JP2020115188A - Fixing device and image forming apparatus - Google Patents

Abstract

To allow stable temperature control of heating elements to improve fixability.SOLUTION: A bi-directional thyristor 56 is controlled such that when power is supplied to heating elements 54b1, 54b2, a CPU 94 supplies a first power to the heating elements 54b1, 54b2 so that a detected temperature becomes a target temperature, and when power is supplied to a heating element 54b3 (or heating element 54b4), the CPU supplies a second power larger than the first power to the heating element 54b3 (or heating element 54b4) so that the detected temperature becomes the target temperature.SELECTED DRAWING: Figure 7

Description

The present invention relates to a fixing device and an image forming apparatus, and more particularly to a fixing device in an image forming apparatus that uses an electrophotographic recording system such as a laser printer, a copying machine, and a facsimile.

Conventionally, a fixing device of an image forming apparatus fixes an unfixed image (toner image) formed on a recording sheet by an image forming unit such as an electrophotographic process onto the recording sheet. In the image forming apparatus, a heat roller type fixing device using a halogen heater as a heat source and a film heating type fixing device using a ceramic heater as a heat source are used. In an image forming apparatus having a fixing device using such a heater as a heat source, a recording paper (hereinafter, referred to as small size paper) having a width shorter than the length of the heating element is passed through the fixing nip portion (hereinafter, referred to as passing paper). ), the following phenomenon may occur. That is, in the area where the heating element is generating heat and the area where the recording paper does not pass (hereinafter referred to as the non-paper passing area), compared to the area where the recording paper passes (hereinafter referred to as the paper passing area). There is a possibility that a phenomenon in which the temperature rises (hereinafter, referred to as non-sheet passing portion temperature rise) will occur. If the temperature becomes too high in the non-sheet passing area, it may affect surrounding members such as a member that supports the heater. In the past, in order to mitigate this effect, the throughput was reduced by detecting/predicting the temperature of the edge or according to the width size of the recording paper. In order to suppress such a decrease in throughput, a fixing device including a plurality of heating elements having different lengths and a switching relay capable of exclusively switching between heating elements that generate heat is disclosed (for example, Patent Document 1). 1). With this configuration, the heating element having a length matching the width of the recording paper to be passed is selectively used to prevent the temperature rise in the non-sheet passing portion, and the control for lowering the throughput is also unnecessary. This makes it possible to achieve high productivity even for small size paper. However, in such a configuration, the following problems occur when power is excessively supplied to one of the heating elements due to a malfunction of the device and only a part of the substrate is excessively heated. That is, only a part of the substrate whose temperature has risen greatly expands, and the difference in expansion from the part where the temperature has not risen causes strain (stress) in the substrate, possibly deforming the substrate.

Therefore, for example, a heater having the following configuration has been studied. For example, for a heating element that requires a high power supply capability, prepare two heating elements having substantially the same length in the longitudinal direction and arrange them at positions where the distance from the center is substantially equal in the lateral direction of the substrate. To do. This prevents thermal stress from occurring. On the other hand, for a heating element that does not require high power supply capacity, the resistance value of the heating element is increased to reduce the power supply capacity. This reduces the thermal stress.

Japanese Patent Laid-Open No. 2001-100558

However, in the configuration in which the difference in power supply capacity between the heating elements is large, if the same power control is continued regardless of the heating elements, excess or deficiency of the supplied power occurs, and the temperature detected by the thermistor (hereinafter, The detected temperature) may not follow the target temperature. If the detected temperature does not follow the target temperature, the fixability may be impaired.

The present invention has been made under such circumstances, and an object of the present invention is to enable stable temperature control of a heating element and improve fixing property.

In order to solve the problems described above, the present invention has the following configurations.

(1) A fixing device for fixing an unfixed toner image carried on a recording material, the first heating element having a first resistance value, and the first heating element having a resistance value larger than the first resistance value. A second heating element having a resistance value of 2; connection means for supplying a current to the first heating element or the second heating element, or for interrupting the supply of the current; A switching unit that switches a power supply path for supplying power to the second heating element, a detection unit that detects the temperature of the fixing device, and a detection temperature that is detected by the detection unit become a target temperature. Controlling means for controlling the first heating element so that the detected temperature reaches the target temperature when the first heating element is supplied with electric power. Power of the second heating element is supplied to the second heating element so that the detected temperature reaches the target temperature. A fixing device characterized by controlling the connecting means so as to supply electric power.

(2) An image comprising: an image forming unit that forms an unfixed toner image on a recording material; and the fixing device according to (1) that fixes the unfixed toner image on the recording material. Forming equipment.

According to the present invention, it is possible to stably control the temperature of the heating element and improve the fixing property.

Overall configuration diagram of the image forming apparatus of Examples 1 to 3 Control block diagram of the image forming apparatus of Embodiments 1 to 3 FIG. 3 is a diagram showing a fixing device and a heater of Examples 1 to 3. The figure which shows the heater of Examples 1-3 The figure which shows the electric power supply to the heater of Examples 1-3. Time chart showing operation of the image forming apparatus for comparison with the first embodiment Time chart showing the operation of the image forming apparatus of the first embodiment Flowchart showing the control of the triac of the image forming apparatus of the first embodiment. A time chart showing the operation of the image forming apparatus according to the second embodiment. Flowchart showing control of the triac of the image forming apparatus of the second embodiment. Flowchart showing control of the triac of the image forming apparatus of the third embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, passing a sheet through the fixing nip portion is called passing a sheet. In addition, an area in which the paper is not passed in the area where the heating element is generating heat is referred to as a non-paper passing area (or a non-paper passing area), and an area in which the paper is passed (or a paper passing area). Paper section). Further, a phenomenon in which the temperature of the non-sheet passing area becomes higher than that of the sheet passing area is called non-sheet passing portion temperature rise.

[Image forming device]
FIG. 1 is a configuration diagram illustrating an in-line type color image forming apparatus, which is an example of an image forming apparatus including the fixing device according to the first exemplary embodiment. The operation of the electrophotographic color image forming apparatus will be described with reference to FIG. The first station is a station for forming a yellow (Y) toner image, and the second station is a station for forming a magenta (M) toner image. Further, the third station is a station for forming a cyan (C) color toner image, and the fourth station is a station for forming a black (K) color toner image.

In the first station, the photosensitive drum 1a which is the image carrier is an OPC photosensitive drum. The photosensitive drum 1a is formed by laminating a plurality of functional organic materials, such as a carrier generation layer that sensitizes and generates charges on a metal cylinder, and a charge transport layer that transfers generated charges, and the outermost layer is an electrical layer. It has low conductivity and is almost insulating. The charging roller 2a, which is a charging unit, is brought into contact with the photosensitive drum 1a, and the surface of the photosensitive drum 1a is uniformly charged while being rotated by the rotation of the photosensitive drum 1a. A voltage in which a DC voltage or an AC voltage is superimposed is applied to the charging roller 2a, and discharge is generated from a nip portion between the charging roller 2a and the surface of the photosensitive drum 1a in a minute air gap on the upstream side and the downstream side in the rotation direction. As a result, the photosensitive drum 1a is charged. The cleaning unit 3a is a unit that cleans the toner remaining on the photosensitive drum 1a after transfer, which will be described later. The developing unit 8a, which is a developing unit, includes a developing roller 4a, a non-magnetic one-component toner 5a, and a developer coating blade 7a. The photosensitive drum 1a, the charging roller 2a, the cleaning unit 3a, and the developing unit 8a are an integral type process cartridge 9a that is detachable from the image forming apparatus.

