JP2010008857A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device for preventing over-current of an induction coil on an energizing circuit, and also, preventing the temperature drop of a heated member upon changing the width of a magnetic circuit exerting on the member to be heated, and to provide an image forming apparatus equipped with the fixing device. <P>SOLUTION: In the fixing device and the image forming apparatus, upon changing the width of the magnetic circuit that exerts on the member to be heated by moving a magnetic member, a switching control of the power supply to the induction coil is continued (whether to apply induction heating (c)), and the magnetic member is continuously moved (whether to drive a core drive motor (d)) under the condition that the magnetic flux of the magnetic circuit exerting on the member to be heated is within a predetermined range near zero. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an induction heating type fixing device and an image forming apparatus including the same, and more particularly to a technique for changing the width of a heating target region of a member to be heated (such as a fixing roller) to be heated by induction. .

In general, an image forming apparatus such as a printer, a copying machine, a facsimile machine, or a multifunction machine of these uses an induction heating type fixing device that heats a heated member such as a fixing roller by electromagnetic induction. Specifically, in the fixing device, an induction coil and a magnetic core are disposed so as to face the maximum sheet passing area of a heated member made of a magnetic material. A magnetic flux is generated by passing a high-frequency current through the induction coil. As a result, the magnetic flux generated by the induction coil is guided to the member to be heated through the magnetic circuit formed by the magnetic core, and the member to be heated is heated by the eddy current (induction current) generated by electromagnetic induction by the magnetic flux. The
Conventionally, in such an induction heating type fixing device, when a paper having a size smaller than the maximum paper passing area is passed, in order to maintain the paper passing area through which the paper passes at a predetermined fixing temperature, There is a problem that overheating occurs in a non-sheet passing region near the end of the heated member through which the sheet does not pass. Therefore, a configuration in which the width of the heating target region in the heated member can be changed according to the paper size has been studied.
For example, in the fixing device disclosed in Patent Document 1, a plurality of magnetic cores having different lengths are rotatably arranged so that any one of them can be opposed to the induction coil. In such a configuration, by changing the magnetic core used for induction heating, the width of the magnetic flux acting on the member to be heated can be changed, and the width of the region to be heated in the member to be heated can be changed.
JP 2005-308783 A

By the way, although the patent document 1 does not mention the relationship between the switching of the magnetic core and the energization of the induction coil, first, the energization of the induction coil is continued even when the magnetic core is switched. Think about the case.
In this case, the inductance component on the current-carrying circuit of the induction coil may fluctuate rapidly due to changes in the characteristics of the magnetic circuit due to the movement of the magnetic core (sudden decrease in saturation magnetic flux, etc.), and an abnormal current increase may occur on the current-carrying circuit. is there. This is a particular problem when the magnetic core moves while the magnetic flux (magnetic flux density) existing on the magnetic circuit formed of the magnetic core is large.
On the other hand, when switching the magnetic core, consider the case where the energization to the induction coil is stopped. In this case, the temperature of the member to be heated decreases during switching of the magnetic core. For this reason, it is necessary to raise the heated member again to a predetermined fixing temperature after switching the magnetic core, and the waiting time until the start of printing becomes longer, and the printing speed (productivity) decreases. For example, when executing continuous printing of a plurality of originals in which A3 size and A4 size are mixed, a waiting time is required every time the A3 size and A4 size are switched. In particular, the smaller the heat capacity of the member to be heated, the more rapidly the temperature of the member to be heated decreases in a short time, so that the problem that the waiting time for reheating becomes longer appears. Also, in the heating control that is started after switching the magnetic core, problems such as overshoot occur in feedback control based on the temperature of the heated member detected by a predetermined temperature sensor, and it may take more time to stabilize the temperature. There is.
Accordingly, the present invention has been made in view of the above circumstances, and its object is to prevent overcurrent on the energizing circuit of the induction coil when changing the width of the magnetic circuit acting on the member to be heated. Another object of the present invention is to provide a fixing device capable of preventing a temperature drop of a heated member and an image forming apparatus provided with the fixing device.

In order to achieve the above object, the present invention provides an induction coil for generating magnetic flux upon receiving power supply, power supply control means for controlling switching of power supplied from an AC power source to the induction coil, and generated by the induction coil. A fixing device comprising: a magnetic member that forms a magnetic circuit that guides the magnetic flux to a member to be heated; and a magnetic path width changing unit that changes the width of the magnetic circuit by moving the magnetic member, When the width of the magnetic circuit is changed by the magnetic path width changing means, the magnetic flux of the magnetic circuit that is periodically changed by the switching control of the power supply control means is changed while the switching control by the power supply control means is executed. By causing the magnetic member to move by the magnetic path width changing means on condition that it is within a predetermined range in the vicinity of 0 set in advance, the magnetic circuit of the magnetic circuit There configured as characterized thereby intermittently change the width of the magnetic circuit for each fall within the periodically said predetermined range.
In the fixing device configured as described above, it is possible to prevent the temperature of the heated member from being lowered when the width of the magnetic circuit is changed and to allow a fixing operation after the width of the magnetic circuit is changed. It can be shortened. In addition, since the magnetic member is moved only when the magnetic flux of the magnetic circuit is within a predetermined range near the zero, a change in the characteristics of the magnetic circuit due to the movement of the magnetic member (such as a sudden decrease in saturation magnetic flux) occurs. The influence of the induction coil on the energizing circuit can be reduced, and the occurrence of overcurrent can be prevented.

