Fixing device and image forming apparatus

Abstract

To switch the control of power supply to a heater from power supply to one heating element to alternate power supply to a plurality of heating elements based on a detection temperature according to the state of a fixing device.SOLUTION: A fixing device comprises: a heater 54 that has heating elements 54b1, 54b3 on a substrate 54a, and heats a fixing film 51; a fixing temperature sensor 59 that detects the temperature of the heater 54; triacs 56a, 56b that switch a power supply path from an AC power supply 55 to the heating element 54b1 or the heating element 54b3; and a CPU 94 that controls a heating element switcher 57 to supply power to the heating element 54b1 or the heating element 54b3. The CPU 94 performs first control of supplying power to the heating element 54b1 (S100), and when the temperature detected by the fixing temperature sensor 59 reaches a threshold temperature (S101), switches to second control of alternately supplying power to the heating element 54b1 or the heating element 54b3 (S102-S107).SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to a fixing device and an image forming apparatus equipped with the fixing device. [Background technology]

[0002] In a fixing device of an image forming apparatus that forms an image on a recording material, the toner image is fixed to the recording material by passing through a fixing nip formed by the contact between a fixing film that heats the toner image on the recording material and a pressure roller that presses the toner image. When continuous printing is performed on recording materials narrower than the longitudinal width of the heater (heating device) that heats the fixing film, a phenomenon known as non-paper-passing area temperature rise occurs in the fixing nip area (non-paper-passing area) where the recording material does not pass. If the temperature rise in the non-paper-passing area becomes significant, the fixing device's fixing components, such as the fixing film and pressure roller, may be damaged by the temperature rise. Patent Document 1, for example, proposes a configuration that reduces the temperature rise in the non-paper-passing area of the fixing device by switching between multiple heating elements with different longitudinal lengths in the heater. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-100558 Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-mentioned method, when the temperature of the central portion of the heater in the fixing device in the longitudinal direction reaches a predetermined temperature, the control switches between the use of multiple heating elements. However, depending on the state of the fixing device, it may take time for the temperature of the end portion of the heater in the longitudinal direction to rise to a temperature suitable for image formation, or conversely, a temperature rise may occur in the non-paper passing portion. Therefore, there is a need for temperature control of the heater according to the state.

[0005] The present invention was made under these circumstances, and aims to switch the power supply to the heater from supplying power to one heating element to supplying power alternately to multiple heating elements based on the detected temperature corresponding to the state of the fixing device. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0007] (1) A fixing device for fixing an unfixed toner image on a recording material to the recording material, the fixing device having a heating rotor, a first heating element on a substrate, and a second heating element having a length in the longitudinal direction shorter than that of the first heating element, the fixing device further comprising: a heater for heating the heating rotor; a first temperature detection means for detecting the temperature of the heater; a pressure rotor forming a nip portion with the heating rotor; a switching means for switching a power supply path from an AC power source to the first heating element or the second heating element; and a control means for controlling the switching means to supply power to the first heating element or the second heating element, the control means performing a first control for supplying power to the first heating element, and a second control for supplying power to the first heating element when the temperature detected by the first temperature detection means reaches a threshold temperature. When it is detected that the recording material has reached the nip portion, a second control for alternately supplying power to the first heating element or the second heating element; Start A fixing device characterized by:

[0008] (2) An image forming apparatus comprising: an image forming means for forming an unfixed toner image on a recording material; and the fixing device according to (1) above for fixing the unfixed toner image on the recording material. [Effects of the Invention]

[0009] According to the present invention, the power supply to the heater can be switched from supplying power to one heat generating element to supplying power alternately to a plurality of heat generating elements based on the detected temperature according to the state of the fixing device. [Brief explanation of the drawings]

[0010] [Figure 1]Overall configuration diagram of the image forming apparatus according to the first to fourth embodiments [Figure 2] FIG. 1 is a block diagram showing the configuration of a control unit of an image forming apparatus according to first and second embodiments. [Figure 3] Schematic cross-sectional view illustrating the configuration of the fixing device according to Examples 1 to 4. [Figure 4] Schematic diagram showing the configuration of the heater in Examples 1 to 4. [Figure 5] Schematic diagram showing a cross section of the heater of Examples 1 to 4 [Figure 6] FIG. 2 is a schematic diagram illustrating the configuration of a power control circuit of the fixing device according to the first and second embodiments. [Figure 7] 1 is a flowchart showing a control sequence for supplying power to a heating element according to a first embodiment; [Figure 8] FIG. 10 is a diagram illustrating the timing for starting switching of the heating elements in the first embodiment. [Figure 9] FIG. 10 is a diagram illustrating the timing for starting switching of the heating elements in the first embodiment. [Figure 10] 10 is a flowchart showing a control sequence for supplying power to a heating element according to a second embodiment. [Figure 11] FIG. 10 is a diagram illustrating the timing for starting switching of the heating elements in the second embodiment. [Figure 12] FIG. 10 is a block diagram showing the configuration of a control unit of an image forming apparatus according to third and fourth embodiments. [Figure 13] FIG. 10 is a schematic diagram illustrating the configuration of a power control circuit of a fixing device according to third and fourth embodiments. [Figure 14] Flowchart showing a voltage calculation sequence in the third and fourth embodiments [Figure 15] 10 is a flowchart showing a control sequence for supplying power to a heating element according to a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following examples, passing a recording material through the fixing nip portion of a fixing device is referred to as "paper passing." Furthermore, an area of the fixing nip portion where the recording material does not pass, which corresponds to an area where the heating element generates heat, is referred to as a non-paper passing area (or non-paper passing portion), and an area where the recording material passes is referred to as a paper passing area (or paper passing portion). Furthermore, the phenomenon in which the non-paper passing area of the fixing nip portion has a higher temperature than the paper passing area is referred to as a non-paper passing portion temperature rise. [Example]

[0012] [Configuration of image forming device] FIG. 1 is a cross-sectional view showing the configuration of an in-line color image forming apparatus, which is an image forming apparatus equipped with the fixing device of the first embodiment. The configuration of the electrophotographic color image forming apparatus will be described using FIG. 1. 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. The third station is a station for forming a cyan (C) toner image, and the fourth station is a station for forming a black (K) toner image.

[0013] In the first station, the photosensitive drum 1a, which serves as an image carrier, is an OPC photosensitive drum. The photosensitive drum 1a is a metal cylinder with multiple layers of functional organic materials laminated on it, including a carrier generation layer that generates charge upon photosensitivity and a charge transport layer that transports the generated charge. The outermost layer has low electrical conductivity and is substantially insulated. The charging roller 2a, which serves as a charging means, contacts the photosensitive drum 1a and rotates in accordance with the rotation of the photosensitive drum 1a, uniformly charging the surface of the photosensitive drum 1a. A voltage consisting of a DC voltage or a superimposed AC voltage is applied to the charging roller 2a, generating discharges in the small air gaps upstream and downstream of the rotational direction of the photosensitive drum 1a from the nip between the charging roller 2a and the surface of the photosensitive drum 1a. This charges the photosensitive drum 1a. The cleaning unit 3a cleans toner remaining on the photosensitive drum 1a after the primary transfer described below. The developing unit 8a, which is the developing means, stores non-magnetic single-component toner 5a and has a developing roller 4a and a developer application blade 7a. The photosensitive drum 1a, charging roller 2a, cleaning unit 3a, and developing unit 8a are housed in an integrated process cartridge 9a (image forming section) that is detachably attached to the image forming apparatus.

[0014] The exposure device 11a, which serves as an exposure means, is composed of a scanner unit or LED (light-emitting diode) array that reflects laser light from a rotating polygon mirror and scans the photosensitive drum 1a. The scanning beam 12a, modulated based on an image signal, is irradiated onto the photosensitive drum 1a. The charging roller 2a is connected to a charging high-voltage power supply 20a, which supplies voltage to the charging roller 2a. The developing roller 4a is connected to a developing high-voltage power supply 21a, which supplies voltage to the developing roller 4a. The primary transfer roller 10a is connected to a primary transfer high-voltage power supply 22a, which supplies voltage to the primary transfer roller 10a. The above describes the configuration of the first station, and the second, third, and fourth stations have similar configurations. In the second, third, and fourth stations, components having the same functions as those in the first station are designated by the same reference numerals, with the suffixes b, c, and d added to the reference numerals for each station. In the following description, the suffixes a, b, c, and d will be omitted except when describing a specific station.

