Image forming device

Abstract

To reduce flicker compared to before.SOLUTION: An image forming apparatus has: a plurality of heaters including a first heater and a second heater; and fixing means that fixes a toner image to a sheet by using heat supplied from at least one heater of the plurality of heaters. The operating period of the first heater has an initial stage during which power supplied from a power supply is gradually increased, and an end stage during which the power supplied from the power supply is gradually decreased. The operating period of the second heater has an initial stage during which the power supplied from the power supply is gradually increased, and an end stage during which the power supplied from the power supply is gradually decreased. The initial stage of the second heater is started before the end stage of the first heater ends. That is, part of the end stage of the first heater overlaps part of the initial stage of the second heater.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus having a fixing device for fixing a toner image onto a sheet. [Background technology]

[0002] The fixing device applies heat to the toner image and the sheet using multiple heaters to fix the toner image on the sheet. If the heaters can reach the target temperature in a short time, the user's waiting time will be reduced. For this reason, a large current is supplied to the heaters from an AC power source. Here, if multiple heaters are turned on at the same time, a so-called flicker phenomenon may occur. The flicker phenomenon is a phenomenon in which the voltage of the AC power source fluctuates due to an inrush current generated in an electrical device connected to the AC power source, and the operation of other devices connected to the AC power source is affected. A typical flicker phenomenon is the flickering of lighting devices. According to Patent Document 1, it has been proposed to prevent the operation periods of multiple heaters from overlapping, and to use soft start and soft stop to gradually increase or decrease the power supplied to each heater. This is said to reduce harmonic current. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2010-96969 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in Patent Document 1, when the first heater is turned off, the second heater is immediately turned on, so flicker still occurs. Therefore, an object of the present invention is to reduce flicker further than in the past. [Means for solving the problem]

[0005] The present invention relates to, for example, an image forming means for forming an image on a sheet; A fixing unit for fixing the image on the sheet, The first one is heated by the current. One heater and 、 Generates heat when supplied with electric current Second heater and 、 of A fixing means having A detection means for detecting the temperature of the fixing means and, system With the means, The control means controlling a first current supplied to the first heater and a second current supplied to the second heater so that the temperature of the fixing unit detected by the detection unit is maintained at a target temperature; When the first current is supplied to the first heater, the first current is gradually increased during a first period. Gradually increasing death , supplying the first current with a first duty cycle during a second time period following the first time period and ceasing to supply the first current during a third time period following the second time period; Gradually decreased height , gradually increasing the second current during a fourth time period when the second current is supplied to the second heater, supplying the second current with a second duty cycle during a fifth time period following the fourth time period, and gradually decreasing the second current to stop supplying the second current during a sixth time period following the fifth time period; controlling the supply of the second current so that a portion of the fourth period overlaps with the third period; The fourth period begins at the beginning of the third period. period It starts after a predetermined time has elapsed from The present invention provides an image forming apparatus characterized by the above-mentioned. Effect of the Invention

[0006] According to the present invention, flicker is reduced further than in the past. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating an image forming apparatus. [Diagram 2] FIG. 2 is a diagram illustrating a fixing device. [Diagram 3] Diagram explaining the controller involved in heater control [Figure 4] Diagram explaining the changes in heater current, voltage and resistance [Diagram 5] Diagram explaining slow start control and slow end control [Figure 6] FIG. 1 is a diagram for explaining flicker caused by driving multiple heaters. [Figure 7] FIG. 1 is a diagram explaining flicker caused by alternating operation of multiple heaters. [Figure 8]FIG. 1 is a diagram for explaining overlapping of a part of a slow start period and a part of a slow end period; [Figure 9] Diagram illustrating power orientation of multiple heaters [Figure 10] Diagram explaining the functions of a CPU [Figure 11] Flowchart showing a heater control method [Figure 12] Flowchart showing a heater control method [Figure 13] Diagram explaining the controller that controls three heaters DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0009] [Image forming equipment] As shown in FIG. 1, the image forming apparatus 100 is an electrophotographic printer equipped with four image forming stations. The image forming apparatus 100 may be commercialized as a copier, a multifunction machine, a facsimile machine, or the like. Here, the first station forms a yellow (y) image. The second station forms a magenta (m) image. The third station forms a cyan (c) image. The fourth station forms a black (k) image. The operations and configurations of the four stations are the same or similar. Therefore, when matters common to the four colors are described, the letters ymck are omitted from the reference symbols. The technical concept of the present invention is also applicable to monochrome printers.

[0010] The photosensitive drum 101 is a photoconductor and an image carrier that rotates while carrying an electrostatic latent image and a toner image. The charging roller 102 is a charging member that uniformly charges the surface of the photosensitive drum 101. The exposure device 103 irradiates the photosensitive drum 101 with laser light E according to an image signal to form an electrostatic latent image on the surface of the photosensitive drum 101. The developing device 104 forms a toner image by attaching toner to the electrostatic latent image. The primary transfer roller 105 transfers the toner image from the photosensitive drum 101 to the intermediate transfer belt 107. That is, a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image are transferred to the intermediate transfer belt 107 in order to form a full-color image. The intermediate transfer belt 107 rotates to transport the toner image to a secondary transfer section. A pair of secondary transfer rollers 109 is provided in the secondary transfer section.