The present invention further comprises temperature detection means for detecting the temperature of the heated member or the temperature of the second heated member heated by heat transfer from the heated member, and the power supply control means However, it is suitable for the configuration in which the switching control is performed based on the temperature detected by the temperature detecting means. In this configuration, since the temperature change of the heated member during movement of the magnetic member is suppressed to a small level, feedback control based on the detected temperature of the temperature detecting means after the magnetic member movement causes overshoot and the like. Problems can be avoided and the time to temperature stabilization can be shortened.
Here, an example of a method for determining whether or not the magnetic flux of the magnetic circuit is within the predetermined range will be described. For example, a rectifier circuit for half-wave rectification or full-wave rectification of the AC voltage output from the AC power supply, and zero-cross detection means for detecting a zero-cross point periodically generated in the output voltage from the AC power supply or the rectifier circuit; If the power supply control means is configured to perform switching control of power supplied from the rectifier circuit to the induction coil, from the time of detection of the zero cross point by the zero cross detection means until a predetermined time has elapsed. During this period, it is possible to determine that the magnetic flux of the magnetic circuit is within the predetermined range. In the above configuration, the magnetic flux of the magnetic circuit changes in a hysteresis curve according to the output voltage from the rectifier circuit, and the magnetic flux is the smallest at the zero cross point that is periodically generated in the output voltage from the rectifier circuit. Because it becomes.
Further, a configuration in which magnetic flux detection means for detecting magnetic flux acting on the magnetic circuit is provided and it is possible to determine whether or not the magnetic flux detected by the magnetic flux detection means is within the predetermined range.

On the other hand, as a configuration that enables the width of the magnetic circuit to be changed, the magnetic member includes a plurality of magnetic cores having different lengths in the longitudinal direction of the heated member, and the plurality of magnetic cores in the circumferential direction. The magnetic path width changing means drives the rotation of the support member to move the positions of the plurality of magnetic cores, thereby reducing the width of the magnetic circuit. It is possible to change. Accordingly, the width of the magnetic circuit can be changed by rotating the support member.
By the way, the present invention may be understood as an image forming apparatus including the fixing device. In this case, each component provided in the fixing device is not limited to the fixing device, but may be disposed in the image forming apparatus.

  According to the present invention, it is possible to prevent the temperature of the heated member from being lowered when the width of the magnetic circuit is changed, and to shorten the time until the fixing operation becomes possible after the width of the magnetic circuit is changed. . In addition, since the magnetic member is moved only when the magnetic flux of the magnetic circuit is within a predetermined range near the zero, a change in the characteristics of the magnetic circuit due to the movement of the magnetic member (such as a sudden decrease in saturation magnetic flux) occurs. The influence of the induction coil on the energizing circuit can be reduced, and the occurrence of overcurrent can be prevented.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings so that the present invention can be understood. The following embodiment is an example embodying the present invention, and does not limit the technical scope of the present invention.
FIG. 1 is a block diagram showing a schematic configuration of the copying machine X according to the embodiment of the present invention. FIG. 2 is a block diagram schematically showing a schematic configuration of the fixing device 5 according to the embodiment of the present invention. FIG. 3 is a diagram for explaining the configuration of the induction heating unit 54 provided in the fixing device 5 according to the embodiment of the present invention, and FIG. 4 shows how the magnetic flux changes in the fixing device 5 according to the embodiment of the present invention. FIG. 5 is a timing chart for explaining the magnetic path width changing process executed in the fixing device 5 according to the embodiment of the present invention.
First, a schematic configuration of the copying machine X according to the embodiment of the present invention will be described with reference to FIG.
As shown in FIG. 1, a copying machine X according to an embodiment of the present invention includes an operation display unit 1, an image reading unit 2, an image processing unit 3, an image forming unit 4, a fixing device 5, a control unit 6, and the like. In general, it is structured. The copying machine X has various other constituent elements that a general electrophotographic copying machine has, but since these are not different from the conventional ones, description thereof is omitted here. The present invention is not limited to the copying machine X, and can also be applied to, for example, an electrophotographic image forming apparatus such as a printer apparatus, a facsimile apparatus, and a multifunction machine having these functions and a scanner function.