[0015] The intermediate transfer belt 13 is supported by three rollers that serve as tensioning members: a secondary transfer opposing roller 15, a tension roller 14, and an auxiliary roller 19. Only the tension roller 14 is subjected to a force in the direction of tensioning the intermediate transfer belt 13 by a spring (not shown), thereby maintaining an appropriate tension on the intermediate transfer belt 13. The secondary transfer opposing roller 15 rotates by receiving rotational drive from a main motor 99 (see FIG. 2), causing the intermediate transfer belt 13 wound around its periphery to rotate. The intermediate transfer belt 13 moves at approximately the same speed in the direction of the arrow (e.g., clockwise in FIG. 1) as the photosensitive drums 1a-1d (e.g., counterclockwise in FIG. 1). The primary transfer roller 10 is positioned opposite the photosensitive drum 1 across the intermediate transfer belt 13 and rotates in response to the movement of the intermediate transfer belt 13. The position where the photosensitive drum 1 and the primary transfer roller 10 abut across the intermediate transfer belt 13 is called the primary transfer position. The auxiliary roller 19, tension roller 14, and secondary transfer opposing roller 15 are electrically grounded. Note that the primary transfer rollers 10b to 10d of the second to fourth stations have the same configuration as the primary transfer roller 10a of the first station, so a description thereof will be omitted.

[0016] [Image formation operation] Next, the image forming operation of the image forming apparatus shown in FIG. 1 will be described. When the image forming apparatus receives a print command while in standby mode, it starts the image forming operation. The photosensitive drum 1, intermediate transfer belt 13, etc. begin to rotate in the direction of the arrow in the figure at a predetermined process speed driven by a main motor 99 (see FIG. 2). The photosensitive drum 1a is uniformly charged by a charging roller 2a to which a voltage is applied from a charging high-voltage power supply 20a. Then, an electrostatic latent image based on image information is formed by a scanning beam 12a irradiated from an exposure device 11a. The toner 5a in the development unit 8a is negatively charged by a developer application blade 7a and applied to the development roller 4a. A predetermined development voltage is then applied to the development roller 4a from a development high-voltage power supply 21a. When the photosensitive drum 1a rotates and the electrostatic latent image formed on the photosensitive drum 1a reaches the development roller 4a, the electrostatic latent image is visualized by the adhesion of negative toner, and a toner image of a first color (e.g., Y (yellow)) is formed on the photosensitive drum 1a. The stations (process cartridges 9b-9d) for the other colors M (magenta), C (cyan), and K (black) operate in the same way. An electrostatic latent image is formed on each of the photosensitive drums 1a-1d by a scanning beam 12 from an exposure device 11, while a write start signal from a controller (not shown) is delayed at a timing corresponding to the distance between the primary transfer positions of each color. A high DC voltage of opposite polarity to that of the toner is applied to each of the primary transfer rollers 10a-10d. As a result, the toner images on the photosensitive drums 1a-1d are transferred in sequence to the intermediate transfer belt 13 (hereinafter referred to as primary transfer), forming a multiple toner image on the intermediate transfer belt 13.

[0017] Thereafter, in synchronization with the formation of the toner image, paper P, which is a recording material loaded in cassette 16 (paper supply unit), is fed by paper feed roller 17, which is rotationally driven by a paper feed solenoid (not shown). The fed paper P is transported by a transport roller (not shown) to registration rollers (hereinafter referred to as registration rollers) 18. In synchronization with the toner image on intermediate transfer belt 13, paper P is transported by registration roller 18 to a transfer nip portion, which is a contact portion between intermediate transfer belt 13 and secondary transfer roller 25. A voltage of opposite polarity to that of the toner is applied to secondary transfer roller 25 by secondary transfer high voltage power supply 26, and the four-color multi-toner image carried on intermediate transfer belt 13 is transferred all at once onto paper P (recording material) (hereinafter referred to as secondary transfer). Meanwhile, after secondary transfer is completed, any toner remaining on intermediate transfer belt 13 is cleaned by cleaning unit 27. After the secondary transfer is completed, the paper P is transported to a fixing device 50, which is a fixing means, and the paper P on which the toner image is fixed is discharged as an image-formed product (print, copy) onto a discharge tray 30. The fixing film 51, nip forming member 52, pressure roller 53, and heater 54 of the fixing device 50 will be described later.

[0018] [Image forming device control block] 2 is a block diagram showing the configuration of the control unit of the image forming apparatus, and the printing operation of the image forming apparatus will be described with reference to this diagram. PC 110, which is a host computer, sends a print command containing image data of the print image and print information to a video controller 91 inside the image forming apparatus.

[0019] The video controller 91 converts image data received from the PC 110 into exposure data and transfers it to an exposure control device 93 in the engine controller 92, while also sending a print command to the CPU 94. The exposure control device 93 is controlled by the CPU 94, and controls the exposure device 11, which turns the laser light on and off in accordance with the exposure data. When the CPU 94, which is a control means, receives a print command from the video controller 91, it starts the image formation operation.

[0020] The engine controller 92 is equipped with a CPU 94, a memory 95, etc. The CPU 94 operates according to a program stored in advance in the memory 95. The CPU 94 also has a timer for measuring time, and the memory 95 stores various information for controlling the fixing device 50, which will be described later. The high-voltage power supply 96 is made up of the above-mentioned charging high-voltage power supply 20, developing high-voltage power supply 21, primary transfer high-voltage power supply 22, and secondary transfer high-voltage power supply 26. The fixing power control device 97 is made up of a bidirectional thyristor (hereinafter referred to as a triac) 56, which is a supply control unit, and a heating element switch 57 (see FIG. 6), which is a switching unit that exclusively selects a heating element to which power is supplied. The fixing power control device 97 selects a heating element to which power is supplied in the fixing device 50 and determines the amount of power to supply.

[0021] The drive device 98 is composed of a main motor 99, a fixing motor 100, etc. The sensor 101 is composed of a fixing temperature sensor 59, which is a temperature detection means for detecting the temperature of the fixing device 50, a paper width sensor 31, etc., which detects the width of the paper P, and the detection result of the sensor 101 is sent to the CPU 94. The CPU 94 acquires the detection result of the sensor 101 and controls the exposure device 11, the high-voltage power supply 96, the fixing power control device 97, and the drive device 98 based on the detection result. In this way, the CPU 94 performs the formation of an electrostatic latent image, the transfer of the developed toner image to the paper P, the fixing of the transferred toner image to the paper P, etc., and controls the image forming process in which image data received from the PC 110 is printed as a toner image on the paper P. Note that the image forming apparatus to which the present invention is applied is not limited to the image forming apparatus with the configuration described in FIG. 1, but may be any image forming apparatus capable of printing on paper P of different widths and equipped with a fixing device 50 having a heater 54, which will be described later.

[0022] [Configuration of fixing device] Next, the configuration of the fixing device 50 that controls the heating device (heater) that heats the toner image on the paper P with a heat generating element will be described with reference to Figure 3. Here, the "longitudinal direction" refers to the direction of the rotation axis of the pressure roller 53, which is approximately perpendicular to the transport direction of the paper P, which will be described later. The length of the paper P in the direction (longitudinal direction) that is approximately perpendicular to the transport direction of the paper P is called the paper width.

[0023] 3 is a cross-sectional view illustrating the configuration of fixing device 50. In fixing device 50, paper P carrying an unfixed toner image T is transported from the left side of the figure in the direction of the arrow toward fixing nip N formed by a fixing film 51 (hereinafter referred to as film 51) and a pressure roller 53 in contact with each other. In fixing nip N, fixing film 51 is sandwiched between pressure roller 53 and heater 54. Paper P is heated while being transported from left to right in fixing nip N, thereby fixing the toner image T to paper P. Fixing device 50 is composed of cylindrical film 51, nip forming member 52 that holds film 51, pressure roller 53 that forms fixing nip N together with film 51, and heater 54 (heater unit), which is a heating device that heats paper P.

[0024] The film 51 is a fixing film that serves as a heating rotor. The film 51 uses, for example, polyimide as a base layer, and an elastic layer made of silicone rubber and a release layer made of PFA are formed on the base layer. Grease is applied to the inner surface of the film 51 to reduce the frictional force that occurs between the film 51 and the nip forming member 52 and heater 54 as the film 51 rotates.

[0025] The nip forming member 52 guides the film 51 from the inside and forms a fixing nip N between the film 51 and the pressure roller 53. The nip forming member 52 is a rigid, heat-resistant, and heat-insulating member made of a liquid crystal polymer or the like. The film 51 is fitted onto the nip forming member 52. The pressure roller 53 is a roller serving as a pressure rotating body and is composed of a core metal 53a, an elastic layer 53b, and a release layer 53c. The pressure roller 53 is rotatably supported at both longitudinal ends and is driven to rotate by a fixing motor 100 (FIG. 2). When the pressure roller 53 rotates, the film 51 is rotated. A heater 54, which is a heating member, is disposed in the internal space of the fixing film 51, is supported by the nip forming member 52, and is in contact with the inner surface of the film 51. Details of the heater 54 will be described later.

