Image forming apparatus

Abstract

To prevent an excessive increase in the temperature of a heater occurring due to energization of the heater when a nip pressure is small.SOLUTION: An image forming apparatus comprises a heater, an endless belt, a pressure rotating body, a nip pressure changing mechanism, and a control unit. The heater has a substrate and a resistance heating element. The belt rotates around the heater. The pressure rotating body sandwiches the belt with the heater to form a nip part. The nip pressure changing mechanism changes a nip pressure at the nip part to a first nip pressure and a second nip pressure smaller than the first nip pressure. The control unit determines whether the nip pressure is the first nip pressure or the second nip pressure (S2), and when determining that the nip pressure is the second nip pressure (S2: No), controls output from the heater to be not more than a second upper limit that is smaller than a first upper limit being an upper limit when determining that the nip pressure is the first nip pressure (S4-S7).SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus equipped with a fixing device that clamps a belt between a heater and a pressurizing rotating body. [Background technology]

[0002] Conventionally, an image forming apparatus is known that includes a heater, a belt, a pressure roller that sandwiches the belt between the heater and the roller, and a pressure detection switch that detects when the belt and pressure roller are pressurized (see Patent Document 1). In this technology, when the belt and pressure roller are pressurized, the pressure detection switch turns ON, enabling power supply to the heater. When the pressure is released, the pressure detection switch turns OFF, cutting off the power supply to the heater. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2005-321511 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Incidentally, in conventional technology, power supply to the heater is cut off when the nip pressure is low, so it is not possible to fix the image to the sheet when the nip pressure is low. On the other hand, if the system is configured to supply power when the nip pressure is low, heat transfer from the heater to the belt and pressure roller may not be performed properly, causing heat to build up in the heater and potentially leading to overheating.

[0005] Therefore, the present invention aims to suppress the overheating of the heater caused by energizing the heater when the nip pressure is low. [Means for solving the problem]

[0006] To solve the aforementioned problems, the image forming apparatus according to the present invention comprises a heater, an endless belt, a pressurized rotating body, a nip pressure changing mechanism, and a control unit. The heater consists of a circuit board and a resistive heating element. The belt rotates around the heater. The pressurized rotating body forms a nip section by sandwiching a belt between it and the heater. The nip pressure changing mechanism changes the nip pressure at the nip section between a first nip pressure and a second nip pressure that is smaller than the first nip pressure. The control unit determines whether the nip pressure is the first nip pressure or the second nip pressure. If it determines that the nip pressure is the second nip pressure, it controls the heater output to be less than or equal to the second upper limit, which is smaller than the first upper limit, which is the upper limit when the nip pressure is determined to be the first nip pressure.

[0007] With this configuration, when the nip pressure is low, the heater output is limited to below the second upper limit, thus suppressing the heater overheating that can occur when the heater is energized when the nip pressure is low.

[0008] Furthermore, the image forming apparatus may be equipped with a temperature sensor to detect the heater temperature, and the control unit may control the heater output based on the deviation between the target temperature and the temperature detected by the temperature sensor, increasing the heater output as the deviation increases.

[0009] Furthermore, the control unit may change the heater output by changing the duty cycle of the power supply to the heater, and if it determines that the nip pressure is the second nip pressure, it may set the duty cycle to be less than or equal to the second duty cycle upper limit, which is smaller than the first duty cycle upper limit, which is the upper limit when it is determined that the nip pressure is the first nip pressure.

[0010] With this configuration, when the nip pressure is low, the heater is not energized with a large duty cycle, so the heater output can be limited to below the second upper limit when the nip pressure is low.

[0011] Further, the image forming apparatus includes a temperature sensor that detects the temperature of the heater, wherein the control unit calculates an operation amount including a sum of a proportional term proportional to a deviation between a target temperature and a temperature detected by the temperature sensor, and a derivative term proportional to a derivative value of the deviation, and is configured to increase the duty ratio of energization to the heater as the absolute value of the operation amount increases; and when the nip pressure is the second nip pressure, the coefficient of the derivative term may be changed to make the absolute value of the operation amount smaller than that when the nip pressure is the first nip pressure.

