JP2006011327A - Image forming method and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent responsiveness in temperature control from being spoiled even when the number of passing sheets of paper, the condition of paper kinds and an external condition are varied in a system where supply power to a heating means is controlled at or under a maximum supply feasible current value. <P>SOLUTION: A power control means controls power supplied to a heating means including a heating element based on temperature information from a temperature detection means, and compares detected temperature from the temperature detection means with previously set target temperature (S3), and arithmetically calculates electric energy being the sum of electric energy being a manipulated variable corresponding to proportional control and electric energy corresponding to integral control (S4 to S14) based on a PI control logical expression. When the arithmetically calculated electric energy is larger than maximum supply feasible electric energy decided in the power control means (S16), the arithmetic calculation of the electric energy corresponding to the integral control is not updated (S17). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrophotographic image forming method and an image forming apparatus, and more specifically, temperature control of a heater in a heat fixing apparatus for heat fixing a non-fixed toner image formed and supported on a recording material. The present invention relates to an image forming method and an image forming apparatus including a unit.

  Conventionally, an image forming apparatus using an electrophotographic process is known. In this image forming apparatus, an unfixed image (toner image) formed on transfer paper by an image forming means such as an electrophotographic process is heated. The image is fixed on the transfer paper by a fixing device. As the heat fixing device, for example, a heat roller type heat fixing device using a halogen heater as a heat source and a film heating type heat fixing device using a ceramic surface heater as a heat source described in Patent Documents 1 to 16 are known. It has been. In general, such a heater is supplied with electric power from an AC power supply via a switching element such as a triac.

  In a fixing device using a halogen heater as a heat source, the temperature of the fixing device is detected by a temperature detecting element such as a thermistor temperature sensitive element. Based on the detected temperature, the switching element is turned on / off by the sequence controller, that is, the power supply to the halogen heater is turned on / off, and the temperature of the fixing device is controlled to the target temperature. Is done.

  On the other hand, in a fixing device using a ceramic surface heater as a heat source, PI (proportional integration) or PID (PID) is calculated from a temperature difference between a temperature detected by a temperature detection element by a sequence controller and a preset target temperature. A power ratio supplied to the ceramic surface heater, which is an operation amount, is calculated based on a proportional-integral-derivative) control calculation formula. The corresponding phase angle or wave number is determined from the calculated power ratio, the switching element is turned on / off at the determined phase or wave number, and the temperature of the fixing device is temperature controlled.

In the case of PI control, the calculated power ratio D is a power ratio that is an operation amount corresponding to proportional control, Dp, a power ratio that is an operation amount corresponding to integral control at time t is Di (t), and a temperature detection means. Assuming that the temperature detected from Tn is Tn, the target temperature is Tt, and the control interval is Δt, the following equation (1) is obtained.
D = Dp + Di (t) = Ap × (Tt−Tn) + Di (t−Δt) + ΔDi (t, Tt−Tn)
... (1)

  However, Ap is a coefficient in proportional control, and ΔDi (t, Tt−Tn) is an increment of the power ratio Di (t) corresponding to the integral control at time t. That is, the switching element is on / off controlled by the power ratio D calculated from the deviation between the detected temperature and the target temperature, and the temperature of the fixing device is temperature controlled.

  A heat roller fixing type heat fixing device basically includes a heating roller (fixing roller) as a heating rotator and an elastic pressure roller as a pressure rotator in pressure contact therewith. The heat roller fixing type heat fixing device rotates the pair of rollers to form and carry an unfixed image (toner image) on the pressure nip portion (fixing nip portion) of the two roller pairs. A recording material (transfer material sheet, electrostatic recording paper, electro-fax paper, printing paper, etc.) is introduced and passed through the pressure nip portion. In this way, the heat roller fixing type heat fixing device heats an unfixed image as a permanently fixed image on the surface of a recording material (hereinafter referred to as a transfer material) by heat from the heat roller and pressure applied at the pressure nip. Fix with pressure.

  Further, film heating type heat fixing devices (on-demand fixing devices) are proposed in, for example, Patent Documents 17 to 20 and the like. In these on-demand fixing devices, a heat-resistant film (fixing film), which is a heating rotator, is brought into close contact with a heating rotator (elastic roller) and is slid and conveyed. Next, the on-demand fixing device introduces a transfer material carrying an unfixed image into a press nip formed by a heating body and a pressure rotating body with the heat-resistant fixing film interposed therebetween, together with the heat-resistant film. Transport. The on-demand fixing device fixes the unfixed image as a permanent image on the transfer material by the heat from the heating body applied via the heat resistant film and the pressure applied to the pressure nip.

  In the film heating type heating apparatus, a low heat capacity linear heating body can be used as the heating body, and a thin film having a low heat capacity can be used as the film. Therefore, the heating device of the film heating method can save power and shorten the wait time (quick start property). In addition, a film heating type heating apparatus is known as a film driving method in which a driving roller is provided on the inner surface of the film, and a method in which a film is driven by a frictional force with a pressure rotating body using a pressure rotating body as a driving roller. Yes. However, in recent years, a pressurizing rotating body drive system having a low number of components and a low cost is often used.

JP-A-63-313182 Japanese Patent Laid-Open No. 2-157878 JP-A-4-44075 JP-A-4-44076 JP-A-4-44077 JP-A-4-44078 JP-A-4-44079 JP-A-4-44080 JP-A-4-44081 JP-A-4-44082 JP-A-4-44083 JP-A-4-204980 JP-A-4-204981 JP-A-4-204982 JP-A-4-204983 JP-A-4-204984 JP-A-63-313182 Japanese Patent Laid-Open No. 2-157878 JP-A-4-44075 JP-A-4-204980

  The AC power supply for supplying power to the ceramic surface heater has a wide power supply voltage range of, for example, 85V to 140V or 187V to 264V. For this reason, the power supplied to the ceramic surface heater energized by full lighting is approximately 2.7 times the power at the highest voltage with respect to the power at the lowest voltage in the power supply voltage range of 85V to 140V. Similarly, the supplied power is approximately double the power at the highest voltage with respect to the power at the lowest voltage in the power supply voltage range of 187V to 264V.

  Further, the current supplied to the ceramic surface heater is controlled so that the sequence controller reaches a predetermined temperature. For this reason, the greater the thickness of the paper that is passed through the fixing device, the greater the supplied power, that is, the current. Therefore, depending on the type of paper, more power than necessary is supplied to the ceramic surface heater. Therefore, it is necessary to control the power supplied to the ceramic surface heater below the maximum supplyable current value to the ceramic surface heater of the fixing device.