[0026] [Overview of the heater section] Next, the heater 54, which is a heating unit, will be described. FIG. 4 is a schematic diagram showing the configuration of the heater 54, in which the heating elements are arranged, as viewed from the pressure roller 53 side shown in FIG. 3. In FIG. 4, reference line a is the longitudinal center line of the heating elements 54b1a, 54b1b, 54b2, and 54b3, and is also the longitudinal center line (paper width direction) of the paper P transported to the fixing nip N of the fixing device 50. As shown in FIG. 4, the heater 54 includes a substrate 54a, heating elements 54b1a, 54b1b, 54b2, and 54b3, a conductor 54c, contacts 54d1 to 54d4, and a protective glass layer 54e. The conductor 54c is the portion painted black in the figure. In this embodiment, the substrate 54a is made of alumina (Al2O3), a ceramic material. Widely known ceramic substrates include alumina (Al2O3), aluminum nitride (AlN), zirconia (ZrO2), and silicon carbide (SiC). Among these, alumina (Al2O3) is inexpensive and easily available. Substrate 54a may also be made of a metal, which offers excellent strength. When using a metal substrate, stainless steel (SUS) is preferred due to its cost and strength. Whether the substrate is ceramic or metal, if it is conductive, an insulating layer may be provided. Heating elements 54b1a, 54b1b, 54b2, and 54b3, conductor 54c, and contacts 54d1 to 54d4 are arranged on substrate 54a, and a protective glass layer 54e is coated on top of these to ensure insulation between the heating elements and film 51.

[0027] Each heating element has a different length in the longitudinal direction (the length in the left-right direction in FIG. 4). The length L1 of heating elements 54b1a and 54b1b is 222 mm, the length L2 of heating element 54b2 is 188 mm, and the length L3 of heating element 54b3 is 154 mm. The length L1, L2, and L3 are in the order L1 > L2 > L3. For example, when the paper P used is A4 size, heating elements 54b1a and 54b1b are used. When the paper P used is B5 size, heating element 54b2 is mainly used. When the paper P used is A5 size, heating element 54b3 is mainly used. The heating elements are arranged in the short-side direction (the vertical direction in FIG. 4) in the order of heating elements 54b1a, 54b2, 54b3, and 54b1b.

[0028] As shown in FIG. 4, heating element 54b1a (first heating element) and heating element 54b1b (fourth heating element) are electrically connected at one end to contact 54d2 (first contact) and at the other end to contact 54d4 (fourth contact) via conductor 54c. Heating element 54b2 is electrically connected at one end to contact 54d2 and at the other end to contact 54d3 (third contact) via conductor 54c. Similarly, heating element 54b3 is electrically connected at one end to contact 54d1 (second contact) and at the other end to contact 54d3 via conductor 54c. As shown in FIG. 4, heating elements 54b1a and 54b1b have the same longitudinal length L1, and these two heating elements 54b1a and 54b1b are always used simultaneously. Hereinafter, the pair of heating elements 54b1a and 54b1b will be collectively referred to as heating element 54b1. The resistance values of the heating elements are 10.7Ω for heating element 54b1 (combined resistance value of heating elements 54b1a and 54b1b), 24.1Ω for heating element 54b2 (second heating element), and 24.1Ω for heating element 54b3 (third heating element). When the power supply voltage of AC power supply 55 (see FIG. 6) is 120V, the maximum average power of each heating element is 1346W for heating element 54b1, and 598W for heating elements 54b2 and 54b3.

[0029] In FIG. 4, the fixing temperature sensor 59 is enclosed by a dashed line. The dashed line indicates that the fixing temperature sensor 59 is located on the back surface of the substrate 54a (opposite the surface on which the heating elements 54b1, 54b2, and 54b3 are located) and indicates the position where the fixing temperature sensor 59 abuts against the substrate 54a. A thermistor 59a that detects the temperature of the fixing temperature sensor 59 is located on the longitudinal center line of the heating elements 54b1, 54b2, and 54b3 and on the reference line a, which is the center line of the paper P being conveyed to the fixing device 50. In this embodiment, the fixing temperature sensor 59, which serves as the first temperature detection means, is located at the longitudinal center of the heater 54, but the location of the fixing temperature sensor 59 is not limited to this position. While the effects of the present invention can be obtained as long as the fixing temperature sensor 59 is located within the fixing nip N, it is more preferable that the fixing temperature sensor 59 be located inside the longitudinal direction of the heating element 54b3, which is a heating element with a short longitudinal length.

[0030] [Heater section configuration] FIG. 5 is a schematic diagram showing a cross section of the heater 54 shown in FIG. 4 when the heater 54 is cut along the center line (reference line a in FIG. 4) in the longitudinal direction of the paper P conveyed to the fixing device 50. The fixing temperature sensor 59, which is a temperature detection means for detecting the temperature of the heater 54, is composed of the following components: a thermistor 59a, a holder 59b, ceramic paper 59c that blocks heat conduction between the holder 59b and thermistor 59a, and an insulating resin sheet 59d that physically and electrically protects the thermistor 59a. The thermistor 59a is a temperature detection element whose resistance value changes in response to the temperature of the heater 54, and whose output voltage changes. The thermistor 59a is connected to the CPU 94 by a dumet wire (not shown) and wiring, and outputs a voltage corresponding to the temperature of the heater 54 to the CPU 94. The CPU 94 controls the temperature of the heater 54 based on the temperature detection result of the fixing temperature sensor 59 (thermistor 59a). The fixing temperature sensor 59 is disposed on the surface of the substrate 54a opposite to the surface on which the heating elements 54b1, 54b2, and 54b3 covered with the protective glass layer 54e are disposed, and is in contact with the substrate 54a.

[0031] [Power control circuit] 6 is a schematic diagram showing the configuration of a power control circuit of the fixing device 50. The fixing device 50 of this embodiment switches the heating element to which power is supplied depending on the size of the paper P, thereby forming a desired temperature distribution in the longitudinal direction of the heater 54.

[0032] The power control circuit of the fixing device 50 includes triacs 56a and 56b, which are switching means for connecting or disconnecting the power supply path, a heating element switch 57, a triac state detection unit 58, and a relay 60 (second relay) that cuts off the power supply to all heating elements. The triacs 56a and 56b connect or disconnect the power supply path from the AC power supply 55 to each of the heating elements 54b1, 54b2, and 54b3. In this embodiment, the heating element switch 57 is configured as a C-contact relay (hereinafter referred to as relay 57). The triac state detection unit 58 monitors the on / off states of the triacs 56a and 56b.

[0033] The triac 56a (first switch) connects (ON state) or disconnects (OFF state) the power supply path between the AC power supply 55 and the contact 54d4 of the heater 54. On the other hand, the triac 56b (second switch) connects (ON state) or disconnects (OFF state) the power supply path between the AC power supply 55 and the contact 54d3 of the heater 54 via the relay 57, or between the AC power supply 55 and the contact 54d1 of the heater 54. The relay 57 (first relay) is switchable to connect the contact 54d3 of the heater 54 to the triac 56b or the AC power supply 55.

[0034] For example, when power is to be supplied from AC power supply 55 to heating element 54b1, triac 56a is turned on to connect AC power supply 55 to contact 54d4 of heater 54, and triac 56b is turned off. This connects heating element 54b1 (54b1a, 54b1b) to AC power supply 55 via contacts 54d2 and 54d4 of heater 54. When power is to be supplied from AC power supply 55 to heating element 54b2, triac 56b is turned on to connect AC power supply 55 to relay 57, and relay 57 is controlled to connect contact 54d3 of heater 54 to triac 56b, and triac 56a is turned off. As a result, one end of the heating element 54b2 is connected to the AC power supply 55 via the contact 54d3 of the heater 54, the relay 57, and the triac 56b, and the other end of the heating element 54b2 is connected to the AC power supply 55 via the contact 54d2 of the heater 54.

[0035] Furthermore, when power is supplied from AC power supply 55 to heating element 54b3, triac 56b is turned on and relay 57 is controlled to connect contact 54d3 of heater 54 to AC power supply 55, and triac 56a is turned off. As a result, one end of heating element 54b3 is connected to AC power supply 55 via contact 54d3 of heater 54 and relay 57, and the other end of heating element 54b3 is connected to AC power supply 55 via contact 54d1 of heater 54 and triac 56b. The on / off operation of triacs 56a and 56b is performed by commands (control signals) from CPU 94.

[0036] The triac state detection unit 58 detects the on / off states of the triacs 56a and 56b. For example, if the triacs 56a and 56b are simultaneously turned on due to an unexpected malfunction of the CPU 94, the triac state detection unit 58 sets the relay 60 to the off state and forcibly cuts off the power supply from the AC power supply 55 to the fixing device 50 (heater 54). This ensures that only one of the triacs 56a and 56b is turned on, or both are turned off, thereby preventing a malfunction of the fixing device 50.

[0037] In this way, triacs 56a and 56b, triac state detection unit 58, and relay 57 operate as a switching unit that switches the connection of the power supply path so that power is supplied from AC power supply 55 to only one of three heating elements 54b1, 54b2, and 54b3. In this embodiment, a switching unit with such a configuration is used, but the configuration for controlling the power supply path is not limited to the above-described configuration as long as power can be supplied to only one of the heating elements.

