Image forming apparatus and control method thereof

Abstract

To prevent the stop time of a heater from becoming long in a case where a fuser is maintained in a stand-by state.SOLUTION: An image formation apparatus comprises: a toner image formation unit which forms a toner image on a sheet; a fuser which has a heater and fixes the toner image on the sheet; a temperature sensor which senses the temperature of the fuser; and a control unit. The control unit performs electric conduction to the heater during a heating period shorter than a control cycle for each prescribed control cycle when a sensed temperature T is less than a target stand-by temperature TR in stand-by control of maintaining the temperature of the fuser at the target stand-by temperature on the basis of the sensed temperature sensed by the temperature sensor, and changes the electric conduction amount to be conducted during the next heating period to the electric conduction amount larger than that in the previous heating period (S132) when the sensed temperature sensed after the time of elapse of the control cycle is less than the target stand-by temperature (S120, No).SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus having a fixing device that fixes a toner image on a sheet, and a control method thereof. [Background technology]

[0002] Conventionally, there is known a technique in which, while the fixing unit is in standby mode, power is supplied to the heater when the detected temperature of the fixing unit falls below a lower limit temperature, and power is stopped when the detected temperature reaches an upper limit temperature (Patent Documents 1 and 2).

[0003] In the technology of Patent Document 1, a non-energized period is provided after the heater is energized, and if the detected temperature falls below the lower limit temperature during the non-energized period, the upper limit temperature is increased, and if the detected temperature after the non-energized period has elapsed is higher than the lower limit temperature, the upper limit temperature is decreased.

[0004] Furthermore, in the technology of Patent Document 2, when the detected temperature falls below the lower limit temperature (first temperature), the duty ratio of the heater is determined according to the deviation between the upper limit temperature (second temperature) and the detected temperature. If the detected peak temperature is higher than the target peak temperature, the duty ratio according to the next deviation is made smaller than the duty ratio according to the current deviation, and if the detected peak temperature is lower than the target peak temperature, the duty ratio according to the next deviation is made larger than the duty ratio according to the current deviation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 7-114296 [Patent Document 2] Japanese Patent Publication No. 2020-20988 Summary of the Invention [Problem to be solved by the invention]

[0006] The heater temperature is significantly different from the fuser temperature (e.g., the temperature of the heat roller surface). When the heater is turned on, its temperature is much higher than that of the fuser, and when it is turned off, it approaches the fuser temperature. Therefore, even if power is turned off to the heater because the fuser temperature reaches its upper limit, the fuser temperature rises due to residual heat from the heater and then gradually decreases. The peak temperature of the fuser at this time varies depending on, for example, the heat storage state of the fuser itself and the components surrounding the fuser, the surrounding environment, and other factors. Therefore, if the peak temperature becomes high, it takes a long time for the detected temperature to decrease to the lower limit. If a long time has passed since the heater was turned off, the heater temperature drops too much, reducing the heater's resistance and resulting in a large inrush current the next time the heater is turned on. This increase in inrush current can lead to problems such as flicker and power supply voltage noise.

[0007] The present invention has been made in view of the above background, and has as its object to prevent the heater from being stopped for a long period of time when the fixing unit is kept in a standby state. [Means for solving the problem]

[0008] An image forming apparatus that solves the above-mentioned problems includes a toner image forming unit that forms a toner image on a sheet, a fixing unit that has a heater and fixes the toner image on the sheet, a temperature sensor that detects the temperature of the fixing unit, and a control unit. In standby control that maintains the temperature of the fixing unit at a target standby temperature based on the detected temperature detected by the temperature sensor, if the detected temperature falls below the target standby temperature, the control unit energizes the heater for a heating period shorter than the control period at each predetermined control period, and if the detected temperature at the end of the control period is below the target standby temperature, changes the amount of electricity to be supplied during the next heating period to an amount of electricity greater than that of the previous heating period.

[0009] In addition, a control method for an image forming apparatus that solves the above-mentioned problems is a control method for an image forming apparatus that includes a toner image forming unit that forms a toner image on a sheet, a fixing unit that has a heater and fixes the toner image on the sheet, a temperature sensor that detects the temperature of the fixing unit, and a control unit. In this method, the control unit executes standby control to maintain the temperature of the fixing unit at a target standby temperature based on the detected temperature detected by the temperature sensor. In the standby control, when the detected temperature falls below the target standby temperature, the control unit energizes the heater for a heating period shorter than the control period at each predetermined control cycle, and when the detected temperature at the end of the control cycle is below the target standby temperature, changes the amount of energization to be applied during the next heating period to an amount of energization greater than that of the previous heating period.

[0010] According to these configurations, it is possible to supply the necessary amount of current for each control cycle, and it is possible to prevent the heater from being stopped for a long time.

[0011] During standby control, if the detected temperature at the end of a control cycle is equal to or higher than the target standby temperature and lower than a threshold temperature higher than the target standby temperature, the control unit can set the amount of power supplied in the next heating period to the same as that in the previous heating period.

[0012] If the detected temperature at the end of a control cycle is equal to or higher than the target standby temperature and lower than a threshold temperature higher than the target standby temperature, it is highly likely that the amount of current supplied during the heating period of the previous control cycle was appropriate, and therefore, by setting the amount of current supplied during the next heating period to the same as that of the previous heating period, the detected temperature can be maintained close to the target standby temperature.

[0013] In the standby control, if the detected temperature at the end of the control cycle is equal to or higher than the threshold temperature, the control unit can change the amount of power applied in the next heating period to a smaller amount of power applied in the previous heating period.