Sequence controller determines the maximum available power ratio _{D limit,} if the power ratio D is calculated from the target temperature and the detected temperature is greater than the maximum available power ratio _{D limit} the control for supplying power ratio as _{D limit} To do.

If D <D _limit , the detected temperature T approaches the target temperature Tt as time passes. However, when D> D _{limit and the} controlled power ratio is fixed to the maximum _supplyable power ratio D _limit , a phenomenon may occur in which the detected temperature Tn does not increase with respect to the target temperature Tt. . In this case, since there is a temperature deviation, the integration increment ΔDi (t, Tt−Tn) in the above equation (1) is continuously added.

  Normally, the integral control increment ΔDi (t, Tt−Tn) is set to be very small as compared with the proportional control Dp. However, if the temperature deviation does not shrink for a long time, the integral control increment ΔDi (t, Tt−Tn) is set. The power ratio Di (t) continues to increase gradually.

  In such a state, when the target temperature falls due to the conditions of the number of sheets to be passed and the paper type, or when the power supply voltage of the AC power supply supplied due to external conditions decreases, the temperature detected by the temperature detecting means is the target temperature. When the above change occurs and the temperature deviation amount is reversed, the supplied power ratio must be reduced.

  However, although the power ratio Dp of the proportional control immediately takes a value that reflects an actual temperature deviation, the power ratio ΔDi (t, Tt−Tn) of the integral control remains in an increased state. Even if the power ratio Dp becomes small (negative value), a power ratio that is larger than the originally required power ratio is supplied, and power control, that is, temperature control is performed until the power ratio of integral control can be absorbed. There will be a delay.

  The present invention has been made in view of such a problem. The object of the present invention is to calculate the power ratio corresponding to the integral control when it is larger than the upper limit power ratio or smaller than the lower limit power ratio. An object of the present invention is to provide an image forming method and an image forming apparatus in which the responsiveness of temperature control is not impaired without updating.

  Another object of the present invention is to provide an image forming method and an image forming apparatus in which the power supplied to the heating means is controlled below the maximum suppliable current value and the responsiveness of temperature control is not impaired.

  In addition, the power supplied to the heating means is controlled below the maximum suppliable current value, so that the responsiveness of temperature control is not impaired even if the number of sheets passed, paper type conditions, and external conditions fluctuate. It is an object to provide a forming method and an image forming apparatus.

  In addition, in a system in which the power supplied to the heating means is controlled below the maximum suppliable current value, the responsiveness of the temperature control is not impaired even if the number of sheets passed, the condition of the paper type, and external conditions fluctuate, An object of the present invention is to provide an image forming method and an image forming apparatus which perform stable temperature control with good followability to a target temperature.

  In addition, the power supplied to the ceramic surface heater is controlled below the maximum current that can be supplied to the ceramic surface heater of the fuser, and even if the number of sheets passed, the type of paper, and external conditions vary, the temperature An object of the present invention is to provide an image forming method and an image forming apparatus which perform power control without impairing control responsiveness.

  The present invention has been made in order to achieve such an object. The invention according to claim 1 is directed to a heating unit having a heating element that generates heat by supplying electric power, and electric power for supplying electric power to the heating unit. In an image forming method in an image forming apparatus, comprising: a supply unit; a temperature detection unit that detects a temperature of the heating unit; and a power control unit that controls a supply amount of power from the power supply unit to the heating unit. The power control means controls the temperature of the heating means by controlling the power supply means with a power ratio that is a ratio to the power when the half wave or full wave of the commercial power supply voltage is fully lit. Compared with the temperature detected by the temperature detection means and a preset target temperature, the power ratio and the integral control corresponding to the proportional control are based on the PI control logical expression. When the power ratio calculation step of calculating the sum of the power ratio and the power ratio that is the operation amount, and the power ratio calculated in the power ratio calculation step is larger than the upper limit power ratio determined in the power control means, Alternatively, when the power ratio is smaller than the lower limit power ratio, it has a calculation update stop step that does not update the calculation of the power ratio corresponding to the integral control. With such a configuration, it is possible to provide an image forming method in which the responsiveness of temperature control is not impaired.

  The invention described in claim 2 is the first invention set forth in the invention described in claim 1, further comprising current detection means for detecting the current supplied to the heating means. A step of supplying power to the heating means at a power ratio, a step of detecting a current value detected by the current detection means, and a maximum supply capable of being supplied to the heating means preset in the power control means Based on the comparison between the current value and the current value detected by the current detection means, and the comparison between the maximum supplyable current value that can be supplied and the detected current value, power is supplied to the heating means. A step of calculating a maximum suppliable power ratio when power is applied, a power control step of controlling power supplied to the heating means by the power supply means below the maximum suppliable power ratio, and the power A calculation update stop step that does not update the calculation of the power ratio corresponding to the integral control when the power ratio calculated in the calculation step is larger than the maximum suppliable power ratio determined in the power control means; It is characterized by that.

  According to a third aspect of the present invention, in the second aspect of the present invention, the temperature detected by the temperature detection means for the calculation of the power ratio corresponding to the integral control for a predetermined period is the target temperature. When the temperature detected by the temperature detecting means is continuously higher than the target temperature, the predetermined amount of power ratio is reduced. When the power ratio calculated by the power ratio calculating step is larger than the maximum suppliable power ratio determined by the power control means, the power ratio is calculated in the power ratio corresponding to the integral control. It is characterized in that only the update that reduces the number is performed.

  The invention according to claim 4 is the invention according to claim 2 or 3, wherein the power control means calculates a power ratio supplied to the heating means and determines a phase angle corresponding to the power ratio. And a temperature control step of controlling the temperature of the heating means by controlling the power to be supplied by phase control.

  Further, the invention according to claim 5 is a heating unit having a heating element that generates heat by supplying power, a power supply unit that supplies power to the heating unit, and a temperature detection unit that detects the temperature of the heating unit. An image forming apparatus comprising: a power control unit that controls a supply amount of power from the power supply unit to the heating unit; wherein the power control unit sets the power supply unit to a predetermined power. Compared with the temperature detected by the temperature detecting means and a preset target temperature, the power ratio corresponding to proportional control and the operation amount corresponding to integral control are compared. When the power ratio calculation means for calculating the power ratio of the sum of the power ratio and the power ratio calculated by the power ratio calculation means is larger than the upper limit power ratio determined by the power control means, Properly if less than the lower limit power ratio is characterized in that an arithmetic update stopping means does not update the calculation of the power ratio corresponding to the integral control. With such a configuration, it is possible to provide an image forming apparatus in which the responsiveness of temperature control is not impaired.