[0011] The paper feed cassette 111 is a paper feed container capable of storing a large number of sheets P. The pickup roller 112 sends out the sheet P from the paper feed cassette 111 to the transport path. The paper feed roller 113 transports the sheet P further downstream while preventing double feeding of the sheets P. Downstream means downstream in the transport direction of the sheets P. The registration roller 114 is a transport roller that prevents the sheets P from skewing. The leading edge of the sheets P in the transport direction of the sheets P is abutted against the registration roller 114, whereby the skewing of the sheets P is corrected. The sheets P are then transported to a secondary transfer unit.

[0012] In the secondary transfer portion, a pair of secondary transfer rollers 109 transfers the toner image from the intermediate transfer belt 107 to the sheet P. A fixing device 120 applies heat and pressure to the sheet P and the toner image to fix the toner image onto the sheet P. Conveying rollers 115, 116, and 117 are disposed downstream of the fixing device 120, and convey the sheet P to a discharge roller 118. The discharge roller 118 discharges the sheet P to the outside of the image forming apparatus 100 (e.g., a sheet tray).

[0013] [Fixing device] As shown in FIG. 2, the fixing device 120 has a heating unit 200 with an endless rotatable fixing belt 210 as a heat transmission medium at its center. In FIG. 2, the Z direction is the height direction, and the X direction is the direction parallel to the conveying direction of the sheet P. The fixing belt 210 is stretched around a pad 220, a heating roller 240, and a tension roller 250. The heating roller 240 is a heating rotatable body having heaters 241a and 241b therein. The heaters 241a and 241b are heat sources such as halogen heaters. The halogen heater is a heater having a halogen lamp as a heat generating body. The heating roller 240 heats the fixing belt 210. The heating roller 240 rotates by a rotational force supplied from a motor or the like. The tension roller 250 is a tension roller that applies a predetermined tension to the fixing belt 210. The tension roller 250 is biased by an elastic body (e.g., a spring) supported by a frame (not shown) of the heating unit 200. The tension of the spring is, for example, 50 N. The tension roller 250 rotates following the fixing belt 210. The pad 220 supports the inner peripheral surface of the fixing belt 210 by a metal stay 260. The pad 220 cooperates with the pressure roller 230 to pinch the fixing belt 210. A so-called substantially planar nip portion N is formed between the pad 220 and the pressure roller 230. At least one of the pressure roller 230 and the pad 220 may be biased by a biasing mechanism (not shown) so that the nip portion N of a predetermined length and width is formed. When the sheet P to which the toner image has been transferred passes through the nip portion N, pressure and heat are applied to the sheet P and the toner image. As a result, the toner image is fixed on the sheet P.

[0014] The fixing belt 210 has thermal conductivity and heat resistance. The fixing belt 210 has a thin cylindrical shape, and the inner diameter is, for example, 120 mm. The fixing belt 210 may adopt a three-layer structure having a base layer, an elastic layer provided on the outer periphery of the base layer, and a release layer provided on the outer periphery of the elastic layer. The thickness of the base layer is, for example, 60 μm. The material of the base layer is, for example, polyimide resin (PI). The thickness of the elastic layer is, for example, 300 μm. The material of the base layer is, for example, silicone rubber. The thickness of the release layer is, for example, 30 μm. The material of the release layer is, for example, fluororesin. For example, PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin) or the like can be adopted as the fluororesin.

[0015] The material of the pad 220 is, for example, LCP (liquid crystal polymer) resin. The heating roller 240 may be a stainless steel pipe. The outer diameter of the pipe may be, for example, 40 mm. The thickness of the pipe may be, for example, 1 mm. One or more heaters other than the heater 241a and the heater 241b may be disposed inside the pipe. Heat supplied from a plurality of heaters including the heater 241a and the heater 241b is propagated from the heating roller 240 to the fixing belt 210, and further propagated from the fixing belt 210 to the sheet P and the toner image. The tension roller 250 may also be formed of a stainless steel pipe. The outer diameter of the pipe is, for example, 40 mm. The thickness of the pipe is, for example, 1 mm. The end of the pipe may be rotatably supported by a bearing (not shown).

[0016] The pressure roller 230 is, for example, a roller having an elastic layer and a release layer. The elastic layer is provided on the outer periphery of the rotation shaft of the pressure roller 230. Furthermore, a release layer is provided on the outer periphery of the elastic layer. The material of the rotation shaft may be metal (e.g., stainless steel). The thickness of the elastic layer is, for example, 5 mm. The material of the elastic layer is, for example, conductive silicone rubber. The thickness of the release layer is, for example, 50 μm. The material of the release layer is, for example, a fluororesin such as PFA.

[0017] [controller] 3 is a diagram illustrating a controller that controls the heaters 241a and 241b. Here, a CPU 307 controls the heaters 241a and 241b in accordance with a control program stored in a ROM area of ​​a memory 309. The memory 309 may include a non-volatile memory (ROM), a volatile memory (RAM), a solid state drive (SSD), a hard disk drive (HDD), and the like.

[0018] AC power supplied from an AC power source 301 such as a commercial power source is supplied to heaters 241a and 241b via switches 302a and 302b such as relays and switches 304a and 304b such as triacs. The switches 302a and 302b are main switches, and are controlled to be always on when heating the fixing device 120. The operation modes of the image forming apparatus 100 include an image formation mode in which an image is formed, and a standby mode in which an image is not formed. In both the image formation mode and the standby mode, the switches 302a and 302b are on. When the image forming apparatus 100 is stopped (shut down) by a user, the switches 302a and 302b are off.

