Fixing device, and image forming apparatus

Abstract

To provide a fixing device that can prevent damage to a fixing member caused by excessive heating, and an image forming apparatus.SOLUTION: A fixing device comprises: a motor 47 that rotates a fixing belt; a heater 42 that heats the fixing belt according to power supply; a temperature sensor 54 that detects the temperature of the heater 42; and a protection circuit 49 including a power supply stopping unit that stops power supply to the heater 42 when the temperature detected by the temperature sensor 54 exceeds a threshold, and a threshold setting unit that changes the threshold from a first temperature to a second temperature higher than the first temperature in an acceleration period during which the rotation speed of the motor 47 increases, and changes the threshold from the second temperature to the first temperature at the start of a deceleration period during which the rotation speed of the motor 47 decreases.SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

[0002] An electrophotographic image forming apparatus includes a fixing device. For example, the fixing device includes a fixing member, a motor, a heater, a temperature sensor, and a power supply stopping unit. The fixing member fixes a toner image transferred onto a sheet onto the sheet. The motor rotates the fixing member. The heater heats the fixing member in response to power supply. The temperature sensor detects the temperature of the apparatus itself. The power supply stopping unit stops power supply to the heater when the temperature detected by the temperature sensor exceeds a preset threshold.

[0003] Also, a fixing device is known as related art that can change the threshold value from a first temperature to a second temperature higher than the first temperature when the rotation speed of the motor increases beyond a predetermined reference speed, and can change the threshold value from the second temperature to the first temperature when the rotation speed of the motor decreases beyond the reference speed (see Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-321573 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the fixing device according to the related art described above, if power continues to be supplied to the heater even after the motor starts to decelerate due to runaway of the CPU that controls the drive of the heater, the fixing member may be excessively heated before the rotation speed of the motor decreases beyond the reference speed, which may result in damage to the fixing member.

[0006] SUMMARY OF THE INVENTION An object of the present invention is to provide a fixing device and an image forming apparatus that can prevent damage to a fixing member caused by excessive heating. [Means for solving the problem]

[0007] A fixing device according to one aspect of the present invention includes a fixing member, a motor, a heater, a temperature sensor, a power supply stopping unit, and a threshold setting unit. The fixing member fixes a toner image transferred onto a sheet to the sheet. The motor rotates the fixing member. The heater heats the fixing member in response to power supply. The temperature sensor detects the temperature of the device itself. The power supply stopping unit stops power supply to the heater when the temperature detected by the temperature sensor exceeds a predetermined threshold. The threshold setting unit changes the threshold from a predetermined first temperature to a second temperature higher than the first temperature during an acceleration period in which the rotation speed of the motor increases, and changes the threshold from the second temperature to the first temperature at the start of a deceleration period in which the rotation speed of the motor decreases.

[0008] According to another aspect of the present invention, there is provided an image forming apparatus including an image forming unit, the image forming unit including the fixing device and forming an image on the sheet. [Effects of the Invention]

[0009] According to the present invention, it is possible to prevent damage to the fixing member caused by excessive heating. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the configuration of an image forming apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing the system configuration of the image forming apparatus according to the embodiment of the present invention. [Figure 3] FIG. 3 is a diagram showing the configuration of the fixing device of the image forming apparatus according to the embodiment of the present invention. [Figure 4]FIG. 4 is a diagram showing the configuration of a heater in the image forming apparatus according to the embodiment of the present invention. [Figure 5] FIG. 5 is a diagram showing the configuration of a heater drive circuit of the image forming apparatus according to the embodiment of the present invention. [Figure 6] FIG. 6 is a diagram showing the configuration of a protection circuit of an image forming apparatus according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Note that the following embodiment is an example of a specific embodiment of the present invention and does not limit the technical scope of the present invention.

[0012] [Configuration of image forming apparatus 100] First, the configuration of an image forming apparatus 100 according to an embodiment of the present invention will be described with reference to Figures 1 and 2. Here, Figure 1 is a cross-sectional view showing the configuration of the image forming apparatus 100.

