Fixing device and image forming apparatus

Abstract

To accurately control the temperature of a fixing device to prevent an increase in temperature of a non-paper feed part.SOLUTION: A fixing device comprises: a fixing film 51; a heater 54 that has heating elements 54b1, 54b3 and heats the fixing film 51; a pressure roller 53; a thermistor 59a 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 elements 54b1, 54b3; and a CPU 94 that controls the triacs 56a, 56b to supply power to the heating elements 54b1, 54b3. The CPU 94 performs first power control (S101-S103) of, based on the temperature and a target temperature of the heater 54, determining the amount of power supplied per unit time to the heating elements 54b1, 54b3 and supplying power, and second power control (S105-S108) of controlling the ratio between the amount of power supplied to the heating element 54b1 and the amount of power supplied to the heating element 54b3 or controlling the ratio between the period for supplying power to the heating element 54b1 and the period for supplying power to the heating element 54b3 in the first power control.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, when continuously printing on paper whose width is narrower than the longitudinal heater width of the heater (heating device) that heats the paper, a phenomenon called non-paper passing area temperature rise occurs, in which the temperature gradually rises in the area of the heater where the paper does not pass. If the temperature rise in the non-paper passing area becomes significant, the fixing components of the fixing device, such as the fixing film and pressure roller, may be damaged by the temperature rise. Therefore, for example, Patent Document 1 proposes a configuration that reduces the temperature rise in the non-paper passing area in the fixing device by switching the heat generation ratio between the center and end portions of the heater in the longitudinal direction. [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, temperature control is performed to maintain the temperature of the longitudinal end portion of the fixing device within a certain range relative to the temperature of the longitudinal center portion, but there is a demand for more accurate temperature control.

[0005] The present invention has been made under such circumstances, and an object thereof is to accurately control the temperature of a fixing device so as to prevent a temperature rise in non-paper passing areas. [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 toner on a recording material by heating the toner thereon, the fixing device comprising: a cylindrical film; a first heating element; and a second heating element having a length in the longitudinal direction shorter than that of the first heating element; a heater disposed in the internal space of the film for heating the film; a pressure roller forming a nip with the film; a detection means for detecting the temperature of the heater; 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. a control means for controlling the supply of power to the first heating element and the second heating element by phase control; and wherein the control means determines an amount of power to be supplied to the first heating element and the second heating element per unit period based on the temperature of the heater detected by the detection means and a target temperature of the heater, and performs a first power control to supply power to the first heating element or the second heating element, and a second power control to supply power to the second heating element until an integrated value of the amount of power supplied to the second heating element becomes equal to or greater than a planned amount of power after supplying the amount of power supply determined by the first power control to the first heating element for the unit period. stomach , The first power control and the second power control are executed periodically at the unit period, and power is supplied to the first heating element or the second heating element during the unit period. In the first power control, the amount of power supplied to the first heating element and the second heating element is determined so that the ratio of the power duty per unit length in the longitudinal direction of the second heating element to the first heating element becomes a value close to 1. A fixing device characterized by:

[0008] (2) An image forming apparatus comprising: an image forming section for forming an image on a recording material; a paper feeding section for feeding the recording material to the image forming section; and the fixing device described in (1). [Effects of the Invention]

[0009] According to the present invention, the temperature of the fixing device can be controlled with high precision so as not to cause a temperature rise in the non-sheet passing portion. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view showing the configuration of an image forming apparatus according to 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 to fourth 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] Schematic diagram showing the configuration of a power control circuit of the fixing device according to Examples 1 to 4. [Figure 7] 1 is a flowchart showing a power supply control sequence for a heating element according to a first embodiment. [Figure 8] FIG. 1 is a diagram illustrating a state of power supply to a heating element in the first embodiment. [Figure 9] 10 is a flowchart showing a power supply control sequence for a heating element according to a second embodiment. [Figure 10] FIG. 10 is a diagram illustrating the state of power supply to the heating element in the second embodiment. [Figure 11] 10 is a flowchart showing a power supply control sequence for a heating element according to a third embodiment. [Figure 12] 10 is a flowchart showing a power supply control sequence for 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 where a heating element generates heat and where the recording material does not pass 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 temperature of the non-paper passing area is higher than that of the paper passing area is referred to as a non-paper passing portion temperature rise. [Example]

[0012] [Overall 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 example of 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. Components with 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] 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 the main motor 99. The photosensitive drum 1a is uniformly charged by the charging roller 2a, to which a voltage is applied from the charging high-voltage power supply 20a. Then, an electrostatic latent image based on image information is formed by the scanning beam 12a irradiated from the exposure device 11a. The toner 5a in the development unit 8a is negatively charged by the developer application blade 7a and applied to the development roller 4a. A predetermined development voltage is then applied to the development roller 4a from the 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] [Control block diagram of image forming device] 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 40. 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 54d1-54d4 are arranged on substrate 54a, and a protective glass layer 54e is coated on top of them 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 elements 54b1a and 54b1b 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 (first heating element). The resistance values of the heating elements are as follows: heating element 54b1 is 10.7Ω (combined resistance value of heating elements 54b1a and 54b1b), heating element 54b2 (third heating element) is 24.1Ω, and heating element 54b3 (second heating element) is 24.1Ω.

[0029] 4, the fixing temperature sensor 59 is surrounded by a dashed line. The dashed line indicates that the fixing temperature sensor 59 is disposed on the rear surface of the substrate 54a (opposite the surface on which the heating elements 54b1, 54b2, and 54b3 are disposed) and indicates the position where the fixing temperature sensor 59 abuts against the substrate 54a. The thermistor 59a that detects the temperature of the fixing temperature sensor 59 is disposed 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 transported to the fixing device 50.

[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] Furthermore, it is preferable to keep the switching period of the switching unit, i.e., the transient period during which power is not supplied to either heating element, as short as possible. The reason for this is that a long switching period of the heating elements during printing of paper P can cause an unintended drop in the temperature of the heater 54, potentially resulting in insufficient melting of the toner on the paper P. In this embodiment, the heating elements are switched during printing of paper P by switching the on / off states of triacs 56a and 56b. Therefore, in this embodiment, the time required to switch the on / off states of triacs 56a and 56b is nearly zero. This allows power to be supplied via triac 56a during a half-wave period (half cycle of the power supply frequency) of the voltage waveform of AC power supply 55, and then via triac 56b during the next half-wave period (half cycle). On the other hand, if relay 57 is used to switch heating elements during printing, it is difficult to shorten the switching time of the heating elements. This is because it takes about 100 msec (milliseconds) to switch relay 57, and it is necessary to provide a period to prevent contact welding that occurs when current flows during switching of relay 57. When an experiment was conducted in which the switching time of relay 57 was intentionally lengthened, it was found that when the switching time of relay 57 exceeded 320 msec, the toner on paper P did not melt sufficiently. Therefore, even when relay 57 is used to switch heating elements while printing on paper P, it is preferable to set the switching time of relay 57 to 320 msec or less.

[0039] [First power control] In this embodiment, a "first power control" is performed to bring the temperature of the heater 54 detected by the fixing temperature sensor 59 closer to the target temperature, and a "second power control" is performed to bring the distribution of power to the multiple heating elements closer to the target power. First, the first power control will be described. In the first power control, the amount of power to be supplied to the heating elements is calculated periodically based on the difference between the target temperature of the heater 54 and the temperature detected by the fixing temperature sensor 59. Specifically, the CPU 94 periodically calculates the amount of power required to raise the temperature of the heater 54 to the target temperature suitable for forming an image on the paper P based on temperature information of the heater 54 detected by the thermistor 59a, which is a temperature detection means. In this embodiment, power is supplied to the heating elements by phase control of the AC power supply 55.