[0014] If the detected temperature at the end of a control cycle is equal to or higher than the threshold temperature, it is highly likely that the amount of current supplied during the heating period of the previous control cycle was too large. Therefore, by changing the amount of current supplied during the next heating period to a smaller amount than that supplied during the previous heating period, the detected temperature can be maintained close to the target standby temperature.

[0015] When the standby control is started, the control unit can start a control cycle based on the fact that the detected temperature has become lower than the target standby temperature.

[0016] With this configuration, if the temperature of the fixing unit is high when standby control is started, the control cycle will start after waiting until the detected temperature reaches the target standby temperature, thereby preventing the temperature of the fixing unit from rising unnecessarily.

[0017] In the standby control, when the detected temperature becomes equal to or higher than the threshold temperature, it is preferable that the control unit does not start the next control cycle until the detected temperature becomes lower than the target standby temperature.

[0018] With this configuration, it is possible to prevent the temperature of the fixing unit from becoming unnecessarily high.

[0019] When the control unit changes the amount of current to a larger amount, it can change the heating period to a period longer than the previous heating period, and when the control unit changes the amount of current to a smaller amount, it can change the heating period to a period shorter than the previous heating period.

[0020] The control unit may energize the heater during the heating period at a set duty ratio, and when changing the amount of energization to a larger amount, may change the duty ratio to a value larger than that of the previous heating period, and when changing the amount of energization to a smaller amount, may change the duty ratio to a value smaller than that of the previous heating period.

[0021] The control unit may control the energization of the heater during the heating period by wave number control at a set duty ratio.

[0022] When controlling the energization of the heater by wave number control, the control unit may change the amount of energization by changing the number of energizations of a predetermined energization pattern of wave number control, assuming that the pattern is performed once.

[0023] When the control unit first starts standby control after the image forming apparatus is turned on, it is desirable that the control unit sets the amount of current for the first heating period to the minimum value that can be set.

[0024] According to this configuration, an excessive amount of heat is not supplied to the fixing unit, and therefore it is possible to prevent the heater from being stopped for a long time.

[0025] In the standby control, when the detected temperature at the end of a control cycle is lower than the target standby temperature, the control unit may increase the amount of change in the amount of power as the deviation between the target standby temperature and the detected temperature increases.

[0026] With this configuration, if the amount of current supplied during the heating period of the previous control cycle was insufficient, the amount of current supplied can be quickly brought closer to an appropriate amount.

[0027] The fixing unit can have a heating member that is heated by a heater and includes a rotating member that is rotatable around the heater, and a pressure member that sandwiches the sheet between the heating member and the heating member. In this case, the control unit can rotate the rotating member when fixing the toner image on the sheet, and can prevent the rotating member from rotating when executing standby control.

[0028] The control unit controls the heater so that the detected temperature becomes a target fixing temperature when the toner image is fixed on the sheet by the fixing unit. The target standby temperature is lower than the target fixing temperature. [Effects of the Invention]

[0029] According to the present invention, when the fixing unit is kept in a standby state, it is possible to prevent the heater from being stopped for a long period of time. [Brief explanation of the drawings]

[0030] [Figure 1] 1 is a diagram illustrating a laser printer according to an embodiment of the present invention. [Figure 2] FIG. 2 is a perspective view showing the arrangement of sensors on a nip plate. [Figure 3] FIG. 2 is a block diagram showing the configuration of a control unit. [Figure 4] FIG. 10 is a diagram showing an example of a current pattern in wave number control. [Figure 5] 4 is a flowchart showing a standby control process in the first embodiment. [Figure 6] 5 is a time chart showing an example of heater operation and temperature change during standby control in the first embodiment. [Figure 7] 10 is an example of a table for setting an increase in the number of heating cycles in the second embodiment. [Figure 8] 10 is a flowchart showing a standby control process in the second embodiment. [Figure 9] 10 is an example of a table for setting a duty ratio in the third embodiment. [Figure 10] 10 is a flowchart showing a standby control process in a third embodiment. [Figure 11] 10 is a time chart showing an example of heater operation and temperature change during standby control in the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0031] Next, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. As shown in FIG. 1, the image forming apparatus 1 is a laser printer that forms an image on a sheet S, and includes a main body housing 2, a supply unit 3, a process unit PR, a fixing unit 8, and a control unit 100.

[0032] The supply unit 3 is a mechanism for supplying sheets S to the process unit PR and is located at the bottom of the main body housing 2. The supply unit 3 includes a supply tray 31 that stores sheets S, a sheet pressure plate 32, and a supply mechanism 33. The supply mechanism 33 includes a pickup roller 33A, a separation roller 33B, a first conveyor roller 33C, and a registration roller 33D. In the supply unit 3, the sheets S in the supply tray 31 are pulled toward the pickup roller 33A by the sheet pressure plate 32 and sent to the separation roller 33B by the pickup roller 33A. The sheets S are separated into single sheets by the separation roller 33B and conveyed by the first conveyor roller 33C. The registration roller 33D aligns the leading edge of the sheets S and then conveys the sheets S toward the process unit PR. Herein, the direction in which the sheets S are conveyed is referred to as the conveyance direction, and the direction perpendicular to the conveyance direction within the plane of the sheets S is referred to as the width direction. Hereinafter, the width direction of the sheets S will be simply referred to as the "width direction."

[0033] The process unit PR has a function of forming a toner image on the sheet S supplied from the supply unit 3. The process unit PR is a toner image forming unit. The process unit PR includes an exposure device 4 and a process cartridge 5.

[0034] The exposure device 4 is disposed in the upper part of the main body housing 2, and includes a laser light source (not shown), a polygon mirror (shown without reference numerals), a lens, a reflecting mirror, etc. In the exposure device 4, a laser beam based on image data is emitted from the laser light source, and is scanned across the surface of the photosensitive drum 61, thereby exposing the surface of the photosensitive drum 61.

[0035] The process cartridge 5 is disposed below the exposure device 4, and is detachable from the main body housing 2 through an opening formed when a front cover 21 provided on the main body housing 2 is opened. The process cartridge 5 includes a drum unit 6 and a developing unit 7.

[0036] The drum unit 6 includes a photosensitive drum 61, a charger 62, and a transfer roller 63. The development unit 7 is detachable from the drum unit 6, and includes a development roller 71, a supply roller 72, a layer thickness regulating blade 73, a toner storage section 74 that stores dry toner, and an agitator 75.

[0037] In the process cartridge 5, the surface of the photosensitive drum 61 is uniformly charged by the charger 62 and then exposed to laser light from the exposure device 4, thereby forming an electrostatic latent image based on image data on the photosensitive drum 61. The toner in the toner storage section 74 is supplied to the developing roller 71 via the supply roller 72 while being agitated by the agitator 75, and as the developing roller 71 rotates, the toner enters between the developing roller 71 and the layer thickness regulating blade 73 and is carried on the developing roller 71 as a thin layer of a uniform thickness.

[0038] The toner carried on the developing roller 71 is supplied from the developing roller 71 to the electrostatic latent image formed on the photosensitive drum 61. This makes the electrostatic latent image visible, and a toner image is formed on the photosensitive drum 61. Thereafter, the sheet S supplied from the supply unit 3 is transported between the photosensitive drum 61 and the transfer roller 63, and the toner image formed on the photosensitive drum 61 is transferred onto the sheet S.

[0039] The fixing device 8 fixes the toner image on the sheet S. The fixing device 8 has a heater H1, a heating member 81 that is heated by the heater H1 and includes a rotating member 81A that is rotatable around the heater H1, and a pressure member 82 that sandwiches the sheet S between the heating member 81 and the heating member 81. The heater H1 is a resistance heating type heater, and in this embodiment, a halogen heater is used as an example.

[0040] The rotating member 81A is a rotatable endless belt. The rotating member 81A has a base material made of metal, resin, or the like, and a release layer covering the outer surface of the base material. Inside the heating member 81, a heater H1 that heats the heating member 81 and a nip plate NP are provided. The heater H1 is a halogen lamp that emits light and generates heat when energized, and heats the rotating member 81A by radiant heat. The heater H1 is disposed inside the rotating member 81A along the width direction.

[0041] The pressure member 82 is a rotatable pressure roller, and has an elastic layer made of elastically deformable rubber or the like on its surface.

[0042] The nip plate NP is a plate-shaped member that receives radiant heat from the heater H1, and is disposed inside the heating member 81 so that the inner peripheral surface of the heating member 81 is in sliding contact with the lower surface of the nip plate NP. The nip plate NP sandwiches the heating member 81 between itself and the pressure member 82. In the fixing device 8, the sheet S onto which the toner image has been transferred is conveyed between the heating member 81 and the pressure member 82, whereby the toner image is thermally fixed onto the sheet S. The sheet S onto which the toner image has been thermally fixed is discharged onto the discharge tray 22 by the second conveyor roller 23 and the discharge roller 24.

[0043] As shown in FIG. 2, the nip plate NP has a central detection portion 131 that protrudes from an end portion in the conveyance direction of the sheet S, and an edge detection portion 132. The central detection portion 131 is located in the center in the width direction. The edge detection portion 132 is located at an end portion in the width direction. A central temperature sensor ST1 is arranged opposite the central detection portion 131. An edge temperature sensor ST2 is arranged opposite the edge detection portion 132. The central temperature sensor ST1 is an example of a temperature sensor that detects the temperature of the fixing unit 8.

[0044] The central temperature sensor ST1 is a sensor that detects the temperature in the width direction at the center of the heating member 81. The central temperature sensor ST1 is capable of detecting the temperature of the central portion of the heating member 81 by detecting the temperature of the nip plate NP through contact or non-contact with the central detection portion 131 of the nip plate NP.

[0045] The edge temperature sensor ST2 is a sensor that detects the temperature of the edge of the heating member 81 in the width direction. The edge temperature sensor ST2 is capable of detecting the temperature of the edge of the heating member 81 by detecting the temperature of the nip plate NP through contact or non-contact with the edge detection unit 132 of the nip plate NP. More specifically, in the width direction, the edge temperature sensor ST2 is located outside the maximum area SW of the sheet S that can be fixed by the fixing unit 8. The edge temperature sensor ST2 may be located within the range of the area SW in the width direction.

[0046] The central temperature sensor ST1 and the end temperature sensor ST2 may be, for example, a thermistor.

[0047] 3, the control unit 100 includes an ASIC 110 and a current supply circuit 120. The ASIC 110 includes a CPU 111, a heater controller 112, and storage units such as a ROM 113 and a RAM 114. The current supply circuit 120 is a circuit including a switching circuit that switches the input AC voltage between a current supply state and a current non-supply state, and is connected to the heater H1 and the ASIC 110.

[0048] The CPU 111 is implemented as a function within the ASIC 110. The CPU 111 outputs a target temperature of the detected temperature T detected by the central temperature sensor ST1 to the heater controller 112. Each target temperature is a command value in feedback processing when the heater controller 112 controls the power supply to the heater H1.

[0049] The heater controller 112 is a function or circuit built into the ASIC 110, and when executing print control, energizes the heater H1 by controlling the energization circuit 120 so that the detected temperature T detected by the central temperature sensor ST1 becomes the target temperature. More specifically, the heater controller 112 performs feedback processing to determine the duty ratio of the AC voltage applied to the heater H1 based on the detected temperature T detected by the central temperature sensor ST1 and the target temperature, and to control the energization circuit 120 at the determined duty ratio. The feedback processing performed by the heater controller 112 may be implemented on a chip external to the ASIC 110, or may be executed by the CPU 111.

[0050] Furthermore, the heater controller 112 energizes the heater H1 at a duty ratio and for a heating time instructed by the CPU 111 during standby control for maintaining the temperature of the fixing unit 8 at the target standby temperature TR.

[0051] The control unit 100 executes control by performing various arithmetic operations based on the print job output from an external computer, the temperatures detected by the central temperature sensor ST1 and the edge temperature sensor ST2, and the programs and data stored in the storage unit. In other words, the control unit 100 functions as a means for executing various controls by operating according to the programs.

[0052] When fixing a toner image on a sheet S, the control unit 100 controls the supply unit 3, the process unit PR, and the fixing unit 8 to perform print control. When performing print control, the control unit 100 controls the supply of electricity to the heater H1 so that the detected temperature T becomes the target fixing temperature. When fixing a toner image on a sheet S, the control unit 100 rotates the rotating member 81A.

[0053] Furthermore, the control unit 100 executes standby control to maintain the temperature of the fixing unit 8 at a target standby temperature TR based on the detected temperature T detected by the central temperature sensor ST1. The standby control maintains the fixing unit 8 at a predetermined target standby temperature TR that is higher than room temperature but lower than the target fixing temperature so that printing can be started promptly. The control unit 100 transitions to standby control, for example, when the image forming apparatus 1 is turned on or when print control is completed. If a print job is not input even after a predetermined time has elapsed since the start of standby control, the control unit 100 stops powering the heater H1 of the fixing unit 8 and transitions to sleep mode.

[0054] During standby control, which maintains the temperature of the fixing unit 8 at a target standby temperature TR based on the detected temperature T detected by the central temperature sensor ST1, the control unit 100 energizes the heater H1 for a heating period shorter than the control period at each predetermined control period when the detected temperature T falls below the target standby temperature TR. The control period is a period of a fixed length. The control period is, for example, approximately 0.5 to 5 seconds, and is determined appropriately depending on, for example, an operation test of the image forming apparatus 1. The shorter the control period, the shorter the period during which the heater H1 is stopped, which can prevent the temperature of the heater H1 from becoming too low. The control unit 100 energizes the heater H1 during the heating period between control periods. When executing standby control, the control unit 100 does not rotate the rotary member 81A.

[0055] The amount of heat required for the image forming apparatus 1 to maintain the fuser 8 at the target standby temperature TR, i.e., the amount of power supplied to the heater H1, varies depending on factors such as the ambient temperature, the temperature of the fuser 8, the power supply voltage, and variations in the heating capacity of the heater H1. Therefore, the control unit 100 appropriately changes the amount of power supplied to the heater H1 during the heating period between control cycles. There are two ways to change the amount of power: changing the output (amount of power supplied per unit time) of the heater H1 without changing the length of the heating period, or changing the length of the heating period without changing the output of the heater H1. In this embodiment, we will explain how to change the amount of power supplied to the heater H1 by changing the length of the heating period. That is, when the control unit 100 increases the amount of power supplied, it lengthens the heating period from the previous heating period, and when it decreases the amount of power supplied, it shortens the heating period from the previous heating period.

[0056] The control unit 100 can control the energization of the heater H1 during the heating period using wave number control at a set duty ratio. In this embodiment, the output of the heater H1 is not changed, so the duty ratio is constant. The control unit 100 changes the amount of energization by changing the number of energizations of a predetermined energization pattern of wave number control, assuming that the number of energizations of the predetermined energization pattern is one. That is, the control unit 100 changes the heating period by changing the number of energizations of the energization pattern. For example, as shown in FIG. 4, the control unit 100 energizes the heater H1 only during the first half-wave of three consecutive half-waves of the AC voltage, thereby achieving a duty ratio of 33%. The heating period is changed by changing the number of times this energization pattern is repeated. Note that hatched areas in FIG. 4 indicate energization. In this embodiment, the minimum number of repetitions is 2 (this number of repetitions is referred to as the "heating count"). When starting standby control for the first time after the image forming apparatus 1 is powered on, the control unit 100 sets the amount of current for the first heating period to the minimum value that can be set, that is, the number of heatings i = 2. Also, when starting standby control in a case other than after the image forming apparatus 1 is powered on, the control unit 100 sets the number of heatings i to the minimum, 2.

[0057] If the detected temperature T is lower than the target standby temperature TR when the control cycle has elapsed, the control unit 100 changes the amount of current to be applied during the next heating period to a higher amount than that during the previous heating period. If the detected temperature T is lower than the target standby temperature TR when the control cycle has elapsed, this is because it is highly likely that the amount of current applied during the previous heating period was insufficient to maintain the temperature of the fixing unit 8 at the target standby temperature TR.

[0058] During standby control, if the detected temperature T at the end of a control cycle is equal to or higher than the target standby temperature TR and lower than a threshold temperature TU higher than the target standby temperature TR, the control unit 100 sets the amount of power supplied in the next heating period to the same as that in the previous heating period. If the detected temperature T is equal to or higher than the target standby temperature TR and lower than the threshold temperature TU at the end of a control cycle, it is highly likely that the amount of power supplied in the previous heating period was appropriate for maintaining the temperature of the fixing unit 8 at the target standby temperature TR.

[0059] During standby control, if the detected temperature T at the end of a control cycle is equal to or higher than the threshold temperature TU, the control unit 100 changes the amount of power supplied in the next heating period to a smaller amount of power supplied in the previous heating period. If the detected temperature T is equal to or higher than the threshold temperature TU at the end of a control cycle, it is highly likely that the amount of power supplied in the previous heating period was too high to maintain the temperature of the fixing unit 8 at the target standby temperature TR.

[0060] When standby control is started, the control unit 100 starts a control cycle based on the detected temperature T becoming lower than the target standby temperature TR. In other words, if the detected temperature T is equal to or higher than the target standby temperature TR when standby control is started, the control unit 100 waits until the detected temperature T becomes lower than the target standby temperature TR, and then starts the first control cycle.

[0061] Furthermore, in the standby control, when the detected temperature T becomes equal to or higher than the threshold temperature TU, the control unit 100 does not start the next control cycle until the detected temperature T becomes lower than the target standby temperature TR.

[0062] An example of the standby control process of the control unit 100 for realizing the above-described control unit 100 will be described with reference to Fig. 5. As shown in Fig. 5, when the control unit 100 starts standby control, it first sets the number of heating cycles i to 2, which is the minimum value and also serves as the initial value (S101). Then, the control unit 100 determines whether the detected temperature T is less than the target standby temperature TR (S102). If the control unit 100 determines that the detected temperature T is not less than the target standby temperature TR (No), it waits until the detected temperature T becomes less than the target standby temperature TR.

[0063] If the control unit 100 determines that the detected temperature T is lower than the target standby temperature TR (Yes), it determines whether the detected temperature T is higher than the lower limit temperature TL (S103). If the detected temperature T is not higher than the lower limit temperature TL (No), for example, if the fixing unit 8 is completely cooled, the control unit 100 energizes the heater H1 at 100% duty until the detected temperature T reaches the target standby temperature TR in order to quickly raise the temperature of the fixing unit 8 (S104), and then returns to step S102 to repeat the process.

[0064] If the control unit 100 determines in step S103 that the detected temperature T is greater than the lower limit temperature TL (Yes), it starts a control cycle and energizes the heater H1 by repeating the energization pattern i times (S110). Thereafter, the control unit 100 determines whether the control cycle has elapsed since the start of the control cycle, that is, since energization of the heater H1 began (S111). If the control unit 100 determines that the control cycle has not elapsed (No), it waits until the control cycle has elapsed.

[0065] If the control unit 100 determines that the control period has elapsed (Yes), it determines whether the detected temperature T is equal to or higher than the target standby temperature TR (S120). If the control unit 100 determines that the detected temperature T is not equal to or higher than the target standby temperature TR, that is, if the detected temperature T is lower than the target standby temperature TR (No), it increments the number of heating cycles i by 2 (S132) and returns to step S102 to repeat the process.

[0066] If the control unit 100 determines in step S120 that the detected temperature T is equal to or higher than the target standby temperature TR (Yes), it further determines whether the detected temperature T is lower than the threshold temperature TU (S141). If the control unit 100 determines that the detected temperature T is lower than the threshold temperature TU (Yes), it returns to step S102 and repeats the process. If the control unit 100 determines that the detected temperature T is not lower than the threshold temperature TU, that is, that the detected temperature T is equal to or higher than the threshold temperature TU (No), and the control unit 100 determines that the number of heating times i is equal to or higher than the threshold temperature TU, and if the number of heating times i is equal to the lower limit of 2 (Yes in S142), the control unit 100 returns to step S102 and repeats the process because the number of heating times i cannot be reduced any further. On the other hand, if the number of heating times i is not 2 (No in S142), it decreases the number of heating times i by 2 (S143), and then returns to step S102 and repeats the process.

[0067] An example of the operation of the heater H1 and the change in the detected temperature T when standby control is performed using the above process will be described. As shown by the solid line in FIG. 6, if the detected temperature T is higher than the target standby temperature TR when standby control is started at time t0, the control unit 100 waits until the detected temperature T reaches the target standby temperature TR (waiting for the temperature to drop). When the detected temperature T reaches the target standby temperature TR (t1), the control unit 100 starts a control cycle. The control unit 100 repeats the energization pattern with the smallest number of heating cycles i (=2) and energizes the heater H1 by duty control. Note that in FIG. 6, the period when the heater H1 is ON (heating period) is shown as a continuous ON cycle, but in detail, the heater H1 is repeatedly turned ON and OFF in small increments, repeating the 33% duty energization pattern shown in FIG. 4 for the number of heating cycles i. If the amount of heating is insufficient with the initial number of heating cycles i, such as when the ambient temperature is low, the detected temperature T drops as shown in FIG. 6. If the detected temperature T is lower than the target standby temperature TR at time t2 after the first control period, the control unit 100 increases the number of heating cycles i by 2, repeats the energization pattern four times in the second control period, and energizes the heater H1 for a longer period than in the first control period. This suppresses the temperature drop in the second control period compared to the first control period. If the detected temperature T is still lower than the target standby temperature TR after the second control period, the control unit 100 further increases the number of heating cycles i by 2, and repeats the energization pattern six times in the third control period. This increases the temperature of the fuser 8 in the third control period, and the temperature of the fuser 8 rises to, for example, a temperature equal to or higher than the threshold temperature TU. When the detected temperature T becomes equal to or higher than the threshold temperature TU, the control unit 100 waits until the detected temperature T reaches the target standby temperature TR (t4 to t5, waiting for a temperature drop). Then, in the next fourth control period, the number of heating cycles i is reduced by 2 from the previous fourth control period, to 4, and at time 5, the energization pattern is repeated four times to duty-control the heater H1. If the detected temperature T is equal to or higher than the target standby temperature TR and lower than the threshold temperature TU at the time when control cycle 4 has elapsed (t6), the heater H1 is duty-controlled by repeating the current application pattern four times in the next control cycle without changing the number of heating cycles i.

[0068] As another example of operation, if the detected temperature T drops to the lower limit temperature TL after standby control begins due to extremely low ambient temperature around the image forming apparatus 1, as shown by the dashed line in FIG. 6, heater H1 starts to be energized at 100% duty when the detected temperature T drops to the lower limit temperature TL, and continues to heat until the detected temperature T reaches the target standby temperature TR (t7). After that, the detected temperature T rises due to residual heat from heater H1 and exceeds the threshold temperature TU (t8). Therefore, the next control cycle is not started until the detected temperature T drops to the target standby temperature TR, and the system waits for the temperature to drop. Once the detected temperature T drops to the target standby temperature TR (t9), the control cycle is started.

[0069] In this way, according to the image forming apparatus 1 of this embodiment, after starting standby control, it is possible to supply the required amount of current for each control cycle, and since the temperature of the fixing device 8 does not rise too much, it is possible to prevent the heater H1 from being stopped for a long time.

[0070] If the detected temperature T at the end of the control cycle is equal to or higher than the target standby temperature TR and lower than the threshold temperature TU, the amount of power supplied in the next heating period can be set to the same as that in the previous heating period, thereby maintaining the detected temperature T at a temperature close to the target standby temperature TR.

[0071] Furthermore, if the detected temperature T at the end of the control period is equal to or higher than the threshold temperature TU, the amount of power supplied in the next heating period can be changed to a smaller amount than that supplied in the previous heating period, thereby maintaining the detected temperature T at a temperature close to the target standby temperature TR.

[0072] Furthermore, if the temperature of the fixing unit 8 is high when the control unit 100 starts standby control, the control unit 100 waits until the detected temperature T reaches the target standby temperature TR before starting the control cycle, thereby preventing the temperature of the fixing unit 8 from becoming unnecessarily high.

[0073] Furthermore, during standby control, when the detected temperature T becomes equal to or higher than the threshold temperature TU, the control unit 100 does not start the next control cycle until the detected temperature T becomes lower than the target standby temperature TR, thereby preventing the temperature of the fixing unit 8 from becoming unnecessarily high.

[0074] [Second embodiment] Next, a second embodiment will be described. In this embodiment, the same parts as those in the first embodiment will be denoted by the same reference numerals in the drawings and their description will be omitted, and only the differences will be described in detail. The image forming apparatus 1 according to the second embodiment differs from the first embodiment in that, in standby control, when the detected temperature T at the end of a control cycle is lower than the target standby temperature TR, the amount of change in the amount of power supplied is not constant, but increases as the deviation between the target standby temperature TR and the detected temperature T increases.

[0075] For example, the control unit 100 stores the table shown in Fig. 7. Fig. 7 is a table showing the relationship between the deviation TR-T and the increase in the number of heating times i. When the deviation TR-T between the target standby temperature TR and the detected temperature T is 0 to 2°C, the increase in the number of heating times i is 2. When the deviation TR-T is 2 to 4°C, the increase in the number of heating times i is 4. When the deviation TR-T is 4 to 6°C, the increase in the number of heating times i is 6. When the deviation TR-T is 6 to 8°C, the increase in the number of heating times i is 8. When the deviation TR-T is 8°C or more, the increase in the number of heating times i is 10. Note that here, the notation 2 to 4°C means, for example, not less than 2°C and not more than 4°C, but it may also be greater than 2°C and not more than 4°C.

[0076] As shown in FIG. 8, in this embodiment, if the detected temperature T at the end of the control period is lower than the target standby temperature TR (S120, No) and higher than the lower limit temperature TL (S131, Yes), i is increased according to the magnitude of the deviation TR-T in accordance with the table of FIG. 7 (S232).

[0077] With this configuration, if the deviation TR-T between the detected temperature T and the target standby temperature TR at the end of the control cycle is large, the next amount of current can be increased according to the deviation TR-T. Therefore, if the amount of current during the heating period of the previous control cycle was insufficient, it can be quickly brought closer to an appropriate amount of current.

[0078] [Third embodiment] Next, a third embodiment will be described. In this embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals in the drawings and their description will be omitted, and only the differences will be described in detail. In the image forming apparatus 1 according to the third embodiment, when changing the amount of power supplied during a heating period, the output of the heater H1 is changed without changing the length of the heating period. Specifically, the control unit 100 supplies power to the heater H1 during the heating period at a set duty ratio, and when changing the amount of power to a higher amount, the control unit 100 changes the duty ratio to a value larger than that of the previous heating period, and when changing the amount of power to a lower amount, the control unit 100 changes the duty ratio to a value smaller than that of the previous heating period.

[0079] For example, the control unit 100 stores the table shown in Fig. 9. Fig. 9 is a table showing the relationship between heating intensity j and duty ratio, where the duty ratio increases as the heating intensity j increases, such that when heating intensity j is 1, the duty ratio is 33%, when heating intensity j is 2, the duty ratio is 40%, and when heating intensity j is 8, the duty ratio is 100%.