The control unit 6 includes a CPU, peripheral devices such as a ROM and a RAM, and executes the process according to a predetermined program stored in the ROM while developing the copy machine X on the RAM. Control all over. When the invention is regarded as an invention of the image forming apparatus, a processing function of a CPU 71 of a heating control unit 7 described later provided in the fixing device 5 may be achieved by the control unit 6.
The operation display unit 1 includes a liquid crystal display that displays various types of information according to instructions from the control unit 6, a touch panel for performing operation input to the control unit 6, and the like.
The image reading unit 2 reads an image of a document set on a document table or an ADF (automatic conveyance device), and the image data read by the image reading unit 2 is input to the image processing unit 3. The
The image processing unit 3 performs various operations on image data of a document read by the image reading unit 2 and image data of a document input from an information processing apparatus (not shown) via a communication network such as a LAN. Image processing is performed. The image data that has been subjected to image processing by the image processing unit 3 is input to the image forming unit 4.
The image forming unit 4 includes a photosensitive drum, a charger, a developing device, an LSU, and the like. A toner image (developer) is applied to a sheet based on image data of a document input from the image processing unit 3. To form.
The fixing device 5 melts and fixes the toner image on the paper on which the toner image is formed by the image forming unit 4. The copying machine X according to the embodiment of the present invention is characterized by the configuration and operation related to the fixing device 5 and will be described in detail below.

As shown in FIG. 2, the fixing device 5 includes a fixing roller 51 and a pressure roller that rotate while pressing a sheet conveyed on the sheet conveying path 50 after the toner image is transferred in the image forming unit 4. A heating roller 53 (an example of a member to be heated) that is rotationally driven by a driving force transmitted from the fixing roller 51 via a fixing belt 52 stretched between the fixing roller 51 and the fixing roller 51; An induction heating unit 54 having an induction coil 54a and a magnetic core 54b (an example of a magnetic member) for induction heating the roller 53, and a core drive used for changing the width of a magnetic circuit formed by the magnetic core 54b A motor 55 (an example of a magnetic path width changing unit) and a heating control circuit 7 that controls the heating operation of the heating roller 53 in the fixing device 5 are provided.
The fixing roller 51 is connected to driving means such as a stepping motor (not shown), and is rotationally driven by the driving means. When the fixing roller 51 is rotationally driven, the fixing belt 52 travels by the driving force, and the heating roller 53 rotates by the driving force transmitted from the fixing belt 52. The heating roller 53 is made of a magnetic material such as iron or a magnetic shunt alloy.

FIG. 3 is a schematic cross-sectional view showing a schematic configuration of the induction heating unit 54.
The induction coil 54a is wound in the axial direction (the depth direction in FIG. 3) of the heating roller 53, and is substantially the same as the width in the longitudinal direction of the heating roller 53 (approximately the same width as the maximum sheet passing width). It is formed in width. The induction coil 54a is supplied with electric power and generates a magnetic flux, and the presence or absence of a high-frequency current at both ends thereof is switched by an IGBT 73 described later.
The magnetic core 54b forms a magnetic circuit (see thick arrow in FIG. 3) that guides the magnetic flux generated by the induction coil 54a to the heating roller 53.
In the fixing device 5, the magnetic flux generated by the induction coil 54 a is guided to the heating roller 53 by the magnetic core 54 b, and an eddy current (inductive current) is generated on the surface of the heating roller 53. The roller 53 is heated. As a result, the fixing belt 52 is heated by the heat transfer from the heating roller 53, and the adhering toner is melted and fixed on the paper contacting the fixing belt 52. It is also conceivable that the fixing belt 52 is made of a magnetic member, and the fixing belt 52 is directly heated by a magnetic flux acting from the magnetic core 54b. Further, the fixing roller 51 may be directly induction-heated without the heating roller 53 or the fixing belt 52. In this case, the fixing belt 52 and the fixing roller 51 can be regarded as an example of a member to be heated.
Furthermore, the induction heating unit 54 employs a configuration that can adjust the width of the heating region in the heating roller 53 when the heating roller 53 is induction heated.

As shown in FIG. 3, the magnetic core 54b has a center core 541 and an outer core 542 made of a ferromagnetic material such as ferrite.
The outer core 542 is composed of a pair of U-shaped and inverted U-shaped magnetic cores surrounding both sides of the heating roller 53. The outer core 542 has a plurality of plate-like cores formed over the entire width of the heating roller 53 in the longitudinal direction or arranged at predetermined intervals over the entire width of the heating roller 53 in the longitudinal direction. Is.
The center core 541 is disposed between a pair of magnetic cores in the outer core 542, and forms a magnetic circuit with the pair of magnetic cores. The center core 541 includes a plurality of magnetic cores 541a to 541c having different lengths in the longitudinal direction of the heating roller 53, and a rotating shaft 541d (an example of a support member) that supports the magnetic cores 541a to 541c in parallel in the circumferential direction. ). For example, each of the magnetic cores 541a, 541b, and 541c has a length corresponding to the width of each of A3, B4, and A4.
The magnetic cores 541a to 541c are respectively configured so that the copying machine X is configured such that the sheet passing through the fixing device 5 passes through the center of the fixing belt 52. The center of the fixing belt 52 is arranged so as to correspond to the center of the fixing belt 52. On the other hand, when the copying machine X is configured such that a sheet passing through the fixing device 5 passes through one end of the fixing belt 52, the magnetic cores 541a to 541c are respectively Is arranged so that the end of the fixing belt is aligned with one end of the fixing belt 52.
The rotary shaft 541d is connected to the core drive motor 55. Therefore, in the fixing device 5, the center shaft 541 is rotated by rotationally driving the rotating shaft 541 d by the core driving motor 55, and the positions of the magnetic cores 541 a to 541 c are changed. The width of the magnetic circuit acting on the roller 53 can be changed. Thereby, the heating target area in the heating roller 53 is changed. Here, the rotational drive of the core drive motor 55 is controlled by a CPU 71 provided in the heating control unit 7.

