Inkjet recording device

Abstract

The number of heaters used to heat the belt is reduced, allowing each heater to operate at a suitable temperature. [Solution] The total duty cycle is calculated according to the temperature difference between the belt temperature detected by the belt temperature sensor and the target temperature (S2), and the heaters to be used for heating are determined from among the multiple heaters based on the total duty cycle (S3, S5, S7). The smaller the total duty cycle, i.e., the smaller the temperature difference between the belt temperature and the target temperature, the fewer heaters are used for heating (S4, S6, S7). A PWM signal with a duty cycle corresponding to the total duty cycle is generated for the heaters to be used for heating, and a PWM signal with a duty cycle of "0%" is generated for the heaters that are not used for heating, and these are output individually to the corresponding FETs. By outputting a PWM signal with a duty cycle of "0%", the FETs are not turned on and off according to the PWM signal, which can suppress the shortening of the heater's lifespan.

Description

Technical Field

[0001] The present invention relates to an inkjet recording apparatus that forms an image on a sheet with ink.

Background Art

[0002] In an inkjet recording apparatus that forms an image on a sheet with ink, a method has been proposed in which heat and pressure are applied to the sheet using a pair of belts that abut against each other to fix the ink to the sheet. By using a pair of belts, it is possible to secure the nip length of the fixing nip portion that sandwiches and conveys the sheet and fixes the ink to the sheet, thereby improving the fixing property of the ink to the sheet.

[0003] In an inkjet recording apparatus, in order to raise the belt to the target temperature as quickly as possible, a belt with a low heat capacity is directly heated by a plurality of halogen heaters. The temperatures of these heaters are appropriately adjusted according to the detection results of a temperature sensor that detects the belt temperature. Conventionally, as in the apparatus described in Patent Document 1, the temperature of the heater is adjusted by rapidly turning on and off the switching element of the power supply circuit that supplies the voltage for operating the heater according to a PWM (Pulse Width Modulation) signal (PWM control). The heater temperature fluctuates up and down according to the duty ratio of the PWM signal.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In standby mode, when the image forming operation can be started immediately on the sheet, and when forming images on sheets with a low basis weight (e.g., thin paper), the target temperature of the belt is set lower than during normal image forming, and the heater is adjusted to a lower temperature. Also, when the belt temperature approaches the target temperature, the heater is adjusted to a lower temperature. Conventionally, in order to lower the heater temperature, the temperature of all multiple heaters is adjusted according to a PWM signal with a small duty cycle. However, since all multiple heaters are kept heated at a temperature lower than the temperature suitable for use, there was a tendency for the heater life to be shortened.

[0006] In view of the above problems, the present invention aims to provide an inkjet recording apparatus that uses a belt to fix ink to a sheet, and reduces the number of heaters used from among the multiple heaters that heat the belt, thereby enabling each heater to operate at a temperature suitable for use. [Means for solving the problem]

[0007] An inkjet recording apparatus according to one embodiment of the present invention comprises: an image forming unit that ejects ink to form an image on a sheet; an endless first belt; an endless second belt that contacts the first belt and forms a nip portion that grips, transports, and heats the sheet on which the image has been formed by the image forming unit; a plurality of heating elements arranged on the inner circumference side of the first belt along the sheet transport direction and heating the first belt; a power supply circuit that supplies power to the plurality of heating elements; a temperature detection unit that detects the temperature of the first belt; and a control unit that generates a plurality of PWM signals for each of the plurality of heating elements to control the power supply circuit by pulse width modulation so that the temperature of the first belt detected by the temperature detection unit becomes a predetermined target temperature, and can individually control the power supplied from the power supply circuit to the plurality of heating elements by the generated plurality of PWM signals, wherein the control unit generates heat from two or more of the heating elements when the temperature difference between the temperature of the first belt and the target temperature is greater than or equal to a threshold, and generates heat from fewer of the heating elements when the temperature difference is less than the threshold than the number of heating elements when the temperature difference is greater than or equal to the threshold. [Effects of the Invention]

[0008] According to the present invention, in a configuration in which ink is fixed to a sheet using a belt, by reducing the number of heating elements that generate heat among the multiple heating elements that heat the belt, each heating element can be operated at a temperature suitable for use. [Brief explanation of the drawing]

[0009] [Figure 1] A schematic diagram showing the inkjet recording apparatus of this embodiment. [Figure 2] A schematic diagram showing the fixing module. [Figure 3] A schematic diagram showing the heating section. [Figure 4] (a) A diagram showing the heater temperature sensor, (b) A diagram explaining the field of view of the heater temperature sensor, (c) A graph explaining the relationship between temperature measurement accuracy and field of view. [Figure 5] (a) A diagram illustrating the heating intensity of the heater at a position in the belt width direction, (b) A graph showing the change in belt temperature over time. [Figure 6] (a) Block diagram showing the heater temperature control system, (b) Block diagram showing the power supply circuit that supplies power to the lower heater. [Figure 7] A flowchart showing the heater control process of the first embodiment. [Figure 8] A flowchart illustrating a conventional heater control process. [Figure 9] A graph showing the time variation of belt temperature and the duty cycle of the PWM signals output to each heater in a conventional example. [Figure 10] A graph showing the time variation of the belt temperature and the duty cycle of the PWM signals output to each heater in this embodiment. [Figure 11] A flowchart showing the heater control process of the second embodiment. [Modes for carrying out the invention]

[0010] [First Embodiment] <Inkjet recording device> Embodiments of the present invention will now be described in detail with reference to the drawings. Figure 1 is a schematic diagram showing an inkjet recording apparatus of this embodiment. The inkjet recording apparatus 1 shown in Figure 1 is a so-called sheet-fed inkjet recording apparatus that forms an ink image on a sheet S using two liquids, a reaction solution and ink. The sheet S can be any recording material that can accept ink, such as paper like plain paper or cardboard, plastic film such as an overhead projector sheet, specially shaped recording material such as an envelope or index paper, or cloth.

[0011] As shown in Figure 1, the inkjet recording device 1 comprises a paper feed module 1000, a print module 2000, a drying module 3000, a fixing module 4000, a cooling module 5000, a reversing module 6000, and a loading module 7000. The sheets S supplied from the paper feed module 1000 undergo various processes as they are transported along the transport path within each module, and are finally discharged into the loading module 7000.

[0012] The paper feed module 1000, print module 2000, drying module 3000, fuser module 4000, cooling module 5000, inversion module 6000, and stacking module 7000 may each have a separate housing, and these housings may be connected to constitute the inkjet recording device 1. Alternatively, the paper feed module 1000, print module 2000, drying module 3000, fuser module 4000, cooling module 5000, inversion module 6000, and stacking module 7000 may be arranged in a single housing.

[0013] The paper feeding module 1000 has storage bins 1500a, 1500b, and 1500c for storing the sheet S, and the storage bins 1500a to 1500c are provided so as to be pullable out toward the front side of the apparatus for storing the sheet S. The sheet S is fed one by one by a separation belt and a conveyance roller in each of the storage bins 1500a to 1500c and conveyed to the printing module 2000. Note that the number of the storage bins 1500a to 1500c is not limited to three, and there may be one, two, four or more.