[0012] Further, when receiving a print command, the control unit may determine whether the nip pressure is the first nip pressure or the second nip pressure.

[0013] Further, a plurality of the resistance heating elements are arranged side by side on the substrate in a sheet conveyance direction, and the length of the nip portion in the conveyance direction when the nip pressure is the second nip pressure may be smaller than the length from the upstream end of the resistance heating element located most upstream in the conveyance direction to the downstream end of the resistance heating element located most downstream in the conveyance direction.

[0014] With this configuration, when the nip pressure is low, heat is easily trapped in the heater, and by limiting the output of the heater to the second upper limit or less, excessive temperature rise of the heater can be suppressed.

[0015] Further, the image forming apparatus may include a sensor that detects information for determining whether the nip pressure is the first nip pressure or the second nip pressure. Effects of the Invention

[0016] According to the present invention, excessive temperature rise of the heater caused by energizing the heater when the nip pressure is low can be suppressed. Brief Description of the Drawings

[0017] [Figure 1] It is a diagram showing a configuration of a laser printer according to an embodiment. [Figure 2] It is a cross-sectional view (a) showing a fixing device, and an enlarged cross-sectional view (b) showing the periphery of the heater. [Figure 3] Figure (a) shows the side of the heater where the resistance heating element is arranged, Figure (b) shows the heater and heat conduction member as seen from the back of the heater, and Figure (c) shows a cross-sectional view of the heater etc. cut along the longitudinal direction. [Figure 4] (a) is a cross-sectional view showing the nip pressure changing mechanism when the nip pressure is the first nip pressure, and (b) is a cross-sectional view showing a magnified view of the heater area. [Figure 5] (a) is a cross-sectional view showing the nip pressure changing mechanism when the nip pressure is the second nip pressure, and (b) is a cross-sectional view showing a magnified view of the heater area. [Figure 6] This is a flowchart showing the operation of the control unit. [Figure 7] Figure (a) shows the first table, and Figure (b) shows the second table. [Figure 8] This flowchart shows the operation of the control unit in relation to the modified example. [Figure 9] Figure (a) shows the third table, and Figure (b) shows the fourth table. [Modes for carrying out the invention]

[0018] Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. As shown in Figure 1, a laser printer 100, which is an example of an image forming apparatus, comprises a main body housing 120, a supply unit 130, an exposure unit 140, a process cartridge 150, a fuser unit 1, and a control unit 500.

[0019] The main casing 120 includes a front cover 121 that opens and closes a first opening H1, a manual feed tray 122, a rear cover 123 that opens and closes a second opening H2, and an output tray 124. The first opening H1 is an opening through which a process cartridge 150 can pass. The manual feed tray 122 is a tray used when printing on sheets S, such as thick paper like postcards, transported along a straight transport path. In the following description, printing performed along a straight transport path will also be referred to as "straight printing".

[0020] The second opening H2 is an opening through which the sheet S discharged from the fuser 1 passes when straight printing is performed. The rear cover 123 is in an open state when straight printing is performed and supports the sheet S discharged from the second opening H2. The discharge tray 124 is a tray for supporting the sheet S discharged from inside the main housing 120 when the rear cover 123 is closed.

[0021] The laser printer 100 is further equipped with a cover sensor SE1 that detects when the rear cover 123 is closed. The information detected by the cover sensor SE1 is output to the control unit 500.

[0022] The supply unit 130 is a mechanism that supplies sheets S to the photosensitive drum 151, which will be described later. The supply unit 130 comprises a supply tray 131 for holding the sheets S, a pressing plate 132, and a supply mechanism 133. The sheets S in the supply tray 131 are pushed upward by the pressing plate 132 and separated one by one by the supply mechanism 133 and supplied to the process cartridge 150.

[0023] The exposure apparatus 140 includes a laser light source (not shown), a polygon mirror, lenses, a reflector, and the like. The exposure apparatus 140 exposes the surface of the photosensitive drum 151 with laser light based on image data emitted from the laser light source.

