JP2015068899A - Fixing unit, and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fixing unit that controls a current so that no unnecessary heavy current flows in a fixing heater.SOLUTION: A fixing unit includes: a fixing heater 305 heated by power supplied from an AC power source 406, for heating a sheet with a toner image transcripted thereon; a temperature detection element 405 for detecting a temperature of the fixing heater 305; and a control circuit 404 for controlling a power-feeding time to the fixing heater 305 per given power-feeding time on the basis of the temperature detected by the temperature detection element 405 and a target temperature of the fixing heater 305. The control circuit 404 gradually increases a power-feeding time after starting feeding of an AC current to the fixing heater 305 by the given power-feeding time, and determines an upper limit of a power-feeding time for the fixing heater 305 thereafter, on the basis of the power-feeding time when a temperature detected by the temperature detection element 405 reaches the target temperature.

Description

  The present invention relates to a fixing device that is used in an image forming apparatus such as a copying machine or a multifunction peripheral and fixes a toner image onto a sheet.

  An image forming apparatus includes: a developing unit that develops a latent image formed on a photoconductor to form an image on paper; a transfer unit that transfers an image (toner image) on the developed photoconductor to paper; And a fixing device for fixing the toner image. As a fixing method using a fixing device, there are a heat roller method using a halogen heater as a heat source and a film heating method using a ceramic surface heating heater as a heat source. In any system, a fixing heater such as a halogen heater or a ceramic surface heating heater generates heat when an AC voltage is applied by a switching element such as a bidirectional thyristor and an AC current is applied.

  The fixing device includes a temperature detection element such as a thermistor in order to detect the temperature of a fixing heater or other temperature control target. The fixing device performs temperature control by adjusting the amount of alternating current applied to the fixing heater according to the detection result of the temperature detection element. The energization amount of the alternating current to the fixing heater is adjusted by phase control for changing the energization start timing in the AC half wave, wave number control for turning on / off the current for each AC half wave, or the like.

  The output (heat generation amount) required for the fixing heater is mainly determined according to the fixing property of the toner image transferred to the paper. The output of the fixing heater is determined by using the applied AC voltage and the resistance value of the electric resistance of the fixing heater as parameters. Since the AC voltage is determined by the area where the fixing device is used, the output of the fixing heater is adjusted by the resistance value of the fixing heater.

  In determining the output of the fixing heater, it is necessary to consider variations in the AC voltage and the electric resistance of the fixing heater. For example, the resistance value of the fixing heater is determined so that the necessary minimum output can be secured even when the AC voltage decreases and the electric resistance of the fixing heater increases. The output of the fixing heater is determined so that sufficient heat can be obtained to fix the toner image even when the AC voltage and the electric resistance of the fixing heater vary greatly.

  However, when the AC voltage increases and the electrical resistance of the fixing heater decreases, the current flowing through the fixing heater increases and the output increases. When a large current flows through the fixing heater, a structure for preventing destruction due to the large current, for example, a fuse having a large rating must be selected, leading to an increase in the size and cost of the fixing device. In addition, it is necessary to suppress an unnecessary large current because it may affect the distribution system of the AC power supply, such as voltage fluctuation and unnecessary operation of the protection system.

  On the other hand, an increase in the temperature of the fixing device can be cited as one of the bottlenecks of the time taken to start the operation of the image forming apparatus. In order for the image forming apparatus to start up quickly, it is necessary to increase the output of the fixing heater, that is, to flow a large amount of current. Therefore, at the start of the operation of the image forming apparatus, it is required to flow as much current as possible to the fixing device in the normal range in accordance with the status of the parameters that determine the output of the fixing heater such as the alternating voltage and resistance value at that time.

  Patent Document 1 discloses an image forming apparatus that controls a current by providing a circuit that detects a current flowing through a fixing heater. Further, Patent Document 2 discloses an image forming apparatus in which a circuit for detecting the voltage of an AC power supply is provided to control the current flowing in the fixing heater and the time until the operation of the image forming apparatus is started as short as possible.

JP 2006-113117 A JP 2007-240719 A

  Since each of the image forming apparatuses disclosed in Patent Documents 1 and 2 has a configuration for detecting a current and a voltage flowing through the fixing heater, it tends to be difficult to suppress the cost of the fixing device itself.

  In order to solve the above problems, it is a main object of the present invention to provide a fixing device that controls current so that an unnecessary large current does not flow through a fixing heater without providing a current detection circuit or the like.