[0014] The printing module 2000 as an image forming unit has a pre-image registration correction unit (not shown), a print belt unit 2010, and a recording unit 2020. The sheet S conveyed from the paper feeding module 1000 is corrected in inclination and position by the pre-image registration correction unit and conveyed to the print belt unit 2010. The recording unit 2020 is disposed at a position facing the print belt unit 2010 with respect to the conveyance path. The recording unit 2020 forms an image by discharging ink onto the sheet S from above by a recording head with respect to the conveyed sheet S. A plurality of recording heads for discharging ink are arranged along the conveyance direction of the sheet S. In the present embodiment, in addition to four colors of Y (yellow), M (magenta), C (cyan), and Bk (black), there are a total of five line type recording heads corresponding to the reaction liquid. The sheet S is adsorbed and conveyed by the print belt unit 2010, thereby ensuring a clearance with the recording head.

[0015] Note that the number of ink colors and the number of recording heads are not limited to the five described above. The inkjet method can adopt a method using a heating element, a method using a piezo element, a method using an electrostatic element, a method using a MEMS element, and the like. Each color of ink is supplied from an ink tank (not shown) to the recording head via an ink tube. The ink contains a resin component of "0.1 mass% to 20.0 mass%", water, a water-soluble organic solvent, a colorant, a wax, an additive, etc. based on the total mass of the ink.

[0016] When the sheet S on which an image is formed by the recording unit 2020 is conveyed by the print belt unit 2010, deviation and color density of the image formed on the sheet S are detected by an in-line scanner (not shown) disposed on the downstream side of the recording unit 2020 in the sheet conveyance direction. Based on the deviation and color density of this image, correction of the image, density, etc. to be formed on the sheet S is performed.

[0017] The drying module 3000 includes a decoupling unit 3200, a drying belt unit 3300, and a hot air blowing unit 3400. The drying module 3000 reduces the liquid components of the ink and the reaction liquid applied to the sheet S in order to enhance the fixing property of the ink to the sheet S by the subsequent fixing module 4000. The sheet S on which an image is formed is conveyed to the decoupling unit 3200 disposed within the drying module 3000. In the decoupling unit 3200, a frictional force is generated between the sheet S and the belt by the wind pressure of the wind blown from above, and the sheet S is conveyed by the belt. In this way, by conveying the sheet S placed on the belt by the frictional force, displacement of the sheet S when the sheet S is conveyed across the print belt unit 2010 and the decoupling unit 3200 is prevented. The sheet S conveyed from the decoupling unit 3200 is adsorbed and conveyed by the drying belt unit 3300, and the ink and the reaction liquid applied to the sheet S are dried by hot air being blown from the hot air blowing unit 3400 disposed above the belt.

[0018] In this way, the drying module 3000 heats the ink and reaction solution applied to the sheet S, promoting the evaporation of moisture, which suppresses the occurrence of so-called cockling, where the sheet S stretches locally and wrinkles due to the absorption of the ink applied to the sheet S. Any device capable of heating and drying can be used for the drying module 3000, but a hot air dryer or heater is preferred. As for the heater, heating by electric heating wire or infrared heater is preferred from the standpoint of safety and energy efficiency. In addition to the method of applying hot air, the drying method may also be configured by combining a method of irradiating the sheet surface with electromagnetic waves (ultraviolet or infrared rays, etc.) or a conduction heat transfer method by contact with a heating element.

[0019] The fixing module 4000 has a fixing belt unit 4100. The fixing belt unit 4100 receives the sheet S conveyed from the drying module 3000, fixes the ink to the sheet S by passing it between a heated upper belt unit and a lower belt unit, and then hands it over to the cooling module 5000. A detailed explanation of the fixing belt unit 4100 will be given later.

[0020] The cooling module 5000 has multiple cooling units 5100, which cool the high-temperature sheet S transported from the fixing module 4000. The cooling units 5100, for example, draw outside air into the cooling box with a fan to increase the pressure inside the cooling box, and then blow air out from the cooling box through nozzles under pressure onto the sheet S to cool it. The cooling units 5100 are positioned on both sides of the transport path of the sheet S, and cool both sides of the sheet S.

[0021] The cooling module 5000 is provided with a transport path switching unit 5002. The transport path switching unit 5002 switches the transport path of the sheet S depending on whether the sheet S is being transported to the inversion module 6000 or to the double-sided transport path for double-sided printing in which images are formed on both sides of the sheet S.

[0022] The inversion module 6000 has an inversion unit 6400. The inversion unit 6400 inverts the front and back sides of the conveyed sheet S, changing the orientation of the sheet S when it is discharged to the loading module 7000. The loading module 7000 has a top tray 7200 and a loading unit 7500, and loads the sheet S conveyed from the inversion module 6000.

[0023] During double-sided printing, the sheet S is transported by the transport path switching unit 5002 to the transport path below the cooling module 5000. The sheet S then passes through the double-sided transport path of the fuser module 4000, drying module 3000, print module 2000, and paper feed module 1000, and is returned to the print module 2000. The double-sided transport unit of the fuser module 4000 is provided with a reversal unit 4200 that reverses the front and back sides of the sheet S. Once returned to the print module 2000, an image is formed on the other side where no image has been formed, and the sheet S is discharged to the loading module 7000 via the drying module 3000, fuser module 4000, cooling module 5000, and reversal module 6000.

[0024] <Fuser Module> Next, the fixing module 4000 will be explained using Figure 2. As shown in Figure 2, the fixing module 4000 includes an upper belt unit 10 and a lower belt unit 20. The sheet S is held and transported by the upper belt unit 10 and the lower belt unit 20, and pressure and heat are applied during this process to fix the image formed by the ink onto the sheet S.

[0025] The upper belt unit 10 is positioned vertically above the lower belt unit 20. The upper belt unit 10 includes an upper belt 30 as the first belt, a plurality of tension rollers that rotatably tension the upper belt 30, heating units 117, 127, 137 that heat the upper belt 30, an upper belt temperature sensor 310 as a temperature detection unit (first temperature detection unit) that detects the temperature of the upper belt 30, and upper heater temperature sensors 210, 220, 230. On the other hand, the lower belt unit 20 includes a lower belt 40 as the second belt, a plurality of tension rollers that rotatably tension the lower belt 40, heating units 147, 157 that heat the lower belt 40, a lower belt temperature sensor 320 (second temperature detection unit) that detects the temperature of the lower belt 40, lower heater temperature sensors 240, 250, and a pad 423. The pad 423 is arranged to form a fixing nip portion N with the upper belt 30 via the lower belt 40.

[0026] The sheet S is held and conveyed by the fixing nip section N between the upper belt unit 10 and the lower belt unit 20. The pressure of the fixing nip section N is determined by the tension and thickness of the upper belt 30 and the curvature of the pad 423. If the pressure of the fixing nip section N is too high, the ink from the sheet S may adhere to the upper belt unit 10, and the ink may peel off the sheet S. Therefore, the pressure is preferably set to "1 Pa or more and 2000 Pa or less", and more preferably to "1 Pa or more and 200 Pa or less".