[0024] The process cartridge 150 is detachable from the main body housing 120 through the first opening H1. The process cartridge 150 includes a photosensitive drum 151, a charger 152, a developing roller 153, and a transfer roller 154.

[0025] The charger 152 charges the surface of the photosensitive drum 151. The exposure device 140 exposes the charged surface of the photosensitive drum 151 to form an electrostatic latent image on the surface of the photosensitive drum 151.

[0026] The developing roller 153 supplies toner from the process cartridge 150 to the electrostatic latent image on the photosensitive drum 151. This forms a toner image on the photosensitive drum 151. Subsequently, the sheet S supplied from the supply unit 130 passes between the photosensitive drum 151 and the transfer roller 154, transferring the toner image on the photosensitive drum 151 to the sheet S.

[0027] The fixing device 1 is a device that fixes the toner image onto the sheet S. The sheet S, on which the toner image has been fixed, is discharged onto the discharge tray 124 by the discharge roller 125.

[0028] As shown in Figure 2(a), the fixing device 1 comprises a heating unit 2, a pressure roller 3 as an example of a pressurized rotating body, and a frame 4.

[0029] The pressure roller 3 is a rotatable roller. The pressure roller 3 has a cylindrical shaft 3A and a cylindrical roller portion 3B. The shaft 3A is made of, for example, metal. The roller portion 3B is made of, for example, rubber. The roller portion 3B covers a part of the shaft 3A.

[0030] The heating unit 2 includes a heater 10, a holder 20, a heat conductive member 30 shown in Figure 2(b), a stay ST, a belt BL, and a temperature sensor SE2 shown in Figure 2(b). The heater 10 heats the belt BL, and heats the sheet S via the belt BL. The temperature sensor SE2 detects the temperature of the heater 10. The temperature sensor SE2 outputs the detected temperature to the control unit 500. The temperature sensor SE2 is in contact with the heat conductive member 30.

[0031] As shown in Figure 2(b), the heater 10 forms a nip portion NP by sandwiching the belt BL between itself and the pressure roller 3. The heater 10 has a substrate 11, a resistance heating element 12 placed on the substrate 11, and a cover 13. The substrate 11 is made of an elongated rectangular plate of ceramic made of aluminum oxide. The heater 10 is a so-called ceramic heater. The resistance heating element 12 is formed on one side of the substrate 11 by printing. As shown in Figure 3(a), in this embodiment, two resistance heating elements 12 are provided. The two resistance heating elements 12 are each elongated in the longitudinal direction of the heater 10 and are arranged parallel to each other in the short direction perpendicular to the longitudinal direction. In other words, the two resistance heating elements 12 are arranged on the substrate 11 side by side in the conveying direction of the sheet S. In the following description, the conveying direction of the sheet S at the nip portion NP will also be simply referred to as the "conveying direction". A conductor 19A is connected to one end 12A of each resistive heating element 12, and power supply terminals 18 for supplying power to the resistive heating elements 12 are provided at each end of the conductor 19A.

[0032] The power supply terminal 18 is electrically connected to the resistive heating element 12 via a conductor 19A. The power supply terminal 18 is located at one end 11E in the longitudinal direction of the circuit board 11. As shown in Figure 3(c), a connector C that supplies electricity to the heater 10 is connected to the power supply terminal 18. The connector C is detachable from one end in the longitudinal direction of the heater 10. Electricity is supplied to the power supply terminal 18 from the connector C. For convenience, the resistive heating element 12, cover 13, and belt BL are not shown in Figure 3(c).

[0033] Furthermore, the other ends 12B of each resistance heating element 12 are connected to each other by a conductor 19B. The number of resistance heating elements 12 is not particularly limited. In addition, the heat distribution in the longitudinal direction may be adjusted by providing resistance heating elements in which the heat generation in the longitudinal center is greater than that in the longitudinal ends, and resistance heating elements in which the heat generation in the longitudinal ends is greater than that in the longitudinal center, and by individually controlling each resistance heating element.

[0034] As shown in Figure 2(b), the cover 13 covers the resistance heating element 12. The cover 13 is made of, for example, glass.