[0013] For ease of explanation, the vertical direction in the installation state where image forming apparatus 100 is usable (the state shown in FIG. 1) is defined as the up-down direction D1. Also, the front-to-back direction D2 is defined with the left side of image forming apparatus 100 shown in FIG. 1 as the front (front face). Also, the left-to-right direction D3 is defined with the front face of image forming apparatus 100 in the installation state as the reference point.

[0014] Image forming apparatus 100 is a printer having a print function that forms an image based on image data. Note that the present invention can be applied to fax machines, copy machines, multifunction machines, and the like that form images using an electrophotographic method.

[0015] 1 and 2, the image forming apparatus 100 includes an image forming unit 1, a sheet conveying unit 2, an operation and display unit 3, a storage unit 4, and a control unit 5. The image forming unit 1, the sheet conveying unit 2, the storage unit 4, and the control unit 5 are housed in a housing 101 (see FIG. 1) of the image forming apparatus 100. The housing 101 is formed in a substantially rectangular parallelepiped shape. The operation and display unit 3 and a sheet receiving unit 102 (see FIG. 1) onto which sheets on which images have been formed by the image forming apparatus 100 are discharged are formed on the top of the housing 101.

[0016] The operation display unit 3 is a user interface of the image forming apparatus 100. The operation display unit 3 has a display unit such as a liquid crystal display that displays various information in response to control instructions from the control unit 5, and an operation unit such as operation keys or a touch panel that inputs various information to the control unit 5 in response to user operations.

[0017] The storage unit 4 is a non-volatile storage device, such as a flash memory.

[0018] The control unit 5 performs overall control of the image forming apparatus 100. As shown in FIG. 2, the control unit 5 includes a CPU 11, a ROM 12, and a RAM 13. The CPU 11 is a processor that executes various types of arithmetic processing. The ROM 12 is a non-volatile storage device that pre-stores information such as control programs for causing the CPU 11 to execute various processes. The RAM 13 is a volatile or non-volatile storage device used as a temporary storage memory (work area) for the various processes executed by the CPU 11. In the control unit 5, the CPU 11 executes various control programs pre-stored in the ROM 12. In this way, the control unit 5 performs overall control of the image forming apparatus 100. The control unit 5 may be configured with an electronic circuit such as an integrated circuit (ASIC). The control unit 5 may also be a control unit provided separately from a main control unit that performs overall control of the image forming apparatus 100.

[0019] The image forming unit 1 is capable of forming an image on a sheet by electrophotography based on image data input from an external information processing device such as a personal computer. As shown in Fig. 1, the image forming unit 1 includes a photosensitive drum 21, a charging device 22, an optical scanning device 23, a developing device 24, a transfer roller 25, a cleaning device 26, and a fixing device 27.

[0020] The photosensitive drum 21 is rotatably supported by the housing 101. The photosensitive drum 21 receives a rotational driving force transmitted from a motor (not shown) and rotates in the direction of the arrow shown in FIG.

[0021] The charging device 22 charges the surface of the photosensitive drum 21 .

[0022] The optical scanning device 23 irradiates the surface of the photosensitive drum 21, which has been charged by the charging device 22, with light based on image data. An electrostatic latent image is formed on the surface of the photosensitive drum 21 by the optical scanning device 23.

[0023] The developing device 24 uses a developer containing toner to develop the electrostatic latent image formed on the surface of the photosensitive drum 21. A toner image is formed on the surface of the photosensitive drum 21 by the developing device 24.

[0024] The transfer roller 25 transfers the toner image formed on the surface of the photosensitive drum 21 onto the sheet being conveyed by the sheet conveying section 2 toward the fixing device 27 .

[0025] The cleaning device 26 cleans the surface of the photosensitive drum 21 after the toner image has been transferred by the transfer roller 25 .

[0026] The fixing device 27 heats the sheet onto which the toner image has been transferred, thereby fixing the toner image to the sheet.