The exposure device 11a, which is an exposure unit, is composed of a scanner unit or an LED (light emitting diode) array for scanning a laser beam with a polygon mirror, and irradiates the photosensitive drum 1a with a scanning beam 12a modulated based on an image signal. Further, the charging roller 2a is connected to a charging high voltage power source 20a which is a means for supplying a voltage to the charging roller 2a. The developing roller 4a is connected to a developing high voltage power source 21a which is a voltage supplying means for the developing roller 4a. The primary transfer roller 10a is connected to a primary transfer high voltage power source 22a which is a voltage supply means for the primary transfer roller 10a. The above is the configuration of the first station, and the second, third, and fourth stations have the same configuration. With respect to the other stations, parts having the same functions as those of the first station are designated by the same reference numerals, and the suffixes of the reference numerals are denoted by b, c, d for each station. In the following description, the subscripts a, b, c, d will be omitted unless a specific station is described.

The intermediate transfer belt 13 is supported by three rollers of a secondary transfer counter roller 15, a tension roller 14, and an auxiliary roller 19 as a stretching member. A force is applied to the intermediate transfer belt 13 by a spring only in the tension roller 14, so that an appropriate tension force is maintained on the intermediate transfer belt 13. The secondary transfer counter roller 15 is rotated by receiving a rotational drive from a main motor (not shown), and the intermediate transfer belt 13 wound around the outer periphery is rotated. The intermediate transfer belt 13 moves at substantially the same speed in the forward direction (eg, clockwise direction in FIG. 1) with respect to the photosensitive drums 1a to 1d (eg, counterclockwise rotation in FIG. 1). Further, the intermediate transfer belt 13 rotates in the arrow direction (clockwise direction), the primary transfer roller 10 is arranged on the opposite side of the photosensitive drum 1 with the intermediate transfer belt 13 interposed therebetween, and the intermediate transfer belt 13 moves. Followed by the rotation. The position where the photosensitive drum 1 and the primary transfer roller 10 are in contact with each other across the intermediate transfer belt 13 is called a primary transfer position. The auxiliary roller 19, the tension roller 14, and the secondary transfer counter roller 15 are electrically grounded. The primary transfer rollers 10b to 10d of the second to fourth stations have the same structure as the primary transfer roller 10a of the first station, and therefore the description thereof will be omitted.

Next, the image forming operation of the image forming apparatus of the first embodiment will be described. When the image forming apparatus receives the print command in the standby state, it starts the image forming operation. The photosensitive drum 1, the intermediate transfer belt 13 and the like start to rotate in the arrow direction at a predetermined process speed by a main motor (not shown). The photosensitive drum 1a is uniformly charged by the charging roller 2a to which a voltage is applied by the charging high-voltage power source 20a, and then an electrostatic latent image according to image information is formed by the scanning beam 12a emitted from the exposure device 11a. To be done. The toner 5a in the developing unit 8a is negatively charged by the developer coating blade 7a and is coated on the developing roller 4a. Then, a predetermined developing voltage is supplied to the developing roller 4a from the developing high voltage power source 21a. When the photosensitive drum 1a rotates and the electrostatic latent image formed on the photosensitive drum 1a reaches the developing roller 4a, the electrostatic latent image is visualized by the negative toner adhering to the photosensitive drum 1a. A toner image of the first color (for example, Y (yellow)) is formed. The other color M (magenta), C (cyan), and K (black) stations (process cartridges 9b to 9d) operate in the same manner. An electrostatic latent image by exposure is formed on each of the photosensitive drums 1a to 1d while delaying a writing signal from a controller (not shown) at a constant timing according to the distance between the primary transfer positions of the respective colors. A DC high voltage having a polarity opposite to that of the toner is applied to each of the primary transfer rollers 10a to 10d. Through the above steps, the toner images are sequentially transferred to the intermediate transfer belt 13 (hereinafter referred to as primary transfer), and a multiple toner image is formed on the intermediate transfer belt 13.

Thereafter, in accordance with the formation of the toner image, the paper P which is the recording material loaded in the cassette 16 is fed (picked up) by the paper feed roller 17 which is rotationally driven by a paper feed solenoid (not shown). .. The fed paper P is conveyed to a registration roller (hereinafter referred to as a registration roller) 18 by a conveyance roller. The sheet P is conveyed to the transfer nip portion, which is a contact portion between the intermediate transfer belt 13 and the secondary transfer roller 25, by the registration roller 18 in synchronization with the toner image on the intermediate transfer belt 13. A voltage having a polarity opposite to that of the toner is applied to the secondary transfer roller 25 by the secondary transfer high-voltage power supply 26, and the multicolor toner images of four colors carried on the intermediate transfer belt 13 are collectively printed on the paper P (recording). (On the material) (hereinafter referred to as secondary transfer). The member (for example, the photosensitive drum 1) that has contributed until the unfixed toner image is formed on the sheet P functions as an image forming unit. On the other hand, the toner remaining on the intermediate transfer belt 13 after the secondary transfer is finished is cleaned by the cleaning unit 27. After the secondary transfer is completed, the paper P is conveyed to a fixing device 50 that is a fixing unit, receives the fixing of the toner image, and is discharged to the discharge tray 30 as an image formed product (print, copy). The film 51, the nip forming member 52, the pressure roller 53, and the heater 54 of the fixing device 50 will be described later.

[Block diagram of image forming apparatus]
FIG. 2 is a block diagram for explaining the operation of the image forming apparatus, and the printing operation of the image forming apparatus will be described with reference to this figure. The PC 110, which is a host computer, is responsible for outputting a print command to the video controller 91 inside the image forming apparatus and transferring the image data of the print image to the video controller 91.

The video controller 91 converts the image data from the PC 110 into exposure data and transfers it to the exposure control device 93 in the engine controller 92. The exposure control device 93 is controlled by the CPU 94 to turn on/off the exposure data and control the exposure device 11. Upon receiving the print command, the CPU 94, which is the control means, starts the image forming sequence.

The engine controller 92 is equipped with a CPU 94, a memory 95, etc., and performs a preprogrammed operation. The high voltage power source 96 is composed of the charging high voltage power source 20, the developing high voltage power source 21, the primary transfer high voltage power source 22, and the secondary transfer high voltage power source 26 described above. The power control unit 97 includes a bidirectional thyristor (hereinafter referred to as a triac) 56, a heating element switch 57 as a switching unit that exclusively selects a heating element that supplies power, and the like. The power control unit 97 selects a heating element that generates heat in the fixing device 50 and determines the amount of power to be supplied. The drive device 98 is composed of a main motor 99, a fixing motor 100 and the like. The sensor 101 includes a fixing temperature sensor 59 for detecting the temperature of the fixing device 50, a paper presence sensor 102 having a flag for detecting the presence or absence of the paper P, and the detection result of the sensor 101 is transmitted to the CPU 94. The CPU 94 acquires the detection result of the sensor 101 in the image forming apparatus, and controls the exposure device 11, the high-voltage power supply 96, the power control unit 97, and the drive device 98. As a result, the CPU 94 forms an electrostatic latent image, transfers the developed toner image, fixes the toner image on the paper P, and the like, and in the image forming process in which the exposure data is printed on the paper P as a toner image. Take control. The image forming apparatus to which the present invention is applied is not limited to the image forming apparatus having the configuration described with reference to FIG. 1, it is possible to print the paper P having different widths, and has the heater 54 described later. Any image forming apparatus including the fixing device 50 may be used.

[Fixing device]
Next, the configuration of the fixing device 50 according to the first exemplary embodiment will be described with reference to FIG. Here, the longitudinal direction is a rotation axis direction of the pressure roller 53 that is substantially orthogonal to a conveyance direction of the sheet P described later. The length of the paper P in the direction (longitudinal direction) substantially orthogonal to the transport direction is called the width. FIG. 3 is a schematic sectional view of the fixing device 50.

The sheet P holding the unfixed toner image Tn from the left side of FIG. 3 is heated while being conveyed from the left side to the right side in the figure in the fixing nip portion N, so that the toner image Tn is fixed to the sheet P. The fixing device 50 according to the first exemplary embodiment includes a cylindrical film 51, a nip forming member 52 that holds the film 51, a pressure roller 53 that forms a fixing nip portion N together with the film 51, and a sheet P for heating. And a heater 54.