  The fixing device of the present invention that solves the above-described problems is a heater that heats paper that is supplied with electric power from an AC power source and that has a toner image transferred thereon, temperature detection means that detects the temperature of the heater, and Control means for controlling the energization time per predetermined period to the heater based on the temperature detected by the temperature detection means and the target temperature of the heater, and the control means has a predetermined energization time. The energization time is gradually increased after energization of the alternating current to the heater is started, and the subsequent heaters are based on the energization time when the temperature detected by the temperature detection means reaches the target temperature. The upper limit of the energization time is determined.

  According to the present invention, it is possible to determine the upper limit value of the energization time to the heater without providing a special detection circuit, and to control the current so that a large current does not flow to the heater.

1 is an internal configuration diagram of an image forming apparatus. The block diagram of a fixing device. The circuit block diagram of an electricity supply circuit. (A)-(d) is an illustration figure of the waveform of each part of an electricity supply circuit. The figure showing the relationship between supplied electric power and a phase angle. The circuit block diagram of a control circuit. The flowchart showing a temperature control process. (A)-(g) is a figure showing dispersion | variation in electric power, an electric current, a phase angle, etc. by the dispersion | variation in the resistance of a power supply voltage and a fixing heater. The flowchart of the process which determines a minimum phase angle. The flowchart showing a temperature control process. The flowchart of the process which determines a minimum phase angle. The figure showing the relationship between electric power and temperature. The figure showing the relationship of several electric power with respect to phase angle (alpha). The figure showing the minimum phase angle to the maximum electric power. The flowchart of the process which determines a minimum phase angle.

  Hereinafter, embodiments will be described in detail with reference to the drawings.

<Image forming apparatus>
FIG. 1 is an internal configuration diagram of an electrophotographic image forming apparatus. In order to form an image on the paper S, the image forming apparatus performs each process of latent image formation, development, transfer, and fixing. The image forming apparatus forms a color image in which four colors of yellow, magenta, cyan, and black are superimposed.

  In the latent image forming process, first, the surfaces of the photosensitive drums 11a to 11d are uniformly charged by the chargers 12a to 12d. The photosensitive drums 11a to 11d whose surfaces are charged are irradiated with laser light from the semiconductor lasers 13a to 13d. A latent image is formed on the photosensitive drums 11a to 11d by the disappearance of the charge in the portion irradiated with the laser beam.

  In the development process, a developer such as toner is attached to the latent images formed on the photosensitive drums 11a to 11d from the developing units 14a to 14d. The latent image is developed by the adhesion of the developer, and toner images are formed on the photosensitive drums 11a to 11d.

  In the transfer process, the toner images formed on the photosensitive drums 11a to 11d are sequentially transferred onto the intermediate transfer belt 30 by the transfer units 35a to 35d so that the toner images overlap each other. By transferring the toner images in a superimposed manner, a color toner image is formed on the intermediate transfer belt 30. The toner image transferred to the intermediate transfer belt 30 is transferred to the paper S at a time in the secondary transfer portions 34 and 36. The sheet S is conveyed from the sheet cassette 21 to the secondary transfer units 34 and 36 through the sheet conveyance path 24 in accordance with the timing at which the toner image transferred to the intermediate transfer belt 30 is conveyed to the secondary transfer units 34 and 36. The

  In the fixing process, the sheet S on which the toner image is transferred is conveyed to the fixing device 40. The fixing device 40 heats and pressurizes the paper S to fix the toner image on the paper S. Thereby, the image formation on the paper S is completed. The paper S for which the fixing process has been completed is discharged to the paper discharge unit 29.

<Fixing device>
FIG. 2 is a configuration diagram of the fixing device 40. The fixing device 40 includes a fixing film 301, a pressure member 302, a fixing heater 305, a lower roller 307, and a temperature detection element 405. The fixing device 40 sandwiches the paper S between the fixing film 301 and the lower roller 307 during the fixing process. As the fixing film 301 rotates in the direction of arrow B and the lower roller 307 rotates in the direction of arrow C, the sheet S is conveyed through the fixing device 40 in the direction of arrow A. A contact portion between the fixing film 301 and the lower roller is called a nip portion.

  The fixing heater 305 generates heat when energized and heats the sheet S in the nip portion. The fixing heater 305 generates heat so that the fixing film 301 reaches a temperature higher than that necessary for fixing the toner image on the paper S. The temperature of the fixing film 301 is detected by the temperature detection element 405. The detection result is transmitted to a control circuit which will be described later, and is used for temperature control of the fixing heater 305 by the control circuit.

  The pressure member 302 presses the fixing heater 305 and the fixing film 301 in the direction of the lower roller 307 at the nip portion. Since the sheet S is conveyed while being sandwiched between the fixing film 301 and the lower roller 307, the pressure member 302 applies pressure to the sheet S. The fixing device 40 fixes the toner image on the paper S by heating by the fixing heater 305 and pressing by the pressure member 302.