[0027] The sheet conveying unit 2 conveys a sheet on which an image is formed by the image forming unit 1. As shown in Fig. 1, the sheet conveying unit 2 includes a paper feed cassette 31, a sheet conveying path 32, a paper feed unit 33, a pair of registration rollers 34, and a pair of paper discharge rollers 35.

[0028] The paper feed cassette 31 stores sheets on which images are formed by the image forming unit 1. As shown in FIG. 1, the paper feed cassette 31 is provided at the bottom of the housing 101. For example, the paper feed cassette 31 stores sheet materials such as paper, coated paper, postcards, envelopes, and transparencies. The paper feed cassette 31 has a lift plate that lifts up the multiple sheets stored therein.

[0029] The sheet transport path 32 is a passageway along which a sheet travels from the paper feed cassette 31 to the sheet receiving section 102 via the transfer roller 25 and the fixing device 27. The sheet transport path 32 is provided with a plurality of roller pairs, including a registration roller pair 34 and a paper discharge roller pair 35. In the sheet transport path 32, a sheet discharged from the paper feed cassette 31 is transported by the plurality of roller pairs in a transport direction D4 (see FIG. 1 ) toward the sheet receiving section 102. The sheet transport path 32 is formed by a pair of transport guide members provided inside the housing 101.

[0030] The paper feed unit 33 feeds sheets stored in the paper feed cassette 31 one by one to the sheet transport path 32. The paper feed unit 33 includes a pickup roller, a paper feed roller, and a retard roller. The pickup roller rotates while contacting the upper surface of the topmost sheet among the multiple sheets lifted by the lift plate of the paper feed cassette 31, thereby feeding the sheet to the paper feed roller. The paper feed roller rotates while contacting the upper surface of the sheet fed by the pickup roller, thereby feeding the sheet to the sheet transport path 32. The retard roller is biased from below the paper feed roller toward the paper feed roller. When multiple overlapping sheets are fed by the pickup roller, the retard roller separates the sheets other than the topmost sheet from the overlapping multiple sheets.

[0031] The pair of resist rollers 34 transports the sheet to the transfer position in accordance with the timing at which the toner image formed on the surface of the photosensitive drum 21 is transported to the transfer position by the transfer roller 25 by the rotation of the photosensitive drum 21.

[0032] The pair of paper discharge rollers 35 discharges the sheet on which the toner image has been fixed by the fixing device 27 to the sheet receiving section 102 .

[0033] [Configuration of fixing device 27] Next, the configuration of the fixing device 27 will be described with reference to Figs. 3 to 6. Fig. 3 is a cross-sectional view showing the configuration of the fixing device 27. Fig. 4 is a cross-sectional view showing the configuration of the heater 42. Fig. 5 is a diagram showing the configuration of a heater drive circuit 48. Fig. 6 is a diagram showing the configuration of a protection circuit 49. In Fig. 5, the analog signal S11, the pulse signal S12, the digital signal S13, the pulse signal S14, and the digital signal S15 are indicated by dashed lines with arrows.

[0034] 3, the fixing device 27 includes a fixing belt 41, a heater 42, a support portion 43, a pressing member 44, and a pressure roller 45. The fixing device 27 also includes a motor 47, a heater driving circuit 48, and a protection circuit 49 shown in FIG.

[0035] The fixing belt 41 is heated to a predetermined fixing temperature, such as 180°C, by a heater 42. The fixing belt 41 contacts a sheet in a heated state to fix a toner image transferred to the sheet. As shown in FIG. 3, the fixing belt 41 is endless. The fixing belt 41 is flexible. The fixing belt 41 includes a base layer, an elastic layer disposed on the outer peripheral surface of the base layer, and a release layer disposed on the outer peripheral surface of the elastic layer. The base layer is formed of a metal material such as stainless steel or a nickel alloy. The elastic layer is formed of a material such as silicone rubber. The release layer is formed of a fluorine-based resin material such as PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin). The fixing belt 41 is elongated along the left-right direction D3. The size of the fixing belt 41 in the left-right direction D3 is determined based on the maximum size of a sheet on which an image can be formed using the image forming apparatus 100. The fixing belt 41 is an example of a fixing member of the present invention. The fixing member of the present invention is not limited to a belt-shaped member, but may be a roller-shaped member or the like.

