JP7779159B2 - Image forming device - Google Patents

Description

This application relates to an image forming apparatus equipped with a fixing device that fixes a developer image onto a sheet.

Patent Document 1 describes an image forming device that executes standby control 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 first temperature, and power is stopped when the detected temperature of the fixing unit exceeds a second temperature higher than the first temperature. More specifically, in this image forming device, standby control detects a peak temperature, which is the maximum detected temperature after power to the heater is stopped, and if the detected peak temperature is higher than a target peak temperature, the next second temperature is set lower than the current second temperature, and if the detected peak temperature is lower than the target peak temperature, the next second temperature is set higher than the current second temperature.

Japanese Patent Application Laid-Open No. 2020-20988

In conventional image forming devices, during standby control to maintain the temperature of the fixing unit at a target standby temperature, if the temperature falls below the target standby temperature, current is passed through the heater by performing wave number control, which switches the switching element between on and off. The temperature of the fixing unit and the resistance value of the heater decrease at different rates, with the heater's resistance value tending to decrease more rapidly than the temperature of the fixing unit. Therefore, if there is a long period of time between the image forming device performing wave number control and the image forming device performing wave number control again, current will suddenly flow through the heater when the heater's resistance value has sufficiently decreased, causing terminal noise in the terminal connecting the heater to the AC power source that supplies AC voltage.

The purpose of this application is to provide technology that can reduce terminal noise generated at the terminals connecting the heater to an AC power source that supplies AC voltage.

In order to achieve the above object, the image forming apparatus of the present application comprises a developer image forming unit that forms a developer image on a sheet, a fixing unit that is connected via terminals to an AC power source that supplies AC voltage and has a heater that is heated by the AC voltage and fixes the developer image on the sheet, a temperature sensor that detects the temperature of the fixing unit, a switching element that is provided between the AC power source and the heater and switches between an ON state in which the AC power source and the heater are electrically connected and an OFF state in which the AC power source and the heater are not electrically connected, and a control unit, and the control unit controls the temperature of the fixing unit based on the detected temperature detected by the temperature sensor. In standby control that maintains the temperature at a target standby temperature, wave number control is performed in which a control cycle is defined as a plurality of consecutive half-waves of the AC voltage, and the switching element is turned on for some of the half-waves within that control cycle and turned off for the remaining half-waves. If the time that has elapsed since wave number control was performed is less than a first predetermined time and the detected temperature is below the target standby temperature, wave number control is performed a first predetermined number of times, and if the time that has elapsed since wave number control was performed is equal to or greater than the first predetermined time and the detected temperature is equal to or greater than the target standby temperature, wave number control is performed a second predetermined number of times.

As a result, if the elapsed time since wave number control was executed is equal to or greater than the first predetermined time and the detected temperature is equal to or greater than the target standby temperature, wave number control is executed before the heater's resistance value has dropped sufficiently, causing current to flow through the heater before the heater's resistance value has dropped sufficiently, preventing terminal noise caused by a sudden current flowing through the heater.

Furthermore, if the time elapsed since wave number control was executed is equal to or greater than a first predetermined time and the detected temperature is equal to or greater than the target standby temperature, the control unit may execute wave number control a second predetermined number of times, and then, when the detected temperature falls below the target standby temperature, reduce the first predetermined number of times and execute wave number control again.

As a result, when the detected temperature is equal to or higher than the target standby temperature, executing wave number control the second predetermined number of times means that current will flow through the heater at temperatures above the target standby temperature. Therefore, when the image forming device executes wave number control when the detected temperature falls below the target standby temperature, it can prevent the detected temperature from rising too high by reducing the first predetermined number of times and executing wave number control.

Furthermore, if the time elapsed since wave number control was performed is less than a second predetermined time that is shorter than the first predetermined time, and the detected temperature is less than the target standby temperature, the control unit may increase the first predetermined number of times and perform wave number control.

As a result, if the time elapsed since wave number control was performed is less than a second predetermined time, which is shorter than the first predetermined time, it becomes difficult to maintain the target standby temperature. Therefore, by increasing the first predetermined number of times and performing wave number control, the image forming device can make it easier to maintain the temperature of the fixing unit at the target standby temperature.

Furthermore, if the time elapsed since wave number control was performed is equal to or greater than the second predetermined time and less than the first predetermined time, and the detected temperature is less than the target standby temperature, the control unit may perform wave number control without changing the first predetermined number of times.

