JP2012252258A - Fixing device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fixing device that can shorten a start time of first print while suppressing the occurrence of overshoot more than a conventional one, and an image forming apparatus using the fixing device.SOLUTION: In a warm-up period of time for heating a heating roller 51 to a fixing temperature (150°C), a control part 60 performs first power control for supplying first power P1 (1,350 W) until the surface temperature of the heating roller 51 reaches a predetermined temperature (140°C) lower than the fixing temperature and, until the surface temperature of the heating roller 51 reaches the fixing temperature after reaching the predetermined temperature, the control part 60 performs second power control for supplying second power P2 corrected and determined according to a surface temperature Ta1 of the heating roller 51 and an ambient temperature Tb1 thereof at the start of warm up and a rise gradient Ka of the surface temperature of the heating roller 51 and a rise gradient Kb of the ambient temperature thereof for a predetermined period of time (2 s) during the first power control.

Description

  The present invention relates to a fixing device and an image forming apparatus using the same, and more particularly to a temperature control technique of the fixing device.

2. Description of the Related Art Image forming apparatuses such as printers and copiers include a fixing device that heats and pressurizes a toner image on a recording sheet with a heating roller.
Usually, the heating roller is heated by a heat source such as a halogen heater, and its temperature is controlled by the control unit so as to reach a predetermined fixing temperature.
FIG. 13 is a graph showing the transition of the surface temperature of the heating roller by controlling the temperature of the heating roller in a conventional fixing device, the horizontal axis is the elapsed time from the start of power supply to the heat source, and the vertical axis is the heating roller. The surface temperature of is shown.

In the conventional example shown in FIG. 13, during the warm-up period in which the surface temperature of the heating roller is increased to the fixing temperature, constant power control for supplying a constant power to the heat source is executed, and the surface temperature of the heating roller reaches the fixing temperature. Therefore, a configuration is adopted in which the fixing temperature is maintained by switching to PID control.
In the constant power control here, in order to make the warm-up period as short as possible, it is common to use as much power as possible. For this reason, when switching from constant power control to PID control, overshoot (such as a portion indicated by symbol a) occurs due to residual heat.

Once such overshoot occurs, an undershoot occurs this time in an attempt to correct it, and these alternately occur and the surface temperature of the heating roller is unstable for a while (see section b). . Therefore, it takes time to converge to the target fixing temperature, and the waiting time until the start of the first page print (first print) becomes longer.
In particular, in the warm-up from the standby state, the inside of the fixing device has been warmed to some extent. Therefore, as with the warm-up from the power ON, if large power is continuously supplied until the fixing temperature is reached, overshoot due to residual heat will occur. The possibility is even higher.

  Therefore, in order to suppress the occurrence of overshoot, as shown in FIG. 14, a fixing device configured to switch from constant power control to PID control at a predetermined temperature lower than the fixing temperature has been proposed (for example, a patent). Reference 1).

JP 2009-31637 A JP 2005-257945 A Japanese Patent Laid-Open No. 2004-70041

In the example of temperature control shown in FIG. 14, since switching to PID control is performed earlier at a predetermined temperature lower than the fixing temperature, it is possible to surely prevent the occurrence of overshoot.
However, in PID control, the rate of temperature increase slows down as the target fixing temperature is approached (see the portion indicated by symbol c in FIG. 14), so the time until the fixing temperature is reached is slow. It is difficult to shorten the first print start time.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a fixing device and an image forming apparatus using the same that can suppress the occurrence of overshoot and can shorten the first print start time as compared with the conventional one.

  In order to achieve the above object, a fixing device according to the present invention forms a fixing nip by pressing a pressing member on the peripheral surface of a heating rotator, and passes a recording sheet carrying a toner image in the fixing nip. A fixing unit for fixing the heating rotary member by receiving power supply, a power supply unit for supplying power to the heating unit, and a first temperature detecting unit for detecting a surface temperature of the heating rotary member. Temperature detection means, second temperature detection means for detecting the ambient temperature around the heating rotator, and the power supply means so that the detected surface temperature of the heating rotator becomes a fixing temperature necessary for fixing. Control means for controlling heating by the heating means via the heating means, wherein the control means has a surface temperature of the heating rotator that is higher than the fixing temperature during a warm-up period in which the heating rotator is heated to a fixing temperature. Low Until the predetermined temperature is reached, the first power control is performed to control the power supply means to supply the first power to the heating means, and after the surface temperature of the heating rotator becomes the predetermined temperature, Until the fixing temperature is reached, the detection result of the second temperature detection means at the start of warm-up and the change of the detection result of the second temperature detection means during a predetermined period when the first power control is executed. The second power control for controlling the power supply means to supply the second power determined according to the rate to the heating means is performed.

  According to the above configuration, in the warm-up period, the first power control for controlling the first power to be supplied to the heating unit until the surface temperature of the heating rotator becomes a predetermined temperature lower than the fixing temperature. After the surface temperature of the heating rotator reaches the predetermined temperature, the second power control is executed until the fixing temperature is reached until the second power is supplied to the heating means. The second electric power is provided with second temperature detecting means for detecting the ambient temperature around the heating rotator, and the detection result of the second temperature detecting means at the start of warm-up and the first temperature control time. It is determined according to the rate of change of the detection result of the second temperature detection means during the predetermined period.

  The temperature control of the heating rotator during warm-up is considered to be greatly influenced by the heat storage state inside the fixing device, and the heat storage state can be estimated from the ambient temperature around the heating rotator. By determining the second power based on the detection result of the temperature detecting means 2, it is possible to supply appropriate power in consideration of the heat storage state of the heating rotator, and the power supplied during warm-up is too large. Shortening the first print start time without overshooting or when the power is switched to PID control immediately before, so that the supply power becomes too small and the fixing temperature is not delayed more than necessary. Can be achieved.

In addition, the control means decreases the second power as the detection result of the second temperature detection means at the start of warm-up is higher and as the change rate of the detection result of the second temperature detection means during the predetermined period is larger. It is desirable to control so that it becomes.
Further, when determining the second power, the control means further refers to the detection result of the first temperature detection means at the start of warm-up and the rate of change of the detection result of the first temperature detection means during a predetermined period. It is desirable to determine the second power.

  In addition, a sheet detection unit that detects that the recording sheet passes through the fixing nip is provided, and the control unit performs a period from the completion of warm-up until the first recording sheet is detected in the fixing nip by the sheet detection unit. Control of the power supply means by PID control for maintaining the surface temperature of the heating rotator at the fixing temperature is executed, and while the first recording sheet passes through the fixing nip, the second temperature detection means at the start of warm-up And a third power control for controlling the power supply means to supply the heating means with the third power determined in accordance with the detection result and the rate of change of the detection result of the second temperature detection means during the predetermined period. It is desirable to execute.