[0040] In this embodiment, PI control is used to calculate the amount of power. The power calculation using PI control is performed periodically, with one cycle being two half-waves of the voltage waveform of the AC power supply 55 (one cycle of the power supply frequency). In PI control, the CPU 94 compares the temperature of the heater 54 detected by thermistor 59a with the target temperature for each cycle and determines the values of the proportional and integral terms in the PI control based on the magnitude of the difference between the two temperatures. Here, the proportional term is a value proportional to the magnitude of the temperature difference, and the integral term is a value corresponding to the integrated value of the temperature difference. The CPU 94 determines the amount of power to be supplied to the heating element based on the values of the proportional and integral terms. In this embodiment, the values of the proportional and integral terms are set in advance for each heating element based on the magnitude of the temperature difference, and the values of the proportional and integral terms of the selected heating element are used to perform calculations for PI control, which is the first type of power control. The following description uses heating elements 54b1 and 54b3 as heating elements to which power is supplied when printing on A5-sized paper P.

[0041] Specifically, the PI control of this embodiment will be described. n is the timing (cycle) number for performing the PI control, and P is the proportional term corresponding to the timing number. n (unit:%), integral term is I n (Unit: %). The power duty D input to the heating element by PI control n(unit: %) is expressed by the following (Equation 1) to (Equation 3). D n =P n +I n (100%≧P n +I n ≧0%) (Equation 1) D n =100 (P n +I n >100%) (Equation 2) D n =0 (0%>P n +I n (when ) (Equation 3)

[0042] Here, the power duty D n represents the input ratio of the amount of power that is determined by how much power is supplied with respect to the AC voltage waveform of the AC power supply 55. n can take on a value between 0% and 100% depending on the power supply pattern determined by phase control. From (Equation 2), the proportional term P n , integral term I n When the value obtained by adding the above is greater than 100%, the power duty D n On the other hand, from (Equation 3), the proportional term P n , integral term I n When the value obtained by adding n The power supply pattern to the heating element by phase control is stored in advance in the memory 95, which is a storage unit. The CPU 94 n In response to the request, the corresponding power supply pattern is selected from the memory 95, and power is supplied to the heating element in accordance with the selected power supply pattern.

[0043] When printing on paper P begins, first, the initial value I0 of the output value of integral term I, which is controlled by integral control, is determined. Table 1 shows the initial values I0 (unit: %) of integral term I of heating elements 54b1 and 54b3. As shown in Table 1, the initial value I0 of heating element 54b1 is 32.5%, and the initial value I0 of heating element 54b3 is 50%. The ratio IP will be described later.

[0044] [Table 1]

[0045] The temperature of the heater 54 is detected by thermistor 59a every two half-wave periods of the voltage waveform (one period of the power supply frequency), and the difference ΔT between the target temperature of the heater 54 and the temperature detected by thermistor 59a (= target temperature - temperature detected by thermistor 59a) is calculated. Table 2 shows the proportional terms P of the heating elements 54b1 and 54b3 corresponding to the calculated difference ΔT (unit: °C). n In Table 2, the proportional terms P of the heating elements 54b1 and 54b3 are calculated for each 1°C change in the difference ΔT from -15°C to 15°C. n For example, when the temperature difference ΔT between the two is −10° C., the proportional term P n is -27.5%, and the proportional term P n Similarly, when the temperature difference ΔT between the two is 5°C, the proportional term P n is 15%, and the proportional term P n The ratio PP will be explained later.

[0046] [Table 2]

[0047] The CPU 94 also calculates an integrated value ΔT by integrating the temperature difference ΔT calculated over two half-wave periods. V is stored in the memory 95. The CPU 94 calculates the integral term I n is calculated using the following (Equation 4). I n =I n-1 +ΔI...(Formula 4)

[0048] For example, the integral term I1 is the integral term I nUsing the initial value I0, the value of ΔI is calculated as I1=I0+ΔI. The value of ΔI is calculated by integrating the difference ΔT. V Table 3 shows the integrated value ΔT V 3 is a table showing the values of ΔI of the heating elements 54b1 and 54b3 according to the temperature (unit: °C). V If the value is equal to or greater than -400 and less than 400, the value of ΔI of the heating elements 54b1 and 54b3 is 0%. V When the value of ΔI is 400 or more, the value of ΔI of the heating element 54b1 is 5%, and the value of ΔI of the heating element 54b3 is 10%. V If the value of is less than -400, the value of ΔI of the heating element 54b1 is -5%, and the value of ΔI of the heating element 54b3 is -10%.

[0049] [Table 3]

[0050] By the above-mentioned processing, the proportional term P of each heating element is n , integral term I n The value of the proportional term P is determined. n , integral term I n The value of the power duty D n is determined, and the determined power duty D n In this embodiment, the power corresponding to the power duty D n The determination of is done in two half-wave periods, and the determined power duty D n The power based on the power duty D is supplied for the next two half-wave periods. n The power corresponding to the power duty D is supplied to the heating element selected by the switching unit. In this embodiment, the timing for switching the heating element to which power is supplied is determined based on the timing for updating the PI control, i.e., the power duty D n This is timed to coincide with the update timing.

[0051] [Example of power supply control to heating elements] Next, we will explain how to control the power supply to the heating elements when printing on actual paper P. Table 4 explains how the amount of power supplied to the heating elements is determined over time when printing on paper P.

[0052] [Table 4]

[0053] Table 4 consists of the following items from top to bottom: Timing n, time (unit: sec (seconds)), target temperature (unit: °C), thermistor detection value (unit: °C), difference ΔT (unit: °C), integrated value ΔT V (unit: °C). Furthermore, Table 4 is composed of the selected heating element to be supplied with power, the power calculation results (proportional term P, integral term I, Duty (power duty)) of heating elements 54b1 and 54b3, the actual power duty, and ratio DP. The ratio DP will be described later.

[0054] In Table 4, when the timing n is 1 (time is 0 seconds), the thermistor detection value indicating the temperature of the heater 54 detected by the thermistor 59a is 215°C, and the difference ΔT from the target temperature of 220°C is 5°C (=220°C-215°C). V is 380°C. For the power calculation of heating element 54b1, the proportional term I is 15% when the difference ΔT is 5°C according to Table 2, and the integral term I is 0% according to Table 3, so I = I0 + ΔI = 32.5% according to Equation 4. As a result, the power duty (Duty) is 47.5% (= 15% + 32.5%). Similarly, for the power calculation of heating element 54b3, the proportional term I is 22.5% when the difference ΔT is 5°C according to Table 2, and the integral term I is 0% according to Table 3, so I = I0 + ΔI = 50% according to Equation 4. As a result, the power duty (Duty) is 72.5% (= 22.5% + 50%). Since the selected heating element when timing n is 1 is 54b1, the power duty actually supplied to heater 54 is 47.5%.

