JP5910368B2 - Fixing apparatus and image forming apparatus - Google Patents

Description

  The present invention relates to a fixing device and an image forming apparatus that heat-fix an unfixed image on a recording sheet using a fixing member that includes a magnetic shunt alloy layer that generates heat by electromagnetic induction and has a Curie temperature higher than the fixing temperature. In particular, the present invention relates to a technique for suppressing power consumption required for electromagnetic induction heating.

In recent years, from the viewpoint of power saving, an electromagnetic induction heating type fixing device has been used as a fixing device in an image forming apparatus such as a printer or a copying machine.
Such an electromagnetic induction heating type fixing device supplies a high-frequency current from a high-frequency power supply circuit (inverter circuit) to an exciting coil and generates an eddy current in a heat generating layer of a belt-shaped fixing member by an alternating magnetic field generated thereby. It is configured to generate Joule heat.

However, there is a possibility that the temperature of the portion where the recording sheet is not passed continues to rise and the temperature is excessively increased. To prevent this, for example, in Patent Document 1, the Curie temperature is higher than the fixing temperature by a predetermined temperature. A fixing device that heats a fixing member using a heating element including a magnetic shunt alloy layer is disclosed.
In the fixing device according to Patent Document 1, for example, when continuous printing is performed, if the temperature of the heating element in the non-sheet passing region where heat is not taken away by the recording sheet rises to the Curie temperature, the heating element becomes ferromagnetic. It changes to paramagnetism, and the density of magnetic flux passing through this portion decreases rapidly, so that the amount of heat generation decreases. As a result, the temperature of the non-sheet passing region is prevented from excessively rising, and the fixing member is prevented from being deteriorated by heat.

JP 2008-70757 A

  Usually, in an electromagnetic induction type fixing device, the surface temperature of the sheet passing area of the heating element is detected by a temperature sensor, and it is necessary to maintain the surface temperature of the heating element at the target fixing temperature based on the detection result. Supply power (hereinafter referred to as “target supply power”) is determined, and control is performed by outputting a control frequency to the high-frequency power supply circuit while performing feedback control to supply this target supply power to the exciting coil.

However, in the fixing device including the magnetic shunt alloy layer in the heating element as described above, when the feedback control is performed, for example, after the small size paper is continuously printed and the temperature of the non-sheet passing region reaches the Curie temperature, the large size It has been found that when paper is passed through, the power supplied to the exciting coil thereafter is not stable, and overshoot exceeding the target power supply may be repeated.
24A and 24B are graphs for explaining the above phenomenon.

FIG. 24A shows the surface temperature of a non-sheet passing region (hereinafter simply referred to as “end region”) when a small-size sheet is passed through the fixing member from the start of warm-up by temperature control of a conventional fixing device. The change of Tp is shown, the horizontal axis indicates the elapsed time (seconds) from the start of warm-up, and the vertical axis indicates the surface temperature (° C.) of the end region.
FIG. 24B is a graph showing how the output (supply power) of the exciting coil changes with the passage of time in FIG. 24A, and the vertical axis indicates the exciting coil. Supply power (W) is shown.

In this example, it is assumed that during the execution of the print job, feedback control is performed with the target power supply being set to 800 W, and the end region has reached the Curie temperature while the small-size paper job is repeated.
As shown in FIG. 24 (a), after printing a small size paper, when the large size paper is printed, both end portions of the large size paper come into contact with the end region, which has been a non-sheet passing region, and heat is generated. Therefore, the surface temperature Tp falls below the Curie temperature (220 ° C. here) (reference numeral v1). As a result, the magnetic shunt alloy layer in the end region shifts from paramagnetism to ferromagnetism, so that the inductance of the exciting coil that is magnetically coupled with this also changes and its output increases rapidly. I understood.

The power control unit tries to suppress the increase in the output of the exciting coil by executing the feedback control, but cannot sufficiently cope with the sudden output change due to the response delay (time lag). As a result, as shown in FIG. Thus, the output of the exciting coil increases rapidly (portion indicated by reference sign w1).
The surface temperature Tp in the end region rises and overshoots against such a sudden increase in output, and reaches the Curie temperature again (position V2 in FIG. 24), which reduces the magnetic permeability of the magnetic shunt alloy layer. The output of the exciting coil decreases because of a decrease. In this state, if the large-size paper is deprived of heat, the temperature in the end region falls below the Curie temperature, the magnetic permeability is recovered, the output increases, and the vicious cycle is repeated again.

As described above, excessive overshoot is repeated in the vicinity of the Curie temperature higher than the target fixing temperature, and there is a problem that unnecessary power consumption is generated.
The present invention has been made in view of the above-described problems, and in an electromagnetic induction type fixing device using a magnetic shunt alloy layer to prevent excessive temperature rise in a non-sheet passing region, overshoot is repeatedly performed. An object of the present invention is to provide a fixing device and an image forming apparatus that can suppress wasteful power consumption due to the above.

  In order to achieve the above object, a fixing device according to the present invention is a recording sheet in which a pressing member is pressed against the surface of a fixing member that generates heat by electromagnetic induction by an exciting coil to form a nip portion and pass through the nip portion. The fixing device is configured to thermally fix the unfixed image on the upper side and suppress the temperature rise in the non-sheet passing region of the fixing member by using a magnetic shunt alloy layer in which the Curie temperature is set to be equal to or higher than the target fixing temperature. Temperature detecting means for detecting the temperature of the recording sheet passing area of the fixing member, and after the temperature of the non-sheet passing area of the recording sheet of the fixing member reaches the Curie temperature, A determination means for determining whether or not at least a part of the region is at a temperature lowering timing lower than the Curie temperature, and a parameter value for controlling power supplied to the exciting coil are determined. Power control means and power supply means for supplying power to the exciting coil in accordance with the determined parameter value, the power control means after the temperature of the non-sheet passing region has reached the Curie temperature, the determination means Until it is determined that the temperature lowering timing is reached, the target supply power to be supplied to the exciting coil is determined based on the detection result of the temperature detection means, and the parameter value is adjusted so as to maintain the target supply power The first control for feedback control is executed, and when the determination means determines that the temperature lowering timing is reached, the value of the parameter is set to the fluctuation amount of the power supplied to the exciting coil. Control is performed by switching to a preset fixed value so as to be smaller than the fluctuation amount based on the response delay of feedback control in the control. And executes the second control.

Here, a second temperature detection unit that detects the temperature of the non-sheet passing region of the fixing member is provided, and the determination unit may determine the temperature decrease timing based on the detection result by the second temperature detection unit. Good.
Further, the determination unit may determine whether or not it is a temperature decrease timing based on a change amount of a parameter value when the power supplied to the exciting coil is feedback-controlled in the first control.

  Here, the power supply means includes an LC resonance circuit and a switching element for turning on / off current supply to the LC resonance circuit, and the parameter is a control frequency for controlling on / off of the switching element, In the LC resonance circuit in the first control, the determination means is a switching element corresponding to the amount of change in time when the absolute value of the resonance waveform of the output voltage is a voltage equal to or higher than a predetermined value, or the resonance waveform. The temperature decrease timing may be determined based on the amount of change in the time when is off.

The power control means may return to the first control when a predetermined time has elapsed after switching from the first control to the second control.
Further, the power control means has a difference between a target supply power in the first control and a power supplied to the exciting coil after switching from the first control to the second control within a certain range. After that, the control may return to the first control.

Here, in the second control, the power control means may control the power supplied to the exciting coil by changing the parameter value stepwise to a preset fixed value.
Here, the determination unit includes a size acquisition unit that acquires a size in the width direction of the recording sheet to be passed, and at least a part of the non-sheet passing region of the fixing member with respect to the first size recording sheet. May be determined to be the temperature decrease timing only when the second size recording sheet having a width smaller than the Curie temperature and larger than the first size recording sheet is passed. .

  Further, the present invention may be an image forming apparatus including the fixing device having the above-described configuration.

  According to the fixing device configured as described above, when it is determined that the temperature of the region of the fixing member that has reached the Curie temperature falls and falls below the Curie temperature (temperature drop timing), the power supply control to the excitation coil is fed back. From control (first control), the parameter value for supply power control is switched to a fixed value for control (second control). This fixed value is a value set in advance so that the fluctuation amount of the power supplied to the exciting coil is smaller than the fluctuation amount based on the response delay of the feedback control in the first control. The fluctuation of the power supplied to the exciting coil can be made smaller than in the case of. As a result, it is possible to reduce the overshoot exceeding the target supply power, and it is possible to suppress the occurrence of useless power consumption.