Further, the control means increases the third detection value as the detection results of the first and second temperature detection means at the start of warm-up are higher and the change rate of the detection result of the second temperature detection means during the predetermined period increases. It is desirable to have low power.
Further, when determining the third power, the control means further refers to the detection result of the first temperature detection means at the start of warm-up and the rate of change of the detection result of the first temperature detection means during a predetermined period. It is desirable to determine the third power.

The heating rotator preferably has a configuration in which a heat generating layer is included in the outer peripheral portion thereof.
The invention of the present application may be an image forming apparatus provided with the fixing device having the above-described configuration, whereby the same effects as those of the fixing device having the above-described configuration can be obtained.

1 is a schematic diagram illustrating a configuration of a printer according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view illustrating a main part of a fixing unit in the printer. It is a figure explaining the positional relationship of the temperature sensor which a fixing part has. It is a block diagram which shows the relationship between the structure of a control part, and the main component used as the control object by a control part. It is a figure which shows the specific example of the temperature control by a control part. It is a flowchart which shows the operation | movement of the temperature control of the heating roller by a control part. It is a flowchart which shows the subroutine of a warm-up process. 6 is a flowchart illustrating a subroutine of power control processing when executing a first print job. (A) is a figure which shows the correction factor table h1, (b) is a figure which shows the correction factor table h2. (A) is a figure which shows the correction factor table h3, (b) is a figure which shows the correction factor table h4. It is a figure which shows the temperature control in the fixing device which concerns on a prior art example. It is a figure which shows the other specific example of the temperature control by a control part. It is a figure which shows the temperature control in the fixing device which concerns on a prior art example. It is a figure which shows the temperature control in the fixing device which concerns on a prior art example.

<Embodiment>
Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described with reference to the drawings by taking a tandem color printer (hereinafter simply referred to as “printer”) as an example.
FIG. 1 is a schematic diagram illustrating a configuration of a printer according to an embodiment of the present invention.
As shown in FIG. 1, the printer 1 includes an image processing unit 3, a paper feeding unit 4, a fixing unit 5, a paper sensor 7, and a control unit 60. When this printer 1 is connected to a network (for example, a LAN) and receives a print job execution instruction from an external terminal device, it forms toner images of yellow, magenta, cyan, and black colors based on the instruction. Then, these are multiplex-transferred to form a color image.

Hereinafter, the reproduction colors of cyan, magenta, yellow, and black are represented as C, M, Y, and K, and the C, M, Y, and K are added as subscripts to the numbers of the components related to the reproduction colors.
The image processing unit 3 includes image forming units 3Y, 3M, 3C, and 3K, an exposure unit 10, an intermediate transfer belt 11, and the like.
The image forming unit 3Y includes a photosensitive drum 31Y, a charger 32Y, a developing unit 33Y, a primary transfer roller 34Y, a cleaner 35Y for cleaning the photosensitive drum 31Y, and the like disposed around the photosensitive drum 31Y. A Y-color toner image is formed on the body drum 31Y. The other image forming units 3M to 3K have a configuration such as the chargers 32M to 32K as in the image forming unit 3Y except that the color of toner is different. Omitted.

The intermediate transfer belt 11 is an endless belt, is stretched around a driving roller 12 and a driven roller 13, and is driven to rotate in the direction of arrow A.
The exposure unit 10 includes a light emitting element such as a laser diode, emits a laser beam L for image formation of Y to K colors by a drive signal from the control unit 60, exposes and scans the photosensitive drums 31Y to 31K, and is charged. Electrostatic latent images are formed on the photosensitive drums 31Y to 31K charged by the devices 32Y to 32K.

On the intermediate transfer belt 11, the toner images formed in the respective image forming units are transferred in a multiple manner.
The paper feeding unit 4 includes a paper feeding cassette 41 that stores the recording sheets P, a feeding roller 42 that feeds the recording sheets P in the paper feeding cassette 41 one by one onto the conveyance path 43, and two feeding of the fed recording sheets P. A timing roller pair 44 and the like for taking the timing of feeding to the next transfer position 46 are provided. The toner image on the intermediate transfer belt 11 is secondarily transferred to the recording sheet P fed from the paper feeding unit 4 by the action of electrostatic force by the secondary transfer roller 45 at the secondary transfer position 46. The recording sheet P that has passed the secondary transfer position 46 is further conveyed to the fixing unit 5, and the toner image (unfixed image) on the recording sheet P is fixed to the recording sheet P by heating and pressing in the fixing unit 5. After that, the sheet is discharged onto the discharge tray 72 via the discharge roller pair 71.

The paper sensor 7 is an optical sensor that detects the passage of the recording sheet P conveyed to the fixing unit 5.
The control unit 60 controls the image processing unit 3, the sheet feeding unit 4, and the fixing unit 5, and details thereof will be described later.
In the present embodiment, the system speed of the printer 1 is 280 [mm / sec], and the print speed is 60 [sheets / min].
<Fixing part>
FIG. 2 is a schematic cross-sectional view showing the main part of the fixing unit 5.

As shown in FIG. 2, the fixing unit 5 includes a heating roller 51 as a heating rotator, a pressure roller 52, a magnetic flux generation unit 53, and temperature sensors s <b> 1 and s <b> 2 inside the housing 59.
The heating roller 51 is formed by forming an elastic layer 512 on the peripheral surface of a cylindrical cored bar 511, and a fixing belt 550 is further attached to the outer peripheral surface of the elastic layer 512. Both ends in the axial direction of the cored bar 511 are rotatably supported by a bearing portion (not shown) provided in the housing 59.

The fixing belt 550 has a three-layer structure including an electromagnetic induction heat generating layer 513, an elastic body layer 514, and a release layer 515.
The electromagnetic induction heat generating layer 513 is made of nickel or the like, and generates heat by the magnetic flux generated from the magnetic flux generating unit 53. The elastic body layer 514 is an elastic member made of heat-resistant silicone rubber or the like, and plays a role of improving the adhesion between the recording sheet P and the surface of the heating roller 51. The release layer 515 is made of PFA (tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer) or the like and plays a role of improving the release property of the surface of the heating roller.