[0038] In this embodiment, the usage ratio of the heating elements 54b1, 54b2, and 54b3 (which is also the ratio of power supply to the heating elements 54b1, 54b2, and 54b3) is controlled according to the size of the paper P to form a desired temperature distribution in the longitudinal direction of the heater 54. The CPU 94 calculates the amount of power required to set the heater 54 to a desired temperature from temperature information detected by the fixing temperature sensor 59 (thermistor 59a). In this embodiment, PI control is used, but the control method is not limited to PI control.

[0039] To achieve the desired heating element usage ratio, the CPU 94 operates the triacs 56a and 56b and the heating element switch 57 to allocate the usage time (time ratio of power supply) of each of the heating elements 54b1, 54b2, and 54b3. The heating elements are switched every cycle of the power supply frequency of the AC power supply 55. For example, if the usage ratio (power supply ratio) of the heating elements 54b1a and 54b1b is set to 2 and the usage ratio (power supply ratio) of the heating element 54b2 is set to 8, the AC power supply 55 is connected to the heating element 54b1 for 1 cycle x 2 = 2 cycles. Then, the heating element to which power is supplied is switched, and the AC power supply 55 is connected to the heating element 54b2 for 1 cycle x 8 = 8 cycles. This operation is repeated until the AC power supply 55 is connected to the heating element 54b1 again. In this embodiment, the usage ratio (power supply ratio) can be switched in increments of 1 from 10:0 to 0:10.

[0040] In this embodiment, as described above, the desired usage ratio (power supply ratio) of the heating elements is realized by allocating the power supply time to the desired heating elements, but the method is not limited to this. The amount of power supplied to each heating element may be distributed by time, voltage, or current, or a combination of these. For example, a triac may be provided for each heating element as the heating element control means, and the CPU 94 may switch the on / off state of each triac to control the amount of current supplied to each heating element, thereby realizing the desired power supply ratio to each heating element. Furthermore, the resolution of the usage ratio (power supply ratio) (10:0 to 0:10) is not limited to this.

[0041] [Count temperature prediction method] Next, we will explain the count temperature prediction method, which is a temperature prediction means for predicting the temperature of each component of the fixing device 50. In this embodiment, the temperature of each component of the fixing device 50 (e.g., the film 51, the pressure roller 53, the nip forming member 52, etc.) is predicted using a count value. The count value is updated by the CPU 94 and increments by +1 each time a sheet of paper P is fixed in the fixing device 50. The more sheets of paper P fixed in the fixing device 50, the larger the count value. Meanwhile, in the standby state after the fixing process is completed, the components of the fixing device 50 naturally cool, so the count value also decreases over time. Specifically, the cooling characteristics of each component of the fixing device 50 are determined in advance, and the count value is decreased using an arithmetic equation that uses elapsed time as a variable. This method of managing the count value and predicting the temperature of each component of the fixing device 50 is called the count temperature prediction method.

[0042] The CPU 94 refers to the period from the count value of 0 to the first count value as Zone 1, and the period from the first count value to the second count value as Zone 2, and changes the frequency of switching the heating elements according to the zone number. The number of zones is not limited to two; three or more may be provided. In this embodiment, the first count value is 30, the second count value is 100, and the third count value is 200, and the zones are divided into four zones: Zone 1, Zone 2, Zone 3, and Zone 4. When printing begins in a Cold state (count value 0) where the fixing device 50 is at room temperature, the count value reaches the first count value of 30 after 30 sheets have been printed. Therefore, Zone 1 ends when the fixing process for the 30th sheet P is completed, and Zone 2 is switched to for the 31st sheet P and onwards.

[0043] Next, the actual printing operation of this embodiment will be described. Here, a case where A5-size paper P is continuously printed will be described. In this embodiment, the fixing device 50 performs the fixing operation on the A5-size paper P by switching between the heating element 54b1 having the largest longitudinal length (width) and the heating element 54b3 having a longitudinal width corresponding to the paper width of the A5-size paper P. When B5-size paper P is continuously printed, the fixing device 50 performs the fixing operation on the paper P by switching between the heating element 54b1 having the largest longitudinal length (width) and the heating element 54b2 having a longitudinal width corresponding to the paper width of the B5-size paper P. Similarly, when A4-size or letter-size paper P is continuously printed, the fixing device 50 performs the fixing operation on the paper P using only the heating element 54b1 having the largest longitudinal length (width). In the following, printing on A5-size paper P will be used as an example of printing on paper P.

[0044] When the above-mentioned zone number is small, the components of the fixing device 50 are in a low temperature state, and in this case, more power is supplied to the heating element 54b1, which is the heating element with the longest longitudinal length. The reason for this is to melt the grease in the film 51 uniformly in the longitudinal direction of the fixing nip N. If there are low-temperature areas in the longitudinal direction of the film 51 due to temperature unevenness, the grease will not melt uniformly, and the sliding resistance of the film 51 will not be uniform in the longitudinal direction, which may result in deformation of the film 51.

[0045] On the other hand, the higher the zone number, the higher the temperature of each component of the fixing device 50. In this case, a fixed proportion of power is supplied to the heating element 54b1, and more power is supplied to the heating element 54b3. This reduces the temperature at the longitudinal ends of the heating element, preventing uneven sliding resistance of the film 51 and deformation of the film 51. However, if the temperature at the longitudinal ends of the heating element becomes too high, it may exceed the heat resistance temperature of the film 51 and damage the film 51. Furthermore, if the temperature at the longitudinal ends of the heating element is too low or too high compared to the temperature at the center, it may lead to uneven temperature distribution of the paper P passing through the fixing nip N. As a result, heat may be supplied to the toner on the paper P at the edge of the paper P passing through the fixing nip N, resulting in poor image quality. Therefore, when printing on narrow paper P, it is preferable to keep the temperature difference between the paper-passing area of the film 51 through which the paper P passes and the non-paper-passing area within an appropriate range.

[0046] [Use ratio of heating elements in each zone and amount of power supplied] Table 1 below shows the usage ratio (power supply ratio) of heating element 54b1 to heating element 54b3 in each zone when printing A5-sized paper P, and the maximum average power consumption for power supply voltages of 120V and 110V. The usage ratio indicates the usage ratio of heating element 54b1 to heating element 54b3. When the AC voltage of AC power supply 55 is 120V, the maximum average power for heating element 54b1 is 1346W and for heating element 54b3 is 598W. When the AC voltage of AC power supply 55 is 110V, the maximum average power for heating element 54b1 is 1131W and for heating element 54b3 is 502W. When the usage ratio of heating elements is controlled as shown in Table 1, the maximum average power also changes depending on the usage ratio of the heating elements. For example, if the power supply voltage is 120V and the location is zone 4, then 1346W (heating element 54b1) x (2 / 10) + 598W (heating element 54b3) x (8 / 10) = 747.6W ≒ 748W. Similarly, if the power supply voltage is 110V and the location is zone 3, then 1131W (heating element 54b1) x (4 / 10) + 502W (heating element 54b3) x (6 / 10) = 753.6W ≒ 754W.

[0047] [Table 1]

[0048] Even if the heater 54 of the fixing device 50 has been sufficiently warmed up, if the paper P does not pass through the fixing nip N for a long period of time, it is preferable to control the temperature of the heater 54 using a heating element with a long longitudinal width (length) (heating element 54b1 in this embodiment). This is because, since the paper P that absorbs the heat of the heater 54 does not pass through the fixing nip N, uniformly heating the fixing nip N across the longitudinal width prevents the temperature difference between the paper passing area and the non-paper passing area from widening. Furthermore, using a heating element with a long longitudinal width (length) allows for a large power supply across the entire longitudinal width of the fixing nip N, allowing the temperature of the heater 54 to reach the target temperature more quickly.

[0049] On the other hand, if the paper P does not pass through the fixing nip N for a long period of time, switching between a heating element with a long longitudinal width (e.g., heating element 54b1) and a heating element with a short longitudinal width (e.g., heating element 54b3) to supply power may result in a drop in the temperature of the non-paper passing area of the fixing nip N. Furthermore, controlling the switching of the heating elements may reduce the average amount of power supplied, which may lengthen the time it takes for the heater 54 to reach the target temperature. In this embodiment, the following temperature control of the heater 54 is performed during the warm-up period before the paper P reaches the fixing nip N, or when there is an interval between the time when the paper P passes through the fixing nip N and the time when the subsequent paper P reaches the fixing nip N. That is, the temperature control of the heater 54 is performed using only heating element 54b1, which is the heating element with a long longitudinal width (length), regardless of the zone determined based on the count value described above.

[0050] [Control sequence of power supply to heating element] 7 is a flowchart showing the control sequence for supplying power to heating elements 54b1 and 54b3 when a print job for printing on A5-sized paper P is executed. The process in FIG. 7 is started when the print job starts and is executed by CPU 94. The period for supplying power to heating elements 54b1 and 54b3 is determined based on the usage ratio (also the power supply ratio) of heating elements 54b1 and 54b3 according to the zone corresponding to the count value shown in Table 1 above. Note that the count value that determines the zone is updated in a separate process.

