Image forming apparatus and its control method

Abstract

To prevent a stop time of a heater from getting longer when maintaining a fuser in a standby state.SOLUTION: In standby control of maintaining a temperature of a fuser at a target standby temperature on the basis of, a detection temperature detected by a temperature sensor, when a temperature fluctuation time CT in a previous control cycle is less than a first threshold (S131, Yes), a control unit of an image forming apparatus increases the amount of energization En+1 in a next control cycle compared with a previous amount of energization En (S132). When the temperature fluctuation time CT in the previous control cycle is equal to or more than a second threshold larger than the first threshold (S133, Yes), the control unit reduces the amount of energization En+1 in the next control cycle compared with the previous amount of energization En (S135). When the temperature fluctuation time CT in the previous control cycle is equal to or more than the first threshold and less than the second threshold (S133, No), the control unit makes the amount of energization En+1 in the next control cycle the same as the previous amount of energization En.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus equipped with a fuser for fixing a toner image onto a sheet, and to a control method thereof. [Background technology]

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

[0003] In the technology described in Patent Document 1, a period of de-energization is provided after the heater is energized. If the detected temperature falls below the lower limit temperature during the de-energization period, the upper limit temperature is raised. If the detected temperature at the end of the de-energization period is higher than the lower limit temperature, the upper limit temperature is lowered.

[0004] Furthermore, in the technology described in Patent Document 2, when the detected temperature drops below the lower limit temperature (first temperature), the heater duty cycle 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 cycle corresponding to the next deviation is made smaller than the duty cycle corresponding to the current deviation. If the detected peak temperature is lower than the target peak temperature, the duty cycle corresponding to the next deviation is made larger than the duty cycle corresponding 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 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Incidentally, the heater temperature differs significantly from the fuser temperature. Here, "fuser temperature" refers to the temperature of the heat roller surface, for example, depending on the location of the temperature sensor installed in the fuser. When power is supplied to the heater, the heater temperature is much higher than the fuser temperature, and after power is cut off, it approaches the fuser temperature. Therefore, even if power to the heater is cut off based on the temperature detected by the temperature sensor installed in the fuser reaching its upper limit, the temperature detected by the temperature sensor will rise due to residual heat from the heater, and then gradually decrease. The peak temperature of the fuser at this time will vary depending on factors such as the heat storage state of the fuser itself and the surrounding components, and the surrounding environment. Therefore, if the peak temperature becomes high, it will take a long time for the detected temperature to drop to the lower limit. As time passes after power is cut off to the heater, the heater temperature decreases, and as the heater temperature decreases, the electrical resistance of the heater decreases. If a long time passes between the deactivation of the heater and the detection temperature reaching the lower limit, the heater temperature drops too low, resulting in a reduced heater resistance. This increases the inrush current flowing through the heater when power is reactivated. This increase in inrush current can lead to problems such as flicker and power supply voltage noise.

[0007] This invention has been made in view of the above background, and aims to suppress the prolonged heater stop time when the fuser is kept in a standby state. [Means for solving the problem]

[0008] An image forming apparatus that solves the aforementioned problems comprises a toner image forming unit that forms a toner image on a sheet, a fuser having a heater that fixes the toner image to the sheet, a temperature sensor that detects the temperature of the fuser, and a control unit. In standby control, which maintains the fuser temperature at a target standby temperature based on the temperature detected by the temperature sensor, the control unit starts powering the heater when the detected temperature falls below the target standby temperature. The control unit supplies power to the heater by amount E for the set heating period. nenergizes, and after the heating period elapses, if the detected temperature is equal to or higher than the target standby temperature, a control cycle of waiting until the temperature drops to the target standby temperature is repeated. The control unit defines the time from when energization of the heater is started to when one control cycle ends as a temperature fluctuation time, and if the temperature fluctuation time of the previous control cycle is less than a first threshold value, the energization amount E of the next control cycle n+1 is set to be larger than the previous energization amount E n . On the other hand, when the temperature fluctuation time of the previous control cycle is equal to or greater than a second threshold value that is larger than the first threshold value, the control unit sets the energization amount E of the next control cycle n+1 to be smaller than the previous energization amount E n . Further, when the temperature fluctuation time of the previous control cycle is equal to or greater than the first threshold value and less than the second threshold value, the control unit sets the energization amount E of the next control cycle n+1 to be the same as the previous energization amount E n .

[0009] A control method for an image forming apparatus that solves the above-described problem is a control method for an image forming apparatus including: a toner image forming unit that forms a toner image on a sheet; a fixing device that has a heater and fixes the toner image onto the sheet; and a control unit. In standby control for maintaining the temperature of the fixing device at a target standby temperature, the control unit starts energizing the heater when the temperature of the fixing device becomes less than the target standby temperature. The control unit energizes the heater with the energization amount E n for a set heating period, and after the heating period elapses, if the temperature of the fixing device becomes equal to or higher than the target standby temperature, the control unit repeats a control cycle of waiting until the temperature drops to the target standby temperature. Then, with the time from when energization of the heater is started to when one control cycle ends being defined as a temperature fluctuation time, if the temperature fluctuation time of the previous control cycle is less than a first threshold value, the energization amount E of the next control cycle n+1 is set to be larger than the previous energization amount E n . On the other hand, when the temperature fluctuation time of the previous control cycle is equal to or greater than a second threshold value that is larger than the first threshold value, the control unit sets the energization amount E of the next control cycle n+1 to be smaller than the previous energization amount E nto be smaller than that. Further, when the temperature fluctuation time of the previous control cycle is not less than the first threshold and less than the second threshold, the control unit sets the energization amount E of the next control cycle n+1 to be the same as the previous energization amount E n .

[0010] According to this configuration, as control cycles are repeated, energization is performed with an appropriate energization amount such that the temperature fluctuation time of one control cycle approaches a time that is not less than the first threshold and less than the second threshold. Therefore, an increase in the stop time of the heater can be suppressed. This makes it possible to suppress an increase in inrush current flowing through the heater when energization of the heater is started. Further, if the control cycle becomes excessively short, the long-term flicker index (Plt) may be degraded. With the above configuration, since the temperature fluctuation time of one control cycle approaches a time that is not less than the first threshold and less than the second threshold, degradation of the long-term flicker index can be suppressed.

[0011] After the heating period has elapsed, if the temperature of the fixing device does not reach or exceed the target standby temperature even after a predetermined period has elapsed from the start of the heating period, the control unit energizes the heater to start the next control cycle, and sets the energization amount E of the next control cycle n+1 to be larger than the previous energization amount E n .

[0012] According to this configuration, even when the temperature of the fixing device does not reach or exceed the target standby temperature after a predetermined period has elapsed from the start of the heating period because the environmental temperature is extremely low, the temperature of the fixing device can be brought close to the target standby temperature.

[0013] When changing the energization amount to a larger energization amount, the control unit may change the heating period to a longer period than the heating period of the previous control cycle, and when changing the energization amount to a smaller energization amount, the control unit may change the heating period to a shorter period than the heating period of the previous control cycle.

[0014] In this case, the control unit may control the energization of the heater during the heating period by wave number control, and change the amount of energization by changing the number of energization cycles of the predetermined energization pattern for the wave number control, counting one predetermined energization pattern as one time.

[0015] When the temperature fluctuation time of the previous control cycle is equal to or greater than a third threshold value that is larger than the second threshold value, the control unit may provide a preheating period before the heating period in the next control cycle, and after performing energization at a first intensity in the preheating period, perform energization at a second intensity that is higher than the first intensity in the heating period.

[0016] According to this configuration, even when the temperature fluctuation time becomes long and the temperature of the heater drops, the heater is first energized at the first intensity, which is lower than the heating intensity in the heating period of the previous control cycle, and then energized at the second intensity, which is higher than the first intensity. For this reason, power is gradually supplied to the heater whose resistance has decreased due to the temperature drop, and the temperature rises gradually, so that an increase in the inrush current flowing through the heater when energization of the heater is started can be suppressed.

[0017] When providing the preheating period, the control unit may control the energization of the heater during the heating period by wave number control, and control the energization of the heater during the preheating period by phase control.

[0018] The control unit may energize the heater during the heating period at a set duty ratio; when changing the energization amount to a larger energization amount, change the duty ratio to a value larger than that in the heating period of the previous control cycle, and when changing the energization amount to a smaller energization amount, change the duty ratio to a value smaller than that in the heating period of the previous control cycle.

[0019] As described above, the energization amount during the heating period can also be changed by changing the duty ratio.

[0020] When the control unit initiates standby control after the power to the image forming apparatus is turned on, it is desirable to set the amount of current supplied during the heating period of the first control cycle to the minimum possible value.

[0021] With this configuration, an excessive amount of heat is not supplied to the fuser, thus preventing the heater from taking too long.

[0022] The control unit, if the temperature fluctuation time of the previous control cycle is less than the first threshold, will adjust the amount of change in the energized amount E as the temperature fluctuation time decreases. n+1 -E n You can make it larger.

[0023] With this configuration, if the amount of current supplied during the heating period of the previous control cycle was insufficient, it is possible to quickly adjust it to an appropriate amount of current.

[0024] The fuser may include a heating element that is heated by a heater, which includes a rotating element that can rotate around the heater, and a pressing element that sandwiches a sheet between the heating element and the heating element. In this case, the control unit can rotate the rotating member when fixing the toner image to the sheet, and refrain from rotating the rotating member when performing standby control.

[0025] The control unit performs print control when fixing the toner image to the sheet using the fuser, controlling the power supply to the heater so that the detected temperature reaches the target fixing temperature. The target standby temperature is lower than the target fixing temperature.

[0026] In another configuration, the control unit may, after the heating period has elapsed and the detected temperature has risen to or above the target standby temperature, wait until the detected temperature drops to the target standby temperature. If the detected temperature after a second predetermined period has elapsed from the start of the heating period is higher than the target standby temperature, the control unit may energize the heater at a third intensity during the execution of the temperature reduction wait.

[0027] According to this, it is possible to suppress the heater temperature from dropping too low, and therefore, when the heater is energized in the next control cycle, it is possible to suppress the inrush current flowing to the heater from becoming too large.

[0028] When energizing the heater at the third intensity, the control unit may control the energization by wavenumber control with a duty cycle of 33%. [Effects of the Invention]

[0029] According to the present invention, when the fuser is kept in a standby state, it is possible to suppress the heater from becoming idle for an extended period. [Brief explanation of the drawing]

[0030] [Figure 1] This figure shows a laser printer according to this embodiment. [Figure 2] This is a perspective view showing the arrangement of sensors on a nip plate. [Figure 3] This is a block diagram showing the configuration of the control unit. [Figure 4] (a) A diagram showing an example of a current flow pattern in wavenumber control, and (b) A diagram showing an example of a voltage waveform when a preheating period for phase control is provided. [Figure 5] This is a flowchart showing the standby control process in the first embodiment. [Figure 6] This is a subroutine that updates the number of heating cycles. [Figure 7] This is a time chart showing an example of heater operation and temperature change in standby control in the first embodiment. [Figure 8] This is an example of a subroutine for updating the number of heating cycles in the second embodiment. [Figure 9] This is an example of a table for setting the duty cycle in the third embodiment. [Figure 10] This is a flowchart showing the standby control process in the third embodiment. [Figure 11]This is an example of a subroutine for updating the number of heating cycles in the third embodiment. [Figure 12] This is a time chart showing an example of heater operation and temperature changes in standby control in the third embodiment. [Figure 13] This is a flowchart showing the standby control process in the fourth embodiment. [Figure 14] This is a time chart showing an example of heater operation and temperature change in standby control in the fourth embodiment. [Modes for carrying out the invention]

[0031] Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. As shown in Figure 1, the image forming apparatus 1 is a laser printer that forms an image on a sheet S, and comprises a main body housing 2, a supply unit 3, a process unit PR, a fuser 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 in the lower part of the main housing 2. The supply unit 3 comprises a supply tray 31 for holding sheets S, a sheet pressing plate 32, and a supply mechanism 33. The supply mechanism 33 comprises a pickup roller 33A, a separation roller 33B, a first transport roller 33C, and a registration roller 33D. In the supply unit 3, the sheets S in the supply tray 31 are pushed towards the pickup roller 33A by the sheet pressing plate 32, and then sent to the separation roller 33B by the pickup roller 33A. The sheets S are separated into individual sheets by the separation roller 33B and transported by the first transport roller 33C. The registration roller 33D aligns the leading edge of the sheets S and then transports the sheets S toward the process unit PR. Here, the direction in which the sheets S are transported is called the transport direction, and the direction perpendicular to the transport direction within the plane of the sheets S is called the width direction. Hereafter, the width direction of the sheets S will simply be referred to as the "width direction".

[0033] The process unit PR has the 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 comprises an exposure device 4 and a process cartridge 5.

[0034] The exposure device 4 is located at the top of the main housing 2 and includes a laser light source (not shown), a polygon mirror (with the symbols omitted), a lens, a reflector, etc. In the exposure device 4, laser light based on image data is emitted from the laser light source and scanned on the surface of the photoreceptor drum 61, thereby exposing the surface of the photoreceptor drum 61.

[0035] The process cartridge 5 is located below the exposure device 4 and can be attached to and detached from the main body 2 through an opening created when the front cover 21 on the main body 2 is opened. The process cartridge 5 comprises a drum unit 6 and a developing unit 7.

[0036] The drum unit 6 comprises a photoreceptor drum 61, a charger 62, and a transfer roller 63. The developing unit 7 is detachable from the drum unit 6 and comprises a developing roller 71, a supply roller 72, a layer thickness regulating blade 73, a toner storage section 74 for storing dry toner, and an agitator 75.

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

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

[0039] The fuser 8 fixes the toner image onto the sheet S. The fuser 8 includes a heater H1, a heating member 81 heated by the heater H1, the heating member 81 including a rotating member 81A that can rotate around the heater H1, and a pressing 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, it is a halogen heater as an example.

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

[0041] The pressurizing member 82 is a rotatable pressurizing 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 positioned inside the heating member 81 such that the inner surface of the heating member 81 slides against the nip plate NP's lower surface. The nip plate NP sandwiches the heating member 81 between itself and the pressurizing member 82. In the fuser 8, the sheet S on which the toner image has been transferred is transported between the heating member 81 and the pressurizing member 82, thereby heat-fixing the toner image onto the sheet S. The sheet S on which the toner image has been heat-fixed is discharged onto the discharge tray 22 by the second transport roller 23 and the discharge roller 24.

[0043] As shown in Figure 2, the nip plate NP has a central detection section 131 and an end detection section 132 that protrude from the end of the sheet S in the transport direction. The central detection section 131 is located in the center in the width direction. The end detection section 132 is located at the end in the width direction. A central temperature sensor ST1 is positioned opposite the central detection section 131. An end temperature sensor ST2 is positioned opposite the end detection section 132. The central temperature sensor ST1 is an example of a temperature sensor that detects the temperature of the fuser 8.

[0044] The central temperature sensor ST1 is a sensor that detects the temperature of the central part of the heating element 81 in the width direction. The central temperature sensor ST1 can detect the temperature of the central part of the heating element 81 by detecting the temperature of the nip plate NP by contact or non-contact with the central detection part 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 element 81 in the width direction. The edge temperature sensor ST2 can detect the temperature of the edge of the heating element 81 by detecting the temperature of the nip plate NP by contact or non-contact with the edge detection part 132 of the nip plate NP. Specifically, in the width direction, the edge temperature sensor ST2 is located outside the largest area SW of the sheet S that can be fixed by the fuser 8. The edge temperature sensor ST2 may also be located within the area SW in the width direction.

[0046] For example, thermistors can be used as the central temperature sensor ST1 and the end temperature sensor ST2.

[0047] As shown in Figure 3, the control unit 100 includes an ASIC 110 and a power supply circuit 120. The ASIC 110 includes a CPU 111, a heater controller 112, and a storage unit such as a ROM 113 or RAM 114. The power supply circuit 120 is a circuit that includes a switching circuit, etc., that switches the input AC voltage between a powered state and a non-powered 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 the 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 the feedback process when the heater controller 112 performs power supply control to the heater H1.

[0049] The heater controller 112 is a function or circuit built into the ASIC 110. When performing print control, it controls the power supply circuit 120 so that the detected temperature T detected by the central temperature sensor ST1 becomes the target fixing temperature, thereby supplying power to the heater H1. Specifically, the heater controller 112 determines the duty cycle of the AC voltage supplied to the heater H1 based on the detected temperature T detected by the central temperature sensor ST1 and the target fixing temperature, and performs feedback processing to control the power supply circuit 120 with the determined duty cycle. Note that the feedback processing performed by the heater controller 112 may be implemented on an external chip outside the ASIC 110, or it may be executed by the CPU 111.

[0050] Furthermore, in standby control to maintain the temperature of the fuser 8 at the target standby temperature TR, the heater controller 112 supplies power to the heater H1 with the duty cycle and heating time instructed by the CPU 111.

[0051] The control unit 100 performs various calculations based on the print job output from an external computer, the temperature detected by the central temperature sensor ST1 and the end temperature sensors ST2, and the programs and data stored in the memory unit, thereby executing control. In other words, the control unit 100 functions as a means of performing various controls by operating according to the program.

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

[0053] Furthermore, the control unit 100 performs standby control to maintain the temperature of the fuser 8 at the target standby temperature TR based on the detected temperature T detected by the central temperature sensor ST1. Standby control is a control that maintains the fuser 8 at a predetermined target standby temperature TR that is higher than room temperature and lower than the target fixing temperature, so that printing can be started quickly. The control unit 100 switches to standby control, for example, when the power to the image forming apparatus 1 is turned on and the temperature of the fuser is raised to the fixing temperature, but no print job is received after a predetermined time has elapsed, or when print control is completed. If no print job is input even after a predetermined time has elapsed since the start of standby control, the control unit 100 cuts off power to the heater H1 of the fuser 8 and switches to sleep mode.

[0054] In standby control, which maintains the temperature of the fuser 8 at the target standby temperature TR based on the detected temperature T detected by the central temperature sensor ST1, the control unit 100 starts energizing the heater H1 when the detected temperature T falls below the target standby temperature TR. Then, the control unit 100 energizes the heater H1 by amount E for the set heating period. n The system is energized, and after the heating period has elapsed, if the detected temperature T rises above the target standby temperature TR, the system repeats a control cycle in which it waits until the temperature drops to the target standby temperature TR. The duration of this control cycle is the period from when the heater H1 is energized until the detected temperature T drops to the target standby temperature TR. In this specification, the waiting period after the heating period has elapsed, when the detected temperature T rises above the target standby temperature TR, is referred to as "temperature drop waiting." In other words, temperature drop waiting is a control method in which, during the control cycle, the system waits from the time the detected temperature T exceeds the target standby temperature TR until it drops to the target standby temperature TR.

[0055] The amount of current E supplied during the heating period. n Normally, the current flow rate E n The amount of current supplied is such that the temperature of the fuser 8 rises above the target standby temperature TR. Exceptionally, if the temperature of the environment in which the image forming apparatus 1 is installed is extremely low, the amount of current supplied E n Even when the heater H1 is energized, the detected temperature T may not reach the target standby temperature TR. In such cases, if the temperature does not reach the target standby temperature after a predetermined period has elapsed since the start of the heating period, the control unit 100 energizes the heater H1 and starts the next control cycle. The control unit 100 then sets the energization amount E for the next control cycle. n+1 The previous current supply amount E n To make it larger than [the specified value]. In this case, the duration of the control cycle is the period from when power is supplied to heater H1 until a predetermined period has elapsed. Note that when heater H1 is powered, power is repeatedly supplied and desupplied in small increments by duty cycle control, but the short periods of power desupply in the duty cycle control are included in the heating period.

[0056] The aforementioned predetermined period is a period of a fixed length. The predetermined period is, for example, about 0.5 to 5 seconds, and is appropriately determined according to the operation test of the image forming apparatus 1, etc. The length of the time set as the heating period is shorter than the length of the predetermined period, and the control unit 100 energizes the heater H1 during the heating period within the predetermined period. Furthermore, when the control unit 100 is performing standby control, it does not rotate the rotating member 81A.

[0057] The control unit 100 then defines the time from when power is supplied to the heater H1 until the end of one control cycle as the temperature fluctuation time CT. If the temperature fluctuation time CT of the previous control cycle is less than the first threshold CT1, the power supply amount E of the next control cycle is determined. n+1 The previous current supply amount E n To make it larger than that. On the other hand, if the temperature fluctuation time CT of the previous control cycle is greater than or equal to the second threshold CT2 which is greater than the first threshold CT1, the control unit 100 will set the energization amount E of the next control cycle. n+1The previous current supply amount E n To make it smaller than. Also, if the temperature fluctuation time CT of the previous control cycle is greater than or equal to the first threshold CT1 and less than the second threshold CT2, the control unit 100 will set the energization amount E of the next control cycle. n+1 The previous current supply amount E n Make it the same.

[0058] The amount of heat required by the image forming apparatus 1 to maintain the fuser 8 at the target standby temperature TR, i.e., the amount of current supplied to the heater H1, varies depending on 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 current supplied to the heater H1 during the heating period between control cycles. There are two ways to change the amount of current supplied: changing the output of the heater H1 (amount of current supplied per unit time) without changing the length of the heating period, and changing the length of the heating period without changing the output of the heater H1. In this embodiment, the case in which the amount of current supplied to the heater H1 is changed by changing the length of the heating period will be described. That is, when the control unit 100 changes the amount of current supplied to a larger amount, it changes the heating period to a longer period than the previous heating period, and when the amount of current supplied to a smaller amount, it changes the heating period to a shorter period than the previous heating period.

[0059] The control unit 100 can control the energization of the heater H1 during the heating period by wavenumber control at a set duty cycle. In this embodiment, the output of the heater H1 is not changed, so the duty cycle is constant. The control unit 100 changes the amount of energization by changing the number of times a predetermined energization pattern of wavenumber control is performed, with one set of this pattern being considered as one. That is, the control unit 100 changes the heating period by changing the number of times the energization pattern is performed. For example, as shown in Figure 4(a), the control unit 100 energizes the heater only during the period of the first of three consecutive half-waves of the AC voltage, resulting in a duty cycle of 33%. The heating period is then changed by changing the number of times this energization pattern is repeated. Note that the hatched areas in Figure 4 indicate energization. In this embodiment, the minimum number of repetitions is 2 (this number of repetitions is called the "heating cycle"). When the control unit 100 starts standby control for the first time after the power to the image forming apparatus 1 is turned on, it sets the amount of power supplied during the initial heating period to the minimum possible value, for example, the number of heating cycles i=2. In addition, when the control unit 100 starts standby control at times other than after the power to the image forming apparatus 1 has been turned on, it sets the number of heating cycles i to the minimum value of 2.

[0060] If the temperature fluctuation time CT becomes long, the temperature of heater H1 may decrease, potentially reducing its electrical resistance. If power is then supplied to heater H1, a large inrush current may flow. Therefore, when the temperature fluctuation time CT becomes long, the control unit 100 gradually supplies power to heater H1, allowing its temperature to rise before proceeding with normal heating.

[0061] To explain in detail, the temperature of heater H1 decreases as time passes after the heating period ends and heater H1 is no longer powered. The detected temperature T also decreases as time passes after the heating period ends and heater H1 is no longer powered, but the rate at which it decreases is easily affected by the environment in which the image forming apparatus 1 is installed. For example, if the temperature of the environment in which the image forming apparatus 1 is installed is high, the detected temperature T will not decrease easily, and the temperature fluctuation time CT will become longer. Furthermore, the resistance of heater H1 decreases as the temperature of heater H1 decreases. Therefore, if the temperature fluctuation time CT becomes long, there is a risk that a large inrush current will flow through heater H1 when power is turned on to heater H1 in the next heating period. Therefore, to avoid a large inrush current flowing through heater H1, the control unit 100 energizes heater H1 during a period separate from the heating period if the temperature fluctuation time CT becomes long. This raises the temperature of heater H1 and prevents its resistance from becoming too low. In the following, "energizing heater H1 to raise its temperature during a period separate from the heating period" will be referred to as "preheating," and "the period for performing preheating" will be referred to as the "preheating period."

[0062] Specifically, if the temperature fluctuation time CT of the previous control cycle is greater than or equal to the third threshold CT3 (which is greater than the second threshold CT2), the control unit 100 sets a preheating period before the heating period in the next control cycle. During the preheating period, the unit applies current at a first intensity lower than the heating intensity of the heating period in the previous control cycle, and then applies current at a second intensity stronger than the first intensity during the heating period. In this embodiment, when a preheating period is set, in order to prevent the total amount of current applied during the preheating period and the heating period from becoming too large, if the number of heating cycles i is greater than the initial value of 2 and a preheating period is set, the actual number of heating cycles is adjusted to be less than the value of i. Also, in this embodiment, when a preheating period is set and preheating is performed, the temperature fluctuation time CT is counted during the preheating period as well.

[0063] Here, heating intensity refers to power per unit time, and when power is supplied with an AC voltage, it refers to the duty cycle of the AC voltage supplied to the heater H1. In this embodiment, when the temperature fluctuation time CT is greater than or equal to the third threshold CT3, as shown in Figure 4(b), the control unit 100 controls the supply of power to the heater H1 during the heating period by wavenumber control and controls the supply of power to the heater during the preheating period by phase control. Here, phase control is a method of controlling the duty cycle of the voltage supplied to the load (heater H1 in this embodiment) by controlling the firing phase for each half-cycle of the AC voltage. Furthermore, wavenumber control is a method of controlling the duty cycle of the voltage supplied to the load (heater H1 in this embodiment) by controlling the ratio of the number of half-waves supplied to the load to the number of half-waves not supplied within a predetermined period of the AC voltage. As an example, during the preheating period, the phase angle of the phase control is set to be larger than half of half a half wave (90°) of the AC voltage, so that the duty cycle is smaller compared to the heating period. In the example in Figure 4(b), the phase control of half a wave is repeated six times during the preheating period. Note that while Figure 4(b) shows an example of voltage change, in terms of current change, during the first half wave of the preheating period, a large current flows through heater H1 because heater H1 has cooled and its electrical resistance has decreased, and the current flowing through heater H1 decreases with each subsequent half wave.

[0064] An example of the standby control process of the control unit 100, which realizes the control unit 100 as described above, will be explained with reference to Figure 5. As shown in Figure 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 the initial value (S110). Then, it starts counting the temperature fluctuation time CT (S111) and waits until the detected temperature T falls below the target standby temperature TR (S112, No). When the control unit 100 determines that the detected temperature T has fallen below the target standby temperature TR (S112, Yes), it updates the number of heating cycles i (S130).

[0065] The update of the heating count i is performed based on the temperature fluctuation time CT counted from the start of heating in the control cycle. Specifically, as shown in Figure 6, the control unit 100 determines whether the temperature fluctuation time CT is less than the first threshold CT1 (S131). If it determines that the temperature fluctuation time CT is less than the first threshold CT1 (Yes), it increases the heating count i by 2 (S132) and terminates the subroutine for updating the heating count i. On the other hand, if the control unit 100 determines that the temperature fluctuation time CT is not less than the first threshold CT1, that is, that the temperature fluctuation time CT is greater than or equal to the first threshold CT1 (No), it further determines whether the temperature fluctuation time CT is greater than or equal to the second threshold CT2 (S133). If the control unit 100 determines that the temperature fluctuation time CT is not greater than or equal to the second threshold CT2 (No), that is, if the temperature fluctuation time CT is greater than or equal to the first threshold CT1 and less than the second threshold CT2, it is considered that the previous energization amount En was relatively appropriate, and therefore terminates the subroutine for updating the heating count i without changing the heating count i. If the control unit 100 determines that the temperature fluctuation time CT is greater than or equal to the second threshold CT2 (Yes), it determines whether the number of heating cycles i is greater than the lower limit of 2 (S134). If it is greater (Yes), it reduces the number of heating cycles i by 2 (S135) and terminates the subroutine for updating the number of heating cycles i. On the other hand, if the control unit 100 determines that the number of heating cycles i is not greater than the lower limit of 2 (S134, No), it terminates the subroutine for updating the number of heating cycles i without changing the number of heating cycles i.

[0066] Returning to Figure 5, the control unit 100 updates the number of heating cycles i (S130), resets the temperature fluctuation time CT, and starts counting the time for the temperature fluctuation time CT (S140). At this time, the temperature fluctuation time CT before the reset is stored separately as the previous temperature fluctuation time CT. Then, it determines whether the previous temperature fluctuation time CT is greater than or equal to the third threshold CT3 (S150). If it is determined that the previous temperature fluctuation time CT is greater than or equal to the third threshold CT3 (Yes), a preheating period is provided before the heating period. That is, the heater H1 is energized for a period of 6 half-waves in phase control (S151). Then, the control unit 100 proceeds to step S160. Also, if the control unit 100 determines in step S150 that the previous temperature fluctuation time CT is not greater than or equal to the third threshold CT3 (No), it proceeds to step S160 without providing a preheating period.

[0067] Subsequently, the control unit 100 repeats the energizing pattern i times to energize the heater H1 and heat it (S160). Then, the control unit 100 determines whether a predetermined time has elapsed since the start of heating, that is, since the start of energizing the heater H1 (S170). If the control unit 100 determines that the predetermined period has not elapsed (No), it waits until the predetermined period has elapsed.

[0068] If the control unit 100 determines that a predetermined period has elapsed (Yes), it determines whether the detected temperature T is equal to or greater than the target standby temperature TR (S171). If the control unit 100 determines that the detected temperature T is not equal to or greater than the target standby temperature TR, that is, if the detected temperature T is less than the target standby temperature TR (No), it terminates the control cycle and proceeds to update the heating count i in step S130 in order to start the next heating cycle. On the other hand, if it determines that the detected temperature T is equal to or greater than the target standby temperature TR (Yes), it returns to step S112 and waits until the detected temperature T drops to the target standby temperature TR (S112, No). Once the detected temperature T drops to the target standby temperature TR (S112, Yes), it proceeds to update the heating count i in step S130 in order to start the next heating cycle.

[0069] The control unit 100 repeats the process in steps S112 to S171 until a condition for terminating standby control is met, such as receiving a new print job.

[0070] An example of the operation of heater H1 and the change in detected temperature T when standby control is performed using the above process will be explained. As shown by the solid line in Figure 7, when standby control is started at time t0, for example, when print control is completed, the control unit 100 waits until the detected temperature T reaches the target standby temperature TR. When the detected temperature T reaches the target standby temperature TR (t1), the control unit 100 starts the control cycle. The control unit 100 repeats the energizing pattern with the fewest number of heating cycles i (=2) and energizes the heater H1 by duty cycle control. In Figure 7, the period during which the heater H1 is ON (heating period) is shown as a continuous ON state, but in detail, the 33% duty cycle energizing pattern shown in Figure 4 is repeated for the number of heating cycles i, with frequent ON and OFF cycles. In the first control cycle 1, for example, if the amount of current supplied is insufficient for the ambient temperature, the temperature fluctuation time CT from when the detected temperature T exceeds the target standby temperature TR once until it falls back to the target standby temperature TR will be smaller than the first threshold CT1. In this case, the control unit 100 increases the number of heating cycles i by 2 and applies power by repeating the energizing pattern 4 times in control cycle 2 (t2). At the end of control cycle 2 (t3), for example, the temperature fluctuation time CT is less than the first threshold CT1, and the number of heating cycles i is increased by 2 in the same way as in control cycle 1. Then, in control cycle 3, power is applied by repeating the energizing pattern 6 times (t3). As a result, when the amount of current is sufficient, the temperature fluctuation time CT becomes longer, for example, greater than or equal to the second threshold CT2. In this case, the control unit 100 decreases the number of heating cycles i by 2. Then, in the next control cycle 4, power is applied by repeating the energizing pattern 4 times (t4). As a result, for example, the temperature fluctuation time CT in control cycle 4 is greater than or equal to the first threshold CT1 and less than the second threshold, so the control unit 100 starts heating in the next control cycle without changing the number of heating cycles i (t5).

[0071] If the standby control continues for a while and the ambient temperature changes, the number of heating cycles i may become 8, for example, as in control cycle 11 (t11). Then, if the temperature fluctuation time CT in control cycle 11 becomes long and exceeds the third threshold CT3, the control unit 100 reduces the number of heating cycles i by 2 to 6, and in the next control cycle 12, it provides a preheating period before the heating period and energizes the heater H1 with 6 half-wave phase control (t12). Immediately after the preheating period, it energizes by wavenumber control by repeating the energizing pattern 6 times. As a result, in control cycle 12, the current flows gradually and the high inrush current is suppressed. If the temperature fluctuation time CT in control cycle 12 exceeds the second threshold CT2, the control unit 100 reduces the number of heating cycles i by 2, and in the next control cycle 13, it energizes by wavenumber control by repeating the energizing pattern 4 times (t13). In control cycle 13, if the temperature fluctuation time CT is greater than or equal to the first threshold CT1 and less than the second threshold, the control unit 100 starts heating in the next control cycle without changing the number of heating cycles i (t14).

[0072] As another example of operation, in cases where the ambient temperature is extremely low, as shown by the dashed line in Figure 7, even if the energizing pattern is repeated twice at time t1, the detected temperature T may not reach the target standby temperature TR within a predetermined period. In this case, the control unit 100 starts the next control cycle when the predetermined period has elapsed, increases the number of heating cycles i by 2, and energizes by repeating the energizing pattern four times.

[0073] Furthermore, although not shown in the diagram, as another example of operation, if it takes time for the detected temperature T to drop to the target standby temperature TR from time t0, and the CT count reaches or exceeds the third threshold CT3, a preheating period is provided before the initial heating period. In this case, since the number of heating cycles i is 2, the heater H1 is energized after the preheating period by repeating the energizing pattern twice.

[0074] In this way, according to the image forming apparatus 1 of this embodiment, after starting standby control, if the length of the temperature fluctuation time CT, which is the time taken for one control cycle, is less than the first threshold CT1, the amount of current supplied for the next control cycle is increased; if the temperature fluctuation time CT is equal to or greater than the second threshold CT2, the amount of current supplied for the next control cycle is decreased; and if the temperature fluctuation time CT is equal to or greater than the first threshold CT1 and less than the second threshold CT2, the amount of current supplied for the next control cycle is made the same as the amount of current supplied for the previous control cycle. As a result, as the control cycle is repeated, the current is supplied at an appropriate amount so that the temperature fluctuation time CT of one control cycle approaches the time between the first threshold CT1 and less than the second threshold CT2. Therefore, according to this embodiment, it is possible to suppress the heater H1 from becoming too slow, thereby suppressing the inrush current from becoming too large when the heater H1 is energized. Furthermore, if the control cycle becomes excessively short, the long-term flicker index (Plt) may worsen, but in this embodiment, the temperature fluctuation time CT of one control cycle approaches a time between the first threshold CT1 and the second threshold CT2, so it is possible to suppress the worsening of the long-term flicker index.

[0075] Furthermore, if the temperature of the fuser 8 does not reach the target standby temperature TR after the heating period has elapsed and a predetermined period has elapsed from the start of the heating period, the control unit 100 energizes the heater H1 and starts the next control cycle. Therefore, even if the ambient temperature is extremely low and the temperature of the fuser 8 does not reach the target standby temperature TR after the heating period has elapsed and a predetermined period has elapsed from the start of the heating period, the temperature of the fuser 8 can be brought closer to the target standby temperature TR.

[0076] Furthermore, if the temperature fluctuation time CT is greater than or equal to the third threshold CT3, the control unit 100 applies current at a first intensity lower than the heating intensity during the heating period of the previous control cycle in the next control cycle, and then applies current at a second intensity stronger than the first intensity during the heating period. As a result, by gradually applying current to the heater H1, whose resistance has decreased due to the drop in temperature, the temperature of the heater H1 gradually rises, thus suppressing a large inrush current.

[0077] Furthermore, when the control unit 100 starts standby control for the first time after the power to the image forming apparatus 1 is turned on, it sets the amount of current supplied during the heating period of the first control cycle to the minimum possible value. Therefore, since an excessive amount of heat is not supplied to the fuser 8, it is possible to suppress the heater H1 from becoming idle for a long time.

[0078] [Second Embodiment] Next, a second embodiment will be described. In this embodiment, the same parts as in the first embodiment are denoted by the same reference numerals in the drawings and their descriptions are omitted, and only the differences will be described in detail. In the control unit 100 of the image forming apparatus 1 according to the second embodiment, in standby control, if the temperature fluctuation time CT of the previous control cycle is less than the first threshold CT1, the smaller the temperature fluctuation time CT, the greater the change in the amount of current supplied E. n+1 -E n The difference from the first embodiment is that the [value] is increased.

[0079] For example, the control unit 100 processes the subroutine for updating the number of heating cycles i as shown in Figure 8. The control unit 100 determines whether the temperature fluctuation time CT is less than the first threshold CT1 (S131), and if it determines that the temperature fluctuation time CT is not less than the first threshold CT1 (No), it processes in the same manner as in the first embodiment (S133~S135).

[0080] If the control unit 100 determines that the temperature fluctuation time CT is less than the first threshold CT1 (S131, Yes), it determines whether the temperature fluctuation time CT is less than the fourth threshold CT4 (S136). The fourth threshold CT4 is a smaller value than the first threshold CT1. If the temperature fluctuation time CT is not less than the fourth threshold CT4, that is, if the temperature fluctuation time CT is greater than or equal to the fourth threshold CT4 and less than the first threshold (No), it increases the number of heating cycles i by 2 (S132) and terminates the subroutine for updating the number of heating cycles i.

[0081] On the other hand, in step S136, if the control unit 100 determines that the temperature fluctuation time CT is less than the fourth threshold CT4 (Yes), it further determines whether the temperature fluctuation time CT is less than the fifth threshold CT5 (S137). The fifth threshold CT5 is a smaller value than the fourth threshold CT4. If the temperature fluctuation time CT is not less than the fifth threshold CT5, that is, if the temperature fluctuation time CT is greater than or equal to the fifth threshold CT5 and less than the fourth threshold CT4 (No), the control unit 100 increases the number of heating cycles i by 4 (S138) and terminates the subroutine for updating the number of heating cycles i. If the control unit 100 determines that the temperature fluctuation time CT is less than the fifth threshold CT5 (Yes), it increases the number of heating cycles i by 6 (S139) and terminates the subroutine for updating the number of heating cycles i.

[0082] According to this process, if the temperature fluctuation time CT is less than the first threshold CT1, the number of heating cycles i is increased by 2 if the temperature fluctuation time CT is between the fourth threshold CT4 and the first threshold CT1, the number of heating cycles i is increased by 4 if the temperature fluctuation time CT is between the fifth threshold CT5 and the fourth threshold CT4, and the number of heating cycles i is increased by 6 if it is less than the fifth threshold CT5. Therefore, if the amount of current supplied during the heating period of the previous control cycle was insufficient, the number of heating cycles i is increased in proportion to the degree of insufficient current supply, thus quickly approaching the appropriate amount of current supply.

[0083] [Third Embodiment] Next, a third embodiment will be described. In this embodiment, the same reference numerals are used in the drawings for parts that are the same as in the first embodiment, and their descriptions are omitted. Only the differences will be described in detail. In the image forming apparatus 1 according to the third embodiment, when changing the amount of current supplied during the heating period, the output of the heater H1 is changed without changing the length of the heating period. Specifically, the control unit 100 supplies current to the heater H1 during the heating period at a set duty cycle. When changing the amount of current supplied to a larger amount, the duty cycle is changed to a value larger than that of the previous heating period. When changing the amount of current supplied to a smaller amount, the duty cycle is changed to a value smaller than that of the previous heating period.

[0084] For example, the control unit 100 stores the table shown in Figure 9. Figure 9 is a table showing the relationship between the heating intensity variable j and the duty cycle. When the heating intensity variable j is 1, the duty cycle is 33%, when the heating intensity variable j is 2, the duty cycle is 40%, ... when the heating intensity variable j is 8, the duty cycle is 100%, and so on. In this relationship, the duty cycle increases as the heating intensity variable j increases.

[0085] Then, as shown in Figure 10, the control unit 100 sets the heating intensity variable j to its initial value of 1 during standby control (S210). After processing steps S111 to S112, the heating intensity variable j is updated (S230).

[0086] As shown in Figure 11, when updating the heating intensity variable j, the control unit 100 determines whether the temperature fluctuation time CT is less than the first threshold CT1 (S231). If it determines that the temperature fluctuation time CT is less than the first threshold CT1 (Yes), it determines whether the heating intensity variable j is at the upper limit of 8 (S232). If it is at the upper limit of 8 (Yes), it terminates the subroutine for updating the heating intensity variable j without increasing the heating intensity variable j. On the other hand, if the heating intensity variable j is not 8 (No), it increases the heating intensity variable j by 1 (S233) and terminates the subroutine for updating the heating intensity variable j.

[0087] On the other hand, if the control unit 100 determines in step S231 that the temperature fluctuation time CT is not less than the first threshold CT1, that is, that the temperature fluctuation time CT is greater than or equal to the first threshold CT1 (No), then it further determines whether the temperature fluctuation time CT is greater than or equal to the second threshold CT2 (S234). If the control unit 100 determines that the temperature fluctuation time CT is not greater than or equal to the second threshold CT2 (No), that is, if the temperature fluctuation time CT is greater than or equal to the first threshold CT1 and less than the second threshold CT2, then the previous energization amount E nSince this is considered relatively appropriate, the subroutine for updating the heating intensity variable j is terminated without changing the heating intensity variable j. If the control unit 100 determines that the temperature fluctuation time CT is greater than or equal to the second threshold CT2 (Yes), it determines whether the heating intensity variable j is at the lower limit of 1 (S235). If it is at the lower limit of 1 (Yes), the subroutine for updating the heating intensity variable j is terminated without decreasing the heating intensity variable j. On the other hand, if the heating intensity variable j is not 1 (No), the heating intensity variable j is decreased by 1 (S236), and the subroutine for updating the heating intensity variable j is terminated.

[0088] Returning to Figure 10, the control unit 100 updates the heating intensity variable j (S230), then performs the processing in steps S140 to S151 as in the first embodiment, and then heats for a constant heating period with a duty cycle set according to the heating intensity variable j (S260). After that, after steps S170 and S171, it returns to step S112 or step S230 and repeats the process.

[0089] This process allows the amount of current supplied during the heating period to be changed by altering the duty cycle. For example, as shown in Figure 12, the heating period of heater H1 during the control period can be kept constant, while the heating intensity variable j can be changed to maintain the temperature of the fuser 8 near the target standby temperature TR. In other words, similar to the first embodiment, if the amount of heat supplied to the fuser 8 is insufficient, the amount of heat supplied can be increased by increasing the output of heater H1, and if the amount of heat supplied to the fuser 8 is too much, the amount of heat supplied can be decreased by lowering the output of heater H1. This prevents the fuser 8 from receiving an excessive amount of heat, thus suppressing the heater H1 from being shut down for extended periods. In Figure 12, the period during which heater H1 is ON (heating period) is shown as a continuous ON state, but in detail, it is repeatedly switched ON and OFF in small increments according to the set duty cycle.

[0090] [Fourth Embodiment] Next, the fourth embodiment will be described. In this embodiment, the same parts as in the first embodiment are denoted by the same reference numerals in the drawings and their descriptions are omitted, and only the differences will be described in detail. In the first to third embodiments, in order to suppress the increase in inrush current caused by the decrease in the temperature of the heater H1 and the decrease in the resistance value of the heater H1 when the temperature fluctuation time CT is long, a preheating period was provided immediately before the heating period in which the heater H1 was energized with less power than the power during the heating period. In contrast, in the fourth embodiment, excessive inrush current is suppressed by suppressing the temperature drop of the heater H1 in advance. Specifically, after the heating period has elapsed and the detected temperature T has risen to or above the target standby temperature TR, the control unit 100 waits during the temperature drop standby period until the detected temperature T drops to the target standby temperature TR, and if the detected temperature T after a second predetermined period has elapsed from the start of the heating period is higher than the target standby temperature TR, the control unit 100 energizes the heater H1 at a third intensity during the execution of the temperature drop standby. In short, if the waiting period for temperature reduction in one control cycle is prolonged, preheating is performed after the end of the heating period of that control cycle, before the detected temperature T drops to the target waiting temperature TR. This raises the temperature of heater H1, suppressing the decrease in the resistance value of heater H1, and preventing the inrush current when energizing heater H1 in the next control cycle from becoming too large. In preheating, the control unit 100 energizes twice with wavenumber control at a duty cycle of 33%, as an example of the third intensity. That is, the energizing pattern shown in Figure 4(a), in which energizing is performed with one half-wave out of three, is repeated twice.

[0091] In the fourth embodiment, as shown in Figure 13, if step S112 is determined to be No, the control unit 100 determines whether the temperature fluctuation time CT has become an integer multiple of the second predetermined period CTP (S201). If the control unit 100 determines that the temperature fluctuation time CT has become an integer multiple of the second predetermined period CTP (S201, Yes), it energizes the heater H1 twice with wavenumber control at a duty cycle of 33% (S202). If the control unit 100 determines in step S201 that the temperature fluctuation time CT is not an integer multiple of the second predetermined period CTP (S201, No), it returns to step S112 without energizing the heater H1. Similarly, after energizing in step S202, it returns to step S112. Note that in the control of this embodiment, the processing in steps S150 and S151 in the first embodiment (Figure 5) is omitted.

[0092] With this control, as shown in the control cycle 11 in Figure 14, if the heating during the heating period becomes excessive and the temperature fluctuation time CT becomes long, and the detected temperature T does not drop to the target standby temperature TR even after the second predetermined period CTP has elapsed, preheating is performed by energizing the heater H1 twice with wavenumber control of a duty cycle of 33% (t21). This preheating is performed each time the second predetermined period CTP has elapsed as long as the detected temperature T has not dropped to the target standby temperature TR. This prevents the temperature of the heater H1 from dropping too low between the time the detected temperature T drops to the target standby temperature TR and the start of the next control cycle, and prevents the inrush current when the heater H1 is energized in the next control cycle from becoming too large. In this embodiment, preheating is performed each time the second predetermined period CTP has elapsed from the start of the heating period, but preheating may also be performed only when the second predetermined period CTP has elapsed from the start of the heating period. For example, in step S201, preheating may be performed only when it is determined that the temperature fluctuation time CT = CTP. Alternatively, the determination in step S201 may be made when CT ≥ CTP. In this case, a separate flag may be provided to indicate whether or not preheating has been performed, so that preheating in step S202 is performed only once in a single control cycle.

[0093] Although embodiments have been described above, the present invention is not limited to these embodiments. Specific configurations can be modified as appropriate without departing from the spirit of the invention.

[0094] For example, in the above embodiment, the heater was energized with an AC voltage, but it may also be energized with a DC voltage. Furthermore, when energized with a DC voltage, the amount of current may be changed by duty cycle control, or the amount of current may be changed by changing the voltage.

[0095] Furthermore, in the above embodiment, the rotating member of the heating element was not rotated when standby control was performed, but the invention is not limited to this, and the rotating member may be rotated when standby control is performed.

[0096] Furthermore, although an endless belt was exemplified as the rotating member in the above embodiment, the invention is not limited thereto, and for example, the rotating member may be a roller. Also, although a pressure roller was exemplified as the pressurizing member in the above embodiment, the invention is not limited thereto, and for example, the pressurizing member may be a pressurizing unit including an endless pressure belt.

[0097] Furthermore, although the above embodiment provided the temperature sensor to detect the temperature of the heating element, it is not limited to this, and may be provided to detect the temperature of a part of the fuser other than the heating element, such as a pressurizing element. Also, the temperature sensor may be a temperature sensor other than a thermistor. In addition, the temperature sensor may be a non-contact type temperature sensor or a contact type temperature sensor.

[0098] Furthermore, although a halogen heater utilizing radiant heat was exemplified as the heater in the above embodiment, the invention is not limited to this, and other heaters such as ceramic heaters or carbon heaters that utilize the heat generated by a resistor may also be used. In addition, the heater may be placed outside the heating element rather than inside it.

[0099] Furthermore, while the above embodiment exemplifies an image forming apparatus that forms monochrome images on a sheet, it is not limited to this, and may also be a printer configured to form color images on a sheet, for example. Also, the image forming apparatus is not limited to a printer, and may be a copier or multifunction device equipped with a document reading device such as a flatbed scanner, for example.

[0100] Furthermore, the elements described in the above-mentioned embodiments and modifications can be combined as appropriate. [Explanation of Symbols]

[0101] 1. Image forming apparatus 8. Fuser 81 Heating element 81A Rotating Member 82 Pressurizing member 100 Control Unit 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 fuser unit having a heater and fixing the toner image onto the sheet, A temperature sensor for detecting the temperature of the fuser, It comprises a control unit and, The control unit, In standby control that maintains the temperature of the fuser at a target standby temperature based on the temperature detected by the temperature sensor, When the detected temperature falls below the target standby temperature, the heater is energized. During the set heating period, the amount of current E supplied to the heater ｎ The system is powered on, and after the heating period has elapsed, if the detected temperature rises above the target standby temperature, the system repeats a control cycle that waits until the temperature drops to the target standby temperature. The time from when the heater is energized until one control cycle is completed is defined as the temperature fluctuation time. If the temperature fluctuation time of the previous control cycle is less than the first threshold, the current E of the next control cycle will be... ｎ＋１ The previous current supply amount E ｎ Make it larger than that, If the temperature fluctuation time of the previous control cycle is greater than or equal to the second threshold, which is greater than the first threshold, then the current E of the next control cycle will be applied. ｎ＋１ The previous current supply amount E ｎ To make it smaller than, If the temperature fluctuation time of the previous control cycle is greater than or equal to the first threshold and less than the second threshold, the current E of the next control cycle will be applied. ｎ＋１ The previous current supply amount E ｎ An image forming apparatus characterized by being the same as the image forming apparatus.

2. Said control unit starts a next control cycle by energizing said heater if the temperature does not reach or exceed said target standby temperature even after a predetermined period has elapsed from the start of the heating period after the heating period has elapsed, and sets the energization amount E of said next control cycle ｎ＋１ to be larger than the previous energization amount E ｎ The image forming apparatus according to claim 1, characterized in that:

3. The control unit, When changing the current flow to a larger current flow, the heating period should be changed to a longer period than the heating period of the previous control cycle. The image forming apparatus according to claim 1, characterized in that when the amount of current supplied is changed to a smaller amount of current supplied, the heating period is changed to a period shorter than the heating period of the previous control cycle.

4. The control unit, The image forming apparatus according to claim 3, characterized in that the energization of the heater during the heating period is controlled by wavenumber control, and the amount of energization is changed by changing the number of energization cycles of a predetermined energization pattern of wavenumber control, with the number of energization cycles of the energization pattern being considered as one cycle.

5. The control unit, The image forming apparatus according to claim 1, characterized in that, if the temperature fluctuation time of the previous control cycle is greater than or equal to a third threshold which is greater than the second threshold, in the next control cycle, a preheating period is provided before the heating period, and during the preheating period, current is applied at a first intensity which is less than the heating intensity of the heating period of the previous control cycle, and then during the heating period, current is applied at a second intensity which is stronger than the first intensity.

6. The control unit, The energization of the heater during the heating period is controlled by wavenumber control. The image forming apparatus according to claim 5, characterized in that the energization of the heater during the preheating period is controlled by phase control.

7. The control unit, During the heating period, the heater is energized at a set duty cycle. When changing the current flow to a larger current flow, the duty cycle is changed to a value greater than the heating period of the previous control cycle. The image forming apparatus according to claim 1, characterized in that when the amount of current supplied is changed to a smaller amount, the duty cycle is changed to a value smaller than the heating period of the previous control cycle.

8. The control unit, The image forming apparatus according to any one of claims 1 to 7, characterized in that, 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 supplied during the heating period of the first control cycle is set to the minimum settable value.

9. The control unit, If the temperature fluctuation time of the previous control cycle is less than the first threshold, the smaller the temperature fluctuation time, the greater the change in the amount of current supplied E. ｎ＋１ -E ｎ An image forming apparatus according to any one of claims 1 to 7, characterized by increasing the size of the image.

10. The aforementioned fuser is A heating member heated by the heater, the heating member including a rotating member that can rotate around the heater, It has a pressing member that sandwiches a sheet between the heating member and the pressing member, The control unit, When fixing the toner image onto the sheet, the rotating member is rotated. The image forming apparatus according to any one of claims 1 to 7, characterized in that the rotating member is not rotated when the standby control described above is performed.

11. The control unit, When fixing the toner image onto the sheet using the fuser, print control is performed to control the power supply to the heater so that the detected temperature becomes the target fixing temperature. The image forming apparatus according to any one of claims 1 to 7, characterized in that the target standby temperature is lower than the target fixing temperature.

12. The control unit, The image forming apparatus according to claim 1, characterized in that, after the heating period has elapsed and the detected temperature has risen to or above the target standby temperature, during the temperature reduction waiting period in which the device waits until the detected temperature drops to the target standby temperature, if the detected temperature after a second predetermined period has elapsed from the start of the heating period is higher than the target standby temperature, the heater is energized at a third intensity during the execution of the temperature reduction waiting period.

13. The control unit, The image forming apparatus according to claim 12, characterized in that the energization of the heater at the third intensity is controlled by wavenumber control with a duty cycle of 33%.

14. A toner image forming unit that forms a toner image on a sheet, A fuser unit having a heater and fixing the toner image onto the sheet, A control method for an image forming apparatus comprising a control unit, The control unit, in standby control for maintaining the temperature of the fuser at a target standby temperature, When the temperature of the fuser falls below the target standby temperature, the heater is energized. During the set heating period, the amount of current E supplied to the heater ｎ The system is energized, and after the heating period has elapsed, if the temperature of the fuser rises above the target standby temperature, the control cycle is repeated until the temperature drops to the target standby temperature. The time from when the heater is energized until one control cycle is completed is defined as the temperature fluctuation time. If the temperature fluctuation time of the previous control cycle is less than the first threshold, the current E of the next control cycle will be... ｎ＋１ The previous current supply amount E ｎ Make it larger than that, If the temperature fluctuation time of the previous control cycle is greater than or equal to the second threshold, which is greater than the first threshold, then the current E of the next control cycle will be applied. ｎ＋１ The previous current supply amount E ｎ To make it smaller than, If the temperature fluctuation time of the previous control cycle is greater than or equal to the first threshold and less than the second threshold, the current E of the next control cycle will be applied. ｎ＋１ The previous current supply amount E ｎ A control method for an image forming apparatus, characterized by making it the same as [the specified method].

15. If, after the heating period has elapsed, the control unit does not reach the target standby temperature even after a predetermined period has elapsed from the start of the heating period, it energizes the heater to start the next control cycle, and the amount of current E of the next control cycle ｎ＋１ The previous current supply amount E ｎ A control method for an image forming apparatus according to claim 14, characterized in that it is made larger than [the specified value].

16. The control unit, When changing the current flow to a larger current flow, the heating period should be changed to a longer period than the heating period of the previous control cycle. The control method for an image forming apparatus according to claim 14, characterized in that when the amount of current supplied is changed to a smaller amount of current supplied, the heating period is changed to a shorter period than the heating period of the previous control cycle.

17. The control unit, The control method for an image forming apparatus according to claim 16, characterized in that the energization of the heater during the heating period is controlled by wavenumber control, and the amount of energization is changed by changing the number of energization cycles of a predetermined energization pattern of wavenumber control, with the number of cycles of the energization pattern being considered as one cycle.

18. The control unit, The control method for an image forming apparatus according to claim 14, characterized in that, if the temperature fluctuation time of the previous control cycle is greater than or equal to a third threshold greater than the second threshold, in the next control cycle, a preheating period is provided before the heating period, and during the preheating period, current is applied at a first intensity that is less than the heating intensity of the heating period of the previous control cycle, and then during the heating period, current is applied at a second intensity that is stronger than the first intensity.

19. The control unit, The energization of the heater during the heating period is controlled by wavenumber control. The control method for an image forming apparatus according to claim 18, characterized in that the energization of the heater during the preheating period is controlled by phase control.

20. The control unit, During the heating period, the heater is energized at a set duty cycle. When changing the current flow to a larger current flow, the duty cycle is changed to a value greater than the heating period of the previous control cycle. The control method for an image forming apparatus according to claim 14, characterized in that when the amount of current supplied is changed to a smaller amount, the duty cycle is changed to a value smaller than the heating period of the previous control cycle.

21. The control unit, The control method for an image forming apparatus according to any one of claims 14 to 20, characterized in that, 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 supplied during the heating period of the first control cycle is set to the minimum settable value.

22. The control unit, If the temperature fluctuation time of the previous control cycle is less than the first threshold, the smaller the temperature fluctuation time, the greater the change in the amount of current supplied E. ｎ＋１ -E ｎ A control method for an image forming apparatus according to any one of claims 14 to 20, characterized by increasing the value.

23. The aforementioned fuser is A heating member heated by the heater, the heating member including a rotating member that can rotate around the heater, It has a pressing member that sandwiches a sheet between the heating member and the pressing member, The control unit, When fixing the toner image onto the sheet, the rotating member is rotated. The control method for an image forming apparatus according to any one of claims 14 to 20, characterized in that the rotating member is not rotated when the standby control described above is performed.

24. The control unit, When fixing the toner image onto the sheet using the fuser, print control is performed to control the power supply to the heater so that the temperature of the fuser reaches the target fixing temperature. The control method for an image forming apparatus according to any one of claims 14 to 20, characterized in that the target standby temperature is lower than the target fixing temperature.

25. The control unit, The control method for an image forming apparatus according to claim 14, characterized in that, after the heating period has elapsed and the temperature of the fuser has risen to or above the target standby temperature, during the temperature reduction waiting period in which the temperature of the fuser is reduced to the target standby temperature, if the temperature of the fuser after a second predetermined period has elapsed from the start of the heating period is higher than the target standby temperature, the heater is energized at a third intensity during the execution of the temperature reduction waiting period.

26. The control unit, The control method for an image forming apparatus according to claim 25, characterized in that the energization of the heater at the third intensity is controlled by wavenumber control with a duty cycle of 33%.

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