[0036] The pressure roller 45 is provided at a position where it can come into contact with the outer peripheral surface 41A (see FIG. 3) of the fixing belt 41. Specifically, as shown in FIG. 3, the pressure roller 45 is provided below the fixing belt 41. The pressure roller 45 is elongated along the left-right direction D3. The pressure roller 45 includes a shaft portion 45A and an elastic layer 45B. The shaft portion 45A is formed in a cylindrical shape from a metal material. The elastic layer 45B is formed on the outer periphery of the shaft portion 45A from an elastic material. The shaft portion 45A is rotatably supported by a pair of side plates provided inside the housing 101. The pressure roller 45 receives a rotational driving force supplied from a motor 47 and rotates in a rotation direction D5 (see FIG. 3).

[0037] The heater 42 heats the fixing belt 41 in response to power supply. As shown in Fig. 3, the heater 42 is provided inside the fixing belt 41, facing the pressure roller 45 with the fixing belt 41 in between. The heater 42 is elongated in the left-right direction D3, and extends across both outer sides of the fixing belt 41 in the left-right direction D3.

[0038] As shown in FIG. 4, the heater 42 includes a substrate 51, a resistance heating element 52, a protective layer 53, and a temperature sensor 54.

[0039] The substrate 51 is a flat member that is long in the left-right direction D3. The substrate 51 is made of a material that has excellent heat resistance, electrical insulation, and low heat capacity. For example, the substrate 51 is made of ceramic such as alumina. The size of the substrate 51 in the left-right direction D3 is larger than that of the fixing belt 41. The substrate 51 is disposed so as to extend on both outer sides of the fixing belt 41 in the left-right direction D3. Therefore, both end portions of the substrate 51 in the left-right direction D3 protrude outward from the fixing belt 41 in the left-right direction D3.

[0040] 3 and 4, the lower surface of substrate 51 faces inner circumferential surface 41B (see FIG. 3) of fixing belt 41. As shown in FIG. 4, resistance heating element 52 is disposed on the lower surface of substrate 51. An area of ​​the lower surface of substrate 51 facing inner circumferential surface 41B of fixing belt 41 is covered with protective layer 53 (see FIG. 4). Protective layer 53 is made of an electrically insulating material such as glass.

[0041] As shown in FIGS. 3 and 4, the upper surface of the substrate 51 faces the bottom surface of the recess 43A of the support portion 43. As shown in FIG. 4, a temperature sensor 54 is disposed on the upper surface of the substrate 51. The temperature sensor 54 detects the temperature of the fixing device 27. Specifically, the temperature sensor 54 detects the temperature of the heater 42 and outputs an analog signal S11 (see FIG. 5) having a voltage corresponding to the detected temperature. For example, the voltage of the analog signal S11 decreases as the temperature detected by the temperature sensor 54 increases. The analog signal S11 output from the temperature sensor 54 is input to the control unit 5 and the protection circuit 49.

[0042] The resistance heating element 52 generates heat in response to power supplied from a commercial power source 200 (see FIG. 5). The resistance heating element 52 is formed in a strip shape that is elongated in the left-right direction D3 and has a predetermined thickness in a direction perpendicular to the lower surface of the substrate 51. For example, the resistance heating element 52 is formed from a material such as silver-palladium (Ag / Pd). The size of the resistance heating element 52 in the left-right direction D3 is smaller than that of the fixing belt 41. The resistance heating element 52 is disposed inside the opposing region on the lower surface of the substrate 51.