In addition, the control unit may start standby control and, before performing the first wave number control, perform phase control in which the switching element is turned on for some waveforms within a half wave of the AC voltage and turned off for the remaining waveforms.

This allows flicker to be prevented by performing phase control before the first wave number control when starting standby control.

1 is a side cross-sectional view of a main part of a laser printer according to an embodiment of the present application. FIG. 4 is a perspective view showing the arrangement of sensors on a nip plate. FIG. 2 is a circuit diagram showing a schematic configuration of a heater control device. 4 is a time chart of signals related to power supply control. 10A and 10B are diagrams showing examples of voltage waveforms when a preheating period for phase control is not provided (FIG. 10A) and when a preheating period for phase control is provided (FIG. 10B). 4 is a flowchart showing the procedure of a control process executed by the control device of FIG. 3, particularly by a CPU. 7 is a flowchart showing a detailed procedure of a standby process included in the control process of FIG. 6; 7 is a flowchart showing detailed procedures of a first number setting process ((a)) and a second number setting process ((b)) included in the control process of FIG. 6; FIG. 3 is a diagram showing an example of a transition of a temperature detected by a temperature sensor shown in FIG. 2 .

Embodiments of the present application will be described in detail below with reference to the accompanying drawings. FIG. 1 shows a cross-sectional side view of the main components of a laser printer 1 according to one embodiment of the present application. The laser printer 1 comprises a main body housing 2, a supply unit 3, a process unit PR, and a fixing unit 8.

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 conveyance roller 33C, and a registration roller 33D. In the supply unit 3, 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 conveyance 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."

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 is equipped with an exposure device 4 and a process cartridge 5.

The exposure device 4 is located at the top 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, and other components. In the exposure device 4, laser light based on image data is emitted from the laser light source and scanned across the surface of the photosensitive drum 61, thereby exposing the surface of the photosensitive drum 61.

The process cartridge 5 is located below the exposure device 4 and is detachable from the main body casing 2 through an opening created when the front cover 21 on the main body casing 2 is opened. The process cartridge 5 includes a drum unit 6 and a developing unit 7.

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 control blade 73, a toner storage section 74 that stores dry toner, and an agitator 75.

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, forming an electrostatic latent image based on image data on the photosensitive drum 61. The toner in the toner storage compartment 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 carried on the developing roller 71 as a thin layer of a uniform thickness.

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. Then, 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.

The fixing device 8 fixes the toner image on the sheet S. The fixing device 8 includes a heater H1 and a heater H1
The heating member 81 is heated by a heater H1, and includes a rotating member 81A that can rotate 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, is a halogen heater, for example.

The rotating member 81A is a rotatable, endless belt. It 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 powered, radiating heat to heat the rotating member 81A. The heater H1 is positioned inside the rotating member 81A along the width direction.

The pressure member 82 is a rotatable pressure roller with an elastic layer made of elastically deformable rubber or the like on its surface.

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 so that the inner peripheral surface of the heating member 81 is in sliding contact with its underside. The nip plate NP sandwiches the heating member 81 between itself and the pressure member 82. In the fixing device 8, the sheet S with the transferred toner image is transported between the heating member 81 and the pressure member 82, whereby the toner image is thermally fixed onto the sheet S. The sheet S with the thermally fixed toner image is discharged onto the discharge tray 22 by the second transport roller 23 and discharge roller 24.

As shown in FIG. 2, the nip plate NP has a central detection portion 131 that protrudes from the edge of the sheet S in the transport direction, 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 the edge 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.

The central temperature sensor ST1 is a sensor that detects the temperature of the central portion of the heating member 81 in the width direction. The central temperature sensor ST1 is able to detect 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.

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 able to detect 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 portion 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 also be located within the range of the area SW in the width direction.

The central temperature sensor ST1 and the end temperature sensor ST2 can be, for example, a thermistor.

3 shows a schematic configuration of the heater control device 100. The heater control device 100 is composed of the heater H1, a power supply board 100A, a main board 100B, etc. The power supply board 100A is provided with a pair of terminals Te, and each terminal Te is connected to a power supply line PL, which is then connected to a commercial power supply (not shown) via a power plug 200. When the power supply board 100A is connected to the commercial power supply, an AC voltage V of, for example, 100V is applied to the pair of terminals Te.
is supplied.