[0019] The switch 304a is turned on / off by the CPU 307 to control the power supplied to the heater 241a. The switch 304b is turned on / off by the CPU 307 to control the power supplied to the heater 241b. The switches 304a and 304b may be switching elements such as triacs, thyristors, transistors, and IGBTs (insulated gate bipolar transistors). However, the switches 304a and 304b can be any switching elements that can be controlled by the CPU 307 and have performance (rated voltage, rated current) that matches the power consumption of the heaters 241a and 241b.

[0020] The temperature sensor 306 is a thermistor or the like that detects the temperature of the heating roller 240. The CPU 307 turns on / off the switches 304a, 304b so that the temperature of the heating roller 240 is maintained at the target temperature. The target temperature is stored in a ROM area of ​​the memory 309. The target temperature may be changed according to the size and basis weight of the sheet P. The memory 309 stores control information indicating the power supply pattern of the heaters 241a, 241b. The CPU 307 may select the power supply pattern according to the temperature detected by the temperature sensor 306.

[0021] [Temperature dependence of heater resistance] Figure 4(A) shows the change in the AC voltage applied to the halogen heater. Figure 4(B) shows the change in the resistance value of the halogen heater. Figure 4(C) shows the change in the current flowing through the halogen heater. When the halogen heater is switched from off to on, a large current called an inrush current flows through the halogen heater. The resistance value of the halogen heater has temperature dependency characteristics. No AC current flows through the halogen heater before time t0. Therefore, the temperature of the halogen heater matches the ambient temperature. Therefore, the resistance value of the halogen heater is relatively low, as shown in Figure 4(B). When the halogen heater is switched from off to on at time t0, an AC current flows into the halogen heater, as shown in Figure 4(C). At this time, the resistance value of the halogen heater is still low, so an inrush current occurs. Because an inrush current flows at time t0, the AC voltage (power supply voltage) input to the halogen heater drops, as shown in the circled area in Figure 4(A). This causes flicker. Therefore, in order to switch the heaters 241a and 241b from off to on, some measure must be taken to reduce this flicker.

[0022] [Anti-flicker] Fig. 5(A) is a diagram explaining the slow start control. Slow start control refers to a control that gradually increases the power supplied to the halogen heater when the halogen heater is switched from off to on. Here, the current flow time (current flow phase angle) is gradually increased for each half cycle (half wave) of the AC. This suppresses the inrush current and reduces AC voltage fluctuations and flicker.

[0023] Fig. 5(B) is a diagram explaining the slow-end control. The slow-end control is a control that gradually reduces the power supplied to the halogen heater in order to switch the halogen heater from on to off. Here, the current flow time (current flow phase angle) is gradually reduced for each half cycle (half wave) of the AC. This reduces AC voltage fluctuations and flicker caused by the halogen heater.

[0024] FIG. 6(A) shows a change in input current (heater current) when the slow start control and the slow end control are applied to the heaters 241a and 241b. FIG. 6(B) shows a change in input voltage (power supply voltage) when the slow start control and the slow end control are applied to the heaters 241a and 241b. Here, the heaters 241a and 241b are simultaneously turned on at time t1 and time t4. The heaters 241a and 241b are simultaneously turned off at time t3 and time t7. The period from time t0 to time t1 and the period from time t4 to time t5 are periods in which the slow start control is applied, and may be called the slow start period. The period from time t2 to time t3 and the period from time t5 to time t7 are periods in which the slow end control is applied, and may be called the slow end period. The period from time t1 to time t2 and the period from time t5 to time t6 may be called the steady period. The period from time t0 to time t3 and the period from time t4 to time t7 are called operation periods. In Fig. 6A, the slow start period, the slow end period, and the steady period are represented as Tstart, Tend, and Tstatic, respectively. Note that Tstart, Tend, and Tstatic may be understood as variables indicating the time lengths of the slow start period, the slow end period, and the steady period.

[0025] As shown in Figures 6(A) and 6(B), flicker is reduced by applying the slow start control and the slow end control. However, since the heaters 241a and 241b connected to the same power system are turned on / off simultaneously, the flicker does not disappear completely.

[0026] FIG. 7(A) shows the change in input current when the slow start control and the slow end control are applied to the heaters 241a and 241b. FIG. 7(B) shows the change in input voltage when the slow start control and the slow end control are applied to the heaters 241a and 241b. Here, the operation period of the heater 241a and the operation period of the heater 241b are shifted so as not to overlap. That is, when the heater 241a is turned off, the heater 241b is switched on. This reduces the voltage fluctuation of the AC power supply 301 in the period from time t0 to time t1 and the period from time t6 to time t7, and reduces flicker. However, new flicker occurs in the circled parts.

[0027] Fig. 8(A) shows the change in input current when the slow start control and the slow end control are applied to the heaters 241a and 241b. Fig. 8(B) shows the change in input voltage when the slow start control and the slow end control are applied to the heaters 241a and 241b. Fig. 8(C) shows the temperature detected by the temperature sensor 306. Compared to Fig. 7(A), in Fig. 8(A), the slow end period of the heater 241a and the slow start period of the heater 241b overlap.