Next, returning to FIG. 2, the heating control unit 7 that controls the heating operation of the heating roller 53 in the fixing device 5 will be described.
As shown in FIG. 2, the heating controller 7 includes a primary power source 70, a CPU 71, an IGBT gate drive circuit 72, an IGBT 73 (insulated gate bipolar transistor), a fixing high temperature detection circuit 74, a protection operation circuit 75, a relay coil 76, and a relay. A contact 77, a thermistor 78, a thermostat 79, a zero-cross circuit 80 (an example of zero-cross detection means), and the like are included.
The primary power source 70 has a half-wave rectifier circuit that half-wave rectifies an AC voltage supplied from a commercial AC power source (not shown) to which the copying machine X is connected, and the half-wave rectified voltage ( (Hereinafter referred to as “half-wave rectified voltage”). The primary power source 70 may have a full-wave rectifier circuit that full-wave rectifies the AC voltage from the commercial AC power source.

The CPU 71 executes predetermined control processing (arithmetic processing) based on a control program and various parameters stored in a ROM or RAM (not shown) provided in the heating control unit 7, thereby causing the control unit 6, the respective components provided in the heating control unit 7 are controlled. Of course, a configuration having an electric circuit such as an ASIC capable of executing similar processing instead of the CPU 71 may be used.
Specifically, the CPU 71 changes the width of the magnetic circuit acting on the heating roller 53 by controlling the rotational drive of the core drive motor 55 as described above, so that the heating target region in the heating roller 53 is changed. To change. Here, the core drive motor 55 is, for example, a stepping motor that rotates by a predetermined amount in accordance with the input number of steps. In this case, the CPU 71 controls the amount of rotation of the core drive motor 55 according to the number of steps input to the core drive motor 55. Of course, other drive means such as a servo motor may be used for the core drive motor 55. The magnetic circuit width changing process (hereinafter referred to as “magnetic path width changing process”) performed by the CPU 71 will be described in detail later.
Further, the CPU 71 stores the relationship between the rotation amount of the core drive motor 55 and the rotation position of the center core 541 rotated by the core drive motor 55, and controls the rotation amount of the core drive motor 55. By doing so, the positions of the magnetic cores 541a to 541c in the center core 541 can be arranged at arbitrary positions.

Further, the CPU 71 energizes the relay coil 76 and closes the relay contact 77 when starting the induction heating by the induction heating unit 54 in accordance with an instruction from the control unit 6, whereby the primary power source The power supply from 70 to the IGBT 73 is started.
The CPU 71 performs switching control of power supplied from the primary power supply 70 to the induction coil 54a by inputting a switching signal to the gate terminal of the IGBT 73 via the IGBT gate driving circuit 72. Here, the CPU 71 when executing such processing corresponds to power supply control means.
At this time, the CPU 71 performs PWM control of the switching signal based on the detected temperature from the thermistor 78 (an example of temperature detecting means) that detects the temperature of the fixing belt 52 (an example of the second heated member). By controlling the feedback, the power supplied to the induction coil 54a is controlled to control the heating intensity of the heating roller 53. The thermistor 78 may detect the temperature of the heating roller 53.

The IGBT gate drive circuit 72 includes drive elements such as transistors and IGBTs, and a switching signal for controlling the drive of the IGBT 73 is input to the gate terminal of the IGBT 73 based on a switching signal input from the CPU 71. To do.
The IGBT 73 is controlled for driving based on a switching signal input from the CPU 71 to the gate terminal via the IGBT gate driving circuit 72. The IGBT 73 is connected to the induction coil 54a of the induction heating unit 54. By performing switching control, the power supplied to the induction coil 54a is adjusted. Specifically, the IGBT 73 establishes an energization path from the primary power source 70 to the induction coil 54a when current is input to its gate terminal, and interrupts the energization path when no current is input. Instead of the IGBT 73, for example, a switching element such as a transistor or an FET may be used.