FIG. 2 is a diagram illustrating a configuration of a printer according to the present embodiment. FIG. 2 is a partial cross-sectional perspective view showing a configuration of a fixing device according to an embodiment of the present invention. It is sectional drawing of the principal part of the said fixing device. It is a figure which shows the relationship between the structure of the control part in the said printer, and the main component used as the control object of a control part. It is a circuit diagram which shows the outline | summary of IH power supply. (A) is a figure which shows the specific example of the control frequency table which divided | segmented the correspondence of control frequency and target supply electric power into less than Curie temperature (A column) and more than Curie temperature (B column), (b) is a graph of the control frequency table of (a). (A) is a graph showing how the central region temperature Ts and end region temperature Tp of the fixing belt change, and (b) is a graph showing how the control frequency Fr and supply power Pw change. It is a figure which shows the specific example of the fixed control frequency table which shows the correspondence of target supply electric power and a fixed frequency. It is a flowchart which shows the control content in the temperature control process which concerns on this Embodiment. It is a flowchart which shows the control content of the feedback control process in the flowchart of FIG. It is a flowchart which shows the control content of the determination process of whether it is the timing which falls below the Curie temperature in the flowchart of FIG. It is a graph for demonstrating the threshold control frequency in the 1st modification of the determination process whether it is the timing which falls below Curie temperature. It is a figure which shows the specific example of the threshold value control frequency table in the said 1st modification. It is a flowchart which shows the control content of the determination process whether it is the timing which falls below the Curie temperature which concerns on a 1st modification. (A)-(c) is a figure for demonstrating operation | movement of LC resonance circuit of IH power supply, respectively. It is a figure which shows the relationship between the switching signal of ON / OFF by a control frequency, and the change of a resonance waveform. (A) (b) is a figure which shows the change of the resonance waveform when Curie temperature is reached and when it is less than Curie temperature, respectively. It is a figure which shows the specific example of the threshold value resonance time table used in the 2nd modification of the determination process whether it is the timing which falls below Curie temperature. It is a flowchart which shows the control content of the determination process whether it is the timing which falls below the Curie temperature which concerns on a 2nd modification. It is a flowchart which shows the control content of the determination process whether it is the timing which falls below the Curie temperature which concerns on another modification. It is a figure which shows the specific example of the fixed control frequency table which concerns on a modification. It is a flowchart which shows the control content of the fixed frequency control process which concerns on a modification. It is a graph which shows the mode of the change of the center area | region temperature Ts and the edge part area | region temperature Tp of a fixing belt for demonstrating the feedback control process which concerns on a modification. It is a graph which shows an example of the mode of the change of the surface temperature Tp of the edge part area | region of a fixing member, and the electric power Pw supplied to an exciting coil for demonstrating the feedback control process which concerns on a prior art example.

Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described by taking as an example a case where it is applied to a tandem color printer (hereinafter simply referred to as “printer”).
(1) Configuration of Printer FIG. 1 is a diagram illustrating a configuration of the printer 1 according to the present embodiment.
As shown in FIG. 1, the printer 1 includes an image process unit 3, a paper feed unit 4, a fixing device 5, a control unit 60, and the like.

  When the printer 1 is connected to a network (for example, a LAN) and receives a print instruction from an external terminal device (not shown) or the operation panel 7 (see FIG. 4), the printer 1 can display yellow, magenta, cyan, and black. A toner image of each color is formed, and these are multiplex-transferred onto a recording sheet to form a full-color image, thereby executing a printing process on the recording sheet. Hereinafter, the reproduction colors of yellow, magenta, cyan, and black are expressed as Y, M, C, and K, and Y, M, C, 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, a secondary transfer roller 45, and the like. Since the image forming units 3Y, 3M, 3C, and 3K have the same configuration, the configuration of the image forming unit 3Y will be mainly described below.
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. Then, a Y-color toner image is formed on the photosensitive drum 31Y. The developing device 33Y faces the photosensitive drum 31Y and conveys charged toner to the photosensitive drum 31Y. The other image forming units 3M, 3C, and 3K have the same configuration. In FIG. 1, for the sake of simplicity, the reference numerals of the constituent members of the image forming units 3M, 3C, and 3K are 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 C. A cleaner 21 for removing toner remaining on the intermediate transfer belt is disposed in the vicinity of the driven roller 13.
The exposure unit 10 includes a light emitting element such as a laser diode, emits laser light L for forming images of Y to K colors in response to a drive signal from the control unit 60, and each of the image forming units 3Y, 3M, 3C, and 3K. The photosensitive drum is exposed and scanned. By this exposure scanning, for example, in the image forming unit 3Y, an electrostatic latent image is formed on the photosensitive drum 31Y charged by the charger 32Y. Similarly, electrostatic latent images are formed on the photosensitive drums of the image forming units 3M, 3C, and 3K.

  The electrostatic latent image formed on each photoconductor drum is developed by each developing unit of the image forming units 3Y, 3M, 3C, and 3K, and a toner image of a corresponding color is formed on each photoconductor drum. The formed toner images are sequentially shifted on the intermediate transfer belt 11 at different timings so as to be superimposed at the same position on the intermediate transfer belt 11 by the primary transfer rollers of the image forming units 3Y, 3M, 3C, and 3K. Primary transfer is performed to form a color toner image.

  The paper feed unit 4 is provided in each of the paper feed cassettes 40 and 41 for storing the recording sheets S, and the paper feed cassettes 40 and 41, and a feeding roller 42 for feeding the recording sheets S one by one on the transport path 43. A timing roller 44 that feeds the fed recording sheet S to the secondary transfer position 46 at a predetermined timing is provided. The sheet feeding cassettes 40 and 41 store recording sheets of different sizes (here, A4 vertical size and A3 vertical size).

  The timing roller 44 moves the recording sheet S in accordance with the timing at which the toner image primarily transferred onto the intermediate transfer belt 11 is conveyed to the secondary transfer position 46 so as to be superimposed at the same position on the intermediate transfer belt 11. Transport to the secondary transfer position 46. Then, the color toner images on the intermediate transfer belt 11 are collectively transferred onto the recording sheet S by the secondary transfer roller 45 at the secondary transfer position 46.

The recording sheet S on which the toner image is secondarily transferred is further conveyed to the fixing device 5, and the toner image (unfixed image) on the recording sheet S is heated and pressed in the fixing device 5 to heat the recording sheet S. After fixing, the paper is discharged onto a paper discharge tray 72 by a discharge roller 71.
An operation panel 7 (FIG. 4) is provided at an easily operable position on the upper front surface of the printer 1. The operation panel 7 includes a plurality of input keys and a liquid crystal display unit, and a touch panel is provided on the surface of the liquid crystal display unit. An instruction from the user is accepted by a touch input from the touch panel or a key input from an input key, and is notified to the control unit 60.

The control unit 60 controls the image processing unit 3 and the paper feeding unit 4 in a unified manner so as to execute a smooth printing operation.
(2) Configuration of Fixing Device FIG. 2 is a partial cross-sectional perspective view showing the configuration of the fixing device 5, and FIGS. 3 (a) and 3 (b) are cross-sectional views of the main part thereof. FIG. 3A shows a cross-sectional view of the fixing device 5, and FIG. 3B is a partial cross-sectional view showing a detailed structure of the fixing belt 155 (portion indicated by a dotted rectangle D in FIG. 3A). Indicates.

As shown in FIG. 2, the fixing device 5 is an electromagnetic induction heating type fixing device, and includes a fixing roller 150, a fixing belt 155, a guide plate 156, a pressure roller 160, a magnetic flux generator 170, a central server. It includes a mr 180 and an end thermistor 181.
The fixing roller 150 is configured by covering the periphery of a long and cylindrical cored bar 152 with an elastic body layer 153, and as shown in the cross-sectional view of the main part of FIG. It is arranged on the inner side of the circuit. As the size of the fixing roller 150, for example, one having an outer diameter of 36 mm can be used.

The cored bar 152 is a member that supports the fixing roller 150, and is formed of a cylindrical body having an outer diameter of about 20 mm, for example. As a material constituting the cored bar 152, for example, aluminum, iron, stainless steel, or the like can be used.
The elastic layer 153 prevents the heat generated by the fixing belt 155 from escaping to the cored bar 152 and, as shown in FIG. 3A, the pressure roller 160 and the fixing nip (155n) via the fixing belt 155. It is a layer for forming. The thickness of the elastic body layer 153 can be 8 mm, for example. As a material constituting the elastic body layer 153, a material having high heat resistance and high heat insulation is desirable. For example, a foaming elastic body such as silicone rubber or fluororubber can be used.

The fixing belt 155 is an endless belt, and as shown in FIG. 3B, a magnetic shunt alloy layer 155a, an elastic body layer 155b, and a release layer 155c are laminated in this order from the belt inner peripheral side. Configured. A heat generating layer made of nickel, copper, or the like may be provided between the magnetic shunt alloy layer 155a and the elastic body layer 155b.
The magnetic shunt alloy layer 155a is a ferromagnetic material until reaching the Curie temperature, and has a property of becoming a paramagnetic material when the Curie temperature is reached. This is a layer that suppresses the temperature rise of the fixing belt 155 due to electromagnetic induction when the temperature is raised and reaches the Curie temperature.

  The thickness of the magnetic shunt alloy layer 155a can be about 30 μm, for example. As a material constituting the magnetic shunt alloy layer 155a, for example, an alloy of nickel and iron can be used. The Curie temperature of the magnetic shunt alloy layer 155a is set to a desired temperature by adjusting the mixing ratio of nickel and iron. Moreover, it is good also as using the alloy of nickel, iron, and chromium as a material which comprises the magnetic shunt alloy layer 155a.