The film 51, which is the first rotating body, is a fixing film as a heating rotating body. In Example 1, for example, polyimide is used as the base layer. An elastic layer made of silicone rubber and a release layer made of PFA are used on the base layer. In order to reduce the frictional force generated between the film 51 and the nip forming member 52 and the heater 54 due to the rotation of the film 51, grease is applied to the inner surface of the film 51.

The nip forming member 52 plays a role of guiding the film 51 from the inside and forming a fixing nip portion N between the film 51 and the pressure roller 53. The nip forming member 52 is a member having rigidity, heat resistance, and heat insulation, and is made of liquid crystal polymer or the like. The film 51 is fitted onto the nip forming member 52. The pressure roller 53, which is the second rotating body, is a roller serving as a pressure rotating body. The pressure roller 53 includes a core metal 53a, an elastic layer 53b, and a release layer 53c. The pressure roller 53 is rotatably held at both ends, and is rotationally driven by a fixing motor (not shown). Further, the film 51 is driven to rotate by the rotation of the pressure roller 53. The heater 54, which is a heating member, is held by the nip forming member 52 and is in contact with the inner surface of the film 51. The substrate 54a, the heating elements 54b1 to 54b4, the protective glass layer 54e, and the fixing temperature sensor 59 will be described later.

[heater]
FIG. 4 shows the configuration of the heater 54 of the first embodiment, and the details will be described below. The substrate 54a is a plate-shaped ceramic substrate formed of an Al2O3 material or the like, and the dimensions are, for example, thickness t=1 mm, width W=6.3 mm, and length l=280 mm. On the substrate 54a, heating elements 54b1, 54b2, 54b3, 54b4, a conductor 54c which is a conductive path, and contacts 54d1, 54d2, 54d3, 54d4 for supplying electric power are formed by a printing process. Hereinafter, the plurality of heating elements 54b1 to 54b4 may be collectively referred to as the heating element 54b. 4, the heating element 54b is shown in white, the conductor 54c is shown in diagonal lines, and the contacts 54d1 to 54d4 are shown in black.

The heating element 54b is the heating element 54b1 having the longest length in the longitudinal direction (hereinafter also referred to as width), the heating element 54b3 having the second largest width, the heating element 54b4 having the third largest width, and the heating element 54b2 having the longest width. Are arranged at regular intervals in the order of. The heating element 54b1 and the heating element 54b2 have substantially the same width. The interval between the heating elements 54b is, for example, 0.7 mm in the first embodiment. In the first embodiment, for example, the heating elements 54b1 and 54b2 have a thickness t=10 μm, a width W=0.7 mm, and a first length L=222 mm. In Example 1, the dimensions of the heating element 54b3 are, for example, a thickness t=10 μm, a width W=0.7 mm, and a length l=188 mm, which is a second length shorter than the first length. In Example 1, the dimensions of the heating element 54b4 are, for example, a thickness t=10 μm, a width W=0.7 mm, and a length l=154 mm, which is a second length shorter than the first length.

The heating elements 54b1 and 54b2, which are the first heating elements, have a length 1=222 mm and are used when printing A4 paper having a width of 210 mm. The second heating element and the third heating element 54b3, which are the third heating element, have a length l=188 mm and are used when printing B5 paper having a width of 182 mm. The heating element 54b4, which is the second heating element and the fourth heating element, has a length l=154 mm and is used when printing A5 paper having a width of 148.5 mm.

The heating element 54b is a conductive material whose main component is silver and palladium, and the conductor 54c and the contacts 54d1 to 54d4 are made of a conductive material whose main component is silver. The electric resistance between both ends of the heating element 54b in the longitudinal direction is 20Ω for both the longest heating elements 54b1 and 54b2, 20Ω for the heating element 54b3 having the second length, and 20Ω for the heating element 54b4 having the third length. To do. One ends of the longest heating elements 54b1 and 54b2 are electrically connected with a common contact 54d1 and the other ends are electrically connected with a common contact 54d2. The combined resistance value of the longest heating elements 54b1 and 54b2 between the contacts 54d1 and 54d2 is 10Ω because the heating elements 54b1 and 54b2 are connected in parallel. Thus, the combined resistance value of the heating elements 54b1 and 54b2 is 10Ω, which is smaller than the resistance value (20Ω) of the heating elements 54b3 and 54b4.

As described above, the heater 54 has the heating element 54b1 and the heating element 54b2 having a length substantially the same as the length of the heating element 54b1 in the longitudinal direction. Further, the heater 54 includes heating elements 54b3 and 54b4 having a shorter length in the longitudinal direction than the heating elements 54b1 and 54b2. The heating element 54b1 is provided at one end of the substrate 54a in the lateral direction, and the heating element 54b2 is provided at the other end of the substrate 54a in the lateral direction. The heating elements 54b3 and 54b4 are provided between the heating elements 54b1 and 54b2 in the lateral direction of the substrate 54a.

Further, in the first embodiment, the contact 54d1 which is the first contact is a contact to which one end of each of the heating elements 54b1 and 54b2 is electrically connected. The contact 54d2, which is the second contact, is a contact to which the other ends of the heating element 54b1, the heating element 54b2, and the heating element 54b3 are electrically connected. The contact 54d3, which is the third contact, is a contact to which one end of the heating element 54b3 and one end of the heating element 54b4 are electrically connected. The contact 54d4, which is the fourth contact, is a contact to which the other end of the heating element 54b4 is electrically connected.

In the first embodiment, the width W of the heating element 54b is the same as 0.7 mm. However, depending on the performance required of the fixing device 50, in order to form the heating element 54b having the same width W, the conductive material may be made conductive. In some cases, it is difficult to select the material for the material. In that case, the width W of the heating element 54b may be changed according to the performance required of the fixing device 50.

(About heating elements 54b1 and 54b2)
The features of the longest heating elements 54b1 and 54b2 in the heater 54 will be described below. If the fixing device 50 can quickly reach a sufficiently heated and fixable state (hereinafter, also referred to as a sheet-passable state), the printed matter can be promptly provided to the user. Therefore, it is preferable to maximize the power supply capability of the longest heating elements 54b1 and 54b2 capable of heating the entire region in the longitudinal direction so that any size of paper P may be selected. The heating elements 54b3, 54b4, which are shorter than the longest heating elements 54b1, 54b2 in the longitudinal direction, are used after the fixing device 50 is sufficiently heated by the longest heating elements 54b1, 54b2. Therefore, it suffices to supplement the amount of electric power for fixing the toner image to the paper P when passing the paper. Therefore, when the heating elements 54b3 and 54b4 are used, the longest heating elements 54b1 and 54b2 have high power supply capability. In comparison, it is preferable to have a low power supply capacity.

That is, when electric power is supplied to the heating elements 54b1 and 54b2, the CPU 94 controls to supply the first electric power to the heating elements 54b1 and 54b2 so that the temperature detected by the fixing temperature sensor 59 reaches the target temperature. To do. When electric power is supplied to the heating element 54b3 (or the heating element 54b4), the CPU 94 applies the first power to the heating element 54b3 (or the heating element 54b4) so that the temperature detected by the fixing temperature sensor 59 becomes the target temperature. The control is performed so as to supply more second power.

The fact that the longest heating elements 54b1 and 54b2 have a high power supply capability means that the risk of deformation of the substrate 54a is large if the longest heating elements 54b1 and 54b2 are excessively supplied with power due to a device failure. is there. In the first embodiment, the longest heating elements 54b1 and 54b2 are composed of two, one heating element 54b1 is arranged at one end of the substrate 54a in the lateral direction, and the other heating element 54b2 is shorted to the substrate 54a. It is placed at the other end in the hand direction. Thus, the two longest heating elements 54b1 and 54b2 are arranged so as to be symmetrical in the lateral direction of the substrate 54a.