[0055] Next, when the timing n is 2 (time is 0.020 seconds), the thermistor detection value indicating the temperature of the heater 54 detected by the thermistor 59a is 216°C, and the difference ΔT from the target temperature of 220°C is 4°C (=220°C-216°C). V is 384°C (=380°C + 4°C). Regarding the power calculation for heating element 54b1, from Table 2, the proportional term P is 12.5% when the difference ΔT is 4°C. Regarding the integral term I, from Table 3, ΔI is 0%, so from Equation 4, I1 = I0 + ΔI = 32.5%. As a result, the power duty (Duty) is 45% (=12.5% + 32.5%). Similarly, regarding the power calculation for heating element 54b3, from Table 2, the proportional term P is 17.5% when the difference ΔT is 4°C. Regarding the integral term I, from Table 3, ΔI is 0%, so from Equation 4, I1 = I0 + ΔI = 50%. As a result, the power duty (Duty) is 67.5% (=17.5% + 50%). Since the selected heating element when timing n is 1 is 54b3, the power duty actually supplied to heater 54 is 67.5%.

[0056] When the timing n is 7 (the time is 0.120 seconds), the thermistor detection value indicating the temperature of the heater 54 detected by the thermistor 59a is 217°C, and the difference ΔT from the target temperature of 220°C is 3°C (=220°C-217°C). V However, according to Table 3 above, the integrated value ΔT V When the temperature is 400°C or higher, ΔI is not 0%, but is 5% in the case of the heating element 54b1 and 10% in the case of the heating element 54b3. V is reset once the integrated value exceeds 400°C. Vis 1 (=401-400). For the power calculation of heating element 54b1, from Table 2, the proportional term P is 10% when the difference ΔT is 3°C, and from Table 3, ΔI is 5% for the integral term I, so from equation 4, I7 = I6 + ΔI = 32.5% + 5% = 37.5%. As a result, the power duty (Duty) is 47.5% (=10% + 37.5%). Similarly, for the power calculation of heating element 54b3, from Table 2, the proportional term P is 15% when the difference ΔT is 3°C, and from Table 3, ΔI is 10%, so from equation 4, I = I6 + ΔI = 50% + 10% = 60%. As a result, the power duty (Duty) is 75% (=15% + 60%). When timing n is 7, the selected heating element is 54b1, so the power duty actually supplied to the heater 54 is 47.5%.

[0057] [Power ratio supplied to heating element] In order to stabilize the temperature of heater 54 detected by thermistor 59a by approaching the target temperature using PI control, which is the first type of power control, the following is important: Even when the power supply destination is switched to a heating element with a different resistance value, it is important that the amount of power supplied to the central portion of heater 54 in the longitudinal direction where thermistor 59 is located does not change suddenly and is supplied stably. In other words, even when the heating element to which power is supplied is switched, it is important that the power supplied per unit length in the longitudinal direction of the heating element does not change suddenly.

[0058] Here, the amount of power W supplied per unit length in the longitudinal direction of the heating element is expressed by the following (Equation 5), where V is the AC voltage of AC power supply 55, R is the resistance value of the heating element, L is the length (width) of the heating element in the longitudinal direction, and D is the power duty. W=(V 2 / R)×D×(1 / L)(W / m) =(V 2 ×D) / (R×L) (Formula 5)

[0059] In order to prevent a sudden change in the amount of power W supplied per unit length of the heating element in the longitudinal direction even when the heating element is switched, it is necessary to set a power duty D for each heating element according to the product of the resistance value R and length L of the heating element, as shown in equation (5).

[0060] Here, the resistance value of heating element 54b1 is R1, its longitudinal length is L1, its proportional term is P1, its initial value of the integral term is I01, and its power duty is D1. Similarly, the resistance value of heating element 54b3 is R2, its longitudinal length is L2, its proportional term is P2, its initial value of the integral term is I02, and its power duty is D2. Furthermore, the ratio of the proportional term P per unit length of the power duty supplied to heating element 54b1 to the proportional term P per unit length of the power duty supplied to heating element 54b3 is PP. Similarly, the ratio of the initial value I0 per unit length of the integral term of the power duty supplied to heating element 54b1 to the initial value I0 per unit length of the integral term of the power duty supplied to heating element 54b3 is I0P. Furthermore, the ratio of the value per unit length of the power duty D supplied to heating element 54b1 to the value per unit length of the power duty D supplied to heating element 54b3 is DP.

[0061] The ratio PP of the proportional term P, the ratio I0P of the initial value I0 of the integral term, and the ratio DP of the power duty D per unit length are respectively expressed as the following (Equation 6), (Equation 7), and (Equation 8). PP=(P1 / (R1×L1)) / (P2 / (R2×L2)) =(P1 / P2)×((R2×L2) / (R1×L1)) (Equation 6) I0P=(I01 / (R1×L1)) / (I02 / (R2×L2)) =(I01 / I02)×((R2×L2) / (R1×L1))...(Formula 7) DP=(D1 / (R1×L1)) / (D2 / (R2×L2)) =(D1 / D2)×((R2×L2) / (R1×L1)) (Equation 8)

[0062] In order to prevent the amount of power W per unit length in the longitudinal direction of the heating element, expressed by (Equation 5), from changing abruptly when the heating element is switched, it is preferable to make the value of the ratio DP shown in (Equation 8) as close to 1 as possible. The ratio I0P shown in Table 1 and the ratio PP shown in Table 2 indicate the ratio I0P calculated from (Equation 7) and the ratio PP calculated from (Equation 6), respectively. As shown in Tables 1 and 2, the values of the ratio I0P and the ratio PP are close to 1. As such, since the values of the ratio I0P and the ratio PP are close to 1, it can be seen that the values of the proportional term P and the integral term I0 do not change abruptly before and after switching of the heating element, regardless of the difference ΔT between the temperature of the heater 54 and the target temperature. Furthermore, since the proportional term ratio PP and the integral term ratio I0P are close to 1, the ratio DP of the power duty D, which is expressed as the sum of the proportional term P and the integral term I, also approaches 1.

[0063] However, the following two cases must be considered as factors that cause the ratio DP of the power duty D to deviate from 1. In the first case, the sum of the proportional term P and the integral term I exceeds 100% or becomes less than 0%, resulting in a power duty D of 100% or 0%. A case in which the power duty D becomes 100% is considered to be when the heater 54 is being started up while the fixing device 50 is being heated from a temperature close to room temperature to the target temperature. On the other hand, a case in which the power duty D becomes 0% is considered to be when the temperature detected by the thermistor 59a of the heater 54 significantly overshoots the target temperature, such as immediately after the fixing device 50 has finished starting up. Thus, the first case occurs during a transitional period, such as when the fixing device 50 is being heated up. Therefore, as time passes and the temperature detected by the thermistor 59a approaches the target temperature to some extent, the value of the power duty D is controlled between 0% and 100%, and the ratio DP of the power duty for each heating element approaches 1.

[0064] The second case in which the ratio DP of the power duty D is shifted away from 1 is when the integral term I changes. As shown in Equation 4 above, the integral term I may change over time. However, with the configuration of this embodiment, even if the integral term I changes, the change in the ratio DP of the power duty D is limited, and the ratio DP falls between 0.8 and 1.2, suppressing abrupt changes in power. This has been confirmed through experiments using the set values shown in Tables 1 and 3. Furthermore, using the configuration of this embodiment, experiments were conducted in which the ratio of the proportional term P and the integral term I for each heating element was changed to intentionally change the value of the power duty ratio DP. Experimental results showed that when the power duty ratio DP was less than 0.7 or greater than 1.3, the temperature of the heater 54 detected by the thermistor 59a did not approach the target temperature, and the detected temperature repeatedly rose and fell. In this way, even when switching the heating element, in order to stabilize the temperature control of the heater 54, it is preferable to keep the value of the power duty ratio DP between 0.7 and 1.3.