The Curie temperature may be equal to or higher than the fixing temperature. However, if the temperature difference from the fixing temperature is too small, the temperature increase rate of the fixing belt 155 is greatly reduced until the temperature of the fixing belt 155 reaches the fixing temperature, and the heat is reduced. Since the warm-up time until the fixing operation is started becomes long, the temperature difference is desirably 30 ° C. or more, for example.
On the other hand, if the Curie temperature is set too high than the fixing temperature, the heat resistance exceeds the heat resistance temperature of the fixing belt 155 and the durability deteriorates. Therefore, it is desirable that the temperature is at least lower than the heat resistance temperature of the fixing belt 155. Needless to say.

In the present embodiment, the Curie temperature of the magnetic shunt alloy layer 155a is set to a temperature (for example, about 220 ° C.) that is about 40 ° C. higher than the control target temperature (for example, about 180 ° C.) during fixing. . Here, the heat resistance temperature of the fixing belt 155 is about 240 ° C.
The elastic body layer 155b is a layer for transferring heat uniformly and flexibly to the toner image on the recording sheet S. By providing the elastic body layer 155b, the toner image can be prevented from being crushed or the toner image can be melted non-uniformly, and image noise can be prevented from being generated. The thickness of the elastic body layer 155b can be about 200 μm, for example. As a material constituting the elastic body layer 155b, a rubber material or a resin material having heat resistance and elasticity can be used. For example, silicone rubber or fluorine rubber can be used.

  The release layer 155 c is an outermost layer of the fixing belt 155 and is a layer for improving the release property between the fixing belt 155 and the recording sheet S. The thickness of the release layer 155c is 5 to 100 μm, desirably 10 to 50 μm. As the material constituting the release layer 155c, a material that can withstand use at the fixing temperature and has excellent release properties with respect to the toner can be used. For example, PFA (tetrafluoroethylene / perfluoroalkoxyethylene copolymer), PTFE (tetrafluoroethylene), FEP (tetrafluoroethylene / hexafluoroethylene copolymer), PFEP (tetrafluoroethylene / hexafluoroethylene) A fluororesin such as a propylene fluoride copolymer) can be used.

  The guide plate 156 is a plate for guiding the fixing belt 155 that is driven to rotate in the rotating direction. The guide plate 156 is disposed inside the circulation path of the fixing belt 155 at a position facing the magnetic flux generator 170 via the fixing belt 155, is curved along the curvature of the fixing belt 155, and is driven to rotate. , The relative position between the fixing belt 155 and the magnetic flux generator 170 is regulated while guiding the fixing belt 155 in its circumferential direction. As a material constituting the guide plate 156, for example, a non-magnetic low resistance material such as copper or aluminum can be used.

  Instead of forming the magnetic shunt alloy layer 155a on the fixing belt 155, a magnetic shunt alloy layer is provided on the guide plate 156 or the fixing roller 150, and the fixing belt 155 has copper, nickel or the like instead of the magnetic shunt alloy layer 155a. It is good also as providing the heat generating layer comprised by these. Even in such a configuration, similarly to the case where the magnetic shunt alloy layer 155a is provided on the fixing belt 155, the guide plate 156 or the fixing roller 150 is heated by electromagnetic induction and the curie via the guide plate 156 or the fixing roller 150. The temperature of the fixing belt 155 is raised until the temperature is reached, and when the Curie temperature is reached, the temperature rise of the fixing belt can be suppressed.

  The pressure roller 160 is configured by laminating a release layer 163 around a cylindrical cored bar 161 via an elastic body layer 162, and is disposed outside the circulation path of the fixing belt 155. By pressing the fixing roller 150 with the pressure roller 160 from the outside of the fixing belt 155 via the fixing belt 155, a fixing having a predetermined width in the circumferential direction between the pressure roller 160 and the outer surface of the fixing belt 155. A nip 155n is formed.

The metal core 161 is a member that supports the pressure roller 160 and is made of a material having heat resistance and strength. As a material of the core metal 161, for example, aluminum, iron, stainless steel, or the like can be used.
The elastic body layer 162 is an elastic body such as silicone rubber or fluorine rubber, and is made of a material having high heat resistance within a thickness range of 1 to 20 mm. The release layer 163 is a layer for improving the release property between the pressure roller 160 and the recording sheet S, and can be composed of the same material and thickness as the release layer 155c. As the size of the pressure roller 160, for example, a roller having an outer diameter of about 35 mm can be used.

Returning to FIG. 2, the magnetic flux generator 170 has a coil bobbin 171, a skirt core 172, an exciting coil 173, a core 174, and a cover 175, and the fixing belt 155 is outside the circulation path of the fixing belt 155. With respect to the position facing the pressure roller 160 with the gap therebetween, it is arranged along the width direction of the fixing belt 155 slightly upstream in the circumferential direction from here.
The exciting coil 173 generates magnetic flux for electromagnetic induction heating of the magnetic shunt alloy layer 155 a of the fixing belt 155, and is wound around the coil bobbin 171. The alternating magnetic flux generated from the exciting coil 173 is guided to the fixing belt 155 by the core 174 and the bottom core 172, and penetrates the portion of the magnetic shunt alloy layer 155a of the fixing belt 155 that mainly faces the magnetic flux generating portion 170. An eddy current is generated in the portion to heat the magnetic shunt alloy layer 155a itself, and the temperature of the fixing belt 155 is increased.

  As the fixing belt 155 rises in temperature, the pressure roller 160 in contact with the fixing belt 155 at the fixing nip 155n also rises in temperature. A central thermistor 180 and an end thermistor 181 for detecting the surface temperature of the fixing belt 155 are disposed in the vicinity of the center and the end in the width direction of the fixing belt 155. In this embodiment, the end thermistor 181 has a difference between the sheet passing area of the maximum size recording sheet (A3 portrait) and the sheet passing area of the small size recording sheet (A4 portrait) of the fixing belt 155. It is arranged at a position for detecting the temperature of the region P (hereinafter referred to as “end region P”).

Based on the detection signals from the central thermistor 180 and the end thermistor 181, the controller 60 controls the exciting coil so that the surface temperature of the fixing belt 155 becomes the target temperature via the IH power source 190 (see FIG. 4). The amount of power supplied to 173 is controlled.
(3) Configuration of Control Unit FIG. 4 is a 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 the figure, the control unit 60 includes a CPU (Central Processing Unit) 601, a communication interface (I / F) unit 602, a ROM (Read Only Memory) 603, a RAM (Random Access Memory) 604, and image data storage. A unit 605, a printing condition storage unit 606, a target supply power table storage unit 607, a fixed control frequency table storage unit 608, and the like.

The communication I / F unit 602 is an interface for connecting to a LAN such as a LAN card or a LAN board.
The ROM 603 stores a program for controlling the image processing unit 3, the paper feeding unit 4, the IH power source 190, the operation panel 7, and the like.
The RAM 604 is used as a work area when the CPU 601 executes a program.

The image data storage unit 605 stores image data for printing received via the communication I / F unit 602.
The print condition storage unit 606 extracts print conditions from the print job data received from the external terminal and stores them as a print job list. Here, it is assumed that the “printing condition” includes information such as the number of printed recording sheets and the size of the recording sheet.

The target supply power table storage unit 607 stores a target supply power table. In this table, the power to be supplied to the excitation coil 173 for setting the detected temperature of the sheet passing area and the temperature of the sheet passing area of the fixing belt 155 to the target fixing temperature (the temperature to be a control target at the time of fixing). Stored in association with (target supply power).
The CPU 601 refers to the print job list from the print condition storage unit 606 and acquires the size of the recording sheet of the print job being executed. At the same time, based on the acquired recording sheet size and the surface temperature of the sheet passing area of the fixing belt 155 detected by the central thermistor 180, the surface of the sheet passing area is referred to by referring to the target supply power table storage unit 607. A target supply power to be supplied to the exciting coil 173 to determine the temperature as the target fixing temperature is determined, and the value is notified to the IH power source 190.

  In the IH power supply 190, feedback control (first control) is performed so that the power supplied to the excitation coil 173 approaches the determined target supply power. When the temperature of the end region P of the fixing belt 155 reaches the Curie temperature and then falls below the Curie temperature, the fixed frequency control (the second frequency control is performed using a fixed frequency determined in advance from the feedback control). Control). Details will be described later.

The fixed control frequency table storage unit 608 stores a fixed frequency table for determining the fixed frequency when the power control supplied to the exciting coil 173 is switched to the fixed frequency control.
As will be described later, the IH power source 190 includes an LC resonance circuit and supplies the target supply power notified from the control unit 60 to the excitation coil 173.

  The control unit 60 reads out a necessary program from the ROM 603, and based on the image data stored in the image data storage unit 605, controls the image processing unit 3, the paper feeding unit 4 and the like in accordance with the printing conditions to perform printing. Make the job run smoothly. Further, when executing the print job, the temperature of the fixing belt 155 is controlled by accurately controlling the IH power source 190 based on the detection results of the central thermistor 180 and the end thermistor 181.

(4) Configuration of IH Power Supply 190 FIG. 5 is a circuit diagram showing an outline of the IH power supply 190.
As shown in the figure, the IH power supply 190 includes a frequency control unit 191, a switching element 192, a capacitor 193, a coil 194, a diode bridge 195, a voltage detection unit 196, a current detection unit 197, a coil 198, a noise filter 199, and the like. The

  The noise filter 199 removes various noise components included in the power supplied from the AC power supply 200. The diode bridge 195 rectifies the AC of the AC power supply 200 after noise removal. The positive side voltage is applied to the connection part P1 of the capacitor 193 and the excitation coil 173 in the LC resonance circuit composed of the capacitor 193 and the excitation coil 173 via the coil 194, and the other negative side voltage is applied from the IGBT, for example. The switching element 192 is applied to the emitter side.