[0027] However, if the curvature of the pad 423 is made too large, the difference in the transport path between the front and back of the sheet S will increase, and there is a risk that the sheet S will rub against the upper belt 30 when passing through the fixing nip section N. Alternatively, if the curvature of the pad 423 is made too large, there is a risk that the sheet S will curl along the curved shape of the pad 423. To prevent these issues, it is desirable that the radius of curvature of the pad 423 be "5000 mm" or more. Furthermore, from the standpoint of manufacturing precision, it is desirable that the radius of curvature of the pad 423 be "100000 mm" or less. Therefore, in this embodiment, the tension of the upper belt 30 is set to "200 N", the thickness of the upper belt 30 to "0.3 mm", the radius of curvature of the pad 423 to "30000 mm", and the pressure at the fixing nip section N to "approximately 16 Pa".

[0028] This allows for uniform pressure to be applied to the sheet S as it passes through the fixing nip section N, even when a long fixing nip section N is formed in the sheet transport direction (arrow H direction). As a result, the contact time between the sheet S and the upper belt 30 is ensured while the temperature of the upper belt 30 is at the melting point of the wax contained in the ink or the boiling point of water, so the sheet S is sufficiently heated.

[0029] However, if a sufficiently heated sheet S continues to be transported through the fixing nip section N, there is a risk that ink may peel off the sheet S and adhere to the upper belt 30, or that the upper belt 30 and the sheet S may rub against each other, causing the image to become distorted. Therefore, it is necessary to limit the time that the sheet S passes through the fixing nip section N. For example, the time it takes from when the leading edge of the sheet S enters the entrance of the fixing nip section N until the trailing edge of the sheet S exits the exit of the fixing nip section N is preferably "0.5s to 4s". In this embodiment, as an example, a pad 423 with a length of "900mm" in the sheet transport direction is used, the transport speed of the sheet S is set to "700mm / s", and the time that the sheet S passes through the fixing nip section N is set to "approximately 1.3s".

[0030] Furthermore, since moisture is necessary for the ink to penetrate the sheet S, the upper belt 30 and lower belt 40 should be made of a material that does not allow moisture to pass through, so that moisture evaporated from the heated surface of the sheet S does not escape through the upper belt 30 or lower belt 40. In this embodiment, taking into account heat resistance, sliding properties, airtightness, and durability, endless belts with a thickness of "approximately 0.4 mm" made of a glass fiber base material with a PTFE (polytetrafluoroethylene) coating on the surface were used as the upper belt 30 and lower belt 40.

[0031] Of the multiple tension rollers provided on the upper belt unit 10 and the lower belt unit 20, one roller in each unit is a drive roller 610, 620 that drives the upper belt 30 and the lower belt 40, respectively. When the drive rollers 610, 620 are rotated by a drive motor (not shown), the upper belt 30 and the lower belt 40 are rotated by the frictional force between the roller surface and the inner surface of the belt.

[0032] The driven rollers 430 and 440 rotate in response to the rotation of the upper belt 30 and the lower belt 40. Rotation detection sensors 410 and 420 are positioned on the rotation axes of these driven rollers 430 and 440. The rotation detection sensors 410 and 420 are elements composed of magnets whose magnetic force switches in the direction of rotation of the driven rollers 430 and 440. By detecting the change in the N and S poles generated by the rotation using a Hall sensor (not shown), the rotation of the upper belt 30 and the lower belt 40 can be detected. In this embodiment, the rotation detection sensors are elements made of magnets, but a transmissive sensor that detects changes in light blocking and light transmission using a physical flag with an edge in the direction of rotation of the driven rollers 430 and 440 may also be used.

[0033] The upper belt unit 10 and the lower belt unit 20 each have heating sections 117, 127, 137 and heating sections 147, 157, each having multiple heaters. The heating sections 117, 127, and 137 of the upper belt unit 10 are located on the fixing nip section N side of the upper belt unit 10 and have heaters for heating the upper belt 30 from the inner circumference side. The heating sections 117, 127, and 137 efficiently transfer heat to the sheet S by directly heating the portion of the upper belt 30 corresponding to the fixing nip section N. In the heating sections 117, 127, and 137, the voltage supplied to the heaters is controlled according to the detection result of the upper belt temperature sensor 310, which detects the temperature of the upper belt 30, thereby adjusting the heater temperature so that the temperature of the upper belt 30 reaches a predetermined target temperature.

[0034] The heating sections 147 and 157 of the lower belt unit 20 are located on the side opposite to the fixing nip section N and have heaters for heating the lower belt 40 from the inner circumference. Since a pad 423 is provided on the fixing nip section N side of the lower belt unit 20, the heating sections 147 and 157 cannot be located on the fixing nip section N side to directly heat the portion of the lower belt 40 corresponding to the fixing nip section N. Therefore, the heating sections 147 and 157 are located on the side opposite to the fixing nip section N, along the lower belt 40, as close as possible to the entrance of the fixing nip section N, so that heat can be efficiently transferred to the sheet S via the lower belt 40. In the heating sections 147 and 157, the voltage supplied to the heater is controlled according to the detection result of the lower belt temperature sensor 320, which detects the temperature of the lower belt 40, thereby adjusting the heater temperature so that the temperature of the lower belt 40 reaches a predetermined target temperature.

[0035] The target temperatures for the upper belt 30 and the lower belt 40 are set to predetermined temperatures depending on whether the machine is in standby mode, during image formation on plain paper (normal operation), during image formation on a sheet S with a lower basis weight than plain paper (e.g., thin paper), or during image formation on a sheet with a higher basis weight than plain paper (e.g., thick paper). For example, during image formation on plain paper, the target temperature is set to the standard target temperature of "95°C". In contrast, when the machine is in standby mode and ready to immediately begin image formation on the sheet S, the target temperature is set to the same "95°C" as during image formation on plain paper. When forming an image on a sheet S with a lower basis weight than plain paper (e.g., thin paper), the target temperature is set to a lower temperature than during image formation on plain paper, ranging from "85°C to 95°C" depending on the basis weight. When forming an image on a sheet S with a higher basis weight than plain paper (e.g., thick paper), the target temperature is set to a higher temperature than during image formation on plain paper, ranging from "105°C or higher" depending on the basis weight.

[0036] Furthermore, when the rotation detection sensors 410 and 420 detect that the upper belt 30 and lower belt 40 have stopped rotating, the voltage supply to the heater is stopped, thereby stopping heating by the heating units 117, 127, 137 and 147, 157. This prevents localized heating of the upper belt 30 and lower belt 40 when they are stopped.

[0037] <Heating section> Next, the configuration of the heating sections (117, 127, 137, 147, 157) described above will be explained using Figure 3. Since the heating sections 117, 127, and 137 of the upper belt unit 10 and the heating sections 147 and 157 of the lower belt unit 20 have similar configurations, the heating section 117 of the upper belt unit 10 will be used as a representative example in the following explanation.