[0065] As described above, in this embodiment, the power supply (power supply amount) to the heating element is changed by PI control, which is the first power control. Specifically, the ON / OFF ratio (power duty) of the power supply is changed by phase control. This controls the temperature of the heater 54 of the fixing device 50. In this embodiment, the amount of power per unit length of the heating element is expressed by the resistance value, longitudinal length, and power duty D of the heating element. The power duty is determined by PI control, which is the first power control, and the magnitude of the power duty D is changed according to the resistance value and longitudinal length (width) of the heating element. Specifically, the greater the product of the resistance value and longitudinal width of the heating element, the greater the value of the power duty D. The value of the ratio DP of the power duty D described above is set as close to 1 as possible. This configuration prevents abrupt changes in the amount of power supplied to the heating element when switching between heating elements, making it possible to stabilize the temperature of the fixing device 50.

[0066] In addition, in this embodiment, PI control with a cycle unit of two half waves is used, but the update cycle and control method of PI control are not limited to this. In addition, in this embodiment, a method of changing the ON / OFF ratio of power supply by phase control is used, but the method of controlling power supply is not limited to this. For example, the amount of power supply may be changed by providing a current limiting circuit to limit the amplitude of the current supplied from the AC power supply 55. In addition, the selection of the heating element by the switching unit is performed by the second power control described below, which will be described later.

[0067] [Count temperature prediction method] Next, we will explain the count temperature prediction method, which is a 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 is incremented 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, during standby after the fixing process is completed, each component of the fixing device 50 cools naturally, so the count value is also decremented 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 to predict the temperature of each component of the fixing device 50 is called the count temperature prediction method.

[0068] 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.

[0069] Here, a case where A5-sized paper P is continuously printed will be described. In this embodiment, 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 54b3 having a longitudinal width corresponding to the paper width of the A5-sized paper P. When B5-sized 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-sized paper P. Similarly, when A4-sized or letter-sized 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-sized paper P will be used as an example of printing on paper P.

[0070] 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.

[0071] 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.

[0072] [Second power control] Therefore, in this embodiment, the second power control is performed by changing the time allocation of power supply to each heating element, so that the temperature difference in the longitudinal direction of the film 51 of the fixing device 50 falls within a predetermined range. Specifically, power is supplied to the heating element 54b1 for a first period, which is a predetermined period. After the first period has elapsed, power is supplied to the heating element 54b3 corresponding to the A5-size paper P for a period that is a predetermined multiple of the first period. In this embodiment, the first period is set to two half-waves (one power supply cycle) of the voltage waveform of the AC power supply, which has the same time width (period) as the PI control cycle. Thus, in this embodiment, the power supply amount by the PI control, which is the first power control, is updated every first period (two half-wave cycles), and power is supplied to the heating element 54b1 for the first period, followed by power supply to the heating element 54b3 for a period that is a predetermined multiple of the first period.

[0073] Table 5 shows the zone numbers determined based on the counter values and the time ratios of the periods during which power is supplied to heating element 54b3 relative to heating element 54b1 in the corresponding zones. The time ratio X in Table 5 indicates the value of a predetermined multiple of the unit period (two half-wave periods of the AC power supply voltage waveform), which is the first period during which power is supplied to heating element 54b1, when the second period during which power is supplied to heating element 54b3 is a predetermined multiple of the unit period. As can be seen from Table 5, in zone 1, the time ratio X is 0, so no power is supplied to heating element 54b3. However, in zone 2, the time ratio is 1, so power is supplied to heating element 54b3 for the same time (first period) as heating element 54b1. In zones 2 and 3, power is supplied to heating element 54b3 for three and five times the first period of heating element 54b1, respectively, depending on the value of the time ratio X. In this embodiment, the PI control cycle and the period during which power is supplied to heating element 54b1 are both the same first period, but they do not necessarily have to have the same time duration, and for example, the period during which power is supplied to heating element 54b1 may be a period that is a predetermined multiple of the first period. In this way, in this embodiment, the time ratio X is adjusted according to the zone number, thereby bringing the power distribution closer to the target value and controlling the temperature of film 51 in the longitudinal direction.

[0074] [Table 5]

[0075] [Control sequence of power supply to heating element] FIG. 7 is a flowchart showing the control sequence for supplying power to heating elements 54b1 and 54b3 when a print job for printing A5-sized paper P is executed. The process in FIG. 7 is initiated when the print job is started and executed by the CPU 94. The power supply period for heating element 54b3 is determined based on the time ratio corresponding to the zone corresponding to the counter value shown in Table 5 above. The counter value is updated by the CPU 94, but this is assumed to be performed by a separate process not shown in FIG. 7. Furthermore, the process of supplying power to heating element 54b1, then switching to heating element 54b3, and then switching back to heating element 54b1 is referred to as a set, and the number of sets is represented as n. Furthermore, the number of times the proportional term P and integral term I, which are the set values for PI control, are updated within one set is referred to as the control number, and the control number is represented as m. m begins with 0.

[0076] When a print job starts, in step (hereinafter referred to as S) 100, the CPU 94 sets the set number n to 0. In S101, the CPU 94 adds 1 to the set number n to update the set number n and sets the control number m to 0. In S102, the CPU 94 determines the power duty D by PI control, which calculates the values of the proportional term P and integral term I for the heating element 54b1 based on the difference ΔT between the temperature of the heater 54 detected by thermistor 59a and the target temperature. In S103, the CPU 94 switches the power supply destination to the heating element 54b1, selects a power supply pattern from memory 95 according to the power duty D calculated in S102, and supplies power for a predetermined unit period (here, two half-wave periods) (indicated as the unit period in the figure). In S104, the CPU 94 determines whether the print job has ended. If it has not ended, the process proceeds to S105. If it has ended, the process ends.

[0077] In S105, the CPU 94 compares the value of the time ratio X set for the zone corresponding to the count value in Table 5 with the value of the control number m. If the CPU 94 determines that the value of the control number m is equal to or greater than the value of the time ratio X (m≧X), the process returns to S101. If the CPU 94 determines that the value of the control number m is less than the value of the time ratio X, the process proceeds to S106. In S106, the CPU 94 determines the power duty D by PI control, which calculates the values of the proportional term P and the integral term I for the heating element 54b3 based on the difference ΔT between the temperature of the heater 54 detected by thermistor 59a and the target temperature. In S107, the CPU 94 switches the power supply destination to the heating element 54b3, selects from memory 95 a power supply pattern corresponding to the power duty D calculated in S107, and supplies power for a predetermined unit period (here, the period of two half waves) (indicated as a unit period in the figure). In S108, the CPU 94 adds 1 to the control number m to update the control number m. In S109, the CPU 94 determines whether the print job has finished, and if it determines that the print job has not finished, returns the process to S105, and if it determines that the print job has finished, ends the process.