The collector side of the switching element 192 is connected to the other connection part P2 of the capacitor 193 and the exciting coil 173.
The frequency control unit 191 includes a CPU 1911 and a control frequency table storage unit 1912.
FIG. 6A shows a specific example of the control frequency table stored in the control frequency table storage unit 1912.

  In the control frequency table of FIG. 6A, the power supplied from the IH power source 190 to the exciting coil 173 is controlled to the target supply power, so that the correspondence between the target supply power and the control frequency as a parameter given to the switching element 192 is used. This indicates a relationship, when the surface temperature of the entire fixing belt 155 is lower than the Curie temperature (column A), and when the temperature of the non-sheet-passing area (end area P) of the fixing belt 155 is equal to or higher than the Curie temperature ( (B column). This table is obtained by a designer in advance by experiments or the like, mainly corresponding to a small-sized recording sheet (A4 longitudinal). Such a table may be provided for each recording sheet size.

In the table of FIG. 6A, for convenience, the surface temperature of the sheet passing area of the fixing belt 155 is within a range required for temperature adjustment (temperature adjustment) after reaching the target temperature (for example, 180 ° C.) at the time of fixing. Only the target supply power value (500 W to 1200 W) is disclosed.
The CPU 1911 of the frequency control unit 191 receives the notification of the target supply power determined by the control unit 60 and controls the switching element 192 with reference to the control frequency table in the control frequency table storage unit 1912. The frequency is determined, and the switching element 192 is turned on / off at the determined control frequency. Thereby, control is performed so that the power supplied to the excitation coil 173 becomes the target power supply. The IH power source 190 is configured to increase the power supply as the control frequency is lower.

In a normal case (when the temperature of the non-sheet passing area does not reach the Curie temperature), for example, when receiving a notification of 800 W as the target supply power from the control unit 60, the CPU 1911 of the frequency control unit 191 causes the CPU 1911 to perform FIG. The control frequency is determined to be 53.3 kHz with reference to column A, and output to the switching element 192.
In FIG. 6A, the target supply power is shown at intervals of 50 W, but when the target supply power within the interval is notified from the control unit 60, the following is performed.

For example, when the notified target supply power is 820 W, the control frequency 53.3 kHz corresponding to 800 W in the A column and the control frequency 52.1 kHz corresponding to 850 W in the A column are proportionally distributed, and the target supply A control frequency of 52.8 kHz corresponding to the power of 820 W can be obtained.
Thereafter, the CPU 1911 calculates the power supplied to the excitation coil 173 based on the detection result of the voltage and current actually supplied to the excitation coil 173 by the voltage detection unit 196 and the current detection unit 197, and the coil Feedback control is performed to adjust the control frequency so that the supplied power is maintained at the target supplied power notified from the control unit 60.

  However, when the temperature of the non-sheet passing region reaches the Curie temperature, a phenomenon occurs in which the output of the exciting coil 173 decreases even if the control frequency is the same. This is because, when the surface temperature of the non-sheet passing area reaches the Curie temperature, the non-sheet passing area becomes a paramagnetic material, and among the magnetic flux generated from the exciting coil 173, the magnetic flux guided to the non-sheet passing area is small. As a result, the amount of magnetic flux returning to the exciting coil 173 increases, so that the inductance of the exciting coil 173 increases. As a result, a phase difference occurs between the voltage and current supplied from the IH power source 190, and the reactive power This is because it becomes larger.

FIG. 6B is a graph of the control frequency table of FIG.
In FIG. 6B, the vertical axis represents the control frequency (kHz) and the horizontal axis represents the target supply power (W). For example, when the control frequency is 53.3 kHz, the surface temperature of the entire fixing belt 155 is less than the Curie temperature. In contrast to the target power supply at 800 W, when the temperature of the non-sheet-passing region becomes equal to or higher than the Curie temperature, the power supply is lowered to about 300 W (see the prediction line L indicated by the broken line). That is, when the control frequency is continuously maintained at 53.3 kHz, when the temperature of the non-sheet passing region reaches the Curie temperature, the supplied power is reduced from around 800 W to around 300 W. Similarly, power reduction occurs at other control frequencies.

Feedback control is performed so as not to cause such a rapid power reduction, but a time lag still occurs until the power is restored. During this time, the temperature of the fixing belt 155 decreases and the fixing temperature is partially increased. This may cause a problem that the fixing property and glossiness are lower than those of other portions.
Therefore, when the temperature of the non-sheet passing region reaches the Curie temperature, the CPU 1911 refers to the B column in FIG. 6A and switches to the control frequency (33.4 kHz) corresponding to the target supply power (800 W). The output is made to the switching element 192. By switching the control frequency at once, the rapid decrease in power is suppressed, the time lag until the power is restored is shortened, and the problem of fixing property and glossiness due to the temperature decrease of the fixing belt 155 occurs. I am trying not to.

Thereafter, while the temperature of the non-sheet passing region is at the Curie temperature, the CPU 1911 calculates the supply power of the excitation coil 173 and maintains the target supply power as before the Curie temperature is reached. Continue feedback control to adjust.
FIG. 7A shows the surface temperature of the central region (hereinafter referred to as “central region temperature”) Ts and the surface temperature of the end region P (hereinafter referred to as “end region temperature”) Tp in the width direction of the fixing belt 155. It is a graph which shows the mode of a change of roughly. FIG. 7A shows a state in which a print job for a large size recording sheet (A3 portrait) is executed after a job for continuous printing on a small size recording sheet (A4 portrait) is performed. Yes.

In the figure, the vertical axis represents the surface temperature (° C.) of the central region and the end region P, and the horizontal axis represents the elapsed time (seconds) from the start of warm-up control for raising the temperature of the fixing belt 155 to the target fixing temperature. ).
FIG. 7B is a graph schematically showing how the control frequency Fr and the supplied power Pw actually supplied to the exciting coil change at this time. The vertical axis represents the supplied power (W) on the left side, the control frequency (kHz) on the right side, and the elapsed time (seconds) similar to FIG. 7A.

  First, as shown in FIG. 7A, by the warm-up control, the central region temperature Ts reaches the target fixing temperature of 180 ° C., and continuous printing of the small size recording sheet is started (time t1), and the non-sheet passing region The difference between the end region temperature Tp of the end region P and the central region temperature Ts increases, and at a time t2 after a while has elapsed from the time t1 (here, at the time of printing the second sheet), the end region Consider a case where the temperature Tp reaches the Curie temperature (about 220 ° C.).

  After the warm-up control (before time t1), the frequency control unit 191 receives a notification of, for example, 800 W as the target supply power to the excitation coil 173 from the control unit 60, and receives the control frequency table (A) in FIG. The control frequency (53.3 kHz) corresponding to 800 W in the column) is read out, and the switching element 192 is feedback-controlled based on the control frequency (see FIG. 7B).

  At time t2, this control frequency is switched to a control frequency (33.4 kHz) corresponding to 800 W in the control frequency table (column B), and feedback control of the switching element 192 is continued. Note that a slight time lag occurs until the control frequency reaches 33.4 kHz, and a slight decrease in power is caused thereby (part indicated by reference numeral X1), but there is no particular problem in terms of fixing properties and glossiness. Yes.

Then, after the small size recording sheet is printed, when the large size recording sheet is passed through the fixing nip 155n (time t3), the recording sheet passes through the end region P and takes heat away. Then, the end region temperature Tp decreases and falls below the Curie temperature (time t4).
Usually, magnetic permeability of the magnetic shunt alloy gradually increases as the temperature drops from the Curie temperature, and the permeability changes abruptly at a certain temperature (for example, a temperature lowered by 4 ° C. from the Curie temperature). Returning to a magnetic material (hereinafter, the temperature of the magnetic shunt alloy when it completely returns to its original ferromagnetic state is called “ferromagnetic recovery temperature.” The temperature difference between the Curie temperature and the ferromagnetic recovery temperature is Although it depends on the type of alloy, it is approximately 10 ° C.).

  Therefore, when the temperature of the end region P starts to fall below the Curie temperature at time t3, the output of the exciting coil 173 temporarily increases. However, after that, when it is determined that the end region temperature Tp falls below the Curie temperature by a method described later (time t4), the supply power control in the IH power source 190 is changed from feedback control to fixed frequency control. I try to switch.

In this fixed frequency control, the control frequency output to the switching element 192 is forcibly changed to a fixed frequency set in advance so that overshoot of the supplied power can be suppressed in anticipation of the increase in power. Control is performed so as to fix the fixed frequency until the target supply power is stabilized.
FIG. 8 shows a specific example of the fixed control frequency table stored in the fixed control frequency table storage unit 608.