On the other hand, the fixing high temperature detection circuit 74 determines whether or not the temperature detected by the thermistor 78 has reached a preset abnormal high temperature, and the determination result is input to the protection operation circuit 75. The
The protection operation circuit 75 switches the establishment / cutoff of the input path of the switching signal to the IGBT 73 according to the determination result input from the fixing high temperature detection circuit 74. Specifically, the protection operation circuit 75 inputs a switching signal to the IGBT 73 when the fixing high temperature detection circuit 74 determines that the temperature detected by the thermistor 78 has reached a preset abnormal high temperature. The induction heating is forcibly stopped by blocking the path.
Further, the thermostat 79 is provided in the vicinity of the fixing belt 52 together with the thermistor 78, and operates when the temperature of the fixing belt 52 reaches a preset upper limit temperature, from the primary power source 70 to the IGBT 73. This switch forcibly cuts off the power supply path to the. In general, the upper limit temperature is set higher than the abnormally high temperature of the fixing high temperature detection circuit 74. That is, the thermostat 79 is provided in order to ensure the minimum safety in the fixing device 5.

The zero-cross circuit 80 detects a zero-cross point periodically generated in the half-wave rectified voltage output from the primary power supply 70, and the detection result is input to the CPU 71. The zero-cross circuit 80 detects both that the half-wave rectified voltage from the primary power supply 70 has reached 0 and that the half-wave rectified voltage has started to rise from 0 as a zero-cross point. That is, two zero cross points are detected for each cycle of the AC voltage supplied from the AC power source.
The zero cross circuit 80 may be a conventionally known circuit using, for example, a Hall element or a photocoupler, and the description thereof is omitted here. Further, the zero cross circuit 80 may detect a zero cross point based on an AC voltage from a commercial AC power source supplied to the primary side instead of a half-wave rectified voltage on the secondary side of the primary power source 70. Good. This is because the zero cross points on the primary side and the secondary side of the primary power supply 70 are the same.

Here, with reference to FIG. 4, a change mode of the magnetic flux (magnetic flux density) of the magnetic circuit formed by the magnetic core 54 b will be described. The horizontal axis of FIG. 4 indicates the magnetic field strength H (A / m), and the vertical axis indicates the magnetic flux density B (Wb / m ² ).
First, when a current starts to flow through the induction coil 54a, the magnetic flux of the magnetic circuit formed by the magnetic core 54b gradually increases and reaches a saturation magnetic flux as shown in FIG. At this time, the half-wave rectified voltage output from the primary power supply 70 changes from the zero cross point toward the peak value.
When the half-wave rectified voltage output from the primary power supply 70 decreases from the peak value toward the zero-cross point, the magnetic flux of the magnetic circuit gradually decreases as shown in FIG. The residual magnetic flux is reached at the point. This residual magnetic flux is the magnetic flux remaining in the magnetic circuit even when the power supplied to the induction coil 54a becomes zero.
Thereafter, when the half-wave rectified voltage output from the primary power source 70 increases from the zero cross point toward the peak value, the magnetic flux of the magnetic circuit gradually increases and becomes a saturated magnetic flux as shown in FIG. To reach.

  As described above, in the magnetic circuit, the magnetic flux changes in a hysteresis curve by the periodic change of the half-wave rectified voltage output from the primary power supply 70. At this time, in the fixing device 5, the magnetic flux acting on the magnetic circuit is the smallest value at the zero cross point (see FIG. 4 (2)) periodically generated in the half-wave rectified voltage output from the primary power supply 70. That is, it becomes a residual magnetic flux. Then, from the time of detection of the zero cross point to the time after the elapse of the predetermined time T1, the magnetic flux of the magnetic circuit is maintained within a predetermined range near the residual magnetic flux that is closest to zero. Here, the predetermined time T1 is measured in advance by experiments or the like, and is stored in advance in a storage means such as a ROM that can be referred to by the CPU 71. The predetermined range means that the current is supplied to the induction coil 54a when the center core 541 is rotated when the switching control of the IGBT 73 is executed to control the current supply to the induction coil 54a. A range that is preset as a value that can prevent overcurrent on the circuit.

Hereinafter, an example of the magnetic path width changing process executed by the CPU 71 in the copying machine X will be described with reference to the timing chart of FIG. 5A is a half-wave rectified voltage from the primary power source 70, FIG. 5B is a detection result of the zero-cross circuit 80 (“1” indicates detection of the zero-cross point), and FIG. The presence / absence of switching control and FIG. 5 (d) show the presence / absence of driving of the core drive motor 55, respectively. In FIG. 5, t1 to t6 indicate the detection time of the zero cross point in the half-wave rectified voltage, and t11 to t16 indicate the time after the predetermined time T1 has elapsed from each of t1 to t6.
In the magnetic path width changing process, for example, since the paper size is switched from A3 size to A4 size during continuous printing in the copying machine X, the heating target area of the heating roller 53 in the fixing device 5 needs to be changed. In this case, it is executed by the CPU 71. Here, it is assumed that the magnetic path width changing process is started at time t1 shown in FIG. Before the time t1 shown in FIG. 5, the control of the induction heating of the heating roller 53 by the CPU 71 is executed as necessary as described above.