[0051] When a print job is started, in step (hereinafter referred to as S) 100, the CPU 94 starts supplying power (first control) to the heating element 54b1 based on PI control. More specifically, the CPU 94 turns on the triac 56a and turns off the triac 56b to supply power from the AC power supply 55 to the heating element 54b1. In S101, the CPU 94 acquires the temperature of the heater 54 from the fixing temperature sensor 59 and determines whether the temperature of the heater 54 has reached a threshold temperature at which to start switching control of the heating element (second control). If the CPU 94 determines that the temperature of the heater 54 has reached the threshold temperature, it proceeds to S102, and if it determines that the temperature of the heater 54 has not reached the threshold temperature, it returns to S101.

[0052] In S102, the CPU 94 determines a zone based on the count value and obtains the usage ratio (also the power supply period ratio) of the heating elements 54b1 and 54b3 in the determined zone from Table 1. In S103, the CPU 94 starts the supply of power to the heating element 54b1 based on PI control, and also resets and starts the timer. In S104, the CPU 94 refers to the timer and determines whether the power supply period to the heating element 54b1 has elapsed. If the CPU 94 determines that the power supply period to the heating element 54b1 has elapsed, the process proceeds to S105, and if it determines that the power supply period to the heating element 54b1 has not elapsed, the process returns to S104.

[0053] In S105, the CPU 94 starts supplying power to the heating element 54b3 based on PI control. Specifically, the CPU 94 turns off the triac 56a, turns on the triac 56b, and switches the heating element switch 57 to connect the contact 54d3 of the heater 54 to the AC power supply 55, thereby supplying power from the AC power supply 55 to the heating element 54b3. The CPU 94 also resets and starts the timer. In S106, the CPU 94 refers to the timer and determines whether the power supply period to the heating element 54b3 has elapsed. If the CPU 94 determines that the power supply period to the heating element 54b3 has elapsed, the process proceeds to S107. If the CPU 94 determines that the power supply period to the heating element 54b3 has not elapsed, the process returns to S106. In S107, the CPU 94 determines whether the print job has ended. If the CPU 94 determines that the print job has ended, the process ends. If the CPU 94 determines that the print job has not ended, the process returns to S102.

[0054] [Heating element switching start timing] (When the power supply voltage is 120V) Next, the timing at which heating element switching is initiated during paper feed in this embodiment will be described. FIG. 8 is a diagram illustrating the relationship between the heating elements to which power is supplied at a power supply voltage of 120V and the temperature detected by the fixing temperature sensor 59 when printing A5-sized paper P in zone 4. From top to bottom, FIG. 8 shows the heating elements to which power is supplied (referred to as "powered heating elements" in the figure) and the timing at which the A5-sized paper P arrives at the fixing nip N (referred to as "paper arrival timing" in the figure). Furthermore, the graph at the bottom of FIG. 8 shows the temperature change of the heater 54 detected by the fixing temperature sensor 59 after the image forming apparatus starts a printing operation. The vertical axis of the graph in FIG. 8 represents the temperature of the heater 54 detected by the fixing temperature sensor 59 (referred to as "thermistor detected temperature" in the figure) (unit: °C), and the horizontal axis represents the elapsed time (unit: seconds) since the start of the printing operation.

[0055] In this embodiment, the target temperature of the heater 54 is 220°C, as shown in the graph of FIG. 8 . The threshold temperature of the heater 54 (referred to as thermistor threshold temperature in the graph) at which switching control of the heating elements 54b1 and 54b3 begins is set to 210°C. Hereinafter, the threshold temperature of the heater 54 at which switching control of the heating elements begins is also referred to as the thermistor threshold temperature or threshold temperature. The threshold temperature is experimentally determined based on the temperature rise in the non-sheet-passing area of the heater 54, which will be described below, and how well the heater 54 follows the target temperature. If the timing at which switching control of the heating elements begins is delayed, the temperature of the non-sheet-passing area of the fixing nip N increases. If the temperature rise in the non-sheet-passing area of the fixing nip N becomes significant, deformation of the fixing film 51 may occur. Therefore, the timing of switching the heating elements must be adjusted so that the temperature of the heater 54 is kept low enough to allow for a sufficient margin relative to the temperature of the non-sheet-passing area of the fixing nip N, which would cause deformation of the fixing film 51. Furthermore, if the timing for starting the switching control of the heating element is too early, the maximum amount of power that can be supplied to the heating element 54b1 will decrease, which may delay the timing at which the heater 54 reaches the target temperature. Therefore, it is necessary to adjust the timing for switching the heating element within a range that does not delay the timing at which the heater 54 reaches the target temperature. Furthermore, the warmer the heater 54 of the fixing device 50 is, the more likely the temperature of the non-paper passing area of the fixing nip N will rise, and the earlier the timing at which the heater 54 reaches the target temperature will be. Therefore, in such cases, it is preferable to lower the thermistor threshold temperature of the heater 54. In this embodiment, the thermistor threshold temperature differs depending on the zone determined based on the count value, and the larger the zone number, the lower the thermistor threshold temperature is.

[0056] In this embodiment, the heater 54 is heated by the heating element 54b1, which has a longer longitudinal length. After the fixing temperature sensor 59 detects that the temperature of the heater 54 has reached the threshold temperature, control is performed to alternate between the heating element 54b1 and the heating element 54b3, which has a shorter longitudinal length. First, a warm-up operation to warm up the fixing device 50 is initiated simultaneously with the start of the printing operation. Specifically, power supply to the heating element 54b1 and rotation of the pressure roller 53 are initiated simultaneously with the start of the printing operation. The CPU 94 supplies power to the heating element 54b1 so that the temperature detected by the fixing temperature sensor 59 quickly approaches the target temperature (first control). As shown in the graph in FIG. 8, 4 seconds after the start of power supply, the temperature detected by the fixing temperature sensor 59 reaches the threshold temperature of 210°C, and control to alternately switch between the heating elements 54b1 and 54b3 (second control) is initiated. Note that the leading edge of the A5-sized paper P enters the fixing nip N 4.5 seconds after the start of power supply. As described above, when the AC power supply 55 has a power supply voltage of 120 V, the maximum average power is 1,346 W when power is supplied using only the heating element 54b1, and the maximum average power is 748 W when switching between the heating elements 54b1 and 54b3 in zone 4. Therefore, when switching from control using only the heating element 54b1 to control using both the heating elements 54b1 and 54b3, the maximum amount of power supplied to the heater 54 decreases. However, in this embodiment, the heater 54 is sufficiently heated with the maximum amount of power that can be supplied being large until the temperature detected by the fixing temperature sensor 59 reaches the threshold temperature, and then control is started to switch between the heating elements 54b1 and 54b3 to supply power. Therefore, even after the maximum amount of power decreases, the temperature of the heater 54 does not decrease, and the temperature of the central portion of the heater 54 in the longitudinal direction tracks the target temperature, preventing image defects due to insufficient temperature in the fixing device 50.

[0057] (When the power supply voltage is 110V) Next, printing operations when the power supply voltage is low will be described using FIG. 9. FIG. 9 is a diagram illustrating the relationship between the heating elements supplied with power at a power supply voltage of 110V and the temperature detected by the fixing temperature sensor 59 when printing A5-sized paper P in zone 4. FIG. 9 is a diagram of the same configuration as FIG. 8, and an explanation of how to read the diagram will be omitted. The target temperature of the heater 54 is 220°C, as in FIG. 8. The threshold temperature of the heater 54 (referred to as thermistor threshold temperature in the graph) at which switching control of the heating elements 54b1 and 54b3 begins is set to 210°C.

[0058] When the power supply voltage is 110V, as in the case of a 120V power supply, power supply from AC power supply 55 to heating element 54b1 and rotation of pressure roller 53 begin simultaneously with the start of printing. Compared to FIG. 8, which shows a 120V power supply voltage, the graph in FIG. 9, which shows a 110V power supply voltage, reveals that the temperature detected by fixing temperature sensor 59 rises more slowly. At 4.5 seconds after the start of printing, when A5-sized paper P enters the fixing nip N, the temperature detected by fixing temperature sensor 59 (thermistor detected temperature) is 201°C, which does not yet reach the thermistor threshold temperature of 210°C. Five seconds after the start of printing, the temperature detected by fixing temperature sensor 59 reaches the thermistor threshold temperature of 210°C, and power supply control begins, switching between heating elements 54b1 and 54b3.