[0035] As shown in Figure 2(a), the holder 20 supports the heater 10 and also guides the belt BL. The holder 20 is made of, for example, resin.

[0036] The stay ST supports the holder 20. The stay ST is made of, for example, metal.

[0037] The belt BL is endless and made of metal or resin. The belt BL rotates around the heater 10 while being guided by the holder 20. The belt BL has an outer circumferential surface and an inner circumferential surface. The outer circumferential surface is in contact with the pressure roller 3 or the sheet S to be heated. The inner circumferential surface is in contact with the heater 10.

[0038] The heat conduction member 30 is a member that conducts heat in the longitudinal direction of the heater 10 to equalize the temperature of the heater 10 in the longitudinal direction. The heat conduction member 30 is a plate-shaped member that is located between the heater 10 and the holder 20 and is in contact with the other surface of the substrate 11. When the heating unit 2 sandwiches the sheet S between the pressure roller 3, the heat conduction member 30 is sandwiched between the heater 10 and the holder 20. The heat conduction member 30 is made of, for example, aluminum.

[0039] As shown in Figures 3(a) and 3(b), one end 12A and the other end 12B of the resistance heating element 12 are located outside the maximum width W1 of the sheet usable by the heating unit 2 in the longitudinal direction, and inside the one end 30A and the other end 30B of the heat conductive member 30. In other words, in the longitudinal direction, the length of the heat conductive member 30 is longer than the length of the resistance heating element 12.

[0040] In the longitudinal direction, the length of the substrate 11 is longer than the length of the heat conductive member 30. One end 30A of the heat conductive member 30 is located inward in the longitudinal direction than one end 11A of the substrate 11. The other end 30B of the heat conductive member 30 is located outward in the longitudinal direction than the other end 11B of the substrate 11.

[0041] As shown in Figure 4(a), the fixing device 1 further includes a nip pressure changing mechanism NM. The nip pressure changing mechanism NM is a mechanism that changes the nip pressure at the nip section NP between a first nip pressure and a second nip pressure that is smaller than the first nip pressure. The nip pressure changing mechanism NM includes a shaft SF, a pressure arm 60, a pressure spring 70, and a cam 80. The frame 4 supports the pressure spring 70 and also rotatably supports the pressure arm 60 and the cam 80.

[0042] The pressure arm 60, pressure spring 70, and cam 80 are positioned at one end and the other end of the frame 4 in the axial direction of the pressure roller 3. In the following description, "axial direction of the pressure roller 3" will also be simply referred to as "axial direction." Since the structure of one side of the nip pressure changing mechanism NM in the axial direction and the structure of the other side are substantially the same, the following description will mainly refer to the structure of the one side in the axial direction.

[0043] The shaft SF extends in the axial direction. The shaft SF is made of, for example, metal. Cams 80 are fixed to each axial end of the shaft SF. The cams 80 rotate as the shaft SF rotates. The frame 4 rotatably supports the shaft SF.

[0044] The pressurizing arm 60 is an arm that presses the heating unit 2 toward the pressurizing roller 3. The frame 4 rotatably supports the pressurizing arm 60.

[0045] The pressure spring 70 is a tension coil spring that biases the pressure arm 60 toward the pressure roller 3. One end of the pressure spring 70 is attached to the pressure arm 60, and the other end is attached to the frame 4.

[0046] The cam 80 is capable of pressing the pressurizing arm 60 against the biasing force of the pressurizing spring 70. Specifically, the cam 80 is rotatable between a first position shown in Figure 4(a) and a second position shown in Figure 5(a). The cam 80 rotates when a driving force is supplied from a motor (not shown).

[0047] When cam 80 is in the first position, the nip pressure is the first nip pressure. When cam 80 is in the second position, the nip pressure is the second nip pressure, which is smaller than the first nip pressure.

[0048] As shown in Figure 4(b), the nip width Ln, which is the length of the nip section NP in the conveying direction, is greater than the length Lr from the upstream end E1 of the resistance heating element 12 located furthest upstream in the conveying direction to the downstream end E2 of the resistance heating element 12 located furthest downstream in the conveying direction, when the nip pressure is the first nip pressure. The two resistance heating elements 12 are located within the range of the nip section NP in the conveying direction.

