting rate determination unit 54b determines a lighting duty corresponding to the amount of power to be supplied to the heater 53 in order to set the surface temperature of the fixing roller 51b to a predetermined target temperature, based on the surface temperature of the fixing roller 51b. Then, the lighting rate determination unit 54b notifies the determined lighting duty to the heater supply voltage control unit 54c. The lighting rate determination unit 54b may determine the lighting duty from the surface temperature of the fixing roller 51b by referring to a table that stores the relationship between the surface temperature of the fixing roller 51b and the lighting duty.

[0027] The lighting duty determined based on the surface temperature of the fixing roller 51b is used, for example, to light the heater 53 from the start to the end of a printing operation. For example, when the surface temperature of the fixing roller 51b is low, a relatively large lighting duty is selected, and the amount of power supplied is increased, thereby raising the surface temperature of the fixing roller 51b in a short period of time. Also, when the surface temperature of the fixing roller 51b is high due to repeated printing operations, a relatively small lighting duty is selected, and the amount of power supplied is reduced.

[0028] The heater supply voltage control unit 54c outputs a timing signal to the triac 56 to turn the triac 56 on or off in accordance with the lighting pattern of the heater 53 that corresponds to the determined lighting duty. At this time, the heater supply voltage control unit 54c outputs the timing signal to the triac 56 based on the zero-cross timing of the AC voltage detected by the zero-cross detection unit 57. As a result, an AC current having a conduction angle that corresponds to the desired lighting pattern is supplied to the heater 53.

[0029] It should be noted that a pattern table can be used that stores time information (lighting patterns) indicating the duration of voltage supply to the heater 53 in each of the four half-wave periods within a control cycle for each lighting duty. The pattern table is stored in a read-only memory (ROM) or random access memory (RAM) that can be read by a CPU provided in the fixing device 50. The ROM and RAM may be provided inside the fixing device 50 or outside the fixing device 50. When the ROM and RAM are provided inside the fixing device 50, the ROM and RAM may store a program for heater control executed by the CPU.

[0030] 10 shows examples of lighting patterns of the heater 53 in the control cycle of the AC voltage supplied to the heater 53. Fig. 10(A) shows an example of a lighting pattern with a lighting duty of 40%, and Fig. 10(B) shows an example of a lighting pattern with a lighting duty of 80%, but the lighting patterns are not limited to these. The control unit 54 performs control to supply AC voltage of a predetermined conduction angle to the heater 53, with two cycles (four half waves) of the AC voltage supplied from the AC power supply 58 as a control cycle. In FIG. 10, the voltage value supplied to the heater 53 by the AC power supply 58 is represented by a sine wave, and four half-wave periods of the AC voltage that are consecutive in time order in a control period (2T) are represented by half-wave periods a, b, c, and d. The value of the control period is not limited, but for example, when a 50 Hz commercial power supply is used as the AC power supply 58, the control period (2T) can be set to 40 msec. In the figure, the colored areas indicate the periods during which power is supplied to the heater 53. A timing signal is output from the heater supply voltage control unit 54c to the triac 56 at the timing corresponding to this area.

[0031] Figure 3 is a graph showing the change in the value detected by the temperature sensor after starting printing from standby mode. This graph shows the case where a thermopile is used as the temperature sensor, and in Figure 3, "sensor output A" shows normal output, and "sensor output B" shows abnormal output. Thermopiles are equipped with an ASIC (Application Specific Integrated Circuit) for data processing, and noise in the ASIC's power supply voltage can cause noise to appear in the output value of the temperature sensor, resulting in an abnormal output such as that shown in "Sensor Output B."

[0032] However, although "sensor output B" has a large amount of noise, it shows that the target temperature is being followed, and the noise is not thought to indicate a malfunction of the object being measured, such as the heater. If the abnormal value indicated by the sensor output is due to a malfunction on the temperature sensor side, rather than a malfunction of the heater, etc., it is undesirable to stop the operation of the fixing device and cause downtime because an abnormal output value has been detected. Therefore, in the fixing device according to the present invention, when a temperature abnormality is detected, it is determined whether there is an abnormality on the temperature sensor side as follows, and if it is determined that there is an abnormality in the temperature sensor, the fixing device is allowed to continue operating, thereby reducing downtime.