[0078] [Example of power supply to a heating element] Next, a power control method in this embodiment will be described. In this embodiment, after power is supplied to the heating element 54b1 for a certain period (first period), the state is switched to enable power supply to the heating element 54b3, which has a shorter longitudinal length than the heating element 54b1. Then, when the power supply period to the heating element 54b3 becomes a predetermined multiple (X times) of the power supply period to the heating element 54b1, the state is switched back to enable power supply to the heating element 54b1, and power is supplied to the heating element 54b1. The following describes an example of continuous printing of A5-sized paper P when the count value is in zone 4 shown in Table 5. As shown in Table 5, the time ratio X for zone 4 is such that the power supply period to the heating element 54b3 is 5 times the power supply period to the heating element 54b1 (X=5). Furthermore, the amount of power supplied to the heating element 54b1 in one cycle unit (here, the period of two half waves) is calculated as WL n,m The amount of power supplied to the heating element 54b3 is expressed as WS n,m It is expressed as:

[0079] A specific example of power supply will be described with reference to Fig. 8. Fig. 8 is a diagram illustrating the state of power supply to heating element 54b1 and heating element 54b3 using the AC voltage waveform of AC power supply 55. In Fig. 8, "heating element 54b1" indicates the state of power supply to heating element 54b1, and "heating element 54b3" indicates the state of power supply to heating element 54b3. In Fig. 8, the horizontal axis indicates time, and the portion indicated by the solid line in the AC voltage waveform indicates a state in which power is supplied based on a power supply pattern corresponding to the power duty, while the portion indicated by the dashed line indicates a state in which power is not being supplied.

[0080] Here, the voltage of the AC power supply is represented as V, and the time of the minimum cycle unit (two half waves) is represented as S (sec (seconds)). When the number of sets n is 1 and the control m is 0, the heating element 54b1 is supplied with an amount of power with a power duty D of 35% under PI control, and the amount of power per unit length WL n,m is calculated from (Equation 5) as follows: WL n,m =(V 2 / R)×D×(1 / L)×S WL 1,0 =(V 2 / 10Ω)×(1 / 222mm)×S×35% ≒0.16V 2 S (W·sec / m) is calculated.

[0081] Electric power WL 1,0 After the heating element 54b1 is turned on, power is supplied to the heating element 54b3 from the next control when the control number m is 1. When the control number m is 1, the power duty D is 80% by PI control, and the power amount per unit length WS n,m is calculated from (Equation 5) as follows: WS n,m =(V 2 / R)×D×(1 / L)×S WS 1,1 =(V 2 / 30Ω)×(1 / 154mm)×S×80% ≒0.17V 2 S (W·sec / m) is calculated.

[0082] Thereafter, the PI control of the heating element 54b3 is performed until the control number m reaches 5, and power is supplied to the heating element 54b3. As shown in FIG. 8, the heating element 54b3 is supplied with 0.14V 2 S, 0.05V 2 S, 0.11V 2 S, 0.07V 2 The amount of power supplied to the heating element 54b3 is S. Since the PI control is performed in two half-wave cycles, the power duty changes each time. Therefore, the amount of power supplied to the heating element 54b3 when the control number m is 1 to 5 also changes.

[0083] Then, the power supply destination is switched back to the heating element 54b1, and a set with the set number n being 2 is started. During the period when the set number n is 1, the amount of power supplied to the heating element 54b1 is 0.16V. 2 S, and the amount of power supplied to the heating element 54b3 is 0.54V 2 As a result, the ratio of the amounts of power supplied to the heating element 54b1:the heating element 54b3 is 0.16V. 2 S:0.54V 2 S=1:3.4.

[0084] [Power ratio measurement] To confirm the power ratios described above, an experiment was conducted under the following conditions. In the circuit shown in FIG. 6, ammeters were installed between triac 56a and heating element 54b1, and between triac 56b and heating elements 54b2 and 54b3 to measure the current flowing through each heating element of heater 54. To stabilize the heat output of fixing device 50, the interval between feeding sheets P from cassette 16 was kept constant, and multiple continuous printings of sheets P were performed. In this example, printing 20 sheets of paper P constituted one print job, and the interval between print jobs was set to 3 minutes. As a result, the temperature near the longitudinal center of the pressure roller of fixing device 50 before printing was approximately 90°C each time, stabilizing the temperature of fixing device 50. The zone determined by the count value at that time was zone 4. In this stable state, the current value during continuous printing was measured, and the amount of power W supplied to heating elements 54b1 and 54b3 was calculated based on the previously measured resistance values of heating elements 54b1 and 54b3 of heater 54. The current measurement value was also calculated as the average current value for multiple sheets of paper P. By repeating the above process, the ratio of the amounts of power supplied to each heating element was calculated.

[0085] Table 6 summarizes the results of the above-described experiment conducted three times. In Table 6, the measured power ratio indicates the ratio of the amount of power supplied to heating element 54b1 to the amount of power supplied to heating element 54b3 in each experiment, and the film edge temperature indicates the maximum temperature (unit: °C) at the edge of film 51. Table 6 shows that the maximum temperature at the edge of film 51 falls within a certain range. In this way, by performing the above-described control, it is possible to control the temperature at the center of the fixing film 51 in the longitudinal direction of fixing device 50 to fall within a certain range, and also control the temperature at the edge to fall within a certain range.

[0086] [Table 6]

[0087] [Flicker] Depending on the frequency of switching the heating elements, flickering may increase. Flickering refers to a phenomenon in which, when a common AC power source supplies power to a lighting device and a heating device, a sudden change in current to the heating device causes a fluctuation in the lighting voltage, resulting in flickering of the lighting. In this embodiment, reducing the frequency of switching the heating elements reduces the frequency of current changes, potentially reducing flickering. On the other hand, reducing the frequency of switching the heating elements may result in an increase or decrease in the temperature of the longitudinal end of the film 51. For example, when the AC power supply cycle is 50 Hz, the maximum time that one heating element is connected is 0.1 seconds (for heating element 54b3). Intentionally reducing the switching frequency by extending the time that power is continuously supplied to one heating element may result in a decrease in the temperature of the longitudinal end of the film 51, resulting in insufficient melting of the toner on the paper P if the time that power is supplied to heating element 54b3 exceeds 32 seconds. Therefore, even when reducing the frequency of switching the heating elements, it is preferable to limit the time that power is supplied to one heating element to 32 seconds or less. In this embodiment, the period for supplying power to the heating element 54b3 is set based on the period for supplying power to the heating element 54b1, but conversely, the period for supplying power to the heating element 54b1 may be set based on the period for supplying power to the heating element 54b3. In such a case, the temperature of the central portion in the longitudinal direction of the fixing film 51 of the fixing device 50 can be controlled to fall within a certain range, and the temperature of the end portions can also be controlled to fall within a certain range.

[0088] As described above, according to this embodiment, the temperature of the fixing device can be controlled with high precision so that the temperature of the non-sheet passing portion does not rise. [Example]

[0089] In the first embodiment, a first power control is described in which PI control is performed to bring the temperature detected by the thermistor closer to a target temperature, and a second power control is performed in which the amount of power is brought closer to a target value by changing the time allocation for supplying power to each heating element. In the second embodiment, the first control is performed in the same manner as in the first embodiment, and the second control is described in which the amount of power actually supplied to each heating element is brought closer to the target amount of power. Note that the configuration of the image forming apparatus including the fixing device in this embodiment is 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 their description will be omitted here.

[0090] [Second power control] In this embodiment, the second power control is characterized by adjusting the integrated amount of power supplied to the heating elements by the first power control (PI control) to approach a target power distribution value. Specifically, the temperature of the longitudinal region of the film 51 of the fixing device 50 is controlled by adjusting the ratio of the integrated amount of power supplied to the heating element 54b1, which has the longest longitudinal length, and the heating element 54b3, which corresponds to the width of A5 size paper. Here, the "integrated amount of power" refers to the integrated amount of power per unit length of the heating element, calculated based on the resistance value, longitudinal length (width), and power duty of the heating element described in the first embodiment. By controlling the integrated amount of power per unit length, it is possible to more accurately control the heat generation amount of the longitudinal region, regardless of the resistance value or longitudinal width of the heating element.