[0028] FIG. 9 shows the orientation of heater power (heater output) of heaters 241a and 241b. Here, the orientation of heater power refers to the distribution of heat generation performance in the extension direction (axial direction) of heaters 241a and 241b. In FIG. 9, arrow Y indicates the extension direction of heaters 241a and 241b. Y0 to Y1 indicate one end region of heaters 241a and 241b. Y1 to Y2 indicate the center region of heaters 241a and 241b. Y2 to Y3 indicate the other end region of heaters 241a and 241b. The heat generation capacity (heater power) of the center region of heater 241a is higher than the heat generation capacity of both end regions of heater 241a. The heat generation capacity of the center region of heater 241b is lower than the heat generation capacity of both end regions of heater 241b. By alternately turning on the heaters 241a and 241b, the entire surface area of ​​the heating roller 240 in the extending direction (Y direction) is uniformly heated.

[0029] As shown in FIG. 8C, at time t0, the temperature near the heating roller 240 is below the start threshold th0. Therefore, the CPU 307 starts turning on the heater 241a. As shown in FIG. 8A, in the slow start period from time t0 to time t1, the CPU 307 executes the slow start control. Thereby, as shown in FIG. 8B, the inrush current flowing to the heater 241a is suppressed. Here, the slow start control is a control that gradually expands the width of the energization period in each half cycle of the AC voltage. The energization period may be called a duty or a current phase angle. In the slow start period, the current energized to the heater 241a is gradually increased. Therefore, the inrush current flowing to the heater 241a is suppressed, and flicker is reduced. For example, when the slow start period is 1 second, the duty of the heater 241a may be expanded by 10% every 100 milliseconds. The duty is gradually increased by the slow start control, and the temperature in the vicinity of the heating roller 240 also gradually increases.

[0030] At time t1, the slow start control ends. As shown in FIG. 8A, the slow end period is a period from time t2 to time t4. The length of the slow start period may be a predetermined period stored in memory 309. The period from time t1 to time t2 is a steady period. In the steady period, the duty is fixed to a predetermined value stored in memory 309. The length of the steady period is also fixed to a predetermined value stored in memory 309. As a result, as shown in FIG. 8C, the temperature near the heating roller 240 further increases. While maintaining the duty at a predetermined value, the CPU 307 monitors whether the temperature detected by the temperature sensor 306 has reached the target temperature th1. Here, it is assumed that the temperature near the heating roller 240 has not reached the target temperature even after a predetermined time (steady period) has elapsed. Therefore, at time t2, the CPU 307 switches the control of the heater 241a from steady control to slow end control. In the slow end control, the duty is gradually narrowed. For example, in the slow-end control, the duty of the AC voltage in each half cycle is gradually reduced. As a result, in the slow-end control, the current flowing through the heater 241a is gradually reduced. This suppresses abrupt changes in the current and reduces flicker. For example, when the slow-end period is 1 second, the duty of the heater 241a is reduced by 10% every 100 milliseconds.

[0031] According to FIG. 8C, at time t3, which is before time t4 when the slow end period ends, the CPU 307 starts turning on the heater 241b. In order to suppress the inrush current flowing through the heater 241b, the CPU 307 also applies the slow start control to the heater 241b for a predetermined time. The slow start period of the heater 241b is a predetermined period from time t3 to time t5. Due to the slow start control, the duty of the heater 241b is gradually increased, and the temperature in the vicinity of the heating roller 240 gradually rises. At time t5, the CPU 307 ends the slow start control of the heater 241b. That is, the CPU 307 switches the control of the heater 241b from the slow start control to the steady control. In the steady control, the duty is maintained constant.

[0032] In the period from time t2 to time t4, the CPU 307 overlaps the slow end period of the heater 241a and the slow start period of the heater 241b. As a result, as shown in FIG. 8B, the fluctuation of the power supply voltage caused by the on / off of the heater 241a and the off / on of the heater 241b is reduced. In other words, flicker is reduced. Here, the time t3 at which the slow start period of the heater 241b starts is an arbitrary timing within the slow end period of the heater 241b. Specifically, the time t3 is selected by experiment or simulation so that the flicker is sufficiently small.

[0033] During the execution of the steady control, the CPU 307 judges whether the temperature detected by the temperature sensor 306 has reached the target temperature th1. In FIG. 6C, the temperature near the heating roller 240 reaches the target temperature at time t6, which is before the time when the steady period reaches its limit value. As a result, the CPU 307 switches the control of the heater 241b from the steady control to the slow-end control. The slow-end period (the period from time t6 to time t7) of the heater 241b is also constant. The duty of the heater 241b is gradually reduced by the slow-end control. At time t7, the CPU 307 ends the slow-end control. Thereafter, the temperature near the heating roller 240 gradually decreases. The power supply to the heaters 241a and 241b is stopped until the temperature near the heating roller 240 reaches the start threshold th0. When the temperature near the heating roller 240 reaches the start threshold th0 at time t8, the CPU 307 starts turning on the heater 241a again. Thereafter, the control from time t0 to time t8 is repeated.