[0049] As shown in Figure 5(b), when the nip pressure is the second nip pressure, the nip width Ln is smaller than the length Lr from the upstream end E1 of the resistance heating element 12 located furthest upstream in the conveying direction to the downstream end E2 of the resistance heating element 12 located furthest downstream in the conveying direction. The nip portion NP is located within the range from the upstream end E1 to the downstream end E2 in the conveying direction.

[0050] The heating unit 2 has side guides SG at each axial end. The side guides SG support the axial ends of the stay ST. The side guides SG are movably supported on the frame 4. The pressure arm 60 presses the side guides SG toward the pressure roller 3.

[0051] The control unit 500 includes a CPU, ROM, RAM, non-volatile memory, etc., and is configured to perform various controls based on a pre-prepared program. The control unit 500 has a function to determine whether or not the rear cover 123 is closed based on information from the cover sensor SE1. If the control unit 500 determines that the rear cover 123 is closed, it controls the motor for rotating the cam 80, thereby setting the nip pressure to the first nip pressure using the nip pressure changing mechanism NM. If the control unit 500 determines that the rear cover 123 is not closed, it sets the nip pressure to the second nip pressure using the nip pressure changing mechanism NM.

[0052] The control unit 500 has a function to determine whether the nip pressure is the first nip pressure or the second nip pressure when it receives a print command. Specifically, when the control unit 500 receives a print command, it determines whether the rear cover 123 is closed or not based on information from the cover sensor SE1. If the control unit 500 determines that the rear cover 123 is closed, it determines that the nip pressure is the first nip pressure. If the control unit 500 determines that the rear cover 123 is not closed, it determines that the nip pressure is the second nip pressure. In this embodiment, the cover sensor SE1 corresponds to a sensor that detects information for determining whether the nip pressure is the first nip pressure or the second nip pressure.

[0053] The control unit 500 has the function of controlling the output of the heater 10 based on the deviation ΔT between the target temperature Tt and the temperature T detected by the temperature sensor SE2. The deviation ΔT is calculated, for example, by subtracting the detected temperature T from the target temperature Tt. The control unit 500 increases the output of the heater 10 as the deviation ΔT increases.

[0054] The control unit 500 has a function to change the output of the heater 10 by changing the duty cycle of the power supply to the heater 10. When the control unit 500 determines that the nip pressure is the first nip pressure, it determines the duty cycle based on the first table shown in Figure 7(a) and the deviation ΔT. The first duty cycle upper limit, which is the upper limit of the duty cycle in the first table, is set to 100%.

[0055] When the control unit 500 determines that the nip pressure is the second nip pressure, it determines the duty cycle based on the second table shown in Figure 7(b) and the deviation ΔT. The upper limit of the second duty cycle, which is the upper limit of the duty cycle in the second table, is 57%.

[0056] In other words, when the control unit 500 determines that the nip pressure is the second nip pressure, it sets the duty cycle so that it is less than or equal to the second duty cycle upper limit (57%), which is smaller than the first duty cycle upper limit (100%). By setting the duty cycle in this way, when the control unit 500 determines that the nip pressure is the second nip pressure, it controls the output of the heater 10 so that it is less than or equal to the second upper limit, which is smaller than the first upper limit, which is the upper limit when the nip pressure is determined to be the first nip pressure.

[0057] Next, the operation of the control unit 500 will be described in detail. As shown in Figure 6, the control unit 500 determines whether or not there is a print command (S1). If it is determined in step S1 that there is no print command (No), the control unit 500 terminates this process.

[0058] If it is determined in step S1 that there is a print command (Yes), the control unit 500 determines whether the nip pressure is the first nip pressure based on the information from the cover sensor SE1 (S2). If it is determined in step S2 that the nip pressure is the first nip pressure (Yes), the control unit 500 selects the first table shown in Figure 7(a) (S3).