As shown in FIG. 5, in the magnetic path width changing process, the CPU 71 continues the switching control for outputting a switching signal to the IGBT 73 (FIG. 5C), and detects the zero cross point by the zero cross circuit 80. Whether or not the core drive motor 55 is driven is controlled based on the time point (FIG. 5D).
Specifically, when the zero-crossing point is detected by the zero-crossing circuit 80 at time t1 in FIG. 5, the CPU 71 performs the magnetic circuit from the time t1 to t11 after a predetermined time T1 has elapsed (see FIG. 4). Is determined to be within a predetermined range near the residual magnetic flux, and the core drive motor 55 is rotated by outputting a drive signal to the core drive motor 55 (FIG. 5D). For example, when the core drive motor 55 is a stepping motor, the CPU 71 inputs a predetermined number of step signals to the core drive motor 55 to drive the core drive motor 55 for a predetermined time T1. Accordingly, the center core 541 is rotated by a predetermined amount by the core drive motor 55.
Thereafter, until the center core 541 is rotated to a desired position, for example, the magnetic core forming the magnetic circuit is changed from the magnetic core 541a corresponding to the A3 size to the magnetic core corresponding to the A4 size. Until the operation is switched to 541c, the center core 541 is gradually rotated while the induction heating of the heating roller 53 is continued by repeatedly executing the above-described processing.

As described above, in the fixing device 5, when the width of the magnetic circuit acting on the heating roller 53 is changed by rotating the center core 541 by the core driving motor 55, the magnetic path is changed by the CPU 71. By executing the width changing process, the magnetic flux of the magnetic circuit that periodically changes by the switching control is executed within the predetermined range in the vicinity of 0 (remaining in this embodiment) while executing the switching control. When the center core 541 is rotationally moved by the core drive motor 55 on the condition that the magnetic flux in the magnetic circuit periodically falls within the predetermined range. The width of the magnetic circuit is changed intermittently. Here, the CPU 71 when executing the processing corresponds to the magnetic path width change control means.
Therefore, since the induction heating of the heating roller 53 is continued even when the center core 541 is rotated, the temperature drop of the heating roller 53 during the rotation of the center core 541 can be prevented. Accordingly, it is possible to shorten the time until the fixing operation is possible after the center core 541 is rotated.
In addition, since the rotation of the center core 541 is performed only while the magnetic flux of the magnetic circuit is in a range as small as possible, the rotation of the center core 541 causes the change in the characteristics of the magnetic circuit to the induction coil 54a. It is possible to prevent the occurrence of overcurrent and transient voltage on the energization circuit.

By the way, in the present embodiment, by rotating the center core 541, the width of the magnetic circuit acting on the heating roller 53 is increased depending on which of the magnetic cores 541a to 541c having different lengths is selected. Although the case of changing is described, the present invention is not limited to this, and the center core 541 may have a shape in which the width of the magnetic circuit acting on the heating roller 53 varies with rotation. For example, it is conceivable that the center core 541 is formed to have a shape in which both ends of a cylinder are cut obliquely and to have a substantially trapezoidal shape when viewed from the circumferential direction of the heating roller 53. In such a configuration, the width of the magnetic circuit acting on the heating roller 53 can be changed by rotating the center core 541.
Further, the present invention is not limited to changing the width of the magnetic circuit acting on the heating roller by rotating the center core 541 to move the position of each of the magnetic cores 541a to 541c, but acting on the heating roller 53. If the width of the magnetic circuit can be changed, for example, a long magnetic core is configured to be slidable in the longitudinal direction of the heating roller 53 so that the magnetic core can be slid. Also good. In addition, a configuration in which the width of the magnetic circuit can be changed by expanding and contracting a magnetic core formed so as to be stretchable by a double structure or the like can be considered as another embodiment.

In the above embodiment, the case where the primary power source 70 outputs a half-wave rectified voltage obtained by half-wave rectifying the AC voltage from the commercial AC power source has been described as an example, but the scope of application of the present invention is limited thereto. Not limited. Specifically, the primary power source 70 outputs the AC voltage from the commercial AC power source as it is, or outputs the AC voltage after raising or lowering the AC voltage to a predetermined voltage value. Can be considered.
Further, instead of the IGBT 73, a switching element such as a plurality of IGBTs is provided, and a half-wave rectified voltage, a radio-wave rectified voltage supplied from the primary power supply 70, a DC voltage after smoothing, or the like can be used. It is also conceivable to provide an inverter circuit that converts to an alternating voltage and outputs it.
In these configurations, an AC voltage is applied to the induction coil 54a. For this reason, since the magnetic flux characteristics of the magnetic circuit acting on the heating roller 53 also change, it is necessary to change the timing for driving the core drive motor 55 in the magnetic path width changing process described above.
Hereinafter, in the first embodiment, application of the present invention to a configuration in which an AC voltage is applied to the induction coil 54a will be described. Here, the fixing device having a configuration in which an AC voltage is applied to the induction coil 54a is referred to as a fixing device 5 ', and the same reference numerals are used to describe the same configuration as the fixing device 5 described in the above embodiment. Is omitted.