[0033] The fixing device 50 according to the present invention includes a pair of fixing rollers 51a and 51b, a heating element 53 for heating the fixing roller 51b, temperature detection means (temperature sensors) 52a and 52b for detecting the temperature of the fixing roller surface, and a control unit 54 for controlling the power supplied to the heating element (heater) 53 for each of a plurality of half-wave periods within a control cycle including a predetermined number of AC voltage cycles, thereby changing the lighting rate of the heating element 53. The control unit 54 determines whether or not there is an abnormality in the temperature detection means 52a based on a temperature difference ΔT between the current temperature of the fixing roller surface detected by the temperature detection means 52a and the temperature in the previous control cycle. If an abnormality is detected in the temperature detection means 52a, the operation of the device is not stopped, and the target period for calculating the average temperature of the fixing roller surface is changed. Specifically, the target period for calculating the average temperature of the fixing roller surface is lengthened. This makes it possible to reduce downtime.

[0034] [Temperature sensor abnormality detection] The following describes examples of detecting abnormalities in the temperature sensor 52a (1) during the period from when the device is turned on until the device is fully started up, and (2) during the period from when the device is switched to standby mode until the heater is turned on again.

[0035] (1) The period from when the device is turned on until the device is fully started up Figure 4 is a graph showing the target temperature and sensor output from when the power is turned on until the equipment is fully started up. Sensor output C shows the change in temperature detected for each control cycle, and sensor output D is the temperature change curve obtained by fitting. In the figure, "Th1" indicates a threshold value that serves as a criterion for determining abnormality.

[0036] 4, immediately after the power is turned on, the fixing roller 51b is at room temperature, so the heater 53 is turned on to reach the target temperature. As a result, the detected temperature tends to rise, and the rate of rise is predictable. The temperature difference ΔT is calculated based on the temperature detected for each control cycle. Specifically, if the current cycle is N (N is an integer), the temperature difference ΔT is obtained by calculating the difference between the temperature detected in cycle N and the temperature detected in the previous control cycle (cycle N-1).

[0037] For example, the maximum temperature rise per 100 ms is set to 10°C. If the fluctuation range due to noise is ±3°C and the fluctuation range due to temperature unevenness of the fixing roller 51b is ±5°C, a temperature rise exceeding 15°C (ΔT > +15°C) is used as the threshold to determine whether the temperature sensor is abnormal. Similarly, a temperature drop exceeding 8°C (ΔT < -8°C) is used as the threshold to determine whether the temperature sensor is abnormal. In this way, the threshold value that serves as a reference for determining whether or not there is an abnormality in the temperature sensor 52a with respect to the temperature difference ΔT has an upper limit value for a temperature rise and a lower limit value for a temperature fall. If these thresholds are exceeded a predetermined number of times (for example, three times), it is determined that the temperature sensor is abnormal.

[0038] It is preferable that the temperature sensor 52a has a function of notifying the user and a service person when it is determined that the temperature sensor 52a has an abnormality.

[0039] FIG. 5 is a flowchart showing an example of a flow for detecting an abnormality in the temperature detection means at start-up. First, the power supply to the device is turned on (step S001), and it is determined whether the heater 53 is turned on (lighted) (step S002). After the heater 53 is turned on, the difference ΔT between the current detected temperature and the detected temperature in the previous control cycle is calculated (step S003). This process is repeated until the start-up is complete (step S004). During the period until the start-up is completed, it is determined whether a temperature increase exceeding the threshold value, in this embodiment 15°C (ΔT > +15°C) or a temperature decrease exceeding 8°C (ΔT < -8°C) has been detected three or more times (S005). If a temperature change exceeding the threshold value has been detected three or more times, it is determined that there is an abnormality in the temperature sensor 52a (step S006), and if the detection has occurred less than three times, it is determined that the temperature sensor 52a is normal (step S007). If it is determined in step S006 that an abnormality has occurred, the process proceeds to control for when an abnormality is detected, which will be described later.

[0040] If an abnormality is determined in step S006, the control proceeds to the control for when an abnormality is detected, which will be described later, and the user is notified of this by a display means, etc. Also, the abnormality state is notified to a service person by a notification means provided in the device.

[0041] (2) The period from when the system transitions to standby mode until the heater is turned on again Figure 6 is a graph showing the target temperature and sensor output when the printer transitions from the paper feed state to the standby state. Sensor output E shows the change in temperature detected at each control cycle, and sensor output F is the curve of the temperature change obtained by fitting. In the figure, "Th2" indicates a threshold value that serves as a criterion for determining abnormality.