[0059] If it is determined in step S2 that the nip pressure is not the first nip pressure (No), then the control unit 500 selects the second table shown in Figure 7(b) because the nip pressure is the second nip pressure (S4). After step S3 or step S4, the control unit 500 obtains the temperature of the heater 10 from the temperature sensor SE2 (S5).

[0060] After step S5, the control unit 500 calculates the deviation ΔT between the target temperature Tt and the temperature T detected by the temperature sensor SE2 (S6). After step S6, the control unit 500 sets the duty cycle based on the table selected in step S3 or step S4 and the deviation ΔT, and controls the power supply to the heater 10 with the set duty cycle (S7).

[0061] After step S7, the control unit 500 determines whether or not printing has finished (S8). If it is determined in step S8 that printing has not finished (No), the control unit 500 returns to the process of step S5.

[0062] If it is determined in step S8 that printing has finished (Yes), the control unit 500 stops supplying power to the heater 10 (S9) and terminates this process.

[0063] Next, a specific example of the operation of the control unit 500 will be described. When the user opens the rear cover 123 shown in Figure 1 to perform straight printing, the control unit 500 controls the nip pressure changing mechanism NM shown in Figure 4(a) based on information from the cover sensor SE1, thereby changing the nip pressure from the first nip pressure to the second nip pressure.

[0064] When the control unit 500 receives a print command while the nip pressure is at the second nip pressure, it proceeds with processing in the order of steps S1:Yes → S2:No → S4~S7. As a result, the output of the heater 10 is limited to the second upper limit (57%) or less.

[0065] Here, as shown in Figure 5(b), when the nip pressure is the second nip pressure, the nip width Ln is small, so the amount of heat transferred from the heater 10 to the pressure roller 3 via the belt BL is small, and heat tends to accumulate in the heater 10. In particular, as in this embodiment, when a part of the resistance heating element 12 extends outside the range of the nip portion NP, heat tends to accumulate in the heater 10. However, as mentioned above, when the nip pressure is the second nip pressure, the output of the heater 10 is limited to below the second upper limit, so heat accumulation in the heater 10 is suppressed.

[0066] As described above, the following effects can be obtained according to this embodiment. When the nip pressure is low, the output of the heater 10 is limited to below the second upper limit, thereby suppressing the overheating of the heater 10 that can occur when the heater 10 is energized when the nip pressure is low.

[0067] When the nip pressure is low, power is not supplied with a large duty ratio, so that the output of the heater 10 can be limited to be equal to or less than the second upper limit when the nip pressure is low.

[0068] Note that the present invention is not limited to the above embodiment, and can be used in various forms as exemplified below. In the following description, the same reference signs are assigned to configurations substantially similar to those in the above embodiment, and the description thereof is omitted.

[0069] In the above embodiment, the duty ratio is set based on the deviation ΔT, but the present invention is not limited thereto. For example, the duty ratio may be set based on a manipulated variable U including the sum of a proportional term proportional to the deviation ΔT and a derivative term proportional to the derivative value of the deviation. Specifically, the control unit 500 calculates the manipulated variable U by the following formula (1). U n =K p ΔT n +K d D n ···(1) D n =(T n -T n-1 ) / ts ts: control cycle (time)

[0070] Here, K p is a proportional gain as a preset fixed value. K d is a derivative gain, which is a value selected from K1 and K2 in accordance with the nip pressure as described later. D is the derivative value of the deviation ΔT. The subscript n attached to each variable indicates that the variable is the current value, and n-1 indicates that it is the previous value.

[0071] K p ΔT n is the proportional term, and K d D n is the derivative term. The positive / negative relationship between the proportional term and the derivative term is set such that they cancel each other out in the process where the detected temperature T rises toward the target temperature Tt. In the following specific example, K p is positive, K d is negative, ΔTn and D n is set to be positive in the process where the detected temperature T increases toward the target temperature Tt.

[0072] Note that, contrary to the following specific example, K p may be negative, and K d may be positive. Further, when ΔT n and D n are set to be negative in the process where the detected temperature T increases toward the target temperature Tt, for example, even in the case where ΔT=T n-1 -T n , it is only necessary to reverse the positive / negative relationship between K p and K d .