As shown in the figure, the fixed control frequency table stores a target supply power and a fixed frequency corresponding thereto. For example, when the target supply power is 800 W, the fixed frequency when switching to the fixed frequency control is 53.3 kHz.
In the present embodiment, the value of the fixed frequency corresponding to each target supply power is the same as the control frequency in the control frequency table (column A) in FIG. 6A, but the present invention is limited to this. is not. At least overshoot is suppressed if the value can reduce the fluctuation of the power supplied to the exciting coil 173 due to the response delay (time lag) when the end region temperature Tp is lower than the Curie temperature in the conventional feedback control. This is because it can contribute to power saving. However, from the viewpoint of avoiding the occurrence of uneven fixing and uneven gloss, temperature fluctuations in the end region of the fixing belt before the temperature drops below the Curie temperature and due to fluctuations in the power supplied to the exciting coil at the ferromagnetic return temperature It is desirable that the value of the fixed control frequency is set so that the amount can be suppressed. This value can be obtained in advance by experiments or the like according to the specifications of the model.

  Returning to FIG. 7B, at time t3, when the end region temperature Tp falls below the Curie temperature, the output of the exciting coil 173 increases, but at the time t4, the frequency control unit 191 (FIG. 5) The fixed frequency (53.3 kHz) corresponding to 800 W in the fixed control frequency table of FIG. 8 is read, and the control frequency output to the switching element 192 is switched to the read fixed frequency to be high. Note that even if the temperature of the end region P is lower than the Curie temperature, the permeability does not become so high as long as the temperature is close to the Curie temperature. (Portion indicated by symbol X2) occurs (time t4 to t5). This is because the amount of decrease in power supply due to the increase in the control frequency is still greater than the amount of increase in power supply due to changes in the permeability of the end region P.

Thereafter, as the temperature of the end region P decreases from the Curie temperature and approaches the ferromagnetic return temperature, the permeability of the end region P gradually increases and the supplied power is restored (time t5). ~ T6).
The inventors of the present invention temporarily reduce the surface temperature of the fixing belt due to such a temporary drop in power supply (the portion indicated by symbol Y in FIG. 7A). If the temperature is higher than the fixing temperature (for example, 170 ° C.), there is no problem in fixing properties, and if the temperature drop is within 10 ° C., for example, it has been confirmed that the occurrence of uneven gloss is suppressed. .

  Such fixed frequency control is temporary control for avoiding a large power fluctuation when the temperature of the region once reaching the Curie temperature is lower than the Curie temperature, and the supplied power is within a predetermined set power range. It is desirable to perform feedback control promptly after recovery. In this embodiment, the set power range in this case is set to ± 10 W of the target supply power.

Therefore, when the output of the excitation coil 173 recovers to the set power range (position K in the figure) (time t6), the control is switched to feedback control.
Since the supply power is switched to feedback control while the supply power is increasing, the supply power immediately after switching is somewhat unstable (the part surrounded by a broken line), but compared to the conventional case where overshoot was repeated The fluctuation amount and period of power can be greatly reduced, and wasteful power consumption can be significantly suppressed.

(5) Temperature Control Processing FIG. 9 shows the control contents in the temperature control processing executed to maintain the fixing belt 155 at the fixing temperature when a print job is executed after warm-up in this embodiment. It is a flowchart, and is executed by the control unit 60 and the frequency control unit 191 of the IH power supply 190.

  This flowchart is executed as a subroutine of a main flowchart (not shown) for controlling the overall operation of the printer 1, and after the printer 1 is turned on or after a print job is accepted and the sleep mode is canceled. Alternatively, after the standby state is canceled, the control is started after the control for increasing the temperature of the fixing belt 155 to the target fixing temperature (180 ° C.) is executed.

In the stage immediately after the start of temperature control, the end region temperature Tp of the fixing belt 155 is still sufficiently lower than the Curie temperature, so the column A is selected in the control frequency table of FIG. 6A (step S1). A feedback control process for maintaining the surface temperature of the sheet passing area of the fixing belt 155 at the target fixing temperature is executed (step S2).
FIG. 10 is a flowchart showing the control contents in the subroutine of the feedback control process.

First, the detected temperature of the central thermistor 180 is acquired (step S21), and the excitation coil 173 is maintained to maintain the target fixing temperature by referring to the table in the target supply power table storage unit 607 from the comparison result with the target fixing temperature. The target supply power to be supplied to the device is determined (step S22).
The determined target supply power value is notified to the CPU 1911 in the frequency control unit 191 of the IH power supply 190, and the CPU 1911 compares the determined target supply power value with the current target supply power value ( Step S23).

When the value of the target supply power has changed (step S23: YES), it corresponds to the determined target supply power with reference to the currently selected control frequency table (A column in FIG. 6A). The control frequency is acquired, and the switching element 192 is turned on / off (step S24).
When the value of the target supply power has not changed (step S23: NO), the CPU 1911 monitors the supply power to the excitation coil 173 by sampling the detection values of the voltage detection unit 196 and the current detection unit 197. Feedback control is performed by adjusting the control frequency so that this power is maintained at the target supply power (step S25).

Here, the current target supply power value is stored in, for example, the storage area of the RAM 604, and is updated with the determined target supply power value after step S24 is executed.
After step S24 or S25, the process returns to the flowchart of FIG.
Then, in step S3 of FIG. 9, it is determined whether or not the temperature adjustment should be terminated. For example, when the print job being executed ends or when a predetermined time has elapsed after the end of the print job, it is determined that the temperature adjustment should be ended, and the process returns to a main flowchart (not shown).

If the temperature adjustment is not yet finished (step S3: NO), if the end region temperature Tp of the fixing belt 155 has not reached the Curie temperature (step S4: NO), the process returns to step S2 and the feedback control process is performed. repeat.
If the Curie temperature is reached (step S4: YES), this time, the B column of the control frequency table is selected (step S5), and the surface temperature of the sheet passing area of the fixing belt 155 is set to the target fixing temperature as in step S2. A subroutine (FIG. 10) of the feedback control process for maintaining is executed (step S6). This is because, as described above, the corresponding control frequency varies greatly even with the same target supply power depending on whether or not the end region temperature Tp is equal to or higher than the Curie temperature. In step S24 of FIG. 10, the value of the control frequency is acquired with reference to the B column of the control frequency table.

If the temperature adjustment is not yet finished (step S7: NO), a determination process is performed to determine whether or not it is the timing when the end region temperature Tp of the fixing belt 155 decreases and falls below the Curie temperature (step S8). .
FIG. 11 is a flowchart showing the control contents in a subroutine for determining whether or not the timing is lower than the Curie temperature.

  First, the end region temperature Tp detected by the end thermistor 181 is acquired (step S31), and it is determined whether the temperature Tp is equal to or lower than a preset threshold temperature (Tp ≦ 218 ° C.) (step S31). S32). This threshold temperature is preferably lower than the Curie temperature and higher than the temperature at which the magnetic permeability rapidly increases. This is because it is desirable to switch to the fixed frequency control at an earlier point of time when returning to ferromagnetism as much as possible in order to avoid the repetition of the overshoot described above. From this point of view, it can be said that the determination of whether or not the timing is lower than the Curie temperature is more accurately the determination of the timing of starting the temperature drop from the Curie temperature (hereinafter also referred to as “temperature drop timing”).

If Tp ≦ 218 ° C. (step S32: YES), it is determined that it is the temperature drop timing at which the temperature will drop thereafter, and the flag F1 = 1 is set (step S33), and Tp ≦ 218. If it is not ° C. (step S32: NO), the flag F1 = 0 is set (step S34).
Thereafter, the process returns to the flowchart of FIG.

Then, in step S9 of FIG. 9, it is determined whether or not F = 1. If F = 1 is not satisfied (step S9: NO), the temperature drop timing is not yet reached, so the process returns to step S6. The feedback control process is repeated.
In step S9, when F = 1 (step S9: YES), it is determined that the temperature is lower than the Curie temperature, and the control unit 60 determines from the fixed frequency table (FIG. 8) in the fixed control frequency table storage unit 608. The CPU 1911 is instructed to acquire a fixed frequency corresponding to the target supply power currently notified to the CPU 1911 and switch the control frequency to the fixed frequency. The CPU 1911 stops the feedback control according to the instruction and controls the switching element 192 on / off with the instructed fixed frequency (step S10).

Thereafter, the supply power to the exciting coil 173 is detected from the detection values of the voltage detection unit 196 and the current detection unit 197, and whether the detection power is within a predetermined set power range (in this case, within ± 10 W of the target supply power). It is determined whether or not (step S11).
If it is determined in step S11 that the detected power is within the set power range (step S11: YES), the process returns to step S1 to return to the feedback control, and the column A in the control frequency table of FIG. Select and execute the feedback control process of step S2. Thereafter, while circulating the above steps, if it is determined in step S3 or S7 that the temperature adjustment has been completed, the temperature adjustment process is terminated and the process returns to a main flowchart (not shown).

  In this way, in the fixed frequency control, the time required until the supplied power is restored to within the set power range can be obtained in advance by experiments or the like, so whether or not the detected power in step S11 is within the set power range. Instead of this determination, the elapsed time since switching to fixed frequency control is measured, and it is determined whether this time has passed a standby time that has been determined in advance as the excitation coil output is stable. When the time has elapsed, the process may return to step S1. This standby time is about several seconds, for example, and is stored in the ROM 603 in advance.