Further, the respective heating elements 54b1 and 54b2 are electrically connected by the common contacts 54d1 and 54d2, and the two heating elements 54b1 and 54b2 are configured to be supplied with electric power at substantially the same time. Accordingly, when electric power is supplied to the longest heating elements 54b1 and 54b2, both ends of the heater 54 in the short-side direction always generate heat, so that the supplied amount of power can be dispersed, and the short-side direction can be distributed. The temperature gradient of the substrate 54a can be reduced.

As described above, the fixing device 50 is made to reach the sheet-passable state in a short time, and even if an excessive power supply state occurs due to a device failure, the temperature gradient of the substrate 54a in the lateral direction is reduced. Therefore, the risk of deformation of the substrate 54a can be reduced.

(Regarding the heating elements 54b3 and 54b4)
Next, the features of the two types of heating elements 54b3 and 54b4 that are not the longest will be described below. The heating element 54b3 and the heating element 54b4 are electrically connected at one end by one contact 54d3. On the other hand, at the other end of the heating element 54b3 and the heating element 54b4, the heating element 54b3 is electrically connected to the contact 54d2, and the heating element 54b4 is electrically connected to the contact 54d4. That is, one of the heating element 54b3 and the heating element 54b4 is configured to generate heat.

As described above, the heating element 54b3 is used when printing B5 paper, and the heating element 54b4 is used when printing A5 paper. The width of the sheet P (hereinafter referred to as the sheet width) and the lengths of the heating elements 54b3 and 54b4 in the longitudinal direction are substantially the same, and the area where the heating elements 54b3 and 54b4 generate heat (hereinafter also referred to as the heating area) is large. The sheet P passes through the portion. For this reason, most of the heat generated from the heating elements 54b3 and 54b4 can be applied to the sheet P, so that the temperature rise in the non-sheet passing area where the sheet P does not pass can be suppressed. This makes it possible to maintain high productivity. Further, since the longest heating elements 54b1 and 54b2 are responsible for heating the fixing device 50 to a sheet-passable state, the non-longest heating elements 54b3 and 54b4 are for fixing the toner image to the sheet P at the time of sheet feeding. The amount of electric power should be supplemented. Therefore, the power supply capability of the heating elements 54b3, 54b4 that are not the longest can be reduced, and the temperature rise of the heating elements 54b3, 54b4 at the time of abnormality can be reduced.

Further, the above-mentioned two kinds of heat generating elements 54b3 and 54b4 are arranged between the longest heat generating element 54b1 and the heat generating element 54b2, and the heat generating elements 54b3 and 54b3 are arranged as close as possible to the center of the substrate 54a in the lateral direction. 54b4 is arranged. Thereby, it is possible to raise the temperature substantially equally at both ends of the first end which is one end in the lateral direction of the substrate 54a and the second end which is the other end, The temperature gradient of the substrate 54a in the lateral direction can be reduced.

From the above, the power supply capability of the non-longest heating elements 54b3, 54b4 is reduced, and the non-longest heating elements 54b3, 54b4 are arranged as symmetrically as possible in the lateral direction of the substrate 54a. As a result, even if an excessive power supply state occurs due to a device failure, the temperature gradient in the short-side direction of the substrate 54a can be reduced, so that the risk of deformation of the substrate 54a can be reduced. In addition, only the longest heating elements 54b1 and 54b2 that require high power supply capacity are two, and the non-longest heating elements 54b3 and 54b4 are the minimum required ones, while considering the symmetry in the lateral direction. Thus, it is possible to reduce the size of the substrate 54a at the same time.

The fixing temperature sensor 59, which is a detection unit, is arranged on the surface of the substrate 54a opposite to the protective glass layer 54e (see FIG. 3), and is installed at the center position of the heating element 54b in the longitudinal direction. Is in contact with. The fixing temperature sensor 59 is, for example, a thermistor, detects the temperature of the heater 54, and outputs the detection result to the CPU 94.

(Aim of configuration of heating element 54b)
Since the longest heating elements 54b1 and 54b2 have the maximum power supply capability among the plurality of heating elements 54b, it is common to heat the fixing device 50 to a paper-passable state in a short time, as described above. This configuration is also used in the first embodiment. On the other hand, a high power supply capability means that excessive power supply is likely due to a malfunction of the device, and the risk of deformation of the substrate 54a is high. In the first embodiment, in order to achieve both high power supply capability and reduction of deformation risk of the substrate 54a, the longest heating element has two heating elements 54b1 and 54b2, and the power is dispersed, that is, the power density. To reduce Further, one heating element 54b1 is arranged at one end of the substrate 54a in the lateral direction, and the other heating element 54b2 is arranged at the other end (the opposite end) of the substrate 54a. Then, the two heating elements 54b1 and 54b2 are electrically connected by the common contacts 54d1 and 54d4 so that the two longest heating elements 54b1 and 54b2 are always supplied with electric power at the same time. As a result, only one end (a part) of the substrate 54a in the lateral direction does not generate heat, the temperature distribution of the substrate 54a can be made uniform, and the risk of deformation of the substrate 54a can be reduced.

Further, in the first embodiment, one non-longest heating element is provided for each of the two levels of the heating element 54b3 and the heating element 54b4. This improves the productivity when printing on a plurality of sizes of paper (narrow paper, small size paper) having a width narrower than the maximum size paper P corresponding to the heating elements 54b1 and 54b2. The non-longest heating elements 54b3, 54b4 are set to have a lower power supply capability than the longest heating elements 54b1, 54b2, and the shortest direction between the two longest heating elements 54b1, 54b2 on the substrate 54a. Place it in the center of. As a result, the substrate 54a can be heated relatively uniformly and the temperature distribution of the substrate 54a can be made gentle compared to the case where the substrate 54a is arranged at one end in the short side direction. The risk of deformation of the substrate 54a can be reduced. The longest heating elements 54b1 and 54b2 that require a high power supply capacity are composed of two, and the non-longest heating elements 54b3 and 54b4 are the minimum one per length level of the heating elements. As a result, the deformation of the substrate 54a is suppressed, the productivity when printing on a plurality of small size papers is increased, and the size of the substrate 54a is minimized.

[Power supply circuit]
FIG. 5 shows a power supply circuit of the first embodiment, and this circuit is used to carry out comparative verification between the first embodiment and the first comparative example. Each power supply circuit will be described below. In the first embodiment of FIG. 5, the contacts 54d1 to 54d4 are connected to the heating element switch 57 for switching the power supply path. Since the heating element 54b that generates heat is switched by switching the power supply path by the heating element switch 57, switching the power supply path is also referred to as switching the heating element 54b. In the first embodiment, the heating element switching unit 57 is specifically the electromagnetic relays 57a and 57b having the C-contact configuration.

The electromagnetic relay 57a has a contact 57a1 connected to the AC power supply 55 via the triac 56 at the first pole, a contact 57a2 connected to the contact 54d1, and a contact 57a3 connected to the contact 54d3. Under the control of the engine controller 92, the electromagnetic relay 57a is in one of a state in which the contact 57a1 and the contact 57a2 are connected and a state in which the contact 57a1 and the contact 57a3 are connected. The electromagnetic relay 57b has a contact 57b1 connected to the second pole of the AC power supply 55, a contact 57b2 connected to the contact 54d2, and a contact 57b3 connected to the contact 54d4. Under the control of the engine controller 92, the electromagnetic relay 57b is in one of a state in which the contact 57b1 and the contact 57b2 are connected and a state in which the contact 57b1 and the contact 57b3 are connected. The triac 56, which is a connecting unit, supplies a current to a predetermined heating element 54b among the plurality of heating elements 54b or cuts off the supply of the current.

FIG. 5 shows the non-operation of the electromagnetic relays 57a and 57b. The electromagnetic relay 57a is connected with the contacts 57a1 and 57a2, and the electromagnetic relay 57b is connected with the contacts 57b1 and 57b2. When the electromagnetic relays 57a and 57b are not operating, electric power is supplied between the contact 54d1 and the contact 54d2, so that the longest heating elements 54b1 and 54b2 generate heat.