[0042] As shown in the graph in Figure 6, the target temperature during paper feed is high (approximately 150°C), and the target temperature in standby mode is low (approximately 60°C). The heater 53 turns off (not lit) when the printer enters standby mode. This causes the detected temperature to tend to decrease, and the rate of decrease is predictable. The temperature difference ΔT is calculated based on the temperature detected for each control cycle, as in (1) above. Specifically, if the current cycle is N (N is an integer), the temperature difference ΔT is obtained by calculating the difference between the temperature detected in cycle N and the temperature detected in the previous control cycle, cycle N-1.

[0043] For example, the maximum temperature drop per 100 ms is set to 5° C. In the standby state, the rotation of the fixing roller 51b is stopped, and there is no need to take temperature unevenness into consideration. If the fluctuation range due to noise is ±3°C, a temperature drop of more than 8°C (ΔT<-8°C) is used as the threshold to determine whether the temperature sensor is abnormal. Similarly, a temperature rise of more than 3°C (ΔT>+3°C) is used as the threshold to determine whether the temperature sensor is abnormal. In this way, the threshold value that serves as a reference for determining whether or not there is an abnormality in the temperature sensor 52a with respect to the temperature difference ΔT has an upper limit value for a temperature rise and a lower limit value for a temperature fall. If these thresholds are exceeded a predetermined number of times (for example, three times), it is determined that the temperature sensor is abnormal.

[0044] FIG. 7 is a flowchart showing an example of a flow of detecting an abnormality in the temperature detection means when transitioning to the standby state. First, the system transitions from a paper passing state to a standby state (step S101), and determines whether the heater 53 has been turned off (not lit) (step S102). After the heater 53 has been turned off, the system calculates the difference ΔT between the current detected temperature and the detected temperature in the previous control cycle (step S103). This process is repeated until the heater 53 is turned on again (step S104). Until the heater 53 is turned on again, it is determined whether a temperature drop exceeding the threshold value, in this embodiment 8°C (ΔT<-8°C) or a temperature rise exceeding 3°C (ΔT>+3°C) has been detected three or more times (S105). If a temperature change exceeding the threshold value has been detected three or more times, it is determined that the temperature sensor 52a is abnormal (step S106), and if the detection is less than three times, it is determined that the temperature sensor 52a is normal (step S107).

[0045] If an abnormality is determined in step S106, the control proceeds to the control for when an abnormality is detected, which will be described later, and the user is notified of this by a display means, etc. The abnormality state is also notified to a service person by a notification means provided in the device.

[0046] [Control after temperature sensor abnormality detection] Control that is performed after it is determined that the temperature sensor 52a is abnormal includes a first embodiment in which the target period for which the temperature sensor 52a calculates the average temperature of the fixing roller surface is changed according to the value of the temperature difference ΔT, and the number of copies per unit time is reduced according to the value of the temperature difference ΔT, and the lighting rate of the heater 53 is reduced; and a second embodiment in which the number of copies per unit time is reduced, and the lighting rate of the heater 53 is set to a predetermined value corresponding to the operating state of the device.

[0047] 1) First embodiment In a first embodiment of control when an abnormality in the temperature sensor is detected, (1) the target period for calculating the average temperature, (2) the number of copies per unit time (CPM), and (3) the maximum heater lighting rate are changed according to the value of the difference ΔT between the current detected temperature and the detected temperature in the previous control cycle. FIG. 8 is a table showing an example of values ​​of (1) the target period (average period) for calculating the average temperature, (2) the number of copies per unit time (CPM), and (3) the maximum heater lighting rate, which are controlled according to the value of ΔT in this embodiment.

[0048] (1) Period for calculating average temperature FIG. 9 shows an explanation of the target period for calculating the average temperature when normal and when an abnormality is detected. As shown in Figure 9(A), under normal control conditions when no abnormality is detected in the temperature sensor (example of Figure 8: ΔT = 0 to 10°C), the temperature detection unit 54a averages the temperature Tm1 detected by the temperature sensor 52a every fixed time (20 ms) over a predetermined period (100 ms) to calculate an average temperature Tm2. On the other hand, as shown in FIG. 9(B), when ΔT=10 to 20°C, which is outside the normal range, the temperature detection unit 54a averages the temperatures Tm3 detected by the temperature sensor 52a every fixed time (20 ms) over a predetermined period (400 ms) to calculate an average temperature Tm4. Similarly, when ΔT=20 to 30°C, the averaging period is set to 1000 ms. It is believed that the larger the value of ΔT, the greater the noise. Therefore, by extending the period covered by the average temperature calculation, the influence of noise can be reduced (noise removal).