  As described above, according to the present embodiment, in the fixing belt 155 having the magnetic shunt alloy layer 155a, the temperature in the region that has reached the Curie temperature is lowered and lower than the Curie temperature during execution of feedback control. When it is determined that the lowering timing is reached, the fixed frequency control is executed to switch to a preset fixed frequency in anticipation of an increase in the output of the exciting coil 173 when the magnetic permeability of the magnetic shunt alloy is completely restored to ferromagnetic after that. In addition, the degree of overshoot exceeding the target supply power, which has been caused by the time lag in the feedback control, can be reduced. Therefore, the effect that the useless power consumption by overshoot can be suppressed can be acquired.

In the present embodiment, when the control unit 60 and the frequency control unit 191 execute the corresponding steps of the flowcharts of FIGS. 9 and 10, the control unit 60 and the frequency control unit 191 function as “power control means” in the present invention, and correspond to the flowcharts of FIG. 11. When executing this step, it functions as the “determination means” in the present invention.
<Modification>
The present invention is not limited to the above-described embodiment, and the following modifications can be considered.

(1) Modified example of determination process of whether timing is lower than Curie temperature (1)
In the temperature adjustment process in the above embodiment, the end region P of the fixing belt 155 is below the Curie temperature when the end region temperature Tp detected by the end thermistor 181 is equal to or lower than a predetermined threshold temperature (218 ° C.). It was determined that it was a temperature drop timing (refer to the determination process of whether or not the timing is lower than the Curie temperature in FIG. 11).

Hereinafter, a modified example in which the temperature drop timing in the end region can be determined without the end thermistor 181 will be described.
(1-1) First Modification In this modification, whether or not the end region temperature Tp of the fixing belt 155 falls below the Curie temperature due to a change in the control frequency output from the CPU 1911 of the IH power supply 190 to the switching element 192. Judgment is made.

FIG. 12 is a graph showing the relationship between the power supplied to the exciting coil 173 and the control frequency.
The horizontal axis indicates the control frequency [kHz], and the vertical axis indicates the power [W] supplied to the exciting coil 173.
A line 61 indicates a relationship between the control frequency and the supplied power in a state where the entire fixing belt 155 is maintained at the target fixing temperature, and a line 62 indicates a non-sheet passing region when the sheet is continuously passed in an A4 vertical size. The relationship between the control frequency and the supplied power when the temperature Tp of (the entire end region P) reaches the Curie temperature is shown.

As shown in the figure, when the same electric power is supplied, the direction when the fixing belt 155 reaches the Curie temperature (line 62) is greater than when the entire fixing belt 155 is maintained at the target fixing temperature (line 62). Line 61), the control frequency is low.
Therefore, for example, when a line 63 passing through a point that is proportionally distributed between the line 61 and the line 62 with a predetermined ratio with respect to the same supply power is considered, and the control frequency becomes higher than the line 63 by feedback control. It is possible to determine that the temperature is lower than the Curie temperature (hereinafter, the line 63 is referred to as a “threshold line”, and a value at each supply power of the threshold line 63 is referred to as a “threshold control frequency”).

This threshold line 63 can be obtained in advance by experiments. For example, as in the above-described embodiment, the supply power value when the end region temperature Tp of the fixing belt 155 is lower than the Curie temperature by a predetermined value (218 ° C. in the above) is detected for a plurality of control frequencies. Thus, the approximate power can be obtained by plotting the supply power of these detected values on a graph.
Since the threshold control frequency can be obtained for each target supply power based on the obtained threshold line 63, in this modification, the control frequency generated by the CPU 1911 is monitored, and this exceeds the predetermined threshold frequency. Sometimes it is determined that the temperature is falling.

  Specifically, for example, when the end region temperature Tp of the fixing belt 155 is equal to or higher than the Curie temperature, when the target supply power notified from the control unit 60 is 800 W, the CPU 1911 has 33.4 kHz as described above. The switching element 192 is controlled at the control frequency. When the end region temperature Tp is lower than the Curie temperature and the magnetic permeability is increased, the coil supply power is gradually increased. Increase the frequency.

Immediately after the temperature drops below the Curie temperature, the degree of change in permeability is still moderate, the degree of change in the power supplied to the exciting coil is also gentle, and feedback control can be followed. When the control frequency becomes 36.4 kHz, it is possible to determine that it is the temperature drop timing.
As described above, the threshold control frequency corresponding to each target supply power is obtained in advance using the graph of FIG. 12, and the threshold control frequency table is created and stored in the ROM 603.

FIG. 13 is a diagram showing an example of the threshold control frequency table. In the present embodiment, the range of the target supply power is shown in the range of 500 W to 1000 W used during temperature control, but of course it is not limited to this range.
FIG. 14 is a flowchart showing the control contents in the determination process of whether or not the timing is lower than the Curie temperature according to the present modification.

First, the control part 60 acquires the control frequency Fa currently given to the switching element 192 from CPU1911 (step S41).
Next, the threshold control frequency Ft corresponding to the target supply power notified to the current CPU 1911 is acquired with reference to the threshold control frequency table shown in FIG. 13 (step S42).
Then, it is determined whether or not the current control frequency Fa is equal to or higher than the threshold control frequency Ft (Fa ≧ Ft) (step S43).

If Fa ≧ Ft, it is considered that the temperature is lower than the Curie temperature. Therefore, the flag F = 1 is set (step S43: YES, step S44). If Fa ≧ Ft, the temperature is not lower than the Curie temperature. The flag F is set to 0 (step S43: NO, step S45).
When the above process is completed, the process returns to the flowchart of FIG. 9, and the flag is determined in step S9. If F = 1 (step S9: YES), the control is switched to the control based on the fixed frequency (step S10).

Note that the determination in step S4 in FIG. 9 can also be easily performed by monitoring the control frequency in the same manner as described above. Specifically, for example, when the target supply power is 800 W, if the current control frequency Fa is 33.4 kHz or less (see the line 62 in FIG. 12), the temperature of the non-sheet passing area is the Curie temperature. Can be determined to have reached.
(1-2) Second Modification Also, when the end region temperature Tp of the fixing belt 155 approaches the Curie temperature, the resonance value generated in the LC resonance circuit of the IH power supply 190 is changed by the change in the reactor value of the exciting coil 173. Even when the temperature falls below the Curie temperature, the reactor value of the exciting coil 173 changes and the resonance waveform of the LC resonance circuit of the IH power supply 190 changes. Therefore, it is also possible to determine the temperature drop timing below the Curie temperature using the parameter indicating this change state.

15 (a) to 15 (c) and FIG. 16 show the resonance waveforms generated in the LC resonance circuit composed of the capacitor 193 and the exciting coil 173 of the IH power supply 190 (in this modification, the collector side (point P2) of the switching element 192). It is a figure for demonstrating the voltage change in.
The voltage Vdc output from the diode bridge 195 is applied between the point P1 and the emitter of the switching element 192 (see FIG. 5). In this state, when the switching element 192 is turned on by the control frequency from the CPU 1911, As shown in FIG. 15A, a current Ic1 and a current Ic2 flow out between the collector and the emitter of the exciting coil 173 and the switching element 192, respectively (see the part of FIG. 16A).

Thereafter, when the switching element 192 is turned off, as shown in FIG. 15B, the current in the exciting coil 173 flows into the capacitor 193, so that the potential at the point P2 gradually increases (in FIG. 16). (See part (b)).
When the charging of the capacitor 193 is completed, the amount of electricity charged in the capacitor 193 is discharged, a current in the reverse direction flows through the exciting coil 173, and the potential at the point P2 drops (see the part (c) in FIG. 16). ).

Then, at the timing when the control frequency changes from OFF to ON, the electrical energy stored in the exciting coil 173 passes through a diode D (not shown) built in the switching element 192, and a regenerative current flows ( FIG. 16 (d) portion).
Thereafter, the above-described FIGS. 16A to 16C are repeated according to the ON / OFF change of the switching control signal according to the control frequency.

As described above, the resonance waveform (change in voltage Vce) in the IH power supply 190 changes in a mountain shape when the switching signal is OFF. When the switching control signal is ON, the voltage has a predetermined value (referred to as “V0”).
Then, as the end region temperature Tp of the fixing belt 155 approaches the Curie temperature and the magnetism changes, the reactor value of the exciting coil 173 changes and the time for generating the resonance waveform becomes longer, and when the temperature falls below the Curie temperature. The time for generating the resonance waveform tends to be shortened.

FIGS. 17A and 17B show changes in the resonance waveform when the Curie temperature is reached and when it is below the Curie temperature, respectively.
In the example shown in the figure, the time for generating one resonance waveform at the Curie temperature (hereinafter referred to as “LC resonance time”) was 9 μs (see FIG. 17A). When it falls below, it is shortened to 7 μs (see FIG. 17B).

Therefore, for each target supply power, a threshold resonance time at a time point that is considered to be lower than the Curie temperature by an experiment or the like is obtained in advance, and it is determined that the LC resonance time is lower than the Curie temperature when the LC resonance time becomes the threshold value. it can.
Therefore, in this modification, in the IH power supply 190, as shown by a two-dot chain line in FIG. 5, a circuit is formed such that the potential at the point P2 is input to the CPU 1911, and the CPU 1911 detects the LC resonance time. I have to. The LC resonance time is measured, for example, by comparing the potential at the point P2 with the reference potential “Vo” by a comparator inside the CPU 1911 and measuring the time when the absolute value of the potential at the point P2 is larger than the absolute value of the reference potential. Can be obtained.