The pressure roller 52 is formed by laminating an elastic body layer 522 and a release layer 523 in this order on the peripheral surface of a cylindrical cored bar 521, and is pressed against the heating roller 51 by a pressing means such as a spring (not shown). ing. Thus, the fixing nip portion N is formed between the heating roller 51 and the heating roller 51.
In addition, the pressure roller 52 is rotatably supported by a bearing portion (not shown) provided on the housing 59 at both ends in the axial direction of the core metal 521, and uses a motor (not shown) as a power source, such as a gear, a belt, or the like. It is rotationally driven in the direction of arrow C through the power transmission mechanism. The heating roller 51 is driven to rotate in the direction of arrow B following the rotation of the pressure roller 52.

The magnetic flux generator 53 includes an electromagnetic induction coil 531, a core 532, a coil bobbin 533, and a cover 534, and is arranged along the axial direction of the heating roller 51.
The coil bobbin 533 has a portion facing the surface of the heating roller 51 curved in an arc shape along the circumferential direction of the heating roller, and a predetermined interval, for example, 3 mm, is provided between the coil bobbin 533 and the circumferential surface of the heating roller 51. Is fixed.

The electromagnetic induction coil 531 is wound around the coil bobbin 533 so as to extend along the width direction of the recording sheet P and to have a circular cross section. The electromagnetic induction coil 531 is supplied with high frequency power and generates an alternating magnetic flux for generating heat in the electromagnetic induction heat generating layer 513 of the heating roller 51.
Each of the cores 532 is made of high-permeability ferrite or the like, and efficiently guides the magnetic flux generated by the electromagnetic induction coil 531 to the heating roller 51.

The magnetic flux guided to the surface of the heating roller 51 passes mainly through a portion of the electromagnetic induction heat generating layer 513 of the heating roller 51 that faces the magnetic flux generating portion 53, and an eddy current is generated in this portion to generate the electromagnetic induction heat generating layer 513. Induction heating. Thereby, the heating roller 51 is heated.
The temperature sensor s <b> 1 is made of a contact thermistor and is provided in contact with the surface of the heating roller 51. The temperature sensor s 1 detects the surface temperature of the heating roller 51 and outputs it to the control unit 60.

  On the other hand, the temperature sensor s2 is composed of a non-contact thermistor and is disposed at a position farther from the heating roller 51 than the temperature sensor s1. This temperature sensor s2 is for estimating the warming state inside the fixing portion 5 due to heat radiation from the heating roller 51, that is, the heat storage state, and detects the ambient temperature around the heating roller 51 (hereinafter referred to as the ambient temperature). To the control unit 60.

The temperature sensor s2 is provided at a position away from the surface of the heating roller 51 by about 1.8 to 5.0 [mm] so that the temperature influence of the heating roller 51 is not too strong or too weak. .
The controller 60 controls the temperature of the heating roller 51 described later based on the temperature information from the temperature sensors s1 and s2.

Further, as shown in FIG. 3, the temperature sensor s <b> 1 is arranged at the approximate center in the rotation axis J direction of the heating roller 51, the temperature sensor s <b> 2 is arranged at a position close to the temperature sensor s <b> 1, and each is a housing 59. Is held by a frame (not shown).
<Control unit>
FIG. 4 is a block diagram illustrating a relationship between the configuration of the control unit 60 and main components that are controlled by the control unit 60.

As shown in FIG. 6, the control unit 60 includes a CPU (Central Processing Unit) 601, a ROM (Read Only Memory) 602, a RAM (Random Access Memory) 603, a communication interface (I / F) unit 604, and an image data storage unit. 605 and the like.
The CPU 601 executes a program for controlling the image processing unit 3, the paper feeding unit 4, the fixing unit 5, the high frequency inverter 6, the paper sensor 7, the operation panel 8, and the like. The ROM 602 is a storage that stores various programs executed by the CPU 601. The RAM 603 is a work area when the CPU 601 executes a program. The communication I / F unit 604 is an interface for connecting to a LAN such as a LAN card or a LAN board. The image data storage unit 605 stores image data for printing input via the communication I / F unit 604 or an image reading unit (not shown).

  The fixing unit 5 includes a current detection unit 54 that detects a current supplied to the electromagnetic induction coil 531 of the magnetic flux generation unit 53 and a voltage detection unit 55 that detects a voltage. The current detection unit 54 outputs a signal indicating the magnitude of the detected current to the control unit 60. The voltage detection unit 55 outputs a signal indicating the magnitude of the detected voltage to the control unit 60. The control unit 60 obtains the power value supplied to the electromagnetic induction coil 531 from the signals output from the current detection unit 54 and the voltage detection unit 55.

The high frequency inverter 6 outputs high frequency power (for example, 30 [kHz], 100 to 1500 [W]) supplied to the electromagnetic induction coil 531. The control unit 60 controls the high-frequency inverter 6 to change the amount of electric power supplied to the electromagnetic induction coil 531 and perform temperature control so that the surface temperature of the heating roller 51 approaches the fixing temperature.
The sheet sensor 7 outputs a Hi level signal to the control unit 60 while detecting the recording sheet P passing the detection position D (see FIG. 2), and outputs a Low level signal while not detecting it. Output to the controller 60. The control unit 60 recognizes the rising edge (change point from Low level to Hi level) and the falling edge (change point from Hi level to Low level) of the output of the paper sensor 7, so It is detected that the end passes the detection position D.

The operation panel 8 is disposed at an easy-to-operate position on the upper portion of the printer 1 and includes a liquid crystal display for displaying a print setting operation screen and a print result, a touch panel stacked on the liquid crystal display, operation buttons, and the like. The control unit 60 can accept various instructions from the user via the operation panel 8, and causes the corresponding program to be executed.
<Temperature control of heating roller>
Next, an outline of temperature control of the heating roller 51 executed by the control unit 60 will be described with reference to FIG.

FIG. 5 is a diagram showing a specific example of temperature control by the control unit 60, and shows the relationship between the power supplied to the electromagnetic induction coil 531 and the surface temperature of the heating roller 51.
As shown in FIG. 5, the control unit 60 sets the heating roller 51 to a predetermined temperature (140 [° C.] in this example) lower than the fixing temperature in the warm-up period in which the heating roller 51 is raised to the fixing temperature (150 [° C.]). Until this time, the first power control for supplying the first power P1 to the electromagnetic induction coil 531 is executed, and after reaching a predetermined temperature, the electromagnetic induction coil 531 has a second power until the fixing temperature is reached. Second power control for supplying power P2 is executed.