[0034] According to FIG. 8A, the change in the input current during the slow start period of the heater 241a is different from the change in the input current during the slow start period of the heater 241b. This is because the temperature of the heater 241a at the start timing of the slow start period of the heater 241a is lower than the temperature of the heater 241b at the start timing of the slow start period of the heater 241b. As shown in FIG. 4B, the resistance values ​​of the heaters 241a and 241b have temperature dependency. Therefore, the current flowing when the temperatures of the heaters 241a and 241b are high is smaller than the current flowing when the temperatures of the heaters 241a and 241b are low. In other words, the change in the input current during the slow start period of the heater 241a is different from the change in the input current during the slow start period of the heater 241b.

[0035] [CPU functions] FIG. 10 shows functions that the CPU 307 realizes according to a control program. The setting unit 1000 sets the target temperature th1 and the start threshold th0, etc., in the temperature determination unit 1001, and sets the length of a period (period threshold) in the period determination unit 1003. Here, the length of a period is a threshold used for period determination in the period determination unit 1003, such as the length of a slow start period, the length of a steady period, and the length of a slow end period. The target temperature th1, the start threshold th0, and the length of each period are stored in the ROM area of ​​the memory 309. The temperature determination unit 1001 compares the detected temperature acquired by the temperature sensor 306 with the threshold set by the setting unit 1000, and outputs the comparison result to the energization control unit 1004. The period determination unit 1003 compares the timer value of the timer 1002 with the threshold set by the setting unit 1000, and outputs the comparison result to the energization control unit 1004. The energization control unit 1004 controls the current supplied to the heaters 241a and 241b based on the determination result of the temperature determination unit 1001 and the determination result of the period determination unit 1003. For example, the energization control unit 1004 selects one of the slow start control, the steady control, and the slow end control as the control applied to the heaters 241a and 241b. The setting unit 1000 sets the change rate (% / sec) of the duty applied in the slow start control, the constant duty (%) applied in the steady control, and the change rate (% / sec) of the duty applied in the slow end control in the energization control unit 1004. The setting unit 1000 may acquire the change rate (% / sec) of the duty applied in the slow start control, the duty (%) applied in the steady control, and the change rate (% / sec) of the duty applied in the slow end control from the memory 309. The unit of the change rate of the duty may be % / half cycle.

[0036] [flowchart] 11 and 12, the heater control executed by the CPU 307 will be described. When the image forming apparatus 100 is started up by receiving power from the AC power supply 301, the CPU 307 executes the following process according to a control program stored in the memory 309.

[0037] In S1101, the CPU 307 (the power supply control unit 1004) starts slow start control of the heater 241a. The power supply control unit 1004 gradually increases the duty for each half cycle of the AC at a rate of change set by the setting unit 1000. Note that a zero-cross detection circuit for identifying the half cycle of the AC supplied from the AC power supply 301 may be connected to the CPU 307.

[0038] S1102 is a step that is executed when the slow-end control of the heater 241b is already being executed. When the slow-end control of the heater 241b is not being executed, S1102 is skipped. In S1102, the CPU 307 (power control unit 1004) ends the slow-end control of the heater 241b. For example, the CPU 307 (period determination unit 1003) determines whether the execution period of the slow-end control has reached a predetermined period. When the execution period of the slow-end control measured by the timer 1002 reaches the predetermined period set by the setting unit 1000, the power control unit 1004 ends the slow-end control of the heater 241b.

[0039] In S1103, the CPU 307 (power supply control unit 1004) ends the slow start control of the heater 241a. For example, the CPU 307 (period determination unit 1003) determines whether the execution period of the slow start control has reached a predetermined period. When the execution period of the slow start control measured by the timer 1002 reaches the predetermined period set by the setting unit 1000, the power supply control unit 1004 ends the slow start control of the heater 241a.

[0040] In S1104, the CPU 307 (the power supply control unit 1004) starts steady-state control of the heater 241a. For example, the power supply control unit 1004 outputs a control signal to the switch 304a so that power is supplied to the heater 241a at a constant duty set by the setting unit 1000.

[0041] In S1105, the CPU 307 (period determination unit 1003) determines whether or not the steady period has ended based on the timer value acquired from the timer 1002. For example, the period determination unit 1003 determines whether or not the timer value has reached the steady period set by the setting unit 1000. If the timer value has not reached the steady period, the CPU 307 advances the process to S1111.

[0042] In S1111, CPU 307 (temperature determination unit 1001) determines whether the detected temperature is equal to or higher than target temperature th1. If the detected temperature is not equal to or higher than target temperature th1, CPU 307 advances the process to S1105. If the detected temperature is equal to or higher than target temperature th1, CPU 307 advances the process to S1112. In other words, if the temperature of heating roller 240 reaches target temperature th1 during the steady period of heater 241a, CPU 307 advances the process to S1112. This is also the case where the steady period does not reach a predetermined maximum value (limit time). In this way, the slow start period and the slow end period are each constant periods, but the steady period may be short.

[0043] At S1112, the CPU 307 (current control unit 1004) ends the steady-state control of the heater 241a. At S1113, the CPU 307 (current control unit 1004) starts slow-end control of the heater 241a. At S1114, the CPU 307 (current control unit 1004) ends the slow-end control of the heater 241a. For example, the CPU 307 (period determination unit 1003) determines whether the execution period of the slow-end control has reached a predetermined period. When the execution period of the slow-end control measured by the timer 1002 reaches the predetermined period set by the setting unit 1000, the current control unit 1004 ends the slow-end control of the heater 241a.