When the electromagnetic relays 57a and 57b are operated, the contact 57a1 and the contact 57a3 are connected to the electromagnetic relay 57a, and the contact 57b1 and the contact 57b3 are connected to the electromagnetic relay 57b. During operation of the electromagnetic relays 57a and 57b, electric power is supplied between the contact 54d3 and the contact 54d4, so that only the heating element 54b4 generates heat. When only the electromagnetic relay 57a is operated, the contact 57a1 and the contact 57a3 are connected to the electromagnetic relay 57a, and the contact 57b1 and the contact 57b2 are connected to the electromagnetic relay 57b. When only the electromagnetic relay 57a operates, electric power is supplied between the contact 54d3 and the contact 54d2, so that only the heating element 54b3 generates heat. The electromagnetic relays 57a and 57b may use a contact switch of an a-contact type electromagnetic relay or a b-contact type electromagnetic relay, or a solid-state relay (SSR), photo-moss relay, triac, or other non-contact switch. It doesn't matter.

[Control of current amount (or power amount) flowing through heating element (duty control)]
Next, the advantages of the image forming apparatus according to the first embodiment will be described with reference to FIGS. 6 and 7. In the first embodiment, at the timing when the heating element 54b to be used is switched, the current amount (electric power amount) flowing through the heating element 54b is changed from the current amount according to the heating element 54b (first heating element) before being changed. After that, the current amount (electric power amount) is switched according to the heating element 54b (second heating element). The amount of electric current (the amount of electric power) supplied to the heating element 54b is controlled by the duty of the triac 56. Therefore, specifically, the duty of the triac 56 is set to the duty corresponding to the changed heating element 54b at the timing when the heating element 54b to be used is switched. This eliminates the excess or deficiency of electric power and enables stable temperature control of the heating element 54b.

(Control of Comparative Example)
First, as a comparative example, an example in which the triac 56 is controlled with the same duty regardless of the heating element 54b used will be described with reference to FIG. After that, the behavior of the image forming apparatus according to the first exemplary embodiment will be described with reference to FIG. 7. FIG. 6 is a diagram of a case in which, in the configuration of FIGS. 1 to 5, printing is performed on six sheets P in the order of the A4 width sheet α1 and the B5 width sheets β1 to β5 in one job. In FIG. 6, (i) shows the transmission and reception of commands between the video controller 91 and the CPU 94, and (ii) shows the paper P passed through the fixing nip portion N. (Iii) shows the state of the electromagnetic relay 57a, and shows the case where the triac 56 is connected to the contact 54d1 and the contact 54d3 by the electromagnetic relay 57a. (Iv) shows the state of the electromagnetic relay 57b, and shows the case where the triac 56 is connected to the contact 54d2 and the contact 54d4 by the electromagnetic relay 57b. (V) shows to which heating element 54b the triac 56 is connected. (Vi) indicates the duty (%) of the triac 56 controlled by the CPU 94. Note that the triac 56 is set to 100% when it is always on, and 0% when it is normally off. (Vii) indicates the temperature detected by the fixing temperature sensor 59 (hereinafter referred to as the thermistor temperature), and TE2 indicates the target value (target) of the fixing temperature. TE1 indicates room temperature.

In the state before receiving the print instruction, the electromagnetic relay 57a is connected to the contact 54d1 and the electromagnetic relay 57b is connected to the contact 54d2, whereby the triac 56 is connected to the heating elements 54b1 and 54b2. In this state, at timing t601, the CPU 94 receives from the video controller 91 a print instruction to print on the six sheets P in the order of the A4 width sheet α1 and the B5 width sheet β1 to β5. The CPU 94 sets the triac 56 to the conductive state (ON) at the timing t601 (FIG. 6(vi)). The CPU 94 turns on the triac 56 with a 100% duty (fixed duty) from the start of printing until the target temperature TE2 is reached so that the target temperature TE2, which is a fixing temperature, can be reached as quickly as possible. to continue.

Then, at timing t602 when the thermistor temperature reaches the target temperature TE2, the CPU 94 starts PI control. The CPU 94 sets the optimum duty X for maintaining the target temperature TE2 by using the heating elements 54b1 and 54b2 as the duty of the triac 56 when the PI control is started (t602). The duty X is stored in the memory 95 which is the first storage means. In the memory 95, for example, the heating element 54b1 and the duty X of the triac 56 corresponding to the heating element 54b1 are stored in association with each other. Similarly, regarding the other heating elements 54b2 to 54b4, the other heating elements 54b2 to 54b4 and the duty X of the triac 56 corresponding to the other heating element 54b are stored in the memory 95 in association with each other.

The sheet α1 is fed and conveyed such that the thermistor temperature reaches the fixing nip portion N while the thermistor temperature is at the target temperature TE2. When the rear end of the sheet α1 passes through the fixing nip portion N, the CPU 94 uses the heating element 54b3 that can perform the fixing process on the sheet β without causing the temperature increase in the non-sheet passing portion. Perform the following control. That is, the CPU 94 connects the electromagnetic relay 57a to the contact 54d3 at the timing t603. Then, the CPU 94 uses the heating element 54b3 to perform the fixing process on the sheets β1 to β5.

As described with reference to FIG. 4, the resistance value (20Ω) of the heating element 54b3 is larger than the combined resistance value (10Ω) of the heating elements 54b1 and 54b2. Therefore, the optimum duty for maintaining the thermistor temperature when the heating elements 54b1 and 54b2 are used at the target temperature TE2 is insufficient when the heating element 54b3 is used, resulting in a shortage of electric power. Then, as the paper β1 passes through the fixing nip portion N, heat is taken by the paper β1, so that after the timing t604, the thermistor temperature starts to significantly decrease. When the thermistor temperature starts to decrease, the duty of the triac 56 gradually increases due to the PI control (FIG. 6(vi)), and thus the decrease in the thermistor temperature is suppressed (FIG. 6(vii)). However, the thermistor temperature does not easily return to the target temperature TE2, and as a result, the fixing properties of the sheets β1 to β5 may be impaired.

(Control of Example 1)
Next, the image forming apparatus according to the first exemplary embodiment will be described with reference to FIG. 7. 7(i) to (vii) are the same as FIGS. 6(i) to (vii), and the description of how to read the drawings is omitted. Further, in the following description, a portion overlapping with FIG. 6 will not be described. The CPU 94 receives the print instruction at timing t701, and the leading end of the paper α1 reaches the fixing nip portion N at timing t702. In FIG. 7, at timing t703, the heating elements 54b1 and 54b2 are switched to the heating element 54b3. In the image forming apparatus of the first embodiment, the CPU 94 controls as follows at the timing t703 when the electromagnetic relay 57a is connected to the contact 54d3 and the heating element 54b3 is connected. That is, the CPU 94 changes the setting to the duty Y which is the optimum duty of the triac 56 to maintain the thermistor temperature at the target temperature TE2 when the heating element 54b3 is used (FIG. 7(vi)). This point is different from the comparative example (FIG. 6(vi)).

The duty of the triac 56 required to maintain the thermistor temperature at the target temperature TE2 by using each heating element 54b largely depends on the resistance value of the heating element 54b. When the combined resistance value of the heating elements 54b1 and 54b2 is 10Ω and the resistance value of the heating element 54b3 is 20Ω, the duty Y of the triac 56 has the relationship shown in the following equation (1).
Duty Y = Duty X x (resistance value of heating element 54b3 / combined resistance value of heating elements 54b1 and 54b2)
= Duty X x (20 [Ω] ÷ 10 [Ω])
= 2 x duty X... Formula (1)

By the CPU 94 driving the triac 56 with this duty Y, the image forming apparatus of the first embodiment does not run out of power even when performing the fixing process using the heating element 54b3. Therefore, even after timing t704, the thermistor temperature can be maintained at the target temperature TE2 (FIG. 7(vii)). Then, printing can be performed on the papers β1 to β5 without impairing the fixing property. The duty Y is also stored in the memory 95, like the duty X. That is, in the memory 95, each heating element 54b and the duty Y of the triac 56 corresponding to each heating element 54b are stored in association with each other.