  According to a sixth aspect of the present invention, in the fifth aspect of the invention, the predetermined power is a power when a half wave or a full wave of a commercial power supply voltage is fully lit, and the power ratio calculating means Is characterized in that the power ratio is calculated based on the PI control logical expression.

  The invention according to claim 7 is the invention according to claim 5, further comprising current detection means for detecting a current supplied to the heating means, wherein the power control means is the power control means. Power can be supplied to the heating means at a set first power ratio, and the current value detected from the current detection means can be detected and supplied to the heating means preset in the power control means Compared with the maximum suppliable current value, the maximum suppliable electric power ratio in the case of supplying electric power to the heating means is calculated, and the electric power supplied by the electric power supply means to the heating means is equal to or less than the maximum suppliable electric power ratio. If the power ratio calculated by the power ratio calculating means is larger than the maximum suppliable power ratio determined by the power control means, the calculation of the power ratio corresponding to the integral control is updated. Do Characterized by comprising a free operation update stopping means. With such a configuration, it is possible to provide an image forming apparatus in which the power supplied to the heating unit is controlled below the maximum suppliable current value and the responsiveness of the temperature control is not impaired.

  Further, the invention according to claim 8 is the invention according to claim 7, wherein the temperature detected by the temperature detecting means for the calculation of the power ratio corresponding to the integral control is continuously detected for a predetermined period. If the temperature detected by the temperature detection means is continuously higher than the target temperature, update is performed to decrease the predetermined amount of power ratio. If the power ratio calculated by the power ratio calculating means is greater than the maximum suppliable power ratio determined by the power control means, the power ratio is updated in the calculation of the power ratio corresponding to the integral control. The update means which performs only is provided. With such a configuration, the power supplied to the heating means is controlled below the maximum suppliable current value, and the responsiveness of the temperature control is not impaired even if the number of sheets passed, the type of paper, or external conditions fluctuate. An image forming apparatus can be provided.

  The invention according to claim 9 is the invention according to claim 7 or 8, wherein the power control means calculates a power ratio supplied to the heating means and determines a phase angle corresponding to the power ratio. The power supplied is controlled by phase control to control the temperature of the heating means. With such a configuration, an image forming apparatus that performs stable temperature control with good followability with respect to a target temperature without losing the responsiveness of temperature control even when the number of sheets to be passed, the type of paper, and external conditions fluctuate. Can be provided.

  The invention according to claim 10 is the invention according to any one of claims 5 to 9, wherein the heating means is formed on an insulating substrate and one or both surfaces of the insulating substrate. A heating member that is slidable with the heating unit, a rotatable pressing member that is pressed to form a nip portion with the heating unit via the film, and an unfixed image carried thereon. And a fixing device for fixing the recording medium by heating the heating element while passing the recording medium through the nip portion. With this configuration, the power supplied to the ceramic surface heater is controlled below the maximum current that can be supplied to the ceramic surface heater of the fuser, and the number of sheets that pass through, the conditions of the paper type, and external conditions vary. However, it is possible to provide an image forming apparatus that performs power control without impairing the responsiveness of temperature control.

  According to the present invention, a heating unit having a heating element that generates heat when power is supplied, a power supply unit that supplies power to the heating unit, a temperature detection unit that detects the temperature of the heating unit, and a heating unit from the power supply unit to the heating unit. In the image forming method in the image forming apparatus comprising the power control means for controlling the amount of power supplied to the power, the power control means turns on the power supply means when the commercial power supply voltage half-wave or full-wave is fully lit. The temperature of the heating means is controlled by controlling the power ratio, which is a ratio to the above. The temperature detected by the temperature detecting means is compared with a preset target temperature based on the PI control logical expression. The power ratio calculation step for calculating the power ratio of the sum of the power ratio corresponding to the proportional control and the power ratio corresponding to the integral control is calculated in the power ratio calculation step. If the power ratio is larger than the upper limit power ratio determined by the power control means or smaller than the lower limit power ratio, the calculation update stop step is not performed so that the calculation of the power ratio corresponding to the integral control is not performed. Further, it is possible to provide an image forming method in which the responsiveness of temperature control is not impaired.

  In addition, a heating unit having a heating element that generates heat when power is supplied, a power supply unit that supplies power to the heating unit, a temperature detection unit that detects the temperature of the heating unit, and a power supply unit that supplies power to the heating unit. Power control means for controlling the supply amount, and the power control means is preset with a temperature detected by the temperature detection means in order to control the power supply means by a power ratio that is a ratio to a predetermined power. Compared with the target temperature, the power ratio calculating means for calculating the power ratio of the sum of the power ratio corresponding to the proportional control and the power ratio corresponding to the integral control, and the power ratio calculating means If the calculated power ratio is larger than the upper limit power ratio determined by the power control means or smaller than the lower limit power ratio, the calculation of the power ratio corresponding to the integral control is not updated. Since a update stopping means, it is possible to provide an image forming apparatus responsiveness of the temperature control is not impaired.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic configuration diagram of an image forming apparatus using an electrophotographic process according to the present invention, and shows, for example, a laser printer. The laser beam printer main body 101 has a cassette 102 for storing the recording paper S, a cassette presence sensor 103 for detecting the presence of the recording paper S in the cassette 102, and a cassette size sensor 104 for detecting the size of the recording paper S in the cassette 102. A paper feed roller 105 for feeding out the recording paper S from the cassette 102 is provided (consisting of a plurality of micro switches).

  A registration roller pair 106 that synchronously conveys the recording paper S is provided downstream of the paper supply roller 105. An image forming unit 108 that forms a toner image on the recording paper S based on the laser beam from the laser scanner unit 107 is provided downstream of the registration roller pair 106. A fixing device 109 for thermally fixing the toner image formed on the recording paper S is provided downstream of the image forming unit 108.

  Downstream of the fixing device 109, a paper discharge sensor 110 that detects the conveyance state of the paper discharge unit, a paper discharge roller 111 that discharges the recording paper S, and a stacking tray 112 on which the recording paper S that has been recorded are stacked are provided. ing. The conveyance reference of the recording paper S is set so as to be centered with respect to the length of the recording paper S in the direction orthogonal to the conveyance direction of the image forming apparatus, that is, the width of the recording paper S.