After the warm-up is completed, the process shifts to PID control for maintaining the heating roller 51 at the fixing temperature. In addition, the control unit 60 temporarily supplies the third power P3 to the electromagnetic induction coil 531 while the first recording sheet P passes through the fixing nip N after the PID control transition. Execute power control.
The first electric power P1 supplied in the first electric power control is a large electric power as long as possible in the specifications of the fixing unit 5 so that the temperature increase rate of the heating roller 51 can be increased and the warm-up period can be shortened. Set to a value.

On the other hand, if the second power P2 of the second power control is too large, there is a high possibility that overshoot will occur. Conversely, if the second power P2 is too small, the temperature rise rate becomes too slow and the warm-up period becomes long. Since there is a fear, it is necessary to set an optimal power value so that the warm-up period can be suppressed from being prolonged while suppressing the overshoot.
Even with the same power value, overshoot may occur depending on the heat storage state. For example, in the warm-up from the standby state, since the inside of the fixing unit 5 is warmed to some extent, the heating roller 51 is more likely to be heated than the case where the power is turned on in the morning and the warm-up is started. Is likely to occur. Therefore, the magnitude of the second electric power P2 is preferably determined in consideration of the heat storage state inside the fixing unit 5. A method for determining the second power P2 will be described later.

The third power P3 of the third power control temporarily suppresses the heating of the heating roller 51 in order to suppress the heat from being absorbed by the passed recording sheet P and the temperature of the heating roller 51 from greatly decreasing. The power value is set to a value that increases.
Note that the amount of change in the temperature of the heating roller 51 when the heat is absorbed by the recording sheet P varies depending on the heat storage state inside the fixing unit 5 including the heating roller 51, so the magnitude of the third power P <b> 3 is set. Also in the determination, it is preferable to determine in consideration of the heat storage state inside the fixing unit 5. A method for determining the third power P3 will also be described later.
(Temperature control operation)
FIG. 6 is a flowchart showing the contents of the temperature control of the heating roller 51 by the control unit 60.

As shown in FIG. 6, when the printer 1 is turned on (step S101), the control unit 60 performs a warm-up process (step S102).
《Warm-up process》
FIG. 7 shows a subroutine for warm-up processing.
As shown in FIG. 7, the control unit 60 first obtains the surface temperature Ta1 and the ambient temperature Tb1 of the heating roller 51 at the start of warm-up from the temperature sensors s1 and s2 (step S201) and stores them in the RAM 603. To do.

Next, time measurement is started (step S202), and first power control for supplying the first power P1 (here, 1350 [W]) to the electromagnetic induction coil 531 is executed (step S203).
Until the measurement time passes a predetermined period (2 [seconds]) or until the surface temperature of the heating roller 51 reaches a predetermined temperature (140 [° C.]) or more (step S204: No), the first of step S203 Maintain power control.

  If the answer is Yes in step S204, the control unit 60 acquires the surface temperature Ta2 and the ambient temperature Tb2 of the heating roller 51 at this time from the temperature sensors s1 and s2, and sets the measurement time as Tm. From the equation (2), the rising gradient Ka of the surface temperature of the heating roller 51 and the rising gradient Kb of the ambient temperature from the warm-up start time are calculated (step S205).

Ka = (Ta2-Ta1) / Tm (1)
Kb = (Tb2-Tb1) / Tm (2)
The rising gradients Ka and Kb calculated in this way are stored in the RAM 603.
The time (predetermined period) for measuring the rising temperature of the surface temperature and the ambient temperature of the heating roller 51 immediately after the start of warm-up is 2 seconds in this embodiment, but it depends on the specifications of the fixing unit and the like. It can be set appropriately. In addition, the “predetermined temperature” here is a temperature set as a timing for switching from the first power control to the second power control. If this temperature is close to the fixing temperature, the temperature is over after the warm-up is completed. If the possibility of shooting is high and the distance is too far, the timing for switching to the second power control is too early and the warm-up time cannot be shortened, so about 5 [° C.] to 20 [° C.] above the fixing temperature. It is desirable to set it to a low temperature.

Next, the control unit 60 determines the second power P2.
In the present embodiment, 90 [%] (1215 [W]) of the first power P1 is set as the initial value of the second power P2, and the heating roller 51 is set to the initial value of the second power P2. A method of determining the second electric power P2 by correcting based on the surface temperature Ta1 and the rising gradient Ka of the surface temperature, the ambient temperature Tb1 and the rising gradient Kb of the ambient temperature is adopted. In the correction, correction rate tables h1 and h2 shown in FIG. 9 are used.

Here, the correction rate tables h1 and h2 will be described briefly.
The correction rate table h1 in FIG. 9A shows the correlation between the detection result by the temperature sensor s1 that detects the surface temperature of the heating roller 51 and the power correction rate. The surface temperature Ta1 of the heating roller 51 is shown, and the row shows the rising gradient Ka of the surface temperature.

  In this correction factor table h1, the correction factor is set such that the higher the surface temperature Ta1 of the heating roller 51, the smaller the second power P2. This is because it can be estimated that the higher the surface temperature Ta1 before heating, that is, the higher the surface temperature Ta1 before heating, the greater the amount of heat stored in the heating roller 51. In this case, the heating roller 51 is heated by heating. This is because it is easy to do.

  In addition, the correction factor is set so that the second power P2 decreases as the rising gradient Ka of the surface temperature increases. This indicates that when the heating amount of the heating roller 51 is the same, the larger the rising gradient Ka, the smaller the heat radiation amount of the heating roller 51, and from this it can be estimated that the inside of the heating roller 51 and its surroundings are warmed. In this case, the temperature of the heating roller 51 is easily raised, so that the power to be supplied is reduced.

Similarly, the correction rate table h2 in FIG. 9B shows the correlation between the detection result by the temperature sensor s2 that detects the ambient temperature in the fixing unit (the temperature around the heating roller 51) and the power correction rate. The vertical row shows the ambient temperature Tb1 of the heating roller 51 at the start of warm-up, and the horizontal row shows the rising gradient Kb of the ambient temperature.
Since the temperature rise around the heating roller 51 is smaller than that of the surface of the heating roller 51, the range of the rising gradient Kb in the row is narrower than the range of the rising gradient Ka in FIG.

In the correction rate table h2, the correction rate is set such that the higher the ambient temperature Tb1 of the heating roller 51, that is, the warmer the inside of the fixing unit 5, the smaller the second power P2. Further, the correction rate is set so that the second power P2 decreases as the ambient temperature rise gradient Kb increases.
This is because the temperature inside the fixing unit 5 is more likely to be raised as the rising gradient Kb is larger. Therefore, it is predicted that the inside of the fixing unit 5 is heated immediately by heating the heating roller 51. This is because the temperature of the heating roller 51 is easily increased as the inside warms up.