[0073] The control unit 500 is configured to increase the duty ratio of energization to the heater 100 as the absolute value of the operation amount U increases. When the nip pressure is the second nip pressure, the control unit 500 is configured to reduce the absolute value of the operation amount U by changing the coefficient K of the differential term d compared to when the nip pressure is the first nip pressure.

[0074] Specifically, when the control unit 500 determines that the nip pressure is the first nip pressure, the coefficient K d is set to -K1. Further, when the control unit 500 determines that the nip pressure is the second nip pressure, the coefficient K d is set to -K2. K1 and K2 are positive values, and satisfy the relationship K1 < K2.

[0075] When the control unit 500 determines that the nip pressure is the first nip pressure, the control unit 500 determines the duty ratio based on the third table shown in FIG. 9(a) and the operation amount U. The first duty upper limit value, which is the upper limit value of the duty ratio in the third table, is 100%.

[0076] When the control unit 500 determines that the nip pressure is the second nip pressure, it determines the duty cycle based on the fourth table shown in Figure 9(b) and the manipulated variable U. The second duty cycle upper limit, which is the upper limit of the duty cycle in the fourth table, is 57%.

[0077] The control unit 500 in this configuration performs the process shown in Figure 8. Since the process shown in Figure 8 is a modified version of the process shown in Figure 6, the same reference numerals are used for the same processes as in Figure 6, and their explanations are omitted.

[0078] As shown in Figure 8, if the control unit 500 determines in step S2 that the nip pressure is the first nip pressure (Yes), it selects the third table shown in Figure 9(a) (S21). After step S21, the control unit 500 determines the coefficient K of the differential term. d Set to -K1 (S22).

[0079] If in step S2 it is determined that the nip pressure is not the first nip pressure (No), the control unit 500 selects the fourth table shown in Figure 9(b) (S23). After step S23, the control unit 500 determines the coefficient K of the differential term. d Set to -K2 (S24). In other words, when the nip pressure is the second nip pressure, the control unit 500 sets the coefficient K of the derivative term. d The absolute value of is made larger than when the nip pressure is the first nip pressure.

[0080] After step S22 or step S24, the control unit 500 executes the process of step S5. After step S6, the control unit 500 calculates the manipulated variable U using equation (1) (S25). After step S25, the control unit 500 sets the duty cycle based on the table selected in step S22 or step S24 and the manipulated variable U, and controls the energization of the heater 10 with the set duty cycle (S26). After step S26, the control unit 500 executes the process of step S8.

[0081] In this configuration as well, when the nip pressure is low, the output of the heater 10 is limited to below the second upper limit, so it is possible to suppress the overheating of the heater 10 that occurs when the heater 10 is energized when the nip pressure is low.

[0082] In the above embodiment, a pressure roller 3 was exemplified as the pressure rotating body, but the present invention is not limited thereto. If the structure on the pressure side includes an endless pressure belt and a pad that sandwiches the pressure belt between the heating unit, the pressure rotating body may be the pressure belt.

[0083] The nip pressure changing mechanism is not limited to the structure of the embodiment described above. The nip pressure changing mechanism may, for example, be able to switch the nip pressure in three or more stages. The nip pressure changing mechanism may have a link mechanism that rotates the cam 80 between a first position and a second position in conjunction with the opening and closing operation of the rear cover 123 that opens and closes the second opening H2. In this case, when the rear cover 123 is closed, the cam 80 is rotated to the first position via the link mechanism, and when the rear cover 123 is open, the cam 80 is rotated to the second position via the link mechanism.

[0084] The sensor that detects information to determine whether the nip pressure is the first nip pressure or the second nip pressure is not limited to the cover sensor SE1, but may also be, for example, a sensor that detects the position of the heating unit, or a sensor that detects the attachment or detachment of the process cartridge 150 to the main housing 120. If the sensor is one that detects the attachment or detachment of the process cartridge 150 to the main housing 120, the nip pressure changing mechanism can rotate the cam 80 between the first position and the second position in conjunction with the attachment or detachment of the process cartridge 150 to the main housing 120.