[0059] As shown in Table 1, when the power supply voltage is 110 V, the maximum average power is 1131 W when power supply control is performed using only heating element 54b1. On the other hand, the maximum average power is 628 W when power supply control is performed by switching between heating elements 54b1 and 54b3 in zone 4. When switching from control using only heating element 54b1 to control using both heating elements 54b1 and 54b3, the maximum amount of power supplied to heater 54 decreases. However, in this embodiment, power supply control is performed by switching between heating elements 54b1 and 54b3 after the heater 54 is sufficiently heated until the temperature of heater 54 detected by fixing temperature sensor 59 reaches the threshold temperature. As a result, even after the maximum amount of power supplied to heater 54 decreases, the temperature of heater 54 does not decrease, and the temperature of the longitudinal center of heater 54 tracks the target temperature. This prevents image defects due to insufficient temperature of heater 54 in fixing device 50. In addition, the temperature rise in the longitudinal non-paper passing area of the fixing nip N, where A5 size paper P did not pass, remained within a certain range, and no image defects occurred at the edge of the A5 size paper P due to excessive heat.

[0060] As described above, according to this embodiment, the power supply to the heater can be switched from supplying power to one heating element to supplying power alternately to multiple heating elements based on the detected temperature corresponding to the state of the fixing device. [Example]

[0061] In the first embodiment, when the temperature of the heater detected by the fixing temperature sensor reaches a threshold temperature, switching control of the power supply to the heating element is started. In the second embodiment, an example will be described in which switching control of the power supply to the heating element is started when the temperature of the heater detected by the fixing temperature sensor reaches the threshold temperature and the leading edge of the paper reaches the fixing nip. Note that the configurations of the image forming apparatus and fixing device 50 are the same as in the first embodiment, and the same devices and members are designated by the same reference numerals as in the first embodiment, and description thereof will be omitted.

[0062] [Control sequence of power supply to heating element] 10 is a flowchart showing the control sequence for supplying power to heating elements 54b1 and 54b3 when a print job is executed to print on A5-sized paper P. The process in FIG. 10 is started when the print job starts and is executed by CPU 94. The period during which power is supplied to heating elements 54b1 and 54b3 is determined based on the usage ratio (also the power supply ratio) of heating elements 54b1 and 54b3 according to the zone corresponding to the count value shown in Table 1 above. Note that the count value that determines the zone is updated in a separate process.

[0063] The process of S200 is the same as S100 in FIG. 7 of the first embodiment, and therefore a description thereof will be omitted here. In S201, the CPU 94 determines whether the leading edge of the A5-sized paper P has reached the fixing nip N. In this embodiment, the CPU 94 calculates the time required for the paper P to reach the fixing nip N based on the distance of the conveyance path from the cassette 16 to the fixing nip N and the conveyance speed of the paper P, and resets and starts a timer. The CPU 94 then determines whether the paper P has reached the fixing nip N by referring to the timer and determining whether the required time has elapsed. It is assumed that the distance of the conveyance path from the cassette 16 to the fixing nip N and the conveyance speed of the paper P are stored in advance in the memory 95. If the CPU 94 determines that the required time has elapsed based on the timer, it determines that the paper P has reached the fixing nip N and proceeds to S202. On the other hand, if the CPU 94 determines that the required time has not elapsed, it determines that the paper P has not reached the fixing nip N, and returns the process to S201. The processes of S202 to S208 are the same as the processes of S101 to S107 in Fig. 7 of the first embodiment, and therefore a description thereof will be omitted here. Note that the order of the processes of S201 and S202 may be reversed.

[0064] [Heating element switching start timing] Next, the timing for starting switching of the heating elements when paper is passing through in this embodiment will be described. Figure 11 is a diagram illustrating the relationship between the heating elements supplied with power at a power supply voltage of 120V and the temperature detected by the fixing temperature sensor 59 when printing A5-sized paper P in zone 4. Figure 11 is a diagram of the same configuration as Figure 8 of embodiment 1, and an explanation of how to read the figure will be omitted. Note that the target temperature of the heater 54 in this embodiment is 220°C, the same as in embodiment 1. Also, the threshold temperature of the heater 54 (referred to as thermistor threshold temperature in the graph) at which switching control of the heating elements 54b1 and 54b3 begins is set to 210°C.

[0065] In this embodiment, the heater 54 is heated by the heating element 54b1, which has a longer longitudinal length. After the fixing temperature sensor 59 detects that the temperature of the heater 54 has reached a threshold temperature, the heating element 54b1 and the heating element 54b3, which has a shorter longitudinal length, are alternately switched over. First, a warm-up operation to warm up the fixing device 50 is initiated simultaneously with the start of the printing operation. Specifically, power supply to the heating element 54b1 and rotation of the pressure roller 53 are initiated simultaneously with the start of the printing operation. As shown in the graph in FIG. 11, 4 seconds after the start of the printing operation, the temperature of the heater 54 detected by the fixing temperature sensor 59 (thermistor detection temperature) reaches the thermistor threshold temperature of 210°C. Then, 4.5 seconds after the start of the printing operation, the leading edge of the A5-sized paper P reaches the fixing nip N, and control to alternately switch the power supply between the heating elements 54b1 and 54b3 is initiated. Thus, in Example 2, unlike Example 1, even if the temperature of the heater 54 detected by the fixing temperature sensor 59 reaches the thermistor threshold temperature, the heating element switching control is not started until the A5 size paper P reaches the fixing nip portion N.

[0066] As described above, if the switching control of the heating element is performed when the paper P is not passing through the fixing nip N, the temperature of the longitudinal ends of the fixing nip N decreases relative to the longitudinal center. Furthermore, when the switching control of the heating element is initiated, the maximum amount of power that can be supplied to the heater 54 decreases, which may delay the timing at which the heater 54 reaches its target temperature. In this embodiment, during warm-up of the fixing device 50, the switching control of the power supply to the heating element is initiated after the paper P arrives at the fixing nip N, thereby preventing a decrease in the temperature of the longitudinal ends of the heater 54. Furthermore, since the maximum amount of power that can be supplied to the heater 54 can be maintained high for a longer period of time, the timing at which the heater 54 reaches its target temperature increases. Furthermore, when the power supply voltage is low, such as 110 V, if the temperature of the heater 54 has not yet reached the thermistor threshold temperature when the leading edge of the paper P reaches the fixing nip N, the same control as described in the first embodiment with reference to FIG. 9 can be performed. As a result, the temperature drop in the center of the heater 54 in the longitudinal direction is suppressed, and the temperature rise in the longitudinal ends is kept within a certain range, preventing image defects at the ends of A5 size paper P due to excessive heat. In this embodiment, in addition to the effects of embodiment 1, it is possible to prevent a temperature drop at the longitudinal ends of the fixing nip portion N during warm-up of the fixing device 50, and to speed up the timing at which the target temperature is reached during warm-up.

[0067] As described above, according to this embodiment, the power supply to the heater can be switched from supplying power to one heating element to supplying power alternately to multiple heating elements based on the detected temperature corresponding to the state of the fixing device. [Example]

[0068] In the third embodiment, an embodiment will be described in which the temperature of the fixing device is accurately controlled by correcting the threshold temperature (also called thermistor threshold temperature) at which switching control of the heating element is initiated according to the ambient temperature and power supply voltage of the image forming device and the basis weight of the paper used.

[0069] [Image forming device control block] Fig. 12 is a block diagram showing the configuration of the control unit of the image forming apparatus of this embodiment. Fig. 12 differs from Fig. 2 of the first embodiment in that an environmental temperature sensor 106 and a current detection circuit 107 are added. The other members and devices are the same as those shown in Fig. 2 of the first embodiment, and the same members and devices are designated by the same reference numerals, and the description thereof will be omitted here.

[0070] An environmental temperature sensor 106, which is a second temperature detection means, is provided at a position not affected by heat generated by the heater 54 of the fixing device 50, detects the temperature around the image forming apparatus (hereinafter referred to as environmental temperature), and outputs the detected environmental temperature to the CPU 94. The fixing power control device 97 also has a current detection circuit 107. The current detection circuit 107 detects the current supplied from the AC power supply 55 to the heater 54 of the fixing device 50, and outputs the detected current value to the CPU 94.

[0071] [Power control circuit] FIG. 13 is a schematic diagram showing the configuration of a power control circuit of the fixing device 50 of this embodiment. FIG. 13 differs from FIG. 6 of the first embodiment in that a current detection circuit 107 is added. Other circuits and components shown in FIG. 13 are similar to those in FIG. 6 of the first embodiment, and the same circuits and components are designated by the same reference numerals, and their description will be omitted. The current detection circuit 107 is provided in the current path between the AC power supply 55 and the triacs 56a and 56b, and detects the value of the current supplied to the heating element of the heater 54 of the fixing device 50. The current detection circuit 107 outputs the detected current value to the CPU 94.

[0072] [Calculating power supply voltage] The CPU 94 calculates the power supply voltage, which is the AC voltage value supplied from the AC power supply 55, based on the current value acquired from the current detection circuit 107. FIG. 14 is a flowchart showing a voltage calculation sequence in which the CPU 94 calculates the power supply voltage. The processing in FIG. 14 is started when the image forming apparatus is powered on, and is executed by the CPU 94. Note that the memory 95 stores a resistance value of the heating element 54b1 that has been measured in advance.