[0038] As shown in Figure 3, the heating section 117 has two heaters 110a and 110b and a reflector 115. The two heaters 110a and 110b are halogen heaters that emit infrared rays and generate heat when lit, and are formed in an elongated shape so as to extend in the belt width direction intersecting the sheet conveying direction (arrow H direction), with both ends supported by support parts not shown. It is preferable that the heaters 110a and 110b are halogen heaters with different maximum voltages, that is, different heating temperatures.

[0039] These heaters 110a and 110b are covered by a reflector 115. The reflector 115 reflects the heat (infrared radiation) emitted by the heaters 110a and 110b to heat the inner surface of the upper belt 30 directly below the heaters 110a and 110b. In order to efficiently heat the upper belt 30, the reflector 115 is made of, for example, a mirror-finished aluminum material and is formed into a parabolic shape having a part of a parabola with the reflector vertex 115a as its vertex. The parabolic shape formed from the reflector vertex 115a in the direction of the upper belt 30 extends to the end point of the reflector parabola 115c, and thereafter has a reflector straight section 115b that extends almost vertically toward the inner surface of the upper belt 30. When forming the reflector 115 into the parabolic shape described above, it may be approximated by forming it into a polygonal shape consisting of multiple line segments due to manufacturing constraints of the parts. The reflector straight section 115b is preferably made as short as possible and may not be provided at all. However, providing the reflector straight section 115b makes it easier to secure space for the upper heater temperature sensor 210, which will be described later.

[0040] Heaters 110a and 110b are both positioned closer to the upper belt 30 than the reflector focal point 115d (the focal point of the parabola) which is determined by the reflector apex 115a and the end point 115c of the reflector parabola. Furthermore, heaters 110a and 110b are positioned at different heights in the vertical direction, with the heater 110a, which has a higher maximum voltage and therefore a higher heating temperature, positioned lower and closer to the upper belt 30 than the heater 110b, which has a lower maximum voltage and therefore a lower heating temperature. By positioning heaters 110a and 110b closer to the upper belt 30 than the reflector focal point 115d in this way, it is possible to reduce the proportion of heat emitted from heaters 110a and 110b that is reflected by the reflector 115, thereby increasing the heating efficiency of the upper belt 30. Furthermore, by arranging the two heaters 110a and 110b with a difference in height in the vertical direction, it is possible to create a difference in the uneven distribution of heat when each heater 110a and 110b is lit individually. This suppresses localized heat concentration when both heaters 110a and 110b are lit simultaneously.

[0041] <Heater temperature sensor> Next, the heater temperature sensors (210-250) will be explained using Figures 4(a) to 4(c) with reference to Figure 3. Figure 4(a) shows the configuration of the heater temperature sensor, with a top view in the upper section and a side view in the lower section. Figure 4(b) is a diagram to explain the field of view of the heater temperature sensor. Figure 4(c) is a diagram to explain the relationship between temperature measurement accuracy and field of view. In the following explanation, to make it easier to understand, the upper heater temperature sensor 210 will be used as a representative example. The other heater temperature sensors (220, 230, 240, 250) are the same as the upper heater temperature sensor 210, so their explanation will be omitted.

[0042] As shown in Figure 3, the upper heater temperature sensor 210 is positioned near the outside of the reflector 115 because it needs to directly detect the temperature of the belt area of ​​the upper belt 30 that is heated by the heaters 110a and 110b. The upper heater temperature sensor 210 detects the temperature of the belt area heated by the heaters 110a and 110b. It is positioned as a safety sensor that stops heating by the heaters 110a and 110b when the temperature of the belt area detected by the upper heater temperature sensor 210 (referred to as the belt temperature) exceeds, for example, "200°C".

[0043] The above temperature of "200°C" is the upper limit temperature to prevent thermal deformation of the upper belt 30, and is preset according to the material of the upper belt 30. In other words, under normal circumstances, when the upper belt 30 is rotating normally, the temperature of the belt area is maintained at "approximately 130°C" or lower, so the upper heater temperature sensor 210 will not detect a temperature of "200°C" or higher. Suppose that the upper belt 30 stops rotating due to a malfunction or other reason while the heaters 110a and 110b are overheating, and the rotation detection sensor 420 fails to detect that the upper belt 30 has stopped rotating. In that case, the heaters 110a and 110b will continue to locally heat the same part of the upper belt 30 that has stopped rotating, causing that part to become hot. Therefore, by using the upper heater temperature sensor 210 to detect the temperature at that location (belt area), even if the upper belt 30 stops rotating unintentionally, the heaters 110a and 110b are emergency-stopped before the upper belt 30 becomes hot and deforms due to heat, thereby preventing the upper belt 30 from being damaged by heat deformation.

[0044] As shown in Figure 4(a), the upper heater temperature sensor 210 consists of a package 3801 with a sensor module (not shown) built inside, mounted on a substrate 3800. In this embodiment, the package 3801 is positioned at the very edge of the substrate 3800 among the components mounted on the substrate 3800. The side of the package 3801 opposite the substrate 3800 is the detection surface, and a detection window 3802 that allows infrared rays to pass through is formed on the detection surface. The upper heater temperature sensor 210 absorbs infrared rays emitted from the upper belt 30 of the object to be measured through the detection window 3802, and converts the energy of the absorbed infrared rays into an electrical signal by the sensor module, thereby enabling non-contact temperature detection. The upper heater temperature sensor 210 can then output the electrical signal converted by the sensor module from a connector 3806.

[0045] The detection window 3802 not only allows infrared rays to pass into the package 3801, but also acts as a lens. As shown in Figure 4(b), the upper heater temperature sensor 210 has a constant field of view 3804 and detects the temperature of the object to be measured 3803 within the field of view 3804 without contact. When the object to be measured 3003 is located on the center line 3805 of the field of view 3804, the temperature measurement accuracy is assumed to be "100%", as shown in Figure 4(c). Next, without changing the distance from the upper heater temperature sensor 210, the object to be measured 3803 is moved left or right from the center line 3805. In this embodiment, the field of view 304 is defined as the angle θ formed by the object to be measured 3803 and the center line 3805 when the temperature measurement accuracy decreases to "50%" due to the movement. Note that here, the field of view 304 is defined as the angle θ when the temperature measurement accuracy decreases to "50%", but this is just an example and is not limited to a temperature measurement accuracy of "50%". The field of view 304 may be, for example, the angle θ when the temperature measurement accuracy decreases to "40%", or the angle θ when the temperature measurement accuracy decreases to "60%".

[0046] <Heater> Next, the heater 110a (110b) will be explained using Figures 5(a) and 5(b). Figure 5(a) is a diagram for explaining the heating intensity of the heater 110a (110b) at a position in the belt width direction. The upper section shows the heater 110a (110b) and the upper belt 30 as viewed from the upstream side to the downstream side in the sheet conveying direction, and the lower section shows the heating intensity of the heater 110a (110b) at a position in the belt width direction. The belt width direction is the direction that intersects the sheet conveying direction and is indicated by the arrow X in the figure. As shown in Figure 5(a), in this embodiment, in order to suppress uneven heating in the belt width direction, the infrared rays emitted from the heater 110a (110b) are distributed so that the heating intensity is higher in the belt end regions 2501 and 2503 than in the belt central region 2502.