[0043] The support portion 43 supports the heater 42. As shown in FIG. 3, the support portion 43 is provided on the inner side of the fixing belt 41. The support portion 43 is elongated in the left-right direction D3 and extends to both outer sides of the fixing belt 41 in the left-right direction D3. A recess 43A corresponding to the shape of the heater 42 is formed in the bottom of the support portion 43. The heater 42 is fitted into the recess 43A.

[0044] The pressing member 44 presses the support portion 43 toward the pressure roller 45. As shown in FIG. 3 , the pressing member 44 is provided inside the fixing belt 41, facing the pressure roller 45 with the support portion 43 interposed therebetween. The pressing member 44 is elongated in the left-right direction D3 and extends across both outer sides of the fixing belt 41 in the left-right direction D3. Both ends of the pressing member 44 in the left-right direction D3 are urged toward the pressure roller 45 by urging members (not shown). As a result, the pressing member 44 presses the support portion 43 toward the pressure roller 45. As the support portion 43 is pressed toward the pressure roller 45, the heater 42 supported by the support portion 43 is pressed toward the pressure roller 45.

[0045] Heater 42 is pressed against inner circumferential surface 41B of fixing belt 41 by being pressed toward pressure roller 45 by pressing member 44. As a result, a fixing nip portion 46 is formed between fixing belt 41 and pressure roller 45, where a toner image transferred to a sheet is fixed to the sheet. In this specification, the area where fixing belt 41 and pressure roller 45 contact each other is defined as fixing nip portion 46. Note that a lubricant such as fluorine grease is applied between heater 42 and inner circumferential surface 41B of fixing belt 41.

[0046] Fixing belt 41 is sandwiched between heater 42 and pressure roller 45. When pressure roller 45 rotates in rotation direction D5, fixing belt 41 rotates along belt rotation direction D6 (see FIG. 3) following the rotation of pressure roller 45.

[0047] The support portion 43 includes a pair of guide portions 43B that contact the inner circumferential surface 41B of the fixing belt 41 to guide the running of the fixing belt 41. The pair of guide portions 43B are provided at both ends of the support portion 43 in the front-rear direction D2. The pair of guide portions 43B guide the fixing belt 41 to run along a predetermined running path.

[0048] The pressure roller 45 may be biased toward the heater 42. In this case, the pressing member 44 does not need to be biased by the biasing member.

[0049] The motor 47 rotates the fixing belt 41. Specifically, the motor 47 supplies a rotational driving force to the pressure roller 45 to rotate the pressure roller 45, thereby rotating the fixing belt 41. The motor 47 transitions from a stopped state to a driven state when the signal level of a digital signal S13 (see FIG. 5) input from the control unit 5 switches from low to high. In the driven state, power supply from a power source (not shown) to the motor 47 is controlled so that the rotation speed of the motor 47 becomes a predetermined specific speed. Furthermore, the motor 47 transitions from the driven state to the stopped state when the signal level of the digital signal S13 input from the control unit 5 switches from high to low. The motor 47 outputs a pulse signal S14 (see FIG. 5) having a frequency corresponding to the rotation speed while rotating. The pulse signal S14 output from the motor 47 is input to a protection circuit 49.

[0050] The heater drive circuit 48 drives the heater 42. As shown in FIG.

[0051] 5, the switching element 61 is provided in the current path between the commercial power supply 200 and the heater 42. The switching element 61 is a semiconductor switch that can switch between connecting and disconnecting the current path between the commercial power supply 200 and the heater 42 in response to an input of a pulse signal S12 (see FIG. 5) having a preset duty ratio that is output from the control unit 5. For example, the switching element 61 is a triac.

[0052] The control unit 5 controls the driving of the heater 42 based on the temperature detected by the temperature sensor 54. Specifically, the control unit 5 sets the duty ratio of the pulse signal S12 based on the analog signal S11 input from the temperature sensor 54 so that the temperature of the fixing belt 41 becomes the fixing temperature.