[0073] When the image forming apparatus is powered on, power is supplied to fixing device 50, and an operation of rotating pressure roller 53 (pre-rotation) is performed. In S31, during pre-rotation, CPU 94 turns on triac 56a and turns off triac 56b, and supplies power from AC power supply 55 to heating element 54b1 at a duty of 80%. In S32, CPU 94 acquires current value I output from AC power supply 55 from current detection circuit 107. In S33, CPU 94 acquires resistance value R of heating element 54b1 from memory 95. In S34, CPU 94 uses current value I acquired in S32 and resistance value R acquired in S33 to calculate power supply voltage V (= current value I × resistance value R) supplied from AC power supply 55, stores the calculated power supply voltage V in memory 95, and ends the process. In this embodiment, the power supply voltage is calculated based on the actually measured current value I and resistance value R. However, for example, a method may be used in which the power supply voltage is predicted from the temperature rise of the heater 54 detected by the fixing temperature sensor 59 relative to the amount of power supplied to the heater 54.

[0074] [Thermistor threshold temperature correction] In this embodiment, in order to accurately control the temperature of the fixing device 50, the threshold temperature (thermistor threshold temperature) at which switching control of the heating elements described above is initiated is corrected according to the ambient temperature and power supply voltage of the image forming apparatus and the basis weight of the paper being used. Here, the thermistor threshold temperature at which switching control of the heating elements described in the first and second embodiments is initiated is defined as the reference thermistor threshold temperature Ta (unit: °C), and the correction amount by which the reference thermistor threshold temperature Ta is corrected is defined as the thermistor threshold correction amount Tb (unit: °C). Then, assuming that the corrected thermistor threshold temperature is defined as the thermistor threshold temperature Ts (unit: °C), the thermistor threshold temperature Ts in this embodiment is expressed by the following (Equation 1). Thermistor threshold temperature Ts = Reference thermistor threshold temperature Ta + Thermistor threshold correction amount Tb ...(Formula 1)

[0075] The reference thermistor threshold temperature Ta is determined by the environmental temperature T (unit: °C) detected by the environmental temperature sensor 106 and the power supply voltage V (unit: V) calculated by the voltage calculation sequence described above, and is expressed by the following (Equation 2): Reference thermistor threshold temperature Ta = 220 - (ambient temperature T-23) -(power supply voltage V-120) / 2 (equation 2)

[0076] As described above, the higher the environmental temperature T is than the predetermined temperature (23°C), the lower the reference thermistor threshold temperature Ta. This is because the higher the environmental temperature T is than the predetermined temperature (23°C), the more allowance there is for a temperature drop in the center of the heater 54 in the longitudinal direction. Furthermore, the higher the power supply voltage V is than the predetermined voltage (120V), the lower the reference thermistor threshold temperature Ta. This is because the higher the power supply voltage V is than the predetermined voltage (120V), the greater the maximum amount of power supplied to the heater 54, and therefore the temperature in the center of the heater 54 is less likely to drop in the longitudinal direction even if heating element switching control is started at a low temperature.

[0077] The thermistor threshold correction amount Tb also varies depending on the basis weight (weight per unit area) of the paper P. Table 2 shows the relationship between the basis weight X of the paper P and the thermistor threshold correction amount Tb. 2 ) and the thermistor threshold correction amount Tb (unit: °C).

[0078] [Table 2]

[0079] As shown in Table 2, the greater the basis weight X, the greater the thermistor threshold correction amount Tb. This is because the smaller the basis weight of the paper P, the less heat is removed from the fixing device 50 (the fixing nip N) when the paper P passes through the fixing nip N. Therefore, even if the switching control of the heating element is started at a low temperature, the temperature at the center of the longitudinal direction of the fixing nip N is less likely to drop. The method of calculating the thermistor threshold temperature Ts in this embodiment using the above-mentioned (Equation 1) was experimentally confirmed by changing the environmental temperature T, power supply voltage V, and basis weight X of the paper P.

[0080] For example, if the ambient temperature T is 30°C, the power supply voltage V is 110V, and the basis weight of A5 size paper P is 80g / m 2 In this case, the reference thermistor threshold temperature Ta is calculated using (Equation 2) as follows: Reference thermistor threshold temperature Ta = 220 - (30 - 23) - (110 - 120) / 2 =218 (unit: °C). Also, the thermistor threshold correction amount Tb (unit: °C) is +5°C according to Table 2. Therefore, the thermistor threshold temperature Ts is calculated from (Equation 1) Thermistor threshold temperature Ts = Reference thermistor threshold temperature Ta + Thermistor threshold correction amount Tb =218+5=223 (unit: °C).

[0081] As described above, in this embodiment, the thermistor threshold temperature Ts is calculated as an absolute value, but it may also be a relative value relative to the target temperature of the heater 54, for example. Furthermore, in this embodiment, the thermistor threshold temperature Ts is corrected based on the basis weight of the paper P, but the thermistor threshold temperature Ts may also be corrected based on an index representing other characteristics of the paper P, such as the surface roughness of the paper P. As described above, in this embodiment, in addition to the effects of the second embodiment, the thermistor threshold temperature is finely corrected based on the ambient temperature, power supply voltage, and paper basis weight. This makes it possible to more accurately control the temperature of the longitudinal center of the heater while also adjusting the temperature of the longitudinal end portions of the fixing nip N.

[0082] As described above, according to this embodiment, the power supply to the heater can be switched from supplying power to one heating element to supplying power alternately to multiple heating elements based on the detected temperature corresponding to the state of the fixing device. [Example]

[0083] In the fourth embodiment, the heater temperature reaches a threshold temperature (thermistor threshold temperature) at which switching control of the heating element is initiated, and after a waiting time set according to the ambient temperature, power supply voltage, and paper basis weight has elapsed, switching control of the heating element is initiated. Note that the configurations of the image forming apparatus and fixing device 50 are the same as those in the third embodiment, and the same devices and members are designated by the same reference numerals as in the third embodiment, and description thereof will be omitted.

[0084] [Waiting time calculation] If the heater 54 starts switching the heating elements immediately after reaching the target temperature, the fixing device 50 will not have enough heat storage capacity. This will result in a decrease in the average power supplied to the heater 54 due to the switching control of the heating elements, resulting in a decrease in the temperature at the longitudinal center of the fixing nip N. As a result, image defects may occur due to insufficient heat output from the heater 54. For example, when the ambient temperature T of the image forming apparatus is low, the heater 54 dissipates more heat than when the ambient temperature T is high, which may result in a decrease in the temperature at the longitudinal center of the heater 54. Furthermore, when the power supply voltage V supplied from the AC power supply 55 is low, the maximum power output of the AC power supply 55 is reduced, which may result in a decrease in the temperature at the longitudinal center of the heater 54. Furthermore, when the basis weight of the paper P is high, the amount of heat absorbed by the paper P from the fixing nip N as it passes through the fixing nip N increases, which may result in a decrease in the temperature at the longitudinal center of the fixing nip N. In this embodiment, when the above-mentioned ambient temperature T is low, the power supply voltage V is low, or the basis weight of the paper P is large, a waiting time is provided after the heater 54 reaches the target temperature before the start of heat element switching control. This allows the fixing device 50 to store heat with a sufficient amount of power, and suppresses a temperature drop in the center of the fixing nip N in the longitudinal direction.

[0085] [Waiting time calculation] Next, a method for calculating the waiting time Tw from when the heater 54 reaches the target temperature until switching control of the heating element starts will be described. If the reference waiting time is a reference waiting time Tn (unit: seconds) and the correction amount for correcting the reference waiting time Tn is a waiting time correction amount Tm (unit: seconds), the waiting time Tw (unit: seconds) is expressed by the following (Equation 3). Waiting time Tw = Reference waiting time Tn + Waiting time correction amount Tm (Equation 3)

[0086] The reference waiting time Tn is determined by the environmental temperature T (unit: °C) detected by the environmental temperature sensor 106 and the power supply voltage V (unit: V) calculated by the voltage calculation sequence described in the third embodiment, and is expressed by the following (Equation 4): Standard standby time Tn=2-(environmental temperature T-23) / 7 -(power supply voltage V-120) / 14 (equation 4)

[0087] Furthermore, the standby time correction amount Tm varies depending on the basis weight (weight per unit area) of the paper P. Table 3 shows the relationship between the basis weight X of the paper P and the standby time correction amount Tm. 2 ) and the waiting time correction amount Tm (unit: seconds) according to the

[0088] [Table 3]

[0089] The waiting time Tw is limited to a value between 0 and 4 seconds. The calculation method for the waiting time Tw using the above-mentioned (Equation 3) was confirmed experimentally by changing the environmental temperature, power supply voltage, and basis weight of the paper P.