[0047] Figure 5(b) shows the change in belt temperature over time when the upper belt 30 is continuously heated by the heater 110a (110b). The horizontal axis represents time, and the vertical axis represents belt temperature. The solid line 2504 represents the temperature rise in the belt end regions 2501 and 2503, and the dashed line 2505 represents the temperature rise in the belt center region 2502. As described above, the infrared rays emitted from the heater 110a (110b) are distributed so that the heating intensity is higher in the belt end regions 2501 and 2503 than in the belt center region 2502. Therefore, the temperature rise in the belt end regions 2501 and 2503, shown by the solid line 2504, is steeper than that in the belt center region 2502, shown by the dashed line 2505. The dashed line (2506) shown in the figure represents the upper temperature limit (e.g., 200°C) to prevent thermal deformation of the upper belt 30. A threshold is set to emergency shut off the heaters 110a and 110b so that the upper belt 30 does not exceed the upper temperature limit of 2506°C.

[0048] <Heater temperature control system> Next, the temperature control system for controlling the temperatures of the upper heaters 110, 120, and 130 of the upper belt unit 10 and the lower heaters 140 and 150 of the lower belt unit 20 will be explained with reference to Figure 2 and using Figures 6(a) and 6(b). The inkjet recording apparatus 1 of this embodiment is equipped with a heater control unit 101 that can control the temperatures of the upper heaters 110, 120, and 130, which are heating elements (first heating elements), and the lower heaters 140 and 150, which are second heating elements.

[0049] In this embodiment, the upper belt unit 10 and the lower belt unit 20 differ in the number of heating elements, with 3 (117, 127, 137) and 2 (147, 157), respectively, and are otherwise similar in configuration except that the lower belt unit 20 has a pad 423. Therefore, to make the explanation easier to understand, unless otherwise specified, the upper belt unit 10 will be used as an example below.

[0050] As shown in Figure 6(a), the heater control unit 101 includes a power supply circuit 1010, a CPU (Central Processing Unit) 1100 that executes programs such as the "heater control processing" described later, a ROM (Read Only Memory) 1200 that stores these programs, and a RAM (Random Access Memory) 1201 that serves as a work area when executing programs. The power supply circuit 1010 includes a relay circuit 1400 and a plurality of FETs (Field Effect Transistors) 111, 112, and 113, each corresponding to the upper heaters 110, 120, and 130. The relay circuit 1400 rectifies the AC voltage supplied from the AC power supply 1300 and outputs it to the FETs 111, 112, and 113.

[0051] The CPU 1100, acting as the control unit, performs PWM (Pulse Width Modulation) control based on the detection result of the upper belt temperature sensor 310 in order to stabilize the temperature of the upper belt 30. FETs 111, 112, and 113 adjust the voltage supplied from the relay circuit 1400 to the upper heaters 110, 120, and 130 based on the PWM signal generated by the CPU 1100. The voltage supplied to the upper heaters 110, 120, and 130 is adjusted by switching the FETs 111, 112, and 113 on and off according to the PWM signal generated by the CPU 1100. Each of the FETs 111, 112, and 113 can output pulse-width modulated voltages to multiple upper heaters 110, 120, and 130 according to the duty cycle (= pulse width / period) of the PWM signal.

[0052] The CPU 1100 includes a voltage control unit 1101 that generates and outputs a PWM signal, a duty cycle calculation unit 1102 that calculates the duty cycle of the PWM signal, and a timing unit 1103. The duty cycle calculation unit 1102 calculates the duty cycle of the PWM signal sent to FETs 111, 112, and 113, respectively. The duty cycle calculation unit 1102 calculates the target duty cycle using PI control or other methods based on the temperature difference between the temperature of the upper belt 30 detected by the upper belt temperature sensor 310 and the target temperature of the upper belt 30. The larger the temperature difference between the temperature of the upper belt 30 and the target temperature of the upper belt 30, the larger the duty cycle calculated. The duty cycle calculation unit 1102 then calculates the total duty cycle by adding up the duty cycles of the PWM signals calculated for each upper heater 110, 120, and 130 (each heating element). This total duty cycle reflects the temperature difference between the temperature of the upper belt 30 and the target temperature; the larger the temperature difference between the upper belt 30 and the target temperature, the larger the total duty cycle.

[0053] Furthermore, since the duty cycles of the upper heaters 110, 120, and 130, calculated from the temperature difference between the temperature of the upper belt 30 and the target temperature of the upper belt 30, are each 100% or less, if there are three upper heaters 110, 120, and 130, the total duty cycle is a maximum of 300%. Also, in this embodiment, the duty cycle calculated from the temperature difference between the temperature of the upper belt 30 and the target temperature of the upper belt 30 is the same value for each of the upper heaters 110, 120, and 130.

[0054] The voltage control unit 1101 determines which heaters will be used for heating (to be lit) based on the total duty cycle calculated by the duty cycle calculation unit 1102. As will be explained in more detail later (see Figure 7), the voltage control unit 1101 generates and outputs individual PWM signals for the heaters selected for heating from among the upper heaters 110, 120, and 130, with duty cycles determined according to the total duty cycle.

[0055] On the other hand, for the upper heaters 110, 120, and 130 that are not subject to heat generation, a PWM signal with a duty cycle of "0%" is generated and output individually. In this embodiment, FETs 111, 112, and 113 to which a PWM signal with a duty cycle of "0%" is output remain in the off state without being controlled on or off according to the PWM signal. Therefore, the upper heaters 110, 120, and 130 corresponding to FETs 111, 112, and 113 to which a PWM signal with a duty cycle of "0%" is output do not receive voltage and do not light up, thus generating no heat. In this specification, a PWM signal with a duty cycle of "0%" includes PWM signals with a duty cycle of "0-2%" in which FETs 111, 112, and 113 are not turned on or off.

[0056] As shown in Figure 6(b), the temperature control system of the lower belt unit 20 has the same configuration as the power supply circuit 1010 (first power supply circuit) described above, and includes a power supply circuit 1011 (second power supply circuit) that has a relay circuit 1401 and a plurality of FETs 211, 212 and supplies power to the lower heaters 140, 150. Based on the detection result of the lower belt temperature sensor 320, the CPU 1100 can generate and output a PWM signal with a duty cycle determined according to the total duty cycle of the lower heaters 140, 150 to the plurality (in this case, two) of FETs 211, 212 in the power supply circuit 1011.

[0057] <Heater control processing> Next, the "heater control process" of this embodiment will be explained using Figures 7 to 10 with reference to Figures 2 and 6. Figure 7 shows a flowchart of the heater control process of the first embodiment. The "heater control process" is executed by the CPU 1100 when the inkjet recording device 1 is powered on.

[0058] As shown in Figure 7, the CPU 1100 drives the drive motor (not shown) for the upper belt 30 to rotate the upper belt 30 and starts heating the upper belt 30 with the upper heaters 110, 120, and 130 (S1). The CPU 1100 then uses a duty cycle calculation unit 1102 to calculate the total duty cycle by adding the duty cycles of the upper heaters 110, 120, and 130, which are determined according to the temperature difference between the temperature of the upper belt 30 detected by the upper belt temperature sensor 310 and the target temperature of the upper belt 30 (S2).