[0091] Table 7 shows the zone numbers determined based on the counter values of the count temperature prediction method described in Example 1, and the ratios (power ratios) of the amounts of power supplied to heating element 54b3 relative to heating element 54b1 in the corresponding zones. The power ratio X in Table 7 indicates the multiple of the amount of power supplied to heating element 54b3 relative to the amount of power supplied to heating element 54b1 in a unit period. As can be seen from Table 7, in Zone 1, power ratio X is 0, so no power is supplied to heating element 54b3. However, in Zone 2, power ratio X is 1, so the amount of power supplied to heating element 54b3 is the same as that of heating element 54b1. In Zones 2 and 3, the amounts of power supplied to heating element 54b3 are three and five times the amount of power supplied to heating element 54b1, respectively, depending on the value of power ratio X. While this example uses the ratio of the amount of power per unit length of the heating element as an index, it is sufficient to control the ratio of the integrated amounts of power of multiple heating elements.

[0092] [Table 7]

[0093] [Control sequence of power supply to heating element] FIG. 9 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 of FIG. 9 is started when the print job starts and executed by CPU 94. The amount of power supplied to heating element 54b3 is determined based on the power ratio according to the zone corresponding to the counter value in Table 7. The counter value is updated by CPU 94, but this is assumed to be performed by a separate process not shown in FIG. 9. As with FIG. 7 of the first embodiment, the number of sets is represented as n and the number of controls is represented as m.

[0094] When a print job is started, the CPU 94 sets the set number n to 0 in S200.

[0095] In S201, the CPU 94 adds 1 to the number of sets n to update the number of sets n, and sets the number of controls m to 0. In S202, the CPU 94 determines the power duty D by PI control, which calculates the values of the proportional term P and integral term I for the heating element 54b1, based on the difference ΔT between the temperature of the heater 54 detected by the thermistor 59a and the target temperature. Then, the CPU 94 calculates the amount of power WL per unit length of the heating element 54b1. n,0 In S203, the CPU 94 switches the power supply destination to the heating element 54b1, selects from the memory 95 a power supply pattern corresponding to the power duty D calculated in S202, and supplies power for a predetermined unit cycle period (here, two half-wave periods) (shown as unit period in the drawing). In S204, the CPU 94 determines whether the print job has ended, and if it has not, proceeds to S205, and if it has ended, ends the process.

[0096] In S205, the CPU 94 acquires the value of the power ratio X set in the zone corresponding to the count value in Table 7, and calculates the amount of power WSpre to be supplied to the heating element 54b3 in the same set. n Determine the amount of power WSpre n WL n,0 The power consumption is X times that of the CPU94. n =Amount of power WL supplied to heating element 54b1 n,0 Using the formula × X, the planned power consumption WSpre n The CPU 94 also calculates the integrated power amount WSall, which indicates the total (integrated amount) of the power amounts input to the heating elements 54b3 in the same set. n Set to 0.

[0097] In S206, the CPU 94 calculates the expected power consumption WSpre n and the cumulative power consumption WSall n The CPU 94 compares the cumulative power consumption WSall n is the planned power consumption WSpre n More than (more than planned power consumption) n ≦WSall n), the process returns to S201. n is the planned power consumption WSpre n If it is determined that the difference is less than the predetermined value, the process proceeds to S207.

[0098] In S207, the CPU 94 determines the power duty D by PI control, which calculates the values of the proportional term P and integral term I for the heating element 54b3, based on the difference ΔT between the temperature of the heater 54 detected by the thermistor 59a and the target temperature. Then, the CPU 94 determines the amount of power WS per unit length of the heating element 54b3. n,m In S208, the CPU 94 switches the power supply destination to the heating element 54b3, selects a power supply pattern from the memory 95 according to the power duty D calculated in S207, and supplies power for a predetermined unit cycle period (here, a period of two half waves) (shown as unit period in the drawing). In S209, the CPU 94 adds 1 to the control number m to update the control number m. The CPU 94 also calculates the integrated power amount WSall n Power consumption WS n,m Adding these, the total power consumption WSall n In S210, the CPU 94 determines whether the print job has been completed, and if it has not been completed, returns the process to S206, and if it has been completed, ends the process.

[0099] [Example of power supply to a heating element] Next, the power control method in this embodiment will be described. In this embodiment, after power is supplied to the heating element 54b1 for a certain period of time, the system switches to a state in which power can be supplied to the heating element 54b3, which has a smaller heat generation amount at its longitudinal end than the heating element 54b1. Then, when the amount of power supplied to the heating element 54b3 (integrated power amount) becomes a predetermined multiple (X times) of the amount of power supplied to the heating element 54b1, the system switches back to a state in which power can be supplied to the heating element 54b1, and power is supplied to the heating element 54b1. The following describes an example of continuous printing of A5-sized paper P when the count value is in zone 4 shown in Table 7. As shown in Table 7, the time ratio X in the case of zone 4 is such that the amount of power supplied to the heating element 54b3 is 5 times (X=5) the amount of power supplied to the heating element 54b1. Furthermore, the amount of power supplied to the heating element 54b1 in one cycle unit (here, the period of two half waves) is set to WL n,m The amount of power supplied to the heating element 54b3 is expressed as WS n,m It is expressed as:

[0100] A specific example of power supply will be described with reference to FIG. 10. FIG. 10 is a diagram illustrating the power supply state to heating element 54b1 and heating element 54b3 using the AC voltage waveform of AC power supply 55. In FIG. 10, "heating element 54b1" indicates the power supply state to heating element 54b1, and "heating element 54b3" indicates the power supply state to heating element 54b3. In FIG. 10, the horizontal axis indicates time, and the portion indicated by the solid line in the AC voltage waveform indicates the state in which power is supplied based on a power supply pattern corresponding to the power duty, while the portion indicated by the dashed line indicates the state in which power is not being supplied. In addition, WSall n represents the cumulative amount of power supplied to the heating element 54b3 in the same set n.

[0101] Here, the voltage of the AC power supply is represented as V, and the time of the minimum cycle unit (two half waves) is represented as S (sec (seconds)). When the number of sets n is 1 and the control m is 0, the heating element 54b1 is supplied with an amount of power with a power duty D of 35% under PI control, and the amount of power per unit length WL n,m is calculated as follows: WL n,m =(V 2 / R)×D×(1 / L)×S WL 1,0 =(V 2 / 10Ω)×(1 / 222mm)×S×35% ≒0.16V 2 S (W·sec / m) is calculated.

[0102] Electric power WL 1,0 After the heating element 54b1 is turned on, power is supplied to the heating element 54b3 from the next control when the control number m is 1. When the control number m is 1, the power duty D is 80% by PI control, and the power amount per unit length WS n,m is calculated as follows: WS n,m =(V 2 / R)×D×(1 / L)×S WS 1,1 =(V 2 / 30Ω)×(1 / 154mm)×S×80% ≒0.17V 2 S (W·sec / m) is calculated.

[0103] The total power input to the heating element 54b3 is WSall n Then, WSall n =WS n,1 +WS n,2 +WS n,3 … It is expressed as:

[0104] After that, CPU94 continues PI control, and WSall1=X×WL 1,0 =5×WL 1,0 ≒0.79V 2 S (W·sec / m). Then, the power supply destination is switched back to the heating element 54b1, and a set with the set number n of 2 is started. During the period when the set number n is 1, the amount of power supplied to the heating element 54b1 is 0.16V. 2 S, and the amount of power supplied to the heating element 54b3 is 0.81V 2S, and the ratio of the amount of power supplied to heating element 54b3 to that of heating element 54b1 is about 5. By performing the above-described control in this embodiment, the ratio of the amount of power actually supplied to the heating elements and the value of the power ratio set by power ratio X become closer.