Such correction factor tables h1 and h2 are obtained by the present inventors through a simulation using an actual printer 1 according to the present embodiment.
Returning to FIG. 7, the control unit 60 acquires a correction rate (hereinafter referred to as a correction rate r1) corresponding to the surface temperature Ta1 and the rising gradient Ka from the correction rate table h1, and the ambient temperature Tb1 from the correction rate table h2. Then, a correction rate corresponding to the rising gradient Kb (hereinafter referred to as a correction rate r2) is acquired (step S206).

Then, the second power P2 is corrected and determined by the acquired correction factors r1 and r2 (step S207). Specifically, using the sum of the correction rate r1 and the correction rate r2 multiplied by a value calculated by multiplying the correction constant α (α ≧ 1) set based on the specification of the fixing unit 5, The second power P2 is corrected by the following equations (3) and (4).
Correction power = 1215 [W] × (r1 + (r2 × α)) (3)
P2 = 1215 [W] + correction power (4)
The correction constant α is set such that the easiness of warming inside the fixing unit 5 is specifically estimated from, for example, the heat capacities of the pressure roller 52 and the housing 59, and becomes larger as the heat becomes easier. By multiplying the correction constant α thus set by the correction rate r2, the correction power calculated based on the ambient temperature Tb1 and the rising gradient Kb is finely adjusted, so that overshoot occurs after the warm-up is completed. I try to suppress it.

The correction constant α is set to 1.05 in the present embodiment.
Thereafter, if the surface temperature of the heating roller 51 is not 140 [° C.] or higher (step S208: No), the first power control is maintained until it reaches 140 [° C.] (step S209). If the temperature is 140 [° C.] or higher (step S208: Yes), second power control for supplying the second power P2 determined in step S207 to the electromagnetic induction coil 531 is executed (step S210). This second power control is maintained until the surface temperature of the heating roller 51 reaches 150 [° C.] or higher (step S211: No).

If the surface temperature of the heating roller 51 reaches 150 [° C.] or higher (step S211: Yes), the warm-up process is terminated and the process returns to the flowchart of FIG.
Returning to FIG. 6, after the warm-up process in step S102, the control unit 60 executes PID control (step S103).
If there is a print job (step S104: Yes), the control unit 60 performs power control processing when the first print job is executed (step S105). Here, if there is no print job (step S104: No), the printer 1 is put into a standby state (step S110). In this standby state, power supply to the electromagnetic induction coil 531 is stopped. While there is no print job (step S111: No), this standby state is maintained, and if a print job is accepted (step S111: Yes), the control unit 60 returns to step S102 and starts warm-up processing.
<< Power control processing during the first print job execution >>
FIG. 8 shows a subroutine of power control processing when the first print job is executed.

  As shown in FIG. 8, the control unit 60 maintains the PID control until the first recording sheet P is passed through the fixing nip N (step S302: No) (step S301). Here, the “first recording sheet P” means the first recording sheet P after the warm-up is completed. In step S302, whether or not the first recording sheet P is passed through the fixing nip N can be determined using the output information of the paper sensor 7 as follows.

  As described above, the control unit 60 can detect that the leading edge of the recording sheet P passes the detection position D of the paper sensor 7 by recognizing the rising edge of the output of the paper sensor 7. Further, the time (t1) until the leading edge of the recording sheet P reaches the fixing nip N from the detection position D is equal to the conveyance distance L1 (see FIG. 2) from the detection position D to immediately before the fixing nip N and the recording sheet P. Can be obtained from the transport speed v. Accordingly, the control unit 60 checks whether the time (t1) has elapsed from the time when the leading edge of the first recording sheet P has passed the detection position D, whereby the first recording sheet P is detected. It is possible to determine whether or not the paper has been passed.

When it is determined that the first recording sheet P has been passed (step S302: Yes), the control unit 60 determines from the output signal from the current detection unit 54 and the output signal from the voltage detection unit 55. The obtained current power value is set to the third power P3 (step S303).
Next, the controller 60 controls the third electric power P3 so as to temporarily increase the heating of the heating roller 51 according to the surface temperature Ta1 and the rising gradient Ka of the heating roller 51 and the ambient temperature Tb1 and the rising gradient Kb. Correct. In this correction, the correction factor is acquired from the correction factor tables h3 and h4 shown in FIG. 10 and used.

The correction rate table h3 in FIG. 10A shows the correction rate corresponding to the surface temperature Ta1 and the rising gradient Ka of the heating roller 51, and the correction rate table h4 in FIG. The correction factors corresponding to the ambient temperature Tb1 and the rising gradient Kb are shown.
In the correction rate table h3, the correction rate is set such that the higher the surface temperature Ta1 of the heating roller 51 and the higher the surface temperature increase gradient Ka, the smaller the second power P2. Also in the correction rate table h4, the correction rate is set such that the higher the ambient temperature Tb1 of the heating roller 51 is, the greater the ambient temperature rising gradient Kb is, and the second power P2 is smaller.

  When the correction factors in the correction factor tables h3 and h4 are compared, the correction factor table h4 has a larger value as a whole. This is because the surface temperature of the heating roller 51 has already become substantially the fixing temperature (150 [° C.]), and a part of the heat absorbed by the recording sheet P passed through the fixing nip N is part of the fixing unit 5. This is because, as the amount of heat released increases, the heat of the heating roller 51 is absorbed by the recording sheet P, and the temperature drop becomes more significant. Therefore, it is more important to consider the heat storage state inside the fixing unit 5. .

The correction rate tables h3 and h4 are also obtained by the present inventors through simulation using the actual printer 1 according to the present embodiment, similarly to the correction rate tables h1 and h2.
Returning to FIG. 8, the control unit 60 obtains a correction rate (hereinafter referred to as a correction rate r3) corresponding to the surface temperature Ta1 and the rising gradient Ka from the correction rate table h3, and the ambient temperature Tb1 from the correction rate table h4. Then, a correction rate corresponding to the rising gradient Kb (hereinafter referred to as a correction rate r4) is acquired (step S304).

Then, the third power P3 is corrected and determined by the acquired correction factors r3 and r4 (step S305). Specifically, using the sum obtained by adding the correction rate r3 and the correction rate r4 and the value calculated by multiplying the correction constant α, the following equation (5), (6) The power P3 is corrected.
Correction power = P3 × (r3 + (r4 × α)) before correction (5)
P3 = P3 before correction + correction power (6)
Here, P3 before correction is the current power value set in step S303.