  The laser scanner 107 emits a laser beam modulated based on an image signal (image signal VDO) sent from an external device 131 (to be described later), and a photosensitive drum 117 (to be described later) from the laser unit 113. A polygon motor 114 for scanning upward, an imaging lens 115, a folding mirror 116, and the like are included.

  The image forming unit 108 includes a photosensitive drum 117, a primary charging roller 119, a developing device 120, a transfer charging roller 121, a cleaner 122, and the like necessary for a known electrophotographic process. The fixing device 109 includes a fixing film 109a, an elastic pressure roller 109b, a ceramic surface heater 109c provided inside the fixing film, and a thermistor 109d that detects the surface temperature of the ceramic surface heater 109c.

  The main motor 123 applies a driving force to the paper supply roller 105 via the paper supply roller clutch 124 and to the registration roller pair 106 via the registration roller 125. The main motor 123 also applies driving force to each unit of the image forming unit 108 including the photosensitive drum 117, the fixing device 109, and the paper discharge roller 111.

  The engine controller 126 controls the electrophotographic process by the laser scanner unit 107, the image forming unit 108, and the fixing device 109, and controls the conveyance of the recording paper in the laser beam printer main body 101.

  The video controller 127 is connected to an external device 131 such as a personal computer via a general-purpose interface (Centronics, RS232C, etc.) 130. The video controller 127 expands the image information sent from the general-purpose interface into bit data, and sends the bit data to the engine controller 126 as a VDO signal.

  That is, the image forming apparatus of the present invention includes a ceramic surface heater (heating means) 109c having a heating element that generates heat when power is supplied, and triacs 4 and 13 (power supply means) for supplying power to the ceramic surface heater 109c. 2), a thermistor 109d (temperature detection means) for detecting the temperature of the ceramic surface heater 109c, and an engine controller 126 for controlling the amount of power supplied from the triacs 4 and 13 to the ceramic surface heater 109c. (Power control means; including 11 in FIG. 2).

  The engine controller 126 includes a power ratio calculation unit 28 and a calculation update stop unit 29. The power ratio calculation unit 28 controls the triacs 4 and 13 from the thermistor 109d in order to control the triacs 4 and 13 with a power ratio that is a ratio to a predetermined power. Comparing the detected temperature with a preset target temperature, calculating the power ratio of the sum of the power ratio, which is the operation amount corresponding to proportional control, and the power ratio, which is the operation amount corresponding to integral control It is.

  Further, the calculation update stop unit 29 corresponds to the integral control when the power ratio calculated by the power ratio calculation unit 28 is larger than the upper limit power ratio determined by the engine controller 126 or smaller than the lower limit power ratio. The power ratio calculation is not updated. With such a configuration, it is possible to provide an image forming apparatus in which the responsiveness of temperature control is not impaired.

  FIG. 2 shows a ceramic heater driving circuit and control circuit according to the present invention. In the figure, reference numeral 1 denotes an AC power source for the laser beam printer. The AC power source 1 is connected to the heating element 3 and the heating element 20 constituting the ceramic surface heater 109c (24) via the AC filter 2. Power supply to the heating element 3 is performed by energization and interruption of the triac 4. Supply of electric power to the heating element 20 is performed by energization and interruption of the triac 13.

  Reference numerals 5 and 6 are bias resistors for the triac 4, and 7 is a phototriac coupler for securing a creepage distance between the primary and secondary. When the light emitting diode of the phototriac coupler 7 is energized, the triac 4 is turned on. Reference numeral 8 denotes a resistor for limiting the current of the phototriac coupler 7. Reference numeral 9 denotes a transistor which controls on / off of the phototriac coupler 7. The transistor 9 operates according to the ON1 signal from the engine controller 126 (11) via the resistor 10.

  Reference numerals 14 and 15 are bias resistors for the triac 13. Reference numeral 16 denotes a phototriac coupler for securing a creepage distance between the primary and secondary. When the light emitting diode of the phototriac coupler 16 is energized, the triac 13 is turned on. Reference numeral 17 denotes a resistor for limiting the current of the phototriac coupler 16. A transistor 18 controls on / off of the phototriac coupler 16. The transistor 18 operates according to the ON2 signal from the engine controller 126 (11) via the resistor 19.

  Reference numeral 12 denotes a zero cross detection circuit connected to the AC power source 1 via the AC filter 2. The zero cross detection circuit 12 notifies the engine controller 126 as a pulse signal (hereinafter referred to as a ZEROX signal) that the commercial power supply voltage is a voltage equal to or lower than a certain threshold value. The engine controller 126 detects the edge of the pulse of the ZEROX signal, and performs on / off control of the triac 4 or 13 by phase control or wave number control.

  The heater current controlled by the triacs 4 and 13 and supplied to the heating elements 3 and 20 is converted into a voltage by the current transformer 25 and input to the current detection circuit 27 through the bleeder resistor 26. The current detection circuit 27 converts the voltage-converted heater current waveform into an average value or an effective value, and A / D-inputs it to the engine controller 126 as an HCRRT signal.

  Reference numeral 109d denotes a thermistor for detecting the temperature of the ceramic surface heater 109c formed by the heating elements 3 and 20. The thermistor 109d (21) is disposed on the ceramic surface heater 109c via an insulator having a withstand voltage so as to ensure an insulation distance from the heating elements 3 and 20. The temperature detected by the thermistor 109d is detected as a partial pressure of the resistor 22 and the thermistor 109d, and A / D is input to the engine controller 126 as a TH signal.

  The temperature of the ceramic surface heater 109c is monitored by the engine controller 126 as a TH signal. The temperature of the ceramic surface heater 109c is compared with the set temperature of the ceramic surface heater 109c set in the engine controller 126. As a result, the engine controller 126 calculates a power ratio to be supplied to the heating elements 3 and 20 constituting the ceramic surface heater 109c, and the supplied power ratio is set to a phase angle (phase control) or a wave number (wave number control). ). Depending on the control conditions, the engine controller 126 sends an ON1 signal to the transistor 9 or an ON2 signal to the transistor 18.

  When calculating the power ratio supplied to the heating elements 3 and 20, the engine controller 126 calculates the upper limit power ratio based on the HCRRT signal notified from the current detection circuit, and the power less than the upper limit power ratio. Is controlled to be energized. For example, in the case of phase control, the engine controller 126 has a control table as shown in Table 1 below, and the engine controller 126 performs control based on this control table.