[0044] In S1115, the CPU 307 (temperature determination unit 1001) determines whether the detected temperature is equal to or lower than the start threshold value th0. If the detected temperature is equal to or lower than the start threshold value th0, the CPU 307 advances the process to S1101 again.

[0045] On the other hand, in S1105, the timer value may reach the maximum value for the steady period before the detected temperature acquired by the temperature sensor 306 reaches the target temperature th1. In this case, the CPU 307 advances the process from S1105 to S1106.

[0046] In S1106, the CPU 307 (power control unit 1004) ends the steady-state control of the heater 241a. In S1107, the CPU 307 (power control unit 1004) starts the slow-end control of the heater 241a. In S1108, the CPU 307 (period determination unit 1003) waits for the start timing of the slow-start control of the heater 241b based on the timer value acquired from the timer 1002. The start timing corresponds to the time t3 shown in FIG. 8A. The period from the time t2 to the time t3 is the predetermined period (standby time or delay time). The period determination unit 1003 measures the elapsed time from the time t2, which is the start timing of the slow-end control of the heater 241a, using the timer 1002. When the elapsed time reaches the predetermined period, that is, when the time t3 arrives, the CPU 307 advances the process to S1201.

[0047] In S1201, the CPU 307 (power supply control unit 1004) starts the slow start control of the heater 241b. That is, the slow start control of the heater 241b is started before the slow end control of the heater 241a ends. This causes the slow end period of the heater 241a and the slow start period of the heater 241b to overlap, thereby reducing flicker. The power supply control unit 1004 gradually increases the duty of the heater 241b for each half cycle of the AC current at the rate of change set by the setting unit 1000.

[0048] In S1202, the CPU 307 (power supply control unit 1004) ends the slow-end control of the heater 241a. For example, the CPU 307 (period determination unit 1003) determines whether the execution period of the slow-end control has reached a predetermined period. When the execution period of the slow-end control measured by the timer 1002 reaches the predetermined period set by the setting unit 1000, the power supply control unit 1004 ends the slow-end control of the heater 241a.

[0049] In S1203, the CPU 307 (power supply control unit 1004) ends the slow start control of the heater 241b. For example, the CPU 307 (period determination unit 1003) determines whether the execution period of the slow start control has reached a predetermined period. When the execution period of the slow start control measured by the timer 1002 reaches the predetermined period set by the setting unit 1000, the power supply control unit 1004 ends the slow start control of the heater 241b.

[0050] In S1204, the CPU 307 (the power supply control unit 1004) starts the steady-state control of the heater 241b. The power supply control unit 1004 outputs a control signal to the switch 304b so that power is supplied to the heater 241b at a constant duty set by the setting unit 1000.

[0051] In S1205, the CPU 307 (period determination unit 1003) determines whether or not the steady period has ended based on the timer value acquired from the timer 1002. The period determination unit 1003 determines whether or not the timer value has reached the maximum value for the steady period set by the setting unit 1000. If the timer value has not reached the maximum value for the steady period, the CPU 307 advances the process to S1211.

[0052] In S1211, the CPU 307 (temperature determination unit 1001) determines whether the detected temperature has reached or exceeded the target temperature th1. If the detected temperature has not reached or exceeded the target temperature th1, the CPU 307 advances the process to S1205. If the detected temperature has reached or exceeded the target temperature th1, the CPU 307 advances the process to S1212. In other words, if the temperature of the heating roller 240 reaches the target temperature th1 during the steady-state period of the heater 241b, the CPU 307 advances the process to S1212.

[0053] At S1212, the CPU 307 (power control unit 1004) ends the steady-state control of the heater 241b. At S1213, the CPU 307 (power control unit 1004) starts slow-end control of the heater 241b. At S1214, the CPU 307 (power control unit 1004) ends the slow-end control of the heater 241b. For example, the CPU 307 (period determination unit 1003) determines whether the execution period of the slow-end control has reached a predetermined period. When the execution period of the slow-end control measured by the timer 1002 reaches the predetermined period set by the setting unit 1000, the power control unit 1004 ends the slow-end control of the heater 241b. As a result, the heaters 241a and 241b stop, and the temperature of the heating roller 240 decreases.

[0054] In S1215, the CPU 307 (temperature determination unit 1001) determines whether the detected temperature is equal to or lower than the start threshold value th0. If the detected temperature is equal to or lower than the start threshold value th0, the CPU 307 advances the process to S1101 again.

[0055] On the other hand, there may be a case where the timer value reaches the steady period in S1205 before the detected temperature acquired by the temperature sensor 306 reaches the target temperature th1. In this case, the CPU 307 advances the process from S1205 to S1206.

[0056] In S1206, the CPU 307 (power control unit 1004) ends the steady-state control of the heater 241b. In S1207, the CPU 307 (power control unit 1004) starts the slow-end control of the heater 241b. In S1208, the CPU 307 (period determination unit 1003) waits for the start timing of the slow-start control of the heater 241a based on the timer value acquired from the timer 1002. This start timing is the same as the start timing of the slow-start control of the heater 241b. In other words, the start timing is a timing at which the slow-start control of the heater 241b starts before the slow-end control of the heater 241b ends. This causes the slow-end period of the heater 241b to overlap with the slow-start period of the heater 241a. In other words, flicker is reduced. The period determination unit 1003 measures the elapsed time from the start timing of the slow-end control of the heater 241b using the timer 1002. When the elapsed time reaches the predetermined period, the CPU 307 advances the process to step S1101.