[0105] [Power ratio measurement] In this example, an experiment was also conducted under the same conditions as in Example 1, and the amount of power supplied to the heating element and the temperature of the non-paper passing area of the film 51 were measured. Table 8 summarizes the results of three experiments. In Table 8, the measured power ratio indicates the ratio of the amount of power supplied to the heating element 54b1 to the amount of power supplied to the heating element 54b3 in each experiment, and the film edge temperature indicates the maximum temperature (unit: °C) at the edge of the film 51. Table 8 shows that a power ratio (1:5) close to the target value was achieved, and the maximum temperature at the edge of the film 51 was within a certain range. In this way, by performing the control described above, the temperature at the longitudinal center of the fixing film 51 of the fixing device 50 can be controlled to fall within a certain range, while the temperature at the edge can also be accurately controlled.

[0106] [Table 8]

[0107] As described above, according to this embodiment, the temperature of the fixing device can be controlled with high precision so that the temperature of the non-sheet passing portion does not rise. [Example]

[0108] In Example 2, when it is detected that the cumulative amount of power supplied to the second heating element exceeds a predetermined multiple of the amount of power supplied to the first heating element, control is performed to terminate the power supply to the second heating element. Therefore, the amount of power supplied to the second heating element becomes greater than the predetermined multiple of the amount of power supplied to the first heating element. In Example 3, an example is described in which the cumulative amount of power supplied to the second heating element, when the cumulative amount of power supplied to the second heating element planned in a certain set exceeds the predetermined multiple, is reduced from the power amount in the next set, thereby adjusting the cumulative amount of power supplied to the second heating element. Note that the configuration of the image forming apparatus including the fixing device in this example is the same as in Examples 1 and 2, and the same devices and components are designated by the same reference numerals as in Example 1, and their description will be omitted here.

[0109] [Control sequence of power supply to heating element] FIG. 11 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 of FIG. 11 is started when the print job starts and executed by CPU 94. The amount of power supplied to heating element 54b3 is determined based on the power ratio according to the zone corresponding to the counter value in Table 7 of the second embodiment. The counter value is updated by CPU 94, but this is assumed to be performed by a separate process not shown in FIG. 11. Also, as with FIG. 9 of the second embodiment, the number of sets is represented as n and the number of controls is represented as m.

[0110] In FIG. 11, the processes of S300 to S305 are the same as the processes of S200 to S205 in FIG. 9 of the second embodiment, and therefore the description thereof will be omitted here.

[0111] In S306, the CPU 94 calculates the integrated amount of power WSall supplied to the heating element 54b3 in the same set n. n The planned amount of power to be supplied to the heating element 54b3 is calculated as follows: That is, the planned amount of power is calculated by multiplying the amount of power WL supplied to the heating element 54b1 by the amount of power WL supplied to the heating element 54b2. n,0 The planned amount of power WSpre is calculated by multiplying the amount of power by the power ratio X. nThe amount of power supplied to the heating element 54b3 in excess of the planned amount in the previous set (n-1) is calculated by subtracting the amount of power supplied to the heating element 54b3 in excess of the planned amount in the previous set (n-1). (n-1) -WSpre (n-1) Therefore, the planned power amount in set n is expressed as WSpre n -(WSall (n-1) -WSpre (n-1) ) The CPU 94 calculates the integrated power consumption WSall n is the planned power consumption (WSpre n -(WSall (n-1) -WSpre (n-1) If it is determined that the amount of power consumed is equal to or greater than the predetermined amount of power consumed, the process proceeds to S307, and if it is determined that the amount of power consumed is less than the predetermined amount of power consumed, the process proceeds to S308.

[0112] In step S307, the CPU 94 calculates the excess of the planned power amount in the set n as WSall n -WSpre n The calculation is performed by the following formula, and the result is stored in memory 95 for reference in the process of S306 in the next set (n+1), and the process returns to S301. The processes of S308 to S311 are the same as the processes of S207 to S210 in FIG. 9 of the second embodiment, and therefore a description thereof will be omitted here.

[0113] [Power ratio measurement] In this example, an experiment was also conducted under the same conditions as in Example 2, and the amount of power supplied to the heating element and the temperature of the non-paper passing area of film 51 were measured. Table 9 is a table summarizing the results of three experiments conducted as described above. In Table 9, the measured power ratio indicates the ratio of the amount of power supplied to heating element 54b1 to the amount of power supplied to heating element 54b3 in each experiment, and the film edge temperature indicates the maximum temperature (unit: °C) at the edge of film 51. The measured power ratio values shown in Table 9 vary less than the measured power ratios shown in Table 8 of Example 2, and as a result, the temperature variation at the edge of film 51 is also smaller than in Example 2.

[0114] [Table 9]

[0115] As described above, according to this embodiment, the temperature of the fixing device can be controlled with high precision so that the temperature of the non-sheet passing portion does not rise. [Example]

[0116] In the third embodiment, with the above-described configuration, when the feeding interval of the paper P fed from the cassette 16 is constant and continuous printing is performed on the paper P, it is possible to stabilize the temperature distribution at the longitudinal end of the film 51. However, if the interval between the preceding paper P and the succeeding paper P (hereinafter referred to as the paper interval) becomes long, the temperature at the longitudinal end of the film 51 may decrease, which may result in a deterioration in image quality. In this embodiment, control of the heating element when the paper interval becomes long will be described. Note that the configuration of the image forming apparatus including the fixing device in this embodiment is the same as 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 here.

[0117] [Second power control] As the gap between the preceding and succeeding sheets increases, the amount of heat absorbed by the A5-sized paper P passing through the fixing nip N from the film 51 decreases, resulting in excessive heat at the longitudinal center of the film 51. However, in practice, the temperature of the heater 54 is controlled to maintain a constant temperature at the center of the heater 54 based on the temperature detected by the thermistor 59a located in the longitudinal center of the heater 54. Therefore, if the ratio X of the integrated power of the heating elements 54b1 and 54b3 is kept constant when the gap between sheets increases, the temperature at the longitudinal ends of the film 51 will decrease. For example, a longer gap between sheets may occur when the video controller 91 takes time to convert image data received from the PC 110 into exposure data for transmission to the exposure control device 93. In this case, the CPU 94 increases the gap between sheets P fed from the cassette 16 and adjusts the timing so that the leading edge of the paper P is aligned with the color image formed on the intermediate transfer belt 13.

[0118] Therefore, in this embodiment, when the paper gap becomes long, if power is being supplied to heating element 54b3, the CPU 94 switches the power supply destination to heating element 54b1, which heats the entire length evenly, and controls the supply of power to heating element 54b1 during the paper gap. This heats the entire length of the fixing nip N during the long paper gap, during which heat is not absorbed by A5-sized paper P from the film 51, thereby preventing a drop in temperature at the end of the film 51 in the length direction. Also, in this embodiment, when a long paper gap is reached during the period when power is being supplied to heating element 54b3, if the integrated power consumption of heating element 54b3 has not reached X times the power consumption of heating element 54b1, the CPU 94 controls the supply of power to the next set to make up for the shortfall in the integrated power consumption.

[0119] [Control sequence of power supply to heating element] FIG. 12 is a flowchart showing the control sequence for supplying power to the heating elements 54b1 and 54b3 when a print job for printing A5-sized paper P is executed, as shown in FIG. 11 of the third embodiment, with the addition of processing for when the paper interval is long.