  Further, a thermostat for preventing overheating when the electric power is supplied to the heating elements 3 and 20 and the power supply control means (engine controller 126) breaks down and the heating elements 3 and 20 reach a thermal runaway. 23 is arranged on the ceramic surface heater 109c. When the heating elements 3 and 20 reach thermal runaway due to the failure of the power supply control means and the thermostat 23 reaches a predetermined temperature or more, the thermostat 23 is opened and the energization of the heating elements 3 and 20 is interrupted.

  3A and 3B are structural views of the ceramic surface heater shown in FIG. 1, FIG. 3A shows a cross section of the ceramic surface heater, and FIG. 3B shows a nip side view. . In addition, the code | symbol b has shown the surface in which the heat generating bodies 32 (3) and 33 (20) are formed, and the code | symbol c has shown the surface opposite to the surface which the code | symbol b shows.

The ceramic surface heater 109c includes a ceramic insulating substrate 31 such as SiC, AlN, Al ₂ O _3, heating elements 3 and 20 formed on the surface of the insulating substrate 31 by paste printing or the like, and two heat generations. It comprises a protective layer 34 such as glass that protects the body. On the protective layer 34, the thermistor 109d and the thermostat 23 for detecting the temperature of the ceramic surface heater 109c are symmetrically positioned with respect to the recording paper transport reference a1 (the center line in the length direction of the heat generating portions 32a and 33a). And is disposed at a position on the inner side of the minimum recording paper width capable of passing paper.

  The heating element 3 includes a part 32a that generates heat when power is supplied, a conductive part 32b that connects the electrode parts 32c and 32d and the heating element, and electrode parts 32c and 32d to which power is supplied via a connector. It is configured. The heating element 20 includes a portion 33a that generates heat when power is supplied, a conductive portion 33b that is connected to the electrode portions 32c and 33d, and electrode portions 32c and 33d that are supplied with power via a connector. . The electrode portion 32 c is connected to the two heating elements 3 and 20 and serves as a common electrode for the heating elements 3 and 20. In addition, a glass layer may be formed on the surface facing the insulating substrate 31 on which the heating elements 3 and 20 are printed in order to improve the slidability.

  A HOT side terminal of the AC power source 1 is connected to the common electrode 32 c via the thermostat 23. The electrode portion 32 d is connected to the triac 4 that controls the heating element 3, and is connected to the neutral terminal of the AC power supply 1. The electrode portion 33 d is electrically connected to the triac 13 that controls the heating element 20, and is connected to the neutral terminal of the AC power supply 1.

  The ceramic surface heater 109c (24) is supported by a film guide 62 as shown in FIGS. 4 (a) and 4 (b). 109a (61) is a fixing film made of a cylindrical heat-resistant material, and is externally fitted to a film guide 62 that supports a ceramic surface heater 109c on the lower surface side. A ceramic surface heater 109c on the lower surface of the film guide 62 and an elastic pressure roller 109b (63) as a pressure member are interposed between the fixing film 109a and a predetermined resistance against the elasticity of the elastic pressure roller 109b. A fixing nip portion having a predetermined width as a heating portion is formed by pressure contact with a pressing force.

  The thermostat 23 is brought into contact with the surface of the insulating substrate 31 or the surface of the protective layer 34 of the ceramic surface heater 109c. The thermostat 23 is corrected in position by the film guide 62, and the thermosensitive surface of the thermostat 23 is in contact with the surface of the ceramic surface heater 109c. Although not shown, the thermistor 109d is also in contact with the surface of the ceramic surface heater 109c. As shown in FIGS. 4A and 4B, the ceramic surface heater 109c may have the heating elements 3 and 20 on the side opposite to the nip part or the heating element on the nip part side. In order to improve the slidability of the fixing film 109a, slidable grease may be applied to the interface between the fixing film 109a and the ceramic surface heater 109c.

FIG. 5 is a flowchart illustrating the control sequence of the fixing device by the engine controller according to the first embodiment.
When the engine controller 126 issues a request to start power supply to the ceramic surface heater 109c (S1), the integration cycle counter tise of the integration control is set to 0 (S2). The electric power supplied to the heating elements 3 and 20 is controlled by PI control based on information from the TH signal so that the predetermined temperature set in the engine controller 126 is reached. The target predetermined temperature Tt and the detected temperature Tn from the TH signal are compared (S3), the proportional control value is set based on the difference (S4), and the duty to be supplied is calculated (S5 to S14). The power ratio corresponding to the proportional control is Dp, the power ratio corresponding to the integral control at time t is Di (t), the temperature detected by the temperature detecting means is Tn, the target temperature is Tt, When the control interval is Δt, for example, it is determined by the following equation (2).
D = Dp + Di (t) = Ap × (Tt−Tn) + Di (t−Δt) + ΔDi (t, Tt−Tn)
Dp = 2.5% × (Tt−Tn)
ΔDi (t, Tt−Tn) = + 2.5% (when Tt> Tn continuously for a predetermined time ti)
= −2.5% (when Tt ≦ Tn continuously for a predetermined time ti)
... (2)

  However, Ap is a coefficient in proportional control, and ΔDi (t, Tt−Tn) is an increment of the power ratio Di (t) corresponding to the integral control at time t.

  That is, the engine controller 126 compares the set target temperature Tt and the detected temperature Tn from the TH signal (S3), calculates the power ratio Dp corresponding to proportional control (S4), and Di (corresponding to integral control). t) is calculated (S5 to S13). When the target temperature Tt is compared with the detected temperature Tn and the target temperature Tt is higher than the detected temperature Tn (S5), it is determined whether or not the control temperature Δt was in the same state before (S6). If they are the same, the integration cycle counter tset is incremented. If the target temperature Tt is lower than the detected temperature Tn in a different state, the state setting is changed to a state where the target temperature Tt is higher than the detected temperature Tn (ip = 1) (S7), and the integration cycle counter tset is reset. (S10).

  Similarly, when the target temperature Tt is equal to or lower than the detected temperature Tn (S5), it is determined whether the state is the same before the control interval Δt (S8). If the target temperature Tt is different from the detected temperature Tn, the state setting is changed to a state where the target temperature Tt is equal to or lower than the detected temperature Tn (ip = 0) (S9), and the integration cycle counter tset is reset. (S10).

  When the integration cycle counter tise has reached the predetermined integration cycle time ti (S12), if ip = 1 (continuous Tt> Tn), 2.5% is added to Di (t), and ip = 0. If so (continuous Tt ≦ Tn), 2.5% is reduced to Di (t) (S13). If the integration cycle counter tise has not reached the predetermined integration cycle time ti (S12), Di (t) is not updated.