Here, FIG. 6 is a diagram for explaining a magnetic flux change mode in the fixing device 5 ′ according to the first embodiment of the present invention, and FIG. 7 is a diagram illustrating a magnetic field executed in the fixing device 5 ′ according to the first embodiment of the present invention. It is a timing chart for explaining road width change processing. The horizontal axis in FIG. 6 indicates the magnetic field strength H (A / m), and the vertical axis indicates the magnetic flux density B (Wb / m ² ). Note that (a1) in FIG. 7 indicates the AC voltage output from the primary power supply 70.
As shown in FIG. 6, first, when a positive current starts to flow through the induction coil 54a, the magnetic flux of the magnetic circuit formed by the magnetic core 54b gradually increases as shown in (4) of FIG. To reach the positive saturation magnetic flux. At this time, the AC voltage output from the primary power source 70 changes from the zero cross point toward the peak value.
When the AC voltage output from the primary power supply 70 decreases from the peak value toward the zero cross point, the magnetic flux of the magnetic circuit gradually decreases as shown in FIG. 6 (5), and at the zero cross point. The residual magnetic flux is reached. This residual magnetic flux is the magnetic flux remaining in the magnetic circuit even when the power supplied to the induction coil 54a becomes zero.
Thereafter, when the AC voltage output from the primary power supply 70 rises from the zero cross point toward the negative peak value, the magnetic flux of the magnetic circuit gradually increases in the negative direction as shown in FIG. The saturation magnetic flux in the negative direction is reached.

When the AC voltage output from the primary power source 70 increases from the negative peak value toward the zero cross point, the magnetic flux in the negative direction of the magnetic circuit gradually decreases as shown in (7) of FIG. The residual magnetic flux is reached at the zero cross point.
Further, when the AC voltage output from the primary power supply 70 increases from the zero cross point toward the positive peak value, the magnetic flux of the magnetic circuit gradually increases in the positive direction as shown in FIG. Thus, the saturation magnetic flux in the positive direction is reached.

As described above, in the magnetic circuit, the magnetic flux changes in a hysteresis curve shown in FIG. 6 due to a periodic change in the AC voltage output from the primary power supply 70.
At this time, in the fixing device 5, the magnetic circuit has the smallest magnetic flux in the vicinity of 0 after the elapse of the predetermined time T <b> 2 and the predetermined time T <b> 3 after the detection of the zero cross point of the AC voltage output from the primary power supply 70. It becomes the value of.
Therefore, in the fixing device 5 ′, the magnetic flux of the magnetic circuit is within a predetermined range near 0 after the predetermined time T 2 has elapsed from when the zero cross point was detected until the predetermined time T 3 has elapsed. It is set as a condition.

Then, the CPU 71 determines that the condition that the magnetic flux of the magnetic circuit is within a predetermined range near 0 is satisfied from the predetermined time T2 to the predetermined time T3, and drives the core driving motor 55. To control.
Specifically, as shown in FIG. 7, in the magnetic path width changing process, the CPU 71 detects that the zero cross point periodically generated in the AC voltage (FIG. 7 (a1)) output from the primary power source 70 is the zero cross. When detected by the circuit 80 (time point t1), from the time point t1 after t12 after a predetermined time T2 elapses until t13 when the predetermined time T3 elapses, that is, the magnetic flux of the magnetic circuit is within a predetermined range near zero. During a certain period (see FIG. 6), the core drive motor 55 is rotated by outputting a drive signal to the core drive motor 55 (FIG. 7D).
Thereafter, until the center core 541 is rotated to a desired position, for example, the magnetic core forming the magnetic circuit is changed from the magnetic core 541a corresponding to the A3 size to the magnetic core corresponding to the A4 size. The center core 541 is gradually rotated while the induction heating of the heating roller 53 is continued by repeatedly executing the process until switching to 541c.
Thereby, also in the fixing device 5 ′, the rotation of the center core 541 is performed on condition that the magnetic flux of the magnetic circuit is within a predetermined range near 0. Therefore, the magnetic circuit by the rotation of the center core 541 is performed. It is possible to prevent an overcurrent or the like on the energization circuit to the induction coil 54a due to a change in the characteristics of the current. Of course, the temperature drop of the heating roller 53 during the rotation of the center core 541 is also prevented.