[0053] As shown in FIG. 5, the relay 62 is provided in the current path between the commercial power supply 200 and the heater 42. The relay 62 interrupts the current path between the commercial power supply 200 and the heater 42 in response to an input of a digital signal S15 (see FIG. 5) output from the protection circuit 49. For example, the relay 62 conducts the current path between the commercial power supply 200 and the heater 42 when the signal level of the digital signal S15 output from the protection circuit 49 is low. Furthermore, the relay 62 interrupts the current path between the commercial power supply 200 and the heater 42 when the signal level of the digital signal S15 output from the protection circuit 49 is high. Note that the heater drive circuit 48 may include a switching element (different from the switching element 61) instead of the relay 62.

[0054] In the fixing device 27, the motor 47 is shifted from the stopped state to the driven state after power supply to the heater 42 is started. As a result, the lubricant is heated before the motor 47 starts to be driven, reducing the sliding resistance between the heater 42 and the fixing belt 41, thereby reducing the load on the motor 47. Also, in the fixing device 27, the motor 47 is shifted from the driven state to the stopped state after power supply to the heater 42 is stopped. As a result, the supply of thermal energy from the heater 42 is stopped before the rotation speed of the fixing belt 41 decelerates, thereby preventing the fixing belt 41 from becoming excessively hot after the rotation speed of the fixing belt 41 decelerates.

[0055] The protection circuit 49 uses a relay 62 to stop the power supply to the heater 42 .

[0056] Specifically, the protection circuit 49 includes a power supply stopping unit 71 and a threshold setting unit 72 shown in FIG.

[0057] The power supply stopping unit 71 stops the power supply to the heater 42 when the temperature detected by the temperature sensor 54 exceeds a preset threshold value.

[0058] As shown in FIG. 6, the power supply stopping unit 71 includes a comparator 81. The comparator 81 outputs a digital signal S15 (see FIG. 6) according to the result of comparison between the voltage of the analog signal S11 (see FIG. 6) output from the temperature sensor 54 and a reference voltage Vref (see FIG. 6) corresponding to the threshold value. Specifically, when the voltage of the analog signal S11 output from the temperature sensor 54 is higher than the reference voltage Vref, the comparator 81 outputs a low-level digital signal S15. On the other hand, when the voltage of the analog signal S11 output from the temperature sensor 54 is lower than the reference voltage Vref, the comparator 81 outputs a high-level digital signal S15.

[0059] Here, a fixing device is known as a related art that can change the threshold value from a first temperature to a second temperature higher than the first temperature when the rotation speed of motor 47 increases beyond a predetermined reference speed, and can change the threshold value from the second temperature to the first temperature when the rotation speed of motor 47 decreases beyond the reference speed.

[0060] However, in the fixing device according to the related art described above, if power supply to heater 42 continues even after motor 47 starts to decelerate due to runaway of CPU 11 of control unit 5 or the like, fixing belt 41 may be excessively heated before the rotation speed of motor 47 decreases beyond the reference speed, which may result in damage to fixing belt 41.

[0061] In contrast, in the image forming apparatus 100 according to the embodiment of the present invention, damage to the fixing belt 41 caused by excessive heating can be suppressed, as will be described below.

[0062] The threshold setting unit 72 changes the threshold from the first temperature to the second temperature, which is higher than the first temperature, during an acceleration period in which the rotation speed of the motor 47 increases, and changes the threshold from the second temperature to the first temperature at the start of a deceleration period in which the rotation speed of the motor 47 decreases.

[0063] For example, when the rotation speed of the motor 47 increases beyond the reference speed, the threshold setting unit 72 changes the threshold from the first temperature to the second temperature. Also, when the motor 47 transitions from the driving state to the stopped state, the threshold setting unit 72 changes the threshold from the second temperature to the first temperature.

[0064] For example, the first temperature is 100° C., and the second temperature is 230° C. The first temperature and the second temperature may be arbitrarily set temperatures.

[0065] As shown in FIG. 6, the threshold setting unit 72 includes a speed determination unit 91, an AND circuit 92, and a reference voltage switching circuit 93.