  The sum of the power ratio Dp corresponding to the proportional control calculated by the above-described calculation and the power ratio Di (t) corresponding to the integral control becomes the power ratio (duty) D based on the PI control equation (S14). If necessary, the harmonic countermeasure is corrected and the power ratio (duty) D is changed (S15).

Then, as compared to the upper limit duty ratio _{D limit} that is calculated in the engine controller 126 sets (S16), the duty D is equal to or larger than the upper limit duty _{D limit,} the charged duty D and the upper limit duty _{D limit} (S17). At this time, if the target temperature Tt is greater than the detected temperature Tn, ip = 1 (S18), the integration cycle counter tset is reset (S19). If the target temperature Tt is equal to or lower than the detected temperature Tn and ip = 0 (S18), it is left as it is.

  The engine controller 126 sends ON1 and ON2 signals using the ZEROX signal as a trigger at a phase angle α corresponding to the input duty D determined based on Table 1, and power is supplied to the ceramic surface heater 109c for temperature control. Perform (S20).

The current value I in the HCRRT signal notified from the current detection circuit 27 is detected when energization is performed at the above-described charging duty D. In the engine controller 126, an upper limit power duty D _limit that can be energized is calculated from the detected current value I, the input duty D, and a preset energizable current value I _limit (S21). When the current value notified to the engine controller 126 by the current detection circuit 27 is an effective value, D _limit is calculated by the following equation.
D _limit = (I _limit / I1) ² × D1

The current value I _limit is set to an allowable current value that can be supplied to the heater by subtracting the current supplied to a portion other than the heater from the rated current of the connected commercial power source.

  Next, if there is a request to end the temperature control of the heater (S22), the temperature control is ended, and if there is no request, temperature control based on the PI control equation is continued.

FIG. 6 is a schematic diagram of the temperature control waveform of the first embodiment shown in FIG. The waveform of the conventional example is indicated by a suffix (_B). The input duty calculated based on the PI control equation is indicated by Dpi and Dpi_B, and the input duty corresponding to the integral control is indicated by Di and Di_B. Conventionally, when the input duty Dpi calculated based on the PI control equation is equal to or greater than the upper limit duty D _limit and the actual temperature Tn does not reach the target temperature, the input duty Di_B corresponding to the integral control is It gradually increases.

  When the target temperature is lowered in this state, the making duty D cannot be lowered, and the control is performed at a temperature Tn_B higher than the target temperature Tt.

As described above, in the first embodiment, when the making duty Dpi calculated based on the PI control equation is equal to or higher than the upper limit duty D _limit , the actual detected temperature Tn is steady with respect to the target temperature Tt. Even if it is small, the update control for increasing the input duty Di corresponding to the integral control is not performed. By this control, even when the target temperature is lowered, it is possible to follow the change and reduce the input duty D. The actual temperature Tn can be controlled to the target temperature Tt by the control of the first embodiment. The same control is possible even when there is only one heating element.

  FIG. 1 is a schematic configuration diagram of an image forming apparatus, a ceramic heater driving circuit and a control circuit, a structure diagram of a ceramic surface heater, and a schematic configuration diagram of a fixing device according to a second embodiment of the present invention. The same as 4.

FIG. 7 is a flowchart for explaining a fixing device control sequence by the engine controller according to the second embodiment.
When the engine controller 126 issues a request to start power supply to the ceramic surface heater 109c (S1), the integration cycle counter tise of the integration control is set to 0 (S2). The electric power supplied to the heating elements 3 and 20 is controlled by PI control based on information from the TH signal so that the predetermined temperature set in the engine controller 126 is reached. The target predetermined temperature Tt and the detected temperature Tn from the TH signal are compared (S3), a proportional control value is set based on the difference (S4), and the duty to be supplied is calculated (S5 to S13). The power ratio corresponding to the proportional control is Dp, the power ratio corresponding to the integral control at time t is Di (t), the temperature detected by the temperature detecting means is Tn, the target temperature is Tt, When the control interval is Δt, for example, it is determined by the following equation (3).
D = Dp + Di (t) = Ap × (Tt−Tn) + Di (t−Δt) + ΔDi (t, Tt−Tn)
Dp = 2.5% × (Tt−Tn)
ΔDi (t, Tt−Tn) = + 2.5% (when Tt> Tn continuously for a predetermined time ti)
= −2.5% (when Tt ≦ Tn continuously for a predetermined time ti)
... (3)

  However, Ap is a coefficient in proportional control, and ΔDi (t, Tt−Tn) is an increment of the power ratio Di (t) corresponding to the integral control at time t.

  That is, the engine controller 126 compares the set target temperature Tt and the detected temperature Tn from the TH signal (S3), calculates the power ratio Dp corresponding to proportional control (S4), and Di (corresponding to integral control). t) is calculated (S5 to S13). When the target temperature Tt is compared with the detected temperature Tn and the target temperature Tt is higher than the detected temperature Tn (S5), it is determined whether or not the control temperature Δt was in the same state before (S6). If they are the same, the integration period counter tset is incremented (S11). If the target temperature Tt is lower than the detected temperature Tn in a different state, the state setting is changed to a state where the target temperature Tt is higher than the detected temperature Tn (ip = 1) (S7), and the integration cycle counter tset is reset. (S10).

  Similarly, when the target temperature Tt is equal to or lower than the detected temperature Tn (S5), it is determined whether the state is the same before the control interval Δt (S8). If the target temperature Tt is different from the detected temperature Tn, the state setting is changed to a state where the target temperature Tt is equal to or lower than the detected temperature Tn (ip = 0) (S9), and the integration cycle counter tset is reset. (S10).

  When the integration cycle counter tise has reached the predetermined integration cycle time ti (S12), if ip = 1 (continuous Tt> Tn), 2.5% is added to Di (t), and ip = 0. If so (continuous Tt ≦ Tn), 2.5% is reduced to Di (t) (S13). If the integration cycle counter tise has not reached the predetermined integration cycle time ti (S12), Di (t) is not updated.

  Di (t) corresponding to the integration control is compared with Dlimit-Dc (S23), and if it is larger than Dlimit-Dc, Di (t) = Dlimit-Dc is set (S24). However, Dc is an appropriate correction coefficient.

  The sum of the power ratio Dp corresponding to the proportional control calculated by the above-described calculation and the power ratio Di (t) corresponding to the integral control becomes the power ratio (duty) D based on the PI control equation (S14). If necessary, the harmonic countermeasure is corrected and the power ratio (duty) D is changed (S15).