[0120] 12, the processes of S400 to S405 are the same as the processes of S300 to S305 in FIG. 11 of the third embodiment, and therefore a description thereof will be omitted here. In S406, the CPU 94 determines whether the paper interval is longer than a predetermined time (has the long paper interval been reached?), and if it is determined that the paper interval is longer than the predetermined time (long paper interval has been reached), the process proceeds to S414, and if it is determined that the paper interval is within the predetermined time, the process proceeds to S407. The processes of S407 to S411 are the same as the processes of S306 to S310 in FIG. 11 of the third embodiment, and therefore a description thereof will be omitted here.

[0121] In S412, the CPU 94 determines whether the print job has ended, and if it has not ended, the process proceeds to S413, and if it has ended, the process ends. In S413, the CPU 94 determines whether the paper gap is longer than a predetermined time (long paper gap reached?), and if it has determined that the paper gap is longer than the predetermined time (long paper gap reached), the process proceeds to S414, and if it has determined that the paper gap is within the predetermined time, the process returns to S407.

[0122] In S414, the CPU 94 calculates the shortfall in the integrated amount of power to be supplied to the heating element 54b3 in the current set n by WSall n -WSpre n The calculation is performed using the formula (WSall n -WSpre n ) becomes a negative value. Also, in the process of S406, if it is determined that the paper interval is longer than the predetermined time (long paper interval has been reached), WSall n The value of is 0, and the expression (WSall n -WSpre n The shortfall in the integrated power consumption calculated by WSpre n The value is the same as the value of

[0123] In S415, the CPU 94 determines the power duty D by PI control, which calculates the values of the proportional term P and integral term I for the heating element 54b1, based on the difference ΔT between the temperature of the heater 54 detected by the thermistor 59a and the target temperature. Then, the CPU 94 calculates the amount of power WL per unit length of the heating element 54b1. n,0 In S416, the CPU 94 switches the power supply destination to the heating element 54b1, selects from the memory 95 a power supply pattern corresponding to the power duty D calculated in S415, and supplies power for a predetermined unit cycle period (here, a period of two half waves). In S417, the CPU 94 determines whether the leading edge of the paper P fed from the cassette 16 has been detected, and if it determines that the leading edge of the paper P has been detected, the process returns to S401, and if it determines that the leading edge of the paper P has not been detected, the process returns to S415.

[0124] The process shown in FIG. 12 differs from the third embodiment only in the process when the added paper gap is long. As described above, regardless of whether the heating element 54b1 or the heating element 54b3 is being controlled, if a long paper gap occurs, the CPU 94 switches the power supply destination to the heating element 54b1, which has a larger longitudinal length (width), and performs control. In this case, control by the CPU 94 starts from S414. First, when a long paper gap occurs, the CPU 94 calculates the difference between the planned amount of power for the heating element 54b3 and the amount of power actually supplied (WSall n -WSpre n) In addition, when the process of S414 is directly shifted from the control of the heating element 54b1, WSpre n =0, so the difference is WSall n Thereafter, control is performed so that power is supplied only to the heating element 54b1 until the leading edge of the next sheet of paper P reaches the fixing nip N. By performing such control, it is possible to prevent the temperature of the longitudinal end of the film 51 from decreasing. Then, when the CPU 94 detects that the leading edge of the next sheet of paper P has reached the fixing nip N, it starts a new set n.

[0125] As described above, according to this embodiment, the temperature of the fixing device can be controlled with high precision so that the temperature of the non-sheet passing portion does not rise. [Explanation of symbols]

[0126] 51 Fixing film 53 Pressure roller 54 Heater 54b1, 54b3 Heating element 56a, 56b triac 59a Thermistor 94 CPU

Claims

1. A fixing device that fixes toner on a recording material by heating the toner, A cylindrical film, a heater having a first heating element and a second heating element whose length in the longitudinal direction is shorter than that of the first heating element, the heater being disposed in the internal space of the film and heating the film; a pressure roller that forms a nip portion with the film; a detection means for detecting the temperature of the heater; 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, the control means performing phase control of the power supply to the first heating element and the second heating element; Equipped with The control means a first power control for determining an amount of power to be supplied to the first heating element and the second heating element per unit period based on the temperature of the heater detected by the detection means and a target temperature of the heater, and supplying power to the first heating element or the second heating element; and a second power control for supplying power to the second heating element until an integrated value of the amount of power supplied to the second heating element becomes equal to or greater than a planned amount of power after the amount of power supply determined by the first power control has been supplied to the first heating element for the unit period, executing the first power control and the second power control in a cycle of the unit period, and supplying power to the first heating element or the second heating element for the unit period; A fixing device characterized in that, in the first power control, the amount of power supplied to the first heating element and the second heating element is determined so that the ratio of the power duty per unit length in the longitudinal direction of the second heating element to the first heating element is close to 1.

2. 2. The fixing device according to claim 1, wherein the power duty of each of the first and second heat generating elements increases as the product of the resistance value and the length in the longitudinal direction of each heat generating element increases.

3. 3. The fixing device according to claim 2, wherein the expected amount of power is an amount obtained by multiplying the amount of power supplied to the first heating element in the unit period by a predetermined factor according to the temperature of the film.

4. the amount of power supplied to the second heating element is determined by the first power control for each unit period; 4. The fixing device according to claim 3, wherein the control means calculates the integrated value by adding up the amount of power supplied to the second heating element for each unit period, and supplies power to the second heating element for successive unit periods until the calculated integrated value becomes equal to or greater than the planned amount of power.

5. The fixing device according to claim 3 or claim 4, characterized in that, when the integrated value of the amount of power supplied to the second heating element exceeds the planned amount of power due to the second power control, the control means reduces the excess amount of power from the planned amount of power to be supplied to the second heating element next time.

6. the temperature of the film is predicted based on a count value that is updated in accordance with the number of sheets of recording material that pass through the nip portion; 6. The fixing device according to claim 3, wherein the count value increases when a recording material passes through the nip portion and decreases as time passes without the recording material passing through the nip portion.

7. the heater has a third heating element having a length in a longitudinal direction shorter than that of the second heating element, the first heating element is a pair of heating elements having substantially the same length in the longitudinal direction, 7. The fixing device according to claim 1, wherein the first heating element, the second heating element, the third heating element, and the first heating element are arranged in this order in the short-side direction of the substrate having the heater.

8. The heater is a first contact point at which one end of the first heating element and one end of the second 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 at which the other end of the second heating element and the other end of the third heating element are electrically connected; a fourth contact point to which the other end of the first heating element is electrically connected; 8. The fixing device according to claim 7, further comprising:

9. 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; 9. The fixing device according to claim 8, 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.

10. 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, 10. The fixing device according to claim 9, wherein the detection unit drives the second relay to cut off the power supply path when the first switch connects the AC power supply and the fourth contact and the second switch connects the AC power supply and the first relay.

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

12. 12. The fixing device according to claim 1, wherein the detecting means is a thermistor.

13. 13. 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.

14. an image forming section that forms an image on a recording material; a paper feed section that feeds recording material to the image forming section; The fixing device according to any one of claims 1 to 13, An image forming apparatus comprising:

15. 15. The image forming apparatus according to claim 14, wherein, if the recording material fed from the paper feed unit is not detected within a predetermined time, the control means continuously supplies the first heating element with the amount of power per unit period determined by the first power control for the unit period until the recording material fed from the paper feed unit is detected.

Images