As described above with reference to FIGS. 6 and 7, the CPU 94 performs the following control when switching from the heating elements 54b1 and 54b2 to the heating element 54b3 by the heating element switch 57. That is, the current amount (power amount) supplied to the heating element 54b3 by the triac 56 is controlled so as to be the current amount (power amount) according to the heating element 54b3. In the memory 95, for example, the heating elements 54b1 and 54b2 and the amount of electric current (power amount) supplied to the heating elements 54b1 and 54b2 by the triac 56 are stored in association with each other. Similarly, for the other heating elements 54b3, 54b4, the other heating elements 54b3, 54b4 and the current amount (electric power amount) supplied to the other heating elements 54b3, 54b4 by the triac 56 are associated and stored in the memory 95. ing. Thus, at the timing when the heating element 54b used for the fixing process is changed from the first heating element to the second heating element, the duty of the triac 56 is changed according to the changed second heating element. Set to. As a result, the image forming apparatus according to the first exemplary embodiment has an advantage of eliminating excess or deficiency of electric power and enabling stable temperature control of the heating element 54b.

[Control Flowchart of Triac of Image Forming Apparatus of First Embodiment]
FIG. 8 is a control flowchart of the triac 56 of the image forming apparatus according to the first embodiment. The control after the CPU 94 receives the print instruction from the video controller 91 (after the timing t701 in FIG. 7) will be described. In step (hereinafter referred to as S) 801, the CPU 94 drives the triac 56 with a duty of 100%. In S802, the CPU 94 determines whether the thermistor temperature has become equal to or higher than the target temperature TE2. If the CPU 94 determines in S802 that the thermistor temperature is lower than the target temperature TE2, the process returns to S802, and if the CPU 94 determines that the thermistor temperature is equal to or higher than the target temperature TE2, the process proceeds to S803. In S803, the CPU 94 reads the duty X of the triac 56 corresponding to the heating element 54b from the memory 95, and drives the triac 56 with the read duty X.

In step S<b>804, the CPU 94 determines whether it is the timing (paper size change timing) when the size of the paper P (more specifically, the width of the paper P) changes. If the CPU 94 determines in S804 that it is not the timing for changing the size of the paper P, the process proceeds to S806. If it is determined that it is the timing for changing the size of the paper P, the CPU 94 advances the process to S805. In step S805, the CPU 94 switches the heating element switching unit 57 to the heating element 54b suitable for the size of the paper P. Then, the CPU 94 reads the duty Y of the triac 56 corresponding to the switched heating element 54b from the memory 95, and drives the triac 56 with the read duty Y. In S806, the CPU 94 determines whether printing is completed. If the CPU 94 determines in step S806 that printing has not ended, the process returns to step S804. If it determines that printing has ended, the process ends.

As described above, the image forming apparatus according to the first embodiment sets the duty of the triac 56 to the duty Y corresponding to the changed heating element 54b at the timing when the heating element 54b used for the fixing process is changed. .. And by this, excess and deficiency of electric power are eliminated, and stable temperature control of the heating element 54b becomes possible.

As described above, according to the first embodiment, it is possible to stably control the temperature of the heating element and improve the fixability.

The image forming apparatus according to the first exemplary embodiment resets the duty of the triac 56 to a duty suitable for the changed heating element 54b at the timing when the heating element 54b used for the fixing process is changed. This eliminates the excess or deficiency of electric power and enables stable temperature control of the heating element 54b. In the first embodiment, as described with reference to FIG. 7, the case where the heating element 54b is switched according to the change in the paper size during continuous printing has been described. On the other hand, in the second embodiment, an image forming apparatus of the second embodiment will be described using a case where the heating element 54b is switched during continuous printing of the same paper size. When the CPU 94 switches from the first heating element 54b to the second heating element 54b, the current amount (power amount) (specifically, the amount of electric power) supplied to the first heating element 54b by the triac 56 that is the connecting means (specifically, The duty) is stored in the memory 95. The CPU 94 is supplied to the first heating element 54b stored in the memory 95 when the heating element switching device 57 switches (returns) the second heating element 54b to the first heating element 54b. The amount of electric current (the amount of electric power) (specifically, the duty) is read. Then, the CPU 94 controls the triac 56 so that the read current amount (power amount), that is, the read duty.

First, the reason why the heating element 54b is switched despite continuous printing of the same paper size will be described. For example, in the case where continuous printing of the paper P of A5 width is performed, the image forming apparatus basically performs the fixing process using the heating element 54b3 which is optimal for printing the paper P of A5 width. However, if the heating element 54b3 is continuously used, the temperature of the end portion of the fixing nip portion N in the longitudinal direction is not controlled, and the temperature difference between the end portion and the central portion of the fixing nip portion N in the longitudinal direction increases. .. If this temperature difference becomes equal to or larger than a predetermined temperature difference, and the expansion degree of the fixing device 50 also changes to some extent, the fixing device 50 may be damaged. Therefore, the image forming apparatus also uses the heating elements 54b1 and 54b2 used for the fixing process of the A4 width paper P at a predetermined frequency even in the case where the A5 width paper P is continuously printed. By utilizing the temperature rise in the non-sheet passing portion by using the heating elements 54b1 and 54b2, the end portion of the fixing nip portion N is heated, and the temperature difference between the end portion and the central portion in the longitudinal direction of the fixing nip portion N is small. To be This is the reason why the heating element 54b is switched even when the paper P of A5 width is continuously printed.

Then, in the case where the heating element 54b is switched at a specific interval (predetermined frequency), the image forming apparatus of the second embodiment stores the duty of the triac 56 before switching the heating element 54b in the memory 95. .. Then, when returning the heating element 54b to the heating element 54b before switching (for example, returning to the heating element 54b3 for the A5 width), the triac 56 is driven (turned on) with the duty stored in the memory 95. As a result, the thermistor temperature is more stably maintained at the target temperature TE2, and sufficient fixing property is secured.

(Control of Example 2)
The operation of the image forming apparatus according to the second exemplary embodiment will be described in detail below with reference to FIG. FIG. 9 shows a case where the fixing process is performed on twelve sheets P from A5 width sheets γ1 to γ12. 9(i) to (vii) are similar to FIGS. 7(i) to (vii), and a description of how to read the figures will be omitted. The image forming apparatus according to the second exemplary embodiment is an image forming apparatus having the same configuration as that of the first exemplary embodiment illustrated in FIGS.

When the CPU 94 receives the print instruction at the timing t801, the heating element capable of performing the fixing process on the sheet P of A5 width without generating the temperature rise in the non-sheet passing portion of the end portion in the longitudinal direction of the fixing nip portion N. The following control is performed in order to use 54b3. That is, the CPU 94 connects the electromagnetic relay 57a to the contact 54d3 and the electromagnetic relay 57b to the contact 57d4 at the timing t801.

Similar to the description of FIG. 7, the CPU 94 drives the triac 56 with a fixed duty of 100% until the thermistor temperature reaches the target temperature TE2. When the thermistor temperature reaches the target temperature TE2 at the timing t802, the CPU 94 starts PI control. At timing t802, the CPU 94 sets the duty of the triac 56 to the duty Z capable of maintaining the thermistor temperature at the target temperature TE2 by using the heating element 54b3.

The duty Z is largely due to the resistance value of the heating element 54b, like the relationship between the duty X and the duty Y described in the equation (1). The combined resistance value of the heating elements 54b1 and 54b2 is 10Ω, the resistance value of the heating element 54b3 is 20Ω, and the duty Z has a relationship as shown in the following expression (2).
Duty Z = Duty X x (20 [Ω] ÷ 10 [Ω])
=2 x duty X ... Formula (2)
The duty Z is stored in the memory 95 as in the first embodiment.