The measurement of this time can be easily obtained by counting the internal clock, and the LC resonance time obtained thereby is transmitted to the control unit 60 as needed. Further, a threshold resonance time table as shown in FIG. 18 is created in advance and stored in the ROM 603.
FIG. 19 is a flowchart showing the control content of the determination process of whether or not the timing is lower than the Curie temperature according to the present modification.

First, the control unit 60 updates the LC resonance time transmitted from the CPU 1911 as needed, stores it in the RAM 604, and acquires the updated latest LC resonance time as the current LC resonance time Ra (step S51).
Next, the threshold resonance time Rt corresponding to the target supply power currently notified to the CPU 1911 is acquired with reference to the threshold resonance time table shown in FIG. 18 (step S52).

Then, it is determined whether or not the current LC resonance time Ra is equal to or less than the threshold resonance time Rt (Ra ≦ Rt) (step S53).
If Ra ≦ Rt, it is considered that the temperature is lower than the Curie temperature, so the flag F = 1 is set (step S53: YES, step S54), and if Ra ≦ Rt, the temperature is not lower than the Curie temperature. In order to show, flag F = 0 is set (step S53: NO, step S55).

When the above process is completed, the process returns to the flowchart of FIG. 10, and the flag is determined in step S9. If F = 1 (step S9: YES), the control is switched to the control based on the fixed frequency (step S10).
As shown in FIGS. 17A and 17B, the LC resonance time is equal to the OFF time of the switching control signal (hereinafter referred to as “OFF time”). It can be determined whether the temperature is below. The OFF time can be easily obtained by, for example, detecting the OFF state of the control frequency generated by the CPU 1911 and counting the clock.

  The control unit 60 acquires the OFF time from the CPU 1911, and when the OFF time becomes equal to or greater than the threshold OFF time obtained from the threshold OFF time table (not shown) created in the same manner as in FIG. It may be determined that the temperature is lower. The flowchart at this time is substantially the same as that in FIG. 19 except that the LC resonance time is replaced with the OFF time, and thus the illustration is omitted.

In the case of this modification, the determination in step S4 in FIG. 9 can be made by monitoring the OFF time of the switching control signal. Specifically, for example, if the OFF time of the switching control signal is 9 μs or more, it can be determined that the temperature of the non-sheet passing region has reached the Curie temperature.
The threshold tables in the first and second modifications have been described on the assumption that the recording sheet has a specific size (A4 longitudinal size), but the power supplied to the exciting coil 173 is fed back. In order to determine whether the temperature is lower than the Curie temperature using the change in the parameters during the control (control frequency in the first modification, LC resonance time or OFF time in the second modification) as an index, Is also applicable in common.

  Of course, the amount of change in the reactor value of the exciting coil 173 when the temperature reaches the Curie temperature and when the temperature falls below the Curie temperature differs depending on the size of the non-sheet passing area (see FIG. 12). If the threshold value table is created for each size and the control unit 60 executes the temperature adjustment process by referring to the corresponding table according to the printing conditions of the print job currently being executed, Detailed temperature control can be performed.

(2) Modified example of determination process of whether timing is lower than Curie temperature (part 2)
In the above embodiment, when the end region temperature Tp is equal to or lower than the threshold temperature (218 ° C.), it is determined that the timing immediately falls below the Curie temperature and immediately switches to fixed frequency control. A reconfirmation step may be provided before this.
Even when printing of a small size recording sheet continues, for example, if a large amount of paper is passed, the heat of the non-paper passing area moves to the paper passing area, and the temperature of the edge area P temporarily falls below the Curie temperature. This is because there is a possibility that the temperature immediately returns to the Curie temperature. In this case, if switching to fixed frequency control immediately, the magnetic shunt alloy layer remains paramagnetic and the frequency is increased. As a result, the supplied power is greatly reduced, and in the worst case, uneven fixing may occur.

Therefore, in this modification, when it is detected that the end region temperature Tp is equal to or lower than the threshold temperature (218 ° C.) (step S32 in FIG. 11), the size of the recording sheet is further confirmed, and then the Curie is confirmed. The temperature drop timing below the temperature is judged.
FIG. 20 shows an example of a subroutine for determining whether or not the timing is lower than the Curie temperature in the case of implementing this modification, and the same contents as those in the flowchart of FIG. 11 are given the same step numbers. ing.

As shown in the drawing, after it is determined in step S32 that the surface temperature Tp of the end region has become 218 ° C. or lower, the flag F1 = 1 is not set immediately, but the recording sheet of the print job currently being executed It is determined whether or not the size in the width direction is larger than the size Sa in the width direction of the recording sheet of the print job so far (step S321).
This determination is made, for example, by updating the recording sheet size Sa of the completed print job every time the print job is completed and storing the updated recording sheet size Sa in the RAM 604, from the print condition storage unit. It can be easily done by reading and comparing the two.

  In this way, when the size in the width direction of the recording sheet of the currently executing print job is larger than the width direction size Sa of the recording sheet in the previous print job (step S321: YES), this causes the end portion. Since the heat of the region is taken away, it is considered that the surface temperature continues to fall below the Curie temperature. In this case, since it can be determined that the temperature is not temporarily lowered, the process proceeds to step S33 and the flag F is set to “1”.

On the other hand, if the size in the width direction of the recording sheet of the print job currently being executed is not larger than the size Sa in the width direction of the recording sheet in the previous print job (step S321: NO), the temperature is temporarily reduced. After the determination, the process moves to step S34, and the flag F is set to “0”. Thereafter, the process returns to the flowchart of FIG.
According to this modification, it is possible to more reliably determine that the temperature of the end region is decreasing from the Curie temperature, and to switch to fixed frequency control at an appropriate timing.

(3) Modified example of fixed frequency control In the above embodiment, a case has been described in which only one fixed frequency is set when switching to fixed frequency control in each target supply power. However, the present invention is not limited to this. Alternatively, the fixed frequency may be switched and controlled in stages.
For example, as shown in FIG. 21, a fixed control frequency table in which a first fixed frequency corresponding to the target supply power and a second fixed frequency higher than the first fixed frequency are set is prepared.

FIG. 22 shows a flowchart of the temperature adjustment process in this case, and for the convenience of explanation, only the parts different from the flowchart of FIG. 9 are shown.
As shown in FIG. 22, after step S9 (see FIG. 9), instead of steps S10 and S11, first, a first fixed frequency corresponding to the target supply power is obtained from the fixed frequency table, and feedback control is performed. Switching to fixed frequency control using the first fixed frequency (step S91).

  Thereafter, the end region temperature Tp detected by the end portion thermistor 181 is acquired (step S92), and the temperature Tp is lower than a threshold temperature for determining whether the temperature falls below the Curie temperature (218 ° C. in this example). The fixed frequency control using the first fixed frequency is continued until the temperature becomes equal to or lower than the first temperature T1 (step S93: NO). The first temperature T1 is set to a temperature (216 ° C. in this example) at a timing at which the permeability of the end region P rapidly changes (becomes higher).

  If the end region temperature Tp is equal to or lower than the first temperature T1 (step S93: YES), the second fixed frequency corresponding to the target supply power is acquired from the fixed frequency table, and the second fixed frequency corresponding to the target supply power is obtained from the first fixed frequency. Switching to fixed frequency control with a fixed frequency of 2 (step S94). Then, the end region temperature Tp detected by the end thermistor 181 is acquired (step S95), and the temperature Tp is equal to or lower than the second temperature T2 that is lower than the first temperature T1 (step S96). : NO), the fixed frequency control by the second fixed frequency is continued. The second temperature T2 is a temperature close to the ferromagnetic return temperature (about 210 ° C. in this example), and the permeability of the end region P approaches the permeability of the ferromagnetic material, and the change becomes gentle. A stable temperature is set. In this example, the second temperature T2 is set to 212 ° C., and this temperature is included in the temperature range when the power supplied to the exciting coil 173 returns to the above-described set power range.

When the end region temperature Tp becomes equal to or lower than the second temperature T2 (step S96: YES), the process returns to step S1 and switches to feedback control.
Thus, by switching the frequency to be fixed stepwise, the control frequency when the permeability of the end region P is still low (the end region temperature Tp is close to the Curie temperature) immediately after switching to the fixed frequency control. Compared with a case where the power supply is rapidly changed, it is possible to suppress a temporary drop in the supplied power (see FIG. 7B). As a result, it is possible to ensure that fixing unevenness and gloss unevenness do not occur at the time of switching to fixed frequency control, and this is effective particularly when accuracy is required, such as when reproducing a precise color image.

In step S93 and step S96, the end region temperature Tp is seen to determine whether to switch the fixed frequency or return to the feedback control. However, the present invention is not limited to this. For example, the power supplied to the excitation coil 173 You may make it make the said judgment by seeing a change. Further, the frequency to be fixed may be switched to three or more stages.
(4) Other Modifications (4-1) The values of each table shown in the above embodiment and modification are only shown as an example in the present embodiment, and depending on the specifications of the device to be applied, etc. It should be changed as appropriate.

Further, instead of the table showing the correspondence (FIG. 6A, FIG. 8, FIG. 13, FIG. 18 etc.), the relational expression showing the correspondence is stored and the control shown in each table based on this. You may make it acquire the value of various control parameters, such as a frequency and various threshold values.
(4-2) In the determination of whether or not it is the temperature lowering timing lower than the Curie temperature as shown in the above embodiment and modification, the end area P of the fixing belt 155 (the sheet passing area of the A3 size recording sheet) A configuration may be adopted in which it is determined whether or not the temperature of at least a part of the difference area of the A4 size recording sheet) is the temperature drop timing.