Also, here, the correction constant α is multiplied by the correction rate r4, whereby the correction power calculated based on the ambient temperature Tb1 and the rising gradient Kb is finely adjusted to heat the first recording sheet P. Although the heat of the roller 51 is absorbed, the temperature is largely prevented from decreasing (undershoot).
Thereafter, third power control for supplying the third power P3 determined in step S305 to the electromagnetic induction coil 531 is executed (step S306).

  Then, until the first recording sheet P passes through the fixing nip portion N (step S307: No), the third power control in step S306 is maintained, and if it passes (step S307: Yes), PID control is performed. Return (step S308). In step S307, whether or not the first recording sheet P has passed through the fixing nip N can be determined using the output information of the sheet sensor 7 as follows.

  The controller 60 can detect that the trailing edge of the recording sheet P passes the detection position D of the paper sensor 7 by recognizing the falling edge of the output of the paper sensor 7. Also, the time (t2) until the trailing edge of the recording sheet P reaches the fixing nip N from the detection position D, the transport distance L2 (see FIG. 2) from the detection position D to immediately after the fixing nip N, and the recording sheet. It can be obtained from the transport speed v of P. Accordingly, the control unit 60 checks whether or not the time (t2) has elapsed since the trailing edge of the first recording sheet P has passed the detection position D, thereby determining the first recording sheet P. Can be determined whether or not.

If the print job is finished (step S309: Yes), the power control process at the time of executing the first print job is finished, and the process returns to the flowchart of FIG.
Returning to FIG. 6, after the power control process at the time of executing the first print job in step S105, if there is no other print job (step S106: No), the printer 1 is set in the standby state (step S110).

If there is another print job (step S106: Yes), PID control is maintained (step S107) until the print job is completed (step S108: No).
Steps subsequent to step S106 are continued until the printer 1 is turned off (step S109: No).
If a trouble such as a jam occurs during the execution of the print job and the printing operation is interrupted, the control unit 60 returns to step S102 and starts the warm-up process after the trouble is solved.
<Specific examples and effects of temperature control>
Next, two specific examples of temperature control in the present embodiment will be described.

First, FIG. 5 is a specific example when the printer 1 is turned on in the morning and the warm-up is started.
In the specific example of FIG. 5, the surface temperature Ta1 of the heating roller 51 at the start of warm-up is 23 [° C.], and the surface temperature Ta2 after 2 [seconds] is 73 [° C.]. Therefore, the rising gradient Ka of the surface temperature is 25 [° C./second].

Although not shown, the ambient temperature Tb1 of the heating roller 51 at the start of warm-up is 23 [° C.], the ambient temperature Tb2 after 2 [seconds] is 35 [° C.], and the ambient temperature rise gradient Kb Is 6 [° C./sec].
From these, the second power P2 is determined as follows.
First, the correction factor r1 is a correction factor (−5 [%] corresponding to the surface temperature Ta1 (23 [° C.]) and the rising gradient Ka (25 [° C./second]) in the correction factor table h1 of FIG. ]). The correction rate r2 is a correction rate (−10 [%) corresponding to the surface temperature Tb1 (23 [° C.]) and the rising gradient Kb (6 [° C./sec]) in the correction rate table h2 of FIG. 9B. ]).

The sum of the acquired correction factor r1 and the value calculated by multiplying the correction factor r2 by the correction constant α (1.05) is −15.5 [%], and the correction power is obtained (−188 [- W] = 1215 [W] × (−15.5 [%])). It is determined by correcting the second power P2 with the calculated correction power (1027 [W] = 1215 [W] -188 [W]).
In this specific example, after the surface temperature of the heating roller 51 reaches a predetermined temperature (140 [° C.]) and until the fixing temperature (150 [° C.]) is reached, the second power P2 thus determined is determined. As shown in FIG. 5, the occurrence of overshoot after completion of warm-up can be suppressed. Therefore, the surface temperature of the heating roller 51 after completion of warm-up is stable, and printing can be started. Further, it is possible to suppress the warm-up period from being prolonged as compared with the case where the PID control is performed after the surface temperature of the heating roller 51 reaches a predetermined temperature (140 [° C.]). Thus, the first print start time can be shortened.

Further, the third power P3 is determined as follows.
First, the correction rate r3 is a correction rate (0 [%]) corresponding to the surface temperature Ta1 (23 [° C.]) and the rising gradient Ka (25 [° C./second]) in the correction rate table h3 of FIG. ) The correction rate r4 is a correction rate (20 [%]) corresponding to the surface temperature Tb1 (23 [° C.]) and the rising gradient Kb (6 [° C./second]) in the correction rate table h4 of FIG. )

In this specific example, the amount of power supplied to the electromagnetic induction coil 531 until the first recording sheet P after completion of warm-up is passed is 998 [W].
The sum of the acquired correction factor r3 and the value calculated by multiplying the correction factor r4 by the correction constant α (1.05) is 21.0 [%], and the magnitude of the supplied power at the above timing (998) [W]), the corrected power is obtained (210 [W] = 998 [W] × 21.0 [%]). It is determined by correcting the third power P3 with the obtained corrected power (1208 [W] = 998 [W] +210 [W]).

In this specific example, while the first recording sheet P passes through the fixing nip portion N, the third power P3 thus determined is supplied, so that the heat of the heating roller 51 is absorbed by the first recording sheet P. However, as shown in FIG. 5, although the surface temperature of the heating roller 51 is slightly reduced, it is possible to suppress a large decrease.
In this specific example, when the second and subsequent recording sheets P pass through the fixing nip portion N, the PID control is restored, but the second and subsequent recording sheets P are heated by the heating roller 51. It is possible to suppress the surface temperature of the heating roller 51 from being reduced due to the absorption of water. This is because the supply power is temporarily increased by supplying the third power P3 while the first recording sheet P passes through the fixing nip N, and then the PID control is restored. However, the responsiveness of the PID control can be improved by increasing the supply power, so that even if the surface temperature of the heating roller 51 starts to decrease, the power necessary to return to the fixing temperature is supplied immediately. It is because it is made.

  On the other hand, in the case of the conventional fixing device, as shown in FIG. 11, after the warm-up is completed, the first recording is performed under PID control in which a small amount of power is supplied to maintain the surface temperature of the heating roller at the fixing temperature. Since the sheet passes through the fixing nip portion, heat is absorbed by the recording sheet, and the surface temperature of the heating roller is greatly reduced. In addition, in this case, since the power supplied immediately before the first recording sheet is passed through the fixing nip is small, in PID control, the power necessary to return the surface temperature of the heating roller to the fixing temperature is immediately supplied. In addition, since the second and subsequent recording sheets P pass through the fixing nip and heat is absorbed, it takes time to return to the fixing temperature. For this reason, fixing failure may occur while the surface temperature of the heating roller is decreasing.