Then, as compared to the upper limit duty ratio _{D limit} that is calculated in the engine controller 126 sets (S16), the duty D is equal to or larger than the upper limit duty _{D limit,} the charged duty D and the upper limit duty _{D limit} (S17).

  The engine controller 126 sends ON1 and ON2 signals using the ZEROX signal as a trigger at a phase angle α corresponding to the input duty D determined based on Table 1, and power is supplied to the ceramic surface heater 109c for temperature control. Perform (S20).

The current value I in the HCRRT signal notified from the current detection circuit 27 is detected when energization is performed at the above-described charging duty D. In the engine controller 126, an upper limit power duty D _limit that can be energized is calculated from the detected current value I, the input duty D, and a preset energizable current value I _limit (S21). When the current value notified to the engine controller 126 by the current detection circuit 27 is an effective value, D _limit is calculated by the following equation.
D _limit = (I _limit / I1) ² × D1

The current value I _limit is set to an allowable current value that can be supplied to the heater by subtracting the current supplied to a portion other than the heater from the rated current of the connected commercial power source.

  Next, if there is a request to end the temperature control of the heater (S22), the temperature control is ended, and if there is no request, temperature control based on the PI control equation is continued.

FIG. 8 is a schematic diagram of a temperature control waveform of the second embodiment shown in FIG. The waveform of the conventional example is indicated by a suffix (_B). The input duty calculated based on the PI control equation is indicated by Dpi and Dpi_B, and the input duty corresponding to the integral control is indicated by Di and Di_B. Conventionally, when the input duty Dpi calculated based on the PI control equation is equal to or greater than the upper limit duty D _limit and the actual temperature Tn does not reach the target temperature, the input duty Di_B corresponding to the integral control is It gradually increases.

  When the target temperature is lowered in this state, the making duty D cannot be lowered, and the control is performed at a temperature Tn_B higher than the target temperature Tt.

As described above, in the second embodiment, when the duty Di corresponding to the integration is not less than the upper limit duty D _limit -Dc, even if the actual detected temperature Tn is constantly smaller than the target temperature Tt, Update control for increasing the input duty Di corresponding to the integral control is not performed. By this control, even when the target temperature is lowered, it is possible to follow the change and reduce the input duty D. The actual temperature Tn can be controlled to the target temperature Tt by the control of the second embodiment. Even when power is supplied at the upper limit duty D _limit , if the duty Di corresponding to the integral control is extremely small, it can be increased to a predetermined amount, and responsiveness can be improved. . The same control is possible even when there is only one heating element.

  4 is a schematic configuration diagram of an image forming apparatus according to a third embodiment of the present invention, a ceramic heater driving circuit and control circuit, a structural diagram of a ceramic surface heater, and a schematic configuration diagram of a fixing device. It is the same.

FIG. 9 is a flowchart illustrating the control sequence of the fixing device by the engine controller according to the third embodiment.
When the engine controller 126 issues a request to start power supply to the ceramic surface heater 109c (S1), PI is determined based on information from the TH signal so that the predetermined temperature set in the engine controller 126 is reached. The electric power supplied to the heating elements 3 and 20 is controlled by the control. The target predetermined temperature Tt is compared with the detected temperature Tn from the TH signal (S3), the proportional control value is set based on the difference (S4), and the integral control value is set (S25) and supplied. Duty is calculated. The power ratio corresponding to the proportional control is Dp, the power ratio corresponding to the integral control at time t is Di (t), the temperature detected by the temperature detecting means is Tn, and the target temperature is Tt. Then, for example, it is determined by the following equation (4).
D = Dp + Di (t) = Ap × (Tt-Tn) + Ai / ti × Σ t-ti t (Tt-Tn)
... (4)

  Here, Ap is a coefficient in proportional control, Ai is a coefficient in integral control, and ti is an integration period.

  The sum of the power ratio Dp corresponding to the proportional control and the power ratio Di (t) corresponding to the integral control calculated by the above formula (4) becomes the power ratio (duty) D based on the PI control formula (S14). . If necessary, the harmonic countermeasure is corrected and the power ratio (duty) D is changed (S15).

Then, as compared to the upper limit duty ratio _{D limit} that is calculated in the engine controller 126 sets (S16), the duty D is equal to or larger than the upper limit duty _{D limit,} the charged duty D and the upper limit duty _{D limit} (S17). At this time, if the target temperature Tt is higher than the detected temperature Tn (S26), the duty increment Ai / ti × (Tt−Tn) corresponding to the integral control is reduced (S27).

  The engine controller 126 sends ON1 and ON2 signals using the ZEROX signal as a trigger at a phase angle α corresponding to the input duty D determined based on Table 1, and power is supplied to the ceramic surface heater 109c for temperature control. Perform (S20).

The current value I in the HCRRT signal notified from the current detection circuit 27 is detected when energization is performed at the above-described charging duty D. In the engine controller 126, an upper limit power duty D _limit that can be energized is calculated from the detected current value I, the input duty D, and a preset current value I _limit that can be energized (S21). When the current value notified to the engine controller 126 by the current detection circuit 27 is an effective value, D _limit is calculated by the following equation.
D _limit = (I _limit / I1) ² × D1

The current value I _limit is set to an allowable current value that can be supplied to the heater by subtracting the current supplied to a portion other than the heater from the rated current of the connected commercial power source.

  Next, if there is a request to end the temperature control of the heater (S22), the temperature control is ended, and if there is no request, temperature control based on the PI control equation is continued.

As described above, in the third embodiment, when the input duty Dpi calculated based on the PI control equation is equal to or higher than the upper limit duty D _limit , the actual detected temperature Tn is smaller than the target temperature Tt. The update control for increasing the input duty Di corresponding to the integral control is not performed. By this control, even when the target temperature is lowered, it is possible to follow the change and reduce the input duty D. By the control of the third embodiment, the actual temperature Tn can be controlled to the target temperature Tt. Further, detailed integration control is possible by this control, and responsiveness can be improved.

Further, as in the second embodiment, when the duty Di corresponding to the integration is equal to or higher than the upper limit duty D _limit -Dc, even if the actual detected temperature Tn is smaller than the target temperature Tt, the integration control is supported. Even if the update control for increasing the input duty Di to be performed is performed, the same effect can be obtained. The same control is possible even when there is only one heating element.