<Energization circuit>
FIG. 3 is a circuit configuration diagram of an energization circuit that energizes the fixing heater 305. The energization circuit supplies a current to the fixing heater 305. The energization circuit includes a zero cross detection circuit 401, a bidirectional thyristor 402, a gate 403, and a control circuit 404, and is connected to an AC power supply 406, a fixing heater 305, and a temperature detection element 405. The AC power source 406 supplies power to other components in the image forming apparatus. The energization circuit is built in the fixing device 40, for example.

  The zero cross detection circuit 401 detects a timing (zero cross) at which the polarity of the AC power supply voltage output from the AC power supply 406 changes, and outputs a detection pulse signal (zero cross signal). The zero cross signal is input to the control circuit 404. The control circuit 404 outputs a control signal for controlling the gate 403 of the bidirectional thyristor 402 according to the zero cross signal and the detection result of the temperature detection element 405. The bidirectional thyristor 402 performs energization control between the nodes T <b> 1 and T <b> 2 according to a control signal input to the gate 403. The fixing heater 305 generates heat when the nodes T1 and T2 are electrically connected and an alternating current is applied. The bidirectional thyristor 402 continues to be conductive until the polarity of the voltage between the nodes T1 and T2 (the polarity of the power supply voltage output from the AC power supply 406) changes after the nodes T1 and T2 are made conductive.

  FIG. 4 is an exemplary diagram of waveforms of voltages, signals, and currents of the respective parts of the energization circuit. FIG. 4A shows a waveform of a power supply voltage that is an AC voltage output from the AC power supply 406. FIG. 4B shows a zero cross signal. FIG. 4C shows the control signal. FIG. 4D shows the current flowing through the fixing heater 305.

  At the timing when the polarity of the AC power supply voltage changes (180 °, 360 °, 540 °, 720 °: FIG. 4A), the zero-cross detection circuit 401 outputs a zero-cross signal that is a pulse signal (FIG. 4 ( b)). In response to the zero-cross signal and the detection result of the temperature detection element 405, the control circuit 404 outputs a control signal (FIG. 4C). The time from the input of the zero cross signal to the output of the control signal is represented by a phase angle α described later. In response to the control signal, the bidirectional thyristor 402 conducts between the nodes T1 and T2. Due to the conduction between the nodes T1 and T2, an AC current is supplied from the AC power source 406 to the fixing heater 305 (FIG. 4D). The bidirectional thyristor 402 makes the node T1 and T2 non-conductive when the polarity between the nodes T1 and T2 (the polarity of the power supply voltage output from the AC power supply 406) is reversed. Therefore, the current flowing through the fixing heater 305 becomes zero until the next control signal is input after the timing when the polarity changes (180 °, 360 °, 540 °). The control circuit 404 changes the phase angle α according to the detection result of the temperature detection element 405, whereby the time (current amount) during which the current flows through the fixing heater 305 changes. Therefore, the amount of electric power supplied to the fixing heater 305 is changed by the AC half wave of the AC voltage, and the temperature of the fixing heater 305 that generates heat by energization can be controlled.

  In the present embodiment, as the energization control method, a phase control method is used that adjusts the timing of the start of energization of AC current within the AC half wave (1/2 cycle) of the AC voltage supplied from the AC power supply 406. Although it is possible to change the timing of starting energization for each AC half-wave, since the power source is connected to the AC device, in many cases, the energizing current control is performed every two AC half-waves (one cycle) of the AC voltage. Done.

  FIG. 5 is a diagram showing the relationship between the electric power P supplied to the fixing heater 305 per predetermined period and the phase angle α. The electric power P is represented by the current and voltage flowing through the fixing heater 305. FIG. 5 represents the power ratio when the energization is started after the time of the phase angle α has elapsed from the zero cross signal, based on the power when the energization is fully energized for a half cycle (predetermined period) of the AC voltage. The power supplied to the fixing heater 305 is 100% when the phase angle α is “0 °”, gradually decreases as the phase angle α increases, and when the phase angle α is “180 °”. It becomes zero. As described above, the power P supplied to the fixing heater 305 is determined by the energization time. Since the power supply voltage is a periodic wave, the relationship with the power P is represented by the phase angle α. However, when current control is actually performed, it is necessary to convert it into time. For example, since the power supply frequency is 50 Hz in the East Japan region, 360 ° = 20 [ms].