Therefore, since the fixing unit 5 can suppress a decrease in the surface temperature of the heating roller 51 during printing, fixing defects can be suppressed more than in the past.
FIG. 12 is a specific example when the warm-up is started from the standby state.
In the specific example of FIG. 12, the surface temperature Ta1 of the heating roller 51 at the start of warm-up is 56 [° C.], and the surface temperature Ta2 after 2 [seconds] is 110 [° C.]. Therefore, the rising gradient Ka of the surface temperature is 27 [° C./second].

Although not shown, the ambient temperature Tb1 of the heating roller 51 at the start of warm-up is 56 [° C.], the ambient temperature Tb2 after 2 seconds is 72 [° C.], and the ambient temperature rise gradient Kb Is 8 [° C./second].
As described above, the surface temperature Ta1 and the ambient temperature Tb1 are both higher than the specific example of FIG. 5, that is, the heating roller 51 and the inside of the fixing unit 5 are stored in the standby state rather than in the morning. I understand that.

Accordingly, the second power P2 is determined as follows.
First, the correction rate r1 is a correction rate (−10 [%] corresponding to the surface temperature Ta1 (56 [° C.]) and the rising gradient Ka (27 [° C./sec]) in the correction rate table h1 of FIG. ]). The correction rate r2 is a correction rate (−20 [%) corresponding to the surface temperature Tb1 (56 [° C.]) and the rising gradient Kb (8 [° C./sec]) in the correction rate table h2 of FIG. 9B. ]).

The sum of the correction rate r1 obtained in this way and the value calculated by multiplying the correction rate r2 by the correction constant α (1.05) is −31.0 [%], and the correction power is obtained (−377). [W] = 1215 [W] × (−31.0 [%])). It is determined by correcting the second power P2 with the calculated correction power (838 [W] = 1215 [W] -377 [W]).
Even when the warming-up is started from the standby state, after the surface temperature of the heating roller 51 reaches a predetermined temperature (140 [° C.]), it is determined in this way until the fixing temperature (150 [° C.]) is reached. By supplying the second power P2, it is possible to suppress the occurrence of overshoot after the warm-up is completed, as shown in FIG. In addition, it is possible to suppress the warm-up period from being prolonged as compared with the case where the PID control is executed after the surface temperature of the heating roller 51 reaches a predetermined temperature (140 [° C.]). The start time can be shortened.

Further, the third power P3 is determined as follows.
First, the correction rate r3 is a correction rate (−5 [%) corresponding to the surface temperature Ta1 (56 [° C.]) and the rising gradient Ka (27 [° C./sec]) in the correction rate table h3 of FIG. ]). The correction rate r4 is a correction rate (10 [%]) corresponding to the surface temperature Tb1 (56 [° C.]) and the rising gradient Kb (8 [° C./second]) in the correction rate table h4 of FIG. )

In this specific example, the amount of power supplied to the electromagnetic induction coil 531 until the first recording sheet P after warm-up is completed is 832 [W].
The obtained correction rate r3 and the value calculated by multiplying the correction rate r4 by the correction constant α (1.05) are combined to obtain a total of 5.5 [%], and the magnitude of the supplied power at the timing (832). [W]), the corrected power is obtained (46 [W] = 832 [W] × 5.5 [%]). It is determined by correcting the third power P3 with the obtained corrected power (878 [W] = 832 [W] +46 [W]).

Also in this specific example, while the first recording sheet P passes through the fixing nip portion N, the third electric power P3 determined in this way is supplied, and as shown in FIG. Since it can suppress that the surface temperature of 51 falls significantly, a fixing defect can be suppressed.
[Modification]
As described above, the present invention has been described based on the embodiment. However, the present invention is not limited to the above-described embodiment, and the following modifications can be implemented.
(1) In the above-described embodiment, a configuration in which the second power P2 is corrected and determined with reference to the surface temperature Ta1 and the rising gradient Ka of the heating roller 51, and the ambient temperature Tb1 and the rising gradient Kb of the heating roller 51. Although shown, it is not limited to this.

  For example, the second power P2 may be corrected and determined with reference to only the ambient temperature Tb1 and the rising gradient Kb of the heating roller 51. Also in this case, since the heat storage state inside the fixing unit 5 can be estimated to some extent by the ambient temperature Tb1 and the rising gradient Kb of the heating roller 51, the second electric power P2 is obtained according to the heat storage state inside the fixing unit 5. By determining, it is possible to suppress the warm-up period from becoming longer while suppressing overshoot than before.

However, in this case, since the correction rate of the correction rate table h1 in FIG. 9A is not used for correcting the second power P2, the value of the correction rate in the correction rate table h2 in FIG. It is necessary to review.
This modification is particularly effective in the case of a fixing device configured to insert a pressing roller having a small diameter inside the heat generating belt. In this case, the heat capacity of the heating rotator itself is reduced, and the influence of the remaining heat on the temperature control is reduced.
(2) Further, the configuration for correcting and determining the third electric power P3 is also limited to the configuration that refers to the surface temperature Ta1 and the rising gradient Ka of the heating roller 51 and the ambient temperature Tb1 and the rising gradient Kb of the heating roller 51. Instead of this, only the ambient temperature Tb1 and the rising gradient Kb may be referred to.