The CPU 94 uses the heating element 54b3 to perform the fixing process on the paper P after the paper γ1. The paper γ1 is fed and conveyed so as to reach the fixing nip portion N while the thermistor temperature is maintained at the target temperature TE2, and then 10 sheets of paper P are conveyed to the paper γ10 and pass through the fixing nip portion N. To do.

Here, in order to prevent the above-described damage of the fixing device 50, when a predetermined number of sheets, for example, ten sheets P in the second embodiment are continuously printed, the CPU 94 causes the electromagnetic relay 57a to the contact 57d1 and the electromagnetic relay 57a. 57b is switched to the contact 57d2. As a result, the CPU 94 originally brings the triac 56 to the heating elements 54b1 and 54b2 for the sheet P of A4 width. As shown in FIG. 9, at the timing t803 when the trailing edge of the sheet γ10 passes through the fixing nip portion N, the heating element 54b3 is switched to the heating elements 54b1 and 54b2 (FIG. 9(v)).

The sheet γ11 is subjected to a fixing process using the heating elements 54b1 and 54b2, and at this time, the end portion in the longitudinal direction of the fixing nip portion N is warmed by the temperature rise in the non-sheet passing portion. As a result, the temperature difference between the end portion and the central portion of the fixing nip portion N in the longitudinal direction is reduced. In the image forming apparatus of the second embodiment, the predetermined frequency is once every 10 sheets, and the heating elements 54b1 and 54b2 are switched to the heating elements 54b1 and 54b2 at this frequency, and the heating elements 54b1 and 54b2 are used for the fixing process. This can prevent the temperature difference between the end portion and the central portion of the fixing nip portion N in the longitudinal direction from exceeding a predetermined temperature difference. The predetermined frequency is determined according to the temperature difference between the central portion and the end portion in the longitudinal direction of the fixing nip portion N.

Further, similar to the operation example described with reference to FIG. 7, the CPU 94 performs the same control as that of the first embodiment at the timing t803 when switching to the heating elements 54b1 and 54b2. That is, the CPU 94 reads the optimum duty X from the memory 95 using the heating elements 54b1 and 54b2 to maintain the thermistor temperature at the target temperature TE2, and drives the triac 56 with the duty X. This allows the thermistor temperature to be maintained at the target temperature TE2 even after switching to the heating elements 54b1 and 54b2 (FIG. 9(vii)).

Then, in the operation example of the second embodiment, the CPU 94 performs the following operation at the timing t803 when the heating element 54b used for the heating elements 54b1 and 54b2 is switched from the heating element 54b3. That is, the CPU 94 stores the duty P of the triac 56 used for the heating element 54b3 in the memory 95 serving as the second storage unit at the timing t803. The duty P is a duty adjusted by the PI control from the duty Z, and at the switching time point (t803), the duty according to the optimum electric energy for maintaining the thermistor temperature at the target temperature TE2 by using the heating element 54b3. It can be said that The following factors are conceivable as examples of factors that change the optimum amount of electric power for maintaining the thermistor temperature at the target temperature TE2. For example, as the members of the fixing device 50 become warmer as the continuous printing progresses, the heat energy of the warmed member can be used for the fixing process, and as a result, the required amount of electric power, that is, the duty of the triac 56 becomes smaller. And so on.

Then, after printing the paper γ11 with the heating elements 54b1 and 54b2, specifically, at the timing t804 when the rear end of the paper γ11 passes through the fixing nip portion N, the heating element switch 57 again switches to the heating element 54b3. The heating element 54b3 is used for the paper γ12, and the CPU 94 reads the duty P stored in the memory 95 and sets the duty P as the duty of the triac 56. At this time, if the triac 56 is driven by the duty Z instead of the duty P, the electric power becomes excessive and the thermistor temperature may become higher than the target temperature TE2. Therefore, when returning from the heating elements 54b1 and 54b2 to the heating element 54b3, by driving the triac 56 with the duty P, it becomes possible to maintain the thermistor temperature at the target temperature TE2. In the second embodiment, the heating elements 54b1 and 54b2 are used to perform the fixing process on one sheet of paper γ11 in order to utilize the temperature rise in the non-sheet passing portion, and then the sheet is returned to the heating element 54b3. Not limited. Regarding the number of sheets of paper γ to be subjected to the fixing process using the heating elements 54b1 and 54b2, the temperature difference between the end portion and the central portion of the fixing nip portion N reaches a temperature difference within a range that does not affect the fixing device 50. You can use any number.

As described above, the image forming apparatus according to the second embodiment stores the duty of the triac 56 before the change in the memory 95 when switching the heating element 54b. Then, when returning to the heating element 54b used when the duty is stored (used again), the triac 56 is turned on with the stored duty. This makes it possible to maintain the thermistor temperature at the target temperature TE2 more stably.

In the image forming apparatus of the second embodiment, the absolute value of the duty immediately before the switching is stored and used, but the relative value of the duty before and after the switching may be stored and used. In this case, when the heating element 54b after switching is switched to the heating element 54b before switching again (returning to the original state), the duty is calculated by subtracting the relative value of the stored duty from the duty being output. Specifically, the CPU 94 stores the relative value ΔD (=Z−P) between the duty Z of the triac 56 with respect to the heating element 54b3 and the duty P of the triac 56 at the timing t803 in the memory 95. Then, at timing t804, the CPU 94 reads the relative value ΔD from the memory 95, subtracts the relative value ΔD from the duty Z of the triac 56 with respect to the heating element 54b3, and calculates the duty P (Z−ΔD=Z−(Z− P)=P).

[Control Flowchart of Triac of Image Forming Apparatus of Second Embodiment]
FIG. 10 is a control flowchart of the triac 56 of the image forming apparatus according to the second embodiment. The control after the CPU 94 receives the print instruction from the video controller 91 (after the timing t801 in FIG. 9) will be described. Note that the processing of S901 to S903 is the same as the processing of S801 to S803 in FIG.

In S904, the CPU 94 determines whether or not it is the timing to switch the heating element 54b (heating element switching timing). For example, the CPU 94 has a counter that counts the paper P that is being continuously printed, and the counter manages the number of printed sheets. When the CPU 94 determines that the number of printed sheets has reached a predetermined number (for example, 10 sheets), it determines that it is time to switch the heating element 54b. If the CPU 94 determines in S904 that the timing for switching the heating element 54b has not come, the process proceeds to S909, and if it determines that the timing for switching has arrived, the process proceeds to S905.

In step S<b>905, the CPU 94 stores the duty P currently used for driving the triac 56 in the memory 95 in association with the heating element 54 b currently used for the fixing process (the heating element 54 b before switching). In S906, the CPU 94 refers to the memory 95 and determines whether or not there is the duty P stored in association with the switched heating element 54b. If the CPU 94 determines in step S906 that the memory 95 has the duty P, the process proceeds to step S907; otherwise, the process proceeds to step S908.

In S907, the CPU 94 drives the triac 56 by using the duty P stored in the memory 95 and associated with the switched heating element 54b. The process of S907 corresponds to the operation at timing t804 in FIG. 9, for example. In S908, the CPU 94 drives the triac 56 by using the duty X stored in the memory 95 corresponding to the switched heating element 54b, and advances the process to S909. The process of S908 corresponds to the operation at timing t803 in FIG. 9, for example. Note that the processing of S909 is the same as the processing of S806 in FIG. If the determination in S909 is NO, the process returns to S904.

As described above, in the image forming apparatus according to the second embodiment, the duty of the triac 56 is reset to the duty corresponding to the changed heating element 54b at the timing when the heating element 54b used for the fixing process is changed. .. Further, the duty before the change is stored at the time of the change, and the triac 56 is driven by using the duty stored when returning to the heating element 54b before the change. As a result, excess and deficiency of electric power are eliminated, and stable temperature control of the heating element is possible.

As described above, according to the second embodiment, it is possible to stably control the temperature of the heating element and improve the fixing property.