[0085] In the above embodiment, the present invention was applied to a laser printer 100, but the present invention is not limited thereto and may be applied to other image forming devices, such as copiers and multifunction printers.

[0086] The elements described in the above embodiments and modifications may be implemented in any combination. [Explanation of Symbols]

[0087] 3. Pressure roller 10 Heaters 11 circuit boards 12 Resistive heating element 100 laser printers 500 Control Unit BL belt NM Nip Pressure Adjustment Mechanism NP Nipple

Claims

1. A heater having a circuit board and a resistive heating element, An endless belt that rotates around the heater, A pressurized rotating body that forms a nip portion by sandwiching the belt between itself and the heater, A nip pressure changing mechanism that changes the nip pressure in the nip section to a first nip pressure and a second nip pressure smaller than the first nip pressure, A temperature sensor that detects the heater temperature, It comprises a control unit and, The control unit, The output of the heater is controlled based on the deviation between the target temperature and the temperature detected by the temperature sensor, and the larger the deviation, the greater the output of the heater. Determine whether the nip pressure is the first nip pressure or the second nip pressure. If it is determined that the nip pressure is the first nip pressure, the output of the heater is controlled to be less than or equal to the first upper limit. If it is determined that the nip pressure is the second nip pressure, the output of the heater is controlled to be less than or equal to the second upper limit. The image forming apparatus is characterized in that the second upper limit is smaller than the first upper limit.

2. The control unit, By changing the duty cycle of the power supply to the heater, the output of the heater is changed. The image forming apparatus according to claim 1, characterized in that, when it is determined that the nip pressure is the second nip pressure, the duty cycle is set to be less than or equal to the second duty cycle upper limit, which is smaller than the first duty cycle upper limit, which is the upper limit when it is determined that the nip pressure is the first nip pressure.

3. The control unit is The system is configured to calculate a manipulated variable that includes the sum of a proportional term proportional to the deviation between the target temperature and the temperature detected by the temperature sensor, and a differential term proportional to the differential value of the deviation, and to increase the duty cycle of supplying power to the heater as the absolute value of the manipulated variable increases. The image forming apparatus according to claim 1, characterized in that, when the nip pressure is the second nip pressure, the absolute value of the manipulated variable is made smaller than when the nip pressure is the first nip pressure by changing the coefficient of the derivative term.

4. The image forming apparatus according to claim 1, characterized in that the control unit determines whether the nip pressure is the first nip pressure or the second nip pressure when it receives a print command.

5. The resistive heating elements are arranged in a plurality in the direction of sheet transport on the substrate. The image forming apparatus according to claim 1, characterized in that when the nip pressure is the second nip pressure, the length of the nip portion in the transport direction is smaller than the length from the upstream end of the resistance heating element located furthest upstream in the transport direction to the downstream end of the resistance heating element located furthest downstream in the transport direction.

6. The image forming apparatus according to any one of claims 1 to 5, further comprising a sensor for detecting information for determining whether the nip pressure is the first nip pressure or the second nip pressure.

7. A heater having a substrate and a resistive heating element, An endless belt that rotates around the heater, A pressurized rotating body that forms a nip portion by sandwiching the belt between itself and the heater, A nip pressure changing mechanism that changes the nip pressure in the nip section to a first nip pressure and a second nip pressure smaller than the first nip pressure, The main body housing has an opening through which the sheet discharged from the nip portion passes, A rear cover that opens and closes the aforementioned opening, A cover sensor that detects the opening and closing of the rear cover, It comprises a control unit and, The control unit, If it is determined that the rear cover is closed based on the information from the cover sensor, the nip pressure changing mechanism sets the nip pressure to the first nip pressure and controls the output of the heater to be less than or equal to the first upper limit value. If it is determined that the rear cover is not closed based on the information from the cover sensor, the nip pressure changing mechanism sets the nip pressure to the second nip pressure and controls the heater output to be below the second upper limit. The image forming apparatus is characterized in that the second upper limit is smaller than the first upper limit.

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