[0066] The speed determination unit 91 determines whether or not the rotation speed of the motor 47 exceeds the reference speed, which is slower than the specific speed.

[0067] Specifically, the speed determination unit 91 is an electronic circuit that outputs a low-level digital signal S16 (see FIG. 6) when the frequency of the pulse signal S14 input from the motor 47 is equal to or lower than the frequency corresponding to the reference speed, and outputs a high-level digital signal S16 when the frequency of the pulse signal S14 input from the motor 47 exceeds the frequency corresponding to the reference speed.

[0068] The AND circuit 92 receives the digital signal S13 (see FIG. 5) output from the control unit 5 and the digital signal S16 (see FIG. 6) output from the speed determination unit 91. The AND circuit 92 outputs a high-level digital signal S17 (see FIG. 6) only when both the input digital signals S13 and S16 are high.

[0069] In other words, when the motor 47 is in the driving state and the rotation speed of the motor 47 exceeds the reference speed, the AND circuit 92 outputs a high-level digital signal S17. On the other hand, when the motor 47 is in the stopped state or the rotation speed of the motor 47 is equal to or lower than the reference speed, the AND circuit 92 outputs a low-level digital signal S17.

[0070] That is, the digital signal S17 output from the AND circuit 92 switches from low level to high level during an acceleration period in which the rotation speed of the motor 47 increases. Also, the digital signal S17 output from the AND circuit 92 switches from high level to low level at the start of a deceleration period in which the rotation speed of the motor 47 decreases (when the state of the motor 47 switches from the driving state to the stopped state).

[0071] The reference voltage switching circuit 93 switches the reference voltage Vref between a first voltage corresponding to the first temperature and a second voltage corresponding to the second temperature in response to a change in the signal level of the digital signal S17 output from the AND circuit 92.

[0072] As shown in FIG. 6, the reference voltage switching circuit 93 includes resistors R1 to R5 and a transistor TR1.

[0073] The transistor TR1 is a PNP type transistor.

[0074] 6, the base terminal of the transistor TR1 is connected to the output terminal of the AND circuit 92 via a resistor R5, and the base terminal of the transistor TR1 is connected to the emitter terminal of the transistor TR1 via a resistor R4.

[0075] 6, the collector terminal of the transistor TR1 is connected to a power supply PW1 via a resistor R1. The collector terminal of the transistor TR1 is also connected to a non-inverting input terminal of a comparator 81 via a resistor R2. The collector terminal of the transistor TR1 is also connected to ground via resistors R2 and R3.

[0076] As shown in FIG. 6, the emitter terminal of the transistor TR1 is connected to a power supply PW1.

[0077] In the reference voltage switching circuit 93, the transistor TR1 is in an on state when the signal level of the digital signal S17 output from the AND circuit 92 is low. When the transistor TR1 is in an on state, the voltage output from the power supply PW1 is divided into the first voltage by the resistors R2 and R3 and input to the non-inverting input terminal of the comparator 81.

[0078] In the reference voltage switching circuit 93, the transistor TR1 is in an off state when the signal level of the digital signal S17 output from the AND circuit 92 is high. When the transistor TR1 is in an off state, the voltage output from the power supply PW1 is divided by the resistors R1, R2, and R3 into the second voltage that is lower than the first voltage, and is input to the non-inverting input terminal of the comparator 81.

[0079] In this way, in the image forming apparatus 100, the threshold value is changed from the first temperature to the second temperature during the acceleration period in which the rotation speed of the motor 47 increases. This makes it possible to prevent the fixing belt 41 from being excessively heated when the motor 47 is stopped or rotating at a low speed, compared to a configuration in which the threshold value is always the second temperature.