Also in this case, the heat storage state inside the fixing unit 5 is estimated from the ambient temperature Tb1 and the rising gradient Kb of the heating roller 51, and the third power P3 is determined according to the heat storage state inside the fixing unit 5, thereby This is because it is possible to suppress the surface temperature of the heating roller 51 from greatly decreasing during print output, and to suppress poor fixing. In addition, it is necessary to review the correction factor in the correction factor table h4 in FIG.
(3) In the above embodiment, the configuration in which 90% of the first power P1 is set as the initial value of the second power P2 is shown, but the present invention is not limited to this. It is necessary to review the correction factors in the correction factor tables h1 and h2 according to the initial value of the second power P2.
(4) The correction factor and the correction constant α in the correction factor tables h1 to h4 in the above embodiment are preferably set according to the specifications of the fixing unit.
(5) In the above embodiment, the configuration in which the third power control for supplying the third power P3 is executed only while the first recording sheet P passes through the fixing nip portion N has been described. It is not limited to. For example, the third power control may be performed while the second or third recording sheet P passes through the fixing nip portion N. In this case, heat is generated by the recording sheet P. It can suppress that the temperature of the heating roller 51 falls by being absorbed.
(6) In the above embodiment, the configuration of the temperature sensor s1 made of a contact type thermistor is shown to detect the surface temperature of the heating roller 51. However, the present invention is not limited to this. For example, it is good also as a structure which detects the surface temperature of a heating roller using a non-contact type thermopile. Configurations such as the type, number, and arrangement of temperature sensors that detect the surface temperature of the heating roller can be appropriately selected according to the specifications of the fixing unit. When a plurality of temperature sensors are provided, when determining the second power P2 and the third power P3, only the detection results of a specific temperature sensor may be used, or the detection results of a plurality of temperature sensors may be used. An average may be used.
(7) In the above embodiment, the configuration of the temperature sensor s2 composed of a non-contact type thermistor is shown to detect the ambient temperature of the heating roller 51, but the present invention is not limited to this. Any device that can detect the atmospheric temperature may be used. Configurations such as the type, number, and arrangement of temperature sensors that detect the ambient temperature around the heating roller can be appropriately selected according to the specifications of the fixing unit. When a plurality of temperature sensors are provided, when determining the second power P2 and the third power P3, only the detection results of a specific temperature sensor may be used, or the detection results of a plurality of temperature sensors may be used. An average may be used.
(8) In the above embodiment, the configuration using the heating roller 51 provided with the fixing belt 550 having the electromagnetic induction heat generating layer 513 is shown as the heating rotator. However, the present invention is not limited to this. For example, a separate roller can be loosely or tightly fitted inside the fixing belt having the electromagnetic induction heat generating layer. When the fixing belt is used, a fixing member that receives the pressing force from the pressure roller 52 may be disposed inside the fixing belt instead of the roller.
(9) In the above embodiment, the configuration of the fixing belt 550 having the three-layer structure of the electromagnetic induction heat generating layer 513, the elastic body layer 514, and the release layer 515 is shown, but the present invention is not limited to this. For example, a low resistance layer made of a low resistance material such as Cu, Ag, or Au can be interposed between the electromagnetic induction heat generating layer 513 and the elastic body layer 514. In this case, however, the thickness of the low-resistance layer is preferably about several [μm] to several tens [μm], and even better heat generation efficiency can be obtained by inserting the low-resistance layer.
(10) In the above embodiment, a tandem color printer is used as the image forming apparatus. However, the scope of the present invention is not limited to this, and a copying machine having a fixing unit using a resistance heating element, It can be applied to facsimile machines, printers, and the like.

  Further, the contents of the above-described embodiment and modification examples may be combined as much as possible.

  The present invention relates to a fixing unit and an image forming apparatus using the fixing unit, and is particularly applicable to a temperature control technique for the fixing device.

DESCRIPTION OF SYMBOLS 1 Printer 5 Fixing part 6 High frequency inverter 7 Paper sensor 51 Heating roller 52 Pressure roller 53 Magnetic flux generation part 54 Current detection part 55 Voltage detection part 59 Case 60 Control part 513 Electromagnetic induction heating layer 531 Electromagnetic induction coil 550 Fixing belt P1 1st 1 power P2 2nd power
P3 Third power
Ta1, Ta2 surface temperature (heating roller)
Tb1, Tb2 ambient temperature (atmosphere temperature around the heating roller)
Ka rising gradient (rising gradient of heating roller surface temperature)
Kb rising gradient (rising gradient of the temperature around the heating roller)
s1 Temperature sensor (first temperature detection means)
s2 Temperature sensor (second temperature detection means)

Claims (8)

A fixing device that forms a fixing nip by pressing a pressing member on a peripheral surface of a heating rotator, and passes a recording sheet carrying a toner image in the fixing nip to fix it.
Heating means for receiving power supply and heating the heating rotator;
Power supply means for supplying power to the heating means;
First temperature detecting means for detecting a surface temperature of the heating rotating body;
Second temperature detecting means for detecting an ambient temperature around the heating rotator;
Control means for controlling heating by the heating means via the power supply means so that the detected surface temperature of the heating rotator becomes a fixing temperature necessary for fixing;
With
The control means includes
In the warm-up period for heating the heating rotator to the fixing temperature,
Until the surface temperature of the heating rotator reaches a predetermined temperature lower than the fixing temperature, the first power control is performed to control the power supply means to supply the first power to the heating means,
After the surface temperature of the heating rotator reaches the predetermined temperature and until the fixing temperature is reached, the detection result of the second temperature detection means at the start of warm-up and the execution of the first power control Performing second power control for controlling the power supply means to supply the second power determined according to the rate of change of the detection result of the second temperature detection means to the heating means during a predetermined period of time. A fixing device characterized by the above. The control means increases the second electric power as the detection result of the second temperature detection means at the start of warm-up is higher and as the change rate of the detection result of the second temperature detection means during the predetermined period is larger. The fixing device according to claim 1, wherein the fixing device is controlled to be low. In determining the second power, the control means further determines a change rate of the detection result of the first temperature detection means at the start of warm-up and the detection result of the first temperature detection means during the predetermined period. The fixing device according to claim 1, wherein the second power is determined with reference to the fixing device. A sheet detecting means for detecting the recording sheet passing through the fixing nip;
The control means includes
During the period from the completion of warm-up until the first recording sheet is detected at the fixing nip by the sheet detecting means, the power supply means is controlled by PID control that maintains the surface temperature of the heating rotator at the fixing temperature. And
While the first recording sheet passes through the fixing nip, the change rate of the detection result of the second temperature detection unit at the start of warm-up and the detection result of the second temperature detection unit during the predetermined period 4. The fixing device according to claim 1, wherein third power control is performed to control the power supply unit to supply the third power determined accordingly to the heating unit. 5. The control means increases the detection results of the first and second temperature detection means at the start of warm-up and increases the change rate of the detection results of the second temperature detection means during the predetermined period. 5. The fixing device according to claim 4, wherein the power of 3 is low. In determining the third power, the control means further calculates a change rate of the detection result of the first temperature detection means at the start of warm-up and the detection result of the first temperature detection means during the predetermined period. The fixing device according to claim 4, wherein the third power is determined with reference to the fixing device. The fixing device according to claim 1, wherein the heating rotator includes a heat generating layer on an outer peripheral portion thereof.   An image forming apparatus comprising the fixing device according to claim 1.

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