  The fixing heater 305 keeps the temperature of the surface of the fixing film 301 constant. The temperature detection element 405 detects the temperature of the fixing film 301. If the temperature detected by the temperature detection element 405 is higher than a predetermined temperature, the control circuit 404 increases the phase angle α to shorten the energization time and reduce the power, thereby lowering the temperature of the fixing film 301. If the temperature detected by the temperature detecting element 405 is lower than the predetermined temperature, the control circuit 404 increases the power by increasing the energization time by decreasing the phase angle α, and increases the temperature of the fixing film 301. The predetermined temperature is a temperature necessary for fixing the toner image on the paper S.

  The temperature detection element 405 is realized using, for example, a thermistor. The thermistor has a characteristic that the resistance value decreases as the temperature of the object to be measured increases, and the resistance value increases as the temperature decreases. In FIG. 3, the temperature detection element 405 is configured by connecting a thermistor and a fixed resistor in series. Since the resistance value of the thermistor depends on the temperature, the temperature of the fixing heater 305 can be detected from the voltage value obtained by dividing the DC voltage Vcc by the thermistor and the fixed resistor output from the temperature detection element 405.

<Temperature control>
FIG. 6 is a circuit configuration diagram of the control circuit 404. The control circuit 404 includes a voltage / temperature conversion circuit 4041, a temperature / power ratio calculation circuit 4042, a power ratio / phase angle calculation circuit 4043, a phase angle / time calculation circuit 4044, and a control signal output circuit 4045. FIG. 7 is a flowchart showing a temperature control process for keeping the temperature of the fixing heater 305 constant by the control circuit 404 as described above.

  The control circuit 404 converts the voltage value output from the temperature detection element 405 into a temperature by the voltage / temperature conversion circuit 4041 (S101). The voltage / temperature conversion circuit 4041 has a table representing the relationship between the voltage value output from the temperature detection element 405 and the temperature, and converts the voltage value into temperature by referring to this table.

  The temperature / power ratio calculation circuit 4042 determines the power to be supplied to the fixing heater 305 according to the temperature converted by the voltage / temperature conversion circuit 4041 and the target temperature of the fixing heater 305 (S102). The temperature / power ratio calculation circuit 4042 calculates the power from the temperature using, for example, a predetermined formula. The power ratio / phase angle calculation circuit 4043 calculates the phase angle α corresponding to the power calculated by the temperature / power ratio calculation circuit 4042 (S103). The power ratio / phase angle calculation circuit 4043 calculates the phase angle α from the power according to the relationship of FIG. The phase angle / time calculation circuit 4044 calculates the time from the zero cross signal according to the phase angle α calculated by the temperature / power ratio calculation circuit 4042 (S104). For example, if the power supply frequency is 50 Hz, the phase angle / time calculation circuit 4044 calculates time from the phase angle α as 360 ° = 20 [ms]. A table that directly determines the phase angle α and time from the temperature information about the fixing heater 305 may be used.

  The control signal output circuit 4045 outputs a control signal after the time calculated by the phase angle / time calculation circuit 4044 after the zero cross signal is input from the zero cross detection circuit 401 (S105). In response to the control signal, the bidirectional thyristor 402 applies an alternating current to the fixing heater 305 to heat the fixing heater 305. By repeating the processes of S101 to S106, the temperature of the fixing heater 305 can be maintained.

<Power supplied to the fixing heater>

  The power supplied to the fixing heater 305 is determined so that the fixing film 301 is heated to a temperature at which the toner image can be fixed on the paper even when the image forming apparatus is placed in the most severe environment. In the present embodiment, the minimum power required for this is 700 [W]. The power is proportional to the square of the applied voltage and inversely proportional to the resistance value. For this reason, variations in applied voltage and resistance become variations in power. FIG. 8 is a diagram illustrating variations in power, current, phase angle, and the like when the power supply voltage is 100 ± 10 [V] and the accuracy of the resistance value of the fixing heater 305 is 10 ± 1 [Ω].

  FIG. 8A shows power (maximum power) when the phase angle α is “0 °”. FIG. 8B shows the current (maximum current) when the phase angle α is “0 °”. When the power supply voltage, which is the most difficult condition to generate heat, is 90 [V] and the resistance value of the fixing heater 305 is 11.0 [Ω], the power is 736 [W], which satisfies the necessary power. When the power supply voltage is 110 [V] and the resistance value of the fixing heater 305 is 9.0 [Ω], which is the most heat-generating condition, the power is 1344 [W], which is twice the required minimum power. Compared with current, the minimum current is 8.2 [A], whereas the maximum current is 12.2 [A], and the maximum current flows to the fixing heater 305 by 4.0 [A]. Become.

  FIG. 8C shows the current flowing through the fixing heater 305 when the power is kept constant at 700 [W]. The power supply voltage is 110 [V], the resistance value of the fixing heater 305 is 9.0 [Ω], and 110 / 9.0 = 12.2 [A], not 8.8 [A]. It shows that the voltage needs to be reduced to 80 [V] equivalently by the directional thyristor 402. The bidirectional thyristor 402 reduces the voltage even under other conditions.

  Since the required power is 700 [W], when the power supply voltage is 90 [V] and the resistance value of the fixing heater 305 is 11.0 [Ω], according to FIGS. There are extra [W] and 0.2 [A]. When the power supply voltage is 110 [V] and the resistance value of the fixing heater 305 is 9.0 [Ω], there is an excess of 644 [W] and 3.4 [A]. FIG. 8D shows a current difference (excess current) between FIG. 8B and FIG. 8C. FIG. 8E shows the current ratio of FIG. 8C, and FIG. 8F shows the power ratio with respect to 700 [W].

  If the fixing device 40 is configured including this excess, the cost increases due to the increase in circuit size, the improvement in temperature resistance, the increase in the size of protective parts such as fuses, and the like. In addition, when a large amount of current flows, there is a possibility of affecting other electrical devices connected to the AC power source 406. However, in order to quickly increase the temperature of the fixing device 40, which is one of the bottlenecks in the start-up time of the image forming apparatus, there is a conflicting requirement that a large amount of current must be passed through the fixing heater 305 to increase the amount of heat generation. There is. Therefore, conventionally, the current is set to the maximum within a range where the cost does not increase, or set to the maximum within a range not affecting the power source.

  In order to suppress such power and current, a device for monitoring the voltage and current of the AC power source 406 has been provided. However, since the addition of such a device leads to an increase in cost, a means capable of suppressing power and current without providing a special device is desired.

  The control circuit 404 controls temperature fluctuations of the fixing film 301 due to fluctuations in the power supply voltage, fluctuations in the resistance of the fixing heater 305, fluctuations in heat generation factors of the fixing heater 305, and the like by increasing or decreasing the phase angle α. That is, the control circuit 404 adjusts the temperature of the fixing film 301 by adjusting the energization time.

  FIG. 8G shows the phase angle α for making the power constant at 700 [W]. When the power supply voltage is 100 [V] and the resistance value of the fixing heater 305 is 10.0 [Ω], the phase angle α is set to “71” so that the current becomes 8.4 [A] (see FIG. 8C). Keep “°”. When the power supply voltage is increased from 100 [V] to 110 [V], the current flowing through the fixing heater 305 is increased by 1.1 times to 9.3 [A], and the power (heat generation amount) is 1.2 times 840. Increase to [W]. As a result, the temperature of the fixing film 301 increases. When the temperature detection element 405 that detects the temperature of the fixing film 301 detects that the temperature of the fixing film 301 is higher than the temperature necessary for fixing the toner image, the control circuit 404 increases the phase angle α to increase the current. The temperature of the fixing film 301 is decreased by decreasing the temperature. The control circuit 404 finally changes the phase angle α from “71 °” to “83 °” so that the power is kept constant at 700 [W] and the temperature of the fixing film 301 is necessary for fixing the toner image. Use a suitable temperature.

  When the power supply voltage is 100 [V] and the resistance value of the fixing heater 305 is 10.0 [Ω] from the state where the resistance value is increased to 11.0 [Ω] due to heat generation or the like, the current flowing through the fixing heater 305 is 1 /1.1 times to 7.6 [A], and the power (heat generation amount) also decreases to 636 [W]. As a result, the temperature of the fixing film 301 decreases. When the temperature detection element 405 detects that the temperature of the fixing film 301 is lower than the temperature necessary for fixing the toner image, the control circuit 404 increases the current by reducing the phase angle α to increase the fixing heater 305. Increase the temperature. The control circuit 404 finally changes the phase angle α from “71 °” to “63 °” so that the power is kept constant at 700 [W] and the temperature of the fixing film 301 is necessary for fixing the toner image. Use a suitable temperature.

  The temperature control has been described in two examples when the power supply voltage changes and when the resistance value changes. The control circuit 404 adjusts the phase angle α by feedback control in accordance with the amount of change from the desired power in order to keep the temperature of the fixing film 301 constant, including factors that cause other temperature changes. That is, the phase angle α gives an index of variation from a desired value. The phase angle α is a control input for controlling the temperature of the fixing film 301, and it is easy to grasp its value.

  For the above reasons, focusing on the phase angle α, it is possible to obtain an index of the current and power flowing through the fixing heater 305 without adding a special device for monitoring the voltage and current applied to the fixing heater 305. It becomes possible.

[First Embodiment]
FIG. 9 is a flowchart of processing for determining the lower limit of the phase angle α by the image forming apparatus as described above. The phase angle α is a parameter that determines the power P supplied to the fixing heater 305. As shown in FIG. 5, the power increases as the phase angle α decreases. Therefore, by determining the lower limit of the phase angle (hereinafter referred to as “lower limit phase angle αL”), the upper limit of power P (hereinafter referred to as “upper limit power”) is determined.

  First, the control circuit 404 controls the temperature of the fixing heater 305 by energizing with the phase angle α0 (S201). As described above, the current flowing through the fixing heater 305 varies in accordance with changes in the power supply voltage and the resistance value of the fixing heater 305. The phase angle α0 is “88 °” for achieving a power of 700 [W] when the power supply voltage is 110 [V], the resistance value of the fixing heater 305 is 9.0 [Ω], which is the most current flowing condition (FIG. 8 (g)). By setting the phase angle α 0 in this way, it is possible to avoid a large current exceeding a predetermined value from flowing through the fixing heater 305. The control circuit 404 performs temperature control so that power equal to or less than the power by the phase angle α 0 is supplied to the fixing heater 305. If the phase angle is greater than or equal to the phase angle α0, the current can be reliably suppressed even if the power supply voltage or the resistance value of the fixing heater 305 fluctuates within an assumed range. If conditions such as the power supply voltage is 110 [V] and the resistance value of the fixing heater 305 is 9.0 [Ω], the temperature of the fixing film 301 reaches the temperature necessary for fixing the toner image after a predetermined time has elapsed. To do.

  However, when the conditions change, for example, when the power supply voltage is 90 [V] and the resistance value of the fixing heater 305 is 11.0 [Ω], the power is (95.1% −52.1% =) 43% ( The fixing film 301 does not reach the temperature necessary for fixing the toner image due to the shortage. Similarly, when the power supply voltage is 100 [V] and the resistance value of the fixing heater 305 is 10.0 [Ω], the power is (70.0% −52.1% =) 17.9% (FIG. 8 (f ) Reference) Due to the shortage, the fixing film 301 does not reach the temperature necessary for fixing the toner image. If the temperature does not reach the temperature required for fixing the toner image after a predetermined time has elapsed after the energization of the fixing heater 305 (S202: N), the control circuit 404 decreases the phase angle α0 to reduce the phase. The angle αi is updated (S203). As the phase angle α decreases, the upper limit value of the supplied power increases. The phase angle α0 is reduced by, for example, Δα (αi = α0−Δα).

  The control circuit 404 controls the temperature of the fixing heater 305 based on the updated phase angle αi (S204). The control circuit 404 repeatedly performs the processes of S202 to S204. By repeating, the phase angle αi is decreased step by step by Δα. The control circuit 404 determines the lower limit of the phase angle αi for setting the fixing heater 305 to a temperature necessary for fixing the toner image by repeating the processes of S202 to S204. When the fixing heater 305 reaches the temperature necessary for fixing the toner image (S202: Y), the control circuit 404 determines the phase angle αi at this time as the lower limit phase angle αL. The control circuit 404 performs subsequent temperature control of the fixing heater 305 based on the lower limit phase angle αL (S205).

  The lower limit phase angle αL determined in this way gives the minimum power necessary to bring the fixing film 301 to a temperature necessary for fixing the toner image, based on the power supply voltage at that time and the resistance value of the fixing heater 305. In the subsequent temperature control of the fixing film 301, the phase angle is controlled so as not to fall below the lower limit phase angle αL. FIG. 10 is a flowchart showing such a temperature control process. The temperature control process shown in FIG. 7 is different in that the processes in S301 and S302 are performed between the processes in S104 and S105, and the other processes are the same.

  After calculating the phase angle α according to the power (S104), the control circuit 404 determines whether or not the calculated phase angle α is smaller than the lower limit phase angle αL (S301). If small (S301: Y), the control circuit 404 changes the phase angle α calculated in S104 to the lower limit phase angle αL (S302), and converts the lower limit phase angle αL to time (S105). When the phase angle α is equal to or larger than the lower limit phase angle αL (S301: N), the control circuit 404 converts the phase angle α into time (S105).

  Since it takes time to raise the temperature of the fixing film 301 to the target temperature, this time reduction is desired. In order to shorten this time, it is effective to reduce the heat capacity of the fixing device 40 and improve the output of the fixing heater 305. Since the lower limit phase angle αL as described above provides the minimum power required to reach the target temperature, the time to reach the target cannot be further shortened.

  If a sudden power supply voltage or environmental change occurs after determining the lower limit of the phase angle α, the determined lower limit phase angle αL cannot cope with the change. For this purpose, the lower limit phase angle αL may have a margin (the lower limit phase angle αL may be further reduced by a predetermined amount). That is, in order to provide a margin for the upper limit power, a value obtained by adding a predetermined amount to the power set as the upper limit in the processing of FIG. 9 is set as a new upper limit power. FIG. 11 is a flowchart of processing for determining such a lower limit phase angle αL. 9 is different from the process in S205 in that the process in S205 is replaced with the process in S401, and the other processes are the same.

  The control circuit 404 gives a margin to the electric power by giving a margin to the phase angle αi when the fixing heater 305 reaches a temperature necessary for fixing the toner image. That is, the upper limit value of the energization time is increased by a margin (margin time). Therefore, the control circuit 404 changes the phase angle αL when the temperature of the fixing heater 305 reaches a predetermined temperature to the lower limit phase angle (αL−δα), and performs subsequent temperature control (S401). In the temperature control, if the phase angle α calculated according to the power is smaller than the lower limit phase angle (αL−δα) as in the processing of S301 and S302 in FIG. 10, the phase angle α is set to the lower limit phase angle (αL−δα). Change to Note that the control circuit 404 may change the lower limit phase angle so that the power becomes constant, in addition to uniformly reducing δα from the phase angle.

  It is effective to determine the lower limit phase angle (upper limit power) at a timing when a parameter that greatly affects the temperature control of the fixing heater 305 is changed. This timing may be, for example, before the image forming apparatus is first turned on to perform image forming processing, before the energization circuit is turned on to start temperature control of the fixing heater 305, immediately after the image forming apparatus is installed, etc. It is.

[Second Embodiment]
As described above, the current flowing through the fixing heater 305 varies in accordance with changes in the power supply voltage and the resistance value of the fixing heater 305. In the second embodiment, a large current does not flow through the fixing heater 305 even when the power supply voltage, which is the most current flowing condition, is 110 [V], and the resistance value of the fixing heater 305 is 9.0 [Ω]. Energization is performed at a constant phase angle α0 such that the temperature of 301 does not rise excessively. The temperature T0 of the fixing film 301 after a predetermined time has elapsed from the start of energization is measured.

  FIG. 12 is a diagram illustrating the relationship between the power supplied to the fixing device 40 and the temperature of the fixing film 301. With respect to the electric power P supplied to the fixing device 40, the temperature of the fixing film 301 changes exponentially to the right. With such a relationship, the measured temperature T 0 of the fixing film 301 can be converted into the electric power P supplied to the fixing device 40.

  The phase angle α and the power P are in the relationship shown in FIG. FIG. 5 represents an ideal power P with respect to the phase angle α. When the power supply voltage or the resistance value of the fixing heater 305 changes to change the power supplied from the AC power supply, the relationship between the phase angle α and the power P becomes a different curve. FIG. 13 shows the relationship between the phase angle α and the power P with respect to the fluctuation (power fluctuation) of the power supplied from the AC power supply. The control circuit 404 stores data indicating characteristics of a plurality of phase angles α and power P in a plurality of situations in which fluctuations in power supplied from the AC power supply are different in a memory in the control circuit, and the phase angle α0 and power Characteristic data of the phase angle α and the power P that coincide with or closest to P0 is selected. That is the characteristic of the phase angle α and power P of the image forming apparatus at that time.

  For example, when the power P0 = 350 [W] is obtained with the phase angle α0 = 110 °, the curve indicated by the arrow is selected according to FIG. Here, a plurality of characteristic data of the phase angle α and the power P are stored and one of them is selected, but since the relationship between the phase angle α and the power P is uniquely determined, the phase angle α and A relationship with the power P may be obtained.

If the electric power P supplied to the fixing heater 305 is k (> 0) times the ideal electric power Pnomi, the phase angle α and the electric power P satisfy the following expression.
P = {(kPnomi) · (π−α + (sin2α) / 2)} / π

“K” represents the ratio of increase / decrease in the power P to the ideal power Pnomi. If the phase angle α is “α0” and the power P is “P0”, “k” can be expressed by the following equation.
k = P0π / {Pnomi · (π−α0 + (sin2α) / 2)}

  As described above, the characteristics of the phase angle α and the power P supplied to the fixing device 40 are expressed. In the second embodiment, the power P supplied to the fixing device 40 is limited according to the state of the image forming apparatus. The temperature control until the fixing film 301 is brought into a steady state requires more time than other components of the image forming apparatus. In order to shorten this time, it is effective to reduce the heat capacity of the fixing device 40 and improve the output of the fixing heater 305. In this embodiment, the output of the fixing heater 305 is improved. From such a viewpoint, the lower limit phase angle αL with respect to the maximum power Pmax is determined.

  FIG. 14 is a diagram illustrating the lower limit phase angle αL with respect to the maximum power Pmax. When the temperature of the fixing heater 305 is controlled, the control is performed in the range of the lower limit phase angle αL to the phase angle 180 °. Thereby, the electric power P can be limited below the maximum electric power Pmax. In the example of FIG. 14, the maximum power Pmax is 900 [W], and the lower limit phase angle αL for this is “68 °”.

  FIG. 15 is a flowchart showing the process for determining the lower limit phase angle αL.

  The control circuit 404 energizes the fixing heater 305 at the phase angle α0 (S501) and waits for a predetermined time (S502). After a predetermined time has elapsed, the control circuit 404 detects the temperature T0 of the fixing heater 305 from the detection result of the temperature detection element 405 (S503).

  The control circuit 404 converts the detected temperature T0 into electric power P0 according to the relationship shown in FIG. 12 (S504). The control circuit 404 selects the phase angle-power characteristic closest to the phase angle α0 and the power P0, for example, using FIG. 15 (S505). The control circuit 404 calculates a lower limit phase angle αL corresponding to the maximum power Pmax according to the selected phase angle-power characteristic. In the subsequent temperature control, the phase angle is performed in the range of “αL to 180 °”.

  The lower limit phase angle αL (upper limit power) determined in this manner gives the power supply voltage at that time, the resistance value of the fixing heater 305, and other minimum power for setting the fixing film 301 to a predetermined temperature. Subsequent temperature control is performed using the minimum power as the upper limit power and the lower limit phase angle αL. Similar to the first embodiment, during the temperature control of the fixing film 301, the phase angle α below the phase angle lower limit αL is corrected to the phase angle lower limit αL (S301 and S302 in FIG. 10).

  As in the first embodiment, the process of determining the lower limit phase angle (upper limit power) is effective when performed at a timing at which a parameter that greatly affects the temperature control of the fixing heater 305 is changed.

  DESCRIPTION OF SYMBOLS 40 ... Fixing device, 301 ... Fixing film, 302 ... Pressure member, 305 ... Fixing heater, 307 ... Lower roller, 405 ... Temperature detection element, 402 ... Bidirectional thyristor, 403 ... Gate, 406 ... AC power supply, S ... Paper

Claims (9)

A heater that heats the paper on which the toner image is transferred by supplying power from an AC power source;
Temperature detecting means for detecting the temperature of the heater;
Control means for controlling the energization time per predetermined period to the heater based on the temperature detected by the temperature detection means and the target temperature of the heater;
The control means gradually increases the energization time after starting energization of the alternating current to the heater in a predetermined energization time, and the temperature detected by the temperature detection means becomes the target temperature. A fixing device, wherein an upper limit of energization time to the heater is determined based on energization time.   The control means increases the energization time when the temperature detected by the temperature detection means after the elapse of a predetermined time after starting energization of the heater in the predetermined energization time does not reach the target temperature. The fixing device according to claim 1, wherein:   The fixing device according to claim 2, wherein the control unit increases the energization time every predetermined time until the temperature detected by the temperature detection unit reaches the target temperature. The control means controls the energization time in the AC half-wave period of the AC power supply,
The fixing device according to claim 1. The control means controls the energization time by adjusting the time from the timing when the polarity of the AC power supply changes to the start of energization of the AC current.
The fixing device according to claim 1.   The fixing unit according to claim 1, wherein the control unit determines, as the upper limit, an energization time when the temperature detected by the temperature detection unit reaches the target temperature. vessel.   The said control means determines the energization time which added predetermined margin time to the energization time when the temperature detected by the said temperature detection means becomes the said target temperature as said upper limit. The fixing device according to any one of 5. A heater that heats the paper on which the toner image is transferred by supplying power from an AC power source;
Temperature detecting means for detecting the temperature of the heater;
Control means for controlling the energization time per predetermined period to the heater based on the temperature detected by the temperature detection means and the target temperature of the heater;
Corresponding to the situation where power fluctuations of the AC power supply are different, storage means for storing a plurality of characteristic data indicating characteristics of energization time to the heater and power supplied to the heater;
Have
The control means is based on the temperature detected by the temperature detection means after a predetermined time has elapsed since the start of energization of the alternating current to the heater in a predetermined energization time and the characteristic data stored in the storage means. A fixing device, wherein an upper limit of energization time to the heater is determined based on selected characteristic data.   An image forming apparatus comprising the fixing device according to claim 1.

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