[0057] In this way, the CPU 307 overlaps a part of the slow end period of one heater with a part of the slow start period of the other heater. This makes it possible to reduce the change in the input current of the heaters 241a and 241b, and to reduce the fluctuation in the power supply voltage (AC voltage) of the AC power supply 301. In other words, flicker is reduced.

[0058] Up to this point, the case where two heaters 241a and 241b are connected to one power system (AC power supply 301) has been described. However, the above embodiment is also applicable to the case where three or more heaters are connected to one power system (AC power supply 301). For example, as shown in FIG. 13, there may be a heater 241c to which power is supplied from the AC power supply 301 via a switch 304c controlled by the CPU 307. In this case, the CPU 307 may operate the three heaters 241a, 241b, and 241c in order one by one. For example, the slow end period of the heater 241a that operates first and the slow start period of the heater 241b that operates next partially overlap. Furthermore, the slow end period of the heater 241b and the slow start period of the heater 241c partially overlap. Furthermore, the slow end period of the heater 241c and the slow start period of the heater 241a partially overlap. Alternatively, the heaters 241a and 241c may be turned on / off simultaneously. In this case, the slow end periods of the heaters 241a and 241c partially overlap with the slow start period of the heater 241b. The slow end period of the heater 241b partially overlaps with the slow start periods of the heaters 241a and 241c.

[0059] <Technical ideas derived from examples> [Points 1, 13] As shown in FIG. 2 and FIG. 3, the heaters 241a and 241b are an example of a plurality of heaters including a first heater and a second heater. The heating roller 240 and the fixing belt 210 are an example of a fixing means for fixing a toner image to a sheet using heat supplied from at least one of the plurality of heaters. The CPU 307 is an example of a control means for controlling the power supplied from a power source to the plurality of heaters. The slow start period of the heater 241a is an example of an initial period during which the power supplied from the power source is gradually increased during the operation period of the first heater. The steady period of the heater 241a is an example of a steady period during which the power supplied from the power source is controlled to be constant. The slow end period of the heater 241a is an example of an end period during which the power supplied from the power source is gradually decreased. The slow start period of the heater 241b is an example of an initial period during which the power supplied from the power source is gradually increased during the operation period of the second heater. The steady period of the heater 241b is an example of a steady period during which the power supplied from the power source is controlled to be constant. The slow end period of heater 241b is an example of an end period in which the power supplied from the power source is gradually reduced. As shown in FIG. 8(A) and other figures, the end period (slow end period) of the operation period of the first heater and the start period (slow start period) of the operation period of the second heater at least partially overlap. In other words, a part of the slow end period of one heater and a part of the slow start period of the other heater overlap. This further reduces flicker compared to conventional techniques.

[0060] [Point 2] The CPU 307 starts the operation period of the second heater before the operation period of the first heater ends. As a result, a part of the end of the operation period of the first heater may overlap a part of the start of the operation period of the second heater. Basically, the slow start period of the second heater starts during the slow end period of the first heater. Furthermore, the slow start period of the second heater may start during the steady period of the first heater.

[0061] [Point 3] The temperature sensor 306 is an example of a detection unit that detects the temperature of the fixing unit (temperature around the fixing unit). There are cases where the temperature detected by the detection unit does not reach the target temperature until a predetermined period has elapsed since the operation of the first heater started (Yes in S1105). In this case, the CPU 307 may transition the first heater from the steady state to the end state. In such a case, the assistance of the heater 241b is required.

[0062] [Point 4] There are cases where the temperature detected by the detection means reaches the target temperature by the time a predetermined period has elapsed since the second heater started operating (Yes in S1211). In this case, the CPU 307 transitions the second heater from the steady state to the end state. In other words, when the target temperature is reached, the first heater and the second heater are each stopped via the slow end period. This reduces flicker.

[0063] [Point 5] There may be cases where the detected temperature has not reached the target temperature by the time a predetermined period has elapsed since the start of operation of the second heater (if Yes in S1205). In this case, the CPU 307 may be configured to transition the second heater from the steady period to the end period, and transition the first heater back to the start period before the end period of the second heater ends.

[0064] [Point 6] The multiple heaters may further include a third heater (e.g., heater 241c). The operation period of the third heater includes an initial period in which the power supplied from the power source is gradually increased, a steady period in which the power supplied from the power source is controlled to be constant, and an end period in which the power supplied from the power source is gradually decreased. There are cases in which the detected temperature does not reach the target temperature until a predetermined period has elapsed from the timing when the operation of the second heater is started. In this case, the CPU 307 may transition the second heater from the steady period to the end period, and transition the third heater to the initial period before the end period of the second heater ends. In this way, even in a case in which the three heaters are turned on one by one in sequence, this embodiment can reduce flicker.

[0065] [Point 7] There may be cases where the detected temperature does not reach the target temperature until a predetermined period has elapsed since the second heater started operating. In this case, the CPU 307 may transition the second heater from the steady state to the end state, and transition the first heater and the third heater to the start state before the end state of the second heater ends. In this way, the start state of one heater group and the end state of the other heater group may be partially overlapped, thereby reducing flicker.