In the image forming apparatus according to the second embodiment, when the heating element 54b is switched, the duty of the triac 56 at that time is stored in the memory 95. When the heating element 54b used when the duty is stored is used again, the duty stored in the memory 95 is used to drive (turn on) the triac 56 to stabilize the thermistor temperature. It has become possible to maintain the target temperature TE2.

In the image forming apparatus of the third embodiment, when the heating element 54b is switched, the duty of the triac 56 after switching is based on the duty of the triac 56 before switching and the ratio of the resistance value of the heating element 54b before and after switching. To calculate. Then, the calculated duty is used to drive the triac 56 after switching the heating element 54b. As described in the first and second embodiments, the duty of the triac 56 required to maintain the thermistor temperature at the target temperature TE2 for each heating element 54b is largely due to the resistance value of the heating element 54b. From this, the duty of the triac 56 can be calculated based on the resistance value of the heating element 54b, and the calculated duty is used for driving the triac 56. This makes it possible to stably maintain the thermistor temperature at the target temperature TE2. In particular, in the case where one type of heating element 54b (for example, heating element 54b3) is used for a long time and then switched to another heating element 54b (for example, heating elements 54b1 and 54b2), the following is performed. Issues may occur. That is, there is a possibility that the deviation from the stored duty may increase with the passage of time. When there is such a possibility, the image forming apparatus according to the third exemplary embodiment is effective.

That is, in the third embodiment, when switching from the first heating element 54b to the second heating element 54b, the CPU 94 controls as follows. That is, the CPU 94 determines the second value based on the amount of current supplied to the first heating element 54b by the triac 56, the resistance value of the first heating element 54b, and the resistance value of the second heating element 54b. The amount of current supplied to the heating element 54b is calculated. Then, the CPU 94 controls the triac 56 so that the obtained current amount is obtained. Here, as in the first and second embodiments, the amount of electric current (the amount of electric power) supplied to the heating element 54b can be translated into a duty for controlling the triac 56.

[Control Flowchart of Triac of Image Forming Apparatus of Third Embodiment]
The difference from the image forming apparatus of the second embodiment is that the duty of the triac 56 after switching is calculated and used, and this will be described below using a flowchart. The image forming apparatus according to the third exemplary embodiment is an image forming apparatus having the same configuration as that of the first exemplary embodiment illustrated in FIGS. In addition, FIG. 9 is referred to for the time chart of the third embodiment. FIG. 11 is a control flowchart of the triac 56 of the image forming apparatus according to the third embodiment. When the CPU 94 receives the print instruction from the video controller 91, the processing after S1001 is started. Note that the processes of S1001 to S1004 are the same as the processes of S901 to S904 in FIG. In S1005, the CPU 94 calculates the duty of the triac 56 after switching the heating element 54b. The CPU 94 drives the triac 56 after switching the heating element 54b with the calculated duty.

The duty of the triac 56 after switching the heating element 54b can be calculated using the following equation (3).
Duty after switching = Duty before switching
× Resistance value of heating element after switching
÷ Resistance value of heating element before switching...Equation (3)
Note that the processing of S1006 is the same as the processing of S909 of FIG.

As described above, the image forming apparatus of the third embodiment calculates the duty after switching the heating element 54b in the following manner when switching the heating element 54b. That is, the duty of the triac 56 after switching is calculated based on the duty of the triac 56 before switching and the ratio of the resistance value of the heating element 54b before switching to the resistance value of the heating element 54b after switching. The CPU 94 drives the triac 56 using the calculated duty. This makes it possible to stably maintain the thermistor temperature at the target temperature TE2.

As described above, according to the third embodiment, it is possible to stably control the temperature of the heating element and improve the fixability.

50 fixing device 54b heating element 56 bidirectional thyristor 57 heating element switching device 59 fixing temperature sensor 94 CPU

Claims (15)

A fixing device for fixing an unfixed toner image carried on a recording material,
A first heating element having a first resistance value, and a second heating element having a second resistance value having a resistance value larger than the first resistance value;
Connection means for supplying an electric current to the first heating element or the second heating element, or for interrupting the supply of the electric current;
Switching means for switching a power supply path for supplying power to the first heating element or the second heating element;
Detection means for detecting the temperature of the fixing device,
Control means for controlling the detected temperature detected by the detecting means to be a target temperature;
Equipped with
When electric power is being supplied to the first heating element, the control means supplies first power to the first heating element so that the detected temperature reaches the target temperature, and When the electric power is supplied to the heating element, the connection is made such that the second heating element is supplied with the second electric power larger than the first electric power so that the detected temperature becomes the target temperature. A fixing device characterized by controlling means. The first heating element has a first length,
The fixing device according to claim 1, wherein the second heating element has a second length that is shorter than the first length. The said 2nd heat generating body contains the 3rd heat generating body and the 4th heat generating body whose length in a longitudinal direction is shorter than the said 3rd heat generating body, The 2nd heat generating body of Claim 2 characterized by the above-mentioned. Fixing device. A substrate on which the first heating element, the third heating element, and the fourth heating element are arranged,
The first heating element includes one of the first heating elements arranged at one end portion in the lateral direction of the substrate and the other of the first heating element arranged at the other end portion. And
The one first heating element, the third heating element, the fourth heating element, and the other first heating element are arranged in this order in the lateral direction. The fixing device according to item 3. A first contact to which one end of each of the one first heating element and the other first heating element is electrically connected;
A second contact to which the other end of the one first heating element, the other first heating element and the third heating element is electrically connected;
A third contact to which one end of each of the third heating element and the fourth heating element is electrically connected,
A fourth contact to which the other end of the fourth heating element is electrically connected;
The fixing device according to claim 4, further comprising: A first storage unit that stores the first heating unit or the second heating unit and the electric power supplied to the first heating unit or the second heating unit by the connecting unit in association with each other. The fixing device according to any one of claims 1 to 5. The control means is stored in the first storage means when the switching means switches from the first heating element to the second heating element or from the second heating element to the first heating element. 7. The fixing device according to claim 6, wherein the electric power supplied to the heat generating element after the switching associated with the heat generating element after the switching is read, and the connecting unit is controlled so as to obtain the read power. apparatus. A second storage means,
The control means is
When switching from the first heating element to the second heating element or from the second heating element to the first heating element by the switching means, the connecting means supplies the heating element before switching to the heating element before switching. Storing the electric power in the second storage means,
When the switching means returns the second heating element to the first heating element or the first heating element to the second heating element, the pre-switching stored in the second storage means 7. The fixing device according to claim 6, wherein the electric power supplied to the heating element is read, and the connecting unit is controlled so that the electric power supplied to the read heating element before the switching becomes the same. The control means is
When switching from the second heating element to the first heating element or from the first heating element to the second heating element by the switching means, the connection means supplies the heating element before switching to the heating element before switching. Based on the power and the resistance value of the heating element after switching and the resistance value of the heating element before switching, the power to be supplied to the heating element after switching is obtained, and the connecting means is set to the obtained power. The fixing device according to claim 6, wherein the fixing device is controlled. When the fixing process is continuously performed using the second heating element, the control unit switches the second heating element to the first heating element by the switching unit at a predetermined frequency. The fixing device according to claim 8 or 9. The connection means is a bidirectional thyristor,
The control means controls the electric power supplied to the first heating element or the second heating element by controlling the duty of the bidirectional thyristor. The fixing device according to any one of items. A first rotating body heated by the heater;
A second rotating body that forms a nip portion with the first rotating body;
The fixing device according to any one of claims 1 to 11, further comprising: The fixing device according to claim 12, wherein the first rotating body is a film. The heater is provided so as to contact the inner surface of the film,
The fixing device according to claim 13, wherein the nip portion is formed by the heater and the second rotating body via the film. Image forming means for forming an unfixed toner image on the recording material,
The fixing device according to claim 1, which fixes an unfixed toner image on a recording material,
An image forming apparatus comprising: