JP2007047473A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus in which temperature control can be exerted as intended even when the frequency of an AC power source is changed. <P>SOLUTION: The image forming apparatus includes: a zero-cross detecting means for comparing a voltage from a power source with a predetermined threshold, outputting a positive signal when the voltage is higher than the threshold, and outputting a negative voltage when the voltage is lower than the threshold; a pulse width detecting means for detecting pulse widths of the positive and negative pulse widths; and a heat fixing device that heats an image formed on a recording material, thereby fixing it to the recording material. The image forming apparatus exerts frequency control for the supply of power to the heat fixing device. The image forming apparatus controls the supply of power to the heat fixing device: at a second timing after a first predetermined time determined based on the positive pulse width is elapsed from a first timing at which the zero-cross signal is changed from the negative pulse width to the positive pulse width; and a fourth timing after a second predetermined time determined based on the negative pulse width has elapsed from a third timing at which the zero-cross signal is changed from the positive pulse width to the negative pulse width. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus capable of performing original temperature control without depending on the state of a commercial power source.

  Conventionally, in an image forming apparatus such as a copying machine, a printer, and a facsimile using an electrophotographic process, heating is performed to thermally fix an unfixed image carried on a recording material such as a transfer material or photosensitive paper by a transfer method or a direct method. As a device, a heat roller finger type heat fixing device using a halogen heater as a heat source and a film heating type heat fixing device using a ceramic heater as a heat source are known. In general, the heat fixing device is connected to an AC power source by meeting switching elements such as a triac, and is supplied with electric power by the AC power source. The heating device is provided with a temperature detection element such as a thermistor temperature sensing element. Based on the temperature detected by the temperature detection element, the image forming apparatus performs on / off control of the switching element to supply power to the heat fixing device. The supply is controlled so that the temperature of the thermal fixing device becomes an arbitrary target temperature.

As a representative example of the on / off control of the heat fixing element, there is a wave number control. The wave number control includes a point at which the power supply voltage of the AC power supply is switched from positive to negative or from negative to positive, and a signal that informs whether the power supply voltage is larger or smaller than a predetermined threshold (hereinafter referred to as zero crossing). Signal) and trigger wave number control. For example, in Japanese Patent Laid-Open No. 2001-052841, the cycle of the zero cross signal is measured, and the power supply to the thermal fixing device is controlled at the timing when the zero cross signal rises or falls, and when half the cycle has elapsed. Is going.
JP 2001-052841 A

  However, if the commercial power supply has a frequency fluctuation due to some cause, it may be different from the period recognized by the image forming apparatus, so that it may not be possible to perform the intended power supply.

  Therefore, in the first invention according to the present application, in wave number control, zero crossing is performed after T1 / n hours from the rising timing of the zero cross signal based on the positive pulse width T1 of the zero cross signal and the negative pulse width T2 of the zero cross signal. After T2 / n time from the signal falling timing, power control to the thermal fixing device is performed.

  In the second invention, in the first invention, the constant n for determining the power supply timing to the heat fixing device is 1/4 <n <3/4.

  As described above, according to the present invention, it is possible to provide an image forming apparatus that can perform original temperature control even if the frequency fluctuates greatly. Furthermore, when the heater drive timing is set to T1 × n hours after the rising timing of the zero cross signal and T2 × n hours after the falling timing of the zero cross signal, 1/4 <n <3/4 is set. The original temperature control can be performed without depending on the slice voltage level of the zero cross detection circuit.

Example 1
FIG. 1 shows the configuration of an image forming apparatus used in Embodiment 1 of the present invention. The surface of the photosensitive drum 101 is uniformly charged to a certain polarity by the charging roller 102, and then only the area under the exposure of the photosensitive drum 101 is neutralized by an exposure unit 103 such as a laser to form a latent image. The toner 105 of the developing device 104 is frictionally charged with the same polarity as the charging surface of the photosensitive drum 101 between the developing blade 104a and the developing sleeve 104b. Then, by applying a DC and AC bias to the developing gap portion where the photosensitive drum 101 and the developing sleeve 104b face each other, the toner 105 is selectively applied to the latent image forming portion of the photosensitive drum 101 while floating and vibrating by the action of electrolysis. After adhering, it is conveyed by the rotation of the transfer roller 106 and the photosensitive drum 101.

  On the other hand, the paper is set in the paper feed cassette 107, conveyed by the paper feed roller 108, and a high voltage having a polarity opposite to that of the toner 105 is applied by the transfer roller 106 on the back of the paper. The paper to which the high voltage is applied electrostatically attracts the toner 105 to the paper side and transfers the image. Further, the reverse side of the sheet is applied with a polarity opposite to that of the toner 105, and a transfer charge is applied to keep the transferred toner 105 held. The sheet on which the image is transferred is heated and pressed by the fixing device 109 to fix the image, and is discharged by the FD roller 110.

  FIG. 2 shows a configuration of a driving circuit of the fixing device 109 and a zero cross detection circuit. In FIG. 2, reference numeral 201 denotes an AC power supply supplied to the image forming apparatus. The image forming apparatus controls the power of the heater 204 via the triac 202 and the relay 203 to cause the heater 204 to generate heat. The resistors 205 and 206 are triac bypass resistors. The phototriac coupler 207 is a device for securing primary and secondary creepage distances, and the triac 202 is turned on by energizing the light emitting diode of the phototriac coupler 207. The resistor 208 is a resistor for limiting the current to the phototriac coupler 207, and the current to the phototriac coupler 207 is on / off controlled by the transistor 209. The phototriac coupler 207 has a zero-cross detection function. When the phototriac coupler 207 is on at the zero-cross point of the applied AC power supply, a half-wave after the zero-cross point is energized. The subsequent half wave is cut off. The transistor 209 operates according to a heater drive signal from the CPU 210. The heater driving signal generates a driving pattern by the CPU 210 based on the detected temperature detected by the thermistor 211. The relay 203 is ON / OFF controlled by the transistor 212. The transistor 212 operates in accordance with a relay drive signal from the CPU 210.

Next, the zero cross detection circuit will be described. The diode bridge 213 performs half-wave rectification of the AC power supply 201, and the half-wave rectified voltage is divided by resistors 214 and 215 and applied to the light emitting side of the photocoupler 216. When the applied voltage reaches the slice voltage Vf corresponding to the operating voltage of the light emitting diode in the photocoupler 216, the photocoupler 216 emits light, the light receiving side transistor is turned on, and the zero cross signal is negative. Become. When the voltage applied to the photocoupler 216 is equal to or lower than the slice voltage Vf, the light emitting diode in the photocoupler 216 is turned off, the light receiving side transistor is turned off, and the zero cross signal becomes positive.
FIG. 3 is a timing chart showing signal changes when energization control of the heater 204 is performed. In FIG. 3, the time of the positive pulse of the zero cross signal is T301, and the time of the negative pulse is T311. The timing for determining whether the heater 204 is on or off is the timing at which T321 has elapsed from the rising edge of the zero cross signal and the timing at which T331 has elapsed from the falling edge of the zero cross signal. It is assumed that T321 and T331 are n times T301 and T311, respectively.

  The above control is performed by an external interrupt that generates an interrupt when the zero-cross signal rises and falls, a measurement timer that generates an interrupt at a fixed period, a heater drive signal output start timing, and a heater drive signal generation timer. . FIG. 4 is a flowchart showing processing when an external interrupt occurs. When an external interrupt occurs, it is determined whether the rising edge detection flag is on (T401). This flag is inverted when a rising edge is detected when the rising edge detection flag is on, or when a falling edge is detected when the rising edge flag is off, and the initial value is on.

  If the rising edge detection flag is ON, it is determined whether the current zero cross signal is positive (T402). If the zero cross signal is negative, the process is interrupted and waits until an interrupt occurs again. If the zero cross signal is positive, the heater drive signal generation timer is turned on (T403). The set time of the timer at this time is a product of a value of a positive pulse width measurement counter described later and an arbitrary constant n. Then, the positive pulse width measurement flag is turned on (T404), the rising edge detection flag is turned off (T405), the pulse width measurement completion flag is turned on (T406), and the process ends.

  If the rising edge detection flag is off, it is determined whether the current zero cross signal is negative (T407). If the zero cross signal is positive, the process is interrupted and waits until an interrupt occurs again. If the zero cross signal is negative, the heater drive signal generation timer is turned on (T408). The set time of the timer at this time is a product of a negative pulse width measurement counter value to be described later and an arbitrary constant n. Then, the positive pulse width measurement flag is turned off (T409), the rising edge detection flag is turned on (T410), the pulse width measurement completion flag is turned on (T411), and the process ends.

  FIG. 5 is a flowchart showing the processing of the measurement timer. When an interrupt is generated by the measurement timer, the state of the positive pulse width measurement flag is determined (T501).

  When the positive pulse width measurement flag is ON, the positive pulse width measurement counter is incremented (T502). If the measurement completion flag is on (T503), the negative pulse width measurement counter is buffered (T504), and the measurement completion flag is turned off (T505).

  If the positive pulse width measurement flag is OFF, the negative pulse width measurement counter is incremented (T506). If the measurement completion flag is on, the positive pulse width measurement counter is buffered (T507), and the measurement completion flag is turned off (T509).

  FIG. 6 is a flowchart showing the processing of the heater drive signal generation timer. When an interrupt is generated by the heater drive signal generation timer, it is determined whether to turn on or off the next half wave (T601). When the next half wave is turned on, the heater drive signal is turned on (T602), and when the next half wave is turned off, the heater drive signal is turned off (T603).

  By using the control method described above, the time from the output start timing of the heater drive signal to the next zero cross point of the AC power supply can be arbitrarily set. For example, the heater drive signal output start timing is the timing when half of the positive pulse width T301 has elapsed since the rise of the zero cross signal and the timing when half of the negative pulse width T311 has elapsed since the falling of the zero cross signal. In this case, the margin to the zero cross point can be obtained as a margin for a quarter cycle of the current AC power supply. Even if the frequency of the AC power supply fluctuates, the heater can be driven as originally intended if the fluctuation amount of the period is 1/4 or less of the original period.

(Example 2)
In the zero cross detection circuit used in the first invention, the zero cross signal becomes positive when the input AC power source reaches a certain slice voltage Vf, and the zero cross signal becomes negative when the slice voltage Vf or less. Become. However, the slice voltage Vf varies depending on the circuit configuration of the zero cross detection circuit, and is not necessarily constant due to an error inherent in the component. Therefore, in the second invention according to the present application, the constant n for determining the heater drive timing in the first invention is set to 1/4 <n <3/4, so that the first invention does not depend on the slice voltage Vf. Control in the first invention is performed.

  A second invention according to the present application will be described with reference to FIG. The AC power supply 201 applied to the image forming apparatus has a peak voltage of V701 and a cycle of T701, and the time from the timing at which the AC power supply becomes the slice voltage Vf to the zero cross point of the AC power supply 201 is T702. . In order to drive the heater as originally intended, the heater driving start timing T703 must be later than the zero cross point T704 of the AC power supply 201 and earlier than the zero cross point T705 of the AC power supply. Similarly, the heater driving start timing T706 must be later than the zero cross point T705 of the AC power supply 201 and earlier than the zero cross point T707 of the AC power supply. Therefore, from the above conditions, the following expression is established.

  In the above equations, T321 and T331 are n times T301 and T302, respectively, so Equations 710, 711, and 712 are as follows.

  T301 and T311 have the following relationship with respect to the period T701 of the AC power supply 201 and the time T702 from the rising timing of the zero cross detection signal to the zero cross point of the AC power supply.

  Therefore, Expression 713, Expression 714, and Expression 715 are as follows based on Expression 716 and Expression 717.

  The above equation can be summarized as follows for n.

  From Equation 721, Equation 722, and Equation 723, an arbitrary constant n satisfies the following conditions.

  Here, since the zero-crossing detection circuit rises until the voltage of the AC power supply becomes V701, the following condition is satisfied.

  Therefore, the following condition is satisfied when Expression 724 is taken as the limit value using Expression 725.

Image forming apparatus used in embodiment 1 Heater drive circuit and zero-cross detection circuit used in Example 1 Timing chart of heater drive signal used in Example 1 Flow chart used in Example 1 Flow chart used in Example 1 Flow chart used in Example 1 Timing chart used in Example 2

Explanation of symbols

T301 Positive pulse width of zero-cross signal T311 Negative pulse width of zero-cross signal T321 Time from rising of zero-cross signal to heater driving timing T331 Time from falling of zero-crossing signal to heater driving timing

Claims (2)

  Zero cross detection means for comparing a voltage from an AC power source with a predetermined threshold value and outputting a positive signal when the voltage is larger than the threshold value and a negative signal when the voltage is smaller, and the positive and negative signals A pulse width detecting means for detecting the pulse width of the recording medium, a heat fixing device for heating and fixing the image formed on the recording material, and an image for performing power supply to the heat fixing device by wave number control. In the forming apparatus, a second timing after a first predetermined time determined based on a positive pulse width from a first timing at which the zero cross signal changes from negative to positive, and a third timing at which the zero cross signal changes from positive to negative. To control the power supplied to the thermal fixing device at a fourth timing after a second predetermined time determined based on the negative pulse width.   In the image forming apparatus, the first predetermined time is greater than 1/4 of the positive pulse width and less than 3/4, and the second predetermined time is greater than 1/4 of the negative pulse width and less than 3/4. The image forming apparatus according to claim 1.

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