In the embodiment and the first embodiment, the case where the drive timing of the core drive motor 55 is determined based on the zero cross point is described as an example. However, it acts on the magnetic circuit of the heating roller 53. A configuration including a magnetic flux detection circuit (an example of magnetic flux detection means) for detecting magnetic flux (magnetic flux density) is also conceivable as another embodiment. Specifically, it is conceivable that the magnetic flux detection circuit detects a magnetic flux by detecting a current induced in a detection coil provided on the magnetic circuit using, for example, a Hall element. In addition, any other known detection circuit may be adopted as the magnetic flux detection circuit.
According to such a configuration, the CPU 71 can determine whether or not the magnetic flux of the magnetic circuit is within a predetermined range near 0 based on the detection result by the magnetic flux detection circuit. Based on this, the drive timing of the core drive motor 55 can be controlled. That is, the core drive motor 55 is rotated intermittently by rotating the core drive motor 55 on condition that the magnetic flux detected by the magnetic flux detection circuit periodically falls within a predetermined range near zero. Just do it.
Accordingly, the temperature of the heating roller 53 is prevented from being lowered and the center core 541 is rotated to change the width of the magnetic circuit while preventing an overcurrent on the energizing circuit to the induction coil 54a. Can do.

1 is a block diagram showing a schematic configuration of a copier according to an embodiment of the present invention. 1 is a block diagram schematically showing a schematic configuration of a fixing device according to an embodiment of the present invention. The figure for demonstrating the structure of the induction heating part provided in the fixing device which concerns on embodiment of this invention. The figure for demonstrating the change aspect of the magnetic flux in the fixing device which concerns on embodiment of this invention. 6 is a timing chart for explaining magnetic path width changing processing executed in the fixing device according to the embodiment of the present invention. FIG. 4 is a diagram for explaining a magnetic flux change mode in the fixing device 5 ′ according to the first exemplary embodiment of the present invention. 6 is a timing chart for explaining a magnetic path width changing process executed in the fixing device 5 ′ according to the first embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Operation display part 2 ... Image reading part 3 ... Image processing part 4 ... Image forming part 5 ... Fixing device 6 ... Control part 7 ... Heating control part 50 ... Paper conveyance path 51 ... Fixing roller 51a ... Pressure roller 52 ... Fixing Belt 53 ... Heating roller (an example of a member to be heated)
54 ... induction heating section 54a ... induction coil 54b ... magnetic core 541 ... center core 541a-541c ... magnetic core 541d ... rotating shaft (an example of a support member)
542 ... External core 55 ... Core drive motor (an example of magnetic path width changing means)
70 ... Primary power supply 71 ... CPU
72 ... IGBT gate drive circuit 73 ... IGBT
74 ... High temperature fixing detection circuit 75 ... Protection operation circuit 76 ... Relay coil 77 ... Relay contact 78 ... Thermistor 79 ... Thermostat 80 ... Zero cross circuit (an example of zero cross detection means)
X: Copier (an example of an image forming apparatus)

Claims (6)

An induction coil that receives power supply to generate magnetic flux, power supply control means that performs switching control of power supplied from an AC power source to the induction coil, and a magnetic circuit that guides the magnetic flux generated by the induction coil to a member to be heated. A fixing device comprising: a magnetic member to be formed; and magnetic path width changing means for changing the width of the magnetic circuit by moving the magnetic member,
When the width of the magnetic circuit is changed by the magnetic path width changing means, the magnetic flux of the magnetic circuit is periodically changed by the switching control of the power supply control means while executing the switching control by the power supply control means. When the magnetic member is moved by the magnetic path width changing means on the condition that is within a predetermined range near 0 set in advance, the magnetic flux of the magnetic circuit periodically falls within the predetermined range. A fixing device comprising a magnetic path width change control means for changing the width of the magnetic circuit intermittently every time. A temperature detecting means for detecting a temperature of the heated member or a temperature of a second heated member heated by heat transfer from the heated member;
The fixing device according to claim 1, wherein the power supply control unit performs the switching control based on a temperature detected by the temperature detection unit. A rectifier circuit for half-wave rectifying or full-wave rectifying the AC voltage output from the AC power supply; and zero-cross detecting means for detecting a zero-cross point periodically generated in the output voltage from the AC power supply or the rectifier circuit. Prepared,
The power supply control means performs switching control of power supplied from the rectifier circuit to the induction coil;
2. The magnetic path width change control means determines that the magnetic flux of the magnetic circuit is within the predetermined range from a time point when a zero cross point is detected by the zero cross detection means until a predetermined time has elapsed. The fixing device according to any one of 2. Magnetic flux detecting means for detecting the magnetic flux acting on the magnetic circuit,
The fixing device according to claim 1, wherein the magnetic path width change control unit determines whether or not the magnetic flux detected by the magnetic flux detection unit is within the predetermined range. The magnetic member includes a plurality of magnetic cores having different lengths in the longitudinal direction of the member to be heated, and a support member that supports the plurality of magnetic cores in parallel in the circumferential direction.
The magnetic path width changing means changes the width of the magnetic circuit by moving the positions of the plurality of magnetic cores by rotationally driving the support member. Fixing device.   An image forming apparatus comprising the fixing device according to claim 1.

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