[0059] Based on the calculated total duty cycle, the CPU 1100 generates and outputs PWM signals with duty cycles according to Table 1 shown below for each of the upper heaters 110, 120, and 130 (S3~S7). As shown in Table 1, in this embodiment, the duty cycle of the PWM signal for the heaters subject to heat generation is set to "30%" or more (a predetermined value or more). [Table 1]

[0060] If the total duty cycle is less than "61%" (YES in S3), the CPU 1100 determines, according to Table 1, that the heater to be subjected to heat generation operation is one of the upper heaters 110-130 (S4). In this case, the CPU 1100 generates a PWM signal with the duty cycle set to "total duty cycle" and outputs it to the FET 113 corresponding to the upper heater 130. The CPU 1100 also generates a PWM signal with the duty cycle set to "0%" and outputs it to the FETs 111 and 112 corresponding to the upper heaters 110 and 120. In other words, it outputs PWM signals with different duty cycles. Upper heaters 110 and 120, to which a PWM signal with a duty cycle of "0%" is output, do not receive voltage and therefore do not light up, thus generating no heat. Upper heater 130, to which a PWM signal with a duty cycle of "total duty cycle" is output, receives voltage and lights up, thus generating heat.

[0061] If the total duty cycle is 61% or more and less than 91% (YES in S5), the CPU 1100 determines, according to Table 1, that the two heaters to be used for heating are upper heaters 120 and 130 (S6). In this case, the CPU 1100 generates a PWM signal with a duty cycle set to "total duty cycle / 2", or "total duty cycle / number of heaters used", and outputs it to FETs 112 and 113, which correspond to upper heaters 120 and 130, respectively. The CPU 1100 also generates a PWM signal with a duty cycle set to "0%" and outputs it to FET 111, which corresponds to upper heater 110. In other words, it outputs PWM signals with different duty cycles. When a PWM signal with a duty cycle of "0%" is output, heater 110 does not receive voltage and does not light up, so it does not generate heat. When a PWM signal with a "total duty cycle / 2" is output, heaters 120 and 130 receive voltage and light up, so they generate heat.

[0062] If the total duty cycle is greater than "91%" (NO in S5), the CPU 1100 determines, according to Table 1, that the three heaters to be used for heating are upper heaters 110, 120, and 130 (S7). In this case, the CPU 1100 generates a PWM signal with a duty cycle set to "total duty cycle / 3 (number of heaters used)" and outputs it to FETs 111, 112, and 113, which correspond to upper heaters 110, 120, and 130, respectively. That is, it outputs a PWM signal with the same duty cycle. Therefore, voltage is supplied to all upper heaters 110-130, causing them to light up and generate heat.

[0063] After the processing of steps S4, S6, and S7 described above, the CPU 1100 determines whether or not to stop heating the upper belt 30 by the upper heaters 110-130 (S8). If the heating of the upper belt 30 by the upper heaters 110-130 is stopped (YES in S8), the CPU 1100 terminates this heater control process. On the other hand, if the heating of the upper belt 30 by the upper heaters 110-130 is not stopped (NO in S8), the CPU 1100 returns to the processing of step S2 and repeatedly executes the processing of steps S2-S8.

[0064] Thus, when the temperature difference between the temperature of the upper belt 30 and the target temperature is greater than or equal to a threshold, two or more upper heaters 120 and 130 are activated. When the temperature difference between the temperature of the upper belt 30 and the target temperature is less than the threshold, only a smaller number of upper heaters 110 are activated compared to when the temperature difference is greater than or equal to the threshold. As shown in Table 1, a predetermined number of upper heaters 110, 120, and 130 are activated according to the temperature difference.

[0065] Here, we have used the case of controlling the upper heaters 110, 120, and 130 as an example, but as shown in Table 1, the CPU 1100 performs the same control on the lower heaters 140 and 150 simultaneously with the upper heaters 110-130. If the total duty cycle is less than "61%", the CPU 1100 determines the heater to be used for heating operation to be the lower heater 150, according to Table 1. In this case, the CPU 1100 generates a PWM signal with the duty cycle set to "total duty cycle" and outputs it to the FET corresponding to the lower heater 150. The CPU 1100 also generates a PWM signal with the duty cycle set to "0%" and outputs it to the FET corresponding to the lower heater 140. If the total duty cycle is greater than "61%", the CPU 1100 determines the heaters to be used for heating operation to be the lower heaters 140 and 150, according to Table 1. In this case, the CPU1100 generates a PWM signal with a duty cycle set to "total duty cycle / 2 (number of heaters used)" and outputs it to the FETs corresponding to the lower heaters 140 and 150, respectively.

[0066] <Comparison with conventional examples> Next, we will compare this embodiment with a conventional example. Figure 8 is a flowchart of the "heater control process" in the conventional example. In the conventional example, the "heater control process" is executed by a CPU capable of PWM control when the inkjet recording device 1 is powered on.

[0067] As shown in Figure 8, the CPU drives the drive motor (not shown) for the upper belt 30 to rotate the upper belt 30 and starts heating the upper belt 30 with the upper heaters 110, 120, and 130 (S11). The CPU then calculates the duty cycle of each of the upper heaters 110, 120, and 130 from the temperature difference between the temperature of the upper belt 30 detected by the upper belt temperature sensor 310 and the target temperature of the upper belt 30 (S12). The CPU generates a PWM signal set to the calculated duty cycle and outputs it to the FETs 111, 112, and 113 corresponding to the upper heaters 110, 120, and 130, respectively, so that voltage is supplied to all of the upper heaters 110 to 130, causing them to light up and generate heat (S13). Subsequently, the CPU determines whether or not to stop heating the upper belt 30 by the upper heaters 110-130 (S14). If it decides to stop heating the upper belt 30 by the upper heaters 110-130 (YES in S14), the heater control process ends. On the other hand, if it does not decide to stop heating the upper belt 30 by the upper heaters 110-130 (NO in S14), the CPU returns to the process in step S12 and repeatedly executes the processes in steps S12-S14.

[0068] Figure 9 shows the time variation of the belt temperature and the duty cycle of the PWM signal output to each heater (specifically, the FET) in a conventional example. Figure 10 shows the time variation of the belt temperature and the duty cycle of the PWM signal output to each heater (specifically, the FET) in this embodiment. From top to bottom, these are the cases for upper heater 110, upper heater 120, and upper heater 130. In Figures 9 and 10, the time variation of the belt temperature is represented by a solid line, and the time variation of the duty cycle of each heater is represented by a dotted line. Here, it is assumed that the target temperature of the upper belt 30 is set to "95°C".