[0049] (2) Number of copies per unit time (CPM: Copies per Minutes) If the target period for calculating the average temperature in (1) above is set too long, the temperature tracking ability will decrease, which may cause temperature ripples and lead to a decrease in print quality. Therefore, in order to prevent a decline in print quality, control is performed to reduce the number of copies per unit time (CPM down). As shown in Figure 8, the CPM is 60 ppm (Pages Per Minute) under normal conditions (ΔT = 0 to 10°C) when no abnormalities are found in the temperature sensor. In contrast, when ΔT = 10 to 20°C, the CPM is reduced to 15 ppm, and when ΔT = 20 to 30°C, the CPM is reduced to 10 ppm.

[0050] If the temperature sensor 52a is determined to be abnormal, the printing operation will be carried out, but in order to perform CPM down control, it is preferable to notify the user of this fact using a display means on the device, etc. It is also preferable to notify the service technician so that the CPM setting will be restored to the normal setting at the next maintenance.

[0051] (3) Maximum heater lighting rate If the target period for calculating the average temperature in (1) above is set too long, the temperature tracking ability will decrease, and if a sudden temperature rise occurs due to the heater 53, the heater will be turned on excessively, which may result in a fire or other problem in the device. Therefore, by lowering the maximum lighting rate of the heater 53, it is possible to prevent a sudden rise in temperature. The lighting rate of the heater 53 is determined for each control period of the AC voltage by the lighting rate determination unit 54b. For example, when the control period of the AC voltage is set to two periods (2T) and the lighting duty is set to 40% or 80%, power is supplied and the heater 53 is turned on as shown in FIG.

[0052] As shown in Figure 8, the maximum lighting rate is 100% under normal conditions (ΔT = 0 to 10°C) when no abnormalities are detected in the temperature sensor. In contrast, when ΔT = 10 to 20°C, the maximum lighting rate is reduced to 50%, and when ΔT = 20 to 30°C, the maximum lighting rate is reduced to 30%. The maximum lighting rate of the heater is the maximum value of the lighting duty, and it is controlled so as not to light up more than the set lighting duty. In other words, when ΔT=10 to 20°C, lighting will not be performed with a lighting duty of 50% or more, and when ΔT=20 to 30°C, lighting will not be performed with a lighting duty of 30% or more.

[0053] According to the first embodiment, in which the target period for which the temperature sensor 52a calculates the average temperature of the fixing roller surface is changed according to the value of the temperature difference ΔT, and CPM down control and control to reduce the lighting rate of the heater 53 are performed according to the value of the temperature difference ΔT, it is possible to prevent a deterioration in print quality even if temperature tracking ability is reduced by extending the target period for calculating the average temperature.

[0054] 2) Second embodiment In a second embodiment of control when an abnormality in the temperature sensor is detected, the value of the difference ΔT between the current detected temperature and the detected temperature in the previous control cycle is not used, and fixed lighting control is performed so that the heater 53 has a predetermined lighting duty depending on the operating state of the fixing device.

[0055] In fixed lighting control, the detected temperature is not used, so it is difficult to accurately adjust the temperature of the fixing roller 51b to the target temperature. Therefore, by reducing the number of copies per unit time (CPM down) in advance, it is possible to control the temperature even if temperature ripple occurs. The CPM down level is also fixed.

[0056] 11 is a table showing an example of heater fixed lighting control when an abnormality occurs in the temperature detection means. In this example of the present embodiment, CPM is set to 10 ppm. When the fixing device is in a standby state, the heater 53 is not turned on (the lighting duty is 0%). In the "paper passing preparation" state before paper starts passing through the fixing device, the heater 53 is turned on for a certain period with a lighting duty of 20%. Next, in the "before paper passing" state after paper starts passing and before the recording medium reaches the fixing roller 51, the lighting duty is set to 30%. When the recording medium reaches the fixing roller 51, heat is absorbed by the recording medium, causing the temperature of the fixing roller 51 to drop, so the heater 53 is turned on with a high lighting duty. Then, in the "paper passing" state when the recording medium passes through the nip of the fixing roller 51, the lighting duty is set to 25%. When the printer returns to the standby state after paper has been fed, the lighting duty is also set to 0% of the standby state. By performing fixed lighting control in this way, it becomes possible to continue the printing operation without monitoring the temperature of the fixing roller 51b.