This is because if at least a part of the region enters the temperature decrease timing, the remaining region also enters the temperature decrease timing. By executing, overshoot exceeding the target supply power can be further suppressed.
(4-3) In the above embodiment, the control frequency is used as a parameter for controlling the power supplied from the IH power source 190 to the exciting coil 173. However, the present invention is not limited to this, and the circuit configuration of the IH power source is used. The type of parameter can change accordingly.

For example, in the case of a power supply circuit related to PWM (Pulse Width Modulation) control, the drive pulse width, on-duty ratio, and the like are parameters.
(4-4) In the above embodiment, the target supply power table storage unit 607 has shown a configuration in which a target supply power table that is commonly used regardless of the size of the recording sheet to be printed is stored. A target supply power table may be provided for each recording sheet size to be stored. When the recording sheet size is different, the amount of heat taken away from the fixing belt is different, so that by providing a target power supply table for each size, the temperature control of the fixing belt can be performed with higher accuracy.

(4-5) In the above embodiment, the target fixing temperature at the time of execution of the print job is fixed to 180 ° C. and the temperature control is executed. However, the target fixing temperature is appropriately changed according to the contents of the print job to be executed. It doesn't matter.
For example, when small-size recording sheets are continuously printed with the target fixing temperature of 180 ° C., the temperature of the paper passing area is adjusted to approximately 180 ° C., but the temperature of the non-paper passing area is It gradually rises and eventually reaches the Curie temperature (220 ° C.).

Next, when printing a large size recording sheet, the temperature difference between the paper passing area and the non-passing area of the previous small size recording sheet becomes the temperature difference in the paper passing area recording of the large size recording sheet as it is, This may cause uneven fixing and uneven gloss.
Therefore, if you have already received several print jobs and you know that you want to execute a print job for a large-sized recording sheet after a print job for a small-sized recording sheet, During the execution of a sheet print job, gradually increase the target power supply to raise the temperature of the sheet passing area, and before the start of passing a large size recording sheet, If the temperature control is performed so that the temperature difference in the non-sheet passing region is further reduced, the above-described problems of fixing unevenness and gloss unevenness do not occur when printing a large size recording sheet.

FIG. 23 is a graph showing an example of temperature control in such a case.
In the example shown in the figure, the end region temperature Tp has already reached the Curie temperature during continuous printing of a small size recording sheet.
The target fixing temperature Tx increases stepwise by 10 ° C. every time one small size recording sheet is passed, and accordingly, the power supplied to the exciting coil is increased and the temperature detected by the central temperature sensor ( Hereinafter, the “center temperature Ts” is gradually increased.

This target fixing temperature rises to approximately the same temperature as the Curie temperature when a small size recording sheet is passed immediately before passing a large size recording sheet. The temperature Ts becomes equal to the end region temperature Tp.
As a result, there is no temperature difference in the main scanning direction of the fixing belt in the sheet passing area of the large size recording sheet, so that no fixing unevenness and gloss unevenness occur. However, the center temperature Ts does not necessarily have to be raised to the Curie temperature, and may be any temperature difference from the end region temperature Tp within a range that does not cause fixing unevenness (for example, a temperature difference of about 10 ° C.).

Note that after the central portion temperature Ts becomes equal to the end region temperature Tp, the target fixing temperature is gradually decreased for each sheet passing from the time of passing the first large size recording sheet in the sheet passing region. It is desirable to finally reach 180 ° C.
When a large size recording sheet is continuously passed, heat is removed almost evenly in the main scanning direction in the passing area. Therefore, even if the target fixing temperature is lowered, the large size recording sheet is kept within the passing area of the large size recording sheet. This is because there is no risk of temperature difference, and if the target fixing temperature is kept at 220 ° C., power is wasted.

When a large-size recording sheet is passed, if the end region temperature Tp reaches a temperature lowering timing lower than the Curie temperature (position Z in FIG. 23), the same fixed frequency control as in the above embodiment is performed.
(4-6) In the fixing device 5, the pressing member that presses the fixing belt 155 to form the nip portion is not limited to the pressure roller, and may be a long pad-shaped member.

Needless to say, the means for detecting the temperature of the fixing member is not limited to the thermistor, and may be, for example, an infrared sensor.
Further, as an example of an image forming apparatus to which the fixing device 5 is applied, a tandem type color printer has been described. However, as described above, an electromagnetic induction method is used in a non-sheet passing region using a magnetic shunt alloy. As long as the image forming apparatus includes a fixing device having a configuration for preventing excessive temperature rise, a monochrome printer may be used, and of course, a copying machine, a facsimile machine, a multifunction machine, and the like can be applied.

  You may combine the said embodiment and each modification as much as possible.

  The present invention relates to an electromagnetic induction heating type fixing device using a magnetic shunt alloy layer and an image forming apparatus using the fixing device, and is particularly suitable as a technique for suppressing power consumption required for electromagnetic induction heating.

DESCRIPTION OF SYMBOLS 1 Printer 3 Image process part 4 Paper feed part 5 Fixing device 7 Operation panel 10 Exposure part 40,41 Paper feed cassette 60 Control part 150 Fixing roller 155 Fixing belt 155a Magnetic shunt alloy layer 155b Elastic body layer 155c Release layer 155n Fixing nip 160 Pressure roller 170 Magnetic flux generator 171 Coil bobbin 172 Bottom core 173 Excitation coil 180 Central thermistor 181 End thermistor 190 IH power supply 191 Frequency controller 192 Switching element 193 Capacitor 195 Diode bridge 196 Voltage detector 197 Current detector 200 AC Power supply 601 CPU
605 Image data storage unit 606 Print condition storage unit 607 Target supply power table storage unit 608 Fixed control frequency table storage unit 1911 CPU
1912 Control frequency table storage unit

Claims (9)

The pressing member is pressed against the surface of the fixing member that is electromagnetically induced by the exciting coil to form a nip portion, and the unfixed image on the recording sheet passed through the nip portion is thermally fixed, and the Curie temperature is set as a target. A fixing device having a configuration in which a temperature rise in a non-sheet passing region of a fixing member is suppressed using a magnetic shunt alloy layer set to a fixing temperature or higher,
Temperature detecting means for detecting the temperature of the sheet passing area of the recording sheet in the fixing member;
Determining whether or not at least a part of the non-sheet passing area is at a temperature lowering timing below the Curie temperature after the temperature of the non-sheet passing area of the recording sheet on the fixing member reaches the Curie temperature Means,
Power control means for determining a parameter value for controlling power supplied to the exciting coil;
Power supply means for supplying power to the exciting coil according to the value of the determined parameter;
With
The power control means includes
After the temperature of the non-sheet passing region reaches the Curie temperature, the target supply power to be supplied to the excitation coil is determined based on the detection result by the temperature detection unit until the determination unit determines that the temperature lowering timing is reached. Determining, adjusting the value of the parameter so as to maintain the target supply power, and performing feedback control to execute the first control,
When it is determined by the determination means that the temperature lowering timing, the value of the parameter is set so that the fluctuation amount of the power supplied to the exciting coil is larger than the fluctuation amount based on the response delay of the feedback control in the first control. A fixing device that executes a second control for switching to a fixed value set in advance so as to be small. A second temperature detecting means for detecting the temperature of the non-sheet passing area of the fixing member;
The fixing device according to claim 1, wherein the determination unit determines the temperature decrease timing based on a detection result by the second temperature detection unit.   The determination means determines whether or not it is the temperature decrease timing based on a change amount of a parameter value when feedback control is performed on electric power supplied to the exciting coil in the first control. Item 4. The fixing device according to Item 1. The power supply means includes an LC resonance circuit and a switching element for turning on / off current supply to the LC resonance circuit, and the parameter is a control frequency for controlling on / off of the switching element,
In the LC resonance circuit in the first control, the determination means corresponds to a change amount of time or a resonance waveform in which an absolute value of the resonance waveform of the output voltage is a voltage equal to or higher than a predetermined value. The fixing device according to claim 1, wherein the temperature decrease timing is determined based on a change amount of time when the switching element is off. The power control means includes
5. The fixing device according to claim 1, wherein after the first control is switched to the second control, the control returns to the first control when a predetermined time elapses. The power control means includes
The first control after the difference between the target supply power in the first control and the power supplied to the exciting coil after switching from the first control to the second control is within a certain range. The fixing device according to any one of claims 1 to 4, wherein the fixing device is restored to. The power control means includes
7. The electric power supplied to the exciting coil is controlled by changing the value of the parameter stepwise up to the preset fixed value in the second control. 8. The fixing device described. The determination means includes
Having size acquisition means for acquiring the size in the width direction of the recording sheet to be passed,
The fixing member has a second size recording sheet in which at least a part of the non-sheet passing region with respect to the first size recording sheet is below the Curie temperature and the width is larger than that of the first size recording sheet. The fixing device according to claim 1, wherein it is determined that the temperature lowering timing is reached only when the sheet is passed.   An image forming apparatus comprising the fixing device according to claim 1.

Images