[0069] As shown in Figure 9, while the temperature of the upper belt 30 is rising to the target temperature of 95°C, the upper heaters 110, 120, and 130 are all supplied with maximum voltage via a PWM signal with a duty cycle of 100%, generating heat. However, as time progresses, the temperature of the upper belt 30 rises, and when the temperature difference between the temperature of the upper belt 30 and the target temperature exceeds a predetermined threshold, a PWM signal with a gradually decreasing duty cycle is output to all of the upper heaters 110, 120, and 130 for the duration (100 seconds) until the temperature of the upper belt 30 reaches the target temperature. In other words, the voltage supplied to the upper heaters 110, 120, and 130 is lowered to reduce the temperature of the heat emitted from the upper heaters 110, 120, and 130, preventing the temperature of the upper belt 30 from overshooting and exceeding the target temperature.

[0070] In the conventional example, after the temperature of the upper belt 30 reaches the target temperature (after 100 seconds), a PWM signal with a reduced duty cycle is continuously output to all of the upper heaters 110, 120, and 130. This is to generate relatively low-temperature heat in the upper heaters 110, 120, and 130 to maintain the temperature of the upper belt 30 at the target temperature, and a PWM signal with a duty cycle of about 10% is output. As described above, the temperature control of the upper heaters 110, 120, and 130 is performed by rapidly switching the FETs 111, 112, and 113 (see Figure 6) of the power supply circuit 1010 that supplies voltage on and off according to the PWM signal. In other words, conventionally, the FETs 111, 112, and 113 were constantly switching on and off according to the PWM signal, and the upper heaters 110, 120, and 130 were always in a heated state. Therefore, the lifespan of the upper heaters 110, 120, and 130 tended to be shortened. Furthermore, if a halogen heater is temperature-controlled by a PWM signal with a duty cycle of less than 30%, the temperature of the glass tube that makes up the halogen heater will fall below 250°C. In this case, tungsten-halogen deposits will adhere to the bulb wall, causing blackening, which may shorten the lifespan of the halogen heater.

[0071] As shown in Figure 10, in this embodiment, as in the conventional example, the upper heaters 110, 120, and 130 are all supplied with maximum voltage via a PWM signal with a duty cycle of 100% to generate heat until the temperature of the upper belt 30 reaches the target temperature of 95°C. However, as time passes, the temperature of the upper belt 30 rises, and when the temperature difference between the temperature of the upper belt 30 and the target temperature exceeds a predetermined threshold, a PWM signal with a gradually decreasing duty cycle is output to all of the upper heaters 110, 120, and 130 for the duration (100 seconds) until the temperature of the upper belt 30 reaches the target temperature. In other words, the voltage supplied to the upper heaters 110, 120, and 130 is lowered to reduce the temperature of the heat emitted from the upper heaters 110, 120, and 130, so that the temperature of the heat emitted from the upper heaters 110, 120, and 130 does not overshoot and exceed the target temperature.

[0072] However, in this embodiment, after the temperature of the upper belt 30 reaches the target temperature (after 100 seconds), unlike the conventional example, a PWM signal with a duty cycle of approximately 40% is output to the upper heaters 120 and 130, supplying voltage and generating heat. At this time, the upper heater 110 outputs a PWM signal with a duty cycle of 0%, effectively supplying no voltage and generating no heat. Subsequently, only the upper heater 130 outputs a PWM signal with a duty cycle of 30% or more, supplying voltage and generating heat. At this time, the upper heaters 110 and 120 output a PWM signal with a duty cycle of 0%, effectively supplying no voltage and generating no heat. In this embodiment, in order to maintain the temperature of the upper belt 30 at the target temperature, a PWM signal with a duty cycle of 30% or more is continuously output only to the upper heater 130. In this case, the upper belt 30 is heated with the same amount of heat as when a PWM signal with a duty cycle of approximately 10% is output to the upper heaters 110, 120, and 130 in the conventional example, and the upper belt 30 is maintained at the target temperature.

[0073] As described above, in this embodiment, in both the upper belt unit 10 and the lower belt unit 20, in order to reduce the number of heaters that generate heat among the multiple heaters that heat the belt, a PWM signal with a duty cycle of "0%" is output to the heaters that do not generate heat. By outputting a PWM signal with a duty cycle of "0%" and controlling the power supply circuit 1010, the FETs 111, 112, and 113 are not turned on and off according to the PWM signal, thereby preventing the shortening of the lifespan of the upper heaters 110, 120, and 130.

[0074] Furthermore, conventionally, even with a duty cycle of less than 30%, a low duty cycle PWM signal was output to regulate the temperature of the halogen heater, which shortened the lifespan of the halogen heater. However, in this embodiment, a PWM signal with a duty cycle of less than 30% is not output, thus further suppressing the shortening of the halogen heater's lifespan.

[0075] In the embodiment described above, the upper heater 130 and lower heater 150, located at the upstream end in the sheet transport direction, are prioritized for illumination (see Table 1). However, the upper heater 110 and lower heater 140, located at the downstream end, may also be prioritized for illumination. Furthermore, when the total duty cycle is 61% or more, a PWM signal with the same duty cycle (total duty cycle / number of heaters used) is generated and output for each heater subject to heat generation. However, PWM signals with different duty cycles may be generated and output for each heater.

[0076] [Second Embodiment] Next, the "heater control process" of the second embodiment will be explained using Figure 11 with reference to Figure 6. In the case of the "heater control process" of the first embodiment described above (see Figure 7), the upper heater 130 and the lower heater 150 are given priority in lighting (see Table 1), resulting in a longer cumulative lighting time than the other heaters (110, 120, 140), and thus faster deterioration. Therefore, in the second embodiment, the upper heaters 110, 120, 130 and the lower heaters 140, 150 are lit on an average basis so that these heaters deteriorate at the same rate. This way, the lifespan of each heater is approximately the same, allowing the user to perform maintenance and replacement on multiple heaters at the same time. Here again, the case of controlling the upper heaters 110, 120, and 130 will be used as an example for explanation.

[0077] As shown in Figure 11, the CPU 1100 drives the drive motor (not shown) for the upper belt 30 to rotate the upper belt 30 and starts heating the upper belt 30 with the upper heaters 110, 120, and 130 (S21). The CPU 1100 then calculates the total duty cycle by adding the duty cycles of the upper heaters 110, 120, and 130, which are determined according to the temperature difference between the temperature of the upper belt 30 detected by the upper belt temperature sensor 310 and the target temperature of the upper belt 30 (S22).

[0078] If the total duty cycle is less than "61%" (YES in S23), the CPU 1100 generates and outputs a PWM signal of the total duty cycle (total ratio) for the heater with the smallest cumulative time among the upper heaters 110 to 130 (heater_Tmin) (S24). In other words, voltage is supplied to one of the upper heaters 110 to 130, causing it to light up and generate heat. The CPU 1100 counts the lighting time of the heater that output the PWM signal using the timing unit 1103, and measures the cumulative time of the heater that output the PWM signal (S25). After that, the CPU 1100 proceeds to the processing in step S31.