[0090] For example, if the ambient temperature T is 30°C, the power supply voltage V is 110V, and the basis weight of A5 size paper P is 80g / m 2 In this case, the reference waiting time Tn is calculated using (Equation 4) as follows: Standard waiting time Tn = 1-(30-23) / 7-(110-120) / 14 =0.71 (seconds). Also, the waiting time correction amount Tm (unit: seconds) is +0.5 seconds according to Table 3. Therefore, the waiting time Tw is calculated from (Equation 3) as follows: The waiting time Tw = 0.71 + 0.5 = 1.21 (unit: seconds).

[0091] [Control sequence of power supply to heating element] FIG. 15 is a flowchart showing a control sequence for supplying power to the heating elements 54b1 and 54b3 when a print job for printing A5-sized paper P is executed in this embodiment. The process of FIG. 15 is started when the print job is started and executed by the CPU 94. The power supply period to the heating elements 54b1 and 54b3 is determined based on the usage ratio (also the power supply ratio) of the heating elements 54b1 and 54b3 corresponding to the zone corresponding to the count value shown in Table 1 of the first embodiment. Note that the count value determining the zone is updated in a separate process. Also, it is assumed that the power supply voltage V calculated by the voltage calculation sequence described in the third embodiment is stored in memory 95. Furthermore, it is assumed that the data of Table 3, which is a table showing the relationship between the basis weight X of the paper P and the standby time correction amount Tm, is stored in memory 95. It is also assumed that the basis weight of each size of paper P is stored in memory 95.

[0092] The processes of S400 to S402 are the same as those of S200 to S202 in Fig. 10 of the second embodiment, and therefore description thereof will be omitted here. In S403, the CPU 94 calculates the above-mentioned standby time Tw. In detail, the CPU 94 acquires the environmental temperature T detected by the environmental temperature sensor 106, acquires the power supply voltage V from the memory 95, and calculates the reference standby time Tn using the above-mentioned (Equation 4). Furthermore, the CPU 94 determines the standby time correction amount Tm based on the data in Table 3 stored in the memory 95 and the basis weight of the A5-sized paper P, and calculates the standby time Tw using the above-mentioned (Equation 3).

[0093] In S404, the CPU 94 resets and starts the timer. In S405, the CPU 94 refers to the timer and determines whether the waiting time Tw has elapsed. If the CPU 94 determines that the waiting time Tw has elapsed, the process proceeds to S406, and if the CPU 94 determines that the waiting time Tw has not elapsed, the process returns to S405. The processes of S406 to S411 are the same as those of S203 to S208 in FIG. 10 of the second embodiment, and therefore will not be described here.

[0094] In this way, in this embodiment, in addition to the effects of embodiment 2, by setting the waiting time until the switching control of the heating element starts after the thermistor threshold temperature is reached, it is possible to perform accurate temperature control of the heater 54. Furthermore, by finely changing the waiting time depending on the ambient temperature, power supply voltage, and paper basis weight, it is possible to perform even more accurate temperature control of the heater 54.

[0095] As described above, according to this embodiment, the power supply to the heater can be switched from supplying power to one heating element to supplying power alternately to multiple heating elements based on the detected temperature corresponding to the state of the fixing device. [Explanation of symbols]

[0096] 51 Fixing film 54 Heater 54b1, 54b3 Heating element 56a, 56b triac 57 Heating element switch 59 Fixing temperature sensor 94 CPU

Claims

1. A fixing device that fixes an unfixed toner image on a recording material to the recording material, A heating rotor; a heater having a first heating element on a substrate and a second heating element having a length in a longitudinal direction shorter than that of the first heating element, the heater heating the heating rotator; a first temperature detection means for detecting the temperature of the heater; a pressure rotating body that forms a nip portion with the heating rotating body; a switching means for switching a power supply path from an AC power source to the first heating element or the second heating element; a control means for controlling the switching means to supply power to the first heating element or the second heating element; Equipped with The control means performs a first control to supply power to the first heating element, and when it detects that the temperature detected by the first temperature detection means reaches a threshold temperature and that the recording material has reached the nip portion, it starts a second control to supply power alternately to the first heating element or the second heating element.

2. a temperature predicting unit that predicts a temperature of the fixing device based on a count value that increases when a recording material passes through the nip portion and decreases as time passes without the recording material passing through the nip portion; 2. The fixing device according to claim 1, wherein the threshold temperature is determined based on the count value of the temperature predicting means.

3. 3. The fixing device according to claim 2, wherein the threshold temperature decreases as the count value of the temperature predicting means increases.

4. 4. The fixing device according to claim 3, wherein a time ratio during which the AC power source supplies power to the first heating element and the second heating element is determined based on the count value.

5. The heater includes a third heating element having a length in the longitudinal direction shorter than that of the second heating element; a fourth heating element having a length in the longitudinal direction substantially equal to that of the first heating element, 5. The fixing device according to claim 1, wherein the first heat generating element, the second heat generating element, the third heat generating element, and the fourth heat generating element are arranged in this order in the short-side direction of the substrate.

6. The heater is a first contact point to which one ends of the first heating element, the second heating element, and the fourth heating element are electrically connected; a second contact point to which one end of the third heating element is electrically connected; a third contact point to which the other ends of the second heating element and the third heating element are electrically connected; a fourth contact point to which the other ends of the first heating element and the fourth heating element are electrically connected; 6. The fixing device according to claim 5, further comprising:

7. the switching means includes a first switch, a second switch, and a first relay; the first switch connects or disconnects the AC power supply and the fourth contact; the second switch connects or disconnects the AC power supply to or from the first relay and the AC power supply to or from the second contact; 7. The fixing device according to claim 6, wherein the first relay is capable of switching between a connection between the second switch and the third contact and a connection between the AC power source and the third contact.

8. a detection unit that detects the states of the first switch and the second switch; a second relay that connects or disconnects a power supply path between the AC power source and the first contact, 8. The fixing device according to claim 7, wherein the detection unit drives the second relay to cut off the power supply path when the detection unit detects that the first switch connects the AC power supply to the fourth contact and that the second switch connects the AC power supply to the first relay.

9. 9. The fixing device according to claim 8, wherein the first switch and the second switch are bidirectional thyristors.

10. 10. The fixing device according to claim 9, wherein the first temperature detecting means is a thermistor.

11. the heating rotatable body is a cylindrical film, the pressure rotating body is a pressure roller that forms the nip portion with the film, 11. The fixing device according to claim 1, wherein the heater is disposed in the internal space of the film, the film is sandwiched between the heater and the pressure roller, and the image on the recording material is heated through the film at the nip portion.

12. an image forming means for forming an unfixed toner image on a recording material; a fixing device according to any one of claims 1 to 11, which fixes an unfixed toner image on a recording material; An image forming apparatus comprising:

13. a second temperature detection unit for detecting an ambient temperature of the image forming apparatus; 13. The image forming apparatus according to claim 12, wherein the threshold temperature is corrected to be lower when the environmental temperature is higher than a predetermined temperature, and is corrected to be higher when the environmental temperature is lower than the predetermined temperature.

14. a current detection unit that detects the current supplied from the AC power source to the fixing device; 13. The image forming apparatus according to claim 12, wherein the threshold temperature is corrected based on a power supply voltage calculated based on a current value detected by the current detection unit.

15. 15. The image forming apparatus according to claim 14, wherein the threshold temperature is corrected to be higher when the power supply voltage is lower than a predetermined voltage, and is corrected to be lower when the power supply voltage is higher than the predetermined voltage.

16. 13. The image forming apparatus according to claim 12, wherein the threshold temperature is corrected to be lower if the basis weight of the recording material is smaller than a predetermined basis weight, and is corrected to be higher if the basis weight of the recording material is larger than the predetermined basis weight.

17. 13. The image forming apparatus according to claim 12, wherein the control unit starts the second control when a standby time has elapsed after detecting that the temperature detected by the first temperature detection unit has reached the threshold temperature and that the recording material has reached the nip portion.

18. a second temperature detection unit for detecting an ambient temperature of the image forming apparatus; 18. The image forming apparatus according to claim 17, wherein the standby time is corrected to be shorter when the environmental temperature is higher than a predetermined temperature, and is corrected to be longer when the environmental temperature is lower than the predetermined temperature.

19. a current detection unit that detects the current supplied from the AC power source to the fixing device; 18. The image forming apparatus according to claim 17, wherein the standby time is corrected based on a power supply voltage calculated based on the current value detected by the current detection unit.

20. 20. The image forming apparatus according to claim 19, wherein the standby time is corrected to be longer when the power supply voltage is lower than a predetermined voltage, and is corrected to be shorter when the power supply voltage is higher than the predetermined voltage.

21. 18. The image forming apparatus according to claim 17, wherein the waiting time is corrected to be shorter if the basis weight of the recording material is smaller than a predetermined basis weight, and is corrected to be longer if the basis weight of the recording material is larger than the predetermined basis weight.

Images