The power supply board 100A is equipped with a line filter 101, an AC/DC converter 102, a zero-cross detection circuit 105, a triac 106, and a changeover switch 107. As shown in Figure 3, the AC/DC converter 102 is connected to the power supply line PL via a connection line L2 that is separate from the connection line L1 to which the changeover switch 107 is connected.

The main board 100B is equipped with a DC/DC converter 103, a control device 104, and other components.

Heater H1 generates heat in response to the application of AC voltage V. Temperature sensor ST can be either the central temperature sensor ST1 or the edge temperature sensor ST2 located near heater H1, and is therefore designated by the symbol "ST." Temperature sensor ST outputs the temperature of the fixing unit 8 as a detected temperature T to control device 104. AC/DC converter 102 converts, for example, 100 V AC voltage into 24 V DC voltage. DC/DC converter 103 converts the 24 V DC voltage from AC/DC converter 102 into 3.3 V DC voltage and supplies this DC voltage to various components including control device 104.

As shown in FIG. 3, the zero-cross detection circuit 105 outputs the zero-cross pulse signal Sr to the control device 104. The control device 104 determines the voltage level of the zero-cross pulse signal Sr, thereby detecting the rising and falling edges of the zero-cross pulse signal Sr.

The control device 104 also has a CPU 104A and memory 104B. Memory 104B includes, for example, RAM, ROM, flash memory, etc., and stores information related to control and processing. Memory 104B also stores control programs that execute the control processes described below (see Figures 6 to 8). CPU 104A performs various controls of the laser printer 1 by executing the control programs stored in memory 104B.

The control device 104 adjusts the time that the input voltage V is applied to the heater H1 based on the zero-cross pulse signal Sr. Specifically, the control device 104 generates a trigger pulse signal Sb (see FIG. 4) based on, for example, the falling edge of the zero-cross pulse signal Sr and outputs it to the triac 106. The triac 106 is connected between the power supply line PL and the heater H1. The triac 106 turns on in response to the trigger pulse signal Sb output from the control device 104 and turns off when a reverse voltage is applied or the current is reduced to zero. In accordance with this operation, the triac 106 controls the time that the input voltage V is applied to the heater H1. This time is, for example, from the rising edge of the trigger pulse signal Sb to the zero-cross point of the input voltage V, as shown in the waveform of the heater voltage Vh in FIG. 4. The control device 104 can control the temperature of the fixing device 8 by the heater H1, for example, by changing the period Tw2, which is the period from the falling edge of the zero-cross pulse signal Sr to the rising edge of the trigger pulse signal Sb.

For example, by setting the period Tw2 to zero, the control device 104 can continuously apply the heater voltage Vh to the heater H1 (application with a 100% energization ratio). Alternatively, the control device 104 can perform phase control or wave number control on the sine-wave heater voltage Vh to intermittently apply the heater voltage Vh to the heater H1. The phase control here refers to, for example, control in which power supply to the heater H1 during a half cycle of the input voltage V begins at a timing that corresponds to the phase angle of the input voltage V and is a predetermined time after the zero-cross timing ZC at the start of the half cycle, and continues until the zero-cross timing ZC at the end of the half cycle. In phase control, the energization time of the heater H1 is controlled not by the wave number but by the conduction phase angle (also known as the firing angle) α. The conduction phase angle α is the phase at which the triac 106 begins to conduct. Wave number control is a control for changing the number of half cycles (wave number) of power supply to the heater H1 during a half cycle of the input voltage V, from the zero-cross timing ZC at the start of the half cycle to the zero-cross timing ZC at the end of the half cycle, by increasing or decreasing the number of half cycles. Therefore, the control device 104 may perform intermittent application (application with a current conduction ratio of less than 100%) based on the zero-cross pulse signal Sr. As a result, the heater H1 is applied with the heater voltage Vh at a specific phase and wave number.

The selector switch 107 shown in FIG. 3 has an input terminal connected to the power supply line PL and an output terminal connected to the heater H1 and zero-cross detection circuit 105. The control device 104 can switch the connection between the input voltage V and the heater H1 and zero-cross detection circuit 105 by switching the selector switch 107 on and off. The selector switch 107 is, for example, a semiconductor switch such as a transistor or a mechanical switch such as a relay.

As described above, the control device 104 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 device 104 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 corresponding energization pattern is one. That is, the control device 104 changes the heating period by changing the number of energizations of the energization pattern. For example, as shown in FIG. 5(a), the control device 104 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 then changed by changing the number of times this energization pattern is repeated. Note that in FIG. 5, hatched areas indicate energization, and non-hatched areas indicate no energization. In this embodiment, the minimum number of repetitions is 1 (this number of repetitions is referred to as the "heating count"). After performing wave number control of the number of heating cycles, the control device 104 maintains the OFF control of the selector switch 107. In Figure 5, the heating period indicates the period during which wave number control is performed, and the non-heating period indicates the period during which the control device 104 maintains the OFF control of the selector switch 107. Note that in the example of Figure 5, waveform control is performed with the number of heating cycles = 2 during the heating period.

In this embodiment, when standby control is initiated, as shown in FIG. 5(b), the control device 104 performs phase control to energize heater H1 during the preheating period and wave number control to energize heater H1 during the heating period. The phase angle of the phase control is set to be greater than half (90°) of the half-wave of the AC voltage so that the maximum voltage is smaller than when no preheating period is provided. In the example of FIG. 5(b), half-wave phase control is repeated six times during the preheating period. The reason for providing a preheating period when standby control is initiated in this manner is to prevent flicker from occurring when standby control is initiated.

The control process executed by the laser printer 1 configured as described above will be explained in detail with reference to Figures 6 to 9. Figure 6 shows the procedure for the control process executed by the control device 104, particularly the CPU 104A. The control process begins, for example, when the laser printer 1 is powered on. Hereinafter, in the explanation of each process procedure, steps will be abbreviated as "S."

In FIG. 6, first, CPU 104A executes preparation processing (S10). Examples of preparation processing include initializing memory 104B and various ports, and reading and setting various default values stored in memory 104B.

Next, CPU 104A executes standby processing (S12). Details of standby processing will be described later using Figures 7 to 9. When standby processing ends, if there is a print command (S14: YES), CPU 104A executes print control in accordance with that print command (S16) and then returns to standby processing. On the other hand, if there is no print command (S14: NO), CPU 104A continues executing standby processing until a predetermined time has elapsed (S18: NO), and once the predetermined time has elapsed (S18: YES), executes sleep control (S20). In other words, the predetermined time is the time for which standby processing continues before entering sleep control.

The CPU 104A then continues to execute sleep control until sleep is released (S22: NO), and when sleep is released (S22: YES), it returns processing to standby processing.

Figure 7 shows the detailed procedure for standby processing. In Figure 7, the CPU 104A first sets the number of heating times m to 2 (S30). The number of heating times m varies depending on the first and second number setting processes described below with reference to Figure 8. In other words, in the processing of S30, the number of heating times m is set to an initial value.

Next, the CPU 104A waits while the temperature T detected by the temperature sensor ST is equal to or greater than the target standby temperature TR (S32: NO), and when the detected temperature T becomes less than the target standby temperature TR (S32: YES), the process proceeds to S34. Here, a specific example of the target standby temperature TR is 122°C, but is not limited to this.

In S34, CPU 104A executes energization control for heater H1 during the preheating period and the heating period, as described above with reference to FIG. 5(b). Specifically, CPU 104A repeats half-wave phase control six times during the preheating period, and repeats wave number control with a duty ratio of 33% twice during the heating period, while maintaining the off control of selector switch 107. Then, CPU 104A starts counting time on timer t (S36). If the control device 104 has a timer function, that function can be used as timer t. If the control device 104 does not have a timer function, a software timer can be constructed on memory 104B and used.

Next, the CPU 104A determines whether a trigger for terminating standby processing has occurred (S38), and if a trigger for terminating standby processing has occurred (S38: YES), the standby processing is terminated. Here, a trigger for terminating standby processing is, for example, the receipt of the print command described above.

On the other hand, if no trigger for terminating standby processing has occurred (S38: NO), CPU 104A makes the same determination as in S32 above, i.e., determines whether detected temperature T < target standby temperature TR (S40). If detected temperature T ≥ target standby temperature TR (S40: NO), CPU 104A determines whether the time counted by timer t ≥ 4 seconds (S50). If the time counted by timer t is < 4 seconds (S50: NO), CPU 104A returns to S38 above. On the other hand, if the time counted by timer t ≥ 4 seconds (S50: YES), CPU 104A repeats wave number control with a duty ratio of 33% for heater H1 twice and maintains the off control of selector switch 107 (S52).