[0057] With the above-described configuration, the fixing device of this embodiment can continue to operate even if a temperature abnormality is detected, thereby reducing downtime. [Explanation of symbols]

[0058] 10 Scanner unit 20 Engine section 30 Paper tray 40 Transport Route 50 Fixing device 51a, 51b Fixing rollers 52a, 52b Temperature detection means (temperature sensor) 53 Heating element (heater) 54 Control Unit 54a Temperature detection unit 54b Lighting rate determination section 54c Heater supply voltage control section 55 Relay 56 Triac 57 Zero cross detection unit 58 AC power supply 60 Output tray 100 Image forming device [Prior art documents] [Patent documents]

[0059] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-203386

Claims

1. a fixing roller; a heating member for heating the fixing roller; a temperature detection means for detecting the temperature of the surface of the fixing roller; and a control unit for controlling the power supplied to the heating member for each of a plurality of half-wave periods within a control period including a predetermined number of periods of an AC voltage, and changing the lighting rate of the heating member; determining whether or not there is an abnormality in the temperature detection means based on a temperature difference ΔT between the current temperature of the surface of the fixing roller detected by the temperature detection means and the temperature in the immediately previous control cycle; If it is determined that the temperature detection means has an abnormality, changing a target period for calculating the average temperature of the surface of the fixing roller without stopping the operation of the device; The temperature detecting means changes a target period for calculating the average temperature of the surface of the fixing roller in accordance with the value of the temperature difference ΔT; and A fixing device characterized by performing control to reduce the number of copies per unit time and control to reduce the lighting rate of the heating member in accordance with the value of the temperature difference ΔT.

2. A fixing device comprising: a fixing roller; a heating element for heating the fixing roller; temperature detection means for detecting the temperature of the surface of the fixing roller; and a control unit for controlling the power supplied to the heating element for each of a plurality of half-wave periods within a control period including a predetermined number of periods of an AC voltage, and for changing the lighting rate of the heating element; determining whether or not there is an abnormality in the temperature detection means based on a temperature difference ΔT between the current temperature of the surface of the fixing roller detected by the temperature detection means and the temperature in the immediately previous control cycle; If it is determined that the temperature detection means has an abnormality, changing a target period for calculating the average temperature of the surface of the fixing roller without stopping the operation of the device; Reduce the number of copies per unit time, and A fixing device, characterized in that the lighting rate of the heating member is set to a predetermined value corresponding to the operating state of the device.

3. A fixing device comprising: a fixing roller; a heating element for heating the fixing roller; temperature detection means for detecting the temperature of the surface of the fixing roller; and a control unit for controlling the power supplied to the heating element for each of a plurality of half-wave periods within a control period including a predetermined number of periods of an AC voltage, and for changing the lighting rate of the heating element; determining whether or not there is an abnormality in the temperature detection means based on a temperature difference ΔT between the current temperature of the surface of the fixing roller detected by the temperature detection means and the temperature in the immediately previous control cycle; a threshold value serving as a reference for determining whether or not there is an abnormality in the temperature detection means with respect to the temperature difference ΔT has an upper limit value for a temperature rise and a lower limit value for a temperature fall, A fixing device characterized in that, when it is determined that the temperature detection means has an abnormality, the operation of the device is not stopped, and a target period for calculating the average temperature of the surface of the fixing roller is changed.

4. 3. The fixing device according to claim 1, further comprising a function of notifying a user and a service person when it is determined that the temperature detection means has an abnormality.

5. A fixing device as described in Claim 3, characterized in that it has a function of notifying a user and a service person if it is determined that the temperature detection means has an abnormality.

6. 5. The fixing device according to claim 1, wherein a threshold value serving as a reference for determining whether or not the temperature detecting means is abnormal with respect to the temperature difference ΔT has an upper limit value for a temperature rise and a lower limit value for a temperature drop.

7. An image forming apparatus comprising the fixing device according to any one of claims 1 to 6.

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