[0066] [Point 8] The CPU 307 may set the energization time (duty) for energizing the first heater and the second heater with the AC supplied from the power source as a unit of a half cycle. The half cycle of the AC can be detected by a zero-cross detection circuit or the like. Therefore, by adopting the half cycle of the AC as a control unit, the CPU 307 can easily accurately control the amount of power supplied to the heaters.

[0067] [Point 9] The resistance value of each of the multiple heaters may increase with increasing temperature. In such a heater, when the temperature of the heater is low, the resistance value of the heater is also low. In other words, an inrush current is likely to flow through the heater, and flicker is likely to occur. Therefore, flicker may be reduced by applying this embodiment to a heater having such temperature-dependent characteristics.

[0068] [Points 10 and 11] As illustrated in FIG. 9, the distribution of the heat generating capacity of the first heater in the extension direction of the first heater and the distribution of the heat generating capacity of the second heater in the extension direction of the second heater may be different. Generally, the sheet P is centered and conveyed so as to pass through the center of the conveying path. Therefore, when the size of the sheet P is small, a large amount of heat is required in the central region of the heating roller 240, and a small amount of heat is required in both end regions of the heating roller 240. On the other hand, when the size of the sheet P is large, the entire area of ​​the heating roller 240 is used to heat the sheet P. In other words, the entire area of ​​the heating roller 240 needs to be heated uniformly. In this way, by combining multiple heaters with different orientations, it is possible to support sheets P of various sizes. In particular, when the fixing means is heated uniformly in the extension direction of the fixing means by alternately operating the first heater and the second heater, flicker is likely to occur. Therefore, by applying the above embodiment, flicker is reduced.

[0069] [Point 12] The heater output of the first heater and the heater output of the second heater may be different. As shown in FIG. 9, the heater 241a for heating the central region is used more frequently than the heater 241b for heating the end region. That is, the heater 241a is used primarily, and the heater 241b is used auxiliary. Therefore, the heater output of the heater 241a is larger than the heater output of the heater 241b.

[0070] [Points 14 and 15] Each of the heaters may have a lamp that outputs light. Each of the heaters may be a halogen heater. In this way, a heater that uses a lamp as a heating element has a temperature-dependent resistance value. In other words, an inrush current is likely to occur when the heater is started. Therefore, by applying this embodiment, flicker is reduced.

[0071] [Point 16] 2, the fixing unit may have a cylindrical rotating body (e.g., heating roller 240). A plurality of heaters may be disposed inside the rotating body. This makes it possible to effectively utilize the limited space inside image forming apparatus 100. Also, it becomes possible to efficiently heat the rotating body.

[0072] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0073] 241a to 241c: heaters, 210: fixing belt, 240: heating roller, 307: CPU

Claims

1. An image forming means for forming an image on a sheet; A fixing unit for fixing the image on the sheet, a first heater that generates heat when supplied with an electric current; a fixing means having a second heater which generates heat when supplied with current; A detection means for detecting the temperature of the fixing means; A control means, The control means controlling a first current supplied to the first heater and a second current supplied to the second heater so that the temperature of the fixing unit detected by the detection unit is maintained at a target temperature; gradually increasing the first current during a first time period when the first current is supplied to the first heater, supplying the first current with a first duty cycle during a second time period following the first time period, and gradually decreasing the first current to stop supplying the first current during a third time period following the second time period; gradually increasing the second current during a fourth time period when the second current is supplied to the second heater, supplying the second current with a second duty cycle during a fifth time period following the fourth time period, and gradually decreasing the second current to stop supplying the second current during a sixth time period following the fifth time period; controlling the supply of the second current so that a portion of the fourth period overlaps with the third period; The fourth period begins after a predetermined time has elapsed from the start of the third period.

1. An image forming apparatus comprising:

2. 2. The image forming apparatus according to claim 1, wherein the control means controls the supply of the first current so that a part of the first period overlaps with the sixth period.

3. The image forming apparatus of claim 1 or 2, characterized in that the control means is configured to transition the first heater from the second period to the third period when the temperature detected by the detection means has not reached the target temperature by the time a predetermined time has elapsed from the time when the first heater starts operating.

4. 4. The image forming apparatus according to claim 3, wherein the control means is configured to transition the second heater from the fifth period to the sixth period when the temperature detected by the detection means reaches the target temperature by the time a predetermined time has elapsed since the second heater started operating.

5. 5. The image forming apparatus according to claim 4, wherein the control means is configured to transition the second heater from the fifth period to the sixth period when the temperature detected by the detection means has not reached the target temperature by the time a predetermined time has elapsed since the second heater started operating, and to transition the first heater back to the first period after the sixth period of the second heater has ended.

6. 6. The image forming apparatus according to claim 1, wherein the control unit sets a power supply time for passing the AC current through the first heater in units of a half cycle of the AC current supplied to the first heater as the current, and sets a power supply time for passing the AC current through the second heater in units of a half cycle of the AC current supplied to the second heater as the current.

7. 7. An image forming apparatus according to claim 1, wherein a distribution of the heat generating capacity of the first heater in the extension direction of the first heater is different from a distribution of the heat generating capacity of the second heater in the extension direction of the second heater.

8. 2. The image forming apparatus according to claim 1, wherein the control means alternately operates the first heater and the second heater.

9. 9. The image forming apparatus according to claim 1, wherein the third period of the first heater is completed during the fourth period of the second heater.

Images