[0080] 10, in standby control, the control unit 100 sets the heating intensity j to an initial value of 1 (S301). When the control cycle starts, heating is performed for a certain heating period at a duty ratio set according to the heating intensity j (S310). If the detected temperature T at the end of the control cycle is lower than the target standby temperature TR (S120, No), the control unit 100 determines whether the heating intensity j is at its upper limit of 8 (S332). If it is 8 (Yes), the control unit 100 returns to step S102 without increasing the heating intensity j. On the other hand, if the heating intensity j is not 8 (No), the control unit 100 increases the heating intensity j by 1 (S333) and returns to step S102.

[0081] Furthermore, if the detected temperature T at the end of the control cycle is equal to or higher than the target standby temperature TR (S120, Yes) and equal to or higher than the threshold temperature TU (S141, No), it is determined whether the heating intensity j is equal to the lower limit of 1 (S342), and if it is equal to the lower limit of 1 (Yes), the process returns to step S102 without decreasing the heating intensity j. On the other hand, if the heating intensity j is not 1 (No), the heating intensity j is decreased by 1 (S343) and the process returns to step S102.

[0082] According to this process, as shown in FIG. 11 , the temperature of the fixing device 8 can be maintained near the target standby temperature TR by changing the heating intensity j while keeping the heating period of the heater H1 constant during the control cycle. Note that, although FIG. 11 also shows the period during which the heater H1 is ON (heating period) as if it were continuously ON, in detail, the heater H1 is repeatedly turned ON and OFF in small increments according to the set duty ratio. In this embodiment, as in the first embodiment, if the amount of heat supplied to the fixing device 8 is insufficient, the output of the heater H1 can be increased to increase the amount of heat supplied, and if the amount of heat supplied to the fixing device 8 is too great, the output of the heater H1 can be decreased to decrease the amount of heat supplied. This prevents excessive heat from being supplied to the fixing device 8, thereby preventing the heater H1 from being stopped for long periods of time.

[0083] Although the embodiment has been described above, the present invention is not limited to the above embodiment, and specific configurations can be appropriately changed without departing from the spirit of the invention.

[0084] For example, in the above embodiment, the heater is energized with an AC voltage, but it may be energized with a DC voltage. When energized with a DC voltage, the amount of energization may be changed by duty control, or the amount of energization may be changed by changing the voltage.

[0085] In addition, in the above embodiment, the rotating member of the heating member is not rotated when standby control is executed, but this is not limited to this, and the rotating member may be rotated when standby control is executed.

[0086] In the above embodiment, an endless belt is used as the rotating member, but this is not limiting, and the rotating member may be, for example, a roller. In the above embodiment, a pressure roller is used as the pressure member, but this is not limiting, and the pressure member may be, for example, a pressure unit including an endless pressure belt.

[0087] In the above embodiment, the temperature sensor is provided to detect the temperature of the heating member, but this is not limiting. For example, the temperature sensor may be provided to detect the temperature of a portion other than the heating portion of the fixing unit, such as a pressure member. The temperature sensor may also be provided to directly detect the temperature of the heater. The temperature sensor may also be a temperature sensor other than a thermistor. The temperature sensor may also be a non-contact type temperature sensor or a contact type temperature sensor.

[0088] In the above embodiment, a halogen heater that uses radiant heat is used as the heater, but the heater is not limited to this and may be, for example, a ceramic heater or a carbon heater that uses heat generated by a resistor. Also, the heater may be disposed outside the heating element instead of inside the heating element.

[0089] In the above embodiment, the image forming apparatus is exemplified as an image forming apparatus that forms a monochrome image on a sheet, but is not limited to this and may be, for example, a printer configured to be able to form a color image on a sheet.Furthermore, the image forming apparatus is not limited to a printer and may be, for example, a copier or multifunction peripheral equipped with a document reading device such as a flatbed scanner.

[0090] Furthermore, the elements described in the above-described embodiment and modified examples can be implemented in appropriate combinations. [Explanation of symbols]

[0091] 1. Image forming device 8 Fixing unit 81 Heating element 81A Rotating member 82 Pressure member 100 control section H1 heater PR Process Department S seat ST1 Central Temperature Sensor

Claims

1. a toner image forming unit that forms a toner image on a sheet; a fixing device having a heater and fixing the toner image on the sheet; a temperature sensor for detecting the temperature of the fixing unit; a control unit, The control unit a standby control for maintaining the temperature of the fixing unit at a target standby temperature based on the detected temperature detected by the temperature sensor, When the detected temperature is lower than the target standby temperature, energizing the heater for a heating period shorter than the control period at each predetermined control period; When the detected temperature at the time of the lapse of the control period is lower than the target standby temperature, the amount of current to be supplied during the next heating period is changed to an amount of current to be supplied greater than that during the previous heating period; When the standby control is started, the amount of current supplied during the first heating period is set to a settable minimum value.

2. In the standby control, the control unit 2. The image forming apparatus according to claim 1, wherein, when the detected temperature at the time when the control cycle has elapsed is equal to or higher than the target standby temperature and is lower than a threshold temperature higher than the target standby temperature, the amount of power supplied in the next heating period is set to the same as that in the previous heating period.

3. In the standby control, the control unit 3. The image forming apparatus according to claim 2, wherein, when the detected temperature at the time when the control period has elapsed is equal to or higher than the threshold temperature, the amount of power applied in the next heating period is changed to a smaller amount of power applied in the previous heating period.

4. The control unit 4. The image forming apparatus according to claim 1, wherein when the standby control is started, the control cycle is started based on the detected temperature becoming lower than the target standby temperature.