[0080] Furthermore, in image forming apparatus 100, the threshold value is changed from the second temperature to the first temperature at the start of the deceleration period in which the rotation speed of motor 47 is reduced. This makes it possible to forcibly stop power supply to heater 42 at the start of the deceleration period, even if CPU 11 is running out of control. Therefore, compared to a configuration in which the threshold value is changed from the second temperature to the first temperature when the rotation speed of motor 47 decreases beyond the reference speed, it is possible to prevent fixing belt 41 from being excessively heated before the rotation speed of motor 47 decreases beyond the reference speed. Therefore, it is possible to prevent damage to fixing belt 41 due to excessive heating.

[0081] The threshold setting unit 72 may change the threshold from the first temperature to the second temperature when a predetermined time has elapsed since the motor 47 shifted from the stopped state to the driven state. For example, the threshold setting unit 72 may include, instead of the speed determination unit 91, a delay circuit that delays the input digital signal S13 by a predetermined time and outputs the delayed digital signal S13.

[0082] Furthermore, the threshold setting unit 72 may change the threshold value stepwise from the first temperature to the second temperature during the acceleration period. For example, the threshold setting unit 72 may change the threshold value from the first temperature to a third temperature between the first temperature and the second temperature when the rotation speed of the motor 47 increases beyond a first speed that is slower than the reference speed, and may change the threshold value from the third temperature to the second temperature when the rotation speed of the motor 47 increases beyond a second speed that is faster than the reference speed and slower than the specific speed.

[0083] The heater 42 may also include a plurality of temperature sensors 54 spaced apart in the left-right direction D3. In this case, the protection circuit 49 may include a power supply stopping unit 71 corresponding to each of the temperature sensors 54.

[0084] Furthermore, the temperature sensor 54 may detect the temperature of a component other than the heater 42 in the fixing device 27. For example, the temperature sensor 54 may detect the temperature of the fixing belt 41.

[0085] [Notes on the Invention] The following is a summary of the invention extracted from the above-described embodiment. Note that the configurations and processing functions described in the following supplementary notes can be selected and combined as desired.

[0086] <Appendix 1> a fixing device comprising: a fixing member that fixes a toner image transferred onto a sheet onto the sheet; a motor that rotates the fixing member; a heater that heats the fixing member in response to power supply; a temperature sensor that detects the temperature of the device itself; a power supply stopping unit that stops power supply to the heater when the temperature detected by the temperature sensor exceeds a predetermined threshold; and a threshold setting unit that changes the threshold from a predetermined first temperature to a second temperature higher than the first temperature during an acceleration period in which the rotation speed of the motor increases, and changes the threshold from the second temperature to the first temperature at the start of a deceleration period in which the rotation speed of the motor decreases.

[0087] <Appendix 2> An image forming apparatus including the fixing device according to claim 1 and an image forming section that forms an image on the sheet. [Explanation of symbols]

[0088] 1 Image forming unit 2 Sheet transport section 3 Operation display section 4 Storage section 5. Control section 21 Photosensitive drum 22 Charging device 23 Optical scanning device 24 Developing device 25 Transfer roller 26 Cleaning device 27 Fixing device 41 Fixing belt 42 Heater 43 Support part 44 Pressing member 45 Pressure Roller 46 Fixing nip 47 Motor 48 Heater drive circuit 49 Protection circuit 54 Temperature Sensor 61 Switching element 62 Relay 71 Power supply stop section 72 Threshold setting unit 100 Image forming device

Claims

1. a fixing member that fixes the toner image transferred onto the sheet; a motor that rotates the fixing member; a heater that heats the fixing member in response to power supply; a temperature sensor for detecting the temperature of the device itself; a power supply stopping unit that stops power supply to the heater when the temperature detected by the temperature sensor exceeds a preset threshold; a threshold setting unit that changes the threshold from a predetermined first temperature to a second temperature higher than the first temperature during an acceleration period in which the rotation speed of the motor increases, and changes the threshold from the second temperature to the first temperature at the start of a deceleration period in which the rotation speed of the motor decreases; A fixing device comprising:

2. an image forming unit including the fixing device according to claim 1 and forming an image on the sheet; Image forming device.

Images