9A shows an example of the transition of the detected temperature T when the process of S52 is executed. In FIG. 9A, time t11 is the time after the CPU 104A has executed the wave number control m times at a duty ratio of 33% (S44 in FIG. 7, which will be described later). For example, this is the case when m is increased to m=6 by the second number setting process, which will be described later with reference to FIG. 8B. At time t11, the CPU 104A executes the wave number control m=6 times, and then maintains the OFF control of the selector switch 107. Then, the detected temperature T gradually rises, and the selector switch 107
Since the OFF control of the wave number control is maintained, the wave number control value decreases. At time t12, 4 seconds (s) have passed since time t11, which corresponds to the determination in S50 being YES. At time t12, the CPU 104A has performed the wave number control N (= 2) times.

In S52, even if the detected temperature T is greater than or equal to the target standby temperature TR, the heater H1 is energized because the temperature of the fixing unit 8, i.e., the detected temperature T detected by the temperature sensor ST, and the heater's resistance decrease at different rates, with the heater's resistance decreasing more rapidly than the temperature of the fixing unit 8. Furthermore, when the resistance of the heater H1 has not yet decreased, i.e., when 4 seconds (s) or more have elapsed since time t11, the CPU 104A executes wave number control a fixed number N (= 2) of times and maintains the OFF control of the selector switch 107. This allows current to flow through the heater H1 once before the resistance of the heater H1 has decreased completely, thereby preventing terminal noise from occurring at the terminal Te connecting the heater H1 to the AC power supply that supplies the AC voltage V.

Returning to FIG. 7, next, CPU 104A executes the first number setting process (S54). FIG. 8(a) shows the detailed procedure of the first number setting process. In FIG. 8(a), CPU 104A first reduces the number of heating cycles m by two (S60). Next, CPU 104A determines whether the number of heating cycles m after the reduction of two is equal to or less than 0 (S62). If the number of heating cycles m is equal to or less than 0 (S62: YES), CPU 104A sets the number of heating cycles m to 1 (S64) and then terminates the first number setting process. On the other hand, if the number of heating cycles m is greater than 0 (S62: NO), CPU 104A terminates the first number setting process. The reason for executing the first number setting process to reduce the number of heating cycles m in this manner is to prevent the detected temperature T from becoming higher than necessary, since the detected temperature T is at or above the target standby temperature TR when the predetermined time of 4 seconds has elapsed and power supply control to heater H1 is being performed.

Returning to FIG. 7, the CPU 104A then resets the timer t and starts timing it (S46). The CPU 104A then returns the process to S38 above.

On the other hand, if the determination in S40 above is that the detected temperature T is less than the target standby temperature TR (S40: YES), the CPU 104A executes a second count setting process (S42). The CPU 104A then repeats wave number control with a duty ratio of 33% for the heater H1 m times, while maintaining the OFF control of the selector switch 107 (S44). The CPU 104A then proceeds to S46 above.

Figure 8(b) shows the detailed procedure for the second number of times setting process. In Figure 8(b), the CPU 104A first determines whether the time counted by timer t is less than 2 seconds (S70). If this determination is true (S70: YES), the CPU 104A increments the number of heating times m by one (S72) and then terminates the second number of times setting process. On the other hand, if the time counted by timer t is greater than or equal to 2 seconds (S70: NO), the CPU 104A terminates the second number of times setting process.

FIG. 9B shows an example of the transition of the detected temperature T when the process of S72 is executed. In FIG. 9B, time t21 is, for example, the time after the CPU 104A has executed wave number control m times at a duty ratio of 33% (S44). For example, this is the case when m=2, shortly after the start of standby processing. At time t21, the CPU 104A executes wave number control m=2 times and then maintains the OFF control of the selector switch 107. Then, the detected temperature T gradually rises, and then, because the OFF control of the selector switch 107 is maintained, it falls. Time t22 indicates the time when the detected temperature T is lower than the target standby temperature TR, and this is the case when less than 2 seconds (s) have elapsed since the time t21 (S44) of the previous power supply control. Since the time from when CPU 104A executes wave number control m times at time t21 until when the detected temperature T becomes less than the target standby temperature TR at time t22 is short, CPU 104A executes wave number control frequently. Therefore, if less than 2 seconds (s) have elapsed since the previous power supply control at time t21 (S44), CPU 104A increases the number of heating cycles (S72).