1 is a schematic configuration diagram of an image forming apparatus using an electrophotographic process according to the present invention. FIG. 2 is a diagram illustrating a driving circuit and a control circuit of a fixing device according to the present invention. It is the schematic of the ceramic heater which is a heating means in this invention. (A), (b) is a schematic block diagram of the fixing device in this invention. FIG. 6 is a flowchart for explaining a fixing device control sequence by the engine controller according to the first embodiment. FIG. 6 is a schematic diagram of a temperature control waveform of fixing control of the fixing device in Embodiment 1 shown in FIG. 5. FIG. 9 is a flowchart for explaining a fixing device control sequence by an engine controller according to a second embodiment. FIG. 8 is a schematic diagram of a temperature control waveform of fixing control of the fixing device in the embodiment 2 illustrated in FIG. 7. FIG. 9 is a flowchart for explaining a fixing device control sequence by an engine controller according to a third embodiment.

Explanation of symbols

24, 109c Ceramic heater 11, 126 Engine controller 21, 109d Temperature detection element 27 for temperature control 27 Current detection circuit 28 Power ratio calculation unit 29 Calculation update stop unit 62, 109b Fixing film 63, 109c Pressure roller 101 Image forming apparatus 109 Thermal fixing device

Claims (10)

Heating means having a heating element that generates heat by supplying power, power supply means for supplying power to the heating means, temperature detection means for detecting the temperature of the heating means, and from the power supply means to the heating means In an image forming method in an image forming apparatus comprising a power control unit that controls a supply amount of power,
The power control means controls the temperature of the heating means by controlling the power supply means with a power ratio that is a ratio to the power when the half wave or full wave of the commercial power supply voltage is fully lit. ,
Comparing the temperature detected by the temperature detection means with a preset target temperature, based on the PI control logical expression, the power ratio corresponding to proportional control and the operation amount corresponding to integral control A power ratio calculation step of calculating a power ratio of the sum with the power ratio;
When the power ratio calculated in the power ratio calculating step is larger than the upper limit power ratio determined by the power control means, or smaller than the lower limit power ratio, the calculation of the power ratio corresponding to the integral control is updated. An image forming method comprising: an operation update stop step that is not performed. Current detection means for detecting the current supplied to the heating means;
Supplying power to the heating means at a first power ratio set in the power control means;
Detecting a current value detected by the current detection means;
Comparing the maximum suppliable current value that can be supplied to the heating means preset in the power control means and the current value detected by the current detection means;
Calculating a maximum suppliable power ratio when supplying electric power to the heating unit based on a comparison between the suppliable maximum suppliable current value and the detected current value;
A power control step for controlling the power supplied by the power supply means to the heating means below the maximum supplyable power ratio;
Calculation update stop step that does not update calculation of the power ratio corresponding to the integral control when the power ratio calculated in the power ratio calculation step is larger than the maximum suppliable power ratio determined in the power control means. The image forming method according to claim 1, further comprising: The calculation of the power ratio corresponding to the integral control is performed by updating the predetermined ratio for a predetermined period when the temperature detected from the temperature detecting means is continuously smaller than the target temperature continuously for a predetermined period. If the temperature continuously detected from the temperature detecting means is always higher than the target temperature, update to reduce the power ratio of a predetermined amount,
When the power ratio calculated by the power ratio calculating step is larger than the maximum suppliable power ratio determined by the power control means, only the update for reducing the power ratio is performed in the calculation of the power ratio corresponding to the integral control. The image forming method according to claim 2, wherein:   The power control means calculates a power ratio supplied to the heating means and determines a phase angle corresponding to the power ratio, and controls the temperature of the heating means by controlling the supplied power by phase control. The image forming method according to claim 2, further comprising a temperature control step. Heating means having a heating element that generates heat by supplying power, power supply means for supplying power to the heating means, temperature detection means for detecting the temperature of the heating means, and from the power supply means to the heating means An image forming apparatus including a power control unit that controls a supply amount of power.
The power control means is
In order to control the power supply means with a power ratio that is a ratio with respect to a predetermined power, the temperature detected from the temperature detection means is compared with a preset target temperature with an operation amount corresponding to proportional control. A power ratio calculation means for calculating a power ratio of the sum of the power ratio and the power ratio which is an operation amount corresponding to integral control;
When the power ratio calculated by the power ratio calculating means is larger than the upper limit power ratio determined by the power control means, or smaller than the lower limit power ratio, the calculation of the power ratio corresponding to the integral control is updated. An image forming apparatus comprising: an operation update stopping unit that does not perform the operation.   The predetermined power is power when a half wave or full wave of a commercial power supply voltage is fully lit, and the power ratio calculation means calculates a power ratio based on a PI control logical expression. The image forming apparatus according to claim 5. Current detection means for detecting the current supplied to the heating means;
The power control means is
Power is supplied to the heating unit at a first power ratio set in the power control unit, a current value detected from the current detection unit is detected, and the heating set in the power control unit in advance is detected. A maximum supplyable power ratio in the case of supplying power to the heating means, compared with a maximum supplyable current value that can be supplied to the means, and the power supply means is less than the maximum supplyable power ratio and the power supply means Controlling the power supplied to the means,
When the power ratio calculated by the power ratio calculating means is larger than the maximum suppliable power ratio determined by the power control means, calculation update stopping means that does not update the calculation of the power ratio corresponding to the integral control is provided. The image forming apparatus according to claim 5, wherein the image forming apparatus is provided. The calculation of the power ratio corresponding to the integral control is updated by adding a predetermined amount of power ratio when the temperature detected from the temperature detecting means is always lower than the target temperature continuously for a predetermined period. Then, if the temperature detected from the temperature detection means is always higher than the target temperature, update to reduce the power ratio of a predetermined amount,
When the power ratio calculated by the power ratio calculation means is larger than the maximum suppliable power ratio determined by the power control means, only the update to reduce the power ratio is performed in the calculation of the power ratio corresponding to the integral control. The image forming apparatus according to claim 7, further comprising an update unit that performs the update.   The power control means calculates a power ratio supplied to the heating means, determines a phase angle corresponding to the power ratio, and controls the temperature of the heating means by controlling the supplied power by phase control. The image forming apparatus according to claim 7 or 8, wherein The heating means includes an insulating substrate and one or more heating elements formed on one or both surfaces of the insulating substrate;
A film that slides with the heating means;
A rotatable pressure member that is pressed to form a nip portion with the heating means via the film;
A fixing device that fixes a recording medium carrying an unfixed image to the recording medium by heating the heating element while passing through the nip portion. An image forming apparatus according to claim 1.

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