5. In the standby control, the control unit 4. The image forming apparatus according to claim 2, wherein when the detected temperature is equal to or higher than the threshold temperature, the next control cycle is not started until the detected temperature becomes lower than the target standby temperature.

6. The control unit When changing the amount of current to a larger amount of current, change the heating period to a period longer than the previous heating period, 6. The image forming apparatus according to claim 1, wherein when the amount of energization is changed to a smaller amount of energization, the heating period is changed to a period shorter than the previous heating period.

7. The control unit energizing the heater during the heating period at a set duty ratio; When changing the amount of current to a larger amount, change the duty ratio to a value larger than that of the previous heating period, 6. The image forming apparatus according to claim 1, wherein when the amount of current is changed to a smaller amount, the duty ratio is changed to a value smaller than that of the previous heating period.

8. The control unit 6. The image forming apparatus according to claim 1, wherein the energization of the heater during the heating period is controlled by wave number control at a set duty ratio.

9. The control unit 9. The image forming apparatus according to claim 8, wherein the amount of current flow is changed by changing the number of times of current flow in a predetermined current flow pattern of wave number control, assuming that the current flow pattern is performed once.

10. The control unit 10. The image forming apparatus according to claim 1, wherein when the standby control is started for the first time after the power of the image forming apparatus is turned on, the amount of current flowing during the initial heating period is set to the minimum value that can be set.

11. The control unit An image forming apparatus according to any one of claims 1 to 10, characterized in that, in the standby control, if the detected temperature at the time the control cycle has elapsed is lower than the target standby temperature, the greater the deviation between the target standby temperature and the detected temperature, the greater the amount of change in the amount of power supplied.

12. The fixing device is a heating member that is heated by the heater and includes a rotating member that is rotatable around the heater; a pressure member that sandwiches the sheet between the pressure member and the heating member, The control unit When the toner image is fixed to the sheet, the rotating member is rotated, 12. The image forming apparatus according to claim 1, wherein the rotating member is not rotated when the standby control is executed.

13. The control unit When a toner image is fixed on a sheet by the fixing device, a print control is executed to control energization to the heater so that the detected temperature becomes a target fixing temperature; 13. The image forming apparatus according to claim 1, wherein the target standby temperature is lower than the target fixing temperature.

14. a toner image forming unit that forms a toner image on a sheet; a fixing device having a heater and fixing the toner image on the sheet; a temperature sensor for detecting the temperature of the fixing unit; A control method for an image forming apparatus including a control unit, The control unit executes standby control to maintain the temperature of the fixing unit at a target standby temperature based on the detected temperature detected by the temperature sensor; In the standby control, When the detected temperature is lower than the target standby temperature, energizing the heater for a heating period shorter than the control period at each predetermined control period; When the detected temperature at the time of the lapse of the control period is lower than the target standby temperature, the amount of current to be supplied during the next heating period is changed to an amount of current to be supplied greater than that during the previous heating period; The control method for an image forming apparatus, wherein when the standby control is started, the amount of current applied during the first heating period is set to a settable minimum value.

15. In the standby control, the control unit 15. The control method for an image forming apparatus according to claim 14, wherein, when the detected temperature at the end of the control period is equal to or higher than the target standby temperature and is lower than a threshold temperature higher than the target standby temperature, the amount of power supplied in the next heating period is set to the same as that in the previous heating period.

16. In the standby control, the control unit 16. The control method for an image forming apparatus according to claim 15, wherein, if the detected temperature at the time when the control period has elapsed is equal to or higher than the threshold temperature, the amount of current applied in the next heating period is changed to a smaller amount of current applied in the previous heating period.

17. The control unit 17. The control method for an image forming apparatus according to claim 14, wherein, when the standby control is started, the control cycle is started based on the detected temperature becoming lower than the target standby temperature.

18. In the standby control, the control unit 17. The control method for an image forming apparatus according to claim 15, wherein, when the detected temperature is equal to or higher than the threshold temperature, the next control cycle is not started until the detected temperature becomes lower than the target standby temperature.

19. The control unit When changing the amount of current to a larger amount of current, change the heating period to a period longer than the previous heating period, 18. The control method for an image forming apparatus according to claim 14, wherein when the amount of current is changed to a smaller amount of current, the heating period is changed to a period shorter than the previous heating period.

20. The control unit energizing the heater during the heating period at a set duty ratio; When changing the amount of current to a larger amount, change the duty ratio to a value larger than that of the previous heating period, 18. The method for controlling an image forming apparatus according to claim 14, wherein when the amount of current is changed to a smaller amount, the duty ratio is changed to a value smaller than that of the previous heating period.

21. The control unit 18. The method for controlling an image forming apparatus according to claim 14, wherein the energization of the heater during the heating period is controlled by wave number control at a set duty ratio.

22. The control unit 22. The control method for an image forming apparatus according to claim 21, wherein the amount of current flow is changed by changing the number of times of current flow in a predetermined current flow pattern of wave number control, assuming that the current flow pattern is performed once.

23. The control unit 23. The control method for an image forming apparatus according to claim 14, wherein when the standby control is started for the first time after the image forming apparatus is turned on, the amount of current flowing during the initial heating period is set to a settable minimum value.

24. The control unit 23. The control method for an image forming apparatus according to claim 14, wherein, in the standby control, if the detected temperature at the time when the control cycle has elapsed is lower than the target standby temperature, the greater the deviation between the target standby temperature and the detected temperature, the greater the amount of change in the amount of power supplied.

Images