In the transition of the detected temperature T in Figure 9(a), time t13 indicates the time when the detected temperature T is less than the target standby temperature TR. At time t13, less than 2 seconds (s) have passed since the previous power control at time t12 (S52), so the number of heating cycles m has been increased. At this time, the number of heating cycles m has decreased by two at time t12 (first number of cycles setting process: S60) and increased by one at time t13 (second number of cycles setting process: S72), so it has decreased by one compared to the number of heating cycles m at time t11.

Figure 9(c) shows an example of the progression of the detected temperature T when the process of S72 is skipped. In Figure 9(c), time t31 is the time after the CPU 104A has executed wave number control m times at a duty ratio of 33% (S44), for example. Then, the detected temperature T gradually rises, and then, because the selector switch 107 remains off, it drops. Time t32 indicates the time when the detected temperature T is less than the target standby temperature TR. Because time t32 is more than two seconds (s) after the previous power control at time t31, the number of heating cycles m is maintained unchanged. In this way, if the detected temperature T falls below the target standby temperature TR when more than two seconds (s) have passed since the previous power control, it is determined that the target standby temperature TR can be maintained with the current number of heating cycles of heater H1.

As described above, the laser printer 1 of this embodiment is equipped with a process cartridge 5 that forms a developer image on a sheet S, a fuser 8 that is connected via terminal Te to an AC power supply that supplies AC voltage V and has a heater H1 that is heated by the AC voltage V and that fuses the developer image on the sheet S, a temperature sensor ST that detects the temperature of the fuser 8, a triac 106 that is provided between the AC power supply and the heater H1 and switches between an ON state in which the AC power supply and the heater H1 are electrically connected and an OFF state in which the AC power supply and the heater H1 are not electrically connected, and a CPU 104A.

In standby control to maintain the temperature of the fixing unit 8 at the target standby temperature TR based on the detected temperature T detected by the temperature sensor ST, the CPU 104A performs wave number control, which sets the triac 106 to the on state for some half-waves within a control cycle consisting of multiple consecutive half-waves of the AC voltage V and sets the triac 106 to the off state for the remaining half-waves.If the elapsed time since the wave number control was performed is less than 4 seconds and the detected temperature T is less than the target standby temperature TR, the wave number control is performed m times for heating.If the elapsed time since the wave number control was performed is 4 seconds or more and the detected temperature T is equal to or greater than the target standby temperature TR, the wave number control is performed twice.

In this way, in the laser printer 1 of this embodiment, if 4 seconds or more have elapsed since wave number control was performed and the detected temperature T is equal to or higher than the target standby temperature TR, wave number control is performed before the resistance value of heater H1 has dropped sufficiently. As a result, current flows through heater H1 before the resistance value of heater H1 has dropped sufficiently, preventing terminal noise caused by a sudden current flowing through heater H1.

In this embodiment, the laser printer 1 is an example of an "image forming device." The process unit PR is an example of a "developer image forming unit." The triac 106 is an example of a "switching element." The CPU 104A is an example of a "control unit." The number of heating cycles m is an example of a "first predetermined number of cycles." Two times is an example of a "second predetermined number of cycles." Four seconds (s) is an example of a "first predetermined time." Note that the first predetermined number of cycles and the second predetermined number of cycles may be the same or different.

Furthermore, if 4 seconds or more have elapsed since wave number control was performed and the detected temperature T is equal to or higher than the target standby temperature TR, the CPU 104A performs wave number control twice, and then, when the detected temperature T falls below the target standby temperature TR, reduces the number of heating cycles m and performs wave number control.

As a result, when the detected temperature T is equal to or higher than the target standby temperature TR and wave number control is performed twice, current flows through the heater H1 at a temperature equal to or higher than the target standby temperature TR. Therefore, when the laser printer 1 performs wave number control when the detected temperature T falls below the target standby temperature TR, it is possible to prevent the detected temperature T from rising too high by reducing the number of heating cycles m and performing wave number control.

Furthermore, if the elapsed time since wave number control was performed is less than 2 seconds (less than 4 seconds) and the detected temperature T is below the target standby temperature TR, the CPU 104A increases the number of heating cycles m and performs wave number control. Incidentally, 2 seconds is an example of a "second predetermined time."

As a result, if the elapsed time since wave number control was performed is less than 2 seconds (less than 4 seconds), it becomes difficult to maintain the target standby temperature TR. Therefore, by increasing the number of heating cycles m and performing wave number control, the laser printer 1 can more easily maintain the temperature of the fixing unit 8 at the target standby temperature TR.