[0079] If the total duty cycle is 61% or more and less than 91% (YES in S26), the CPU 1100 generates and outputs a PWM signal of "total duty cycle (total ratio) / 2 (number of heaters used)" for the heaters with the first and second shortest cumulative time among the upper heaters 110 to 130 (heater_Tmin, heater_Tmin+1) (S27). In other words, voltage is supplied to two of the upper heaters 110 to 130, causing them to light up and generate heat. The CPU 1100 counts the lighting time of each of the two heaters that output PWM signals using the timing unit 1103, and measures the cumulative time of each of the two heaters that output PWM signals (S28). After that, the CPU 1100 proceeds to the processing in step S31.

[0080] If the total duty cycle is greater than "91%" (NO in S26), the CPU 1100 generates and outputs a PWM signal for all of the upper heaters 110-130 (heater_Tmin, heater_Tmin+1, heater_Tmin+2) with a value of "total duty cycle (total ratio) / 3 (number of heaters used)" (S29). In other words, voltage is supplied to all of the upper heaters 110-130, causing them to light up and generate heat. The CPU 1100 counts the lighting time for each of the three heaters that output PWM signals using the timing unit 1103, and measures the cumulative time for all heaters that output PWM signals (S30). After that, the CPU 1100 proceeds to the processing in step S31.

[0081] When image formation is complete (S31), the CPU 1100 sets the order in which to light up the upper heaters 110-130 according to the cumulative time of each heater (S32). Here, the upper heaters 110-130 are divided and set to be lit in order of increasing cumulative time of each heater: the heater with the highest priority (first) to light up (heater_Tmin), the heater with the next highest priority (second) to light up (heater_Tmin+1), and the heater with the last highest priority (third) to light up (heater_Tmin+2).

[0082] Subsequently, the CPU 1100 determines whether or not to stop heating the upper belt 30 by the upper heaters 110-130 (S33). If the heating of the upper belt 30 by the upper heaters 110-130 is stopped (YES in S33), the CPU 1100 terminates this heater control process. On the other hand, if the heating of the upper belt 30 by the upper heaters 110-130 is not stopped (NO in S33), the CPU 1100 returns to the process in step S22 and repeatedly executes the processes in steps S22-S32.

[0083] Furthermore, when counting the illumination time of each heater (S25, S28, S30), if the heater is illuminated with a low duty cycle, even if the illumination time is the same as when it is illuminated with a high duty cycle, the cumulative time may be calculated to be shorter than the actual illumination time counted. In other words, the CPU 1100 may weight the time to be counted according to the duty cycle of the PWM signal when calculating the cumulative time.

[0084] As described above, in the second embodiment, as in the first embodiment described above, the power supply circuit 1010 is controlled by a PWM signal with a duty cycle of "0%" for heaters that do not generate heat, so that FETs 111, 112, and 113 are not switched on or off, thereby suppressing the shortening of the lifespan of the heaters 110, 120, and 130. In addition, in the second embodiment, since heaters that have been used less are used preferentially according to the cumulative time, the lifespans of multiple heaters will be around the same time, so that the user can perform maintenance and replacement of multiple heaters at the same time. [Explanation of Symbols]

[0085] 1... Inkjet recording device, 30... First belt (upper belt), 40... Second belt (lower belt), 110, 120, 130... Heating element (first heating element, upper heater), 140, 150... Second heating element (lower heater), 310... Temperature detection unit (first temperature detection unit, upper belt temperature sensor), 320... Second temperature detection unit (lower belt temperature sensor), 1010... Power supply circuit (first power supply circuit), 1011... Second power supply circuit (power supply circuit), 1100... Control unit (CPU), 1103... Timing unit, 2000... Image forming unit (print module), N... Nip unit (fixing nip unit), S... Sheet

Claims

1. An image forming unit that ejects ink to form an image on a sheet, The endless first belt, An endless second belt that contacts the first belt and forms a nip portion for gripping, transporting, and heating a sheet on which an image has been formed by the image forming unit, A plurality of heating elements are arranged on the inner circumference of the first belt along the sheet conveying direction and heat the first belt, A power supply circuit that supplies power to the plurality of heating elements, A temperature detection unit for detecting the temperature of the first belt, The system includes a control unit that generates a plurality of PWM signals for each of the plurality of heating elements, which control the power supply circuit by pulse width modulation so that the temperature of the first belt detected by the temperature detection unit becomes a predetermined target temperature, and which can individually control the power supplied from the power supply circuit to the plurality of heating elements using the generated plurality of PWM signals, The control unit, based on the plurality of PWM signals, If the temperature difference between the temperature of the first belt and the target temperature is greater than or equal to a threshold, two or more of the heating elements are made to generate heat. When the temperature difference is smaller than the threshold, fewer heating elements are generated than when the temperature difference is greater than or equal to the threshold. An inkjet recording apparatus characterized by the following features.

2. The control unit generates a plurality of PWM signals with different duty cycles based on at least the temperature difference between the temperature of the first belt and the target temperature. The inkjet recording apparatus according to feature 1.

3. The control unit generates heat in a predetermined number of heating elements according to the temperature difference. The inkjet recording apparatus according to feature 1.

4. The control unit generates a PWM signal with a duty cycle of "0%" with respect to the heating element that is not generating heat among the plurality of heating elements. The inkjet recording apparatus according to feature 1.

5. The control unit generates the PWM signal with the same duty cycle of a predetermined value or more with respect to the heating element that generates heat among the plurality of heating elements, according to the temperature difference. The inkjet recording apparatus according to feature 1.

6. The aforementioned predetermined value is "30%" or more. The inkjet recording apparatus according to feature 5.

7. The system includes a timing unit that counts the time each of the multiple heating elements is generating heat and measures the cumulative time. The plurality of heating elements are heated in order from the heating element with the smallest cumulative time measured by the timing unit. The inkjet recording apparatus according to feature 1.

8. The aforementioned heating element is a halogen heater that generates heat by emitting infrared radiation. The inkjet recording apparatus according to feature 1.

9. The aforementioned multiple heating elements are, firstly, heating elements. The aforementioned power supply circuit is the first power supply circuit, The temperature detection unit is a first temperature detection unit, A plurality of second heating elements are arranged on the inner circumference of the second belt along the sheet conveying direction and heat the second belt, A second power supply circuit that supplies power to the plurality of second heating elements, It comprises a second temperature detection unit that detects the temperature of the second belt, The control unit generates a plurality of second PWM signals for each of the plurality of second heating elements to control the second power supply circuit by pulse width modulation so that the temperature of the second belt detected by the second temperature detection unit becomes the target temperature, and the power supplied from the second power supply circuit to the plurality of second heating elements can be individually controlled by the plurality of second PWM signals generated. The control unit, based on the plurality of second PWM signals, If the temperature difference between the temperature of the second belt and the target temperature is greater than or equal to the threshold, two or more of the second heating elements are made to generate heat. When the temperature difference between the temperature of the second belt and the target temperature is less than the threshold, the second heating element is heated in fewer units than when the temperature difference between the temperature of the second belt and the target temperature is greater than or equal to the threshold. The inkjet recording apparatus according to feature 1.

10. The control unit generates a plurality of second PWM signals with different duty cycles based on at least the temperature difference between the temperature of the second belt and the target temperature. The inkjet recording apparatus according to feature 9.

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