Furthermore, if the elapsed time since wave number control was performed is 2 seconds or more but less than 4 seconds, and the detected temperature T is lower than the target standby temperature TR, the CPU 104A performs wave number control without changing the heating count m.

In addition, before starting standby control and performing the first wave number control, CPU 104A performs phase control in which triac 106 is turned on for some waveforms within a half-wave of the AC voltage and turned off for the remaining waveforms.

This allows flicker to be prevented by performing phase control before the first wave number control when starting standby control.

The present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the invention.

(1) In the above embodiment, the heater H1 is energized with an AC voltage V, but it may also be energized with a DC voltage. Furthermore, 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.

(2) The number of heating cycles m, the six half-waves used in phase control, the duty ratio used in wave number control, and the 4 seconds or 2 seconds compared with the time counted by timer t are merely examples, and other values may be used.

(3) In the above embodiment, the temperature sensor was configured to detect the temperature of the heating member, but this is not limited to this. For example, the temperature sensor may be configured to detect the temperature of a portion other than the heating portion of the fixing unit, such as a pressure member. Furthermore, the temperature sensor may be a temperature sensor other than a thermistor. Furthermore, the temperature sensor may be a non-contact temperature sensor or a contact temperature sensor.

(4) In the above embodiment, a halogen heater that uses radiant heat is used as the heater H1, but this is not limited thereto, and the heater may be, for example, a ceramic heater or a carbon heater that uses heat generated by a resistor. Furthermore, the heater may be disposed outside the heating element, rather than inside the heating element.

(5) In the above embodiment, an image forming device that forms a monochrome image on a sheet was used as an example of the image forming device, but this is not limited to this and may be, for example, a printer configured to form a color image on a sheet. Furthermore, the image forming device is not limited to a printer and may be, for example, a copier or multifunction device equipped with a document reading device such as a flatbed scanner.

1...laser printer, 8...fixing unit, 104...controller, 104A...CPU, 104B...memory, 105...zero-cross detection circuit, 106...triac, 107...changeover switch, H1...heater, PR...processing unit, ST...temperature sensor.

Claims (5)

a developer image forming unit that forms a developer image on a sheet;
a fixing device that is connected to an AC power source that supplies an AC voltage via terminals and has a heater that is heated by the AC voltage, and that fixes the developer image on the sheet;
a temperature sensor for detecting the temperature of the fixing unit;
a switching element that is provided between the AC power supply and the heater and that switches between an ON state in which the AC power supply and the heater are electrically connected and an OFF state in which the AC power supply and the heater are not electrically connected;
A control unit;
Equipped with
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,
a control cycle is defined as a plurality of successive half waves of the AC voltage, and wave number control is performed in which the state of the switching element is set to the on state for some half waves within the control cycle and the state of the switching element is set to the off state for the remaining half waves;
When the elapsed time since the wave number control was performed is less than a first predetermined time and the detected temperature is less than the target standby temperature, the wave number control is performed a first predetermined number of times;
When the elapsed time since the wave number control was performed is equal to or longer than the first predetermined time and the detected temperature is equal to or higher than the target standby temperature, the wave number control is performed a second predetermined number of times.
An image forming apparatus characterized by: When the elapsed time since the wave number control was performed is equal to or longer than the first predetermined time and the detected temperature is equal to or higher than the target standby temperature, the control unit performs the wave number control the second predetermined number of times, and then, when the detected temperature becomes lower than the target standby temperature, reduces the first predetermined number of times and performs the wave number control.
2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium. When the elapsed time since the wave number control was performed is less than a second predetermined time that is shorter than the first predetermined time, and the detected temperature is less than the target standby temperature, the control unit increases the first predetermined number of times and performs the wave number control.
3. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium. When the elapsed time since the wave number control was performed is equal to or longer than the second predetermined time and shorter than the first predetermined time, and the detected temperature is lower than the target standby temperature, the control unit performs the wave number control without changing the first predetermined number of times.
4. The image forming apparatus according to claim 3, wherein the image forming apparatus is a recording medium. the control unit starts the standby control, and before executing the first wave number control, executes phase control to set the state of the switching element to the on state for a part of waveforms within the half wave of the AC voltage, and to set the state of the switching element to the off state for the remaining waveforms.
5. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.